The ligand binding site of the angiotensin AT1 receptor

Bidirectional modulation of exocytosis by angiotensin II involves multiple G-protein-regulated transduction pathways in chromaffin cells

Angiotensin II (AngII) receptors couple to a multitude of different types of G-proteins resulting in activation of numerous signaling pathways. In this study we examined the consequences of this promiscuous G-protein coupling on secretion. Chromaffin cells were voltage-clamped at -80 mV in perforated-patch configuration, and Ca(2+)-dependent exocytosis was evoked with brief voltage steps to +20 mV. Vesicle fusion was monitored by changes in membrane capacitance (DeltaC(m)), and released catecholamine was detected with single-cell amperometry. Ca(2+) signaling was studied by recording voltage-dependent Ca(2+) currents (I(Ca)) and by measuring intracellular Ca(2+) ([Ca(2+)](i)) with fura-2 AM. AngII inhibited I(Ca) (IC(50) = 0.3 nm) in a voltage-dependent, pertussis toxin (PTX)-sensitive manner consistent with G(i/o)-protein coupling to Ca(2+) channels. DeltaC(m) was modulated bi-directionally; subnanomolar AngII inhibited depolarization-evoked exocytosis, whereas higher concentrations, in spite of I(Ca) inhibition, potentiated DeltaC(m) fivefold (EC(50) = 3.4 nm). Potentiation of exocytosis by AngII involved activation of phospholipase C (PLC) and Ca(2+) mobilization from internal stores. PTX treatment did not affect AngII-dependent Ca(2+) mobilization or facilitation of exocytosis. However, protein kinase C (PKC) inhibitors decreased the facilitatory effects but not the inhibitory effects of AngII on stimulus-secretion coupling. The AngII type 1 receptor (AT1R) antagonist losartan blocked both inhibition and facilitation of secretion by AngII. The results of this study show that activation of multiple types of G-proteins and transduction pathways by a single neuromodulator acting through one receptor type can produce concentration-dependent, bi-directional regulation of exocytosis.

Excitatory peptides and osmotic pressure modulate mechanosensitive cation channels in concert

Behavioral and neuroendocrine responses underlying systemic osmoregulation are synergistically controlled by osmoreceptors and neuropeptides released within the hypothalamus. Although mechanisms underlying osmoreception are understood, the cellular basis for the integration of osmotic and peptidergic signals remains unknown. Here we show that the excitatory effects of angiotensin II, cholecystokinin and neurotensin on supraoptic neurosecretory neurons are due to the stimulation of the stretch-inactivated cation channels responsible for osmoreception. This molecular convergence underlies the facilitatory effects of neuropeptides on responses to osmotic stimulation and provides a basis for the gating effects of plasma osmolality on the responsiveness of osmoregulatory neurons to peptidergic stimulation.

Role of transmembrane helix IV in G-protein specificity of the angiotensin II type 1 receptor

G-protein activation by G-protein coupled receptors (GPCRs) is accomplished through proper interaction with the cytoplasmic loops rather than through sequence-specific interactions. However, the mechanism by which a specific G-protein is selected by a GPCR is not known. In the current model of GPCR activation, agonist binding modulates helix-helix interactions, which is necessary for fully determining G-protein specificity and stimulation of GDP/GTP exchange. In this study, we report that a single-residue deletion in transmembrane helix IV leads the angiotensin II type 1 (AT(1)) receptor chimera CR17 to retain GTP-sensitive high affinity for the agonist angiotensin II but results in complete inactivation of intracellular inositol phosphate production. The agonist dissociation profile of CR17 in the presence of guanosine 5'-3-O-(thio)triphosphate suggests that the activation-induced conformational changes of the chimeric receptor itself remain intact. Insertion of an alanine at position 149 (CR17triangle down149A) in this chimera rescued the inactive phenotype, restoring intracellular inositol phosphate production by the chimera. This finding suggests that in the wild-type AT(1) receptor the orientation of transmembrane helix IV-residues following Cys(149) is a key determinant for effectively distinguishing among various structurally similar G-proteins. The results emphasize that the contacts within the membrane-embedded portion of transmembrane helix IV in the AT(1) receptor is important for specific G-protein selection.

Modelling G-protein-coupled receptors for drug design

The G-protein coupled receptors form a large and diverse multi-gene superfamily with many important physiological functions. As such, they have become important targets in pharmaceutical research. Molecular modelling and site-directed mutagenesis have played an important role in our increasing understanding of the structural basis of drug action at these receptors. Aspects of this understanding, how these techniques can be used within a drug-design programme, and remaining challenges for the future are reviewed.

