1
|
Ohno K, Ohkawara B, Shen XM, Selcen D, Engel AG. Clinical and Pathologic Features of Congenital Myasthenic Syndromes Caused by 35 Genes-A Comprehensive Review. Int J Mol Sci 2023; 24:ijms24043730. [PMID: 36835142 PMCID: PMC9961056 DOI: 10.3390/ijms24043730] [Citation(s) in RCA: 34] [Impact Index Per Article: 34.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2023] [Revised: 02/09/2023] [Accepted: 02/09/2023] [Indexed: 02/16/2023] Open
Abstract
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.
Collapse
Affiliation(s)
- Kinji Ohno
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Japan
- Correspondence: (K.O.); (A.G.E.)
| | - Bisei Ohkawara
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya 466-8550, Japan
| | - Xin-Ming Shen
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN 55905, USA
| | - Duygu Selcen
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN 55905, USA
| | - Andrew G. Engel
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN 55905, USA
- Correspondence: (K.O.); (A.G.E.)
| |
Collapse
|
2
|
Grassi F, Fucile S. Calcium influx through muscle nAChR-channels: One route, multiple roles. Neuroscience 2019; 439:117-124. [PMID: 30999028 DOI: 10.1016/j.neuroscience.2019.04.011] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2019] [Revised: 04/01/2019] [Accepted: 04/04/2019] [Indexed: 01/31/2023]
Abstract
Although Ca2+ influx through muscle nAChR-channels has been described over the past 40 years, its functions remain still poorly understood. In this review we suggest possible roles of Ca2+ entry at all stages of muscle development, summarizing the evidence present in literature. nAChRs are expressed in myoblasts prior to fusion, and can be activated in the absence of an ACh-releasing nerve terminal, with Ca2+ influx likely contributing to regulate cell fusion. Upon establishment of nerve-muscle contact, Ca2+ influx contributes to orchestrate the signaling required for the correct formation of the neuromuscular junction. Finally, in the mature synapse, Ca2+ entry through postsynaptic nAChR-channels - highly Ca2+ permeable, in particular in humans - acts on K+ and Na+ channels to shape endplate excitability. However, when genetic defects cause excessive channel activation, Ca2+ influx becomes toxic and causes endplate myopathy. Throughout the review, we highlight how Ricardo Miledi has contributed to construct this whole body of knowledge, from the initial description of Ca2+ permeability of endplate nAChR channels, to the rationale for the treatment of endplate excitotoxic damage under pathological conditions. This article is part of a Special Issue entitled: SI: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.
Collapse
Affiliation(s)
- Francesca Grassi
- Department of Physiology and Pharmacology, Sapienza University, piazzale Aldo Moro 5, 00185, Rome, Italy.
| | - Sergio Fucile
- Department of Physiology and Pharmacology, Sapienza University, piazzale Aldo Moro 5, 00185, Rome, Italy; IRCCS Neuromed, Viale dell'Elettronica, 86077, Pozzilli, Italy
| |
Collapse
|
3
|
Fucile S. The Distribution of Charged Amino Acid Residues and the Ca 2+ Permeability of Nicotinic Acetylcholine Receptors: A Predictive Model. Front Mol Neurosci 2017; 10:155. [PMID: 28611586 PMCID: PMC5447003 DOI: 10.3389/fnmol.2017.00155] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2017] [Accepted: 05/08/2017] [Indexed: 11/13/2022] Open
Abstract
Nicotinic acetylcholine receptors (nAChRs) are cation-selective ligand-gated ion channels exhibiting variable Ca2+ permeability depending on their subunit composition. The Ca2+ permeability is a crucial functional parameter to understand the physiological role of nAChRs, in particular considering their ability to modulate Ca2+-dependent processes such as neurotransmitter release. The rings of extracellular and intracellular charged amino acid residues adjacent to the pore-lining TM2 transmembrane segment have been shown to play a key role in the cation selectivity of these receptor channels, but to date a quantitative relationship between these structural determinants and the Ca2+ permeability of nAChRs is lacking. In the last years the Ca2+ permeability of several nAChR subtypes has been experimentally evaluated, in terms of fractional Ca2+ current (Pf, i.e., the percentage of the total current carried by Ca2+ ions). In the present study, the available Pf-values of nAChRs are used to build a simplified modular model describing the contribution of the charged residues in defined regions flanking TM2 to the selectivity filter controlling Ca2+ influx. This model allows to predict the currently unknown Pf-values of existing nAChRs, as well as the hypothetical Ca2+ permeability of subunit combinations not able to assemble into functional receptors. In particular, basing on the amino acid sequences, a Pf > 50% would be associated with homomeric nAChRs composed by different α subunits, excluding α7, α9, and α10. Furthermore, according to the model, human α7β2 receptors should have Pf-values ranging from 3.6% (4:1 ratio) to 0.1% (1:4 ratio), much lower than the 11.4% of homomeric α7 nAChR. These results help to understand the evolution and the function of the large diversity of the nicotinic receptor family.
