The critical regulator of embryonic hematopoiesis, SCL, is vital in the adult for megakaryopoiesis, erythropoiesis, and lineage choice in CFU-S12

Deletion of Mtg16, a target of t(16;21), alters hematopoietic progenitor cell proliferation and lineage allocation

While a number of DNA binding transcription factors have been identified that control hematopoietic cell fate decisions, only a limited number of transcriptional corepressors (e.g., the retinoblastoma protein [pRB] and the nuclear hormone corepressor [N-CoR]) have been linked to these functions. Here, we show that the transcriptional corepressor Mtg16 (myeloid translocation gene on chromosome 16), which is targeted by t(16;21) in acute myeloid leukemia, is required for hematopoietic progenitor cell fate decisions and for early progenitor cell proliferation. Inactivation of Mtg16 skewed early myeloid progenitor cells toward the granulocytic/macrophage lineage while reducing the numbers of megakaryocyte-erythroid progenitor cells. In addition, inactivation of Mtg16 impaired the rapid expansion of short-term stem cells, multipotent progenitor cells, and megakaryocyte-erythroid progenitor cells that is required under hematopoietic stress/emergency. This impairment appears to be a failure to proliferate rather than an induction of cell death, as expression of c-Myc, but not Bcl2, complemented the Mtg16(-/-) defect.

Differential use of SCL/TAL-1 DNA-binding domain in developmental hematopoiesis

Dissecting the molecular mechanisms used by developmental regulators is essential to understand tissue specification/differentiation. SCL/TAL-1 is a basic helix-loop-helix transcription factor absolutely critical for hematopoietic stem/progenitor cell specification and lineage maturation. Using in vitro and forced expression experimental systems, we previously suggested that SCL might have DNA-binding-independent functions. Here, to assess the requirements for SCL DNA-binding activity in vivo, we examined hematopoietic development in mice carrying a germline DNA-binding mutation. Remarkably, in contrast to complete absence of hematopoiesis and early lethality in scl-null embryos, specification of hematopoietic cells occurred in homozygous mutant embryos, indicating that direct DNA binding is dispensable for this process. Lethality was forestalled to later in development, although some mice survived to adulthood. Anemia was documented throughout development and in adulthood. Cellular and molecular studies showed requirements for SCL direct DNA binding in red cell maturation and indicated that scl expression is positively autoregulated in terminally differentiating erythroid cells. Thus, different mechanisms of SCL's action predominate depending on the developmental/cellular context: indirect DNA binding activities and/or sequestration of other nuclear regulators are sufficient in specification processes, whereas direct DNA binding functions with transcriptional autoregulation are critically required in terminal maturation processes.

Ablation of Gata1 in adult mice results in aplastic crisis, revealing its essential role in steady-state and stress erythropoiesis

The transcription factor Gata1 is expressed in several hematopoietic lineages and plays essential roles in normal hematopoietic development during embryonic stages. The lethality of Gata1-null embryos has precluded determination of its role in adult erythropoiesis. Here we have examined the effects of Gata1 loss in adult erythropoiesis using conditional Gata1 knockout mice expressing either interferon- or tamoxifen-inducible Cre recombinase (Mx-Cre and Tx-Cre, respectively). Mx-Cre-mediated Gata1 recombination, although incomplete, resulted in maturation arrest of Gata1-null erythroid cells at the proerythroblast stage, thrombocytopenia, and excessive proliferation of megakaryocytes in the spleen. Tx-Cre-mediated Gata1 recombination resulted in depletion of the erythroid compartment in bone marrow and spleen. Formation of the early and late erythroid progenitors in bone marrow was significantly reduced in the absence of Gata1. Furthermore, on treatment with a hemolytic agent, these mice failed to activate a stress erythropoietic response, despite the rising erythropoietin levels. These results indicate that, in addition to the requirement of Gata1 in adult megakaryopoiesis, Gata1 is necessary for steady-state erythropoiesis and for erythroid expansion in response to anemia. Thus, ablation of Gata1 in adult mice results in a condition resembling aplastic crisis in human.

Gene Ontology-driven transcriptional analysis of CD34+ cell-initiated megakaryocytic cultures identifies new transcriptional regulators of megakaryopoiesis

Differentiation of hematopoietic stem and progenitor cells is an intricate process controlled in large part at the level of transcription. While some key megakaryocytic transcription factors have been identified, the complete network of megakaryocytic transcriptional control is poorly understood. Using global gene expression microarray analysis, Gene Ontology-based functional annotations, and a novel interlineage comparison with parallel, isogenic granulocytic cultures as a negative control, we closely examined the mRNA level of transcriptional regulators in megakaryocytes derived from human mobilized peripheral blood CD34(+) hematopoietic cells. This approach identified 199 differentially expressed transcription factors or transcriptional regulators. We identified and detailed the transcriptional kinetics of most known megakaryocytic transcription factors including GATA1, FLI1, and MAFG. Furthermore, many genes with transcription factor activity or transcription factor binding activity were identified in megakaryocytes that had not previously been associated with that lineage, including BTEB1, NR4A2, FOXO1A, MEF2C, HDAC5, VDR, and several genes associated with the tumor suppressor p53 (HIPK2, FHL2, and TADA3L). Protein expression and nuclear localization were confirmed in megakaryocytic cells for four of the novel candidate megakaryocytic transcription factors: FHL2, MXD1, E2F3, and RFX5. In light of the hypothesis that transcription factors expressed in a particular differentiation program are important contributors to such a program, these data substantially expand our understanding of transcriptional regulation in megakaryocytic differentiation of stem and progenitor cells.

Lyl1 interacts with CREB1 and alters expression of CREB1 target genes

The basic helix-loop-helix (bHLH) transcription factor family contains key regulators of cellular proliferation and differentiation as well as the suspected oncoproteins Tal1 and Lyl1. Tal1 and Lyl1 are aberrantly over-expressed in leukemia as a result of chromosomal translocations, or other genetic or epigenetic events. Protein-protein and protein-DNA interactions described so far are mediated by their highly homologous bHLH domains, while little is known about the function of other protein domains. Hetero-dimers of Tal1 and Lyl1 with E2A or HEB, decrease the rate of E2A or HEB homo-dimer formation and are poor activators of transcription. In vitro, these hetero-dimers also recognize different binding sites from homo-dimer complexes, which may also lead to inappropriate activation or repression of promoters in vivo. Both mechanisms are thought to contribute to the oncogenic potential of Tal1 and Lyl1. Despite their bHLH structural similarity, accumulating evidence suggests that Tal1 and Lyl1 target different genes. This raises the possibility that domains flanking the bHLH region, which are distinct in the two proteins, may participate in target recognition. Here we report that CREB1, a widely-expressed transcription factor and a suspected oncogene in acute myelogenous leukemia (AML) was identified as a binding partner for Lyl1 but not for Tal1. The interaction between Lyl1 and CREB1 involves the N terminal domain of Lyl1 and the Q2 and KID domains of CREB1. The histone acetyl-transferases p300 and CBP are recruited to these complexes in the absence of CREB1 Ser 133 phosphorylation. In the Id1 promoter, Lyl1 complexes direct transcriptional activation. We also found that in addition to Id1, over-expressed Lyl1 can activate other CREB1 target promoters such as Id3, cyclin D3, Brca1, Btg2 and Egr1. Moreover, approximately 50% of all gene promoters identified by ChIP-chip experiments were jointly occupied by CREB1 and Lyl1, further strengthening the association of Lyl1 with Cre binding sites. Given the newly recognized importance of CREB1 in AML, the ability of Lyl1 to modulate promoter responses to CREB1 suggests that it plays a role in the malignant phenotype by occupying different promoters than Tal1.

