Hsf1 promotes hematopoietic stem cell fitness and proteostasis in response to ex vivo culture stress and aging I

Maintaining proteostasis is key to resisting stress and to promoting healthy aging. Proteostasis is necessary to preserve stem cell function, but little is known about the mechanisms that regulate proteostasis during stress in stem cells, and whether disruptions in proteostasis contribute stem cell aging is largely unexplored. We determined that ex vivo cultured mouse and human hematopoietic stem cells (HSCs) rapidly increase protein synthesis. This challenge to HSC proteostasis was associated with nuclear accumulation of Hsf1, and deletion of Hsf1 impaired HSC maintenance ex vivo. Strikingly, supplementing cultures with small molecules that enhance Hsf1 activation partially suppressed protein synthesis, rebalanced proteostasis, and supported retention of HSC serial reconstituting activity. Although Hsf1 was dispensable for young adult HSCs in vivo, Hsf1 deficiency increased protein synthesis and impaired the reconstituting activity of middle-aged HSCs. Hsf1 thus promotes proteostasis and the regenerative activity of HSCs in response to culture stress and aging. Overall design: RNA-seq experiment to examine the effects of cell culture on hrmatopoietic stem cell (HSC) gene expression. Also tested is the effect of 17-AAG on wild-type and Hsf1-deficient HSCs.

维持蛋白质稳态（proteostasis）是抵抗应激、促进健康衰老的关键。蛋白质稳态对于维持干细胞功能不可或缺，但目前对于干细胞在应激状态下调控蛋白质稳态的分子机制仍知之甚少，且蛋白质稳态失衡是否会导致干细胞衰老这一问题，也尚未得到充分探索。本研究发现，体外（ex vivo）培养的小鼠与人类造血干细胞（hematopoietic stem cells, HSCs）会快速提升蛋白质合成速率。这种对造血干细胞蛋白质稳态的应激挑战，与热休克因子1（Heat shock factor 1, Hsf1）的核内积累相关；而敲除Hsf1会损害体外培养的造血干细胞的维持能力。值得注意的是，向培养体系中添加可增强Hsf1激活的小分子化合物，能够部分抑制蛋白质合成、重新平衡蛋白质稳态，并维持造血干细胞的连续造血重建活性。尽管在体内环境中，Hsf1对于年轻成年小鼠的造血干细胞并非必需，但Hsf1缺失会提升中年造血干细胞的蛋白质合成速率，并损害其造血重建活性。由此可见，Hsf1可在培养应激与衰老过程中，维持造血干细胞的蛋白质稳态并保障其再生活性。实验整体设计：通过RNA测序（RNA-seq）实验，探究细胞培养对造血干细胞基因表达的影响；同时检测17-AAG对野生型与Hsf1敲除型造血干细胞的作用。