Decoding pathogenesis of slow-channel congenital myasthenic syndromes using recombinant expression and mice models.
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. [P R Health Sci J 2010;1:4-17]
Key words: Acetylcholine receptor, congenital myasthenic syndromes, slow-channel congenital myasthenic syndromes
Si bien es considerada como una de las enfermedades huerfanas, los sindromes de miastenia congenitas en los casos mas benignos somete a aquellos que lo padecen a debilidades musculares, mientras que en los casos mas severos puede causar desgastes musculares progresivos que sentencian a los pacientes a permanecer en silla de ruedas o, incluso, a enfrentar la muerte. En comparacion con otras enfermedades neurologicas mas comunes, los sindromes de miastenia congenita son poco frecuentes. Sin embargo, investigaciones exhaustivas sobre la misma son llevadas a cabo en laboratorios como el nuestro. Entre los diversos desordenes neuromusculares de miastenia congenita, nos enfocamos en el sindrome de miastenia congenita del canal lento (SCS por sus siglas en ingles), el cual es causado por mutaciones en genes que codifican a las diferentes subunidades del receptor de acetilcolina. El estudio del SCS ha evolucionado de estudios clinicos electrofisiologicos a sistemas de expresion in vitro y modelos de ratones transgenicos. El presente resumen evalua los enfoques metodologicos que son comunmente empleados para determinar los danos sinapticos en los SCS y ademas proveer perspectivas para nuevos enfoques. Entre las metodologias electrofisiologicas tipicamente empleadas por medicos para diagnosticar pacientes se incluyen la electromiografia, mientras que muestras musculares del paciente son utilizadas para evaluaciones intracelulares, grabaciones de datos unitarios y experimentos de enlazamiento de toxinas. Sistemas de expresion in vitro permiten el estudio de una mutacion en particular sin la necesidad de intervenir con el paciente. De la misma forma, estos sistemas in vitro han sido implicados usualmente en el desarrollo de estrategias terapeuticas, tales como los tratamientos basados en quinidina y fluoxetinas y, mas recientemente, en interferencia de ARN. Un avance importante en el estudio del SCS ha sido el desarrollo de ratones transgenicos que llevan consigo las mutaciones que causan el SCS. Estos modelos de ratones transgenicos han sido una pieza clave en la elucidacion de la patogenesis de las mutaciones del SCS por conectar los receptores IP-3 a la sobrecarga de calcio en la placa motora, ademas de las caspasas y calpeinas a las caracteristicas patogenicas del SCS, conocidas como las miopatias de la placa motora. Finalmente, se resume nuestra experiencia con pacientes en los cuales se sospecha SCS desde una perspectiva local y comentamos sobre un aspecto de la contribucion de nuestro grupo al estudio del SCS.
|Article Type:||Perspectiva general de la enfermedad/trastorno|
Ratones como animales de laboratorio
Ratones como animales de laboratorio (Analisis de casos)
Otero-Cruz, Jose David
Baez-Pagan, Carlos Alberto
Grajales-Reyes, Gary Emanuel
Ramirez-Ordonez, Rosaura Teresa
Luciano, Carlos A.
Gomez, Christopher Manuel
Lasalde-Dominicci, Jose Antonio
|Publication:||Name: Puerto Rico Health Sciences Journal Publisher: Universidad de Puerto Rico, Recinto de Ciencias Medicas Audience: Academic Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2010 Universidad de Puerto Rico, Recinto de Ciencias Medicas ISSN: 0738-0658|
|Issue:||Date: March, 2010 Source Volume: 29 Source Issue: 1|
|Geographic:||Geographic Name: Puerto Rico|
Congenital myasthenic syndromes (CMS) are a group of orphan
disorders of neuromuscular transmission characterized by muscle weakness
and fatigability in the axial and limb muscles (with hypotonia in
early-onset forms), the ocular muscles (leading to ptosis and
ophthalmoplegia), and the facial and bulbar musculature (affecting
sucking and swallowing, and leading to dysphonia). The severity and
course of the disease is highly variable, ranging from minor symptoms to
progressive disabling weakness. CMS is due to a defect in an essential
component of the transmission at the neuromuscular junction (NMJ) (1-3).
Investigation of CMS has provided insights that have greatly enhanced
our understanding of the development, maintenance, and function of the
NMJ. The development of CMS animal models as a complementary approach to
investigate the mechanisms for endplate degeneration in the
aforementioned syndrome has confirmed the value of transgenic animal
models in studying NMJ disorders. Understanding the molecular mechanisms
leading to the pathogenesis of different forms of CMS is essential to
develop new strategies for therapeutic intervention.
The study of CMS has been facilitated by the accessibility of the NMJ and the sophisticated techniques available for its study. Clinical electrophysiological studies have demonstrated the impairment of neuromuscular transmission in CMS in vivo. Pre- or post-synaptic defect localization have been determined by in vitro electrophysiological recordings of live biopsied muscle (4). An increasing number of known molecular targets in the NMJ have identified the genetic lesion and the type of CMS by ultrastructural and cytochemical studies, bungarotoxin-binding, molecular genetic test, and antibody binding assays (1, 3).
Many of the CMS cases reported are developmental disorders, resulting from absence or loss of function of a protein critical for NMJ development (2). These disorders are recessively inherited, usually non-progressive and result in fatigability and weakness, impaired skeletal muscle development and sometimes congenital joint and skeletal deformities. In most cases of CMS, the NMJ is frequently under-developed, providing a relatively forthright basis for the weakness. The exception to this is the slow-channel congenital myasthenic syndrome (SCS) because it increases the synaptic response to acetylcholine neurotransmitter. SCS is the focus of this review. SCS is usually an autosomal dominant inherited disorder caused by mutation in the genes encoding for the alphal, betal, delta, or epsilon subunit of the acetylcholine receptor (Table 1). The nature of the progressive weakness, impaired neuromuscular transmission, and degeneration of the NMJ in SCS has proven to be a complex interplay between developmental and degenerative processes acting at pre- and post-synaptic sites (5). The characterization of each of these processes may shed some light on the mechanisms governing both synaptic plasticity and synaptic disease. Several independent laboratories are working to elucidate the pathogenesis mechanisms and therapeutic strategies for the progressive synaptic degeneration seen in SCS.
