Activation of recombinant mouse acetylcholine receptors by acetylcholine, carbamylcholine and tetramethylammonium

Distinct functional roles for the M4 α-helix from each homologous subunit in the hetero-pentameric ligand-gated ion channel nAChR

The outermost lipid-exposed α-helix (M4) in each of the homologous α, β, δ, and γ/ε subunits of the muscle nicotinic acetylcholine receptor (nAChR) has previously been proposed to act as a lipid sensor. However, the mechanism by which this sensor would function is not clear. To explore how the M4 α-helix from each subunit in human adult muscle nAChR influences function, and thus explore its putative role in lipid sensing, we functionally characterized alanine mutations at every residue in αM4, βM4, δM4, and εM4, along with both alanine and deletion mutations in the post-M4 region of each subunit. Although no critical interactions involving residues on M4 or in post-M4 were identified, we found that numerous mutations at the M4–M1/M3 interface altered the agonist-induced response. In addition, homologous mutations in M4 in different subunits were found to have different effects on channel function. The functional effects of multiple mutations either along M4 in one subunit or at homologous positions of M4 in different subunits were also found to be additive. Finally, when characterized in both Xenopus oocytes and human embryonic kidney 293T cells, select αM4 mutations displayed cell-specific phenotypes, possibly because of the different membrane lipid environments. Collectively, our data suggest different functional roles for the M4 α-helix in each heteromeric nAChR subunit and predict that lipid sensing involving M4 occurs primarily through the cumulative interactions at the M4–M1/M3 interface, as opposed to the alteration of specific interactions that are critical to channel function.

Conformational transitions and ligand-binding to a muscle-type nicotinic acetylcholine receptor

Fast synaptic communication requires receptors that respond to the presence of neurotransmitter by opening an ion channel across the post-synaptic membrane. The muscle-type nicotinic acetylcholine receptor from the electric fish, Torpedo, is the prototypic ligand-gated ion channel, yet the structural changes underlying channel activation remain undefined. Here we use cryo-EM to solve apo and agonist-bound structures of the Torpedo nicotinic receptor embedded in a lipid nanodisc. Using both a direct biochemical assay to define the conformational landscape and molecular dynamics simulations to assay flux through the pore, we correlate structures with functional states and elucidate the motions that lead to pore activation of a heteromeric nicotinic receptor. We highlight an underappreciated role for the complementary subunit in channel gating, establish the structural basis for the differential agonist affinities of α/δ versus α /γ sites, and explain why nicotine is less potent at muscle nicotinic receptors compared to neuronal ones.

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

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

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc/.

Analysis of neuronal nicotinic acetylcholine receptor α4β2 activation at the single-channel level

The neuronal nicotinic acetylcholine receptor α4β2 forms pentameric proteins with two alternate stoichiometries. The high-sensitivity receptor is related to (α4)2(β2)3 stoichiometry while the low-sensitivity receptor to (α4)3(β2)2 stoichiometry. Both subtypes share two binding sites at the α4((+))/β2((-)) interface with high affinity for agonists. (α4)3(β2)2 has an additional binding site at the α4((+))/α4((-)) interface with low affinity for agonists. We investigated activation kinetics of both receptor subtypes by patch-clamp recordings of single-channel activity in the presence of several concentrations of acetylcholine (0.5 to 300μM). We used kinetic software to fit these data with kinetic models. We found that the high-sensitivity subtype correlates with the low-conductance channel (g-70=29pS) and does not activate with high efficacy. On the contrary, the low-sensitivity subtype correlated with a high-conductance channel (g-70=44pS) and exhibited higher activation efficacy. Opening events of individual nAChRs at high agonist concentrations occurred in clusters, which allowed us to determine kinetic constants for the activation of the triliganded receptor. Our kinetic modeling identified an intermediate state, between resting and open conformation of the receptor. Binding of the third molecule increases the efficacy of receptor activation by favoring the transition between resting and intermediate state around 18 times. The low rate for this transition in the diliganded receptor explains the action of acetylcholine as partial agonist when it binds to the high-affinity sites. The presence of the third binding site emerges as a potent modulator of nicotinic receptor α4β2 activation which may display different functions depending on agonist concentration.

The atypical cation-conduction and gating properties of ELIC underscore the marked functional versatility of the pentameric ligand-gated ion-channel fold

The superfamily of pentameric ligand-gated ion channels (pLGICs) is unique among ionotropic receptors in that the same overall structure has evolved to generate multiple members with different combinations of agonist specificities and permeant-ion charge selectivities. However, aside from these differences, pLGICs have been typically regarded as having several invariant functional properties. These include pore blockade by extracellular quaternary-ammonium cations in the micromolar-to-millimolar concentration range (in the case of the cation-selective members), and a gain-of-function phenotype, which manifests as a slower deactivation time course, as a result of mutations that reduce the hydrophobicity of the transmembrane pore lining. Here, we tested this notion on three distantly related cation-selective members of the pLGIC superfamily: the mouse muscle nicotinic acetylcholine receptor (nAChR), and the bacterial GLIC and ELIC channels. Remarkably, we found that, whereas low millimolar concentrations of TMA(+) and TEA(+) block the nAChR and GLIC, neither of these two quaternary-ammonium cations blocks ELIC at such concentrations; instead, both carry measurable inward currents when present as the only cations on the extracellular side. Also, we found that, whereas lidocaine binding speeds up the current-decay time courses of the nAChR and GLIC in the presence of saturating concentrations of agonists, the binding of lidocaine to ELIC slows this time course down. Furthermore, whereas mutations that reduce the hydrophobicity of the side chains at position 9' of the M2 α-helices greatly slowed the deactivation time course of the nAChR and GLIC, these mutations had little effect--or even sped up deactivation--when engineered in ELIC. Our data indicate that caution should be exercised when generalizing results obtained with ELIC to the rest of the pLGICs, but more intriguingly, they hint at the possibility that ELIC is a representative of a novel branch of the superfamily with markedly divergent pore properties despite a well-conserved three-dimensional architecture.