Protein Stability and Transcription Factor Complex Assembly Determined by the SCL-LMO2 Interaction

Gene expression programs are established by networks of interacting transcription factors. The basic helix-loop-helix factor SCL and the LIM-only protein LMO2 are components of transcription factor complexes that are essential for hematopoiesis. Here we show that LMO2 and SCL are predominant interaction partners in hematopoietic cells and that this interaction occurs through a conserved interface residing in the loop and helix 2 of SCL. This interaction nucleates the assembly of SCL complexes on DNA and is required for target gene induction and for the stimulation of erythroid and megakaryocytic differentiation. We also demonstrate that SCL determines LMO2 protein levels in hematopoietic cells and reveal that interaction with SCL prevents LMO2 degradation by the proteasome. We propose that the SCL-LMO2 interaction couples protein stabilization with higher order protein complex assembly, thus providing a powerful means of modulating the stoichiometry and spatiotemporal activity of SCL complexes. This interaction likely provides a rate-limiting step in the transcriptional control of hematopoiesis and leukemia, and similar mechanisms may operate to control the assembly of diverse protein modules.

Novel role for EKLF in megakaryocyte lineage commitment

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

The SCL +40 enhancer targets the midbrain together with primitive and definitive hematopoiesis and is regulated by SCL and GATA proteins

The SCL/Tal-1 gene encodes a basic helix-loop-helix transcription factor with key roles in hematopoietic and neural development. SCL is expressed in, and required for, both primitive and definitive erythropoiesis. Thus far, we have identified only one erythroid SCL enhancer. Located 40 kb downstream of exon 1a, the +40 enhancer displays activity in primitive erythroblasts. We demonstrate here that a 3.7-kb fragment containing this element also targets expression to the midbrain, a known site of endogenous SCL expression. Although the 3.7-kb construct was active in primitive, but not definitive, erythroblasts, a larger 5.0-kb fragment, encompassing the 3.7-kb region, was active in both fetal and adult definitive hematopoietic cells. This included Ter119+ erythroid cells along with fetal liver erythroid and myeloid progenitors. Unlike two other SCL hematopoietic enhancers (+18/19 and -4), +40 enhancer transgenes were inactive in the endothelium. A conserved 400-bp core region, essential for both hematopoietic and midbrain +40 enhancer activity in embryos, relied on two GATA/E-box motifs and was bound in vivo by GATA-1 and SCL in erythroid cells. These results suggest a model in which the SCL +18/19 and/or -4 enhancers initiate SCL expression in early mesodermal derivatives capable of generating blood and endothelium, with subsequent activation of the +40 enhancer via an autoregulatory loop.

Aberrant mast-cell differentiation in mice lacking the stem-cell leukemia gene

The stem cell leukemia (SCL) gene encodes a basic helix-loop-helix transcription factor expressed in erythroid, megakaryocyte, and mast-cell lineages. SCL is essential for growth of megakaryocyte and erythroid progenitors. We have used a conditional knockout of SCL (SCL(-/Delta)) to examine its function in mast cells, critical effectors of the immune system. SCL(-/Delta) mice had markedly increased numbers of mast-cell progenitors (MCPs) within the peritoneal fluid, bone marrow, and spleen. Fractionation of bone marrow myeloid progenitors demonstrated that these MCPs were present in the megakaryocyte-erythroid-restricted cell fraction. In contrast, unilineage MCPs from control mice were present in the cell fraction with granulocyte-macrophage potential. The aberrant mast-cell differentiation of SCL(-/Delta) megakaryocyte-erythroid progenitors was associated with increased expression of GATA-2. Despite increased numbers of MCPs in SCL(-/Delta) mice, numbers of mature tissue mast cells were not increased unless SCL(-/Delta) mice were treated with IL-3 and stem-cell factor. In part, this may be due to a requirement for SCL in normal mast-cell maturation: SCL(-/Delta) mast cells had reduced expression of the high-affinity IgE receptor and mast cell proteases, MCP-5 and MCP-6. Together, these studies suggest that loss of SCL leads to aberrant mast-cell differentiation of megakaryocyte-erythroid progenitors.

From hematopoietic stem cells to platelets

Megakaryocytopoiesis is the process that leads to the production of platelets. This process involves the commitment of multipotent hematopoietic stem cells toward megakaryocyte (MK) progenitors, the proliferation and differentiation of MK progenitors, the polyploidization of MK precursors and the maturation of MK. Mature MK produce platelets by cytoplasmic fragmentation occurring through a dynamic and regulated process, called proplatelet formation, and consisting of long pseudopodial elongations that break in the blood flow. Recent insights have demonstrated that the MK and erythroid lineages are tightly associated at both the cellular and molecular levels, especially in the transcription factors that regulate their differentiation programs. Megakaryocytopoiesis is regulated by two types of transcription factors, those regulating the differentiation process, such as GATA-1, and those regulating proplatelet formation, such as NF-E2. The humoral factor thrombopoietin (TPO) is the primary regulator of MK differentiation and platelet production through the stimulation of its receptor MPL. Numerous acquired or congenital pathologies of the MK lineage are now explained by molecular abnormalities in the activity of the transcription factors involved in megakaryocytopoiesis, in the Tpo or c-mpl genes, as well as in signaling molecules associated with MPL. The recent development of MPL agonists may provide efficient agents for the treatment of some thrombocytopenias.

The zebrafish lysozyme C promoter drives myeloid-specific expression in transgenic fish

Background

How different immune cell compartments contribute to a successful immune response is central to fully understanding the mechanisms behind normal processes such as tissue repair and the pathology of inflammatory diseases. However, the ability to observe and characterize such interactions, in real-time, within a living vertebrate has proved elusive. Recently, the zebrafish has been exploited to model aspects of human disease and to study specific immune cell compartments using fluorescent reporter transgenic lines. A number of blood-specific lines have provided a means to exploit the exquisite optical clarity that this vertebrate system offers and provide a level of insight into dynamic inflammatory processes previously unavailable.

Results

We used regulatory regions of the zebrafish lysozyme C (lysC) gene to drive enhanced green fluorescent protein (EGFP) and DsRED2 expression in a manner that completely recapitulated the endogenous expression profile of lysC. Labeled cells were shown by co-expression studies and FACS analysis to represent a subset of macrophages and likely also granulocytes. Functional assays within transgenic larvae proved that these marked cells possess hallmark traits of myelomonocytic cells, including the ability to migrate to inflammatory sources and phagocytose bacteria.

Conclusion

These reporter lines will have utility in dissecting the genetic determinants of commitment to the myeloid lineage and in further defining how lysozyme-expressing cells participate during inflammation.

