Identification of conserved aromatic residues essential for agonist binding and second messenger production at 5-hydroxytryptamine2A receptors

Synthesis and Structure-Activity Relationships of 2,5-Dimethoxy-4-Substituted Phenethylamines and the Discovery of CYB210010: A Potent, Orally Bioavailable and Long-Acting Serotonin 5-HT2 Receptor Agonist

Structure-activity studies of 4-substituted-2,5-dimethoxyphenethylamines led to the discovery of 2,5-dimethoxy-4-thiotrifluoromethylphenethylamines, including CYB210010, a potent and long-acting serotonin 5-HT2 receptor agonist. CYB210010 exhibited high agonist potency at 5-HT2A and 5-HT2C receptors, modest selectivity over 5-HT2B, 5-HT1A, 5-HT6, and adrenergic α2A receptors, and lacked activity at monoamine transporters and over 70 other proteins. CYB210010 (0.1-3 mg/kg) elicited a head-twitch response (HTR) and could be administered subchronically at threshold doses without behavioral tolerance. CYB210010 was orally bioavailable in three species, readily and preferentially crossed into the CNS, engaged frontal cortex 5-HT2A receptors, and increased the expression of genes involved in neuroplasticity in the frontal cortex. CYB210010 represents a new tool molecule for investigating the therapeutic potential of 5-HT2 receptor activation. In addition, several other compounds with high 5-HT2A receptor potency, yet with little or no HTR activity, were discovered, providing the groundwork for the development of nonpsychedelic 5-HT2A receptor ligands.

The molecular basis of the antidepressant action of the magic mushroom extract, psilocin

Magic mushrooms, and their extract psilocybin, are well-known for their psychedelic properties and recreational use. Psilocin, the bio-active form of psilocybin, can potentially treat various psychiatric diseases. Psilocin putatively exerts its psychedelic effect as an agonist to the serotonin 2A receptor (5-HT2AR), which is also the receptor for the neurological hormone serotonin. The two key chemical differences between the two molecules are first, that the primary amine in serotonin is replaced with a tertiary amine in psilocin, and second, the hydroxyl group is substituted differently on the aromatic ring. Here, we find that psilocin can bind to 5-HT2AR with an affinity higher than serotonin, and provide the molecular logic behind the higher binding affinity of psilocin using extensive molecular dynamics simulations and free energy calculations. The binding free energy of psilocin is dependent upon the protonation states of the ligands, as well as that of the key residue in the binding site: Aspartate 155. We find that the tertiary amine of psilocin, and not the altered substitution of the hydroxyl group in the ring is responsible for the increased affinity of psilocin. We propose design rules for effective antidepressants based on molecular insights from our simulations.

Structural basis for recognition of antihistamine drug by human histamine receptor

The histamine receptors belong to the G protein-coupled receptor (GPCR) superfamily, and play important roles in the regulation of histamine and other neurotransmitters in the central nervous system, as potential targets for the treatment of neurologic and psychiatric disorders. Here we report the crystal structure of human histamine receptor H₃R bound to an antagonist PF-03654746 at 2.6 Å resolution. Combined with the computational and functional assays, our structure reveals binding modes of the antagonist and allosteric cholesterol. Molecular dynamic simulations and molecular docking of different antihistamines further elucidate the conserved ligand-binding modes. These findings are therefore expected to facilitate the structure-based design of novel antihistamines.

In vitro assays for the functional characterization of (psychedelic) substances at the serotonin receptor 5-HT2A R

Serotonergic psychedelics are substances that induce alterations in mood, perception, and thought, and have the activation of serotonin (5-HT) 2A receptors (5-HT_2A Rs) as a main pharmacological mechanism. Besides their appearance on the (illicit) drug market, e.g. as new psychoactive substances, their potential therapeutic application is increasingly explored. This group of substances demonstrates a broad structural variety, leading to insufficiently described structure-activity relationships, hence illustrating the need for better functional characterization. This review therefore elaborates on the in vitro molecular techniques that have been used the most abundantly for the characterization of (psychedelic) 5-HT_2A R agonists. More specifically, this review covers assays to monitor the canonical G protein signaling pathway (e.g. measuring G protein recruitment/activation, inositol phosphate accumulation, or Ca²⁺ mobilization), assays to monitor non-canonical G protein signaling (such as arachidonic acid release), assays to monitor β-arrestin recruitment or signaling, and assays to monitor receptor conformational changes. In particular, focus lies on the mechanism behind the techniques, and the specific advantages and challenges that are associated with these. Additionally, several variables are discussed that one should consider when attempting to compare functional outcomes from different studies, both linked to the specific assay mechanism and linked to its specific execution, as these may heavily impact the assay outcome.

Molecular insights into psychedelic drug action

A confluence of factors has renewed interest in the scientific understanding and translational potential of psychedelic drugs such as lysergic acid diethylamide (LSD), mescaline, and psilocybin: the desire for additional approaches to mental health care, incremental progress in basic and clinical research, and the reconsideration and relaxation of existing drug policies. With the United States Food and Drug Administration's designation of psilocybin as a "Breakthrough Therapy" for treatment-resistant depression, a new path has been forged for the conveyance of psychedelics to the clinic. Essential to the further development of such applications, however, is a clearer understanding of how these drugs exert their effects at the molecular level. Here we review the current knowledge regarding the molecular details of psychedelic drug actions and suggest that these discoveries can facilitate new insights into their hallucinogenic and therapeutic mechanisms.

Convergent olfactory trace amine-associated receptors detect biogenic polyamines with distinct motifs via a conserved binding site

Biogenic amines activate G-protein-coupled receptors (GPCRs) in the central nervous system in vertebrate animals. Several biogenic amines, when excreted, stimulate trace amine-associated receptors (TAARs), a group of GPCRs in the main olfactory epithelium, and elicit innate behaviors. How TAARs recognize amines with varying numbers of amino groups is largely unknown. We reasoned that a comparison between lamprey and mammalian olfactory TAARs, which are thought to have evolved independently but show convergent responses to polyamines, may reveal structural determinants of amine recognition. Here, we demonstrate that sea lamprey TAAR365 (sTAAR365) responds strongly to biogenic polyamines cadaverine, putrescine, and spermine, and shares a similar response profile as a mammalian TAAR (mTAAR9). Docking and site-directed mutagenesis analyses show that both sTAAR365 and mTAAR9 recognize the two amino groups of cadaverine with the conserved Asp^3.32 and Tyr^6.51 residues. sTAAR365, which has remarkable sensitivity for cadaverine (EC₅₀ = 4 nM), uses an extra residue, Thr^7.42, to stabilize ligand binding. These cadaverine recognition sites also interact with amines with four and three amino groups (spermine and spermidine, respectively). Glu^7.36 of sTAAR365 cooperates with Asp^3.32 and Thr^7.42 to recognize spermine, whereas mTAAR9 recognizes spermidine through an additional aromatic residue, Tyr^7.43. These results suggest a conserved mechanism whereby independently evolved TAAR receptors recognize amines with two, three, or four amino groups using the same recognition sites, at which sTAAR365 and mTAAR9 evolved distinct motifs. These motifs interact directly with the amino groups of the polyamines, a class of potent and ecologically important odorants, mediating olfactory signaling.

