The channel opening rate of adult- and fetal-type mouse muscle nicotinic receptors activated by acetylcholine

Different Time Courses of Mono- and Bi-Liganded Bursts of Channel Openings of Adult nAChR Molecules Formed by the Reactions of Transmembrane Regions

We recorded transmembrane currents through single nicotinic acetylcholine receptors (nAChRs) in cell-attached patches at high temporal resolutions from cultured and transiently transfected HEK 293 cells. Receptor activation was elicited by acetylcholine (ACh) or epibatidine (Ebd) at concentrations ranging from 0.01 to 100 µM, binding to one (Rαδ or Rαε) or both extracellular ligand binding sites (Rαδ+αε). Agonist binding to Rαδ resulted in very short openings with mean durations of (τo1 < 5 µs), while the binding to Rαε produced short (τo2 = 37 µs) and intermediate openings (τo3 = 187 µs). Binding at both sites (Rαδ+αε) generated long openings (τo4 = 752 µs). All durations are noted in brackets since missed closures could shorten the results. Mono-liganded bursts were elicited at 0.01 µM ACh or Ebd, lasted less than a millisecond, displayed the typical current amplitude, and were interrupted by frequent microsecond-scale closures (µBs) that often did not reach the zero current. In contrast, bi-liganded bursts exhibited classical full amplitudes and long open states lasting up to several milliseconds, interspersed with rare µB closures of a similar duration to those observed in mono-liganded bursts.

Full and partial agonists evoke distinct structural changes in opening the muscle acetylcholine receptor channel

The muscle acetylcholine (ACh) receptor transduces a chemical into an electrical signal, but the efficiency of transduction, or efficacy, depends on the particular agonist. It is often presumed that full and partial agonists elicit the same structural changes after occupancy of their binding sites but with differing speed and efficiency. In this study, we tested the alternative hypothesis that full and partial agonists elicit distinct structural changes. To probe structural changes, we substituted cysteines for pairs of residues that are juxtaposed in the three-dimensional structure and recorded agonist-elicited single-channel currents before and after the addition of an oxidizing reagent. The results revealed multiple cysteine pairs for which agonist-elicited channel opening changes after oxidative cross-linking. Moreover, we found that the identity of the agonist determined whether cross-linking affects channel opening. For the αD97C/αY127C pair at the principal face of the subunit, cross-linking markedly suppressed channel opening by full but not partial agonists. Conversely, for the αD97C/αK125C pair, cross-linking impaired channel opening by the weak agonist choline but not other full or partial agonists. For the αT51C/αK125C pair, cross-linking enhanced channel opening by the full agonist ACh but not other full or partial agonists. At the complementary face of the subunit, cross-linking between pairs within the same β hairpin suppressed channel opening by ACh, whereas cross-linking between pairs from adjacent β hairpins was without effect for all agonists. In each case, the effects of cross-linking were reversed after addition of a reducing reagent, and receptors with single cysteine substitutions remained unaltered after addition of either oxidizing or reducing reagents. These findings show that, in the course of opening the receptor channel, different agonists elicit distinct structural changes.

Improved resolution of single channel dwell times reveals mechanisms of binding, priming, and gating in muscle AChR

The acetylcholine receptor (AChR) from vertebrate skeletal muscle initiates voluntary movement, and its kinetics of activation are crucial for maintaining the safety margin for neuromuscular transmission. Furthermore, the kinetic mechanism of the muscle AChR serves as an archetype for understanding activation mechanisms of related receptors from the Cys-loop superfamily. Here we record currents through single muscle AChR channels with improved temporal resolution approaching half an order of magnitude over our previous best. A range of concentrations of full and partial agonists are used to elicit currents from human wild-type and gain-of-function mutant AChRs. For each agonist-receptor combination, rate constants are estimated from maximum likelihood analysis using a kinetic scheme comprised of agonist binding, priming, and channel gating steps. The kinetic scheme and rate constants are tested by stochastic simulation, followed by incorporation of the experimental step response, sampling rate, background noise, and filter bandwidth. Analyses of the simulated data confirm all rate constants except those for channel gating, which are overestimated because of the established effect of noise on the briefest dwell times. Estimates of the gating rate constants were obtained through iterative simulation followed by kinetic fitting. The results reveal that the agonist association rate constants are independent of agonist occupancy but depend on receptor state, whereas those for agonist dissociation depend on occupancy but not on state. The priming rate and equilibrium constants increase with successive agonist occupancy, and for a full agonist, the forward rate constant increases more than the equilibrium constant; for a partial agonist, the forward rate and equilibrium constants increase equally. The gating rate and equilibrium constants also increase with successive agonist occupancy, but unlike priming, the equilibrium constants increase more than the forward rate constants. As observed for a full and a partial agonist, the gain-of-function mutation affects the relationship between rate and equilibrium constants for priming but not for channel gating. Thus, resolving brief single channel currents distinguishes priming from gating steps and reveals how the corresponding rate and equilibrium constants depend on agonist occupancy.

Muscarinic inhibition of nicotinic transmission in rat sympathetic neurons and adrenal chromaffin cells

Little is known about the interactions between nicotinic and muscarinic acetylcholine receptors (nAChRs and mAChRs). Here we report that methacholine (MCh), a selective agonist of mAChRs, inhibited up to 80% of nicotine-induced nAChR currents in sympathetic superior cervical ganglion neurons and adrenal chromaffin cells. The muscarine-induced inhibition (MiI) substantially reduced ACh-induced membrane currents through nAChRs and quantal neurotransmitter release. The MiI was time- and temperature-dependent. The slow recovery of nAChR current after washout of MCh, as well as the high value of Q10 (3.2), suggested, instead of a direct open-channel blockade, an intracellular metabotropic process. The effects of GTP-γ-S, GDP-β-S and pertussis toxin suggested that MiI was mediated by G-protein signalling. Inhibitors of protein kinase C (bisindolymaleimide-Bis), protein kinase A (H89) and PIP2 depletion attenuated the MiI, indicating that a second messenger pathway is involved in this process. Taken together, these data suggest that mAChRs negatively modulated nAChRs via a G-protein-mediated second messenger pathway. The time dependence suggests that MiI may provide a novel mechanism for post-synaptic adaptation in all cells/neurons and synapses expressing both types of AChRs.

Mechanism of activation of the prokaryotic channel ELIC by propylamine: a single-channel study

Prokaryotic channels, such as Erwinia chrysanthemi ligand-gated ion channel (ELIC) and Gloeobacter violaceus ligand-gated ion channel, give key structural information for the pentameric ligand-gated ion channel family, which includes nicotinic acetylcholine receptors. ELIC, a cationic channel from E. chrysanthemi, is particularly suitable for single-channel recording because of its high conductance. Here, we report on the kinetic properties of ELIC channels expressed in human embryonic kidney 293 cells. Single-channel currents elicited by the full agonist propylamine (0.5-50 mM) in outside-out patches at -60 mV were analyzed by direct maximum likelihood fitting of kinetic schemes to the idealized data. Several mechanisms were tested, and their adequacy was judged by comparing the predictions of the best fit obtained with the observable features of the experimental data. These included open-/shut-time distributions and the time course of macroscopic propylamine-activated currents elicited by fast theta-tube applications (50-600 ms, 1-50 mM, -100 mV). Related eukaryotic channels, such as glycine and nicotinic receptors, when fully liganded open with high efficacy to a single open state, reached via a preopening intermediate. The simplest adequate description of their activation, the "Flip" model, assumes a concerted transition to a single intermediate state at high agonist concentration. In contrast, ELIC open-time distributions at saturating propylamine showed multiple components. Thus, more than one open state must be accessible to the fully liganded channel. The "Primed" model allows opening from multiple fully liganded intermediates. The best fits of this type of model showed that ELIC maximum open probability (99%) is reached when at least two and probably three molecules of agonist have bound to the channel. The overall efficacy with which the fully liganded channel opens was ∼ 102 (∼ 20 for α1β glycine channels). The microscopic affinity for the agonist increased as the channel activated, from 7 mM for the resting state to 0.15 mM for the partially activated intermediate state.

