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

Dual-Specificity Tyrosine Phosphorylation-Regulated Kinase 1B Inhibition Promotes Megakaryocyte Polyploidization and Platelet Production

Platelets are produced from mature megakaryocytes which undergo polyploidization and proplatelet formation. Cell-cycle regulation plays a crucial role in megakaryocyte terminal differentiation especially in polyploidization. Dual-specificity tyrosine phosphorylation-regulated kinase 1B (DYRK1B) controls cell-cycle progression in cancer cells. The objective of this study was to determine DYRK1B function in megakaryocyte maturation and platelet production. A DYRK1B knock-out mouse was generated with increased peripheral platelet count compared with the wild type mouse without affecting megakaryocyte numbers in bone marrow. Polyploidy and proplatelet formations were significantly enhanced when DYRK1B was depleted in vitro. DYRK1B inhibition promoted megakaryocyte maturation by simultaneously upregulating cyclin D1 and downregulating P27. Furthermore, there was platelet restoration in two mice disease models of transient thrombocytopenia. In summary, DYRK1B plays an important role in megakaryocyte maturation and platelet production by interacting with cyclin D1 and P27. DYRK1B inhibition has potential therapeutic value in transient thrombocytopenia treatment. Graphic Abstract.

The Y14-p53 regulatory circuit in megakaryocyte differentiation and thrombocytopenia

Thrombocytopenia-absent radius (TAR) syndrome is caused by RBM8A insufficiency. We generated megakaryocyte-specific Rbm8a knockout (Rbm8aKO^MK) mice that exhibited marked thrombocytopenia, internal hemorrhage, and splenomegaly, providing evidence that genetic deficiency of Rbm8a causes a disorder of platelet production. Rbm8aKO^MK mice accumulated low-ploidy immature megakaryocytes in the bone marrow and exhibited defective platelet activation and aggregation. Accordingly, depletion of Y14 (RBM8A) in human erythroleukemia (HEL) cells compromised phorbol-ester-induced polyploidization. Notably, Y14/RBM8A deficiency induced both p53 and p21 in megakaryocytes and HEL cells. Treatment with a p53 inhibitor restored ex vivo differentiation of Rbm8aKO^MK megakaryocytes and unexpectedly activated Y14 expression in HEL cells. Trp53 knockout partially restored megakaryocyte differentiation by reversing cell-cycle arrest and increased platelet counts of Rbm8aKO^MK, indicating that excess p53 in part accounts for thrombocytopenia in TAR syndrome. This study provides evidence for the role of the Y14-p53 circuit in platelet production and a potential therapeutic strategy.

The Severe Fever with Thrombocytopenia Syndrome Virus NSs Protein Interacts with CDK1 To Induce G2 Cell Cycle Arrest and Positively Regulate Viral Replication

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a newly identified phlebovirus associated with severe hemorrhagic fever in humans. While many viruses subvert the host cell cycle to promote viral growth, it is unknown whether this is a strategy employed by SFTSV. In this study, we investigated how SFTSV manipulates the cell cycle and the effect of the host cell cycle on SFTSV replication. Our results suggest that cells arrest at the G₂/M transition following infection with SFTSV. The accumulation of cells at the G₂/M transition did not affect virus adsorption and entry but did facilitate viral replication. In addition, we found that SFTSV NSs, a nonstructural protein that forms viroplasm-like structures in the cytoplasm of infected cells and promotes virulence by modulating the interferon response, induces a large number of cells to arrest at the G₂/M transition by interacting with CDK1. The interaction between NSs and CDK1, which is inclusion body dependent, inhibits formation and nuclear import of the cyclin B1-CDK1 complex, thereby leading to cell cycle arrest. Expression of a CDK1 loss-of-function mutant reversed the inhibitive effect of NSs on the cell cycle, suggesting that this protein is a potential antiviral target. Our study provides new insight into the role of a specific viral protein in SFTSV replication, indicating that NSs induces G₂/M arrest of SFTSV-infected cells, which promotes viral replication.IMPORTANCE Severe fever with thrombocytopenia syndrome virus (SFTSV) is a tick-borne pathogen that causes severe hemorrhagic fever. Although SFTSV poses a serious threat to public health and was recently isolated, its pathogenesis remains unclear. In particular, the relationship between SFTSV infection and the host cell cycle has not been described. Here, we show for the first time that both asynchronized and synchronized SFTSV-susceptible cells arrest at the G₂/M checkpoint following SFTSV infection and that the accumulation of cells at this checkpoint facilitates viral replication. We also identify a key mechanism underlying SFTSV-induced G₂/M arrest, in which SFTSV NSs interacts with CDK1 to inhibit formation and nuclear import of the cyclin B1-CDK1 complex, thus preventing it from regulating cell cycle progression. Our study highlights the key role that NSs plays in SFTSV-induced G₂/M arrest.

