1
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Dissanayake KN, Redman RR, Mackenzie H, Eddleston M, Ribchester RR. "Calcium bombs" as harbingers of synaptic pathology and their mitigation by magnesium at murine neuromuscular junctions. Front Mol Neurosci 2022; 15:937974. [PMID: 35959105 PMCID: PMC9361872 DOI: 10.3389/fnmol.2022.937974] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2022] [Accepted: 07/04/2022] [Indexed: 12/24/2022] Open
Abstract
Excitotoxicity is thought to be an important factor in the onset and progression of amyotrophic lateral sclerosis (ALS). Evidence from human and animal studies also indicates that early signs of ALS include degeneration of motor nerve terminals at neuromuscular junctions (NMJs), before degeneration of motor neuron cell bodies. Here we used a model of excitotoxicity at NMJs in isolated mouse muscle, utilizing the organophosphorus (OP) compound omethoate, which inhibits acetylcholinesterase activity. Acute exposure to omethoate (100 μM) induced prolonged motor endplate contractures in response to brief tetanic nerve stimulation at 20-50 Hz. In some muscle fibers, Fluo-4 fluorescence showed association of these contractures with explosive increases in Ca2+ ("calcium bombs") localized to motor endplates. Calcium bombs were strongly and selectively mitigated by increasing Mg2+ concentration in the bathing medium from 1 to 5 mM. Overnight culture of nerve-muscle preparations from WldS mice in omethoate or other OP insecticide components and their metabolites (dimethoate, cyclohexanone, and cyclohexanol) induced degeneration of NMJs. This degeneration was also strongly mitigated by increasing [Mg2+] from 1 to 5 mM. Thus, equivalent increases in extracellular [Mg2+] mitigated both post-synaptic calcium bombs and degeneration of NMJs. The data support a link between Ca2+ and excitotoxicity at NMJs and suggest that elevating extracellular [Mg2+] could be an effective intervention in treatment of synaptic pathology induced by excitotoxic triggers.
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Affiliation(s)
- Kosala N. Dissanayake
- Euan MacDonald Centre for Motor Neurone Disease Research, The University of Edinburgh, Edinburgh, United Kingdom,Centre for Discovery Brain Sciences, The University of Edinburgh, Edinburgh, United Kingdom
| | - Robert R. Redman
- Euan MacDonald Centre for Motor Neurone Disease Research, The University of Edinburgh, Edinburgh, United Kingdom,Centre for Discovery Brain Sciences, The University of Edinburgh, Edinburgh, United Kingdom
| | - Harry Mackenzie
- Euan MacDonald Centre for Motor Neurone Disease Research, The University of Edinburgh, Edinburgh, United Kingdom,Centre for Discovery Brain Sciences, The University of Edinburgh, Edinburgh, United Kingdom
| | - Michael Eddleston
- Clinical Pharmacology, Toxicology and Therapeutics, Centre for Cardiovascular Science, Queen’s Medical Research Institute, The University of Edinburgh, Edinburgh, United Kingdom
| | - Richard R. Ribchester
- Euan MacDonald Centre for Motor Neurone Disease Research, The University of Edinburgh, Edinburgh, United Kingdom,Centre for Discovery Brain Sciences, The University of Edinburgh, Edinburgh, United Kingdom,*Correspondence: Richard R. Ribchester,
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2
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Fross S, Mansel C, McCormick M, Vohra BPS. Tributyltin Alters Calcium Levels, Mitochondrial Dynamics, and Activates Calpains Within Dorsal Root Ganglion Neurons. Toxicol Sci 2021; 180:342-355. [PMID: 33481012 DOI: 10.1093/toxsci/kfaa193] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Tributyltin (TBT) remains a global health concern. The primary route of human exposure to TBT is either through ingestion or skin absorption, but TBT's effects on the peripheral nervous system have still not been investigated. Therefore, we exposed in vitro sensory dorsal root ganglion (DRG) neurons to TBT at a concentration of 50-200 nM, which is similar to the observed concentrations of TBT in human blood samples. We observed that TBT causes extensive axon degeneration and neuronal death in the DRG neurons. Furthermore, we discovered that TBT causes an increase in both cytosolic and mitochondrial calcium levels, disrupts mitochondrial dynamics, decreases neuronal ATP levels, and leads to the activation of calpains. Additional experiments demonstrated that inhibition of calpain activation prevented TBT-induced fragmentation of neuronal cytoskeletal proteins and neuronal cell death. Thus, we conclude that calpain activation is the key executioner of TBT-induced peripheral neurodegeneration.
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Affiliation(s)
- Shaneann Fross
- Department of Biology, William Jewell College, Liberty, Missouri 64068, USA
| | - Clayton Mansel
- Department of Biology, William Jewell College, Liberty, Missouri 64068, USA
| | - Madison McCormick
- Department of Biology, William Jewell College, Liberty, Missouri 64068, USA
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3
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Kudryavtsev D, Isaeva A, Barkova D, Spirova E, Mukhutdinova R, Kasheverov I, Tsetlin V. Point Mutations of Nicotinic Receptor α1 Subunit Reveal New Molecular Features of G153S Slow-Channel Myasthenia. Molecules 2021; 26:molecules26051278. [PMID: 33652901 PMCID: PMC7956382 DOI: 10.3390/molecules26051278] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2020] [Revised: 02/18/2021] [Accepted: 02/23/2021] [Indexed: 11/16/2022] Open
Abstract
Slow-channel congenital myasthenic syndromes (SCCMSs) are rare genetic diseases caused by mutations in muscle nicotinic acetylcholine receptor (nAChR) subunits. Most of the known SCCMS-associated mutations localize at the transmembrane region near the ion pore. Only two SCCMS point mutations are at the extracellular domains near the acetylcholine binding site, α1(G153S) being one of them. In this work, a combination of molecular dynamics, targeted mutagenesis, fluorescent Ca2+ imaging and patch-clamp electrophysiology has been applied to G153S mutant muscle nAChR to investigate the role of hydrogen bonds formed by Ser 153 with C-loop residues near the acetylcholine-binding site. Introduction of L199T mutation to the C-loop in the vicinity of Ser 153 changed hydrogen bonds distribution, decreased acetylcholine potency (EC50 2607 vs. 146 nM) of the double mutant and decay kinetics of acetylcholine-evoked cytoplasmic Ca2+ rise (τ 14.2 ± 0.3 vs. 34.0 ± 0.4 s). These results shed light on molecular mechanisms of nAChR activation-desensitization and on the involvement of such mechanisms in channelopathy genesis.
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Affiliation(s)
- Denis Kudryavtsev
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya 16/10, 117997 Moscow, Russia; (A.I.); (E.S.); (R.M.); (I.K.); (V.T.)
- Correspondence:
| | - Anastasia Isaeva
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya 16/10, 117997 Moscow, Russia; (A.I.); (E.S.); (R.M.); (I.K.); (V.T.)
- Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Russia
| | - Daria Barkova
- Biological Department, Lomonosov Moscow State University, 119991 Moscow, Russia;
| | - Ekaterina Spirova
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya 16/10, 117997 Moscow, Russia; (A.I.); (E.S.); (R.M.); (I.K.); (V.T.)
| | - Renata Mukhutdinova
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya 16/10, 117997 Moscow, Russia; (A.I.); (E.S.); (R.M.); (I.K.); (V.T.)
- Biological Department, Lomonosov Moscow State University, 119991 Moscow, Russia;
| | - Igor Kasheverov
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya 16/10, 117997 Moscow, Russia; (A.I.); (E.S.); (R.M.); (I.K.); (V.T.)
- Institute of Molecular Medicine, Sechenov First Moscow State Medical University, Trubetskaya str. 8, bld. 2, 119991 Moscow, Russia
| | - Victor Tsetlin
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya 16/10, 117997 Moscow, Russia; (A.I.); (E.S.); (R.M.); (I.K.); (V.T.)
