Dr. Sanford Bernstein's Lab Personnel Home Page: Abstract Section

Aging Cell. Mar 20:e14134 (2024)

Aging-affiliated post-translational modifications of skeletal muscle myosin affect biochemical properties, myofibril structure, muscle function, and proteostasis.

Neal CL, Kronert WA, Camillo JRT, Suggs JA, Huxford T, Bernstein SI.

Abstract
The molecular motor myosin is post-translationally modified in its globular head, its S2 hinge, and its thick filament domain during human skeletal muscle aging. To determine the importance of such modifications, we performed an integrative analysis of transgenic Drosophila melanogaster expressing myosin containing post-translational modification mimic mutations. We determined effects on muscle function, myofibril structure, and myosin biochemistry. Modifications in the homozygous state decreased jump muscle function by a third at 3 weeks of age and reduced indirect flight muscle function to negligible levels in young flies, with severe effects on flight muscle myofibril assembly and/or maintenance. Expression of mimic mutations in the heterozygous state or in a wild-type background yielded significant, but less severe, age-dependent effects upon flight muscle structure and function. Modification of the residue in the globular head disabled ATPase activity and in vitro actin filament motility, whereas the S2 hinge mutation reduced actin-activated ATPase activity by 30%. The rod modification diminished filament formation in vitro. The latter mutation also reduced proteostasis, as demonstrated by enhanced accumulation of polyubiquitinated proteins. Overall, we find that mutation of amino acids at sites that are chemically modified during human skeletal muscle aging can disrupt myosin ATPase, myosin filament formation, and/or proteostasis, providing a mechanistic basis for the observed muscle defects. We conclude that age-specific post-translational modifications present in human skeletal muscle are likely to act in a dominant fashion to affect muscle structure and function and may therefore be implicated in degeneration and dysfunction associated with sarcopenia.

Biology (Basel). 2022 Jul 29;11(8):1137.

Myosin Transducer Inter-Strand Communication Is Critical for Normal ATPase Activity and Myofibril Structure.

Kronert WA, Hsu KH, Madan A, Sarsoza F, Cammarato A, Bernstein SI.

The R249Q mutation in human β-cardiac myosin results in hypertrophic cardiomyopathy. We previously showed that inserting this mutation into Drosophila melanogaster indirect flight muscle myosin yields mechanical and locomotory defects. Here, we use transgenic Drosophila mutants to demonstrate that residue R249 serves as a critical communication link within myosin that controls both ATPase activity and myofibril integrity. R249 is located on a β-strand of the central transducer of myosin, and our molecular modeling shows that it interacts via a salt bridge with D262 on the adjacent β-strand. We find that disrupting this interaction via R249Q, R249D or D262R mutations reduces basal and actin-activated ATPase activity, actin in vitro motility and flight muscle function. Further, the R249D mutation dramatically affects myofibril assembly, yielding abnormalities in sarcomere lengths, increased Z-line thickness and split myofibrils. These defects are exacerbated during aging. Re-establishing the β-strand interaction via a R249D/D262R double mutation restores both basal ATPase activity and myofibril assembly, indicating that these properties are dependent upon transducer inter-strand communication. Thus, the transducer plays an important role in myosin function and myofibril architecture.

International Journal of Molecular Sciences. 2022; 23(5):2533. https://doi.org/10.3390/ijms23052533

The R369 Myosin Residue within Loop 4 Is Critical for Actin Binding and Muscle Function in Drosophila.

Trujillo AS, Hsu KH, Viswanathan MC, Cammarato A, Bernstein SI.

The myosin molecular motor interacts with actin filaments in an ATP-dependent manner to yield muscle contraction. Myosin heavy chain residue R369 is located within loop 4 at the actin-tropomyosin interface of myosin's upper 50 kDa subdomain. To probe the importance of R369, we introduced a histidine mutation of that residue into Drosophila myosin and implemented an integrative approach to determine effects at the biochemical, cellular, and whole organism levels. Substituting the similarly charged but bulkier histidine residue reduces maximal actin binding in vitro without affecting myosin ATPase activity. R369H mutants exhibit impaired flight ability that is dominant in heterozygotes and progressive with age in homozygotes. Indirect flight muscle ultrastructure is normal in mutant homozygotes, suggesting that assembly defects or structural deterioration of myofibrils are not causative of reduced flight. Jump ability is also reduced in homozygotes. In contrast to these skeletal muscle defects, R369H mutants show normal heart ultrastructure and function, suggesting that this residue is differentially sensitive to perturbation in different myosin isoforms or muscle types. Overall, our findings indicate that R369 is an actin binding residue that is critical for myosin function in skeletal muscles, and suggest that more severe perturbations at this residue may cause human myopathies through a similar mechanism.

Mol Biol Cell. 2021 Aug 19;32(18):1690-1706.

Myosin dilated cardiomyopathy mutation S532P disrupts actomyosin interactions, leading to altered muscle kinetics, reduced locomotion, and cardiac dilation in Drosophila.

Adriana S. Trujillo, Karen H. Hsu, Joy Puthawala, Meera C. Viswanathan, Amy Loya, Thomas C. Irving, Anthony Cammarato, Douglas M. Swank, and Sanford I. Bernstein.

Dilated cardiomyopathy (DCM), a life-threatening disease characterized by pathological heart enlargement, can be caused by myosin mutations that reduce contractile function. To better define the mechanistic basis of this disease, we employed the powerful genetic and integrative approaches available in Drosophila melanogaster. To this end, we generated and analyzed the first fly model of human myosin-induced DCM. The model reproduces the S532P human β-cardiac myosin heavy chain DCM mutation, which is located within an actin binding region of the motor domain. In concordance with the mutation's location at the actomyosin interface, steady-state ATPase and muscle mechanics experiments revealed that the S532P mutation reduces the rates of actin-dependent ATPase activity and actin binding and increases the rate of actin detachment. The depressed function of this myosin form reduces the number of cross-bridges during active wing beating, the power output of indirect flight muscles, and flight ability. Further, S532P mutant hearts exhibit cardiac dilation that is mutant gene dose-dependent. Our study shows that Drosophila can faithfully model various aspects of human DCM phenotypes and suggests that impaired actomyosin interactions in S532P myosin induce contractile deficits that trigger the disease.

Biophys J. 2021 Mar 2;120(5):844-854.

Prolonged myosin binding increases muscle stiffness in Drosophila models of Freeman-Sheldon syndrome.

Kaylyn M Bell, Alice Huang, William A Kronert, Sanford I Bernstein, Douglas M Swank.

Freeman-Sheldon syndrome (FSS) is characterized by congenital contractures resulting from dominant point mutations in the embryonic isoform of muscle myosin. To investigate its disease mechanism, we used Drosophila models expressing FSS myosin mutations Y583S or T178I in their flight and jump muscles. We isolated these muscles from heterozygous mutant Drosophila and performed skinned fiber mechanics. The most striking mechanical alteration was an increase in active muscle stiffness. Y583S/+ and T178I/+ fibers' elastic moduli increased 70 and 77%, respectively. Increased stiffness contributed to decreased power generation, 49 and 66%, as a result of increased work absorbed during the lengthening portion of the contractile cycle. Slower muscle kinetics also contributed to the mutant phenotype, as shown by 17 and 32% decreases in optimal frequency for power generation, and 27 and 41% slower muscle apparent rate constant 2πb. Combined with previous measurements of slower in vitro actin motility, our results suggest a rate reduction of at least one strongly bound cross-bridge cycle transition that increases the time myosin spends strongly bound to actin, ton. Increased ton was further supported by decreased ATP affinity and a 16% slowing of jump muscle relaxation rate in T178I heterozygotes. Impaired muscle function caused diminished flight and jump ability of Y583S/+ and T178I/+ Drosophila. Based on our results, assuming that our model system mimics human skeletal muscle, we propose that one mechanism driving FSS is elevated muscle stiffness arising from prolonged ton in developing muscle fibers.

J Biol Chem. 2020 Oct 16;295(42):14522-14535.

Alternative N-terminal regions of Drosophila myosin heavy chain II regulate communication of the purine binding loop with the essential light chain.

Marieke J Bloemink, Karen H Hsu, Michael A Geeves, Sanford I Bernstein.

We investigated the biochemical and biophysical properties of one of the four alternative exon-encoded regions within the Drosophila myosin catalytic domain. This region is encoded by alternative exons 3a and 3b and includes part of the N-terminal β-barrel. Chimeric myosin constructs (IFI-3a and EMB-3b) were generated by exchanging the exon 3-encoded areas between native slow embryonic body wall (EMB) and fast indirect flight muscle myosin isoforms (IFI). We found that this exchange alters the kinetic properties of the myosin S1 head. The ADP release rate (k-D ) in the absence of actin is completely reversed for each chimera compared with the native isoforms. Steady-state data also suggest a reciprocal shift, with basal and actin-activated ATPase activity of IFI-3a showing reduced values compared with wild-type (WT) IFI, whereas for EMB-3b these values are increased compared with wild-type (WT) EMB. In the presence of actin, ADP affinity (KAD ) is unchanged for IFI-3a, compared with IFI, but ADP affinity for EMB-3b is increased, compared with EMB, and shifted toward IFI values. ATP-induced dissociation of acto-S1 (K1k +2 ) is reduced for both exon 3 chimeras. Homology modeling, combined with a recently reported crystal structure for Drosophila EMB, indicates that the exon 3-encoded region in the myosin head is part of the communication pathway between the nucleotide binding pocket (purine binding loop) and the essential light chain, emphasizing an important role for this variable N-terminal domain in regulating actomyosin crossbridge kinetics, in particular with respect to the force-sensing properties of myosin isoforms.

Skelet Muscle. 2020 Aug 15;10(1):24.

Drosophila myosin mutants model the disparate severity of type 1 and type 2B distal arthrogryposis and indicate an enhanced actin affinity mechanism.

Yiming Guo, William A Kronert, Karen H Hsu, Alice Huang, Floyd Sarsoza, Kaylyn M Bell, Jennifer A Suggs, Douglas M Swank, Sanford I Bernstein.

Background: Distal arthrogryposis (DA) is a group of autosomal dominant skeletal muscle diseases characterized by congenital contractures of distal limb joints. The most common cause of DA is a mutation of the embryonic myosin heavy chain gene, MYH3. Human phenotypes of DA are divided into the weakest form-DA1, a moderately severe form-DA2B (Sheldon-Hall Syndrome), and a severe DA disorder-DA2A (Freeman-Sheldon Syndrome). As models of DA1 and DA2B do not exist, their disease mechanisms are poorly understood.

Methods: We produced the first models of myosin-based DA1 (F437I) and DA2B (A234T) using transgenic Drosophila melanogaster and performed an integrative analysis of the effects of the mutations. Assessments included lifespan, locomotion, ultrastructural analysis, muscle mechanics, ATPase activity, in vitro motility, and protein modeling.

Results: We observed significant defects in DA1 and DA2B Drosophila flight and jump ability, as well as myofibril assembly and stability, with homozygotes displaying more severe phenotypes than heterozygotes. Notably, DA2B flies showed dramatically stronger phenotypic defects compared to DA1 flies, mirroring the human condition. Mechanical studies of indirect flight muscle fibers from DA1 heterozygotes revealed reduced power output along with increased stiffness and force production, compared to wild-type controls. Further, isolated DA1 myosin showed significantly reduced myosin ATPase activity and in vitro actin filament motility. These data in conjunction with our sinusoidal analysis of fibers suggest prolonged myosin binding to actin and a slowed step associated with Pi release and/or the power stroke. Our results are supported by molecular modeling studies, which indicate that the F437I and A234T mutations affect specific amino acid residue interactions within the myosin motor domain that may alter interaction with actin and nucleotide.

Conclusions: The allele-specific ultrastructural and locomotory defects in our Drosophila DA1 and DA2B models are concordant with the differential severity of the human diseases. Further, the mechanical and biochemical defects engendered by the DA1 mutation reveal that power production, fiber stiffness, and nucleotide handling are aberrant in F437I muscle and myosin. The defects observed in our DA1 and DA2B Drosophila models provide insight into DA phenotypes in humans, suggesting that contractures arise from prolonged actomyosin interactions.

J Mol Biol. 2020 Jan 17;432(2):427-447.

X-ray Crystallographic and Molecular Dynamic Analyses of Drosophila melanogaster Embryonic Muscle Myosin Define Domains Responsible for Isoform-Specific Properties.

Caldwell JT, Mermelstein DJ, Walker RC, Bernstein SI, Huxford T.

Drosophila melanogaster is a powerful system for characterizing alternative myosin isoforms and modeling muscle diseases, but high-resolution structures of fruit fly contractile proteins have not been determined. Here we report the first x-ray crystal structure of an insect myosin: the D melanogaster skeletal muscle myosin II embryonic isoform (EMB). Using our system for recombinant expression of myosin heavy chain (MHC) proteins in whole transgenic flies, we prepared and crystallized stable proteolytic S1-like fragments containing the entire EMB motor domain bound to an essential light chain. We solved the x-ray crystal structure by molecular replacement and refined the resulting model against diffraction data to 2.2 angstrom resolution. The protein is captured in two slightly different renditions of the rigor-like conformation with a citrate of crystallization at the nucleotide binding site and exhibits structural features common to myosins of diverse classes from all kingdoms of life. All atom molecular dynamics simulations on EMB in its nucleotide-free state and a derivative homology model containing 61 amino acid substitutions unique to the indirect flight muscle isoform (IFI) suggest that differences in the identity of residues within the relay and the converter that are encoded for by MHC alternative exons 9 and 11, respectively, directly contribute to increased mobility of these regions in IFI relative to EMB. This suggests the possibility that alternative folding or conformational stability within these regions contribute to the observed functional differences in Drosophila EMB and IFI myosins.

J Physiol. 2019 May;597(9):2403-2420.

The R249Q hypertrophic cardiomyopathy myosin mutation decreases contractility in Drosophila by impeding force production.

Bell KM, Kronert WA, Huang A1, Bernstein SI, Swank DM.

KEY POINTS:
Hypertrophic cardiomyopathy (HCM) is a genetic disease that causes thickening of the heart's ventricular walls and is a leading cause of sudden cardiac death. HCM is caused by missense mutations in muscle proteins including myosin, but how these mutations alter muscle mechanical performance in largely unknown. We investigated the disease mechanism for HCM myosin mutation R249Q by expressing it in the indirect flight muscle of Drosophila melanogaster and measuring alterations to muscle and flight performance. Muscle mechanical analysis revealed R249Q decreased muscle power production due to slower muscle kinetics and decreased force production; force production was reduced because fewer mutant myosin cross-bridges were bound simultaneously to actin. This work does not support the commonly proposed hypothesis that myosin HCM mutations increase muscle contractility, or causes a gain in function; instead, it suggests that for some myosin HCM mutations, hypertrophy is a compensation for decreased contractility.

ABSTRACT:
Hypertrophic cardiomyopathy (HCM) is an inherited disease that causes thickening of the heart's ventricular walls. A generally accepted hypothesis for this phenotype is that myosin heavy chain HCM mutations increase muscle contractility. To test this hypothesis, we expressed an HCM myosin mutation, R249Q, in Drosophila indirect flight muscle (IFM) and assessed myofibril structure, skinned fibre mechanical properties, and flight ability. Mechanics experiments were performed on fibres dissected from 2-h-old adult flies, prior to degradation of IFM myofilament structure, which started at 2 days old and increased with age. Homozygous and heterozygous R249Q fibres showed decreased maximum power generation by 67% and 44%, respectively. Decreases in force and work and slower overall muscle kinetics caused homozygous fibres to produce less power. While heterozygous fibres showed no overall slowing of muscle kinetics, active force and work production dropped by 68% and 47%, respectively, which hindered power production. The muscle apparent rate constant 2πb decreased 33% for homozygous but increased for heterozygous fibres. The apparent rate constant 2πc was greater for homozygous fibres. This indicates that R249Q myosin is slowing attachment while speeding up detachment from actin, resulting in less time bound. Decreased IFM power output caused 43% and 33% decreases in Drosophila flight ability and 19% and 6% drops in wing beat frequency for homozygous and heterozygous flies, respectively. Overall, our results do not support the increased contractility hypothesis. Instead, our results suggest the ventricular hypertrophy for human R249Q mutation is a compensatory response to decreases in heart muscle power output.

J Cancer Educ. 2019 Nov 14.

Educating the Next Generation of Undergraduate URM Cancer Scientists: Results and Lessons Learned from a Cancer Research Partnership Scholar Program.

Gaida E, Barrios AJ, Wolkowicz R, Crowe SE, Bernstein SI, Quintana Serrano MA, Dumbauld JN, Pakiz B, Cripps RM, Arredondo EM, Martinez ME, Madanat H.

To improve cancer disparities among under-represented minority (URM) populations, better representation of URM individuals in cancer research is needed. The San Diego State University and University of California San Diego Moores Cancer Center Partnership is addressing cancer disparities through an educational program targeting undergraduate URM students. The Partnership provides a paid intensive summer research internship enriched with year-round activities that include educational sessions, a journal club, mentorship, social activities, and poster sessions and presentations. Program evaluation through follow-up surveys, focus groups, and other formal and informal feedback, including advisory and program steering committees, are used to improve the program. Long-term follow-up among scholars (minimum of 10 years) provides data to evaluate the program's long-term impact on scholars' education and career path. Since 2016, 63 URM undergraduate students participated in the scholar program. At the year-2 follow-up (2016 cohort; n = 12), 50% had completed their Graduate Record Examination (GRE) and/or applied to graduate or medical school. Lessons learned during the course of the program led to implementation of changes to provide a better learning experience and increase overall program satisfaction. Updates were made to recruitment timeline, improvements of the recruitment processes, refinement of the program contracts and onboarding meetings, identification of essential program coordinator skills and responsibilities, adjustments to program components, and establishment of a well-mapped and scheduled evaluation plan. The Partnership identified best practices and lessons learned for implementing lab-based internship scholar programs in biomedical and public health fields that could be considered in other programs.

