Identification of sites of incorporation in the nicotinic acetylcholine receptor of a photoactivatible general anesthetic

Fishing for allosteric sites on GABAA receptors

GABA(A) receptors have structural and functional homology with a super-family of cys-loop ligand-gated ion channel receptors including the nicotinic acetylcholine receptors. Amino acid residues involved in ligand-binding pockets are homologous among super-family members, leading to the multiple-loop model of binding sites situated at subunit interfaces, validated by structural studies on the nicotinic acetylcholine receptor and water-soluble snail acetylcholine binding protein. This article will briefly review the literature on the agonist binding sites on the receptor super-family, and then describe the current situation for attempts to identify sites for allosteric modulators on the GABA(A) receptors. A combination of mutagenesis and photoaffinity labeling with anesthetic ligands has given some leads in this endeavor. Current work by others and ourselves focuses on three putative sites for modulators: (1) within the ion channel domain TM2, near the extracellular end; (2) the agonist binding sites and homologous pockets at other subunit interfaces of the pentameric receptor; and (3) on the linker region stretching from the agonist site loop C to the top of the TM1 region. It is likely that concrete structural information will be forthcoming soon.

Identification of binding sites in the nicotinic acetylcholine receptor for [3H]azietomidate, a photoactivatable general anesthetic

To identify binding domains in a ligand-gated ion channel for etomidate, an intravenous general anesthetic, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with a photoactivatable analog, [(3)H]azietomidate. Based upon the inhibition of binding of the noncompetitive antagonist [(3)H]phencyclidine, azietomidate and etomidate bind with 10-fold higher affinity to nAChRs in the desensitized state (IC(50) = 70 microm) than in the closed channel state. In addition, both drugs between 0.1 and 1 mm produced a concentration-dependent enhancement of [(3)H]ACh equilibrium binding affinity, but they inhibited binding at higher concentrations. UV irradiation resulted in preferential [(3)H]azietomidate photoincorporation into the nAChR alpha and delta subunits. Photolabeled amino acids in both subunits were identified in the ion channel domain and in the ACh binding sites by Edman degradation. Within the nAChR ion channel in the desensitized state, there was labeling of alphaGlu-262 and deltaGln-276 at the extracellular end and deltaSer-258 and deltaSer-262 toward the cytoplasmic end. Within the acetylcholine binding sites, [(3)H]azietomidate photolabeled alphaTyr-93, alphaTyr-190, and alphaTyr-198 in the site at the alpha-gamma interface and deltaAsp-59 (but not the homologous position, gammaGlu-57). Increasing [(3)H]azietomidate concentration from 1.8 to 150 microm increased the efficiency of incorporation into amino acids within the ion channel by 10-fold and in the ACh sites by 100-fold, consistent with higher affinity binding within the ion channel. The state dependence and subunit selectivity of [(3)H]azietomidate photolabeling are discussed in terms of the structures of the nAChR transmembrane and extracellular domains.

Identification of nicotinic acetylcholine receptor amino acids photolabeled by the volatile anesthetic halothane

To identify inhalational anesthetic binding domains in a ligand-gated ion channel, we photolabeled nicotinic acetylcholine receptor (nAChR)-rich membranes from Torpedo electric organ with [(14)C]halothane and determined by Edman degradation some of the photolabeled amino acids in nAChR subunit fragments isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography. Irradiation at 254 nm for 60 s in the presence of 1 mM [(14)C]halothane resulted in incorporation of approximately 0.5 mol of (14)C/mol of subunit, with photolabeling distributed within the nAChR extracellular and transmembrane domains, primarily at tyrosines. GammaTyr-111 in ACh binding site segment E was labeled, while alphaTyr-93 in segment A was not. Within the transmembrane domain, alphaTyr-213 within alphaM1 and deltaTyr-228 within deltaM1 were photolabeled, while no labeled amino acids were identified within the deltaM2 ion channel domain. Although the efficiency of photolabeling at the subunit level was unaffected by agonist, competitive antagonist, or isoflurane, state-dependent photolabeling was seen in a delta subunit fragment beginning at deltaPhe-206. Labeling of deltaTyr-212 in the extracellular domain was inhibited >90% by d-tubocurarine, whereas addition of either carbamylcholine or isoflurane had no effect. Within M1, the level of photolabeling of deltaTyr-228 with [(14)C]halothane was increased by carbamylcholine (90%) or d-tubocurarine (50%), but it was inhibited by isoflurane (40%). Within the structure of the nAChR transmembrane domain, deltaTyr-228 projects into an extracellular, water accessible pocket formed by amino acids from the deltaM1-deltaM3 alpha-helices. Halothane photolabeling of deltaTyr-228 provides initial evidence that halothane and isoflurane bind within this pocket with occupancy or access increased in the nAChR desensitized state compared to the closed channel state. Halothane binding at this site may contribute to the functional inhibition of nAChRs.

