Human megakaryocyte biology and pathophysiology

Bone marrow pathology predicts mortality in chronic hemodialysis patients

Introduction. A bone marrow biopsy is a useful procedure for the diagnosis and staging of various hematologic and systemic diseases. The objective of this study was to investigate whether the findings of bone marrow studies can predict mortality in chronic hemodialysis patients. Methods. Seventy-eight end-stage renal disease patients on maintenance hemodialysis underwent bone marrow biopsies between 2000 and 2011, with the most common indication being unexplained anemia followed by unexplained leukocytosis and leukopenia. Results. The survivors had a higher incidence of abnormal megakaryocyte distribution (P = 0.001), band and segmented cells (P = 0.021), and lymphoid cells (P = 0.029) than the nonsurvivors. The overall mortality rate was 38.5% (30/78), and the most common cause of mortality was sepsis (83.3%) followed by respiratory failure (10%). In multivariate Cox regression analysis, both decreased (OR 3.714, 95% CI 1.671–8.253, P = 0.001) and absent (OR 9.751, 95% CI 2.030–45.115, P = 0.004) megakaryocyte distribution (normal megakaryocyte distribution as the reference group), as well as myeloid/erythroid ratio (OR 1.054, CI 1.012–1.098, P = 0.011), were predictive of mortality. Conclusion. The results of a bone marrow biopsy can be used to assess the pathology, and, in addition, myeloid/erythroid ratio and abnormal megakaryocyte distribution can predict mortality in chronic hemodialysis patients.

Analysis of citations to biomedical articles affected by scientific misconduct

We describe the ongoing citations to biomedical articles affected by scientific misconduct, and characterize the papers that cite these affected articles. The citations to 102 articles named in official findings of scientific misconduct during the period of 1993 and 2001 were identified through the Institute for Scientific Information Web of Science database. Using a stratified random sampling strategy, we performed a content analysis of 603 of the 5,393 citing papers to identify indications of awareness that the cited articles affected by scientific misconduct had validity issues, and to examine how the citing papers referred to the affected articles. Fewer than 5% of citing papers indicated any awareness that the cited article was retracted or named in a finding of misconduct. We also tested the hypothesis that affected articles would have fewer citations than a comparison sample; this was not supported. Most articles affected by misconduct were published in basic science journals, and we found little cause for concern that such articles may have affected clinical equipoise or clinical care.

Megakaryocyte development and platelet production

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, and the proliferation, maturation and terminal differentiation of the megakaryocytic progenitors. Circulating levels of thrombopoietin (TPO), the primary growth-factor for the megakaryocyte (MK) lineage, induce concentration-dependent proliferation and maturation of MK progenitors by binding to the c-Mpl receptor and signalling induction. Decreased platelet turnover rates results in increased concentration of free TPO, enabling the compensatory response of marrow MKs to increased platelet production. C-Mpl activity is orchestrated by a complex cascade of signalling molecules that induces the action of specific transcription factors to drive MK proliferation and maturation. Mature MKs form proplatelet projections that are fragmented into circulating particles. Newly developed thrombopoietic agents operating via c-Mpl receptor may prove useful in supporting platelet production in thrombocytopenic state. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease state, and the new approaches to thrombopoietic therapy.

Functional expression of TRAIL and TRAIL-R2 during human megakaryocytic development

The expression and function of surface TRAIL and TRAIL receptors were investigated in primary megakaryocytic cells, generated in serum-free liquid phase from peripheral human CD34(+) cells. The surface expression of both TRAIL and "death receptor" TRAIL-R2 became detectable starting from the early phase of megakaryocytic differentiation (day 6 of culture) and persisted at later (days10-14) culture times. On the other hand, "death receptor" TRAIL-R1, "decoy receptors" TRAIL-R3, and TRAIL-R4 were barely detectable or undetectable at any time point examined. Addition of recombinant TRAIL at day 6 of culture increased the rate of spontaneous apoptosis of CD34(+)/CD41(dim) megakaryoblasts and it significantly decreased the total output of mature megakaryocytic cells evaluated after additional 4-8 days of culture. Conversely, addition in culture of TRAIL-R2-Fc chimera, which blocked the interaction between endogenous TRAIL and TRAIL-R2 on the surface of cultured megakaryocytic cells, increased the total megakaryocytic cell count. In addition, recombinant TRAIL promoted a small but reproducible increase of maturation in the surviving megakaryocytic cell population, evaluated by both phenotypic analysis and morphology. A similar pro-maturation effect was observed when TRAIL was added to bone marrow-derived CD61(+) megakaryocytic cells. Thus, our data suggest a role of TRAIL as a regulator of megakaryocytopoiesis.

