Negative cross-talk between hematopoietic regulators: GATA proteins repress PU.1

The order of expression of transcription factors directs hierarchical specification of hematopoietic lineages

The mechanism of lineage specification in multipotent stem cells has not been fully understood. We recently isolated progenitors with the eosinophil, basophil, or mast cell lineage potential, all of which originate from granulocyte/monocyte progenitors (GMPs). By using these prospectively purified progenitors, we show here that the expression timing of GATA-2 and CCAAT enhancer-binding protein alpha (C/EBPalpha) can differentially control their lineage commitment. The expression of GATA-2 instructed C/EBPalpha-expressing GMPs to commit exclusively into the eosinophil lineage, while it induced basophil and/or mast cell lineage commitment if C/EBPalpha was suppressed at the GMP stage. Furthermore, simply by switching the order of C/EBPalpha and GATA-2 transduction, even lymphoid-committed progenitors recaptured these developmental processes to be reprogrammed into each of these lineages. We propose that the order of expression of key transcription factors is critical for their interplay to selectively drive lineage specification programs, by which stem cells could generate multiple lineage cells in a hierarchical manner.

Blood Cells and Blood Cell Development in the Animal Kingdom

Recent findings strongly suggest that the molecular pathways involved in the development and function of blood cells are highly conserved among vertebrates and various invertebrate phyla. This has led to a renewed interest regarding homologies between blood cell types and their developmental origin among different animals. One way to address these areas of inquiry is to shed more light on the biology of blood cells in extant invertebrate taxa that have branched off the bilaterian tree in between insects and vertebrates. This review attempts, in a broadly comparative manner, to update the existing literature that deals with early blood cell development. I begin by providing a brief survey of the different types of blood cell lineages among metazoa. There is now good reason to believe that, in vertebrates and invertebrates alike, blood cell lineages diverge from a common type of progenitor cell, the hemocytoblast. I give a synopsis of the origin and determination of the hematocytoblast, beginning with a look at the hematopoietic organs that house hemocytoblasts in adult animals, followed by a more detailed overview of the embryonic development of the hematopoietic organ. Finally, I compare the process of blood lineage diversification in vertebrates and Drosophila.

Multilineage transcriptional priming and determination of alternate hematopoietic cell fates

Hematopoietic stem cells and their progenitors exhibit multilineage patterns of gene expression. Molecular mechanisms underlying the generation and refinement of these patterns during cell fate determination remain unexplored because of the absence of suitable experimental systems. Using PU.1(-/-) progenitors, we demonstrate that at subthreshold levels, this Ets transcription factor regulates a mixed pattern (macrophage/neutrophil) of gene expression within individual myeloid progenitors. Increased PU.1 levels refine the pattern and promote macrophage differentiation by modulating a novel regulatory circuit comprised of counter antagonistic repressors, Egr-1,2/Nab-2 and Gfi-1. Egr-1 and Egr-2 function redundantly to activate macrophage genes and to repress the neutrophil program. These results are used to assemble and mathematically model a gene regulatory network that exhibits both graded and bistable behaviors and accounts for the onset and resolution of mixed lineage patterns during cell fate determination.

B-Lymphoma cells with epigenetic silencing of Pax5 trans-differentiate into macrophages, but not other hematopoietic lineages

In mice, zygotic or pro-B-cell-specific knock-out of the Pax5 gene allows differentiation of pro-B-cells into all hematopoietic lineages. We previously generated and characterized a murine B-cell lymphoma, dubbed Myc5, whose cells spontaneously lose Pax5 expression when cultured in vitro, but regain it when re-injected into syngeneic mice. In cultured Myc5 cells, the loss of Pax5 correlates with the acquisition of myeloid markers, such as CD11b and F4/80. Here, we sought to determine whether these cells are truly B-macrophage-restricted or, like Pax5-null progenitors, can give rise to additional hematopoietic lineages. In vitro differentiation assays with various cytokines showed that Myc5 cells do not differentiate into NK cells, dendritic cells, neutrophils, or osteoclasts. At the same time, in the presence of macrophage colony-stimulating factor (M-CSF), they readily phagocytose latex beads and provide T-cell help. Both phenomena are indicative of the bona fide macrophage phenotype. Conversely, enforced Pax5 re-expression in macrophage-like Myc5 cells led to down-regulation of the M-CSF receptor and re-acquisition of some B-cell surface markers (e.g., CD79a) and lineage-specific transcription factors (e.g., IRF4 and Blimp). Retrovirally encoded Pax5 also restored expression of several master B-cell differentiation proteins, such as the IL-7 receptor and transcription factor E2A. In contrast, levels of EBF were unaffected by Pax5 suggesting that EBF acts exclusively upstream of Pax5 and might contribute to Pax5 expression. Indeed, transduction with an EBF-encoding retrovirus partly reactivated endogenous Pax5. Our data reveal the complex relationship between B-cell-specific transcription factors and suggest the existence of numerous feedback mechanisms.

