The relationship between Mll1 and Notch signaling in epithelial stem cell homeostasis

混合谱系白血病（Mll 1）是六种H3 K4甲基转移酶的哺乳动物家族的创始成员。它被发现是早发性白血病中突变的主要基因，随后被发现是建立和维持造血干细胞所必需的。然而，尽管Mll 1广泛表达，但其在非造血组织中的作用仍在很大程度上未被探索。在我们之前的DFG项目中，我们发现成年小鼠Mll 1的缺失导致小肠功能的快速衰竭。值得注意的是，观察到的肠缺陷-分泌细胞的扩增以及干细胞隔室的耗尽-重现了肠隐窝干细胞中的Notch信号传导阻断。因此，我们的目标是破译Mll 1在肠道系统中的作用及其与Notch信号的关系。我们提出，Mll 1是表达Notch信号通路的组分所必需的，或者是Notch转录应答的Notch靶基因所物理需要的。在小鼠和类器官中使用条件诱变，我们将通过转录组分析和染色质免疫沉淀研究来鉴定Mll 1靶基因。我们还将研究Mll 1在建立肠干细胞中的胚胎作用，并确定Mll 1是否可以在去除后重新表达后在成人中重新建立功能性隐窝。除了他莫昔芬诱导的Cre/loxP条件突变，我们将探索一种新的配体诱导的功能丧失策略在小鼠中的应用，基于生长素诱导的降解决定子。我们的初步发现将Mll 1与肠干细胞中的Notch信号传导联系起来，这对Notch信号传导也涉及的其他上皮干细胞隔室具有影响。Mll 1和Notch信号之间的联系在不同的上皮隔室可能是所有上皮干细胞的基础和中央上皮肿瘤发生的机制。

Mixed lineage leukemia (Mll1) is the founding member of the mammalian family of six H3K4 methyltransferases. It was discovered as the main gene mutated in early onset leukemia and subsequently found to be required for the establishment and maintenance of hematopoietic stem cells. However, although ubiquitously expressed, the roles of Mll1 in non-hematopoietic tissues remain largely unexplored. In our previous DFG project, we found that loss of Mll1 in adult mice results in rapid failure of small intestinal function. Remarkably, the observed intestinal defect - an expansion of secretory cells together with a depletion of the stem cell compartment - recapitulated Notch signaling blockage in intestinal crypt stem cells. Consequently, we aim to decipher the role of Mll1 in the intestinal system and its relationship to Notch signaling. We propose that either Mll1 is required for expression of a component(s) of the Notch signaling pathway or is physically required at Notch target genes for Notch transcriptional responses. Using conditional mutagenesis in mice and organoids, we will identify Mll1 target genes by transcriptome profiling and chromatin immunoprecipitation studies. We will also investigate the embryonic role of Mll1 in the establishment of intestinal stem cells and determine whether Mll1 can re-establish functional crypts in adults upon re-expression after removal. In addition to tamoxifen-induced Cre/loxP conditional mutagenesis, we will explore the application of a new ligand-inducible loss-of-function strategy in mice based on the auxin-inducible degron. Our primary discovery connecting Mll1 to Notch signaling in intestinal stem cells has implications for other epithelial stem cell compartments where Notch signaling is also implicated. The link between Mll1 and Notch signaling in different epithelial compartments could be fundamental to all epithelial stem cells and central to mechanisms of epithelial tumorigenesis.