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Abstract
Congenital myasthenic syndromes (CMS) constitute a heterogenous group of inherited disorders in which neuromuscular transmission is compromised by one or more specific mechanisms. Clinical evidence for the diagnosis of a CMS includes a history of increased fatigable weakness since infancy or early childhood, a decremental EMG response, and the absence of acetylcholine receptor (AChR) antibodies. There has been rapid progress in understanding of the molecular basis of CMS. Mutation analysis of the AChR subunits has revealed numerous disease-associated mutations. These mutations alter the response to acetylcholine. It is decreased in the fast-channel syndromes and in primary AChR deficiency; and it is increased in the slow-channel syndrome due to prolonged open-time of the AChR. Acetylcholinesterase deficiency is associated with mutations in the gene encoding the collagenic tail subunit of the enzyme. Mutations in the gene encoding for choline acetyltransferase causes the CMS associated with episodic apnea.
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Affiliation(s)
- Joern P Sieb
- Department of Neurology, Max Planck Institute of Psychiatry, Munich, Germany
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52
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Ohno K, Engel AG, Shen XM, Selcen D, Brengman J, Harper CM, Tsujino A, Milone M. Rapsyn mutations in humans cause endplate acetylcholine-receptor deficiency and myasthenic syndrome. Am J Hum Genet 2002; 70:875-85. [PMID: 11791205 PMCID: PMC379116 DOI: 10.1086/339465] [Citation(s) in RCA: 171] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2001] [Accepted: 01/04/2002] [Indexed: 01/22/2023] Open
Abstract
Congenital myasthenic syndromes (CMSs) stem from genetic defects in endplate (EP)-specific presynaptic, synaptic, and postsynaptic proteins. The postsynaptic CMSs identified to date stem from a deficiency or kinetic abnormality of the acetylcholine receptor (AChR). All CMSs with a kinetic abnormality of AChR, as well as many CMSs with a deficiency of AChR, have been traced to mutations in AChR-subunit genes. However, in a subset of patients with EP AChR deficiency, the genetic defect has remained elusive. Rapsyn, a 43-kDa postsynaptic protein, plays an essential role in the clustering of AChR at the EP. Seven tetratricopeptide repeats (TPRs) of rapsyn subserve self-association, a coiled-coil domain binds to AChR, and a RING-H2 domain associates with beta-dystroglycan and links rapsyn to the subsynaptic cytoskeleton. Rapsyn self-association precedes recruitment of AChR to rapsyn clusters. In four patients with EP AChR deficiency but with no mutations in AChR subunits, we identify three recessive rapsyn mutations: one patient carries L14P in TPR1 and N88K in TPR3; two are homozygous for N88K; and one carries N88K and 553ins5, which frameshifts in TPR5. EP studies in each case show decreased staining for rapsyn and AChR, as well as impaired postsynaptic morphological development. Expression studies in HEK cells indicate that none of the mutations hinders rapsyn self-association but that all three diminish coclustering of AChR with rapsyn.
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Affiliation(s)
- Kinji Ohno
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN 55905, USA
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53
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Ohno K, Engel AG. Congenital myasthenic syndromes: genetic defects of the neuromuscular junction. Curr Neurol Neurosci Rep 2002; 2:78-88. [PMID: 11898587 DOI: 10.1007/s11910-002-0057-7] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
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 the resynthesis of ACh. Insufficient resynthesis of ACh is now known to be caused by mutations that reduce the expression, catalytic efficiency, or both of choline acetyltransferase. The synaptic CMS are caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent ColQ from associating with catalytic subunits or from insertion into the synaptic basal lamina. With one exception, postsynaptic CMS identified to date are associated with a kinetic abnormality or decreased expression of the acetylcholine receptor (AChR). Numerous mutations have now been identified in subunits of AChR that alter the kinetics or surface expression of the receptor. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most mutations that reduce surface expression of AChR reside in the receptor's epsilon subunit and are partially compensated by residual expression of the fetal-type gamma subunit. Null mutations in both alleles of other AChR subunits are likely lethal, owing to absence of a substituting subunit.
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Affiliation(s)
- Kinji Ohno
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA.