- Institute of Engineering Physics for Biomedicine, MePhi, 115409 Moscow, Russia
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4
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Grajales-Reyes JG, García-González A, María-Ríos JC, Grajales-Reyes GE, Delgado-Vélez M, Báez-Pagán CA, Quesada O, Gómez CM, Lasalde-Dominicci JA. A Panel of Slow-Channel Syndrome Mice Reveals a Unique Locomotor Behavioral Signature. J Neuromuscul Dis 2019; 4:341-347. [PMID: 29036836 DOI: 10.3233/jnd-170226] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
Muscle nicotinic acetylcholine receptor (nAChR) mutations can lead to altered channel kinetics and neuromuscular junction degeneration, a neurodegenerative disorder collectively known as slow-channel syndrome (SCS). A multivariate analysis using running wheels was used to generate activity profiles for a variety of SCS models, uncovering unique locomotor patterns for the different nAChR mutants. Particularly, the αL251T and ɛL269F mutations exhibit decreased event distance, duration, and velocity over a period of 24 hours. Our approach suggests a robust relationship between the pathophysiology of SCS and locomotor activity.
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Affiliation(s)
- José G Grajales-Reyes
- Department of Biology, University of Puerto Rico, Río Piedras Campus, San Juan, PR, USA
| | | | - José C María-Ríos
- Department of Biology, University of Puerto Rico, Río Piedras Campus, San Juan, PR, USA
| | - Gary E Grajales-Reyes
- Department of Biology, University of Puerto Rico, Río Piedras Campus, San Juan, PR, USA
| | - Manuel Delgado-Vélez
- Department of Biology, University of Puerto Rico, Río Piedras Campus, San Juan, PR, USA
| | - Carlos A Báez-Pagán
- Department of Biology, University of Puerto Rico, Río Piedras Campus, San Juan, PR, USA
| | - Orestes Quesada
- Department of Physical Sciences, University of Puerto Rico, Río Piedras Campus, San Juan, PR, USA
| | | | - José A Lasalde-Dominicci
- Department of Biology, University of Puerto Rico, Río Piedras Campus, San Juan, PR, USA.,Department of Chemistry, University of Puerto Rico, Río Piedras Campus, San Juan, PR, USA
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5
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Gómez-Canela C, Prats E, Lacorte S, Raldúa D, Piña B, Tauler R. Metabolomic changes induced by nicotine in adult zebrafish skeletal muscle. ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 2018; 164:388-397. [PMID: 30142605 DOI: 10.1016/j.ecoenv.2018.08.042] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/23/2018] [Revised: 08/11/2018] [Accepted: 08/14/2018] [Indexed: 06/08/2023]
Abstract
Acute exposure to nicotinic agonists induces myotoxicity in zebrafish embryos. The main goal of this work was to evaluate the potential myotoxicity of nicotine acetylcholine receptor agonists on adult zebrafish muscle tissue by using nicotine as a model compound. Liquid chromatography coupled to high resolution mass spectrometry (LC-HRMS) datasets were processed with different chemometric tools based on the selection of Regions of Interest (ROI) and Multivariate Curve-Resolution (ROI-MCR procedure) Alternating Least Squares (ALS) for the analysis of different exposure experiments. Analysis of Variance Simultaneous Component Analysis (ASCA) of changes on metabolite peak profile areas showed significant nicotine concentration and exposure time-dependent changes, clearly differentiating between exposed and non-exposed samples and between short (2 h) and long exposure times (6 h or 24 h). Most of the changes observed in the concentrations of different metabolites are probably secondary to the observed hyperlocomotion, as they have been also observed in humans after strenuous muscular exercise. The absence of myotoxicity might be related with the reduced calcium permeability of adult muscle-type nicotinic acetylcholine receptors (nAChRs).
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Affiliation(s)
- Cristian Gómez-Canela
- Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain.
| | - Eva Prats
- Centre d'Investigació I Desenvolupament, CID-CSIC, Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain
| | - Silvia Lacorte
- Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain
| | - Demetrio Raldúa
- Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain
| | - Benjamí Piña
- Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain
| | - Romà Tauler
- Department of Environmental Chemistry, IDAEA-CSIC, Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain
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6
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Animal Models of the Neuromuscular Junction, Vitally Informative for Understanding Function and the Molecular Mechanisms of Congenital Myasthenic Syndromes. Int J Mol Sci 2018; 19:ijms19051326. [PMID: 29710836 PMCID: PMC5983836 DOI: 10.3390/ijms19051326] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2018] [Revised: 04/24/2018] [Accepted: 04/25/2018] [Indexed: 01/16/2023] Open
Abstract
The neuromuscular junction is the point of contact between motor nerve and skeletal muscle, its vital role in muscle function is reliant on the precise location and function of many proteins. Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders of neuromuscular transmission with 30 or more implicated proteins. The use of animal models has been instrumental in determining the specific role of many CMS-related proteins. The mouse neuromuscular junction (NMJ) has been extensively studied in animal models of CMS due to its amenability for detailed electrophysiological and histological investigations and relative similarity to human NMJ. As well as their use to determine the precise molecular mechanisms of CMS variants, where an animal model accurately reflects the human phenotype they become useful tools for study of therapeutic interventions. Many of the animal models that have been important in deconvolving the complexities of neuromuscular transmission and revealing the molecular mechanisms of disease are highlighted.
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7
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Wang S, Seaberg B, Paez-Colasante X, Rimer M. Defective Acetylcholine Receptor Subunit Switch Precedes Atrophy of Slow-Twitch Skeletal Muscle Fibers Lacking ERK1/2 Kinases in Soleus Muscle. Sci Rep 2016; 6:38745. [PMID: 27934942 PMCID: PMC5146667 DOI: 10.1038/srep38745] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2016] [Accepted: 11/15/2016] [Indexed: 01/10/2023] Open
Abstract
To test the role of extracellular-signal regulated kinases 1 and 2 (ERK1/2) in slow-twitch, type 1 skeletal muscle fibers, we studied the soleus muscle in mice genetically deficient for myofiber ERK1/2. Young adult mutant soleus was drastically wasted, with highly atrophied type 1 fibers, denervation at most synaptic sites, induction of “fetal” acetylcholine receptor gamma subunit (AChRγ), reduction of “adult” AChRε, and impaired mitochondrial biogenesis and function. In weanlings, fiber morphology and mitochondrial markers were mostly normal, yet AChRγ upregulation and AChRε downregulation were observed. Synaptic sites with fetal AChRs in weanling muscle were ~3% in control and ~40% in mutants, with most of the latter on type 1 fibers. These results suggest that: (1) ERK1/2 are critical for slow-twitch fiber growth; (2) a defective γ/ε-AChR subunit switch, preferentially at synapses on slow fibers, precedes wasting of mutant soleus; (3) denervation is likely to drive this wasting, and (4) the neuromuscular synapse is a primary subcellular target for muscle ERK1/2 function in vivo.
