Characterization of the hematopoietic transcription factor NF-E2 in primary murine megakaryocytes

Investigation of the mechanism of action of Shengxuexiaoban Capsules against primary immune thrombocytopenia using network pharmacology and experimental validation

BACKGROUND

Shengxuexiaoban Capsules (SC) is a classical prescription in traditional Chinese medicine (TCM) and has been clinically adopted in the treatment of primary immune thrombocytopenia (ITP) in China. However, the underlying mechanisms of the actions of SC on ITP remain clear.

METHODS

A network pharmacology approach was adopted to investigate the underlying molecular mechanism of SC in treating ITP, and the effects of SC on the proliferation, differentiation, and apoptosis of megakaryocyte (MK) and on the ITP animal model were investigated.

RESULTS

Network pharmacology analysis found 128 active compounds and 268 targets of these compounds in SC, as well as 221 ITP-related targets. The topological analysis found a central network containing 82 genes, which were significantly associated with the regulation of transcription, cell proliferation, apoptosis processes, the PI3K-AKT signaling pathway, the MAPK signaling pathway, and the ERK1 and ERK2 cascades. It showed that SC increased the proliferation and differentiation of MK, but had no significant impact on MK apoptosis in vivo. The addition of SC increased the gene expression of several potential targets, including STAT3, KDR, CASP3, and TGFB1. In addition, SC administration elevated the protein expression of p-AKT and inhibit the protein expression of p-ERK, but has no impact on the protein expression of p-P38. Moreover, SC could improve haemogram parameters, coagulation indicators, and the proliferation and differentiation of MK in the ITP animal model.

CONCLUSIONS

The present study systematically elucidated the underlying mechanisms of SC against ITP and provided an efficient strategy to discover the pharmacological mechanism of TCM. It may strengthen the understanding of SC and facilitate more application of this formula in the treatment of ITP.

Adipocyte Fatty Acid Transfer Supports Megakaryocyte Maturation

Megakaryocytes (MKs) come from a complex process of hematopoietic progenitor maturation within the bone marrow that gives rise to de novo circulating platelets. Bone marrow microenvironment contains a large number of adipocytes with a still ill-defined role. This study aims to analyze the influence of adipocytes and increased medullar adiposity in megakaryopoiesis. An in vivo increased medullar adiposity in mice caused by high-fat-diet-induced obesity is associated to an enhanced MK maturation and proplatelet formation. In vitro co-culture of adipocytes with bone marrow hematopoietic progenitors shows that delipidation of adipocytes directly supports MK maturation by enhancing polyploidization, amplifying the demarcation membrane system, and accelerating proplatelet formation. This direct crosstalk between adipocytes and MKs occurs through adipocyte fatty acid transfer to MKs involving CD36 to reinforce megakaryocytic maturation. Thus, these findings unveil an influence of adiposity on MK homeostasis based on a dialogue between adipocytes and MKs.

The molecular basis of platelet biogenesis, activation, aggregation and implications in neurological disorders

Platelets are anucleated blood constituents, vital for hemostasis and involved in the pathophysiology of several cardiovascular, neurovascular diseases as well as inflammatory processes and metastasis. Over the past few years, the molecular processes that regulate the function of platelets in hemostasis and thrombosis have emerged revealing platelets to be perhaps more complex than may have been expected. The most understood part of platelets is to respond to a blood vessel injury by altering shape, secreting granule contents, and aggregating. These responses, while advantageous for hemostasis, can become detrimental when they root ischemia or infarction. Only a few transcription and signaling factors involved in platelet biogenesis have been identified till date. Platelets encompass an astonishingly complete array of organelles and storage granules including mitochondria, lysosomes, alpha granules, dense granules, a dense tubular system (analogous to the endoplasmic reticulum of nucleated cells); a highly invaginated plasma membrane system known as the open canalicular system (OCS) and large fields of glycogen. Platelets as a model cells to study neurological disorders have been recommended by several researchers since several counterparts exist between platelets and the brain, which make them interesting for studying the neurobiology of various neurological disorders. This review has been compiled with an aim to integrate the latest research on platelet biogenesis, activation and aggregation focusing on the molecular pathways that power and regulate these processes. The dysregulation of important molecular players affecting fluctuating platelet biology and thereby resulting in neurovascular diseases has also been discussed.

Nuclear Factor Erythroid 2 Regulates Human HSC Self-Renewal and T Cell Differentiation by Preventing NOTCH1 Activation

Nuclear factor erythroid-derived 2 (NF-E2) has been associated with megakaryocyte maturation and platelet production. Recently, an increased in NF-E2 activity has been implicated in myeloproliferative neoplasms. Here, we investigate the role of NF-E2 in normal human hematopoiesis. Knockdown of NF-E2 in the hematopoietic stem and progenitor cells (HSPCs) not only reduced the formation of megakaryocytes but also drastically impaired hematopoietic stem cell activity, decreasing human engraftment in immunodeficient (NSG) mice. This phenotype is likely to be related to both increased cell proliferation (p21-mediated) and reduced Notch1 protein expression, which favors HSPC differentiation over self-renewal. Strikingly, although NF-E2 silencing in HSPCs did not affect their myeloid and B cell differentiation in vivo, it almost abrogated T cell production in primary hosts, as confirmed by in vitro studies. This effect is at least partly due to Notch1 downregulation in NF-E2-silenced HSPCs. Together these data reveal that NF-E2 is an important driver of human hematopoietic stem cell maintenance and T lineage differentiation.

MicroRNAs 10a and 10b Regulate the Expression of Human Platelet Glycoprotein Ibα for Normal Megakaryopoiesis

MicroRNAs are a class of small non-coding RNAs that bind to the three prime untranslated region (3′-UTR) of target mRNAs. They cause a cleavage or an inhibition of the translation of target mRNAs, thus regulating gene expression. Here, we employed three prediction tools to search for potential miRNA target sites in the 3′-UTR of the human platelet glycoprotein (GP) 1BA gene. A luciferase reporter assay shows that miR-10a and -10b sites are functional. When miR-10a or -10b mimics were transfected into the GP Ibβ/GP IX-expressing cells, along with a DNA construct harboring both the coding and 3′-UTR sequences of the human GP1BA gene, we found that they inhibit the transient expression of GP Ibα on the cell surface. When the miR-10a or -10b mimics were introduced into murine progenitor cells, upon megakaryocyte differentiation, we found that GP Ibα mRNA expression was markedly reduced, suggesting that a miRNA-induced mRNA degradation is at work. Thus, our study identifies GP Ibα as a novel target of miR-10a and -10b, suggesting that a drastic reduction in the levels of miR-10a and -10b in the late stage of megakaryopoiesis is required to allow the expression of human GP Ibα and the formation of the GP Ib-IX-V complex.

NF-E2, FLI1 and RUNX1 collaborate at areas of dynamic chromatin to activate transcription in mature mouse megakaryocytes

Mutations in mouse and human Nfe2, Fli1 and Runx1 cause thrombocytopenia. We applied genome-wide chromatin dynamics and ChIP-seq to determine these transcription factors’ (TFs) activities in terminal megakaryocyte (MK) maturation. Enhancers with H3K4me2-marked nucleosome pairs were most enriched for NF-E2, FLI and RUNX sequence motifs, suggesting that this TF triad controls much of the late MK program. ChIP-seq revealed NF-E2 occupancy near previously implicated target genes, whose expression is compromised in Nfe2-null cells, and many other genes that become active late in MK differentiation. FLI and RUNX were also the motifs most enriched near NF-E2 binding sites and ChIP-seq implicated FLI1 and RUNX1 in activation of late MK, including NF-E2-dependent, genes. Histones showed limited activation in regions of single TF binding, while enhancers that bind NF-E2 and either RUNX1, FLI1 or both TFs gave the highest signals for TF occupancy and H3K4me2; these enhancers associated best with genes activated late in MK maturation. Thus, three essential TFs co-occupy late-acting cis-elements and show evidence for additive activity at genes responsible for platelet assembly and release. These findings provide a rich dataset of TF and chromatin dynamics in primary MK and explain why individual TF losses cause thrombopocytopenia.

