Consequences of GATA-1 Deficiency in Megakaryocytes and Platelets

The CXCR1/CXCR2 Inhibitor Reparixin Alters the Development of Myelofibrosis in the Gata1 low Mice

A major role for human (h)CXCL8 (interleukin-8) in the pathobiology of myelofibrosis (MF) has been suggested by observations indicating that MF megakaryocytes express increased levels of hCXCL8 and that plasma levels of this cytokine in MF patients are predictive of poor patient outcomes. Here, we demonstrate that, in addition to high levels of TGF-β, the megakaryocytes from the bone marrow of the Gata1 low mouse model of myelofibrosis express high levels of murine (m)CXCL1, the murine equivalent of hCXCL8, and its receptors CXCR1 and CXCR2. Treatment with the CXCR1/R2 inhibitor, Reparixin in aged-matched Gata1 low mice demonstrated reductions in bone marrow and splenic fibrosis. Of note, the levels of fibrosis detected using two independent methods (Gomori and reticulin staining) were inversely correlated with plasma levels of Reparixin. Immunostaining of marrow sections indicated that the bone marrow from the Reparixin-treated group expressed lower levels of TGF-β1 than those expressed by the bone marrow from vehicle-treated mice while the levels of mCXCL1, and expression of CXCR1 and CXCR2, were similar to that of vehicle-treated mice. Moreover, immunofluorescence analyses performed on bone marrow sections from Gata1 low mice indicated that treatment with Reparixin induced expression of GATA1 while reducing expression of collagen III in megakaryocytes. These data suggest that in Gata1low mice, Reparixin reduces fibrosis by reducing TGF-β1 and collagen III expression while increasing GATA1 in megakaryocytes. Our results provide a preclinical rationale for further evaluation of this drug alone and in combination with current JAK inhibitor therapy for the treatment of patients with myelofibrosis.

hGATA1 Under the Control of a μLCR/β-Globin Promoter Rescues the Erythroid but Not the Megakaryocytic Phenotype Induced by the Gata1 low Mutation in Mice

The phenotype of mice carrying the Gata1^low mutation that decreases expression of Gata1 in erythroid cells and megakaryocytes, includes anemia, thrombocytopenia, hematopoietic failure in bone marrow and development of extramedullary hematopoiesis in spleen. With age, these mice develop myelofibrosis, a disease sustained by alterations in stem/progenitor cells and megakaryocytes. This study analyzed the capacity of hGATA1 driven by a μLCR/β-globin promoter to rescue the phenotype induced by the Gata1^low mutation in mice. Double hGATA1/Gata1^low/0 mice were viable at birth with hematocrits greater than those of their Gata1^low/0 littermates but platelet counts remained lower than normal. hGATA1 mRNA was expressed by progenitor and erythroid cells from double mutant mice but not by megakaryocytes analyzed in parallel. The erythroid cells from hGATA1/Gata1^low/0 mice expressed greater levels of GATA1 protein and of α- and β-globin mRNA than cells from Gata1^low/0 littermates and a reduced number of them was in apoptosis. By contrast, hGATA1/Gata1^low/0 megakaryocytes expressed barely detectable levels of GATA1 and their expression of acetylcholinesterase, Von Willebrand factor and platelet factor 4 as well as their morphology remained altered. In comparison with Gata1^+/0 littermates, Gata1^low/0 mice contained significantly lower total and progenitor cell numbers in bone marrow while the number of these cells in spleen was greater than normal. The presence of hGATA1 greatly increased the total cell number in the bone marrow of Gata1^low/0 mice and, although did not affect the total cell number of the spleen which remained greater than normal, it reduced the frequency of progenitor cells in this organ. The ability of hGATA1 to rescue the hematopoietic functions of the bone marrow of the double mutants was confirmed by the observation that these mice survive well splenectomy and did not develop myelofibrosis with age. These results indicate that hGATA1 under the control of µLCR/β-globin promoter is expressed in adult progenitors and erythroid cells but not in megakaryocytes rescuing the erythroid but not the megakaryocyte defect induced by the Gata1^low/0 mutation.

GATA1 pathogenic variants disrupt MYH10 silencing during megakaryopoiesis

BACKGROUND

GATA1 is an essential transcription factor for both polyploidization and megakaryocyte (MK) differentiation. The polyploidization defect observed in GATA1 variant carriers is not well understood.

OBJECTIVE

To extensively phenotype two pedigrees displaying different variants in the GATA1 gene and determine if GATA1 controls MYH10 expression levels, a key modulator of MK polyploidization.

METHOD

A total of 146 unrelated propositi with constitutional thrombocytopenia were screened on a multigene panel. We described the genotype-phenotype correlation in GATA1 variant carriers and investigated the effect of these novel variants on MYH10 transcription using luciferase constructs.

RESULTS

The clinical profile associated with the p.L268M variant localized in the C terminal zinc finger was unusual in that the patient displayed bleeding and severe platelet aggregation defects without early-onset thrombocytopenia. p.N206I localized in the N terminal zinc finger was associated, on the other hand, with severe thrombocytopenia (15G/L) in early life. High MYH10 levels were evidenced in platelets of GATA1 variant carriers. Analysis of MKs anti-GATA1 chromatin immunoprecipitation-sequencing data revealed two GATA1 binding sites, located in the 3' untranslated region and in intron 8 of the MYH10 gene. Luciferase reporter assays showed their respective role in the regulation of MYH10 gene expression. Both GATA1 variants significantly alter intron 8 driven MYH10 transcription.

CONCLUSION

The discovery of an association between MYH10 and GATA1 is a novel one. Overall, this study suggests that impaired MYH10 silencing via an intronic regulatory element is the most likely cause of GATA1-related polyploidization defect.

New Insights of Human Parvovirus B19 in Modulating Erythroid Progenitor Cell Differentiation

Human parvovirus B19 (B19), a human pathogen of the erythroparvovirus genus, is responsible for a variety of diseases. B19 cause less symptoms in healthy individuals, also cause acute and chronic anemia in immunodeficiency patients. Transient aplastic crisis and pure red cell aplasia are two kinds of anemic hemogram, respectively, in acute and chronic B19 infection phase, especially occurring in patients with a shortened red cell survival or with immunodeficiency. In addition, B19-infected pregnant women may cause hydrops fetalis or fetal loss. B19 possesses high affinity to bone marrow and fetal liver due to its extremely restricted cytotoxicity to erythroid progenitor cells (EPCs) mediated by viral proteins. The nonstructural protein NS1 is considered to be the major pathogenic factor, which has been shown to inhibit the differentiation and maturation of EPCs through inducing viral DNA damage responses and cell cycle arrest. The time phase property of NS1 activity during DNA replication and conformity to transient change of hemogram are suggestive of its role in regulating differentiation of hematopoietic cells, which is not completely understood. In this review, we summarized the bridge between B19 NS1 and Notch signaling pathway or transcriptional factors GATA, which play an important role in erythroid cell proliferation and differentiation, to provide a new insight of the potential mechanism of B19-induced differential inhibition of EPCs.

