Molecular analysis of protein assembly in muscle development

Striated Preferentially Expressed Protein Kinase (SPEG) in Muscle Development, Function, and Disease

Mutations in striated preferentially expressed protein kinase (SPEG), a member of the myosin light chain kinase protein family, are associated with centronuclear myopathy (CNM), cardiomyopathy, or a combination of both. Burgeoning evidence suggests that SPEG plays critical roles in the development, maintenance, and function of skeletal and cardiac muscles. Here we review the genotype-phenotype relationships and the molecular mechanisms of SPEG-related diseases. This review will focus on the progress made toward characterizing SPEG and its interacting partners, and its multifaceted functions in muscle regeneration, triad development and maintenance, and excitation-contraction coupling. We will also discuss future directions that are yet to be investigated including understanding of its tissue-specific roles, finding additional interacting proteins and their relationships. Understanding the basic mechanisms by which SPEG regulates muscle development and function will provide critical insights into these essential processes and help identify therapeutic targets in SPEG-related disorders.

Genetic Control of Muscle Diversification and Homeostasis: Insights from Drosophila

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

Self-organizing motors divide active liquid droplets

The cytoskeleton is a collection of protein assemblies that dynamically impose spatial structure in cells and coordinate processes such as cell division and mechanical regulation. Biopolymer filaments, cross-linking proteins, and enzymatically active motor proteins collectively self-organize into various precise cytoskeletal assemblies critical for specific biological functions. An outstanding question is how the precise spatial organization arises from the component macromolecules. We develop a system to investigate simple physical mechanisms of self-organization in biological assemblies. Using a minimal set of purified proteins, we create droplets of cross-linked biopolymer filaments. Through the addition of enzymatically active motor proteins, we construct composite assemblies, evocative of cellular structures such as spindles, where the inherent anisotropy drives motor self-organization, droplet deformation, and division into two droplets. These results suggest that simple physical principles underlie self-organization in complex biological assemblies and inform bioinspired materials design.

Protein aggregation in cooked pork products: New details on the supramolecular organization

The molecular weight distribution of protein aggregates from raw meat and cooked pork products was assessed by size exclusion-high performance liquid chromatography (SE-HPLC). Electrophoretic analysis under reducing conditions showed that the high molecular weight SE-HPLC peak (peak 1) of the cooked products contained protein aggregates in addition to high molecular weight muscle proteins, while the second peak (peak 2) still contained aggregates and <50 kDa proteins. The protein aggregates composition was investigated by HPLC-tandem mass spectrometry. Different classes of proteins were identified and the cooked products showed a more complex composition and organization, according to the muscle structure and the technological procedures, respectively. The key role of actin in the building of the protein networks was also confirmed. The different multi-protein systems found in the cooked products suggest protein re-organization in heat-induced supramolecular structures, which might be responsible for the texture and the structural properties of the final products.

Dynamics of myosin replacement in skeletal muscle cells

Highly organized thick filaments in skeletal muscle cells are formed from ~300 myosin molecules. Each thick-filament-associated myosin molecule is thought to be constantly exchanged. However, the mechanism of myosin replacement remains unclear, as does the source of myosin for substitution. Here, we investigated the dynamics of myosin exchange in the myofibrils of cultured myotubes by fluorescent recovery after photobleaching and found that myofibrillar myosin is actively replaced with an exchange half-life of ~3 h. Myosin replacement was not disrupted by the absence of the microtubule system or by actomyosin interactions, suggesting that known cytoskeletal systems are dispensable for myosin substitution. Intriguingly, myosin replacement was independent of myosin binding protein C, which links myosin molecules together to form thick filaments. This implies that an individual myosin molecule rather than a thick filament functions as an exchange unit. Furthermore, the myosin substitution rate was decreased by the inhibition of protein synthesis, suggesting that newly synthesized myosin, as well as preexisting cytosolic myosin, contributes to myosin replacement in myofibrils. Notably, incorporation and release of myosin occurred simultaneously in myofibrils, but rapid myosin release from myofibrils was observed without protein synthesis. Collectively, our results indicate that myosin shuttles between myofibrils and the nonmyofibrillar cytosol to maintain a dynamic equilibrium in skeletal muscle cells.

Actin scaffolding by clathrin heavy chain is required for skeletal muscle sarcomere organization

Clathrin heavy chain contributes to the formation and maintenance of the contractile apparatus in skeletal muscle through interactions with costameric proteins.

The ubiquitous clathrin heavy chain (CHC), the main component of clathrin-coated vesicles, is well characterized for its role in intracellular membrane traffic and endocytosis from the plasma membrane (PM). Here, we demonstrate that in skeletal muscle CHC regulates the formation and maintenance of PM–sarcomere attachment sites also known as costameres. We show that clathrin forms large coated lattices associated with actin filaments and the muscle-specific isoform of α-actinin at the PM of differentiated myotubes. Depletion of CHC in myotubes induced a loss of actin and α-actinin sarcomeric organization, whereas CHC depletion in vivo induced a loss of contractile force due to the detachment of sarcomeres from the PM. Our results suggest that CHC contributes to the formation and maintenance of the contractile apparatus through interactions with costameric proteins and highlight an unconventional role for CHC in skeletal muscle that may be relevant to pathophysiology of neuromuscular disorders.

Tropomyosin is required for cardiac morphogenesis, myofibril assembly, and formation of adherens junctions in the developing mouse embryo

BACKGROUND

We explored a function for tropomyosin (TM) in mammalian myofibril assembly and cardiac development by analyzing a deletion in the mouse TPM1 gene targeting αTM1, the major striated muscle TM isoform.

RESULTS

Mice lacking αTM1 are embryonic lethal at E9.5 with enlarged, misshapen, and non-beating hearts characterized by an abnormally thin myocardium and reduced trabeculae. αTM1-deficient cardiomyocytes do not assemble striated myofibrils, instead displaying aberrant non-striated F-actin fibrils with α-actinin puncta dispersed irregularly along their lengths. αTM1's binding partner, tropomodulin1 (Tmod1), is also disorganized, and both myomesin-containing thick filaments as well as titin Z1Z2 fail to assemble in a striated pattern. Adherens junctions are reduced in size in αTM1-deficient cardiomyocytes, α-actinin/F-actin adherens belts fail to assemble at apical cell-cell contacts, and cell contours are highly irregular, resulting in abnormal cell shapes and a highly folded cardiac surface. In addition, Tmod1-deficient cardiomyocytes exhibit failure of α-actinin/F-actin adherens belt assembly.

CONCLUSIONS

Absence of αTM1 resulting in unstable F-actin may preclude sarcomere formation and/or lead to degeneration of partially assembled sarcomeres due to unregulated actomyosin interactions. Our data also identify a novel αTM1/Tmod1-based pathway stabilizing F-actin at cell-cell junctions, which may be required for maintenance of cell shapes during embryonic cardiac morphogenesis.

