RESUMEN
We have found that mutant acetylcholine receptor channels (AChRs) that cause slow-channel congenital myasthenic syndromes are activated by serum and that the high frequency of openings in serum is reduced by treatment with choline oxidase. Thus, slow-channel congenital myasthenic syndrome AChRs at the neuromuscular junction are likely to be activated both by steady exposure to serum choline and by transient exposure to synaptically released transmitter. Single-channel kinetic analyses indicate that the increased response to choline is caused by a reduced intrinsic stability of the closed channel. The results suggest that a mutation that destabilizes the inactive conformation of the AChR, together with the sustained exposure of endplates to serum choline, results in continuous channel activity that contributes to the pathophysiology of the disease.
Asunto(s)
Colina/sangre , Mutación , Miastenia Gravis/genética , Receptores Colinérgicos/genética , Receptores Colinérgicos/fisiología , Animales , Línea Celular , Humanos , Riñón , Potenciales de la Membrana , Ratones , Miastenia Gravis/congénito , Miastenia Gravis/fisiopatología , Técnicas de Placa-Clamp , Mutación Puntual , Síndrome , TransfecciónRESUMEN
Congenital myasthenic syndromes (CMS) can arise from presynaptic, synaptic, or postsynaptic defects. Mutations of the acetylcholine receptor (AChR) that increase or decrease the synaptic response to acetylcholine (ACh) are a common cause of the postsynaptic CMS. An increased response occurs in the slow-channel syndromes. Here, dominant mutations in different AChR subunits and in different domains of the subunits prolong the activation episodes of AChR by either delaying channel closure or increasing the affinity of AChR for ACh. A decreased synaptic response to ACh occurs with recessive, loss-of-function mutations. Missense mutations in the low-affinity, fast-channel syndrome and in a disorder associated with mode-switching kinetics of AChR result in brief activation episodes and reduce the probability of channel opening. Mutations causing premature termination of the translational chain or missense mutations preventing the assembly or glycosylation of AChR curtail the expression of AChR. These mutations are concentrated in the epsilon subunit, probably because substitution of the fetal gamma for the adult epsilon subunit can rescue humans from fatal null mutations in epsilon. Recent molecular genetic studies have also elucidated the pathogenesis of the CMS caused by absence of the asymmetric form of acetylcholinesterase from the synaptic basal lamina. Endplate acetylcholinesterase deficiency is now known to be caused by mutations in the collagenic tail subunit of the asymmetric enzyme that prevents the association of the collagenic tail subunit with the catalytic subunit or its insertion into the basal lamina.
Asunto(s)
Miastenia Gravis/congénito , Humanos , Mutación , Miastenia Gravis/genética , Miastenia Gravis/patología , Receptores Colinérgicos/deficiencia , Receptores Colinérgicos/genética , Sinapsis/patología , SíndromeAsunto(s)
Miastenia Gravis/congénito , 4-Aminopiridina/análogos & derivados , 4-Aminopiridina/uso terapéutico , Adulto , Amifampridina , Estimulantes del Sistema Nervioso Central/uso terapéutico , Niño , Inhibidores de la Colinesterasa , Edrofonio , Inhibidores Enzimáticos/uso terapéutico , Efedrina/uso terapéutico , Humanos , Lactante , Recién Nacido , Masculino , Miastenia Gravis/diagnóstico , Miastenia Gravis/terapia , Canales de Potasio , Bromuro de Piridostigmina/uso terapéutico , Quinidina/uso terapéutico , Síndrome , TimectomíaRESUMEN
Four, three- to four-week-old Brahman calves, with a common sire and a common maternal grandsire, developed progressive muscular weakness which was exacerbated by exercise and improved with rest. Although the calves remained alert and continued sucking, their neuromuscular condition deteriorated and they had to be euthanased within four to six weeks. The one calf in which an attempt was made to diagnose the condition responded quickly, but transiently, to the intravenous administration of edrophonium chloride and for a longer period to oral pyridostigmine bromide. Repeated stimulation of a peripheral nerve resulted in a decrease in the amplitude of the evoked muscle action potential. A definitive diagnosis could not be made but the condition showed many similarities to congenital myasthenia gravis, which has not previously been described in cattle.
