Cysteine modification reveals which subunits form the ligand binding site in human heteromeric 5-HT3AB receptors

Mutations of the nACh Receptor M4 Helix Reveal Different Phenotypes in Different Expression Systems: Could Lipids be Responsible?

The role of the outermost helix (M4) in the pentameric ligand-gated ion channel (pLGIC) family is currently not fully understood. It is known that M4 is important for receptor assembly, possibly via interactions with neighboring M1 and M3 helices. M4 can also transmit information on the lipid content of the membrane to the gating mechanism, and it may form a link to the extracellular domain via the Cys-loop. Our previous study examining the α4β2 nACh receptor M4 helix using HEK cells indicated M4 here is more sensitive to change than those of other pLGIC. Many of these other studies, however, were performed in Xenopus oocytes. Here we examine the nine previously identified nonfunctional α4β2 nACh receptor M4 mutant receptors using this system. The data reveal that seven of these mutant receptors do function when expressed in oocytes, with only 2, the conserved Asp at the intracellular end of M4 and a Phe in the center, having a similar phenotype (nonfunctional) in both HEK cells and oocytes. The oocyte data are more consistent with studies in other pLGIC and demonstrate the importance of the expression system used. Of the many differences between these two expression systems, we suggest that the different lipid content of the plasma membrane is a possible candidate for explaining these discrepancies.

Tapping into 5-HT3 Receptors to Modify Metabolic and Immune Responses

5-hydroxytryptamine type 3 (5-HT₃) receptors are ligand gated ion channels, which clearly distinguish their mode of action from the other G-protein coupled 5-HT or serotonin receptors. 5-HT₃ receptors are well established targets for emesis and gastrointestinal mobility and are used as adjunct targets in treating schizophrenia. However, the distribution of these receptors is wider than the nervous system and there is potential that these additional sites can be targeted to modulate inflammatory and/or metabolic conditions. Recent progress in structural biology and pharmacology of 5-HT₃ receptors have provided profound insights into mechanisms of their action. These advances, combined with insights into clinical relevance of mutations in genes encoding 5-HT₃ subunits and increasing understanding of their implications in patient's predisposition to diseases and response to the treatment, open new avenues for personalized precision medicine. In this review, we recap on the current status of 5-HT₃ receptor-based therapies using a biochemical and physiological perspective. We assess the potential for targeting 5-HT₃ receptors in conditions involving metabolic or inflammatory disorders based on recent findings, underscoring the challenges and limitations of this approach.

Structure, Function and Physiology of 5-Hydroxytryptamine Receptors Subtype 3

5-hydroxytryptamine receptor subtype 3 (5-HT₃R) is a pentameric ligand-gated ion channel (pLGIC) involved in neuronal signaling. It is best known for its prominent role in gut-CNS signaling though there is growing interest in its other functions, particularly in modulating non-serotonergic synaptic activity. Recent advances in structural biology have provided mechanistic understanding of 5-HT₃R function and present new opportunities for the field. This chapter gives a broad overview of 5-HT₃R from a physiological and structural perspective and then discusses the specific details of ion permeation, ligand binding and allosteric coupling between these two events. Biochemical evidence is summarized and placed within a physiological context. This perspective underscores the progress that has been made as well as outstanding challenges and opportunities for future 5-HT₃R research.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

The binding orientations of structurally-related ligands can differ; A cautionary note

Crystal structures can identify ligand-receptor interactions and assist the development of novel therapeutics, but experimental challenges sometimes necessitate the use of homologous proteins. Tropisetron is an orthosteric ligand at both 5-HT₃ and α7 nACh receptors and its binding orientation has been determined in the structural homologue AChBP (pdbid: 2WNC). Co-crystallisation with a structurally-related ligand, granisetron, reveals an almost identical orientation (pdbid; 2YME). However, there is a >1000-fold difference in the affinity of tropisetron at 5-HT₃ versus α7 nACh receptors, and α7 nACh receptors do not bind granisetron. These striking pharmacological differences prompt questions about which receptor the crystal structures most closely represent and whether the ligand orientations are correct. Here we probe the binding orientation of tropisetron and granisetron at 5-HT₃ receptors by in silico modelling and docking, radioligand binding on cysteine-substituted 5-HT₃ receptor mutants transiently expressed in HEK 293 cells, and synthetic modification of the ligands. For 15 of the 23 cysteine substitutions, the effects on tropisetron and granisetron were different. Structure-activity relationships on synthesised derivatives of both ligands were also consistent with different orientations, revealing that contrary to the crystallographic evidence from AChBP, the two ligands adopt different orientations in the 5-HT₃ receptor binding site. Our results show that even quite structurally similar molecules can adopt different orientations in the same binding site, and that caution may be needed when using homologous proteins to predict ligand binding.

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The drugs granisetron and tropisetron are structurally similar.

