Involvement of cell surface phosphatidylinositol-anchored glycoproteins in cell-cell adhesion of chick embryo myoblasts

Regulation of pre-fusion events: recruitment of M-cadherin to microrafts organized at fusion-competent sites of myogenic cells

Background

Previous research indicates that the membrane ruffles and leading edge of lamellipodia of myogenic cells contain presumptive fusion sites. A micrometer-sized lipid raft (microraft) is organized at the presumptive fusion site of mouse myogenic cells in a cell-contact independent way and serves as a platform tethering adhesion proteins that are relevant to cell fusion. However, the mechanisms underlying recruitment of adhesion proteins to lipid rafts and microraft organization remain unknown.

Results

Here we show that small G-protein Rac1 was required for microraft organization and subsequent cell fusion. However, Rac1 activity was unnecessary for recruitment of M-cadherin to lipid rafts. We found that p120 catenin (p120) binds to M-cadherin exclusively in lipid rafts of differentiating myogenic cells. The Src kinase inhibitor SU6656 prevented p120 binding to M-cadherin and their recruitment to lipid rafts, then suppressed microraft organization, membrane ruffling, and myogenic cell fusion. Suppression of membrane ruffling in SU6656-treated cells was partially restored by pretreatment with the protein tyrosine phosphatase inhibitor vanadate. The present analyses using an antibody to tyrosine phosphorylated p120 suggest that Src family kinases play a role in binding of p120 to M-cadherin and the recruitment of M-cadherin to lipid rafts through phosphorylation of putative substrates other than p120.

Conclusions

The present study showed that the procedure establishing fusion-competent sites consists of two sequential events: recruitment of adhesion complexes to lipid rafts and organization of microrafts. The recruitment of M-cadherin to lipid rafts depended on interaction with p120 catenin, whereas the organization of microrafts was controlled by a small G protein, Rac1.

Dynamic clustering and dispersion of lipid rafts contribute to fusion competence of myogenic cells

Recent research indicates that the leading edge of lamellipodia of myogenic cells (myoblasts and myotubes) contains presumptive fusion sites, yet the mechanisms that render the plasma membrane fusion-competent remain largely unknown. Here we show that dynamic clustering and dispersion of lipid rafts contribute to both cell adhesion and plasma membrane union during myogenic cell fusion. Adhesion-complex proteins including M-cadherin, beta-catenin, and p120-catenin accumulated at the leading edge of lamellipodia, which contains the presumptive fusion sites of the plasma membrane, in a lipid raft-dependent fashion prior to cell contact. In addition, disruption of lipid rafts by cholesterol depletion directly prevented the membrane union of myogenic cell fusion. Time-lapse recording showed that lipid rafts were laterally dispersed from the center of the lamellipodia prior to membrane fusion. Adhesion proteins that had accumulated at lipid rafts were also removed from the presumptive fusion sites when lipid rafts were laterally dispersed. The resultant lipid raft- and adhesion complex-free area at the leading edge fused with the opposing plasma membrane. These results demonstrate a key role for dynamic clustering/dispersion of lipid rafts in establishing fusion-competent sites of the myogenic cell membrane, providing a novel mechanistic insight into the regulation of myogenic cell fusion.

Localized cyclic AMP-dependent protein kinase activity is required for myogenic cell fusion

Multinucleated myotubes are formed by fusion of mononucleated myogenic progenitor cells (myoblasts) during terminal skeletal muscle differentiation. In addition, myoblasts fuse with myotubes, but terminally differentiated myotubes have not been shown to fuse with each other. We show here that an adenylate cyclase activator, forskolin, and other reagents that elevate intracellular cyclic AMP (cAMP) levels induced cell fusion between small bipolar myotubes in vitro. Then an extra-large myotube, designated a "myosheet," was produced by both primary and established mouse myogenic cells. Myotube-to-myotube fusion always occurred between the leading edge of lamellipodia at the polar end of one myotube and the lateral plasma membrane of the other. Forskolin enhanced the formation of lamellipodia where cAMP-dependent protein kinase (PKA) was accumulated. Blocking enzymatic activity or anchoring of PKA suppressed forskolin-enhanced lamellipodium formation and prevented fusion of multinucleated myotubes. Localized PKA activity was also required for fusion of mononucleated myoblasts. The present results suggest that localized PKA plays a pivotal role in the early steps of myogenic cell fusion, such as cell-to-cell contact/recognition through lamellipodium formation. Furthermore, the localized cAMP-PKA pathway might be involved in the specification of the fusion-competent areas of the plasma membrane in lamellipodia of myogenic cells.

