Differentiation between binding sites for angiotensin II and nonpeptide antagonists on the angiotensin II type 1 receptors

Shared determinants of receptor binding for subtype selective, and dual endothelin-angiotensin antagonists on the AT1 angiotensin II receptor

Site-directed interspecies amino acid exchange was used to compare the binding determinants of a novel dual endothelial-angiotensin receptor ligand, L-746,072, with type-1 angiotensin receptor (AT1) selective antagonists on AT receptors expressed in COS cells. These studies suggest that residues on AT receptors which are non-conserved between amphibian and mammalian species play a greater role in subtype selective ligand recognition than for dual receptor ligands. These data also support the hypothesis that a common non-peptide binding site exists within transmembrane domains on peptidergic receptors.

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.

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.

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.

Atypical angiotensin II receptors coupled to phosphoinositide turnover/calcium signalling in catfish hepatocytes

In catfish (Ictalurus punctatus) hepatocytes angiotensin II induced an immediate increase in cytosolic Ca2+ concentration. Other angiotensin analogues also induced this effect including: human angiotensin II, fish angiotensin II, human angiotensin III, human angiotensin I, fish angiotensin I and saralasin. CGP 42112A induced a very small effect at the highest concentration tested and angiotensin IV was without effect. Angiotensin II also increased the resynthesis of phosphatidylinositol and the production of IP3. These physiological effects were not blocked by losartan (AT1-selective antagonist) or PD 123177 (AT2-selective antagonist). [125I]Angiotensin II bound to liver plasma membranes in a saturable fashion with high affinity (K(D) 2.7 nM) and a B(max) of 185 fmol/mg of protein. Binding competition experiments showed the following order of potency: human angiotensin II = fish angiotensin II > human angiotensin III > or = human angiotensin I = fish angiotensin I. These sites were insensitive to losartan or PD 123177. The data indicate that the angiotensin II receptors expressed in catfish hepatocytes are coupled to the phosphoinositide turnover/calcium mobilization signal transduction pathway and are atypical receptors, i.e., pharmacologically distinct from mammalian AT1 and AT2 receptors.

Identification of domains conferring ligand binding specificity to the prostanoid receptor. Studies on chimeric prostacyclin/prostaglandin D receptors

To identify domains conferring ligand binding specificity to prostanoid receptors, we constructed a series of chimeric receptors by successively replacing the regions from the carboxyl-terminal tail of mouse prostacyclin (prostaglandin I (PGI)) receptor (mIP) with the corresponding regions of the mouse PGD receptor (mDP). The mIP receptor expressed in COS 7 cells bound [3H]iloprost, a PGI2 analog, and [3H]PGE1 with Kd values of 13 and 27 nM, respectively. This receptor did not bind [3H]PGD2, [3H]PGE2, and [3H]PGF2alpha. The mDP receptor bound only [3H]PGD2 with a Kd value of 43 nM. The chimeric IPN-VII/DPC receptor with replacement of the carboxyl tail of the mIP receptor with that of the mDP receptor showed 12-16-fold higher affinities for [3H]iloprost and [3H]PGE1 than the mIP receptor. The region extending from the sixth transmembrane domain to the carboxyl terminus of the mIP receptor was next replaced with the corresponding region of the mDP receptor. This chimeric IPN-V/DPVI-C receptor acquired the ability to bind [3H]PGD2 and [3H]PGE2 without decreasing the affinities of the mIP receptor to [3H]iloprost and [3H]PGE1. These binding characteristics did not change when the fourth and fifth transmembrane domains of the mIP receptor were further replaced with the corresponding regions of the mDP receptor. However, when the first extracellular to second intracellular loop of the mIP receptor containing the third transmembrane domain was further replaced with those of the mDP receptor, the affinities for [3H]PGE1, [3H]PGE2, and [3H]iloprost were markedly decreased, whereas that for [3H]PGD2 was increased by about 2-fold. [3H]PGF2alpha showed no affinity for the mIP, mDP, and all the chimeric receptors. These results suggest that the sixth to seventh transmembrane domain of the mIP receptor confers the specificity of this receptor to bind selectively to PGE1 and not to PGE2 and that the third transmembrane domain of the mDP receptor confers the selective binding of PGD2 to this receptor.

Characterization of the binding and activity of a high affinity, pseudoirreversible morpholino tachykinin NK1 receptor antagonist