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54
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Brownlow S, Webster R, Croxen R, Brydson M, Neville B, Lin JP, Vincent A, Newsom-Davis J, Beeson D. Acetylcholine receptor δ subunit mutations underlie a fast-channel myasthenic syndrome and arthrogryposis multiplex congenita. J Clin Invest 2001. [DOI: 10.1172/jci200112935] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
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55
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Brownlow S, Webster R, Croxen R, Brydson M, Neville B, Lin JP, Vincent A, Newsom-Davis J, Beeson D. Acetylcholine receptor delta subunit mutations underlie a fast-channel myasthenic syndrome and arthrogryposis multiplex congenita. J Clin Invest 2001; 108:125-30. [PMID: 11435464 PMCID: PMC209343 DOI: 10.1172/jci12935] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
Limitation of movement during fetal development may lead to multiple joint contractures in the neonate, termed arthrogryposis multiplex congenita. Neuromuscular disorders are among the many different causes of reduced fetal movement. Many congenital myasthenic syndromes (CMSs) are due to mutations of the adult-specific epsilon subunit of the acetylcholine receptor (AChR), and, thus, functional deficits do not arise until late in gestation. However, an earlier effect on the fetus might be predicted with some defects of other AChR subunits. We studied a child who presented at birth with joint contractures and was subsequently found to have a CMS. Mutational screening revealed heteroallelic mutation within the AChR delta subunit gene, delta 756ins2 and delta E59K. Expression studies demonstrate that delta 756ins2 is a null mutation. By contrast, both fetal and adult AChR containing delta E59K have shorter than normal channel activations that predict fast decay of endplate currents. Thus, delta E59K causes dysfunction of fetal as well as the adult AChR and would explain the presence of joint contractures on the basis of reduced fetal movement. This is the first report of the association of AChR gene mutations with arthrogryposis multiplex congenita. It is probable that mutations that severely disrupt function of fetal AChR will underlie additional cases.
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Affiliation(s)
- S Brownlow
- Neurosciences Group, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Headington, Oxford, United Kingdom
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56
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Changeux J, Edelstein SJ. Allosteric mechanisms in normal and pathological nicotinic acetylcholine receptors. Curr Opin Neurobiol 2001; 11:369-77. [PMID: 11399437 DOI: 10.1016/s0959-4388(00)00221-x] [Citation(s) in RCA: 89] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Recent chemical and advanced structural studies on site-directed and naturally occurring pathological mutants of individual members of the multigene family of nicotinic acetylcholine receptors have yielded structure-function relationships supporting indirect 'allosteric' interactions between the acetylcholine-binding sites and the ion channel in signal transduction.
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Affiliation(s)
- J Changeux
- Laboratoire de Neurobiologie Moléculaire, Institut Pasteur, 25 rue du Dr Roux, 75724 Cedex 15, Paris, France.
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57
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Engel AE. 73(rd) ENMC International Workshop: congenital myasthenic syndromes. 22-23 October, 1999, Naarden, The Netherlands. Neuromuscul Disord 2001; 11:315-21. [PMID: 11297949 DOI: 10.1016/s0960-8966(00)00189-9] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Affiliation(s)
- A E Engel
- Department of Neurology, Mayo Clinic, Rochester, MN 55905, USA
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58
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Abstract
OBJECTIVE To review the structure and function of membrane ion channels with special emphasis on inherited nervous system channel disorders or channelopathies. RESULTS Channels are pores in the cell membrane. Through these pores ions flow across the membrane and depolarize or hyperpolarize the cell. Channels can be classified into 3 types: non-gated, directly gated and second messenger gated channels. Among the important directly gated channels are voltage gated (Na(+), K(+), Ca(2+), Cl(-)) and ligand gated (ACh, Glutamate, GABA, Glycine) channels. Channels are macromolecular protein complexes within the lipid membrane. They are divided into distinct protein units called subunits. Each subunit has a specific function and is encoded by a different gene. The following inherited channelopathies are described. (1) Sodium channelopathies: familial generalized epilepsy with febrile seizures plus, hyperkalemic periodic paralysis, paramyotonias, hypokalemic periodic paralysis; (2) potassium channelopathies: benign infantile epilepsy, episodic ataxia type 1; (3) calcium channelopathies: episodic ataxia type 2, spinocerebellar ataxia type 6, familial hemiplegic migraine, hypokalemic periodic paralysis, central core disease, malignant hyperthermia syndrome, congenital stationary night blindness; (4) chloride channelopathies: myotonia congenitas; (5) ACh receptor channelopathies: autosomal dominant frontal lobe nocturnal epilepsy, congenital myasthenic syndromes; (6) glycine receptor channelopathies: hyperekplexia. CONCLUSIONS Studies of human inherited channelopathies have clarified the functions of many ion channels. More than one gene may regulate a function in a channel, thus different genetic mutations may manifest with the same disorder. The complex picture of the genetic and molecular structures of channels will require frequent updates.