Hyaluronan Depolymerization by Megakaryocyte Hyaluronidase-2 Is Required for Thrombopoiesis

Hyaluronan is the predominant glycosaminoglycan component of the extracellular matrix with an emerging role in hematopoiesis. Modulation of hyaluronan polymer size is responsible for its control over cellular functions, and the balance of hyaluronan synthesis and degradation determines its molecular size. Although two active somatic hyaluronidases are expressed in mammals, only deficiency in hyaluronidase-2 (Hyal-2) results in thrombocytopenia of unknown mechanism. Our results reveal that Hyal-2 knockout mice accumulate hyaluronan within their bone marrow and within megakaryocytes, the cells responsible for platelet generation. Proplatelet formation by Hyal-2 knockout megakaryocytes was disrupted because of abnormal formation of the demarcation membrane system, which was dilated and poorly developed. Importantly, peptide-mediated delivery of exogenous hyaluronidase rescued deficient proplatelet formation in murine and human megakaryocytes lacking Hyal-2. Together, our data uncover a previously unsuspected mechanism of how hyaluronan and Hyal-2 control platelet generation.

Regulation and function of the NFE2 transcription factor in hematopoietic and non-hematopoietic cells

The NFE2 transcription factor was identified over 25 years ago. The NFE2 protein forms heterodimers with small MAF proteins, and the resulting complex binds to regulatory elements in a large number of target genes. In contrast to other CNC transcription family members including NFE2L1 (NRF1), NFE2L2 (NRF2) and NFE2L3 (NRF3), which are widely expressed, earlier studies had suggested that the major sites of NFE2 expression are hematopoietic cells. Based on cell culture studies it was proposed that this protein acts as a critical regulator of globin gene expression. However, the knockout mouse model displayed only mild erythroid abnormalities, while the major phenotype was a defect in megakaryocyte biogenesis. Indeed, absence of NFE2 led to severely impaired platelet production. A series of recent data, also summarized here, shed new light on the various functional roles of NFE2 and the regulation of its activity. NFE2 is part of a complex regulatory network, including transcription factors such as GATA1 and RUNX1, controlling megakaryocytic and/or erythroid cell function. Surprisingly, it was recently found that NFE2 also has a role in non-hematopoietic tissues, such as the trophoblast, in which it is also expressed, as well as the bone, opening the door to new research areas for this transcription factor. Additional data showed that NFE2 function is controlled by a series of posttranslational modifications. Important strides have been made with respect to the clinical significance of NFE2, linking this transcription factor to hematological disorders such as polycythemias.

Developmental transcriptome analysis of human erythropoiesis

To globally survey the changes in transcriptional landscape during terminal erythroid differentiation, we performed RNA sequencing (RNA-seq) on primary human CD34(+) cells after ex vivo differentiation from the earliest into the most mature erythroid cell stages. This analysis identified thousands of novel intergenic and intronic transcripts as well as novel alternative transcript isoforms. After rigorous data filtering, 51 (presumptive) novel protein-coding transcripts, 5326 long and 679 small non-coding RNA candidates remained. The analysis also revealed two clear transcriptional trends during terminal erythroid differentiation: first, the complexity of transcript diversity was predominantly achieved by alternative splicing, and second, splicing junctional diversity diminished during erythroid differentiation. Finally, 404 genes that were not known previously to be differentially expressed in erythroid cells were annotated. Analysis of the most extremely differentially expressed transcripts revealed that these gene products were all closely associated with hematopoietic lineage differentiation. Taken together, this study will serve as a comprehensive platform for future in-depth investigation of human erythroid development that, in turn, may reveal new insights into multiple layers of the transcriptional regulatory hierarchy that controls erythropoiesis.

Rho kinase inhibition drives megakaryocyte polyploidization and proplatelet formation through MYC and NFE2 downregulation

The processes of megakaryocyte polyploidization and demarcation membrane system (DMS) formation are crucial for platelet production, but the mechanisms controlling these processes are not fully determined. Inhibition of Rho kinase (ROCK) signalling leads to increased polyploidization in umbilical cord blood-derived megakaryocytes. To extend these findings we determined the effect of ROCK inhibition on development of the DMS and on proplatelet formation. The underlying mechanisms were explored by analysing the effect of ROCK inhibition on the expression of MYC and NFE2, which encode two transcription factors critical for megakaryocyte development. ROCK inhibition promoted DMS formation, and increased proplatelet formation and platelet release. Rho kinase inhibition also downregulated MYC and NFE2 expression in mature megakaryocytes, and this down-regulation correlated with increased proplatelet formation. Our findings suggest a model whereby ROCK inhibition drives polyploidization, DMS growth and proplatelet formation late in megakaryocyte maturation through downregulation of MYC and NFE2 expression.

JAK2V617F leads to intrinsic changes in platelet formation and reactivity in a knock-in mouse model of essential thrombocythemia

The principal morbidity and mortality in patients with essential thrombocythemia (ET) and polycythemia rubra vera (PV) stems from thrombotic events. Most patients with ET/PV harbor a JAK2V617F mutation, but its role in the thrombotic diathesis remains obscure. Platelet function studies in patients are difficult to interpret because of interindividual heterogeneity, reflecting variations in the proportion of platelets derived from the malignant clone, differences in the presence of additional mutations, and the effects of medical treatments. To circumvent these issues, we have studied a JAK2V617F knock-in mouse model of ET in which all megakaryocytes and platelets express JAK2V617F at a physiological level, equivalent to that present in human ET patients. We show that, in addition to increased differentiation, JAK2V617F-positive megakaryocytes display greater migratory ability and proplatelet formation. We demonstrate in a range of assays that platelet reactivity to agonists is enhanced, with a concomitant increase in platelet aggregation in vitro and a reduced duration of bleeding in vivo. These data suggest that JAK2V617F leads to intrinsic changes in both megakaryocyte and platelet biology beyond an increase in cell number. In support of this hypothesis, we identify multiple differentially expressed genes in JAK2V617F megakaryocytes that may underlie the observed biological differences.

Overexpression of nuclear distribution protein (hNUDC) causes pro-apoptosis and differentiation in Dami megakaryocytes

OBJECTIVES

Overexpression of hNUDC, a member of the nuclear distribution protein family, reduces cell population growth in prostate cancer cell lines, concurrent with induced morphological change and enhanced polyploidization. These phenomena are also closely associated with terminal phases of megakaryocyte maturation.

MATERIALS AND METHODS

In Dami cells, MTT and trypan blue assays were used to investigate cell viability and proliferation effects of hNUDC, and flow cytometry was used to analyse cell cycle and DNA content. Real-time RT-PCR was employed to detect mRNA expression. Activations of caspase-3, ERK, Akt and Stat-5 were determined by immunoblotting. May-Grünwald-Giemsa staining was performed to reveal cell morphology.

RESULTS AND CONCLUSION

Functional studies using adenovirus-mediated hNUDC overexpression led to inhibition of megakaryocyte proliferation via cell cycle arrest in G2/M transition phase. This process could have been be mediated by upregulation of p21 and downregulation of its downstream targets, including cyclin B1, cyclin B2 and c-myc. Enhanced apoptosis in turn ensued, characterized by increased caspase-3 activation, upregulation of pro-apoptotic Bax and downregulation of anti-apoptotic Bcl-2. Furthermore, hNUDC overexpression elevated the level of megakaryocyte maturation, associated with increased polyploidy, cell morphological changes and increased expression of cell surface differentiation markers, including CD10, CD44, CD41 and CD61. Our results further suggest that the ERK signalling pathway was involved in hNUDC overexpression-induced apoptosis. Taken together, this study provides experimental evidence for overexpression of hNUDC in Dami cells and suggests that activation of apoptotic machinery may be involved in megakaryocytic differentiation.

