Double duty for mammary stem cell niche The stem cell niche is a complex local signaling microenvironment that regulates stem cell activity for tissue and organ maintenance and regeneration. As well as responding locally, during puberty, the mammary gland stem cell niche also responds to systemic hormonal signals. Zhao et al. have found that Gli2, a transcriptional effector of Hedgehog signaling, coordinates the niche-signaling program and activates expression of receptors for the mammatrophic hormones estrogen and growth hormone throughout the mammary gland (see the Perspective by Robertson). Disease may result not only from stem cell defects, but also from dysregulation of the microenvironment. Science, this issue p. eaal3485; see also p. 250 The GLI2 transcription factor regulates stromal cell expression of signals controlling breast epithelial stem cell activity. INTRODUCTION The stem cell niche is a complex signaling microenvironment that acts locally to sustain stem cell activity in tissue maintenance and regeneration. Although the cellular constitution and signaling activity of the stem cell niche is coming into focus in a variety of tissues, genetic regulatory factors that specify the niche are less clear. The activity of such factors is particularly intriguing in organs such as the breast, where the niche provides local signals for tissue homeostasis but also must be entrained by circulating hormones that induce the dramatic changes of puberty. RATIONALE The epithelial stem cells of a variety of organs respond to secreted signals generated in subjacent stromal cells, in a manner often dependent on activity of the Hedgehog (Hh) signaling pathway. Gli2, which encodes the major transcriptional effector of Hh signaling, is expressed in a subset of stromal cells adjacent to the ductal epithelium of the mouse mammary gland and is highly expressed at terminal end buds and end structures of the pubertal and adult mammary gland, respectively, which represent the sites of mammary epithelial stem cells. RESULTS Stromal ablation of a conditional Gli2fl allele with Fsp1Cre, a stromally expressed recombinase allele (producing Gli2∆S mice), caused a delay in mammary ductal development, reduced the number of mammary gland stromal cells and volume of extracellular matrix, and caused abnormal mammary duct distension. Stromal Gli2 ablation did not alter development of the ovary or pituitary, nor their production of mammatrophic hormones such as estrogen or growth hormone, but did affect ductal regeneration, as indicated by a fivefold decrease in outgrowth efficiency of mammary stem cells (MaSCs) transplanted into Gli2∆S mammary glands. These findings suggest that Gli2 specifies a stromal niche signaling program that critically regulates MaSC activity. FACS-isolated mammary stromal cells showed Gli2-dependent expression of factors that stimulate epithelial stem cell renewal, ductal outgrowth, and morphogenesis, including specific members of the IGF, WNT, FGF, and HGF families of secreted peptides. Single-cell analysis showed expression of these factors in a subset of stromal cells in a manner dependent on Gli2 function. Receptors for the mammatrophic hormones estrogen and growth hormone were also expressed in a Gli2-dependent manner, and estrogen and growth hormone treatment of cultured stromal cells induced these secreted peptide factors, suggesting that a critical role of Gli2 in mammary stromal niche cells is to render these cells responsive to systemic mammatrophic hormones. Supporting this conclusion, mammary gland implants of polymer fragments releasing IGF1 and WNT2 rescued Gli2∆S ductal growth phenotypes, whereas growth hormone–releasing polymer did not. CONCLUSION We find that the Hedgehog pathway transcriptional effector GLI2 specifies a stromal cell niche signaling program that supports mammary epithelial stem cells in pubertal and virgin adult mice. This program includes expression of factors such as IGF and WNT, and GLI2 acts in part by causing expression of receptors for hormones such as estrogen and growth hormone, thus entraining local stem cell niche activity to systemic mammatrophic hormones. Our work illustrates the central role of the stromal niche in controlling epithelial stem cell activity and suggests that niche failure may underlie pathogenesis of certain diseases, including the deficient breast development and hormonal insensitivity associated with the human disorder, combined pituitary hormone deficiency. Stromal GLI2 coordinates mammary stem cell niche signaling program. Transcriptional regulation by Hedgehog pathway effector GLI2 in stromal cells of the mammary gland coordinates a hormone-responsive niche signaling program that directs epithelial stem cell activity during the changes of puberty (schematic, left). Gli2-deficient stromal cells in mouse mammary are unable to support normal ductal outgrowth and morphogenesis of transplanted, green fluorescent protein–labeled stem cells (right). The stem cell niche is a complex local signaling microenvironment that sustains stem cell activity during organ maintenanceorgan maintenance and regeneration. The mammary gland niche must support its associated stem cells while also responding to systemic hormonal regulation that triggers pubertal changes. We find that Gli2, the major Hedgehog pathway transcriptional effector, acts within mouse mammary stromal cells to direct a hormone-responsive niche signaling program by activating expression of factors that regulate epithelial stem cells as well as receptors for the mammatrophic hormones estrogen and growth hormone. Whereas prior studies implicate stem cell defects in human disease, this work shows that niche dysfunction may also cause disease, with possible relevance for human disorders and in particular the breast growth pathogenesis associated with combined pituitary hormone deficiency.

