Potent inhibition of HIV-1 infectivity in macrophages and lymphocytes by a novel CCR5 antagonist

The chemokine receptors CXCR4 and CCR5 have recently been shown to act as coreceptors, in concert with CD4, for human immunodeficiency virus-type 1 (HIV-1) infection. RANTES and other chemokines that interact with CCR5 and block infection of peripheral blood mononuclear cell cultures inhibit infection of primary macrophages inefficiently at best. If used to treat HIV-1-infected individuals, these chemokines could fail to influence HIV replication in nonlymphocyte compartments while promoting unwanted inflammatory side effects. A derivative of RANTES that was created by chemical modification of the amino terminus, aminooxypentane (AOP)-RANTES, did not induce chemotaxis and was a subnanomolar antagonist of CCR5 function in monocytes. It potently inhibited infection of diverse cell types (including macrophages and lymphocytes) by nonsyncytium-inducing, macrophage-tropic HIV-1 strains. Thus, activation of cells by chemokines is not a prerequisite for the inhibition of viral uptake and replication. Chemokine receptor antagonists like AOP-RANTES that achieve full receptor occupancy at nanomolar concentrations are strong candidates for the therapy of HIV-1-infected individuals.

[7TM receptors--from the olfactory system and new hormones against HIV infection]

It has recently become evident that all types of chemical messengers, hormones and transmitters act through membrane receptors which constitute our largest superfamily of proteins, i.e. the G protein-coupled receptors. These proteins, which are characterized by having seven transmembrane segments (7TM), also act as, for example sensors for light and odor components in our sensory systems. Already today monoamine 7TM receptors are the target for many drugs; however, the development of non-peptide ligands for a variety of peptide receptors indicates that probably all 7TM receptors can become pharmacotherapeutic targets. The discovery that chemokine receptors function as the crucial cofactors for cell entry of HIV-1 suggests that antagonists or agonists for one or more chemokine 7TM receptor could be interesting agents against AIDS. The occurrence of a multitude of orphan 7TM receptors without known ligand indicates, that surprisingly large areas within endocrinology and neuroscience are still today waiting to be characterized.

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.

Engineering of metal-ion sites as distance constraints in structural and functional analysis of 7TM receptors

G-protein-coupled receptors with their seven transmembrane (7TM) segments constitute the largest superfamily of proteins known. Unfortunately, still only relatively low resolution structures derived from electron cryo-microscopy analysis of 2D crystals are available for these proteins. We have used artificially designed Zn(II) metal-ion binding sites to probe 7TM receptors structurally and functionally and to define some basic distance constraints for molecular modeling. In this way, the relative helical rotation and vertical translocation of transmembrane helices TM-II, TM-III, TM-V, and TM-VI of the tachykinin NK-1 receptor have been restricted. Collectively, these zinc sites constitute a basic network of distance constraints that limit the degrees of freedom of the interhelical contact faces in molecular models of 7TM receptors. The construction of artificially designed metal-ion sites is discussed also in the context of probes for conformational changes occurring during receptor activation.

Septide and neurokinin A are high-affinity ligands on the NK-1 receptor: evidence from homologous versus heterologous binding analysis

The three main tachykinins, substance P, neurokinin A (NKA), and neurokinin B, are believed to be selective ligands for respectively the NK-1, NK-2 and NK-3 receptors. However, NKA also has actions which cannot be mediated through its normal NK-2 receptor and the synthetic peptide [pGlu6,Pro9]-Substance P9-11--called septide--is known to have tachykinin-like actions despite its apparent lack of binding to any known tachykinin receptor. In the cloned NK-1 receptor expressed in COS-7 cells NKA and septide as expected were poor competitors for radiolabeled substance P. However, by using radiolabeled NKA and septide directly, it was found that both peptides in homologous binding assays as well as in competition against each other in fact bound to the NK-1 receptor with high affinity: Kd values of 0.51 +/- 0.15 nM (NKA) and 0.55 +/- 0.03 nM (septide). It is concluded that NKA and septide are high-affinity ligands for the NK-1 receptor but that they are poor competitors for substance P, which in contrast competes very well for binding with both NKA and septide.

