Differentiation-dependent interactions between RUNX-1 and FLI-1 during megakaryocyte development

Wnt/β-Catenin-Signaling Modulates Megakaryopoiesis at the Megakaryocyte-Erythrocyte Progenitor Stage in the Hematopoietic System

The bone marrow (BM) hematopoietic system (HS) gives rise to blood cells originating from hematopoietic stem cells (HSCs), including megakaryocytes (MKs) and red blood cells (erythrocytes; RBCs). Many steps of the cell-fate decision remain to be elucidated, being important for cancer treatment. To explore the role of Wnt/β-catenin for MK and RBC differentiation, we activated β-catenin signaling in platelet-derived growth factor b (Pdgfb)-expressing cells of the HS using a Cre-lox approach (Ctnnb1BM-GOF). FACS analysis revealed that Pdgfb is mainly expressed by megakaryocytic progenitors (MKPs), MKs and platelets. Recombination resulted in a lethal phenotype in mutants (Ctnnb1BM-GOFwt/fl, Ctnnb1BM-GOFfl/fl) 3 weeks after tamoxifen injection, showing an increase in MKs in the BM and spleen, but no pronounced anemia despite reduced erythrocyte counts. BM transplantation (BMT) of Ctnnb1BM-GOF BM into lethally irradiated wildtype recipients (BMT-Ctnnb1BM-GOF) confirmed the megakaryocytic, but not the lethal phenotype. CFU-MK assays in vitro with BM cells of Ctnnb1BM-GOF mice supported MK skewing at the expense of erythroid colonies. Molecularly, the runt-related transcription factor 1 (RUNX1) mRNA, known to suppress erythropoiesis, was upregulated in Ctnnb1BM-GOF BM cells. In conclusion, β-catenin activation plays a key role in cell-fate decision favoring MK development at the expense of erythroid production.

Apoptosis in megakaryocytes: Safeguard and threat for thrombopoiesis

Platelets, generated from precursor megakaryocytes (MKs), are central mediators of hemostasis and thrombosis. The process of thrombopoiesis is extremely complex, regulated by multiple factors, and related to many cellular events including apoptosis. However, the role of apoptosis in thrombopoiesis has been controversial for many years. Some researchers believe that apoptosis is an ally of thrombopoiesis and platelets production is apoptosis-dependent, while others have suggested that apoptosis is dispensable for thrombopoiesis, and is even inhibited during this process. In this review, we will focus on this conflict, discuss the relationship between megakaryocytopoiesis, thrombopoiesis and apoptosis. In addition, we also consider why such a vast number of studies draw opposite conclusions of the role of apoptosis in thrombopoiesis, and try to figure out the truth behind the mystery. This review provides more comprehensive insights into the relationship between megakaryocytopoiesis, thrombopoiesis, and apoptosis and finds some clues for the possible pathological mechanisms of platelet disorders caused by abnormal apoptosis.

Transcription factor Fli-1 as a new target for antitumor drug development

The transcription factor Friend leukemia virus integration 1 (Fli-1) belonging to the E26 Transformation-Specific (ETS) transcription factor family is not only expressed in normal cells such as hematopoietic stem cells and vascular endothelial cells, but also abnormally expressed in various malignant tumors including Ewing sarcoma, Merkel cell sarcoma, small cell lung carcinoma, benign or malignant hemangioma, squamous cell carcinoma, adenocarcinoma, bladder cancer, leukemia, and lymphoma. Fli-1 binds to the promoter or enhancer of the target genes and participates in a variety of physiological and pathological processes of tumor cells, including cell growth, proliferation, differentiation, and apoptosis. The expression of Fli-1 gene is related to the specific biological functions and characteristics of the tissue in which it is located. In tumor research, Fli-1 gene is used as a specific marker for the occurrence, metastasis, efficacy, and prognosis of tumors, thus, a potential new target for tumor diagnosis and treatment. These studies indicated that Fli-1 may be a specific candidate for antitumor drug development. Recent studies identified small molecules regulating Fli-1 thanks to our screened strategy of natural products and their derivatives. Therefore, in this review, the advanced research on Fli-1 as a target for antitumor drug development is analyzed in different cancers. The inhibitors and agonists of Fli-1 that regulate its expression are introduced and their clinical applications in the treatment of cancer, thus providing new therapeutic strategies.

Current insights into the role of Fli-1 in hematopoiesis and malignant transformation

Fli-1, a member of the ETS family of transcription factors, was discovered in 1991 through retroviral insertional mutagenesis as a driver of mouse erythroleukemias. In the past 30 years, nearly 2000 papers have defined its biology and impact on normal development and cancer. In the hematopoietic system, Fli-1 controls self-renewal of stem cells and their differentiation into diverse mature blood cells. Fli-1 also controls endothelial survival and vasculogenesis, and high and low levels of Fli-1 are implicated in the auto-immune diseases systemic lupus erythematosus and systemic sclerosis, respectively. In addition, aberrant Fli-1 expression is observed in, and is essential for, the growth of multiple hematological malignancies and solid cancers. Here, we review the historical context and latest research on Fli-1, focusing on its role in hematopoiesis, immune response, and malignant transformation. The importance of identifying Fli-1 modulators (both agonists and antagonists) and their potential clinical applications is discussed.

The genome-wide impact of trisomy 21 on DNA methylation and its implications for hematopoiesis

Down syndrome is associated with genome-wide perturbation of gene expression, which may be mediated by epigenetic changes. We perform an epigenome-wide association study on neonatal bloodspots comparing 196 newborns with Down syndrome and 439 newborns without Down syndrome, adjusting for cell-type heterogeneity, which identifies 652 epigenome-wide significant CpGs (P < 7.67 × 10⁻⁸) and 1,052 differentially methylated regions. Differential methylation at promoter/enhancer regions correlates with gene expression changes in Down syndrome versus non-Down syndrome fetal liver hematopoietic stem/progenitor cells (P < 0.0001). The top two differentially methylated regions overlap RUNX1 and FLI1, both important regulators of megakaryopoiesis and hematopoietic development, with significant hypermethylation at promoter regions of these two genes. Excluding Down syndrome newborns harboring preleukemic GATA1 mutations (N = 30), identified by targeted sequencing, has minimal impact on the epigenome-wide association study results. Down syndrome has profound, genome-wide effects on DNA methylation in hematopoietic cells in early life, which may contribute to the high frequency of hematological problems, including leukemia, in children with Down syndrome.

Down syndrome has a high co-morbidity with immune and hematopoietic disorders. Here, the authors perform an epigenome-wide association study in newborns with and without Down syndrome to find differential methylation across the genome, including in hematopoietic regulators RUNX1 and FLI1.

