Seeking structural specificity: direct modulation of pentameric ligand-gated ion channels by alcohols and general anesthetics

Abstract

Alcohols and other anesthetic agents dramatically alter neurologic function in a wide range of organisms, yet their molecular sites of action remain poorly characterized. Pentameric ligand-gated ion channels, long implicated in important direct effects of alcohol and anesthetic binding, have recently been illuminated in renewed detail thanks to the determination of atomic-resolution structures of several family members from lower organisms. These structures provide valuable models for understanding and developing anesthetic agents and for allosteric modulation in general. This review surveys progress in this field from function to structure and back again, outlining early evidence for relevant modulation of pentameric ligand-gated ion channels and the development of early structural models for ion channel function and modulation. We highlight insights and challenges provided by recent crystal structures and resulting simulations, as well as opportunities for translation of these newly detailed models back to behavior and therapy.

Fig. 1.

Spectrum of sites implicated in direct modulation of pentameric ligand-gated ion channels by alcohols and other anesthetic agents. Ribbons depict crystal structure of two neighboring subunits of the prokaryotic GLIC protein, viewed from the channel pore (transparent cylinder). Putative binding sites are indicated by colored ribbons and labeled according to at least one of the GABA_A, glycine (Gly), or nicotinic acetylcholine (ACh) receptor subtypes in which they were implicated. Residues in the M2 helix, many of which have been implicated in multiple subtypes, are labeled in prime notation for ease of comparison. (A) Competitive alcohol/benzodiazepine site, defined by the amino acid residue equivalent to position 100 in the α6 GABA_A receptor subunit (purple). (B) Bromoform molecule (blue sphere) cocrystallized in the extracellular domains of the right-hand ELIC subunit (Protein Data Bank [PDB] ID 3ZKR) superimposed on the GLIC backbone (blue). (C) Amino acid residue 52 in extracellular loop 2 of the left-hand subunit at which mutations influence alcohol modulation of α1 glycine receptors (cyan). (D) Intrasubunit anesthetic binding site(s), defined by the structure of one propofol molecule (green spheres) cocrystallized in the right-hand GLIC subunit (PDB ID 3P50), along with amino acid positions that influenced anesthetic modulation in GLIC (M2 18′; M3 255) or were photolabeled by propofol derivatives (δ M1 232, 236; M2 18′) or sensitive to saturation transfer from halothane (α4 M1 222; β2 M1 212, 224, 231; M2 3′, 22′) in nicotinic receptors (green). (E) Intersubunit pore-facing potentiation site, defined by the structure of one ethanol molecule (yellow spheres) cocrystallized with an ethanol-sensitive GLIC variant (PDB ID 4HFE), along with amino acid positions that influenced alcohol and anesthetic modulation in multiple receptor subtypes (M2 14′–17′, 19′) or were sensitive to halothane saturation transfer in nicotinic receptors (α4 M1 221; M2 20′) (yellow). (F) Pore-blocking anesthetic binding site(s), defined by the structure of bromoform (orange sphere) cocrystallized with ELIC (PDB ID 3P4W), along with amino acid positions photolabeled by propofol derivatives in nicotinic receptors (M2 2′, 6′, 9′, 10′, 13′) (orange). Additional residues (M2 16′, 17′, 20′) influencing anesthetic modulation of nicotinic receptors are depicted in E (yellow). (G) Intersubunit membrane-facing anesthetic site(s), defined by the structure of bromoform (red sphere) cocrystallized with ELIC (PDB ID 3P4W), along with amino acid positions involved in anesthetic fluorescence quenching in ELIC (M1 221, 225) as well as GLIC, photolabeling of GABA_A receptors by anesthetic derivatives (α1 M1 236; M3 294; β3 M3 290), or alcohol and anesthetic modulation of α1 glycine (M1 229; M3 288) and GABA_A receptors (red).