Molecular and cellular mechanisms of axon guidance during visual system development in zebrafish

During embryonic development of vision, newly formed retinal ganglion cells (RGCs) in the developing eye send axons over long distances and complex anatomic terrain in order to form precise connections with specific targets in the brain. In the visual system of vertebrates, only RGC axons form the retinotectal (RT) projection, the fully formed connection between the retina and its primary target in the brain, the optic tectum. In forming the RT projection, accurate navigation of RGC axons is accomplished through a series of complex molecular signaling interactions that occur between the leading process, or growth cone of the axon, and the variety of cell types over which it must pass. A wide variety of molecules that orchestrate axon guidance in nervous systems of diverse species continue to be identified, but many remain largely unknown. Here I use the well characterized zebrafsh visual system to investigate RGC axon pathfinding in the genetic mutant space cadet. I show that in space cadet mutant zebrafish larvae, RGC axons fail to navigate properly at two critical choice points along the retinotectal projection as the result of a primary defect in axon guidance. The first navigational choice point affected occurs at the optic nerve head (ONH), preventing RGC axons from recognizing the exit point from the eye. The second guidance error that space cadet RGC axons make enroute to their target occurs at the optic chiasm. Affected RGC axons entering the chiasm turn sharply backward and project improperly into the ipsilateral optic tract. Analysis of chimeric embryos demonstrates that space cadet activity is required cell autonomously within RGC growth cones in order to navigate properly. Furthermore, I demonstrate that the chemokine SDF1a signals through its receptor CXCR4b to modulate slit/robo repulsion during RGC axon guidance, and which may also impinge on space cadet activity. I have mapped the space cadet mutant locus to a small pericentromeric region of chromosome 21, and analysis of all transcription units (predicted and known) in the interval indicates that the mutation affects a gene likely to be novel to vertebrate RGC axon guidance