GATA-4 Incompletely Substitutes for GATA-1 in Promoting Both Primitive and Definitive Erythropoiesis in Vivo

Vertebrate GATA transcription factors have been classified into two subgroups; GATA-1, GATA-2, and GATA-3 are expressed in hematopoietic cells, whereas GATA-4, GATA-5, and GATA-6 are expressed in mesoendoderm-derived tissues. We previously discovered that expression of GATA-2 or GATA-3 under the transcriptional control for the Gata1 gene eliminates lethal anemia in Gata1 germ line mutant mice (Gata1.05/Y). Here, we show that the GATA-4 expression by the same regulatory cassette prolongs the life span of Gata1.05/Y embryos from embryonic day 12.5 to 15.5 but fails to abrogate its embryonic lethality. Gata1.05/Y mice bearing the GATA-4 transgene showed impaired maturation of both primitive and definitive erythroid cells and defective erythroid cell expansion in fetal liver. Moreover, the incidence of apoptosis was observed prominently in primitive erythroid cells. In contrast, a GATA-4-GATA-1 chimeric protein prepared by linking the N-terminal region of GATA-4 to the C-terminal region of GATA-1 significantly promoted the differentiation and survival of primitive erythroid cells, although this protein is still insufficient for rescuing Gata1.05/Y embryos from lethal anemia. These data thus show a functional incompatibility between hematopoietic and endodermal GATA factors in vivo and provide evidence indicating specific roles of the C-terminal region of GATA-1 in primitive erythropoiesis.

Gene expression during terminal erythroid differentiation

PURPOSE OF REVIEW

Expression profiling is a powerful technique to sample cell state. This review shows how expression profiling is being applied to the study of erythroid differentiation.

RECENT FINDINGS

Expression-based studies of multipotential hematopoietic progenitor cells has shown that these cells express lineage-restricted genes from multiple lineages at low levels, and that they are in effect 'primed' to develop into all hematopoietic cell types. Expression profiling of oligopotent and committed progenitor cells has further shown that commitment to the erythroid lineage is associated with a progressive decline in the number of expressed genes. Lineage commitment is regulated by lineage-restricted transcription factors, and studies show that the erythroid transcription factor GATA1, in addition to activating a subset of genes, has global repressive effects on gene expression. Terminal erythroid differentiation is associated with further reduction in the number of expressed genes. The erythroid program is defined by those genes that are still expressed, and their high-level expression depends on specific epigenetic modifications, recruitment of transcription factors, and posttranscriptional effects.

SUMMARY

Expression profiling provides the means to identify novel targets for the therapy of erythrocytes disorders, and to obtain insights into the mechanisms of cellular differentiation.

Activation of the canonical Wnt pathway leads to loss of hematopoietic stem cell repopulation and multilineage differentiation block

Wnt signaling increases hematopoietic stem cell self-renewal and is activated in both myeloid and lymphoid malignancies, indicating involvement in both normal and malignant hematopoiesis. We report here activated canonical Wnt signaling in the hematopoietic system through conditional expression of a stable form of beta-catenin. This enforced expression led to hematopoietic failure associated with loss of myeloid lineage commitment at the granulocyte-macrophage progenitor stage; blocked erythrocyte differentiation; disruption of lymphoid development; and loss of repopulating stem cell activity. Loss of hematopoietic stem cell function was associated with decreased expression of Cdkn1a (encoding the cell cycle inhibitor p21(cdk)), Sfpi1, Hoxb4 and Bmi1 (encoding the transcription factors PU.1, HoxB4 and Bmi-1, respectively) and altered integrin expression in Lin(-)Sca-1(+)c-Kit(+) cells, whereas PU.1 was upregulated in erythroid progenitors. Constitutive activation of canonical Wnt signaling therefore causes multilineage differentiation block and compromised hematopoietic stem cell maintenance.

