Striated muscle cytoarchitecture: an intricate web of form and function

Troponin I is required for myofibrillogenesis and sarcomere formation inDrosophilaflight muscle

Myofibrillar proteins assemble to form the highly ordered repetitive contractile structural unit known as a sarcomere. Studies of myogenesis in vertebrate cell culture and embryonic developmental systems have identified some of the processes involved during sarcomere formation. However, isoform changes during vertebrate muscle development and a lack of mutants have made it difficult to determine how these proteins assemble to form sarcomeres. The indirect flight muscles (IFMs) of Drosophila provide a unique genetic system with which to study myofibrillogenesis in vivo. We show in this paper that neither sarcomeric myosin nor actin are required for myoblast fusion or the subsequent morphogenesis of muscle fibres, i.e. fibre morphogenesis does not depend on myofibrillogenesis. However, fibre formation and myofibrillogenesis are very sensitive to the interactions between the sarcomeric proteins. A troponin I (TnI) mutation, hdp3, leads to an absence of TnI in the IFMs and tergal depressor of trochanter (TDT) muscles due to a transcript-splicing defect. Sarcomeres do not form and the muscles degenerate. TnI is part of the thin filament troponin complex which regulates muscle contraction. The effects of the hdp3 mutation are probably caused by unregulated acto-myosin interactions between the thin and thick filaments as they assemble. We have tested this proposal by using a transgenic myosin construct to remove the force-producing myosin heads. The defects in sarcomeric organisation and fibre degeneration in hdp3 IFMs are suppressed, although not completely, indicating the need for inhibition of muscle contraction during muscle development. We show that mRNA and translated protein products of all the major thin filament proteins are reduced in hdp3 muscles and discuss how this and previous studies of thin filament protein mutants indicate a common co-ordinated control mechanism that may be the primary cause of the muscle defects.

Obscurin regulates the organization of myosin into A bands

Obscurin is a giant sarcomeric protein composed of adhesion modules and signaling domains. It surrounds myofibrils at the level of the Z disk and the M line. To study the role of obscurin during myofibrillogenesis, we used adenovirus-mediated gene delivery to overexpress part of its COOH terminus in primary cultures of postnatal day 1 (P1) skeletal myotubes. Examination of the subcellular distribution of a number of sarcomeric proteins revealed that the organization of myosin into A bands was dramatically reduced. Myosin assembled into A bands normally in mock- or control-infected P1 myotubes. Overexpression of the COOH terminus of obscurin did not affect the organization of other sarcomeric markers, including actin, alpha-actinin, titin, and myomesin. Assembly of myomesin into nascent M lines in treated myotubes suggests that these structures can form independently of A bands. Immunoblot analysis indicated that there was a small ( approximately 20%) but consistent decrease in the amount of myosin expressed in cells infected with the COOH terminus of obscurin. Coimmunoprecipitation experiments in which we used adult skeletal muscle homogenates demonstrated that obscurin exists in a complex with myosin. Thus our findings suggest that the COOH-terminal region of obscurin interacts with sarcomeric myosin and may play a critical role in its ability to assemble into A bands in striated muscle.

Chaperone-mediated folding and assembly of myosin in striated muscle

De novo folding and assembly of striated muscle myosin was analyzed by expressing a GFP-tagged embryonic myosin heavy chain (GFP-myosin) in post-mitotic C2C12 myocytes using replication defective adenoviruses. In the early stages of muscle differentiation, the GFP-myosin accumulates in bright globular foci and short filamentous structures that are later replaced by brightly fluorescent myofibrils. Time-lapse microscopy shows that the intermediates are dynamic and are present in elongating and fusing myocytes and in multinucleated myotubes. Immunostaining reveals the co-localization of the molecular chaperones Hsc70 and Hsp90 with the GFP-myosin in the intermediates, but not in the mature myofibrils. Uninfected cells have similar intermediates suggesting a common pathway for myosin maturation. Two conformation-sensitive antibodies that bind the unfolded motor domain and the coiled-coil conformation of the rod demonstrate that in the intermediates, the myosin rod is folded but the motor domain is not folded. Electron microscopy reveals that the intermediates contain loose filament bundles surrounded by a protein rich matrix. Geldanamycin, a specific inhibitor of Hsp90, reversibly blocks myofibril assembly and triggers accumulation of myosin folding intermediates. We conclude that multimeric complexes of nascent myosin filaments associated with Hsc70 and Hsp90 are intermediates in the folding and assembly pathway of muscle myosin.

