Gene expression profile of megakaryocytes from human cord blood CD34(+) cells ex vivo expanded by thrombopoietin

Human Platelets Contain, Translate, and Secrete Azurocidin; A Novel Effect on Hemostasis

Platelets play a significant role in hemostasis and perform essential immune functions, evidenced by the extensive repertoire of antimicrobial molecules. Currently, there is no clear description of the presence of azurocidin in human platelets. Azurocidin is a 37 kDa cationic protein abundant in neutrophils, with microbicidal, opsonizing, and vascular permeability-inducing activity. Therefore, this work aimed to characterize the content, secretion, translation, and functions of azurocidin in platelets. Our results show the presence of azurocidin mRNA and protein in α-granules of platelet and megakaryoblasts, and stimulation with thrombin, ADP, and LPS leads to the secretion of free azurocidin as well as within extracellular vesicles. In addition, platelets can translate azurocidin in a basal or thrombin-induced manner. Finally, we found that the addition of low concentrations of azurocidin prevents platelet aggregation and activation. In conclusion, we demonstrate that platelets contain, secrete, and translate azurocidin, and this protein may have important implications for hemostasis.

Gene and protein expression in human megakaryocytes derived from induced pluripotent stem cells

BACKGROUND

There is interest in deriving megakaryocytes (MKs) from pluripotent stem cells (iPSC) for biological studies. We previously found that genomic structural integrity and genotype concordance is maintained in iPSC-derived MKs.

OBJECTIVE

To establish a comprehensive dataset of genes and proteins expressed in iPSC-derived MKs.

METHODS

iPSCs were reprogrammed from peripheral blood mononuclear cells (MNCs) and MKs were derived from the iPSCs in 194 healthy European American and African American subjects. mRNA was isolated and gene expression measured by RNA sequencing. Protein expression was measured in 62 of the subjects using mass spectrometry.

RESULTS AND CONCLUSIONS

MKs expressed genes and proteins known to be important in MK and platelet function and demonstrated good agreement with previous studies in human MKs derived from CD34+ progenitor cells. The percent of cells expressing the MK markers CD41 and CD42a was consistent in biological replicates, but variable across subjects, suggesting that unidentified subject-specific factors determine differentiation of MKs from iPSCs. Gene and protein sets important in platelet function were associated with increasing expression of CD41/42a, while those related to more basic cellular functions were associated with lower CD41/42a expression. There was differential gene expression by the sex and race (but not age) of the subject. Numerous genes and proteins were highly expressed in MKs but not known to play a role in MK or platelet function; these represent excellent candidates for future study of hematopoiesis, platelet formation, and/or platelet function.

Beyond COX-1: the effects of aspirin on platelet biology and potential mechanisms of chemoprevention

After more than a century, aspirin remains one of the most commonly used drugs in western medicine. Although mainly used for its anti-thrombotic, anti-pyretic, and analgesic properties, a multitude of clinical studies have provided convincing evidence that regular, low-dose aspirin use dramatically lowers the risk of cancer. These observations coincide with recent studies showing a functional relationship between platelets and tumors, suggesting that aspirin's chemopreventive properties may result, in part, from direct modulation of platelet biology and biochemistry. Here, we present a review of the biochemistry and pharmacology of aspirin with particular emphasis on its cyclooxygenase-dependent and cyclooxygenase-independent effects in platelets. We also correlate the results of proteomic-based studies of aspirin acetylation in eukaryotic cells with recent developments in platelet proteomics to identify non-cyclooxygenase targets of aspirin-mediated acetylation in platelets that may play a role in its chemopreventive mechanism.

