Signaling from Rho to the Actin Cytoskeleton Through Protein Kinases ROCK and LIM-kinase

TLDR

The actin cytoskeleton remodels during cell morphogenesis and motility, a process regulated by the small GTPase Rho, yet the precise mechanisms—particularly involving cofilin inhibition through LIM‑kinase phosphorylation—remain unclear. ROCK, a downstream effector of Rho, phosphorylates and activates LIM‑kinase, which in turn phosphorylates cofilin, rather than ROCK directly targeting cofilin. Phosphorylation of LIM‑kinase by ROCK and the resulting increase in cofilin phosphorylation drive Rho‑mediated actin cytoskeleton reorganization, as shown by LPA‑induced neurite retraction and Y‑27632‑sensitive stress‑fiber formation.

Abstract

The actin cytoskeleton undergoes extensive remodeling during cell morphogenesis and motility. The small guanosine triphosphatase Rho regulates such remodeling, but the underlying mechanisms of this regulation remain unclear. Cofilin exhibits actin-depolymerizing activity that is inhibited as a result of its phosphorylation by LIM-kinase. Cofilin was phosphorylated in N1E-115 neuroblastoma cells during lysophosphatidic acid–induced, Rho-mediated neurite retraction. This phosphorylation was sensitive to Y-27632, a specific inhibitor of the Rho-associated kinase ROCK. ROCK, which is a downstream effector of Rho, did not phosphorylate cofilin directly but phosphorylated LIM-kinase, which in turn was activated to phosphorylate cofilin. Overexpression of LIM-kinase in HeLa cells induced the formation of actin stress fibers in a Y-27632–sensitive manner. These results indicate that phosphorylation of LIM-kinase by ROCK and consequently increased phosphorylation of cofilin by LIM-kinase contribute to Rho-induced reorganization of the actin cytoskeleton.