Mode switching kinetics produced by a naturally occurring mutation in the cytoplasmic loop of the human acetylcholine receptor epsilon subunit

Congenital Myasthenic Syndrome associated with acetylcholine receptor deficiency: case report and review of the literature

INTRODUCTION

Congenital Myasthenic Syndromes are a diverse group of conditions with a broad array of genetic underpinnings and phenotypic presentations. Acetylcholine receptor deficiency is one form that usually involves pathogenic variants in the Cholinergic Receptor Nicotinic Epsilon Subunit (CHRNE) gene encoding the ɛ-subunit of the acetylcholine receptor.

METHODS

We report a case of a 4-year-old male with suspected Congenital Myasthenic Syndrome with Acetylcholine Receptor Deficiency who presented with ocular symptoms and generalized muscle weakness. We additionally summarize published findings regarding the genetic, phenotypic, and clinical considerations of Congenital Myasthenic Syndrome with Acetylcholine Receptor Deficiency.

RESULTS

Exome sequencing revealed biallelic variants in CHRNE gene with a pathogenic frameshift variant and a variant of uncertain significance. After suboptimal response to pyridostigmine and albuterol, the patient experienced benefit with 3,4-DAP. The most commonly reported clinical characteristics in the literature are ptosis, muscle fatigability or weakness, and ophthalmoplegia.

CONCLUSION

We present the case of a patient with biallelic variants in CHRNE gene including a variant of uncertain significance. Evaluation of variants of this gene, including the variant of uncertain significance identified in this case report, through further cases and studies may improve our understanding of Congenital Myasthenic Syndrome with Acetylcholine Receptor deficiency.

HS-AFM single-molecule structural biology uncovers basis of transporter wanderlust kinetics

The Pyrococcus horikoshii amino acid transporter GltPh revealed, like other channels and transporters, activity mode switching, previously termed wanderlust kinetics. Unfortunately, to date, the basis of these activity fluctuations is not understood, probably due to a lack of experimental tools that directly access the structural features of transporters related to their instantaneous activity. Here, we take advantage of high-speed atomic force microscopy, unique in providing simultaneous structural and temporal resolution, to uncover the basis of kinetic mode switching in proteins. We developed membrane extension membrane protein reconstitution that allows the analysis of isolated molecules. Together with localization atomic force microscopy, principal component analysis and hidden Markov modeling, we could associate structural states to a functional timeline, allowing six structures to be solved from a single molecule, and an inward-facing state, IFSopen-1, to be determined as a kinetic dead-end in the conformational landscape. The approaches presented on GltPh are generally applicable and open possibilities for time-resolved dynamic single-molecule structural biology.

Impaired gating of γ- and ε-AChR respectively causes Escobar syndrome and fast-channel myasthenia

OBJECTIVE

To dissect the kinetic defects of acetylcholine receptor (AChR) γ subunit variant in an incomplete form of the Escobar syndrome without pterygium and compare it with those of a variant of corresponding residue in the AChR ε subunit in a congenital myasthenic syndrome (CMS).

METHODS

Whole exome sequencing, α-bungarotoxin binding assay, single channel patch-clamp recordings, and maximum likelihood analysis of channel kinetics.

RESULTS

We identified compound heterozygous variants in AChR γ and ε subunits in three Escobar syndrome (1-3) and three CMS patients (4-6), respectively. Each Escobar syndrome patient carries γP121R along with γV221Afs*44 in patients 1 and 2, and γY63* in patient 3. Three CMS patients share εP121T along with εR20W, εG-8R, and εY15H in patients 4, 5, and 6, respectively. Surface expressions of γP121R- and εP121T-AChR were 80% and 138% of the corresponding wild-type AChR, whereas εR20W, εG-8R, and εY15H reduced receptor expression to 27%, 35%, and 30% of wild-type εAChR, respectively. γV221Afs*44 and γY63* are null variants. Thus, γP121R and εP121T determine the phenotype. γP121R and εP121T shorten channel opening burst duration to 28% and 18% of corresponding wild-type AChR by reducing the channel gating equilibrium constant 44- and 63-fold, respectively.

INTERPRETATION

Similar impairment of channel gating efficiency of a corresponding P121 residue in the acetylcholine-binding site of the AChR γ and ε subunits causes Escobar syndrome without pterygium and fast-channel CMS, respectively, suggesting that therapy for the fast-channel CMS will benefit Escobar syndrome.

Regulation of nicotinic acetylcholine receptors by post-translational modifications

Nicotinic acetylcholine receptors (nAChRs) comprise a family of pentameric ligand-gated ion channels widely distributed in the central and peripheric nervous system and in non-neuronal cells. nAChRs are involved in chemical synapses and are key actors in vital physiological processes throughout the animal kingdom. They mediate skeletal muscle contraction, autonomic responses, contribute to cognitive processes, and regulate behaviors. Dysregulation of nAChRs is associated with neurological, neurodegenerative, inflammatory and motor disorders. In spite of the great advances in the elucidation of nAChR structure and function, our knowledge about the impact of post-translational modifications (PTMs) on nAChR functional activity and cholinergic signaling has lagged behind. PTMs occur at different steps of protein life cycle, modulating in time and space protein folding, localization, function, and protein-protein interactions, and allow fine-tuned responses to changes in the environment. A large body of evidence demonstrates that PTMs regulate all levels of nAChR life cycle, with key roles in receptor expression, membrane stability and function. However, our knowledge is still limited, restricted to a few PTMs, and many important aspects remain largely unknown. There is thus a long way to go to decipher the association of aberrant PTMs with disorders of cholinergic signaling and to target PTM regulation for novel therapeutic interventions. In this review we provide a comprehensive overview of what is known about how different PTMs regulate nAChR.

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.