Identification of ligand effector binding sites in transmembrane regions of the human G protein-coupled C3a receptor

The human C3a anaphylatoxin receptor (C3aR) is a G protein-coupled receptor (GPCR) composed of seven transmembrane alpha-helices connected by hydrophilic loops. Previous studies of chimeric C3aR/C5aR and loop deletions in C3aR demonstrated that the large extracellular loop2 plays an important role in noneffector ligand binding; however, the effector binding site for C3a has not been identified. In this study, selected charged residues in the transmembrane regions of C3aR were replaced by Ala using site-directed mutagenesis, and mutant receptors were stably expressed in the RBL-2H3 cell line. Ligand binding studies demonstrated that R161A (helix IV), R340A (helix V), and D417A (helix VII) showed no binding activity, although full expression of these receptors was established by flow cytometric analysis. C3a induced very weak intracellular calcium flux in cells expressing these three mutant receptors. H81A (helix II) and K96A (helix III) showed decreased ligand binding activity. The calcium flux induced by C3a in H81A and K96A cells was also consistently reduced. These findings suggest that the charged transmembrane residues Arg161, Arg340, and Asp417 in C3aR are essential for ligand effector binding and/or signal coupling, and that residues His81 and Lys96 may contribute less directly to the overall free energy of ligand binding. These transmembrane residues in C3aR identify specific molecular contacts for ligand interactions that account for C3a-induced receptor activation.

Potentiation of Bradykinin Actions by ACE Inhibitors

Angiotensin I-converting enzyme (kininase II; ACE) inhibitors, antibodies to ACE and slowly cleaved substrates of ACE potentiate the effect of bradykinin and its analogs on their B2 receptors independently of blocking peptide metabolism. ACE inhibitors also resensitized the receptors desensitized by the ligand (tachyphylaxis). The studies were performed on isolated organs and cells co-transfected with the receptor and the enzyme or constitutively expressing them. This enhancement of the effect of B2 ligands is attributed to a crosstalk between the enzyme and the receptor, and not to a direct action on the receptors. It might reflect some of the local activities of ACE inhibitors.

Insertion of an electronegative sulfur atom in the side chain of position 5 of angiotensin II: changes in the tachyphylactic properties of the peptide

Angiotensin II (AII) analogs bearing n-Leu, Met or S-substituted groups for cysteine at position 5 were studied regarding their agonistic and tachyphylactic properties. It was shown that these analogs lowered the relative affinity towards the AT1 receptor as determined by contractile responses, which could be due to the removal of the beta-branching residue at position 5. Insertion of a sulfur atom in a different position away from the attached backbone carbon atom presented no significant difference in EC50 values for these analogs. Interestingly, the S-bearing analogs at position 5 were full agonists but the tachyphylactic property was lost, in contrast to [n-Leu5]AII, which still induced reduction of the contractile responses. Nevertheless after replacing the Asp with Sar in position 1 (Sar1) tachyphylaxis was again established. It is concluded that the insertion of Met or an S-substituted cysteine into the side chain at position 5 of AII may promote interactions with its receptor due to the slight electronegative character of the sulfur atom and changes in the restricted conformational freedom of the Ile5 residue in the AII molecule. This was overcome by Sar1, probably through interactions due to its fully protonated N-terminal amino group and favoring the conformation responsible for the tachyphylaxis phenomenon.

Bradykinin-dependent cardioprotective effects of losartan against ischemia and reperfusion in rat hearts

It is unclear whether losartan, an angiotensin II type 1 (AT1) receptor antagonist, protects the heart against acute ischemia-reperfusion injury. Therefore we evaluated cardiac protection conferred by pre- and postischemic treatment as well as by exclusive postischemic treatment with losartan. Furthermore, we sought to determine both the extent of this protection and its dependence on bradykinin in comparison with quinaprilat, a cardioprotective angiotensin-converting enzyme inhibitor. Cardiac protection was assessed as recovery of coronary flow, left ventricular developed pressure, phosphocreatine, and adenosine triphosphate (ATP) in isolated perfused rat hearts after 15 min of global ischemia and 30 min of postischemic reperfusion. We found that, in hearts pre- and postischemically treated with losartan (1 microM) or quinaprilat (0.1 microM), these variables all recovered significantly better than those in untreated control hearts. In hearts that were only postischemically treated with losartan, these variables also recovered significantly better than those in control hearts. In contrast, in hearts treated with the combination of the bradykinin B2 receptor antagonist Hoe 140 with quinaprilat or losartan, the recovery of the variables no longer differed from that in control hearts. In conclusion, losartan protects the heart against acute ischemia-reperfusion injury. This protection can be achieved by pre- and postischemic treatment as well as by exclusive postischemic treatment with losartan. Furthermore, the extent of this protection is equivalent to that conferred by quinaprilat and, unexpectedly, dependent on bradykinin.

