Molecular architecture of the neuromuscular junction

The neuromuscular junction (NMJ) is a complex structure that serves to efficiently communicate the electrical impulse from the motor neuron to the skeletal muscle to signal contraction. Over the last 200 years, technological advances in microscopy allowed visualization of the existence of a gap between the motor neuron and skeletal muscle that necessitated the existence of a messenger, which proved to be acetylcholine. Ultrastructural analysis identified vesicles in the presynaptic nerve terminal, which provided a beautiful structural correlate for the quantal nature of neuromuscular transmission, and the imaging of synaptic folds on the muscle surface demonstrated that specializations of the underlying protein scaffold were required. Molecular analysis in the last 20 years has confirmed the preferential expression of synaptic proteins, which is guided by a precise developmental program and maintained by signals from nerve. Although often overlooked, the Schwann cell that caps the NMJ and the basal lamina is proving to be critical in maintenance of the junction. Genetic and autoimmune disorders are known that compromise neuromuscular transmission and provide further insights into the complexities of NMJ function as well as the subtle differences that exist among NMJ that may underlie the differential susceptibility of muscle groups to neuromuscular transmission diseases. In this review we summarize the synaptic physiology, architecture, and variations in synaptic structure among muscle types. The important roles of specific signaling pathways involved in NMJ development and acetylcholine receptor (AChR) clustering are reviewed. Finally, genetic and autoimmune disorders and their effects on NMJ architecture and neuromuscular transmission are examined.     

Correlation of C3 level with severity of generalized myasthenia gravis

Acute exacerbation of generalized myasthenia gravis (GMG) can cause swallowing impairment, respiratory failure, or death. It is important to identify immunological factors that might be regarded reliably as an index of the patient's clinical condition, response to treatment, and measure of certain immune aberrations of MG. In this study we investigated correlations between complement component C3, acetylcholine receptor antibody (AChRab) titer, and clinical severity of GMG. AChRab titer and C3 concentration were determined by radioimmunoassay and nephelometry, respectively. The clinical severity of GMG was assessed by the quantitative MG score (QMGS) according to Besinger and colleagues. Our findings indicate that the C3 level correlates with clinical severity of AChRab‐positive GMG. Muscle Nerve, 2009     

Protective effect of scFv-DAF fusion protein on the complement attack to acetylcholine receptor: a possible option for treatment of myasthenia gravis

Introduction: Autoantibody‐induced complement activation, which causes disruption of the postsynaptic membrane, is recognized as a key pathogenic factor in myasthenia gravis (MG). Therefore, specific targeting of complement inhibitors to the site of complement activation is a potential therapeutic strategy for treatment of MG. Methods: We assessed expression of single‐chain antibody fragment–decay accelerating factor (scFv‐DAF), comprising a single‐chain fragment scFv1956 based on the rat complement inhibitor DAF in prokaryotic systems, and studied its inhibitory effect on complement deposition in vitro. Results: The recombinant conjugate scFv‐DAF completely retained the wild‐type binding activity of scFv1956 to AChR and inhibited complement activation of DAF in vitro. Conclusions: We found that scFv‐DAF could bind specifically to TE671 cells, and it is significantly more potent at inhibiting complement deposition than the untargeted parent molecule DAF. scFv‐DAF may be a candidate for in vivo protection of the AChR in MG. Muscle Nerve, 2012     

The B-cell repertoire in myasthenia gravis includes all four acetylcholine receptor subunits

By enumerating cells secreting IgG antibodies of particular specificities using an enzyme-linked immunospot (ELISPOT) assay, the B-cell responses to Torpedo acetylcholine receptor (AChR) and its α-, β-, γ- and δ-subunits in peripheral blood from patients with myasthenia gravis (MG), and controls with other neurological diseases (OND) as well as healthy subjects were determined. Compared to controls, the patients with MG had elevated numbers of B cells secreting antibodies against AChR and its α-, β-, γ- and δ-subunits in peripheral blood in parallel. The mean numbers of anti-AChR antibody secreting cells were about 17 per 10⁵ blood MNC, and for the subunits 10 to 15 in MG patients, compared to between 0.8 and 1.9 per 10⁵ blood MNC in OND patients, and 0.1 to 0.3 in healthy controls. Such B cells detected in controls probably represent naturally occurring B cells responded to AChR and its subunits. The finding that most (60%) MG patients had B cells predominantly recognizing the α-subunit may be an indirect argument for the existence of a main immunogenic region (MIR). In the remaining 40% of patients with MG the predominant B-cell responses were directed to β-, γ- or δ-subunit. The data suggest that all four AChR subunits may function as strong immunogens in MG, though the α-subunit may be the major immune target in a substantial proportion of MG patients.     

