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.

Accumulation of DNA damage leading to adult stem cell exhaustion has been proposed to be a principal mechanism of ageing. Here we address this question by taking advantage of the highly specific role of DNA ligase IV in the repair of DNA double-strand breaks by non-homologous end-joining, and by the discovery of a unique mouse strain with a hypomorphic Lig4(Y288C) mutation. The Lig4(Y288C) mouse, identified by means of a mutagenesis screening programme, is a mouse model for human LIG4 syndrome, showing immunodeficiency and growth retardation. Diminished DNA double-strand break repair in the Lig4(Y288C) strain causes a progressive loss of haematopoietic stem cells and bone marrow cellularity during ageing, and severely impairs stem cell function in tissue culture and transplantation. The sensitivity of haematopoietic stem cells to non-homologous end-joining deficiency is therefore a key determinant of their ability to maintain themselves against physiological stress over time and to withstand culture and transplantation.

Sustained blood cell production requires preservation of a quiescent, multipotential stem cell pool that intermittently gives rise to progenitors with robust proliferative potential. The ability of cells to shift from a highly constrained to a vigorously active proliferative state is critical for maintaining stem cells while providing the responsiveness necessary for host defense. The cyclin-dependent kinase inhibitor (CDKI), p21(cip1/waf1) (p21) dominates stem cell kinetics. Here we report that another CDKI, p27(kip1) (p27), does not affect stem cell number, cell cycling, or self-renewal, but markedly alters progenitor proliferation and pool size. Therefore, distinct CDKIs govern the highly divergent stem and progenitor cell populations. When competitively transplanted, p27-deficient stem cells generate progenitors that eventually dominate blood cell production. Modulating p27 expression in a small number of stem cells may translate into effects on the majority of mature cells, thereby providing a strategy for potentiating the impact of transduced cells in stem cell gene therapy.

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

The prospects for using autologous induced pluripotent stem cells (iPSCs) in cell replacement therapy have been tempered by evidence that undifferentiated, syngeneic mouse iPSCs are immunogenic upon transplantation. However, the immunogenicity of more therapeutically relevant differentiated cells remains unexplored. Here, we differentiated mouse iPSCs into embryoid bodies (EBs) or representative cell types spanning the three embryonic germ layers and assessed their immunogenicity in vitro and after their transplantation into syngeneic recipients. We found no evidence of increased T cell proliferation in vitro, rejection of syngeneic iPSC-derived EBs/tissue-specific cells (TSCs) after transplantation, or an antigen-specific secondary immune response. Thus, differentiated cells derived from syngeneic iPSCs do not appear to be rejected after transplantation. We also found little evidence of an immune response to undifferentiated, syngeneic iPSCs. Our data support the idea that differentiated cells generated from autologous iPSCs could be applied for cell replacement therapy without eliciting immune rejection.

Evidence suggests the transcription factor GATA-2 is a critical regulator of murine hematopoietic stem cells. Here, we explore the relation between GATA-2 and cell proliferation and show that inducing GATA-2 increases quiescence (

SummaryThe regulatory pathways necessary for the maintenance of adult hematopoietic stem cells (HSCs) remain poorly defined. By using loss-of-function approaches, we report a selective and cell-autonomous requirement for the p300/CBP-binding transcriptional coactivator Cited2 in adult HSC maintenance. Conditional deletion of Cited2 in the adult mouse results in loss of HSCs causing multilineage bone marrow failure and increased lethality. In contrast, conditional ablation of Cited2 after lineage specification in lymphoid and myeloid lineages has no impact on the maintenance of these lineages. Additional deletion of Ink4a/Arf (encoding p16Ink4a and p19Arf) or Trp53 (encoding p53, a downstream target of p19Arf) in a Cited2-deficient background restores HSC functionality and rescues mice from bone marrow failure. Furthermore, we show that the critical role of Cited2 in primitive hematopoietic cells is conserved in humans. Taken together, our studies provide genetic evidence that Cited2 selectively maintains adult HSC functions, at least in part, via Ink4a/Arf and Trp53.

The zinc finger transcription factor GATA-2 has been implicated in the regulation of hematopoietic stem cells. Herein, we explored the role of GATA-2 as a candidate regulator of the hematopoietic progenitor cell compartment. We showed that bone marrow from GATA-2 heterozygote (GATA-2 ؉/؊ ) mice displayed attenuated granulocyte-macrophage progenitor function in colony-forming cell (CFC) and serial replating CFC assays. This defect was mapped to the Lin ؊ CD117 ؉ Sca-1 ؊ CD34 ؉ CD16/32 high granulocyte-macrophage progenitor (GMP) compartment of GATA-2 ؉/؊ marrow, which was reduced in size and functionally impaired in CFC assays and competitive transplantation. Similar functional impairments were obtained using a RNA interference approach to stably knockdown GATA-2 in wild-type GMP. Although apoptosis and cell-cycle distribution remained unperturbed in GATA-2 ؉/؊ GMP, quiescent cells from GATA-2 ؉/؊ GMP exhibited altered functionality. Gene expression analysis showed attenuated expression of HES-1 mRNA in GATA-2-deficient GMP. Binding of GATA-2 to the HES-1 locus was detected in the myeloid progenitor cell line 32Dcl3, and enforced expression of HES-1 expression in GATA-2 ؉/؊ GMP rectified the functional defect, suggesting that GATA-2 regulates myeloid progenitor function through HES-1. These data collectively point to GATA-2 as a novel, pivotal determinant of GMP cell fate. (Blood. 2008;112:4862-4873)