Myofibrillogenesis in skeletal muscle cells in zebrafish

Does Nebulin Make Tropomyosin Less Dynamic in Mature Myofibrils in Cross-Striated Myocytes?

Myofibrils in vertebrate cardiac and skeletal muscles are characterized by groups of proteins arranged in contractile units or sarcomeres, which consist of four major components – thin filaments, thick filaments, titin and Z-bands. The thin actin/tropomyosin-containing filaments are embedded in the Z-bands and interdigitate with the myosin-containing thick filaments aligned in A-bands. Titin is attached to the Z-band and extends upto the middle of the A-Band. In this mini review, we have addressed the mechanism of myofibril assembly as well as the dynamics and maintenance of the myofibrils in cardiac and skeletal muscle cells. Evidence from our research as well as from other laboratories favors the premyofibril model of myofibrillogenesis. This three-step model (premyofibril to nascent myofibril to mature myofibril) not only provides a reasonable mechanism for sequential interaction of various proteins during assembly of myofibrils, but also suggests why the dynamics of a thin filament protein like tropomyosin is higher in cardiac muscle than in skeletal muscles. The dynamics of tropomyosin not only varies in different muscle types (cardiac vs. skeletal), but also varies during myofibrillogenesis, for example, premyofibril versus mature myofibrils in skeletal muscle. One of the major differences in protein composition between cardiac and skeletal muscle is nebulin localized along the thin filaments (two nebulins/thin filament) of mature myofibrils in skeletal muscle cells, but which is expressed in a minimal quantity (one nebulin/50 actin filaments) in ventricular cardiomyocytes. Interestingly, nebulin is not associated with premyofibrils in skeletal muscle. Our FRAP(Fluorescence Recovery After Photobleaching) results suggest that tropomyosin is more dynamic in premyofibrils than in mature myofibrils in skeletal muscle, and also, the dynamics of tropomyosin in mature myofibrils is significantly higher in cardiac muscle compared to skeletal muscle. Our working hypothesis is that the association of nebulin in mature myofibrils renders tropomyosin less dynamic in skeletal muscle.

Regulation of nonmuscle myosin II during 3-methylcholanthrene induced dedifferentiation of C2C12 myotubes

3-Methylcholanthrene (3MC) induces tumor formation at the site of injection in the hind leg of mice within 110 days. Recent reports reveal that the transformation of normal muscle cells to atypical cells is one of the causes for tumor formation, however the molecular mechanism behind this process is not well understood. Here, we show in an in vitro study that 3MC induces fragmentation of multinucleate myotubes into viable mononucleates. These mononucleates form colonies when they are seeded into soft agar, indicative of cellular transformation. Immunoblot analysis reveals that phosphorylation of myosin regulatory light chain (RLC20) is 5.6±0.5 fold reduced in 3MC treated myotubes in comparison to vehicle treated myotubes during the fragmentation of myotubes. In contrast, levels of myogenic factors such as MyoD, Myogenin and cell cycle regulators such as Cyclin D, Cyclin E1 remain unchanged as assessed by real-time PCR array and reverse transcriptase PCR analysis, respectively. Interestingly, addition of the myosin light chain kinase inhibitor, ML-7, enhances the fragmentation, whereas phosphatase inhibitor perturbs the 3MC induced fragmentation of myotubes. These results suggest that decrease in RLC20 phosphorylation may be associated with the fragmentation step of dedifferentiation.

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.

Myofibrillar misalignment correlated to triad disappearance of mdx mouse gastrocnemius muscle probed by SHG microscopy

We show that the canonical single frequency sarcomeric SHG intensity pattern (SHG-IP) of control muscles is converted to double frequency sarcomeric SHG-IP in preserved mdx mouse gastrocnemius muscles in the vicinity of necrotic fibers. These double frequency sarcomeric SHG-IPs are often spatially correlated to double frequency sarcomeric two-photon excitation fluorescence (TPEF) emitted from Z-line and I-bands and to one centered spot SHG angular intensity pattern (SHG-AIP) suggesting that these patterns are signature of myofibrillar misalignement. This latter is confirmed with transmission electron microscopy (TEM). Moreover, a good spatial correlation between SHG signature of myofibrillar misalignment and triad reduction is established. Theoretical simulation of sarcomeric SHG-IP is used to demonstrate the correlation between change of SHG-IP and -AIP and myofibrillar misalignment. The extreme sensitivity of SHG microscopy to reveal the submicrometric organization of A-band thick filaments is highlighted. This report is a first step toward future studies aimed at establishing live SHG signature of myofibrillar misalignment involving excitation contraction defects due to muscle damage and disease.

Membrane-myofibril cross-talk in myofibrillogenesis and in muscular dystrophy pathogenesis: lessons from the zebrafish

Striated muscle has a highly ordered structure in which specialized domains of the cell membrane involved in force transmission (costameres) and excitation-contraction coupling (T tubules) as well as the internal membranes of the sarcoplasmic reticulum are organized over specific regions of the sarcomere. Optimal muscle function is dependent on this high level of organization but how it established and maintained is not well understood. Due to its ex utero development and transparency, the zebrafish embryo is an excellent vertebrate model for the study of dynamic relationships both within and between cells during development. Transgenic models have allowed the delineation of cellular migration and complex morphogenic rearrangements during the differentiation of skeletal myocytes and the assembly and organization of new myofibrils. Molecular targeting of genes and transcripts has allowed the identification of key requirements for myofibril assembly and organization. With the recent advances in gene editing approaches, the zebrafish will become an increasingly important model for the study of human myopathies and muscular dystrophies. Its high fecundity and small size make it well suited to high-throughput screenings to identify novel pharmacologic and molecular therapies for the treatment of a broad range of neuromuscular conditions. In this review, we examine the lessons learned from the zebrafish model regarding the complex interactions between the sarcomere and the sarcolemma that pattern the developing myocyte and discuss the potential for zebrafish as a model system to examine the pathophysiology of, and identify new treatments for, human myopathies and muscular dystrophies.

