Interaction between FOG-1 and the corepressor C-terminal binding protein is dispensable for normal erythropoiesis in vivo

A novel role for Friend of GATA1 (FOG-1) in regulating cholesterol transport in murine erythropoiesis

Friend of GATA1 (FOG-1) is an essential transcriptional co-factor of the master erythroid transcription factor GATA1. The knockout of the Zfpm1 gene, coding for FOG-1, results in early embryonic lethality due to anemia in mice, similar to the embryonic lethal phenotype of the Gata1 gene knockout. However, a detailed molecular analysis of the Zfpm1 knockout phenotype in erythropoiesis is presently incomplete. To this end, we used CRISPR/Cas9 to knockout Zfpm1 in mouse erythroleukemic (MEL) cells. Phenotypic characterization of DMSO-induced terminal erythroid differentiation showed that the Zfpm1 knockout MEL cells did not progress past the proerythroblast stage of differentiation. Expression profiling of the Zfpm1 knockout MEL cells by RNAseq showed a lack of up-regulation of erythroid-related gene expression profiles. Bioinformatic analysis highlighted cholesterol transport as a pathway affected in the Zfpm1 knockout cells. Moreover, we show that the cholesterol transporters Abca1 and Ldlr fail to be repressed during erythroid differentiation in Zfpm1 knockout cells, resulting in higher intracellular lipid levels and higher membrane fluidity. We also show that in FOG-1 knockout cells, the nuclear levels of SREBP2, a key transcriptional regulator of cholesterol biosynthesis and transport, are markedly increased. On the basis of these findings we propose that FOG-1 (and, potentially, GATA1) regulate cholesterol homeostasis during erythroid differentiation directly through the down regulation of cholesterol transport genes and indirectly, through the repression of the SREBP2 transcriptional activator of cholesterol homeostasis. Taken together, our work provides a molecular basis for understanding FOG-1 functions in erythropoiesis and reveals a novel role for FOG-1 in cholesterol transport.

GATA1 in Normal and Pathologic Megakaryopoiesis and Platelet Development

GATA1 is a highly conserved hematopoietic transcription factor (TF), essential for normal erythropoiesis and megakaryopoiesis, that encodes a full-length, predominant isoform and an amino (N) terminus-truncated isoform GATA1s. It is consistently expressed throughout megakaryocyte development and interacts with its target genes either independently or in association with binding partners such as FOG1 (friend of GATA1). While the N-terminus and zinc finger have classically been demonstrated to be necessary for the normal regulation of platelet-specific genes, murine models, cell-line studies, and human case reports indicate that the carboxy-terminal activation domain and zinc finger also play key roles in precisely controlling megakaryocyte growth, proliferation, and maturation. Murine models have shown that disruptions to GATA1 increase the proliferation of immature megakaryocytes with abnormal architecture and impaired terminal differentiation into platelets. In humans, germline GATA1 mutations result in variable cytopenias, including macrothrombocytopenia with abnormal platelet aggregation and excessive bleeding tendencies, while acquired GATA1s mutations in individuals with trisomy 21 (T21) result in transient abnormal myelopoiesis (TAM) and myeloid leukemia of Down syndrome (ML-DS) arising from a megakaryocyte-erythroid progenitor (MEP). Taken together, GATA1 plays a key role in regulating megakaryocyte differentiation, maturation, and proliferative capacity. As sequencing and proteomic technologies expand, additional GATA1 mutations and regulatory mechanisms contributing to human diseases of megakaryocytes and platelets are likely to be revealed.

Systematic identification and characterization of repressive domains in Drosophila transcription factors

All multicellular life relies on differential gene expression, determined by regulatory DNA elements and DNA-binding transcription factors that mediate activation and repression via cofactor recruitment. While activators have been extensively characterized, repressors are less well studied: the identities and properties of their repressive domains (RDs) are typically unknown and the specific co-repressors (CoRs) they recruit have not been determined. Here, we develop a high-throughput, next-generation sequencing-based screening method, repressive-domain (RD)-seq, to systematically identify RDs in complex DNA-fragment libraries. Screening more than 200,000 fragments covering the coding sequences of all transcription-related proteins in Drosophila melanogaster, we identify 195 RDs in known repressors and in proteins not previously associated with repression. Many RDs contain recurrent short peptide motifs, which are conserved between fly and human and are required for RD function, as demonstrated by motif mutagenesis. Moreover, we show that RDs that contain one of five distinct repressive motifs interact with and depend on different CoRs, such as Groucho, CtBP, Sin3A, or Smrter. These findings advance our understanding of repressors, their sequences, and the functional impact of sequence-altering mutations and should provide a valuable resource for further studies.

Zfp281 (ZBP-99) plays a functionally redundant role with Zfp148 (ZBP-89) during erythroid development

Erythroid maturation requires the concerted action of a core set of transcription factors. We previously identified the Krüppel-type zinc finger transcription factor Zfp148 (also called ZBP-89) as an interacting partner of the master erythroid transcription factor GATA1. Here we report the conditional knockout of Zfp148 in mice. Global loss of Zfp148 results in perinatal lethality from nonhematologic causes. Selective Zfp148 loss within the hematopoietic system results in a mild microcytic and hypochromic anemia, mildly impaired erythroid maturation, and delayed recovery from phenylhydrazine-induced hemolysis. Based on the mild erythroid phenotype of these mice compared with GATA1-deficient mice, we hypothesized that additional factor(s) may complement Zfp148 function during erythropoiesis. We show that Zfp281 (also called ZBP-99), another member of the Zfp148 transcription factor family, is highly expressed in murine and human erythroid cells. Zfp281 knockdown by itself results in partial erythroid defects. However, combined deficiency of Zfp148 and Zfp281 causes a marked erythroid maturation block. Zfp281 physically associates with GATA1, occupies many common chromatin sites with GATA1 and Zfp148, and regulates a common set of genes required for erythroid cell differentiation. These findings uncover a previously unknown role for Zfp281 in erythroid development and suggest that it functionally overlaps with that of Zfp148 during erythropoiesis.

The GATA factor revolution in hematology

The discovery of the GATA binding protein (GATA factor) transcription factor family revolutionized hematology. Studies of GATA proteins have yielded vital contributions to our understanding of how hematopoietic stem and progenitor cells develop from precursors, how progenitors generate red blood cells, how hemoglobin synthesis is regulated, and the molecular underpinnings of nonmalignant and malignant hematologic disorders. This thrilling journey began with mechanistic studies on a β-globin enhancer- and promoter-binding factor, GATA-1, the founding member of the GATA family. This work ushered in the cloning of related proteins, GATA-2-6, with distinct and/or overlapping expression patterns. Herein, we discuss how the hematopoietic GATA factors (GATA-1-3) function via a battery of mechanistic permutations, which can be GATA factor subtype, cell type, and locus specific. Understanding this intriguing protein family requires consideration of how the mechanistic permutations are amalgamated into circuits to orchestrate processes of interest to the hematologist and more broadly.

Exploring the developmental mechanisms underlying Wolf-Hirschhorn Syndrome: Evidence for defects in neural crest cell migration

Wolf-Hirschhorn Syndrome (WHS) is a neurodevelopmental disorder characterized by mental retardation, craniofacial malformation, and defects in skeletal and heart development. The syndrome is associated with irregularities on the short arm of chromosome 4, including deletions of varying sizes and microduplications. Many of these genotypic aberrations in humans have been correlated with the classic WHS phenotype, and animal models have provided a context for mapping these genetic irregularities to specific phenotypes; however, there remains a significant knowledge gap concerning the cell biological mechanisms underlying these phenotypes. This review summarizes literature that has made recent contributions to this topic, drawing from the vast body of knowledge detailing the genetic particularities of the disorder and the more limited pool of information on its cell biology. Finally, we propose a novel characterization for WHS as a pathophysiology owing in part to defects in neural crest cell motility and migration during development.