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Affiliation(s)
- G G Celesia
- Department of Neurology, Loyola University Chicago, Stritch School of Medicine, 2160 S. First Avenue, IL, Maywood, USA.
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59
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Nichols P, Croxen R, Vincent A, Rutter R, Hutchinson M, Newsom-Davis J, Beeson D. Mutation of the acetylcholine receptor ?-subunit promoter in congenital myasthenic syndrome. Ann Neurol 2001. [DOI: 10.1002/1531-8249(199904)45:4<439::aid-ana4>3.0.co;2-w] [Citation(s) in RCA: 65] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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60
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Akk G, Steinbach JH. Structural elements near the C-terminus are responsible for changes in nicotinic receptor gating kinetics following patch excision. J Physiol 2000; 527 Pt 3:405-17. [PMID: 10990529 PMCID: PMC2270086 DOI: 10.1111/j.1469-7793.2000.t01-2-00405.x] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
We have studied the effect of patch excision on the gating kinetics of muscle nicotinic acetylcholine receptors transiently expressed in HEK 293 cells. The experiments were performed on embryonic and adult wild-type, and several mutated, receptors using acetylcholine, carbamylcholine and tetramethylammonium as agonists. We show that patch excision of cell-attached patches into the inside-out configuration led to a reduction of mean open duration in receptors containing a gamma-subunit (embryonic) but not an epsilon-subunit (adult receptors). Kinetic analysis of an embryonic receptor containing a mutated residue, alphaY93F, showed that the reduction in the mean open duration upon patch excision was mainly caused by an increase in the channel closing rate constant. This was confirmed by experiments on embryonic wild-type receptors using carbamylcholine as an agonist with low efficacy. By expressing receptors containing chimeric gamma-epsilon subunits we found that segments of the gamma-subunit corresponding to a region within the M3-M4 linker (the amphipathic helix, HA) and the M4 transmembrane domain were required for the reduction in channel open duration after excision. The results indicate that particular residues in both M4 and HA are required to allow the change in open time after excision. This finding suggests that there is an interaction between these two regions in determining the modulation of gating kinetics.
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Affiliation(s)
- G Akk
- Department of Anesthesiology, Washington University School of Medicine, St Louis, MO 63110, USA.
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61
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Affiliation(s)
- J H Steinbach
- Department of Anesthesiology, Washington University School of Medicine, Saint Louis, Missouri 63110, USA
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62
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Wang HL, Ohno K, Milone M, Brengman JM, Evoli A, Batocchi AP, Middleton LT, Christodoulou K, Engel AG, Sine SM. Fundamental gating mechanism of nicotinic receptor channel revealed by mutation causing a congenital myasthenic syndrome. J Gen Physiol 2000; 116:449-62. [PMID: 10962020 PMCID: PMC2233692 DOI: 10.1085/jgp.116.3.449] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/04/2022] Open
Abstract
We describe the genetic and kinetic defects in a congenital myasthenic syndrome due to the mutation epsilonA411P in the amphipathic helix of the acetylcholine receptor (AChR) epsilon subunit. Myasthenic patients from three unrelated families are either homozygous for epsilonA411P or are heterozygous and harbor a null mutation in the second epsilon allele, indicating that epsilonA411P is recessive. We expressed human AChRs containing wild-type or A411P epsilon subunits in 293HEK cells, recorded single channel currents at high bandwidth, and determined microscopic rate constants for individual channels using hidden Markov modeling. For individual wild-type and mutant channels, each rate constant distributes as a Gaussian function, but the spread in the distributions for channel opening and closing rate constants is greatly expanded by epsilonA411P. Prolines engineered into positions flanking residue 411 of the epsilon subunit greatly increase the range of activation kinetics similar to epsilonA411P, whereas prolines engineered into positions equivalent to epsilonA411 in beta and delta subunits are without effect. Thus, the amphipathic helix of the epsilon subunit stabilizes the channel, minimizing the number and range of kinetic modes accessible to individual AChRs. The findings suggest that analogous stabilizing structures are present in other ion channels, and possibly allosteric proteins in general, and that they evolved to maintain uniformity of activation episodes. The findings further suggest that the fundamental gating mechanism of the AChR channel can be explained by a corrugated energy landscape superimposed on a steeply sloped energy well.
