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

Alternative relay regulates the adenosine triphosphatase activity of Locusta migratoria striated muscle myosin

Locust (Locusta migratoria) has a single striated muscle myosin heavy chain (Mhc) gene, which contains 5 clusters of alternative exclusive exons and 1 differently included penultimate exon. The alternative exons of Mhc gene encode 4 distinct regions in the myosin motor domain, that is, the N-terminal SH3-like domain, one lip of the nucleotide-binding pocket, the relay, and the converter. Here, we investigated the role of the alternative regions on the motor function of locust muscle myosin. Using Sf9-baculovirus protein expression system, we expressed and purified 5 isoforms of the locust muscle myosin heavy meromyosin (HMM), including the major isoform in the thorax dorsal longitudinal flight muscle (FL1) and 4 isoforms expressed in the abdominal intersegmental muscle (AB1 to AB4). Among these 5 HMMs, FL1-HMM displayed the highest level of actin-activated adenosine triphosphatase (ATPase) activity (hereafter referred as ATPase activity). To identify the alternative region(s) responsible for the elevated ATPase activity of FL1-HMM, we produced a number of chimeras of FL1-HMM and AB4-HMM. Substitution with the relay of AB4-HMM (encoded by exon-14c) substantially decreased the ATPase activity of FL1-HMM, and conversely, the relay of FL1-HMM (encoded by exon-14a) enhanced the ATPase activity of AB4-HMM. Mutagenesis showed that the exon-14a-encoded residues Gly474 and Asn509 are responsible for the elevated ATPase activity of FL1-HMM. Those results indicate that the alternative relay encoded by exon-14a/c play a key role in regulating the ATPase activity of FL1-HMM and AB4-HMM.

Spatial and temporal requirement of Mlp60A isoforms during muscle development and function in Drosophila melanogaster

Many myofibrillar proteins undergo isoform switching in a spatio-temporal manner during muscle development. The biological significance of the variants of several of these myofibrillar proteins remains elusive. One such myofibrillar protein, the Muscle LIM Protein (MLP), is a vital component of the Z-discs. In this paper, we show that one of the Drosophila MLP encoding genes, Mlp60A, gives rise to two isoforms: a short (279 bp, 10 kDa) and a long (1461 bp, 54 kDa) one. The short isoform is expressed throughout development, but the long isoform is adult-specific, being the dominant of the two isoforms in the indirect flight muscles (IFMs). A concomitant, muscle-specific knockdown of both isoforms leads to partial developmental lethality, with most of the surviving flies being flight defective. A global loss of both isoforms in a Mlp60A-null background also leads to developmental lethality, with muscle defects in the individuals that survive to the third instar larval stage. This lethality could be rescued partially by a muscle-specific overexpression of the short isoform. Genetic perturbation of only the long isoform, through a P-element insertion in the long isoform-specific coding sequence, leads to defective flight, in around 90% of the flies. This phenotype was completely rescued when the P-element insertion was precisely excised from the locus. Hence, our data show that the two Mlp60A isoforms are functionally specialized: the short isoform being essential for normal embryonic muscle development and the long isoform being necessary for normal adult flight muscle function.

A mosaic of conserved and novel modes of gene expression and morphogenesis in mesoderm and muscle formation of a larval bivalve

The mesoderm gives rise to several key morphological features of bilaterian animals including endoskeletal elements and the musculature. A number of regulatory genes involved in mesoderm and/or muscle formation (e.g., Brachyury (Bra), even-skipped (eve), Mox, myosin II heavy chain (mhc)) have been identified chiefly from chordates and the ecdysozoans Drosophila and Caenorhabditis elegans, but data for non-model protostomes, especially those belonging to the ecdysozoan sister clade, Lophotrochozoa (e.g., flatworms, annelids, mollusks), are only beginning to emerge. Within the lophotrochozoans, Mollusca constitutes the most speciose and diverse phylum. Interestingly, however, information on the morphological and molecular underpinnings of key ontogenetic processes such as mesoderm formation and myogenesis remains scarce even for prominent molluscan sublineages such as the bivalves. Here, we investigated myogenesis and developmental expression of Bra, eve, Mox, and mhc in the quagga mussel Dreissena rostriformis, an invasive freshwater bivalve and an emerging model in invertebrate evodevo. We found that all four genes are expressed during mesoderm formation, but some show additional, individual sites of expression during ontogeny. While Mox and mhc are involved in early myogenesis, eve is also expressed in the embryonic shell field and Bra is additionally present in the foregut. Comparative analysis suggests that Mox has an ancestral role in mesoderm and possibly muscle formation in bilaterians, while Bra and eve are conserved regulators of mesoderm development of nephrozoans (protostomes and deuterostomes). The fully developed Dreissena veliger larva shows a highly complex muscular architecture, supporting a muscular ground pattern of autobranch bivalve larvae that includes at least a velum muscle ring, three or four pairs of velum retractors, one or two pairs of larval retractors, two pairs of foot retractors, a pedal plexus, possibly two pairs of mantle retractors, and the muscles of the pallial line, as well as an anterior and a posterior adductor. As is typical for their molluscan kin, remodelling and loss of prominent larval features such as the velum musculature and various retractor systems appear to be also common in bivalves.

Supplementary information

The online version contains supplementary material available at 10.1007/s13127-022-00569-5.

