Glutamate 172, essential for modulation of L247T alpha7 ACh receptors by Ca2+, lines the extracellular vestibule

Key role of the TM2-TM3 loop in calcium potentiation of the α9α10 nicotinic acetylcholine receptor

The α9α10 nicotinic cholinergic receptor (nAChR) is a ligand-gated pentameric cation-permeable ion channel that mediates synaptic transmission between descending efferent neurons and mechanosensory inner ear hair cells. When expressed in heterologous systems, α9 and α10 subunits can assemble into functional homomeric α9 and heteromeric α9α10 receptors. One of the differential properties between these nAChRs is the modulation of their ACh-evoked responses by extracellular calcium (Ca2+). While α9 nAChRs responses are blocked by Ca2+, ACh-evoked currents through α9α10 nAChRs are potentiated by Ca2+ in the micromolar range and blocked at millimolar concentrations. Using chimeric and mutant subunits, together with electrophysiological recordings under two-electrode voltage-clamp, we show that the TM2-TM3 loop of the rat α10 subunit contains key structural determinants responsible for the potentiation of the α9α10 nAChR by extracellular Ca2+. Moreover, molecular dynamics simulations reveal that the TM2-TM3 loop of α10 does not contribute to the Ca2+ potentiation phenotype through the formation of novel Ca2+ binding sites not present in the α9 receptor. These results suggest that the TM2-TM3 loop of α10 might act as a control element that facilitates the intramolecular rearrangements that follow ACh-evoked α9α10 nAChRs gating in response to local and transient changes of extracellular Ca2+ concentration. This finding might pave the way for the future rational design of drugs that target α9α10 nAChRs as otoprotectants.

Effects of C-Terminal Carboxylation on α-Conotoxin LsIA Interactions with Human α7 Nicotinic Acetylcholine Receptor: Molecular Simulation Studies

α-Conotoxins selectively bind to nicotinic acetylcholine receptors (nAChRs), which are therapeutic targets due to their important role in signaling transmission in excitable cells. A previous experimental study has demonstrated that carboxylation of the C-terminal of α-conotoxin LsIA reduces its potency to inhibit human α7 nAChR relative to naturally amidated LsIA. However, little is known about the contribution of conformational changes in the receptor and interactions, induced by C-terminal amidation/carboxylation of conotoxins, to selective binding to nAChRs, since most conotoxins and some disulfide-rich peptides from other conotoxin subfamilies possess a naturally amidated C-terminal. In this study, we employ homology modeling and molecular dynamics (MD) simulations to propose the determinants for differential interactions between amidated and carboxylated LsIAs with α7 nAChR. Our findings indicate an overall increased number of contacts favored by binding of amidated LsIA versus its carboxylated counterpart. Toxin-receptor pairwise interactions, which may play a role in enhancing the potency of the former, include ARG10-TRP77, LEU141 and CYS17-GLN79 via persistent hydrogen bonds and cation-π interactions, which are weakened in the carboxylated form due to a strong intramolecular salt-bridge formed by ARG10 and carboxylated C-terminus. The binding of amidated LsIA also induces enhanced movements in loop C and the juxtamembrane Cys-loop that are closely associated with receptor function. Additionally, the impacts of binding of LsIA on the overall structure and inter-subunit contacts were examined using inter-residue network analysis, suggesting a clockwise tilting of the α7 C and F loops upon binding to carboxylated LsIA, which is absent for amidated LsIA binding. The predicted molecular mechanism of LsIA binding to the α7 receptor may provide new insights into the important role of the C-terminal in the binding potency of conotoxins at neuronal nAChRs for pharmacological purposes.

Attention-deficit/hyperactivity disorder: neurophysiology, information processing, arousal and drug development

In this review, we draw on literature from both animal and human neurophysiological studies to consider the neurochemical mechanisms underlying attention-deficit/ hyperactivity disorder (ADHD). Psychophysiological and neuropsychological research is used to propose possible etiological endophenotypes of ADHD. These are conceptualized as patients with distinct cortical-arousal, information-processing or maturational abnormalities, or a combination thereof, and how the endophenotypes can be used to help drug development and optimize treatment and management. To illustrate, the paper focuses on neuro- and psychophysiological evidence that suggests cholinergic mechanisms may underlie specific information-processing abnormalities that occur in ADHD. The clinical implications for a cholinergic hypothesis of ADHD are considered, along with its possible implications for treatment and pharmacological development.

