Expression of a developmentally regulated antigen on the surface of skeletal and cardiac muscle cells

Increasing alpha 7 beta 1-integrin promotes muscle cell proliferation, adhesion, and resistance to apoptosis without changing gene expression

The dystrophin-glycoprotein complex maintains the integrity of skeletal muscle by associating laminin in the extracellular matrix with the actin cytoskeleton. Several human muscular dystrophies arise from defects in the components of this complex. The alpha(7)beta(1)-integrin also binds laminin and links the extracellular matrix with the cytoskeleton. Enhancement of alpha(7)-integrin levels alleviates pathology in mdx/utrn(-/-) mice, a model of Duchenne muscular dystrophy, and thus the integrin may functionally compensate for the absence of dystrophin. To test whether increasing alpha(7)-integrin levels affects transcription and cellular functions, we generated alpha(7)-integrin-inducible C2C12 cells and transgenic mice that overexpress the integrin in skeletal muscle. C2C12 myoblasts with elevated levels of integrin exhibited increased adhesion to laminin, faster proliferation when serum was limited, resistance to staurosporine-induced apoptosis, and normal differentiation. Transgenic expression of eightfold more integrin in skeletal muscle did not result in notable toxic effects in vivo. Moreover, high levels of alpha(7)-integrin in both myoblasts and in skeletal muscle did not disrupt global gene expression profiles. Thus increasing integrin levels can compensate for defects in the extracellular matrix and cytoskeleton linkage caused by compromises in the dystrophin-glycoprotein complex without triggering apparent overt negative side effects. These results support the use of integrin enhancement as a therapy for muscular dystrophy.

Role for the α7β1 integrin in vascular development and integrity

The alpha7beta1 integrin is a laminin receptor that has been implicated in muscle disease and the development of neuromuscular and myotendinous junctions. Studies have shown the alpha7beta1 integrin is also expressed in nonskeletal muscle tissues. To identify the expression pattern of the alpha7 integrin in these tissues during embryonic development, alpha7 integrin chain knockout mice were generated by a LacZ knockin strategy. In these mice, expression from the alpha7 promoter is reported by beta-galactosidase. From embryonic day (ED) 11.5 to ED14.5, beta-galactosidase was detected in the developing central and peripheral nervous systems and vasculature. The loss of the alpha7 integrin gene resulted in partial embryonic lethality. Several alpha7 null embryos were identified with cerebrovascular hemorrhages and showed reduced vascular smooth muscle cells and cerebral vascularization. The alpha7 null mice that survived to birth exhibited vascular smooth muscle defects, including hyperplasia and hypertrophy. In addition, altered expression of alpha5 and alpha6B integrin chains was detected in the cerebral arteries of alpha7 null mice, which may contribute to the vascular phenotype. Our results demonstrate for the first time that the alpha7beta1 integrin is important for the recruitment or survival of cerebral vascular smooth muscle cells and that this integrin plays an important role in vascular development and integrity.

Developmentally regulated expression and functional role of alpha7 integrin in the chick embryo

Integrin alpha 7 beta 1 is a specific cellular receptor for laminin. In the present work, we studied the distribution pattern of the alpha 7 subunit by immunofluorescence and immunoprecipitation and the role of the integrin by blocking antibodies in early chick embryos. alpha 7 immunoreactivity was first detectable in the neural plate during neural furrow formation (stage HH5, early neurula, Hamburger & Hamilton 1951) and its expression was upregulated in the neural folds during primary neurulation. The alpha 7 expression domain spanned the entire neural tube by stage HH8 (4 somites), and was then downregulated and confined to the neuroepithelial cells in the germinal region near the lumen and the ventrolateral margins of the neural tube in embryos by the onset of stage HH17 (29 somites). Expression of alpha 7 in the neural tube was transient suggesting that alpha 7 functions during neural tube closure and axon guidance and may not be required for neuronal differentiation or for the maintenance of the differentiated cell types. alpha 7 immunoreactivity was strong in the newly formed epithelial somites, although this expression was restricted only to the myotome in the mature somites. The most intense alpha 7 immunoreactivity was detectable in the paired heart primordia and the endoderm apposing the heart primordia in embryos at stage HH8. In the developing heart, alpha 7 immunoreactivity was: (i) intense in the myocardium; (ii) milder in the endocardial cushions of the ventricle; (iii) intense in the sinus venosus; (iv) distinct in the associated blood vessels; and (v) undetectable in the dorsal mesocardium of embryos at stage HH17. Inhibition of function of alpha 7 by blocking antibodies showed that alpha 7 integrin-laminin signaling may play a critical role in tissue organization of the neural plate and neural tube closure, in tissue morphogenesis of the heart tube but not in the directional migration of pre-cardiac cells, and in somite epithelialization but not in segment formation in presomitic mesoderm. In embryos treated with alpha 7 antibody, the formation of median somites in place of a notochord was intriguing and suggested that alpha 7 integrin-laminin signaling may have played a role in segment re-specification in the mesoderm.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

The muscle integrin binding protein (MIBP) interacts with alpha7beta1 integrin and regulates cell adhesion and laminin matrix deposition

Integrins are alphabeta transmembrane receptors that function in key cellular processes, including cell adhesion, differentiation, and extracellular matrix deposition through interactions with extracellular, membrane, and cytoplasmic proteins. We previously identified and cloned a muscle beta1 integrin cytoplasmic binding protein termed MIBP and found that the expression level of MIBP is critical in the decision-making process of terminal myogenic differentiation. We report here that MIBP interacts with the alpha7beta1 integrin but not the alpha5beta1 integrin in C2C12 myoblasts, suggesting an important role of integrin alpha chains in the regulation of the beta1-MIBP interaction. Furthermore, consistent with its selective binding activity toward the alpha7beta1 laminin receptor, we have found that overexpression of MIBP in C2C12 myoblasts resulted in a significant reduction of cell adhesion to laminin and inhibition of laminin matrix deposition. By contrast, neither cell adhesion to fibronectin nor fibronectin matrix deposition was significantly altered in cells overexpressing MIBP. Finally, we show that both the protein level and tyrosine phosphorylation of paxillin, a key signaling molecule involved in the cellular control of myogenic differentiation, are increased by MIBP. These results suggest that MIBP functions in the control of myogenic differentiation by regulating alpha7beta1 integrin-mediated cell interactions with laminin matrix and intracellular signaling through paxillin.

Alteration of the C-terminal amino acid of tubulin specifically inhibits myogenic differentiation

Detyrosination is an evolutionarily conserved post-translational modification of microtubule polymers that is known to be enhanced during early morphological differentiation of cultured myogenic cells (Gundersen, G. G., Khawaja, S., and Bulinski, J. C. (1989) J. Cell Biol. 109, 2275-2288). We proposed that altering the C terminus of alpha-tubulin by detyrosination plays a role in morphological differentiation. To test our hypothesis, we treated L6 myoblasts with 3-nitrotyrosine (Eiserich, J. P., Estevez, A. G., Bamberg, T. V., Ye, Y. Z., Chumley, P. H., Beckman, J. S., and Freeman, B. A. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6365-6375), a nontoxic inhibitor that resulted in high level inhibition of microtubule detyrosination and low level incorporation of nitrotyrosine into microtubules. Even though microtubule stabilization or modification by acetylation still occurred normally, morphological differentiation was blocked; myoblasts neither elongated significantly nor fused. Nitrotyrosine treatment prevented synthesis or activation of markers of myogenic differentiation, including muscle-specific myosin, alpha-actin, integrin alpha(7), and myogenin. Consistent with this, myoblast integrin beta(1A) remained highly expressed. In contrast, the increase in beta-catenin level characteristic of early myogenesis was unaffected by treatment. These results show that the identity of the C-terminal residue of alpha-tubulin modulates microtubule activity, possibly because binding to or signaling from modified microtubules is required for the myogenic program.

