Merkel cells specify innervating SAI sensory neuron phenotype.

描述（由申请人提供）：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默克尔细胞神经突复合体介导的。这些复合物由缓慢适应的I型（SAI）神经纤维和默克尔细胞组成，存在于手脚无毛皮肤的表皮真皮边界、胡须毛囊和被称为触觉丘的有毛皮肤的特殊区域。皮肤SAI神经支配的发育独立于默克尔细胞的分化，并在默克尔细胞缺失的情况下维持。然而，在缺乏默克尔细胞的小鼠中，SAI事件表现出旺盛的末端分支和典型SAI电生理反应的丧失。这表明默克尔细胞可能在指导这些神经元的分化/成熟中发挥作用，其机制目前尚未明确。尽管触觉具有关键和基本的性质，但它是所有感官中最不为人所知的。因此，控制体感觉神经元分化的分子通路一直是研究的热点。转录因子和神经营养因子受体（NTRs）共同指导背根神经节（DRGs）体感觉神经元的分化。此外，神经营养因子在体感觉神经元存活和神经突生长中起作用。SAI事件中控制这些过程的确切机制以及指导它们的NT信号的明确来源尚不清楚。这对躯体感觉系统中存在固有缺陷的许多人类疾病（自身免疫性疾病、脱髓鞘疾病、遗传异常、感染、损伤、代谢紊乱、有毒物质暴露和血管紊乱）的治疗具有启示意义。本提案的目的是研究指示DRG神经元承担SAI纤维表型不同方面的因素。传统上，ltmr的数量少，它们在高度异质的DRG内的随机分布，以及缺乏分子同一性，阻碍了这种类型的分析；然而，我们的实验室产生了缺乏默克尔细胞的小鼠，为我们提供了一个独特的机会来研究SAI神经元规范、分化和神经支配的外周控制。本研究的目的是：A.研究皮肤发育过程中Merkel细胞在SAI传入分支中的功能。传入神经准确地瞄准缺乏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中SAI传入分支进行定量分析；和野生型动物来定义分支是如何改变的。这将决定神经元是否未能完成向SAI神经元的分化或是否成功

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.