Mol Biol Cell. 2019 Jan 1; 30(1): 30-41.

Reductions in ATPase activity, actin sliding velocity and myofibril stability yield muscle dysfunction in Drosophila models of myosin-based Freeman Sheldon syndrome.

Rao DS, Kronert WA, Guo Y, Hsu KH, Sarsoza F, Bernstein SI

Using Drosophila melanogaster we created the first animal models for myosin-based Freeman Sheldon Syndrome, a dominant form of distal arthrogryposis defined by congenital facial and distal skeletal muscle contractures. Electron microscopy of homozygous mutant indirect flight muscles showed normal (Y583S) or altered (T178I, R672C) myofibril assembly, followed by progressive disruption of the myofilament lattice. In contrast, all alleles permitted normal myofibril assembly in the heterozygous state, but caused myofibrillar disruption during aging. The severity of myofibril defects in heterozygotes correlated with the level of flight impairment. Thus our Drosophila models mimic the human condition, in that Freeman Sheldon Syndrome mutations are dominant and display varied degrees of phenotypic severity. Molecular modeling indicates that the mutations disrupt communication between the nucleotide binding site of myosin and its lever arm that drives force production. Each mutant myosin showed reduced in vitro actin sliding velocity, with the two more severe alleles significantly decreasing the catalytic efficiency of actin-activated ATP hydrolysis. The observed reductions in actin motility and catalytic efficiency may serve as the mechanistic basis of the progressive myofibrillar disarray observed in the Drosophila models as well as the prolonged contractile activity responsible for skeletal muscle contractures in Freeman Sheldon Syndrome patients.

Hum Mol Genet. 2019 Feb 1;28(3):351-371.

Suppression of myopathic lamin mutations by muscle-specific activation of AMPK and modulation of downstream signaling.

Chandran S, Suggs JA, Wang BJ, Han A, Bhide S, Cryderman DE, Moore SA, Bernstein SI, Wallrath LL, Melkani GC.

Laminopathies are diseases caused by dominant mutations in the human LMNA gene encoding A-type lamins. Lamins are intermediate filaments that line the inner nuclear membrane, provide structural support for the nucleus, and regulate gene expression. Drosophila melanogaster models of skeletal muscle laminopathies were developed to investigate the pathological defects caused by mutant lamins and identify potential therapeutic targets. Human disease-causing LMNA mutations were modeled in Drosophila Lamin C (LamC) and expressed in indirect flight muscle (IFM). IFM-specific expression of mutant, but not wild-type LamC, caused held-up wings indicative of myofibrillar defects. Analyses of the muscles revealed cytoplasmic aggregates of nuclear envelope (NE) proteins, nuclear and mitochondrial dysmorphology, myofibrillar disorganization, and up-regulation of the autophagy cargo receptor p62. We hypothesized that the cytoplasmic aggregates of NE proteins trigger signaling pathways that alter cellular homeostasis, causing muscle dysfunction. In support of this hypothesis, transcriptomics data from human muscle biopsy tissue revealed misregulation of the AMPK/4E-BP1/autophagy/proteostatic pathways. S6K mRNA levels were increased and AMPKα and mRNAs encoding downstream targets were decreased in muscles expressing mutant LMNA relative controls. The Drosophila laminopathy models were used to determine if altering the levels of these factors modulated muscle pathology. Muscle-specific over-expression of AMPKα and down-stream targets 4E-BP, Foxo and PGC1α, as well as inhibition of S6K, suppressed the held-up wing phenotype, myofibrillar defects, and LamC aggregation. These findings provide novel insights on mutant LMNA-based disease mechanisms and identify potential targets for drug therapy.

Elife. 2018 Aug 13;7

Prolonged cross-bridge binding triggers muscle dysfunction in a Drosophila model of myosin-based hypertrophic cardiomyopathy.

Kronert WA, Bell KM, Viswanathan MC, Melkani GC, Trujillo AS, Huang A, Melkani A, Cammarato A, Swank DM, Bernstein SI.

K146N is a dominant mutation in human β-cardiac myosin heavy chain, which causes hypertrophic cardiomyopathy. We examined how Drosophila muscle responds to this mutation and integratively analyzed the biochemical, physiological and mechanical foundations of the disease. ATPase assays, actin motility, and indirect flight muscle mechanics suggest at least two rate constants of the cross-bridge cycle are altered by the mutation: increased myosin attachment to actin and decreased detachment, yielding prolonged binding. This increases isometric force generation, but also resistive force and work absorption during cyclical contractions, resulting in decreased work, power output, flight ability and degeneration of flight muscle sarcomere morphology. Consistent with prolonged cross-bridge binding serving as the mechanistic basis of the disease and with human phenotypes, 146N/+ hearts are hypercontractile with increased tension generation periods, decreased diastolic/systolic diameters and myofibrillar disarray. This suggests that screening mutated Drosophila hearts could rapidly identify hypertrophic cardiomyopathy alleles and treatments.

Proc. Natl. Acad. Sci. U.S.A. Feb 27;115(9):E1991-E2000 (2018)

Interacting-heads motif has been conserved as a mechanism of myosin II inhibition since before the origin of animals.

Lee KH, Sulbarán G, Yang S, Mun JY, Alamo L, Pinto A, Sato O, Ikebe M, Liu X, Korn ED, Sarsoza F, Bernstein SI, Padrón R, Craig R.

Electron microscope studies have shown that the switched-off state of myosin II in muscle involves intramolecular interaction between the two heads of myosin and between one head and the tail. The interaction, seen in both myosin filaments and isolated molecules, inhibits activity by blocking actin-binding and ATPase sites on myosin. This interacting-heads motif is highly conserved, occurring in invertebrates and vertebrates, in striated, smooth, and nonmuscle myosin IIs, and in myosins regulated by both Ca2+ binding and regulatory light-chain phosphorylation. Our goal was to determine how early this motif arose by studying the structure of inhibited myosin II molecules from primitive animals and from earlier, unicellular species that predate animals. Myosin II from Cnidaria (sea anemones, jellyfish), the most primitive animals with muscles, and Porifera (sponges), the most primitive of all animals (lacking muscle tissue) showed the same interacting-heads structure as myosins from higher animals, confirming the early origin of the motif. The social amoeba Dictyostelium discoideum showed a similar, but modified, version of the motif, while the amoeba Acanthamoeba castellanii and fission yeast (Schizosaccharomyces pombe) showed no head-head interaction, consistent with the different sequences and regulatory mechanisms of these myosins compared with animal myosin IIs. Our results suggest that head-head/head-tail interactions have been conserved, with slight modifications, as a mechanism for regulating myosin II activity from the emergence of the first animals and before. The early origins of these interactions highlight their importance in generating the inhibited (relaxed) state of myosin in muscle and nonmuscle cells.

Hum Mol Genet. Dec 15;26(24):4799-4813 (2017).

Myosin storage myopathy mutations yield defective myosin filament assembly in vitro and disrupted myofibrillar structure and function in vivo.

Viswanathan MC, Tham RC, Kronert WA, Sarsoza F, Trujillo AS, Cammarato A, Bernstein SI.

Myosin storage myopathy (MSM) is a congenital skeletal muscle disorder caused by missense mutations in the β-cardiac/slow skeletal muscle myosin heavy chain rod. It is characterized by subsarcolemmal accumulations of myosin that have a hyaline appearance. MSM mutations map near or within the assembly competence domain known to be crucial for thick filament formation. Drosophila MSM models were generated for comprehensive physiological, structural, and biochemical assessment of the mutations' consequences on muscle and myosin structure and function. L1793P, R1845W, and E1883K MSM mutant myosins were expressed in an indirect flight (IFM) and jump muscle myosin null background to study the effects of these variants without confounding influences from wild-type myosin. Mutant animals displayed highly compromised jump and flight ability, disrupted muscle proteostasis, and severely perturbed IFM structure. Electron microscopy revealed myofibrillar disarray and degeneration with hyaline-like inclusions. In vitro assembly assays demonstrated a decreased ability of mutant myosin to polymerize, with L1793P filaments exhibiting shorter lengths. In addition, limited proteolysis experiments showed a reduced stability of L1793P and E1883K filaments. We conclude that the disrupted hydropathy or charge of residues in the heptad repeat of the mutant myosin rods likely alters interactions that stabilize coiled-coil dimers and thick filaments, causing disruption in ordered myofibrillogenesis and/or myofibrillar integrity, and the consequent myosin aggregation. Our Drosophila models are the first to recapitulate the human MSM phenotype with ultrastructural inclusions, suggesting that the diminished ability of the mutant myosin to form stable thick filaments contributes to the dystrophic phenotype observed in afflicted subjects.

FEBS Lett. Nov;591(21):3447-3458. (2017).

TRiC/CCT chaperonins are essential for maintaining myofibril organization, cardiac physiological rhythm, and lifespan.

Melkani GC, Bhide S, Han A, Vyas J, Livelo C, Bodmer R, Bernstein SI.

We recently reported that CCT chaperonin subunits are upregulated in a cardiac-specific manner under time-restricted feeding (TRF) [Gill S et al. (2015) Science 347, 1265-1269], suggesting that TRiC/CCT has a heart-specific function. To understand the CCT chaperonin function in cardiomyocytes, we performed its cardiac-specific knock-down in the Drosophila melanogaster model. This resulted in disorganization of cardiac actin- and myosin-containing myofibrils and severe physiological dysfunction, including restricted heart diameters, elevated cardiac dysrhythmia and compromised cardiac performance. We also noted that cardiac-specific knock-down of CCT chaperonin significantly shortens lifespans. Additionally, disruption of circadian rhythm yields further deterioration of cardiac function of hypomorphic CCT mutants. Our analysis reveals that both the orchestration of protein folding and circadian rhythms mediated by CCT chaperonin are critical for maintaining heart contractility.

Dis Model Mech. Jun 1;10(6):761-771. (2017).

A Drosophila model of dominant inclusion body myopathy type 3 shows diminished myosin kinetics that reduce muscle power and yield myofibrillar defects.

Suggs JA, Melkani GC, Glasheen BM, Detor MM, Melkani A, Marsan NP, Swank DM, Bernstein SI.

Individuals with inclusion body myopathy type 3 (IBM3) display congenital joint contractures with early-onset muscle weakness that becomes more severe in adulthood. The disease arises from an autosomal dominant point mutation causing an E706K substitution in myosin heavy chain type IIa. We have previously expressed the corresponding myosin mutation (E701K) in homozygous Drosophila indirect flight muscles and recapitulated the myofibrillar degeneration and inclusion bodies observed in the human disease. We have also found that purified E701K myosin has dramatically reduced actin-sliding velocity and ATPase levels. Since IBM3 is a dominant condition, we now examine the disease state in heterozygote Drosophila in order to gain a mechanistic understanding of E701K pathogenicity. Myosin ATPase activities in heterozygotes suggest that approximately equimolar levels of myosin accumulate from each allele. In vitro actin sliding velocity rates for myosin isolated from the heterozygotes were lower than the control, but higher than for the pure mutant isoform. Although sarcomeric ultrastructure was nearly wild type in young adults, mechanical analysis of skinned indirect flight muscle fibers revealed a 59% decrease in maximum oscillatory power generation and an approximately 20% reduction in the frequency at which maximum power was produced. Rate constant analyses suggest a decrease in the rate of myosin attachment to actin, with myosin spending decreased time in the strongly bound state. These mechanical alterations result in a one-third decrease in wing beat frequency and marginal flight ability. With aging, muscle ultrastructure and function progressively declined. Aged myofibrils showed Z-line streaming, consistent with the human heterozygote phenotype. Based upon the mechanical studies, we hypothesize that the mutation decreases the probability of the power stroke occurring and/or alters the degree of movement of the myosin lever arm, resulting in decreased in vitro motility, reduced muscle power output and focal myofibrillar disorganization similar to that seen in individuals with IBM3.

Aging Cell. Feb;16(1):82-92. (2017).

Expression patterns of cardiac aging in Drosophila.

Cannon L, Zambon AC, Cammarato A, Zhang Z, Vogler G, Munoz M, Taylor E, Cartry J, Bernstein SI, Melov S, Bodmer R.

Aging causes cardiac dysfunction, often leading to heart failure and death. The molecular basis of age-associated changes in cardiac structure and function is largely unknown. The fruit fly, Drosophila melanogaster, is well-suited to investigate the genetics of cardiac aging. Flies age rapidly over the course of weeks, benefit from many tools to easily manipulate their genome, and their heart has significant genetic and phenotypic similarities to the human heart. Here, we performed a cardiac-specific gene expression study on aging Drosophila and carried out a comparative meta-analysis with published rodent data. Pathway level transcriptome comparisons suggest that age-related, extra-cellular matrix remodeling and alterations in mitochondrial metabolism, protein handling, and contractile functions are conserved between Drosophila and rodent hearts. However, expression of only a few individual genes similarly changed over time between and even within species. We also examined gene expression in single fly hearts and found significant variability as has been reported in rodents. We propose that individuals may arrive at similar cardiac aging phenotypes via dissimilar transcriptional changes, including those in transcription factors and micro-RNAs. Finally, our data suggest the transcription factor Odd-skipped, which is essential for normal heart development, is also a crucial regulator of cardiac aging.

Cardiovasc Res.110(2):238-48 (2016).

Profilin modulates sarcomeric organization and mediates cardiomyocyte hypertrophy.

Kooij V, Viswanathan MC, Lee DI2, Rainer PP, Schmidt W, Kronert WA, Harding SE, Kass DA, Bernstein SI, Van Eyk JE, Cammarato A.

AIMS:
Heart failure is often preceded by cardiac hypertrophy, which is characterized by increased cell size, altered protein abundance, and actin cytoskeletal reorganization. Profilin is a well-conserved, ubiquitously expressed, multifunctional actin-binding protein, and its role in cardiomyocytes is largely unknown. Given its involvement in vascular hypertrophy, we aimed to test the hypothesis that profilin-1 is a key mediator of cardiomyocyte-specific hypertrophic remodelling.

METHODS AND RESULTS:
Profilin-1 was elevated in multiple mouse models of hypertrophy, and a cardiomyocyte-specific increase of profilin in Drosophila resulted in significantly larger heart tube dimensions. Moreover, adenovirus-mediated overexpression of profilin-1 in neonatal rat ventricular myocytes (NRVMs) induced a hypertrophic response, measured by increased myocyte size and gene expression. Profilin-1 silencing suppressed the response in NRVMs stimulated with phenylephrine or endothelin-1. Mechanistically, we found that profilin-1 regulates hypertrophy, in part, through activation of the ERK1/2 signalling cascade. Confocal microscopy showed that profilin localized to the Z-line of Drosophila myofibrils under normal conditions and accumulated near the M-line when overexpressed. Elevated profilin levels resulted in elongated sarcomeres, myofibrillar disorganization, and sarcomeric disarray, which correlated with impaired muscle function.

CONCLUSION:
Our results identify novel roles for profilin as an important mediator of cardiomyocyte hypertrophy. We show that overexpression of profilin is sufficient to induce cardiomyocyte hypertrophy and sarcomeric remodelling, and silencing of profilin attenuates the hypertrophic response.

J Mol Biol. 428(11):2446-61 (2016).

A Restrictive Cardiomyopathy Mutation in an Invariant Proline at the Myosin Head/Rod Junction Enhances Head Flexibility and Function, Yielding Muscle Defects in Drosophila.

Achal M, Trujillo AS, Melkani GC, Farman GP, Ocorr K, Viswanathan MC, Kaushik G, Newhard CS, Glasheen BM, Melkani A1, Suggs JA, Moore JR, Swank DM, Bodmer R, Cammarato A, Bernstein SI.

An "invariant proline" separates the myosin S1 head from its S2 tail and is proposed to be critical for orienting S1 during its interaction with actin, a process that leads to muscle contraction. Mutation of the invariant proline to leucine (P838L) caused dominant restrictive cardiomyopathy in a pediatric patient (Karam et al., Congenit. Heart Dis. 3:138-43, 2008). Here, we use Drosophila melanogaster to model this mutation and dissect its effects on the biochemical and biophysical properties of myosin, as well as on the structure and physiology of skeletal and cardiac muscles. P838L mutant myosin isolated from indirect flight muscles of transgenic Drosophila showed elevated ATPase and actin sliding velocity in vitro. Furthermore, the mutant heads exhibited increased rotational flexibility, and there was an increase in the average angle between the two heads. Indirect flight muscle myofibril assembly was minimally affected in mutant homozygotes, and isolated fibers displayed normal mechanical properties. However, myofibrils degraded during aging, correlating with reduced flight abilities. In contrast, hearts from homozygotes and heterozygotes showed normal morphology, myofibrillar arrays, and contractile parameters. When P838L was placed in trans to Mhc(5), an allele known to cause cardiac restriction in flies, it did not yield the constricted phenotype. Overall, our studies suggest that increased rotational flexibility of myosin S1 enhances myosin ATPase and actin sliding. Moreover, instability of P838L myofibrils leads to decreased function during aging of Drosophila skeletal muscle, but not cardiac muscle, despite the strong evolutionary conservation of the P838 residue.

J Biol Chem. 290: 29270–29280 (2015).