Purification of mouse primary myoblasts based on alpha 7 integrin expression

Fundamental insights have come from the study of myogenesis. Primary myoblasts isolated directly from muscle tissue more closely approximate myogenesis than established cell lines. However, contamination of primary muscle cultures with nonmyogenic cells can complicate the results. To overcome this problem, we previously described a method for myoblast purification based on novel culture conditions (T. A. Rando and H. M. Blau, 1994, J. Cell Biol. 125, 1275--1287). Here we report a refinement of this method that leads directly to an enriched population of mouse primary myoblasts, within significantly fewer population doublings. The method described here avoids using adhesion as a criterion for selection. This advance capitalizes on the ability of the antibody CA5.5 to recognize alpha 7 integrin, a muscle-specific cell surface antigen. Enrichment of myoblasts to greater than 95% of the cell population can be achieved by a single round of flow cytometry or magnetic bead separation. This is the first description of a mouse myoblast purification method based on a cell-type-specific antigen. The ease of this procedure for isolating primary myoblasts should expand the opportunities for (1) using these cells in cell transplantation studies in animal models of human disease, (2) isolating and characterizing mutant myoblasts from transgenic animals, and (3) allowing in vitro studies of molecules that regulate muscle cell growth, differentiation, and neoplasia.

Laminin-induced change in conformation of preexisting alpha7beta1 integrin signals secondary myofiber formation

Two distinct populations of myoblasts, distinguishable by alpha7 integrin expression have been hypothesized to give rise to two phases of myofiber formation in embryonic limb development. We show here that alpha7 integrin is detectable far earlier than previously reported on both "primary" and "secondary" lineage myoblasts and myofibers. An antibody (1211) that recognizes an intracellular epitope allowed detection of alpha7 integrin previously missed using an antibody (H36) that recognizes an extracellular epitope. We found that when myoblasts were isolated and cultured from different developmental stages, H36 only detected alpha7 integrin that was in direct contact with its ligand, laminin. Moreover, alpha7 integrin detection by H36 was reversible and highly localized to subcellular points of contact between myoblasts and laminin-coated 2.8-microm microspheres. Prior to secondary myofiber formation in limb embryogenesis, laminin was present but not in close proximity to clusters of primary myofibers that expressed alpha7 integrin detected by antibody 1211 using deconvolution microscopy. These results suggest that the timing of the interaction of preexisting alpha7 integrin with its ligand, laminin, is a major determinant of allosteric changes that result in an activated form of alpha7 integrin capable of transducing signals from the extracellular matrix commensurate with secondary myofiber formation.

Cell therapy attenuates deleterious ventricular remodeling and improves cardiac performance after myocardial infarction

BACKGROUND

Myocardial infarction (MI) promotes deleterious remodeling of the myocardium, resulting in ventricular dilation and pump dysfunction. We examined whether supplementing infarcted myocardium with skeletal myoblasts would (1) result in viable myoblast implants, (2) attenuate deleterious remodeling, and (3) enhance in vivo and ex vivo contractile performance.

METHODS AND RESULTS

Experimental MI was induced by 1-hour coronary ligation followed by reperfusion in adult male Lewis rats. One week after MI, 10(6) myoblasts were injected directly into the infarct region. Three groups of animals were studied at 3 and 6 weeks after cell therapy: noninfarcted control (control), MI plus sham injection (MI), and MI plus cell injection (MI+cell). In vivo cardiac function was assessed by maximum exercise capacity testing and ex vivo function was determined by pressure-volume curves obtained from isolated, red cell-perfused, balloon-in-left ventricle (LV) hearts. MI and MI+cell hearts had indistinguishable infarct sizes of approximately 30% of the LV. At 3 and 6 weeks after cell therapy, 92% (13 of 14) of MI+cell hearts showed evidence of myoblast graft survival. MI+cell hearts exhibited attenuation of global ventricular dilation and reduced septum-to-free wall diameter compared with MI hearts not receiving cell therapy. Furthermore, cell therapy improved both post-MI in vivo exercise capacity and ex vivo LV systolic pressures.

CONCLUSIONS

Implanted skeletal myoblasts form viable grafts in infarcted myocardium, resulting in enhanced post-MI exercise capacity and contractile function and attenuated ventricular dilation. These data illustrate that syngeneic myoblast implantation after MI improves both in vivo and ex vivo indexes of global ventricular dysfunction and deleterious remodeling and suggests that cellular implantation may be beneficial after MI.

Integrins as receptors for laminins

Laminins are a family of trimeric glycoproteins present in the extracellular matrix and the major constituents of basement membranes. Integrins are alpha beta transmembrane receptors that play critical roles in both cell-matrix and cell-cell adhesion. Several members of the integrin family, including alpha 1 beta 1, alpha 2 beta 1, alpha 3 beta 1, alpha 6 beta 1, alpha 7 beta 1 and alpha 6 beta 4 heterodimers serve as laminin receptors on a variety of cell types. This review summarizes recent advances in understanding the involvement of individual integrins in cell interactions with laminins and the roles of laminin-binding integrins in adhesion-mediated events in vertebrates, including embryonic development, cell migration and tumor cell invasiveness, cell proliferation and differentiation, as well as basement membrane assembly. We discuss the regulation of integrin function via alternative splicing of cytoplasmic domains of alpha and beta subunits of the integrin receptors for laminins and present examples of functional collaboration between laminin-binding integrins and non-integrin laminin receptors. Advances in our understanding of the laminin-binding integrins continue to demonstrate the essential roles these receptors play in maintaining cell polarity and tissue architecture.

Role of hyaluronan and CD44 in in vitro branching morphogenesis of ureteric bud cells

Mutual interaction between the metanephric mesenchyme (MM) and the ureteric bud (UB) in the developing kidney leads to branching morphogenesis and the formation of the ureteric tree. A UB-derived cell line, stimulated by conditioned medium derived from an embryonic MM cell line (or, similarly, by 10% fetal calf serum), forms branching tubules under three-dimensional culture conditions (H. Sakurai et al., 1997, Proc. Natl. Acad. Sci. USA 94, 6279-6284). The formation of branching tubules in this simple in vitro system for early nephrogenesis is highly sensitive to the matrix environment, a key component of which is the glycosaminoglycan hyaluronan (HA). Consistent with this, we found that HA in the extracellular environment markedly stimulated the formation of cellular processes and multicellular cords (early steps in branching morphogenesis) and also acted as a cell survival factor. Inhibition of HA binding to the cells by addition of blocking antibodies to CD44, the principal cell surface receptor for HA, or degradation of HA by the addition of Streptomyces hyaluronidase resulted in decreased cell survival and diminished morphogenesis, indicating that the HA-CD44 axis plays a central role in in vitro branching morphogenesis. Analysis of the expression of a large number of genes displayed on a cDNA array revealed that significant changes in gene expression in cells undergoing morphogenesis in the presence of HA were limited to a small subset of genes regulating apoptosis, proliferation, and morphogenesis. This included upregulation by HA of its receptor, CD44, which was found to largely localize to the tips of branching cellular processes. In the embryonic kidney, HA was found near the developing ureteric tree and CD44 was expressed basolaterally in UB-derived structures. In addition, both UB and MM appear to express HA synthase, suggesting their ability to secrete HA. We propose that HA promotes branching morphogenesis by creating a positive feedback loop that results in (1) enhanced interaction of HA-CD44 at branching tips (possibly leading to localization of HA binding morphoregulatory factors at the tips) and (2) an activated transcriptional program favoring cell survival/proliferation and migration/morphogenesis of cells through matrix by the expression of key morphoregulatory molecules. Furthermore, since HA, hyaluronidase, and CD44 have been functionally implicated in branching morphogenesis in this model, and since HA, CD44, and HA synthase are all expressed in an appropriate spatiotemporal fashion in the developing kidney, we propose that these molecules may, together, constitute a morphoregulatory pathway that plays a key role in sequential cycles of branching morphogenesis in the UB.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Cellular localization of alpha3beta1 integrin isoforms in association with myofibrillogenesis during cardiac myocyte development in culture

