Protein kinase Cα and integrin-linked kinase mediate the negative axon guidance effects of Sonic hedgehog

In addition to its role as a morphogen, Sonic hedgehog (Shh) has also been shown to function as a guidance factor that directly acts on the growth cones of various types of axons. However, the noncanonical signaling pathways that mediate the guidance effects of Shh protein remain poorly understood. We demonstrate that a novel signaling pathway consisting of protein kinase C alpha (PKC alpha) and integrin-linked kinase (ILK) mediates the negative guidance effects of high concentration of Shh on retinal ganglion cell (RGC) axons. Shh rapidly increased Ca2+ level and activated PKC alpha and ILK in the growth cones of RGC axons. By in vitro kinase assay, PKC alpha was found to directly phosphorylate ILK on threonine-173 and -181. Inhibition of PKC alpha or expression of a mutant ILK with the PKC alpha phosphorylation sites mutated (ILK-DM), abolished the Shh-induced macropinocytosis, growth cone collapse and repulsive axon turning. In vivo, expression of a dominant negative PKC alpha or ILK-DM disrupted RGC axon pathfinding at the optic chiasm but not the projection toward the optic disk, supporting that this signaling pathway plays a specific role in Shh-mediated negative guidance effects. (c) 2012 Elsevier Inc. All rights reserved.