A Failure to Communicate MYOSIN RESIDUES INVOLVED IN HYPERTROPHIC CARDIOMYOPATHY AFFECT INTERDOMAIN INTERACTION.

Kronert WA, Melkani GC, Melkani A, Bernstein SI.

Our molecular modeling studies suggest a charge-dependent interaction between residues Glu-497 in the relay domain and Arg-712 in the converter domain of human β-cardiac myosin. To test the significance of this putative interaction, we generated transgenic Drosophila expressing indirect flight muscle myosin with charge reversal mutations in the relay (E496R) or converter (R713E). Each mutation yielded dramatic reductions in myosin Ca-ATPase activity (∼80%) as well as in basal (∼67%) and actin-activated (∼84%) Mg-ATPase activity. E496R myosin-induced in vitro actin-sliding velocity was reduced by 71% and R713E myosin permitted no actin motility. Indirect flight muscles of late pupae from each mutant displayed disrupted myofibril assembly, with adults having severely abnormal myofibrils and no flight ability. To understand the molecular basis of these defects, we constructed a putative compensatory mutant that expresses myosin with both E496R and R713E. Intriguingly, ATPase values were restored to ∼73% of wild-type and actin-sliding velocity increased to 40%. The double mutation suppresses myofibril assembly defects in pupal indirect flight muscles and dramatically reduces myofibril disruption in young adults. Although sarcomere organization is not sustained in older flies and flight ability is not restored in homozygotes, young heterozygotes fly well. Our results indicate that this charge-dependent interaction between the myosin relay and converter domains is essential to the mechanochemical cycle and sarcomere assembly. Furthermore, the same inter-domain interaction is disrupted when modeling human β-cardiac myosin heavy chain cardiomyopathy mutations E497D or R712L, implying that abolishing this salt bridge is one cause of the human disease.

J. Biol. Chem.. 291:1763-1773 (2016).

The relay-converter interface influences hydrolysis of ATP by skeletal muscle myosin II.

Bloemink MJ, Melkani GC, Bernstein SI, Geeves MA.

The interface between relay and converter domain of muscle myosin is critical for optimal myosin performance. Using Drosophila melanogaster indirect flight muscle S1 we performed a kinetic analysis of the effect of mutations in the converter and relay domain. Introduction of a mutation (R759E) in the converter domain inhibits the steady-state ATPase of myosin S1, whereas an additional mutation in the relay domain (N509K) is able to restore the ATPase towards wild-type values. The S1- R759E construct showed little effect on most steps of the actomyosin ATPase cycle. The exception was a 25-30% reduction in the rate constant of the hydrolysis step, the step coupled to the cross-bridge recovery stroke and involving a change in conformation at the relay/converter domain interface. Significantly the double mutant restored the hydrolysis step to values similar to the wild-type myosin. Modelling the relay/converter interface suggests a possible interaction between converter residue 759 and relay residue 509 in the actin-detached conformation, which is lost in R759E but is restored in N509K/R759E. This detailed kinetic analysis of Drosophila myosin carrying the R759E mutation shows that the interface between the relay loop and converter domain is important for fine-tuning myosin kinetics, in particular ATP-binding and hydrolysis.

Sci Transl Med. 2015 Jun 17;7(292):292ra99

Vinculin network–mediated cytoskeletal remodeling regulates contractile function in the aging heart.

Kaushik G, Spenlehauer A, Sessions AO, Trujillo AS, Fuhrmann A, Fu Z, Venkatraman V, Pohl D, Tuler J, Wang M, Lakatta EG, Ocorr K, Bodmer R, Bernstein SI, Van Eyk JE, Cammarato A, Engler AJ.

The human heart is capable of functioning for decades despite minimal cell turnover or regeneration, suggesting that molecular alterations help sustain heart function with age. However, identification of compensatory remodeling events in the aging heart remains elusive. We present the cardiac proteomes of young and old rhesus monkeys and rats, from which we show that certain age-associated remodeling events within the cardiomyocyte cytoskeleton are highly conserved and beneficial rather than deleterious. Targeted transcriptomic analysis in Drosophila confirmed conservation and implicated vinculin as a unique molecular regulator of cardiac function during aging. Cardiac-restricted vinculin overexpression reinforced the cortical cytoskeleton and enhanced myofilament organization, leading to improved contractility and hemodynamic stress tolerance in healthy and myosin-deficient fly hearts. Moreover, cardiac-specific vinculin overexpression increased median life span by more than 150% in flies. A broad array of potential therapeutic targets and regulators of age-associated modifications, specifically for vinculin, are presented. These findings suggest that the heart has molecular mechanisms to sustain performance and promote longevity, which may be assisted by therapeutic intervention to ameliorate the decline of function in aging patient hearts.

J Biol Chem. 289: 12779-90 (2014)

Mapping interactions between myosin relay and converter domains that power muscle function.

Kronert WA, Melkani GC, Melkani A, Bernstein SI.

Intra-molecular communication within myosin is essential for its function as motor, but the specific amino acid residue interactions required are unexplored within muscle cells. Using Drosophila melanogaster skeletal muscle myosin, we performed a novel in vivo molecular suppression analysis to define the importance of three relay loop amino acid residues (I508, N509 and D511) in communicating with converter domain residue R759. We find that the N509K relay mutation suppresses defects in myosin ATPase, in vitro motility, myofibril stability and muscle function associated with the R759E converter mutation. Through molecular modeling we define a mechanism for this interaction and suggest why the I508K and D511K relay mutations fail to suppress R759E. Interestingly, I508K disables motor function and myofibril assembly, suggesting productive relay-converter interaction is essential for both processes. We conclude that the putative relay-converter interaction mediated by myosin residues 509 and 759 is critical for the biochemical and biophysical function of skeletal muscle myosin and the normal ultrastructural and mechanical properties of muscle.

Rare Dis. 2: e968003 (2014)

Drosophila as a potential model to ameliorate mutant Huntington-mediated cardiac amyloidosis.

Trujillo AS, Ramos R, Bodmer R, Bernstein SI, Ocorr K, Melkani GC.

Several human diseases, including Huntington's disease (HD), are associated with the expression of mutated, misfolded, and aggregation-prone amyloid proteins. Cardiac disease is the second leading cause of death in HD, which has been mainly studied as a neurodegenerative disease that is caused by expanded polyglutamine repeats in the huntingtin protein. Since the mechanistic basis of mutant HD-induced cardiomyopathy is unknown, we established a Drosophila heart model that exhibited amyloid aggregate-induced oxidative stress, resulting in myofibrillar disorganization and physiological defects upon expression of HD-causing PolyQ expression in cardiomyocytes. Using powerful Drosophila genetic techniques, we suppressed mutant HD-induced cardiomyopathy by modulating pathways associated with folding defects and oxidative stress. In this addendum, we describe additional potential molecular players that might be associated with HD cardiac amyloidosis. Drosophila, with its high degree of conservation to the human genome and many techniques to manipulate its gene expression, will be an excellent model for the suppression of cardiac amyloidosis linked to other polyglutamine expansion repeat disorders.

Front Physiol. 5: 416 (2014)

X-ray diffraction from flight muscle with a headless myosin mutation: implications for interpreting reflection patterns.

Iwamoto H, Trombitás K, Yagi N, Suggs JA, Bernstein SI.

Fruit fly (Drosophila melanogaster) is one of the most useful animal models to study the causes and effects of hereditary diseases because of its rich genetic resources. It is especially suitable for studying myopathies caused by myosin mutations, because specific mutations can be induced to the flight muscle-specific myosin isoform, while leaving other isoforms intact. Here we describe an X-ray-diffraction-based method to evaluate the structural effects of mutations in contractile proteins in Drosophila indirect flight muscle. Specifically, we describe the effect of the headless myosin mutation, Mhc10-Y97, in which the motor domain of the myosin head is deleted, on the X-ray diffraction pattern. The loss of general integrity of the filament lattice is evident from the pattern. A striking observation, however, is the prominent meridional reflection at d = 14.5 nm, a hallmark for the regularity of the myosin-containing thick filament. This reflection has long been considered to arise mainly from the myosin head, but taking the 6th actin layer line reflection as an internal control, the 14.5-nm reflection is even stronger than that of wild-type muscle. We confirmed these results via electron microscopy, wherein image analysis revealed structures with a similar periodicity. These observations have major implications on the interpretation of myosin-based reflections.

Int Rev Cell Mol Biol. 313: 103-44 (2014)

The UNC-45 myosin chaperone: from worms to flies to vertebrates.

Lee CF, Melkani GC, Bernstein SI.

UNC-45 (uncoordinated mutant number 45) is a UCS (UNC-45, CRO1, She4p) domain protein that is critical for myosin stability and function. It likely aides in folding myosin during cellular differentiation and maintenance, and protects myosin from denaturation during stress. Invertebrates have a single unc-45 gene that is expressed in both muscle and nonmuscle tissues. Vertebrates possess one gene expressed in striated muscle (unc-45b) and another that is more generally expressed (unc-45a). Structurally, UNC-45 is composed of a series of α-helices connected by loops. It has an N-terminal tetratricopeptide repeat domain that binds to Hsp90 and a central domain composed of armadillo repeats. Its C-terminal UCS domain, which is also comprised of helical armadillo repeats, interacts with myosin. In this chapter, we present biochemical, structural, and genetic analyses of UNC-45 in Caenorhabditis elegans, Drosophila melanogaster, and various vertebrates. Further, we provide insights into UNC-45 functions, its potential mechanism of action, and its roles in human disease.

Anat Rec. 297: 1637-49 (2014)

Getting folded: chaperone proteins in muscle development, maintenance and disease.

Smith DA, Carland CR, Guo Y, Bernstein SI.

Chaperone proteins are critical for protein folding and stability, and hence are necessary for normal cellular organization and function. Recent studies have begun to interrogate the role of this specialized class of proteins in muscle biology. During development, chaperone-mediated folding of client proteins enables their integration into nascent functional sarcomeres. In addition to assisting with muscle differentiation, chaperones play a key role in the maintenance of muscle tissues. Furthermore, disruption of the chaperone network can result in neuromuscular disease. In this review, we discuss how chaperones are involved in myofibrillogenesis, sarcomere maintenance, and muscle disorders. We also consider the possibilities of therapeutically targeting chaperones to treat muscle disease.

PLoS Genet. 10: e1004024 (2013)

Huntington's disease induced cardiac amyloidosis is reversed by modulating protein folding and oxidative stress pathways in the Drosophila heart.

Melkani GC, Trujillo AS, Ramos R, Bodmer R, Bernstein SI, Ocorr K.

Amyloid-like inclusions have been associated with Huntington's disease (HD), which is caused by expanded polyglutamine repeats in the Huntingtin protein. HD patients exhibit a high incidence of cardiovascular events, presumably as a result of accumulation of toxic amyloid-like inclusions. We have generated a Drosophila model of cardiac amyloidosis that exhibits accumulation of PolyQ aggregates and oxidative stress in myocardial cells, upon heart-specific expression of Huntingtin protein fragments (Htt-PolyQ) with disease-causing poly-glutamine repeats (PolyQ-46, PolyQ-72, and PolyQ-102). Cardiac expression of GFP-tagged Htt-PolyQs resulted in PolyQ length-dependent functional defects that included increased incidence of arrhythmias and extreme cardiac dilation, accompanied by a significant decrease in contractility. Structural and ultrastructural analysis of the myocardial cells revealed reduced myofibrillar content, myofibrillar disorganization, mitochondrial defects and the presence of PolyQ-GFP positive aggregates. Cardiac-specific expression of disease causing Poly-Q also shortens lifespan of flies dramatically. To further confirm the involvement of oxidative stress or protein unfolding and to understand the mechanism of PolyQ induced cardiomyopathy, we co-expressed expanded PolyQ-72 with the antioxidant superoxide dismutase (SOD) or the myosin chaperone UNC-45. Co-expression of SOD suppressed PolyQ-72 induced mitochondrial defects and partially suppressed aggregation as well as myofibrillar disorganization. However, co-expression of UNC-45 dramatically suppressed PolyQ-72 induced aggregation and partially suppressed myofibrillar disorganization. Moreover, co-expression of both UNC-45 and SOD more efficiently suppressed GFP-positive aggregates, myofibrillar disorganization and physiological cardiac defects induced by PolyQ-72 than did either treatment alone. Our results demonstrate that mutant-PolyQ induces aggregates, disrupts the sarcomeric organization of contractile proteins, leads to mitochondrial dysfunction and increases oxidative stress in cardiomyocytes leading to abnormal cardiac function. We conclude that modulation of both protein unfolding and oxidative stress pathways in the Drosophila heart model can ameliorate the detrimental PolyQ effects, thus providing unique insights into the genetic mechanisms underlying amyloid-induced cardiac failure in HD patients.

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PLoS Genet. 9: e1003544 (2013)

The NADPH metabolic network regulates human alpha B-crystallin cardiomyopathy and reductive stress in Drosophila melanogaster.

Xie , H. B., A. Cammarato, N. S. Rajasekaran, H. Zhang, J. A. Suggs, H.-C. Lin, S. I. Bernstein, I. J. Benjamin and K. G. Golic.

Dominant mutations in the alpha-B crystallin (CryAB) gene are responsible for a number of inherited human disorders, including cardiomyopathy, skeletal muscle myopathy, and cataracts. The cellular mechanisms of disease pathology for these disorders are not well understood. Among recent advances is that the disease state can be linked to a disturbance in the oxidation/reduction environment of the cell. In a mouse model, cardiomyopathy caused by the dominant CryABR120G missense mutation was suppressed by mutation of the gene that encodes glucose 6-phosphate dehydrogenase (G6PD), one of the cell's primary sources of reducing equivalents in the form of NADPH. Here, we report the development of a Drosophila model for cellular dysfunction caused by this CryAB mutation. With this model, we confirmed the link between G6PD and mutant CryABpathology by finding that reduction of G6PD expression suppressed the phenotype while overexpression enhanced it. Moreover, we find that expression of mutant CryAB in the Drosophila heart impaired cardiac function and increased heart tube dimensions, similar to the effects produced in mice and humans, and that reduction of G6PD ameliorated these effects. Finally, to determine whether CryAB pathology responds generally to NADPH levels we tested mutants or RNAi-mediated knockdowns of phosphogluconate dehydrogenase (PGD), isocitrate dehydrogenase (IDH), and malic enzyme (MEN), the other major enzymatic sources of NADPH, and we found that all are capable of suppressing CryABR120G pathology, confirming the link between NADP/H metabolism and CryAB.

J Mol Biol. 416:543-557 (2012)

Alternative relay and converter domains tune native muscle myosin isoform function in Drosophila.

Kronert WA, Melkani GC, Melkani A, Bernstein SI.

Myosin isoforms help define muscle-specific contractile and structural properties. Alternative splicing of myosin heavy chain gene transcripts in Drosophila melanogaster yields muscle-specific isoforms and highlights alternative domains that fine-tune myosin function. To gain insight into how native myosin is tuned, we expressed three embryonic myosin isoforms in indirect flight muscles lacking endogenous myosin. These isoforms differ in their relay and/or converter domains. We analyzed isoform-specific ATPase activities, in vitro actin motility and myofibril structure/stability. We find that dorsal acute body wall muscle myosin (EMB-9c11d) shows a significant increase in MgATPase V(max) and actin sliding velocity, as well as abnormal myofibril assembly compared to cardioblast myosin (EMB-11d). These properties differ as a result of alternative exon-9-encoded relay domains that are hypothesized to communicate signals among the ATP-binding pocket, actin-binding site and the converter domain. Further, EMB-11d shows significantly reduced levels of basal Ca- and MgATPase as well as MgATPase V(max) compared to embryonic body wall muscle isoform (EMB) (expressed in a multitude of body wall muscles). EMB-11d also induces increased actin sliding velocity and stabilizes myofibril structure compared to EMB. These differences arise from exon-11-encoded alternative converter domains that are proposed to reposition the lever arm during the power and recovery strokes. We conclude that relay and converter domains of native myosin isoforms fine-tune ATPase activity, actin motility and muscle ultrastructure. This verifies and extends previous studies with chimeric molecules and indicates that interactions of the relay and converter during the contractile cycle are key to myosin-isoform-specific kinetic and mechanical functions.

Mol Biol Cell. 23:2057-65 (2012).

Expression of the inclusion body myopathy 3 mutation in Drosophila depresses myosin function and stability and recapitulates muscle inclusions and weakness.

Wang Y, Melkani GC, Suggs JA, Melkani A, Kronert WA, Cammarato A, Bernstein SI.

Hereditary myosin myopathies are characterized by variable clinical features. Inclusion body myopathy 3 (IBM-3) is an autosomal dominant disease associated with a missense mutation (E706K) in the myosin heavy chain IIa gene. Adult patients experience progressive muscle weakness. Biopsies reveal dystrophic changes, rimmed vacuoles with cytoplasmic inclusions, and focal disorganization of myofilaments. We constructed a transgene encoding E706K myosin and expressed it in Drosophila (E701K) indirect flight and jump muscles to establish a novel homozygous organism with homogeneous populations of fast IBM-3 myosin and muscle fibers. Flight and jump abilities were severely reduced in homozygotes. ATPase and actin sliding velocity of the mutant myosin were depressed >80% compared with wild-type myosin. Light scattering experiments and electron microscopy revealed that mutant myosin heads bear a dramatic propensity to collapse and aggregate. Thus E706K (E701K) myosin appears far more labile than wild-type myosin. Furthermore, mutant fly fibers exhibit ultrastructural hallmarks seen in patients, including cytoplasmic inclusions containing aberrant proteinaceous structures and disorganized muscle filaments. Our Drosophila model reveals the unambiguous consequences of the IBM-3 lesion on fast muscle myosin and fibers. The abnormalities observed in myosin function and muscle ultrastructure likely contribute to muscle weakness observed in our flies and patients.

