Drosophila muscle regulation characterized by electron microscopy and three-dimensional reconstruction of thin filament mutants

Regular Exercise Rescues Heart Function Defects and Shortens the Lifespan of Drosophila Caused by dMnM Downregulation

Although studies have shown that myomesin 2 (MYOM2) mutations can lead to hypertrophic cardiomyopathy (HCM), a common cardiovascular disease that has a serious impact on human life, the effect of MYOM2 on cardiac function and lifespan in humans is unknown. In this study, dMnM (MYOM2 homologs) knockdown in cardiomyocytes resulted in diastolic cardiac defects (diastolic dysfunction and arrhythmias) and increased cardiac oxidative stress. Furthermore, the knockdown of dMnM in indirect flight muscle (IFM) reduced climbing ability and shortened lifespan. However, regular exercise significantly ameliorated diastolic cardiac dysfunction, arrhythmias, and oxidative stress triggered by dMnM knockdown in cardiac myocytes and also reversed the reduction in climbing ability and shortening of lifespan caused by dMnM knockdown in Drosophila IFM. In conclusion, these results suggest that Drosophila cardiomyocyte dMnM knockdown leads to cardiac functional defects, while dMnM knockdown in IFM affects climbing ability and lifespan. Furthermore, regular exercise effectively upregulates cardiomyocyte dMnM expression levels and ameliorates cardiac functional defects caused by Drosophila cardiomyocyte dMnM knockdown by increasing cardiac antioxidant capacity. Importantly, regular exercise ameliorates the shortened lifespan caused by dMnM knockdown in IFM.

Troponin Revealed: Uncovering the Structure of the Thin Filament On-Off Switch in Striated Muscle

Recently, our understanding of the structural basis of troponin-tropomyosin's Ca2+-triggered regulation of striated muscle contraction has advanced greatly, particularly via cryo-electron microscopy data. Compelling atomic models of troponin-tropomyosin-actin were published for both apo- and Ca2+-saturated states of the cardiac thin filament. Subsequent electron microscopy and computational analyses have supported and further elaborated the findings. Per cryo-electron microscopy, each troponin is highly extended and contacts both tropomyosin strands, which lie on opposite sides of the actin filament. In the apo-state characteristic of relaxed muscle, troponin and tropomyosin hinder strong myosin-actin binding in several different ways, apparently barricading the actin more substantially than does tropomyosin alone. The troponin core domain, the C-terminal third of TnI, and tropomyosin under the influence of a 64-residue helix of TnT located at the overlap of adjacent tropomyosins are all in positions that would hinder strong myosin binding to actin. In the Ca2+-saturated state, the TnI C-terminus dissociates from actin and binds in part to TnC; the core domain pivots significantly; the N-lobe of TnC binds specifically to actin and tropomyosin; and tropomyosin rotates partially away from myosin's binding site on actin. At the overlap domain, Ca2+ causes much less tropomyosin movement, so a more inhibitory orientation persists. In the myosin-saturated state of the thin filament, there is a large additional shift in tropomyosin, with molecular interactions now identified between tropomyosin and both actin and myosin. A new era has arrived for investigation of the thin filament and for functional understandings that increasingly accommodate the recent structural results.

TNNT2 mutations in the tropomyosin binding region of TNT1 disrupt its role in contractile inhibition and stimulate cardiac dysfunction

Muscle contraction is regulated by the movement of end-to-end-linked troponin-tropomyosin complexes over the thin filament surface, which uncovers or blocks myosin binding sites along F-actin. The N-terminal half of troponin T (TnT), TNT1, independently promotes tropomyosin-based, steric inhibition of acto-myosin associations, in vitro. Recent structural models additionally suggest TNT1 may restrain the uniform, regulatory translocation of tropomyosin. Therefore, TnT potentially contributes to striated muscle relaxation; however, the in vivo functional relevance and molecular basis of this noncanonical role remain unclear. Impaired relaxation is a hallmark of hypertrophic and restrictive cardiomyopathies (HCM and RCM). Investigating the effects of cardiomyopathy-causing mutations could help clarify TNT1's enigmatic inhibitory property. We tested the hypothesis that coupling of TNT1 with tropomyosin's end-to-end overlap region helps anchor tropomyosin to an inhibitory position on F-actin, where it deters myosin binding at rest, and that, correspondingly, cross-bridge cycling is defectively suppressed under diastolic/low Ca2+ conditions in the presence of HCM/RCM lesions. The impact of TNT1 mutations on Drosophila cardiac performance, rat myofibrillar and cardiomyocyte properties, and human TNT1's propensity to inhibit myosin-driven, F-actin-tropomyosin motility were evaluated. Our data collectively demonstrate that removing conserved, charged residues in TNT1's tropomyosin-binding domain impairs TnT's contribution to inhibitory tropomyosin positioning and relaxation. Thus, TNT1 may modulate acto-myosin activity by optimizing F-actin-tropomyosin interfacial contacts and by binding to actin, which restrict tropomyosin's movement to activating configurations. HCM/RCM mutations, therefore, highlight TNT1's essential role in contractile regulation by diminishing its tropomyosin-anchoring effects, potentially serving as the initial trigger of pathology in our animal models and humans.

