HMGA2 Maintains Hematopoietic Stem Cell Via Pleiotropic Regulation of the Transcription in Stress Conditions

High-mobility group AT-hook 2 (Hmga2), an epigenetic modifier, opens the chromatin and modulate the transcription. Hmga2 is highly expressed in fetal and adult hematopoietic stem cells (HSCs). Hmga2 over-expression has been shown to promote self-renewal of HSC, however the molecular mechanism of how Hmga2 enhanced the self-renewal of HSC is still unclear. In this study, we assessed the function of Hmga2 in HSCs in steady and stress conditions by utilizing new Hmga2 conditional knock-in (KI) mouse and Hmga2 conditional knock-out mouse, which were crossed with either Cre-ERT2 mouse or Vav1-iCre mouse. Hmga2 KI mice showed a mild elevation in platelet counts, but did not develop malignancies in one year observation period. We performed a competitive transplantation assay by using purified HSCs, and found that wild-type HSCs diminished the repopulating capacity at the tertiary transplantation, Hmga2 KI HSCs maintained higher chimerism in myeloid cells and platelets in the PB and HSCs in the BM. We found that Hmga2 KO cells reduced the repopulating capacity, compared to wild-type cells. Thus, the expression of Hmga2 is critical for the self-renewal of HSC upon the transplantation. By performing RNA-sequencing of HSCs in homeostatic condition, we found that Hmga2 KI HSCs showed positive enrichments in cell cycle and proliferative signature, but maintained a stem cell signature, compared to wild-type HSCs. Since Hmga2 has been shown to globally open the chromatin in neural stem cells, we performed ATAC-sequence analysis in HSCs. Notably, Hmga2 KI HSCs showed 539 opened and 387 closed chromatin in H3K27ac-marked active regulatory regions, compared to wild-type HSCs. Among these opened genes by Hmga2, we generated a virus vector for fifteen genes, which were highly expressed in Hmga2 KI HSCs, and found that ectopic expression of Igf2bp2, an RNA binding protein, increased self-renewal capacity of HSC, but did not induce the enhanced production of myeloid cells and platelets that were observed in Hmga2 KI cells, in in vitro and in vivo settings. Indeed, Hmga2-ChIP-sequencing revealed that Hmga2 was directly bound to a proximal region of the Igf2bp2 gene, and CRISPR/Cas9-mediated deletion of the Igf2bp2 gene canceled the enhanced self-renewal capacity of Hmga2 KI HSCs, indicating that the Hmga2-Igf2bp2 axis is critical for the self-renewal of HSC.

We next assessed function of Hmga2 in stress hematopoiesis after in vivo treatment of 5-FU. Hmga2 KI mice showed faster recoveries of reduction of platelets in the PB and increased CD41+HSCs and megakaryocyte progenitors in the BM in twelve days, in which WT mice reduced numbers of those cells in this condition. RNA-sequencing revealed that Hmga2 KI HSCs maintained expression levels of genes in stem cell- and proliferation-signatures at 3 days and 6 days post 5-FU injection, compared to WT HSCs that reduced expression of stem cell genes but activated inflammatory response genes. In contrast, Hgma2 KO HSCs enhanced expression of inflammatory response genes post the 5-FU injection, indicating Hmga2 represses expression of inflammatory response genes in the stress condition. Indeed, Hmga2-ChIP-sequencing revealed that Hmga2 was bound to larger numbers of genes involved in inflammatory responses in Kit+ cells post the 5-FU treatment from those in the control cells. Given increased expression of and a post-translational modification of Hmga2 protein in Kit+ cells post the 5-FU treatment, the remodeling of Hmga2 binding regions was appeared to depend on the modification of Hmga2 at downstream of the stress signal. Thus, Hmga2 directly activates Igf2bp2 to enhance the self-renew of HSC, but also represses the inflammatory response, leading to the enhanced megakaryopoiesis in the stress condition.