Beyond the neuromuscular junction: dysfunction of spinal synaptic targets of motoneurons in Amyotrophic Lateral Sclerosis

每年有超过5000人被诊断患有肌萎缩侧索硬化症（ALS）。其结果总是致命的，从诊断开始的中位生存时间只有4-5年。最初的症状包括肌肉无力、痉挛和抽搐。ALS是一种中枢神经系统疾病，主要针对运动神经元。运动神经元位于脊髓中，并将它们的长突起发送到全身，与不同的肌肉群接触。通过这些接触，运动神经元激活导致肌肉收缩，并执行所有运动任务，从行走到抓握，咀嚼，甚至呼吸。在疾病的早期症状阶段，运动神经元与肌肉的接触变得无效，阻碍了运动任务的成功执行。随后是进行性运动神经元死亡，导致更严重的运动缺陷，无法行走，承重或处理物体。退化沿着脊髓发展，并延伸到负责呼吸所需的膈肌收缩的运动神经元。这通常是疾病的最后阶段，患者保持完全意识，但逐渐无法独立呼吸。目前没有治愈方法，唯一批准的治疗方法（如利鲁唑，一种降低运动神经元兴奋性的化合物）只能略微延长预期寿命。这种疾病的原因尚不清楚，但至少20%的患者有家族史，证明是遗传性的。事实上，家族研究导致了对影响运动神经元健康的蛋白质的几种突变的鉴定。这一发现带来了一个重大突破：这些突变被用于开发该疾病的动物模型。我们建议使用其中两种动物模型，它们彼此非常不同，但其症状密切反映了相应受影响的人类患者的ALS症状和时间进程。以前的研究集中在分析运动神经元中可能导致退化和死亡的细胞事件。虽然已经确定了一些关键事件，因果关系证明很难查明，因此很难制定治疗策略。在我们的建议中，我们假设运动神经元变性不仅是由于发生在单细胞内的分子水平上的事件，也是由于调节运动控制的电路内的事件。特别是，我们将专注于运动神经元和一类特殊的抑制性神经元之间的局部电路称为伦肖细胞，具有限制和微调运动神经元的兴奋的作用。这些回路的中断可能会导致恶性循环，其中运动神经元变得过度兴奋和退化，这是它们接收到的过度、未受抑制的兴奋量的结果。之前有一些证据表明，在受影响的人类和突变的动物中，这种回路都受到了破坏，我们的研究将量化这种破坏的程度和时间过程。我们的研究有双重目的：首先，通过了解局部电路如何以及何时受到影响，我们可能能够设计出一种诊断测试，可以在明显症状出现之前揭示疾病的早期阶段。这可能会打开更长的治疗时间窗。其次，局部回路中断在疾病发展中的重要性的证明可以为开发旨在保护和增强正常局部回路功能的新疗法开辟道路。

Every year more than 5000 people are diagnosed with Amyotrophic Lateral Sclerosis (ALS). Its outcome is always fatal, with a median survival time from diagnosis of only 4-5 years. The initial symptoms include muscle weakness, cramps, and twitching. ALS is a disease of the central nervous system that mostly targets motoneurons. Motoneurons are located in the spinal cord and send their long processes throughout the body making contacts with distinct muscle groups. Through these contacts motoneuron activation leads to muscle contraction and to execution of all motor tasks, from walking to grasping, chewing, and even breathing. During the early symptomatic stages of the disease, contacts made by motoneurons onto muscles become ineffective, hampering successful execution of motor tasks. This is followed by progressive motoneuron death, leading to more severe motor deficits, inability to walk, bear weight, or handle objects. The degeneration progresses along the spinal cord and extends to motoneurons responsible for the contraction of the diaphragm necessary for respiration. This is normally the final stage of the disease, in which the patient maintains full awareness, but becomes progressively incapable of unassisted breathing.Currently there is no cure and the only approved treatments (such as riluzole, a compound that reduces the excitability of motoneurons) only marginally prolong the life expectancy. The causes of the disease are unknown, but in at least 20% of patients there is a family history that proves a hereditary trait. Indeed, familial studies led to the identification of several mutations in proteins that affect the health of motoneurons. A major breakthrough resulted from this knowledge: these mutations were used to develop animal models of the disease.We are proposing to use two of these animal models that are quite different from each other, but whose symptoms closely mirror the symptoms and time course of ALS in the corresponding affected human patients. Previous research has focussed on the analysis of cellular events in motoneurons that could lead to degeneration and death. While a number of key events have been identified, the cause-effect relationships proved hard to pinpoint, thus making it difficult to develop therapeutic strategies.In our proposal we hypothesize that motoneuron degeneration is not only due to events occurring at the molecular level within single cells, but also due to events within circuits regulating motor control. In particular, we will focus on local circuits between motoneurons and a special class of inhibitory neurons called Renshaw cells that have the role of limiting and fine tuning the excitation of motoneurons. Disruption of these circuits may cause a vicious cycle in which motoneurons become over excitable and degenerate as a consequence of the excessive, unchecked amount of excitation they receive. There is some previous evidence of disruption of this circuit in both affected humans and mutated animals, and our research will quantify the extent and time course of this disruption. Our investigation has a dual purpose: first, by understanding how and when the local circuits are affected, we may be able to devise a diagnostic test that could reveal early stages of the disease, well before clear symptoms appear. This would potentially open a longer therapeutic time window. Second, the demonstration of the importance of the disruption of local circuits in the development of the disease could open the way to the development of new therapies aimed at preserving and enhancing normal local circuit function.