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Crystals of them bound to AChBP suggest they have similar binding orientations.

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At 5-HT₃R, the effects of mutagenesis indicate that their orientations differ.

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SAR on both of these drugs also supports different orientations.

Novel mechanism of modulation at a ligand-gated ion channel; action of 5-Cl-indole at the 5-HT3 A receptor

BACKGROUND AND PURPOSE

The 5-HT₃ receptor is a prototypical member of the Cys-loop ligand-gated ion channel (LGIC) superfamily and an established therapeutic target. In addition to activation via the orthosteric site, receptor function can be modulated by allosteric ligands. We have investigated the pharmacological action of Cl-indole upon the 5-HT₃ A receptor and identified that this positive allosteric modulator possesses a novel mechanism of action for LGICs.

EXPERIMENTAL APPROACH

The impact of Cl-indole upon the 5-HT₃ receptor was assessed using single cell electrophysiological recordings and [³ H]-granisetron binding in HEK293 cells stably expressing the 5-HT₃ receptor.

KEY RESULTS

Cl-indole failed to evoke 5-HT₃ A receptor-mediated responses (up to 30 μM) or display affinity for the [³ H]-granisetron binding site. However, in the presence of Cl-indole, termination of 5-HT application revealed tail currents mediated via the 5-HT₃ A receptor that were independent of the preceding 5-HT concentration but were antagonized by the 5-HT₃ receptor antagonist, ondansetron. These tail currents were absent in the 5-HT₃ AB receptor. Furthermore, the presence of 5-HT revealed a concentration-dependent increase in the affinity of Cl-indole for the orthosteric binding site of the human 5-HT₃ A receptor.

CONCLUSIONS AND IMPLICATIONS

Cl-indole acts as both an orthosteric agonist and an allosteric modulator, but the presence of an orthosteric agonist (e.g. 5-HT) is a prerequisite to reveal both actions. Precedent for ago-allosteric action is available, yet the essential additional presence of an orthosteric agonist is now reported for the first time. This widening of the pharmacological mechanisms to modulate LGICs may offer further therapeutic opportunities.

5-HT3 Receptor Brain-Type B-Subunits are Differentially Expressed in Heterologous Systems

Genes for five different 5-HT₃ receptor subunits have been identified. Most of the subunits have multiple isoforms, but two isoforms of the B subunits, brain-type 1 (Br1) and brain-type 2 (Br2) are of particular interest as they appear to be abundantly expressed in human brain, where 5-HT3B subunit RNA consists of approximately 75% 5-HT3Br2, 24% 5-HT3Br1, and <1% 5-HT3B. Here we use two-electrode voltage-clamp, radioligand binding, fluorescence, whole cell, and single channel patch-clamp studies to characterize the roles of 5-HT3Br1 and 5-HT3Br2 subunits on function and pharmacology in heterologously expressed 5-HT₃ receptors. The data show that the 5-HT3Br1 transcriptional variant, when coexpressed with 5-HT3A subunits, alters the EC₅₀, n_H, and single channel conductance of the 5-HT₃ receptor, but has no effect on the potency of competitive antagonists; thus, 5-HT₃ABr1 receptors have the same characteristics as 5-HT₃AB receptors. There were some differences in the shapes of 5-HT₃AB and 5-HT₃ABr1 receptor responses, which were likely due to a greater proportion of homomeric 5-HT₃A versus heteromeric 5-HT₃ABr1 receptors in the latter, as expression of the 5-HT3Br1 compared to the 5-HT3B subunit is less efficient. Conversely, the 5-HT3Br2 subunit does not appear to form functional channels with the 5-HT3A subunit in either oocytes or HEK293 cells, and the role of this subunit is yet to be determined.

Allosteric activation of the 5-HT3AB receptor by mCPBG

The 5-HT3AB receptor contains three A and two B subunits in an A-A-B-A-B order. However, serotonin function at the 5-HT3AB receptor has been shown to depend solely on the A-A interface present in the homomeric receptor. Using mutations at sites on both the primary (E122) and complementary (Y146) faces of the B subunit, we demonstrate that meta-chlorophenyl biguanide (mCPBG), a 5-HT3 selective agonist, is capable of binding to and activating the 5-HT3AB receptor at all five subunit interfaces of the heteromer. Further, mCPBG is capable of allosterically modulating the activity of serotonin from these sites. While these five binding sites are similar enough that they conform to a monophasic dose - response relationship, we uncover subtle differences in the heteromeric binding sites. We also find that the A-A interface appears to contribute disproportionately to the efficacy of 5-HT3AB receptor activation.

