Role of the second and third extracellular loops of the histamine H4 receptor in receptor activation

Histamine via histamine H1 receptor enhances the muscarinic receptor-induced calcium response to acetylcholine in an enterochromaffin cell model

As a prerequisite for serotonin secretion, the P‐STS ileal enterochromaffin cell line responds to acetylcholine (ACh) stimulation with an increase in intracellular calcium mediated by the muscarinic ACh receptor M3 (M3R). Histamine increases intracellular calcium via histamine H1 receptor (H1R) in P‐STS cells and pre‐incubation with histamine specifically augments the response to ACh but not to epinephrine or nicotine. We aimed to elucidate whether histamine receptors are involved in this synergism. Astonishingly, HEK‐293 T cells—known to express M3R, but only a very low amount of histamine receptor messenger RNA—showed a similar enhancement of the calcium response to ACh by pre‐incubation with histamine. Despite the much lower level of H1R protein detected in HEK‐293 T cells as compared to P‐STS cells, in both cell lines pre‐treatment with H1R antagonists inhibited the synergism between histamine and ACh. No indication for an involvement of histamine H2 or H4 receptors in the synergism was found. Furthermore, pre‐incubation with the cAMP‐inducing compound forskolin had no influence on the intracellular calcium response to ACh. Serotonin secretion from P‐STS cells was increased after challenge with ACh and histamine added simultaneously compared to ACh alone, suggesting that histamine increases ACh‐induced serotonin secretion from enterochromaffin cells. In conclusion, our data suggest that histamine enhances the M3R‐mediated intracellular calcium response to ACh via activation of H1R. This probably increases serotonin secretion from enterochromaffin cells and thereby affects intestinal motility in histamine intolerance, food allergies and irritable bowel syndrome.

A Dynamic, Split-Luciferase-Based Mini-G Protein Sensor to Functionally Characterize Ligands at All Four Histamine Receptor Subtypes

In drug discovery, assays with proximal readout are of great importance to study target-specific effects of potential drug candidates. In the field of G protein-coupled receptors (GPCRs), the determination of GPCR-G protein interactions and G protein activation by means of radiolabeled GTP analogs ([³⁵S]GTPγS, [γ-³²P]GTP) has widely been used for this purpose. Since we were repeatedly faced with insufficient quality of radiolabeled nucleotides, there was a requirement to implement a novel proximal functional assay for the routine characterization of putative histamine receptor ligands. We applied the split-NanoLuc to the four histamine receptor subtypes (H₁R, H₂R, H₃R, H₄R) and recently engineered minimal G (mini-G) proteins. Using this method, the functional response upon receptor activation was monitored in real-time and the four mini-G sensors were evaluated by investigating selected standard (inverse) agonists and antagonists. All potencies and efficacies of the studied ligands were in concordance with literature data. Further, we demonstrated a significant positive correlation of the signal amplitude and the mini-G protein expression level in the case of the H₂R, but not for the H₁R or the H₃R. The pEC₅₀ values of histamine obtained under different mini-G expression levels were consistent. Moreover, we obtained excellent dynamic ranges (Z’ factor) and the signal spans were improved for all receptor subtypes in comparison to the previously performed [³⁵S]GTPγS binding assay.

[3H]UR-DEBa176: A 2,4-Diaminopyrimidine-Type Radioligand Enabling Binding Studies at the Human, Mouse, and Rat Histamine H4 Receptors