Connectivity and orientation of the seven helical bundle in the tachykinin NK-1 receptor probed by zinc site engineering

A high affinity, tridentate metal ion site has been constructed previously by His substitutions in an antagonist binding site located between transmembrane segment (TM)-V and TM-VI in the substance P NK-1 receptor. Here, an attempt is made to probe helix-helix interactions systematically in the NK-1 receptor by engineering of bis-His Zn(II) sites. His residues were introduced at selected positions individually and in combinations in the exterior segments of TM-II, III and V in both the wild-type background and after Ala substitution of naturally occurring His residues, and the increase in the affinity for Zn(II) was monitored in competition binding experiments with iodinated substance P or a tritiated non-peptide antagonist. In this way, two high affinity bis-His sites were constructed between position 193 in TM-V (Glu193, G1uV:01) and position 109 in TM-III (Asn1O9, AsnIII:05) as well as between the neighboring, naturally occurring His108 in TM-III (HisIII:04) and position 92 in TM-II (Tyr92, TyrII:24), respectively. Functionally, the coordination of zinc ions at these two sites blocked the receptor as it antagonized the substance P-induced increase in phosphatidylinositol turnover. It is concluded that the bis-His zinc sites from the central TM-III helix to TM-II and -V, respectively, together with the interconnected, previously constructed tridentate site between TM-V and -VI, constitute a basic network of distance constraints for the molecular models of receptors with seven transmembrane segments which, for example, strongly support an anti-clockwise orientation of the seven helical bundle as viewed from the extracellular space.

Radioligand-dependent discrepancy in agonist affinities enhanced by mutations in the kappa-opioid receptor

A series of kappa/mu receptor chimeras and a number of kappa receptors substituted in the second transmembrane segment (TM-II) were investigated using as radioligands, respectively, the kappa-selective agonist [3H]C1977 and the nonselective opioid antagonist [3H]diprenorphine (DIP). All of the receptor constructs bound [3H]DIP with similar and high affinity, whereas the apparent affinity of the nonpeptide agonist C1977, when estimated in competition binding with the antagonist [3H]DIP, was impaired between 42- and > 500-fold in the kappa/mu chimeras and between 64- and 153-fold in three of the kappa receptor mutants that had been substituted in the TM-II segment. However, homologous competition binding experiments, using [3H]C1977 as radioligand, showed that the high affinity binding of this nonpeptide agonist was in fact not impaired in four of the kappa/mu chimeras and in three TM-II substituted kappa receptors compared with the wild-type kappa receptor. In all cases in which mutations decreased the apparent affinity of C1977 without affecting its actual affinity, as determined in homologous assays using [3H]C1977, the calculated number of receptor sites (Bmax) was decreased. In three of the kappa/mu constructs, binding of [3H]C1977 was undetectable, indicating that in these chimeras the affinity of the nonpeptide agonist had actually been affected. Also, for the kappa-selective peptide agonist dynorphin A(1-8), the measured affinity for the receptor mutants was strongly dependent on whether it was determined using the antagonist [3H]DIP or the agonist [3H]C1977 in that < or = 800-fold higher Ki values were determined in competition with the antagonist. It is concluded that mutations in the kappa-opioid receptor can cause large discrepancies between the affinity determined for agonists in homologous versus heterologous competition binding assays and that this pattern, which is compatible with a partial uncoupling of receptors, is observed in surprisingly many types of receptor mutations.

Construction of a high affinity zinc switch in the kappa-opioid receptor

Very limited structural information is available concerning the superfamily of G-protein-coupled receptors with their seven-transmembrane segments. Recently a non-peptide antagonist site was structurally and functionally replaced by a metal ion site in the tachykinin NK-1 receptor. Here, this Zn(II) site is transferred to the kappa-opioid receptor by substituting two residues at the outer portion of transmembrane V (TM-V), Asp223 and Lys227, and one residue at the top of TM-VI, Ala298, with histidyl residues. The histidyl residues had no direct effect on the binding of either the non-peptide antagonist [3H]diprenorphine or the non-peptide agonist, [3H]CI977, just as these mutations/substitutions did not affect the apparent affinity of a series of other peptide and non-peptide ligands when tested in competition binding experiments. However, zinc ions in a dose-dependent manner prevented binding of both agonist and antagonist ligands with an apparent affinity for the metal ion, which gradually was built up to 10(-6) M. This represents an increase in affinity for the metal ion of about 1000-fold as compared with the wild-type kappa receptor and is specific for Zn(II) as the affinity for e.g. Cu(II) was almost unaffected. The direct transfer of this high affinity metal ion switch between two only distantly related receptors indicates a common overall arrangement of the seven-helix bundle among receptors of the rhodopsin family.

