Myofibrillogenesis in the developing chicken heart: assembly of Z-disk, M-line and the thick filaments

Independent regulation of Z-lines and M-lines during sarcomere assembly in cardiac myocytes revealed by the automatic image analysis software sarcApp

Sarcomeres are the basic contractile units within cardiac myocytes, and the collective shortening of sarcomeres aligned along myofibrils generates the force driving the heartbeat. The alignment of the individual sarcomeres is important for proper force generation, and misaligned sarcomeres are associated with diseases, including cardiomyopathies and COVID-19. The actin bundling protein, α-actinin-2, localizes to the 'Z-Bodies" of sarcomere precursors and the 'Z-Lines' of sarcomeres, and has been used previously to assess sarcomere assembly and maintenance. Previous measurements of α-actinin-2 organization have been largely accomplished manually, which is time-consuming and has hampered research progress. Here, we introduce sarcApp, an image analysis tool that quantifies several components of the cardiac sarcomere and their alignment in muscle cells and tissue. We first developed sarcApp to utilize deep learning-based segmentation and real space quantification to measure α-actinin-2 structures and determine the organization of both precursors and sarcomeres/myofibrils. We then expanded sarcApp to analyze 'M-Lines' using the localization of myomesin and a protein that connects the Z-Lines to the M-Line (titin). sarcApp produces 33 distinct measurements per cell and 24 per myofibril that allow for precise quantification of changes in sarcomeres, myofibrils, and their precursors. We validated this system with perturbations to sarcomere assembly. We found perturbations that affected Z-Lines and M-Lines differently, suggesting that they may be regulated independently during sarcomere assembly.

A comprehensive review of computational and image analysis techniques for quantitative evaluation of striated muscle tissue architecture

Unbiased evaluation of morphology is crucial to understanding development, mechanics, and pathology of striated muscle tissues. Indeed, the ability of striated muscles to contract and the strength of their contraction is dependent on their tissue-, cellular-, and cytoskeletal-level organization. Accordingly, the study of striated muscles often requires imaging and assessing aspects of their architecture at multiple different spatial scales. While an expert may be able to qualitatively appraise tissues, it is imperative to have robust, repeatable tools to quantify striated myocyte morphology and behavior that can be used to compare across different labs and experiments. There has been a recent effort to define the criteria used by experts to evaluate striated myocyte architecture. In this review, we will describe metrics that have been developed to summarize distinct aspects of striated muscle architecture in multiple different tissues, imaged with various modalities. Additionally, we will provide an overview of metrics and image processing software that needs to be developed. Importantly to any lab working on striated muscle platforms, characterization of striated myocyte morphology using the image processing pipelines discussed in this review can be used to quantitatively evaluate striated muscle tissues and contribute to a robust understanding of the development and mechanics of striated muscles.

Mechanobiology of muscle and myofibril morphogenesis

Muscles generate forces for animal locomotion. The contractile apparatus of muscles is the sarcomere, a highly regular array of large actin and myosin filaments linked by gigantic titin springs. During muscle development many sarcomeres assemble in series into long periodic myofibrils that mechanically connect the attached skeleton elements. Thus, ATP-driven myosin forces can power movement of the skeleton. Here we review muscle and myofibril morphogenesis, with a particular focus on their mechanobiology. We describe recent progress on the molecular structure of sarcomeres and their mechanical connections to the skeleton. We discuss current models predicting how tension coordinates the assembly of key sarcomeric components to periodic myofibrils that then further mature during development. This requires transcriptional feedback mechanisms that may help to coordinate myofibril assembly and maturation states with the transcriptional program. To fuel the varying energy demands of muscles we also discuss the close mechanical interactions of myofibrils with mitochondria and nuclei to optimally support powerful or enduring muscle fibers.

A computational approach to quantitatively define sarcomere dimensions and arrangement in skeletal muscle

BACKGROUND AND OBJECTIVE

The skeletal muscle is composed of integrated tissues mainly composed of myofibers i.e., long, cylindrical syncytia, whose cytoplasm is mostly occupied by parallel myofibrils. In section, each myofibril is organized in serially end-to-end arranged sarcomeres connected by Z lines. In muscle disorders, these structural and functional units can undergo structural alterations in terms of Z-line and sarcomere lengths, as well as lateral alignment of Z-line among adjacent myofibrils. In this view, objectifying alterations of the myofibril and sarcomere architecture would provide a solid foundation for qualitative observations. In this work, specific quantitative parameters characterizing the sarcomere and myofibril arrangement were defined using a computerized analysis of ultrastructural images.

METHODS

computerized analysis was carried out on transmission electron microscopy pictures of the murine vastus lateralis muscle. Samples from both euploid (control) and trisomic (showing myofiber alterations) Ts65Dn mice were used. Two routines were written in MATLAB to measure specific structural parameters on sarcomeres and myofibrils. The output included the Z-line, M-line, and sarcomere lengths, the Aspect Ratio (AsR) and Curviness (Cur) sarcomere shape parameters, myofibril axis (α angle), and the H parameter (evaluation of sequence of Z-lines of adjacent myofibrils).

RESULTS

Both routines worked well in control (euploid) skeletal muscle yielding consistent quantitative data of sarcomere and myofibril structural organization. In comparison with euploid, trisomic muscle showed statistically significant lower Z-line length, similar M-line length, and statistically significant lower sarcomere length. Both AsR and Cur were statistically significantly lower in trisomic muscle, suggesting the sarcomere is barrel-shaped in the latter. The angle (α) distribution showed that the sarcomere axes are almost parallel in euploid muscle, while a large variability occurs in trisomic tissue. The mean value of H was significantly higher in trisomic versus euploid muscle indicating that Z-lines are not perfectly aligned in trisomic muscle.

CONCLUSIONS

Our procedure allowed us to accurately extract and quantify sarcomere and myofibril parameters from the high-resolution electron micrographs thereby yielding an effective tool to quantitatively define trisomy-associated muscle alterations. These results pave the way to future objective quantification of skeletal muscle changes in pathological conditions.

SHORT ABSTRACT

The skeletal muscle is composed of integrated tissues mainly composed of myofibers i.e., long, cylindrical syncytia, whose cytoplasm is mostly occupied by parallel myofibrils organized in serially end-to-end arranged sarcomeres. Several pieces of evidence have highlighted that in muscle disorders and diseases the sarcomere structure may be altered. Therefore, objectifying alterations of the myofibril and sarcomere architecture would provide a solid foundation for qualitative observations. A computerized analysis was carried out on transmission electron microscopy images of euploid (control) and trisomic (showing myofiber alterations) skeletal muscle. Two routines were written in MATLAB to measure nine sarcomere and myofibril structural parameters. Our computational method confirmed and expanded on previous qualitative ultrastructural findings defining several trisomy-associated skeletal muscle alterations. The proposed procedure is a potentially useful tool to quantitatively define skeletal muscle changes in pathological conditions involving the sarcomere.

Disruption of Drosophila larval muscle structure and function by UNC45 knockdown

Background

Proper muscle function is heavily dependent on highly ordered protein complexes. UNC45 is a USC (named since this region is shared by three proteins UNC45/CRO1/She4P) chaperone that is necessary for myosin incorporation into the thick filaments. UNC45 is expressed throughout the entire Drosophila life cycle and it has been shown to be important during late embryogenesis when initial muscle development occurs. However, the effects of UNC45 manipulation at later developmental times, after muscle development, have not yet been studied.

