The UNC-4 homeodomain protein regulates the specificity of synaptic input as well as the strength of synaptic output in the C. elegans motor circuit. UNC-4 is expressed in embryonic DA and larval VA motor neurons. In
unc-4 mutants, VAs are miswired with inputs normally reserved for their VB sister cells; both DAs and VAs show decreased numbers of synaptic vesicles (Lickteig et al., 2001). These effects also depend on UNC-37, a Groucho homolog, that functions as a transcriptional co-repressor with UNC-4. We have now adopted microarray-based strategies to identify
unc-4-regulated genes that govern these motor neuron traits.We used a microarray-based strategy, MAPCeL, (see Fox et al., this meeting), to identify
unc-4 and
unc-37 regulated transcripts in
unc-4::GFP neurons (i.e. DA motor neurons) isolated from primary cultures. ~90 genes with elevated expression in both
unc-4 and
unc-37 mutant cells were detected. A comparison to independent microarray data derived from postembryonic VA motor neurons by the mRNA tagging strategy (Von Stetina et al., this meeting) has revealed that
ceh-12, the C. elegans homolog of the HB9 homeodomain transcription factor, is negatively regulated by
unc-4 in both DA and VA motor neurons. In flies and vertebrates, Hb9 specifies motor neuron fate and axonal trajectory and thus is a strong candidate for an
unc-4-regulated gene that controls synaptic choice. We have shown that
ceh-12::GFP is exclusively expressed in the VB motor neurons in the wild type. In
unc-4 and
unc-37 mutants,
ceh-12::GFP is now also detected in DA and VA motor neurons. Moreover, ectopic expression of CEH-12 in the VAs is sufficient to induce the Unc-4 movement phenotype. Conversely, the
ceh-12 null mutant,
gk391, is a partial suppressor of the
unc-4 movement defect. These data suggest that
ceh-12 is negatively regulated in VA motor neurons to block the creation of VB type inputs. It will be important to establish if
ceh-12 is also necessary for the synaptic vesicle defect exhibited in all
unc-4 motor neurons. We expect that analysis of other candidate
unc-4 target genes in the MAPCeL data set will reveal additional
unc-4 and potentially
ceh-12 regulated genes in this pathway.