Molecular Analysis of the Interaction between the Hematopoietic Master Transcription Factors GATA-1 and PU.1

GATA-1 and PU.1 are transcription factors that control erythroid and myeloid development, respectively. The two proteins have been shown to function in an antagonistic fashion, with GATA-1 repressing PU.1 activity during erythropoiesis and PU.1 repressing GATA-1 function during myelopoiesis. It has also become clear that this functional antagonism involves direct interactions between the two proteins. However, the molecular basis for these interactions is not known, and a number of inconsistencies exist in the literature. We have used a range of biophysical methods to define the molecular details of the GATA-1-PU.1 interaction. A combination of NMR titration data and extensive mutagenesis revealed that the PU.1-Ets domain and the GATA-1 C-terminal zinc finger (CF) form a low affinity interaction in which specific regions of each protein are implicated. Surprisingly, the interaction cannot be disrupted by single alanine substitution mutations, suggesting that binding is distributed over an extended interface. The C-terminal basic tail region of CF appears to be sufficient to mediate an interaction with PU.1-Ets, and neither acetylation nor phosphorylation of a peptide corresponding to this region disrupts binding, indicating that the interaction is not dominated by electrostatic interactions. The CF basic tail shares significant sequence homology with the PU.1 interacting motif from c-Jun, suggesting that GATA-1 and c-Jun might compete to bind PU.1. Taken together, our data provide a molecular perspective on the GATA-1-PU.1 interaction, resolving several issues in the existing data and providing insight into the mechanisms through which these two proteins combine to regulate blood development.

Erythroblast transformation by the friend spleen focus-forming virus is associated with a block in erythropoietin-induced STAT1 phosphorylation and DNA binding and correlates with high expression of the hematopoietic phosphatase SHP-1

Infection of mice with Friend spleen focus-forming virus (SFFV) results in a multistage erythroleukemia. In the first stage, the SFFV envelope glycoprotein interacts with the erythropoietin receptor and a short form of the receptor tyrosine kinase sf-Stk, resulting in constitutive activation of signal transducing molecules and the development of erythropoietin (Epo)-independent erythroid hyperplasia and polycythemia. The second stage results from the outgrowth of a rare virus-infected erythroid cell that expresses nonphysiological levels of the myeloid transcription factor PU.1. These cells exhibit a differentiation block and can be grown as murine erythroleukemia (MEL) cell lines. In this study, we examined SFFV MEL cells to determine whether their transformed phenotype was associated with a block in the activation of any Epo signal-transducing molecules. Our studies indicate that Epo- or SFFV-induced activation of STAT1/3 DNA binding activity is blocked in SFFV MEL cells. The block is at the level of tyrosine phosphorylation of STAT1, although Jak2 phosphorylation is not blocked in these cells. In contrast to Epo, alpha interferon can induce STAT1 phosphorylation and DNA binding in SFFV MEL cells. The SFFV-transformed cells were shown to express elevated levels of the hematopoietic phosphatase SHP-1, and treatment of the cells with a phosphatase inhibitor restored STAT1 tyrosine phosphorylation. MEL cells derived from Friend murine leukemia virus (MuLV) or ME26 MuLV-infected mice, which do not express PU.1, express lower levels of SHP-1 and are not blocked in STAT1/3 DNA-binding activity. Our studies suggest that SFFV-infected erythroid cells become transformed when differentiation signals activated by STAT1/3 are blocked due to high SHP-1 levels induced by inappropriate expression of the PU.1 protein.

Spi-1/PU.1 Oncoprotein Affects Splicing Decisions in a Promoter Binding-dependent Manner

The expression of the Spi-1/PU.1 transcription factor is tightly regulated as a function of the hematopoietic lineage. It is required for myeloid and B lymphoid differentiation. When overexpressed in mice, Spi-1 is associated with the emergence of transformed proerythroblasts unable to differentiate. In the course of a project undertaken to characterize the oncogenic function of Spi-1, we found that Spi-1 interacts with proteins of the spliceosome in Spi-1-transformed proerythroblasts and participates in alternative splice site selection. Because Spi-1 is a transcription factor, it could be hypothesized that these two functions are coordinated. Here, we have developed a system allowing the characterization of transcription and splicing from a single target. It is shown that Spi-1 is able to regulate alternative splicing of a pre-mRNA for a gene whose transcription it regulates. Using a combination of Spi-1 mutants and Spi-1-dependent promoters, we demonstrate that Spi-1 must bind and transactivate a given promoter to favor the use of the proximal 5' alternative site. This establishes that Spi-1 affects splicing decisions in a promoter binding-dependent manner. These results provide new insight into how Spi-1 may act in the blockage of differentiation by demonstrating that it can deregulate gene expression and also modify the nature of the products generated from target genes.