Structure of a Hallucinogen-Activated Gq-Coupled 5-HT2A Serotonin Receptor

Hallucinogens like lysergic acid diethylamide (LSD), psilocybin, and substituted N-benzyl phenylalkylamines are widely used recreationally with psilocybin being considered as a therapeutic for many neuropsychiatric disorders including depression, anxiety, and substance abuse. How psychedelics mediate their actions-both therapeutic and hallucinogenic-are not understood, although activation of the 5-HT_2A serotonin receptor (HTR2A) is key. To gain molecular insights into psychedelic actions, we determined the active-state structure of HTR2A bound to 25-CN-NBOH-a prototypical hallucinogen-in complex with an engineered Gαq heterotrimer by cryoelectron microscopy (cryo-EM). We also obtained the X-ray crystal structures of HTR2A complexed with the arrestin-biased ligand LSD or the inverse agonist methiothepin. Comparisons of these structures reveal determinants responsible for HTR2A-Gαq protein interactions as well as the conformational rearrangements involved in active-state transitions. Given the potential therapeutic actions of hallucinogens, these findings could accelerate the discovery of more selective drugs for the treatment of a variety of neuropsychiatric disorders.

Computational Methods for the Identification of Molecular Targets of Toxic Food Additives. Butylated Hydroxytoluene as a Case Study

Butylated hydroxytoluene (BHT) is one of the most commonly used synthetic antioxidants in food, cosmetic, pharmaceutical and petrochemical products. BHT is considered safe for human health; however, its widespread use together with the potential toxicological effects have increased consumers concern about the use of this synthetic food additive. In addition, the estimated daily intake of BHT has been demonstrated to exceed the recommended acceptable threshold. In the present work, using BHT as a case study, the usefulness of computational techniques, such as reverse screening and molecular docking, in identifying protein-ligand interactions of food additives at the bases of their toxicological effects has been probed. The computational methods here employed have been useful for the identification of several potential unknown targets of BHT, suggesting a possible explanation for its toxic effects. In silico analyses can be employed to identify new macromolecular targets of synthetic food additives and to explore their functional mechanisms or side effects. Noteworthy, this could be important for the cases in which there is an evident lack of experimental studies, as is the case for BHT.

Revised Pharmacophore Model for 5-HT2A Receptor Antagonists Derived from the Atypical Antipsychotic Agent Risperidone

Pharmacophore models for 5-HT_2A receptor antagonists consist of two aromatic/hydrophobic regions at a given distance from a basic amine. We have previously shown that both aromatic/hydrophobic moieties are unnecessary for binding or antagonist action. Here, we deconstructed the 5-HT_2A receptor antagonist/serotonin-dopamine antipsychotic agent risperidone into smaller structural segments that were tested for 5-HT_2A receptor affinity and function. We show, again, that the entire risperidone structure is unnecessary for retention of affinity or antagonist action. Replacement of the 6-fluoro-3-(4-piperidinyl)-1,2-benz[ d]isoxazole moiety by isosteric tryptamines resulted in retention of affinity and antagonist action. Additionally, 3-(4-piperidinyl)-1,2-benz[ d]isoxazole (10), which represents less than half the structural features of risperidone, retains both affinity and antagonist actions. 5-HT_2A receptor homology modeling/docking studies suggest that 10 binds in a manner similar to risperidone and that there is a large cavity to accept various N₄-substituted analogues of 10 such as risperidone and related agents. Alterations of this "extended" moiety improve receptor binding and functional potency. We propose a new risperidone-based pharmacophore for 5-HT_2A receptor antagonist action.

Aminergic GPCR-Ligand Interactions: A Chemical and Structural Map of Receptor Mutation Data

The aminergic family of G protein-coupled receptors (GPCRs) plays an important role in various diseases and represents a major drug discovery target class. Structure determination of all major aminergic subfamilies has enabled structure-based ligand design for these receptors. Site-directed mutagenesis data provides an invaluable complementary source of information for elucidating the structural determinants of binding of different ligand chemotypes. The current study provides a comparative analysis of 6692 mutation data points on 34 aminergic GPCR subtypes, covering the chemical space of 540 unique ligands from mutagenesis experiments and information from experimentally determined structures of 52 distinct aminergic receptor-ligand complexes. The integrated analysis enables detailed investigation of structural receptor-ligand interactions and assessment of the transferability of combined binding mode and mutation data across ligand chemotypes and receptor subtypes. An overview is provided of the possibilities and limitations of using mutation data to guide the design of novel aminergic receptor ligands.

Synthesis and evaluation of nuciferine and roemerine enantiomers as 5-HT2 and α1 receptor antagonists

In this study, the (S)-enantiomers of the aporphine alkaloids, nuciferine and roemerine, were prepared via a synthetic route involving catalytic asymmetric hydrogenation and both stereoisomers were evaluated in vitro for functional activity at human 5-HT2 and adrenergic α1 receptor subtypes using a transforming growth factor-α shedding assay. Both enantiomers of each of the compounds were found to act as antagonists at 5-HT2 and α1 receptors. (R)-roemerine was the most potent compound at 5-HT2A and 5-HT2C receptors (pKb = 7.8-7.9) with good selectivity compared to (S)-roemerine at these two receptors and compared to its activity at 5-HT2B, α1A, α1B and α1D receptors.