Palladin promotes assembly of non-contractile dorsal stress fibers through VASP recruitment

Stress fibers are major contractile actin structures in non-muscle cells, where they have an important role in adhesion, morphogenesis and mechanotransduction. Palladin is a multidomain protein, which associates with stress fibers in a variety of cell-types. However, the exact role of palladin in stress fiber assembly and maintenance has remained obscure, and whether it functions as an actin filament cross-linker or scaffolding protein was unknown. We demonstrate that palladin is specifically required for assembly of non-contractile dorsal stress fibers, and is consequently essential for generation of stress fiber networks and regulation of cell morphogenesis in osteosarcoma cells migrating in three-dimensional collagen matrix. Importantly, we reveal that palladin is necessary for the recruitment of vasodilator stimulated phosphoprotein (VASP) to dorsal stress fibers, and that it promotes stress fiber assembly through VASP. Both palladin and VASP display similar rapid dynamics at dorsal stress fibers, suggesting that they associate with stress fibers as a complex. Thus, palladin functions as a dynamic scaffolding protein, which promotes the assembly of dorsal stress fibers by recruiting VASP to these structures.

The titin A-band rod domain is dispensable for initial thick filament assembly in zebrafish

The sarcomeres of skeletal and cardiac muscle are highly structured protein arrays, consisting of thick and thin filaments aligned precisely to one another and to their surrounding matrix. The contractile mechanisms of sarcomeres are generally well understood, but how the patterning of sarcomeres is initiated during early skeletal muscle and cardiac development remains uncertain. Two of the most widely accepted hypotheses for this process include the "molecular ruler" model, in which the massive protein titin defines the length of the sarcomere and provides a scaffold along which the myosin thick filament is assembled, and the "premyofibril" model, which proposes that thick filament formation does not require titin, but that a "premyofibril" consisting of non-muscle myosin, α-actinin and cytoskeletal actin is used as a template. Each model posits a different order of necessity of the various components, but these have been difficult to test in vivo. Zebrafish motility mutants with developmental defects in sarcomere patterning are useful for the elucidation of such mechanisms, and here we report the analysis of the herzschlag mutant, which shows deficits in both cardiac and skeletal muscle. The herzschlag mutant produces a truncated titin protein, lacking the C-terminal rod domain that is proposed to act as a thick filament scaffold, yet muscle patterning is still initiated, with grossly normal thick and thin filament assembly. Only after embryonic muscle contraction begins is breakdown of sarcomeric myosin patterning observed, consistent with the previously noted role of titin in maintaining the contractile integrity of mature sarcomeres. This conflicts with the "molecular ruler" model of early sarcomere patterning and supports a titin-independent model of thick filament organization during sarcomerogenesis. These findings are also consistent with the symptoms of human titin myopathies that exhibit a late onset, such as tibial muscular dystrophy.

Heart-specific stiffening in early embryos parallels matrix and myosin expression to optimize beating

In development and differentiation, morphological changes often accompany mechanical changes [1], but it is unclear whether or when cells in embryos sense tissue elasticity. The earliest embryo is uniformly pliable, while adult tissues vary widely in mechanics from soft brain and stiff heart to rigid bone [2]. However, cell sensitivity to microenvironment elasticity is debated based in part on results from complex three-dimensional culture models [3]. Regenerative cardiology provides strong motivation to clarify any cell-level sensitivities to tissue elasticity because rigid postinfarct regions limit pumping by the adult heart [4]. Here, we focus on the spontaneously beating embryonic heart and sparsely cultured cardiomyocytes, including cells derived from pluripotent stem cells. Tissue elasticity, Et, increases daily for heart to 1-2 kPa by embryonic day 4 (E4), and although this is ~10-fold softer than adult heart, the beating contractions of E4 cardiomyocytes prove optimal at ~Et,E4 both in vivo and in vitro. Proteomics reveals daily increases in a small subset of proteins, namely collagen plus cardiac-specific excitation-contraction proteins. Rapid softening of the heart's matrix with collagenase or stiffening it with enzymatic crosslinking suppresses beating. Sparsely cultured E4 cardiomyocytes on collagen-coated gels likewise show maximal contraction on matrices with native E4 stiffness, highlighting cell-intrinsic mechanosensitivity. While an optimal elasticity for striation proves consistent with the mathematics of force-driven sarcomere registration, contraction wave speed is linear in Et as theorized for excitation-contraction coupled to matrix elasticity. Pluripotent stem cell-derived cardiomyocytes also prove to be mechanosensitive to matrix and thus generalize the main observation that myosin II organization and contractile function are optimally matched to the load contributed by matrix elasticity.

