The Rho small GTPase regulates myosin II activity through the phosphorylation of the myosin light chain (MLC) by activating Rho‐kinase, which is a target of Rho. Several lines of evidence point to an important role of Rho in the action of lysophosphatidic acid (LPA) and thrombin in provoking neurite retraction in N1E‐115 neuroblastoma cells.

Here we examined whether Rho‐kinase and myosin II are involved in neurite retraction in N1E‐115 cells. We showed that the expression of constitutively active forms of Rho‐kinase induced neurite retraction in N1E‐115 cells and MLC phosphorylation in NIH 3T3 cells, whereas the expression of dominant negative forms of Rho‐kinase inhibited the LPA‐induced neurite retraction in N1E‐115 cells and the serum‐induced MLC phosphorylation in NIH 3T3 cells. The expression of mutant MLC^T18D,S19D (substitution of Thr and Ser by Asp), which is known to lead to the activation of myosin ATPase and a conformational change of myosin II when reconstituted with myosin heavy chains in vitro, also promoted neurite retraction.

These results indicate that Rho‐kinase is involved in the LPA‐induced neurite retraction downstream of Rho, and that myosin II activation promotes neurite retraction downstream of Rho and Rho‐kinase.