Analysis of the activation mechanism of the guinea-pig Histamine H1-receptor

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Comparison of the pharmacological properties of human and rat histamine H(3)-receptors

Ligand pharmacology of histamine H(3)-receptors is species-dependent. In previous studies, two amino acids in transmembrane domain 3 (TM III) were shown to play a significant role. In this study, we characterized human and rat histamine H(3)-receptors (hH(3)R and rH(3)R, respectively), co-expressed with mammalian G proteins in Sf9 insect cell membranes. We compared a series of imidazole-containing H(3)R ligands in radioligand binding and steady-state GTPase assays. H(3)Rs similarly coupled to Gα(i/o)-proteins. Affinities and potencies of the agonists histamine, N(α)-methylhistamine and R-(α)-methylhistamine were in the same range. Imetit was only a partial agonist. The pharmacology of imetit and proxifan was similar at both species. However, impentamine was more potent and efficacious at rH(3)R. The inverse agonists ciproxifan and thioperamide showed higher potency but lower efficacy at rH(3)R. Clobenpropit was not species-selective. Strikingly, imoproxifan was almost full agonist at hH(3)R, but an inverse agonist at rH(3)R. Imoproxifan was docked into the binding pocket of inactive and active hH(3)R- and rH(3)R-models and molecular dynamic simulations were performed. Imoproxifan bound to hH(3)R and rH(3)R in E-configuration, which represents the trans-isomer of the oxime-moiety as determined in crystallization studies, and stabilized active hH(3)R-, but inactive rH(3)R-conformations. Large differences in electrostatic surfaces between TM III and TM V cause differential orientation of the oxime-moiety of imoproxifan, which then differently interacts with the rotamer toggle switch Trp(6.48) in TM VI. Collectively, the substantial species differences at H(3)Rs are explained at a molecular level by the use of novel H(3)R active-state models.

Interactions of histamine H1-receptor agonists and antagonists with the human histamine H4-receptor

The human histamine H(4)-receptor (hH(4)R) possesses high constitutive activity and, like the human H(1)-receptor (hH(1)R), is involved in the pathogenesis of type-I allergic reactions. The study aims were to explore the value of dual H(1)/H(4)R antagonists as antiallergy drugs and to address the question of whether H(1)R ligands bind to hH(4)R. In an acute murine asthma model, the H(1)R antagonist mepyramine and the H(4)R antagonist 1-[(5-chloro-1H-indol-2-yl)carbonyl]-4-methyl-piperazine (JNJ 7777120) exhibited synergistic inhibitory effects on eosinophil accumulation in the bronchoalveolar lavage fluid. At the hH(4)R expressed in Sf9 insect cells, 18 H(1)R antagonists and 22 H(1)R agonists showed lower affinity to hH(4)R than to hH(1)R as assessed in competition binding experiments. For a small number of H(1)R antagonists, hH(4)R partial agonism was observed in the steady-state GTPase assay. Most compounds were neutral antagonists or inverse agonists. Twelve phenylhistamine-type hH(1)R partial agonists were also hH(4)R partial agonists. Four histaprodifen-type hH(1)R partial agonists were hH(4)R inverse agonists. Dimeric histaprodifen was a more efficacious hH(4)R inverse agonist than the reference compound thioperamide. Suprahistaprodifen was the only histaprodifen acting as hH(4)R partial agonist. Suprahistaprodifen was docked into the binding pocket of inactive and active hH(4)R models in two different orientations, predominantly stabilizing the active state of hH(4)R. Collectively, the synergistic effects of H(1)R and H(4)R antagonists in an acute asthma model and the overlapping interaction of structurally diverse H(1)R ligands with hH(1)R and hH(4)R indicate that the development of dual H(1)R/H(4)R antagonists is a worthwhile and technically feasible goal for the treatment of type-I allergic reactions.

