Rho GTPases in growth cone guidance.

It is now well established that the small GTPases of the Rho family--Rac, Cdc42 and Rho--regulate growth cone morphology. Less clear is their role in guiding the growth cone. Do they act permissively, providing the dynamic actin structures needed for guidance? Or do they act instructively, transducing specific guidance signals? Recent studies have provided the first strong evidence for an instructive role: extracellular guidance cues can modulate Rho GTPase activities in vitro, and Rho GTPase activators function in growth cone guidance in vivo. The pathways linking Rho GTPases and the actin cytoskeleton are also rapidly coming into view, revealing further points of regulation by extracellular guidance cues. The growth cone is therefore guided by signals transduced both via and independently of Rho GTPases.