中文翻译：

---

基质 Gli2 活性协调乳腺上皮干细胞的利基信号程序

乳腺干细胞生态位的双重职责 干细胞生态位是一个复杂的局部信号微环境，可调节干细胞活性以进行组织和器官的维持和再生。除了局部反应，在青春期，乳腺干细胞生态位也对全身激素信号作出反应。赵等人。发现 Gli2 是 Hedgehog 信号转导的转录效应子，它协调生态位信号传递程序并激活整个乳腺中乳腺激素雌激素和生长激素受体的表达（参见 Robertson 的观点）。疾病不仅可能由干细胞缺陷引起，还可能由微环境失调引起。科学，这个问题 p。eaal3485; 另见第。250 GLI2 转录因子调节基质细胞表达控制乳腺上皮干细胞活性的信号。引言 干细胞生态位是一个复杂的信号微环境，在组织维持和再生过程中发挥局部作用以维持干细胞活性。尽管干细胞生态位的细胞构成和信号活动在各种组织中成为焦点，但指定生态位的遗传调节因素还不太清楚。这些因子的活动在诸如乳房的器官中特别有趣，其中的生态位为组织稳态提供局部信号，但也必须由循环激素携带，这些激素会引起青春期的剧烈变化。基本原理 各种器官的上皮干细胞对下层基质细胞产生的分泌信号作出反应，以通常依赖于 Hedgehog (Hh) 信号通路活性的方式。Gli2 编码 Hh 信号传导的主要转录效应子，在与小鼠乳腺导管上皮相邻的基质细胞亚群中表达，并分别在青春期和成年乳腺的末端芽和末端结构中高度表达，代表乳腺上皮干细胞的位点。结果 用 Fsp1Cre 对条件性 Gli2fl 等位基因进行基质消融，Fsp1Cre 是一种基质表达的重组酶等位基因（产生 Gli2ΔS 小鼠），导致乳腺导管发育延迟，减少乳腺基质细胞数量和细胞外基质体积，并导致异常乳腺导管扩张。基质 Gli2 消融没有改变卵巢或垂体的发育，它们也不产生诸如雌激素或生长激素之类的乳腺激素，但确实影响了导管再生，如移植到 Gli2ΔS 乳腺中的乳腺干细胞 (MaSC) 的生长效率降低了五倍所示。这些发现表明 Gli2 指定了一个严格调节 MaSC 活动的基质生态位信号程序。FACS 分离的乳腺基质细胞显示出刺激上皮干细胞更新、导管生长和形态发生的因子的 Gli2 依赖性表达，包括分泌肽的 IGF、WNT、FGF 和 HGF 家族的特定成员。单细胞分析显示这些因子在基质细胞亚群中的表达依赖于 Gli2 功能。乳腺激素雌激素和生长激素的受体也以 Gli2 依赖性方式表达，雌激素和生长激素处理培养的基质细胞会诱导这些分泌肽因子，这表明 Gli2 在乳腺基质生态位细胞中的关键作用是使这些细胞对全身性乳腺激素产生反应。支持这一结论的是，释放 IGF1 和 WNT2 的聚合物片段的乳腺植入物挽救了 Gli2ΔS 导管生长表型，而释放生长激素的聚合物则没有。结论 我们发现 Hedgehog 通路转录效应子 GLI2 指定了一种基质细胞生态位信号传导程序，该程序支持青春期和处女成年小鼠的乳腺上皮干细胞。该程序包括 IGF 和 WNT 等因子的表达，而 GLI2 的部分作用是通过引起雌激素和生长激素等激素受体的表达，从而将局部干细胞生态位活动引入全身性乳腺激素。我们的工作说明了基质生态位在控制上皮干细胞活性方面的核心作用，并表明生态位失败可能是某些疾病发病机制的基础，包括与人类疾病、垂体激素缺乏症相关的乳房发育缺陷和激素不敏感。基质 GLI2 协调乳腺干细胞生态位信号程序。Hedgehog 通路效应子 GLI2 在乳腺基质细胞中的转录调控协调激素反应性生态位信号传导程序，该程序在青春期变化期间指导上皮干细胞活动（示意图，左）。小鼠乳腺中 Gli2 缺陷的基质细胞无法支持移植的正常导管生长和形态发生，绿色荧光蛋白标记的干细胞（右）。干细胞生态位是一个复杂的局部信号微环境，在器官维护和再生过程中维持干细胞活性。乳腺生态位必须支持其相关的干细胞，同时还要响应触发青春期变化的全身激素调节。我们发现主要 Hedgehog 通路转录效应子 Gli2 在小鼠乳腺基质细胞内起作用，通过激活调节上皮干细胞的因子的表达以及乳腺激素雌激素和生长激素的受体来指导激素响应性生态位信号传导程序。虽然先前的研究表明人类疾病中存在干细胞缺陷，但这项工作表明生态位功能障碍也可能导致疾病，