Characterizing new fluorescent tools for studying 5-HT₃ receptor pharmacology

The pharmacological characterization of ligands depends upon the ability to accurately measure their binding properties. Fluorescence provides an alternative to more traditional approaches such as radioligand binding. Here we describe the binding and spectroscopic properties of eight fluorescent 5-HT3 receptor ligands. These were tested on purified receptors, expressed receptors on live cells, or in vivo. All compounds had nanomolar affinities with fluorescent properties extending from blue to near infra-red emission. A fluorescein-derivative had the highest affinity as measured by fluorescence polarization (FP; 1.14 nM), flow cytometry (FC; 3.23 nM) and radioligand binding (RB; 1.90 nM). Competition binding with unlabeled 5-HT3 receptor agonists (5-HT, mCPBG, quipazine) and antagonists (granisetron, palonosetron, tropisetron) yielded similar affinities in all three assays. When cysteine substitutions were introduced into the 5-HT3 receptor binding site the same changes in binding affinity were seen for both granisetron and the fluorescein-derivative, suggesting that they both adopt orientations that are consistent with co-crystal structures of granisetron with a homologous protein (5HTBP). As expected, in vivo live imaging in anaesthetized mice revealed staining in the abdominal cavity in intestines, but also in salivary glands. The unexpected presence of 5-HT3 receptors in mouse salivary glands was confirmed by Western blots. Overall, these results demonstrate the wide utility of our new high-affinity fluorescently-labeled 5-HT3 receptor probes, ranging from in vitro receptor pharmacology, including FC and FP ligand competition, to live imaging of 5-HT3 expressing tissues.

Discriminating between 5-HT₃A and 5-HT₃AB receptors

The 5-HT3B subunit was first cloned in 1999, and co-expression with the 5-HT3A subunit results in heteromeric 5-HT₃AB receptors that are functionally distinct from homomeric 5-HT₃A receptors. The affinities of competitive ligands at the two receptor subtypes are usually similar, but those of non-competitive antagonists that bind in the pore often differ. A competitive ligand and allosteric modulator that distinguishes 5-HT₃A from 5-HT₃AB receptors has recently been described, and the number of non-competitive antagonists identified with this ability has increased in recent years. In this review, we discuss the differences between 5-HT₃A and 5-HT₃AB receptors and describe the possible sites of action of compounds that can distinguish between them.

The Concise Guide to PHARMACOLOGY 2013/14: ligand-gated ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full.

Ligand-gated ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets.

It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Prolonged inhibition of 5-HT₃ receptors by palonosetron results from surface receptor inhibition rather than inducing receptor internalization

BACKGROUND AND PURPOSE

The 5-HT₃ receptor antagonist palonosetron is an important treatment for emesis and nausea during cancer therapy. Its clinical efficacy may result from its unique binding and clearance characteristics and receptor down-regulation mechanisms. We investigated the mechanisms by which palonosetron exerts its long-term inhibition of 5-HT₃ receptors for a better understanding of its clinical efficacy.

EXPERIMENTAL APPROACH

Cell surface receptors (recombinantly expressed 5HT₃A or 5HT₃AB in COS-7 cells) were monitored using [³H]granisetron binding and ELISA after exposure to palonosetron. Receptor endocytosis was investigated using immunofluorescence microscopy.

KEY RESULTS

Chronic exposure to palonosetron reduced the number of available cell surface [³H]granisetron binding sites. This down-regulation was not sensitive to either low temperature or pharmacological inhibitors of endocytosis (dynasore or nystatin) suggesting that internalization did not play a role. This was corroborated by our observation that there was no change in cell surface 5-HT₃ receptor levels or increase in endocytic rate. Palonosetron exhibited slow dissociation from the receptor over many hours, with a significant proportion of binding sites being occupied for at least 4 days. Furthermore, our observations suggest that chronic receptor down-regulation involved interactions with an allosteric binding site.

CONCLUSIONS AND IMPLICATIONS

Palonosetron acts as a pseudo-irreversible antagonist causing prolonged inhibition of 5-HT₃ receptors due to its very slow dissociation. In addition, an irreversible binding mode persists for at least 4 days. Allosteric receptor interactions appear to play a role in this phenomenon.

A single channel mutation alters agonist efficacy at 5-HT3A and 5-HT3AB receptors

BACKGROUND AND PURPOSE

5-HT₃ receptors are composed of 5-HT₃A subunits (homomeric receptors), or combinations of 5-HT₃A and other 5-HT₃ receptor subunits (heteromeric receptors, the best studied of which are 5-HT₃AB receptors). Here we explore the effects of partial agonists at 5-HT₃A and 5-HT₃AB receptors, and the importance of a channel-lining residue in determining the efficacy of activation.

EXPERIMENTAL APPROACH

Wild type and mutant 5-HT₃A and 5-HT₃AB receptors were expressed in Xenopus oocytes and examined using two-electrode voltage-clamp, or expressed in HEK293 cells and examined using [³H]granisetron binding.

