Congenital myasthenic syndromes (CMS) result from reduced cholinergic transmission at neuromuscular junctions (NMJs). While the etiology of CMS varies, the disease is characterized by muscle weakness. To date, it remains unknown if CMS causes long-term and irreversible changes to skeletal muscles. In this study, we examined skeletal muscles in a mouse line with reduced expression of Vesicular Acetylcholine Transporter (VAChT, mouse line herein called VAChT-KD^HOM). We examined this mouse line for several reasons. First, VAChT plays a central function in loading acetylcholine (ACh) into synaptic vesicles and releasing it at NMJs, in addition to other cholinergic nerve endings. Second, loss of function mutations in VAChT causes myasthenia in humans. Importantly, VAChT-KD^HOM present with reduced ACh and muscle weakness, resembling CMS. We evaluated the morphology, fiber type (myosin heavy chain isoforms), and expression of muscle-related genes in the extensor digitorum longus (EDL) and soleus muscles. This analysis revealed that while muscle fibers atrophy in the EDL, they hypertrophy in the soleus muscle of VAChT-KD^HOM mice. Along with these cellular changes, skeletal muscles exhibit altered levels of markers for myogenesis (Pax-7, Myogenin, and MyoD), oxidative metabolism (PGC1-α and MTND1), and protein degradation (Atrogin1 and MuRF1) in VAChT-KD^HOM mice. Importantly, we demonstrate that deleterious changes in skeletal muscles and motor deficits can be partially reversed following the administration of the cholinesterase inhibitor, pyridostigmine in VAChT-KD^HOM mice. These findings reveal that fast and slow type muscles differentially respond to cholinergic deficits. Additionally, this study shows that the adverse effects of cholinergic transmission, as in the case of CMS, on fast and slow type skeletal muscles are reversible.

先天性肌无力综合征 (CMS) 是由神经肌肉接头 (NMJ) 胆碱能传递减少引起的。虽然 CMS 的病因各不相同，但该疾病的特点是肌肉无力。迄今为止，尚不清楚 CMS 是否会导致骨骼肌发生长期且不可逆转的变化。^{在这项研究中，我们检查了水泡乙酰胆碱转运蛋白（VAChT，本文中称为 VAChT-KD HOM}的小鼠品系）表达减少的小鼠品系的骨骼肌。我们出于多种原因检查了该小鼠品系。首先，除了其他胆碱能神经末梢之外，VAChT 在将乙酰胆碱 (ACh) 加载到突触小泡中并在 NMJ 处释放方面发挥着核心功能。其次，VAChT 的功能缺失突变会导致人类肌无力。重要的是，VAChT-KD ^HOM表现为乙酰胆碱减少和肌肉无力，类似于 CMS。我们评估了趾长伸肌 (EDL) 和比目鱼肌的形态、纤维类型（肌球蛋白重链亚型）和肌肉相关基因的表达。该分析表明，虽然 EDL 中的肌纤维萎缩，但 VAChT-KD ^HOM小鼠的比目鱼肌却肥大。^{除了这些细胞变化外，VAChT-KD HOM}小鼠的骨骼肌还表现出肌生成（Pax-7、Myogenin 和 MyoD）、氧化代谢（PGC1-α 和 MTND1）和蛋白质降解（Atrogin1 和 MuRF1）标记物水平的改变。^{重要的是，我们证明在 VAChT-KD HOM}小鼠中给予胆碱酯酶抑制剂吡斯的明后，骨骼肌和运动缺陷的有害变化可以部分逆转。这些发现表明，快型和慢型肌肉对胆碱能缺陷的反应不同。此外，这项研究表明，胆碱能传递（如 CMS 的情况）对快型和慢型骨骼肌的不利影响是可逆的。