HES and Mox genes are expressed during early mesoderm formation in a mollusk with putative ancestral features

The mesoderm is considered the youngest of the three germ layers. Although its morphogenesis has been studied in some metazoans, the molecular components underlying this process remain obscure for numerous phyla including the highly diverse Mollusca. Here, expression of Hairy and enhancer of split (HES), Mox, and myosin heavy chain (MHC) was investigated in Acanthochitona fascicularis, a representative of Polyplacophora with putative ancestral molluscan features. While AfaMHC is expressed throughout myogenesis, AfaMox1 is only expressed during early stages of mesodermal band formation and in the ventrolateral muscle, an autapomorphy of the polyplacophoran trochophore. Comparing our findings to previously published data across Metazoa reveals Mox expression in the mesoderm in numerous bilaterians including gastropods, polychaetes, and brachiopods. It is also involved in myogenesis in molluscs, annelids, tunicates, and craniates, suggesting a dual role of Mox in mesoderm and muscle formation in the last common bilaterian ancestor. AfaHESC2 is expressed in the ectoderm of the polyplacophoran gastrula and later in the mesodermal bands and in putative neural tissue, whereas AfaHESC7 is expressed in the trochoblasts of the gastrula and during foregut formation. This confirms the high developmental variability of HES gene expression and demonstrates that Mox and HES genes are pleiotropic.

Cofilin Loss in Drosophila Muscles Contributes to Muscle Weakness through Defective Sarcomerogenesis during Muscle Growth

Sarcomeres, the fundamental contractile units of muscles, are conserved structures composed of actin thin filaments and myosin thick filaments. How sarcomeres are formed and maintained is not well understood. Here, we show that knockdown of Drosophila cofilin (DmCFL), an actin depolymerizing factor, disrupts both sarcomere structure and muscle function. The loss of DmCFL also results in the formation of sarcomeric protein aggregates and impairs sarcomere addition during growth. The activation of the proteasome delays muscle deterioration in our model. Furthermore, we investigate how a point mutation in CFL2 that causes nemaline myopathy (NM) in humans affects CFL function and leads to the muscle phenotypes observed in vivo. Our data provide significant insights to the role of CFLs during sarcomere formation, as well as mechanistic implications for disease progression in NM patients.

How sarcomeres are added and maintained in a growing muscle cell is unclear. Balakrishnan et al. observed that DmCFL loss in growing muscles affects sarcomere size and addition through unregulated actin polymerization. This results in a collapse of sarcomere and muscle structure, formation of large protein aggregates, and muscle weakness.

Genetic Control of Muscle Diversification and Homeostasis: Insights from Drosophila

In the fruit fly, Drosophila melanogaster, the larval somatic muscles or the adult thoracic flight and leg muscles are the major voluntary locomotory organs. They share several developmental and structural similarities with vertebrate skeletal muscles. To ensure appropriate activity levels for their functions such as hatching in the embryo, crawling in the larva, and jumping and flying in adult flies all muscle components need to be maintained in a functionally stable or homeostatic state despite constant strain. This requires that the muscles develop in a coordinated manner with appropriate connections to other cell types they communicate with. Various signaling pathways as well as extrinsic and intrinsic factors are known to play a role during Drosophila muscle development, diversification, and homeostasis. In this review, we discuss genetic control mechanisms of muscle contraction, development, and homeostasis with particular emphasis on the contractile unit of the muscle, the sarcomere.

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

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 Å 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.

The load dependence of muscle's force-velocity curve is modulated by alternative myosin converter domains

The hyperbolic shape of the muscle force-velocity relationship (FVR) is characteristic of all muscle fiber types. The degree of curvature of the hyperbola varies between muscle fiber types and is thought to be set by force-dependent properties of different myosin isoforms. However, the structural elements in myosin and the mechanism that determines force dependence are unresolved. We tested our hypothesis that the myosin converter domain plays a critical role in the force-velocity relationship (FVR) mechanism. Drosophila contains a single myosin heavy chain gene with five converters encoded by alternative exons. We measured FVR properties of Drosophila jump muscle fibers from five transgenic lines each expressing a single converter. Consistent with our hypothesis, we observed up to 2.4-fold alterations in FVR curvature. Maximum shortening velocity (v0) and optimal velocity for maximum power generation were also altered, but isometric tension and maximum power generation were unaltered. Converter 11a, normally found in the indirect flight muscle (IFM), imparted the highest FVR curvature and v0, whereas converter 11d, found in larval body wall muscle, imparted the most linear FVR and slowest v0. Jump distance strongly correlated with increasing FVR curvature and v0, meaning flies expressing the converter from the IFM jumped farther than flies expressing the native jump muscle converter. Fitting our data with Huxley's two-state model and a biophysically based four-state model suggest a testable hypothesis that the converter sets muscle type FVR curvature by influencing the detachment rate of negatively strained myosin via changes in the force dependence of product release.

