Structure of the human histamine H1 receptor complex with doxepin

Evolutionary history of histamine receptors: Early vertebrate origin and expansion of the H3-H4 subtypes

Histamine receptors belonging to the superfamily of G protein-coupled receptors (GPCRs) mediate the diverse biological effects of biogenic histamine. They are classified into four phylogenetically distinct subtypes H₁-H₄, each with a different binding affinity for histamine and divergent downstream signaling pathways. Here we present the evolutionary history of the histamine receptors using a phylogenetic approach complemented with comparative genomics analyses of the sequences, gene structures, and synteny of gene neighborhoods. The data indicate the earliest emergence of histamine-mediated GPCR signaling by a H₂ in a prebilaterian ancestor. The analyses support a revised classification of the vertebrate H₃-H₄ receptor subtypes. We demonstrate the presence of the H₄ across vertebrates, contradicting the currently held notion that H₄ is restricted to mammals. These non-mammalian vertebrate H₄ orthologs have been mistaken for H₃. We also identify the presence of a new H₃ subtype (H₃B), distinct from the canonical H₃ (H₃A), and propose that the H₃A, H₃B, and H₄ likely emerged from a H₃ progenitor through the 1R/2R whole genome duplications in an ancestor of the vertebrates. It is apparent that the ability of the H₁, H₂, and H_3-4 to bind histamine was acquired convergently. We identified genomic signatures suggesting that the H₁ and H₃-H₄ shared a last common ancestor with the muscarinic receptor in a bilaterian predecessor whereas, the H₂ and the α-adrenoreceptor shared a progenitor in a prebilaterian ancestor. Furthermore, site-specific analysis of the vertebrate subtypes revealed potential residues that may account for the functional divergence between them.

Supramolecular Complexes of β-Cyclodextrin with Clomipramine and Doxepin: Effect of the Ring Substituent and Component of Drugs on Their Inclusion Topologies and Structural Flexibilities

Depression is a global threat. Tricyclic antidepressants (TCAs) are still efficacious in treating depression, albeit with more side effects. Cyclodextrins (CDs) with a suitable nanocavity are potential drug carriers and can enhance the drug bioavailability. Aiming for an atomistic understanding of the CD encapsulation facilitating the improvement of drug stability and the reduction of side effects, a comprehensive study series of the β-CD-TCA inclusion complexes through single crystal X-ray diffraction and density functional theory (DFT) calculation was undertaken. This work reports the supramolecular complexes of β-CD with two pivotal TCAs, clomipramine (CPM; 1) and doxepin (DXP; 2). The different inclusion topologies of the β-CD-TCA complexes were notable. X-ray analysis revealed that, in 1, the CPM B-ring (without chloro group) was entrapped in the β-CD cavity, whereas, in 2, the E-DXP A-ring and the Z-DXP B-ring were disordered in the cavity, yielding energetically favorable complexes primarily maintained by intermolecular C-H⋯π interactions, as indicated by DFT calculation. Because both wings of TCAs were similar, an alternative inclusion scenario of the A-ring was evidenced crystallographically in four other TCA complexes. The enhanced TCA thermodynamic stabilities via CD inclusion complexation helped to reduce the side effects and to increase the bioavailability. Moreover, the scrutinization of six TCAs in different lattice circumstances revealed the greater TCA structural flexibilities for their optimum pharmacological activity while binding with proteins.

Molecular Mechanism for Ligand Recognition and Subtype Selectivity of α2C Adrenergic Receptor

Adrenergic G-protein-coupled receptors (GPCRs) mediate different cellular signaling pathways in the presence of endogenous catecholamines and play important roles in both physiological and pathological conditions. Extensive studies have been carried out to investigate the structure and function of β adrenergic receptors (βARs). However, the structure of α adrenergic receptors (αARs) remains to be determined. Here, we report the structure of the human α_2C adrenergic receptor (α_2CAR) with the non-selective antagonist, RS79948, at 2.8 Å. Our structure, mutations, modeling, and functional experiments indicate that a α_2CAR-specific D206^ECL2-R409^ECL3-Y405^6.58 network plays a role in determining α₂ adrenergic subtype selectivity. Furthermore, our results show that a specific loosened helix at the top of TM4 in α_2CAR is involved in receptor activation. Together, our structure of human α_2CAR-RS79948 provides key insight into the mechanism underlying the α₂ adrenergic receptor activation and subtype selectivity.

A structure-kinetic relationship study using matched molecular pair analysis

The lifetime of a binary drug–target complex is increasingly acknowledged as an important parameter for drug efficacy and safety. With a better understanding of binding kinetics and better knowledge about kinetic parameter optimization, intentionally induced prolongation of the drug–target residence time through structural changes of the ligand could become feasible. In this study we assembled datasets from 21 publications and the K4DD (Kinetic for Drug Discovery) database to conduct large scale data analysis. This resulted in 3812 small molecules annotated to 78 different targets from five protein classes (GPCRs: 273, kinases: 3238, other enzymes: 240, HSPs: 160, ion channels: 45). Performing matched molecular pair (MMP) analysis to further investigate the structure–kinetic relationship (SKR) in this data collection allowed us to identify a fundamental contribution of a ligand's polarity to its association rate, and in selected cases, also to its dissociation rate. However, we furthermore observed that the destabilization of the transition state introduced by increased polarity is often accompanied by simultaneous destabilization of the ground state resulting in an unaffected or even worsened residence time. Supported by a set of case studies, we provide concepts on how to alter ligands in ways to trigger on-rates, off-rates, or both.

A large-scale study employing matched molecular pair (MMP) analysis to uncover the contribution of a compound's polarity to its association and dissociation rates.

Histaminergic neurons in the tuberomammillary nucleus as a control centre for wakefulness

Histamine plays pleiotropic roles as a neurotransmitter in the physiology of brain function, this includes the maintenance of wakefulness, appetite regulation and memory retrieval. Since numerous studies have revealed an association between histaminergic dysfunction and diverse neuropsychiatric disorders, such as Alzheimer's disease and schizophrenia, a large number of compounds acting on the brain histamine system have been developed to treat neurological disorders. In 2016, pitolisant, which was developed as a histamine H₃ receptor inverse agonist by Schwartz and colleagues, was launched for the treatment of narcolepsy, emphasising the prominent role of brain histamine on wakefulness. Recent advances in neuroscientific techniques such as chemogenetic and optogenetic approaches have led to remarkable progress in the understanding of histaminergic neural circuits essential for the control of wakefulness. In this review article, we summarise the basic knowledge about the histaminergic nervous system and the mechanisms underlying sleep/wake regulation that are controlled by the brain histamine system. LINKED ARTICLES: This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.4/issuetoc.