J Cell Mol Med. 16:1656-1662.(2012)

Measuring passive myocardial stiffness in Drosophila melanogaster to investigate diastolic dysfunction.

Kaushik G, Zambon AC, Fuhrmann A, Bernstein SI, Bodmer R, Engler AJ, Cammarato A.

Aging is marked by a decline in left ventricular diastolic function, which encompasses abnormalities in diastolic relaxation, chamber filling and/or passive myocardial stiffness. Genetic tractability and short life span make Drosophila melanogaster an ideal organism to study the effects of aging on heart function, including senescent-associated changes in gene expression and in passive myocardial stiffness. However, use of the Drosophila heart tube to probe deterioration of diastolic performance is subject to at least two challenges: the extent of genetic homology to mammals and the ability to resolve mechanical properties of the bilayered fly heart, which consists of a ventral muscle layer that covers the contractile cardiomyocytes. Here we argue for wide-spread use of Drosophila as a novel myocardial aging model by 1) describing diastolic dysfunction in flies, 2) discussing how critical pathways involved in dysfunction are conserved across species, and 3) demonstrating the advantage of an atomic force microscopy-based analysis method to measure stiffness of the multilayered Drosophila heart tube versus isolated myocytes from other model systems. By using powerful Drosophila genetic tools we aim to efficiently alter changes observed in factors that contribute to diastolic dysfunction to understand how one might improve diastolic performance at advanced ages in humans.

Methods. 56:25-32. (2012)

Transgenic expression and purification of myosin isoforms using the Drosophila melanogaster indirect flight muscle system.

Caldwell JT, Melkani GC, Huxford T, Bernstein SI.

Biophysical and structural studies on muscle myosin rely upon milligram quantities of extremely pure material. However, many biologically interesting myosin isoforms are expressed at levels that are too low for direct purification from primary tissues. Efforts aimed at recombinant expression of functional striated muscle myosin isoforms in bacterial or insect cell culture have largely met with failure, although high level expression in muscle cell culture has recently been achieved at significant expense. We report a novel method for the use of strains of the fruit fly Drosophila melanogaster genetically engineered to produce histidine-tagged recombinant muscle myosin isoforms. This method takes advantage of the single muscle myosin heavy chain gene within the Drosophila genome, the high level of expression of accessible myosin in the thoracic indirect flight muscles, the ability to knock out endogenous expression of myosin in this tissue and the relatively low cost of fruit fly colony production and maintenance. We illustrate this method by expressing and purifying a recombinant histidine-tagged variant of embryonic body wall skeletal muscle myosin II from an engineered fly strain. The recombinant protein shows the expected ATPase activity and is of sufficient purity and homogeneity for crystallization. This system may prove useful for the expression and isolation of mutant myosins associated with skeletal muscle diseases and cardiomyopathies for their biochemical and structural characterization.

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J Mol Biol. 414:477-84 (2011)

Structural basis for myopathic defects engendered by alterations in the myosin rod.

Cammarato A, Li XE, Reedy MC, Lee CF, Lehman W, Bernstein SI.

While mutations in the myosin subfragment 1 motor domain can directly disrupt the generation and transmission of force along myofibrils and lead to myopathy, the mechanism whereby mutations in the myosin rod influences mechanical function is less clear. Here, we used a combination of various imaging techniques and molecular dynamics simulations to test the hypothesis that perturbations in the myosin rod can disturb normal sarcomeric uniformity and, like motor domain lesions, would influence force production and propagation. We show that disrupting the rod can alter its nanomechanical properties and, in vivo, can drive asymmetric myofilament and sarcomere formation. Our imaging results indicate that myosin rod mutations likely disturb production and/or propagation of contractile force. This provides a unifying theory where common pathological cascades accompany both myosin motor and specific rod domain mutations. Finally, we suggest that sarcomeric inhomogeneity, caused by asymmetric thick filaments, could be a useful index of myopathic dysfunction.

Biophys J. 2011 10:1114-22

Disrupting the myosin converter-relay interface impairs Drosophila indirect flight muscle performance.

Ramanath S, Wang Q, Bernstein SI, Swank DM.

Structural interactions between the myosin converter and relay domains have been proposed to be critical for the myosin power stroke and muscle power generation. We tested this hypothesis by mutating converter residue 759, which interacts with relay residues I508, N509, and D511, to glutamate (R759E) and determined the effect on Drosophila indirect flight muscle mechanical performance. Work loop analysis of mutant R759E indirect flight muscle fibers revealed a 58% and 31% reduction in maximum power generation (P(WL)) and the frequency at which maximum power (f(WL)) is generated, respectively, compared to control fibers at 15°C. Small amplitude sinusoidal analysis revealed a 30%, 36%, and 32% reduction in mutant elastic modulus, viscous modulus, and mechanical rate constant 2πb, respectively. From these results, we infer that the mutation reduces rates of transitions through work-producing cross-bridge states and/or force generation during strongly bound states. The reductions in muscle power output, stiffness, and kinetics were physiologically relevant, as mutant wing beat frequency and flight index decreased about 10% and 45% compared to control flies at both 15°C and 25°C. Thus, interactions between the relay loop and converter domain are critical for lever-arm and catalytic domain coordination, high muscle power generation, and optimal Drosophila flight performance.

J Biol Chem. 286: 28435-28443 (2011)

Two Drosophila myosin transducer mutants with distinct cardiomyopathies have divergent ADP and actin affinities.

Bloemink M.J., Melkani G.C., Dambacher C.M., Bernstein S.I., Geeves M.A.

Two Drosophila myosin II point mutations (D45 and Mhc5) generate Drosophila cardiac phenotypes that are similar to dilated or restrictive human cardiomyopathies. Our homology models suggest the mutations (A261T in D45, G200D in Mhc5) could stabilize (D45) or destabilize (Mhc5) loop 1 of myosin, a region known to influence ADP release. To gain insight into the molecular mechanism that causes the cardiomyopathic phenotypes to develop, we determined whether kinetic properties of the mutant molecules have been altered. We used myosin subfragment 1 (S1) carrying either of the two mutations (S1A261T and S1G200D) from the indirect flight muscles of Drosophila. The kinetic data show that the two point mutations have an opposite effect on the enzymatic activity of S1. S1A261T is less active (reduced ATPase, higher ADP affinity for S1 and actoS1 and reduced ATP-induced dissociation of acto-S1) whereas S1G200D shows increased enzymatic activity (enhanced ATPase, reduced ADP affinity for both S1 and actoS1). The opposite changes in the myosin properties are consistent with the induced cardiac phenotypes for S1A261T (dilated) and S1G200D (restrictive). Our results provide novel insights into the molecular mechanisms that cause different cardiomyopathy phenotypes for these mutants. In addition we report that S1A261T weakens the affinity of S1.ADP for actin while S1G200D increases it. This may account for the suppression (A261T) or enhancement (G200D) of the skeletal muscle hypercontraction phenotype induced by the troponin I held-up2 mutation in Drosophila.

PLoS One 6: e22579 (2011)

The UNC-45 Chaperone Is Critical for Establishing Myosin-Based Myofibrillar Organization and Cardiac Contractility in the Drosophila Heart Model.

Melkani, G. C., R. Bodmer, K. Ocorr and S.I. Bernstein

UNC-45 is a UCS (UNC-45/CRO1/She4P) class chaperone necessary for myosin folding and/or accumulation, but its requirement for maintaining cardiac contractility has not been explored. Given the prevalence of myosin mutations in eliciting cardiomyopathy, chaperones like UNC-45 are likely to be equally critical in provoking or modulating myosin-associated cardiomyopathy. Here, we used the Drosophila heart model to examine its role in cardiac physiology, in conjunction with RNAi-mediated gene silencing specifically in the heart in vivo. Analysis of cardiac physiology was carried out using high-speed video recording in conjunction with movement analysis algorithms. unc-45 knockdown resulted in severely compromised cardiac function in adults as evidenced by prolonged diastolic and systolic intervals, and increased incidence of arrhythmias and extreme dilation; the latter was accompanied by a significant reduction in muscle contractility. Structural analysis showed reduced myofibrils, myofibrillar disarray, and greatly decreased cardiac myosin accumulation. Cardiac unc-45 silencing also dramatically reduced life-span. In contrast, third instar larval and young pupal hearts showed mild cardiac abnormalities, as severe cardiac defects only developed during metamorphosis. Furthermore, cardiac unc-45 silencing in the adult heart (after metamorphosis) led to less severe phenotypes. This suggests that UNC-45 is mostly required for myosin accumulation/folding during remodeling of the forming adult heart. The cardiac defects, myosin deficit and decreased life-span in flies upon heart-specific unc-45 knockdown were significantly rescued by UNC-45 over-expression. Our results are the first to demonstrate a cardiac-specific requirement of a chaperone in Drosophila, suggestive of a critical role of UNC-45 in cardiomyopathies, including those associated with unfolded proteins in the failing human heart. The dilated cardiomyopathy phenotype associated with UNC-45 deficiency is mimicked by myosin knockdown suggesting that UNC-45 plays a crucial role in stabilizing myosin and possibly preventing human cardiomyopathies associated with functional deficiencies of myosin.

PLoS One 6: e18497. (2011)

A mighty small heart: the cardiac proteome of adult Drosophila melanogaster.

Cammarato, A., C. H. Ahrens, N. N. Alayari, E. Qeli, J. Rucker, M. C. Reedy, C. M. Zmasek, M. Gucek, R. N. Cole, J. E. Van Eyk, R. Bodmer, B. O'Rourke, S. I. Bernstein and D. B. Foster.

Drosophila melanogaster is emerging as a powerful model system for the study of cardiac disease. Establishing peptide and protein maps of the Drosophila heart is central to implementation of protein network studies that will allow us to assess the hallmarks of Drosophila heart pathogenesis and gauge the degree of conservation with human disease mechanisms on a systems level. Using a gel-LC-MS/MS approach, we identified 1228 protein clusters from 145 dissected adult fly hearts. Contractile, cytostructural and mitochondrial proteins were most abundant consistent with electron micrographs of the Drosophila cardiac tube. Functional/Ontological enrichment analysis further showed that proteins involved in glycolysis, Ca(2+)-binding, redox, and G-protein signaling, among other processes, are also over-represented. Comparison with a mouse heart proteome revealed conservation at the level of molecular function, biological processes and cellular components. The subsisting peptidome encompassed 5169 distinct heart-associated peptides, of which 1293 (25%) had not been identified in a recent Drosophila peptide compendium. PeptideClassifier analysis was further used to map peptides to specific gene-models. 1872 peptides provide valuable information about protein isoform groups whereas a further 3112 uniquely identify specific protein isoforms and may be used as a heart-associated peptide resource for quantitative proteomic approaches based on multiple-reaction monitoring. In summary, identification of excitation-contraction protein landmarks, orthologues of proteins associated with cardiovascular defects, and conservation of protein ontologies, provides testimony to the heart-like character of the Drosophila cardiac tube and to the utility of proteomics as a complement to the power of genetics in this growing model of human heart disease.

Structure 19: 397-408. (2011)

X-ray crystal structure of the UCS domain-containing UNC-45 myosin chaperone from Drosophila melanogaster.

Lee, C. F., A. V. Hauenstein, J. K. Fleming, W. C. Gasper, V. Engelke, B. Sankaran, S. I. Bernstein and T. Huxford .

UCS proteins, such as UNC-45, influence muscle contraction and other myosin-dependent motile processes. We report the first X-ray crystal structure of a UCS domain-containing protein, the UNC-45 myosin chaperone from Drosophila melanogaster (DmUNC-45). The structure reveals that the central and UCS domains form a contiguous arrangement of 17 consecutive helical layers that arrange themselves into five discrete armadillo repeat subdomains. Small-angle X-ray scattering data suggest that free DmUNC-45 adopts an elongated conformation and exhibits flexibility in solution. Protease sensitivity maps to a conserved loop that contacts the most carboxy-terminal UNC-45 armadillo repeat subdomain. Amino acid conservation across diverse UCS proteins maps to one face of this carboxy-terminal subdomain, and the majority of mutations that affect myosin-dependent cellular activities lie within or around this region. Our crystallographic, biophysical, and biochemical analyses suggest that DmUNC-45 function is afforded by its flexibility and by structural integrity of its UCS domain.

J. Cell Sci. 24: 699-705. (2011)

Drosophila UNC-45 accumulates in embryonic blastoderm and in muscles and is essential for muscle myosin stability.

Lee, C. F., G. C. Melkani, Q. Yu, J. A. Suggs, W. A. Kronert, Y. Suzuki, L. Hipolito, M. G. Price, H. F. Epstein and S. I. Bernstein

UNC-45 is a chaperone that facilitates folding of myosin motor domains. We have used Drosophila melanogaster to investigate the role of UNC-45 in muscle development and function. Drosophila UNC-45 (dUNC-45) is expressed at all developmental stages. It colocalizes with non-muscle myosin in embryonic blastoderm of 2-hour-old embryos. At 14 hours, it accumulates most strongly in embryonic striated muscles, similarly to muscle myosin. dUNC-45 localizes to the Z-discs of sarcomeres in third instar larval body-wall muscles. We produced a dunc-45 mutant in which zygotic expression is disrupted. This results in nearly undetectable dUNC-45 levels in maturing embryos as well as late embryonic lethality. Muscle myosin accumulation is robust in dunc-45 mutant embryos at 14 hours. However, myosin is dramatically decreased in the body-wall muscles of 22-hour-old mutant embryos. Furthermore, electron microscopy showed only a few thick filaments and irregular thick-thin filament lattice spacing. The lethality, defective protein accumulation, and ultrastructural abnormalities are rescued with a wild-type dunc-45 transgene, indicating that the mutant phenotypes arise from the dUNC-45 deficiency. Overall, our data indicate that dUNC-45 is important for myosin accumulation and muscle function. Furthermore, our results suggest that dUNC-45 acts post-translationally for proper myosin folding and maturation.

J. Mol. Biol. 398: 625-632. (2010)

Mutating the converter-relay interface of Drosophila myosin perturbs ATPase activity, actin motility, myofibril stability and flight ability.

Kronert, W. A., G. C. Melkani, A. Melkani and S. I. Bernstein.

We used an integrative approach to probe the significance of the interaction between the relay loop and converter domain of the myosin molecular motor from Drosophila melanogaster indirect flight muscle. During the myosin mechanochemical cycle, ATP-induced twisting of the relay loop is hypothesized to reposition the converter, resulting in cocking of the contiguous lever arm into the pre-power stroke configuration. The subsequent movement of the lever arm through its power stroke generates muscle contraction by causing myosin heads to pull on actin filaments. We generated a transgenic line expressing myosin with a mutation in the converter domain (R759E) at a site of relay loop interaction. Molecular modeling suggests that the interface between the relay loop and converter domain of R759E myosin would be significantly disrupted during the mechanochemical cycle. The mutation depressed calcium as well as basal and actin-activated MgATPase (V(max)) by approximately 60% compared to wild-type myosin, but there is no change in apparent actin affinity (K(m)). While ATP or AMP-PNP (adenylyl-imidodiphosphate) binding to wild-type myosin subfragment-1 enhanced tryptophan fluorescence by approximately 15% or approximately 8%, respectively, enhancement does not occur in the mutant. This suggests that the mutation reduces lever arm movement. The mutation decreases in vitro motility of actin filaments by approximately 35%. Mutant pupal indirect flight muscles display normal myofibril assembly, myofibril shape, and double-hexagonal arrangement of thick and thin filaments. Two-day-old fibers have occasional "cracking" of the crystal-like array of myofilaments. Fibers from 1-week-old adults show more severe cracking and frayed myofibrils with some disruption of the myofilament lattice. Flight ability is reduced in 2-day-old flies compared to wild-type controls, with no upward mobility but some horizontal flight. In 1-week-old adults, flight capability is lost. Thus, altered myosin function permits myofibril assembly, but results in a progressive disruption of the myofilament lattice and flight ability. We conclude that R759 in the myosin converter domain is essential for normal ATPase activity, in vitro motility and locomotion. Our results provide the first mutational evidence that intramolecular signaling between the relay loop and converter domain is critical for myosin function both in vitro and in muscle.

Biochem. Biophys. Res. Comm. 396: 317-322. (2010)

Drosophila UNC-45 prevents heat-induced aggregation of skeletal muscle myosin and facilitates refolding of citrate synthase.

Melkani, G. C., C. F. Lee, A. Cammarato and S. I. Bernstein.

UNC-45 belongs to the UCS (UNC-45, CRO1, She4p) domain protein family, whose members interact with various classes of myosin. Here we provide structural and biochemical evidence that Escherichia coli-expressed Drosophila UNC-45 (DUNC-45) maintains the integrity of several substrates during heat-induced stress in vitro. DUNC-45 displays chaperone function in suppressing aggregation of the muscle myosin heavy meromyosin fragment, the myosin S-1 motor domain, alpha-lactalbumin and citrate synthase. Biochemical evidence is supported by electron microscopy, which reveals the first structural evidence that DUNC-45 prevents inter- or intra-molecular aggregates of skeletal muscle heavy meromyosin caused by elevated temperatures. We also demonstrate for the first time that UNC-45 is able to refold a denatured substrate, urea-unfolded citrate synthase. Overall, this in vitro study provides insight into the fate of muscle myosin under stress conditions and suggests that UNC-45 protects and maintains the contractile machinery during in vivo stress.

