Genetic analysis of axon outgrowth in Caenorhabditis elegans

Axon polarity, guidance, target recognition, and retrograde signaling are highly dynamic and tightly regulated cellular processes. In order to understand the phases that regulate axon outgrowth and relation to functional output, we have focused our work on the GABAergic motor neurons AVL and DVB in Caenorhabditis elegans. The enteric muscles form synaptic connections onto AVL and DVB mediating the expulsion step of defecation behavior.;We began with an investigation into the nature of cues that mediate neural outgrowth defects in AVL and DVB by testing known axon outgrowth and defecation defective mutants. A variety of pathways, represented by a limited number of mutants, mediate axonal outgrowth defects in AVL and DVB including mutations in neurotransmission, cytoskeletal architecture, and signal transduction. Next, we conducted a forward genetic screen for the ectopic/erratic neurite outgrowth (eno) phenotype in the AVL/DVB neurons. Whereas previously characterized axonal outgrowth and synaptic activity mutants display pleiotropic defects, our screen was biased toward the selection of mutants that displayed "wild type like" appearance and behavior in order to retrieve genes involved in uncharacterized contexts and in cell type specific roles. We have isolated 12 eno mutants which have been categorized into at least 11 complementation groups and 5 distinct classes.;The pre-selection of mutants that are healthy in appearance from our screen has enabled the isolation of a unique allele, ot1, which codes for the SCF F-box E3 ubiquitin ligase lin-23. lin-23 has previously been shown to negatively regulate postembryonic cell divisions. We show that lin-23 mutants display a dramatic overgrowth of the AVL neuron within the tail. Further investigation reveals abnormal termination of outgrowth in a subset of neurons, including ventral nerve cord motor neurons, head motor neurons, and mechanosensory neurons. Our isolated allele of lin-23, a point mutation in the C-terminal tail of the protein, displays axonal outgrowth defects similar to those observed in null alleles of this gene, but does not affect cell cycle regulation. We have thus separated a role for LIN-23 into two distinct processes, cell cycle control and axon patterning.