Determination of peptide contact points in the human angiotensin II type I receptor (AT1) with photosensitive analogs of angiotensin II

To identify ligand-binding domains of Angiotensin II (AngII) type 1 receptor (AT1), two different radiolabeled photoreactive AngII analogs were prepared by replacing either the first or the last amino acid of the octapeptide by p-benzoyl-L-phenylalanine (Bpa). High yield, specific labeling of the AT1 receptor was obtained with the 125I-[Sar1,Bpa8]AngII analog. Digestion of the covalent 125I-[Sar1,Bpa8]AngII-AT1 complex with V8 protease generated two major fragments of 15.8 kDa and 17.8 kDa, as determined by SDS-PAGE. Treatment of the [Sar1,Bpa8]AngII-AT1 complex with cyanogen bromide produced a major fragment of 7.5 kDa which, upon further digestion with endoproteinase Lys-C, generated a fragment of 3.6 kDa. Since the 7.5-kDa fragment was sensitive to hydrolysis by 2-nitro-5-thiocyanobenzoic acid, we circumscribed the labeling site of 125I-[Sar1,Bpa8]AngII within amino acids 285 and 295 of the AT1 receptor. When the AT1 receptor was photolabeled with 125I-[Bpa1]AngII, a poor incorporation yield was obtained. Cleavage of the labeled receptor with endoproteinase Lys-C produced a glycopeptide of 31 kDa, which upon deglycosylation showed an apparent molecular mass of 7.5 kDa, delimiting the labeling site of 125I-[Bpa1]AngII within amino acids 147 and 199 of the AT1 receptor. CNBr digestion of the hAT1 I165M mutant receptor narrowed down the labeling site to the fragment 166-199. Taken together, these results indicate that the seventh transmembrane domain of the AT1 receptor interacts strongly with the C-terminal amino acid of [Sar1, Bpa8]AngII interacts with the second extracellular loop of the AT1 receptor.

Regulation of angiotensin II type 1 (AT1) receptor function

The type 1 angiotensin receptor (AT1) mediates the important biological actions of the peptide hormone, angiotensin II (AngII), by activating an array of intracellular signaling pathways. The unique temporal arrangement and duration of AngII-stimulated signals suggests a hierarchy of post-AT1 receptor binding events that permits activation of selective effector pathways. Moreover, it predicts that the coupling of AT1 receptors is tightly regulated, allowing cells to differentiate acute responses from those requiring longer periods of stimulation. Recent studies have concentrated on delineating the molecular processes involved in modulating AT1 receptor activity. In addition to AT1 receptor modification (phosphorylation), trafficking (internalization and degradation) and interaction with regulatory intracellular proteins, other processes may include receptor dimerization, cross-regulation by other receptor systems, and receptor isomerization between activated and non-activated forms. This review focuses on recent advances in this area of research, highlighting directions for future investigation.

Angiotensin receptors in the nervous system

In addition to its traditional role as a circulating hormone, angiotensin is also involved in local functions through the activity of tissue renin-angiotensin systems that occur in many organs, including the brain. In the brain, both systemic and presumptive neurally derived angiotensin and angiotensin metabolites act through specific receptors to modulate many functions. This review examines the distribution of these specific angiotensin receptors and discusses evidence regarding the function of angiotensin peptides in various brain regions. Angiotensin AT1 and AT2 receptors occur in characteristic distributions that are highly correlated with the distribution of angiotensin-like immunoreactivity in nerve terminals. Acting through the AT1 receptor in the brain, angiotensin has effects on fluid and electrolyte homeostasis, neuroendocrine systems, autonomic pathways regulating cardiovascular function and behavior. Angiotensin AT1 receptors are also found in many afferent and efferent components of the peripheral autonomic nervous system. The role of the AT2 receptor in the brain is less well understood, although recent knockout studies point to an involvement with behavioral and cardiovascular functions. In addition to the AT1 and AT2 receptors, receptors for other fragments of angiotensin have been proposed. The AT4 binding site, which binds angiotensin, has a widespread distribution in the brain quite distinct from that of the AT1 and AT2 receptors. It is associated with many cholinergic neuronal groups and also several sensory nuclei, but its function remains to be determined. Our discovery that another brain-derived peptide binds to the AT4 binding site in the brain and may represent the native ligand is discussed. Overall, the distribution of angiotensin receptors in the brain indicate that they play diverse and important physiological roles in the nervous system.

