Identification and functional characterization of a novel subtype of neuromedin U receptor

Neuromedin U is a bioactive peptide isolated originally from the porcine spinal cord. We recently identified neuromedin U as the cognate ligand for the orphan G protein-coupled receptor FM-3. In this study, we isolated cDNA coding for a novel G protein-coupled receptor, TGR-1, which was highly homologous with FM-3. We found that neuromedin U specifically and clearly elevated the extracellular acidification rates, arachidonic acid metabolite release, and intracellular Ca(2+) mobilization in Chinese hamster ovary cells expressing TGR-1. Radiolabeled neuromedin U specifically bound with high affinity to membrane fractions prepared from these cells. These results show that TGR-1, like FM-3, is a specific and functional receptor for neuromedin U. We analyzed TGR-1 mRNA tissue distribution in rats using quantitative reverse transcription-polymerase chain reaction and found it to considerably differ from that of FM-3 mRNA. TGR-1 mRNA was primarily expressed in the uterus, suggesting that TGR-1 mediates the contractile activity of neuromedin U in this tissue. The identification of specific and functional receptor subtypes for neuromedin U will facilitate the study of their physiological roles and the search for their specific agonists and antagonists.