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Query: UNIPROT:P06889 (Mol)
 630,302 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Stable cell lines that express the canine-derived A2a adenosine receptor (A2aAR) have been generated. Using a previously characterized anti-A2aAR antibody probe, we have identified the recombinant receptor protein and examined the desensitization process of this G protein-coupled receptor. Agonist exposure induced a rapid desensitization of A2aAR-stimulated adenylyl cyclase activity. This was associated with reduced affinity of the receptor for the A2aAR-selective agonist [3H]CGS21680 and agonist-stimulated phosphorylation of the receptor protein. Agonist-stimulated A2aAR sequestration into a light membrane fraction was also detected over the same time frame but, whereas inhibition of this process did not affect the extent of desensitization, the rapid recovery normally observed after short term agonist exposure was dramatically reduced. Long term agonist treatment resulted in the down-regulation of A2aARs and up-regulation of Gi alpha 2 and Gi alpha 3, as determined by immunoblotting. Recovery of A2aAR function after agonist removal required several hours and was associated with the return of receptor levels to control values. In contrast, inactivation of Gi proteins by pertussis toxin treatment did not alter the extent of agonist-induced desensitization observed. Neither short nor long term desensitization could be mimicked by elevation of intracellular cAMP levels alone. Therefore, these data suggest that A2aAR desensitization is mediated by multiple, temporally distinct, agonist-dependent processes. Agonist-stimulated phosphorylation of the receptor may induce short term desensitization by impairing receptor-Gs coupling, whereas long term down-regulation of receptor number and up-regulation of inhibitory G proteins mediate long term adaptation.
Mol Pharmacol 1994 Jun
PMID:Desensitization of the canine A2a adenosine receptor: delineation of multiple processes. 802 2

A new member of the G protein-coupled receptor superfamily has been isolated from an ovine genomic library with a probe generated by the application of the PCR technique, using cDNA synthesized on a mRNA template isolated from the ovine pars tuberalis. This genomic clone encodes a novel receptor of 325 amino acids with seven transmembrane domains. These domains share homology with other members of this family, but the best homology is with the recently cloned human MC-1 (50% in the transmembrane domains) and MC-3 (69% in the transmembrane domains) MSH receptors and the human ACTH (42% in the transmembrane domains) receptor. When this receptor was expressed in Cos7 cells, it was able to bind a potent analogue of alpha-MSH, [Nle4,D-Phe7]-alpha-MSH (NDP-MSH), with high affinity. This binding could be displaced by pro-opiomelanocortin-derived and related peptides, with the order of potency NDP-MSH > alpha-MSH = ACTH > beta-MSH and with no effect of gamma-MSH, delta-MSH or beta-endorphin. The expressed receptor was demonstrated to be functionally coupled to the adenylate cyclase second messenger pathway, with alpha-MSH, beta-MSH and ACTH stimulating cyclic AMP production. The amount of the mRNA for this receptor was found to be very low. The tissue distribution of this receptor could only be observed using the reverse transcription-PCR technique and the receptor was found to be present in a number of somatic tissues. These data indicate that this is a new and distinct member of the melanocortin receptor family.
J Mol Endocrinol 1994 Apr
PMID:Cloning and expression of a new member of the melanocyte-stimulating hormone receptor family. 806 Apr 85

