FOG acts as a repressor of red blood cell development in Xenopus

Transcriptional regulation of erythropoiesis. Fine tuning of combinatorial multi-domain elements

Haematopoiesis, the differentiation of haematopoietic stem cells and progenitors into various lineages, involves complex interactions of transcription factors that modulate the expression of downstream genes and mediate proliferation and differentiation signals. Commitment of pluripotent haematopoietic stem cells to the erythroid lineage induces erythropoiesis, the production of red blood cells. This process involves a concerted progression through an erythroid burst forming unit (BFU-E), an erythroid colony forming unit (CFU-E), proerythroblast and an erythroblast. The terminally differentiated erythrocytes, in mammals, lose their nucleus yet function several more months. A well-coordinated cohort of transcription factors regulates the formation, survival, proliferation and differentiation of multipotent progenitor into the erythroid lineage. Here, we discuss broad-spectrum factors essential for self-renewal and/or differentiation of multipotent cells as well as specific factors required for proper erythroid development. These factors may operate solely or as part of transcriptional complexes, and exert activation or repression. Sequence comparisons reveal evolutionarily conserved modular composition for these factors; X-ray crystallography demonstrates that they include multidomain elements (e.g. HLH or zinc finger motifs), consistent with their complex interactions with other proteins. Finally, transfections and genomic studies show that the timing of each factor's expression during the hematopoietic process, the cell lineages affected and the existing combination of other factors determine the erythroid cell fate.

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

The hematopoietic, zinc-finger protein FOG-1 is essential for the development of the erythroid and megakaryocytic lineages. FOG-1's function in hematopoiesis is dependent on its ability to interact with the transcription factor GATA-1. FOG-1 has also been observed to interact with the corepressor molecule C-terminal binding protein (CtBP) through a peptide motif shared by all FOG family members. In this study, we confirmed that FOG-1 and CtBP interact by coimmunoprecipitation. We further demonstrate that a FOG-1 mutant unable to interact with CtBP has increased erythropoietic (but not megakaryocytic) rescue (relative to the wild type) of a FOG-1(-/-) cell line. To analyze further the physiological role of the FOG-1-CtBP interaction, we generated knock-in mice that express a FOG-1 variant unable to bind CtBP. Surprisingly, these mice are normal and fertile. Furthermore, erythropoiesis at all stages of development is normal in these mice. Erythrocyte production is similar in mutant and wild-type mice even under conditions of erythropoietic stress stimulated by either exogenously added erythropoietin or phenylhydrazine-induced anemia. Thus, despite conservation of the FOG-CtBP interaction site, the in vivo function of FOG-1 in erythroid development is not affected by its inability to interact with the corepressor CtBP.

Friend of GATA is expressed in naive Th cells and functions as a repressor of GATA-3-mediated Th2 cell development

The commitment of naive T cells to polarized Th cells requires specific changes in their transcription factors. Retrovirally overexpressed GATA-3 has been reported to induce the Th2 cytokine profile in developing Th1 cells. In this study, we examined the role of the N-terminal finger (Nf) of GATA-3 in Th2 cell development. The Nf, as well as the C-terminal finger and the transactivation domain, is critical for the induction of the Th2 phenotype. Using the GATA-3-Nf as a bait, our yeast two-hybrid screening identified friend of GATA (FOG) in the Th2 cell-specific library. Naive T cells express significant levels of FOG mRNA, which was rapidly down-regulated upon commitment to both Th1 and Th2 lineages. In reporter assays, FOG blocked the GATA-3-mediated activation of several cytokine promoters. Finally, retroviral expression of FOG in developing Th2 cells suppressed both IL-4 and IL-5 and allowed for IFN-gamma production, which was accompanied by a significant level of T-bet mRNA expression. Serial deletion mutation analysis indicated that the N-terminal region, but not the consensus C-terminal binding protein-binding motif, of FOG is critical for the effects. Our results clearly indicate that 1) FOG is a repressor of GATA-3 in naive T cells and 2) the down-regulation of FOG induces Th2 cell differentiation by releasing GATA-3 from its repression.

CtBP, an unconventional transcriptional corepressor in development and oncogenesis

CtBP family proteins are conserved among vertebrates and invertebrates and function as transcriptional corepressors. They repress transcription in a histone deacetylase-dependent or -independent manner. CtBPs play important roles during development and oncogenesis. In this review, their unusual properties, the mechanisms of transcriptional repression, regulation, and their biological functions are discussed.

