Human megakaryocytes. I. Characterization of the membrane and cytoplasmic components of isolated marrow megakaryocytes

RepID represses megakaryocytic differentiation by recruiting CRL4A-JARID1A at DAB2 promoter

BACKGROUND

Megakaryocytes (MKs) are platelet precursors, which arise from hematopoietic stem cells (HSCs). While MK lineage commitment and differentiation are accompanied by changes in gene expression, many factors that modulate megakaryopoiesis remain to be uncovered. Replication initiation determinant protein (RepID) which has multiple histone-code reader including bromodomain, cryptic Tudor domain and WD40 domains and Cullin 4-RING E3 ubiquitin ligase complex (CRL4) recruited to chromatin mediated by RepID have potential roles in gene expression changes via epigenetic regulations. We aimed to investigate whether RepID-CRL4 participates in transcriptional changes required for MK differentiation.

METHODS

The PCR array was performed using cDNAs derived from RepID-proficient or RepID-deficient K562 erythroleukemia cell lines. Correlation between RepID and DAB2 expression was examined in the Cancer Cell Line Encyclopedia (CCLE) through the CellMinerCDB portal. The acceleration of MK differentiation in RepID-deficient K562 cells was determined by estimating cell sizes as well as counting multinucleated cells known as MK phenotypes, and by qRT-PCR analysis to validate transcripts of MK markers using phorbol 12-myristate 13-acetate (PMA)-mediated MK differentiation condition. Interaction between CRL4 and histone methylation modifying enzymes were investigated using BioGRID database, immunoprecipitation and proximity ligation assay. Alterations of expression and chromatin binding affinities of RepID, CRL4 and histone methylation modifying enzymes were investigated using subcellular fractionation followed by immunoblotting. RepID-CRL4-JARID1A-based epigenetic changes on DAB2 promoter were analyzed by chromatin-immunoprecipitation and qPCR analysis.

RESULTS

RepID-deficient K562 cells highly expressing MK markers showed accelerated MKs differentiation exhibiting increases in cell size, lobulated nuclei together with reaching maximum levels of MK marker expression earlier than RepID-proficient K562 cells. Recovery of WD40 domain-containing RepID constructs in RepID-deficient background repressed DAB2 expression. CRL4A formed complex with histone H3K4 demethylase JARID1A in soluble nucleus and loaded to the DAB2 promoter in a RepID-dependent manner during proliferation condition. RepID, CRL4A, and JARID1A were dissociated from the chromatin during MK differentiation, leading to euchromatinization of the DAB2 promoter.

CONCLUSION

This study uncovered a role for the RepID-CRL4A-JARID1A pathway in the regulation of gene expression for MK differentiation, which can form the basis for the new therapeutic approaches to induce platelet production. Video Abstract.

RepID represses megakaryocytic differentiation by recruiting CRL4A-JARID1A at DAB2 promoter

Background Megakaryocytes (MKs) are platelet precursors, which arise from hematopoietic stem cells (HSCs). While MK lineage commitment and differentiation are accompanied by changes in gene expression, many factors that modulate megakaryopoiesis remain to be uncovered. Replication origin binding protein (RepID) which has multiple histone-code reader including bromodomain, cryptic Tudor domain and WD40 domains and Cullin 4-RING ubiquitin ligase complex (CRL4) recruited to chromatin mediated by RepID have potential roles in gene expression changes via epigenetic regulations. We aimed to investigate whether RepID-CRL4 participates in transcriptional changes required for MK differentiation. Methods The PCR array was performed using cDNAs derived from RepID-proficient or RepID-deficient K562 erythroleukemia cell lines. Correlation between RepID and DAB2 expression was examined in the Cancer Cell Line Encyclopedia (CCLE) through the CellMinerCDB portal. The acceleration of MK differentiation in RepID-deficient K562 cells was determined by estimating cell sizes as well as counting multinucleated cells known as MK phenotypes, and by qRT-PCR analysis to validate transcripts of MK markers using phorbol 12-myristate 13-acetate (PMA)-mediated MK differentiation condition. Interaction between CRL4 and histone methylation modifying enzymes were investigated using BioGRID database, immunoprecipitation and proximity ligation assay. Alterations of expression and chromatin binding affinities of RepID, CRL4 and histone methylation modifying enzymes were investigated using subcellular fractionation followed by immunoblotting. RepID-CRL4-JARID1A-based epigenetic changes on DAB2 promoter were analyzed by chromatin-immunoprecipitation and qPCR analysis. Results RepID-deficient K562 cells highly expressing MK markers showed accelerated MKs differentiation exhibiting increases in cell size, lobulated nuclei together with reaching maximum levels of MK marker expression earlier than RepID-proficient K562 cells. Recovery of WD40 domain-containing RepID constructs in RepID-deficient background repressed DAB2 expression. CRL4A formed complex with histone H3K4 demethylase JARID1A in soluble nucleus and loaded to the DAB2 promoter in a RepID-dependent manner during proliferation condition. RepID, CRL4A, and JARID1A were dissociated from the chromatin during MK differentiation, leading to euchromatinization of the DAB2 promoter. Conclusion This study uncovered a role for the RepID-CRL4A-JARID1A pathway in the regulation of gene expression for MK differentiation, which can form the basis for the new therapeutic approaches to induce platelet production.