Navigating Transcriptional Coregulator Ensembles to Establish Genetic Networks: A GATA Factor Perspective

Complex developmental programs require orchestration of intrinsic and extrinsic signals to control cell proliferation, differentiation, and survival. Master regulatory transcription factors are vital components of the machinery that transduce these stimuli into cellular responses. This is exemplified by the GATA family of transcription factors that establish cell type-specific genetic networks and control the development and homeostasis of systems including blood, vascular, adipose, and cardiac. Dysregulated GATA factor activity/expression underlies anemia, immunodeficiency, myelodysplastic syndrome, and leukemia. Parameters governing the capacity of a GATA factor expressed in multiple cell types to generate cell type-specific transcriptomes include selective coregulator usage and target gene-specific chromatin states. As knowledge of GATA-1 mechanisms in erythroid cells constitutes a solid foundation, we will focus predominantly on GATA-1, while highlighting principles that can be extrapolated to other master regulators. GATA-1 interacts with ubiquitous and lineage-restricted transcription factors, chromatin modifying/remodeling enzymes, and other coregulators to activate or repress transcription and to maintain preexisting transcriptional states. Major unresolved issues include: how does a GATA factor selectively utilize diverse coregulators; do distinct epigenetic landscapes and nuclear microenvironments of target genes dictate coregulator requirements; and do gene cohorts controlled by a common coregulator ensemble function in common pathways. This review will consider these issues in the context of GATA factor-regulated hematopoiesis and from a broader perspective.

FOG-2 mediated recruitment of the NuRD complex regulates cardiomyocyte proliferation during heart development

FOG-2 is a multi-zinc finger protein that binds the transcriptional activator GATA4 and modulates GATA4-mediated regulation of target genes during heart development. Our previous work has demonstrated that the Nucleosome Remodeling and Deacetylase (NuRD) complex physically interacts with FOG-2 and is necessary for FOG-2 mediated repression of GATA4 activity in vitro. However, the relevance of this interaction for FOG-2 function in vivo has remained unclear. In this report, we demonstrate the importance of FOG-2/NuRD interaction through the generation and characterization of mice homozygous for a mutation in FOG-2 that disrupts NuRD binding (FOG-2(R3K5A)). These mice exhibit a perinatal lethality and have multiple cardiac malformations, including ventricular and atrial septal defects and a thin ventricular myocardium. To investigate the etiology of the thin myocardium, we measured the rate of cardiomyocyte proliferation in wild-type and FOG-2(R3K5A) developing hearts. We found cardiomyocyte proliferation was reduced by 31±8% in FOG-2(R3K5A) mice. Gene expression analysis indicated that the cell cycle inhibitor Cdkn1a (p21(cip1)) is up-regulated 2.0±0.2-fold in FOG-2(R3K5A) hearts. In addition, we demonstrate that FOG-2 can directly repress the activity of the Cdkn1a gene promoter, suggesting a model by which FOG-2/NuRD promotes ventricular wall thickening by repression of this cell cycle inhibitor. Consistent with this notion, the genetic ablation of Cdkn1a in FOG-2(R3K5A) mice leads to an improvement in left ventricular function and a partial rescue of left ventricular wall thickness. Taken together, our results define a novel mechanism in which FOG-2/NuRD interaction is required for cardiomyocyte proliferation by directly down-regulating the cell cycle inhibitor Cdkn1a during heart development.

Hematopoietic overexpression of FOG1 does not affect B-cells but reduces the number of circulating eosinophils

We have identified expression of the gene encoding the transcriptional coactivator FOG-1 (Friend of GATA-1; Zfpm1, Zinc finger protein multitype 1) in B lymphocytes. We found that FOG-1 expression is directly or indirectly dependent on the B cell-specific coactivator OBF-1 and that it is modulated during B cell development: expression is observed in early but not in late stages of B cell development. To directly test in vivo the role of FOG-1 in B lymphocytes, we developed a novel embryonic stem cell recombination system. For this, we combined homologous recombination with the FLP recombinase activity to rapidly generate embryonic stem cell lines carrying a Cre-inducible transgene at the Rosa26 locus. Using this system, we successfully generated transgenic mice where FOG-1 is conditionally overexpressed in mature B-cells or in the entire hematopoietic system. While overexpression of FOG-1 in B cells did not significantly affect B cell development or function, we found that enforced expression of FOG-1 throughout all hematopoietic lineages led to a reduction in the number of circulating eosinophils, confirming and extending to mammals the known function of FOG-1 in this lineage.

Friend of GATA and GATA-6 modulate the transcriptional up-regulation of hepcidin in hepatocytes during inflammation

Hepcidin is an antimicrobial peptide hormone that plays a central role in the metabolism of iron and its expression in the liver can be induced through two major pathways: the inflammatory pathway, mainly via IL-6; and the iron-sensing pathway, mediated by BMP-6. GATA-proteins are group of evolutionary conserved transcriptional regulators that bind to the consensus motif-WGATAR-in the promoter region. In hepatoma cells, GATA-proteins 4 and 6 in conjunction with the co-factor friend of GATA (FOG) were shown to modulate the transcription of HAMP. However, it is unclear as to which of the GATA-proteins drive the expression of HAMP in vivo. In this study, using in vitro and in vivo approaches, we investigated the relevance of GATA and FOG proteins in the expression of hepcidin following treatment with IL-6 and BMP-6. We found that treatment of Huh7 cells with either IL-6 or BMP-6 increased the HAMP promoter activity. The HAMP promoter activity following treatment with IL-6 or BMP-6 was further increased by co-transfection of the promoter with GATA proteins 4 and 6. However, co-transfection of the HAMP promoter with FOG proteins 1 or 2 repressed the promoter response to treatments with either IL-6 or BMP-6. The effects of both GATA and FOG proteins on the promoter activity in response to IL-6 or BMP-6 treatment were abrogated by mutation of the GATA response element-TTATCT-in the HAMP promoter region -103/-98. In vivo, treatment of mice with lipopolysaccharide led to a transient increase of Gata-6 expression in the liver that was positively correlated with the expression of hepcidin. Our results indicate that during inflammation GATA-6 is up-regulated in concert with hepcidin while GATA-4 and FOG (1 and 2) are repressed.

Transcriptional mechanisms underlying hemoglobin synthesis

The physiological switch in expression of the embryonic, fetal, and adult β-like globin genes has garnered enormous attention from investigators interested in transcriptional mechanisms and the molecular basis of hemoglobinopathies. These efforts have led to the discovery of cell type-specific transcription factors, unprecedented mechanisms of transcriptional coregulator function, genome biology principles, unique contributions of nuclear organization to transcription and cell function, and promising therapeutic targets. Given the vast literature accrued on this topic, this article will focus on the master regulator of erythroid cell development and function GATA-1, its associated proteins, and its frontline role in controlling hemoglobin synthesis. GATA-1 is a crucial regulator of genes encoding hemoglobin subunits and heme biosynthetic enzymes. GATA-1-dependent mechanisms constitute an essential regulatory core that nucleates additional mechanisms to achieve the physiological control of hemoglobin synthesis.