KEY RESULTS

Dopamine, quipazine and VUF10166 were partial agonists at wild type 5-HT₃A and 5-HT₃AB receptors, with quipazine and VUF10166 causing a long-lived (>20 min) inhibition of subsequent agonist responses. At 5-HT₃A receptors, mCPBG was a partial agonist, but was a superagonist at 5-HT₃AB receptors, as it produced a response 2.6× greater than that of 5-HT. A T6'S substitution in the 5-HT₃A subunit decreased EC₅₀ and increased R_max of dopamine and quipazine at both homomeric and heteromeric receptors. The greatest changes were seen with VUF10166 at 5-HT₃A_T6'SB receptors, where it became a full agonist (EC₅₀ = 7 nM) with an EC₅₀ 58-fold less than 5-HT (EC₅₀ = 0.4 μM) and no longer caused inhibition of subsequent agonist responses.

CONCLUSIONS AND IMPLICATIONS

These results indicate that a mutation in the pore lining domain in both 5-HT₃A and 5-HT₃AB receptors alters the relative efficacy of a series of agonists, changing some (e.g. quipazine) from apparent antagonists to potent and efficacious agonists.

The binding characteristics and orientation of a novel radioligand with distinct properties at 5-HT3A and 5-HT3AB receptors

VUF10166 (2-chloro-3-(4-methyl piperazin-1-yl)quinoxaline) is a ligand that binds with high affinity to 5-HT3 receptors. Here we synthesise [(3)H]VUF10166 and characterise its binding properties at 5-HT3A and 5-HT3AB receptors. At 5-HT3A receptors [(3)H]VUF10166 displayed saturable binding with a Kd of 0.18 nM. Kinetic measurements gave monophasic association (6.25 × 10(7) M(-1) min(-1)) and dissociation (0.01 min(-1)) rates that yielded a similar Kd value (0.16 nM). At 5-HT3AB receptors two association (6.15 × 10(-7), 7.23 M(-1) min(-1)) and dissociation (0.024, 0.162 min(-1)) rates were seen, yielding Kd values (0.38 nM and 22 nM) that were consistent with values obtained in saturation (Kd = 0.74 nM) and competition (Ki = 37 nM) binding experiments respectively. At both receptor types, specific binding was inhibited by classical 5-HT3 receptor-selective orthosteric ligands (5-HT, allosetron, d-tubocurarine, granisetron, mCPBG, MDL72222, quipazine), but not by non-competitive antagonists (bilobalide, ginkgolide B, picrotoxin) or competitive ligands of other Cys-loop receptors (ACh, bicuculline, glycine, gabazine). To explore VUF10166 ligand-receptor interactions we used in silico modelling and docking, and tested the predictions using site directed mutagenesis. The data suggest that VUF10166 adopts a similar orientation to 5-HT3 receptor agonists bound in AChBP (varenicline) and 5HTBP (5-HT) crystal structures.

Natural compounds boldine and menthol are antagonists of human 5-HT3 receptors: implications for treating gastrointestinal disorders

BACKGROUND

Impaired 5-HT3 receptor function is likely involved in the pathogenesis of functional gastrointestinal disorders (FGID) and 5-HT3 receptor antagonists are effective treatments for chemotherapy-induced nausea and vomiting (CINV) and irritable bowel syndrome (IBS). The monoterpene alcohol menthol and the aporphine alkaloid boldine combat symptoms of gastrointestinal diseases; both interact with other members of the Cys-loop ligand-gated ion channel family and may therefore also act on 5-HT3 receptors.

METHODS

The impact of boldine and menthol on human recombinant homomeric 5-HT3 A- and heteromeric 5-HT3 AB receptors in HEK293 cells was determined by radioligand binding, a luminescence-based Ca(2+) assay, and a membrane potential assay. 5-HT3 protein and mRNA expression was assessed in human colon tissue.

KEY RESULTS

Boldine and menthol inhibited the 5-HT-induced activation of 5-HT3 receptors in the low and middle micromolar range, respectively. Boldine was a competitive antagonist of both receptors being 6.5- to 10-fold more potent at 5-HT3 A- vs 5-HT3 AB receptors. Menthol non-competitively and stereoselectively inhibited both receptors: In contrast to (+)-menthol, (-)-menthol was significantly more potent toward 5-HT3 A- vs 5-HT3 AB receptors. We show co-expression of 5-HT3A and 5-HT3B subunits in the human gut epithelium, the lamina propria, the myenteric plexus, and the muscular cell layer.

CONCLUSIONS & INFERENCES

The demonstrated 5-HT3 inhibitory effects may be relevant for boldine's and menthol's alleviating properties on FGID and may encourage clinical studies with the compounds or the plant extracts for CINV and IBS treatment. The found receptor-discriminative properties make boldine and (-)-menthol to potentially useful tools for analyzing structural differences between these receptor subtypes.

