GPR41 and GPR43 are related members of a homologous family of orphan G protein-coupled receptors that are tandemly encoded at a single chromosomal locus in both humans and mice. We identified the acetate anion as an agonist of human GPR43 during routine ligand bank screening in yeast. This activity was confirmed after transient transfection of GPR43 into mammalian cells using Ca 2؉ mobilization and [ 35 S]guanosine 5-O-(3-thiotriphosphate) binding assays and by coexpression with GIRK G protein-regulated potassium channels in Xenopus laevis oocytes. Other short chain carboxylic acid anions such as formate, propionate, butyrate, and pentanoate also had agonist activity. GPR41 is related to GPR43 (52% similarity; 43% identity) and was activated by similar ligands but with differing specificity for carbon chain length, with pentanoate being the most potent agonist. A third family member, GPR42, is most likely a recent gene duplication of GPR41 and may be a pseudogene. GPR41 was expressed primarily in adipose tissue, whereas the highest levels of GPR43 were found in immune cells. The identity of the cognate physiological ligands for these receptors is not clear, although propionate is known to occur in vivo at high concentrations under certain pathophysiological conditions.Within family A of the G protein-coupled receptor (GPCR) 1 gene superfamily (also classified as family 1), there is a phylogenetically related group of ϳ90 receptors that respond to an unusually wide variety of ligand types, considering the relatively close similarity of their primary sequences (1). The group includes receptors that respond to purinergic or pyrimidinergic nucleotides (P2Y 1 , P2Y 2 , P2Y 4 , P2Y 6 , P2Y 11 , P2Y 12 , and P2Y 13 ), modified nucleotides (UDP-glucose), lipids (plateletactivating factor receptor), leukotrienes (BLT 1 and BLT 2 and CysLT 1 and CysLT 2 ), proteases (protease-activated receptor-1-4), chemoattractants (FPR1), and chemokines. To date, these receptors have no clear homologs in invertebrates, unlike the monoamine or neuropeptide receptors, suggesting a relatively recent evolutionary origin (2, 3). At least 50 GPCRs whose cognate ligands are unknown (orphans) (4) are categorized within this group on the basis of sequence homology. Often, these orphans fall into subsets, being more related to each other than to receptors with known ligands; and this, combined with the ligand diversity noted above, makes it difficult to predict the chemical nature of their ligands. One subset comprises GPR40 -43, which were identified as tandemly encoded genes present on cosmids isolated from human chromosomal locus 19q13.1 (5). GPR42 differs from GPR41 at only six amino acid positions; otherwise, the four members of this subfamily share ϳ30% minimum identity. BLAST searches have identified the next most closely related receptors as the proteaseactivated receptors. However, the long N-terminal extracellular domains that serve as protease substrates and that are characteristic of protease-activated receptors are absent in the GPR...

GPR40 is a member of a subfamily of homologous G protein-coupled receptors that include GPR41 and GPR43 and that have no current function or ligand ascribed. Ligand fishing experiments in HEK293 cells expressing human GPR40 revealed that a range of saturated and unsaturated carboxylic acids with carbon chain lengths greater than six were able to induce an elevation of [Ca 2؉ ] i , measured using a fluorometric imaging plate reader. 5,8,11-Eicosatriynoic acid was the most potent fatty acid tested, with a pEC 50 of 5.7. G protein coupling of GPR40 was examined in Chinese hamster ovary cells expressing the G␣ q/i -responsive Gal4-Elk1 reporter system. Expression of human GPR40 led to a constitutive induction of luciferase activity, which was further increased by exposure of the cells to eicosatriynoic acid. Neither the constitutive nor ligandmediated luciferase induction was inhibited by pertussis toxin treatment, suggesting that GPR40 was coupled to G␣ q/11. Expression analysis by quantitative reverse transcription-PCR showed that GPR40 was specifically expressed in brain and pancreas, with expression in rodent pancreas being localized to insulin-producing ␤-cells. These data suggest that some of the physiological effects of fatty acids in pancreatic islets and brain may be mediated through a cell-surface receptor.

