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

Clinical and Pathologic Features of Congenital Myasthenic Syndromes Caused by 35 Genes-A Comprehensive Review

Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders characterized by impaired neuromuscular signal transmission due to germline pathogenic variants in genes expressed at the neuromuscular junction (NMJ). A total of 35 genes have been reported in CMS (AGRN, ALG14, ALG2, CHAT, CHD8, CHRNA1, CHRNB1, CHRND, CHRNE, CHRNG, COL13A1, COLQ, DOK7, DPAGT1, GFPT1, GMPPB, LAMA5, LAMB2, LRP4, MUSK, MYO9A, PLEC, PREPL, PURA, RAPSN, RPH3A, SCN4A, SLC18A3, SLC25A1, SLC5A7, SNAP25, SYT2, TOR1AIP1, UNC13A, VAMP1). The 35 genes can be classified into 14 groups according to the pathomechanical, clinical, and therapeutic features of CMS patients. Measurement of compound muscle action potentials elicited by repetitive nerve stimulation is required to diagnose CMS. Clinical and electrophysiological features are not sufficient to identify a defective molecule, and genetic studies are always required for accurate diagnosis. From a pharmacological point of view, cholinesterase inhibitors are effective in most groups of CMS, but are contraindicated in some groups of CMS. Similarly, ephedrine, salbutamol (albuterol), amifampridine are effective in most but not all groups of CMS. This review extensively covers pathomechanical and clinical features of CMS by citing 442 relevant articles.

Probing function in ligand-gated ion channels without measuring ion transport

Although the functional properties of ion channels are most accurately assessed using electrophysiological approaches, a number of experimental situations call for alternative methods. Here, working on members of the pentameric ligand-gated ion channel (pLGIC) superfamily, we focused on the practical implementation of, and the interpretation of results from, equilibrium-type ligand-binding assays. Ligand-binding studies of pLGICs are by no means new, but the lack of uniformity in published protocols, large disparities between the results obtained for a given parameter by different groups, and a general disregard for constraints placed on the experimental observations by simple theoretical considerations suggested that a thorough analysis of this classic technique was in order. To this end, we present a detailed practical and theoretical study of this type of assay using radiolabeled α-bungarotoxin, unlabeled small-molecule cholinergic ligands, the human homomeric α7-AChR, and extensive calculations in the framework of a realistic five-binding-site reaction scheme. Furthermore, we show examples of the practical application of this method to tackle two longstanding questions in the field: our results suggest that ligand-binding affinities are insensitive to binding-site occupancy and that mutations to amino-acid residues in the transmembrane domain are unlikely to affect the channel's affinities for ligands that bind to the extracellular domain.

The surprising difficulty of "simple" equilibrium binding measurements on ligand-gated ion channels

Godellas and Grosman revisit equilibrium binding assays to shed new light on pentameric ligand-gated ion channels.

Slow Channel Syndrome Revisited: 40 Years Clinical Follow-Up and Genetic Characterization of Two Cases

BACKGROUND

The slow channel syndrome is a rare hereditary disorder caused by a dominant gain-of-function variant in one of the subunits of the acetylcholine receptor at the neuromuscular junction. Patients typically experience axial, limb and particularly extensor finger muscle weakness.

OBJECTIVE

Age at diagnosis is variable and although the long-term prognosis is important for newly diagnosed patients, extensive follow-up studies are rare. We aim to provide answers and perspective for this patient group by presenting an elaborate description of the lifetime follow-up of two slow channel syndrome patients.

METHODS

We describe 40 years follow-up in two, genetically confirmed cases (CHRNA1; c.866G >  T p.(Ser289Ile)(legacy Ser269Ile) and CHRNE; c.721C >  T p.(Leu241Phe)(legacy Leu221Phe) variants).

RESULTS

We find that the disease course has a fluctuating pattern and is only mildly progressive. However, hormonal imbalances, (psychological) stress or excessive hot or cold environments are often aggravating factors. Quinidine and fluoxetine are helpful, but ephedrine and salbutamol may also improve symptoms.

CONCLUSION

Slow channel syndrome is mildly progressive with a fluctuating pattern. The observations reported here provide a lifespan perspective and answers to the most pressing questions about prognosis and treatment options for newly diagnosed patients.

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.

Clinical and genetic characterization of an Italian family with slow-channel syndrome

INTRODUCTION

The slow-channel congenital myasthenic syndrome (SCCMS) is a postsynaptic form of congenital myasthenic syndromes (CMSs), a clinically heterogeneous group of disorders caused by genetic defects leading to an abnormal signal transmission at the endplate.

METHODS

We report clinical and molecular data of a multigenerational family in which the presentation of a progressive proximal-distal weakness with ocular involvement led to a number of different clinical diagnoses.

RESULTS

A comprehensive genetic study which included whole-genome linkage analysis and whole-exome sequencing identified a heterozygous missense substitution (c.721C>T, p.L241F) in the ε subunit of the acetylcholine receptor (CHRNE) that was consistent with clinical weakness in all patients.