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Affiliation(s)
- H L Wang
- Receptor Biology Laboratory, Department of Physiology and Biophysics, Mayo Foundation, Rochester, Minnesota 55905, USA
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63
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Schulz R, Bertrand S, Chamaon K, Smalla KH, Gundelfinger ED, Bertrand D. Neuronal nicotinic acetylcholine receptors from Drosophila: two different types of alpha subunits coassemble within the same receptor complex. J Neurochem 2000; 74:2537-46. [PMID: 10820216 DOI: 10.1046/j.1471-4159.2000.0742537.x] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Although neuronal nicotinic acetylcholine receptors from insects have been reconstituted in vitro more than a decade ago, our knowledge about the subunit composition of native receptors as well as their functional properties still remains limited. Immunohistochemical evidence has suggested that two alpha subunits, alpha-like subunit (ALS) and Drosophila alpha2 subunit (Dalpha2), are colocalized in the synaptic neuropil of the Drosophila CNS and therefore may be subunits of the same receptor complex. To gain further understanding of the composition of these nicotinic receptors, we have examined the possibility that a receptor may imbed more than one alpha subunit using immunoprecipitations and electrophysiological investigations. Immunoprecipitation experiments of fly head extracts revealed that ALS-specific antibodies coprecipitate Dalpha2, and vice versa, and thereby suggest that these two alpha subunits must be contained within the same receptor complex, a result that is supported by investigations of reconstituted receptors in Xenopus oocytes. Discrimination between binary (ALS/beta2 or Dalpha2/beta2) and ternary (ALS/Dalpha2/beta2) receptor complexes was made on the basis of their dose-response curve to acetylcholine as well as their sensitivity to alpha-bungarotoxin or dihydro-beta-erythroidine. These data demonstrate that the presence of the two alpha subunits within a single receptor complex confers new receptor properties that cannot be predicted from knowledge of the binary receptor's properties.
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Affiliation(s)
- R Schulz
- Leibniz Institute for Neurobiology, Otto von Guericke University, Magdeburg, Germany
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64
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Abstract
Much progress has been made in the 26 years since initial studies of the first purified acetylcholine receptors (AChRs) led to the discovery that an antibody-mediated autoimmune response to AChRs causes the muscular weakness and fatigability characteristic of myasthenia gravis (MG) and its animal model, experimental autoimmune myasthenia gravis (EAMG). Now, the structure of muscle AChRs is much better known. Monoclonal antibodies to muscle AChRs, developed as model autoantibodies for studies of EAMG, were used for initial purifications of neuronal AChRs, and now many homologous subunits of neuronal nicotinic AChRs have been cloned. There is a basic understanding of the pathological mechanisms by which autoantibodies to AChRs impair neuromuscular transmission. Immunodiagnostic assays for MG are used routinely. Nonspecific approaches to immunosuppressive therapy have been refined. However, fundamental mysteries remain regarding what initiates and sustains the autoimmune response to muscle AChRs and how to specifically suppress this autoimmune response using a practical therapy. Many rare congenital myasthenic syndromes have been elegantly shown to result from mutations in muscle AChRs. These studies have provided insights into AChR structure and function as well as into the pathological mechanisms of these diseases. Evidence has been found for autoimmune responses even to some central nervous system neurotransmitter receptors, but only one neuronal AChR has so far been implicated in an autoimmune disease. Thus far, only two neuronal AChR mutations have been found to be associated with a rare form of epilepsy, but many more neuronal AChR mutations will probably be found to be associated with disease in the years ahead.
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Affiliation(s)
- J M Lindstrom
- Department of Neuroscience, Medical School of the University of Pennsylvania, Philadelphia, Pennsylvania 19104-6074, USA.