Cardiomyopathy mutations in the tail of β-cardiac myosin modify the coiled-coil structure and affect integration into thick filaments in muscle sarcomeres in adult cardiomyocytes

It is unclear why mutations in the filament-forming tail of myosin heavy chain (MHC) cause hypertrophic or dilated cardiomyopathy as these mutations should not directly affect contraction. To investigate this, we first investigated the impact of five hypertrophic cardiomyopathy-causing (N1327K, E1356K, R1382W, E1555K, and R1768K) and one dilated cardiomyopathy-causing (R1500W) tail mutations on their ability to incorporate into muscle sarcomeres in vivo. We used adenoviral delivery to express full-length wild type or mutant enhanced GFP-MHC in isolated adult cardiomyocytes. Three mutations (N1327K, E1356K, and E1555K) reduced enhanced GFP-MHC incorporation into muscle sarcomeres, whereas the remainder had no effect. No mutations significantly affected contraction. Fluorescence recovery after photobleaching showed that fluorescence recovery for the mutation that incorporated least well (N1327K) was significantly faster than that of WT with half-times of 25.1 ± 1.8 and 32.2 ± 2.5 min (mean ± S.E.), respectively. Next, we determined the effects of each mutation on the helical properties of wild type and seven mutant peptides (7, 11, or 15 heptads long) from the myosin tail by circular dichroism. R1382W and E1768K slightly increased the α-helical nature of peptides. The remaining mutations reduced α-helical content, with N1327K showing the greatest reduction. Only peptides containing residues 1301-1329 were highly α-helical suggesting that this region helps in initiation of coiled coil. These results suggest that small effects of mutations on helicity translate into a reduced ability to incorporate into sarcomeres, which may elicit compensatory hypertrophy.

Study of the effect of myofibrillar misalignment on the sarcomeric SHG intensity pattern

We present a theoretical simulation of the sarcomeric SHG intensity pattern (SHG-IP) that takes into account myofibrillar misalignment that is experimentally observed in SHG images of proteolysed muscles. The model predicts that myofibrillar displacement results in the conversion from one peak (1P) to two peaks (2P) sarcomeric SHG-IP in agreement with experimental results. This study suggests that sarcomeric SHG-IP is a powerful tool for mapping spatial myofibrillar displacement and its related excitation-contraction disruption that could occur during muscle physiological adaptation and disease.

Alp/Enigma family proteins cooperate in Z-disc formation and myofibril assembly

The Drosophila Alp/Enigma family protein Zasp52 localizes to myotendinous junctions and Z-discs. It is required for terminal muscle differentiation and muscle attachment. Its vertebrate ortholog ZASP/Cypher also localizes to Z-discs, interacts with α-actinin through its PDZ domain, and is involved in Z-disc maintenance. Human mutations in ZASP cause myopathies and cardiomyopathies. Here we show that Drosophila Zasp52 is one of the earliest markers of Z-disc assembly, and we use a Zasp52-GFP fusion to document myofibril assembly by live imaging. We demonstrate that Zasp52 is required for adult Z-disc stability and pupal myofibril assembly. In addition, we show that two closely related proteins, Zasp66 and the newly identified Zasp67, are also required for adult Z-disc stability and are participating with Zasp52 in Z-disc assembly resulting in more severe, synergistic myofibril defects in double mutants. Zasp52 and Zasp66 directly bind to α-actinin, and they can also form a ternary complex. Our results indicate that Alp/Enigma family members cooperate in Z-disc assembly and myofibril formation; and we propose, based on sequence analysis, a novel class of PDZ domain likely involved in α-actinin binding.

α-Actinin2 is required for the lateral alignment of Z discs and ventricular chamber enlargement during zebrafish cardiogenesis

α-Actinin2 (Actn2) is a predominant protein in the sarcomere Z disc whose mutation can lead to cardiomyopathy. However, the function of Actn2 in Z-disc assembly and cardiomyopathy in vertebrates remains elusive. We leveraged genetic tools in zebrafish embryos to elucidate the function of Actn2. We identified a single Actn2 homologue expressed in the zebrafish heart and conducted loss-of-function studies by antisense morpholino technology. Although zebrafish Actn2 assembles early into the Z disc, depletion of actn2 did not affect the early steps of sarcomere assembly. Instead, Actn2 is required for Z bodies to register laterally, forming well-aligned Z discs. Presumably as a consequence to this structural defect in the sarcomere, the depletion of Actn2 resulted in reduced cardiac function, primarily characterized as a reduced end-diastolic diameter. The depletion of actn2 also significantly reduced the ventricle chamber size, due to both reduced cardiomyocyte (CM) size and CM number. Interestingly, reduced CM size can be rescued by the cessation of heart contractions. The genetic studies of zebrafish uncovered a function for actn2 in lateral registration of Z body. In actn2 morphant fish, the Z-disc defect sequentially affects cardiac function, which leads to morphological changes in the ventricle through a mechanical force-dependent mechanism.

Characterization and investigation of zebrafish models of filamin-related myofibrillar myopathy

Myofibrillar myopathies are a group of muscle disorders characterized by the disintegration of skeletal muscle fibers and formation of sarcomeric protein aggregates. All the proteins known to be involved in myofibrillar myopathies localize to a region of the sarcomere known as the Z-disk, the site at which defects are first observed. Given the common cellular phenotype observed in this group of disorders, it is thought that there is a common mechanism of pathology. Mutations in filamin C, which has several proposed roles in the development and function of skeletal muscle, can result in filamin-related myofibrillar myopathy. The lack of a suitable animal model system has limited investigation into the mechanism of pathology in this disease and the role of filamin C in muscle development. Here, we characterize stretched out (sot), a zebrafish filamin Cb mutant, together with targeted knockdown of zebrafish filamin Ca, revealing fiber dissolution and formation of protein aggregates strikingly similar to those seen in filamin-related myofibrillar myopathies. Through knockdown of both zebrafish filamin C homologues, we demonstrate that filamin C is not required for fiber specification and that fiber damage is a consequence of muscle activity. The remarkable similarities in the myopathology between our models and filamin-related myofibrillar myopathy makes them suitable for the study of these diseases and provides unique opportunities for the investigation of the function of filamin C in muscle and development of therapies.

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.