The cellular localization of alpha3beta1 integrin isoforms was examined in cultured neonatal myocytes at selected times during development using double immunofluorescence assays. The distribution of alpha3A subunits began as diffuse and patternless, but as the cells matured, the distribution assumed a sarcomeric banding pattern, and alpha3A appeared to be localized in costameres - sarcolemmal regions adjacent to the Z-disks. Alpha-actinin, a component of the Z-disk, was localized in the same intracellular regions. Temporal analysis of the incorporation of the alpha3A subunit and other myofibrillar proteins into sarcomeres revealed that alpha3A was integrated into sarcomeres following incorporation of alpha-actinin and myosin heavy chain (MHC) but prior to that of desmin. This suggests that alpha3A integrins are incorporated into a pre-existing myofibrillar structure, and it is unlikely that alpha3A integrins participate in the initial assembly of myofibrillar proteins. The alpha3B, beta1A and beta1D subunits were also localized in costameres, where they formed alpha3Abeta1A, alpha3Abeta1D and alpha3Bbeta1A heterodimers. The alpha3Bbeta1D heterodimer, however, was not found in cardiac myocytes. The antisera raised against the cytoplasmic domains of alpha3A, alpha3B, beta1A and beta1D caused disruption of sarcomere structure. Thus, the myofibril-extracellular matrix linkages mediated by isoforms of alpha3beta1 integrin may play a crucial role in the stabilization of myofibril assembly and in the maintenance of sarcomere structure. Co-immunoprecipitation experiments revealed that beta1A, but not beta1D, interacts with the Nck signaling protein, suggesting that Nck participates in downstream signaling triggered by beta1A and that the beta1A-mediated signaling pathway is distinct from that of beta1D.

Secondary reduction of alpha7B integrin in laminin alpha2 deficient congenital muscular dystrophy supports an additional transmembrane link in skeletal muscle

The integrins are a large family of heterodimeric transmembrane cellular receptors which mediate the association between the extracellular matrix (ECM) and cytoskeletal proteins. The alpha7beta1 integrin is a major laminin binding integrin in skeletal and cardiac muscle and is thought to be involved in myogenic differentiation and migration processes. The main binding partners of the alpha7 integrin are laminin-1 (alpha1-beta1-gamma1), laminin-2 (alpha2-beta1-gamma1) and laminin-4 (alpha2-beta2-gamma1). Targeted deletion of the gene for the alpha7 integrin subunit (ITGA7) in mice leads to a novel form of muscular dystrophy. In the present study we have investigated the expression of two alternative splice variants, the alpha7B and beta1D integrin subunits, in normal human skeletal muscle, as well as in various forms of muscular dystrophy. In normal human skeletal muscle the expression of the alpha7 integrin subunit appeared to be developmentally regulated: it was first detected at 2 years of age. In contrast, the beta1D integrin could be detected in immature and mature muscle in the sarcolemma of normal fetal skeletal muscle at 18 weeks gestation. The expression of alpha7B integrin was significantly reduced at the sarcolemma in six patients with laminin alpha2 chain deficient congenital muscular dystrophy (CMD) (age >2 years). However, this reduction was not correlated with the amount of laminin alpha2 chain expressed. In contrast, the expression of the laminin alpha2 chain was not altered in the skeletal muscle of the alpha7 knock-out mice. These data argue in favor that there is not a tight correlation between the expression of the alpha7 integrin subunit and that of the laminin alpha2 chain in either human or murine dystrophic muscle. Interestingly, in dystrophinopathies (Duchenne and Becker muscular dystrophy; DMD/BMD) expression of alpha7B was upregulated irrespective of the level of dystrophin expression as shown by a strong sarcolemmal staining pattern even in young boys (age <2 years). The expression of the beta1D integrin subunit was not altered in any of our patients with different types of muscular dystrophy. In contrast, sarcolemmal expression of beta1D integrin was significantly reduced in the alpha7 integrin knock-out mice, whereas the expression of the components of the DGC was not altered. The secondary loss of alpha7B in laminin alpha2 chain deficiency defines a biochemical change in the composition of the plasma membrane resulting from a primary protein deficiency in the basal lamina. These findings, in addition to the occurrence of a muscular dystrophy in alpha7 deficient mice, implies that the alpha7B integrin is an important laminin receptor within the plasma membrane which plays a significant role in skeletal muscle function and stability.

The muscle-specific laminin receptor alpha7 beta1 integrin negatively regulates alpha5 beta1 fibronectin receptor function

alpha7 beta1 is the major integrin complex expressed in differentiated muscle cells where it functions as a laminin receptor. In this work we have expressed the alpha7 integrin subunit in CHO cells to investigate the functional properties of this receptor. After transfection with alpha7 CHO cells acquired the ability to adhere and spread on laminin 1 consistent with the laminin receptor activity of the alpha7 beta1. alpha7 transfectants, however, showed a 70% reduction in the ability to adhere to fibronectin and were unable to assemble a fibronectin matrix. The degree of reduction was inversely related to the level of alpha7 expression. To define the mechanisms underlying this adhesive defect we analyzed surface expression and functional properties of the alpha5 beta1 fibronectin receptor. Although cell surface expression of alpha5 beta1 was reduced by a factor of 20-25% in alpha7 transfectants compared to control untransfected cells, this slight reduction was not sufficient to explain the dramatic reduction in cell adhesion (70%) and matrix assembly (close to 100%). Binding studies showed that the affinity of 125I-fibronectin for its surface receptor was decreased by 50% in alpha7 transfectants, indicating that the alpha5 beta1 integrin is partially inactivated in these cells. Inactivation can be reversed by Mn2+, a cation known to increase integrin affinity for their ligands. In fact, incubation of cells with Mn2+ restored fibronectin binding affinity, adhesion to fibronectin, and assembly of fibronectin matrix in alpha7 transfectants. These data indicate that alpha7 expression leads to the functional down regulation of alpha5beta1 integrin by decreasing ligand binding affinity and surface expression. In conclusion, the data reported establish the existence of a negative cooperativity between alpha7 and alpha5 integrins that may be important in determining functional regulation of integrins during myogenic differentiation.

A functional role for specific spliced variants of the alpha7beta1 integrin in acetylcholine receptor clustering

The clustering of acetylcholine receptors (AChR) on skeletal muscle fibers is an early event in the formation of neuromuscular junctions. Recent studies show that laminin as well as agrin can induce AChR clustering. Since the alpha7beta1 integrin is a major laminin receptor in skeletal muscle, we determined if this integrin participates in laminin and/or agrin-induced AChR clustering. The alternative cytoplasmic domain variants, alpha7A and alpha7B, and the extracellular spliced forms, alpha7X1 and alpha7X2, were studied for their ability to engage in AChR clustering. Immunofluorescence microscopy of C2C12 myofibers shows that the alpha7beta1 integrin colocalizes with laminin-induced AChR clusters and to a much lesser extent with agrin-induced AChR clusters. However, together laminin and agrin promote a synergistic response and all AChR colocalize with the integrin. Laminin also induces the physical association of the integrin and AChR. High concentrations of anti-alpha7 antibodies inhibit colocalization of the integrin with AChR clusters as well as the enhanced response promoted by both laminin and agrin. Engaging the integrin with low concentrations of anti-alpha7 antibody initiates cluster formation in the absence of agrin or laminin. Whereas both the alpha7A and alpha7B cytoplasmic domain variants cluster with AChR, only those isoforms containing the alpha7X2 extracellular domain were active. These results demonstrate that the alpha7beta1 integrin has a physiologic role in laminin-induced AChR clustering, that alternative splicing is integral to this function of the alpha7 chain, and that laminin, agrin, and the alpha7beta1 integrin interact in a common or convergent pathway in the formation of neuromuscular junctions.