Mannose receptor regulates myoblast motility and muscle growth

Myoblast fusion is critical for the formation, growth, and maintenance of skeletal muscle. The initial formation of nascent myotubes requires myoblast-myoblast fusion, but further growth involves myoblast-myotube fusion. We demonstrate that the mannose receptor (MR), a type I transmembrane protein, is required for myoblast-myotube fusion. Mannose receptor (MR)-null myotubes were small in size and contained a decreased myonuclear number both in vitro and in vivo. We hypothesized that this defect may arise from a possible role of MR in cell migration. Time-lapse microscopy revealed that MR-null myoblasts migrated with decreased velocity during myotube growth and were unable to migrate in a directed manner up a chemoattractant gradient. Furthermore, collagen uptake was impaired in MR-null myoblasts, suggesting a role in extracellular matrix remodeling during cell motility. These data identify a novel function for MR during skeletal muscle growth and suggest that myoblast motility may be a key aspect of regulating myotube growth.

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.

Neural cell adhesion molecule (NCAM) and myoblast fusion

Considerable evidence points to an involvement of neural cell adhesion molecule (NCAM) in myoblast fusion. Changes in the level of NCAM expression, isoform specificity, and localization in muscle cells and tissues correspond to key morphogenetic events during muscle differentiation and repair. Furthermore, anti-NCAM antibodies have been shown by others to reduce the rate of myoblast fusion, whereas overexpression of NCAM cDNAs increases the rate of myoblast fusion compared to controls. In this study we have used a novel fusion assay based on intracistronic complementation of lacZ, in combination with fluorescent X-gal histochemistry and immunocytochemistry to assess levels of NCAM expression in individual muscle cells. Our results indicate that a substantial proportion of newly fused myoblasts have NCAM expression levels unchanged from the levels of the surrounding unfused population suggesting that increased expression of NCAM is not required for wild-type myoblasts to fuse. Moreover, pure populations of primary myoblasts isolated from mice homozygous null for NCAM and therefore lacking the molecule, when placed in differentiation medium, consistently fused to form contractile myotubes with kinetics equivalent to wild-type primary myoblasts. We conclude that the increase in expression of NCAM, although typically observed during myogenesis, is not essential to myoblast fusion to form myotubes.

Glycosylphosphatidylinositol-anchored cell surface proteins regulate position-specific cell affinity in the limb bud

Although regional differences in mesenchymal cell affinity in the limb bud represent positional identity, the molecular basis for cell affinity is poorly understood. We found that treatment of the cell surface with bacterial phosphatidylinositol-specific phospholipase C (PI-PLC) could change cell affinity in culture. When PI-PLC was added to the culture medium, segregation of the progress zone (PZ) cells from different stage limb buds was inhibited. Similarly, sorting out of the cells from different positions along the proximodistal (PD) axis of the same stage limb buds was disturbed. Since PI-PLC can remove glycosylphosphatidylinositol (GPI)-anchored membrane bound proteins from the cell surface, the GPI-anchored cell surface proteins may be involved in sorting out. To define the GPI-anchored molecules that determine the segregation of limb mesenchymal cells, we examined the effect of neutralizing antibody on the EphA4 receptor that binds to GPI-anchored cell surface ligands, called ephrin-A. Sorting out of the PZ cells at different stages could be inhibited by the neutralizing antibody to EphA4. These results suggest that EphA4 and its GPI-anchored ligands are, at least in part, involved in sorting out of limb mesenchymal cells with different proximal-distal positional values, and that GPI-anchored cell surface proteins play important roles in determining cell affinity in the limb bud.