ACALCULIA: AN fMRI STUDY WITH IMPLICATIONS WITH RESPECT TO BRAIN PLASTICITY

We report the case of a 44-year-old right-handed man with severe acalculia and 4 normal age-matched controls. The acalculia patient had magnetic resonance findings of a wedge shaped defect in the left posterior temporal-parietal cortex, involving the angular gyrus, and a lacunar infarct of the right thalamus. Functional magnetic resonance exams were performed utilizing four tasks consisting of serial subtracting by sevens, basic calculation, complex calculation, and abstract calculation. Both patient and controls showed specific task-related activation with some differences that may indicate brain plasticity, even though no significant recovery in calculations abilities was observed in the patient     

重症肌无力B细胞免疫应答涉及乙酰胆碱受体的全部亚单位

用酶联免疫斑点（Ｅｌｉｓｐｏｔ）法计数乙酰胆碱受体（ＡＣｈＲ）及其α－、β－、γ－和δ－４种亚单位的特异性ＩｇＧ抗体分泌细胞，评价重症肌无力（ＭＧ）患者外周血Ｂ细胞免疫应答。与其他神经疾病（ＯＮＤ）组相比，ＭＧ组存在显著高水平的ＡＣｈＲ及其４种亚单位的抗体分泌细胞，ＭＧ组４种ＡＣｈＲ亚单位的抗体分泌细胞平均值为１０／１０５～１５／１０５外周血单个核细胞（ＭＮＣ），ＯＮＤ组为０．６／１０５～１．４／１０５ＭＮＣ（Ｐ＜０．０００１）。ＭＧ组ＡＣｈＲ抗体分泌细胞数总是高于任何一种亚单位抗体分泌细胞数，且对α－亚单位的Ｂ细胞免疫应答占有明显优势，这可能是α－亚单位存在主要免疫源区（ＭＩＲ）的间接证据。     

Correlating extent of neuromuscular instability with acetylcholine receptor antibodies

In a retrospective study of 86 patients with myasthenia gravis (MG), we correlated the acetylcholine receptor (AChR) antibody titers with single‐fiber EMG studies to explore whether a relationship exists between these parameters. We found that the AChR antibody titers correlated significantly with the mean of the mean consecutive difference of orbicularis oculi (OO, P < 0.0001) and extensor digitorum communis (EDC, P < 0.0001). The correlation was found to be stronger in OO. The antibody titers also correlated with the percentage of potential pairs with increased jitter in both muscles and, again, the correlation was more significant in OO (P < 0.0001) than in EDC (P = 0.001). We speculate that this relationship is stronger in OO than in the limb muscles, because the architectural and immunological differences in the motor unit render OO more vulnerable and sensitive to disturbances in neuromuscular transmission. Muscle Nerve, 2009     

Complement activation profiles in anti-acetylcholine receptor positive myasthenia gravis

Background and purpose

Complement component 5 (C5) targeting therapies are clinically beneficial in patients with acetylcholine receptor antibody⁺ (AChR-Ab⁺) generalized myasthenia gravis (MG). That clearly implicates antibody-mediated complement activation in MG pathogenesis. Here, classical and alternative complement pathways were profiled in patients from different MG subgroups.

Methods

In a case–control study, concentrations of C3a, C5a and sC5b9 were simultaneously quantified, indicating general activation of the complement system, whether via the classical and lectin pathways (C4a) or the alternative pathway (factors Ba and Bb) in MG patients with AChR or muscle-specific kinase antibodies (MuSK-Abs) or seronegative MG compared to healthy donors.

Results

Treatment-naïve patients with AChR-Ab⁺ MG showed substantially increased plasma levels of cleaved complement components, indicating activation of the classical and alternative as well as the terminal complement pathways. These increases were still present in a validation cohort of AChR-Ab⁺ patients under standard immunosuppressive therapies; notably, they were not evident in patients with MuSK-Abs or seronegative MG. Neither clinical severity parameters (at the time of sampling or 1 year later) nor anti-AChR titres correlated significantly with activated complement levels.

