Differential expression and distribution of chicken skeletal- and smooth-muscle-type alpha-actinins during myogenesis in culture

Muscle transcriptome signature and gene regulatory network analysis in two divergent lines of a hilly bovine species Mithun (Bos frontalis)

A massive bovine, Bos frontalis, also known as Mithun or Gayal, found at higher altitude is very promising meat and milk animal. For candidate gene and marker discovery, RNA-seq data was generated from longissimus dorsi muscle tissues with Illumina-HiSeq. Such markers can be used in future for genetic gain of traits like feed conversion efficiency (FCE) and average daily gain (ADG). Analysis revealed 297differentially expressed genes (DEGs) having 173 up and 124 down-regulated unigenes. Extensive conservation was found in genic region while comparing with Bos taurus. Analysis revealed 57 pathways having 112 enzymes, 72 transcriptional factors and cofactors, 212 miRNAs regulating 71 DEGs, 25,855 SSRs, mithun-specific 104,822 variants and 7288 indels, gene regulatory network (GRN) having 24 hub-genes and transcriptional factors regulating cell proliferation, immune tolerance and myogenesis. This is first report of muscle transcriptome depicting candidate genes with GRN controlling FCE and ADG. Reported putative molecular markers, candidate genes and hub proteins can be valuable genomic resources for association studies in genetic improvement programme.

Pericytes in development and pathology of skeletal muscle

Increasing attention is currently devoted to the multiple roles that pericytes (also defined as mural, Rouget, or perivascular cells) may play during angiogenesis, vascular homeostasis, and pathology. Many recent excellent reviews thoroughly address these topics (see below); hence, we will not discuss them in detail here. However, not much is known about origin, heterogeneity, gene expression, and developmental potential of pericytes during fetal and postnatal development. This is likely because of the paucity of markers expressed by pericytes and the absence of truly unique ones. Thus, in vivo identification and ex perspective isolation are challenging and explain the relative little data available in comparison with neighbor but far more characterized cells such as the endothelium. Despite this preliminary knowledge, we will propose that contribution to growing mesoderm tissues may be an important role for pericytes. Thus, their ability to contribute to tissue regeneration may be a consequence of their role in tissue growth. However, in a severely damaged or diseased tissue, acute or chronic inflammation likely results in the production of signaling molecules that are different from those present in developing tissues, thus explaining why pericytes are easily diverted from a regenerative to a fibrotic fate.

Sarcomeric α-actinins and their role in human muscle disease

In skeletal muscle, the sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-line and crosslink actin thin filaments to maintain the structure of the sarcomere. Based on their known protein binding partners, the sarcomeric α-actinins are likely to have a number of structural, signaling and metabolic roles in skeletal muscle. In addition, the α-actinins interact with many proteins responsible for inherited muscle disorders. In this paper, we explore the role of the sarcomeric α-actinins in normal skeletal muscle and in the pathogenesis of a range of neuromuscular disorders.

Contractile smooth muscle cells derived from hair-follicle stem cells

AIMS

We hypothesized that hair-follicle stem cells can differentiate toward smooth contractile muscle cells, providing an autologous cell source for cardiovascular tissue regeneration.

METHODS AND RESULTS

Smooth muscle progenitor cells (SMPCs) were obtained from ovine hair follicles using a tissue-specific promoter and fluorescence-activated cell sorting. Hair-follicle smooth muscle progenitor cells (HF-SMPCs) expressed several markers of vascular smooth muscle including alpha-actin, calponin, myosin heavy chain (MHC), caldesmon, smoothelin, and SM22. HF-SMPCs were highly proliferative and showed high clonogenic potential without any signs of chromosomal abnormalities as evidenced by karyotype analysis. HF-SMPCs compacted fibrin hydrogels to a similar extent as vascular smooth muscle cells from ovine umbilical veins (V-SMCs), indicating the development of the force-generating machinery. In addition, cylindrical tissue equivalents prepared with HF-SMPCs displayed significant contractility in response to vasoactive agonists including KCl and the thromboxane A2 mimetic U46619, suggesting that these cells had developed receptor and non-receptor-mediated pathways of contractility. Finally, transforming growth factor-beta1 promoted differentiation of HF-SMPCs toward a mature SMC phenotype as suggested by increased expression of MHC and enhanced matrix compaction.

CONCLUSION

Our results suggest that hair follicles may be an easily accessible, autologous, and rich source of functional SMPC for cardiovascular tissue engineering and regenerative medicine.

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) binds to alpha-actinin 1: novel pathways in skeletal muscle?

Background

Hereditary inclusion body myopathy (HIBM) is a rare neuromuscular disorder caused by mutations in GNE, the key enzyme in the biosynthetic pathway of sialic acid. While the mechanism leading from GNE mutations to the HIBM phenotype is not yet understood, we searched for proteins potentially interacting with GNE, which could give some insights about novel putative biological functions of GNE in muscle.

Methodology/Principal Findings

We used a Surface Plasmon Resonance (SPR)-Biosensor based assay to search for potential GNE interactors in anion exchanged fractions of human skeletal muscle primary culture cell lysate. Analysis of the positive fractions by in vitro binding assay revealed α-actinin 1 as a potential interactor of GNE. The direct interaction of the two proteins was assessed in vitro by SPR-Biosensor based kinetics analysis and in a cellular environment by a co-immunoprecipitation assay in GNE overexpressing 293T cells. Furthermore, immunohistochemistry on stretched mouse muscle suggest that both GNE and α-actinin 1 localize to an overlapping but not identical region of the myofibrillar apparatus centered on the Z line.

Conclusions/Significance

The interaction of GNE with α-actinin 1 might point to its involvement in α-actinin mediated processes. In addition these studies illustrate for the first time the expression of the non-muscle form of α-actinin, α-actinin 1, in mature skeletal muscle tissue, opening novel avenues for its specific function in the sarcomere. Although no significant difference could be detected in the binding kinetics of α-actinin 1 with either wild type or mutant GNE in our SPR biosensor based analysis, further investigation is needed to determine whether and how the interaction of GNE with α-actinin 1 in skeletal muscle is relevant to the putative muscle-specific function of α-actinin 1, and to the muscle-restricted pathology of HIBM.

Myogenic stage, sarcomere length, and protease activity modulate localization of muscle-specific calpain

p94/calpain 3 is a Ca(2+)-binding intracellular protease predominantly expressed in skeletal muscles. p94 binds to the N2A and M-line regions of connectin/titin and localizes in the Z-bands. Genetic evidence showing that compromised p94 proteolytic activity leads to muscular dystrophy (limb-girdle muscular dystrophy type 2A) indicates the importance of p94 function in myofibrils. Here we show that a series of p94 splice variants is expressed immediately after muscle differentiation and differentially change localization during myofibrillogenesis. We found that the endogenous N-terminal (but not C-terminal) domain of p94 was not only localized in the Z-bands but also directly bound to sarcomeric alpha-actinin. These data suggest the incorporation of proteolytic N-terminal fragments of p94 into the Z-bands. In myofibrils localization of exogenously expressed p94 shifted from the M-line to N2A as the sarcomere lengthens beyond approximately 2.6 and 2.8 microm for wild-type and proteaseinactive p94, respectively. These data demonstrate for the first time that p94 proteolytic activity is involved in responses to muscle conditions, which may explain why p94 inactivation causes limb-girdle muscular dystrophy.