Hydrophobic Gating of Ion Permeation in Magnesium Channel CorA

Ion channels catalyze ionic permeation across membranes via water-filled pores. To understand how changes in intracellular magnesium concentration regulate the influx of Mg²⁺ into cells, we examine early events in the relaxation of Mg²⁺ channel CorA toward its open state using massively-repeated molecular dynamics simulations conducted either with or without regulatory ions. The pore of CorA contains a 2-nm-long hydrophobic bottleneck which remained dehydrated in most simulations. However, rapid hydration or “wetting” events concurrent with small-amplitude fluctuations in pore diameter occurred spontaneously and reversibly. In the absence of regulatory ions, wetting transitions are more likely and include a wet state that is significantly more stable and more hydrated. The free energy profile for Mg²⁺ permeation presents a barrier whose magnitude is anticorrelated to pore diameter and the extent of hydrophobic hydration. These findings support an allosteric mechanism whereby wetting of a hydrophobic gate couples changes in intracellular magnesium concentration to the onset of ionic conduction.

This study shows how rapid wetting/dewetting transitions in the pores of ion channels participate in the control of biological ion permeation. Ion channels catalyze ionic permeation across non-polar membranes via water-filled pores. However, non-polar stretches or hydrophobic bottlenecks are present in the pores of many ion channels. To clarify the relationship between channel regulation, pore hydration, and ion permeation, we examine how the slow relaxation of magnesium channel CorA from its closed state towards its open state modulates wetting of its hydrophobic bottleneck. Results provide a quantitative description of wetting and dewetting probabilities and kinetics and a quantitative relationship between the extent of pore hydration and the energetics of ion permeation, consistent with a mechanism of hydrophobic gating.

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.

Anesthetic sites and allosteric mechanisms of action on Cys-loop ligand-gated ion channels

PURPOSE

The Cys-loop ligand-gated ion channel superfamily is a major group of neurotransmitter-activated receptors in the central and peripheral nervous system. The superfamily includes inhibitory receptors stimulated by γ-aminobutyric acid (GABA) and glycine and excitatory receptors stimulated by acetylcholine and serotonin. The first part of this review presents current evidence on the location of the anesthetic binding sites on these channels and the mechanism by which binding to these sites alters their function. The second part of the review addresses the basis for this selectivity, and the third part describes the predictive power of a quantitative allosteric model showing the actions of etomidate on γ-aminobutyric acid type A receptors (GABA(A)Rs).

PRINCIPAL FINDINGS

General anesthetics at clinical concentrations inhibit the excitatory receptors and enhance the inhibitory receptors. The location of general anesthetic binding sites on these receptors is being defined by photoactivable analogues of general anesthetics. The receptor studied most extensively is the muscle-type nicotinic acetylcholine receptor (nAChR), and progress is now being made with GABA(A)Rs. There are three categories of sites that are all in the transmembrane domain: 1) within a single subunit's four-helix bundle (intrasubunit site; halothane and etomidate on the δ subunit of AChRs); 2) between five subunits in the transmembrane conduction pore (channel lumen sites; etomidate and alcohols on nAChR); and 3) between two subunits (subunit interface sites; etomidate between the α1 and β2/3 subunits of the GABA(A)R).

CONCLUSIONS

These binding sites function allosterically. Certain conformations of a receptor bind the anesthetic with greater affinity than others. Time-resolved photolabelling of some sites occurs within milliseconds of channel opening on the nAChR but not before. In GABA(A)Rs, electrophysiological data fit an allosteric model in which etomidate binds to and stabilizes the open state, increasing both the fraction of open channels and their lifetime. As predicted by the model, the channel-stabilizing action of etomidate is so strong that higher concentrations open the channel in the absence of agonist. The formal functional paradigm presented for etomidate may apply to other potent general anesthetic drugs. Combining photolabelling with structure-function mutational studies in the context of allosteric mechanisms should lead us to a more detailed understanding of how and where these important drugs act.

Conformational changes in the nicotinic acetylcholine receptor during gating and desensitization

The nicotinic acetylcholine receptor (nAChR) is a member of the important Cys loop ligand-gated ion channel superfamily that modulates neuronal excitability. After they respond to their agonists, their actions are terminated either by removal of ligand or by fast and slow desensitization, processes that play an important role in modulating the duration of conducting states and hence of integrated neuronal behavior. We monitored structural changes occurring during fast and slow desensitization in the transmembrane domain of the Torpedo nAChR using time-resolved photolabeling with the hydrophobic probe 3-(trifluoromethyl)-3-(m-iodophenyl)diazirine (TID). After channel opening, TID photolabels a residue on the delta-subunit's M2-M3 loop and a cluster of four residues on deltaM1 and deltaM2, defining an open state pocket [Arevalo, E., et al. (2005) J. Biol. Chem. 280, 13631-13640]. We now find that photolabeling of this pocket persists during the transition to the fast desensitized state, the extent of photoincorporation decreasing only with the transition to the slow desensitized state. In contrast, the extent of photoincorporation in the channel lumen at the conserved 9'-leucines on the second transmembrane helix (M2-9') decreased successively during the resting to open and open to fast desensitized state transitions, implying that the local conformation is different in each state, a conclusion consistent with the hypothesis that there are separate gates for channel opening and desensitization. Thus, although during fast desensitization there is a conformation change in the channel lumen at the level of M2-9', there is none in the regions of the delta-subunit's M2-M3 loop and the interior of its M1-M4 helix bundle until slow desensitization occurs.

Agonist and blocking actions of choline and tetramethylammonium on human muscle acetylcholine receptors

Choline has been used widely as an agonist for the investigation of gain-of-function mutants of the nicotinic acetylcholine receptor. It is useful because it behaves like a partial agonist. The efficacy of choline is difficult to measure because choline blocks the channel at concentrations about four times lower than those that activate it. We have fitted activation mechanisms to single-channel activity elicited from HEK-expressed human recombinant muscle nicotinic receptors by choline and by tetramethylammonium (TMA). Channel block by the agonist was incorporated into the mechanisms that were fitted, and block was found not to be selective for the open state. The results also suggest that channel block is very fast and that the channel can shut almost as fast as normal when the blocker was bound. Single-channel data are compatible with a mechanism in which choline is actually a full agonist, its maximum response being limited only by channel block. However, they are also compatible with a mechanism incorporating a pre-opening conformation change ('flip') in which choline is a genuine partial agonist. The latter explanation is favoured by concentration jump experiments, and by the fact that only this mechanism fits the TMA data. We propose that choline, like TMA, is a partial agonist because it is very ineffective (approximately 600-fold less than acetylcholine) at eliciting the initial, pre-opening conformation change. Once flipping has occurred, all agonists, even choline, open the channel with similar efficiency.

Detection and trapping of intermediate states priming nicotinic receptor channel opening

In the course of synaptic transmission in the brain and periphery, acetylcholine receptors (AChRs) rapidly transduce a chemical signal into an electrical impulse. The speed of transduction owes in large part to rapid ACh association and dissociation, implying a binding site relatively non-selective for small cations; selective transduction has been supposed to originate from the ability of ACh, over that of other organic cations, to trigger the subsequent channel opening step. However transitions to and from the open state were shown to be similar for agonists with widely different efficacies.1,2,3 Here, by studying mutant AChRs, we find that the ultimate closed to open transition is agonist-independent and preceded by two primed closed states; the first primed state elicits brief openings, whereas the second elicits long-lived openings. Long-lived openings and the associated primed state are detected in the absence and presence of agonist, and exhibit the same kinetic signatures under both conditions. By covalently locking the agonist binding sites in the bound conformation, we find that each site initiates a priming step. Thus a change in binding site conformation primes the AChR for channel opening in a process that enables selective activation by ACh while maximizing speed and efficiency of the biological response.