2(S)-((3,5-Bis(trifluoromethyl)benzyl)-oxy)-3(S)-phenyl-4-((3-oxo-1,2,4- triazol-5-yl)methyl)morpholine (L-742,694) is a selective morpholino tachykinin NK1 receptor antagonist that inhibits the binding of 125I-substance P to the human tachykinin NK1 receptor with a Kd = 37 pM. Increasing concentrations of L-742,694 added to cells 15 min prior to agonist progressively increase the apparent EC50 of substance P for inducing the synthesis of inositol phosphate in Chinese hamster ovary (CHO) cells expressing human tachykinin NK1 receptor and decrease the maximal level of stimulation observed. In contrast, addition of substance P and L-742,694 to the cells at the same time results in an increase in the EC50 for substance P with no decrease in the maximal level of stimulation. The compound also decreases the apparent number of binding sites for 125I-substance P observed by Scatchard analysis. Analysis of the binding of [3H]L-742,694 to the tachykinin NK1 receptor shows that it associates with the receptor with k(a) = 3.98 x 10(8) M(-1) min(-1), and dissociates with k(d) = 0.026 min(-1) and t1/2 = 27 min at 22 degrees C. The slow rate of dissociation of L-742,694 from the tachykinin NK1 receptor and the observation that altering the order of addition of antagonist and substance P attenuates the effect of the antagonist on the maximal activation suggest that L-742,694 is a competitive antagonist that can behave as a pseudoirreversible antagonist under some experimental conditions. L-742,694 has reduced affinity for tachykinin NK1 receptors in which alanine has been substituted for Gln165, His197 or His265 in transmembrane helices 4, 5 and 6, respectively. These three residues have previously been shown to be present in the binding site of tachykinin NK1 receptor antagonists of several structural classes. In addition, L-742,694 inhibits binding of the quinuclidine antagonist (2S,3S)-cis-2-(diphenyl methyl)-N-[(2-iodophenyl)-methyl]-1-azabicyclo[2.2.2]octane 3-amine ([125I]L-703,606) with the same affinity as it inhibits binding of 125I-substance P. These data indicate that L-742,694 binds to the same site within the transmembrane domain of the receptor as previously described competitive antagonists.

AT1 receptor blockade and the sympathoadrenal response to insulin-induced hypoglycemia in humans

An important facilitating effect of angiotensin II on adrenal catecholamine release has been demonstrated in several species. To determine whether specific AT1 receptor blockade affects medullary epinephrine secretion and musculocutaneous norepinephrine release during insulin-induced hypoglycemia, 16 healthy volunteers received losartan vs. placebo followed by an intravenous insulin bolus and measurement of effect variables at short intervals for 150 min. AT1 receptor blockade was effective, as evidenced by substantially increased circulating renin and angiotensin II levels, a 60% inhibition of circulating aldosterone, and an 8.5% decrease of mean arterial pressure over time compared with placebo. Arterial glucose concentration fell to a nadir of 1.9 mM, arterial epinephrine concentration increased 23-fold, forearm musculocutaneous norepinephrine release increased 4-fold, heart rate increased 40%, and forearm blood flow increased 3-fold. All absolute values and the time course of these changes were independent of AT1 receptor blockade. It is concluded that a putative interaction between angiotensin II and the sympathoadrenal axis may not be mediated by AT1 receptors in humans.

The conformational change responsible for AT1 receptor activation is dependent upon two juxtaposed asparagine residues on transmembrane helices III and VII

A model of the angiotensin AT1 receptor and site-directed mutagenesis were used to identify key residues involved in ligand binding. Receptors were stably expressed in human embryonic kidney 293 cells, and their binding properties compared. Wild type receptors exhibited low and high affinity binding sites for peptides. Substitution of Asn111, situated in the third transmembrane helix, resulted in a significant alteration in ligand binding with only high affinity binding of the peptides, angiotensin II, angiotensin III, and [p-amino-Phe6]angiotensin II and a marked loss in the binding affinity of the AT1 receptor selective non-peptide antagonist losartan. From our model it was apparent that Asn111 was in close spatial proximity to Asn295 in the seventh transmembrane helix. Substitution of Asn295, produced identical changes in the receptor's pharmacological profile. Furthermore, the Ser111AT1A and Ser295AT1A mutants did not require the association of a G-protein for high affinity agonist binding. Finally, the Ser295AT1A mutant maintained higher basal generation of inositol trisphosphate than the wild type, indicating constitutive activation. We propose that substitution of these residues causes the loss of an interaction between transmembrane helices III and VII, which allows the AT1 receptor to "relax" into its active conformation.

Dual agonistic and antagonistic property of nonpeptide angiotensin AT1 ligands: susceptibility to receptor mutations

