Desensitization of mutant acetylcholine receptors in transgenic mice reduces the amplitude of neuromuscular synaptic currents

While the slow onset of desensitization of nicotinic acetylcholine receptors (AChRs), relative to the rate of acetylcholine removal, excludes this kinetic state from shaping synaptic responses in normal neuromuscular transmission, its role in neuromuscular disorders has not been examined. The slow-channel congenital myasthenic syndrome (SCCMS) is a disorder caused by point mutations in the AChR subunit-encoding genes leading to kinetically abnormal (slow) channels, reduced miniature endplate current amplitudes (MEPCs), and degeneration of the postsynaptic membrane. Because of this complicated picture of kinetic and structural change in the neuromuscular junction, it is difficult to assess the importance of the multiple factors that may be responsible for the reduced endplate current amplitudes, and ultimately the clinical syndrome. In order to address this we have used a transgenic mouse model for the SCCMS that has slow AChR ion channels and reduced endplate responsiveness in the absence of any of the degenerative changes. We found that the reduction in MEPC amplitudes in these mice could not be explained by either reduced AChR number or by reduced AChR channel conductance. Rather, we found that the mutant AChRs in situ manifested an activity-dependent reduction in sensitivity that caused diminished MEPC and endplate current amplitude with nerve stimulation. This observation demonstrates that the basis for the reduction in MEPC amplitudes in the SCCMS may be multifactorial. Moreover, these findings demonstrate that, under conditions that alter their rate of desensitization, the kinetic properties of nicotinic AChRs can control the strength of synaptic responses.     

Mutations in different functional domains of the human muscle acetylcholine receptor alpha subunit in patients with the slow-channel congenital myasthenic syndrome

Congenital myasthenic syndromes are a group of rare genetic disorders that compromise neuromuscular transmission. A subset of these disorders, the slow-channel congenital myasthenic syndrome (SCCMS), is dominantly inherited and has been shown to involve mutations within the muscle acetylcholine receptor (AChR). We have identified three new SCCMS mutations and a further familial case of the alpha G153S mutation. Single channel recordings from wild-type and mutant human AChR expressed in Xenopus oocytes demonstrate that each mutation prolongs channel activation episodes. The novel mutations alpha V156M, alpha T254I and alpha S269I are in different functional domains of the AChR alpha subunit. Whereas alpha T254I is in the pore-lining region, like five of six previously reported SCCMS mutations, alpha S269I and alpha V156M are in extracellular domains. alpha S269I lies within the short extracellular sequence between M2 and M3, and identifies a new region of muscle AChR involved in ACh binding/channel gating. alpha V156M, although located close to alpha G153S which has been shown to increase ACh binding affinity, appears to alter channel function through a different molecular mechanism. Our results demonstrate heterogeneity in the SCCMS, indicate new regions of the AChR involved in ACh binding/channel gating and highlight the potential role of mutations outside the pore-lining regions in altering channel function in other ion channel disorders.     

Congenital myasthenic syndromes: clinical and genetic analysis of 18 patients

Congenital myasthenic syndromes (CMS) are a group of rare gentic disorders in which neuromuscular transmission is compromised by a variety of mechanisms, other than autoimmunity. Recently, substantial progress has been made by the identification of mutations in acetylcholine receptor (AChR) genes which cause CMS. We report on the clinical and genetic analysis of 18 independent CMS patients. All patients were clinically classified as sporadic cases of CMS (group III according to ENMC consensus). In order to investigate the prevalence of AChR mutations in this group we analyzed structural domains of the AChR genes at strategically important sites - the channel pore-lining regions (M2 domains) of the alpha, beta and epsilon subunits, and the extracellular domain close the acetylcholine (ACh) binding site. All patients showed wild-type sequence in these regions, mutations were not detected. Therefore, we conclude, that point mutations in domains which are known to cause slow channel congenital myasthenic syndromes (SCCMS) are rare in group III-patients in Germany. Determining the genetic defects causing CMS may have implications for diagnosis and genetic counseling of CMS patients. Moreover, this may be important for the therapeutic management of CMS as some patients may profit form quinidine sulfate. Therefore, further efforts will be undertaken to elucidate the underlying defects of CMS.     