Functional conservation of hematopoietic factors in Drosophila and vertebrates

The GATA, Friend of GATA, and Runt homology domain protein families function during hematopoiesis to promote progenitor cell development and regulate lineage commitment and differentiation. The hematopoietic functions of these factors have been remarkably conserved across taxonomic groups, ranging from flies to humans. Furthermore, aspects of hematopoiesis and hemocyte function appear to be conserved. Thus, comparative studies using Drosophila and vertebrate models should enhance our understanding of blood cell development.

Friend of GATA-1 represses GATA-3-dependent activity in CD4+ T cells

The development of naive CD4+ T cells into a T helper (Th) 2 subset capable of producing interleukin (IL)-4, IL-5, and IL-13 involves a signal transducer and activator of transcription (Stat)6-dependent induction of GATA-3 expression, followed by Stat6-independent GATA-3 autoactivation. The friend of GATA (FOG)-1 protein regulates GATA transcription factor activity in several stages of hematopoietic development including erythrocyte and megakaryocyte differentiation, but whether FOG-1 regulates GATA-3 in T cells is uncertain. We show that FOG-1 can repress GATA-3-dependent activation of the IL-5 promoter in T cells. Also, FOG-1 overexpression during primary activation of naive T cells inhibited Th2 development in CD4+ T cells. FOG-1 fully repressed GATA-3-dependent Th2 development and GATA-3 autoactivation, but not Stat6-dependent induction of GATA-3. FOG-1 overexpression repressed development of Th2 cells from naive T cells, but did not reverse the phenotype of fully committed Th2 cells. Thus, FOG-1 may be one factor capable of regulating the Th2 development.

X-linked thrombocytopenia caused by a novel mutation of GATA-1

A family with recessive X-linked thrombocytopenia affecting 4 males in 2 generations, characterized by macrothrombocytopenia, profound bleeding, and mild dyserythropoiesis, is described. Microsatellite linkage analysis identified a region of the X chromosome including the GATA-1 gene, which encodes a critical transcription factor involved in erythrocyte and megakaryocyte development. By sequencing the entire coding region of GATA-1, a 2-base mutation was detected that results in a single amino acid substitution (glycine 208 to serine) within a highly conserved portion of the N-terminal zinc finger domain. Restriction fragment length polymorphism confirmed that this novel mutation segregated with the affected males and female carrier. Although not required for DNA binding, Gly208 of GATA-1 is involved in direct interaction with Friend of GATA-1 (FOG), a cofactor required for normal megakaryocytic and erythroid development. These results demonstrate that the GATA-1-FOG interaction is partially disrupted by the mutation and that the greatest effect involves contact with the FOG zinc finger 9. These findings help describe a novel mutation of GATA-1 in humans as a cause of X-linked thrombocytopenia, and they confirm the vital role played by this transcription factor during in vivo megakaryocyte development.

Endoderm specification and differentiation in Xenopus embryos

It is known from work with amniote embryos that regional specification of the gut requires cell-cell signalling between the mesoderm and the endoderm. In recent years, much of the interest in Xenopus endoderm development has focused on events that occur before gastrulation and this work has led to a different model whereby regional specification of the endoderm is autonomous. In this paper, we examine the specification and differentiation of the endoderm in Xenopus using neurula and tail-bud-stage embryos and we show that the current hypothesis of stable autonomous regional specification is not correct. When the endoderm is isolated alone from neurula and tail bud stages, it remains fully viable but will not express markers of regional specification or differentiation. If mesoderm is present, regional markers are expressed. If recombinations are made between mesoderm and endoderm, then the endodermal markers expressed have the regional character of the mesoderm. Previous results with vegetal explants had shown that endodermal differentiation occurs cell-autonomously, in the absence of mesoderm. We have repeated these experiments and have found that the explants do in fact show some expression of mesoderm markers associated with lateral plate derivatives. We believe that the formation of mesoderm cells by the vegetal explants accounts for the apparent autonomous development of the endoderm. Since the fate map of the Xenopus gut shows that the mesoderm and endoderm of each level do not come together until tail bud stages, we conclude that stable regional specification of the endoderm must occur quite late, and as a result of inductive signals from the mesoderm.