Myocyte differentiation generates nuclear invaginations traversed by myofibrils associating with sarcomeric protein mRNAs

Certain types of cell both in vivo and in vitro contain invaginated or convoluted nuclei. However, the mechanisms and functional significance of the deformation of the nuclear shape remain enigmatic. Recent studies have suggested that three types of cytoskeleton, microfilaments, microtubules and intermediate filaments, are involved in the formation of nuclear invaginations, depending upon cell type or conditions. Here, we show that undifferentiated mouse C2C12 skeletal muscle myoblasts had smoothsurfaced spherical or ellipsoidal nuclei, whereas prominent nuclear grooves and invaginations were formed in multinucleated myotubes during terminal differentiation. Conversion of mouse fibroblasts to myocytes by the transfection of MyoD also resulted in the formation of nuclear invaginations after differentiation. C2C12 cells prevented from differentiation did not have nuclear invaginations, but biochemically differentiated cells without cell fusion exhibited nuclear invaginations. Thus, biochemical differentiation is sufficient for the nuclear deformation. Although vimentin markedly decreased both in the biochemically and in the terminally differentiated cells, exogenous expression of vimentin in myotubes did not rescue nuclei from the deformation. On the other hand, non-striated premyofibrils consisting of sarcomeric actinmyosin filament bundles and cross-striated myofibrils traversed the grooves and invaginations. Time-lapse microscopy showed that the preformed myofibrillar structures cut horizontally into the nuclei. Prevention of myofibril formation retarded the generation of nuclear invaginations. These results indicate that the myofibrillar structures are, at least in part, responsible for the formation of nuclear grooves and invaginations in these myocytes. mRNA of sarcomeric proteins including myosin heavy chain and alpha-actin were frequently associated with the myofibrillar structures running along the nuclear grooves and invaginations. Consequently, the grooves and invaginations might function in efficient sarcomeric protein mRNA transport from the nucleus along the traversing myofibrillar structures for active myofibril formation.

Electrotransfer in differentiated myotubes: a novel, efficient procedure for functional gene transfer

Development of reliable techniques for experimental manipulation of gene expression in multinucleated skeletal muscle fibers is critical for understanding molecular mechanisms involved in both physiology and pathophysiology. At present, viral vectors represent the only method to obtain efficient gene transfer in terminally differentiated myotubes. Here we present an in vitro procedure that relies on the application of a pulsed electric field for transferring naked DNA into differentiated myotubes seeded on coverslips. Compared with standard transfection methods, electroporation was at least 1000 times more efficient, as judged by quantitative determination of luciferase content. Percentage of transfected myotubes averaged around 45%. Moreover, we were successful in transfecting a dominant-negative ADP ribosylation factor 1 (ARF1) mutant, i.e., ARF1N126I, in myotubes, thus interfering with endoplasmic reticulum-Golgi traffic, as indicated by alterations of subcellular distribution of GM130, a cis/medial-Golgi marker. Co-transfection experiments with beta-galactosidase also showed that the ARF1 mutant appeared to inhibit myoblast fusion and could not be used before myotube formation. The present work validates the use of electroporation as a highly efficient approach for gene transfer in fully differentiated myotubes.

Metalloprotease-disintegrin ADAM 12 interacts with alpha-actinin-1

ADAM 12, a member of the ADAM family of proteins (containing A Disintegrin And Metalloprotease domain), has been implicated in differentiation and fusion of myoblasts. While the extracellular domain of ADAM 12 contains an active metalloprotease and a region involved in cell adhesion, the function of the cytoplasmic tail of ADAM 12 has been less clear. Here we show that the cytoplasmic domain of ADAM 12 interacts in vitro and in vivo with alpha-actinin-1, an actin-binding and cross-linking protein. Green fluorescent protein fused to ADAM 12 cytoplasmic domain co-localizes with alpha-actinin-1-containing actin stress fibres in C2C12 cells. The interaction between ADAM 12 and alpha-actinin-1 is direct and involves the 58-amino acid C-terminal fragment of ADAM 12 and the 27 kDa N-terminal domain of alpha-actinin-1. Consistently, expression of the 27 kDa fragment of alpha-actinin-1 in C2C12 cells using a mitochondrial targeting system results in recruitment of the co-expressed ADAM 12 cytoplasmic domain to the mitochondrial surface. Moreover, alpha-actinin-1 co-purifies with a transmembrane, His6-tagged form of ADAM 12 expressed in C2C12 myoblasts, indicating that the transmembrane ADAM 12 forms a complex with alpha-actinin-1 in vivo. These results indicate that the actin cytoskeleton may play a critical role in ADAM 12-mediated cell-cell adhesion or cell signalling during myoblast differentiation and fusion.

Differentiation of mammalian skeletal muscle cells cultured on microcarrier beads in a rotating cell culture system

The growth and repair of adult skeletal muscle are due in part to activation of muscle precursor cells, commonly known as satellite cells or myoblasts. These cells are responsive to a variety of environmental cues, including mechanical stimuli. The overall goal of the research is to examine the role of mechanical signalling mechanisms in muscle growth and plasticity through utilisation of cell culture systems where other potential signalling pathways (i.e. chemical and electrical stimuli) are controlled. To explore the effects of decreased mechanical loading on muscle differentiation, mammalian myoblasts are cultured in a bioreactor (rotating cell culture system), a model that has been utilised to simulate microgravity. C2C12 murine myoblasts are cultured on microcarrier beads in a bioreactor and followed throughout differentiation as they form a network of multinucleated myotubes. In comparison with three-dimensional control cultures that consist of myoblasts cultured on microcarrier beads in teflon bags, myoblasts cultured in the bioreactor exhibit an attenuation in differentiation. This is demonstrated by reduced immunohistochemical staining for myogenin and alpha-actinin. Western analysis shows a decrease, in bioreactor cultures compared with control cultures, in levels of the contractile proteins myosin (47% decrease, p < 0.01) and tropomyosin (63% decrease, p < 0.01). Hydrodynamic measurements indicate that the decrease in differentiation may be due, at least in part, to fluid stresses acting on the myotubes. In addition, constraints on aggregate size imposed by the action of fluid forces in the bioreactor affect differentiation. These results may have implications for muscle growth and repair during spaceflight.

Na+ -transport modulation induces isoform-specific expression of Na+,K+ -Atpase alpha-subunit isoforms in C2C12 skeletal muscle cell

Changes in demands for Na+ transport alter expression of the Na+,K+ -ATPase subunit isoforms. In skeletal muscle, the effects of these changes on expression the alpha2 isoform, the major isoform expressed in differentiated muscle cell, is not known. Therefore, this study examines regulation of the alpha-subunit isoforms by Na+ in the C2C12 skeletal muscle cell that expresses the alpha1 and alpha2 isoforms. Western blot analysis showed that in differentiating C2C12 muscle cell, but not in undifferentiated myoblast, veratridine, a Na+ channel activator, greatly increased expression of the alpha2 isoform; expression of alpha1 was unaltered. Because the level of alpha-actinin was unaltered, the data suggest that veratridine treatment did not significantly alter the progression of cell differentiation. Furthermore, a reduction in Na+ transport by tetrodotoxin again failed to alter expression of alpha1. Thus, in C2C12 skeletal muscle cell, changes in Na+ transport alters expression of the alpha2, but not the alpha1 isoform. These results differ from those observed previously in muscle cells that express only the alpha1 isoform. Because mammalian skeletal muscle expresses both the alpha1- and alpha2-subunit isoforms, the differential regulation that was observed may be physiologically relevant in these muscle cells in vivo.

