Merkel cells specify innervating SAI sensory neuron phenotype.

描述(申请人提供)：皮肤体感系统处理生物体“感觉”的信息，如疼痛、压力、温度和触摸。点、边和曲率的检测是由Merkel细胞轴突复合体介导的。这些复合体由缓慢适应的I型(SAI)神经纤维和默克尔细胞组成，发现于手脚无毛(无毛)皮肤的表皮-真皮边界、胡须毛囊和有毛皮肤的特殊区域，称为触摸穹顶。皮肤SAI神经支配的发展独立于Merkel细胞的分化，并在没有Merkel细胞的情况下维持。然而，在缺乏Merkel细胞的小鼠中，SAI传入细胞表现出活跃的终末分支和典型的SAI电生理反应的丧失。这提示Merkel细胞可能通过目前尚不明确的机制在这些神经元的分化/成熟过程中发挥作用。尽管触觉具有批判性和基础性，但它是所有感官中最不被理解的。因此，控制体感神经元特化和分化的分子通路一直是研究的热点。转录因子和神经营养素受体(NTRs)共同指导背根神经节(DRGs)体感神经元的分化。此外，神经营养因子在躯体感觉神经元存活和轴突生长中发挥作用。在SAI传入中支配这些过程的确切机制，以及指导它们的NT信号的显性来源，尚不清楚。这对于人体感觉系统存在固有缺陷(自身免疫性疾病、脱髓鞘疾病、遗传异常、感染、损伤、代谢紊乱、中毒暴露和血管紊乱)的许多人类疾病的治疗具有重要意义。这项建议的目的是研究指导DRG神经元呈现SAI纤维表型的不同方面的因素。传统上，LTMR的数量很少，它们在高度异质性的DRG中的随机分布，以及缺乏分子同一性，排除了这种类型的分析；然而，我们的实验室已经产生了缺乏Merkel细胞的小鼠，这为我们提供了一个独特的机会来研究SAI神经元的外周控制、分化和神经支配。本研究的目的是：1.研究皮肤发育过程中默克尔细胞在SAI传入分支中的作用。传入神经准确定位于没有默克尔细胞的触觉穹顶，但它们显示出异常的分支。我们将对K14、Atoh1CKO和野生动物的SAI传入分支进行定量分析，以确定分支是如何改变的。这将决定神经元是否未能完成向SAI神经元的分化，或者是否成功 成熟，但在没有默克尔细胞的情况下未能保持充分分化的状态。我们还将删除Merkel细胞中的BDNF，并对Atoh1、BdnfCKO和野生型小鼠的SAI传入分支进行定量分析，以确定Merkel细胞来源的BDNF是否在传入分支模式中发挥作用。这些实验将提供靶神经支配神经元形态的机制。B.探讨默克尔细胞是否是DRG神经元亚型分化和维持所必需的细胞。皮肤中默克尔细胞的丢失会导致A？传入群体反应的重新分布。我们将研究默克尔细胞是否直接分化SAI，或者它们是否需要维持分化状态。此外，我们将使用默克尔依赖的DRG神经元特征的变化来识别SAI分化和维持这一命运的重要分子。这些实验将确定SAI识别过程中涉及的分子级联以及外围设备对这些过程的贡献。

DESCRIPTION (provided by applicant): The cutaneous somatosensory system processes information that organisms "feel", such as pain, pressure, temperature, and touch. Detection of points, edges, and curvature is mediated by the Merkel cell neurite complex. These complexes consist of slowly adapting type I (SAI) nerve fibers and Merkel cells, and are found at the epidermal-dermal border of glabrous (hairless) skin of the hands and feet, whisker follicles, and specialized regions of hairy skin called touch domes. Cutaneous SAI innervation develops independently of Merkel cell differentiation, and is maintained in the absence of Merkel cells. However, SAI afferents exhibit exuberant terminal branching and loss of prototypical SAI electrophysiological responses in mice that lack Merkel cells. This suggests that Merkel cells may play a role in directing differentiation/maturation of these neurons by currently undefined mechanisms. Despite the critical and fundamental nature of touch, it is the least well understood of all the senses. As such, the molecular pathways that control somatosensory neuron specification and differentiation have been the focus of intense study. A combination of transcription factors and neurotrophin receptors (NTRs) directs the differentiation of somatosensory neurons of the dorsal root ganglia (DRGs). In addition, neurotrophins function in somatosensory neuron survival and neurite outgrowth. The exact mechanisms that govern these processes in SAI afferents, and the explicit sources of the NT signals that direct them, are unknown. This has implications for the treatment of a number of human conditions where there are inherent defects in the somatosensory system (autoimmune diseases, demyelinating diseases, genetic abnormalities, infections, injuries, metabolic disorders, toxic exposures, and vascular disorders). The aims of this proposal are designed to examine the factors that instruct DRG neurons to assume different aspects of the SAI fiber phenotype. Traditionally, the small number of LTMRs, their random distribution within the highly heterogeneous DRG, and the lack of molecular identity have precluded this type of analysis; however, our laboratory has generated mice lacking Merkel cells, providing us with a unique opportunity to study peripheral control of SAI neuron specification, differentiation, and innervation. The goals of this proposal are: A. To investigate the function of Merkel cells in SAI afferent branching during skin development. Afferent nerves accurately target touch domes that lack Merkel cells, but they display aberrant branching. We will perform a quantitative analysis of SAI afferent branching in K14; Atoh1CKO and wild-type animals to define how branching is altered. This will determine whether the neuron fails to complete its differentiation to a SAI neuron or whether it successfully matures but fails to maintain the fully differentiated state in the absence of Merkel cells. We wil also delete BDNF from Merkel cells and perform quantitative analysis of SAI afferent branching in Atoh1; BdnfCKO and wildtype mice to define whether Merkel cell-derived BDNF plays a role in afferent branching patterns. These experiments will provide a mechanism whereby target innervation instructs neuron morphology. B. To investigate whether Merkel cells are required for DRG neuron subtype differentiation and maintenance. Loss of Merkel cells in the skin results in redistribution of the A¿ afferent population response. We will examine whether Merkel cells direct differentiation of the SAI or whether they are required for maintenance of the differentiate state. In addition, we will use Merkel dependent-changes in DRG neuron characteristics to identify molecules important in SAI differentiation and maintenance of that fate. These experiments will determine the molecular cascades involved in SAI identity and the peripheral contribution to these processes.