Attenuated influx of calcium ions at nerve endings of csp and shibire mutant Drosophila

Previous work has shown that cysteine-string proteins (csps) are synaptic vesicle proteins that are important for evoked neurotransmitter release at Drosophila neuromuscular junctions. Indirect evidence has implicated csps in a regulatory link between synaptic vesicles and presynaptic calcium (Ca) channels. In this report, we use Ca Crimson to monitor stimulus-dependent changes of cytosolic Ca at motor nerve terminals of csp mutant Drosophila. These mutants display temperature-sensitive (TS) paralysis and a presynaptic failure of evoked synaptic transmission. We show that this TS inhibition of neuromuscular transmission is correlated with a block of Ca ion entry at nerve endings of csp mutants. These data support the hypothesis that csps mediate a regulatory interaction between synaptic vesicles and presynaptic Ca channels. Moreover, these results predict that if one depletes nerve endings of synaptic vesicles, one may see a reduction of presynaptic Ca ion entry. Defects of the dynamin gene in TS shibire mutant Drosophila interfere with synaptic vesicle recycling and lead to an activity-dependent depletion of these organelles. Our results show that Ca influx is blocked at nerve terminals of shibire mutant larvae at the same time that synaptic transmission fails in these organisms. Thus, using two completely independent Drosophila mutants, we demonstrate that synaptic vesicles and csps are vital for the function of presynaptic Ca channels.     

Neuronal nicotinic ACh receptor antibody in subacute autonomic neuropathy and cancer-related syndromes

BACKGROUND: Autoantibodies specific for the acetylcholine receptor (AChR) of skeletal muscle (containing the alpha1 subunit) impair neuromuscular transmission in myasthenia gravis (MG). AChRs mediating fast synaptic transmission through autonomic ganglia are structurally similar to muscle AChR, but contain the alpha3 subunit. We propose that ganglionic AChR autoimmunity may cause dysautonomia. OBJECTIVE: To test serum of patients with autonomic neuropathy for autoantibodies of neuronal ganglionic AChR specificity. METHODS: We developed an immunoprecipitation radioassay by complexing epibatidine (125I-labeled high affinity agonist) to a Triton X-100-solubilized AChR antigen from peripheral neuroblastoma membranes. Monoclonal rat immunoglobulins (IgG) specific for muscle or neuronal AChRs validated the assay's specificity. We tested serum from 52 healthy subjects, 12 patients with subacute autonomic neuropathy, and 248 patients with other neurologic disorders. RESULTS: Twelve patients had antibodies that bound unequivocally to ganglionic AChR. Five had subacute autonomic neuropathy, and three (of six tested) had Isaacs' syndrome; four of these eight had a carcinoma (lung, bladder, rectum, thyroid). The remaining four seropositive patients (two Lambert-Eaton syndrome, one dementia, one sensory neuronopathy) all had Ca2+ channel antibodies and three had small cell lung carcinoma. No healthy subject had ganglionic AChR antibodies, nor did 62 patients with MG and muscle AChR antibodies. CONCLUSION: Neuronal AChR antibodies are a novel serologic marker of neurologic autoimmunity. The pathogenicity of neuronal AChR autoantibodies in autonomic neuropathy, Isaacs' syndrome, or other neurologic disorders remains to be shown, as has been demonstrated for muscle AChR antibodies in MG. An autoimmune and potentially paraneoplastic etiology is implicated in seropositive patients.     

Oxidative stress, mutant SOD1, and neurofilament pathology in transgenic mouse models of human motor neuron disease

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that primarily affects motor neurons in the spinal cord and brain stem. About 10% of all ALS cases are familial (FALS), inherited in an autosomal dominant manner. One fifth of FALS patients carry mutations in the Cu/Zn superoxide dismutase (SOD1) gene, and several lines of transgenic mice have been engineered to express mutant forms of the SOD1 gene that are linked to FALS. Significantly, many of these transgenic lines of mice develop a motor neuron disease (MND) that resembles human FALS. Oxidative stress induced by human SOD1 mutations is believed to play an important role in the pathogenesis of FALS and the FALS-like MND seen in the mutant SOD1 transgenic mice. For example, two lines of these mice showed prominent degeneration of mitochondria and endoplasmic reticulum in spinal cord neurons. Furthermore, recent studies have shown that neurofilament (NF)-rich spheroids. Lewy body-like NF inclusions, altered ubiquitin immunoreactivity, and Golgi fragmentation occur in the spinal cord motoneurons of these mutant SOD1 transgenic mice. Because these lesions recapitulate hallmark abnormalities of human ALS, mutant SOD1 transgenic mice provide a useful model for studies designed to elucidate the pathogenesis of ALS. Furthermore, transgenic mice that overexpress NF proteins also develop a clinical and pathologic phenotype similar to human MND, and polymorphisms in an NF gene have been linked to patients with ALS. Collectively, these observations implicate NF protein abnormalities in the pathogenesis of this disorder. Accordingly, this review summarizes recent insights into mechanisms of motor neuron degeneration in ALS that have emerged from studies of these new animal models of this neurodegenerative disease.     

