Repulsive axonal pathfinding requires the Ena/VASP family of actin regulatory proteins in vertebrates

Abstract

Vertebrate nervous system development requires the careful interpretation of many attractive and repulsive guidance molecules. For the incredibly complicated wiring diagram comprising the vertebrate nervous system to elaborate properly, the highly motile "growth cone" at the tip of an axon must sense extracellular embryonic cues and respond through a number of intracellular interactions leading ultimately to coordinated changes in cytoskeletal morphology and modulation of the axonal path. Here I describe axon pathfinding defects displayed by mice genetically deficient for all three vertebrate Ena/VASP homologues: Mena, VASP, and EVL. As has been reported previously in invertebrates, these defects share phenotypic overlap with those seen in mice genetically deficient for the repulsive guidance molecules Slit and Robo. I find that the pathfinding errors observed in Ena/VASP deficient mice are likely a result of failure to respond to Slit/Robo. Furthermore, based on my findings, I propose a "four-step" model of growth cone responses to repulsive cues. Finally I find that the direct binding of Ena/VASP proteins to Robo seen in invertebrates is conserved and expanded in vertebrates. These interactions appear to be tunable by phosphorylation, suggesting a model by which context dictates the Ena/VASP:Robo interaction, potentially leading to changes in growth cone responsiveness to guidance cues.