Pharmacology and Toxicology of N-Benzylphenethylamine ("NBOMe") Hallucinogens

Serotonergic hallucinogens induce profound changes in perception and cognition. The characteristic effects of hallucinogens are mediated by 5-HT2A receptor activation. One class of hallucinogens are 2,5-dimethoxy-substituted phenethylamines, such as the so-called 2C-X compounds 2,5-dimethoxy-4-bromophenethylamine (2C-B) and 2,5-dimethoxy-4-iodophenethylamine (2C-I). Addition of an N-benzyl group to phenethylamine hallucinogens produces a marked increase in 5-HT2A-binding affinity and hallucinogenic potency. N-benzylphenethylamines ("NBOMes") such as N-(2-methoxybenzyl)-2,5-dimethoxy-4-iodophenethylamine (25I-NBOMe) show subnanomolar affinity for the 5-HT2A receptor and are reportedly highly potent in humans. Several NBOMEs have been available from online vendors since 2010, resulting in numerous cases of toxicity and multiple fatalities. This chapter reviews the structure-activity relationships, behavioral pharmacology, metabolism, and toxicity of members of the NBOMe hallucinogen class. Based on a review of 51 cases of NBOMe toxicity reported in the literature, it appears that rhabdomyolysis is a relatively common complication of severe NBOMe toxicity, an effect that may be linked to NBOMe-induced seizures, hyperthermia, and vasoconstriction.

Activation is Hallucinogenic and Antagonism is Therapeutic: Role of 5-HT2A Receptors in Atypical Antipsychotic Drug Actions

This review summarizes recent studies with 5-hydroxytryptamine2A (5-HT2A) receptors, which represent the major site of action of hallucinogens and a likely site for atypical antipsychotic drug actions. We present evidence demonstrating that atypical antipsychotic drugs, as a group, have a preferentially high affinity for 5-HT2A receptors, compared with their affinities for other neurotransmitter receptors. The 5-HT2A receptor blockade seen with atypical antipsychotic drugs is probably an essential factor in explaining many of the unique features of atypical antipsychotic drugs. Atypical antipsychotic drugs have high affinities for several other 5-HT receptors (5-HT2C, 5-HT6, and 5-HT7), and the potential role of these novel 5-HT receptors in atypical antipsychotic drug action is also summarized.

Psychedelics

Psychedelics (serotonergic hallucinogens) are powerful psychoactive substances that alter perception and mood and affect numerous cognitive processes. They are generally considered physiologically safe and do not lead to dependence or addiction. Their origin predates written history, and they were employed by early cultures in many sociocultural and ritual contexts. After the virtually contemporaneous discovery of (5R,8R)-(+)-lysergic acid-N,N-diethylamide (LSD)-25 and the identification of serotonin in the brain, early research focused intensively on the possibility that LSD and other psychedelics had a serotonergic basis for their action. Today there is a consensus that psychedelics are agonists or partial agonists at brain serotonin 5-hydroxytryptamine 2A receptors, with particular importance on those expressed on apical dendrites of neocortical pyramidal cells in layer V. Several useful rodent models have been developed over the years to help unravel the neurochemical correlates of serotonin 5-hydroxytryptamine 2A receptor activation in the brain, and a variety of imaging techniques have been employed to identify key brain areas that are directly affected by psychedelics. Recent and exciting developments in the field have occurred in clinical research, where several double-blind placebo-controlled phase 2 studies of psilocybin-assisted psychotherapy in patients with cancer-related psychosocial distress have demonstrated unprecedented positive relief of anxiety and depression. Two small pilot studies of psilocybin-assisted psychotherapy also have shown positive benefit in treating both alcohol and nicotine addiction. Recently, blood oxygen level-dependent functional magnetic resonance imaging and magnetoencephalography have been employed for in vivo brain imaging in humans after administration of a psychedelic, and results indicate that intravenously administered psilocybin and LSD produce decreases in oscillatory power in areas of the brain's default mode network.

Similarities between the Binding Sites of SB-206553 at Serotonin Type 2 and Alpha7 Acetylcholine Nicotinic Receptors: Rationale for Its Polypharmacological Profile

Evidence from systems biology indicates that promiscuous drugs, i.e. those that act simultaneously at various protein targets, are clinically better in terms of efficacy, than those that act in a more selective fashion. This has generated a new trend in drug development called polypharmacology. However, the rational design of promiscuous compounds is a difficult task, particularly when the drugs are aimed to act at receptors with diverse structure, function and endogenous ligand. In the present work, using docking and molecular dynamics methodologies, we established the most probable binding sites of SB-206553, a drug originally described as a competitive antagonist of serotonin type 2B/2C metabotropic receptors (5-HT2B/2CRs) and more recently as a positive allosteric modulator of the ionotropic α7 nicotinic acetylcholine receptor (nAChR). To this end, we employed the crystal structures of the 5-HT2BR and acetylcholine binding protein as templates to build homology models of the 5-HT2CR and α7 nAChR, respectively. Then, using a statistical algorithm, the similarity between these binding sites was determined. Our analysis showed that the most plausible binding sites for SB-206553 at 5-HT2Rs and α7 nAChR are remarkably similar, both in size and chemical nature of the amino acid residues lining these pockets, thus providing a rationale to explain its affinity towards both receptor types. Finally, using a computational tool for multiple binding site alignment, we determined a consensus binding site, which should be useful for the rational design of novel compounds acting simultaneously at these two types of highly different protein targets.

Discovery of novel potent and selective ligands for 5-HT2A receptor with quinazoline scaffold

A series of compounds with quinazoline scaffold were designed, synthesized and evaluated as novel potent 5-HT2A receptor ligands. N-(4-Chlorophenyl)-2-(piperazin-1-yl)quinazolin-4-amine (5o) has a Ki value of 14.04 ± 0.21 nM, with a selectivity more than 10,000 fold over 5-HT1A receptors (D1 and D2-like receptors). The functional assay showed that this compound is an antagonist to 5-HT2A receptor with an IC50 value of 1.66 μM.