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Affiliation(s)
- Shuo Wang
- Department of Neuroscience and Experimental Therapeutics, Texas A&M University Health Science Center, Bryan, Texas, USA
| | - Bonnie Seaberg
- Department of Neuroscience and Experimental Therapeutics, Texas A&M University Health Science Center, Bryan, Texas, USA
| | - Ximena Paez-Colasante
- Department of Neuroscience and Experimental Therapeutics, Texas A&M University Health Science Center, Bryan, Texas, USA
| | - Mendell Rimer
- Department of Neuroscience and Experimental Therapeutics, Texas A&M University Health Science Center, Bryan, Texas, USA
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8
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Dubińska-Magiera M, Daczewska M, Lewicka A, Migocka-Patrzałek M, Niedbalska-Tarnowska J, Jagla K. Zebrafish: A Model for the Study of Toxicants Affecting Muscle Development and Function. Int J Mol Sci 2016; 17:E1941. [PMID: 27869769 PMCID: PMC5133936 DOI: 10.3390/ijms17111941] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/09/2016] [Revised: 11/10/2016] [Accepted: 11/14/2016] [Indexed: 01/08/2023] Open
Abstract
The rapid progress in medicine, agriculture, and allied sciences has enabled the development of a large amount of potentially useful bioactive compounds, such as drugs and pesticides. However, there is another side of this phenomenon, which includes side effects and environmental pollution. To avoid or minimize the uncontrollable consequences of using the newly developed compounds, researchers seek a quick and effective means of their evaluation. In achieving this goal, the zebrafish (Danio rerio) has proven to be a highly useful tool, mostly because of its fast growth and development, as well as the ability to absorb the molecules diluted in water through its skin and gills. In this review, we focus on the reports concerning the application of zebrafish as a model for assessing the impact of toxicants on skeletal muscles, which share many structural and functional similarities among vertebrates, including zebrafish and humans.
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Affiliation(s)
- Magda Dubińska-Magiera
- Department of Animal Developmental Biology, Institute of Experimental Biology, University of Wroclaw, 21 Sienkiewicza Street, 50-335 Wroclaw, Poland.
| | - Małgorzata Daczewska
- Department of Animal Developmental Biology, Institute of Experimental Biology, University of Wroclaw, 21 Sienkiewicza Street, 50-335 Wroclaw, Poland.
| | - Anna Lewicka
- Department of Animal Developmental Biology, Institute of Experimental Biology, University of Wroclaw, 21 Sienkiewicza Street, 50-335 Wroclaw, Poland.
| | - Marta Migocka-Patrzałek
- Department of Animal Developmental Biology, Institute of Experimental Biology, University of Wroclaw, 21 Sienkiewicza Street, 50-335 Wroclaw, Poland.
| | - Joanna Niedbalska-Tarnowska
- Department of Animal Developmental Biology, Institute of Experimental Biology, University of Wroclaw, 21 Sienkiewicza Street, 50-335 Wroclaw, Poland.
| | - Krzysztof Jagla
- GReD-Genetics, Reproduction and Development Laboratory, INSERM U1103, CNRS UMR6293, University of Clermont-Auvergne, 28 Place Henri-Dunant, 63000 Clermont-Ferrand, France.
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9
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Menelaou E, Paul LT, Perera SN, Svoboda KR. Motoneuron axon pathfinding errors in zebrafish: differential effects related to concentration and timing of nicotine exposure. Toxicol Appl Pharmacol 2015; 284:65-78. [PMID: 25668718 PMCID: PMC4567840 DOI: 10.1016/j.taap.2015.01.022] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2014] [Revised: 01/27/2015] [Accepted: 01/28/2015] [Indexed: 12/12/2022]
Abstract
Nicotine exposure during embryonic stages of development can affect many neurodevelopmental processes. In the developing zebrafish, exposure to nicotine was reported to cause axonal pathfinding errors in the later born secondary motoneurons (SMNs). These alterations in SMN axon morphology coincided with muscle degeneration at high nicotine concentrations (15-30 μM). Previous work showed that the paralytic mutant zebrafish known as sofa potato exhibited nicotine-induced effects onto SMN axons at these high concentrations but in the absence of any muscle deficits, indicating that pathfinding errors could occur independent of muscle effects. In this study, we used varying concentrations of nicotine at different developmental windows of exposure to specifically isolate its effects onto subpopulations of motoneuron axons. We found that nicotine exposure can affect SMN axon morphology in a dose-dependent manner. At low concentrations of nicotine, SMN axons exhibited pathfinding errors, in the absence of any nicotine-induced muscle abnormalities. Moreover, the nicotine exposure paradigms used affected the 3 subpopulations of SMN axons differently, but the dorsal projecting SMN axons were primarily affected. We then identified morphologically distinct pathfinding errors that best described the nicotine-induced effects on dorsal projecting SMN axons. To test whether SMN pathfinding was potentially influenced by alterations in the early born primary motoneuron (PMN), we performed dual labeling studies, where both PMN and SMN axons were simultaneously labeled with antibodies. We show that only a subset of the SMN axon pathfinding errors coincided with abnormal PMN axonal targeting in nicotine-exposed zebrafish. We conclude that nicotine exposure can exert differential effects depending on the levels of nicotine and developmental exposure window.
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Affiliation(s)
- Evdokia Menelaou
- Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA
| | - Latoya T Paul
- Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA
| | - Surangi N Perera
- Joseph J. Zilber School of Public Health, University of Wisconsin - Milwaukee, Milwaukee, WI 53205, USA
| | - Kurt R Svoboda
- Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA; Joseph J. Zilber School of Public Health, University of Wisconsin - Milwaukee, Milwaukee, WI 53205, USA.
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10
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Zhu H, Grajales-Reyes GE, Alicea-Vázquez V, Grajales-Reyes JG, Robinson K, Pytel P, Báez-Pagán CA, Lasalde-Dominicci JA, Gomez CM. Fluoxetine is neuroprotective in slow-channel congenital myasthenic syndrome. Exp Neurol 2014; 270:88-94. [PMID: 25448156 DOI: 10.1016/j.expneurol.2014.10.008] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2014] [Revised: 09/30/2014] [Accepted: 10/17/2014] [Indexed: 11/19/2022]
Abstract
The slow-channel congenital myasthenic syndrome (SCS) is an inherited neurodegenerative disease that caused mutations in the acetylcholine receptor (AChR) affecting neuromuscular transmission. Leaky AChRs lead to Ca(2+) overload and degeneration of the neuromuscular junction (NMJ) attributed to activation of cysteine proteases and apoptotic changes of synaptic nuclei. Here we use transgenic mouse models expressing two different mutations found in SCS to demonstrate that inhibition of prolonged opening of mutant AChRs using fluoxetine not only improves motor performance and neuromuscular transmission but also prevents Ca(2+) overload, the activation of cysteine proteases, calpain, caspase-3 and 9 at endplates, and as a consequence, reduces subsynaptic DNA damage at endplates, suggesting a long term benefit to therapy. These studies suggest that prolonged treatment of SCS patients with open ion channel blockers that preferentially block mutant AChRs is neuroprotective.
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Affiliation(s)
- Haipeng Zhu
- Department of Neurology, The University of Chicago, Chicago, IL, USA
| | | | | | | | - KaReisha Robinson
- Department of Neurology, The University of Chicago, Chicago, IL, USA
| | - Peter Pytel
- Department of Pathology, The University of Chicago, Chicago, IL, USA
| | - Carlos A Báez-Pagán
- Department of Biology, The University of Puerto Rico, San Juan, Puerto Rico, USA
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11
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Garcia CC, Potian JG, Hognason K, Thyagarajan B, Sultatos LG, Souayah N, Routh VH, McArdle JJ. Acetylcholinesterase deficiency contributes to neuromuscular junction dysfunction in type 1 diabetic neuropathy. Am J Physiol Endocrinol Metab 2012; 303:E551-61. [PMID: 22739110 PMCID: PMC3423102 DOI: 10.1152/ajpendo.00622.2011] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/05/2011] [Accepted: 06/02/2012] [Indexed: 12/19/2022]
Abstract
Diabetic neuropathy is associated with functional and morphological changes of the neuromuscular junction (NMJ) associated with muscle weakness. This study examines the effect of type 1 diabetes on NMJ function. Swiss Webster mice were made diabetic with three interdaily ip injections of streptozotocin (STZ). Mice were severely hyperglycemic within 7 days after the STZ treatment began. Whereas performance of mice on a rotating rod remained normal, the twitch tension response of the isolated extensor digitorum longus to nerve stimulation was reduced significantly at 4 wk after the onset of STZ-induced hyperglycemia. This mechanical alteration was associated with increased amplitude and prolonged duration of miniature end-plate currents (mEPCs). Prolongation of mEPCs was not due to expression of the embryonic acetylcholine receptor but to reduced muscle expression of acetylcholine esterase (AChE). Greater sensitivity of mEPC decay time to the selective butyrylcholinesterase (BChE) inhibitor PEC suggests that muscle attempts to compensate for reduced AChE levels by increasing expression of BChE. These alterations of AChE are attributed to STZ-induced hyperglycemia since similar mEPC prolongation and reduced AChE expression were found for db/db mice. The reduction of muscle end-plate AChE activity early during the onset of STZ-induced hyperglycemia may contribute to endplate pathology and subsequent muscle weakness during diabetes.