Five Alternative Myosin Converter Domains Influence Muscle Power, Stretch Activation, and Kinetics

Muscles have evolved to power a wide variety of movements. A protein component critical to varying power generation is the myosin isoform present in the muscle. However, how functional variation in muscle arises from myosin structure is not well understood. We studied the influence of the converter, a myosin structural region at the junction of the lever arm and catalytic domain, using Drosophila because its single myosin heavy chain gene expresses five alternative converter versions (11a-e). We created five transgenic fly lines, each forced to express one of the converter versions in their indirect flight muscle (IFM) fibers. Electron microscopy showed that the converter exchanges did not alter muscle ultrastructure. The four lines expressing converter versions (11b-e) other than the native IFM 11a converter displayed decreased flight ability. IFM fibers expressing converters normally found in the adult stage muscles generated up to 2.8-fold more power and displayed up to 2.2-fold faster muscle kinetics than fibers with converters found in the embryonic and larval stage muscles. Small changes to stretch-activated force generation only played a minor role in altering power output of IFM. Muscle apparent rate constants, derived from sinusoidal analysis of the chimeric converter fibers, showed a strong positive correlation between optimal muscle oscillation frequency and myosin attachment kinetics to actin, and an inverse correlation with detachment related cross-bridge kinetics. This suggests the myosin converter alters at least two rate constants of the cross-bridge cycle with changes to attachment and power stroke related kinetics having the most influence on setting muscle oscillatory power kinetics.

Myosin II sequences for Lethocerus indicus

We present the genomic and expressed myosin II sequences from the giant waterbug, Lethocerus indicus. The intron rich gene appears relatively ancient and contains six regions of mutually exclusive exons that are alternatively spliced. Alternatively spliced regions may be involved in the asymmetric myosin dimer structure known as the interacting heads motif, as well as stabilizing the interacting heads motif within the thick filament. A lack of negative charge in the myosin S2 domain may explain why Lethocerus thick filaments display a perpendicular interacting heads motif, rather than one folded back to contact S2, as is seen in other thick filament types such as those from tarantula.

The Drosophila indirect flight muscle myosin heavy chain isoform is insufficient to transform the jump muscle into a highly stretch-activated muscle type

Stretch activation (SA) is a delayed increase in force that enables high power and efficiency from a cyclically contracting muscle. SA exists in various degrees in almost all muscle types. In Drosophila, the indirect flight muscle (IFM) displays exceptionally high SA force production (F_SA), whereas the jump muscle produces only minimal F_SA We previously found that expressing an embryonic (EMB) myosin heavy chain (MHC) isoform in the jump muscle transforms it into a moderately SA muscle type and enables positive cyclical power generation. To investigate whether variation in MHC isoforms is sufficient to produce even higher F_SA, we substituted the IFM MHC isoform (IFI) into the jump muscle. Surprisingly, we found that IFI only caused a 1.7-fold increase in F_SA, less than half the increase previously observed with EMB, and only at a high Pi concentration, 16 mM. This IFI-induced F_SA is much less than what occurs in IFM, relative to isometric tension, and did not enable positive cyclical power generation by the jump muscle. Both isometric tension and F_SA of control fibers decreased with increasing Pi concentration. However, for IFI-expressing fibers, only isometric tension decreased. The rate of F_SA generation was ~1.5-fold faster for IFI fibers than control fibers, and both rates were Pi dependent. We conclude that MHC isoforms can alter F_SA and hence cyclical power generation but that isoforms can only endow a muscle type with moderate F_SA Highly SA muscle types, such as IFM, likely use a different or additional mechanism.

Determining exon connectivity in complex mRNAs by nanopore sequencing

Short-read high-throughput RNA sequencing, though powerful, is limited in its ability to directly measure exon connectivity in mRNAs that contain multiple alternative exons located farther apart than the maximum read length. Here, we use the Oxford Nanopore MinION sequencer to identify 7,899 ‘full-length’ isoforms expressed from four Drosophila genes, Dscam1, MRP, Mhc, and Rdl. These results demonstrate that nanopore sequencing can be used to deconvolute individual isoforms and that it has the potential to be a powerful method for comprehensive transcriptome characterization.

The Drosophila Transcription Factors Tinman and Pannier Activate and Collaborate with Myocyte Enhancer Factor-2 to Promote Heart Cell Fate

Expression of the MADS domain transcription factor Myocyte Enhancer Factor 2 (MEF2) is regulated by numerous and overlapping enhancers which tightly control its transcription in the mesoderm. To understand how Mef2 expression is controlled in the heart, we identified a late stage Mef2 cardiac enhancer that is active in all heart cells beginning at stage 14 of embryonic development. This enhancer is regulated by the NK-homeodomain transcription factor Tinman, and the GATA transcription factor Pannier through both direct and indirect interactions with the enhancer. Since Tinman, Pannier and MEF2 are evolutionarily conserved from Drosophila to vertebrates, and since their vertebrate homologs can convert mouse fibroblast cells to cardiomyocytes in different activator cocktails, we tested whether over-expression of these three factors in vivo could ectopically activate known cardiac marker genes. We found that mesodermal over-expression of Tinman and Pannier resulted in approximately 20% of embryos with ectopic Hand and Sulphonylurea receptor (Sur) expression. By adding MEF2 alongside Tinman and Pannier, a dramatic expansion in the expression of Hand and Sur was observed in almost all embryos analyzed. Two additional cardiac markers were also expanded in their expression. Our results demonstrate the ability to initiate ectopic cardiac fate in vivo by the combination of only three members of the conserved Drosophila cardiac transcription network, and provide an opportunity for this genetic model system to be used to dissect the mechanisms of cardiac specification.

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

Intramolecular 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 (Ile(508), Asn(509), and Asp(511)) in communicating with converter domain residue Arg(759). We found that the N509K relay mutation suppressed 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 disabled motor function and myofibril assembly, suggesting that 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.