Multiple phases of chondrocyte enlargement underlie differences in skeletal proportions

The wide diversity of skeletal proportions in mammals is evident upon a survey of any natural history museum's collections and allows us to distinguish between species even when reduced to their calcified components. Similarly, each individual is comprised of a variety of bones of differing lengths. The largest contribution to the lengthening of a skeletal element, and to the differential elongation of elements, comes from a dramatic increase in the volume of hypertrophic chondrocytes in the growth plate as they undergo terminal differentiation. However, the mechanisms of chondrocyte volume enlargement have remained a mystery. Here we use quantitative phase microscopy to show that mammalian chondrocytes undergo three distinct phases of volume increase, including a phase of massive cell swelling in which the cellular dry mass is significantly diluted. In light of the tight fluid regulatory mechanisms known to control volume in many cell types, this is a remarkable mechanism for increasing cell size and regulating growth rate. It is, however, the duration of the final phase of volume enlargement by proportional dry mass increase at low density that varies most between rapidly and slowly elongating growth plates. Moreover, we find that this third phase is locally regulated through a mechanism dependent on insulin-like growth factor. This study provides a framework for understanding how skeletal size is regulated and for exploring how cells sense, modify and establish a volume set point.

Mice lacking the ITIM-containing receptor G6b-B exhibit macrothrombocytopenia and aberrant platelet function

Platelets are highly reactive cell fragments that adhere to exposed extracellular matrix (ECM) and prevent excessive blood loss by forming clots. Paradoxically, megakaryocytes, which produce platelets in the bone marrow, remain relatively refractory to the ECM-rich environment of the bone marrow despite having the same repertoire of receptors as platelets. These include the ITAM (immunoreceptor tyrosine-based activation motif)-containing collagen receptor complex, which consists of glycoprotein VI (GPVI) and the Fc receptor γ-chain, and the ITIM (immunoreceptor tyrosine-based inhibition motif)-containing receptor G6b-B. We showed that mice lacking G6b-B exhibited macrothrombocytopenia (reduced platelet numbers and the presence of enlarged platelets) and a susceptibility to bleeding as a result of aberrant platelet production and function. Platelet numbers were markedly reduced in G6b-B-deficient mice compared to those in wild-type mice because of increased platelet turnover. Furthermore, megakaryocytes in G6b-B-deficient mice showed enhanced metalloproteinase production, which led to increased shedding of cell-surface receptors, including GPVI and GPIbα. In addition, G6b-B-deficient megakaryocytes exhibited reduced integrin-mediated functions and defective formation of proplatelets, the long filamentous projections from which platelets bud off. Together, these findings establish G6b-B as a major inhibitory receptor regulating megakaryocyte activation, function, and platelet production.

Potentiated activation of VLA-4 and VLA-5 accelerates proplatelet-like formation

Fibronectin (FN) plays important roles in the proliferation, differentiation, and maintenance of megakaryocytic-lineage cells through FN receptors. However, substantial role of FN receptors and their functional assignment in proplatelet-like formation (PPF) of megakaryocytes are not yet fully understood. Herein, we investigated the effects of FN receptors on PPF using the CHRF-288 human megakaryoblastic cell line, which expresses VLA-4 and VLA-5 as FN receptors. FN and phorbol 12-myristate 13-acetate (PMA) were essential for inducing PPF in CHRF-288 cells. Blocking experiments using anti-β1-integrin monoclonal antibodies indicated that the adhesive interaction with FN via VLA-4 and VLA-5 were required for PPF. PPF induced by FN plus PMA was accelerated when CHRF-288 cells were enforced adhering to FN by TNIIIA2, a peptide derived from tenascin-C, which we recently found to induce β1-integrin activation. Adhesion to FN enhanced PMA-stimulated activation of extracellular signal-regulated protein kinase 1 (ERK1)/2 and enforced adhesion to FN via VLA-4 and VLA-5 by TNIIIA2-accelerated activation of ERK1/2 with FN plus PMA. However, c-Jun amino-terminal kinase 1 (JNK1), p38, and phosphoinositide-3 kinase (PI3K)/Akt were not stimulated by FN plus PMA, even with TNIIIA2. Thus, the enhanced activation of ERK1/2 by FN, PMA plus TNIIIA2 was responsible for acceleration of PPF with FN plus PMA.

Dasatinib enhances megakaryocyte differentiation but inhibits platelet formation

Dasatinib is a novel, potent, ATP-competitive inhibitor of Bcr-Abl, cKIT, and Src family kinases that exhibits efficacy in patients with imatinib-resistant chronic myelogenous leukemia. Dasatinib treatment is associated with mild thrombocytopenia and an increased risk of bleeding, but its biological effect on megakaryocytopoiesis and platelet production is unknown. In this study, we show that dasatinib causes mild thrombocytopenia in mice without altering platelet half-life, suggesting that it inhibits platelet formation. Conversely, the number of megakaryocytes (MKs) in the bone marrow of dasatinib-treated mice was increased and the ploidy of MKs derived from bone marrow progenitor cells in vitro was elevated in the presence of dasatinib. Furthermore, a significant delay in platelet recovery after immune-induced thrombocytopenia was observed in dasatinib-treated mice even though the number of MKs in the bone marrow was increased relative to controls at all time points. Interestingly, the migration of MKs toward a gradient of stromal cell-derived factor 1α (SDF1α) and the formation of proplatelets in vitro were abolished by dasatinib. We propose that dasatinib causes thrombocytopenia as a consequence of ineffective thrombopoiesis, promoting MK differentiation but also impairing MK migration and proplatelet formation.

Infusion of mature megakaryocytes into mice yields functional platelets

Thrombopoiesis, the process by which circulating platelets arise from megakaryocytes, remains incompletely understood. Prior studies suggest that megakaryocytes shed platelets in the pulmonary vasculature. To better understand thrombopoiesis and to develop a potential platelet transfusion strategy that is not dependent upon donors, of which there remains a shortage, we examined whether megakaryocytes infused into mice shed platelets. Infused megakaryocytes led to clinically relevant increases in platelet numbers. The released platelets were normal in size, displayed appropriate surface markers, and had a near-normal circulating half-life. The functionality of the donor-derived platelets was also demonstrated in vivo. The infused megakaryocytes mostly localized to the pulmonary vasculature, where they appeared to shed platelets. These data suggest that it may be unnecessary to generate platelets from ex vivo grown megakaryocytes to achieve clinically relevant increases in platelet numbers.