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65
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Quiram PA, Ohno K, Milone M, Patterson MC, Pruitt NJ, Brengman JM, Sine SM, Engel AG. Mutation causing congenital myasthenia reveals acetylcholine receptor beta/delta subunit interaction essential for assembly. J Clin Invest 1999; 104:1403-10. [PMID: 10562302 PMCID: PMC409847 DOI: 10.1172/jci8179] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
We describe a severe postsynaptic congenital myasthenic syndrome with marked endplate acetylcholine receptor (AChR) deficiency caused by 2 heteroallelic mutations in the beta subunit gene. One mutation causes skipping of exon 8, truncating the beta subunit before its M1 transmembrane domain, and abolishing surface expression of pentameric AChR. The other mutation, a 3-codon deletion (beta426delEQE) in the long cytoplasmic loop between the M3 and M4 domains, curtails but does not abolish expression. By coexpressing beta426delEQE with combinations of wild-type subunits in 293 HEK cells, we demonstrate that beta426delEQE impairs AChR assembly by disrupting a specific interaction between beta and delta subunits. Studies with related deletion and missense mutants indicate that secondary structure in this region of the beta subunit is crucial for interaction with the delta subunit. The findings imply that the mutated residues are positioned at the interface between beta and delta subunits and demonstrate contribution of this local region of the long cytoplasmic loop to AChR assembly.
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MESH Headings
- Acetylcholinesterase/metabolism
- Alleles
- Amino Acid Sequence
- Animals
- Child
- Codon
- Exons
- Female
- Humans
- Macromolecular Substances
- Male
- Molecular Sequence Data
- Motor Endplate/pathology
- Motor Endplate/physiology
- Muscle, Skeletal/metabolism
- Muscle, Skeletal/pathology
- Muscle, Skeletal/physiopathology
- Myasthenia Gravis, Neonatal/genetics
- Myasthenia Gravis, Neonatal/pathology
- Myasthenia Gravis, Neonatal/physiopathology
- Nuclear Family
- Pedigree
- Protein Structure, Secondary
- Receptors, Cholinergic/chemistry
- Receptors, Cholinergic/genetics
- Receptors, Cholinergic/metabolism
- Reference Values
- Sequence Alignment
- Sequence Deletion
- Sequence Homology, Amino Acid
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Affiliation(s)
- P A Quiram
- Receptor Biology Laboratory, Department of Physiology and Biophysics, Mayo Clinic, Rochester, Minnesota 55905, USA
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66
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Wang HL, Milone M, Ohno K, Shen XM, Tsujino A, Batocchi AP, Tonali P, Brengman J, Engel AG, Sine SM. Acetylcholine receptor M3 domain: stereochemical and volume contributions to channel gating. Nat Neurosci 1999; 2:226-33. [PMID: 10195214 DOI: 10.1038/6326] [Citation(s) in RCA: 94] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
By defining the functional defect in a congenital myasthenic syndrome (CMS), we show that the third transmembrane domain (M3) of the muscle acetylcholine receptor governs the speed and efficiency of gating of its channel. The clinical phenotype of this CMS results from the mutation V285I in M3 of the alpha subunit, which attenuates endplate currents, accelerates their decay and causes abnormally brief acetylcholine-induced single-channel currents. Kinetic analysis of engineered alpha V285I receptors demonstrated a predominant effect on channel gating, with abnormally slow opening and rapid closing rates. Analysis of site-directed mutations revealed stereochemical and volume-dependent contributions of alpha V285 to channel gating. Thus, we demonstrate a functional role for the M3 domain as a key component of the nicotinic acetylcholine receptor channel-gating mechanism.
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Affiliation(s)
- H L Wang
- Department of Physiology and Biophysics, Mayo Foundation, Rochester, Minnesota 55905, USA
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67
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Mammalian nicotinic receptors with alpha7 subunits that slowly desensitize and rapidly recover from alpha-bungarotoxin blockade. J Neurosci 1999. [PMID: 9852571 DOI: 10.1523/jneurosci.18-24-10335.1998] [Citation(s) in RCA: 84] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
One of the most abundant nicotinic receptors in the nervous system is a species that contains the alpha7 gene product, rapidly desensitizes, and binds alpha-bungarotoxin with great affinity. The receptor has a high relative permeability to calcium and performs a variety of functions including presynaptic modulation of transmitter release and postsynaptic generation of synaptic currents. Fast excitatory transmission in mammalian intracardiac ganglia is mediated primarily by nicotinic receptors, and although intracardiac ganglion neurons express the alpha7 gene, no toxin-sensitive response has been detected previously in them. We report here that whole-cell patch-clamp recordings from freshly dissociated intracardiac ganglion neurons reveal a nicotinic response that desensitizes slowly and is blocked by alpha-bungarotoxin in a rapidly reversible manner. The only rat gene previously thought capable of forming such receptors was alpha9, but no evidence suggests that the alpha9 gene is expressed in neurons. We find that reverse transcription (RT)-PCR detects alpha7 but not alpha9 mRNA in the ganglia. In addition, the pharmacology of the nicotinic response is typical of alpha7-containing receptors but differs in several respects from that expected for alpha9. Binding experiments with immunotethered receptors identifies a ganglionic species that contains the alpha7 gene product. Moreover, intracellular perfusion of the cells with an anti-alpha7 monoclonal antibody specifically reduces the amplitude of the toxin-sensitive response. The results indicate that alpha7-containing receptors are responsible for the slowly desensitizing, toxin-reversible response and suggest that the receptors are modified in cell-specific ways to influence their functional properties.