Rational design of new multitarget histamine H3 receptor ligands as potential candidates for treatment of Alzheimer's disease

Design and development of multitarget-directed ligands (MTDLs) has become a very important approach in the search of new therapies for Alzheimer's disease (AD). In our present research, a number of xanthone derivatives were first designed using a pharmacophore model for histamine H₃ receptor (H₃R) antagonists/inverse agonists, and virtual docking was then performed for the enzyme acetylcholinesterase. Next, 23 compounds were synthesised and evaluated in vitro for human H₃R (hH₃R) affinity and inhibitory activity on cholinesterases. Most of the target compounds showed hH₃R affinities in nanomolar range and exhibited cholinesterase inhibitory activity with IC₅₀ values in submicromolar range. Furthermore, the inhibitory effects of monoamine oxidases (MAO) A and B were investigated. The results showed low micromolar and selective human MAO B (hMAO B) inhibition. Two azepane derivatives, namely 23 (2-(5-(azepan-1-yl)pentyloxy)-9H-xanthen-9-one) and 25 (2-(5-(azepan-1-yl)pentyloxy)-7-chloro-9H-xanthen-9-one), were especially very promising and showed high affinity for hH₃R (K_i = 170 nM and 100 nM respectively) and high inhibitory activity for acetylcholinesterase (IC₅₀ = 180 nM and 136 nM respectively). Moreover, these compounds showed moderate inhibitory activity for butyrylcholinesterase (IC₅₀ = 880 nM and 394 nM respectively) and hMAO B (IC₅₀ = 775 nM and 897 nM respectively). Furthermore, molecular docking studies were performed for hH₃R, human cholinesterases and hMAO B to describe the mode of interactions with these biological targets. Next, the two most promising compounds 23 and 25 were selected for in vivo studies. The results showed significant memory-enhancing effect of compound 23 in dizocilpine-induced amnesia in rats in two tests: step-through inhibitory avoidance paradigm (SIAP) and transfer latency paradigm time (TLPT). In addition, favourable analgesic effects of compound 23 were observed in neuropathic pain models. Therefore, compound 23 is a particularly promising structure for further design of new MTDLs for AD.

Benchmarking GPCR homology model template selection in combination with de novo loop generation

G protein-coupled receptors (GPCR) comprise the largest family of membrane proteins and are of considerable interest as targets for drug development. However, many GPCR structures remain unsolved. To address the structural ambiguity of these receptors, computational tools such as homology modeling and loop modeling are often employed to generate predictive receptor structures. Here we combined both methods to benchmark a protocol incorporating homology modeling based on a locally selected template and extracellular loop modeling that additionally evaluates the presence of template ligands during these modeling steps. Ligands were also docked using three docking methods and two pose selection methods to elucidate an optimal ligand pose selection method. Results suggest that local template-based homology models followed by loop modeling produce more accurate and predictive receptor models than models produced without loop modeling, with decreases in average receptor and ligand RMSD of 0.54 Å and 2.91 Å, respectively. Ligand docking results showcased the ability of MOE induced fit docking to produce ligand poses with atom root-mean-square deviation (RMSD) values at least 0.20 Å lower (on average) than the other two methods benchmarked in this study. In addition, pose selection methods (software-based scoring, ligand complementation) selected lower RMSD poses with MOE induced fit docking than either of the other methods (averaging at least 1.57 Å lower), indicating that MOE induced fit docking is most suited for docking into GPCR homology models in our hands. In addition, target receptor models produced with a template ligand present throughout the modeling process most often produced target ligand poses with RMSD values ≤ 4.5 Å and Tanimoto coefficients > 0.6 after selection based on ligand complementation than target receptor models produced in the absence of template ligands. Overall, the findings produced by this study support the use of local template homology modeling in combination with de novo ECL2 modeling in the presence of a ligand from the template crystal structure to generate GPCR models intended to study ligand binding interactions.

Roles of Lys191 and Lys179 in regulating thermodynamic binding forces of ligands to determine their binding affinity for human histamine H1 receptors

Docking simulations based on the crystal structure of human histamine H₁ receptors have predicted crucial roles of Lys191^5.39 and Lys179^ECL2, which exist at the entrance of the ligand-binding pocket, in increasing the H₁-receptor selectivity for carboxylated second-generation antihistamines via electrostatic interaction. In this study, we evaluated the roles of Lys191^5.39 and Lys179^ECL2 in regulating the thermodynamic binding forces of non-carboxylated and carboxylated antihistamines that determine their binding affinity for human H₁ receptors. The binding enthalpy and entropy of the 3 sets of non-carboxylated and corresponding carboxylated antihistamines (doxepin and olopatadine, desloratadine and loratadine, and terfenadine and fexofenadine, respectively) were estimated using the van't Hoff equation with the dissociation constants obtained from the displacement curves of the non-carboxylated and carboxylated antihistamines against the binding of [³H]mepyramine to the membrane preparations of Chinese hamster ovary cells expressing human H₁ receptors at various temperatures, ranging from 4 °C to 37 °C. We found that the affinity for carboxylated antihistamines was lower than that for the corresponding non-carboxylated compounds due to lower enthalpy-dependent electrostatic binding forces and/or entropy-dependent hydrophobic binding forces. Mutations of Lys191^5.39 and/or Lys179^ECL2 to alanine mostly increased the binding affinity for antihistamines due to a variety of changes in both enthalpy- and entropy-dependent binding forces. These results suggest that Lys191^5.39 and Lys179^ECL2 may not contribute to selectively increasing the binding affinity for carboxylated antihistamines via electrostatic interaction, but that they can negatively modulate the binding affinity for non-carboxylated and carboxylated antihistamines non-selectively by affecting their electrostatic as well as hydrophobic binding forces.

Asymmetric Catalysis upon Helically Chiral Loratadine Analogues Unveils Enantiomer-Dependent Antihistamine Activity

Analogues of the conformationally dynamic Claritin (loratadine) and Clarinex (desloratadine) scaffolds have been enantio- and chemoselectively N-oxidized using an aspartic acid containing peptide catalyst to afford stable, helically chiral products in up to >99:1 er. The conformational dynamics and enantiomeric stability of the N-oxide products have been investigated experimentally and computationally with the aid of crystallographic data. Furthermore, biological assays show that rigidifying the core structure of loratadine and related analogues through N-oxidation affects antihistamine activity in an enantiomer-dependent fashion. Computational docking studies illustrate the observed activity differences.

Distinct binding of cetirizine enantiomers to human serum albumin and the human histamine receptor H1

Cetirizine, a major metabolite of hydroxyzine, became a marketed second-generation H₁ antihistamine that is orally active and has a rapid onset of action, long duration of effects and a very good safety record at recommended doses. The approved drug is a racemic mixture of (S)-cetirizine and (R)-cetirizine, the latter being the levorotary enantiomer that also exists in the market as a third-generation, non-sedating and highly selective antihistamine. Both enantiomers bind tightly to the human histamine H₁ receptor (hH₁R) and behave as inverse agonists but the affinity and residence time of (R)-cetirizine are greater than those of (S)-cetirizine. In blood plasma, cetirizine exists in the zwitterionic form and more than 90% of the circulating drug is bound to human serum albumin (HSA), which acts as an inactive reservoir. Independent X-ray crystallographic work has solved the structure of the hH₁R:doxepin complex and has identified two drug-binding sites for cetirizine on equine serum albumin (ESA). Given this background, we decided to model a membrane-embedded hH₁R in complex with either (R)- or (S)-cetirizine and also the complexes of both ESA and HSA with these two enantiomeric drugs to analyze possible differences in binding modes between enantiomers and also among targets. The ensuing molecular dynamics simulations in explicit solvent and additional computational chemistry calculations provided structural and energetic information about all of these complexes that is normally beyond current experimental possibilities. Overall, we found very good agreement between our binding energy estimates and extant biochemical and pharmacological evidence. A much higher degree of solvent exposure in the cetirizine-binding site(s) of HSA and ESA relative to the more occluded orthosteric binding site in hH₁R is translated into larger positional fluctuations and considerably lower affinities for these two nonspecific targets. Whereas it is demonstrated that the two known pockets in ESA provide enough stability for cetirizine binding, only one such site does so in HSA due to a number of amino acid replacements. At the histamine-binding site in hH₁R, the distinct interactions established between the phenyl and chlorophenyl moieties of the two enantiomers with the amino acids lining up the pocket and between their free carboxylates and Lys179 in the second extracellular loop account for the improved pharmacological profile of (R)-cetirizine.

