We are using C. elegans to address the fundamental question of how neurons choose synaptic partners. The UNC-4 homeodomain protein is expressed in VA motor neurons where it specifies synaptic connections; in
unc-4 mutants, VAs adopt inputs from interneurons normally reserved for their lineal sisters, the VBs. We have shown that UNC-4 acts with its corepressor partner, UNC-37/Groucho, in VAs to turn off VB-specific genes 1 . We propose that genes negatively regulated by
unc-4 promote VB-type inputs. We have adopted an Affymetrix microarray-based approach to identify these
unc-4 -regulated genes. The mRNA-tagging strategy was used to reveal
unc-4 target genes in the postembyonic VA motor neurons. To find genes that UNC-4 regulates in the embryonically derived DA motor neurons, we relied on transcripts from FACS-isolated
unc-4 ::GFP motor neurons grown in culture (see Fox, et. al., this meeting). We reasoned that authentic
unc-4 regulated genes might be detected in both data sets. A comparison of microarray data obtained from wild type vs
unc-37 mutant A-class motor neurons by the mRNA tagging method revealed ~400 genes that are upregulated in
unc-37(
e262). The sensitivity of this approach to
unc-4 targets is confirmed by the inclusion of two known
unc-4 -regulated genes,
del-1 and
glr-4 , in this data set. Independent microarray data from cultured
unc-4:: GFP neurons identified ~200 genes elevated in
unc-4 and
unc-37 mutants vs wildtype.
ceh-12 , a homolog of the Hb9 homeodomain protein, is upregulated in all three of these
unc-4 and
unc-37 data sets. In other organisms, Hb9 is required to specify motor neuron fate and governs axonal trajectory and synapse formation 2,3,4,5 . Thus,
ceh-12 is an attractive candidate to test for
unc-4 regulation.
ceh-12 ::GFP is exclusively detected in VB motor neurons. In
unc-4 and
unc-37 mutants,
ceh-12 ::GFP is ectopically expressed in VA and DA motor neurons, confirming that this target gene is authentic. Loss-of-function (RNAi) and gain-of-function (
ceh-12 ectopic expression in A-class neurons) experiments are underway to test
ceh-12 for a role in the
unc-4 -governed motor neuron circuit. 1. Winnier, A. R. et al. Genes Dev 13 , 2774-86 (1999). 2. Thaler, J. et al. Neuron 23 , 675-87 (1999). 3. Arber, S. et al. Neuron 23 , 659-74 (1999). 4. Broihier, H. T. & Skeath, J. B. Neuron 35 , 39-50 (2002). 5. Odden, J. P., Holbrook, S. & Doe, C. Q. J Neurosci 22 , 9143-9 (2002).