Population heterogeneity among cells of the megakaryocyte lineage

Platelet populations and priming in hematological diseases

In healthy subjects and patients with hematological diseases, platelet populations can be distinguished with different response spectra in hemostatic and vascular processes. These populations partly overlap, and are less distinct than those of leukocytes. The platelet heterogeneity is linked to structural properties, and is enforced by inequalities in the environment. Contributing factors are variability between megakaryocytes, platelet ageing, and positive or negative priming of platelets during their time in circulation. Within a hemostatic plug or thrombus, platelet heterogeneity is enhanced by unequal exposure to agonists, with populations of contracted platelets in the thrombus core, discoid platelets at the thrombus surface, patches of ballooned and procoagulant platelets forming thrombin, and coated platelets binding fibrin. Several pathophysiological hematological conditions can positively or negatively prime the responsiveness of platelet populations. As a consequence, in vivo and in vitro markers of platelet activation can differ in thrombotic and hematological disorders.

Distinct functional effects for dynamin 3 during megakaryocytopoiesis

Dynamin 3 (DNM3) is a member of a family of motor proteins that participate in a number of membrane rearrangements such as cytokinesis, budding of transport vesicles, phagocytosis, and cell motility. Recently, DNM3 was implicated as having a role in megakaryocyte (MK) development. To further investigate the functional role of DNM3 during megakaryocytopoiesis, we introduced sequence-specific short hairpin RNAs (shRNAs) into developing MKs. The results showed that knockdown of DNM3 inhibited a stage of MK development that involved progenitor amplification. This was evident by significant decreases in the number of colony forming unit-megakaryocytes, the total number of nucleated cells, and the number of CD41(+) and CD61(+) MKs produced in culture. Using a styrl membrane dye to quantify the demarcation membrane system (DMS) of terminally differentiated MKs, we found that DNM3 co-localized with the DMS and that DNM3 lentiviral shRNAs precluded the formation of the DMS. Knockdown of dynamin 3 in murine MKs also caused a decrease in the number of morphologically large MKs and the overall size of large MKs was decreased relative to controls. MK protein lysates were used in overlay blots to show that both DNM3 and actin bind to nonmuscle myosin IIA (MYH9). Consistent with these observations, immunofluorescence studies of MKs and proplatelet processes showed co-localization of DNM3 with MYH9. Overall, these studies demonstrate that DNM3 not only participates in MK progenitor amplification, but is also involved in cytoplasmic enlargement and the formation of the DMS.

Comparison of the biological activities of anagrelide and its major metabolites in haematopoietic cell cultures

The platelet-lowering drug anagrelide inhibits bone marrow megakaryocytopoiesis by an unknown mechanism. Recently, it was found that anagrelide is bio-transformed in humans into two major metabolites (6,7-dichloro-3-hydroxy-1,5 dihydro-imidazo[2,1-b]quinazolin-2-one (BCH24426) and 2-amino-5,6-dichloro-3,4,-dihydroquinazoline (RL603). Whether these metabolites have biological activities that may underlie the mode of action of the parent drug is presently unclear. To clarify this question here we have compared the activities of anagrelide, BCH24426 and RL603 on the growth and differentiation of CD34(+) haematopoietic progenitor cells in liquid culture and on the migration of differentiated megakaryocytes. Incubation with either anagrelide, BCH24426 or RL603 did not affect the early expansion of CD34(+) cells stimulated by thrombopoietin. In contrast, both anagrelide and BCH24426 potently inhibited the development of megakaryocytes (IC(50) +/- s.e.m. = 26 +/- 4 and 44 +/- 6 nM, respectively), whereas RL603 showed no significant effect. Anagrelide and BCH24426 did not affect erythroid or myelomonocytic differentiation stimulated by erythropoietin or granulocyte-macrophage colony-stimulating factor, demonstrating the selectivity of these compounds against the megakaryocytic lineage. Neither anagrelide nor its metabolites showed a significant effect on the migratory response of megakaryocytes towards stromal cell-derived factor-1alpha. Although BCH24426 was shown to be considerably more potent than anagrelide as an inhibitor of phosphodiesterase type III (PDEIII) (IC(50) = 0.9 vs 36 nM) this activity did not correlate with the potency of inhibition of megakaryocyte development. Furthermore, other PDEIII inhibitors of widely differing potency were shown to have negligible effects on megakaryocytopoiesis. Taken together our results demonstrate that anagrelide and BCH24426 target a cellular event involved specifically in the megakaryocyte differentiation programme, which is independent of PDEIII inhibition.