In Silico and In Vitro Analysis of Bacoside A Aglycones and Its Derivatives as the Constituents Responsible for the Cognitive Effects of Bacopa monnieri

Bacopa monnieri has been used in Ayurvedic medicine to improve memory and cognition. The active constituent responsible for its pharmacological effects is bacoside A, a mixture of dammarane-type triterpenoid saponins containing sugar chains linked to a steroid aglycone skeleton. Triterpenoid saponins have been reported to be transformed in vivo to metabolites that give better biological activity and pharmacokinetic characteristics. Thus, the activities of the parent compounds (bacosides), aglycones (jujubogenin and pseudojujubogenin) and their derivatives (ebelin lactone and bacogenin A1) were compared using a combination of in silico and in vitro screening methods. The compounds were docked into 5-HT1A, 5-HT2A, D1, D2, M1 receptors and acetylcholinesterase (AChE) using AutoDock and their central nervous system (CNS) drug-like properties were determined using Discovery Studio molecular properties and ADMET descriptors. The compounds were screened in vitro using radioligand receptor binding and AChE inhibition assays. In silico studies showed that the parent bacosides were not able to dock into the chosen CNS targets and had poor molecular properties as a CNS drug. In contrast, the aglycones and their derivatives showed better binding affinity and good CNS drug-like properties, were well absorbed through the intestines and had good blood brain barrier (BBB) penetration. Among the compounds tested in vitro, ebelin lactone showed binding affinity towards M1 (Ki = 0.45 μM) and 5-HT2A (4.21 μM) receptors. Bacoside A and bacopaside X (9.06 μM) showed binding affinity towards the D1 receptor. None of the compounds showed any inhibitory activity against AChE. Since the stimulation of M1 and 5-HT2A receptors has been implicated in memory and cognition and ebelin lactone was shown to have the strongest binding energy, highest BBB penetration and binding affinity towards M1 and 5-HT2A receptors, we suggest that B. monnieri constituents may be transformed in vivo to the active form before exerting their pharmacological activity.

Agonist binding by the β2-adrenergic receptor: an effect of receptor conformation on ligand association-dissociation characteristics

The β₂-adrenergic receptor (β₂-AR), a G protein-coupled receptor (GPCR), is a physiologically important transmembrane protein that is a target for drugs used for treatment of asthma and cardiovascular diseases. Study of the first steps of ligand recognition and the molecular basis of ligand binding to the orthosteric site is essential for understanding the pharmacological properties of the receptor. In this work we investigated the characteristic features of the agonist association–dissociation process to and from the different conformational forms of β₂-AR by use of advanced molecular modeling techniques. The investigation was focused on estimating the free energy profiles (FEPs) corresponding to the process of a full agonist ((R,R)-fenoterol) and an inverse agonist (carazolol) binding and unbinding to and from β₂-AR. The two different conformational forms of β₂-AR, i.e. active β₂-AR–PDB: 3P0G and inactive β₂-AR–PDB: 2RH1 were included in this stage of the study. We revealed several significant qualitative differences between FEPs characteristic of both conformational forms. Both FEPs suggest the existence of three transient binding sites in the extracellular domain of β₂-AR. Comparison of the residues surrounding these transient binding sites in both β₂-AR states revealed the importance of the aromatic residues F194, H93^2.64, H296^6.58, and H178 (extracellular part of β₂-AR) in the early stages of the binding process. In addition, slightly different exit and entry paths are preferred by the ligand molecule in the extracellular part of β₂-AR, depending on the conformation of the receptor.

Structure and function of serotonin G protein-coupled receptors

Serotonin receptors are prevalent throughout the nervous system and the periphery, and remain one of the most lucrative and promising drug discovery targets for disorders ranging from migraine headaches to neuropsychiatric disorders such as schizophrenia and depression. There are 14 distinct serotonin receptors, of which 13 are G protein-coupled receptors (GPCRs), which are targets for approximately 40% of the approved medicines. Recent crystallographic and biochemical evidence has provided a converging understanding of the basic structure and functional mechanics of GPCR activation. Currently, two GPCR crystal structures exist for the serotonin family, the 5-HT1B and 5-HT2B receptor, with the antimigraine and valvulopathic drug ergotamine bound. The first serotonin crystal structures not only provide the first evidence of serotonin receptor topography but also provide mechanistic explanations into functional selectivity or biased agonism. This review will detail the findings of these crystal structures from a molecular and mutagenesis perspective for driving rational drug design for novel therapeutics incorporating biased signaling.

Pharmacological and signalling properties of a D2-like dopamine receptor (Dop3) in Tribolium castaneum

Dopamine is an important neurotransmitter in the central nervous system of vertebrates and invertebrates. Despite their evolutionary distance, striking parallels exist between deuterostomian and protostomian dopaminergic systems. In both, signalling is achieved via a complement of functionally distinct dopamine receptors. In this study, we investigated the sequence, pharmacology and tissue distribution of a D2-like dopamine receptor from the red flour beetle Tribolium castaneum (TricaDop3) and compared it with related G protein-coupled receptors in other invertebrate species. The TricaDop3 receptor-encoding cDNA shows considerable sequence similarity with members of the Dop3 receptor class. Real time qRT-PCR showed high expression in both the central brain and the optic lobes, consistent with the role of dopamine as neurotransmitter. Activation of TricaDop3 expressed in mammalian cells increased intracellular Ca(2+) signalling and decreased NKH-477 (a forskolin analogue)-stimulated cyclic AMP levels in a dose-dependent manner. We studied the pharmacological profile of the TricaDop3 receptor and demonstrated that the synthetic vertebrate dopamine receptor agonists, 2 - amino- 6,7 - dihydroxy - 1,2,3,4 - tetrahydronaphthalene hydrobromide (6,7-ADTN) and bromocriptine acted as agonists. Methysergide was the most potent of the antagonists tested and showed competitive inhibition in the presence of dopamine. This study offers important information on the Dop3 receptor from Tribolium castaneum that will facilitate functional analyses of dopamine receptors in insects and other invertebrates.

Aromatic interactions impact ligand binding and function at serotonin 5-HT2C G protein-coupled receptors: Receptor homology modeling, ligand docking, and molecular dynamics results validated by experimental studies

The serotonin (5-hydroxytryptamine, 5-HT) 5-HT2 G protein-coupled receptor (GPCR) family consists of types 2A, 2B, and 2C that share ~75% transmembrane (TM) sequence identity. Agonists for 5-HT2C receptors are under development for psychoses, whereas, at 5-HT2A receptors, antipsychotic effects are associated with antagonists-in fact, 5-HT2A agonists can cause hallucinations and 5-HT2B agonists cause cardiotoxicity. It is known that 5-HT2A TM6 residues W6.48, F6.51, and F6.52 impact ligand binding and function, however, ligand interactions with these residues at the 5-HT2C receptor has not been reported. To predict and validate molecular determinants for 5-HT2C-specific activation, results from receptor homology modeling, ligand docking, and molecular dynamics (MD) simulation studies were compared with experimental results for ligand binding and function at wild type and W6.48A, F6.51A, and F6.52A point-mutated 5-HT2C receptors.

Molecular interactions between general anesthetics and the 5HT2B receptor

BACKGROUND

Serotonin modulates many processes through a family of seven serotonin receptors. However, no studies have screened for interactions between general anesthetics currently in clinical use and serotonergic G-protein-coupled receptors (GPCRs). Given that both intravenous and inhalational anesthetics have been shown to target other classes of GPCRs, we hypothesized that general anesthetics might interact directly with some serotonin receptors and thus modify their function.

