骨髓微环境中血管生成素样蛋白2对造血干细胞干性维持的作用和意义

Increasing evidences suggest that the stemness (self-renewal and differentiation) of hematopoietic stem cells is precisely regulated by many types of stromal cells found in the bone marrow microenvironment (niche) and their secretary factors. Our previous studies, for the first time, identified a membrane receptor for several angiopoietin-like proteins (Angptls) which has been shown to play an important role in the regulation of the stemness of hematopoietic stem cells and leukemia stem cells. In addition, our current work indicates that certain member of Angptls, such as Angptl2, may be an important component of the bone marrow niche. Based on these results, we decide to analyze the expression pattern of Angptl2 in different types of niche cells. Meanwhile, we also try to elucidate the underlying signal pathways and potential targets for the maintenance of stemness of hematopoietic stem cells by using of the Angptl2 conditional knockout mouse we have constructed. On the other side, studies indicate that the pathological bone marrow niche may also affect the cell fates of both hematopoietic stem cells and leukemia stem cells. By taking advantage of our established mouse leukemia model, we will unravel the roles of Angptl2 in the regulation of the stemness in both hematopoietic stem cells and leukemia stem cells under this pathological situation. The possible interaction and significance among the pathological bone marrow niche, hematopoietic stem cells and leukemia stem cells will be studied as well. This work will be greatly helpful to understand the molecular mechanisms in maintenance of the stemness and malignant transformation in hematopoietic stem cells. It may also open a new avenue to develop strategies for ex vivo expansion of hematopoietic stem cells and targeting to leukemia stem cells.

越来越多的证据提示骨髓微环境（Niche）中的各种细胞及其分泌成分对成体造血干细胞的干性维持（即自我更新和分化）起着精细调控作用。我们前期研究首次证实了血管生成素样蛋白调控造血干细胞和白血病干细胞干性的膜受体，目前初步结果也表明血管生成素样蛋白家族成员（如Angptl2）可能参与了骨髓Niche的组成。据此，本项目拟利用已构建的Angptl2条件性敲除小鼠，阐明Angptl2在骨髓Niche中的表达和分布、及其调控造血干细胞干性维持的信号通路和潜在靶点；此外，鉴于疾病状态下骨髓Niche可能发生重塑并影响着造血干细胞和白血病干细胞的命运，我们也将利用小鼠白血病模型，初步探讨病理性骨髓Niche中Angptl2与造血干细胞和白血病干细胞的相互影响和临床意义。这些问题的解决，不仅有助于理解造血干细胞干性维持和恶性转化的分子基础，也为造血干细胞的体外扩增与应用及白血病干细胞的靶向清除提供新思路。

课题围绕骨髓微环境中血管生成素样蛋白2（Angiopoietin-like protein 2, Angptl2）对造血干细胞（Hematopoietic stem cells, HSCs）干性维持的作用，利用分子生物学、基因敲除小鼠、动物疾病模型等手段，揭示了血管内皮来源的Angptl2是骨髓Niche的重要组成成分，并在维持HSCs自我更新中发挥重要作用。特异敲除血管内皮中的Angptl2可导致HSCs重建骨髓能力的显著降低以及在血管周微环境定位能力的丧失。在MLL-AF9诱导的白血病模型中，内皮来源的Angptl2也能促进白血病干细胞（leukemia stem cells, LSCs）的增殖并加速白血病的发生。机制上，Angptl2通过G0S2精细调控HSCs的干性和功能。我们也进一步拓展揭示了Angptl2等成分也可通过表面分子受体（如JAM3）介导的信号通路调控LSCs干性维持的新形式和新规律。考虑到Angptl2是一种重要的脂代谢调控因子，我们进一步探讨了骨髓微环境中相关成分（如Angptl2）调控HSCs/LSCs的代谢调控规律，初步发现：Angptl2可能参与了糖代谢的调控；并进一步拓展揭示了LSCs以糖酵解作为能量的主要来源，PDK2调控糖酵解以维持LSCs归巢和对称分裂新机制；同时，我们还发现了HSCs支链氨基酸（branched-chain amino acids，BCAAs）独特代谢规律，即BCAAs降解限速酶PPM1K通过维持适当的BCAAs水平以避免MEIS1/p21的泛素化降解，进而维持HSCs糖酵解和静息状态的代谢新途径。有意思的是，我们的工作也表明LSCs更依赖于BCAAs代谢以维持其致病能力。这些问题的阐明，不仅有助于理解骨髓微环境中HSCs干性维持和恶性转化的微环境组成成分和分子基础，也为造血干细胞的体外扩增与应用及白血病干细胞的靶向清除提供了新思路。