Determinants of lymphoid-myeloid lineage diversification

In recent years, investigators have made great progress in delineating developmental pathways of several lymphoid and myeloid lineages and in identifying transcription factors that establish and maintain their fate. However, the developmental branching points between these two large cell compartments are still controversial, and little is known about how their diversification is induced. Here, we give an overview of determinants that play a role at lymphoid-myeloid junctures, in particular transcription factors and cytokine receptors. Experiments showing that myeloid lineages can be reversibly reprogrammed into one another by transcription factor network perturbations are used to highlight key principles of lineage commitment. We also discuss experiments showing that lymphoid-to-myeloid but not myeloid-to-lymphoid conversions can be induced by the enforced expression of a single transcription factor. We close by proposing that this asymmetry is related to a higher complexity of transcription factor networks in lymphoid cells compared with myeloid cells, and we suggest that this feature must be considered when searching for mechanisms by which hematopoietic stem cells become committed to lymphoid lineages.

Zipzap/p200 is a novel zinc finger protein contributing to cardiac gene regulation

Serum response factor (SRF) plays an important role in the regulation of immediate-early genes and muscle-specific genes, while SRF cofactors may contribute significantly to assist in tissue-specific, development-stage related regulation of SRF-target genes. We recently cloned a novel SRF cofactor, termed zipzap/p200, which is a zinc finger protein yet to be characterized. We determined that zipzap/p200 is a 200-kDa protein with two classic C2H2 zinc fingers at the carboxyl terminus where the nucleotide sequence was highly conserved among human, mouse, and rat. The zipzap gene was expressed in multiple tissues and at multiple ages, including the fetal and adult heart. The zipzap protein interacted with SRF in vivo and was found in protein complexes containing SRF and other SRF cofactors, including p49/strap and Nkx2.5. Zipzap/p200 activated the promoter of cardiac genes and potentiated the effect of myocardin on ANF promoter activity. Therefore, zipzap may serve as a transcription co-activator for the regulation of cardiac gene expression. Our data support the notion that a number of SRF cofactors may participate in gene regulation and thereby contribute to the delicate control of gene expression in complex biological processes.

PU.1 phosphorylation correlates with hydroquinone-induced alterations in myeloid differentiation and cytokine-dependent clonogenic response in human CD34+ hematopoietic progenitor cells

The transcriptional regulatory factor PU.1 is important for the regulation of a diverse group of hematopoietic and myeloid genes. Posttranslational phosphorylation of PU.1 has been demonstrated in the regulation of a variety of promoters in normal cells. In leukemia cells, differing patterns of PU.1 phosphorylation have been described among acute myelogenous leukemia (AML) subtypes. Therefore, we hypothesized that modulation of PU.1-dependent gene expression might be a molecular mediator of alterations in myeloid cell growth and differentiation that have been demonstrated to be early events in benzene-induced leukemogenesis. We found that freshly isolated human CD34(+) hematopoietic progenitor cells (HPC) exhibit multiple PU.1-DNA binding species that represent PU.1 proteins in varying degrees of phosphorylation states as determined by phosphatase treatment in combination with electrophoretic mobility shift assay (EMSA). Maturation of granulocyte and monocyte lineages is also accompanied by distinct changes in PU.1-DNA binding patterns. Experiments reveal that increasing doses of the benzene metabolite, hydroquinone (HQ) induce a time-and dose-dependent alteration in the pattern of PU.1-DNA binding in cultured human CD34(+) cells, corresponding to hyperphosphorylation of the PU.1 protein. HQ-induced alterations in PU.1-DNA binding are concomitant with a sustained immature CD34(+) phenotype and cytokine-dependent enhanced clonogenic activity in cultured human HPC. These results suggest that HQ induces a dysregulation in the external signals modulating PU.1 protein phosphorylation and this dysregulation may be an early event in the generation of benzene-induced AML.