Distinct functions for different scl isoforms in zebrafish primitive and definitive hematopoiesis

The stem-cell leukemia (SCL, also known as TAL1) gene encodes a basic helix-loop-helix transcription factor that is essential for the initiation of primitive and definitive hematopoiesis, erythrocyte and megakarocyte differentiation, angiogenesis, and astrocyte development. Here we report that the zebrafish produces, through an alternative promoter site, a novel truncated scl (tal1) isoform, scl-beta, which manifests a temporal and spatial expression distinct from the previously described full-length scl-alpha. Functional analysis reveals that while scl-alpha and -beta are redundant for the initiation of primitive hematopoiesis, these two isoforms exert distinct functions in the regulation of primitive erythroid differentiation and definitive hematopoietic stem cell specification. We further demonstrate that differences in the protein expression levels of scl-alpha and -beta, by regulating their protein stability, are likely to give rise to their distinct functions. Our findings suggest that hematopoietic cells at different levels of hierarchy are likely governed by a gradient of the Scl protein established through temporal and spatial patterns of expression of the different isoforms.

Cre-loxP-mediated recombination between the SIL and SCL genes leads to a block in T-cell development at the CD4- CD8- to CD4+ CD8+ transition

In the most common form of stem cell leukemia (SCL) gene rearrangement, an interstitial deletion of 82 kb brings SCL under the control of regulatory elements that normally govern expression of the ubiquitously expressed SCL interrupting locus (SIL) gene, which is located directly upstream of SCL. To investigate the effect of this fusion in a mouse model, a bacterial artificial chromosome (BAC) clone containing both human SIL and SCL genes was isolated, and loxP sites were inserted into intron 1 of both the SIL and SCL genes, corresponding to the sites at which recombination occurs in human T-cell acute lymphocytic leukemia patients. This BAC clone was used to generate transgenic SILloxloxSCL mice. These transgenic mice were subsequently bred to Lck-Cre mice that express the Cre recombinase specifically in the thymus. The BAC transgene was recombined between the two loxP sites in over 50% of the thymocytes from SILloxloxSCL/Cre double-transgenic mice, bringing the SCL gene under the direct control of SIL regulatory elements. Aberrant SCL gene expression in the thymus was verified by reverse transcription-polymerase chain reaction. Using FACS analysis, we found that mice carrying both SILloxloxSCL and Cre transgenes have increased CD4-/CD8- thymocytes compared with transgene-negative mice. In the spleen, these transgenic mice show a marked reduction in the number of mature CD4+ or CD8+ cells. These results demonstrate that conditional activation of SCL under control of SIL regulatory elements can impair normal T-cell development.

Gradients of antigen expression and developmental potential in hematopoiesis

Prospective isolation of hematopoietic stem and progenitor cell subsets depends upon the premise that expression of combinations of surface antigens reflects developmental potential. During the process of differentiation, however, the loss of antigens associated with stem cells and the concomitant gain of those associated with progenitor cells often occurs as a continuum rather than by discrete binary steps. Coupled with the fact that assay conditions can profoundly influence the developmental fates of prospectively isolated cells, gradients of antigen expression during differentiation have led to a variety of interpretations of lineage commitment in hematopoiesis.

Transcription factors in myeloid development: balancing differentiation with transformation

In recent years, great progress has been made in elucidating the progenitor-cell hierarchy of the myeloid lineage. Transcription factors have been shown to be key determinants in the orchestration of myeloid identity and differentiation fates. Most transcription factors show cell-lineage-restricted and stage-restricted expression patterns, indicating the requirement for tight regulation of their activities. Moreover, if dysregulated or mutated, these transcription factors cause the differentiation block observed in many myeloid leukaemias. Consequently, therapies designed to restore defective transcription factor functions are an attractive option in the treatment of myeloid and other human cancers.

TAL-1/SCL and its partners E47 and LMO2 up-regulate VE-cadherin expression in endothelial cells

The basic helix-loop-helix TAL-1/SCL essential for hematopoietic development is also required during vascular development for embryonic angiogenesis. We reported that TAL-1 acts positively on postnatal angiogenesis by stimulating endothelial morphogenesis. Here, we investigated the functional consequences of TAL-1 silencing in human primary endothelial cells. We found that TAL-1 knockdown caused the inhibition of in vitro tubulomorphogenesis, which was associated with a dramatic reduction in vascular endothelial cadherin (VE-cadherin) at intercellular junctions. Consistently, silencing of TAL-1 as well as of its cofactors E47 and LMO2 down-regulated VE-cadherin at both the mRNA and the protein level. Endogenous VE-cadherin transcription could be activated in nonendothelial HEK-293 cells by the sole concomitant ectopic expression of TAL-1, E47, and LMO2. Transient transfections in human primary endothelial cells derived from umbilical vein (HUVECs) demonstrated that VE-cadherin promoter activity was dependent on the integrity of a specialized E-box associated with a GATA motif and was maximal with the coexpression of the different components of the TAL-1 complex. Finally, chromatin immunoprecipitation assays showed that TAL-1 and its cofactors occupied the VE-cadherin promoter in HUVECs. Together, these data identify VE-cadherin as a bona fide target gene of the TAL-1 complex in the endothelial lineage, providing a first clue to TAL-1 function in angiogenesis.

The paralogous hematopoietic regulators Lyl1 and Scl are coregulated by Ets and GATA factors, but Lyl1 cannot rescue the early Scl-/- phenotype

Transcription factors are key regulators of hematopoietic stem cells (HSCs), yet the molecular mechanisms that control their expression are largely unknown. Previously, we demonstrated that expression of Scl/Tal1, a transcription factor required for the specification of HSCs, is controlled by Ets and GATA factors. Here we characterize the molecular mechanisms controlling expression of Lyl1, a paralog of Scl also required for HSC function. Two closely spaced promoters directed expression to hematopoietic progenitor, megakaryocytic, and endothelial cells in transgenic mice. Conserved binding sites required for promoter activity were bound in vivo by GATA-2 and the Ets factors Fli1, Elf1, Erg, and PU.1. However, despite coregulation of Scl and Lyl1 by the same Ets and GATA factors, Scl expression was initiated prior to Lyl1 in embryonic stem (ES) cell differentiation assays. Moreover, ectopic expression of Scl but not Lyl1 rescued hematopoietic differentiation in Scl-/- ES cells, thus providing a molecular explanation for the vastly different phenotypes of Scl-/- and Lyl1-/- mouse embryos. Furthermore, coregulation of Scl and Lyl1 later during development may explain the mild phenotype of Scl-/- adult HSCs.

A critical role for the transcription factor Scl in platelet production during stress thrombopoiesis

The generation of platelets from megakaryocytes in the steady state is regulated by a variety of cytokines and transcription factors, including thrombopoietin (TPO), GATA-1, and NF-E2. Less is known about platelet production in the setting of stress thrombopoiesis, a pivotal event in the context of cytotoxic chemotherapy. Here we show in mice that the transcription factor Scl is critical for platelet production after chemotherapy and in thrombopoiesis induced by administration of TPO. Megakaryocytes from these mice showed appropriate increases in number and ploidy but failed to shed platelets. Ultrastructural examination of Scl-null megakaryocytes revealed a disorganized demarcation membrane and reduction in platelet granules. Quantitative real-time polymerase chain reaction showed that Scl-null platelets lacked NF-E2, and chromatin immunoprecipitation analysis demonstrated Scl binding to the NF-E2 promoter in the human megakaryoblastic-cell line Meg-01, along with its binding partners E47, Lmo2, and the cofactors Ldb1 and GATA-2. These findings suggest that Scl acts up-stream of NF-E2 expression to control megakaryocyte development and platelet release in settings of thrombopoietic stress.