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Affiliation(s)
- Carmen C Garcia
- Dept. of Pharmacology and Physiology, New Jersey Medical School-UMDNJ, MSB-I626, 185 South Orange Ave., Newark, NJ 07101-1709, USA
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12
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Transgenic mouse model reveals an unsuspected role of the acetylcholine receptor in statin-induced neuromuscular adverse drug reactions. THE PHARMACOGENOMICS JOURNAL 2012; 13:362-8. [PMID: 22688219 DOI: 10.1038/tpj.2012.21] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/17/2012] [Revised: 04/13/2012] [Accepted: 04/13/2012] [Indexed: 01/11/2023]
Abstract
High cholesterol levels are an established risk factor for cardiovascular disease (CVD), the world's leading cause of death. Inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (statins) are prescribed to lower serum cholesterol levels and reduce the risk of CVD. Despite the success of statins, many patients abandon treatment owing to neuromuscular adverse drug reactions (ADRs). Genome-wide association studies have identified the single-nucleotide polymorphism (SNP) rs4149056 in the SLCO1B1 gene as being associated with an increased risk for statin-induced ADRs. By studying slow-channel syndrome transgenic mouse models, we determined that statins trigger ADRs in mice expressing the mutant allele of the rs137852808 SNP in the nicotinic acetylcholine receptor (nAChR) α-subunit gene CHRNA1. Mice expressing this allele show a remarkable contamination of end-plates with caveolin-1 and develop early signs of neuromuscular degeneration upon statin treatment. This study demonstrates that genes coding for nAChR subunits may contain variants associated with statin-induced ADRs.
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13
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Chevessier F, Peter C, Mersdorf U, Girard E, Krejci E, McArdle JJ, Witzemann V. A new mouse model for the slow-channel congenital myasthenic syndrome induced by the AChR εL221F mutation. Neurobiol Dis 2011; 45:851-61. [PMID: 22178625 DOI: 10.1016/j.nbd.2011.10.024] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2011] [Revised: 09/29/2011] [Accepted: 10/28/2011] [Indexed: 11/24/2022] Open
Abstract
We have generated a new mouse model for congenital myasthenic syndromes by inserting the missense mutation L221F into the ε subunit of the acetylcholine receptor by homologous recombination. This mutation has been identified in man to cause a mild form of slow-channel congenital myasthenic syndrome with variable penetrance. In our mouse model we observe as in human patients prolonged endplate currents. The summation of endplate potentials may account for a depolarization block at increasing stimulus frequencies, moderate reduced muscle strength and tetanic fade. Calcium and intracellular vesicle accumulation as well as junctional fold loss and organelle degeneration underlying a typical endplate myopathy, were identified. Moreover, a remodeling of neuromuscular junctions occurs in a muscle-dependent pattern expressing variable phenotypic effects. Altogether, this mouse model provides new insight into the pathophysiology of congenital myasthenia and serves as a new tool for deciphering signaling pathways induced by excitotoxicity at peripheral synapses.
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Affiliation(s)
- Frédéric Chevessier
- Department of Molecular Neurobiology, Max Planck Institute for Medical Research, Jahnstrasse 29, 69120 Heidelberg, Germany
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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.
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Affiliation(s)
- Vanessa Piccari
- Dipartimento di Fisiologia e Farmacologia, Sapienza Università di Roma, P.le A. Moro 5, I-00185 Roma, Italy
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Otero-Cruz JD, Báez-Pagán CA, Dorna-Pérez L, Grajales-Reyes GE, Ramírez-Ordoñez RT, Luciano CA, Gómez CM, Lasalde-Dominicci JA. Decoding pathogenesis of slow-channel congenital myasthenic syndromes using recombinant expression and mice models. PUERTO RICO HEALTH SCIENCES JOURNAL 2010; 29:4-17. [PMID: 20222328 PMCID: PMC2929179] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
Despite the fact that they are orphan diseases, congenital myasthenic syndromes (CMS) challenge those who suffer from it by causing fatigable muscle weakness, in the most benign cases, to a progressive wasting of muscles that may sentence patients to a wheelchair or even death. Compared to other more common neurological diseases, CMS are rare. Nevertheless, extensive research in CMS is performed in laboratories such as ours. Among the diverse neuromuscular disorders of CMS, we are focusing in the slow-channel congenital myasthenic syndrome (SCS), which is caused by mutations in genes encoding acetylcholine receptor subunits. The study of SCS has evolved from clinical electrophysiological studies to in vitro expression systems and transgenic mice models. The present review evaluates the methodological approaches that are most commonly employed to assess synaptic impairment in SCS and also provides perspectives for new approaches. Electrophysiological methodologies typically employed by physicians to diagnose patients include electromyography, whereas patient muscle samples are used for intracellular recordings, single-channel recordings and toxin binding experiments. In vitro expression systems allow the study of a particular mutation without the need of patient intervention. Indeed, in vitro expression systems have usually been implicated in the development of therapeutic strategies such as quinidine- and fluoxetine-based treatments and, more recently, RNA interference. A breakthrough in the study of SCS has been the development of transgenic mice bearing the mutations that cause SCS. These transgenic mice models have actually been key in the elucidation of the pathogenesis of the SCS mutations by linking IP-3 receptors to calcium overloading, as well as caspases and calpains to the hallmark of SCS, namely endplate myopathy. Finally, we summarize our experiences with suspected SCS patients from a local perspective and comment on one aspect of the contribution of our group in the study of SCS.
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Voigt T. Early effects of carbachol on the morphology of motor endplates of mammalian skeletal muscle fibers. Muscle Nerve 2009; 41:399-405. [PMID: 19882636 DOI: 10.1002/mus.21508] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Long-term disturbance of the calcium homeostasis of motor endplates (MEPs) causes necrosis of muscle fibers. The onset of morphological changes in response to this disturbance, particularly in relation to the fiber type, is presently unknown. Omohyoid muscles of mice were incubated for 1-30 minutes in 0.1 mM carbachol, an acetylcholine agonist that causes an inward calcium current. In these muscles, the structural changes of the sarcomeres and the MEP sarcoplasm were evaluated at the light- and electron-microscopic level. Predominantly in type I fibers, carbachol incubation resulted in strong contractures of the sarcomeres underlying the MEPs. Owing to these contractures, the usual beret-like form of the MEP-associated sarcoplasm was deformed into a mushroom-like body. Consequently, the squeezed MEPs partially overlapped the adjacent muscle fiber segments. There are no signs of contractures below the MEPs if muscles were incubated in carbachol in calcium-free Tyrode's solution. Carbachol induced inward calcium current and produced fiber-type-specific contractures. This finding points to differences in the handling of calcium in MEPs. Possible mechanisms for these fiber-type-specific differences caused by carbachol-induced calcium entry are assessed.
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Affiliation(s)
- Tilman Voigt
- Institute of Anatomy, University of Fribourg, Switzerland.
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17
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Millar NS. A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors. Biochem Pharmacol 2009; 78:766-76. [PMID: 19540210 DOI: 10.1016/j.bcp.2009.06.015] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2009] [Revised: 06/08/2009] [Accepted: 06/10/2009] [Indexed: 11/18/2022]
Abstract
Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.
