Distinct functions of Rac1 and Cdc42 during axon guidance and growth cone morphogenesis in Drosophila

Rho family small GTPases are thought to be key molecules in the regulation of cytoskeletal organization, especially for actin filaments. In order to examine the functions of Rac1 and Cdc42 in axon guidance at the midline of the central nervous system in Drosophila embryos, we either activated or inactivated Rac1 and Cdc42 in all postmitotic neurons. We found that the phenotypes of Cdc42 activation and Rac1 inactivation were similar to those of roundabout mutants, in that many extra axons crossed the midline. We also found that Rac1 inactivation is dominant over Roundabout receptor activation. Our observations indicate that Rac1 and Cdc42 have distinct functions in downstream signalling events triggered by Roundabout receptors. In order to further examine the functional difference between Rac1 and Cdc42 in the growth cone morphogenesis, we used primary embryonic cultures to closely observe neurite formation. We showed that activation of Rac1 and Cdc42 has distinct effects on neurite formation, particularly on growth cone morphology and the actin filaments within. Both Rac1 and Cdc42 activation induced large growth cones and long filopodia, but Cdc42 did so more efficiently than Rac1. Only Rac1 activation, however, induced thick actin bundles in the filopodia. We also found a clear difference between Rac1 and Cdc42 in terms of the response to an inhibitor of actin polymerization. Our results suggest that Cdc42 is specifically involved in the regulation of actin filaments in growth cones, whereas Rac1 is involved in additional functions.