Acetylcholine receptor structure and function
A principal component of the NMJ is the acetylcholine receptor (AChR), a ligand-gated ion channel consisting of homologous subunits in the stoichiometry of 2a1(?1Se in the adult form or 2[[alpha].sub.1][[beta].sub.1][delta][epsilon] in the fetal form. The AChR belongs to the superfamily of the ligand-gated ion channel receptors that are of fundamental importance in the chemical synaptic transmission throughout the nervous system (6-8). The AChR is composed of four homologous membrane-spanning subunits forming a cation-selective ion channel (9). Each subunit contains a large hydrophilic extracellular N-terminus, four putative transmembrane domains (M1-M4), a large cytoplasmic loop between the M3 and M4, and a short extracellular C-terminus (10). The Ml and M2 domains form the ion channel pore (11-12), whereas the M3 and M4 domains have the largest contact with the membrane lipids (13-14) and are distant from the ion channel pore and the ligand-binding sites (Figure 1). The AChR at the endplate (EP) of NMJ is activated by binding of the chemical neurotransmitter acetylcholine (ACh) released by the motor neuron. This produces an endplate potential (EPP) that spreads across the surface of the muscle fiber into the transverse tubules, eliciting the release of calcium from the sarcoplasmic reticulum, thus initiating muscle contraction (15).
Slow-Channel Congenital Myasthenic Syndrome
SCS, usually an autosomal dominant disorder of postsynaptic CMS, is caused by a kinetic abnormality of the AChR which comprises the healthy safety margin of neuromuscular transmission. SCS is characterized by the slow decay of endplate currents (Figure 2), destabilization of the closed channel state, prolonged activation episodes of AChR, and an endplate myopathy caused by cationic overloading of the muscle fibers. The precise diagnosis of SCS requires a multidisciplinary team comprising neurologists, geneticists, biochemists, and electrophysiologists to correlate the clinical phenotypes with in vitro electrophysiological, morphological, and molecular genetic studies.
Unlike other CMS, SCS tipically shows an autosomal dominant inheritance (Table 2). The clinical phenotype of SCS is variable as patients may express a severe early onset to moderate late onset (5, 16). In most cases, SCS patients present a severe weakness of the cervical, scapular, wrist, and finger extensor muscles (17). However, in selected cases ptosis, ophthalmoparesis, dysarthria, dysphagia, proximal limb weakness, and respiratory insufficiency may also occur.
[FIGURE 1 OMITTED]
SCS is etiologically produced by mutations in the genes encoding AChR subunits that cause a kinetic abnormality of the AChR (18-19). The SCS mutations produce an increase in the channel opening rate, a decrease in the channel closing rate, or an increase in the affinity of ACh for the AChR, resulting in the stabilization of the open state or the destabilization of the closed state of the AChR (18, 20-21) (Figure 3). The net effect of the mutation is a gain of function that abnormally prolongs channel opening events, slows the decay of endplate currents and potentials, and permits the cationic overload of the synaptic region ofthe muscle fiber. The prolonged opening episodes of the AChR channel lead to: 1) a slow decay of the synaptic potential, eventually causing a depolarization block, 2) evoke a repetitive compound muscle action potential (CMAP) caused by the EPP outlasting the absolute refractory period of the muscle fiber, 3) a cationic overload of the muscle endplate mainly co ntributed by the excessive accumulation of [Ca.sup.2+], and 4) an endplate myopathy characterized by degeneration of the junctional folds and static weakness (5, 19, 21-22). The endplate myopathy is mainly attributed to the excessive accumulation of [Ca.sup.2+] in muscle fiber and is characterized by the ultrastructure remodeling of the endplate, which consists in the widening of the synaptic space, calcium deposits, destruction of the junctional folds, vacuolizations, and tubular aggregates.
Diagnosis of SCS should be considered in any person that has been considered for myasthenia gravis (MG). Although there is a rare autoimmune form of the disease, SCS patients typically have no serum anti-AChR antibodies (23). The presence of a positive family history for autosomal dominant inheritance of a myasthenic syndrome should strongly suggest a SCS. In these patients and those with sporadic disease, the presence of repetitive CMAPs evoked by single nerve stimuli, particularly with decremental responses to repetitive stimulation in small muscles of the upper limb, strongly suggest the disease. Anti-cholinesterase toxicity and the condition of congenital endplate acetylcholinesterase must be excluded. In vitro electrophysiological studies, as described above, definitively establish the diagnosis. Genetic diagnosis, required to identify the responsible mutation, is available only on a research basis.
Diagnosis of SCS can be challenging for several reasons. The wide range in clinical presentation from severe congenital weakness to subtle fatigability noted in the fifth decade means that there is no classical presentation. Patients with SCS have been mistaken for having a variety of other disorders including MG, mitochondrial myopathy, and oculopharyngeal dystrophy. Difficulties relating predominantly to muscle sampling are sometimes encountered in eliciting repetitive CMAPs; the classic feature suggesting prolonged action of ACh. Both electrophysiological studies of biopsied muscle and molecular genetic testing are only available on a research basis.
At present, SCS patients are treated with quinidine sulfate and fluoxetine hydrochloride (24-31). Both therapeutic medications are long-lived open-channel blockers of AChR that shorten the duration of channel opening events in a concentration dependent manner. Quinidine is orally administrated at a dose of200 mg two to three times daily for one week in adults. After the first week, the dose is gradually increased to maintain a serum level of 1.0 - 2.5 [micro]g/mL (3.0 - 7.5 [micro]M/L) (27-28). In children, quinidine is administrated at a dose of 15 - 60 mg/ kg/day in four to six divided doses. Fluoxetine, an alternative drug to treat SCS patients allergic to quinidine, is less prescribed and less effective than quinidine, but is eliminated more slowly, thus providing a more sustained serum levels (29). Fluoxetine is also orally administrated until a total daily dose of 80 to 120 mg (serum fluoxetine + norfluoxetine levels 8 to 11 [micro]M/L) in adults (29). However, the maximal dose of fluoxetine for children has not been established hitherto. The therapy progress requires monitoring of routine laboratory analysis, physical, and electrophysiological examination (31). The physical evaluation includes measures of the arm elevation time, the number of times a patient can rise from squatting or from a low stool, the degree of eyelid ptosis, the number of steps they can climb or the distance they can walk before having to rest, the degree of weakness of selected muscles, maximal inspiratory and expiratory pressures, and maximal expiratory flow (27-28). The needle electromyographic (EMG) analysis includes measures of amplitude and area of the motor unit action potentials and CMAP (5).