Two nonpeptide ligands that differ chemically by only a single methyl group but have agonistic (L-162,782) and antagonistic (L-162,389) properties in vivo were characterized on the cloned angiotensin AT1 receptor. Both compounds bound with high affinity (K(I) = 8 and 28 nM, respectively) to the AT1 receptor expressed transiently in COS-7 cells as determined in radioligand competition assays. L-162,782 acted as a powerful partial agonist, stimulating phosphatidylinositol turnover with a bell-shaped dose-response curve to 64% of the maximal level reached in response to angiotensin II. Surprisingly, L-162,389 also stimulated phosphatidylinositol turnover, albeit only to a small percentage of the angiotensin response. The prototype nonpeptide AT1 agonist L-162,313 gave a response of approximately 50%. The apparent EC50 values for all three compounds in stimulating phosphatidylinositol turnover were similar, approximately 30 nM, corresponding to their binding affinity. Each of the three compounds also acted as angiotensin antagonists, yet in this capacity the compounds differed markedly, with IC50 values ranging from 1.05 x 10(-7) M for L-162,389 to 6.5 x 10(-6) for L-162,782. A series of point mutations in the transmembrane segments (TMs) of the AT1 receptor had only minor effect on the binding affinity of the nonpeptide compounds, with the exception of A104V at the top of TM III, which selectively impaired the binding of L-162,782 and L-162,389. Substitutions in the middle of TM III, VI, or VII, which did not affect the binding affinity of the compounds, impaired or eliminated the agonistic efficacy of the nonpeptides but with only minor or no effect on the angiotensin potency or efficacy. Thus, in the N295D rat AT1 construct, L-162,782, L-162,313, and L-162,389 all antagonized the angiotensin-induced phosphatidylinositol turnover with surprisingly similar IC50 values (90-180 nM), and they all bound with unaltered, high affinity (22-36 nM). However, L-162,313 and L-162,782 could stimulate phosphatidylinositol turnover to only 20% of that of angiotensin. It is concluded that minor chemical modifications of either the compound or the receptor can dramatically alter the agonistic efficacy of biphenyl imidazole compounds on the AT1 receptor without affecting their affinity, as determined in binding assays, and that a number of substitutions in the middle of the TM segments affect the efficacy of nonpeptide agonists as opposed to angiotensin.

Mutations in the B2 bradykinin receptor reveal a different pattern of contacts for peptidic agonists and peptidic antagonists

The B2 bradykinin receptor, a seven-helix transmembrane receptor, binds the inflammatory mediator bradykinin (BK) and the structurally related peptide antagonist HOE-140. The binding of HOE-140 and the binding of bradykinin are mutually exclusive and competitive. Fifty-four site-specific receptor mutations were made. BK's affinity is reduced 2200-fold by F261A, 490-fold by T265A, 60-fold by D286A, and 3-10-fold by N200A, D268A, and Q290A. In contrast, HOE-140 affinity is reduced less than 7-fold by F254A, F261A, Y297A, and Q262A. The almost complete discordance of mutations that affect BK binding versus HOE-140 binding is surprising, but it was paralleled by the effect of single changes in BK and HOE-140. [Ala9]BK and [Ala6]BK are reduced in receptor binding affinity 27,000- and 150-fold, respectively, while [Ala9]HOE-140 affinity is reduced 7-fold and [Ala6]HOE-140 affinity is unchanged. NMR spectroscopy of all of the peptidic analogs of BK or HOE-140 revealed a beta-turn at the C terminus. Models of the receptor-ligand complex suggested that bradykinin is bound partially inside the helical bundle of the receptor with the amino terminus emerging from the extracellular side of helical bundle. In these models a salt bridge occurs between Arg9 and Asp286; the models also place Phe8 in a hydrophobic pocket midway through the transmembrane region. Models of HOE-140 binding to the receptor place its beta-turn one alpha-helical turn deeper and closer to helix 7 and helix 1 as compared with bradykinin-receptor complex models.

Identification of nonconserved amino acids in the AT1 receptor which comprise a general binding site for biphenylimidazole antagonists

Mutational analysis based on pharmacological differences between mammalian and amphibian angiotensin II receptors (AT receptors) previously led to construction of a mutant receptor that gained > 25000-fold affinity for the biphenylimidazole, Losartan. This variant frog receptor also bound with high affinity other nonpeptides in the biphenylimidazole chemical class according to the following rank order of potency (expressed in Fmut values=mutant IC50/rAT1b IC50): Losartan, 0.91; L-162,389, 1.0; L-163,491, 1.9; L-158,809, 3.5; L-163,017, 3.9; SC-51,316, 3.9. In contrast, the imidazoleacrylic acids, SKF-108,566 (Fmut= 160) and SB-203,220 (Fmut = 170), bound with markedly less affinity. Thus, nonconserved residues determining the molecular requirements for biphenylimidazole recognition are conserved in general, but are not identical to nonconserved residues necessary for high affinity binding of imidazoleacrylic acids.

Polar residues in the transmembrane domains of the type 1 angiotensin II receptor are required for binding and coupling. Reconstitution of the binding site by co-expression of two deficient mutants

Type 1 angiotensin receptors (AT1) are G-protein coupled receptors, mediating the physiological actions of the vasoactive peptide angiotensin II. In this study, the roles of 7 amino acids of the rat AT1A receptor in ligand binding and signaling were investigated by performing functional assays of individual receptor mutants expressed in COS and Chinese hamster ovary cells. Substitutions of polar residues in the third transmembrane domain with Ala indicate that Ser105, Ser107, and Ser109 are not essential for maintenance of the angiotensin II binding site. Replacement of Asn111 or Ser115 does not alter the binding affinity for peptidic analogs, but modifies the ability of the receptor to interact with AT1 (DuP753)- or AT2 (CGP42112A)-specific ligands. These 2 residues are probably involved in determining the binding specificity for these analogs. The absence of G-protein coupling to the Ser115 mutant suggests that this residue, in addition to previously identified residues, Asp74 and Tyr292, participates in the receptor activation mechanism. Finally, Lys102 (third helix) and Lys199 (fifth helix) mutants do not bind angiotensin II or different analogs. Co-expression of these two deficient receptors permitted the restoration of a normal binding site. This effect was not due to homologous recombination of the cDNAs but to protein trans-complementation.