DISCUSSION

SCCMS is characterized by a broad and heterogeneous clinical phenotype in which disease onset, symptoms, severity, and progression can be highly variable even between family members. The identification of a CHRNE mutation allowed to make the definitive diagnosis of CMS in this family and contributed to define the clinical spectrum of this disease.

Bayesian Statistical Inference in Ion-Channel Models with Exact Missed Event Correction

The stochastic behavior of single ion channels is most often described as an aggregated continuous-time Markov process with discrete states. For ligand-gated channels each state can represent a different conformation of the channel protein or a different number of bound ligands. Single-channel recordings show only whether the channel is open or shut: states of equal conductance are aggregated, so transitions between them have to be inferred indirectly. The requirement to filter noise from the raw signal further complicates the modeling process, as it limits the time resolution of the data. The consequence of the reduced bandwidth is that openings or shuttings that are shorter than the resolution cannot be observed; these are known as missed events. Postulated models fitted using filtered data must therefore explicitly account for missed events to avoid bias in the estimation of rate parameters and therefore assess parameter identifiability accurately. In this article, we present the first, to our knowledge, Bayesian modeling of ion-channels with exact missed events correction. Bayesian analysis represents uncertain knowledge of the true value of model parameters by considering these parameters as random variables. This allows us to gain a full appreciation of parameter identifiability and uncertainty when estimating values for model parameters. However, Bayesian inference is particularly challenging in this context as the correction for missed events increases the computational complexity of the model likelihood. Nonetheless, we successfully implemented a two-step Markov chain Monte Carlo method that we called "BICME", which performs Bayesian inference in models of realistic complexity. The method is demonstrated on synthetic and real single-channel data from muscle nicotinic acetylcholine channels. We show that parameter uncertainty can be characterized more accurately than with maximum-likelihood methods. Our code for performing inference in these ion channel models is publicly available.

Impaired NGF/TrkA Signaling Causes Early AD-Linked Presynaptic Dysfunction in Cholinergic Primary Neurons

Alterations in NGF/TrkA signaling have been suggested to underlie the selective degeneration of the cholinergic basal forebrain neurons occurring in vivo in AD (Counts and Mufson, 2005; Mufson et al., 2008; Niewiadomska et al., 2011) and significant reduction of cognitive decline along with an improvement of cholinergic hypofunction have been found in phase I clinical trial in humans affected from mild AD following therapeutic NGF gene therapy (Tuszynski et al., 2005, 2015). Here, we show that the chronic (10–12 D.I.V.) in vitro treatment with NGF (100 ng/ml) under conditions of low supplementation (0.2%) with the culturing serum-substitute B27 selectively enriches the basal forebrain cholinergic neurons (+36.36%) at the expense of other non-cholinergic, mainly GABAergic (−38.45%) and glutamatergic (−56.25%), populations. By taking advantage of this newly-developed septo-hippocampal neuronal cultures, our biochemical and electrophysiological investigations demonstrate that the early failure in excitatory neurotransmission following NGF withdrawal is paralleled by concomitant and progressive loss in selected presynaptic and vesicles trafficking proteins including synapsin I, SNAP-25 and α-synuclein. This rapid presynaptic dysfunction: (i) precedes the commitment to cell death and is reversible in a time-dependent manner, being suppressed by de novo external administration of NGF within 6 hr from its initial withdrawal; (ii) is specific because it is not accompanied by contextual changes in expression levels of non-synaptic proteins from other subcellular compartments; (ii) is not secondary to axonal degeneration because it is insensible to pharmacological treatment with known microtubule-stabilizing drug such paclitaxel; (iv) involves TrkA-dependent mechanisms because the effects of NGF reapplication are blocked by acute exposure to specific and cell-permeable inhibitor of its high-affinity receptor. Taken together, this study may have important clinical implications in the field of AD neurodegeneration because it: (i) provides new insights on the earliest molecular mechanisms underlying the loss of synaptic/trafficking proteins and, then, of synapes integrity which occurs in vulnerable basal forebrain population at preclinical stages of neuropathology; (ii) offers prime presynaptic-based molecular target to extend the therapeutic time-window of NGF action in the strategy of improving its neuroprotective in vivo intervention in affected patients.

Mutations Causing Slow-Channel Myasthenia Reveal That a Valine Ring in the Channel Pore of Muscle AChR is Optimized for Stabilizing Channel Gating

We identify two novel mutations in acetylcholine receptor (AChR) causing a slow-channel congenital myasthenia syndrome (CMS) in three unrelated patients (Pts). Pt 1 harbors a heterozygous βV266A mutation (p.Val289Ala) in the second transmembrane domain (M2) of the AChR β subunit (CHRNB1). Pts 2 and 3 carry the same mutation at an equivalent site in the ε subunit (CHRNE), εV265A (p.Val285Ala). The mutant residues are conserved across all AChR subunits of all species and are components of a valine ring in the channel pore, which is positioned four residues above the leucine ring. Both βV266A and εV265A reduce the amino acid size and lengthen the channel opening bursts by fourfold by enhancing gating efficiency by approximately 30-fold. Substitution of alanine for valine at the corresponding position in the δ and α subunit prolongs the burst duration four- and eightfold, respectively. Replacing valine at ε codon 265 either by a still smaller glycine or by a larger leucine also lengthens the burst duration. Our analysis reveals that each valine in the valine ring contributes to channel kinetics equally, and the valine ring has been optimized in the course of evolution to govern channel gating.