Actin stress fibers--assembly, dynamics and biological roles

Actin filaments assemble into diverse protrusive and contractile structures to provide force for a number of vital cellular processes. Stress fibers are contractile actomyosin bundles found in many cultured non-muscle cells, where they have a central role in cell adhesion and morphogenesis. Focal-adhesion-anchored stress fibers also have an important role in mechanotransduction. In animal tissues, stress fibers are especially abundant in endothelial cells, myofibroblasts and epithelial cells. Importantly, recent live-cell imaging studies have provided new information regarding the mechanisms of stress fiber assembly and how their contractility is regulated in cells. In addition, these studies might elucidate the general mechanisms by which contractile actomyosin arrays, including muscle cell myofibrils and cytokinetic contractile ring, can be generated in cells. In this Commentary, we discuss recent findings concerning the physiological roles of stress fibers and the mechanism by which these structures are generated in cells.

α-Actinin-2 deficiency results in sarcomeric defects in zebrafish that cannot be rescued by α-actinin-3 revealing functional differences between sarcomeric isoforms

α-Actinins are actin-binding proteins that can be broadly divided into Ca(2+)-sensitive cytoskeletal and Ca(2+)-insensitive sarcomeric isoforms. To date, little is known about functional differences between the isoforms due to their indistinguishable activities in most in vitro assays. To identify functional differences in vivo between sarcomeric isoforms, we employed computational and molecular approaches to characterize the zebrafish (Danio rerio) genome, which contains orthologoues of each human α-actinin gene, including duplicated copies of actn3. Each isoform exhibits a distinct and unique pattern of gene expression as assessed by mRNA in situ hybridization, largely sharing similar expression profiles as seen in humans. The spatial conservation of expression of these genes from lower invertebrates to humans suggests that regulation and subsequent functions of these genes are conserved during evolution. Morpholino-based knockdown of the sarcomeric isoform, actn2, leads to skeletal muscle, cardiac, and ocular defects evident over the first week of development. Remarkably, despite the high degree of sequence conservation between actn2 and actn3, the phenotypes of α-actinin-2 deficient zebrafish can be rescued by overexpression of α-actinin-2 but not by α-actinin-3 mRNAs from zebrafish or human. These data provide functional evidence that the primary sequences of α-actinin-2 and α-actinin-3 evolved differences to optimize their functions.

Analysis of myotilin turnover provides mechanistic insight into the role of myotilinopathy-causing mutations

MFM (myofibrillar myopathies) are caused by mutations in several sarcomeric components, including the Z-disc protein myotilin. The morphological changes typical of MFM include Z-disc alterations and aggregation of dense filamentous sarcomeric material. The causes and mechanisms of protein aggregation in myotilinopathies and other forms of MFM remain unknown, although impaired degradation may explain, in part, the abnormal protein accumulation. In the present paper we have studied the mechanisms regulating myotilin turnover, analysed the consequences of defective myotilin degradation and tested whether disease-causing myotilin mutations result in altered protein turnover. The results indicate that myotilin is a substrate for the Ca(2+)-dependent protease calpain and identify two calpain cleavage sites in myotilin by MS. We further show that myotilin is degraded by the proteasome system in transfected COS7 cells and in myotubes, and that disease-causing myotilinopathy mutations result in reduced degradation. Finally, we show that proteolysis-inhibitor-induced reduction in myotilin turnover results in formation of intracellular myotilin and actin-containing aggregates, which resemble those seen in diseased muscle cells. These findings identify for the first time biological differences between wt (wild-type) and mutant myotilin. The present study provides novel information on the pathways controlling myotilin turnover and on the molecular defects associated with MFM.

Obscurin depletion impairs organization of skeletal muscle in developing zebrafish embryos

During development, skeletal myoblasts differentiate into myocytes and skeletal myotubes with mature contractile structures that are precisely oriented with respect to surrounding cells and tissues. Establishment of this highly ordered structure requires reciprocal interactions between the differentiating myocytes and the surrounding extracellular matrix to form correctly positioned and well-organized attachments from the skeletal muscle to the bony skeleton. Using the developing zebrafish embryo as a model, we examined the relationship between new myofibril assembly and the organization of the membrane domains involved in cell-extracellular matrix interactions. We determined that depletion of obscurin, a giant muscle protein, resulted in irregular cell morphology and disturbed extracellular matrix organization during skeletal muscle development. The resulting impairment of myocyte organization was associated with disturbance of the internal architecture of the myocyte suggesting that obscurin participates in organizing the internal structure of the myocyte and translating those structural cues to surrounding cells and tissues.

Myotilin dynamics in cardiac and skeletal muscle cells

Myotilin cDNA has been cloned for the first time from chicken muscles and sequenced. Ectopically expressed chicken and human YFP-myotilin fusion proteins localized in avian muscle cells in the Z-bodies of premyofibrils and the Z-bands of mature myofibrils. Fluorescence recovery after photobleaching experiments demonstrated that chicken and human myotilin were equally dynamic with 100% mobile fraction in premyofibrils and Z-bands of mature myofibrils. Seven myotilin mutants cDNAs (S55F, S55I, T57I, S60C, S60F, S95I, R405K) with known muscular dystrophy association localized in mature myofibrils in the same way as normal myotilin without affecting the formation and maintenance of myofibrils. N- and C-terminal halves of human myotilin were cloned and expressed as YFP fusions in myotubes and cardiomyocytes. N-terminal myotilin (aa 1-250) localized weakly in Z-bands with a high level of unincorporated protein and no adverse effect on myofibril structure. C-terminal myotilin (aa 251-498) localized in Z-bands and in aggregates. Formation of aggregated C-terminal myotilin was accompanied by the loss of Z-band localization of C-terminal myotilin and partial or complete loss of alpha-actinin from the Z-bands. In regions of myotubes with high concentrations of myotilin aggregates there were no alpha-actinin positive Z-bands or organized F-actin. The dynamics of the C-terminal-myotilin and N-terminal myotilin fragments differed significantly from each other and from full-length myotilin. In contrast, no significant changes in dynamics were detected after expression in myotubes of myotilin mutants with single amino acid changes known to be associated with myopathies.