Delineation of the peptide binding site of the human galanin receptor

Galanin, a neuroendocrine peptide of 29 amino acids, binds to Gi/Go-coupled receptors to trigger cellular responses. To determine which amino acids of the recently cloned seven-transmembrane domain-type human galanin receptor are involved in the high-affinity binding of the endogenous peptide ligand, we performed a mutagenesis study. Mutation of the His264 or His267 of transmembrane domain VI to alanine, or of Phe282 of transmembrane domain VII to glycine, results in an apparent loss of galanin binding. The substitution of Glu271 to serine in the extracellular loop III of the receptor causes a 12-fold loss in affinity for galanin. We combined the mutagenesis results with data on the pharmacophores (Trp2, Tyr9) of galanin and with molecular modelling of the receptor using bacteriorhodopsin as a model. Based on these studies, we propose a binding site model for the endogenous peptide ligand in the galanin receptor where the N-terminus of galanin hydrogen bonds with Glu271 of the receptor, Trp2 of galanin interacts with the Zn2+ sensitive pair of His264 and His267 of transmembrane domain VI, and Tyr9 of galanin interacts with Phe282 of transmembrane domain VII, while the C-terminus of galanin is pointing towards the N-terminus of th

Proposals for the angiotensin II receptor-bound conformation by comparative computer modeling of AII and cyclic analogs

A conformational search using high-temperature molecular dynamics on angiotensin II(AII) and on two cyclic S-S bridged analogs, namely [Hcy3,5]AII and [Cys3,5]AII, in conjunction with a cluster analysis based on the similarities of the three-dimensional patterns of the binding and activation elements, had led to putative AII receptor-bound conformations. These conformations are characterized by a compact folded shape of the peptide backbone, and by particular relative positions of the four pharmacophore groups, namely the aromatic moieties of the Tyr4, His6 and Phe8 residues, and the C-terminal carboxyl group. This compact folded shape, arising from attractive electrostatic interactions between the desolvated N- and C-terminal groups, is similar to the crystallographically determined conformation of AII bound to the antibody Fab receptor.

Interaction of Phe8 of angiotensin II with Lys199 and His256 of AT1 receptor in agonist activation

The acidic pharmacophores of selective ligands bind to Lys199 and His256 of the AT1 receptor (Noda, K., Saad, Y., Kinoshita, A., Boyle, T. P., Graham, R. M., Husain, A., and Karnik, S. (1995) J. Biol. Chem. 270, 2284-2289). In this report we examine how interactions between these residues and agonists activate inositol phosphate production in transiently transfected COS-1 cells. [Sar1] angiotensin (Ang II) II and [Sar1]Ang II-amide stimulated a 5-fold inositol phosphate response from wild-type AT1 receptor. The peptide antagonist [Sar1,Ile8]Ang II and the non-peptide agonist L-162,313 produced a partial but saturating response. Stimulation of wild-type receptor by [Sar1]Ang II-amide and the mutant K199Q and K199A receptors by [Sar1]Ang II demonstrates that AT1 receptor activation is not critically dependent on the ion-pairing of the alpha-COOH group of Ang II with Lys199. The mutation of His256 produced diminished inositol phosphate response without commensurate change in binding affinity of ligands. The His256 side chain is critical for maximal activation of the AT1 receptor, although isosteric Gln substitution is sufficient for preserving the affinity for Phe8-substituted analogues of [Sar1]Ang II. Therefore, AT1 receptor activation requires interaction of Phe8 side chain of Ang II with His256, which is achieved by docking the alpha-COOH group of Phe8 to Lys199. Furthermore, non-peptide agonists interact with Lys199 and His256 in a similar fashion.

Fluorescent labeling of purified beta 2 adrenergic receptor. Evidence for ligand-specific conformational changes

The purpose of the present study was to develop an approach to directly monitor structural changes in a G protein-coupled receptor in response to drug binding. Purified human beta 2 adrenergic receptor was covalently labeled with the cysteine-reactive, fluorescent probe N,N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1,3-diazol-4- yl)ethylenediamine (IANBD). IANBD is characterized by a fluorescence which is highly sensitive to the polarity of its environment. We found that the full agonist, isoproterenol, elicited a stereoselective and dose-dependent decrease in fluorescence from IANBD-labeled beta 2 receptor. The change in fluorescence could be plotted against the concentration of isoproterenol as a simple hyperbolic binding isotherm demonstrating interaction with a single binding site in the receptor. The ability of several adrenergic antagonists to reverse the response confirmed that this binding site is identical to the well described binding site in the beta 2 receptor. Comparison of the response to isoproterenol with a series of adrenergic agonists, having different biological efficacies, revealed a linear correlation between biological efficacy and the change in fluorescence. This suggests that the agonist-mediated decrease in fluorescence from IANBD-labeled beta 2 receptor is due to the same conformational change as involved in receptor activation and G protein coupling. In contrast to agonists, negative antagonists induced a small but significant increase in base-line fluorescence. Despite the small amplitude of this response, it supports the notion that antagonists by themselves may alter receptor structure. In conclusion, our data provide the first direct evidence for ligand-specific conformational changes occurring in a G protein-coupled receptor. Furthermore, the data demonstrate the potential of fluorescence spectroscopy as a tool for further delineating the molecular mechanisms of drug action at G protein-coupled receptors.