Differences in sequence homology between human (h), mouse (m), and rat (r) histamine H₄ receptors (H₄R) cause discrepancies regarding affinities, potencies, and/or efficacies of ligands and therefore compromise translational animal models and the applicability of radioligands. Aiming at a radioligand enabling robust and comparative binding studies at the h/m/rH₄Rs, 2,4-diaminopyrimidines were synthesized and pharmacologically investigated. The most notable compounds identified were two (partial) agonists with comparable potencies at the h/m/rH₄Rs: UR-DEBa148 (N-neopentyl-4-(1,4,6,7-tetrahydro-5H-imidazo[4,5-c]pyridin-5-yl)pyrimidin-2-amine bis(2,2,2-trifluoroacetate), 43), the most potent [pEC₅₀ (reporter gene assay) = 9.9/9.6/10.3] compound in the series being slightly G-protein biased and UR-DEBa176 [(R)-4-[3-(dimethylamino)pyrrolidin-1-yl]-N-neopentylpyrimidin-2-amine bis(2,2,2-trifluoroacetate), 46, pEC₅₀ (reporter gene assay) = 8.7/9.0/9.2], a potential "cold" form of a tritiated H₄R ligand. After radiolabeling, binding studies with [³H]UR-DEBa176 ([³H]46) at the h/m/rH₄Rs revealed comparable K_d values (41/17/22 nM), low nonspecific binding (11-17%, ∼K_d), and fast associations/dissociations (25-30 min) and disclosed [³H]UR-DEBa176 as useful molecular tool to determine h/m/rH₄R binding affinities for H₄R ligands.

Hybrid approach to structure modeling of the histamine H3 receptor: Multi-level assessment as a tool for model verification

The crucial role of G-protein coupled receptors and the significant achievements associated with a better understanding of the spatial structure of known receptors in this family encouraged us to undertake a study on the histamine H3 receptor, whose crystal structure is still unresolved. The latest literature data and availability of different software enabled us to build homology models of higher accuracy than previously published ones. The new models are expected to be closer to crystal structures; and therefore, they are much more helpful in the design of potential ligands. In this article, we describe the generation of homology models with the use of diverse tools and a hybrid assessment. Our study incorporates a hybrid assessment connecting knowledge-based scoring algorithms with a two-step ligand-based docking procedure. Knowledge-based scoring employs probability theory for global energy minimum determination based on information about native amino acid conformation from a dataset of experimentally determined protein structures. For a two-step docking procedure two programs were applied: GOLD was used in the first step and Glide in the second. Hybrid approaches offer advantages by combining various theoretical methods in one modeling algorithm. The biggest advantage of hybrid methods is their intrinsic ability to self-update and self-refine when additional structural data are acquired. Moreover, the diversity of computational methods and structural data used in hybrid approaches for structure prediction limit inaccuracies resulting from theoretical approximations or fuzziness of experimental data. The results of docking to the new H3 receptor model allowed us to analyze ligand-receptor interactions for reference compounds.

Molecular Modelling Approaches for the Analysis of Histamine Receptors and Their Interaction with Ligands

Several experimental techniques to analyse histamine receptors are available, e.g. pharmacological characterisation of known or new compounds by different types of assays or mutagenesis studies. To obtain insights into the histamine receptors on a molecular and structural level, crystal structures have to be determined and molecular modelling studies have to be performed. It is widely accepted to generate homology models of the receptor of interest based on an appropriate crystal structure as a template and to refine the resulting models by molecular dynamic simulations. A lot of modelling techniques, e.g. docking, QSAR or interaction fingerprint methods, are used to predict binding modes of ligands and pharmacological data, e.g. affinity or even efficacy. However, within the last years, molecular dynamic simulations got more and more important: First of all, molecular dynamic simulations are very helpful to refine the binding mode of a ligand to a histamine receptor, obtained by docking studies. Furthermore, with increasing computational performance it got possible to simulate complete binding pathways of ions or ligands from the aqueous extracellular phase into the allosteric or orthosteric binding pocket of histamine receptors.