Main results

UNC45 was knocked down with RNAi in a manner that permitted survival to the pupal stage, allowing for characterization of sarcomere organization in the well-studied third instar larvae. Second harmonic generation (SHG) microscopy revealed changes in the striated pattern of body wall muscles as well as a reduction of signal intensity. This observation was confirmed with immunofluorescence and electron microscopy imaging, showing diminished UNC45 signal and disorganization of myosin and z-disk proteins. Concomitant alterations in both synaptic physiology and locomotor function were also found. Both nerve-stimulated response and spontaneous vesicle release were negatively affected, while larval movement was impaired.

Conclusions

This study highlights the dependency of normal sarcomere structure on UNC45 expression. We confirm the known role of UNC45 for myosin localization and further show the I-Z-I complex is also disrupted. This suggests a broad need for UNC45 to maintain sarcomere integrity. Newly discovered changes in synaptic physiology reveal the likely presence of a homeostatic response to partially maintain synaptic strength and muscle function.

Progressive Myofibril Reorganization of Human Cardiomyocytes on a Dynamic Nanotopographic Substrate

Cardiomyocyte (CM) alignment with striated myofibril organization is developed during early cardiac organogenesis. Previous work has successfully achieved in vitro CM alignment using a variety of biomaterial scaffolds and substrates with static topographic features. However, the cellular processes that occur during the response of CMs to dynamic surface topographic changes, which may provide a model of in vivo developmental progress of CM alignment within embryonic myocardium, remains poorly understood. To gain insights into these cellular processes involved in the response of CMs to dynamic topographic changes, we developed a dynamic topographic substrate that employs a shape memory polymer coated with polyelectrolyte multilayers to produce a flat-to-wrinkle surface transition when triggered by a change in incubation temperature. Using this system, we investigated cellular morphological alignment and intracellular myofibril reorganization in response to the dynamic wrinkle formation. Hence, we identified the progressive cellular processes of human-induced pluripotent stem cell-CMs in a time-dependent manner, which could provide a foundation for a mechanistic model of cardiac myofibril reorganization in response to extracellular microenvironment changes.

The M-band: The underestimated part of the sarcomere

The sarcomere is the basic unit of the myofibrils, which mediate skeletal and cardiac Muscle contraction. Two transverse structures, the Z-disc and the M-band, anchor the thin (actin and associated proteins) and thick (myosin and associated proteins) filaments to the elastic filament system composed of titin. A plethora of proteins are known to be integral or associated proteins of the Z-disc and its structural and signalling role in muscle is better understood, while the molecular constituents of the M-band and its function are less well defined. Evidence discussed here suggests that the M-band is important for managing force imbalances during active muscle contraction. Its molecular composition is fine-tuned, especially as far as the structural linkers encoded by members of the myomesin family are concerned and depends on the specific mechanical characteristics of each particular muscle fibre type. Muscle activity signals from the M-band to the nucleus and affects transcription of sarcomeric genes, especially via serum response factor (SRF). Due to its important role as shock absorber in contracting muscle, the M-band is also more and more recognised as a contributor to muscle disease.

Defective sarcomere organization and reduced larval locomotion and fish survival in slow muscle heavy chain 1 (smyhc1) mutants

Zebrafish skeletal muscles are broadly divided into slow-twitch and fast-twitch muscle fibers. The slow fibers, which express a slow fiber-specific myosin heavy chain 1 (Smyhc1), are the first group of muscle fibers formed during myogenesis. To uncover Smyhc1 function in muscle growth, we generated three mutant alleles with reading frame shift mutations in the zebrafish smyhc1 gene using CRISPR. The mutants showed shortened sarcomeres with no thick filaments and M-lines in slow fibers of the mutant embryos. However, the formation of slow muscle precursors and expression of other slow muscle genes were not affected and fast muscles appeared normal. The smyhc1 mutant embryos and larvae showed reduced locomotion and food intake. The mutant larvae exhibited increased lethality of incomplete penetrance. Approximately 2/5 of the homozygous mutants were viable and grew into reproductive adults. These adult mutants displayed a typical pattern of slow and fast muscle fiber distribution, and regained normal slow muscle formation. Together, our studies indicate that Smyhc1 is essential for myogenesis in embryonic slow muscles, and loss of Smyhc1 results in defective sarcomere assembly, reduces larval motility and fish survival, but has no visible impact on muscle growth in juvenile and adult zebrafish that escape the larval lethality.

Temporal Impact of Substrate Anisotropy on Differentiating Cardiomyocyte Alignment and Functionality

Anisotropic biomaterials can affect cell function by driving cell alignment, which is critical for cardiac engineered tissues. Recent work, however, has shown that pluripotent stem cell-derived cardiomyocytes may self-align over long periods of time. To determine how the degree of biomaterial substrate anisotropy impacts differentiating cardiomyocyte structure and function, we differentiated mouse embryonic stem cells to cardiomyocytes on nonaligned, semialigned, and aligned fibrous substrates and evaluated cell alignment, contractile displacement, and calcium transient synchronicity over time. Although cardiomyocyte gene expression was not affected by fiber alignment, we observed gradient- and threshold-based differences in cardiomyocyte alignment and function. Cardiomyocyte alignment increased with the degree of fiber alignment in a gradient-based manner at early time points and in a threshold-based manner at later time points. Calcium transient synchronization tightly followed cardiomyocyte alignment behavior, allowing highly anisotropic biomaterials to drive calcium transient synchronization within 8 days, while such synchronized cardiomyocyte behavior required 20 days of culture on nonaligned biomaterials. In contrast, cardiomyocyte contractile displacement had no directional preference on day 8 yet became anisotropic in the direction of fiber alignment on aligned fibers by day 20. Biomaterial anisotropy impact on differentiating cardiomyocyte structure and function is temporally dependent. Impact Statement This work demonstrates that biomaterial anisotropy can quickly drive desired pluripotent stem cell-derived cardiomyocyte structure and function. Such an understanding of matrix anisotropy's time-dependent influence on stem cell-derived cardiomyocyte function will have future applications in the development of cardiac cell therapies and in vitro cardiac tissues for drug testing. Furthermore, our work has broader implications concerning biomaterial anisotropy effects on other cell types in which function relies on alignment, such as myocytes and neurons.

Muscle-specific stress fibers give rise to sarcomeres in cardiomyocytes

The sarcomere is the contractile unit within cardiomyocytes driving heart muscle contraction. We sought to test the mechanisms regulating actin and myosin filament assembly during sarcomere formation. Therefore, we developed an assay using human cardiomyocytes to monitor sarcomere assembly. We report a population of muscle stress fibers, similar to actin arcs in non-muscle cells, which are essential sarcomere precursors. We show sarcomeric actin filaments arise directly from muscle stress fibers. This requires formins (e.g., FHOD3), non-muscle myosin IIA and non-muscle myosin IIB. Furthermore, we show short cardiac myosin II filaments grow to form ~1.5 μm long filaments that then 'stitch' together to form the stack of filaments at the core of the sarcomere (i.e., the A-band). A-band assembly is dependent on the proper organization of actin filaments and, as such, is also dependent on FHOD3 and myosin IIB. We use this experimental paradigm to present evidence for a unifying model of sarcomere assembly.