The binding site of neuropeptide vasopressin V1a receptor. Evidence for a major localization within transmembrane regions

To identify receptor functional domains underlying binding of the neurohypophysial hormones vasopressin (AVP) and oxytocin (OT), we have constructed a three-dimensional (3D) model of the V1a vasopressin receptor subtype and docked the endogenous ligand AVP. To verify and to refine the 3D model, residues likely to be involved in agonist binding were selected for site-directed mutagenesis. Our experimental results suggest that AVP, which is characterized by a cyclic structure, could be completely buried into a 15-20-A deep cleft defined by the transmembrane helices of the receptor and interact with amino acids located within this region. Moreover, the AVP-binding site is situated in a position equivalent to that described for the cationic neurotransmitters. Since all mutated residues are highly conserved in AVP and OT receptors, we propose that the same agonist-binding site is shared by all members of this receptor family. In contrast, the affinity for the antagonists tested, including those with a structure closely related to AVP, is not affected by mutations. This indicates a different binding mode for agonists and antagonists in the vasopressin receptor.

Genetic transfer of a nonpeptide antagonist binding site to a previously unresponsive angiotensin receptor

Mutational analysis based on the pharmacological differences between mammalian and amphibian angiotensin II receptors (AT receptors) previously identified 7 aa residues located in transmembrane domains (TMs) III (Val-108), IV (Ala-163), V (Pro-192, Thr-198), VI (Ser-252), and VII (Leu-300, Phe-301) of the rat AT receptor type 1b (rAT1b receptor) that significantly influenced binding of the nonpeptide antagonist Losartan. Further studies have shown that an additional 6 residues in the rAT1b receptor TMs II (Ala-73), III (Ser-109, Ala-114, Ser-115), VI (Phe-248), and VII (Asn-295) are important in Losartan binding. The 13 residues required for Losartan binding in the mammalian receptor were exchanged for the corresponding amino acids in the Xenopus AT receptor type a (xATa receptor) to generate a mutant amphibian receptor that bound Losartan with the same affinity as the rAT1b receptor (Losartan IC50 values: rAT1b, 2.2 +/- 0.2 nM: xATa, > 50 microM; mutant, 2.0 +/- 0.1 nM). To our knowledge, this is the first report of a gain-of-function mutant in which the residues crucial to formation of a ligand binding site in a mammalian peptide hormone receptor were transferred to a previously unresponsive receptor by site-directed mutagenesis. Ala substitutions and comparison of mammalian and amphibian combinatorial mutants indicated that TM III in the rAT1b receptor plays a key role in Losartan binding. Identification of residues involved in nonpeptide ligand binding will facilitate studies aimed at elucidating the chemical basis for ligand recognition in the AT receptor and peptide hormone receptors in general.

A topographical model of mu-opioid and brain somatostatin receptor selective ligands. NMR and molecular dynamics studies

We have refined the 1H NMR-based conformations of the mu-opioid receptor selective peptides related to somatostatin of general formula Xxx-Yyy1-Cys-Zzz-D-Trp-Lys(Orn)5-Thr-Pen-Thr8- NH2, where Xxx, Yyy, Zzz are 0, D-Phe and Tyr for 1; 0, D-Tic and Tyr for 2; Gly, D-Tic and Tyr for 3; and 0, D-Phe and Tic for 4, respectively, (Kazmierski et al., J. Am. Chem. 113, 2275-2283), using a molecular-dynamics approach. We present evidence that the NMR data are compatible with beta II'-, gamma- and gamma'-turns for the central tetrapeptide Tyr-D-Trp-Lys/Orn-Thr. Based on detailed structural and topographical considerations, we suggest that the mu-opioid receptor selectivity of 2 is due to a particular spatial arrangement of aromatic side chains of D-Tic1 and Tyr3 (7.5 A), and that the opioid receptor recognition domain is located in the N-terminal part of the peptide while the somatostatin receptor recognition domain is determined by the central, turn forming part of this class of cyclic peptides. A model for a mu-opioid selective ligand has emerged from these studies that shows excellent structural similarities to rigid opioid alkaloids.