METHODS

Radioligand binding assays were performed to screen serotonin receptors for interactions with propofol and isoflurane as well as for affinity determinations. Docking calculations using the crystal structure of 5-HT2B were performed to computationally confirm the binding assay results and locate anesthetic binding sites.

RESULTS

The 5-HT2B class of receptors interacted significantly with both propofol and isoflurane in the primary screen. The affinities for isoflurane and propofol were determined to be 7.78 and .95 μM, respectively, which were at or below the clinical concentrations for both anesthetics. The estimated free energy derived from docking calculations for propofol (-6.70 kcal/mol) and isoflurane (-5.10 kcal/mol) correlated with affinities from the binding assay. The anesthetics were predicted to dock at a pharmacologically relevant binding site of 5HT2B.

CONCLUSIONS

The molecular interactions between propofol and isoflurane with the 5-HT2B class of receptors were discovered and characterized. This finding implicates the serotonergic GPCRs as potential anesthetic targets.

Pharmacological characterization of a 5-HT1-type serotonin receptor in the red flour beetle, Tribolium castaneum

Serotonin (5-hydroxytryptamine, 5-HT) is known for its key role in modulating diverse physiological processes and behaviors by binding various 5-HT receptors. However, a lack of pharmacological knowledge impedes studies on invertebrate 5-HT receptors. Moreover, pharmacological information is urgently needed in order to establish a reliable classification system for invertebrate 5-HT receptors. In this study we report on the molecular cloning and pharmacological characterization of a 5-HT1 receptor from the red flour beetle, Tribolium castaneum (Trica5-HT1). The Trica5-HT1 receptor encoding cDNA shows considerable sequence similarity with members of the 5-HT1 receptor class. Real time PCR showed high expression in the brain (without optic lobes) and the optic lobes, consistent with the role of 5-HT as neurotransmitter. Activation of Trica5-HT1 in mammalian cells decreased NKH-477-stimulated cyclic AMP levels in a dose-dependent manner, but did not influence intracellular Ca(2+) signaling. We studied the pharmacological profile of the 5-HT1 receptor and demonstrated that α-methylserotonin, 5-methoxytryptamine and 5-carboxamidotryptamine acted as agonists. Prazosin, methiothepin and methysergide were the most potent antagonists and showed competitive inhibition in presence of 5-HT. This study offers important information on a 5-HT1 receptor from T. castaneum facilitating functional research of 5-HT receptors in insects and other invertebrates. The pharmacological profiles may contribute to establish a reliable classification scheme for invertebrate 5-HT receptors.

Homology models of melatonin receptors: challenges and recent advances

Melatonin exerts many of its actions through the activation of two G protein-coupled receptors (GPCRs), named MT1 and MT2. So far, a number of different MT1 and MT2 receptor homology models, built either from the prototypic structure of rhodopsin or from recently solved X-ray structures of druggable GPCRs, have been proposed. These receptor models differ in the binding modes hypothesized for melatonin and melatonergic ligands, with distinct patterns of ligand-receptor interactions and putative bioactive conformations of ligands. The receptor models will be described, and they will be discussed in light of the available information from mutagenesis experiments and ligand-based pharmacophore models. The ability of these ligand-receptor complexes to rationalize structure-activity relationships of known series of melatonergic compounds will be commented upon.

Life beyond kinases: structure-based discovery of sorafenib as nanomolar antagonist of 5-HT receptors

Of great interest in recent years has been computationally predicting the novel polypharmacology of drug molecules. Here, we applied an "induced-fit" protocol to improve the homology models of 5-HT(2A) receptor, and we assessed the quality of these models in retrospective virtual screening. Subsequently, we computationally screened the FDA approved drug molecules against the best induced-fit 5-HT(2A) models and chose six top scoring hits for experimental assays. Surprisingly, one well-known kinase inhibitor, sorafenib, has shown unexpected promiscuous 5-HTRs binding affinities, K(i) = 1959, 56, and 417 nM against 5-HT(2A), 5-HT(2B), and 5-HT(2C), respectively. Our preliminary SAR exploration supports the predicted binding mode and further suggests sorafenib to be a novel lead compound for 5HTR ligand discovery. Although it has been well-known that sorafenib produces anticancer effects through targeting multiple kinases, carefully designed experimental studies are desirable to fully understand whether its "off-target" 5-HTR binding activities contribute to its therapeutic efficacy or otherwise undesirable side effects.

Ligand-dependent conformations and dynamics of the serotonin 5-HT(2A) receptor determine its activation and membrane-driven oligomerization properties

From computational simulations of a serotonin 2A receptor (5-HT_2AR) model complexed with pharmacologically and structurally diverse ligands we identify different conformational states and dynamics adopted by the receptor bound to the full agonist 5-HT, the partial agonist LSD, and the inverse agonist Ketanserin. The results from the unbiased all-atom molecular dynamics (MD) simulations show that the three ligands affect differently the known GPCR activation elements including the toggle switch at W6.48, the changes in the ionic lock between E6.30 and R3.50 of the DRY motif in TM3, and the dynamics of the NPxxY motif in TM7. The computational results uncover a sequence of steps connecting these experimentally-identified elements of GPCR activation. The differences among the properties of the receptor molecule interacting with the ligands correlate with their distinct pharmacological properties. Combining these results with quantitative analysis of membrane deformation obtained with our new method (Mondal et al, Biophysical Journal 2011), we show that distinct conformational rearrangements produced by the three ligands also elicit different responses in the surrounding membrane. The differential reorganization of the receptor environment is reflected in (i)-the involvement of cholesterol in the activation of the 5-HT_2AR, and (ii)-different extents and patterns of membrane deformations. These findings are discussed in the context of their likely functional consequences and a predicted mechanism of ligand-specific GPCR oligomerization.

The 5-HT_2A receptor for the neurotransmitter serotonin (5-HT) belongs to family A (rhodopsin-like) G-protein coupled receptors (GPCRs), one of the most important classes of membrane proteins that are targeted by an extensive and diverse collection of external stimuli. Recently we learned that different ligands targeting the same GPCR can elicit different biological responses, but the mechanisms remain unknown. We address this fundamental question for the serotonin 5-HT_2A receptor, because it is known to respond to the binding of structurally diverse ligands by producing similar stimuli in the cell, and to the binding of quite similar ligands with dramatically different responses. Molecular dynamics simulations of molecular models of the serotonin 5-HT_2A receptor in complex with pharmacologically distinct ligands show the dynamic rearrangements of the receptor molecule to be different for these ligands, and the nature and extents of the rearrangements reflect the known pharmacological properties of the ligands as full, partial or inverse activators of the receptor. The different rearrangements of the receptor molecule are shown to produce different rearrangements of the surrounding membrane, a remodeling of the environment that can have differential ligand-determined effects on receptor function and association in the cell's membrane.