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

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

Potential new therapeutic modality revealed through agent-based modeling of the neuromuscular junction and acetylcholinesterase inhibition

Background

One of the leading causes of death and illness within the agriculture industry is through unintentionally ingesting or inhaling organophosphate pesticides. OP intoxication directly inhibits acetylcholinesterase, resulting in an excitatory signaling cascade leading to fasciculation, loss of control of bodily fluids, and seizures.

Methods

Our model was developed using a discrete, rules-based modeling approach in NetLogo. This model includes acetylcholinesterase, the nicotinic acetylcholine receptor responsible for signal transduction, a single release of acetylcholine, organophosphate inhibitors, and a theoretical novel medical countermeasure. We have parameterized the system considering the molecular reaction rate constants in an agent-based approach, as opposed to apparent macroscopic rates used in differential equation models.

Results

Our model demonstrates how the cholinergic crisis can be mitigated by therapeutic intervention with an acetylcholinesterase activator. Our model predicts signal rise rates and half-lives consistent with in vitro and in vivo data in the absence and presence of inhibitors. It also predicts the efficacy of theoretical countermeasures acting through three mechanisms: increasing catalytic turnover of acetylcholine, increasing acetylcholine binding affinity to the enzyme, and decreasing binding rates of inhibitors.

Conclusion

We present a model of the neuromuscular junction confirming observed acetylcholine signaling data and suggesting that developing a countermeasure capable of reducing inhibitor binding, and not activator concentration, is the most important parameter for reducing organophosphate (OP) intoxication.

Electronic supplementary material

The online version of this article (doi:10.1186/1742-4682-11-42) contains supplementary material, which is available to authorized users.

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.

A mouse model of the slow channel myasthenic syndrome: Neuromuscular physiology and effects of ephedrine treatment

In the slow channel congenital myasthenic syndrome mutations in genes encoding the muscle acetylcholine receptor give rise to prolonged ion channel activations. The resulting cation overload in the postsynaptic region leads to damage of synaptic structures, impaired neuromuscular transmission and fatigable muscle weakness. Previously we identified and characterised in detail the properties of the slow channel syndrome mutation εL221F. Here, using this mutation, we generate a transgenic mouse model for the slow channel syndrome that expresses mutant human ε-subunits harbouring an EGFP tag within the M3-M4 cytoplasmic region, driven by a ~1500 bp region of the CHRNB promoter. Fluorescent mutant acetylcholine receptors are assembled, cluster at the motor endplates and give rise to a disease model that mirrors the human condition. Mice demonstrate mild fatigable muscle weakness, prolonged endplate and miniature endplate potentials, and variable degeneration of the postsynaptic membrane. We use our model to investigate ephedrine as a potential treatment. Mice were assessed before and after six weeks on oral ephedrine (serum ephedrine concentration 89 ± 3 ng/ml) using an inverted screen test and in vivo electromyography. Treated mice demonstrated modest benefit for screen hang time, and in measures of compound muscle action potentials and mean jitter that did not reach statistical significance. Ephedrine and salbutamol show clear benefit when used in the treatment of DOK7 or COLQ congenital myasthenic syndromes. Our results highlight only a modest potential benefit of these β2-adrenergic receptor agonists for the treatment of the slow channel syndrome.

Single-channel properties of α3β4, α3β4α5 and α3β4β2 nicotinic acetylcholine receptors in mice lacking specific nicotinic acetylcholine receptor subunits

Previous attempts to measure the functional properties of recombinant nicotinic acetylcholine receptors (nAChRs) composed of known receptor subunits have yielded conflicting results. The use of knockout mice that lack α5, β2, α5β2 or α5β2α7 nAChR subunits enabled us to measure the single-channel properties of distinct α3β4, α3β4α5 and α3β4β2 receptors in superior cervical ganglion (SCG) neurons. Using this approach, we found that α3β4 receptors had a principal conductance level of 32.6 ± 0.8 pS (mean ± SEM) and both higher and lower secondary conductance levels. α3β4α5 receptors had the same conductance as α3β4 receptors, but differed from α3β4 receptors by having an increased channel open time and increased burst duration. By contrast, α3β4β2 receptors differed from α3β4 and α3β4α5 receptors by having a significantly smaller conductance level (13.6 ± 0.5 pS). After dissecting the single-channel properties of these receptors using our knockout models, we then identified these properties – and hence the receptors themselves – in wild-type SCG neurons. This study is the first to identify the single-channel properties of distinct neuronal nicotinic receptors in their native environment.

