Spatio-temporal regulation of Rac1 and Cdc42 activity during nerve growth factor-induced neurite outgrowth in PC12 cells. 
Neurite outgrowth is an important process in the formation of neuronal networks.  It is widely accepted that Rac1 and Cdc42, members of the Rho GTPase family, positively regulate neurite extension through reorganization of the actin cytoskeleton; however, it remains largely unknown when and where Rac1 and Cdc42 are activated during neuritogenesis.  This study visualized the spatio-temporal regulation of Rac1 and Cdc42 activities during nerve growth factor (NGF)-induced neurite outgrowth in living PC12 cells by using probes based on the principle of fluorescence resonance energy transfer (FRET).  Immediately after the addition of NGF, Rac1 and Cdc42 were transiently activated in broad areas of the cell periphery; a repetitive activation and inactivation cycle was then observed at the motile tips of protrusions.  This localized activation, which was more evident in PC12 cells treated with NGF for more than 24 h, might facilitate neurite extension, because the expression of constitutively active mutants of Rac1 and Cdc42 abrogated NGF-induced neurite outgrowth.  FRET imaging also delineated a difference between the localization of activated Rac1 and that of Cdc42 within the neurite tips.  Experiments with dominant-negative mutants suggested that Rac1 and Cdc42 were activated by a common guanine nucleotide exchange factor(s) in an early stage of the activation phase.  Therefore, the difference between Rac1- and Cdc42-activated areas possibly came from the differential localization between Rac1-specific GTPase-activation proteins (GAPs) and Cdc42-specific GAPs.  It was concluded that the localized activation of Rac1 and Cdc42 was caused by both guanine nucleotide exchange factors and GAPs, and was important for neurite extension.