Interaction of d-tubocurarine analogs with the mouse nicotinic acetylcholine receptor. Ligand orientation at the binding site

Structure-Function of Neuronal Nicotinic Acetylcholine Receptor Inhibitors Derived From Natural Toxins

Neuronal nicotinic acetylcholine receptors (nAChRs) are prototypical cation-selective, ligand-gated ion channels that mediate fast neurotransmission in the central and peripheral nervous systems. nAChRs are involved in a range of physiological and pathological functions and hence are important therapeutic targets. Their subunit homology and diverse pentameric assembly contribute to their challenging pharmacology and limit their drug development potential. Toxins produced by an extensive range of algae, plants and animals target nAChRs, with many proving pivotal in elucidating receptor pharmacology and biochemistry, as well as providing templates for structure-based drug design. The crystal structures of these toxins with diverse chemical profiles in complex with acetylcholine binding protein (AChBP), a soluble homolog of the extracellular ligand-binding domain of the nAChRs and more recently the extracellular domain of human α9 nAChRs, have been reported. These studies have shed light on the diverse molecular mechanisms of ligand-binding at neuronal nAChR subtypes and uncovered critical insights useful for rational drug design. This review provides a comprehensive overview and perspectives obtained from structure and function studies of diverse plant and animal toxins and their associated inhibitory mechanisms at neuronal nAChRs.

A structural and mutagenic blueprint for molecular recognition of strychnine and d-tubocurarine by different cys-loop receptors

Cys-loop receptors (CLR) are pentameric ligand-gated ion channels that mediate fast excitatory or inhibitory transmission in the nervous system. Strychnine and d-tubocurarine (d-TC) are neurotoxins that have been highly instrumental in decades of research on glycine receptors (GlyR) and nicotinic acetylcholine receptors (nAChR), respectively. In this study we addressed the question how the molecular recognition of strychnine and d-TC occurs with high affinity and yet low specificity towards diverse CLR family members. X-ray crystal structures of the complexes with AChBP, a well-described structural homolog of the extracellular domain of the nAChRs, revealed that strychnine and d-TC adopt multiple occupancies and different ligand orientations, stabilizing the homopentameric protein in an asymmetric state. This introduces a new level of structural diversity in CLRs. Unlike protein and peptide neurotoxins, strychnine and d-TC form a limited number of contacts in the binding pocket of AChBP, offering an explanation for their low selectivity. Based on the ligand interactions observed in strychnine- and d-TC-AChBP complexes we performed alanine-scanning mutagenesis in the binding pocket of the human α1 GlyR and α7 nAChR and showed the functional relevance of these residues in conferring high potency of strychnine and d-TC, respectively. Our results demonstrate that a limited number of ligand interactions in the binding pocket together with an energetic stabilization of the extracellular domain are key to the poor selective recognition of strychnine and d-TC by CLRs as diverse as the GlyR, nAChR, and 5-HT₃R.

Ligand-gated ion channels play an important role in fast electrochemical signaling in the brain. Cys-loop receptors are a class of pentameric ligand-gated ion channels that are activated by specific neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), glycine (Gly), and γ-aminobutyric acid (GABA). Each type of cys-loop receptor contains an extracellular domain that specifically recognizes only one of these four neurotransmitters and opens an ion-conducting channel pore upon ligand binding. In this study, we investigated the poor specificity with which two potent neurotoxic inhibitors, namely strychnine and d-tubocurarine, are recognized by different cys-loop receptors. Using X-ray crystallography we solved 3-dimensional structures of strychnine or d-tubocurarine in complex with ACh binding protein (AChBP), a well-recognized structural homolog of the nicotinic ACh receptor. Based on ligand-receptor interactions observed in AChBP structures we designed mutant GlyR and α7 nAChR to identify hot spots in the binding pocket of these receptors that define potent inhibition by strychnine and d-tubocurarine, respectively. Combined, our results offer detailed understanding of the molecular recognition of antagonists that have high affinity but poor specificity for different cys-loop receptors.