Dengue and the Lectin Pathway of the Complement System

Dengue is a mosquito-borne viral disease causing significant health and economic burdens globally. The dengue virus (DENV) comprises four serotypes (DENV1-4). Usually, the primary infection is asymptomatic or causes mild dengue fever (DF), while secondary infections with a different serotype increase the risk of severe dengue disease (dengue hemorrhagic fever, DHF). Complement system activation induces inflammation and tissue injury, contributing to disease pathogenesis. However, in asymptomatic or primary infections, protective immunity largely results from the complement system’s lectin pathway (LP), which is activated through foreign glycan recognition. Differences in N-glycans displayed on the DENV envelope membrane influence the lectin pattern recognition receptor (PRR) binding efficiency. The important PRR, mannan binding lectin (MBL), mediates DENV neutralization through (1) a complement activation-independent mechanism via direct MBL glycan recognition, thereby inhibiting DENV attachment to host target cells, or (2) a complement activation-dependent mechanism following the attachment of complement opsonins C3b and C4b to virion surfaces. The serum concentrations of lectin PRRs and their polymorphisms influence these LP activities. Conversely, to escape the LP attack and enhance the infectivity, DENV utilizes the secreted form of nonstructural protein 1 (sNS1) to counteract the MBL effects, thereby increasing viral survival and dissemination.

Endocytosed factor V is trafficked to CD42b+ proplatelet extensions during differentiation of human umbilical cord blood-derived megakaryocytes

Plasma- and platelet-derived factor Va are essential for thrombin generation catalyzed by the prothrombinase complex; however, several observations demonstrate that the platelet-derived cofactor, which is formed following megakaryocyte endocytosis and modification of the plasma procofactor, factor V, is more hemostatically relevant. Factor V endocytosis, as a function of megakaryocyte differentiation and proplatelet formation, was assessed by flow cytometry and microscopy in CD34⁺ hematopoietic progenitor cells isolated from human umbilical cord blood and cultured for 12 days in the presence of cytokines to induce ex vivo differentiation into megakaryocytes. Expression of an early marker of megakaryocyte differentiation, CD41, endocytosis of factor V, and the percentage of CD41⁺ cells that endocytosed factor V increased from days 6 to 12 of differentiation. In contrast, statistically significant decreases in expression of the stem cell marker, CD34, and in the percentage of CD34⁺ cells that endocytosed factor V were observed. A statistically significant increase in the expression of CD42b, a late marker of megakaryocyte differentiation, was also observed over time, such that by Day 12, all CD42b⁺ cells endocytosed factor V and expressed CD41. This endocytosed factor V was trafficked to proplatelet extensions and was localized in a punctate pattern in the cytoplasm consistent with its storage in α-granules. In conclusion, loss of CD34 and expression of CD42b define cells capable of factor V endocytosis and trafficking to proplatelet extensions during differentiation of megakaryocytes ex vivo from progenitor cells isolated from umbilical cord blood.

Purification of human megakaryocytes using a glycoprotein Ib dependent agglutination technique

In order to study normal and abnormal human megakaryocytopoiesis we modified a glycoprotein Ib dependent megakaryocyte purification technique originally described in guinea pigs. After Percoll density centrifugation, 2 ml bone marrow aspirates were subjected to 3 steps of bovine plasma induced cell clumping, which should selectively agglutinate glycoprotein Ib positive cells. In 13 experiments the resulting samples contained 28-102 x 10(3) megakaryocytes (median 70) with a median purity of 79%. Megakaryocyte recovery ranged from 30-86% (median 51%). The DNA profile showed median values of 10% for 8 N, 44% for 16 N and 43% for 32 N megakaryocytes. The described method is relatively simple, inexpensive and gives results comparable with more elaborate techniques.

MBL2 gene polymorphisms protect against development of thrombocytopenia associated with severe dengue phenotype

Dengue disease can clinically evolve from an asymptomatic and mild disease, known as dengue fever (DF), to a severe disease known as dengue hemorrhagic fever (DHF). Recent evidence has shown how host genetic factors can be correlated with severe dengue susceptibility or protection. Many of these genes, such as CD209, TNF-a, vitamin D receptor, and FC gamma receptor IIA, are components of the innate immune system, suggesting that innate responses might have a role in dengue pathogenesis. MBL2 gene polymorphisms have been shown to modulate susceptibility or protection in many viral diseases. We investigated the involvement of MBL2 gene in the dengue clinical outcome through the analysis of MBL2 exon 1 polymorphisms (at codons 52, 54, and 57) known to be associated with reduced serum levels of the MBL protein. The genotypes of 110 well-characterized dengue-positive patients were statistically analyzed to establish possible correlations between MBL2 polymorphisms and parameters such as sex, type of infection (primary or secondary response), race/ethnicity, course of infection, and age. We found significant correlations between wild-type AA MBL2 genotype and age as associated risk factors for development of dengue-related thrombocytopenia.

Expression and function of NJ-1 surface antigen in megakaryopoiesis

Immunostaining with NJ-1 monoclonal antibody (MoAb) revealed that NJ-1 is expressed on megakaryocytes (MKs). NJ-1-positive and lineage-negative progenitor cells have a higher potency to proliferate and differentiate into MKs. MKs were divided into NJ-1(+)MKs and NJ-1(-)MKs. NJ-1(+)MKs are immature MKs because of their low potential to generate pro-platelets. When cultured CD41-positive MK cells were analyzed with RT-PCR, we found that the expression of NJ-1 is down-regulated. NJ-1(+)MKs have a high adherent potential to endothelial cells comparing with NJ-1(-)MKs, and this binding ability could be inhibited by the NJ-1-Fc fusion protein. We hypothesize that NJ-1(+)MKs are immature MKs and the NJ-1 molecule is involved in MK adhesion to endothelial cells.