Dependence of AT1 angiotensin receptor function on adjacent asparagine residues in the seventh transmembrane helix

For several G protein-coupled receptors, amino acids in the seventh transmembrane helix have been implicated in ligand binding and receptor activation. The function of this region in the AT1 angiotensin receptor was further investigated by mutation of two conserved polar residues (Asn294 and Asn295) and the adjacent Phe293 residue. Analysis of the properties of the mutant receptors expressed in COS-7 cells revealed that alanine replacement of Phe293 had no major effect on AT1 receptor function. Substitution of the adjacent Asn294 residue with alanine (N294A) reduced receptor binding affinities for angiotensin II, two nonpeptide agonists (L-162,313 and L-163,491), and the AT1-selective nonpeptide antagonist losartan but not that for the peptide antagonist [Sar1, Ile8]angiotensin II. The N294A receptor also showed impaired G protein coupling and severely attenuated inositol phosphate generation. In contrast, alanine replacement of Asn295 decreased receptor binding affinities for all angiotensin II ligands but did not impair signal transduction. Additional substitutions of Asn295 with a variety of amino acids did not identify specific structural elements for ligand binding. These findings indicate that Asn295 is required for the integrity of the intramembrane binding pocket of the AT1a receptor but is not essential for signal generation. They also demonstrate the importance of transmembrane helices in the formation of the binding site for nonpeptide AT1 receptor agonists. We conclude that the Asn294 residue of the AT1 receptor is an essential determinant of receptor activation and that the adjacent Asn295 residue is required for normal ligand binding.

Angiotensin, thirst, and sodium appetite

Angiotensin (ANG) II is a powerful and phylogenetically widespread stimulus to thirst and sodium appetite. When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in NaCl intake. Drinking is vigorous, highly motivated, and rapidly completed. The amounts of water taken within 15 min or so of injection can exceed what the animal would spontaneously drink in the course of its normal activities over 24 h. The increase in NaCl intake is slower in onset, more persistent, and affected by experience. Increases in circulating ANG II have similar effects on drinking, although these may be partly obscured by accompanying rises in blood pressure. The circumventricular organs, median preoptic nucleus, and tissue surrounding the anteroventral third ventricle in the lamina terminalis (AV3V region) provide the neuroanatomic focus for thirst, sodium appetite, and cardiovascular control, making extensive connections with the hypothalamus, limbic system, and brain stem. The AV3V region is well provided with angiotensinergic nerve endings and angiotensin AT1 receptors, the receptor type responsible for acute responses to ANG II, and it responds vigorously to the dipsogenic action of ANG II. The nucleus tractus solitarius and other structures in the brain stem form part of a negative-feedback system for blood volume control, responding to baroreceptor and volume receptor information from the circulation and sending ascending noradrenergic and other projections to the AV3V region. The subfornical organ, organum vasculosum of the lamina terminalis and area postrema contain ANG II-sensitive receptors that allow circulating ANG II to interact with central nervous structures involved in hypovolemic thirst and sodium appetite and blood pressure control. Angiotensin peptides generated inside the blood-brain barrier may act as conventional neurotransmitters or, in view of the many instances of anatomic separation between sites of production and receptors, they may act as paracrine agents at a distance from their point of release. An attractive speculation is that some are responsible for long-term changes in neuronal organization, especially of sodium appetite. Anatomic mismatches between sites of production and receptors are less evident in limbic and brain stem structures responsible for body fluid homeostasis and blood pressure control. Limbic structures are rich in other neuroactive peptides, some of which have powerful effects on drinking, and they and many of the classical nonpeptide neurotransmitters may interact with ANG II to augment or inhibit drinking behavior. Because ANG II immunoreactivity and binding are so widely distributed in the central nervous system, brain ANG II is unlikely to have a role as circumscribed as that of circulating ANG II. Angiotensin peptides generated from brain precursors may also be involved in functions that have little immediate effect on body fluid homeostasis and blood pressure control, such as cell differentiation, regeneration and remodeling, or learning and memory. Analysis of the mechanisms of increased drinking caused by drugs and experimental procedures that activate the renal renin-angiotensin system, and clinical conditions in which renal renin secretion is increased, have provided evidence that endogenously released renal renin can generate enough circulating ANG II to stimulate drinking. But it is also certain that other mechanisms of thirst and sodium appetite still operate when the effects of circulating ANG II are blocked or absent, although it is not known whether this is also true for angiotensin peptides formed in the brain. Whether ANG II should be regarded primarily as a hormone released in hypovolemia helping to defend the blood volume, a neurotransmitter or paracrine agent with a privileged role in the neural pathways for thirst and sodium appetite of all kinds, a neural organizer especially in sodium appetit

Mutational analysis of the angiotensin type 2 receptor: contribution of conserved amino acids in the region of the sixth transmembrane domain

