Pharmacological Differences between Human and Guinea Pig Histamine H1 Receptors: Asn84 (2.61) as Key Residue within an Additional Binding Pocket in the H1 Receptor

We tested several histamine H(1) receptor (H(1)R) and antagonists for their differences in agonists binding affinities between human and guinea pig H(1)Rs transiently expressed in African green monkey kidney (COS-7) cells. Especially, the bivalent agonist histaprodifen-histamine dimer (HP-HA) shows a higher affinity for guinea pig than for human H(1)Rs. Based on the structure of HP-HA, we have further identified VUF 4669 [7-(3-(4-(hydroxydiphenylmethyl)piperidin-1-yl)propoxy)-4-oxochroman-2-carboxylic acid] as a guinea pig-preferring H(1)R antagonist, demonstrating that the concept of species selectivity is not limited to agonists. To delineate the molecular mechanisms behind the observed species selectivity, we have created mutant human H(1)Rs in which amino acids were individually replaced by their guinea pig H(1)R counterparts. Residue Asn(84) (2.61) in transmembrane domain (TM) 2 seemed to act as a selectivity switch in the H(1)R. Molecular modeling and site-directed mutagenesis studies suggest that Asn(84) interacts with the conserved Tyr(458) (7.43) in TM7. Our data provide the first evidence that for some H(1)R ligands, the binding pocket is not only limited to TMs 3, 4, 5, and 6 but also comprises an additional pocket formed by TMs 2 and 7.

Homology modelling and binding site mapping of the human histamine H1 receptor

Three-dimensional model of the human histamine H1 receptor was developed by homology modelling using the high resolution structure of bovine rhodopsin as template. Genetic algorithm based docking calculations were used to identify the role of several amino acids having an effect on agonist or antagonist binding. Binding mode analyses of mepyramine, desloratidine, loratidine and acrivastine allowed us to rationalise their binding affinity. Binding site mapping resulted in seven new potential aromatic interaction points (Tyr 108, Phe 184, Phe 190, Phe 199, Phe 424, Trp 428, Tyr 431), that took part in forming the lipophilic pocket of the antagonist binding cavity.

Two threonine residues and two serine residues in the second and third intracellular loops are both involved in histamine H1receptor downregulation

Human histamine H1 receptor (H1R) contains five possible phosphorylation residues (Thr140, Thr142, Ser396, Ser398 and Thr478) and the substitution of all these five residues to alanine completely impairs agonist-induced receptor downregulation. In the present study, to determine which residue(s) are responsible for receptor downregulation, we used mutant H1Rs in which single or multiple residues were substituted with alanine. The results suggested that two groups, i.e., residues Thr140 and Thr142, and residues Ser396 and Ser398, independently contributed to H1R downregulation. Thr140 and Ser398 mainly contributed to downregulation, and Thr142 or Ser396 had a slight inhibitory effect on Thr140- or Ser398-mediated process, respectively.

Mepyramine, a histamine H1 receptor inverse agonist, binds preferentially to a G protein-coupled form of the receptor and sequesters G protein

Accurate characterization of the molecular mechanisms of the action of ligands is an extremely important issue for their appropriate research, pharmacological, and therapeutic uses. In view of this fact, the aim of the present work was to investigate the mechanisms involved in the actions of mepyramine at the guinea pig H(1) receptor stably expressed in Chinese hamster ovary cells. We found that mepyramine is able to decrease the basal constitutive activity of the guinea pig H(1) receptor, to bind with high affinity to a G(q/11) protein-coupled form of the receptor and to promote a G protein-coupled inactive state of the H(1) receptor that interferes with the G(q/11)-mediated signaling of the endogenously expressed ATP receptor, as predicted by the Cubic Ternary Complex Model of receptor occupancy. The effect of mepyramine on ATP-induced signaling was specifically neutralized by Galpha(11) overexpression, indicating that mepyramine is able to reduce G protein availability for other non-related receptors associated with the same signaling pathway. Finally, we found a loss of mepyramine efficacy in decreasing basal levels of intracellular calcium at high Galpha(11) expression levels, which can be theoretically explained in terms of high H(1) receptor constitutive activity. The whole of the present work sheds new light on H(1) receptor pharmacology and the mechanisms H(1) receptor inverse agonists could use to exert their observed negative efficacy.