Recently, inositol hexakisphosphate (phytic acid) was shown to bind to photoreceptor arrestin and block its interaction with rhodopsin. Such an interaction might predict that inositol polyphosphates could alter G protein-coupled receptor desensitization. To investigate the possible roles of higher inositol polyphosphates on receptor desensitization, we have expressed the rat substance P receptor in Xenopus laevis oocytes. The functional expression of substance P receptor was monitored by voltage-clamp recording of substance P-induced Ca(2+)-dependent Cl- currents. When control oocytes were stimulated with substance P (30 nM), after 10 min of washing the second responses to substance P were approximately 15% of the first responses. Cytosolic injection of inositol pentakisphosphate (100 microM) or inositol hexakisphosphate (100 microM) inhibited the reduction of the second substance P-induced current responses, maintaining the second responses to 57-58% of the initial responses. The protective effects of inositol pentakisphosphate and inositol hexakisphosphate against agonist-induced desensitization were concentration and time dependent and structurally specific, in that inositol hexasulfate and inositol tris- and tetrakisphosphate isomers were inactive. Microinjection of inositol hexakisphosphate did not (a) change the potency of substance P or the sensitivity of the expressed substance P receptor to substance P, (b) inhibit 12-O-tetradecanoylphorbol-13-acetate-induced loss of substance P-induced current responses, or (c) alter the currents elicited by microinjection of inositol-1,4,5-trisphosphate. These results suggest that inositol pentakisphosphate and inositol hexakisphosphate have specific inhibitory effects on the agonist-induced loss of responsiveness of the rat substance P receptor. Moreover, these protective effects of inositol hexakisphosphate against desensitization were also observed with the endogenous lysophosphatidic acid/phosphatidic acid receptor, indicating that this mechanism is not specific to ectopic receptors. These results suggest that inositol pentakisphosphate and inositol hexakisphosphate may be novel pharmacological tools for the study of agonist-induced desensitization.
Mol Pharmacol 1994 Aug
PMID:Attenuation of agonist-induced desensitization of the rat substance P receptor by microinjection of inositol pentakis-and hexakisphosphates in Xenopus laevis oocytes. 807

Activation of the pituitary gonadotropin-releasing hormone receptor, a member of the seven-transmembrane G protein-coupled receptor (GPCR) family, triggers a cascade of events leading to gonadotropin release and stimulation of the reproductive system. An unusual feature of this receptor, observed in mice, rats, and humans, is the presence of Asn87 in the second putative transmembrane helix at the location of a highly conserved aspartate in the GPCR family and of Asp318 in the putative seventh transmembrane helix where nearly all other GPCRs have asparagine. The possibility that these residues interact was suggested by this reciprocal pattern and by a three-dimensional model of the gonadotropin-releasing hormone receptor and was investigated by site-directed mutagenesis. Replacing Asn87 in the second transmembrane domain by aspartate eliminated detectable ligand binding. A second mutation, generating the double-mutant receptor Asp87Asn318, recreated the arrangement found in other GPCRs and re-established high affinity agonist and antagonist binding. The restoration of binding by a reciprocal mutation indicates that these two specific residues in helices 2 and 7 are adjacent in space and provides an empirical basis to refine the model of the transmembrane helix bundle of the receptor.
Mol Pharmacol 1994 Feb
PMID:A reciprocal mutation supports helix 2 and helix 7 proximity in the gonadotropin-releasing hormone receptor. 811 67

Metabotropic glutamate receptors (mGluRs) are G protein-linked receptors that operate through the formation of different second messengers. Utilizing quantitative autoradiographic techniques, we have characterized [3H]glutamate binding to mGluRs in discrete regions of adult rat brain. [3H]Glutamate binding, in the presence of high concentrations of alpha-amino-3-hydroxymethyl-4-isoxazolepropionic acid (10 microM), N-methyl-D-aspartate (100 microM), and 2.5 mM calcium chloride (CaCl2), was saturable. Scatchard plots were linear in all regions examined and revealed similar affinity constants of about 500 nM. The largest number of sites was found in the outer cerebral cortical layers (10 pmol/mg of protein). [3H]Glutamate binding was displaced by quisqualate, trans-1-amino-1,3-cyclopentane dicarboxylic acid (t-ACPD) (racemic mixture), and (1S,3R)-ACPD but not by (1R,3S)-ACPD. The guanine nucleotide analogue guanosine-5'-O-(3-thio) triphosphate (100 microM) reduced the binding by affecting the affinity but not the total number of sites, as predicted for G protein-coupled receptor sites. Quisqualate displacement curves were always biphasic and resolved two binding sites, with Ki values in the low nanomolar (15 nM) and micromolar (63 microM) ranges. (1S,3R)-ACPD displaced [3H]glutamate binding both in the absence and in the presence of 2.5 microM quisqualate, suggesting that both high and low affinity quisqualate sites are linked to mGluRs. (1S,3R)-ACPD competition curves were broad (Hill coefficient = 0.73) but monophasic under both conditions, with Ki values in the micromolar range (14-116 microM), suggesting that (1S,3R)-ACPD acts on the two quisqualate sites with similar apparent affinities. The regional distributions of the two sites were different. The highest levels of the high affinity quisqualate binding site were found in the cerebellar molecular layer. The highest levels of the low affinity quisqualate binding sites were found in the outer cerebral cortex. The pharmacological profile and regional distribution suggest that the high and low affinity quisqualate-sensitive components of [3H]glutamate binding sites might correspond to the mGluR1/mGluR5 and mGluR2/mGluR3 subgroups of cloned mGluRs, respectively.
Mol Pharmacol 1994 Apr
PMID:Metabotropic glutamate receptor heterogeneity in rat brain. 818 41