Combinatorial regulation of tissue specification by GATA and FOG factors

The development of complex organisms requires the formation of diverse cell types from common stem and progenitor cells. GATA family transcriptional regulators and their dedicated co-factors, termed Friend of GATA (FOG) proteins, control cell fate and differentiation in multiple tissue types from Drosophila to man. FOGs can both facilitate and antagonize GATA factor transcriptional regulation depending on the factor, cell, and even the specific gene target. In this review, we highlight recent studies that have elucidated mechanisms by which FOGs regulate GATA factor function and discuss how these factors use these diverse modes of gene regulation to control cell lineage specification throughout metazoans.

A microdeletion proximal of the critical deletion region is associated with mild Wolf-Hirschhorn syndrome

It is generally accepted that the facial phenotype of Wolf-Hirschhorn syndrome is caused by deletions of either Wolf-Hirschhorn critical regions 1 or 2 (WHSCR 1-2). Here, we identify a 432 kb deletion located 600 kb proximal to both WHSCR1-2 in a patient with a WHS facial phenotype. Seven genes are underlying this deletion region including FAM193a, ADD1, NOP14, GRK4, MFSD10, SH3BP2, TNIP2. The clinical diagnosis of WHS facial phenotype was confirmed by 3D facial analysis using dense surface modeling. Our results suggest that the WHSCR1-2 flanking sequence contributes directly or indirectly to the severity of WHS. Sequencing the Wolf-Hirschhorn syndrome candidate 1 and 2 genes did not reveal any mutations. Long range position effects of the deletion that could influence gene expression within the WHSCR were excluded in EBV cell lines derived from patient lymphoblasts. We hypothesize that either (1) this locus harbors regulatory sequences which affect gene expression in the WHSCR1-2 in a defined temporal and spatial developmental window or (2) that this locus is additive to deletions of WHSCR1-2 increasing the phenotypic expression.

Friend of GATA (FOG) interacts with the nucleosome remodeling and deacetylase complex (NuRD) to support primitive erythropoiesis in Xenopus laevis

Friend of GATA (FOG) plays many diverse roles in adult and embryonic hematopoiesis, however the mechanisms by which it functions and the roles of potential interaction partners are not completely understood. Previous work has shown that overexpression of FOG in Xenopus laevis causes loss of blood suggesting that in contrast to its role in mammals, FOG might normally function to repress erythropoiesis in this species. Using loss-of-function analysis, we demonstrate that FOG is essential to support primitive red blood cell (RBC) development in Xenopus. Moreover, we show that it is specifically required to prevent excess apoptosis of circulating primitive RBCs and that in the absence of FOG, the pro-apoptotic gene Bim-1 is strongly upregulated. To identify domains of FOG that are essential for blood development and, conversely, to begin to understand the mechanism by which overexpressed FOG represses primitive erythropoiesis, we asked whether FOG mutants that are unable to interact with known co-factors retain their ability to rescue blood formation in FOG morphants and whether they repress erythropoiesis when overexpressed in wild type embryos. We find that interaction of FOG with the Nucleosome Remodeling and Deacetylase complex (NuRD), but not with C-terminal Binding Protein, is essential for normal primitive RBC development. In contrast, overexpression of all mutant and wild type constructs causes a comparable repression of primitive erythropoiesis. Together, our data suggest that a requirement for FOG and its interaction with NuRD during primitive erythropoiesis are conserved in Xenopus and that loss of blood upon FOG overexpression is due to a dominant-interfering effect.

Friend of GATA suppresses the GATA-induced transcription of hepcidin in hepatocytes through a GATA-regulatory element in the HAMP promoter

Hepcidin is an antimicrobial peptide hormone involved in the metabolism of iron, encoded for by the HAMP gene mainly in hepatocytes. It's expressed at lower levels in other cells such as the macrophages. The mechanisms that determine tissue-specific expression of hepcidin remain unclear. GATA- and its co-factor Friend of GATA (FOG) modulate the tissue-specific transcription of other genes involved in the metabolism of iron. GATA proteins are group of evolutionary conserved transcriptional regulators that bind to the consensus motif -WGATAR- in the promoter. We characterized a 1.3 kb fragment of the 5'-flanking sequence of the HAMP gene in Huh7 cells, which express HAMP. Transfection of 5'-deletions of the HAMP promoter in Huh7 cells revealed two regions, -932/-878 and -155/-96, that when deleted decreased promoter activity. Using site-directed mutations in the HAMP promoter region -155/-96 we identified two subregions, -138/-125 and -103/-98, which when mutated suppressed promoter activity by 70 and 90% respectively. Site -103/-98 with a sequence -TTATCT- to which endogenous GATA proteins 4 and 6 bind and transactivate HAMP is a GATA-regulatory element (RE). Mutation of the GATA-RE abrogated binding of GATA proteins 4 and 6 to the promoter and blunted the GATA transactivation of HAMP. FOG proteins 1 and 2 suppressed the endogenous and exogenous GATA activation of the HAMP promoter. We concluded that the GATA-RE, -TTATCT- in the HAMP promoter region -103/-98 is crucial for the GATA-4 and GATA-6 driven transcription of hepcidin in Huh7 cells and that FOG proteins moderate the transcription by suppressing the GATA transactivation of HAMP.

The role of the GATA2 transcription factor in normal and malignant hematopoiesis

Hematopoiesis involves an elaborate regulatory network of transcription factors that coordinates the expression of multiple downstream genes, and maintains homeostasis within the hematopoietic system through the accurate orchestration of cellular proliferation, differentiation and survival. As a result, defects in the expression levels or the activity of these transcription factors are intimately linked to hematopoietic disorders, including leukemia. The GATA family of nuclear regulatory proteins serves as a prototype for the action of lineage-restricted transcription factors. GATA1 and GATA2 are expressed principally in hematopoietic lineages, and have essential roles in the development of multiple hematopoietic cells, including erythrocytes and megakaryocytes. Moreover, GATA2 is crucial for the proliferation and maintenance of hematopoietic stem cells and multipotential progenitors. In this review, we summarize the current knowledge regarding the biological properties and functions of the GATA2 transcription factor in normal and malignant hematopoiesis.

GATA switches as developmental drivers

Transcriptional networks orchestrate complex developmental processes. Such networks are commonly instigated by master regulators of development. Considerable progress has been made in elucidating GATA factor-dependent genetic networks that control blood cell development. GATA-2 is required for the genesis and/or function of hematopoietic stem cells, whereas GATA-1 drives the differentiation of hematopoietic progenitors into a subset of the blood cell lineages. GATA-1 directly represses Gata2 transcription, and this involves GATA-1-mediated displacement of GATA-2 from chromatin, a process termed a GATA switch. GATA switches occur at numerous loci with critical functions, indicating that they are widely utilized developmental control tools.