IL-12 is involved in the induction of experimental autoimmune myasthenia gravis, an antibody-mediated disease

IL-12 has been shown to be involved in the pathogenesis of Th1-mediated autoimmune diseases, but its role in antibody-mediated autoimmune pathologies is still unclear. We investigated the effects of exogenous and endogenous IL-12 in experimental autoimmune myasthenia gravis (EAMG). EAMG is an animal model for myasthenia gravis, a T cell-dependent, autoantibody-mediated disorder of neuromuscular transmission caused by antibodies to the muscle nicotinic acetylcholine receptor (AChR). Administration of IL-12 with Torpedo AChR (ToAChR) to C57BL/6 (B6) mice resulted in increased ToAChR-specific IFN-gamma production and increased anti-ToAChR IgG2a serum antibodies compared with B6 mice primed with ToAChR alone. These changes were associated with earlier and greater neurophysiological evidence of EAMG in the IL-12-treated mice, and reduced numbers of AChR. By contrast, when IL-12-deficient mice were immunized with ToAChR, ToAChR-specific Th1 cells and anti-ToAChR IgG2a serum antibodies were reduced compared to ToAChR-primed normal B6 mice, and the IL-12-deficient mice showed almost no neurophysiological evidence of EAMG and less reduction in AChR. These results indicate an important role of IL-12 in the induction of an antibody-mediated autoimmune disease, suggest that Th1-dependent complement-fixing IgG2a anti-AChR antibodies are involved in the pathogenesis of EAMG, and help to account for the lack of correlation between anti-AChR levels and clinical disease seen in many earlier studies.     

Acetylcholine receptor epsilon-subunit deletion causes muscle weakness and atrophy in juvenile and adult mice

In mammalian muscle a postnatal switch in functional properties of neuromuscular transmission occurs when miniature end plate currents become shorter and the conductance and Ca2+ permeability of end plate channels increases. These changes are due to replacement during early neonatal development of the gamma-subunit of the fetal acetylcholine receptor (AChR) by the epsilon-subunit. The long-term functional consequences of this switch for neuromuscular transmission and motor behavior of the animal remained elusive. We report that deletion of the epsilon-subunit gene caused in homozygous mutant mice the persistence of gamma-subunit gene expression in juvenile and adult animals. Neuromuscular transmission in these animals is based on fetal type AChRs present in the end plate at reduced density. Impaired neuromuscular transmission, progressive muscle weakness, and atrophy caused premature death 2 to 3 months after birth. The results demonstrate that postnatal incorporation into the end plate of epsilon-subunit containing AChRs is essential for normal development of skeletal muscle.     

Mutation in the M1 domain of the acetylcholine receptor alpha subunit decreases the rate of agonist dissociation

We describe the kinetic consequences of the mutation N217K in the M1 domain of the acetylcholine receptor (AChR) alpha subunit that causes a slow channel congenital myasthenic syndrome (SCCMS). We previously showed that receptors containing alpha N217K expressed in 293 HEK cells open in prolonged activation episodes strikingly similar to those observed at the SCCMS end plates. Here we use single channel kinetic analysis to show that the prolonged activation episodes result primarily from slowing of the rate of acetylcholine (ACh) dissociation from the binding site. Rate constants for channel opening and closing are also slowed but to much smaller extents. The rate constants derived from kinetic analysis also describe the concentration dependence of receptor activation, revealing a 20-fold shift in the EC50 to lower agonist concentrations for alpha N217K. The apparent affinity of ACh binding, measured by competition against the rate of 125I-alpha-bungarotoxin binding, is also enhanced 20-fold by alpha N217K. Both the slowing of ACh dissociation and enhanced apparent affinity are specific to the lysine substitution, as the glutamine and glutamate substitutions have no effect. Substituting lysine for the equivalent asparagine in the beta, epsilon, or delta subunits does not affect the kinetics of receptor activation or apparent agonist affinity. The results show that a mutation in the amino-terminal portion of the M1 domain produces a localized perturbation that stabilizes agonist bound to the resting state of the AChR.     

