Distinctions in ligand binding sites on the nicotinic acetylcholine receptor

The conformation of acetylcholine at its target site in the membrane-embedded nicotinic acetylcholine receptor

The conformation of the neurotransmitter acetylcholine bound to the fully functional nicotinic acetylcholine receptor embedded in its native membrane environment has been characterized by using frequency-selective recoupling solid-state NMR. Six dipolar couplings among five resolved (13)C-labeled atoms of acetylcholine were measured. Bound acetylcholine adopts a bent conformation characterized with a quaternary ammonium-to-carbonyl distance of 5.1 A. In this conformation, and with its orientation constrained to that previously determined by us, the acetylcholine could be docked satisfactorily in the agonist pocket of the agonist-bound, but not the agonist-free, crystal structure of a soluble acetylcholine-binding protein from Lymnaea stagnali. The quaternary ammonium group of the acetylcholine was determined to be within 3.9 A of five aromatic residues and its acetyl group close to residues C187/188 of the principle and residue L112 of the complementary subunit. The observed >C O chemical shift is consistent with H bonding to the nicotinic acetylcholine receptor residues gammaY116 and deltaT119 that are homologous to L112 in the soluble acetylcholine-binding protein.

Development of ultra short-acting muscle relaxant agents: History, research strategies, and challenges

Author has reviewed the literature and his own work related to the chemistry, pharmacology, and clinical aspects of new muscle relaxants. Emphasis has been placed on the basic science concepts and technologies (e.g. structure-activity relationships, nicotinic receptor pharmacology, and investigation of side effects) behind the development of rapidly and short acting nondepolarizing muscle relaxants.

Conformation, action, and mechanism of action of neuromuscular blocking muscle relaxants

Since curare was introduced into clinical anaesthesia in 1942, efforts to create better neuromuscular blocking (NMB) muscle relaxants have continued. Today, muscle relaxation remains a mainstay of modern anaesthesia and intensive care. Through manipulation of the traditional structure-action relationships, many new and improved muscle relaxants have been created, and several have been brought to clinical use. However, structure-action relationship is inconsistent and has its limits. Using computer-aided molecular conformational analyses, the conformation-action relationships of NMB agents of various chemical classes have been explored. Conformation, no less than structure, of the NMB agents has shed new light on their mechanisms of action. By reflection, the conformations also suggest new details of the topology of the receptive sites of the nicotinic acetylcholine receptor modeled for the motor endplate of the skeletal muscle.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Molecular approaches to receptors as targets for drug discovery

The cloning of a great number of receptors and channels has revealed that many of these targets for drug discovery can be grouped into superfamilies based on sequence and structural similarities. This review presents an overview of how molecular biological approaches have revealed a plethora of receptor subtypes, led to new definitions of subtypes and isoforms, and played a role in the development of high selective drugs. Moreover, the diversity of subtypes has molded current views of the structure and function of receptor families. Practical difficulties and limitations inherent in the characterization of the ligand binding and signaling properties of expressed recombinant receptors are discussed. The importance of evaluating drug-receptor interactions that differ with temporally transient and distinct receptor conformational states is emphasized. Structural motifs and signal transduction features are presented for the following major receptor superfamilies: ligand-gated ion channel, voltage-dependent ion channel, G-protein coupled, receptor tyrosine-kinase, receptor protein tyrosine-phosphatase, cytokine and nuclear hormone. In addition, a prototypic receptor is analyzed to illustrate functional properties of a given family. The review concludes with a discussion of future directions in receptor research that will impact drug discovery, with a specific focus on orphan receptors as targets for drug discovery. Methods for classifying orphan receptors based upon homologies with members of existing superfamilies are presented together with molecular approaches to the greater challenge of defining their physiological roles. Besides revealing new orphan receptors, the human genome sequencing project will result in the identification of an abundance of novel receptors that will be molecular targets for the development of highly selective drugs. These findings will spur the discovery and development of an exciting new generation of receptor-subtype specific drugs with enhanced therapeutic specificity.

