RHOA SIGNALING VIA SERUM RESPONSE FACTOR PLAYS AN OBLIGATORY ROLE IN MYOGENIC DIFFERENTIATION

Serum response factor (SRF) plays a central role during myogenesis, be ing required for the expression of striated alpha-actin genes. As show n here, the small GTPase RhoA dependent activation of SRF results in t he expression of muscle specific genes, thereby promoting myogenic dif ferentiation in myoblast cell lines. Co-expression of activated V14-Rh oA and SRF results in an approximately 10-fold activation of the skele tal alpha-actin promoter in replicating myoblasts, while SRFpm1, a dom inant negative SRF mutant, blocks RhoA dependent skeletal alpha-actin promoter activity. Serum withdrawal further potentiates RhoA- and SRF- mediated activation of alpha-actin promoter to about 30-fold in differ entiated myotubes. In addition, the proximal SRE1 in the skeletal alph a-actin promoter is sufficient to mediate RhoA signaling via SRF. Furt hermore, SRFpm1 and to a lesser extent dominant negative N19-RhoA inhi bit myoblast fusion, postreplicative myogenic differentiation, and exp ression of direct SRF targets such as skeletal alpha-actin and indirec t targets such as myogenin and alpha-myosin heavy chain. Moreover, Rho A also stimulates the autoregulatable murine SRF gene promoter in myob lasts, and the expression level of SRF is reduced in myoblasts overexp ressing N19-RhoA Our study supports the concept that RhoA signaling vi a SRF serves as an obligatory muscle differentiation regulatory pathwa y.