Voltage dependence of mouse acetylcholine receptor gating: different charge movements in di-, mono- and unliganded receptors

Adenosine Promotes Endplate nAChR Channel Activity in Adult Mouse Skeletal Muscle Fibers via Low Affinity P1 Receptors

Adenosine is a powerful modulator of skeletal neuromuscular transmission, operating via inhibitory or facilitatory purinergic-type P1 receptors. To date, studies have been focused mainly on the effect of adenosine on presynaptic P1 receptors controlling transmitter release. In this study, using two-microelectrode voltage-clamp and single-channel patch-clamp recording techniques, we have explored potential postsynaptic targets of adenosine and their modulatory effect on nicotinic acetylcholine receptor (nAChR)-mediated synaptic responses in adult mouse skeletal muscle fibers in vitro. In the whole-mount neuromuscular junction (NMJ) preparation, adenosine (100 μM) significantly reduced the frequency of the miniature endplate currents (MEPCs) and slowed their rising and decay time. Consistent with a postsynaptic site of action, adenosine and the potent P1 receptor agonist NECA significantly increased the open probability, the frequency and the open time of single nAChR channels, recorded at the endplate region. Using specific ligands for the P1 receptor subtypes, we found that the low-affinity P1 receptor subtype A_2B was responsible for mediating the effects of adenosine on the nAChR channel openings. Our data suggest that at the adult mammalian NMJ, adenosine acts not only presynaptically to modulate acetylcholine transmitter release, but also at the postsynaptic level, to enhance the activity of nAChRs. Our findings open a new scenario in understanding of purinergic regulation of nAChR activity at the mammalian endplate region.

Structural correlates of affinity in fetal versus adult endplate nicotinic receptors

Adult-type nicotinic acetylcholine receptors (AChRs) mediate signalling at mature neuromuscular junctions and fetal-type AChRs are necessary for proper synapse development. Each AChR has two neurotransmitter binding sites located at the interface of a principal and a complementary subunit. Although all agonist binding sites have the same core of five aromatic amino acids, the fetal site has ∼30-fold higher affinity for the neurotransmitter ACh. Here we use molecular dynamics simulations of adult versus fetal homology models to identify complementary-subunit residues near the core that influence affinity, and use single-channel electrophysiology to corroborate the results. Four residues in combination determine adult versus fetal affinity. Simulations suggest that at lower-affinity sites, one of these unsettles the core directly and the others (in loop E) increase backbone flexibility to unlock a key, complementary tryptophan from the core. Swapping only four amino acids is necessary and sufficient to exchange function between adult and fetal AChRs.

Adult and fetal nicotinic acetylcholine receptors (AChRs) have different functional requirements and affinity for ACh. Here, the authors use molecular dynamics and electrophysiology to investigate this affinity, and identify four amino acids that when swapped exchange function between adult and fetal AChRs.

Fluoxetine prevents acetylcholine-induced excitotoxicity blocking human endplate acetylcholine receptor

INTRODUCTION

Fluoxetine is an open channel blocker of fetal muscle acetylcholine (ACh) receptor (AChR) and slow-channel mutant AChRs. It is used commonly to treat patients with slow-channel congenital myasthenic syndromes. Fluoxetine effects on adult wild-type endplate AChR are less characterized, although muscle AChR isoforms are differentially modulated by some drugs.

METHODS

Excitotoxicity assays and patch clamp recordings were performed in human embryonic kidney 293 (HEK) cells expressing wild-type or slow-channel mutant human AChRs.

RESULTS

Fluoxetine (2-10 μM) abolished ACh-induced death and decreased ACh-activated whole-cell currents in cells expressing all AChR types. In outside-out patches, fluoxetine rapidly curtailed ACh evoked unitary activity and macroscopic currents. The effect was increased if fluoxetine was applied before ACh.

CONCLUSIONS

Fluoxetine is an open channel blocker, but it also affects AChR in the closed state. AChR blockade likely underlies the rescue of HEK cells from ACh-induced death.

Mechanistic and structural determinants of NMDA receptor voltage-dependent gating and slow Mg2+ unblock

NMDA receptor (NMDAR)-mediated currents depend on membrane depolarization to relieve powerful voltage-dependent NMDAR channel block by external magnesium (Mg(o)(2+)). Mg(o)(2+) unblock from native NMDARs exhibits a fast component that is consistent with rapid Mg(o)(2+) -unbinding kinetics and also a slower, millisecond time scale component (slow Mg(o)(2+) unblock). In recombinant NMDARs, slow Mg(o)(2+) unblock is prominent in GluN1/2A (an NMDAR subtype composed of GluN1 and GluN2A subunits) and GluN1/2B receptors, with slower kinetics observed for GluN1/2B receptors, but absent from GluN1/2C and GluN1/2D receptors. Slow Mg(o)(2+) unblock from GluN1/2B receptors results from inherent voltage-dependent gating, which increases channel open probability with depolarization. Here we examine the mechanisms responsible for NMDAR subtype dependence of slow Mg(o)(2+) unblock. We demonstrate that slow Mg(o)(2+) unblock from GluN1/2A receptors, like GluN1/2B receptors, results from inherent voltage-dependent gating. Surprisingly, GluN1/2A and GluN1/2B receptors exhibited equal inherent voltage dependence; faster Mg(o)(2+) unblock from GluN1/2A receptors can be explained by voltage-independent differences in gating kinetics. To investigate the absence of slow Mg(o)(2+) unblock in GluN1/2C and GluN1/2D receptors, we examined the GluN2 S/L site, a site responsible for several NMDAR subtype-dependent channel properties. Mutating the GluN2 S/L site of GluN2A subunits from serine (found in GluN2A and GluN2B subunits) to leucine (found in GluN2C and GluN2D) greatly diminished both voltage-dependent gating and slow Mg(o)(2+) unblock. Therefore, the residue at the GluN2 S/L site governs the expression of both slow Mg(o)(2+) unblock and inherent voltage dependence.