Proposed megakaryocytic regulon of p53: the genes engaged to control cell cycle and apoptosis during megakaryocytic differentiation

During endomitosis, megakaryocytes undergo several rounds of DNA synthesis without division leading to polyploidization. In primary megakaryocytes and in the megakaryocytic cell line CHRF, loss or knock-down of p53 enhances cell cycling and inhibits apoptosis, leading to increased polyploidization. To support the hypothesis that p53 suppresses megakaryocytic polyploidization, we show that stable expression of wild-type p53 in K562 cells (a p53-null cell line) attenuates the cells' ability to undergo polyploidization during megakaryocytic differentiation due to diminished DNA synthesis and greater apoptosis. This suggested that p53's effects during megakaryopoiesis are mediated through cell cycle- and apoptosis-related target genes, possibly by arresting DNA synthesis and promoting apoptosis. To identify candidate genes through which p53 mediates these effects, gene expression was compared between p53 knock-down (p53-KD) and control CHRF cells induced to undergo terminal megakaryocytic differentiation using microarray analysis. Among substantially downregulated p53 targets in p53-KD megakaryocytes were cell cycle regulators CDKN1A (p21) and PLK2, proapoptotic FAS, TNFRSF10B, CASP8, NOTCH1, TP53INP1, TP53I3, DRAM1, ZMAT3 and PHLDA3, DNA-damage-related RRM2B and SESN1, and actin component ACTA2, while antiapoptotic CKS1B, BCL2, GTSE1, and p53 family member TP63 were upregulated in p53-KD cells. Additionally, a number of cell cycle-related, proapoptotic, and cytoskeleton-related genes with known functions in megakaryocytes but not known to carry p53-responsive elements were differentially expressed between p53-KD and control CHRF cells. Our data support a model whereby p53 expression during megakaryopoiesis serves to control polyploidization and the transition from endomitosis to apoptosis by impeding cell cycling and promoting apoptosis. Furthermore, we identify a putative p53 regulon that is proposed to orchestrate these effects.

Thrombocytogenesis by megakaryocyte; Interpretation by protoplatelet hypothesis

Serial transmission electron microscopy of human megakaryocytes (MKs) revealed their polyploidization and gradual maturation through consecutive transition in characteristics of various organelles and others. At the beginning of differentiation, MK with ploidy 32N, e.g., has 16 centrosomes in the cell center surrounded by 32N nucleus. Each bundle of microtubules (MTs) emanated from the respective centrosome supports and organizes 16 equally volumed cytoplasmic compartments which together compose one single 32N MK. During the differentiation, single centriole separated from the centriole pair, i.e., centrosome, migrates to the most periphery of the cell through MT bundle, corresponding to a half of the interphase array originated from one centrosome, supporting one "putative cytoplasmic compartment" (PCC). Platelet demarcation membrane (DM) is constructed on the boundary surface between neighbouring PCCs. Matured PCC, composing of a tandem array of platelet territories covered by a sheet of DM is designated as protoplatelet. Eventually, the rupture of MK results in release of platelets from protoplatelets. (Communicated by Tadamitsu Kishimoto, M.J.A.).

