Association of plectin with Z-discs is a prerequisite for the formation of the intermyofibrillar desmin cytoskeleton

Plectin is a high-molecular mass protein (approximately 500 kd) that binds actin, intermediate filaments, and microtubules. Mutations of the plectin gene cause a generalized blistering skin disorder and muscular dystrophy. In adult muscle, plectin is colocalized with desmin at structures forming the intermyofibrillar scaffold and beneath the plasma membrane. To study the involvement of plectin in myofibrillogenesis, we analyzed the spatial and temporal expression patterns of plectin in cultured differentiating human skeletal muscle cells and its relationship to desmin intermediate filaments during this process. Northern and Western blot analyses demonstrated that at least two different plectin isoforms are expressed at all developmental stages from proliferating myoblasts to mature myotubes. Using immunocytochemistry, we show that the localization of plectin dramatically changes from a network-like distribution into a cross-striated distribution during maturation of myocytes. Double immunofluorescence experiments revealed that desmin and plectin are colocalized in premyofibrillar stages and in mature myotubes. Interestingly, plectin was often found to localize to the periphery of Z-discs during the actual alignment of neighboring myofibrils, and an obvious cross-striated plectin staining pattern was observed before desmin was localized in the Z-disc region. We conclude that the association of plectin with Z-discs is an early event in the lateral alignment of myofibrils that precedes the formation of the intermyofibrillar desmin cytoskeleton.

Characterization of muscle filamin isoforms suggests a possible role of gamma-filamin/ABP-L in sarcomeric Z-disc formation

Filamin, also called actin binding protein-280, is a dimeric protein that cross-links actin filaments in the cortical cytoplasm. In addition to this ubiquitously expressed isoform (FLN1), a second isoform (ABP-L/gamma-filamin) was recently identified that is highly expressed in mammalian striated muscles. A monoclonal antibody was developed, that enabled us to identify filamin as a Z-disc protein in mammalian striated muscles by immunocytochemistry and immunoelectron microscopy. In addition, filamin was identified as a component of intercalated discs in mammalian cardiac muscle and of myotendinous junctions in skeletal muscle. Northern and Western blots showed that both, ABP-L/gamma-filamin mRNA and protein, are absent from proliferating cultured human skeletal muscle cells. This muscle specific filamin isoform is, however, up-regulated immediately after the induction of differentiation. In cultured myotubes, ABP-L/gamma-filamin localises in Z-discs already at the first stages of Z-disc formation, suggesting that ABP-L/gamma-filamin might play a role in Z-disc assembly.

Modulation of sarcomere organization during embryonic stem cell-derived cardiomyocyte differentiation

Myofibrillogenesis - sarcomeres - mouse embryonic stem cells - cardiomyocytes - beta1 integrin Mouse embryonic stem (ES) cells, when cultivated as embryoid bodies, differentiate in vitro into cardiomyocytes of ventricle-, atrium- and pacemaker-like cell types characterized by developmentally controlled expression of cardiac-specific genes, structural proteins and ion channels. Using this model system, we show here, (I) that during cardiac myofibrillogenesis sarcomeric proteins are organized in a developmentally regulated manner following the order: titin (Z-disk), alpha-actinin, myomesin, titin (M-band), myosin heavy chain, alpha-actin, cardiac troponin T and M-protein, recapitulating the sarcomeric organization in the chicken embryonal heart in vivo. Our data support the view that the formation of I-Z-I complexes is developmentally delayed with respect to A-band assembly. We show (2) that the process of cardiogenic differentiation in vitro is influenced by medium components: Using a culture medium supplemented with glucose, amino acids, vitamins and selenium ions, we were able to increase the efficiency of cardiac differentiation of wild-type, as well as of beta1 integrin-deficient (beta1-/-) ES cells, and to improve the degree of organization of sarcomeric structures in wild-type and in beta1-/- cardiac cells. The data demonstrate the plasticity of cardiogenesis during the differentiation of wild-type and of genetically modified ES cells.