In Vitro Expression Studies
The identification of a SCS is a clinical process. Physicians evaluate symptoms such as ocular and other muscle weakness, perform tests as EMGs, and check for anti-AChR antibodies. If anti-AChRs antibodies are not detected, congenital myasthenic syndromes are often suspected. On a research basis, a genetic analysis is then required to test this possibility. In addition, intracellular recordings as those required to probe miniature endplate potentials (MEPP), miniature endplate currents (MEPC), endplate potentials (EPP) and endplate currents (EPC) are performed directly on patient muscle biopsies. Patch clamp experiments to determine single-channel kinetics and conductance may also be performed on isolated muscle fibers from patients. Nevertheless, in vitro expression studies, more often than not, follow the clinical and ex vivo studies. These studies include the expression of recombinant AChR engineered to contain the mutation of interest in a suitable expression system such as Xenopus laevis oocytes and Human Embryonic Kidney293 cells (HEK 293 cells). The characterization of AChR SCS mutations is facilitated by in vitro expression studies as the number ofinvolved researchers is not limited by access to patients and because X. laevis oocytes and HEK 293 cells are amenable to patch clamping and toxin binding assays among other commonly employed techniques. An additional advantage of the in vitro studies is that they have facilitated the development of strategies to selectively silence the AChR mutations that cause SCS in an allele-specific manner through RNA interference. In the following paragraphs, a set of the methodological approaches most frequently followed to study SCS using in vitro expression systems and the more recent efforts to selectively silence pathogenic AChR mutations are briefly discussed in the context of the progress that has been achieved.
[FIGURE 2 OMITTED]
AChR mutations that cause SCS are characterized by prolonged openings and as such, a direct approach for their identification is through the use of the patch clamp technique. Developed in 1976 by German scientists Erwin Neher and Bert Sakmann (32), the patch clamp technique enables the researcher to probe the activity of individual ion channels and to interpret the single-channel data in terms of mechanism. Indeed, not too long after the development of the technique, the necessary theory to interpret single-channel events in terms of functional mechanisms began to arise by David Colquhoun and Alan G. Hawkes (33-34). Soon after developing the patch clamp technique, Neher and Sakmann realized its full potential and in 1981 released a paper describing several patch clamp configuration, all of which are frequently used up to these days (35). Among these, the cell-attached configurations has been commonly used to study SCS as open times are readily measured (18-20, 36-43). Ed win Neher and Bert Sachmann were awarded the Nobel Prize in Physiology or Medicine in 1991 for "their discoveries concerning the function of single ion channels in cells".
In the mid 1990's, an increasing number of AChR mutations began to emerge as cause for SCS. Among these, the [epsilon]T264P AChR mutation was identified by Andrew G. Engel's group in a twenty-year-old woman suffering from myasthenia syndromes since birth (18). Patch clamp experiments on intercostals muscle fibers revealed markedly prolonged channel openings; an observation later confirmed during single-channel recordings in HEK 293 cells expressing the [epsilon]T264P AChR mutation. Indeed, while both single-channel recordings in muscle fibers and HEK 293 cells revealed prolonged open times, the mean duration did result shorter in the patient than in the expression system. Furthermore, the amplitudes of the single-channel events in the patient were lower as well. However, as the [epsilon]T264P was engineered in the mouse AChR rather than human, minor discrepancies in the data could be explained by differences in species (18). Interestingly, while recording single-channel activity in the absence of ACh, the authors also found that the [epsilon]T264P expressed in HEK 293 cells displayed an unusually high rate of spontaneous openings (18).
Another AChR mutation identified in a patient displaying myasthenic symptoms and characterized through patch clamp experiments in transfected HEK 293 cells is the [alpha]G153S (20). Similar to the [epsilon]T264P mutation, the aG153S was engineered into the mouse AChR and displayed prolonged openings as determined by single-channel recordings. Furthermore, single-channel events evoked by various ACh concentrations were fitted into a kinetic scheme to estimate rate constants thus revealing a remarkable decrease in the rate of agonist dissociation when compared to the wild-type (WT) AChR. Such a decrease in the rate of agonist dissociation would cause the ion channel to open repeatedly during ACh occupancy (20). The prolonged single-channel activations episodes displayed by the [alpha]N217K AChR mutation expressed in HEK 293 cells were also shown to result from a decrease in the rate of agonist dissociation (38). The [alpha]N217K had been previously shown to elicit prolonged openings through single-channel recordings along with two other AChR mutations identified in patients presenting myasthenic syndromes, namely [epsilon]L269F and [beta]V266M (21). The [delta]Q267E AChR mutation was also studied by Engel et al., 1996; however, it did not evoke prolonged openings (21). Additional AChR mutations expressed in HEK 293 cells and characterized using the patch clamp technique include the [alpha]V249F, which showed prolonged openings and increased rate of spontaneous openings (19), and two mutations along the e subunit, namely [epsilon]L78P and [epsilon]L221F, both of which revealed significant increases in the time constant of the longest component of the burst length distributions as compared to the WT (40).
[FIGURE 3 OMITTED]
Acetylcholine receptor mutations that cause SCS are located along transmembrane domains M1 and M2, the M2-M3 linker, and the N-terminal (Table 2). However, a mutation located in M4 was recently identified by Andrew Engel's group (42). In this paper, the authors report a 24-year-old patient displaying bilateral lid ptosis, fatigable weakness of the lower extremities, high-arched palate, pectus carinatum, moderate limitation of eye movements, weak orbicularis oculi muscles, and selectively severe distal upper limb and extensor hallucis longus weakness. Furthermore, CMAP showed decremental response upon repetitive nerve stimulation. Repetitive action potentials were also elicited by a single stimulation. The patient did not present anti-AChR antibodies thus ruling out MG (42). Upon genetic testing, a cysteine to tryptophan substitution was identified in codon 418 of the a subunit ([alpha]C418W). The [alpha]C418W AChR mutation was engineered into the human a subunit cDNA, coexpressed with the remaining subunits in HEK 293 cells and patch-clamp experiments were performed, thus revealing an increase in the mean duration of the longest open component and bursts (42). An in-depth single-channel analysis of single-channel currents elicited by the weak-agonist choline also showed a fourfold increase the rate constant of channel opening and a sixfold decrease in the channel closing rate (42).
