GPR139 Signals Through Gq/11 to Oppose Mu Opioid Receptor Signaling

Abstract

Signaling through G protein coupled receptors (GPCRs) is highly regulated and shaped not only by how and when a GPCR engages its ligand, but also through complex and varied protein interactions at the level of the receptor and at downstream effectors. Opioid receptors are an excellent example of a GPCR system that is finely regulated in the central nervous system. The mu opioid receptor is the primary receptor that modulates acute pain responses and has been heavily targeted therapeutically for its anti‐nociceptive properties. The duration and extent of endogenous MOR signaling is modulated by available endogenous opioid peptides, by β‐arrrestin recruitment and subsequent receptor desensitization, by regulator of G protein signaling (RGS) control of Gαo activation, and by interaction with other GPCRs that may positively or negatively alter signaling. Using an unbiased forward genetic screen in MOR‐expressing c. elegans, we uncovered another possible regulator of MOR signaling: the orphan GPCR GPR139. Studies using GPR139 knockout mice demonstrate hypersensitivity to opioid analgesics across multiple measures of antinociception. However, the exact molecular mechanism(s) of how GPR139 activation and signaling can modulate MOR downstream signaling is unknown. We ran a battery of in vitro signaling assays to determine how GPR139 signals in cells and how that could negatively modulate MOR signaling. GPR139 G protein coupling was tested using every G alpha protein and was found to couple to both Gq/11 and Gi/o family of G proteins. We examined the effect of GPR139 on multiple parameters of MOR signaling and trafficking and found that GPR139‐Gq signals in an opposing manner to MOR‐Go signaling at multiple downstream effectors in vitro. Further, MOR‐mediated changes in habenular neuron firing are blunted by dynamic GPR139 activation and can be rescued by Gαq inhibition. Overall, this research examines the molecular underpinnings of GPR139 negative regulation of MOR, explores mechanisms of GPCR cross‐regulation, and evaluates the possibility of therapeutically targeting GPR139 to increase the safety and efficacy of opioids.Support or Funding InformationNational Institute on Drug Abuse (NIDA) NRSA F32DA047771 to HMSNational Institute on Drug Abuse (NIDA) DA042746 to KAM