Thick filament assembly occurs after the formation of a cytoskeletal scaffold

The development of myofibrils involves the formation of contractile filaments and their assembly into the strikingly regular structure of the sarcomere. We analysed this assembly process in cultured human skeletal muscle cells and in rat neonatal cardiomyocytes by immunofluorescence microscopy using antibodies directed against cytoskeletal and contractile proteins. In particular, the question in which temporal order the respective proteins are integrated into developing sarcomeres was addressed. Although sarcomeric myosin heavy chain is expressed as one of the first myofibrillar proteins, its characteristic A band arrangement is reached at a very late stage. In contrast, titin, then myomesin and finally C-protein (MyBP-C) gradually form a regularly arranged scaffold on stress fiber-like structures (SFLS), on non-striated myofibrils (NSMF) and on nascent striated myofibrils (naSMF). Immediately subsequent to the completion of sarcomere cytoskeleton formation, the labeling pattern of myosin changes from the continuous staining of SFLS to the periodic staining characteristic for mature myofibrils. This series of events can be seen most clearly in the skeletal muscle cell cultures and--probably due to a faster developmental progression less well in cardiomyocytes. We therefore conclude that the correct assembly of a cytoskeletal scaffold is a prerequisite for correct thick filament assembly and for the integration of the contractile apparatus into the myofibril.

Immunogold EM reveals a close association of plectin and the desmin cytoskeleton in human skeletal muscle

Plectin is a multifunctional cytoskeletal linker protein with an intermediate filament-binding site and sequence elements with high homology to actin-binding domains. Mutations of the human plectin gene as well as the targeted inactivation of its murine analog cause a generalized blistering skin disorder and muscular dystrophy, thus implying its essential role in cells that are exposed to mechanical stress. In the present study we report the characterization of two new domain-specific plectin antibodies as well as ultrastructural localization of plectin in normal human skeletal muscle. Using immunogold electron microscopy, we localized plectin at three prominent sites: 1) Plectin is found at regularly spaced intervals along the cytoplasmic face of the plasma membrane. 2) It is distinctly localized at filamentous bridges between Z-lines of peripheral myofibrils and the sarcolemma and 3) at structures forming the intermyofibrillar scaffold. At the latter two locations, plectin and desmin were found to colocalize. Our ultrastructural analysis suggests that plectin may have a central role in the structural and functional organization of the intermediate filament cytoskeleton in mature human skeletal muscle.

M band proteins myomesin and skelemin are encoded by the same gene: analysis of its organization and expression

The complete exon-intron organization of the murine gene encoding sarcomeric myomesin has been determined. The gene is composed of 38 exons and 37 introns, spanning approximately 105 kb of DNA. Intron positions and phases are essentially identical to those identified in M-protein. They are related to the modular structure of myomesin, which is composed almost entirely of immunoglobulin and fibronectin type III domains. Nearly all repeats follow a two exon-one domain structure. The start and end of each domain are defined by introns in phase I, while internal introns are more divergent in position and very rarely use phase I. Genomic Southern blotting and reverse transcription-polymerase chain reaction revealed that differential splicing of a single exon gives rise to two polypeptides, described in the literature as myomesin and skelemin, respectively. A single transcriptional start point was detected in both skeletal and cardiac muscle. Analysis of the presumptive promoter region revealed several potential regulatory elements. CAT expression assays using promoter deletion constructs identified three regions that seem to be important for the muscle-specific transcriptional activation of the myomesin gene. These results provide the basis for a comparative analysis of the regulation of myomesin and M-protein genes in vivo.

Structural basis for activation of the titin kinase domain during myofibrillogenesis

The giant muscle protein titin (connectin) is essential in the temporal and spatial control of the assembly of the highly ordered sarcomeres (contractile units) of striated muscle. Here we present the crystal structure of titin's only catalytic domain, an autoregulated serine kinase (titin kinase). The structure shows how the active site is inhibited by a tyrosine of the kinase domain. We describe a dual mechanism of activation of titin kinase that consists of phosphorylation of this tyrosine and binding of calcium/calmodulin to the regulatory tail. The serine kinase domain of titin is the first known non-arginine-aspartate kinase to be activated by phosphorylation. The phosphorylated tyrosine is not located in the activation segment, as in other kinases, but in the P + 1 loop, indicating that this tyrosine is a binding partner of the titin kinase substrate. Titin kinase phosphorylates the muscle protein telethonin in early differentiating myocytes, indicating that this kinase may act in myofibrillogenesis.

