Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors

Abstract

General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface.

Mapping General Anesthetic Sites in Heteromeric γ-Aminobutyric Acid Type A Receptors Reveals a Potential For Targeting Receptor Subtypes

“…In cationic channels, crystallography reveals general anesthetics binding in intra-subunit pockets within the four helix bundles of GLIC (61). In muscle type acetylcholine receptors, inhibitory sites are in the channel lumen, and four helix bundles (62–64). …”

Photolabeling a Nicotinic Acetylcholine Receptor (nAChR) with an (α4)3(β2)2 nAChR-Selective Positive Allosteric Modulator

“…Photoaffinity labeling and protein microsequencing techniques have been used extensively for identification of amino acids contributing to drug binding sites in many targets (Vodovozova, 2007;Das, 2011), including muscle-type nAChR binding sites for orthosteric ligands, channel blockers, and allosteric modulators (Hamouda et al, 2014;Forman et al, 2015). Photoaffinity labeling identifies amino acid residues within the protein structure that are in direct contact with a bound drug and differentiates them from amino acids not contributing to the binding site but important for drug action, which can be identified by mutational analyses.…”

Nicotinic acetylcholine receptors at the single‐channel level

“…An inter‐subunit transmembrane site between transmembrane helices of adjacent subunits has been identified as a modulatory site for ethanol, general anaesthetics and ivermectin in pentameric ligand‐gated ion channels (Hibbs and Gouaux, ; Lynagh and Lynch, ; Sauguet et al, , ). Photoreactive anaesthetics label homologous residues of the α/γ interface in Torpedo nACh receptors (Husain et al, ; Nirthanan et al, ; Forman et al, ). This site has been proposed for binding of short chain alcohols that positively modulate Torpedo and neuronal nACh receptors by stabilizing the open state (Nagata et al, ; Forman and Zhou, ; Zuo et al, ).…”