Effect of increased HoxB4 on human megakaryocytic development

In order to produce clinically useful quantities of platelets ex vivo we may need to firstly enhance early self-renewal of hematopoietic stem cells (HSCs) and/or megakaryocyte (Mk) progenitors. The homeodomain transcription factor HoxB4 has been shown to be an important regulator of stem cell renewal and hematopoiesis; however, its effect on megakaryopoiesis is unclear. In this study, we investigated the effect of HoxB4 overexpression or RNA silencing on megakaryocytic development in the human TF1 progenitor cell line; we then used recombinant tPTD-HoxB4 fusion protein to study the effect of exogenous HoxB4 on megakaryocytic development of human CD34 positively-selected cord blood cells. We found that ectopic HoxB4 in TF1 cells increased the antigen expression of CD61and CD41a, increased the gene expression of thrombopoietin receptor (TpoR), Scl-1, Cyclin D1, Fog-1 and Fli-1 while it decreased c-Myb expression. HoxB4 RNA silencing in TF1 cells decreased the expression of CD61 and CD41a and decreased Fli-1 expression while it increased the expression of c-Myb. Recombinant tPTD-HoxB4 fusion protein increased the percentages and absolute numbers of CD41a and CD61 positive cells during megakaryocytic differentiation of CD34 positively-selected cord blood cells and increased the numbers of colony-forming unit-megakaryocyte (CFU-Mk). Adding tPTD-HoxB4 fusion protein increased the gene expression of TpoR, Cyclin D1, Fog-1 and Fli-1 while it inhibited c-Myb expression. Our data suggest that increased HoxB4 enhanced early megakaryocytic development in human TF1 cells and CD34 positively-selected cord blood cells primarily by upregulating TpoR and Fli-1 expression and downregulating c-Myb expression. Increasing HoxB4 expression or adding recombinant HoxB4 protein might be a way to expand Mks for the production of platelets for use in transfusion medicine.

The microtubule modulator RanBP10 plays a critical role in regulation of platelet discoid shape and degranulation

Terminally mature megakaryocytes undergo dramatic cellular reorganization to produce hundreds of virtually identical platelets. A hallmark feature of this process is the generation of an elaborate system of branched protrusions called proplatelets. We recently identified RanBP10 as a tubulin-binding protein that is concentrated along polymerized microtubules in mature megakaryocytes. RanBP10 depletion in vitro caused the disturbance of polymerized filaments. Here we study the function of RanBP10 in vivo by generating deficient mice using a gene-trap approach. Mutant mice show normal platelet counts, and fetal liver-derived megakaryocytes reveal only slightly reduced proplatelet formation. However, ultrastructural analysis unveiled a significantly increased geometric axis ratio for resting platelets, and many platelets exhibited disorders in microtubule filament numbers and localization. Mutant mice showed a markedly prolonged bleeding time. Granule release, a process that depends on internal contraction of the microtubule marginal coil, also was reduced. Flow cytometry analysis revealed reduced expression of CD62P and CD63 after PAR4-peptide stimulation. These data suggest that RanBP10 plays an essential role in hemostasis and in maintaining microtubule dynamics with respect to both platelet shape and function.

Critical role for ERK1/2 in bone marrow and fetal liver-derived primary megakaryocyte differentiation, motility, and proplatelet formation

Objective

Megakaryopoiesis and platelet formation is a multistep process through which hematopoietic progenitor cells develop into mature megakaryocytes (MKs) and form proplatelets. The present study investigates the regulation of different steps of megakaryopoiesis (i.e., differentiation, migration, and proplatelet formation) by extracellar signal-regulated kinase (ERK)1/2 and p38 mitogen-activated protein kinase (MAPK) in two models of primary murine MKs derived from bone marrow (BM) cells and fetal liver (FL) cells.

Materials and Methods

A preparation of MKs was generated from BM obtained from femora and tibiae of C57BL6 mice. FL-derived MKs were obtained from the liver of mouse fetuses aged 13 to 15 days.

Results

For both cell populations, activation of MEK-ERK1/2 pathway by thrombopoietin was found to have a critical role in MK differentiation, regulating polyploidy and surface expression of CD34, GPIIb, and GPIb. The MEK-ERK1/2 pathway plays a major role in migration of BM-derived MKs toward a stromal-cell−derived factor 1α (SDF1α) gradient, whereas unexpectedly, FL-derived cells fail to migrate in response to the chemokine due to negligible expression of its receptor, CXCR4. The MEK-ERK1/2 pathway also plays a critical role in the generation of proplatelets. In contrast, p38MAPK pathway was not involved in any of these processes.

Conclusion

This report demonstrates a critical role of MEK-ERK1/2 pathway in MK differentiation, motility, and proplatelet formation. This study highlights several differences between BM- and FL-derived MKs, which are discussed.

Identification of Tspan9 as a novel platelet tetraspanin and the collagen receptor GPVI as a component of tetraspanin microdomains

Platelets are essential for wound healing and inflammatory processes, but can also play a deleterious role by causing heart attack and stroke. Normal platelet activation is dependent on tetraspanins, a superfamily of glycoproteins that function as 'organisers' of cell membranes by recruiting other receptors and signalling proteins into tetraspanin-enriched microdomains. However, our understanding of how tetraspanin microdomains regulate platelets is hindered by the fact that only four of the 33 mammalian tetraspanins have been identified in platelets. This is because of a lack of antibodies to most tetraspanins and difficulties in measuring mRNA, due to low levels in this anucleate cell. To identify potentially platelet-expressed tetraspanins, mRNA was measured in their nucleated progenitor cell, the megakaryocyte, using serial analysis of gene expression and DNA microarrays. Amongst 19 tetraspanins identified in megakaryocytes, Tspan9, a previously uncharacterized tetraspanin, was relatively specific to these cells. Through generating the first Tspan9 antibodies, Tspan9 expression was found to be tightly regulated in platelets. The relative levels of CD9, CD151, Tspan9 and CD63 were 100, 14, 6 and 2 respectively. Since CD9 was expressed at 49000 cell surface copies per platelet, this suggested a copy number of 2800 Tspan9 molecules. Finally, Tspan9 was shown to be a component of tetraspanin microdomains that included the collagen receptor GPVI (glycoprotein VI) and integrin alpha6beta1, but not the von Willebrand receptor GPIbalpha or the integrins alphaIIbbeta3 or alpha2beta1. These findings suggest a role for Tspan9 in regulating platelet function in concert with other platelet tetraspanins and their associated proteins.

Novel approach for formation of platelet-like particles from mouse embryonic stem cells without using feeder cells

Megakaryocytes (MKs) and platelet-like particles (PLPs) have generally been obtained by culturing embryonic stem (ES) cells over feeder cells. However, using feeder cells need many labor-consuming processes and the MK and PLP fractions obtained are often contaminated by such cells and their fragments. Here we describe our new culture system for differentiating mouse ES cells to MKs and PLPs without using feeder cells. ES cells are differentiated to cells with MK-like morphology and properties, including proplatelet formation, high ploidy (>8N), and CD41 expression. The culture medium contained PLPs expressing platelet glycoproteins, CD41 and GPIb. Integrin alpha(IIb)beta(3) of PLPs can be activated by thrombin. Addition of the metalloproteinase inhibitor TAPI-2 to the culture increased the surface expression of GPIbalpha and augmented the adhesion of PLPs to immobilized von Willebrand factor through decreasing the shedding of GPIbalpha. Thus our mouse ES cells culture system is a suitable and efficient method for obtaining MKs and functional PLPs that obviates the need for feeder cells.

RanBP10 is a cytoplasmic guanine nucleotide exchange factor that modulates noncentrosomal microtubules

Microtubule spindle assembly in mitosis is stimulated by Ran.GTP, which is generated along condensed chromosomes by the guanine nucleotide exchange factor (GEF) RCC1. This relationship suggests that similar activities might modulate other microtubule structures. Interphase microtubules usually extend from the centrosome, although noncentrosomal microtubules function in some differentiated cells, including megakaryocytes. In these cells, platelet biogenesis requires massive mobilization of microtubules in the cell periphery, where they form proplatelets, the immediate precursors of platelets, in the apparent absence of centrioles. Here we identify a cytoplasmic Ran-binding protein, RanBP10, as a factor that binds beta-tubulin and associates with megakaryocyte microtubules. Unexpectedly, RanBP10 harbors GEF activity toward Ran. A point mutation in the candidate GEF domain abolishes exchange activity, and our results implicate RanBP10 as a localized cytoplasmic Ran-GEF. RNA interference-mediated loss of RanBP10 in cultured megakaryocytes disrupts microtubule organization. These results lead us to propose that spatiotemporally restricted generation of cytoplasmic Ran.GTP may influence organization of the specialized microtubules required in thrombopoiesis and that RanBP10 might serve as a molecular link between Ran and noncentrosomal microtubules.

