Mechanisms that regulate erythroid differentiation of hematopoietic stem cells

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Hematopoietic stem cells (HSCs) are capable of regenerating the entire hematopoietic system. This capacity is fully unleashed upon bone marrow transplantation, however, recent studies including ours indicate that HSCs have a much more limited contribution to hematopoiesis during homeostasis. While stress from sources like chemotherapies or inflammation increases the contribution from HSCs to multiple hematopoietic lineages, little is known about how HSCs respond to stressors that cause anemia. Identifying the progenitor cell population and signals that promote erythroid regeneration may lead to novel strategies to better harness HSCs for the treatment of anemia and regenerative medicine overall. Using a HSC lineage tracing model, we found that hemolytic anemia specifically enhances erythroid contribution by HSCs, indicating that HSCs respond to erythroid stress by initiating erythropoiesis. After hemolytic anemia, HSCs expressed more erythropoiesis-related genes and exhibited enhanced erythroid differentiation in vitro and in vivo. Interestingly, HSCs had increased iron content after hemolytic anemia and iron chelation prevented erythroid-biased differentiation. Iron is a cofactor for the iron(II)/α-ketoglutarate-dependent dioxygenase TET2 that demethylates DNA, and erythropoiesis is associated with DNA demethylation. We found that TET2 protein, but not mRNA, is increased in HSCs during anemia and deletion of Tet2 suppressed the enhanced erythroid differentiation of HSCs we observed following hemolytic anemia. We thus hypothesized that HSCs respond to anemia by increasing iron uptake, TET2 expression, and DNA demethylation, thereby increasing the expression of erythropoiesis genes. In Aim 1, we will study the heterogeneity of splenic HSCs during anemia and identify the erythroid-biased fraction of HSCs. In Aim 2, we will examine the role of TET2 in promoting erythroid commitment of splenic HSC. In Aim 3, we will investigate the mechanism by which TET2 protein is stabilized in HSCs during anemia. Completion of this study will provide novel insights into the differences between bone marrow and splenic HSCs in responding to anemia, and the mechanism by which iron and TET2 instructs HSCs to become committed to the erythroid lineage.