J. Vis. Exp. 2009 31: 1435.

Semi-automated Optical Heartbeat Analysis of small hearts.

Ocorr K., M. Fink, A. Cammarato, S. I. Bernstein and R. Bodmer.

We have developed a method for analyzing high speed optical recordings from Drosophila, zebrafish and embryonic mouse hearts (Fink, et. al., 2009). Our Semi-automatic Optical Heartbeat Analysis (SOHA) uses a novel movement detection algorithm that is able to detect cardiac movements associated with individual contractile and relaxation events. The program provides a host of physiologically relevant readouts including systolic and diastolic intervals, heart rate, as well as qualitative and quantitative measures of heartbeat arrhythmicity. The program also calculates heart diameter measurements during both diastole and systole from which fractional shortening and fractional area changes are calculated. Output is provided as a digital file compatible with most spreadsheet programs. Measurements are made for every heartbeat in a record increasing the statistical power of the output. We demonstrate each of the steps where user input is required and show the application of our methodology to the analysis of heart function in all three genetically tractable heart models.

Biophys J. 2009 96:4132-4143.

Alternative S2 hinge regions of the myosin rod affect myofibrillar structure and myosin kinetics.

Miller, M. S., C. M. Dambacher, A. F. Knowles, J. M. Braddock, G. P. Farman, T. C. Irving, D. M. Swank, S. I. Bernstein and D. W. Maughan.

The subfragment 2/light meromyosin "hinge" region has been proposed to significantly contribute to muscle contraction force and/or speed. Transgenic replacement of the endogenous fast muscle isovariant hinge A (exon 15a) in Drosophila melanogaster indirect flight muscle with the slow muscle hinge B (exon 15b) allows examination of the structural and functional changes when only this region of the myosin molecule is different. Hinge B was previously shown to increase myosin rod length, increase A-band and sarcomere length, and decrease flight performance compared to hinge A. We applied additional measures to these transgenic lines to further evaluate the consequences of modifying this hinge region. Structurally, the longer A-band and sarcomere lengths found in the hinge B myofibrils appear to be due to the longitudinal addition of myosin heads. Functionally, hinge B, although a significant distance from the myosin catalytic domain, alters myosin kinetics in a manner consistent with this region increasing myosin rod length. These structural and functional changes combine to decrease whole fly wing-beat frequency and flight performance. Our results indicate that this hinge region plays an important role in determining myosin kinetics and in regulating thick and thin filament lengths as well as sarcomere length.

J Mol Biol. 2009 389:707-721.

Alternative exon 9-encoded relay domains affect more than one communication pathway in the Drosophila myosin head.

Bloemink M. J., C. M. Dambacher, A. F. Knowles , G. C. Melkani, M. A. Geeves and S. I. Bernstein.

We investigated the biochemical and biophysical properties of one of the four alternative regions within the Drosophila myosin catalytic domain: the relay domain encoded by exon 9. This domain of the myosin head transmits conformational changes in the nucleotide-binding pocket to the converter domain, which is crucial to coupling catalytic activity with mechanical movement of the lever arm. To study the function of this region, we used chimeric myosins (IFI-9b and EMB-9a), which were generated by exchange of the exon 9-encoded domains between the native embryonic body wall (EMB) and indirect flight muscle isoforms (IFI). Kinetic measurements show that exchange of the exon 9-encoded region alters the kinetic properties of the myosin S1 head. This is reflected in reduced values for ATP-induced actomyosin dissociation rate constant (K(1)k(+2)) and ADP affinity (K(AD)), measured for the chimeric constructs IFI-9b and EMB-9a, compared to wild-type IFI and EMB values. Homology models indicate that, in addition to affecting the communication pathway between the nucleotide-binding pocket and the converter domain, exchange of the relay domains between IFI and EMB affects the communication pathway between the nucleotide-binding pocket and the actin-binding site in the lower 50-kDa domain (loop 2). These results suggest an important role of the relay domain in the regulation of actomyosin cross-bridge kinetics.

J. Mol. Biol. 2008 379:443-456.

Alternative relay domains of Drosophila melanogaster myosin differentially affect ATPase activity, in vitro motility, myofibril structure and muscle function.

Kronert, W. A., C. M. Dambacher, A. F. Knowles, D. M. Swank and S. I. Bernstein.

The relay domain of myosin is hypothesized to function as a communication pathway between the nucleotide-binding site, actin-binding site and the converter domain. In Drosophila melanogaster, a single myosin heavy chain gene encodes three alternative relay domains. Exon 9a encodes the indirect flight muscle isoform (IFI) relay domain, whereas exon 9b encodes one of the embryonic body wall isoform (EMB) relay domains. To gain a better understanding of the function of the relay domain and the differences imparted by the IFI and the EMB versions, we constructed two transgenic Drosophila lines expressing chimeric myosin heavy chains in indirect flight muscles lacking endogenous myosin. One expresses the IFI relay domain in the EMB backbone (EMB-9a), while the second expresses the EMB relay domain in the IFI backbone (IFI-9b). Our studies reveal that the EMB relay domain is functionally equivalent to the IFI relay domain when it is substituted into IFI. Essentially no differences in ATPase activity, actin-sliding velocity, flight ability at room temperature or muscle structure are observed in IFI-9b compared to native IFI. However, when the EMB relay domain is replaced with the IFI relay domain, we find a 50% reduction in actin-activated ATPase activity, a significant increase in actin affinity, abolition of actin sliding, defects in myofibril assembly and rapid degeneration of muscle structure compared to EMB. We hypothesize that altered relay domain conformational changes in EMB-9a impair intramolecular communication with the EMB-specific converter domain. This decreases transition rates involving strongly bound actomyosin states, leading to a reduced ATPase rate and loss of actin motility.

J. Mol. Biol. 2008 381:519-528.

Similarities and differences between frozen-hydrated, rigor acto-S1 complexes of insect flight and chicken skeletal muscles.

Littlefield, K. P., J. S. Chappie, A. B. Ward, M. K. Reedy, S. I. Bernstein, R. A. Milligan and M. C. Reedy.

The structure and function of myosin crossbridges in asynchronous insect flight muscle (IFM) have been elucidated in situ using multiple approaches. These include generating "atomic" models of myosin in multiple contractile states by rebuilding the crystal structure of chicken subfragment 1 (S1) to fit IFM crossbridges in lower-resolution electron microscopy tomograms and by "mapping" the functional effects of genetically substituted, isoform-specific domains, including the converter domain, in chimeric IFM myosin to sequences in the crystal structure of chicken S1. We prepared helical reconstructions (approximately 25 A resolution) to compare the structural characteristics of nucleotide-free myosin0 S1 bound to actin (acto-S1) isolated from chicken skeletal muscle (CSk) and the flight muscles of Lethocerus (Leth) wild-type Drosophila (wt Dros) and a Drosophila chimera (IFI-EC) wherein the converter domain of the indirect flight muscle myosin isoform has been replaced by the embryonic skeletal myosin converter domain. Superimposition of the maps of the frozen-hydrated acto-S1 complexes shows that differences between CSk and IFM S1 are limited to the azimuthal curvature of the lever arm: the regulatory light-chain (RLC) region of chicken skeletal S1 bends clockwise (as seen from the pointed end of actin) while those of IFM S1 project in a straight radial direction. All the IFM S1s are essentially identical other than some variation in the azimuthal spread of density in the RLC region. This spread is most pronounced in the IFI-EC S1, consistent with proposals that the embryonic converter domain increases the compliance of the IFM lever arm affecting the function of the myosin motor. These are the first unconstrained models of IFM S1 bound to actin and the first direct comparison of the vertebrate and invertebrate skeletal myosin II classes, the latter for which, data on the structure of discrete acto-S1 complexes, are not readily available.

Mol. Biol. Cell 2008 19:553-562.

Myosin transducer mutations differentially affect motor function, myofibril structure and the performance of skeletal and cardiac muscles.

Cammarato, A., C. M. Dambacher, A. F. Knowles, W. A. Kronert, R. Bodmer, K. Ocorr and S. I. Bernstein.

Striated muscle myosin is a multidomain ATP-dependent molecular motor. Alterations to various domains affect the chemomechanical properties of the motor, and they are associated with skeletal and cardiac myopathies. The myosin transducer domain is located near the nucleotide-binding site. Here, we helped define the role of the transducer by using an integrative approach to study how Drosophila melanogaster transducer mutations D45 and Mhc(5) affect myosin function and skeletal and cardiac muscle structure and performance. We found D45 (A261T) myosin has depressed ATPase activity and in vitro actin motility, whereas Mhc(5) (G200D) myosin has these properties enhanced. Depressed D45 myosin activity protects against age-associated dysfunction in metabolically demanding skeletal muscles. In contrast, enhanced Mhc(5) myosin function allows normal skeletal myofibril assembly, but it induces degradation of the myofibrillar apparatus, probably as a result of contractile disinhibition. Analysis of beating hearts demonstrates depressed motor function evokes a dilatory response, similar to that seen with vertebrate dilated cardiomyopathy myosin mutations, and it disrupts contractile rhythmicity. Enhanced myosin performance generates a phenotype apparently analogous to that of human restrictive cardiomyopathy, possibly indicating myosin-based origins for the disease. The D45 and Mhc(5) mutations illustrate the transducer's role in influencing the chemomechanical properties of myosin and produce unique pathologies in distinct muscles. Our data suggest Drosophila is a valuable system for identifying and modeling mutations analogous to those associated with specific human muscle disorders.

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Biophys. J. 2008 95:5228-5237.

Alternative versions of the myosin relay domain differentially respond to load to influence Drosophila muscle kinetics.

Yang, C., S. Ramanath, W. A. Kronert, S. I. Bernstein, D. W. Maughan, D. M. Swank.

We measured the influence of alternative versions of the Drosophila melanogaster myosin heavy chain relay domain on muscle mechanical properties. We exchanged relay domain regions (encoded by alternative versions of exon 9) between an embryonic (EMB) isoform and the indirect flight muscle isoform (IFI) of myosin. Previously, we observed no effect of exchanging the EMB relay domain region into the flight muscle isoform (IFI-9b) on in vitro actin motility velocity or solution ATPase measurements compared to IFI. However, in indirect flight muscle fibers, IFI-9b exhibited decreased maximum power generation (P(max)) and optimal frequency of power generation (f(max)) to 70% and 83% of IFI fiber values. The decrease in muscle performance reduced the flight ability and wing-beat frequency of IFI-9b Drosophila compared to IFI Drosophila. Previously, we found that exchanging the flight muscle specific relay domain into the EMB isoform (EMB-9a) prevented actin movement in the in vitro motility assay compared to EMB, which does support actin movement. However, in indirect flight muscle fibers EMB-9a was a highly effective motor, increasing P(max) and f(max) 2.5-fold and 1.4-fold, respectively, compared to fibers expressing EMB. We propose that the oscillatory load EMB-9a experiences in the muscle fiber reduces a high activation energy barrier between two strongly bound states of the cross-bridge cycle, thereby promoting cross-bridge cycling. The IFI relay domain's enhanced sensitivity to load increases cross-bridge kinetics, whereas the EMB version is less load-sensitive.

J. Mol. Biol. 2007 368:1051-1066.

A variable domain near the ATP binding site in Drosophila muscle myosin is part of the communication pathway between the nucleotide and actin-binding sites.

Miller, B. M., M. J., Bloemink, M. Nyitrai, S. I. Bernstein and M. A. Geeves.

Drosophila expresses several muscle myosin isoforms from a single gene by alternatively splicing six of the 19 exons. Here we investigate exon 7, which codes for a region in the upper 50 kDa domain near the nucleotide-binding pocket. This region is of interest because it is also the place where a large insert is found in myosin VI and where several cardiomyopathy mutations have been identified in human cardiac myosin. We expressed and purified chimeric muscle myosins from Drosophila, each varying at exon 7. Two chimeras exchanged the entire exon 7 domain between the indirect flight muscle (IFI, normally containing exon 7d) and embryonic body wall muscle (EMB, normally containing exon 7a) isoforms to create IFI-7a and EMB-7d. The second two chimeras replaced each half of the exon 7a domain in EMB with the corresponding portion of exon 7d to create EMB-7a/7d and EMB-7d/7a. Transient kinetic studies of the motor domain from these myosin isoforms revealed changes in several kinetic parameters between the IFI or EMB isoforms and the chimeras. Of significance were changes in nucleotide binding, which differed in the presence and absence of actin, consistent with a model in which the exon 7 domain is part of the communication pathway between the nucleotide and actin-binding sites. Homology models of the structures suggest how the exon 7 domain might modulate this pathway.

J. Mol. Biol. 2007 367: 1312-1329.

Alternative S2 hinge regions of the myosin rod differentially affect muscle function, myofibril dimensions and myosin tail length.

Suggs, J. A., A. Cammarato, W. A. Kronert, M. Nikkhoy, C. M. Dambacher, A. Megighian and S. I. Bernstein.

Muscle myosin heavy chain (MHC) rod domains intertwine to form alpha-helical coiled-coil dimers; these subsequently multimerize into thick filaments via electrostatic interactions. The subfragment 2/light meromyosin "hinge" region of the MHC rod, located in the C-terminal third of heavy meromyosin, may form a less stable coiled-coil than flanking regions. Partial "melting" of this region has been proposed to result in a helix to random-coil transition. A portion of the Drosophila melanogaster MHC hinge is encoded by mutually exclusive alternative exons 15a and 15b, the use of which correlates with fast (hinge A) or slow (hinge B) muscle physiological properties. To test the functional significance of alternative hinge regions, we constructed transgenic fly lines in which fast muscle isovariant hinge A was switched for slow muscle hinge B in the MHC isoforms of indirect flight and jump muscles. Substitution of the slow muscle hinge B impaired flight ability, increased sarcomere lengths by approximately 13% and resulted in minor disruption to indirect flight muscle sarcomeric structure compared with a transgenic control. With age, residual flight ability decreased rapidly and myofibrils developed peripheral defects. Computational analysis indicates that hinge B has a greater coiled-coil propensity and thus reduced flexibility compared to hinge A. Intriguingly, the MHC rod with hinge B was approximately 5 nm longer than myosin with hinge A, consistent with the more rigid coiled-coil conformation predicted for hinge B. Our study demonstrates that hinge B cannot functionally substitute for hinge A in fast muscle types, likely as a result of differences in the molecular structure of the rod, subtle changes in myofibril structure and decreased ability to maintain sarcomere structure in indirect flight muscle myofibrils. Thus, alternative hinges are important in dictating the distinct functional properties of myosin isoforms and the muscles in which they are expressed.

Gene Expr. Patterns 2007 7: 413-422.

Transcriptional regulation of the Drosophila melanogaster muscle myosin heavy-chain gene.

Hess, N. K., P. A. Singer, K. Trinh, M. Nikkhoy and S. I. Bernstein.

We show that a 2.6kb fragment of the muscle myosin heavy-chain gene (Mhc) of Drosophila melanogaster (containing 458 base pairs of upstream sequence, the first exon, the first intron and the beginning of the second exon) drives expression in all muscles. Comparison of the minimal promoter to Mhc genes of 10 Drosophila species identified putative regulatory elements in the upstream region and in the first intron. The first intron is required for expression in four small cells of the tergal depressor of the trochanter (jump) muscle and in the indirect flight muscle. The 3'-end of this intron is important for Mhc transcription in embryonic body wall muscle and contains AT-rich elements that are protected from DNase I digestion by nuclear proteins of Drosophila embryos. Sequences responsible for expression in embryonic, adult body wall and adult head muscles are present both within and outside the intron. Elements important for expression in leg muscles and in the large cells of the jump muscle flank the intron. We conclude that multiple transcriptional regulatory elements are responsible for Mhc expression in specific sets of Drosophila muscles.

Biophys. J. 2006 91: 4500-4506.

Passive stiffness in Drosophila indirect flight muscle reduced by disrupting paramyosin phosphorylation, but not by embryonic myosin S2 hinge substitution.

Hao, Y., M. S. Miller, D. M. Swank, H. Liu, S. I. Bernstein, D. W. Maughan and G. H. Pollack.

High passive stiffness is one of the characteristic properties of the asynchronous indirect flight muscle (IFM) found in many insects like Drosophila. To evaluate the effects of two thick filament protein domains on passive sarcomeric stiffness, and to investigate their correlation with IFM function, we used microfabricated cantilevers and a high resolution imaging system to study the passive IFM myofibril stiffness of two groups of transgenic Drosophila lines. One group (hinge-switch mutants) had a portion of the endogenous S2 hinge region replaced by an embryonic version; the other group (paramyosin mutants) had one or more putative phosphorylation sites near the N-terminus of paramyosin disabled. Both transgenic groups showed severely compromised flight ability. In this study, we found no difference (compared to the control) in passive elastic modulus in the hinge-switch group, but a 15% reduction in the paramyosin mutants. All results were corroborated by muscle fiber mechanics experiments performed on the same lines. The fact that myofibril elasticity is unaffected by hinge switching implies alternative S2 hinges do not critically affect passive sarcomere stiffness. In contrast, the mechanical defects observed upon disrupting paramyosin phosphorylation sites in Drosophila suggests that paramyosin phosphorylation is important for maintaining high passive stiffness in IFM myofibrils, probably by affecting paramyosin's interaction with other sarcomeric proteins.

In Nature's Versatile Engine: Insect Flight Muscle Inside and Out. (J. Vigoreaux, ed.). Landes Biosciences, Georgetown TX. 2006 62-75.