A Novel Homozygous Variant in the CHRNE Gene in 2 Siblings with Congenital Myasthenic Syndrome

Cholinergic receptor nicotinic epsilon (CHRNE) subunit mutations cause postsynaptic type of congenital myasthenic syndrome either as a primary acetylcholine-receptor deficiency or abnormal channel kinetics in the receptor. We report a novel homozygous variant (c.322C > T, p.Pro108Ser) in the epsilon subunit causing primary acetylcholine-receptor deficiency in two siblings. Two siblings presented with fatigable weakness. Both siblings had whole exome sequencing showing a homozygous variant (c.322C > T, p.Pro108Ser) of unknown significance in the epsilon subunit. Electromyography/nerve conduction study with repetitive nerve stimulation on one sibling showed a defect in neuromuscular junction transmission. Pseudoephedrine and fluoxetine for suspected slow-channel congenital myasthenic syndrome yielded no improvement. A trial of pyridostigmine led to clinical improvement. Given the clinical presentation, consanguinity, homozygous genetic variant, and response to pyridostigmine, we rationalize the homozygous variant (c.322C > T, p.Pro108Ser) in cholinergic receptor nicotinic epsilon subunit causes the primary acetylcholine-receptor deficiency congenital myasthenic syndrome.

Congenital Myasthenic Syndromes or Inherited Disorders of Neuromuscular Transmission: Recent Discoveries and Open Questions

Congenital myasthenic syndromes (CMS) form a heterogeneous group of rare diseases characterized by fatigable muscle weakness. They are genetically-inherited and caused by defective synaptic transmission at the cholinergic neuromuscular junction (NMJ). The number of genes known to cause CMS when mutated is currently 30, and the relationship between fatigable muscle weakness and defective functions is quite well-understood for many of them. However, some of the most recent discoveries in individuals with CMS challenge our knowledge of the NMJ, where the basis of the pathology has mostly been investigated in animal models. Frontier forms between CMS and congenital myopathy, which have been genetically and clinically identified, underline the poorly understood interplay between the synaptic and extrasynaptic molecules in the neuromuscular system. In addition, precise electrophysiological and histopathological investigations of individuals with CMS suggest an important role of NMJ plasticity in the response to CMS pathogenesis. While efficient drug-based treatments are already available to improve neuromuscular transmission for most forms of CMS, others, as well as neurological and muscular comorbidities, remain resistant. Taken together, the available pathological data point to physiological issues which remain to be understood in order to achieve precision medicine with efficient therapeutics for all individuals suffering from CMS.

Congenital Myasthenic Syndromes in 2018

PURPOSE OF REVIEW

Summarize features of the currently recognized congenital myasthenic syndromes (CMS) with emphasis on novel findings identified in the past 6 years.

RECENT FINDINGS

Since the last review of the CMS in this journal in 2012, several novel CMS were identified. The identified disease proteins are SNAP25B, synaptotagmin 2, Munc13-1, synaptobrevin-1, GFPT1, DPAGT1, ALG2, ALG14, Agrin, GMPPB, LRP4, myosin 9A, collagen 13A1, the mitochondrial citrate carrier, PREPL, LAMA5, the vesicular ACh transporter, and the high-affinity presynaptic choline transporter. Exome sequencing has provided a powerful tool for identifying novel CMS. Identifying the disease genes is essential for determining optimal therapy. The landscape of the CMS is still unfolding.

CHRNE compound heterozygous mutations in congenital myasthenic syndrome: A case report

RATIONALE

Congenital myasthenic syndrome (CMSs) are a group of rare genetic disorders of the neurological junction, which can result in structural or functional weakness. Here, we characterized a case of CMS in order to clarify the diagnosis and expand the understanding of it. The molecular diagnosis had implications for choice of treatment and genetic counseling.

PATIENT CONCERNS

A 3-year-old male patient with CMS had ptosis and limb weakness for 2 months after birth. Clinical course and electrophysiological, imaging, and genetic findings were assessed. Protein structure/function was predicted. A novel mutation of c.295C>T (exon 4) and another known mutation of c.442T>A (exon 5) were found in CHRNE. Both mutations localized in conserved sequences. The c.442T>A (p.C148S) missense mutation in CHRNE was predicted to be damaging/deleterious. The iterative threading assembly refinement (I-TASSER) server generated vastly different 3-dimensional (3D) atomic models based on protein sequences from wide-type and novel nonsense mutation of c.295C>T (p.R99X) in CHRNE.

DIAGNOSES

The diagnosis of CMS with CHRNE mutations in Han Chinese was confirmed.

INTERVENTIONS

The patient was given prednisone (10 mg, once daily, taken orally) and pyridostigmine (15 mg, three times a day, taken orally).

OUTCOMES

The patient had a moderate response to prednisone and pyridostigmine.

LESSONS

We expanded the genotype and phenotype of CMS with CHRNE mutations in Han Chinese and provided new insights into the molecular mechanism of CMS and help to the diagnosis and treatment of CMS.

Genetic basis and phenotypic features of congenital myasthenic syndromes

The congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. The disease proteins reside in the nerve terminal, the synaptic basal lamina, or in the postsynaptic region, or at multiple sites at the neuromuscular junction as well as in other tissues. Targeted mutation analysis by Sanger or exome sequencing has been facilitated by characteristic phenotypic features of some CMS. No fewer than 20 disease genes have been recognized to date. In one-half of the currently identified probands, the disease stems from mutations in genes encoding subunits of the muscle form of the acetylcholine receptor (CHRNA1, CHRNB, CHRNAD1, and CHRNE). In 10-14% of the probands the disease is caused by mutations in RAPSN, DOK 7, or COLQ, and in 5% by mutations in CHAT. Other less frequently identified disease genes include LAMB2, AGRN, LRP4, MUSK, GFPT1, DPAGT1, ALG2, and ALG 14 as well as SCN4A, PREPL, PLEC1, DNM2, and MTM1. Identification of the genetic basis of each CMS is important not only for genetic counseling and disease prevention but also for therapy, because therapeutic agents that benefit one type of CMS can be harmful in another.