Large-scale overproduction, functional purification and ligand affinities of the His-tagged human histamine H1 receptor

This report describes an efficient strategy for amplified functional purification of the human H1 receptor after heterologous expression in Sf9 cells. The cDNA encoding a C-terminally histidine-tagged (10xHis) human histamine H1 receptor was used to generate recombinant baculovirus in a Spodoptera frugiperda-derived cell line (IPLB-Sf9). As judged from its ligand affinity profile, functional receptor could be expressed at high levels (30-40 pmol per 10(6) cells). Rapid proteolysis in the cell culture led to limited fragmentation, without loss of ligand binding, but could be efficiently suppressed by including the protease inhibitor leupeptin during cell culture and all subsequent manipulations. Effective solubilization of functional receptor with optimal recovery and stability required the use of dodecylmaltoside as a detergent in the presence of a high concentration of NaCl and of a suitable inverse agonist. Efficient purification of solubilized receptor could be achieved by affinity chromatography over nickel(II) nitrilotriacetic acid resin. Functional membrane reconstitution of purified H1 receptor was accomplished in mixed soybean lipids (asolectin). The final proteoliposomic H1 receptor preparation has a purity greater than 90% on a protein basis and displays a ligand binding affinity profile very similar to the untagged receptor expressed in COS-7 cells. In conclusion, we are able to produce pharmacologically viable H1 receptor in a stable membrane environment allowing economic large-batch operation. This opens the way to detailed studies of structure-function relationships of this medically and biologically important receptor protein by 3D-crystallography, FT-IR spectroscopy and solid-state NMR spectroscopy.

Identification of Amino Acid Residues Responsible for Agonist-Induced Down-Regulation of Histamine H1 Receptors

The histamine H(1) receptor (H1R) level is dynamically regulated in vivo under various physiological and pathological conditions. The H1R regulation may consist of various processes, and this study focused on the process of receptor trafficking, that is, receptor internalization to endosomes and the following receptor degradation. First, we identified five possible phosphorylation residues of human H1R, Thr(140), Thr(142), Ser(396), Ser(398), and Thr(478), based on in vitro phosphorylation studies. Then to determine the role of these residues, we constructed a mutant H1R in which all of these five residues were substituted with alanine. Both wild-type and the mutant receptors expressed in Chinese hamster ovary (CHO) cells had similar values of K(d) for [(3)H]mepyramine binding and K(i) for histamine, and these cells showed similar levels of histamine-stimulated inositol phosphate formation. Both types of H1Rs were internalized essentially in the same way upon stimulation with histamine (100 microM) for 30 min. However, down-regulation of the mutant H1R was completely impaired, whereas that of wild-type H1R occurred by approximately 60% by the treatment with 100 microM histamine for 24 h. These results suggest that these residues are responsible for receptor down-regulation but not for receptor internalization. Possibly, phosphorylation of the residues is required for receptor transport from endosomes to lysosomes.

Analysis of histamine and modeling of ligand-receptor interactions in the histamine H 1 receptor for Magic Angle Spinning NMR studies

OBJECTIVE AND DESIGN

Investigation of the principles of ligand-receptor interaction in histamine receptors can help to provide a solid foundation for structure-based drug design. Stable isotope labelling of the ligand 'Histamine' has been performed and 1D (13)C CP MAS and 2D Radio Frequency Dipolar Recoupling (RFDR) spectra for the ligand are presented. Hyperfine signals were well spread and did not suffer from any sizable line broadening. The production of H(1) receptor for Magic Angle Spinning NMR studies is currently in progress.

TREATMENT

An agonist binding domain is proposed using homology modeling, database searches and mutagenesis data for the H(1) receptor.

METHODS

Homology modeling, Database searches for Expressed sequence Tag (ESTs), Magic Angle Spinning Nuclear Magnetic Resonance analysis of the ligand histamine.