Myosin.

Miller, B. M. and S. I. Bernstein.

The molecular motor myosin, composed of two heavy chains and four light chains, is responsible for defining both structural and mechanical properties of insect flight muscle. Myosin polymerizes into thick filaments that are a major component of the sarcomeric units of myofibrils. In the presence of Ca2+, the globular head of myosin interacts with actin-containing thin filaments to generate force and movement in an ATP-dependent fashion. While myosin biochemical properties have been studied in only a few insects to date, the tools of molecular genetics have revealed that multiple isoforms of insect myosin exist in a single species with specialized isoforms accumulating in flight muscles. In at least some insect species, isoforms of myosin heavy chain and the essential light chain arise from the process of alternative splicing of transcripts from a single gene. Mutations in Drosophila myosin, in conjunction with molecular modeling, implicate particular amino acid residues in thick filament assembly, sarcomere stability and ATPase activity. Molecular genetic approaches and transgenic technology in Drosophila are proving powerful in demonstrating how structural elements of myosin affect functional properties at the biochemical, fiber and whole organism levels. These integrative studies show that properties of the indirect flight muscle are critically dependent on the specific myosin isoform expressed.

Biophys. J. 2006 90; 2427-2435.

An alternative domain near the ATP binding pocket of Drosophila myosin affects muscle fiber kinetics.

Swank, D. M., J. Braddock, W. Brown, H. Lesage, S. I. Bernstein and D. W. Maughan.

We examined the importance of alternative versions of a region near the ATP binding site of Drosophila myosin heavy chain for muscle mechanical properties. Previously, we exchanged two versions of this region (encoded by alternative exon 7s) between the indirect flight muscle myosin isoform (IFI) and an embryonic myosin isoform (EMB) and found, surprisingly, that in vitro solution actin-activated ATPase rates were increased (higher Vmax) by both exon exchanges. Here we examined the effect of increased ATPase rate on indirect flight muscle (IFM) fiber mechanics and Drosophila locomotion. IFM expressing EMB with the exon 7a domain replaced by the IFM specific exon 7d domain (EMB-7d) exhibited 3.2-fold greater maximum oscillatory power (Pmax) and 1.5-fold greater optimal frequency of power generation (fmax) versus fibers expressing EMB. In contrast, IFM expressing IFI with the exon 7d region replaced by the EMB exon 7a region (IFI-7a), showed no change in Pmax, fmax, step response, or isometric muscle properties compared to native IFI fibers. A slight decrement in IFI-7a flight ability was observed, suggesting a negative influence of the increased ATPase rate on Drosophila locomotion, perhaps due to energy supply constraints. Our results show that exon 7 plays a substantial role in establishing fiber speed and flight performance, and that the limiting step that sets ATPase rate in Drosophila myosin has little to no direct influence in setting fmax for fast muscle fiber types.

J. Mol. Biol. 2006 358: 635-645.

alphaB-Crystallin maintains skeletal muscle myosin enzymatic activity and prevents its aggregation under heat-shock stress.

Melkani, G. C., A. Cammarato and S. I. Bernstein

Here, we provide functional and direct structural evidence that alphaB-crystallin, a member of the small heat-shock protein family, suppresses thermal unfolding and aggregation of the myosin II molecular motor. Chicken skeletal muscle myosin was thermally unfolded at heat-shock temperature (43 degrees C) in the absence and in the presence of alphaB-crystallin. The ATPase activity of myosin at 25 degrees C was used as a parameter to monitor its unfolding. Myosin retained only 65% and 8% of its ATPase activity when incubated at heat-shock temperature for 15min and 30min, respectively. However, 84% and 58% of the myosin ATPase activity was maintained when it was incubated with alphaB-crystallin under the same conditions. Furthermore, actin-stimulated ATPase activity of myosin was reduced by approximately 90%, when myosin was thermally unfolded at 43 degrees C for 30min, but was reduced by only approximately 42% when it was incubated with alphaB-crystallin under the same conditions. Light-scattering assays and bound thioflavin T fluorescence indicated that myosin aggregates when incubated at 43 degrees C for 30min, while alphaB-crystallin suppressed this thermal aggregation. Photo-labeled bis-ANS alphaB-crystallin fluorescence studies confirmed the transient interaction of alphaB-crystallin with myosin. These findings were further supported by electron microscopy of rotary shadowed molecules. This revealed that approximately 94% of myosin molecules formed inter and intra-molecular aggregates when incubated at 43 degrees C for 30min. alphaB-Crystallin, however, protected approximately 48% of the myosin molecules from thermal aggregation, with protected myosin appearing identical to unheated molecules. These results are the first to show that alphaB-crystallin maintains myosin enzymatic activity and prevents the aggregation of the motor under heat-shock conditions. Thus, alphaB-crystallin may be critical for nascent myosin folding, promoting myofibrillogenesis, maintaining cytoskeletal integrity and sustaining muscle performance, since heat-shock temperatures can be produced during multiple stress conditions or vigorous exercise.

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Proc. Natl. Acad. Sci. U.S.A. 2005 102: 10522-10527.

Paramyosin phosphorylation site disruption affects indirect flight muscle stiffness and power generation in Drosophila melanogaster.

Liu, H., M. S. Miller, D. M. Swank, W. A. Kronert, D. W. Maughan, and S. I. Bernstein.

The phosphoprotein paramyosin is a major structural component of invertebrate muscle thick filaments. To investigate the importance of paramyosin phosphorylation, we produced transgenic Drosophila melanogaster in which one, three, or four phosphorylatable serine residues in the N-terminal nonhelical domain were replaced by alanines. Depending on the residues mutated, transgenic lines were either unaffected or severely flight impaired. Flight-impaired strains had decreases in the most acidic paramyosin isoforms, with a corresponding increase in more basic isoforms. Surprisingly, ultrastructure of indirect flight muscle myofibrils was normal, indicating N-terminal phosphorylation is not important for myofibril assembly. However, mechanical studies of active indirect flight muscle fibers revealed that phosphorylation site mutations reduced elastic and viscous moduli by 21-59% and maximum power output by up to 42%. Significant reductions also occurred under relaxed and rigor conditions, indicating that the phosphorylation-dependent changes are independent of strong crossbridge attachment and likely arise from alterations in thick filament backbone properties. Further, normal crossbridge kinetics were observed, demonstrating that myosin motor function is unaffected in the mutants. We conclude that N-terminal phosphorylation of Drosophila paramyosin is essential for optimal force and oscillatory power transduction within the muscle fiber and is key to the high passive stiffness of asynchronous insect flight muscles. Phosphorylation may reinforce interactions between myosin rod domains, enhance thick filament connections to the central M-line of the sarcomere and/or stabilize thick filament interactions with proteins that contribute to fiber stiffness.

J. Mol. Biol. 2005 353: 14-25.

An alternative domain near the nucleotide-binding site of Drosophila muscle myosin affects ATPase kinetics.

Miller, B. M., S. Zhang, J. A. Suggs, D. M. Swank, K. P. Littlefield, A. F. Knowles and S.I. Bernstein.

In Drosophila melanogaster expression of muscle myosin heavy chain isoforms occurs by alternative splicing of transcripts from a single gene. The exon 7 domain is one of four variable regions in the catalytic head and is located near the nucleotide-binding site. To ascribe a functional role to this domain, we created two chimeric myosin isoforms (indirect flight isoform-exon 7a and embryonic-exon 7d) that differ from the native indirect flight muscle and embryonic body-wall muscle isoforms only in the exon 7 region. Germline transformation and subsequent expression of the chimeric myosins in the indirect flight muscle of myosin-null Drosophila allowed us to purify the myosin for in vitro studies and to assess in vivo structure and function of transgenic muscles. Intriguingly, in vitro experiments show the exon 7 domain modulates myosin ATPase activity but has no effect on actin filament velocity, a novel result compared to similar studies with other Drosophila variable exons. Transgenic flies expressing the indirect flight isoform-exon 7a have normal indirect flight muscle structure, and flight and jump ability. However, expression of the embryonic-exon 7d chimeric isoform yields flightless flies that show improvements in both the structural stability of the indirect flight muscle and in locomotor abilities as compared to flies expressing the embryonic isoform. Overall, our results suggest the exon 7 domain participates in the regulation of the attachment of myosin to actin in order to fine-tune the physiological properties of Drosophila myosin isoforms.

Biophys. J. 2004 87: 1805-1814.

Alternative N-terminal regions of Drosophila myosin heavy chain tune cross-bridge kinetics for optimal muscle power output.

Swank, D.M., W.A. Kronert, S.I. Bernstein and D.W. Maughan.

We assessed the influence of alternative versions of a region near the N-terminus of Drosophila myosin heavy chain on muscle mechanical properties. Previously, we exchanged N-terminal regions (encoded by alternative exon 3s) between an embryonic (EMB) isoform and the indirect flight muscle isoform (IFI) of myosin, and demonstrated that it influences solution ATPase rates and in vitro actin sliding velocity. Because each myosin is expressed in Drosophila indirect flight muscle, in the absence of other myosin isoforms, this allows for muscle mechanical and whole organism locomotion assays. We found that exchanging the flight muscle specific exon 3 region into the embryonic isoform (EMB-3b) increased maximum power generation (P(max)) and optimal frequency of power generation (f(max)) threefold and twofold compared to fibers expressing EMB, whereas exchanging the embryonic exon 3 region into the flight muscle isoform (IFI-3a) decreased P(max) and f(max) to approximately 80% of IFI fiber values. Drosophila expressing IFI-3a exhibited a reduced wing beat frequency compared to flies expressing IFI, which optimized power generation from their kinetically slowed flight muscle. However, the slower wing beat frequency resulted in a substantial loss of aerodynamic power as manifest in decreased flight performance of IFI-3a compared to IFI. Thus the N-terminal region is important in tuning myosin kinetics to match muscle speed for optimal locomotory performance.

J. Muscle Res. Cell Motil. 2004 25: 359-366.

Passive stiffness of Drosophila IFM myofibrils: A novel, high accuracy measurement method.

Hao, Y., S. I. Bernstein and G.H. Pollack.

As the smallest muscle-cell substructure that retains the intact contractile apparatus, the single myofibril is considered the optimal specimen for muscle mechanics, although its small size also poses some technical difficulties. Myofibrils from Drosophila indirect flight muscle (IFM) are particularly difficult to study because their high passive stiffness makes them hard to handle, and too resistant to stretch to produce enough elongation for the accurate measurement of sarcomere length change. In this study, we devised a novel method for accurate stiffness measurement of single relaxed myofibrils using microfabricated cantilevers and phase contrast microscopy. A special experimental protocol was developed to minimize errors, and some data analysis strategies were used to identify and exclude spurious data. Remarkably consistent results were obtained from Drosophila IFM myofibrils. This novel, high accuracy method is potentially an effective tool for detecting small passive stiffness change in muscle mutants.

Curr. Biol. 2003 13: R525-R527

UCS proteins: managing the myosin motor.

Yu, Q. and S. I. Bernstein.

Recent studies indicate that myosin molecular motors interact inside cells with proteins containing a conserved 'UCS' domain. This appears to ensure proper folding of myosin heads so that they can perform their ATP-dependent actin-based motor functions.

J. Biol. Chem. 2003 278: 50293-50300.

Kinetic analysis of Drosophila muscle myosin isoforms suggests a novel mode of mechanochemical coupling.

Miller, B.M., M. Nyitrai, S. I. Bernstein, and M. A. Geeves.

The molecular mechanism of myosin function was addressed by measuring transient kinetic parameters of naturally occurring and chimeric Drosophila muscle myosin isoforms. We assessed the native embryonic isoform, the native indirect flight muscle isoform, and two chimeric isoforms containing converter domains exchanged between the indirect flight muscle and embryonic isoforms. Myosin was purified from the indirect flight muscles of transgenic flies, and S1 was produced by alpha-chymotryptic digestion. Previous studies in vertebrate and scallop myosins have shown a correlation between actin filament velocity in motility assays and cross-bridge detachment rate, specifically the rate of ADP release. In contrast, our study showed no correlation between the detachment rate and actin filament velocity in Drosophila myosin isoforms and further that the converter domain does not significantly influence the biochemical kinetics governing the detachment of myosin from actin. We suggest that evolutionary pressure on a single muscle myosin gene may maintain a fast detachment rate in all isoforms. As a result, the attachment rate and completion of the power stroke or the equilibrium between actin.myosin.ADP states may define actin filament velocity for these myosin isoforms.

J Biol Chem 2003 May 9;278(19):17475-82

Variable N-terminal Regions of Muscle Myosin Heavy Chain Modulate ATPase Rate and Actin Sliding Velocity.

Swank DM, Knowles AF, Kronert WA, Suggs JA, Morrill GE, Nikkhoy M, Manipon GG, Bernstein SI.

We integratively assessed the function of alternative versions of a region near the N terminus of Drosophila muscle myosin heavy chain (encoded by exon 3a or 3b). We exchanged the alternative exon 3 regions between an embryonic isoform and the indirect flight muscle isoform. Each chimeric myosin was expressed in Drosophila indirect flight muscle, in the absence of other myosin isoforms, allowing for purified protein analysis and whole organism locomotory studies. The flight muscle isoform generates higher in vitro actin sliding velocity and solution ATPase rates than the embryonic isoform. Exchanging the embryonic exon 3 region into the flight muscle isoform decreased ATPase rates to embryonic levels but did not affect actin sliding velocity or flight muscle ultrastructure. Interestingly, this swap only slightly impaired flight ability. Exchanging the flight muscle-specific exon 3 region into the embryonic isoform increased actin sliding velocity 3-fold and improved indirect flight muscle ultrastructure integrity but failed to rescue the flightless phenotype of flies expressing embryonic myosin. These results suggest that the two structural versions of the exon 3 domain independently influence the kinetics of at least two steps of the actomyosin cross-bridge cycle.

Am J Physiol Cell Physiol 2003 Apr;284(4):C1031-8

The converter domain modulates kinetic properties of Drosophila myosin.

Littlefield KP, Swank DM, Sanchez BM, Knowles AF, Warshaw DM, Bernstein SI.

Recently the converter domain, an integral part of the "mechanical element" common to all molecular motors, was proposed to modulate the kinetic properties of Drosophila chimeric myosin isoforms. Here we investigated the molecular basis of actin filament velocity (V(actin)) changes previously observed with the chimeric EMB-IC and IFI-EC myosin proteins [the embryonic body wall muscle (EMB) and indirect flight muscle isoforms (IFI) with genetic substitution of the IFI and EMB converter domains, respectively]. In the laser trap assay the IFI and IFI-EC myosins generate the same unitary step displacement (IFI = 7.3 +/- 1.0 nm, IFI-EC = 5.8 +/- 0.9 nm; means +/- SE). Thus converter-mediated differences in the kinetics of strong actin-myosin binding, rather than the mechanical capabilities of the protein, must account for the observed V(actin) values. Basal and actin-activated ATPase assays and skinned fiber mechanical experiments definitively support a role for the converter domain in modulating the kinetic properties of the myosin protein. We propose that the converter domain kinetically couples the P(i) and ADP release steps that occur during the cross-bridge cycle.

Nat Cell Biol 2002 Apr;4(4):312-6

The myosin converter domain modulates muscle performance.

Swank DM, Knowles AF, Suggs JA, Sarsoza F, Lee A, Maughan DW, Bernstein SI.

Myosin is the molecular motor that powers muscle contraction as a result of conformational changes during its mechanochemical cycle. We demonstrate that the converter, a compact structural domain that differs in sequence between Drosophila melanogaster myosin isoforms, dramatically influences the kinetic properties of myosin and muscle fibres. Transgenic replacement of the converter in the fast indirect flight muscle with the converter from an embryonic muscle slowed muscle kinetics, forcing a compensatory reduction in wing beat frequency to sustain flight. Conversely, replacing the embryonic converter with the flight muscle converter sped up muscle kinetics and increased maximum power twofold, compared to flight muscles expressing the embryonic myosin isoform. The substitutions also dramatically influenced in vitro actin sliding velocity, suggesting that the converter modulates a rate-limiting step preceding cross-bridge detachment. Our integrative analysis demonstrates that isoform-specific differences in the myosin converter allow different muscle types to meet their specific locomotion demands.

J. Muscle Res. Cell Motil. (2001) 22: 287-299.

Overexpression of miniparamyosin causes dysfunction and age-dependent myofibril degeneration in the indirect flight muscles of Drosophila melanogaster.

Arredondo, J.J., M. Mardahl-Dumesnil, R.M. Cripps, M. Cervera and S.I. Bernstein.

Miniparamyosin (mPM) is a protein of invertebrate muscle thick filaments. Its similarity to paramyosin (PM) suggests that it regulates thick filament and myofibril assembly. To determine its role in muscle structure and function we overexpressed mPM in muscles of Drosophila melanogaster. Surprisingly, myofibrils accumulating excess mPM assemble nearly normally, with thick filament electron density and sarcomere length unaffected. Myofibrils in some indirect flight muscle groups are misaligned and young flies exhibit a moderate level of flight impairment. This phenotype is exacerbated with age. Transgenic flies undergo progressive myofibril deterioration that increases flight muscle dysfunction. Our observations indicate that the correct stoichiometry of mPM is important for maintenance of myofibril integrity and for the proper function of the flight musculature.

J. Biol. Chem. (2001) 276: 15117-15124.

Alternative exon-encoded regions of Drosophila myosin heavy chain modulate ATPase rates and actin sliding velocity.

Swank, D.M., M.L. Bartoo, A.F. Knowles, C. Iliffe, S.I. Bernstein, J.E. Molloy and J.C. Sparrow.

