Homology-based cloning approaches and genome-sequencing efforts have revealed the existence of a large number of human genes encoding ‘orphan’ G-protein-coupled receptors (GPCRs), receptors that bind unidentified natural ligands. Discovery of these natural ligands is the first necessary step in understanding the biological significance of the orphan GPCRs. We¹ and others² have developed an approach by which to successfully isolate endogenous ligands from complex tissue libraries. This approach, referred to as the orphan-receptor strategy³, uses orphan receptors as baits to isolate their native ligand from tissue extracts and has been successfully applied to identify new neuropeptides^4,5,6. Here we apply this strategy to the orphan receptor GPR14 and show that it binds the bioactive peptide known as urotensin II.

As GPR14 is similar to the somatostatin receptors we decided to screen somatostatin-like, cysteine-bridge-containing peptides on GPR14 and to compare their biological activities and physicochemical properties with that of the endogenous compound identified in our purification scheme. Peptides tested were human melanin-concentrating hormone (MCH), somatostatin-14 (SST-14) and cortistatin-14 and urotensin II. Whereas MCH was inactive, SST-14 and the related peptide cortistatin activated GPR14, albeit only at very high concentrations (Table 1). In contrast, urotensin II induced a robust increase in [Ca²⁺]_i, with a median effective concentration (EC₅₀) of 0.14 nM (Table 1), showing a potency more than 25,000-fold greater than that of SST-14.

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