Expression and function of the collagen receptor GPVI during megakaryocyte maturation

In this report, the expression and function of the platelet collagen receptor glycoprotein VI (GPVI) were studied in human megakaryocytes during differentiation and maturation of mobilized blood and cord blood derived CD34(+) cells. By flow cytometry, using an anti-GPVI monoclonal antibody or convulxin, a GPVI-specific ligand, GPVI was detected only on CD41(+) cells including some CD41(+)/CD34(+) cells, suggesting expression at a stage of differentiation similar to CD41. These results were confirmed at the mRNA level using reverse transcription-polymerase chain reaction. GPVI expression was low during megakaryocytic differentiation but increased in the more mature megakaryocytes (CD41(high)). As in platelets, megakaryocyte GPVI associates with the Fc receptor gamma chain (FcRgamma). The FcR gamma chain was detected at the RNA and protein level at all stages of megakaryocyte maturation preceding the expression of GPVI. The other collagen receptor, alpha(2)beta(1) integrin (CD49b/CD29), had a pattern of expression similar to GPVI. Megakaryocytic GPVI was recognized as a 55-kDa protein by immunoblotting and ligand blotting, and thus it presented a slightly lower apparent molecular mass than platelet GPVI (58 kDa). Megakaryocytes began to adhere to immobilized convulxin via GPVI after only 8-10 days of culture, at a time when megakaryocytes were maturing. At this stage of maturation, they also adhered to immobilized collagen by alpha(2)beta(1) integrin-dependent and -independent mechanisms. Convulxin induced a very similar pattern of protein tyrosine phosphorylation in megakaryocytes and platelets including Syk, FcRgamma, and PLC(gamma)2. Our results showed that GPVI is expressed early during megakaryocytic differentiation but functionally allows megakaryocyte adherence to collagen only at late stages of differentiation when its expression increases.

Different expression of CD41 on human lymphoid and myeloid progenitors from adults and neonates

The glycoprotein (Gp) IIb/IIIa integrin, also called CD41, is the platelet receptor for fibrinogen and several other extracellular matrix molecules. Recent evidence suggests that its expression is much wider in the hematopoietic system than was previously thought. To investigate the precise expression of the CD41 antigen during megakaryocyte (MK) differentiation, CD34(+) cells from cord blood and mobilized blood cells from adults were grown for 6 days in the presence of stem cell factor and thrombopoietin. Two different pathways of differentiation were observed: one in the adult and one in the neonate cells. In the neonate samples, early MK differentiation proceeded from CD34(+)CD41(-) through a CD34(-)CD41(+)CD42(-) stage of differentiation to more mature cells. In contrast, in the adult samples, CD41 and CD42 were co-expressed on a CD34(+) cell. The rare CD34(+)CD41(+)CD42(-) cell subset in neonates was not committed to MK differentiation but contained cells with all myeloid and lymphoid potentialities along with long-term culture initiating cells (LTC-ICs) and nonobese diabetic/severe combined immune-deficient repopulating cells. In the adult samples, the CD34(+)CD41(+)CD42(-) subset was enriched in MK progenitors, but also contained erythroid progenitors, rare myeloid progenitors, and some LTC-ICs. All together, these results demonstrate that the CD41 antigen is expressed at a low level on primitive hematopoietic cells with a myeloid and lymphoid potential and that its expression is ontogenically regulated, leading to marked differences in the surface antigenic properties of differentiating megakaryocytic cells from neonates and adults. (Blood. 2001;97:2023-2030)

Megakaryocytes endocytose insulin-like growth factor (IGF) I and IGF-binding protein-3: a novel mechanism directing them into alpha granules of platelets

Lysis of platelets releases the contents of the alpha-granules, which contain growth factors, including insulin-like growth factor I (IGF-I) and IGF-binding protein-3 (IGFBP-3). We investigated the mechanism by which IGF-I and IGFBP-3 appeared in the alpha-granules with a goal of modulating their levels in platelets to affect platelet functions. Reverse transcription-PCR was initially used to test whether megakaryocytes contained IGFBP-3 and IGF-I messenger RNA transcripts. We found that megakaryocytes did not express the IGFBP-3 gene, but did have IGF-I messenger RNA. We subsequently investigated whether they incorporated IGFBP-3 and IGF-I by the process of endocytosis and packaged them into the alpha-granules. This hypothesis was tested in two ways. 1) We examined whether during pregnancy in the rat the alpha-granule content for IGFBP-3 paralleled the changes in plasma IGFBP-3 levels caused by the pregnancy-induced IGFBP-3 protease. The alpha-granule contents of both IGFBP-3 and IGF-I declined in parallel to the plasma changes in pregnant rats and returned to normal postpartum. As the binding protein protease acts extracellularly, endocytosis of the IGF-I:IGFBP-3 complex from the extracellular fluid by megakaryocytes was suggested. 2) We tested whether an IGF-I:IGFBP-3 complex comprised of human IGF-I and IGFBP-3 (recombinant 28.7 kDa) injected i.v. appeared in rat platelet alpha-granules. Hypophysectomized rats were injected i.v. with 5.24 mg of a 1:1 complex of IGF-I:IGFBP-3. After 24 h, platelet lysates were prepared and analyzed for IGFBP-3 by Western ligand blotting, and IGF-I was determined by RIA. Platelet lysates of the treated animals showed a prominent new band at approximately 28 kDa, whereas control rats were negative. In addition, the alpha-granule IGF-I concentration increased from 0.38 to 1.9 ng/1 x 10(9) platelets. These results indicate that the IGF-I:IGFBP-3 complex is taken up by megakaryocytes and packaged into the alpha-granules of platelets and demonstrate how the contents of IGF-I and IGFBP-3 in platelets can be modulated by their plasma concentrations. As reverse transcription-PCR has shown that the IGF-I, but not the IGFBP-3, gene is expressed by megakaryocytes, there may be two mechanisms for directing IGF-I into the alpha-granules of platelets.