An embryonic myosin isoform enables stretch activation and cyclical power in Drosophila jump muscle

The mechanism behind stretch activation (SA), a mechanical property that increases muscle force and oscillatory power generation, is not known. We used Drosophila transgenic techniques and our new muscle preparation, the jump muscle, to determine if myosin heavy chain isoforms influence the magnitude and rate of SA force generation. We found that Drosophila jump muscles show very low SA force and cannot produce positive power under oscillatory conditions at pCa 5.0. However, we transformed the jump muscle to be moderately stretch-activatable by replacing its myosin isoform with an embryonic isoform (EMB). Expressing EMB, jump muscle SA force increased by 163% and it generated net positive power. The rate of SA force development decreased by 58% with EMB expression. Power generation is Pi dependent as >4 mM Pi was required for positive power from EMB. Pi increased EMB SA force, but not wild-type SA force. Our data suggest that when muscle expressing EMB is stretched, EMB is more easily driven backward to a weakly bound state than wild-type jump muscle. This increases the number of myosin heads available to rapidly bind to actin and contribute to SA force generation. We conclude that myosin heavy chain isoforms influence both SA kinetics and SA force, which can determine if a muscle is capable of generating oscillatory power at a fixed calcium concentration.

Post-transcriptional regulation of myotube elongation and myogenesis by Hoi Polloi

Striated muscle development requires the coordinated expression of genes involved in sarcomere formation and contractility, as well as genes that determine muscle morphology. However, relatively little is known about the molecular mechanisms that control the early stages of muscle morphogenesis. To explore this facet of myogenesis, we performed a genetic screen for regulators of somatic muscle morphology in Drosophila, and identified the putative RNA-binding protein (RBP) Hoi Polloi (Hoip). Hoip is expressed in striated muscle precursors within the muscle lineage and controls two genetically separable events: myotube elongation and sarcomeric protein expression. Myotubes fail to elongate in hoip mutant embryos, even though the known regulators of somatic muscle elongation, target recognition and muscle attachment are expressed normally. In addition, a majority of sarcomeric proteins, including Myosin Heavy Chain (MHC) and Tropomyosin, require Hoip for their expression. A transgenic MHC construct that contains the endogenous MHC promoter and a spliced open reading frame rescues MHC protein expression in hoip embryos, demonstrating the involvement of Hoip in pre-mRNA splicing, but not in transcription, of muscle structural genes. In addition, the human Hoip ortholog NHP2L1 rescues muscle defects in hoip embryos, and knockdown of endogenous nhp2l1 in zebrafish disrupts skeletal muscle development. We conclude that Hoip is a conserved, post-transcriptional regulator of muscle morphogenesis and structural gene expression.

Myosin isoform switching during assembly of the Drosophila flight muscle thick filament lattice

During muscle development myosin molecules form symmetrical thick filaments, which integrate with the thin filaments to produce the regular sarcomeric lattice. In Drosophila indirect flight muscles (IFMs) the details of this process can be studied using genetic approaches. The weeP26 transgenic line has a GFP-encoding exon inserted into the single Drosophila muscle myosin heavy chain gene, Mhc. The weeP26 IFM sarcomeres have a unique MHC-GFP-labelling pattern restricted to the sarcomere core, explained by non-translation of the GFP exon following alternative splicing. Characterisation of wild-type IFM MHC mRNA confirmed the presence of an alternately spliced isoform, expressed earlier than the major IFM-specific isoform. The two wild-type IFM-specific MHC isoforms differ by the presence of a C-terminal 'tailpiece' in the minor isoform. The sequential expression and assembly of these two MHCs into developing thick filaments suggest a role for the tailpiece in initiating A-band formation. The restriction of the MHC-GFP sarcomeric pattern in weeP26 is lifted when the IFM lack the IFM-specific myosin binding protein flightin, suggesting that it limits myosin dissociation from thick filaments. Studies of flightin binding to developing thick filaments reveal a progressive binding at the growing thick filament tips and in a retrograde direction to earlier assembled, proximal filament regions. We propose that this flightin binding restricts myosin molecule incorporation/dissociation during thick filament assembly and explains the location of the early MHC isoform pattern in the IFM A-band.

Mechanical analysis of Drosophila indirect flight and jump muscles

The genetic advantages of Drosophila make it a very appealing choice for investigating muscle development, muscle physiology and muscle protein structure and function. To take full advantage of this model organism, it has been vital to develop isolated Drosophila muscle preparations that can be mechanically evaluated. We describe techniques to isolate, prepare and mechanically analyze skinned muscle fibers from two Drosophila muscle types, the indirect flight muscle and the jump muscle. The function of the indirect flight muscle is similar to vertebrate cardiac muscle, to generate power in an oscillatory manner. The indirect flight muscle is ideal for evaluating the influence of protein mutations on muscle and cross-bridge stiffness, oscillatory power, and deriving cross-bridge rate constants. Jump muscle physiology and structure are more similar to skeletal vertebrate muscle than indirect flight muscle, and it is ideal for measuring maximum shortening velocity, force-velocity characteristics and steady-state power generation.

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

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.

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

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.

