• Alternative enhancer usage and targeted polycomb marking hallmark promoter choice during t cell differentiation

      Maqbool, M. A.; Pioger, L.; El Aabidine, A. Z.; Karasu, N.; Molitor, A. M.; Dao, L. T. M.; Charbonnier, G.; van Laethem, F.; Fenouil, R.; Koch, F.; et al. (2020)
      During thymic development and upon peripheral activation, T cells undergo extensive phenotypic and functional changes coordinated by lineage-specific developmental programs. To characterize the regulatory landscape controlling T cell identity, we perform a wide epigenomic and transcriptional analysis of mouse thymocytes and naive CD4 differentiated T helper cells. Our investigations reveal a dynamic putative enhancer landscape, and we could validate many of the enhancers using the high-throughput CapStarr sequencing (CapStarr-seq) approach. We find that genes using multiple promoters display increased enhancer usage, suggesting that apparent "enhancer redundancy" might relate to isoform selection. Furthermore, we can show that two Runx3 promoters display long-range interactions with specific enhancers. Finally, our analyses suggest a novel function for the PRC2 complex in the control of alternative promoter usage. Altogether, our study has allowed for the mapping of an exhaustive set of active enhancers and provides new insights into their function and that of PRC2 in controlling promoter choice during T cell differentiation. Keywords: CapSTARR-seq; T cell enhancerome; enhancer and promoter usage; enhancer redundancy; long-distance enhancer-promoter interactions.
    • Blood cell generation from the hemangioblast.

      Lancrin, Christophe; Sroczynska, Patrycja; Serrano, Alicia G; Gandillet, Arnaud; Ferreras, Cristina; Kouskoff, Valerie; Lacaud, Georges; Cancer Research UK, Stem Cell Biology Group, Paterson Institute for Cancer Research, University of Manchester, Wilmslow Road, Manchester, M20 4BX, UK. (2010-02)
      Understanding how blood cells are generated is important from a biological perspective but also has potential implications in the treatment of blood diseases. Such knowledge could potentially lead to defining new conditions to amplify hematopoietic stem cells (HSCs) or could translate into new methods to produce HSCs, or other types of blood cells, from human embryonic stem cells or induced pluripotent stem cells. Additionally, as most key transcription factors regulating early hematopoietic development have also been implicated in various types of leukemia, understanding their function during normal development could result in a better comprehension of their roles during abnormal hematopoiesis in leukemia. In this review, we discuss our current understanding of the molecular and cellular mechanisms of blood development from the earliest hematopoietic precursor, the hemangioblast, a precursor for both endothelial and hematopoietic cell lineages.
    • Concise review: recent advances in the in vitro derivation of blood cell populations.

      Batta, Kiran; Menegatti, Sara; Garcia-Alegria, Eva; Florkowska, Magdalena; Lacaud, Georges; Kouskoff, Valerie; Cancer Research UK Stem Cell Biology Group, Cancer Research UK Manchester (2016-07-07)
      : Hematopoietic cell-based therapies are currently available treatment options for many hematological and nonhematological disorders. However, the scarcity of allogeneic donor-derived cells is a major hurdle in treating these disorders. Embryonic stem cell-based directed differentiation and direct reprogramming of somatic cells provide excellent tools for the potential generation of hematopoietic stem cells usable in the clinic for cellular therapies. In addition to blood stem cell transplantation, mature blood cells such as red blood cells, platelets, and engineered T cells have also been increasingly used to treat several diseases. Besides cellular therapies, induced blood progenitor cells generated from autologous sources (either induced pluripotent stem cells or somatic cells) can be useful for disease modeling of bone marrow failures and acquired blood disorders. However, although great progress has been made toward these goals, we are still far from the use of in vitro-derived blood products in the clinic. We review the current state of knowledge on the directed differentiation of embryonic stem cells and the reprogramming of somatic cells toward the generation of blood stem cells and derivatives.
    • Contrasting effects of Sox17- and Sox18-sustained expression at the onset of blood specification.