Cannabinoids and Cannabinoid Receptors: The Story so Far

Like most modern molecular biology and natural product chemistry, understanding cannabinoid pharmacology centers around molecular interactions, in this case, between the cannabinoids and their putative targets, the G-protein coupled receptors (GPCRs) cannabinoid receptor 1 (CB₁) and cannabinoid receptor 2 (CB₂). Understanding the complex structure and interplay between the partners in this molecular dance is required to understand the mechanism of action of synthetic, endogenous, and phytochemical cannabinoids. This review, with 91 references, surveys our understanding of the structural biology of the cannabinoids and their target receptors including both a critical comparison of the extant crystal structures and the computationally derived homology models, as well as an in-depth discussion about the binding modes of the major cannabinoids. The aim is to assist in situating structural biochemists, synthetic chemists, and molecular biologists who are new to the field of cannabis research.

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Cannabinoid research has greatly expanded

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Structural biology and computational chemistry jointly provide mechanistic insight

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Structural data are being generated at an exponentially increasing rate

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Phytocannabinoid targeting of other GPCR receptors deserves investigation

Overview of Neurological Mechanism of Pain Profile Used for Animal "Pain-Like" Behavioral Study with Proposed Analgesic Pathways

Pain is the most common sensation installed in us naturally which plays a vital role in defending us against severe harm. This neurological mechanism pathway has been one of the most complex and comprehensive topics but there has never been an elaborate justification of the types of analgesics that used to reduce the pain sensation through which specific pathways. Of course, there have been some answers to curbing of pain which is a lifesaver in numerous situations-chronic and acute pain conditions alike. This has been explored by scientists using pain-like behavioral study methodologies in non-anesthetized animals since decades ago to characterize the analgesic profile such as centrally or peripherally acting drugs and allowing for the development of analgesics. However, widely the methodology is being practiced such as the tail flick/Hargreaves test and Von Frey/Randall-Selitto tests which are stimulus-evoked nociception studies, and there has rarely been a complete review of all these methodologies, their benefits and its downside coupled with the mechanism of the action that is involved. Thus, this review solely focused on the complete protocol that is being adapted in each behavioral study methods induced by different phlogogenic agents, the different assessment methods used for phasic, tonic and inflammatory pain studies and the proposed mechanism of action underlying each behavioral study methodology for analgesic drug profiling. It is our belief that this review could significantly provide a concise idea and improve our scientists' understanding towards pain management in future research.

Understanding Membrane Protein Drug Targets in Computational Perspective

Membrane proteins play crucial physiological roles in vivo and are the major category of drug targets for pharmaceuticals. The research on membrane protein is a significant part in the drug discovery. The biological process is a cycled network, and the membrane protein is a vital hub in the network since most drugs achieve the therapeutic effect via interacting with the membrane protein. In this review, typical membrane protein targets are described, including GPCRs, transporters and ion channels. Also, we conclude network servers and databases that are referring to the drug, drug-target information and their relevant data. Furthermore, we chiefly introduce the development and practice of modern medicines, particularly demonstrating a series of state-of-the-art computational models for the prediction of drug-target interaction containing network-based approach and machine-learningbased approach as well as showing current achievements. Finally, we discuss the prospective orientation of drug repurposing and drug discovery as well as propose some improved framework in bioactivity data, created or improved predicted approaches, alternative understanding approaches of drugs bioactivity and their biological processes.

Elucidation of Inverse Agonist Activity of Bilastine

H1-antihistamines antagonize histamine and prevent it from binding to the histamine H1 receptor (H1R). Some of them also act as inverse agonists, which are more potent than pure antagonists because they suppress the constitutive H1R activity. Bilastine is a non-sedative antihistamine which is one of the most satisfy the requirements for oral antihistamines. However, there is no information to show the inverse agonist activity of bilastine including inositol phosphates accumulation, and its inverse agonist activity is yet to be elucidated. Here we evaluated whether bilastine has inverse agonist activity or not. Intracellular calcium concentration was measured using Fluo-8. Inositol phosphates accumulation was assayed using [3H]myo-inositol. The H1R mRNA level was measured using real-time RT-PCR. At rest, Ca2+ oscillation was observed, indicating that H1R has intrinsic activity. Bilastine attenuated this fluorescence oscillation. Bilastine suppressed the increase in IPs formation in a dose-dependent manner and it was about 80% of the control level at the dose of 3 μM. Bilastine also suppressed histamine-induced increase in IPs formation to the control level. Furthermore, bilastine suppressed basal H1R gene expression in a dose-dependent manner. Data suggest that bilastine is an inverse agonist. Preseasonal prophylactic administration with bilastine could down-regulate basal H1R gene expression in the nasal mucosa and ameliorate the nasal symptoms during the peak pollen period.

Antiedematogenic and Anti-Inflammatory Activity of the Monoterpene Isopulegol and Its β-Cyclodextrin (β-CD) Inclusion Complex in Animal Inflammation Models

Isopulegol (ISO) is an alcoholic monoterpene widely found in different plant species, such as Melissa officinalis, and has already been reported to have a number of pharmacological properties. Like other terpenes, ISO is a highly volatile compound that is slightly soluble in water, so its inclusion into cyclodextrins (CDs) is an interesting approach to increase its solubility and bioavailability. Thus, our aim was to evaluate the antiedematogenic and anti-inflammatory activity of isopulegol and a β-cyclodextrin-isopulegol inclusion complex (ISO/β-CD) in rodent models. For the anti-inflammatory activity evaluation, antiedematogenic plethysmometry and acute (peritonitis and pleurisy), as well as chronic (cotton pellet-induced granuloma) anti-inflammatory models, were used. The docking procedure is used to evaluate, analyze, and predict their binding mode of interaction with H1 and Cox-2 receptors. The animals (n = 6) were divided into groups: ISO and ISO/β-CD, negative control (saline), and positive control (indomethacin and promethazine). ISO and ISO/β-CD were able to reduce acute inflammatory activity by decreasing albumin extravasation, leukocyte migration, and MPO concentration, and reducing exudate levels of IL-1β and TNF-α. ISO and ISO/β-CD significantly inhibited edematogenic activity in carrageenan- and dextran-induced paw edema. Moreover, both significantly reduced chronic inflammatory processes, given the lower weight and protein concentration of granulomas in the foreign body granulomatous inflammation model. The results suggest that the inclusion of ISO in β-cyclodextrins improves its pharmacological properties, with the histamine and prostaglandin pathways as probable mechanisms of inhibition, and also reinforces the anti-inflammatory profile of this terpene.

HMF causes anaphylactic symptoms by acting as a H1 receptor agonist

5-Hydroxymethylfurfural (HMF) can readily form by acid-catalyzed transformations of various sugars such as fructose, sucrose and to a lesser degree glucose, and is known to widely exist in various sugar-containing consumer products. Thus the potential health effect of HMF has been a subject of intensive studies. There have been earlier reports of HMF's undesirable effects at or above high micromolar concentrations. In this study, HMF is found to stimulate the H₁ receptor in vivo and in vitro. When assessed in cell culture and animal models, HMF was found to cause deformation of in cell culture studies of HUVECs at 50 μM, to increase the vascular permeability of paw skin at 1.0 mg/mL, and trigger symptoms of anaphylaxis in animal models at 32.5 μg/kg. At the molecular level, HMF was found to induce the release of NO and related cytokines, and trigger H₁ receptor-mediated inflammatory responses. Mutation studies also suggest the binding sites for HMF on the H₁ receptor. The findings described suggest the need for close monitoring of HMF contents in consumer products and their related side effects.

