GPR which is also known

GPR40, which is also known as FFA receptor 1 (FFAR1 or FFA1), was identified as an orphan receptor in the search for novel human galanin receptor (GALR) subtypes in 1997. Using reverse pharmacology approaches measuring calcium transients, GPR40 were deorphanized and characterized as being activated by saturated and unsaturated medium- and long-chain fatty acids at physiologically relevant concentrations.9, 11 It was independently deorphanized using high-throughput reporter screening assay by a third group. Of the saturated FFAs, pentadecanoic (C15) and palmitic (C16) acids are the most potent, whereas for the unsaturated FFAs, there is no major effect in chain length and/or degree of saturation on binding affinity or potency. GPR40 is a promiscuous receptor that can be activated by several endogenous ligands, different from most of the typical GPCRs that are activated by one or a few endogenous ligands. In the primary sequence, GPR40 is highly related to other receptors within the same family, exhibiting high homology with GPR41 (FFAR3) and GPR43 (FFAR2),10, 14 whereas it has little homology with GPR119 and GPR120 (FFAR4).
Molecular Cloning and Tissue Distribution of GPR40
Physiology of GPR40
Pharmacology of GPR40
Pathophysiology of GPR40 Up to now, human genomic studies have identified three nucleotide substitutions in GPR40 that result in coding variations, including one polymorphism, R211H, and two mutations, D175N and G180S (Fig. 3.2). In 2005, Hamid et al. studied the potential relationship between human GPR40 gene variation and insulin resistance in T2DM patients. Using direct sequencing in Danish Caucasian subjects, they identified two single nucleotide substitutions in GPR40 that result in R211H polymorphism located in the third intracellular loop and D175N mutation located in the second extracellular loop. The D175N mutation was found in heterozygous form in both normal Cy5 NHS ester (non-sulfonated) tolerance subjects and T2DM patients. Further sequence analysis of the family members of one T2DM patient harboring the D175N mutation found three more members carrying the heterozygous mutation and observed no cosegregation of this mutation with T2DM. The R211H polymorphism, detected in both T2DM patients and control subjects with a similar frequency (24%), was found to not associate with changes in insulin secretion or pancreatic β-cell function. Functional studies by measuring inositol phosphate turnover showed that R211H GPR40 has similar signaling properties (EC50 and maximal response) as the wild-type receptor, whereas D175N mutant has decreased maximal signaling in response to eicosatriynoic acid by 39%, although the EC50 is similar to wild-type receptor. No data on the expression and ligand binding properties of these receptors were reported. However, the GPR40 R211H polymorphism was reported to affect the variation of insulin secretory capacity in other ethnic groups. The allele frequency of H211 in Japanese was 78.4%.102, 103 By comparing clinical and metabolic parameters among Japanese men, Ogawa et al. found that histidine homozygotes have significantly higher serum insulin levels, homeostasis model of insulin resistance, and pancreatic β-cell function than arginine homozygotes do. Therefore, the authors suggested that genetic variation of GPR40 might alter the insulin secretory capacity. Another GPR40 mutation, G180S located in TM5, was identified in heterozygous form in the Sicilian population in Italy. The frequency of G180S is 0.75%, which, interestingly, increases with the severity of obesity. Subjects carrying GPR40 G180S mutation have significantly decreased insulin secretion than those carrying wild-type GPR40 during oral glucose tolerance test. Functional studies showed that G180S mutants have similar expression levels (both total and cell surface) as the wild-type receptor but diminished increase of intracellular Ca concentrations in response to oleic acid. Whether this defect is due to ligand binding or G protein coupling/activation is not clear. Binding experiments were not performed.