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. Our work has 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 work has also ruled out roles for these VB genes in synaptic choice. To identify the missing
unc-4 target genes, we have now adopted microarray-based strategies for profiling VA motor neurons. We used mRNA-tagging (Roy, et al 2002) to isolate VA-specific transcripts from wildtype and
unc-37 mutant animals; ~250 transcripts are upregulated in
unc-37 animals. The large number of
unc-37 upregulated genes is consistent with the known role for UNC-37/Groucho in mediating repression by diverse classes of transcription factors. To identify the subset of
unc-4 target genes in this list, we compared these data to
unc-4 regulated genes detected in microarray experiments with FACS isolated embryonic
unc-4::GFP neurons (see Fox, et. al., this meeting). This comparison revealed that
ceh-12, the C. elegans homolog of the HB9 homeodomain, is upregulated in both datasets.
ceh-12::GFP is exclusively expressed in VB motor neurons in wildtype but also ectopically expressed in VA and DA motor neurons in
unc-4 and
unc-37 mutants. Hb9 functions in mammals and flies to specify motor neuron traits, including axon trajectory and synapse formation (Thaler et al 1999, Odden et al 2002). Thus, CEH-12 is a strong candidate for an UNC-4 target gene that regulates synaptic choice. To test this idea, we used the
unc-4 promoter to drive CEH-12 expression in wildtype 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. To demonstrate that
ceh-12 is also necessary for the generation of VB-type inputs, we showed that
ceh-12 (
gk391) is a partial suppressor of Unc-4 movement. 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. Additional analysis of our microarray results is likely to reveal these genes.