In Silico Analysis, Synthesis, and Biological Evaluation of Triazole Derivatives as H1 Receptor Antagonist

INTRODUCTION

Histamine, a biological amine, is considered as a principal mediator of many pathological processes regulating several essential events in allergies and autoimmune diseases. Numerous derivatives have been developed that strive with histamine at the H1 receptor and prevent binding of histamine at the H1 receptor, thereby preventing allergic reactions. Molecules containing a triazole ring fused with six-membered ring systems are found to possess broad applications in the field of medicine and industry. The present study is an attempt to characterize the impact of the nature of the substituent introduced at 5 positions of the-4H-1,2,4-triazole-3-thiol on their capacities to bind with the H1 receptor.

METHODS

Molecular docking (PDB ID: 3RZE) revealed that synthesized derivatives and target proteins were actively involved in binding with Tyr-108, Thr-112, Ala-216, and Phe-432 subunits. A pharmacophore model, new 5-(4-substituted phenyl)-4-(phenylamino)-4-H-1,2,4-triazole-3- thiols (5a-5h) were designed and evaluated for H1-blocking activity using isolated segments from the guinea pig ileum.

RESULTS

According to in silico analysis, all the compounds have a topological polar surface area (TPSA) less than 140 Å squared, so they tend to easily penetrate cell membranes. The results show that most of the compounds are non-inhibitors of CYP450 substrates that play a fundamental role in drug metabolism. Compounds 5d (50.53±12.03), 5h (50.62±12.33) and 7a (55.07±12.41) are more active than others.

CONCLUSION

Finally, these derivatives were screened for H1 receptor antagonist activity using guinea pig ileum, taking chlorpheniramine maleate as a standard. Most of the compounds were found to possess better antihistamine activity.

Lipid-Protein Interactions Are a Unique Property and Defining Feature of G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are membrane-bound proteins that depend on their lipid environment to carry out their physiological function. Combined efforts from many theoretical and experimental studies on the lipid-protein interaction profile of several GPCRs hint at an intricate relationship of these receptors with their surrounding membrane environment, with several lipids emerging as particularly important. Using coarse-grained molecular dynamics simulations, we explore the lipid-protein interaction profiles of 28 different GPCRs, spanning different levels of classification and conformational states and totaling to 1 ms of simulation time. We find a close relationship with lipids for all GPCRs simulated, in particular, cholesterol and phosphatidylinositol phosphate (PIP) lipids, but the number, location, and estimated strength of these interactions is dependent on the specific GPCR as well as its conformational state. Although both cholesterol and PIP lipids bind specifically to GPCRs, they utilize distinct mechanisms. Interactions with PIP lipids are mediated by charge-charge interactions with intracellular loop residues and stabilized by one or both of the transmembrane helices linked by the loop. Interactions with cholesterol, on the other hand, are mediated by a hydrophobic environment, usually made up of residues from more than one helix, capable of accommodating its ring structure and stabilized by interactions with aromatic and charged/polar residues. Cholesterol binding to GPCRs occurs in a small number of sites, some of which (like the binding site on the extracellular side of transmembrane 6/7) are shared among many class A GPCRs. Combined with a thorough investigation of the local membrane structure, our results provide a detailed picture of GPCR-lipid interactions. Additionally, we provide an accompanying website to interactively explore the lipid-protein interaction profile of all GPCRs simulated to facilitate analysis and comparison of our data.

Multi targeted cell membrane chromatography: A comprehensive method for screening the anaphylactoid components from complex samples

Anaphylactoid reactions induced by drugs are serious and can be life-threatening, and screening the anaphylactoid ingredients especially in complex samples is challenging. Here, a multi targeted cell membrane chromatography method, based on the Mas-related G protein coupled receptor X2, Fc epsilon RI and H₁ receptors, online coupled with LC-MS system provides a comprehensive solution for screening the anaphylactoid components from complex samples. The validation including selectivity and suitability of this system has been evaluated, and it shows promising results. With optimized conditions, this method has been utilized to screen the anaphylactoid ingredients from Shenmai Injection. Ginsenoside Rb₁, ginsenoside Rb₂, ginsenoside Rc, ginsenoside Rd and 20(S)-ginsenoside Rg3 were identified as anaphylactoid components. The anaphylactoid effects of these five components were further verified by the in vitro sensitization assay, showing promising effects on some or all sensitization cells. In conclusion, the multi targeted cell membrane chromatography online coupled with LC-MS system developed throughout this study could be used to fully screen anaphylactoid components in complex samples. Moreover, it also provides new insights for drug quality control.

Performance of virtual screening against GPCR homology models: Impact of template selection and treatment of binding site plasticity

Rational drug design for G protein-coupled receptors (GPCRs) is limited by the small number of available atomic resolution structures. We assessed the use of homology modeling to predict the structures of two therapeutically relevant GPCRs and strategies to improve the performance of virtual screening against modeled binding sites. Homology models of the D2 dopamine (D2R) and serotonin 5-HT2A receptors (5-HT2AR) were generated based on crystal structures of 16 different GPCRs. Comparison of the homology models to D2R and 5-HT2AR crystal structures showed that accurate predictions could be obtained, but not necessarily using the most closely related template. Assessment of virtual screening performance was based on molecular docking of ligands and decoys. The results demonstrated that several templates and multiple models based on each of these must be evaluated to identify the optimal binding site structure. Models based on aminergic GPCRs showed substantial ligand enrichment and there was a trend toward improved virtual screening performance with increasing binding site accuracy. The best models even yielded ligand enrichment comparable to or better than that of the D2R and 5-HT2AR crystal structures. Methods to consider binding site plasticity were explored to further improve predictions. Molecular docking to ensembles of structures did not outperform the best individual binding site models, but could increase the diversity of hits from virtual screens and be advantageous for GPCR targets with few known ligands. Molecular dynamics refinement resulted in moderate improvements of structural accuracy and the virtual screening performance of snapshots was either comparable to or worse than that of the raw homology models. These results provide guidelines for successful application of structure-based ligand discovery using GPCR homology models.

Expression and purification of recombinant G protein-coupled receptors: A review

Given their extensive role in cell signalling, GPCRs are significant drug targets; despite this, many of these receptors have limited or no available prophylaxis. Novel drug design and discovery significantly rely on structure determination, of which GPCRs are typically elusive. Progress has been made thus far to produce sufficient quantity and quality of protein for downstream analysis. As such, this review highlights the systems available for recombinant GPCR expression, with consideration of their advantages and disadvantages, as well as examples of receptors successfully expressed in these systems. Additionally, an overview is given on the use of detergents and the styrene maleic acid (SMA) co-polymer for membrane solubilisation, as well as purification techniques.

Ethno-medicinal study of Artemisia ordosica Krasch. (traditional Chinese/Mongolian medicine) extracts for the treatment of allergic rhinitis and nasosinusitis

ETHNOPHARMACOLOGICAL RELEVANCE

Artemisia ordosica Krasch. (AOK) has been used for rheumatic arthritis, cold headache, sore throat, etc. in traditional Chinese/Mongolian medicine and is used for nasosinusitis by local Mongolian "barefoot" doctors. Up to now, their mechanisms are still unclear.

