Desensitization of diliganded mouse muscle nicotinic acetylcholine receptor channels

Dynamics of receptor activation by agonists

How do agonists turn on receptors? The model system we have used to address this question is the adult-type skeletal muscle nicotinic acetylcholine receptor. This ligand-gated ion channel has two orthosteric sites (for neurotransmitters) in the extracellular domain linked to an allosteric site (a gate) in the transmembrane domain. The goal of this perspective is to summarize how measurements of agonist binding energy reveal the dynamics of the neurotransmitter sites and the fundamental link between binding and gating.

Mutational analysis to explore long-range allosteric couplings involved in a pentameric channel receptor pre-activation and activation

Pentameric ligand-gated ion channels (pLGICs) mediate chemical signaling through a succession of allosteric transitions that are yet not completely understood as intermediate states remain poorly characterized by structural approaches. In a previous study on the prototypic bacterial proton-gated channel GLIC, we generated several fluorescent sensors of the protein conformation that report a fast transition to a pre-active state, which precedes the slower process of activation with pore opening. Here, we explored the phenotype of a series of allosteric mutations, using simultaneous steady-state fluorescence and electrophysiological measurements over a broad pH range. Our data, fitted to a three-state Monod-Wyman-Changeux model, show that mutations at the subunit interface in the extracellular domain (ECD) principally alter pre-activation, while mutations in the lower ECD and in the transmembrane domain principally alter activation. We also show that propofol alters both transitions. Data are discussed in the framework of transition pathways generated by normal mode analysis (iModFit). It further supports that pre-activation involves major quaternary compaction of the ECD, and suggests that activation involves principally a reorganization of a ‘central gating region’ involving a contraction of the ECD β-sandwich and the tilt of the channel lining M2 helix.

Pathways for nicotinic receptor desensitization

Nicotinic acetylcholine receptors (AChRs) are ligand-gated ion channels that generate transient currents by binding agonists and switching rapidly between closed- and open-channel conformations. Upon sustained exposure to ACh, the cell response diminishes slowly because of desensitization, a process that shuts the channel even with agonists still bound. In liganded receptors, the main desensitization pathway is from the open-channel conformation, but after agonists dissociate the main recovery pathway is to the closed-channel conformation. In this Viewpoint, I discuss two mechanisms that can explain the selection of different pathways, a question that has puzzled the community for 60 yr. The first is based on a discrete-state model (the “prism”), in which closed, open, and desensitized conformational states interconnect directly. This model predicts that 5% of unliganded AChRs are desensitized. Different pathways are taken with versus without agonists because ligands have different energy properties (φ values) at the transition states of the desensitization and recovery reactions. The second is a potential energy surface model (the “monkey saddle”), in which the states connect indirectly at a shared transition state region. Different pathways are taken because agonists shift the position of the gating transition state relative to the point where gating and desensitization conformational trajectories intersect. Understanding desensitization pathways appears to be a problem of kinetics rather than of thermodynamics. Other aspects of the two mechanisms are considered, as are experiments that may someday distinguish them.

Fast desensitization of acetylcholine receptors induced by a spider toxin

Nicotinic acetylcholine receptors (nAChRs) are members of the “cys-loop” ligand-gated ion channel superfamily that play important roles in both the peripheral and central system. At the neuromuscular junction, the endplate current is induced by ACh binding and nAChR activation, and then, the current declines to a small steady state, even though ACh is still bound to the receptors. The kinetics of nAChRs with high affinity for ACh but no measurable ion conductance is called desensitization. This adopted desensitization of nAChR channel currents might be an important mechanism for protecting cells against uncontrolled excitation. This study aimed to show that Grammostola spatulata toxin (GsMTx4), which was first purified and characterized from the venom of the tarantula Grammostola spatulata (now genus Phixotricus), can facilitate the desensitization of nAChRs in murine C2C12 myotubes. To examine the details, muscle-type nAChRs, which are expressed heterologously in HEK293T cells, were studied. A single channel current was recorded under the cell-attached configuration, and the channel activity (NP_o) decayed much faster after the addition of GsMTx-4 to the pipette solution. The channel kinetics were further analyzed, and GsMTx-4 affected the channel activity of nAChRs by prolonging the closing time without affecting channel conductance or opening activity. The interaction between nAChRs embedded in the lipid membrane and toxin inserted into the membrane may contribute to the conformational change in the receptor and thus change the channel activity. This new property of GsMTx-4 may lead to a better understanding of the desensitization of ligand-gated channels and disease therapy.

Agonist efficiency from concentration-response curves: Structural implications and applications

Agonists are evaluated by a concentration-response curve (CRC), with a midpoint (EC₅₀) that indicates potency, a high-concentration asymptote that indicates efficacy, and a low-concentration asymptote that indicates constitutive activity. A third agonist attribute, efficiency (η), is the fraction of binding energy that is applied to the conformational change that activates the receptor. We show that η can be calculated from EC₅₀ and the asymptotes of a CRC derived from either single-channel or whole-cell responses. For 20 agonists of skeletal muscle nicotinic receptors, the distribution of η-values is bimodal with population means at 51% (including acetylcholine, nornicotine, and dimethylphenylpiperazinium) and 40% (including epibatidine, varenicline, and cytisine). The value of η is related inversely to the size of the agonist's headgroup, with high- versus low-efficiency ligands having an average volume of 70 vs. 102 Å³. Most binding site mutations have only a small effect on acetylcholine efficiency, except for αY190A (35%), αW149A (60%), and those at αG153 (42%). If η is known, the EC₅₀ and high-concentration asymptote can be calculated from each other. Hence, an entire CRC can be estimated from the response to a single agonist concentration, and efficacy can be estimated from EC₅₀ of a CRC that has been normalized to 1. Given η, the level of constitutive activity can be estimated from a single CRC.

The desensitization pathway of GABAA receptors, one subunit at a time

GABAA receptors mediate most inhibitory synaptic transmission in the brain of vertebrates. Following GABA binding and fast activation, these receptors undergo a slower desensitization, the conformational pathway of which remains largely elusive. To explore the mechanism of desensitization, we used concatemeric α1β2γ2 GABAA receptors to selectively introduce gain-of-desensitization mutations one subunit at a time. A library of twenty-six mutant combinations was generated and their bi-exponential macroscopic desensitization rates measured. Introducing mutations at the different subunits shows a strongly asymmetric pattern with a key contribution of the γ2 subunit, and combining mutations results in marked synergistic effects indicating a non-concerted mechanism. Kinetic modelling indeed suggests a pathway where subunits move independently, the desensitization of two subunits being required to occlude the pore. Our work thus hints towards a very diverse and labile conformational landscape during desensitization, with potential implications in physiology and pharmacology.

Reduced Activation of the Synaptic-Type GABAA Receptor Following Prolonged Exposure to Low Concentrations of Agonists: Relationship between Tonic Activity and Desensitization

Synaptic GABA_A receptors are alternately exposed to short pulses of a high, millimolar concentration of GABA and prolonged periods of low, micromolar concentration of the transmitter. Prior work has indicated that exposure to micromolar concentrations of GABA can both activate the postsynaptic receptors generating sustained low-amplitude current and desensitize the receptors, thereby reducing the peak amplitude of subsequent synaptic response. However, the precise relationship between tonic activation and reduction of peak response is not known. Here, we have measured the effect of prolonged exposure to GABA or the combination of GABA and the neurosteroid allopregnanolone, which was intended to desensitize a fraction of receptors, on a subsequent response to a high concentration of agonist in human α1β3γ2L receptors expressed in Xenopus oocytes. We show that the reduction in the peak amplitude of the post-exposure test response correlates with the open probability of the preceding desensitizing response. Curve fitting of the inhibitory relationship yielded an IC₅₀ of 12.5 µM and a Hill coefficient of -1.61. The activation and desensitization data were mechanistically analyzed in the framework of a three-state Resting-Active-Desensitized model. Using the estimated affinity, efficacy, and desensitization parameters, we calculated the amount of desensitization that would accumulate during a long (2-minute) application of GABA or GABA plus allopregnanolone. The results indicate that accumulation of desensitization depends on the level of activity rather than agonist or potentiator concentration per se. We estimate that in the presence of 1 µM GABA, approximately 5% of α1β3γ2L receptors are functionally eliminated because of desensitization. SIGNIFICANCE STATEMENT: We present an analytical approach to quantify and predict the loss of activatable GABA_A receptors due to desensitization in the presence of transmitter and the steroid allopregnanolone. The findings indicate that the peak amplitude of the synaptic response is influenced by ambient GABA and that changes in ambient concentrations of the transmitter and other GABAergic agents can modify tonically and phasically activated synaptic receptors in opposite directions.