Time-resolved photolabeling of the nicotinic acetylcholine receptor by [3H]azietomidate, an open-state inhibitor

Azietomidate is a photoreactive analog of the general anesthetic etomidate that acts as a nicotinic acetylcholine receptor (nAChR) noncompetitive antagonist. We used rapid perfusion electrophysiological techniques to characterize the state dependence and kinetics of azietomidate inhibition of Torpedo californica nAChRs and time-resolved photolabeling to identify the nAChR binding sites occupied after exposure to [(3)H]azietomidate and agonist for 50 ms (open state) or at equilibrium (desensitized state). Azietomidate acted primarily as an open channel inhibitor characterized by a bimolecular association rate constant of k(+) = 5 x 10(5) M(-1) s(-1) and a dissociation rate constant of <3s(-1). Azietomidate at 10 microM, when perfused with acetylcholine (ACh), inhibited the ACh response by approximately 50% after 50 ms; when preincubated for 10 s, it decreased the peak initial response by approximately 15%. Comparison of the kinetics of recovery of ACh responses after exposure to ACh and azietomidate or to ACh alone indicated that at subsecond times, azietomidate inhibited nAChRs without enhancing the kinetics of agonist-induced desensitization. In nAChRs frozen after 50-ms exposure to agonist and [(3)H]azietomidate, amino acids were photolabeled in the ion channel [position M2-20 (alphaGlu-262, betaAsp-268, deltaGln-276)], in deltaM1 (deltaCys-236), and in alphaMA/alphaM4 (alphaGlu-390, alphaCys-412) that were also photolabeled in nAChRs in the equilibrium desensitized state at approximately half the efficiency. These results identify azietomidate binding sites at the extracellular end of the ion channel, in the delta subunit helix bundle, and in the nAChR cytoplasmic domain that seem similar in structure and accessibility in the open and desensitized states of the nAChR.

Agonist-dependent single channel current and gating in alpha4beta2delta and alpha1beta2gamma2S GABAA receptors

The family of γ-aminobutyric acid type A receptors (GABA_ARs) mediates two types of inhibition in the mammalian brain. Phasic inhibition is mediated by synaptic GABA_ARs that are mainly comprised of α₁, β₂, and γ₂ subunits, whereas tonic inhibition is mediated by extrasynaptic GABA_ARs comprised of α_4/6, β₂, and δ subunits. We investigated the activation properties of recombinant α₄β₂δ and α₁β₂γ_2S GABA_ARs in response to GABA and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3(2H)-one (THIP) using electrophysiological recordings from outside-out membrane patches. Rapid agonist application experiments indicated that THIP produced faster opening rates at α₄β₂δ GABA_ARs (β ∼1600 s⁻¹) than at α₁β₂γ_2S GABA_ARs (β ∼ 460 s⁻¹), whereas GABA activated α₁β₂γ_2S GABA_ARs more rapidly (β ∼1800 s⁻¹) than α₄β₂δ GABA_ARs (β < 440 s⁻¹). Single channel recordings of α₁β₂γ_2S and α₄β₂δ GABA_ARs showed that both channels open to a main conductance state of ∼25 pS at −70 mV when activated by GABA and low concentrations of THIP, whereas saturating concentrations of THIP elicited ∼36 pS openings at both channels. Saturating concentrations of GABA elicited brief (<10 ms) openings with low intraburst open probability (P_O ∼ 0.3) at α₄β₂δ GABA_ARs and at least two “modes” of single channel bursting activity, lasting ∼100 ms at α₁β₂γ_2S GABA_ARs. The most prevalent bursting mode had a P_O of ∼0.7 and was described by a reaction scheme with three open and three shut states, whereas the “high” P_O mode (∼0.9) was characterized by two shut and three open states. Single channel activity elicited by THIP in α₄β₂δ and α₁β₂γ_2S GABA_ARs occurred as a single population of bursts (P_O ∼0.4–0.5) of moderate duration (∼33 ms) that could be described by schemes containing two shut and two open states for both GABA_ARs. Our data identify kinetic properties that are receptor-subtype specific and others that are agonist specific, including unitary conductance.

Agonist-induced hump current production in heterologously-expressed human alpha4beta2-nicotinic acetylcholine receptors

AIM

To characterize the functional and pharmacological features of agonist-induced hump currents in human alpha4beta2-nicotinic acetylcholine receptors (nAChR).

METHODS

Whole-cell and outside-out patch recordings were performed using human alpha4beta2-nAChR heterologously expressed in stably-transfected, native nAChR-null subclonal human epithelial 1 (SH-EP1) cells. RT-PCR was used to test the mRNA expression of transfected nAChR. Homology modeling and acetylcholine (ACh) docking were applied to show the possible ACh-binding site in the channel pore.

RESULTS

The rapid exposure of 10 mmol/L ACh induced an inward current with a decline from peak to steady-state. However, after the removal of ACh, an additional inward current, called phumpq current, reoccurred. The ability of agonists to produce these hump currents cannot be easily explained based on drug size, charge, acute potency, or actions as full or partial agonists. Hump currents were associated with a rebound increase in whole-cell conductance, and they had voltage dependence-like peak currents induced by agonist action. Hump currents blocked by the alpha4beta2-nAChR antagonist dihydro-beta-erythroidine were reduced when alpha4beta2-nAChR were desensitized, and were more pronounced in the absence of external Ca2+. Outside-out single-channel recordings demonstrated that compared to 1 micromol/L nicotine, 100 micromol/L nicotine reduced channel current amplitude, shortened the channel mean open time, and prolonged the channel mean closed time, supporting an agonist-induced open-channel block before hump current production. A docking model also simulated the agonist-binding site in the channel pore.

CONCLUSION

These results support the hypothesis that hump currents reflect a rapid release of agonists from the alpha4beta2-nAChR channel pore and a rapid recovery from desensitized alpha4beta2-nAChR.

Differential effects of serotonin and dopamine on human 5-HT3A receptor kinetics: interpretation within an allosteric kinetic model

Serotonin type 3 (5-HT3) receptors are members of the pentameric Cys-loop superfamily of receptors that modulate synaptic neurotransmission. In response to agonist binding and unbinding, members of this superfamily undergo a series of conformational transitions that define their functional properties. In this study, we report the results of electrophysiological studies using rapid solution exchange designed to characterize and compare the actions of the high-efficacy agonist serotonin and the low-efficacy agonist dopamine on human 5-HT3A receptors expressed in human embryonic kidney HEK293 cells. In the case of serotonin, receptor activation rates varied with agonist concentration, and deactivation occurred as a single-exponential process with a rate that was similar to the maximal rate of desensitization. Receptors recovered slowly from long desensitizing pulses of serotonin with a sigmoidal time course. In the case of dopamine, receptor activation rates were independent of agonist concentration, receptor deactivation occurred as a complex process that was significantly faster than the maximal rate of desensitization, and recovery from desensitization occurred more quickly than with 5-HT and its time course was not sigmoidal. We developed an allosteric kinetic model for 5-HT3A receptor activation, deactivation, desensitization, and resensitization. Interpretation of our results within the context of this model indicated that the distinct modulatory actions of serotonin versus dopamine are largely attributable to the vastly different rates with which these two agonists induce channel opening and dissociate from open and desensitized states.