Asunto(s)
Enfermedades de los Bovinos/congénito , Miastenia Gravis/veterinaria , Animales , Bovinos , Femenino , Miastenia Gravis/congénitoRESUMEN
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.
Asunto(s)
Mutación , Miastenia Gravis/congénito , Miastenia Gravis/genética , Estructura Secundaria de Proteína , Receptores Colinérgicos/genética , Mutación del Sistema de Lectura , Humanos , Modelos Moleculares , Miastenia Gravis/fisiopatología , Receptores Colinérgicos/biosíntesis , Receptores Colinérgicos/química , Eliminación de Secuencia , SíndromeAsunto(s)
Mutación , Miastenia Gravis/genética , Miastenia Gravis/fisiopatología , Receptores Colinérgicos/genética , Potenciales Evocados , Mutación del Sistema de Lectura , Genes Recesivos , Humanos , Modelos Moleculares , Placa Motora/fisiología , Placa Motora/fisiopatología , Miastenia Gravis/congénito , Técnicas de Placa-Clamp , Mutación Puntual , Conformación Proteica , Receptores Colinérgicos/química , SíndromeAsunto(s)
Cromosomas Humanos Par 17 , Miastenia Gravis/genética , Miastenia Gravis/fisiopatología , Adolescente , Adulto , Niño , Preescolar , Mapeo Cromosómico , Electrodiagnóstico , Femenino , Genes Dominantes , Genes Recesivos , Ligamiento Genético , Variación Genética , Humanos , Escala de Lod , Masculino , Repeticiones de Microsatélite , Persona de Mediana Edad , Miastenia Gravis/congénito , Linaje , Receptores Colinérgicos/deficiencia , Receptores Colinérgicos/genética , SíndromeAsunto(s)
Placa Motora/fisiopatología , Músculo Esquelético/fisiopatología , Miastenia Gravis/genética , Miastenia Gravis/fisiopatología , Receptores Colinérgicos/fisiología , Potenciales de Acción , Animales , Bungarotoxinas/metabolismo , Electromiografía , Humanos , Ratones , Ratones Transgénicos , Placa Motora/patología , Músculo Esquelético/patología , Miastenia Gravis/congénito , Unión Neuromuscular/fisiopatología , Técnicas de Placa-Clamp , Nervio Frénico/fisiopatología , Receptores Colinérgicos/genética , SíndromeAsunto(s)
Placa Motora/fisiopatología , Mutación , Miastenia Gravis/genética , Receptores Colinérgicos/genética , Secuencia de Aminoácidos , Potenciales Evocados , Exones , Femenino , Humanos , Sustancias Macromoleculares , Masculino , Fibras Musculares Esqueléticas/patología , Músculo Esquelético/patología , Miastenia Gravis/congénito , Miastenia Gravis/fisiopatología , Núcleo Familiar , Técnicas de Placa-Clamp , Linaje , Receptores Colinérgicos/química , Receptores Colinérgicos/deficienciaAsunto(s)
Miastenia Gravis/genética , Receptores Colinérgicos/genética , Eliminación de Secuencia , Secuencia de Bases , Exones , Femenino , Humanos , Sustancias Macromoleculares , Masculino , Repeticiones de Microsatélite , Miastenia Gravis/congénito , Miastenia Gravis/fisiopatología , Linaje , Receptores Colinérgicos/química , Receptores Colinérgicos/deficiencia , SíndromeAsunto(s)
Empalme Alternativo , Mutación del Sistema de Lectura , Miastenia Gravis/genética , Receptores Colinérgicos/genética , Adolescente , Adulto , Niño , Femenino , Humanos , Sustancias Macromoleculares , Masculino , Modelos Moleculares , Miastenia Gravis/congénito , Núcleo Familiar , Linaje , Conformación Proteica , Receptores Colinérgicos/deficiencia , Receptores Colinérgicos/metabolismo , Síndrome , TurquíaAsunto(s)
Antagonistas Colinérgicos/farmacología , Miastenia Gravis/fisiopatología , Quinidina/farmacología , Receptores Colinérgicos/fisiología , Línea Celular , Humanos , Activación del Canal Iónico/efectos de los fármacos , Activación del Canal Iónico/fisiología , Miastenia Gravis/congénito , Mutación Puntual , Receptores Colinérgicos/química , Receptores Colinérgicos/efectos de los fármacos , Proteínas Recombinantes/efectos de los fármacos , Proteínas Recombinantes/metabolismo , Síndrome , TransfecciónRESUMEN
The slow-channel congenital myasthenic syndrome (SCCMS) is caused by gain of function mutations in subunits of the end-plate acetylcholine receptor (AChR). The mutations prolong the opening episodes of the AChR channel, leading to a depolarization block and an end-plate myopathy. Because levels of quinidine sulfate attainable in clinical practice shorten the opening episodes of genetically engineered mutant SCCMS receptors in vitro, we tested the notion that the drug can be of benefit in SCCMS. We treated 6 SCCMS patients with quinidine sulfate in an open-label trial, using objective clinical measures of muscle strength and repetitive stimulation studies as end points. One patient became allergic to quinidine after 7 days. The remaining patients tolerated the drug well and after 30 days of continuous therapy showed statistically significant improvement in muscle strength and in decrement of the compound muscle action potential elicited by rapid rates of stimulation.
