During nervous system development, axons must navigate a variety of complex environments to reach their synaptic targets, creating functional neural networks. The growth cones of early extending axons interact with attractive and repulsive cues in the surrounding tissue. Later extending axons can interact directly with these earlier "pioneering" axons, following the path that has already been laid out. The first axon to extend along the ventral nerve cord (VNC) in C. elegans is that of the AVG neuron. AVG pioneers the right VNC and approximately 50 axons follow it, most notably motor neuron and interneuron axons. When AVG is ablated or fails to differentiate, these axons are dramatically misguided. The initial cues that guide AVG axon extension are currently unknown, since the known axon guidance cues have either no or only minimal AVG axon navigation defects.
nid-1 is a basement membrane component that is enriched along the VNC. In the absence of
nid-1, AVG is misguided at a 12% penetrance. We conducted an enhancer screen in a
nid-1 mutant background and identified several mutants that shown increased AVG axon navigation defects. In a wildtype background these mutants do not have any AVG defects, indicating that the genes are needed in conjunction with
nid-1 for correct AVG axon extension. Among the genes identified in this enhancer screen is F49E7.2, an enzyme homologous to Phosphatidylinositol glycan (GPI) anchor biosynthesis class X (PIG-X). 50% of F49E7.2
(hd145)
nid-1(
cg119) double mutant animals show AVG axon guidance defects. This suggests that one or more GPI-anchored proteins are required for AVG axon navigation. We identified the IgCAMs
wrk-1,
rig-5 and
rig-6 as likely candidate genes, as double mutants with
nid-1 show enhanced AVG defects similar to F49E7.2
(hd145) mutants. Genetic interaction studies suggest that
rig-5 and
rig-6 act together in the same genetic pathway, whereas
wrk-1 acts in a separate pathway. Our study establishes a novel role for three IgCAMs in the
nid-1 dependent navigation of a pioneer axon.