A conserved NPLFY sequence contributes to agonist binding and signal transduction but is not an internalization signal for the type 1 angiotensin II receptor

A conserved NPX2-3Y sequence that is located in the seventh transmembrane helix of many G protein-coupled receptors has been predicted to participate in receptor signaling and endocytosis. The role of this sequence (NPLFY) in angiotensin II receptor function was studied in mutant and wild-type rat type 1a angiotensin II receptors transiently expressed in COS-7 cells. The ability of the receptor to interact with G proteins and to stimulate inositol phosphate responses was markedly impaired by alanine replacement of Asn298 and was reduced by replacement of Pro299 or Tyr302. The F301A mutant receptor exhibited normal G protein coupling and inositol phosphate responses, and the binding of the peptide antagonist, [Sar1,Ile8]angiotensin II, was only slightly affected. However, its affinity for angiotensin II and the nonpeptide antagonist losartan was reduced by an order of a magnitude, suggesting that angiotensin II and losartan share an intramembrane binding site, possibly through their aromatic moieties. None of the agonist-occupied mutant receptors, including Y302A and triple alanine replacements of Phe301, Tyr302, and Phe304, showed substantial changes in their internalization kinetics. These findings demonstrate that the NPLFY sequence of the type 1a angiotensin II receptor is not an important determinant of agonist-induced internalization. However, the Phe301 residue contributes significantly to agonist binding, and Asn298 is required for normal receptor activation and signal transduction.

A computer modeling postulated mechanism for angiotensin II receptor activation

The angiotensin II receptor of the AT1-type has been modeled starting from the experimentally determined three-dimensional structure of bacteriorhodopsin as the template. Intermediate 3D structures of rhodopsin and beta 2-adrenergic receptors were built because no direct sequence alignment is possible between the AT1 receptor and bacteriorhodopsin. Docking calculations were carried out on the complex of the modeled receptor with AII, and the results were used to analyze the binding possibilities of DuP753-type antagonistic non-peptide ligands. We confirm that the positively charged Lys199 on helix 5 is crucial for ligand binding, as in our model; the charged side chain of this amino acid interacts strongly with the C-terminal carboxyl group of peptide agonists or with the acidic group at the 2'-position of the biphenyl moiety of DuP753-type antagonists. Several other receptor residues which are implicated in the binding of ligands and the activation of receptor by agonists are identified, and their functional role is discussed. Therefore, a plausible mechanism of receptor activation is proposed. The three-dimensional docking model integrates most of the available experimental observations and helps to plan pertinent site-directed mutagenesis experiments which in turn may validate or modify the present model and the proposed mechanism of receptor activation.

Mutagenesis and the molecular modeling of the rat angiotensin II receptor (AT1)

The molecular interaction involved in the ligand binding of the rat angiotensin II receptor (AT1A) was studied by site-directed mutagenesis and receptor model building. The three-dimensional structure of AT1A was constructed on the basis of a multiple amino acid sequence alignment of seven transmembrane domain receptors and angiotensin II receptors and after the beta 2 adrenergic receptor model built on the template of the bacteriorhodopsin structure. These data indicated that there are conserved residues that are actively involved in the receptor-ligand interaction. Eleven conserved residues in AT1, His166, Arg167, Glu173, His183, Glu185, Lys199, Trp253, His256, Phe259, Thr260, and Asp263, were targeted individually for site-directed mutation to Ala. Using COS-7 cells transiently expressing these mutated receptors, we found that the binding of angiotensin II was not affected in three of the mutations in the second extracellular loop, whereas the ligand binding affinity was greatly reduced in mutants Lys199-->Ala, Trp253-->Ala, Phe259-->Ala, Asp263-->Ala, and Arg167-->Ala. These amino acid residues appeared to provide binding sites for Ang II. The molecular modeling provided useful structural information for the peptide hormone receptor AT1A. Binding of EXP985, a nonpeptide angiotensin II antagonist, was found to be involved with Arg167 but not Lys199.

The docking of Arg2 of angiotensin II with Asp281 of AT1 receptor is essential for full agonism

The structural model of AT1 angiotensin receptor contains seven-transmembrane alpha-helices with three interhelical loops on either side of the membrane. The angiotensin II binding pocket within the receptor is not clearly defined. We showed earlier that Lys199 in transmembrane-helix-5 of the AT1 receptor binds the COOH-terminal alpha-carboxyl group of angiotensin II (Noda, K., Saad, Y., Kinoshita, A., Boyle, T. P., Graham, R. M., Husain, A., and Karnik, S. S. (1995) J. Biol. Chem. 270, 2284-2289). We now show that His183 and Asp281, both located in the extracellular domain of the AT1 receptor, are involved in binding the NH2-terminal Asp1 and Arg2 residues of angiotensin II, respectively. The Asp1/His183 interaction appears to be weak and is unlikely to be important for agonism. But the loss of Arg2/Asp281 interaction leads to partial agonism of the receptor. The action of non-peptide agonists is not affected by Asp281 mutations. These results suggest that several independent interactions between angiotensin II and AT1 receptor are necessary for full agonism. Since L-162,313 the non-peptide agonist of the AT1 receptor is a partial agonist that does not make contact with Asp281, we speculate that the degree of agonism may be increased if it is redesigned to make contacts with Asp281.