Interaction of hCLIM1, an enigma family protein, with alpha-actinin 2

Enigma proteins are proteins that possess a PDZ domain at the amino terminal and one to three LIM domains at the carboxyl terminal. They are cytoplasmic proteins that are involved with the cytoskeleton and signal transduction pathway. By virtue of the two protein interacting domains, they are capable of protein-protein interactions. Here we report a study on a human Enigma protein hCLIM1, in particular. Our study describes the interaction of the human 36 kDa carboxyl terminal LIM domain protein (hCLIM1), the human homologue of CLP36 in rat, with alpha-actinin 2, the skeletal muscle isoform of alpha-actinin. hCLIM1 protein was shown to interact with alpha-actinin 2 by yeast two-hybrid screening and immunochemical analyses. Yeast two-hybrid analyses also demonstrated that the LIM domain of hCLIM1 binds to the EF-hand region of alpha-actinin 2, defining a new mode of LIM domain interactions. Immunofluorescent study demonstrates that hCLIM1 colocalizes with alpha-actinin at the Z-disks in human myocardium. Taken together, our experimental results suggest that hCLIM1is a novel cytoskeletal protein and may act as an adapter that brings other proteins to the cytoskeleton.

Fast skeletal muscle isoforms exhibit the highest incorporation level into myofibrils and stress fibers among members of myosin alkali light chain isoform family

Isoproteins of myosin alkali light chain (LC) were co-expressed in cultured chicken cardiomyocytes and fibroblasts and their incorporation levels into myofibrils and stress fibers were compared among members of the LC isoform family. In order to distinguish each isoform from the other, cDNAs of LC isoforms were tagged with different epitopes. Expressed LCs were detected with antibodies to the tags and their distribution was analyzed by confocal microscopy. In cardiomyocytes, the incorporation level of LC into myofibrils was shown to increase in the order from nonmuscle isoform (LC3nm), to slow skeletal muscle isoform (LC1sa), to slow skeletal/ventricular muscle isoform (LC1sb), and to fast skeletal muscle isoforms (LC1f and LC3f). Thus, the hierarchal order of the LC affinity for the cardiac myosin heavy chain (MHC) is identical to that obtained in the rat (Komiyama et al., 1996. J. Cell Sci., 109: 2089-2099), suggesting that this order may be common for taxonomic animal classes. In fibroblasts, the affinity of LC for the nonmuscle MHC in stress fibers was found to increase in the order from LC3nm, to LC1sb, to LC1sa, and to LC1f and LC3f. This order for the nonmuscle MHC is partly different from that for the cardiac MHC. This indicates that the order of the affinity of LC isoproteins for MHC varies depending on the MHC isoform. Further, for both the cardiac and nonmuscle MHCs, the fast skeletal muscle LCs exhibited the highest affinity. This suggests that the fast skeletal muscle LCs may be evolved isoforms possessing the ability to associate tightly with a variety of MHC isoforms.

Isoforms of alpha-actinin from cardiac, smooth, and skeletal muscle form polar arrays of actin filaments

We have used a positively charged lipid monolayer to form two-dimensional bundles of F-actin cross-linked by alpha-actinin to investigate the relative orientation of the actin filaments within them. This method prevents growth of the bundles perpendicular to the monolayer plane, thereby facilitating interpretation of the electron micrographs. Using alpha-actinin isoforms isolated from the three types of vertebrate muscle, i.e., cardiac, skeletal, and smooth, we have observed almost exclusively cross-linking between polar arrays of filaments, i.e., actin filaments with their plus ends oriented in the same direction. One type of bundle can be classified as an Archimedian spiral consisting of a single actin filament that spirals inward as the filament grows and the bundle is formed. These spirals have a consistent hand and grow to a limiting internal diameter of 0.4-0.7 microm, where the filaments appear to break and spiral formation ceases. These results, using isoforms usually characterized as cross-linkers of bipolar actin filament bundles, suggest that alpha-actinin is capable of cross-linking actin filaments in any orientation. Formation of specifically bipolar or polar filament arrays cross-linked by alpha-actinin may require additional factors that either determine the filament orientation or restrict the cross-linking capabilities of alpha-actinin.

CLP-36 PDZ-LIM protein associates with nonmuscle alpha-actinin-1 and alpha-actinin-4

The PDZ-LIM family of proteins (Enigma/LMP-1, ENH, ZASP/Cypher, RIL, ALP, and CLP-36) has been suggested to act as adapters that direct LIM-binding proteins to the cytoskeleton. Most interactions of PDZ-LIM proteins with the cytoskeleton have been identified in striated muscle, where several PDZ-LIM proteins are predominantly expressed. By contrast, CLP-36 mRNA is expressed in several nonmuscle tissues, and here we demonstrate high expression of CLP-36 in epithelial cells by in situ hybridization analysis. Our subcellular localization studies indicate that in nonmuscle cells, CLP-36 protein localizes to actin stress fibers. This localization is mediated via the PDZ domain of CLP-36 that associates with the spectrin-like repeats of alpha-actinin. Interestingly, immunoprecipitation and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis indicate that both nonmuscle alpha-actinin-1 and alpha-actinin-4 form complexes with CLP-36. The high expression of alpha-actinin-4 in the colon, together with these results, suggests a specific function for the alpha-actinin-4-CLP-36 complex in the colonic epithelium. More generally, results presented here demonstrate that the association of PDZ-LIM proteins with the cytoskeleton extends to the actin stress fibers of nonmuscle cells.

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.

Plectin is a linker of intermediate filaments to Z-discs in skeletal muscle fibers

Plectin is a versatile linker protein which is associated with various types of cytoskeletal components and/or filaments including intermediate filaments, and its deficiency causes the disruption of myofibrils, or muscular dystrophy. To better understand the functional role of plectin in skeletal muscle fibers, we have examined the topological and structural relationships of plectin to intermediate filaments and Z-discs in rat diaphragm muscles by confocal and immunoelectron microscopy. Immunofluorescence analysis revealed that plectin was colocalized with desmin at the periphery of Z-discs. This plectin localization around Z-discs was constantly maintained irrespective of the contracted or extended state of the muscle fibers, suggesting either direct or indirect association of plectin with Z-discs. Immunogold labeling in skinned muscle fibers clearly demonstrated that plectin-labeled fine threads linked desmin intermediate filaments to Z-discs and connected intermediate filaments to each other. These results indicate that through plectin threads desmin intermediate filaments form lateral linkages among adjacent Z-discs, preventing individual myofibrils from disruptive contraction and ensuring effective force generation.