Unique general anesthetic binding sites within distinct conformational states of the nicotinic acetylcholine receptor

General anesthesia is a complex behavioral state provoked by the pharmacological action of a broad range of structurally different hydrophobic molecules called general anesthetics (GAs) on receptor members of the genetically linked ligand-gated ion channel (LGIC) superfamily. This superfamily includes nicotinic acetylcholine (AChRs), type A and C gamma-aminobutyric acid (GABAAR and GABACR), glycine (GlyR), and type 3 5-hydroxytryptamine (5-HT3R) receptors. This review focuses on recent advances in the localization of GA binding sites on conformationally and compositionally distinct AChRs. The experimental evidence outlined in this review suggests that: 1. Several neuronal-type AChRs might be targets for the pharmacological action of distinct GAs. 2. The molecular components of a specific GA binding site on a certain receptor subtype are different from the structural determinants of the locus for the same GA on a different receptor subtype. 3. There are unique binding sites for distinct GAs in the same receptor protein. 4. A GA can activate, potentiate, or inhibit an ion channel, indicating the existence of more than one binding site for the same GA. 5. The affinity of a specific GA depends on the conformational state of the receptor. 6. GAs inhibition channels by at least two mechanisms, an open-channel-blocking and/or an allosteric mechanism. 7. Certain GAs may inhibit AChR function by competing for the agonist binding sites or by augmenting the desensitization rate.

2-(3-Methyl-3H-diaziren-3-yl)ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate: a derivative of the stereoselective general anesthetic etomidate for photolabeling ligand-gated ion channels

To locate general anesthetic binding sites on ligand-gated ion channels, a diazirine derivative of the potent intravenous anesthetic, R-(+)-etomidate (2-ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate), has been synthesized and characterized. R-(+)-Azietomidate [2-(3-methyl-3H-diaziren-3-yl)ethyl 1-(1-phenylethyl)-1H-imidazole-5-carboxylate] anesthetizes tadpoles with an EC(50) of 2.2 microM, identical to that of R-(+)-etomidate. At this concentration both agents equally enhanced GABA-induced currents and decreased binding of the caged-convulsant [(35)S]TBPS to GABA(A) receptors. In all of the above actions R-(+)-azietomidate is about an order of magnitude more potent than S-(-)-azietomidate, an enantioselectivity comparable to etomidate's. R-(+)-Azietomidate also inhibits acetylcholine-induced currents in nicotinic acetylcholine receptors, with about twice the potency of the parent compound. [(3)H]Azietomidate photoincorporated into Torpedo nicotinic acetylcholine receptor-rich membranes. Desensitization decreased photoincorporation into the delta-subunit and increased that into the alpha-subunit. The latter increase was confined to a proteolytic fragment containing the first three transmembrane segments. Thus, R-(+)-azietomidate is a potent stereoselective general anesthetic and an effective photolabel.

Geometric isomers of a photoactivable general anesthetic delineate a binding site on adenylate kinase

General anesthetics are a class of drugs whose mode of action is poorly understood. Here, two photoactivable general anesthetics, n-octan-1-ol geometric isomers bearing a diazirine group on either the third or seventh carbon (3- and 7-azioctanol, respectively), were used to locate and delineate an anesthetic site on adenylate kinase. Each photoincorporated at a mole ratio of 1:1 as determined by mass spectrometry. The photolabeled kinase was subjected to tryptic digest, and the fragments were separated by chromatography and sequenced by mass spectrometry. 3-Azioctanol photolabeled His-36, whereas its isomer, 7-azioctanol, photolabeled Asp-41. Inspection of the known structure of adenylate kinase shows that the side chains of these residues are within approximately 5 A of each other. This distance matches the separation of the 3- and 7-positions of an extended aliphatic chain. The alkanol site so-defined spans two domains of adenylate kinase. His-36 is part of the CORE domain, and Asp-41 belongs to the nucleotide monophosphate binding domain. Upon ligand binding the nucleotide monophosphate binding domain rotates relative to the CORE domain, causing a conformational change that might be expected to affect alkanol binding. Indeed, the substrate-mimicking inhibitor adenosine-(5')-pentaphospho-(5')-adenosine (Ap5A) reduced the photoincorporation of 3-[(3)H]azioctanol by 75%.