Blood platelet formation at a glance

The main function of blood platelets is to ensure hemostasis and prevent hemorrhages. The 1011 platelets needed daily are produced in a well-orchestrated process. However, this process is not yet fully understood and in vitro platelet production is still inefficient. Platelets are produced in the bone marrow by megakaryocytes, highly specialized precursor cells that extend cytoplasmic projections called proplatelets (PPTs) through the endothelial barrier of sinusoid vessels. In this Cell Science at a Glance article and the accompanying poster we discuss the mechanisms and pathways involved in megakaryopoiesis and platelet formation processes. We especially address the – still underestimated – role of the microenvironment of the bone marrow, and present recent findings on how PPT extension in vivo differs from that in vitro and entails different mechanisms. Finally, we recapitulate old but recently revisited evidence that – although bone marrow does produce megakaryocytes and PPTs – remodeling and the release of bona fide platelets, mainly occur in the downstream microcirculation.

A Forward Genetic Screen Targeting the Endothelium Reveals a Regulatory Role for the Lipid Kinase Pi4ka in Myelo- and Erythropoiesis

Given its role as the source of definitive hematopoietic cells, we sought to determine whether mutations initiated in the hemogenic endothelium would yield hematopoietic abnormalities or malignancies. Here, we find that endothelium-specific transposon mutagenesis in mice promotes hematopoietic pathologies that are both myeloid and lymphoid in nature. Frequently mutated genes included previously recognized cancer drivers and additional candidates, such as Pi4ka, a lipid kinase whose mutation was found to promote myeloid and erythroid dysfunction. Subsequent validation experiments showed that targeted inactivation of the Pi4ka catalytic domain or reduction in mRNA expression inhibited myeloid and erythroid cell differentiation in vitro and promoted anemia in vivo through a mechanism involving deregulation of AKT, MAPK, SRC, and JAK-STAT signaling. Finally, we provide evidence linking PI4KAP2, previously considered a pseudogene, to human myeloid and erythroid leukemia.

RUNX1-ETO: Attacking the Epigenome for Genomic Instable Leukemia

Oncogenic fusion protein RUNX1-ETO is the product of the t(8;21) translocation, responsible for the most common cytogenetic subtype of acute myeloid leukemia. RUNX1, a critical transcription factor in hematopoietic development, is fused with almost the entire ETO sequence with the ability to recruit a wide range of repressors. Past efforts in providing a comprehensive picture of the genome-wide localization and the target genes of RUNX1-ETO have been inconclusive in understanding the underlying mechanism by which it deregulates native RUNX1. In this review; we dissect the current data on the epigenetic impact of RUNX1 and RUNX1-ETO. Both share similarities however, in recent years, research focused on epigenetic factors to explain their differences. RUNX1-ETO impairs DNA repair mechanisms which compromises genomic stability and favors a mutator phenotype. Among an increasing pool of mutated factors, regulators of DNA methylation are frequently found in t(8;21) AML. Together with the alteration of both, histone markers and distal enhancer regulation, RUNX1-ETO might specifically disrupt normal chromatin structure. Epigenetic studies on the fusion protein uncovered new mechanisms contributing to leukemogenesis and hopefully will translate into clinical applications.

Targeting the CALR interactome in myeloproliferative neoplasms

Mutations in the ER chaperone calreticulin (CALR) are common in myeloproliferative neoplasm (MPN) patients, activate the thrombopoietin receptor (MPL), and mediate constitutive JAK/STAT signaling. The mechanisms by which CALR mutations cause myeloid transformation are incompletely defined. We used mass spectrometry proteomics to identify CALR-mutant interacting proteins. Mutant CALR caused mislocalization of binding partners and increased recruitment of FLI1, ERP57, and CALR to the MPL promoter to enhance transcription. Consistent with a critical role for CALR-mediated JAK/STAT activation, we confirmed the efficacy of JAK2 inhibition on CALR-mutant cells in vitro and in vivo. Due to the altered interactome induced by CALR mutations, we hypothesized that CALR-mutant MPNs may be vulnerable to disruption of aberrant CALR protein complexes. A synthetic peptide designed to competitively inhibit the carboxy terminal of CALR specifically abrogated MPL/JAK/STAT signaling in cell lines and primary samples and improved the efficacy of JAK kinase inhibitors. These findings reveal what to our knowledge is a novel potential therapeutic approach for patients with CALR-mutant MPN.

TCF-1 and HEB cooperate to establish the epigenetic and transcription profiles of CD4+CD8+ thymocytes

Thymocyte development requires a complex orchestration of multiple transcription factors. Ablating either TCF-1 or HEB in CD4⁺CD8⁺ thymocytes elicits similar developmental outcomes including increased proliferation, decreased survival, and fewer late Tcra rearrangements. Here, we provide a mechanistic explanation for these similarities by showing that TCF-1 and HEB share ~7,000 DNA-binding sites genome wide and promote chromatin accessibility. The binding of both TCF-1 and HEB was required at these shared sites for epigenetic and transcriptional gene regulation. Binding of TCF-1 and HEB to their conserved motifs in the enhancer regions of genes associated with T cell differentiation promoted their expression. Binding to sites lacking conserved motifs in the promoter regions of cell-cycle-associated genes limited proliferation. TCF-1 displaced nucleosomes, allowing for chromatin accessibility. Importantly, TCF-1 inhibited Notch signaling and consequently protected HEB from Notch-mediated proteasomal degradation. Thus, TCF-1 shifts nucleosomes and safeguards HEB, thereby enabling their cooperation in establishing the epigenetic and transcription profiles of CD4⁺CD8⁺ thymocytes.

Genome wide mapping of ETV6 binding sites in pre-B leukemic cells

Genetic alterations in the transcriptional repressor ETV6 are associated with hematological malignancies. Notably, the t(12;21) translocation leading to an ETV6-AML1 fusion gene is the most common genetic alteration found in childhood acute lymphoblastic leukemia. Moreover, most of these patients also lack ETV6 expression, suggesting a tumor suppressor function. To gain insights on ETV6 DNA-binding specificity and genome wide transcriptional regulation capacities, we performed chromatin immunoprecipitation experiments coupled to deep sequencing in a t(12;21)-positive pre-B leukemic cell line. This strategy led to the identification of ETV6-bound regions that were further associated to gene expression. ETV6 binding is mostly cell type-specific as only few regions are shared with other blood cell subtypes. Peaks localization and motif enrichment analyses revealed that this unique binding profile could be associated with the ETV6-AML1 fusion protein specific to the t(12;21) background. This study underscores the complexity of ETV6 binding and uncovers ETV6 transcriptional network in pre-B leukemia cells bearing the recurrent t(12;21) translocation.