Tescalcin is an essential factor in megakaryocytic differentiation associated with Ets family gene expression

We show here that the process of megakaryocytic differentiation requires the presence of the recently discovered protein tescalcin. Tescalcin is dramatically upregulated during the differentiation and maturation of primary megakaryocytes or upon PMA-induced differentiation of K562 cells. This upregulation requires sustained signaling through the ERK pathway. Overexpression of tescalcin in K562 cells initiates events of spontaneous megakaryocytic differentiation, such as expression of specific cell surface antigens, inhibition of cell proliferation, and polyploidization. Conversely, knockdown of this protein in primary CD34+ hematopoietic progenitors and cell lines by RNA interference suppresses megakaryocytic differentiation. In cells lacking tescalcin, the expression of Fli-1, Ets-1, and Ets-2 transcription factors, but not GATA-1 or MafB, is blocked. Thus, tescalcin is essential for the coupling of ERK cascade activation with the expression of Ets family genes in megakaryocytic differentiation.

Novel role for EKLF in megakaryocyte lineage commitment

Megakaryocytes and erythroid cells are thought to derive from a common progenitor during hematopoietic differentiation. Although a number of transcriptional regulators are important for this process, they do not explain the bipotential result. We now show by gain- and loss-of-function studies that erythroid Krüppel-like factor (EKLF), a transcription factor whose role in erythroid gene regulation is well established, plays an unexpected directive role in the megakaryocyte lineage. EKLF inhibits the formation of megakaryocytes while at the same time stimulating erythroid differentiation. Quantitative examination of expression during hematopoiesis shows that, unlike genes whose presence is required for establishment of both lineages, EKLF is uniquely down-regulated in megakaryocytes after formation of the megakaryocyte-erythroid progenitor. Expression profiling and molecular analyses support these observations and suggest that megakaryocytic inhibition is achieved, at least in part, by EKLF repression of Fli-1 message levels.

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.

Dosage-dependent requirement for mouse Vezf1 in vascular system development

Vezf1 is an early development gene that encodes a zinc finger transcription factor. In the developing embryo, Vezf1 is expressed in the yolk sac mesoderm and the endothelium of the developing vasculature and, in addition, in mesodermal and neuronal tissues. Targeted inactivation of Vezf1 in mice reveals that it acts in a closely regulated, dose-dependent fashion on the development of the blood vascular and lymphatic system. Homozygous mutant embryos display vascular remodeling defects and loss of vascular integrity leading to localized hemorrhaging. Ultrastructural analysis shows defective endothelial cell adhesion and tight junction formation in the mutant vessels. Moreover, in heterozygous embryos, haploinsufficiency is observed that is characterized by lymphatic hypervascularization associated with hemorrhaging and edema in the jugular region; a phenotype reminiscent of the human congenital lymphatic malformation syndrome cystic hygroma.

Small Maf proteins serve as transcriptional cofactors for keratinocyte differentiation in the Keap1-Nrf2 regulatory pathway

The small Maf proteins, MafF, MafG, and MafK, possess a leucine zipper (Zip) domain that is required for homodimer or heterodimer complex formation with other bZip transcription factors. In this study we sought to determine the identity of the specific constituent that collaboratively interacts with Nrf2 to bind to the Maf recognition element in vivo. Studies in vitro suggested that Nrf2 forms heterodimers with small Maf proteins and then bind to Maf recognition elements, but the bona fide partner molecules supporting Nrf2 activity in vivo have not been definitively identified. Nrf2 activity is usually suppressed by a cytoplasmic repressor, Keap1, so disruption of the keap1 gene causes constitutive activation of Nrf2. Nrf2 hyperactivity results in hyperproliferation of keratinocytes in the esophagus and forestomach leading to perinatal lethality. However, simultaneous disruption of nrf2 rescued keap1-null mice from the lethality. We exploited this system to investigate whether small Mafs are required for Nrf2 function. We generated keap1 and small maf compound mutant mice and examined whether keratinocyte abnormalities persisted in these animals. The data show that loss of mafG and mafF in the keap1-null mice reversed the lethal keratinocyte dysfunction and rescued the keap1-null mutant mice from perinatal lethality. This rescue phenotype of mafG::mafF::keap1 triple compound mutant mice phenocopies that of the nrf2::keap1 compound mutant mice, indicating that the small Maf proteins MafG and MafF must functionally cooperate with Nrf2 in vivo.

Megakaryocyte-osteoblast interaction revealed in mice deficient in transcription factors GATA-1 and NF-E2

Mice deficient in GATA-1 or NF-E2 have a 200-300% increase in bone volume and formation parameters. Osteoblasts and osteoclasts generated in vitro from mutant and control animals were similar in number and function. Osteoblast proliferation increased up to 6-fold when cultured with megakaryocytes. A megakaryocyte-osteoblast interaction plays a role in the increased bone formation in these mice.

INTRODUCTION

GATA-1 and NF-E2 are transcription factors required for the differentiation of megakaryocytes. Mice deficient in these factors have phenotypes characterized by markedly increased numbers of immature megakaryocytes, a concomitant drastic reduction of platelets, and a striking increased bone mass. The similar bone phenotype in both animal models led us to explore the interaction between osteoblasts and megakaryocytes.

MATERIALS AND METHODS

Histomorphometry, microCT, and serum and urine biochemistries were used to assess the bone phenotype in these mice. Wildtype and mutant osteoblasts were examined for differences in proliferation, alkaline phosphatase activity, and osteocalcin secretion. In vitro osteoclast numbers and resorption were measured. Because mutant osteoblasts and osteoclasts were similar to control cells, and because of the similar bone phenotype, we explored the interaction between cells of the osteoblast lineage and megakaryocytes.

RESULTS

A marked 2- to 3-fold increase in trabecular bone volume and bone formation indices were observed in these mice. A 20- to 150-fold increase in trabecular bone volume was measured for the entire femoral medullary canal. The increased bone mass phenotype in these animals was not caused by osteoclast defects, because osteoclast number and function were not compromised in vitro or in vivo. In contrast, in vivo osteoblast number and bone formation parameters were significantly elevated. When wildtype or mutant osteoblasts were cultured with megakaryocytes from GATA-1- or NF-E2-deficient mice, osteoblast proliferation increased over 3- to 6-fold by a mechanism that required cell-to-cell contact.

CONCLUSIONS

These observations show an interaction between megakaryocytes and osteoblasts, which results in osteoblast proliferation and increased bone mass, and may represent heretofore unrecognized anabolic pathways in bone.

Interferon regulatory factor-2 drives megakaryocytic differentiation

IRFs [IFN (interferon) regulatory factors] constitute a family of transcription factors involved in IFN signalling and in the development and differentiation of the immune system. IRF-2 has generally been described as an antagonist of IRF-1-mediated transcription of IFN and IFN-inducible genes; however, it has been recently identified as a transcriptional activator of some genes, such as those encoding histone H4, VCAM-1 (vascular cell adhesion molecule-1) and Fas ligand. Biologically, IRF-2 plays an important role in cell growth regulation and has been shown to be a potential oncogene. Studies in knock-out mice have also implicated IRF-2 in the differentiation and functionality of haematopoietic cells. Here we show that IRF-2 expression in a myeloid progenitor cell line leads to reprogramming of these cells towards the megakaryocytic lineage and enables them to respond to thrombopoietin, as assessed by cell morphology and expression of specific differentiation markers. Up-regulation of transcription factors involved in the development of the megakaryocytic lineage, such as GATA-1, GATA-2, FOG-1 (friend of GATA-1) and NF-E2 (nuclear factor-erythroid-2), and transcriptional stimulation of the thrombopoietin receptor were also demonstrated. Our results provide evidence for a key role for IRF-2 in the induction of a programme of megakaryocytic differentiation, and reveal a remarkable functional diversity of this transcription factor in the regulation of cellular responses.