Molecular basis for the selective interaction of synthetic agonists with the human histamine H1-receptor compared with the guinea pig H1-receptor

Previous studies revealed that phenylhistamines and histaprodifens possess higher potency and affinity at guinea pig histamine H(1)-receptor (gpH(1)R) than at human histamine H(1)-receptor (hH(1)R). However, we recently identified an imidazolylpropylguanidine [N(1)-(3-cyclohexylbutanoyl)-N(2)-[3-(1H-imidazol-4-yl)-propyl]guanidine (UR-AK57)] with higher potency and efficacy at hH(1)R compared with gpH(1)R. The aim of this study was to reveal the molecular basis for the species differences of synthetic ligands. We studied 11 novel phenylhistamines and phenoprodifens. H(1)R species isoforms were expressed in Sf9 insect cells, and [(3)H]mepyramine competition binding and GTPase assays were performed. We identified bulky phenylhistamines with higher potency and affinity at hH(1)R compared with gpH(1)R. Molecular dynamics simulations of ligand-H(1)R interactions revealed four potential binding modes for phenylhistamines possessing an additional histamine moiety; the terminal histamine moiety showed a high flexibility in the binding pocket. There are striking similarities in ligand properties in bulky phenylhistamines and UR-AK57. Comparison of bulky phenylhistamine binding mode with binding mode of UR-AK57 suggests that only one of these four binding modes should be established. The higher potency is explained by more effective van der Waals interaction of the compounds with Asn(2.61) (hH(1)R) relative to Ser(2.61) (gpH(1)R). In addition, two stable binding modes for phenoprodifens with different orientations in the binding-pocket were identified. Depending on phenoprodifen orientation, the highly conserved Trp(6.48), part of the toggle switch involved in receptor activation, was found in an inactive or active conformation, respectively. We identified the first phenylhistamines with higher potency at hH(1)R than at gpH(1)R and obtained insight into the binding mode of bulky phenylhistamines and imidazolylpropylguanidines.

Ligand-specific contribution of the N terminus and E2-loop to pharmacological properties of the histamine H1-receptor

There are species differences between human histamine H(1) receptor (hH(1)R) and guinea pig (gp) histamine H(1) receptor (gpH(1)R) for phenylhistamines and histaprodifens. Several studies showed participation of the second extracellular loop (E2-loop) in ligand binding for some G protein-coupled receptors (GPCRs). Because there are large species differences in the amino acid sequence between hH(1)R and gpH(1)R for the N terminus and E2-loop, we generated chimeric hH(1)Rs with gp E2-loop (h(gpE2)H(1)R) and gp N terminus and gp E2-loop (h(gpNgpE2)H(1)R). hH(1)R, gpH(1)R, and chimeras were expressed in Sf9 insect cells. [(3)H]Mepyramine binding assays and steady-state GTPase assays were performed. In the series hH(1)R > h(gpE2)H(1)R > h(gpNgpE2)H(1)R, we observed a significant decrease in potency of histamine 1 in the GTPase assay. For phenoprodifen 5 and the chiral phenoprodifens 6R and 6S, a significant decrease in affinity and potency was found in the series hH(1)R > h(gpE2)H(1)R > h(gpNgpE2)H(1)R. In addition, we constructed new active-state H(1)R models based on the crystal structure of the human beta(2)-adrenergic receptor (hbeta(2)AR). Compared with the H(1)R active-state models based on the crystal structure of bovine rhodopsin, the E2-loop differs in its contact to the ligand bound in the binding pocket. In the bovine rhodopsin-based model, the backbone carbonyl of Lys187 (gpH(1)R) interacts with large histaprodifens in the binding pocket, but in the hbeta(2)AR-based model, Lys187 (gpH(1)R) is located distantly from the binding pocket. In conclusion, the differences in N terminus and E2-loop between hH(1)R and gpH(1)R exert an influence on affinity and/or potency for histamine and phenoprodifens 5, 6R, and 6S.