Characterizing transient hematopoietic multipotent and megakaryocyte progenitor cells during postnatal development

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Abstract Fetal hematopoiesis has properties distinct from adult hematopoiesis, explaining the unique features of infant myeloproliferative disorders and leukemia. Adult-type hematopoietic stem cells (HSCs) are generated in the major arteries in the aorta-gonad-mesonephros region at embryonic day (E)9.5-E11.5 in mice, colonize the fetal liver (FL) and fetal bone marrow (BM), and remain in adult BM for an organism’s lifetime. Previous studies hypothesized that the fetal-to-adult hematopoietic transition is a gradual maturation process of adult type HSCs. However, two fetal-type hematopoietic progenitors (HPCs) originate from the yolk sac hematopoiesis (E7.5 and E8.5) that also colonize the FL and fetal BM, overlap with maturing HSCs, and are the primary contributors to hematopoiesis during fetal development. Increased evidence suggests this transition is a combined process of declining fetal-type HPCs and maturation of adult-type HSCs. However, the cellular and molecular mechanisms regulating the hematopoietic transition are still largely unknown. Green fluorescent protein (GFP) expression in Ctnnal1-GFP reporter mice was shown to specifically label HSCs in young adults, but not downstream multipotent progenitors (MPPs) or megakaryocyte progenitors (MkPs). In addition to HSCs in neonatal mice, I found GFP labels a subset of MPP1, MPP2 and MkPs. These GFP+ HPCs gradually decline until absent by week 4 post-birth, while GFP- MPP1, MPP2, and MkPs remain. As adult HSC maturation occurs during this period, GFP+ subsets represent novel developmentally restricted HPCs with unknown origin, function, and regulatory mechanisms. In vitro study demonstrates that GFP+ MPP1, MPP2 and MkPs are distinct from their GFP- counterparts and display many properties of fetal type HPCs. For example, although both GFP+ and GFP- MPP2s differentiate to monocytes, granulocytes and megakaryocytes (Mk), GFP+ MPP2s are biased to monocyte and Mk differentiation like fetal HPCs, while GFP- MPP2s are biased to granulocytic differentiation. Compared to Mks produced by GFP- MkPs, GFP+ MkPs produce smaller, lower ploidy Mks similar to Mks found in fetal BM. I hypothesize that Ctnnal1-GFP+ MPP1, MPP2, and MkPs are phenotypically and functionally distinct from their Ctnnal1-GFP- counterparts, are derived from fetal type HPCs independent of adult HSCs, and represent novel transient populations that are capable of initiating infant hematological diseases. I intend to address this hypothesis by (1) fully characterizing the biology and function of GFP+ MPP1, MPP2, and MkPs; and (2) determining where they arise and if they are derived from fetal type HPCs independent of adult HSCs. To do so, I will use three mouse models: Ctnna1-GFP mice for in vitro and flow cytometry experiments, Ctnna1-GFP/Rosa26-tdTomato mice for transplantations, and Cdh5Cre-Ert/Ctnnal1- GFP/Rosa26-tdTomato mice to determine the origin of GFP+ cells. Success of this study will not only help to understand the mechanism of fetal to adult hematopoietic transition, but also serve as a model for hypothesis generation related to the development of infant/pediatric leukemia.