Voltage- and calcium-dependent gating of TMEM16A/Ano1 chloride channels are physically coupled by the first intracellular loop

Ca(2+)-activated Cl(-) channels (CaCCs) are exceptionally well adapted to subserve diverse physiological roles, from epithelial fluid transport to sensory transduction, because their gating is cooperatively controlled by the interplay between ionotropic and metabotropic signals. A molecular understanding of the dual regulation of CaCCs by voltage and Ca(2+) has recently become possible with the discovery that Ano1 (TMEM16a) is an essential subunit of CaCCs. Ano1 can be gated by Ca(2+) or by voltage in the absence of Ca(2+), but Ca(2+)- and voltage-dependent gating are very closely coupled. Here we identify a region in the first intracellular loop that is crucial for both Ca(2+) and voltage sensing. Deleting (448)EAVK in the first intracellular loop dramatically decreases apparent Ca(2+) affinity. In contrast, mutating the adjacent amino acids (444)EEEE abolishes intrinsic voltage dependence without altering the apparent Ca(2+)affinity. Voltage-dependent gating of Ano1 measured in the presence of intracellular Ca(2+) was facilitated by anions with high permeability or by an increase in [Cl(-)](e). Our data show that the transition between closed and open states is governed by Ca(2+) in a voltage-dependent manner and suggest that anions allosterically modulate Ca(2+)-binding affinity. This mechanism provides a unified explanation of CaCC channel gating by voltage and ligand that has long been enigmatic.

Glycine hinges with opposing actions at the acetylcholine receptor-channel transmitter binding site

The extent to which agonists activate synaptic receptor-channels depends on both the intrinsic tendency of the unliganded receptor to open and the amount of agonist binding energy realized in the channel-opening process. We examined mutations of the nicotinic acetylcholine receptor transmitter binding site (α subunit loop B) with regard to both of these parameters. αGly147 is an "activation" hinge where backbone flexibility maintains high values for intrinsic gating, the affinity of the resting conformation for agonists and net ligand binding energy. αGly153 is a "deactivation" hinge that maintains low values for these parameters. αTrp149 (between these two glycines) serves mainly to provide ligand binding energy for gating. We propose that a concerted motion of the two glycine hinges (plus other structural elements at the binding site) positions αTrp149 so that it provides physiologically optimal binding and gating function at the nerve-muscle synapse.

Subunit symmetry at the extracellular domain-transmembrane domain interface in acetylcholine receptor channel gating

Transmitter molecules bind to synaptic acetylcholine receptor channels (AChRs) to promote a global channel-opening conformational change. Although the detailed mechanism that links ligand binding and channel gating is uncertain, the energy changes caused by mutations appear to be more symmetrical between subunits in the transmembrane domain compared with the extracellular domain. The only covalent connection between these domains is the pre-M1 linker, a stretch of five amino acids that joins strand β10 with the M1 helix. In each subunit, this linker has a central Arg (Arg(3')), which only in the non-α-subunits is flanked by positively charged residues. Previous studies showed that mutations of Arg(3') in the α-subunit alter the gating equilibrium constant and reduce channel expression. We recorded single-channel currents and estimated the gating rate and equilibrium constants of adult mouse AChRs with mutations at the pre-M1 linker and the nearby residue Glu(45) in non-α-subunits. In all subunits, mutations of Arg(3') had similar effects as in the α-subunit. In the ε-subunit, mutations of the flanking residues and Glu(45) had only small effects, and there was no energy coupling between εGlu(45) and εArg(3'). The non-α-subunit Arg(3') residues had Φ-values that were similar to those for the α-subunit. The results suggest that there is a general symmetry between the AChR subunits during gating isomerization in this linker and that the central Arg is involved in expression more so than gating. The energy transfer through the AChR during gating appears to mainly involve Glu(45), but only in the α-subunits.

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.

Computational study of the transmembrane domain of the acetylcholine receptor

The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel protein whose transmembrane domain (TM-domain) is believed to be responsible for channel gating via a hydrophobic effect. In this work, we perform molecular dynamics and Brownian dynamics simulations to investigate the effect of transmembrane potential on the conformation and water occupancy of TM-domain, and the resulting ion permeation events. The results show that the behavior of the hydrophobic gate is voltage-dependent. Large hyperpolarized membrane potential can change the conformation of TM-domain and water occupancy in this region, which may enable ion conduction. An electrostatic gating mechanism is also proposed from our simulations, which seems to play a role in addition to the well-known hydrophobic effect.

Aromatic Residues {epsilon}Trp-55 and {delta}Trp-57 and the Activation of Acetylcholine Receptor Channels

The two transmitter binding sites of the neuromuscular acetylcholine (ACh) receptor channel contain several aromatic residues, including a tryptophan located on the complementary, negative face of each binding pocket. These two residues, Trp-55 in the epsilon subunit and Trp-57 in the delta subunit, were mutated (AEFHILRVY), and for most constructs the rate constants for acetylcholine binding and channel gating were estimated by using single channel kinetic analyses. The rate constants for unliganded channel opening and closing were also estimated for some mutants. From these measurements we calculated all of the equilibrium constants of the "allosteric" cycle as follows: diliganded gating, unliganded gating, dissociation from the C(losed) conformation, and dissociation from the O(pen) conformation. The results indicate the following. (i) These aromatic side chains play a relatively minor role in ACh receptor channel activation. (ii) The main consequence of mutations is to reduce the affinity of the O conformation of the binding site for ACh, with the effect being greater at the epsilon subunit. (iii) In epsilon (but not delta) the aromatic nature of the side chain is important in determining affinity, to a slightly greater degree in the O conformation. Phi value analyses (of both tryptophan residues) show Phi approximately 1 for both the ACh binding and diliganded gating reactions. (iv) This suggests that the structural boundaries of the dynamic elements of the gating conformational change may not be subunit-delimited, and (v) the mutated tryptophan residues experience energy changes that occur relatively early in both the ligand-binding and channel-gating reactions.

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.

Voltage-dependent gating of NR1/2B NMDA receptors

Ligand-gated ion channels are activated by agonist binding, but may also be modulated by membrane voltage. N-Methyl-d-aspartate receptors (NMDARs) exhibit especially strong voltage dependence due to channel block by external Mg(2+) (Mg(o)(2+)). Here we demonstrate that activity of NMDARs composed of NR1 and NR2B subunits (NR1/2B receptors) is enhanced by depolarization even in 0 Mg(o)(2+), causing slow current relaxations in response to rapid voltage changes. We present a kinetic model of receptor activation that incorporates voltage-dependent gating-associated NR2B subunit conformational changes. The model accurately reproduces current relaxations during depolarizations and subsequent repolarizations in 0 Mg(o)(2+). Model simulations in physiological Mg(o)(2+) concentrations show that voltage-dependent receptor gating also underlies the slow component of Mg(o)(2+) unblock, a phenomenon that previously was shown to influence Mg(o)(2+) unblock kinetics during dendritic spikes. We propose that voltage-dependent gating of NR1/2B receptors confers enhanced voltage and time dependence on NMDAR-mediated signalling.

