Ubiquitination of RhoA by Smurf1 promotes neurite outgrowth.

The Rho-family of small GTPases consists of essential regulators of neurite outgrowth, axonal pathfinding, and dendritic arborization. Previous work has demonstrated in non-neuronal cell types that Smurf1, an E3 ubiquitin ligase, regulates cell polarity and protrusive activity via PKCzeta-dependent recruitment to cellular protrusion sites, and subsequent ubiquitination and proteasomal degradation of RhoA. In this study, we show that Smurf1 enhances neurite outgrowth in Neuro2a neuroblastoma cells. We demonstrate that RhoA is ubiquitinated, and that Smurf1 and RhoA physically interact in vivo. Interestingly, Smurf1 overexpression in Neuro2a cells dramatically reduces RhoA protein levels during dibutyric cyclic AMP, but not retinoic acid induced neurite outgrowth. This Smurf1-dependent reduction in RhoA protein levels was abrogated using the general proteasome inhibitor MG132, suggesting that RhoA is targeted for ubiquitination and degradation via Smurf1. Together, our data suggest that localized regulation of different subsets of Rho GTPases by specific guidance signals results in an intracellular asymmetry of RhoA activity, which could regulate neurite outgrowth and guidance.