Using C. elegans Genetics to Identify Regulators of μ‐Opioid Receptor Signaling

Abstract

Opioids exert their analgesic effects by activating the μ‐opioid receptor (MOR), a G protein‐coupled receptor expressed in the nervous system. In addition to clinical pain relief, activation of MOR produces euphoria and leads to dependence. Despite significant process in understanding molecular mechanisms of MOR signaling, the regulatory mechanisms of MOR function are not fully understood. To better understand MOR regulation and to uncover genes influencing MOR signaling, we developed a genetic behavioral platform utilizing nematode C. elegans. Mammalian MOR was transgenically expressed in the nervous system of C. elegans endowing the resulting model (tgMOR) with opioid sensitivity. We validated tgMOR platform showing conservation of pharmacological properties and regulatory mechanisms of MOR mediated behavioral responsiveness. We then performed a large scale forward genetic screen isolating a population of mutants with altered opioid sensitivity. Integrating whole genome sequencing, CRISPR/Cas9 gene editing and transgenic rescue, we identified a set of candidate negative modulators of MOR signaling. One of these genes encoded an orphan GPCR, FRPR‐13. Our phylogenic and functional analysis revealed that mammalian ortholog of FRPR‐13 is GPR139. We found GPR139 to be extensively co‐expressed with MOR in the mammalian nervous system. Knockout of GPR139 in mice enhanced opioid analgesia and reward, but diminished withdrawal. These observations suggest the existence of an “anti‐opioid” system that regulates the extent of MOR signaling in vivo. Our findings further showcase the utility of C. elegans as a scalable platform for genetic discovery of novel GPCR signaling principles.Support or Funding InformationThis work was supported by an NIH Cutting Edge Basic Research Award (R21DA040406) to B.G. and K.A.M., DA036596 to K.A.M., and an NIH Center of Biomedical Research Excellence Grant (P20GM103638) to University of Kansas Genome Sequencing Core.