Skeletal muscle mitochondrial volume and myozenin-1 protein differences exist between high versus low anabolic responders to resistance training

Background

We sought to examine how 12 weeks of resistance exercise training (RET) affected skeletal muscle myofibrillar and sarcoplasmic protein levels along with markers of mitochondrial physiology in high versus low anabolic responders.

Methods

Untrained college-aged males were classified as anabolic responders in the top 25th percentile (high-response cluster (HI); n = 13, dual x-ray absorptiometry total body muscle mass change (Δ) = +3.1 ± 0.3 kg, Δ vastus lateralis (VL) thickness = +0.59 ± 0.05 cm, Δ muscle fiber cross sectional area = +1,426 ± 253 μm²) and bottom 25th percentile (low-response cluster (LO); n = 12, +1.1 ± 0.2 kg, +0.24 ± 0.07 cm, +5 ± 209 μm²; p < 0.001 for all Δ scores compared to HI). VL muscle prior to (PRE) and following RET (POST) was assayed for myofibrillar and sarcoplasmic protein concentrations, myosin and actin protein content, and markers of mitochondrial volume. Proteins related to myofibril formation, as well as whole lysate PGC1-α protein levels were assessed.

Results

Main effects of cluster (HI > LO, p = 0.018, Cohen’s d = 0.737) and time (PRE > POST, p = 0.037, Cohen’s d = −0.589) were observed for citrate synthase activity, although no significant interaction existed (LO PRE = 1.35 ± 0.07 mM/min/mg protein, LO POST = 1.12 ± 0.06, HI PRE = 1.53 ± 0.11, HI POST = 1.39 ± 0.10). POST myofibrillar myozenin-1 protein levels were up-regulated in the LO cluster (LO PRE = 0.96 ± 0.13 relative expression units, LO POST = 1.25 ± 0.16, HI PRE = 1.00 ± 0.11, HI POST = 0.85 ± 0.12; within-group LO increase p = 0.025, Cohen’s d = 0.691). No interactions or main effects existed for other assayed markers.

Discussion

Our data suggest myofibrillar or sarcoplasmic protein concentrations do not differ between HI versus LO anabolic responders prior to or following a 12-week RET program. Greater mitochondrial volume in HI responders may have facilitated greater anabolism, and myofibril myozenin-1 protein levels may represent a biomarker that differentiates anabolic responses to RET. However, mechanistic research validating these hypotheses is needed.

Facilitation of ischaemia-induced ventricular fibrillation by catecholamines is mediated by β1 and β2 agonism in the rat heart in vitro

BACKGROUND AND PURPOSE

Antiarrhythmic β-blockers are used in patients at risk of myocardial ischaemia, but the survival benefit and mechanisms are unclear. We hypothesized that β-blockers do not prevent ventricular fibrillation (VF) but instead inhibit the ability of catecholamines to facilitate ischaemia-induced VF, limiting the scope of their usefulness.

EXPERIMENTAL APPROACH

ECGs were analysed from ischaemic Langendorff-perfused rat hearts perfused with adrenoceptor antagonists and/or exogenous catecholamines (CATs: 313 nM noradrenaline + 75 nM adrenaline) in a blinded and randomized study. Ischaemic zone (IZ) size was deliberately made small or large.

KEY RESULTS

In rat hearts with large IZs, ischaemia-induced VF incidence was high in controls. Atenolol, butoxamine and trimazosin did not affect VF at concentrations with β1 -, β2 - or α1 - adrenoceptor specificity and selectivity (confirmed in separate rat aortae myography experiments). In hearts with small IZs and low baseline incidence of ischaemia-induced VF, CATs, delivered to the uninvolved zone (UZ), increased ischaemia-induced VF incidence. This effect was not mimicked by atrial pacing, hence, not due to sinus tachycardia. However, the CATs-facilitated increase in ischaemia-induced VF was inhibited by atenolol and butoxamine (but not trimazosin), indicative of β1 - and β2 - but not α1 -adrenoceptor involvement (confirmed by immunoblot analysis of downstream phosphoproteins). CATs did not facilitate VF in low-flow globally ischaemic hearts, which have no UZ.

CONCLUSIONS AND IMPLICATIONS

Catecholamines facilitated ischaemia-induced VF when risk was low, acting via β1 - and β2 - adrenoceptors located in the UZ. There was no scope for facilitation when VF risk was high (large IZ), which may explain why β-blockers have equivocal effectiveness in humans.

Polarization-resolved microscopy reveals a muscle myosin motor-independent mechanism of molecular actin ordering during sarcomere maturation

Sarcomeres are stereotyped force-producing mini-machines of striated muscles. Each sarcomere contains a pseudocrystalline order of bipolar actin and myosin filaments, which are linked by titin filaments. During muscle development, these three filament types need to assemble into long periodic chains of sarcomeres called myofibrils. Initially, myofibrils contain immature sarcomeres, which gradually mature into their pseudocrystalline order. Despite the general importance, our understanding of myofibril assembly and sarcomere maturation in vivo is limited, in large part because determining the molecular order of protein components during muscle development remains challenging. Here, we applied polarization-resolved microscopy to determine the molecular order of actin during myofibrillogenesis in vivo. This method revealed that, concomitantly with mechanical tension buildup in the myotube, molecular actin order increases, preceding the formation of immature sarcomeres. Mechanistically, both muscle and nonmuscle myosin contribute to this actin order gain during early stages of myofibril assembly. Actin order continues to increase while myofibrils and sarcomeres mature. Muscle myosin motor activity is required for the regular and coordinated assembly of long myofibrils but not for the high actin order buildup during sarcomere maturation. This suggests that, in muscle, other actin-binding proteins are sufficient to locally bundle or cross-link actin into highly regular arrays.

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

Asb2α-Filamin A Axis Is Essential for Actin Cytoskeleton Remodeling During Heart Development

RATIONALE

Heart development involves differentiation of cardiac progenitors and assembly of the contractile sarcomere apparatus of cardiomyocytes. However, little is known about the mechanisms that regulate actin cytoskeleton remodeling during cardiac cell differentiation.

OBJECTIVE

The Asb2α (Ankyrin repeat-containing protein with a suppressor of cytokine signaling box 2) CRL5 (cullin 5 RING E3 ubiquitin ligase) triggers polyubiquitylation and subsequent degradation by the proteasome of FLNs (filamins). Here, we investigate the role of Asb2α in heart development and its mechanisms of action.

METHODS AND RESULTS

Using Asb2 knockout embryos, we show that Asb2 is an essential gene, critical to heart morphogenesis and function, although its loss does not interfere with the overall patterning of the embryonic heart tube. We show that the Asb2α E3 ubiquitin ligase controls Flna stability in immature cardiomyocytes. Importantly, Asb2α-mediated degradation of the actin-binding protein Flna marks a previously unrecognized intermediate step in cardiac cell differentiation characterized by cell shape changes and actin cytoskeleton remodeling. We further establish that in the absence of Asb2α, myofibrils are disorganized and that heartbeats are inefficient, leading to embryonic lethality in mice.

CONCLUSIONS

These findings identify Asb2α as an unsuspected key regulator of cardiac cell differentiation and shed light on the molecular and cellular mechanisms determining the onset of myocardial cell architecture and its link with early cardiac function. Although Flna is known to play roles in cytoskeleton organization and to be required for heart function, this study now reveals that its degradation mediated by Asb2α ensures essential functions in differentiating cardiac progenitors.