Conclusions

Markers indicative of complement activation are prominently increased in patients with AChR-Ab MG despite standard immunosuppressive therapies. Complement inhibition proximal to C5 cleavage should be explored for its potential therapeutic benefits in AChR-Ab⁺ MG.     

Myasthenia gravis associated with etanercept therapy

Etanercept is an antagonist of tumor necrosis factor alpha that was developed to treat rheumatoid arthritis. In this report we present a patient who developed myasthenia gravis while taking etanercept and had resolution of symptoms after stopping it. This is the first report of this potential side effect and is of additional interest, because etanercept has been proposed as a treatment for myasthenia gravis. Muscle Nerve, 2009     

Single-fiber electromyography in experimental autoimmune myasthenia gravis

The sensitivity of stimulated single-fiber electromyography in the detection of early abnormalities in neuromuscular transmission in experimental autoimmune myasthenia gravis (EAMG) was tested. Increased jitter and blocking were seen up to 3 weeks before clinical illness or decrement developed. Stimulation at 10 Hz appeared more sensitive in detection of abnormalities than stimulation at 3 or 5 Hz. Jitter values did not correlate with anti-Torpedo acetylcholine receptor (AChR), nor with anti-rat AChR antibody titer. No correlation was found between jitter and AChR loss or AChR-antibody complexes in muscle. It is concluded that, in addition to AChR loss and the presence of AChR-antibody complexes, other factors must determine the neuromuscular dysfunction in EAMG and possibly myasthenia gravis.     

Clinical findings in MuSK‐antibody positive myasthenia gravis: A U.S. experience

We performed a retrospective chart review on 53 muscle‐specific kinase antibody (MuSK‐Ab)‐positive myasthenia gravis (MG) patients at nine university‐based centers in the U.S. Of these, 66% were Caucasian, 85% were women, and age of onset was 9–79 years. Twenty‐seven patients were nonresponsive to anticholinesterase therapy. Myasthenia Gravis Foundation of America improvement status was achieved in 53% patients on corticosteroids, 51% with plasma exchange, and in 20% on intravenous immunoglobulin (IVIG). Thymectomy was beneficial in 7/18 patients at 3 years. Long‐term (≥3 years) outcome was very favorable in 58% of patients who achieved remission and/or minimal manifestation status. Overall, 73% improved. There was one MG‐related death. This survey reinforces several cardinal features of MuSK‐Ab‐positive MG, including prominent bulbar involvement and anticholinesterase nonresponsiveness. Facial or tongue atrophy was rare. Most patients respond favorably to immunotherapy. The best clinical response was to corticosteroids and plasma exchange, and the poorest response was to IVIG. Long‐term outcome is favorable in about 60% of cases. Muscle Nerve, 2009     

Age-related susceptibility to experimental autoimmune myasthenia gravis: Immunological and electrophysiological aspects

Susceptibility to experimental autoimmune myasthenia gravis (EAMG) was found to decrease with aging in both Lewis and Brown Norway (BN) rats. In this study, the difference in susceptibility between young and aged Lewis and BN rats was used to analyze factors determining the clinical severity of EAMG. The incidence and severity of muscular weakness did not correlate with acetylcholine receptor (AChR) loss nor with the ability of antibodies to interfere with AChR function. Aged rats showed significantly lower anti-rat AChR antibody titers than young rats and developed less severe or no clinical signs of disease. In individual young or aged rats, however, no significant correlation was found between the clinical signs of disease and anti-rat AChR titer. Neuromuscular transmission was found to change with aging as measured by single-fiber electromyography (SFEMG). In aged BN rats, increased jitter and blockings were found even before EAMG induction. Despite this disturbed neuromuscular transmission, these aged BN rats were clinically resistant against induction of EAMG. The results of this study indicate that the age-related susceptibility to EAMG is influenced by factors determined by the immune attack as well as mechanisms at the level of the neuromuscular junction. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1091–1101, 1997     