RESULTS

The three-dimensional receptor model and mutagenesis studies suggest that the amine of the agonist histamine may form an ion pair with the TM III Asp, whereas the imidazole ring of histamine may associate with TM V Asp and Thr.

CONCLUSIONS

Homology modeling studies confirms the absence of TM VIII in the H(1) receptor. According to the model the histamine in particular interacts with the transmembrane (TM) regions of the H(1) receptor structure, in particular TM helix III and V. This is in line with recent mutagenesis studies. Database search methods for ESTs have been used for electronic prediction of tissue distribution of H(1) receptor expression. The results indicate that the H(1) expression is highest in heart and skeletal muscle, which may be of importance for drug targeting.

Predictive QSAR modeling based on diversity sampling of experimental datasets for the training and test set selection

One of the most important characteristics of Quantitative Structure Activity Relashionships (QSAR) models is their predictive power. The latter can be defined as the ability of a model to predict accurately the target property (e.g., biological activity) of compounds that were not used for model development. We suggest that this goal can be achieved by rational division of an experimental SAR dataset into the training and test set, which are used for model development and validation, respectively. Given that all compounds are represented by points in multidimensional descriptor space, we argue that training and test sets must satisfy the following criteria: (i) Representative points of the test set must be close to those of the training set; (ii) Representative points of the training set must be close to representative points of the test set; (iii) Training set must be diverse. For quantitative description of these criteria, we use molecular dataset diversity indices introduced recently (Golbraikh, A., J. Chem. Inf. Comput. Sci., 40 (2000) 414-425). For rational division of a dataset into the training and test sets, we use three closely related sphere-exclusion algorithms. Using several experimental datasets, we demonstrate that QSAR models built and validated with our approach have statistically better predictive power than models generated with either random or activity ranking based selection of the training and test sets. We suggest that rational approaches to the selection of training and test sets based on diversity principles should be used routinely in all QSAR modeling research.

Multiple differences in agonist and antagonist pharmacology between human and guinea pig histamine H1-receptor

Species isoforms of histamine H2-, H3-, and H4-receptors differ in their pharmacological properties. The study aim was to dissect differences between the human H1R (hH1R) and guinea pig H1R (ghH1R). We coexpressed hH1R and gpH1R with regulators of G-protein signaling in Sf9 insect cells and analyzed the GTPase activity of Gq-proteins. Small H1R agonists showed similar effects at hH1R and gpH1R, whereas bulkier 2-phenylhistamines and histaprodifens were up to approximately 10-fold more potent at gpH1R than at hH1R. Most 2-phenylhistamines and histaprodifens were more efficacious at gpH1R than at hH1R. Several first-generation H1R antagonists were approximately 2-fold, and arpromidine-type H1R antagonists up to approximately 10-fold more potent at gpH1R than at hH1R. [3H]Mepyramine competition binding studies confirmed the potency differences of the GTPase studies. Phe-153-->Leu-153 or Ile-433-->Val-433 exchange in hH1R (hH1R-->gpH1R) resulted in poor receptor expression, low [3H]mepyramine affinity, and functional inactivity. The Phe-153-->Leu-153/Ile-433-->Val-433 double mutant expressed excellently but only partially changed the pharmacological properties of hH1R. Small H1R agonists and 2-phenylhistamines interacted differentially with human and guinea pig H2R in terms of potency and efficacy, respectively. Our data show the following: 1) there are differences in agonist- and antagonist-pharmacology of hH1R and gpH1R encompassing diverse classes of bulky ligands. These differences may be explained by higher conformational flexibility of gpH1R relative to hH1R; 2) Phe-153 and Ile-433 are critical for proper folding and expression of hH1R; and 3) H2R species isoforms distinguish between H1R agonists.