The alpha7beta1 integrin mediates adhesion and migration of skeletal myoblasts on laminin

Many aspects of myogenesis are believed to be regulated by myoblast interactions with specific components of the extracellular matrix. For example, laminin has been found to promote adhesion, migration, and proliferation of mammalian myoblasts. Based on affinity chromatography, the alpha7beta1 integrin has been presumed to be the major receptor mediating myoblast interactions with laminin. We have prepared a monoclonal antibody, O26, that specifically reacts with both the X1 and the X2 extracellular splice variants of the alpha7 integrin chain. This antibody completely and selectively blocks adhesion and migration of rat L8E63 myoblasts on laminin-1, but not on fibronectin. In contrast, a polyclonal antibody to the fibronectin receptor, alpha5beta1 integrin, blocks myoblast adhesion on fibronectin, but not on laminin-1. The alpha7beta1 integrin also binds to a mixture of laminin-2 and laminin-4, the major laminin isoforms in developing and adult skeletal muscle, but O26 is a much less potent inhibitor of myoblast adhesion on the laminin-2/4 mixture than on laminin-1. Based on affinity chromatography, we suggest that this may be due to higher affinity binding of alpha7X1 to laminin-2/4 than to laminin-1.

Identification and characterization of the cell type-specific and developmentally regulated alpha7 integrin gene promoter

Expression of alpha7 is mainly confined to skeletal and cardiac muscle in which it appears to be the major laminin-binding integrin. When myoblasts differentiate to myotubes, alpha7 mRNA and protein expression is up-regulated. To explore the mechanisms involved in the tissue-specific and developmentally regulated expression of alpha7, we isolated and characterized a genomic clone containing approximately 2.8 kilobase pairs (kb) of the 5'-flanking region of the murine alpha7 gene. The 5'-flanking region lacks both TATA and CCAAT boxes but contains five putative Sp1 binding sites located in a CpG island. Two transcription start sites, located near an initiator-like sequence, are 176 and 170 base pairs upstream of the translation start site. There are numerous binding sites for developmental and cell type-specific transcription factors, including AP-1, AP-2, GATA, and several AT-rich sites. There are also eight consensus E-boxes that bind the basic helix-loop-helix family of muscle-specific transcription factors. The approximately 2.8-kb 5'-flanking region was an active promoter in C2C12 skeletal myoblasts and exhibited increased expression upon conversion to myotubes but was inactive in HtLM2 cells, a mouse breast carcinoma epithelial cell line that does not express alpha7. Deletion analysis identified both positive and negative regulatory elements within the approximately 2.8-kb fragment. In 10T1/2 fibroblasts the approximately 2.8-kb alpha7 promoter was trans-activated by the myogenic basic helix-loop-helix proteins myogenin and MyoD but not by MRF4 and myf5. These results suggest that muscle-specific transcription factors play a role in regulating the cell-type expression of the alpha7 gene during development.

Differential expression of alpha-6 and other subunits of laminin binding integrins during development of the murine heart

The development of the heart from a single heart tube to a four chambered organ with two separated unidirectional flows is a highly complex process. Events like looping, septation, tissue remodelling, and development of valves take place in a time period in which the heart already exerts its pump function. Adhesion of cells to each other and to their extracellular matrix as well as the capability to migrate in such a dynamic environment are extremely important. Integrins and extracellular matrix components have already been implicated in this process. In this report, we describe in detail the differential expression of the alpha-6 integrin subunit during late murine heart development, e.g., in the process from looping to the end of septation. We compare mRNA and protein expression patterns with those of beta-1 and other subunits of laminin-binding integrins, alpha-3 and alpha-7. We show a constant and high expression of alpha-6 in the atrial myocardium and a decrease in expression in the ventricular trabecular myocardium. The compact myocardial wall and the ventricular septum do not express alpha-6, except for the myocardium of the distal outflow tract at early stages. Moreover, we describe expression of this integrin subunit in the endocardial cushions that contribute to the development of the atrioventricular and semilunar valves. We propose a role for the alpha-6-beta-1 laminin receptor in the adhesion of cells to their extracellular matrix at sites of high stress due to cardiac contraction or blood flow induced shear stress. Moreover, site specific endothelial expression within the heart and surrounding extracardiac tissue is discussed. This study suggests a distinct role for alpha-6-beta-1 in the heart and provides insight concerning probably important roles of integrins and their extracellular matrix ligands during embryonic development.

Localization of the alpha 7 integrin gene (ITGA7) on human chromosome 12q13: clustering of integrin and Hox genes implies parallel evolution of these gene families

Expression of the alpha 7 integrin gene (ITGA7) is developmentally regulated during the formation of skeletal muscle. Increased levels of expression and production of isoforms containing different cytoplasmic and extracellular domains accompany myogenesis. To determine whether a single or multiple alpha 7 genes underlie the structural diversity in this alpha chain that accompanies development, we have examined the rat and human genomes by Southern blotting and in situ hybridization. Our results demonstrate that there is only one alpha 7 gene in both the rat and the human genomes. In the human, ITGA7 is present on chromosome 12q13. Phylogenetic analysis of the integrin alpha chain sequences suggests that the early integrin genes evolved in two pathways to form the I-integrins and the non-I-integrins. The I-integrin alpha chains contain an additional sequence of approximately 180 amino acids and arose as a result of an early insertion into the non-I-gene. The I-chain subfamily further evolved by duplications within the same chromosome. The non-I-integrin alpha chain genes are localized in clusters on chromosomes 2, 12, and 17, and this closely coincides with the localization of the human homeobox gene clusters. Non-I-integrin alpha chain genes appear to have evolved in parallel and in proximity to the Hox clusters. Thus, the Hox genes that underlie the design of body structure and the Integrin genes that underlie informed cell-cell and cell-matrix interactions appear to have evolved in parallel and coordinate fashions.

Expression of alpha 7 integrin cytoplasmic domains during skeletal muscle development: alternate forms, conformational change, and homologies with serine/threonine kinases and tyrosine phosphatases

We recently reported the cloning and sequencing of the alpha 7 integrin chain and its regulated expression during the development of skeletal muscle (Song et al. (1992) J. Cell Biol. 117, 643–657). The alpha 7 chain is expressed during the development of the myogenic lineage and on adult muscle fibers and this suggests that it participates in multiple and diverse functions at different times during muscle development. One interesting portion of this isoform is its cytoplasmic domain; comprised of 77 amino acids it is the largest in the alpha chains thus reported. In these experiments we begin to study the potential functions of the alpha 7 cytoplasmic domain by analyzing homologies between the rat and human sequences, by immunologic studies using an anti-cytoplasmic domain antiserum, and by identifying two alternate forms. In keeping with the nomenclature used to describe the alpha 3 and alpha 6 alternate cytoplasmic domains, we refer to the originally reported species as alpha 7B and the two additional forms as alpha 7A and alpha 7C. These three cytoplasmic domains likely arise as a consequence of alternate splicing. A splice site at the junctions of the transmembrane and cytoplasmic domains is used to generate the alpha 3, alpha 6 and alpha 7 A and B forms. The alpha 7A form RNA contains an additional 113 nucleotides compared to the B form, and a common coding region in the A and B form RNAs is used in alternate reading frames. Part of the coding region of alpha 7B appears to be used as the 3′-untranslated region of the alpha 7A form. The alpha 7C mRNA is 595 nucleotides smaller than the alpha 7B RNA and part of the 3′-untranslated region of the alpha 7B isoform is used as coding sequence in alpha 7C. There is developmental specificity in expression of these alternate mRNAs: alpha 7A and alpha 7C transcripts are found upon terminal myogenic differentiation whereas alpha 7B is present earlier in replicating cells and diminishes upon differentiation. We suggest this selective expression of the alpha 7 cytoplasmic domains underlies the diversity in function of the alpha 7 beta 1 integrin at different stages of muscle development. Immunochemical analyses indicate that the alpha 7B cytoplasmic domain undergoes a change in conformation in response to binding laminin or upon crosslinking initiated with antibody reactive with the integrin extracellular domain. Crosslinking also promotes association of the integrin with the cell cytoskeleton.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial and temporal dissection of immediate and early events following cadherin-mediated epithelial cell adhesion

