Toward understanding the molecular mechanisms of Merkel cell fate determination

总结 皮肤上皮为身体提供基本的保护和感觉功能。它包括 作为屏障的表皮，对于热保护很重要的毛囊，以及 默克尔细胞，其是神经支配的机械感觉细胞，其介导质地的光触摸感觉， 形状识别虽然表皮和毛囊的发育和稳态良好， 虽然研究了很多，但对默克尔细胞的生物学还没有很好的了解。 我们对控制默克尔细胞的机制的许多知识来自于对小鼠 背部皮肤，其中默克尔细胞被组织在称为触摸圆顶的新月形结构中， 位于初级毛囊周围。我们最近的研究表明毛囊与 默克尔细胞发育，因为基因突变废除了小鼠毛发形成的早期阶段， 导致默克尔细胞的丢失。我们进一步研究了这一现象，通过进行谱系追踪， 实验中，我们发现Sox9+胚胎毛囊干细胞，这是已知的产生头发， 毛囊和成体毛囊干细胞也产生默克尔细胞。我们还分析了 控制Sox 9+细胞向默克尔细胞谱系的特化，并显示成纤维细胞的重要性。 生长因子（Fgf）信号在这个过程中。事实上，我们发现表皮Fgf受体2（FgfR2）的缺失， 在毛发形态发生之前，不影响Sox 9+细胞的外观或毛发的发育 卵泡，但导致默克尔细胞的损失。综上所述，我们假设Sox 9+细胞是胚胎细胞， 产生毛囊和默克尔细胞谱系的多能干细胞，以及FgfR2介导的 信号传导功能促进Sox 9+细胞分化为默克尔细胞。 这次竞争性更新将侧重于测试上述假设。在目标1中，我们将检验 用于默克尔细胞形成的Sox 9+细胞。我们将进行体内谱系追踪和离体活体成像分析 以证实Sox 9+细胞是默克尔细胞前体。我们还将进行条件性消融， 转录因子Sox 9，已知其对于维持Sox 9+胚胎毛囊是必需的 干细胞，并分析对默克尔细胞形成的影响。在目标2中，我们将研究分子 FgfR2介导的默克尔细胞发育控制的机制。我们将鉴定FgfR2介导的 细胞内信号传导途径，这是至关重要的默克尔细胞的形成，通过分析等位基因系列的 阻止效应蛋白结合的FGFR 2的条件小鼠突变体。接下来我们将挑选候选人 执行Fgfr 2转录应答的转录因子并测试它们对默克尔细胞的意义 阵总之，这些研究将确定信号和转录之间的动态相互作用， 控制默克尔细胞命运决定的过程。

Summary The skin epithelium provides essential protective and sensory functions for the body. It consists of the epidermis, which serves as a barrier, the hair follicles, which are important for thermal protection, and the Merkel cells, which are innervated mechanosensory cells that mediate light touch sensations for texture and shape recognition. While the development and homeostasis of the epidermis and hair follicles have been well studied, the biology of the Merkel cells is not as well understood. Much of our knowledge on the mechanisms controlling Merkel cells comes from the analysis of murine dorsal skin, in which Merkel cells are organized in crescent-shaped structures called touch domes, and are located around primary hair follicles. Our recent studies revealed close relationship between hair follicle and Merkel cell development, as genetic mutations that abrogate early stages of hair formation in the mouse also result in the loss of Merkel cells. We further investigated this phenomenon and, by performing lineage tracing experiments, we showed that Sox9+ embryonic hair follicle stem cells, which are known to give rise to the hair follicles and adult hair follicle stem cells, also give rise to Merkel cells. We also dissected the mechanisms controlling the specification of Sox9+ cells to the Merkel cell lineage and showed the importance of fibroblast growth factor (Fgf) signaling in this process. Indeed, we found that epidermal loss of Fgf receptor 2 (FgfR2) prior to hair morphogenesis does not affect the appearance of Sox9+ cells or the development of the hair follicles, but leads to loss of Merkel cells. Taken together, we hypothesize that Sox9+ cells are embryonic multipotent stem cells that give rise to both the hair follicle and Merkel cell lineages, and FgfR2-mediated signaling functions to promote the differentiation of Sox9+ cells into Merkel cells. This competitive renewal will focus on testing the above hypothesis. In Aim 1, we will test the significance of Sox9+ cells for Merkel cell formation. We will perform in vivo lineage tracing and ex vivo live imaging assays to confirm that Sox9+ cells are Merkel cell precursors. We will also perform conditional ablation of the transcription factor Sox9, which is known to be essential for the maintenance of Sox9+ embryonic hair follicle stem cells, and analyze the effect on Merkel cell formation. In Aim 2, we will investigate the molecular mechanisms of FgfR2-mediated control of Merkel cell development. We will identify FgfR2-mediated intracellular signaling pathways that are critical for Merkel cell formation by analyzing an allelic series of conditional mouse mutants of fgfr2 that prevent the binding of effector proteins. We will next select candidate transcriptional factors that execute Fgfr2 transcriptional responses and test their significance for Merkel cell formation. In summary, these studies will define the dynamic interplay between signaling and transcriptional processes in the control of Merkel cell fate determination.