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Affiliation(s)
- Neil S Millar
- Department of Neuroscience, Physiology and Pharmacology, University College London, London WC1E 6BT, UK.
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18
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Uncoupling nicotine mediated motoneuron axonal pathfinding errors and muscle degeneration in zebrafish. Toxicol Appl Pharmacol 2008; 237:29-40. [PMID: 18694773 DOI: 10.1016/j.taap.2008.06.025] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2008] [Revised: 06/16/2008] [Accepted: 06/16/2008] [Indexed: 11/23/2022]
Abstract
Zebrafish embryos offer a unique opportunity to investigate the mechanisms by which nicotine exposure impacts early vertebrate development. Embryos exposed to nicotine become functionally paralyzed by 42 hpf suggesting that the neuromuscular system is compromised in exposed embryos. We previously demonstrated that secondary spinal motoneurons in nicotine-exposed embryos were delayed in development and that their axons made pathfinding errors (Svoboda, K.R., Vijayaraghaven, S., Tanguay, R.L., 2002. Nicotinic receptors mediate changes in spinal motoneuron development and axonal pathfinding in embryonic zebrafish exposed to nicotine. J. Neurosci. 22, 10731-10741). In that study, we did not consider the potential role that altered skeletal muscle development caused by nicotine exposure could play in contributing to the errors in spinal motoneuron axon pathfinding. In this study, we show that an alteration in skeletal muscle development occurs in tandem with alterations in spinal motoneuron development upon exposure to nicotine. The alteration in the muscle involves the binding of nicotine to the muscle-specific AChRs. The nicotine-induced alteration in muscle development does not occur in the zebrafish mutant (sofa potato, [sop]), which lacks muscle-specific AChRs. Even though muscle development is unaffected by nicotine exposure in sop mutants, motoneuron axonal pathfinding errors still occur in these mutants, indicating a direct effect of nicotine exposure on nervous system development.
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Jin TE, Wernig A, Witzemann V. Changes in acetylcholine receptor function induce shifts in muscle fiber type composition. FEBS J 2008; 275:2042-54. [DOI: 10.1111/j.1742-4658.2008.06359.x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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20
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Groshong JS, Spencer MJ, Bhattacharyya BJ, Kudryashova E, Vohra BP, Zayas R, Wollmann RL, Miller RJ, Gomez CM. Calpain activation impairs neuromuscular transmission in a mouse model of the slow-channel myasthenic syndrome. J Clin Invest 2007; 117:2903-12. [PMID: 17853947 PMCID: PMC1974862 DOI: 10.1172/jci30383] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2006] [Accepted: 06/26/2007] [Indexed: 11/17/2022] Open
Abstract
The slow-channel myasthenic syndrome (SCS) is a hereditary disorder of the acetylcholine receptor (AChR) of the neuromuscular junction (NMJ) that leads to prolonged AChR channel opening, Ca(2+) overload, and degeneration of the NMJ. We used an SCS transgenic mouse model to investigate the role of the calcium-activated protease calpain in the pathogenesis of synaptic dysfunction in SCS. Cleavage of a fluorogenic calpain substrate was increased at the NMJ of dissociated muscle fibers. Inhibition of calpain using a calpastatin (CS) transgene improved strength and neuromuscular transmission. CS caused a 2-fold increase in the frequency of miniature endplate currents (MEPCs) and an increase in NMJ size, but MEPC amplitudes remained reduced. Persistent degeneration of the NMJ was associated with localized activation of the non-calpain protease caspase-3. This study suggests that calpain may act presynaptically to impair NMJ function in SCS but further reveals a role for other cysteine proteases whose inhibition may be of additional therapeutic benefit in SCS and other excitotoxic disorders.
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Affiliation(s)
- Jason S. Groshong
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
| | - Melissa J. Spencer
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
| | - Bula J. Bhattacharyya
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
| | - Elena Kudryashova
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
| | - Bhupinder P.S. Vohra
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
| | - Roberto Zayas
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
| | - Robert L. Wollmann
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
| | - Richard J. Miller
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
| | - Christopher M. Gomez
- Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
UCLA, Los Angeles, California, USA.
Feinberg School of Medicine, Northwestern University, Chicago, Illinois, USA.
Washington University School of Medicine, St. Louis, Missouri, USA.
Department of Neurology, The University of Chicago, Chicago, Illinois, USA
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Müller JS, Mihaylova V, Abicht A, Lochmüller H. Congenital myasthenic syndromes: spotlight on genetic defects of neuromuscular transmission. Expert Rev Mol Med 2007; 9:1-20. [PMID: 17686188 DOI: 10.1017/s1462399407000427] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
The neuromuscular junction (NMJ) is a complex structure that efficiently communicates the electrical impulse from the motor neuron to the skeletal muscle to induce muscle contraction. Genetic and autoimmune disorders known to compromise neuromuscular transmission are providing further insights into the complexities of NMJ function. Congenital myasthenic syndromes (CMSs) are a genetically and phenotypically heterogeneous group of rare hereditary disorders affecting neuromuscular transmission. The understanding of the molecular basis of the different types of CMSs has evolved rapidly in recent years. Mutations were first identified in the subunits of the nicotinic acetylcholine receptor (AChR), but now mutations in ten different genes - encoding post-, pre- or synaptic proteins - are known to cause CMSs. Pathogenic mechanisms leading to an impaired neuromuscular transmission modify AChRs or endplate structure or lead to decreased acetylcholine synthesis and release. However, the genetic background of many CMS forms is still unresolved. A precise molecular classification of CMS type is of paramount importance for the diagnosis, counselling and therapy of a patient, as different drugs may be beneficial or deleterious depending on the molecular background of the particular CMS.
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Affiliation(s)
- Juliane S Müller
- Friedrich-Baur-Institute, Department of Neurology, Ludwig-Maximilians-University, Munich, Germany
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Zayas R, Groshong JS, Gomez CM. Inositol-1,4,5-triphosphate receptors mediate activity-induced synaptic Ca2+ signals in muscle fibers and Ca2+ overload in slow-channel syndrome. Cell Calcium 2006; 41:343-52. [PMID: 16973214 DOI: 10.1016/j.ceca.2006.07.007] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2006] [Revised: 06/03/2006] [Accepted: 07/15/2006] [Indexed: 11/21/2022]
Abstract
Strict control of calcium entry through excitatory synaptic receptors is important for shaping synaptic responses, gene expression, and cell survival. Disruption of this control may lead to pathological accumulation of Ca2+. The slow-channel congenital myasthenic syndrome (SCS), due to mutations in muscle acetylcholine receptor (AChR), perturbs the kinetics of synaptic currents, leading to post-synaptic Ca2+ accumulation. To understand the regulation of calcium signaling at the neuromuscular junction (NMJ) and the etiology of Ca2+ overload in SCS we studied the role of sarcoplasmic Ca2+ stores in SCS. Using fura-2 loaded dissociated fibers activated with acetylcholine puffs, we confirmed that Ca2+ accumulates around wild type NMJ and discovered that Ca2+ accumulates significantly faster around the NMJ of SCS transgenic dissociated muscle fibers. Additionally, we determined that this process is dependant on the activation, altered kinetics, and movement of Ca2+ ions through the AChR, although, surprisingly, depletion of intracellular stores also prevents the accumulation of this cation around the NMJ. Finally, we concluded that the sarcoplasmic reticulum is the main source of Ca2+ and that inositol-1,4,5-triphosphate receptors (IP3R), and to a lesser degree L-type voltage gated Ca2+ channels, are responsible for the efflux of this cation from intracellular stores. These results suggest that a signaling system mediated by the activation of AChR, Ca2+, and IP3R is responsible for localized Ca2+ signals observed in muscle fibers and the Ca2+ overload observed in SCS.