Biosynthesis of neuropeptide Y in porcine tissues and generation of N-terminal fragments in neuroblastoma cell lines

The biosynthesis of neuropeptide Y (NPY) was investigated to determine the efficiency of synthesis and processing of the precursor. In brain tissues examined, the major product was amidated NPY(1-36). Although this was also the major product in adrenal and heart atrium, a minor portion of the immunoreactivity was identified as unprocessed precursor. NPY degradation was investigated using SK-N-MC and SMS-MSN cells in conjunction with iodinated NPY tracers, labeled in either the tyrosine-1 or the tyrosine-36 position. Similar patterns of degradation were observed with both cell lines, and it would appear that the initial proteolytic attack on NPY(1-36) generates predominately N-terminal fragments.

Analysis of selective binding epitopes for the kappa-opioid receptor antagonist nor-binaltorphimine

The structural determinants for the selective binding of the nonpeptide opioid receptor antagonist nor-binaltorphimine (nor-BNI) to the kappa-opioid receptor were characterized using a systematic series of chimeras between the kappa receptor and the homologous mu-opioid receptor. All 10 chimeric constructs bound the nonselective antagonists (-)-naloxone and diprenorphine with similar affinities, as did the two wild-type receptors. Introduction of amino-terminal segments of increasing length, extending to and including transmembrane segment VI, from the mu receptor into the kappa receptor did not impair the high affinity binding of nor-BNI, and neither did introduction of the intracellular carboxyl-terminal extension of the mu receptor. In contrast, nor-BNI binding was impaired > or = 600-fold in constructs in which extracellular loop 3 and transmembrane segment VII originated from the mu receptor. The exchange of a single residue within this region, Glu297, for lysine, the corresponding residue from the mu receptor, reduced the binding affinity of nor-BNI 142-fold, without affecting the binding the nonselective compounds (-)-naloxone and diprenorphine. It is concluded that the selective binding of nor-BNI to the kappa-opioid receptor is determined by nonconserved residues located in extracellular loop 3 and transmembrane segment VII and that Glu297, located just outside transmembrane segment VI, plays a major role in the kappa-selective binding characteristics of nor-BNI.

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.

Interaction between the nonpeptide angiotensin antagonist SKF-108,566 and histidine 256 (HisVI:16) of the angiotensin type 1 receptor

His256 (HisVI:16) of transmembrane segment (TM)-VI of the rat angiotensin type 1 (AT1) receptor was targeted for mutagenesis to investigate its potential involvement in ligand binding. Substitution of His256 with alanine, phenylalanine, glutamine, or isoleucine did not affect the binding of either angiotensin II or nine different biphenylimidazole AT1 antagonists. In contrast, the binding affinity of the prototype imidazoleacrylic acid antagonist SKF-108,566 was reduced 15-fold by the exchange of His256 with alanine. Substitution of His256 with either isoleucine or phenylalanine yielded similar results, whereas a glutamine residue was able to substitute for His256, suggesting that the epsilon-nitrogen of His256 could be involved in the interaction with the imidazoleacrylic acid. To identify the chemical groups on SKF-108,566 that interact with His256 and with Asn295, a previously identified interaction point for nonpeptide antagonists located in TM-VII, we tested the binding of 15 analogs of SKF-108,566 in which different chemical moieties were systematically exchanged. The results indicated that the carboxyphenyl group of SKF-108,566 interacts with the imidazole side chain of His256. The data did not point to any particular contact group on the antagonist for Asn295. It is concluded that the imidazoleacrylic acid antagonists share some interactions in TM-VII of the AT1 receptor with the biphenylimidazole antagonists, but the binding of the imidazoleacrylic acid compounds is uniquely dependent on His256 in TM-VI, possibly through the carboxyphenyl moiety.

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.