Contractile activity regulates isoform expression and polysialylation of NCAM in cultured myotubes: involvement of Ca2+ and protein kinase C

Muscle development involves a series of complex cell-cell interactions that are mediated, at least in part, by several different cell adhesion molecules. Previous work from this lab showed that the different isoforms of NCAM and its level of polysialylation are developmentally regulated during chick myogenesis in vivo and that this regulation is important for normal muscle development. Using developing chick secondary myotubes grown in culture, we show here that both the polysialylation of NCAM and the developmental switch in isoform expression are regulated by activity and that Ca2+ entry through voltage-gated channels and the subsequent activation of protein kinase C are required for the developmental changes in NCAM isoform synthesis. Specifically, PSA expression was shown to be developmentally regulated with high expression being temporally correlated with the onset of spontaneous contractile activity. Furthermore, blocking contractile activity caused a decrease in PSA expression, while increasing activity with electrical stimulation resulted in its up-regulation. Immunoblot and metabolic labeling studies indicated that dividing myoblasts synthesize primarily 145-kD NCAM, newly formed, spontaneously contracting myotubes synthesize 130-, 145-, and 155-kD NCAM isoforms, while older, more mature myotubes primarily synthesize the glycosylphosphatidylinositol-anchored 130-kD isoform which, in contrast to the other three isoforms, had a high rate of turnover. This developmental switch in NCAM isoform expression could be inhibited with Ca2+ channel blockers and inhibitors of protein kinase C. Taken together, these results suggest that Ca2+ ions and protein kinase C are involved in a second messenger cascade coupling membrane depolarization with transcriptional factors that regulate NCAM isoform synthesis and polysialylation.

Regulation and activity-dependence of N-cadherin, NCAM isoforms, and polysialic acid on chick myotubes during development

Muscle development in vivo involves a complex sequence of cell-cell interactions in which secondary myotubes first form in association with primary myotubes and subsequently separate from them. We show here that during this process N-cadherin and the different structural forms of NCAM are regulated in a pattern that involves both temporal changes in expression and localization to particular regions of the muscle cell surface. In particular, levels of N-cadherin on maturing myotubes are decreased, and the form of NCAM synthesized by the muscle changes from a transmembrane non-polysialylated to a lipid-linked polysialylated membrane protein. Moreover, while NCAM was distributed on all myotube surfaces, the polysialyated form of NCAM was restricted to regions of the myotube surface that had recently separated from neighboring cells. We previously found that blockade of nerve-induced activity by d-Tubocurarine perturbed muscle cell interactions, resulting in a failure of myotubes to separate. We now show that this activity blockade also alters adhesion molecule expression. First, N-cadherin was no longer down-regulated in maturing myotubes, and its persistence on the surfaces of mature myotubes may partly explain their failure to separate. Secondly, the developmental switch from transmembrane to lipid-linked NCAM did not occur, and polysialylated NCAM was no longer formed. As the unusual physical properties of PSA have been proposed to impede cell-cell interactions, this alteration would also be expected to compromise cell separation. Together, these results suggest that the regulated expression of both N-cadherin and NCAM isoforms including their polysialylation, is an essential mechanism for the normal separation of secondary myotubes from primary myotubes.

Differential effects of over-expressed neural cell adhesion molecule isoforms on myoblast fusion

We have used a transfection based approach to analyze the role of neural cell adhesion molecule (NCAM) in myogenesis at the stage of myoblast fusion to form multinucleate myotubes. Stable cell lines of myogenic C2 cells were isolated that express the transmembrane 140- or 180-kD NCAM isoforms or the glycosylphosphatidylinositol (GPI) linked isoforms of 120 or 125 kD. We found that expression of the 140-kD transmembrane isoform led to a potent enhancement of myoblast fusion. The 125-kD GPI-linked NCAM also enhanced the rate of fusion but less so when a direct comparison of cell surface levels of the 140-kD transmembrane form was carried out. While the 180-kD transmembrane NCAM isoform was effective in promoting C2 cell fusion similar to the 140-kD isoform, the 120-kD isoform did not have an effect on fusion parameters. It is possible that these alterations in cell fusion are associated with cis NCAM interactions in the plane of the membrane. While all of the transfected human NCAMs (the transmembrane 140- and 180-kD isoforms and the 125- and 120-kD GPI isoforms) could be clustered in the plane of the plasma membrane by species-specific antibodies there was a concomitant clustering of the endogenous mouse NCAM protein in all cases except with the 120-kD human isoform. These studies show that different isoforms of NCAM can undergo specific interactions in the plasma membrane which are likely to be important in fusion. While the transmembrane and the 125-kD GPI-anchored NCAMs are capable of enhancing fusion the 120-kD GPI NCAM is not. Thus it is likely that interactions associated with NCAM intracellular domains and also the muscle specific domain (MSD) region in the extracellular domain of the GPI-linked 125-kD NCAM are important. In particular this is the first role ascribed to the O-linked carbohydrate containing MSD region which is specifically expressed in skeletal muscle.