Inhibition of the prokaryotic pentameric ligand-gated ion channel ELIC by divalent cations

The modulation of pentameric ligand-gated ion channels (pLGICs) by divalent cations is believed to play an important role in their regulation in a physiological context. Ions such as calcium or zinc influence the activity of pLGIC neurotransmitter receptors by binding to their extracellular domain and either potentiate or inhibit channel activation. Here we have investigated by electrophysiology and X-ray crystallography the effect of divalent ions on ELIC, a close prokaryotic pLGIC homologue of known structure. We found that divalent cations inhibit the activation of ELIC by the agonist cysteamine, reducing both its potency and, at higher concentrations, its maximum response. Crystal structures of the channel in complex with barium reveal the presence of several distinct binding sites. By mutagenesis we confirmed that the site responsible for divalent inhibition is located at the outer rim of the extracellular domain, at the interface between adjacent subunits but at some distance from the agonist binding region. Here, divalent cations interact with the protein via carboxylate side-chains, and the site is similar in structure to calcium binding sites described in other proteins. There is evidence that other pLGICs may be regulated by divalent ions binding to a similar region, even though the interacting residues are not conserved within the family. Our study provides structural and functional insight into the allosteric regulation of ELIC and is of potential relevance for the entire family.

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

Role of loop 9 on the function of neuronal nicotinic receptors

We have studied the role of loop 9 in the function of neuronal nicotinic receptors. By systematically mutating the residues in the loop we have determined that the most important amino acids determining the coupling of binding to gating are the ones closer to the transmembrane region. Single mutations at location E173 in homomeric alpha7 receptors destroyed their function by completely abolishing the current while preserving the expression at the membrane. In contrast, heteromeric receptor alpha3beta4 with the same mutations retained some function. We conclude that loop 9 has a different role in the function of homomeric and heteromeric receptors.

A residue in loop 9 of the beta2-subunit stabilizes the closed state of the GABAA receptor

In gamma-aminobutyric acid type A (GABA(A)) receptors, the structural elements that couple ligand binding to channel opening remain poorly defined. Here, site-directed mutagenesis was used to determine if Loop 9 on the non-GABA binding site interface of the beta2-subunit may be involved in GABA(A) receptor activation. Specifically, residues Gly(170)-Gln(185) of the beta2-subunit were mutated to alanine, co-expressed with wild-type alpha1- and gamma2S-subunits in human embryonic kidney (HEK) 293 cells and assayed for their activation by GABA, the intravenous anesthetic propofol and the endogenous neurosteroid pregnanolone using whole cell macroscopic recordings. Three mutants, G170A, V175A, and G177A, produced 2.5-, 6.7-, and 5.6-fold increases in GABA EC(50) whereas one mutant, Q185A, produced a 5.2-fold decrease in GABA EC(50). None of the mutations affected the ability of propofol or pregnanolone to potentiate a submaximal GABA response, but the Q185A mutant exhibited 8.3- and 3.5-fold increases in the percent direct activation by propofol and pregnanolone, respectively. Mutant Q185A receptors also had an increased leak current that was sensitive to picrotoxin, indicating an increased gating efficiency. Further Q185E, Q185L, and Q185W substitutions revealed a strong correlation between the hydropathy of the amino acid at this position and the GABA EC(50). Taken together, these results indicate that beta2 Loop 9 is involved in receptor activation by GABA, propofol, and pregnanolone and that beta2(Q185) participates in hydrophilic interactions that are important for stabilizing the closed state of the GABA(A) receptor.