Decoding the signaling of a GPCR heteromeric complex reveals a unifying mechanism of action of antipsychotic drugs

Atypical antipsychotic drugs, such as clozapine and risperidone, have a high affinity for the serotonin 5-HT(2A) G protein-coupled receptor (GPCR), the 2AR, which signals via a G(q) heterotrimeric G protein. The closely related non-antipsychotic drugs, such as ritanserin and methysergide, also block 2AR function, but they lack comparable neuropsychological effects. Why some but not all 2AR inhibitors exhibit antipsychotic properties remains unresolved. We now show that a heteromeric complex between the 2AR and the G(i)-linked GPCR, metabotropic glutamate 2 receptor (mGluR2), integrates ligand input, modulating signaling output and behavioral changes. Serotonergic and glutamatergic drugs bind the mGluR2/2AR heterocomplex, which then balances Gi- and Gq-dependent signaling. We find that the mGluR2/2AR-mediated changes in Gi and Gq activity predict the psychoactive behavioral effects of a variety of pharmocological compounds. These observations provide mechanistic insight into antipsychotic action that may advance therapeutic strategies for disorders including schizophrenia and dementia.

Molecular basis of ligand dissociation in β-adrenergic receptors

The important and diverse biological functions of β-adrenergic receptors (βARs) have promoted the search for compounds to stimulate or inhibit their activity. In this regard, unraveling the molecular basis of ligand binding/unbinding events is essential to understand the pharmacological properties of these G protein-coupled receptors. In this study, we use the steered molecular dynamics simulation method to describe, in atomic detail, the unbinding process of two inverse agonists, which have been recently co-crystallized with β₁ and β₂ARs subtypes, along four different channels. Our results indicate that this type of compounds likely accesses the orthosteric binding site of βARs from the extracellular water environment. Importantly, reconstruction of forces and energies from the simulations of the dissociation process suggests, for the first time, the presence of secondary binding sites located in the extracellular loops 2 and 3 and transmembrane helix 7, where ligands are transiently retained by electrostatic and Van der Waals interactions. Comparison of the residues that form these new transient allosteric binding sites in both βARs subtypes reveals the importance of non-conserved electrostatic interactions as well as conserved aromatic contacts in the early steps of the binding process.

Molecular dynamics simulations and docking studies on 3D models of the heterodimeric and homodimeric 5-HT2A receptor subtype

Background: G-protein coupled receptors may exist as functional homodimers, heterodimers and even as higher aggregates. In this work, we investigate the 5-HT2A receptor, which is a known target for antipsychotic drugs. Recently, 5-HT2A has been shown to form functional homodimers and heterodimers with the mGluR2 receptor. The objective of this study is to build up 3D models of the 5-HT2A/mGluR2 heterodimer and of the 5-HT2A-5-HT2A homodimer, and to evaluate the impact of the dimerization interface on the shape of the 5-HT2A binding pocket by using molecular dynamics simulations and docking studies. Results and discussion: The heterodimer, homodimer and monomeric 5-HT2A receptors were simulated by molecular dynamics for 40 ns each. The trajectories were clustered and representative structures of six clusters for each system were generated. Inspection of the these representative structures clearly indicate an effect of the dimerization interface on the topology of the binding pocket. Docking studies allowed to generate receiver operating characteristic curves for a set of 5-HT2A ligands, indicating that different complexes prefer different classes of 5-HT2A ligands. Conclusion: This study clearly indicates that the presence of a dimerization interface must explicitly be considered when studying G-protein coupled receptors known to exist as dimers. Molecular dynamics simulation and cluster analysis are appropriate tools to study the phenomenon.

Theoretical studies on the interaction of partial agonists with the 5-HT2A receptor

A series of 51 5-HT(2A) partial agonistic arylethylamines (primary or benzylamines) from different structural classes (indoles, methoxybenzenes, quinazolinediones) was investigated by fragment regression analysis (FRA), docking and 3D-QSAR approaches. The data, pEC(50) values and intrinsic activities (E(max)) on rat arteries, show high variability of pEC(50) from 4 to 10 and of E(max) from 15 to 70%. FRA indicates which substructures affect potency or intrinsic activity. The high contribution of halogens in para position of phenethylamines to pEC(50) points to a specific hydrophobic pocket. Other results suggest the significance of hydrogen bonds of the aryl moiety for activation and the contrary effect of benzyl groups on affinity (increasing) and intrinsic activity (decreasing). Results from fragment regression and data on all available mutants were considered to derive a common binding site at the rat 5-HT(2A) receptor. After generation and MD simulations of a receptor model based on the β(2)-adrenoceptor structure, typical derivatives were docked, leading to the suggestion of common interactions, e.g., with serines in TM3 and TM5 and with a cluster of aromatic amino acids in TM5 and TM6. The whole series was aligned by docking and minimization of the complexes. The pEC(50) values correlate well with Sybyl docking energies and hydrophobicity of the aryl moieties. With this alignment, CoMFA and CoMSIA approaches based on a training set of 36 and a test set of 15 compounds were performed. The correlation of pEC(50) with steric, electrostatic, hydrophobic and H-bond acceptor fields resulted in sufficient fit (q (2): 0.75-0.8, r (2): 0.92-0.95) and predictive power (r (pred) (2) : 0.85-0.88). The important interaction regions largely reflect the patterns provided by the putative binding site. In particular, the fit of the aryl moieties and benzyl substituents to two hydrophobic pockets is evident.