      Serrano, Alicia G; Gandillet, Arnaud; Pearson, Stella; Lacaud, Georges; Kouskoff, Valerie; Stem Cell Haematopoiesis Group, Cancer Research, UK. (2010-05-13)
      We have previously shown that Sox7 was transiently expressed at the onset of blood specification and was implicated in the regulation of cell survival, proliferation, and maturation of hematopoietic precursors. Here, we assessed, using embryonic stem cell differentiation as a model system, whether Sox17 and Sox18, 2 close homologs of Sox7, may act similarly to Sox7 at the onset of hematopoietic development. Sox18-enforced expression led to the enhanced proliferation of early hematopoietic precursors while blocking their maturation, phenotype highly reminiscent of Sox7-enforced expression. In striking contrast, Sox17-enforced expression dramatically increased the apoptosis of these early precursors. Similarly to Sox7, Sox18 was transiently expressed during early hematopoiesis, but its expression was predominantly observed in CD41(+) cells, contrasting with Sox7, mostly expressed in Flk1(+) cells. Conversely, Sox17 remained marginally expressed during blood specification. Overall, our data uncover contrasting effect and expression pattern for Sox18 and Sox17 at the onset of hematopoiesis specification.
    • Cooperative binding of AP-1 and TEAD4 modulates the balance between vascular smooth muscle and hemogenic cell fate.

      Obier, N; Cauchy, P; Assi, S; Gilmour, J; Lie-A-Ling, Michael; Lichtinger, M; Hoogenkamp, M; Noailles, L; Cockerill, P; Lacaud, Georges; et al. (2016-10-17)
      The transmission of extracellular signals into the nucleus involves inducible transcription factors, but how different signaling pathways act in a cell type-specific fashion is poorly understood. Here, we studied the regulatory role of the AP-1 transcription factor family in blood development using embryonic stem cell differentiation coupled with genome-wide transcription factor binding and gene expression analyses. AP-1 factors respond to MAP kinase signaling and are comprised of dimers of FOS, ATF and JUN proteins. To examine genes regulated by AP-1 and to examine how it interacts with other inducible transcription factors we abrogated its global DNA-binding activity using a dominant negative FOS peptide. We show that FOS and JUN bind to and activate a specific set of vascular genes and that AP-1 inhibition shifts the balance between smooth muscle and hematopoietic differentiation towards blood. Further, AP-1 is required for de novo binding of TEAD4, a transcription factor connected to Hippo signaling. Our bottom-up approach demonstrates that AP-1 and TEAD4 associated cis-regulatory elements comprise hubs for multiple signaling responsive transcription factors and defines the cistrome regulating vascular and hematopoietic development by extrinsic signals.
    • Developmental-stage-dependent transcriptional response to leukaemic oncogene expression.

      Regha, K; Assi, S; Tsoulaki, Olga; Gilmour, J; Lacaud, Georges; Bonifer, C; School of Cancer Sciences, Institute for Biomedical Research, University of Birmingham (2015)
      Acute myeloid leukaemia (AML) is characterized by a block in myeloid differentiation the stage of which is dependent on the nature of the transforming oncogene and the developmental stage of the oncogenic hit. This is also true for the t(8;21) translocation that gives rise to the RUNX1-ETO fusion protein and initiates the most common form of human AML. Here we study the differentiation of mouse embryonic stem cells expressing an inducible RUNX1-ETO gene into blood cells as a model, combined with genome-wide analyses of transcription factor binding and gene expression. RUNX1-ETO interferes with both the activating and repressive function of its normal counterpart, RUNX1, at early and late stages of blood cell development. However, the response of the transcriptional network to RUNX1-ETO expression is developmental stage specific, highlighting the molecular mechanisms determining specific target cell expansion after an oncogenic hit.
    • A developmentally regulated heparan sulfate epitope defines a subpopulation with increased blood potential during mesodermal differentiation.