Cell-cell adhesion is at the top of a molecular cascade of protein interactions that leads to the remodeling of epithelial cell structure and function. The earliest events that initiate this cascade are poorly understood. Using high resolution differential interference contrast microscopy and retrospective immunohistochemistry, we observed that cell-cell contact in MDCK epithelial cells consists of distinct stages that correlate with specific changes in the interaction of E-cadherin with the cytoskeleton. We show that formation of a stable contact is preceded by numerous, transient contacts. During this time and immediately following formation of a stable contact, there are no detectable changes in the distribution, relative amount, or Triton X-100 insolubility of E-cadherin at the contact. After a lag period of approximately 10 min, there is a rapid acquisition of Triton X-100 insolubility of E-cadherin localized to the stable contact. Significantly, the total amount of E-cadherin at the contact remains unchanged during this time. The increase in the Triton X-100 insoluble pool of E-cadherin does not correlate with changes in the distribution of actin or fodrin, suggesting that the acquisition of the Triton X-100 insolubility is due to changes in E-cadherin itself, or closely associated proteins such as the catenins. The 10 minute lag period, and subsequent prompt and localized nature of E-cadherin reorganization indicate a form of signaling is occurring.

H36-alpha 7 is a novel integrin alpha chain that is developmentally regulated during skeletal myogenesis

H36 is a 120,000-D membrane glycoprotein that is expressed during the differentiation of skeletal muscle. H36 cDNA clones were isolated from a lambda UniZapXR rat myotube cDNA library and sequenced. The deduced amino acid sequence demonstrates that H36 is a novel integrin alpha chain that shares extensive homology with other alpha integrins that includes: (a) the GFFKR sequence found in all alpha integrins; (b) a single membrane spanning region; (c) conservation of 18 of 22 cysteines; and (d) a protease cleavage site found in the non-I region integrin alpha chains. The cytoplasmic domain of H36 is unique and additional regions of nonhomology further indicate H36 is distinct from all other alpha chains. In keeping with current nomenclature we designate this alpha chain alpha 7. Northern blots demonstrate that expression of H36-alpha 7 mRNA is regulated both early in the development of the myogenic lineage and later, during terminal differentiation. Detection of H36-alpha 7 mRNA coincides with conversion of H36- myogenic precursor cells to H36+ cells. H36-alpha 7 mRNA is present in replicating myoblasts: expression increases upon terminal differentiation and is markedly reduced in developmentally defective myoblasts. In addition, H36-alpha 7 mRNA is not detected in C3H10T1/2 cells. It is in myotubes derived from myoblasts obtained by treatment of 10T1/2 cells with azacytidine or transfection with MRF4. Immunoblots and immunofluorescence demonstrate that the H36-alpha 7 chain is associated with integrin beta 1. Affinity chromatography demonstrates that H36-alpha 7 beta 1 selectively binds to laminin. The expression of H36-alpha 7 on secondary myoblasts during the development of the limb in vivo corresponds with the appearance of laminin in the limb, with the responsiveness of secondary myoblast proliferation to laminin, and with the onset of increased muscle mass, suggesting that H36-alpha 7 modulates this stage in limb development. We conclude that H36-alpha 7 is a novel alpha integrin laminin binding protein whose expression is developmentally regulated during skeletal myogenesis.

Expression and function of heterotypic adhesion molecules during differentiation of human skeletal muscle in culture

The infiltration of skeletal muscle by leukocytes occurs in a variety of myopathies and frequently accompanies muscle degeneration and regeneration. The latter involves development of new myofibers from precursor myoblasts, and so infiltrating cells may interact with muscle at all stages of differentiation. The authors have investigated the surface expression of ligands for T-cell adhesion during the differentiation of human skeletal muscle in vitro. Myoblasts expressed low levels of ICAM-1 (CD54), which remained constant during muscle cell differentiation and could be induced by cytokines such as gamma-interferon. It is therefore likely that ICAM-1 is involved in the invasive accumulation of lymphocytes during skeletal muscle inflammation. In contrast, LFA-3 (CD58) was expressed at higher levels than ICAM-1 on myoblasts, decreased significantly during myogenesis, and was unaffected by immune mediators. Both ICAM-1 and LFA-3 were able to mediate T cell binding to myoblasts, whereas adhesion to myotubes was independent of the LFA-3 ligand. Although expressed throughout myogenesis, human leukocyte antigen class I and CD44 did not appear to mediate T cell binding. The expression of ligands that facilitate interaction of myogenic cells with lymphocytes may have important implications for myoblast transplantation.

In vitro development of precursor cells in the myogenic lineage

Expression of the muscle-specific integral membrane protein H36 and the intermediate filament protein desmin, detected by immunofluorescence, was used to identify cells at distinct stages in the skeletal myogenic lineage. These proteins were coordinately expressed in cultures of rat hindlimb myoblasts from 17- and 19-day fetuses and newborn pups, and in satellite cells from juveniles. Both H36+ and desmin+ cells were present in cultures from 13.5- and 15-day embryonic hindlimbs, but desmin expression was more prevalent: H36-/desmin+ myoblasts predominate during this early stage of development. H36 was not detected in Day 12 embryo hindlimb bud cells in vivo nor in cultures soon after plating. Initially, only 1% of the Day 12 limb bud cells expressed desmin. When these cells were serially passaged every 3-4 days, cells with all three possible myogenic phenotypes developed: that is, H36+/desmin-, H36+/desmin+, and H36-/desmin+ cells. There was a progressive increase in the frequency of H36+ cells, with 75% of cells positive by passage 6 (Day 27 in vitro). The maximum frequency of cells that expressed desmin occurred in passage 5 (Day 23 in vitro). These results demonstrate that precursors to the cells that express H36 and desmin are present in the 12-day embryo hindlimb bud and that the transition from H36-/desmin- precursors to cells with a myogenic phenotype can occur in vitro. MyoD1 and myogenin were not detected in these cells, suggesting that the initial expression of H36 and desmin in the myogenic lineage may precede and/or is independent of these regulatory proteins. The conversion of precursor cells in the 12-day limb bud to a more advanced stage of development serves to define additional cells in the myogenic lineage. The ability to monitor in vitro these stages of development affords the opportunity to study how they are regulated.

