Project 3: Neuronal Morphogenesis and the Microtubule Cytoskeleton
Our studies of the LAR receptor phosphatase led us to the discovery that the LAR pathway regulates synaptic growth and the morphogenesis of the active zone – a structure that orchestrates neurotransmitter release at chemical synapses. We have defined factors upstream and downstream of LAR in this context, and the machinery appears to be highly conserved. Upstream, LAR interacts with synaptic heparan sulfate proteoglycans that control distinct aspects of synapse morphogenesis or function. Downstream, LAR activity is mediated by a pathway linking the phosphatase the intracellular tyrosine kinase Abl. Genetic screens for Abl effectors in Drosophila identified the both actin regulatory factors and the microtubule plus-tip interacting protein (MT+TIP) CLASP as a protein required for Abl function in vivo. Our subsequent biochemical and functional studies showed that CLASP associates with and is phosophorylated by CLASP in mammalian cells, suggesting conservation in the guidance machinery. We have used genetic and proteomic tools to define a network of functional partners for CLASP, and find not only additional MT+TIPs, but also MT-actin cross-linking factors suggesting that CLASP and Abl are involved in the coordination of the two major polymer systems. Our recent analysis of the Abl and CLASP-interactors revealed a novel growth cone-specific role for the MT polymerase (XMAP215) that appears to involve coordination and coupling between the MT and actin polymer networks. We are currently pursuing additional CLASP and Abl interacting components.