Activation and block of the adult muscle-type nicotinic receptor by physostigmine: single-channel studies

The plant-derived acetylcholinesterase inhibitor physostigmine has previously been shown to act on the nicotinic acetylcholine receptor (nAChR) causing either direct activation or potentiation of currents elicited by low concentrations of nicotinic agonists, or, at higher concentrations, channel block. We examined mouse adult-type muscle nAChR activation by physostigmine and found that channel activation by physostigmine exhibits many characteristics common with channel activity elicited by nicotinic agonists. Single-channel conductance was indistinguishable, and mutants known to slow channel closing in the presence of nicotinic agonists had a similar effect in the presence of physostigmine. However, physostigmine is a very inefficacious agonist. The presence of physostigmine did not alter the effective opening rate for a subsaturating dosage of carbachol, suggesting that physostigmine does not interact with the nicotinic agonist binding site. Mutations to a residue (alphaLys125) previously identified as part of the putative binding site for physostigmine reduced the duration of openings elicited by physostigmine, but the effects were generally small and, in most cases, nonsignificant. At higher concentrations, physostigmine blocked channel activity. Block manifested as a reduction in the mean open time and the emergence of a closed state, with a mean duration of 3 to 7 ms. The properties of block were consistent with two equivalent blocking sites per receptor with microscopic binding and unbinding rate constants for physostigmine of 20 microM(-1) s(-1) and 450 s(-1) (K(D) = 23 microM). These observations indicate that physostigmine is able to activate muscle nAChR by interacting with a site other than the nicotinic ligand binding site.

Mode of cembranoid action on embryonic muscle acetylcholine receptor

The mechanism of eupalmerin acetate (EUAC) actions on the embryonic muscle nicotinic acetylcholine receptor (nAChR) in BC3H-1 cells was studied by using whole-cell and single-channel patch-clamp current measurements. With whole-cell currents, EUAC did not act as an agonist on this receptor. Coapplication of 30 microM EUAC with 50 microM, 100 microM, or 500 microM carbamoylcholine (CCh) reversibly inhibited the current amplitude, whereas, with 20 microM CCh, current was increased above control values in the presence of EUAC. EUAC concentration curves (0.01-40 microM) obtained with 100 microM and 500 microM CCh displayed slope coefficients, n(H), significantly smaller than one, suggesting that EUAC bound to several sites with widely differing affinities on the receptor molecule. The apparent rate of receptor desensitization in the presence of EUAC and CCh was either slower than or equal to that obtained with CCh alone. The major finding from single-channel studies was that EUAC did not affect single-channel conductance or the ability of CCh to interact with the receptor. Instead, EUAC acted by increasing the channel closing rate constant. The results are not consistent with the competitive model for EUAC inhibition, with the sequential open-channel block model, or with inhibition by increased desensitization. The data are best accounted for by a model in which EUAC acts by closed-channel block at low concentrations, by positive modulation at intermediate concentrations, and by negative allosteric modulation of the open channel at high concentrations.

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.

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.

Functional asymmetry of transmembrane segments in nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are heteropentameric ion channels that open upon activation to a single conducting state. The second transmembrane segments of each subunit were identified as channel-forming elements, but their respective contribution in the gating process remains unclear. Moreover, the detailed impact of variations of the membrane potential, such as occurring during an action potential, on the transmembrane domains, is unknown. Residues at the 12' position, close to the center of each second transmembrane segment, play a key role in channel gating. We examined their functional symmetry by substituting a lysine to that position of each subunit and measuring the electrical activity of single channels. For 12' lysines in the alpha, gamma and delta subunits rapid transitions between an intermediate and large conductance appeared, which are interpreted as single lysine protonation events. From the kinetics of these transitions we calculated the pK (a) values of respective lysines and showed that they vary differently with membrane hyperpolarization. Respective mutations in beta or epsilon subunits gave receptors with openings of either intermediate or large conductance, suggesting extreme pK (a) values in two open state conformations. The results demonstrate that these parts of the highly homologous transmembrane domains, as probed by the 12' lysines, sense unequal microenvironments and are differently affected by physiologically relevant voltage changes. Moreover, observation of various gating events for mutants of alpha subunits suggests that the open channel pore exists in multiple conformations, which in turn supports the notion of functional asymmetry of the channel.

Galantamine activates muscle-type nicotinic acetylcholine receptors without binding to the acetylcholine-binding site

Galantamine (Reminyl; Janssen Pharmaceutica, Titusville, NJ) belongs to a class of acetylcholinesterase inhibitors approved for symptomatic treatment of Alzheimer's disease. The drug presumably acts by raising and prolonging the profile of acetylcholine (ACh) via an inhibitory effect on the esterase. However, there is also evidence demonstrating that galantamine can activate the nicotinic ACh receptor or modulate its activation by ACh. In this study, we have examined the ability of galantamine to directly activate the muscle-type nicotinic ACh receptor or to modulate receptor activation by selected nicotinic agonists. Studies of direct activation by galantamine demonstrated that this ligand is a low-efficacy agonist of the muscle-type ACh receptor. Point mutations in the M2-M3 linker (alphaS269I) and the M2 transmembrane domain (epsilonT264P) had similar effects on receptor activation by galantamine and nicotinic agonists, suggesting that the general features of receptor activation by galantamine are similar to that in the presence of ACh. Experiments performed in the simultaneous presence of galantamine and various nicotinic ligands showed that channel activation by the nicotinic ligands studied (ACh, carbachol, and choline) was not affected by the presence of galantamine at concentrations up to 100 microm. In addition, galantamine did not reduce the initial rate of binding for 125I-alpha-bungarotoxin. These results demonstrate that galantamine does not interfere with the occupation of the nicotinic agonist binding site by ACh, carbachol, or choline. We conclude that galantamine activates the muscle-type ACh receptor by interacting with a binding site that is distinct from the site for nicotinic agonists.

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.

Refined structure of the nicotinic acetylcholine receptor at 4A resolution

We present a refined model of the membrane-associated Torpedo acetylcholine (ACh) receptor at 4A resolution. An improved experimental density map was obtained from 342 electron images of helical tubes, and the refined structure was derived to an R-factor of 36.7% (R(free) 37.9%) by standard crystallographic methods, after placing the densities corresponding to a single molecule into an artificial unit cell. The agreement between experimental and calculated phases along the helical layer-lines was used to monitor progress in the refinement and to give an independent measure of the accuracy. The atomic model allowed a detailed description of the whole receptor in the closed-channel form, including the ligand-binding and intracellular domains, which have not previously been interpreted at a chemical level. We confirm that the two ligand-binding alpha subunits have a different extended conformation from the three other subunits in the closed channel, and identify several interactions on both pairs of subunit interfaces, and within the alpha subunits, which may be responsible for their "distorted" structures. The ACh-coordinating amino acid side-chains of the alpha subunits are far apart in the closed channel, indicating that a localised rearrangement, involving closure of loops B and C around the bound ACh molecule, occurs upon activation. A comparison of the structure of the alpha subunit with that of AChBP having ligand present, suggests how the localised rearrangement overcomes the distortions and initiates the rotational movements associated with opening of the channel. Both vestibules of the channel are strongly electronegative, providing a cation-stabilising environment at either entrance of the membrane pore. Access to the pore on the intracellular side is further influenced by narrow lateral windows, which would be expected to screen out electrostatically ions of the wrong charge and size.