Collapse
Affiliation(s)
- Sergio Fucile
- Department of Physiology and Pharmacology "V. Erspamer", Sapienza Università di RomaRome, Italy.,Molecular Pathology, Istituto Neurologico Mediterraneo (IRCCS), Parco TecnologicoPozzilli, Italy
| |
Collapse
|
4
|
Pissulin CNA, de Souza Castro PAT, Codina F, Pinto CG, Vechetti-Junior IJ, Matheus SMM. GaAs laser therapy reestablishes the morphology of the NMJ and nAChRs after injury due to bupivacaine. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY 2017; 167:256-263. [DOI: 10.1016/j.jphotobiol.2016.12.024] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/20/2016] [Accepted: 12/19/2016] [Indexed: 12/18/2022]
|
5
|
Shen XM, Okuno T, Milone M, Otsuka K, Takahashi K, Komaki H, Giles E, Ohno K, Engel AG. Mutations Causing Slow-Channel Myasthenia Reveal That a Valine Ring in the Channel Pore of Muscle AChR is Optimized for Stabilizing Channel Gating. Hum Mutat 2016; 37:1051-9. [PMID: 27375219 DOI: 10.1002/humu.23043] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2016] [Revised: 06/18/2016] [Accepted: 06/23/2016] [Indexed: 12/22/2022]
Abstract
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.
Collapse
Affiliation(s)
- Xin-Ming Shen
- Department of Neurology, Mayo Clinic, Rochester, Minnesota. ,
| | - Tatsuya Okuno
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | | | - Kenji Otsuka
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | - Koji Takahashi
- Department of Child Neurology, National Center Hospital of Neurology and Psychiatry, Tokyo, Japan
| | - Hirofumi Komaki
- Department of Child Neurology, National Center Hospital of Neurology and Psychiatry, Tokyo, Japan
| | | | - Kinji Ohno
- Department of Neurology, Mayo Clinic, Rochester, Minnesota.,Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | - Andrew G Engel
- Department of Neurology, Mayo Clinic, Rochester, Minnesota
| |
Collapse
|
6
|
Ohno K, Ohkawara B, Ito M. Recent advances in congenital myasthenic syndromes. ACTA ACUST UNITED AC 2016. [DOI: 10.1111/cen3.12316] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Affiliation(s)
- Kinji Ohno
- Division of Neurogenetics; Center for Neurological Diseases and Cancer; Nagoya University Graduate School of Medicine; Nagoya Japan
| | - Bisei Ohkawara
- Division of Neurogenetics; Center for Neurological Diseases and Cancer; Nagoya University Graduate School of Medicine; Nagoya Japan
| | - Mikako Ito
- Division of Neurogenetics; Center for Neurological Diseases and Cancer; Nagoya University Graduate School of Medicine; Nagoya Japan
| |
Collapse
|
7
|
RamaKrishnan AM, Sankaranarayanan K. Understanding autoimmunity: The ion channel perspective. Autoimmun Rev 2016; 15:585-620. [PMID: 26854401 DOI: 10.1016/j.autrev.2016.02.004] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2016] [Accepted: 01/29/2016] [Indexed: 12/11/2022]
Abstract
Ion channels are integral membrane proteins that orchestrate the passage of ions across the cell membrane and thus regulate various key physiological processes of the living system. The stringently regulated expression and function of these channels hold a pivotal role in the development and execution of various cellular functions. Malfunction of these channels results in debilitating diseases collectively termed channelopathies. In this review, we highlight the role of these proteins in the immune system with special emphasis on the development of autoimmunity. The role of ion channels in various autoimmune diseases is also listed out. This comprehensive review summarizes the ion channels that could be used as molecular targets in the development of new therapeutics against autoimmune disorders.