The nature of sites of general anaesthetic action

The molecular nature of the site of general anaesthesia has long been sought through the process of comparing the in vivo potencies of general anaesthetics with their physical properties, particularly their ability to dissolve in solvents of various polarities. This approach has led to the conclusion that the site of general anaesthesia is largely apolar but contains a strong polar component. However, there is growing evidence that several physiological targets underlie general anaesthesia, and that different agents may act selectively on subsets of these targets. Consequently research now focuses on the details of general-anaesthetic-protein interactions. There are large amounts of structural data that identify cavities where anaesthetics bind on soluble proteins that are readily crystallizable. These proteins serve as models, having no role in anaesthesia. Two problems make studies of the more likely targets--excitable membrane proteins--difficult. One is that they rarely crystallize and the other is that the sites have their highest affinity for general anaesthetics when the channels are in the open state. Such states rarely exist for more than tens of milliseconds. Crystallographers are making progress with the first problem, whilst anaesthesia researchers have developed a number of strategies for addressing the second. Some of these (kinetic analysis, site-directed mutagenesis) provide indirect evidence for sites and their nature, whilst others seek direct identification of sites by employing newly developed general anaesthetics that are photoaffinity labels. Such studies on acetylcholine, glycine and GABA receptors point to the existence of sites located within the plane of the membrane either within the ion channel lumen (acetylcholine receptor), or on the outer side of the alpha-helix lining that lumen (GABAA and glycine receptors). Bound anaesthetics generally exert their actions on ion channels by binding to allosteric sites whose topology varies from one conformation to another, but definitive proof for this mechanism remains elusive.

Cembranoid and long-chain alkanol sites on the nicotinic acetylcholine receptor and their allosteric interaction

Long-chain alkanols are general anesthetics which can also act as uncharged noncompetitive inhibitors of the peripheral nicotinic acetylcholine receptor (AChR) by binding to one or more specific sites on the AChR. Cembranoids are naturally occurring, uncharged noncompetitive inhibitors of peripheral and neuronal AChRs, which have no demonstrable general anesthetic activity in vivo. In this study, [3H]tenocyclidine ([3H]TCP), an analogue of the cationic noncompetitive inhibitor phencyclidine (PCP), was used to characterize the cembranoid and long-chain alkanol sites on the desensitized Torpedo californica AChR and to investigate if these sites interact. These studies confirm that there is a single cembranoid site which sterically overlaps the [3H]TCP channel site. This cembranoid site probably also overlaps the sites for the cationic noncompetitive inhibitors, procaine and quinacrine. Evidence is also presented for one or more allosteric cembranoid sites which negatively modulate cembranoid affinity for the inhibitory site. In contrast, long-chain alkanols inhibit [3H]TCP binding through an allosteric mechanism involving two or more alkanol sites which display positive cooperativity toward each other. Double inhibitor studies show that the cembranoid inhibitory site and the alkanol sites are not independent of each other but interfere allosterically with each other's inhibition of [3H]TCP binding. The simplest models consistent with the observed data are presented and discussed.

Structure of ligand-gated ion channels: critical assessment of biochemical data supports novel topology

Rapid signaling across the synaptic junction is partially mediated by the ligand-gated ion channel superfamily (LGICS), which includes inhibitory glycine and GABA receptors and excitatory acetylcholine and serotonin receptors. The glycine receptor (GlyR) can assemble as homopentamers of alpha subunits, and baculovirus expression systems are capable of overexpressing large quantities of active receptors. Limited proteolysis coupled to mass spectrometry on reconstituted alpha1 GlyR homopentamers identified proteolytic cleavages within proposed transmembrane domains postulated to fold as bilayer-spanning alpha helices in the "classical" model and identified unexpected membrane-associated regions in the N-terminal domain (J. F. Leite et al., 2000, J. Biol. Chem. 275, 13683-13689). In this review, optimized sequence alignments were used to integrate these proteolysis data with biochemical information determined in studies of all the LGICS members in order to construct a novel topological model.

