Despite the functional importance of electrical synapses, the molecular mechanisms that direct the formation of neuron-specific gap junctions remain largely unknown. To address this question, we identified targets of the UNC-4 transcription factor that controls connectivity in the C. elegans motor circuit. UNC-4 functions in VA motor neurons to direct the formation of gap junctions on the VA axon with the interneuron AVA (VA-AVA). Locomotion is disrupted in
unc-4 mutants because VAs are miswired with electrical input from the interneuron AVB (VA-AVB) which aberrantly form on the VA soma. Thus, UNC-4 controls both the specificity and subcellular placement of electrical synapses. We determined that UNC-4 blocks expression of two antagonists of cAMP, the phosphodiesterase, PDE-1, and the Go/Gi-coupled GPCR, FRPR-17, to prevent assembly of ectopic VA-AVB gap junctions. This finding suggests that cAMP signaling promotes the formation of functional wild-type VA-AVA gap junctions. We validated this hypothesis by showing that optogenetic elevation of cAMP rescues the Unc-4 movement defect and thus restores VA-AVA circuit function. In addition, forced depletion of cAMP in VAs phenocopies
unc-4 mutants. Because gap junction placement is shifted from the VA axon to cell soma in
unc-4 mutants, we reasoned that trafficking of gap junction components could be perturbed. Live-cell imaging of the gap junction protein, GFP-UNC-9, confirmed that trafficking into the VA axon is strikingly impaired in
unc-4 mutants. Genetic activation of cAMP signaling is sufficient to restore GFP-UNC-9 trafficking in VAs. Thus, we propose that cAMP directs both the specificity and placement of electrical synapses by activating mechanisms that transport gap junction components into the VA axonal compartment. Although studies in cultured cells have implicated cAMP in gap junction assembly, our in vivo experiments now firmly establish that cAMP regulates the biogenesis of electrical synapses in an intact nervous system.