To investigate the molecular functions of the regions encoded by alternative exons from the single Drosophila myosin heavy chain gene, we made the first kinetic measurements of two muscle myosin isoforms that differ in all alternative regions. Myosin was purified from the indirect flight muscles of wild-type and transgenic flies expressing a major embryonic isoform. The in vitro actin sliding velocity on the flight muscle isoform (6.4 microns x s(-1) at 22 degrees C) is among the fastest reported for a type II myosin and was 9-fold faster than with the embryonic isoform. With smooth muscle tropomyosin bound to actin, the actin sliding velocity on the embryonic isoform increased 6-fold, whereas that on the flight muscle myosin slightly decreased. No difference in the step sizes of Drosophila and rabbit skeletal myosins were found using optical tweezers, suggesting that the slower in vitro velocity with the embryonic isoform is due to altered kinetics. Basal ATPase rates for flight muscle myosin are higher than those of embryonic and rabbit myosin. These differences explain why the embryonic myosin cannot functionally substitute in vivo for the native flight muscle isoform, and demonstrate that one or more of the five myosin heavy chain alternative exons must influence Drosophila myosin kinetics.

Mech. Dev. (2001) 101: 35-39.

Spatially and temporally regulated expression of myosin heavy chain alternative exons during embryogenesis of Drosophila.

Zhang, S. and S.I. Bernstein.

We used alternative exon-specific probes to determine the accumulation of transcripts encoding myosin heavy chain (MHC) isoforms in Drosophila melanogaster embryos. Six isoforms accumulate in body wall muscles. Transverse (external) muscles express a different major form than intermediate and internal muscles, suggesting different properties. Cardioblasts express one of the somatic muscle transcripts; visceral muscles express at least two transcript types. The pharyngeal muscle accumulates a unique Mhc transcript, suggesting unique contractile abilities. Mhc transcription begins in stage 12 in visceral and somatic muscles, but as late as stage 15 in cardioblasts. This is the first study of myosin isoform localization during insect embryogenesis, and forms the basis for transgenic and biochemical experiments designed to determine how MHC domains regulate muscle physiology.

J. Biol. Chem. (2001) 276: 8278-8287.

Control of Drosophila paramyosin/miniparamyosin gene expression: differential regulatory mechanisms for muscle-specific transcription.

Arredondo, J.J., R.M. Ferreres, M. Maroto, R.M. Cripps, R. Marco, S. I. Bernstein and M. Cervera.

To define the transcriptional mechanisms contributing to stage- and tissue-specific expression of muscle genes, we performed transgenic analysis of Drosophila paramyosin gene regulation. This gene has two promoters, one for paramyosin and one for miniparamyosin, which are active in partially overlapping domains. Regions between 0.9 and 1.7 kilobases upstream of each initiation site contribute to the temporal and spatial expression patterns. By comparing the Drosophila melanogaster and Drosophila virilis promoters, conserved binding sites were found for known myogenic factors, including one MEF2 site and three E boxes. In contrast with previous data, our experiments with the paramyosin promoter indicate that the MEF2 site is essential but not sufficient for proper paramyosin gene transcription. Mutations in the three E boxes, on the other hand, do not produce any effect in embryonic/larval muscles. Thus MEF2 site- and E box binding proteins can play different roles in the regulation of different muscle-specific genes. For the miniparamyosin promoters, several conserved sequences were shown to correspond to functionally important regions. Our data further show that the two promoters work independently. Even when both promoters are active in the same muscle fiber, the transcription driven by one of the promoters is not affected by transcription driven by the other.

Microsc. Res. Tech. (2000) 50: 430-442.

Determining structure/function relationships for sarcomeric myosin heavy chain by genetic and transgenic manipulation of Drosophila.

Swank, D.M., L. Wells, W.A. Kronert, G.E. Morrill and S.I. Bernstein.

Drosophila melanogaster is an excellent system for examining the structure/function relationships of myosin. It yields insights into the roles of myosin in assembly and stability of myofibrils, in defining the mechanical properties of muscle fibers, and in dictating locomotory abilities. Drosophila has a single gene encoding muscle myosin heavy chain (MHC), with alternative RNA splicing resulting in stage- and tissue-specific isoform production. Localization of the alternative domains of Drosophila MHC on a three-dimensional molecular model suggests how they may determine functional differences between isoforms. We are testing these predictions directly by using biophysical and biochemical techniques to characterize myosin isolated from transgenic organisms. Null and missense mutations help define specific amino acid residues important in actin binding and ATP hydrolysis and the function of MHC in thick filament and myofibril assembly. Insights into the interaction of thick and thin filaments result from studying mutations in MHC that suppress ultrastructural defects induced by a troponin I mutation. Analysis of transgenic organisms expressing engineered versions of MHC shows that the native isoform of myosin is not critical for myofibril assembly but is essential for muscle function and maintenance of muscle integrity. We show that the C-terminus of MHC plays a pivotal role in the maintenance of muscle integrity. Transgenic studies using headless myosin reveal that the head is important for some, but not all, aspects of myofibril assembly. The integrative approach described here provides a multi-level understanding of the function of the myosin molecular motor.

EMBO J. 1999 18: 1793-1804.

Assembly of thick filaments and myofibrils occurs in the absence of the myosin head.

Cripps, R.M., J.A. Suggs and S.I. Bernstein.

We investigated the importance of the myosin head in thick filament formation and myofibrillogenesis by generating transgenic Drosophila lines expressing either an embryonic or an adult isoform of the myosin rod in their indirect flight muscles. The headless myosin molecules retain the regulatory light-chain binding site, the alpha-helical rod and the C-terminal tailpiece. Both isoforms of headless myosin co-assemble with endogenous full-length myosin in wild-type muscle cells. However, rod polypeptides interfere with muscle function and cause a flightless phenotype. Electron microscopy demonstrates that this results from an antimorphic effect upon myofibril assembly. Thick filaments assemble when the myosin rod is expressed in mutant indirect flight muscles where no full-length myosin heavy-chain is produced. These filaments show the characteristic hollow cross-section observed in wild type. The headless thick filaments can assemble with thin filaments into hexagonally packed arrays resembling normal myofibrils. However, thick filament length as well as sarcomere length and myofibril shape are abnormal. Therefore, thick filament assembly and many aspects of myofibrillogenesis are independent of the myosin head and these processes are regulated by the myosin rod and tailpiece. However, interaction of the myosin head with other myofibrillar components is necessary for defining filament length and myofibril dimensions.

J Cell Biol 1999 Mar 8;144(5):989-1000.

Specific myosin heavy chain mutations suppress troponin I defects in Drosophila muscles.

Kronert, W.A., A. Acebes, A. Ferrus and S.I. Bernstein.

We show that specific mutations in the head of the thick filament molecule myosin heavy chain prevent a degenerative muscle syndrome resulting from the hdp2 mutation in the thin filament protein troponin I. One mutation deletes eight residues from the actin binding loop of myosin, while a second affects a residue at the base of this loop. Two other mutations affect amino acids near the site of nucleotide entry and exit in the motor domain. We document the degree of phenotypic rescue each suppressor permits and show that other point mutations in myosin, as well as null mutations, fail to suppress the hdp2 phenotype. We discuss mechanisms by which the hdp2 phenotypes are suppressed and conclude that the specific residues we identified in myosin are important in regulating thick and thin filament interactions. This in vivo approach to dissecting the contractile cycle defines novel molecular processes that may be difficult to uncover by biochemical and structural analyses. Our study illustrates how expression of genetic defects are dependent upon "genetic background", and therefore could have implications for understanding gene interactions in human disease.

Genetics 1999 151:263-276.

The role of evolutionarily-conserved sequences in alternative splicing at the 3' end of Drosophila melanogaster myosin heavy chain RNA.

Hodges, D., R.M. Cripps, M. O'Connor, and S.I. Bernstein.

Exon 18 of the muscle myosin heavy chain gene (Mhc) of Drosophila melanogaster is excluded from larval transcripts, but included in most adult transcripts. To identify cis-acting elements regulating this alternative RNA splicing, we sequenced the 3' end of Mhc from the distantly related species D. virilis. Three non-coding regions are conserved: 1) The non-consensus splice junctions at either end of exon 18. 2) Exon 18 itself. 3) A 30 nucleotide, pyrimidine-rich sequence located about 40 nt upstream of the 3' splice site of exon 18. We generated transgenic flies expressing Mhc mini-genes designed to test the function of these regions. Improvement of both splice sites of adult-specific exon 18 toward the consensus sequence switches the splicing pattern to include exon 18 in all larval transcripts. Thus non-consensus splice junctions are critical to stage-specific exclusion of this exon. Deletion of nearly all of exon 18 does not affect stage-specific utilization. However, splicing of transcripts lacking the conserved pyrimidine sequence is severely disrupted in adults. Disruption is not rescued by insertion of a different polypyrimidine tract, suggesting that the conserved pyrimidine-rich sequence interacts with tissue-specific splicing factors to activate utilization of the poor splice sites of exon 18 in adult muscle.

J Mol Biol 1997 Aug 8;271(1):1-6

Fine tuning a molecular motor: the location of alternative domains in the Drosophila myosin head.

Bernstein SI, Milligan RA

Myosin isoform sequence variation is likely critical for generating differences in contraction velocity and force production exhibited by the various skeletal muscles in an animal. To examine how myosin heavy chain (MHC) isoform diversity could affect physiological function, we studied the locations of structural differences in the motor domains of muscle MHCs from Drosophila melanogaster. Drosophila has only one muscle Mhc gene. Isoform variation is achieved by alternative splicing of a limited number of exons, clearly delineating the domains of MHC that are critical for muscle-specific functions. There are four alternative regions that contribute to the motor domain of Drosophila myosin. We used the X-ray structure of chicken skeletal S1 as a framework to examine the locations of these four regions. One lies near the ATP-binding pocket in a position where amino acid changes might be expected to modulate entry or exit of the nucleotide. Interestingly, the other three are clustered at the distal end of the molecule, surrounding the reactive cysteine SH1 and the pivot point about which the light chain-containing region swings. These observations underscore the importance of this region, distant from the site of ATP entry and the actin binding interface, as a part of the molecule where modulation of function can be achieved.

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EMBO J 1996 Sep 2;15(17):4454-4459

Myosin heavy chain isoforms regulate muscle function but not myofibril assembly.

Wells L, Edwards KA, Bernstein SI

Myosin heavy chain (MHC) is the motor protein of muscle thick filaments. Most organisms produce many muscle MHC isoforms with temporally and spatially regulated expression patterns. This suggests that isoforms of MHC have different characteristics necessary for defining specific muscle properties. The single Drosophila muscle Mhc gene yields various isoforms as a result of alternative RNA splicing. To determine whether this multiplicity of MHC isoforms is critical to myofibril assembly and function, we introduced a gene encoding only an embryonic MHC into Drosophila melanogaster. The embryonic transgene acts in a dominant antimorphic manner to disrupt flight muscle function. The transgene was genetically crossed into an MHC null background. Unexpectedly, transformed flies expressing only the embryonic isoform are viable. Adult muscles containing embryonic MHC assemble normally, indicating that the isoform of MHC does not determine the dramatic ultrastructural variation among different muscle types. However, transformed flies are flightless and show reduced jumping and mating ability. Their indirect flight muscle myofibrils progressively deteriorate. Our data show that the proper MHC isoform is critical for specialized muscle function and myofibril stability.

J Mol Biol 1995 May 26;249(1):111-125

Defects in the Drosophila myosin rod permit sarcomere assembly but cause flight muscle degeneration.

Kronert WA, O'Donnell PT, Fieck A, Lawn A, Vigoreaux JO, Sparrow JC, Bernstein SI

We have determined the molecular and ultrastructural defects associated with three homozygous-viable myosin heavy chain mutations of Drosophila melanogaster. These mutations cause a dominant flightless phenotype but allow relatively normal assembly of indirect flight muscle myofibrils. As adults age, the contents of the indirect flight muscle myofibers are pulled to one end of the thorax. This apparently results from myofibril "hyper-contraction", and leads to sarcomere rupture and random myofilament orientation. All three mutations cause single amino acid changes in the light meromyosin region of the myosin rod. Two change the same glutamic acid to a lysine residue and the third affects an amino acid five residues away, substituting histidine for arginine. Both affected residues are conserved in muscle myosins, cytoplasmic myosins and paramyosins. The mutations are associated with age-dependent, site-specific degradation of myosin heavy chain and failure to accumulate phosphorylated forms of flightin, an indirect flight muscle-specific protein previously localized to the thick filament. Given the repeating nature of the hydrophobic and charged amino acid residues of the myosin rod and the near-normal assembly of myofibrils in the indirect flight muscle of these mutants, it is remarkable that single amino acid changes in the rod cause such severe defects. It is also interesting that these severe defects are not apparent in other muscles. These phenomena likely arise from the highly organized nature and rigorous performance requirements of indirect flight muscle, and perhaps from the interaction of myosin with flightin, a protein specific to this muscle type.

Trends Cardiovasc Med. 1994 Nov-Dec;4(6):243-50

Genetic and transgenic approaches to dissecting muscle development and contractility using the Drosophila model system.

Becker KD, Bernstein SI.

Both genetic and transgenic analyses of Drosophila melanogaster, the common fruit fly, are providing important insights into the mechanisms of muscle cell determination and development, myofibril assembly, and muscle contraction. This model system affords tremendous advantages such as ease of isolating mutants defective in these processes, determining the identity of affected genes, and analyzing protein function by transformation with in vitro mutagenized versions of such genes. These approaches have identified a series of proteins that are critical to mesoderm and muscle determination, many of which are likely to serve similar roles in vertebrates. The effects of mutating structural protein genes upon myofibril assembly and function in Drosophila help to define the differential roles of contractile protein isoforms and the importance of proper protein stoichiometry for physiologic function. These studies may also provide insight into the role of structural proteins in vertebrate contractility.

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J Cell Biol 1994 Aug;126(3):689-699

Transformation of Drosophila melanogaster with the wild-type myosin heavy-chain gene: rescue of mutant phenotypes and analysis of defects caused by overexpression.

Cripps RM, Becker KD, Mardahl M, Kronert WA, Hodges D, Bernstein SI

We have transformed Drosophila melanogaster with a genomic construct containing the entire wild-type myosin heavy-chain gene, Mhc, together with approximately 9 kb of flanking DNA on each side. Three independent lines stably express myosin heavy-chain protein (MHC) at approximately wild-type levels. The MHC produced is functional since it rescues the mutant phenotypes of a number of different Mhc alleles: the amorphic allele Mhc1, the indirect flight muscle and jump muscle-specific amorphic allele Mhc10, and the hypomorphic allele Mhc2. We show that the Mhc2 mutation is due to the insertion of a transposable element in an intron of Mhc. Since a reduction in MHC in the indirect flight muscles alters the myosin/actin protein ratio and results in myofibrillar defects, we determined the effects of an increase in the effective copy number of Mhc. The presence of four copies of Mhc results in overabundance of the protein and a flightless phenotype. Electron microscopy reveals concomitant defects in the indirect flight muscles, with excess thick filaments at the periphery of the myofibrils. Further increases in copy number are lethal. These results demonstrate the usefulness and potential of the transgenic system to study myosin function in Drosophila. They also show that overexpression of wild-type protein in muscle may disrupt the function of not only the indirect flight but also other muscles of the organism.

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J Mol Biol 1994 Feb 25;236(3):697-702

A charge change in an evolutionarily-conserved region of the myosin globular head prevents myosin and thick filament accumulation in Drosophila.

Kronert WA, O'Donnell PT, Bernstein SI

We have determined the molecular lesion in Mhc9, a homozygous-viable mutant of the Drosophila muscle myosin heavy chain gene. This mutation is in an adult-specific alternative exon (exon 9a) which encodes a portion of the myosin head that is highly conserved among both cytoplasmic and muscle myosins of all organisms. The mutation results in a charge change in the evolutionarily invariant amino acid residue 482. The phenotype of the homozygous mutant is identical to that of an organism having a stop codon within alternative exon 9a, i.e. lack of thick filaments in the indirect flight muscles and a greatly reduced number of thick filaments in the small cells of the jump muscles. This phenotype correlates with the known expression pattern of exon 9a. Genetic, biochemical and ultrastructural analyses show that the failure to accumulate thick filaments in the mutant is not a result of aberrant interactions with thin filaments and that the mutant myosin heavy chain does not poison assembly of wild-type thick filaments. Our results, in conjunction with recent structural and mutant studies by others, indicate that residue 482 is important for generating ATPase activity and for myosin stability in muscle.

Dev Biol. 1992 Dec;154(2):231-44

Genetic approaches to understanding muscle development.

Epstein HF, Bernstein SI.

The analysis of both naturally occurring and experimentally induced mutants has greatly advanced our understanding of muscle development. Molecular biological techniques have led to the isolation of genes associated with inherited human diseases that affect muscle tissues. Analysis of the encoded proteins in conjunction with the mutant phenotypes can provide powerful insights into the function of the protein in normal muscle development. Systematic searches for muscle mutations have been made in experimental systems, most notably the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. In addition, known muscle protein genes from other organisms have been used to isolate homologs from genetically manipulatable organisms, allowing mutant analysis and the study of protein function in vivo. Mutations in transcription factor genes that affect mesoderm development have been isolated and genetic lesions affecting myofibril assembly have been identified. Genetic experiments inducing mutations and rescuing them by transgenic methods have uncovered functions of myofibrillar protein isoforms. Some isoforms perform muscle-specific functions, whereas others appear to be replaceable by alternative isoforms. Mutant analysis has also uncovered a relationship between proteins at the cell membrane and the assembly and alignment of the myofibrillar apparatus. We discuss examples of each of these genetic approaches as well as the developmental and evolutionary implications of the results.