Effect of Mesenchymal Stem Cell-derived Microvesicles on Megakaryocytic Differentiation of CD34+ Hematopoietic Stem Cells

Purpose: Mesenchymal stem cells (MSCs) release hematopoietic cytokines, growth factors, and Microvesicles (MVs) supporting the hematopoietic stem cells (HSCs). MVs released from various cells, playing a crucial role in biological functions of their parental cells. MSC-derived MVs contain microRNAs and proteins with key roles in the regulation of hematopoiesis. Umbilical cord blood (UCB) is a source for transplantation but the long-term recovery of platelets is a main problem. Therefore, we intend to show that MSC-MVs are able to improve the differentiation of UCB-derived CD34⁺ cells to megakaryocyte lineage.

Methods: In this descriptive study, MSCs were cultured in DMEM to collect the culture supernatant, which was ultracentrifuged for the isolation of MVs. HSCs were isolated from UCB using MACS method and cultured in IMDM supplemented with cytokines and MVs in three different conditions. Megakaryocyte differentiation was evaluated through the expression of specific markers and genes after 72 hours, and the data was analyzed by t test (P<0.05).

Results: The expression of specific megakaryocyte markers (CD41 and CD61) in the presence of different concentrations of MSC-MVs did not show any significant difference. Also, the expression of specific genes of megakaryocyte lineage was compared with control group. The expression of GATA2 and c-Mpl was significantly increased, GATA1 was not significantly decreased, and FLI1 was significantly decreased.

Conclusion: MSC-MVs could improve the expression of specific megakaryocyte genes; however, there was no significant expression of CD markers. Further studies, including the evaluation of late stages of megakaryocyte differentiation, are required to evaluate platelet production and shedding

Shared roles for Scl and Lyl1 in murine platelet production and function

The stem cell leukemia (Scl or Tal1) protein forms part of a multimeric transcription factor complex required for normal megakaryopoiesis. However, unlike other members of this complex such as Gata1, Fli1, and Runx1, mutations of Scl have not been observed as a cause of inherited thrombocytopenia. We postulated that functional redundancy with its closely related family member, lymphoblastic leukemia 1 (Lyl1) might explain this observation. To determine whether Lyl1 can substitute for Scl in megakaryopoiesis, we examined the platelet phenotype of mice lacking 1 or both factors in megakaryocytes. Conditional Scl knockout (KO) mice crossed with transgenic mice expressing Cre recombinase under the control of the mouse platelet factor 4 (Pf4) promoter generated megakaryocytes with markedly reduced but not absent Scl These Pf4Sclc-KO mice had mild thrombocytopenia and subtle defects in platelet aggregation. However, Pf4Sclc-KO mice generated on an Lyl1-null background (double knockout [DKO] mice) had severe macrothrombocytopenia, abnormal megakaryocyte morphology, defective pro-platelet formation, and markedly impaired platelet aggregation. DKO megakaryocytes, but not single-knockout megakaryocytes, had reduced expression of Gata1, Fli1, Nfe2, and many other genes that cause inherited thrombocytopenia. These gene expression changes were significantly associated with shared Scl and Lyl1 E-box binding sites that were also enriched for Gata1, Ets, and Runx1 motifs. Thus, Scl and Lyl1 share functional roles in platelet production by regulating expression of partner proteins including Gata1. We propose that this functional redundancy provides one explanation for the absence of Scl and Lyl1 mutations in inherited thrombocytopenia.

The Pleiotropic Effects of GATA1 and KLF1 in Physiological Erythropoiesis and in Dyserythropoietic Disorders

In the last few years, the advent of new technological approaches has led to a better knowledge of the ontogeny of erythropoiesis during development and of the journey leading from hematopoietic stem cells (HSCs) to mature red blood cells (RBCs). Our view of a well-defined hierarchical model of hematopoiesis with a near-homogeneous HSC population residing at the apex has been progressively challenged in favor of a landscape where HSCs themselves are highly heterogeneous and lineages separate earlier than previously thought. The coordination of these events is orchestrated by transcription factors (TFs) that work in a combinatorial manner to activate and/or repress their target genes. The development of next generation sequencing (NGS) has facilitated the identification of pathological mutations involving TFs underlying hematological defects. The examples of GATA1 and KLF1 presented in this review suggest that in the next few years the number of TF mutations associated with dyserythropoietic disorders will further increase.

Enhancer function regulated by combinations of transcription factors and cofactors

Regulation of the expression of diverse genes is essential for making possible the complexity of higher organisms, and the temporal and spatial regulation of gene expression allows for the alteration of cell types and growth patterns. A critical component of this regulation is the DNA sequence-specific binding of transcription factors (TFs). However, most TFs do not independently participate in gene transcriptional regulation, because they lack an effector function. Instead, TFs are thought to work by recruiting cofactors, including Mediator complex (Mediator), chromatin-remodeling complexes (CRCs), and histone-modifying complexes (HMCs). Mediator associates with the majority of transcribed genes and acts as an integrator of multiple signals. On the other hand, CRCs and HMCs are selectively recruited by TFs. Although all the pairings between TFs and CRCs or HMCs are not fully known, there are a growing number of established TF-CRC and TF-HMC combinations. In this review, we focused on the most important of these pairings and discuss how they control gene expression.

GATA1 insufficiencies in primary myelofibrosis and other hematopoietic disorders: consequences for therapy

INTRODUCTION

GATA1, the founding member of a family of transcription factors, plays important roles in the development of hematopoietic cells of several lineages. Although loss of GATA1 has been known to impair hematopoiesis in animal models for nearly 25 years, the link between GATA1 defects and human blood diseases has only recently been realized. Areas covered: Here the current understanding of the functions of GATA1 in normal hematopoiesis and how it is altered in disease is reviewed. GATA1 is indispensable mainly for erythroid and megakaryocyte differentiation. In erythroid cells, GATA1 regulates early stages of differentiation, and its deficiency results in apoptosis. In megakaryocytes, GATA1 controls terminal maturation and its deficiency induces proliferation. GATA1 alterations are often found in diseases involving these two lineages, such as congenital erythroid and/or megakaryocyte deficiencies, including Diamond Blackfan Anemia (DBA), and acquired neoplasms, such as acute megakaryocytic leukemia (AMKL) and the myeloproliferative neoplasms (MPNs). Expert commentary: Since the first discovery of GATA1 mutations in AMKL, the number of diseases that are associated with impaired GATA1 function has increased to include DBA and MPNs. With respect to the latter, we are only just now appreciating the link between enhanced JAK/STAT signaling, GATA1 deficiency and disease pathogenesis.

Transcriptional and Microenvironmental Regulation of Lineage Ambiguity in Leukemia

Leukemia is characterized by the uncontrolled production of leukemic cells and impaired normal hematopoiesis. Although the combination of chemotherapies and hematopoietic stem cell transplantation has significantly improved the outcome of leukemia patients, a proportion of patients still suffer from relapse after treatment. Upon relapse, a phenomenon termed “lineage switch” is observed in a subset of leukemia patients, in which conversion of lymphoblastic leukemia to myeloid leukemia or vice versa is observed. A rare entity of leukemia called mixed-phenotype acute leukemia exhibits co-expression of markers representing two or three lineages. These two phenotypes regarding the lineage ambiguity suggest that the fate of some leukemia retain or acquire a certain degree of plasticity. Studies using animal models provide insight into how lineage specifying transcription factors can enforce or convert a fate in hematopoietic cells. Modeling lineage conversion in normal hematopoietic progenitor cells may improve our current understanding of how lineage switch occurs in leukemia. In this review, we will summarize the role of transcription factors and microenvironmental signals that confer fate plasticity to normal hematopoietic progenitor cells, and their potential to regulate lineage switching in leukemias. Future efforts to uncover the mechanisms contributing to lineage conversion in both normal hematopoiesis and leukemia may pave the way to improve current therapeutic strategies.

