PROJECT SUMMARY: Motor neurons are the only neuronal cell type with cell bodies located within the central nervous system (CNS) and axons that project out into the periphery. Once outside of the CNS, motor axons bundle, but do not intermingle, with sensory axons as they co-extend towards peripheral targets. To establish this distinct projection pattern, motor neurons rely on combinations of molecular signaling pathways that direct them during different stages of their development. The mechanisms that constrain motor neuron cell bodies within the spinal cord while allowing motor axons to exit into the periphery are still not completely understood. I have found that a cell adhesion molecule, Transient Axonal Glycoprotein type-1 (TAG-1), is a fundamental, multifunctional regulator of motor neuron development and circuit formation. Multiple neural cell types transiently express TAG-1 during development, and while TAG-1 is expressed on motor neuron cell bodies and axons during early stages of differentiation and axon outgrowth, the function of TAG-1 in motor neurons is not known. An examination of TAG- 1 knockout mice revealed three major defects in motor neurons: (1) motor axon bundles (ventral roots) are expanded, (2) motor neuron cell bodies aberrantly leave the spinal cord, and (3) motor axons have severe guidance defects and invade dorsal root ganglia. Utilizing a combination of novel mouse genetics, whole embryo imaging, and in vitro assays, I will investigate the molecular mechanisms of TAG-1 function in motor circuit formation. I will also determine whether TAG-1 is required cell autonomously to regulate motor neuron migration, axonal growth and guidance. This proposal will elucidate the function of TAG-1 in motor neurons and uncover the fundamental mechanisms that regulate early motor neuron development. Overall, understanding the mechanisms that regulate neural circuit formation can inform future therapeutic interventions for re- establishing proper neural circuits after physical injury, neurodegeneration, or developmental mis-wiring.