Comparative structure analysis of the ETSi domain of ERG3 and its complex with the E74 promoter DNA sequence

ERG3 (ETS-related gene) is a member of the ETS (erythroblast transformation-specific) family of transcription factors, which contain a highly conserved DNA-binding domain. The ETS family of transcription factors differ in their binding to promoter DNA sequences, and the mechanism of their DNA-sequence discrimination is little known. In the current study, crystals of the ETSi domain (the ETS domain of ERG3 containing a CID motif) in space group P41212 and of its complex with the E74 DNA sequence (DNA9) in space group C2221 were obtained and their structures were determined. Comparative structure analysis of the ETSi domain and its complex with DNA9 with previously determined structures of the ERGi domain (the ETS domain of ERG containing inhibitory motifs) in space group P65212 and of the ERGi-DNA12 complex in space group P41212 were performed. The ETSi domain is observed as a homodimer in solution as well as in the crystallographic asymmetric unit. Superposition of the structure of the ETSi domain on that of the ERGi domain showed a major conformational change at the C-terminal DNA-binding autoinhibitory (CID) motif, while minor changes are observed in the loop regions of the ETSi-domain structure. The ETSi-DNA9 complex in space group C2221 forms a structure that is quite similar to that of the ERG-DNA12 complex in space group P41212. Upon superposition of the complexes, major conformational changes are observed at the 5' and 3' ends of DNA9, while the conformation of the core GGA nucleotides was quite conserved. Comparison of the ETSi-DNA9 structure with known structures of ETS class 1 protein-DNA complexes shows the similarities and differences in the promoter DNA binding and specificity of the class 1 ETS proteins.

In vitro large scale production of megakaryocytes to functional platelets from human hematopoietic stem cells

Megakaryocytopoiesis results in the formation of platelets, which are essential for hemostasis. Decreased production or increased destruction of platelets can cause thrombocytopenia, in which platelet transfusion is the mode of treatment. The present study is aimed in generation of megakaryocytes (MKs) and platelet from human hematopoietic stem cells (HSCs). The purity of HSCs was assessed through Flow cytometry and immunocytochemistry (ICC) studies. These pure HSCs were induced with thrombopoietin (TPO), similarly with Andrographis paniculata extract (APE) for 21 days to generate MKs. The APE is mainly composed of andrographolide which stimulates TPO from the liver, and this binds to CD110 present on the surface of HSCs and triggers the proliferation of HSCs and initiate higher MKs population subsequently, a large number of platelets. The results of the present study showed increased proliferation of HSCs grown in the presence of APE and revealed a high population of CD41a and CD42b positive MKs as enumerated by Flow cytometry compared with TPO induced MKs. These results also concurred with qRT-PCR and western blot analysis. The scanning electron microscopy (SEM) revealed the morphology of differentiated MKs and platelets were similar to human blood platelets. The differentiated MKs in APE exhibited polyploidy up to 32 N while TPO induced MKs showed polyploidy of 8 N, these results corroborated with colony forming unit assay. On thrombin stimulation, high expression of P-selectin (CD62p) and fibrinogen binding were detected in APE induced platelets. Autologous transplantation of platelets generated from APE may be a useful option in thrombocytopenia condition.

The nonreceptor tyrosine kinase c-Abl phosphorylates Runx1 and regulates Runx1-mediated megakaryocyte maturation

The transcription factor Runx1 is an essential regulator of definitive hematopoiesis, megakaryocyte (MK) maturation, and lymphocyte differentiation. Runx1 mutations that interfere with its transcriptional activity are often present in leukemia patients. Recent work demonstrated that the transcriptional activity of Runx1 is regulated by kinase-mediated phosphorylation. In this study, we showed that c-Abl, but not Arg tyrosine kinase, associated with Runx1 both in cultured cells and in vitro. c-Abl-mediated tyrosine phosphorylation in the Runx1 transcription inhibition domain negatively regulated the transcriptional activity of Runx1 and inhibited Runx1-mediated MK maturation. Consistent with these findings, increased numbers of MKs were detected in the spleens and bone marrow of abl gene conditional knockout mice. Our findings demonstrate an important role of c-Abl kinase in Runx1-mediated MK maturation and platelet formation and provide a potential mechanism of Abl kinase-regulated hematopoiesis.

Single-cell transcriptomics reveals a new dynamical function of transcription factors during embryonic hematopoiesis

Recent advances in single-cell transcriptomics techniques have opened the door to the study of gene regulatory networks (GRNs) at the single-cell level. Here, we studied the GRNs controlling the emergence of hematopoietic stem and progenitor cells from mouse embryonic endothelium using a combination of single-cell transcriptome assays. We found that a heptad of transcription factors (Runx1, Gata2, Tal1, Fli1, Lyl1, Erg and Lmo2) is specifically co-expressed in an intermediate population expressing both endothelial and hematopoietic markers. Within the heptad, we identified two sets of factors of opposing functions: one (Erg/Fli1) promoting the endothelial cell fate, the other (Runx1/Gata2) promoting the hematopoietic fate. Surprisingly, our data suggest that even though Fli1 initially supports the endothelial cell fate, it acquires a pro-hematopoietic role when co-expressed with Runx1. This work demonstrates the power of single-cell RNA-sequencing for characterizing complex transcription factor dynamics.

A census of P. longum's phytochemicals and their network pharmacological evaluation for identifying novel drug-like molecules against various diseases, with a special focus on neurological disorders

Piper longum (P. longum, also called as long pepper) is one of the common culinary herbs that has been extensively used as a crucial constituent in various indigenous medicines, specifically in traditional Indian medicinal system known as Ayurveda. For exploring the comprehensive effect of its constituents in humans at proteomic and metabolic levels, we have reviewed all of its known phytochemicals and enquired about their regulatory potential against various protein targets by developing high-confidence tripartite networks consisting of phytochemical—protein target—disease association. We have also (i) studied immunomodulatory potency of this herb; (ii) developed subnetwork of human PPI regulated by its phytochemicals and could successfully associate its specific modules playing important role in diseases, and (iii) reported several novel drug targets. P10636 (microtubule-associated protein tau, that is involved in diseases like dementia etc.) was found to be the commonly screened target by about seventy percent of these phytochemicals. We report 20 drug-like phytochemicals in this herb, out of which 7 are found to be the potential regulators of 5 FDA approved drug targets. Multi-targeting capacity of 3 phytochemicals involved in neuroactive ligand receptor interaction pathway was further explored via molecular docking experiments. To investigate the molecular mechanism of P. longum’s action against neurological disorders, we have developed a computational framework that can be easily extended to explore its healing potential against other diseases and can also be applied to scrutinize other indigenous herbs for drug-design studies.

A screen for Fli-1 transcriptional modulators identifies PKC agonists that induce erythroid to megakaryocytic differentiation and suppress leukemogenesis

The ETS-related transcription factor Fli-1 affects many developmental programs including erythroid and megakaryocytic differentiation, and is frequently de-regulated in cancer. Fli-1 was initially isolated following retrovirus insertional mutagenesis screens for leukemic initiator genes, and accordingly, inhibition of this transcription factor can suppress leukemia through induction of erythroid differentiation. To search for modulators of Fli-1, we hereby performed repurposing drug screens with compounds isolated from Chinese medicinal plants. We identified agents that can transcriptionally activate or inhibit a Fli-1 reporter. Remarkably, agents that increased Fli-1 transcriptional activity conferred a strong anti-cancer activity upon Fli-1-expressing leukemic cells in culture. As opposed to drugs that suppress Fli1 activity and lead to erythroid differentiation, growth suppression by these new Fli-1 transactivating compounds involved erythroid to megakaryocytic conversion (EMC). The identified compounds are structurally related to diterpene family of small molecules, which are known agonists of protein kinase C (PKC). In accordance, these PKC agonists (PKCAs) induced PKC phosphorylation leading to activation of the mitogen-activated protein kinase (MAPK) pathway, increased cell attachment and EMC, whereas pharmacological inhibition of PKC or MAPK diminished the effect of our PKCAs. Moreover, in a mouse model of leukemia initiated by Fli-1 activation, the PKCA compounds exhibited strong anti-cancer activity, which was accompanied by increased presence of CD41/CD61 positive megakaryocytic cells in leukemic spleens. Thus, PKC agonists offer a novel approach to combat Fli-1-induced leukemia, and possibly other cancers,by inducing EMC in part through over-activation of the PKC-MAPK-Fli-1 pathway.