Alpha-actinin in different invertebrate muscle cell types of Drosophila melanogaster, the earthworm Eisenia foetida, and the snail Helix aspersa

The presence and distribution of alpha-actinin has been studied in several invertebrate muscle cell types. These comprised transversely striated muscle (flight muscle) from the fruit fly Drosophila melanogaster, transversely striated muscle (heart muscle) from the snail Helix aspersa, obliquely striated muscle (body wall muscle) from the earthworm Eisenia foetida, smooth muscle (retractor muscle) from H. aspersa, and smooth muscle (outer muscular layer of the pseudoheart) from E. foetida. The study was carried by means of Western blot analysis, ELISA, and immunohistochemical electron microscopy, using anti alpha-actinin antibody. Immunoreaction for a protein with the same molecular weight as that of mammalian alpha-actinin was detected in all muscle types studied, although the amount and intensity of immunoreaction varied among them. In the insect muscle, immunolabelling was found along the whole Z-line. In both the transversely striated muscle from the snail and the obliquely striated muscle from the earthworm, immunolabelling did not occupy the whole Z-line but showed discontinuous, orderly arranged patches along the Z-line course. In the two smooth muscles studied (snail and earthworm), immunolabelling was limited to small patches which did not show an apparently ordered distribution. Since it is assumed that alpha-actinin is located at the anchorage sites for actin filaments, present observations suggest that, only in the Drosophila muscle, actin filaments are parallelly arranged in all their course, whereas in the other invertebrate muscles studied these filaments converge on discontinuously distributed anchorage sites.

Human skeletal muscle-specific alpha-actinin-2 and -3 isoforms form homodimers and heterodimers in vitro and in vivo

Alpha-actinins belong to a family of actin-binding and crosslinking proteins and are expressed in many different cell types. Multiple isoforms of alpha-actinin are found in humans and are encoded by at least four distinct genes. Human skeletal muscle contains two sarcomeric isoforms, alpha-actinin-2 and -3. Previous studies have shown that the alpha-actinins function as anti-parallel homodimers but the question of heterodimer formation between two different isoforms expressed in the same cell type has not been explored. To address this issue, we expressed both alpha-actinin-2 and -3 in vitro and were able to detect their interaction by both blot overlay and co-immunoprecipitation methods. We were also able to demonstrate the presence of heterodimers in vivo in human skeletal muscle and in COS-1 cells transiently transfected with both isoforms. Our results clearly demonstrate the potential for alpha-actinin isoforms to form heterodimers which might have unique functional characteristics.

Identification of a novel marker for primordial smooth muscle and its differential expression pattern in contractile vs noncontractile cells

The assembly of the vessel wall from its cellular and extracellular matrix components is an essential event in embryogenesis. Recently, we used the descending aorta of the embryonic quail to define the morphological events that initiate the formation of a multilayered vessel wall from a nascent endothelial cell tube (Hungerford, J.E., G.K. Owens, W.S. Argraves, and C.D. Little. 1996. Dev. Biol. 178:375-392). We generated an mAb, 1E12, that specifically labels smooth muscle cells from the early stages of development to adulthood. The goal of our present study was to characterize further the 1E12 antigen using both cytological and biochemical methods. The 1E12 antigen colocalizes with the actin cytoskeleton in smooth muscle cells grown on planar substrates in vitro; in contrast, embryonic vascular smooth muscle cells in situ contain 1E12 antigen that is distributed in threadlike filaments and in cytoplasmic rosette-like patterns. Initial biochemical analysis shows that the 1E12 mAb recognizes a protein, Mr = 100,000, in lysates of adult avian gizzard. An additional polypeptide band, Mr = 40,000, is also recognized in preparations of lysate, when stronger extraction conditions are used. We have identified the 100-kD polypeptide as smooth muscle alpha-actinin by tandem mass spectroscopy analysis. The 1E12 antibody is an IgM isotype. To prepare a more convenient 1E12 immunoreagent, we constructed a single chain antibody (sFv) using recombinant protein technology. The sFv recognizes a single 100-kD protein in gizzard lysates. Additionally, the recombinant antibody recognizes purified smooth muscle alpha-actinin. Our results suggest that the 1E12 antigen is a member of the alpha-actinin family of cytoskeletal proteins; furthermore, the onset of its expression defines a primordial cell restricted to the smooth muscle lineage.

The intracompartmental sorting of myosin alkali light chain isoproteins reflects the sequence of developmental expression as determined by double epitope-tagging competition

In order to compare within the same cell the various degrees of specificity of myosin alkali light chain (MLC) isoproteins sorting to sarcomeres, a competition assay was established using double epitope tagging. Various combinations of two different MLC isoform cDNAs tagged with either a vesicular stomatitis virus VSV-G (VSV) or a medium T (mT) protein epitope were co-expressed in cultured cardiomyocytes from adult and neonatal rat ventricles. Expressed isoproteins were detected by means of anti-VSV and anti-mT antibodies and their sorting patterns were analyzed by confocal microscopy. The sorting specificity of MLC isoforms to sarcomeric sites was shown to increase in the order MLC3nm, to ML1sa, to MLC1sb, to MLC1f and MLC3f following the sequence of developmental expression. Expressed fast skeletal muscle isoforms (MLC1f and MLC3f) were always localized at the A-bands of myofibrils, while nonmuscle type (MLC3nm) was distributed throughout the cytoplasm. The slow skeletal muscle type (MLC1sa) showed a weak sarcomeric pattern if it was co-expressed with MLC3nm, but it was distributed throughout the cytoplasm when expressed in combination with MLC1f, MLC3f or the slow skeletal/ventricular muscle isoform (MLC1sb). The MLC1sb was localized at the A-bands when it was co-expressed with MLC3nm or MLC1sa, while it was also distributed to the cytoplasm if co-expressed with MLC1f or MLC3f. Further, expression of chimeric cDNAs revealed that the N-terminal lobe of each isoprotein is responsible for the isoform-specific sorting pattern.

Involvement of transglutaminase in myofibril assembly of chick embryonic myoblasts in culture

Involvement of transglutaminase in myofibrillogenesis of chick embryonic myoblasts has been investigated in vitro. Both the activity and protein level of transglutaminase initially decreased to a minimal level at the time of burst of myoblast fusion but gradually increased thereafter. The localization of transglutaminase underwent a dramatic change from the whole cytoplasm in a diffuse pattern to the cross-striated sarcomeric A band, being strictly colocalized with the myosin thick filaments. For a brief period prior to the appearance of cross-striation, transglutaminase was localized in nonstriated filamental structures that coincided with the stress fiber-like structures. When 12-o-tetradecanoyl phorbol acetate was added to muscle cell cultures to induce the sequential disassembly of thin and thick filaments, transglutaminase was strictly colocalized with the myosin thick filaments even in the myosacs, of which most of the thin filaments were disrupted. Moreover, monodansylcadaverine, a competitive inhibitor of transglutaminase, reversibly inhibited the myofibril maturation. In addition, myosin heavy chain behaved as one of the potential intracellular substrates for transglutaminase. The cross-linked myosin complex constituted approximately 5% of the total Triton X-100-insoluble pool of myosin molecules in developing muscle cells, and its level was reduced to below 1% upon treatment with monodansylcadaverine. These results suggest that transglutaminase plays a crucial role in myofibrillogenesis of developing chick skeletal muscle.

