Delineation of the functional properties and the mechanism of action of TMPPAA, an allosteric agonist and positive allosteric modulator of 5-HT3 receptors

Allosteric Modulators of Serotonin Receptors: A Medicinal Chemistry Survey

Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter regulating numerous physiological functions, and its dysregulation is a crucial component of the pathological processes of schizophrenia, depression, migraines, and obesity. 5-HT interacts with 14 different receptors, of which 5-HT1A-1FRs, 5-HT2A-CRs, and 5-HT4-7Rs are G protein-coupled receptors (GPCRs), while 5-HT3R is a ligand-gated ion channel. Over the years, selective orthosteric ligands have been identified for almost all serotonin receptors, yielding several clinically relevant drugs. However, the high degree of homology between 5-HTRs and other GPCRs means that orthosteric ligands can have severe side effects. Thus, there has recently been increased interest in developing safer ligands of GPCRs, which bind to less conserved, more specific sites, distinct from that of the receptor's natural ligand. The present review describes the identification of allosteric ligands of serotonin receptors, which are largely natural compounds (oleamide, cannabidiol, THC, and aporphine alkaloids), complemented by synthetic modulators developed in large part for the 5-HT2C receptor. The latter are positive allosteric modulators sought after for their potential as drugs preferable over the orthosteric agonists as antiobesity agents for their potentially safer profile. When available, details on the interactions between the ligand and allosteric binding site will be provided. An outlook on future research in the field will also be provided.

Structural mechanisms of α7 nicotinic receptor allosteric modulation and activation

The α7 nicotinic acetylcholine receptor is a pentameric ligand-gated ion channel that plays an important role in cholinergic signaling throughout the nervous system. Its unique physiological characteristics and implications in neurological disorders and inflammation make it a promising but challenging therapeutic target. Positive allosteric modulators overcome limitations of traditional α7 agonists, but their potentiation mechanisms remain unclear. Here, we present high-resolution structures of α7-modulator complexes, revealing partially overlapping binding sites but varying conformational states. Structure-guided functional and computational tests suggest that differences in modulator activity arise from the stable rotation of a channel gating residue out of the pore. We extend the study using a time-resolved cryoelectron microscopy (cryo-EM) approach to reveal asymmetric state transitions for this homomeric channel and also find that a modulator with allosteric agonist activity exploits a distinct channel-gating mechanism. These results define mechanisms of α7 allosteric modulation and activation with implications across the pentameric receptor superfamily.

Phylogenetic analyses of 5-hydroxytryptamine 3 (5-HT3) receptors in Metazoa

The 5-hydroxytrptamine 3 (5-HT3) receptor is a member of the 'Cys-loop' family and the only pentameric ligand gated ion channel among the serotonin receptors. 5-HT3 receptors play an important role in controlling growth, development, and behaviour in animals. Several 5-HT3 receptor antagonists are used to treat diseases (e.g., irritable bowel syndrome, nausea and emesis). Humans express five different subunits (A-E) enabling a variety of heteromeric receptors to form but all contain 5HT3A subunits. However, the information available about the 5-HT3 receptor subunit occurrence among the metazoan lineages is minimal. In the present article we searched for 5-HT3 receptor subunit homologs from different phyla in Metazoa. We identified more than 1000 5-HT3 receptor subunits in Metazoa in different phyla and undertook simultaneous phylogenetic analysis of 526 5HT3A, 358 5HT3B, 239 5HT3C, 70 5HT3D, and 173 5HT3E sequences. 5-HT3 receptor subunits were present in species belonging to 11 phyla: Annelida, Arthropoda, Chordata, Cnidaria, Echinodermata, Mollusca, Nematoda, Orthonectida, Platyhelminthes, Rotifera and Tardigrada. All subunits were most often identified in Chordata phylum which was strongly represented in searches. Using multiple sequence alignment, we investigated variations in the ligand binding region of the 5HT3A subunit protein sequences in the metazoan lineage. Several critical amino acid residues important for ligand binding (common structural features) are commonly present in species from Nematoda and Platyhelminth gut parasites through to Chordata. Collectively, this better understanding of the 5-HT3 receptor evolutionary patterns raises possibilities of future pharmacological challenges facing Metazoa including effects on parasitic and other species in ecosystems that contain 5-HT3 receptor ligands.