A role for actin flexibility in thin filament-mediated contractile regulation and myopathy

Striated muscle contraction is regulated by the translocation of troponin-tropomyosin strands over the thin filament surface. Relaxation relies partly on highly-favorable, conformation-dependent electrostatic contacts between actin and tropomyosin, which position tropomyosin such that it impedes actomyosin associations. Impaired relaxation and hypercontractile properties are hallmarks of various muscle disorders. The α-cardiac actin M305L hypertrophic cardiomyopathy-causing mutation lies near residues that help confine tropomyosin to an inhibitory position along thin filaments. Here, we investigate M305L actin in vivo, in vitro, and in silico to resolve emergent pathological properties and disease mechanisms. Our data suggest the mutation reduces actin flexibility and distorts the actin-tropomyosin electrostatic energy landscape that, in muscle, result in aberrant contractile inhibition and excessive force. Thus, actin flexibility may be required to establish and maintain interfacial contacts with tropomyosin as well as facilitate its movement over distinct actin surface features and is, therefore, likely necessary for proper regulation of contraction.

The α-cardiac actin M305L hypertrophic cardiomyopathy-causing mutation is located near residues that help confine tropomyosin to an inhibitory position along thin filaments. Here the authors assessed M305L actin in vivo, in vitro, and in silico to characterize emergent pathological properties and define the mechanistic basis of disease.

Multi-modal and multiscale imaging approaches reveal novel cardiovascular pathophysiology in Drosophila melanogaster

Establishing connections between changes in linear DNA sequences and complex downstream mesoscopic pathology remains a major challenge in biology. Herein, we report a novel, multi-modal and multiscale imaging approach for comprehensive assessment of cardiovascular physiology in Drosophila melanogaster We employed high-speed angiography, optical coherence tomography (OCT) and confocal microscopy to reveal functional and structural abnormalities in the hdp² mutant, pre-pupal heart tube and aorta relative to controls. hdp² harbor a mutation in wupA, which encodes an ortholog of human troponin I (TNNI3). TNNI3 variants frequently engender cardiomyopathy. We demonstrate that the hdp² aortic and cardiac muscle walls are disrupted and that shorter sarcomeres are associated with smaller, stiffer aortas, which consequently result in increased flow and pulse wave velocities. The mutant hearts also displayed diastolic and latent systolic dysfunction. We conclude that hdp² pre-pupal hearts are exposed to increased afterload due to aortic hypoplasia. This may in turn contribute to diastolic and subtle systolic dysfunction via vascular-heart tube interaction, which describes the effect of the arterial loading system on cardiac function. Ultimately, the cardiovascular pathophysiology caused by a point mutation in a sarcomeric protein demonstrates that complex and dynamic micro- and mesoscopic phenotypes can be mechanistically explained in a gene sequence- and molecular-specific manner.

The actin 'A-triad's' role in contractile regulation in health and disease

Striated muscle contraction is regulated by Ca²⁺ -dependent modulation of myosin cross-bridge binding to F-actin by the thin filament troponin (Tn)-tropomyosin (Tm) complex. In the absence of Ca²⁺ , Tn binds to actin and constrains Tm to an azimuthal location where it sterically occludes myosin binding sites along the thin filament surface. This limits force production and promotes muscle relaxation. In addition to Tn-actin interactions, inhibitory Tm positioning requires associations between other thin filament constituents. For example, the actin 'A-triad', composed of residues K326, K328 and R147, forms numerous, highly favourable electrostatic contacts with Tm that are critical for establishing its inhibitory azimuthal binding position. Here, we review recent findings, including the identification and interrogation of modifications within and proximal to the A-triad that are associated with disease and/or altered muscle behaviour, which highlight the surface feature's role in F-actin-Tm interactions and contractile regulation.

Identifying Differential Gene Expression in Wing Polymorphism of Adult Males of the Largest Water Strider: De novo Transcriptome Assembly for Gigantometra gigas (Hemiptera: Gerridae)

Wing polymorphism is common in a wide variety of insect species. However, few studies have reported on adaptations in the wing polymorphism of insects at molecular level, in particular for males. Thus, the adaptive mechanisms need to be explored. The remarkable variability in wing morphs of insects is well represented in the water striders (Hemiptera: Gerridae). Within this family, Gigantometra gigas (China, 1925), the largest water strider known worldwide, displays macropterous and apterous males. In the present study, we used de novo transcriptome assembly to obtain gene expression information and compared body and leg-component lengths of adult males in different wing morphs. The analyses in both gene expression and phenotype levels were used for exploring the adaptive mechanism in wing polymorphism of G. gigas. After checking, a series of highly expressed structural genes were found in macropterous morphs, which were related to the maintenance of flight muscles and the enhancement of flight capacity, whereas in the apterous morphs, the imaginal morphogenesis protein-Late 2 (Imp-L2), which might inhibit wing development and increase the body size of insects, was still highly expressed in the adult stage. Moreover, body and leg-component lengths were significantly larger in apterous than in macropterous morphs. The larger size of the apterous morphs and the differences in highly expressed genes between the two wing morphs consistently demonstrate the adaptive significance of wing polymorphism in G. gigas. These results shed light on the future loss-of-function research of wing polymorphism in G. gigas.

Distortion of the Actin A-Triad Results in Contractile Disinhibition and Cardiomyopathy

Striated muscle contraction is regulated by the movement of tropomyosin over the thin filament surface, which blocks or exposes myosin binding sites on actin. Findings suggest that electrostatic contacts, particularly those between K326, K328, and R147 on actin and tropomyosin, establish an energetically favorable F-actin-tropomyosin configuration, with tropomyosin positioned in a location that impedes actomyosin associations and promotes relaxation. Here, we provide data that directly support a vital role for these actin residues, termed the A-triad, in tropomyosin positioning in intact functioning muscle. By examining the effects of an A295S α-cardiac actin hypertrophic cardiomyopathy-causing mutation, over a range of increasingly complex in silico, in vitro, and in vivo Drosophila muscle models, we propose that subtle A-triad-tropomyosin perturbation can destabilize thin filament regulation, which leads to hypercontractility and triggers disease. Our efforts increase understanding of basic thin filament biology and help unravel the mechanistic basis of a complex cardiac disorder.