The 5-HT3AB receptor shows an A3B2 stoichiometry at the plasma membrane

The 5-HT3AB receptor is the best-characterized heteropentameric 5-HT3 receptor. Under conditions of heterologous expression, the 5-HT3AB receptor shows a single functionally resolvable population, suggesting the presence of a unique subunit stoichiometry; however, conflicting previous reports have suggested two different possible stoichiometries. Here we isolate plasma membrane sheets containing assembled receptors from individual HEK293T cells. We then determine the stoichiometry of 5-HT3AB receptors on the plasma membrane by fluorescence methods, employing meCFP- and meYFP-labeled A and B subunits. Over a wide range of cDNA transfection ratios, fluorescence intensity ratios are closest to values that correspond to a subunit ratio of A3B2. Förster resonance energy transfer (family FRET) efficiencies provide minor corrections (3-6%) to the subunit ratios and provide independent support for a predominantly A3B2 stoichiometry on the plasma membrane sheets. Twin FRET efficiencies support these data, also suggesting that the two B subunits are nonadjacent in most of the heteropentamers. The high-frequency variant HTR3B p.Y129S (c.386A>C, rs11767445), linked to psychiatric disease, also forms A3B2 receptors on the plasma membrane. The 5-HT3B Y129S, subunit incorporates in a slightly (11-14%) more efficient manner than the common variant. In general, most of the subunits reside within the cell. In contrast to the findings for the plasma membrane, the relative abundances and FRET characteristics of intracellular subunits depend strongly on the transfection ratio. The straightforward and unambiguous combination of plasma membrane-sheet isolation, fluorescence intensity ratios, and FRET is a generally promising procedure for determining stoichiometry of proteins on the plasma membrane.

Exploring a potential palonosetron allosteric binding site in the 5-HT(3) receptor

Palonosetron (Aloxi) is a potent second generation 5-HT(3) receptor antagonist whose mechanism of action is not yet fully understood. Palonosetron acts at the 5-HT(3) receptor binding site but recent computational studies indicated other possible sites of action in the extracellular domain. To test this hypothesis we mutated a series of residues in the 5-HT3A receptor subunit (Tyr(73), Phe(130), Ser(163), and Asp(165)) and in the 5-HT3B receptor subunit (His(73), Phe(130), Glu(170), and Tyr(143)) that were previously predicted by in silico docking studies to interact with palonosetron. Homomeric (5-HT(3)A) and heteromeric (5-HT(3)AB) receptors were then expressed in HEK293 cells to determine the potency of palonosetron using both fluorimetric and radioligand methods to test function and ligand binding, respectively. The data show that the substitutions have little or no effect on palonosetron inhibition of 5-HT-evoked responses or binding. In contrast, substitutions in the orthosteric binding site abolish palonosetron binding. Overall, the data support a binding site for palonosetron at the classic orthosteric binding pocket between two 5-HT3A receptor subunits but not at allosteric sites previously identified by in silico modelling and docking.

Importance of recognition loops B and D in the activation of human 5-HT₃ receptors by 5-HT and meta-chlorophenylbiguanide

The 5-HT₃ receptor is a cation selective member of the pentameric Cys-loop ligand-gated ion channels. While five subunits are known to exist, only two receptor subtypes have been significantly characterized: the homomeric receptor consisting of five A subunits and the heteromeric receptor containing both A and B subunits. The agonist recognition and activation of these receptors is orchestrated by six recognition loops three, A-C, on the principal subunit, and three, D-F, on the complementary subunit. In this study we have focused on the B loop of the principal subunit and loop D of the complementary subunit where aligned amino acids differ between the two subunits. A mutational analysis has been carried out using both 5-HT and m-chlorophenylbiguanide (mCPBG) to characterize receptor activation in the mutant receptors using two-electrode voltage clamp in Xenopus oocytes. The results show that the B loop W178I mutation of the 5-HT3A subunit markedly reduces the efficacy of mCPBG in both the homomeric and heteromeric receptors, while activation by 5-HT remains intact. Replacement of the D loop amino acid triplet RQY of the 5-HT3A subunit, with the aligned residues from the 5-HT3B subunit, QEV, converts 5-HT to a weak partial agonist in both the homomer and heteromer, but does not compromise activation by mCPBG. Exchange of the RQY triplet for the 5-HT3B subunit homologue, QEV, increases the Hill coefficient and decreases the EC₅₀ of this mutant when expressed with the wild type 5-HT3A subunit.

