HSCs maintain the circulating blood cell population. Defects in the orderly pattern of hematopoietic cell division and differentiation can lead to leukemia, myeloproliferative disorders, or marrow failure; however, the factors that control this pattern are incompletely understood. Geminin is an unstable regulatory protein that regulates the extent of DNA replication and is thought to coordinate cell division with cell differentiation. Here, we set out to determine the function of Geminin in hematopoiesis by deleting the Geminin gene (Gmnn) from mouse bone marrow cells. This severely perturbed the pattern of blood cell production in all 3 hematopoietic lineages (erythrocyte, megakaryocyte, and leukocyte). Red cell production was virtually abolished, while megakaryocyte production was greatly enhanced. Leukocyte production transiently decreased and then recovered. Stem and progenitor cell numbers were preserved, and Gmnn–/– HSCs successfully reconstituted hematopoiesis in irradiated mice. CD34+Gmnn–/– leukocyte precursors displayed DNA overreplication and formed extremely small granulocyte and monocyte colonies in methylcellulose. While cultured Gmnn–/– megakaryocyte-erythrocyte precursors did not form erythroid colonies, they did form greater than normal numbers of megakaryocyte colonies. Gmnn–/– megakaryocytes and erythroblasts had normal DNA content. These data led us to postulate that Geminin regulates the relative production of erythrocytes and megakaryocytes from megakaryocyte-erythrocyte precursors by a replication-independent mechanism.
Kathryn M. Shinnick, Elizabeth A. Eklund, Thomas J. McGarry
Submitter: Thomas J. McGarry | t-mcgarry@northwestern.edu
Authors: Kathryn Shinnick, Elizabeth Eklund
Northwestern University, Feinberg Cardiovascular Research Institute
Published March 31, 2011
Dr. Taraviras’s group has elegantly and convincingly demonstrated that Geminin-/- T lymphocytes show a reduced capacity to proliferate after stimulation (1). The cells enter S phase normally but linger in S/G2/M phase and exit the cell cycle without over-replicating their DNA. Despite these defects, mice with Geminin-/- lymphocytes show only moderately reduced numbers of T cells and their differentiation pattern is roughly normal. Our findings with Geminin-/- white blood cells are very similar (2). Geminin-/- granulocyte and monocyte precursors form smaller colonies than wild-type cells when cultured in vitro. A significant fraction of these colony cells (15-25%) have DNA content >4n, strongly suggesting that their poor growth is due to over-replication. Nevertheless, mice transplanted with Geminin-/- hematopoietic cells have normal white counts and the pattern of white cell differentiation is largely normal. We hypothesize that Geminin is dispensable for cell proliferation in both lymphocytes and leukocytes under baseline conditions. We suggest that when the cells are stimulated to divide rapidly, either by antibodies that engage surface receptors or by exposure to growth factors, Geminin-independent mechanisms become inadequate to control replication and cell proliferation is compromised. In support of this hypothesis, we have also observed that Geminin-/- hematopoietic stem cells (HSCs) engraft poorly but are able to survive indefinitely when generated in situ (2). We hypothesize that after transplantation the HSCs are stressed to divide more rapidly than usual and become dependent on Geminin. We entirely agree that different types of cells will have different requirements for Geminin. The megakaryocyte/erythrocyte lineage is clearly more dependent on Geminin for normal proliferation and differentiation (2). Moreover, embryonic stem (ES) cells are not produced at all in Geminin-/- early embryos (3, 4). In contrast, our group has recently found that Geminin-/- neural stem cells show no defects in proliferation or differentiation (5). Further studies are clearly needed to elucidate how Geminin regulates differentiation and self-renewal in different cell types.
Submitter: Stavros Taraviras | taraviras@med.upatras.gr
Department of Physiology, Medical School, University of Patras, 26504 Rio, Patras, Greece.
Published December 13, 2010
In their recent publication, Schinnick et al., 2010 studied mice in which Geminin has been inactivated in adult haematopoietic stem cells (HSCs) (1). They have used the interferon inducible Mx1 promoter to drive Cre recombinase expression that permits deletion of a gene in several cell types including the HSCs (2). In the absence of Geminin, red blood cells show a dramatic reduction while leukocytes are only transiently reduced. On the contrary, megakaryocytes show increased numbers in mutant mice, while the effects in the lymphoid lineage have not been examined. Furthermore, they suggest that, in the absence of Geminin adult HSCs and progenitor cells numbers are not significantly altered (1).
These findings are reinforcing previous data from our team and others indicating that Geminin is not necessary for every cell division (3). We have shown that conditional inactivation of Geminin in the lymphoid system appears not to be essential for the maintenance and differentiation processes of the T cell lineage in the thymus, since all progenitor populations are present. A small reduction in cell number of progenitor T cell is observed, suggesting that Geminin does not have a major role in cellular divisions of these cells in the thymus. However, T cells lacking Geminin expression show a defect in proliferation both in vitro, following T cell receptor activation, and in vivo, following adoptive transfer into lymphopenic recipient mice. Furthermore, we show that activated T cells lacking Geminin are able to enter S phase, but show reduced ability to perform multiple rounds of division, possibly due to an arrest in G2 phase (3). Similar data have been suggested from work in different tumorigenic cell lines (4-6).
Conditional inactivation of Geminin in the lymphoid (3) and bone marrow cells (1) give alternative views on the role of Geminin in maintaining genomic stability and controlling progenitor cell proliferation and cell fate acquisition. Further studies are required in order to understand the role of Geminin and why is required for self-renewal and differentiation in certain cell types and not in others.