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The role of integrins in muscle differentiation was addressed by ectopic expression of integrin alpha subunits in primary quail skeletal muscle, a culture system particularly amenable to efficient transfection and expression of exogenous genes. Ectopic expression of either the human alpha5 subunit or the chicken alpha6 subunit produced contrasting phenotypes. The alpha5-transfected myoblasts remain in the proliferative phase and are differentiation inhibited even in confluent cultures. In contrast, myoblasts that overexpress the alpha6 subunit exhibit inhibited proliferation and substantial differentiation. Antisense suppression of endogenous quail alpha6 expression inhibits myoblast differentiation resulting in sustained proliferation. These effects of ectopic alpha subunit expression are mediated, to a large extent, by the cytoplasmic domains. Ectopic expression of chimeric alpha subunits, alpha5ex/6cyto and alpha6ex/5cyto, produced phenotypes opposite to those observed with ectopic alpha5 or alpha6 expression. Myoblasts that express alpha5ex/6cyto show decreased proliferation while differentiation is partially restored. In contrast, the alpha6ex/5cyto transfectants remain in the proliferative phase unless allowed to become confluent for at least 24 h. Furthermore, expression of human alpha5 subunit cytoplasmic domain truncations, before and after the conserved GFFKR motif, shows that this sequence is important in alpha5 regulation of differentiation. Ectopic alpha5 and alpha6 expression also results in contrasting responses to the mitogenic effects of serum growth factors. Myoblasts expressing the human alpha5 subunit differentiate only in the absence of serum while differentiation of untransfected and alpha6-transfected myoblasts is insensitive to serum concentration. Addition of individual, exogenous growth factors to alpha5-transfected myoblasts results in unique responses that differ from their effects on untransfected cells. Both bFGF or TGFbeta inhibit the serum-free differentiation of alpha5-transfected myoblasts, but differ in that bFGF stimulates proliferation whereas TGF-beta inhibits it. Insulin or TGF-alpha promote proliferation and differentiation of alpha5-transfected myoblasts; however, insulin alters myotube morphology. TGF-alpha or PDGF-BB enhance muscle alpha-actinin organization into myofibrils, which is impaired in differentiated alpha5 cultures. With the exception of TGF-alpha, these growth factor effects are not apparent in untransfected myoblasts. Finally, myoblast survival under serum-free conditions is enhanced by ectopic alpha5 expression only in the presence of bFGF and insulin while TGF-alpha and TGF-beta promote survival of untransfected myoblasts. Our observations demonstrate (1) a specificity for integrin alpha subunits in regulating myoblast proliferation and differentiation; (2) that the ratio of integrin expression can affect the decision to proliferate or differentiate; (3) a role for the alpha subunit cytoplasmic domain in mediating proliferative and differentiative signals; and (4) the regulation of proliferation, differentiation, cytoskeletal assembly, and cell survival depend critically on the expression levels of different integrins and the growth factor environment in which the cells reside.

Integrin alpha subunit ratios, cytoplasmic domains, and growth factor synergy regulate muscle proliferation and differentiation.