Dibenzo[b,f][1,4]oxazepines and dibenzo[b,e]oxepines: Influence of the chlorine substitution pattern on the pharmacology at the H1R, H4R, 5-HT2AR and other selected GPCRs

Inspired by VUF6884 (7-Chloro-11-(4-methylpiperazin-1-yl)dibenzo[b,f][1,4]oxazepine), reported as a dual H₁/H₄ receptor ligand (pK_i: 8.11 (human H₁R (hH₁R)), 7.55 (human H₄R (hH₄R))), four known and 28 new oxazepine and related oxepine derivatives were synthesised and pharmacologically characterized at histamine receptors and selected aminergic GPCRs. In contrast to the oxazepine series, within the oxepine series, the new compounds showed high affinity to the hH₁R (pK_i: 6.8-8.7), but no or moderate affinity to the hH₄R (pK_i:≤5.3). For one oxepine derivative (1-(2-Chloro-6,11-dihydrodibenzo[b,e]oxepin-11-yl)-4-methylpiperazine), the enantiomers were separated and the R-enantiomer was identified as the eutomer at the hH₁R (pK_i: 8.83 (R), 7.63 (S)) and the guinea-pig H₁R (gpH₁R) (pK_i: 8.82 (R), 7.41 (S)). Molecular dynamic studies suggest that the tricyclic core of the compounds is bound in a similar mode into the binding pocket, as described for doxepine in the hH₁R crystal structure. Moreover, docking studies of all oxepine derivatives at the hH₁R indicate that the oxygen and the position of the chlorine in the tricyclic core determines, if the R- or the S-enantiomer is the eutomer. For some of the oxazepines and oxepines the affinity to other aminergic GPCRs is in the same range as to hH₁R or hH₄R, thus, those compounds have to be classified as dirty drugs. However, one oxazepine derivative (3,7-Dichloro-11-(4-methylpiperazin-1-yl)dibenzo[b,f][1,4]oxazepine was identified as dual hH₁/h5-HT_2A receptor ligand (pK_i: 9.23 (hH₁R), 8.74 (h5-HT_2AR), ≤7 at other analysed GPCRs), whereas one oxepine derivative (1-(3,8-Dichloro-6,11-dihydrodibenzo[b,e]oxepin-11-yl)-4-methylpiperazine) was identified as selective hH₁R antagonist (pK_i: 8.44 (hH₁R), ≤6.7 at other analyzed GPCRs). Thus, the pharmacological results suggest that the oxazepine/oxepine moiety and additionally the chlorine substitution pattern toggles receptor selectivity and specificity.

2,4-Diaminopyrimidines as dual ligands at the histamine H1 and H4 receptor-H1/H4-receptor selectivity

Distinct diaminopyrimidines, for example, 4-(4-methylpiperazin-1-yl)-5,6-dihydrobenzo[h]quinazolin-2-amine are histamine H4 receptor (H4R) antagonists and show high affinity to the H4R, but only a moderate affinity to the histamine H1 receptor (H1R). Within previous studies it was shown that an aromatic side chain with a distinct distance to the basic amine and aromatic core is necessary for affinity to the human H1R (hH1R). Thus, a rigid aminopyrimidine with a tricyclic core was used as a lead structure. There, (1) the flexible aromatic side chain was introduced, (2) the substitution pattern of the pyrimidine core was exchanged and (3) rigidity was decreased by opening the tricyclic core. Within the present study, two compounds with similar affinity in the one digit μM range to the human H1R and H4R were identified. While the affinity at the hH1R increased about 4- to 8-fold compared to the parent diaminopyrimidine, the affinity to the hH4R decreased about 5- to 8-fold. In addition to the parent diaminopyrimidine, two selected compounds were docked into the H1R and H4R and molecular dynamic studies were performed to predict the binding mode and explain the experimental results on a molecular level. The two new compounds may be good lead structures for the development of dual H1/H4 receptor ligands with affinities in the same range.

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.