Expression of the TAL1/SCL transcription factor in physiological and pathological vascular processes

The TAL1/SCL transcription factor is essential for haematopoietic commitment and vascular remodelling during embryonic development. To help clarify its role in postnatal vascular processes, we characterized the expression of mouse Tal1 protein by immunocytochemistry in several experimental models of blood vessel formation. In adult mice, Tal1 protein was expressed in rare microvascular endothelial cells and in extravascular cells provisionally identified as endothelial progenitors from their morphology, proximity to vessels and expression of vascular endothelial growth factor receptor-2. The number of Tal1-expressing endothelial cells increased significantly but transiently in all the models-hormone-induced ovulation, wound healing and tumour development. Finally, Tal1 protein was detected in the nuclei of newly formed lymphatic endothelial cells in tumour-bearing animals. These results show that TAL1 is expressed by vascular endothelial cells and endothelial progenitors at sites of physiological and pathological neovascularization and suggest a role for this transcription factor in adult vasculogenesis. This work also provides the first evidence for TAL1 expression in lymphangiogenesis.

The transcription factor ZBP-89 controls generation of the hematopoietic lineage in zebrafish and mouse embryonic stem cells

Hematopoietic development is closely linked to that of blood vessels and the two processes are regulated in large part by transcription factors that control cell fate decisions and cellular differentiation. Both blood and blood vessels derive from a common progenitor, termed the hemangioblast, but the factor(s) specifying the development and differentiation of this stem cell population into the hematopoietic and vascular lineages remain ill defined. Here, we report that knockdown of the Krüppel-like transcription factor ZBP-89 in zebrafish embryos results in a bloodless phenotype, caused by disruption of both primitive and definitive hematopoiesis, while leaving primary blood vessel formation intact. Injection of ZBP-89 mRNA into cloche zebrafish embryos, which lack both the hematopoietic and endothelial lineages, rescues hematopoiesis but not vasculogenesis. Injection of mRNA for Stem Cell Leukemia (SCL), a transcription factor that directs hemangioblast development into blood cell precursors, rescues the bloodless phenotype in ZBP-89 zebrafish morphants. Forced expression of ZBP-89 induces the expansion of hematopoietic progenitors in wild-type zebrafish and in mouse embryonic stem cell cultures but inhibits angiogenesis in vivo and in vitro. These findings establish a unique regulatory role for ZBP-89, positioned at the interface between early blood and blood vessel development.

Low SCL/TAL1 expression reveals its major role in adult hematopoietic myeloid progenitors and stem cells

Stem cell leukemia/T cell acute leukemia 1 (SCL/TAL1) plays a key role in the development of murine primitive hematopoiesis but its functions in adult definitive hematopoiesis are still unclear. Using lentiviral delivery of TAL1-directed shRNA in human hematopoietic cells, we show that decreased expression of TAL1 induced major disorders at different levels of adult hematopoietic cell development. Erythroid and myeloid cell production in cultures was dramatically decreased in TAL1-directed shRNA-expressing cells, whereas lymphoid B-cell development was normal. These results confirm the role of TAL1 in the erythroid compartment and show TLA1's implication in the function of myeloid committed progenitors. Moreover, long-term cultures and transplantation of TAL1-directed shRNA-expressing CD34+ cells into irradiated nonobese diabetic-severe combined immunodeficient (NOD-SCID) mice led to dramatically low levels of human cells of all lineages including the B-lymphoid lineage, strongly suggesting that TAL1 has a role in the early commitment of hematopoietic stem cells (HSCs) in humans. Cultures and transplantation experiments performed with mouse Sca1+ cells gave identical results. Altogether, these observations definitively show that TAL1 participates in the regulation of hematopoiesis from HSCs to myeloid progenitors, and pinpoint TAL1 as a master protein of human and murine adult hematopoiesis.

The SCL relative LYL-1 is required for fetal and adult hematopoietic stem cell function and B-cell differentiation

Hematopoietic stem cells (HSCs) arise, self-renew, or give rise to all hematopoietic lineages through the effects of transcription factors activated by signaling cascades. Lyl-1 encodes a transcription factor containing a basic helix-hoop-helix (bHLH) motif closely related to scl/tal, which controls numerous decisions in embryonic and adult hematopoiesis. We report here that Lyl-1 null mice are viable and display normal blood cell counts, except for a reduced number of B cells resulting from a partial block after the pro-B stage. Nevertheless, the deletion of Lyl-1 results in a diminution in the frequency of immature progenitors (Lin–, CD34–, sca-1+, c-kit+ [LSK], and LSK-side population [LSK-SP]) and in S12 colony-forming unit (CFU-S12) and long-term culture-initiating cell (LTC-IC) content in embryonic day 14 fetal liver (E14 FL) and adult bone marrow (BM). More important, Lyl-1–/– E14 FL cells and BM are severely impaired in their competitive reconstituting abilities, especially with respect to B and T lineage reconstitution. Thus, ablation of Lyl-1 quantitatively and functionally affects HSCs, a cell population that transcribes Lyl-1 more actively than their differentiated progenies. Our results demonstrate for the first time that Lyl-1 functions are important for HSC properties and B-cell differentiation and that they are largely distinct from scl functions.

Coordination of erythropoiesis by the transcription factor c-Myb

The involvement of the transcription factor c-Myb in promoting the proliferation and inhibition of erythroid cell differentiation has been established in leukemia cell models. The anemia phenotype observed in c-myb knockout and knockdown mice highlights a critical role for c-Myb in erythropoiesis. However, determining the reason for the failure of erythropoiesis in these mice and the precise function of c-Myb in erythroid progenitors remains elusive. We examined erythroid development under conditions of reduced c-Myb protein levels and report an unexpected role for c-Myb in the promotion of commitment to the erythroid lineage and progression to erythroblast stages. c-myb knockdown erythroid colony-forming unit (CFU-E) stage progenitors displayed an immature phenotype and aberrant expression of several hematopoietic regulators. To extend our findings, we analyzed the response of normal enriched erythroid progenitors to inducible disruption of a floxed c-myb allele. In agreement with the c-myb knockdown phenotype, we show that c-Myb is strictly required for expression of the c-Kit receptor in erythroid cells.

C/EBPalpha determines hematopoietic cell fate in multipotential progenitor cells by inhibiting erythroid differentiation and inducing myeloid differentiation

C/EBPalpha is an essential transcription factor required for myeloid differentiation. While C/EBPalpha can act as a cell fate switch to promote granulocyte differentiation in bipotential granulocyte-macrophage progenitors (GMPs), its role in regulating cell fate decisions in more primitive progenitors is not known. We found increased numbers of erythroid progenitors and erythroid cells in C/EBPalpha(-/-) fetal liver (FL). Also, enforced expression of C/EBPalpha in hematopoietic stem cells resulted in a loss of erythroid progenitors and an increase in myeloid cells by inhibition of erythroid development and inducing myeloid differentiation. Conditional expression of C/EBPalpha in murine erythroleukemia (MEL) cells induced myeloid-specific genes, while inhibiting erythroid-specific gene expression including erythropoietin receptor (EpoR), which suggests a novel mechanism to determine hematopoietic cell fate. Thus, C/EBPalpha functions in hematopoietic cell fate decisions by the dual actions of inhibiting erythroid and inducing myeloid gene expression in multipotential progenitors.