Transcriptome analysis of IFM-specific actin and myosin nulls in Drosophila melanogaster unravels lesion-specific expression blueprints across muscle mutations

Muscle contraction is a highly fine-tuned process that requires the precise and timely construction of large protein sub-assemblies to form sarcomeres. Mutations in many genes encoding constituent proteins of this macromolecular machine result in defective functioning of the muscle tissue. However, the pathways underlying muscle degeneration, and manifestation of myopathy phenotypes are not well understood. In this study, we explored transcriptional alterations that ensue from the absence of the two major muscle proteins - myosin and actin - using the Drosophila indirect flight muscles. Our aim was to understand how the muscle tissue responds as a whole to the absence of either of the major scaffold proteins, whether the responses are generic to the tissue; or unique to the thick versus thin filament systems. Our results indicated that muscles respond by altering gene transcriptional levels in multiple systems active in muscle remodelling, protein degradation and heat shock responses. However, there were some responses that were filament-specific signatures of muscle degeneration, like immune responses, metabolic alterations and alterations in expression of muscle structural genes and mitochondrial ribosomal genes. These general and filament-specific changes in gene expression may be of relevance to human myopathies.

Thin Filament Structure and the Steric Blocking Model

By interacting with the troponin-tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling "steric model" began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin-binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X-ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three-dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low-Ca2+, tropomyosin was shown to occupy a "blocked-state" position on the filament, and switched-on in a two-step process, involving first a movement of tropomyosin away from the majority of the myosin-binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+-regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model.

Pseudo-acetylation of K326 and K328 of actin disrupts Drosophila melanogaster indirect flight muscle structure and performance

In striated muscle tropomyosin (Tm) extends along the length of F-actin-containing thin filaments. Its location governs access of myosin binding sites on actin and, hence, force production. Intermolecular electrostatic associations are believed to mediate critical interactions between the proteins. For example, actin residues K326, K328, and R147 were predicted to establish contacts with E181 of Tm. Moreover, K328 also potentially forms direct interactions with E286 of myosin when the motor is strongly bound. Recently, LC-MS/MS analysis of the cardiac acetyl-lysine proteome revealed K326 and K328 of actin were acetylated, a post-translational modification (PTM) that masks the residues' inherent positive charges. Here, we tested the hypothesis that by removing the vital actin charges at residues 326 and 328, the PTM would perturb Tm positioning and/or strong myosin binding as manifested by altered skeletal muscle function and structure in the Drosophila melanogaster model system. Transgenic flies were created that permit tissue-specific expression of K326Q, K328Q, or K326Q/K328Q acetyl-mimetic actin and of wild-type actin via the UAS-GAL4 bipartite expression system. Compared to wild-type actin, muscle-restricted expression of mutant actin had a dose-dependent effect on flight ability. Moreover, excessive K328Q and K326Q/K328Q actin overexpression induced indirect flight muscle degeneration, a phenotype consistent with hypercontraction observed in other Drosophila myofibrillar mutants. Based on F-actin-Tm and F-actin-Tm-myosin models and on our physiological data, we conclude that acetylating K326 and K328 of actin alters electrostatic associations with Tm and/or myosin and thereby augments contractile properties. Our findings highlight the utility of Drosophila as a model that permits efficient targeted design and assessment of molecular and tissue-specific responses to muscle protein modifications, in vivo.

A cis-regulatory mutation in troponin-I of Drosophila reveals the importance of proper stoichiometry of structural proteins during muscle assembly

Rapid and high wing-beat frequencies achieved during insect flight are powered by the indirect flight muscles, the largest group of muscles present in the thorax. Any anomaly during the assembly and/or structural impairment of the indirect flight muscles gives rise to a flightless phenotype. Multiple mutagenesis screens in Drosophila melanogaster for defective flight behavior have led to the isolation and characterization of mutations that have been instrumental in the identification of many proteins and residues that are important for muscle assembly, function, and disease. In this article, we present a molecular-genetic characterization of a flightless mutation, flightless-H (fliH), originally designated as heldup-a (hdp-a). We show that fliH is a cis-regulatory mutation of the wings up A (wupA) gene, which codes for the troponin-I protein, one of the troponin complex proteins, involved in regulation of muscle contraction. The mutation leads to reduced levels of troponin-I transcript and protein. In addition to this, there is also coordinated reduction in transcript and protein levels of other structural protein isoforms that are part of the troponin complex. The altered transcript and protein stoichiometry ultimately culminates in unregulated acto-myosin interactions and a hypercontraction muscle phenotype. Our results shed new insights into the importance of maintaining the stoichiometry of structural proteins during muscle assembly for proper function with implications for the identification of mutations and disease phenotypes in other species, including humans.