Molecular characterization of specific H1-receptor agonists histaprodifen and its Nalpha-substituted analogues on bovine aortic H1-receptors

We determined the molecular properties of the selective and potent H(1)-receptor agonist histaprodifen and its N(alpha) substituted analogues: methyl-, dimethyl-, and imidazolylethyl-histaprodifen (suprahistaprodifen). All derivatives show high affinity for (3)H-mepyramine labeled bovine aortic H(1)-receptor binding sites with the following order of potency: suprahistaprodifen > dimethylhistaprodifen > methylhistaprodifen > histaprodifen > histamine. Suprahistaprodifen and dimethylhistaprodifen were the most potent displacers of (3)H-mepyramine binding (K(i)=4.3 and 4.9 nM, respectively). Histaprodifen, methylhistaprodifen and suprahistaprodifen binding was differentially influenced by GTP, whereas dimethylhistaprodifen was not affected. All drugs, except dimethylhistaprodifen, were activators of G-proteins. Their order of potency was suprahistaprodifen > histamine > histaprodifen > methylhistaprodifen. Their effect on G-protein activation was abolished by the addition of the H(1)-receptor antagonist triprolidine (10 microM), which given alone did not activate G-proteins. Our data suggest that histaprodifens are potent but heterogeneous H(1)-receptor ligands with diverse effects on the molecular level in our model system. While the histaprodifen, methylhistaprodifen and suprahistaprodifen data are in agreement with their agonistic nature, as shown in the functional studies performed on different species (rat and guinea pig H(1)-receptor), dimethylhistaprodifen behaved as an antagonist in our study.

Identification of Bphs, an autoimmune disease locus, as histamine receptor H1

Bphs controls Bordetella pertussis toxin (PTX)-induced vasoactive amine sensitization elicited by histamine (VAASH) and has an established role in autoimmunity. We report that congenic mapping links Bphs to the histamine H1 receptor gene (Hrh1/H1R) and that H1R differs at three amino acid residues in VAASH-susceptible and -resistant mice. Hrh1-/- mice are protected from VAASH, which can be restored by genetic complementation with a susceptible Bphs/Hrh1 allele, and experimental allergic encephalomyelitis and autoimmune orchitis due to immune deviation. Thus, natural alleles of Hrh1 control both the autoimmune T cell and vascular responses regulated by histamine after PTX sensitization.

Predictive QSAR modeling based on diversity sampling of experimental datasets for the training and test set selection

One of the most important characteristics of Quantitative Structure Activity Relashionships (QSAR) models is their predictive power. The latter can be defined as the ability of a model to predict accurately the target property (e.g., biological activity) of compounds that were not used for model development. We suggest that this goal can be achieved by rational division of an experimental SAR dataset into the training and test set, which are used for model development and validation, respectively. Given that all compounds are represented by points in multidimensional descriptor space, we argue that training and test sets must satisfy the following criteria: (i) Representative points of the test set must be close to those of the training set; (ii) Representative points of the training set must be close to representative points of the test set; (iii) Training set must be diverse. For quantitative description of these criteria, we use molecular dataset diversity indices introduced recently (Golbraikh, A., J. Chem. Inf. Comput. Sci., 40 (2000) 414-425). For rational division of a dataset into the training and test sets, we use three closely related sphere-exclusion algorithms. Using several experimental datasets, we demonstrate that QSAR models built and validated with our approach have statistically better predictive power than models generated with either random or activity ranking based selection of the training and test sets. We suggest that rational approaches to the selection of training and test sets based on diversity principles should be used routinely in all QSAR modeling research.

Tick histamine-binding proteins: lipocalins with a second binding cavity

Tick histamine-binding proteins (HBPs) are lipocalins with two binding pockets. One of these binds histamine with a high affinity and is found at the position expected from other lipocalins, adjacent to the omega-loop at the open-end of the beta-barrel. A second binding cavity, which is a low-affinity site for histamine in one of the HBPs, is located at the end of the barrel that is closed off in other lipocalins. In order to create the second site, the 'closed-end' region has undergone a major reconstruction. Typical lipocalin characteristics, such as the 3(10) helix and a structural cluster of highly conserved residues, have been lost, while an alpha-helix now shields the cavity from the exterior. The prominence of acidic residues in the binding pockets is another distinctive characteristic of HBPs. Whereas most lipocalins have highly hydrophobic binding cavities designed to bind lipophilic compounds, HBPs have evolved to trap cationic, hydrophilic molecules.