As indicated above, X. laevis oocytes have also been employed for in vitro expression studies and, as HEK 293 cells, oocytes are amenable for single-channel recordings provided the vitelline membrane is removed before attempting to form gigaseals. Indeed, patch clamp recordings of the ([beta]L263M mutation identified in a 32-year-old patient and expressed in X. laevis oocytes revealed an eightfold increase in channel open time (36). The [alpha]G153S mutation previously identified in a patient and characterized in vitro using HEK 293 cells (20), was also studied through single-channel recordings in the X. laevis oocyte expression system (37) thus confirming the previously observed prolonged openings. Three novel SCS mutations were also described in the aforementioned publication (37), namely [alpha]V156M, [alpha]T254I, and [alpha]S269I, all of which displayed prolonged channel activations, although a detailed kinetic analysis of the single-channel data was not carried out and thus rate constants were not estimated. Nevertheless, differences in the single-channel events within bursts evoked by the [alpha]G153S, [alpha]V156M, [alpha]T254I, and [alpha]S269I AChR mutations suggested different mechanisms responsible for the long duration of bursts (37). Besides, the [delta]S268F and [beta]V229F SCS mutations were also characterized using the X. laevis oocyte expression system by means of patch clamp recordings (39, 41). Single-channel currents evoked by the [delta]S268F mutation revealed a more than sevenfold increase in open time while displaying a similar single-channel conductance as the WT (39) whereas an increased open time constant and burst durations where displayed by the [beta]V229F and similar current amplitudes as the WT (41). Single-channel recordings performed at high ACh concentrations also revealed a decreased closing rate and increased effective opening rate in the SCS mutation [beta]229F (41).
Toxin binding assays
Toxin binding assays are used as means to estimate the cell-surface expression of the AChR in a given expression system. As [alpha]-bungarotoxin ([alpha]-BgTx) tightly binds AChR (44), exposing cells expressing the AChR to [[sup.125]I]-labeled [alpha]-BgTx ([[sup.125]I][alpha]-BgTx) and probing the remaining radioactivity upon extensive washing, constitutes an effective and sensitive methodological approach to quantify the cell-surface expression levels of the AChR. For instance, the surface expression ofthe [epsilon]T264P AChR mutation engineered into the mouse AChR and expressed in HEK 293 cells was as estimated as ranging from 40% to 80% that of the WT receptor (18). Unlike the [epsilon]T264P AChR mutation, the [epsilon]L78P and [epsilon]L221F mutations expressed in HEK 293 cells and the [beta]V229F mutation expressed in X. laevis oocytes did not exhibit reduced surface expression levels as determined by [[sup.125]I] [alpha]-BgTx binding assays, thus suggesting their normal assembly and oligomerization (40-41). These results also led the authors to conclude that the phenotype resulting from these AChR mutations probably results from altered AChR function rather than number (40). The [alpha]C418W AChR expressed in HEK 293 cells was also studied by [[sup.125]I][alpha]-BgTx binding assays (42). Indeed, these experiments showed that the surface expression of the mutant AChR was reduced to 70% of the WT. Furthermore, toxin binding assays, among other methodological approaches, were used to measure the downregulation ofthe AChR brought upon by short interference RNA (42).
As described above, Shen et al. characterized the [alpha]C418W AChR mutation expressed in HEK 293 cells through single-channel recordings and toxin binding assays. In addition, the authors employed a relatively new technique founded on the recently discovered post-transcriptional gene silencing by double-stranded RNA (dsRNA) known as RNA interference (RNAi) (45). RNAi is a gene-silencing mechanism initiated with the cleavage of dsRNA by the Dicer complex into 21-23 base pair fragments called short interfering RNA (siRNA) which upon being incorporated into RISC protein, are unwound and used as template to recognize the target mRNA. Depending on complementarity, this may result in the cleavage of the target mRNA. Cleaved mRNA is recognized as an intruder and degraded by a nuclease thus preventing its translation (46). Andrew Z. Fire and Craig C. Mello received the Nobel Prize in Physiology or Medicine, in 2006 for their discovery of RNAi.
The authors used this novel technology to selectively silence the [alpha]C418W AChR expressed in HEK 293 cells. To this aim, siRNAs were designed and transfected together with WT or mutant [alpha] cDNA in addition to [beta], [epsilon], and [delta] cDNAs in a 2:1:1:1 ratio. The effect of siRNA on transcription and cell-surface expression was subsequently assessed by RT-PCR and toxin-binding experiments, respectively. Indeed, the authors were able to suppress the expression of the [alpha]C418W AChR to only 5% of its expression in HEK 293 cells not treated with siRNA, whereas the expression of the WT upon siRNA treatment remained at 84% of the non-treated control (42). Furthermore, the effect of siRNA on protein expression was corroborated using patch clamp experiments in the cell-attached configuration (35). For these experiments, a desensitizing concentration of ACh (10 [micro]M) was used to observe clusters of single-channel events separated by relatively long periods of inactivity. The intracluster events are assumed to be evoked by a single channel, and the fraction of time spent in the open state with respect to the length of the cluster is known as the open probability ([P.sub.open]) (47). The [P.sub.open] determined using HEK 293 cells expressing the WT or mutant AChR differed markedly and thus, the activity of both WT and [alpha]C418W AChRs co-expressed in HEK 293 cells could be monitored independently to assess the selective downregulation of the mutant AChR. Using this approach, the authors were able to confirm the results gathered through toxin binding and RT-PCR experiments, demonstrating the RNAi-mediated effective and selective downregulation of the mutant AChR over the WT (42).