Myofibrillogenesis regulator 1 induces hypertrophy by promoting sarcomere organization in neonatal rat cardiomyocytes

Human myofibrillogenesis regulator 1, a novel 17-kDa protein, is closely involved in cardiac hypertrophy. We studied the molecular mechanism that links MR-1 to hypertrophic response. Hypertrophic hallmarks such as cell size and [³H]-leucine incorporation were significantly increased when MR-1 was transfected into cardiomyocytes for 48 h. However, sarcomere organization was promoted when MR-1 was transfected for 8 h. The finding that cardiac hypertrophy was induced long after increase of sarcomere organization indicates that the promoted sarcomere organization may be one of the crucial factors causing hypertrophy. Furthermore, when MR-1 was transfected into cardiomyocytes, the nuclear localization of myomesin-1 was shifted to the cytoplasm. Transfection with small ubiquitin-like modifier-1 (SUMO-1) mimicked the effect of MR-1 inducing translocation of myomesin-1. However, transfection with SUMO-1 in MR-1-silenced cardiomyocytes failed to induce translocation and sarcomere organization, even though SUMO-1 expression was at the same level. Overexpression of MR-1 may induce cardiomyocyte hypertrophy via myomesin-1-mediated sarcomere organization.

Thin-filament length correlates with fiber type in human skeletal muscle

Force production in skeletal muscle is proportional to the amount of overlap between the thin and thick filaments, which, in turn, depends on their lengths. Both thin- and thick-filament lengths are precisely regulated and uniform within a myofibril. While thick-filament lengths are essentially constant across muscles and species (∼1.65 μm), thin-filament lengths are highly variable both across species and across muscles of a single species. Here, we used a high-resolution immunofluorescence and image analysis technique (distributed deconvolution) to directly test the hypothesis that thin-filament lengths vary across human muscles. Using deltoid and pectoralis major muscle biopsies, we identified thin-filament lengths that ranged from 1.19 ± 0.08 to 1.37 ± 0.04 μm, based on tropomodulin localization with respect to the Z-line. Tropomodulin localized from 0.28 to 0.47 μm further from the Z-line than the NH(2)-terminus of nebulin in the various biopsies, indicating that human thin filaments have nebulin-free, pointed-end extensions that comprise up to 34% of total thin-filament length. Furthermore, thin-filament length was negatively correlated with the percentage of type 2X myosin heavy chain within the biopsy and shorter in type 2X myosin heavy chain-positive fibers, establishing the existence of a relationship between thin-filament lengths and fiber types in human muscle. Together, these data challenge the widely held assumption that human thin-filament lengths are constant. Our results also have broad relevance to musculoskeletal modeling, surgical reattachment of muscles, and orthopedic rehabilitation.

Abnormal actomyosin assembly in proliferating and differentiating myoblasts upon expression of a cytosolic DMPK isoform

DMPK, the product of the mutated gene in myotonic dystrophy type 1, belongs to the subfamily of Rho-associated serine-threonine protein kinases, whose members play a role in actin-based cell morphodynamics. Not much is known about the physiological role of differentially localized individual DMPK splice isoforms. We report here that prominent stellar-shaped stress fibers are formed during early and late steps of differentiation in DMPK-deficient myoblast-myotubes upon complementation with the short cytosolic DMPK E isoform. Expression of DMPK E led to an increased phosphorylation status of MLC2. We found no such effects with vectors that encode a mutant DMPK E which was rendered enzymatically inactive or any of the long C-terminally anchored DMPK isoforms. Presence of stellar structures appears associated with changes in cell shape and motility and a delay in myogenesis. Our data strongly suggest that cytosolic DMPK participates in remodeling of the actomyosin cytoskeleton in developing skeletal muscle cells. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

I-Z-I complexes in congenital myopathy

A 3-month-old boy with hypotonia at birth succumbed to a congenital myopathy. The major finding in his muscle biopsy corresponded to I-Z-I complexes described previously in embryonic skeletal muscle. A few previous myopathy cases have described findings suggestive of I-Z-I-like complexes. A mutation affecting mononuclear myoblasts or early myotubes was suspected, although an acquired lesion could not be ruled out. The findings may also have been altered by secondary events in this unusual case.

Differential expression of two cardiac myosin-binding protein-C isoforms in developing chicken cardiac and skeletal muscle cells

Myosin-binding protein-C (MyBP-C), also known as C-protein, is a major myosin-binding protein characteristic of striated muscle, and plays a critical role in myofibril organization, especially in registration of thick filaments in the sarcomeres during myofibrillogenesis. We previously demonstrated that cardiac-type MyBP-C is involved early in the process of myofibrillogenesis in both cardiac and skeletal muscle during chicken muscle development. Two variants (type I and type II) have been detected in chicken cardiac MyBP-C; they differ only in the presence or absence of a sequence of 15 amino acid residues (termed P-seq) that includes a phosphorylation site for cyclic AMP-dependent kinase in the cardiac MyBP-C motif ( Yasuda et al, 1995 ). Therefore, types I and II are regarded as phosphorylatable and non-phosphorylatable isoforms, respectively. In this study, an antibody specific for P-seq was prepared. With this and other monoclonal antibodies to cardiac MyBP-C (C-315), expression and localization of the two MyBP-C isoforms in developing chicken cardiac and skeletal muscle were examined by immunocytochemistry and immunoblotting. The results showed that type I is predominantly expressed in the heart and is localized in myofibrils of both atrial and ventricular muscles through development. In contrast, type II is mainly expressed in embryonic skeletal muscle, although type I is faintly expressed in cultured skeletal muscle. These observations were confirmed by RT-PCR.

Loss of Smyhc1 or Hsp90alpha1 function results in different effects on myofibril organization in skeletal muscles of zebrafish embryos

BACKGROUND

Myofibrillogenesis requires the correct folding and assembly of sarcomeric proteins into highly organized sarcomeres. Heat shock protein 90alpha1 (Hsp90alpha1) has been implicated as a myosin chaperone that plays a key role in myofibrillogenesis. Knockdown or mutation of hsp90alpha1 resulted in complete disorganization of thick and thin filaments and M- and Z-line structures. It is not clear whether the disorganization of these sarcomeric structures is due to a direct effect from loss of Hsp90alpha1 function or indirectly through the disorganization of myosin thick filaments.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we carried out a loss-of-function analysis of myosin thick filaments via gene-specific knockdown or using a myosin ATPase inhibitor BTS (N-benzyl-p-toluene sulphonamide) in zebrafish embryos. We demonstrated that knockdown of myosin heavy chain 1 (myhc1) resulted in sarcomeric defects in the thick and thin filaments and defective alignment of Z-lines. Similarly, treating zebrafish embryos with BTS disrupted thick and thin filament organization, with little effect on the M- and Z-lines. In contrast, loss of Hsp90alpha1 function completely disrupted all sarcomeric structures including both thick and thin filaments as well as the M- and Z-lines.

CONCLUSION/SIGNIFICANCE

Together, these studies indicate that the hsp90alpha1 mutant phenotype is not simply due to disruption of myosin folding and assembly, suggesting that Hsp90alpha1 may play a role in the assembly and organization of other sarcomeric structures.

