Identification of Rho GTPases implicated in terminal differentiation of muscle cells in ascidia

Background information. Members of the Rho GTPase family mediate changes in the actin cytoskeleton and are also implicated in developmental processes, including myogenesis. Nevertheless, a comprehensive analysis of these proteins during myofibrillogenesis has never been performed in any organism. Results. Using the ascidian model to identify the role of Rho GTPases on myofibrillogenesis, we show that transcripts for all Rho GTPases are detected in muscle cells of the embryo. We find that activation of RhoA, TC10 and Cdc42 (cell division cycle 42) disturbs the polarity of muscle cells, whereas that of other Rho GTPases induced cell positioning defects. Moreover, dominant negative version of five Rho GTPases, RhoA, Rac2, RCL2 (Rac- and Cdc42-like 2), TC10 and WRCH (Wnt-1 responsive Cdc42 homologue), impaired the formation of mature myofibrils. Conclusions. Taken together, our results show that several Rho GTPase-dependent pathways are required to control the spatial localization of muscle cells in the embryo and to coordinate myofibril assembly. This stresses the importance of analysing the entire Rho family when studying a new biological process.