A novel mode of stimulating platelet formation activity in megakaryocytes with peanut skin extract

We report in this study novel biochemical activities of peanut skin extract (PEXT) on thrombocytopoiesis. Peanut skin, derived from Arachis hypogaea L., is a traditional Chinese medicine that is used to treat chronic hemorrhage. We have shown that oral administration of PEXT increases the peripheral platelet levels in mice. Recently, we reported a liquid culture system that is useful for investigating megakaryocytopoiesis and thrombocytopoiesis from human CD34⁺ cells. In this liquid culture system, PEXT was shown to enhance the formation of CD41⁺/DAPI^- cells (platelets), but had no effect on the formation of CD41⁺/DAPI⁺ cells (megakaryocytes) or on the DNA content. Furthermore, PEXT selectively stimulated proplatelet formation from cultured mature megakaryocytes and phorbol 12-myristate 13 acetate (PMA)-induced formation of platelet-like particles from Meg01 cells. Despite having no influence on the formation of megakaryocyte colony forming units (CFUs), PEXT increased the size of megakaryocytes during their development from CD34⁺ cells. PEXT showed no effect on the GATA-1 and NF-E2 mRNA levels, which are known to play an important role in thrombocytopoiesis and, based on the results of a pMARE-Luc (pGL3-MARE-luciferase) assay, had no influence on NF-E2 activation in Meg01 cells. These results suggest that PEXT accelerates proplatelet formation from megakaryocytes but does not influence the development of hematopoietic stem cells into megakaryocytes.

The thrombopoietin/MPL axis is activated in the Gata1low mouse model of myelofibrosis and is associated with a defective RPS14 signature

Myelofibrosis (MF) is characterized by hyperactivation of thrombopoietin (TPO) signaling, which induces a RPS14 deficiency that de-regulates GATA1 in megakaryocytes by hampering its mRNA translation. As mice carrying the hypomorphic Gata1^low mutation, which reduces the levels of Gata1 mRNA in megakaryocytes, develop MF, we investigated whether the TPO axis is hyperactive in this model. Gata1^low mice contained two times more Tpo mRNA in liver and TPO in plasma than wild-type littermates. Furthermore, Gata1^low LSKs expressed levels of Mpl mRNA (five times greater than normal) and protein (two times lower than normal) similar to those expressed by LSKs from TPO-treated wild-type mice. Gata1^low marrow and spleen contained more JAK2/STAT5 than wild-type tissues, an indication that these organs were reach of TPO-responsive cells. Moreover, treatment of Gata1^low mice with the JAK inhibitor ruxolitinib reduced their splenomegaly. Also in Gata1^low mice activation of the TPO/MPL axis was associated with a RSP14 deficiency and a discordant microarray ribosome signature (reduced RPS24, RPS26 and SBDS expression). Finally, electron microscopy revealed that Gata1^low megakaryocytes contained poorly developed endoplasmic reticulum with rare polysomes. In summary, Gata1^low mice are a bona fide model of MF, which recapitulates the hyperactivation of the TPO/MPL/JAK2 axis observed in megakaryocytes from myelofibrotic patients.

Transcription factor defects causing platelet disorders

Recent years have seen increasing recognition of a subgroup of inherited platelet function disorders which are due to defects in transcription factors that are required to regulate megakaryopoiesis and platelet production. Thus, germline mutations in the genes encoding the haematopoietic transcription factors RUNX1, GATA-1, FLI1, GFI1b and ETV6 have been associated with both quantitative and qualitative platelet abnormalities, and variable bleeding symptoms in the affected patients. Some of the transcription factor defects are also associated with an increased predisposition to haematologic malignancies (RUNX1, ETV6), abnormal erythropoiesis (GATA-1, GFI1b, ETV6) and immune dysfunction (FLI1). The persistence of MYH10 expression in platelets is a surrogate marker for FLI1 and RUNX1 defects. Characterisation of the transcription factor defects that give rise to platelet function disorders, and of the genes that are differentially regulated as a result, are yielding insights into the roles of these genes in platelet formation and function.

Repercussion of Megakaryocyte-Specific Gata1 Loss on Megakaryopoiesis and the Hematopoietic Precursor Compartment

During hematopoiesis, transcriptional programs are essential for the commitment and differentiation of progenitors into the different blood lineages. GATA1 is a transcription factor expressed in several hematopoietic lineages and essential for proper erythropoiesis and megakaryopoiesis. Megakaryocyte-specific genes, such as GP1BA, are known to be directly regulated by GATA1. Mutations in GATA1 can lead to dyserythropoietic anemia and pseudo gray-platelet syndrome. Selective loss of Gata1 expression in adult mice results in macrothrombocytopenia with platelet dysfunction, characterized by an excess of immature megakaryocytes. To specifically analyze the impact of Gata1 loss in mature committed megakaryocytes, we generated Gata1-Lox|Pf4-Cre mice (Gata1cKO^MK). Consistent with previous findings, Gata1cKO^MK mice are macrothrombocytopenic with platelet dysfunction. Supporting this notion we demonstrate that Gata1 regulates directly the transcription of Syk, a tyrosine kinase that functions downstream of Clec2 and GPVI receptors in megakaryocytes and platelets. Furthermore, we show that Gata1cKO^MK mice display an additional aberrant megakaryocyte differentiation stage. Interestingly, these mice present a misbalance of the multipotent progenitor compartment and the erythroid lineage, which translates into compensatory stress erythropoiesis and splenomegaly. Despite the severe thrombocytopenia, Gata1cKO^MK mice display a mild reduction of TPO plasma levels, and Gata1cKO^MK megakaryocytes show a mild increase in Pf4 mRNA levels; such a misbalance might be behind the general hematopoietic defects observed, affecting locally normal TPO and Pf4 levels at hematopoietic stem cell niches.

Thrombin Maybe Plays an Important Role in MK Differentiation into Platelets

OBJECTIVES

After development and differentiation, megakaryocytes (MKs) can produce platelets. As is well known, thrombopoietin (TPO) can induce MKs to differentiate. The effect of thrombin on MKs differentiation is not clear. In this study, we used a human megakaryoblastic leukemia cell line (Meg-01) to assess the effect of thrombin on MKs differentiation.

METHODS

In order to interrogate the role of thrombin in Meg-01 cells differentiation, the changes of morphology, cellular function, and expression of diverse factors were analyzed.

RESULTS

The results show that thrombin suppresses Meg-01 cells proliferation and induces apoptosis and cell cycle arrest. Thrombin upregulates the expression of CD41b, which is one of the most important MK markers. Globin transcription factor 1 (GATA-1), an important transcriptional regulator, controls MK development and maturation. The expression of GATA-1 is also upregulated by thrombin in Meg-01 cells. The expression of B-cell lymphoma 2 (Bcl-2), an apoptosis-inhibitory protein, is downregulated by thrombin. Phosphorylated protein kinase B (p-AKT) and phosphorylated extracellular signal-regulated kinase (p-ERK) were upregulated by thrombin in Meg-01 cells. All the results are consistent with Meg-01 cells treated with TPO.