Gene expression profiling of normal and malignant CD34-derived megakaryocytic cells

Gene expression profiles of bone marrow (BM) CD34-derived megakaryocytic cells (MKs) were compared in patients with essential thrombocythemia (ET) and healthy subjects using oligonucleotide microarray analysis to identify differentially expressed genes and disease-specific transcripts. We found that proapoptotic genes such as BAX, BNIP3, and BNIP3L were down-regulated in ET MKs together with genes that are components of the mitochondrial permeability transition pore complex, a system with a pivotal role in apoptosis. Conversely, antiapoptotic genes such as IGF1-R and CFLAR were up-regulated in the malignant cells, as was the SDF1 gene, which favors cell survival. On the basis of the array results, we characterized apoptosis of normal and ET MKs by time-course evaluation of annexin-V and sub-G1 peak DNA stainings of immature and mature MKs after culture in serum-free medium with an optimal thrombopoietin concentration, and annexin-V-positive MKs only, with decreasing thrombopoietin concentrations. ET MKs were more resistant to apoptosis than their normal counterparts. We conclude that imbalance between proliferation and apoptosis seems to be an important step in malignant ET megakaryocytopoiesis.

Cyclooxygenase-2 expression is induced during human megakaryopoiesis and characterizes newly formed platelets

Cyclooxygenase (COX)-1 or -2 and prostaglandin (PG) synthases catalyze the formation of various PGs and thromboxane (TX) A(2). We have investigated the expression and activity of COX-1 and -2 during human megakaryocytopoiesis. We analyzed megakaryocytes from bone marrow biopsies and derived from thrombopoietin-treated CD34(+) hemopoietic progenitor cells in culture. Platelets were obtained from healthy donors and patients with high platelet regeneration because of immune thrombocytopenia or peripheral blood stem cell transplantation. By immunocytochemistry, COX-1 was observed in CD34(+) cells and in megakaryocytes at each stage of maturation, whereas COX-2 was induced after 6 days of culture, and remained detectable in mature megakaryocytes. CD34(+) cells synthesized more PGE(2) than TXB(2) (214 +/- 50 vs. 30 +/- 10 pg/10(6) cells), whereas the reverse was true in mature megakaryocytes (TXB(2) 8,440 +/- 2,500 vs. PGE(2) 906 +/- 161 pg/10(6) cells). By immunostaining, COX-2 was observed in <10% of circulating platelets from healthy controls, whereas up to 60% of COX-2-positive platelets were found in patients. A selective COX-2 inhibitor reduced platelet production of both PGE(2) and TXB(2) to a significantly greater extent in patients than in healthy subjects. Finally, we found that COX-2 and the inducible PGE-synthase were coexpressed in mature megakaryocytes and in platelets. We conclude that both COX-isoforms contribute to prostanoid formation during human megakaryocytopoiesis and that COX-2-derived PGE(2) and TXA(2) may play an unrecognized role in inflammatory and hemostatic responses in clinical syndromes associated with high platelet turnover.

Interleukin-4 downregulates nuclear factor-erythroid 2 (NF-E2) expression in primary megakaryocytes and in megakaryoblastic cell lines