In fact, this methodological approach had been developed previously to selectively silence the SCS AChR mutation [alpha]S226F expressed in HEK 293 cells (48). The underlying rationale behind this study, as in (42), is that as little as only one mismatch between the designed siRNA and the target sequence is sufficient to significantly reduce the silencing effect (49). Thus, on the basis of this discrimination, the cell-surface expression of mutant AChR could be reduced over the WT. By utilizing siRNA bearing a mismatch in position 10 to the WT sequence, the authors were able to reduce the surface expression of the [alpha]S226F AChR to 28% of normalized surface levels, whereas the WT expression was reduced to 71% of normalized surface levels as determined by radioactively-labeled toxin binding assays. Moreover, the authors also probed the short hairpin RNAs (shRNA) to selectively silence mutant AChR (48). Sequences were designed and, utilizing the reporter gene DsRed2 as a model system initially, they discovered that, by placing a nucleotide mismatch at position 9, the ability of the shRNA to silence the expression of the reporter gene was almost obliterated. Using this knowledge, the authors set out to selectively silence the SCS AChR mutation [alpha]S226F. To this aim, HEK 293 cells were co-transfected with either the WT or mutant a subunit cDNA in addition to [beta], [epsilon], and [delta] subunit cDNA and the shRNA molecules designed to selectively down-regulate the mutant AChR. Cell-surface AChR expression was quantified by radioactively-labeled toxin-binding assays and demonstrated that the expression of the [alpha]S226F was reduced to 38% of normalized surface levels, whereas in the WT the expression was only reduced to 83% of normalized surface levels. The selective down-regulation of the [alpha]S226F AChR by either siRNA or shRNA was confirmed qualitatively by immunofluorescence microscopy. These experiments demonstrated the feasibility of RNAi-based selective silencing of pathogenic mutant AChRs as a therapeutic approach to treat SCS.
Transgenic Mice Models
Not long after the first AChR mutations were identified in SCS patients, transgenic mice expressing AChR mutants were generated by microinjection of single-cell mouse embryos as models for SCS (50). The first of these transgenic mice expressed the [delta]S262T AChR mutation (50). While this particular mutation is not known to be a spontaneously occurring mutation; it served, nevertheless, as model for SCS. Indeed, the success of this pioneering work paved the way for the generation of transgenic mice models expressing SCS mutations identified in patients (22, 51-54) with particular emphasis on the [epsilon]L269F, which is among the first reported SCS mutations (21, 55). Methodological approaches employed on transgenic mice are not all that different to those employed on human muscle biopsies. In essence, intracellular recordings, single-channel recordings, toxin binding experiments, light microscopy, and electron microscopy, among others, are commonly employed. In addition, more recent articles have made use of additional methodological approaches to study the pathogenesis of the hallmark of SCS, namely endplate myopathy (52-54).
As stated herein, CMAPs typical of SCS show two distinctive features, namely repetitive muscle action potentials upon a single stimulus to nerve and a decremental response evoked by repetitive nerve stimulation (37, 39, 50). Thus far, the [epsilon]L269F-transgenic has been characterized in terms of these two parameters through hindlimb and forelimb recordings (22, 54) while the [delta]S262T-transgenic has been characterized in terms of repetitive muscle action potentials (50). For hindlimp recordings, animals were anesthetized and the sciatic nerve exposed to enable stimulation while responses from the gastrocnemius and intrinsic hindpaw muscles were recorded separately. Alternatively, forelimb recordings were achieved by placing the stimulus electrode into the brachial plexus (22). Repetitive CMAPs were observed in the [epsilon]L269F-transgenic, being more prominent in the gastrocnemius and absent or diminished in the intrinsic hindpaw muscles. Indeed, up to two additional action potentials were observed in the gastrocnemius, an observation rationalized by the authors as the result of the prolongation of the endplate potential beyond the muscle fiber refractory period. Repetitive CMAPs were also observed in the [delta]S262T-transgenic (50). In contrast, repetitive nerve stimulation in the [epsilon]L269F-transgenic did not show decremental response in the gastrocnemius while intrinsic muscles of the hindpaws elicited decremental CMAPs, thus indicating a reduced safety margin of neuromuscular transmission (22). Repetitive nerve stimulation studies were also employed to study the effect of transgenic expression of calpastatin (CS), the natural inhibitor of calpain, in the decremental response observed in the [epsilon]L269F-transgenic. Indeed, decremental responses in CS/ [epsilon]L269F mice were significantly lower when compared to the [epsilon]L269F-transgenic (54). Further details on this subject will be presented in the following sections.
MEPCs reflect the activation of postsynaptic AChRs by quantal release of ACh. Analogous to the exponential distribution of ion channel open times, miniature currents decay exponentially with rates defined by the time constant t. MEPCs are analyzed in terms of their decay rate, amplitude, and frequency. While MEPCs are recorded in the absence of nerve stimulation, EPCs are typically recorded upon stimulation of the phrenic nerve. Several publications present results from transgenic mice MEPC and EPC recordings (50, 56), whereas others have focused on MEPC recordings (22, 51, 54). In essence, MEPC and EPC recordings of the [delta]S262T-transgenic (38-39) and MEPC recordings of [epsilon]L269F (22, 51, 54), [alpha]C418W (51), and [alpha]L251T (51) transgenics have been reported.
MEPC recordings from three different lines of [delta]S262T-transgenics with different levels of the transgene mRNA in skeletal muscle as determined by RNA blot analysis, revealed that all three lines displayed prolonged decay phases and diminished amplitudes when compared to control mice (50). EPC recordings of the [delta]S262T-transgenic evoked by phrenic nerve stimulation showed prolongation of the currents (50) and reduction in amplitude (56). Moreover, a greater proportion of the transgenic mice MEPCs displayed bi-exponential decays (25%) when compared to WT (6%). Indeed, this particular difference proved to be important, as reported on a paper published shortly after, to help explain the reduced MEPC amplitudes in terms of an activity-dependent reduction in sensitivity as neither a reduction in the number of endplate AChRs nor ion channel conductance was observed in the [delta]S262T-transgenic (56). Other transgenic mice models studied employing intracellular recordings methods, namely [epsilon]L269F (22, 51, 54), [alpha]C418W (51), and [alpha]L251T (51), also displayed the typical MEPC prolonged durations which increased successively in transgenic lines [delta]S262T, [alpha]C418W, [alpha]L251T and [epsilon]L269F . Furthermore, while differences in MEPC amplitude seldom reached statistical significance (statistical significance was only reached in the [epsilon]L269F transgenic), the trend strongly reflected an inverse proportionality between mice strength and MEPC amplitude, thus suggesting the neuromuscular origin of the muscle weakness in these transgenic lines (51).
