UNC-4 controls synaptic specificity by modulating antagonistic Wnt pathways in the C. elegans motor circuit

Abstract

Coordinated movement depends on specific connections between neurons, yet mechanisms that govern synaptic specificity are poorly understood. The simple, well-defined motor circuit of the nematode, C. elegans provides an optimal model system to study how neurons choose synaptic partners. Here, interneurons AVA, AVD, and AVE synapse with VA and DA motor neurons to establish the backward motor circuit, whereas interneurons AVB and PVC make connections with VB and DB motor neurons in the forward circuit. Mutations in the UNC-4 homeodomain transcription factor miswire VA motor neurons with inputs normally reserved for VBs, thereby disrupting backward locomotion. The Miller lab has shown that UNC-4 functions in post-synaptic VA motor neurons to block expression of VB genes that specify these aberrant connections. One VB gene, the transcription factor, CEH-12/HB9, is required for miswiring posterior VA motor neurons. My dissertation addresses two important questions: (1) What mechanism limits ceh-12/HB9 expression to posterior VAs and (2) What additional unc-4 pathway genes regulate inputs onto anterior VAs? First, my results establish that a posterior source of the diffusible ligand, EGL-20/Wnt is required for expression of CEH-12/HB9 in VAs. Genetic results indicate that the frizzled receptors, MOM-5 and MIG-1, respond to EGL-20/Wnt and drive ceh-12 expression through a canonical Wnt pathway. This also revealed a parallel pathway, involving LIN-44/Wnt and LIN-17/Frz, which antagonizes ceh-12 expression and promotes the creation of VA-type inputs. We hypothesize that UNC-4 inhibits expression of MIG-1 and MOM-5 to prevent posterior VAs from responding to EGL-20/Wnt. UNC-4 effectively biases overall Wnt signaling to favor the LIN-44/LIN-17 pathway. Second, I utilized genetic screens designed to isolate mutants that function in parallel to ceh-12. This approach revealed 22 independent blr (backward locomotion restored) mutations that map to 16 genetic linkage groups. Detailed phenotypic characterization of six blr mutants confirmed at least three genetic loci, blr-1, blr-3, blr-15, function in parallel to ceh-12 to regulate the specificity of interneuron input to VAs. Future molecular identification of these unc-4 pathway genes should provide key insights into the mechanism of synaptic specificity.