Cellular pathways and molecular regulation of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are defined by their unique properties to self-renew and differentiate into all hematopoietic cell lineages. Residing in the bone marrow (BM), adult HSCs are organized in a hierarchy of cells that differ in their self-renewal and proliferative capacity. Exact relationships of the various cells in the hematopoietic hierarchy, their potential to commit to certain lineages and early commitment steps are still unclear. According to classical model the first commitment step from HSCs is believed to be a divergence into the common lymphoid progenitor (CLP) and the common myeloid progenitor (CMP). Recently the alternative model has been suggested by introducing the lymphoid-primed multipotent progenitors (LMPPs) with sustained granulocyte/macrophage (GM) and lymphoid potential, but little or no megakaryocyte-erythroid (MkE) potential. In this work we further characterized LMPPs and established the multilineage transcriptional priming and lineage potentials of recently identified distinct and hierarchically related long term HSCs (LSKCD34-Flt3-), short term HSCs (LSKCD34+Flt3-), and LMPPs (LSKCD34+Flt3hi) in mouse fetal liver (FL) and adult BM providing evidence for a hierarchical organization of transcriptional lineage priming within these multipotent stem and progenitor cell compartments and demonstrating that LMPPs are defined already during fetal development. Furthermore, MkE potential segregated almost entirely with LSKFlt3hi cells expressing the thrombopoietin receptor (THPOR), whereas LSKFlt3hiTHPO- LMPPs lacked significant MkE potential, but sustained GM and lymphoid potentials. Gradual increase of transcriptional lymphoid-priming in subfractions of LMPPs (GFP-, GFPlo, GFPint, GFPhi) isolated from BM of Rag1GFP reporter mouse, occur in the presence of maintained GM lineage priming, but gradually reduced GM lineage potential. The transcription factor PAX5, a critical regulator of B-cell lineage commitment, has recently been shown to be expressed in myeloid malignancies, particularly in a subset of acute myeloid leukemia (AML) with translocation t(8;21). Herein, we demonstrate that overexpression of PAX5 in LSK cells can promote survival and proliferation of biphenotypic (B220+GR-1/MAC-1+) myeloid progenitors coexpressing, at the single-cell level, myeloid genes and B-cell-associated PAX5 target genes, indicating potential relevance of current observation to dysregulated PAX5 expression seen in AML. This work has also investigated the regulatory role of β-catenin (involved in the Wnt pathway) previously shown to increase self-renewal and expansion of HSCs. We found that constitutive activation of β-catenin can result in loss of repopulating HSC activity and depletion of the HSCs.