Deconstructing Spasticity after Spinal Cord Injury

本研究的目的是研究痉挛背后的细胞和分子机制，以及 为未来临床转化研究奠定基础。大多数美国退伍军人患有 SCI 经历临床上显着的痉挛，这可能会扰乱康复并对生活质量产生负面影响， 例如，流动性、个人卫生、亲密关系（Holtz 等人，2017 年；Skold 等人，1999 年；Walter 等人， 2002）。目前的痉挛管理策略只是姑息治疗，未能解决根本原因。 由于非特异性作用或长期使用，现有的治疗方案也存在很高的不良反应风险 （Adams 等人，2005 年；Kheder 等人，2012 年）。开发更好的 SCI 治疗方法面临的一个主要障碍是 诱发性痉挛是指缺乏对损伤如何导致残疾的机制认识。 为了推进改善痉挛管理的循证调查，我们将开展 实验有两个目标： 在目标 1 中，我们将实施条件敲除研究，以了解 Rac1 特别在运动神经元和星形胶质细胞中，导致 SCI 后痉挛。我们之前的工作表明 药物抑制剂可以阻断运动神经元中 Rac1 调节的树突棘重塑，并减少 痉挛（Bandaru 等，2015；Zhao 等，2016）。然而，迄今为止我们的研究依赖于使用 药理学 Rac1 抑制剂 NSC23766，这使我们无法确定该药物对 神经元。还不清楚为什么 NSC23766 只能部分恢复正常反射输出，并且剂量- 有限的副作用阻碍了长期治疗。因此，为了阐明 Rac1 信号传导在 神经元和星形胶质细胞，我们将使用 1) 病毒介导的 Cre-Lox 系统来敲除 Rac1 表达 运动神经元，2) 星形胶质细胞中特别缺乏 Rac1 的转基因小鼠。星形胶质细胞是不可或缺的 突触可塑性并维持神经元过度兴奋，但尚未在以下背景下进行研究 SCI后痉挛。我们将使用电生理学和行为测试来测量诱发的 H 反射 兴奋性和痉挛。为了控制运动功能的其他变化，我们还将监测粗大运动 功能。为了评估与痉挛和其他解剖学变化相关的树突棘发育不全，我们 将在“清除”的脊髓组织中进行图像分析。 在目标 2 中，为了奠定临床转化的基础，我们还将评估以下两种方法的可行性： 针对 Rac1 通路的翻译相关方法。具体来说，首先我们将评估 基于病毒的基因治疗“平台”可降低 Rac1 表达并缓解痉挛。我们有 以前使用病毒传递定制的 shRNA 构建体来有效靶向错误表达的蛋白质 并改善受伤或疾病后的神经性疼痛（Samad 等人，2013；Tan 等人，2015）。在第二个 方法，我们将确定“重新利用”罗米地辛的潜在效用，罗米地辛是一种临床可用的药物 破坏 PAK1，一种连接 Rac1 与树突棘重组的下游效应子 (Hayashi et al., 2007)。 总之，这项研究的结果不仅可以提高对机制的理解 痉挛并推动该领域走向临床应用，但也有可能扩展到 SCI 之外， 多发性硬化症、创伤性脑损伤、中风等疾病在美国退伍军人中广泛流行。

The goal of this study is to investigate the cellular and molecular mechanisms underlying spasticity, and establish the groundwork for future translational studies in the clinic. A majority of US Veterans with SCI experience clinically significant spasticity, which can disrupt rehabilitation and negatively impact quality-of-life, e.g., mobility, personal hygiene, intimate relationships (Holtz et al., 2017; Skold et al., 1999; Walter et al., 2002). Current spasticity management strategies are palliative, and fail to address the underlying cause. Available treatment options also carry high risk for adverse effects due to non-specific action or long-term use (Adams et al., 2005; Kheder et al., 2012). A major hurdle facing the development of better treatments for SCI- induced spasticity is a lack of mechanistic insight into how injury leads to disability. To advance an evidence-based investigation toward improving spasticity management, we will carry out experiments with two objectives: In Objective 1, we will implement conditional knockout studies to understand the mechanistic contribution of Rac1 specifically in motor neurons and astrocytes to spasticity after SCI. Our previous work demonstrates that pharmacological inhibitors can block Rac1-regulated dendritic spine remodeling in motor neurons and reduce spasticity (Bandaru et al., 2015; Zhao et al., 2016). However, our studies thus far have relied upon the use of a pharmacological Rac1-inhibitor, NSC23766, which precluded our ability to determine the drug’s direct action on neurons. It is also unclear why NSC23766 rendered only partial restoration of normal reflex output, and dose- limiting side effects have prevented longer-term treatment. Thus, to clarify the contribution of Rac1 signaling in neurons and astrocytes, we will use a 1) virally-mediated Cre-Lox system to knockout Rac1 expression in motor neurons, and 2) transgenic mice lacking Rac1 specifically in astrocytes. Astrocytes are integral to synaptic plasticity and maintain neuronal hyperexcitability, but have not been studied within the context of spasticity after SCI. We will use electrophysiological and behavioral tests to measure evoked H-reflex excitability and spasticity. To control for other changes in motor function, we will also monitor gross locomotor function. To assess dendritic spine dysgenesis associated with spasticity, and other anatomical changes, we will perform image analyses in “cleared” spinal cord tissue. In Objective 2, to establish the groundwork for clinical translation, we will also assess the feasibility of two translationally-relevant approaches targeting the Rac1-pathway. Specifically, first we will assess the utility of a viral-based gene therapy “platform” to knockdown Rac1 expression and alleviate spasticity. We have previously used viral-delivery of custom-made shRNA constructs to effectively target misexpressed proteins and modify neuropathic pain after injury or disease (Samad et al., 2013; Tan et al., 2015). In the second approach, we will determine the potential utility of “repurposing” romidepsin, a clinically available drug to disrupt PAK1, a downstream effector linking Rac1 to dendritic spine reorganization (Hayashi et al., 2007). In summary, findings from this study could be expected to not only improve the mechanistic understanding of spasticity and advance the field toward clinical application, but also potentially extend beyond SCI, to conditions such as MS, TBI, stroke that are widely prevalent among US Veterans.