Collapse
Affiliation(s)
| | - Kavitha Sankaranarayanan
- AU-KBC Research Centre, Madras Institute of Technology, Anna University, Chrompet, Chennai 600 044, India.
| |
Collapse
|
8
|
Lipovsek M, Fierro A, Pérez EG, Boffi JC, Millar NS, Fuchs PA, Katz E, Elgoyhen AB. Tracking the molecular evolution of calcium permeability in a nicotinic acetylcholine receptor. Mol Biol Evol 2014; 31:3250-65. [PMID: 25193338 PMCID: PMC4245820 DOI: 10.1093/molbev/msu258] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Nicotinic acetylcholine receptors are a family of ligand-gated nonselective cationic channels that participate in fundamental physiological processes at both the central and the peripheral nervous system. The extent of calcium entry through ligand-gated ion channels defines their distinct functions. The α9α10 nicotinic cholinergic receptor, expressed in cochlear hair cells, is a peculiar member of the family as it shows differences in the extent of calcium permeability across species. In particular, mammalian α9α10 receptors are among the ligand-gated ion channels which exhibit the highest calcium selectivity. This acquired differential property provides the unique opportunity of studying how protein function was shaped along evolutionary history, by tracking its evolutionary record and experimentally defining the amino acid changes involved. We have applied a molecular evolution approach of ancestral sequence reconstruction, together with molecular dynamics simulations and an evolutionary-based mutagenesis strategy, in order to trace the molecular events that yielded a high calcium permeable nicotinic α9α10 mammalian receptor. Only three specific amino acid substitutions in the α9 subunit were directly involved. These are located at the extracellular vestibule and at the exit of the channel pore and not at the transmembrane region 2 of the protein as previously thought. Moreover, we show that these three critical substitutions only increase calcium permeability in the context of the mammalian but not the avian receptor, stressing the relevance of overall protein structure on defining functional properties. These results highlight the importance of tracking evolutionarily acquired changes in protein sequence underlying fundamental functional properties of ligand-gated ion channels.
Collapse
Affiliation(s)
- Marcela Lipovsek
- Instituto de Investigaciones en Ingeniería Genética y Biología Molecular, Dr. Héctor N Torres, Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina
| | - Angélica Fierro
- Department of Organic Chemistry, Faculty of Chemistry, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Edwin G Pérez
- Department of Organic Chemistry, Faculty of Chemistry, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Juan C Boffi
- Instituto de Investigaciones en Ingeniería Genética y Biología Molecular, Dr. Héctor N Torres, Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina
| | - Neil S Millar
- Department of Neuroscience, Physiology and Pharmacology, University College London, London, United Kingdom
| | - Paul A Fuchs
- Department of Otolaryngology, Head and Neck Surgery, and Center for Hearing and Balance, The Johns Hopkins University School of Medicine, Baltimore, MD
| | - Eleonora Katz
- Instituto de Investigaciones en Ingeniería Genética y Biología Molecular, Dr. Héctor N Torres, Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina Departamento de Fisiología, Biología Molecular y Celular, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina
| | - Ana Belén Elgoyhen
- Instituto de Investigaciones en Ingeniería Genética y Biología Molecular, Dr. Héctor N Torres, Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina Departamento de Farmacología, Facultad de Medicina, Universidad de Buenos Aires, Buenos Aires, Argentina
| |
Collapse
|
9
|
Deflorio C, Palma E, Conti L, Roseti C, Manteca A, Giacomelli E, Catalano M, Limatola C, Inghilleri M, Grassi F. Riluzole blocks human muscle acetylcholine receptors. J Physiol 2012; 590:2519-28. [PMID: 22431338 DOI: 10.1113/jphysiol.2012.230201] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Riluzole, the only drug available against amyotrophic lateral sclerosis (ALS), has recently been shown to block muscle ACh receptors (AChRs), raising concerns about possible negative side-effects on neuromuscular transmission in treated patients. In this work we studied riluzole's impact on the function of muscle AChRs in vitro and on neuromuscular transmission in ALS patients, using electrophysiological techniques. Human recombinant AChRs composed of α(1)β(1)δ subunits plus the γ or ε subunit (γ- or ε-AChR) were expressed in HEK cells or Xenopus oocytes. In both preparations, riluzole at 0.5 μm, a clinically relevant concentration, reversibly reduced the amplitude and accelerated the decay of ACh-evoked current if applied before coapplication with ACh. The action on γ-AChRs was more potent and faster than on ε-AChRs. In HEK outside-out patches, riluzole-induced block of macroscopic ACh-evoked current gradually developed during the initial milliseconds of ACh presence. Single channel recordings in HEK cells and in human myotubes from ALS patients showed that riluzole prolongs channel closed time, but has no effect on channel conductance and open duration. Finally, compound muscle action potentials (CMAPs) evoked by nerve stimulation in ALS patients remained unaltered after a 1 week suspension of riluzole treatment. These data indicate that riluzole, while apparently safe with regard to synaptic transmission, may affect the function of AChRs expressed in denervated muscle fibres of ALS patients, with biological consequences that remain to be investigated.
