Toward understanding the molecular mechanisms of Merkel cell fate determination

摘要 皮肤上皮为身体提供基本的保护和感觉功能。它由 作为屏障的表皮，对热保护很重要的毛囊，以及 默克尔细胞，这是被神经支配的机械感觉细胞，为质地和 形状识别。而表皮和毛囊的发育和动态平衡一直很好 经过研究，默克尔细胞的生物学还没有得到很好的理解。 我们关于控制默克尔细胞的机制的大部分知识来自于对小鼠的分析 背部皮肤，其中默克尔细胞组织成新月形结构，称为触觉穹顶，并 位于初级毛囊周围。我们最近的研究表明，毛囊和 默克尔细胞发育，因为基因突变消除了小鼠早期的毛发形成 导致默克尔细胞的损失。我们进一步研究了这一现象，并通过进行血统追踪 实验中，我们发现了Sox9+胚胎毛囊干细胞，这是已知的产生头发的 毛囊和成人毛囊干细胞，也产生默克尔细胞。我们还剖析了它们的机制。 控制Sox9+细胞对Merkel细胞谱系的指定并显示成纤维细胞的重要性 生长因子(FGF)在此过程中的信号转导。事实上，我们发现成纤维细胞生长因子受体2(FGFR2)的表皮丢失 毛发形态发生前不影响Sox9+细胞的出现或毛发的发育 卵泡，但会导致默克尔细胞的丧失。综上所述，我们假设Sox9+细胞是胚胎 同时产生毛囊和默克尔细胞系的多潜能干细胞以及FGFR2介导的 信号转导功能促进Sox9+细胞分化为Merkel细胞。 这场竞争性更新将集中检验上述假设。在目标1中，我们将测试其重要性 用于默克尔细胞形成的Sox9+细胞。我们将进行体内血统追踪和体外活体成像分析 以确认Sox9+细胞是默克尔细胞前体细胞。我们还将进行有条件的消融 转录因子Sox9，已知对Sox9+胚胎毛囊的维持至关重要 干细胞，并分析其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Merkel细胞发育调控机制。我们将确定FGFR2介导的 通过分析一系列对默克尔细胞形成至关重要的等位基因 阻止效应蛋白结合的FGFR2的条件小鼠突变体。我们下一步将选出候选人 执行FGFR2转录反应的转录因子及其对Merkel细胞的意义 队形。总之，这些研究将定义信号和转录之间的动态相互作用 在控制默克尔细胞命运的过程中。

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.