The essential haematopoietic transcription factor Scl is also critical for neuronal development

Abstract The basic helix-loop-helix (bHLH) transcription factor Scl displays tissue-restricted expression and is critical for the establishment of the haematopoietic system; loss of Scl results in embryonic death due to absolute anaemia. Scl is also expressed in neurons of the mouse diencephalon, mesencephalon and metencephalon; however, its requirement in those sites remains to be determined. Here we report conditional deletion of Scl in neuronal precursor cells using the Cre/LoxP system. Neuronal-Scl deleted mice died prematurely, were growth retarded and exhibited an altered motor phenotype characterized by hyperactivity and circling. Moreover, ablation of Scl in the nervous system affected brain morphology with abnormal neuronal development in brain regions known to express Scl under normal circumstances; there was an almost complete absence of Scl-null neurons in the hindbrain and partial loss of Scl-null neurons in the thalamus and midbrain from early neurogenesis onwards. Our results demonstrate a crucial role for Scl in the development of Scl-expressing neurons, including gamma-aminobutyric acid (GABA)ergic interneurons. Our study represents one of the first demonstrations of functional overlap of a single bHLH protein that regulates neural and haematopoietic cell development. This finding underlines Scl's critical function in cell fate determination of mesodermal as well as neuroectodermal tissues.

Absence of the transcription factor CCAAT enhancer binding protein alpha results in loss of myeloid identity in bcr/abl-induced malignancy

The lineage-determining transcription factor CCAAT enhancer binding protein alpha (C/EBPalpha) is required for myeloid differentiation. Decreased function or expression of C/EBPalpha is often found in human acute myeloid leukemia. However, the precise impact of C/EBPalpha deficiency on the maturation arrest in leukemogenesis is not well understood. To address this question, we used a murine transplantation model of a bcr/abl-induced myeloproliferative disease. The expression of bcr/abl in C/EBPalphapos fetal liver cells led to a chronic myeloid leukemia-like disease. Surprisingly, bcr/abl-expressing C/EBPalpha-/- fetal liver cells failed to induce a myeloid disease in transplanted mice, but caused a fatal, transplantable erythroleukemia instead. Accordingly, increased expression of the transcription factors SCL and GATA-1 in hematopoietic precursor cells of C/EBPalpha-/-R01-EY-11298 ) fetal livers was found. The mechanism for the lineage shift from myeloid to erythroid leukemia was studied in a bcr/abl-positive cell line. Consistent with findings of the transplant model, expression of C/EBPalpha and GATA-1 was inversely correlated. Id1, an inhibitor of erythroid differentiation, was identified as a critical direct target of C/EBPalpha. Down-regulation of Id1 by RNA interference impaired C/EBPalpha-induced granulocytic differentiation. Taken together, our study provides evidence that myeloid lineage identity of malignant hematopoietic progenitor cells requires the residual expression of C/EBPalpha.

Transcriptional activators, repressors, and epigenetic modifiers controlling hematopoietic stem cell development

Hematopoietic stem cells (HSCs) are pluripotent cells that give rise to all of the circulating blood cell types. Their unique ability to self-renew while generating differentiated daughter cells permits HSCs to sustain blood cell production throughout life. In mammals, the pool of HSCs shifts from early sites in the aorta-gonad-mesonephros region and the placenta to the fetal liver and ultimately bone marrow. During the past decade, a map of transcriptional activators and repressors that regulate gene expression in HSCs, their precursors and their progeny, at distinct stages of development has been drafted. These factors control a program that first establishes the pool of HSCs in the fetus, and later guides decisions between quiescence, self-renewal, and lineage commitment with progressive differentiation to maintain homeostasis. Continuing studies of the regulatory mechanisms that control HSC gene expression followed by the identification of specific loci that are activated or silenced during the life of an HSC will help to further elucidate longstanding issues in HSC decisions to self-renew or to differentiate, and to define the origins of and connections between distinct HSC pools and their precursors.

ETO2 coordinates cellular proliferation and differentiation during erythropoiesis

The passage from proliferation to terminal differentiation is critical for normal development and is often perturbed in malignancies. To define the molecular mechanisms that govern this process during erythropoiesis, we have used tagging/proteomics approaches and characterized protein complexes nucleated by TAL-1/SCL, a basic helix-loop-helix transcription factor that specifies the erythrocytic lineage. In addition to known TAL-1 partners, GATA-1, E2A, HEB, LMO2 and Ldb1, we identify the ETO2 repressor as a novel component recruited to TAL-1 complexes through interaction with E2A/HEB. Ectopic expression and siRNA knockdown experiments in hematopoietic progenitor cells show that ETO2 actively represses erythroid TAL-1 target genes and governs the expansion of erythroid progenitors. At the onset of erythroid differentiation, a change in the stoichiometry of ETO2 within the TAL-1 complex activates the expression of known erythroid-specific TAL-1 target genes and of Gfi-1b and p21(Cip), encoding two essential regulators of erythroid cell proliferation. These results suggest that the dynamics of ETO2 recruitment within nuclear complexes couple cell proliferation to cell differentiation and determine the onset of terminal erythroid maturation.

PU.1/Spi-1 Binds to the Human TAL-1 Silencer to Mediate its Activity

The TAL-1/SCL gene encodes a basic helix-loop-helix (bHLH) transcription factor essential for primitive hematopoiesis and for adult erythroid and megakaryocytic development. Activated transcription of TAL-1 as a consequence of chromosomal rearrangements is associated with a high proportion of human T cell acute leukemias, showing that appropriate control of TAL-1 is crucial for the formation and subsequent fate of hematopoietic cells. Hence, the knowledge of the mechanisms, which govern the pattern of TAL-1 expression in hematopoiesis, is of great interest. We previously described a silencer in the 3'-untranslated region of human TAL-1, the activity of which is mediated through binding of a tissue-specific 40 kDa nuclear protein to a new DNA recognition motif, named tal-RE. Here, we show that tal-RE-binding activity, high in immature human hematopoietic progenitors is down regulated upon erythroid and megakaryocytic differentiation. This expression profile helped us to identify that PU.1/Spi-1 binds to the tal-RE sequences in vitro and occupies the TAL-1 silencer in vivo. By expressing a mutant protein containing only the ETS domain of PU.1 in human erythroleukemic HEL cells, we demonstrated that PU.1 mediates the transcriptional repression activity of the silencer. We found that ectopic PU.1 is not able to induce silencing activity in PU.1-negative Jurkat T cells, indicating that PU.1 activity, although necessary, is not sufficient to confer transcriptional repression activity to the TAL-1 silencer. Finally, we showed that the silencer is also active in TAL-1-negative myeloid HL60 cells that express PU.1 at high levels. In summary, our study shows that PU.1, in addition to its positive role in TAL-1 expression in early hematopoietic progenitors, may also act as a mediator of TAL-1 silencing in some hematopoietic lineages.

