Members of the G protein coupled receptor (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.
The majority of deorphanized GPCRs demonstrate coupling to G , however a limited number of techniques allow the detection of intrinsically small constitutive activity associated with G protein activation which represents a significant barrier in our ability to study orphan GPCR signaling. Using luciferase reporter assays, we effectively detected constitutive G , G , and G protein signaling by unliganded receptors, and introducing various G protein chimeras, we provide a novel, highly-sensitive tool capable of identifying G coupling in unliganded orphan GPCRs.
Using this approach, we measured the constitutive activity of the entire class C GPCR family that includes 8 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 8 orphan GPCRs (GPR22, GPR137b, GPR88, GPR156, GPR158, GPR179, GPRC5D, and GPRC6A) previously linked to pathophysiological processes.
We provide a new platform that could be utilized in ongoing studies in orphan receptor signaling and deorphanization efforts.

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