Conotoxins targeting nicotinic acetylcholine receptors: an overview

Marine snails of the genus Conus are a large family of predatory gastropods with an unparalleled molecular diversity of pharmacologically active compounds in their venom. Cone snail venom comprises of a rich and diverse cocktail of peptide toxins which act on a wide variety of ion channels such as voltage-gated sodium- (Na_V), potassium- (K_V), and calcium- (Ca_V) channels as well as nicotinic acetylcholine receptors (nAChRs) which are classified as ligand-gated ion channels. The mode of action of several conotoxins has been the subject of investigation, while for many others this remains unknown. This review aims to give an overview of the knowledge we have today on the molecular pharmacology of conotoxins specifically interacting with nAChRs along with the structure–function relationship data.

Agonists binding nicotinic receptors elicit specific channel-opening patterns at αγ and αδ sites

'Embryonic' muscle-type nicotinic acetylcholine receptor channels (nAChRs) bind ligands at interfaces of α- and γ- or δ-subunits. αγ and αδ sites differ in affinity, but their contributions to opening the channel have remained elusive. We compared high-resolution patch clamp currents evoked by epibatidine (Ebd), carbamylcholine (CCh) and acetylcholine (ACh). Ebd binds with 75-fold higher affinity at αγ than at αδ sites, whereas CCh and ACh prefer αδ sites. Similar short (τO1), intermediate (τO2) and long (τO3) types of opening were observed with all three agonists. τO2 openings were maximally prevalent at low Ebd concentrations, binding at αγ sites. By contrast, τO1 openings appear to be generated at αδ sites. In addition, two types of burst appeared: short bursts of an average of 0.75 ms (τB1) that should arise from the αγ site, and long bursts of 12-25 ms (τB2) in duration arising from double liganded receptors. Limited by the temporal resolution, the closings within bursts were invariant at 3 μs. Corrected for missed closings, in the case of ACh the openings within long bursts lasted 170 μs and those in short bursts about 30 μs. Blocking αδ sites with α-conotoxin M1 (CTx) eliminated both τO1 and τB2 and left only τO2 and the short τB1 bursts, as expected. Furthermore we found desensitization when the receptors bound ACh only at the αγ site. When CTx was applied to 'embryonic' mouse endplates, monoquantal current rise times were increased, and amplitude and decay time constants were reduced, as expected. Thus the αγ and αδ sites of nAChRs elicit specific channel-opening patterns.

Structure-activity relationships of acetylcholine derivatives with Lucilia cuprina nicotinic acetylcholine receptor α1 and α2 subunits in chicken β2 subunit hybrid receptors in comparison with chicken nicotinic acetylcholine receptor α4/β2

Insect nicotinic acetylcholine (ACh) receptors (nAChRs) are the targets of several insecticide classes. In the present study, we report the gene identification and cloning of nAChR α1 and α2 subunits (Lcα1 and Lcα2) from the sheep blowfly Lucilia cuprina. Xenopus oocytes voltage clamp experiments as hybrids with the chicken β2 nAChR (Ggβ2) subunit resulted in ACh-gated ion channels with distinct dose-response curves for Lcα1/Ggβ2 (effective concentration 50% [EC50 ] = 80 nM; nH  = 1.05), and Lcα2/Ggβ2 (EC50  = 5.37 μM, nH  = 1.46). The neonicotinoid imidacloprid was a potent agonist for the α-bungarotoxin-sensitive Lcα1/Ggβ2 (EC50 ∼ 20 nM), while the α-bungarotoxin-resistant Lcα2/Ggβ2 showed a 30-fold lower sensitivity to this insecticide (EC50  = 0.62 μM). Thirteen close derivatives of ACh were analysed in EC50 , Hill coefficient and maximum current (relative to ACh) determinations for Lcα1/Ggβ2 and Lcα2/Ggβ2 and the chicken Ggα4/Ggβ2 nAChRs, and comparisons relative to ACh allowed the definition of novel structure-activity and structure-selectivity relationships. In the case of N-ethyl-acetylcholine, the EC50 of the chicken Ggα4/Ggβ2 rose by a factor of 1000, while for both Lcα1/Ggβ2 and Lcα2/Ggβ2, potency remained unchanged. Further derivatives with insect nAChR selectivity potential were acetyl-α-methylcholine and trimethyl-(3-methoxy-3-oxopropyl)ammonium, followed by acetylhomocholine and trimethyl-(4-oxopentyl) ammonium. Our results may provide guidance for the identification or design of insect-specific nAChR agonists using structure-based or in silico methods.

An integrated catch-and-hold mechanism activates nicotinic acetylcholine receptors

In neuromuscular acetylcholine (ACh) receptor channels (AChRs), agonist molecules bind with a low affinity (LA) to two sites that can switch to high affinity (HA) and increase the probability of channel opening. We measured (by using single-channel kinetic analysis) the rate and equilibrium constants for LA binding and channel gating for several different agonists of adult-type mouse AChRs. Almost all of the variation in the equilibrium constants for LA binding was from differences in the association rate constants. These were consistently below the limit set by diffusion and were substantially different even though the agonists had similar sizes and the same charge. This suggests that binding to resting receptors is not by diffusion alone and, hence, that each binding site can undergo two conformational changes ("catch" and "hold") that connect three different structures (apo-, LA-bound, and HA-bound). Analyses of ACh-binding protein structures suggest that this binding site, too, may adopt three discrete structures having different degrees of loop C displacement ("capping"). For the agonists we tested, the logarithms of the equilibrium constants for LA binding and LA↔HA gating were correlated. Although agonist binding and channel gating have long been considered to be separate processes in the activation of ligand-gated ion channels, this correlation implies that the catch-and-hold conformational changes are energetically linked and together comprise an integrated process having a common structural basis. We propose that loop C capping mainly reflects agonist binding, with its two stages corresponding to the formation of the LA and HA complexes. The catch-and-hold reaction coordinate is discussed in terms of preopening states and thermodynamic cycles of activation.