Molecular cloning of low-Ca(2+)-sensitive-type non-muscle alpha-actinin

We previously reported the purification and characterization of a novel non-muscle alpha-actinin from chicken lung [Imamura, M. & Masaki, T, (1992) J. Biol. Chem. 267, 25927-25933]. The Ca2+ sensitivity of the lung alpha-actinin for the interaction with polymerized actin (F-actin) was much lower than those of the other reported non-muscle alpha-actinins. Here, we isolated a cDNA clone encoding the novel alpha-actinin by screening a chicken lung lambda g11 cDNA library with antibody specific for the low-Ca(2+)-sensitive alpha-actinin. The deduced amino acid sequence of the lung alpha-actinin showed 76%, 82% and 83% identity to those of chicken skeletal muscle, smooth-muscle and fibroblast-type alpha-actinin, respectively. Marked difference in the structure between the lung-type and the other alpha-actinins was found in the extreme NH2-terminal and in the COOH-terminal half; in the third and fourth regions of four spectrin-like repeats, and in two Ca(2+)-binding EF-hand consensus regions. The NH2-terminal-side EF-hand contained a notable defect in one of the five oxygen-containing amino acid side chains involved in chelating Ca2+, suggesting that the lower Ca2+ sensitivity of the lung alpha-actinin is ascribable to this defect. Northern blot analysis showed that the expression pattern of lung-type alpha-actinin mRNA in various non-muscle tissues differed from that of the other known non-muscle-type (fibroblast-type) alpha-actinin. The present results clearly demonstrate the existence of two structurally and functionally different types of non-muscle alpha-actinin; high-Ca(2+)-sensitive-type (NM1) and low-Ca(2+)-sensitive-type (NM2) alpha-actinin.

The muscle Z band: lessons in stress management

Two of the most characteristic features of striated muscle are (i) its ability to contract and generate tension when activated and (ii) its ability to return to its original length and form after contraction or stretching ceases. These two properties are to a large extent the primary manifestations of separate sets of filament systems: contractile actin and myosin filaments and viscoelastic titin and intermediate filaments. Z bands function as a common link that mechanically integrates contractile and elastic elements and as such they play a fundamental role in transmission of active and passive forces. Differences in Z band structure have been described for distinct classes of muscle and fibre types. The diversity in Z band architecture has been built around its phylogenetically conserved role as an actin-anchoring structure. Novel proteins are likely to account for structural and functional differences seen across the phyla.

Switching of the dominant calcium sequestering protein during skeletal muscle differentiation

A major Ca(2+)-storing protein in endoplasmic reticulum (ER) of non-muscle cells is calreticulin (CR), which is considered to be functionally homologous to calsequestrin. Calsequestrin is a Ca(2+)-binding protein in sarcoplasmic reticulum (SR) of striated muscle, which stores Ca2+ during muscle relaxation. In order to investigate the expression and distribution of calsequestrin and calreticulin during skeletal muscle differentiation, cultured chick embryonic skeletal muscles were observed by immunofluorescence using anti-calsequestrin, anti-calreticulin, anti-desmin, and anti-sarcomeric myosin antibodies and rhodamine-phalloidin. Within 6 hours in culture, myoblasts started to express desmin. Desmin-positive cells demonstrated the reticular staining of calreticulin, as did desmin-negative cells. Around fusion, calsequestrin and sarcomeric myosin started to appear in desmin-positive cells. The expression of calsequestrin slightly preceded that of sarcomeric myosin. As the myotubes matured, the fluorescent dots of calsequestrin increased and spread to the cell periphery along the myofibrils, while the reticular pattern of calreticulin gradually disappeared. Double labeling showed that calsequestrin colocalized with calreticulin. In mature myotubes, anti-calsequestrin staining demonstrated many dots along myofibrils, whereas calreticulin was barely seen except at the perinuclear region. These results suggest that the expression of calsequestrin and calreticulin are switched during skeletal muscle differentiation.

Distribution of connectin (titin), nebulin and alpha-actinin at myotendinous junctions of chicken pectoralis muscles: an immunofluorescence and immunoelectron microscopic study

The distribution of connectin (titin), nebulin and alpha-actinin in the areas of myotendinous junctions of chicken pectoralis muscles was examined by immunocytochemical methods. Staining with antibodies against connectin (4C9, SM1 and P1200) and nebulin formed 'doublets' flanking nonterminal Z-bands; near the end of muscle fibres 'singlets' were seen within the terminal sarcomere on the side adjacent to the terminal Z-bands. The apical regions of muscle processes, where no myosin filaments are present although actin filaments exist, were reactive with anti-nebulin but not with anti-connectin. Antibodies against pectoralis (skeletal muscle type) alpha-actinin stained non terminal Z-bands and that against gizzard (smooth muscle type) the sarcolemma. Terminal Z-bands were unreactive with both of these antibodies. These findings indicate that, although terminal and nonterminal Z-bands differ in their molecular composition, connectin and nebulin filaments appear to link myosin and actin filaments, respectively, to both Z-band types.

Dynamics of actin and assembly of connectin (titin) during myofibrillogenesis in embryonic chick cardiac muscle cells in vitro

Immunogold electron microscopy of cardiac myocytes microinjected with biotin-labeled actin showed that gold labeling was first found around the A band level of myofibrils at their proximal parts. This observation suggests that polymerization of actin and/or the addition of newly formed actin filaments occurs preferentially in association with myosin filaments to increase the myofibrillar girth. At the distal portions of developing myofibrils, their terminal ends were initially labeled, suggesting that continued reorganization and/or de novo formation of myofibrils occurs at these locations. Soon, gold particles were seen along the termini of growing myofibrils. This appears to indicate that actin subunits are added at the membrane-associated ends of preexisting actin filaments to increase the length of myofibrils. Adhesion plaque proteins, e.g., vinculin, do not appear to play any role in assembling actin monomers at these sites on the inner surface of the sarcolemma. Immunofluorescence and immunoelectron microscopy of cardiomyocytes double-stained with antibodies against two distant domains of connectin (titin) filaments and other sarcomeric proteins showed that these domains of connectin filaments and myosin were synthesized almost simultaneously on large polyribosomes and/or associated immediately after the synthesis of these molecules. Connectin and myosin bands were formed after alpha-actinin striations (Z bands) were seen on preformed I-Z-I-like structures.(ABSTRACT TRUNCATED AT 250 WORDS)

A chick skeletal-muscle alpha-actinin gene gives rise to two alternatively spliced isoforms which differ in the EF-hand Ca(2+)-binding domain

A chick non-muscle alpha-actinin cDNA probe encoding the EF-hand region of molecule was used to screen a lambda gt10 chick brain cDNA library from 14-day embryos. A partial 2.1-kb alpha-actinin cDNA was isolated (8W cDNA) which encoded a protein identical to chick skeletal-muscle alpha-actinin, except in the C-terminal part of the first EF hand. In the variant, the 22 residues found in the skeletal-muscle isoform were replaced by a stretch of 26 unique residues. Analysis of the structure of the skeletal-muscle alpha-actinin gene showed that the region of divergence was encoded by two exons which are alternatively spliced. Quantitative reverse transcriptase/polymerase chain reaction (RT/PCR) was used to investigate the levels of the alpha-actinin transcripts in various tissues. The skeletal-muscle alpha-actinin variant was expressed at low levels in brain, liver and spleen, but could not be detected in skeletal muscle. Surprisingly, skeletal-muscle alpha-actinin mRNA was also expressed in brain, liver and spleen. The RT/PCR products were authenticated by using diagnostic restriction enzyme sites and by sequencing. The splice variant derived from the skeletal-muscle alpha-actinin gene was also detected in a variety of cDNA libraries from both adult and embryonic tissues by PCR. Although a transcript encoding this alpha-actinin splice variant is expressed in non-muscle tissues, neither of the two EF-hands would be predicted to be functional, making it unlikely to be a typical non-muscle isoform which are calcium-sensitive with respect to binding actin. The two vertebrate non-muscle alpha-actinins sequenced to date also have a spacer of five amino acids between the two EF hands, whereas in the variant, the spacer is just four residues in length. Further analysis will be required before this alpha-actinin isoform, which we refer to as SKv, can be classified as muscle or non-muscle alpha-actinin. We propose a new nomenclature to describe the various alpha-actinin genes and their transcripts.