Cloning, sequence and functional analysis of goat ATP-binding cassette transporter G2 (ABCG2)

ATP-binding cassette transporter G2 (ABCG2) gene encodes a protein that has a wide variety of substrates and is responsible for the active secretion of clinically and toxicologically important molecules into milk. Although known in many species, this marks the first time this gene product has been reported in goats. In this study, we cloned and sequenced goat ABCG2 gene complete coding sequence and predicted its putative translated protein structure with implicative functional domains. One six-transmembrane span on C-terminal region and at least one coiled-coil domain on N-terminal were predicted and compared primarily with those of other closely related species. In addition, three conserved cysteines (in positions 595, 606, and 611) were determined toward the C-terminal of goat's ABCG2. Two known functional motifs were identified in goat's protein through comparative studies with other species. The goat ABCG2 relative expression profile revealed that the gene expression was a function of lactation stage and parallel to goat lactation curve.

Focal adhesion kinase is essential for costamerogenesis in cultured skeletal muscle cells

A central question in muscle biology is how costameres are formed and become aligned with underlying myofibrils in mature tissues. Costameres are composed of focal adhesion proteins, including vinculin and paxillin, and anchor myofibril Z-bands to the sarcolemma. In the present study, we investigated the process of costamere formation ("costamerogenesis") in differentiating primary mouse myoblasts. Using vinculin and paxillin as costameric markers, we found that two additional focal adhesion components, alpha5beta1 integrin and focal adhesion kinase (FAK), are associated with costameres. We have characterized costamerogenesis as occurring in three distinct stages based on the organizational pattern of these costameric proteins. We show that both costamerogenesis and myofibrillogenesis are initiated at sites of membrane contacts with the extracellular matrix and that their maturation is tightly coupled. To test the importance of FAK signaling in these processes, we analyzed cells expressing a dominant negative form of FAK (dnFAK). When cells expressing dnFAK were induced to differentiate, both costamerogenesis and myofibrillogenesis were disrupted although the expression of constituent proteins was not inhibited. Likewise, inhibiting FAK activity by reducing FAK levels using an siRNA approach also resulted in an inhibition of costamerogenesis and myofibrillogenesis. The relationship between costamere and myofibril formation was tested further by treating myotube cultures with potassium or tetrodotoxin to block contraction and disrupt myofibril organization. This also resulted in inhibition of costamere maturation. We present a model of costamerogenesis whereby signaling through FAK is essential for both normal costamerogenesis and normal myofibrillogenesis which are tightly coupled during skeletal myogenesis.

Inflammatory cardiomyopathy: there is a specific matrix destruction in the course of the disease

Cardiomyopathies are responsible for a high proportion of cases of congestive heart failure and sudden death, as well as for the need for transplantation. Understanding of the causes of these disorders has been sought in earnest over the past decade. We hypothesized that DCM is a disease of the cytoskeleton/sarcolemma, which affects the sarcomere. Evaluation of the sarcolemma in DCM and other forms of systolic heart failure demonstrates membrane disruption; and, secondarily, the extracellular matrix architecture is also affected. Disruption of the links from the sarcolemma to ECM at the dystrophin C-terminus and those to the sarcomere and nucleus via N-terminal dystrophin interactions could lead to a "domino effect" disruption of systolic function and development of arrhythmias. We also have suggested that dystrophin mutations play a role in idiopathic DCM in males. The T-cap/MLP/alpha-actinin/titin complex appears to stabilize Z-disc function via mechanical stretch sensing. Loss of elasticity results in the primary defect in the endogenous cardiac muscle stretch sensor machinery. The over-stretching of individual myocytes leads to activation of cell death pathways, at a time when stretch-regulated survival cues are diminished due to defective stretch sensing, leading to progression of heart failure. Genetic DCM and the acquired disorder viral myocarditis have the same clinical features including heart failure, arrhythmias, and conduction block, and also similar mechanisms of disease based on the proteins targeted. In dilated cardiomyopathy, the process of progressive ventricular dilation and changes of the shape of the ventricle to a more spherical shape, associated with changes in ventricular function and/or hypertrophy, occurs without known initiating disturbance. In those cases in which resolution of cardiac dysfunction does not occur, chronic DCM results. It has been unclear what the underlying etiology of this long-term sequela could be, but viral persistence and autoimmunity have been widely speculated.

Microtubule-dependent transport and organization of sarcomeric myosin during skeletal muscle differentiation

It has been proposed that microtubules (MTs) participate in skeletal muscle cell differentiation. However, it is still unclear how this happens. To examine whether MTs could participate directly in the organization of thick and thin filaments into sarcomeres, we observed the concomitant reorganization and dynamics of MTs with the behavior of sarcomeric actin and myosin by time-lapse confocal microscopy. Using green fluorescent protein (GFP)-EB1 protein to label MT plus ends, we determined that MTs become organized into antiparallel arrays along fusing myotubes. Their dynamics and orientation was found to be different across the thickness of the myotubes. We observed fast movements of Dsred-myosin along GFP-MTs. Comparison of GFP-EB1 and Dsred-myosin dynamics revealed that myosin moved toward MT plus ends. Immuno-electron microscopy experiments confirmed that myosin was actually associated with MTs in myotubes. Finally, we confirmed that MTs were required for the stabilization of myosin-containing elements prior to incorporation into mature sarcomeres. Collectively, our results strongly suggest that MTs become organized into a scaffold that provides directional cues for the positioning and organization of myosin filaments during sarcomere formation.

Myosin phosphatase and myosin phosphorylation in differentiating C2C12 cells

C2C12 cells offer a useful model to study the differentiation of non-muscle cells to skeletal muscle cells. Myosin phosphorylation and changes in related enzymes, with an emphasis on myosin phosphatase (MP) were analyzed over the first 6 days of C2C12 differentiation. There was a transition from myosin phosphatase target subunit 1 (MYPT1), predominant in the non-muscle cells to increased expression of MYPT2. Levels of MYPT1/2 were estimated, and both isoforms were higher in non- or partially differentiated cells compared to the concentrations in the differentiated isolated myotubes from day 6. A similar profile of expression was estimated for the type 1 protein phosphatase catalytic subunit, delta isoform (PP1c delta). Phosphatase activities, using phosphorylated smooth and skeletal muscle myosins, were estimated for total cell lysates and isolated myotubes. In general, smooth muscle myosin was the preferred substrate. Although the expression of MYPT1/2 and PP1c delta was considerably reduced in isolated myotubes the phosphatase activities were not reduced to corresponding levels. Most of the MP activity was due to PP1c, as indicated by okadaic acid. In spite of relatively high expression of MYPT1/2 and PP1c delta, marked phosphorylation of non-muscle myosin (over 50% of total myosin) was observed at day 2 (onset of expression of muscle-specific proteins) and both mono- and diphosphorylated light chains were observed. Partial inhibition of MLCK by 1-(5-chloronaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine HCl (ML-9) or by a construct designed from the autoinhibitory domain of MLCK, resulted in an increase in small myotubes (3-5 nuclei) after 3 days of differentiation and a decrease in larger myotubes (compared to control). The effect of ML-9 was not due to a reduction in intracellular Ca2+ levels. These results suggest that phosphorylation of non-muscle myosin is important in growth of myotubes, either in the fusion process to form larger myotubes or indirectly, by its role in sarcomere organization.

