Spaulding Emily L, Sleigh James N, Morelli Kathryn H, Pinter Martin J, Burgess Robert W, Seburn Kevin L
The Jackson Laboratory, Bar Harbor, Maine 04609, Graduate School of Biomedical Science and Engineering, University of Maine, Orono, Maine 04469.
Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford OX3 9DU, United Kingdom, and.
J Neurosci. 2016 Mar 16;36(11):3254-67. doi: 10.1523/JNEUROSCI.1762-15.2016.
Patients with Charcot-Marie-Tooth Type 2D (CMT2D), caused by dominant mutations in Glycl tRNA synthetase (GARS), present with progressive weakness, consistently in the hands, but often in the feet also. Electromyography shows denervation, and patients often report that early symptoms include cramps brought on by cold or exertion. Based on reported clinical observations, and studies of mouse models of CMT2D, we sought to determine whether weakened synaptic transmission at the neuromuscular junction (NMJ) is an aspect of CMT2D. Quantal analysis of NMJs in two different mouse models of CMT2D (Gars(P278KY), Gars(C201R)), found synaptic deficits that correlated with disease severity and progressed with age. Results of voltage-clamp studies revealed presynaptic defects characterized by: (1) decreased frequency of spontaneous release without any change in quantal amplitude (miniature endplate current), (2) reduced amplitude of evoked release (endplate current) and quantal content, (3) age-dependent changes in the extent of depression in response to repetitive stimulation, and (4) release failures at some NMJs with high-frequency, long-duration stimulation. Drugs that modify synaptic efficacy were tested to see whether neuromuscular performance improved. The presynaptic action of 3,4 diaminopyridine was not beneficial, whereas postsynaptic-acting physostigmine did improve performance. Smaller mutant NMJs with correspondingly fewer vesicles and partial denervation that eliminates some release sites also contribute to the reduction of release at a proportion of mutant NMJs. Together, these voltage-clamp data suggest that a number of release processes, while essentially intact, likely operate suboptimally at most NMJs of CMT2D mice.
We have uncovered a previously unrecognized aspect of axonal Charcot-Marie-Tooth disease in mouse models of CMT2D. Synaptic dysfunction contributes to impaired neuromuscular performance and disease progression. This suggests that drugs which improve synaptic efficacy at the NMJ could be considered in treating the pathophysiology of CMT2D patients.
由甘氨酰tRNA合成酶（GARS）显性突变引起的2D型夏科-马里-图思病（CMT2D）患者表现为进行性肌无力，主要累及手部，但足部也常受累。肌电图显示失神经支配，患者常报告早期症状包括因寒冷或劳累引发的痉挛。基于已报道的临床观察以及CMT2D小鼠模型的研究，我们试图确定神经肌肉接头（NMJ）处突触传递减弱是否为CMT2D的一个特征。对两种不同的CMT2D小鼠模型（Gars(P278KY)、Gars(C201R)）的NMJ进行量子分析，发现突触缺陷与疾病严重程度相关且随年龄进展。电压钳研究结果揭示了突触前缺陷，其特征为：（1）自发释放频率降低，量子幅度（微小终板电流）无任何变化；（2）诱发释放（终板电流）幅度和量子含量降低；（3）对重复刺激的抑制程度随年龄变化；（4）在一些NMJ处，高频、长时间刺激会导致释放失败。测试了改变突触效能的药物，以观察神经肌肉功能是否改善。3,4 - 二氨基吡啶的突触前作用并无益处，而作用于突触后的毒扁豆碱确实改善了功能。较小的突变型NMJ，其囊泡数量相应减少且部分失神经支配消除了一些释放位点，这也导致一部分突变型NMJ的释放减少。总之，这些电压钳数据表明，虽然许多释放过程基本完整，但在CMT2D小鼠的大多数NMJ处可能运作欠佳。
我们在CMT2D小鼠模型中发现了轴索性夏科-马里-图思病一个此前未被认识的特征。突触功能障碍导致神经肌肉功能受损和疾病进展。这表明改善NMJ处突触效能的药物可考虑用于治疗CMT2D患者的病理生理学。