Collapse
Affiliation(s)
- Cristina Deflorio
- Department of Physiology and Pharmacology, Sapienza University, Rome, Italy
| | | | | | | | | | | | | | | | | | | |
Collapse
|
10
|
Piccari V, Deflorio C, Bigi R, Grassi F, Fucile S. Modulation of the Ca(2+) permeability of human endplate acetylcholine receptor-channel. Cell Calcium 2011; 49:272-8. [PMID: 21470676 DOI: 10.1016/j.ceca.2011.03.002] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2010] [Revised: 02/02/2011] [Accepted: 03/07/2011] [Indexed: 11/16/2022]
Abstract
In slow-channel congenital myasthenic syndrome, point mutations of the endplate acetylcholine receptor (AChR) prolong channel openings, leading to excessive Ca(2+) entry with ensuing endplate degeneration and myasthenic symptoms. The Ca(2+) permeability of the human endplate AChR-channel is quite high, and is further increased by two slow-channel mutations in its ɛ subunit, worsening the pathological cascade. To gain further support to the hypothesis that the ɛ subunit plays a crucial role in controlling Ca(2+) permeability of endplate AChR-channel, in this work we measured the fractional Ca(2+) current (P(f), i.e., the percentage of the total current carried by Ca(2+) ions) of a panel of AChR carrying slow-channel mutations in the α, β and ɛ subunits detected in patients (α(N217K), α(S226Y), α(C418W), β(V266A), β(V266M), ɛ(I257F), ɛ(V265A) and ɛ(L269F)). We confirm that only mutations in the ɛ subunit altered Ca(2+) permeability of AChR-channels, with ɛ(L269F) increasing P(f) (10% vs. 7% of wild type AChR) and ɛ(I257F) decreasing it (to 4.6%). We also found that, for ɛ(L269F)-AChR, the Ca(2+) permeability and ACh-induced cell death can be normalized by clinically relevant concentrations of salbutamol or verapamil, providing the first evidence that the Ca(2+) permeability of AChR-channels can be modulated and this treatment may provide protection against excitotoxic insults.
Collapse
Affiliation(s)
- Vanessa Piccari
- Dipartimento di Fisiologia e Farmacologia, Sapienza Università di Roma, P.le A. Moro 5, I-00185 Roma, Italy
| | | | | | | | | |
Collapse
|
11
|
Mutant human β4 subunit identified in amyotrophic lateral sclerosis patients impairs nicotinic receptor function. Pflugers Arch 2010; 461:225-33. [PMID: 21107856 DOI: 10.1007/s00424-010-0905-2] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2010] [Revised: 10/19/2010] [Accepted: 11/08/2010] [Indexed: 10/18/2022]
Abstract
Recently identified mutations in the genes encoding the neuronal nicotinic ACh receptor (nAChR) subunits in patients affected by sporadic amyotrophic lateral sclerosis (sALS) may represent a factor which enhances disease susceptibility, in particular in association with ambient causes such as cigarette smoking. In this work, we characterize the functional properties of nAChRs containing the β4R349C subunit, the mutation most frequently encountered in sALS patients. The mutation was coexpressed with wild-type α3 or α4 subunits or with mutant α4R487Q subunit, which has been detected in one patient together with β4R349C mutation. None of the functional parameters examined showed differences between α4β4 and α4R487Qβ4 nAChRs. By contrast, β4R349C mutation, independent of the companion α subunit, caused the reduction in potency of both ACh and nicotine, decreased the density of whole-cell current evoked by maximal transmitter concentrations, and altered the kinetics of ACh-evoked whole-cell currents. These data confirm that sALS-associated mutations in nicotinic subunits may markedly perturb cholinergic transmission in individuals bearing the mutations.
Collapse
|
12
|
Moriconi C, Di Castro MA, Fucile S, Eusebi F, Grassi F. Mechanism of verapamil action on wild-type and slow-channel mutant human muscle acetylcholine receptor. J Neurochem 2010; 114:1231-40. [PMID: 20533996 DOI: 10.1111/j.1471-4159.2010.06842.x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Verapamil, a Ca(2+) channel blocker widely used in clinical practice, also affects the properties of frog and mouse muscle acetylcholine receptor (AChR). Here, we examine the mechanism of action of verapamil on human wild-type and slow-channel mutant muscle AChRs harboring in any subunit a valine-to-alanine mutation of 13' residue of the pore-lining M2 transmembrane segment. Verapamil, after a pre-treatment of 0.5-10 s, accelerated the decay of whole-cell or macroscopic outside-out currents within milliseconds of ACh application even at clinically attainable doses. Recordings of unitary events in the cell-attached and outside-out configurations showed that verapamil does not alter single-channel conductance, but reduces channel open probability, by prolonging the dwell time into the closed state for wild-type and all mutant AChR. The duration of channel openings decreased only for the epsilonV265A-AChR, by shortening the longest exponential component of the open-time distribution. These results provide a rationale for the therapeutic use of verapamil in the slow-channel syndrome and emphasize the major role played by epsilon subunit in controlling the functional properties of human muscle AChR, as revealed by the peculiar alterations imparted by mutations in this subunit.