The first structure-activity relationship study of oxytocin as a positive allosteric modulator for the µ opioid receptor

Positive allosteric modulators (PAMs) of G protein-coupled receptors (GPCRs) have drawn attention as novel drug candidates. PAMs can enhance the activities of endogenous agonists which are not only secreted at appropriate times and in parts of the body, but also are immediately metabolized. Therefore, they are expected to show fewer side effects than exogeneous orthosteric ligands. Recently, we have reported that oxytocin (OT) functioned as a PAM of the μ opioid receptor (MOR) which was one of the most potent targets for analgesics. OT is thus thought to be a useful compound for the development of novel analgesics. In this study, several OT analogs were synthesized and evaluated with an intact cell-based assay to investigate the crucial structures of OT for exerting the PAM activity. The assay results indicated that the cyclic structure formed by an intramolecular disulfide bond and the three C-terminal residues containing a small Gly residue of OT were essential for their function as a MOR-PAM. Intriguingly, two analogs having an amide or an ethylene tether instead of the intramolecular disulfide bridge did not have any PAM effects. The results suggested that the disulfide linkage of OT would be a key structure for exerting the PAM activity at the MOR.

Discovery and functional characterization of N-(thiazol-2-yl)-benzamide analogs as the first class of selective antagonists of the Zinc-Activated Channel (ZAC)

The Zinc-Activated Channel (ZAC) is an atypical member of the Cys-loop receptor (CLR) superfamily of pentameric ligand-gated ion channels, with its very different endogenous agonists and signalling properties. In this study, a compound library screening at ZAC resulted in the identification of 2-(5-bromo-2-chlorobenzamido)-4-methylthiazole-5-methyl ester (1) as a novel ZAC antagonist. The structural determinants for ZAC activity in 1 were investigated by functional characterization of 61 analogs at ZAC expressed in Xenopus oocytes by two-electrode voltage clamp electrophysiology, and couple of analogs exerting more potent ZAC inhibition than 1 were identified (IC₅₀ values: 1-3 μM). 1 and N-(4-(tert-butyl)thiazol-2-yl)-3-fluorobenzamide (5a, TTFB) were next applied in studies of the functional properties and the mode of action of this novel class of ZAC antagonists. TTFB was a roughly equipotent antagonist of Zn⁺- and H⁺-evoked ZAC signaling and of spontaneous ZAC activity, and the slow on-set of its channel block suggested that its ZAC inhibition is state-dependent. TTFB was found to be a selective ZAC antagonist, exhibiting no significant agonist, antagonist or modulatory activity at 5-HT₃A, α₃β₄ nicotinic acetylcholine, α₁β₂γ_2S GABA_A or α₁ glycine receptors at 30 μM. 1 displayed largely non-competitive antagonism of Zn²⁺-induced ZAC signalling, and TTFB was demonstrated to target the transmembrane and/or intracellular domains of the receptor, which collectively suggests that the N-(thiazol-2-yl)-benzamide analog acts a negative allosteric modulator of ZAC. We propose that this first class of selective ZAC antagonists could constitute useful pharmacological tools in future explorations of the presently poorly elucidated physiological functions governed by this CLR.