Measurements of the Timescale and Conformational Space of AMPA Receptor Desensitization

Ionotropic glutamate receptors are ligand-gated ion channels that mediate excitatory synaptic transmission in the central nervous system. Desensitization of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype after glutamate binding appears critical for brain function and involves rearrangement of the ligand binding domains (LBDs). Recently, several full-length structures of ionotropic glutamate receptors in putative desensitized states were published. These structures indicate movements of the LBDs that might be trapped by cysteine cross-links and metal bridges. We found that cysteine mutants at the interface between subunits A and C and lateral zinc bridges (between subunits C and D or A and B) can trap freely desensitizing receptors in a spectrum of states with different stabilities. Consistent with a close approach of subunits during desensitization processes, the introduction of bulky amino acids at the A-C interface produced a receptor with slow recovery from desensitization. Further, in wild-type GluA2 receptors, we detected the population of a stable desensitized state with a lifetime around 1 s. Using mutations that progressively stabilize deep desensitized states (E713T and Y768R), we were able to selectively protect receptors from cross-links at both the diagonal and lateral interfaces. Ultrafast perfusion enabled us to perform chemical modification in less than 10 ms, reporting movements associated to desensitization on this timescale within LBD dimers in resting receptors. These observations suggest that small disruptions of quaternary structure are sufficient for fast desensitization and that substantial rearrangements likely correspond to stable desensitized states that are adopted relatively slowly on a timescale much longer than physiological receptor activation.

Obtaining transition rates from single-channel data without initial parameter seeding

Background and Purpose: Ion-channels are membrane proteins that can adopt several distinct structural conformations. Some of the conformations are open and allow the passage of ions through the membrane; others are closed and hinder ion flow. Patch-clamp recordings of single ion-channels show if a channel is open or closed, but does not immediately reveal the underlying mechanism, which typically includes several open and closed conformations.With kinetic analysis of single-channel data, sequences of observed open and closed times are fitted to proposed schemes to deduct the underlying kinetics of the ion-channel. Current programs to perform kinetic analysis uses initial parameter guessing. Here an alternative approach that uses a global fitting procedure and no initial parameter seeding is developed and tested.Methods: Different fitting algorithms that use variations and combinations of Simplex-optimization, Genetic Algorithm and Particle Swarm are tested against simulated data with brief events removed as in real resolution limited data.Results: A two-step fitting algorithm that uses Particle Swarm optimization to find initial parameters and then a modified Simplex approach to fine-adjust the initial parameters successfully find the correct rates used for data simulation.Conclusions: SCAIM (Single Channel Analysis in MATLAB) facilitate the deduction of kinetic schemes underlying single-channel data.

Cryo-EM structures of a lipid-sensitive pentameric ligand-gated ion channel embedded in a phosphatidylcholine-only bilayer

The lipid dependence of the nicotinic acetylcholine receptor from the Torpedo electric organ has long been recognized, and one of the most consistent experimental observations is that, when reconstituted in membranes formed by zwitterionic phospholipids alone, exposure to agonist fails to elicit ion-flux activity. More recently, it has been suggested that the bacterial homolog ELIC (Erwinia chrysanthemi ligand-gated ion channel) has a similar lipid sensitivity. As a first step toward the elucidation of the structural basis of this phenomenon, we solved the structures of ELIC embedded in palmitoyl-oleoyl-phosphatidylcholine- (POPC-) only nanodiscs in both the unliganded (4.1-Å resolution) and agonist-bound (3.3 Å) states using single-particle cryoelectron microscopy. Comparison of the two structural models revealed that the largest differences occur at the level of loop C-at the agonist-binding sites-and the loops at the interface between the extracellular and transmembrane domains (ECD and TMD, respectively). On the other hand, the transmembrane pore is occluded in a remarkably similar manner in both structures. A straightforward interpretation of these findings is that POPC-only membranes frustrate the ECD-TMD coupling in such a way that the "conformational wave" of liganded-receptor gating takes place in the ECD and the interfacial M2-M3 linker but fails to penetrate the membrane and propagate into the TMD. Furthermore, analysis of the structural models and molecular simulations suggested that the higher affinity for agonists characteristic of the open- and desensitized-channel conformations results, at least in part, from the tighter confinement of the ligand to its binding site; this limits the ligand's fluctuations, and thus delays its escape into bulk solvent.

Efficiency measures the conversion of agonist binding energy into receptor conformational change

Receptors alternate between resting↔active conformations that bind agonists with low↔high affinity. Here, we define a new agonist attribute, energy efficiency (η), as the fraction of ligand-binding energy converted into the mechanical work of the activation conformational change. η depends only on the resting/active agonist-binding energy ratio. In a plot of activation energy versus binding energy (an "efficiency" plot), the slope gives η and the y intercept gives the receptor's intrinsic activation energy (without agonists; ΔG₀). We used single-channel electrophysiology to estimate η for eight different agonists and ΔG₀ in human endplate acetylcholine receptors (AChRs). From published equilibrium constants, we also estimated η for agonists of K_Ca1.1 (BK channels) and muscarinic, γ-aminobutyric acid, glutamate, glycine, and aryl-hydrocarbon receptors, and ΔG₀ for all of these except K_Ca1.1. Regarding AChRs, η is 48-56% for agonists related structurally to acetylcholine but is only ∼39% for agonists related to epibatidine; ΔG₀ is 8.4 kcal/mol in adult and 9.6 kcal/mol in fetal receptors. Efficiency plots for all of the above receptors are approximately linear, with η values between 12% and 57% and ΔG₀ values between 2 and 12 kcal/mol. Efficiency appears to be a general attribute of agonist action at receptor binding sites that is useful for understanding binding mechanisms, categorizing agonists, and estimating concentration-response relationships.

The dual-gate model for pentameric ligand-gated ion channels activation and desensitization

Pentameric ligand-gated ion channels (pLGICs) mediate fast neurotransmission in the nervous system. Their dysfunction is associated with psychiatric, neurological and neurodegenerative disorders such as schizophrenia, epilepsy and Alzheimer's disease. Understanding their biophysical and pharmacological properties, at both the functional and the structural level, thus holds many therapeutic promises. In addition to their agonist-elicited activation, most pLGICs display another key allosteric property, namely desensitization, in which they enter a shut state refractory to activation upon sustained agonist binding. While the activation mechanisms of several pLGICs have been revealed at near-atomic resolution, the structural foundation of desensitization has long remained elusive. Recent structural and functional data now suggest that the activation and desensitization gates are distinct, and are located at both sides of the ion channel. Such a 'dual gate mechanism' accounts for the marked allosteric effects of channel blockers, a feature illustrated herein by theoretical kinetics simulations. Comparison with other classes of ligand- and voltage-gated ion channels shows that this dual gate mechanism emerges as a common theme for the desensitization and inactivation properties of structurally unrelated ion channels.

Emerging Molecular Mechanisms of Signal Transduction in Pentameric Ligand-Gated Ion Channels

Nicotinic acetylcholine, serotonin type 3, γ-amminobutyric acid type A, and glycine receptors are major players of human neuronal communication. They belong to the family of pentameric ligand-gated ion channels, sharing a highly conserved modular 3D structure. Recently, high-resolution structures of both open- and closed-pore conformations have been solved for a bacterial, an invertebrate, and a vertebrate receptor in this family. These data suggest that a common gating mechanism occurs, coupling neurotransmitter binding to pore opening, but they also pinpoint significant differences among subtypes. In this Review, we summarize the structural and functional data in light of these gating models and speculate about their mechanistic consequences on ion permeation, pathological mutations, as well as functional regulation by orthosteric and allosteric effectors.