Conformational changes in alpha 7 acetylcholine receptors underlying allosteric modulation by divalent cations

Allosteric modulation of membrane receptors is a widespread mechanism by which endogenous and exogenous agents regulate receptor function. For example, several members of the nicotinic receptor family are modulated by physiological concentrations of extracellular calcium ions. In this paper, we examined conformational changes underlying this modulation and compare these with changes evoked by ACh. Two sets of residues in the α7 acetylcholine receptor extracellular domain were mutated to cysteine and analyzed by measuring the rates of modification by the thiol-specific reagent 2-aminoethylmethane thiosulfonate. Using Ba²⁺ as a surrogate for Ca²⁺, we found a divalent-dependent decrease the modification rates of cysteine substitutions at M³⁷ and M⁴⁰, residues at which rates were also slowed by ACh. In contrast, Ba²⁺ had no significant effect at N⁵²C, a residue where ACh increased the rate of modification. Thus divalent modulators cause some but not all of the conformational effects elicited by agonist. Cysteine substitution of either of two glutamates (E⁴⁴ or E¹⁷²), thought to participate in the divalent cation binding site, caused a loss of allosteric modulation, yet Ba²⁺ still had a significant effect on modification rates of these residues. In addition, the effect of Ba²⁺ at these residues did not appear to be due to direct occlusion. Our data demonstrate that modulation by divalent cations involves substantial conformational changes in the receptor extracellular domain. Our evidence also suggests the modulation occurs via a binding site distinct from one which includes either (or both) of the conserved glutamates at E⁴⁴ or E¹⁷².

Binding-gating coupling in a nondesensitizing alpha7 nicotinic receptor A single channel pharmacological study

The highly conserved alphaLys145 has been suggested to play an important role in the early steps of activation of the nicotinic acetylcholine receptor (nAChR) by acetylcholine. Both macroscopic and single channel currents were recorded in the slowly desensitizing mutants L248T- and K145A-L248T-alpha7 receptors expressed in Xenopus oocytes. On ACh-evoked currents, substitution of Lys145 by alanine showed the same effects that in wild type receptors: moderately decreased gating function and a more-than-expected loss of ACh potency, thus validating the experimental model. Single channel analysis quantitatively agreed with macroscopic data and revealed that impaired gating function in the double mutant alpha7K145A/L248T is the consequence of a slower opening rate, beta. Several nicotinic agonists were also studied, showing important features. Particularly, dimethylphenylpiperazinium (DMPP), acting as an antagonist in alpha7K145A, became a full agonist in alpha7K145A/L248T. Single channel analysis of DMPP-evoked currents showed effects of Lys145 removal similar to those observed with ACh. Data suggest that alpha7Lys145 facilitates the early steps of channel activation. Moreover, the slowly desensitizing mutant alpha7L248T could be an interesting tool for the study of channel activation in alpha7 receptors. Nevertheless, its extensively altered pharmacology precludes the simple extrapolation of pharmacological data obtained in singly mutated alpha7 receptors.

Positive allosteric modulation of the alpha7 nicotinic acetylcholine receptor: ligand interactions with distinct binding sites and evidence for a prominent role of the M2-M3 segment

The alpha7 nicotinic acetylcholine receptor (nAChR), a homopentameric, rapidly activating and desensitizing ligand-gated ion channel with relatively high degree of calcium permeability, is expressed in the mammalian central nervous system, including regions associated with cognitive processing. Selective agonists targeting the alpha7 nAChR have shown efficacy in animal models of cognitive dysfunction. Use of positive allosteric modulators selective for the alpha7 receptor is another strategy that is envisaged in the design of active compounds aiming at improving attention and cognitive dysfunction. The recent discovery of novel positive allosteric modulators such as 1-(5-chloro-2-hydroxyphenyl)-3-(2-chloro-5-trifluoromethylphenyl)urea (NS-1738) and 1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)urea (PNU-120596) that are selective for the alpha7 nAChRs but display significant phenotypic differences in their profile of allosteric modulation, suggests that these molecules may act at different sites on the receptor. Taking advantage of the possibility to obtain functional receptors by the fusion of proteins domains from the alpha7 and the 5-HT(3) receptor, we examined the structural determinants required for positive allosteric modulation. This strategy revealed that the extracellular N-terminal domain of alpha7 plays a critical role in allosteric modulation by NS-1738. In addition, alpha7-5HT(3) chimeras harboring the M2-M3 segment showed that spontaneous activity in response to NS-1738, which confirmed the critical contribution of this small extracellular segment in the receptor gating. In contrast to NS-1738, positive allosteric modulation by PNU-120596 could not be restored in the alpha7-5HT(3) chimeras but was selectively observed in the reverse 5HT(3)-alpha7 chimera. All together, these data illustrate the existence of distinct allosteric binding sites with specificity of different profiles of allosteric modulators and open new possibilities to investigate the alpha7 receptor function.