Tropomodulin capping of actin filaments in striated muscle development and physiology

Efficient striated muscle contraction requires precise assembly and regulation of diverse actin filament systems, most notably the sarcomeric thin filaments of the contractile apparatus. By capping the pointed ends of actin filaments, tropomodulins (Tmods) regulate actin filament assembly, lengths, and stability. Here, we explore the current understanding of the expression patterns, localizations, and functions of Tmods in both cardiac and skeletal muscle. We first describe the mechanisms by which Tmods regulate myofibril assembly and thin filament lengths, as well as the roles of closely related Tmod family variants, the leiomodins (Lmods), in these processes. We also discuss emerging functions for Tmods in the sarcoplasmic reticulum. This paper provides abundant evidence that Tmods are key structural regulators of striated muscle cytoarchitecture and physiology.

Imaging of zebrafish in vivo with second-harmonic generation reveals shortened sarcomeres associated with myopathy induced by statin

We employed second-harmonic generation (SHG) imaging and the zebrafish model to investigate the myopathy caused by statin in vivo with emphasis on the altered microstructures of the muscle sarcomere, the fundamental contractile element of muscles. This approach derives an advantage of SHG imaging to observe the striated skeletal muscle of living zebrafish based on signals produced mainly from the thick myosin filament of sarcomeres without employing exogenous labels, and eliminates concern about the distortion of muscle structures caused by sample preparation in conventional histological examination. The treatment with statin caused a significantly shortened sarcomere relative to an untreated control (1.73±0.09 µm vs 1.91±0.08 µm, P<0.05) while the morphological integrity of the muscle fibers remained largely intact. Mechanistic tests indicated that this microstructural disorder was associated with the biosynthetic pathway of cholesterol, or, specifically, with the impaired production of mevalonate by statins. This microstructural disorder exhibited a strong dependence on both the dosage and the duration of treatment, indicating a possibility to assess the severity of muscle injury according to the altered length of the sarcomeres. In contrast to a conventional assessment of muscle injury using clinical biomarkers in blood, such as creatine kinase that is released from only disrupted myocytes, the ability to determine microstructural modification of sarcomeres allows diagnosis of muscle injury before an onset of conventional clinical symptoms. In light of the increasing prevalence of the incidence of muscle injuries caused by new therapies, our work consolidates the combined use of the zebrafish and SHG imaging as an effective and sensitive means to evaluate the safety profile of new therapeutic targets in vivo.

Biochemical and cell biological analysis of actin in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has long been a useful model organism for muscle research. Its body wall muscle is obliquely striated muscle and exhibits structural similarities with vertebrate striated muscle. Actin is the core component of the muscle thin filaments, which are highly ordered in sarcomeric structures in striated muscle. Genetic studies have identified genes that regulate proper organization and function of actin filaments in C. elegans muscle, and sequence of the worm genome has revealed a number of conserved candidate genes that may regulate actin. To precisely understand the functions of actin-binding proteins, such genetic and genomic studies need to be complemented by biochemical characterization of these actin-binding proteins in vitro. This article describes methods for purification and biochemical characterization of actin from C. elegans. Although rabbit muscle actin is commonly used to characterize actin-binding proteins from many eukaryotic organisms, we detect several quantitative differences between C. elegans actin and rabbit muscle actin, highlighting that use of actin from an appropriate source is important in some cases. Additionally, we describe probes for cell biological analysis of actin in C. elegans.

Growth and the regulation of myotomal muscle mass in teleost fish

Teleost muscle first arises in early embryonic life and its development is driven by molecules present in the egg yolk and modulated by environmental stimuli including temperature and oxygen. Several populations of myogenic precursor cells reside in the embryonic somite and external cell layer and contribute to muscle fibres in embryo, larval, juvenile and adult stages. Many signalling proteins and transcription factors essential for these events are known. In all cases, myogenesis involves myoblast proliferation, migration, fusion and terminal differentiation. Maturation of the embryonic muscle is associated with motor innervation and the development of a scaffold of connective tissue and complex myotomal architecture needed to generate swimming behaviour. Adult muscle is a heterogeneous tissue composed of several cell types that interact to affect growth patterns. The development of capillary and lymphatic circulations and extramuscular organs--notably the gastrointestinal, endocrine, neuroendocrine and immune systems--serves to increase information exchange between tissues and with the external environment, adding to the complexity of growth regulation. Teleosts often exhibit an indeterminate growth pattern, with body size and muscle mass increasing until mortality or senescence occurs. The dramatic increase in myotomal muscle mass between embryo and adult requires the continuous production of muscle fibres until 40-50% of the maximum body length is reached. Sarcomeric proteins can be mobilised as a source of amino acids for energy metabolism by other tissues and for gonad generation, requiring the dynamic regulation of muscle mass throughout the life cycle. The metabolic and contractile phenotypes of muscle fibres also show significant plasticity with respect to environmental conditions, migration and spawning. Many genes regulating muscle growth are found as multiple copies as a result of paralogue retention following whole-genome duplication events in teleost lineages. The extent to which indeterminate growth, ectothermy and paralogue preservation have resulted in modifications of the genetic pathways regulating muscle growth in teleosts compared to mammals largely remains unknown. This review describes the use of compensatory growth models, transgenesis and tissue culture to explore the mechanisms of muscle growth in teleosts and provides some perspectives on future research directions.