Force Generation via β-Cardiac Myosin, Titin, and α-Actinin Drives Cardiac Sarcomere Assembly from Cell-Matrix Adhesions

Truncating mutations in the sarcomere protein titin cause dilated cardiomyopathy due to sarcomere insufficiency. However, it remains mechanistically unclear how these mutations decrease sarcomere content in cardiomyocytes. Utilizing human induced pluripotent stem cell-derived cardiomyocytes, CRISPR/Cas9, and live microscopy, we characterize the fundamental mechanisms of human cardiac sarcomere formation. We observe that sarcomerogenesis initiates at protocostameres, sites of cell-extracellular matrix adhesion, where nucleation and centripetal assembly of α-actinin-2-containing fibers provide a template for the fusion of Z-disk precursors, Z bodies, and subsequent striation. We identify that β-cardiac myosin-titin-protocostamere form an essential mechanical connection that transmits forces required to direct α-actinin-2 centripetal fiber assembly and sarcomere formation. Titin propagates diastolic traction stresses from β-cardiac myosin, but not α-cardiac myosin or non-muscle myosin II, to protocostameres during sarcomerogenesis. Ablating protocostameres or decoupling titin from protocostameres abolishes sarcomere assembly. Together these results identify the mechanical and molecular components critical for human cardiac sarcomerogenesis.

Quaking RNA-Binding Proteins Control Early Myofibril Formation by Modulating Tropomyosin

Skeletal muscle contraction is mediated by myofibrils, complex multi-molecular scaffolds structured into repeated units, the sarcomeres. Myofibril structure and function have been extensively studied, but the molecular processes regulating its formation within the differentiating muscle cell remain largely unknown. Here we show in zebrafish that genetic interference with the Quaking RNA-binding proteins disrupts the initial steps of myofibril assembly without affecting early muscle differentiation. Using RNA sequencing, we demonstrate that Quaking is required for accumulation of the muscle-specific tropomyosin-3 transcript, tpm3.12. Further functional analyses reveal that Tpm3.12 mediates Quaking control of myofibril formation. Moreover, we identified a Quaking-binding site in the 3' UTR of tpm3.12 transcript, which is required in vivo for tpm3.12 accumulation and myofibril formation. Our work uncovers a Quaking/Tpm3 pathway controlling de novo myofibril assembly. This unexpected developmental role for Tpm3 could be at the origin of muscle defects observed in human congenital myopathies associated with tpm3 mutation.

Mechanical tension and spontaneous muscle twitching precede the formation of cross-striated muscle in vivo

Muscle forces are produced by repeated stereotypical actomyosin units called sarcomeres. Sarcomeres are chained into linear myofibrils spanning the entire muscle fiber. In mammalian body muscles, myofibrils are aligned laterally, resulting in their typical cross-striated morphology. Despite this detailed textbook knowledge about the adult muscle structure, it is still unclear how cross-striated myofibrils are built in vivo. Here, we investigate the morphogenesis of Drosophila abdominal muscles and establish them as an in vivo model for cross-striated muscle development. By performing live imaging, we find that long immature myofibrils lacking a periodic actomyosin pattern are built simultaneously in the entire muscle fiber and then align laterally to give mature cross-striated myofibrils. Interestingly, laser micro-lesion experiments demonstrate that mechanical tension precedes the formation of the immature myofibrils. Moreover, these immature myofibrils do generate spontaneous Ca²⁺-dependent contractions in vivo, which, when chemically blocked, result in cross-striation defects. Taken together, these results suggest a myofibrillogenesis model in which mechanical tension and spontaneous muscle twitching synchronize the simultaneous self-organization of different sarcomeric protein complexes to build highly regular cross-striated myofibrils spanning the length of large muscle fibers.

Summary: In Drosophila, immature myofibrils are built simultaneously across an entire muscle fiber, and then self-organize in a manner dependent on spontaneous contractions and mechanical tension.

Titin and Nebulin in Thick and Thin Filament Length Regulation

In this review we discuss the history and the current state of ideas related to the mechanism of size regulation of the thick (myosin) and thin (actin) filaments in vertebrate striated muscles. Various hypotheses have been considered during of more than half century of research, recently mostly involving titin and nebulin acting as templates or 'molecular rulers', terminating exact assembly. These two giant, single-polypeptide, filamentous proteins are bound in situ along the thick and thin filaments, respectively, with an almost perfect match in the respective lengths and structural periodicities. However, evidence still questions the possibility that the proteins function as templates, or scaffolds, on which the thin and thick filaments could be assembled. In addition, the progress in muscle research during the last decades highlighted a number of other factors that could potentially be involved in the mechanism of length regulation: molecular chaperones that may guide folding and assembly of actin and myosin; capping proteins that can influence the rates of assembly-disassembly of the myofilaments; Ca²⁺ transients that can activate or deactivate protein interactions, etc. The entire mechanism of sarcomere assembly appears complex and highly dynamic. This mechanism is also capable of producing filaments of about the correct size without titin and nebulin. What then is the role of these proteins? Evidence points to titin and nebulin stabilizing structures of the respective filaments. This stabilizing effect, based on linear proteins of a fixed size, implies that titin and nebulin are indeed molecular rulers of the filaments. Although the proteins may not function as templates in the assembly of the filaments, they measure and stabilize exactly the same size of the functionally important for the muscles segments in each of the respective filaments.

A Zebrafish Model for a Human Myopathy Associated with Mutation of the Unconventional Myosin MYO18B

Myosin 18B is an unconventional myosin that has been implicated in tumor progression in humans. In addition, loss-of-function mutations of the MYO18B gene have recently been identified in several patients exhibiting symptoms of nemaline myopathy. In mouse, mutation of Myo18B results in early developmental arrest associated with cardiomyopathy, precluding analysis of its effects on skeletal muscle development. The zebrafish, frozen (fro) mutant was identified as one of a group of immotile mutants in the 1996 Tübingen genetic screen. Mutant embryos display a loss of birefringency in their skeletal muscle, indicative of disrupted sarcomeric organization. Using meiotic mapping, we localized the fro locus to the previously unannotated zebrafish myo18b gene, the product of which shares close to 50% identity with its human ortholog. Transcription of myo18b is restricted to fast-twitch myocytes in the zebrafish embryo; consistent with this, fro mutant embryos exhibit defects specifically in their fast-twitch skeletal muscles. We show that sarcomeric assembly is blocked at an early stage in fro mutants, leading to the disorganized accumulation of actin, myosin, and α-actinin and a complete loss of myofibrillar organization in fast-twitch muscles.

The sarcomeric cytoskeleton: from molecules to motion

Highly ordered organisation of striated muscle is the prerequisite for the fast and unidirectional development of force and motion during heart and skeletal muscle contraction. A group of proteins, summarised as the sarcomeric cytoskeleton, is essential for the ordered assembly of actin and myosin filaments into sarcomeres, by combining architectural, mechanical and signalling functions. This review discusses recent cell biological, biophysical and structural insight into the regulated assembly of sarcomeric cytoskeleton proteins and their roles in dissipating mechanical forces in order to maintain sarcomere integrity during passive extension and active contraction. α-Actinin crosslinks in the Z-disk show a pivot-and-rod structure that anchors both titin and actin filaments. In contrast, the myosin crosslinks formed by myomesin in the M-band are of a ball-and-spring type and may be crucial in providing stable yet elastic connections during active contractions, especially eccentric exercise.

