THE RHO-FAMILY G-PROTEINS PLAY A CRITICAL ROLE IN MUSCLE DIFFERENTIATION

The Rho family GTP-binding proteins play a critical role in a variety of cytoskeleton-dependent cell functions. In this study,,ve examined t he role of Rho family G proteins in muscle differentiation. Dominant n egative forms of Rho family proteins and RhoGDI, a GDP dissociation in hibitor, suppressed transcription of muscle-specific genes, while muta tionally activated forms of Rho family proteins strongly activated the ir transcription. C2C12 cells overexpressing RhoGDI (C2C12RhoGDI cells ) did not differentiate into myotubes, and expression levels of myogen in, MRF4, and contractile protein genes but not MyoD and myf5 genes we re markedly reduced in C2C12RhoGDI cells. The promoter activity of the myogenin gene nas suppressed by dominant negative mutants of Rho fami ly proteins and was reduced in C2C12RhoGDI cells. Expression of myocyt e enhancer binding factor 2 (MEF2), which has been reported to be requ ired for the expression of the myogenin gene, was reduced at the mRNA and protein levels in C2C12RhoGDI cells. These results suggest that th e Rho family proteins play a critical role in muscle differentiation, possibly by regulating the expression of the myogenin and MEF2 genes.