Free-energy landscapes of ion-channel gating are malleable: changes in the number of bound ligands are accompanied by changes in the location of the transition state in acetylcholine-receptor channels

Acetylcholine-receptor channels (AChRs) are allosteric membrane proteins that mediate synaptic transmission by alternatively opening and closing ("gating") a cation-selective transmembrane pore. Although ligand binding is not required for the channel to open, the binding of agonists (for example, acetylcholine) increases the closed right harpoon over left harpoon open equilibrium constant because the ion-impermeable --> ion-permeable transition of the ion pathway is accompanied by a low-affinity --> high-affinity change at the agonist-binding sites. The fact that the gating conformational change of muscle AChRs can be kinetically modeled as a two-state reaction has paved the way to the experimental characterization of the corresponding transition state, which represents a snapshot of the continuous sequence of molecular events separating the closed and open states. Previous studies of fully (di) liganded AChRs, combining single-channel kinetic measurements, site-directed mutagenesis, and data analysis in the framework of the linear free-energy relationships of physical organic chemistry, have suggested a transition-state structure that is consistent with channel opening being an asynchronous conformational change that starts at the extracellular agonist-binding sites and propagates toward the intracellular end of the pore. In this paper, I characterize the gating transition state of unliganded AChRs, and report a remarkable difference: unlike that of diliganded gating, the unliganded transition state is not a hybrid of the closed- and open-state structures but, rather, is almost indistinguishable from the open state itself. This displacement of the transition state along the reaction coordinate obscures the mechanism underlying the unliganded closed right harpoon over left harpoon open reaction but brings to light the malleable nature of free-energy landscapes of ion-channel gating.

Loose protein packing around the extracellular half of the GABA(A) receptor beta1 subunit M2 channel-lining segment

GABA(A) receptors are ligand-gated ion channels formed by the pseudosymmetrical assembly of five homologous subunits around the central channel axis. The five M2 membrane-spanning segments largely line the channel. In the present work we probed the water surface accessibility of the beta(1) subunit M2 segment using the substituted cysteine accessibility method. We assayed the reaction of the negatively charged sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS(-)), by its effect on subsequent currents elicited by EC(50) and saturating GABA concentrations. pCMBS(-), applied with GABA, reacted with 14 of the 19 residues tested. At the M2 cytoplasmic end from 2' to 6' only beta(1)A252C (2') and beta(1)T256C (6') were pCMBS(-)-reactive in the presence of GABA. We infer that the M2 segments are tightly packed in this region. Toward the extracellular half of M2 all residues from beta(1)T262C (12') through beta(1)E270C (20') reacted with pCMBS(-) applied with GABA. We infer that this region is highly mobile and loosely packed against the rest of the protein. Based on differences in pCMBS(-) reaction rates two domains can be distinguished on the putative channel-lining side of M2. A faster reacting domain includes the 2', 9', 12', 13', and 16' residues. The slower reacting face contains the 6', 10', and 14' residues. We hypothesize that these may represent the channel-lining faces in the closed and open states and that gating involves an 80-100 degrees rotation of the M2 segments. These results are consistent with the loose packing of the M2 segments inferred from the structure of the homologous Torpedo nicotinic acetylcholine receptor.

Conformation-dependent hydrophobic photolabeling of the nicotinic receptor: electrophysiology-coordinated photochemistry and mass spectrometry

We characterized the differential accessibility of the nicotinic acetylcholine receptor alpha1 subunit in the open, closed, and desensitized states by using electrophysiology-coordinated photolabeling by several lipophilic probes followed by mass spectrometric analysis. Voltage-clamped oocytes expressing receptors were preincubated with one of the lipophilic probes and were continually exposed to acetylcholine; UV irradiation was applied during 500-ms pulses to + 40 or to -140 mV (which produced closed or approximately 50% open receptors, respectively). In the open state, there was specific probe incorporation within the N-terminal domain at residues that align with the beta8-beta9 loop of the acetylcholine-binding protein. In the closed state, probe incorporation was identified at several sites of the N-terminal domain within the conserved cysteine loop (residues 128-142), the cytoplasmic loop (M3-M4), and M4. The labeling pattern in the M4 region is consistent with previous results, further defining the lipid-exposed face of this transmembrane alpha-helix. These results show regions within the N-terminal domain that are involved in gating-dependent conformational shifts, confirm that the cysteine loop resides at or near the protein-membrane interface, and show that segments of the M3-M4 loop are near to the lipid bilayer.

The role of loop 5 in acetylcholine receptor channel gating

Nicotinic acetylcholine receptor channel (AChR) gating is an organized sequence of molecular motions that couples a change in the affinity for ligands at the two transmitter binding sites with a change in the ionic conductance of the pore. Loop 5 (L5) is a nine-residue segment (mouse alpha-subunit 92-100) that links the beta4 and beta5 strands of the extracellular domain and that (in the alpha-subunit) contains binding segment A. Based on the structure of the acetylcholine binding protein, we speculate that in AChRs L5 projects from the transmitter binding site toward the membrane along a subunit interface. We used single-channel kinetics to quantify the effects of mutations to alphaD97 and other L5 residues with respect to agonist binding (to both open and closed AChRs), channel gating (for both unliganded and fully-liganded AChRs), and desensitization. Most alphaD97 mutations increase gating (up to 168-fold) but have little or no effect on ligand binding or desensitization. Rate-equilibrium free energy relationship analysis indicates that alphaD97 moves early in the gating reaction, in synchrony with the movement of the transmitter binding site (Phi = 0.93, which implies an open-like character at the transition state). alphaD97 mutations in the two alpha-subunits have unequal energetic consequences for gating, but their contributions are independent. We conclude that the key, underlying functional consequence of alphaD97 perturbations is to increase the unliganded gating equilibrium constant. L5 emerges as an important and early link in the AChR gating reaction which, in the absence of agonist, serves to increase the relative stability of the closed conformation of the protein.