Expression of sarcomeric proteins and assembly of myofibrils in the putative myofibroblast cell line BHK-21/C13

The expression and organization patterns of several myofibrillar proteins were analysed in the putative myofibroblast cell line BHK-21/C13. Although this cell line originates from renal tissue, the majority of the cells express titin. In these cells, titin is, under standard culture conditions, detected in myofibril-like structures (MLSs), where it alternates with non-muscle myosin (NMM). Expression of sarcomeric myosin heavy chain (sMyHC) is observed in a small minority of cells, while other sarcomeric proteins, such as nebulin, myosin binding protein C (MyBP-C), myomesin and M-protein are not expressed at all. By changing the culture conditions in a way equal to conditions that induce differentiation of skeletal muscle cells, a process reminiscent of sarcomerogenesis in vitro is induced. Within one day after the switch to a low-nutrition medium, myofibrillar proteins can be detected in a subset of cells, and after two to five days, all myofibrillar proteins examined are organized in typical sarcomeric patterns. Frequently, cross-striations are visible with phase contrast optics. Transfection of these cells with truncated myomesin fragments showed that a specific part of the myomesin molecule, known to contain a titin-binding site, binds to MLSs, whereas other parts do not. These results demonstrate that this cell line could serve as a powerful model to study the assembly of myofibrils. At the same time, its transfectability offers an invaluable tool for in vivo studies concerning binding properties of sarcomeric proteins.

Two immunoglobulin-like domains of the Z-disc portion of titin interact in a conformation-dependent way with telethonin

The giant muscle protein titin/connectin plays a crucial role in myofibrillogenesis as a molecular ruler for sarcomeric protein sorting. We describe here that the N-terminal titin immunoglobulin domains Z1 and Z2 interact specifically with telethonin in yeast two-hybrid analysis and protein binding assays. Immunofluorescence with antibodies against the N-terminal region of titin and telethonin detects both proteins at the Z-disc of human myotubes. Longer titin fragments, comprising a serine-proline-rich phosphorylation site and the next domain, do not interact. The interaction of telethonin with titin is therefore conformation-dependent, reflecting a possible phosphorylation regulation during myofibrillogenesis.

Mapping of a myosin-binding domain and a regulatory phosphorylation site in M-protein, a structural protein of the sarcomeric M band

The myofibrils of cross-striated muscle fibers contain in their M bands cytoskeletal proteins whose main function seems to be the stabilization of the three-dimensional arrangement of thick filaments. We identified two immunoglobin domains (Mp2-Mp3) of M-protein as a site binding to the central region of light meromyosin. This binding is regulated in vitro by phosphorylation of a single serine residue (Ser76) in the immediately adjacent amino-terminal domain Mp1. M-protein phosphorylation by cAMP-dependent kinase A inhibits binding to myosin LMM. Transient transfection studies of cultured cells revealed that the myosin-binding site seems involved in the targeting of M-protein to its location in the myofibril. Using the same method, a second myofibril-binding site was uncovered in domains Mp9-Mp13. These results support the view that specific phosphorylation events could be also important for the control of sarcomeric M band formation and remodeling.

Structure and expression of the gene encoding murine M-protein, a sarcomere-specific member of the immunoglobulin superfamily

The complete exon-intron organization of the murine gene encoding M-protein, a structural protein of sarcomeric myofibrils, was determined. The gene is composed of 37 exons and 36 introns, spanning approximately 75 kb of DNA. Intron positions are related to the modular structure of M-protein, which is composed essentially of immunoglobulin and fibronectin type III domains. Almost all repeats follow a two exon-one domain structure. The beginning and end of each domain are defined by introns in phase I; internal introns are more divergent in position and very rarely use phase I. A single transcriptional start point was detected in both skeletal and cardiac muscle. Analysis of the prospective promoter region revealed several potential regulatory elements. CAT expression assays using promoter deletion constructs identified three regions that seem to be most important for the muscle-specific transcription activation of the M-protein gene. These results provide the first complete characterization of a gene for a member of the intracellular branch of the immunoglobulin superfamily.