GATA proteins work together with friend of GATA (FOG) and C-terminal binding protein (CTBP) co-regulators to control adipogenesis

GATA transcription factors have been implicated in controlling adipogenesis in Drosophila and in mammals. In mammals, both GATA2 and GATA3 have been shown to be present in preadipocytes, and their silencing allows the onset of adipogenesis. Overexpression of GATA proteins blocks adipogenesis in cellular assays. GATA factors have been found to operate through recruiting cofactors of the Friend of GATA (FOG) family. FOG proteins, in turn, recruit co-regulators, including C-terminal binding proteins (CTBPs). We have investigated whether FOGs and CTBPs influence adipogenesis. We found that both FOG1 and FOG2 are expressed in cells prior to adipogenesis but are down-regulated as adipogenesis proceeds. Overexpression of FOG1 or FOG2 interferes with adipogenesis. Mutant versions of FOG2 unable to bind CTBP or GATA proteins are impaired in their inability to inhibit adipogenesis. Finally, a mutant version of GATA2, unable to associate with FOGs, also displays abnormal activity and causes enhanced cell proliferation. These results implicate FOGs and CTBPs as partners of GATA proteins in the control of adipocyte proliferation and differentiation.

Role of the GATA-1/FOG-1/NuRD pathway in the expression of human beta-like globin genes

The human beta-globin genes are expressed in a developmentally controlled fashion. Studies on the molecular mechanisms underlying the stage-specific regulation of globin genes have been fueled by the clinical benefit of elevated fetal gamma-globin expression in patients with sickle cell anemia and thalassemia. Recent reports suggested a role of the hematopoietic transcription factor GATA-1, its cofactor FOG-1, and the associated chromatin remodeling complex NuRD in the developmental silencing of HBG1 and HBG2 gene expression. To examine whether FOG-1 via NuRD controls HBG1 and HBG2 silencing in vivo, we created mice in which the FOG-1/NuRD complex is disrupted (A. Miccio et al., EMBO J. 29:442-456, 2010) and crossed these with animals carrying the entire human beta-globin gene locus as a transgene. We found that the FOG-1/NuRD interaction is dispensable for the silencing of human HBG1 and HBG2 expression. In addition, mutant animals displayed normal silencing of the endogenous embryonic globin genes. In contrast, a significant reduction of adult-type human and murine globin gene expression was found in adult bone marrows of mutant animals. These results suggest that, unexpectedly, NuRD is required for FOG-1-dependent activation of adult-type globin gene expression but is dispensable for human gamma-globin silencing in vivo.

Controlling hematopoiesis through sumoylation-dependent regulation of a GATA factor

GATA factors establish transcriptional networks that control fundamental developmental processes. Whereas the regulator of hematopoiesis GATA-1 is subject to multiple posttranslational modifications, how these modifications influence GATA-1 function at endogenous loci is unknown. We demonstrate that sumoylation of GATA-1 K137 promotes transcriptional activation only at target genes requiring the coregulator Friend of GATA-1 (FOG-1). A mutation of GATA-1 V205G that disrupts FOG-1 binding and K137 mutations yielded similar phenotypes, although sumoylation was FOG-1 independent, and FOG-1 binding did not require sumoylation. Both mutations dysregulated GATA-1 chromatin occupancy at select sites, FOG-1-dependent gene expression, and were rescued by tethering SUMO-1. While FOG-1- and SUMO-1-dependent genes migrated away from the nuclear periphery upon erythroid maturation, FOG-1- and SUMO-1-independent genes persisted at the periphery. These results illustrate a mechanism that controls trans-acting factor function in a locus-specific manner, and differentially regulated members of the target gene ensemble reside in distinct subnuclear compartments.

FOG-1-mediated recruitment of NuRD is required for cell lineage re-enforcement during haematopoiesis

The transcriptional co-factor Friend of GATA1 (FOG-1) has been shown to interact with subunits of the nucleosome remodelling and histone deacetylase (NuRD) complex through a specific motif located at its N-terminus. To test the importance of FOG-1/NuRD interaction for haematopoiesis in vivo, we generated mice with a mutation that specifically disrupts FOG-1/NuRD interaction (FOG-1(R3K5A)). Homozygous FOG-1(R3K5A) mice were found to have splenomegaly, extramedullary erythropoiesis, granulocytosis and thrombocytopaenia secondary to a block in megakaryocyte maturation. FOG-1(R3K5A/R3K5A) megakaryocytes and erythroid progenitors expressed increased levels of GATA2, showing that FOG-1/NuRD interaction is required for the earlier described 'GATA Switch'. In addition, ablation of FOG-1/NuRD interaction led to inappropriate expression of mast cell and eosinophil-specific genes in the megakaryocyte and erythroid lineages. Chromatin immunoprecipitation experiments revealed that the NuRD complex was not properly recruited to a mast cell gene promoter in FOG-1(R3K5A/R3K5A) megakaryocytes, suggesting that FOG-1/NuRD interaction is required for the direct suppression of mast cell gene expression. Taken together, these results underscore the importance of the FOG-1/NuRD interaction for the re-enforcement of lineage commitment during erythropoiesis and megakaryopoiesis in vivo.

NuRD mediates activating and repressive functions of GATA-1 and FOG-1 during blood development

GATA transcription factors interact with FOG proteins to regulate tissue development by activating and repressing transcription. FOG-1 (ZFPM1), a co-factor for the haematopoietic factor GATA-1, binds to the NuRD co-repressor complex through a conserved N-terminal motif. Surprisingly, we detected NuRD components at both repressed and active GATA-1/FOG-1 target genes in vivo. In addition, while NuRD is required for transcriptional repression in certain contexts, we show a direct requirement of NuRD also for FOG-1-dependent transcriptional activation. Mice in which the FOG-1/NuRD interaction is disrupted display defects similar to germline mutations in the Gata1 and Fog1 genes, including anaemia and macrothrombocytopaenia. Gene expression analysis in primary mutant erythroid cells and megakaryocytes (MKs) revealed an essential function for NuRD during both the repression and activation of select GATA-1/FOG-1 target genes. These results show that NuRD is a critical co-factor for FOG-1 and underscore the versatile use of NuRD by lineage-specific transcription factors to activate and repress gene transcription in the appropriate cellular and genetic context.

Erythropoiesis: model systems, molecular regulators, and developmental programs

Human erythropoiesis is a complex multistep developmental process that begins at the level of pluripotent hematopoietic stem cells (HSCs) at bone marrow microenvironment (HSCs niche) and terminates with the production of erythrocytes (RBCs). This review covers the basic and contemporary aspects of erythropoiesis. These include the: (a) cell-lineage restricted pathways of differentiation originated from HSCs and going downward toward the blood cell development; (b) model systems employed to study erythropoiesis in culture (erythroleukemia cell lines and embryonic stem cells) and in vivo (knockout animals: avian, mice, zebrafish, and xenopus); (c) key regulators of erythropoiesis (iron, hypoxia, stress, and growth factors); (d) signaling pathways operating at hematopoietic stem cell niche for homeostatic regulation of self renewal (SCF/c-kit receptor, Wnt, Notch, and Hox) and for erythroid differentiation (HIF and EpoR). Furthermore, this review presents the mechanisms through which transcriptional factors (GATA-1, FOG-1, TAL-1/SCL/MO2/Ldb1/E2A, EKLF, Gfi-1b, and BCL11A) and miRNAs regulate gene pattern expression during erythroid differentiation. New insights regarding the transcriptional regulation of alpha- and beta-globin gene clusters were also presented. Emphasis was also given on (i) the developmental program of erythropoiesis, which consists of commitment to terminal erythroid maturation and hemoglobin production, (two closely coordinated events of erythropoieis) and (ii) the capacity of human embryonic and umbilical cord blood (UCB) stem cells to differentiate and produce RBCs in culture with highly selective media. These most recent developments will eventually permit customized red blood cell production needed for transfusion.