The CtBP family: enigmatic and enzymatic transcriptional co-repressors

Transcription factors that associate with DNA sequences in promoters and enhancers often recruit co-regulators that modulate their activity. Many of these co-regulators have intrinsic enzymatic activity and influence gene expression by modifying chromatin and altering its structure. Recently, a new family of co-repressors, the C-terminal binding proteins, has been described. These proteins recognize Pro-X-Asp-Leu-Ser (PXDLS) motifs in DNA-binding proteins and function as transcriptional co-repressors in Drosophila, Xenopus and mammals. The precise mechanisms by which they influence transcription are still under investigation. CtBP proteins dimerize and can contact histone deacetylases; hence they may operate by linking deacetylases to DNA-bound factors. But it appears that CtBP proteins also have intrinsic enzymatic activity. They have significant homology to D-isomer-specific 2-hydroxy acid dehydrogenases, and remarkably one family member, rat CtBP, has been shown to have a second role, functioning as an acyl transferase in Golgi maintenance. These observations raise the possibility that CtBP proteins might regulate gene expression directly by means of their enzymatic activities, in addition to serving as simple bridging proteins. Supplementary material for this article can be found on the BioEssays homepage at http://www.interscience.wiley.com/jpages/0265-9247/suppmat/v23_8.684.

Friend of GATA 2 physically interacts with chicken ovalbumin upstream promoter-TF2 (COUP-TF2) and COUP-TF3 and represses COUP-TF2-dependent activation of the atrial natriuretic factor promoter

Friend of GATA (FOG)-2 is a multi-zinc finger transcriptional corepressor protein that binds specifically to GATA4. Gene targeting studies have demonstrated that FOG-2 is required for normal cardiac morphogenesis, including the development of the coronary vasculature, left ventricular compact zone, and heart valves. To better understand the molecular mechanisms by which FOG-2 regulates these cardiac developmental programs, we screened a mouse day 11 embryo library using a yeast two-hybrid interaction trap with the fifth and sixth zinc fingers of FOG-2 as bait. Using this approach, we isolated clones encoding the orphan nuclear receptors chicken ovalbumin upstream promoter-transcription factor (COUP-TF) 2 and COUP-TF3. COUP-TF2-null embryos die during embryonic development with defective angiogenesis and cardiac defects, a pattern that partly resembles the FOG-2-null phenotype. The interaction between COUP-TF2 and FOG-2 in mammalian cells was confirmed by co-immunoprecipitation of these proteins from transfected COS-7 cells. The sites of binding interaction between COUP-TF2 and FOG-2 were mapped to zinc fingers 5 and 6 and fingers 7 and 8 of FOG-2 and to the carboxyl terminus of the COUP-TF proteins. Binding to COUP-TF2 was specific because FOG-2 did not interact with the ligand-binding domains of retinoid X receptor alpha, glucocorticoid receptor, and peroxisome proliferating antigen receptor gamma, which are related to the COUP-TF proteins. Full-length FOG-2 markedly enhanced transcriptional repression by GAL4-COUP-TF2(117-414), but not by a COUP-TF2 repression domain mutant. Moreover, FOG-2 repressed COUP-TF2dependent synergistic activation of the atrial natriuretic factor promoter by both GATA4 and the FOG-2-independent mutant GATA4-E215K. Taken together, these findings suggest that FOG-2 functions as a corepressor for both GATA and COUP-TF proteins.

Conserved cardiogenic functions of the multitype zinc-finger proteins: U-shaped and FOG-2

Multitype zinc-finger proteins murine Friend of GATA-2 (FOG-2) and Drosophila U-shaped (Ush) are required for heart development. Both FOG proteins participate in signal transduction pathways that are essential for cardiogenesis. FOG-2 regulates signaling from the myocardium, which is required for the production of the coronary vasculature. Ush functions in a common pathway with the Heartless (Htl) fibroblast growth factor (FGF) receptor to control mesodermal cell migration, which is required for cardiogenic cell fate commitment. In vitro studies have demonstrated that both FOG proteins repress GATA factor transcriptional activation of cardiac promoters. These similarities provide further evidence for the conservation of gene functions during cardiogenesis in Drosophila and higher eukaryotes.