Localization of anti-clathrin antibody in the sarcomere and sensitivity of myofibril structure to chloroquine suggest a role for clathrin in myofibril assembly

Immunofluorescence microscopy has been used to demonstrate that X22, a monoclonal antibody specific for clathrin heavy chain, localizes in repetitive bands that appear soon after the fusion of skeletal myoblasts into multinucleate fibers. This organization has been found in cultures containing myotubes that develop in vitro from explants of newborn rat hindlimb cells and in myotubes derived from the L8E63 myogenic line. Bands were also prominent in skinned fibers prepared from adult rat soleus muscle and in cardiac myocytes grown in vitro from 4-day heart ventricles. Immunofluorescence banding was localized in the sarcomere as a doublet, with one element on either side of the Z line. Evidence that supports the conclusion that the reaction with X22 antibody is specific and indicative of the localization of clathrin in the sarcomere includes: (1) Identical titration of X22 antibody reactivity with the determinant in coated vesicles and in the sarcomere. (2) Conditions (eg., pH and Tris) that disrupt clathrin baskets or prevent its assembly likewise disrupt the localization of X22 in bands. (3) Chloroquine inhibits both the normal trafficking of clathrin in the cell and X22 banding in the sarcomere. (4) Immunoblot analysis of myotube lysates reveals a single band with an electrophoretic mobility identical to the 180,000-Da clathrin heavy chain. (5) The assembly of clathrin into sarcomeric bands occurs early in the development of the myofibrillar apparatus. Quantitation of the appearance of X22 banding in primary cultures of myotubes indicates that it precedes that of other myofibrillar proteins and that assembly takes place in the following order: X22, titin, myosin heavy chain, actin, and desmin. The assembly of myosin, titin, and actin into sarcomeric bands, as well as X22, is inhibited by chloroquine. Upon prolonged exposure to chloroquine previously assembled proteins are drastically reduced or no longer evident in the sarcomere. On the basis of these results and considering the role of clathrin in intracellular transport and its capacity to interact with actin and alpha-actinin, we suggest that clathrin may have diverse roles in the assembly, integrity, and functioning of the sarcomere and its integration with the sarcolemma. The early organization of X22 into bands further suggests that clathrin may also function early in the assembly of the contractile system.

Antibodies to 100- and 60-kDa surface proteins inhibit substratum attachment and differentiation of rodent skeletal myoblasts

A rabbit polyclonal antiserum was raised against membrane vesicles shed from the surface of fusing L6 rat myoblasts. In immunoblots the antiserum recognized fibronectin, a protein of approximately 100,000 Da (100-kDa), and a protein of approximately 60,000 Da (60 kDa). If added prior to cellular alignment, immunoglobulins from this serum inhibited fusion of both rat (L6) and mouse (C2) myoblasts in a dose-dependent fashion. To determine which component of this serum was responsible for fusion inhibition, antibodies against fibronectin, the 100- and 60-kDa proteins were microaffinity purified and tested, individually, for their effects on myoblast fusion. Antibodies against fibronectin had no effect on fusion. Antibodies against the 100-kDa protein released most cells from the substratum. Antibodies against the 60-kDa protein completely inhibited fusion. Fusion inhibition was accompanied by a corresponding inhibition of expression of two differentiation markers, creatine phosphokinase and the acetylcholine receptor. The 60-kDa protein was found, by immunoblot analysis, in smooth muscle-like cells (BC3H1 cells) and in variant L6 cells that do not differentiate and do not fuse. However, in the differentiation incompetent cells, the 60-kDa antigen appeared to be present in reduced amount. Indirect immunofluorescence of unpermeabilized L6 cells revealed alterations in the distribution of all three antigens during development. Fibronectin first appeared in long fibrillar arrays above the surface of cells that were beginning to align and fuse; fibronectin was not present on myotubes. The 100-kDa protein was seen initially in prominent fibrillar projections at the tips of prefusion myoblasts. During fusion the antigen was observed at sites of cell-cell contact and on extracellular vesicles. The 100-kDa protein appeared to be less abundant on myotubes. The 60-kDa protein first appeared in regions of cell-cell contact on cells that were beginning to align and fuse. As. fusion progressed, the 60-kDa protein was also found in extracellular vesicles. The 60-kDa protein was not observed on myotubes. As a result of this study we have identified two previously undescribed cell surface proteins involved in rodent skeletal myogenesis. The first is an approximately 100-kDa protein involved in early interactions of skeletal myoblasts with their substratum. The second is an approximately 60-kDa protein involved in myoblast differentiation. Both proteins are shed from the myoblast surface during myotube formation.

Evidence for export of a muscle lectin from cytosol to extracellular matrix and for a novel secretory mechanism

A soluble lactose-binding lectin with subunit Mr of 14,500 is believed to function by interacting with extracellular glycoconjugates, because it has been detected extracellularly by immunohistochemistry. This localization has been questioned, however, since the lectin lacks a secretion signal sequence, which challenges the contention that it is secreted. We have demonstrated externalization of this lectin from C2 mouse muscle cells by both immunoprecipitation of metabolically labeled protein and immunohistochemical localization. We further show that externalization of the lectin is a developmentally regulated process that accompanies myoblast differentiation and that the lectin codistributes with laminin in myotube extracellular matrix. Immunohistochemical localization during intermediate stages of externalization suggests that the lectin becomes concentrated in evaginations of plasma membrane, which pinch off to form labile lectin-rich extracellular vesicles. This suggests a possible mechanism for lectin export from the cytosol to the extracellular matrix.

Effects of cesium on in vitro myoblast differentiation: an electron microscopic study

This paper describes the microscopic evidence supporting a cesium-induced delay in the fusion of chick embryo myoblast membranes during in vitro myogenic differentiation. We have recently demonstrated that the sharp decrease in the conductivity and permittivity of the membranes of these myogenic cells at the time of fusion is delayed 30 h by the addition of cesium to the culture medium (Santini et al., Biochim. Biophys. Acta 945:56-64; 1988). We report here that this delay in fusion is substantiated by direct microscopic observation and that cesium also induces ultrastructural changes in the myoblast cells themselves. Possible mechanisms by which cesium may cause both the delay in fusion as well as the ultrastructural changes observed are discussed.

A unique subset of developmentally regulated surface proteins turns over rapidly during fusion of the L6 rat myoblast cell line

We have previously identified several developmentally regulated surface polypeptides in the L6 rat myoblast cell line, on the basis of their susceptibility to lactoperoxidase catalyzed iodination. An analysis of the turnover rates of these polypeptides now indicates that while the bulk of the iodinated polypeptides have a half-life of 20-30 h, four low-molecular-weight polypeptides have half lives of 2-7 h. The half-lives of all of the rapid turnover class surface polypeptides were greatly increased in cultures where fusion was inhibited by chloroquine and in nonfusing variants of the L6 cell line. In contrast, inhibition of fusion by the metalloendoprotease inhibitor 1, 10-phenanthroline did not alter the turnover of any iodinatable surface proteins. We propose that some or all of the rapid turnover class of polypeptides may be surface receptors which control cell surface alterations involved in the acquisition of fusion competence or in fusion itself.

Membrane-cytoskeleton associations during myogenesis deviate from traditional definitions

Plasma membrane-cytoskeleton associations involving four membrane proteins (A5, H58, H36, and I20) were studied in developing L8E63 rat skeletal muscle cells using immunofluorescence microscopy and photometry on the basis of three criteria: Triton-insolubility, colocalization with cytoskeletal components, and sensitivity to cytoskeleton-directed drugs. The results presented demonstrate that there are developmental stage-specific associations between membrane proteins and the cytoskeleton during skeletal myogenesis. Several inconsistencies were found with traditional expectations of membrane-cytoskeleton associations. For example, although A5 is Triton-insoluble and sensitive to cytochalasin, its distribution generally does not correspond with any known cytoskeletal structure. Furthermore, the topography of A5 is dependent on the integrity of the plasma membrane. H36 and I20 are completely soluble in Triton and therefore by accepted definitions would not be expected to be associated with any cytoskeletal component. Yet H36 and actin codisrupt in the presence of cytochalasin, while I20, whose distribution does not correspond with microtubules, is uniquely sensitive to their disruption. These results demonstrate that (i) neither Triton-solubility nor colocalization alone predicts all membrane-cytoskeleton associations; some associations between the membrane and cytoskeleton are unstable in nonionic detergent; (ii) the native distribution of proteins in the membrane may not reflect their cytoskeletal associations; and (iii) the topography of some membrane proteins with no apparent association with the cytoskeleton may be greatly influenced by the cell cytoskeleton.

