Reciprocal Activation of GATA-1 and PU.1 Marks Initial Specification of Hematopoietic Stem Cells into Myelo-Erythroid and Myelo-Lymphoid Lineages.

Understanding how multipotent cells commit to each of their terminal fate potentials is an important aspect of stem cell biology. In adult murine hematopoiesis, HSCs with long-term self-renewal potential reside within the Lin ⁻Sca-1⁺c-Kit⁺ (LSK) fraction having CD34⁻, Thy1^lo, and Flt3/Flk2⁻phenotypes. The LSK cells having CD34⁺, Thy1⁻, and/or Flt3⁺ phenotypes are capable of multi-lineage reconstitution only for a short-term, and therefore should contain multipotent progenitors (MPPs). In terms of developmental steps downstream of MPPs, there has been a controversy. The existence of prospectively isolatable progenitors capable of generating only myeloerythroid cells (common myeloid progenitor: CMP) or only lymphocytes (common lymphoid progenitors: CLP) outside the LSK fraction suggests that the first commitment step after the MPP stage is the strict bifurcation of lymphoid vs. myeloid pathway. Recently, however, several studies have suggested that the lineage commitment could occur within the LSK fraction, preceding the proposed bifurcation of myeloid and lymphoid pathways. For example, MPPs expressing Flt3 at a high level retained granulocytes/monocytes (GM) but not megakaryocyte/erythrocyte (MegE) potential together with lymphoid potential, suggesting the existence of common progenitor for GM and lymphoid lineages within the LSK fraction. Based on this data, the coupled loss of self-renewal activity and MegE potential in the early HSC commitment has been proposed. How can we reconcile the controversy in the early hematopoietic lineage map? By utilizing mice having GATA-1 or PU.1 transcriptional reporters, we here present a high-resolution map containing new lineage-restricted progenitor populations within the MPP population of CD34⁺ LSK phenotype. Initial upregulation of GATA-1 and PU.1 occurred independently at the MPP stage. The GATA-1⁺ MPP displayed potent myeloerythroid potential without giving rise to lymphocytes, whereas the PU.1⁺ MPP showed granulocyte/monocyte/lymphoid-restricted progenitor activity without megakaryocyte/erythroid differentiation. Furthermore, GATA-1⁺ and PU.1⁺ MPPs possessed huge expansion potential, and differentiated into the original CMPs and CLPs, respectively. Thus, the reciprocal activation of GATA-1 and PU.1 primarily organizes hematopoietic lineage fate decision to form the earliest hematopoietic branchpoint that comprises new, isolatable myeloerythroid and myelolymphoid progenitor populations.