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Mech Dev 1992 May;37(3):127-140

Suboptimal 5' and 3' splice sites regulate alternative splicing of Drosophila melanogaster myosin heavy chain transcripts in vitro.

Hodges D, Bernstein SI

Using a Drosophila cell-free system, we have analyzed the regulation of alternative splicing of Drosophila muscle myosin heavy chain (MHC) transcripts. Splicing of MHC 3' end transcripts results in exclusion of adult-specific alternative exon 18, as is observed in embryonic and larval muscle in vivo. Mutations that strengthen either the 5' or the 3' splice sites of exon 18 do not promote inclusion of this exon. However, strengthening both splice junctions results in efficient removal of both introns and completely inhibits skip splicing. Our data suggest that the affinity of exons 17 and 19, as well as failure of constitutive splicing factors to recognize exon 18 splice sites, causes the exclusion of exon 18 in wild-type transcripts processed in vitro.

J Cell Biol 1992 Feb;116(3):669-681

Analysis of Drosophila paramyosin: identification of a novel isoform which is restricted to a subset of adult muscles.

Becker KD, O'Donnell PT, Heitz JM, Vito M, Bernstein SI

In this report we show that Drosophila melanogaster muscles contain the standard form of the thick filament protein paramyosin, as well as a novel paramyosin isoform, which we call miniparamyosin. We have isolated Drosophila paramyosin using previously established methods. This protein is approximately 105 kD and cross-reacts with polyclonal antibodies made against Caenorhabditis elegans or Heliocopris dilloni paramyosin. The Heliocopris antibody also cross-reacts with a approximately 55-kD protein which may be miniparamyosin. We have cloned and sequenced cDNA's encoding both Drosophila isoforms. Standard paramyosin has short nonhelical regions at each terminus flanking the expected alpha-helical heptad repeat seen in other paramyosins and in myosin heavy chains. The COOH-terminal 363 amino acids are identical in standard and miniparamyosin. However, the smaller isoform has 114 residues at the NH2 terminus that are unique as compared to the current protein sequence data base. The paramyosin gene is located at chromosome position 66E1. It appears to use two promoters to generate mRNA's that have either of two different 5' coding sequences joined to common 3' exons. Each protein isoform is encoded by two transcripts that differ only in the usage of polyadenylation signals. This results in four size classes of paramyosin mRNA which are expressed in a developmentally regulated pattern consistent with that observed for other muscle-specific RNA's in Drosophila. In situ hybridization to Drosophila tissue sections shows that standard paramyosin is expressed in all larval and adult muscle tissues whereas miniparamyosin is restricted to a subset of the adult musculature. Thus miniparamyosin is a novel muscle-specific protein that likely plays a role in thick filament structure or function in some adult muscles of Drosophila.

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EMBO J 1991 Sep;10(9):2479-2488

Muscle-specific accumulation of Drosophila myosin heavy chains: a splicing mutation in an alternative exon results in an isoform substitution.

Kronert WA, Edwards KA, Roche ES, Wells L, Bernstein SI

We show that the molecular lesions in two homozygousviable mutants of the Drosophila muscle myosin heavy chain gene affect an alternative exon (exon 9a) which encodes a portion of the myosin head that is highly conserved among both cytoplasmic and muscle myosins of all organisms. In situ hybridization and Northern blotting analysis in wild-type organisms indicates that exon 9a is used in indirect flight muscles whereas both exons 9a and 9b are utilized in jump muscles. Alternative exons 9b and 9c are used in other larval and adult muscles. One of the mutations in exon 9a is a nonsense allele that greatly reduces myosin RNA stability. It prevents thick filament accumulation in indirect flight muscles and severely reduces the number of thick filaments in a subset of cells of the jump muscles. The second mutation affects the 5' splice site of exon 9a. This results in production of an aberrantly spliced transcript in indirect flight muscles, which prevents thick filament accumulation. Jump muscles of this mutant substitute exon 9b for exon 9a and consequently have normal levels of thick filaments in this muscle type. This isoform substitution does not obviously affect the ultrastructure or function of the jump muscle. Analysis of this mutant illustrates that indirect flight muscles and jump muscles utilize different mechanisms for alternative RNA splicing.

Dev Biol 1991 Aug;146(2):339-344

Developmentally regulated alternative splicing of Drosophila myosin heavy chain transcripts: in vivo analysis of an unusual 3' splice site.

Hess NK, Bernstein SI

The 3' penultimate exon (exon 18) of transcripts from the muscle myosin heavy chain (MHC) gene of Drosophila melanogaster is excluded from mRNAs of embryonic and larval muscle, while it is included in mRNAs of adult thoracic muscles. By transforming organisms with the MHC gene 5' end, linked in-frame to the MHC gene 3' end, we were able to generate correct tissue-specific expression of this minigene and stage-specific splicing of exon 18, indicating that all the cis-acting sequences necessary for alternative splicing are contained within the construct. The 3' splice site that precedes exon 18 is unusually purine-rich, may form a stem-loop structure with the 5' splice site following exon 18, and is conserved relative to the splice site of an alternative exon of the Drosophila alkali myosin light chain gene. We converted the MHC gene 3' splice junction to a consensus splice site and also inserted the branchpoint and 3' splice site of a constitutively spliced intron in its place. These alterations had no effect on the splicing pathway in vivo, ruling out the possibility that the unusual splice junction, or secondary structures that involve this splice junction, directly regulate alternative splicing of exon 18.

Genes Dev 1990 Jun;4(6):885-895

Alternative myosin hinge regions are utilized in a tissue-specific fashion that correlates with muscle contraction speed.

Collier VL, Kronert WA, O'Donnell PT, Edwards KA, Bernstein SI

By comparing the structure of wild-type and mutant muscle myosin heavy chain (MHC) genes of Drosophila melanogaster, we have identified the defect in the homozygous-viable, flightless mutant Mhc10. The mutation is within the 3' splice acceptor of an alternative exon (exon 15a) that encodes the central region of the MHC hinge. The splice acceptor defect prevents the accumulation of mRNAs containing exon 15a, whereas transcripts with a divergent copy of this exon (exon 15b) are unaffected by the mutation. In situ hybridization and Northern blot analysis of wild-type organisms reveals that exon 15b is used in larval MHCs, whereas exons 15a and/or 15b are used in adult tissues. Because Mhc10 mutants fail to accumulate transcripts encoding MHC protein with hinge region a, analysis of their muscle-specific reduction in thick filament number serves as a sensitive assay system for determining the pattern of accumulation of MHCs with alternative hinge regions. Electron microscopic comparisons of various muscles from wild-type and Mhc10 adults reveals that those that contract rapidly or develop high levels of tension utilize only hinge region a, those that contract at moderate rates accumulate MHCs of both types, and those that are slowly contracting have MHCs with hinge region b. The presence of alternative hinge-coding exons and their highly tissue-specific usage suggests that this portion of the MHC molecule is important to the isoform-specific properties of MHC that lead to the different physiological and ultrastructural characteristics of various Drosophila muscle types. The absence of other alternative exons in the rod-coding region, aside from those shown previously to encode alternative carboxyl termini, demonstrates that the bulk of the myosin rod is not involved in the generation of isoform-specific properties of the MHC molecule.

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Genes Dev 1989 Aug;3(8):1233-1246

Ultrastructural and molecular analyses of homozygous-viable Drosophila melanogaster muscle mutants indicate there is a complex pattern of myosin heavy-chain isoform distribution.

O'Donnell PT, Collier VL, Mogami K, Bernstein SI

We describe the ultrastructural and initial molecular characterization of four homozygous-viable, dominant-flightless mutants of Drosophila melanogaster. Genetic mapping indicates that these mutations are inseparable from the known muscle myosin heavy-chain (MHC) allele Mhc1, and each mutation results in a muscle-specific reduction in MHC protein accumulation. The indirect flight muscles (IFMs) of each of these homozygous mutants fail to accumulate MHC, lack thick filaments, and do not display normal cylindrical myofibrils. As opposed to the null phenotype observed in the IFM, normal amounts of MHC accumulate in the leg muscles of three of these mutants, whereas the fourth mutant shows a 45% reduction in leg muscle MHC. The ultrastructure of the tergal depressor of the trochanter muscle TDT, or jump muscle) is normal in one mutant, completely lacks thick filaments in a second mutant, and displays a reduction of thick filaments in two mutants. The thick filament reduction in this latter class of mutants is limited to the four smaller anterior cells of the TDT, indicating that the TDT is a mixed fiber-type muscle. Because all isoforms of muscle MHC are encoded by alternative splicing of transcripts from a single gene, our results suggest that there is a complex pattern of MHC isoform accumulation in Drosophila. The phenotypes of the homozygous-viable mutants provide evidence for the differential localization of MHC isoforms in different muscles, within the same muscle, and even within a single muscle cell. The mutant characteristics also suggest that the use of some alternative exons is shared among the IFM, TDT, and additional muscles whereas the use of others is unique to the IFM.

J Cell Biol 1988 Dec;107(6 Pt 2):2601-2612

Molecular and ultrastructural defects in a Drosophila myosin heavy chain mutant: differential effects on muscle function produced by similar thick filament abnormalities.

O'Donnell PT, Bernstein SI

We have determined the molecular defect of the Drosophila melanogaster myosin heavy chain (MHC) mutation Mhc and the mutation's effect on indirect flight muscle, jump muscle, and larval intersegmental muscle. We show that the Mhc1 mutation is essentially a null allele which results in the dominant-flightless and recessive-lethal phenotypes associated with this mutant (Mogami, K., P. T. O'Donnell, S. I. Bernstein, T. R. F. Wright, C. P. Emerson, Jr. 1986. Proc. Natl. Acad. Sci. USA. 83:1393-1397). The mutation is a 101-bp deletion in the MHC gene which removes most of exon 5 and the intron that precedes it. S1 nuclease mapping indicates that mutant transcripts follow two alternative processing pathways. Both pathways result in the production of mature transcripts with altered reading frames, apparently yielding unstable, truncated MHC proteins. Interestingly, the preferred splicing pathway uses the more distal of two available splice donor sites. We present the first ultrastrutural characterization of a completely MHC-null muscle and show that it lacks any discernable thick filaments. Sarcomeres in these muscles are completely disorganized suggesting that thick filaments play a critical role in sarcomere assembly. To understand why the Mhc1 mutation severely disrupts indirect flight muscle and jump muscle function in heterozygotes, but does not seriously affect the function of other muscle types, we examined the muscle ultrastructure of Mhc1/+ heterozygotes. We find that these organisms have a nearly 50% reduction in the number of thick filaments in indirect flight muscle, jump muscle, and larval intersegmental muscle. In addition, aberrantly shaped thick filaments are common in the jump muscle and larval intersegmental muscle. We suggest that the differential sensitivity of muscle function to the Mhc1 mutation is a consequence of the unique myofilament arrays in each of these muscles. The highly variable myofilament array of larval intersegmental muscle makes its function relatively insensitive to changes in thick filament number and morphology. Conversely, the rigid double hexagonal lattice of the indirect flight muscle, and the organized lattice of the jump muscle cannot be perturbed without interfering with the specialized and evolutionarily more complex functions they perform.

J Biol Chem 1988 Jul 5;263(19):9079-9082

Altered turnover of allelic variants of hypoxanthine phosphoribosyltransferase is associated with N-terminal amino acid sequence variation.

Johnson GG, Kronert WA, Bernstein SI, Chapman VM, Smith KD

The results of our previous studies suggested that differences in the primary structures of the hypoxanthine phosphoribosyltransferase (HPRT) A and B proteins (EC 2.4.2.8) of mice are associated with altered turnover of these proteins in reticulocytes. On the basis of nucleotide sequence comparisons of their corresponding cDNAs, we show here that the HPRT A and B proteins differ at two positions; there is an alanine/proline substitution at amino acid position 2 and a valine/alanine substitution at amino acid position 29 (HPRT A/B proteins, respectively; total protein length, 218 amino acids). On the basis of results obtained from sequencing of the N termini of the purified HPRT A and B proteins, we also show that these amino acid substitutions are associated with differences in processing of the proteins; HPRT B, which is encoded as N-terminal Met-Pro, has a free N-terminal proline residue; HPRT A, which is encoded as N-terminal Met-Ala, lacks a free N-terminal alpha-amino group and is presumed to be acetylated following removal of the N-terminal methionine (i.e. AcO-Ala). These observations are discussed in reference to the idea that the N terminus of a protein plays a role in determining the rate at which the protein is degraded in erythroid cells.

J Biol Chem 1987 Aug 5;262(22):10741-10747

Analysis of the 5' end of the Drosophila muscle myosin heavy chain gene. Alternatively spliced transcripts initiate at a single site and intron locations are conserved compared to myosin genes of other organisms.

Wassenberg DR 2d, Kronert WA, O'Donnell PT, Bernstein SI

We have localized the transcription start site of the Drosophila melanogaster muscle myosin heavy chain (MHC) gene and find that all forms of the alternatively spliced MHC mRNA initiate at the same location. Therefore the alternative inclusion/exclusion of the 3' penultimate exon in transcripts from this gene (Bernstein, S.I., Hansen, C.J., Becker, K.D., Wassenberg, D.R., II, Roche, E.S., Donady, J.J., and Emerson, C. P., Jr. (1986) Mol. Cell. Biol. 6, 2511-2519; Rozek, C.E., and Davidson, N. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 2128-2134) does not result from the use of different 5' transcription initiation sites. This gene is the first invertebrate MHC gene shown to have TATA and CAAT box consensus sequences and a noncoding 5' exon, properties that are shared with some vertebrate and invertebrate contractile protein genes. The intron that splits the 5' noncoding region of the Drosophila MHC gene contains no major conserved elements relative to other Drosophila contractile protein genes. The introns within the coding region near the 5' end of the Drosophila MHC gene are located at the same sites as nematode and vertebrate MHC gene introns, indicating that these MHC genes are derived from a common ancestral sequence. The putative ATP binding domain encoded in the fourth exon of the Drosophila MHC gene is highly conserved relative to vertebrate, invertebrate, and non-muscle MHC genes suggesting that each of these myosins bind ATP by the same mechanism. Two divergent copies of the third exon are present within the 5' region of the Drosophila MHC gene, suggesting that alternative splicing produces MHC isoforms with different globular head regions.

Mol Cell Biol 1986 Jul;6(7):2511-2519

Alternative RNA splicing generates transcripts encoding a thorax-specific isoform of Drosophila melanogaster myosin heavy chain.

Bernstein SI, Hansen CJ, Becker KD, Wassenberg DR 2d, Roche ES, Donady JJ, Emerson CP Jr

Genomic and cDNA sequencing studies show that transcripts from the muscle myosin heavy-chain (MHC) gene of Drosophila melanogaster are alternatively spliced, producing RNAs that encode at least two MHC isoforms with different C termini. Transcripts encoding an MHC isoform with 27 unique C-terminal amino acids accumulate during both larval and adult muscle differentiation. Transcripts for the second isoform encode one unique C-terminal amino acid and accumulate almost exclusively in pupal and adult thoracic segments, the location of the indirect flight muscles. The 3' splice acceptor site preceding the thorax-specific exon is unusually purine rich and thus may serve as a thorax-specific splicing signal. We suggest that the alternative C termini of these two MHC isoforms control myofilament assembly and may play a role in generating the distinctive myofilament organizations of flight muscle and other muscle types.

Proc Natl Acad Sci U S A 1986 Mar;83(5):1393-1397

Mutations of the Drosophila myosin heavy-chain gene: effects on transcription, myosin accumulation, and muscle function.

Mogami K, O'Donnell PT, Bernstein SI, Wright TR, Emerson CP Jr

Mutations of the myosin heavy-chain (MHC) gene of Drosophila melanogaster were identified among a group of dominant flightless and recessive lethal mutants (map position 2-52, 36A8-B1,2). One mutation is a 0.1-kilobase deletion in the 5' region of the MHC gene and reduces MHC protein in the leg and thoracic muscles of heterozygotes to levels found in 36AC haploids. Three mutations are insertions of 8-to 10-kilobase DNA elements within the MHC gene and produce truncated MHC transcripts. Heterozygotes of these insertional mutations possess levels of MHC intermediate between those of haploids and diploids. An additional mutation has no gross alteration of the MHC gene or its RNA transcripts. Although leg and larval muscles function normally in each mutant heterozygote, indirect flight muscles are defective and possess disorganized myofibrils. Homozygous mutants die during embryonic or larval development and display abnormal muscle function prior to death. These findings provide direct genetic evidence that the MHC gene at 36B (2L) is essential for both larval and adult muscle development and function. The results are consistent with the previous molecular evidence that Drosophila, unlike other organisms, has only a single muscle MHC gene per haploid genome. Quantitative expression of both copies of the MHC gene is required for function of indirect flight muscle, whereas expression of a single MHC gene is sufficient for function of larval muscles and adult tubular muscles.

Nature 1983 Mar 6;302(5907):393-397

Drosophila muscle myosin heavy chain encoded by a single gene in a cluster of muscle mutations.

Bernstein SI, Mogami K, Donady JJ, Emerson CP Jr

Drosophila muscle myosin heavy chain is encoded by a single-copy gene which is transcribed during both larval and adult development. This myosin gene maps to a chromosomal locus distant from any of the actin genes, but is within a cluster of flight muscle mutations.