Raman spectroscopic studies on screening of myopathies

Myopathies are among the major causes of mortality in the world. There is no complete cure for this heterogeneous group of diseases, but a sensitive, specific, and fast diagnostic tool may improve therapy effectiveness. In this study, Raman spectroscopy is applied to discriminate between muscle mutants in Drosophila on the basis of associated changes at the molecular level. Raman spectra were collected from indirect flight muscles of mutants, upheld(1) (up(1)), heldup(2) (hdp(2)), myosin heavy chain(7) (Mhc(7)), actin88F(KM88) (Act88F(KM88)), upheld(101) (up(101)), and Canton-S (CS) control group, for both 2 and 12 days old flies. Difference spectra (mutant minus control) of all the mutants showed an increase in nucleic acid and β-sheet and/or random coil protein content along with a decrease in α-helix protein. Interestingly, the 12th day samples of up(1) and Act88F(KM88) showed significantly higher levels of glycogen and carotenoids than CS. A principal components based linear discriminant analysis classification model was developed based on multidimensional Raman spectra, which classified the mutants according to their pathophysiology and yielded an overall accuracy of 97% and 93% for 2 and 12 days old flies, respectively. The up(1) and Act88F(KM88) (nemaline-myopathy) mutants form a group that is clearly separated in a linear discriminant plane from up(101) and hdp(2) (cardiomyopathy) mutants. Notably, Raman spectra from a human sample with nemaline-myopathy formed a cluster with the corresponding Drosophila mutant (up(1)). In conclusion, this is the first demonstration in which myopathies, despite their heterogeneity, were screened on the basis of biochemical differences using Raman spectroscopy.

A Drosophila melanogaster model of diastolic dysfunction and cardiomyopathy based on impaired troponin-T function

RATIONALE

Regulation of striated muscle contraction is achieved by Ca2+ -dependent steric modulation of myosin cross-bridge cycling on actin by the thin filament troponin-tropomyosin complex. Alterations in the complex can induce contractile dysregulation and disease. For example, mutations between or near residues 112 to 136 of cardiac troponin-T, the crucial TnT1 (N-terminal domain of troponin-T)-tropomyosin-binding region, cause cardiomyopathy. The Drosophila upheld(101) Glu/Lys amino acid substitution lies C-terminally adjacent to this phylogenetically conserved sequence.

OBJECTIVE

Using a highly integrative approach, we sought to determine the molecular trigger of upheld(101) myofibrillar degeneration, to evaluate contractile performance in the mutant cardiomyocytes, and to examine the effects of the mutation on the entire Drosophila heart to elucidate regulatory roles for conserved TnT1 regions and provide possible mechanistic insight into cardiac dysfunction.

METHODS AND RESULTS

Live video imaging of Drosophila cardiac tubes revealed that the troponin-T mutation prolongs systole and restricts diastolic dimensions of the heart, because of increased numbers of actively cycling myosin cross-bridges. Elevated resting myocardial stiffness, consistent with upheld(101) diastolic dysfunction, was confirmed by an atomic force microscopy-based nanoindentation approach. Direct visualization of mutant thin filaments via electron microscopy and 3-dimensional reconstruction resolved destabilized tropomyosin positioning and aberrantly exposed myosin-binding sites under low Ca2+ conditions.

CONCLUSIONS

As a result of troponin-tropomyosin dysinhibition, upheld(101) hearts exhibited cardiac dysfunction and remodeling comparable to that observed during human restrictive cardiomyopathy. Thus, reversal of charged residues about the conserved tropomyosin-binding region of TnT1 may perturb critical intermolecular associations required for proper steric regulation, which likely elicits myopathy in our Drosophila model.

Flight muscle-specific Pro-Ala-rich extension of troponin is important for maintaining the insect-type myofilament lattice integrity

Insect flight muscle (IFM) can oscillate at frequencies up to 1000Hz, owing to its capability of stretch activation (SA). It is a highly specialized form of cross striated muscles, and its peculiar features include the IFM-specific isoform of troponin-I (troponin-H or TnH) with an unusually long Pro-Ala-rich extension at the C-terminus. Although we have shown that this extension does not directly take part in SA, questions remain as to what its real role is and why it is expressed only in IFM. Here we explored the structural role of the extension, be comparing X-ray diffraction patterns and electron micrographs of bumblebee IFM fibers before and after enzymatic removal of the extension. The removal had a dramatic effect on diffraction patterns: In IFMs in general, the equatorial 2,0 reflection is much stronger than the 1,1 reflection, but after removal, their intensities became almost equal (stronger 1,1 is a feature of vertebrate skeletal muscle). Electron micrographs revealed that a substantial fraction of the thin filaments showed a tendency to move towards the vertebrate position (the trigonal position between three thick filaments), while the rest of the thin filaments remained in their original insect position (midway between two neighboring thick filaments). Therefore, one of the roles of the extension is suggested to keep the filament lattice in the correct configuration for IFM. This insect-type lattice structure is preserved among IFMs from varied insect orders but not in body muscles, suggesting that the maintenance of this lattice structure is important for flight functions.