      Baldwin, Rebecca J; Ten Dam, Gerdy; Van Kuppevelt, Toin H; Lacaud, Georges; Gallagher, John T; Kouskoff, Valerie; Merry, Catherine L R; University of Manchester and Cancer Research UK Department of Medical Oncology, Manchester, United Kingdom. (2008-12)
      Heparan sulfate (HS) is a mandatory coreceptor for many growth factors and morphogens involved in embryonic development; its bioactivity is dictated by complex sulfation motifs embedded within the polymer chain. Using a panel of HS-specific antibodies we have identified a unique HS epitope recognized by antibody HS4C3 that is selectively expressed during differentiation of embryonic stem (ES) cells along the mesodermal lineage to the hemangioblast stage. The appearance of this high-affinity HS4C3-binding (HS4C3(high)) epitope is transient; the epitope is specifically expressed within the emerging Brachyury(+) (Bry(+)) population and marks those cells that will become fetal liver kinase 1 (Flk1)(+). Fluorescence-activated cell sorting (FACS) separation and colony forming assays revealed that HS4C3(high)/Flk1(+) cells have a dramatically increased potential to form both blast and endothelial colonies, both of which depend upon the HS-binding growth factor vascular endothelial growth factor. Critically, expression of this HS epitope is tightly regulated, disappearing from the cell surface as the resultant hematopoietic lineages mature, in a similar manner to protein markers Bry and Flk1. In vivo studies showed a remarkable correlation with in vitro findings, with expression of HS4C3-binding epitopes restricted to newly formed mesodermal tissues during gastrulation. We believe this is the first time a defined HS epitope has been implicated in a specific developmental pathway and that this provides, in addition, a novel enrichment technique for the isolation of hemangioblasts from mixed differentiated ES cell cultures.
    • The differential activities of Runx1 promoters define milestones during embryonic hematopoiesis.

      Sroczynska, Patrycja; Lancrin, Christophe; Kouskoff, Valerie; Lacaud, Georges; Cancer Research UK Stem Cell Biology Group, Paterson Institute for Cancer Research, University of Manchester, Manchester, United Kingdom. (2009-12-17)
      The transcription factor RUNX1/AML1 is a master regulator of hematopoietic development. Its spatiotemporal expression is tightly regulated during embryonic development and is under the control of 2 alternative promoters, distal and proximal. Despite the functional significance of Runx1, the relative and specific activities of these 2 promoters remain largely uncharacterized. To investigate these activities, we introduced 2 reporter genes under the control of the proximal and distal promoters in embryonic stem cell and transgenic mouse lines. Our study reveals that both in vitro and in vivo the proximal Runx1 isoform marks a hemogenic endothelium cell population, whereas the subsequent expression of distal Runx1 defines fully committed definitive hematopoietic progenitors. Interestingly, hematopoietic commitment in distal Runx1 knockout embryos appears normal. Altogether, our data demonstrate that the differential activities of the 2 Runx1 promoters define milestones of hematopoietic development and suggest that the proximal isoform plays a critical role in the generation of hematopoietic cells from hemogenic endothelium. Identification and access to the discrete stages of hematopoietic development defined by the activities of the Runx1 promoters will provide the opportunity to further explore the cellular and molecular mechanisms of hematopoietic development.
    • Dynamic gene regulatory networks drive hematopoietic specification and differentiation.

      Goode, D; Obier, N; Vijayabaskar, M; Lie-A-Ling, Michael; Lilly, Andrew J; Hannah, R; Lichtinger, M; Batta, Kiran; Florkowska, Magdalena; Patel, Rahima; et al. (2016-03-07)
      Metazoan development involves the successive activation and silencing of specific gene expression programs and is driven by tissue-specific transcription factors programming the chromatin landscape. To understand how this process executes an entire developmental pathway, we generated global gene expression, chromatin accessibility, histone modification, and transcription factor binding data from purified embryonic stem cell-derived cells representing six sequential stages of hematopoietic specification and differentiation. Our data reveal the nature of regulatory elements driving differential gene expression and inform how transcription factor binding impacts on promoter activity. We present a dynamic core regulatory network model for hematopoietic specification and demonstrate its utility for the design of reprogramming experiments. Functional studies motivated by our genome-wide data uncovered a stage-specific role for TEAD/YAP factors in mammalian hematopoietic specification. Our study presents a powerful resource for studying hematopoiesis and demonstrates how such data advance our understanding of mammalian development.
    • Early chromatin unfolding by RUNX1: a molecular explanation for differential requirements during specification versus maintenance of the hematopoietic gene expression program.