alpha 1-Adrenergic receptors (ARs) are members of the G protein-coupled receptor superfamily. alpha 1-AR subtypes mediate the effects of the sympathetic nervous system, especially those involved in cardiac homeostasis. To investigate signal transduction by a novel subtype (alpha 1D), which we recently cloned, and to compare it with that by the previously characterized alpha 1B-AR, we assessed the ability of each subtype to activate polyphosphoinositide (PI) metabolism, cAMP accumulation, and arachidonic acid release in Chinese hamster ovary (CHO) and COS-1 cells expressing these subtypes after stable or transient transfection, respectively. In COS-1 and CHO cells, both the alpha 1D- and alpha 1B-AR were found to couple to PI hydrolysis through a pertussis toxin-insensitive G protein. Both alpha 1-AR subtypes also increased intracellular cAMP by an indirect mechanism, although this effect was observed only in COS-1 cells and not in CHO cells. Interestingly, alpha 1-AR-stimulated arachidonic acid release was also demonstrated for both subtypes in COS-1 cells. This release was mediated through phospholipase A2 activation and involved a pertussis toxin-sensitive G protein. alpha 1-AR-stimulated arachidonic acid release was dependent upon extracellular calcium and was inhibited by 1 microM nifedipine. Inhibitors of protein kinase C, phospholipase C, and diacylglycerol lipase did not alter alpha 1-AR-stimulated release of arachidonic acid. These findings indicate that in COS-1 cells alpha 1-AR-stimulated arachidonic acid release is most likely coupled to dihydropyridine-sensitive L-type calcium channels via a pertussis toxin-sensitive G protein. The influx of extracellular calcium then stimulates phospholipase A2 to release arachidonic acid. alpha 1-AR-stimulated arachidonic acid release could also be demonstrated in CHO cells and was pertussis toxin sensitive but nifedipine insensitive. These cells were also unresponsive to Bay K8644, indicating a lack of voltage-sensitive calcium channels in CHO cells. Nevertheless, alpha 1-AR activation increased intracellular Ca2+ levels, as assessed by fura-2 fluorescence studies. Neomycin blocked both alpha 1-AR-stimulated PI hydrolysis and increases in intracellular Ca2+ levels but did not inhibit the increase in arachidonic acid release. Taken together, these data indicate that in CHO cells alpha 1-ARs can couple directly to phospholipase A2 activation via a pertussis toxin-sensitive pathway. Thus, in these model systems we demonstrate for the first time that a single alpha 1-AR subtype can activate multiple distinct signal transduction pathways, in which receptor-effector coupling is modulated by distinct G proteins.
Mol Pharmacol 1993 Oct
PMID:Coupling of expressed alpha 1B- and alpha 1D-adrenergic receptor to multiple signaling pathways is both G protein and cell type specific. 823 29

