Exercise and NT-3-mediated lumbar motoneuron plasticity and recovery after SCI

脊髓损伤（SCI）是影响美国军队受伤人员的最严重的致残性疾病之一。 不幸的是，SCI患者没有有效的治疗方法。开发新的修复策略， 减轻SCI的破坏性并将其转化为临床是迫切的医疗需求， 我们的SCI退伍军人的生活质量。腰椎运动神经元（MN）是运动的最终共同通路， 输出到后肢。这些MN的任何损伤都可能导致后肢瘫痪和肌肉萎缩。的 腰髓的直接损伤或腰髓的间接损伤可能会损害腰髓神经元， 颈椎或胸椎水平的腰髓以上（称为水平以上损伤）。对于后者，腰椎MN 没有直接受到创伤的伤害，但它们经历了严重的树突萎缩和突触剥离， 去神经支配的脊髓上和脊髓本体轴突。这种改变的MN形态学和突触变化可能 导致对后肢肌肉的运动输出受损并因此导致运动功能受损。虽然大多数 脊髓损伤的研究主要集中在损伤部位的再生或保护， 研究探索了腰椎MN回路的调节如何影响病理和功能 一级以上SCI的后果。我们研究的目的是了解腰椎MN是如何改变的 解剖学和功能后，一个以上的水平脊髓损伤，以及如何有益的恢复治疗影响他们的 重组和功能性后果。神经营养因子是调节神经元生长的蛋白质家族， 存活、神经突生长、突触可塑性和神经传递。其中，神经营养素-3（NT-3）发挥着重要的作用。 通过促进轴突生长和多个脊髓通路中的突触可塑性，在运动恢复中发挥特殊作用。 外源性给予NT-3已被认为是SCI的一种潜在治疗方法。这允许 我们提出了第一个假设，即从MN中释放逆行运输的NT-3将导致 MN池周围局部NT-3水平升高，促进腰椎运动回路重塑，以及 增强在高于水平的SCI之后的生理和行为恢复。我们和其他人还表明， 单独的运动训练可改善SCI后的协调运动功能。运动训练也 有助于增加促进神经元存活和可塑性的脊髓内神经营养因子的水平， 神经元回路的重组，以及突触功能和行为的改善。所以我们 我提出第二个假设，即运动训练可能通过以下方式协同增强NT-3的作用： 重建备用的下行脊髓回路并促进其功能连接的形成 腰椎MN。使用成年小鼠T9中度挫伤SCI模型和腺相关病毒 血清型2编码NT-3的载体（AAV 2-NT-3）基因转移的方法，我们提出了3个具体的目的， 确定NT-3促进SCI后恢复的机制以及NT-3的长期疗效 的 （一） D 使用临床上可行的输送途径进行治疗，（2）确定运动训练是否会增强 NT-3对脊髓损伤后腰椎MN回路重塑和功能恢复的影响， (3)确定腰椎MN的特定下行通路在其调节 腰椎神经回路和功能恢复后的最佳治疗。完成本提案将不会 仅允许我们揭示NT-3/运动训练介导的MN回路重塑的基本机制 而且还发现了针对后肢运动恢复的新的治疗策略。

Spinal cord injury (SCI) is among the most disabling conditions affecting wounded members of the U.S. military. Unfortunately, no effective treatment has been available for SCI patients. Developing novel repair strategies to mitigate the devastating nature of SCI and translating them clinically are urgent medical needs that will improve quality of life of our veterans with SCI. The lumbar motoneurons (MNs) are the final common pathway for motor output to the hindlimbs. Any impairment of these MNs can cause hindlimb paralysis and muscle atrophy. The lumbar MNs could be impaired by a direct injury to the lumbar cord or by an indirect injury occurring at levels above the lumbar cord at cervical or thoracic levels (called above-level injuries). For the latter, the lumbar MNs are not directly injured by the trauma, but they undergo profound dendritic atrophy and synaptic stripping from denervated supraspinal and propriospinal axons. Such altered MN morphological and synaptic changes could result in impaired motor outputs to hindlimb muscles and therefore impaired locomotor functions. While most SCI studies have been focused on the regeneration or protection of injured spinal cord at the site of injury, few studies have explored how modulation of lumbar MN circuitry would affect pathological and functional consequences after an above-level SCI. The goal of our research is to understand how lumbar MNs are altered anatomically and functionally after an above-level SCI and how a beneficial restorative treatment affects their reorganization and functional consequences. Neurotrophins are a family of proteins that regulate neuronal survival, neurite outgrowth, synaptic plasticity and neurotransmission. Among them, Neurotrophin-3 (NT-3) plays a particular role in motor restoration by promoting axon growth and synaptic plasticity in multiple spinal pathways. Exogenous administration of NT-3 has been proposed as one potential therapeutic treatment for SCI. This allows us to propose the first hypothesis that the release of retrogradely transported NT-3 from MNs will result in an elevation of local NT-3 levels around the MN pools, promoting remodeling of lumbar motor circuitry, and enhancing physiological and behavioral recoveries following an above-level SCI. We and others also showed exercise training alone improved coordinated motor function following SCIs. Exercise training also contributed to the increased levels of intraspinal neurotrophic factors that promote neuronal survival and plasticity, to the reorganization of neuronal circuitry, and to improvements in synaptic function and behavior. Therefore, we propose the second hypothesis that exercise training will synergistically enhance the effect of NT-3 perhaps by remodeling the spared descending spinal circuits and facilitating the formation of their functional connections with lumbar MNs. Using an adult mouse T9 moderate contusive SCI model and an adeno-associated virus serotype 2 vector encoding NT-3 (AAV2-NT-3) gene transfer approach, we propose 3 Specific Aims to etermine the mechanism by which NT-3 improves recovery after SCI and the long-term efficacy of the NT-3 that (1) d treatment using a clinically feasible delivery route, (2) determine whether exercise training will enhance the effects of NT-3 on the remodeling of lumbar MN circuitry and functional recovery after an above-level SCI, and (3) determine the functional roles of specific descending pathways to lumbar MNs in their ability to modulate lumbar neural circuitry and functional recovery after the optimal treatment. Completion of this proposal will not only allow us to reveal fundamental mechanisms of NT-3/exercise training-mediated remodeling of MN circuitry but also to identify new therapeutic strategies targeting hindlimb locomotor recovery.