The pre-M1 segment of the alpha1 subunit is a transduction element in the activation of the GABAA receptor

The binding of the neurotransmitter GABA induces conformational changes in the GABAA receptor (GABAAR), leading to the opening of a gate that controls ion permeation through an integral transmembrane pore. A number of structural elements within each subunit, located near the membrane interface, are believed to undergo relative movements during this activation process. In this study, we explored the functional role of the beta-10 strand (pre-M1 segment), which connects the extracellular domain to the transmembrane domain. In alpha1beta2gamma2s GABAARs, analysis of the 12 residues of the beta-10 strand in the alpha1 subunit proximal to the first transmembrane domain identified two residues, alpha1V212 and alpha1K220, in which mutations produced rightward shifts in the GABA concentration-response relationship and also reduced the relative efficacy of the partial agonist, piperidine-4-sulphonic acid. Ultra-fast agonist techniques were applied to mutant alpha1(K220A)beta2gamma2s GABAARs and revealed that the macroscopic functional deficit in this mutant could be attributed to a slowing of the opening rate constant, from approximately 1500 s(-1) in wild-type (WT) channels to approximately 730 s(-1) in the mutant channels, and a reduction in the time spent in the active state for the mutant. These changes were accompanied by a decrease in agonist affinity, with half-maximal activation rates achieved at 0.77 mM GABA in WT and 1.4 mM GABA in the alpha1(K220A)beta2gamma2s channels. The beta-10 strand (pre-M1 segment) emerges, from this and other studies, as a key functional component in the activation of the GABAAR.

Recent advances in Cys-loop receptor structure and function

Throughout the nervous system, moment-to-moment communication relies on postsynaptic receptors to detect neurotransmitters and change the membrane potential. For the Cys-loop superfamily of receptors, recent structural data have catalysed a leap in our understanding of the three steps of chemical-to-electrical transduction: neurotransmitter binding, communication between the binding site and the barrier to ions, and opening and closing of the barrier. The emerging insights might be expected to explain how mutations of receptors cause neurological disease, but the opposite is generally true. Namely, analyses of disease-causing mutations have clarified receptor structure-function relationships as well as mechanisms governing the postsynaptic response.

Understanding ion channel selectivity and gating and their role in cellular signalling

Ion channels play an essential role in the communication between and within cells. Here some of the different ion channel proteins and the roles they perform are introduced, before a discussion of the mechanisms by which they discriminate between different ion types and open and close to allow the passage of ions at the appropriate times.

Plasticity of acetylcholine receptor gating motions via rate-energy relationships

Like other protein conformational changes, ion channel gating requires the protein to achieve a high-energy transition-state structure. It is not known whether ion channel gating takes place on a broad energy landscape on which many alternative transition state structures are accessible, or on a narrow energy landscape where only a few transition-state structures are possible. To address this question, we measured how rate-equilibrium free energy relationships (REFERs) for di-liganded and unliganded acetylcholine receptor gating vary as a function of the gating equilibrium constant. A large slope for the REFER plot indicates an openlike transition state, whereas a small slope indicates a closedlike transition state. Due to this relationship between REFERs and transition-state structure, the sensitivity of the REFER slope to mutation-induced energetic perturbations allows estimation of the breadth of the energy landscape underlying gating. The relatively large sensitivity of di-liganded REFER slopes to energetic perturbations suggests that the motions underlying di-liganded gating take place on a broad, shallow energy landscape where many alternative transition-state structures are accessible.

Role of aspartate 298 in mouse 5-HT3A receptor gating and modulation by extracellular Ca2+

The TM2-TM3 extracellular loop is critical for activation of the Cys-loop family of ligand-gated ion channels. The contribution of aspartate 298 (D298), an amino acid that links the transmembrane domain 2 (TM2) to the TM2-TM3 loop, in mouse 5-hydroxytryptamine(3A) (5-HT(3A)) receptor function was probed with site-directed mutagenesis in the present study. This negatively charged residue was replaced with an alanine to neutralize the charge, with a glutamate to conserve the charge, or with an arginine to reverse the charge. Human embryonic kidney 293 (HEK 293) cells transfected with the wild-type and mutant receptors were studied by combining whole-cell patch-clamp recording with fast agonist application. The D-->A or D-->R mutations resulted in a receptor with reduced 5-HT potency, and accelerated kinetics of desensitization and deactivation. In addition, the efficacy of partial agonists was reduced by the D-->A mutation. The D-->E mutation produced a receptor with properties similar to those of the wild-type receptor. In addition, the potential role of this residue in modulation of the receptor by extracellular calcium ([Ca(2)(+)](o)) was investigated. Increasing [Ca(2)(+)](o) inhibited 5-HT-activated currents and altered receptor kinetics in a similar manner in the wild-type and D298E receptors, and this alteration was eliminated by the D-->A and D-->R mutations. Our data suggest that the charge at D298 participates in transitions between functional states of the 5-HT(3A) receptor, and provide evidence that the charge of the side-chain at residue D298 contributes to channel gating kinetics and is crucial for Ca(2)(+) modulation.

Activation of heteroliganded mouse muscle nicotinic receptors

The activation of the mouse muscle-type nicotinic acetylcholine receptor was studied in the presence of carbachol, and in the simultaneous presence of carbachol and choline. The channel currents were recorded under steady-state conditions using cell-attached single-channel patch clamp, and during transient exposures to the agonists using a piezo-driven fast application system. The presence of choline resulted in inhibition of currents elicited by carbachol. The inhibitory effect of choline manifested as a reduction in the effective opening rate (increase in the mean intracluster closed time duration) in single-channel recordings. In the fast application experiments, the peak current amplitude was reduced and the current rise time increased when choline was co-applied with carbachol. The data were analysed according to a model in which receptor interactions with carbachol and choline resulted in three types of ligation: receptors occupied by two carbachol molecules, receptors occupied by two choline molecules, and receptors in which one agonist binding site was occupied by carbachol and the other by choline, i.e. heteroliganded receptors. All three agonist-bound receptor populations could open albeit with different efficacies. The affinity of the resting receptor to choline was estimated to be 1-2 mm, and heteroliganded receptors opened with an opening rate constant of approximately 3000 s(-1). The results of the analysis suggest that the presence of choline in the neuromuscular junction in vivo has little effect on the time course of synaptic currents. Nevertheless, the contribution of heteroliganded receptors should be taken into consideration when the receptor is exposed simultaneously to two or more agonists.

Ligand-induced conformational change in the alpha7 nicotinic receptor ligand binding domain

Molecular dynamics simulations of a homology model of the ligand binding domain of the alpha7 nicotinic receptor are conducted with a range of bound ligands to induce different conformational states. Four simulations of 15 ns each are run with no ligand, antagonist d-tubocurarine (dTC), agonist acetylcholine (ACh), and agonist ACh with potentiator Ca(2+), to give insight into the conformations of the active and inactive states of the receptor and suggest the mechanism for conformational change. The main structural factor distinguishing the active and inactive states is that a more open, symmetric arrangement of the five subunits arises for the two agonist simulations, whereas a more closed and asymmetric arrangement results for the apo and dTC cases. Most of the difference arises in the lower portion of the ligand binding domain near its connection to the adjacent transmembrane domain. The transfer of the more open state to the transmembrane domain could then promote ion flow through the channel. Variation in how subunits pack together with no ligand bound appears to give rise to asymmetry in the apo case. The presence of dTC expands the receptor but induces rotations in alternate directions in adjacent subunits that lead to an asymmetric arrangement as in the apo case. Ca(2+) appears to promote a slightly greater expansion in the subunits than ACh alone by stabilizing the C-loop and ACh positions. Although the simulations are unlikely to be long enough to view the full conformational changes between open and closed states, a collection of different motions at a range of length scales are observed that are likely to participate in the conformational change.

