Transmission of chemical neuronal impulses depends on fusion of neurotransmitter vesicles with the presynaptic membrane. Here we present evidence that expression of vesicular proteins is regulated in specific cholinergic motor neurons by
unc-4 and
unc-37 . The transcription factors UNC-4 (homeobox protein) and UNC-37 (Groucho) are required for proper differentiation of DA and VA motor neurons ("
unc-4 motor neurons"). In
unc-4 and
unc-37 loss-of-function mutants, we observe decreased levels of vesicular proteins in
unc-4 motor neurons.
unc-17 encodes the vesicular acetylcholine transporter (VAChT) and
cha-1 encodes choline acetyltransferase (ChAT). Both of these proteins are associated with synaptic vesicles and are required for cholinergic function.
unc-17 and
cha-1 are coordinately transcribed from a common promoter. In
unc-4 and
unc-37 mutants, antibody staining of UNC-17 and ChAT is substantially reduced in
unc-4 motor neurons. However,
unc-4 and
unc-37 mutations do not decrease expression of a GFP reporter gene driven by the
unc-17-
cha-1 promoter. Since the levels of UNC-17 and ChAT proteins are decreased in
unc-4 and
unc-37 mutants whereas
unc-17-
cha-1 transcription is not affected, we hypothesize that
unc-4 and
unc-37 are regulating
unc-17 and
cha-1 via a post-transcriptional mechanism. This model is also consistent with the observation that
unc-4 and
unc-37 are required for maintaining normal levels of the vesicular proteins, synaptotagmin, VAMP, and RAB-3. The concomitant effects of
unc-4 and
unc-37 mutations on at least five different vesicular proteins may mean that UNC-4 and UNC-37 regulate expression of gene products involved in general aspects of vesicle synthesis, assembly, and/or stability. Since UNC-4 and UNC-37 are hypothesized to function as transcriptional repressors, we further propose that UNC-4 and UNC-37 may negatively regulate a gene that destabilizes vesicular proteins in
unc-4 motor neurons. A major goal of this work is to define the mechanism whereby
unc-4 and
unc-37 regulate neurotransmitter vesicle proteins in cholinergic motor neurons.