1017 – LIFE STAGE-DEPENDENT ALTERATION IN THE REGULATION OF HEMATOPOIETIC STEM AND PROGENITOR POPULATIONS VIA THROMBOPOIETIN SIGNALING

Hematopoietic stem cells (HSCs) are the origin of all blood cells and maintained for throughout life to ensure the homeostasis of the blood system. HSCs exhibit an innate capacity for self-renewal and differentiation, yet their cell fate differs during life-stages. HSCs are predominantly proliferative in the fetal liver yet enter cell cycle dormancy in the adult bone marrow (BM). Systemically and locally produced cytokines specify the cell fate of hematopoietic lineages within the bone marrow (BM). Deficiency in the cytokine thrombopoietin (Thpo) and its receptor Mpl results in an age-progressive decline in hematopoietic stem and progenitor cell (HSPC) and megakaryocytes (MK) number. While HSPC-driven hematopoiesis in developmental and adult-stages differ in proliferative and differentiative nature, no study has addressed whether and how hematopoietic cell sensitivity of Thpo/Mpl signaling varies during these stages. We thus generated a Mpl reporter mouse (Mpl-eGFP and MplCreERT2;CAG-LSL-EGFP) to monitor, induce and trace Mpl expression on hematopoietic cells in the fetal liver (FL)(E14.5) and adult BM. While Mpl was expressed in HSCs, multipotent progenitor (MPP) cells and lineage-specific progenitor cells in the FL, Mpl expression was restricted to HSC and early MPP populations upon short-term (7days after tamoxifen administration) labeling in the adult BM. Furthermore, platelet-biased HSCs but not MK erythroid progenitor (MEP) cells expressed Mpl with short term labeling. We next assessed the effect of inducing deletion of Thpo on hematopoietic cell populations in the BM using Thpofl/fl;CreERT+ (Thpo cKO) mice. Thpo cKO mice exhibited thrombocytopenia and significant decrease in plasma Thpo. BM HSCs, platelet-biased HSCs and MKP numbers sharply and significantly decreased at day7 while other progenitor cell populations retained their number until day 28 of Thpo deletion. Difference in sensitivity to Thpo may shape life-stage dependent functional variation of HSPCs and guide the transition from developmental to adult hematopoiesis.