The intrinsic energy of the gating isomerization of a neuromuscular acetylcholine receptor channel

Nicotinic acetylcholine receptor (AChR) channels at neuromuscular synapses rarely open in the absence of agonists, but many different mutations increase the unliganded gating equilibrium constant (E₀) to generate AChRs that are active constitutively. We measured E₀ for two different sets of mutant combinations and by extrapolation estimated E₀ for wild-type AChRs. The estimates were 7.6 and 7.8 × 10⁻⁷ in adult-type mouse AChRs (−100 mV at 23°C). The values are in excellent agreement with one obtained previously by using a completely different method (6.5 × 10⁻⁷, from monoliganded gating). E₀ decreases with depolarization to the same extent as does the diliganded gating equilibrium constant, e-fold with ∼60 mV. We estimate that at −100 mV the intrinsic energy of the unliganded gating isomerization is +8.4 kcal/mol (35 kJ/mol), and that in the absence of a membrane potential, the intrinsic chemical energy of this global conformational change is +9.4 kcal/mol (39 kJ/mol). Na⁺ and K⁺ in the extracellular solution have no measureable effect on E₀, which suggests that unliganded gating occurs with only water occupying the transmitter binding sites. The results are discussed with regard to the energy changes in receptor activation and the competitive antagonism of ions in agonist binding.

End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease

The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.

Design and control of acetylcholine receptor conformational change

Allosteric proteins use energy derived from ligand binding to promote a global change in conformation. The "gating" equilibrium constant of acetylcholine receptor-channels (AChRs) is influenced by ligands, mutations, and membrane voltage. We engineered AChRs to have specific values of this constant by combining these perturbations, and then calculated the corresponding values for a reference condition. AChRs were designed to have specific rate and equilibrium constants simply by adding multiple, energetically independent mutations with known effects on gating. Mutations and depolarization (to remove channel block) changed the diliganded gating equilibrium constant only by changing the unliganded gating equilibrium constant (E(0)) and did not alter the energy from ligand binding. All of the tested perturbations were approximately energetically independent. We conclude that naturally occurring mutations mainly adjust E(0) and cause human disease because they generate AChRs that have physiologically inappropriate values of this constant. The results suggest that the energy associated with a structural change of a side chain in the gating isomerization is dissipated locally and is mainly independent of rigid body or normal mode motions of the protein. Gating rate and equilibrium constants are estimated for seven different AChR agonists using a stepwise engineering approach.

The gating isomerization of neuromuscular acetylcholine receptors

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

Probing protein packing surrounding the residues in and flanking the nicotinic acetylcholine receptor M2M3 loop

Nicotinic acetylcholine receptors (nAChR) are cation-selective, ligand-gated ion channels of the cysteine (Cys)-loop gene superfamily. The recent crystal structure of a bacterial homolog from Erwinia chrysanthemi (ELIC) agrees with previous structures of the N-terminal domain of AChBP (acetylcholine-binding protein) and of the electron-microscopy-derived Torpedo nAChR structure. However, the ELIC transmembrane domain is significantly more tightly packed than the corresponding region of the Torpedo nAChR. We investigated the tightness of protein packing surrounding the extracellular end of the M2 transmembrane segment and around the loop connecting the M2 and M3 segments using the substituted cysteine accessibility method. The M2 20' to 27' residues were highly water accessible and the variation in reaction rates were consistent with this region being alpha-helical. At all positions tested, the presence of ACh changed methanethiosulfonate ethylammonium (MTSEA) modification rates by <10-fold. In the presence of ACh, reaction rates for residues in the last extracellular alpha-helical turn of M2 and in the M2M3 loop increased, whereas rates in the penultimate alpha-helical turn of M2 decreased. Only three of eight M2M3 loop residues were accessible to MTSEA in both the presence and absence of ACh. We infer that the protein packing around the M2M3 loop is tight, consistent with its location at the interdomain interface where it is involved in the transduction of ligand binding in the extracellular domain to gating in the transmembrane domain. Our data indicate that the Torpedo nAChR transmembrane domain structure is a better model than the ELIC structure for eukaryotic Cys-loop receptors.

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.

Morantel allosterically enhances channel gating of neuronal nicotinic acetylcholine alpha 3 beta 2 receptors

We studied allosteric potentiation of rat alpha3beta2 neuronal nicotinic acetylcholine receptors (nAChRs) by the anthelmintic compound morantel. Macroscopic currents evoked by acetylcholine (ACh) from nAChRs expressed in Xenopus laevis oocytes increase up to 8-fold in the presence of low concentrations of morantel (< or =10 microM); the magnitude of the potentiation depends on both agonist and modulator concentrations. It is noteworthy that the potentiated currents exceed the maximum currents achieved by saturating (millimolar) concentrations of agonist. Studies of macroscopic currents elicited by prolonged drug applications (100-300 s) indicate that morantel does not increase alpha3beta2 receptor activity by reducing slow (> or =1 s) desensitization. Instead, using outside-out patch-clamp recordings, we demonstrate that morantel increases the frequency of single-channel openings and alters the bursting characteristics of the openings in a manner consistent with enhanced channel gating; these results quantitatively explain the macroscopic current potentiation. Morantel is a very weak agonist alone, but we show that the classic competitive antagonist dihydro-beta-erythroidine inhibits morantel-evoked currents noncompetitively, indicating that morantel does not bind to the canonical ACh binding sites.