Structural mechanisms of activation and desensitization in neurotransmitter-gated ion channels

Ion channels gated by neurotransmitters are present across metazoans, in which they are essential for brain function, sensation and locomotion; closely related homologs are also found in bacteria. Structures of eukaryotic pentameric cysteine-loop (Cys-loop) receptors and tetrameric ionotropic glutamate receptors in multiple functional states have recently become available. Here, I describe how these studies relate to established ideas regarding receptor activation and how they have enabled decades' worth of functional work to be pieced together, thus allowing previously puzzling aspects of receptor activity to be understood.

Crystal structure and dynamics of a lipid-induced potential desensitized-state of a pentameric ligand-gated channel

Desensitization in pentameric ligand-gated ion channels plays an important role in regulating neuronal excitability. Here, we show that docosahexaenoic acid (DHA), a key ω−3 polyunsaturated fatty acid in synaptic membranes, enhances the agonist-induced transition to the desensitized state in the prokaryotic channel GLIC. We determined a 3.25 Å crystal structure of the GLIC-DHA complex in a potentially desensitized conformation. The DHA molecule is bound at the channel-periphery near the M4 helix and exerts a long-range allosteric effect on the pore across domain-interfaces. In this previously unobserved conformation, the extracellular-half of the pore-lining M2 is splayed open, reminiscent of the open conformation, while the intracellular-half is constricted, leading to a loss of both water and permeant ions. These findings, in combination with spin-labeling/EPR spectroscopic measurements in reconstituted-membranes, provide novel mechanistic details of desensitization in pentameric channels.

DOI:http://dx.doi.org/10.7554/eLife.23886.001

The nerve cells (or neurons) in the brain communicate with each other by releasing chemicals called neurotransmitters that bind to ion channels on neighboring neurons. This ultimately causes ions to flow in or out of the receiving neuron through these ion channels; this ion flow determines how the neuron responds.

One family of ion channels that is found at the junction between neurons, and between neurons and muscle fibers, is known as the pentameric ligand-gated ion channels (or pLGICs). These channels act as ‘gates’ that open to allow ions through them when a neurotransmitter binds to the channel. In addition to the open ‘active’ state, the channels can take on two different ‘inactive’ states that do not allow ions to pass through the channel: a closed (resting) state and a desensitized state (that is still bound to the neurotransmitter). Understanding how channels switch between these states is important for designing drugs that correct problems that cause the channels to work incorrectly.

Problems that affect the desensitized state have been linked to neurological disorders such as epilepsy. Medically important molecules such as anesthetics and alcohols are thought to affect desensitization, and drugs that target desensitized ion channels may present ways of treating neurological disorders with fewer side effects.

Docosahexaenoic acid (DHA) is an abundant lipid molecule that is present in the membranes of neurons. It is one of the key ingredients in fish oil supplements and is thought to enhance learning and memory. DHA affects the desensitization of pLGICs but it is not clear exactly how it does so.

Basak et al. now show that DHA affects a bacterial pLGIC in the same way as it affects human channels – by enhancing desensitization. Using a technique called X-ray crystallography to analyze the channel while bound to DHA revealed a previously unobserved channel structure. The DHA molecule binds to a site at the edge of the channel and causes a change in its structure that leaves the upper part of the channel open while the lower part is constricted. Basak et al. predict that molecules such as anesthetics target this desensitized state.

The next step will be to obtain the structures of bacterial and human pLGIC channels in a natural membrane environment. This will allow us to better understand the changes in structure that the channels go through as they transmit signals between neurons, and so help in the development of new treatments for neurological disorders.

DOI:http://dx.doi.org/10.7554/eLife.23886.002

Molecular recognition at cholinergic synapses: acetylcholine versus choline

KEY POINTS

Neuromuscular acetylcholine (ACh) receptors have a high affinity for the neurotransmitter ACh and a low affinity for its metabolic product choline. At each transmitter binding site three aromatic groups determine affinity, and together provide ∼50% more binding energy for ACh than for choline. Deprotonation of αY190 by a nearby lysine strengthens the interaction between this aromatic ring and both ACh and choline. H-bonds position ACh and choline differently in the aromatic cage to generate the different affinities.

ABSTRACT

Acetylcholine (ACh) released at the vertebrate nerve-muscle synapse is hydrolysed rapidly to choline (Cho), so endplate receptors (AChRs) are exposed to high concentrations of both of these structurally related ligands. To understand how these receptors distinguish ACh and Cho, we used single-channel electrophysiology to measure resting affinities (binding free energies) of these and other agonists in adult-type mouse AChRs having a mutation(s) at the transmitter-binding sites. The aromatic rings of αY190, αW149 and αY198 each provide ∼50% less binding energy for Cho compared to ACh. At αY198 a phenylalanine substitution had no effect, but at αY190 this substitution caused a large, agonist-independent loss in binding energy that depended on the presence of αK145. The results suggest that (1) αY190 is deprotonated by αK145 to strengthen the interaction between this benzene ring and the agonist's quaternary ammonium (QA) and (2) AChRs respond strongly to ACh because an H-bond positions the QA to interact optimally with the rings, and weakly to Cho because a different H-bond tethers the ligand to misalign the QA and form weaker interactions with the aromatic groups. The results suggest that the difference in ACh versus Cho binding energies is determined by different ligand positions within a fixed protein structure.

Modal affinities of endplate acetylcholine receptors caused by loop C mutations

Modal activity at the nicotinic acetylcholine receptor, in which open channel probability switches reversibly between discrete values, arises from changes in the resting affinity at the agonist site.

The time course of the endplate current is determined by the rate and equilibrium constants for acetylcholine receptor (AChR) activation. We measured these constants in single-channel currents from AChRs with mutations at the neurotransmitter-binding sites, in loop C. The main findings are: (a) Almost all perturbations of loop C generate heterogeneity in the channel open probability (“modes”). (b) Modes are generated by different affinities for ACh that can be either higher or lower than in the wild-type receptors. (c) The modes are stable, in so far as each receptor maintains its affinity for at least several minutes. (d) Different agonists show different degrees of modal activity. With the loop C mutation αP197A, there are four modes with ACh but only two with partial agonists. (e) The affinity variations arise exclusively from the αδ-binding site. (f) Substituting four γ-subunit residues into the δ subunit (three in loop E and one in the β5–β5′ linker) reduces modal activity. (g) At each neurotransmitter-binding site, affinity is determined by a core of five aromatic residues. Modes are eliminated by an alanine mutation at δW57 but not at the other aromatics. (h) Modes are eliminated by a phenylalanine substitution at all core aromatics except αY93. The results suggest that, at the αδ agonist site, loop C and the complementary subunit surface can each adopt alternative conformations and interact with each other to influence the position of δW57 with respect to the aromatic core and, hence, affinity.