Cytokines, endothelium, and adhesive molecules in pathologic thrombopoiesis

Clonal thrombocytosis (CT) associated with myeloproliferative disorders (MPD) is believed to be secondary to autonomous unregulated platelet production. Secondary or reactive thrombocytosis (RT) can be observed in a number of clinical circumstances and may be related to persistent production of some thrombopoietic factors acting on megakaryocytes (MK). The goal of this study is to assess the serum concentrations of these cytokines in control subjects and patients with MPD associated with thrombocythemia, RT, and autoimmune thrombocytopenic purpura (ATP). Eleven patients with MPD, five with chronic myeloid leukemia (CML), three with polycythemia vera (PCV), two with essential thrombocythemia (ET), one with myelofibrosis, 15 with RT, eight with ATP, and 12 healthy volunteers were enrolled in the study. Serum interleukin (IL)-1beta, IL-6, tumor necrosis factor-alpha (TNF), fibronectin, intracellular adhesion molecule-1 (ICAM-1), and thrombomodulin (TM) were measured in these groups. Interleukin- 1beta, IL-6, and TNF levels were high in patients with RT and ATP, suggesting that these cytokines act on early uncommitted progenitors, promoting commitment along the MK lineage and leading to thrombocytosis or compensation for thrombocytopenia. TM was significantly increased in patients with MPD compared to all other groups, probably indicating the presence of subclinical endothelial damage. Fibronectin levels were high in MPD and RT patients. This finding can be secondary to high platelet turnover in these patients. We found that ICAM-1 levels were high in patients with clonal thrombocytosis. ICAM-1 can be one of the factors initiating the events ultimately leading to clonal thrombocytosis. Thrombocythemia associated with MPD is an autonomous phenomenon not regulated by cytokines.

Megakaryocytopoiesis in vitro: from the stem cells' perspective

Megakaryocytopoiesis is a complex network regulated by different megakaryocyte (MK)-stimulating factors (i.e., thrombopoietin [TPO], stem cell factor [SCF], interleukin 3 [IL-3], IL-6, IL-11 and GM-CSF). Although all of these factors can affect human and murine megakaryocytopoiesis at different levels of MK development, the effect on very primitive hematopoietic stem cells (HSC) is not well understood. We have further characterized the in vitro biological activity of recombinant murine TPO, SCF and IL-3 on the maturation and proliferation of MK progenitors from different murine primitive hematopoietic cells in a fibrin clot system under serum-free conditions. Neither TPO nor SCF alone induced MK colony formation (CFU-MK) from Lin- Sca+ cells. However, isolated large and mature MKs were observed in the presence of TPO. In contrast, IL-3 exerted a potent effect on CFU-MK formation from Lin- Sca+ cells. On this population of HSC, a significant increase of large MK colonies with mature MK were obtained under those conditions in which TPO was combined with IL-3 or SCF plus IL-3. Similar results were obtained with murine bone marrow cells enriched by primitive progenitors from day 3 post-5-fluorouracil treated mice (5-FUBMC). In contrast, TPO-sensitive precursors were detected in fetal liver cells (FLC). These cells differentiate and proliferate to MK progenitors in the presence of TPO. A significant increase in the number of CFU-MK was induced when TPO was combined with either IL-3 or SCF. On these populations of primitive hematopoietic progenitors, IL-3 induced both the proliferation and differentiation of MK progenitors. Because erythropoietin and TPO share similarities between their molecules and their receptors, we studied whether these growth factors may modulate megakaryocytopoiesis from FLC. Flow cytometry analysis of FLC expressing erythroid markers demonstrated that these cells expressed c-Mpl receptor. In our in vitro studies, although EPO by itself did not induce MK colonies from FLC, it enhanced the proliferative activity of TPO. High ploidy and proplatelet-shedding MK were observed in Lin- Sca+ cells, 5-FUBMC and FLC stimulated with TPO alone or in combination with other MK-stimulating factors. Based on these observations, we propose that TPO, IL-3 and SCF constitute early MK-acting factors with differential proliferative and differentiative activities on murine stem cells. TPO by itself does not appear to be involved in the proliferation of MK progenitors from bone marrow HSC. TPO appears to induce in these cells the commitment toward MK differentiation. However, this growth factor may enhance the proliferative activity of IL-3. IL-3 is an early MK-stimulating factor able to induce in vitro the proliferation and differentiation of MK progenitors from HSC.