Roles of amino acids and subunits in determining the inhibition of nicotinic acetylcholine receptors by competitive antagonists

BACKGROUND

Binding sites for agonists and competitive antagonists (nondepolarizing neuromuscular blocking agents) are located at the alpha-delta and alpha-epsilon subunit interfaces of adult nicotinic acetylcholine receptors. Most information about the amino acids that participate in antagonist binding comes from binding studies with (+)-tubocurarine and metocurine. These bind selectively to the alpha-epsilon interface but are differentially sensitive to mutations. To test the generality of this observation, the authors measured current inhibition by five competitive antagonists on wild-type and mutant acetylcholine receptors.

METHODS

HEK293 cells were transfected with wild-type or mutant (alphaY198F, epsilonD59A, epsilonD59N, epsilonD173A, epsilonD173N, deltaD180K) mouse muscle acetylcholine receptor complementary DNA. Outside-out patches were excised and perfused with acetylcholine in the absence and presence of antagonist. Concentration-response curves were constructed to determine antagonist IC50. An antagonist-removal protocol was used to determine dissociation and association rates.

RESULTS

Effects of mutations were antagonist specific. alphaY198F decreased the IC50 of (+)-tubocurarine 10-fold, increased the IC50 of vecuronium 5-fold, and had smaller effects on other antagonists. (+)-Tubocurarine was the most sensitive antagonist to epsilonD173 mutations. epsilonD59 mutations had large effects on metocurine and cisatracurium. deltaD180K decreased inhibition by pancuronium, vecuronium, and cisatracurium. Inhibition by these antagonists was increased for receptors containing two delta subunits but no epsilon subunit. Differences in IC50 arose from differences in both dissociation and association rates.

CONCLUSION

Competitive antagonists exhibited different patterns of sensitivity to mutations. Except for pancuronium, the antagonists were sensitive to mutations at the alpha-epsilon interface. Pancuronium, vecuronium, and cisatracurium were selective for the alpha-delta interface. This suggests the possibility of synergistic inhibition by pairs of antagonists.

Mapping the structural requirements for nicotinic acetylcholine receptor activation by using tethered alkyltrimethylammonium agonists and antagonists

A molecule as simple in structure as tetramethylammonium gates the nicotinic acetylcholine receptor (nAChR) with high efficacy. To compare the structure of the nAChR transmitter binding site in the open channel state with that of the ACh binding protein, we determined the efficacy of nAChR gating by -S(CH(2))(n)N(CH(3))(3)(+) (n = 1-4) tethered to substituted cysteines at positions in the alpha subunits or gamma and delta subunits predicted to contribute to the ACh binding sites in mutant Torpedo nAChRs expressed in Xenopus oocytes. For tethered thiocholine [-S(CH(2))(2)N(CH(3))(3)(+)], we previously reported that within alpha195-201 gating was observed only at alphaY198C while at alphaY93C it acted as an antagonist. We now show that within alpha191-194, thiocholine activates when tethered at alphaCys192 or alphaCys193. Thiocholine also activates when tethered at alphaY190C or alphaW149C in nAChRs containing a beta subunit mutation (betaL257S) that destabilizes the closed channel, but not from gammaW55C/deltaW57C, where longer adducts can activate. When tethered at positions in binding site segment E, thiocholine activates only from gammaL119C/deltaL121C, where the shorter -S(CH(2))(1)N(CH3)(3)(+) acts as an antagonist. Longer adducts tethered at gammaL109C/deltaL111C or gammaL119C/deltaL121C also activate, but less efficiently. The length requirements for efficient gating by tethered agonists agree closely with predictions based upon the structure of the agonist site in a nAChR homology model derived from the ACh binding protein structure, which suggests that this structure is an excellent model of the nAChR agonist binding site in the open channel conformation. The inability of thiocholine to activate from alphaY93C, which is not predicted by the model, is discussed in terms of the structure of the nAChR in the closed state.

Estimating binding affinities of the nicotinic receptor for low-efficacy ligands using mixtures of agonists and two-dimensional concentration-response relationships

The phenomenon of ligand-induced ion channel gating hinges upon the ability of a receptor channel to bind ligand molecules with conformation-specific affinities. However, our understanding of this fundamental phenomenon is notably limited, not only because the changes in binding site structure and ligand conformation that occur upon gating are largely unknown but, also, because the strength of these ligand–receptor interactions are experimentally elusive. Both high- and low-efficacy ligands pose a number of analytical and experimental challenges that can render the estimation of their conformation-specific binding affinities impossible. In this paper, we present a novel assay that overcomes some of the hurdles presented by weak agonists of the muscle nicotinic receptor and allows the estimation of their closed-state affinities. The method, which we have termed the “activation-competition” assay, consists of a single-channel concentration–response assay performed in the presence of a binary mixture of ligands of widely different efficacies. By plotting the channel response (i.e., the open probability) as a function of the concentration of each agonist in the mixture, interpreting the observed response in the framework of a plausible kinetic scheme, and fitting the open probability surface with the corresponding function, the affinities of the closed receptor for the two agonists can be simultaneously extracted as free parameters. Here, we applied this methodology to estimate the closed-state affinity of the muscle nicotinic receptor for choline (a very weak agonist) using acetylcholine (ACh) as the partner in the mixture. We estimated the dissociation equilibrium constant of choline (K_D) from the wild type's closed state to be 4.1 ± 0.5 mM (and that of ACh to be 106 ± 6 μM). We also discuss the use of accurate estimates of affinities for low-efficacy agonists as a tool to discriminate between binding and gating effects of mutations, and in the context of the rational design of therapeutic drugs.

