| Description |
Development of the nervous system involves establishment of precise long distance connections between distinct single cells called neurons. During development, long axons extend from the cell body and grow, following a very specific pathway through a complex environment to their target. The growth cone is a dynamic structure with finger-like filopodia that sense guidance cues in the surrounding environment through receptors. External guidance cues can be attractive or repulsive, and growth cone turning in response to a guidance cue is driven by actin dynamics, with increased polymerization during attractive turning and increased disassembly during repulsive turning. Therefore, regulators of actin dynamics such as actin-binding proteins are the targets of signaling in the growth cone initiated by external guidance cues. The growth cone changes its behavior very rapidly in response to guidance cues, even as it becomes further and further from the cell body. Growth cones have the ability to act autonomously to guidance cues, in order to continue to react quickly without delay. Local mRNA translation in growth cones has an important role in growth cone behavior, giving the ability to respond to external guidance cues without communication with the cell body. Local translation is regulated by RNA-binding proteins and directly influences actin dynamics that are important for growth cone behavior. While in vitro studies have revealed a wealth of knowledge about the mechanisms involved with local translation during axon guidance, the requirement for the function of RNA-binding proteins in vivo has not been tested extensively. In this dissertation, an in vivo local translation timelapse assay, performed in the zebrafish retinotectal system, demonstrates that the zebrafish β- actin3'UTR is sufficient to target Kaede expression to RGC growth cones. Also, Igf2bp1 is shown to be the zebrafish ZBP1 ortholog, and a bipartite "zipcode element," required for interaction with ZBP1, is identified in the β-actin3'UTR. I also present the first evidence to date that Igf2bp1/ZBP1 function is required for axon guidance, and furthermore provide data that suggest that Igf2bp1 function may be required for axon specification or elongation, contrary to the common belief that ZBP1 function is required only for axon branching and arbor formation, and that local β-actin translation is only required for growth cone turning during axon guidance. |