Telethonin deficiency is associated with maladaptation to biomechanical stress in the mammalian heart

RATIONALE

Telethonin (also known as titin-cap or t-cap) is a 19-kDa Z-disk protein with a unique β-sheet structure, hypothesized to assemble in a palindromic way with the N-terminal portion of titin and to constitute a signalosome participating in the process of cardiomechanosensing. In addition, a variety of telethonin mutations are associated with the development of several different diseases; however, little is known about the underlying molecular mechanisms and telethonin's in vivo function.

OBJECTIVE

Here we aim to investigate the role of telethonin in vivo and to identify molecular mechanisms underlying disease as a result of its mutation.

METHODS AND RESULTS

By using a variety of different genetically altered animal models and biophysical experiments we show that contrary to previous views, telethonin is not an indispensable component of the titin-anchoring system, nor is deletion of the gene or cardiac specific overexpression associated with a spontaneous cardiac phenotype. Rather, additional titin-anchorage sites, such as actin-titin cross-links via α-actinin, are sufficient to maintain Z-disk stability despite the loss of telethonin. We demonstrate that a main novel function of telethonin is to modulate the turnover of the proapoptotic tumor suppressor p53 after biomechanical stress in the nuclear compartment, thus linking telethonin, a protein well known to be present at the Z-disk, directly to apoptosis ("mechanoptosis"). In addition, loss of telethonin mRNA and nuclear accumulation of this protein is associated with human heart failure, an effect that may contribute to enhanced rates of apoptosis found in these hearts.

CONCLUSIONS

Telethonin knockout mice do not reveal defective heart development or heart function under basal conditions, but develop heart failure following biomechanical stress, owing at least in part to apoptosis of cardiomyocytes, an effect that may also play a role in human heart failure.

Titin visualization in real time reveals an unexpected level of mobility within and between sarcomeres

Contrary to prior models in which titin serves as a stable scaffold in sarcomeres, sarcomeric and soluble titin exchange dynamically in myofibers when calcium levels are low.

The giant muscle protein titin is an essential structural component of the sarcomere. It forms a continuous periodic backbone along the myofiber that provides resistance to mechanical strain. Thus, the titin filament has been regarded as a blueprint for sarcomere assembly and a prerequisite for stability. Here, a novel titin-eGFP knockin mouse provided evidence that sarcomeric titin is more dynamic than previously suggested. To study the mobility of titin in embryonic and neonatal cardiomyocytes, we used fluorescence recovery after photobleaching and investigated the contribution of protein synthesis, contractility, and calcium load to titin motility. Overall, the kinetics of lateral and longitudinal movement of titin-eGFP were similar. Whereas protein synthesis and developmental stage did not alter titin dynamics, there was a strong, inhibitory effect of calcium on titin mobility. Our results suggest a model in which the largely unrestricted movement of titin within and between sarcomeres primarily depends on calcium, suggesting that fortification of the titin filament system is activity dependent.

Micromechanical function of myofibrils isolated from skeletal and cardiac muscles of the zebrafish

The zebrafish is a potentially important and cost-effective model for studies of development, motility, regeneration, and inherited human diseases. The object of our work was to show whether myofibrils isolated from zebrafish striated muscle represent a valid subcellular contractile model. These organelles, which determine contractile function in muscle, were used in a fast kinetic mechanical technique based on an atomic force probe and video microscopy. Mechanical variables measured included rate constants of force development (k_ACT) after Ca²⁺ activation and of force decay (τ_REL⁻¹) during relaxation upon Ca²⁺ removal, isometric force at maximal (F_max) or partial Ca²⁺ activations, and force response to an external stretch applied to the relaxed myofibril (F_pass). Myotomal myofibrils from larvae developed greater active and passive forces, and contracted and relaxed faster than skeletal myofibrils from adult zebrafish, indicating developmental changes in the contractile organelles of the myotomal muscles. Compared with murine cardiac myofibrils, measurements of adult zebrafish ventricular myofibrils show that k_ACT, F_max, Ca²⁺ sensitivity of the force, and F_pass were comparable and τ_REL⁻¹ was smaller. These results suggest that cardiac myofibrils from zebrafish, like those from mice, are suitable contractile models to study cardiac function at the sarcomeric level. The results prove the practicability and usefulness of mechanical and kinetic investigations on myofibrils isolated from larval and adult zebrafish muscles. This novel approach for investigating myotomal and myocardial function in zebrafish at the subcellular level, combined with the powerful genetic manipulations that are possible in the zebrafish, will allow the investigation of the functional primary consequences of human disease–related mutations in sarcomeric proteins in the zebrafish model.

The actin associated protein palladin in smooth muscle and in the development of diseases of the cardiovasculature and in cancer

Palladin is an actin associated protein serving as a cytoskeleton scaffold, and actin cross linker, localizing at stress fibers, focal adhesions, and other actin based structures. Recent studies showed that palladin plays a critical role in smooth muscle differentiation, migration, contraction, and more importantly contributes to embryonic development. This review will focus on the functions and possible mechanisms of palladin in smooth muscle and in pathological conditions such as cardiovascular diseases and cancers.

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.

Troponin T is essential for sarcomere assembly in zebrafish skeletal muscle

In striated muscle, the basic contractile unit is the sarcomere, which comprises myosin-rich thick filaments intercalated with thin filaments made of actin, tropomyosin and troponin. Troponin is required to regulate Ca(2+)-dependent contraction, and mutant forms of troponins are associated with muscle diseases. We have disrupted several genes simultaneously in zebrafish embryos and have followed the progression of muscle degeneration in the absence of troponin. Complete loss of troponin T activity leads to loss of sarcomere structure, in part owing to the destructive nature of deregulated actin-myosin activity. When troponin T and myosin activity are simultaneously disrupted, immature sarcomeres are rescued. However, tropomyosin fails to localise to sarcomeres, and intercalating thin filaments are missing from electron microscopic cross-sections, indicating that loss of troponin T affects thin filament composition. If troponin activity is only partially disrupted, myofibrils are formed but eventually disintegrate owing to deregulated actin-myosin activity. We conclude that the troponin complex has at least two distinct activities: regulation of actin-myosin activity and, independently, a role in the proper assembly of thin filaments. Our results also indicate that sarcomere assembly can occur in the absence of normal thin filaments.