Significance of troponin dynamics for Ca2+-mediated regulation of contraction and inherited cardiomyopathy

Ca(2+) dissociation from troponin causes cessation of muscle contraction by incompletely understood structural mechanisms. To investigate this process, regulatory site Ca(2+) binding in the NH(2)-lobe of subunit troponin C (TnC) was abolished by mutagenesis, and effects on cardiac troponin dynamics were mapped by hydrogen-deuterium exchange (HDX)-MS. The findings demonstrate the interrelationships among troponin's detailed dynamics, troponin's regulatory actions, and the pathogenesis of cardiomyopathy linked to troponin mutations. Ca(2+) slowed HDX up to 2 orders of magnitude within the NH(2)-lobe and the NH(2)-lobe-associated TnI switch helix, implying that Ca(2+) greatly stabilizes this troponin regulatory region. HDX of the TnI COOH terminus indicated that its known role in regulation involves a partially folded rather than unfolded structure in the absence of Ca(2+) and actin. Ca(2+)-triggered stabilization extended beyond the known direct regulatory regions: to the start of the nearby TnI helix 1 and to the COOH terminus of the TnT-TnI coiled-coil. Ca(2+) destabilized rather than stabilized specific TnI segments within the coiled-coil and destabilized a region not previously implicated in Ca(2+)-mediated regulation: the coiled-coil's NH(2)-terminal base plus the preceding TnI loop with which the base interacts. Cardiomyopathy-linked mutations clustered almost entirely within influentially dynamic regions of troponin, and many sites were Ca(2+)-sensitive. Overall, the findings demonstrate highly selective effects of regulatory site Ca(2+), including opposite changes in protein dynamics at opposite ends of the troponin core domain. Ca(2+) release triggers an intramolecular switching mechanism that propagates extensively within the extended troponin structure, suggests specific movements of the TnI inhibitory regions, and prominently involves troponin's dynamic features.

Isolation, electron microscopy and 3D reconstruction of invertebrate muscle myofilaments

Understanding the molecular mechanism of muscle contraction and its regulation has been greatly influenced and aided by studies of myofilament structure in invertebrate muscles. Invertebrates are easily obtained and cover a broad spectrum of species and functional specializations. The thick (myosin-containing) filaments from some invertebrates are especially stable and simple in structure and thus much more amenable to structural analysis than those of vertebrates. Comparative studies of invertebrate filaments by electron microscopy and image processing have provided important generalizations of muscle molecular structure and function. This article reviews methods for preparing thick and thin filaments from invertebrate muscle, for imaging filaments by electron microscopy, and for determining their three dimensional structure by image processing. It also highlights some of the key insights into filament function that have come from these studies.

Regulating the contraction of insect flight muscle

The rapid movement of the wings in small insects is powered by the indirect flight muscles. These muscles are capable of contracting at up to 1,000 Hz because they are activated mechanically by stretching. The mechanism is so efficient that it is also used in larger insects like the waterbug, Lethocerus. The oscillatory activity of the muscles occurs a low concentration of Ca(2+), which stays constant as the muscles contract and relax. Activation by stretch requires particular isoforms of tropomyosin and the troponin complex on the thin filament. We compare the tropomyosin and troponin of Lethocerus and Drosophila with that of vertebrates. The characteristics of the flight muscle regulatory proteins suggest ways in which stretch-activation works. There is evidence for bridges between troponin on thin filaments and myosin crossbridges on the thick filaments. Recent X-ray fibre diffraction results suggest that a pull on the bridges activates the thin filament by shifting tropomyosin from a blocking position on actin. The troponin bridges are likely to contain extended sequences of tropomyosin or troponin I (TnI). Flight muscle has two isoforms of TnC with different Ca(2+)-binding properties: F1 TnC is needed for stretch-activation and F2 TnC for isometric contractions. In this review, we describe the structural changes in both isoforms on binding Ca(2+) and TnI, and discuss how the steric model of muscle regulation can apply to insect flight muscle.

A method to measure myocardial calcium handling in adult Drosophila

RATIONALE

Normal cardiac physiology requires highly regulated cytosolic Ca(2+) concentrations and abnormalities in Ca(2+) handling are associated with heart failure. The majority of approaches to identifying the components that regulate intracellular Ca(2+) dynamics rely on cells in culture, mouse models, and human samples. However, a genetically robust system for unbiased screens of mutations that affect Ca(2+) handling remains a challenge.

OBJECTIVE

We sought to develop a new method to measure myocardial Ca(2+) cycling in adult Drosophila and determine whether cardiomyopathic fly hearts recapitulate aspects of diseased mammalian myocardium.

METHODS AND RESULTS

Using engineered transgenic Drosophila that have cardiac-specific expression of Ca(2+)-sensing fluorescent protein, GCaMP2, we developed methods to measure parameters associated with myocardial Ca(2+) handling. The following key observations were identified: (1) Control w(1118) Drosophila hearts have readily measureable Ca(2+)-dependent fluorescent signals that are dependent on L-type Ca(2+) channels and SR Ca(2+) stores and originate from rostral and caudal pacemakers. (2) A fly mutant, held-up(2) (hdp(2)), that has a point mutation in troponin I and has a dilated cardiomyopathic phenotype demonstrates abnormalities in myocardial Ca(2+) handling that include increases in the duration of the 50% rise in intensity to peak intensity, the half-time of fluorescence decline from peak, the full duration at half-maximal intensity, and decreases in the linear slope of decay from 80% to 20% intensity decay. (3) Hearts from hdp(2) mutants had reductions in caffeine-induced Ca(2+) increases and reductions in ryanodine receptor (RyR) without changes in L-type Ca(2+) channel transcripts in comparison with w(1118).

CONCLUSIONS

Our results show that the cardiac-specific expression of GCaMP2 provides a means of characterizing propagating Ca(2+) transients in adult fly hearts. Moreover, the adult fruit fly heart recapitulates several aspects of Ca(2+) regulation observed in mammalian myocardium. A mutation in Drosophila that causes an enlarged cardiac chamber and impaired contractile function is associated with abnormalities in the cytosolic Ca(2+) transient as well as changes in transcript levels of proteins associated with Ca(2+) handling. This new methodology has the potential to permit an examination of evolutionarily conserved myocardial Ca(2+)-handing mechanisms by applying the vast resources available in the fly genomics community to conduct genetic screens to identify new genes involved in generated Ca(2+) transients and arrhythmias.