Calcium modulation of 5-HT3 receptor binding and function

Calcium modulates the 5-HT3 receptor response by reducing peak current amplitude and increasing rates of activation, deactivation and desensitisation, but the binding site(s) and mechanism(s) of this modulation are unknown. Here we study residues that may be involved in calcium binding in two partially overlapping regions of the extracellular domain (E213-E215-E218 and D204-E218-V219). The modulatory effects of calcium were assessed by radioligand binding and whole-cell patch-clamp. Comparisons of [3H]granisetron binding showed an increase in Kd in 10mM calcium that was abolished by the substitutions E213Q, E215Q, D204N and V219L. E218Q mutant receptors displayed no specific binding or function, and immunofluorescence showed that they did not reach the cell surface. E213Q increased inherent rates of desensitisation, but the relative effects of calcium on these rates, and on the reduction in current amplitude, were similar to wild type receptors. Current responses and calcium-mediated effects at E215Q mutant receptors were indistinguishable from wild type. D204N and V219L mutants were non-functional. A calcium impermeable mutant (E277A/S297R) revealed no changes in peak amplitude or kinetics with increased calcium. Our results are consistent with residues D204, E218 and V219 participating in receptor assembly, structure and/or trafficking to the plasma membrane, and we speculate that this might rely upon the stabilising effect of bound calcium. E213, E215, D204 and V219 may contribute to a calcium binding site that is responsible for the calcium-mediated effects on ligand binding. However, the major site for calcium-dependent modulation of the 5-HT3 current is located within the ion channel or cell interior.

Role of negatively charged amino acids in beta 4 F-loop in activation and desensitization of alpha 3 beta 4 rat neuronal nicotinic receptors

The role of negatively charged amino acids in the F-loop of the beta 4 subunit in channel activation and desensitization was studied using the patch-clamp technique. The selected amino acids were changed to their neutral analogs via point mutations. Whole-cell currents were recorded in COS cells transiently transfected with the alpha 3 beta 4 nicotinic acetylcholine receptor. The application of acetylcholine (ACh), nicotine (Nic), cytisine (Cyt), carbamylcholine (CCh) and epibatidine (Epi) to cells clamped at -40 mV produced inward currents which displayed biphasic desensitization. The EC50 of Epi and Nic were increased by a factor of 3-6 due to mutations D191N or D192N. Only Epi remained an agonist in the double-mutated receptors with EC50 increased 17-fold. The interaction of the receptors with the competitive antagonist (+)tubocurarine (TC) was weakened almost 3-fold in the double-mutated receptors. The mutations increased the proportion of the slower desensitization component and increased the response plateau, resulting in decreased receptor desensitization. The double mutation substantially accelerated the return from long-term desensitization induced by Epi.

Agonist-driven conformational changes in the inner beta-sheet of alpha7 nicotinic receptors

Cys-loop ligand-gated ion channels assemble as pentameric proteins, and each monomer contributes two structural elements: an extracellular ligand-binding domain (LBD) and a transmembrane ion channel domain. Models of receptor activation include rotational movements of subunits leading to opening of the ion channel. We tested this idea using substituted cysteine accessibility to track conformational changes in the inner beta sheet of the LBD. Using a nondesensitizing chick alpha7 background (L(247)T), we constructed 18 consecutive cysteine replacement mutants (Leu(36) to Ile(53)) and tested each for expression of acetylcholine (ACh)-evoked currents and functional sensitivity to thiol modification. We measured rates of modification in the presence and absence of ACh to identify conformational changes associated with receptor activation. Resting modification rates of eight substituted cysteines in the beta1 and beta2 strands and the sequence between them (loop 2) varied over several orders of magnitude, suggesting substantial differences in the accessibility or electrostatic environment of individual side chains. These differences were in general agreement with structural models of the LBD. Eight of 18 cysteine replacements displayed ACh-dependent changes in modification rates, indicating a change in the accessibility or electrostatic environment of the introduced cysteine during activation. We were surprised that the effects of agonist exposure were difficult to reconcile with rotational models of activation. Acetylcholine reduced the modification rate of M(40)C but increased it at N(52)C despite the close physical proximity of these residues. Our results suggest that models that depend strictly on rigid-body rotation of the LBD may provide an incomplete description of receptor activation.