Stem cell autotomy and niche interaction in different systems

The best known cases of cell autotomy are the formation of erythrocytes and thrombocytes (platelets) from progenitor cells that reside in special niches. Recently, autotomy of stem cells and its enigmatic interaction with the niche has been reported from male germline stem cells (GSCs) in several insect species. First described in lepidopterans, the silkmoth, followed by the gipsy moth and consecutively in hemipterans, foremost the milkweed bug. In both, moths and the milkweed bug, GSCs form finger-like projections toward the niche, the apical cells (homologs of the hub cells in Drosophila). Whereas in the milkweed bug the projection terminals remain at the surface of the niche cells, in the gipsy moth they protrude deeply into the singular niche cell. In both cases, the projections undergo serial retrograde fragmentation with progressing signs of autophagy. In the gipsy moth, the autotomized vesicles are phagocytized and digested by the niche cell. In the milkweed bug the autotomized vesicles accumulate at the niche surface and disintegrate. Autotomy and sprouting of new projections appears to occur continuously. The significance of the GSC-niche interactions, however, remains enigmatic. Our concept on the signaling relationship between stem cell-niche in general and GSC and niche (hub cells and cyst stem cells) in particular has been greatly shaped by Drosophila melanogaster. In comparing the interactions of GSCs with their niche in Drosophila with those in species exhibiting GSC autotomy it is obvious that additional or alternative modes of stem cell-niche communication exist. Thus, essential signaling pathways, including niche-stem cell adhesion (E-cadherin) and the direction of asymmetrical GSC division - as they were found in Drosophila - can hardly be translated into the systems where GSC autotomy was reported. It is shown here that the serial autotomy of GSC projections shows remarkable similarities with Wallerian axonal destruction, developmental axon pruning and dying-back degeneration in neurodegenerative diseases. Especially the hypothesis of an existing evolutionary conserved “autodestruction program” in axons that might also be active in GSC projections appears attractive. Investigations on the underlying signaling pathways have to be carried out. There are two other well known cases of programmed cell autotomy: the enucleation of erythroblasts in the process of erythrocyte maturation and the segregation of thousands of thrombocytes (platelets) from one megakaryocyte. Both progenitor cell types - erythroblasts and megakaryocytes - are associated with a niche in the bone marrow, erythroblasts with a macrophage, which they surround, and the megakaryocytes with the endothelial cells of sinusoids and their extracellular matrix. Although the regulatory mechanisms may be specific in each case, there is one aspect that connects all described processes of programmed cell autotomy and neuronal autodestruction: apoptotic pathways play always a prominent role. Studies on the role of male GSC autotomy in stem cell-niche interaction have just started but are expected to reveal hitherto unknown ways of signal exchange. Spermatogenesis in mammals advance our understanding of insect spermatogenesis. Mammal and insect spermatogenesis share some broad principles, but a comparison of the signaling pathways is difficult. We have intimate knowledge from Drosophila, but of almost no other insect, and we have only limited knowledge from mammals. The discovery of stem cell autotomy as part of the interaction with the niche promises new general insights into the complicated stem cell-niche interdependence.

Cell-autonomous function of Runx1 transcriptionally regulates mouse megakaryocytic maturation

RUNX1 transcription factor (TF) is a key regulator of megakaryocytic development and when mutated is associated with familial platelet disorder and predisposition to acute myeloid leukemia (FPD-AML). We used mice lacking Runx1 specifically in megakaryocytes (MK) to characterized Runx1-mediated transcriptional program during advanced stages of MK differentiation. Gene expression and chromatin-immunoprecipitation-sequencing (ChIP-seq) of Runx1 and p300 identified functional Runx1 bound MK enhancers. Runx1/p300 co-bound regions showed significant enrichment in genes important for MK and platelet homeostasis. Runx1 occupied genomic regions were highly enriched in RUNX and ETS motifs and to a lesser extent in GATA motif. Megakaryocytic specificity of Runx1/P300 bound enhancers was validated by transfection mutagenesis and Runx1/P300 co-bound regions of two key megakaryocytic genes Nfe2 and Selp were tested by in vivo transgenesis. The data provides the first example of genome wide Runx1/p300 occupancy in maturating primary FL-MK, unravel the Runx1-regulated program controlling MK maturation in vivo and identify a subset of its bona fide regulated genes. It advances our understanding of the molecular events that upon RUNX1mutations in human lead to the predisposition to familial platelet disorders and FPD-AML.

Multiploid CD61+ cells are the pre-dominant cell lineage infected during acute dengue virus infection in bone marrow

Depression of the peripheral blood platelet count during acute infection is a hallmark of dengue. This thrombocytopenia has been attributed, in part, to an insufficient level of platelet production by megakaryocytes that reside in the bone marrow (BM). Interestingly, it was observed that dengue patients experience BM suppression at the onset of fever. However, few studies focus on the interaction between dengue virus (DENV) and megakaryocytes and how this interaction can lead to a reduction in platelets. In the studies reported herein, BM cells from normal healthy rhesus monkeys (RM) and humans were utilized to identify the cell lineage(s) that were capable of supporting virus infection and replication. A number of techniques were employed in efforts to address this issue. These included the use of viral RNA quantification, nonstructural protein and infectivity assays, phenotypic studies utilizing immunohistochemical staining, anti-differentiation DEAB treatment, and electron microscopy. Cumulative results from these studies revealed that cells in the BM were indeed highly permissive for DENV infection, with human BM having higher levels of viral production compared to RM. DENV-like particles were predominantly observed in multi-nucleated cells that expressed CD61+. These data suggest that megakaryocytes are likely the predominant cell type infected by DENV in BM, which provides one explanation for the thrombocytopenia and the dysfunctional platelets characteristic of dengue virus infection.

Platelet mRNA: the meaning behind the message

PURPOSE OF REVIEW

It is now well appreciated that megakaryocytes invest platelets with a diverse repertoire of messenger RNAs (mRNAs), which are competent for translation. Herein we describe what is currently known regarding the expression, function, and clinical significance of mRNAs in platelets.

RECENT FINDINGS

Although mRNA was detected in platelets nearly 30 years ago, we are only beginning to understand the roles of mRNA in platelet biology and human disease. Recent studies have shown that megakaryocytes specifically sort, rather than randomly transfer, mRNA to platelets during thrombopoiesis. As a result, platelets are released into the circulation with thousands of mRNAs. The emergence of next-generation RNA sequencing has demonstrated that platelet mRNAs possess classic structural features, which include untranslated regions and open reading frames. There is also growing evidence that platelet mRNA expression patterns are altered in human disease.