Angiotensin II (AngII) mediates its physiological actions through two receptor subtypes: the Type 1 (AT1) and Type 2 (AT2) receptors. The subtypes have identical affinities for AngII, while sharing only 34% homology. Mutagenesis has focused mainly on the AT1 receptor, identifying residues important for AngII binding. In contrast, relatively little is known of the binding mechanism of the AT2 receptor. It has been hypothesized that residues that are conserved between the two subtypes that have been shown to be important in the AT1 receptor may also contribute to AngII binding in the AT2 receptor as well. To test this hypothesis, the role of two conserved residues in the sixth transmembrane domain of the AT2 receptor in ligand binding were investigated: tryptophan 269 and aspartate 279. In contrast to the AT1 receptor, mutation of Trp269 in the AT2 receptor to an alanine had no effect on AngII binding, while mutation of Asp279 to alanine similarly impaired AngII binding in both receptors. However, the more sterically conservative substitution of Asp279 to asparagine in the AT2 receptor showed near wild type affinity. Based on this finding, we mutated Asp263 in the AT1 receptor to asparagine. Subsequent studies indicated that this more conservative mutation had no effect on AngII binding to the AT1 receptor. Collectively, these results demonstrate that although there may be commonalities in ligand binding between the AT1 and AT2 AngII receptors, there are also clear differences.

Residues Val254, His256, and Phe259 of the angiotensin II AT1 receptor are not involved in ligand binding but participate in signal transduction

The role of the external third of helix VI of the angiotensin II (AII) AT1 receptor for the interaction with its ligand and for the subsequent signal transduction was investigated by individually replacing residues 252-256 by Ala, and residues 259 or 261 by Tyr, and permanently transfecting the resulting mutants to Chinese hamster ovary (CHO) cells. Binding experiments showed no great changes in affinity of any of the mutants for AII, [Sar1]-AII, or [Sar1, Leu8]-AII, but the affinity for the nonpeptide antagonist DuP753 was significantly decreased. The inositol phosphate response to AII was remarkably decreased in mutants V254A, H256A, and F259Y. These results indicate that AT1 residues Val254, His256, and Phe259 are not involved in ligand binding but participate in signal transduction. Based in these results and in others from the literature, it is suggested that, in addition to the His256 imidazole ring, the Phe259 aromatic ring interacts with the AII's Phe8, thus contributing to the signal-triggering mechanism.

Requirement of membrane-proximal amino acids in the carboxyl-terminal tail for expression of the rat AT1a angiotensin receptor

A series of deletion mutants was created to analyze the function of the membrane-proximal region of the cytoplasmic tail of the rat type 1a (AT1a) angiotensin receptor. In transiently transfected COS-7 cells, the truncated mutant receptors showed a progressive decrease in surface expression, with no major change in binding affinity for the peptide antagonist, [Sar1,Ile8]angiotensin II. In parallel with the decrease in receptor expression, a progressive decrease in angiotensin II-induced inositol phosphate responses was observed. Alanine substitutions in the region 307-311 identified the highly conserved phenylalanine309 and adjacent lysine residues as significant determinants of AT1a receptor expression.

CCR5 coreceptor utilization involves a highly conserved arginine residue of HIV type 1 gp120

The seven-transmembrane CCR5 was recently found to double as a coreceptor for a genetically diverse family of human and nonhuman primate lentiviruses. Paradoxically, the main region of the envelope protein believed to be involved in CCR5 utilization was mapped to hypervariable region 3, or V3, of the envelope glycoprotein gp120. In this study, we addressed the question of whether functional convergence in CCR5 utilization is mediated by certain V3 residues that are highly conserved among HIV type 1 (HIV-1), HIV type 2, and simian immunodeficiency virus. Site-directed mutagenesis carried out on three such V3 residues revealed that the Arg-298 of HIV-1 gp120 has an important role in CCR5 utilization. In contrast, no effect was observed for the other residues we tested. The inability of Arg-298 mutants to use CCR5 was not attributed to global alteration of gp120 conformation. Neither the expression, processing, and incorporation of mutant envelope proteins into virions, nor CD4 binding were significantly affected by the mutations. This interpretation is further supported by the finding that alanine substitutions of five residues immediately adjacent to the arginine residue had no effect on CCR5 utilization. Taken together, our data strongly suggests that the highly conserved Arg-298 residue identified in the V3 of HIV-1 has a significant role in CCR5 utilization, and may represent an unusually conserved target for future anti-viral designs.

