During nervous system development, axons navigate complex environments to reach synaptic targets. Growth cones of early extending axons must interact with guidance cues in surrounding tissue, while later extending axons can interact directly with earlier "pioneering" axons, "following" their path. This is how the ventral nerve cord (VNC) is formed in Caenorhabditis elegans. The first axon to extend along the VNC is that of the AVG neuron, which pioneers the right VNC axon tract. Motoneuron and interneuron axons of the motor circuit follow the AVG pioneer axon. In the absence of AVG, many VNC follower axons are misguided, causing locomotion defects. The mechanisms for AVG axon guidance are largely unknown. To uncover genes controlling AVG axon guidance, we conducted an enhancer screen in a sensitized
nid-1 mutant background. NID-1 is a basement membrane component enriched along the VNC. In the absence of
nid-1, the AVG axon is misguided at a low penetrance. From this screen, we previously identified several genes, including
aex-3, a
rab-3 guanine exchange factor.
aex-3 and
rab-3 genetically interact with
unc-5, a receptor for the guidance cue UNC-6/netrin. Here we describe a new gene in this pathway:
ccd-5 is a putative
cdk-5 binding partner that acts in a pathway with
cdk-5,
aex-3,
unc-18, and
unc-5.
ccd-5 nid-1 double mutants exhibit elevated navigation defects of AVG, command interneuron, and motoneuron axons. Navigation defects of interneuron and motoneuron axons significantly correlate with AVG pioneer axon defects. This suggests that most of the follower axon defects observed are a secondary effect of the AVG defects, and that
ccd-5 specifically affects pioneer axon navigation. To determine the functional effects of these navigation defects, we used a multi-worm tracker to assess locomotion, responsiveness, and habituation. We found the
ccd-5 single mutants have no significant behaviour defects, and
ccd-5 nid-1 double mutants are less responsive to mechanosensory stimuli than
nid-1 single mutants. These surprisingly minor behavioral defects indicate either a high tolerance for axon guidance defects within the motor circuit and/or an ability to maintain synaptic connections among commonly misguided axons.