Isoform transitions of the myosin binding protein C family in developing human and mouse muscles: lack of isoform transcomplementation in cardiac muscle

Mutations in the gene for the cardiac isoform of myosin binding protein C (MyBP-C) have been identified as the cause of chromosome 11-associated autosomal-dominant familial hypertrophic cardiomyopathy (FHC). Most mutations produce a truncated polypeptide that lacks the sarcomeric binding region. We have now investigated the expression pattern of the cardiac and skeletal isoforms of cMyBP-C in mice and humans by in situ hybridization and immunofluorescence microscopy using specific antibodies and probes. We demonstrate that the cardiac isoform is expressed only in cardiac muscle throughout development. The slow and fast isoforms of MyBP-C remain specific for skeletal muscle, where they can be coexpressed. Immunological evidence also suggests that an embryonic isoform of MyBP-C precedes the expression of slow MyBP-C in developing skeletal muscle. This suggests that transcomplementation of MyBP-C isoforms is possible in skeletal but not cardiac muscle.

Assembly of titin, myomesin and M-protein into the sarcomeric M band in differentiating human skeletal muscle cells in vitro

Immunochemical experiments and in vitro binding studies have revealed that titin/connectin, the elastic protein that spans the whole distance of a half-sarcomere, associates with several myosin-binding proteins of the sarcomeric A and M band. Two of these proteins, M-protein and myomesin, anchor titin in the region of the M band. A detailed molecular map describing the arrangement of titin, M-protein and myomesin in this part of the sarcomere was recently proposed. Furthermore, specific binding sites between the molecules were identified. How these polypeptides function in the assembly of the sarcomeric M band region has, however, remained unclear. Here we describe the distribution patterns of different epitopes recognized by newly developed antibodies against the extreme carboxyterminal portion of titin that is anchored in the M band, during the in vitro differentiation of human skeletal muscle cells. In contrast to a set of antibodies directed against Z band, I band and A band titin epitopes, anti-M band titin did not stain titin aggregates or titin in non-striated myofibrils (NSMF). The M band epitopes were only revealed in their characteristic sarcomeric locations, and were obviously not accessible in the non-striated part of nascent myofibrils, or during earlier developmental stages. We speculate that this phenomenon is associated with "immature" tertiary/quarternary structures of M band titin that avoid preliminary binding of M band proteins. In contrast to earlier observations on myofibrillogenesis in the mouse embryo, all the titin epitopes studied were simultaneously detected in their specific positions. Thus, sarcomere assembly in the widely used in vitro model systems seems to proceed at a much higher speed than in vivo. Similarly, myomesin and M-protein were only perceptible in striated myofibrils. While myomesin antibodies stained myofibrils at the time-point of appearance of the first titin striations, the incorporation of M-protein was found to be slightly delayed. In several myotubes no expression of M-protein was observed even during mature stages. These observations indicate its less important general role in the process of myofibrillogenesis. Furthermore, the relative number of M-protein negative myotubes varied in cultures derived from different muscles. This confirms the observation that cultured satellite cells are predestined to form a certain type of myofibers.

Molecular structure of the sarcomeric M band: mapping of titin and myosin binding domains in myomesin and the identification of a potential regulatory phosphorylation site in myomesin

The M band of sarcomeric muscle is a highly complex structure which contributes to the maintenance of the regular lattice of thick filaments. We propose that the spatial coordination of this assembly is regulated by specific interactions of myosin filaments, the M band protein myomesin and the large carboxy-terminal region of titin. Corresponding binding sites between these proteins were identified. Myomesin binds myosin in the central region of light meromyosin (LMM, myosin residues 1506-1674) by its unique amino-terminal domain My1. A single titin immunoglobulin domain, m4, interacts with a myomesin fragment spanning domains My4-My6. This interaction is regulated by phosphorylation of Ser482 in the linker between myomesin domains My4 and My5. Myomesin phosphorylation at this site by cAMP-dependent kinase and similar or identical activities in muscle extracts block the association with titin. We propose that this demonstration of a phosphorylation-controlled interaction in the sarcomeric cytoskeleton is of potential relevance for sarcomere formation and/or turnover. It also reveals how binding affinities of modular proteins can be regulated by modifications of inter-domain linkers.