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.

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

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

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

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

The Agonist Paradox: Agonists and Antagonists of Acetylcholine Receptors and Opioid Receptors

In contrast to antagonists, agonists tend to induce considerable conformational changes in their receptors, resulting in opening of ion channels, either directly or via secondary messengers. These conformational transformations require great energy expenses. However, the experimentally determined free energies of complexation between agonists and receptors are often relatively smaller than those for the corresponding antagonists. To rationalize this so-called 'agonist paradox', which has not been clarified in the literature, we have developed an alternative model. Our model may help to discriminate between agonists and antagonists of the acetylcholine (ACh) and mu-opioid receptors. For this purpose, a series of ligands (1-18) have been analyzed both in structural terms and with respect to complexation geometry within the anionic binding sites of these two receptor types.

A human congenital myasthenia-causing mutation (epsilon L78P) of the muscle nicotinic acetylcholine receptor with unusual single channel properties

A mutation in the epsilon subunit of the human nicotinic acetylcholine receptor (epsilonL78P) is known to cause a congenital slow channel myasthenic syndrome. We have investigated the changes in receptor function that result in the mutant receptor producing prolonged endplate currents, and consequent muscle damage. The rate constants for channel gating and for the binding and dissociation of acetylcholine were investigated by analysis of single ion channel recordings. A conventional mechanism with two non-equivalent binding sites, and variations upon this mechanism, were fitted to data using a maximum likelihood method that uses the Hawkes-Jalali-Colquhoun (HJC) treatment of missed brief events. The mutant receptor produced prolonged activations, bursts of openings that cause a slow decay of simulated synaptic currents. The main reason for the longer bursts of openings seen with mutant receptor was a decrease in the rate of ACh dissociation from diliganded receptors, though the lifetime of individual openings was somewhat increased too. As well as producing long bursts, the mutant receptor also produced many very short openings, though these carry little current. The burst structure for the mutant receptor at low ACh concentration is unusual in that most long bursts appear to start in a very brief monoliganded open state that then usually binds another ACh molecule to produce a long diliganded activation. The first opening is so short that it will usually be missed (together with the shut time that follows it), so the true burst length is likely to be underestimated.

A fluorophore attached to nicotinic acetylcholine receptor beta M2 detects productive binding of agonist to the alpha delta site

To study conformational transitions at the muscle nicotinic acetylcholine (ACh) receptor (nAChR), a rhodamine fluorophore was tethered to a Cys side chain introduced at the beta 19' position in the M2 region of the nAChR expressed in Xenopus oocytes. This procedure led to only minor changes in receptor function. During agonist application, fluorescence increased by (Delta F/F) approximately 10%, and the emission peak shifted to lower wavelengths, indicating a more hydrophobic environment for the fluorophore. The dose-response relations for Delta F agreed well with those for epibatidine-induced currents, but were shifted approximately 100-fold to the left of those for ACh-induced currents. Because (i) epibatidine binds more tightly to the alpha gamma-binding site than to the alpha delta site and (ii) ACh binds with reverse-site selectivity, these data suggest that Delta F monitors an event linked to binding specifically at the alpha delta-subunit interface. In experiments with flash-applied agonists, the earliest detectable Delta F occurs within milliseconds, i.e., during activation. At low [ACh] (< or = 10 microM), a phase of Delta F occurs with the same time constant as desensitization, presumably monitoring an increased population of agonist-bound receptors. However, recovery from Delta F is complete before the slowest phase of recovery from desensitization (time constant approximately 250 s), showing that one or more desensitized states have fluorescence like that of the resting channel. That conformational transitions at the alpha delta-binding site are not tightly coupled to channel activation suggests that sequential rather than fully concerted transitions occur during receptor gating. Thus, time-resolved fluorescence changes provide a powerful probe of nAChR conformational changes.

Mechanistic Diversity Underlying Fast Channel Congenital Myasthenic Syndromes

A host of missense mutations in muscle nicotinic receptor subunits have been identified as the cause of congenital myasthenic syndromes (CMS). Two classes of CMS phenotypes have been identified: slow channel myasthenic syndromes (SCCMSs) and fast channel myasthenic syndromes (FCCMSs). Although both have similar phenotypic consequences, they are physiologic opposites. Expression of the FCCMS phenotype requires the missense mutation to be accompanied by a second mutation, either a null or a missense mutation, in the second allele encoding the same receptor subunit. This seemingly rare scenario has arisen with surprisingly high incidence over the past few years, and analyses of the syndromes have revealed a diverse array of mechanisms underlying the pathology. This review focuses on new mechanisms underlying the FCCMS.

Life at the top: the transition state of AChR gating

Most neurotransmitter receptors belong to either the pentameric nicotinoid receptor family or the tetrameric glutamatergic receptor family. The muscle nicotinic acetylcholine receptor (AChR), the prototype of the nicotinoid receptor family, gates by switching between a closed configuration (in which ion permeation is forbidden) and an open configuration (which allows ions to pass through). Rate-equilibrium linear free energy relationship analysis has allowed us to explore the transition state that links these two stable conformations. A series of point mutations were made to individual AChR residues, and the ensuing changes in the rate constants of channel opening and closing for the fully liganded receptor were determined. These experiments suggest that gating occurs approximately as a reversible, solitary conformational wave that propagates between the neurotransmitter binding site and the membrane domain, along the long axis of the receptor. A detailed knowledge of the gating mechanism can serve as a basis for understanding the shape of the postsynaptic ion current and for the differences in synaptic responses among different ligand-gated channels.