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Affiliation(s)
- Roberto Zayas
- Department of Neuroscience and Neurology, University of Minnesota, Minneapolis, MN 55455, USA
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Zayas R, Lasalde-Dominicci J, Gomez CM. Macroscopic properties of spontaneous mutations in slow-channel syndrome: correlation by domain and disease severity. Synapse 2006; 60:441-9. [PMID: 16881075 DOI: 10.1002/syn.20317] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The slow-channel syndrome (SCS) is a neuromuscular disorder characterized by fatigability, progressive weakness, and degeneration of the neuromuscular junction. The SCS is caused by missense mutations in the four subunits of the skeletal muscle acetylcholine receptor (AChR), which leads to altered channel gating, prolonged neuromuscular postsynaptic currents, and impaired neuromuscular transmission. Although a diverse set of mutations in different functional domains of the AChR appear to be associated with symptoms of widely ranging severity, there is as yet no mutant channel property or combination that explains the variations in disease severity. By observing the recovery time of AChR from desensitization, the authors determined that this process is significantly enhanced in SCS channels. In addition, as expected, the authors found that SCS macroscopic decay currents in transfected HEK293 cells are slower than wild type currents. While slight differences in relative Ca(2+) permeability between some SCS mutations were identified, they did not correlate with apparent disease severity. These results suggest that of the different AChR kinetic features studied, only recovery from desensitization and slow postsynaptic currents correlate with the severity observed in the different mutations of this syndrome.
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Affiliation(s)
- Roberto Zayas
- Department of Neuroscience and Neurology, University of Minnesota, Minneapolis, 55455, USA
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Abstract
Ion channelopathies are a diverse array of human disorders caused by mutations in ion channel genes. This review focuses on the pathogenic mechanisms of channelopathies affecting skeletal muscle and brain arising from mutations of voltage-gated ion channels and fast ligand-gated ion channels expressed at the surface membrane. Derangements in channel function alter the electrical excitability of the cell and thereby increase susceptibility to transient symptomatic attacks including myasthenia, periodic paralysis, myotonic stiffness, seizures, headache, dyskinesia, or episodic ataxia. Although these disorders are rare, they stand out as exemplary cases for which disease pathogenesis can be traced from a point mutation to altered protein function, to altered cellular activity, and to clinical phenotype. The study of these disorders has provided insights on channel structure-function relations, the physiological roles of ion channels, and rational approaches toward therapeutic intervention for many disorders of cellular excitability.
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Affiliation(s)
- Stephen C Cannon
- Department of Neurology, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA.
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Vohra BPS, Groshong JS, Zayas R, Wollmann RL, Gomez CM. Activation of apoptotic pathways at muscle fiber synapses is circumscribed and reversible in a slow-channel syndrome model. Neurobiol Dis 2006; 23:462-70. [PMID: 16815027 DOI: 10.1016/j.nbd.2006.04.018] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2006] [Revised: 03/30/2006] [Accepted: 04/05/2006] [Indexed: 10/24/2022] Open
Abstract
In the slow-channel syndrome (SCS) mutant acetylcholine receptors elicit calcium overload and myonuclear degeneration at the neuromuscular junction (NMJ), without muscle fiber death. Activated caspases are present at SCS motor endplates. We hypothesized that SCS represents a limited form of apoptosis. We found condensed chromatin and occasional single-strand DNA nicks in degenerating synaptic nuclei. Cleaved forms of caspases-3 and -9 were present in mouse SCS muscle homogenates and were specifically localized to NMJs. Finally, interruption of cholinergic activity by axotomy markedly reduced NMJ caspase activity and improved the morphological features of apoptosis at NMJs. These results demonstrate that in SCS processes leading to apoptosis may remain compartmentalized and reversible. Use of cysteine protease inhibitors may aid in treatment of this and other dystrophic muscle and excitotoxic disorders. Identification of extrasynaptic factors that prevent the spread of apoptosis in SCS muscle fibers may aid in developing treatments for neurological disorders characterized by excitotoxicity or apoptosis.
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Affiliation(s)
- Bhupinder P S Vohra
- Washington University School of Medicine, Department of Pediatrics, St. Louis, MO 63110, USA
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26
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Navedo MF, Lasalde-Dominicci JA, Báez-Pagán CA, Díaz-Pérez L, Rojas LV, Maselli RA, Staub J, Schott K, Zayas R, Gomez CM. Novel beta subunit mutation causes a slow-channel syndrome by enhancing activation and decreasing the rate of agonist dissociation. Mol Cell Neurosci 2006; 32:82-90. [PMID: 16624571 PMCID: PMC4636114 DOI: 10.1016/j.mcn.2006.02.004] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2005] [Revised: 02/08/2006] [Accepted: 02/21/2006] [Indexed: 11/19/2022] Open
Abstract
We traced the cause of a slow-channel syndrome (SCS) in a patient with progressive muscle weakness, repetitive compound muscle action potential and prolonged low amplitude synaptic currents to a V --> F substitution in the M1 domain of the beta subunit (betaV229F) of the muscle acetylcholine receptor (AChR). In vitro expression studies in Xenopus oocytes indicated that the novel mutation betaV229F expressed normal amounts of AChRs and decreased the ACh EC50 by 10-fold compared to wild type. Kinetic analysis indicated that the mutation displayed prolonged mean open duration and repeated openings during activation. Prolonged openings caused by the betaV229F mutation were due to a reduction in the channel closing rate and an increase in the effective channel opening rate. Repeated openings of the channel during activation were caused by a significant reduction in the agonist dissociation constant. In addition, the betaV229F mutation produced an increase in calcium permeability. The kinetic and permeation studies presented in this work are sufficient to explain the consequences of the betaV229F mutation on the miniature endplate currents and thus are direct evidence that the betaV229F mutation is responsible for compromising the safety margin of neuromuscular transmission in the patient.
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Affiliation(s)
- Manuel F. Navedo
- Department of Physiology and Biophysics, University of Washington, Seattle, WA 98195-7290, USA
| | | | | | - Luzed Díaz-Pérez
- Department of Biology, University of Puerto Rico, San Juan, PR 00931, USA
| | - Legier V. Rojas
- Department of Physiology, Universidad Central del Caribe, Bayamón, PR 00960, USA
| | | | - Julie Staub
- Department of Neurology and Neuroscience, MMC 295 420 Delaware St. SE University of Minnesota, Minneapolis, MN 55455, USA
| | - Kelly Schott
- Department of Neurology and Neuroscience, MMC 295 420 Delaware St. SE University of Minnesota, Minneapolis, MN 55455, USA
| | - Roberto Zayas
- Department of Neurology and Neuroscience, MMC 295 420 Delaware St. SE University of Minnesota, Minneapolis, MN 55455, USA
| | - Christopher M. Gomez
- Department of Neurology and Neuroscience, MMC 295 420 Delaware St. SE University of Minnesota, Minneapolis, MN 55455, USA
- Corresponding author. (C.M. Gomez)
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Fucile S, Sucapane A, Grassi F, Eusebi F, Engel AG. The human adult subtype ACh receptor channel has high Ca2+ permeability and predisposes to endplate Ca2+ overloading. J Physiol 2006; 573:35-43. [PMID: 16527851 PMCID: PMC1779694 DOI: 10.1113/jphysiol.2006.108092] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
Slow-channel congenital myasthenic syndrome, caused by mutations in subunits of the endplate ACh receptor (AChR), results in prolonged synaptic currents and excitotoxic injury of the postsynaptic region by Ca2+ overloading. The Ca2+ overloading could be due entirely to the prolonged openings of the AChR channel or could be abetted by enhanced Ca2+ permeability of the mutant channels. We therefore measured the fractional Ca2+ current, defined as the percentage of the total ACh-evoked current carried by Ca2+ ions (Pf), for AChRs harbouring the alphaG153S or the alphaV249F slow-channel mutation, and for wild-type human AChRs in which Pf has not yet been determined. Experiments were performed in transiently transfected GH4C1 cells and human myotubes with simultaneous recording of ACh-evoked whole-cell currents and fura-2 fluorescence signals. We found that the Pf of the wild-type human endplate AChR was unexpectedly high (Pf approximately 7%), but neither the alphaV249F nor the alphaG153S mutation altered Pf. Fetal human AChRs containing either the wild-type or the mutated alpha subunit had a much lower Pf (2-3%). We conclude that the Ca2+ permeability of human endplate AChRs is higher than that reported for any other human nicotinic AChR, with the exception of alpha7-containing AChRs (Pf > 10%); and that neither the alphaG153S nor the alphaV249F mutations affect the Pf of fetal or adult endplate AChRs. However, the intrinsically high Ca2+ permeability of human AChRs probably predisposes to development of the endplate myopathy when opening events of the AChR channel are prolonged by altered AChR-channel kinetics.