Over-expression of N-type calcium channels in cortical neurons from a mouse model of Amyotrophic Lateral Sclerosis

Voltage-gated Ca(2+) channels (VGCCs) mediate calcium entry into neuronal cells in response to membrane depolarisation and play an essential role in a variety of physiological processes. In Amyotrophic Lateral Sclerosis (ALS), a fatal neurodegenerative disease caused by motor neuron degeneration in the brain and spinal cord, intracellular calcium dysregulation has been shown, while no studies have been carried out on VGCCs. Here we show that the subtype N-type Ca(2+) channels are over expressed in G93A cultured cortical neurons and in motor cortex of G93A mice compared to Controls. In fact, by western blotting, immunocytochemical and electrophysiological experiments, we observe higher membrane expression of N-type Ca(2+) channels in G93A neurons compared to Controls. G93A cortical neurons filled with calcium-sensitive dye Fura-2, show a net calcium entry during membrane depolarization that is significantly higher compared to Control. Analysis of neuronal vitality following the exposure of neurons to a high K(+) concentration (25 mM, 5h), shows a significant reduction of G93A cellular survival compared to Controls. N-type channels are involved in the G93A higher mortality because ω-conotoxin GVIA (1 μM), which selectively blocks these channels, is able to abolish the higher G93A mortality when added to the external medium. These data provide robust evidence for an excess of N-type Ca(2+) expression in G93A cortical neurons which induces a higher mortality following membrane depolarization. These results may be central to the understanding of pathogenic pathways in ALS and provide novel molecular targets for the design of rational therapies for the ALS disorder.

Effect of ifenprodil on GluN1/GluN2B N-methyl-D-aspartate receptor gating

Ifenprodil is an allosteric inhibitor of GluN1/GluN2B N-methyl-D-aspartate receptors. Despite its widespread use as a prototype for drug development and a subtype-selective tool for physiologic experiments, its precise effect on GluN1/GluN2B gating is yet to be fully understood. Interestingly, recent crystallographic evidence identified that ifenprodil, unlike zinc, binds at the interface of the GluN1/GluN2B amino terminal domain dimer by an induced-fit mechanism. To delineate the effect of this unique binding on GluN1/GluN2B receptor gating, we recorded steady-state currents from cell-attached and outside-out patches. At pH 7.9 in cell-attached patches, ifenprodil increased the occupancy of the long-lived shut conformations, thereby reducing the open probability of the receptor with no change in the mean open time. In addition, ifenprodil selectively affected the area of shut time constants, but not the time constants themselves. Kinetic analyses suggested that ifenprodil prevents the transition of the receptor to an open state and increases its dwell time in an intrinsically occurring closed conformation or desensitized state. We found distinct differences in the action of ifenprodil at GluN1/GluN2B in comparison with previous studies on the effect of zinc on GluN1/GluN2A gating, which may arise due to their unique binding sites. Our data also uncover the potential pH-dependent action of ifenprodil on gating. At a low pH (pH 7.4), but not pH 7.9, ifenprodil reduces the mean open time of GluN1/GluN2B receptors, which may be responsible for its usefulness as a context-dependent inhibitor in conditions like ischemia and stroke, when the pH of the extracellular milieu becomes acidic.

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.

A new mouse model for the slow-channel congenital myasthenic syndrome induced by the AChR εL221F mutation

We have generated a new mouse model for congenital myasthenic syndromes by inserting the missense mutation L221F into the ε subunit of the acetylcholine receptor by homologous recombination. This mutation has been identified in man to cause a mild form of slow-channel congenital myasthenic syndrome with variable penetrance. In our mouse model we observe as in human patients prolonged endplate currents. The summation of endplate potentials may account for a depolarization block at increasing stimulus frequencies, moderate reduced muscle strength and tetanic fade. Calcium and intracellular vesicle accumulation as well as junctional fold loss and organelle degeneration underlying a typical endplate myopathy, were identified. Moreover, a remodeling of neuromuscular junctions occurs in a muscle-dependent pattern expressing variable phenotypic effects. Altogether, this mouse model provides new insight into the pathophysiology of congenital myasthenia and serves as a new tool for deciphering signaling pathways induced by excitotoxicity at peripheral synapses.

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.

Phenotypic heterogeneity in a large Thai slow-channel congenital myasthenic syndrome kinship

The slow-channel congenital myasthenic syndrome (SCCMS) is an autosomal dominant neuromuscular disorder caused by mutations in different subunits of the acetylcholine receptor (AChR). We here report our clinical findings in three generations of a large Thai kinship suffering from SCCMS and trace the disease to the p.Gly153Ser mutation in the AChR α subunit. The same mutation had previously been reported only in Caucasian but not in Asian patients. The clinical features include ptosis, ophthalmoparesis, and weakness of the cervical and finger extensor muscles as well as marked phenotypic heterogeneity.

Position 552 in a FMRFamide-gated Na(+) channel affects the gating properties and the potency of FMRFamide

FMRFamide-gated Na(+) channel (FaNaC) is a peptide-gated sodium channel in the epithelial Na(+) channel/degenerin family. Although there are some data on the location of the putative peptide binding site, there is no structural information on the activation gating of FaNaC. Here, we addressed the function of a conserved aspartate residue in the second transmembrane domain of FaNaC. We used Aplysia kurodai FaNaC (AkFaNaC) and examined the function of the aspartate (D552) by site-directed mutagenesis and electrophysiological recording in Xenopus oocytes. We found that the macroscopic activation, desensitization, and potency of FMRFamide and its modification by external Ca(2+) and Mg(2+) are greatly affected by physicochemical properties of the amino acid at position 552. We conclude that D552 is situated in a key position that affects the gating properties of FaNaC.