Genetic regulatory networks programming hematopoietic stem cells and erythroid lineage specification

Erythroid cell production results from passage through cellular hierarchies dependent on differential gene expression under the control of transcription factors responsive to changing niches. We have constructed Genetic Regulatory Networks (GRNs) describing this process, based predominantly on mouse data. Regulatory network motifs identified in E. coli and yeast GRNs are found in combination in these GRNs. Feed-forward motifs with autoregulation generate forward momentum and also control its rate, which is at its lowest in hematopoietic stem cells (HSCs). The simultaneous requirement for multiple regulators in multi-input motifs (MIMs) provides tight control over expression of target genes. Combinations of MIMs, exemplified by the SCL/LMO2 complexes, which have variable content and binding sites, explain how individual regulators can have different targets in HSCs and erythroid cells and possibly also how HSCs maintain stem cell functions while expressing lineage-affiliated genes at low level, so-called multi-lineage priming. MIMs combined with cross-antagonism describe the relationship between PU.1 and GATA-1 and between two of their target genes, Fli-1 and EKLF, with victory for GATA-1 and EKLF leading to erythroid lineage specification. These GRNs are useful repositories for current regulatory information, are accessible in interactive form via the internet, enable the consequences of perturbation to be predicted, and can act as seed networks to organize the rapidly accumulating microarray data.

Notch Signaling Requires GATA-2 to Inhibit Myelopoiesis from Embryonic Stem Cells and Primary Hemopoietic Progenitors

The bone marrow and thymus, although both hemopoietic environments, induce very distinct differentiation outcomes. The former supports hemopoietic stem cell self-renewal and multiple hemopoietic lineages, while the latter supports T lymphopoiesis almost exclusively. This distinction suggests that the thymic environment acts to restrict the hemopoietic fates available to thymic immigrants. In this study, we demonstrate that the addition of the Notch ligand Delta-like-1 (Dll-1) to an in vitro system that otherwise supports myelopoiesis, greatly reduces the myelopoietic potential of stem cells or uncommitted progenitors. In contrast, committed myeloid progenitors mature regardless of the presence of Dll-1. The block in myelopoiesis is the direct result of Notch signaling within the hemopoietic progenitor, and Dll-1-induced signals cause a rapid increase in the expression of the zinc finger transcription factor GATA-2. Importantly, in the absence of GATA-2, Dll-1-induced signals fail to inhibit commitment to the myeloid fate. Taken together, our results support a role for GATA-2 in allowing Dll-1 to restrict non-T cell lineage differentiation outcomes.

Differential binding of erythroid Krupple-like factor to embryonic/fetal globin gene promoters during development

The competition model for beta-like globin gene switching during development predicts that differential binding of transcription factors to globin gene promoters and/or proximal enhancers regulate the competitive interactions of globin gene family members with the powerful locus control region (LCR). Direct interactions of individual genes with the LCR are essential for high level expression in erythroid cells. In this paper, we have demonstrated, by chromatin immunoprecipitation, that erythroid-Krupple-like factor (EKLF) binds to embryonic/fetal globin gene promoters in primitive (but not in definitive) erythroid cells. EKLF binds strongly to adult globin gene promoters and to LCR sequences HS4, HS3, HS2, and HS1 in both primitive and definitive erythroid cells. Trimethylation of histone H3K4 and H3K27 at the embryonic/fetal and adult globin gene promoters is equivalent in definitive cells; therefore, the differential binding of EKLF to these promoters does not appear to result from changes in chromatin configuration. Interestingly, the level of EKLF in definitive cells is 3-fold higher than the level in primitive cells. These results suggest that temporal-specific changes in EKLF abundance result in differential binding of this essential erythroid transcription factor to embryonic/fetal globin gene promoters during development and that these changes in EKLF binding specificity mediate the competitive interactions of globin gene family members with the LCR.

A versatile tool for conditional gene expression and knockdown

Drug-inducible systems allowing the control of gene expression in mammalian cells are invaluable tools for genetic research, and could also fulfill essential roles in gene- and cell-based therapy. Currently available systems, however, often have limited in vivo functionality because of leakiness, insufficient levels of induction, lack of tissue specificity or prohibitively complicated designs. Here we describe a lentiviral vector-based, conditional gene expression system for drug-controllable expression of polymerase (Pol) II promoter-driven transgenes or Pol III promoter-controlled sequences encoding small inhibitory hairpin RNAs (shRNAs). This system has great robustness and versatility, governing tightly controlled gene expression in cell lines, in embryonic or hematopoietic stem cells, in human tumors xenotransplanted into nude mice, in the brain of rats injected intraparenchymally with the vector, and in transgenic mice generated by infection of fertilized oocytes. These results open up promising perspectives for basic or translational research and for the development of gene-based therapeutics.