Small molecule regulators of Rb-E2F pathway as modulators of transcription

The retinoblastoma tumor suppressor protein, Rb, plays a major role in the regulation of mammalian cell cycle progression. It has been shown that Rb function is essential for the proper modulation of G1/S transition and inactivation of Rb contributes to deregulated cell proliferation. Rb exerts its cell cycle regulatory functions mainly by targeting the E2F family of transcription factors and Rb has been shown to physically interact with E2Fs 1, 2 and 3, repressing their transcriptional activity. Multiple genes involved in DNA synthesis and cell cycle progression are regulated by E2Fs, and Rb prevents their expression by inhibiting E2F activity, inducing growth arrest. It has been established that inactivation of Rb by phosphorylation, mutation, or by the interaction of viral oncoproteins leads to a release of the repression of E2F activity, facilitating cell cycle progression. Rb-mediated repression of E2F activity involves the recruitment of a variety of transcriptional co-repressors and chromatin remodeling proteins, including histone deacetylases, DNA methyltransferases and Brg1/Brm chromatin remodeling proteins. Inactivation of Rb by sequential phosphorylation events during cell cycle progression leads to a dissociation of these co-repressors from Rb, facilitating transcription. It has been found that small molecules that prevent the phosphorylation of Rb prevent the dissociation of certain co-repressors from Rb, especially Brg1, leading to the maintenance of Rb-mediated transcriptional repression and cell cycle arrest. Such small molecules have anti-cancer activities and will also act as valuable probes to study chromatin remodeling and transcriptional regulation.

Contributions of TRAIL-mediated megakaryocyte apoptosis to impaired megakaryocyte and platelet production in immune thrombocytopenia

Recent in vitro studies provide evidence for autoantibody-induced suppression of megakaryocytopoiesis and show a reduction in megakaryocyte production and maturation in the presence of immune thrombocytopenia (ITP) plasma. Here, we present CD34(+) cells from healthy umbilical cord blood mononuclear cells cultured in medium containing thrombopoietin, stem cell factor, interleukin-3, and 10% plasma from either ITP patients or healthy subjects. The quantity, quality, and apoptosis of megakaryocytes were measured. We observed that most ITP plasma boosted megakaryocyte quantity but impaired quality, resulting in significantly less polyploidy cells (N ≥ 4) and platelet release. In these megakaryocytes, we found a lower percentage of cell apoptosis, a lower expression of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), and a higher expression of Bcl-xL. Furthermore, there was a decrease of sTRAIL in ITP plasma and in cell culture supernatants of this group compared with the control group. Our findings suggest that decreased apoptosis of megakaryocytes also contributes to in vitro dysmegakaryocytopoiesis and reduced platelet production. The abnormal expression of sTRAIL in plasma and TRAIL and Bcl-xL in megakaryocytes may play a role in the pathogenesis of impaired megakaryocyte apoptosis in ITP.

Gender dependence for a subset of the low-abundance signaling proteome in human platelets

The incidence of cardiovascular diseases is ten-times higher in males than females, although the biological basis for this gender disparity is not known. However, based on the fact that antiplatelet drugs are the mainstay for prevention and therapy, we hypothesized that the signaling proteomes in platelets from normal male donors might be more activated than platelets from normal female donors. We report here that platelets from male donors express significantly higher levels of signaling cascade proteins than platelets from female donors. In silico connectivity analysis shows that the 24 major hubs in platelets from male donors focus on pathways associated with megakaryocytic expansion and platelet activation. By contrast, the 11 major hubs in platelets from female donors were found to be either negative or neutral for platelet-relevant processes. The difference may suggest a biological mechanism for gender discrimination in cardiovascular disease.