Induction and Myofibrillar Targeting of CARP, and Suppression of the Nkx2.5 Pathway in the MDM Mouse with Impaired Titin-based Signaling

Muscular dystrophy with myositis (mdm) is a recessive mouse mutation that is caused by a small deletion in the giant elastic muscle protein titin. Homozygous mdm/mdm mice develop a progressive muscular dystrophy, leading to death at approximately 2 months of age. We surveyed the transcriptomes of skeletal muscles from 24 day old homozygous mdm/mdm and +/+ wild-type mice, an age when MDM animals have normal passive and active tensions and sarcomeric structure. Of the 12488 genes surveyed (U74 affymetrix array), 75 genes were twofold to 30-fold differentially expressed, including CARP (cardiac ankyrin repeat protein), ankrd2/Arpp (a CARP-like protein) and MLP (muscle LIM protein), all of which associate with the titin filament system. The four genes most strongly affected (eightfold to 30-fold change) were all members of the CARP-regulated Nkx-2.5-dependent signal pathway, and CARP mRNA level was 30-fold elevated in MDM skeletal muscle tissues. The CARP protein overexpressed in MDM became associated with the I-band region of the sarcomere. The mdm mutation excises the C-terminal portion of titin's N2A region, abolishing its interaction with p94/calpain-3 protease. Thus, the composition of the titin N2A protein complex is altered in MDM by incorporation of CARP and loss of p94/calpain-3. These changes were absent from the following control tissues (1). cardiac muscles from homozygous mdm/mdm animals, (2). skeletal and cardiac muscle from heterozygous mdm/+ animals, and (3). dystrophic muscles from MDX mice. Thus, the altered composition of the titin N2A complex is specific for the titin-based skeletal muscular dystrophy in MDM.

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.

An ?tropomyosin mutation alters dimer preference in nemaline myopathy

Nemaline myopathy is a human neuromuscular disorder associated with muscle weakness, Z-line accumulations (rods), and myofibrillar disorganization. Disease-causing mutations have been identified in genes encoding muscle thin filament proteins: actin, nebulin, slow troponin T, betaTropomyosin, and alphaTropomyosin(slow). Skeletal muscle expresses three tropomyosin (Tm) isoforms from separate genes: alphaTm(fast)(alphaTm, TPM1), betaTm (TPM2), and alphaTm(slow) (gammaTm, TPM3). In this article, we show that the level of betaTm, but not alphaTm(fast) protein, is reduced in human patients with mutations in alphaTm(slow) and in a transgenic mouse model of alphaTm(slow)(Met9Arg) nemaline myopathy. A postnatal time course of Tm expression in muscles of the mice indicated that the onset of alphaTm(slow)(Met9Arg) expression coincides with the decline of betaTm. Reduction of betaTm levels is independent of the degree of pathology (rods) within a muscle and is detected before the onset of muscle weakness. Thus, reduction in the level of betaTm represents an early clinical diagnostic marker for alphaTm(slow)-based mutations. Examinations of tropomyosin dimer formation using either recombinant proteins or sarcomeric extracts show that the mutation reduces the formation of the preferred alpha/beta heterodimer. We suggest this perturbation of tropomyosin isoform levels and dimer preference alters sarcomeric thin filament dynamics and contributes to muscle weakness in nemaline myopathy.

Myofibrillogenesis in the developing chicken heart: Role of actin isoforms and of the pointed end actin capping protein tropomodulin during thin filament assembly

Recently, important differences between myofibrillogenesis in cultured cardiomyocytes vs. the three-dimensional setting in situ could be determined. We investigated thin filament assembly in situ by confocal microscopy of whole-mount preparations of immunostained embryonic chicken hearts. Of interest, a distinct localisation of different actin isoforms was observed in immature thin filaments. Cardiac alpha-actin is restricted to filaments with a length comparable to mature thin filaments as soon as the first contractions occur, while vascular alpha-actin makes up filaments that extend toward the M-band. The pointed-end actin filament capping protein tropomodulin can be found initially in close association with the plasma membrane, but attains its mature localisation pattern at the ends of the thin filaments only comparatively late during myofibrillogenesis. Thus tropomodulin acts as a length stabilising element of actin filaments also in developing cardiomyocytes in situ, but plays an additional role together with membrane-associated actin filaments in the earliest steps of myofibril assembly.