Translational isoforms of FOG1 regulate GATA1-interacting complexes

Erythropoietic and megakaryocytic programs are directed by the transcription factor GATA1. Friend of GATA1 (FOG1), a protein interaction partner of GATA1, is critical for GATA1 function in multiple contexts. Previous work has shown that FOG1 recruits two multi-protein complexes, the nucleosome remodeling domain (NuRD) complex and a C-terminal binding protein (CTBP)-containing complex, into association with GATA1 to mediate activation and repression of target genes. To elucidate mechanisms that might differentially regulate the association of FOG1, as well as GATA1, with these two complexes, we characterized a previously unrecognized translational isoform of FOG1. We found that an N-terminally truncated version of FOG1 is produced from an internal ATG and that this isoform, designated FOG1S, lacks the nucleosome remodeling domain-binding domain, altering the complexes with which it interacts. Both isoforms interact with the C-terminal binding protein complex, which we show also contains lysine-specific demethylase 1 (LSD1). FOG1S is preferentially excluded from the nucleus by unknown mechanisms. These data reveal two novel mechanisms for the regulation of GATA1 interaction with FOG1-dependent protein complexes through the production of two translational isoforms with differential interaction profiles and independent nuclear localization controls.

Mouse models of Wolf-Hirschhorn syndrome

Subtelomeric deletion syndromes represent a significant cause of mental retardation and craniofacial disease. However, for most of these syndromes the pathogenic genes have yet to be identified. Currently there is every indication that identification of these genes will be a slow process if we continue to rely strictly upon clinical data. An alternative approach is the use of mouse models to complement the patient studies. Wolf-Hirschhorn syndrome (WHS), caused by deletions in 4p16.3, is the first recognized subtelomeric deletion syndrome. As with other syndromes of this class, WHS has not yet been subjected to an intensive, systematic analysis using mouse models. Nonetheless, a significant number of targeted mutations have been introduced into mouse genomic region, 5B1, which is orthologous to 4p16.3. Included among these mutations are a series of deletions approximating the deletions in some patients. The mouse lines carrying these deletions display a remarkable concordance of phenotypes with the human patient's characteristics, strongly indicating that the mouse models can be used to phenocopy WHS. In this review, we will catalog the currently existing targeted mutations in mice in the regions orthologous to the WHS critical regions. For each mutation we will discuss the resulting phenotype and its potential relevance to the pathogenesis of the syndrome. Further, we will describe how the phenotypes of some of the mutations suggest new directions for the clinical studies. Finally we will outline approaches for the efficient creation of new mouse models of WHS going forward.

CtBP is required for proper development of peripheral nervous system in Drosophila

C-terminal binding protein (CtBP) is an evolutionarily and functionally conserved transcriptional corepressor known to integrate diverse signals to regulate transcription. Drosophila CtBP (dCtBP) regulates tissue specification and segmentation during early embryogenesis. Here, we investigated the roles of dCtBP during development of the peripheral nervous system (PNS). Our study includes a detailed quantitative analysis of how altered dCtBP activity affects the formation of adult mechanosensory bristles. We found that dCtBP loss-of-function resulted in a series of phenotypes with the most prevalent being supernumerary bristles. These dCtBP phenotypes are more complex than those caused by Hairless, a known dCtBP-interacting factor that regulates bristle formation. The emergence of additional bristles correlated with the appearance of extra sensory organ precursor (SOP) cells in earlier stages, suggesting that dCtBP may directly or indirectly inhibit SOP cell fates. We also found that development of a subset of bristles was regulated by dCtBP associated with U-shaped through the PxDLS dCtBP-interacting motif. Furthermore, the double bristle with sockets phenotype induced by dCtBP mutations suggests the involvement of this corepressor in additional molecular pathways independent of both Hairless and U-shaped. We therefore propose that dCtBP is part of a gene circuitry that controls the patterning and differentiation of the fly PNS via multiple mechanisms.

Drosophila C-terminal binding protein, dCtBP is required for sensory organ prepattern and sharpens proneural transcriptional activity of the GATA factor Pnr

The peripheral nervous system is required for animals to detect and to relay environmental stimuli to central nervous system for the information processing. In Drosophila, the precise spatial and temporal expression of two proneural genes achaete (ac) and scute (sc), is necessary for development of the sensory organs. Here we present an evidence that the transcription co-repressor, dCtBP acts as a negative regulator of sensory organ prepattern. Loss of dCtBP function mutant exhibits ectopic sensory organs, while overexpression of dCtBP results in a dramatic loss of sensory organs. These phenotypes are correlated with mis-emerging of sensory organ precursors and perturbated expression of proneural transcription activator Ac. Mammalian CtBP-1 was identified via interaction with the consensus motif PXDLSX(K/R) of adenovirus E1A oncoprotein. We demonstrated that dCtBP binds directly to PLDLS motif of Drosophila Friend of GATA-1 protein, U-shaped and sharpens the adult sensory organ development. Moreover, we found that dCtBP mediates multivalent interaction with the GATA transcriptional activator Pannier and acts as a direct co-repressor of the Pannier-mediated activation of proneural genes. We demonstrated that Pannier genetically interacts with dCtBP-interacting protein HDAC1, suggesting that the dCtBP-dependent regulation of Pannier activity could utilize a repressive mechanism involving alteration of local chromatine structure.

Gene expression profiling related to the enhanced erythropoiesis in mouse bone marrow cells

Peroxiredoxin II knockout (Prdx II(-/-)) mice had a spontaneous phenotype of hemolytic anemia. In this study, we found that Ter-119(+)CD71(+) cells increased in Prdx II(-/-) mice bone marrow (BM) at 8 weeks of age. We examined the differential expression profiles to bone marrow cells (BMCs) between Prdx II(+/+) and Prdx II(-/-) mice using a cDNA microarray. We identified the 136 candidates were differentially expressed a greater twofold increase or decrease than EPO receptor. In this study, we focused on the up-regulated NBPs during erythropoietic differentiation. According to cDNA microarray results, six NBPs except zfp-127 were up-regulated during erythropoiesis in Prdx II(-/-) mice. Among the six candidates, eIF3-p44, hnRNPH1, G3bp, and Zfpm-1 were dramatically increased at day 7 of the in vitro erythropoietic differentiation of human CD34(+) cells. However, DJ-1 and Rbm3 were slightly increased only at day 12. Our results suggest that up-regulated NBPs might be involved during erythropoietic differentiation.

Antagonism of FOG-1 and GATA factors in fate choice for the mast cell lineage

The zinc finger transcription factor GATA-1 requires direct physical interaction with the cofactor friend of GATA-1 (FOG-1) for its essential role in erythroid and megakaryocytic development. We show that in the mast cell lineage, GATA-1 functions completely independent of FOG proteins. Moreover, we demonstrate that FOG-1 antagonizes the fate choice of multipotential progenitor cells for the mast cell lineage, and that its down-regulation is a prerequisite for mast cell development. Remarkably, ectopic expression of FOG-1 in committed mast cell progenitors redirects them into the erythroid, megakaryocytic, and granulocytic lineages. These lineage switches correlate with transcriptional down-regulation of GATA-2, an essential mast cell GATA factor, via switching of GATA-1 for GATA-2 at a key enhancer element upstream of the GATA-2 gene. These findings illustrate combinatorial control of cell fate identity by a transcription factor and its cofactor, and highlight the role of transcriptional networks in lineage determination. They also provide evidence for lineage instability during early stages of hematopoietic lineage commitment.