Magnetic activated cell sorting (MACS)--a new immunomagnetic method for megakaryocytic cell isolation: comparison of different separation techniques

Megakaryocytes are difficult to isolate because of their fragility, their tendency to aggregate, and their varying sizes. For purification of cells at different stages of maturation and of different sizes (ploidy classes) we developed an immunomagnetic cell sorting method (MACS) to enrich the whole spectrum of the megakaryocytic cell lineage. The use of small magnetic beads coupled to various antibodies and labelling with fluorescent antibodies allowed direct analysis of enrichment and evaluation of the isolated fraction without further staining or detachment procedures. CD 61 (Y2/51), a monoclonal antibody directed against platelet glycoprotein IIIa, was employed to perform the separation procedure. An enrichment up to 47% of CD 61-positive cells with an average of 37% and a recovery rate of 37% was obtained by using the MACS technique. Pre-enrichment by Percoll density centrifugation, followed by MACS separation, resulted in an enrichment of 65% and a recovery rate of 67%. The relative amount of small megakaryocytic cells in only MACS-enriched cell populations, however, was higher than in Percoll/MACS fractions. As a parameter of vitality we tested cytokine secretion of the enriched megakaryocytes in reverse haemolytic plaque assays. Secretion of IL-1, IL-6, GM-CSF, and PDGF with and without stimulation by phorbol myristate acetate was demonstrable at the single cell level.

Regulation of megakaryocytopoiesis

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

Distribution of bacterial endotoxin in human and rabbit blood and effects of stroma-free hemoglobin

Bacterial endotoxin (lipopolysaccharide [LPS]) is known to interact with numerous components of blood, including erythrocytes, mononuclear cells, platelets, neutrophils, lipoproteins, and plasma proteins. The relative affinities of LPS for these elements, and the distribution of LPS between them, are unknown. Cross-linked stroma-free hemoglobin (SFH), a potential substitute for erythrocyte transfusion, produces in vivo toxicity in animals consistent with significant LPS contamination. Therefore, we studied the distribution of LPS in human and rabbit blood and examined whether the presence of SFH altered LPS distribution. In either the presence or absence of SFH, LPS was associated predominantly with high-density lipoproteins and apoproteins. There was lesser binding to low- and very-low-density lipoproteins. Examination of the apoprotein pool by column chromatography and density centrifugation demonstrated that LPS in this fraction was predominantly protein bound. Binding of LPS to SFH resulted in dissociation of a portion of the LPS into low-molecular-weight complexes. Cell-bound LPS was only 2 to 16% of the total and was unaffected by SFH. The distribution among blood cells demonstrated predominant binding to platelets in human blood but predominant binding to erythrocytes in rabbit blood. Cellular distribution was not significantly altered by SFH.

Replication of Colorado tick fever virus within human hematopoietic progenitor cells

Significant neutropenia, as well as thrombocytopenia and a mild anemia, occurs in patients infected with Colorado tick fever virus. In this study, human bone marrow CD34+ cells and KG-1a cells, a human hematopoietic progenitor cell line, were infected in vitro with Colorado tick fever virus. The time course and morphological appearance of viral replication in human progenitor cells were similar to those seen in erythroblasts and in HEL cells and suggest one possible mechanism for the clinical hematologic findings.

Population heterogeneity among cells of the megakaryocyte lineage

Understanding the developmental steps in megakaryocyte differentiation requires information regarding the microenvironmental influences which direct or permit the growth and differentiation of these cells. The megakaryocyte microenvironment, like other lineages, is a complex structure comprised of the various megakaryocytic cells, the extracellular matrix (ECM) surrounding them, and the hematopoietic stromal cells which elaborate both growth factors and ECM. As a result, definition of the minimal essential requirements for megakaryocyte development is difficult. The intricacies of megakaryocyte development are further complicated by the cellular heterogeneity of both mature megakaryocytes and their precursors, as well as a differential responsiveness of these cells to hematopoietic growth factors. This review focuses on defining the various subpopulations of megakaryocytic cells and examining their functional distinctions and in vitro responsiveness to various stimuli.

HLA-DR expression by platelets in acute idiopathic thrombocytopenic purpura

Induction of expression of MHC class II antigens on the surface of cells that do not ordinarily express these proteins has been implicated in the pathogenesis of autoimmunity in diabetes mellitus and autoimmune thyroiditis. Platelets express class I but not class II HLA antigens. In this report, we describe a child with acute idiopathic thrombocytopenic purpura who at the time of the thrombocytopenic episode had class II (HLA-DR) antigens on his platelets. Following recovery, the HLA-DR antigens were no longer present on the platelets. We postulated that class II had been induced on his megakaryocytes by a cytokine such as interferon gamma, and that the induced expression of class II antigens contributed to the autoimmune disorder. To substantiate this possibility we next studied class I and II antigen expression on an erythroleukaemia cell line (HEL), which has many megakaryocytic features. Following treatment of HEL cells with interferon gamma, class I expression was increased and HLA-DR antigens were induced. These observations suggest that cytokine-mediated induced HLA-DR expression may contribute to the pathogenesis of a subset of thrombocytopenias.

Identification of a normal human bone marrow cell population co-expressing megakaryocytic and erythroid markers in culture

A population of haematopoietic cells co-expressing glycoprotein IIIa (GPIIIa), which has been shown to be present in the megakaryocyte-platelet lineage, and glycophorin A, which has been shown to be specific for the erythroid lineage, has been identified in normal bone marrow cultures using double immunofluorescence staining. The cells showing this phenotype have the size of lymphocytes and appear at an early time (day 1 to d 6) of culture. We have also detected a type of mixed megakaryocyte (MK) cluster containing this phenotype on d 3 or 4 in most normal marrow samples. Such a cell phenotype was not detected after 6 d of culture. It is thus the first time that this phenotype, although already described for several human neoplastic cell lines, has been observed in several normal human bone marrow cultures. Cells expressing this phenotype may represent haematopoietic cells, common to megakaryocytic and erythroid lineages, at a very early stage of differentiation.