Replicating myoblasts express a muscle-specific phenotype

During the terminal stage of skeletal myogenesis, myoblasts stop replicating, fuse to form multinucleate fibers, and express the genes that encode the proteins that convey contractile capacity. Because of this dramatic shift in proliferative state, morphology, and gene expression, it has been possible to readily identify and quantitate terminally differentiating myoblasts. In contrast, it is not clear whether the proliferating cells that give rise to postmitotic myoblasts are equally distinct in their phenotype and in fact whether distinct stages in skeletal myogenesis precede the onset of terminal differentiation. To address these questions, monoclonal antibodies and immunofluorescence microscopy were used to determine that replicating myoblasts from newborn rats do express a muscle-specific phenotype. To identify replicating cells, incorporation of 5-bromo-2'-deoxyuridine (BrdUrd) into DNA was assayed by using anti-BrdUrd antibody. The developmentally regulated, muscle-specific, integral membrane protein H36 and the intermediate-filament protein desmin were scored as markers of the myogenic phenotype. The percentage of BrdUrd+ (i.e., proliferative) cells among H36+ and desmin+ myoblasts was equal to the percentage of BrdUrd+ cells in the entire population, indicating that the expression of H36 and desmin is uniformly characteristic of replicating myoblasts. Inhibition of protein synthesis before and during growth in BrdUrd did not alter the frequency of desmin and H36 immunofluorescence in BrdUrd+ cells. Thus, desmin and H36 were present in the replicating myoblasts prior to the onset of growth in BrdUrd. These results were confirmed using H36+ cells selected by flow cytometry: these purified H36+ myoblasts replicate, express desmin, and differentiate. Similar results were obtained with mouse myoblasts. Desmin expression in these mammalian cells differs from that in chicken embryo myoblasts: only a small proportion of replicating chicken embryo myoblasts express desmin. That replicating mammalian myoblasts have a muscle-specific phenotype serves to define a distinct stage in myogenic development and a specific cell in the myogenic lineage. Further, it implies that there is a regulatory event activated during myogenesis that precedes terminal differentiation and that is required for expression of those genes whose products distinguish the replicating myoblast.

Preferential expression of a 130,000-Da cell surface protein by vascular wall cells in vitro and in vivo

Monoclonal antibodies have been raised against cell surface proteins of cultured bovine retinal pericytes. One antibody was selected, designated PC4, which preferentially stained primary cultures of bovine pericytes and smooth muscle cells, but not endothelial cells and fibroblasts. In freshly plated cells a homogeneous cell surface staining was observed, whereas in well-spread cells the antigen was concentrated at cell attachment sites. The antigen remained at these sites after spontaneous detachment of the cells. PC4 monoclonal antibodies reacted with a major protein of 130,000 Da and two minor antigens of 75,000 and 70,000 Da in immunoblots of extracts from cultured pericytes and smooth muscle cells and from fibroblasts cultured for an extended period of time. In frozen sections of bovine tissues the antigen was found in the vascular wall. There was no staining of skeletal muscle cells or duodenal smooth muscle cells, indicating that the antigen may be a specific component of the vascular wall.

A cell surface phosphoprotein of 48 kDa specific for myoblast fusion

The data we present here permit us to affirm that a 48 kDa phosphoprotein is the target of extracellular Ca2+ during fusion. It is detected only in fusion-competent L6 myoblasts and not in the fusion-defective spontaneous stable variants we isolated. The phosphorylation of this protein species can be totally inhibited by culturing myoblasts in a medium containing low Ca2+ concentrations (0.250 mM). However, under such conditions myoblasts do not fuse, but withdraw from the cell cycle and accumulate the muscle isoform of creatine kinase (M-CK). The results we have obtained support the following conclusions: (1) in fusion-competent cells, overall Ca2+-dependent phosphorylation of cell surface proteins appears to be necessary, but is not sufficient by itself for myoblast fusion; (2) the phosphorylation of a 48 kDa protein species is required for cell fusion; and (3) the phosphorylation of this 48 kDa protein is independent of other main events of cellular differentiation.

Isolation of human myoblasts with the fluorescence-activated cell sorter

We have established procedures for the rapid and efficient purification of human myoblasts using the fluorescence-activated cell sorter. Our approach capitalizes on the specific reaction of monoclonal antibody 5.1H11 with a human muscle cell surface antigen. For each of the five samples analyzed, an enrichment of myoblasts to greater than 99% of the cell population was immediately achieved. Following 3 to 4 weeks of additional growth in vitro, sorted myoblast cultures remained 97% pure. Differentiation of the sorted myoblast cultures, assessed by creatine kinase activity and isozyme content, was comparable to that of pure myoblast cultures obtained by cloning, and was significantly greater than that of mixed fibroblast and myoblast cultures. An average of 10(4) viable myoblasts can be obtained per 0.1 g tissue, each with the potential to undergo approximately 40 cell divisions. Accordingly, if only two-thirds of this proliferative capacity is utilized, the potential yield approximates 10(12) myoblasts, equivalent to 1 kg of cells. Human myogenesis in vitro is no longer limited by cell number and is now amenable to molecular and biochemical analysis on a large scale.+

Expression of a single transfected cDNA converts fibroblasts to myoblasts

5-azacytidine treatment of mouse C3H10T1/2 embryonic fibroblasts converts them to myoblasts at a frequency suggesting alteration of one or only a few closely linked regulatory loci. Assuming such loci to be differentially expressed as poly(A)+ RNA in proliferating myoblasts, we prepared proliferating myoblast-specific, subtracted cDNA probes to screen a myocyte cDNA library. Based on a number of criteria, three cDNAs were selected and characterized. We show that expression of one of these cDNAs transfected into C3H10T1/2 fibroblasts, where it is not normally expressed, is sufficient to convert them to stable myoblasts. Myogenesis also occurs, but to a lesser extent, when this cDNA is expressed in a number of other cell lines. The major open reading frame encoded by this cDNA contains a short protein segment similar to a sequence present in the myc protein family.

Differential expression and modulation of Thy-1 on myoblast clones

In the preceding paper [J. S. Schweitzer, M. A. Dichter, and S. J. Kaufman, 1987, Exp. Cell Res. 172, 1] we demonstrated that expression of Thy-1 antigen by rat skeletal muscle myoblasts in culture is modulated by fibroblasts. Growth of myoblasts with myofibroblasts, or in medium in which fibroblasts have been grown, causes a sharp reduction in the percentage of myoblasts that express Thy-1 antigen on their membrane. In this paper we demonstrate that there are two populations of myoblasts that can be distinguished by their capacity to respond to the Thy-1-modulating factor (TMF). The majority of Thy-1-positive or -negative myoblasts grown at clonal densities are responsive to TMF. A second myoblast clonal type is Thy-1 positive and fuses to form contractile myotubes, but is insensitive to TMF and has a more fibroblast-like morphology. These clonal phenotypes are stable upon passage in vitro and may represent distinct subsets of the myogenic lineage.

Fibroblasts modulate expression of Thy-1 on the surface of skeletal myoblasts

Thy-1 antigen is a well-characterized cell-surface glycoprotein known to be variably expressed in many different tissues, including lymphocytes, brain, and muscle. Its function remains unknown. In skeletal muscle, both in vivo and in vitro, the antigen has been reported on immature but not on adult tissue, and its disappearance corresponds roughly to the time of myoblast fusion. Using monoclonal H36 antibody to identify myoblasts unambiguously, we demonstrate here that Thy-1 is expressed only on a small (less than 1%) fraction of rat skeletal muscle myoblasts in heterogeneous primary cultures, but the number of myoblasts that express Thy-1 rises to a steady level of about 70% when fibroblasts are removed from secondary cultures. Restitution of fibroblasts or growth of purified myoblasts in medium conditioned by fibroblasts greatly suppresses this increase in myoblast Thy-1 expression. Thus an interaction between fibroblasts and myoblasts, mediated by a soluble nondialyzable molecule, modulates expression of Thy-1 on the myoblast outer membrane.