Differential requirements for Myocyte Enhancer Factor-2 during adult myogenesis in Drosophila

Identifying the genetic program that leads to formation of functionally and morphologically distinct muscle fibers is one of the major challenges in developmental biology. In Drosophila, the Myocyte Enhancer Factor-2 (MEF2) transcription factor is important for all types of embryonic muscle differentiation. In this study we investigated the role of MEF2 at different stages of adult skeletal muscle formation, where a diverse group of specialized muscles arises. Through stage- and tissue-specific expression of Mef2 RNAi constructs, we demonstrate that MEF2 is critical at the early stages of adult myoblast fusion: mutant myoblasts are attracted normally to their founder cell targets, but are unable to fuse to form myotubes. Interestingly, ablation of Mef2 expression at later stages of development showed MEF2 to be more dispensable for structural gene expression: after myoblast fusion, Mef2 knockdown did not interrupt expression of major structural gene transcripts, and myofibrils were formed. However, the MEF2-depleted fibers showed impaired integrity and a lack of fibrillar organization. When Mef2 RNAi was induced in muscles following eclosion, we found no adverse effects of attenuating Mef2 function. We conclude that in the context of adult myogenesis, MEF2 remains an essential factor, participating in control of myoblast fusion, and myofibrillogenesis in developing myotubes. However, MEF2 does not show a major requirement in the maintenance of muscle structural gene expression. Our findings point to the importance of a diversity of regulatory factors that are required for the formation and function of the distinct muscle fibers found in animals.

Regulated functional alternative splicing in Drosophila

Alternative splicing expands the coding capacity of metazoan genes, and it was largely genetic studies in the fruit-fly Drosophila melanogaster that established the principle that regulated alternative splicing results in tissue- and stage-specific protein isoforms with different functions in development. Alternative splicing is particularly prominent in germ cells, muscle and the central nervous system where it modulates the expression of various proteins including cell-surface molecules and transcription factors. Studies in flies have given us numerous insights into alternative splicing in terms of upstream regulation, the exquisite diversity of their forms and the key differential cellular functions of alternatively spliced gene products. The current inundation of transcriptome sequencing data from Drosophila provides an unprecedented opportunity to gain a comprehensive view of alternative splicing.

The influence of myosin converter and relay domains on cross-bridge kinetics of Drosophila indirect flight muscle

We are investigating the influence of the converter and relay domains on elementary rate constants of the actomyosin cross-bridge cycle. The converter and relay domains vary between Drosophila myosin heavy chain isoforms due to alternative mRNA splicing. Previously, we found that separate insertions of embryonic myosin isoform (EMB) versions of these domains into the indirect flight muscle (IFM) myosin isoform (IFI) both decreased Drosophila IFM power and slowed muscle kinetics. To determine cross-bridge mechanisms behind the changes, we employed sinusoidal analysis while varying phosphate and MgATP concentrations in skinned Drosophila IFM fibers. Based on a six-state cross-bridge model, the EMB converter decreased myosin rate constants associated with actin attachment and work production, k(4), but increased rates related to cross-bridge detachment and work absorption, k(2). In contrast, the EMB relay domain had little influence on kinetics, because only k(4) decreased. The main alteration was mechanical, in that work production amplitude decreased. That both domains decreased k(4) supports the hypothesis that these domains are critical to lever-arm-mediated force generation. Neither domain significantly influenced MgATP affinity. Our modeling suggests the converter domain is responsible for the difference in rate-limiting cross-bridge steps between EMB and IFI myosin--i.e., a myosin isomerization associated with MgADP release for EMB and Pi release for IFI.

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

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.

Organ-associated muscles in Aedes albopictus (Diptera: Culicidae) respond differentially to Sindbis virus

Differential host cell responses to the alphavirus Sindbis were observed in visceral muscles of the adult female mosquito Aedes albopictus. Following intrathoracic inoculation with SIN, muscles associated with the midgut, hindgut, and ovary resulted in clearance, persistence, and refractoriness to virus, respectively. Prominent sarcomeres characteristic of myofilaments were identified in muscles associated with these three organs by phalloidin labeling of actin, confirming these cells as muscle. The location of virus antigen mimicked the distribution of actin in both mid- and hindgut-associated muscles. Furthermore, these myofilaments remained intact following virus clearance from midgut muscles and during virus persistence in hindgut muscles. Changes in the temporal onset of virus antigen following high titer inoculum compared with standard titer inoculum was observed in anterior midgut muscles, but not in muscles associated with the posterior midgut or hindgut. Muscle bundles closely approximated the gut surface, while a wispy association was displayed at the ovary surface. Prominent ultrastructural differences were observed in the basal lamina attached to the gut compared with the ovary. Additionally, ultrastructural evidence for virus-associated pathology was observed in gut-associated muscles and gut epithelium. Visceral muscles, all composed of the same tissue type, but associated to three different organs in the insect abdomen, responded differentially to Sindbis. We speculate that variations in structure, function or physiology and ultrastructure inherent to insect host cells or organs interactions reflect the complicated milieu of the organism and contribute to differential virus phenotypic expression in muscle cells.

From action potential to contraction: neural control and excitation-contraction coupling in larval muscles of Drosophila

The neuromuscular system of Drosophila melanogaster has been studied for many years for its relative simplicity and because of the genetic and molecular versatilities. Three main types of striated muscles are present in this dipteran: fibrillar muscles, tubular muscles and supercontractile muscles. The visceral muscles in adult flies and the body wall segmental muscles in embryos and larvae belong to the group of supercontractile muscles. Larval body wall muscles have been the object of detailed studies as a model for neuromuscular junction function but have received much less attention with respect to their mechanical properties and to the control of contraction. In this review we wish to assess available information on the physiology of the Drosophila larval muscular system. Our aim is to establish whether this system has the requisites to be considered a good model in which to perform a functional characterization of Drosophila genes, with a known muscular expression, as well as Drosophila homologs of human genes, the dysfunction of which, is known to be associated with human hereditary muscle pathologies.

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

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.