Histaprodifens: synthesis, pharmacological in vitro evaluation, and molecular modeling of a new class of highly active and selective histamine H(1)-receptor agonists

A new class of histamine analogues characterized by a 3, 3-diphenylpropyl substituent at the 2-position of the imidazole nucleus has been prepared outgoing from 4,4-diphenylbutyronitrile (4b) via cyclization of the corresponding methyl imidate 5b with 2-oxo-4-phthalimido-1-butyl acetate or 2-oxo-1,4-butandiol in liquid ammonia, followed by standard reactions. The title compounds displayed partial agonism on contractile H(1) receptors of the guinea-pig ileum and endothelium-denuded aorta, respectively, except 10 (histaprodifen; 2-[2-(3, 3-diphenylpropyl)-1H-imidazol-4-yl]ethanamine) which was a full agonist in the ileum assay. While 10 was equipotent with histamine (1), methylhistaprodifen (13) and dimethylhistaprodifen (14) exceeded the functional potency of 1 by a factor of 3-5 (13) and 2-3 (14). Compounds 10 and 13-17 relaxed precontracted rat aortic rings (intact endothelium) with relative potencies of 3.3- up to 28-fold (compared with 1), displaying partial agonism as well. Agonist effects were sensitive to blockade by the selective H(1)-receptor antagonist mepyramine (pA(2) approximately 9 (guinea-pig) and pA(2) approximately 8 (rat aorta)). The affinity of 10 and 13-17 for guinea-pig H(1) receptors increased 20- to 100-fold compared with 1. Two lower homologues of 10 were weak partial H(1)-receptor agonists while two higher homologues of 10 were silent antagonists endowed with micromolar affinity for rat and guinea-pig H(1) receptors. In functional selectivity experiments, 10, 13, and 14 did not stimulate H(2), H(3), and several other neurotransmitter receptors. They displayed only low to moderate affinity for these sites (pA(2) < 6). For a better understanding of structure-activity relationships, the interaction of 1 and 10, 13 and 14 within the transmembrane (TM) domains of the human histamine H(1) receptor were studied using molecular dynamics simulations. Remarkable differences were found between the binding modes of 10, 13, and 14 and that of 1. The imidazole ring of 10, 13, and 14 was placed 'upside down' compared with 1, making the interaction of the N(pi)-atom with Tyr431 possible. This new orientation was mainly caused by the space filling substitution at the 2-position of the imidazole ring and influenced the location of the protonated N(alpha)-atom which was positioned more between TM III and TM VI. This orientation can explain both the increased relative potency and the maximum effect of 10, 13, and 14 compared with 1. Compound 13 (methylhistaprodifen; N(alpha)-methyl-2-[2-(3, 3-diphenylpropyl)-1H-imidazol-4-yl]ethanamine) is the most potent histamine H(1)-receptor agonist reported so far in the literature and may become a valuable tool for the study of physiological and pathophysiological H(1)-receptor-mediated effects.

Mutational analysis of the antagonist-binding site of the histamine H(1) receptor

We combined in a previously derived three-dimensional model of the histamine H(1) receptor (Ter Laak, A. M., Timmerman, H., Leurs, H., Nederkoorn, P. H. J., Smit, M. J., and Donne-Op den Kelder, G. M. (1995) J. Comp. Aid. Mol. Design. 9, 319-330) a pharmacophore for the H(1) antagonist binding site (Ter Laak, A. M., Venhorst, J., Timmerman, H., and Donné-Op de Kelder, G. M. (1994) J. Med. Chem. 38, 3351-3360) with the known interacting amino acid residue Asp(116) (in transmembrane domain III) of the H(1) receptor and verified the predicted receptor-ligand interactions by site-directed mutagenesis. This resulted in the identification of the aromatic amino acids Trp(167), Phe(433), and Phe(436) in transmembrane domains IV and VI of the H(1) receptor as probable interaction points for the trans-aromatic ring of the H(1) antagonists. Subsequently, a specific interaction of carboxylate moieties of two therapeutically important, zwitterionic H(1) antagonists with Lys(200) in transmembrane domain V was predicted. A Lys(200) --> Ala mutation results in a 50- (acrivastine) to 8-fold (d-cetirizine) loss of affinity of these zwitterionic antagonists. In contrast, the affinities of structural analogs of acrivastine and cetirizine lacking the carboxylate group, triprolidine and meclozine, respectively, are unaffected by the Lys(200) --> Ala mutation. These data strongly suggest that Lys(200), unique for the H(1) receptor, acts as a specific anchor point for these "second generation" H(1) antagonists.