Contractile activity is required for Z-disc sarcomere maturation in vivo

Sarcomere structure underpins structural integrity, signaling, and force transmission in the muscle. In embryos of the frog Xenopus tropicalis, muscle contraction begins even while sarcomerogenesis is ongoing. To determine whether contractile activity plays a role in sarcomere formation in vivo, chemical tools were used to block acto-myosin contraction in embryos of the frog X. tropicalis, and Z-disc assembly was characterized in the paralyzed dicky ticker mutant. Confocal and ultrastructure analysis of paralyzed embryos showed delayed Z-disc formation and defects in thick filament organization. These results suggest a previously undescribed role for contractility in sarcomere maturation in vivo. genesis 53:299–307, 2015. © 2015 The Authors. Genesis Published by Wiley Periodicals, Inc.

Myosin VI localization and expression in striated muscle pathology

Myosin VI (MVI) is a unique unconventional myosin translocating, unlike other myosins, towards the minus end of actin filaments. It is involved in numerous cellular processes such as endocytosis, intracellular trafficking, cell migration, and transcription. In mammalian skeletal muscles it localizes mainly to sarcoplasmic reticulum and is also present within the muscle nuclei and at the neuromuscular junction (Karolczak et al. Histochem Cell Biol 2013; 23:219-228). We have also shown that in denervated rat hindlimb muscle the MVI expression level is significantly increased and its localization is changed, indicating an important role of MVI in striated muscle pathology. Here, we addressed this problem by examining the distribution and expression levels of myosin VI in biopsies of skeletal muscles from patients with different myopathies. We found that, particularly in myopathies associated with fiber atrophy, the amount of MVI was enhanced and its localization in affected fibers was changed. Also, since a mutation within the human MVI gene was shown to be associated with cardiomyopathy, we assessed MVI localization and expression level in cardiac muscle using wild type and MLP(-/-) mice, a dilated cardiomyopathy model. No significant difference in MVI expression level was observed for both types of animals. MVI was found at intercalated discs and also at the sarcoplasmic reticulum. In the knockout mice, it was also present in ring-like structures surrounding the nuclei. The data indicate that in striated muscle MVI could be engaged in sarcoplasmic reticulum maintenance and/or functioning, vesicular transport, signal transmission and possibly in gene transcription.

Effect of substrate mechanics on cardiomyocyte maturation and growth

Cardiac tissue engineering constructs are a promising therapeutic treatment for myocardial infarction, which is one of the leading causes of death. In order to further advance the development and regeneration of engineered cardiac tissues using biomaterial platforms, it is important to have a complete overview of the effects that substrates have on cardiomyocyte (CM) morphology and function. This article summarizes recent studies that investigate the effect of mechanical cues on the CM differentiation, maturation, and growth. In these studies, CMs derived from embryos, neonates, and mesenchymal stem cells were seeded on different substrates of various elastic modulus. Measuring the contractile function by force production, work output, and calcium handling, it was seen that cell behavior on substrates was optimized when the substrate stiffness mimicked that of the native tissue. The contractile function reflected changes in the sarcomeric protein confirmation and organization that promoted the contractile ability. The analysis of the literature also revealed that, in addition to matrix stiffness, mechanical stimulation, such as stretching the substrate during cell seeding, also played an important role during cell maturation and tissue development.

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

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

Cytoskeleton and Adhesion in Myogenesis

The function of muscle is to contract, which means to exert force on a substrate. The adaptations required for skeletal muscle differentiation, from a prototypic cell, involve specialization of housekeeping cytoskeletal contracting and supporting systems into crystalline arrays of proteins. Here I discuss the changes that all three cytoskeletal systems (microfilaments, intermediate filaments, and microtubules) undergo through myogenesis. I also discuss their interaction, through the membrane, to extracellular matrix and to other cells, where force will be exerted during contraction. The three cytoskeletal systems are necessary for the muscle cell and must exert complementary roles in the cell. Muscle is a responsive system, where structure and function are integrated: the structural adaptations it undergoes depend on force production. In this way, the muscle cytoskeleton is a portrait of its physiology. I review the cytoskeletal proteins and structures involved in muscle function and focus particularly on their role in myogenesis, the process by which this incredible muscle machine is made. Although the focus is on skeletal muscle, some of the discussion is applicable to cardiac and smooth muscle.

Glucocorticoids and foetal heart maturation; implications for prematurity and foetal programming

Glucocorticoids are steroid hormones, essential in mammals to prepare for life after birth. Blood levels of glucocorticoids (cortisol in most mammals including humans; corticosterone in rats and mice) rise dramatically shortly before birth. This is mimicked clinically in the routine administration of synthetic glucocorticoids to pregnant women threatened by a preterm birth or to preterm infants to improve neonatal survival. Whilst effects on lung are well documented and essential for postnatal survival, those on heart are less well known. In this study, we review recent evidence for a crucial role of glucocorticoids in late gestational heart maturation. Either insufficient or excessive glucocorticoid exposure before birth may alter the normal glucocorticoid-regulated trajectory of heart maturation with potential life-long consequences.

Tension and force-resistant attachment are essential for myofibrillogenesis in Drosophila flight muscle

BACKGROUND

Higher animals generate an elaborate muscle-tendon network to perform their movements. To build a functional network, developing muscles must establish stable connections with tendons and assemble their contractile apparatuses. Current myofibril assembly models do not consider the impact of muscle-tendon attachment on myofibrillogenesis. However, if attachment and myofibrillogenesis are not properly coordinated, premature muscle contractions can destroy an unstable myotendinous system, leading to severe myopathies.

RESULTS

Here, we use Drosophila indirect flight muscles to investigate how muscle-tendon attachment and myofibrillogenesis are coordinated. We find that flight muscles first stably attach to tendons and then assemble their myofibrils. Interestingly, this myofibril assembly is triggered simultaneously throughout the entire muscle, suggesting a self-assembly mechanism. By applying laser-cutting experiments, we show that muscle attachment coincides with an increase in mechanical tension before periodic myofibrils can be detected. We manipulated tension buildup within the myotendinous system either by genetically compromising attachment initiation and integrin recruitment to the myotendinous junction or by optically severing tendons from muscle. Both treatments cause strong myofibrillogenesis defects. We find that myosin motor activity is required for both tension formation and myofibril assembly, suggesting that myofibril assembly itself contributes to tension buildup.

CONCLUSIONS

Our results demonstrate that force-resistant attachment enables a stark tension increase in the myotendinous system. Subsequently, this tension increase triggers simultaneous myofibril self-assembly throughout the entire muscle fiber. As myofibril and sarcomeric architecture as well as their molecular components are evolutionarily conserved, we propose a similar tension-based mechanism to regulate myofibrillogenesis in vertebrates.