Phospholipid composition of rat megakaryocytes and its rearrangement in platelets

Rat platelets and their megakaryocyte precursors were examined for phospholipid composition. (1) The phospholipid composition of rat megakaryocytes, which were enriched and prepared from bone marrow cells, was almost identical to that of platelets. (2) The subclass composition of choline-containing glycerophospholipids (CGP) of rat megakaryocytes differed significantly from that of platelets: 1-alkenyl-2-acyl glycerophosphocholine (GPC) in megakaryocytes accounted for 29% of the total, whereas that in platelets was only 7%. (3) Rat platelets contained a larger amount of arachidonic acid than megakaryocytes, especially in ethanolamine-containing glycerophospholipids (EGP). (4) [32P]Phosphoric acid was significantly incorporated into megakaryocytes, whereas platelets showed little incorporation. On the other hand, the uptake of [3H]arachidonic acid into platelet phospholipids was about 15-times higher than that observed with megakaryocytes. (5) As reported previously for other blood cells, such as neutrophils and macrophages, the radioactivity of labeled arachidonic acid incorporated into CGP of platelets decreased, whereas that incorporated into EGP increased during a subsequent chase period. Hardly any such change was observed with megakaryocytes. These results suggest that the phospholipid composition of rat platelets is mainly determined at the time of thrombopoiesis, whereas the composition of molecular species is remodeled during circulation after thrombopoiesis.

Immunomagnetic bead isolation of megakaryocytes from guinea-pig bone marrow: effect of recombinant interleukin-6 on size, ploidy and cytoplasmic fragmentation

Guinea-pig bone marrow megakaryocytes were isolated using an antibody to platelet glycoprotein Ib and a second antibody conjugated to magnetic beads. The procedure yielded an average of 644,800 megakaryocytes from two guinea-pigs with an average viability of 83%. All of the platelet glycoprotein Ib positive cells also expressed the platelet glycoprotein IIb-IIIa complex. The size and ploidy of megakaryocytes isolated by this technique were analysed in the presence of 10 ng/ml of interleukin-6 (IL-6). Without IL-6 megakaryocyte size increased significantly after 24 h, but an even larger increase in size occurred in the presence of IL-6. The modal ploidy class was 16N with an average of 19% 2N, 2.6% 4N, 16.4% 8N, 50.8% 16N and 11.1% 32N cells as determined by flow cytometry. Measurements made by microspectrophotometry were in close agreement. After 24 h incubation there was a significant rise in the percentage of 2N and 32N cells. The ploidy distribution after 24 h with IL-6 was the same as the control. Megakaryocytes cultured in the absence of serum on collagen gels did not form pseudopods and fragment, as occurs with serum (Leven et al, 1987). Addition of IL-6 to the serum-free cultures caused megakaryocytes to form extensive proplatelet extensions. We conclude that large numbers of pure guinea-pig bone marrow megakaryocytes can be isolated by immunomagnetic bead selection, including low ploidy immature megakaryocytes. Spontaneous maturation occurred as evidenced by the increase in megakaryocyte size and ploidy. IL-6 altered megakaryocyte size and morphology but not ploidy, indicating that these different characteristics of megakaryocytes may be regulated separately.

Platelet alpha-granule fibrinogen, albumin, and immunoglobulin G are not synthesized by rat and mouse megakaryocytes

It has been assumed that endogenous synthesis by the platelet precursor cell, the bone marrow megakaryocyte, is the major source of platelet alpha-granule protein. To test this hypothesis, we used mRNA phenotyping to detect in megakaryocytes the presence of mRNA transcripts specific for various proteins. Our results indicate that megakaryocytes synthesize platelet factor 4, a protein relatively specific for platelets, but do not express mRNA transcripts for the fibrinogen, albumin, or IgG found in alpha-granules. We have previously shown that megakaryocytes endocytose circulating proteins, including fibrinogen, albumin, and IgG, and incorporate them into alpha-granules. Thus, platelets appear to contain a unique type of secretory granule whose contents originate by both endogenous synthesis and endocytosis from plasma. Under basal conditions, the source of alpha-granule fibrinogen is plasma.

Expression of CD4 by human megakaryocytes

The CD4 antigen, which serves as the receptor for human immunodeficiency virus type 1 (HIV-1) on T cells, has been detected on human megakaryocytes. Recent evidence of impaired thrombopoiesis in HIV-1-related thrombocytopenia suggested that these cells could be directly infected by the virus and prompted a search for a receptor on megakaryocytes of normal subjects that could permit entry of HIV-1. Bone marrow specimens from uninfected normal control subjects were centrifuged over Ficoll-Hypaque (1.077 g/ml) and analyzed by three-color analysis with a flow cytometer utilizing monoclonal antibodies against CD4 and a glycoprotein present on the surface of megakaryocytes and platelets (GPIIb/IIIa; CD41), as well as 7-aminoactinomycin D, a stain for DNA. Cells presumed to be megakaryocytes were identified by having a DNA content greater than tetraploid and staining brightly with anti-CD41. Approximately 0.4% of the nucleated cells of the marrow met these criteria. Twenty-five percent of these megakaryocytes stained as brightly as CD4+ T cells. Several clones of antibody recognizing different epitopes of the CD4 molecule gave similar results. Platelets were CD4-. Staining of megakaryocytes with anti-CD4 was confirmed by direct microscopic examination of Percoll-gradient-enriched megakaryocytes employing two-color (CD4-phycoerythrin and CD41-fluorescein) immunofluorescence analysis and phase-contrast microscopy. The proportion of double-labeled cells among 112 phase-contrast-identifiable megakaryocytes from five bone marrow specimens varied between 20% and 26% with a mean and SD of 22% +/- 2.5%. Thus some human megakaryocytes express CD4 on their surface that should be capable of binding the HIV-1 gp120 envelope protein. This could serve as a portal of entry for HIV-1.