Molecular properties and signalling pathways of the histamine H1 receptor

With cloning of the gene encoding the histamine H1 receptor, a new area of histamine research has become reality. Finally, it seems feasible to study the target of the therapeutically important clans of antihistamine. Expression of the genes in mammalian cells allows detailed investigations of the various signal transduction routes of the histamine H1 receptor. Moreover, using molecular biological techniques, it is now possible to investigate ligand receptor interaction at the molecular level. Studies with mutant H1 receptors have shown that H1 antagonists bind to a specific amino acid residues in TM3 and 5. It is expected that these new developments will provide much fundamental knowledge on the ligand interaction with the H1 receptor.

Tick histamine-binding proteins: isolation, cloning, and three-dimensional structure

High-affinity histamine-binding proteins (HBPs) were discovered in the saliva of Rhipicephalus appendiculatus ticks. Their ability to outcompete histamine receptors indicates that they suppress inflammation during blood feeding. The crystal structure of a histamine-bound HBP, determined at 1.25 A resolution, reveals a lipocalin fold novel in containing two binding sites for the same ligand. The sites are orthogonally arranged and highly rigid and form an internal surface of unusual polar character that complements the physicochemical properties of histamine. As soluble receptors of histamine, HBPs offer a new strategy for controlling histamine-based diseases.

Unique binding pocket for KW-4679 in the histamine H1 receptor

The histamine H1 receptor has an aspartate (Asp) residue in transmembrane helix 3 (TM3), which is well-conserved among biogenic amine receptors. The Asp residue is one of the most crucial amino acids for ligand binding. The tested histamine H1 receptor antagonists with tri- and tetracyclic structures were not selective for histamine H1 receptors and showed affinity for several other biogenic amine receptors. In contrast, KW-4679 ((Z)-11-[3-(dimethylamino)propylidene]-6,11-dihydrodibenz[b, e]oxepin-2-acetic acid hydrochloride), a tricyclic compound, was a selective histamine H1 receptor antagonist. [3H]KW-4679 had high affinity (Kd value of 2.5 +/- 0.12 nM) for wild-type human histamine H1 receptors. In the [3H]KW-4679 binding assay, replacement of Asp107 by alanine by site-directed mutagenesis greatly reduced the affinities (280-2100-fold) of tri- and tetracyclic compounds, whereas this mutation led to a comparatively small reduction (14-fold) in KW-4679 affinity. These results demonstrate that the tested tri- and tetracyclic histamine H1 receptor antagonists which have a tight interaction with the Asp residue are not selective for the histamine H1 receptor. Furthermore, the high selectivity of KW-4679 might be explained by a unique binding pocket, which consists of the Asp residue and other acceptor sites, in the histamine H1 receptor.

Lysine200 located in the fifth transmembrane domain of the histamine H1 receptor interacts with histamine but not with all H1 agonists