The patch clamp electrophysiological technique (32) has been employed in the study of various transgenic lines. For example, flexor digitorum brevis muscles isolated by blunt dissection from [delta]S262T-transgenic mice and dissociated through 0.2% collagenase two-hour incubations, followed by gentle trituration (56) were used for patch clamp experiments in the cell-attached configuration (35). Single-channel recordings were performed on single fibers bathed in Tyrode's solution at different membrane potentials to determine the amplitude of single-channel events (current) as function of membrane potential and thus calculate single-channel conductance. The aim of these experiments was to assess the possibility of a decreased conductance as cause for the reduced MEPC amplitudes displayed by the [delta]S262T-transgenic mice. The best linear fit of the current-voltage plots revealed a conductance of 61.8 pS for the transgenic and 61.4 pS for the control thus ruling out changes in single-channel conductance as the underlying cause for the reduced MEPC amplitudes (56). Open time histograms were also generated for the transgenic and the time constants were compared to those obtained while recording single-channel events from X. laevis oocytes expressing the [delta]S262T AChR mutant (50). The similarity between the time constants determined in both expression systems suggested the predominance of the [delta]S262T AChR in the [delta]S262T-transgenic mice endplates (56).
An additional transgenic mouse model employed in patch experiments is the [epsilon]L269F -transgenic. Open time histograms generated using single-channel data recorded from [epsilon]L269F dissociated fibers revealed channels with prolonged open durations (22). In fact, single-channel recordings exhibited channel events of both short and long duration presumably evoked by single- and double-ligand occupancy AChR openings, and resembled the previously reported findings in a SCS patient bearing the [epsilon]L269F mutation (21).
Electron micrographs of transgenic mice endplates enable researchers to study potential ultrastructural abnormalities (22, 51-52, 54, 56), as classical SCS are commonly associated with endplate myopathy, which consists of striking degenerative changes ofthe postsynaptic membrane (5). To this end, muscles are fixed in 2% glutaraldehyde, osmicated and embedded in epon following standard procedures in electron microscopy (56). An outstanding result was reported by Christopher M. Gomez's group in which the [delta]S262T-transgenic endplate displayed no recognizable pathological changes or ultrastructural abnormalities (22). Because MEPCs in this particular transgenic line have similar reductions in amplitude as other transgenic mice that display marked endplate myopathy (22), the [delta]S262T-transgenic enabled the study of additional factors that contribute to MEPC reduction, which eventually led to insights into the role played by abnormal desensitization in determining the amplitude of endplate currents (56). The important role of desensitization was confirmed a few years later in SCS spontaneous mutations (43). Nevertheless, ultrastructural abnormalities are a common (5) feature of SCS cases as demonstrated by the degenerating neuromuscular synapse from [epsilon]L269F mice displayed in Figure 4 (22).
Additional SCS transgenic mice studied using electron microscopy include the [alpha]C418W- and [alpha]L251T-transgenics, being the [epsilon]L269F the most extensively studied (22, 51, 52, 54). Similar to the [delta]S262T-transgenic NMJ, the [alpha]C418W-transgenic displayed essentially normal NMJ with the exception of a few vacuoles observed in the endplates (51). However, both [alpha]L251T- and [epsilon]L269F-transgenics had numerous endplates displaying marked ultrastructural changes such as collections of membrane-bound structures which in some cases filled the junctional sarcoplasm and that seemed as to arise from dilated sarcoplasmic reticulum. Other structural abnormalities present in both transgenic lines included degenerating nuclei, dilated or degenerating mitochondria, and membranous or granular debris (51). It is noteworthy that 66% of the [epsilon]L269F endplates displayed abnormalities which may be observed even in two-day-old mice (52) compared to 56% in the [alpha]L251T mice (51). Also, endplate denervation in the [epsilon]L269F-transgenic caused improvement of ultrastructural abnormalities as determined by electron microscopy (42).
The degeneration of motor endplates discussed above, and present in many SCS cases, has been attributed to [Ca.sup.2+] overload in the sarcoplasm and the concomitant activation of [Ca.sup.2+]-sensitive degradative enzymes (5), thus sparking the interest ofresearchers in making use and developing metho-dological approaches to assess [Ca.sup.2+] overloading in endplates (22, 51-53). For instance, histochemical methods relying on the Karnovsky and Roots cholinesterase method to localize endplates (57) and the glyoxal bis-(2 hydroxyanil) (GBHA) (58) or von Kossa (59) methods to localize [Ca.sup.2+] were used to probe calcium deposits in [epsilon]L269F transgenic endplates (22). These experiments revealed [Ca.sup.2+] staining over regions that co-localized with acetylcholinesterase, resembling the results obtained in the SCS patient bearing the [epsilon]L269F AChR mutation (5, 55). Using the same approach, the proportion of endplates overloaded with [Ca.sup.2+] was assessed on various transgenic mice lines revealing no GBHA staining in either control mice or the [delta]S262T-transgenic, whereas the [alpha]C418W-, and [alpha]L251T-transgenic displayed some [Ca.sup.2+] overloading, and the [epsilon]L269F -transgenic showed a significantly greater proportion of [Ca.sup.2+]-overloaded endplates compared to the other transgenic lines (51). Indeed, endplates from three-day-old [epsilon]L269F mice displayed intense GBHA staining (52). It was later revealed, using an epifluorescence microscopy-based approach, that the sarcoplasmic reticulum is the main source of the observed [Ca.sup.2+] and implicating inositol-1,4,5-triphosphate receptors and L-type voltage gated [Ca.sup.2+] channels as responsible for its efflux (53). Interestingly, it was also established that a 30-minute standardized exercise protocol results in a remarkable increase in the proportion of endplates overloaded with [Ca.sup.2+] in the [alpha]C418W- [alpha]L251T- and [epsilon]L269F-transgenic lines (51).
