Neil P. Rodrigues

Relative quiescence is a defining characteristic of hematopoietic stem cells, while their progeny have dramatic proliferative ability and inexorably move toward terminal differentiation. The quiescence of stem cells has been conjectured to be of critical biologic importance in protecting the stem cell compartment, which we directly assessed using mice engineered to be deficient in the G1 checkpoint regulator, cyclin-dependent kinase inhibitor, p21cip1/waf1 (p21). In the absence of p21, hematopoietic stem cell proliferation and absolute number were increased under normal homeostatic conditions. Exposing the animals to cell cycle-specific myelotoxic injury resulted in premature death due to hematopoietic cell depletion. Further, self-renewal of primitive cells was impaired in serially transplanted bone marrow from p21-/- mice, leading to hematopoietic failure. Therefore, p21 is the molecular switch governing the entry of stem cells into the cell cycle, and in its absence, increased cell cycling leads to stem cell exhaustion. Under conditions of stress, restricted cell cycling is crucial to prevent premature stem cell depletion and hematopoietic death.

The zinc finger transcription factor GATA-2 plays a fundamental role in generating hematopoietic stem-cells in mammalian development. Less well defined is whether GATA-2 participates in adult stem-cell regulation, an issue we addressed using GATA-2 heterozygote mice that express reduced levels of GATA-2 in hematopoietic cells. While GATA-2+/- mice demonstrated decreases in some colony-forming progenitors, the most prominent changes were observed within the stem-cell compartment. Heterozygote bone marrow had a lower abundance of Lin(-)c-kit(+)Sca-1(+)CD34- cells and performed poorly in competitive transplantation and quantitative week-5 cobblestone area-forming cell (CAFC) assays. Furthermore, a stem-cell-enriched population from GATA1+/- marrow was more quiescent and exhibited a greater frequency of apoptotic cells associated with decreased expression of the anti-apoptotic gene Bcl-xL. Yet the self-renewal potential of the +/- stem-cell compartment, as judged by serial transplantations, was unchanged. These data indicate compromised primitive cell proliferation and survival in the setting of a lower GATA-2 gene dose without a change in the differentiation or self-renewal capacity of the stem-cells that remain. Thus, GATA-2 dose regulates adult stem-cell homeostasis by affecting select aspects of stem cell function.