Distinct activities of GABA agonists at synaptic- and extrasynaptic-type GABAA receptors

The activation characteristics of synaptic and extrasynaptic GABA(A) receptors are important for shaping the profile of phasic and tonic inhibition in the central nervous system, which will critically impact on the activity of neuronal networks. Here, we study in isolation the activity of three agonists, GABA, muscimol and 4,5,6,7-tetrahydoisoxazolo[5,4-c]pyridin-3(2H)-one (THIP), to further understand the activation profiles of alpha 1 beta 3 gamma 2, alpha 4 beta 3 gamma 2 and alpha 4 beta 3 delta receptors that typify synaptic- and extrasynaptic-type receptors expressed in the hippocampus and thalamus. The agonists display an order of potency that is invariant between the three receptors, which is reliant mostly on the agonist dissociation constant. At delta subunit-containing extrasynaptic-type GABA(A) receptors, both THIP and muscimol additionally exhibited, to different degrees, superagonist behaviour. By comparing whole-cell and single channel currents induced by the agonists, we provide a molecular explanation for their different activation profiles. For THIP at high concentrations, the unusual superagonist behaviour on alpha 4 beta 3 delta receptors is a consequence of its ability to increase the duration of longer channel openings and their frequency, resulting in longer burst durations. By contrast, for muscimol, moderate superagonist behaviour was caused by reduced desensitisation of the extrasynaptic-type receptors. The ability to specifically increase the efficacy of receptor activation, by selected exogenous agonists over that obtained with the natural transmitter, may prove to be of therapeutic benefit under circumstances when synaptic inhibition is compromised or dysfunctional.

What have we learned from the congenital myasthenic syndromes

The congenital myasthenic syndromes have now been traced to an array of molecular targets at the neuromuscular junction encoded by no fewer than 11 disease genes. The disease genes were identified by the candidate gene approach, using clues derived from clinical, electrophysiological, cytochemical, and ultrastructural features. For example, electrophysiologic studies in patients suffering from sudden episodes of apnea pointed to a defect in acetylcholine resynthesis and CHAT as the candidate gene (Ohno et al., Proc Natl Acad Sci USA 98:2017-2022, 2001); refractoriness to anticholinesterase medications and partial or complete absence of acetylcholinesterase (AChE) from the endplates (EPs) has pointed to one of the two genes (COLQ and ACHE ( T )) encoding AChE, though mutations were observed only in COLQ. After a series of patients carrying mutations in a disease gene have been identified, the emerging genotype-phenotype correlations provided clues for targeted mutation analysis in other patients. Mutations in EP-specific proteins also prompted expression studies that proved pathogenicity, highlighted important functional domains of the abnormal proteins, and pointed to rational therapy.

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

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

What single-channel analysis tells us of the activation mechanism of ligand-gated channels: the case of the glycine receptor

Glycine receptors are, in several ways, the member of the nicotinic superfamily that is best-suited for single-channel recording. That means that they are ideal for testing ideas about how activation proceeds in a ligand-gated ion channel from the binding of the agonist to the opening of the channel. This review describes the quantitative characterization by single-channel analysis of a novel activation mechanism for the glycine receptor. The favourable properties of the glycine receptor allowed the first detection of a conformation change that follows the binding of the agonist but precedes the opening of the channel. We used the term 'flipping' to describe this pre-opening conformational change. The 'flipped' state has a binding affinity higher than the resting state, but lower than the open state. This increased affinity presumably reflects a structural change near the agonist binding site, possibly the 'capping' of the C-loop. The significance of the 'flip' activation mechanism goes beyond understanding the behaviour and the structure-function relation of glycine channels, as this mechanism can be applied also to other members of the superfamily, such as the muscle nicotinic receptor. The 'flip' mechanism has thrown light on the question of why partial agonists are not efficacious at keeping the channel open, a question that is fundamental to rational drug design. In both muscle nicotinic and glycine receptors, partial agonists are as good as full agonists at opening the channel once flipping has occurred, but are not as effective as full agonists in eliciting this early conformational change.

