The formin mDia regulates GSK3β through novel PKCs to promote microtubule stabilization but not MTOC reorientation in migrating fibroblasts

In migrating cells, external signals polarize the microtubule (MT) cytoskeleton by stimulating the formation of oriented, stabilized MTs and inducing the reorientation of the MT organizing center (MTOC). Glycogen synthase kinase 3 beta (GSK3 beta) has been implicated in each of these processes, although whether it regulates both processes in a single system and how its activity is regulated are unclear. We examined these issues in wound-edge, serum-starved NIH 3 beta fibroblasts where MT stabilization and MTOC reorientation are triggered by lysophosphatidic acid (LPA), but are regulated independently by distinct Rho GTPase-signaling pathways. In the absence of other treatments, the GSK3 beta inhibitors, LiCl or SB216763, induced the formation of stable MTs, but not MTOC reorientation, in starved fibroblasts. Overexpression of GSK3 beta in starved fibroblasts inhibited LPA-induced stable MTs without inhibiting MTOC reorientation. Analysis of factors involved in stable MT formation (Rho, mDia, and EB1) showed that GSK3 beta functioned upstream of EB1, but downstream of Rho-mDia. mDia was both necessary and sufficient for inducing stable MTs and for up-regulating GSK3 beta phosphorylation on Ser9, an inhibitory site. mDia appears to regulate GSK3 beta through novel class PKCs because PKC inhibitors and dominant negative constructs of novel PKC isoforms prevented phosphorylation of GSK3 beta Ser9 and stable MT formation. Novel PKCs also interacted with mDia in vivo and in vitro. These results identify a new activity for the formin mDia in regulating GSK3 beta through novel PKCs and implicate novel PKCs as new factors in the MT stabilization pathway.