In vitro profiling of orphan G protein coupled receptor (GPCR) constitutive activity

Background and Purpose Members of the GPCR family are targeted by a significant fraction of the available FDA‐approved drugs. However, the physiological role and pharmacological properties of many GPCRs remain unknown, representing untapped potential in drug design. Of particular interest are ~100 less‐studied GPCRs known as orphans because their endogenous ligands are unknown. Intriguingly, disease‐causing mutations identified in patients, together with animal studies, have demonstrated that many orphan receptors play crucial physiological roles and, thus, represent attractive drug targets. Experimental Approach The majority of deorphanized GPCRs demonstrate coupling to G i/o . However, a limited number of techniques allow the detection of intrinsically small constitutive activity associated with G i/o protein activation, which represents a significant barrier in our ability to study orphan GPCR signalling. Using luciferase reporter assays, we effectively detected constitutive G s , G q and G 12/13 protein signalling by unliganded receptors and introducing various G protein chimeras, we provide a novel, highly sensitive tool capable of identifying G i/o coupling in unliganded orphan GPCRs. Key Results Using this approach, we measured the constitutive activity of the entire class C GPCR family that includes eight orphan receptors and a subset of 20 prototypical class A GPCR members, including 11 orphans. Excitingly, this approach illuminated the G protein coupling profile of eight orphan GPCRs (GPR22, GPR137b, GPR88, GPR156, GPR158, GPR179, GPRC5D and GPRC6A) previously linked to pathophysiological processes. Conclusion and Implications We provide a new platform that could be utilized in ongoing studies in orphan receptor signalling and de‐orphanization efforts.