Signal transduction and activator of transcription (STAT) protein-dependent activation of angiotensinogen promoter: a cellular signal for hypertrophy in cardiac muscle

The role of the peptide hormone angiotensin (AngII) in promoting myocardial hypertrophy is well documented. Our studies demonstrate that AngII uses a signaling pathway in cardiac myocytes in which the promoter of the gene encoding its prohormone, angiotensinogen, serves as the target site for activated signal transduction and activator of transcription (STAT) proteins. Gel mobility-shift assay revealed that STAT3 and STAT6 are selectively activated by AngII treatment of cardiomyocytes in culture and bind to a sequence motif (St-domain) in the angiotensinogen promoter to activate its transcription in transient transfection assay. We have also observed a dramatic increase in the St-domain binding activity of STAT proteins in the hypertrophied heart of the genetically hypertensive rat relative to that of the aged-matched normotensive strain WKY, providing a compelling argument in favor of the linkage of STAT pathway to the heart tissue autocrine AngII loop. These studies thus uncover a mechanism by which the activation of a selective set of STATs underlies mobilization of the gene activation program intrinsic to cardiac hypertrophy.

Identification of binding domains of the growth hormone-releasing hormone receptor by analysis of mutant and chimeric receptor proteins

The hypothalamic peptide GH-releasing hormone (GHRH) stimulates the release of GH from the pituitary through binding and activation of the GHRH receptor, which belongs to the family of G protein-coupled receptors. The objective of this study was to identify regions of the receptor critical for interaction with the ligand by expressing and analyzing truncated and chimeric epitope-tagged GHRH receptors. Two truncated receptors, GHRHdeltaN, in which part of the N-terminal domain between the putative signal sequence and the first transmembrane domain was deleted, and GHRHdeltaC, which was truncated downstream of the first intracellular loop, were generated. Both the receptors were deficient in ligand binding, indicating that neither the N-terminal extracellular domain (N terminus) nor the membrane-spanning domains with the associated extracellular loops (C terminus) are alone sufficient for interaction with GHRH. In subsequent studies, chimeric proteins between the receptors for GHRH and vasoactive intestinal peptide (VIP) or secretin were generated, using the predicted start of the first transmembrane domain as the junction for the exchange of the N terminus between receptors. The chimeras having the N terminus of the GHRH receptor and the C terminus of either the VIP or secretin receptor (GNVC and GNSC) did not bind GHRH or activate adenylate cyclase after GHRH treatment. The reciprocal chimeras having the N terminus of either the VIP or secretin receptors and the C terminus of the GHRH receptor (VNGC and SNGC) bound GHRH and stimulated cAMP accumulation after GHRH treatment. These results suggest that although the N-terminal extracellular domain is essential for ligand binding, the transmembrane domains and associated extracellular loop regions of the GHRH receptor provide critical information necessary for specific interaction with GHRH.

Intrinsic ANG II type 1 receptor stimulation contributes to recovery of postischemic mechanical function

To determine whether intrinsic angiotensin II (ANG II) type 1 receptor (AT1-R) stimulation modulates recovery of postischemic mechanical function, we studied the effects of selective AT1-R blockade with losartan on proton production from glucose metabolism and recovery of function in isolated working rat hearts perfused with Krebs-Henseleit buffer containing palmitate, glucose, and insulin. Aerobic perfusion (50 min) was followed by global, no-flow ischemia (30 min) and reperfusion (30 min) in the presence (n = 10) or absence (n = 14) of losartan (1 mumol/l) or the cardioprotective adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA, 0.5 mumol/l, n = 11). During reperfusion in untreated hearts (controls), left ventricular (LV) minute work partially recovered to 38% of aerobic baseline, whereas proton production increased to 155%. Compared with controls, CHA improved recovery of LV work to 79% and reduced proton production to 44%. Losartan depressed recovery of LV work to 0% without altering proton production. However, exogenous ANG II (1-100 nmol/l) in combination with losartan restored recovery of LV work during reperfusion in a concentration-dependent manner, suggesting that postischemic recovery of function depends on intrinsic AT1-R stimulation.

Mutational analysis of the angiotensin II type 2 receptor: contribution of conserved extracellular amino acids

While much work has been done examining the ligand-binding characteristics of the AT1 receptor, very little attention has been focused on the AT2 receptor. Both receptors bind angiotensin II (AngII) with identical affinities, but share only 34% homology. Although it is tempting to assume that conserved residues between the two subtypes are responsible for the binding of AngII, there is little data to support this view. To determine the commonalities in ligand binding of the AT1 and AT2 receptors, we have chosen several conserved extracellular amino acids which have been shown to be important in AngII binding [1,2] to the AT1 receptor for mutational studies of the AT2 receptor. Specifically, we have mutated tyrosine108 in extracellular loop 1 (ECL1), arginine182 in ECL2, and aspartate297 in ECL3 of the AT2 receptor in order to determine their contribution to AngII binding. In the AT2 receptor, mutation of tyrosine108 to an alanine resulted in a receptor with wild-type binding for AngII, while mutation of either arginine182 or aspartate297 drastically impaired AngII binding ( > 100 nM). These results demonstrate both similarities as well as clear differences between receptor subtypes in the contributions to AngII binding of several conserved extracellular amino acid residues.