SUMMARY

Intense investigation of platelet mRNA has shed considerable light on predicted functions of platelets and identified previously unrecognized attributes of platelets. Lessons learned from platelet mRNA is presented in this review.

Novel pathways to erythropoiesis induced by dimerization of intracellular C-Mpl in human hematopoietic progenitors

The cytokine thrombopoietin (Tpo) plays a critical role in hematopoiesis by binding to the extracellular domain and inducing homodimerization of the intracellular signaling domain of its receptor, c-Mpl. Mpl homodimerization can also be accomplished by binding of a synthetic ligand to a constitutively expressed fusion protein F36VMpl consisting of a ligand binding domain (F36V) and the intracellular signaling domain of Mpl. Unexpectedly, in contrast to Tpo stimulation, robust erythropoiesis is induced after dimerization of F36VMpl in human CD34+ progenitor cells. The goal of this study was to define the hematopoietic progenitor stages at which dimerization of intracellular Mpl induces erythropoiesis and the downstream molecular events that mediate this unanticipated effect. Dimerization (in the absence of erythropoietin and other cytokines) in human common myeloid progenitors and megakaryocytic erythroid progenitors caused a significant increase in CD34+ cells (p < .01) and induced all stages of erythropoiesis including production of enucleated red blood cells. In contrast, erythropoiesis was not seen with Tpo stimulation. CD34+ cell expansion was the result of increased cell cycling and survival (p < .05). Microarray profiling of CD34+ cells demonstrated that a unique transcriptional pattern is activated in progenitors by F36VMpl dimerization. Ligand-inducible dimerization of intracellular Mpl in human myeloerythroid progenitors induces progenitor expansion and erythropoiesis through molecular mechanisms that are not shared by Tpo stimulation of endogenous Mpl.

Platelets and platelet-like particles mediate intercellular RNA transfer

The role of platelets in hemostasis and thrombosis is clearly established; however, the mechanisms by which platelets mediate inflammatory and immune pathways are less well understood. Platelets interact and modulate the function of blood and vascular cells by releasing bioactive molecules. Although the platelet is anucleate, it contains transcripts that may mirror disease. Platelet mRNA is only associated with low-level protein translation; however, platelets have a unique membrane structure allowing for the passage of small molecules, leading to the possibility that its cytoplasmic RNA may be passed to nucleated cells. To examine this question, platelet-like particles with labeled RNA were cocultured with vascular cells. Coculture of platelet-like particles with activated THP-1, monocytic, and endothelial cells led to visual and functional RNA transfer. Posttransfer microarray gene expression analysis of THP-1 cells showed an increase in HBG1/HBG2 and HBA1/HBA2 expression that was directly related to the transfer. Infusion of wild-type platelets into a TLR2-deficient mouse model established in vivo confirmation of select platelet RNA transfer to leukocytes. By specifically transferring green fluorescent protein, we also observed external RNA was functional in the recipient cells. The observation that platelets possess the capacity to transfer cytosolic RNA suggests a new function for platelets in the regulation of vascular homeostasis.

Novel anti-bacterial activities of β-defensin 1 in human platelets: suppression of pathogen growth and signaling of neutrophil extracellular trap formation

Human β-defensins (hBD) are antimicrobial peptides that curb microbial activity. Although hBD's are primarily expressed by epithelial cells, we show that human platelets express hBD-1 that has both predicted and novel antibacterial activities. We observed that activated platelets surround Staphylococcus aureus (S. aureus), forcing the pathogens into clusters that have a reduced growth rate compared to S. aureus alone. Given the microbicidal activity of β-defensins, we determined whether hBD family members were present in platelets and found mRNA and protein for hBD-1. We also established that hBD-1 protein resided in extragranular cytoplasmic compartments of platelets. Consistent with this localization pattern, agonists that elicit granular secretion by platelets did not readily induce hBD-1 release. Nevertheless, platelets released hBD-1 when they were stimulated by α-toxin, a S. aureus product that permeabilizes target cells. Platelet-derived hBD-1 significantly impaired the growth of clinical strains of S. aureus. hBD-1 also induced robust neutrophil extracellular trap (NET) formation by target polymorphonuclear leukocytes (PMNs), which is a novel antimicrobial function of β-defensins that was not previously identified. Taken together, these data demonstrate that hBD-1 is a previously-unrecognized component of platelets that displays classic antimicrobial activity and, in addition, signals PMNs to extrude DNA lattices that capture and kill bacteria.

Platelets are small cells in the bloodstream whose primary function is to stop bleeding. In addition to their clotting functions, we show that human platelets stall bacterial growth. This inhibitory property of platelets is due to β-defensin 1, a small antimicrobial protein that kills bacteria. β-defensin 1 also induces white blood cells to discharge spider-like webs that trap and kill bacteria. Together, these findings indicate that human platelets use β-defensin 1 to fight off bacterial infection.