Mapping spatial relationships between residues in the ligand-binding domain of the 5-HT3 receptor using a molecular ruler

The serotonin 5-HT(3) receptor (5-HT(3)R) is a member of the Cys-loop ligand-gated ion channel family. We used a combination of site-directed mutagenesis, homology modeling, and ligand-docking simulations to analyze antagonist-receptor interactions. Mutation of E236, which is near loop C of the binding site, to aspartate prevents expression of the receptor on the cell surface, and no specific ligand binding can be detected. On the other hand, mutation to glutamine, asparagine, or alanine produces receptors that are expressed on the cell surface, but decreases receptor affinity for the competitive antagonist d-tubocurarine (dTC) 5-35-fold. The results of a double-mutant cycle analysis employing a panel of dTC analogs to identify specific points of interactions between the dTC analogs and E236 are consistent with E236 making a direct physical interaction with the 12 -OH of dTC. dTC is a rigid molecule of known three-dimensional structure. Together with previous studies linking other regions of dTC to specific residues in the binding site, these data allow us to define the relative spatial arrangement of three different residues in the ligand-binding site: R92 (loop D), N128 (loop A), and E236 (near loop C). Molecular modeling employing these distance constraints followed by molecular-dynamics simulations produced a dTC/receptor complex consistent with the experimental data. The use of the rigid ligands as molecular rulers in conjunction with double-mutant cycle analysis provides a means of mapping the relative positions of various residues in the ligand-binding site of any ligand-receptor complex, and thus is a useful tool for delineating the architecture of the binding site.

Site selectivity of competitive antagonists for the mouse adult muscle nicotinic acetylcholine receptor

The muscle-type nicotinic acetylcholine receptor has two nonidentical binding sites for ligands. The selectivity of acetylcholine and the competitive antagonists (+)-tubocurarine and metocurine for adult mouse receptors is known. Here, we examine the site selectivity for four other competitive antagonists: cisatracurium, pancuronium, vecuronium, and rocuronium. We rapidly applied acetylcholine to outside-out patches from transfected BOSC23 cells and measured macroscopic currents. We have reported the IC(50) of the antagonists individually in prior publications. Here, we determined inhibition by pairs of competitive antagonists. At least one antagonist was present at a concentration producing > or =67% receptor inhibition. Metocurine shifted the apparent IC(50) of (+)-tubocurarine in quantitative agreement with complete competitive antagonism. The same was observed for pancuronium competing with vecuronium. However, pancuronium and vecuronium each shifted the apparent IC(50) of (+)-tubocurarine less than expected for complete competition but more than expected for independent binding. The situation was similar for cisatracurium and (+)-tubocurarine or metocurine. Cisatracurium did not shift the apparent IC(50) of pancuronium or vecuronium, indicating independent binding of these two pairs. The data were fit to a two-site, two-antagonist model to determine the antagonist binding constants for each site, L(alphaepsilon) and L(alphadelta). We found L(alphaepsilon)/L(alphadelta) = 0.22 (range, 0.14-0.34), 20 (9-29), 21 (4-36), and 1.5 (0.3-2.9) for cisatracurium, pancuronium, vecuronium, and rocuronium, respectively. The wide range of L(alphaepsilon)/L(alphadelta) for some antagonists may reflect experimental uncertainties in the low affinity site, relatively poor selectivity (rocuronium), or possibly that the binding of an antagonist at one site affects the affinity of the second site.

Electrostatic steering at acetylcholine binding sites

The electrostatic environments near the acetylcholine binding sites on the nicotinic acetylcholine receptor (nAChR) and acetylcholinesterase were measured by diffusion-enhanced fluorescence energy transfer (DEFET) to determine the influence of long-range electrostatic interactions on ligand binding kinetics and net binding energy. Changes in DEFET from variously charged Tb3+ -chelates revealed net potentials of -20 mV at the nAChR agonist sites and -14 mV at the entrance to the AChE active site, in physiological ionic strength conditions. The potential at the alphadelta-binding site of the nAChR was determined independently in the presence of d-tubocurarine to be -14 mV; the calculated potential at the alphagamma-site was approximately threefold stronger than at the alphadelta-site. By determining the local potential in increasing ionic strength, Debye-Hückel theory predicted that the potentials near the nAChR agonist binding sites are constituted by one to three charges in close proximity to the binding site. Examination of the binding kinetics of the fluorescent acetylcholine analog dansyl-C6-choline at ionic strengths from 12.5 to 400 mM revealed a twofold decrease in association rate. Debye-Hückel analysis of the kinetics revealed a similar charge distribution as seen by changes in the potentials. To determine whether the experimentally determined potentials are reflected by continuum electrostatics calculations, solutions to the nonlinear Poisson-Boltzmann equation were used to compute the potentials expected from DEFET measurements from high-resolution models of the nAChR and AChE. These calculations are in good agreement with the DEFET measurements for AChE and for the alphagamma-site of the nAChR. We conclude that long-range electrostatic interactions contribute -0.3 and -1 kcal/mol to the binding energy at the nAChR alphadelta- and alphagamma-sites due to an increase in association rates.