Agrin-LRP4-MuSK signaling as a therapeutic target for myasthenia gravis and other neuromuscular disorders

INTRODUCTION

Signal transduction at the neuromuscular junction (NMJ) is compromised in a diverse array of diseases including myasthenia gravis, Lambert-Eaton myasthenic syndrome, Isaacs' syndrome, congenital myasthenic syndromes, Fukuyama-type congenital muscular dystrophy, amyotrophic lateral sclerosis, and sarcopenia. Except for sarcopenia, all are orphan diseases. In addition, the NMJ signal transduction is impaired by tetanus, botulinum, curare, α-bungarotoxin, conotoxins, organophosphate, sarin, VX, and soman to name a few. Areas covered: This review covers the agrin-LRP4-MuSK signaling pathway, which drives clustering of acetylcholine receptors (AChRs) and ensures efficient signal transduction at the NMJ. We also address diseases caused by autoantibodies against the NMJ molecules and by germline mutations in genes encoding the NMJ molecules. Expert opinion: Representative small compounds to treat the defective NMJ signal transduction are cholinesterase inhibitors, which exert their effects by increasing the amount of acetylcholine at the synaptic space. Another possible therapeutic strategy to enhance the NMJ signal transduction is to increase the number of AChRs, but no currently available drug has this functionality.

Phenotypic heterogeneity in two large Roma families with a congenital myasthenic syndrome due to CHRNE 1267delG mutation. A long-term follow-up

Congenital myasthenic syndromes (CMS) are a heterogeneous group of genetic disorders. Mutations in CHRNE are one of the most common cause of them and the ɛ1267delG frameshifting mutation is described to be present on at least one allele of 60% of patients with CHRNE mutations. We present a comprehensive description of the heterogeneous clinical features of the CMS caused by the homozygous 1267delG mutation in the AChR Ɛ subunit in nine members of two large Gipsy kindreds. Our observations indicate that founder Roma mutation 1267delG leads to a phenotype further characterized by ophthalmoplegia, bilateral ptosis, and good response to pyridostigmine and 3,4-DAP; but also by facial weakness, bulbar symptoms, neck muscle weakness, and proximal limb weakness that sometimes entails the loss of ambulation. Interestingly, we found in our series a remarkable proportion of patients with a progressive or fluctuating course of the disease. This finding is in some contrast with previous idea that considered this form of CMS as benign, non progressive, and with a low impact on the capacity of ambulation.

Congenital myasthenic syndromes: pathogenesis, diagnosis, and treatment

The congenital myasthenic syndromes (CMS) are a diverse group of genetic disorders caused by abnormal signal transmission at the motor endplate, a special synaptic contact between motor axons and each skeletal muscle fibre. Most CMS stem from molecular defects in the muscle nicotinic acetylcholine receptor, but they can also be caused by mutations in presynaptic proteins, mutations in proteins associated with the synaptic basal lamina, defects in endplate development and maintenance, or defects in protein glycosylation. The specific diagnosis of some CMS can sometimes be reached by phenotypic clues pointing to the mutated gene. In the absence of such clues, exome sequencing is a useful technique for finding the disease gene. Greater understanding of the mechanisms of CMS have been obtained from structural and electrophysiological studies of the endplate, and from biochemical studies. Present therapies for the CMS include cholinergic agonists, long-lived open-channel blockers of the acetylcholine receptor ion channel, and adrenergic agonists. Although most CMS are treatable, caution should be exercised as some drugs that are beneficial in one syndrome can be detrimental in another.

Congenital myasthenic syndrome in Japan: ethnically unique mutations in muscle nicotinic acetylcholine receptor subunits

Congenital myasthenic syndromes (CMS) are caused by mutations in genes expressed at the neuromuscular junction. Most CMS patients have been reported in Western and Middle Eastern countries, and only four patients with COLQ mutations have been reported in Japan. We here report six mutations in acetylcholine receptor (AChR) subunit genes in five Japanese patients. Five mutations are novel, and one mutation is shared with a European American patient but with a different haplotype. Among the observed mutations, p.Thr284Pro (p.Thr264Pro according to the legacy annotation) in the epsilon subunit causes a slow-channel CMS. Five other mutations in the delta and epsilon subunits are splice site, frameshift, null, or missense mutations causing endplate AChR deficiency. We also found a heteroallelic p.Met465Thr in the beta subunit in another patient. p.Met465Thr, however, was likely to be polymorphism, because single channel recordings showed mild shortening of channel openings without affecting cell surface expression of AChR, and the minor allelic frequency of p.Met465Thr was 5.1% in the Japanese population. Lack of shared mutant alleles between the Japanese and the other patients suggests that most mutations described here are ethnically unique or de novo in each family.

A single mutation in the acetylcholine receptor δ-subunit causes distinct effects in two types of neuromuscular synapses

Mutations in AChR subunits, expressed as pentamers in neuromuscular junctions (NMJs), cause various types of congenital myasthenic syndromes. In AChR pentamers, the adult ε subunit gradually replaces the embryonic γ subunit as the animal develops. Because of this switch in subunit composition, mutations in specific subunits result in synaptic phenotypes that change with developmental age. However, a mutation in any AChR subunit is considered to affect the NMJs of all muscle fibers equally. Here, we report a zebrafish mutant of the AChR δ subunit that exhibits two distinct NMJ phenotypes specific to two muscle fiber types: slow or fast. Homozygous fish harboring a point mutation in the δ subunit form functional AChRs in slow muscles, whereas receptors in fast muscles are nonfunctional. To test the hypothesis that different subunit compositions in slow and fast muscles underlie distinct phenotypes, we examined the presence of ε/γ subunits in NMJs using specific antibodies. Both wild-type and mutant larvae lacked ε/γ subunits in slow muscle synapses. These findings in zebrafish suggest that some mutations in human congenital myasthenic syndromes may affect slow and fast muscle fibers differently.

