Nicotinic receptors in wonderland

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To understand how snake neurotoxins interact with nicotinic acetylcholine receptors, we have elaborated an experimentally based model of the ␣-cobratoxin-␣7 receptor complex. This model was achieved by using (i) a three-dimensional model of the ␣7 extracellular domain derived from the crystallographic structure of the homologous acetylcholine-binding protein, (ii) the previously solved x-ray structure of the toxin, and (iii) nine pairs of residues identified by cycle-mutant experiments to make contacts between the ␣-cobratoxin and ␣7 receptor. Because the receptor loop F occludes entrance of the toxin binding pocket, we submitted this loop to a dynamics simulation and selected a conformation that allowed the toxin to reach its binding site. T he ␣-neurotoxins from snake venom are potent antagonists that block nicotinic acetylcholine receptors (AChRs) and hence affect synaptic transmission (1-3). Despite many studies (reviewed in ref. 4), the molecular process associated with this efficient blockage remains unclear. To approach this question, we previously studied ␣-cobratoxin (␣-Cbtx), an ␣͞K neurotoxin that binds to both muscular and homopentameric neuronal receptors (␣7 and ␣8) with high affinities (4). This toxin, similar to other snake neurotoxins, is folded into three adjacent loops rich in ␤-sheet that emerge from a small globular core in which four disulfide bonds are located (5). By mutational analyses, the residues by which ␣-Cbtx interacts with the muscular-type or neuronal ␣7 receptors were identified previously (6, 7). The present study shows how functional residues account for the antagonistic properties of the toxin toward the ␣7 neuronal receptor. The ␣7 AChR possesses five identical ␣7 subunits (8) that offer five ligand-binding sites located at the interface of two subunits (9). These sites include residues located on the different functional loops described previously on the principal ␣7 (ϩ) face, loops A, B, and C and on the complementary ␣7 (Ϫ) face, loops D, E, and F (refs. 10-13; see Fig. 1). Until now, the residues of the ␣7 receptor involved in snake toxin binding have remained unknown.The aim of the present paper is fourfold. First, by an extensive mutational study we have identified ␣7 receptor residues involved in the interaction with ␣-Cbtx. Second, by using a double-mutant cycle approach we have disclosed several pairs of interacting residues in the toxin-receptor complex. Third, by using the three-dimensional (3D) structure of an AChBP that is similar functionally and structurally to the N-terminal domain of an AChR ␣-subunit (14), we used a 3D model for the ␣7 subunit extracellular region obtained by comparative modeling [see accompanying paper on page 3210 (15)]. Fourth, by using this model, a molecular dynamics simulation of the loop F region, and the constraints derived from our pairwise analysis, we propose an experimentally based 3D model of the complex between the ␣-Cbtx and ␣7 receptor, which explains the antagonistic properties of the snake toxin toward the neuronal recepto...

Section: Discussionsupporting

confidence: 89%

Experimentally based model of a complex between a snake toxin and the α7 nicotinic receptor

Fruchart-Gaillard

Gilquin

Antil-Delbeke

et al. 2002

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“…The high affinities mentioned above correspond to those measured at equilibrium on nAChRs in a desensitized state. This observation is consistent with the hypothesis that AChBP was crystallized in a compact conformation that corresponds to a desensitized state for nAChR (45). A model of interaction between ␣-bungarotoxin and AChBP has been presented by Harel et al (46), on the basis of the crystallized-compact-form of AChBP.…”

Section: Model Of Chick ␣7supporting

confidence: 85%

Models of the extracellular domain of the nicotinic receptors and of agonist- and Ca ²⁺ -binding sites

