The central long-term goal of the project is to understand molecular signaling mechanisms that underly the development of neural connectivity. Many extracellular guidance cues have been identified in the last decade, but the specific molecular pathways involved in their actions are not yet well understood, and are currently an interesting and important area of research. Our main focus is on ephrins and their Eph receptors, which have important functions in axon guidance, dendrite development, and synapse formation. Aim 1 is to investigate downstream signaling mechanisms. A key feature of the ephrins is that they signal bi-directionally, with a 'forward'signal through the receptor, and a 'reverse'signal transduced through the ligand. Reverse signaling pathways have been identified for the transmembrane ephrin-Bs, but signaling by the lipid-anchored ephrin-As is less well understood, and will be a focus of this Aim. Aim 2 is to study upstream regulation of Eph receptors. The focus will be on RNA-based mechanisms that have recently been identified in axons;RNA-based regulation is currently an exciting area of biology generally, and these studies will delineate novel mechanisms in neural development. Aim 3 is to characterize interactions among receptor-like protein tyrosine phosphatases, Eph receptor kinases, and their common theme of regulatory interactions with heparan sulfate proteoglycans. While our primary goal is to elucidate basic molecular mechanisms, the work has broader implications for understanding and treatment of disease. Ephrins and receptor-like protein tyrosine phosphatases have been identified as regulators of CNS axon regeneration. Beyond axonal connectivity, ephrins have important roles in other aspects of biology, including vascular development, cancer, and stem cell biology.