Seronegative myasthenia gravis: disease severity and prognosis

Around 10-20% of myasthenia gravis (MG) patients do not have acetylcholine receptor (AChR) antibodies (seronegative), of whom some have antibodies to a membrane-linked muscle specific kinase (MuSK). To examine MG severity and long-term prognosis in seronegative MG compared with seropositive MG, and to look specifically at anti-AChR antibody negative and anti-MuSK antibody negative patients. Seventeen consecutive seronegative non-thymomatous MG patients and 34 age and sex matched contemporary seropositive non-thymomatous MG controls were included in a retrospective follow-up study for a total period of 40 years. Clinical criteria were assessed each year, and muscle antibodies were assayed. There was no difference in MG severity between seronegative and seropositive MG. However, when thymectomized patients were excluded from the study at the year of thymectomy, seropositive MG patients had more severe course than seronegative (P < 0.001). One seropositive patient died from MG related respiratory insufficiency. The need for thymectomy in seronegative MG was lower than in seropositive MG. None of the seronegative patients had MuSK antibodies. This study shows that the presence of AChR antibodies in MG patients correlates with a more severe MG. With proper treatment, especially early thymectomy for seropositive MG, the outcome and long-term prognosis is good in patients with and without AChR antibodies.     

Low conversion rate of ocular to generalized myasthenia gravis in Singapore

Introduction: Ocular myasthenia gravis (OMG) is a common condition of the neuromuscular junction that may convert to generalized myasthenia gravis (GMG). Our aim in this study was to determine the conversion rate and predictive factors for generalization in OMG, in an Asian population. Methods: The investigation consisted of a retrospective study of OMG patients with a minimum 2 years of follow‐up. Results: Among 191 patients with OMG, 155 had the minimum 2‐year follow‐up. The conversion rate at median follow‐up (40.8 months) was 10.6% (95% confidence interval 7.9%–13.3%), and at the 2‐year follow‐up it was 7.7% (95% confidence interval 5.6%–9.8%). At baseline, the predictive factors for generalization were positive acetylcholine receptor antibodies (hazard ratio 3.71, P = 0.024), positive repetitive nerve stimulation (RNS) studies (hazard ratio 4.42, P = 0.003), and presence of radiologically presumed or pathologically confirmed thymoma (hazard ratio 3.10, P = 0.013). Discussion: The conversion rate of OMG to GMG in Asian patients is low, as predicted by presence of acetylcholine receptor antibodies, presence of thymoma, and positive RNS studies. Muscle Nerve 57 : 756–760, 2018     

Immunochemical properties of junctional and extrajunctional acetylcholine receptor

Immunological assays were performed to compare two distinct forms of the nicotinic acetylcholine receptor (AChR): junctional (JR) and extrajunctional receptor (EJR). Antibodies from myasthenia gravis patients' sera inhibited the binding of [¹²⁵I]α-bungarotoxin (BGT), to EJR more effectively than binding to JR. Immunological differences between JR and EJR were confirmed by other assay methods. In all cases, EJR appeared to have antigenic determinants not found on JR. It was established that enzymatic removal of carbohydrates from EJR caused it to more closely resemble JR. Thus differences between JR and EJR may be due, in part, to carbohydrate residues found on EJR that are absent on JR. The extent of antibody binding to EJR was examined by gel filtration methods. Immunochemical studies of bands from SDS gels showed that antibodies are present in myasthenic serum which react with the 3 subunits (42, 53, 64 kdaltons) of AChR to varying degrees.     

白细胞介素-6与重症肌无力患者临床特点的关系

目的　探讨白细胞介素 6(IL 6)与重症肌无力(MG)患者临床特点的关系。方法　采用双抗体 夹心ELISA法检测36例MG患者及20名健康对照者的血清IL 6和乙酰胆碱受体抗体(AchRAb)水平,并分 析其与MG临床特点的关系。结果　MG患者血清IL 6水平高于健康对照者(P<0.01);AchRAb阳性患者 高于阴性患者(P<0.05);全身型患者高于眼肌型患者(P<0.05);病情重者高于病情轻者(P<0.05);急性 期高于非急性期(P<0.01);预后差者高于预后好者(P<0.05);伴胸腺异常者高于胸腺正常者(P<0.05)。 结论　IL 6与MG临床特点相关,在MG发病机制中起重要作用。血清IL 6水平可间接反映体内免疫功能紊 乱的程度,对判断MG患者病情、预后和指导治疗有重要的参考价值。