Functional interaction between losartan and central tachykinin NK3 receptors in the conscious rat

1. The cardiovascular and behavioural effects elicted by the intracerebroventricular (i.c.v.) injection of substance P (SP), neurokinin A (NKA), [MePhe7]neurokinin B ([MePhe7]NKB) or angiotensin II (AII) in the conscious rat were assessed before and 5 min after i.c.v. pretreatment with antagonists selective for angiotensin AT1 (losartan and its active metabolite EXP 3174), angiotensin AT2 (PD 123,319) or tachykinin NK3 (R 486) receptors. 2. I.c.v. administration of 25 pmol AII evoked an increase in mean arterial blood pressure (MAP) and water intake behaviour, accompanied by a transient bradycardia, whereas 25 pmol [MePhe7]NKB caused a transient increase in MAP and heart rate (HR) concurrently with marked wet dog shake behaviour. At the same dose, SP and NKA were more potent than [MePhe7]NKB in increasing MAP and HR, but did not produce water intake or wet dog shake behaviours. 3. Losartan (650 pmol, i.c.v.) reduced significantly the cardiovascular and behavioural responses to AII or [MePhe7]NKB, but not to SP or NKA. While 65 pmol losartan was inactive, 260 pmol inhibited selectively the central effects of AII. Whereas EXP 3174 (6.5 nmol) blocked both AII and [MePhe7]NKB-mediated responses, the dose of 650 pmol blocked only the responses to AII. 4. The central responses to AII and [MePhe7]NKB were not affected by PD 123,319 (650 pmol). On the other hand, the [MePhe7]NKB-induced central effects were significantly reduced by R 486 (650 pmol). The NK3-selective antagonist had no effect against AII. 5. This study provides functional evidence, to support earlier binding data, that losartan (and to some extent its active metabolite EXP 3174) interact with the tachykinin NK3 receptor in rat brain. However,the cardiovascular and behavioural responses induced by central tachykinin agonists (SP, NKA and[MePhe7]NKB) and All are mediated by unrelated mechanisms.

Conversion of antagonist-binding site to metal-ion site in the tachykinin NK-1 receptor

Mutational analysis of the tachykinin NK-1 (refs 1-7), NK-2 (ref. 8) and angiotensin AT-1 (refs 9, 10) receptors indicates that non-peptide antagonists act through residues located between the seven transmembrane segments, whereas natural peptide agonists bind mainly to residues scattered in the exterior part of the receptor. The presumed contact points for the prototype NK-1 antagonist CP96,345 cluster on opposing faces of the outer portions of transmembrane helices V and VI (refs 1-5). Here we show that systematic introduction of histidyl residues at this antagonist-binding site in the human NK-1 receptor gradually converts it into a high-affinity metal-ion-binding site without affecting agonist binding. In a double mutant with histidine residues substituted at the top of transmembrane segments V and VI, respectively, Zn2+ inhibits binding of radiolabelled agonist peptide and efficiently blocks phosphoinositol turnover induced by substance P. We propose that Zn2+ and CP96,345 act as 'allosteric competitive' antagonists by stabilizing inactive conformations of the mutant and the wild-type receptor respectively. Introduction of metal-ion-binding sites could be used as a general tool in the structural and functional characterization of helix-helix interactions in G-protein-coupled receptors, as well as in other membrane proteins.

Tetrazole and carboxylate groups of angiotensin receptor antagonists bind to the same subsite by different mechanisms

To identify specific interactions between either the tetrazole or carboxylate pharmacophores of non-peptide antagonists and the rat AT1 receptor, 6 basic residues were examined by site-directed mutagenesis. Three of the mutants (H183Q, H256Q, and H272Q) appeared to be like wild type. Lys102 and Arg167 mutants displayed reduced binding of the non-peptide antagonist losartan. Examination of their properties employing group-specific angiotensin II analogues indicated that their effects on binding were indirect. Interestingly, the affinity of losartan was not altered by a K199Q mutation, but the same mutation reduced the affinity of angiotensin II, the antagonist [Sar1,Ile8]angiotensin II, and several carboxylate analogues of losartan. An Ala199 substitution reduced the affinity of peptide analogues to a larger extent as compared to the affinity of losartan. Thus, the crucial acidic pharmacophores of angiotensin and losartan appear to occupy the same space within the receptor pocket, but the protonated amino group of Lys199 is not essential for binding the tetrazole anion. The binding of the tetrazole moiety with the AT1 receptor involves multiple contacts with residues such as Lys199 and His256 that constitute the same subsite of the ligand binding pocket. However, this interaction does not involve a conventional salt bridge, but rather an unusual lysine-aromatic interaction.