Asunto(s)
Miastenia Gravis/tratamiento farmacológico , Quinidina/uso terapéutico , Potenciales de Acción/efectos de los fármacos , Adolescente , Adulto , Niño , Estimulación Eléctrica , Femenino , Humanos , Masculino , Nervio Mediano/efectos de los fármacos , Nervio Mediano/fisiopatología , Persona de Mediana Edad , Músculo Esquelético/efectos de los fármacos , Músculo Esquelético/fisiopatología , Miastenia Gravis/congénito , Miastenia Gravis/genética , Estudios Prospectivos , Receptores Colinérgicos/efectos de los fármacos , Receptores Colinérgicos/genética , Receptores Colinérgicos/fisiología , Proteínas Recombinantes/efectos de los fármacos , Proteínas Recombinantes/metabolismo , SíndromeRESUMEN
We studied two families with five affected members suffering from ptosis and slowly progressive limb-girdle muscle weakness. All patients had abnormal decremental response on low-frequency nerve stimulation, but there were no repetitive responses to single stimuli. The patients improved on anti-acetylcholinesterase drugs. Intercostal muscle was obtained for special studies from one patient of each family. In vitro microelectrode studies were done in Patient 1. Miniature end-plate potentials were of low amplitude, and the quantal content of the evoked end-plate potentials was normal. Light microscopy revealed a marked type 1 fiber predominance. Acetylcholinesterase reactivity was dispersed over increased length of individual fibers in Patient 2. On morphometry of the end-plate ultrastructure, the number of secondary synaptic clefts per neuromuscular junction and the expansion of the postsynaptic area were markedly reduced. In Patient 1, but not in Patient 2, the envelopment of the nerve terminal by Schwann cell was increased. Acetylcholine-receptor (AChR) density was reduced as judged by the reduced immunoreactivity to antibodies against different receptor subunits. Immunohistochemical analysis of proteins known to be involved in orchestrating the end-plate structure showed deficiency of the AChR-associated protein utrophin. These patients appear to have a defect in the development or maintenance of the postsynaptic clefts; whether this defect results from or causes a reduced expression of utrophin or AChR is unclear.