Fast x-ray recordings reveal dynamic action of contractile and regulatory proteins in stretch-activated insect flight muscle

To assess the ability of the thin-filament regulatory system to control each stretch-activation (SA) event in the fast beating of asynchronous insect flight muscle (IFM), we obtained fast (3.4 ms/frame) and semistatic (> or = 50 ms) x-ray diffraction recordings for IFM fibers from bumblebees (beating at 170 Hz) and compared the results with those acquired in giant waterbugs (20-30 Hz) and crane flies (40 Hz, semistatic only). In contrast to the well-documented large SA force of waterbug IFMs, the SA force of bumblebee and crane fly IFMs was small compared to their large isometric force. In semistatic recordings, step-stretched bumblebee and crane fly IFMs showed smaller net SA-associated intensity changes in reflections that report myosin attachment to actin and tropomyosin movement toward its activating position. However, fast recordings on bumblebee IFMs showed a fast and large temporary reversal of intensities in these reflections, suggesting that the myosin heads supporting isometric force are dynamically replaced by SA-supporting heads, and that tropomyosin moves to and back from its inactivating position in milliseconds. In waterbug IFMs, the fast temporary reversal of intensities was not obvious. The observed rates of the attachment/detachment of myosin heads and the motion of tropomyosin are fast enough for the thin-filament regulatory system to control each SA event in fast-beating insects.

In vitro motility of native thin filaments from Drosophila indirect flight muscles reveals that the held-up 2 TnI mutation affects calcium activation

A procedure for the isolation of regulated native thin filaments from the indirect flight muscles (IFM) of Drosophila melanogaster is described. These are the first striated invertebrate thin filaments to show Ca-regulated in vitro motility. Regulated native thin filaments from wild type and a troponin I mutant, held-up-2, were compared by in vitro motility assays that showed that the mutant troponin I caused activation of motility at pCa values higher than wild type. The held-up2 mutation, in the sole troponin I gene (wupA) in the Drosophila genome, is known to cause hypercontraction of the IFM and other muscles in vivo leading to their eventual destruction. The mutation causes substitution of alanine by valine at a homologous and completely conserved troponin I residue (A25) in the vertebrate skeletal muscle TnI isoform. The effects of the held-up 2 mutation on calcium activation of thin filament in vitro motility are discussed with respect to its effects on hypercontraction and dysfunction. Previous electron microscopy and 3-dimensional reconstruction studies showed that the tropomyosin of held-up 2 thin filaments occupies positions associated with the so-called 'closed' state, but independently of calcium concentration. This is discussed with respect to calcium dependent regulation of held-up-2 thin filaments in in vitro motility.

Electron microscopy and three-dimensional reconstruction of native thin filaments reveal species-specific differences in regulatory strand densities

Throughout the animal kingdom striated muscle contraction is regulated by the thin filament troponin-tropomyosin complex. Homologous regulatory components are shared among vertebrate and arthropod muscles; however, unique protein extensions and/or components characterize the latter. The Troponin T (TnT) isoforms of Drosophila indirect flight and tarantula femur muscle for example contain distinct C-terminal extensions and are approximately 20% larger overall than their vertebrate counterpart. Using electron microscopy and three-dimensional helical reconstruction of native Drosophila, tarantula and frog muscle thin filaments we have identified species-specific differences in tropomyosin regulatory strand densities. The strands on the arthropod thin filaments were significantly larger in diameter than those from vertebrates, although not significantly different from each other. These findings reflect differences in the regulatory troponin-tropomyosin complex, which are likely due to the larger TnT molecules aligning and extending along much of the tropomyosin strands' length. Such an arrangement potentially alters the physical properties of the regulatory strands and may help establish contractile characteristics unique to certain arthropod muscles.

Evidence for unique structural change of thin filaments upon calcium activation of insect flight muscle

Upon activation of living or skinned vertebrate skeletal muscle fibers, the sixth X-ray layer-line reflection from actin (6th ALL) is known to intensify, without a shift of its peak position along the layer line. Since myosin attachment to actin is expected to shift the peak towards the meridian, this intensification is considered to reflect the structural change of individual actin monomers in the thin filament. Here, we show that the 6th ALL of skinned insect flight muscles (IFMs) is rather weakened upon isometric calcium activation, and its peak shifts away from the meridian. This suggests that the actin monomers in the two types of muscles change their structures in substantially different manners. The changes that occurred in the 6th ALL of IFM were not diminished by lowering the temperature from 20 to 5 degrees C, while active force was greatly reduced. The inclusion of 100 microM blebbistatin (a myosin inhibitor) did not affect the changes either. This suggests that calcium binding to troponin C, rather than myosin binding to actin, causes the structural change of IFM actin.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Reverse actin sliding triggers strong myosin binding that moves tropomyosin

