PLR-1, a putative E3 ubiquitin ligase and AEX-3, the GDP/GTP exchange factor homologue for RAB-3, respectively regulate cell polarity and axon navigation of the ventral nerve cord pioneer AVG in Caenorhabditis elegans

Accurate and precise neuronal circuit formation is the hallmark of a functional nervous system. During development neurons extend axons and dendrites that have to reach their appropriate targets. This process is highly regulated and is achieved by using a set of conserved guidance cues and receptors. ‘Pioneer’ neurons extend axons first and are closely followed by the late outgrowing axons called ‘followers’ to extend upon. In Caenorhabditis elegans, the AVG axon pioneers the right axon tract of the ventral nerve cord (VNC). The molecular basis for the navigation of the AVG axon is largely unknown. The aim of this study was to identify novel regulators of AVG axon navigation. In genetic screens for AVG axon outgrowth and guidance defects we identified alleles of plr-1 that reversed the polarity of AVG neuron and also caused outgrowth and navigation defects in the AVG axon and several other neuronal and non-neuronal cells. plr-1 is predicted to encode a putative transmembrane E3 ligase, widely expressed during the development including in the AVG neuron. plr-1 and its vertebrate homologues control Wnt signalling by removing the frizzled receptors from the cell surface. We have shown that mutations in a gene reducing Wnt-signalling as well as mutations in unc-53 and unc-73 suppress the AVG polarity reversal defects, but not the other defects seen in plr-1 mutants. This suggests that plr-1 has Wnt dependent and Wnt independent functions.Simple genetic screens have not yielded mutants with penetrant AVG axon navigation defects except plr-1. In enhancer screens for AVG axon navigation defects in a nid-1 mutant background we isolated several candidate mutants including an allele of aex-3. aex-3 mutant animals show penetrant AVG axon navigation defects as well as follower axon navigation defects in the VNC, which are nid-1 dependent. AEX-3 is a GDP/GTP exchange factor for RAB-3 and RAB-27 GTPases. Our genetic interaction data suggests that AEX-3 regulates RAB-3 and not RAB-27. We also show that aex-3 acts along with unc-31/CAPS, ida-1/IA-2 and unc-64/Syntaxin in the same genetic pathway for AVG navigation. Moreover, our genetic interaction data suggests that AEX-3 might regulate the transport of the Netrin receptor UNC-5 in the growth cone.