CWN-2 is one of five C. elegans Wnt ligands, with known roles in neuron migration, axon guidance and nerve ring placement (reviewed in (Sawa and Korswagen, 2013)). CWN-2 is highly expressed in the SMDD neurons (Kennerdell et al., 2009; Taylor et al., 2019), and therefore we were interested in determining whether CWN-2 regulates SMDD axonal development.We used the
cwn-2(
ok895) deletion allele to analyze SMDD axonal development. We found that loss of
cwn-2 caused a defective SMDD axonal phenotype, where axons do not extend along the dorsal sublateral cord, suggesting that the axons did not exit the nerve ring due to defects in axon outgrowth or guidance (Figure 1A-C). To ensure that this phenotype was due to the loss of CWN-2 function, we transgenically expressed
cwn-2 cDNA driven by the
ctbp-1a promoter, which drives expression in SMDD and ten other head neurons (Sherry et al., 2020). Expressing
cwn-2 using this promoter partially rescues the defective axonal phenotype (Figure 1C). This suggests that CWN-2 controls SMDD development in an autocrine manner and/or non-autonomously from other head neurons.Next, we investigated which Wnt receptor controls SMDD axonal development. There are six Wnt receptors encoded in the C. elegans genome: four Frizzled receptors (LIN-17, CFZ-2, MIG-1 and MOM-5,), one Ror receptor (CAM-1) and one Ryk receptor (LIN-18) (Sawa and Korswagen, 2013). We analyzed the effect of loss-of-function mutations for each receptor and found that loss of
cam-1, but not the other receptors, caused defective SMDD axonal development (Figure 1D). CAM-1 was previously shown to function as a CWN-2 receptor for SIA, SIB and RMED/V posterior-directed neurite outgrowth (Kennerdell et al., 2009; Song et al., 2010; Wang and Ding, 2018). The SIA, SIB, RMED/V and SMDD motor neuron cell bodies are all positioned in the nerve ring and extend posteriorly-directed axons (White et al., 1986). Therefore, our data show that CWN-2 and CAM-1 have a common role in controlling axon extension in posteriorly-directed lateral or sublateral axons.