Approaching the 5-HT₃ receptor heterogeneity by computational studies of the transmembrane and intracellular domains

5-hydroxytryptamine type-3 receptor (5-HT₃), an important target of many neuroactive drugs, is a cation selective transmembrane pentamer whose functional stoichiometries and subunit arrangements are still debated, due to the extreme complexity of the system. The three dimensional structure of the 5-HT₃R subunits has not been solved so far. Moreover, most of the available structural and functional data is related to the extracellular ligand-binding domain, whereas the transmembrane and the intracellular receptor domains are far less characterised, although they are crucial for receptor function. Here, for the first time, 3D homology models of the transmembrane and the intracellular receptor domains of all the known human 5-HT₃ subunits have been built and assembled into homopentameric (5-HT(3A)R, 5-HT(3B)R, 5-HT(3C)R, 5-HT(3D)R and 5-HT(3E)R) and heteropentameric receptors (5-HT(3AB), 5-HT(3AC), 5-HT(3AD) and 5-HT(3AE)), on the basis of the known three-dimensional structures of the nicotinic-acetylcholine receptor and of the ligand gated channel from Erwinia chrysanthemi. The comparative analyses of sequences, modelled structures, and computed electrostatic properties of the single subunits and of the assembled pentamers shed new light both on the stoichiometric composition and on the physicochemical requirements of the functional receptors. In particular, it emerges that a favourable environment for the crossing of the pore at the transmembrane and intracellular C terminus domain levels by Ca²⁺ ions is granted by the maximum presence of two B subunits in the 5-HT₃ pentamer.

Agonists and antagonists induce different palonosetron dissociation rates in 5-HT₃A and 5-HT₃AB receptors

Palonosetron is a potent 5-HT₃ receptor antagonist with a unique structure and some unusual properties. Here we explore the properties of palonosetron at heterologously expressed 5-HT₃A and 5-HT₃AB receptors. We used receptors expressed in HEK293 cells, and functionally analysed them using a membrane potential sensitive dye in a Flexstation, which revealed IC₅₀s of 0.24 nM and 0.18 nM for 5-HT₃A and 5-HT₃AB receptors respectively. Radioligand binding studies with [³H]palonosetron revealed similar K_ds: 0.34 nM for 5-HT₃A and 0.15 nM for 5-HT₃AB receptors. Kinetic studies showed palonosetron association and dissociation rates were slightly faster in 5-HT₃AB than 5-HT₃A receptors, and for both subtypes dissociation rates were ligand-dependent, with antagonists causing more rapid dissociation than agonists. Similar ligand effects were not observed for [³H]granisetron dissociation studies. These data support previous studies which show palonosetron has actions distinct to other 5-HT₃ receptor antagonists, and the slow rates observed for agonist induced dissociation (t_1/2 > 10 h) could at least partly explain the long duration of palonosetron effects in vivo.

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Palonosetron IC₅₀s and K_ds for 5-HT₃A and 5-HT₃AB receptors in HEK293 cells are similar.

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Palonosetron association and dissociation rates are slower at 5-HT₃A than 5-HT₃AB receptors.

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Agonist-induced palonosetron dissociation rates are slower than those for antagonists in both 5-HT₃A and 5-HT₃AB receptors.

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Agonist- and antagonist-induced granisetron dissociation rates are similar in both 5-HT₃A and 5-HT₃AB receptors.

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Palonosetron and granisetron have distinct actions.

Structure-activity relationships of quinoxaline-based 5-HT3A and 5-HT3AB receptor-selective ligands

Until recently, discriminating between homomeric 5-HT3A and heteromeric 5-HT3AB receptors was only possible with ligands that bind in the receptor pore. This study describes the first series of ligands that can discriminate between these receptor types at the level of the orthosteric binding site. During a recent fragment screen, 2-chloro-3-(4-methylpiperazin-1-yl)quinoxaline (VUF10166) was identified as a ligand that displays an 83-fold difference in [(3)H]granisetron binding affinity between 5-HT3A and 5-HT3AB receptors. Fragment hit exploration, initiated from VUF10166 and 3-(4-methylpiperazin-1-yl)quinoxalin-2-ol, resulted in a series of compounds with higher affinity at either 5-HT3A or 5-HT3AB receptors. These ligands reveal that a single atom is sufficient to change the selectivity profile of a compound. At the extremes of the new compounds were 2-amino-3-(4-methylpiperazin-1-yl)quinoxaline, which showed 11-fold selectivity for the 5-HT3A receptor, and 2-(4-methylpiperazin-1-yl)quinoxaline, which showed an 8.3-fold selectivity for the 5-HT3AB receptor. These compounds represent novel molecular tools for studying 5-HT3 receptor subtypes and could help elucidate their physiological roles.

Bacterially expressed human serotonin receptor 3A is functionally reconstituted in proteoliposomes

Human serotonin receptor 3A (5-HT3A) is a ligand-gated ion channel regulated by serotonin. A fusion protein (P9-5-HT3A) of 5-HT3A with the P9 protein, a major envelope protein of bacteriophage phi6, was highly expressed in the membrane fraction of Escherichia coli, and the expressed protein was purified to homogeneity using an affinity chromatography. P9-5-HT3A was observed as mixed oligomers in detergents. The purified P9-5-HT3A was efficiently reconstituted into proteoliposomes, and the serotonin-dependent ion-channel activity of P9-5-HT3A was observed by measuring the increased fluorescence of Fluo-3 attributed to the formation of a complex with the Ca(2+) ions released from the proteoliposomes. Alanine substitution for Trp178 of 5-HT3A abolished the serotonin-dependent ion-channel activity, confirming the importance of Trp178 as a ligand-binding site. Furthermore, the ion-channel activity of the reconstituted P9-5-HT3A was effectively blocked by treatment with ondansetron, an antagonist of 5-HT3A. The bacterial expression system of human 5-HT3A and the proteoliposomes reconstituted with 5-HT3A would provide biophysical and structural analyses of 5-HT3A.