Identification of peptide binding residues in the extracellular domains of the AT1 receptor

To locate essential determinants for angiotensin II binding, we have performed a systematic mutational analysis of the exterior domain of the AT1 receptor. Receptor mutants, deficient in peptide binding, were analyzed using radiolabeled nonpeptide ligand as an important tool. Two independent strategies for mutagenesis were employed: conservative segment exchange and point mutagenesis of evolutionarily conserved residues. Results from the conservative segment exchange in which 6-17 residues were replaced with chemically similar, yet different, amino acid sequences of the same length suggested that important peptide ligand binding epitopes are located in the N-terminal extension of the AT1 receptor, in particular adjacent to the top of transmembrane segment I (TM-I), and in the third extracellular loop, close to the top of TM-VII. The substitution of residues from either of these regions resulted in a 5,000-20,000-fold decrease in affinity for the peptide agonist angiotensin II (AII) and the peptide antagonist [Sar1,Leu8]AII without affecting the binding of nonpeptide antagonists. Alanine substitution of evolutionarily conserved residues demonstrated that peptide binding was dependent on several residues in the N-terminal extension, near the top of TM-I, a tyrosine residue located in extracellular loop 1, close to TM-II, and 2 aspartate residues positioned in extracellular loop 3 on the same face of an alpha-helical extension of TM-VII. In all cases the binding of nonpeptide antagonist was unaffected by these substitutions. It is concluded that important epitopes involved in angiotensin II binding are located around the top of transmembrane segments I, II, and VII which conceivably are in close spatial proximity in the folded receptor structure.

Glucagon and glucagon-like peptide 1: selective receptor recognition via distinct peptide epitopes

Glucagon and glucagon-like peptide 1 (GLP-1) are homologous peptide hormones that are recognized by likewise homologous, but highly selective receptors. Analogs of glucagon and GLP-1, in which the divergent residues were systematically exchanged, were employed to identify the structural requirements for their selective receptor recognition. Substitutions in the NH2-terminal part of the glucagon molecule with the corresponding GLP-1 residues, as for example in [Ala2,Glu3]-glucagon and [Val10,Ser12]glucagon, reduced the binding affinity for the glucagon receptor several hundred-fold without increasing the affinity for the GLP-1 receptor. In contrast, introduction of GLP-1 residues into the far COOH-terminal part of the glucagon molecule, e.g. [Val27,Lys28,Gly29,Arg30]glucagon, had a minimal effect on recognition of the glucagon receptor, but improved the affinity of the analog for the GLP-1 receptor up to 200-fold. Similarly, substitutions in especially the far COOH-terminal part of the GLP-1 molecule with the corresponding glucagon residues, e.g. des-Arg30-[Met27,Asn28,Thr29]GLP-1, decreased the affinity for the GLP-1 receptor several hundred-fold (IC50 = 0.4-190 nM) without increasing the affinity for the glucagon receptor. Conversely, substitutions in the NH2-terminal part of the GLP-1 molecule impaired the affinity for the GLP-1 receptor only moderately. We conclude that the selective recognition of the glucagon and GLP-1 receptors is determined by residues located at opposite ends of the homologous peptide ligands. This conclusion is supported by the observation that a "chimeric" peptide consisting of the NH2-terminal part of the glucagon molecule joined to the COOH-terminal part of the GLP-1 molecule was recognized with high affinity by both receptors.

Mutations along transmembrane segment II of the NK-1 receptor affect substance P competition with non-peptide antagonists but not substance P binding

Mutational analysis of the NK-1 receptor indicates that residues involved in non-peptide antagonist binding cluster around the outer portion of transmembrane segments (TM) V and VI. In contrast mutations affecting the binding of the natural peptide agonist, substance P, are scattered in the exterior part of the receptor. Recently it was reported that a number of mutations in TM-II also seriously impair substance P binding. Here we confirm that Ala substitutions for these residues located on a hydrophilic helical face of TM-II basically eliminate substance P binding to the NK-1 receptor, provided that a radiolabeled non-peptide antagonist is used as radioligand. Surprisingly, radiolabeled substance P bound well to all these mutant receptors and was displaced with only slightly reduced affinity by the unlabeled peptide and by the non-peptide antagonists. The wild-type homologous NK-2 receptor displayed properties similar to those observed in the mutated NK-1 receptors, i.e. concomitant high affinity binding of radiolabeled agonist peptide (in this case neurokinin A), yet low affinity, G-protein independent competition of unlabeled peptide with radiolabeled non-peptide antagonist. It is concluded that substitutions in TM-II of the NK-1 receptor do not affect the high affinity binding of substance P but instead block the ability of the peptides to compete for non-peptide antagonist binding. It is suggested that certain mutations can impair interchange between receptor conformations that each bind different ligands with high affinity.

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.