The contributing effects of muscle, nerve and the NMJ to SMA pathology

描述(申请人提供)：近端脊髓性肌萎缩症(SMA)是一种常见的神经肌肉疾病，由运动神经元存活1(SMN1)基因突变及其翻译产物SMN蛋白水平不足引起。SMA是导致儿童死亡的最常见的遗传原因。在SMA小鼠和人类患者中，这种疾病的特征包括脊髓运动神经元丢失和骨骼肌萎缩。基于这些特征，人们普遍认为运动神经元选择性地易受SMN减少的影响，肌肉萎缩是神经变性的继发性后果。尽管有这些长期持有的信念，关于运动神经元是否真的唯一容易受到SMN自主作用细胞水平降低的影响的激烈辩论仍在继续。或者，与运动神经元密切相关的其他细胞类型的初级效应可能会引发神经退化。如果SMN确实在运动神经元内发挥作用，以确保它们的健康和生存，目前还不清楚为什么它们而不是其他细胞对蛋白质水平的降低如此敏感。为了更好地了解SMA的分子和细胞原因，已经产生了从基因上模拟人类情况的小鼠模型。在NIH的这份资金申请中，我们概述了三个相关的实验，目的是确定SMA是否是一种完全由运动神经元健康决定的疾病，以及将SMN蛋白的正常水平恢复到这种细胞类型是否足以完全改善疾病表型。我们建议用两种方式回答这个问题。首先，我们将选择性地将SMN恢复到SMA小鼠的运动神经元，并询问这是否导致完全的表型纠正。其次，我们将选择性地消耗健康小鼠运动神经元和两个相关组织-肌肉和神经胶质细胞中的SMN蛋白，并询问这种操作在多大程度上造成神经肌肉病理。在第二组实验中，我们将确定为什么SMN蛋白不足会导致神经肌肉系统的选择性退化。为了回答这个问题，我们将研究减少SMN对SMA小鼠神经肌肉突触发育的影响。如果减少的SMN扰乱了突触及其组成蛋白的发育，这对确保正常的神经肌肉功能至关重要，这将解释人类疾病特有的神经肌肉病理。鉴于SMA在人类中的发生率很高，缺乏有效的治疗方法，并由此给社会带来负担，因此必须尽可能及时地回答这里提出的问题。公共卫生相关性：脊髓性肌萎缩症是一种毁灭性的神经退行性疾病，是婴幼儿的主要基因杀手。SMA目前无法治疗。了解SMA导致神经肌肉衰竭和死亡的原因对于设计适当的治疗方法很重要。在这项提案中，我们将使用人类状况的小鼠模型来确定哪些细胞类型导致神经肌肉衰竭以及它们为什么退化。我们相信，我们的结果将对SMA成功疗法的设计产生深远影响。

DESCRIPTION (provided by applicant): Proximal spinal muscular atrophy (SMA) is a common neuromuscular disorder caused by mutations in the Survival of Motor Neuron 1 (SMN1) gene and insufficient levels of its translated product, the SMN protein. SMA is the most common genetic cause of childhood mortality. Hallmarks of the disease in SMA mice and human patients include spinal motor neuron loss and skeletal muscle atrophy. Based on these characteristics it is widely believed that motor neurons are selectively vulnerable to reduced SMN and that muscle atrophy is a secondary consequence of neurodegeneration. These long-held beliefs notwithstanding, there continues to be a vigorous debate about whether motor neurons are indeed uniquely susceptible to reduced levels of SMN acting cell autonomously within them. Alternatively, neurodegeneration could be triggered by primary effects on some other cell type closely associated with motor neurons. If SMN does function within motor neurons to ensure their health and survival, it is not clear why they and not other cells are so sensitive to reduced levels of the protein. To better understand the molecular and cellular causes of SMA, mouse models that genetically mimic the human condition have been generated. In this application for funding to the NIH, we have outlined experiments described in three related aims to determine if SMA is a disease dictated exclusively by the health of the motor neurons and whether restoring normal levels of the SMN protein to this cell type is sufficient to completely ameliorate the disease phenotype. We propose to answer this question in two ways. Firstly, we will restore SMN selectively to the motor neurons of mice with SMA and ask if this results in complete phenotypic correction. Secondly, we will selectively deplete the SMN protein in the motor neurons and two associated tissues, muscle and glia, of healthy mice and ask to what extent such manipulations create neuromuscular pathology. In a second set of experiments, we will determine why insufficient SMN protein causes a selective degeneration of the neuromuscular system. To answer this question, we will look at the effects of reduced SMN on the development of the nerve-muscle synapse of SMA mice. If reduced SMN disrupts the development of this synapse and its constituent proteins which are crucial in ensuring proper nerve-muscle function, it will explain the neuromuscular pathology so characteristic of the human disease. Given the high frequency of SMA among humans, the lack of an effective treatment and the consequent burden it places on society, it is imperative that questions such as those posed here be answered in as timely a manner as possible. PUBLIC HEALTH RELEVANCE: Spinal muscular atrophy is a devastating neurodegenerative disease and the leading genetic killer of infants and toddlers. SMA is not presently treatable. Understanding why SMA results in neuromuscular failure and death is important to designing an appropriate treatment. In this proposal, we will use mouse models of the human condition to determine which cell types contribute to neuromuscular failure and why they degenerate. We believe our results will profoundly impact the design of successful therapies for SMA.