Serotonin signals through postsynaptic Gα_q, Trio RhoGEF, and DAG to promote C. elegans egg-laying circuit activity and behavior

Abstract

Activated Gα_q signals through Phospholipase-Cβ (PLCβ) 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 PLCβ and Trio mediate serotonin and Gα_q signaling through independent or related biochemical pathways. Using genetic rescue experiments, we find that PLCβ functions in neurons while Trio RhoGEF functions in both neurons and the postsynaptic vulval muscles. While Gα_q, PLCβ, and Trio RhoGEF mutants all fail to lay eggs in response to serotonin, optogenetic stimulation of the serotonin releasing HSN command neurons restores egg laying only in PLCβ mutants. PLCβ mutants showed vulval muscle Ca²⁺ transients while strong Gα_q and Trio RhoGEF mutants had little or no vulval muscle Ca²⁺ activity. Treatment with Phorbol esters that mimic Diacylglycerol (DAG), a product of PIP₂ hydrolysis, rescued egg-laying circuit activity and behavior defects of Gα_q signaling mutants, suggesting both Phospholipase C and Rho signaling promote synaptic transmission and egg-laying via DAG production. DAG activates effectors including UNC-13, however, we find that phorbol esters, but not serotonin, stimulate egg laying in unc-13 and PLCβ mutants. These results support a model where serotonin signaling through Gα_q, PLCβ, and UNC-13 promote neurotransmitter release, and that serotonin also signals through Gα_q, 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.