Distinct effects of Rac1 on differentiation of primary avian myoblasts

Rho family GTPases have been implicated in the regulation of the actin cyto skeleton in response to extracellular cues and in the transduction of signa ls from the membrane to the nucleus. Their role in development and cell dif ferentiation, however, is little understood. Here we show that the transien t expression of constitutively active Rac1 and Cdc42 in unestablished avian myoblasts is sufficient to cause inhibition of myogenin expression and blo ck of the transition to the myocyte compartment, whereas activated RhoA. af fects myogenic differentiation only marginally. Activation of c-Jun N-termi nal kinase (JNK) appears not to be essential for block of differentiation b ecause, although Rac1 and Cdc42 GTPases modestly activate JNK in quail myob lasts, a Rac1 mutant defective for JNK activation can still inhibit myogeni c differentiation. Stable expression of active Rac1, attained by infection with a recombinant retrovirus, is permissive for terminal differentiation, but the resulting myotubes accumulate severely reduced levels of muscle-spe cific proteins. This inhibition is the consequence of posttranscriptional e vents and suggests the presence of a novel level of regulation of myogenesi s. We also show that myotubes expressing constitutively active Rac1 fail to assemble ordered sarcomeres. Conversely, a dominant-negative Rac1 variant accelerates sarcomere maturation and inhibits v-Src-induced selective disas sembly of I-Z-I complexes. Collectively, our findings provide a role for Ra d during skeletal muscle differentiation and strongly suggest that Rac1 is required downstream of v-Src in the signaling pathways responsible for the dismantling of tissue-specific supramolecular structures.