[FIGURE 4 OMITTED]
Personal Experience With Congenital Myasthenic Syndromes In Puerto Rico: A Local Perspective
CMS in Puerto Rico are rare, although the precise prevalence is not known. Over the last 10 years, one of the authors (CAL-R) has been able to identify five patients with features consistent with a CMS. These patients present ptosis and external ophthalmoplegia as the most prominent feature, although muscle weakness and fatigue are also prominent. In some instances, they have experienced episodes of respiratory insufficiency requiring temporary mechanical ventilation, although in our experience, this has been rare. With careful inquiry, it is possible to determine that the symptoms started during the neonatal period or during early childhood with delayed motor milestones. In most instances, the patients have been referred for the possibility of MG and the diagnosis of a CMS has been suspected on the basis of the early age at onset of symptoms and the lack or minimal response to immune suppressant therapies. The evaluation of these patients has included a thorough evaluation for possible Congenital Myopathies that may result in similar symptoms and the exclusion of autoimmune myasthenia with negative acetylcholine receptor or muscle specific kinase antibodies. The response to pyridostigmine has been variable indicating the possibility of different mutations within the group of patients. In some instances, we have been able to identify electrophysiologic features suggestive of a SCS mutation, such as the presence of a repetitive CMAP on the motor nerve conduction studies, with an abnormal decremental response on repetitive nerve stimulation. (Figure 5A and B). It is likely that many more of these patients are misdiagnosed and have not been identified. Although treatment for these syndromes is limited, an active effort to identify these patients is necessary to optimize their care and to avoid the needless exposure to immunotherapies with multiple and serious side effects.
Electron microscopy micrographs such as those displayed in Figure 4 have had an important role in the study of endplate myopathies which are considered the hallmark of SCS. In spite of the inherent high resolution of electron microscopy, alternative imaging strategies must be thoroughly examined in order to more effectively monitor endplate changes associated to SCS. For instance, changes in the amount of endplates per muscle fiber or the extent of AChR expression within endplates among other valuable criteria are not easily examined through EM. To surpass these limitations, light microscopy-based experimental approaches have been developed in our laboratory to monitor endplates of transgenic mice bearing SCS mutations with respect to WT FVB mice. While the achievable resolution in light microscopy is much lower than that of electron microscopy, the optical sectioning capability of confocal microscopy and state-of-the-art three-dimensional analysis software packages such as Imaris x64 6.2.0 (www.bitplane.com) enable a detailed analysis of endplate morphology (based on the fluorescence of labeled-a-bungarotoxin) in the different transgenic lines bearing SCS mutations that we currently work with (Figure 6). Using this methodological approach, it is also possible to quantify the effects of any given variable on the number of transgenic mice endplates, the volume of endplates, and the number of AChRs. Furthermore, a common feature of degenerating NMJ, namely the widening of the synaptic space may also be studied using the optical sectioning capability of confocal microscopy. Certainly, by fluorescently labeling the pre-synaptic and post-synaptic membranes, endplate plasticity may be studied in SCS transgenics and compared to WT mice. The possibility that the expression of AChR displaying altered kinetics may concomitantly change endplate plasticity should provide additional insight into the pathogenesis of SCS and increase our understanding of the NMJ in general. Thus far, a significant number of novel observations have been enabled by this approach thus underscoring its importance in years to come.
[FIGURE 5 OMITTED]
The investigation of CMS has enhanced our understanding of the development, maintenance, and function of the NMJ. The development of CMS animal models to investigate the mechanisms for endplate degeneration has been a major breakthrough for the elucidation of molecular mechanisms leading to the CMS phenotype. All clinical, electrophysiological, pathologic, and biochemical aspects of the human disease process in CMS are fully replicated in the transgenic models. Moreover, the heterogeneous phenotypes of CMS are also replicated in the transgenic mice model. Several aspects of the CMS must be fully elucidated to define more efficient strategies for therapeutic intervention. The first aspect that deserves special attention is the alteration of endplate plasticity in the CMS: Is pre-synaptic feedback (i.e., decrease in release sites) playing a compensation role to attenuate the gain-of-function AChRmutation? What types of post-synaptic compensations are evident in the NMJ in response to a CMS (i.e. reduction of nAChR expression in endplate). Are there any environmental factors that can exacerbate or attenuate phenotypes within the same CMS? It is also important to understand the factors that regulate the expression of mutant AChR in the CMS endplate. Understanding the molecular mechanisms leading to synaptic remodeling of the NMJ in different forms of CMS is essential to develop new strategies for therapeutic intervention.
The study of SCS has led to a remarkably detailed explanation for the pathogenesis of the electrophysiological, pathological, and genetic basis of the disease. This understanding of SCS has led to design of therapeutic interventions that modulate the mutant AChR function,, as well as insights into further therapy development. These insights may have additional implications into understanding the pathological processes that pay a role in other disorders, such as epilepsy, stroke, Alzheimer's disease, and myopathies. The diagnosis remains challenging due to failure to suspect the disease and the lack of clinically available genetic studies.
[FIGURE 6 OMITTED]
Ach--acetylcholine, AchR--acetylcholine receptor, [alpha]-BgTx--[alpha]- bungarotoxin, [[sup.125]I]-[alpha]-BgTx--[[sup.125]I-labeled [alpha]-bungarotoxin, CA--calpastatin, CMAP--compound muscle action potential, CMS--congenital myasthenic syndrome, dsRNA--double-stranded RNA, EMG--electromyography, EP--endplate, EPC--endplate current, EPP--endplate potentials, GBHA--glyoxal bis-(2-hydroxyanil), HEK--human embryonic kidney, MEPC miniature endplate current, MEPP--miniature endplate potential, MG--myasthenia gravis, NMJ--neuromuscular junction, RNAi--RNA interference, SCS--slow-channel congenital myasthenic syndrome, shRNA--short hairpin RNA, WT--wild-type
This work was partly supported by grants from the National Institutes of Health to J. A. Lasalde-Dominicci (grant no. 2RO1GM56371-12 and SNRP U54NS0430311), to C. M. Gomez (grant no. 2RO1-N33202), and to C. A. Luciano (grant no. P20RR011126). We especially thank Zayra E. Marrero Burgos for proofreading the manuscript. The authors have no conflict of interest to disclose.
