A Critical Role for PU.1 in Homing and Long-Term Engraftment by Hematopoietic Stem Cells in the Bone Marrow

Genetic programs regulating HSC specification, maintenance and expansion

All mature blood cells originate from a small population of self-renewing pluripotent hematopoietic stem cells (HSCs). The capacity to self-renew characterizes all stem cells, whether normal or neoplastic. Interestingly, recent studies suggest that self-renewal is essential for tumor cell maintenance, implicating that this process has therapeutic relevance. Unfortunately, the molecular bases for self-renewal of vertebrate cells remain poorly defined. This article will focus on the developmental mechanisms underlying fetal and adult HSC homeostasis. Specifically, distinctions between genetic programs regulating HSC specification (identity), self-renewal (in both fetal and adult) and differentiation/commitment will be discussed with a special emphasis on transcriptional and chromatin regulators.

Platelet-derived microparticles bind to hematopoietic stem/progenitor cells and enhance their engraftment

Because human CD34+ and murine Sca-1+hematopoietic stem–progenitor cells (HSPCs) express platelet-binding sialomucin P-selectin (CD162) and integrin Mac-1 (CD11b–CD18) antigen, it was inferred that these cells might interact with platelets. As a result of this interaction, microparticles derived from platelets (PMPs) may transfer many platelet antigens (CD41, CD61, CD62, CXCR4, PAR-1) to the surfaces of HSPCs. To determine the biologic significance of the presence of PMPs on human CD34+ and murine Sca-1+ cells, their expressions on mobilized peripheral blood (mPB) and on nonmobilized PB- and bone marrow (BM)–derived CD34+ cells were compared. In addition, the effects of PMPs on the proliferation of CD34+ and Sca-1+ cells and on adhesion of HSPCs to endothelium and immobilized SDF-1 were studied. Finally, the hematopoietic reconstitution of lethally irradiated mice receiving transplanted BM mononuclear cells covered or not covered with PMPs was examined. It was found that PMPs are more numerous on mPB than on BM CD34+cells, do not affect the clonogenicity of human and murine HSPCs, and increase adhesion of these cells to endothelium and immobilized SDF-1. Moreover, murine BM cells covered with PMPs engrafted lethally irradiated mice significantly faster than those not covered, indicating that PMPs play an important role in the homing of HSPCs. This could explain why in a clinical setting human mPB HSPCs (densely covered with PMPs) engraft more rapidly than BM HSPCs (covered with fewer PMPs). These findings indicate a new role for PMPs in stem cell transplantation and may have clinical implications for the optimization of transplantations.

Mouse models in the study of the Ets family of transcription factors

The Ets family of transcription factors is one of a growing number of master regulators of development. This family was originally defined by the presence of a conserved DNA binding domain, the Ets domain. To date, nearly 30 members of this family have been identified and implicated in a wide range of physiological and pathological processes. Despite the likely importance of Ets-family members, each of their precise roles has not been delineated. Herein, we describe the elucidation of essential functions of a few of these family members in vivo using knockout mouse models. Of the knockouts generated to date, the majority shows important functions in hematopoiesis, ranging from PU.1, a principle regulator of myelo-lymphopoiesis, to Spi-B which regulates the proper function of terminally differentiated cells. Ets1 was shown to be of intermediate importance as a regulator of pan-lymphoid development. Other Ets family members such as Fli1 and TEL1 display distinct and/or overlapping functions in vasculo/angiogenesis, hemostasis and hematopoiesis. The remaining knockouts generated, Ets2 and Er81, show non-hematopoietic defects related to extraembryonic development and neurogenesis, respectively. The pioneering group of knockout models described reveals only the most distinct functions of each of these Ets family members. A better understanding of the roles and hierarchies of Ets family members in cellular differentiation will come with the generation of new null alleles in previously untargeted family members, more mutant alleles in members already disrupted, double knockouts, ES cell differentiation and chimera rescue experiments, and tissue-specific inducible knockouts.

T Cell Development in PU.1-Deficient Mice

These studies address the role of PU.1 in T cell development through the analysis of PU.1−/− mice. We show that the majority of PU.1−/− thymocytes are blocked in differentiation prior to T cell commitment, and contain a population of thymocyte progenitors with the cell surface phenotype of CD44+, HSAbright, c-kitint, Thy-1−, CD25−, Sca-1−, CD4−, and CD8−. These cells correspond in both number and cell surface phenotype with uncommitted thymocyte progenitors found in wild-type fetal thymus. RT-PCR analysis demonstrated that PU.1 is normally expressed in this early progenitor population, but is down-regulated during T cell commitment. Rare PU.1−/− thymi, however, contained small numbers of thymocytes expressing markers of T cell commitment. Furthermore, almost 40% of PU.1−/− thymi placed in fetal thymic organ culture are capable of T cell development. Mature PU.1−/− thymocytes generated during organ culture proliferated and produced IL-2 in response to stimulation through the TCR. These data demonstrate that PU.1 is not absolutely required for T cell development, but does play a role in efficient commitment and/or early differentiation of most T progenitors.