Amyotrophic lateral sclerosis (ALS) is a devastating motoneuron disease, in which lower motoneurons lose control of skeletal muscles. Degeneration of neuromuscular junctions (NMJs) occurs at the initial stage of ALS. Dipeptide repeat proteins (DPRs) from G4C2 repeat-associated non-ATG (RAN) translation are known to cause C9orf72-associated ALS (C9-ALS). However, DPR inclusion burdens are weakly correlated with neurodegenerative areas in C9-ALS patients, indicating that DPRs may exert cell non-autonomous effects, in addition to the known intracellular pathological mechanisms. Here, we report that poly-GA, the most abundant form of DPR in C9-ALS, is released from cells. Local administration of poly-GA proteins in peripheral synaptic regions causes muscle weakness and impaired neuromuscular transmission in vivo. The NMJ structure cannot be maintained, as evidenced by the fragmentation of postsynaptic acetylcholine receptor (AChR) clusters and distortion of presynaptic nerve terminals. Mechanistic study demonstrated that extracellular poly-GA sequesters soluble Agrin ligands and inhibits Agrin-MuSK signaling. Our findings provide a novel cell non-autonomous mechanism by which poly-GA impairs NMJs in C9-ALS. Thus, targeting NMJs could be an early therapeutic intervention for C9-ALS.
肌萎缩性侧索硬化症 (ALS) 是一种致死性的运动神经元退行性疾病，病人的运动神经元逐渐病变死亡，导致无法控制骨骼肌。神经肌肉接头(NMJ)是运动神经元与骨骼肌之间的突触连接，其退化在ALS的初始阶段就开始发生。第九号染色体C9orf72基因的内含子区域GGGGCC的异常重复，是导致ALS的最主要遗传因素。GGGGCC重复能够不依赖ATG翻译启动子，产生毒性的二肽重复蛋白(DPR)，其中多聚甘氨酸-丙氨酸重复多肽蛋白（poly-GA）是病人中含量最高的DPR。我们发现在肌肉突触局部注射poly-GA蛋白，会导致肌肉无力并损害神经肌肉的信息传递，同时还影响了NMJ结构的维持。我们进一步发现poly-GA会结合凝集素Agrin，进而抑制Agrin-MuSK信号通路，从而导致乙酰胆碱受体解聚。我们的研究说明poly-GA可以通过细胞非自主机制来损害NMJ和运动功能，针对C9-ALS的NMJ早期靶向治疗干预可能是一个有前景的方向。.
Keywords: Agrin; Amyotrophic lateral sclerosis; Neuromuscular junction; Poly-Gly-Ala.