Cells in dengue virus infection in vivo

Dengue has been recognized as one of the most important vector-borne emerging infectious diseases globally. Though dengue normally causes a self-limiting infection, some patients may develop a life-threatening illness, dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS). The reason why DHF/DSS occurs in certain individuals is unclear. Studies in the endemic regions suggest that the preexisting antibodies are a risk factor for DHF/DSS. Viremia and thrombocytopenia are the key clinical features of dengue virus infection in patients. The amounts of virus circulating in patients are highly correlated with severe dengue disease, DHF/DSS. Also, the disturbance, mainly a transient depression, of hematological cells is a critical clinical finding in acute dengue patients. However, the cells responsible for the dengue viremia are unresolved in spite of the intensive efforts been made. Dengue virus appears to replicate and proliferate in many adapted cell lines, but these in vitro properties are extremely difficult to be reproduced in primary cells or in vivo. This paper summarizes reports on the permissive cells in vitro and in vivo and suggests a hematological cell lineage for dengue virus infection in vivo, with the hope that a new focus will shed light on further understanding of the complexities of dengue disease.

Study of gene expression profile during cord blood-associated megakaryopoiesis

AIMS

To study the gene profile in cord blood (CB)-associated megakaryopoiesis.

METHODS

In vitro differentiation of megakaryocytes (Mks) was carried out using human CB CD34(+) cells under the stimulation of recombinant human interleukin-3, stem cell factor and thrombopoietin for 7 d, followed by thrombopoietin only for further 3 d. Lineage-specific differentiation of Mk was examined by the expression of CD41 using flow cytometry and confocal microscopy. Total cellular RNA was extracted from day-0 CD34(+), day-10 CD41(+) and CD41(-) populations were isolated by immunomagnetic sorting respectively. Microarray was performed, and the data were analyzed using the GeneChip Operating System, Spotfire software and Genomatix BiblioSphere.

RESULTS

Flow cytometric analysis showed 19.44 +/- 3.05% CD41(+) cells at day 10 of culture. The purity of CD41(+) population was enriched to 95.70 +/- 4.19% after sorting. Gene expression profiling revealed an upregulation of 285 and downregulation of 53 unique genes in the CD41(+) cells compared with CD41(-) and CD34(+) cells. Platelet-associated genes, such as thrombospondin 1, platelet glycoprotein IIIa, etc., were highly expressed in CD41(+) cells but not in CD41(-) cells and CD34(+) cells. Moreover, some genes that have not been reported to be associated with CB-derived megakaryopoiesis, such as Cbl-interacting proteins Sts-1, protocadherin 21, etc., are found to be highly expressed in the CD41(+) cells from this study.

CONCLUSIONS

This study reveals a global gene expression profile of in vitro human CB-derived megakaryopoiesis at day 10. Some of these genes may play regulatory roles during the development of CB-derived megakaryopoiesis.

Comparative, genome-scale transcriptional analysis of CHRF-288-11 and primary human megakaryocytic cell cultures provides novel insights into lineage-specific differentiation

OBJECTIVES

Little is known about the transcriptional events underlying megakaryocytic (Mk) differentiation. We sought to identify genes and pathways previously unassociated with megakaryopoiesis and to evaluate the CHRF-288-11 (CHRF) megakaryoblastic cell line as a model system for investigating megakaryopoiesis.

METHODS

Using DNA microarrays, Q-RT-PCR, and protein-level assays, we compared the dynamic gene expression pattern of phorbol ester-induced differentiation of CHRF cells to cytokine-induced Mk differentiation of human mobilized peripheral blood CD34(+) cells.

RESULTS

Transcriptional patterns of well-known Mk genes were similar between the two systems. CHRF cells constitutively express some early Mk genes including GATA-1. Expression patterns of apoptosis-related genes suggested that increased p53 activity is involved in Mk apoptosis, and this was confirmed by p53-DNA-binding activity data and flow-cytometric analysis of the p53 target gene BBC3. Certain Rho and G-protein-coupled-receptor signaling pathway components were upregulated, including genes not previously associated with Mk cells. Ontological analysis revealed upregulation of defense-response genes, including both known and candidate platelet-derived contributors to inflammation. Upregulation of interferon-responsive genes occurred in the cell line, but not in the primary cells, likely due to a known genetic mutation in the JAK2/STAT5 signaling pathway.

CONCLUSIONS

This analysis of megakaryopoiesis, which integrates dynamic gene expression data with protein abundance and activity assays, has identified a number of genes and pathways that may help govern megakaryopoiesis. Furthermore, the transcriptional data support the hypothesis that CHRF cells resemble an early Mk phenotype and, with certain limitations, exhibit genuine transcriptional features of Mk differentiation upon treatment with phorbol esters.

A systems-biology analysis of isogenic megakaryocytic and granulocytic cultures identifies new molecular components of megakaryocytic apoptosis

Background

The differentiation of hematopoietic stem cells into platelet-forming megakaryocytes is of fundamental importance to hemostasis. Constitutive apoptosis is an integral, yet poorly understood, facet of megakaryocytic (Mk) differentiation. Understanding Mk apoptosis could lead to advances in the treatment of Mk and platelet disorders.