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

Agonist-induced displacement of quinacrine from its binding site on the nicotinic acetylcholine receptor: plausible agonist membrane partitioning mechanism

It was previously demonstrated that high concentrations of cholinergic agonists such as acetylcholine (ACh), carbamylcholine (CCh), suberyldicholine (SubCh) and spin-labelled acetylcholine (SL-ACh) displaced quinacrine from its high-affinity binding site located at the lipid-protein interface of the nicotinic acetylcholine receptor (AChR) (Anas, H. R. and Johnson, D. A. (1995) Biochemistry, 34, 1589-1595). In order to account for the agonist self-inhibitory binding site which overlaps, at least partially, with the quinacrine binding site, we determined the partition coefficient (Kp) of these agonists relative to the local anaesthetic tetracaine in AChR native membranes from Torpedo californica electric organ by examining (1) the ability of tetracaine and SL-ACh to quench membrane-partitioned 1-pyrenedecanoic acid (C10-Py) monomer fluorescence, and (2) the ability of ACh, CCh and SubCh to induce an increase in the excimer/monomer ratio of C10-Py-labelled AChR membrane fluorescence. To further assess the differences in agonist accessibility to the quinacrine binding site, we calculated the agonist concentration in the lipid membrane (CM) at an external agonist concentration high enough to inhibit 50% of quinacrine binding (IC50), which in turn was obtained by agonist back titration of AChR-bound quinacrine. Initial experiments established that high agonist concentrations do not affect either transmembrane proton concentration equilibria (pH) of AChR membrane suspension or AChR-bound quinacrine fluorescence spectra. The agonist membrane partitioning experiments indicated relatively small (< or = 20) Kp values relative to tetracaine. These values follow the order: SL-ACh>SubCh>>CCh-ACh. A direct correlation was observed between Kp and the apparent inhibition constant (Ki) for agonists to displace AChR-bound quinacrine. Particularly, agonist with high KpS such as SL-ACh and SubCh showed low Ki values, and this relationship was opposite for CCh and ACh. The calculated CM values indicated significant (between 7 and 54 mM) agonist accessibility to lipid membrane. By themselves, these results support the conjecture that agonist self-inhibition seems to be mediated by the quinacrine binding site via a membrane approach mechanism. The existence of an agonist self-inhibitory binding site, not located in the channel lumen would indicate an allosteric mechanism of ion channel inhibition; however, we can not discard that the process of agonist self-inhibition can also be mediated by a steric blockage of the ion channel.

Effects of halothane on the nicotinic acetylcholine receptor from Torpedo californica

To determine whether the binding of anesthetics to key membrane receptors is a plausible mode of action, we modeled the effect of the general anesthetic halothane in the nicotinic acetylcholine receptor membrane system isolated from Torpedo californica. Our results demonstrated that halothane inhibits the binding of [3H]phencyclidine ([3H]PCP) to the acetylcholine receptor. The inhibition was reversible, concentration dependent, and had an equilibrium dissociation constant (Kd) of 2.2% atm halothane at 25 degrees. Double-reciprocal plots of the halothane effects at various phencyclidine (PCP) concentrations imply that, under equilibrium conditions, halothane inhibits [3H]PCP binding competitively. In contrast, results from kinetic studies showed that the rate of PCP dissociation is highly sensitive to halothane with EC50 = 0.8% atm halothane in nitrogen. Several possible interpretations are discussed; however, the basic observation was that the kinetics of [3H]PCP binding to the nicotinic acetylcholine receptor was affected by halothane at low concentrations in this model system.

The histrionicotoxin-sensitive ethidium binding site is located outside of the transmembrane domain of the nicotinic acetylcholine receptor: a fluorescence study

A novel, relatively photostable, long-wavelength fluorescent membrane probe, N-(Texas Red sulfonyl)-5(and 6)-dodecanoylamine (C12-Texas Red), was synthesized and used as an electronic energy acceptor for Förster fluorescence resonance energy transfer (FRET) between ethidium bound to a histrionicotoxin-sensitive binding site on the Torpedo nicotinic acetylcholine receptor (AChR) and the lipid membrane surface. FRET from membrane-partitioned 5-(N-dodecanoylamino)fluorescein (C12-fluorescein) to the membrane-partitioned C12-Texas Red was also determined with a parallel set of cuvettes to (1) compare FRET results with a donor in a known position in the membrane and (2) assess the surface density of the membrane-partitioned C12-Texas Red. Stern-Volmer analysis of the FRET results showed that C12-Texas Red quenched membrane-partitioned C12-fluorescein fluorescence 2.9 times more effectively than it quenched the receptor-bound ethidium fluorescence even though the Förster critical distances for the two donor-acceptor pairs were very similar (49.9 and 54.3 A, respectively). Analysis of the ethidium to C12-Texas Red FRET as a function of acceptor surface density with the assumptions that the donor is attached along the major axis of symmetry of a cylindrical protein embedded perpendicularly into the membrane (On-Axis FRET model) suggested that the distance of closest approach between the receptor-bound ethidium and the membrane surface was approximately 52 A. Because the minimum distance between the surface of the lipid-membrane domain and the major symmetry axis of the AChR is approximately 28 A, the FRET results strongly suggest that the ethidium binding site is not located near the entrance of the luminal transmembrane domain is generally assumed.(ABSTRACT TRUNCATED AT 250 WORDS)