AIM

To evaluate the in vivo anti-inflammatory and allergic rhinitis (AR) alleviating effect as well as in vitro antimicrobial activities of AOK extracts to verify its ethno-medicinal claims.

MATERIALS AND METHODS

Crude extracts (methanol/95%-ethanol/ethyl acetate) of AOK root/stem/leaf and fractions (petroleum ether/ethyl acetate/n-butanol/aqueous) of AOK root extract were prepared. Xylene-induced ear swelling model in mouse and ovalbumin (OVA)-induced AR model in guinea pig were established. Ear swelling degrees of mice were measured. The numbers of rubbing movement and sneezes of guinea pigs were counted to evaluate the symptoms of AR. The serum levels of histamine, INF-γ, IL-2/4/10, and VCAM-1 were measured by ELISA assay. The histological changes of nasal mucosa were investigated by light microscope after H&E staining. Antimicrobial activities of AOK extracts were also tested. LC-MS/MS analysis was performed to characterize the constituents of active extract and molecular docking was conducted to predict the biological mechanism.

RESULTS

In ear-swelling model, extract (100.00 mg/kg) from the ethyl acetate layer of 95% ethanol (100.00 mg/kg) showed better swelling inhibition in mice than positive control (dexamethasone, 191.91 mg/kg). In AR model, extract from the ethyl acetate layer of 95% ethanol significantly alleviated the AR symptoms in guinea pigs, decreased the serum levels of histamine, INF-γ, IL-2/4/10, and VCAM-1, and reduced the infiltration of eosinophil in nasal mucosa. For Staphylococcus aureus, the ethyl acetate extract of AOK stem showed the highest inhibition (MIC=1.25 mg/mL), for Escherichia coli, n-butanol layer of 95% ethanol extract of AOK root showed the highest inhibition (MIC=15.00 mg/mL), for Candida glabrata, 95%-ethyl acetate extract of AOK leaf showed the best inhibition (MIC=0.064 mg/mL), while ethyl acetate and n-butanol layers showed similar inhibition on MRSA (MIC=7.50 mg/mL). LC-MS/MS characterization showed that dicaffeoylquinic acids account for more than 30% of ethyl acetate layer of AOK extract. Dicaffeoylquinic acids bind with histamine-1 receptor with high affinities and interesting modes.

CONCLUSIONS

Extracts from AOK had interesting anti-inflammatory activity in mice, alleviating effect against OVA-induced AR in guinea pigs, and antimicrobial activities in vitro, which support the ethno-medicinal use of it. The main constituents in ethyl acetate layer of AOK root extract are dicaffeoylquinic acids and could bind with histamine-1 receptor well. These findings highlighted the importance of natural product chemistry study of AOK.

Precuneus stimulation alters the neural dynamics of autobiographical memory retrieval

Autobiographical memory (AM) unfolds over time, but little is known about the dynamics of its retrieval. Space-based models of memory implicate the hippocampus, retrosplenial cortex, and precuneus in early memory computations. Here we used transcranial magnetic stimulation (TMS) and magnetoencephalography (MEG) to investigate the causal role of the precuneus in the dynamics of AM retrieval. During early memory search and construction, precuneus stimulation compared to vertex stimulation led to delayed evoked neural activity within 1000 ​ms after cue presentation. During later memory elaboration, stimulation led to decreased sustained positivity. We further identified a parietal late positive component during memory elaboration, the amplitude of which was associated with spatial perspective during recollection. This association was disrupted following precuneus stimulation, suggesting that this region plays an important role in the neural representation of spatial perspective during AM. These findings demonstrate a causal role for the precuneus in early AM retrieval, during memory search before a specific memory is accessed, and in spatial context reinstatement during the initial stages of memory elaboration and re-experiencing. By utilizing the high temporal resolution of MEG and the causality of TMS, this study helps clarify the neural correlates of early naturalistic memory retrieval.

Combinatorial allosteric modulation of agonist response in a self-interacting G-protein coupled receptor

The structural plasticity of G-protein coupled receptors (GPCRs) enables the long-range transmission of conformational changes induced by specific orthosteric site ligands and other pleiotropic factors. Here, we demonstrate that the ligand binding cavity in the sphingosine 1-phosphate receptor S1PR1, a class A GPCR, is in allosteric communication with both the β-arrestin-binding C-terminal tail, and a receptor surface involved in oligomerization. We show that S1PR1 oligomers are required for full response to different agonists and ligand-specific association with arrestins, dictating the downstream signalling kinetics. We reveal that the active form of the immunomodulatory drug fingolimod, FTY720-P, selectively harnesses both these intramolecular networks to efficiently recruit β-arrestins in a stable interaction with the receptor, promoting deep S1PR1 internalization and simultaneously abrogating ERK1/2 phosphorylation. Our results define a molecular basis for the efficacy of fingolimod for people with multiple sclerosis, and attest that GPCR signalling can be further fine-tuned by the oligomeric state.

Patrone et al study the mechanism by which fingolimod, a drug used for multiple sclerosis, and agonist to G-coupled receptor S1PR1, compared to the endogenous ligand S1P. They find that whereas S1P binds a S1PR1 dimer, the action of fingolimod is dependent on receptor oligomerisation, which affects β-arrestin binding, internalisation and signaling.

Designing, docking and molecular dynamics simulation studies of novel cloperastine analogues as anti-allergic agents: homology modeling and active site prediction for the human histamine H1 receptor

The present study predicts a three-dimensional model for the histamine H1 receptor and the design of antihistamine inhibitors using cloperastine as the core molecule by docking studies. In this work, we predicted a three-dimensional structure of the histamine H1 receptor using the MODELLER9V7 software. The protein structure was developed based on the crystal structure of the histamine H1 receptor, the lysozyme chimera of Escherichia virus T4 (PDB ID: 3RZE_A) target collected from the PDB data bank. Using molecular dynamics simulation methods, the final predicted structure is obtained and further analyzed by VERIFY3D and PROCHECK programs, confirming that the final model is reliable. The drug derivatives of cloperastine were designed and docking was performed with the designed ligands along with the drug. The predicted model of the histamine H1 receptor structure is stable and confirms that it is a reliable structure for docking studies. The results indicate that MET 183, THR 184 and ILE 187 in the histamine H1 receptor are important determinant residues for binding as they have strong hydrogen bonding with cloperastine derivatives. The drug derivatives were docked to the histamine H1 receptor protein by hydrogen bonding interactions and these interactions played an important role in the binding studies. The molecule 1-{2-[(4-chlorophenyl) (phenyl) methoxy] ethyl}-4-methylenepiperidine showed the best docking results with the histamine H1 receptor. The docking results predicted the best compounds, which may act as better drugs than cloperastine and in the future, these may be developed for anti-allergy therapy.

The present study predicts a three-dimensional model for the histamine H1 receptor and the design of antihistamine inhibitors using cloperastine as the core molecule by docking studies.

Helix 8 is the essential structural motif of mechanosensitive GPCRs

G-protein coupled receptors (GPCRs) are versatile cellular sensors for chemical stimuli, but also serve as mechanosensors involved in various (patho)physiological settings like vascular regulation, cardiac hypertrophy and preeclampsia. However, the molecular mechanisms underlying mechanically induced GPCR activation have remained elusive. Here we show that mechanosensitive histamine H₁ receptors (H₁Rs) are endothelial sensors of fluid shear stress and contribute to flow-induced vasodilation. At the molecular level, we observe that H₁Rs undergo stimulus-specific patterns of conformational changes suggesting that mechanical forces and agonists induce distinct active receptor conformations. GPCRs lacking C-terminal helix 8 (H8) are not mechanosensitive, and transfer of H8 to non-responsive GPCRs confers, while removal of H8 precludes, mechanosensitivity. Moreover, disrupting H8 structural integrity by amino acid exchanges impairs mechanosensitivity. Altogether, H8 is the essential structural motif endowing GPCRs with mechanosensitivity. These findings provide a mechanistic basis for a better understanding of the roles of mechanosensitive GPCRs in (patho)physiology.