      Hoogenkamp, Maarten; Lichtinger, Monika; Krysinska, Hanna; Lancrin, Christophe; Clarke, Deborah; Williamson, Andrew J K; Mazzarella, Luca; Ingram, Richard; Jorgensen, Helle; Fisher, Amanda; et al. (2009-07-09)
      At the cellular level, development progresses through successive regulatory states, each characterized by their specific gene expression profile. However, the molecular mechanisms regulating first the priming and then maintenance of gene expression within one developmental pathway are essentially unknown. The hematopoietic system represents a powerful experimental model to address these questions and here we have focused on a regulatory circuit playing a central role in myelopoiesis: the transcription factor PU.1, its target gene colony-stimulating-factor 1 receptor (Csf1r), and key upstream regulators such as RUNX1. We find that during ontogeny, chromatin unfolding precedes the establishment of active histone marks and the formation of stable transcription factor complexes at the Pu.1 locus and we show that chromatin remodeling is mediated by the transient binding of RUNX1 to Pu.1 cis-elements. By contrast, chromatin reorganization of Csf1r requires prior expression of PU.1 together with RUNX1 binding. Once the full hematopoietic program is established, stable transcription factor complexes and active chromatin can be maintained without RUNX1. Our experiments therefore demonstrate how individual transcription factors function in a differentiation stage-specific manner to differentially affect the initiation versus maintenance of a developmental program.
    • Early human hemogenic endothelium generates primitive and definitive hematopoiesis in vitro.

      Garcia-Alegria, E; Menegatti, S; Fadlullah, Muhammad Z H; Menendez, P; Lacaud, Georges; Kouskoff, Valerie; Developmental Haematopoiesis Group, Faculty of Biology, Medicine and Health, The University of Manchester, Manchester M13 9PT, UK (2018)
      The differentiation of human embryonic stem cells (hESCs) to hematopoietic lineages initiates with the specification of hemogenic endothelium, a transient specialized endothelial precursor of all blood cells. This in vitro system provides an invaluable model to dissect the emergence of hematopoiesis in humans. However, the study of hematopoiesis specification is hampered by a lack of consensus in the timing of hemogenic endothelium analysis and the full hematopoietic potential of this population. Here, our data reveal a sharp decline in the hemogenic potential of endothelium populations isolated over the course of hESC differentiation. Furthermore, by tracking the dynamic expression of CD31 and CD235a at the onset of hematopoiesis, we identified three populations of hematopoietic progenitors, representing primitive and definitive subsets that all emerge from the earliest specified hemogenic endothelium. Our data establish that hemogenic endothelium populations endowed with primitive and definitive hematopoietic potential are specified simultaneously from the mesoderm in differentiating hESCs.
    • Embryonic stem cell-derived hemangioblasts remain epigenetically plastic and require PRC1 to prevent neural gene expression.

      Mazzarella, L; Jørgensen, H; Soza-Ried, J; Terry, A; Pearson, Stella; Lacaud, Georges; Kouskoff, Valerie; Merkenschlager, M; Fisher, A; Lymphocyte Development Group, Medical Research Council Clinical Sciences Centre, Faculty of Medicine, Imperial College London, London, United Kingdom. (2011-01-06)
      Many lineage-specific developmental regulator genes are transcriptionally primed in embryonic stem (ES) cells; RNA Pol(II) is bound at their promoters but is prevented from productive elongation by the activity of polycomb repressive complexes (PRC) 1 and 2. This epigenetically poised state is thought to enable ES cells to rapidly execute multiple differentiation programs and is recognized by a simultaneous enrichment for trimethylation of lysine 4 and trimethylation of lysine 27 of histone H3 (bivalent chromatin) across promoter regions. Here we show that the chromatin profile of this important cohort of genes is progressively modified as ES cells differentiate toward blood-forming precursors. Surprisingly however, neural specifying genes, such as Nkx2-2, Nkx2-9, and Sox1, remain bivalent and primed even in committed hemangioblasts, as conditional deletion of PRC1 results in overt and inappropriate expression of neural genes in hemangioblasts. These data reinforce the importance of PRC1 for normal hematopoietic differentiation and reveal an unexpected epigenetic plasticity of mesoderm-committed hemangioblasts.
    • Endoglin expression in blood and endothelium is differentially regulated by modular assembly of the Ets/Gata hemangioblast code.