Transcriptional control of erythropoiesis: emerging mechanisms and principles

Transcriptional networks orchestrate fundamental biological processes, including hematopoiesis, in which hematopoietic stem cells progressively differentiate into specific progenitors cells, which in turn give rise to the diverse blood cell types. Whereas transcription factors recruit coregulators to chromatin, leading to targeted chromatin modification and recruitment of the transcriptional machinery, many questions remain unanswered regarding the underlying molecular mechanisms. Furthermore, how diverse cell type-specific transcription factors function cooperatively or antagonistically in distinct cellular contexts is poorly understood, especially since genes in higher eukaryotes commonly encompass broad chromosomal regions (100 kb and more) and are littered with dispersed regulatory sequences. In this article, we describe an important set of transcription factors and coregulators that control erythropoiesis and highlight emerging transcriptional mechanisms and principles. It is not our intent to comprehensively survey all factors implicated in the transcriptional control of erythropoiesis, but rather to underscore specific mechanisms, which have potential to be broadly relevant to transcriptional control in diverse systems.

U-shaped protein domains required for repression of cardiac gene expression in Drosophila

U-shaped is a zinc finger protein that functions predominantly as a negative transcriptional regulator of cell fate determination during Drosophila development. In the early stages of dorsal vessel formation, the protein acts to control cardioblast specification, working as a negative attenuator of the cardiogenic GATA factor Pannier. Pannier and the homeodomain protein Tinman normally work together to specify heart cells and activate cardioblast gene expression. One target of this positive regulation is a heart enhancer of the D-mef2 gene and U-shaped has been shown to antagonize enhancer activation by Pannier and Tinman. We have mapped protein domains of U-shaped required for its repression of cardioblast gene expression. Such studies showed GATA factor interacting zinc fingers of U-shaped are required for enhancer repression, as well as three small motifs that are likely needed for co-factor binding and/or protein modification. These analyses have also allowed for the definition of a 253 amino acid interval of U-shaped that is essential for its nuclear localization. Together, these findings provide molecular insights into the function of U-shaped as a negative regulator of heart development in Drosophila.

An alternative transcript of the FOG-2 gene encodes a FOG-2 isoform lacking the FOG repression motif

The FOG family of transcriptional co-factors is composed of two members in mammals: FOG-1 and FOG-2. Both have been shown to bind to GATA factors and function as transcriptional co-repressors in specific cell and promoter contexts. We have previously defined a novel repression domain localized to the N-terminus of each FOG family member, the FOG repression motif, which is necessary for FOG-mediated transcriptional repression. In this report, we describe the identification and characterization of a novel isoform of FOG-2 lacking the FOG repression motif. In contrast to full-length FOG-2, this isoform is expressed predominately in the embryonic and adult heart. It can bind GATA4 avidly, but is unable to repress GATA4-mediated activation of cardiac-restricted gene promoters. Together, these results suggest that FOG-2 repressive activity may be modulated by the generation of isoforms of FOG-2 lacking the FOG repression motif.

The suppression of zfpm-1 accelerates the erythropoietic differentiation of human CD34+ cells

Erythropoiesis is a complex multistage process for the differentiation of mature erythrocytes from hematopoietic stem cells. The function of several transcription factors has been reported in hematopoietic stem cell differentiation. However, the molecular basis governing its functional behavior is unclear. In this study, we characterized the role of Zfpm-1 during the erythropoietic differentiation of human hematopoietic stem cells. To verify the function of Zfpm-1 during erythropoietic differentiation, we established human CD34+ cell culture system by using human umbilical cord blood. At day 7 of the human CD34+ cell differentiation process to proerythocytes, Zfpm-1 was initially up-regulated and then dramatically down-regulated at day 9. The Zfpm-1 siRNA transfected HSCs contained 20% more GPA+ cells than the mock transfected cells, and showed repressed expression of the hematopoietic transcription factors, c-myc and c-myb, but increased expression of GATA-1. In contrast, the Zfpm-1 gain-of-function is the opposite of loss-of-function results above.

Role of the C-terminal binding protein PXDLS motif binding cleft in protein interactions and transcriptional repression

C-terminal binding proteins (CtBPs) are multifunctional proteins that can mediate gene repression. CtBPs contain a cleft that binds Pro-X-Asp-Leu-Ser (PXDLS) motifs. PXDLS motifs occur in numerous transcription factors and in effectors of gene repression, such as certain histone deacetylases. CtBPs have been depicted as bridging proteins that self-associate and link PXDLS-containing transcription factors to PXDLS-containing chromatin-modifying enzymes. CtBPs also recruit effectors that do not contain recognizable PXDLS motifs. We have investigated the importance of the PXDLS binding cleft to CtBP's interactions with various partner proteins and to its ability to repress transcription. We used CtBP cleft mutant and cleft-filled fusion derivatives to distinguish between partner proteins that bind in the cleft and elsewhere on the CtBP surface. Functional assays demonstrate that CtBP mutants that carry defective clefts retain repression activity when fused to heterologous DNA-binding domains. This result suggests that the cleft is not essential for recruiting effectors. In contrast, when tested in the absence of a fused DNA-binding domain, disruption of the cleft abrogates repression activity. These results demonstrate that the PXDLS binding cleft is functionally important but suggest that it is primarily required for localization of the CtBP complex to promoter-bound transcription factors.

Calsenilin interacts with transcriptional co-repressor C-terminal binding protein(s)

Calsenilin/potassium channel-interacting protein (KChIP)3/ downstream regulatory element sequence antagonist modulator (DREAM) is a neuronal calcium-binding protein that has been shown to have multiple functions in the cell, including the regulation of presenilin processing, repression of transcription and modulation of A-type potassium channels. To gain a better understanding of the precise role of calsenilin in specific cellular compartments, an interactor hunt for proteins that bind to the N-terminal domain of calsenilin was carried out. Using a yeast two-hybrid system and co-immunoprecipitation studies, we have identified the transcriptional co-repressor C-terminal binding protein (CtBP)2 as an interactor for calsenilin and have shown that the two proteins can interact in vivo. In co-immunoprecipitation studies, calsenilin also interacted with CtBP1, a CtBP2 homolog. Our data also showed a calsenilin-dependent increase in c-fos protein levels in CtBP knockout fibroblasts, suggesting that CtBP may modulate the transcriptional repression of c-fos by calsenilin. Furthermore, the finding that histone deacetylase protein and activity were associated with the calsenilin-CtBP immunocomplex suggests a mechanism by which calsenilin-CtBP may act to repress transcription. Finally, we demonstrated that calsenilin and CtBP are present in synaptic vesicles and can interact in vivo.