Variegation of the phenotype induced by the Gata1low mutation in mice of different genetic backgrounds

All mice harboring the X-linked Gata1low mutation in a predominantly CD1 background are born anemic and thrombocytopenic. They recover from anemia at 1 month of age but remain thrombocytopenic all their life and develop myelofibrosis, a syndrome similar to human idiopathic myelofibrosis, at 12 months. The effects of the genetic background on the myelofibrosis developed by Gata1low mice was assessed by introducing the mutation, by standard genetic approaches, in the C57BL/6 and DBA/2 backgrounds and by analyzing the phenotype of the different mutants at 12 to 13 (by histology) and 16 to 20 (by cytofluorimetry) months of age. Although all the Gata1low mice developed fibrosis at 12 to 13 months, variegations were observed in the severity of the phenotype expressed by mutants of different backgrounds. In C57BL/6 mice, the mutation was no longer inherited in a Mendelian fashion, and fibrosis was associated with massive osteosclerosis. Instead, DBA/2 mutants, although severely anemic, expressed limited fibrosis and osteosclerosis and did not present tear-drop poikilocytes in blood or extramedullary hemopoiesis in liver up to 20 months of age. We propose that the variegation in myelofibrosis expressed by Gata1low mutants of different strains might represent a model to study the variability of the clinical picture of the human disease.

ETO-2 associates with SCL in erythroid cells and megakaryocytes and provides repressor functions in erythropoiesis

Lineage specification and cellular maturation require coordinated regulation of gene expression programs. In large part, this is dependent on the activator and repressor functions of protein complexes associated with tissue-specific transcriptional regulators. In this study, we have used a proteomic approach to characterize multiprotein complexes containing the key hematopoietic regulator SCL in erythroid and megakaryocytic cell lines. One of the novel SCL-interacting proteins identified in both cell types is the transcriptional corepressor ETO-2. Interaction between endogenous proteins was confirmed in primary cells. We then showed that SCL complexes are shared but also significantly differ in the two cell types. Importantly, SCL/ETO-2 interacts with another corepressor, Gfi-1b, in red cells but not megakaryocytes. The SCL/ETO-2/Gfi-1b association is lost during erythroid differentiation of primary fetal liver cells. Genetic studies of erythroid cells show that ETO-2 exerts a repressor effect on SCL target genes. We suggest that, through its association with SCL, ETO-2 represses gene expression in the early stages of erythroid differentiation and that alleviation/modulation of the repressive state is then required for expression of genes necessary for terminal erythroid maturation to proceed.

RFP represses transcriptional activation by bHLH transcription factors

Basic helix-loop-helix (bHLH) transcription factors play a pivotal role in the regulation of tumorigenesis, and also in a wide range of other developmental processes in diverse species from yeast to humans. Here we demonstrate for the first time that Ret finger protein (RFP), a member of the TRIM family of proteins initially identified as a recombined transforming gene from a human lymphoma, is a novel interaction partner for four different bHLH proteins (SCL, E47, MyoD and mASH-1), but does not interact with GATA-1 or PU.1. Interaction with SCL required the B-box and first coiled-coil region of RFP together with the bHLH domain of SCL. RFP was able to repress transcriptional activation by E47, MyoD and mASH-1, but not by members of several other transcription factor families. Transcriptional repression by RFP was trichostatin A sensitive and did not involve an Id-like mechanism or ubiquitination with subsequent degradation of bHLH proteins. Instead, our results suggest that bHLH transcription factors are regulated by a previously undescribed interaction with RFP, which functions to recruit HDAC and/or Polycomb proteins and thus repress target genes of bHLH proteins. These results reveal an unexpected link between the bHLH and TRIM protein families.

The proto-oncogene ERG in megakaryoblastic leukemias

Aneuploidy is one of the hallmarks of cancer. Acquired additions of chromosome 21 are a common finding in leukemias, suggesting a contributory role to leukemogenesis. About 10% of patients with a germ line trisomy 21 (Down syndrome) are born with transient megakaryoblastic leukemia. We and others have shown acquired mutations in the X chromosome gene GATA1 in all these cases. The gene or genes on chromosome 21 whose overexpression promote the megakaryoblastic phenotype are presently unknown. We propose that ERG, an Ets transcription factor situated on chromosome 21, is one such candidate. We show that ERG is expressed in hematopoietic stem cells, megakaryoblastic cell lines, and in primary leukemic cells from Down syndrome patients. ERG expression is induced upon megakaryocytic differentiation of the erythroleukemia cell lines K562 and UT-7, and forced expression of ERG in K562 cells induces erythroid to megakaryoblastic phenotypic switch. We also show that ERG activates the gpIb megakaryocytic promoter and binds the gpIIb promoter in vivo. Furthermore, both ERG and ETS2 bind in vivo the hematopoietic enhancer of SCL/TAL1, a key regulator of hematopoietic stem cell and megakaryocytic development. We propose that trisomy 21 facilitates the occurrence of megakaryoblastic leukemias through a shift toward the megakaryoblastic lineage caused by the excess expression of ERG, and possibly by other chromosome 21 genes, such as RUNX1 and ETS2, in hematopoietic progenitor cells, coupled with a differentiation arrest caused by the acquisition of mutations in GATA1.

Childhood and adolescent non-Hodgkin lymphoma: new insights in biology and critical challenges for the future

Pediatric non-Hodgkin lymphoma (NHL) is a common and fascinating group of diseases with distinctive underlying genetic events that characterize the major histologic subtypes: diffuse large B-cell lymphoma, Burkitt lymphoma, anaplastic large cell lymphoma and lymphoblastic lymphoma. With systematic improvements in therapy over recent decades, the vast majority of children with NHL of all subtypes are now cured. The similarities and differences between adult and childhood presentations of disease, and whether or not some subtypes of NHL and leukemia are the same or different disease entities, are interesting questions that will be addressed with advances in our understanding of the molecular and genetic bases of these diseases. As is the case with other pediatric malignancies, growing emphasis is now being placed on the development of less toxic, targeted therapeutic approaches, and this review highlights some of the biological discoveries that will potentially open these avenues.

Role of transcription factors C/EBPalpha and PU.1 in normal hematopoiesis and leukemia

Differentiation of hematopoietic stem and progenitor cells is under strict control of a regulatory network orchestrated by lineage-specific transcription factors. A block in normal differentiation is a major contributing factor in the development of solid tumors and leukemias. Cells from patients with acute myeloid leukemia (AML) frequently harbor mutated or dysregulated transcription factor genes, suggesting their involvement in leukemogenesis. As a consequence, these alterations diminish the pool of available molecules of a small number of critical transcription factors, such as CCAAT enhancer binding proteins, PU.1, GATA-1, and AML-1. In this review, we focus on the mechanisms of how this functional pool of transcription factors is maintained during normal and malignant hematopoiesis, including direct protein-protein interactions, competition for DNA binding, and the control of transcription factor genes by proximal and distal regulatory elements. Results of recent studies of mice carrying hypomorphic PU.1 alleles have indicated that reduction in the expression of a single transcription factor is capable of predisposing mice to AML. The implications of these findings for the study of hematopoiesis in the future as well as novel approaches to more disease-specific therapies are discussed.

