Sema4D/plexin-B1 activates GSK-3beta through R-Ras GAP activity, inducing growth cone collapse.

Plexins are receptors for the axonal guidance molecules known as semaphorins, and the semaphorin 4D (Sema4D) receptor plexin-B1 induces repulsive responses by functioning as an R-Ras GTPase-activating protein (GAP). Here we characterized the downstream signalling of plexin-B1-mediated R-Ras GAP activity, inducing growth cone collapse. Sema4D suppressed R-Ras activity in hippocampal neurons, in parallel with dephosphorylation of Akt and activation of glycogen synthase kinase (GSK)-3beta. Ectopic expression of the constitutively active mutant of Akt or treatment with GSK-3 inhibitors suppressed the Sema4D-induced growth cone collapse. Constitutive activation of phosphatidylinositol-3-OH kinase (PI(3)K), an upstream kinase of Akt and GSK-3beta, also blocked the growth cone collapse. The R-Ras GAP activity was necessary for plexin-B1-induced dephosphorylation of Akt and activation of GSK-3beta and was also required for phosphorylation of a downstream kinase of GSK-3beta, collapsin response mediator protein-2. Plexin-A1 also induced dephosphorylation of Akt and GSK-3beta through its R-Ras GAP activity. We conclude that plexin-B1 inactivates PI(3)K and dephosphorylates Akt and GSK-3beta through R-Ras GAP activity, inducing growth cone collapse.