Gating dynamics of the acetylcholine receptor extracellular domain

We used single-channel recording and model-based kinetic analyses to quantify the effects of mutations in the extracellular domain (ECD) of the alpha-subunit of mouse muscle-type acetylcholine receptors (AChRs). The crystal structure of an acetylcholine binding protein (AChBP) suggests that the ECD is comprised of a beta-sandwich core that is surrounded by loops. Here we focus on loops 2 and 7, which lie at the interface of the AChR extracellular and transmembrane domains. Side chain substitutions in these loops primarily affect channel gating by either decreasing or increasing the gating equilibrium constant. Many of the mutations to the beta-core prevent the expression of functional AChRs, but of the mutants that did express almost all had wild-type behavior. Rate-equilibrium free energy relationship analyses reveal the presence of two contiguous, distinct synchronously-gating domains in the alpha-subunit ECD that move sequentially during the AChR gating reaction. The transmitter-binding site/loop 5 domain moves first (Phi = 0.93) and is followed by the loop 2/loop 7 domain (Phi = 0.80). These movements precede that of the extracellular linker (Phi = 0.69). We hypothesize that AChR gating occurs as the stepwise movements of such domains that link the low-to-high affinity conformational change in the TBS with the low-to-high conductance conformational change in the pore.

Evidence for inhibitory effect of the agonist gaboxadol at human α1β2γ2S GABAA receptors

Gaboxadol (THIP; 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) is an agonist at GABA(A) receptors. THIP concentrations (0.01-50 mM) were applied rapidly to Sf9 cells expressing the human alpha(1)beta(2)gamma(2S) GABA(A) receptors. The EC(50) values for the peak current in THIP alone or THIP plus 1 microM diazepam were 154 and 53 microM, respectively. In supersaturating THIP (10-50 mM) the rate of current decay increased and an off-current developed when THIP was rapidly removed. The mean currents measured over the first 4 s in 10 mM and higher THIP concentrations were 0.6 or less of the 1 mM THIP mean current. Diazepam (1 microM) increased the 4 s mean current when evoked by 10 to 20 mM THIP but not 50 mM THIP. No similar effects on the current time-course were recorded in supersaturating gamma-aminobutyric acid (GABA) concentrations (50 and 80 mM). The results demonstrate an inhibitory as well as agonist effect of THIP at alpha(1)beta(2)gamma(2S) GABA(A) receptors.

Dependence of miniature endplate current on kinetic parameters of acetylcholine receptors activation: a model study

Mathematical modeling was applied to study the dependence of miniature endplate current (MEPC) amplitude and temporal parameters on the values of the rate constants of acetylcholine binding to receptors (k+) when cholinesterase was either active or inactive. The simulation was performed under two different sets of parameters describing acetylcholine receptor activation--one with high and another with low probability (Pohigh and Polow) of receptor transition into the open state after double ligand binding. The dependence of model MEPC amplitudes, rise times, and decay times on k+ differs for set Polow and set Pohigh. The main outcome is that for set Pohigh, the rise time is significantly longer at low values of k+ because of the prolongation of ACh diffusion time to the receptor. For the set Polow, the rise time is shorter at low values of k+, which can be explained by the small probability of AChR forward isomerization after ACh binding and faster MEPC's peak formation.

Saturation and self-inhibition of rat hippocampal GABA(A) receptors at high GABA concentrations

Current responses to ultrafast gamma-aminobutyric acid (GABA) applications were recorded from excised patches in rat hippocampal neurons to study the gating properties of GABA(A) receptors at GABA concentrations close to saturating ones and higher. The amplitude of currents saturated at approximately 1 mm, while the onset rate of responses reached saturation at 4-6 mm GABA. At high GABA concentrations (> 10 mm), the amplitude of current responses was reduced in a dose-dependent manner with a half-blocking GABA concentration of approximately 50 mm. The peak reduction at high GABA doses was accompanied by a tendency to increase the steady-state to peak ratio. At concentrations higher than 30 mm, this effect took the form of a rebound current, i.e. during the prolonged GABA applications, the current firstly declined due to desensitization onset and then, instead of decreasing towards a steady-state value, clearly increased. Both the self-inhibition of GABA(A) receptors by high GABA doses and rebound were clearly voltage dependent, being larger at positive holding potentials. The fast desensitization component accelerated with depolarization at all saturating [GABA] tested. The rebound phenomenon indicates that the self-block of GABAA receptors is state dependent, and suggests that the sojourn in the desensitized conformation provides a 'rescue' from the block. We propose that high GABA concentrations inhibit the receptors by direct occlusion of the channel pore having no effect on the receptor gating.

Analysis of a slow desensitized state of recombinant adult-type nicotinic acetylcholine receptor channels

A characteristic feature of the kinetics of nicotinic acetylcholine receptor (nAChR) channels is fast and nearly complete desensitization with a time course between 10 and 100 ms and recovery from desensitization in the range of some hundred ms. In the present study we used a piezo-driven system for ultra-fast solution exchange, analysed the recovery from the fast desensitized state of mouse recombinant adult-type nAChR channels and found no difference to that of embryonic-type channels. By double pulse experiments with application of pulses with a saturating concentration of 1 mm acetylcholine (ACh) with increasing duration of the first pulse and a constant interval between pulses we detected a second slow desensitized state which was entered with a time constant of 2835 ms. Recovery from the slow desensitized state proceeded with a single exponential with a time constant of 16134 ms. The experimental data were interpreted by the addition of a transition from the desensitized state with two bound ACh molecules to a slow desensitized state to the well known circular kinetic scheme of activation and desensitization of nAChR channels. This slow desensitized state might play a role in muscle fatigue or in pathological states like myasthenic syndromes.

Desensitization of diliganded mouse muscle nicotinic acetylcholine receptor channels

Nicotinic ACh receptor channels (AChRs) exposed to high concentrations of ACh adopt 'desensitized' conformations that have a high affinity for the transmitter and no measurable ion conductance. Single-channel currents elicited by 0.1 or 1 mM ACh were recorded from human embryonic kidney (HEK) cells that had been transiently transfected with mouse alpha, beta, delta, and epsilon subunits. On the time scale of approximately 0.1 ms to approximately 1 h, apparent open intervals are described by a single exponential component, and shut intervals associated with desensitization are described by the sum of four or five exponential components. The kinetic behaviour appeared to be stationary and homogeneous. Desensitization rate constants were estimated by kinetic modelling of currents from cell-attached and outside-out patches (where the number of channels in the patch was measured). A single AChR recovered from the longest-lived desensitized state only after approximately 5 min. The occupancy of an AChR for each of the desensitized states was calculated as a function of time after the continuous application of a pulse of saturating ACh. The longest-lived desensitized state accounted for 90 % of the total only after several seconds. The fractional recovery from desensitization (during a 200 ms wash period) decreased as the duration of the desensitizing pulse increased, suggesting that recovery is slower from the longer-lived desensitized states. The free energy landscape for the AChR desensitization reaction in cell-attached patches exhibited an initial destabilization, followed by a plateau region of gradually increasing stability, followed by a deep well.

Control of kinetic properties of GluR2 flop AMPA-type channels: impact of R/G nuclear editing

The GluR2 flop subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors greatly determines calcium permeability and kinetic properties of heteromeric AMPA subunit assemblies. Post-transcriptional editing of this subunit at the Q/R/N site controls calcium permeability whereas editing at the R/G site is involved in the regulation of biophysical properties. We used patch-clamp techniques with ultrafast solution exchange to examine the kinetics of recombinant human homomeric GluR2 flop channels transiently expressed in HEK293 cells [edited at the R/G site and Q/R/N site (GR), and unedited (RN) and edited (GN) at the R/G site both with asparagine (N) at the Q/R/N site]. The time constant of desensitization after application of 10 mm glutamate was 1.38 +/- 0.05 ms (n = 10), 5.53 +/- 0.57 ms (n = 7) and 1.33 +/- 0.06 ms (n = 12) for the GluR2 flop GR, RN and GN channels, respectively. The time constant of resensitization was 75 ms for the GluR2 flop RN and 30 ms for the GN channels. The dose-dependence of the peak current amplitude, kinetics of activation and deactivation, and peak open probability did not differ between RN and GN channels. The study shows that desensitization and resensitization kinetics of homomeric GluR2 flop channels are controlled by a single amino acid exchange (glycine by arginine) at the R/G site. Quantitative analysis by computer simulation using a circular kinetic scheme allows the prediction of the main experimental results.

