Regulation of Sensory-Motor Connectivity by Semaphorin-Plexin Signaling

描述（由申请人提供）：神经元以精确的特异性建立突触连接，以组装神经回路。虽然控制初始轴突轨迹的分子相对来说已经被很好地理解了，但是在目标区域内突触连接是如何精确形成的在很大程度上是未知的。这项提议的目的是了解突触形成的分子基础和突触特异性在发育中的小鼠脊髓。脊髓中的脊髓反射回路是研究突触形成和突触特异性的一个很好的模型系统，因为它相对简单，并且基于先前的解剖学和电生理学研究提供了丰富的知识。腹侧灰质中的细胞体组成“运动神经元池”，将轴突投射到特定的肌肉。在小鼠脊髓的四肢水平上大约有50个这样的运动神经元池。“本体感觉神经元”以本体感觉纤维支配这些肌肉，其细胞体位于背根神经节（DRG），并将轴突投射到脊髓，与适当的运动神经元池终止并形成突触。我们的初步数据强烈表明，信号配体的信号蛋白家族及其受体丛蛋白控制着突触的形成和感觉-运动连接的突触特异性。首先，在所有semas和plexins中，只有plexinA1、plexinD1和sema6B在本体感觉神经元中表达高度富集。其次，sema6D及其受体plexinA1在突触发生时由运动和本体感觉神经元表达。第三，plexinD1由本体感觉神经元亚群表达，而其配体sema3E由运动神经元亚群表达。我们假设sema-plexin组合控制着发育中的脊髓突触的形成和突触特异性。第一个目的是研究sema6D-plexinA1信号是否调节感觉-运动连接的突触形成。第二个和第三个目标将检查sema3E- plexinD1和plexinA4-sema6B信号是否控制感觉-运动连接的突触特异性。我们将通过结合解剖、电生理、行为和体外分析以及小鼠遗传学来解决这些问题。公共卫生相关性：脊髓是中枢神经系统（CNS）中与运动行为有关的区域。脊髓运动神经元的功能障碍发生在许多神经系统疾病中，如肌萎缩侧索硬化症（ALS）和脊髓性肌萎缩症（SMA）。这些运动神经元疾病是一组进行性疾病，破坏控制基本肌肉活动的细胞，如说话、行走、呼吸和吞咽。来自大脑神经细胞的信息被传递到脊髓和脑干的神经细胞，并从它们传递到特定的肌肉。运动神经元的活动也受外周区背根神经节本体感觉神经元的控制。因此，为了开发运动神经元疾病的治疗方法，了解运动神经元是如何被外周（由本体感觉神经元）和中枢（由大脑皮层神经元）控制的是很重要的。此外，脊髓损伤引起运动神经元的功能障碍，由于许多其他人类神经系统疾病是由神经回路形成的缺陷引起的，了解神经回路形成的机制将揭示疾病的潜在原因。在这个提议中，我们将验证我们的假设，即信号传导-神经丛信号控制小鼠脊髓中正确的感觉-运动连接。我们相信，更好地了解感觉-运动连接形成的分子机制可能有助于推进神经系统疾病的诊断、治疗和预防。

DESCRIPTION (provided by applicant): Neurons make synaptic connections with a precise specificity in order to assemble neural circuits. Although molecules that control initial axonal trajectories are relatively well understood, it is largely unknown how precise synaptic connections are formed within the target area. The goal of this proposal is to understand the molecular basis of synapse formation and synaptic specificity in the developing mouse spinal cord. The spinal reflex circuit in the spinal cord is an excellent model system to study synapse formation and synaptic specificity because of its relative simplicity and availability of abundant knowledge based on previous anatomical and electrophysiological studies. Cell bodies in the ventral gray matter are grouped into "motor neuron pools", which project axons to specific muscles. There are approximately fifty such motor neuron pools at the levels of the limbs in the mouse spinal cord. "Proprioceptive sensory neurons", which innervate these muscles with proprioceptive fibers, have their cell bodies in the dorsal root ganglia (DRG), and project axons into the spinal cord, which terminate and make synapses with the appropriate motor neuron pools. Our preliminary data strongly suggest that the semaphorin (sema) family of signaling ligands, and their receptors, the plexins, control synapse formation and synaptic specificity of the sensory-motor connections. First, of all semas and plexins, expression of only plexinA1, plexinD1, and sema6B is highly enriched in proprioceptive sensory neurons. Second, sema6D and its receptor plexinA1 are expressed by motor and proprioceptive sensory neurons when synaptogenesis is occurring. Third, plexinD1 is expressed by subsets of proprioceptive sensory neurons, while its ligand sema3E is expressed by subsets of motor neurons. We hypothesize that sema-plexin combinations contro l synapse formation and synaptic specificity in the developing spinal cord. The first aim will examine whether sema6D-plexinA1 signaling regulates synapse formation of sensory-motor connections. The second and third aims will examine if sema3E- plexinD1 and plexinA4-sema6B signaling control synaptic specificity of sensory-motor connections. We will address these issues by using a combination of anatomical, electrophysiological, behavioral, and in vitro analyses together with mouse genetics. PUBLIC HEALTH RELEVANCE: The spinal cord represents the region of the central nervous system (CNS) concerned with motor behavior. Malfunction of the motor neurons in the spinal cord occurs in a number of neurological disorders such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). These motor neuron diseases are a group of progressive disorders that destroy cells that control essential muscle activity such as speaking, walking, breathing, and swallowing. Messages from nerve cells in the brain are transmitted to nerve cells in the spinal cord as well as the brain stem and from them to particular muscles. The activity of motor neurons is also controlled by proprioceptive sensory neurons of the dorsal root ganglia (DRG) in the peripheral regions. Therefore, it is important to understand how motor neurons are controlled peripherally (by proprioceptive sensory neurons) as well as centrally (by cortical neurons in the brain) in order to develop the therapy for motor neurons diseases. Furthermore, spinal cord injury causes disfunction of motor neurons, and since many other human neurological disorders are caused by defects in neural circuit formation, understanding the mechanism of neural circuit formation will uncover an underlying cause of the diseases. In this proposal, we will test our hypothesis that semaphorin-plexin signaling controls correct sensory-motor connectivity in the mouse spinal cord. We believe that better understanding of the molecular mechanisms underlying the formation of sensory-motor connections is likely to contribute to advancing diagnosis, therapy, and prevention of neurological disorders.