      Pimanda, John E; Chan, Wan Y I; Wilson, Nicola K; Smith, Aileen M; Kinston, Sarah; Knezevic, Kathy; Janes, Mary E; Landry, Josette-Renee; Kolb-Kokocinski, Anja; Frampton, Jonathan; et al. (2008-12-01)
      Endoglin is an accessory receptor for TGF-beta signaling and is required for normal hemangioblast, early hematopoietic, and vascular development. We have previously shown that an upstream enhancer, Eng -8, together with the promoter region, mediates robust endothelial expression yet is inactive in blood. To identify hematopoietic regulatory elements, we used array-based methods to determine chromatin accessibility across the entire locus. Subsequent transgenic analysis of candidate elements showed that an endothelial enhancer at Eng +9 when combined with an element at Eng +7 functions as a strong hemato-endothelial enhancer. Chromatin immunoprecipitation (ChIP)-chip analysis demonstrated specific binding of Ets factors to the promoter as well as to the -8, +7+9 enhancers in both blood and endothelial cells. By contrast Pu.1, an Ets factor specific to the blood lineage, and Gata2 binding was only detected in blood. Gata2 was bound only at +7 and GATA motifs were required for hematopoietic activity. This modular assembly of regulators gives blood and endothelial cells the regulatory freedom to independently fine-tune gene expression and emphasizes the role of regulatory divergence in driving functional divergence.
    • Endoglin potentiates nitric oxide synthesis to enhance definitive hematopoiesis.

      Nasrallah, R; Knezevic, K; Thai, T; Thomas, S; Göttgens, B; Lacaud, Georges; Kouskoff, Valerie; Pimanda, J; Lowy Cancer Research Centre and the Prince of Wales Clinical School, UNSW Australia, Sydney, NSW 2052, Australia Cancer research UK Manchester Institute, The University of Manchester, Manchester, M20 4BX, United Kingdom (2015-05-15)
      During embryonic development, hematopoietic cells develop by a process of endothelial-to hematopoietic transition of a specialized population of endothelial cells. These hemogenic endothelium (HE) cells in turn develop from a primitive population of FLK1(+) mesodermal cells. Endoglin (ENG) is an accessory TGF-β receptor that is enriched on the surface of endothelial and hematopoietic stem cells and is also required for the normal development of hemogenic precursors. However, the functional role of ENG during the transition of FLK1(+) mesoderm to hematopoietic cells is ill defined. To address this we used a murine embryonic stem cell model that has been shown to mirror the temporal emergence of these cells in the embryo. We noted that FLK1(+) mesodermal cells expressing ENG generated fewer blast colony-forming cells but had increased hemogenic potential when compared with ENG non-expressing cells. TIE2(+)/CD117(+) HE cells expressing ENG also showed increased hemogenic potential compared with non-expressing cells. To evaluate whether high ENG expression accelerates hematopoiesis, we generated an inducible ENG expressing ES cell line and forced expression in FLK1(+) mesodermal or TIE2(+)/CD117(+) HE cells. High ENG expression at both stages accelerated the emergence of CD45(+) definitive hematopoietic cells. High ENG expression was associated with increased pSMAD2/eNOS expression and NO synthesis in hemogenic precursors. Inhibition of eNOS blunted the ENG induced increase in definitive hematopoiesis. Taken together, these data show that ENG potentiates the emergence of definitive hematopoietic cells by modulating TGF-β/pSMAD2 signalling and increasing eNOS/NO synthesis.
    • ETV2 expression marks blood and endothelium precursors, including hemogenic endothelium, at the onset of blood development.

      Wareing, Sarah; Eliades, Alexia; Lacaud, Georges; Kouskoff, Valerie; Cancer Research UK Stem Cell Hematopoiesis Group, Paterson Institute for Cancer Research, University of Manchester, Manchester, UK. (2012-09)
      ETV2 has been identified as an important player in embryonic hematopoiesis. However, the cell populations in which this transcription factor is expressed and operates during blood specification remain to be fully characterized. Here we address these issues using ES cells and a transgenic mouse line expressing green fluorescent protein (GFP) under the control of ETV2 regulatory elements, allowing us to observe the tight association between ETV2 expression and the initiation of hematopoiesis.
    • Expression of the leukemia oncogene Lmo2 is controlled by an array of tissue-specific elements dispersed over 100 kb and bound by Tal1/Lmo2, Ets, and Gata factors.

