Identification of a proline-rich inositol polyphosphate 5-phosphatase (PIPP) collapsin response mediator protein 2 (CRMP2) complex that regulates neurite elongation

Neuron polarization is essential for the formation of one axon and multiple dendrites, establishing the neuronal circuitry. Phosphoinositide-3-kinase (PI3K) signaling promotes axon selection and elongation. Here we report in hippocampal neurons siRNA knockdown of the proline-rich inositol-polyphosphate 5-phosphatase (PIPP), which degrades PI3K-generated PtdIns(3,4,5)P(3), results in multiple hyper-elongated axons consistent with a polarization defect. We identify CRMP2, which regulates axon selection by promoting WAVE1 delivery via Kinesin-1 motors to the axon growth cone, as a PIPP-interacting protein by Y2H-screening, direct binding studies and coimmunoprecipitation of an endogenous PIPP, CRMP2 and Kinesin-1 complex from brain lysates. The C-terminal growth cone-targeting domain of PIPP facilitates its interaction with CRMP2. PIPP growth cone localization is CRMP2-dependent. PIPP knockdown in PC12 cells promotes neurite elongation, WAVE1 and Kinesin-1 growth cone localization, whilst knockdown of CRMP2 exhibits the opposite phenotype, with shorter neurites and decreased WAVE1/Kinesin-1 at the growth cone. In contrast CRMP2 overexpression promotes neurite elongation, a phenotype rescued by full-length PIPP, or expression of the CRMP2-binding PIPP domain. Therefore this study identifies PIPP and CRMP2 exert opposing roles in promoting axon selection and neurite elongation and the complex between these proteins serves to regulate the localization of effectors that promote neurite extension.