The synapse-associated protein rapsyn regulates tyrosine phosphorylation of proteins colocalized at nicotinic acetylcholine receptor clusters

Protein tyrosine phosphorylation has been suggested to play an important role in the clustering of the nicotinic acetylcholine receptor (AChR) at the developing neuromuscular junction. Recent studies have shown that the 43-kDa synapse-associated protein rapsyn induces clustering of the AChR in heterologous expression systems. In this study we examined whether tyrosine phosphorylation is involved in this rapsyn-induced AChR clustering. Rapsyn-induced AChR clusters in fibroblasts contain phosphotyrosine, as detected using immunofluorescent labeling with anti-phosphotyrosine antibodies. No anti-phosphotyrosine staining of rapsyn clusters is seen in the absence of AChR expression, indicating that the AChR is required for the appearance of phosphotyrosine at clusters. In addition, coexpression of rapsyn with the AChR induces the tyrosine phosphorylation of the beta amd delta subunits of the AChR. Surprisingly, mutation of the tyrosine phosphorylation sites in the AChR did not inhibit rapsyn-induced clustering of the AChR and clusters of the mutant AChRs still contained high levels of phosphotyrosine. Experiments with single AChR subunits demonstrate that the alpha subunit of the AChR appears to be necessary and sufficient for codistribution of phosphotyrosine with rapsyn-induced clusters of AChR subunits. Finally, transfection of cells with rapsyn activates cellular protein tyrosine kinase activity, resulting in the tyrosine phosphorylation of several membrane-associated proteins. These results suggest that rapsyn may therefore regulate clustering at least in part by regulating the tyrosine phosphorylation of cellular proteins.     

Lidocaine block of LQT-3 mutant human Na+ channels

In transiently transfected mammalian cells we have identified pharmacological consequences of a naturally occurring deletion mutation, delta KPQ, of the human heart Na+ channel alpha subunit that previously has been linked to one form of the long QT syndrome, an inherited heart disease. Our results show that the Class IB antiarrhythmic agent lidocaine blocks maintained inward current through and slows recovery from inactivation of delta KPQ-encoded Na+ channels. Block is greater for maintained than for peak current. Because incomplete inactivation of mutant Na+ channels is now thought to underlie the prolonged ventricular action potential, which is the phenotype of this disease, and we find that the delta KPQ mutation speeds the recovery from inactivation of drug-free mutant channels, our results provide evidence, for the first time, that clinically relevant dysfunctional properties of an ion channel can be selectively targeted on the basis of the molecular properties conferred on the channel by an inherited genetic disorder.     

Association of the mitochondrial 8344 MERRF mutation with maternally inherited spinocerebellar degeneration and Leigh disease

We report previously undescribed or atypical clinical and biochemical manifestations of the mitochondrial DNA MERRF mutation at nucleotide 8344 in members of a multigenerational family with maternally inherited, highly variable neurodegenerative disorder. The more profound neurologic abnormalities include Leigh disease, spinocerebellar degeneration, and atypical Charcot-Marie-Tooth disease.     

The Th2 cytokine IL-4 is not required for the progression of antibody-dependent autoimmune myasthenia gravis

Experimental autoimmune myasthenia gravis (EAMG), a disorder of the neuromuscular junction, is mediated by autoantibodies against muscle nicotinic acetylcholine receptor (AChR). The roles of IFN-gamma (Th1) and IL-4 (Th2) cytokines in the initiation and progression of this disease are not fully understood. Recently, we have demonstrated that IFN-gamma is necessary for the initiation of tAChR-induced EAMG in mice. However, the role of IL-4 in the progression of clinical EAMG remained undetermined. In this study we have addressed the contribution of IL-4 in the disease progression in IL-4(-/-) C57BL/6j mice whose IL-4 gene has been disrupted. Following immunization with Torpedo (t) AChR, the IL-4(-/-) mice readily developed signs of muscle weakness and succumbed to clinical EAMG with kinetics similar to the susceptibility of IL-4(+/+) mice. The tAChR-primed lymph node cells from IL-4(-/-) mice vigorously proliferated to tAChR and to its dominant alpha146-162 sequence associated with disease pathogenesis. However, these T cells secreted higher levels of IFN-gamma and IL-2, suggesting the development of a Th1 default pathway in these mice. Nevertheless, the IL-4 mutation had no effect on the recruitment of CD4+ Vbeta6+ T cells specific to the dominant tAChR alpha146-162 sequence in vivo. Immune sera from IL-4(-/-) mice showed a dramatic increase in mouse AChR-specific IgG2a levels followed by a concomitant decrease in IgG1 levels, but these mice did not exhibit an accelerated disease. In conclusion, we have demonstrated for the first time that IL-4 is not required either for the generation of a pathogenic anti-AChR humoral immune response or for progression of clinical EAMG in mice.     