Cyclin D-Cdk4 is regulated by GATA-1 and required for megakaryocyte growth and polyploidization

Endomitosis is a unique form of cell cycle used by megakaryocytes, in which the latter stages of mitosis are bypassed so that the cell can increase its DNA content and size. Although several transcription factors, including GATA-1 and RUNX-1, have been implicated in this process, the link between transcription factors and polyploidization remains undefined. Here we show that GATA-1-deficient megakaryocytes, which display reduced size and polyploidization, express nearly 10-fold less cyclin D1 and 10-fold increased levels of p16 compared with their wild-type counterparts. We further demonstrate that cyclin D1 is a direct GATA-1 target in megakaryocytes, but not erythroid cells. Restoration of cyclin D1 expression, when accompanied by ectopic overexpression of its partner Cdk4, resulted in a dramatic increase in megakaryocyte size and DNA content. However, terminal differentiation was not rescued. Of note, polyploidization was only modestly reduced in cyclin D1-deficient mice, likely due to compensation by elevated cyclin D3 expression. Finally, consistent with an additional defect conferred by increased levels of p16, inhibition of cyclin D-Cdk4 complexes with a TAT-p16 fusion peptide significantly blocked polyploidization of wild-type megakaryocytes. Together, these data show that GATA-1 controls growth and polyploidization by regulating cyclin D-Cdk4 kinase activity.

Molecular control of megakaryopoiesis and thrombopoiesis

Megakaryopoiesis and subsequent thrombopoiesis occur through complex biologic steps: megakaryocyte precursors that developed from hematopoietic stem cells initially proliferate, then differentiate into mature polyploid megakaryocytes, and finally release platelets. Although a number of growth factors can augment megakaryopoiesis in vitro, thrombopoietin is a physiologic and the most potent regulator of megakaryopoiesis in vitro and in vivo. Thrombopoietin induces the growth of megakaryocyte precursors through activation of multiple signaling cascades, including Ras/mitogen-activated protein kinase (MAPK), signal transducers and activators of transcription 5 (STAT5), phosphatidylinositol 3-kinase (PI3-K)/Akt, and protein kinase C, whereas it induces megakaryocytic maturation primarily through the Ras/MAPK pathway. During the maturation step, megakaryocytes undergo polyploidization characterized by repeated rounds of DNA replication without concomitant cell division. During these rounds of replication, cytokinesis is neglected because of the down-regulated expression of AIM-1, and DNA replication occurs through the increased expression of D-type cyclins. As for transcriptional regulation during megakaryopoiesis, GATA-1 plays a central role in the lineage commitment of hematopoietic stem cells toward erythroid/megakaryocytic lineage and subsequent maturation. p45 NF-E2 is essential for platelet release from terminally differentiated megakaryocytes. At present, mutations of GATA-1, AML1, and HOXA11 genes have been found in hereditary diseases accompanying thrombocytopenia among humans.

Different ploidy levels of megakaryocytes generated from peripheral or cord blood CD34+ cells are correlated with different levels of platelet release

Ploidy could be the key to understanding megakaryocyte (MK) biology and platelet production. Human CD34+ cells purified from umbilical cord blood (CB) and peripheral blood (PB) were investigated on their capability to give rise, in a serum-free medium containing thrombopoietin, to MKs and platelets. CB-MKs showed reduced polyploidization and platelet number compared with PB-MKs, but a similar membrane phenotype. Most CB-MKs showed a 2N content of DNA (approximately 80%) and only 2.6% had 8N, whereas 40% of the PB cells had 8N or more. Platelets were substantially released in PB culture from day 12; at day 14 the CB-derived MKs were able to release platelets although at a reduced level (approximately 35%), correlating with their reduced size. A direct correlation was demonstrated by sorting polyploid cells from PB-MKs and evaluating the platelets released in the supernatant. Furthermore, the study analyzed the expression and distribution of cyclin D3 and cyclin B1. Cyclin D3 protein was increased in PB in comparison to CB-MKs; in PB culture most cells rapidly became positive, whereas in CB-derived cells cyclin D3 expression was evident only from day 9 and in a reduced percentage. Cyclin B1 was essentially localized at the nuclear level in the CB and was expressed during the whole culture. In PB-MKs, at day 9, a reduction was observed, correlating with an advanced ploidy state. The data indicate the inability of the CB-MKs to progress in the endomitotic process and a direct correlation between DNA content and platelet production.