The central Z-disk region of titin is assembled from a novel repeat in variable copy numbers

The giant sarcomeric protein titin (also described as connectin) is composed mainly of immunoglobulin (Ig)-like and fibronectin type III (fn3)-like domains arranged consecutively. At both ends of the molecule, these domains are interrupted by sequence insertions. The amino terminus of titin is localized in the Z-disk, a structure of great variability in different muscle types. We have determined the ultrastructural position of sequences in this region of the molecule in skeletal and cardiac muscle by immunoelectron microscopy using antibodies directed against unique epitopes. Titin molecules entering the Z-disk from two half sarcomeres do not significantly overlap, showing that the amino terminus is at the centre of the Z-disk. A serine/proline rich site, which can be phosphorylated by kinases in developing muscle tissues, was identified near the amino terminus of titin. Sequence analysis revealed the presence of a novel 45 residue repeat (‘Z-repeats’) in this region of the molecule. The number of titin Z-repeats varies due to differential splicing. We propose that this mechanism is a means of assembling Z-disks of variable thickness and mechanical strength.

The structure of the sarcomeric M band: localization of defined domains of myomesin, M-protein, and the 250-kD carboxy-terminal region of titin by immunoelectron microscopy

The M band of vertebrate cross-striated myofibrils has remained an enigmatic structure. In addition to myosin thick filaments, two major structural proteins, myomesin and M-protein, have been localized to the M band. Also, titin is expected to be anchored in this structure. To begin to understand the molecular layout of these three proteins, a panel of 16 polyclonal and monoclonal antibodies directed against unique epitopes of defined sequence was assembled, and immunoelectron microscopy was used to locate the position of the epitopes at the sarcomere level. The results allow the localization and orientation of defined domains of titin, myomesin, and M-protein at high resolution. The 250-kD carboxy-terminal region of titin clearly enters the M band with the kinase domain situated approximately 52 nm from the central M1-line. The positions of three additional epitopes are compatible with the view that the titin molecule reaches approximately 60 nm into the opposite sarcomere half. Myomesin also seems to bridge the central M1-line and is oriented parallel to the long axis of the myofibril. The neighboring molecules are oriented in an antiparallel and staggered fashion. The amino-terminal portion of the protein, known to contain a myosin binding site, seems to adopt a specific three-dimensional arrangement. While myomesin is present in both slow and fast fibers, M-protein is restricted to fast fibers. It appears to be organized in a fundamentally different manner: the central portion of the polypeptide is around the M1-line, while the terminal epitopes seem to be arranged along thick filaments. This orientation fits the conspicuously stronger M1-lines in fast twitch fibers. Obvious implications of this model are discussed.

Integration of titin into the sarcomeres of cultured differentiating human skeletal muscle cells

Titin is amongst the first sarcomeric proteins to be detected in the process of myofibrillogenesis of striated muscle. During embryogenesis this high molecular weight protein is initially observed in a punctate staining pattern in immunohistochemical studies, while during maturation titin organizes into a cross-striated pattern. The dynamic process of titin assembly up to its integration into the sarcomeres of cultured human skeletal muscle cells has been studied in subsequent stages of differentiation with antibodies to four well-defined titin epitopes. Since in maturated muscle cells these epitopes are clearly distinguishable on the extended titin molecule we wondered how these epitopes reorganize during myofibrillogenesis, and whether such a reorganization would reveal important clues about its supramolecular organization during development. Immunofluorescence staining of postmitotic mononuclear myoblasts indicate that the investigated epitopes of the titin molecule are displayed in a punctate pattern with neighboring, but clearly separate spots in the cytoplasm of the cells. During elongation and fusion of the cells, these titin spots associate with stress fiber-like structures to finally reach their position at either the Z-line, the A-I junction or the A-band. We propose that during this transition the large titin molecule is unfolded, with the amino terminus of the molecule migrating in the direction of the Z-line and the carboxy terminus moving towards the M-line. In maturated, fused myotubes the final cross-striated patterns of all investigated titin epitopes are observed. While this process of unfolding of the titin molecule progresses, other compounds of the Z-line and the A-band migrate to their specific positions in the nascent sarcomere. A-band components such as sarcomeric myosin and C-protein, are also observed as dot-like aggregates during initial stages of muscle cell differentiation and organize into a cross-striated pattern in the sarcomere virtually simultaneously with titin. The Z-line associated component desmin organizes into a cross-striated pattern at a later stage.