SCL/TAL1 expression level regulates human hematopoietic stem cell self-renewal and engraftment

The fate of hematopoietic stem cells (HSCs) is regulated through a combinatorial action of proteins that determine their self-renewal and/or their commitment to differentiation. Stem cell leukemia/T-cell acute lymphoblastic leukemia 1 (SCL/TAL1), a basic helix-loop-helix (bHLH) transcription factor, plays key roles in controlling the development of primitive and definitive hematopoiesis during mouse development but its function in adult HSCs is still a matter of debate. We report here that the lentiviral-mediated enforced expression of TAL1 in human CD34+ cells marginally affects in vitro the differentiation of committed progenitors, whereas in vivo the repopulation capacity of the long-term SCID (severe combined immunodeficient) mouse–repopulating cells (LT-SRCs) is enhanced. As a consequence, the production of SRC-derived multipotent progenitors as well as erythroid- and myeloid-differentiated cells is increased. Looking at the lymphoid compartment, constitutive TAL1-enforced expression impairs B- but not T-cell differentiation. Expression of a mutant TAL1 protein that cannot bind DNA specifically impairs human LT-SRC amplification, indicating a DNA-binding dependent effect of TAL1 on primitive cell populations. These results indicate that TAL1 expression level regulates immature human hematopoietic cell self-renewal and that this regulation requires TAL1 DNA-binding activity.

SCL expression at critical points in human hematopoietic lineage commitment

The stem cell leukemia (SCL or tal-1) gene was initially identified as a translocation partner in a leukemia that possessed both lymphoid and myeloid differentiation potential. Mice that lacked SCL expression showed a complete block in hematopoiesis; thus, SCL was associated with hematopoietic stem cell (HSC) function. More recent studies show a role for SCL in murine erythroid differentiation. However, the expression pattern and the role of SCL during early stages of human hematopoietic differentiation are less clear. In this study we chart the pattern of human SCL expression from HSCs, through developmentally sequential populations of lymphoid and myeloid progenitors to mature cells of the hematopoietic lineages. Using recently defined surface immunophenotypes, we fluorescence-activated cell-sorted (FACS) highly purified populations of primary human hematopoietic progenitors for reverse transcription-polymerase chain reaction (RT-PCR) analysis of SCL expression. Our data show that SCL mRNA is easily detectable in all hematopoietic populations with erythroid potential, including HSCs, multipotential progenitors, common myeloid progenitors, megakaryocyte/erythrocyte progenitors, and nucleated erythroid lineage cells. SCL mRNA expression was present but rapidly downregulated in the common lymphoid progenitor and granulocyte/monocyte progenitor populations that lack erythroid potential. SCL expression was undetectable in immature cells of nonerythroid lineages, including pro-B cells, early thymic progenitors, and myeloid precursors expressing the M-CSF receptor. SCL expression was also absent from all mature cells of the nonerythroid lineages. Although low levels of SCL were detected in lymphoid- and myeloid-restricted progenitors, our studies show that abundant SCL expression is normally tightly linked with erythroid differentiation potential.

Functional but abnormal adult erythropoiesis in the absence of the stem cell leukemia gene

Previous studies have indicated that the stem cell leukemia gene (SCL) is essential for both embryonic and adult erythropoiesis. We have examined erythropoiesis in conditional SCL knockout mice for at least 6 months after loss of SCL function and report that SCL was important but not essential for the generation of mature red blood cells. Although SCL-deleted mice were mildly anemic with increased splenic erythropoiesis, they responded appropriately to endogenous erythropoietin and hemolytic stress, a measure of late erythroid progenitors. However, SCL was more important for the proliferation of early erythroid progenitors because the predominant defects in SCL-deleted erythropoiesis were loss of in vitro growth of the burst-forming erythroid unit and an in vivo growth defect revealed by transplant assays. With respect to erythroid maturation, SCL-deleted proerythroblasts could generate more mature erythroblasts and circulating red blood cells. However, SCL was required for normal expression of TER119, one of the few proposed target genes of SCL. The unexpected finding that SCL-independent erythropoiesis can proceed in the adult suggests that alternate factors can replace the essential functions of SCL and raises the possibility that similar mechanisms also explain the relatively minor defects previously observed in SCL-null hematopoietic stem cells.

Mechanisms of thrombopoiesis

Megakaryocytes (MKs) expand and differentiate over several days in response to thrombopoietin (Tpo) before releasing innumerable blood platelets. The final steps in platelet assembly and release represent a unique cellular transformation that is orchestrated by a range of transcription factors, signaling molecules, and cytoskeletal elements. Here we review recent advances in the physiology and molecular basis of MK differentiation. Genome-wide approaches, including transcriptional profiling and proteomics, have been used to identify novel platelet products and differentiation markers. The extracellular factors, stromal-derived factor (SDF)-1 chemokine and fibroblast growth factor (FGF)-4 direct MK interactions with the bone marrow stroma and regulate cytokine-independent cell maturation. An abundance of bone marrow MKs induce pathologic states, including excessive bone formation and myelofibrosis, and the basis for these effects is now better appreciated. We review the status of transcription factors that control MK differentiation, with special emphasis on nuclear factor-erythroid 2 (NF-E2) and its two putative target genes, beta1-tubulin and 3-beta-hydroxysteroid reductase. MKs express steroid receptors and some estrogen ligands, which may constitute an autocrine loop in formation of proplatelets, the cytoplasmic protrusions within which nascent blood platelets are assembled. Finally, we summarize our own studies on cellular and molecular facets of proplatelet formation and place the findings within the context of outstanding questions about mechanisms of thrombopoiesis.

Transgenic analysis of the stem cell leukemia +19 stem cell enhancer in adult and embryonic hematopoietic and endothelial cells

Appropriate transcriptional regulation is critical for the biological functions of many key regulatory genes, including the stem cell leukemia (SCL) gene. As part of a systematic dissection of SCL transcriptional regulation, we have previously identified a 5,245-bp SCL +18/19 enhancer that targeted embryonic endothelium together with embryonic and adult hematopoietic progenitors and stem cells (HSCs). This enhancer is proving to be a powerful tool for manipulating hematopoietic progenitors and stem cells, but the design and interpretation of such transgenic studies require a detailed understanding of enhancer activity in vivo. In this study, we demonstrate that the +18/19 enhancer is active in mast cells, megakaryocytes, and adult endothelium. A 644-bp +19 core enhancer exhibited similar temporal and spatial activity to the 5,245-bp +18/19 fragment both during development and in adult mice. Unlike the +18/19 enhancer, the +19 core enhancer was only active in adult mice when linked to the eukaryotic reporter gene human placental alkaline phosphatase. Activity of a single core enhancer in HSCs, endothelium, mast cells, and megakaryocytes suggests possible overlaps in their respective transcriptional programs. Moreover, activity in a proportion of thymocytes and other SCL-negative cell types suggests the existence of a silencer elsewhere in the SCL locus.

Transcriptional regulation of the SCL locus: identification of an enhancer that targets the primitive erythroid lineage in vivo

The stem cell leukemia (SCL) gene, also known as TAL-1, encodes a basic helix-loop-helix protein that is essential for the formation of all hematopoietic lineages, including primitive erythropoiesis. Appropriate transcriptional regulation is essential for the biological functions of SCL, and we have previously identified five distinct enhancers which target different subdomains of the normal SCL expression pattern. However, it is not known whether these SCL enhancers also regulate neighboring genes within the SCL locus, and the erythroid expression of SCL remains unexplained. Here, we have quantitated transcripts from SCL and neighboring genes in multiple hematopoietic cell types. Our results show striking coexpression of SCL and its immediate downstream neighbor, MAP17, suggesting that they share regulatory elements. A systematic survey of histone H3 and H4 acetylation throughout the SCL locus in different hematopoietic cell types identified several peaks of histone acetylation between SIL and MAP17, all of which corresponded to previously characterized SCL enhancers or to the MAP17 promoter. Downstream of MAP17 (and 40 kb downstream of SCL exon 1a), an additional peak of acetylation was identified in hematopoietic cells and was found to correlate with expression of SCL but not other neighboring genes. This +40 region is conserved in human-dog-mouse-rat sequence comparisons, functions as an erythroid cell-restricted enhancer in vitro, and directs beta-galactosidase expression to primitive, but not definitive, erythroblasts in transgenic mice. The SCL +40 enhancer provides a powerful tool for studying the molecular and cellular biology of the primitive erythroid lineage.