DISCUSSION AND CONCLUSION

In conclusion, all these data indicate that thrombin maybe plays an important role in MK differentiation into platelets. However, whether the platelet-like particles are certainly platelets remains unknown.

RUNX1B Expression Is Highly Heterogeneous and Distinguishes Megakaryocytic and Erythroid Lineage Fate in Adult Mouse Hematopoiesis

The Core Binding Factor (CBF) protein RUNX1 is a master regulator of definitive hematopoiesis, crucial for hematopoietic stem cell (HSC) emergence during ontogeny. RUNX1 also plays vital roles in adult mice, in regulating the correct specification of numerous blood lineages. Akin to the other mammalian Runx genes, Runx1 has two promoters P1 (distal) and P2 (proximal) which generate distinct protein isoforms. The activities and specific relevance of these two promoters in adult hematopoiesis remain to be fully elucidated. Utilizing a dual reporter mouse model we demonstrate that the distal P1 promoter is broadly active in adult hematopoietic stem and progenitor cell (HSPC) populations. By contrast the activity of the proximal P2 promoter is more restricted and its upregulation, in both the immature Lineage- Sca1high cKithigh (LSK) and bipotential Pre-Megakaryocytic/Erythroid Progenitor (PreMegE) populations, coincides with a loss of erythroid (Ery) specification. Accordingly the PreMegE population can be prospectively separated into "pro-erythroid" and "pro-megakaryocyte" populations based on Runx1 P2 activity. Comparative gene expression analyses between Runx1 P2+ and P2- populations indicated that levels of CD34 expression could substitute for P2 activity to distinguish these two cell populations in wild type (WT) bone marrow (BM). Prospective isolation of these two populations will enable the further investigation of molecular mechanisms involved in megakaryocytic/erythroid (Mk/Ery) cell fate decisions. Having characterized the extensive activity of P1, we utilized a P1-GFP homozygous mouse model to analyze the impact of the complete absence of Runx1 P1 expression in adult mice and observed strong defects in the T cell lineage. Finally, we investigated how the leukemic fusion protein AML1-ETO9a might influence Runx1 promoter usage. Short-term AML1-ETO9a induction in BM resulted in preferential P2 upregulation, suggesting its expression may be important to establish a pre-leukemic environment.

P-Selectin Sustains Extramedullary Hematopoiesis in the Gata1 low Model of Myelofibrosis

Splenomegaly is a major manifestation of primary myelofibrosis (PMF) contributing to clinical symptoms and hematologic abnormalities. The spleen from PMF patients contains increased numbers of hematopoietic stem cells (HSC) and megakaryocytes (MK). These MK express high levels of P-selectin (P-sel) that, by triggering neutrophil emperipolesis, may cause TGF-β release and disease progression. This hypothesis was tested by deleting the P-sel gene in the myelofibrosis mouse model carrying the hypomorphic Gata1(low) mutation that induces megakaryocyte abnormalities that recapitulate those observed in PMF. P-sel(null) Gata1(low) mice survived splenectomy and lived 3 months longer than P-sel(WT) Gata1(low) littermates and expressed limited fibrosis and osteosclerosis in the marrow or splenomegaly. Furthermore, deletion of P-sel disrupted megakaryocyte/neutrophil interactions in spleen, reduced TGF-β content, and corrected the HSC distribution that in Gata1(low) mice, as in PMF patients, is abnormally expanded in spleen. Conversely, pharmacological inhibition of TGF-β reduced P-sel expression in MK and corrected HSC distribution. Spleens, but not marrow, of Gata1(low) mice contained numerous cKIT(pos) activated fibrocytes, probably of dendritic cell origin, whose membrane protrusions interacted with MK establishing niches hosting immature cKIT(pos) hematopoietic cells. These activated fibrocytes were not detected in spleens from P-sel(null) Gata1(low) or TGF-β-inhibited Gata1(low) littermates and were observed in spleen, but not in marrow, from PMF patients. Therefore, in Gata1(low) mice, and possibly in PMF, abnormal P-sel expression in MK may mediate the pathological cell interactions that increase TGF-β content in MK and favor establishment of a microenvironment that supports myelofibrosis-related HSC in spleen.

Differential regulation of MCM7 and its intronic miRNA cluster miR-106b-25 during megakaryopoiesis induced polyploidy

Megakaryocytes exit from mitotic cell cycle and enter a phase of repeated DNA replication without undergoing cell division, in a process termed as endomitosis of which little is known. We studied the expression of a DNA replication licensing factor mini chromosome maintenance protein 7 (MCM7) and its intronic miR-106b-25 cluster during mitotic and endo-mitotic cycles in megakaryocytic cell lines and in vitro cultured megakaryocytes obtained from human cord blood derived CD34(+) cells. Our results show that contrary to mitotic cell cycle, endomitosis proceeds with an un-coupling of the expression of MCM7 and miR-106b-25. This was attributed to the presence of a transcript variant of MCM7 which undergoes nonsense mediated decay (NMD). Additionally, miR-25 which was up regulated during endomitosis was found to promote megakaryopoiesis by inhibiting the expression of PTEN. Our study thus highlights the importance of a transcript variant of MCM7 destined for NMD in the modulation of megakaryopoiesis.

Regulation of the genetic code in megakaryocytes and platelets

Platelets are generated from nucleated precursors referred to as megakaryocytes. The formation of platelets is one of the most elegant and unique developmental processes in eukaryotes. Because they enter the circulation without nuclei, platelets are often considered simple, non-complex cells that have limited functions beyond halting blood flow. However, emerging evidence over the past decade demonstrates that platelets are more sophisticated than previously considered. Platelets carry a rich repertoire of messenger RNAs (mRNAs), microRNAs (miRNAs), and proteins that contribute to primary (adhesion, aggregation, secretion) and alternative (immune regulation, RNA transfer, translation) functions. It is also becoming increasingly clear that the 'genetic code' of platelets changes with race, genetic disorders, or disease. Changes in the 'genetic code' can occur at multiple points including megakaryocyte development, platelet formation, or in circulating platelets. This review focuses on regulation of the 'genetic code' in megakaryocytes and platelets and its potential contribution to health and disease.

Inducible Gata1 suppression expands megakaryocyte-erythroid progenitors from embryonic stem cells

Transfusion of donor-derived platelets is commonly used for thrombocytopenia, which results from a variety of clinical conditions and relies on a constant donor supply due to the limited shelf life of these cells. Embryonic stem (ES) and induced pluripotent stem (iPS) cells represent a potential source of megakaryocytes and platelets for transfusion therapies; however, the majority of current ES/iPS cell differentiation protocols are limited by low yields of hematopoietic progeny. In both mice and humans, mutations in the gene-encoding transcription factor GATA1 cause an accumulation of proliferating, developmentally arrested megakaryocytes, suggesting that GATA1 suppression in ES and iPS cell-derived hematopoietic progenitors may enhance megakaryocyte production. Here, we engineered ES cells from WT mice to express a doxycycline-regulated (dox-regulated) shRNA that targets Gata1 transcripts for degradation. Differentiation of these cells in the presence of dox and thrombopoietin (TPO) resulted in an exponential (at least 10¹³-fold) expansion of immature hematopoietic progenitors. Dox withdrawal in combination with multilineage cytokines restored GATA1 expression, resulting in differentiation into erythroblasts and megakaryocytes. Following transfusion into recipient animals, these dox-deprived mature megakaryocytes generated functional platelets. Our findings provide a readily reproducible strategy to exponentially expand ES cell-derived megakaryocyte-erythroid progenitors that have the capacity to differentiate into functional platelet-producing megakaryocytes.