A sarcomeric alpha-actinin truncated at the carboxyl end induces the breakdown of stress fibers in PtK2 cells and the formation of nemaline-like bodies and breakdown of myofibrils in myotubes

In many nonmuscle cells, nonsarcomeric alpha-actinin is distributed in the dense bodies of stress fibers, adhesion plaques, and adherens junctions. In striated muscle, a sarcomeric isoform of alpha-actinin (s-alpha-actinin) is found in the Z-bands of myofibrils and subsarcolemmal adhesion plaques. To understand the role(s) of the alpha-actinin isoforms in the assembly and maintenance of such cytoskeletal structures, full-length or truncated s-alpha-actinin cDNAs were expressed in PtK2 cells and in primary skeletal myogenic cells. We found the following. (i) In transfected PtK2 cells the truncated s-alpha-actinin was rapidly incorporated into preexisting dense bodies, adhesion plaques, and adherens junctions. With time these structures collapsed, and the affected cells detached from the substrate. (ii) In myotubes the truncated s-alpha-actinin was incorporated into nascent Z-bands. Many of these progressively hypertrophied, forming nemaline-like bodies. With time the affected myofibrils fragmented, and the myotubes detached from the substrate. (iii) In both cell types the truncated s-alpha-actinin was significantly more disruptive of the cytoskeletal structures than the full-length molecule. (iv) Pools of "over-expressed" full-length or truncated protein did not self-aggregate into homogeneous, amorphous complexes; rather the exogenous proteins selectively colocalized with the same cohort of cytoskeletal proteins with which the endogenous alpha-actinin normally associates. The similarity among the hypertrophied Z-bands in transfected myotubes, the nemaline bodies in patients with nemaline myopathies, and the streaming Z-bands seen in various muscle pathologies raises the possibility that the genetically determined nemaline bodies and the pathologically induced Z-band alterations may reflect primary and/or post-translational modifications of s-alpha-actinin.

The alpha 5 beta 1 integrin associates with a dystrophin-containing lattice during muscle development

The organization of the alpha 5 beta 1 integrin on skeletal muscle was studied in culture and in sections from adult and embryonic tissue using monoclonal antibodies specific for the alpha 5 subunit. The alpha 5 beta 1 integrin showed changes in organization and in the molecules with which it colocalizes. On early myoblasts, possessing a fibroblast-like morphology, the alpha 5 integrin organization was indistinguishable from that on fibroblasts; it was expressed prominently and localized in numerous focal contacts around the cell periphery. In bipolar myoblasts and early myotubes, the alpha 5 integrin was expressed only weakly and localized in a small number of focal contact-like structures. As myogenesis proceeded there was an apparent increase in integrin expression and a change in organization. In addition to the focal contact-like structures that persist throughout myogenesis in vitro, a dense lattice-like structure of integrin appeared. Fibrillar fibronectin, talin, and non-muscle alpha-actinin did not colocalize with the alpha 5 beta 1 integrin in the lattice structure as they did in the focal contact-like structures. However, dystrophin, which displayed a diffuse distribution earlier, now colocalized with the alpha 5 beta 1 integrin in the punctate lattice. Coincident with the registration of myofibrils into visible sarcomeres, the prominent dense, lattice structure disappeared leaving the focal contact-like structures as the only regions of organized alpha 5 beta 1 integrin. Despite the presence of the beta 1 integrin in neuromuscular or myotendinous junctions in vivo and on myotubes in vitro, the alpha 5 beta 1 integrin was not present in either junction. These observations suggest that the alpha 5 beta 1 integrin is involved in the adhesion of muscle to the extracellular matrix, the organization of the dystrophin-containing lattice, and the organization of nascent myofibrils which emanate from the focal contact- and stress fiber-like structures in muscle. Other integrins appear to anchor myofibrils at the myotendinous and neuromuscular junctions.

The vinculin/sarcomeric-alpha-actinin/alpha-actin nexus in cultured cardiac myocytes

Experiments are described supporting the proposition that the assembly of stress fibers in non-muscle cells and the assembly of myofibrils in cardiac cells share conserved mechanisms. Double staining with a battery of labeled antibodies against membrane-associated proteins, myofibrillar proteins, and stress fiber proteins reveals the following: (a) dissociated, cultured cardiac myocytes reconstitute intercalated discs consisting of adherens junctions (AJs) and desmosomes at sites of cell-cell contact and sub-sarcolemmal adhesion plaques (SAPs) at sites of cell-substrate contact; (b) each AJ or SAP associates proximally with a striated myofibril, and conversely every striated myofibril is capped at either end by an AJ or a SAP; (C) the invariant association between a given myofibril and its SAP is especially prominent at the earliest stages of myofibrillogenesis; nascent myofibrils are capped by oppositely oriented SAPs; (d) the insertion of nascent myofibrils into AJs or into SAPs invariably involves vinculin, alpha-actin, and sarcomeric alpha-actinin (s-alpha-actinin); (e) AJs are positive for A-CAM but negative for talin and integrin; SAPs lack A-CAM but are positive for talin and integrin; (f) in cardiac cells all alpha-actinin-containing structures invariably are positive for the sarcomeric isoform, alpha-actin and related sarcomeric proteins; they lack non-s-alpha-actinin, gamma-actin, and caldesmon; (g) in fibroblasts all alpha-actinin-containing structures are positive for the non-sarcomeric isoform, gamma-actin, and related non-sarcomeric proteins, including caldesmon; and (h) myocytes differ from all other types of adherent cultured cells in that they do not assemble authentic stress fibers; instead they assemble stress fiber-like structures of linearly aligned I-Z-I-like complexes consisting exclusively of sarcomeric proteins.

Transfection of chicken skeletal muscle alpha-actinin cDNA into nonmuscle and myogenic cells: dimerization is not essential for alpha-actinin to bind to microfilaments

alpha-Actinins from striated muscle, smooth muscle, and nonmuscle cells are distinctive in their primary structure and Ca2+ sensitivity for the binding to F-actin. We isolated alpha-actinin cDNA clones from a cDNA library constructed from poly(A)+ RNA of embryonic chicken skeletal muscle. The amino acid sequence deduced from the nucleotide sequence of these cDNAs was identical to that of adult chicken skeletal muscle alpha-actinin. To examine whether the differences in the structure and Ca2+ sensitivity of alpha-actinin molecules from various tissues are responsible for their tissue-specific localization, the cDNA cloned into a mammarian expression vector was transfected into cell lines of mouse fibroblasts and skeletal muscle myoblasts. Immunofluorescence microscopy located the exogenous alpha-actinin by use of an antibody specific for skeletal muscle alpha-actinin. When the protein was expressed at moderate levels, it coexisted with endogenous alpha-actinin in microfilament bundles in the fibroblasts or myoblasts and in Z-bands of sarcomeres in the myotubes. These results indicate that Ca2+ sensitivity or insensitivity of the molecules does not determine the tissue-specific localization. In the cells expressing high levels of the exogenous protein, however, the protein was diffusely present and few microfilament bundles were found. Transfection with cDNAs deleted in their 3' portions showed that the expressed truncated proteins, which contained the actin-binding domain but lacked the domain responsible for dimerization, were able to localize, though less efficiently in microfilament bundles. Thus, dimer formation is not essential for alpha-actinin molecules to bind to microfilaments.