Calcium transients regulate patterned actin assembly during myofibrillogenesis

The highly ordered arrangement of sarcomeric myosin during striated muscle development requires spontaneous calcium (Ca(2+)) transients. Here, we show that blocking transients also compromises patterned assembly of actin thin filaments, titin, and capZ. Because a conserved temporal assembly pattern has been described for these proteins, selective inhibitors of either thick or thin filament formation were used to determine their relative temporal interdependencies. For example, inhibition of myosin light chain kinase (MLCK) by application of a specific inhibitory peptide or phorbol myistate acetate (PMA) disrupts myosin assembly without significantly affecting formation of actin bands. The MLCK inhibitor ML-7, however, disrupted actin as well as myosin. Surprisingly, agents that interfere with actin dynamics, such as cytochalasin D, produced only minor organizational disruptions in actin, capZ, and titin staining. However, cytochalasin D and other actin disrupting compounds significantly perturbed myosin organization. The results indicate that (1) Ca(2+) transients regulate one or more of the earliest steps in sarcomere formation, (2) mature actin filaments can assemble independently of myosin band formation, and (3) myosin thick filament assembly is extremely sensitive to disruption of either the actin or titin filament systems.

Aberrant myofibril assembly in tropomodulin1 null mice leads to aborted heart development and embryonic lethality

Tropomodulin1 (Tmod1) caps thin filament pointed ends in striated muscle, where it controls filament lengths by regulating actin dynamics. Here, we investigated myofibril assembly and heart development in a Tmod1 knockout mouse. In the absence of Tmod1, embryonic development appeared normal up to embryonic day (E) 8.5. By E9.5, heart defects were evident, including aborted development of the myocardium and inability to pump, leading to embryonic lethality by E10.5. Confocal microscopy of hearts of E8–8.5 Tmod1 null embryos revealed structures resembling nascent myofibrils with continuous F-actin staining and periodic dots of α-actinin, indicating that I-Z-I complexes assembled in the absence of Tmod1. Myomesin, a thick filament component, was also assembled normally along these structures, indicating that thick filament assembly is independent of Tmod1. However, myofibrils did not become striated, and gaps in F-actin staining (H zones) were never observed. We conclude that Tmod1 is required for regulation of actin filament lengths and myofibril maturation; this is critical for heart morphogenesis during embryonic development.

Microtextured substrata alter gene expression, protein localization and the shape of cardiac myocytes

Many of the experiments designed to understand fundamental principles in cardiac physiology are performed in vitro using myocytes isolated from adult or neonatal hearts. However, these cells have probably lost some of their original properties in culture prior to study. Our objective is to recapitulate cardiac myocyte structure and function by growing cells on microtextured silicone substrata produced by photolithography and microfabrication techniques. Myocytes are plated on nontextured, micropegged (5 microm high), microgrooved (parallel grooves with a depth of 5 microm) or combination (micropegged and grooved) substrata. Myocytes plated on microtextured surfaces display a change in cell shape with an increase in myofibrillar height and a decrease in cell area. This shape change did not affect the stoichiometry of the myofibrillar proteins but did elicit microenvironmental remodeling of proteins that mechanically attach the cell to its surroundings. Cells terminate in a sarcomeric striation on the vertical interface of the peg whereas on nontextured surfaces they end in long nonstriated cables. Vinculin, a focal adhesion protein, was found to decrease in expression on combination surfaces as compared to nontextured substrata. A three-dimensional microtextured substratum appears to reintroduce a more physiological microarchitecture for tissue culture that may have potential uses in biological research as well as in tissue engineering and diagnostic applications.

Cadherin-mediated adhesion is essential for myofibril continuity across the plasma membrane but not for assembly of the contractile apparatus

The strong coordinated contraction of heart muscle is dependent on the correct alignment and connection of the myofibrils across the plasma membrane. Previous studies indicate that N-cadherin is involved in cardiac myocyte adhesion and myofibrillogenesis. To investigate whether N-cadherin is specifically required for normal myocyte structure and function, we cultured myocytes from wild-type, N-cadherin-null and mutant embryos expressing the epithelial cadherin E-cadherin. In contrast to previous studies in chicken using N-cadherin-perturbing antibodies, our in vitro studies with mouse cells demonstrate that N-cadherin is not required for myofibrillogenesis, but is critical for myofibril organization. That is, N-cadherin-deficient myocytes beat and myofibrils were well formed; however, alignment of the myofibrils through regions of cell-cell contact was lost, resulting in their random orientation. Gap junctions were perturbed in the N-cadherin-null myocytes. By contrast, focal contacts appeared normal in the mutant cells. Furthermore, E-cadherin restored normal cell morphology and behavior to the N-cadherin-deficient myocytes, including proper alignment of the myofibrils. We conclude that a different adhesive system, most probably integrin, is responsible for myofibrillogenesis in the N-cadherin-null myocytes.

Drosophila paramyosin is important for myoblast fusion and essential for myofibril formation

Paramyosin is a major structural protein of thick filaments in invertebrate muscles. Coiled-coil dimers of paramyosin form a paracrystalline core of these filaments, and the motor protein myosin is arranged on the core surface. To investigate the function of paramyosin in myofibril assembly and muscle contraction, we functionally disrupted the Drosophila melanogaster paramyosin gene by mobilizing a P element located in its promoter region. Homozygous paramyosin mutants die at the late embryo stage. Mutants display defects in both myoblast fusion and in myofibril assembly in embryonic body wall muscles. Mutant embryos have an abnormal body wall muscle fiber pattern arising from defects in myoblast fusion. In addition, sarcomeric units do not assemble properly and muscle contractility is impaired. We confirmed that these defects are paramyosin-specific by rescuing the homozygous paramyosin mutant to adulthood with a paramyosin transgene. Antibody analysis of normal embryos demonstrated that paramyosin accumulates as a cytoplasmic protein in early embryo development before assembling into thick filaments. We conclude that paramyosin plays an unexpected role in myoblast fusion and is important for myofibril assembly and muscle contraction.

Binding of an ankyrin-1 isoform to obscurin suggests a molecular link between the sarcoplasmic reticulum and myofibrils in striated muscles

Assembly of specialized membrane domains, both of the plasma membrane and of the ER, is necessary for the physiological activity of striated muscle cells. The mechanisms that mediate the structural organization of the sarcoplasmic reticulum with respect to the myofibrils are, however, not known. We report here that ank1.5, a small splice variant of the ank1 gene localized on the sarcoplasmic reticulum membrane, is capable of interacting with a sequence of 25 aa located at the COOH terminus of obscurin. Obscurin is a giant sarcomeric protein of approximately 800 kD that binds to titin and has been proposed to mediate interactions between myofibrils and other cellular structures. The binding sites and the critical aa required in the interaction between ank1.5 and obscurin were characterized using the yeast two-hybrid system, in in vitro pull-down assays and in experiments in heterologous cells. In differentiated skeletal muscle cells, a transfected myc-tagged ank1.5 was found to be selectively restricted near the M line region where it colocalized with endogenous obscurin. The M line localization of ank1.5 required a functional obscurin-binding site, because mutations of this domain resulted in a diffused distribution of the mutant ank1.5 protein in skeletal muscle cells. The interaction between ank1.5 and obscurin represents the first direct evidence of two proteins that may provide a direct link between the sarcoplasmic reticulum and myofibrils. In keeping with the proposed role of obscurin in mediating an interaction with ankyrins and sarcoplasmic reticulum, we have also found that a sequence with homology to the obscurin-binding site of ank1.5 is present in the ank2.2 isoform, which in striated muscles has been also shown to associate with the sarcoplasmic reticulum. Accordingly, a peptide containing the COOH terminus of ank2.2 fused with GST was found to bind to obscurin. Based on reported evidence showing that the COOH terminus of ank2.2 is necessary for the localization of ryanodine receptors and InsP3 receptors in the sarcoplasmic reticulum, we propose that obscurin, through multiple interactions with ank1.5 and ank2.2 isoforms, may assemble a large protein complex that, in addition to a structural function, may play a role in the organization of specific subdomains in the sarcoplasmic reticulum.