Properties of the human muscle nicotinic receptor, and of the slow-channel myasthenic syndrome mutant epsilonL221F, inferred from maximum likelihood fits

The mechanisms that underlie activation of nicotinic receptors are investigated using human recombinant receptors, both wild type and receptors that contain the slow channel myasthenic syndrome mutation, epsilonL221F. The method uses the program HJCFIT, which fits the rate constants in a specified mechanism directly to a sequence of observed open and shut times by maximising the likelihood of the sequence with exact correction for missed events. A mechanism with two different binding sites was used. The rate constants that apply to the diliganded receptor (opening, shutting and total dissociation rates) were estimated robustly, being insensitive to the exact assumptions made during fitting, as expected from simulation studies. They are sufficient to predict the main physiological properties of the receptors. The epsilonL221F mutation causes an approximately 4-fold reduction in dissociation rate from diliganded receptors, and a smaller increase in opening rate and mean open time. These are sufficient to explain the approximately 6-fold slowing of decay of miniature synaptic currents seen in patients. The distinction between the two binding sites was less robust, the estimates of rate constants being dependent to some extent on assumptions, e.g. whether an extra short-lived shut state was included or whether the EC50 was constrained. The results suggest that the two binding sites differ by roughly 10-fold in the affinity of the shut receptor for ACh in the wild type, and that in the epsilonL221F mutation the lower affinity is increased so the sites become more similar.

Congenital myasthenic syndromes: progress over the past decade

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic basal lamina, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A basal lamina CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.

The nicotinic receptor ligand binding domain

The ligand binding domain (LBD) of the nicotinic acetylcholine receptor has served as a prototype for understanding molecular recognition in the family of neurotransmitter-gated ion channels. During the past fifty years, studies progressed from fundamental electrophysiological analyses of ACh-evoked ion flow, to biochemical purification of the receptor protein, pharmacological measurements of ligand binding, molecular cloning of receptor subunits, site-directed mutagenesis combined with functional analysis and recently, atomic structural determination. The emerging picture of the nicotinic receptor LBD is a specialized pocket of aromatic and hydrophobic residues formed at interfaces between protein subunits that changes conformation to convert agonist binding into gating of an intrinsic ion channel.

Naturally occurring mutations at the acetylcholine receptor binding site independently alter ACh binding and channel gating

By defining functional defects in a congenital myasthenic syndrome (CMS), we show that two mutant residues, located in a binding site region of the acetylcholine receptor (AChR) epsilon subunit, exert opposite effects on ACh binding and suppress channel gating. Single channel kinetic analysis reveals that the first mutation, epsilon N182Y, increases ACh affinity for receptors in the resting closed state, which promotes sequential occupancy of the binding sites and discloses rate constants for ACh occupancy of the nonmutant alphadelta site. Studies of the analogous mutation in the delta subunit, deltaN187Y, disclose rate constants for ACh occupancy of the nonmutant alpha epsilon site. The second CMS mutation, epsilon D175N, reduces ACh affinity for receptors in the resting closed state; occupancy of the mutant site still promotes gating because a large difference in affinity is maintained between closed and open states. epsilon D175N impairs overall gating, however, through an effect independent of ACh occupancy. When mapped on a structural model of the AChR binding site, epsilon N182Y localizes to the interface with the alpha subunit, and epsilon D175 to the entrance of the ACh binding cavity. Both epsilon N182Y and epsilon D175 show state specificity in affecting closed relative to desensitized state affinities, suggesting that the protein chain harboring epsilon N182 and epsilon D175 rearranges in the course of receptor desensitization. The overall results show that key residues at the ACh binding site differentially stabilize the agonist bound to closed, open and desensitized states, and provide a set point for gating of the channel.

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.

Microscopic kinetics and energetics distinguish GABA(A) receptor agonists from antagonists

Although agonists and competitive antagonists presumably occupy overlapping binding sites on ligand-gated channels, these interactions cannot be identical because agonists cause channel opening whereas antagonists do not. One explanation is that only agonist binding performs enough work on the receptor to cause the conformational changes that lead to gating. This idea is supported by agonist binding rates at GABA(A) and nicotinic acetylcholine receptors that are slower than expected for a diffusion-limited process, suggesting that agonist binding involves an energy-requiring event. This hypothesis predicts that competitive antagonist binding should require less activation energy than agonist binding. To test this idea, we developed a novel deconvolution-based method to compare binding and unbinding kinetics of GABA(A) receptor agonists and antagonists in outside-out patches from rat hippocampal neurons. Agonist and antagonist unbinding rates were steeply correlated with affinity. Unlike the agonists, three of the four antagonists tested had binding rates that were fast, independent of affinity, and could be accounted for by diffusion- and dehydration-limited processes. In contrast, agonist binding involved additional energy-requiring steps, consistent with the idea that channel gating is initiated by agonist-triggered movements within the ligand binding site. Antagonist binding does not appear to produce such movements, and may in fact prevent them.

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.

