Electrophysiological phenotypes in a Drosophila model of SMA

描述（由申请人提供）：脊髓性肌萎缩症（SMA）是人类中最普遍的儿童遗传性疾病之一，是由于端粒SMN1基因的缺失和与SMN1相似的着丝粒同源物SMN2的部分恢复，但由于剪接错误，产生的功能SMN蛋白很少。主要症状包括神经肌肉系统进行性衰竭、运动神经元丧失、肌肉无力和萎缩。目前缺乏治疗方案，因此，它们的发展需要在研究上积极投资，以了解潜在的疾病机制。小鼠SMA模型依赖于补充了人类SMN2基因的缺失突变体，以及最近的组织特异性敲除SMN。总的来说，使用该模型系统的实验基本上支持了SMA的神经元起源。在果蝇中，SMN单一同源基因的突变表现为生存能力降低、活动能力缺陷和神经-肌肉连接处（NMJ）突触前接触数量的畸变。重要的是，果蝇的SMA表型似乎强烈依赖于肌肉中的SMN功能，并且来自脊椎动物模型的几份报告强调了“仅神经元”模型的不足，这表明肌肉在SMA病理生理中不是被动的参与者。事实上，SMA可以被视为一种由NMJ突触维护不当引起的疾病。次优的NMJ可能在突触传递上有缺陷，导致异常的运动行为。然而，目前还缺乏对SMN突变进行彻底的电生理评估，以了解肌肉中SMN减少的相对重要性。由于最近对RNAi等位基因的描述，这种分析现在是可行的，这些等位基因可用于选择性地敲除肌肉组织中的SMN。此外，已经进行了遗传筛选，以鉴定修饰SMN依赖性表型的基因，以寻找可能被操纵作为SMA治疗的潜在遗传元件。其中一种候选途径是FGF通路，它可以修复由肌肉中SMN缺失引起的突触结构缺陷，尽管目前尚不清楚FGF是否也可以修复SMA的电生理缺陷。在本提案中，我们通过首先检查由果蝇肌肉特异性SMN丢失引起的电生理和行为表型来解决这些悬而未决的问题。接下来，我们评估FGF信号通路在挽救这些表型中的功效。因此，该项目首次集中评估了肌肉组织中SMN缺失所导致的电生理后果。

DESCRIPTION (provided by applicant): Spinal muscular atrophy (SMA), one of the most widespread childhood genetic disorders in humans, is due to the loss of the telomeric SMN1 gene and partial rescue by the centromeric homolog SMN2 which is similar to SMN1 but that, due to splicing errors, makes very little functional SMN protein. Predominant symptoms include progressive failure of the neuro-muscular system, motor neuron loss, muscular weakness and atrophy. Therapeutic options are currently absent, and, therefore, their development requires aggressive investments in research to understand underlying disease mechanisms. Mouse models of SMA rely on deletion mutants that are supplemented with the human SMN2 gene, and also more recently tissue specific knock outs of SMN. Collectively, experiments using this model system have essentially supported a neuronal origin of SMA. In Drosophila, mutations in the single homolog of SMN display reduced viability, mobility defects and aberrations in the number of pre-synaptic contacts at the neuro-muscular junction (NMJ). Importantly, SMA phenotypes in flies seem to rely strongly on SMN function in muscle, and together with several reports from vertebrate models that highlight the insufficiency of a "neuron-only" model, suggest that the muscle is not a passive player in SMA pathophysiology. Indeed SMA can be viewed as a disease that arises from improper synaptic maintenance at the NMJ. A sub-optimal NMJ is likely to be defective in synaptic transmission, leading to abnormal motor behavior. However, a thorough electrophysiological evaluation of SMN mutations to understand the relative importance of reduced SMN in muscles has been lacking. Such analysis is now feasible due to the recent description of RNAi alleles that can be used to selectively knock down SMN in muscle tissue. Additionally, genetic screens have been conducted to identify genes that modify SMN dependent phenotypes in search of potential genetic elements that may be manipulated as therapy for SMA. One such candidate, the FGF pathway rescues structural defects at the synapse that arise from loss of SMN in muscles, though it is unclear whether electrophysiological defects in SMA will also be rescued by FGF. In this proposal, we address these outstanding questions by first examining electrophysiological and behavioral phenotypes that are caused by muscle specific loss of SMN in Drosophila. We next evaluate the efficacy of the FGF signaling pathway in rescuing these phenotypes. Thus, this project represents the first focused assessment of electrophysiological consequences that result from loss of SMN in the muscle tissue. PUBLIC HEALTH RELEVANCE: Spinal muscular atrophy (SMA) leads to neuron loss and muscle atrophy, though the relative contribution of these two tissues to disease pathology is unclear. We propose to make use of powerful genetic models of SMA in Drosophila to assay neuro-muscular transmission defects that arise from loss of SMN in muscles. We will also evaluate a signaling pathway (FGF) for its ability to rescue SMN derived electrophysiological and behavioral phenotypes.