The transcriptional factor nuclear factor-erythroid 2 (NF-E2) is one of the few transcription factors known to be functionally linked to the megakaryocytic lineage, where it regulates terminal megakaryocyte maturation and platelet formation. However, the regulation of NF-E2 expression in megakaryocytic cells has not been extensively evaluated. In particular, no data have been reported on the effect of negative regulators of megakaryocytopoiesis on NF-E2 expression. This study investigated the in vitro effects of two negative regulators of megakaryocytopoiesis, such as interleukin-4 (IL-4) and transforming growth factor-beta1 (TGF-beta1) on the expression of NF-E2 transcription factor in megakaryoblastic cell lines (Hel and MK1) and in normal CD34-derived megakaryocytic cells. For this purpose, we used quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) to detect mRNA NF-E2 isoforms (a and f) and flow-cytometry analysis to evaluate NF-E2 protein expression. Our results demonstrated that TGF-beta1 did not inhibit NF-E2 mRNA and protein expression of either maturating or fully mature normal megakaryocytic cells as well as that of the two cell lines. By contrast, IL-4 downmodulates the expression of NF-E2 transcription factor at both mRNA and protein levels in normal maturating megakaryocytic cells and in the megakaryoblastic cell lines. NF-E2 expression of normal mature megakaryocytes was not affected by IL-4. Thus, the results of the present investigation demonstrate that NF-E2 transcription factor is involved not only in terminal megakaryocyte maturation but also in the negative regulation of the early phase of megakaryocyte development.

Ultrastructural characterization of maturation, platelet release, and senescence of human cultured megakaryocytes

The aim of this study was to evaluate the ultrastructural features of human megakaryocytes cultured in vitro. For this purpose, pluripotent CD34(+) (cluster of differentiation 34) hematopoietic progenitor cells, obtained from the peripheral blood of healthy adult donors, were differentiated along the megakaryocytic lineage in liquid cultures by the addition of the megakaryocyte-specific growth factor thrombopoietin (TPO, 100 ng/ml). After only 6-8 days, virtually all of the CD34-derived cells expressed the early megakaryocytic CD61 antigen, while, after 15-16 days, most cells also expressed the late megakaryocytic CD42a antigen. Ultrastructural analysis of cells obtained after 7 days of culture showed aspects typical of developing megakaryocytes (MK), such as formation of platelet territories and cytoplasmic fragmentation. At later (15-16 day) culture times, two distinct cell populations were observed: fully developed megakaryocytes releasing platelets into the culture medium and senescent megakaryocytes, characterized by morphological features of apoptosis. Analysis of DNA fragmentation in these cells revealed that apoptosis in megakaryocytes occurred in the absence of the internucleosomic cleavage, which is characteristic of most, but not all, types of apoptosis in cells of hematopoietic origin. On the other hand, flow cytometry of the DNA content of senescent megakaryocytes showed a subdiploid peak that was likely due to a loss of micronuclei during processing.

All-trans retinoic acid at low concentration directly stimulates normal adult megakaryocytopoiesis in the presence of thrombopoietin or combined cytokines

In order to investigate the direct effects of retinoids on normal adult hematopoietic progenitors, purified CD34+ cells were seeded in serum-free cultures in the presence of pharmacological (10(-6)) M or physiological (10(-12)) M concentrations of all-trans retinoic acid (ATRA) and 9-cis retinoic acid (9-cis RA) plus combinations of specific cytokines. 10(-6) M ATRA and 9-cis RA significantly decreased the number of granulomacrophagic, erythroid and megakaryocytic (CFU-meg) progenitors. On the other hand, 10(-12) M ATRA significantly promoted the growth of CFU-meg, in the presence either of thrombopoietin or of IL-3+ GM-CSF, and induced a reproducible stimulation of the immature CD34+DR- subset. In conclusion, our findings suggest that retinoic acids probably play a direct role in normal adult hematopoietic development at both physiological and pharmacological concentrations. The stimulatory effect on megakaryocytopoiesis should be considered in the perspective of a potential use of low-dose ATRA, combined with thrombopoietin or other cytokines, in pathological conditions where the megakaryocytic compartment is impaired and the stimulation of megakaryocytopoiesis is requested.