Arg/Abl-binding protein, a Z-body and Z-band protein, binds sarcomeric, costameric, and signaling molecules

ArgBP2 (Arg/Abl-Binding Protein) is expressed at high levels in the heart and is localized in the Z-bands of mature myofibrils. ArgBP2 is a member of a small family of proteins that also includes vinexin and CAP (c-Cbl-associated protein), all characterized by having one sorbin homology (SOHO) domain and three C-terminal SH3 domains. Antibodies directed against ArgBP2 also react with the Z-bodies of myofibril precursors: premyofibrils and nascent myofibrils. Expression in cardiomyocytes of plasmids encoding Yellow Fluorescent Protein (YFP) fused to either full length ArgBP2, the SOHO, mid-ArgBP or the SH3 domains of ArgBP2 led to Z-band targeting of the fusion proteins, whereas an N-terminal fragment lacking these domains did not target to Z-bands. Although ArgBP2 is not found in skeletal muscle cells, YFP-ArgBP2 did target to Z-bodies and Z-bands in cultured myotubes. GST-ArgBP2-SH3 bound actin, α-actinin and vinculin proteins in blot overlays, cosedimentation assays, and EM negative staining techniques. Over-expression of ArgBP2 and ArgBP2-SH3 domains, but not YFP alone, led to loss of myofibrils in cardiomyocytes. Fluorescence recovery after photobleaching was used to measure the rapid dynamics of both the full length and some truncated versions of ArgBP2. Our results indicate that ArgBP2 may play an important role in the assembly and maintenance of myofibrils in cardiomyocytes.

Dynamic regulation of sarcomeric actin filaments in striated muscle

In striated muscle, the actin cytoskeleton is differentiated into myofibrils. Actin and myosin filaments are organized in sarcomeres and specialized for producing contractile forces. Regular arrangement of actin filaments with uniform length and polarity is critical for the contractile function. However, the mechanisms of assembly and maintenance of sarcomeric actin filaments in striated muscle are not completely understood. Live imaging of actin in striated muscle has revealed that actin subunits within sarcomeric actin filaments are dynamically exchanged without altering overall sarcomeric structures. A number of regulators for actin dynamics have been identified, and malfunction of these regulators often result in disorganization of myofibril structures or muscle diseases. Therefore, proper regulation of actin dynamics in striated muscle is critical for assembly and maintenance of functional myofibrils. Recent studies have suggested that both enhancers of actin dynamics and stabilizers of actin filaments are important for sarcomeric actin organization. Further investigation of the regulatory mechanism of actin dynamics in striated muscle should be a key to understanding how myofibrils develop and operate.

Knockdown and overexpression of Unc-45b result in defective myofibril organization in skeletal muscles of zebrafish embryos

Background

Unc-45 is a myosin chaperone and a Hsp90 co-chaperone that plays a key role in muscle development. Genetic and biochemical studies in C. elegans have demonstrated that Unc-45 facilitates the process of myosin folding and assembly in body wall muscles. Loss or overexpression of Unc-45 in C. elegans results in defective myofibril organization. In the zebrafish Danio rerio, unc-45b, a homolog of C. elegans unc-45, is expressed in both skeletal and cardiac muscles. Earlier studies indicate that mutation or knockdown of unc-45b expression in zebrafish results in a phenotype characterized by a loss of both thick and thin filament organization in skeletal and cardiac muscle. The effects of unc-45b knockdown on other sarcomeric structures and the phenotype of Unc-45b overexpression, however, are poorly understood in vertebrates.

Results

Both knockdown and overexpression provide useful tools to study gene function during animal development. Using such methods, we characterized the role of Unc-45b in myofibril assembly of skeletal muscle in Danio rerio. We showed that, in addition to thick and thin filament defects, knockdown of unc-45b expression disrupted sarcomere organization in M-lines and Z-lines of skeletal muscles in zebrafish embryos. Western blotting analysis showed that myosin protein levels were significantly decreased in unc-45b knockdown embryos. Similarly, embryos overexpressing Unc-45b also exhibited severely disorganized myosin thick filaments. Disruption of thick filament organization by Unc-45b overexpression depends on the C-terminal UCS domain in Unc-45b required for interaction with myosin. Deletion of the C-terminal UCS domain abolished the disruptive activity of Unc-45b in myosin thick filament organization. In contrast, deletion of the N-terminal TPR domain required for binding with Hsp90α had no effect.

Conclusion

Collectively, these studies indicate that the expression levels of Unc-45b must be precisely regulated to ensure normal myofibril organization. Loss or overexpression of Unc-45b leads to defective myofibril organization.

Distinct roles for telethonin N-versus C-terminus in sarcomere assembly and maintenance

The N-terminus of telethonin forms a unique structure linking two titin N-termini at the Z-disc. While a specific role for the C-terminus has not been established, several studies indicate it may have a regulatory function. Using a morpholino approach in Xenopus, we show that telethonin knockdown leads to embryonic paralysis, myocyte defects, and sarcomeric disruption. These myopathic defects can be rescued by expressing full-length telethonin mRNA in morpholino background, indicating that telethonin is required for myofibrillogenesis. However, a construct missing C-terminal residues is incapable of rescuing motility or sarcomere assembly in cultured myocytes. We, therefore, tested two additional constructs: one where four C-terminal phosphorylatable residues were mutated to alanines and another where terminal residues were randomly replaced. Data from these experiments support that the telethonin C-terminus is required for assembly, but in a context-dependent manner, indicating that factors and forces present in vivo can compensate for C-terminal truncation or mutation.