GPCRs are versatile cellular sensors for chemical stimuli but the molecular mechanisms underlying mechanically induced GPCR activation have remained elusive. Here authors identify the C-terminal helix 8 (H8) as the essential structural motif endowing H₁R and other GPCRs with mechanosensitivity.

4-(3-Aminoazetidin-1-yl)pyrimidin-2-amines as High-Affinity Non-imidazole Histamine H3 Receptor Agonists with in Vivo Central Nervous System Activity

Despite the high diversity of histamine H3 receptor (H3R) antagonist/inverse agonist structures, partial or full H3R agonists have typically been imidazole derivatives. An in-house screening campaign intriguingly afforded the non-imidazole 4-(3-azetidin-1-yl)pyrimidin-2-amine 11b as a partial H3R agonist. Here, the design, synthesis, and structure-activity relationships of 11b analogues are described. This series yields several non-imidazole full agonists with potencies varying with the alkyl substitution pattern on the basic amine following the in vitro evaluation of H3R agonism using a cyclic adenosine monophosphate response element-luciferase reporter gene assay. The key compound VUF16839 (14d) combines nanomolar on-target activity (pKi = 8.5, pEC50 = 9.5) with weak activity on cytochrome P450 enzymes and good metabolic stability. The proposed H3R binding mode of 14d indicates key interactions similar to those attained by histamine. In vivo evaluation of 14d in a social recognition test in mice revealed an amnesic effect at 5 mg/kg intraperitoneally. The excellent in vitro and in vivo pharmacological profiles and the non-imidazole structure of 14d make it a promising tool compound in H3R research.

Covalent Inhibition of the Histamine H3 Receptor

Covalent binding of G protein-coupled receptors by small molecules is a useful approach for better understanding of the structure and function of these proteins. We designed, synthesized and characterized a series of 6 potential covalent ligands for the histamine H₃ receptor (H₃R). Starting from a 2-amino-pyrimidine scaffold, optimization of anchor moiety and warhead followed by fine-tuning of the required reactivity via scaffold hopping resulted in the isothiocyanate H₃R ligand 44. It shows high reactivity toward glutathione combined with appropriate stability in water and reacts selectively with the cysteine sidechain in a model nonapeptide equipped with nucleophilic residues. The covalent interaction of 44 with H₃R was validated with washout experiments and leads to inverse agonism on H₃R. Irreversible binder 44 (VUF15662) may serve as a useful tool compound to stabilize the inactive H₃R conformation and to study the consequences of prolonged inhibition of the H₃R.

Crystal structure of dopamine receptor D4 bound to the subtype selective ligand, L745870

Multiple subtypes of dopamine receptors within the GPCR superfamily regulate neurological processes through various downstream signaling pathways. A crucial question about the dopamine receptor family is what structural features determine the subtype-selectivity of potential drugs. Here, we report the 3.5-angstrom crystal structure of mouse dopamine receptor D4 (DRD4) complexed with a subtype-selective antagonist, L745870. Our structure reveals a secondary binding pocket extended from the orthosteric ligand-binding pocket to a DRD4-specific crevice located between transmembrane helices 2 and 3. Additional mutagenesis studies suggest that the antagonist L745870 prevents DRD4 activation by blocking the relative movement between transmembrane helices 2 and 3. These results expand our knowledge of the molecular basis for the physiological functions of DRD4 and assist new drug design.

Basal Histamine H4 Receptor Activation: Agonist Mimicry by the Diphenylalanine Motif

Histamine H4 receptor (H4 R) orthologues are G-protein-coupled receptors (GPCRs) that exhibit species-dependent basal activity. In contrast to the basally inactive mouse H4 R (mH4 R), human H4 R (hH4 R) shows a high degree of basal activity. We have performed long-timescale molecular dynamics simulations and rigidity analyses on wild-type hH4 R, the experimentally characterized hH4 R variants S179M, F169V, F169V+S179M, F168A, and on mH4 R to investigate the molecular nature of the differential basal activity. H4 R variant-dependent differences between essential motifs of GPCR activation and structural stabilities correlate with experimentally determined basal activities and provide a molecular explanation for the differences in basal activation. Strikingly, during the MD simulations, F16945.55 dips into the orthosteric binding pocket only in the case of hH4 R, thus adopting the role of an agonist and contributing to the stabilization of the active state. The results shed new light on the molecular mechanism of basal H4 R activation that are of importance for other GPCRs.

Agonist binding of human mu opioid receptors expressed in the yeast Pichia pastoris: Effect of cholesterol complementation

This study compared pharmacological profiles between human mu opioid receptors (hMOR) overexpressed in the SH-SY5Y neuroblastoma cell line (SH-hMOR) and the methylotrophic yeast Pichia pastoris (Pp-hMOR). Affinity determinations were performed by direct binding with the tritiated agonist DAMGO and antagonist diprenorphine (DIP). Additionally, displacement of these drugs with agonists (morphine and DAMGO) and antagonists (β-funaltrexamine, naloxone and diprenorphine) was examined. Tritiated DAMGO could bind to membranes prepared from Pp-hMOR, although the receptor was not coupled with G-proteins. The data obtained with this yeast strain suggested that only 7.5% of receptors were in a high-affinity-state conformation. This value was markedly less than that estimated in SH-hMOR membranes, which reached 50%. Finally, to understand the pharmacological discrepancies between Pp-hMOR and SH-hMOR, the role of sterols was evaluated. The major sterol in P. pastoris is ergosterol, while hMOR naturally functions in a cholesterol-containing membrane environment. Cell membranes were sterol-depleted or cholesterol-loaded with methyl-β-cyclodextrine. The results indicated that cholesterol must be present to ensure Pp-hMOR function. The proportion of high-affinity-state conformation was reversibly increased by cholesterol complementation.

Harnessing Ion-Binding Sites for GPCR Pharmacology

Endogenous ions play important roles in the function and pharmacology of G-protein coupled receptors (GPCRs). Historically the evidence for ionic modulation of GPCR function dates to 1973 with studies of opioid receptors, where it was demonstrated that physiologic concentrations of sodium allosterically attenuated agonist binding. This Na+-selective effect was distinct from effects of other monovalent and divalent cations, with the latter usually counteracting sodium's negative allosteric modulation of binding. Since then, numerous studies documenting the effects of mono- and divalent ions on GPCR function have been published. While ions can act selectively and nonselectively at many sites in different receptors, the discovery of the conserved sodium ion site in class A GPCR structures in 2012 revealed the unique nature of Na+ site, which has emerged as a near-universal site for allosteric modulation of class A GPCR structure and function. In this review, we synthesize and highlight recent advances in the functional, biophysical, and structural characterization of ions bound to GPCRs. Taken together, these findings provide a molecular understanding of the unique roles of Na+ and other ions as GPCR allosteric modulators. We will also discuss how this knowledge can be applied to the redesign of receptors and ligand probes for desired functional and pharmacological profiles. SIGNIFICANCE STATEMENT: The function and pharmacology of GPCRs strongly depend on the presence of mono and divalent ions in experimental assays and in living organisms. Recent insights into the molecular mechanism of this ion-dependent allosterism from structural, biophysical, biochemical, and computational studies provide quantitative understandings of the pharmacological effects of drugs in vitro and in vivo and open new avenues for the rational design of chemical probes and drug candidates with improved properties.

