Rac is required for the survival of cortical neurons.

Rac1, a member of small Rho GTPases, is involved in diverse cellular processes in neuronal cells. Rac1 plays especially important roles during development, and its roles have been extensively studied using Rac1-deficient mice. Rac3, a close homolog of Rac1, is ubiquitously expressed in the nervous system and may therefore compensate for Rac1 in Rac1-deficient cells. Exploration of the roles of Rac in neurons may therefore be difficult. We thus deleted both Rac1 and Rac3 in cortical neurons. Rac-deficient cerebral cortices formed slightly hypoplastic but almost normally layered structures at birth, but cortical neurons underwent apoptosis soon after birth. Rac-deficient cortical neurons had poor survivability and there was reduction in the length and the number of neurites in vitro. Activation of Pak1, a downstream effector of Rac, in Rac-deficient cortical neurons rescued the survivability in vitro. Pak1-activated Rac-deficient neurons had numerous dendrites, but no axons. Restoration of p35, a regulator of Cdk5, partly rescued the survivability of Rac-deficient neurons both in vitro and in vivo. Expression of p35 also partly rescued the length and the number of neurites in Rac-deficient neurons in vitro. Rac was shown to be indispensable for the survival of cortical neurons, and Pak1 and Cdk5/p35 work as downstream effectors of Rac to promote neuronal survival.