The UCS family of myosin chaperones

The canonical UCS (UNC-45/Cro1/She4p) protein, Caenorhabditis elegans UNC-45, was one of the earliest molecules to be shown genetically to be necessary for sarcomere assembly. Genetic analyses of homologues in several fungal species indicate that the conserved UCS domain functionally interacts with conventional type II and unconventional type V myosins. In C. elegans and other invertebrate species, UNC-45 and its orthologues interact with both sarcomeric and non-sarcomeric myosins whereas, in vertebrates, there are two UNC-45 isoforms: a general cell (GC) and a striated muscle (SM) isoform. Although the mechanism of action of UCS proteins is unknown, recent biochemical studies suggest that they may act as molecular chaperones that facilitate the folding and/or maturation of myosin.

Characterization of a TM-4 type tropomyosin that is essential for myofibrillogenesis and contractile activity in embryonic hearts of the Mexican axolotl

A striated muscle isoform of a Tropomyosin (TM-4) gene was characterized and found to be necessary for contractile function in embryonic heart. The full-length clone of this isoform was isolated from the Mexican axolotl (Ambystoma mexicanum) and named Axolotl Tropomyosin Cardiac-3 (ATmC-3). The gene encoded a cardiac-specific tropomyosin protein with 284 amino acid residues that demonstrated high homology to the Xenopus cardiac TM-4 type tropomyosin. Northern blot analysis indicates a transcript of approximately 1.25 kb in size. RT-PCR and in situ hybridization demonstrated that this isoform is predominantly in cardiac tissue. Our laboratory uses an animal model that carries a cardiac lethal mutation (gene c), this mutation results in a greatly diminished level of tropomyosin protein in the ventricle. Transfection of ATmC-3 DNA into mutant hearts increased tropomyosin levels and promoted myofibrillogenesis. ATmC-3 expression was blocked in normal hearts by transfection of exon-specific anti-sense oligonucleotide (AS-ODN). RT-PCR confirmed lower transcript expression of ATmC-3 and in vitro analysis confirmed the specificity of the ATmC-3 exon 2 anti-sense oligonucleotide. These AS-ODN treated hearts also had a disruption of myofibril organization and disruption of synchronous contractions. These results demonstrated that a striated muscle isoform of the TM-4 gene was expressed embryonically and was necessary for normal structure and function of the ventricle.

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

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

Muscle-specific RING finger-1 interacts with titin to regulate sarcomeric M-line and thick filament structure and may have nuclear functions via its interaction with glucocorticoid modulatory element binding protein-1

The COOH-terminal A168-170 region of the giant sarcomeric protein titin interacts with muscle-specific RING finger-1 (MURF-1). To investigate the functional significance of this interaction, we expressed green fluorescent protein fusion constructs encoding defined fragments of titin's M-line region and MURF-1 in cardiac myocytes. Upon expression of MURF-1 or its central region (containing its titin-binding site), the integrity of titin's M-line region was dramatically disrupted. Disruption of titin's M-line region also resulted in a perturbation of thick filament components, but, surprisingly, not of the NH2-terminal or I-band regions of titin, the Z-lines, or the thin filaments. This specific phenotype also was caused by the expression of titin A168-170. These data suggest that the interaction of titin with MURF-1 is important for the stability of the sarcomeric M-line region.MURF-1 also binds to ubiquitin-conjugating enzyme-9 and isopeptidase T-3, enzymes involved in small ubiquitin-related modifier-mediated nuclear import, and with glucocorticoid modulatory element binding protein-1 (GMEB-1), a transcriptional regulator. Consistent with our in vitro binding data implicating MURF-1 with nuclear functions, endogenous MURF-1 also was detected in the nuclei of some myocytes. The dual interactions of MURF-1 with titin and GMEB-1 may link myofibril signaling pathways (perhaps including titin's kinase domain) with muscle gene expression.

Molecular dissection of the interaction of desmin with the C-terminal region of nebulin

In vertebrate skeletal muscle, ultrastructural studies have suggested that the Z-line and extracellular intermediate filaments are linked, although a structural basis for this has remained elusive. We searched for potential novel ligands of the Z-line portion of nebulin by a yeast two-hybrid (Y2H) approach. This identified that the nebulin modules M160 to M170 interact with desmin. In desmin, deletion series experiments assigned a 19-kDa central coiled-coil domain as the nebulin-binding site. The specific interactions of nebulin and desmin were confirmed in vitro by GST pull-down experiments. In situ, the nebulin modules M176 to M181 colocalize with desmin in a Z-line-associated, striated pattern as shown by immunofluorescence studies. Our data are consistent with a model that desmin attaches directly to the Z-line through its interaction with the nebulin repeats M163-M170. This interaction may link myofibrillar Z-discs to the intermediate filament system, thereby forming a lateral linkage system which contributes to maintain adjacent Z-lines in register.

Myopalladin, a novel 145-kilodalton sarcomeric protein with multiple roles in Z-disc and I-band protein assemblies

We describe here a novel sarcomeric 145-kD protein, myopalladin, which tethers together the COOH-terminal Src homology 3 domains of nebulin and nebulette with the EF hand motifs of alpha-actinin in vertebrate Z-lines. Myopalladin's nebulin/nebulette and alpha-actinin-binding sites are contained in two distinct regions within its COOH-terminal 90-kD domain. Both sites are highly homologous with those found in palladin, a protein described recently required for actin cytoskeletal assembly (Parast, M.M., and C.A. Otey. 2000. J. Cell Biol. 150:643-656). This suggests that palladin and myopalladin may have conserved roles in stress fiber and Z-line assembly. The NH(2)-terminal region of myopalladin specifically binds to the cardiac ankyrin repeat protein (CARP), a nuclear protein involved in control of muscle gene expression. Immunofluorescence and immunoelectron microscopy studies revealed that myopalladin also colocalized with CARP in the central I-band of striated muscle sarcomeres. Overexpression of myopalladin's NH(2)-terminal CARP-binding region in live cardiac myocytes resulted in severe disruption of all sarcomeric components studied, suggesting that the myopalladin-CARP complex in the central I-band may have an important regulatory role in maintaining sarcomeric integrity. Our data also suggest that myopalladin may link regulatory mechanisms involved in Z-line structure (via alpha-actinin and nebulin/nebulette) to those involved in muscle gene expression (via CARP).