Agonist activation of a nicotinic acetylcholine receptor

How does an agonist activate a receptor? In this article I consider the activation process in muscle nicotinic acetylcholine receptors (AChRs), a prototype for understanding the energetics of binding and gating in other ligand-gated ion channels. Just as movements that generate gating currents activate voltage-gated ion channels, movements at binding sites that generate an increase in affinity for the agonist activate ligand-gated ion channels. The main topics are: i) the schemes and intermediate states of AChR activation, ii) the energy changes of each of the steps, iii) the sources of the energies, iv) the three kinds of AChR agonist binding site and v) the correlations between binding and gating energies. The binding process is summarized as sketches of different conformations of an agonist site. The results suggest that agonists lower the free energy of the active conformation of the protein in stages by establishing favorable, local interactions at each binding site, independently. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Bispyridinium Compounds Inhibit Both Muscle and Neuronal Nicotinic Acetylcholine Receptors in Human Cell Lines

Standard treatment of poisoning by organophosphorus anticholinesterases uses atropine to reduce the muscarinic effects of acetylcholine accumulation and oximes to reactivate acetylcholinesterase (the effectiveness of which depends on the specific anticholinesterase), but does not directly address the nicotinic effects of poisoning. Bispyridinium molecules which act as noncompetitive antagonists at nicotinic acetylcholine receptors have been identified as promising compounds and one has been shown to improve survival following organophosphorus poisoning in guinea-pigs. Here, we have investigated the structural requirements for antagonism and compared inhibitory potency of these compounds at muscle and neuronal nicotinic receptors and acetylcholinesterase. A series of compounds was synthesised, in which the length of the polymethylene linker between the two pyridinium moieties was increased sequentially from one to ten carbon atoms. Their effects on nicotinic receptor-mediated calcium responses were tested in muscle-derived (CN21) and neuronal (SH-SY5Y) cells. Their ability to inhibit acetylcholinesterase activity was tested using human erythrocyte ghosts. In both cell lines, the nicotinic response was inhibited in a dose-dependent manner and the inhibitory potency of the compounds increased with greater linker length between the two pyridinium moieties, as did their inhibitory potency for human acetylcholinesterase activity in vitro. These results demonstrate that bispyridinium compounds inhibit both neuronal and muscle nicotinic receptors and that their potency depends on the length of the hydrocarbon chain linking the two pyridinium moieties. Knowledge of structure-activity relationships will aid the optimisation of molecular structures for therapeutic use against the nicotinic effects of organophosphorus poisoning.

Activation of endplate nicotinic acetylcholine receptors by agonists

The interaction of a small molecule made in one cell with a large receptor made in another is the signature event of cell signaling. Understanding the structure and energy changes associated with agonist activation is important for engineering drugs, receptors and synapses. The nicotinic acetylcholine receptor (AChR) is a ∼300kD ion channel that binds the neurotransmitter acetylcholine (ACh) and other cholinergic agonists to elicit electrical responses in the central and peripheral nervous systems. This mini-review is in two sections. First, general concepts of skeletal muscle AChR operation are discussed in terms of energy landscapes for conformational change. Second, adult vs. fetal AChRs are compared with regard to interaction energies between ACh and agonist-site side chains, measured by single-channel electrophysiology and molecular dynamics simulations. The five aromatic residues that form the core of each agonist binding site can be divided into two working groups, a triad (led by αY190) that behaves similarly at all sites and a coupled pair (led by γW55) that has a large influence on affinity only in fetal AChRs. Each endplate AChR has 5 homologous subunits, two of α(1) and one each of β, δ, and either γ (fetal) or ϵ (adult). These nicotinic AChRs have only 2 functional agonist binding sites located in the extracellular domain, at αδ and either αγ or αϵ subunit interfaces. The receptor undergoes a reversible, global isomerization between structures called C and O. The C shape does not conduct ions and has a relatively low affinity for ACh, whereas O conducts cations and has a higher affinity. When both agonist sites are empty (filled only with water) the probability of taking on the O conformation (PO) is low, <10(-6). When ACh molecules occupy the agonist sites the C→O opening rate constant and C↔O gating equilibrium constant increase dramatically. Following a pulse of ACh at the nerve-muscle synapse, the endplate current rises rapidly to reach a peak that corresponds to PO ∼0.96.

Function of the M1 π-helix in endplate receptor activation and desensitization

KEY POINTS

A conserved proline in M1 causes a kink between α and π helical segments. The kink is under greater tension in the resting versus active conformation. The kink and the agonist do not interact directly. The π-helix separates the gating functions of the extracellular and transmembrane domains. Mutations of the conserved proline and propofol increase desensitization.

ABSTRACT

Nicotinic acetylcholine receptors (AChRs) switch on/off to generate transient membrane currents (C↔O; closed-open 'gating') and enter/recover from long-lived, refractory states (O↔D; 'desensitization'). The M1 transmembrane helix of the muscle endplate AChR is linked to a β-strand of the extracellular domain that extends to a neurotransmitter binding site. We used electrophysiology to measure the effects of mutations of amino acids that are located at a proline kink in M1 that separates π and α helices, in both α (N217, V218 and P221) and non-α subunits. In related receptors, the kink is straighter and more stable in O vs. C structures (gating is 'spring-loaded'). None of the AChR kink mutations had a measureable effect on agonist affinity but many influenced the allosteric gating constant substantially. Side chains in the M1 α-helix experience extraordinarily large energy differences between C and O structures, probably because of a ∼2 Å displacement and tilt of M2 relative to M1. There is a discrete break in the character of the gating transition state between αN217 and αV218, indicating that the π-helix is a border between extracellular- and transmembrane-domain function. Mutations of the conserved M1 proline, and the anaesthetic propofol, increase a rate constant for desensitization. The results suggest that straightening of the M1 proline kink triggers AChR desensitization.

Kinetic properties and open probability of α7 nicotinic acetylcholine receptors

The alpha7 nicotinic acetylcholine receptor (nAChR) has some peculiar kinetic properties. From the literature of α7 nAChR-mediated currents we concluded that experimentally measured kinetic properties reflected properties of the solution exchange system, rather than genuine kinetic properties of the receptors. We also concluded that all experimentally measured EC50 values for agonists must inherently be inaccurate. The aim of this study was to assess the undistorted kinetic properties of α7 nAChRs, and to construct an improved kinetic model, which can also serve as a basis of modeling the effect of the positive allosteric modulator PNU-120596, as it is described in the accompanying paper. Agonist-evoked currents were recorded from GH4C1 cells stably transfected with pCEP4/rat α7 nAChR using patch-clamp and fast solution exchange. We used two approaches to circumvent the problem of insufficient solution exchange rate: extrapolation and kinetic modeling. First, using different solution exchange rates we recorded evoked currents, and extrapolated their amplitude and kinetics to instantaneous solution exchange. Second, we constructed a kinetic model that reproduced concentration-dependence and solution exchange rate-dependence of receptors, and then we simulated receptor behavior at experimentally unattainably fast solution exchange. We also determined open probabilities during choline-evoked unmodulated and modulated currents using nonstationary fluctuation analysis. The peak open probability of 10 mM choline-evoked currents was 0.033 ± 0.006, while in the presence of choline (10 mM) and PNU-120596 (10 μM), it was increased to 0.599 ± 0.058. Our kinetic model could adequately reproduce low open probability, fast kinetics, fast recovery and solution exchange rate-dependent kinetics.

Intermediate closed state for glycine receptor function revealed by cysteine cross-linking

Pentameric ligand-gated ion channels (pLGICs) mediate signal transmission by coupling the binding of extracellular ligands to the opening of their ion channel. Agonist binding elicits activation and desensitization of pLGICs, through several conformational states, that are, thus far, incompletely characterized at the structural level. We previously reported for GLIC, a prokaryotic pLGIC, that cross-linking of a pair of cysteines at both sides of the extracellular and transmembrane domain interface stabilizes a locally closed (LC) X-ray structure. Here, we introduced the homologous pair of cysteines on the human α1 glycine receptor. We show by electrophysiology that cysteine cross-linking produces a gain-of-function phenotype characterized by concomitant constitutive openings, increased agonist potency, and equalization of efficacies of full and partial agonists. However, it also produces a reduction of maximal currents at saturating agonist concentrations without change of the unitary channel conductance, an effect reversed by the positive allosteric modulator propofol. The cross-linking thus favors a unique closed state distinct from the resting and longest-lived desensitized states. Fitting the data according to a three-state allosteric model suggests that it could correspond to a LC conformation. Its plausible assignment to a gating intermediate or a fast-desensitized state is discussed. Overall, our data show that relative movement of two loops at the extracellular-transmembrane interface accompanies orthosteric agonist-mediated gating.