Results

We used a Gene-ontology-driven microarray-based transcriptional analysis coupled with protein-level and activity assays to identify genes and pathways involved in Mk apoptosis. Peripheral blood CD34⁺ hematopoietic progenitor cells were induced to either Mk differentiation or, as a negative control without observable apoptosis, granulocytic differentiation. Temporal gene-expression data were analyzed by a combination of intra- and inter-culture comparisons in order to identify Mk-associated genes. This novel approach was first applied to a curated set of general Mk-related genes in order to assess their dynamic transcriptional regulation. When applied to all apoptosis associated genes, it revealed a decrease in NF-κB signaling, which was explored using phosphorylation assays for IκBα and p65 (RELA). Up-regulation was noted among several pro-apoptotic genes not previously associated with Mk apoptosis such as components of the p53 regulon and TNF signaling. Protein-level analyses probed the involvement of the p53-regulated GADD45A, and the apoptosis signal-regulating kinase 1 (ASK1). Down-regulation of anti-apoptotic genes, including several of the Bcl-2 family, was also detected.

Conclusion

Our comparative approach to analyzing dynamic large-scale transcriptional data, which was validated using a known set of Mk genes, robustly identified candidate Mk apoptosis genes. This led to novel insights into the molecular mechanisms regulating apoptosis in Mk cells.

NF-E2-mediated enhancement of megakaryocytic differentiation and platelet production in vitro and in vivo

OBJECTIVE

NF-E2 is a prime regulator of megakaryocyte (MK) terminal differentiation and platelet release. By overexpressing the p45 subunit of NF-E2, we aim to increase the proportion of mature MKs and the potential for platelet production in vitro and in vivo.

METHODS

Retroviral vectors expressing p45-NF-E2 together with the enhanced green fluorescent protein (eGFP) were used to transduce murine bone marrow cells (BMCs). Aspects of MK differentiation, proliferation, proplatelet, and platelet production were evaluated.

RESULTS

Compared to controls, a higher proportion of BMCs overexpressing p45-NF-E2 were found to express the MK markers CD41, CD42a, and CD42b, with some effect on cell proliferation. Early MK differentiation, characterized by colony-forming unit (CFU)-MK formation, was enhanced by p45-NF-E2 overexpression at the expense of CFU-granulocyte macrophage development. An increased number of acetylcholinesterase(+) MKs was also observed in NF-E2(++) cultures. Although endomitosis was found not to be affected, the resultant upregulation of NF-E2 target genes was also followed by significant increases in proplatelet and functional platelet production. Transplantation of enriched MK progenitor cells overexpressing p45-NF-E2 into lethally irradiated mice resulted in a threefold increase in eGFP(+)/NF-E2(++) platelet production in vivo over 10 days, although no appreciable expansion in their number was observed over 32 days.

CONCLUSION

These results suggest that enforced expression of p45-NF-E2 selectively enhances many aspects of MK differentiation, including MK maturation, proplatelet formation, and platelet release. In addition, p45 overexpression increases MK commitment during early megakaryopoiesis, while inhibiting white blood cell differentiation.

Idiopathic thrombocytopenic purpura and dysmegakaryocytopoiesis

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder characterized with thrombocytopenia, primarily caused by platelet destruction. However, the studies of platelet kinetics show platelet turn over are normal or decreased, suggesting that reduced platelet production may lead to severity of ITP. We review recent research progress on abnormal cell events involved in megakaryocytopoiesis contributing to thrombocytopenia.

CD41+/CD45+Cells Without Acetylcholinesterase Activity Are Immature and a Major Megakaryocytic Population in Murine Bone Marrow

Murine megakaryocytes (MKs) are defined by CD41/CD61 expression and acetylcholinesterase (AChE) activity; however, their stages of differentiation in bone marrow (BM) have not been fully elucidated. In murine lineage-negative (Lin(-))/CD45(+) BM cells, we found CD41(+) MKs without AChE activity (AChE(-)) except for CD41(++) MKs with AChE activity (AChE(+)), in which CD61 expression was similar to their CD41 level. Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs could differentiate into AChE(+), with an accompanying increase in CD41/CD61 during in vitro culture. Both proplatelet formation (PPF) and platelet (PLT) production for Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs were observed later than for Lin(-)/CD41(++)/CD45(+)/AChE(+) MKs, whereas MK progenitors were scarcely detected in both subpopulations. GeneChip and semiquantitative polymerase chain reaction analyses revealed that the Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs are assigned at the stage between the progenitor and PPF preparation phases in respect to the many MK/PLT-specific gene expressions, including beta1-tubulin. In normal mice, the number of Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs was 100 times higher than that of AChE(+) MKs in BM. When MK destruction and consequent thrombocytopenia were caused by an antitumor agent, mitomycin-C, Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs led to an increase in AChE(+) MKs and subsequent PLT recovery with interleukin-11 administration. It was concluded that MKs in murine BM at least in part consist of immature Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs and more differentiated Lin(-)/CD41(++)/CD45(+)/AChE(+) MKs. Immature Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs are a major MK population compared with AChE(+) MKs in BM and play an important role in rapid PLT recovery in vivo.

