Neurotrophin 3 and regulation of proprioceptor subtype identity and connectivity

描述(申请人提供)：本体感觉神经元(PSN)通过向中枢和脊髓运动神经元提供反馈信号，将肌肉活动的状态传达给中枢和脊髓运动神经元，在完善脊髓运动系统的输出方面发挥关键作用。不同的PSN亚型参与特定肌肉骨骼任务(例如姿势控制、膝关节屈曲、脚踝伸展等)的特定脊椎回路。感觉-运动连接的这种精确度被认为在很大程度上取决于不同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同一性的建立方式知之甚少。我们最近的研究挑战了主流观点，我们的研究表明，PSN亚型特征的某些方面是由神经营养素3(NT3)介导的分级信号而不是内在的转录决定因素。这个想法建立在几个观察的基础上，最引人注目的发现是，当PSN建立其亚型认同时，胚胎肌肉在不同肌肉中NT3的表达水平存在差异。假设NT3信号强度的变化直接导致PSN亚型特征导致两个预测。首先，如果分级的NT3信号驱动PSN亚型多样性，NT3应该在PSN中引起不同的分子反应，从而支配表达不同水平NT3的肌肉靶标。其次，基于PSN身份与脊髓连接模式内在联系的概念，NT3信号水平的变化应该导致PSN连接模式的改变。提案中的实验旨在测试这些预期。与我们的预测一致，在初步研究中，我们发现了几个分子标记，它们在支配NT3高水平的PSN亚群和支配NT3低水平肌肉靶标的PSN亚群中都有不同的表达。这些分子标记不仅为PSN亚型同一性的各个方面提供了新的见解，而且重要的是，它们是评估NT3在调节PSN多样性中的作用的强大工具(目标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-构建已定义的NT3低和NT3高PSN亚群的脊髓连接模式的解剖框架，并研究NT3信号在建立这些模式中的作用。最终，这些分析应该会导致对PSN亚型的基本分子和网络特征的新见解，并可能开始揭示构成脊髓感觉-运动回路的组织规则。

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.