Molecular modeling studies give hint for the existence of a symmetric hβ2R-Gαβγ-homodimer

Conformational Restriction and Enantioseparation Increase Potency and Selectivity of Cyanoguanidine-Type Histamine H4 Receptor Agonists

2-Cyano-1-[4-(1H-imidazol-4-yl)butyl]-3-[2-(phenylsulfanyl)ethyl]guanidine (UR-PI376, 1) is a potent and selective agonist of the human histamine H4 receptor (hH4R). To gain information on the active conformation, we synthesized analogues of 1 with a cyclopentane-1,3-diyl linker. Affinities and functional activities were determined at recombinant hHxR (x: 1-4) subtypes on Sf9 cell membranes (radioligand binding, [(35)S]GTPγS, or GTPase assays) and in part in luciferase assays on human or mouse H4R (HEK-293 cells). The most potent H4R agonists among 14 racemates were separated by chiral HPLC, yielding eight enantiomerically pure compounds. Configurations were assigned based on X-ray structures of intermediates and a stereocontrolled synthetic pathway. (+)-2-Cyano-1-{[trans-(1S,3S)-3-(1H-imidazol-4-yl)cyclopentyl]methyl}-3-[2-(phenylsulfanyl)ethyl]guanidine ((1S,3S)-UR-RG98, 39a) was the most potent H4R agonist in this series (EC50 11 nM; H4R vs H3R, >100-fold selectivity; H1R, H2R, negligible activities), whereas the optical antipode proved to be an H4R antagonist ([(35)S]GTPγS assay). MD simulations confirmed differential stabilization of the active and inactive H4R state by the enantiomers.

Multi-Component Protein - Protein Docking Based Protocol with External Scoring for Modeling Dimers of G Protein-Coupled Receptors

In order to apply structure-based drug design techniques to GPCR complexes, it is essential to model their 3D structure. For this purpose, a multi-component protocol was derived based on protein-protein docking which generates populations of dimers compatible with membrane integration, considering all reasonable interfaces. At the next stage, we applied a scoring procedure based on up to eleven different parameters including shape or electrostatics complementarity. Two methods of consensus scoring were performed: (i) average scores of 100 best scored dimers with respect to each interface, and (ii) frequencies of interfaces among 100 best scored dimers. In general, our multi-component protocol gives correct indications for dimer interfaces that have been observed in X-ray crystal structures of GPCR dimers (opsin dimer, chemokine CXCR4 and CCR5 dimers, κ opioid receptor dimer, β1 adrenergic receptor dimer and smoothened receptor dimer) but also suggests alternative dimerization interfaces. Interestingly, at times these alternative interfaces are scored higher than the experimentally observed ones suggesting them to be also relevant in the life cycle of studied GPCR dimers. Further results indicate that GPCR dimer and higher-order oligomer formation may involve transmembrane helices (TMs) TM1-TM2-TM7, TM3-TM4-TM5 or TM4-TM5-TM6 but not TM1-TM2-TM3 or TM2-TM3-TM4 which is in general agreement with available experimental and computational data.

Molecular determinants for the high constitutive activity of the human histamine H4 receptor: functional studies on orthologues and mutants

BACKGROUND AND PURPOSE

Some histamine H4 receptor ligands act as inverse agonists at the human H4 receptor (hH4 R), a receptor with exceptionally high constitutive activity, but as neutral antagonists or partial agonists at the constitutively inactive mouse H4 receptor (mH4 R) and rat H4 receptor (rH4 R). To study molecular determinants of constitutive activity, H4 receptor reciprocal mutants were constructed: single mutants: hH4 R-F169V, mH4 R-V171F, hH4 R-S179A, hH4 R-S179M; double mutants: hH4 R-F169V+S179A, hH4 R-F169V+S179M and mH4 R-V171F+M181S.

EXPERIMENTAL APPROACH

Site-directed mutagenesis with pVL1392 plasmids containing hH4 or mH4 receptors were performed. Wild-type or mutant receptors were co-expressed with Gαi2 and Gβ1 γ2 in Sf9 cells. Membranes were studied in saturation and competition binding assays ([(3) H]-histamine), and in functional [(35) S]-GTPγS assays with inverse, partial and full agonists of the hH4 receptor.

KEY RESULTS

Constitutive activity decreased from the hH4 receptor via the hH4 R-F169V mutant to the hH4 R-F169V+S179A and hH4 R-F169V+S179M double mutants. F169 alone or in concert with S179 plays a major role in stabilizing a ligand-free active state of the hH4 receptor. Partial inverse hH4 receptor agonists like JNJ7777120 behaved as neutral antagonists or partial agonists at species orthologues with lower or no constitutive activity. Some partial and full hH4 receptor agonists showed decreased maximal effects and potencies at hH4 R-F169V and double mutants. However, the mutation of S179 in the hH4 receptor to M as in mH4 receptor or A as in rH4 receptor did not significantly reduce constitutive activity.

CONCLUSIONS AND IMPLICATIONS

F169 and S179 are key amino acids for the high constitutive activity of hH4 receptors and may also be of relevance for other constitutively active GPCRs.

LINKED ARTICLES

This article is part of a themed issue on Histamine Pharmacology Update published in volume 170 issue 1. To view the other articles in this issue visit http://onlinelibrary.wiley.com/doi/10.1111/bph.2013.170.issue-1/issuetoc.

Insights into a defined secondary binding region on β-adrenoceptors and putative roles in ligand binding and drug design

Putative roles of a secondary binding region shared among beta-adrenoceptors.

Mathematical analysis of the sodium sensitivity of the human histamine H3 receptor

Purpose

It was shown by several experimental studies that some G protein coupled receptors (GPCR) are sensitive to sodium ions. Furthermore, mutagenesis studies or the determination of crystal structures of the adenosine A_2A or δ-opioid receptor revealed an allosteric Na⁺ binding pocket near to the highly conserved Asp^2.50. Within a previous study, the influence of NaCl concentration onto the steady-state GTPase activity at the human histamine H₃ receptor (hH₃R) in presence of the endogenous histamine or the inverse agonist thioperamide was analyzed. The purpose of the present study was to examine and quantify the Na⁺-sensitivity of hH₃R on a molecular level.

Methods

To achieve this, we developed a set of equations, describing constitutive activity and the different ligand-receptor equilibria in absence or presence of sodium ions. Furthermore, in order to gain a better understanding of the ligand- and Na⁺-binding to hH₃R on molecular level, we performed molecular dynamic (MD) simulations.

Results

The analysis of the previously determined experimental steady-state GTPase data with the set of equations presented within this study, reveals that thioperamide binds into the orthosteric binding pocket of the hH₃R in absence or presence of a Na⁺ in its allosteric binding site. However, the data suggest that thioperamide binds preferentially into the hH₃R in absence of a sodium ion in its allosteric site. These experimental results were supported by MD simulations of thioperamide in the binding pocket of the inactive hH₃R. Furthermore, the MD simulations revealed two different binding modes for thioperamide in presence or absence of a Na⁺ in its allosteric site.

Conclusion

The mathematical model presented within this study describes the experimental data regarding the Na⁺-sensitivity of hH₃R in an excellent manner. Although the present study is focused onto the Na⁺-sensitivity of the hH₃R, the resulting equations, describing Na⁺- and ligand-binding to a GPCR, can be used for all other ion-sensitive GPCRs.