      Landry, Josette-Renee; Bonadies, Nicolas; Kinston, Sarah; Knezevic, Kathy; Wilson, Nicola K; Oram, S Helen; Janes, Mary E; Piltz, Sandie; Hammett, Michelle; Carter, Jacinta; et al. (2009-06-04)
      The Lmo2 gene encodes a transcriptional cofactor critical for the development of hematopoietic stem cells. Ectopic LMO2 expression causes leukemia in T-cell acute lymphoblastic leukemia (T-ALL) patients and severe combined immunodeficiency patients undergoing retroviral gene therapy. Tightly controlled Lmo2 expression is therefore essential, yet no comprehensive analysis of Lmo2 regulation has been published so far. By comparative genomics, we identified 17 highly conserved noncoding elements, 9 of which revealed specific acetylation marks in chromatin-immunoprecipitation and microarray (ChIP-chip) assays performed across 250 kb of the Lmo2 locus in 11 cell types covering different stages of hematopoietic differentiation. All candidate regulatory regions were tested in transgenic mice. An extended LMO2 proximal promoter fragment displayed strong endothelial activity, while the distal promoter showed weak forebrain activity. Eight of the 15 distal candidate elements functioned as enhancers, which together recapitulated the full expression pattern of Lmo2, directing expression to endothelium, hematopoietic cells, tail, and forebrain. Interestingly, distinct combinations of specific distal regulatory elements were required to extend endothelial activity of the LMO2 promoter to yolk sac or fetal liver hematopoietic cells. Finally, Sfpi1/Pu.1, Fli1, Gata2, Tal1/Scl, and Lmo2 were shown to bind to and transactivate Lmo2 hematopoietic enhancers, thus identifying key upstream regulators and positioning Lmo2 within hematopoietic regulatory networks.
    • Expression of the MOZ-TIF2 oncoprotein in mice represses senescence.

      Largeot, Anne; Perez-Campo, Flor-Maria; Marinopoulou, Elli; Lie-A-Ling, Michael; Kouskoff, Valerie; Lacaud, Georges; Cancer Research UK Stem Cell Biology Group, CR-UK Manchester Institute, The University of Manchester, Wilmslow road, Manchester (2016-02-05)
      The MOZ-TIF2 translocation, which fuses monocytic leukemia zinc finger protein (MOZ) histone acetyltransferase (HAT) with the nuclear co-activator TIF2, is associated the development of acute myeloid leukemia. We recently found that in the absence of MOZ HAT activity, p16(INK4a) transcriptional levels are significantly increased, triggering an early entrance into replicative senescence. Because oncogenic fusion proteins must bypass cellular safeguard mechanisms, such as senescence and apoptosis, to induce leukemia, we hypothesized that this repressive activity of MOZ over p16(INK4a) transcription could be preserved, or even reinforced, in MOZ leukemogenic fusion proteins, such as MOZ-TIF2. We describe here that, indeed, MOZ-TIF2 silences expression of the CDKN2A locus (p16(INK4a) and p19(ARF)), inhibits the triggering of senescence, and enhances proliferation, providing conditions favorable to the development of leukemia. Furthermore, we describe that abolishing the MOZ HAT activity of the fusion protein leads to a significant increase in expression of the CDKN2A locus and the number of hematopoietic progenitors undergoing senescence. Finally, we report that inhibition of senescence by MOZ-TIF2 is associated with increased apoptosis, suggesting a role for the fusion protein in p53 apoptosis-versus-senescence balance. Our results underscore the importance of the HAT activity of MOZ, preserved in the fusion protein, for repression of CDKN2A locus transcription and the subsequent block of senescence, a necessary step for the survival of leukemic cells.
    • Feedback regulation of p38 activity via ATF2 is essential for survival of embryonic liver cells.

      Breitwieser, Wolfgang; Lyons, Steve; Flenniken, Ann Marie; Ashton, Garry; Bruder, Gail; Willington, Mark; Lacaud, Georges; Kouskoff, Valerie; Jones, Nic; Cell Regulation Department, Paterson Institute for Cancer Research, University of Manchester, Manchester M20 4BX, United Kingdom. (2007-08-15)
      The ATF2 transcription factor is phosphorylated by the stress-activated mitogen-activated protein kinases (MAPKs) JNK and p38. We show that this phosphorylation is essential for ATF2 function in vivo, since a mouse carrying mutations in the critical phosphorylation sites has a strong phenotype identical to that seen upon deletion of the DNA-binding domain. In addition, combining this mutant with a knockout of the ATF2 homolog, ATF7, results in embryonic lethality with severe abnormalities in the developing liver and heart. The mutant fetal liver is characterized by high levels of apoptosis in developing hepatocytes and haematopoietic cells. Furthermore, we observe a significant increase in active p38 due to loss of a negative feedback loop involving the ATF2-dependent transcriptional activation of MAPK phosphatases. In embryonic liver cells, this increase drives apoptosis, since it can be suppressed by chemical inhibition of p38. Our findings demonstrate the importance of finely regulating the activities of MAPKs during development.
    • The Flk1-Cre-Mediated Deletion of ETV2 Defines Its Narrow Temporal Requirement During Embryonic Hematopoietic Development.