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Affiliation(s)
- Sergio Fucile
- Pasteur Institute -Cenci Bolognetti Foundation & Department of Human Physiology and Pharmacology & Centre of Excellence for Biology and Molecular Medicine, University of Rome La Sapienza, Piazzale Aldo Moro 5; I-00185 Rome, Italy
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Fidzianska A, Ryniewicz B, Shen XM, Engel AG. IBM-type inclusions in a patient with slow-channel syndrome caused by a mutation in the AChR epsilon subunit. Neuromuscul Disord 2005; 15:753-9. [PMID: 16198106 DOI: 10.1016/j.nmd.2005.07.009] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2005] [Revised: 07/03/2005] [Accepted: 07/18/2005] [Indexed: 11/25/2022]
Abstract
We report a patient with a slow-channel congenital myasthenic syndrome who carries a novel slow-channel mutation in the epsilon subunit of the acetylcholine receptor and has tubulofilamentous inclusion bodies, in skeletal muscle of the type observed in hereditary and sporadic inclusion body myositis. Ultrastructural analysis of a muscle specimen obtained at the age of 9 years showed an endplate myopathy typical of the slow-channel syndrome. Twenty years later, a second muscle specimen again showed the endplate myopathy as well numerous nuclear and cytoplasmic tubulofilamentous inclusion bodies. Molecular genetic studies revealed a novel valine to phenylalanine mutation (epsilonV259F) in the M2 domain of the acetylcholine receptor. Coexistence of the slow-channel syndrome with a feature of IBM has not been observed before.
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MESH Headings
- Acetylcholine/pharmacology
- Adult
- Bungarotoxins/pharmacokinetics
- Cell Line
- DNA Mutational Analysis/methods
- Dose-Response Relationship, Drug
- Female
- Humans
- Iodine Isotopes/pharmacokinetics
- Membrane Potentials/drug effects
- Membrane Potentials/physiology
- Microscopy, Electron, Transmission/methods
- Molecular Sequence Data
- Muscle, Skeletal/metabolism
- Muscle, Skeletal/pathology
- Muscle, Skeletal/ultrastructure
- Mutation
- Myasthenic Syndromes, Congenital/complications
- Myasthenic Syndromes, Congenital/genetics
- Myasthenic Syndromes, Congenital/pathology
- Myositis, Inclusion Body/complications
- Myositis, Inclusion Body/genetics
- Myositis, Inclusion Body/pathology
- Patch-Clamp Techniques/methods
- Protein Binding/drug effects
- Radioligand Assay/methods
- Receptors, Nicotinic/genetics
- Transfection/methods
- Valine/genetics
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Affiliation(s)
- Anna Fidzianska
- Neuromuscular Unit, Medical Research Centre, Pol. Ac. Sci. Pawinskiego 5, 02-106 Warsaw, Poland.
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29
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Selective DNAzyme-mediated cleavage of AChR mutant transcripts by targeting the mutation site or through mismatches in the binding arm. JOURNAL OF RNAI AND GENE SILENCING : AN INTERNATIONAL JOURNAL OF RNA AND GENE TARGETING RESEARCH 2005; 1:32-7. [PMID: 19771202 PMCID: PMC2737196] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/27/2005] [Revised: 07/07/2005] [Accepted: 07/08/2005] [Indexed: 10/30/2022]
Abstract
Many dominantly inherited disorders are caused by missense amino acid substitutions resulting from a single nucleotide exchange in the encoding gene. For these disorders, where proteins expressed from the mutant alleles are often pathogenic and present throughout life, gene silencing, through intervention at the mRNA level, holds promise as a therapeutic approach. We have used mutations that underlie the slow channel congenital myasthenic syndrome (SCCMS) as a model system to study allele-specific gene silencing of RNA transcripts by DNAzymes. We tested the ability of DNAzymes to give allele-specific cleavage for i) mutations that create cleavage sites, and ii) mutations located close to a DNAzyme cleavage site that create a potential mismatch in the binding arms. For both we demonstrate selective cleavage of mutant transcripts under simulated physiological conditions. For DNAzymes with binding arm mismatches the degree of selectivity for mutant over wild type may be enhanced by optimising the mismatch position as well as the binding arm length. The optimal sites for mismatches are 1.1 and 1.2 in arm I, and 16.2 in arm II. Asymmetric binding arm DNAzymes with a shorter arm I are more discriminative. Our results show it should be possible to apply DNAzyme-mediated cleavage of mutant alleles even when the mutant does not itself create a putative cleavage site. This therapeutic approach may be well suited to dominantly inherited disorders such as SCCMS, where loss of some wild type transcripts is unlikely to have pathogenic consequences.
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30
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Abdelgany A, Ealing J, Wood M, Beeson D. Selective cleavage of AChR cRNAs harbouring mutations underlying the slow channel myasthenic syndrome by hammerhead ribozymes. JOURNAL OF RNAI AND GENE SILENCING : AN INTERNATIONAL JOURNAL OF RNA AND GENE TARGETING RESEARCH 2005; 1:26-31. [PMID: 19771201 PMCID: PMC2737193] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/27/2005] [Revised: 07/07/2005] [Accepted: 07/08/2005] [Indexed: 10/26/2022]
Abstract
Slow channel congenital myasthenic syndrome (SCCMS) is a dominant disorder caused by missense mutations in muscle acetylcholine receptors (AChR). Expression from mutant alleles causes prolonged AChR ion-channel activations. This 'gain of function' results in excitotoxic damage due to excess entry of calcium ions that manifests as an endplate myopathy. The biology of SCCMS provides a model system to investigate the potential of catalytic nucleic acids for therapy in dominantly inherited disorders involving single missense mutations. Hammerhead ribozymes can catalytically cleave RNA transcripts in a sequence-specific manner. We designed hammerhead ribozymes to target transcripts from four SCCMS mutations, alphaT254I, alphaS226F, alphaS269I and epsilonL221F. Ribozymes were incubated with cRNA transcripts encoding wild type and mutant AChR subunits. The ribozymes efficiently cleaved the mutant allele cRNA transcripts but left the wild type cRNA intact. Cleavage efficiency was optimised for alphaS226F. We were able to demonstrate robust catalytic activity under simulated physiological conditions and at high Ca(2+) concentrations, which is likely to be accumulated at the endplate region of the SCCMS patient muscles. These results demonstrate the potential for gene therapy applications of ribozymes to specifically down-regulate expression of mutant alleles in dominantly inherited disorders.