The Friend of GATA proteins U-shaped, FOG-1, and FOG-2 function as negative regulators of blood, heart, and eye development in Drosophila

Friend of GATA (FOG) proteins regulate GATA factor-activated gene transcription. During vertebrate hematopoiesis, FOG and GATA proteins cooperate to promote erythrocyte and megakaryocyte differentiation. The Drosophila FOG homologue U-shaped (Ush) is expressed similarly in the blood cell anlage during embryogenesis. During hematopoiesis, the acute myeloid leukemia 1 homologue Lozenge and Glial cells missing are required for the production of crystal cells and plasmatocytes, respectively. However, additional factors have been predicted to control crystal cell proliferation. In this report, we show that Ush is expressed in hemocyte precursors and plasmatocytes throughout embryogenesis and larval development, and the GATA factor Serpent is essential for Ush embryonic expression. Furthermore, loss of ush function results in an overproduction of crystal cells, whereas forced expression of Ush reduces this cell population. Murine FOG-1 and FOG-2 also can repress crystal cell production, but a mutant version of FOG-2 lacking a conserved motif that binds the corepressor C-terminal binding protein fails to affect the cell lineage. The GATA factor Pannier (Pnr) is required for eye and heart development in Drosophila. When Ush, FOG-1, FOG-2, or mutant FOG-2 is coexpressed with Pnr during these developmental processes, severe eye and heart phenotypes result, consistent with a conserved negative regulation of Pnr function. These results indicate that the fly and mouse FOG proteins function similarly in three distinct cellular contexts in Drosophila, but may use different mechanisms to regulate genetic events in blood vs. cardial or eye cell lineages.

Pannier is a transcriptional target and partner of Tinman during Drosophila cardiogenesis

During Drosophila embryogenesis, the homeobox gene tinman is expressed in the dorsal mesoderm where it functions in the specification of precursor cells of the heart, visceral, and dorsal body wall muscles. The GATA factor gene pannier is similarly expressed in the dorsal-most part of the mesoderm where it is required for the formation of the cardial cell lineage. Despite these overlapping expression and functional properties, potential genetic and molecular interactions between the two genes remain largely unexplored. Here, we show that pannier is a direct transcriptional target of Tinman in the heart-forming region. The resulting coexpression of the two factors allows them to function combinatorially in the regulation of cardiac gene expression, and a physical interaction of the proteins has been demonstrated in cultured cells. We also define functional domains of Tinman and Pannier that are required for their synergistic activation of the D-mef2 differentiation gene in vivo. Together, these results provide important insights into the genetic mechanisms controlling heart formation in the Drosophila model system.

Acetylation of adenovirus E1A regulates binding of the transcriptional corepressor CtBP

Adenovirus E1A mediates its effects on cellular transformation and transcription by interacting with critical cellular proteins involved in cell growth and differentiation. The amino terminus of E1A binds to CBP/p300 and associated histone acetyltransferases such as P/CAF. The carboxyl terminus binds to the carboxyl-terminal binding protein (CtBP), which associates with histone deacetylases. We show that 12S E1A can be acetylated by p300 and P/CAF and map one of the acetylation sites to Lys-239. This Lys residue is adjacent to the consensus CtBP binding motif, PXDLS. Mutation of Lys-239 to Gln or Ala blocks CtBP binding in vitro and disrupts the E1A-CtBP interaction in vivo. Peptide competition assays demonstrated that the interaction of E1A with CtBP is also blocked by Lys-239 acetylation. Supporting a functional role for Lys-239 in CtBP binding, mutation of this residue to Ala decreases the ability of E1A to block cAMP-regulated enhancer (CRE)-binding protein (CREB)-stimulated gene expression. Finally, we demonstrate that Lys-239 is acetylated in cells by using an antibody directed against an acetyl-Lys-239 E1A peptide. CtBP interacts with a wide variety of other transcriptional repressors through the PXDLS motif, and, in many instances, this motif is followed by a Lys residue. We suggest that acetylation of this residue by histone acetyltransferases, and the consequent disruption of repressor complexes, might be a general mechanism for gene activation.

Solution structures of two CCHC zinc fingers from the FOG family protein U-shaped that mediate protein-protein interactions

BACKGROUND

Zinc finger domains have traditionally been regarded as sequence-specific DNA binding motifs. However, recent evidence indicates that many zinc fingers mediate specific protein-protein interactions. For instance, several zinc fingers from FOG family proteins have been shown to interact with the N-terminal zinc finger of GATA-1.