Modulation of stretch-induced myocyte remodeling and gene expression by nitric oxide: a novel role for lipoma preferred partner in myofibrillogenesis

Prolonged hemodynamic load as a result of hypertension eventually leads to maladaptive cardiac adaptation and heart failure. The signaling pathways that underlie these changes are still poorly understood. The adaptive response to mechanical load is mediated by mechanosensors that convert the mechanical stimuli into a biological response. We examined the effect of cyclic mechanical stretch on myocyte adaptation using neonatal rat ventricular myocytes with 10% (adaptive) or 20% (maladaptive) maximum strain at 1 Hz for 48 h to mimic in vivo mechanical stress. Cells were also treated with and without nitro-L-arginine methyl ester (L-NAME), a general nitric oxide synthase (NOS) inhibitor to suppress NO production. Maladaptive 20% mechanical stretch led to a significant loss of intact sarcomeres that were rescued by L-NAME (P < 0.05; n ≥ 5 cultures). We hypothesized that the mechanism was through NO-induced alteration of myocyte gene expression. L-NAME upregulated the mechanosensing proteins muscle LIM protein (MLP; by 100%; P < 0.05; n = 5 cultures) and lipoma preferred partner (LPP), a novel cardiac protein (by 80%; P < 0.05; n = 4 cultures). L-NAME also significantly altered the subcellular localization of LPP and MLP in a manner that favored growth and adaptation. These findings suggest that NO participates in stretch-mediated adaptation. The use of isoform selective NOS inhibitors indicated a complex interaction between inducible NOS and neuronal NOS isoforms regulate gene expression. LPP knockdown by small intefering RNA led to formation of α-actinin aggregates and Z bodies showing that myofibrillogenesis was impaired. There was an upregulation of E3 ubiquitin ligase (MUL1) by 75% (P < 0.05; n = 5 cultures). This indicates that NO contributes to stretch-mediated adaptation via the upregulation of proteins associated with mechansensing and myofibrillogenesis, thereby presenting potential therapeutic targets during the progression of heart failure.

Myosin filament assembly onto myofibrils in live neonatal cardiomyocytes observed by TPEF-SHG microscopy

AIMS

Understanding myofibrillogenesis is essential for elucidating heart muscle formation, development, and remodelling in response to physiological stimulation. Here, we report the dynamic assembly process of contractile myosin filaments onto myofibrils in a live cardiomyocyte culture during myofibrillogenesis.

METHODS AND RESULTS

Utilizing a custom-built, two-photon excitation fluorescence and second harmonic generation imaging system equipped with an on-stage incubator, we observed new sarcomere additions in rat neonatal cardiomyocytes during 10 h of on-stage incubation. The new sarcomere additions occurred at the side of existing myofibrils, where we observed mature myofibrils acting as templates, or at the interstice of several separated myofibrils.

CONCLUSIONS

During sarcomeric addition, myosin filaments are assembled onto the premyofibril laterally. This lateral addition, which proceeds stepwise along the axial direction, plays an important role in the accumulation of Z-bodies to form mature Z-disks and in the regulation of sarcomeric alignment during maturation.

Sarcomeric pattern formation by actin cluster coalescence

Contractile function of striated muscle cells depends crucially on the almost crystalline order of actin and myosin filaments in myofibrils, but the physical mechanisms that lead to myofibril assembly remains ill-defined. Passive diffusive sorting of actin filaments into sarcomeric order is kinetically impossible, suggesting a pivotal role of active processes in sarcomeric pattern formation. Using a one-dimensional computational model of an initially unstriated actin bundle, we show that actin filament treadmilling in the presence of processive plus-end crosslinking provides a simple and robust mechanism for the polarity sorting of actin filaments as well as for the correct localization of myosin filaments. We propose that the coalescence of crosslinked actin clusters could be key for sarcomeric pattern formation. In our simulations, sarcomere spacing is set by filament length prompting tight length control already at early stages of pattern formation. The proposed mechanism could be generic and apply both to premyofibrils and nascent myofibrils in developing muscle cells as well as possibly to striated stress-fibers in non-muscle cells.

Muscle contraction driving voluntary movements and the beating of the heart relies on the contraction of highly regular bundles of actin and myosin filaments, which share a periodic, sarcomeric pattern. We know little about the mechanisms by which these ‘biological crystals’ are assembled and it is a general question how order on a scale of 100 micrometers can emerge from the interactions of micrometer-sized building blocks, such as actin and myosin filaments. In our paper, we consider a computational model for a bundle of actin filaments and discuss physical mechanisms by which periodic order emerges spontaneously. Mutual crosslinking of actin filaments results in the formation and coalescence of growing actin clusters. Active elongation and shrinkage dynamics of actin filaments generates polymerization forces and causes local actin flow that can act like a conveyor belt to sort myosin filaments in place.

Muscle diseases in the zebrafish

Animal models in biomedical research are important for understanding the pathological mechanisms of human diseases at a molecular and cellular level. Several aspects of mammalian animals, however, may limit their use in modelling neuromuscular disorders. Many attributes of zebrafish (Danio rerio) are complementary to mammalian experimental systems, establishing the zebrafish as a powerful model organism in disease biology. This review focuses on a number of key studies using the zebrafish to model hereditary muscle diseases with additional emphasis on recent advances in zebrafish functional genomics and drug discovery. Increasing research in zebrafish disease models, combined with knowledge from mammalian models, will bring novel insights into the disease pathogenesis of neuromuscular disorders, as well as facilitate the development of effective therapeutic strategies.

Myofibrillogenesis in live neonatal cardiomyocytes observed with hybrid two-photon excitation fluorescence-second harmonic generation microscopy

We developed a hybrid two-photon excitation fluorescence-second harmonic generation (TPEF-SHG) imaging system with an on-stage incubator for long-term live-cell imaging. Using the imaging system, we observed the addition of new sarcomeres during myofibrillogenesis while a cardiomyocyte was spreading on the substrate. The results suggest that the TPEF-SHG imaging system with an on-stage incubator is an effective tool for investigation of dynamic myofibrillogenesis.

Knockdown of embryonic myosin heavy chain reveals an essential role in the morphology and function of the developing heart

The expression and function of embryonic myosin heavy chain (eMYH) has not been investigated within the early developing heart. This is despite the knowledge that other structural proteins, such as alpha and beta myosin heavy chains and cardiac alpha actin, play crucial roles in atrial septal development and cardiac function. Most cases of atrial septal defects and cardiomyopathy are not associated with a known causative gene, suggesting that further analysis into candidate genes is required. Expression studies localised eMYH in the developing chick heart. eMYH knockdown was achieved using morpholinos in a temporal manner and functional studies were carried out using electrical and calcium signalling methodologies. Knockdown in the early embryo led to abnormal atrial septal development and heart enlargement. Intriguingly, action potentials of the eMYH knockdown hearts were abnormal in comparison with the alpha and beta myosin heavy chain knockdowns and controls. Although myofibrillogenesis appeared normal, in knockdown hearts the tissue integrity was affected owing to apparent focal points of myocyte loss and an increase in cell death. An expression profile of human skeletal myosin heavy chain genes suggests that human myosin heavy chain 3 is the functional homologue of the chick eMYH gene. These data provide compelling evidence that eMYH plays a crucial role in important processes in the early developing heart and, hence, is a candidate causative gene for atrial septal defects and cardiomyopathy.

Striated acto-myosin fibers can reorganize and register in response to elastic interactions with the matrix

The remarkable striation of muscle has fascinated many for centuries. In developing muscle cells, as well as in many adherent, nonmuscle cell types, striated, stress fiberlike structures with sarcomere-periodicity tend to register: Based on several studies, neighboring, parallel fibers at the basal membrane of cultured cells establish registry of their respective periodic sarcomeric architecture, but, to our knowledge, the mechanism has not yet been identified. Here, we propose for cells plated on an elastic substrate or adhered to a neighboring cell, that acto-myosin contractility in striated fibers close to the basal membrane induces substrate strain that gives rise to an elastic interaction between neighboring striated fibers, which in turn favors interfiber registry. Our physical theory predicts a dependence of interfiber registry on externally controllable elastic properties of the substrate. In developing muscle cells, registry of striated fibers (premyofibrils and nascent myofibrils) has been suggested as one major pathway of myofibrillogenesis, where it precedes the fusion of neighboring fibers. This suggests a mechanical basis for the optimal myofibrillogenesis on muscle-mimetic elastic substrates that was recently observed by several groups in cultures of mouse-, human-, and chick-derived muscle cells.