Asymmetric transmitter binding sites of fetal muscle acetylcholine receptors shape their synaptic response

Neuromuscular acetylcholine receptors (AChRs) have two transmitter binding sites: at α-δ and either α-γ (fetal) or α-ε (adult) subunit interfaces. The γ-subunit of fetal AChRs is indispensable for the proper development of neuromuscular synapses. We estimated parameters for acetylcholine (ACh) binding and gating from single channel currents of fetal mouse AChRs expressed in tissue-cultured cells. The unliganded gating equilibrium constant is smaller and less voltage-dependent than in adult AChRs. However, the α-γ binding site has a higher affinity for ACh and provides more binding energy for gating compared with α-ε; therefore, the diliganded gating equilibrium constant at -100 mV is comparable for both receptor subtypes. The -2.2 kcal/mol extra binding energy from α-γ compared with α-δ and α-ε is accompanied by a higher resting affinity for ACh, mainly because of slower transmitter dissociation. End plate current simulations suggest that the higher affinity and increased energy from α-γ are essential for generating synaptic responses at low pulse [ACh].

An outline of desensitization in pentameric ligand-gated ion channel receptors

Pentameric ligand-gated ion channel (pLGIC) receptors exhibit desensitization, the progressive reduction in ionic flux in the prolonged presence of agonist. Despite its pathophysiological importance and the fact that it was first described over half a century ago, surprisingly little is known about the structural basis of desensitization in this receptor family. Here, we explain how desensitization is defined using functional criteria. We then review recent progress into reconciling the structural and functional basis of this phenomenon. The extracellular-transmembrane domain interface is a key locus. Activation is well known to involve conformational changes at this interface, and several lines of evidence suggest that desensitization involves a distinct conformational change here that is incompatible with activation. However, major questions remain unresolved, including the structural basis of the desensitization-induced agonist affinity increase and the mechanism of pore closure during desensitization.

The energetic consequences of loop 9 gating motions in acetylcholine receptor-channels

Acetylcholine receptor-channels (AChRs) mediate fast synaptic transmission between nerve and muscle. In order to better-understand the mechanism by which this protein assembles and isomerizes between closed- and open-channel conformations we measured changes in the diliganded gating equilibrium constant (E(2)) consequent to mutations of residues at the C-terminus of loop 9 (L9) in the α and ε subunits of mouse neuromuscular AChRs. These amino acids are close to two interesting interfaces, between the extracellular and transmembrane domain within a subunit (E–T interface) and between primary and complementary subunits (P–C interface). Most α subunit mutations modestly decreased E(2) (mainly by slowing the channel-opening rate constant) and sometimes produced AChRs that had heterogeneous gating kinetic properties. Mutations in the ε subunit had a larger effect and could either increase or decrease E(2), but did not induce kinetic heterogeneity. There are broad-but-weak energetic interactions between αL9 residues and others at the αE–T interface, as well as between the εL9 residue and others at the P–C interface (in particular, the M2–M3 linker). These interactions serve, in part, to maintain the structural integrity of the AChR assembly at the E–T interface. Overall, the energy changes of L9 residues are significant but smaller than in other regions of the protein.

Role of acetylcholinesterase on the structure and function of cholinergic synapses: insights gained from studies on knockout mice

Electrophysiological and ultrastructural studies were performed on phrenic nerve-hemidiaphragm preparations isolated from wild-type and acetylcholinesterase (AChE) knockout (KO) mice to determine the compensatory mechanisms manifested by the neuromuscular junction to excess acetylcholine (ACh). The diaphragm was selected since it is the primary muscle of respiration, and it must adapt to allow for survival of the organism in the absence of AChE. Nerve-elicited muscle contractions, miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) were recorded by conventional electrophysiological techniques from phrenic nerve-hemidiaphragm preparations isolated from 1.5- to 2-month-old wild-type (AChE(+/+)) or AChE KO (AChE(-/-)) mice. These recordings were chosen to provide a comprehensive assessment of functional alterations of the diaphragm muscle resulting from the absence of AChE. Tension measurements from AChE(-/-) mice revealed that the amplitude of twitch tensions was potentiated, but tetanic tensions underwent a use-dependent decline at frequencies below 70 Hz and above 100 Hz. MEPPs recorded from hemidiaphragms of AChE(-/-) mice showed a reduction in frequency and a prolongation in decay (37%) but no change in amplitude compared to values observed in age-matched wild-type littermates. In contrast, MEPPs recorded from hemidiaphragms of wild-type mice that were exposed for 30 min to the selective AChE inhibitor 5-bis(4-allyldimethyl-ammoniumphenyl)pentane-3-one (BW284C51) exhibited a pronounced increase in amplitude (42%) and a more marked prolongation in decay (76%). The difference between MEPP amplitudes and decays in AChE(-/-) hemidiaphragms and in wild-type hemidiaphragms treated with BW284C51 represents effective adaptation by the former to a high ACh environment. Electron microscopic examination revealed that diaphragm muscles of AChE(-/-) mice had smaller nerve terminals and diminished pre- and post-synaptic surface contacts relative to neuromuscular junctions of AChE(+/+) mice. The morphological changes are suggested to account, in part, for the ability of muscle from AChE(-/-) mice to function in the complete absence of AChE.

Desensitization of neurotransmitter-gated ion channels during high-frequency stimulation: a comparative study of Cys-loop, AMPA and purinergic receptors

Changes in synaptic strength allow synapses to regulate the flow of information in the neural circuits in which they operate. In particular, changes lasting from milliseconds to minutes (‘short-term changes') underlie a variety of computational operations and, ultimately, behaviours. Most studies thus far have attributed the short-term type of plasticity to activity-dependent changes in the dynamics of neurotransmitter release (a presynaptic mechanism) while largely dismissing the role of the loss of responsiveness of postsynaptic receptor channels to neurotransmitter owing to entry into desensitization. To better define the response of the different neurotransmitter-gated ion channels (NGICs) to repetitive stimulation without interference from presynaptic variables, we studied eight representative members of all three known superfamilies of NGICs in fast-perfused outside-out patches of membrane. We found that the responsiveness of all tested channels (two nicotinic acetylcholine receptors, two glycine receptors, one GABA receptor, two AMPA-type glutamate receptors and one purinergic receptor) declines along trains of brief neurotransmitter pulses delivered at physiologically relevant frequencies to an extent that suggests that the role of desensitization in the synaptic control of action-potential transmission may be more general than previously thought. Furthermore, our results indicate that a sizable fraction (and, for some NGICs, most) of this desensitization occurs during the neurotransmitter-free interpulse intervals. Clearly, an incomplete clearance of neurotransmitter from the synaptic cleft between vesicle-fusion events need not be invoked to account for NGIC desensitization upon repetitive stimulation.

Acetylcholine receptor channels activated by a single agonist molecule

The neuromuscular acetylcholine receptor (AChR) is an allosteric protein that alternatively adopts inactive versus active conformations (R<-->R). The R shape has a higher agonist affinity and ionic conductance than R. To understand how agonists trigger this gating isomerization, we examined single-channel currents from adult mouse muscle AChRs that isomerize normally without agonists but have only a single site able to use agonist binding energy to motivate gating. We estimated the monoliganded gating equilibrium constant E(1) and the energy change associated with the R versus R change in affinity for agonists. AChRs with only one operational binding site gave rise to a single population of currents, indicating that the two transmitter binding sites have approximately the same affinity for the transmitter ACh. The results indicated that E(1) approximately 4.3 x 10(-3) with ACh, and approximately 1.7 x 10(-4) with the partial-agonist choline. From these values and the diliganded gating equilibrium constants, we estimate that the unliganded AChR gating constant is E(0) approximately 6.5 x 10(-7). Gating changes the stability of the ligand-protein complex by approximately 5.2 kcal/mol for ACh and approximately 3.3 kcal/mol for choline.

The gating isomerization of neuromuscular acetylcholine receptors

Acetylcholine receptor-channels are allosteric proteins that isomerize ('gate') between conformations that have a low vs. high affinity for the transmitter and conductance for ions. In order to comprehend the mechanism by which the affinity and conductance changes are linked it is of value to know the magnitude, timing and distribution of energy flowing through the system. Knowing both the di- and unliganded gating equilibrium constants (E(2) and E(0)) is a foundation for understanding the AChR gating mechanism and for engineering both the ligand and the protein to operate in predictable ways. In adult mouse neuromuscular receptors activated by acetylcholine, E(2) = 28 and E(0) approximately 6.5 x 10(7). At each (equivalent) transmitter binding site acetylcholine provides approximately 5.2 kcal mol(1) to motivate the isomerization. The partial agonist choline provides approximately 3.3 kcal mol(1). The relative time of a residue's gating energy change is revealed by the slope of its rate-equilibrium constant relationship. A map of this parameter suggests that energy propagates as a conformational cascade between the transmitter binding sites and the gate region. Although gating energy changes are widespread throughout the protein, some residues are particularly sensitive to perturbations. Several specific proposals for the structural events that comprise the gating conformational cascade are discussed.