FOG-1 represses GATA-1-dependent FcepsilonRI beta-chain transcription: transcriptional mechanism of mast-cell-specific gene expression in mice

Cell-type-specific transcription of mouse high-affinity IgE receptor (FcepsilonRI) beta-chain is positively regulated by the transcription factor GATA-1. Although GATA-1 is expressed in erythroid cells, megakaryocytes, and mast cells, the expression of mouse FcepsilonRI beta-chain is restricted to mast cells. In the present study, we characterized the role of GATA-associated cofactor FOG-1 in the regulation of the FcepsilonRI beta-chain promoter. The expression levels of FOG-1, GATA-1, and beta-chain in each hematopoietic cell line were analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting. FOG-1 expression was higher in the beta-chain-negative hematopoietic progenitor cell line Ba/F3 than in the beta-chain-positive mast cell line PT18. By contrast, GATA-1 expression was similar when comparing the 2 cell lines. A transient reporter assay demonstrated that the beta-chain promoter functioned in PT18 but not in Ba/F3 and that the transcription activity of the beta-chain promoter in PT18 was markedly suppressed by overexpression of FOG-1. Although the activity of the beta-chain promoter, which was upregulated by coexpression of GATA-1, was significantly suppressed by coexpression of FOG-1 in the simian kidney CV-1 cells (beta-chain(-), GATA-1(-), and FOG-1(-)), the transactivation of the beta-chain promoter by the GATA-1 mutant V205G, which cannot bind FOG-1, was not affected by coexpression of FOG-1. Further, overexpression of FOG-1 in PT18 resulted in decreases in cell surface expression of FcepsilonRI and beta-chain transcription. Finally, suppression of FOG-1 expression using an siRNA approach resulted in increased beta-chain promoter activity in Ba/F3. These results suggest that FOG-1 expression level regulates the GATA-1-dependent FcepsilonRI beta-chain promoter.

Fog1 is required for cardiac looping in zebrafish

To further our understanding of FOG gene function during cardiac development, we utilized zebrafish to examine FOG's role in the early steps of heart morphogenesis. We identified fragments of three fog genes in the zebrafish genomic database and isolated full-length coding sequences for each of these genes by using a combination of RT-PCR and 5'-RACE. One gene was similar to murine FOG-1 (fog1), while the remaining two were similar to murine FOG-2 (fog2a and fog2b). All Fog proteins were able to physically interact with GATA4 and function as transcriptional co-repressors. Whole-mount in situ hybridization revealed fog1 expression in the heart, the hematopoietic system, and the brain, while fog2a and fog2b expression was restricted to the brain. Injection of zebrafish embryos with a morpholino directed against fog1 resulted in embryos with a large pericardial effusion and an unlooped heart tube. This looping defect could be rescued by co-injection of mRNA encoding murine FOG-1, but not by mRNA encoding FOG-1 lacking the FOG repression motif. Taken together, these results demonstrate the importance of FOG proteins for zebrafish cardiac development and suggest a previously unappreciated role for FOG proteins in heart looping that is dependent on the FOG repression motif.

Developmental control via GATA factor interplay at chromatin domains

Despite the extraordinary task of packaging mammalian DNA within the constraints of a cell nucleus, individual genes assemble into cell type-specific chromatin structures with high fidelity. This chromatin architecture is a crucial determinant of gene expression signatures that distinguish specific cell types. Whereas extensive progress has been made on defining biochemical and molecular mechanisms of chromatin modification and remodeling, many questions remain unanswered about how cell type-specific chromatin domains assemble and are regulated. This mini-review will discuss emerging studies on how interplay among members of the GATA family of transcription factors establishes and regulates chromatin domains. Dissecting mechanisms underlying the function of hematopoietic GATA factors has revealed fundamental insights into the control of blood cell development from hematopoietic stem cells and the etiology of pathological states in which hematopoiesis is perturbed.

Hematopoietic development from human embryonic stem cell lines

The most common human cell-based therapy applied today is hematopoietic stem cell (HSC) transplantation. Currently, human bone marrow, mobilized peripheral blood, and umbilical cord blood represent the major sources of transplantable HSCs, but their availability for use is limited by both compatibility between donor and recipient and required quantity. Although increasing evidence suggests that somatic HSCs can be expanded to meet current needs, their in vivo potential is concomitantly compromised after ex vivo culture. In contrast, human embryonic stem cells (hESC) possess indefinite proliferative capacity in vitro and have been shown to differentiate into the hematopoietic cell fate, giving rise to erythroid, myeloid, and lymphoid lineages using a variety of differentiation procedures. Human ESC-derived hematopoietic cells emerge from a subset of embryonic endothelium expressing PECAM-1, Flk-1, and VE-Cadherin, but lacking CD45 (CD45negPFV). These CD45negPFV precursors are exclusively responsible for hematopoietic potential of differentiated hESCs. hESC-derived hematopoietic cells show similar clonogenic capacity and primitive phenotype to somatic sources of hematopoietic progenitors and possess limited in vivo repopulating capacity in immunodeficient mice, suggestive of HSC function. Here, we will review current progress in studies of hESC-derived hematopoietic cells and discuss the potential precincts and applications.

GATA Transcription Factors in Hematologic Disease

GATA family transcription factors play essential roles in broad developmental settings. GATA-1, one of the hematopoietically expressed members, is required for normal erythroid and megakaryocytic differentiation. Over the past few years, mutations in the gene encoding GATA-1 have been linked to several human hematologic disorders, including X-linked dyserythropoietic anemia and thrombocytopenia, X-linked thrombocytopenia and beta-thalassemia, and Down syndrome acute megakaryoblastic leukemia. This review summarizes the role of GATA-1 during normal hematopoiesis and discusses how disease-associated mutations may affect its function.

FOG-1 recruits the NuRD repressor complex to mediate transcriptional repression by GATA-1

Transcription factor GATA-1 and its cofactor FOG-1 coordinate erythroid cell maturation by activating erythroid-specific genes and repressing genes associated with the undifferentiated state. Here we show that FOG-1 binds to the NuRD corepressor complex in vitro and in vivo. The interaction is mediated by a small conserved domain at the extreme N-terminus of FOG-1 that is necessary and sufficient for NuRD binding. This domain defines a novel repression module found in diverse transcriptional repressors. NuRD is present at GATA-1/FOG-1-repressed genes in erythroid cells in vivo. Point mutations near the N-terminus of FOG-1 that abrogate NuRD binding block gene repression by FOG-1. Finally, the ability of GATA-1 to repress transcription was impaired in erythroid cells expressing mutant forms of FOG-1 that are defective for NuRD binding. Together, these studies show that FOG-1 and likely other FOG-like proteins are corepressors that link GATA factors to histone deacetylation and nucleosome remodeling.

Coregulation of GATA factors by the Friend of GATA (FOG) family of multitype zinc finger proteins

The Friend of GATA (FOG) family of proteins is an evolutionarily conserved class of large multitype zinc finger cofactors that bind to the amino zinc finger of GATA transcription factors and modulate their activity. Two FOG genes have been identified in mammals, both of which interact with each of the six known vertebrate GATA factors in vitro. Physical interaction between FOG and GATA proteins in vivo is essential for the development of a broad array of tissues, reflecting the overlapping expression patterns of these factors. In this review, we will discuss the identification and characterization of FOG proteins, their role in human disease, and recent studies that shed new light on their function and regulation.

GATA1 in normal and malignant hematopoiesis

In the late 1980s, several research groups independently discovered the founding member of the GATA family of transcription factors, GATA-1. Each group had evidence that GATA-1 played an important role in erythroid gene expression, but little did they know that it would turn out to be a key regulator of development of not only red blood cells, but of several other hematopoietic cell types as well. Furthermore, few would have guessed that missense mutations in GATA1 would cause inherited blood disorders, while acquired mutations would be found associated with essentially all cases of acute megakaryoblastic leukemia (AMKL) in children with Down syndrome (DS). With respect to the latter disorder, the presence of a GATA1 mutation is now arguably the defining feature of this leukemia. In this review, I will summarize our current knowledge of the role of GATA-1 in normal development, and discuss how mutations in GATA1 lead to abnormal and malignant hematopoiesis.