Pharmacological profile of TH-142177, a novel orally active AT1-receptor antagonist

The pharmacological properties of TH-142177 (N-n-butyl-N-[2'-(1-H-tetrazole-5-yl) biphenyl-4-yl]-methyl-(N-carboxymethyl-benzylamino)-acetamide), a novel antagonist of the angiotensin II (AII) AT1 receptor, were studied in vitro and in vivo, and compared to those of losartan. In the rat isolated aorta, TH-142177 produced parallel shifts to the right of the concentration-response curves for AII-induced contractions without affecting the maximal response (pA2 = 9.07). The inhibitory potency of TH-142177 in the aorta was about three times greater than that of losartan. TH-142177 completely inhibited the specific binding of [125I]AII to AT1 receptor in rat aortic membranes (Ki = 1.6 x 10(-8) M), whereas specific [125I]AII binding to AT2 receptor in bovine cerebellum and human myocardium was not affected by concentrations of TH-142177 up to 10(-5) M. Losartan also inhibited the [125I]AII binding to rat aortic membranes (Ki = 2.2 x 10(-8) M). Following the intravenous administration to anesthetized normotensive rats, TH-142177 dose-dependently inhibited the increase in systolic blood pressure induced by an intravenous bolus injection of AII that was 1.5 times less potent than losartan. Furthermore, the oral administration of TH-142177 to conscious renal hypertensive rats exerted a dose-dependent reduction of systolic blood pressure without significantly effecting the heart rate. TH-142177 was at least three times more potent than losartan. These results demonstrate that TH-142177 is a potent and selective antagonist of AT1 receptors and by oral administration has a long-lasting antihypertensive activity.

Nonpeptide antagonists of neuropeptide receptors: tools for research and therapy

The recent development of selective and highly potent nonpeptide antagonists for peptide receptors has constituted a major breakthrough in the field of neuropeptide research. Following the discovery of the first nonpeptide antagonists for peptide receptors ten years ago, numerous other antagonists have been developed for most neuropeptide families. These new, metabolically stable compounds, orally active and capable of crossing the blood-brain barrier, offer clear advantages over the previously available peptide antagonists. Nonpeptide antagonists have provided valuable tools to investigate peptide receptors at the molecular, pharmacological and anatomical levels, and have considerably advanced our understanding of the pathophysiological roles of peptides in the CNS and periphery. Evidence from animal and clinical studies suggests that nonpeptide antagonists binding to peptide receptors could be useful for the treatment of disease states associated with high levels of neuropeptides. In this article Catalina Batancur, Mounia Azzi and William Rostène will address the recent developments in nonpeptide antagonists for neuropeptide receptors, with a particular focus on their CNS actions.

Modes of peptide binding in G protein-coupled receptors

The G-protein coupled seven transmembrane domain receptors bind a wide variety of ligands of different molecular size ranging from small monoamines to large neuropeptides and peptide hormones. This review summarises data from studies on the localisation of the binding site for a few neuropeptides in their receptors and compares this to the binding pockets for non peptide ligands. The main conclusion is that neuropeptide binding involves residues on the top of several transmembrane domains and in extracellular loops of the receptors while the non peptide type ligands to the same receptors tend to bind deeper in the plane of the membrane, between several transmembrane domains--similarly to monoamines. Thus the antagonism exerted by most of the non peptide type ligands is an allosteric phenomenon whereby binding of these to another site than the peptide binding site stablises a "non agonist" binding, and for signalling inactive, conformation of the 7 TM receptor.

A review of mutagenesis studies of angiotensin II type 1 receptor, the three-dimensional receptor model in search of the agonist and antagonist binding site and the hypothesis of a receptor activation mechanism

OBJECTIVE

To seek the mechanism whereby agonists, competitive antagonists and insurmountable antagonists affect the receptor function differently, by reviewing recent mutagenesis studies of angiotensin II type 1 receptor (AT1) in which the binding of the agonist and antagonists and receptor signaling were affected.

