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. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Keywords: Alcohol; Allosterism; Barbiturates; Mutagenesis; Photolabel; Propofol.

Figure 1. Transmembrane anesthetic sites on nAChRs

A structural model of Torpedo nAChR (Hamouda et al., 2014) showing peptide backbone as ribbons. Molecular graphics and analyses were performed with the UCSF Chimera package (Pettersen et al., 2004). A) A side view depicts the transmembrane domain (TMD) and extra-cellular domain (ECD). B) A view from the extracellular space. Subunits are colored as follows: α, yellow; β, red; γ, green; δ, purple. C) The TMD viewed from above, depicting three anesthetic sites: the ion channel, an intra-subunit helix bundle, and an inter-subunit site.

Figure 2. Anesthetic photolabeling in the nAChR ion channel

The Torpedo nAChR TMD structure is depicted as backbone ribbons, viewed from the membrane with the γ and one α subunit removed. Anesthetic photolabeled residues are shown as ball-and-stick structures, colored according to which anesthetics modified them: azi-etomidate, TDBzl-etomidate, or TFD-etomidate = gold; propofol or mTFD-MPAB = light blue.

Figure 3. The γ+/α-inter-subunit site

The Torpedo nAChR TMD structure is viewed from the membrane. Anesthetic photolabeled residues are shown as ball-and-stick structures, colored according to which anesthetics modified them: azi-etomidate, TDBzl-etomidate, or TFD-etomidate = gold; mTFD-MPAB = light blue.

Figure 4. The δ-subunit four-helix bundle site

The nAChR TMD structure is depicted as backbone ribbons, viewed from above. The δ subunit four-helix bundle is shown along with another perspective on the γ+/α-interfacial site. Anesthetic photolabeled residues are shown as ball-and-stick structures, colored according to which anesthetics modified them: halothane = green; azi-etomidate = gold; TID or aziPm = dark blue; azi-octanol = cyan.