Activation and block of mouse muscle-type nicotinic receptors by tetraethylammonium

We have studied the activation and inhibition of the mouse muscle adult-type nicotinic acetylcholine receptor by tetraethylammonium (TEA) and related quaternary ammonium derivatives. The data show that TEA is a weak agonist of the nicotinic receptor. No single-channel clusters were observed at concentrations as high as 5 mM TEA or in the presence of a mutation which selectively increases the efficacy of the receptor. When coapplied with 1 mM carbamylcholine (CCh), TEA decreased the effective opening rate demonstrating that it acts as a competitive antagonist of CCh-mediated activation. Kinetic analysis of currents elicited by CCh and TEA allowed an estimate of receptor affinity for TEA of about 1 mM, while an upper limit of 10 s-1 could be set for the wild-type channel-opening rate constant for receptors activated by TEA alone. At millimolar concentrations, TEA inhibited nicotinic receptor currents by depressing the single-channel amplitude. The effect had an IC50 of 2-3 mM, depending on the conditions of the experiment, and resembled a standard open-channel block. However, the decrease in channel amplitudes was not accompanied by an increase in the mean burst duration, indicating that a linear open-channel blocking mechanism is not applicable. Upon studying block by other nicotinic receptor ligands it was found that block by CCh, tetramethylammonium and phenyltrimethylammonium can be accounted for by the sequential blocking mechanism while block in the presence of methyltriethylammonium, ethyltrimethylammonium or choline was inconsistent with such a mechanism. A mechanism in which receptors blocked by TEA can close would account for the experimental findings.

Properties of neuronal alpha7 mutant nicotinic acetylcholine receptors gated by bicuculline

We have shown previously that mutating to threonine the leucine residue in the M2 domain of the alpha7 nicotinic acetylcholine receptor (human L248T, L248T; chick L247T, L247T) converts bicuculline (BIC) from an antagonist into an agonist. In this work we studied the functional properties of the BIC-activated channels and report that, in Xenopus oocytes injected with L248T subunit cDNA, BIC activates single-channel currents that have similar conductances, but shorter mean burst duration, than the channels activated by ACh. In contrast, both the conductance and kinetics of the channels activated by either ACh or BIC are substantially the same in oocytes expressing L247T receptors. We have also shown previously that if Cys 189 and 190, which are thought to be at or near the transmitter binding site, are additionally mutated to Ser, the new mutant receptor (L247T-C189S-C190S) has a reduced affinity for ACh. We now find that the EC(50) in the BIC dose-current response relation, as well the characteristics of the channels activated by BIC, are similar in oocytes expressing either L247T or L247T-C189S-C190S receptors. On the other hand, ACh activation of L247T-C189S-C190S receptors gates channels whose mean open time and burst duration are much shorter than those of ACh-gated L247T-channels. Therefore, the gating kinetics of both L248T and L247R-C189S-C190S receptor-channels change when BIC is replaced by ACh; and we conclude that both ACh and BIC activate mutant alpha7 receptors with different patterns of activation.

Anesthetic and ethanol effects on spontaneously opening glycine receptor channels

Strychnine-sensitive glycine receptors mediate inhibitory neurotransmission occurring in the brain stem and spinal cord. Alcohols, volatile anesthetics and inhaled drugs of abuse are positive allosteric modulators of glycine receptor function, normally enhancing function only in the presence of glycine. A complication in studying allosteric actions on ligand-gated ion channels is in the dissection of their effects on neurotransmitter binding from their effects on channel opening. Mutation of an aspartate residue at position 97 to arginine in the glycine receptor alpha1 subunit simulated the effects of glycine binding, producing receptors that exhibited tonic channel opening in the absence of neurotransmitter; i.e. these receptors demonstrated a dissociation of channel opening from neurotransmitter binding. In these receptors, ethanol, enflurane, chloroform, halothane, 1,1,1-trichloroethane and toluene elicited inward currents in the absence of glycine. We previously identified mutations on ligand-gated ion channels that eliminate ethanol, anesthetic and inhalant actions (such as S267I on alpha1 glycine receptors). The double mutant (D97R and S267I) receptors were both constitutively active and resistant to the enhancing effects of ethanol and enflurane. These data demonstrate that ethanol and volatile anesthetics can affect glycine receptor channel opening independently of their effects on enhancing neurotransmitter binding.

Mechanism of tacrine block at adult human muscle nicotinic acetylcholine receptors

We used single-channel kinetic analysis to study the inhibitory effects of tacrine on human adult nicotinic receptors (nAChRs) transiently expressed in HEK 293 cells. Single channel recording from cell-attached patches revealed concentration- and voltage-dependent decreases in mean channel open probability produced by tacrine (IC(50) 4.6 microM at -70 mV, 1.6 microM at -150 mV). Two main effects of tacrine were apparent in the open- and closed-time distributions. First, the mean channel open time decreased with increasing tacrine concentration in a voltage-dependent manner, strongly suggesting that tacrine acts as an open-channel blocker. Second, tacrine produced a new class of closings whose duration increased with increasing tacrine concentration. Concentration dependence of closed-times is not predicted by sequential models of channel block, suggesting that tacrine blocks the nAChR by an unusual mechanism. To probe tacrine's mechanism of action we fitted a series of kinetic models to our data using maximum likelihood techniques. Models incorporating two tacrine binding sites in the open receptor channel gave dramatically improved fits to our data compared with the classic sequential model, which contains one site. Improved fits relative to the sequential model were also obtained with schemes incorporating a binding site in the closed channel, but only if it is assumed that the channel cannot gate with tacrine bound. Overall, the best description of our data was obtained with a model that combined two binding sites in the open channel with a single site in the closed state of the receptor.

Serotonin antagonizes the human neuronal alpha7 nicotinic acetylcholine receptor and becomes an agonist after L248T alpha7 mutation

The effects of serotonin (5-hydroxytryptamine or 5HT) on chick alpha7 nicotinic receptors have already been described. However similar studies on human alpha7 receptors have been lacking. To begin to fill this deficiency, studies were made on wild-type and mutant human alpha7 (halpha7) receptors expressed in Xenopus oocytes or human BOSC 23 cells. In oocytes wild-type halpha7 receptors were blocked by 5HT, and this block was voltage-dependent. In contrast, 5HT acted as an agonist on halpha7-mutant receptors (L248T). Outside-out membrane-patches from BOSC 23 cells expressing halpha7-mutant receptors exhibited spontaneous channel openings of two conductance levels (59 pS and 76 pS) and short mean open time (0.9 ms). halpha7-Mutant channels activated by nicotine or 5HT displayed similar conductances and high Ca(2+) permeability; but longer duration (2.7 ms) than the spontaneous openings. Mutations at Cys190 and Cys191, in the extracellular N-terminus of the human alpha7 gene, did not prevent receptor expression and incorporation in the oocyte membrane (determined by alpha-bungarotoxin binding). However, both 5HT and nicotine were incapable of gating the channels, indicating that the mutated Cys residues are in, or near, the 5HT- and nicotine-binding site. This is the first report that alpha7 receptors have spontaneous openings; and that 5HT is an agonist of halpha7-mutant receptors, and an antagonist of halpha7-wild-type receptors, through interactions at, or near the acetylcholine-binding sites.

