PROJECT SUMMARY Determining how neurons are assembled into functional circuits will provide insight into developmental disorders of the nervous system and may suggest therapeutic approaches to promote nerve regeneration. To navigate to their correct targets, axons must modulate their responses to extracellular cues, and regulated intracellular protein trafficking plays a pivotal role in this process. For example, commissural axons cross the midline despite the presence of repellant ligands in order to establish connections that are essential for coordinated motor behavior. In Drosophila, the endosomal protein Commissureless (Comm) prevents commissural axons from prematurely responding to the repellant Slit, by inhibiting surface expression of the Slit receptor Roundabout1 (Robo1). In mammals, Robo receptors are also negatively regulated in commissural axons prior to midline crossing, but the mechanisms are unknown. Unlike Slit and Robo, comm is not conserved in vertebrates; however, our preliminary data indicate that the vertebrate Nedd-4 interacting proteins (Ndfip1 and Ndfip2) can act analogously to Comm to regulate the trafficking and stability of human Robo receptors in vitro, and that loss of Ndfip1or Ndfip2 function in vivo in mice results in increased expression of Robo receptors and defects in axon guidance. We will test the hypothesis that Ndfip proteins control axon guidance in the developing brain and spinal cord by recruiting Robo receptors to endosomes and triggering their degradation through interactions with Nedd-4 E3 ubiquitin ligases. In aim 1, we will use molecular, cell biological and biochemical approaches to: 1) determine whether Ndfip proteins exhibit differential effects on intracellular trafficking of Robo receptors or other axon guidance receptors, 2) delimit the sequences that are necessary and sufficient to mediate interactions between Ndfip proteins and Robo family receptors, 3) characterize the role of HECT E3 ligase activity on receptor trafficking and 4) identify the specific Nedd4 family ligase(s) that is required for Robo receptor regulation. Aim 2 will explore the embryonic expression patterns and in vivo requirements for Ndfip proteins during commissural axon guidance by examining the trajectory of commissural axons in Ndfip1 and Ndfip2 single and double mutants, using 1) immunofluorescence for pre and post-crossing commissural axon markers, and 2) unilateral lipophilic dye tracing experiments. In addition, we will generate conditional knockouts of Ndfip1, Nedd4-1 and Nedd4-2 using Cre-lines specific for commissural neurons to investigate requirements for Nedd4-1 and Nedd4-2 in spinal commissural axon guidance. Aim 3 will assess the in vivo links between Ndfip proteins and Robo receptors by 1) testing whether neurons cultured from Ndfip mutants exhibit altered repulsive responses to exogenously added Slit proteins and 2) examining genetic interactions between Ndfip and Robo mutants. Finally, a biochemical screen...