The atypical protein kinase C (aPKC) in complex with PAR3 and PAR6 is required for axon-dendrite differentiation, but the upstream factors responsible for regulating its activity are largely unknown. Here, we report that in cultured hippocampal neurons aPKC is directly regulated by Dishevelled (Dvl), an immediate downstream effector of Wnt. We found that downregulation of Dvl abrogated axon differentiation, whereas Dvl overexpression resulted in multiple axon formation. Interestingly, Dvl was associated with aPKC and this interaction resulted in aPKC stabilization and activation. Furthermore, the multiple axon formation resulting from Dvl overexpression was attenuated by expressing a dominant-negative aPKC in these neurons and overexpression of aPKC prevented the loss of axon caused by Dvl downregulation. Finally, Wnt5a, a noncanonical Wnt, activated aPKC and promoted axon differentiation. The Wnt5a effect on axon differentiation was attenuated by downregulating Dvl or inhibiting aPKC. Thus, Dvl-aPKC interaction can promote axon differentiation mediated by the PAR3-PAR6-aPKC complex.

The PAR-3͞PAR-6͞atypical PKC (aPKC) complex is required for axon-dendrite specification of hippocampal neurons. However, the downstream effectors of this complex are not well defined. In this article, we report a role for microtubule affinity-regulating kinase (MARK)͞PAR-1 in axon-dendrite specification. Knocking down MARK2 expression with small interfering RNAs induced formation of multiple axon-like neurites and promoted axon outgrowth. Ectopic expression of MARK2 caused phosphorylation of tau (S262) and led to loss of axons, and this phenotype was rescued by expression of PAR-3, PAR-6, and aPKC. In contrast, the polarity defects caused by an MARK2 mutant (T595A), which is not responsive to aPKC, were not rescued by the PAR-3͞PAR-6͞aPKC complex. Moreover, polarity was abrogated in neurons overexpressing a mutant of MARK2 with a deleted kinase domain but an intact aPKC-binding domain. Finally, suppression of MARK2 rescued the polarity defects induced by a dominant-negative aPKC mutant. These results suggest that MARK2 is involved in neuronal polarization and functions downstream of the PAR-3͞PAR-6͞aPKC complex. We propose that aPKC in complex with PAR-3͞PAR-6 negatively regulates MARK(s), which in turn causes dephosphorylation of microtubule-associated proteins, such as tau, leading to the assembly of microtubules and elongation of axons.polarity complex ͉ partition-defective protein 1b ͉ axon specification