Anthracyclines induce calpain-dependent titin proteolysis and necrosis in cardiomyocytes

Titin, the largest myofilament protein, serves as a template for sarcomere assembly and acts as a molecular spring to contribute to diastolic function. Titin is known to be extremely susceptible to calcium-dependent protease degradation in vitro. We hypothesized that titin degradation is an early event in doxorubicin-induced cardiac injury and that titin degradation occurs by activation of the calcium-dependent proteases, the calpains. Treatment of cultured adult rat cardiomyocytes with 1 or 3 micromol/liter doxorubicin for 24 h resulted in degradation of titin in myocyte lysates, which was confirmed by a reduction in immunostaining of an antibody to the spring-like (PEVK) domain of titin at the I-band of the sarcomere. The elastic domain of titin appears to be most susceptible to proteolysis because co-immunostaining with an antibody to titin at the M-line was preserved, suggesting targeted proteolysis of the spring-like domain of titin. Doxorubicin treatment for 1 h resulted in approximately 3-fold increase in calpain activity, which remained elevated at 48 h. Co-treatment with calpain inhibitors resulted in preservation of titin, reduction in myofibrillar disarray, and attenuation of cardiomyocyte necrosis but not apoptosis. Co-treatment with a caspase inhibitor did not prevent the degradation of titin, which precludes caspase-3 as an early mechanism of titin proteolysis. We conclude that calpain activation is an early event after doxorubicin treatment in cardiomyocytes and appears to target the degradation of titin. Proteolysis of the spring-like domain of titin may predispose cardiomyocytes to diastolic dysfunction, myofilament instability, and cell death by necrosis.

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.

Myosin storage myopathy associated with a heterozygous missense mutation in MYH7

Myosin constitutes the major part of the thick filaments in the contractile apparatus of striated muscle. MYH7 encodes the slow/beta-cardiac myosin heavy chain (MyHC), which is the main MyHC isoform in slow, oxidative, type 1 muscle fibers of skeletal muscle. It is also the major MyHC isoform of cardiac ventricles. Numerous missense mutations in the globular head of slow/beta-cardiac MyHC are associated with familial hypertrophic cardiomyopathy. We identified a missense mutation, Arg1845Trp, in the rod region of slow/beta-cardiac MyHC in patients with a skeletal myopathy from two different families. The myopathy was characterized by muscle weakness and wasting with onset in childhood and slow progression, but no overt cardiomyopathy. Slow, oxidative, type 1 muscle fibers showed large inclusions consisting of slow/beta-cardiac MyHC. The features were similar to a previously described entity: hyaline body myopathy. Our findings indicate that the mutated residue of slow/beta-cardiac MyHC is essential for the assembly of thick filaments in skeletal muscle. We propose the term myosin storage myopathy for this disease.

The Muscle Ankyrin Repeat Proteins: CARP, ankrd2/Arpp and DARP as a Family of Titin Filament-based Stress Response Molecules

CARP, ankrd-2/Arpp, and DARP, are three members of a conserved gene family, referred to here as MARPs (muscle ankyrin repeat proteins). The expression of MARPs is induced upon injury and hypertrophy (CARP), stretch or denervation (ankrd2/Arpp), and during recovery following starvation (DARP), suggesting that they are involved in muscle stress response pathways. Here, we show that MARP family members contain within their ankyrin repeat region a binding site for the myofibrillar elastic protein titin. Within the myofibril, MARPs, myopalladin, and the calpain protease p94 appear to be components of a titin N2A-based signaling complex. Ultrastructural studies demonstrated that all three endogenous MARP proteins co-localize with I-band titin N2A epitopes in adult heart muscle tissues. In cultured fetal rat cardiac myocytes, passive stretch induced differential distribution patterns of CARP and DARP: staining for both proteins was increased in the nucleus and at the I-band region of myofibrils, while DARP staining also increased at intercalated discs. We speculate that the myofibrillar MARPs are regulated by stretch, and that this links titin-N2A-based myofibrillar stress/strain signals to a MARP-based regulation of muscle gene expression.

Selected Contribution: Identification of differentially expressed genes between young and old rat soleus muscle during recovery from immobilization-induced atrophy

After cessation of hindlimb immobilization, which resulted in a 27-37% loss in soleus mass, the atrophied soleus muscle of young but not old rats regrows to its mass before treatment. We hypothesized that during remobilization the mRNA levels of growth potentiating factor(s) would be present in the soleus muscle of young (3- to 4-mo-old) but absent in old (30- to 31-mo-old) Fischer 344 x Brown Norway rats or that mRNAs for growth inhibitory factor(s) would be absent in young but present in old. Gene expression levels of >24,000 transcripts were determined by using Affymetrix RGU34A-C high-density oligonucleotide microarrays in soleus muscles at 3, 6, 10, and 30 days of remobilization after cessation of a 10-day period of hindlimb immobilization. Each muscle sample was applied to an independent set of arrays. Recovery-related differences were determined by using a three-factor ANOVA with a false discovery rate-adjustment of P = 0.01, which yielded 64 significantly different probe sets. Elfin, amphiregulin, and clusterin mRNAs were selected for further confirmation by real-time PCR. Elfin mRNA levels were less in old than in young rats at 6, 10, and 30 days of remobilization. Amphiregulin expression exhibited a unique spike on the 10th day of successful regrowth in young rats but remained unchanged old. Clusterin mRNA was unchanged in young muscles but was elevated on the 3rd, 6th, and 10th days of recovery in old soleus muscles. The mRNAs identified as differentially expressed between young and old recovery may modulate muscle growth that could highlight new candidate mechanisms to explain the failure of old soleus muscle to recover lost muscle mass.