Mouse RUNX1C regulates premegakaryocytic/erythroid output and maintains survival of megakaryocyte progenitors

RUNX1 is crucial for the regulation of megakaryocyte specification, maturation, and thrombopoiesis. Runx1 possesses 2 promoters: the distal P1 and proximal P2 promoters. The major protein isoforms generated by P1 and P2 are RUNX1C and RUNX1B, respectively, which differ solely in their N-terminal amino acid sequences. RUNX1C is the most abundantly expressed isoform in adult hematopoiesis, present in all RUNX1-expressing populations, including the cKit+ hematopoietic stem and progenitor cells. RUNX1B expression is more restricted, being highly expressed in the megakaryocyte lineage but downregulated during erythropoiesis. We generated a Runx1 P1 knock-in of RUNX1B, termed P1-MRIPV This mouse line lacks RUNX1C expression but has normal total RUNX1 levels, solely comprising RUNX1B. Using this mouse line, we establish a specific requirement for the P1-RUNX1C isoform in megakaryopoiesis, which cannot be entirely compensated for by RUNX1B overexpression. P1 knock-in megakaryocyte progenitors have reduced proliferative capacity and undergo increased cell death, resulting in thrombocytopenia. P1 knock-in premegakaryocyte/erythroid progenitors demonstrate an erythroid-specification bias, evident from increased erythroid colony-forming ability and decreased megakaryocyte output. At a transcriptional level, multiple erythroid-specific genes are upregulated and megakaryocyte-specific transcripts are downregulated. In addition, proapoptotic pathways are activated in P1 knock-in premegakaryocyte/erythroid progenitors, presumably accounting for the increased cell death in the megakaryocyte progenitor compartment. Unlike in the conditional adult Runx1 null models, megakaryocytic maturation is not affected in the P1 knock-in mice, suggesting that RUNX1B can regulate endomitosis and thrombopoiesis. Therefore, despite the high degree of structural similarity, RUNX1B and RUNX1C isoforms have distinct and specific roles in adult megakaryopoiesis.

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.

MAPK/ERK2 phosphorylates ERG at serine 283 in leukemic cells and promotes stem cell signatures and cell proliferation

Aberrant ERG (v-ets avian erythroblastosis virus E26 oncogene homolog) expression drives leukemic transformation in mice and high expression is associated with poor patient outcomes in acute myeloid leukemia (AML) and T-acute lymphoblastic leukemia (T-ALL). Protein phosphorylation regulates the activity of many ETS factors but little is known about ERG in leukemic cells. To characterize ERG phosphorylation in leukemic cells, we applied liquid chromatography coupled tandem mass spectrometry and identified five phosphorylated serines on endogenous ERG in T-ALL and AML cells. S283 was distinct as it was abundantly phosphorylated in leukemic cells but not in healthy hematopoietic stem and progenitor cells (HSPCs). Overexpression of a phosphoactive mutant (S283D) increased expansion and clonogenicity of primary HSPCs over and above wild-type ERG. Using a custom antibody, we screened a panel of primary leukemic xenografts and showed that ERG S283 phosphorylation was mediated by mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling and in turn regulated expression of components of this pathway. S283 phosphorylation facilitates ERG enrichment and transactivation at the ERG +85 HSPC enhancer that is active in AML and T-ALL with poor prognosis. Taken together, we have identified a specific post-translational modification in leukemic cells that promotes progenitor proliferation and is a potential target to modulate ERG-driven transcriptional programs in leukemia.

Inefficient megakaryopoiesis in mouse hematopoietic stem-progenitor cells lacking T-bet

Differentiation of hematopoietic stem-progenitor cells (HSPCs) into mature blood lineages results from the translation of extracellular signals into changes in the expression levels of transcription factors controlling cell fate decisions. Multiple transcription factor families are known to be involved in hematopoiesis. Although the T-box transcription factor family is known to be involved in the differentiation of multiple tissues, and expression of T-bet, a T-box family transcription factor, has been observed in HSPCs, T-box family transcription factors do not have a described role in HSPC differentiation. In the current study, we address the functional consequences of T-bet expression in mouse HSPCs. T-bet protein levels differed among HSPC subsets, with highest levels observed in megakaryo-erythroid progenitor cells (MEPs), the common precursor to megakaryocytes and erythrocytes. HSPCs from T-bet-deficient mice exhibited a defect in megakaryocytic differentiation when cultured in the presence of thrombopoietin. In contrast, erythroid differentiation in culture in the presence of erythropoietin was not substantially altered in T-bet-deficient HSPCs. Differences observed with respect to megakaryocyte number and maturity, as assessed by level of expression of CD41 and CD61, and megakaryocyte ploidy, in T-bet-deficient HSPCs were not associated with altered proliferation or survival in culture. Gene expression micro-array analysis of MEPs from T-bet-deficient mice exhibited diminished expression of multiple genes associated with the megakaryocyte lineage. These data advance our understanding of the transcriptional regulation of megakaryopoiesis by supporting a new role for T-bet in the differentiation of MEPs into megakaryocytes.

Structural Basis for Dimerization and DNA Binding of Transcription Factor FLI1

FLI1 (Friend leukemia integration 1) is a metazoan transcription factor that is upregulated in a number of cancers. In addition, rearrangements of the fli1 gene cause sarcomas, leukemias, and lymphomas. These rearrangements encode oncogenic transcription factors, in which the DNA binding domain (DBD or ETS domain) of FLI1 on the C-terminal side is fused to a part of an another protein on the N-terminal side. Such abnormal cancer cell-specific fusions retain the DNA binding properties of FLI1 and acquire non-native protein-protein or protein-nucleic acid interactions of the substituted region. As a result, these fusions trigger oncogenic transcriptional reprogramming of the host cell. Interactions of FLI1 fusions with other proteins and with itself play a critical role in the oncogenic regulatory functions, and they are currently under intense scrutiny, mechanistically and as potential novel anticancer drug targets. We report elusive crystal structures of the FLI1 DBD, alone and in complex with cognate DNA containing a GGAA recognition sequence. Both structures reveal a previously unrecognized dimer of this domain, consistent with its dimerization in solution. The homodimerization interface is helix-swapped and dominated by hydrophobic interactions, including those between two interlocking Phe362 residues. A mutation of Phe362 to an alanine disrupted the propensity of this domain to dimerize without perturbing its structure or the DNA binding function, consistent with the structural observations. We propose that FLI1 DBD dimerization plays a role in transcriptional activation and repression by FLI1 and its fusions at promoters containing multiple FLI1 binding sites.