Inhibition of nicotinic acetylcholine receptors by bicuculline

A study was made on the effects of bicuculline, the classical gamma-aminobutyric acid-A receptor antagonist, on heteromeric mouse muscle alphabetagammadelta, heteromeric neuronal rat alpha2beta4 and alpha4beta2 and homomeric human alpha7 nicotinic acetylcholine receptors (nAChRs), expressed in Xenopus oocytes. Bicuculline reduced the ACh-induced currents in a rapid and reversible way, with IC50 values of 34+/-1.5 microM for mouse muscle alphabetagammadelta and 12.4+/-0.7 and 18+/-1 microM for rat neuronal alpha2beta4 and alpha4beta2 nAChRs, respectively. Therefore, the three types of heteromeric receptors are inhibited by bicuculline but the neuronal alpha2beta4 and alpha4beta2 receptors were more sensitive than the muscle alphabetagammadelta receptor. The Hill coefficients for ACh-current inhibition were close to one for all types of receptors, suggesting a single site of action for bicuculline inhibition of nAChRs. Bicuculline shifted the ACh-dose-current response curve to the right and the maximal current was reduced, a reduction that for the heteromeric receptors was not overcome by high concentrations of ACh. The effect of bicuculline was examined at different membrane potentials, and the ACh-current-membrane potential relationships obtained indicate that the inhibition by bicuculline is voltage-dependent for muscle alphabetagammadelta and neuronal alpha2beta4 and alpha4beta2 nAChRs. All these results are consistent with the notion that bicuculline blocks the heteromeric muscle and neuronal nAChRs in a non-competitive way. Studies were also made on the wild type (wt alpha7) and mutant leu-to-threo (L248T) homomeric human neuronal alpha7-nAChRs. In sharp contrast to the heteromeric ACh receptors examined, bicuculline blocked in a competitive way the homomeric wt alpha7-nAChRs, as evidenced by a parallel shift of the bicuculline dose-ACh-current inhibition on raising the ACh concentration. Moreover, similar to the effects of serotonin on wt and mutant alpha7 ACh receptors, the mutation converted bicuculline from an antagonist into a competitive agonist. All this suggests that bicuculline may serve as a lead molecule to design new anticholinergic substances.

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.

Binding properties of agonists and antagonists to distinct allosteric states of the nicotinic acetylcholine receptor are incompatible with a concerted model

Recent work has shown that the nicotinic acetylcholine receptor (nAChR) can be fixed in distinct conformations by chemical cross-linking with glutardialdehyde, which abolishes allosteric transitions in the protein. Here, two conformations that resemble the desensitized and the resting states were compared with respect to their affinities for different classes of ligands. The same ligands were tested for their ability to convert the nAChR from a conformation with low affinity to a conformation with high affinity for acetylcholine. As expected, agonists were found to bind with higher affinity to the desensitized state-like conformation and to induce a shift of the nAChR to this high affinity state. In contrast, although most antagonists tested bound preferentially to the desensitized receptor as well they failed to induce a change of the affinity for acetylcholine. These observations sharply contradict basic predictions of the concerted model, including the postulate of a preformed equilibrium between the different states of the nAChR in the absence of agonist. With a similar approach we could show that the non-competitive inhibitor ethidium is displaced in a non-allosteric manner by other well characterized channel blockers from the cross-linked nAChR. These results require revision of current models for the mechanisms underlying non-competitive antagonism at the nAChR.

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.

Kinetic, mechanistic, and structural aspects of unliganded gating of acetylcholine receptor channels: a single-channel study of second transmembrane segment 12' mutants

The spontaneous activity of adult mouse muscle acetylcholine receptor channels, transiently expressed in HEK-293 cells, was studied with the patch-clamp technique. To increase the frequency of unliganded openings, mutations at the 12' position of the second transmembrane segment were engineered. Our results indicate that: (a) in both wild type and mutants, a C <--> O kinetic scheme provides a good description of spontaneous gating. In the case of some mutant constructs, however, additional states were needed to improve the fit to the data. Similar additional states were also needed in one of six patches containing wild-type acetylcholine receptor channels; (b) the delta12' residue makes a more pronounced contribution to unliganded gating than the homologous residues of the alpha, beta, and straightepsilon subunits; (c) combinations of second transmembrane segment 12' mutations in the four different subunits appear to have cumulative effects; (d) the volume of the side chain at delta12' is relevant because residues larger than the wild-type Ser increase spontaneous gating; (e) the voltage dependence of the unliganded gating equilibrium constant is the same as that of diliganded gating, but the voltage dependences of the opening and closing rate constants are opposite (this indicates that the reaction pathway connecting the closed and open states of the receptor changes upon ligation); (f) engineering binding-site mutations that decrease diliganded gating (alphaY93F, alphaY190W, and alphaD200N) reduces spontaneous activity as well (this suggests that even in the absence of ligand the opening of the channel is accompanied by a conformational change at the binding sites); and (g) the diliganded gating equilibrium constant is also increased by the 12' mutations. Such increase is independent of the particular ligand used as the agonist, which suggests that these mutations affect mostly the isomerization step, having little, if any, effect on the ligand-affinity ratio.

Mapping the conformational wave of acetylcholine receptor channel gating

Allosteric transitions allow fast regulation of protein function in living systems. Even though the end points of such conformational changes are known for many proteins, the characteristics of the paths connecting these states remain largely unexplored. Rate-equilibrium linear free-energy relationships (LFERs) provide information about such pathways by relating changes in the free energy of the transition state to those of the ground states upon systematic perturbation of the system. Here we present an LFER analysis of the gating reaction pathway of the muscle acetylcholine receptor. We studied the closed <==> open conformational change at the single-molecule level following perturbation by series of single-site mutations, agonists and membrane voltages. This method provided a snapshot of several regions of the receptor at the transition state in terms of their approximate positions along the reaction coordinate, on a scale from 0 (closed-like) to 1 (open-like). The resulting map reveals a spatial gradient of positional values, which suggests that the conformational change proceeds in a wave-like manner, with the low-to-high affinity change at the transmitter-binding sites preceding the complete opening of the pore.