Postnatal suppression of myomesin, muscle creatine kinase and the M-line in rat extraocular muscle

The M-line and its associated creatine kinase (CK) M-isoform (CK-M) are ubiquitous features of skeletal and cardiac muscle. The M-line maintains myosin myofilaments in register, links the contractile apparatus to the cytoskeleton for external force transfer and localizes CK-based energy storage and transfer to the site of highest ATP demand. We establish here that the muscle group responsible for movements of the eye, extraocular muscle (EOM), is divergent from other striated muscles in lacking both an M-line and its associated CK-M. Although an M-line forms during myogenesis, both in vivo and in vitro, it is actively repressed after birth. Transcripts of the major M-line structural proteins, myomesin 1 and myomesin 2, follow the same pattern of postnatal downregulation, while the embryonic heart-specific EH-myomesin 1 transcript is expressed early and retained in adult eye muscle. By immunocytochemistry, myomesin protein is absent from adult EOM sarcomeres. M-line suppression does not occur in organotypic co-culture with oculomotor motoneurons, suggesting that the mechanism for suppression may lie in muscle group-specific activation or workload patterns experienced only in vivo. The M-line is, however, still lost in dark-reared rats, despite the developmental delay this paradigm produces in the visuomotor system and EOMs. EOM was low in all CK isoform transcripts except for the sarcomeric mitochondrial (Ckmt2) isoform. Total CK enzyme activity of EOM was one-third that of hindlimb muscle. These findings are singularly unique among fast-twitch skeletal muscles. Since EOM exhibits isoform diversity for other sarcomeric proteins, the M-line/CK-M divergence probably represents a key physiological adaptation for the unique energetics and functional demands placed on this muscle group in voluntary and reflexive eye movements.

alpha-cardiac actin (ACTC) binds to the band 3 (AE1) cardiac isoform

The band 3 protein is the major integral protein present in the erythrocyte membrane. Two tissue-specific isoforms are also expressed in kidney alpha intercalated cells and in cardiomyocytes. It has been suggested that the cardiac isoform predominantly mediates the anion exchange in cardiomyocytes, but the role of the cytoplasmic domain of the band 3 (CDB3) protein in the cardiac tissue is unknown. In order to characterize novel associations of the CDB3 in the cardiac tissue, we performed the two-hybrid assay, using a bait comprising the region from leu 258 to leu 311 of the erythrocyte band 3, which must also be present in the cardiac isoform. The assay revealed two clones containing the C-terminal region of the alpha-cardiac actin. Immunoprecipitation of whole rat heart using an anti-actin antibody, immunoblotted with anti-human band 3, showed that actin binds to band 3 which was confirmed in the reverse assay. The confocal microscopy showed band 3 in the intercalated discs. Thus, besides the in vivo physical interaction in the Saccharomyces cerevisiae cell, we demonstrated using immunopreciptation that there is a physical association of band 3 with alpha-cardiac actin in cardiomyocyte, and we suggest that the binding occur "in situ," in the intercalated disc, a site of cell-cell contact and attachment of the sarcomere to the plasma membrane.

The complete mouse nebulin gene sequence and the identification of cardiac nebulin

