Up to 50% of patients with myasthenia gravis (MG) without acetylcholine receptor antibodies (AChR-Abs) have antibodies to muscle-specific kinase (MuSK). Most MuSK antibodies (MuSK-Abs) are IgG4 and inhibit agrin-induced MuSK phosphorylation, leading to impaired clustering of AChRs at the developing or mature neuromuscular junction. However, IgG1-3 MuSK-Abs also exist in MuSK-MG patients, and their potential mechanisms have not been explored fully.

C2C12 myotubes were exposed to MuSK-MG plasma IgG1-3 or IgG4, with or without purified agrin. MuSK, Downstream of Kinase 7 (DOK7), and βAChR were immunoprecipitated and their phosphorylation levels identified by immunoblotting. Agrin and agrin-independent AChR clusters were measured by immunofluorescence and AChR numbers by binding of ¹²⁵I-α-bungarotoxin. Transcriptomic analysis was performed on treated myotubes.

IgG1-3 MuSK-Abs impaired AChR clustering without inhibiting agrin-induced MuSK phosphorylation. Moreover, the well-established pathway initiated by MuSK through DOK7, resulting in βAChR phosphorylation, was not impaired by MuSK-IgG1-3 and was agrin-independent. Nevertheless, the AChR clusters did not form, and both the number of AChR microclusters that precede full cluster formation and the myotube surface AChRs were reduced. Transcriptomic analysis did not throw light on the pathways involved. However, the SHP2 inhibitor, NSC-87877, increased the number of microclusters and led to fully formed AChR clusters.

MuSK-IgG1-3 is pathogenic but seems to act through a noncanonical pathway. Further studies should throw light on the mechanisms involved at the neuromuscular junction.

高达 50% 没有乙酰胆碱受体抗体 (AChR-Abs) 的重症肌无力 (MG) 患者具有肌肉特异性激酶 (MuSK) 抗体。大多数 MuSK 抗体 (MuSK-Ab) 是 IgG4，可抑制集聚蛋白诱导的 MuSK 磷酸化，导致发育或成熟神经肌肉接头处的 AChR 聚集受损。然而，MuSK-MG 患者中也存在 IgG1-3 MuSK-Ab，其潜在机制尚未得到充分探索。

将 C2C12 肌管暴露于含有或不含纯化集聚蛋白的 MuSK-MG 血浆 IgG1-3 或 IgG4。对 MuSK、激酶 7 下游 (DOK7) 和 βAChR 进行免疫沉淀，并通过免疫印迹鉴定其磷酸化水平。通过免疫荧光测量集聚蛋白和不依赖集聚蛋白的 AChR 簇，并通过¹²⁵ I-α-银环蛇毒素的结合测量 AChR 数量。对处理过的肌管进行转录组分析。

IgG1-3 MuSK-Ab 会损害 AChR 聚类，但不会抑制集聚蛋白诱导的 MuSK 磷酸化。此外，由 MuSK 通过 DOK7 启动、导致 βAChR 磷酸化的成熟途径不会受到 MuSK-IgG1-3 的损害，并且不依赖于集聚蛋白。然而，AChR 簇并未形成，并且在完整簇形成之前的 AChR 微簇数量和肌管表面 AChR 均减少。转录组分析并没有阐明所涉及的途径。然而，SHP2 抑制剂 NSC-87877 增加了微簇的数量并导致完全形成的 AChR 簇。

MuSK-IgG1-3 具有致病性，但似乎通过非规范途径发挥作用。进一步的研究应该能够阐明神经肌肉接头所涉及的机制。