Energy and structure of the M2 helix in acetylcholine receptor-channel gating

We studied single-channel currents from neuromuscular acetylcholine receptor-channels with mutations in the pore-lining, M2 helix of the epsilon-subunit. Three parameters were quantified: 1), the diliganded gating equilibrium constant (E(2)), which reflects the energy difference between C(losed) and O(pen) conformations; 2), the correlation between the opening rate constant and E(2) on a log-log scale (Phi), which illuminates the energy character of the residue (C- versus O-like) within the C<-->O isomerization process; and 3), the open-channel current amplitude (i(0)), which reports whether a mutation alters the energetics of ion permeation. The largest E(2) changes were observed in the cytoplasmic half of epsilonM2 (5', 9', 12', 13', and 16'), with smaller changes apparent for residues > or =17'. Phi was approximately 0.54 for most epsilonM2 residues, but was approximately 0.32 at the positions that had largest E(2) changes. An arginine substitution reduced i(0) significantly at six positions, with the magnitude of the reduction increasing, 16'-->2'. The measurements suggest that the 9', 12', and 13' residues experience large and late free-energy changes in the channel-opening process. We speculate that in the gating isomerization the pore-facing residues >6' and <16' experience multiple energy perturbations associated with changes in protein structure and, perhaps, hydration.

The local anaesthetics proadifen and adiphenine inhibit nicotinic receptors by different molecular mechanisms

BACKGROUND AND PURPOSE

Many local anaesthetics are non-competitive inhibitors of nicotinic receptors (acetylcholine receptor, AChR). Proadifen induces a high-affinity state of the receptor, but its mechanism of action and that of an analogue, adiphenine, is unknown.

EXPERIMENTAL APPROACH

We measured the effects of proadifen and adiphenine on single-channel and macroscopic currents of adult mouse muscle AChR (wild-type and mutant). We assessed the results in terms of mechanisms and sites of action.

KEY RESULTS

Both proadifen and adiphenine decreased the frequency of ACh-induced single-channel currents. Proadifen did not change cluster properties, but adiphenine decreased cluster duration (36-fold at 100 micromolxL(-1)). Preincubation with proadifen decreased the amplitude (IC(50)= 19 micromolxL(-1)) without changing the decay rate of macroscopic currents. In contrast, adiphenine did not change amplitude but increased the decay rate (IC(50)= 15 micromolxL(-1)). Kinetic measurements demonstrate that proadifen acts on the resting state to induce a desensitized state whose kinetics of recovery resemble those of ACh-induced desensitization. Adiphenine accelerates desensitization from the open state, but previous application of the drug to resting receptors is required. Both drugs stabilize desensitized states, as evidenced by the decrease in the number of clusters elicited by high ACh concentrations. The inhibition by adiphenine is not affected by proadifen, and the mutation alphaE262K decreases the sensitivity of the AChR only for adiphenine, indicating that these drugs act at different sites.

CONCLUSIONS AND IMPLICATIONS

Two analogous local anaesthetics bind to different sites and inhibit AChR activity via different mechanisms and conformational states. These results provide new information on drug modulation of AChR.

Functional equivalence of the nicotinic acetylcholine receptor transmitter binding sites in the open state

The subunits of the muscle-type nicotinic acetylcholine receptor (AChR) are not uniformly oriented in the resting closed conformation: the two alpha subunits are rotated relative to its non-alpha subunits. In contrast, all the subunits overlay well with one another when agonist is bound to the AChR, suggesting that they are uniformly oriented in the open receptor. This gating-dependent increase in orientational uniformity due to rotation of the alpha subunits might affect the relative affinities of the two transmitter binding sites, making the two affinities dissimilar (functionally non-equivalent) in the initial ligand-bound closed state but similar (functionally equivalent) in the open state. To test this hypothesis, we measured single-channel activity of the alphaG153S gain-of-function mutant receptor evoked by choline, and estimated the resting closed-state and open-state affinities of the two transmitter binding sites. Both model-independent analyses and maximum-likelihood estimation of microscopic rate constants indicate that channel opening makes the binding sites' affinities more similar to each other. These results support the hypothesis that open-state affinities to the transmitter binding sites are primarily determined by the alpha subunits.

Unliganded gating of acetylcholine receptor channels

We estimated the unliganded opening and closing rate constants of neuromuscular acetylcholine receptor-channels (AChRs) having mutations that increased the gating equilibrium constant. For some mutant combinations, spontaneous openings occurred in clusters. For 25 different constructs, the unliganded gating equilibrium constant (E(0)) was correlated with the product of the predicted fold-increase in the diliganded gating equilibrium constant caused by each mutation alone. We estimate that (i) E(0) for mouse, wild-type alpha(2)beta delta epsilon AChRs is approximately 1.15 x 10(-7); (ii) unliganded AChRs open for approximately 80 micros, once every approximately 15 min; (iii) the affinity for ACh of the O(pen) conformation is approximately 10 nM, or approximately 15,600 times greater than for the C(losed) conformation; (iv) the ACh-monoliganded gating equilibrium constant is approximately 1.7 x 10(-3); (v) the C-->O isomerization reduces substantially ACh dissociation, but only slightly increases association; and (vi) ACh provides only approximately 0.9 k(B)T more binding energy per site than carbamylcholine but approximately 3.1 k(B)T more than choline, mainly because of a low O conformation affinity. Most mutations of binding site residue alphaW149 increase E(0). We estimate that the mutation alphaW149F reduces the ACh affinity of C only by 13-fold, but of O by 190-fold. Rate-equilibrium free-energy relationships for different regions of the protein show similar slopes (Phi values) for un- vs. diliganded gating, which suggests that the conformational pathway of the gating structural change is fundamentally the same with and without agonists. Agonist binding is a perturbation that (like most mutations) changes the energy, but not the mechanism, of the gating conformational change.

Acetylcholine receptor gating at extracellular transmembrane domain interface: the cys-loop and M2-M3 linker

Acetylcholine receptor channel gating is a propagated conformational cascade that links changes in structure and function at the transmitter binding sites in the extracellular domain (ECD) with those at a “gate” in the transmembrane domain (TMD). We used Φ-value analysis to probe the relative timing of the gating motions of α-subunit residues located near the ECD–TMD interface. Mutation of four of the seven amino acids in the M2–M3 linker (which connects the pore-lining M2 helix with the M3 helix), including three of the four residues in the core of the linker, changed the diliganded gating equilibrium constant (K_eq) by up to 10,000-fold (P272 > I274 > A270 > G275). The average Φ-value for the whole linker was ∼0.64. One interpretation of this result is that the gating motions of the M2–M3 linker are approximately synchronous with those of much of M2 (∼0.64), but occur after those of the transmitter binding site region (∼0.93) and loops 2 and 7 (∼0.77). We also examined mutants of six cys-loop residues (V132, T133, H134, F135, P136, and F137). Mutation of V132, H134, and F135 changed K_eq by 2800-, 10-, and 18-fold, respectively, and with an average Φ-value of 0.74, similar to those of other cys-loop residues. Even though V132 and I274 are close, the energetic coupling between I and V mutants of these positions was small (≤0.51 kcal mol⁻¹). The M2–M3 linker appears to be the key moving part that couples gating motions at the base of the ECD with those in TMD. These interactions are distributed along an ∼16-Å border and involve about a dozen residues.

