ATAC-seq data in project that HMGA2 protects hematopoietic stem cell and drives the self-renewal division in stress condition. Mus musculus

Hematopoietic stem cell (HSC) respond to various stresses and produce mature blood cells; however, it is not clear how HSCs maintain and restore hematopoiesis by determining their self-renewal and differentiation fates. High mobility group AT-hook 2 (Hmga2) is a chromatin modifier protein that is highly expressed in HSCs and regulates transcription of target genes. In steady state, Hmga2 knock-out (KO) mice and Hmga2-over-expressing conditional knock-in (KI) mice both did not show a significant change in hematopoiesis; however, in response to 5-FU injection, Hmga2 KI mice showed enhanced self-renewal of HSCs and rapid hematopoietic recovery increasing platelet counts, while Hmga2 KO mice decreased numbers of HSCs and megakaryocyte progenitor cells and delayed hematopoietic recovery. Because the expression of Hmga2 was able to repress transcription of inflammatory response genes, we assessed Hmga2-binding regions and chromatin accessibility in HSCs post the in vivo injection of 5-FU or in vitro treatment with TNF-a. We found that Hmga2 bound to distinct regions and reduced chromatin accessibility in inflammatory response genes, resulting in the reduced expression of these genes in the stress condition. As the acidic domain in the Hmga2 protein was phosphorylated by Casein kinase 2, the inhibition of phosphorylation in the acidic domain by substitution with alanine resulted in impairments in the stress-induced chromatin binding of Hmga2 and the expansion of HSCs after the stress insults. Thus, Hmga2 dynamically modulated the chromatin accessibility and transcription of genes in HSC via phosphorylation of the Hmga2 protein and protected HSC in stress hematopoiesis.

造血干细胞（Hematopoietic stem cell, HSC）可响应各类应激刺激并分化产生成熟血细胞，但目前尚不明确HSC如何通过调控自我更新与分化命运，维持并重建造血功能。高迁移率族AT钩蛋白2（High mobility group AT-hook 2, Hmga2）是一类在HSC中高表达的染色质修饰蛋白，可调控靶基因的转录。在生理稳态条件下，Hmga2基因敲除（KO）小鼠与条件性Hmga2过表达基因敲入（KI）小鼠的造血功能均未出现显著变化；但在接受5-氟尿嘧啶（5-FU）注射应激后，Hmga2 KI小鼠的HSC自我更新能力增强，造血恢复速度加快，血小板计数升高；而Hmga2 KO小鼠的HSC与巨核系祖细胞数量减少，造血恢复延迟。鉴于Hmga2可抑制炎症应答基因的转录，我们分别在体内注射5-FU或体外经肿瘤坏死因子-α（TNF-α）处理后的HSC中，检测了Hmga2的结合区域与染色质可及性。研究发现，在应激状态下，Hmga2可结合至炎症应答基因的特定区域，降低其染色质可及性，从而抑制这些基因的表达。进一步研究显示，Hmga2蛋白的酸性结构域可被酪蛋白激酶2（Casein kinase 2）磷酸化；若将该结构域的磷酸化位点替换为丙氨酸以阻断磷酸化，则会损伤Hmga2在应激状态下的染色质结合能力，以及应激刺激后HSC的扩增能力。综上，Hmga2可通过自身蛋白的磷酸化动态调控HSC的染色质可及性与基因转录，从而在应激造血过程中保护HSC。