Mapping residues in the ligand-binding domain of the 5-HT(3) receptor onto d-tubocurarine structure

The serotonin 5-HT(3) receptor (5-HT(3)R) is a member of the cys-loop ligand-gated ion channel family. We have used the combination of site-directed mutagenesis, homology modeling of the 5-HT(3)R extracellular domain, and ligand docking simulations as a way to map the architecture of the 5-HT(3)R ligand binding domain. Mutation of Phe226 in loop C of the binding site to tyrosine (F226Y) has no effect on the apparent affinity of the competitive antagonist d-tubocurarine (dTC) for the receptor. On the other hand, replacement of Asn128 in loop A of the binding site with alanine (N128A) increases the apparent affinity of dTC by approximately 10-fold. Double-mutant cycle analysis employing a panel of dTC analogs with substitutions at various positions to identify specific points of interactions between the dTC analogs and Asn128 suggests that Asn128 makes a direct interaction with the 2'N of dTC. Molecular modeling of the 5-HT(3)R extracellular domain using the antagonist-bound conformation of the Aplysia californica acetylcholine binding protein as a template followed by ligand docking simulations produces two classes of structures of the 5-HT(3)R/dTC complex; only one of these has the 2'N of dTC positioned at Asn128 and thus is consistent with the data from this study and previously published data. The use of the rigid dTC analogs as "molecular rulers" in conjunction with double-mutant cycle analysis of mutant receptors can allow the spatial mapping of the position of various residues in the ligand-binding site.

Interaction of d-tubocurarine analogs with mutant 5-HT(3) receptors

d-Tubocurarine is a potent competitive antagonist of both the muscle-type nicotinic acetylcholine receptor (AChR) and the serotonin type-3 receptor (5HT(3)R). We have previously used a series of structural analogs of d-tubocurarine to demonstrate that the ligand-binding domains of both receptors share common structural features. We have now extended these studies to examine the interaction of a series of d-tubocurarine analogs with 5HT(3)Rs containing mutations at either of two residues within the ligand-binding domain of the receptor (W90F and R92A). The W90F mutation results in an approximately 2-4-fold decrease in the affinity of the analogs relative to wild-type receptors, while the R92A results in an approximately 8-10-fold increase in affinity. However, since the effect of a given mutation is more or less equivalent for all analogs, neither residue W90 nor R92 is likely to make a specific interaction with d-tubocurarine itself. Rather, these two residues are likely to play a role in determining both the geometry of the binding site, as well as the overall environment that a ligand encounters in the binding site.

Orientation of d-tubocurarine in the muscle nicotinic acetylcholine receptor-binding site

Ligand modification and receptor site-directed mutagenesis were used to examine binding of the competitive antagonist, d-tubocurarine (dTC), to the muscle-type nicotinic acetylcholine receptor (AChR). By using various dTC analogs, we measured the interactions of specific dTC functional groups with amino acid positions in the AChR gamma-subunit. Because data for mutations at residue gammaTyr(117) were the most consistent with direct interaction with dTC, we focused on that residue. Double mutant thermodynamic cycle analysis showed apparent interactions of gammaTyr(117) with both the 2-N and the 13'-positions of dTC. Examination of a dTC analog with a negative charge at the 13'-position failed to reveal electrostatic interaction with charged side-chain substitutions at gamma117, but the effects of side-chain substitutions remained consistent with proximity of Tyr(117) to the cationic 2-N of dTC. The apparent interaction of gammaTyr(117) with the 13'-position of dTC was likely mediated by allosteric changes in either dTC or the receptor. The data also show that cation-pi electron stabilization of the 2-N position is not required for high affinity binding. Molecular modeling of dTC within the binding pocket of the acetylcholine-binding protein places the 2-N in proximity to the residue homologous to gammaTyr(117). This model provides a plausible structural basis for binding of dTC within the acetylcholine-binding site of the AChR family that appears consistent with findings from photoaffinity labeling studies and with site-directed mutagenesis studies of the AChR.