Voltage dependence of the glycine receptor-channel kinetics in the zebrafish hindbrain

Electrophysiological recordings of outside-out patches to fast-flow applications of glycine were made on patches derived from the Mauthner cells of the 50-h-old zebrafish larva. As for glycinergic miniature inhibitory postsynaptic currents (mIPSCs), depolarizing the patch produced a broadening of the transient outside-out current evoked by short applications (1 ms) of a saturating concentration of glycine (3 mM). When the outside-out patch was depolarized from -50 to +20 mV, the peak current varied linearly with voltage. A 1-ms application of 3 mM glycine evoked currents that activated rapidly and deactivated biexponentially with time constants of approximately 5 and approximately 30 ms (holding potential of -50 mV). These two decay time constants were increased by depolarization. The fast deactivation time constant increased e-fold per 95 mV. The relative amplitude of the two decay components did not significantly vary with voltage. The fast component represented 64.2 +/- 2.8% of the total current at -50 mV and 54.1 +/- 10% at +20 mV. The 20-80% rise time of these responses did not show any voltage dependence, suggesting that the opening rate constant is insensitive to voltage. The 20-80% rise time was 0.2 ms at -70 mV and 0.22 ms at +20 mV. Responses evoked by 100-200 ms application of a low concentration of glycine (0.1 mM) had a biphasic rising phase reflecting the complex gating behavior of the glycine receptor. The time constant of these two components and their relative amplitude did not change with voltage, suggesting that modal shifts in the glycine-activated channel gating mode are not sensitive to the membrane potential. Using a Markov model to simulate glycine receptor gating behavior, we were able to mimic the voltage-dependent change in the deactivation time course of the responses evoked by 1-ms application of 3 mM glycine. This kinetics model incorporates voltage-dependent closing rate constants. It provides a good description of the time course of the onset of responses evoked by the application of a low concentration of glycine at all membrane potentials tested.

Allosteric activation mechanism of the alpha 1 beta 2 gamma 2 gamma-aminobutyric acid type A receptor revealed by mutation of the conserved M2 leucine

A conserved leucine residue in the midpoint of the second transmembrane domain (M2) of the ligand-activated ion channel family has been proposed to play an important role in receptor activation. In this study, we assessed the importance of this leucine in the activation of rat alpha 1 beta 2 gamma 2 GABA receptors expressed in Xenopus laevis oocytes by site-directed mutagenesis and two-electrode voltage clamp. The hydrophobic conserved M2 leucines in alpha1(L263), beta2(L259), and gamma 2(L274) subunits were mutated to the hydrophilic amino acid residue serine and coexpressed in all possible combinations with their wild-type and/or mutant counterparts. The mutation in any one subunit decreased the EC(50) and created spontaneous openings that were blocked by picrotoxin and, surprisingly, by the competitive antagonist bicuculline. The magnitudes of the shifts in GABA EC(50) and picrotoxin IC(50) as well as the degree of spontaneous openings were all correlated with the number of subunits carrying the leucine mutation. Simultaneous mutation of the GABA binding site (beta 2Y157S; increased the EC(50)) and the conserved M2 leucine (beta 2L259S; decreased the EC(50)) produced receptors with the predicted intermediate agonist sensitivity, indicating the two mutations affect binding and gating independently. The results are discussed in light of a proposed allosteric activation mechanism.

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.

Single channel properties of P2X2 purinoceptors

The single channel properties of cloned P2X2 purinoceptors expressed in human embryonic kidney (HEK) 293 cells and Xenopus oocytes were studied in outside-out patches. The mean single channel current-voltage relationship exhibited inward rectification in symmetric solutions with a chord conductance of approximately 30 pS at -100 mV in 145 mM NaCl. The channel open state exhibited fast flickering with significant power beyond 10 kHz. Conformational changes, not ionic blockade, appeared responsible for the flickering. The equilibrium constant of Na+ binding in the pore was approximately 150 mM at 0 mV and voltage dependent. The binding site appeared to be approximately 0.2 of the electrical distance from the extracellular surface. The mean channel current and the excess noise had the selectivity: K+ > Rb+ > Cs+ > Na+ > Li+. ATP increased the probability of being open (Po) to a maximum of 0.6 with an EC50 of 11.2 microM and a Hill coefficient of 2.3. Lowering extracellular pH enhanced the apparent affinity of the channel for ATP with a pKa of approximately 7.9, but did not cause a proton block of the open channel. High pH slowed the rise time to steps of ATP without affecting the fall time. The mean single channel amplitude was independent of pH, but the excess noise increased with decreasing pH. Kinetic analysis showed that ATP shortened the mean closed time but did not affect the mean open time. Maximum likelihood kinetic fitting of idealized single channel currents at different ATP concentrations produced a model with four sequential closed states (three binding steps) branching to two open states that converged on a final closed state. The ATP association rates increased with the sequential binding of ATP showing that the binding sites are not independent, but positively cooperative. Partially liganded channels do not appear to open. The predicted Po vs. ATP concentration closely matches the single channel current dose-response curve.

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.

Backbone mutations in transmembrane domains of a ligand-gated ion channel: implications for the mechanism of gating

An approach to identify backbone conformational changes underlying nicotinic acetylcholine receptor (nAChR) gating was developed. Specific backbone peptide bonds were replaced with an ester, which disrupts backbone hydrogen bonds at the site of mutation. At a conserved proline residue (alphaPro221) in the first transmembrane (M1) domain, the amide-to-ester mutation provides receptors with near-normal sensitivity, although the natural amino acids tested other than Pro produce receptors that gate with a much larger EC50 than normal. Therefore, a backbone hydrogen bond at this site may interfere with normal gating. In the alphaM2 domain, the amide-to-ester mutation yielded functional receptors at 15 positions, 3 of which provided receptors with >10-fold lower EC50 than wild type. These results support a model for gating that includes significant changes of backbone conformation within the M2 domain.