Use of p112-deficient myoblasts to determine the temporal order of the in vitro expression of myogenic components

The present investigation examines the function and site(s) of involvement of an ecto-protein kinase and its substrate protein (a cell surface 112 kDa protein) in the in vitro myogenic pathway. The phosphorylated 112 kDa protein (p112) has recently been shown to be involved in myogenesis. Not much information is currently available on the role of the ecto-protein kinase and the 112 kDa protein in modulating the expression of the myogenic factors and various muscle-specific proteins. Five different p112-deficient rat myoblasts were used to examine the temporal order of the in vitro expression of the myogenic components; namely, L6 myoblasts treated with BrdUrd or phloretin, a conditional p112-defective mutant (clone D1), an ecto-protein kinase-deficient mutant (clone F72), and a mutant defective in the 112 kDa protein (clone D1/S4). All these p112-deficient myoblasts were also impaired in myogenesis. The absence of p112, ecto-protein kinase, and/or the 112 kDa protein was found to have no effect on the Myf-5 mRNA level. However, the expected increase in NCAM and Myf-4 mRNAs was not observed in any of the p112-deficient myoblasts examined. This suggests that the p112 site of action is probably located upstream of the Myf-4 and NCAM sites in the myogenic pathway. While 7-28 fold increases in the MLC, MHC, and TnT transcripts were observed during myogenesis, such increases were not observed in the p112-deficient myoblasts. However, when mutant D1/S4 was transfected with the myf-4 cDNA, expression of Myf-4 in the transfectant resulted in increased level of the MLC, MHC, and TnT mRNAs, and in myotube formation, even though the Myf-5 and NCAM mRNA levels and p112 were not altered. This suggests that p112 may function by activating transcription of Myf-4, which will subsequently promote the expression of muscle-specific proteins and myotube formation. In the absence of p112, Myf-5 cannot activate the expression of Myf-4, NCAM, MLC, MHC, TnT, and myotube formation. If all these components are involved in the same myogenic pathway, then p112 may be acting downstream from Myf-5, and upstream from NCAM and Myf-4.

Glucose induces lipolytic cleavage of a glycolipidic plasma membrane anchor in yeast

In the yeast Saccharomyces cerevisiae an amphiphilic cAMP-binding protein has been found recently to be anchored to plasma membranes by virtue of a glycolipid structure (Müller and Bandlow, 1991a, 1992). The cAMP-binding parameters of this protein are affected by the lipolytic removal of the glycosylphosphatidylinositol (GPI) membrane anchor by exogenous (G)PI-specific phospholipases C or D (PLC or PLD) (Müller and Bandlow, 1993) suggesting a regulatory role of glycolipidic membrane anchorage. Here we report that transfer of yeast cells from lactate to glucose medium results in the conversion of the amphiphilic form of the cAMP receptor protein into a hydrophilic version accompanied by the rapid loss of fatty acids from the GPI anchor of the [14C]palmitic acid-labeled protein. Analysis of the cleavage site identifies [14C]inositol phosphate as the major product after treatment of the soluble, [14C]inositol-labeled protein with nitrous acid which destroys the glucosamine constituent of the anchor. Together with the observed cross-reactivity of the hydrophilic fragment with antibodies directed against the cross-reacting determinant of soluble trypanosomal variable surface glycoproteins (i.e., myo-inositol-1,2-cyclic phosphate) this demonstrates that, in membrane release, the initial cleavage event is catalyzed by an intrinsic GPI-PLC activated upon transfer of cells to glucose medium. Release from the plasma membrane in soluble form requires, in addition, the presence of high salt or alpha-methyl mannopyranoside, or the removal of the carbohydrate moieties, because otherwise the protein remains associated with the membrane presumably at least in part via its N-glycosidic carbohydrate side chains. The data point to the possibility that cleavage of the anchor could play a role in the transfer of the signal for the nutritional situation to the interior of the cell.

