Muscle and neuromuscular junctions in spinal muscular atrophy

描述（由申请人提供）：脊髓性肌萎缩症（SMA）是一种目前无法治疗的常染色体隐性运动神经元疾病，是婴儿死亡的主要遗传原因。SMA是由运动神经元存活蛋白（SMN）缺乏引起的。我们的长期研究目标是了解SMA的潜在发病机制，以便为这种疾病制定有效的治疗策略。最近的研究表明，SMA开始是因为肌肉和运动神经末梢的内在异常;然而，这些缺陷的性质仍然未知。组蛋白去乙酰化酶（HDAC）抑制剂已被证明可以增加SMA小鼠的存活率;但目前还不清楚这些药物如何改善肌肉和/或运动神经元功能。在SMA小鼠的初步研究中，我们已经表明，当小鼠极度虚弱时，几乎没有结构性去神经支配。然而，有广泛的不成熟和营养不良的肌纤维以及简化的神经肌肉接头（NMJ）。用泛HDAC抑制剂处理的小鼠显示出存活期的显著延长、运动功能的增加以及肌纤维的大小和成熟度的改善，而运动神经元数量没有变化。基于这些初步数据，我们假设SMN缺乏导致肌肉成熟停滞和/或NMJ传递失败，这可以用HDAC抑制剂克服，HDAC抑制剂可以加速SMA运动单元的发育。我们进一步假设，HDAC亚型特异性药物将对SMA运动单元具有不同的生物学效应，这将为这些化合物的治疗机制提供重要的见解。我们将通过以下方式检验这些假设：1）通过研究培养的SMA肌肉细胞和通过在条件性SMA小鼠中的肌肉组织中特异性地拯救SMN表达来确定肌肉发育的缺陷是否有助于SMA，2）通过检查SMA小鼠中NMJ的电生理学和形态学来确定SMA的虚弱是否是由于神经肌肉传递的损伤，和3）通过用广泛活性和HDAC同种型特异性HDAC抑制剂处理SMA肌细胞和SMA小鼠，表征HDAC抑制剂促进肌肉和NMJ成熟并改善小鼠中SMA的能力。

DESCRIPTION (provided by applicant): Spinal muscular atrophy (SMA) is a currently untreatable, autosomal recessive motor neuron disease that is the leading inherited cause of infant mortality. SMA is caused by deficiency of the survival motor neuron (SMN) protein. Our long term research goal is to understand the underlying pathogenesis of SMA in order to develop effective treatment strategies for this disease. Recent studies suggest that SMA begins because of intrinsic abnormalities of both muscle and motor nerve terminals; however the nature of these defects remains unknown. Histone deacetylase (HDAC) inhibitors have been shown to increase survival of SMA mice; but it is unclear how these drugs improve muscle and/or motor neuron function. In preliminary studies in SMA mice, we have shown that at a time when the mouse is profoundly weak, there is little structural denervation. However there are widespread immature and hypotrophic myofibers as well as simplified neuromuscular junctions (NMJs). Mice treated with a pan- HDAC inhibitor show a substantial extension of survival, increase in motor function, and improvement in the size and maturity of myofibers, without a change in motor neuron number. Based on these preliminary data, we hypothesize that SMN deficiency causes an arrest of muscle maturation and/or a failure of NMJ transmission that can be overcome with HDAC inhibitors, which accelerate the development of the SMA motor unit. We further hypothesize that HDAC isoform-specific drugs will have distinct biological effects on the SMA motor unit that will provide crucial insights into the therapeutic mechanism of these compounds. We will test these hypotheses by: 1) establishing whether or not a defect of muscle development contributes to SMA by studying cultured SMA muscle cells and by rescuing SMN expression specifically in muscle tissue in conditional SMA mice, 2) determining whether or not weakness in SMA is due to an impairment of neuromuscular transmission by examining the electrophysiology and morphology of the NMJs in SMA mice, and 3) characterizing the ability of HDAC inhibitors to facilitate muscle and NMJ maturation and ameliorate SMA in mice by treating SMA muscle cells and SMA mice with broadly active and HDAC-isoform specific HDAC inhibitors.