Aromatics at the murine nicotinic receptor agonist binding site: mutational analysis of the alphaY93 and alphaW149 residues

1. Two aromatic residues of the muscle nicotinic receptor putative agonist binding site, a tyrosine in position alpha93 and a tryptophan in position alpha149, were mutated to phenylalanine and the effects of the mutations on receptor properties were investigated using single-channel patch clamp. 2. The alphaY93F mutation reduced the receptor affinity by approximately 4-fold and the channel opening rate constant by 48-fold. The alphaW149F mutation reduced the receptor affinity by approximately 12-fold and the channel opening rate constant by 93-fold. 3. The kinetic properties of hybrid receptors that contained one wild-type and one mutated alpha subunit were also examined. Only one type of hybrid receptor activity was detected. The hybrid receptors had a channel opening rate constant intermediate to those of the wild-type and mutant receptors. It was concluded that the ligand binding sites in the mutated muscle nicotinic receptor contributed equally to channel gating. In the case of the alphaW149F mutation, the presence of the mutation in one of the binding sites had no effect on the binding properties of the other, non-mutated, site. 4. The mutant channel opening and closing rate constants were also estimated in the presence of tetramethylammonium. The data suggested significant interaction between the acetyl group of acetylcholine and the alphaY93 residue.

Open probability of homomeric murine 5-HT3A serotonin receptors depends on subunit occupancy

1. The time course of macroscopic current responses of homomeric murine serotonin 5-HT3A receptors was studied in whole cells and excised membrane patches under voltage clamp in response to rapid application of serotonin. 2. Serotonin activated whole cell currents with an EC(50) value for the peak response of 2 microM and a Hill slope of 3.0 (n = 12), suggesting that the binding of at least three agonist molecules is required to open the channel. 3. Homomeric 5-HT3A receptors in excised membrane patches had a slow activation time course (mean +/- S.E.M. 10-90 % rise time 12.5 +/- 1.6 ms; n = 9 patches) for 100 microM serotonin. The apparent activation rate was estimated by fitting an exponential function to the rising phase of responses to supramaximal serotonin to be 136 s(-1). 4. The 5-HT3A receptor response to 100 microM serotonin in outside-out patches (n = 19) and whole cells (n = 41) desensitized with a variable rate that accelerated throughout the experiment. The time course for desensitization was described by two exponential components (for patches tau(slow) 1006 +/- 139 ms, amplitude 31 %; tau(fast) 176 +/- 25 ms, amplitude 69 %). 5. Deactivation of the response following serotonin removal from excised membrane patches (n = 8) and whole cells (n = 29) was described by a dual exponential time course with time constants similar to those for desensitization (for patches tau(slow) 838 +/- 217 ms, 55 % amplitude; tau(fast) 213 +/- 44 ms, 45 % amplitude). 6. In most patches (6 of 8), the deactivation time course in response to a brief 1-5 ms pulse of serotonin was similar to or slower than desensitization. This suggests that the continued presence of agonist can induce desensitization with a similar or more rapid time course than agonist unbinding. The difference between the time course for deactivation and desensitization was voltage independent over the range -100 to -40 mV in patches (n = 4) and -100 to +50 mV in whole cells (n = 4), suggesting desensitization of these receptors in the presence of serotonin does not reflect a voltage-dependent block of the channel by agonist. 7. Simultaneously fitting the macroscopic 5-HT3A receptor responses in patches to submaximal (2 microM) and maximal (100 microM) concentrations of serotonin to a variety of state models suggests that homomeric 5-HT3A receptors require the binding of three agonists to open and possess a peak open probability greater than 0.8. Our modelling also suggests that channel open probability varies with the number of serotonin molecules bound to the receptor, with a reduced open probability for fully liganded receptors. Increasing the desensitization rate constants in this model can generate desensitization that is more rapid than deactivation, as observed in a subpopulation of our patches.

Deactivation and desensitization of mouse embryonic- and adult-type nicotinic receptor channel currents

Recombinant nicotinic acetylcholine receptor (nAChR) channels transiently expressed in HEK293 cells were investigated using the patch-clamp technique in the cell-attached and outside-out modes for single-channel analysis and ultra-fast agonist application to multiple channels. Deactivation (current decay after removal of agonist) and desensitization (current decay in the presence of agonist) were analyzed at embryonic- (gamma) and adult-type (epsilon) nAChR channels. Time constants of desensitization were similar for both receptor types (epsilon: 53.1+/-16.9 ms; gamma: 49.2+/-15.7 ms) and corresponded to the mean duration of clusters of single channel openings activated by pulses of 1 mM ACh. Deactivation showed distinct characteristics. Time constants were 1.76+/-0.16 ms for epsilon- and 3.19+/-0.18 ms for gamma-nAChR channels, corresponding to mean burst duration analyzed from single channels in the same preparation (epsilon: 1.85+/-1.2 ms, gamma: 3.85+/-2.1 ms). It is assumed that differences in deactivation are of functional relevance at the muscle endplate.

Sex differences in the acetylcholine receptor kinetics of postnatal and denervated rat muscle

Single-channel recording from visualised endplates in freshly dissociated muscles from postnatal and denervated rat muscle revealed the presence of a low conductance, fetal type of acetylcholine receptor. Kinetic analysis showed a main component in the burst durations with a mean of 10.8 +/- 2.7 ms (n = 29). Receptors from female rats had an additional 27.3 +/- 5.5 ms (n = 5) kinetic component which was found in one-third of the 15 female endplates. Recordings from male and female denervated muscles gave more homogeneous kinetics with single time constants of 7.2 +/- 1.3 and 7.4 +/- 1.3 ms, respectively. It is concluded that the acetylcholine receptor channels present during early development are different from those of denervated muscle.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Structural elements near the C-terminus are responsible for changes in nicotinic receptor gating kinetics following patch excision

We have studied the effect of patch excision on the gating kinetics of muscle nicotinic acetylcholine receptors transiently expressed in HEK 293 cells. The experiments were performed on embryonic and adult wild-type, and several mutated, receptors using acetylcholine, carbamylcholine and tetramethylammonium as agonists. We show that patch excision of cell-attached patches into the inside-out configuration led to a reduction of mean open duration in receptors containing a gamma-subunit (embryonic) but not an epsilon-subunit (adult receptors). Kinetic analysis of an embryonic receptor containing a mutated residue, alphaY93F, showed that the reduction in the mean open duration upon patch excision was mainly caused by an increase in the channel closing rate constant. This was confirmed by experiments on embryonic wild-type receptors using carbamylcholine as an agonist with low efficacy. By expressing receptors containing chimeric gamma-epsilon subunits we found that segments of the gamma-subunit corresponding to a region within the M3-M4 linker (the amphipathic helix, HA) and the M4 transmembrane domain were required for the reduction in channel open duration after excision. The results indicate that particular residues in both M4 and HA are required to allow the change in open time after excision. This finding suggests that there is an interaction between these two regions in determining the modulation of gating kinetics.