Conserved alternative splicing and expression patterns of arthropod N-cadherin

Metazoan development requires complex mechanisms to generate cells with diverse function. Alternative splicing of pre-mRNA not only expands proteomic diversity but also provides a means to regulate tissue-specific molecular expression. The N-Cadherin gene in Drosophila contains three pairs of mutually-exclusive alternatively-spliced exons (MEs). However, no significant differences among the resulting protein isoforms have been successfully demonstrated in vivo. Furthermore, while the N-Cadherin gene products exhibit a complex spatiotemporal expression pattern within embryos, its underlying mechanisms and significance remain unknown. Here, we present results that suggest a critical role for alternative splicing in producing a crucial and reproducible complexity in the expression pattern of arthropod N-Cadherin. We demonstrate that the arthropod N-Cadherin gene has maintained the three sets of MEs for over 400 million years using in silico and in vivo approaches. Expression of isoforms derived from these MEs receives precise spatiotemporal control critical during development. Both Drosophila and Tribolium use ME-13a and ME-13b in “neural” and “mesodermal” splice variants, respectively. As proteins, either ME-13a- or ME-13b-containing isoform can cell-autonomously rescue the embryonic lethality caused by genetic loss of N-Cadherin. Ectopic muscle expression of either isoform beyond the time it normally ceases leads to paralysis and lethality. Together, our results offer an example of well-conserved alternative splicing increasing cellular diversity in metazoans.

Animal development requires complex mechanisms to generate many different types of cells. Alternative splicing is a process by which a single gene could produce several protein variants under particular circumstances. It is a useful means to generate a diversified set of proteins in different cell types. In this report, we showed that the alternative splicing of the arthropod N-Cadherin gene has been maintained for over 400 million years. The switch of expression patterns of two distinct variants is also well conserved in arthropods. As proteins, these two N-Cadherin splice variants have similar ability to rescue the embryonic lethality caused by genetic loss of N-Cadherin. However, when the expression of either isoforms was prolonged in muscles where the endogenous expression ceased beyond certain stages, it leads to larval lethality, suggesting the importance of precise spatiotemporal regulation of N-Cadherin splice-variant expression. This finding is particularly important because it offers an example of well-conserved alternative splicing increasing cellular diversity in animals.

Non-autonomous modulation of heart rhythm, contractility and morphology in adult fruit flies

The outermost layer of the vertebrate heart originates from migratory mesothelial cells (epicardium) that give rise to coronary vascular smooth muscles and fibroblasts. The role of the epicardium in myocardial morphogenesis and establishment of normal heart function is still largely unknown. Here, we use Drosophila to investigate non-autonomous influences of epicardial-like tissue surrounding the heart tube on the structural and functional integrity of the myocardium. It has previously been shown that during Drosophila heart formation, mesodermal expression of the homeobox transcription factor even-skipped (eve) is required for specification of a subset of non-myocardial progenitors in the precardiac mesoderm. These progenitors may share some similarities with the vertebrate epicardium. To investigate a non-autonomous epicardial-like influence on myocardial physiology, we studied the consequences of reduced mesodermal Eve expression and epi/pericardial cell numbers on the maturation of the myocardial heart tube, its contractility, and acquisition of a normal heart rhythm in the Drosophila model. Targeting the eve repressor ladybird early (lbe) with the minimal eve mesodermal enhancer efficiently eliminates the mesodermal Eve lineages. These flies exhibit defects in heart structure, including a reduction in systolic and diastolic diameter (akin to 'restrictive cardiomyopathy'). They also exhibit an elevated incidence of arrhythmias and intermittent asystoles, as well as compromised performance under stress. These abnormalities are restored by eve reexpression or by lbe-RNAi co-overexpression. The data suggest that adult heart function in Drosophila is likely to be modulated non-autonomously, possibly by paracrine influences from neighboring cells, such as the epi/pericardium. Thus, Drosophila may serve as a model for finding genetic effectors of epicardial-myocardial interactions relevant to higher organisms.

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

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.

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

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.

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

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.

Comparative genomic analysis of the arthropod muscle myosin heavy chain genes allows ancestral gene reconstruction and reveals a new type of 'partially' processed pseudogene

BACKGROUND

Alternative splicing of mutually exclusive exons is an important mechanism for increasing protein diversity in eukaryotes. The insect Mhc (myosin heavy chain) gene produces all different muscle myosins as a result of alternative splicing in contrast to most other organisms of the Metazoa lineage, that have a family of muscle genes with each gene coding for a protein specialized for a functional niche.

RESULTS

The muscle myosin heavy chain genes of 22 species of the Arthropoda ranging from the waterflea to wasp and Drosophila have been annotated. The analysis of the gene structures allowed the reconstruction of an ancient muscle myosin heavy chain gene and showed that during evolution of the arthropods introns have mainly been lost in these genes although intron gain might have happened in a few cases. Surprisingly, the genome of Aedes aegypti contains another and that of Culex pipiens quinquefasciatus two further muscle myosin heavy chain genes, called Mhc3 and Mhc4, that contain only one variant of the corresponding alternative exons of the Mhc1 gene. Mhc3 transcription in Aedes aegypti is documented by EST data. Mhc3 and Mhc4 inserted in the Aedes and Culex genomes either by gene duplication followed by the loss of all but one variant of the alternative exons, or by incorporation of a transcript of which all other variants have been spliced out retaining the exon-intron structure. The second and more likely possibility represents a new type of a 'partially' processed pseudogene.

