GSK3 Regulates Myelin-Dependent Axon Outgrowth Inhibition through CRMP4

Myelin-associated inhibitors (MAIs) contribute to failed regeneration in the CNS. The intracellular signaling pathways through which MAIs block axonal repair remain largely unknown. Here, we report that the kinase GSK3 is directly phosphorylated and inactivated by MAIs, consequently regulating protein–protein interactions that are critical for myelin-dependent inhibition. Inhibition of GSK3 mimics the neurite outgrowth inhibitory effect of myelin. The inhibitory effects of GSK3 inhibitors and myelin are not additive indicating that GSK3 is a major effector of MAIs. Consistent with this, overexpression of GSK3 attenuates myelin inhibition. MAI-dependent phosphorylation and inactivation of GSK3 regulate phosphorylation of CRMP4, a cytosolic regulator of myelin inhibition, and its ability to complex with RhoA. Introduction of a CRMP4 antagonist attenuates the neurite outgrowth inhibitory properties of GSK3 inhibitors. We describe the first example of GSK3 inactivation in response to inhibitory ligands and link the neurite outgrowth inhibitory effects of GSK3 inhibition directly to CRMP4. These findings raise the possibility that GSK3 inhibition will not effectively promote long-distance CNS regeneration following trauma such as spinal cord injury.