The anthelmintic pyrantel acts as a low efficacious agonist and an open-channel blocker of mammalian acetylcholine receptors

Pyrantel is an anthelmintic which acts as an agonist of nicotinic receptors (AChRs) of nematodes and exerts its therapeutic effects by depolarizing their muscle membranes. Here we explore at the single-channel level the action of pyrantel at mammalian muscle AChR. AChR currents are elicited by pyrantel. However, openings do not appear in clearly identifiable clusters over a range of pyrantel concentrations (1-300 microM). The mean open time decreases as a function of concentration, indicating an additional open-channel block. Single-channel recordings in the presence of high ACh concentrations and pyrantel demonstrate that the anthelmintic acts as a high-affinity open-channel blocker. When analyzed in terms of a sequential blocking scheme, the calculated forward rate constant for the blocking process is 8x10(7) M(-1) x s(-1), the apparent dissociation constant is 8 microM at a membrane potential of -70 mV and the process is voltage dependent. Pyrantel displaces alpha-bungarotoxin binding but the concentration dependence of equilibrium binding is shifted towards higher concentrations with respect to that of ACh binding. Thus, by acting at the binding site pyrantel activates mammalian AChRs with low efficacy, and by sterical blockade of the pore, the activated channels are then rapidly inhibited.

Choline and acetylcholine have similar kinetic properties of activation and desensitization on the alpha7 nicotinic receptors in rat hippocampal neurons

The alpha7-type nicotinic acetylcholine receptor (nAChR) was recently found to be both fully activated and desensitized by choline, in addition to ACh. In order to understand the combined effects of the two agonists on alpha7 nAChR-mediated neuronal signaling, the kinetics of the receptor-channel's interaction with ACh and choline was examined. To this end, whole-cell and single-channel currents evoked by fast-switching pulses of the agonists were recorded in rat hippocampal neurons in culture. Currents evoked by equieffective concentrations of choline and ACh were very similar, except that choline-evoked currents decayed more quickly to the baseline after removal of the agonist, and that recovery from desensitization was faster with choline. The conductance of channels activated by choline and ACh was 91.5+/-8.5 and 82.9+/-11.6 pS, respectively. The mean apparent channel open times were close to 100 micros, with both agonists. After a 4-s exposure to concentrations up to 80 microM ACh or 600 microM choline, the extent of desensitization and the cumulative charge flow carried by the channels increased in the same proportion, until reaching a maximum. At higher concentrations of either agonist, the cumulative charge started decreasing with concentration, reflecting further desensitization. Kinetic modeling suggested that alpha7 nAChRs have at least two non-equivalent paths to desensitized states, and that choline dissociates faster than ACh from the receptor. Our results established that the main difference between choline and ACh is of affinity, and support the concept that the switching of endogenous agonist may change the desensitization-resensitization dynamics of alpha7 nAChRs.

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.

Localization of agonist and competitive antagonist binding sites on nicotinic acetylcholine receptors

Identification of all residues involved in the recognition and binding of cholinergic ligands (e.g. agonists, competitive antagonists, and noncompetitive agonists) is a primary objective to understand which structural components are related to the physiological function of the nicotinic acetylcholine receptor (AChR). The picture for the localization of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are located mainly on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are identical, the observed high and low affinity for different ligands on the receptor is conditioned by the interaction of the alpha subunit with other non-alpha subunits. This molecular interaction takes place at the interface formed by the different subunits. For example, the high-affinity acetylcholine (ACh) binding site of the muscle-type AChR is located on the alphadelta subunit interface, whereas the low-affinity ACh binding site is located on the alphagamma subunit interface. Regarding homomeric AChRs (e.g. alpha7, alpha8, and alpha9), up to five binding sites may be located on the alphaalpha subunit interfaces. From the point of view of subunit arrangement, the gamma subunit is in between both alpha subunits and the delta subunit follows the alpha aligned in a clockwise manner from the gamma. Although some competitive antagonists such as lophotoxin and alpha-bungarotoxin bind to the same high- and low-affinity sites as ACh, other cholinergic drugs may bind with opposite specificity. For instance, the location of the high- and the low-affinity binding site for curare-related drugs as well as for agonists such as the alkaloid nicotine and the potent analgesic epibatidine (only when the AChR is in the desensitized state) is determined by the alphagamma and the alphadelta subunit interface, respectively. The case of alpha-conotoxins (alpha-CoTxs) is unique since each alpha-CoTx from different species is recognized by a specific AChR type. In addition, the specificity of alpha-CoTxs for each subunit interface is species-dependent. In general terms we may state that both alpha subunits carry the principal component for the agonist/competitive antagonist binding sites, whereas the non-alpha subunits bear the complementary component. Concerning homomeric AChRs, both the principal and the complementary component exist on the alpha subunit. The principal component on the muscle-type AChR involves three loops-forming binding domains (loops A-C). Loop A (from mouse sequence) is mainly formed by residue Y(93), loop B is molded by amino acids W(149), Y(152), and probably G(153), while loop C is shaped by residues Y(190), C(192), C(193), and Y(198). The complementary component corresponding to each non-alpha subunit probably contributes with at least four loops. More specifically, the loops at the gamma subunit are: loop D which is formed by residue K(34), loop E that is designed by W(55) and E(57), loop F which is built by a stretch of amino acids comprising L(109), S(111), C(115), I(116), and Y(117), and finally loop G that is shaped by F(172) and by the negatively-charged amino acids D(174) and E(183). The complementary component on the delta subunit, which corresponds to the high-affinity ACh binding site, is formed by homologous loops. Regarding alpha-neurotoxins, several snake and alpha-CoTxs bear specific residues that are energetically coupled with their corresponding pairs on the AChR binding site. The principal component for snake alpha-neurotoxins is located on the residue sequence alpha1W(184)-D(200), which includes loop C. In addition, amino acid sequence 55-74 from the alpha1 subunit (which includes loop E), and residues gammaL(119) (close to loop F) and gammaE(176) (close to loop G) at the low-affinity binding site, or deltaL(121) (close to the homologous region of loop G) at the high-affinity binding site, are i

Acetylcholine and epibatidine binding to muscle acetylcholine receptors distinguish between concerted and uncoupled models