CONCLUSION

Based on the comparative genomic analysis of the alternatively spliced arthropod muscle myosin heavy chain genes we propose that the splicing process operates sequentially on the transcript. The process consists of the splicing of the mutually exclusive exons until one exon out of the cluster remains while retaining surrounding intronic sequence. In a second step splicing of introns takes place. A related mechanism could be responsible for the splicing of other genes containing mutually exclusive exons.

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

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.

Stereotypic founder cell patterning and embryonic muscle formation in Drosophila require nautilus (MyoD) gene function

nautilus is the only MyoD-related gene in Drosophila. Nautilus expression begins around stage 9 at full germ-band extension in a subset of mesodermal cells organized in a stereotypic pattern in each hemisegment. The muscle founder cell marker Duf-LacZ, produced by the enhancer trap line rP298LacZ, is coexpressed in numerous Nautilus-positive cells when founders first appear. Founders entrain muscle identity through the restricted expression of transcription factors such as S59, eve, and Kr, all of which are observed in subsets of the nautilus expressing founders. We inactivated the nautilus gene using homology-directed gene targeting and Gal4/UAS regulated RNAi to determine whether loss of nautilus gene activity affected founder cell function. Both methods produced a range of defects that included embryonic muscle disruption, reduced viability and female sterility, which could be rescued by hsp70-nautilus cDNA transgenes. Our results demonstrate Nautilus expression marks early founders that give rise to diverse muscle groups in the embryo, and that nautilus gene activity is required to seed the correct founder myoblast pattern that prefigures the muscle fiber arrangement during embryonic development.

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

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.

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

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.

Diversity in transcriptional start site selection and alternative splicing affects the 5'-UTR of mouse striated muscle myosin transcripts

We have analyzed nearly 2,000 myosin heavy chain gene (Myh) clones representing over 30 different transcripts from seven of eight striated muscle Myh genes expressed in mouse. We also report the transcriptional start sites (TSS) for the mouse developmental Myh genes. The data reveal a previously unknown diversity of TSSs and 5'-end alternative splicing in these transcripts. The cardiac Myh6 gene had two major TSSs. Use of the major downstream site led to an alternatively spliced second exon. Each of the other Myh genes had one major TATA-directed TSS and one or more minor alternative TSSs, some associated with alternative splicing. The minor transcripts were associated with polysomes and their spatial-temporal expression largely mirrored that of the major transcripts in wild-type, Myh1 null, Myh4 null, injured, and uninjured muscle, except that one form of Myh7, detected in heart, was not detected in diaphragm, and the ratio of the two major Myh6 transcripts varied in some circumstances. These findings indicate that alternative TSS usage and alternative splicing in the 5'-UTR are a general feature of murine Myh gene expression and that Myh gene regulation is more complex than previously appreciated.

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

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.

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

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.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Myofilin, a protein in the thick filaments of insect muscle

Thick filaments in striated muscle are myosin polymers with a length and diameter that depend on the fibre type. In invertebrates, the length of the thick filaments varies widely in different muscles and additional proteins control filament assembly. Thick filaments in asynchronous insect flight muscle have an extremely regular structure, which is likely to be essential for the oscillatory contraction of these muscles. The factors controlling the assembly of thick filaments in insect flight muscle are not known. We previously identified a thick filament core protein, zeelin 1, in Lethocerus flight and non-flight muscles. This has been sequenced, and the corresponding proteins in Drosophila and Anopheles have been identified. The protein has been re-named myofilin. Zeelin 2, which is on the outside of Lethocerus flight muscle thick filaments, has been sequenced and because of the similarity to Drosophila flightin, is re-named flightin. In Drosophila flight muscle, myofilin has a molecular weight of 20 kDa and is one of five isoforms produced from a single gene. In situ hybridisation of Drosophila embryos showed that myofilin RNA is first expressed late in embryogenesis at stage 15, a little later than myosin. Antibody to myofilin labelled the entire A-band, except for the H-zone, in cryosections of flight and non-flight muscle. The periodicity of myofilin in Drosophila flight muscle thick filaments was found to be 30 nm by measuring the spacing of gold particles in labelled cryosections; this is about twice the 14.5 nm spacing of myosin molecules. The molar ratio of myofilin to myosin in indirect flight muscle is 1:2, which is the same as that of flightin. We propose a model for the association of these proteins in thick filaments, which is consistent with the periodicity and stoichiometry. Myofilin is probably needed for filament assembly in all muscles, and flightin for stability of flight muscle thick filaments in adult flies.

Isolation and kinetic characterisation of myosin and myosin S1 from the Drosophila indirect flight muscles

The potential to explore myosin function through the alternative exons and mutations of the single muscle myosin heavy chain gene, Mhc of Drosophila requires detailed kinetic analysis of the myosins. We have obtained microgram quantities of enzymatically active Drosophila myosin and subfragment 1 (S1) from dissected indirect flight muscles. Using recent developments in stopped-flow and flash-photolysis methods combined with fluorescent/light scattering technologies we have determined some of the key kinetic parameters of actin-myosin and myosin-nucleotide interactions. The rate of ATP-induced dissociation of actin from Drosophila myosin (0.23 microM(-1) s(-1)) and subfragment 1 (S1, 0.82 microM(-1) s(-1)) are both fast and similar to values measured for mammalian skeletal muscle myosins and S1 fragments respectively. The ATP-induced cross bridge dissociation of Drosophila acto.S1 is expected to be fast since, for a rapidly contracting muscle like the Drosophila flight muscle, the post power stroke cross bridge must detach rapidly from actin or become a drag on the contracting filament. ATP-induced detachment is preceded by ADP release and this is proposed as the rate-limiting step that defines muscle shortening velocity. We show that the affinity of ADP for acto.S1 at 400 microM is 2-3 fold weaker than fast vertebrate myosins. This leads to an estimate of the ADP release rate constant of 4000 s(-1). We show that this predicts a maximum shortening velocity very similar to that obtained from in vivo estimates of indirect flight muscle shortening. The data is therefore compatible with ADP dissociation limiting the in vivo shortening velocity.

