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

Structural basis for partial agonism in 5-HT3A receptors

Hyperactivity of serotonin 3 receptors (5-HT3R) underlies pathologies associated with irritable bowel syndrome and chemotherapy-induced nausea and vomiting. Setrons, a class of high-affinity competitive antagonists, are used in the treatment of these conditions. Although generally effective for chemotherapy-induced nausea and vomiting, the use of setrons for treating irritable bowel syndrome has been impaired by adverse side effects. Partial agonists are now being considered as an alternative strategy, with potentially less severe side effects than full antagonists. However, a structural understanding of how these ligands work is lacking. Here, we present high-resolution cryogenic electron microscopy structures of the mouse 5-HT3AR in complex with partial agonists (SMP-100 and ALB-148471) captured in pre-activated and open-like conformational states. Molecular dynamics simulations were used to assess the stability of drug-binding poses and interactions with the receptor over time. Together, these studies reveal mechanisms for the functional differences between orthosteric partial agonists, full agonists and antagonists of the 5-HT3AR.

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.

Signal transduction through Cys-loop receptors is mediated by the nonspecific bumping of closely apposed domains

One of the most fundamental questions in the field of Cys-loop receptors (pentameric ligand-gated ion channels, pLGICs) is how the affinity for neurotransmitters and the conductive/nonconductive state of the transmembrane pore are correlated despite the ∼60-Å distance between the corresponding domains. Proposed mechanisms differ, but they all converge into the idea that interactions between wild-type side chains across the extracellular-transmembrane-domain (ECD-TMD) interface are crucial for this phenomenon. Indeed, the successful design of fully functional chimeras that combine intact ECD and TMD modules from different wild-type pLGICs has commonly been ascribed to the residual conservation of sequence that exists at the level of the interfacial loops even between evolutionarily distant parent channels. Here, using mutagenesis, patch-clamp electrophysiology, and radiolabeled-ligand binding experiments, we studied the effect of eliminating this residual conservation of sequence on ion-channel function and cell-surface expression. From our results, we conclude that proper state interconversion ("gating") does not require conservation of sequence-or even physicochemical properties-across the ECD-TMD interface. Wild-type ECD and TMD side chains undoubtedly interact with their surroundings, but the interactions between them-straddling the interface-do not seem to be more important for gating than those occurring elsewhere in the protein. We propose that gating of pLGICs requires, instead, that the overall structure of the interfacial loops be conserved, and that their relative orientation and distance be the appropriate ones for changes in one side to result in changes in the other, in a phenomenon akin to the nonspecific "bumping" of closely apposed domains.

Large Intracellular Domain-Dependent Effects of Positive Allosteric Modulators on Glycine Receptors

Glycine receptors (GlyRs) are members of the pentameric ligand-gated ionic channel family (pLGICs) and mediate fast inhibitory neurotransmission in the brain stem and spinal cord. The function of GlyRs can be modulated by positive allosteric modulators (PAMs). So far, it is largely accepted that both the extracellular (ECD) and transmembrane (TMD) domains constitute the primary target for many of these PAMs. On the other hand, the contribution of the intracellular domain (ICD) to the PAM effects on GlyRs remains poorly understood. To gain insight about the role of the ICD in the pharmacology of GlyRs, we examined the contribution of each domain using a chimeric receptor. Two chimeras were generated, one consisting of the ECD of the prokaryotic homologue Gloeobacter violaceus ligand-gated ion channel (GLIC) fused to the TMD of the human α₁GlyR lacking the ICD (Lily) and a second with the ICD (Lily-ICD). The sensitivity to PAMs of both chimeric receptors was studied using electrophysiological techniques. The Lily receptor showed a significant decrease in the sensitivity to four recognized PAMs. Remarkably, the incorporation of the ICD into the Lily background was sufficient to restore the wild-type α₁GlyR sensitivity to these PAMs. Based on these data, we can suggest that the ICD is necessary to form a pLGIC having full sensitivity to positive allosteric modulators.