Human H1 receptor (HRH1) gene polymorphism is associated with the severity of side effects after desloratadine treatment in Chinese patients with chronic spontaneous uticaria

H1 nonsedating antihistamines, such as desloratadine, are first-line treatment options for chronic spontaneous urticaria (CSU). However, desloratadine induces various degrees of sedation side effect in CSU patients, and no biomarkers currently exist for predicting the severity of such side effect. Herein, we evaluated the association between HRH1 gene rs901865 polymorphism and the severity of sedation side effect following desloratadine therapy in patients with CSU. We found that 20 of the 114 patients (17.50%) showed sedation side effect after desloratadine treatment, and 3 patients (2.63%) experienced serious sleepiness. The frequency of HRH1 rs901865 G allele was significantly higher in patients who experienced sedation than in patients with rs901865 A allele (p = 0.0009). Moreover, patients with the rs901865 G/G genotype suffered a more serious sedation side effect than patients with the rs901865 G/A genotype (p = 0.005). These results provide evidence that the HRH1 rs901865 G/G polymorphism is associated with severe sedation side effect after desloratadine treatment. Thus, the HRH1 rs901865 allele may potentially be used as a biomarker for predicting the severity of sedation side effect in patients suffering from CSU and treated with desloratadine.

High-throughput identification of peptide agonists against GPCRs by co-culture of mammalian reporter cells and peptide-secreting yeast cells using droplet microfluidics

Since G-protein coupled receptors (GPCRs) are linked to various diseases, screening of functional ligands against GPCRs is vital for drug discovery. In the present study, we developed a high-throughput functional cell-based assay by combining human culture cells producing a GPCR, yeast cells secreting randomized peptide ligands, and a droplet microfluidic device. We constructed a reporter human cell line that emits fluorescence in response to the activation of human glucagon-like peptide-1 receptor (hGLP1R). We then constructed a yeast library secreting an agonist of hGLP1R or randomized peptide ligands. We demonstrated that high-throughput identification of functional ligands against hGLP1R could be performed by co-culturing the reporter cells and the yeast cells in droplets. We identified functional ligands, one of which had higher activity than that of an original sequence. The result suggests that our system could facilitate the discovery of functional peptide ligands of GPCRs.

Fexofenadine inhibits TNF signaling through targeting to cytosolic phospholipase A2 and is therapeutic against inflammatory arthritis

OBJECTIVE

Tumour necrosis factor alpha (TNF-α) signalling plays a central role in the pathogenesis of various autoimmune diseases, particularly inflammatory arthritis. This study aimed to repurpose clinically approved drugs as potential inhibitors of TNF-α signalling in treatment of inflammatory arthritis.

METHODS

In vitro and in vivo screening of an Food and Drug Administration (FDA)-approved drug library; in vitro and in vivo assays for examining the blockade of TNF actions by fexofenadine: assays for defining the anti-inflammatory activity of fexofenadine using TNF-α transgenic (TNF-tg) mice and collagen-induced arthritis in DBA/1 mice. Identification and characterisation of the binding of fexofenadine to cytosolic phospholipase A2 (cPLA2) using drug affinity responsive target stability assay, proteomics, cellular thermal shift assay, information field dynamics and molecular dynamics; various assays for examining fexofenadine inhibition of cPLA2 as well as the dependence of fexofenadine's anti-TNF activity on cPLA2.

RESULTS

Serial screenings of a library composed of FDA-approved drugs led to the identification of fexofenadine as an inhibitor of TNF-α signalling. Fexofenadine potently inhibited TNF/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-ĸB) signalling in vitro and in vivo, and ameliorated disease symptoms in inflammatory arthritis models. cPLA2 was isolated as a novel target of fexofenadine. Fexofenadine blocked TNF-stimulated cPLA2 activity and arachidonic acid production through binding to catalytic domain 2 of cPLA2 and inhibition of its phosphorylation on Ser-505. Further, deletion of cPLA2 abolished fexofenadine's anti-TNF activity.

CONCLUSION

Collectively, these findings not only provide new insights into the understanding of fexofenadine action and underlying mechanisms but also provide new therapeutic interventions for various TNF-α and cPLA2-associated pathologies and conditions, particularly inflammatory rheumatic diseases.

Directed Evolution of P450 BM3 towards Functionalization of Aromatic O-Heterocycles

The O-heterocycles, benzo-1,4-dioxane, phthalan, isochroman, 2,3-dihydrobenzofuran, benzofuran, and dibenzofuran are important building blocks with considerable medical application for the production of pharmaceuticals. Cytochrome P450 monooxygenase (P450) Bacillus megaterium 3 (BM3) wild type (WT) from Bacillus megaterium has low to no conversion of the six O-heterocycles. Screening of in-house libraries for active variants yielded P450 BM3 CM1 (R255P/P329H), which was subjected to directed evolution and site saturation mutagenesis of four positions. The latter led to the identification of position R255, which when introduced in the P450 BM3 WT, outperformed all other variants. The initial oxidation rate of nicotinamide adenine dinucleotide phosphate (NADPH) consumption increased ≈140-fold (WT: 8.3 ± 1.3 min-1; R255L: 1168 ± 163 min-1), total turnover number (TTN) increased ≈21-fold (WT: 40 ± 3; R255L: 860 ± 15), and coupling efficiency, ≈2.9-fold (WT: 8.8 ± 0.1%; R255L: 25.7 ± 1.0%). Computational analysis showed that substitution R255L (distant from the heme-cofactor) does not have the salt bridge formed with D217 in WT, which introduces flexibility into the I-helix and leads to a heme rearrangement allowing for efficient hydroxylation.

Route to Prolonged Residence Time at the Histamine H1 Receptor: Growing from Desloratadine to Rupatadine

Drug–target binding kinetics are an important predictor of in vivo drug efficacy, yet the relationship between ligand structures and their binding kinetics is often poorly understood. We show that both rupatadine (1) and desloratadine (2) have a long residence time at the histamine H₁ receptor (H₁R). Through development of a [³H]levocetirizine radiolabel, we find that the residence time of 1 exceeds that of 2 more than 10-fold. This was further explored with 22 synthesized rupatadine and desloratadine analogues. Methylene-linked cycloaliphatic or β-branched substitutions of desloratadine increase the residence time at the H₁R, conveying a longer duration of receptor antagonism. However, cycloaliphatic substituents directly attached to the piperidine amine (i.e., lacking the spacer) have decreased binding affinity and residence time compared to their methylene-linked structural analogues. Guided by docking studies, steric constraints within the binding pocket are hypothesized to explain the observed differences in affinity and binding kinetics between analogues.

Molecular pharmacology of metabotropic receptors targeted by neuropsychiatric drugs

Metabotropic receptors are responsible for so-called 'slow synaptic transmission' and mediate the effects of hundreds of peptide and non-peptide neurotransmitters and neuromodulators. Over the past decade or so, a revolution in membrane-protein structural determination has clarified the molecular determinants responsible for the actions of these receptors. This Review focuses on the G protein-coupled receptors (GPCRs) that are targets of neuropsychiatric drugs and shows how insights into the structure and function of these important synaptic proteins are accelerating understanding of their actions. Notably, elucidating the structure and function of GPCRs should enhance the structure-guided discovery of novel chemical tools with which to manipulate and understand these synaptic proteins.

