Multiple axon guidance pathways asymmetrically distribute axons in the ventral nerve cord of Caenorhabditis elegans

Early during development neurons project small filamentous processes, axons and dendrites, that extend and eventually connect with other cells and tissues. These processes can grow over long distances and allow for transmission of information between cells. The proper functioning of our nervous system is dependent on these same processes correctly navigating to specific end targets. This is achieved through guidance cues in the environment which interact with receptors on the extending processes allowing them to be steered in the correct direction. Unfortunately, due to the high complexity of most vertebrate nervous systems our understanding of how axons and dendrites use these cues to navigate is still very limited. The aim of this thesis was to discover novel genes regulating axon guidance to shine additional light on how axons navigate during development. Normally axons of the ventral nerve cord in the nematode Caenorhabditis elegans are invariably sorted asymmetrically. Animals with mutations impacting function in individual axon guidance signaling pathways show no or only very low penetrance of disruption of VNC asymmetry. Here genetic screens successfully isolated four mutants in which asymmetry between major longitudinal axon tracts is disrupted. One of these four mutants include a novel allele of the gene col-99 which encodes a previously uncharacterized transmembrane collagen with vertebrate homologs. Detailed characterization of animals lacking COL-99 revealed widespread axon guidance defects impacting longitudinal and lateral axon navigation of a variety of neurons. Of the remaining three mutants two were found to be alleles of unc-52 and unc-34, both previously characterised for roles in axon guidance, while the final mutation remains unidentified. Disruption of any one signaling pathway does not lead to penetrant VNC asymmetry defects suggesting redundancy between parallel signaling pathways here. To better understand how signaling pathways of multiple guidance cues may converge to control guidance at choice points single mutants were crossed into a nid-1 null mutant background and VNC asymmetry phenotypes examined. Previously nid-1 was found to substantially enhance navigation defects of the VNC pioneering neuron AVG when crossed into mutants showing a low penetrance of AVG navigation defects. Double mutants with nid-1 saw defect penetrance significantly increase in several cases indicating parallel signaling pathways. Combination of mutants into triple and quadruple mutant strains showed that UNC-6, SAX-3, and COL-99 represent members of parallel signaling pathways acting redundantly to guide axons in establishment of asymmetry which in addition depends on basement membranes components, including EPI-1. Thus multiple axon guidance signaling pathways, acting in tandem, ensure correct guidance and segregation of axons at the anterior choice point of the VNC establishing VNC asymmetry.