Intracompartmental sorting of essential myosin light chains: molecular dissection and in vivo monitoring by epitope tagging

The isoprotein-specific intracompartmental sorting of the three essential myosin light chains (LCs), the skeletal muscle LC-1f and LC-3f and the nonmuscle LC-3nm, was investigated. Epitope tagging was used to monitor the intracellular localization to different cytoskeletal structures of the exogenously introduced constructs in adult rat cardiomyocytes (ARCs), which exhibit both stress fibers and regenerating myofibrils. LC-1f and LC-3f bind almost exclusively to the sarcomeric myosin heavy chain (MHC) with high affinity, while the LC-3nm interacts with stress fibers and sarcomeres equally well. Sorting appears to be directed by a hierarchical order of different affinities. Domain mapping by deletion and by construction of a LC-1f/3nm chimera suggests that the LCs are composed of three functionally distinct domains: a basal MHC binding site in the C-terminus; the central part, modulating the preferential interaction with MHC isoforms; and the isoprotein-specific N-terminus of the essential LC, which is probably not involved in the sorting process.

Interfiber tension transmission in series-fibered muscles of the cat hindlimb

Several muscles of the cat hindlimb, including biceps femoris and tenuissimus, are composed of short, in-series muscle fibers with tapered intrafascicular terminations. Tension generation and transmission within such muscles requires that active fibers should be mechanically coupled in series via myomyous junctions, specialized connective tissue attachments, or the endomysium. This report establishes that the tapered fibers of the cat biceps femoris and tenuissimus muscles have insignificant numbers of either myomyous or specialized connective tissue junctions. Tension appears to be transmitted in a distributed manner across the plasmalemma of the tapered (and probably the non-tapered) portions of the fibers to the connective tissue of the endomysium, which is therefore an essential series elastic element in these muscles. Subplasmalemmal dense plaques were identified and may play a role in transmembrane force transmission. In addition to the endomysium, passive muscle fibers may also serve to transmit tension between active fibers, and therefore should also be considered to be series elastic elements.

Assembly of connectin (titin) in relation to myosin and alpha-actinin in cultured cardiac myocytes

By using polyclonal and monoclonal antibodies against connectin (titin) which stain the A-I junctional area and the A-band domain (polyclonal anti-connectin and monoclonal 4C9) and the I-band domain (monoclonal SM1), the developmental relationship of this elastic protein with sarcomeric proteins, especially and alpha-actin, was examined in embryonic chick cardiac myocytes in vitro under fluorescence microscopy. During premyofibril stages, I-Z-I proteins were detected first (alpha-actinin dots and diffuse actin [phalloidin and anti-troponin C] staining), and later in these areas connectin and myosin dots appeared with nearly identical distribution. Somewhat later, phalloidin-positive nonstriated fibrils were observed in a straight course. They were always reactive with antibodies against alpha-actinin and troponin C, but unreactive or only weakly reactive with anticonnectin and anti-myosin. Initially, alpha-actinin dots were aligned along these fibrils but did not form striations. As they aggregated to form Z-bands, connectin and myosin started to exhibit typical striation ('doublets' and A-bands, respectively). No difference in the staining pattern was observed with two kinds of monoclonal antibodies against different domains of connectin filaments (4C9 and SM1) at early phases. As myosin staining began to show clear A-bands, connectin epitopes became arranged in polarized positions. We conclude that primitive I-Z-I complexes appear prior to the assembly of connectin and myosin filaments and then connectin filaments, developing intimately and coordinately with myosin, become associated with the alpha-actinin lines. Thus it appears that the putative elastic protein connectin plays some role in integrating myosin filaments with the preexisting I-Z-I brushes. The occasional absence of connectin and A-bands between two Z-bands, beyond both of which clear sarcomeres have been formed, indicates that connectin is not a preformed scaffold of myofibrils on which sarcomeric proteins accumulate.

Differential distribution of subsets of myofibrillar proteins in cardiac nonstriated and striated myofibrils

Cultured cardiac myocytes were stained with antibodies to sarcomeric alpha-actinin, troponin-I, alpha-actin, myosin heavy chain (MHC), titin, myomesin, C-protein, and vinculin. Attention was focused on the distribution of these proteins with respect to nonstriated myofibrils (NSMFs) and striated myofibrils (SMFs). In NSMFs, alpha-actinin is found as longitudinally aligned, irregular approximately 0.3-microns aggregates. Such aggregates are associated with alpha-actin, troponin-I, and titin. These I-Z-I-like complexes are also found as ectopic patches outside the domain of myofibrils in close apposition to the ventral surface of the cell. MHC is found outside of SMFs in the form of discrete fibrils. The temporal-spatial distribution and accumulation of the MHC-fibrils with respect to the I-Z-I-like complexes varies greatly along the length of the NSMFs. There are numerous instances of I-Z-I-like complexes without associated MHC-fibrils, and also cases of MHC-fibrils located many microns from I-Z-I-like complexes. The transition between the terminal approximately 1.7-microns sarcomere of any given SMF and its distal NSMF-tip is abrupt and is marked by a characteristic narrow alpha-actinin Z-band and vinculin positive adhesion plaque. A titin antibody T20, which localizes to an epitope at the Z-band in SMFs, precisely costains the 0.3-microns alpha-actinin aggregates in ectopic patches and NSMFs. Another titin antibody T1, which in SMFs localizes to an epitope at the A-I junction, typically does not stain ectopic patches and NSMFs. Where detectable, the T1-positive material is adjacent to rather than part of the 0.3-microns alpha-actinin aggregates. Myomesin and C-protein are found only in their characteristic sarcomeric locations (even in just perceptible SMFs). These A-band-associated proteins appear to be absent in ectopic patches and NSMFs.