BW373U86 is a potent and highly selective nonpeptidic agonist for delta-opioid receptors. To determine its ability to couple with G protein-linked second messenger systems, this study examined the effects of BW373U86 on the inhibition of adenylyl cyclase and the stimulation of low-Km GTPase activity. In rat striatal membranes, BW373U86 inhibited basal adenylyl cyclase activity in a GTP-dependent manner, with maximal inhibition levels similar to those of the prototypic delta agonist [D-Ser2,Thr6]Leu-enkephalin (DSLET). However, BW373U86 was approximately 100 times more potent than DSLET in inhibiting adenylyl cyclase. Analysis of the inhibitory activity across 10 brain regions revealed that both low and high concentrations of BW373U86 inhibited adenylyl cyclase activity in a manner similar to that of DSLET. Inhibition of adenylyl cyclase by BW373U86 was delta receptor selective, because the delta receptor-selective antagonist naltrindole was significantly more potent than naloxone and the mu receptor-selective antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 was ineffective in blocking BW373U86 inhibition. BW373U86 also inhibited adenylyl cyclase activity in membranes prepared from NG108-15 cells, with an IC50 value 5 times lower than that of DSLET. This increased potency was not observed in concentration-effect curves for agonist-stimulated low-Km GTPase in NG108-15 membranes. BW373U86 is a competitive inhibitor of [3H]diprenorphine at delta receptors of NG108-15 cell membranes. However, unlike DSLET, BW373U86 displacement of [3H]diprenorphine binding to NG108-15 cell membranes was not affected by sodium and guanine nucleotides. This lack of GTP effect on binding apparently produced slow dissociation rates for this agonist, because naltrindole was less potent in blocking BW373U86 inhibition of adenylyl cyclase when membranes were preincubated with this agonist. These results demonstrate the novel finding that the binding of a full agonist to a G protein-coupled receptor is not regulated by GTP, and they also show how the lack of regulation in receptor binding affects agonist potency.
Mol Pharmacol 1993 Oct
PMID:BW373U86: a nonpeptidic delta-opioid agonist with novel receptor-G protein-mediated actions in rat brain membranes and neuroblastoma cells. 823 33

The A2a adenosine receptor, a member of the G protein-coupled receptor family, is important in the regulation of dopaminergic pathways of the brain and in platelet and cardiovascular functions. In this study, the role of extracellular loops in ligand binding to the human A2a receptor was explored through site-directed mutagenesis. Four glutamate/aspartate residues (Glu151, Glu161, Glu169, and Asp170) in the second extracellular loop (E2) and a cysteine residue (Cys262) in the third extracellular loop (E3) were individually replaced with alanine and other amino acids. A proline residue (Pro173) in E2 was mutated to arginine, the homologous amino acid in A3 receptors. The binding properties of the resultant mutant receptors were determined in transfected COS-7 cells. The mutant receptors were tagged at their amino terminus with a hemagglutinin epitope, thus allowing their detection in the plasma membrane with immunological techniques. High affinity specific binding of [3H]2-[4-[(2-carboxyethyl)phenyl]ethyl-amino]-5'-N-ethylcarboxamidoad eno sine (15 nM) and [3H]8-[4-[[[[2-aminoethyl)-amino]carbonyl]methyl]oxy]phenyl]-1,3- dipropylxanthine (4nM), an A2a agonist and antagonist, respectively, was not observed with four of the mutant receptors, E151A, E151Q, E151D, and E169A, although they were well expressed at the cell surface. The E151A and E169A mutant receptors showed nearly full stimulation of adenylyl cyclase at approximately 10(3)-fold higher concentrations of 2-[4-[(2-carboxyethyl)phenyl]ethyl-amino]-5'-N-ethylcarboxamidoadenosine . The E161A mutant receptor showed as increase in affinity for the nonxanthine adenosine antagonist 9-chloro-2-(furyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine(6 fold) but not for other ligands. An E169Q mutant gained affinity (5-22 fold) for adenosine derivatives (agonists) substituted at N6 but not at C2 or C5' positions. Mutant receptors D170K and P173R were similar to wild-type receptors in binding of both agonist and antagonist radioligands. A C262G mutant also resembled the wild-type receptor in radioligand binding, indicating that a potential disulfide bridge with another cysteine residue in proximity is not required for the structural integrity of the receptor. Our data suggest that certain amino acids in the second extracellular loop may be directly or indirectly involved in ligand binding.
Mol Pharmacol 1996 Apr
PMID:Glutamate residues in the second extracellular loop of the human A2a adenosine receptor are required for ligand recognition. 860 97