Megakaryocytic differentiation is accompanied by a reduction in cell migratory potential

Megakaryocytes (MKs) have been found in the peripheral circulation, suggesting that they can migrate out of the bone marrow. In order to evaluate if megakaryocytic differentiation confers a migratory phenotype, we investigated this property in the haematopoietic cell lines MO7e and UT-7/mpl and in CD34+ progenitor cells before and after induction of differentiation by thrombopoietin (TPO). Migration was studied using a bicompartmental culture system in the presence or absence of a bone marrow endothelial cell monolayer. Preincubation with TPO led to a significant reduction in stromal cell-derived factor-1 (SDF-1)-induced migration of MO7e cells (0.7% +/- 0.08% for TPO-treated vs. 2.6% +/- 0.3% for controls P < 0.05). A similar decreased migratory response was seen with UT-7/mpl cells (7.4% +/- 0.4% for TPO-treated vs. 11.1% +/- 0.01% for controls, P<0.05), although these cells did not migrate in response to SDF-1. CD34+ cells partially differentiated with TPO showed decreased migration following further TPO-induced maturation (13.9% +/- 1.8% for TPO-treated vs. 24.1% +/- 1.8% for untreated, P < 0.05). This reduction was more pronounced in the large MK (> or = 4n) fraction. These results demonstrate that megakaryocytic differentiation is accompanied by a partial suppression of the haematopoietic cell migratory phenotype.

Investigation of megakaryopoiesis in myelosuppressed bone marrow using immunogold-silver staining (IGSS)

To determine the frequencies and differential counts of megakaryocytes after cytoreductive treatment in nucleated low-density (1.060 g/ml) bone marrow cells (BMNC) of dogs an immunogold-silver staining (IGSS) technique with the lineage specific monoclonal antibody 2F9 was established. This antibody recognizes the glycoprotein IIb/IIIa complex expressed on the surface of canine megakaryocytes and platelets. The IGSS technique enables not only the detection of megakaryocytes occurring at a low frequency (0.1-0.2%), but also the discrimination between the different maturation stages of megakaryocytes due to cell size, nuclear morphology and cytoplasmic staining. By the use of this technique, small lymphoid megakaryocytic cells were identified. Comparable numbers of megakaryocyte colony-forming cells in 2F9-depleted and nondepleted BMNC suspensions (25.7 +/- 5.0 vs. 25.3 +/- 5.1 Meg-CFC/10(5) BMNC) indicate that these small 2F9 positive cells are nonclonogenic precursors of megakaryoblasts. To prove the applicability of IGSS, serial examinations of bone marrow samples from dogs treated with recombinant human interleukin-6 (IL-6) after exposure to 2.4 Gy total body irradiation (TBI) were performed. The results of the microscopic evaluation indicate that, in the recovery phase after TBI, IL-6 induced an earlier and stronger increase in megakaryocyte frequency in comparison to the control. Interestingly, all maturation stages of the megakaryocytic lineage took part in this IL-6 induced improvement of megakaryocyte recovery.

The regulation of megakaryocytopoiesis

The process of megakaryocytopoiesis begins with the commitment of a pluripotent hematopoietic stem cell to a differentiation pathway that culminates in the release of mature platelets into the circulation. A variety of megakaryocyte precursor cells have been identified after stem cell commitment has occurred and these may be recognized by their morphologic or immunophenotypic characteristics. Megakaryocytopoiesis is regulated by a number of cytokines with either stimulatory or inhibitory effects and by a variety of cell-cell interactions. Some factors potentiating platelet development promote the proliferation of megakaryocyte progenitor cells, while others result in their maturation. Thrombopoietin, a cytokine with specific megakaryocyte maturational activity recently has been identified as the c-Mpl ligand, and it will be evaluated as a therapeutic agent in the setting of thrombocytopenia due to impaired megakaryocytopoiesis.

Regulation of megakaryocytopoiesis

Megakaryocytopoiesis is the cellular developmental process prior to the release of platelets into the circulation. Regulation of megakaryocytopoiesis is a complex phenomenon that begins with commitment of hematopoietic stem cells to the replication and maturation of progenitor cells through endomitosis and megakaryocyte differentiation [1-4]. Platelet production is determined by the number and size of megakaryocytes in the marrow and may be regulated at two levels: at early stages of cell proliferation resulting in increased megakaryocyte numbers, and at later stages by endoreplication which increases DNA content and the size of megakaryocytes [5]. The mature megakaryocyte is a large polyploid cell with a highly defined invaginated membrane (demarcation membrane) and contains the membrane molecules necessary for platelet function [6-9]. Platelet shedding appears to occur by fragmentation of the cytoplasm of the megakaryocyte. Platelet release is thought to occur via transendothelial processes projecting into the vascular compartment [10, 11], although several studies indicate that megakaryocytes lodged in the lungs are capable of platelet formation [12-17]. The factors stimulating megakaryocytopoiesis in the lung have not been well characterized. In the past, the study of megakaryocyte development in vivo and in vitro was hampered by the rarity of megakaryocytes in the bone marrow, the poorly defined cell populations, and inadequate assays. These prior studies of megakaryocyte development have been discussed in the recent past by R. Hoffman [1], N. Williams [3], and M. W. Long [2]. An attempt will be made in this review to highlight and synthesize various new concepts of regulation of megakaryocytopoiesis.