Fundamental gating mechanism of nicotinic receptor channel revealed by mutation causing a congenital myasthenic syndrome

We describe the genetic and kinetic defects in a congenital myasthenic syndrome due to the mutation epsilonA411P in the amphipathic helix of the acetylcholine receptor (AChR) epsilon subunit. Myasthenic patients from three unrelated families are either homozygous for epsilonA411P or are heterozygous and harbor a null mutation in the second epsilon allele, indicating that epsilonA411P is recessive. We expressed human AChRs containing wild-type or A411P epsilon subunits in 293HEK cells, recorded single channel currents at high bandwidth, and determined microscopic rate constants for individual channels using hidden Markov modeling. For individual wild-type and mutant channels, each rate constant distributes as a Gaussian function, but the spread in the distributions for channel opening and closing rate constants is greatly expanded by epsilonA411P. Prolines engineered into positions flanking residue 411 of the epsilon subunit greatly increase the range of activation kinetics similar to epsilonA411P, whereas prolines engineered into positions equivalent to epsilonA411 in beta and delta subunits are without effect. Thus, the amphipathic helix of the epsilon subunit stabilizes the channel, minimizing the number and range of kinetic modes accessible to individual AChRs. The findings suggest that analogous stabilizing structures are present in other ion channels, and possibly allosteric proteins in general, and that they evolved to maintain uniformity of activation episodes. The findings further suggest that the fundamental gating mechanism of the AChR channel can be explained by a corrugated energy landscape superimposed on a steeply sloped energy well.

Asymmetric and independent contribution of the second transmembrane segment 12' residues to diliganded gating of acetylcholine receptor channels: a single-channel study with choline as the agonist

Mutagenesis studies have suggested that the second transmembrane segment (M2) plays a critical role during acetylcholine receptor liganded gating. An adequate description of the relationship between gating and structure of the M2 domain, however, has been hampered by the fact that many M2 mutations increase the opening rate constant to levels that, in the presence of acetylcholine, are unresolvably fast. Here, we show that the use of saturating concentrations of choline, a low-efficacy agonist, is a convenient tool to circumvent this problem. In the presence of 20 mM choline: (a) single-channel currents occur in clusters; (b) fast blockade by choline itself reduces the single-channel conductance by approximately 50%, yet the excess open-channel noise is only moderate; (c) the kinetics of gating are fitted best by a single-step, C <--> O model; and (d) opening and closing rate constants are within a well resolvable range. Application of this method to a series of recombinant adult mouse muscle M2 12' mutants revealed that: (a) the five homologous M2 12' positions make independent and asymmetric contributions to diliganded gating, the delta subunit being the most sensitive to mutation; (b) mutations at delta12' increase the diliganded gating equilibrium constant in a manner that is consistent with the sensitivity of the transition state to mutation being approximately 30% like that of the open state and approximately 70% like that of the closed state; (c) the relationship between delta12' amino acid residue volume, hydrophobicity or alpha-helical tendency, and the gating equilibrium constant of the corresponding mutants is not straightforward; however, (d) rate and equilibrium constants for the mutant series are linearly correlated (on log-log plots), which suggests that the conformational rearrangements upon mutation are mostly local and that the position of the transition state along the gating reaction coordinate is unaffected by these mutations.

Effects of halothane on GABA(A) receptor kinetics: evidence for slowed agonist unbinding

Many anesthetics, including the volatile agent halothane, prolong the decay of GABA(A) receptor-mediated IPSCs at central synapses. This effect is thought to be a major factor in the production of anesthesia. A variety of different kinetic mechanisms have been proposed for several intravenous agents, but for volatile agents the kinetic mechanisms underlying this change remain unknown. To address this question, we used rapid solution exchange techniques to apply GABA to recombinant GABA(A) receptors (alpha(1)beta(2)gamma(2s)) expressed in HEK 293 cells, in the absence and presence of halothane. To differentiate between different microscopic kinetic steps that may be altered by the anesthetic, we studied a variety of measures, including peak concentration-response characteristics, macroscopic desensitization, recovery from desensitization, maximal current activation rates, and responses to the low-affinity agonist taurine. Experimentally observed alterations were compared with predictions based on a kinetic scheme that incorporated two agonist binding steps, and open and desensitized states. We found that, in addition to slowing deactivation after a brief pulse of GABA, halothane increased agonist sensitivity and slowed recovery from desensitization but did not alter macroscopic desensitization or maximal activation rate and only slightly slowed rapid deactivation after taurine application. This pattern of responses was found to be consistent with a reduction in the microscopic agonist unbinding rate (k(off)) but not with changes in channel gating steps, such as the channel opening rate (beta), closing rate (alpha), or microscopic desensitization. We conclude that halothane slows IPSC decay by slowing dissociation of agonist from the receptor.

Effects of halothane on GABA(A) receptor kinetics: evidence for slowed agonist unbinding

Many anesthetics, including the volatile agent halothane, prolong the decay of GABA(A) receptor-mediated IPSCs at central synapses. This effect is thought to be a major factor in the production of anesthesia. A variety of different kinetic mechanisms have been proposed for several intravenous agents, but for volatile agents the kinetic mechanisms underlying this change remain unknown. To address this question, we used rapid solution exchange techniques to apply GABA to recombinant GABA(A) receptors (alpha(1)beta(2)gamma(2s)) expressed in HEK 293 cells, in the absence and presence of halothane. To differentiate between different microscopic kinetic steps that may be altered by the anesthetic, we studied a variety of measures, including peak concentration-response characteristics, macroscopic desensitization, recovery from desensitization, maximal current activation rates, and responses to the low-affinity agonist taurine. Experimentally observed alterations were compared with predictions based on a kinetic scheme that incorporated two agonist binding steps, and open and desensitized states. We found that, in addition to slowing deactivation after a brief pulse of GABA, halothane increased agonist sensitivity and slowed recovery from desensitization but did not alter macroscopic desensitization or maximal activation rate and only slightly slowed rapid deactivation after taurine application. This pattern of responses was found to be consistent with a reduction in the microscopic agonist unbinding rate (k(off)) but not with changes in channel gating steps, such as the channel opening rate (beta), closing rate (alpha), or microscopic desensitization. We conclude that halothane slows IPSC decay by slowing dissociation of agonist from the receptor.

Recovery from open channel block by acetylcholine during neuromuscular transmission in zebrafish

At larval zebrafish neuromuscular junctions (NMJs), miniature end plate currents (mEPCs) recorded in vivo have an unusually fast time course. We used fast-flow application of acetylcholine (ACh) onto outside-out patches to mimic the effect of synaptic release onto small numbers of ACh receptor channels (AChRs). Positively charged ACh acted at hyperpolarized potentials and at millimolar concentrations as a fast ("flickering") open channel blocker of AChRs. Because of filtering, the open channel block resulted in reduced amplitude of single channel currents. Immediately after brief (1 msec) application (without significant desensitization) of millimolar ACh at hyperpolarized potentials, a slower, transient current appeared because of delayed reversal of the block. This rebound current depended on the ACh concentration and resembled in time course the mEPC. A simple kinetic model of the AChR that includes an open channel-blocking step accounted for our single channel results, as well as the experimentally observed slowing of the time course of mEPCs recorded at a hyperpolarized compared with a depolarized potential. Recovery from AChR block is a novel mechanism of synaptic transmission that may contribute in part at all NMJs.

Recovery from open channel block by acetylcholine during neuromuscular transmission in zebrafish

At larval zebrafish neuromuscular junctions (NMJs), miniature end plate currents (mEPCs) recorded in vivo have an unusually fast time course. We used fast-flow application of acetylcholine (ACh) onto outside-out patches to mimic the effect of synaptic release onto small numbers of ACh receptor channels (AChRs). Positively charged ACh acted at hyperpolarized potentials and at millimolar concentrations as a fast ("flickering") open channel blocker of AChRs. Because of filtering, the open channel block resulted in reduced amplitude of single channel currents. Immediately after brief (1 msec) application (without significant desensitization) of millimolar ACh at hyperpolarized potentials, a slower, transient current appeared because of delayed reversal of the block. This rebound current depended on the ACh concentration and resembled in time course the mEPC. A simple kinetic model of the AChR that includes an open channel-blocking step accounted for our single channel results, as well as the experimentally observed slowing of the time course of mEPCs recorded at a hyperpolarized compared with a depolarized potential. Recovery from AChR block is a novel mechanism of synaptic transmission that may contribute in part at all NMJs.