Previously, we have shown that asparagine207 in the fifth transmembrane domain of the histamine H1 receptor is crucial for the binding of the N tau-nitrogen of the imidazole ring of histamine (Leurs et al., Biochem. Biophys. Res. Commun., 201, 295, 1994). In view of the potential interaction of the imidazole ring of histamine with a binding site, formed by asparagine207 and lysine200, we mutated lysine200 in the fifth transmembrane domain of the histamine H1 receptor to a non-functional alanine residue. This mutation did not affect the binding of the tested H1 receptor antagonists but resulted in a 5-fold lower affinity for histamine. The binding of other H1 receptor agonists was not affected. In stably transfected CHO cells histamine was 55-fold less effective in activating the H1Lys200Ala receptor (EC50 = 66 microM) compared to the wild type H1 receptor (EC50 = 1.2 microM). Receptor activation by the 2-methyl and the 2-(3-bromophenyl)-analogues however was hardly affected by the mutation, indicating that the 2-substituent probably prevents the interaction with the lysine200 residue. Finally, the Lys200Ala mutation reduced the production of [3H]inositol phosphates, stimulated by the non-imidazole H1 receptor agonist 2-pyridylethylamine. These data indicate that lysine200 interacts with the N pi-nitrogen of histamine and is important for the activation of the H1 receptor by histamine and the non-imidazole agonist 2-pyridylethylamine.

Modelling and mutation studies on the histamine H1-receptor agonist binding site reveal different binding modes for H1-agonists: Asp116 (TM3) has a constitutive role in receptor stimulation

A modelling study has been carried out, investigating the binding of histamine (Hist), 2-methylhistamine (2-MeHist) and 2-phenylhistamine (2-PhHist) at two postulated agonistic binding sites on transmembrane domain 5 (TM5) of the histamine H1-receptor. For this purpose a conformational analysis study was performed on three particular residues of TM5, i.e., Lys200, Thr203 and Asn207, for which a functional role in binding has been proposed. The most favourable results were obtained for the interaction between Hist and the Lys200/Asn207 pair. Therefore, Lys200 was subsequently mutated and converted to an alanine, resulting in a 50-fold decrease of H1-receptor stimulation by histamine. Altogether, the data suggest that the Lys200/Asn207 pair is important for activation of the H1-receptor by histamine. In contrast, analogues of 2-PhHist seem to belong to a distinct subclass of histamine agonists and an alternative mode of binding is proposed in which the 2-phenyl ring binds to the same receptor location as one of the aromatic rings of classical histamine H1-antagonists. Subsequently, the binding modes of the agonists Hist, 2-MeHist and 2-PhHist and the H1-antagonist cyproheptadine were evaluated in three different seven-alpha-helical models of the H1-receptor built in homology with bacteriorhodopsin, but using three different alignments. Our findings suggest that the position of the carboxylate group of Asp116 (TM3) within the receptor pocket depends on whether an agonist or an antagonist binds to the protein; a conformational change of this aspartate residue upon agonist binding is expected to play an essential role in receptor stimulation.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Site-directed mutagenesis of the histamine H1 receptor: roles of aspartic acid107, asparagine198 and threonine194

Based on structural comparison with other biogenic amine receptors and the histamine H2 receptor, it has been suggested that in the human histamine H1 receptor, Asp107, Thr194, and Asn198 are the residues involved in binding of histamine. We therefore used site-directed mutagenesis to investigate the roles of these three amino acid residues. Asp107 was essential for both agonist and antagonist binding. Asn198 was necessary for agonist but not for antagonist binding. Thr194 was not important for either type of binding. A good correlation was found between agonist binding and receptor activation for all the wild-type and mutant receptors. The results show that the histamine H1 receptor recognizes and is activated by histamine through the interactions of Asp107 and the amino group, and Asn198 and the imidazole ring.

Site-directed mutagenesis of the histamine H1-receptor reveals a selective interaction of asparagine207 with subclasses of H1-receptor agonists

In this study we investigated the role of the threonine203 and the asparagine207 residues in the fifth transmembrane domain of the guinea-pig histamine H1-receptor by site-directed mutagenesis to non-functional alanines. Whereas the threonine203 residue is not important for the action of histamine, the asparagine207 residue appears to be involved in the binding of the N tau-nitrogen atom of histamine and its 2-methyl-analogue. For the 2-phenyl-analogue and non-imidazole H1-receptor agonists, this residue is, however, not essential for binding. On the basis of this study we conclude that different histamine H1-receptor agonists interact in different ways with the H1-receptor protein. Moreover, we speculate that the interaction with the N pi-nitrogen atom is essential for H1-receptor activation.