Selective modulation of specific protein kinase C (PKC) isoforms in primary human megakaryocytic vs. erythroid cells

We have investigated the pattern of expression of classical (alpha, betaI, betaII, gamma), novel (delta) and atypical (zeta) protein kinase C (PKC) isoforms during the course of human hematopoietic differentiation along the closely related megakaryocytic and erythroid lineages. Using in situ immunofluorescence analysis, freshly isolated human pluripotent CD34+ hematopoietic progenitor cells expressed detectable amounts of all the PKC isoforms investigated. On the other hand, clear-cut differences in terms of PKC staining were noticed between cells belonging to the erythroid and megakaryocytic lineages, obtained after 9 days of serum-free liquid culture in the presence of specific growth factors. Specifically, 1) erythroid cells showed a very weak expression of PKC-alpha, -betaI, -betaII, and -gamma, while megakaryocytes showed an enhanced expression of all classical PKC isoforms, predominantly confined to the cytoplasm; 2) the expression of PKC-delta increased in the cytoplasmic and nuclear compartments of both erythroid and megakaryocytic cells with respect to CD34+ cells; and 3) atypical PKC-zeta isoform showed a striking accumulation in the nucleus during both erythroid and megakaryocytic differentiation.

Selective modulation of the cyclin B/CDK1 and cyclin D/CDK4 complexes during in vitro human megakaryocyte development

Mammalian megakaryocyte development is characterized by a progressive accumulation of cells exhibiting a polylobated nucleus with a polyploid DNA content. In this study human megakaryocytes were obtained from CD34+ haemopoietic progenitors by in vitro liquid culture in the presence of 100 ng/ml of recombinant thrombopoietin (TPO). Ultrastructural examination of polyploid megakaryocytes showed the presence of a large number of centrioles, the breakdown of the nuclear envelope, and the progressive chromatin condensation, all aspects characteristic of mitosis. At both indirect immunofluorescence and Western blot analyses, cyclin B and its related cyclin-dependent kinase (CDK)1, which forms the mitosis promoting factor (MPF), showed an increased expression in maturating megakaryoblasts and megakaryocytes (day 8 of culture) with respect to freshly isolated CD34+ progenitors. This expression tended to decline in fully developed megakaryocytes (day 15 of culture). The amount of cyclin D and of the related CDK4, governing the G1 phase of the cell cycle, increased during megakaryocyte development, maintaining high levels of expression also in mature megakaryocytes. These results indicate that megakaryocyte polyploidization depends on a true, although incomplete, mitotic process, and that cyclin D/CDK4 probably plays a crucial role throughout megakaryocytopoiesis.

The Induction of Megakaryocyte Differentiation Is Accompanied by Selective Ser133 Phosphorylation of the Transcription Factor CREB in Both HEL Cell Line and Primary CD34+Cells

The addition of thrombopoietin (TPO) to HEL cells, cultured in a chemically defined serum-free medium, induced a rapid and dose-dependent phosphorylation of the transcription factor CREB on serine133 (PSer133), as detected by Western blot analysis. TPO also significantly increased the transactivation of CRE-dependent promoter, as determined in transient transfection experiments. On the other hand, neither erythropoietin (Epo; 1 to 10 U) nor hemin (10−7 mol/L) were able to significantly stimulate CREB-PSer133 or to activate CRE-promoter in HEL cells. Although pharmacological inhibitors of protein kinase C (chelerytrine and BIM) and protein kinase A (H-89) failed to block the TPO-mediated CREB phosphorylation, a specific inhibitor of the mitogen-activated protein kinases (PD98059) completely blocked the ability of TPO to stimulate CREB-PSer133. Moreover, PD98059 significantly decreased the ability of TPO to upregulate the surface expression of the αIIbβ3 megakaryocytic marker in HEL cells. In parallel, primary CD34+ hematopoietic cells were seeded in liquid cultures supplemented with 100 ng/mL of TPO and examined by immunofluorescence for the coexpression of αIIbβ3 and CREB-PSer133 at various time points. High levels of nuclear CREB-PSer133 were unequivocally demonstrated in αIIbβ3+cells, including morphologically recognizable megakaryocytes. Taken together, these data suggest that CREB plays a role in modulating the expression of genes critical for megakaryocyte differentiation and that the TPO-mediated CREB phosphorylation seems to be regulated via mitogen-activated protein kinases.