Assembly of myofibrils in cardiac muscle cells

How do myofibrils assemble in cardiac muscle cells? When does titin first assemble into myofibrils? What is the role of titin in the formation of myofibrils in cardiac muscle cells? This chapter reviews when titin is first detected in cultured cardiomyocytes that have been freshly isolated from embryonic avian hearts. Our results support a model for myofibrillogenesis that involves three stages of assembly: premyofibrils, nascent myofibrils and mature myofibrils. Titin and muscle thick filaments were first detected associated with the nascent myofibrils. The Z-band targeting site for titin is localized in the N-terminus of titin. This region of titin binds alpha-actinin and less avidly vinculin. Thus the N-terminus of titin via its binding to alpha-actinin, and vinculin could also help mediate the costameric attachment of the Z-bands of mature myofibrils to the nearest cell surfaces.

Probing the functional roles of titin ligands in cardiac myofibril assembly and maintenance

Sarcomeres of cardiac muscle are comprised of numerous proteins organized in an elegantly precise order. The exact mechanism of how these proteins are assembled into myofibrils during heart development is not yet understood, although existing in vitro and in vivo model systems have provided great insight into this complex process. It has been proposed by several groups that the giant elastic protein titin acts as a "molecular template" to orchestrate sarcomeric organization during myofibrillogenesis. Titin's highly modular structure, composed of both repeating and unique domains that interact with a wide spectrum of contractile and regulatory ligands, supports this hypothesis. Recent functional studies have provided clues to the physiological significance of the interaction of titin with several titin-binding proteins in the context of live cardiac cells. Improved models of cardiac myofibril assembly, along with the application of powerful functional studies in live cells, as well as the characterization of additional titin ligands, is likely to reveal surprising new functions for the titin third filament system.

STEM Analysis of Caenorhabditis elegans muscle thick filaments: evidence for microdifferentiated substructures

In the thick filaments of body muscle in Caenorhabditis elegans, myosin A and myosin B isoforms and a subpopulation of paramyosin, a homologue of myosin heavy chain rods, are organized about a tubular core. As determined by scanning transmission electron microscopy, the thick filaments show a continuous decrease in mass-per-length (MPL) from their central zones to their polar regions. This is consistent with previously reported morphological studies and suggests that both their content and structural organization are microdifferentiated as a function of position. The cores are composed of a second distinct subpopulation of paramyosin in association with the alpha, beta, and gamma-filagenins. MPL measurements suggest that cores are formed from seven subfilaments containing four strands of paramyosin molecules, rather than the two originally proposed. The periodic locations of the filagenins within different regions and the presence of a central zone where myosin A is located, implies that the cores are also microdifferentiated with respect to molecular content and structure. This differentiation may result from a novel "induced strain" assembly mechanism based upon the interaction of the filagenins, paramyosin and myosin A. The cores may then serve as "differentiated templates" for the assembly of myosin B and paramyosin in the tapering, microdifferentiated polar regions of the thick filaments.

Assembly of thick, thin, and titin filaments in chick precardiac explants

De novo cardiac myofibril assembly has been difficult to study due to the lack of available cell culture models that clearly and accurately reflect heart muscle development in vivo. However, within precardiac chick embryo explants, premyocardial cells differentiate and commence beating in a temporal pattern that corresponds closely with myocyte differentiation in the embryo. Immunofluorescence staining of explants followed by confocal microscopy revealed that distinct stages of cardiac myofibril assembly, ranging from the earliest detection of sarcomeric proteins to the late appearance of mature myofibrils, were consistently recognized in precardiac cultures. Assembly events involved in the early formation of sarcomeres were clearly visualized and accurately reflected observations described by others during chick heart muscle development. Specifically, the early colocalization of alpha-actinin and titin dots was observed near the cell periphery representing I-Z-I-like complex formation. Myosin-containing thick filaments assembled independently of actin-containing thin filaments and appeared centered within sarcomeres when titin was also linearly aligned at or near cell borders. An N-terminal epitope of titin was detected earlier than a C-terminal epitope; however, both epitopes were observed to alternate near the cell periphery concomitant with the earliest formation of myofibrils. Although vascular actin was detected within cells during early assembly stages, cardiac actin predominated as the major actin isoform in mature thin filaments. Well-aligned thin filaments were also observed in the absence of organized staining for tropomodulin at thin filament pointed ends, suggesting that tropomodulin is not required to define thin filament lengths. Based on these findings, we conclude that the use of the avian precardiac explant system accurately allows for direct investigation of the mechanisms regulating de novo cardiac myofibrillogenesis.

Formation of sarcomeres in developing myotubes: role of mechanical stretch and contractile activation

In a series of experiments, cultured myotubes were exposed to passive stretch or pharmacological agents that block contractile activation. Under these experimental conditions, the formation of Z lines and A bands (morphological structures, resulting from the specific structural alignment of sarcomeric proteins, necessary for contraction) was assessed by immunofluorescence. The addition of an antagonist of the voltage-gated Na(+) channels [tetrodotoxin (TTX)] for 2 days in developing rat myotube cultures led to a nearly total absence of Z lines and A bands. When contractile activation was allowed to resume for 2 days, the Z lines and A bands reappeared in a significant way. The appearance of Z lines or A bands could not be inhibited nor facilitated by the application of a uniaxial passive stretch. Electrical stimulation of the cultures increased sarcomere assembly significantly. Antagonists of L-type Ca(2+) channels (verapamil, nifedipine) combined with electrical stimulation led to the absence of Z lines and A bands to the same degree as the TTX treatment. Western blot analysis did not show a major change in the amount of sarcomeric alpha-actinin nor a shift in myosin heavy chain phenotype as a result of a 2-day passive stretch or TTX treatment. Results of experiments suggest that temporal Ca(2+) transients play an important factor in the assembly and maintenance of sarcomeric structures during muscle fiber development.

Striated muscle preferentially expressed genes alpha and beta are two serine/threonine protein kinases derived from the same gene as the aortic preferentially expressed gene-1

Aortic preferentially expressed gene (APEG)-1 is a 1.4-kilobase pair (kb) mRNA expressed in vascular smooth muscle cells and is down-regulated by vascular injury. An APEG-1 5'-end cDNA probe identified three additional isoforms. The 9-kb striated preferentially expressed gene (SPEG)alpha and the 11-kb SPEGbeta were found in skeletal muscle and heart. The 4-kb brain preferentially expressed gene was detected in the brain and aorta. We report here cloning of the 11-kb SPEGbeta cDNA. SPEGbeta encodes a 355-kDa protein that contains two serine/threonine kinase domains and is homologous to proteins of the myosin light chain kinase family. At least one kinase domain is active and capable of autophosphorylation. In the genome, all four isoforms share the middle three of the five exons of APEG-1, and they differ from each other by using different 5'- and 3'-ends and alternative splicing. We show that the expression of SPEGalpha and SPEGbeta is developmentally regulated in the striated muscle during C2C12 myoblast to myotube differentiation in vitro and cardiomyocyte maturation in vivo. This developmental regulation suggests that both SPEGalpha and SPEGbeta can serve as sensitive markers for striated muscle differentiation and that they may be important for adult striated muscle function.