Noncoordinate developmental regulation of N-cadherin, N-CAM, integrin, and fibronectin mRNA levels during myoblast terminal differentiation

N-cadherin, N-CAM, fibronectin, and beta 1-integrins have been implicated in the control of myoblast fusion to form multinucleate myotubes, a critical step in the terminal differentiation of skeletal muscle. We have analyzed the temporal pattern of expression of mRNA transcripts encoding these adhesion molecules during the terminal differentiation of C2 mouse myoblasts. The accumulation of mRNA transcripts encoding N-cadherin, N-CAM, fibronectin, alpha 5-integrin, and beta 1-integrin subunits was developmentally, but not coordinately, regulated. N-cadherin and integrin subunit expression was maximal in prefusion myoblasts and declined thereafter. In contrast, N-CAM mRNA levels were low in prefusion myoblasts, and increased coincident with the onset of terminal differentiation. Fibronectin mRNA levels were also low in myoblasts, and they did not increase until after cell fusion had occurred. The results indicate that despite their lack of coordinate regulation maximal levels of mRNA transcripts encoding adhesion molecules are present at a stage which corresponds to the peak of the active phase of myoblast fusion.

Release of cell surface-associated basic fibroblast growth factor by glycosylphosphatidylinositol-specific phospholipase C

Heparan sulfate proteoglycans (HSPG) are ubiquitous constituents of mammalian cell surfaces and most extracellular matrices. A portion of the cell surface HSPG is anchored via a covalently linked glycosyl-phosphatidylinositol (Pl) residue, which can be released by treatment with a glycosyl-Pl specific phospholipase C (Pl-PLC). We report that exposure of bovine aortic endothelial and smooth muscle cells to Pl-PLC resulted in release of cell surface-associated, growth-promoting activity that was neutralized by antibasic fibroblast growth factor (bFGF) antibodies. Active bFGF was also released by treating the cells with bacterial heparitinase. Under the same conditions there was no release of mitogenic activity from cells (BHK-21, NIH/3T3, PF-HR9) that expressed little or no bFGF, as opposed to Pl-PLC-mediated release of active bFGF from the same cells transfected with the bFGF gene. The released bFGF competed with recombinant bFGF in a radioreceptor assay. Addition of Pl-PLC to sparsely seeded vascular endothelial cells resulted in a marked stimulation of cell proliferation, but there was no mitogenic effect of Pl-PLC on 3T3 fibroblasts. Studies with exogenously added 125I-bFGF revealed that about 6.5% and 20% of the cell surface-bound bFGF were released by treatment with Pl-PLC and heparitinase, respectively. Both enzymes also released sulfate-labeled heparan sulfate from metabolically labeled 3T3 fibroblasts. Pl-PLC failed to release 125I-bFGF from the subendothelial extracellular matrix (ECM), as compared to release of 60% of the ECM-bound bFGF by heparitinase. Our results indicate that 3-8% of the total cellular content of bFGF is associated with glycosyl-Pl anchored cell surface HSPG. This FGF may exert both autocrine and paracrine effects, provided that it is released by Pl-PLC and adequately presented to high affinity bFGF cell surface receptor sites.

Neural cell adhesion molecule mediates contact-dependent inhibition of growth of near-diploid mouse fibroblast cell line m5S/1M

A near-diploid mouse fibroblast cell line m5S/1M used in this study shows a high sensitivity to contact-dependent inhibition of growth, and the addition of EGF causes both morphological change and loss of contact-dependent inhibition of growth. The m5S/1M cell and its transformants obtained by x-ray irradiation have been used to search for the cell surface glycoproteins that are responsible for the growth regulation via cell-cell interactions. Lectin blotting analyses showed that the expression of the cell surface glycoprotein of 140 kD (140KGP) is highly sensitive to the transformation induced either by x-ray irradiation or by the EGF stimulation. We purified the 140KGP and found that it was composed of two glycoproteins. The major component of 140KGP was identified as neural cell adhesion molecule (NCAM) by amino acid sequence analyses of the peptide fragments and by the cross-reactivity with anti-NCAM mAb, clone H28.1.2.3. Monoclonal antibody against 140KGP (clone LN-10) recognizes all three isoforms of NCAM expressed on m5S/1M cell and showed that the expression of NCAM was highly sensitive to the transformation. Furthermore, the immobilized LN-10 strongly inhibited the growth of actively proliferating m5S/1M cells and the LN-10 in a soluble form showed a significant growth-stimulating effect on the confluent quiescent cultures of m5S/1M cells. The results show that NCAM plays a major role in the contact-dependent inhibition of growth of m5S/1M, and that NCAM might be involved in the regulation of cell growth during embryogenesis and formation of nervous systems.