The Induction of Megakaryocyte Differentiation Is Accompanied by Selective Ser133 Phosphorylation of the Transcription Factor CREB in Both HEL Cell Line and Primary CD34+Cells

The addition of thrombopoietin (TPO) to HEL cells, cultured in a chemically defined serum-free medium, induced a rapid and dose-dependent phosphorylation of the transcription factor CREB on serine133 (PSer133), as detected by Western blot analysis. TPO also significantly increased the transactivation of CRE-dependent promoter, as determined in transient transfection experiments. On the other hand, neither erythropoietin (Epo; 1 to 10 U) nor hemin (10−7 mol/L) were able to significantly stimulate CREB-PSer133 or to activate CRE-promoter in HEL cells. Although pharmacological inhibitors of protein kinase C (chelerytrine and BIM) and protein kinase A (H-89) failed to block the TPO-mediated CREB phosphorylation, a specific inhibitor of the mitogen-activated protein kinases (PD98059) completely blocked the ability of TPO to stimulate CREB-PSer133. Moreover, PD98059 significantly decreased the ability of TPO to upregulate the surface expression of the αIIbβ3 megakaryocytic marker in HEL cells. In parallel, primary CD34+ hematopoietic cells were seeded in liquid cultures supplemented with 100 ng/mL of TPO and examined by immunofluorescence for the coexpression of αIIbβ3 and CREB-PSer133 at various time points. High levels of nuclear CREB-PSer133 were unequivocally demonstrated in αIIbβ3+cells, including morphologically recognizable megakaryocytes. Taken together, these data suggest that CREB plays a role in modulating the expression of genes critical for megakaryocyte differentiation and that the TPO-mediated CREB phosphorylation seems to be regulated via mitogen-activated protein kinases.

Megakaryocyte progenitors derived from bone marrow or G-CSF-mobilized peripheral blood CD34 cells show a distinct phenotype and responsiveness to interleukin-3 (IL-3) and PEG-recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF)

In the present study we investigated the proliferative response of megakaryocyte progenitor cells (CFU-MK) derived from peripheral blood stem cell (PBSC) collections of patients with haematological malignancies and normal donors. Highly purified CD34+ cells and mononuclear cell fractions were assayed in the presence of recombinant interleukin-3 (IL-3) and pegylated-recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF), alone or in combination, and megakaryocyte colony formation was evaluated in the plasma clot. In comparison, steady-state bone marrow samples from normal donors were highly enriched in CD34+ cells and tested with the cytokines studied. Our results showed that IL-3 was able to stimulate CFU-MK colony formation from bone marrow and peripheral blood CD34+ cells. Similarly, PEG-rHuMGDF stimulated, in a dose-response manner, CD34+ cells from the bone marrow. However, normal mobilized peripheral blood CD34+ cells were not induced to generate CFU-MK colonies by PEG-rHuMGDE The same lack of response was observed when patients peripheral blood CD34+ cells primed with chemotherapy plus G-CSF or with G-CSF alone were assessed. In contrast, PEG-rHuMGDF stimulated CFU-MK growth when mononuclear cells, either from the bone marrow or from mobilized peripheral blood, were grown in plasma clot. Moreover, we analysed by flow cytometry the expression of Mpl receptor on the cell membrane of normal mobilized peripheral blood and normal steady-state bone marrow CD34+ cells. Our results showed a reduced expression of Mpl receptor on mobilized peripheral blood progenitor cells in comparison with bone marrow cells.

In Vitro Senescence and Apoptotic Cell Death of Human Megakaryocytes

To investigate the fate of human megakaryocytes, CD34+ hematopoietic progenitor cells were purified from the peripheral blood or bone marrow of healthy donors and seeded in serum-free chemically defined suspension cultures. In the presence of thrombopoietin (TPO; 100 ng/mL), CD34-derived cells showed an eightfold numerical expansion and a progressive maturation along the megakaryocytic lineage. Megakaryocyte maturation was characterized ultrastructurally by the presence of a demarcation membrane system and phenotypically by a high surface expression of αIIbβ3 integrin. The number of mature megakaryocytes peaked at days 12 to 15 of culture. On the other hand, the number of platelets released in the culture supernatant by CD34-derived megakaryocytes peaked at days 18 to 21, when a high percentage of megakaryocytes showed the characteristic features of apoptosis, as evaluated by electron microscopy, terminal deoxynucleotidyl transferase (TdT)-mediated d-UTP-biotin nick end-labeling technique (TUNEL) and uptake of propidium iodide. In other experiments, primary αIIbβ3+ megakaryocytic cells were directly purified from the bone marrow aspirates of normal donors and seeded in serum-free suspension cultures. In the absence of cytokines, αIIbβ3+ megakaryocytes progressively underwent apoptotic cell death. The addition of TPO but not interleukin-3 or erythropoietin showed some protection of αIIbβ3+ cells from apoptosis at early culture times (days 2 to 4), but it did not show any significant effect at later time points. These findings suggest that the terminal phase of the megakaryocyte life span is characterized by the onset of apoptosis, which can be modulated only to a certain extent by TPO.

