TGF signals regulate axonal development through distinct Smad-independent mechanisms

Proper nerve connections form when growing axons terminate at the correct postsynaptic target. Here I show that Transforming growth factor beta (TGF) signals regulate axon growth. In most contexts, TGF signals are tightly linked to Smad transcriptional activity. Although known to exist, how Smad-independent pathways mediate TGF responses in vivo is unclear. In Drosophila mushroom body (MB) neurons, loss of the TGF receptor Baboon (Babo) results in axon overextension. Conversely, misexpression of constitutively active Babo results in premature axon termination. Smad activity is not required for these phenotypes. This study shows that Babo signals require the Rho GTPases Rho1 and Rac, and LIM kinase1 (LIMK1), which regulate the actin cytoskeleton. Contrary to the well-established receptor activation model, in which type 1 receptors act downstream of type 2 receptors, this study shows that the type 2 receptors Wishful thinking (Wit) and Punt act downstream of the Babo type 1 receptor. Wit and Punt regulate axon growth independently, and interchangeably, through LIMK1-dependent and -independent mechanisms. Thus, novel TGF receptor interactions control non-Smad signals and regulate multiple aspects of axonal development in vivo.