Different localizations and cellular behaviors of leiomodin and tropomodulin in mature cardiomyocyte sarcomeres

Leiomodin (Lmod) is a muscle-specific F-actin-nucleating protein that is related to the F-actin pointed-end-capping protein tropomodulin (Tmod). However, Lmod contains a unique ∼150-residue C-terminal extension that is required for its strong nucleating activity. Overexpression or depletion of Lmod compromises sarcomere organization, but the mechanism by which Lmod contributes to myofibril assembly is not well understood. We show that Tmod and Lmod localize through fundamentally different mechanisms to the pointed ends of two distinct subsets of actin filaments in myofibrils. Tmod localizes to two narrow bands immediately adjacent to M-lines, whereas Lmod displays dynamic localization to two broader bands, which are generally more separated from M-lines. Lmod's localization and F-actin nucleation activity are enhanced by interaction with tropomyosin. Unlike Tmod, the myofibril localization of Lmod depends on sustained muscle contraction and actin polymerization. We further show that Lmod expression correlates with the maturation of myofibrils in cultured cardiomyocytes and that it associates with sarcomeres only in differentiated myofibrils. Collectively, the data suggest that Lmod contributes to the final organization and maintenance of sarcomere architecture by promoting tropomyosin-dependent actin filament nucleation.

Paralysis and delayed Z-disc formation in the Xenopus tropicalis unc45b mutant dicky ticker

Background

The protein components of mature skeletal muscle have largely been characterized, but the mechanics and sequence of their assembly during normal development remain an active field of study. Chaperone proteins specific to sarcomeric myosins have been shown to be necessary in zebrafish and invertebrates for proper muscle assembly and function.

Results

The Xenopus tropicalis mutation dicky ticker results in disrupted skeletal muscle myofibrillogenesis, paralysis, and lack of heartbeat, and maps to a missense mutation in the muscle-specific chaperone unc45b. Unc45b is known to be required for folding the head domains of myosin heavy chains, and mutant embryos fail to incorporate muscle myosin into sarcomeres. Mutants also show delayed polymerization of α-actinin-rich Z-bodies into the Z-disks that flank the myosin-containing A-band.

Conclusions

The dicky ticker phenotype confirms that a requirement for myosin-specific chaperones is conserved in tetrapod sarcomerogenesis, and also suggests a novel role for myosin chaperone function in Z-body maturation.

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.

Determination of the mobility of novel and established Caenorhabditis elegans sarcomeric proteins in vivo

A screen was instigated to identify novel protein components of the Caenorhabditis elegans sarcomere. The subcellular localisation of full-length GFP fusion proteins was examined, in transgenic animals, for 62 essentially uncharacterized genes thought to be expressed within bodywall muscle cells. Three genes, T03G6.3, C46G7.2 and K04A8.6, were identified for further study. K04A8.6::GFP only displayed a regular sarcomeric distribution sporadically. However, C46G7.2::GFP localised to the centre of A-bands and dense bodies and T03G6.3::GFP localised in the I-band, of the bodywall muscle sarcomeres, consistently. This success with such a small screen suggests that there are further minor components of the C. elegans sarcomere yet to be discovered. Fluorescence Recovery After Photobleaching (FRAP) was applied to live transgenic individuals to assess the mobility of T03G6.3 and C46G7.2 and other well-known constituents of the sarcomere in vivo. Proteins associated with the thin filaments showed dynamic exchange whilst those associated with thick filaments appeared more static. This is the first demonstration that there are sarcomeric proteins in C. elegans muscle cells in dynamic exchange and that the rates of exchange in vivo correspond in general terms with observations in other experimental systems.

Mechanical stress-induced sarcomere assembly for cardiac muscle growth in length and width

A ventricular myocyte experiences changes in length and load during every beat of the heart and has the ability to remodel cell shape to maintain cardiac performance. Specifically, myocytes elongate in response to increased diastolic strain by adding sarcomeres in series, and they thicken in response to continued systolic stress by adding filaments in parallel. Myocytes do this while still keeping the resting sarcomere length close to its optimal value at the peak of the length-tension curve. This review focuses on the little understood mechanisms by which direction of growth is matched in a physiologically appropriate direction. We propose that the direction of strain is detected by differential phosphorylation of proteins in the costamere, which then transmit signaling to the Z-disc for parallel or series addition of thin filaments regulated via the actin capping processes. In this review, we link mechanotransduction to the molecular mechanisms for regulation of myocyte length and width.

Sequence and expression of the zebrafish alpha-actinin gene family reveals conservation and diversification among vertebrates

Alpha-actinins are actin microfilament crosslinking proteins. Vertebrate actinins fall into two classes: the broadly-expressed actinins 1 and 4 (actn1 and actn4) and muscle-specific actinins, actn2 and actn3. Members of this family have numerous roles, including regulation of cell adhesion, cell differentiation, directed cell motility, intracellular signaling, and stabilization of f-actin at the sarcomeric Z-line in muscle. Here we identify five zebrafish actinin genes including two paralogs of ACTN3. We describe the temporal and spatial expression patterns of these genes through embryonic development. All zebrafish actinin genes have unique expression profiles, indicating specialization of each gene. In particular, the muscle actinins display preferential expression in different domains of axial, pharyngeal, and cranial musculature. There is no identified avian actn3 and approximately 16% of humans are null for ACTN3. Duplication of actn3 in the zebrafish indicates that variation in actn3 expression may promote physiological diversity in muscle function among vertebrates.