Anesthetics and ion channels: molecular models and sites of action

The mechanisms of general anesthesia in the central nervous system are finally yielding to molecular examination. As a result of research during the past several decades, a group of ligand-gated ion channels have emerged as plausible targets for general anesthetics. Molecular biology techniques have greatly accelerated attempts to classify ligand-gated ion channel sensitivity to general anesthetics, and have identified the sites of receptor subunits critical for anesthetic modulation using chimeric and mutated receptors. The experimental data have facilitated the construction of tenable molecular models for anesthetic binding sites, which in turn allows structural predictions to be tested. In vivo significance of a putative anesthetic target can now be examined by targeted gene manipulations in mice. In this review, we summarize from a molecular perspective recent advances in our understanding of mechanisms of action of general anesthetics on ligand-gated ion channels.

Breaking the Meyer-Overton rule: predicted effects of varying stiffness and interfacial activity on the intrinsic potency of anesthetics

Exceptions to the Meyer-Overton rule are commonly cited as evidence against indirect, membrane-mediated mechanisms of general anesthesia. However, another interpretation is possible within the context of an indirect mechanism in which solubilization of an anesthetic in the membrane causes a redistribution of lateral pressures in the membrane, which in turn shifts the conformational equilibrium of membrane proteins such as ligand-gated ion channels. It is suggested that compounds of different stiffness and interfacial activity have different intrinsic potencies, i.e., they cause widely different redistributions of the pressure profile (and thus different effects on protein conformational equilibria) per unit concentration of the compound in the membrane. Calculations incorporating the greater stiffness of perfluoromethylenic chains and the large interfacial attraction of hydroxyl groups predict the higher intrinsic potency of short alkanols than alkanes, the cutoffs in potency of alkanes and alkanols and the much shorter cutoffs for their perfluorinated analogues. Both effects, increased stiffness and interfacial activity, are present in unsaturated hydrocarbon solutes, and the intrinsic potencies are predicted to depend on the magnitude of both effects and on the number and locations of multiple bonds within the molecule. Most importantly, the intrinsic potencies of polymeric alkanols with regularly spaced hydroxyl groups are predicted to rise with increasing chain length, without cutoff; such molecules should serve to distinguish unambiguously between indirect mechanisms and direct binding mechanisms of anesthesia.

Mechanism of action of general anaesthetics--new information from molecular pharmacology

Major progress in our understanding of the mechanisms of anaesthesia has been made during the past year. Several key advances in defining very specific sites of action on ligand-gated ion channels have been described. Furthermore, new techniques have become available for addressing the identification of binding sites and transduction mechanisms on these receptors. The discovery that anaesthetics affect a recently identified family of potassium channels could also lead to major new findings in the next few years.

A transmembrane site determines sensitivity of neuronal nicotinic acetylcholine receptors to general anesthetics

Neuronal nicotinic acetylcholine receptors (nAChRs) are potential targets for a wide variety of general anesthetics. We recently showed that alpha(4)beta(2) nAChRs are more sensitive than alpha(4)beta(4) receptors to the gaseous anesthetics nitrous oxide and xenon. The present study examines chimeric and point mutant rat nAChRs expressed in Xenopus oocytes and identifies a single amino acid residue (beta(2)-Val(253) or beta(4)-Phe(255)) near the middle of the second transmembrane segment (TM2) that determines gaseous anesthetic sensitivity. Mutations of this residue in beta subunits and the homologous residue of alpha(4) subunits (alpha(4)-Val(254)) showed that this position also determines sensitivities of nAChRs to acetylcholine, isoflurane, pentobarbital, and hexanol. In contrast, these mutations did not affect actions of ketamine. The positively charged sulfhydryl-specific reagent methanethiosulfonate ethylammonium reacted with a cysteine introduced at alpha(4)-Val(254) or beta(2)-Val(253), and irreversibly reduced anesthetic sensitivities of nAChRs. Propyl methanethiosulfonate is an anesthetic analog that covalently binds to a TM2 site of gamma-aminobutyric acid(A) and glycine receptors and irreversibly enhances receptor function. However, propyl methanethiosulfonate reversibly inhibited cysteine-substitution mutants at alpha(4)-Val(254) or beta(2)-Val(253) of nAChRs, and did not affect anesthetic sensitivity. Thus, residues alpha(4)-Val(254) and beta(2)-Val(253) alter channel gating and determine anesthetic sensitivity of nAChRs, but are not likely to be anesthetic-binding sites.