Kinetic analysis of megakaryocyte numbers and ploidy levels in developing colonies from mouse bone marrow cells

Checkpoint defects leading to premature mitosis also cause endoreplication of DNA in Aspergillus nidulans

The G2 DNA damage and slowing of S-phase checkpoints over mitosis function through tyrosine phosphorylation of NIMX(cdc2) in Aspergillus nidulans. We demonstrate that breaking these checkpoints leads to a defective premature mitosis followed by dramatic rereplication of genomic DNA. Two additional checkpoint functions, uvsB and uvsD, also cause the rereplication phenotype after their mutation allows premature mitosis in the presence of low concentrations of hydroxyurea. uvsB is shown to encode a rad3/ATR homologue, whereas uvsD displays homology to rad26, which has only previously been identified in Schizosaccharomyces pombe. uvsB(rad3) and uvsD(rad26) have G2 checkpoint functions over mitosis and another function essential for surviving DNA damage. The rereplication phenotype is accompanied by lack of NIME(cyclinB), but ectopic expression of active nondegradable NIME(cyclinB) does not arrest DNA rereplication. DNA rereplication can also be induced in cells that enter mitosis prematurely because of lack of tyrosine phosphorylation of NIMX(cdc2) and impaired anaphase-promoting complex function. The data demonstrate that lack of checkpoint control over mitosis can secondarily cause defects in the checkpoint system that prevents DNA rereplication in the absence of mitosis. This defines a new mechanism by which endoreplication of DNA can be triggered and maintained in eukaryotic cells.

Thrombopoietin-induced polyploidization of bone marrow megakaryocytes is due to a unique regulatory mechanism in late mitosis

Megakaryocytes undergo a unique differentiation program, becoming polyploid through repeated cycles of DNA synthesis without concomitant cell division. However, the mechanism underlying this polyploidization remains totally unknown. It has been postulated that polyploidization is due to a skipping of mitosis after each round of DNA replication. We carried out immunohistochemical studies on mouse bone marrow megakaryocytes during thrombopoietin- induced polyploidization and found that during this process megakaryocytes indeed enter mitosis and progress through normal prophase, prometaphase, metaphase, and up to anaphase A, but not to anaphase B, telophase, or cytokinesis. It was clearly observed that multiple spindle poles were formed as the polyploid megakaryocytes entered mitosis; the nuclear membrane broke down during prophase; the sister chromatids were aligned on a multifaced plate, and the centrosomes were symmetrically located on either side of each face of the plate at metaphase; and a set of sister chromatids moved into the multiple centrosomes during anaphase A. We further noted that the pair of spindle poles in anaphase were located in close proximity to each other, probably because of the lack of outward movement of spindle poles during anaphase B. Thus, the reassembling nuclear envelope may enclose all the sister chromatids in a single nucleus at anaphase and then skip telophase and cytokinesis. These observations clearly indicate that polyploidization of megakaryocytes is not simply due to a skipping of mitosis, and that the megakaryocytes must have a unique regulatory mechanism in anaphase, e.g., factors regulating anaphase such as microtubule motor proteins might be involved in this polyploidization process.

Novel alterations in CDK1/cyclin B1 kinase complex formation occur during the acquisition of a polyploid DNA content

The pathways that regulate the S-phase events associated with the control of DNA replication are poorly understood. The bone marrow megakaryocytes are unique in that they leave the diploid (2C) state to differentiate, synthesizing 4 to 64 times the normal DNA content within a single nucleus, a process known as endomitosis. Human erythroleukemia (HEL) cells model this process, becoming polyploid during phorbol diester-induced megakaryocyte differentiation. The mitotic arrest occurring in these polyploid cells involves novel alterations in the cdk1/cyclin B1 complex: a marked reduction in cdk1 protein levels, and an elevated and sustained expression of cyclin B1. Endomitotic cells thus lack cdk1/cyclin B1-associated H1-histone kinase activity. Constitutive over-expression of cdk1 in endomitotic cells failed to re-initiate normal mitotic events even though cdk1 was present in a 10-fold excess. This was due to an inability of cyclin-B1 to physically associate with cdk1. Nonetheless, endomitotic cyclin B1 possesses immunoprecipitable H1-histone kinase activity, and specifically translocates to the nucleus. We conclude that mitosis is abrogated during endomitosis due to the absence of cdk1 and the failure to form M-phase promoting factor, resulting in a disassociation of mitosis from the completion of S-phase. Further studies on cyclin and its interacting proteins should be informative in understanding endomitosis and cell cycle control.