The N Termini of Friend of GATA (FOG) Proteins Define a Novel Transcriptional Repression Motif and a Superfamily of Transcriptional Repressors

Members of the Friend of GATA (FOG) family of transcriptional co-factors are required for the development of both the cardiovascular and hematopoietic systems. FOG proteins physically interact with members of the GATA family of transcriptional activators and modulate their activity. We have previously shown that FOG-2 can bind to the N-terminal zinc finger of GATA4 and, via this interaction, repress GATA4-mediated transcriptional activation of various cardiac promoters. In this report we further characterize the domain of FOG-2 necessary for repression of GATA4 transcriptional activity. We show that FOG-2-mediated repression is not blocked by the histone deacetylase inhibitor tricostatin A, suggesting that FOG-2 repression of GATA4 occurs via a histone deacetylase independent mechanism. N-terminal deletion mutants of FOG-2 revealed that the first 12 amino acids of FOG-2 are necessary for FOG-2-mediated repression. Fusion of these 12 amino acids to the DNA binding domain of GAL4 demonstrated that this region is sufficient to mediate transcriptional repression even when recruited to a heterologous promoter. Single amino acid substitutions within this N-terminal domain of FOG-2 defined the critical amino acid sequence as RRKQxxPxxI. Interestingly, a search of the NCBI protein data base identified several other partially characterized zinc finger transcriptional repressors from various vertebrate species that contained this motif at their N terminus. Taken together, these observations define a novel transcriptional repression motif and a superfamily of zinc finger transcriptional repressors.

Differentiation status dependent function of FOG-1

The molecular interactions between transcription factors and cofactors play crucial roles in various biological processes, including haematopoiesis. FOG-1 is a cognate cofactor of GATA-1, and the FOG-1/GATA-1 complex is essential for the haematopoietic differentiation of erythroid cells and megakaryocytes. In order to elucidate the biological functions of FOG-1 in the different contexts of cell differentiation, we analysed the effects of FOG-1 expression on haematopoietic cell differentiation, using a combination of in vitro differentiation of mouse embryonic stem (ES) cells and conditional gene expression. FOG-1 suppressed the proliferation of primitive and definitive erythroid cells in all stages of differentiation. However, FOG-1 inhibited and enhanced megakaryopoiesis in the early and late differentiation stages, respectively, through different molecular mechanisms. In addition, FOG-1 inhibited the proliferation of ES cells, the molecular mechanism of which differs from those of erythroid and megakaryocytic cells. These results suggest that FOG-1 functions in a cell differentiation context-dependent manner.

Endothelial and hematopoietic cell fate of human embryonic stem cells originates from primitive endothelium with hemangioblastic properties

The cellular organization and relationships among precursors that initiate embryonic angiogenesis and hematopoiesis in the human have yet to be characterized. Here, we identify a subpopulation of primitive endothelial-like cells derived from human embryonic stem cells (hESCs) that express PECAM-1, Flk-1, and VE-cadherin, but not CD45 (CD45negPFV cells), and that are uniquely responsible for endothelial and hematopoietic development. Molecular profiling of CD45negPFV cells is consistent with endothelial and hematopoietic competency. Clonal isolation demonstrates that the CD45negPFV population includes bipotent cells with endothelial and hematopoietic capacity. We suggest that human hematopoiesis and endothelial maturation originate exclusively from a subset of embryonic endothelium that possesses hemangioblastic properties and offers a model system to study these lineage relationships in the human.

Transforming acidic coiled-coil protein 3 (TACC3) controls friend of GATA-1 (FOG-1) subcellular localization and regulates the association between GATA-1 and FOG-1 during hematopoiesis

Physical association between the transcription factor GATA-1 and the cofactor, Friend of GATA-1 (FOG-1), is essential for the differentiation of two blood cell types, erythroid cells and megakaryocytes. However, little is known regarding the mechanisms that modulate their interaction within cells. In the present study, we have identified TACC3 as a FOG-1-interacting protein. Transforming acidic coiled-coil protein 3 (TACC3), a protein that is highly expressed in hematopoietic cells, has been reported to have a critical role in the expansion of immature hematopoietic progenitors. We show that TACC3 affects FOG-1 nuclear localization, altering the interaction between GATA-1 and FOG-1. However, GATA-1 competes with TACC3 in the interaction with FOG-1. We observe that high levels of TACC3 inhibit the function of FOG-1 as a transcriptional cofactor of GATA-1. Furthermore, forced expression of TACC3 to levels similar to those found in progenitor cells delays terminal maturation of MEL and G1ER cells, two cell models of erythroid cell development. We suggest a role for TACC3 in regulating the cellular distribution of FOG-1 and thus the direct interaction of GATA-1 and FOG-1 as a mechanism to control the transition between expansion of multipotential progenitor cell populations and final stages of erythroid maturation.

Coregulator-dependent facilitation of chromatin occupancy by GATA-1

Coregulator recruitment by DNA-bound factors results in chromatin modification and protein-protein interactions, which regulate transcription. However, the mechanism by which the Friend of GATA (FOG) coregulator mediates GATA factor-dependent transcription is unknown. We showed previously that GATA-1 replaces GATA-2 at an upstream region of the GATA-2 locus, and that this GATA switch represses GATA-2. Genetic complementation analysis in FOG-1-null hematopoietic precursors revealed that FOG-1 is not required for establishment or maintenance of the active GATA-2 domain, but is critical for the GATA switch. Analysis of GATA factor binding to additional loci also revealed FOG-1-dependent GATA switches. Thus, FOG-1 facilitates chromatin occupancy by GATA-1 at sites bound by GATA-2. We propose that FOG-1 is a prototype of a new class of coregulators termed chromatin occupancy facilitators, which confer coregulation in certain contexts via enhancing trans-acting factor binding to chromatin in vivo.

Context-dependent regulation of GATA-1 by friend of GATA-1

The transcription factor GATA-1 and its cofactor, friend of GATA-1 (FOG-1), are essential for normal erythroid development. FOG-1 physically interacts with GATA-1 to augment or inhibit its activity. The mechanisms by which FOG-1 regulates GATA-1 function are unknown. By using an assay that is based on the phenotypic rescue of a GATA-1-null erythroid cell line, we found that a conditional form of GATA-1 (GATA-1-ER) strongly induced histone acetylation at the beta-major globin promoter in vivo, consistent with previous results. In contrast, GATA-1 bearing a point mutation that impairs FOG-1 binding [GATA-1(V205M)-ER] failed to induce high levels of histone acetylation at this site. However, at DNase I-hypersensitive site (HS)3 of the beta-globin locus control region, GATA-1-induced histone acetylation was FOG-1-independent. Because the V205M mutation does not disrupt GATA-1 binding to DNA templates in vitro, we were surprised to find that in vivo GATA-1(V205M)-ER fails to bind the beta-globin promoter. However, at HS3, DNA binding by GATA-1 was FOG-1-independent, thus correlating histone acetylation with GATA-1 occupancy. Examination of additional GATA-1-dependent regulatory elements showed that the interaction with FOG-1 is required for GATA-1 occupancy at select sites, such as HS2, but is dispensable at others, including the FOG-1-independent GATA-1 target gene EKLF. Remarkably, at the GATA-2 gene, which is repressed by GATA-1, interaction with FOG-1 was dispensable for GATA-1 occupancy and was required for transcriptional inhibition and histone deacetylation. These results indicate that FOG-1 employs distinct mechanisms when cooperating with GATA-1 during transcriptional activation and repression.