5-HT(3) receptors

5-Hydroxytryptamine type 3 (5-HT(3)) receptors are cation-selective Cys loop receptors found in both the central and peripheral nervous systems. There are five 5-HT(3) receptor subunits (A-E), and all functional receptors require at least one A subunit. Regions from noncontiguous parts of the subunit sequence contribute to the agonist-binding site, and the roles of a range of amino acid residues that form the binding pocket have been identified. Drugs that selectively antagonize 5-HT(3) receptors (the "setrons") are the current gold standard for treatment of chemotherapy-induced and postoperative nausea and vomiting and have potential for the treatment of a range of other conditions.

Design, synthesis, and structure-activity relationships of highly potent 5-HT₃ receptor ligands

The 5-HT₃ receptor, a pentameric ligand-gated ion channel (pLGIC), is an important therapeutic target. During a recent fragment screen, 6-chloro-N-methyl-2-(4-methyl-1,4-diazepan-1-yl)quinazolin-4-amine (1) was identified as a 5-HT₃R hit fragment. Here we describe the synthesis and structure–activity relationships (SAR) of a series of (iso)quinoline and quinazoline compounds that were synthesized and screened for 5-HT₃R affinity using a [³H]granisetron displacement assay. These studies resulted in the discovery of several high affinity ligands of which compound 22 showed the highest affinity (pK_i > 10) for the 5-HT₃ receptor. The observed SAR is in agreement with established pharmacophore models for 5-HT₃ ligands and is used for ligand–receptor binding mode prediction using homology modeling and in silico docking approaches.

A first step towards the understanding of the 5-HT3 receptor subunit heterogeneity from a computational point of view

The functional serotonin type-3 receptor (5-HT(3)-R), which is the target of many neuroactive drugs, is known to be a homopentamer made of five identical subunits A (5-HT(3A)-R) or a binary heteropentamer made of subunits A and B (5-HT(3A/B)-R) with a still debated arrangement and stoichiometry. This complex picture has been recently further complicated by the discovery of additional 5-HT(3)-R subunits, C, D, and E, which, similarly to the B subunit, are apparently able to form functional receptors only if co-expressed with subunit A. Being the binding site for both serotonin and antagonists (i.e. drugs) located at the extracellular interface between two adjacent subunits, the large variability of the 5-HT(3)-R composition becomes a crucial issue, since it can originate many different interfaces providing non-equivalent ligand binding sites and complicating the pharmacological modulation. Here, the different 5-HT(3)-R interfaces are analysed, on the bases of the structural conformations of previously built 3D homology models and of the known subunit sequences, by addressing their physicochemical characterization. The results confirm the presence of an aromatic cluster located in the core of the A-A interface as a key determinant for having an interface both stable and functional. This is used as a discriminant to make hypotheses about the capability of all the other possible interfaces constituted by the known 5-HT(3)-R sequences A, B, C, D, and E to build active receptors.

Stoichiometry and subunit arrangement of α1β glycine receptors as determined by atomic force microscopy

The glycine receptor is an anion-permeable member of the Cys-loop ion channel receptor family. Synaptic glycine receptors predominantly comprise pentameric α1β subunit heteromers. To date, attempts to define the subunit stoichiometry and arrangement of these receptors have not yielded consistent results. Here we introduced FLAG and six-His epitopes into α1 and β subunits, respectively, and imaged single antibody-bound α1β receptors using atomic force microscopy. This permitted us to infer the number and relative locations of the respective subunits in functional pentamers. Our results indicate an invariant 2α1:3β stoichiometry with a β-α-β-α-β subunit arrangement.