Stimulation of thrombopoiesis in mice by human recombinant interleukin 6

To date, testing of various cytokines for the stimulation of blood cell production has not demonstrated a consistent effect on peripheral platelet levels. In this report, we provide evidence that human recombinant IL-6 increased platelet production in mice, as measured by both peripheral platelet levels and [75Se]selenomethionine (75SeM) incorporation into newly forming platelets. Peripheral white blood cell counts also were increased, but only to a modest extent, and hematocrit values were unchanged. A dose-response relationship between the amount of IL-6 administered and platelet count, 75SeM incorporation, and white blood cell count was demonstrated. Detectable megakaryocyte and granulocyte-macrophage colony-forming cells in mice that had received IL-6 also were increased in both bone marrow and spleen. These results demonstrate the ability of a purified, recombinant protein to stimulate platelet production in vivo.

Synergistic regulation of human megakaryocyte development

Little information exists concerning differing levels of regulation occurring during human megakaryocyte development. We hypothesize that megakaryocytic proliferation and maturation is controlled by two, synergistic regulatory factors. One, megakaryocyte colony-stimulating activity, is an obligate requirement for colony formation and drives the development of relatively immature cells. Megakaryocyte colony-stimulating activity is a functional component of the human recombinant proteins, interleukin 3 or GM-CSF. Human recombinant growth factors, interleukin 1, interleukin 6, or crythropoietin, do not effect megakaryocyte development either alone or in combination with interleukin 3. Full maturation requires a second synergistic activity which increases megakaryocyte number, size, and cytoplasmic and antigenic content. In culture, this synergistic regulator augments maturation by increasing the number of colonies, colony cellularity, and size. In suspension cultures, this cofactor increases megakaryocyte cytoplasmic and antigenic content, and shifts the morphological distribution from immature to mature megakaryocytes. Finally, this activity also increases the number of antigen positive megakaryocytes, either by stimulating proliferation or conversion of antigen-negative to antigen-positive cells. Comparative studies of megakaryocytic regulation suggests that this in vitro regulator mimicks some of the known effects of thrombopoietin in vivo.

Immune regulation of in vitro murine megakaryocyte development. Role of T lymphocytes and Ia antigen expression

Mitogen-activated murine T lymphocytes or T cell hybridomas produce an activity (megakaryocyte [Mk] potentiator activity) that enhances the in vitro growth and development of Mk colonies. This activity was found in optimal concentrations (2.5%) in T cell hybridoma-conditioned medium, and was also produced by feeder layers of concanavalin A-activated T cells. A subpopulation of murine Mk progenitor cells (colony-forming units; CFU-Mk) bears the Ia antigen. Separate experiments indicated that T cell products stimulate CFU-Mk by increasing their basal levels of Ia expression as well as the frequency of cells actively synthesizing DNA. The hypothesis that the expression of this antigen was related to the cell cycle status of these progenitor cells was confirmed in studies that indicated that ablation of actively cycling cells in vivo abrogated the cytotoxic effects of anti-Ia monoclonal antibodies. The interdependence of T cell lymphokine regulation of both Ia expression and cell cycle status was also seen in in vitro experiments in which Ia+ progenitor cells were eliminated by complement-dependent cytotoxicity. The removal of Ia+ cells prevented 5-hydroxyurea-mediated inhibition of cells in S phase. We hypothesize that immune modulation of megakaryocytopoiesis occurs via soluble T cell products that augment Mk differentiation. Further, the mechanism of immune recognition/modulation may occur via Ia antigens present on the surface of these progenitor cells.