Potentiation of acetylcholine receptors by divalent cations

Divalent cations promote activation of several nicotinic acetylcholine receptor (AChR) subtypes, presumably by lowering the energetic barrier between open and closed conformations. In wild-type alpha7 AChRs, binding of calcium to a particular part of the extracellular domain is required for potentiating activation. McLaughlin et al. (p. 16) tested the hypothesis that movements involved in agonist activation and calcium modulation involve a nearby beta sheet by linking strands within this sheet through disulfide bonds formed by replacing adjacent amino acids with cysteines to alter its mobility. These studies are helping to reveal how movements initiated by agonist binding to ACh binding sites are propagated through the extracellular domain of AChRs to regulate opening of the cation channel through the membrane.

Role of the Outer β-Sheet in Divalent Cation Modulation of α7 Nicotinic Receptors

alpha-7 Nicotinic acetylcholine receptors (AChRs) exhibit a positive modulation by divalent cations similar to that observed in other AChRs. In the chick alpha7 AChR, this modulation involves a conserved glutamate in loop 9 (Glu172) that undergoes agonist-dependent movements during activation. From these observations, we hypothesized that movements of the nearby beta-sheet formed by the beta7, beta9, and beta10 strands may be involved in agonist activation and/or divalent modulation. To test this hypothesis, we examined functional properties of cysteine mutations of the beta7 and beta10 strands, alone or in pairs. We postulated that reduced flexibility or mobility of the beta7/beta9/beta10-sheet as a result of introduction of a disulfide bond between the beta strands would alter activation by agonists. Using a nondesensitizing alpha7 mutant background (L247T), we identified one mutant pair, K144C + T198C, that exhibited a unique characteristic: it was fully activated by divalent cations (Ca2+, Ba2+, or Sr2+) in the absence of acetylcholine (ACh). Divalent-evoked currents were blocked by the alpha7 antagonist methyllycaconitine and were abolished when Glu172 was mutated to glutamine. When the K144C + T198C pair was expressed in wild-type alpha7 receptors, activation required both ACh and divalent cations. We conclude that the introduction of a disulfide bond into beta7/beta9/beta10 lowers the energetic barrier between open and closed conformations, probably by reducing the torsional flexibility of the beta-sheet. In this setting, divalent cations, acting at the conserved glutamate in loop 9, act as full agonists or requisite coagonists.

Agonist-induced conformational changes in the extracellular domain of alpha 7 nicotinic acetylcholine receptors

The molecular mechanisms that couple agonist binding to the gating of Cys-loop ionotropic receptors are not well understood. The crystal structure of the acetylcholine (ACh) binding protein has provided insights into the structure of the extracellular domain of nicotinic receptors and a framework for testing mechanisms of activation. Key ligand binding residues are located at the C-terminal end of the beta9 strand. At the N-terminal end of this strand (loop 9) is a conserved glutamate [E172 in chick alpha7 nicotinic acetylcholine receptors (nAChRs)] that is important for modulating activation. We hypothesize that agonist binding induces the movement of loop 9. To test this, we used the substituted-cysteine accessibility method to examine agonist-dependent changes in the modification of cysteines introduced in loop 9 of L247T alpha7 nAChRs. In the absence of agonist, ACh-evoked responses of E172C/L247T alpha7 nAChRs were inhibited by 2-trimethylammonioethylmethane thiosulfonate (MTSET). Agonist coapplication with MTSET reduced the extent and rate of modification. The dose-dependence of ACh activation was nearly identical with that of ACh-dependent protection from modification. ACh increased the inhibition by methanethiosulfonate reagents of N170C and did not change inhibition of G171C receptors. The antagonist dihydro-beta-erythroidine did not mimic the effects of ACh. Combined with a structural model, the data suggest that receptor activation includes subunit rotation and/or intrasubunit conformational changes that move N170 to a more accessible position and E172 to a more protected position away from the vestibule. Thus, loop 9, located near the junction between the extracellular and transmembrane domains, participates in conformational changes triggered by ligand binding.