AT1 RECEPTOR STRUCTURE AND LIGAND BINDING SITES

We built a model of seven transmembrane spanning domains of the AT1 receptors using bacteriorhodopsin as a template. The carboxy terminal of angiotensin II binds to Lys199 in transmembrane domain 5, whereas the guanidinium group of Arg2 binds to Asp281 in transmembrane domain 7. Results of studies using mutagenesis supporting proposed ligand-docking models are discussed. HYPOTHESIS FOR THE LIGAND-INDUCED RECEPTOR SIGNALING MECHANISM: We submit a set of hypotheses for a mechanism whereby the ligand binding induces changes in the receptor conformation by the rotation of transmembrane helices as the initial event for the subsequent activation of a G protein. In this mechanism antagonists are not capable of rotating the helices but agonists are able to do so, which results in the formation of a hydrogen bond between Asp74 in transmembrane domain 2 and Tyr292 in transmembrane domain 7. This mechanism also provides plausible explanation for the activation of monoamine receptors.

COMPETITIVE AND INSURMOUNTABLE ANTAGONISTS

Competitive antagonists share the same binding sites with agonists, but insurmountable antagonists do not, and binding of the latter does not preclude agonist binding, for example, to Asp281.

CONCLUSION

This hypothesis of the intrareceptor signaling mechanism and the receptor model indicate that some amino acid residues essential for the signaling play their roles in the intrareceptor activation mechanism, whereas others participate directly in ligand binding.

Identification of angiotensin II-binding domains in the rat AT2 receptor with photolabile angiotensin analogs

To identify binding domains between angiotensin II (AngII) and its type 2 receptor (AT2), two different radiolabeled photoreactive analogs were prepared by replacing either the first or the last amino acid in the peptide with p-benzoyl-L-phenylalanine (Bpa). Digestion of photolabeled receptors with kallikrein revealed that the two photoreactive analogs label the amino-terminal part of the receptor within the first 182 amino acids. Digestion of 125I-[Bpa1]AngII.AT2 receptor complex with endoproteinase Lys-C produced a glycoprotein of 80 kDa. Deglycosylation of this 80-kDa product decreased its apparent molecular mass to 4.6 kDa and further cleavage of this 4.6-kDa product with V8 protease decreased its molecular mass to 3.6 kDa, circumscribing the labeling site of 125I-[Bpa1]AngII within amino acids 3-30 of AT2 receptor. Treatment of 125I-[Bpa8]AngII.AT2 receptor complex with cyanogen bromide produced two major receptor fragments of 3.6 and 2.6 kDa. Cyanogen bromide hydrolysis of a mutant AT2 receptor produced two major fragments of 12.6 kDa and 2.6 kDa defining the labeling site of 125I-[Bpa8]AngII within residues 129-138 of AT2 receptor. Our results indicate that the amino-terminal tail of the AT2 receptor interacts with the amino-terminal end of AngII, whereas the inner half of the third transmembrane domain of AT2 receptor interacts with the carboxyl-terminal end of AngII.

Opposite effects of angiotensin AT1 and AT2 receptor antagonists on recovery of mechanical function after ischemia-reperfusion in isolated working rat hearts

BACKGROUND

Angiotensin II type 1 (AT1) receptor antagonists, when given over the long term, reduce the deleterious consequences of ischemia-reperfusion injury. Whether short-term administration of AT1 or angiotensin II type 2 (AT2) receptor antagonists is cardioprotective has not been investigated.

METHODS AND RESULTS

The effects of short-term administration of selective AT1 and AT2 receptor antagonists on the recovery of mechanical function during reperfusion after 30 minutes of global, no-flow ischemia were studied in left atrium-perfused isolated working rat hearts. Control hearts (n = 8) showed incomplete recovery of left ventricular minute work (LV work) and cardiac efficiency during reperfusion to 51 +/- 15% and 61 +/- 19% of preischemic levels, respectively. Compared with control hearts, the selective AT2 receptor antagonist PD123,319 (0.3 mumol/L) given before ischemia (n = 7) improved the recovery of LV work and efficiency to 82 +/- 4% and 98 +/- 7% of preischemic levels, respectively (P < .01). In contrast, the selective AT1 antagonist losartan (1 mumol/L) blocked the recovery of LV work and depressed efficiency to 0 +/- 0% and 1 +/- 0% (n = 7) of preischemic levels, respectively (P < .01; n = 7). Neither antagonist altered coronary vascular conductance.

CONCLUSIONS

This is the first demonstration that short-term treatment with a selective AT1 versus AT2 antagonist exerts different effects on recovery of mechanical function after ischemia-reperfusion: the AT2 antagonist was cardioprotective, whereas the AT1 antagonist was not. These data suggest that AT2 antagonists and AT1 agonists may offer novel approaches for the treatment of mechanical dysfunction after ischemia-reperfusion.