The secret life of a megakaryocyte: emerging roles in bone marrow homeostasis control

Megakaryocytes are rare cells found in the bone marrow, responsible for the everyday production and release of millions of platelets into the bloodstream. Since the discovery and cloning, in 1994, of their principal humoral factor, thrombopoietin, and its receptor c-Mpl, many efforts have been directed to define the mechanisms underlying an efficient platelet production. However, more recently different studies have pointed out new roles for megakaryocytes as regulators of bone marrow homeostasis and physiology. In this review we discuss the interaction and the reciprocal regulation of megakaryocytes with the different cellular and extracellular components of the bone marrow environment. Finally, we provide evidence that these processes may concur to the reconstitution of the bone marrow environment after injury and their deregulation may lead to the development of a series of inherited or acquired pathologies.

Are GATA1 mutations occurring at random in Down syndrome transient leukemia?

The somatic mutation theory of cancer proposes that cancer begins with a somatic mutation occurring at random in a single cell that then passes the mutation to its progeny, generating a clone of premalignant cells. This clone leads to a full malignant tumor through additional mutations and selection processes. Strikingly, the best-documented human model of early oncogenesis, i.e., transient myeloproliferative disorder followed by acute megakaryoblastic leukemia (AMKL) in infants with Down syndrome (DS, or trisomy 21), exhibits important discrepancies with the SMT. Somatic mutations in megakaryocytic precursors occur at least 100,000 times more frequently in the GATA1 gene in fetuses with DS compared to the general population. Further, mutations are limited to GATA1 only; the general mutation rate does not significantly differ between individuals with DS and euploid individuals. Importantly, the mutations are also lineage-specific, occurring only in the megakaryocytic lineage, and proliferative anomalies of the megakaryocytic lineage are observed before the occurrence of GATA1 mutations. Thus, GATA1 mutations in fetuses with DS cannot be random events occurring in normal cells. Here, transcription-associated mutagenesis is proposed as the mechanism by which the earliest mutations of AMKL occur in DS. Transcription-associated mutagenesis is observed in non-dividing cells when a gene is over-expressed. The over-expression of GATA1 in the megakaryocytic lineage in DS fetal liver cells is proposed to be the cause of targeted GATA1 somatic mutations. As transcription-associated mutagenesis is a universal process, this mechanism may also apply to early oncogenesis in other situations, including after birth and following exposure to a carcinogenic agent. Thus, this hypothesis represents a new avenue for understanding and exploring oncogenesis in the context of DS and in other disease states.

Preclinical models for drug selection in myeloproliferative neoplasms

The discovery that an abnormally activated JAK-STAT signaling pathway is central to the pathogenesis of myeloproliferative neoplasms has promoted the clinical development of small-molecule JAK2 inhibitors. These agents have shown remarkable efficacy in disease control, but do not induce molecular remission; on the other hand, interferon holds the promise to target the putative hematopoietic progenitor cell initiating the disease. The presence of additional molecular abnormalities indicates a high molecular complexity of myeloproliferative neoplasms, and the need for simultaneously targeting different targets. Several drugs are currently under study as single agents and in combination. This review briefly describes the several in vitro and in vivo models of myeloproliferative neoplasms that are being used as preclinical models for drug development.

Distinct, strict requirements for Gfi-1b in adult bone marrow red cell and platelet generation

Strict, lineage-intrinsic requirement for continuous adult Gfi-1b expression at two distinct critical stages of erythropoiesis and megakaryopoiesis.

The zinc finger transcriptional repressor Gfi-1b is essential for erythroid and megakaryocytic development in the embryo. Its roles in the maintenance of bone marrow erythropoiesis and thrombopoiesis have not been defined. We investigated Gfi-1b’s adult functions using a loxP-flanked Gfi-1b allele in combination with a novel doxycycline-inducible Cre transgene that efficiently mediates recombination in the bone marrow. We reveal strict, lineage-intrinsic requirements for continuous adult Gfi-1b expression at two distinct critical stages of erythropoiesis and megakaryopoiesis. Induced disruption of Gfi-1b was lethal within 3 wk with severely reduced hemoglobin levels and platelet counts. The erythroid lineage was arrested early in bipotential progenitors, which did not give rise to mature erythroid cells in vitro or in vivo. Yet Gfi-1b^−/− progenitors had initiated the erythroid program as they expressed many lineage-restricted genes, including Klf1/Eklf and Erythropoietin receptor. In contrast, the megakaryocytic lineage developed beyond the progenitor stage in Gfi-1b’s absence and was arrested at the promegakaryocyte stage, after nuclear polyploidization, but before cytoplasmic maturation. Genome-wide analyses revealed that Gfi-1b directly regulates a wide spectrum of megakaryocytic and erythroid genes, predominantly repressing their expression. Together our study establishes Gfi-1b as a master transcriptional repressor of adult erythropoiesis and thrombopoiesis.

Lessons in platelet production from inherited thrombocytopenias

Our knowledge of the cellular and molecular mechanisms of platelet production has greatly expanded in recent years due to the opportunity to culture in vitro megakaryocytes and to create transgenic animals with specific genetic defects that interfere with platelet biogenesis. However, in vitro models do not reproduce the complexity of the bone marrow microenvironment where megakaryopoiesis takes place, and experience shows that what is seen in animals does not always happen in humans. So, these experimental models tell us what might happen in humans, but does not assure us that these events really occur. In contrast, inherited thrombocytopenias offer the unique opportunity to verify in humans the actual effects of abnormalities in specific molecules on platelet production. There are currently 20 genes whose defects are known to result in thrombocytopenia and, on this basis, this review tries to outline a model of megakaryopoiesis based on firm evidence. Inherited thrombocytopenias have not yet yielded all the information they can provide, because nearly half of patients have forms that do not fit with any known disorder. So, further investigation of inherited thrombocytopenias will advance not only the knowledge of human illnesses, but also our understanding of human platelet production.

Pathological interactions between hematopoietic stem cells and their niche revealed by mouse models of primary myelofibrosis

Primary myelofibrosis (PMF) belongs to the Philadelphia-negative myeloproliferative neoplasms and is a hematological disorder caused by abnormal function of the hematopoietic stem cells. The disease manifests itself with a plethora of alterations, including anemia, splenomegaly and extramedullary hematopoiesis. Its hallmarks are progressive marrow fibrosis and atypical megakaryocytic hyperplasia, two distinctive features used to clinically monitor disease progression. In an attempt to investigate the role of abnormal megakaryocytopoiesis in the pathogenesis of PMF, several transgenic mouse models have been generated. These models are based either on mutations that interfere with the extrinsic (thrombopoietin and its receptor, MPL) and intrinsic (the GATA1 transcription factor) control of normal megakaryocytopoiesis, or on known genetic lesions associated with the human disease. Here we provide an up-to-date review on the insights into the pathobiology of human PMF achieved by studying these animal models, with particular emphasis on results obtained with Gata1(low) mice.