Discovery of a novel allosteric modulator of 5-HT3 receptors: inhibition and potentiation of Cys-loop receptor signaling through a conserved transmembrane intersubunit site

The ligand-gated ion channels in the Cys-loop receptor superfamily mediate the effects of neurotransmitters acetylcholine, serotonin, GABA, and glycine. Cys-loop receptor signaling is susceptible to modulation by ligands acting through numerous allosteric sites. Here we report the discovery of a novel class of negative allosteric modulators of the 5-HT(3) receptors (5-HT(3)Rs). PU02 (6-[(1-naphthylmethyl)thio]-9H-purine) is a potent and selective antagonist displaying IC(50) values of ~1 μM at 5-HT(3)Rs and substantially lower activities at other Cys-loop receptors. In an elaborate mutagenesis study of the 5-HT(3)A receptor guided by a homology model, PU02 is demonstrated to act through a transmembrane intersubunit site situated in the upper three helical turns of TM2 and TM3 in the (+)-subunit and TM1 and TM2 in the (-)-subunit. The Ser(248), Leu(288), Ile(290), Thr(294), and Gly(306) residues are identified as important molecular determinants of PU02 activity with minor contributions from Ser(292) and Val(310), and we propose that the naphthalene group of PU02 docks into the hydrophobic cavity formed by these. Interestingly, specific mutations of Ser(248), Thr(294), and Gly(306) convert PU02 into a complex modulator, potentiating and inhibiting 5-HT-evoked signaling through these mutants at low and high concentrations, respectively. The PU02 binding site in the 5-HT(3)R corresponds to allosteric sites in anionic Cys-loop receptors, which emphasizes the uniform nature of the molecular events underlying signaling through the receptors. Moreover, the dramatic changes in the functional properties of PU02 induced by subtle changes in its binding site bear witness to the delicate structural discrimination between allosteric inhibition and potentiation of Cys-loop receptors.

VUF10166, a novel compound with differing activities at 5-HT₃A and 5-HT₃AB receptors

The actions of a novel, potent 5-HT₃ receptor ligand, [2-chloro-(4-methylpiperazine-1-yl)quinoxaline (VUF10166)], were examined at heterologously expressed human 5-HT₃A and 5-HT₃AB receptors. VUF10166 displaced [³H]granisetron binding to 5-HT₃A receptors expressed in human embryonic kidney cells with high affinity (K(i) = 0.04 nM) but was less potent at 5-HT₃AB receptors (K(i) = 22 nM). Dissociation of [³H]granisetron in the presence of VUF10166 was best fit with a single time constant (t(1/2) = 53 min) at 5-HT₃A receptors, but with two time constants (t(1/2) = 55 and 2.4 min) at 5-HT₃AB receptors. Electrophysiological studies in oocytes revealed that VUF10166 inhibited 5-HT-induced responses at 5-HT₃A receptors at nanomolar concentrations, but inhibition and recovery were too slow to determine an IC₅₀. At 5-HT₃AB receptors, inhibition and recovery were faster, yielding an IC₅₀ of 40 nM. Cysteine substitutions in the complementary (-), but not the principal (+), face of the 5-HT₃B subunit produced heteromeric receptors in which the actions of VUF10166 resembled those at homomeric receptors. At 5-HT₃A receptors, VUF10166 at higher concentrations also behaved as a partial agonist (EC₅₀ = 5.2 μM; R(max) = 0.24) but did not elicit significant responses at 5-HT₃AB receptors at ≤100 μM. Thus, we propose that VUF10166 binds to the common A+A- site of both receptor types and to a second A+B- modulatory site in the heteromeric receptor. The ability of VUF10166 to distinguish between 5-HT₃A and 5-HT₃AB receptors could help evaluate differences between these receptor types and has potential therapeutic value.

A single amino acid determines the toxicity of Ginkgo biloba extracts

Ginkgo biloba extracts are currently used for a wide range of health-related conditions. Some of the medical benefits of these extracts are controversial, but their lack of toxicity in humans is not in doubt. These extracts are, however, highly toxic to insects. Their active components (ginkgolides and bilobalide) have structures similar to the convulsant picrotoxin, a GABA_A receptor antagonist, so their lack of toxicity in mammals is puzzling. Here, we show that the different compositions of insect and vertebrate GABA receptor pores are responsible for the differing toxicities. Insect GABA receptors contain Ala at their 2′ position in the pore. Substitution with Val, which is the equivalent residue in vertebrate GABA_A receptor α-subunits, decreases ginkgolide potency by up to 10,000-fold. The reverse mutation in vertebrate GABA_A α1 subunits increased the sensitivity of α1β2 and α1β2γ2 receptors to ginkgolides. Mutant cycle analysis demonstrates a strong interaction between the ginkgolides and the 2′ residue, a result supported by in silico docking of compounds into a model of the pore. We conclude that the insecticidal activity of G. biloba extracts can be attributed to their effects at insect GABA receptors, and the presence of a Val at the 2′ position in vertebrate GABA_A receptors explains why these compounds are not similarly toxic to humans.—Thompson, A. J., McGonigle, I., Duke, R., Johnston, G. A. R., Lummis, S. C. R. A single amino acid determines the toxicity of Ginkgo biloba extracts.

High-affinity fluorescent ligands for the 5-HT(3) receptor

The synthesis, photophysical and biological characterization of a small library of fluorescent 5-HT(3) receptor ligands is described. Several of these novel granisetron conjugates have high quantum yields and show high affinity for the human 5-HT(3)AR.