A laminin substrate promotes myogenesis in rat skeletal muscle cultures: analysis of replication and development using antidesmin and anti-BrdUrd monoclonal antibodies

Cells from newborn rat hindlimb show greatly enhanced myogenicity when grown on surfaces coated with poly-L-lysine followed by laminin (PLL/Lam) instead of the collagens routinely used. Coating with poly-L-lysine (PLL) alone or with PLL followed by collagen does not enhance myogenicity. Both myogenic and nonmyogenic cells, as distinguished by a monoclonal antibody specific for desmin, attach equally well to collagen- and laminin-coated surfaces, but there is a two- to five-fold increase in the number of myogenic cells on PLL/Lam by 72 hr, followed by increased myotube formation. To determine whether this increase in myogenic cells was a consequence of a selective increase in proliferation on PLL/Lam, incorporation of 5-bromodeoxyuridine into DNA followed by labeling with anti-BrdUrd antibody was used as an index of cell proliferation. The results indicate that desmin is expressed in replicating rat myoblasts, and that replication of myogenic cells is greatly enhanced on laminin compared to collagen. The rate of replication of nonmyogenic cells is the same on both substrates. Addition of 10 micrograms/ml laminin to the medium of cells seeded on PLL or collagen has no effect on myogenicity. We conclude that a laminin substrate enhances skeletal myogenesis in vitro by promoting selectively the replication of myoblasts. Cultures prepared from fetuses at 17 and 19 days gestation also show enhanced myogenicity when grown on PLL/Lam, while those from 15-day fetuses do not. Growth and development of fetal myoblasts on collagen were very poor, whereas myoblasts from the newborn rat do proliferate and differentiate on this substrate. Thus myogenic cells at different stages of fetal and neonatal development may require and respond to different extracellular environments. Myotube formation in the E63 clone of L8 rat myoblasts is inhibited by PLL/Lam.

Identification of sarcolemma-associated antigens with differential distributions on fast and slow skeletal muscle fibers

We have identified three sarcolemma-associated antigens, including two antigens that are differentially distributed on skeletal muscle fibers of the fast, fast/slow, and slow types. Monoclonal antibodies were prepared using partially purified membranes of adult chicken skeletal muscles as immunogens and were used to characterize three antigens associated with the sarcolemma of muscle fibers. Immunofluorescence staining of cryosections of adult and embryonic chicken muscles showed that two of the three antigens differed in expression by fibers depending on developmental age and whether the fibers were of the fast, fast/slow, or slow type. Fiber type was assigned by determining the content of fast and slow myosin heavy chain. MSA-55 was expressed equally by fibers of all types. In contrast, MSA-slow and MSA-140 differed in their expression by muscle fibers depending on fiber type. MSA-slow was detected exclusively at the periphery of fast/slow and slow fibers, but was not detected on fast fibers. MSA-140 was detected on all fibers but fast/slow and slow fibers stained more intensely suggesting that these fiber types contain more MSA-140 than fast fibers. These sarcolemma-associated antigens were developmentally regulated in ovo and in vitro. MSA-55 and MSA-140 were detected on all primary muscle fibers by day 8 in ovo of embryonic development, whereas MSA-slow was first detected on muscle fibers just before hatching. Those antigens expressed by fast fibers (MSA-55 and MSA-140) were expressed only after myoblasts differentiated into myotubes, but were not expressed by fibroblasts in cell culture. Each antigen was also detected in one or more nonskeletal muscle cell types: MSA-55 and MSA-slow in cardiac myocytes and smooth muscle of gizzard (but not vascular structures) and MSA-140 in cardiac myocytes and smooth muscle of vascular structures. MSA-55 was identified as an Mr 55,000, nonglycosylated, detergent-soluble protein, and MSA-140 was an Mr 140,000, cell surface protein. The Mr of MSA-slow could not be determined by immunoblotting or immunoprecipitation techniques. These findings indicate that muscle fibers of different physiological function differ in the components associated with the sarcolemma. While the function of these sarcolemma-associated antigens is unknown, their regulated appearance during development in ovo and as myoblasts differentiate in culture suggests that they may be important in the formation, maturation, and function of fast, fast/slow, and slow muscle fibers.

Histological characterization of a monoclonal antibody raised against the branchial arches of the chick embryo: reactivity with myogenic lineages and a few non-mesodermal derivatives

Monoclonal antibody A7H2 has been selected from a library of monoclonals raised against the branchial arch cells of 3-day-old chicken embryos, and its histological distribution was examined at different embryonic stages and levels. The immunoreactivity appeared during neurulation and was almost general in extraembryonic areas. As the embryo formed, A7H2-fluorescence disappeared from most tissues, but was concentrated in the splanchnomesoderm-derived smooth muscle lineages and epithelial linings of the coelomic cavity. A strong and durable reactivity was also observed in the myogenic condensations constituting the axis of the branchial arches, whereas myotomal cells of the differentiating somites were labelled more weakly and for a shorter time. Interestingly, non-mesodermal fluorescence was restricted to the branchial arch epithelium and some ectodermal placodes, to the thinning-out zones of the neural tube destined to form the choroid plexus, and to the pharyngeal and cloacal extremities of the digestive tract.

Endocytosis of alpha 2-macroglobulin is developmentally regulated during myogenesis

A monoclonal antibody, H143, reacts with an intracellular antigen present and accumulated in E63 rat myoblasts. H143 is directed against a species-specific determinant on purified equine serum alpha 2-macroglobulin. Immunofluorescence analyses of differentiating myoblasts grown in horse serum demonstrate that the capacity to take up alpha 2-macroglobulin is stage-specific: the rapid uptake of alpha 2-macroglobulin characteristic of myoblasts ceases prior to their fusion to form multinucleate fibers (myotubes). Neither rat fibroblasts nor a developmentally defective mutant of E63 exhibit this change in alpha 2-macroglobulin uptake. The temperature and calcium requirements for the uptake of H143 antigen, and its accumulation as effected by lysosomotropic amines, indicate that alpha 2-macroglobulin is taken up by myoblasts via a developmentally regulated endocytic process. Electron microscopy using equine alpha 2-macroglobulin labeled with colloidal gold supports this finding.

DNA replication and differentiation in rat myoblasts studied with monoclonal antibodies against 5-bromodeoxyuridine, actin, and alpha 2-macroglobulin

During the differentiation of skeletal muscle, mononucleate myoblasts proliferate, then stop replicating, spontaneously fuse, and express a large number of genes which encode the muscle phenotype. We have used monoclonal antibodies specific for 5-bromodeoxyuridine, myoactin, and equine alpha 2-macroglobulin to follow and establish the sequence of events that surround the transition from a replicating to a differentiating population. Triple-label immunofluorescence microscopy was used to visualize the changes in DNA synthesis, formation of myoactin fibers, and the cessation of endocytosis of alpha 2-macroglobulin that accompany myogenesis. Our results indicate that myoblasts cease actively endocytosing alpha 2-macroglobulin after stopping DNA synthesis but prior to fusion. Formation of myoactin fibers rarely occurs in mononucleate myoblasts and only in post-mitotic cells, but they are common in multinucleate myotubes. We suggest that the regulation of DNA synthesis is critical to normal myogenesis and that detection of incorporated BrdUrd by immunofluorescence, in conjunction with other antibodies and nucleic acid probes, is a convenient method with which to study and sequence the molecular events in single cells as they relate to the transition in DNA synthesis that accompanies differentiation.