Statistical analysis of modal gating in ion channels

Ion channels regulate the concentrations of ions within cells. By stochastically opening and closing its pore, they enable or prevent ions from crossing the cell membrane. However, rather than opening with a constant probability, many ion channels switch between several different levels of activity even if the experimental conditions are unchanged. This phenomenon is known as modal gating: instead of directly adapting its activity, the channel seems to mix sojourns in active and inactive modes in order to exhibit intermediate open probabilities. Evidence is accumulating that modal gating rather than modulation of opening and closing at a faster time scale is the primary regulatory mechanism of ion channels. However, currently, no method is available for reliably calculating sojourns in different modes. In order to address this challenge, we develop a statistical framework for segmenting single-channel datasets into segments that are characteristic for particular modes. The algorithm finds the number of mode changes, detects their locations and infers the open probabilities of the modes. We apply our approach to data from the inositol-trisphosphate receptor. Based upon these results, we propose that mode changes originate from alternative conformational states of the channel protein that determine a certain level of channel activity.

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.

Current status of the congenital myasthenic syndromes

Congenital myasthenic syndromes (CMS) are heterogeneous disorders in which the safety margin of neuromuscular transmission is compromised by one or more specific mechanisms. Clinical, electrophysiologic, and morphologic studies have paved the way for detecting CMS-related mutations in proteins residing in the nerve terminal, the synaptic basal lamina, and in the postsynaptic region of the motor endplate. The disease proteins identified to date include choline acetyltransferase (ChAT), the endplate species of acetylcholinesterase (AChE), β2-laminin, the acetylcholine receptor (AChR), rapsyn, plectin, Na(v)1.4, the muscle specific protein kinase (MuSK), agrin, downstream of tyrosine kinase 7 (Dok-7), and glutamine-fructose-6-phosphate transaminase 1 (GFPT1). Myasthenic syndromes associated with centronuclear myopathies were recently recognized. Analysis of properties of expressed mutant proteins contributed to finding improved therapy for most CMS. Despite these advances, the molecular basis of some phenotypically characterized CMS remains elusive. Moreover, other types of CMS and disease genes likely exist and await discovery.

Molecular dissection of Cl--selective Cys-loop receptor points to components that are dispensable or essential for channel activity

Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) that bind neurotransmitters to open an intrinsic transmembrane ion channel pore. The recent crystal structure of a prokaryotic pLGIC from the cyanobacterium Gloeobacter violaceus (GLIC) revealed that it naturally lacks an N-terminal extracellular α helix and an intracellular domain that are typical of eukaryotic pLGICs. GLIC does not respond to neurotransmitters acting at eukaryotic pLGICs but is activated by protons. To determine whether the structural differences account for functional differences, we used a eukaryotic chimeric acetylcholine-glutamate pLGIC that was modified to carry deletions corresponding to the sequences missing in the prokaryotic homolog GLIC. Deletions made in the N-terminal extracellular α helix did not prevent the expression of receptor subunits and the appearance of receptor assemblies on the cell surface but abolished the capability of the receptor to bind α-bungarotoxin (a competitive antagonist) and to respond to the neurotransmitter. Other truncated chimeric receptors that lacked the intracellular domain did bind ligands; displayed robust acetylcholine-elicited responses; and shared with the full-length chimeric receptor similar anionic selectivity, effective open pore diameter, and unitary conductance. We suggest that the integrity of the N-terminal α helix is crucial for ligand accommodation because it stabilizes the intersubunit interfaces adjacent to the neurotransmitter-binding pocket(s). We also conclude that the intracellular domain of the chimeric acetylcholine-glutamate receptor does not modulate the ion channel conductance and is not involved in positioning of the pore-lining helices in the conformation necessary for coordinating a Cl- ion within the intracellular vestibule of the ion channel pore.

Clinical features in a series of fast channel congenital myasthenia syndrome

Fast channel congenital myasthenic syndromes are rare, but frequently result in severe weakness. We report a case of 12 fast channel patients to highlight clinical features and management difficulties. Patients were diagnosed through genetic screening and identification of mutations shown to cause fast channel syndrome. Data was obtained from clinical notes, history, examination and follow up. Patterns of muscle weakness involved limb, trunk, bulbar, respiratory, facial and extraocular muscles. Patients responded to treatment with anticholinesterase medication and 3,4-diaminopyridine. Fast channel syndrome contrasted with AChR deficiency in the occurrence of severe respiratory crises in infancy and childhood. The death of two children even when on treatment and the family histories of sibling deaths re-inforces the need for accurate genetic diagnosis, optimised pharmacological treatment and additional supportive measures to manage acute respiratory crises. Referral to a specialist paediatric respiratory centre and regular resuscitation training for parents are recommended.

Engineering a prokaryotic Cys-loop receptor with a third functional domain

Prokaryotic members of the Cys-loop receptor ligand-gated ion channel superfamily were recently identified. Previously, Cys-loop receptors were only known from multicellular organisms (metazoans). Contrary to the metazoan Cys-loop receptors, the prokaryotic ones consist of an extracellular (ECD) and a transmembrane domain (TMD), lacking the large intracellular domain (ICD) present in metazoa (between transmembrane segments M3 and M4). Using a chimera approach, we added the 115-amino acid ICD from mammalian serotonin type 3A receptors (5-HT(3A)) to the prokaryotic proton-activated Gloeobacter violaceus ligand-gated ion channel (GLIC). We created 12 GLIC-5-HT(3A)-ICD chimeras by replacing a variable number of amino acids in the short GLIC M3M4 linker with the entire 5-HT(3A)-ICD. Two-electrode voltage clamp recordings after expression in Xenopus laevis oocytes showed that only two chimeras were functional and produced currents upon acidification. The pH(50) was comparable with wild-type GLIC. 5-HT(3A) receptor expression can be inhibited by the chaperone protein RIC-3. We have shown previously that the 5-HT(3A)-ICD is required for the attenuation of 5-HT-induced currents when RIC-3 is co-expressed with 5-HT(3A) receptors in X. laevis oocytes. Expression of both functional 5-HT(3A) chimeras was inhibited by RIC-3 co-expression, indicating appropriate folding of the 5-HT(3A)-ICD in the chimeras. Our results indicate that the ICD can be considered a separate domain that can be removed from or added to the ECD and TMD while maintaining the overall structure and function of the ECD and TMD.