Stromal miR-200s contribute to breast cancer cell invasion through CAF activation and ECM remodeling

The activation of cancer-associated fibroblasts (CAFs) is a key event in tumor progression, and alternative extracellular matrix (ECM) proteins derived from CAFs induce ECM remodeling and cancer cell invasion. Here we found that miR-200 s, which are generally downregulated in activated CAFs in breast cancer tissues and in normal fibroblasts (NFs) activated by breast cancer cells, are direct mediators of NF reprogramming into CAFs and of ECM remodeling. NFs with downregulated miR-200 s displayed the traits of activated CAFs, including accelerated migration and invasion. Ectopic expression of miR-200 s in CAFs at least partially restored the phenotypes of NFs. CAF activation may be governed by the targets of miR-200 s, Fli-1 and TCF12, which are responsible for cell development and differentiation; Fli-1 and TCF12 were obviously elevated in CAFs. Furthermore, miR-200 s and their targets influenced collagen contraction by CAFs. The upregulation of fibronectin and lysyl oxidase directly by miR-200 or indirectly through Fli-1 or TCF12 contributed to ECM remodeling, triggering the invasion and metastasis of breast cancer cells both in vitro and vivo. Thus, these data provide important and novel insights into breast CAF activation and ECM remodeling, which trigger tumor cell invasion.

RUNX1 represses the erythroid gene expression program during megakaryocytic differentiation

The activity of antagonizing transcription factors represents a mechanistic paradigm of bidirectional lineage-fate control during hematopoiesis. At the megakaryocytic/erythroid bifurcation, the cross-antagonism of krueppel-like factor 1 (KLF1) and friend leukemia integration 1 (FLI1) has such a decisive role. However, how this antagonism is resolved during lineage specification is poorly understood. We found that runt-related transcription factor 1 (RUNX1) inhibits erythroid differentiation of murine megakaryocytic/erythroid progenitors and primary human CD34(+) progenitor cells. We show that RUNX1 represses the erythroid gene expression program during megakaryocytic differentiation by epigenetic repression of the erythroid master regulator KLF1. RUNX1 binding to the KLF1 locus is increased during megakaryocytic differentiation and counterbalances the activating role of T-cell acute lymphocytic leukemia 1 (TAL1). We found that corepressor recruitment by RUNX1 contributes to a block of the KLF1-dependent erythroid gene expression program. Our data indicate that the repressive function of RUNX1 influences the balance between erythroid and megakaryocytic differentiation by shifting the balance between KLF1 and FLI1 in the direction of FLI1. Taken together, we show that RUNX1 is a key player within a network of transcription factors that represses the erythroid gene expression program.

Transcription factor-mediated reprogramming toward hematopoietic stem cells

De novo generation of human hematopoietic stem cells (HSCs) from renewable cell types has been a long sought-after but elusive goal in regenerative medicine. Paralleling efforts to guide pluripotent stem cell differentiation by manipulating developmental cues, substantial progress has been made recently toward HSC generation via combinatorial transcription factor (TF)-mediated fate conversion, a paradigm established by Yamanaka's induction of pluripotency in somatic cells by mere four TFs. This review will integrate the recently reported strategies to directly convert a variety of starting cell types toward HSCs in the context of hematopoietic transcriptional regulation and discuss how these findings could be further developed toward the ultimate generation of therapeutic human HSCs.

Megakaryopoiesis and platelet production: insight into hematopoietic stem cell proliferation and differentiation

Hematopoietic stem cells (HSCs) undergo successive lineage commitment steps to generate megakaryocytes (MKs) in a process referred to as megakaryopoiesis. MKs undergo a unique differentiation process involving endomitosis to eventually produce platelets. Many transcription factors participate in the regulation of this complex progress. Chemokines and other factors in the microenvironment where megakaryopoiesis and platelet production occur play vital roles in the regulation of HSC lineage commitment and MK maturation; among these factors, thrombopoietin (TPO) is the most important. Endomitosis is a vital process of MK maturation, and granules that are formed in MKs are important for platelet function. Proplatelets are firstly generated from mature MKs and then become platelets. The proplatelet production process was verified by novel studies that revealed that the mechanism is partially regulated by the invaginated membrane system (IMS), microtubules and Rho GTPases. The extracellular matrices (ECMs) and shear stress also affect and regulate the process while the mature MKs migrate from the marrow to the sub-endothelium region near the venous sinusoids leading to the release of platelets into the circulation. This review describes the entire process of megakaryopoiesis in detail, illustrates both the transcriptional and microenvironmental regulation of MKs and provides insight into platelet biogenesis.

Novel function of PITH domain-containing 1 as an activator of internal ribosomal entry site to enhance RUNX1 expression and promote megakaryocyte differentiation

INTRODUCTION

Altered gene expression coincides with leukemia development and may affect distinct features of leukemic cells. PITHD1 was significantly downregulated in leukemia and upregulated upon PMA induction in K562 cells undergoing megakaryocyte differentiation. We aimed to study the function of PITHD1 in megakaryocyte differentiation.

MATERIALS AND METHODS

K562 cells and fetal liver cells were used for either overexpression or downregulation of PITHD1 by retroviral or lentiviral transduction. FACS was used to detect the expression of CD41 and CD42 to measure megakaryocyte differentiation in these cells. Western blot and quantitative RT-PCR were used to measure gene expression. Dual luciferase assay was used to detect promoter or internal ribosomal entry site (IRES) activity.

RESULTS

Ectopic expression of PITHD1 promoted megakaryocyte differentiation and increased RUNX1 expression while PITHD1 knockdown showed an opposite phenotype. Furthermore, PITHD1 efficiently induced endogenous RUNX1 expression and restored megakaryocyte differentiation suppressed by a dominant negative form of RUNX1. PITHD1 regulated RUNX1 expression at least through two distinct mechanisms: increasing transcription activity of proximal promoter and enhancing translation activity of an IRES element in exon 3. Finally, we confirmed the function of PITHD1 in regulating RUNX1 expression and megakaryopoiesis in mouse fetal liver cells.

CONCLUSION AND SIGNIFICANCE

PITHD1 was a novel activator of IRES and enhanced RUNX1 expression that subsequently promoted megakaryocyte differentiation. Our findings shed light on understanding the mechanisms underlying megakaryopoiesis or leukemogenesis.