Metabolites of Vinca Alkaloid Vinblastine: Tubulin Binding and Activation of Nausea-Associated Receptors

Vinblastine (VLB) is an antimitotic drug that binds to the vinca site of tubulin. The molecule possesses a high molecular weight and a complex chemical structure with many possibilities of metabolization. Despite advances in drug discovery research in reducing drug toxicity, the cause and mechanism of VLB-induced adverse drug reactions (ADRs) remains poorly understood. VLB is metabolized to at least 35 known metabolites, which have been identified and collected in this present work. This study also explores how VLB metabolites affect nausea-associated receptors such as muscarinic, dopaminergic, and histaminic. The metabolites have stronger binding interactions than acetylcholine (ACh) for muscarinic M₁, M₄, and M₅ receptors and demonstrate similar binding profiles to that of the natural substrate, ACh. The affinities of VLB metabolites to dopaminergic and histaminic receptors, their absorption, distribution, metabolism, excretion, toxicity properties, and the superiority of VLB to ACh for binding to M₅R, indicate their potential to trigger activation of nausea-associated receptors during chemotherapy with VLB. It has been shown that metabolite 20-hydroxy-VLB (metabolite 10) demonstrates a stronger binding affinity to the vinca site of tubulin than VLB; however, they have similar modes of action. VLB and metabolite 10 have similar gastric solubility (FaSSGF), intestinal solubility (FeSSIF), and log P values. Metabolite 10 has a more acceptable pharmacokinetic profile than VLB, a better gastric and intestinal solubility. Furthermore, metabolite 10 was found to be less bound to plasma proteins than VLB. These are desired and essential features for effective drug bioavailability. Metabolite 10 is not a substrate of CYP2D6 and thus is less likely to cause drug-drug interactions and ADRs compared to its parent drug. The hydroxyl group added upon metabolism of VLB suggests that it can also be a reasonable starting compound for designing the next generation of antimitotic drugs to overcome P-glycoprotein-mediated multidrug resistance, which is often observed with vinca alkaloids.

Ligand-guided homology modeling drives identification of novel histamine H3 receptor ligands

In this study, we report a ligand-guided homology modeling approach allowing the analysis of relevant binding site residue conformations and the identification of two novel histamine H3 receptor ligands with binding affinity in the nanomolar range. The newly developed method is based on exploiting an essential charge interaction characteristic for aminergic G-protein coupled receptors for ranking 3D receptor models appropriate for the discovery of novel compounds through virtual screening.

Structural modifications and in vitro pharmacological evaluation of 4-pyridyl-piperazine derivatives as an active and selective histamine H3 receptor ligands

A novel series of 4-pyridylpiperazine derivatives with varying alkyl linker length and eastern part substituents proved to be potent histamine H₃ receptor (hH₃R) ligands in the nanomolar concentration range. While paying attention to their alkyl linker length, derivatives with a six methylene linker tend to be more potent than their five methylene homologues. Moreover, in the case of both phenoxyacetyl- and phenoxypropionyl- derivatives, an eight methylene linkers possess lower activity than their seven methylene homologues. However, in global analysis of collected data on the influence of alkyl linker length, a three methylene homologues appeared to be of highest hH₃R affinity among all described 4-pyridylpiperazine derivatives from our group up to date. In the case of biphenyl and benzophenone derivatives, compounds with para- substituted second aromatic ring were of higher affinity than their meta analogues. Interestingly, benzophenone derivative 18 showed the highest affinity among all tested compounds (hH₃R K_i = 3.12 nM). The likely protein-ligand interactions, responsible for their high affinity were demonstrated using molecular modeling techniques. Furthermore, selectivity, intrinsic activity at H₃R, as well as drug-like properties of selected ligands were evaluated using in vitro methods.

Probe dependency in the determination of ligand binding kinetics at a prototypical G protein-coupled receptor

Drug-target binding kinetics are suggested to be important parameters for the prediction of in vivo drug-efficacy. For G protein-coupled receptors (GPCRs), the binding kinetics of ligands are typically determined using association binding experiments in competition with radiolabelled probes, followed by analysis with the widely used competitive binding kinetics theory developed by Motulsky and Mahan. Despite this, the influence of the radioligand binding kinetics on the kinetic parameters derived for the ligands tested is often overlooked. To address this, binding rate constants for a series of histamine H₁ receptor (H₁R) antagonists were determined using radioligands with either slow (low k_off) or fast (high k_off) dissociation characteristics. A correlation was observed between the probe-specific datasets for the kinetic binding affinities, association rate constants and dissociation rate constants. However, the magnitude and accuracy of the binding rate constant-values was highly dependent on the used radioligand probe. Further analysis using recently developed fluorescent binding methods corroborates the finding that the Motulsky-Mahan methodology is limited by the employed assay conditions. The presented data suggest that kinetic parameters of GPCR ligands depend largely on the characteristics of the probe used and results should therefore be viewed within the experimental context and limitations of the applied methodology.

Phenothiazine antipsychotics exhibit dual properties in pseudo-allergic reactions: Activating MRGPRX2 and inhibiting the H1 receptor

Phenothiazines are a class of antipsychotics that share the same tricyclic structure and are widely used in clinical settings. Adverse reactions from these drugs, however, have been regularly reported, with allergic skin reactions noted in some cases. Nevertheless, the mechanisms underlying anaphylaxis by these drugs have not been described. In the present study, we found that phenothiazine antipsychotics increased calcium mobilization and activated mast cells to release β-hexosaminidase, histamine, and tumor necrosis factor-α via Mas-related G-protein-coupled receptor member X2 (MRGPRX2) in vitro. In addition, they induced histamine release in serum via Mrgprb2 in C57BL/6 mice without Evans blue extravasation or paw swell. Further experiments indicated these drugs had good interaction with the histamine H₁ receptor (H1R) and show an anti-calcium mobilization effect on H₁R-HEK293 cells, which confirmed a potential antagonist effect of these drugs on the H₁R. The molecular docking and activity experiments indicated that the N-methyl substitution on the side chain of these drugs played a significant role in activating MRGPRX2, while the phenothiazine tricyclic ring was associated with the inhibiting effect on the H₁R. Therefore, due to their dual properties of increasing histamine levels without obvious allergic symptoms, clinicians should be highly vigilant for damage from histamine accumulation and long-term inflammatory reactions during the clinical use of phenothiazine antipsychotics.

A Metadynamics-Based Protocol for the Determination of GPCR-Ligand Binding Modes

G protein-coupled receptors (GPCRs) are a main drug target and therefore a hot topic in pharmaceutical research. One important prerequisite to understand how a certain ligand affects a GPCR is precise knowledge about its binding mode and the specific underlying interactions. If no crystal structure of the respective complex is available, computational methods can be used to deduce the binding site. One of them are metadynamics simulations which have the advantage of an enhanced sampling compared to conventional molecular dynamics simulations. However, the enhanced sampling of higher-energy states hampers identification of the preferred binding mode. Here, we present a novel protocol based on clustering of multiple walker metadynamics simulations which allows identifying the preferential binding mode from such conformational ensembles. We tested this strategy for three different model systems namely the histamine H₁ receptor in combination with its physiological ligand histamine, as well as the β2 adrenoceptor with its agonist adrenaline and its antagonist alprenolol. For all three systems, the proposed protocol was able to reproduce the correct binding mode known from the literature suggesting that the approach can more generally be applied to the prediction of GPCR ligand binding in future.