A novel human G protein-coupled receptor named AXOR12, exhibiting 81% homology to the rat orphan receptor GPR54, was cloned from a human brain cDNA library. Heterologous expression of AXOR12 in mammalian cells permitted the identification of three surrogate agonist peptides, all with a common C-terminal amidated motif. High potency agonism, indicative of a cognate ligand, was evident from peptides derived from the gene KiSS-1, the expression of which prevents metastasis in melanoma cells. Quantitative reverse transcriptase-polymerase chain reaction was used to study the expression of AXOR12 and KiSS-1 in a variety of tissues. The highest levels of expression of AXOR12 mRNA were observed in brain, pituitary gland, and placenta. The highest levels of KiSS-1 gene expression were observed in placenta and brain. A polyclonal antibody raised to the C terminus of AXOR12 was generated and used to show localization of the receptor to neurons in the cerebellum, cerebral cortex, and brainstem. The biological significance of these expression patterns and the nature of the putative cognate ligand for AXOR12 are discussed.The G protein-coupled receptors (GPCRs) 1 form a large family of membrane bound proteins that share a unique structural feature comprising seven transmembrane ␣-helices. These molecules act as receptors for a diverse range of extracellular signaling molecules including small molecules (amino acids and biogenic amines), lipids, small bioactive peptides, and large polypeptides (1). They have been used successfully as drug targets by the pharmaceutical industry for a number of years. Attention has focused on a number of proteins that are known to be GPCRs through structural homology but for which no ligand has been identified: so-called orphan receptors. At the same time as the recent discovery of new GPCRs, there has been a renewed focus on discovering potential novel peptides that may act as endogenous ligands for these receptors.Here, we describe the cloning of a novel human orphan receptor, a class I GPCR with sequence similarity to receptors for the neuropeptide galanin. This receptor was given the name AXOR12 in accordance with its position in a series of receptors identified in our organization. AXOR12 has a high degree of homology to the rat orphan receptor GPR54 (2) (81% amino acid identity), which suggests that these two receptors may be orthologs. To identify a ligand for AXOR12, we expressed this receptor in mammalian cells and screened the transfected cells in a functional assay against a library rich in known and putative peptide transmitters. Although there was no activity in response to galanin, we identified three peptides that acted as low potency agonists of AXOR12. These peptides were all derived from invertebrates and shared a C-terminal LRF-or LRW-amide motif.During the preparation of this article, a search of patent literature revealed the existence of additional high potency agonists with sequence similarities to the surrogate agonists identified from the screen. These peptides were deri...

Nicotinic acid has been used clinically for over 40 years in the treatment of dyslipidemia producing a desirable normalization of a range of cardiovascular risk factors, including a marked elevation of high density lipoprotein and a reduction in mortality. The precise mechanism of action of nicotinic acid is unknown, although it is believed that activation of a G i -G proteincoupled receptor may contribute. Utilizing available information on the tissue distribution of nicotinic acid receptors, we identified candidate orphan receptors. The selected orphan receptors were screened for responses to nicotinic acid, in an assay for activation of G i -G proteins. Here we describe the identification of the G protein-coupled receptor HM74 as a low affinity receptor for nicotinic acid. We then describe the subsequent identification of HM74A in follow-up bioinformatics searches and demonstrate that it acts as a high affinity receptor for nicotinic acid and other compounds with related pharmacology. The discovery of HM74A as a molecular target for nicotinic acid may facilitate the discovery of superior drug molecules to treat dyslipidemia.

Uridine 5-diphosphoglucose (UDP-glucose) has a well established biochemical role as a glycosyl donor in the enzymatic biosynthesis of carbohydrates. It is less well known that UDP-glucose may possess pharmacological activity, suggesting that a receptor for this molecule may exist. Here, we show that UDP-glucose, and some closely related molecules, potently activate the orphan G protein-coupled receptor KIAA0001 heterologously expressed in yeast or mammalian cells. Nucleotides known to activate P2Y receptors were inactive, indicating the distinctly novel pharmacology of this receptor. The receptor is expressed in a wide variety of human tissues, including many regions of the brain. These data suggest that some sugar-nucleotides may serve important physiological roles as extracellular signaling molecules in addition to their familiar role in intermediary metabolism.