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

混合谱系白血病（Mll1）是哺乳动物六种H3K4甲基转移酶家族的创始成员。它被发现是早发性白血病的主要突变基因，随后被发现是造血干细胞建立和维持所必需的。然而，尽管Mll1在非造血组织中普遍表达，但其作用在很大程度上仍未被探索。在我们之前的DFG项目中，我们发现成年小鼠Mll1的缺失会导致小肠功能的快速衰竭。值得注意的是，观察到的肠道缺陷——分泌细胞的扩增和干细胞室的耗竭——再现了肠隐窝干细胞中Notch信号通路的阻塞。因此，我们的目标是破译Mll1在肠道系统中的作用及其与Notch信号的关系。我们认为，要么Mll1是Notch信号通路某个组分表达所必需的，要么是Notch靶基因在Notch转录应答中所必需的。利用小鼠和类器官的条件诱变，我们将通过转录组分析和染色质免疫沉淀研究来鉴定Mll1靶基因。我们还将研究Mll1在肠干细胞建立中的胚胎作用，并确定Mll1在去除后重新表达后是否可以在成人中重建功能隐窝。除了他莫昔芬诱导的Cre/loxP条件诱变外，我们还将探索一种新的基于生长素诱导退化的配体诱导功能丧失策略在小鼠中的应用。我们的主要发现将肠干细胞中的Mll1与Notch信号连接起来，这对Notch信号也涉及的其他上皮干细胞区室也有影响。不同上皮细胞室中Mll1和Notch信号之间的联系可能是所有上皮干细胞的基础，也是上皮肿瘤发生机制的核心。

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.