Heritable ER stress impairs mitochondrial metabolism and leads to exhaustion of low-dose irradiated hematopoietic stem cells [ATACseqHSC]

How hematopoietic stem cells (HSC) respond to low levels of stress such as exposure to very low doses of radiation currently used in medicine, is largely unknown. Here, we show that 20mGy-irradiated HSC (20mGy-HSC) exhibit, when they proliferate, oxidative stress with increased mitochondrial Reactive Oxygen Species and metabolism together with mitochondrial Ca2+ overload. These mitochondrial defects are due to an immediate and sustained endoplasmic reticulum (ER) stress adapted by proliferative 20mGy-HSC through activation of the eIF2?-ATF4 branch of the UPR. The ER stress is heritable and leads, in long-term quiescent 20mGy-HSC, to activation of the IRE1?-Xbp1 branch of UPR, which fails to restore ER homeostasis resulting in HSC self-renewal deficiency and exhaustion. Finally, we show that this heritable ER stress leads to a global DNA hypomethylation, partially reverted by early inhibition of ER stress. Our studies illuminate how an adaptive ER stress response can lead to mitochondrial defects and HSC deficiency. Overall design: ATAC-seq in HSC isolated from bone marrow of mice transplanted with sham or 20mGy-irradiated LSK

造血干细胞（hematopoietic stem cells, HSC）对医学临床当前使用的极低剂量辐射等低水平应激的响应机制，目前仍未完全阐明。本研究证实，经20mGy辐射的造血干细胞（20mGy-HSC）在增殖过程中会表现出氧化应激，伴随线粒体活性氧（mitochondrial Reactive Oxygen Species）水平升高、代谢异常以及线粒体钙超载。上述线粒体缺陷源于即时且持续的内质网（endoplasmic reticulum, ER）应激，而增殖状态的20mGy-HSC可通过激活未折叠蛋白反应（unfolded protein response, UPR）的eIF2α-ATF4通路来适应该内质网应激。这种内质网应激具有可遗传性，并在长期静息的20mGy-HSC中触发UPR的IRE1α-XBP1通路激活，但该通路无法恢复内质网稳态，最终导致造血干细胞自我更新缺陷与功能耗竭。最后，本研究发现这种可遗传性内质网应激会引发全基因组DNA低甲基化，且可通过早期抑制内质网应激得到部分逆转。本研究阐明了适应性内质网应激反应如何介导线粒体缺陷与造血干细胞功能异常。整体实验设计：对接受假照射或20mGy辐射的LSK细胞移植的小鼠骨髓中分离的造血干细胞开展转座酶可及性测序分析（Assay for Transposase-Accessible Chromatin using sequencing, ATAC-seq）检测。