Neurotrophin 3 and regulation of proprioceptor subtype identity and connectivity

描述（由申请人提供）：本体感觉神经元（pSN）通过提供反馈信号在改善脊髓运动系统的输出中起关键作用，所述反馈信号将肌肉活动的状态传递到中枢和脊髓运动神经元。不同的pSN亚型与专用于特定肌肉骨骼任务（例如姿势控制、膝关节屈曲、踝关节伸展等）的选定脊髓回路接合。这种感觉-运动连接的精确性被认为在很大程度上取决于各种本体感受器亚型之间的分子差异，但令人惊讶的是，人们对本体感受器亚型身份的建立方式知之甚少。 与流行的观点不同，我们最近的研究表明，pSN亚型的某些方面的特征是由神经营养因子3（NT 3）的分级信号介导的，而不是由内在的转录决定簇。这个想法是基于几个观察结果，最值得注意的发现是，当pSN建立其亚型身份时，胚胎肌肉在NT 3表达水平上表现出逐肌肉差异。 假设NT 3信号强度的变化直接pSN亚型特征导致两种预测。首先，如果分级的NT 3信号传导驱动pSN亚型多样性，则NT 3应该在pSN中引起不同的分子反应，所述pSN支配表达不同水平的NT 3的肌肉靶标。第二，基于pSN身份与脊髓连接模式内在相关的概念，NT 3信号传导水平的变化应导致pSN连接模式的改变。提案中的实验旨在测试这些期望。 与我们的预测一致，在初步研究中，我们确定了几个分子标记物，它们在神经支配NT 3 high和神经支配NT 3low肌肉靶点的pSN亚群之间差异表达。这些分子标记物不仅提供了对pSN亚型身份的各个方面的新见解，而且重要的是，它们是评估NT 3在调节pSN多样性中的作用的有力工具（目的1）。此外（目标2），我们将利用新开发的策略-基于重组狂犬病病毒的顺行跨突触转移-构建定义的NT 3low和NT 3 high pSN子集的脊髓连接模式的解剖框架，并检查NT 3信号在建立这些模式中的作用。 最终，这些分析将导致对pSN亚型的主要分子和网络特征的新见解，并可能开始揭示脊髓感觉运动回路形成的组织规则。

DESCRIPTION (provided by applicant): Proprioceptive sensory neurons (pSNs) serve a key role in refining the output of the spinal motor system through the provision of feedback signals that convey the state of muscle activity to central and spinal motor neurons. Distinct pSN subtypes engage with select spinal circuits dedicated to specific musculoskeletal tasks (e.g. postural control, knee flexion, ankle extension etc). This precision in sensory-motor connectivity is presumed to rest in large part on the molecular distinctions between the various proprioceptor subtypes, yet surprisingly little is known of the way in which proprioceptor subtype identity is established. Challenging prevailing views, our recent studies suggest that certain aspects pSN subtype character are mediated by graded signaling by neurotrophin 3 (NT3) rather than by intrinsic transcriptional determinants. This idea is founded on several observations, most notably the finding that embryonic muscles exhibit muscle- by-muscle differences in NT3 expression levels at the time when pSNs establish their subtype identity. The hypothesis that variations in the strength of NT3 signaling direct pSN subtype character leads to two predictions. First, if graded NT3 signaling drives pSN subtype diversity, NT3 should elicit distinct molecular responses in pSNs that innervate muscle targets expressing different levels of NT3. Second, based on the notion that pSN identity is inherently linked to spinal connectivity patterns, changes in NT3 signaling levels should result in alterations in pSN connectivity patterns. The experiments in the proposal are designed to test these expectations. In agreement with our predictions, in preliminary studies, we identified several molecular markers that are differentiall expressed between pSN subsets that innervate NT3high -and those that innervate NT3low muscle targets. These molecular markers not only provide new insights into the various aspects of pSN subtype identity, but importantly, are powerful tools through which to assess the role of NT3 in regulating pSN diversity (Aim 1). In addition (Aim 2), we will take advantage of newly developed strategies - based on anterograde transsynaptic transfer of recombinant rabies virus - to construct an anatomical framework of the spinal connectivity patterns of defined NT3low and NT3high pSN subsets, and examine the role of NT3 signaling in establishing these patterns. Ultimately, these analysis' should lead to new insights in cardinal molecular and network features of pSN subtypes and may begin to reveal the organizational rules that underlie the formation of spinal sensory- motor circuits.