Actin/myosin interactions in vertebrate striated muscles are believed to be regulated by the "steric blocking" mechanism whereby the binding of calcium to the troponin complex allows tropomyosin (TM) to change position on actin, acting as a molecular switch that blocks or allows myosin heads to interact with actin. Movement of TM during activation is initiated by interaction of Ca(2+) with troponin, then completed by further displacement by strong binding cross-bridges. We report x-ray evidence that TM in insect flight muscle (IFM) moves in a manner consistent with the steric blocking mechanism. We find that both isometric contraction, at high [Ca(2+)], and stretch activation, at lower [Ca(2+)], develop similarly high x-ray intensities on the IFM fourth actin layer line because of TM movement, coinciding with x-ray signals of strong-binding cross-bridge attachment to helically favored "actin target zones." Vanadate (Vi), a phosphate analog that inhibits active cross-bridge cycling, abolishes all active force in IFM, allowing high [Ca(2+)] to elicit initial TM movement without cross-bridge attachment or other changes from relaxed structure. However, when stretched in high [Ca(2+)], Vi-"paralyzed" fibers produce force substantially above passive response at pCa approximately 9, concurrent with full conversion from resting to active x-ray pattern, including x-ray signals of cross-bridge strong-binding and TM movement. This argues that myosin heads can be recruited as strong-binding "brakes" by backward-sliding, calcium-activated thin filaments, and are as effective in moving TM as actively force-producing cross-bridges. Such recruitment of myosin as brakes may be the major mechanism resisting extension during lengthening contractions.

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

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.

Acetylation regulates tropomyosin function in the fission yeast Schizosaccharomyces pombe

Tropomyosin is an evolutionarily conserved alpha-helical coiled-coil protein that promotes and maintains actin filaments. In yeast, Tropomyosin-stabilised filaments are used by molecular motors to transport cargoes or to generate motile forces by altering the dynamics of filament growth and shrinkage. The Schizosaccharomyces pombe tropomyosin Cdc8 localises to the cytokinetic actomyosin ring during mitosis and is absolutely required for its formation and function. We show that Cdc8 associates with actin filaments throughout the cell cycle and is subjected to post-translational modification that does not vary with cell cycle progression. At any given point in the cell cycle 80% of Cdc8 molecules are acetylated, which significantly enhances their affinity for actin. Reconstructions of electron microscopic images of actin-Cdc8 filaments establish that the majority of Cdc8 strands sit in the 'closed' position on actin filaments, suggesting a role in the regulation of myosin binding. We show that Cdc8 regulates the equilibrium binding of myosin to actin without affecting the rate of myosin binding. Unacetylated Cdc8 isoforms bind actin, but have a reduced ability to regulate myosin binding to actin. We conclude that although acetylation of Cdc8 is not essential, it provides a regulatory mechanism for modulating actin filament integrity and myosin function.

The regulation of myosin binding to actin filaments by Lethocerus troponin

Lethocerus indirect flight muscle has two isoforms of troponin C, TnC-F1 and F2, which are unusual in having only a single C-terminal calcium binding site (site IV, isoform F1) or one C-terminal and one N-terminal site (sites IV and II, isoform F2). We show here that thin filaments assembled from rabbit actin and Lethocerus tropomyosin (Tm) and troponin (Tn) regulate the binding of rabbit myosin to rabbit actin in much the same way as the mammalian regulatory proteins. The removal of calcium reduces the rate constant for S1 binding to regulated actin about threefold, independent of which TmTn is used. This is consistent with calcium removal causing the TmTn to occupy the B or blocked state to about 70% of the total. The mid point pCa for the switch differed for TnC-F1 and F2 (pCa 6.9 and 6.0, respectively) consistent with the reported calcium affinities for the two TnCs. Equilibrium titration of S1 binding to regulated actin filaments confirms calcium regulated binding of S1 to actin and shows that in the absence of calcium the three actin filaments (TnC-F1, TnC-F2 and mammalian control) are almost indistinguishable in terms of occupancy of the B and C states of the filament. In the presence of calcium TnC-F2 is very similar to the control with approximately 80% of the filament in the C-state and 10-15% in the fully on M-State while TnC-F1 has almost 50% in each of the C and M states. This higher occupancy of the M-state for TnC-F1, which occurs above pCa 6.9, is consistent with this isoform being involved in the calcium activation of stretch activation. However, it leaves unanswered how a C-terminal calcium binding site of TnC can activate the thin filament.

Aberrant splicing of an alternative exon in the Drosophila troponin-T gene affects flight muscle development

During myofibrillogenesis, many muscle structural proteins assemble to form the highly ordered contractile sarcomere. Mutations in these proteins can lead to dysfunctional muscle and various myopathies. We have analyzed the Drosophila melanogaster troponin T (TnT) up1 mutant that specifically affects the indirect flight muscles (IFM) to explore troponin function during myofibrillogenesis. The up1 muscles lack normal sarcomeres and contain "zebra bodies," a phenotypic feature of human nemaline myopathies. We show that the up(1) mutation causes defective splicing of a newly identified alternative TnT exon (10a) that encodes part of the TnT C terminus. This exon is used to generate a TnT isoform specific to the IFM and jump muscles, which during IFM development replaces the exon 10b isoform. Functional differences between the 10a and 10b TnT isoforms may be due to different potential phosphorylation sites, none of which correspond to known phosphorylation sites in human cardiac TnT. The absence of TnT mRNA in up1 IFM reduces mRNA levels of an IFM-specific troponin I (TnI) isoform, but not actin, tropomyosin, or troponin C, suggesting a mechanism controlling expression of TnT and TnI genes may exist that must be examined in the context of human myopathies caused by mutations of these thin filament proteins.