The extracellular loop 2 (ECL2) of the human histamine H4 receptor substantially contributes to ligand binding and constitutive activity

In contrast to the corresponding mouse and rat orthologs, the human histamine H4 receptor (hH4R) shows extraordinarily high constitutive activity. In the extracellular loop (ECL), replacement of F169 by V as in the mouse H4R significantly reduced constitutive activity. Stabilization of the inactive state was even more pronounced for a double mutant, in which, in addition to F169V, S179 in the ligand binding site was replaced by M. To study the role of the FF motif in ECL2, we generated the hH4R-F168A mutant. The receptor was co-expressed in Sf9 insect cells with the G-protein subunits Gαi2 and Gβ1γ2, and the membranes were studied in [3H]histamine binding and functional [35S]GTPγS assays. The potency of various ligands at the hH4R-F168A mutant decreased compared to the wild-type hH4R, for example by 30- and more than 100-fold in case of the H4R agonist UR-PI376 and histamine, respectively. The high constitutive activity of the hH4R was completely lost in the hH4R-F168A mutant, as reflected by neutral antagonism of thioperamide, a full inverse agonist at the wild-type hH4R. By analogy, JNJ7777120 was a partial inverse agonist at the hH4R, but a partial agonist at the hH4R-F168A mutant, again demonstrating the decrease in constitutive activity due to F168A mutation. Thus, F168 was proven to play a key role not only in ligand binding and potency, but also in the high constitutive activity of the hH4R.

The histamine H4 -receptor (H4 R) regulates eosinophilic inflammation in ovalbumin-induced experimental allergic asthma in mice

Via the histamine H4 -receptor (H4 R), histamine promotes the pathogenesis of experimental allergic asthma in mice. Application of H4 R antagonists during sensitization as well as during provocation reduces the severity of the disease. However, the specific cell types functionally expressing H4 R in experimental allergic asthma have not been well characterized in vivo. In this study, we identified the cell type(s) responsible for H4 R activity in experimental asthma and related physiological mechanisms. Using H4 R-deficient mice, we studied the role of H4 R in the sensitization and effector phase. DCs lacking H4 R expression during the in vitro sensitization reaction resulted in effector T cells unable to induce an entire eosinophilic inflammation in the lung upon adoptive transfer in vivo. Recipient mice lacking H4 R expression, which were adoptively transferred with H4 R(+/+) T cells polarized in the presence of H4 R(+/+) DCs, showed reduced signs of inflammation and ameliorated lung function. Here, we provide in vivo evidence that in experimental asthma in mice the H4 R specifically regulates activation of DCs during sensitization, while in the effector phase the H4 R is active in cells involved in the activation of eosinophils, and possibly other cells. A putative therapy targeting the H4 R may be an option for asthma patients developing IL-5-dependent eosinophilia.

Dimeric carbamoylguanidine-type histamine H2 receptor ligands: A new class of potent and selective agonists

The bioisosteric replacement of the acylguanidine moieties in dimeric histamine H2 receptor (H2R) agonists by carbamoylguanidine groups resulted in compounds with retained potencies and intrinsic activities, but considerably improved stability against hydrolytic cleavage. These compounds achieved up to 2500 times the potency of histamine when studied in [(35)S]GTPγS assays on recombinant human and guinea pig H2R. Unlike 3-(imidazol-4-yl)propyl substituted carbamoylguanidines, the corresponding 2-amino-4-methylthiazoles revealed selectivity over histamine receptor subtypes H1R, H3R and H4R in radioligand competition binding studies. H2R binding studies with three fluorescent compounds and one tritium-labeled ligand, synthesized from a chain-branched precursor, failed due to pronounced cellular accumulation and high non-specific binding. However, the dimeric H2R agonists proved to be useful pharmacological tools for functional studies on native cells, as demonstrated for selected compounds by cAMP accumulation and inhibition of fMLP-stimulated generation of reactive oxygen species in human monocytes.

Luciferase reporter gene assay on human, murine and rat histamine H4 receptor orthologs: correlations and discrepancies between distal and proximal readouts