A mutational analysis of the acetylcholine receptor channel transmitter binding site

Mutagenesis and single-channel kinetic analysis were used to investigate the roles of four acetylcholine receptor channel (AChR) residues that are candidates for interacting directly with the agonist. The EC50 of the ACh dose-response curve was increased following alpha-subunit mutations Y93F and Y198F and epsilon-subunit mutations D175N and E184Q. Single-channel kinetic modeling indicates that the increase was caused mainly by a reduced gating equilibrium constant (Theta) in alphaY198F and epsilonD175N, by an increase in the equilibrium dissociation constant for ACh (KD) and a reduction in Theta in alphaY93F, and only by a reduction in KD in epsilonE184Q. This mutation altered the affinity of only one of the two binding sites and was the only mutation that reduced competition by extracellular K+. Additional mutations of epsilonE184 showed that K+ competition was unaltered in epsilonE184D and was virtually eliminated in epsilonE184K, but that neither of these mutations altered the intrinsic affinity for ACh. Thus there is an apparent electrostatic interaction between the epsilonE184 side chain and K+ ( approximately 1.7kBT), but not ACh+. The results are discussed in terms of multisite and induced-fit models of ligand binding to the AChR.

Desensitization of mouse nicotinic acetylcholine receptor channels. A two-gate mechanism

The rate constants of acetylcholine receptor channels (AChR) desensitization and recovery were estimated from the durations and frequencies of clusters of single-channel currents. Diliganded-open AChR desensitize much faster than either unliganded- or diliganded-closed AChR, which indicates that the desensitization rate constant depends on the status of the activation gate rather than the occupancy of the transmitter binding sites. The desensitization rate constant does not change with the nature of the agonist, the membrane potential, the species of permeant cation, channel block by ACh, the subunit composition (epsilon or gamma), or several mutations that are near the transmitter binding sites. The results are discussed in terms of cyclic models of AChR activation, desensitization, and recovery. In particular, a mechanism by which activation and desensitization are mediated by two distinct, but interrelated, gates in the ion permeation pathway is proposed.

A distinct contribution of the delta subunit to acetylcholine receptor channel activation revealed by mutations of the M2 segment

Acetylcholine receptor (AChR) channels with proline (P) mutations in the putative pore-forming domain (at the 12' position of the M2 segment) were examined at the single-channel level. For all subunits (alpha, beta, epsilon, and delta), a 12'P mutation increased the open channel lifetime >5-fold. To facilitate the estimation of binding and gating rate constants, subunits with 12'P mutations were co-expressed with alpha subunits having a binding site mutation that slows channel opening (alphaD200N). In these AChRs, a 12'P mutation in epsilon or beta slowed the closing rate constant approximately 6-fold but had no effect on either the channel opening rate constant or the equilibrium dissociation constant for ACh (Kd). In contrast, a 12'P mutation in delta slowed the channel closing rate constant only approximately 2-fold and significantly increased both the channel opening rate constant and the Kd. Pairwise expression of 12'P subunits indicates that mutations in epsilon and beta act nearly independently, but one in delta reduces the effect of a homologous mutation in epsilon or beta. The results suggest that a 12'P mutation in epsilon and beta has mainly local effects, whereas one in delta has both local and distributed effects that influence both agonist binding and channel gating.

The breakdown of cell membranes by electrical and mechanical stress

We attempted to determine whether mechanical tension and electrical stress couple to cause membrane breakdown in cells. Using cell-attached patches from HEK293 cells, we estimated the mechanically produced tension from the applied pressure and geometry of the patch. Voltage pulses of increasing amplitude were applied until we observed a sudden increase in conductance and capacitance. For pulses of 50 micros duration, breakdown required >0.5 V and was dependent on the tension. For pulses of 50-100 ms duration, breakdown required 0.2-0.4 V and was independent of tension. Apparently two physically different processes can lead to membrane breakdown. We could explain the response to the short, high-voltage pulses if breakdown occurred in the lipid bilayer. The critical electromechanical energy per unit area for breakdown by short pulses was approximately 4 dyne/cm, in agreement with earlier results on bilayers. Our data suggest that, at least in a patch, the bilayer may hold a significant fraction (approximately 40%) of the mean tension. To be compatible with the large, nonlytic area changes of patches, the bilayer appears to be pulled toward the pipette tip, perhaps by hydrophobic forces wetting membrane proteins bound to the glass. Although breakdown voltages for long pulses were in agreement with earlier work on algae, the mechanism(s) for this breakdown remain unclear.

Contribution of the beta subunit M2 segment to the ion-conducting pathway of the acetylcholine receptor

We have applied the substituted-cysteine-accessibility method (SCAM) to the M2 segment and the M1-M2 loop of the acetylcholine (ACh) receptor beta subunit. Each residue from beta P248 to beta D273 was mutated one at a time to Cys, and the mutant beta subunits were expressed together with wild-type alpha, beta, and delta subunits in Xenopus oocytes. For each of the mutants, the ACh-induced current was near wild-type. The accessibility of the substituted Cys was inferred from the irreversible inhibition or potentiation of ACh-induced current by methanethiosulfonate (MTS) derivatives added extracellularly. Inhibition by MTSethylammonium of beta G255C, in the narrow part of the channel, was mainly due to a reduction in the single-channel conductance. Conversely, potentiation by MTSethylammonium of beta V266C, in a wider part of the channel, was mainly due to an increase in channel open-time. Two substituted Cys at the intracellular end of M2 and three at the extracellular end were accessible to MTSethylammonium in the absence of ACh. Three additional Cys in the middle of M2 and three in the M1-M2 loop were accessible in the presence of ACh. In the presence of ACh, the secondary structure of beta M2 is alpha-helical from beta G255 to beta V266 and extended from beta L268 to beta D273. The accessible residues in beta M2 are remarkably hydrophobic, while the accessible residues in the M1-M2 loop are charged. beta M2, like alpha M2, alpha M1, and beta M1, undergoes widespread structural changes concomitant with gating, but the gate itself is close to the intracellular end of the channel. Many aligned residues in the M2 segments of alpha and beta are not identically accessible, indicating that the two subunits contribute differently to the channel lining.

The location of the gate in the acetylcholine receptor channel

The cation-conducting channel of the nicotinic acetylcholine (ACh) receptor is lined by the first (M1) and second (M2) membrane-spanning segments of each of its five subunits. Six consecutive residues, alphaS239 to alphaT244, in the alpha subunit M1-M2 loop and at the intracellular end of M2 were mutated to cysteine. The accessibility of the substituted cysteines were probed with small, cationic, sulfhydryl-specific reagents added extracellularly and intracellularly. In the closed state of the channel, there is a barrier to these reagents added from either side between alphaG240 and alphaT244. ACh induces the removal of this barrier, which acts as an activation gate. The residues alphaG240, alphaE241, alphaK242, and alphaT244 line a narrow part of the channel, in which this gate is located.