Novel potent 5-HT(3) receptor ligands based on the pyrrolidone structure: synthesis, biological evaluation, and computational rationalization of the ligand-receptor interaction modalities

Novel conformationally constrained derivatives of classical 5-HT(3) receptor antagonists were designed and synthesized with the aim of probing the central 5-HT(3) receptor recognition site in a systematic way. The newly-synthesized compounds were tested for their potential ability to inhibit [(3)H]granisetron specific binding to 5-HT(3) receptor in rat cortical membranes. These studies revealed subnanomolar affinity in some of the compounds under study. The most potent ligand in this series was found to be quinuclidine derivative (S)-7i, which showed an affinity comparable with that of the reference ligand granisetron. The potential 5-HT(3) agonist/antagonist activity of some selected compounds was assessed in vitro on the 5-HT(3) receptor-dependent [(14)C]guanidinium uptake in NG 108-15 cells. Both of the tropane derivatives tested in this functional assay (7a and 9a) showed antagonist properties, while the quinuclidine derivatives studied [the enantiomers of compounds 7i, 8g, and 9g, and compound (R)-8h] showed a full range of intrinsic efficacies. Therefore, the functional behavior of these 5-HT(3) receptor ligands appears to be affected by the structural features of both the azabicyclo moiety and the heteroaromatic portion. In agreement with the data obtained on NG 108-15 cells, investigations on the 5-HT(3) receptor-dependent Bezold-Jarisch reflex in urethane-anaesthetized rats confirmed the 5-HT(3) receptor antagonist properties of compounds 7a and (S)-7i showing for these compounds ID(50) values of 2.8 and 181 microg/kg, respectively. Finally, compounds 7a, (S)-7i and 9a (at the doses of 0.01, 1.0, and 0.01 mg/kg ip, respectively) prevented scopolamine-induced amnesia in the mouse passive avoidance test suggestive of a potential usefulness in cognitive disorders for these compounds. Qualitative and quantitative structure-affinity relationship studies were carried out by means of theoretical descriptors derived on a single structure and ad-hoc defined size and shape descriptors (indirect approach). The results showed to be useful in capturing information relevant to ligand-receptor interaction. Additional information derived by the analysis of the energy minimized 3-D structures of the ligand-receptor complexes (direct approach) suggested interesting mechanistic and methodological considerations on the binding mode multiplicity at the 5-HT(3) receptors and on the degree of tolerance allowed in the alignment of molecules for the indirect approach, respectively.

Site-specific charge interactions of alpha-conotoxin MI with the nicotinic acetylcholine receptor

We have tested the importance of charge interactions for alpha-conotoxin MI binding to the nicotinic acetylcholine receptor (AChR). Ionic residues on alpha-conotoxin MI were altered by site-directed mutagenesis or by chemical modification. In physiological buffer, removal of charges at the N terminus, His-5, and Lys-10 had small (2-4-fold) effects on binding affinity to the mouse muscle AChR and the Torpedo AChR. It was also demonstrated that conotoxin had no effect on the conformational equilibrium of either receptor, as assessed by the effects of the noncompetitive antagonist proadifen on conotoxin binding and, conversely, the effect of conotoxin on the affinity of phencyclidine, proadifen, and ethidium. Conotoxin displayed higher binding affinity in low ionic strength buffer; neutralization of Lys-10 and the N terminus by acetylation blocked this affinity shift at the alphadelta site but not at the alphagamma site. It is concluded that Ctx residues Lys-10 and the N terminal interact with oppositely charged receptor residues only at the alphadelta site, and the two sites have distinct arrangements of charged residues. Ethidium fluorescence experiments demonstrated that conotoxin is formally competitive with a small cholinergic ligand, tetramethylammonium. Thus, alpha-conotoxin MI appears to interact with the portion of the binding site responsible for stabilizing agonist cations but does not do so with a cationic residue and is, consequently, incapable of inducing a conformational change.

Interaction of D-tubocurarine analogs with the 5HT3 receptor

D-Tubocurarine is a potent competitive antagonist of two members of the ligand-gated ion channel family, the muscle-type nicotinic acetylcholine receptor (AChR) and serotonin type-3 receptor (5HT3R). We have used a series of analogs of D-tubocurarine to determine the effects of methylation, stereoisomerization and halogenation on the interaction of D-tubocurarine with the 5HT3R. The affinities of the analogs for the 5HT3R span a 200-fold concentration range and fall into three broad groups. The first group, with affinity constants (Ki) < 150 nM, consists of D-tubocurarine and analogs modified at the nitrogens or 7' hydroxyl. The fact that these compounds all have high affinity for the 5HT3R suggests that these portions of the ligand do not make interactions with the receptor that are critical for high-affinity binding. The second group, with Ki's in the 1-5 microM range, consists of analogs modified at the 12'-hydroxyl or the adjacent 13'-carbon, which suggests that this portion of the ligand makes interactions that are important for high-affinity binding. The third, very low affinity, group is a compound with altered stereoconfiguration at the 1 carbon, demonstrating the importance of proper configuration of the antagonist in ligand-receptor interactions. For the most part, this pattern of selectivity is similar to that for the AChR, suggesting that the structures of the ligand-binding sites of these two receptors share common structural features.