Intracellular Binding Site for a Positive Allosteric Modulator of the Dopamine D1 Receptor.

The binding site for DETQ [2-(2,6-dichlorophenyl)-1-((1S,3R)-3-(hydroxymethyl)-5-(2-hydroxypropan-2-yl)-1-methyl-3,4-dihydroisoquinolin-2(1H)-yl)ethan-1-one], a positive allosteric modulator (PAM) of the dopamine D1 receptor, was identified and compared with the binding site for CID 2886111 [N-(6-tert-butyl-3-carbamoyl-4,5,6,7-tetrahydro-1-benzothiophen-2-yl)pyridine-4-carboxamide], a reference D1 PAM. From D1/D5 chimeras, the site responsible for potentiation by DETQ of the increase in cAMP in response to dopamine was narrowed down to the N-terminal intracellular quadrant of the receptor; arginine-130 in intracellular loop 2 (IC2) was then identified as a critical amino acid based on a human/rat species difference. Confirming the importance of IC2, a ß2-adrenergic receptor construct in which the IC2 region was replaced with its D1 counterpart gained the ability to respond to DETQ. A homology model was built from the agonist-state ß2-receptor structure, and DETQ was found to dock to a cleft created by IC2 and adjacent portions of transmembrane helices 3 and 4 (TM3 and TM4). When residues modeled as pointing into the cleft were mutated to alanine, large reductions in the potency of DETQ were found for Val119 and Trp123 (flanking the conserved DRY sequence in TM3), Arg130 (located in IC2), and Leu143 (TM4). The D1/D5 difference was found to reside in Ala139; changing this residue to methionine as in the D5 receptor reduced the potency of DETQ by approximately 1000-fold. None of these mutations affected the activity of CID 2886111, indicating that it binds to a different allosteric site. When combined, DETQ and CID 2886111 elicited a supra-additive response in the absence of dopamine, implying that both PAMs can bind to the D1 receptor simultaneously.

GPR142 Controls Tryptophan-Induced Insulin and Incretin Hormone Secretion to Improve Glucose Metabolism.

GPR142, a putative amino acid receptor, is expressed in pancreatic islets and the gastrointestinal tract, but the ligand affinity and physiological role of this receptor remain obscure. In this study, we show that in addition to L-Tryptophan, GPR142 signaling is also activated by L-Phenylalanine but not by other naturally occurring amino acids. Furthermore, we show that Tryptophan and a synthetic GPR142 agonist increase insulin and incretin hormones and improve glucose disposal in mice in a GPR142-dependent manner. In contrast, Phenylalanine improves in vivo glucose disposal independently of GPR142. Noteworthy, refeeding-induced elevations in insulin and glucose-dependent insulinotropic polypeptide are blunted in Gpr142 null mice. In conclusion, these findings demonstrate GPR142 is a Tryptophan receptor critically required for insulin and incretin hormone regulation and suggest GPR142 agonists may be effective therapies that leverage amino acid sensing pathways for the treatment of type 2 diabetes.

Ectonucleotidase NTPDase3 is abundant in pancreatic ß-cells and regulates glucose-induced insulin secretion.

Extracellular ATP released from pancreatic ß-cells acts as a potent insulinotropic agent through activation of P2 purinergic receptors. Ectonucleotidases, a family of membrane-bound nucleotide-metabolizing enzymes, regulate extracellular ATP levels by degrading ATP and related nucleotides. Ectonucleotidase activity affects the relative proportion of ATP and its metabolites, which in turn will impact the level of purinergic receptor stimulation exerted by extracellular ATP. Therefore, we investigated the expression and role of ectonucleotidases in pancreatic ß-cells. Of the ectonucleotidases studied, only ENTPD3 (gene encoding the NTPDase3 enzyme) mRNA was detected at fairly abundant levels in human and mouse pancreatic islets as well as in insulin-secreting MIN6 cells. ARL67156, a selective ectonucleotidase inhibitor, blocked degradation of extracellular ATP that was added to MIN6 cells. The compound also decreased degradation of endogenous ATP released from cells. Measurements of insulin secretion in MIN6 cells as well as in mouse and human pancreatic islets demonstrated that ARL67156 potentiated glucose-dependent insulin secretion. Downregulation of NTPDase3 expression in MIN6 cells with the specific siRNA replicated the effects of ARL67156 on extracellular ATP hydrolysis and insulin secretion. Our results demonstrate that NTPDase3 is the major ectonucleotidase in pancreatic ß-cells in multiple species and that it modulates insulin secretion by controlling activation of purinergic receptors.

Activation of prostaglandin E receptor 4 triggers secretion of gut hormone peptides GLP-1, GLP-2, and PYY.

Prostaglandins E1 and E2 are synthesized in the intestine and mediate a range of gastrointestinal functions via activation of the prostanoid E type (EP) family of receptors. We examined the potential role of EP receptors in the regulation of gut hormone secretion from L cells. Analysis of mRNA expression in mouse enteroendocrine GLUTag cells demonstrated the abundant expression of EP4 receptor, whereas expression of other EP receptors was much lower. Prostaglandin E1 and E2, nonselective agonists for all EP receptor subtypes, triggered glucagon like peptide 1 (GLP-1) secretion from GLUTag cells, as did the EP4-selective agonists CAY10580 and TCS2510. The effect of EP4 agonists on GLP-1 secretion was blocked by incubation of cells with the EP4-selective antagonist L161,982 or by down-regulating EP4 expression with specific small interfering RNA. Regulation of gut hormone secretion with EP4 agonists was further studied in mice. Administration of EP4 agonists to mice produced a significant elevation of plasma levels of GLP-1, glucagon like peptide 2 (GLP-2) and peptide YY (PYY), whereas gastric inhibitory peptide (GIP) levels were not increased. Thus, our data demonstrate that activation of the EP4 receptor in enteroendocrine L cells triggers secretion of gut hormones.

Regulation of GPR119 receptor activity with endocannabinoid-like lipids.

The GPR119 receptor plays an important role in the secretion of incretin hormones in response to nutrient consumption. We have studied the ability of an array of naturally occurring endocannabinoid-like lipids to activate GPR119 and have identified several lipid receptor agonists. The most potent receptor agonists identified were three N-acylethanolamines: oleoylethanolamine (OEA), palmitoleoylethanolamine, and linoleylethanolamine (LEA), all of which displayed similar potency in activating GPR119. Another lipid, 2-oleoylglycerol (2-OG), also activated GPR119 receptor but with significantly lower potency. Endogenous levels of endocannabinoid-like lipids were measured in intestine in fasted and refed mice. Of the lipid GPR119 agonists studied, the intestinal levels of only OEA, LEA, and 2-OG increased significantly upon refeeding. Intestinal levels of OEA and LEA in the fasted mice were low. In the fed state, OEA levels only moderately increased, whereas LEA levels rose drastically. 2-OG was the most abundant of the three GPR119 agonists in intestine, and its levels were radically elevated in fed mice. Our data suggest that, in lean mice, 2-OG and LEA may serve as physiologically relevant endogenous GPR119 agonists that mediate receptor activation upon nutrient uptake.

Synthesis and SAR of novel histamine H3 receptor antagonists.

The synthesis and biological evaluation of novel tetrahydroisoquinoline, tetrahydroquinoline, and tetrahydroazepine antagonists of the human and rat H(3) receptors are described. The substitution around these rings as well as the nature of the substituent on nitrogen is explored. Several compounds with high affinity and selectivity for the human and rat H(3) receptors are reported.