On the structural basis of modal gating behavior in K(+) channels

Modal-gating shifts represent an effective regulatory mechanism by which ion channels control the extent and time course of ionic fluxes. Under steady-state conditions, the K(+) channel KcsA shows three distinct gating modes, high-P(o), low-P(o) and a high-frequency flicker mode, each with about an order of magnitude difference in their mean open times. Here we show that in the absence of C-type inactivation, mutations at the pore-helix position Glu71 unmask a series of kinetically distinct modes of gating in a side chain-specific way. These gating modes mirror those seen in wild-type channels and suggest that specific interactions in the side chain network surrounding the selectivity filter, in concert with ion occupancy, alter the relative stability of pre-existing conformational states of the pore. The present results highlight the key role of the selectivity filter in regulating modal gating behavior in K(+) channels.

hnRNP H enhances skipping of a nonfunctional exon P3A in CHRNA1 and a mutation disrupting its binding causes congenital myasthenic syndrome

In humans and great apes, CHRNA1 encoding the muscle nicotinic acetylcholine receptor alpha subunit carries an inframe exon P3A, the inclusion of which yields a nonfunctional alpha subunit. In muscle, the P3A(-) and P3A(+) transcripts are generated in a 1:1 ratio but the functional significance and regulation of the alternative splicing remain elusive. An intronic mutation (IVS3-8G>A), identified in a patient with congenital myasthenic syndrome, disrupts an intronic splicing silencer (ISS) and results in exclusive inclusion of the downstream P3A exon. We found that the ISS-binding splicing trans-factor was heterogeneous nuclear ribonucleoprotein (hnRNP) H and the mutation attenuated the affinity of hnRNP for the ISS approximately 100-fold. We next showed that direct placement of hnRNP H to the 3' end of intron 3 silences, and siRNA-mediated downregulation of hnRNP H enhances recognition of exon P3A. Analysis of the human genome suggested that the hnRNPH-binding UGGG motif is overrepresented close to the 3' ends of introns. Pursuing this clue, we showed that alternative exons of GRIP1, FAS, VPS13C and NRCAM are downregulated by hnRNP H. Our findings imply that the presence of the hnRNP H-binding motif close to the 3' end of an intron is an essential but underestimated splicing regulator of the downstream exon.

Mutations in Dalpha1 or Dbeta2 nicotinic acetylcholine receptor subunits can confer resistance to neonicotinoids in Drosophila melanogaster

Resistance to insecticides by modification of their molecular targets is a serious problem in chemical control of many arthropod pests. Neonicotinoids target the nicotinic acetylcholine receptor (nAChR) of arthropods. The spectrum of possible resistance-conferring mutations of this receptor is poorly understood. Prediction of resistance is complicated by the existence of multiple genes encoding the different subunits of this essential component of neurotransmission. We focused on the cluster of three Drosophila melanogaster nAChR subunit genes at cytological region 96A. EMS mutagenesis and selection for resistance to nitenpyram was performed on hybrids carrying a deficiency for this chromosomal region. Two complementation groups were defined for the four strains isolated. Molecular characterisation of the mutations found lesions in two nAChR subunit genes, Dalpha1 (encoding an alpha-type subunit) and Dbeta2 (beta-type). Mutations conferring resistance in beta-type receptors have not previously been reported, but we found several lesions in the Dbeta2 sequence, including locations distant from the predicted neonicotinoid-binding site. This study illustrates that mutations in a single-receptor subunit can confer nitenpyram resistance. Moreover, some of the mutations may protect the insect against nitenpyram by interfering with subunit assembly or channel activation, rather than affecting binding affinities of neonicotinoids to the channel.

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

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

A quantitative description of KcsA gating II: single-channel currents

The kinetic transitions of proton-activated WT KcsA and the noninactivating E71A mutant were studied at the single-channel level in purified, liposome-reconstituted preparations. Single-channel currents were recorded using patch-clamp techniques under nonstationary and steady-state conditions. Maximum-likelihood analyses reveal that the key influence of acidic pH is to increase the frequency of bursting without an effect on the intraburst open and closed dwell times, consistent with the finding from macroscopic currents that protons promote activation without a significant effect on inactivation. However, in steady-conditions of pH, voltage not only alters the burst frequency but also affects their properties, such as the frequency of the flickers and the dwell times of the closed and open states. This is to be expected if voltage modulates pathways connecting open and inactivated states. Upon opening, KcsA can enter at least two closed states that are not part of the activation pathway. The frequency and duration of these closed states was found to be voltage dependent and therefore these are likely to represent short-lived inactivated states. Single-channel recordings of WT KcsA also show varying propensity for the presence of subconductance states. The probability of occurrence of these states did not show clear modulation by voltage or pH and their origin remains unclear and a focus for further investigation. A kinetic model is proposed to describe the gating events in KcsA that recapitulates its macroscopic and single-channel behavior. The model has been constrained by the single-channel analyses presented in this work along with data from macroscopic currents in the preceding paper.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

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.