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Affiliation(s)
- Amr Abdelgany
- Neurosciences Group, Weatherall Institute of Molecular Medicine, University of Oxford, The John Radcliffe Hospital, Oxford OX3 9DS, UK
| | - John Ealing
- Neurosciences Group, Weatherall Institute of Molecular Medicine, University of Oxford, The John Radcliffe Hospital, Oxford OX3 9DS, UK
| | - Matthew Wood
- Department of Human Anatomy and Genetics, South Parks Road, Oxford, OX1 3QX, UK
| | - David Beeson
- Neurosciences Group, Weatherall Institute of Molecular Medicine, University of Oxford, The John Radcliffe Hospital, Oxford OX3 9DS, UK,Correspondence to: David Beeson, , Tel: +44 1865 222311, Fax: +44 1865 222402
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31
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Peter C, Korngreen A, Witzemann V. Mutation of single murine acetylcholine receptor subunits reveals differential contribution of P121 to acetylcholine binding and channel opening. Pflugers Arch 2005; 450:178-84. [PMID: 15864502 DOI: 10.1007/s00424-005-1387-5] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2004] [Accepted: 01/25/2005] [Indexed: 10/25/2022]
Abstract
The nicotinic acetylcholine receptor (AChR) is a heteropentameric, ligand-gated ion channel at the neuromuscular junction, where it is responsible for signal transduction between the motorneuron and the muscle. Point mutations in the subunits of the receptor change the channel's electrophysiological properties and underlie inherited forms of muscle weakness, the congenital myasthenic syndromes. One point mutation (P121L) has been identified in the epsilon-subunit of patients suffering from the fast-channel congenital myasthenic syndrome, which is evoked by reduced AChR openings. We introduced the P121L mutation into all murine AChR subunits and performed electrophysiological studies in Xenopus laevis oocytes. The P121L mutation in the epsilon-subunit of the adult mouse AChR affected ligand binding and channel gating in a manner similar to that described for human AChR. At equivalent positions in the alpha- and beta-subunits, the mutation caused only minor electrophysiological changes. Mutation of the delta-subunit had similar, but less pronounced functional consequences compared to epsilonP121L, reflecting the asymmetry of the acetylcholine binding sites and the dominant effect of the alpha-epsilon site on channel opening.
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Affiliation(s)
- Christoph Peter
- Abt. Zellphysiologie, Max-Planck-Institut für medizinische Forschung, Jahnstr. 29, 69120, Heidelberg, Germany
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Shelley C, Colquhoun D. A human congenital myasthenia-causing mutation (epsilon L78P) of the muscle nicotinic acetylcholine receptor with unusual single channel properties. J Physiol 2005; 564:377-96. [PMID: 15731194 PMCID: PMC1464449 DOI: 10.1113/jphysiol.2004.081497] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
A mutation in the epsilon subunit of the human nicotinic acetylcholine receptor (epsilonL78P) is known to cause a congenital slow channel myasthenic syndrome. We have investigated the changes in receptor function that result in the mutant receptor producing prolonged endplate currents, and consequent muscle damage. The rate constants for channel gating and for the binding and dissociation of acetylcholine were investigated by analysis of single ion channel recordings. A conventional mechanism with two non-equivalent binding sites, and variations upon this mechanism, were fitted to data using a maximum likelihood method that uses the Hawkes-Jalali-Colquhoun (HJC) treatment of missed brief events. The mutant receptor produced prolonged activations, bursts of openings that cause a slow decay of simulated synaptic currents. The main reason for the longer bursts of openings seen with mutant receptor was a decrease in the rate of ACh dissociation from diliganded receptors, though the lifetime of individual openings was somewhat increased too. As well as producing long bursts, the mutant receptor also produced many very short openings, though these carry little current. The burst structure for the mutant receptor at low ACh concentration is unusual in that most long bursts appear to start in a very brief monoliganded open state that then usually binds another ACh molecule to produce a long diliganded activation. The first opening is so short that it will usually be missed (together with the shut time that follows it), so the true burst length is likely to be underestimated.
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Lefebvre JL, Ono F, Puglielli C, Seidner G, Franzini-Armstrong C, Brehm P, Granato M. Increased neuromuscular activity causes axonal defects and muscular degeneration. Development 2004; 131:2605-18. [PMID: 15128655 DOI: 10.1242/dev.01123] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
Before establishing terminal synapses with their final muscle targets,migrating motor axons form en passant synaptic contacts with myotomal muscle. Whereas signaling through terminal synapses has been shown to play important roles in pre- and postsynaptic development, little is known about the function of these early en passant synaptic contacts. Here, we show that increased neuromuscular activity through en passant synaptic contacts affects pre- and postsynaptic development. We demonstrate that in zebrafish twistermutants, prolonged neuromuscular transmission causes motor axonal extension and muscular degeneration in a dose-dependent manner. Cloning of twister reveals a novel, dominant gain-of-function mutation in the muscle-specific nicotinic acetylcholine receptor α-subunit, CHRNA1. Moreover, electrophysiological analysis demonstrates that the mutant subunit increases synaptic decay times, thereby prolonging postsynaptic activity. We show that as the first en passant synaptic contacts form, excessive postsynaptic activity in homozygous embryos severely impedes pre- and postsynaptic development, leading to degenerative defects characteristic of the human slow-channel congenital myasthenic syndrome. By contrast, in heterozygous embryos, transient and mild increase in postsynaptic activity does not overtly affect postsynaptic morphology but causes transient axonal defects, suggesting bi-directional communication between motor axons and myotomal muscle. Together, our results provide compelling evidence that during pathfinding, myotomal muscle cells communicate extensively with extending motor axons through en passant synaptic contacts.
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Affiliation(s)
- Julie L Lefebvre
- Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6058, USA
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Vohra BPS, Groshong JS, Maselli RA, Verity MA, Wollmann RL, Gomez CM. Focal caspase activation underlies the endplate myopathy in slow-channel syndrome. Ann Neurol 2004; 55:347-52. [PMID: 14991812 DOI: 10.1002/ana.10823] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Slow-channel syndrome (SCS) is a progressive neuromuscular disorder caused by abnormal gating of mutant acetylcholine receptors (AChRs) in the neuromuscular junction (NMJ). The pathological hallmark is selective degeneration of the NMJ termed endplate myopathy. Endplate myopathy consists of a combination of ultrastructural abnormalities, including degenerating subsynaptic nuclei, mitochondria, and postsynaptic folds, caused by localized cation overload through mutant AChRs. Because some of these changes resemble those seen in programmed cell death, we evaluated SCS muscle for evidence of focal activation of apoptotic pathways. Using antisera specific for the activated forms of caspases, the family of cysteine proteases that underlies apoptosis, we demonstrated that active forms of initiator and effector caspases are selectively localized at the NMJ in SCS. In comparison with an electron microscopic assessment of the abnormalities seen in endplate myopathy, we found that activated caspases were present at between 15 and 57% of endplates, similar to the proportion of endplates with degenerating mitochondria or vacuoles. This greatly exceeds the number of NMJs exhibiting nuclear degeneration. These findings provide the first evidence supporting the view that caspase activation in human disease can play a prominent role in localized cellular degenerative processes without causing nuclear or cell death.
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Affiliation(s)
- Bhupinder P S Vohra
- Departments of Neurology and Neuroscience, University of Minnesota, Minneapolis, MN 55455, USA
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Abstract
Nicotinic acetylcholine receptors (nAChRs) are expressed in muscle cells and neurons, as well as in an increasing number of other cell types. The nAChR channels are permeable to cations, including Ca(2+). Ca(2+) entry through nAChR channels has been shown to modulate several Ca(2+)-dependent cellular processes, such as neurotransmitter release, synaptic plasticity, and cell motility. The value of Ca(2+) permeability associated to a particular nAChR subtype thus represents an important indication for its physiological role. This review summarizes the quantitative data on Ca(2+) permeability obtained from several nAChR subtypes in native and heterologous systems. Different experimental approaches are compared, and the structural determinants of Ca(2+) permeability are discussed.
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Affiliation(s)
- Sergio Fucile
- Dipartimento di Fisiologia Umana e Farmacologia, Università di Roma La Sapienza, P.le Aldo Moro 5, I-00185 Rome, Italy.
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