Pim-1 induced polyploidy but did not affect megakaryocytic differentiation of K562 cells and CD34+ cells from cord blood

In a previous study, we determined the gene expression profile of both megakaryocytic and non-megakaryocytic lineage cells via serial analysis of gene expression and microarray methods, and demonstrated that Pim-1 was expressed more abundantly in megakaryocytic lineage cells. In this study, we knocked down Pim-1 in K562 cells, as well as in CD34+ cells from cord blood, via RNA interference, in order to analyze the effects of Pim-1 expression on the megakaryocytic differentiation of these cells. We then additionally overexpressed the Pim-1 genes in K562 cells, and conducted a comparison of these effects with those of RNAi cells on the course of megakaryocytic differentiation. The results of this study revealed that Pim-1 knockdown exerted no effects on commitment or differentiation toward megakaryocytic lineage, as evidenced by the detected CD41+ or CD61+ cells, or on the number of megakaryocytic colony forming units. However, Pim-1 knockdown was found to elicit a reduction in CD41+ cells with >4n DNA content, and a concomitant increase in the fraction of cells achieving a ploidy of >4n in the Pim-1 overexpressing population of K562 cells. Collectively, the findings of these studies indicate that the expression of Pim-1 expression is both necessary and sufficient for polyploidization, but is not critical to cytoplasmic differentiation on megakaryopoiesis.

Evidence that the Pim1 kinase gene is a direct target of HOXA9

The HOXA9 homeoprotein exerts dramatic effects in hematopoiesis. Enforced expression of HOXA9 enhances proliferation of primitive blood cells, expands hematopoietic stem cells (HSCs), and leads to myeloid leukemia. Conversely, loss of HOXA9 inhibits proliferation and impairs HSC function. The pathways by which HOXA9 acts are largely unknown, and although HOXA9 is a transcription factor, few direct target genes have been identified. Our previous study suggested that HOXA9 positively regulates Pim1, an oncogenic kinase. The hematologic phenotypes of Hoxa9- and Pim1-deficient animals are strikingly similar. Here we show that HOXA9 protein binds to the Pim1 promoter and induces Pim1 mRNA and protein in hematopoietic cells. Pim1 protein is diminished in Hoxa9(-/-) cells, and Hoxa9 and Pim1 mRNA levels track together in early hematopoietic compartments. Induction of Pim1 protein by HOXA9 increases the phosphorylation and inactivation of the proapoptotic BAD protein, a target of Pim1. Hoxa9(-/-) cells show increased apoptosis and decreased proliferation, defects that are ameliorated by reintroduction of Pim1. Thus Pim1 appears to be a direct transcriptional target of HOXA9 and a mediator of its antiapoptotic and proproliferative effects in early cells. Since HOXA9 is frequently up-regulated in acute myeloid leukemia, Pim1 may be a therapeutic target in human disease.

Expression analysis of primary mouse megakaryocyte differentiation and its application in identifying stage-specific molecular markers and a novel transcriptional target of NF-E2

Megakaryocyte (MK) differentiation is well described in morphologic terms but its molecular counterparts and the basis for platelet release are incompletely understood. We profiled mRNA expression in populations of primary mouse MKs representing successive differentiation stages. Genes associated with DNA replication are highly expressed in young MKs, in parallel with endomitosis. Intermediate stages are characterized by disproportionate expression of genes associated with the cytoskeleton, cell migration, and G-protein signaling, whereas terminally mature MKs accumulate hemostatic factors, including many membrane proteins. We used these expression profiles to extract a reliable panel of molecular markers for MKs of early, intermediate, or advanced differentiation and establish the value of this marker panel using mouse models of defective thrombopoiesis resulting from absence of GATA1, NF-E2, or tubulin beta1. Computational analysis of the promoters of late-expressed MK genes identified new candidate targets for NF-E2, a critical transcriptional regulator of platelet release. One such gene encodes the kinase adaptor protein LIMS1/PINCH1, which is highly expressed in MKs and platelets and significantly reduced in NF-E2-deficient cells. Transactivation studies and chromatin immunoprecipitation implicate Lims1 as a direct target of NF-E2 regulation. Attribution of stage-specific genes, in combination with various applications, thus constitutes a powerful way to study MK differentiation and platelet biogenesis.

A comprehensive proteomics and genomics analysis reveals novel transmembrane proteins in human platelets and mouse megakaryocytes including G6b-B, a novel immunoreceptor tyrosine-based inhibitory motif protein

The platelet surface is poorly characterized due to the low abundance of many membrane proteins and the lack of specialist tools for their investigation. In this study we identified novel human platelet and mouse megakaryocyte membrane proteins using specialist proteomics and genomics approaches. Three separate methods were used to enrich platelet surface proteins prior to identification by liquid chromatography and tandem mass spectrometry: lectin affinity chromatography, biotin/NeutrAvidin affinity chromatography, and free flow electrophoresis. Many known, abundant platelet surface transmembrane proteins and several novel proteins were identified using each receptor enrichment strategy. In total, two or more unique peptides were identified for 46, 68, and 22 surface membrane, intracellular membrane, and membrane proteins of unknown subcellular localization, respectively. The majority of these were single transmembrane proteins. To complement the proteomics studies, we analyzed the transcriptome of a highly purified preparation of mature primary mouse megakaryocytes using serial analysis of gene expression in view of the increasing importance of mutant mouse models in establishing protein function in platelets. This approach identified all of the major classes of platelet transmembrane receptors, including multitransmembrane proteins. Strikingly 17 of the 25 most megakaryocyte-specific genes (relative to 30 other serial analysis of gene expression libraries) were transmembrane proteins, illustrating the unique nature of the megakaryocyte/platelet surface. The list of novel plasma membrane proteins identified using proteomics includes the immunoglobulin superfamily member G6b, which undergoes extensive alternate splicing. Specific antibodies were used to demonstrate expression of the G6b-B isoform, which contains an immunoreceptor tyrosine-based inhibition motif. G6b-B undergoes tyrosine phosphorylation and association with the SH2 domain-containing phosphatase, SHP-1, in stimulated platelets suggesting that it may play a novel role in limiting platelet activation.