Asunto(s)
Proteínas del Citoesqueleto/deficiencia , Proteínas de la Membrana/deficiencia , Placa Motora/química , Miastenia Gravis/congénito , Miastenia Gravis/genética , Receptores Colinérgicos/deficiencia , Adulto , Animales , Proteínas del Citoesqueleto/análisis , Proteínas del Citoesqueleto/genética , Femenino , Humanos , Masculino , Proteínas de la Membrana/análisis , Proteínas de la Membrana/genética , Ratones , Ratones Noqueados , Microscopía Electrónica , Placa Motora/ultraestructura , Miastenia Gravis/patología , Linaje , Receptores Colinérgicos/análisis , Receptores Colinérgicos/genética , Vesículas Sinápticas/ultraestructura , UtrofinaRESUMEN
Nicotinic acetylcholine receptors (AChRs) are a family of acetylcholine-gated cation channels that form the predominant excitatory neurotransmitter receptors on muscles and nerves in the peripheral nervous system. AChRs are also expressed on neurons in lower amounts throughout the central nervous system. AChRs are even being reported on unexpected cell types such as keratinocytes. Structures of these AChRs are being determined with increasing precision, but functions of some orphan subunits are just beginning to be established. Functional roles for postsynaptic AChRs in muscle are well known, but in neurons the post-, peri-, extra-, and presynaptic roles of AChRs are just being revealed. Pathogenic roles of AChRs are being discovered in many diseases involving mechanisms ranging from mutations, to autoimmune responses, to the unknown; involving cell types ranging from muscles, to neurons, to keratinocytes; and involving signs and symptoms ranging from muscle weakness to epilepsy, to neurodegenerative disease, to psychiatric disease, to nicotine addiction. Awareness of AChR involvement in some of these diseases has provoked new interests in development of therapeutic agonists for specific AChR subtypes and the use of expressed cloned AChR subunits as possible immunotherapeutic agents. Highlights of recent developments in these areas will be briefly reviewed.
Asunto(s)
Acetilcolina/fisiología , Receptores Nicotínicos/fisiología , Enfermedad de Alzheimer/metabolismo , Anestésicos/farmacología , Animales , Enfermedades Autoinmunes/metabolismo , Colinérgicos/farmacología , Antagonistas Colinérgicos/farmacología , Epilepsia/genética , Epilepsia/metabolismo , Etanol/farmacología , Femenino , Queratinocitos/metabolismo , Masculino , Modelos Moleculares , Proteínas Musculares/química , Proteínas Musculares/clasificación , Proteínas Musculares/efectos de los fármacos , Proteínas Musculares/genética , Proteínas Musculares/fisiología , Miastenia Gravis/congénito , Miastenia Gravis/metabolismo , Proteínas del Tejido Nervioso/química , Proteínas del Tejido Nervioso/clasificación , Proteínas del Tejido Nervioso/efectos de los fármacos , Proteínas del Tejido Nervioso/genética , Proteínas del Tejido Nervioso/fisiología , Unión Neuromuscular/fisiología , Nicotina/farmacología , Nicotina/uso terapéutico , Enfermedad de Parkinson/metabolismo , Embarazo , Conformación Proteica , Receptores Nicotínicos/química , Receptores Nicotínicos/clasificación , Receptores Nicotínicos/efectos de los fármacos , Receptores Nicotínicos/genética , Esquizofrenia/metabolismo , Fumar/metabolismo , Torpedo , Síndrome de Tourette/tratamiento farmacológico , Síndrome de Tourette/metabolismoRESUMEN
We describe and functionally characterize six mutations of the acetylcholine receptor (AChR) epsilon subunit gene in three congenital myasthenic syndrome patients. Endplate studies demonstrated severe endplate AChR deficiency, dispersed endplate regions and well preserved junctional folds in all three patients. Electrophysiologic studies were consistent with expression of the fetal gamma-AChR at the endplates in one patient, prolongation of some channel events in another and gamma-AChR expression as well as some shorter than normal channel events in still another. Genetic analysis revealed two recessive and heteroallelic epsilon subunit gene mutations in each patient. One mutation in each (epsilonC190T [epsilon R64X], epsilon 127ins5 and epsilon 553del 7) generates a nonsense codon that predicts truncation of the epsilon subunit in its N-terminal, extracellular domain; and one mutation in each generates a missense codon (epsilon R147L, epsilon P245L and epsilon R311W). None of the mutations was detected in 100 controls. Expression studies in HEK cells indicate that the three nonsense mutations are null mutations and that surface expression of AChRs harboring the missense mutations is significantly reduced. Kinetic analysis of AChRs harboring the missense mutations show that epsilon R147L is kinetically benign, epsilon P245L prolongs burst open duration 2-fold by slowing the rate of channel closing and epsilon R311W shortens burst duration 2-fold by slowing the rate of channel opening and speeding the rate of ACh dissociation. The modest changes in activation kinetics are probably overshadowed by reduced expression of the missense mutations. The consequences of the endplate AChR deficiency are mitigated by persistent expression of gamma-AChR, changes in the release of transmitter quanta and appearance of multiple endplate regions on the muscle fiber.