In Vitro Senescence and Apoptotic Cell Death of Human Megakaryocytes

To investigate the fate of human megakaryocytes, CD34+ hematopoietic progenitor cells were purified from the peripheral blood or bone marrow of healthy donors and seeded in serum-free chemically defined suspension cultures. In the presence of thrombopoietin (TPO; 100 ng/mL), CD34-derived cells showed an eightfold numerical expansion and a progressive maturation along the megakaryocytic lineage. Megakaryocyte maturation was characterized ultrastructurally by the presence of a demarcation membrane system and phenotypically by a high surface expression of αIIbβ3 integrin. The number of mature megakaryocytes peaked at days 12 to 15 of culture. On the other hand, the number of platelets released in the culture supernatant by CD34-derived megakaryocytes peaked at days 18 to 21, when a high percentage of megakaryocytes showed the characteristic features of apoptosis, as evaluated by electron microscopy, terminal deoxynucleotidyl transferase (TdT)-mediated d-UTP-biotin nick end-labeling technique (TUNEL) and uptake of propidium iodide. In other experiments, primary αIIbβ3+ megakaryocytic cells were directly purified from the bone marrow aspirates of normal donors and seeded in serum-free suspension cultures. In the absence of cytokines, αIIbβ3+ megakaryocytes progressively underwent apoptotic cell death. The addition of TPO but not interleukin-3 or erythropoietin showed some protection of αIIbβ3+ cells from apoptosis at early culture times (days 2 to 4), but it did not show any significant effect at later time points. These findings suggest that the terminal phase of the megakaryocyte life span is characterized by the onset of apoptosis, which can be modulated only to a certain extent by TPO.

Thrombopoietin Enhances the αIIbβ3-Dependent Adhesion of Megakaryocytic Cells to Fibrinogen or Fibronectin Through PI 3 Kinase

The effect of thrombopoietin (TPO) on the functional activity of surface αIIbβ3 (GPIIbIIIa) was investigated in both primary human megakaryocytic cells, derived from peripheral blood CD34+ cells, and HEL hematopoietic cell line. TPO (100 ng/mL) induced a sixfold to ninefold enhancement of adhesion of both primary megakaryocytic and HEL cells to plates coated with either fibrinogen or fibronectin and a parallel increase of immunoreactivity to the PAC1 monoclonal antibody (MoAb) and fluorescein isothiocyanate-fibrinogen, both of which recognize an activated state of αIIbβ3 . The enhanced adhesion to fibrinogen or fibronectin was mediated by the Arg-Gly-Asp (RGD) recognition sequence of αIIbβ3 , as it was abolished by pretreatment of cells with saturating concentrations of RGDS peptide. A MoAb specific for the αIIb subunit of αIIbβ3 also inhibited cell attachment to fibrinogen or fibronectin, while MoAb to anti-αvβ3 or anti-α5 integrins were completely ineffective, clearly indicating that αIIbβ3 participates in this association. A role for PI 3 kinase (PI 3-K) in the TPO-mediated increase in αIIbβ3 function in megakaryocytic cells was suggested by the ability of the PI 3-K inhibitor wortmannin (100 nmol/L) and antisense oligonucleotides directed against the p85 regulatory subunit of PI 3-K to completely block the TPO-induced increase in αIIbβ3 integrin activity upon TPO stimulation. The modulation of adhesiveness to extracellular matrix proteins containing the RGD motif mediated by TPO likely plays a physiologic role in megakaryocytopoiesis, as pretreatment of CD34+ cells with RGDS or anti-αIIb MoAb significantly reduced the number of megakaryocytic colonies obtained in a fibrinclot semisolid assay.