An azido derivative of [3H2](2S, 3S)-cis-2-(diphenylmethyl)-N-((2-methoxyphenyl) methyl)-1-azabicyclo[2.2.2]octon-3-amine (CP-96,345), a potent nonpeptide antagonist of the substance P (SP) (neurokinin-1) receptor, was synthesized and shown to have an affinity for the human SP receptor similar to that of the parent compound, CP-96,345. When Chinese hamster ovary cells expressing the human SP receptor were photolabeled with this compound and analyzed with the use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography, several radioactive bands were observed, including a major band centered at molecular mass 80 kDa, the expected value for the SP receptor expressed in Chinese hamster ovary cells. Only the labeling of the 80-kDa protein was specific: nonradiolabeled CP-96,345 but not its optical enantiomer, CP-96,344 was a potent inhibitor of photoincorporation. SP prevented photolabeling only at concentrations higher than expected from its binding affinity but similar to those shown in a competition binding assay to displace radioiodinated analogue of CP-96,345. Antiserum generated against a synthetic peptide corresponding to the carboxyl terminus of the human SP receptor immunoprecipitated only the 80-kDa photoaffinity labeled protein, confirming that it is the human SP receptor. Interestingly, a second antiserum that was generated against the third extracellular loop of the G protein-coupled receptor no longer immunoprecipitated the receptor when covalently labeled with [3H2]azido-CP-96,345. This result indicates either that attachment of the antagonist modified the antigenic region directly, suggesting involvement of this domain in the binding of CP-96,345, or that the loss of recognition by the antiserum is secondary to a change in conformation induced by the covalent attachment of the antagonist at a different site.
Mol Pharmacol 1996 May
PMID:Photoaffinity labeling of the human substance P (neurokinin-1) receptor with [3H2]azido-CP-96,345, a photoreactive derivative of a nonpeptide antagonist. 862 30

Opioids are cationic compounds that mediate their biological action through three highly homologous receptors (mu, delta, and kappa) known to belong to the G protein-coupled receptor (GPR) family. The third putative transmembrane domain of opioid receptors contains a conserved aspartate residue that is typically found in biogenic amine binding GPRs and is generally believed to form an ion pair with the cationic neurotransmitters. Using site-directed mutagenesis, we investigated the possibility of an identical role for this residue (Asp128) in the mouse delta-opioid receptor. Removal of the carboxylate group via an aspartate-to-alanine mutation did not modify binding affinity of a representative set of opioid compounds, including bremazocine, diprenorphine, naloxone, Tyr-D-Thr-Gly-Phe-Leu-Thr, [D-Ala2,D-Leu5]enkephalin, cyclic[D-penicillamine2,D-penicillamine5]enkephalin, deltorphin II, (+/-)-4-[(a-R*)-a-[(2S*,5R*)-4-allyl-2,5-di-methyl-1- piperazinyl]-3-hydroxybenzyl]-N,N-diethylbenzamide, and naltrindole. It nevertheless decreased receptor expression level and affected the binding of three agonists ([D-Ala2,D-Leu5]enkephalin, Tyr-D-Thr-Gly-Phe-Leu-Thr, and (+/-)-4-[(a-R*)-a-[(2S*,5R*)-4-allyl-2,5-di- methyl-1-piperazinyl]-3-hydroxybenzyl]-N,N-diethylbenzamide) when the receptor was under Na(+)-induced low affinity state. On the other hand, the aspartate-to-asparagine mutation strongly impaired the binding of all of the above ligands and highlighted differential modes of interaction for alkaloids and peptides. Finally, removal of the homologous carboxylate group in the mouse mu receptor had distinct effects because it dramatically reduced the binding potency of some, but not all, tested ligands. Taken together, these results demonstrate that (i) the direct ligand/receptor interaction previously demonstrated for the beta-adrenergic receptor does not take place in the delta receptor, (ii) Asp128 nevertheless contributes to stabilization of the spatial conformation of the binding pocket, and (iii) these conclusions cannot be extended to the closely related mu receptor.
Mol Pharmacol 1996 Feb
PMID:The conserved aspartate residue in the third putative transmembrane domain of the delta-opioid receptor is not the anionic counterpart for cationic opiate binding but is a constituent of the receptor binding site. 863 52


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