The muscle acetylcholine receptor (AChR) has served as a prototype for understanding allosteric mechanisms of neurotransmitter-gated ion channels. The phenomenon of cooperative agonist binding is described by the model of Monod et al. (Monod, J., Wyman, J., and Changeux, J. P. (1965) J. Mol. Biol. 12, 88-118; MWC model), which requires concerted switching of the two binding sites between low and high affinity states. The present study examines binding of acetylcholine (ACh) and epibatidine, agonists with opposite selectivity for the two binding sites of mouse muscle AChRs. We expressed either fetal or adult AChRs in 293 HEK cells and measured agonist binding by competition against the initial rate of 125I-alpha-bungarotoxin binding. We fit predictions of the MWC model to epibatidine and ACh binding data simultaneously, taking as constants previously determined parameters for agonist binding and channel gating steps, and varying the agonist-independent parameters. We find that the MWC model describes the apparent dissociation constants for both agonists but predicts Hill coefficients that are far too steep. An Uncoupled model, which relaxes the requirement of concerted state transitions, accurately describes binding of both ACh and epibatidine and provides parameters for agonist-independent steps consistent with known aspects of AChR function.

Probing the agonist domain of the nicotinic acetylcholine receptor by cysteine scanning mutagenesis reveals residues in proximity to the alpha-bungarotoxin binding site

We have constructed a series of cysteine-substitution mutants in order to identify residues in the mouse muscle nicotinic acetylcholine receptor (AChR) that are involved in alpha-bungarotoxin (alpha-Bgtx) binding. Following transient expression in HEK 293-derived TSA-201 cells, covalent modification of the introduced cysteines with thiol-specific reagents reveals that alpha subunit residues W187, V188, F189, Y190, and P194 are solvent accessible and are in a position to contribute to the alpha-Bgtx binding site in native receptors. These results with the intact receptor are consistent with NMR studies of an alpha-Bgtx/receptor-dodecapeptide complex [Basus, V., Song., G., and Hawrot, E. (1993) Biochemistry 32, 12290-12298]. We pursued a more detailed analysis of the F189C mutant as this site varies substantially between AChRs that bind Bgtx and certain neuronal AChRs that do not. Treatment of intact cells expressing F189C with either bromoacetylcholine (BrACh) or [2-(trimethylammonium)ethyl] methane-thiosulfonate (MTSET), both methylammonium-containing thiol-modifying reagents with agonist properties, results in a marked decrease ( approximately 55-70%) in the number of alpha-Bgtx binding sites, as measured under saturating conditions. The decrease in sites appears to affect both alpha/gamma and alpha/delta sites to the same extent, as shown for alphaW187C and alphaF189C which were the two mutants examined on this issue. In contrast to the results obtained with MTSET and BrACh, modification with reagents that lack the alkylammonium entity, such as methylmethanethiosulfonate (MMTS), the negatively charged 2-sulfonatoethyl methane-thiosulfonate (MTSES), or the positively charged aminoethyl methylthiosulfonate (MTSEA), has little or no effect on the maximal binding of alpha-Bgtx to the alphaW187C, alphaV188C, or alphaF189C mutant receptors. The striking alkylammonium dependency suggests that an interaction of the tethered modifying group with the negative subsite within the agonist binding domain is primarily responsible for the observed blockade of toxin binding.

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.

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.

Defining affinity with the GABAA receptor

At nicotinic and glutamatergic synapses, the duration of the postsynaptic response depends on the affinity of the receptor for transmitter (Colquhoun et al., 1977;Pan et al., 1993). Affinity is often thought to be determined by the ligand unbinding rate, whereas the binding rate is assumed to be diffusion-limited. In this view, the receptor selects for those ligands that form a stable complex on binding, but binding is uniformly fast and does not itself affect selectivity. We tested these assumptions for the GABAA receptor by dissecting the contributions of microscopic binding and unbinding kinetics for agonists of equal efficacy but of widely differing affinities. Agonist pulses applied to outside-out patches of cultured rat hippocampal neurons revealed that agonist unbinding rates could not account for affinity if diffusion-limited binding was assumed. However, direct measurement of the instantaneous competition between agonists and a competitive antagonist revealed that binding rates were orders of magnitude slower than expected for free diffusion, being more steeply correlated with affinity than were the unbinding rates. The deviation from diffusion-limited binding indicates that a ligand-specific energy barrier between the unbound and bound states determines GABAA receptor selectivity. This barrier and our kinetic observations can be quantitatively modeled by requiring the participation of movable elements within a flexible GABA binding site.

Defining affinity with the GABAA receptor

At nicotinic and glutamatergic synapses, the duration of the postsynaptic response depends on the affinity of the receptor for transmitter (Colquhoun et al., 1977;Pan et al., 1993). Affinity is often thought to be determined by the ligand unbinding rate, whereas the binding rate is assumed to be diffusion-limited. In this view, the receptor selects for those ligands that form a stable complex on binding, but binding is uniformly fast and does not itself affect selectivity. We tested these assumptions for the GABAA receptor by dissecting the contributions of microscopic binding and unbinding kinetics for agonists of equal efficacy but of widely differing affinities. Agonist pulses applied to outside-out patches of cultured rat hippocampal neurons revealed that agonist unbinding rates could not account for affinity if diffusion-limited binding was assumed. However, direct measurement of the instantaneous competition between agonists and a competitive antagonist revealed that binding rates were orders of magnitude slower than expected for free diffusion, being more steeply correlated with affinity than were the unbinding rates. The deviation from diffusion-limited binding indicates that a ligand-specific energy barrier between the unbound and bound states determines GABAA receptor selectivity. This barrier and our kinetic observations can be quantitatively modeled by requiring the participation of movable elements within a flexible GABA binding site.

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.

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.