Gαq signals via Trio RhoGEF to modulate synaptic transmission in a model serotonin motor circuit in C. elegans

Activated Gq signals through Phospholipase-C{beta} (PLC{beta}) and Trio, a Rho GTPase exchange factor (RhoGEF), but how these distinct effector pathways promote cellular responses to neurotransmitters like serotonin remains poorly understood. We used the egg-laying behavior circuit of C. elegans to determine whether PLC{beta} and Trio mediate serotonin and Gq signaling through independent or related biochemical pathways. Our genetic rescue experiments suggest that PLC{beta} functions in neurons while Trio RhoGEF functions in both neurons and the postsynaptic vulval muscles. While Gq, PLC{beta}, and Trio RhoGEF mutants all fail to lay eggs in response to serotonin, optogenetic stimulation of the serotonin-releasing HSN neurons restores egg laying only in PLC{beta} mutants. PLC{beta} mutants showed vulval muscle Ca2+ transients while strong Gq and Trio RhoGEF mutants had little or no vulval muscle Ca2+ activity. Treatment with phorbol 12-myristate 13-acetate (PMA) that mimics 1,2-diacylglycerol (DAG), a product of PIP2 hydrolysis, rescued egg-laying circuit activity and behavior defects of Gq signaling mutants, suggesting both Phospholipase-C and Rho signaling promote synaptic transmission and egg laying via modulation of DAG levels. DAG activates effectors including UNC-13, however we find that phorbol esters, but not serotonin, stimulate egg laying in unc-13 and PLC{beta} mutants. These results support a model where serotonin signaling through Gq, PLC{beta}, and UNC-13 promote neurotransmitter release, and that serotonin also signals through Gq, Trio RhoGEF, and an unidentified, PMA-responsive effector to promote postsynaptic muscle excitability. Thus, the same neuromodulator serotonin can signal in distinct cells and effector pathways to coordinate activation of a motor behavior circuit.