Nature's strategy for optimizing power generation in insect flight muscle

Table 1 summarizes the primary mechanisms most likely responsible for modifying wing beat frequency (WBF) and muscle power in the Drosophila mutants discussed above. The different outcomes reflect different mechanisms that come into play, depending on the protein and site of the mutation. For example, the reduced muscle power and WBF of the RLC phosphorylation site mutant Mlc2(S6sA,S67A) reflect the reduced number of myosin heads available to form working cross-bridges and the concomitant reduction in muscle stiffness. The mixed results of the other mutants are more difficult to explain. For example, while the reduced muscle stiffness of the paramyosin rod mutant pm(S18A) and the projectin mutant bent(D)/+ may in part reflect mutation-related increases in compliance of the thick filaments (pm(S18A)) or connecting filaments (bent(D)/+), the elevated WBF is unexpected because one would expect reduced muscle stiffness to lower WBF rather than raise it. Other aspects of the results are equally baffling. In the case of pm(S18A), e.g., myofilament kinetics are enhanced, opposite to what one would predict from reduced myofilament stiffness (Wang et al. 1999), but consistent with a direct effect of the mutation on cross-bridge kinetics. It is tempting to speculate that the fly increases the resonance frequency of its flight system, perhaps even over-compensating, as a mechanism for bringing the optimum frequency of power output of the flight system in line with the optimum frequency of power output of the myofilaments in order to achieve flight. The fly might accomplish this by voluntarily activating flight control muscles that change the stiffness and shape of the thoracic box (Tu and Dickinson, 1996), thereby significantly changing the basal stiffness of the resonance system. This effective strategy would serve to tune flight system kinetics to that of the actomyosin motor for optimum power transmission. Notably, of the four thick filament mutations listed in Table 1 produce no significant changes in wing beat frequency, three exhibit reduced muscle power, so these flies must make other adjustments to maintain flight competency. These may be additional cases in which the effects of marked changes in cross-bridge kinetics (MHC IFI-EC), cross-bridge deployment (Mlc2(delta2-46), or sarcomere (thick filament) stiffness (pm(S-A4) and Df(3L) fln(1)/+) are ameliorated by the intervention of direct flight muscles. In summary, it may well be that the fly's general response to mutations that alter one component of the flight system is to alter another in order to maintain optimum transmission of power and flight competency. That is, nature's strategy for optimizing power generation throughout the flight system is probably the same as that at the level of the myofibril: that is, strengthen weak links, orient parts for optimum power production, and modify power train proteins through isoform switches or post-translational modifications to assure all components are in tune with one another.

E93K charge reversal on actin perturbs steric regulation of thin filaments

Contraction in striated muscles is regulated by Ca2+-dependent movement of tropomyosin-troponin on thin filaments. Interactions of charged amino acid residues between the surfaces of tropomyosin and actin are believed to play an integral role in this steric mechanism by influencing the position of tropomyosin on the filaments. To investigate this possibility further, thin filaments were isolated from troponin-regulated, indirect flight muscles of Drosophila mutants that express actin with an amino acid charge reversal at residue 93 located at the interface between actin subdomains 1 and 2, in which a lysine residue is substituted for a glutamic acid. Electron microscopy and 3D helical reconstruction were employed to evaluate the structural effects of the mutation. In the absence of Ca2+, tropomyosin was in a position that blocked the myosin-binding sites on actin, as previously found with wild-type filaments. However, in the presence of Ca2+, tropomyosin position in the mutant filaments was much more variable than in the wild-type ones. In most cases (approximately 60%), tropomyosin remained in the blocking position despite the presence of Ca2+, failing to undergo a normal Ca2+-induced change in position. Thus, switching of a negative to a positive charge at position 93 on actin may stabilize negatively charged tropomyosin in the Ca2+-free state regardless of Ca2+ levels, an alteration that, in turn, is likely to interfere with steric regulation and consequently muscle activation. These results highlight the importance of actin's surface charges in determining the distribution of tropomyosin positions on thin filaments derived from troponin-regulated striated muscles.

Digital three-dimensional models of Drosophila development

Digital models of organs, cells and subcellular structures have become important tools in biological and medical research. Reaching far beyond their traditional widespread use as didactic tools, computer-generated models serve as electronic atlases to identify specific elements in complex patterns, and as analytical tools that reveal relationships between such pattern elements that would remain obscure in two-dimensional sections. Digital models also offer the unique opportunity to store and display gene-expression patterns, and pilot studies have been made in several genetic model organisms, including mouse, Drosophila and Caenorhabditis elegans, to construct digital graphic databases intended as repositories for gene-expression data.

Ca-activation and stretch-activation in insect flight muscle

Asynchronous insect flight muscle is specialized for myogenic oscillatory work, but can also produce isometric tetanic contraction. In skinned insect flight muscle fibers from Lethocerus, with sarcomere length monitored by a striation follower, we determined the relation between isometric force (F(0)) at serial increments of [Ca(2+)] and the additional active force recruited at each [Ca(2+)] by a stretch of approximately 12 nm per half-sarcomere (F(SA)). The isometric force-pCa relation shows that 1.5-2 units of pCa are necessary to raise isometric force from its threshold (pCa approximately 6.5) to its maximum (F(0,max)). The amplitude of F(SA) depends only on the preceding baseline level of isometric force, which must reach at least 0.05 F(0,max) to enable stretch-activation. F(SA) rises very steeply to its maximum as F(0) reaches approximately 0.2 F(0,max), then decreases as F(0) increases so as to produce a constant sum (F(0) + F(SA)) = F(max). Thus Ca- and stretch-activation are complementary pathways that trigger a common process of cross-bridge attachment and force production. We suggest that stretch-induced distortion of attached cross-bridges relieves the steric blocking by tropomyosin of additional binding sites on actin, thereby enabling maximum force even at low [Ca(2+)].