By interacting with the C-terminal Phe of apelin, Phe255 and Trp259 in helix VI of the apelin receptor are critical for internalization

Apelin is the endogenous ligand of the orphan seven-transmembrane domain (TM) G protein-coupled receptor APJ. Apelin is involved in the regulation of body fluid homeostasis and cardiovascular functions. We previously showed the importance of the C-terminal Phe of apelin 17 (K17F) in the hypotensive activity of this peptide. Here, we show either by deleting the Phe residue (K16P) or by substituting it by an Ala (K17A), that it plays a crucial role in apelin receptor internalization but not in apelin binding or in Gα(i)-protein coupling. Then we built a homology three-dimensional model of the human apelin receptor using the cholecystokinin receptor-1 model as a template, and we subsequently docked K17F into the binding site. We visualized a hydrophobic cavity at the bottom of the binding pocket in which the C-terminal Phe of K17F was embedded by Trp(152) in TMIV and Trp(259) and Phe(255) in TMVI. Using molecular modeling and site-directed mutagenesis studies, we further showed that Phe(255) and Trp(259) are key residues in triggering receptor internalization without playing a role in apelin binding or in Gα(i)-protein coupling. These findings bring new insights into apelin receptor activation and show that Phe(255) and Trp(259), by interacting with the C-terminal Phe of the pyroglutamyl form of apelin 13 (pE13F) or K17F, are crucial for apelin receptor internalization.

A pivot between helices V and VI near the retinal-binding site is necessary for activation in rhodopsins

Rhodopsins are photoreceptor proteins that have diverged from ligand-binding G protein-coupled receptors (GPCRs). Unlike other GPCRs, rhodopsins contain an intrinsic antagonist, 11-cis-retinal, which is converted to the agonist all-trans-retinal upon absorption of a photon. Through evolution, vertebrate rhodopsins have lost the ability of direct binding to the agonist, but some invertebrate and vertebrate non-visual rhodopsins have retained this ability. Here, we investigated the difference in the agonist-binding state between these rhodopsins to further our understanding of the structural and functional diversity of rhodopsins. Mutational analyses of agonist-binding rhodopsin showed that replacement of Ala-269, one of the residues constituting the antagonist-binding site, with bulky amino acids resulted in a large spectral shift in its active state and a great reduction in G protein activity, whereas these were rescued by subsequent replacement of Phe-208 with smaller amino acids. Although similar replacements in vertebrate rhodopsin did not cause a spectral shift in the active state, a similar reduction in and recovery of G protein activity was observed. Therefore, the agonist is located close to Ala-269 in the agonist-binding rhodopsin, but not in vertebrate rhodopsins, and Ala-269 with Phe-208 acts as a pivot for the formation of the G protein-activating state in both rhodopsins. The positions corresponding to Ala-269 and Phe-208 in other GPCRs have been reported to form part of an agonist-binding site. Therefore, an agonist-binding rhodopsin has the molecular architecture of the agonist-binding site similar to that of a general GPCR, whereas vertebrate rhodopsins changed the architecture, resulting in loss of agonist binding during molecular evolution.

Position 5.46 of the serotonin 5-HT2A receptor contributes to a species-dependent variation for the 5-HT2C agonist (R)-9-ethyl-1,3,4,10b-tetrahydro-7-trifluoromethylpyrazino[2,1-a]isoindol-6(2H)-one: impact on selectivity and toxicological evaluation

Successful development of 5-HT(2C) agonists requires selectivity versus the highly homologous 5-HT(2A) receptor, because agonism at this receptor can result in significant adverse events. (R)-9-Ethyl-1,3,4,10b-tetrahydro-7-trifluoromethylpyrazino[2,1-a]isoindol-6(2H)-one (compound 1) is a potent 5-HT(2C) agonist exhibiting selectivity over the human 5-HT(2A) receptor. Evaluation of the compound at the rat 5-HT(2A) receptor, however, revealed potent binding and agonist functional activity. The physiological consequence of this higher potency was the observation of a significant increase in blood pressure in conscious telemeterized rats that could be prevented by ketanserin. Docking of compound 1 in a homology model of the 5-HT(2A) receptor indicated a possible binding mode in which the ethyl group at the 9-position of the molecule was oriented toward position 5.46 of the 5-HT(2A) receptor. Within the human 5-HT(2A) receptor, position 5.46 is Ser242; however, in the rat 5-HT(2A) receptor, it is Ala242, suggesting that the potent functional activity in this species resulted from the absence of the steric bulk provided by the -OH moiety of the Ser in the human isoform. We confirmed this hypothesis using site-directed mutagenesis through the mutation of both the human receptor Ser242 to Ala and the rat receptor Ala242 to Ser, followed by radioligand binding and second messenger studies. In addition, we attempted to define the space allowed by the alanine by evaluating compounds with larger substitutions at the 9-position. The data indicate that position 5.46 contributed to the species difference in 5-HT(2A) receptor potency observed for a pyrazinoisoindolone compound, resulting in the observation of a significant cardiovascular safety signal.

Synthesis, structure-affinity relationships, and modeling of AMDA analogs at 5-HT2A and H1 receptors: structural factors contributing to selectivity

Histamine H(1) and serotonin 5-HT(2A) receptors present in the CNS have been implicated in various neuropsychiatric disorders. 9-Aminomethyl-9,10-dihydroanthracene (AMDA), a conformationally constrained diarylalkyl amine derivative, has affinity for both of these receptors. A structure-affinity relationship (SAFIR) study was carried out studying the effects of N-methylation, varying the linker chain length and constraint of the aromatic rings on the binding affinities of the compounds with the 5-HT(2A) and H(1) receptors. Homology modeling of the 5-HT(2A) and H(1) receptors suggests that AMDA and its analogs, the parent of which is a 5-HT(2A) antagonist, can bind in a fashion analogous to that of classical H(1) antagonists whose ring systems are oriented toward the fifth and sixth transmembrane helices. The modeled orientation of the ligands are consistent with the reported site-directed mutagenesis data for 5-HT(2A) and H(1) receptors and provide a potential explanation for the selectivity of ligands acting at both receptors.

Molecular dynamics simulation of the heterodimeric mGluR2/5HT(2A) complex. An atomistic resolution study of a potential new target in psychiatric conditions

Homo- and heterodimerization is becoming an assessed concept in G-protein coupled receptor (GPCR) pharmacology, and the notion that GPCRs may dimerize or oligomerize is allowing for a reinterpretation of some inconsistencies or anomalies and is providing medicinal chemists with potentially relevant novel molecular targets for a variety of therapeutic conditions. Recently, it has been reported that two unrelated GPCRs, namely class C metabotropic glutamate receptor type-2 (mGluR2) and class A 5HT(2A) serotoninergic receptor, can heterodimerize at the transmembrane domain level. We performed a 40 ns molecular dynamics simulation of the mGluR2/5HT(2A) heterocomplex constructed around a TM4/TM5 interface and embedded in an explicit phospholipidic bilayer surrounded by water molecules. In a separate experiment, the monomeric 5HT(2A) receptor was simulated for additional 40 ns under the same conditions. The analysis and the comparison of the two simulations allowed us to clearly identify a cross-talk between the two protomers and to put forward an effect of the heterodimerization on the shape of the binding pocket of 5HT(2A). This result provides the first molecular explanation for the reported allosteric effect of mGluR2 on 5HT(2A)-mediated response and suggests that the heterocomplex can be a more suitable target for in silico screening than the monomeric protomers.

