Nerve Growth Factor (NGF) is a polypeptide growth factor that plays a critical role in the differentiation of immature neuroblasts and the subsequent survival of a subpopulation of mature neurons in both the peripheral and central nervous systems. NGF initiates its effects by binding to TrkA, a receptor type tyrosine kinase that when activated, initiates a signal transduction cascade resulting in Ras activation and the induction of early response genes. NGF-induced neurite outgrowth is also accompanied by extensive re-organization of the neuronal cytoskeleton to initiate neurite extension of neurite processes. Indeed, while the shape of filopodia and lamella in growth cones is largely determined by the organization of the actin cytoskeleton, little is known about how NGF and TrkA signaling is linked to the cytoskeleton. Over the past few years, we have been studying the Crk adaptor protein and its role during NGF-induced neuronal differentiation. Upon NGF addition to PC12 cells or primary dorsal root ganglia neurons, c-Crk is rapidly tyrosine phosphorylated at tyrosine 222. Interestingly, overexpression of mutant c-Crk carrying a mutation at Y222 (c-CrkY222F) impairs NGF-mediated neurite outgrowth in PC12 cells and abolishes TrkA- induced tyrosine phosphorylation of the cytoskeletal protein Paxillin. Our results indicate that Crk is involved in a novel aspect of TrkA signaling to adhesion complexes, thereby regulating actin re- organization and cell adhesion. In the present application, we shall study the molecular mechanisms by which c-Crk couples the TrkA receptor to the cytoskeleton, with the long-term goal to understand how NGF promotes differentiation and survival. Specifically, we plan to: (i) determine whether c-CrkY222F is a dominant negative c-Crk protein by impairing NGF-induced cell adhesion, (ii) determine the circuitry involved in NGF-induced tyrosine phosphorylation of c-Crk, (iii) determine the relationship between NGF- and adhesion-induced tyrosine phosphorylation of the c-Crk effector protein Paxillin, (iv) determine the biological role of Paxillin during NGF-mediated responses, and (v) determine the role of c-Crk during neuronal development using transgene approaches.
