Department of Human Genetics, Medical Genetics Centre, Leiden University Medical Centre, Leiden, The Netherlands.
Brain. 2012 Apr;135(Pt 4):1081-101. doi: 10.1093/brain/aws025. Epub 2012 Mar 6.
Myasthenia gravis is a paralytic disorder with autoantibodies against acetylcholine receptors at the neuromuscular junction. A proportion of patients instead has antibodies against muscle-specific kinase, a protein essential for acetylcholine receptor clustering. These are generally of the immunoglobulin-G4 subclass and correlate with disease severity, suggesting specific myasthenogenic activity. However, immunoglobulin-G4 subclass antibodies are generally considered to be 'benign' and direct proof for their pathogenicity in muscle-specific kinase myasthenia gravis (or other immunoglobulin-G4-associated disorders) is lacking. Furthermore, the exact electrophysiological synaptic defects caused at neuromuscular junctions by human anti-muscle-specific kinase autoantibodies are hitherto unknown. We show that purified immunoglobulin-G4, but not immunoglobulin-G1-3, from patients with muscle-specific kinase myasthenia gravis binds to mouse neuromuscular junctions in vitro, and that injection into immunodeficient mice causes paralysis. Injected immunoglobulin-G4 caused reduced density and fragmented area of neuromuscular junction acetylcholine receptors. Detailed electrophysiological synaptic analyses revealed severe reduction of postsynaptic acetylcholine sensitivity, and exaggerated depression of presynaptic acetylcholine release during high-rate activity, together causing the (fatigable) muscle weakness. Intriguingly, compensatory transmitter release upregulation, which is the normal homeostatic response in acetylcholine receptor myasthenia gravis, was absent. This conveys extra vulnerability to neurotransmission at muscle-specific kinase myasthenia gravis neuromuscular junctions. Thus, we demonstrate that patient anti-muscle-specific kinase immunoglobulin-G4 is myasthenogenic, independent of additional immune system components, and have elucidated the underlying electrophysiological neuromuscular junction abnormalities.
重症肌无力是一种神经肌肉接头处乙酰胆碱受体自身抗体介导的瘫痪性疾病。一部分患者存在肌肉特异性激酶抗体，这种蛋白对于乙酰胆碱受体聚集至关重要。这些抗体通常属于免疫球蛋白 G4 亚类，与疾病严重程度相关，提示具有特定的致肌无力作用。然而，免疫球蛋白 G4 亚类抗体通常被认为是“良性的”，缺乏其在肌肉特异性激酶重症肌无力（或其他免疫球蛋白 G4 相关疾病）中的致病性的直接证据。此外，人类抗肌肉特异性激酶自身抗体在神经肌肉接头处引起的确切电生理学突触缺陷迄今尚不清楚。我们发现，来自肌肉特异性激酶重症肌无力患者的纯化免疫球蛋白 G4（而非免疫球蛋白 G1-3）可在体外与小鼠神经肌肉接头结合，并且注射到免疫缺陷小鼠中会引起瘫痪。注射的免疫球蛋白 G4 导致神经肌肉接头乙酰胆碱受体的密度降低和面积碎片化。详细的电生理学突触分析显示，突触后乙酰胆碱敏感性严重降低，在高频率活动期间，突触前乙酰胆碱释放的抑制作用增强，导致（易疲劳性）肌肉无力。有趣的是，在肌肉特异性激酶重症肌无力中，作为正常的代偿性递质释放上调缺失。这使得神经递质传递在肌肉特异性激酶重症肌无力神经肌肉接头处更容易受到影响。因此，我们证明了患者的抗肌肉特异性激酶免疫球蛋白 G4 具有致肌无力作用，不依赖于其他免疫系统成分，并且阐明了潜在的电生理学神经肌肉接头异常。