Non-peptide angiotensin agonist. Functional and molecular interaction with the AT1 receptor

Non-peptide ligands for peptide receptors for the G-protein-coupled type are generally antagonists, except in the opiate system. Recently, it was observed that a subset of biphenylimidazole derivatives surprisingly possessed angiotensin-like activity in vivo. In COS-7 cells transfected with the rat AT1 receptor a prototype of these compounds, L-162,313 stimulated phosphoinositide hydrolysis with an EC50 of 33 +/- 11 nM. The maximal response to the compound was 50% of that of angiotensin II in COS-7 cells but only 3% in stably transfected Chinese hamster ovary cells. The agonistic effect of L-162,313 was blocked by the AT1-specific antagonist L-158,809 and was not observed in untransfected cells. In Chinese hamster ovary cells, L-162,313 also acted as an insurmountable antagonist of the angiotensin stimulated phosphoinositide hydrolysis. In contrast to previously tested non-peptide ligands, L-162,313 bound with reasonably high affinity to the Xenopus laevis AT1 receptor. In the human receptor, the binding of L-162,313 was found to be unaffected by point mutations in transmembrane segments III and VII, which impaired the binding of biphenylimidazole antagonists. Substitutions in the extracellular domains of the human and rat receptor, which impaired the binding of angiotensin II, did not affect the binding of L-162,313. It is concluded that a subset of biphenylimidazole compounds can act as high affinity partial agonists on the AT1 receptor. These compounds have molecular interactions with the receptor which appear to differ both from that of the structurally similar non-peptide antagonists and from that of their functional counterpart, the peptide agonist.

The angiotensin type 1 and type 2 receptor families. Siblings or cousins?

The diverse actions of angiotensin II (AngII) are mediated by cell surface receptors. Molecular cloning techniques have identified two distinct subtypes of AngII receptors referred to as AT1 and AT2. It is now well accepted that multiple forms of the AT1 receptor exist, but similar diversity of the AT2 subtype has not been conclusively demonstrated. Nonetheless, several converging lines of evidence do suggest that multiple AT2 receptors may be present in brain and cultured neuron-like cells lines. For instance, some AT2 receptors are regulated by guanine nucleotides and sulfhydryl-reducing agents, whereas others are insensitive. AT2 receptor populations also exhibit differing pharmacological profiles particularly with respect to their affinity for peptidic and non-peptidic ligands. Moreover, a recently developed anti-AT2 polyclonal antisera reveals a unique pattern of immunohistochemical staining in brain and it does not immunoreact with the recently cloned AT2 receptor. Collectively, these results support the hypothesis of multiple AT2 receptors at least within the CNS. Future studies should reveal whether these putative AT2 receptor subtypes result from unique genes or cell-specific post-translational modifications of a single gene product.

Lysine 182 of endothelin B receptor modulates agonist selectivity and antagonist affinity: evidence for the overlap of peptide and non-peptide ligand binding sites

The potent vasoactive peptide hormone endothelin (ET) binds to receptors which belong to the G-protein coupled receptor family. The availability of non-peptide antagonists for ET receptors allows investigation of the relationship among the binding sites for peptide and non-peptide ligands. In this study, a lysine residue, conserved within transmembrane domain 3 (TM3) of the ETA and ETB receptor subtypes, is implicated in agonist and antagonist binding by its analogous position within TM3 to a binding site aspartate residue conserved within bioactive amine receptors. Replacement of this lysine within hETB by arginine, alanine, methionine, aspartate, or glutamate results in hETB variants with unaltered affinities for agonist peptide ET-1 but which have affinities for peptide agonists ET-2, ET-3, sarafotoxin 6C, and TRL 1736 which are between 1-3 orders of magnitude lower than their corresponding wild-type hETB values. Significantly, the affinities of non-peptide antagonists, (+/-)-SB 209670 and its analogs as well as Ro 46-2005, are abrogated. The results suggest that an interaction of K182 of hETB with the indan 2-carboxyl of (+/-)-SB 209670 may contribute to the high-affinity binding of the diarylindan antagonists. The results indicate that TM3 of hETB is a region of overlap among the binding sites of non-peptide antagonists and the affected peptide agonists.

Mutations in transmembrane segment VII of the AT1 receptor differentiate between closely related insurmountable and competitive angiotensin antagonists

Chimeric constructs between the human and the Xenopus laevis AT1 receptor have demonstrated, that the binding of non-peptide angiotensin antagonists is dependent on non-conserved residues located deep in transmembrane segment VII of the AT1 receptor. Here we have studied four pairs of closely related antagonists each consisting of a competitive and an insurmountable compound differentiated by one out of three different types of minor chemical modifications. None of the antagonists bound to the Xenopus receptor and the binding of all of the compounds to the human receptor was severely impaired by the introduction of non-conserved residues from transmembrane segment VII of the Xenopus receptor. In all four pairs of antagonists the competitive compound was affected more by these substitutions than the corresponding insurmountable one (209 vs. 22, 281 vs. 29, 290 vs. 29 and 992 vs. 325-fold increase in Ki values). A similar pattern was observed in response to substitution of a single non-conserved residue in transmembrane segment VII, Asn295 to Ser. These results indicate that a common molecular mechanism distinguishes the interaction of insurmountable and competitive antagonists with the AT1 receptor.