Interrelation between polyploidization and megakaryocyte differentiation: a gene profiling approach

Polyploidization is a part of the normal developmental process leading to platelet production during megakaryocyte (MK) differentiation. Ploidization is mainly involved in cell enlargement, but it is not clear whether gene expression is modified during MK ploidization. In this study, human MKs were grown from CD34(+) cells in the presence of thrombopoietin and sorted according to their ploidy level. A pangenomic microarray technique was applied to compare gene expression in 2N-, 4N-, 8N-, and 16N-sorted MKs. Using hierarchical clustering, we demonstrated that 2N and 4N MKs or 8N and 16N MKs are 2 different close populations with 105 discriminating genes. In the second approach, we determined the profile of genes that were continuously down- and up-regulated during polyploidization. Among the 100 down-regulated genes, 24 corresponded to genes involved in DNA replication and repair. The great majority of up-regulated genes corresponded to genes directly involved in platelet functions, such as genes encoding specific platelet glycoproteins and alpha-granule proteins, actin and microtubule cytoskeleton, factors involved in signaling, and transport proteins. Together, these results suggest that MK polyploidization per se does not regulate gene expression but is intrinsically included in the differentiation process.

Cord blood in vitro expanded CD41 cells: identification of novel components of megakaryocytopoiesis

BACKGROUND

Megakaryopoiesis represents a multi-step, often unclear, process leading to commitment, differentiation, and maturation of megakaryocytes (MKs) that release platelets.

AIM

To identify the novel genes that might help to clarify the molecular mechanisms of megakaryocytopoiesis and be regarded as potential candidates of inherited platelet defects, global gene expression of hematopoietic lineages was carried out.

METHODS

Human cord blood was used to purify CD34+ stem cells and in vitro expand CD41+ cells and burst-forming unit-erythroid (BFU-E). We investigated the expression profiles of these three hematopoietic lineages in the Affymetrix system and selected genes specifically expressed in MKs by comparing transcripts of the different lineages using the dchip and pam algorithms.

RESULTS

A detailed characterization of MK population showed that 99% of cells expressed the CD41 antigen whereas 73% were recognizable as terminally differentiated fetal MKs. The profile of these cells was compared with that of CD34+ cells and BFU-E allowing us to select 70 transcripts (MK-core), which represent not only the genes with a well-known function in MKs, but also novel genes never detected or characterized in these cells. Moreover, the specific expression was confirmed at both RNA and protein levels, thus validating the 'MK-core' isolated by informatics tools.

CONCLUSIONS

This is a global gene expression that for the first time depicts a well-characterized population of cord blood-derived fetal MKs. Novel genes have been detected, such as those encoding components of the extracellular matrix and basal membrane, which have been found in the cytoplasm of Mks, suggesting that new physiological aspects of MKs should be studied.

The biogenesis of platelets from megakaryocyte proplatelets

Platelets are formed and released into the bloodstream by precursor cells called megakaryocytes that reside within the bone marrow. The production of platelets by megakaryocytes requires an intricate series of remodeling events that result in the release of thousands of platelets from a single megakaryocyte. Abnormalities in this process can result in clinically significant disorders. Thrombocytopenia (platelet counts less than 150,000/microl) can lead to inadequate clot formation and increased risk of bleeding, while thrombocythemia (platelet counts greater than 600,000/microl) can heighten the risk for thrombotic events, including stroke, peripheral ischemia, and myocardial infarction. This Review will describe the process of platelet assembly in detail and discuss several disorders that affect platelet production.

Platelet genomics and proteomics in human health and disease

Proteomic and genomic technologies provide powerful tools for characterizing the multitude of events that occur in the anucleate platelet. These technologies are beginning to define the complete platelet transcriptome and proteome as well as the protein-protein interactions critical for platelet function. The integration of these results provides the opportunity to identify those proteins involved in discrete facets of platelet function. Here we summarize the findings of platelet proteome and transcriptome studies and their application to diseases of platelet function.

