Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their differentiation.

The PU.1 transcription factor is a key regulator of hematopoietic development, but its role at each hematopoietic stage remains unclear. In particular, the expression of PU.1 in hematopoietic stem cells (HSCs) could simply represent "priming" of genes related to downstream myelolymphoid lineages. By using a conditional PU.1 knock-out model, we here show that HSCs express PU.1, and its constitutive expression is necessary for maintenance of the HSC pool in the bone marrow. Bone marrow HSCs disrupted with PU.1 in situ could not maintain hematopoiesis and were outcompeted by normal HSCs. PU.1-deficient HSCs also failed to generate the earliest myeloid and lymphoid progenitors. PU.1 disruption in granulocyte/monocyte-committed progenitors blocked their maturation but not proliferation, resulting in myeloblast colony formation. PU.1 disruption in common lymphoid progenitors, however, did not prevent their B-cell maturation. In vivo disruption of PU.1 in mature B cells by the CD19-Cre locus did not affect B-cell maturation, and PU.1-deficient mature B cells displayed normal proliferation in response to mitogenic signals including the cross-linking of surface immunoglobulin M (IgM). Thus, PU.1 plays indispensable and distinct roles in hematopoietic development through supporting HSC self-renewal as well as commitment and maturation of myeloid and lymphoid lineages.

Enhancement of hematopoietic stem cell repopulating capacity and self-renewal in the absence of the transcription factor C/EBP alpha.

The transcription factor C/EBP alpha is required for granulopoiesis and frequently disrupted in human acute myeloid leukemia (AML). Here, we show disruption of C/EBP alpha blocks the transition from the common myeloid to the granulocyte/monocyte progenitor but is not required beyond this stage for terminal granulocyte maturation. C/EBP alpha-deficient hematopoietic stem cells (HSCs) have increased expression of Bmi-1 and enhanced competitive repopulating activity. Bone marrow in adult C/EBP alpha-deficient mice was filled with myeloblasts, similar to human AML, supporting the notion that disruption of C/EBP alpha cooperates with other events in the development of leukemia. Therefore, C/EBP alpha is not only essential for granulocyte development but, in addition, is a regulator of hematopoietic stem cell activity.