The Role of the E47-p21 Pathway in Long-Term Hematopoietic Stem Cells During Homeostasis and Under Repopulation Stress

[摘要] During hematopoiesis the transcription factor E47 plays two important roles. First, E47 promotes lymphoid lineage development. Second, E47 regulates proliferation of LT-HSCs, bone marrow cells uniquely capable of long-term self-renewal and multilineage reconstitution. Because hyperproliferation of LT-HSCs can be detrimental to its function, LT-HSC proliferation must be tightly regulated.We have previously shown that E47 directly activates the cell cycle inhibitor p21 in LT-HSCs. However, the biological relevance of the E47-p21 pathway to LT-HSC function in vivo has not yet been examined. Here, we used mice with reduced gene dosage in E47 and p21 individually (E47het or p21het) versus in tandem (E47hetp21het) and show that E47hetp21het LT-HSCs exhibit hyperproliferation during homeostasis and under transplantation stress. In serial adoptive transfers that rigorously challenge self-renewal, E47hetp21het LT-HSCs dramatically and progressively decline, indicating the importance of cell-intrinsic E47-p21 pathway in preserving LT-HSC self-renewal under repopulation stress. Transient numeric recovery of downstream progenitors enabled production of functionally competent myeloid cells but not lymphoid cells as common lymphoid progenitors (CLPs) were decreased and peripheral lymphocytes virtually ablated. Thus, we demonstrate a developmental compartment-specific and lineage-specific requirement for the E47-p21 pathway in maintaining LT-HSC and lymphoid lineage cells under hematopoietic repopulation stress in vivo. It is likely that other mechanisms exist by which E47 regulate LT-HSC proliferation. Therefore, we also examined if p15PAF is a direct E47 target gene since E47-deficient and p15PAF-deficient mice share very similar early hematopoietic defects during homeostasis. We show that E47 can activate p15PAF promoter-mediated transcription in a model cell line. Unexpectedly, analysis of transcript levels show a 2-fold increase in E47-deficient progenitors compared to WT, while protein levels were comparable. These findings suggest that E47 is a transcriptional regulator of p15PAF expression that is part of a broader network of transcriptional and post-translational mechanisms that regulate p15PAF expression in primary hematopoietic progenitors. Collectively, these studies demonstrate the importance of the E47-p21 pathway in LT-HSC self-renewal and lymphoid lineage development under transplantation stress and identify p15PAF as a novel E47 target gene. These findings might provide mechanistic insights into preserving and enhancing LT-HSC function for improved therapeutic hematopoietic stem cell transplantation.

[发布机构] the University of Pittsburgh