Polygons and adhesion plaques and the disassembly and assembly of myofibrils in cardiac myocytes

Successive stages in the disassembly of myofibrils and the subsequent assembly of new myofibrils have been studied in cultures of dissociated chick cardiac myocytes. The myofibrils in trypsinized and dispersed myocytes are sequentially disassembled during the first 3 d of culture. They split longitudinally and then assemble into transitory polygons. Multiples of single sarcomeres, the cardiac polygons, are analogous to the transitory polygonal configurations assumed by stress fibers in spreading fibroblasts. They differ from their counterparts in fibroblasts in that they consist of muscle alpha-actinin vertices and muscle myosin heavy chain struts, rather than of the nonmuscle contractile protein isoforms of stress fiber polygons. EM sections reveal the vertices and struts in cardiac polygons to be typical Z and A bands. Most cardiac polygons are eliminated by day 5 of culture. Concurrent with the disassembly and elimination of the original myofibrils new myofibrils are rapidly assembled elsewhere in the same myocyte. Without exception both distal tips of each nascent myofibril terminate in adhesion plaques. The morphology and composition of the adhesion plaques capping each end of each myofibril are similar to those of the termini of stress fibers in fibroblasts. However, whereas the adhesion complexes involving stress fibers in fibroblasts consist of vinculin/nonmuscle alpha-actinin/beta- and gamma-actins, the analogous structures in myocytes involving myofibrils consist of vinculin/muscle alpha-actinin/alpha-actin. The addition of 1.7-2.0 microns sarcomeres to the distal tips of an elongating myofibril, irrespective of whether the myofibril consists of 1, 10, or several hundred tandem sarcomeres, occurs while the myofibril appears to remain linked to its respective adhesion plaques. The adhesion plaques in vitro are the equivalent of the in vivo intercalated discs, both in terms of their molecular composition and with respect to their functioning as initiating sites for the assembly of new sarcomeres. How 1.7-2.0 microns nascent sarcomeres can be added distally during elongation while the tips of the myofibrils remain inserted into submembranous adhesion plaques is unknown.

Differential response of myofibrillar and cytoskeletal proteins in cells treated with phorbol myristate acetate

Muscle-specific and nonmuscle contractile protein isoforms responded in opposite ways to 12-o-tetradecanoyl phorbol-13-acetate (TPA). Loss of Z band density was observed in day-4-5 cultured chick myotubes after 2 h in the phorbol ester, TPA. By 5-10 h, most I-Z-I complexes were selectively deleted from the myofibril, although the A bands remained intact and longitudinally aligned. The deletion of I-Z-I complexes was inversely related to the appearance of numerous cortical, alpha-actinin containing bodies (CABs), transitory structures approximately 3.0 microns in diameter. Each CAB consisted of a filamentous core that costained with antibodies to alpha-actin and sarcomeric alpha-actinin. In turn each CAB was encaged by a discontinuous rim that costained with antibodies to vinculin and talin. Vimentin and desmin intermediate filaments and most cell organelles were excluded from the membrane-free CABs. These curious bodies disappeared over the next 10 h so that in 30-h myosacs all alpha-actin and sarcomeric alpha-actinin structures had been eliminated. On the other hand vinculin and talin adhesion plaques remained prominent even in 72-h myosacs. Disruption of the A bands was first initiated after 15-20 h in TPA (e.g., 15-20-h myosacs). Thick filaments of apparently normal length and structure were progressively released from A segments, and by 40 h all A bands had been broken down into enormous numbers of randomly dispersed, but still intact single thick filaments. This breakdown correlated with the formation of amorphous cytoplasmic aggregates which invariably colocalized antibodies to myosin heavy chain, MLC 1-3, myomesin, and C protein. Complete elimination of all immunoreactive thick filament proteins required 60-72 h of TPA exposure. The elimination of the thick filament-associated proteins did not involve the participation of vinculin or talin. In contrast to its effects on myofibrils, TPA did not induce the disassembly of the contractile proteins in stress fibers and microfilaments either in myosacs or in fibroblastic cells. Similarly, TPA, which rapidly induces the translocation of vinculin and talin to ectopic sites in many types of immortalized cells, had no gross effect on the adhesion plaques of myosacs, primary fibroblastic cells, or presumptive myoblasts. Clearly, the response to TPA of contractile protein and some cytoskeletal isoforms not only varies among phenotypes, but even within the domains of a given myotube the myofibrils respond one way, the stress fibers/microfilaments another.

Detection of a specific isoform of alpha-actinin with antisera directed against dystrophin

We have characterized a protein immunologically related to dystrophin, the protein product of the Duchenne muscular dystrophy gene. We identify this related protein as a fast-twitch glycolytic isoform (mouse extensor digitorum longus-specific) of myofibrillar alpha-actinin. This specific isoform of alpha-actinin exhibits a more restricted pattern of expression in skeletal muscle than fast-twitch-specific isoforms of both myosin and Ca2+-ATPase. Our results provide evidence that dystrophin and myofibrillar alpha-actinin are related proteins, reinforcing the previous data concerning the sequence homologies noted between nonmuscle cytoskeletal alpha-actinin and dystrophin. In addition, we describe the first antisera directed against a specific myofibrillar skeletal muscle isoform of alpha-actinin.

Myofibril assembly is linked with vinculin, alpha-actinin, and cell-substrate contacts in embryonic cardiac myocytes in vitro

The relationship of nascent myofibrils with the accumulation of adhesion plaque proteins and the formation of focal cell contacts was studied in embryonic chick cardiac myocytes in vitro. The cultures were double-stained with various combinations of the specific antiactin drug phalloidin and antibodies against vinculin, alpha-actinin, connectin (titin), myosin heavy chain, fibronectin, and desmin and examined under fluorescence and interference reflection microscopy. In the areas of myofibril assembly, vinculin and alpha-actinin plaques were formed at the ventral sarcolemmae. These areas overlapped with the sites of cell-to-substrate focal contacts and extracellular fibronectin. Because the myofibrils always ran in a straight line between these sites, polarized lines appeared to be generated within the cells in response to their physical (e.g., stress) and/or biochemical environment (e.g., adhesion plaque proteins). The possible presence of other factors cannot be ruled out for the proper alignment of myofibrils. As soon as myofibrils came to span between these adhesion sites, they exhibited typically mature cross-striated characteristics. Thus, the formation of these inferred lines has some relation to, or is in fact necessary for, the maturation of myofibrils, in addition to the directional arrangement of sarcomeric proteins. Additionally, synthesis and distribution of myosin and connectin were tightly linked during early developmental (premyofibril and myofibril) stages. The spatial deployment of desmin was not coupled with vinculin. Thus, connectin and desmin do not appear to form the initial scaffold of sarcomeres.

Primary structure of chicken skeletal muscle and fibroblast alpha-actinins deduced from cDNA sequences

The complete 897-amino-acid sequence of chicken skeletal muscle alpha-actinin and the 856-amino-acid sequence (97% of the entire sequence) of chicken fibroblast alpha-actinin have been determined by cloning and sequencing the cDNAs. Genomic Southern analysis with the cDNA sequences shows that skeletal and fibroblast alpha-actinins are encoded by separate single-copy genes. RNA blot analyzes show that the skeletal alpha-actinin gene is expressed in the pectoralis muscle and that the fibroblast gene is expressed in the gizzard smooth muscle as well as in the fibroblast. The deduced skeletal alpha-actinin molecule has a calculated Mr of 104 x 10(3), and each alpha-actinin can be divided into three domains: (1) the NH2-terminal highly conserved actin-binding domain, which shows similarity to the product of the Duchenne's muscular dystrophy locus; (2) the middle rod-shaped dimer-forming domain, which contains the spectrin-type repeat units; and (3) the COOH-terminal two EF-hand consensus regions. Comparison of the skeletal alpha-actinin sequence with the fibroblast and smooth muscle alpha-actinin sequences demonstrated that the EF-hand structure was conserved in all of these alpha-actinin sequences, despite the reported variability of the Ca2+ sensitivities of the actin-gelation by various alpha-actinin isoforms.