Serum choline activates mutant acetylcholine receptors that cause slow channel congenital myasthenic syndromes

We have found that mutant acetylcholine receptor channels (AChRs) that cause slow-channel congenital myasthenic syndromes are activated by serum and that the high frequency of openings in serum is reduced by treatment with choline oxidase. Thus, slow-channel congenital myasthenic syndrome AChRs at the neuromuscular junction are likely to be activated both by steady exposure to serum choline and by transient exposure to synaptically released transmitter. Single-channel kinetic analyses indicate that the increased response to choline is caused by a reduced intrinsic stability of the closed channel. The results suggest that a mutation that destabilizes the inactive conformation of the AChR, together with the sustained exposure of endplates to serum choline, results in continuous channel activity that contributes to the pathophysiology of the disease.

A re-examination of adult mouse nicotinic acetylcholine receptor channel activation kinetics

1. During routine sequencing of our mouse muscle alpha subunit acetylcholine receptor channel (AChR) cDNA clones, we detected a discrepancy with the GenBank database entry (accession X03986). At nucleotides 1305-7 (residue 433, in the M4 domain) the database lists GTC which encodes a valine, while our putative 'wild-type' cDNA had the nucleotides GCC, which encodes an alanine. No other sequence differences were found. 2. PCR amplification of genomic DNA confirmed that the BALB/C mouse alpha subunit gene has a T nucleotide at position 1306, and, therefore, that the protein has a V at position 433 in the M4 segment. 3. In order to determine the functional consequences of this difference, either wild-type (V433) or mutant (A433) alpha subunits were co-expressed in HEK cells with mouse beta, epsilon and delta subunits. Single-channel currents were recorded in cell-attached patches, and rate and equilibrium constants were estimated from open and closed durations obtained from a range of ACh concentrations. No significant differences were found between the activation rate constants or equilibrium constants of the V433 and A433 variants. 4. Kinetic modelling of alphaV433 AChR suggests that the two transmitter binding sites have similar dissociation equilibrium constants for acetylcholine ( approximately 160 microM in 142 mM extracellular KCl). 5. Diliganded AChRs occupy a closed state that has a lifetime of approximately 1 ms. The rate constants for entering and leaving this state do not vary with the ACh concentration. 6. The kinetics of a mutant AChR that causes a slow channel congenital myaesthenic syndrome, alphaG153S, was re-examined. The properties of this mutant were similar with a V or an A at position alpha433.

Time-resolved photolabeling of membrane proteins: application to the nicotinic acetylcholine receptor

An apparatus has been developed that allows photoaffinity ligands to be crossed-linked to milligram quantities of membrane proteins with maximum attainable yield following contact times of approximately 1 ms. The apparatus consisted of three parts: a conventional rapid mixing unit, a novel freeze-quench unit, and a photolabeling unit. The freeze-quench unit consisted of a rapidly rotating metal disk which was precooled in liquid nitrogen. Correct alignment of the exit jet from the sample mixer allowed up to 2 ml of sample to be frozen in a thin film on the disk. Experiments with colorimetric reactions showed the combined dead time of mixing and freeze-quenching to be submillisecond. Photoincorporation was maximized by prolonged irradiation of the freeze-quenched sample. Using this apparatus we determine the binding kinetics of the resting state channel inhibitor 3-[125I](trifluoromethyl)-3-(m-iodophenyl) diazirine (TID) to nicotinic acetylcholine receptor-rich membranes from Torpedo. The binding kinetics for the 125I-labeled alpha and delta subunits were biphasic; about half the binding was complete by 2.4 ms, and the remainder could be resolved and occurred with a pseudo-first-order rate constant determined at 4 microM [125I]TID of 12.0 +/- 2.3 and 13.6 +/- 4.0 s-1, respectively. This compares well to the same constant determined for the inhibition of agonist-induced cation flux in Torpedo membranes.

Alternative splicing generates multiple mRNA forms of the acetylcholine receptor gamma-subunit in rat muscle

The fetal type acetylcholine receptor, composed of the alphabeta gammadelta subunits, has shown a highly variable channel kinetics during postnatal development. We examine the hypothesis whether such a variability could result from multiple channel forms, differing in the N-terminus of the gamma-subunit. RT-PCR revealed, in addition to the full-length mRNA, three new forms lacking exon 4. One of them in addition lacks 19 nucleotides from exon 5, predicting a complete subunit, with a 43 residues shorter N-terminus. A third one lacking the complete exon 5 predicts a subunit without transmembrane segments. These forms, generated by alternative splicing, may account for the kinetic variability of the acetylcholine receptor channel.

Epibatidine activates muscle acetylcholine receptors with unique site selectivity

We recently showed that at desensitized muscle nicotinic receptors, epibatidine selects by 300-fold between the two agonist binding sites. To determine whether receptors in the resting, activatible state show similar site selectivity, we studied epibatidine-induced activation of mouse fetal and adult receptors expressed in 293 HEK cells. Kinetic analysis of single-channel currents reveals that (-)-epibatidine binds with 15-fold selectivity to sites of adult receptors and 75-fold selectivity to sites of fetal receptors. For each receptor subtype, site selectivity arises solely from different rates of epibatidine dissociation from the two sites. To determine the structural basis for epibatidine selectivity, we introduced mutations into either the gamma or the delta subunit and measured epibatidine binding and epibatidine-induced single-channel currents. Complexes formed by alpha and mutant gamma(K34S+F172I) subunits bind epibatidine with increased affinity compared to alphagamma complexes, whereas the kinetics of alpha2betadeltagamma(K34S+F172I) receptors reveal no change in affinity of the low-affinity site, but increased affinity of the high-affinity site. Conversely, complexes formed by alpha and mutant delta(S36K+I178F) subunits bind epibatidine with decreased affinity compared to alphadelta complexes, whereas the kinetics of alpha2betagammadelta(S36K+I178F) and alpha2betaepsilondelta(S36K+I178F) receptors show markedly reduced sensitivity to epibatidine. The overall data show that epibatidine activates muscle receptors by binding with high affinity to alphagamma and alphaepsilon sites, but with low affinity to the alphadelta site.

Single channel currents at six microsecond resolution elicited by acetylcholine in mouse myoballs

1. A patch-clamp set-up was optimized for low noise and high time resolution. An Axoclamp 200B amplifier was modified to incorporate a Teflon connector to the electrode. An electrode puller was equipped with a hydrogen-oxygen burner to produce quartz-glass pipettes with optimally 0.2 micron openings and 20 MOmega resistance. 2. The r.m.s. (root mean square) noise of sealed pipettes in the bath ranged from 3.6 fA with 100 Hz filter cut-off to 1.5 pA with 61 kHz filter cut-off. At these extremes currents of 17 fA and more than 3 ms, or 9 pA and more than 6 micros could be resolved with a negligible error rate. 3. The system was tested on mouse myoballs, recording 9-10 pA single channel currents on-cell at -200 mV polarization which were elicited by 0.1-5000 microM acetylcholine (ACh). 4. Distributions of open and closed times and of correlations of open times to the preceding closed time defined several open states: single openings with mean durations of 1.2 and 25 micros, from single-liganded receptors, and bursts of 10 ms mean duration containing on average 800 micros openings and 16 micros closings, from double liganded receptors. Above 0.1 mM ACh these openings are interrupted increasingly by on average 18 micros and 72 micros channel blocks by ACh.