Transverse distance between the membrane and the agonist binding sites on the Torpedo acetylcholine receptor: a fluorescence study

Fluorescence dipolar resonance energy transfer between a receptor-bound fluorescent agonist, dansyl-C6-choline, and two membrane-partitioned fluorescent probes, C18-rhodamine and C12-eosin, was used to measure the transverse distance between the acetylcholine (ACh) binding sites on the intact Torpedo nicotinic acetylcholine receptor (nAChR) and the surface of the lipid membrane. Control experiments demonstrated that: (1) dansyl-C6-choline binds to cobra-alpha-toxin sensitive sites on the nAChR with a KD approximately 20 nM, (2) the quantum yield of dansyl-C6-choline increases 3.1-fold upon binding, and (3) the receptor-bound dansyl-C6-choline fluorescence is stable for at least 2 h. The calculated transverse distances between receptor-bound dansyl-C6-choline and the membrane-partitioned acceptors, C12-eosin and C18-rhodamine, were 31 and 39 A, respectively. Therefore, given the dimensions of the extracellular domain of the receptor, the ACh binding sites are located significantly below (approximately 25 A) the extracellular apex of the nAChR. These results are in agreement with the recent proposed location for the ACh binding sites in a pocket within each of the two alpha-subunits, approximately 30 A above the membrane surface (Unwin, N. (1993) J. Mol. Biol. 229: 1101-1124).

Effects of mutations of Torpedo acetylcholine receptor alpha 1 subunit residues 184-200 on alpha-bungarotoxin binding in a recombinant fusion protein

Residues between positions 184 and 200 of the Torpedo acetylcholine receptor alpha 1 subunit were changed by oligonucleotide-directed mutagenesis in a recombinant fusion protein containing residues 166-211. Amino acids were substituted with residues present in the snake alpha subunit, with an alanine, or with a functionally dissimilar residue. The competitive antagonist alpha-bungarotoxin bound to the fusion protein with high apparent affinity (IC50 = 3.2 x 10(-8) M), and binding was competed by agonists and antagonists. Mutation of His-186, Tyr-189, Tyr-190, Cys-192, Cys-193, Pro-194, and Asp-195 greatly reduced or abolished alpha-bungarotoxin binding, while mutation of Tyr-198 reduced binding, indicating these residues play an important role in binding either through functional interaction with neurotoxin residues or by stabilizing the conformation of the binding site. Molecular modeling of acetylcholine receptor residues 184-200 and knowledge of both neurotoxin and receptor residues essential for binding allow analysis of possible structure-function relationships of the interaction of alpha-bungarotoxin with this region of the receptor.

Aromatic trivalent arsenicals: covalent yet reversible reagents for the agonist binding site of nicotinic receptors

The agonist binding site of nicotinic acetylcholine receptors (AChRs) includes a disulfide bond that is easily reduced with dithiothreitol to a pair of thiols, and can be then either reoxidized with dithiobis(nitrobenzoic acid) (DTNB) or irreversibly alkylated with bromoacetylcholine (BAC). Aromatic trivalent arsenicals form stable complexes with pairs of appropriately-spaced thiols, but not single thiols. Furthermore, once complexed in proteins, trivalent arsenicals can be removed with dimercaptans, such as 2,3-dimercaptopropanesulfonic acid (DMPS). In an effort to develop reagents that will covalently, yet reversibly label AChRs, we investigated the effects of two model arsenicals, p-aminophenyldichloroarsine (APA) and 4-bromoacetyl-aminophenylarsenoxide (BAPA) on two types of nicotinic receptors: AChRs from Torpedo electroplax and neuronal receptors from chick retina. APA and BAPA significantly decrease the number of 125I-alpha-bungarotoxin binding sites in reduced Torpedo AChRs. Furthermore, arsenylation of neuronal and Torpedo receptors with APA or BAPA (1) prevents reoxidation with DTNB, (2) is reversible with DMPS, and (3) protects against irreversible alkylation by BAC. In Torpedo receptors, the EC50 of protection against BAC alkylation with APA or BAPA is approximately 30 nM. APA arsenylation of Torpedo receptors persists up to 20 h, but can be reversed at any time with DMPS. These results suggest that heterobifunctional arsenicals could anchor labeling groups in the agonist binding site in order to map the agonist binding site, quantitate receptors, or purify and reconstitute functional receptors.