Inherited platelet function disorders: overview and disorders of granules, secretion, and signal transduction

Inherited disorders of platelet function are characterized by highly variable mucocutaneous bleeding manifestations. The platelet dysfunction arises by diverse mechanisms, including abnormalities in platelet membrane glycoproteins, granules and their contents, platelet signaling and secretion mechanisms: thromboxane production pathways and in platelet procoagulant activities. Platelet aggregation and secretion studies using platelet-rich plasma currently form the primary basis for the diagnosis of an inherited platelet dysfunction. In most such patients, the molecular and genetic mechanisms are unknown. Management of these patients needs to be individualized; therapeutic options include platelet transfusions, 1-desamino-8d-arginine vasopressin (DDAVP), recombinant factor VIIa, and antifibrinolytic agents.

Abnormal P-selectin localization during megakaryocyte development determines thrombosis in the gata1low model of myelofibrosis

Patients with primary myelofibrosis have increased risk for bleeding and thrombosis. It is debated whether propensity to thrombosis is due to increased numbers of platelet microparticles and/or to pathological platelet-neutrophil interactions. Platelet neutrophil interactions are mediated by P-selectin and even though the megakaryocytes of myelofibrosis patients express normal levels of P-selectin, it remains abnormally localized to the demarcation membrane system rather than being assembled into the α-granules in platelets. Mice carrying the hypomorphic Gata1(low) mutation express the same megakaryocyte abnormalities presented by primary myelofibrosis patients, including abnormal P-selectin localization to the DMS and develop with age myelofibrosis, a disease that closely resembles human primary myelofibrosis. Whether these mice would also develop thrombosis has not been investigated as yet. The aim of this study was to determine whether Gata1(low) mice would develop thrombosis with age and, in this case, the role played by P-selectin in the development of the trait. To this aim, Gata1(low) mice were crossed with P-sel(null) mice according to standard genetic protocols and Gata1(low)P-sel(wt), Gata1(low)P-sel(null) and Gata1(WT)P-sel(null) or Gata1(wt)P-sel(wt) (as controls) littermates obtained. It was shown that platelet counts, but not hematocrit, are reduced in Gata1(low) mice. Moreover, platelet microparticles are reduced in Gata1(low) mice and P-selectin positive platelet microparticles were not found. To determine the phenotypic implications of the different mutations, bleeding time was estimated by a tail cut procedure. Mutant mice were sacrificed and presence of thrombosis was determined by immunohistological staining of organs. Gata1(low) mice with or without the P-selectin null trait had a prolonged bleeding time compared to wild type mice. However, in Gata1(low) mice significantly higher frequency of thrombotic events was seen in adult and old Gata1(low) mice compared to Gata1(low)P-sel(null) mice. Thus, presence of the P-selectin null trait rescued Gata1(low) mice from the thrombotic phenotype, but did not change the level of platelet microparticles. Taken together these data indicate that abnormal localization of P-selectin, induced by the Gata1(low) mutation, and thus, increased pathological interactions with leucocytes, is responsible for the increased presence of thrombosis seen in these mice.

Thrombocytopenia induced by the histone deacetylase inhibitor abexinostat involves p53-dependent and -independent mechanisms

Abexinostat is a pan histone deacetylase inhibitor (HDACi) that demonstrates efficacy in malignancy treatment. Like other HDACi, this drug induces a profound thrombocytopenia whose mechanism is only partially understood. We have analyzed its effect at doses reached in patient plasma on in vitro megakaryopoiesis derived from human CD34⁺ cells. When added at day 0 in culture, abexinostat inhibited CFU-MK growth, megakaryocyte (MK) proliferation and differentiation. These effects required only a short incubation period. Decreased proliferation was due to induction of apoptosis and was not related to a defect in TPO/MPL/JAK2/STAT signaling. When added later (day 8), the compound induced a dose-dependent decrease (up to 10-fold) in proplatelet (PPT) formation. Gene profiling from MK revealed a silencing in the expression of DNA repair genes with a marked RAD51 decrease at protein level. DNA double-strand breaks were increased as attested by elevated γH2AX phosphorylation level. Moreover, ATM was phosphorylated leading to p53 stabilization and increased BAX and p21 expression. The use of a p53 shRNA rescued apoptosis, and only partially the defect in PPT formation. These results suggest that HDACi induces a thrombocytopenia by a p53-dependent mechanism along MK differentiation and a p53-dependent and -independent mechanism for PPT formation.

Developmental differences in IFN signaling affect GATA1s-induced megakaryocyte hyperproliferation

About 10% of Down syndrome (DS) infants are born with a transient myeloproliferative disorder (DS-TMD) that spontaneously resolves within the first few months of life. About 20%-30% of these infants subsequently develop acute megakaryoblastic leukemia (DS-AMKL). Somatic mutations leading to the exclusive production of a short GATA1 isoform (GATA1s) occur in all cases of DS-TMD and DS-AMKL. Mice engineered to exclusively produce GATA1s have marked megakaryocytic progenitor (MkP) hyperproliferation during early fetal liver (FL) hematopoiesis, but not during postnatal BM hematopoiesis, mirroring the spontaneous resolution of DS-TMD. The mechanisms that underlie these developmental stage-specific effects are incompletely understood. Here, we report a striking upregulation of type I IFN-responsive gene expression in prospectively isolated mouse BM- versus FL-derived MkPs. Exogenous IFN-α markedly reduced the hyperproliferation FL-derived MkPs of GATA1s mice in vitro. Conversely, deletion of the α/β IFN receptor 1 (Ifnar1) gene or injection of neutralizing IFN-α/β antibodies increased the proliferation of BM-derived MkPs of GATA1s mice beyond the initial postnatal period. We also found that these differences existed in human FL versus BM megakaryocytes and that primary DS-TMD cells expressed type I IFN-responsive genes. We propose that increased type I IFN signaling contributes to the developmental stage-specific effects of GATA1s and possibly the spontaneous resolution of DS-TMD.

Pathogenesis and management of inherited thrombocytopenias: rationale for the use of thrombopoietin-receptor agonists

Knowledge in the field of inherited thrombocytopenias (ITs) has considerably improved over the recent years. In the last 5 years, nine new genes whose mutations are responsible for thrombocytopenia have been identified, and this also led to the recognition of several novel nosographic entities, such as thrombocytopenias deriving from mutations in CYCS, TUBB1, FLNA, ITGA2B/ITGB3, ANKRD26 and ACTN1. The identification of novel molecular alterations causing thrombocytopenia together with improvement of methodologies to study megakaryopoiesis led to considerable advances in understanding pathophysiology of ITs, thus providing the background for proposing new treatments. Thrombopoietin-receptor agonists (TPO-RAs) represent an appealing therapeutic hypothesis for ITs and have been tested in a limited number of patients. In this review, we provide an updated description of pathogenetic mechanisms of thrombocytopenia in the different forms of ITs and recapitulate the current management of these disorders. Moreover, we report the available clinical and preclinical data about the role of TPO-RAs in ITs and discuss the rationale for the use of these molecules in view of pathogenesis of the different forms of thrombocytopenia of genetic origin.