Mechanistic contributions of residues in the M1 transmembrane domain of the nicotinic receptor to channel gating

The nicotinic receptor (AChR) is a pentamer of homologous subunits with an alpha(2)betaepsilondelta composition in adult muscle. Each subunit contains four transmembrane domains (M1-M4). Position 15' of the M1 domain is phenylalanine in alpha subunits while it is isoleucine in non-alpha subunits. Given this peculiar conservation pattern, we studied its contribution to muscle AChR activation by combining mutagenesis with single-channel kinetic analysis. AChRs containing the mutant alpha subunit (alphaF15'I) as well as those containing the reverse mutations in the non-alpha subunits (betaI15'F, deltaI15'F, and epsilonI15'F) show prolonged lifetimes of the diliganded open channel resulting from a slower closing rate with respect to wild-type AChRs. The kinetic changes are not equivalent among subunits, the beta subunit, being the one that produces the most significant stabilization of the open state. Kinetic analysis of betaI15'F of AChR channels activated by the low-efficacious agonist choline revealed a 10-fold decrease in the closing rate, a 2.5-fold increase in the opening rate, a 28-fold increase in the gating equilibrium constant in the diliganded receptor, and a significant increase opening in the absence of agonist. Mutations at betaI15' showed that the structural bases of its contribution to gating is complex. Rate-equilibrium linear free-energy relationships suggest an approximately 70% closed-state-like environment for the beta15' position at the transition state of gating. The overall results identify position 15' as a subunit-selective determinant of channel gating and add new experimental evidence that gives support to the involvement of the M1 domain in the operation of the channel gating apparatus.

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

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

Decremental response to high-frequency trains of acetylcholine pulses but unaltered fractional Ca2+ currents in a panel of "slow-channel syndrome" nicotinic receptor mutants

The slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the neuromuscular junction caused by gain-of-function mutations to the muscle nicotinic acetylcholine (ACh) receptor (AChR). Although it is clear that the slower deactivation time course of the ACh-elicited currents plays a central role in the etiology of this disease, it has been suggested that other abnormal properties of these mutant receptors may also be critical in this respect. We characterized the kinetics of a panel of five SCCMS AChRs (αS269I, βV266M, εL221F, εT264P, and εL269F) at the ensemble level in rapidly perfused outside-out patches. We found that, for all of these mutants, the peak-current amplitude decreases along trains of nearly saturating ACh pulses delivered at physiologically relevant frequencies in a manner that is consistent with enhanced entry into desensitization during the prolonged deactivation phase. This suggests that the increasingly reduced availability of activatable AChRs upon repetitive stimulation may well contribute to the fatigability and weakness of skeletal muscle that characterize this disease. Also, these results emphasize the importance of explicitly accounting for entry into desensitization as one of the pathways for burst termination, if meaningful mechanistic insight is to be inferred from the study of the effect of these naturally occurring mutations on channel function. Applying a novel single-channel–based approach to estimate the contribution of Ca²⁺ to the total cation currents, we also found that none of these mutants affects the Ca²⁺-conduction properties of the AChR to an extent that seems to be of physiological importance. Our estimate of the Ca²⁺-carried component of the total (inward) conductance of wild-type and SCCMS AChRs in the presence of 150 mM Na⁺, 1.8 mM Ca²⁺, and 1.7 mM Mg²⁺ on the extracellular side of cell-attached patches turned out be in the 5.0–9.4 pS range, representing a fractional Ca²⁺ current of ∼14%, on average. Remarkably, these values are nearly identical to those we estimated for the NR1-NR2A N-methyl-d-aspartate receptor (NMDAR), which has generally been considered to be the main neurotransmitter-gated pathway of Ca²⁺ entry into the cell. Our estimate of the rat NMDAR Ca²⁺ conductance (using the same single-channel approach as for the AChR but in the nominal absence of extracellular Mg²⁺) was 7.9 pS, corresponding to a fractional Ca²⁺ current of 13%.

Activation of recombinant NR1/NR2C NMDA receptors

The N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptors comprises both NR1 and NR2 subunits, and plays numerous roles in both physiological and pathophysiological processes in the central nervous system (CNS). NR2C-containing NMDA receptors are most abundant in cerebellum, thalamus and olfactory bulb, and are also expressed in oligodendrocytes and hippocampal interneurons. We have used patch clamp recording to explore the activation properties of recombinant NR1/NR2C receptors expressed in HEK293 cells. NR1/NR2C receptors activated by a maximally effective concentration of glutamate and glycine had two main conductance levels of 45 pS and 28 pS when the extracellular Ca(2+) concentration was 0.5 mm and the holding potential was -80 mV. The occurrence of the lower subconductance state was reduced in the absence of extracellular Ca(2+). The distribution of closed durations recorded from patches with a high probability of containing only one active channel were best fitted by five exponential functions; the apparent open duration histogram could be fitted by two exponential functions (n = 10 patches). The apparent mean open time of NR1/NR2C receptors was brief (0.52 +/- 0.04 ms), suggesting that the stability of the open state of the NR1/NR2C receptors is lower than other NR2-containing receptors. NR1/NR2C open probability was exceptionally low, being 0.011 +/- 0.002 in patches containing a single active receptor (n = 8). Fast agonist concentration jumps were performed on outside out patches with multiple NR1/NR2C channels, which activated with a 10-90% rise time of 3.9 +/- 0.4 ms, faster than other NR2-containing receptors. The deactivation time constant after a brief (5-8 ms) application of a maximally effective concentration of agonists was 319 +/- 34 ms. The majority of the patches also showed a modest level of desensitization that could be described by either a single or a double exponential time course with the fastest time constant between 15 and 47 ms. Conceptual models of activation were fitted using the maximum interval likelihood (MIL) method to the sequence of open and closed durations recorded from outside-out patches that contained one active NR1/NR2C channel. NR1/NR2C receptor properties including modest desensitization and low open probability could be described by gating schemes similar to those previously proposed for other NMDA receptor subunit combinations.