RESULTS

We have used NMR spectroscopy to determine the first structures of two FOG family zinc fingers that are involved in protein-protein interactions: fingers 1 and 9 from U-shaped. These fingers resemble classical TFIIIA-like zinc fingers, with the exception of an unusual extended portion of the polypeptide backbone prior to the fourth zinc ligand. [15N,(1)H]-HSQC titrations have been used to define the GATA binding surface of USH-F1, and comparison with other FOG family proteins indicates that the recognition mechanism is conserved across species. The surface of FOG-type fingers that interacts with GATA-1 overlaps substantially with the surface through which classical fingers typically recognize DNA. This suggests that these fingers could not contact both GATA and DNA simultaneously. In addition, results from NMR, gel filtration, and sedimentation equilibrium experiments suggest that the interactions are of moderate affinity.

CONCLUSIONS

Our results demonstrate unequivocally that zinc fingers comprising the classical betabetaalpha fold are capable of mediating specific contacts between proteins. The existence of this alternative function has implications for the prediction of protein function from sequence data and for the evolution of protein function.

GATA-1- and FOG-dependent activation of megakaryocytic alpha IIB gene expression

FOG is a multitype zinc finger protein that is essential for megakaryopoiesis, binds to the amino-terminal finger of GATA-1, and modulates the transcription of GATA-1 target genes. Presently investigated are effects of FOG and GATA-1 on the transcription of the megakaryocytic integrin gene, alpha IIb. In GATA-1-deficient FDCER cells (in the presence of endogenous FOG), ectopically expressed GATA-1 activated transcription 3-10-fold both from alpha IIb templates and the endogenous alpha IIb gene. The increased expression of FOG increased reporter construct transcription 30-fold overall. Unexpectedly, alphaIIb gene transcription also was stimulated efficiently upon the ectopic expression in of FOG per se. This occurred in the absence of any detectable expression of GATA-1 and was observed in multiple independent sublines for both the endogenous alpha IIb gene and transfected constructs yet proved to depend largely upon conserved GATA elements 457 and 55 base pairs upstream from the transcriptional start site. In 293 cells, FOG plus GATA-1 but not FOG alone only moderately stimulated alpha IIb transcription, and no direct interactions of FOG with the alpha IIb promoter were detectable. Thus, FOG acts in concert with GATA-1 to stimulate alpha IIb expression but also can act via a GATA-1-independent route, which is proposed to involve additional hematopoietic-restricted cofactors (possibly GATA-2).

Antagonism between C/EBPbeta and FOG in eosinophil lineage commitment of multipotent hematopoietic progenitors

The commitment of multipotent cells to particular developmental pathways requires specific changes in their transcription factor complement to generate the patterns of gene expression characteristic of specialized cell types. We have studied the role of the GATA cofactor Friend of GATA (FOG) in the differentiation of avian multipotent hematopoietic progenitors. We found that multipotent cells express high levels of FOG mRNA, which were rapidly down-regulated upon their C/EBPbeta-mediated commitment to the eosinophil lineage. Expression of FOG in eosinophils led to a loss of eosinophil markers and the acquisition of a multipotent phenotype, and constitutive expression of FOG in multipotent progenitors blocked activation of eosinophil-specific gene expression by C/EBPbeta. Our results show that FOG is a repressor of the eosinophil lineage, and that C/EBP-mediated down-regulation of FOG is a critical step in eosinophil lineage commitment. Furthermore, our results indicate that maintenance of a multipotent state in hematopoiesis is achieved through cooperation between FOG and GATA-1. We present a model in which C/EBPbeta induces eosinophil differentiation by the coordinate direct activation of eosinophil-specific promoters and the removal of FOG, a promoter of multipotency as well as a repressor of eosinophil gene expression.

Diversification of haematopoietic stem cells to specific lineages

Diverse types of blood cell (lineages) are produced from rare haematopoietic stem cells that reside in the bone marrow. This process, known as haematopoiesis, provides a valuable model for examining how genetic programs are established and executed in vertebrates, and also how homeostasis of blood formation is altered in leukaemias. So, how does an apparently small group of critical lineage-restricted nuclear regulatory factors specify the diversity of haematopoietic cells? Recent findings not only indicate how this may be achieved but also show the extraordinary plasticity of tissue stem cells in vivo.