Probing the molecular basis for signal transduction through the Zinc-Activated Channel (ZAC)

The molecular basis for the signal transduction through the classical Cys-loop receptors (CLRs) has been delineated in great detail. The Zinc-Activated Channel (ZAC) constitutes a so far poorly elucidated fifth branch of the CLR superfamily, and in this study we explore the molecular mechanisms underlying ZAC signaling in Xenopus oocytes by two-electrode voltage clamp electrophysiology. In studies of chimeric receptors fusing either the extracellular domain (ECD) or the transmembrane/intracellular domain (TMD-ICD) of ZAC with the complementary domains of 5-HT₃A serotonin or α₁ glycine receptors, serotonin and Zn²⁺/H⁺ evoked robust concentration-dependent currents in 5-HT₃A/ZAC- and ZAC/α₁-Gly-expressing oocytes, respectively, suggesting that Zn²⁺ and protons activate ZAC predominantly through its ECD. The molecular basis for Zn²⁺-mediated ZAC signaling was probed further by introduction of mutations of His, Cys, Glu and Asp residues in this domain, but as none of the mutants tested displayed substantially impaired Zn²⁺ functionality compared to wild-type ZAC, the location of the putative Zn²⁺ binding site(s) in the ECD was not identified. Finally, the functional importance of Leu²⁴⁶ (Leu9') in the transmembrane M2 α-helix of ZAC was investigated by Ala, Val, Ile and Thr substitutions. In concordance with findings for this highly conserved residue in classical CLRs, the ZAC^L9'X mutants exhibited left-shifted agonist concentration-response relationships, markedly higher degrees of spontaneous activity and slower desensitization kinetics compared to wild-type ZAC. In conclusion, while ZAC is an atypical CLR in terms of its (identified) agonists and channel characteristics, its signal transduction seems to undergo similar conformational transitions as those in the classical CLR.

Drug Development in Channelopathies: Allosteric Modulation of Ligand-Gated and Voltage-Gated Ion Channels

Ion channels have been characterized as promising drug targets for treatment of numerous human diseases. Functions of ion channels can be fine-tuned by allosteric modulators, which interact with channels and modulate their activities by binding to sites spatially discrete from those of orthosteric ligands. Positive and negative allosteric modulators have presented a plethora of potential therapeutic advantages over traditionally orthosteric agonists and antagonists in terms of selectivity and safety. This thematic review highlights the discovery of representative allosteric modulators for ligand-gated and voltage-gated ion channels, discussing in particular their identifications, locations, and therapeutic uses in the treatment of a range of channelopathies. Additionally, structures and functions of selected ion channels are briefly described to aid in the rational design of channel modulators. Overall, allosteric modulation represents an innovative targeting approach, and the corresponding modulators provide an abundant but challenging landscape for novel therapeutics targeting ligand-gated and voltage-gated ion channels.

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.

Conformational Changes in the 5-HT3A Receptor Extracellular Domain Measured by Voltage-Clamp Fluorometry