Contribution of GATA1 dysfunction to multi-step leukemogenesis

In mammals, hematopoietic homeostasis is maintained by a fine-tuned balance among the self-renewal, proliferation, differentiation and survival of hematopoietic stem cells and their progenies. Each process is also supported by the delicate balance of the expression of multiple genes specific to each process. GATA1 is a transcription factor that comprehensively regulates the genes that are important for the development of erythroid and megakaryocytic cells. Accumulating evidence supports the notion that defects in GATA1 function are intimately linked to hematopoietic disorders. In particular, the somatic mutation of the GATA1 gene, which leads to the production of N-terminally truncated GATA1, contributes to the genesis of transient myeloproliferative disorder and acute megakaryoblastic leukemia in infants with Down syndrome. Similarly, a mutation in the GATA1 regulatory region that reduces GATA1 expression is involved in the onset of erythroid leukemia in mice. In both cases, the accumulation of immature progenitor cells caused by GATA1 dysregulation underlies the pathogenesis of the leukemia. This review provides a summary of multi-step leukemogenesis with a focus on GATA1 dysfunction.

Pluripotent stem cells reveal the developmental biology of human megakaryocytes and provide a source of platelets for clinical application

Human pluripotent stem cells [PSCs; including human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs)] can infinitely proliferate in vitro and are easily accessible for gene manipulation. Megakaryocytes (MKs) and platelets can be created from human ESCs and iPSCs in vitro and represent a potential source of blood cells for transfusion and a promising tool for studying the human thrombopoiesis. Moreover, disease-specific iPSCs are a powerful tool for elucidating the pathogenesis of hematological diseases and for drug screening. In that context, we and other groups have developed in vitro MK and platelet differentiation systems from human pluripotent stem cells (PSCs). Combining this co-culture system with a drug-inducible gene expression system enabled us to clarify the novel role played by c-MYC during human thrombopoiesis. In the next decade, technical advances (e.g., high-throughput genomic sequencing) will likely enable the identification of numerous gene mutations associated with abnormal thrombopoiesis. Combined with such technology, an in vitro system for differentiating human PSCs into MKs and platelets could provide a novel platform for studying human gene function associated with thrombopoiesis.

NPC1 defect results in abnormal platelet formation and function: studies in Niemann-Pick disease type C1 patients and zebrafish

Niemann-Pick type C is a lysosomal storage disease associated with mutations in NPC1 or NPC2, resulting in an accumulation of cholesterol in the endosomal-lysosomal system. Niemann-Pick type C has a clinical spectrum that ranges from a neonatal rapidly fatal disorder to an adult-onset chronic neurodegenerative disease combined with remarkably, in some cases, hematological defects such as thrombocytopenia, anemia and petechial rash. A role of NPC1 in hematopoiesis was never shown. Here, we describe platelet function abnormalities in three unrelated patients with a proven genetic and biochemical NPC1 defect. Their platelets have reduced aggregations, P-selectin expression and ATP secretions that are compatible with the observed abnormal alpha and reduced dense granules as studied by electron microscopy and CD63 staining after platelet spreading. Their blood counts were normal. NPC1 expression was shown in platelets and megakaryocytes (MKs). In vitro differentiated MKs from NPC1 patients exhibit hyperproliferation of immature MKs with different CD63(+) granules and abnormal cellular accumulation of cholesterol as shown by filipin stainings. The role of NPC1 in megakaryopoiesis was further studied using zebrafish with GFP-labeled thrombocytes or DsRed-labeled erythrocytes. NPC1 depletion in zebrafish resulted in increased cell death in the brain and abnormal cellular accumulation of filipin. NPC1-depleted embryos presented with thrombocytopenia and mild anemia as studied by flow cytometry and real-time QPCR for specific blood cell markers. In conclusion, this is the first report, showing a role of NPC1 in platelet function and formation but further studies are needed to define how cholesterol storage interferes with these processes.

Defective nuclear localization of Hsp70 is associated with dyserythropoiesis and GATA-1 cleavage in myelodysplastic syndromes

Normal human erythroid cell maturation requests the transcription factor GATA-1 and a transient activation of caspase-3, with GATA-1 being protected from caspase-3-mediated cleavage by interaction with the chaperone heat shock protein 70 (Hsp70) in the nucleus. Erythroid cell dysplasia observed in early myelodysplastic syndromes (MDS) involves impairment of differentiation and excess of apoptosis with a burst of caspase activation. Analysis of gene expression in MDS erythroblasts obtained by ex vivo cultures demonstrates the down-regulation of a set of GATA-1 transcriptional target genes, including GYPA that encodes glycophorin A (GPA), and the up-regulation of members of the HSP70 family. GATA-1 protein expression is decreased in MDS erythroblasts, but restores in the presence of a pan-caspase inhibitor. Expression of a mutated GATA-1 that cannot be cleaved by caspase-3 rescues the transcription of GATA-1 targets, and the erythroid differentiation, but does not improve survival. Hsp70 fails to protect GATA-1 from caspases because the protein does not accumulate in the nucleus with active caspase-3. Expression of a nucleus-targeted mutant of Hsp70 protects GATA-1 and rescues MDS erythroid cell differentiation. Alteration of Hsp70 cytosolic-nuclear shuttling is a major feature of MDS that favors GATA-1 cleavage and differentiation impairment, but not apoptosis, in dysplastic erythroblasts.

Pleiotropic platelet defects in mice with disrupted FOG1-NuRD interaction

Understanding platelet biology has been aided by studies of mice with mutations in key megakaryocytic transcription factors. We have shown that point mutations in the GATA1 cofactor FOG1 that disrupt binding to the nucleosome remodeling and deacetylase (NuRD) complex have erythroid and megakaryocyte lineages defects. Mice that are homozygous for a FOG1 point mutation (ki/ki), which ablates FOG1-NuRD interactions, have platelets that display a gray platelet syndrome (GPS)-like macrothrombocytopenia. These platelets have few α-granules and an increased number of lysosomal-like vacuoles on electron microscopy, reminiscent of the platelet in patients with GATA1-related X-linked GPS. Here we further characterized the platelet defect in ki/ki mice. We found markedly deficient levels of P-selectin protein limited to megakaryocytes and platelets. Other α-granule proteins were expressed at normal levels and were appropriately localized to α-granule-like structures. Treatment of ki/ki platelets with thrombin failed to stimulate Akt phosphorylation, resulting in poor granule secretion and platelet aggregation. These studies show that disruption of the GATA1/FOG1/NuRD transcriptional system results in a complex, pleiotropic platelet defect beyond GPS-like macrothrombocytopenia and suggest that this transcriptional complex regulates not only megakaryopoiesis but also α-granule generation and signaling pathways required for granule secretion.

c-Myb and GATA-1 alternate dominant roles during megakaryocyte differentiation

BACKGROUND

Transcription factors are essential for blood cell formation. Mice expressing low levels of c-Myb (c-Myb(low)) have an increased number of bone marrow megakaryocytes (MKs) and corresponding thrombocytosis. In contrast, mice engineered to express low levels of GATA-1 (GATA-1(low)) in the megakaryocytic lineage exhibit aberrant megakaryocytopoiesis with hyperproliferation of progenitors and defective terminal differentiation leading to thrombocytopenia. These seemingly opposite roles may affect platelet turnover and thus be of clinical relevance.

