Recent progress in assays for GPCR drug discovery.

G-protein-coupled receptors (GPCRs), also known as seven transmembrane receptors, are the largest family of cell surface receptors in eukaryotes. There are ∼800 GPCRs in human, regulating diverse physiological processes. The GPCRs are the most intensively studied drug targets. Drugs that target GPCRs account for about a quarter of the global market share of therapeutic drugs. Therefore, to develop physiologically relevant and robust assays to search new GPCR ligands or modulators remain the major focus of drug discovery research worldwide. Early functional GPCR assays mainly depend on the measurement of G-protein-mediated second messenger generation. Recent developments in GPCR biology indicate that the signaling of these receptors is much more complex than the oversimplified classical view. The GPCRs have been found to activate multiple G proteins simultaneously and induce β-arrestin-mediated signaling. They have also been found to interact with other cytosolic scaffolding proteins and form dimer or heteromer with GPCRs or other transmembrane proteins. Here, we mainly discuss technologies focused on detecting protein-protein interactions, such as fluorescence resonance energy transfer/bioluminescence resonance energy transfer (FRET/BRET), NanoLuc binary technology (NanoBiT), Tango, etc., and their applications in measuring GPCRs interacting with various signaling partners. In the final part, we also discuss the species differences in GPCRs when using animal models to study the in vivo functions of GPCR ligands, and possible ways to solve this problem with modern genetic tools.