Fast-folding alpha-helices as reversible strain absorbers in the muscle protein myomesin

The highly oriented filamentous protein network of muscle constantly experiences significant mechanical load during muscle operation. The dimeric protein myomesin has been identified as an important M-band component supporting the mechanical integrity of the entire sarcomere. Recent structural studies have revealed a long α-helical linker between the C-terminal immunoglobulin (Ig) domains My12 and My13 of myomesin. In this paper, we have used single-molecule force spectroscopy in combination with molecular dynamics simulations to characterize the mechanics of the myomesin dimer comprising immunoglobulin domains My12-My13. We find that at forces of approximately 30 pN the α-helical linker reversibly elongates allowing the molecule to extend by more than the folded extension of a full domain. High-resolution measurements directly reveal the equilibrium folding/unfolding kinetics of the individual helix. We show that α-helix unfolding mechanically protects the molecule homodimerization from dissociation at physiologically relevant forces. As fast and reversible molecular springs the myomesin α-helical linkers are an essential component for the structural integrity of the M band.

Scaffolds and chaperones in myofibril assembly: putting the striations in striated muscle

Sarcomere assembly in striated muscles has long been described as a series of steps leading to assembly of individual proteins into thick filaments, thin filaments and Z-lines. Decades of previous work focused on the order in which various structural proteins adopted the striated organization typical of mature myofibrils. These studies led to the view that actin and α-actinin assemble into premyofibril structures separately from myosin filaments, and that these structures are then assembled into myofibrils with centered myosin filaments and actin filaments anchored at the Z-lines. More recent studies have shown that particular scaffolding proteins and chaperone proteins are required for individual steps in assembly. Here, we review the evidence that N-RAP, a LIM domain and nebulin repeat protein, scaffolds assembly of actin and α-actinin into I-Z-I structures in the first steps of assembly; that the heat shock chaperone proteins Hsp90 & Hsc70 cooperate with UNC-45 to direct the folding of muscle myosin and its assembly into thick filaments; and that the kelch repeat protein Krp1 promotes lateral fusion of premyofibril structures to form mature striated myofibrils. The evidence shows that myofibril assembly is a complex process that requires the action of particular catalysts and scaffolds at individual steps. The scaffolds and chaperones required for assembly are potential regulators of myofibrillogenesis, and abnormal function of these proteins caused by mutation or pathological processes could in principle contribute to diseases of cardiac and skeletal muscles.

A myomesin mutation associated with hypertrophic cardiomyopathy deteriorates dimerisation properties

Myomesin plays an important structural and functional role in the M-band of striated muscles. The C-terminal domain 13 of myomesin dimerises and forms antiparallel strands which cross-link neighboring Myosin filaments and titin in the M-line of the sarcomeres. These interactions stabilise the contractile apparatus during striated muscle contraction. Since myomesin is an important component of the M-band we screened the myomesin gene for genetic variants in patients with hypertrophic cardiomyopathy (HCM). We identified the missense mutation V1490I in domain 12 of myomesin in a family with inherited HCM. Analytical ultracentrifugation experiments, circular dichroism spectra, and surface plasmon resonance spectroscopy of myomesin fragments were carried out to investigate the effects of the mutation V1490I on structure and function of myomesin domains 11-13 and 12-13. Both the wild type and mutated myomesin domains My11-13 revealed similar secondary structures and formed stable dimers. Mutated myomesin domains My11-13 and My12-13 dimers revealed a reduced thermal stability and a significantly decreased dimerisation affinity, showing disturbed functional properties of V1490I mutated myomesin. However, monomeric myomesin domains My11-12, i.e. without dimerisation domain 13 showed no difference in thermal stability between wild type and V1490I mutated myomesin. In conclusion, the V1490I mutation associated with HCM lead to myomesin proteins with abnormal functional properties which affect dimerisation properties of myomesin domain 13. These effects may contribute to the pathogenesis of HCM.

Dissection of cardiovascular development and disease pathways in zebrafish

The use of animal models in medicine has contributed significantly to the development of drug treatments and surgical procedures for the last century, in particular for cardiovascular disease. In order to model human disease in an animal, an appreciation of the strengths and limitations of the system are required to interpret results and design the logical sequence of steps toward clinical translation. As the world's population ages, cardiovascular disease will become even more prominent and further progress will be essential to stave off what seems destined to become a massive public health issue. Future treatments will require the imaginative application of current models as well as the generation of new ones. In this review, we discuss the resources available for modeling cardiovascular disease in zebrafish and the varied attributes of this system. We then discuss current zebrafish disease models and their potential that has yet to be exploited.

Sarcomere formation occurs by the assembly of multiple latent protein complexes

The stereotyped striation of myofibrils is a conserved feature of muscle organization that is critical to its function. Although most components that constitute the basic myofibrils are well-characterized biochemically and are conserved across the animal kingdom, the mechanisms leading to the precise assembly of sarcomeres, the basic units of myofibrils, are poorly understood. To gain insights into this process, we investigated the functional relationships of sarcomeric protein complexes. Specifically, we systematically analyzed, using either RNAi in primary muscle cells or available genetic mutations, the organization of myofibrils in Drosophila muscles that lack one or more sarcomeric proteins. Our study reveals that the thin and thick filaments are mutually dependent on each other for striation. Further, the tension sensor complex comprised of zipper/Zasp/α-actinin is involved in stabilizing the sarcomere but not in its initial formation. Finally, integrins appear essential for the interdigitation of thin and thick filaments that occurs prior to striation. Thus, sarcomere formation occurs by the coordinated assembly of multiple latent protein complexes, as opposed to sequential assembly.

Muscle functionality relies on the correct assembly of myofibrils, which are composed of tandem arrays of basic functional contractile units called the sarcomeres. Many mutations in genes encoding sarcomeric proteins cause muscle diseases such as congenital myopathy and dilated cardiac hypertrophy. Understanding the process of sarcomere assembly is not only relevant to the understanding of how protein complexes interact to form complex supra-molecular structures, but also of great significance to medicine for muscle diseases. Here, by taking advantage of our newly developed primary muscle cell culture method, we reevaluate sarcomere assembly by systematically analyzing the functional relationship of sarcomeric proteins using RNA interference or genetic ablation techniques. Our analysis leads us to propose a “two-state” model whereby sarcomeric proteins exist either in the “chaotic” state with independently assembled differential functional complexes or the “highly ordered suprastructure” state made from these complexes. Because we fail to detect any previously hypothesized sarcomere assembly intermediates in our system, our data support the model that sarcomere assembly is a highly coordinated process mediated by multiple latent protein complexes and does not occur in a step-wise fashion.

Assembly and dynamics of myofibrils

We review some of the problems in determining how myofibrils may be assembled and just as importantly how this contractile structure may be renewed by sarcomeric proteins moving between the sarcomere and the cytoplasm. We also address in this personal review the recent evidence that indicates that the assembly and dynamics of myofibrils are conserved whether the cells are analyzed in situ or in tissue culture conditions. We suggest that myofibrillogenesis is a fundamentally conserved process, comparable to protein synthesis, mitosis, or cytokinesis, whether examined in situ or in vitro.

