Cytoplasmic p21^Cip1/WAF1 regulates neurite remodeling by inhibiting Rho-kinase activity

p21^Cip1/WAF1 has cell cycle inhibitory activity by binding to and inhibiting both cyclin/Cdk kinases and proliferating cell nuclear antigen. Here we show that p21^Cip1/WAF1 is induced in the cytoplasm during the course of differentiation of chick retinal precursor cells and N1E-115 cells. Ectopic expression of p21^Cip1/WAF1 lacking the nuclear localization signal in N1E-115 cells and NIH3T3 cells affects the formation of actin structures, characteristic of inactivation of Rho. p21^Cip1/WAF1 forms a complex with Rho-kinase and inhibits its activity in vitro and in vivo. Neurite outgrowth and branching from the hippocampal neurons are promoted if p21^Cip1/WAF1 is expressed abundantly in the cytoplasm. These results suggest that cytoplasmic p21^Cip1/WAF1 may contribute to the developmental process of the newborn neurons that extend axons and dendrites into target regions.