RUNX3 facilitates growth of Ewing sarcoma cells

Ewing sarcoma is an aggressive pediatric small round cell tumor that predominantly occurs in bone. Approximately 85% of Ewing sarcomas harbor the EWS/FLI fusion protein, which arises from a chromosomal translocation, t(11:22)(q24:q12). EWS/FLI interacts with numerous lineage-essential transcription factors to maintain mesenchymal progenitors in an undifferentiated state. We previously showed that EWS/FLI binds the osteogenic transcription factor RUNX2 and prevents osteoblast differentiation. In this study, we investigated the role of another Runt-domain protein, RUNX3, in Ewing sarcoma. RUNX3 participates in mesenchymal-derived bone formation and is a context dependent tumor suppressor and oncogene. RUNX3 was detected in all Ewing sarcoma cells examined, whereas RUNX2 was detected in only 73% of specimens. Like RUNX2, RUNX3 binds to EWS/FLI via its Runt domain. EWS/FLI prevented RUNX3 from activating the transcription of a RUNX-responsive reporter, p6OSE2. Stable suppression of RUNX3 expression in the Ewing sarcoma cell line A673 delayed colony growth in anchorage independent soft agar assays and reversed expression of EWS/FLI-responsive genes. These results demonstrate an important role for RUNX3 in Ewing sarcoma.

SCL/TAL1-mediated transcriptional network enhances megakaryocytic specification of human embryonic stem cells

Human embryonic stem cells (hESCs) are a unique in vitro model for studying human developmental biology and represent a potential source for cell replacement strategies. Platelets can be generated from cord blood progenitors and hESCs; however, the molecular mechanisms and determinants controlling the in vitro megakaryocytic specification of hESCs remain elusive. We have recently shown that stem cell leukemia (SCL) overexpression accelerates the emergence of hemato-endothelial progenitors from hESCs and promotes their subsequent differentiation into blood cells with higher clonogenic potential. Given that SCL participates in megakaryocytic commitment, we hypothesized that it may potentiate megakaryopoiesis from hESCs. We show that ectopic SCL expression enhances the emergence of megakaryocytic precursors, mature megakaryocytes (MKs), and platelets in vitro. SCL-overexpressing MKs and platelets respond to different activating stimuli similar to their control counterparts. Gene expression profiling of megakaryocytic precursors shows that SCL overexpression renders a megakaryopoietic molecular signature. Connectivity Map analysis reveals that trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA), both histone deacetylase (HDAC) inhibitors, functionally mimic SCL-induced effects. Finally, we confirm that both TSA and SAHA treatment promote the emergence of CD34(+) progenitors, whereas valproic acid, another HDAC inhibitor, potentiates MK and platelet production. We demonstrate that SCL and HDAC inhibitors are megakaryopoiesis regulators in hESCs.

NLS-tagging: an alternative strategy to tag nuclear proteins

The characterization of transcription factor complexes and their binding sites in the genome by affinity purification has yielded tremendous new insights into how genes are regulated. The affinity purification requires either the use of antibodies raised against the factor of interest itself or by high-affinity binding of a C- or N-terminally added tag sequence to the factor. Unfortunately, fusing extra amino acids to the termini of a factor can interfere with its biological function or the tag may be inaccessible inside the protein. Here, we describe an effective solution to that problem by integrating the ‘tag’ close to the nuclear localization sequence domain of the factor. We demonstrate the effectiveness of this approach with the transcription factors Fli-1 and Irf2bp2, which cannot be tagged at their extremities without loss of function. This resulted in the identification of novel proteins partners and a new hypothesis on the contribution of Fli-1 to hematopoiesis.

Runx1 regulation of Pu.1 corepressor/coactivator exchange identifies specific molecular targets for leukemia differentiation therapy

Gene activation requires cooperative assembly of multiprotein transcription factor-coregulator complexes. Disruption to cooperative assemblage could underlie repression of tumor suppressor genes in leukemia cells. Mechanisms of cooperation and its disruption were therefore examined for PU.1 and RUNX1, transcription factors that cooperate to activate hematopoietic differentiation genes. PU.1 is highly expressed in leukemia cells, whereas RUNX1 is frequently inactivated by mutation or translocation. Thus, coregulator interactions of Pu.1 were examined by immunoprecipitation coupled with tandem mass spectrometry/Western blot in wild-type and Runx1-deficient hematopoietic cells. In wild-type cells, the NuAT and Baf families of coactivators coimmunoprecipitated with Pu.1. Runx1 deficiency produced a striking switch to Pu.1 interaction with the Dnmt1, Sin3A, Nurd, CoRest, and B-Wich corepressor families. Corepressors of the Polycomb family, which are frequently inactivated by mutation or deletion in myeloid leukemia, did not interact with Pu.1. The most significant gene ontology association of Runx1-Pu.1 co-bound genes was with macrophages, therefore, functional consequences of altered corepressor/coactivator exchange were examined at Mcsfr, a key macrophage differentiation gene. In chromatin immunoprecipitation analyses, high level Pu.1 binding to the Mcsfr promoter was not decreased by Runx1 deficiency. However, the Pu.1-driven shift from histone repression to activation marks at this locus, and terminal macrophage differentiation, were substantially diminished. DNMT1 inhibition, but not Polycomb inhibition, in RUNX1-translocated leukemia cells induced terminal differentiation. Thus, RUNX1 and PU.1 cooperate to exchange corepressors for coactivators, and the specific corepressors recruited to PU.1 as a consequence of RUNX1 deficiency could be rational targets for leukemia differentiation therapy.

Thrombocytopenia-associated mutations in the ANKRD26 regulatory region induce MAPK hyperactivation

Point mutations in the 5' UTR of ankyrin repeat domain 26 (ANKRD26) are associated with familial thrombocytopenia 2 (THC2) and a predisposition to leukemia. Here, we identified underlying mechanisms of ANKRD26-associated thrombocytopenia. Using megakaryocytes (MK) isolated from THC2 patients and healthy subjects, we demonstrated that THC2-associated mutations in the 5' UTR of ANKRD26 resulted in loss of runt-related transcription factor 1 (RUNX1) and friend leukemia integration 1 transcription factor (FLI1) binding. RUNX1 and FLI1 binding at the 5' UTR from healthy subjects led to ANKRD26 silencing during the late stages of megakaryopoiesis and blood platelet development. We showed that persistent ANKRD26 expression in isolated MKs increased signaling via the thrombopoietin/myeloproliferative leukemia virus oncogene (MPL) pathway and impaired proplatelet formation by MKs. Importantly, we demonstrated that ERK inhibition completely rescued the in vitro proplatelet formation defect. Our data identify a mechanism for development of the familial thrombocytopenia THC2 that is related to abnormal MAPK signaling.