Platelet-Mimicry of Cancer Cells: Epiphenomenon with Clinical Significance

Stem cell mimicry of cancer cells has been known for a long time and is considered to be responsible for ectopic gene expressions. The stem cell characteristics of tumor cells are shown to be involved in epithelial-mesenchymal transition and in the phenomenon of vascular mimicry. Certain cancer types acquire a geno-phenotype closely resembling the platelets and express several megakaryocytic genes (adhesion receptors alpha IIb beta 3, thrombin receptor and PECAM/CD 31 and/or platelet-type 12-LOX) able to activate the coagulation cascade or the platelets themselves. Here we define these potentials as platelet mimicry of cancer cells typical of pancreatic, breast, prostate, colorectal and urogenital cancers and melanoma. Data all support that platelet mimicry of certain cancer types is an important factor in their hematogenous dissemination and provides an attractive therapeutic target. Besides the long-available preclinical data, clinical trials have only recently provided evidence that targeting platelet mimicry of cancers is an efficient way to prevent tumor progression. The systematic discovery of the markers of platelet mimicry in various cancer types and their molecular targeting may provide new supportive therapeutic modalities for the management of the progressing disease.

Mechanisms of thrombopoiesis

Megakaryocytes (MKs) expand and differentiate over several days in response to thrombopoietin (Tpo) before releasing innumerable blood platelets. The final steps in platelet assembly and release represent a unique cellular transformation that is orchestrated by a range of transcription factors, signaling molecules, and cytoskeletal elements. Here we review recent advances in the physiology and molecular basis of MK differentiation. Genome-wide approaches, including transcriptional profiling and proteomics, have been used to identify novel platelet products and differentiation markers. The extracellular factors, stromal-derived factor (SDF)-1 chemokine and fibroblast growth factor (FGF)-4 direct MK interactions with the bone marrow stroma and regulate cytokine-independent cell maturation. An abundance of bone marrow MKs induce pathologic states, including excessive bone formation and myelofibrosis, and the basis for these effects is now better appreciated. We review the status of transcription factors that control MK differentiation, with special emphasis on nuclear factor-erythroid 2 (NF-E2) and its two putative target genes, beta1-tubulin and 3-beta-hydroxysteroid reductase. MKs express steroid receptors and some estrogen ligands, which may constitute an autocrine loop in formation of proplatelets, the cytoplasmic protrusions within which nascent blood platelets are assembled. Finally, we summarize our own studies on cellular and molecular facets of proplatelet formation and place the findings within the context of outstanding questions about mechanisms of thrombopoiesis.

Transcriptional profiling of human hematopoiesis during in vitro lineage-specific differentiation

To better understand the transcriptional program that a ccompanies orderly lineage-specific hematopoietic differentiation, we performed serial oligonucleotide microarray analysis of human normal CD34+ bone marrow cells during lineage-specific differentiation. CD34+ bone marrow cells isolated from healthy individuals were selectively stimulated in vitro with the cytokines erythropoietin (EPO), thrombopoietin (TPO), granulocyte colony-stimulating factor (G-CSF), and granulocyte macrophage colony-stimulating factor (GM-CSF). Cells from each of the lineages were harvested after 4, 7, and 11 days of culture for expression profiling. Gene expression was analyzed by oligonucleotide microarrays (HG-U133A; Affymetrix, Santa Clara, CA). Experiments were done in triplicates. We identified 258 genes that are consistently upregulated or downregulated during the course of lineage-specific differentiation within each specific lineage (horizontal change). In addition, we identified 52 genes that contributed to a specific expression profile, yielding a genetic signature specific for successive stages of differentiation within each of the three lineages. Analysis of horizontal changes selected 21 continuously upregulated genes for EPO-induced differentiation (including GTPase activator proteins RAP1GA1 and ARHGAP8, which regulate small Rho GTPases), 21 for G-CSF-induced/GM-CSF-induced differentiation, and 91 for TPO-induced differentiation (including DLK1, of which the role in normal hematopoiesis is not defined). During the lineage-specific differentiation, 58 (erythropoiesis), 30 (granulopoiesis), and 37 (thrombopoiesis) genes were significantly downregulated, respectively. The expression of selected genes was confirmed by real-time polymerase chain reaction. Our data encompass the first extensive transcriptional profile of human hematopoiesis during in vitro lineage-specific differentiation.

Gene expression profiling of pediatric acute myelogenous leukemia

Contemporary treatment of pediatric acute myeloid leukemia (AML) requires the assignment of patients to specific risk groups. To explore whether expression profiling of leukemic blasts could accurately distinguish between the known risk groups of AML, we analyzed 130 pediatric and 20 adult AML diagnostic bone marrow or peripheral blood samples using the Affymetrix U133A microarray. Class discriminating genes were identified for each of the major prognostic subtypes of pediatric AML, including t(15;17)[PML-RARα], t(8;21)[AML1-ETO], inv16 [CBFβ-MYH11], MLL chimeric fusion genes, and cases classified as FAB-M7. When subsets of these genes were used in supervised learning algorithms, an overall classification accuracy of more than 93% was achieved. Moreover, we were able to use the expression signatures generated from the pediatric samples to accurately classify adult de novo AMLs with the same genetic lesions. The class discriminating genes also provided novel insights into the molecular pathobiology of these leukemias. Finally, using a combined pediatric data set of 130 AMLs and 137 acute lymphoblastic leukemias, we identified an expression signature for cases with MLL chimeric fusion genes irrespective of lineage. Surprisingly, AMLs containing partial tandem duplications of MLL failed to cluster with MLL chimeric fusion gene cases, suggesting a significant difference in their underlying mechanism of transformation.