On the nature of partial agonism in the nicotinic receptor superfamily

Partial agonists are ligands that bind to receptors but produce only a small maximum response even at concentrations where all receptors are occupied. In the case of ligand-activated ion channels, it has been supposed since 1957 that partial agonists evoke a small response because they are inefficient at eliciting the change of conformation between shut and open states of the channel. We have investigated partial agonists for two members of the nicotinic superfamily-the muscle nicotinic acetylcholine receptor and the glycine receptor-and find that the open-shut reaction is similar for both full and partial agonists, but the response to partial agonists is limited by an earlier conformation change ('flipping') that takes place while the channel is still shut. This has implications for the interpretation of structural studies, and in the future, for the design of partial agonists for therapeutic use.

An intersubunit trigger of channel gating in the muscle nicotinic receptor

Binding of neurotransmitter triggers gating of synaptic receptor channels, but our understanding of the structures that link the binding site to the channel is just beginning to develop. Here, we identify an intersubunit triggering element required for rapid and efficient gating of muscle nicotinic receptors using a structural model of the Torpedo receptor at 4 A resolution, recordings of currents through single receptor channels, measurements of inter-residue energetic coupling, and functional consequences of disulfide trapping. Mutation of the conserved residues, alphaTyr 127, epsilonAsn 39, and deltaAsn 41, located at the two subunit interfaces that form the agonist binding sites, markedly attenuates acetylcholine-elicited channel gating; mutant cycle analyses based on changes in the channel gating equilibrium constant reveal strong energetic coupling among these residues. After each residue is substituted with Cys, oxidizing conditions that promote disulfide bond formation attenuate gating of mutant, but not wild-type receptors. Gating is similarly attenuated when the Cys substitutions are confined to either of the binding-site interfaces, but can be restored by reducing conditions that promote disulfide bond breakage. Thus, the Tyr-Asn pair is an intersubunit trigger of rapid and efficient gating of muscle nicotinic receptors.

Modulation of AMPA Receptors in Cultured Cortical Neurons Induced by the Antiepileptic Drug Levetiracetam

PURPOSE

The present study explores the hypothesis that the antiepileptic mechanism of action of levetiracetam (LEV) is related to effects on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor channels in mouse cortical neurons in culture.

METHODS

The neurons were subjected to the whole-cell configuration of the patch-clamp recording technique and were 8-12 days old in culture.

RESULTS

Kainate elicited concentration-dependent (EC(50)= 80 microM) inward currents in all the patched cells. LEV (5-200 microM) itself did not induce inward or outward currents on all patched neurons, whereas it was effective on the kainate- and AMPA-induced current because it significantly decreased the amplitude of these currents. LEV was also able to significantly decrease the total membrane conductance during kainate perfusion, indicating that its effect was not dependent on the cellular voltage membrane potential. Further evidence that LEV modulated the ionotropic non-NMDA receptors came from the analysis of miniature excitatory postsynaptic currents (mEPSCs). In fact, LEV significantly decreased both the amplitude and the frequency of mEPSCs, as shown by the relative cumulative distributions.

CONCLUSIONS

These results reveal that AMPA receptors are modulated by LEV because a significant decrease in the kainate- and AMPA-induced currents and a decrease in amplitude and in frequency of mEPSCs have been observed in cortical neurons in culture. The described effect of LEV on AMPA receptors in cortical neurons is probably due to the etheromeric composition of the receptors and may be considered as a possible new antiepileptic mechanism of action.

Single-channel study of the spasmodic mutation alpha1A52S in recombinant rat glycine receptors

Inherited defects in glycine receptors lead to hyperekplexia, or startle disease. A mutant mouse, spasmodic, that has a startle phenotype, has a point mutation (A52S) in the glycine receptor alpha1 subunit. This mutation reduces the sensitivity of the receptor to glycine, but the mechanism by which this occurs is not known. We investigated the properties of A52S recombinant receptors by cell-attached patch-clamp recording of single-channel currents elicited by 30-10000 microM glycine. We used heteromeric receptors, which resemble those found at adult inhibitory synapses. Activation mechanisms were fitted directly to single channel data using the HJCFIT method, which includes an exact correction for missed events. In common with wild-type receptors, only mechanisms with three binding sites and extra shut states could describe the observations. The most physically plausible of these, the 'flip' mechanism, suggests that preopening isomerization to the flipped conformation that follows binding is less favoured in mutant than in wild-type receptors, and, especially, that the flipped conformation has a 100-fold lower affinity for glycine than in wild-type receptors. In contrast, the efficacy of the gating reaction was similar to that of wild-type heteromeric receptors. The reduction in affinity for the flipped conformation accounts for the reduction in apparent cooperativity seen in the mutant receptor (without having to postulate interaction between the binding sites) and it accounts for the increased EC50 for responses to glycine that is seen in mutant receptors. This mechanism also predicts accurately the faster decay of synaptic currents that is observed in spasmodic mice.