Enrichment of FLI1 and RUNX1 mutations in families with excessive bleeding and platelet dense granule secretion defects

We analyzed candidate platelet function disorder genes in 13 index cases with a history of excessive bleeding in association with a significant reduction in dense granule secretion and impaired aggregation to a panel of platelet agonists. Five of the index cases also had mild thrombocytopenia. Heterozygous alterations in FLI1 and RUNX1, encoding Friend leukemia integration 1 and RUNT-related transcription factor 1, respectively, which have a fundamental role in megakaryocytopoeisis, were identified in 6 patients, 4 of whom had mild thrombocytopenia. Two FLI1 alterations predicting p.Arg337Trp and p.Tyr343Cys substitutions in the FLI1 DNA-binding domain abolished transcriptional activity of FLI1. A 4-bp deletion in FLI1, and 2 splicing alterations and a nonsense variation in RUNX1, which were predicted to cause haploinsufficiency of either FLI1 or RUNX1, were also identified. Our findings suggest that alterations in FLI1 and RUNX1 may be common in patients with platelet dense granule secretion defects and mild thrombocytopenia.

RUNX1, but not its familial platelet disorder mutants, synergistically activates PF4 gene expression in combination with ETS family proteins

BACKGROUND

Familial platelet disorder (FPD) is a rare autosomal dominant disease characterized by thrombocytopenia and abnormal platelet function. Causal mutations have been identified in the gene encoding runt-related transcription factor 1 (RUNX1) of FPD patients.

OBJECTIVES

To elucidate the role of RUNX1 in the regulation of expression of platelet factor 4 (PF4) and to propose a plausible mechanism underlying RUNX1-mediated induction of the FPD phenotype.

METHODS

We assessed whether RUNX1 and its mutants, in combination with E26 transformation-specific-1 (ETS-1), Core-binding factor subunit beta (CBFβ), and Friend leukemia virus integration 1 (FLI-1), cooperatively regulate PF4 expression during megakaryocytic differentiation. In an embryonic stem cell differentiation system, expression levels of endogenous and exogenous RUNX1 and PF4 were determined by real-time RT-PCR. Promoter activation by the transcription factors were evaluated by reporter gene assays with HepG2 cells. DNA binding activity and protein interaction were analyzed by electrophoretic mobility shift assay and immunoprecipitation assay with Cos-7 cells, respectively. Protein localization was analyzed by immunocytochemistry and Western blotting with Cos-7 cells.

RESULTS

We demonstrated that RUNX1 activates endogenous PF4 expression in megakaryocytic differentiation. RUNX1, but not its mutants, in combination with ETS-1 and CBFβ, or FLI-1, synergistically activated the PF4 promoter. Each RUNX1 mutant harbors various functional abnormalities, including loss of DNA-binding activity, abnormal subcellular localization, and/or alterations of binding affinities for ETS-1, CBFβ, and FLI-1.

CONCLUSIONS

RUNX1, but not its mutants, strongly and synergistically activates PF4 expression along with ETS family proteins. Furthermore, loss of the RUNX1 transcriptional activation function is induced by various functional abnormalities.

MYH10 protein expression in platelets as a biomarker of RUNX1 and FLI1 alterations

RUNX1 gene alterations are associated with acquired and inherited hematologic malignancies that include familial platelet disorder/acute myeloid leukemia, primary or secondary acute myeloid leukemia, and chronic myelomonocytic leukemia. Recently, we reported that RUNX1-mediated silencing of nonmuscle myosin heavy chain IIB (MYH10) was required for megakaryocyte ploidization and maturation. Here we demonstrate that runx1 deletion in mice induces the persistence of MYH10 in platelets, and a similar persistence was observed in platelets of patients with constitutional (familial platelet disorder/acute myeloid leukemia) or acquired (chronic myelomonocytic leukemia) RUNX1 mutations. MYH10 was also detected in platelets of patients with the Paris-Trousseau syndrome, a thrombocytopenia related to the deletion of the transcription factor FLI1 that forms a complex with RUNX1 to regulate megakaryopoiesis, whereas MYH10 persistence was not observed in other inherited forms of thrombocytopenia. We propose MYH10 detection as a new and simple tool to identify inherited platelet disorders and myeloid neoplasms with abnormalities in RUNX1 and its associated proteins.

Tyrosyl phosphorylation toggles a Runx1 switch

The Runx1 transcription factor is post-translationally modified by seryl/threonyl phosphorylation, acetylation, and methylation that control its interactions with transcription factor partners and epigenetic coregulators. In this issue of Genes & Development, Huang and colleagues (pp. 1587-1601) describe how the regulation of Runx1 tyrosyl phosphorylation by Src family kinases and the Shp2 phosphatase toggle Runx1's interactions between different coregulatory molecules.

Transcription factors Fli1 and EKLF in the differentiation of megakaryocytic and erythroid progenitor in 5q- syndrome and in Diamond-Blackfan anemia

Friend leukemia virus integration 1 (Fli1) and erythroid Krüppel-like factor (EKLF) participate under experimental conditions in the differentiation of megakaryocytic and erythroid progenitor in cooperation with other transcription factors, cytokines, cytokine receptors, and microRNAs. Defective erythropoiesis with refractory anemia and effective megakaryopoiesis with normal or increased platelet count is typical for 5q- syndrome. We decided to evaluate the roles of EKLF and Fli1 in the pathogenesis of this syndrome and of another ribosomopathy, Diamond-Blackfan anemia (DBA). Fli1 and EKLF mRNA levels were examined in mononuclear blood and bone marrow cells from patients with 5q- syndrome, low-risk MDS patients with normal chromosome 5, DBA patients, and healthy controls. In 5q- syndrome, high Fli1 mRNA levels in the blood and bone marrow mononuclear cells were found. In DBA, Fli1 expression did not differ from the controls. EKLF mRNA level was significantly decreased in the blood and bone marrow of 5q- syndrome and in all DBA patients. We propose that the elevated Fli1 in 5q- syndrome protects megakaryocytic cells from ribosomal stress contrary to erythroid cells and contributes to effective though dysplastic megakaryopoiesis.

Cofactor-mediated restriction of GATA-1 chromatin occupancy coordinates lineage-specific gene expression

GATA-1 and its cofactor FOG-1 are required for the differentiation of erythrocytes and megakaryocytes. In contrast, mast cell development requires GATA-1 and the absence of FOG-1. Through genome-wide comparison of the chromatin occupancy of GATA-1 and a naturally occurring mutant that cannot bind FOG-1 (GATA-1(V205G)), we reveal that FOG-1 intricately regulates the chromatin occupancy of GATA-1. We identified GATA1-selective and GATA-1(V205G)-selective binding sites and show that GATA-1, in the absence of FOG-1, occupies GATA-1(V205G)-selective sites, but not GATA1-selective sites. By integrating ChIP-seq and gene expression data, we discovered that GATA-1(V205G) binds and activates mast cell-specific genes via GATA-1(V205G)-selective sites. We further show that exogenous expression of FOG-1 in mast cells leads to displacement of GATA-1 from mast cell-specific genes and causes their downregulation. Together these findings establish a mechanism of gene regulation whereby a non-DNA binding cofactor directly modulates the occupancy of a transcription factor to control lineage specification.