Differential assembly of alpha- and gamma-filagenins into thick filaments in Caenorhabditis elegans

Muscle thick filaments are highly organized supramolecular assemblies of myosin and associated proteins with lengths, diameters and flexural rigidities characteristic of their source. The cores of body wall muscle thick filaments of the nematode Caenorhabditis elegans are tubular structures of paramyosin sub-filaments coupled by filagenins and have been proposed to serve as templates for the assembly of native thick filaments. We have characterized alpha- and gamma-filagenins, two novel proteins of the cores with calculated molecular masses of 30,043 and 19,601 and isoelectric points of 10.52 and 11.49, respectively. Western blot and immunoelectron microscopy using affinity-purified antibodies confirmed that the two proteins are core components. Immunoelectron microscopy of the cores revealed that they assemble with different periodicities. Immunofluorescence microscopy showed that alpha-filagenin is localized in the medial regions of the A-bands of body wall muscle cells whereas gamma-filagenin is localized in the flanking regions, and that alpha-filagenin is expressed in 1.5-twofold embryos while gamma-filagenin becomes detectable only in late vermiform embryos. The expression of both proteins continues throughout later stages of development. C. elegans body wall muscle thick filaments of these developmental stages have distinct lengths. Our results suggest that the differential assembly of alpha- and gamma-filagenins into thick filaments of distinct lengths may be developmentally regulated.

Developmental relationship of myosin binding proteins (myomesin, connectin and C-protein) to myosin in chicken somites as studied by immunofluorescence microscopy

The developmental relationship of myosin binding proteins (myomesin, connectin and C-protein) to myosin was studied in chicken cervical somites by immunofluorescence microscopy. Muscle and non-muscle myosins initially appeared as slender rods at the same sites, and then, fused to form non-striated fibrils. As muscle myosin formed striated structures (A bands), non-muscle myosin disappeared from this structure. Myomesin (reactive with monoclonal antibodies MyB4 and MyBB78) and connectin (carboxy terminal region, reactive with monoclonal antibody T51) were seen as dots in the center of these myosin rods. These proteins then formed characteristic mature striations on non-striated fibrils of myosin. Earlier alignment of these myosin binding proteins rather than myosin indicates that the correct assembly of these proteins seems to be related to the formation of initial myosin rods as well as subsequent linear and periodic alignment of myosin molecules to form early A bands. Connectin spots reactive with 9D10 were scattered around myosin rods/myomesin dots/connectin T51 dots. These spots may represent radiating connectin filaments from these rods/dots to link myosin rods to the I-Z-I structures of myofibrils to be incorporated. Since the slow isoform of C-protein formed its characteristic bands ("doublets") prior to H zone formation within A bands by myosin, this isoform may help to precisely align myosin filaments within the A band region. The presence of the slow, then the slow and the cardiac, and finally the co-existence of the slow and the fast isoforms of C-protein may interfere with the incorporation and co-polymerization of non-adult isoforms into myofibrils.

Form follows function: how muscle shape is regulated by work

What determines the shape, size, and force output of cardiac and skeletal muscle? Chicago architect Louis Sullivan (1856-1924), father of the skyscraper, observed that "form follows function." This is as true for the structural elements of a striated muscle cell as it is for the architectural features of a building. Function is a critical evolutionary determinant, not form. To survive, the animal has evolved muscles with the capacity for dynamic responses to altered functional demand. For example, work against an increased load leads to increased mass and cross-sectional area (hypertrophy), which is directly proportional to an increased potential for force production. Thus a cell has the capacity to alter its shape as well as its volume in response to a need for altered force production. Muscle function relies primarily on an organized assembly of contractile and other sarcomeric proteins. From analysis of homogenized cells and molecular and biochemical assays, we have learned about transcription, translation, and posttranslational processes that underlie protein synthesis but still have done little in addressing the important questions of shape or regional cell growth. Skeletal muscles only grow in length as the bones grow; therefore, most studies of adult hypertrophy really only involve increased cross-sectional area. The heart chamber, however, can extend in both longitudinal and transverse directions, and cardiac cells can grow in length and width. We know little about the regulation of these directional processes that appear as a cell gets larger with hypertrophy or smaller with atrophy. This review gives a brief overview of the regulation of cell shape and the composition and aggregation of contractile proteins into filaments, the sarcomere, and myofibrils. We examine how mechanical activity regulates the turnover and exchange of contraction proteins. Finally, we suggest what kinds of experiments are needed to answer these fundamental questions about the regulation of muscle cell shape.

Assembly of smooth muscle myosin by the 38k protein, a homologue of a subunit of pre-mRNA splicing factor-2

Smooth muscle myosin in the dephosphorylated state does not form filaments in vitro. However, thick filaments, which are composed of myosin and myosin-binding protein(s), persist in smooth muscle cells, even if myosin is subjected to the phosphorylation- dephosphorylation cycle. The characterization of telokin as a myosin-assembling protein successfully explained the discrepancy. However, smooth muscle cells that are devoid of telokin have been observed. We expected to find another ubiquitous protein with a similar role, and attempted to purify it from chicken gizzard. The 38k protein bound to both phosphorylated and dephosphorylated myosin to a similar extent. The effect of the myosin-binding activity was to assemble dephosphorylated myosin into filaments, although it had no effect on the phosphorylated myosin. The 38k protein bound to myosin with both COOH-terminal 20 and NH(2)-terminal 28 residues of the 38k protein being essential for myosin binding. The amino acid sequence of the 38k protein was not homologous to telokin, but to human p32, which was originally found in nuclei as a subunit of pre-mRNA splicing factor-2. Western blotting showed that the protein was expressed in various smooth muscles. Immunofluorescence microscopy with cultured smooth muscle cells revealed colocalization of the 38k protein with myosin and with other cytoskeletal elements. The absence of nuclear immunostaining was discussed in relation to smooth muscle differentiation.

Myosin light chain kinase mediates sarcomere organization during cardiac hypertrophy in vitro

During the development of hypertrophy, cardiac myocytes increase organization of the sarcomere, a highly ordered contractile unit in striated muscle cells. Several hypertrophic agonists, such as angiotensin II, phenylephrine, and endothelin-1, have been shown to promote the sarcomere organization. However, the signaling pathway, which links extracellular stimuli to sarcomere organization, has not been clearly demonstrated. Here, we demonstrate that myosin light chain kinase specifically mediates agonist-induced sarcomere organization during early hypertrophic response. Acute administration of a hypertrophic agonist, phenylephrine, or angiotensin II, causes phosphorylation of myosin light chain 2v both in cultured cardiac myocytes and in the adult heart in vivo. We also show that both sarcomere organization and myosin light chain 2v phosphorylation are dependent on the activation of Ca2+/calmodulin pathway, a known activator of myosin light chain kinase. These results define a new and specific role of myosin light chain kinase in cardiac myocytes, which may provide a rapid adaptive mechanism in response to hypertrophic stimuli.

Evidence for differential post-translational modifications of slow myosin heavy chain during murine skeletal muscle development

The contractile properties of muscle fibres are, in part, determined by the myosin heavy chain (MyHC) isoforms they express. Using monoclonal antibodies, we show that at least three forms of slow twitch MyHC accumulate sequentially during mouse fetal development and that slow MyHC maturation in slow fibres occurs before expression of the adult fast MyHCs in fast fibres. Expression of deletion derivatives of beta-cardiac MyHC cDNA shows that the slow MyHC epitopes that are detected in adult but not in young animals are located near the N-terminus. The same N-terminal region of various fast MyHC molecules contains a conserved epitope that can, on occasions, be observed when slow MyHC cDNA is expressed in non-muscle cells. The results raise the possibility that the N-terminal epitopes result from post-translational modification of the MyHC and that a sequence of slow and fast MyHC isoform post-translational modifications plays a significant role during development of murine muscle fibres.