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Jose David Otero-Cruz, Ph D[*]; Carlos Alberto Baez-Pagan, Ph D[*]; Luisamari Dorna-Perez[*]; Gary Emanuel Grajales-Reyes[*]; Rosaura Teresa Ramirez-Ordonez, Ph D[*]; Carlos A. Luciano, MD[dagger]; Christopher Manuel Gomez, MD/Ph D[dagger]; Jose Antonio Lasalde-Dominicci, Ph D[*]
[*] Department of Biology, University of Puerto Rico, Rio Piedras Campus, San Juan, PR 00931; tDepartment of Comparative Medicine, School of Medicine, University of Puerto Rico, Medical Sciences Campus, San Juan, PR 00936; [dagger] Department of Neurology, The University of Chicago, Biological Sciences Division, Chicago, IL 60637, USA
Address correspondence to: Jose A. Lasalde-Dominicci Ph D, Department of Biology, University of Puerto Rico, Rio Piedras Campus, P.O. Box 23360 San Juan, Puerto Rico 00931-3360. Tel: (787) 764-0000 Ext: 1-3023/4887 * Fax: (787) 753-3852 * E-mail: email@example.com
Table 1. Summary of the genes encoding human acetylcholine receptor at the neuromuscular junction. AChR GeneID/MIM Gene Chromosome subunit number * symbol number Alpha ([[alpha].sub.1]) 1134; 100960 CHRNA1 2 Beta ([[Beta].sub.1]) 1140; 100710 CHRNB1 17 Delta ([Delta]) 1144; 100720 CHRND 2 Epsilon ([epsilon]) 1145; 100725 CHRNE 17 AChR Gene map Cloning subunit locus reference Alpha ([[alpha].sub.1]) 2q24-q32 (60-63) Beta ([[Beta].sub.1]) 17p13.1 (62, 64) Delta ([Delta]) 2q33-q34 (62, 65-67) Epsilon ([epsilon]) 17p13-p12 (68) * GeneID in Entrez Gene database and MIM number in Online Mendelian Inheritance in Man database. Table 2. Slow-channel congenital myasthenic syndrome kindred and acetylcholine receptor mutations. Mutation/ Domain DB (1) Onset/Activity (2) [alpha] G153S N-terminal 1 Infant-53/sports [alpha] V156M N-terminal -- 14 [alpha] N217K M1 9, 10 8-57 [alpha] V249F M2 -- Infant/wc 8 [alpha] T254I M2 -- 16 [alpha] S269I M2 -- 23 [alpha] C418W M4 -- Infant-teens [beta] V229F M1 7 18-50 [beta] L262M M2 4 inf/wc 13 [beta] V266M M2 13 Infant [delta] S268F M2 6 Infant/wc teens [epsilon] L78P N-terminal 22-29 [epsilon] L221F M1 -- 17-40s [epsilon] T264P M2 -- Infant/wc teens [epsilon] L269F M2 2, 5 Infant Mutation/ Domain Medical Exam (3) Grade (4) [alpha] G153S N-terminal Ocular, hand, arm 1-2 [alpha] V156M N-terminal No ocular; neck, 1 jaw, limb [alpha] N217K M1 Ocular, hand, arm, 2 neck, gait [alpha] V249F M2 Severe generalized 3 [alpha] T254I M2 No ocular; neck, arm 1 [alpha] S269I M2 Arm, jaw, legs 1 [alpha] C418W M4 Ocular, arm, legs 2 [beta] V229F M1 Hand, neck, arms 1 [beta] L262M M2 Severe generalized, 3 atrophy [beta] V266M M2 Ocular, face, limb 2-4 [delta] S268F M2 No atrophy, severe 4 generalized [epsilon] L78P N-terminal Ocular, face, neck, 1 shoulder, hand, hip [epsilon] L221F M1 Ocular, wrist, hand, 1 neck [epsilon] T264P M2 Severe limb, neck 3 [epsilon] L269F M2 Severe focal, 3 resp insuf. Mutation/ Domain MEPP MEPC decay (6) EpM (7) amplitude (5) [alpha] G153S N-terminal 36-90% 7.8/26.4 Yes [alpha] V156M N-terminal NA NA NA [alpha] N217K M1 43-69% 3.6/22.1 Yes [alpha] V249F M2 44% 2.4/43.9 Yes [alpha] T254I M2 NA NA NA [alpha] S269I M2 Normal 20/28 * NA [alpha] C418W M4 NA NA NA [beta] V229F M1 20% 0.67 2.25/26.1 Yes (ps) [beta] L262M M2 19% 38.6 * Yes [beta] V266M M2 43-69% 3.5/35.6 Yes [delta] S268F M2 50% 31 * Yes (ps) [epsilon] L78P N-terminal NA NA NA [epsilon] L221F M1 NA NA NA [epsilon] T264P M2 Reduced 1.5/16.5 Yes [epsilon] L269F M2 36-69% 3.3/50 Yes Mutation/ Domain [alpha] References BT (8) [alpha] G153S N-terminal Low (20, 37) [alpha] V156M N-terminal NA (37) [alpha] N217K M1 Low (21, 69) [alpha] V249F M2 39% (19) [alpha] T254I M2 NA (37) [alpha] S269I M2 44% (37) [alpha] C418W M4 NA (42) [beta] V229F M1 NA (41, 69) [beta] L262M M2 High (36) [beta] V266M M2 Low (21, 69) [delta] S268F M2 NA (39, 69) [epsilon] L78P N-terminal NA (40) [epsilon] L221F M1 NA (40) [epsilon] T264P M2 39% (18) [epsilon] L269F M2 NA (21, 52, 55) (1) Kindred/Proband number in the SCS database (2) Age or range of age of onset within kindred./Level of activity or disability. wc = wheelchair. (3) Most prominent features of weakness. (4) Score defined as 1: onset after infancy, limb, ocular weakness minimal limitation of activity; 2: onset in infancy, limitation of activity; 3: respiratory insufficiency, confinement to wheelchair; 4: death from respiratory failure. (5) Amplitude of miniature endplate currents or miniature endplate potentials. Shown is percent of control. NA=not available. (6) Time constant(s) of decay of miniature endplate current or potentials in milliseconds (ms). Normal = 2-3.3 ms. * = MEPP. ** = noise analysis. (7) EpM = endplate myopathy confirmed. yes = present; n = absent. ps = presynaptic-predominant endplate myopathy. (8) [alpha]-Bungarotoxin binding sites on endplates. Shown is percent of control, reduced or increased.
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