Single-channel properties of glycine receptors of juvenile rat spinal motoneurones in vitro

An essential step in understanding fast synaptic transmission is to establish the activation mechanism of synaptic receptors. The purpose of this work was to extend our detailed single-channel kinetic characterization of α1β glycine channels from rat recombinant receptors to native channels from juvenile (postnatal day 12–16) rat spinal cord slices. In cell-attached patches from ventral horn neurones, 1 mm glycine elicited clusters of channel openings to a single conductance level (41 ± 1 pS, n=12). This is similar to that of recombinant heteromers. However, fewer than 1 in 100 cell-attached patches from spinal neurones contained glycine channels. Outside-out patches gave a much higher success rate, but glycine channels recorded in this configuration appeared different, in that clusters opened to three conductance levels (28 ± 2, 38 ± 1 and 46 ± 1 pS, n=7, one level per cluster, all levels being detected in each patch). Furthermore, open period properties were different for the different conductances. As a consequence of this, the only recordings suitable for kinetic analysis were the cell-attached ones. Low channel density precluded recording at glycine concentrations other than 1 mm, but the 1 mm data allowed us to estimate the fully bound gating constants by global model fitting of the ‘flip’ mechanism of Burzomato and co-workers. Our results suggest that glycine receptors on ventral horn neurones in the juvenile rat are heteromers and have fast gating, similar to that of recombinant α1β receptors.

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.

Agonist-activated ion channels

This paper looks at ion channels as an example of the pharmacologist's stock in trade, the action of an agonist on a receptor to produce a response. Looked at in this way, ion channels have been helpful because they are still the only system which is simple enough for quantitative investigation of transduction mechanisms. A short history is given of attempts to elucidate what happens between the time when agonist first binds, and the time when the channel opens.

Structure of the first transmembrane domain of the neuronal acetylcholine receptor beta2 subunit

The recent cryoelectron microscopy structure of the Torpedo nicotinic acetylcholine receptor (nAChR) at 4-A resolution shows long helices for all transmembrane (TM) domains. This is in disagreement with several previous reports that the first TM domain of nAChR and other Cys-loop receptors are not entirely helical. In this study, we determined the structure and backbone dynamics of an extended segment encompassing the first TM domain (TM1e) of nAChR beta(2) subunit in dodecylphosphocholine micelles, using solution-state NMR and circular dichroism (CD) spectroscopy. Both CD and NMR results show less helicity in TM1e than in Torpedo nAChR structure (Protein Data Bank: 2BG9). The helical ending residues at the C-terminus are the same in the TM1e NMR structure and the Torpedo nAChR structure, but the helical starting residue (I-217) in TM1e is seven residues closer to the C-terminus. Interestingly, the helical starting residue is two residues before the highly conserved P-219, in accordance with the hypothesis that proline causes helical distortions at three residues preceding it. The NMR relaxation measurements show a dynamics pattern consistent with TM1e structure. The substantial nonhelical content adds greater flexibilities to TM1e, thereby implicating a different molecular basis for nAChR function compared to a longer and more rigid helical TM1.

Analysis of single ion channel data incorporating time-interval omission and sampling

Hidden Markov models are widely used to describe single channel currents from patch-clamp experiments. The inevitable anti-aliasing filter limits the time resolution of the measurements and therefore the standard hidden Markov model is not adequate anymore. The notion of time-interval omission has been introduced where brief events are not detected. The developed, exact solutions to this problem do not take into account that the measured intervals are limited by the sampling time. In this case the dead-time that specifies the minimal detectable interval length is not defined unambiguously. We show that a wrong choice of the dead-time leads to considerably biased estimates and present the appropriate equations to describe sampled data.

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.

Slow-channel mutation in acetylcholine receptor alphaM4 domain and its efficient knockdown

OBJECTIVE

To identify the genetic basis of a slow-channel myasthenic syndrome, characterize functional properties of the mutant receptor, and selectively silence the mutant allele.

METHODS

We performed nutation analysis, cloning, and patch-clamp analysis of the functional properties of the mutant receptor; screening for a small interfering RNA with check plasmid; and assessed of the efficacy of small interfering RNA at the messenger RNA, protein, and functional levels.

RESULTS

We traced the cause of a slow-channel myasthenic syndrome to a C418W mutation in the M4 domain of the acetylcholine receptor alpha subunit. The mutation is the first one to occur spontaneously in an M4 domain of the receptor, and it is positioned within a stripe of hydrophobic residues facing the lipid bilayer. Kinetic analysis shows that alphaC418W enhances the channel opening equilibrium constant 26-fold without altering agonist affinity. Using a check plasmid as a screening tool, we identified a small interfering RNA that markedly suppresses the mutant but not the wild-type allele at the messenger RNA, protein, and functional levels.

INTERPRETATION

alphaC418W occurring in humans causes a slow-channel syndrome by enhancing the relative stability of the channel open state. Efficient and selective knockdown of the mutant allele holds promise of therapeutic gene silencing.