Coordinated movement depends upon the formation of specific synapses in the motor circuit. In C. elegans, the homeodomain transcription factor UNC-4 directs VA motor neurons to connect with AVA interneurons which drive backward locomotion. Backward movement is disrupted in
unc-4 mutants because VA neurons synapse with AVB interneurons instead of with AVA. Previous work demonstrated that UNC-4 antagonizes a posterior EGL-20/Wnt signaling pathway to preserve VA inputs from AVA. In
unc-4 mutants, EGL-20/Wnt drives ectopic expression of the homeodomain transcription factor CEH-12 in posterior VAs which switches synaptic inputs to AVB. We built an endogenous CEH-12::GFP reporter to confirm that EGL-20 is indeed required for ectopic CEH-12::GFP expression in posterior VAs. Surprisingly, in situ hybridization (smFISH) detected ectopic
ceh-12 transcripts in
unc-4 mutant VAs throughout the nerve cord. smFISH also revealed that EGL-20/Wnt is not required for
ceh-12 transcription. Thus, our results suggest that EGL-20/Wnt regulates a post-transcriptional mechanism that limits expression of CEH-12 protein to posterior VA motor neurons. We are now investigating the mechanism of Wnt-dependent post-transcriptional control of a key regulator of synaptic choice.