Novel structural determinants of single-channel conductance in nicotinic acetylcholine and 5-hydroxytryptamine type-3 receptors

Nicotinic ACh (acetylcholine) and 5-HT3 (5-hydroxytryptamine type-3) receptors are cation-selective ion channels of the Cys-loop transmitter-gated ion channel superfamily. Numerous lines of evidence indicate that the channel lining domain of such receptors is formed by the alpha-helical M2 domain (second transmembrane domain) contributed by each of five subunits present within the receptor complex. Specific amino acid residues within the M2 domain have accordingly been demonstrated to influence both single-channel conductance (gamma) and ion selectivity. However, it is now clear from work performed on the homomeric 5-HT3A receptor, heteromeric 5-HT3A/5-HT3B receptor and 5-HT3A/5-HT3B receptor subunit chimaeric constructs that an additional major determinant of gamma resides within a cytoplasmic domain of the receptor termed the MA-stretch (membrane-associated stretch). The MA-stretch, within the M3-M4 loop, is not traditionally thought to be implicated in ion permeation and selection. Here, we describe how such observations extend to a representative neuronal nicotinic ACh receptor composed of alpha4 and beta2 subunits and, by inference, probably other members of the Cys-loop family. In addition, we will attempt to interpret our results within the context of a recently developed atomic scale model of the nicotinic ACh receptor of Torpedo marmorata (marbled electric ray).

Molecular determinants of single-channel conductance and ion selectivity in the Cys-loop family: insights from the 5-HT3 receptor

The molecular determinants of the ionic selectivity and single-channel conductance of the Cys-loop family of transmitter-gated ion channels are beginning to be understood with increasing precision, in part, as a result of the recent availability of refined ultrastructural information for the archetype of the family, the nicotinic acetylcholine receptor (nAChR). Studies of another member of this family, the 5-HT(3) receptor, have now provided insight into the structure of its channel pore, the location of its gate and mechanisms of ion selectivity and translocation. The anomaly of the extremely low single-channel conductance of the homo-oligomeric 5-HT(3A) receptor has recently been solved, revealing that an intracellular domain of the protein is an important determinant of single-channel conductance. Such data are interpreted, in this article, in light of the most recent developments in structural characterization of the nAChR.

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

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

The alpha 1 and alpha 6 subunit subtypes of the mammalian GABA(A) receptor confer distinct channel gating kinetics

The GABA(A) receptors show a large degree of structural heterogeneity, with seven different subunit families, and 16 different subtypes in mammalian species. The alpha family is the largest, with six different subtypes. The alpha1 and alpha6 subtypes are among the most diverse within this family and confer distinct pharmacological properties to recombinant and neuronal receptors. To determine whether different single channel and macroscopic kinetic properties were also associated with these subtypes, the alpha1 or alpha6 subunit was expressed in mammalian cells along with beta3 and gamma2L subunits and the kinetic properties examined with outside-out patch recordings. The alpha1 beta3 gamma2L receptors responded to GABA with long-duration openings organized into multi-opening bursts. In contrast, channel openings of the alpha6 beta3 gamma2L receptors were predominately short in duration and occurred as isolated, single openings. The subunit subtype also affected the deactivation rate of the receptor, which was almost 2-fold slower for alpha6 beta3 gamma2L, compared with the alpha1 beta3 gamma2L isoform. Onset of fast desensitization did not differ between the isoforms. To determine the structural domains responsible for these differences in kinetic properties, we constructed six chimeric subunits, combining different regions of the alpha1 and alpha6 subunits. The properties of the chimeric subunits indicated that structures within the third transmembrane domain (TM3) and the TM3-TM4 intracellular loop conferred differences in single channel gating kinetics that subsequently affected the deactivation rate and GABA EC50. The effect of agonist concentration on the rise time of the current showed that the extracellular N-terminal domain was largely responsible for binding characteristics, while the transmembrane domains determined the activation rate at saturating GABA concentrations. This suggests that subunit structures outside of the agonist binding and pore-lining domains are responsible for the kinetic differences conferred by the alpha1 and alpha6 subtypes. Structural heterogeneity within these transmembrane and intracellular regions can therefore influence the characteristics of the postsynaptic response of GABA(A) receptors with different subunit composition.

Molecular Basis of the Differential Sensitivity of Nematode and Mammalian Muscle to the Anthelmintic Agent Levamisole

Levamisole is an anthelmintic agent that exerts its therapeutic effect by acting as a full agonist of the nicotinic receptor (AChR) of nematode muscle. Its action at the mammalian muscle AChR has not been elucidated to date despite its wide use as an anthelmintic in humans and cattle. By single channel and macroscopic current recordings, we investigated the interaction of levamisole with the mammalian muscle AChR. Levamisole activates mammalian AChRs. However, single channel openings are briefer than those activated by acetylcholine (ACh) and do not appear in clusters at high concentrations. The peak current induced by levamisole is about 3% that activated by ACh. Thus, the anthelmintic acts as a weak agonist of the mammalian AChR. Levamisole also produces open channel blockade of the AChR. The apparent affinity for block (190 microm at -70 mV) is similar to that of the nematode AChR, suggesting that differences in channel activation kinetics govern the different sensitivity of nematode and mammalian muscle to anthelmintics. To identify the structural basis of this different sensitivity, we performed mutagenesis targeting residues in the alpha subunit that differ between vertebrates and nematodes. The replacement of the conserved alphaGly-153 with the homologous glutamic acid of nematode AChR significantly increases the efficacy of levamisole to activate channels. Channel activity takes place in clusters having two different kinetic modes. The kinetics of the high open probability mode are almost identical when the agonist is ACh or levamisole. It is concluded that alphaGly-153 is involved in the low efficacy of levamisole to activate mammalian muscle AChRs.