Nebulin is a giant (M(r) 750-850kDa), modular sarcomeric protein proposed to regulate the assembly, and to specify the precise lengths of actin (thin) filaments in vertebrate skeletal muscles. Nebulin's potential role as a molecular template is based on its structural and biochemical properties. Its central approximately 700kDa portion associates with actin along the entire length of the thin filament, its N-terminal region extends to thin filament pointed ends, and approximately 80kDa of its C-terminal region integrates within the Z-line lattice. Here, we determined the exon/intron organization of the entire mouse nebulin gene, which contains 165 exons in a 202kb segment. We identified 16 novel exons, 15 of which encode nebulin-repeat motifs (12 from its central region and 3 from its Z-line region). One novel exon shares high sequence homology to the 20 residue repeats of the tight-junction protein, ZO-1. RT-PCR analyses revealed that all 16 novel exons are expressed in mouse skeletal muscle. Surprisingly, we also amplified mRNA transcripts from mouse and human heart cDNA using primers designed along the entire length of nebulin. The expression of cardiac-specific nebulin transcripts was confirmed by in situ hybridization in fetal rat cardiomyocytes and in embryonic Xenopus laevis (frog) heart. On the protein level, antibodies specific for skeletal muscle nebulin's N and C-terminal regions stained isolated rat cardiac myofibrils at the pointed and barbed ends of thin filaments, respectively. These data indicate a conserved molecular layout of the nebulin filament systems in both cardiac and skeletal myofibrils. We propose that thin filament length regulation in cardiac and skeletal muscles may share conserved nebulin-based mechanisms, and that nebulin isoform diversity may contribute to thin filament length differences in cardiac and skeletal muscle.

The mode of myofibril remodelling in human skeletal muscle affected by DOMS induced by eccentric contractions

Myofibrillar Z-disc streaming and loss of the desmin cytoskeleton are considered the morphological hallmarks of eccentric contraction-induced injury. The latter is contradicted by recent studies where a focal increase of desmin was observed in biopsies taken from human muscles with DOMS. In order to determine the effects of eccentric contraction-induced alterations of the myofibrillar Z-disc, we examined the distribution of alpha-actinin, the Z-disc portion of titin and the nebulin NB2 region in relation to actin and desmin in DOMS biopsies. In biopsies taken 2-3 days and 7-8 days after exercise, we observed a significantly higher number of fibres showing focal areas lacking staining for alpha-actinin, titin and nebulin than in biopsies taken from control or 1 h after exercise. None of these proteins were part of Z-disc streamings but instead they were found in distinct patterns in areas characterised by altered staining for desmin and actin. These were preferentially seen in regions with increased numbers of sarcomeres in parallel myofibrils. We propose that these staining patterns represent different stages of sarcomere formation. These findings therefore support our previous suggestion that muscle fibres subjected to eccentric contractions adapt to unaccustomed activity by the addition of new sarcomeres.

Mutations in genes encoding fast-twitch contractile proteins cause distal arthrogryposis syndromes

The distal arthrogryposes (DAs) are a group of disorders characterized by multiple congenital contractures of the limbs. We previously mapped a locus for DA type 2B (DA2B), the most common of the DAs, to chromosome 11. We now report that DA2B is caused by mutations in TNNI2 that are predicted to disrupt the carboxy-terminal domain of an isoform of troponin I (TnI) specific to the troponin-tropomyosin (Tc-Tm) complex of fast-twitch myofibers. Because the DAs are genetically heterogeneous, we sought additional candidate genes by examining modifiers of mutant Drosophila isoforms of TnI. One of these modifiers, Tm2, encodes tropomyosin, another component of the Tc-Tm complex. A human homologue of Tm2, TPM2, encodes beta-tropomyosin and maps to the critical interval of DA type 1 (DA1). We discovered that DA1 is caused by substitution of a highly conserved amino acid residue in beta-tropomyosin. These findings suggest that DAs, in general, may be caused by mutations in genes encoding proteins of the contractile apparatus specific to fast-twitch myofibers. This provides a new opportunity to directly study the etiology and pathogenesis of multiple-congenital-contracture syndromes.

Sensing stretch is fundamental

Stretch induces changes in cardiomyocyte biology that are implicated in heart failure, but the mechanism by which stretch is sensed and signals are transduced is unknown. New understanding of the Z disc elements of contractile units are beginning to elucidate the mechanism of stretch sensing and its relation to cardiac adaptation and disease.

The cardiac mechanical stretch sensor machinery involves a Z disc complex that is defective in a subset of human dilated cardiomyopathy

Muscle cells respond to mechanical stretch stimuli by triggering downstream signals for myocyte growth and survival. The molecular components of the muscle stretch sensor are unknown, and their role in muscle disease is unclear. Here, we present biophysical/biochemical studies in muscle LIM protein (MLP) deficient cardiac muscle that support a selective role for this Z disc protein in mechanical stretch sensing. MLP interacts with and colocalizes with telethonin (T-cap), a titin interacting protein. Further, a human MLP mutation (W4R) associated with dilated cardiomyopathy (DCM) results in a marked defect in T-cap interaction/localization. We propose that a Z disc MLP/T-cap complex is a key component of the in vivo cardiomyocyte stretch sensor machinery, and that defects in the complex can lead to human DCM and associated heart failure.