Expression Profiling Reveals Key Regulators of Synaptic Specificity and Function in the <i>C. elegans</i> Motor Circuit

Abstract

Animal movement is controlled by the motor circuit, which features an axial nerve cord where motor neurons transmit signals from the brain to specific muscles. The development of this circuit depends on differential gene expression in the specific cells that contribute to its function. The identification of these genes should lead to a better understanding of the developmental programs that generate each of these essential cell types. To identify the genes that specify this network, I have developed a genomic strategy, MAPCeL (Microarray Profiling <i>C. elegans</i> Cells) and used it to obtain gene expression profiles of the body wall muscle cells and the excitatory cholinergic motor neurons from <i>C. elegans</i>. Bioinformatic analysis and GFP reporters were used to validate MAPCeL profiles obtained from these experiments. In addition, we show that <i>acr-16</i>, a nicotinic acetylcholine receptor gene identified in the MAPCeL profile of body muscle cells, is required for normal locomotion. In a second project, I used MAPCeL to identify genes that regulate synaptic assembly and function in the <i>C. elegans</i> motor circuit. The UNC-4 homeodomain protein is expressed in DA and VA class motor neurons (i.e., A-class motor neurons) where it functions with its transcriptional co-repressor, UNC-37/Groucho to define the specificity of synaptic input and the strength of synaptic output of these motor neurons. In <i>unc-4</i> mutants, VA motor neurons are miswired with inputs normally reserved for their VB sister cells; both DAs and VAs show decreased numbers of synaptic vesicles. We propose that UNC-4 specifies A-motor neuron traits by repressing B-motor neuron genes. Using the MAPCeL approach, I have generated a list of candidate UNC-4 regulated genes and shown that one of these targets, CEH-12/Hb9, functions downstream of UNC-4. In flies and vertebrates, the Hb9 transcription factor is expressed in the developing spinal cord where it specifies motor neuron fate. We show that the <i>C. elegans</i> Hb9 homolog, <i>ceh-12</i>, is normally restricted to VB motor neurons but is also expressed in DAs and VAs in <i>unc-4</i> and <i>unc-37</i> mutants. Ectopic expression of CEH-12 in VAs is sufficient to induce the Unc-4 movement phenotype. Furthermore, <i>ceh-12(0)</i> mutants partially suppress the Unc-4 movement phenotype and fully suppress the synaptic vesicle defect. These data suggest that <i>ceh-12</i> functions downstream of UNC-4 to regulate synaptic input to A-class motor neurons as well as the strength of signaling output to body muscles.