Characterization of a hypercontraction-induced myopathy in Drosophila caused by mutations in Mhc

The Myosin heavy chain (Mhc) locus encodes the muscle-specific motor mediating contraction in Drosophila. In a screen for temperature-sensitive behavioral mutants, we have identified two dominant Mhc alleles that lead to a hypercontraction-induced myopathy. These mutants are caused by single point mutations in the ATP binding/hydrolysis domain of Mhc and lead to degeneration of the flight muscles. Electrophysiological analysis in the adult giant fiber flight circuit demonstrates temperature-dependent seizure activity that requires neuronal input, as genetic blockage of neuronal activity suppresses the electrophysiological seizure defects. Intracellular recordings at the third instar neuromuscular junction show spontaneous muscle movements in the absence of neuronal stimulation and extracellular Ca²⁺, suggesting a dysregulation of intracellular calcium homeostasis within the muscle or an alteration of the Ca²⁺ dependence of contraction. Characterization of these new Mhc alleles suggests that hypercontraction occurs via a mechanism, which is molecularly distinct from mutants identified previously in troponin I and troponin T.

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

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.

Positive autoregulation of the Myocyte enhancer factor-2 myogenic control gene during somatic muscle development in Drosophila

The myocyte enhancer factor-2 (MEF2) transcription factor plays a central role in the activation and maintenance of muscle gene expression in fruit flies and vertebrates. The mechanism of action and downstream target genes of MEF2 have been defined in considerable detail, but relatively little is known about the mechanisms that regulate MEF2 expression during muscle development. Here we demonstrate that MEF2 maintains its own expression in all differentiated muscle cell types during late embryonic and larval development in Drosophila by binding a conserved MEF2 site in a muscle-specific regulatory enhancer. Ectopic expression of Mef2 is sufficient to directly activate this enhancer in some, but not all, non-muscle cells. Furthermore, activation of the Mef2 enhancer normally in muscle cells and ectopically in non-muscle cells is dependent upon the integrity of the MEF2 binding site. These findings suggest an evolutionarily conserved mechanism whereby MEF2 can stabilize the muscle phenotype by sustaining its own expression through a myogenic autoregulatory loop.

Morphology and molecular organization of the adult neuromuscular junction of Drosophila

While the larval neuromuscular junction (NMJ) of Drosophila has emerged as a model system to study synaptic function and development, little attention has been given to the study of the adult NMJ. Here we report an immunocytochemical and morphological characterization of an adult NMJ preparation of the prothorax. All muscles examined were innervated by small, uniform type II terminals (0.5-1.5 microm), a subset of which contained octopamine. Terminals classified as type I varied in their morphology across different muscles, ranging from strings or clusters of boutons (0.8-5.5 microm) to an elongate terminal (80-100 microm long) with few branches and contiguous swellings (3-15 microm) along its length. Analysis of the molecular composition of the NMJs during the first 5 days after eclosion revealed four major findings: 1) type I boutons increase in size during early adulthood; 2) Fasciclin II-immunoreactivity is not detectable at type I terminals, while DLG-immunoreactivity is observed at the synapse; 3) a Shaker-GFP fusion protein that localizes to all type I boutons in the larva is differentially localized at adult prothoracic NMJs; and 4) while all type I terminals contain glutamate, the glutamate receptor subunits, DGluRIIA and DGluRIIB, are expressed and clustered in only a subset of muscles. These findings suggest that maturation of the adult NMJ occurs during early adulthood and that muscle-specific properties may play a role in organizing synaptic components in the adult. Furthermore, these results demonstrate that there are major differences in the molecular organization of the adult and larval NMJs.

Notch and Ras signaling pathway effector genes expressed in fusion competent and founder cells during Drosophila myogenesis

Drosophila muscles originate from the fusion of two types of myoblasts, founder cells (FCs) and fusion-competent myoblasts (FCMs). To better understand muscle diversity and morphogenesis, we performed a large-scale gene expression analysis to identify genes differentially expressed in FCs and FCMs. We employed embryos derived from Toll10b mutants to obtain primarily muscle-forming mesoderm, and expressed activated forms of Ras or Notch to induce FC or FCM fate, respectively. The transcripts present in embryos of each genotype were compared by hybridization to cDNA microarrays. Among the 83 genes differentially expressed, we found genes known to be enriched in FCs or FCMs, such as heartless or hibris, previously characterized genes with unknown roles in muscle development, and predicted genes of unknown function. Our studies of newly identified genes revealed new patterns of gene expression restricted to one of the two types of myoblasts, and also striking muscle phenotypes. Whereas genes such as phyllopod play a crucial role during specification of particular muscles, others such as tartan are necessary for normal muscle morphogenesis.

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

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.

Variable N-terminal regions of muscle myosin heavy chain modulate ATPase rate and actin sliding velocity

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.