The endoplasmic reticulum as a determinant of hematopoietic niche homeostasis

造血干细胞（hsc）在应激反应中产生所有血细胞谱系。骨髓（BM）微环境（也称为骨髓生态位）在生物体的生命周期内维持造血干细胞的这种多系再种群（LTR）能力。众所周知，在慢性应激（感染或炎症）和衰老过程中，造血干细胞的LTR能力（LTR）明显降低。然而，目前尚不清楚来自骨髓基的外部信号如何触发和维持造血干细胞的LTR能力和自我更新。我们已经建立了小鼠模型，其中BM生态位的特异性原发变化导致hsc中LTRA的继发性减少。在我们将要使用的小鼠品系中，我们删除了表达osterix的骨祖细胞（OS1Del/Del小鼠）中的strp1基因。在这个项目中，我们将研究来自这些小鼠的BM小生境细胞的变化如何改变它们与造血干细胞的交流以及它们维持这些细胞LTRA的能力。我们从OS1Del/Del小鼠的转录组中获得的初步数据显示，在分选的骨祖细胞中，内质蛋白酶平衡失调。进一步的验证研究证实，OS1Del/Del小鼠的BM小生境细胞在蛋白质水平上下调了蛋白抑制因子，并显著降低了蛋白质降解。蛋白质静止是一个重要的细胞维持过程，不仅可以去除错误折叠的蛋白质，还可以将蛋白质分类并分泌到细胞外空间。因此，内质蛋白酶抑制可能是调节骨髓小生境细胞和造血干细胞之间通讯的决定性因素。在这个项目中，我们将进一步研究BM小生境细胞的内质蛋白酶抑制，并确定内质蛋白酶抑制过程和分泌组中的关键介质。此外，我们将对OS1Del/Del小鼠进行治疗，目的是恢复BM生态位的内质蛋白酶平衡，提高hsc的LTR能力。这些实验的长期目标是确定针对利基的策略，以防止慢性应激（感染或炎症）和衰老中具有LTR能力的hsc耗尽。

Hematopoietic stem cells (HSCs) produce all blood cell lineages in response to stressors. The bone marrow (BM) microenvironment (also called: the BM niche) maintains this multilineage repopulation (LTR) ability of HSCs for the life time of the organism. It is known that in chronic stress (infections or inflammation) and in aging, the LTR ability (LTR) of HSCs is strongly reduced. However, it remains largely unclear how outside signals from the BM niche trigger and maintain LTR ability and self-renewal of HSCs. We have established mouse models in which specific primary changes in the BM niche cause secondary reduction of LTRA in HSCs. In the mouse strain we will be using, we deleted the Sfrp1 gene in Osterix-expressing osteoprogenitor cells (OS1Del/Del mice). In this project, we will study how the changes in BM niche cells from these mice change their communication with HSCs and their ability to maintain the LTRA of these cells.Our preliminary data from the transcriptome of OS1Del/Del mice shows deregulation of endoplasmic proteostasis in sorted osteoprogenitors. Further validation studies confirmed downregulation of proteostatic factors at the protein level, and a strong reduction of protein degradation in BM niche cells from OS1Del/Del mice. Proteostasis is a critical cellular maintenance process, not only for removing misfolded proteins, but also for sorting of proteins for secretion to the extracellular space. As such, endoplasmic proteostasis would be a determining factor in regulating communication between BM niche cells and HSCs. In this project we will further investigate endoplasmic proteostasis in BM niche cells, as well as identify critical mediators in both the endoplasmic proteostatic process and in the secretome. Furthermore, we will treat OS1Del/Del mice with the aim to restore endoplasmic proteostasis in the BM niche and to improve the LTR ability of HSCs. A long-term goal of these experiments is to identify niche-directed strategies to prevent exhaustion of HSCs with LTR ability in chronic stress (infections or inflammation) and in aging.