Novère

Grütter

Changeux

2002

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We constructed a three-dimensional model of the amino-terminal extracellular domain of three major types of nicotinic acetylcholine receptor, (␣7)5, (␣4)2(␤2)3, and (␣1)2␤1␥␦, on the basis of the recent x-ray structure determination of the molluscan acetylcholine-binding protein. Comparative analysis of the three models reveals that the agonist-binding pocket is much more conserved than the overall structure. Differences exist, however, in the side chains of several residues. In particular, a phenylalanine residue, present in ␤2 but not in ␣7, is proposed to contribute to the high affinity for agonists in receptors containing the ␤2 subunit. The semiautomatic docking of agonists in the ligand-binding pocket of (␣7)5 led to positions consistent with labeling and mutagenesis experiments. Accordingly, the quaternary ammonium head group of nicotine makes a -cation interaction with W148 (␣7 numbering), whereas the pyridine ring is close to both the cysteine pair 189 -190 and the complementary component of the binding site. The intrinsic affinities inferred from docking give a rank order epibatidine > nicotine > acetylcholine, in agreement with experimental values. Finally, our models offer a structural basis for potentiation by external Ca 2؉ .

“…The AChBP from which the LBD was modeled is thought to be in the active͞desensitized state (20,23,49). The TMD is probably in the closed͞desensitized state (19).…”

Section: Homology Model Analysismentioning

confidence: 99%

A gating mechanism proposed from a simulation of a human α7 nicotinic acetylcholine receptor

Law

Henchman

McCammon

2005

132

142

The nicotinic acetylcholine receptor is a well characterized ligandgated ion channel, yet a proper description of the mechanisms involved in gating, opening, closing, ligand binding, and desensitization does not exist. Until recently, atomic-resolution structural information on the protein was limited, but with the production of the x-ray crystal structure of the Lymnea stagnalis acetylcholine binding protein and the EM image of the transmembrane domain of the torpedo electric ray nicotinic channel, we were provided with a window to examine the mechanism by which this channel operates. A 15-ns all-atom simulation of a homology model of the homomeric human ␣7 form of the receptor was conducted in a solvated palmitoyl-2-oleoyl-sn-glycerol-phosphatidylcholine bilayer and examined in detail. The receptor was unliganded. The structure undergoes a twist-to-close motion that correlates movements of the C loop in the ligand binding domain, via the ␤10-strand that connects the two, with the 10°rotation and inward movement of two nonadjacent subunits. The Cys loop appears to act as a stator around which the ␣-helical transmembrane domain can pivot and rotate relative to the rigid ␤-sheet binding domain. The M2-M3 loop may have a role in controlling the extent or kinetics of these relative movements. All of this motion, along with essential dynamics analysis, is suggestive of the direction of larger motions involved in gating of the channel.T he nicotinic acetylcholine receptor is one of the best-studied ligand-gated ion channels (LGICs) (1). It has been found to have a role in a wide range of pathological conditions and diseases including epilepsy (2), schizophrenia (3), Alzheimer's (4), Parkinson's (5), neuromuscular diseases (6), inflammation (7), nicotine addiction (8), and addiction to other drugs such as alcohol (9) and cocaine (10), as well as a role in anesthetic action (11). The nicotinic acetylcholine receptor is a member of a superfamily of pentameric receptors that include the serotonin (excitory), GABA, and glycine (inhibitory) receptors that are all involved in the transmission and modification of electrical signals in excitory cells. The family is known as the Cys-loop LGIC family, because of a conserved pair of linked cysteines in the N-terminal domain of the receptor (12). Within the superfamily, an array of different nicotinic receptor subunits exists (13,14), making different subunit assemblies possible, for different roles, in different cell types. In all of these receptors, the ligand binds at the interface between the ␣ and other subunits of the ligand binding domain (LBD). This event begins a chain reaction of conformational changes within the receptor that transmits the message of ligand binding to the transmembrane domain (TMD), which then opens to allow the passage of ions through the channel. Although the genetics, kinetics, electrophysiology, and many topological aspects are well characterized for the nicotinic receptor (15, 16), the exact nature of the structural rearrangments involved in activ...