Three distinct amine receptors operating at different levels within the locomotory circuit are each essential for the serotonergic modulation of chemosensation in Caenorhabditis elegans

Serotonin modulates behavioral plasticity in both vertebrates and invertebrates and in Caenorhabditis elegans regulates key behaviors, including locomotion, aversive learning and olfaction through at least four different 5-HT receptors. In the present study, we examined the serotonergic stimulation of aversive responses to dilute octanol in animals containing null alleles of these 5-HT receptors. Both ser-1 and mod-1 null animals failed to increase sensitivity to dilute octanol on food/5-HT, in contrast to wild-type, ser-4 or ser-7 null animals. 5-HT sensitivity was restored by the expression of MOD-1 and SER-1 in the AIB or potentially the AIY, and RIA interneurons of mod-1 and ser-1 null animals, respectively. Because none of these 5-HT receptors appear to be expressed in the ASH sensory neurons mediating octanol sensitivity, we identified a 5-HT(6)-like receptor, F16D3.7(SER-5), that was required for food/5-HT-dependent increases in octanol sensitivity. ser-5 null animals failed to increase octanol sensitivity in the presence of food/5-HT and sensitivity could be restored by expression of SER-5 in the ASHs. Similarly, the RNAi knockdown of ser-5 expression in the ASHs of wild-type animals also abolished 5-HT-dependent increases in octanol sensitivity, suggesting that SER-5 modulates the octanol responsiveness of the ASHs directly. Together, these results suggest that multiple amine receptors, functioning at different levels within the locomotory circuit, are each essential for the serotonergic modulation of ASH-mediated aversive responses.

Dual excitatory and inhibitory serotonergic inputs modulate egg laying in Caenorhabditis elegans

Serotonin (5-HT) regulates key processes in both vertebrates and invertebrates. Previously, four 5-HT receptors that contributed to the 5-HT modulation of egg laying were identified in Caenorhabditis elegans. Therefore, to assess potential receptor interactions, we generated animals containing combinations of null alleles for each receptor, especially animals expressing only individual 5-HT receptors. 5-HT-stimulated egg laying and egg retention correlated well with different combinations of predicted excitatory and inhibitory serotonergic inputs. For example, 5-HT did not stimulate egg laying in ser-1, ser-7, or ser-7 ser-1 null animals, and ser-7 ser-1 animals retained more eggs than wild-type animals. In contrast, 5-HT-stimulated egg laying in ser-4;mod-1 animals was greater than in wild-type animals, and ser-4;mod-1 animals retained fewer eggs than wild-type animals. Surprisingly, ser-4;mod-1;ser-7 ser-1 animals retained the same number of eggs as wild-type animals and exhibited significant 5-HT-stimulated egg laying that was dependent on a previously uncharacterized receptor, SER-5. 5-HT-stimulated egg laying was absent in ser-5;ser-4;mod-1;ser-7 ser-1 animals, and these animals retained more eggs than either wild-type or ser-4;mod-1;ser-7 ser-1 animals. The 5-HT sensitivity of egg laying could be restored by ser-5 muscle expression. Together, these results highlight the dual excitatory/inhibitory serotonergic inputs that combine to modulate egg laying.

Potential modes of interaction of 9-aminomethyl-9,10-dihydroanthracene (AMDA) derivatives with the 5-HT2A receptor: a ligand structure-affinity relationship, receptor mutagenesis and receptor modeling investigation

The effects of 3-position substitution of 9-aminomethyl-9,10-dihydroanthracene (AMDA) on 5-HT 2A receptor affinity were determined and compared to a parallel series of DOB-like 1-(2,5-dimethoxyphenyl)-2-aminopropanes substituted at the 4-position. The results were interpreted within the context of 5-HT 2A receptor models that suggest that members of the DOB-like series can bind to the receptor in two distinct modes that correlate with the compounds' functional activity. Automated ligand docking and molecular dynamics suggest that all of the AMDA derivatives, the parent of which is a 5-HT 2A antagonist, bind in a fashion analogous to that for the sterically demanding antagonist DOB-like compounds. The failure of the F340 (6.52)L mutation to adversely affect the affinity of AMDA and the 3-bromo derivative is consistent with the proposed modes of orientation. Evaluation of ligand-receptor complex models suggest that a valine/threonine exchange between the 5-HT 2A and D 2 receptors may be the origin of selectivity for AMDA and two substituted derivatives.

Structural motifs of importance for the constitutive activity of the orphan 7TM receptor EBI2: analysis of receptor activation in the absence of an agonist

The Epstein-Barr induced receptor 2 (EBI2) is a lymphocyte-expressed orphan seven transmembrane-spanning (7TM) receptor that signals constitutively through Galphai, as shown, for instance by guanosine 5'-O-(3-thio)triphosphate incorporation. Two regions of importance for the constitutive activity were identified by a systematic mutational analysis of 29 residues in EBI2. The cAMP response element-binding protein transcription factor was used as a measure of receptor activity and was correlated to the receptor surface expression. PheVI:13 (Phe257), and the neighboring CysVI:12 (Cys256), in the conserved CW/FxP motif in TM 6, acted as negative regulators as Ala substitutions at these positions increased the constitutive activity 5.7- and 2.3-fold, respectively, compared with EBI2 wild type (wt). In contrast, ArgII:20 (Arg87) in TM-2 acted as a positive regulator, as substitution to Ala, but not to Lys, decreased the constitutive activity more than 7-fold compared with wt EBI2. IleIII:03 (Ile106) is located only 4 A from ArgII:20, and a favorable electrostatic interaction with ArgII:20 was created by introduction of Glu in III:03, given that the activity increased to 4.4-fold of that wt EBI2. It is noteworthy that swapping these charges by introduction of Glu in II:20 and Arg in III:03 resulted in a 2.7-fold increase compared with wt EBI2, thereby rescuing the two signaling-deficient single mutations, which exhibited a 3.8- to 4.5-fold decrease in constitutive activity. The uncovering of these molecular mechanisms for EBI2 activation is important from a drug development point of view, in that it may facilitate the rational design and development of small-molecule inverse agonists against EBI2 of putative importance as antiviral- or immune modulatory therapy.