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.

Roads to polyploidy: the megakaryocyte example

Polyploidy, recognized by multiple copies of the haploid chromosome number, has been described in plants, insects, and in mammalian cells such as, the platelet precursors, the megakaryocytes. Several of these cell types reach high ploidy via a different cell cycle. Megakaryocytes undergo an endomitotic cell cycle, which consists of an S phase interrupted by a gap, during which the cells enter mitosis but skip anaphase B and cytokinesis. Here, we review the mechanisms that lead to this cell cycle and to polyploidy in megakaryocytes, while also comparing them to those described for other systems in which high ploidy is achieved. Overall, polyploidy is associated with an orchestrated change in expression of several genes, of which, some may be a result of high ploidy and hence a determinant of a new cell physiology, while others are inducers of polyploidization. Future studies will aim to further explore these two groups of genes.

Kinetics of endomitosis in primary murine megakaryocytes

Megakaryocytes (MKs) develop from diploid progenitor cells via successive rounds of DNA synthesis in the absence of cell division, a process termed endomitosis (EnM). While the mechanism underlying EnM is not known, studies in yeast and leukemic cell lines have suggested that it may be due to reduced levels of cyclin B1 or cdc2, leading to a decrease in mitotic kinase activity. Using flow cytometry to study EnM highly purified marrow-derived MK precursors, we found that: (1) on average, 36% of 8N-32N MKs expressed abundant cyclin B during G2/M. The percentage of cells in G2/M decreased in >64N MKs, suggesting the limit of EnM, (2) the level of cyclin B per G2/M MK increased linearly with ploidy, (3) cyclin B expression oscillated normally in polyploid MKs, (4) MPM-2, a phosphoepitope created by the action of mitotic kinases and specific to M-phase cells, was expressed in a significant fraction of polyploid MKs, and (5) there was an apparent increase of cyclin B in G1-phase in polyploid MKs. This study provides the first qualitative kinetic data regarding the cell cycle status of MKs within individual ploidy classes. It also demonstrates the feasibility of using anti-cyclin B antibody and flow cytometry to resolve G1 from G2/M populations in polyploid MKs. Finally, these findings establish that neither a relative nor absolute deficiency of mitotic kinase components is responsible for EnM, suggesting that the departure from normal cell division kinetics seen in polyploid MKs is likely due to alterations in other cell cycle regulators.

Downregulation of an AIM-1 kinase couples with megakaryocytic polyploidization of human hematopoietic cells

During the late phase of megakaryopoiesis, megakaryocytes undergo polyploidization, which is characterized by DNA duplication without concomitant cell division. However, it remains unknown by which mechanisms this process occurs. AIM-1 and STK15 belong to the Aurora/increase-in-ploidy (Ipl)1 serine/threonine kinase family and play key roles in mitosis. In a human interleukin-3-dependent cell line, F-36P, the expressions of AIM-1 and STK15 mRNA were specifically observed at G2/M phase of the cell cycle during proliferation. In contrast, the expressions of AIM-1 and STK15 were continuously repressed during megakaryocytic polyploidization of human erythro/megakaryocytic cell lines (F-36P, K562, and CMK) treated with thrombopoietin, activated ras (H-ras(G12V)), or phorbol ester. Furthermore, their expressions were suppressed during thrombopoietin-induced polyploidization of normal human megakaryocytes. Activation of AIM-1 by the induced expression of AIM-1(wild-type) canceled TPA-induced polyploidization of K562 cells significantly, whereas that of STK15 did not. Moreover, suppression of AIM-1 by the induced expression of AIM-1 (K/R, dominant-negative type) led to polyploidization in 25% of K562 cells, whereas STK15(K/R) showed no effect. Also, the induced expression of AIM-1(K/R) in CMK cells provoked polyploidization up to 32N. These results suggested that downregulation of AIM-1 at M phase may be involved in abortive mitosis and polyploid formation of megakaryocytes.

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.