Conformational dynamics of the alphaM3 transmembrane helix during acetylcholine receptor channel gating

Muscle acetylcholine receptors are synaptic ion channels that "gate" between closed- and open-channel conformations. We used Phi-value analysis to probe the transition state of the diliganded gating reaction with regard to residues in the M3, membrane-spanning helix of the muscle acetylcholine receptor alpha-subunit. Phi (a fraction between 1 and 0) parameterizes the extent to which a mutation changes the opening versus the closing rate constant and, for a linear reaction mechanism, the higher the Phi-value, the "earlier" the gating motion. In the upper half of alphaM3 the gating motions of all five tested residues were temporally correlated (Phi approximately 0.30) and serve to link structural changes occurring at the middle of the M2, pore-lining helix with those occurring at the interface of the extracellular and transmembrane domains. alphaM3 belongs to a complex and diverse set of synchronously moving parts that change structure relatively late in the channel-opening process. The propagation of the gating Brownian conformational cascade has a complex spatial distribution in the transmembrane domain.

Modulation of proton-induced current fluctuations in the human nicotinic acetylcholine receptor channel

The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel that switches upon activation from a closed state to a full conducting state. We found that the mutation delta S268K, located at 12' position of the second transmembrane domain of the delta subunit of the human nAChR generates a long-lived intermediate conducting state, from which openings to a wild-type like conductance level occur on a submillisecond time scale. Aiming to understand the interplay between structural changes near the 12' position and channel gating, we investigated the influence of various parameters: different ligands (acetylcholine, choline and epibatidine), ligand concentrations, transmembrane voltages and both fetal and adult nAChRs. Since sojourns in the high conductance state are not fully resolved in time, spectral noise analysis was used as a complement to dwell time analysis to determine the gating rate constants. Open channel current fluctuations are described by a two-state Markov model. The characteristic time of the process is markedly influenced by the ligand and the receptor type, whereas the frequency of openings to the high conductance state increases with membrane hyperpolarization. Conductance changes are discussed with regard to reversible transfer reaction of single protons at the lysine 12' side chain.

Desensitization contributes to the synaptic response of gain-of-function mutants of the muscle nicotinic receptor

Although the muscle nicotinic receptor (AChR) desensitizes almost completely in the steady presence of high concentrations of acetylcholine (ACh), it is well established that AChRs do not accumulate in desensitized states under normal physiological conditions of neurotransmitter release and clearance. Quantitative considerations in the framework of plausible kinetic schemes, however, lead us to predict that mutations that speed up channel opening, slow down channel closure, and/or slow down the dissociation of neurotransmitter (i.e., gain-of-function mutations) increase the extent to which AChRs desensitize upon ACh removal. In this paper, we confirm this prediction by applying high-frequency trains of brief (∼1 ms) ACh pulses to outside-out membrane patches expressing either lab-engineered or naturally occurring (disease-causing) gain-of-function mutants. Entry into desensitization was evident in our experiments as a frequency-dependent depression in the peak value of succesive macroscopic current responses, in a manner that is remarkably consistent with the theoretical expectation. We conclude that the comparatively small depression of the macroscopic currents observed upon repetitive stimulation of the wild-type AChR is due, not to desensitization being exceedingly slow but, rather, to the particular balance between gating, entry into desensitization, and ACh dissociation rate constants. Disruption of this fine balance by, for example, mutations can lead to enhanced desensitization even if the kinetics of entry into, and recovery from, desensitization themselves are not affected. It follows that accounting for the (usually overlooked) desensitization phenomenon is essential for the correct interpretation of mutagenesis-driven structure–function relationships and for the understanding of pathological synaptic transmission at the vertebrate neuromuscular junction.

Macroscopic properties of spontaneous mutations in slow-channel syndrome: correlation by domain and disease severity

The slow-channel syndrome (SCS) is a neuromuscular disorder characterized by fatigability, progressive weakness, and degeneration of the neuromuscular junction. The SCS is caused by missense mutations in the four subunits of the skeletal muscle acetylcholine receptor (AChR), which leads to altered channel gating, prolonged neuromuscular postsynaptic currents, and impaired neuromuscular transmission. Although a diverse set of mutations in different functional domains of the AChR appear to be associated with symptoms of widely ranging severity, there is as yet no mutant channel property or combination that explains the variations in disease severity. By observing the recovery time of AChR from desensitization, the authors determined that this process is significantly enhanced in SCS channels. In addition, as expected, the authors found that SCS macroscopic decay currents in transfected HEK293 cells are slower than wild type currents. While slight differences in relative Ca(2+) permeability between some SCS mutations were identified, they did not correlate with apparent disease severity. These results suggest that of the different AChR kinetic features studied, only recovery from desensitization and slow postsynaptic currents correlate with the severity observed in the different mutations of this syndrome.

Site specificity of agonist-induced opening and desensitization of the Torpedo californica nicotinic acetylcholine receptor

Agonist-binding kinetics to the nicotinic acetylcholine receptor (AChR) from Torpedo californica were measured using sequential-mixing stopped-flow fluorescence methods to determine the contribution of each individual site to agonist-induced opening and desensitization. Timed dansyl-C6-choline (DC6C) binding followed by its dissociation upon mixing with high, competing agonist concentrations revealed four kinetic components: an initial, fast fluorescence decay, followed by a transient increase, and then two characteristic decays that reflect dissociation from the desensitized agonist sites. The transient increase resulted from DC6C binding to the open-channel based on its prevention by proadifen, a noncompetitive antagonist. Further characterization of DC6C channel binding by the inhibition of [3H]phencyclidine binding and by equilibrium measurements of DC6C fluorescence yielded KD values of 2-4 microM for the desensitized AChR and approximately 600 microM for the closed state. At this site, DC6C displayed a strongly blue-shifted emission spectrum, higher intrinsic fluorescence, and weaker energy transfer from tryptophans than when bound to either agonist site. The initial, fast fluorescence decay was assigned to DC6C dissociation from the alphadelta site of the AChR in its closed conformation, on the basis of inhibition with the site-selective antagonists d-tubocurarine and alpha-conotoxin MI. Fast decay amplitude data indicated an apparent affinity of 0.9 microM for the closed-state alphadelta site; the closed-state alphagamma-site affinity is inferred to be near 100 microM. These values and the known affinities for the desensitized conformation show that the alphagamma site drives AChR desensitization to a approximately 40-fold greater extent than the alphadelta site, undergoes energetically larger conformational changes, and is the primary determinant of agonist potency.

Physostigmine modulation of acetylcholine currents in COS cells transfected with mouse muscle nicotinic receptor

Physostigmine (Phy), a reversible inhibitor of acetylcholine (ACh) esterase (AChE), may also act as a low potency agonist and a modulator of the nicotinic receptor. The actions of Phy on mouse muscle nicotinic receptors in the COS-7 cell line were studied by the patch-clamp technique. Currents were recorded in the whole-cell mode 3-7 days after cell transfection by plasmids coding alphabetagammadelta combination of receptor subunits. The application of ACh to cells clamped at -10 mV produced inward currents which displayed desensitization. The application of Phy in concentrations up to 1 x 10(-3) M did not give reliable specific whole-cell membrane responses. The application of Phy in concentrations of 10(-6)-10(-4) M together with ACh modulated the amplitude; accelerated desensitization of currents induced by ACh and increased the final extent of desensitization in a concentration-dependent manner. This finding is in contrast to the suppression and slowing down of desensitization by Phy and 1-methyl-galanthamine observed in Torpedo receptors.

Use-dependent inhibition of P2X3 receptors by nanomolar agonist

P2X3 receptors desensitize within 100 ms of channel activation, yet recovery from desensitization requires several minutes. The molecular basis for this slow rate of recovery is unknown. We designed experiments to test the hypothesis that this slow recovery is attributable to the high affinity (< 1 nM) of desensitized P2X3 receptors for agonist. We found that agonist binding to the desensitized state provided a mechanism for potent inhibition of P2X3 current. Sustained applications of 0.5 nM ATP inhibited > 50% of current to repetitive applications of P2X3 agonist. Inhibition occurred at 1000-fold lower agonist concentrations than required for channel activation and showed strong use dependence. No inhibition occurred without previous activation and desensitization. Our data are consistent with a model whereby inhibition of P2X3 by nanomolar [agonist] occurs by the rebinding of agonist to desensitized channels before recovery from desensitization. For several ATP analogs, the concentration required to inhibit P2X3 current inversely correlated with the rate of recovery from desensitization. This indicates that the affinity of the desensitized state and recovery rate primarily depend on the rate of agonist unbinding. Consistent with this hypothesis, unbinding of [32P]ATP from desensitized P2X3 receptors mirrored the rate of recovery from desensitization. As expected, disruption of agonist binding by site-directed mutagenesis increased the IC50 for inhibition and increased the rate of recovery.