[Congenital myasthenic syndromes: phenotypic expression and pathophysiological characterisation]

Congenital Myasthenic Syndromes (CMS) are a heterogeneous group of diseases caused by genetic defects affecting neuromuscular transmission. The twenty five past Years saw major advances in identifying different types of CMS due to abnormal presynaptic, synaptic, and postsynaptic proteins. CMS diagnosis requires two steps: 1) positive diagnosis supported by myasthenic signs beginning in neonatal period, efficacy of anticholinesterase medications, positive family history, negative tests for anti-acetylcholine receptor (AChR) antibodies, electromyographic studies (decremental response at low frequency, repetitive CMAP after one single stimulation); 2) pathophysiological characterisation of CMS implying specific studies: light and electron microscopic analysis of endplate (EP) morphology, estimation of the number of AChR per EP, acetylcholinesterase (AChE) expression, molecular genetic analysis. Most CMS are postsynaptic due to mutations in the AChR subunits genes that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh resulting respectively in Slow Channel Syndrome (characterized by a autosomal dominant transmission, repetitive CMAP, refractoriness to anticholinesterase medication) and fast channel, recessively transmitted. AChR deficiency without kinetic abnormalities is caused by recessive mutations in AChR genes (mostly epsilon subunit) or by primary rapsyn deficiency, a post synaptic protein involved in AChR concentration. Recently, mutations in SCN4A sodium channel have been reported in one patient. AChE deficiency is identified on the following data: recessive transmission, presence of repetitive CMAP, refractoriness to cholinesterase inhibitors, slow pupillary response to light and absent expression of the enzyme at EP. This synaptic CMS is caused by mutations in the collagenic tail subunit (ColQ) that anchors the catalytic subunits in the synaptic basal lamina. The most frequent presynaptic CMS is caused by mutations of choline acetyltransferase. Several CMS are still not characterized. Many EP molecules are potential etiological candidates. In these unidentified cases, other methods of investigations are required: linkage analysis, when sufficient number of informative relatives are available, microelectrophysiological studies performed in intercostal or anconeus muscles. Prognosis of CMS, depending on severity and evolution of symptoms, is difficult to assess, and it cannot not be simply derived from mutation identification. Most patients respond favourably to anticholinesterase medications or to 3,4 DAP which is effective not only in presynaptic but also in postsynaptic CMS. Specific therapies for slow channel CMS are quinidine and fluoxetine that normalize the prolonged opening episodes. Clinical benefits derived from the full characterisation of each case include genetic counselling and specific therapy.

Mutation in the AChR ion channel gate underlies a fast channel congenital myasthenic syndrome

BACKGROUND

Most congenital myasthenic syndromes (CMS) have postsynaptic defects from mutations within the muscle acetylcholine receptor (AChR). Mutations underlying the slow channel syndrome cause a "gain of function" and usually show dominant inheritance, whereas mutations underlying AChR deficiency or the fast channel syndrome cause a "loss of function" and show recessive inheritance.

OBJECTIVE

To characterize the disease mechanism underlying an apparently dominantly inherited CMS that responds to IV edrophonium.

METHODS

DNA from CMS patients was analyzed for mutations by single-strand conformation polymorphism analysis, DNA sequence analysis, and restriction endonuclease digestion. Functional analysis of mutations was by alpha-bungarotoxin binding studies and by patch clamp analysis of mutant AChR expressed in human embryonic kidney cells.

RESULTS

Analysis of muscle biopsies from father and son in an affected kinship showed normal endplate morphology and AChR number but severely reduced miniature endplate potentials. DNA analysis revealed that each harbors a single missense mutation in the AChR alpha-subunit gene, alphaF256L. Expression studies demonstrate this mutation underlies a fast channel phenotype with fewer and shorter ion channel activations. The major effect of alphaF256L, located within the M2 transmembrane domain, is on channel gating, both reducing the opening and increasing the closure rate.

CONCLUSIONS

Mutation alphaF256L results in fast channel kinetics. Expression studies suggest a dominant-negative effect within the AChR pentamer, severely compromising receptor function.

Neurosteroids shift partial agonist activation of GABA(A) receptor channels from low- to high-efficacy gating patterns

Although GABA activates synaptic (alphabetagamma) GABA(A) receptors with high efficacy, partial agonist activation of alphabetagamma isoforms and GABA activation of the primary extrasynaptic (alphabetadelta) GABA(A) receptors are limited to low-efficacy activity, characterized by minimal desensitization and brief openings. The unusual sensitivity of alphabetadelta receptor channels to neurosteroid modulation prompted investigation of whether this high sensitivity was dependent on the delta subunit or the low-efficacy channel function that it confers. We show that the isoform specificity (alphabetadelta > alphabetagamma) of neurosteroid modulation could be reversed by conditions that reversed isoform-specific activity modes, including the use of beta-alanine to achieve increased efficacy with alphabetadelta receptors and taurine to render alphabetagamma receptors low efficacy. We suggest that neurosteroids preferentially enhance low-efficacy GABA(A) receptor activity independent of subunit composition. Allosteric conversion of partial to full agonism may be a general mechanism for reversibly scaling the efficacy of GABA(A) receptors to endogenous partial agonists.

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.

Congenital Myasthenic Syndromes: Multiple Molecular Targets at the Neuromuscular Junction

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic, 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 synaptic 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.

Modal gating of NMDA receptors and the shape of their synaptic response

N-methyl-D-aspartate receptor (NMDAR) channels mediate the slow component of excitatory potentials at glutamatergic synapses. They have complex kinetic behavior, and much remains to be understood about NMDAR gating mechanisms and the molecular events that shape the synaptic current. Here we show that an individual NMDAR produces at least three stable patterns of activity. For all modes, channels gate by the same mechanism and can occupy either of two open states. The relative stability of the open states differs across modes because of a common perturbation to the NMDAR structure that may be subject to cellular control. Simulations indicate that native NMDAR-mediated synaptic responses arise mainly from the most common mode, and that the slow rise and decay of the current can be attributed to multiple transitions between fully liganded open and closed states rather than to agonist dissociation.