N-cadherin-associated proteins in chicken muscle

The development and functional activity of the heart depends on the regulated interaction of cardiac cells. This is in part mediated by cell-cell adhesion molecules such as N-cadherin. N-cadherin belongs to a family of Ca+(+)-dependent, transmembrane, adhesion glycoproteins that promote cell-cell adhesion by molecular self-association extracellularly, and interact intracellularly with the cytoskeleton through highly conserved carboxy-terminal domains. In this paper we show that embryonic chicken cardiac myocytes grown in vitro display Ca+(+)-dependent adhesion and express N-cadherin. When immunoprecipitated from detergent extracts of embryonic chicken cardiac and skeletal muscle cultures, N-cadherin associates with proteins immunologically unrelated to itself. The associated proteins are similar in molecular weight to proteins that coimmunoprecipatate with E-cadherin from human epithelial cells. We postulate that the coimmunoprecipitating proteins are involved in linking the cadherins to the cytoskeleton.

A role for the Ca2(+)-dependent adhesion molecule, N-cadherin, in myoblast interaction during myogenesis

The formation of multinucleate skeletal muscle cells (myotubes) is a Ca2(+)-dependent process involving the interaction and fusion of mononucleate muscle cells (myoblasts). Specific cell-cell adhesion precedes lipid bilayer union during myoblast fusion and has been shown to involve both Ca2(+)-independent (CI)2 and Ca2(+)-dependent (CD) mechanisms. In this paper we present evidence that CD myoblast adhesion involves a molecule similar or identical to two known CD adhesion glycoproteins, N-cadherin and A-CAM. These molecules were previously identified by other laboratories in brain and cardiac muscle, respectively, and are postulated to be the same molecule. Antibodies to N-cadherin and A-CAM immunoblotted a similar band with a molecular weight of approximately 125,000 in extracts of brain, heart, and pectoral muscle isolated from chick embryos and in extracts of muscle cells grown in vitro at Ca2+ concentrations that either promoted or inhibited myotube formation. In assays designed to measure the interaction of fusion-competent myoblasts in suspension, both polyclonal and monoclonal anti-N-cadherin antibodies inhibited CD myoblast aggregation, suggesting that N-cadherin mediates the CD aspect of myoblast adhesion. Anti-N-cadherin also had a partial inhibitory effect on myotube formation likely due to the effect on myoblast-myoblast adhesion. The results indicate that N-cadherin/A-CAM plays a role in myoblast recognition and adhesion during skeletal myogenesis.

Enhanced myogenesis in NCAM-transfected mouse myoblasts

The fusion of mononucleate precursor myoblasts to form the multinucleated skeletal muscle fibre is proceeded by a series of complex cell-cell interactions but the cell-surface molecules involved in these events have not been characterized. During myogenesis in vivo and in vitro, expression of the neural cell adhesion molecule (NCAM) undergoes an isoform transition that precisely correlates with terminal myoblast differentiation and myotube formation. Altered processing of RNA results in the replacement of the transmembrane NCAM (relative molecular mass, 145,000 (145K) in proliferating myoblasts by a predominant 125K NCAM form linked to glycosyl phosphatidylinositol in myotubes. We now report that mouse myoblasts transfected to constitutively express the human muscle-specific 125K glycosylphosphatidylinositol-linked NCAM isoform more readily fuse to form myotubes. This suggests that NCAM plays a part in myoblast fusion and that the isoform switch may promote this function.

A role for the neural cell adhesion molecule, NCAM, in myoblast interaction during myogenesis

The Ca2(+)-independent neural cell adhesion molecule, NCAM, is expressed by both nerve and muscle cells and has been shown to mediate both nerve-nerve and nerve-muscle cell interaction. A role for NCAM in muscle-muscle cell interaction has been proposed but not demonstrated. Here we report evidence that NCAM is expressed by embryonic chick muscle cells during in vitro development and functions together with Ca2(+)-dependent adhesion molecules in mediating myoblast interaction during the formation of multinucleate cells.