Purification and biochemical characterization of myomesin, a myosin-binding and titin-binding protein, from bovine skeletal muscle

We report a method for isolating homogeneous myomesin from mammalian skeletal muscle. The identity of the purified bovine protein was confirmed by its reactivity with myomesin-specific monoclonal antibodies and with polyclonal antibodies raised against peptides derived from the amino-terminal and carboxy-terminal ends of the sequence predicted by the human myomesin cDNA. All partial sequences obtained from bovine myomesin can be aligned along the human sequence predicted by its cloned cDNA. Electron microscopy of myomesin revealed short flexible rods with a molecular length of about 50 nm. Circular dichroism spectra showed a high degree of beta structure as expected for a member of the immunoglobulin superfamily of proteins. Alignment of the sequences of the class I and II domains of myomesin with the sequences of domains of known three-dimensional structure provides a more detailed model of myomesin. In agreement with this view, the cleavage sites observed by limited proteolysis locate primarily between individual domains. In a solid-phase overlay assay myomesin specifically bound to the myosin rod and to light meromyosin (LMM), but not to the carboxy-terminal 30-kDa fragment of LMM. The myosin-binding site seemed to be confined to the amino-terminal 240 residues of the molecule. The cross-reactivity of myomesin with the phosphorylation-dependent monoclonal neurofilament antibody NE14 [Shaw, G.E., Debus, E. & Weber, K. (1984) Eur. J. Cell Biol. 34, 130-136] was analyzed. NE14 reactivity of myomesin was abolished by alkaline phosphatase. Reactivity of the antibody on stable proteolytic fragments of myomesin showed that the phosphorylation site must reside within the carboxy-terminal 60 residues.

The anatomy of a molecular giant: how the sarcomere cytoskeleton is assembled from immunoglobulin superfamily molecules

Cross-striated muscle contains an elastic cytoskeleton comprised of the giant protein titin and several associated proteins. cDNA sequencing revealed that all these proteins are immunoglobulin superfamily members. This modular structure opens the possibility to dissect the proteins involved into functional units and to approach the problem of structure-function correlation at the molecular level.

The globular head domain of titin extends into the center of the sarcomeric M band. cDNA cloning, epitope mapping and immunoelectron microscopy of two titin-associated proteins

Immunoelectron microscopical results have shown that the Z and M bands of the sarcomere are interconnected by the long titin molecules. Here we have characterized by monoclonal antibodies, cDNA cloning and immunoelectron microscopy the two titin-associated proteins (190 and 165 kDa proteins), which seem responsible for the formation of a head structure on one end of the 0.9 micron long titin string. The human 165 kDa (1465 residues) and 190 kDa (1451 residues) proteins have unique N-terminal domains some 110 residues in length. Both proteins show 12 repeat domains with strong homology to either fibronectin type III (motif I) or immunoglobulin C2 (motif II) domains, which are arranged in the order II-II-I-I-I-I-I-II-II-II-II-II. Over these repeat domains the two proteins share 50% sequence identity (70% similarity). Epitopes situated in the C-terminal 138 or in the preceding 206 residues of the 165 kDa protein locate in immunoelectron microscopy to stripes situated 18 or 15 nm from the center of the M band. An epitope situated 277 to 129 residues prior to the C-terminus of the 190 kDa protein (i.e. repeats 10 and 11) locates to the center of the M band. Thus the head structure of the titin molecule extends into the center of the M band. Microsequence data on peptides from the titin-associated bovine 165 kDa protein and from conventionally purified bovine M-protein argue together with the reactivity of the antibodies that 165 kDa protein and M-protein are identical. The integrating structure of the sarcomere, which is based on titin and its side-on (C-protein and 86 kDa protein) or end-on (190 kDa protein and 165 kDa protein) associated proteins arises from muscle-specific members of the superfamily of immunoglobulin-like proteins.