Megakaryocytes of human immunodeficiency virus-infected individuals express viral RNA

The pathogenesis of thrombocytopenia associated with human immunodeficiency virus (HIV) infection is not fully understood. Immune mechanisms provide a partial explanation but fail to account for a lack of compensatory megakaryocytosis, the rapid reversal after treatment with azidothymidine, and the ultrastructural aberrations seen in the megakaryocytes of patients with acquired immunodeficiency syndrome. Therefore, a direct effect of HIV on megakaryocytes was investigated. The bone marrow of HIV seropositive individuals was analyzed ultrastructurally, and the megakaryocytes of 10 thrombocytopenic patients were subjected to in situ hybridization with a HIV RNA probe. The structural aberrations in HIV megakaryocytes were distinct from those in HIV-negative immune thrombocytopenias, and the megakaryocytes of 10 of 10 patients examined by in situ hybridization unambiguously expressed viral RNA. Therefore, it is likely that direct infection of megakaryocytes with HIV-1 is one mechanism for the decrease in platelet production.

Megakaryocyte function and dysfunction

More than a hundred years have passed since platelets were recognized as cells and their haemostatic functions discovered. However, the process of platelet production is still not understood. The location, the mechanism and the regulation of thrombopoiesis remain elusive. Megakaryocytes are known to be the source of platelets. Investigations of megakaryocytes have revealed their normal functions and some of the abnormalities present in various diseases which affect platelets. In recent years, new techniques of cell isolation and tissue culture have been developed and have made possible advances in characterizing megakaryocyte precursors and differentiation. The primary function of megakaryocytes is to synthesize and assemble platelet components and organelles. Although debated for a long time, new data seems to indicate that the lung may be a central locus of platelet production. The new techniques for megakaryocyte investigations have barely begun to be of use in the study of abnormal platelet production in disease.

Neutrophil and monocyte cell surface p150,95 has iC3b-receptor (CR4) activity resembling CR3

Previous investigations of p150,95 (CD11c), the third member of the CD18 membrane glycoprotein family that includes CR3 (Mac-1 or CD11b) and LFA-1 (CD11a), had demonstrated that solubilized p150,95 bound to iC3b-agarose in a manner similar to isolated CR3. The current study showed that membrane surface p150,95 also expressed iC3b-receptor activity and was probably the same as the neutrophil receptor for iC3b- or C3dg-coated erythrocytes (EC3bi or EC3dg) that had been previously designated CR4. Normal neutrophil and macrophage CR4-dependent EC3bi rosettes were inhibited by monoclonal anti-p150,95, and cells from a patient with CD18 deficiency did not form CR4-dependent EC3bi rosettes. With neutrophils that bore large amounts of CR1 and CR3 and little p150,95, EC3bi were found primarily via CR1 and CR3, and demonstration of p150,95-dependent rosettes required large amounts of fixed iC3b, low-ionic strength buffer, and antibody blockade of CR1 and CR3. By contrast, culture-derived macrophages expressed eight times more p150,95 than did monocytes and EC3bi were bound to both p150,95 and CR3 when EC3bi bore small amounts of fixed iC3b and assays were carried out in isotonic buffer. Comparison of the amounts of CR1, CR3, and CR4 in various tissues by immunoperoxidase staining revealed that CR4 was the most abundant C3 receptor molecule on tissue macrophages, and suggested that CR4 might be involved in clearance of C3-opsonized particles or immune complexes.

Acute leukemia of megakaryocyte lineage with tumor formation. An autopsy case of patient with Down's syndrome

In this report we present the autopsy findings of acute megakaryoblastic leukemia with tumor formation in a 2-year-old female infant with Down's syndrome. Chromosomal analysis of blast cells revealed constitutional anomaly of trisomy 21 and two other related types of abnormal clones. Flow cytometric examination revealed blast cells expressing Ia-like or HLA-DR antigens. Postmortem examination showed extensive infiltration of leukemic cells in most of the examined organs, including the bone marrow with myelofibrosis. Tumor masses in the maxillary, frontal and femoral bones and the atria of the heart had undergone massive infiltration of atypical blast cells with an increase in the reticulin network. The final diagnosis was confirmed by ultrastructural cytochemistry of the platelet peroxidase reaction as well as by immunological staining utilizing anti-platelet glycoprotein IIb/IIIa, antiplatelet factor 4 and anti-beta-thromboglobulin antibodies for the blast cells. It seems likely that platelet-derived growth factor, secondary to an increase in the reticulin network, plays a major role in myelofibrosis of acute megakaryoblastic leukemia with tumor formation.

Diploid and tetraploid precursors of megakaryocytes in normal human bone marrow detected by immunofluorescence

A sequential preparation method is described which allows immunological identification, morphological characterization, cytophotometric determination of relative DNA content of the megakaryocyte lineage as well as quantitation of megakaryocyte precursors in human bone marrow aspirates. We compared several monoclonal (anti-GP IIIa and HD 19) and polyclonal (A225, RAHPS) antiplatelet antibodies for immunofluorescent staining. Among the identified cells, a small number of cells showing a diploid and tetraploid DNA content were found which must be regarded as promegakaryoblasts, representing 2.5-4.7% of all megakaryocytes. The heterogenous morphology of these precursors in panoptically stained smears is described.