The role of leukemia inhibitory factor in skeletal muscle regeneration

Although a number of cytokines have been implicated in tissue regeneration, it is unknown which ones actually function in vivo. Here, we use mice with a targeted mutation in the leukemia inhibitory factor (LIF) gene to examine the role of LIF in muscle regeneration. Using a muscle crush model, we show that muscle regeneration in LIF knockout mice is significantly, reduced compared to control littermates. Further, targeted infusion of LIF in both normal and LIF knockout animals stimulated muscle regeneration, but the stimulation observed was much greater in the mutant animals than in controls. In contrast, interleukin-6 and transforming growth factor-alpha, which also stimulate myoblast proliferation in vitro, had no effect on regeneration. These findings demonstrate directly that LIF is involved in regeneration of injured muscle and points to the use of LIF as a therapeutic agent in the treatment of neuromuscular disease.     

Mode switching kinetics produced by a naturally occurring mutation in the cytoplasmic loop of the human acetylcholine receptor epsilon subunit

We describe the genetic and kinetic defects in a congenital myasthenic syndrome caused by heteroallelic mutations of the acetylcholine receptor (AChR) epsilon subunit gene. The mutations are an in-frame duplication of six residues in the long cytoplasmic loop (epsilon1254ins18) and a cysteine-loop null mutation (epsilonC128S). The epsilon1254 ins18 mutation causes mode switching in the kinetics of receptor activation in which three modes activate slowly and inactivate rapidly. The epsilon1245ins18-AChR at the endplate shows abnormally brief activation episodes during steady state agonist application and appears electrically silent during the synaptic response to acetylcholine. The phenotypic consequences are endplate AChR deficiency, simplification of the postsynaptic region, and compensatory expression of fetal AChR that restores electrical activity at the endplate and rescues the phenotype.     

Modified hippocampal long-term potentiation in PKC gamma-mutant mice

Calcium-phospholipid-dependent protein kinase (PKC) has long been suggested to play an important role in modulating synaptic efficacy. We have created a strain of mice that lacks the gamma subtype of PKC to evaluate the significance of this brain-specific PKC isozyme in synaptic plasticity. Mutant mice are viable, develop normally, and have synaptic transmission that is indistinguishable from wild-type mice. Long-term potentiation (LTP), however, is greatly diminished in mutant animals, while two other forms of synaptic plasticity, long-term depression and paired-pulse facilitation, are normal. Surprisingly, when tetanus to evoke LTP was preceded by a low frequency stimulation, mutant animals displayed apparently normal LTP. We propose that PKC gamma is not part of the molecular machinery that produces LTP but is a key regulatory component.     

Intracellular calcium regulates agrin-induced acetylcholine receptor clustering

Agrin is an extracellular matrix protein that directs neuromuscular junction formation. Early signal transduction events in agrin-mediated postsynaptic differentiation include activation of a receptor tyrosine kinase and phosphorylation of acetylcholine receptors (AChRs), but later steps in this pathway are unknown. Here, we have investigated the role of intracellular calcium in agrin-induced AChR clustering on cultured myotubes. Clamping intracellular calcium levels by loading with the fast chelator BAPTA inhibited agrin-induced AChR aggregation. In addition, preexisting AChR aggregates dispersed under these conditions, indicating that the maintenance of AChR clusters is similarly dependent on intracellular calcium fluxes. The decrease in AChR clusters in BAPTA-loaded cells was dose-dependent and reversible, and no change in the number or mobility of AChRs was observed. Clamping intracellular calcium did not block agrin-induced tyrosine phosphorylation of the AChR beta-subunit, indicating that intracellular calcium fluxes are likely to act downstream from or parallel to AChR phosphorylation. Finally, the targets of the intracellular calcium are likely to be close to the calcium source, since agrin-induced AChR clustering was unaffected in cells loaded with EGTA, a slower-binding calcium chelator. These findings distinguish a novel step in the signal transduction mechanism of agrin and raise the possibility that the pathways mediating agrin- and activity-driven changes in synaptic architecture could intersect at the level of intracellular calcium fluxes.