The endoplasmic reticulum as a determinant of hematopoietic niche homeostasis

造血干细胞(HSCs)在应激源作用下产生所有血细胞系。骨髓(BM)微环境(也称为BM生态位)在生物体的整个生命周期中维持着HSCs的这种多系再繁殖(LTR)能力。众所周知，在慢性应激(感染或炎症)和衰老过程中，HSC的LTR能力(LTR)显著降低。然而，在很大程度上仍然不清楚来自BM利基的外部信号如何触发和维持HSC的LTR能力和自我更新。我们已经建立了小鼠模型，在该模型中，BM利基的特定初级变化导致HSCs中LTRA的继发性减少。在我们将要使用的小鼠品系中，我们在表达Osterix的骨祖细胞(OS1Del/Del小鼠)中删除了SFRP1基因。在这个项目中，我们将研究来自这些小鼠的BM利基细胞的变化如何改变它们与HSCs的通信以及它们维持这些细胞的LTRA的能力。我们从OS1Del/Del小鼠的转录组获得的初步数据显示，在分类的骨祖细胞中，内质蛋白平衡的调节被解除。进一步的验证研究证实了蛋白抑制因子在蛋白质水平上的下调，并显著减少了OS1Del/Del小鼠BM利基细胞的蛋白质降解。蛋白质平衡是一个关键的细胞维持过程，不仅是为了去除错误折叠的蛋白质，也是为了将蛋白质分选出来，分泌到细胞外空间。因此，内质蛋白平衡将是调节骨髓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壁龛细胞的内质蛋白平衡，并确定内质蛋白平衡过程和分泌组中的关键介质。此外，我们将治疗OS1Del/Del小鼠，目的是恢复BM利基中的内质蛋白平衡，并提高HSCs的LTR能力。这些实验的一个长期目标是确定利基导向的策略，以防止在慢性应激(感染或炎症)和衰老中具有LTR能力的HSCs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.