Regulation of neurotransmitter vesicles by the homeodomain protein UNC-4 and its transcriptional corepressor UNC-37/Groucho in Caenorhabditis eleganscholinergic motor neurons

Motor neuron function depends on neurotransmitter release from synaptic ves icles (SVs). Here we show that the UNC-4 homeoprotein and its transcription al corepressor protein UNC-37 regulate SV protein levels in specific Caenor habditis elegans motor neurons. UNC-4 is expressed in four classes (DA, VA, VC, and SAB) of cholinergic motor neurons. Antibody staining reveals that five different vesicular proteins (UNC-17, choline acetyltransferase, Synap totagmin, Synaptobrevin, and RAB-3) are substantially reduced in unc-4 and unc-37 mutants in these cells; nonvesicular neuronal proteins (Syntaxin, UN C-18, and UNC-11) are not affected, however. Ultrastructural analysis of VA motor neurons in the mutant unc-4( e120) confirms that SV number in the pr esynaptic zone is reduced (similar to 40%) whereas axonal diameter and syna ptic morphology are not visibly altered. Because the UNC-4-UNC-37 complex h as been shown to mediate transcriptional repression, we propose that these effects are performed via an intermediate gene. Our results are consistent with a model in which this unc-4 target gene ("gene-x") functions at a post -transcriptional level as a negative regulator of SV biogenesis or stabilit y. Experiments with a temperature-sensitive unc-4 mutant show that the adul t level of SV proteins strictly depends on unc-4 function during a critical period of motor neuron differentiation. unc-4 activity during this sensiti ve larval stage is also required for the creation of proper synaptic inputs to VA motor neurons. The temporal correlation of these events may mean tha t a common unc-4-dependent mechanism controls both the specificity of synap tic inputs as well as the strength of synaptic outputs for these motor neur ons.