Role of AP1/NFE2 binding sites in endogenous alpha-globin gene transcription

High-level alpha-globin expression depends on cis-acting regulatory sequences located far upstream of the alpha-globin cluster. Sequences that contain the alpha-globin positive regulatory element (PRE) activate alpha-globin expression in transgenic mice. The alpha-globin PRE contains a pair of composite binding sites for the transcription factors activating protein 1 and nuclear factor erythroid 2 (AP1/NFE2). To determine the role of these binding sites in alpha-globin gene transcription, we mutated the AP1/NFE2 sites in the alpha-globin PRE in mice. We replaced the AP1/NFE2 sites with a neomycin resistance gene (neo) that is flanked by LoxP sites (floxed). Mice with this mutation exhibited increased embryonic death and alpha-thalassemia intermedia. Next, we removed the neo gene by Cre-mediated recombination, leaving a single LoxP site in place of the AP1/NFE2 sites. These mice were phenotypically normal. However, alpha-globin expression, measured by allele-specific RNA polymerase chain reaction (PCR), was decreased 25%. We examined the role of the hematopoietic-restricted transcription factor p45Nfe2 in activating expression through these sites and found that it is not required. Thus, we have demonstrated that AP1/NFE2 binding sites in the murine alpha-globin PRE contribute to long-range alpha-globin gene activation. The proteins that mediate this effect remain to be determined.

A role for Rab27b in NF-E2-dependent pathways of platelet formation

Megakaryocytes release platelets by reorganizing the cytoplasm into proplatelet extensions. Fundamental to this process is the need to coordinate transport of products and organelles in the appropriate abundance to nascent platelets. The importance of the Rab family of small GTPases (guanosine 5'-triphosphatases) in platelet biogenesis is revealed in gunmetal (gm/gm) mice, which show deficient Rab isoprenylation and macrothrombocytopenia with few granules and abnormal megakaryocyte morphology. Although some Rab proteins are implicated in vesicle and organelle transport along microtubules or actin, the role of any Rab protein in platelet biogenesis is unknown. The limited number of Rab proteins with defective membrane association in gm/gm megakaryocytes prominently includes Rab27a and Rab27b. Normal expression of Rab27b is especially increased with terminal megakaryocyte differentiation and dependent on nuclear factor-erythroid 2 (NF-E2), a transcription factor required for thrombopoiesis. Chromatin immunoprecipitation demonstrates recruitment of NF-E2 to the putative Rab27B promoter. Inhibition of endogenous Rab27 function in primary megakaryocytes causes severe quantitative and qualitative defects in proplatelet formation that mimic findings in gm/gm cells. Rab27b localizes to alpha and dense granules in megakaryocytes. These results establish a role for Rab27 in platelet synthesis and suggest that Rab27b in particular may coordinate proplatelet formation with granule transport, possibly by recruiting specific effector pathways.

Picosecond-hetero-FRET microscopy to probe protein-protein interactions in live cells

By using a novel time- and space-correlated single-photon counting detector, we show that fluorescence resonance energy transfer (FRET) between cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) fused to herpes simplex virus thymidine kinase (TK) monomers can be used to reveal homodimerization of TK in the nucleus and cytoplasm of live cells. However, the quantification of energy transfer was limited by the intrinsic biexponential fluorescence decay of the donor CFP (lifetimes of 1.3 +/- 0.2 ns and 3.8 +/- 0.4 ns) and by the possibility of homodimer formation between two TK-CFP. In contrast, the heterodimerization of the transcriptional factor NF-E2 in the nucleus of live cells was quantified from the analysis of the fluorescence decays of GFP in terms of 1) FRET efficiency between GFP and DsRed chromophores fused to p45 and MafG, respectively, the two subunits of NF-E2 (which corresponds to an interchromophoric distance of 39 +/- 1 A); and 2) fractions of GFP-p45 bound to DsRed-MafG (constant in the nucleus, varying in the range of 20% to 70% from cell to cell). The picosecond resolution of the fluorescence kinetics allowed us to discriminate between very short lifetimes of immature green species of DsRed-MafG and that of GFP-p45 involved in FRET with DsRed-MafG.

A transcription-factor-binding surface of coactivator p300 is required for haematopoiesis

The coactivators CBP (Cre-element binding protein (CREB)-binding protein) and its paralogue p300 are thought to supply adaptor molecule and protein acetyltransferase functions to many transcription factors that regulate gene expression. Normal development requires CBP and p300, and mutations in these genes are found in haematopoietic and epithelial tumours. It is unclear, however, which functions of CBP and p300 are essential in vivo. Here we show that the protein-binding KIX domains of CBP and p300 have nonredundant functions in mice. In mice homozygous for point mutations in the KIX domain of p300 designed to disrupt the binding surface for the transcription factors c-Myb and CREB, multilineage defects occur in haematopoiesis, including anaemia, B-cell deficiency, thymic hypoplasia, megakaryocytosis and thrombocytosis. By contrast, age-matched mice homozygous for identical mutations in the KIX domain of CBP are essentially normal. There is a synergistic genetic interaction between mutations in c-Myb and mutations in the KIX domain of p300, which suggests that the binding of c-Myb to this domain of p300 is crucial for the development and function of megakaryocytes. Thus, conserved domains in two highly related coactivators have contrasting roles in haematopoiesis.

Megakaryocytes derived from embryonic stem cells implicate CalDAG-GEFI in integrin signaling

Fibrinogen binding to integrin alphaIIbbeta3 mediates platelet aggregation and requires agonist-induced "inside-out" signals that increase alphaIIbbeta3 affinity. Agonist regulation of alphaIIbbeta3 also takes place in megakaryocytes, the bone marrow cells from which platelets are derived. To facilitate mechanistic studies of inside-out signaling, we describe here the generation of megakaryocytes in quantity from murine embryonic stem (ES) cells. Coculture of ES cells for 8-12 days with OP9 stromal cells in the presence of thrombopoietin, IL-6, and IL-11 resulted in the development of large, polyploid megakaryocytes that produced proplatelets. These cells expressed alphaIIbbeta3 and platelet glycoprotein Ibalpha but were devoid of hematopoietic stem cell, erythrocyte, and leukocyte markers. Mature megakaryocytes, but not megakaryocyte progenitors, specifically bound fibrinogen by way of alphaIIbbeta3 in response to platelet agonists. Retrovirus-mediated expression of the reporter gene, green fluorescent protein, in ES cell-derived megakaryocytes did not affect viability or alphaIIbbeta3 function. On the other hand, retroviral expression of CalDAG-GEFI, a Rap1 exchange factor identified by megakaryocyte gene profiling as a candidate integrin regulator, enhanced agonist-induced activation of Rap1b and fibrinogen binding to alphaIIbbeta3 (P < 0.01). These results establish that ES cells are a ready source of mature megakaryocytes for integrin studies and other biological applications, and they implicate CalDAG-GEFI in inside-out signaling to alphaIIbbeta3.