The investigation of the (patho)physiological role of the histamine H4 receptor (H4R) and its validation as a possible drug target in translational animal models are compromised by distinct species-dependent discrepancies regarding potencies and receptor subtype selectivities of the pharmacological tools. Such differences were extremely pronounced in case of proximal readouts, e. g. [(32)P]GTPase or [(35)S]GTPγS binding assays. To improve the predictability of in vitro investigations, the aim of this study was to establish a reporter gene assay for human, murine and rat H4Rs, using bioluminescence as a more distal readout. For this purpose a cAMP responsive element (CRE) controlled luciferase reporter gene assay was established in HEK293T cells, stably expressing the human (h), the mouse (m) or the rat (r) H4R. The potencies and efficacies of 23 selected ligands (agonists, inverse agonists and antagonists) were determined and compared with the results obtained from proximal readouts. The potencies of the examined ligands at the human H4R were consistent with reported data from [(32)P]GTPase or [(35)S]GTPγS binding assays, despite a tendency toward increased intrinsic efficacies of partial agonists. The differences in potencies of individual agonists at the three H4R orthologs were generally less pronounced compared to more proximal readouts. In conclusion, the established reporter gene assay is highly sensitive and reliable. Regarding discrepancies compared to data from functional assays such as [(32)P]GTPase and [(35)S]GTPγS binding, the readout may reflect multifactorial causes downstream from G-protein activation, e.g. activation/amplification of or cross-talk between different signaling pathways.

Study of structural dynamics of ligand-activated membrane receptors by means of principal component analysis

The structural dynamics of three different ligand-activated G-protein coupled receptors (GPCRs) and the photoreactive receptor rhodopsin from mammals were comparatively studied. As a result, diagrams demonstrating the main structural differences between the studied membrane receptors were obtained. These diagrams represent the projection of the crystal structures of rhodopsin photointermediates and ligand-activated receptors onto the plane defined by the principal components. Thus, we were able to associate the activation process of the receptors with large-scale movements of their individual transmembrane (TM) domains. In addition, the dynamics of extracellular loops of ligand-activated receptors responsible for recognition and initial binding of ligands was studied. Based on these results, two parameters of functionally significant structural dynamics of membrane receptors can be thoroughly analyzed simultaneously - movements of individual TM helices and of extracellular loops.

Molecular and cellular analysis of human histamine receptor subtypes

The human histamine receptors hH(1)R and hH(2)R constitute important drug targets, and hH(3)R and hH(4)R have substantial potential in this area. Considering the species-specificity of pharmacology of H(x)R orthologs, it is important to analyze hH(x)Rs. Here, we summarize current knowledge of hH(x)Rs endogenously expressed in human cells and hH(x)Rs recombinantly expressed in mammalian and insect cells. We present the advantages and disadvantages of the various systems. We also discuss problems associated with the use of hH(x)R antibodies, an issue of general relevance for G-protein-coupled receptors (GPCRs). There is much greater overlap in activity of 'selective' ligands for other hH(x)Rs than the cognate receptor subtype than generally appreciated. Studies with native and recombinant systems support the concept of ligand-specific receptor conformations, encompassing agonists and antagonists. It is emerging that for characterization of hH(x)R ligands, one cannot rely on a single test system and a single parameter. Rather, multiple systems and parameters have to be studied. Although such studies are time-consuming and expensive, ultimately, they will increase drug safety and efficacy.

Species-dependent activities of G-protein-coupled receptor ligands: lessons from histamine receptor orthologs

Histamine is a biogenic amine that exerts its biological effects as a neurotransmitter and local mediator via four histamine receptor (HR) subtypes (H(x)Rs) - H(1)R, H(2)R, H(3)R, and H(4)R - belonging to the superfamily of G-protein-coupled receptors (GPCRs). All four H(x)Rs exhibit pronounced differences in agonist and/or antagonist pharmacology among various species orthologs. The species differences constitute a problem for animal experiments and drug development. This problem applies to GPCRs with diverse ligands. Here, we summarize our current knowledge on H(x)R orthologs as a case study for species-dependent activity of GPCR ligands. We show that species-specific pharmacology also provides unique opportunities to study important aspects of GPCR pharmacology in general, including ligand-binding sites, the roles of extracellular domains in ligand binding and receptor activation, agonist-independent (constitutive) receptor activity, thermodynamics of ligand/receptor interaction, receptor-activation mechanisms, and ligand-specific receptor conformations.