The role of microtubule plus-end binding protein TACC3 during axon outgrowth and guidance

Abstract

Axon guidance is a critical process in forming the connections between a neuron and its target. Development of a properly functioning nervous system relies heavily on how accurately an axon is guided to the right target. Defects in the guidance machinery may result in neurological disorders. The growth cone that is formed at the tip of a growing axon is responsible for navigating axons to their final targets. The growth cone steers the growing axon towards the appropriate direction by integrating extracellular guidance cues received by membrane-associated receptors at the growth cone periphery. Upon receiving guidance cues, a number of intracellular signal transduction pathways are initiated downstream of the guidance receptors, that can promote or halt growth cone advance. The growth cone generates these responses by remodeling its cytoskeletal components, which are actin network in the periphery and microtubules in the growth cone center. In this thesis, we focus on understanding the role of microtubule dynamics regulation within the growth cone as it makes guidance decisions. Specifically, we examine the role of TACC3 as a microtubule plus-end binding protein during axon outgrowth and guidance. We show that TACC3 localizes at microtubule plus-ends in embryonic Xenopus laevis growth cones and regulates microtubule growth parameters. We also show that TACC3 is important for promoting axon outgrowth in cultured neural tube explants. Furthermore, our data suggests that TACC3 affects axon guidance in vivo and ex vivo. Examination of embryos depleted of TACC3 revealed guidance defects in the spinal cord neurons, while TACC3-overexpressing cultured spinal neurons showed increased resistance to Slit2-induced growth cone collapse. Finally, in an attempt to delineate the mechanism behind TACC3-mediated axon guidance under Slit2, we studied the importance of tyrosine phosphorylation induced by Abelson tyrosine kinase. We find that retaining phosphorylatable tyrosines within the TACC domain is important for its microtubule plus-end tracking behavior and its impact on microtubule dynamics regulation, axon outgrowth and guidance. Together, this thesis contributes new insights to the understanding of the role of TACC3 as a microtubule plus-end binding protein and identifies TACC3 as a potential regulator of axon outgrowth and guidance during Xenopus laevis embryonic development.