The 5-hydroxytryptamine (5-HT) type 3 receptor is a member of the cysteine (Cys)-loop receptor super family of ligand-gated ion channels in the nervous system and is a clinical target in a range of diseases. The 5-HT₃ receptor mediates fast serotonergic neurotransmission by undergoing a series of conformational changes initiated by ligand binding that lead to the rapid opening of an intrinsic cation-selective channel. However, despite the availability of high-resolution structures of a mouse 5-HT₃ receptor, many important aspects of the mechanistic basis of 5-HT₃ receptor function and modulation by drugs remain poorly understood. In particular, there is little direct evidence for the specific conformational changes predicted to occur during ligand-gated channel activation and desensitization. In the present study, we used voltage-clamp fluorometry (VCF) to measure conformational changes in regions surrounding the orthosteric binding site of the human 5-HT_3A (h5-HT_3A) receptor during binding of 5-HT and different classes of 5-HT₃ receptor ligands. VCF utilizes parallel measurements of receptor currents with photon emission from fluorescent reporter groups covalently attached to specific positions in the receptor structure. Reporter groups that are highly sensitive to the local molecular environment can, in real time, report conformational changes as changes in fluorescence that can be correlated with changes in receptor currents reporting the functional states of the channel. Within the loop C, D, and E regions that surround the orthosteric binding site in the h5-HT_3A receptor, we identify positions that are amenable to tagging with an environmentally sensitive reporter group that reports robust fluorescence changes upon 5-HT binding and receptor activation. We use these reporter positions to characterize the effect of ligand binding on the local structure of the orthosteric binding site by agonists, competitive antagonists, and allosterically acting channel activators. We observed that loop C appears to show distinct fluorescence changes for ligands of the same class, while loop D reports similar fluorescence changes for all ligands binding at the orthosteric site. In contrast, the loop E reporter position shows distinct changes for agonists, antagonists, and allosteric compounds, suggesting the conformational changes in this region are specific to ligand function. Interpretation of these results within the framework of current models of 5-HT₃ and Cys-loop mechanisms are used to expand the understanding of how ligand binding in Cys-loop receptors relates to channel gating. SIGNIFICANCE STATEMENT: The 5-HT₃ receptor is an important ligand-gated ion channel and drug target in the central and peripheral nervous system. Determining how ligand binding induced conformational changes in the receptor is central for understanding the structural mechanisms underlying 5-HT₃ receptor function. Here, we employ voltage-gated fluorometry to characterize conformational changes in the extracellular domain of the human 5-HT₃ receptor to identify intrareceptor motions during binding of a range of 5-HT₃ receptor agonists and antagonists.

Conformational transitions of the serotonin 5-HT3 receptor

The serotonin 5-HT₃ receptor is a pentameric ligand-gated ion channel (pLGIC). It belongs to a large family of receptors that function as allosteric signal transducers across the plasma membrane^1,2; upon binding of neurotransmitter molecules to extracellular sites, the receptors undergo complex conformational transitions that result in transient opening of a pore permeable to ions. 5-HT₃ receptors are therapeutic targets for emesis and nausea, irritable bowel syndrome and depression³. In spite of several reported pLGIC structures^4-8, no clear unifying view has emerged on the conformational transitions involved in channel gating. Here we report four cryo-electron microscopy structures of the full-length mouse 5-HT₃ receptor in complex with the anti-emetic drug tropisetron, with serotonin, and with serotonin and a positive allosteric modulator, at resolutions ranging from 3.2 Å to 4.5 Å. The tropisetron-bound structure resembles those obtained with an inhibitory nanobody⁵ or without ligand⁹. The other structures include an 'open' state and two ligand-bound states. We present computational insights into the dynamics of the structures, their pore hydration and free-energy profiles, and characterize movements at the gate level and cation accessibility in the pore. Together, these data deepen our understanding of the gating mechanism of pLGICs and capture ligand binding in unprecedented detail.

Characterization of a 5-HT3-ELIC Chimera Revealing the Sites of Action of Modulators

Cys-loop receptors are major sites of action for many important therapeutically active compounds, but the sites of action of those that do not act at the orthosteric binding site or at the pore are mostly poorly understood. To help understand these, we here describe a chimeric receptor consisting of the extracellular domain of the 5-HT₃A receptor and the transmembrane domain of a prokaryotic homologue, ELIC. Alterations of some residues at the coupling interface are required for function, but the resulting receptor expresses well and responds to 5-HT with a lower EC₅₀ (0.34 μM) than that of the 5-HT₃A receptor. Partial agonists and competitive antagonists of the 5-HT₃A receptor activate and inhibit the chimera as expected. Examination of a range of receptor modulators, including ethanol, thymol, 5-hydroxyindole, and 5-chloroindole, which can affect the 5-HT₃A receptor and ELIC, suggest that these compounds act via the transmembrane domain, except for 5-hydroxyindole, which can compete with 5-HT at the orthosteric binding site. The data throw further light on the importance of coupling interface in Cys-loop receptors and provide a platform for examining the mechanism of action of compounds that act in the extracellular domain of the 5-HT₃A receptor and the transmembrane domain of ELIC.