Effects of hypoxia on megakaryocytopoiesis and granulopoiesis

Previous studies have reported that exposure of mice to hypoxic conditions results in a decrease in blood platelets. To further explore the effect of hypoxia on megakaryocytopoiesis, megakaryocytes and their progenitor cells (CFU-M) were studied in hypoxic mice. Mice were exposed to hypoxia by enclosure in cages covered with dimethyl-silicone rubber membranes for up to 10 d. At various times during the hypoxic and ex-hypoxic periods the total megakaryocytes and CFU-M were determined in the humerus and spleen. CFU-M were assayed in the soft gel colony forming assay using pokeweed mitogen-stimulated spleen cell conditioned medium (PWCM) as a source of colony stimulating activity. After 10 d of hypoxia, packed red cell volume (PRCV) increased to 148% of control levels and blood platelets decreased to 40% of controls. Total megakaryocytes and CFU-M per humerus decreased to 18% and 50% of controls respectively. 4 d into the ex-hypoxic phase, PRCV was still increased at 128% of controls while marrow megakaryocytes and CFU-M increased to normal levels. Platelet recovery was somewhat slower, returning to normal by d 6. In contrast to the findings in the marrow, total spleen megakaryocytes and CFU-M increased to about 3- and 5-fold of control levels respectively by 6 d of hypoxia. During the exhypoxic phase, CFU-M decreased to normal on d 4, followed by a rebound of 3-fold control values on d 8. Spleen megakaryocytes decreased more slowly, returning to normal by d 10. A marked granulocytosis was observed during the hypoxic phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Antigenic differences between human platelets and megakaryocytes

To investigate the apparent paradox in the observation that most patients with immune thrombocytopenias have normal or increased numbers of megakaryocytes (MKs), the extent of antigenic cross-reactivity between normal platelets and MK was examined. Indirect immunofluorescence and ultrastructural studies were carried out by means of four antisera specific for platelets: anti-GpIb, anti-GpIIb/IIIa, anti-PLA1, and an antiserum from a patient with quinidine-induced thrombocytopenia. Following incubation of freshly collected marrow with these antisera, MK were first identified by phase-contrast microscopy and then inspected for fluorescence. Almost all MKs were found reactive with the last three antisera, albeit to a variable extent. In contrast, only 24% reacted with anti-GpIb. The pattern of fluorescence, ie, rim, partial or cytoplasmic, appeared to be related to the extent of MK fragmentation. Only rim fluorescence of living MKs could be interpreted to indicate that the platelet epitope was exposed on the surface of the precursor cell. The observations suggest that platelet antigens are variably expressed on the plasma membranes of MKs. In a clinical setting, the heterogeneity among platelet target antigens and the extent to which these are exposed on MKs at various stages of maturation may dictate the severity of the thrombocytopenia and degree of ineffective thrombocytopoiesis.

Biosynthesis of major platelet proteins in human blood platelets

We studied de novo protein biosynthesis in platelets of normal adult donors and in newly formed platelets isolated from splenectomized patients with idiopathic thrombocytopenic purpura (ITP). After metabolic labelling of platelet proteins, performed with different radiolabelled amino acids or carbohydrates, a tenfold increase in incorporation of radioactivity into trichloroacetic-acid-precipitable material was obtained with ITP platelets compared to control platelets. Electron microscopic studies of ITP platelets revealed the presence of rough endoplasmic reticulum and polyribosomes, providing morphological evidence for protein synthesis. SDS-PAGE of radiolabelled ITP platelet proteins followed by autoradiography showed that [35S]methionine and [3H]leucine were incorporated into almost all Coomassie-blue-stained proteins whereas [3H]carbohydrates only labelled a few bands. Using crossed-immunoelectrophoresis we identified some of the labelled platelet compounds and demonstrated that major membrane glycoproteins (GPIb, IIb, IIIa) and alpha-granule proteins, such as fibrinogen, thrombospondin, albumin and von Willebrand factor, were synthesized in newly formed circulating platelets.

Human megakaryocytic progenitor cells

Megakaryocytopoiesis represents one of several differentiation pathways that hematopoietic stem cells may enter. Cells representing intermediate stages of differentiation between pluripotent stem cells and maturing megakaryocytes are called megakaryocytic progenitor cells. They are identified in human bone marrow and peripheral blood by their ability to proliferate in culture (colony forming unit-megakaryocyte, CFU-M); at some point they lose the capacity for cell division and acquire the ability for endoreduplication of DNA, a phenomenon that is unique to the megakaryocyte lineage. This review summarizes current understanding of the biology of human megakaryocytic progenitor cells, including characterization of their proliferation potentials, their antigenic determinants, and the mechanisms that govern their proliferation and maturation. Finally the involvement of CFU-M in various disorders of thrombopoiesis is discussed.

Glycoprotein Ib distribution on the surface of platelets in resting and activation states: an electron microscope study

Using a monoclonal antibody (TM60) against glycoprotein (GP) Ib, we determined immunocytochemically how GPIb is distributed on the platelet surface. When glutaraldehyde-fixed platelets were incubated with TM60, a uniform distribution of ferritin particles which represent the localization of GPIb was observed on the surface membrane of platelets. The particles were distributed at intervals of about 100 nm. The number of ferritin particles on the surface of one side were 2070-4150 (2940 +/- 790; mean +/- S.D., n = 10) under the scanning electron microscope. The distribution of ferritin particles was somewhat disarranged on the surface of unfixed platelets incubated with TM60 compared to that in the fixed platelets. Cluster-like structures of ferritin particles were observed in several places. When platelets were activated with ristocetin or thrombin, the distribution of ferritin particles was disturbed and cluster formation was observed in several places on the surface. These findings suggest that GPIb is uniformly distributed on the surface of platelets in the resting state, and that cluster formation occurs during activation of platelets.