      Wareing, Sarah; Mazan, Andrzej; Pearson, Stella; Göttgens, B; Lacaud, Georges; Kouskoff, Valerie; Cancer Research UK Stem Cell Hematopoiesis Group,Paterson Institute for Cancer Research, University of Manchester, Wilmslow Road, Manchester, United Kingdom. (2012-07)
      During embryonic development, the emergence of hematopoiesis and vasculogenesis is tightly associated, with many transcription factors implicated in both developmental processes. Among those factors, ETV2 acts at the top of the hierarchy and controls the formation of both lineages. However, it is not known at which stage of mesoderm development ETV2 is acting and whether ETV2 activity is further required once mesodermal precursors have been specified to the hematopoietic and endothelial fates. In this study, we characterize the developmental window during which ETV2 expression is required for hematopoietic and endothelial development. Using cre-mediated deletion of ETV2, we demonstrate that ETV2 is acting prior to or at the time of FLK1 expression in mesodermal precursors to initiate the hematopoietic and endothelial program. Using the in vitro differentiation of embryonic stem cells as a model system, we further show that ETV2 re-expression in Etv2(-/-) Flk1-negative precursors drives hematopoiesis specification and switches on the expression of most genes known to be implicated in hematopoietic and endothelial development. Among the downstream targets of ETV2, we identify the transcription factors SCL, GATA2, and FLI1 known to operate a recursive loop controlling hematopoietic development. Surprisingly, SCL re-expression in Etv2(-/-) cells fully rescues hematopoiesis, while the re-expression of FLI1 or GATA2 promotes only a very limited rescue. Altogether, our data establish that ETV2 is required very transiently to specify mesodermal precursors to hematopoiesis and vasculogenesis and that SCL is one of the key downstream targets of ETV2 in controlling hematopoietic specification. STEM Cells2012;30:1521-1531.
    • FOXF1 inhibits hematopoietic lineage commitment during early mesoderm specification.

      Fleury, Maud; Eliades, Alexia; Carlsson, P; Lacaud, Georges; Kouskoff, Valerie; Cancer Research UK Stem Cell Hematopoiesis Group, Cancer Research UK Manchester Institute, The University of Manchester, Wilmslow Road, M20 4BX, UK (2015-08-20)
      The molecular mechanisms orchestrating early mesoderm specification are still poorly understood. In particular, how alternate cell fate decisions are regulated in nascent mesoderm remains mostly unknown. In the present study, we investigated both in vitro in differentiating embryonic stem cells and in vivo in gastrulating embryos the lineage specification of early mesodermal precursors expressing or not the Forkhead transcription factor FOXF1. Our data revealed that FOXF1-expressing mesoderm is derived from FLK1(+) progenitors and that in vitro this transcription factor is expressed in smooth muscle and transiently in endothelial lineages but not in hematopoietic cells. In gastrulating embryos, FOXF1 marks most extra-embryonic mesoderm derivatives including the chorion, the allantois, the amnion and a subset of endothelial cells. Similarly to the in vitro situation, FOXF1 expression is excluded from the blood islands and blood cells. Further analysis revealed an inverse correlation between hematopoietic potential and FOXF1 expression in vivo with increase commitment toward primitive erythropoiesis in Foxf1 deficient embryos while FOXF1-enforced expression in vitro was shown to repress hematopoiesis. Altogether our data establish that, during gastrulation, FOXF1 marks all posterior primitive streak extra-embryonic mesoderm derivatives with the remarkable exception of the blood lineage. Our study further suggests that this transcription factor is implicated in actively restraining the specification of mesodermal progenitors to hematopoiesis.