Proper function of the brain requires that neurons adopt different morphologies and connections. In the nematode C. elegans, VA and VB motor neurons arise from a common precursor cell but adopt different morphologies and accept input from separate sets of command interneurons. In unc-4 mutants, VA motor neurons are miswired with VB-type inputs. We have proposed that miswiring results when VB genes are ectopically expressed in the VAs in unc-4 mutants. Previous work revealed that UNC-4 functions with the UNC-37/Groucho co-repressor protein to repress the VB-specific genes acr-5, del-1, glr-4. However, our genetic data rule out roles for these VB genes in synaptic choice. To identify the missing unc-4 target genes, a microarray-based strategy for profiling VA motor neurons was adopted.
A comparison of VA-specific transcripts isolated by mRNA-tagging from wildtype and unc-37 mutant animals revealed ~250 upregulated transcripts in unc-37 animals. One of these genes, ceh-12, is the C. elegans homolog of HB9, a homeodomain transcription factor with conserved roles in motor neuron fate in flies and vertebrates (Arber et al 1999, Broihier and Skeath 2002). In C. elegans, ceh-12::GFP is exclusively expressed in VB motor neurons in wildtype animals. In unc-4 and unc-37 mutants, ceh-12::GFP is also expressed in VA motor neurons as suggested by the microarray data. Thus, CEH-12 is a strong candidate for an UNC-4 target gene that regulates synaptic choice.
To test this idea, the unc-4 promoter was used to drive CEH-12 expression in wildype VA motor neurons. These animals exhibit an Unc-4 like backward movement defect, as expected for a model in which ectopic CEH-12 is sufficient to impose VB type inputs. In addition, we also showed that ceh-12 deletion mutants are partial suppressors of Unc-4 movement, thereby confirming that CEH-12 is also required for the Unc-4 miswiring defect. We conclude the VB-specific gene, ceh-12, is normally repressed in VA motor neurons to prevent the imposition of VB-type inputs. The incomplete suppression of unc-4, however, suggests that UNC-4 also controls other downstream target genes that function in parallel pathways to regulate synaptic choice.