Modes and models of GABA(A) receptor gating

Upon activation by agonist, the type A gamma-aminobutyric acid receptor (GABAR) 'gates', allowing chloride ions to permeate membranes and produce fast inhibition of neurons. There is no consensus kinetic model for the GABAR gating mechanism. We expressed human alpha(1)beta(1)gamma(2S) GABARs in HEK 293 cells and recorded single channel currents in the cell-attached configuration using various GABA concentrations (50-5000 microm). Closed and open events occurred individually and in clusters that had at least three different modes that were distinguishable by open probability (P(O)): High (P(O)= 0.73), Mid (P(O)= 0.50), and Low (P(O)= 0.21). We used a critical time to isolate shorter bursts of openings and to thus eliminate long-lived, desensitized events. Bursts from all three modes contained three closed and three open components. We employed maximum likelihood fitting, autocorrelation analysis and macroscopic current simulation to distinguish kinetic schemes. The 'core' gating scheme for most models contained two closed states that preceded an open state (C(1) C(2) O(1)). The two best-fitting models had a third closed state connected to C(1) and a second open state (O(2)) connected to C(2). The third open state, whose occupancy varied greatly between modes, could be connected either to O(2) or C(2). We estimated rate constants for two identical, independent GABA binding steps by globally fitting data across GABA concentrations ranging from 50 to 1000 microm. For the most highly ranked model the binding rate constants were: k(+)= 3 microm(-1) s(-1) and k(-)= 272 s(-1) (K(D)= 91 microm).

Dynamics of the acetylcholine receptor pore at the gating transition state

Neuromuscular acetylcholine receptors (AChRs) are ion channels that alternatively adopt stable conformations that either allow (open) or prohibit (closed) ionic conduction. We probed the dynamics of pore (M2) residues at the diliganded gating transition state by using single-channel kinetic and rate-equilibrium free energy relationship (phi-value) analyses of mutant AChRs. The mutations were at the equatorial (9') position of the alpha, beta, and epsilon subunits (n = 15) or at sites between the equator and the extracellular domain in the alpha-subunit (n = 8). We also studied AChRs having only one of the two alpha-subunits mutated. The results indicate that the alpha-subunit, like the delta-subunit, has a region of flexure near the middle of M2, that the two alpha-subunits experience distinct energy barriers to gating at the equator (but not elsewhere), and that the collective subunit motions at the equator are asymmetric during the AChR gating isomerization.

Inhibition of acetylcholine receptor function by seronegative myasthenia gravis non-IgG factor correlates with desensitisation

15% of myasthenia gravis (MG) patients do not have antibodies against the acetylcholine receptor (AChR). Some of these "seronegative" MG patients have antibodies against muscle specific kinase (MuSK), and many have a non-IgG factor that acutely inhibits AChR function in a muscle-like cell line, CN21. Here we show, using mainly one plasma negative for both AChR and MuSK antibodies, that the inhibitory effect of the non-IgG fraction correlates well with the desensitisation caused by 100 microM nicotine, and is found also when AChRs are expressed in a non-muscle cell line (HEK). Moreover, a similar effect was seen with M3C7-a monoclonal antibody against human AChR. The results suggest that, rather than acting indirectly as previously proposed, the SNMG factor may bind directly to an allosteric site that induces or enhances AChR desensitisation.

Initial coupling of binding to gating mediated by conserved residues in the muscle nicotinic receptor

We examined functional consequences of intrasubunit contacts in the nicotinic receptor alpha subunit using single channel kinetic analysis, site-directed mutagenesis, and structural modeling. At the periphery of the ACh binding site, our structural model shows that side chains of the conserved residues alphaK145, alphaD200, and alphaY190 converge to form putative electrostatic interactions. Structurally conservative mutations of each residue profoundly impair gating of the receptor channel, primarily by slowing the rate of channel opening. The combined mutations alphaD200N and alphaK145Q impair channel gating to the same extent as either single mutation, while alphaK145E counteracts the impaired gating due to alphaD200K, further suggesting electrostatic interaction between these residues. Interpreted in light of the crystal structure of acetylcholine binding protein (AChBP) with bound carbamylcholine (CCh), the results suggest in the absence of ACh, alphaK145 and alphaD200 form a salt bridge associated with the closed state of the channel. When ACh binds, alphaY190 moves toward the center of the binding cleft to stabilize the agonist, and its aromatic hydroxyl group approaches alphaK145, which in turn loosens its contact with alphaD200. The positional changes of alphaK145 and alphaD200 are proposed to initiate the cascade of perturbations that opens the receptor channel: the first perturbation is of beta-strand 7, which harbors alphaK145 and is part of the signature Cys-loop, and the second is of beta-strand 10, which harbors alphaD200 and connects to the M1 domain. Thus, interplay between these three conserved residues relays the initial conformational change from the ACh binding site toward the ion channel.

Invariant aspartic Acid in muscle nicotinic receptor contributes selectively to the kinetics of agonist binding

We examined functional contributions of interdomain contacts within the nicotinic receptor ligand binding site using single channel kinetic analyses, site-directed mutagenesis, and a homology model of the major extracellular region. At the principal face of the binding site, the invariant αD89 forms a highly conserved interdomain contact near αT148, αW149, and αT150. Patch-clamp recordings show that the mutation αD89N markedly slows acetylcholine (ACh) binding to receptors in the resting closed state, but does not affect rates of channel opening and closing. Neither αT148L, αT150A, nor mutations at both positions substantially affects the kinetics of receptor activation, showing that hydroxyl side chains at these positions are not hydrogen bond donors for the strong acceptor αD89. However substituting a negative charge at αT148, but not at αT150, counteracts the effect of αD89N, demonstrating that a negative charge in the region of interdomain contact confers rapid association of ACh. Interpreted within the structural framework of ACh binding protein and a homology model of the receptor ligand binding site, these results implicate main chain amide groups in the domain harboring αW149 as principal hydrogen bond donors for αD89. The specific effect of αD89N on ACh association suggests that interdomain hydrogen bonding positions αW149 for optimal interaction with ACh.

Structural dynamics of the M4 transmembrane segment during acetylcholine receptor gating

The transition state structures that link the stable end states of allosteric proteins are largely unresolved. We used single-molecule kinetic analysis to probe the dynamics of the M4 transmembrane segments during the closed<==>open isomerization of the neuromuscular acetylcholine receptor ion channel (AChR). We measured the slopes (phi) of the free energy relationships for 87 mutants, which reveal the open- versus closed-like characters of the mutated residues at the transition state and hence the sequence and organization of gating molecular motions. phi was constant throughout the length of the alpha subunit M4 segment with an average value of 0.54, suggesting that this domain moves as a unit, approximately midway through the reaction. Analysis of a hybrid construct indicates that the two alpha subunits move synchronously. Between subunits, the sequence of M4 motions is alpha-epsilon-beta. The AChR ion channel emerges as a dynamic nanomachine with many moving parts.

Neuronal nicotinic acetylcholine receptors as drug targets

Neuronal nicotinic acetylcholine receptors (nAChRs) are an important class of ion channels that have been associated with a number of neurological conditions. A great deal of research has been focused on attempting to understand the exact physiological role of these receptors. As drug targets, the nAChRs are quite complex, both in their structure (multiple receptor subtypes) and their physiological function. Initially, the difficulty encountered in identifying small-molecule modulators led to doubts about the validity of this class of receptors as drug targets. More recently, in vitro and in vivo data, homology modelling, and the identification of small-molecule agonists, have confirmed nAChRs as valid drug discovery targets. In fact, several compounds are now in clinical development for the treatment of pain, smoking cessation and cognitive disorders.