A Src family kinase-Shp2 axis controls RUNX1 activity in megakaryocyte and T-lymphocyte differentiation

Hematopoietic development occurs in complex microenvironments and is influenced by key signaling events. Yet how these pathways communicate with master hematopoietic transcription factors to coordinate differentiation remains incompletely understood. The transcription factor RUNX1 plays essential roles in definitive hematopoietic stem cell (HSC) ontogeny, HSC maintenance, megakaryocyte (Mk) maturation, and lymphocyte differentiation. It is also the most frequent target of genetic alterations in human leukemia. Here, we report that RUNX1 is phosphorylated by Src family kinases (SFKs) and that this occurs on multiple tyrosine residues located within its negative regulatory DNA-binding and autoinhibitory domains. Retroviral transduction, chemical inhibitor, and genetic studies demonstrate a negative regulatory role of tyrosine phosphorylation on RUNX1 activity in Mk and CD8 T-cell differentiation. We also demonstrate that the nonreceptor tyrosine phosphatase Shp2 binds directly to RUNX1 and contributes to its dephosphorylation. Last, we show that RUNX1 tyrosine phosphorylation correlates with reduced GATA1 and enhanced SWI/SNF interactions. These findings link SFK and Shp2 signaling pathways to the regulation of RUNX1 activity in hematopoiesis via control of RUNX1 multiprotein complex assembly.

The genomic and proteomic blueprint of mouse megakaryocytes derived from embryonic stem cells

BACKGROUND

Platelets are specialized cells, produced by megakaryocytes (MKs) in the bone marrow, which represent the first defense against hemorrhage. There are many diseases where platelet production or function is impaired, with severe consequences for patients. Therefore, new insights into the process of MK differentiation and platelet formation would have a major impact on both basic and clinical research.

OBJECTIVES

Embryonic stem (ES) cells represent a good in vitro model to study the differentiation of MKs, with the possibility of being genetically engineered and constituting an unlimited source of MKs. However, lack of knowledge about the molecular identity of ES-derived MKs (ES-MKs) may prevent any further development and application of this model.

METHODS

This paper presents the first comprehensive transcriptional and proteome profile analyses of mouse ES-MKs in comparison with MKs derived from mouse fetal liver progenitors (FL-MKs).

RESULTS

In ES-MKs we found a down-regulation of cytoskeleton proteins, specific transcription factors and membrane receptors at both transcriptional and protein levels. At the phenotypic level, this molecular blueprint was displayed by ES-MKs' lower polyploidy, lower nuclear/cytoplasm ratio and reduced capacity to form proplatelets and releasing platelets.

CONCLUSIONS

Overall our data demonstrate that ES-MKs represent a useful model to clarify many aspects of both MK physiology and pathological conditions where impaired MK functions are related to defective MK development, as in inherited thrombocytopenias and primary myelofibrosis.

Direct recruitment of polycomb repressive complex 1 to chromatin by core binding transcription factors

Polycomb repressive complexes (PRCs) play key roles in developmental epigenetic regulation. Yet the mechanisms that target PRCs to specific loci in mammalian cells remain incompletely understood. In this study we show that Bmi1, a core component of Polycomb Repressive Complex 1 (PRC1), binds directly to the Runx1/CBFβ transcription factor complex. Genome-wide studies in megakaryocytic cells demonstrate significant chromatin occupancy overlap between the PRC1 core component Ring1b and Runx1/CBFβ and functional regulation of a considerable fraction of commonly bound genes. Bmi1/Ring1b and Runx1/CBFβ deficiencies generate partial phenocopies of one another in vivo. We also show that Ring1b occupies key Runx1 binding sites in primary murine thymocytes and that this occurs via PRC2-independent mechanisms. Genetic depletion of Runx1 results in reduced Ring1b binding at these sites in vivo. These findings provide evidence for site-specific PRC1 chromatin recruitment by core binding transcription factors in mammalian cells.

RUNX1-induced silencing of non-muscle myosin heavy chain IIB contributes to megakaryocyte polyploidization

Megakaryocytes are unique mammalian cells that undergo polyploidization (endomitosis) during differentiation, leading to an increase in cell size and protein production that precedes platelet production. Recent evidence demonstrates that endomitosis is a consequence of a late failure in cytokinesis associated with a contractile ring defect. Here we show that the non-muscle myosin IIB heavy chain (MYH10) is expressed in immature megakaryocytes and specifically localizes in the contractile ring. MYH10 downmodulation by short hairpin RNA increases polyploidization by inhibiting the return of 4N cells to 2N, but other regulators, such as of the G1/S transition, might regulate further polyploidization of the 4N cells. Conversely, re-expression of MYH10 in the megakaryocytes prevents polyploidization and the transition of 2N to 4N cells. During polyploidization, MYH10 expression is repressed by the major megakaryocyte transcription factor RUNX1. Thus, RUNX1-mediated silencing of MYH10 is required for the switch from mitosis to endomitosis, linking polyploidization with megakaryocyte differentiation.

RUNX1 and RUNX1-ETO: roles in hematopoiesis and leukemogenesis

RUNX1 is a transcription factor that regulates critical processes in many aspects of hematopoiesis. RUNX1 is also integral in defining the definitive hematopoietic stem cell. In addition, many hematological diseases like myelodysplastic syndrome and myeloproliferative neoplasms have been associated with mutations in RUNX1. Located on chromosomal 21, the RUNX1 gene is involved in many forms of chromosomal translocations in leukemia. t(8;21) is one of the most common chromosomal translocations found in acute myeloid leukemia (AML), where it results in a fusion protein between RUNX1 and ETO. The RUNX1-ETO fusion protein is found in approximately 12% of all AML patients. In this review, we detail the structural features, functions, and models used to study both RUNX1 and RUNX1-ETO in hematopoiesis over the past two decades.

Genome-wide Runx2 occupancy in prostate cancer cells suggests a role in regulating secretion

Runx2 is a metastatic transcription factor (TF) increasingly expressed during prostate cancer (PCa) progression. Using PCa cells conditionally expressing Runx2, we previously identified Runx2-regulated genes with known roles in epithelial-mesenchymal transition, invasiveness, angiogenesis, extracellular matrix proteolysis and osteolysis. To map Runx2-occupied regions (R2ORs) in PCa cells, we first analyzed regions predicted to bind Runx2 based on the expression data, and found that recruitment to sites upstream of the KLK2 and CSF2 genes was cyclical over time. Genome-wide ChIP-seq analysis at a time of maximum occupancy at these sites revealed 1603 high-confidence R2ORs, enriched with cognate motifs for RUNX, GATA and ETS TFs. The R2ORs were distributed with little regard to annotated transcription start sites (TSSs), mainly in introns and intergenic regions. Runx2-upregulated genes, however, displayed enrichment for R2ORs within 40 kb of their TSSs. The main annotated functions enriched in 98 Runx2-upregulated genes with nearby R2ORs were related to invasiveness and membrane trafficking/secretion. Indeed, using SDS-PAGE, mass spectrometry and western analyses, we show that Runx2 enhances secretion of several proteins, including fatty acid synthase and metastasis-associated laminins. Thus, combined analysis of Runx2's transcriptome and genomic occupancy in PCa cells lead to defining its novel role in regulating protein secretion.