The homeodomain protein Meis1 is essential for definitive hematopoiesis and vascular patterning in the mouse embryo

Homeodomain proteins of the Meis subfamily are expressed dynamically in several organs during embryogenesis and exert potent regulatory activity through their interaction with Hox proteins and other transcription factors. Here we show that Meis1 is expressed in the hematopoietic stem cell (HSC) compartment in the fetal liver, and in the primary sites of definitive hematopoiesis, including the aorta-gonad-mesonephros (AGM) mesenchyme, the hemogenic embryonic arterial endothelium, and hematopoietic clusters within the aorta, vitelline, and umbilical arteries. We inactivated the Meis1 gene in mice and found that Meis1 mutant mice die between embryonic days 11.5 and 14.5, showing internal hemorrhage, liver hypoplasia, and anemia. In Meis1 mutant mouse fetal liver and AGM, HSC compartments are severely underdeveloped and colony-forming potential is profoundly impaired. AGM mesenchymal cells expressing Runx1, an essential factor for definitive HSC specification, are almost absent in mutant mice. In addition, hematopoietic clusters in the dorsal aorta, vitelline, and umbilical arteries are reduced in size and number. These results show a requirement for Meis1 in the establishment of definitive hematopoiesis in the mouse embryo. Meis1 mutant mice also displayed complete agenesis of the megakaryocyte lineage and localized defects in vascular patterning, which may cause the hemorrhagic phenotype.

Effect of transcription-factor concentrations on leukemic stem cells

Increasing evidence suggests that leukemias are sustained by leukemic stem cells. However, the molecular pathways underlying the transformation of normal cells into leukemic stem cells are still poorly understood. The involvement of a small group of key transcription factors into this process was suggested by their frequent mutation or down-regulation in patients with acute myeloid leukemia (AML). Recent findings in mice with hypomorphic transcription-factor genes demonstrated that leukemic stem-cell formation in AML could directly be caused by reduced transcription-factor activity beyond a critical threshold. Most interestingly, those experimental models and the paucity of biallelic null mutations or deletions in transcription-factor genes in patients suggest that AML is generally associated with graded down-regulation rather than complete disruption of transcription factors. Here, we discuss the effects of transcription-factor concentrations on hematopoiesis and leukemia, with a focus on the regulation of transcription-factor gene expression as a major mechanism that alters critical threshold levels during blood development and cancer.

Tracking mesoderm formation and specification to the hemangioblast in vitro

During development of the mouse embryo, blood cells are generated from mesodermal precursors at specific times and locations. Using various in vivo and in vitro systems, we are now starting to understand the cascade of molecular events leading to the commitment of mesoderm and the formation of the first blood precursors, the hemangioblast. The in vitro differentiation of embryonic stem (ES) cells has proved to be an invaluable model to study lineage commitment, because one can access and easily manipulate large quantities of early progenitor cells. To help us track mesodermal subpopulations and study their specification toward blood lineages, we have engineered an ES cell line that expresses the green fluorescence protein exclusively in the mesoderm germ layer, under the control of the Brachyury regulatory sequences.

Scl is required for dorsal aorta as well as blood formation in zebrafish embryos

Blood and endothelial cells arise in close association in developing embryos, possibly from a shared precursor, the hemangioblast, or as hemogenic endothelium. The transcription factor, Scl/Tal1 (stem cell leukemia protein), is essential for hematopoiesis but thought to be required only for remodeling of endothelium in mouse embryos. By contrast, it has been implicated in hemangioblast formation in embryoid bodies. To resolve the role of scl in endothelial development, we knocked down its synthesis in zebrafish embryos where early precursors and later phenotypes can be more easily monitored. With respect to blood, the zebrafish morphants phenocopied the mouse knockout and positioned scl in the genetic hierarchy. Importantly, endothelial development was also clearly disrupted. Dorsal aorta formation was substantially compromised and gene expression in the posterior cardinal vein was abnormal. We conclude that scl is especially critical for the development of arteries where adult hematopoietic stem cells emerge, implicating scl in the formation of hemogenic endothelium.

Loss of Runx1 perturbs adult hematopoiesis and is associated with a myeloproliferative phenotype

Homozygous loss of function of Runx1 (Runt-related transcription factor 1 gene) during murine development results in an embryonic lethal phenotype characterized by a complete lack of definitive hematopoiesis. In light of recent reports of disparate requirements for hematopoietic transcription factors during development as opposed to adult hematopoiesis, we used a conditional gene-targeting strategy to effect the loss of Runx1 function in adult mice. In contrast with the critical role of Runx1 during development, Runx1 was not essential for hematopoiesis in the adult hematopoietic compartment, though a number of significant hematopoietic abnormalities were observed. Runx1 excision had lineage-specific effects on B- and T-cell maturation and pronounced inhibition of common lymphocyte progenitor production. Runx1 excision also resulted in inefficient platelet production. Of note, Runx1-deficient mice developed a mild myeloproliferative phenotype characterized by an increase in peripheral blood neutrophils, an increase in myeloid progenitor populations, and extramedullary hematopoiesis composed of maturing myeloid and erythroid elements. These findings indicate that Runx1 deficiency has markedly different consequences during development compared with adult hematopoiesis, and they provide insight into the phenotypic manifestations of Runx1 deficiency in hematopoietic malignancies.

Differential regulation of granulopoiesis by the basic helix-loop-helix transcriptional inhibitors Id1 and Id2

Inhibitor of DNA binding (Id) proteins function as inhibitors of members of the basic helix-loop-helix family of transcription factors and have been demonstrated to play an important role in regulating lymphopoiesis. However, the role of these proteins in regulation of myelopoiesis is currently unclear. In this study, we have investigated the role of Id1 and Id2 in the regulation of granulopoiesis. Id1 expression was initially up-regulated during early granulopoiesis, which was then followed by a decrease in expression during final maturation. In contrast, Id2 expression was up-regulated in terminally differentiated granulocytes. In order to determine whether Id expression plays a critical role in regulating granulopoiesis, Id1 and Id2 were ectopically expressed in CD34(+) cells by retroviral transduction. Our experiments demonstrate that constitutive expression of Id1 inhibits eosinophil development, whereas in contrast neutrophil differentiation was modestly enhanced. Constitutive Id2 expression accelerates final maturation of both eosinophils and neutrophils, whereas inhibition of Id2 expression blocks differentiation of both lineages. Transplantation of beta2-microglobulin(-/-) nonobese diabetic severe combined immunodeficient (NOD/SCID) mice with CD34(+) cells ectopically expressing Id1 resulted in enhanced neutrophil development, whereas ectopic expression of Id2 induced both eosinophil and neutrophil development. These data demonstrate that both Id1 and Id2 play a critical, although differential role in granulopoiesis.