Protein machines and self assembly in muscle organization

The remarkable order of striated muscle is the result of a complex series of protein interactions at different levels of organization. Within muscle, the thick filament and its major protein myosin are classical examples of functioning protein machines. Our understanding of the structure and assembly of thick filaments and their organization into the regular arrays of the A-band has recently been enhanced by the application of biochemical, genetic, and structural approaches. Detailed studies of the thick filament backbone have shown that the myosins are organized into a tubular structure. Additional protein machines and specific myosin rod sequences have been identified that play significant roles in thick filament structure, assembly, and organization. These include intrinsic filament components, cross-linking molecules of the M-band and constituents of the membrane-cytoskeleton system. Muscle organization is directed by the multistep actions of protein machines that take advantage of well-established self-assembly relationships.

Tropomodulin assembles early in myofibrillogenesis in chick skeletal muscle: evidence that thin filaments rearrange to form striated myofibrils

Actin filament lengths in muscle and nonmuscle cells are believed to depend on the regulated activity of capping proteins at both the fast growing (barbed) and slow growing (pointed) filament ends. In striated muscle, the pointed end capping protein, tropomodulin, has been shown to maintain the lengths of thin filaments in mature myofibrils. To determine whether tropomodulin might also be involved in thin filament assembly, we investigated the assembly of tropomodulin into myofibrils during differentiation of primary cultures of chick skeletal muscle cells. Our results show that tropomodulin is expressed early in differentiation and is associated with the earliest premyofibrils which contain overlapping and misaligned actin filaments. In addition, tropomodulin can be found in actin filament bundles at the distal tips of growing myotubes, where sarcomeric alpha-actinin is not always detected, suggesting that tropomodulin caps actin filament pointed ends even before the filaments are cross-linked into Z bodies by alpha-actinin. Tropomodulin staining exhibits an irregular punctate pattern along the length of premyofibrils that demonstrate a smooth phalloidin staining pattern for F-actin. Strikingly, the tropomodulin dots often appear to be located between the closely spaced, dot-like Z bodies that are stained for (α)-actinin. Thus, in the earliest premyofibrils, the pointed ends of the thin filaments are clustered and partially aligned with respect to the Z bodies (the location of the barbed filament ends). At later stages of differentiation, the tropomodulin dots become aligned into regular periodic striations concurrently with the appearance of striated phalloidin staining for F-actin and alignment of Z bodies into Z lines. Tropomodulin, together with the barbed end capping protein, CapZ, may function from the earliest stages of myofibrillogenesis to restrict the lengths of newly assembled thin filaments by capping their ends; thus, transitions from nonstriated to striated myofibrils in skeletal muscle are likely due principally to filament rearrangements rather than to filament polymerization or depolymerization. Rearrangements of actin filaments capped at their pointed and barbed ends may be a general mechanism by which cells restructure their actin cytoskeletal networks during cell growth and differentiation.

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

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

Muscle assembly: a titanic achievement?

The formation of perfectly aligned myofibrils in striated muscle represents a dramatic example of supramolecular assembly in eukaryotic cells. Recently, considerable progress has been made in deciphering the roles that titin, the third most abundant protein in muscle, has in this process. An increasing number of sarcomeric proteins (ligands) are being identified that bind to specific titin domains. Titin may serve as a molecular blueprint for sarcomere assembly and turnover by specifying the precise position of its ligands within each half-sarcomere in addition to functioning as a molecular spring that maintains the structural integrity of the contracting myofibrils.

The NH2 terminus of titin spans the Z-disc: its interaction with a novel 19-kD ligand (T-cap) is required for sarcomeric integrity

Titin is a giant elastic protein in vertebrate striated muscles with an unprecedented molecular mass of 3-4 megadaltons. Single molecules of titin extend from the Z-line to the M-line. Here, we define the molecular layout of titin within the Z-line; the most NH2-terminal 30 kD of titin is located at the periphery of the Z-line at the border of the adjacent sarcomere, whereas the subsequent 60 kD of titin spans the entire width of the Z-line. In vitro binding studies reveal that mammalian titins have at least four potential binding sites for alpha-actinin within their Z-line spanning region. Titin filaments may specify Z-line width and internal structure by varying the length of their NH2-terminal overlap and number of alpha-actinin binding sites that serve to cross-link the titin and thin filaments. Furthermore, we demonstrate that the NH2-terminal titin Ig repeats Z1 and Z2 in the periphery of the Z-line bind to a novel 19-kD protein, referred to as titin-cap. Using dominant-negative approaches in cardiac myocytes, both the titin Z1-Z2 domains and titin-cap are shown to be required for the structural integrity of sarcomeres, suggesting that their interaction is critical in titin filament-regulated sarcomeric assembly.

A calcium signaling cascade essential for myosin thick filament assembly in Xenopus myocytes

Spontaneous calcium release from intracellular stores occurs during myofibrillogenesis, the process of sarcomeric protein assembly in striated muscle. Preventing these Ca2+ transients disrupts sarcomere formation, but the signal transduction cascade has not been identified. Here we report that specific blockade of Ca2+ release from the ryanodine receptor (RyR) activated Ca2+ store blocks transients and disrupts myosin thick filament (A band) assembly. Inhibition of an embryonic Ca2+/calmodulin-dependent myosin light chain kinase (MLCK) by blocking the ATP-binding site, by allosteric phosphorylation, or by intracellular delivery of a pseudosubstrate peptide, also disrupts sarcomeric organization. The results indicate that both RyRs and MLCK, which have well-described calcium signaling roles in mature muscle contraction, have essential developmental roles during construction of the contractile apparatus.

beta-Filagenin, a newly identified protein coassembling with myosin and paramyosin in Caenorhabditis elegans

Muscle thick filaments are stable assemblies of myosin and associated proteins whose dimensions are precisely regulated. The mechanisms underlying the stability and regulation of the assembly are not understood. As an approach to these problems, we have studied the core proteins that, together with paramyosin, form the core structure of the thick filament backbone in the nematode Caenorhabditis elegans. We obtained partial peptide sequences from one of the core proteins, beta-filagenin, and then identified a gene that encodes a novel protein of 201-amino acid residues from databases using these sequences. beta-Filagenin has a calculated isoelectric point at 10.61 and a high percentage of aromatic amino acids. Secondary structure algorithms predict that it consists of four beta-strands but no alpha-helices. Western blotting using an affinity-purified antibody showed that beta-filagenin was associated with the cores. beta-Filagenin was localized by immunofluorescence microscopy to the A bands of body-wall muscles, but not the pharynx. beta-filagenin assembled with the myosin homologue paramyosin into the tubular cores of wild-type nematodes at a periodicity matching the 72-nm repeats of paramyosin, as revealed by immunoelectron microscopy. In CB1214 mutants where paramyosin is absent, beta-filagenin assembled with myosin to form abnormal tubular filaments with a periodicity identical to wild type. These results verify that beta-filagenin is a core protein that coassembles with either myosin or paramyosin in C. elegans to form tubular filaments.