Molecular control of megakaryopoiesis and thrombopoiesis

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

Molecular and transcriptional regulation of megakaryocyte differentiation

Megakaryocytes, among the rarest of hematopoietic cells, serve the essential function of producing numerous platelets. Genetic studies have recently provided rich insights into the molecular and transcriptional regulation of megakaryocyte differentiation and thrombopoiesis. Three transcription factors, GATA-1, FOG-1, and NF-E2, are essential regulators of distinct stages in megakaryocyte differentiation, extending from the birth of early committed progenitors to the final step of platelet release; a fourth factor, Fli-1, likely also plays an important role. The putative transcriptional targets of these regulators, including the NF-E2-dependent hematopoietic-specific beta-tubulin isoform beta1, deepen our understanding of molecular mechanisms in platelet biogenesis. The study of rare syndromes of inherited thrombocytopenia in mice and man has also refined the emerging picture of megakaryocyte maturation. Synthesis of platelet-specific organelles is mediated by a variety of regulators of intracellular vesicle membrane fusion, and platelet release is coordinated through extensive and dynamic reorganization of the actin and microtubule cytoskeletons. As in other aspects of hematopoiesis, characterization of recurrent chromosomal translocations in human leukemias provides an added dimension to the molecular underpinnings of megakaryocyte differentiation. Long regarded as a mysterious cell, the megakaryocyte is thus yielding many of its secrets, and mechanisms of thrombopoiesis are becoming clearer. Although this review focuses on transcriptional control mechanisms, it also discusses recent advances in broader consideration of the birth of platelets.

Transcription factor BACH1 is recruited to the nucleus by its novel alternative spliced isoform

The transcription factor Bach1 is a member of a novel family of broad complex, tramtrack, bric-a-brac/poxvirus and zinc finger (BTB/POZ) basic region leucine zipper factors. Bach1 forms a heterodimer with MafK, a member of the small Maf protein family (MafF, MafG, and MafK), which recognizes the NF-E2/Maf recognition element, a cis-regulatory motif containing a 12-O-tetradecanoylphorbol-13-acetate-responsive element. Here we describe the gene structure of human BACH1, including a newly identified promoter and an alternatively RNA-spliced truncated form of BACH1, designated BACH1t, abundantly transcribed in human testis. The alternate splicing originated from the usage of a novel exon located 5.6 kilobase pairs downstream of the exon encoding the leucine zipper domain, and produced a protein that contained the conserved BTB/POZ, Cap'n collar, and basic region domains, but lacked the leucine zipper domain essential for NF-E2/Maf recognition element binding. Subcellular localization studies using green fluorescent protein as a reporter showed that full-length BACH1 localized to the cytoplasm, whereas BACH1t accumulated in the nucleus. Interestingly, coexpression of BACH1 and BACH1t demonstrated interaction between the molecules and the induction of nuclear import of BACH1. These results suggested that BACH1t recruits BACH1 to the nucleus through BTB domain-mediated interaction.

Cellular and molecular biology of megakaryocyte differentiation in the absence of lineage-restricted transcription factors

Targeted gene disruption of two distinct lineage-restricted hematopoietic transcription factors has provided useful insights into the transcriptional control of platelet production. Absence of either the basic leucine-zipper protein NF-E2 or of the zinc-finger protein GATA-1 in vivo results in severe thrombocytopenia secondary to distinct patterns of arrested megakaryocyte cytoplasmic maturation; in addition, megakaryocyte-selective loss of GATA-1 expression leads to dysregulated proliferation of progenitor cells. The ultrastructure of the defective megakaryocytes suggests that absence of the respective transcription factors impairs biogenesis of platelet-specific granules and proper development and organization of demarcation membranes. In particular, transcriptional targets of NF-E2 may be implicated in the very final stages of megakaryocyte differentiation, which involve the organization and release of platelets. Preliminary characterization of genes that are downregulated in NF-E2-/- megakaryocytes is in progress and is likely to lead to mechanistic insights into thrombocytopoiesis.

Positive or negative MARE-dependent transcriptional regulation is determined by the abundance of small Maf proteins

The small Maf transcription factor proteins bind to Maf Recognition Elements (MAREs) by dimerizing with CNC proteins or themselves. We undertook experiments to clarify the functional relationship between the small Mafs and their partners in vivo. Embryos expressing abundant transgene-derived MafK died of severe anemia, while lines expressing lower levels of small Maf lived to adulthood. Megakaryocytes from the latter overexpressing lines exhibited reduced proplatelet formation and MARE-dependent transcription, phenocopying mafG null mutant mice. When the mafG null mutants were bred to small Maf-overexpressing transgenic animals, both loss- and gain-of-function phenotypes were reversed. These results provide direct in vivo evidence that transcriptional regulation through MARE elements hinges on an exquisitely sensitive balance of activating CNC molecules and their small Maf partners.

Expression of transcription factors during megakaryocytic differentiation of CD34+ cells from human cord blood induced by thrombopoietin

Although normal megakaryocytic development has been shown to require the presence of functional GATA-1 and NF-E2 transcription factors in vivo, the roles of other members of the GATA binding factors and NF-E2 family during megakaryocytic differentiation are unclear. the present study, the expression of GATA family members, GATA-1 and GATA-2, a GATA-binding factor, EVI-1, the large subunit of NF-E2 factor, p45 and the related factors, Nrf1, Nrf2, Nrf3, BACH1, BACH2, and the small subunit of NF-E2, MAFK and MAFG has been examined in human megakaryocytic and erythroid cells by reverse transcriptase-polymerase chain reaction. CD34+ cells isolated from human cord blood were induced to unilineage megakaryocytic or erythroid differentiation in liquid suspension culture in the presense of thrombopoietin or erythropoietin, respectively. Each lineage was identified by monoclonal antibody against GPIIb/IIIa or glycophorin A. In megakaryocytic culture, p45, Nrf1, Nrf2, BACH1, MAFK and MAFG mRNAs were induced similarly to erythroid culture. Nrf3 mRNA was barely detected in both cultures. BACH2 was induced only in megakaryocytic culture, although the level of expression was low. Furthermore, the profiles of transcription factors involved in hematopoiesis, EVI-1 and Ets-1 mRNAs were induced only in megakaryocytic culture. Megakaryocytic and erythroid differentiation pathways are closely related to each other, and these two lineage cells share a number of lineage-specific transcription factors. However, the results showed that the profile of the expression of these transcription factors in megakaryocytic cells is distinct from that of erythroid lineage. The dynamic changes in the levels of different transcription factors that occur during primary megakaryocytic differentiation suggest that the levels of these factors may influence the progression to specific hematopoietic pathways.

Hematopoietic-specific β1 tubulin participates in a pathway of platelet biogenesis dependent on the transcription factor NF-E2

The cellular and molecular bases of platelet release by terminally differentiated megakaryocytes represent important questions in cell biology and hematopoiesis. Mice lacking the transcription factor NF-E2 show profound thrombocytopenia, and their megakaryocytes fail to produce proplatelets, the microtubule-based precursors of blood platelets. Using mRNA subtraction between normal and NF-E2–deficient megakaryocytes, cDNA was isolated encoding β1 tubulin, the most divergent β tubulin isoform. In NF-E2–deficient megakaryocytes, β1 tubulin mRNA and protein are virtually absent. The expression of β1 tubulin is exquisitely restricted to platelets and megakaryocytes, where it appears late in differentiation and localizes to microtubule shafts and coils within proplatelets. Restoring NF-E2 activity in a megakaryoblastic cell line or in NF-E2–deficient primary megakaryocytes rescues the expression of β1 tubulin. Re-expressing β1 tubulin in isolation does not, however, restore proplatelet formation in the defective megakaryocytes, indicating that other critical factors are required; indeed, other genes identified by mRNA subtraction also encode structural and regulatory components of the cytoskeleton. These findings provide critical mechanistic links between NF-E2, platelet formation, and selected microtubule proteins, and they also provide novel molecular insights into thrombopoiesis.