The sarcomeric Z-disc component myopodin is a multiadapter protein that interacts with filamin and alpha-actinin

Here we introduce myopodin as a novel filamin C binding partner. Corroborative yeast two-hybrid and biochemical analyses indicate that the central part of myopodin that shows high homology to the closely related protein synaptopodin and that is common to all its currently known or predicted variants interacts with filamin C immunoglobulin-like domains 20-21. A detailed characterization of the previously described interaction between myopodin and alpha-actinin demonstrates for the first time that myopodin contains three independent alpha-actinin-binding sites. Newly developed myopodin-specific antibodies reveal expression at the earliest stages of in vitro differentiation of human skeletal muscle cells preceding the expression of sarcomeric alpha-actinin. Myopodin colocalizes with filamin and alpha-actinin during all stages of muscle development. By contrast, colocalization with its previously identified binding partner zyxin is restricted to early developmental stages. Genetic and cellular analyses of skeletal muscle provided direct evidence for an alternative transcriptional start site in exon three, corroborating the expression of a myopodin variant lacking the PDZ domain encoded by exons 1 and 2 in skeletal muscle. We conclude that myopodin is a multiadapter protein of the sarcomeric Z-disc that links nascent myofibrils to the sarcolemma via zyxin, and might play a role in early assembly and stabilization of the Z-disc. Mutations in FLNC, ACTN2 and several other genes encoding Z-disc-related proteins cause myopathy and cardiomyopathy. Its localization and its association with the myopathy-associated proteins filamin C and alpha-actinin make myopodin an interesting candidate for a muscle disease gene.

Phosphorylation and function of cardiac myosin binding protein-C in health and disease

During the past 5 years there has been an increasing body of literature describing the roles cardiac myosin binding protein C (cMyBP-C) phosphorylation play in regulating cardiac function and heart failure. cMyBP-C is a sarcomeric thick filament protein that interacts with titin, myosin and actin to regulate sarcomeric assembly, structure and function. Elucidating the function of cMyBP-C is clinically important because mutations in this protein have been linked to cardiomyopathy in more than sixty million people worldwide. One function of cMyBP-C is to regulate cross-bridge formation through dynamic phosphorylation by protein kinase A, protein kinase C and Ca(2+)-calmodulin-activated kinase II, suggesting that cMyBP-C phosphorylation serves as a highly coordinated point of contractile regulation. Moreover, dephosphorylation of cMyBP-C, which accelerates its degradation, has been shown to associate with the development of heart failure in mouse models and in humans. Strikingly, cMyBP-C phosphorylation presents a potential target for therapeutic development as protection against ischemic-reperfusion injury, which has been demonstrated in mouse hearts. Also, emerging evidence suggests that cMyBP-C has the potential to be used as a biomarker for diagnosing myocardial infarction. Although many aspects of cMyBP-C phosphorylation and function remain poorly understood, cMyBP-C and its phosphorylation states have significant promise as a target for therapy and for providing a better understanding of the mechanics of heart function during health and disease. In this review we discuss the most recent findings with respect to cMyBP-C phosphorylation and function and determine potential future directions to better understand the functional role of cMyBP-C and phosphorylation in sarcomeric structure, myocardial contractility and cardioprotection.

Myofibrillogenesis in skeletal muscle cells in zebrafish

The "premyofibril" model of myofibrillogenesis, based on observations in cultured avian muscle cells, proposes that mature myofibrils are preceded by two intermediary structures: premyofibrils and nascent myofibrils. To determine if this model applies to zebrafish skeletal muscle development, we stained developing embryos with antibodies to sarcomeric alpha-actinin and myosin II. In the youngest muscle cells, sarcomeric alpha-actinin and non-muscle myosin II were each localized in linear arrays of small bands that resembled the premyofibrils in avian myocytes. The distribution of muscle-specific myosin II began as scattered short filaments followed in time by overlapping bundles of filaments and organized A-bands in the older somites. Alpha-actinin organization changed from small z-bodies to beaded Z-bands and ordered Z-bands in myofibrils that extended the length of the elongating somites. In older somites with mature myofibrils, premyofibrils were also present at the ends of the mature myofibrils, suggesting that as the cells and somites grew longer, premyofibrils were involved in the elongation of existing mature myofibrils. Fluorescence Recovery After Photobleaching showed that the exchange of proteins (actin, alpha-actinin, FATZ, myotilin and telethonin) between sarcoplasm and the Z-bands of mature myofibrils in zebrafish resembled that seen for the same proteins in cultured avian myotubes, suggesting that myofibril assembly and maintenance in zebrafish share common properties with avian muscle. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Absence of heartbeat in the Xenopus tropicalis mutation muzak is caused by a nonsense mutation in cardiac myosin myh6

Mechanisms coupling heart function and cardiac morphogenesis can be accessed in lower vertebrate embryos that can survive to swimming tadpole stages on diffused oxygen. Forward genetic screens in Xenopus tropicalis have identified more than 80 mutations affecting diverse developmental processes, including cardiac morphogenesis and function. In the first positional cloning of a mutation in X. tropicalis, we show that non-contractile hearts in muzak (muz) embryos are caused by a premature stop codon in the cardiac myosin heavy chain gene myh6. The mutation deletes the coiled-coil domain responsible for polymerization into thick filaments, severely disrupting the cardiomyocyte cytoskeleton. Despite the lack of contractile activity and absence of a major structural protein, early stages of cardiac morphogenesis including looping and chamber formation are grossly normal. Muz hearts subsequently develop dilated chambers with compressed endocardium and fail to form identifiable cardiac valves and trabeculae.

Expression and alternative splicing of N-RAP during mouse skeletal muscle development

N-RAP alternative splicing and protein localization were studied in developing skeletal muscle tissue from pre- and postnatal mice and in fusing primary myotubes in culture. Messages encoding N-RAP-s and N-RAP-c, the predominant isoforms of N-RAP detected in adult skeletal muscle and heart, respectively, were present in a 5:1 ratio in skeletal muscle isolated from E16.5 embryos. N-RAP-s mRNA levels increased three-fold over the first 3 weeks of postnatal development, while N-RAP-c mRNA levels remained low. N-RAP alternative splicing during myotube differentiation in culture was similar to the pattern observed in embryonic and neonatal muscle, with N-RAP-s expression increasing and N-RAP-c mRNA levels remaining low. In both developing skeletal muscle and cultured myotubes, N-RAP protein was primarily associated with developing myofibrillar structures containing alpha-actinin, but was not present in mature myofibrils. The results establish that N-RAP-s is the predominant spliced form of N-RAP present throughout skeletal muscle development.

Insights into the characteristics of mammalian cardiomyocyte terminal differentiation shown through the study of mice with a dysfunctional c-kit

Mammalian cardiomyocytes withdraw from the cell cycle soon after birth. This process is called terminal differentiation. The c-kit, a receptor tyrosine kinase, is expressed on cardiomyocytes immediately after birth but for only a few days. In mice with genetic c-kit dysfunction, adult cardiomyocytes are phenotypically indistinguishable from those of wild type mice, except that they are capable of proliferation in vivo after acute pressure overload. This review explores the idea that postnatal cardiomyocyte differentiation and cell cycle withdrawal are distinct processes and that terminal differentiation may not simply be due to altered expression of genes that regulate the cell cycle but could involve c-kit induced epigenetic change.