Abstract
G-protein-coupled receptors (GPCRs) have varying and diverse physiological roles, transmitting signals from a range of stimuli, including light, chemicals, peptides, and mechanical forces. More than 130 GPCRs are orphan receptors (i.e., their endogenous ligands are unknown), representing a large untapped reservoir of potential therapeutic targets for pharmaceutical intervention in a variety of diseases. Current deorphanization approaches are slow, laborious, and usually require some in-depth knowledge about the receptor pharmacology. In this study we describe a cell-based assay to identify small molecule probes of orphan receptors that requires no a priori knowledge of receptor pharmacology. Built upon the concept of pharmacochaperones, where cell-permeable small molecules facilitate the trafficking of mutant receptors to the plasma membrane, the simple and robust technology is readily accessible by most laboratories and is amenable to high-throughput screening. The assay consists of a target harboring a synthetic point mutation that causes retention of the target in the endoplasmic reticulum. Coupled with a beta-galactosidase enzyme-fragment complementation reporter system, the assay identifies compounds that act as pharmacochaperones causing forward trafficking of the mutant GPCR. The assay can identify compounds with varying mechanisms of action including agonists and antagonists. A universal positive control compound circumvents the need for a target-specific ligand. The veracity of the approach is demonstrated using the beta-2-adrenergic receptor. Together with other existing assay technologies to validate the signaling pathways and the specificity of ligands identified, this pharmacochaperone-based approach can accelerate the identification of ligands for these potentially therapeutically useful receptors.