OBJECTIVE

To determine how these two transcription factors act together to control megakaryocytopoiesis and platelet formation.

METHODS

We used a combination of cellular and molecular in vitro assays to examine the ability of bone marrow cells from mice expressing low levels of both c-Myb and GATA-1 (referred to as double(low)) to produce MKs and platelets.

RESULTS

Double(low) cells, or those with low GATA-1 levels in which c-Myb is conditionally deleted, lack the hyperproliferative capacity of GATA-1(low) cells, allowing the cells to proceed towards more committed MKs that are, however, impaired in their capacity to produce fully differentiated cells, as confirmed by the abundance of morphologically aberrant cells that lack the ability to form proplatelets.

CONCLUSION

c-Myb and GATA-1 act in concert to achieve correct megakaryocytic differentiation. GATA-1 regulates both the proliferation of megakaryocytic progenitors and their terminal maturation. c-Myb also acts at the level of the progenitor by influencing its commitment to differentiation, but in contrast to GATA-1 it does not have any effect on the process of terminal differentiation.

SCL-mediated regulation of the cell-cycle regulator p21 is critical for murine megakaryopoiesis

Megakaryopoiesis is a complex process that involves major cellular and nuclear changes and relies on controlled coordination of cellular proliferation and differentiation. These mechanisms are orchestrated in part by transcriptional regulators. The key hematopoietic transcription factor stem cell leukemia (SCL)/TAL1 is required in early hematopoietic progenitors for specification of the megakaryocytic lineage. These early functions have, so far, prevented full investigation of its role in megakaryocyte development in loss-of-function studies. Here, we report that SCL critically controls terminal megakaryocyte maturation. In vivo deletion of Scl specifically in the megakaryocytic lineage affects all key attributes of megakaryocyte progenitors (MkPs), namely, proliferation, ploidization, cytoplasmic maturation, and platelet release. Genome-wide expression analysis reveals increased expression of the cell-cycle regulator p21 in Scl-deleted MkPs. Importantly, p21 knockdown-mediated rescue of Scl-mutant MkPs shows full restoration of cell-cycle progression and partial rescue of the nuclear and cytoplasmic maturation defects. Therefore, SCL-mediated transcriptional control of p21 is essential for terminal maturation of MkPs. Our study provides a mechanistic link between a major hematopoietic transcriptional regulator, cell-cycle progression, and megakaryocytic differentiation.

Mouse models of diseases of megakaryocyte and platelet homeostasis

Platelets are the small anuclear blood cells that are the product of megakaryocytopoiesis, the process of hematopoietic stem cell commitment to megakaryocyte production and the differentiation and maturation of these cells for platelet release. Deregulation or disruption of megakaryocytopoiesis can result in platelet deficiencies, the thrombocytopenias, with attendant risk of hemorrhage or thrombocytosis, a pathological excess of platelet numbers. Mouse models, particularly those engineered to carry genetic alterations modeling mutations associated with human disease, have provided important insights into megakaryocytopoiesis and deregulation of this process in disease. This review focuses on mouse models of diseases of altered megakaryocyte and platelet number, illustrating the profound contribution of these models in validating suspected roles of disease-associated genetic alterations, promoting discovery of new links between genetic mutations and specific diseases, and providing unique tools for better understanding of disease pathophysiology and progression, as well as resources to define drug action or develop new therapeutic strategies.

Bidirectional interactions between bone metabolism and hematopoiesis

Interactions between hematopoiesis and bone metabolism have been described in various developmental and pathological situations. Here we review this evidence from the literature with a focus on microenvironmental regulation of hematopoiesis and bone metabolism. Our hypothesis is that this process occurs by bidirectional signaling between hematopoietic and mesenchymal cells through cell adhesion molecules, membrane-bound growth factors, and secreted matrix proteins. Examples of steady-state hematopoiesis and pathologies are presented and support our view that hematopoietic and mesenchymal cell functions are modulated by specific microenvironments in the bone marrow.

Regulatory effects of TLR2 on megakaryocytic cell function

TLR2, a functional, inflammatory-related receptor, is known to be expressed on megakaryocytes and platelets and to lead to infection and immune-mediated activation of platelets; however, the role of this receptor in megakaryocytes is not understood. Using Meg-01 cells and mouse megakaryocytes, we found that NFκB, ERK-MAPK, and PI3K/Akt pathways, known downstream pathways of TLRs, are activated by Pam3CSK4, a TLR2-specific ligand. In addition, transcription factors associated with megakaryocyte maturation, GATA-1, NF-E2, and mammalian target of rapamycin (mTOR), are all increased in the presence of Pam3CSK4. The effect of Pam3CSK4 on megakaryocyte maturation was verified by the increase in DNA content and adhesion to extracellular matrix proteins by TLR2-dependent stimulation. In addition, TLR2 stimulation resulted in an increase in reactive oxygen species (ROS) production. Gene expression and protein levels of GP1b, CD41, MCP-1, COX2, NFκB1, and TLR2 were up-regulated in megakaryocytes after TLR2 stimulation through NFκB, PI3K/Akt, and ERK-MAPK pathways. Treatment of wild-type mice with Pam3CSK4 resulted in a return to normal platelet levels and an increase in megakaryocyte maturation, which did not occur in the TLR2(-/-) mice. Therefore, inflammation, through TLR2, can increase maturation and modulate the phenotype of megakaryocytes, contributing to the interrelationship between inflammation and hemostasis.

Developmental differences in megakaryocytopoiesis are associated with up-regulated TPO signaling through mTOR and elevated GATA-1 levels in neonatal megakaryocytes

Multiple observations support the existence of developmental differences in megakaryocytopoiesis. We have previously shown that neonatal megakaryocyte (MK) progenitors are hyperproliferative and give rise to MKs smaller and of lower ploidy than adult MKs. Based on these characteristics, neonatal MKs have been considered immature. The molecular mechanisms underlying these differences are unclear, but contribute to the pathogenesis of disorders of neonatal megakaryocytopoiesis. In the present study, we demonstrate that low-ploidy neonatal MKs, contrary to traditional belief, are more mature than adult low-ploidy MKs. These mature MKs are generated at a 10-fold higher rate than adult MKs, and result from a developmental uncoupling of proliferation, polyploidization, and terminal differentiation. This pattern is associated with up-regulated thrombopoietin (TPO) signaling through mammalian target of rapamycin (mTOR) and elevated levels of full-length GATA-1 and its targets. Blocking of mTOR with rapamycin suppressed the maturation of neonatal MKs without affecting ploidy, in contrast to the synchronous inhibition of polyploidization and cytoplasmic maturation in adult MKs. We propose that these mechanisms allow fetuses/neonates to populate their rapidly expanding bone marrow and intravascular spaces while maintaining normal platelet counts, but also set the stage for disorders restricted to fetal/neonatal MK progenitors, including the Down syndrome-transient myeloproliferative disorder and the thrombocytopenia absent radius syndrome.