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68
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Ohno K, Anlar B, Ozdirim E, Brengman JM, DeBleecker JL, Engel AG. Myasthenic syndromes in Turkish kinships due to mutations in the acetylcholine receptor. Ann Neurol 1998; 44:234-41. [PMID: 9708546 DOI: 10.1002/ana.410440214] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
We report and functionally characterize five new mutations of the acetylcholine receptor (AChR) in 11 Turkish patients with recessive congenital myasthenic syndromes (CMS) belonging to six families. All mutations are in the epsilon-subunit gene. Parental consanguinity is present in three families. The disease cosegregates with homozygous mutations in five families and with two different heteroallelic mutations in one family. Four mutations are frameshifting, predicting truncation of the epsilon subunit, and one occurs at a splice donor site. Expression of each frameshifting mutation and the likely transcripts of the splice-site mutation in human embryonic kidney 293 cells shows that each mutation is a null mutation. The findings support the notion that loss-of-function mutations of the acetylcholine receptor causing CMS are concentrated in the epsilon subunit, and that such mutations are a frequent cause of CMS.
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Affiliation(s)
- K Ohno
- Department of Neurology, Mayo Clinic, Rochester, MN 55905, USA
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69
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Léna C, Changeux JP. Allosteric nicotinic receptors, human pathologies. JOURNAL OF PHYSIOLOGY, PARIS 1998; 92:63-74. [PMID: 9782446 DOI: 10.1016/s0928-4257(98)80140-x] [Citation(s) in RCA: 71] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
Nicotinic acetylcholine receptors are ligand-gated ion channels present in muscle and brain. These allosteric oligomers may exist in several conformational states which include a resting state, an open-channel state, and a desensitized refractory state. Recent work has shown that point mutations in the nicotinic receptor may, altogether, abolish desensitization, increase apparent affinity for agonists and convert the effect of a competitive antagonist into an agonist response. These pleiotropic effects are interpreted in terms of the allosteric model. This paper reviews recent evidence that such mutations occur spontaneously in humans and may cause diseases such as congenital myasthenia or familial frontal lobe epilepsy. In addition, nicotinic receptors are involved in tobacco smoking. Accumulating evidence, including experiments with knock-out animals, indicates that addiction to nicotine is linked to the activation of beta 2-subunit containing nicotinic receptors in the dopaminergic mesolimbic neurons which are part of the reward systems in the brain. Current research also indicates that nicotinic agonists might serve as therapeutic agents for Alzheimer's disease and Tourette's syndrome, as well as for schizophrenia. This paper extends and updates a recently published review.
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Affiliation(s)
- C Léna
- CNRS UA 1284, Institut Pasteur, Paris, France
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70
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Engel AG, Ohno K, Sine SM. Congenital myasthenic syndromes: experiments of nature. JOURNAL OF PHYSIOLOGY, PARIS 1998; 92:113-7. [PMID: 9782453 DOI: 10.1016/s0928-4257(98)80147-2] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
Congenital myasthenic syndromes (CMS) can arise from presynaptic, synaptic, or postsynaptic defects. Recent studies indicate that mutations in the acetylcholine receptor (AChR) subunit genes are a common cause of the postsynaptic CMS. The mutations, which increase or decrease the response to acetylcholine, are experiments of nature that highlight functionally significant domains of the AChR.
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Affiliation(s)
- A G Engel
- Department of Neurology, Mayo Clinic, Rochester, MN 55905, USA
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71
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Engel AG. Congenital myasthenic syndromes. Eur J Paediatr Neurol 1998; 2:97. [PMID: 10724103 DOI: 10.1016/s1090-3798(98)80048-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Affiliation(s)
- A G Engel
- Department of Neurology, Mayo Clinic, Rochester, MN 55905, USA
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