Hematopoietic-specific β1 tubulin participates in a pathway of platelet biogenesis dependent on the transcription factor NF-E2

The cellular and molecular bases of platelet release by terminally differentiated megakaryocytes represent important questions in cell biology and hematopoiesis. Mice lacking the transcription factor NF-E2 show profound thrombocytopenia, and their megakaryocytes fail to produce proplatelets, the microtubule-based precursors of blood platelets. Using mRNA subtraction between normal and NF-E2–deficient megakaryocytes, cDNA was isolated encoding β1 tubulin, the most divergent β tubulin isoform. In NF-E2–deficient megakaryocytes, β1 tubulin mRNA and protein are virtually absent. The expression of β1 tubulin is exquisitely restricted to platelets and megakaryocytes, where it appears late in differentiation and localizes to microtubule shafts and coils within proplatelets. Restoring NF-E2 activity in a megakaryoblastic cell line or in NF-E2–deficient primary megakaryocytes rescues the expression of β1 tubulin. Re-expressing β1 tubulin in isolation does not, however, restore proplatelet formation in the defective megakaryocytes, indicating that other critical factors are required; indeed, other genes identified by mRNA subtraction also encode structural and regulatory components of the cytoskeleton. These findings provide critical mechanistic links between NF-E2, platelet formation, and selected microtubule proteins, and they also provide novel molecular insights into thrombopoiesis.

CDP/Cut DNA binding activity is down-modulated in granulocytes, macrophages and erythrocytes but remains elevated in differentiating megakaryocytes

DNA binding by the CCAAT-displacement protein, the mammalian homologue of the Drosophila melanogaster Cut protein, was previously found to increase sharply in S phase, suggesting a role for CDP/Cut in cell cycle progression. Genetic studies in Drosophila indicated that cut plays an important role in cell-type specification in several tissues. In the present study, we have investigated CDP/Cut expression and activity in a panel of multipotent hematopoietic cell lines that can be induced to differentiate in vitro into distinct cell types. While CDP/Cut DNA binding activity declined in the pathways leading to macrophages, granulocytes and erythrocytes, it remained elevated in megakaryocytes. CDP/Cut was also highly expressed in primary megakaryocytes isolated from mouse, and some DNA binding activity could be detected. Altogether, these results raise the possibility that CDP/Cut may be a determinant of cell type identity downstream of the myelo-erythroid precursor cell. Another possibility, which does not exclude a role in lineage identity, is that CDP/Cut activity in megakaryocytes is linked to endomitosis. Indeed, elevated CDP/Cut activity in differentiating megakaryocytes and during the S phase of the cell cycle suggests that it may be required for DNA replication.

Regulation of mouse p45 NF-E2 transcription by an erythroid-specific GATA-dependent intronic alternative promoter

The erythroid-enriched transcription factor NF-E2 is composed of two subunits, p45 and p18, the former of which is mainly expressed in the hematopoietic system. We have isolated and characterized the mouse p45 NF-E2 gene; we show here that, similar to the human gene, the mouse gene has two alternative promoters, which are differentially active during development and in different hematopoietic cells. Transcripts from the distal promoter are present in both erythroid and myeloid cells; however, transcripts from an alternative proximal 1b promoter, lying in the first intron, are abundant in erythroid cells, but barely detectable in myeloid cells. During development, both transcripts are detectable in yolk sac, fetal liver, and bone marrow. Transfection experiments show that proximal promoter 1b has a strong activity in erythroid cells, which is completely dependent on the integrity of a palindromic GATA-1 binding site. In contrast, the distal promoter 1a is not active in this assay. When the promoter 1b is placed 3' to the promoter 1a and reporter gene, in an arrangement that resembles the natural one, it acts as an enhancer to stimulate the activity of the upstream promoter la.

Loading of DNA-binding factors to an erythroid enhancer

The HS-40 enhancer is the major cis-acting regulatory element responsible for the developmental stage- and erythroid lineage-specific expression of the human alpha-like globin genes, the embryonic zeta and the adult alpha2/alpha/1. A model has been proposed in which competitive factor binding at one of the HS-40 motifs, 3'-NA, modulates the capability of HS-40 to activate the embryonic zeta-globin promoter. Furthermore, this modulation was thought to be mediated through configurational changes of the HS-40 enhanceosome during development. In this study, we have further investigated the molecular basis of this model. First, human erythroid K562 cells stably integrated with various HS-40 mutants cis linked to a human alpha-globin promoter-growth hormone hybrid gene were analyzed by genomic footprinting and expression analysis. By the assay, we demonstrate that factors bound at different motifs of HS-40 indeed act in concert to build a fully functional enhanceosome. Thus, modification of factor binding at a single motif could drastically change the configuration and function of the HS-40 enhanceosome. Second, a specific 1-bp, GC-->TA mutation in the 3'-NA motif of HS-40, 3'-NA(II), has been shown previously to cause significant derepression of the embryonic zeta-globin promoter activity in erythroid cells. This derepression was hypothesized to be regulated through competitive binding of different nuclear factors, in particular AP1 and NF-E2, to the 3'-NA motif. By gel mobility shift and transient cotransfection assays, we now show that 3'-NA(II) mutation completely abolishes the binding of small MafK homodimer. Surprisingly, NF-E2 as well as AP1 can still bind to the 3'-NA(II) sequence. The association constants of both NF-E2 and AP1 are similar to their interactions with the wild-type 3'-NA motif. However, the 3'-NA(II) mutation causes an approximately twofold reduction of the binding affinity of NF-E2 factor to the 3'-NA motif. This reduction of affinity could be accounted for by a twofold-higher rate of dissociation of the NF-E2-3'-NA(II) complex. Finally, we show by chromatin immunoprecipitation experiments that only binding of NF-E2, not AP1, could be detected in vivo in K562 cells around the HS-40 region. These data exclude a role for AP1 in the developmental regulation of the human alpha-globin locus via the 3'-NA motif of HS-40 in embryonic/fetal erythroid cells. Furthermore, extrapolation of the in vitro binding studies suggests that factors other than NF-E2, such as the small Maf homodimers, are likely involved in the regulation of the HS-40 function in vivo.

Primary megakaryocytes reveal a role for transcription factor NF-E2 in integrin alpha IIb beta 3 signaling

Platelet integrin alphaIIbbeta3 responds to intracellular signals by binding fibrinogen and triggering cytoskeletal reorganization, but the mechanisms of alphaIIbbeta3 signaling remain poorly understood. To better understand this process, we established conditions to study alphaIIbbeta3 signaling in primary murine megakaryocytes. Unlike platelets, these platelet precursors are amenable to genetic manipulation. Cytokine-stimulated bone marrow cultures produced three arbitrary populations of alphaIIbbeta3-expressing cells with increasing size and DNA ploidy: small progenitors, intermediate-size young megakaryocytes, and large mature megakaryocytes. A majority of the large megakaryocytes bound fibrinogen in response to agonists, while almost none of the smaller cells did. Fibrinogen binding to large megakaryocytes was inhibited by Sindbis virus-mediated expression of isolated beta3 integrin cytoplasmic tails. Strikingly, large megakaryocytes from mice deficient in the transcription factor NF-E2 failed to bind fibrinogen in response to agonists, despite normal surface expression of alphaIIbbeta3. Furthermore, while megakaryocytes from wild-type mice spread on immobilized fibrinogen and exhibited filopodia, lamellipodia and Rho-dependent focal adhesions and stress fibers, NF-E2-deficient megakaryocytes adhered poorly. These studies establish that agonist-induced activation of alphaIIbbeta3 is controlled by NF-E2-regulated signaling pathways that mature late in megakaryocyte development and converge at the beta3 cytoplasmic tail. Megakaryocytes provide a physiologically relevant and tractable system for analysis of bidirectional alphaIIbbeta3 signaling.