Incorporation of a circulating protein into megakaryocyte and platelet granules

To determine whether or not proteins circulating in plasma can be incorporated into megakaryocytes and platelets, horseradish peroxidase (HRP) was injected intravenously into guinea pigs and these cells were examined for its uptake by electron microscopy and cytochemistry. Enriched samples of megakaryocytes enabled ultrastructural analysis of large numbers of these rare cells. In megakaryocytes, 50% of alpha granules contained HRP between 75 min and 7 hr after injection. At 24 hr, 25% of the megakaryocyte granules were peroxidase-positive, less were positive by 48 hr, and there were none at 4 days. Thus, the findings demonstrate that a circulating protein can be endocytosed by megakaryocytes and rapidly packaged into alpha granules. Platelet granules also contain HRP by 7 hr after injection, and they can secrete it in response to thrombin. Unfortunately, our present studies do not allow us to distinguish between direct endocytosis by the platelet and/or shedding of new platelets from recently labeled megakaryocytes. It is concluded that while some alpha granule proteins are synthesized by megakaryocytes, others may be acquired from plasma by endocytosis. In addition to providing evidence that some of the proteins of alpha granules may be of exogenous origin, this study has allowed the definition of a pathway whereby plasma proteins may be temporarily sequestered in megakaryocytes before reentering the circulation in platelets.

Megakaryopoiesis disturbances in atherosclerotic rabbits

Rabbits given a hypercholesterolemic diet (500 mg/day) for 6 months and then maintained for another 6 months on a normal diet were found to have developed fibrous lipidic lesions in the aorta. Although circulating platelet levels in these animals were normal there was a reduction in mean megakaryocyte ploidy. The high concentrations of megakaryoblasts in all the sedimentation fractions collected by the 'STAPUT' system suggested an increase in megakaryocyte turnover with activation of committed stem cells. In addition, other defects in maturation of megakaryocytes were observed, such as abnormalities in the demarcation membrane system and granule number. These data reveal that defects in megakaryocyte maturation and turnover may occur during the process of reparative fibrosis of the arterial tree following a period of moderate hypercholesterolemic diet in the rabbit.

Detection of cells of megakaryocyte lineage in haematological malignancies by immuno-alkaline phosphatase labelling cell smears with a panel of monoclonal antibodies

Immuno-alkaline phosphatase staining (by the APAAP technique) has been used to identify promegakaryoblasts in cell smears from 10 cases of leukaemia (three acute leukaemia, seven blast transformations). In all cases promegakaryoblasts were labelled by at least two anti-platelet glycoprotein (gp) antibodies, the highest percentages being obtained with anti-gp IIIa (antibody C17). HLA-DR was expressed by a variable percentage of neoplastic cells in all cases, the T11 (CD2) antigen (sheep red cell receptor) in four of seven cases tested and the p150,95 antigen in three of the six cases tested. In some cases of acute myeloid leukaemia APAAP staining of blood smears revealed circulating promegakaryoblasts and micromegakaryocytes (which superficially resemble small lymphoid cells). It is concluded that immuno-alkaline phosphatase staining of cell smears offers a convenient means of diagnosing acute megakaryoblastic leukaemia in the routine laboratory.

Expression of the fibrinogen genes in rat megakaryocytes

A variety of evidence suggests that megakaryocytes synthesize fibrinogen and comparative immunochemical and structural studies indicate that fibrinogen produced in or associated with megakaryocytes may be different than fibrinogen produced in the liver. Two studies have reported that the gamma' chain, which is produced from the gamma chain gene by alternative splicing, is absent from fibrinogen produced in the megakaryocyte. Since there is only a single gene for each of the three fibrinogen chains the reported structural differences suggest different mechanisms for production of hepatic and megakaryocytic fibrinogen. We have begun an investigation of the varying mechanisms for expression of the fibrinogen genes by examining the structure of fibrinogen mRNA's in the two tissues. Fibrinogen mRNA's of identical length are found in both liver and megakaryocytes. Furthermore, despite the reported absence of the gamma' chain in platelet-associated fibrinogen, we have used a probe specific for the alternative spliced region of the gamma' mRNA to clearly demonstrate this chain in megakaryocyte mRNA. These studies indicate that the gamma' mRNA is either not translated in platelets or that the gamma' chain is unable to associated with the alpha and beta chains to form a mature molecule.

Inverse relationship between megakaryocyte buoyant density and maturity

We examined the relationship between rat megakaryocyte buoyant density and maturation stage in continuous Percoll density gradients. An average of 88% of megakaryocytes had buoyant densities less than 1.054 g/ml. There was an inverse relationship between megakaryocyte buoyant density and maturation. Morphologically mature forms comprised 90% of the megakaryocytes with buoyant densities of 1.030-1.033 g/ml. In contrast, immature morphology was present in three-quarters of megakaryocytes with buoyant densities of 1.042-1.046 g/ml. These morphological findings were confirmed by [3H]thymidine labelling studies. Cell viability assessed by trypan blue exclusion was highest among more dense megakaryocytes of which the majority were immature. The lowest trypan blue exclusion was found in the less dense, predominantly mature megakaryocytes indicating that these cells are more susceptible to membrane damage during marrow suspension. Megakaryocyte DNA content distributions and platelet antigen levels, determined by two-colour flow cytometry, were also related to megakaryocyte density; the more dense megakaryocytes showed an approximately two-fold higher proportion of 8N cells and less platelet antibody binding than did less dense megakaryocytes. These studies suggest that megakaryocytes can be fractionated according to their buoyant densities into immature and mature populations suitable for molecular studies of differentiation.

Fluorescence-activated sorting of individual cells onto poly-L-lysine-coated slide areas

Cells sorted by a fluorescence-activated cell sorter are collected onto small areas of a glass slide. These collection areas have been coated with poly-L-lysine to attach the cells firmly to the glass surface. This simple procedure proved to be suitable to sort single cells and small cell populations with preservation of cytomorphology and viability without modifying the cell sorter. Additional studies on sorted cells may be performed, as shown by peroxidase-anti-peroxidase analysis of cellular antigens and by mRNA in situ hybridization.