Mutation causing severe myasthenia reveals functional asymmetry of AChR signature cystine loops in agonist binding and gating

We describe a highly disabling congenital myasthenic syndrome (CMS) associated with rapidly decaying, low-amplitude synaptic currents, and trace its cause to a valine to leucine mutation in the signature cystine loop (cys-loop) of the AChR alpha subunit. The recently solved crystal structure of an ACh-binding protein places the cys-loop at the junction between the extracellular ligand-binding and transmembrane domains where it may couple agonist binding to channel gating. We therefore analyzed the kinetics of ACh-induced single-channel currents to identify elementary steps in the receptor activation mechanism altered by the alphaV132L mutation. The analysis reveals that alphaV132L markedly impairs ACh binding to receptors in the resting closed state, decreasing binding affinity for the second binding step 30-fold, but attenuates gating efficiency only about twofold. By contrast, mutation of the equivalent valine residue in the delta subunit impairs channel gating approximately fourfold with little effect on ACh binding, while corresponding mutations in the beta and epsilon subunits are without effect. The unique functional contribution of the alpha subunit cys-loop likely owes to its direct connection via a beta strand to alphaW149 at the center of the ligand-binding domain. The overall findings reveal functional asymmetry between cys-loops of the different AChR subunits in contributing to ACh binding and channel gating.

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.

Congenital myasthenic syndrome caused by low-expressor fast-channel AChR delta subunit mutation

OBJECTIVE

To determine the molecular basis of a disabling congenital myasthenic syndrome (CMS) observed in two related and one unrelated Arab kinship.

BACKGROUND

CMS can arise from defects in presynaptic, synaptic basal lamina-associated, or postsynaptic proteins. Most CMS are postsynaptic, and most reside in the AChR epsilon subunit; only two mutations have been reported in the AChR delta subunit to date.

METHODS

Cytochemistry, electron microscopy, alpha-bungarotoxin binding studies, microelectrode and patch-clamp recordings, mutation analysis, mutagenesis, and expression studies in human embryonic kidney cells were employed.

RESULTS

Endplate studies showed AChR deficiency, fast decaying, low-amplitude endplate currents, and abnormally brief channel opening events. Mutation analysis revealed a novel homozygous missense mutation (deltaP250Q) of the penultimate proline in the first transmembrane domain (TMD1) of the AChR delta subunit. Expression studies indicate that deltaP250Q (1) hinders delta/alpha subunit association during early AChR assembly; (2) hinders opening of the doubly occupied closed receptor (A(2)R); and (3) speeds the dissociation of acetylcholine from A(2)R. Mutagenesis studies indicate that deltaP250L also has fast-channel effects, whereas epsilon P245L and epsilon P245Q, identical mutations of the corresponding proline in the epsilon subunit, have mild slow-channel effects.

CONCLUSIONS

deltaP250Q represents the third mutation observed in the AChR delta subunit. The severe phenotype caused by deltaP250Q is attributed to endplate AChR deficiency, fast decay of the synaptic response, and lack of compensatory factors. That the penultimate prolines in TMD1 of the delta and epsilon subunits exert a reciprocal regulatory effect on the length of the channel opening bursts reveals an unexpected functional asymmetry between the two subunits.

The spectrum of congenital myasthenic syndromes

The past decade saw remarkable advances in defining the molecular and genetic basis of the congenital myasthenic syndromes. These advances would not have been possible without antecedent clinical observations, electrophysiologic analysis, and careful morphologic studies that pointed to candidate genes or proteins. For example, a kinetic abnormality of the acetylcholine receptor (AChR) detected at the single channel level pointed to a kinetic mutation in an AChR subunit; endplate AChR deficiency suggested mutations residing in an AChR subunit or in rapsyn; absence of acetylcholinesterase (AChE) from the endplate predicted mutations in the catalytic or collagen-tailed subunit of this enzyme; and a history of abrupt episodes of apnea associated with a stimulation dependent decrease of endplate potentials and currents implicated proteins concerned with ACh resynthesis or vesicular filling. Discovery of mutations in endplate-specific proteins also prompted expression studies that afforded proof of pathogenicity, provided clues for rational therapy, lead to precise structure function correlations, and highlighted functionally significant residues or molecular domains that previous systematic mutagenesis studies had failed to detect. An overview of the spectrum of the congenital myasthenic syndromes suggests that most are caused by mutations in AChR subunits, and particularly in the epsilon subunit. Future studies will likely uncover new types of CMS that reside in molecules governing quantal release, organization of the synaptic basal lamina, and expression and aggregation of AChR on the postsynaptic junctional folds.

Nicotinic receptor M3 transmembrane domain: position 8' contributes to channel gating

The nicotinic acetylcholine receptor (nAChR) is a pentamer of homologous subunits with composition alpha(2)(beta)(epsilon)(delta) in adult muscle. Each subunit contains four transmembrane domains (M1-M4). Position 8' of the M3 domain is phenylalanine in all heteromeric alpha subunits, whereas it is a hydrophobic nonaromatic residue in non-alpha subunits. Given this peculiar conservation pattern, we studied its contribution to muscle nAChR activation by combining mutagenesis with single-channel kinetic analysis. Construction of nAChRs carrying different numbers of phenylalanine residues at 8' reveals that the mean open time decreases as a function of the number of phenylalanine residues. Thus, all subunits contribute through this position independently and additively to the channel closing rate. The impairment of channel opening increases when the number of phenylalanine residues at 8' increases from two (wild-type nAChR) to five. The gating equilibrium constant of the latter mutant nAChR is 13-fold lower than that of the wild-type nAChR. The replacement of (alpha)F8', (beta)L8', (delta)L8', and (epsilon)V8' by a series of hydrophobic amino acids reveals that the structural bases of the observed kinetic effects are nonequivalent among subunits. In the alpha subunit, hydrophobic amino acids at 8' lead to prolonged channel lifetimes, whereas they lead either to normal kinetics (delta and epsilon subunits) or impaired channel gating (beta subunit) in the non-alpha subunits. The overall results indicate that 8' positions of the M3 domains of all subunits contribute to channel gating.