Accurate axon pathfinding is an essential yet highly complicated process during nervous system development. The mechanisms by which axons form complex functional neuronal networks are still a major puzzle and are relevant to understanding how abnormalities in neuronal development arise and also to nerve regeneration therapeutics. My long-term goal is to define the logic by which guidance information is integrated at the level of cytoskeletal dynamics control during axon pathfinding. As a starting point to address this issue, I will study the role of Msps and TACC, two microtubule-associated proteins which have been recently identified as suppressors of the Abl tyrosine kinase effector protein Orbit which regulates microtubule dynamics in the growth cone and mediates midline axon repulsion in the Drosophila nervous system. I will use genetic, biochemical, proteomic, and cell biological assays to investigate Msps, TACC and Orbit function in the growth cone to define the network of interactions which coordinate positive and negative microtubule dynamics during axon guidance. Specifically, I will: 1) define potential genetic pathways of interaction between Msps, TACC, and the Abl Kinase pathway (e.g. Slit, Robo, Orbit), using axonal pathfinding phenotypes in the Drosophila embryonic nervous system as an assay, testing the hypothesis that Msps and TACC function opposite of Orbit and distinguishing between possible genetic models; 2) use biochemical and proteomic analysis to determine if there are direct physical interactions between Msps/TACC, Orbit, and Abl, as well as to expand and define the Msps and TACC interaction networks involved in regulating microtubule dynamics in a Drosophila cell culture line and in neurons; and 3) define the cellular mechanisms of action of Msps and TACC, using highresolution live imaging in Xenopus growth cones. In particular, I will determine if Msps and TACC play a functionally antagonistic role to Orbit, by promoting MT extension towards the growth cone leading edge, or whether they have a different effect on MT dynamics in Xenopus growth cones. Abnormalities in axon guidance have been associated with multiple hereditary neurological disorders and thus this work may shed light on how these defects arise and possibly how to prevent them. Furthermore, mechanisms involved in axon guidance are thought to influence the ability of axons to regenerate after neural injury and so we may be able to use this information to design treatments to allow regeneration in the future. Finally, the proteins studied here are also misregulated in certain cancers. Thus, the research proposed here is of broad biomedical significance. [unreadable] [unreadable]