Collapse
Affiliation(s)
- Claudia Moriconi
- Dipartimento di Fisiologia e Farmacologia, Università La Sapienza, Roma, Italy
| | | | | | | | | |
Collapse
|
13
|
Engel AG, Shen XM, Selcen D, Sine SM. What have we learned from the congenital myasthenic syndromes. J Mol Neurosci 2010; 40:143-53. [PMID: 19688192 PMCID: PMC3050586 DOI: 10.1007/s12031-009-9229-0] [Citation(s) in RCA: 56] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2009] [Accepted: 07/20/2009] [Indexed: 10/20/2022]
Abstract
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.
Collapse
Affiliation(s)
- Andrew G Engel
- Department of Neurology and Muscle Research Laboratory, Mayo Clinic College of Medicine, Rochester, MN 55905, USA.
| | | | | | | |
Collapse
|
14
|
Elenes S, Decker M, Cymes GD, Grosman C. Response to the Letter: “About a new method to measure fractional Ca2+ currents through ligand-gated ion channels”. J Gen Physiol 2009. [PMCID: PMC2737225 DOI: 10.1085/jgp.200910231] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Affiliation(s)
- Sergio Elenes
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801
| | - Michael Decker
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801
| | - Gisela D. Cymes
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801
| | - Claudio Grosman
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801
| |
Collapse
|
15
|
Elenes S, Decker M, Cymes GD, Grosman C. Decremental response to high-frequency trains of acetylcholine pulses but unaltered fractional Ca2+ currents in a panel of "slow-channel syndrome" nicotinic receptor mutants. ACTA ACUST UNITED AC 2009; 133:151-69. [PMID: 19171769 PMCID: PMC2638206 DOI: 10.1085/jgp.200810089] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
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%.
Collapse
Affiliation(s)
- Sergio Elenes
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, 61801, USA
| | | | | | | |
Collapse
|
16
|
Selcen D, Milone M, Shen XM, Harper CM, Stans AA, Wieben ED, Engel AG. Dok-7 myasthenia: phenotypic and molecular genetic studies in 16 patients. Ann Neurol 2008; 64:71-87. [PMID: 18626973 DOI: 10.1002/ana.21408] [Citation(s) in RCA: 84] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
OBJECTIVE Detailed analysis of phenotypic and molecular genetic aspects of Dok-7 myasthenia in 16 patients. METHODS We assessed our patients by clinical and electromyographic studies, by intercostal muscle biopsies for in vitro microelectrode analysis of neuromuscular transmission and quantitative electron microscopy EM of 409 end plates (EPs), and by mutation analysis, and expression studies of the mutants. RESULTS The clinical spectrum varied from mild static limb-girdle weakness to severe generalized progressive disease. The synaptic contacts were single or multiple, and some, but not all, were small. In vitro microelectrode studies indicated variable decreases of the number of released quanta and of the synaptic response to acetylcholine; acetylcholine receptor (AChR) channel kinetics were normal. EM analysis demonstrated widespread and previously unrecognized destruction and remodeling of the EPs. Each patient carries 2 or more heteroallelic mutations: 11 in genomic DNA, 7 of which are novel; and 6 identifiable only in complementary DNA or cloned complementary DNA, 3 of which are novel. The pathogenicity of the mutations was confirmed by expression studies. Although the functions of Dok-7 include AChR beta-subunit phosphorylation and maintaining AChR site density, patient EPs showed normal AChR beta-subunit phosphorylation, and the AChR density on the remaining junctional folds appeared normal. INTERPRETATION First, the clinical features of Dok-7 myasthenia are highly variable. Second, some mutations are complex and identifiable only in cloned complementary DNA. Third, Dok-7 is essential for maintaining not only the size but also the structural integrity of the EP. Fourth, the profound structural alterations at the EPs likely contribute importantly to the reduced safety margin of neuromuscular transmission.
Collapse
Affiliation(s)
- Duygu Selcen
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN 55905, USA
| | | | | | | | | | | | | |
Collapse
|
17
|
|