Phorbol diesters stimulate the development of an early murine progenitor cell. The burst-forming unit-megakaryocyte

When murine (C57BL/6) bone marrow cells are cultivated with WEHI-3 conditioned media, a source of megakaryocyte-colony-stimulating activity (Mk-CSA), and phorbol myristate acetate (PMA), a previously undetected population of megakaryocyte (Mk) progenitor cells is observed. These new Mk colonies are reminiscent of erythroid bursts, in that they contain large numbers (40-500) of Mk and multiple foci (2-7) of development. These burst-forming units, Mk (BFU-Mk), are defined as having greater than or equal to 42 cells/colony and, at least, three foci of Mk development (colonies grown in soft agar cultures, all studies done at limiting dilutions; colonies detected by acetylcholinesterase [ACh-E] staining). CFU-Mk and BFU-Mk require two activities for optimal growth: Mk-CSA and PMA. However, the BFU-Mk require a tenfold greater concentration of PMA for optimal development (10(-6) vs. 10(-7) M). BFU-Mk detection is linear (over a range of 25-100 X 10(3) cells/ml), with the regression line passing through the origin. Bone marrow frequencies of these two progenitor cells are CFU-Mk, 36.7 +/- 2.5, and BFU-Mk, 7.3 +/- 0.7 per 10(5) total nucleated cells (mean +/- SEM; n = 28). The BFU-Mk have a restricted velocity sedimentation range (3.3-4.5 mmh-1 vs. 3.3-6.8 mmh-1 for CFU-Mk). Modal buoyant densities are 1.068 +/- 0.0002 and 1.070 +/- 0.002 for BFU-Mk and CFU-Mk, respectively. Thus, these cells are found among the smallest and less dense of the Mk progenitors, and are not clumps or clusters of CFU-Mk. Kinetic analysis indicates that CFU-Mk require 5-7 d for optimal growth, whereas BFU-Mk require 10-12 d. Examination of the proliferative potential (cells per colony) shows 19.3 +/- 1.5 cells per colony (n = 246 colonies) for day 10 CFU-Mk, vs. 118 +/- 6.0 for day 10 BFU-Mk (n = 163). Analysis of the cellularity/subcolony within each burst indicates 37.0 +/- 2.1 (n = 146) Mk/colony and 3.9 +/- 0.1 subcolonies/burst (n = 100). Finally, greater than 90% of the BFU-Mk contain only ACh-E positive cells, indicating that these are not mixed colonies. These results indicate that the BFU-Mk, compared with the CFU-Mk, require an increased amount of stimulation in order to differentiate, show delayed in vitro development, and have a higher proliferative potential. These data are consistent with the hypothesis that these cells are early progenitor cells in the Mk lineage that antedate the CFU-Mk.

Role of phorbol diesters in in vitro murine megakaryocyte colony formation

In vitro megakaryocyte differentiation is regulated by two activities: a megakaryocyte colony-stimulating activity (Mk-CSA), which is required for proliferation, and an auxiliary factor, megakaryocyte potentiating activity, which plays a role in later differentiation events. Tumor-promoting phorbol esters alter many cellular differentiation-related events. Thus, it was hypothesized that phorbol esters may bring about megakaryocyte differentiation in vitro. 4 beta-Phorbol 12-myristate 13-acetate (PMA), when co-cultured with a source of Mk-CSA, stimulated a threefold increase in colony numbers. Co-culture of PMA and megakaryocyte potentiator activity did not stimulate colony formation, thus eliminating any action of PMA as an Mk-CSA. The direct effect of PMA on the formation of megakaryocyte colonies was established by (a) the function of PMA as a megakaryocyte potentiator in serum-free experiments, (b) the ability of PMA to stimulate megakaryocyte colony formation using bone marrow cells depleted of populations known to produce potentiating activity, (c) the inability of bone marrow adherent cells previously treated with phorbol, 12,13-dibutyrate (PDBu) to augment megakaryocyte colony formation, and (d) the ability of PMA to induce the growth of immature megakaryocytes into large single megakaryocytes. Structure:activity experiments resulted in equivalent activities for PMA and PDBu, whereas the nontumor promoter phorbol 12,13-diacetate and phorbol itself lacked activity. The observations in this study indicate that phorbol esters can bring about megakaryocyte differentiation, and during colony formation, can induce effects identical to those brought about by biological sources of megakaryocyte potentiator activity.