Molecular characterization of the murine homologue of the DC-derived protein DC-SCRIPT

Dendritic cell-specific transcript (DC-SCRIPT) is a putative DC zinc (Zn) finger-type transcription factor described recently in humans. Here, we illustrate that DC-SCRIPT is highly conserved in evolution and report the initial characterization of the murine ortholog of DC-SCRIPT, which is also preferentially expressed in DC as shown by real-time quantitative polymerase chain reaction, and its distribution resembles that of its human counterpart. Studies undertaken in human embryonic kidney 293 cells depict its nuclear localization and reveal that the Zn finger domain of the protein is mainly responsible for nuclear import. The human and the mouse genes are located in syntenic chromosomal regions and exhibit a similar genomic organization with numerous common transcription factor-binding sites in their promoter region, including sites for many factors implicated in haematopoiesis and DC biology, such as Gfi, GATA-1, Spi-B, and c-Rel. Taken together, these data show that DC-SCRIPT is well-conserved in evolution and that the mouse homologue is more than 80% homologous to the human protein. Therefore, mouse models can be used to elucidate the function of this novel DC marker.

Redirecting differentiation of hematopoietic progenitors by a transcription factor, GATA-2

GATA-2 is a zinc finger transcription factor essential for differentiation of immature hematopoietic cells. We analyzed the function of GATA-2 by a combined method of tetracycline-dependent conditional gene expression and in vitro hematopoietic differentiation from mouse embryonic stem (ES) cells using OP9 stroma cells (OP9 system). In the presence of macrophage colony-stimulating factor (M-CSF), the OP9 system induced macrophage differentiation. GATA-2 expression in this system inhibited macrophage differentiation and redirected the fate of hematopoietic differentiation to other hematopoietic lineages. GATA-2 expression commencing at day 5 or day 6 induced megakaryocytic or erythroid differentiation, respectively. Expression levels of PU.1, a hematopoietic transcription factor that interferes with GATA-2, appeared to play a critical role in differentiation to megakaryocytic or erythroid lineages. Transcription of PU.1 was affected by histone acetylation induced by binding of GATA-2 to the PU.1 promoter region. This study demonstrates that the function of GATA-2 is modified in a context-dependent manner by expression of PU.1, which in turn is regulated by GATA-2.

Towards an understanding of lineage specification in hematopoietic stem cells: a mathematical model for the interaction of transcription factors GATA-1 and PU.1

In addition to their self-renewal capabilities, hematopoietic stem cells guarantee the continuous supply of fully differentiated, functional cells of various types in the peripheral blood. The process which controls differentiation into the different lineages of the hematopoietic system (erythroid, myeloid, lymphoid) is referred to as lineage specification. It requires a potentially multi-step decision sequence which determines the fate of the cells and their successors. It is generally accepted that lineage specification is regulated by a complex system of interacting transcription factors. However, the underlying principles controlling this regulation are currently unknown. Here, we propose a simple quantitative model describing the interaction of two transcription factors. This model is motivated by experimental observations on the transcription factors GATA-1 and PU.1, both known to act as key regulators and potential antagonists in the erythroid vs. myeloid differentiation processes of hematopoietic progenitor cells. We demonstrate the ability of the model to account for the observed switching behavior of a transition from a state of low expression of both factors (undifferentiated state) to the dominance of one factor (differentiated state). Depending on the parameter choice, the model predicts two different possibilities to explain the experimentally suggested, stem cell characterizing priming state of low level co-expression. Whereas increasing transcription rates are sufficient to induce differentiation in one scenario, an additional system perturbation (by stochastic fluctuations or directed impulses) of transcription factor levels is required in the other case.

Multistable and multistep dynamics in neutrophil differentiation

Background

Cell differentiation has long been theorized to represent a switch in a bistable system, and recent experimental work in micro-organisms has revealed bistable dynamics in small gene regulatory circuits. However, the dynamics of mammalian cell differentiation has not been analyzed with respect to bistability.

Results

Here we studied how HL60 promyelocytic precursor cells transition to the neutrophil cell lineage after stimulation with the differentiation inducer, dimethyl sulfoxide (DMSO). Single cell analysis of the expression kinetics of the differentiation marker CD11b (Mac-1) revealed all-or-none switch-like behavior, in contrast to the seemingly graduated change of expression when measured as a population average. Progression from the precursor to the differentiated state was detected as a discrete transition between low (CD11b^Low) and high (CD11b^High) expressor subpopulations distinguishable in a bimodal distribution. Hysteresis in the dependence of CD11b expression on DMSO dose suggests that this bimodality may reflect a bistable dynamic. But when an "unswitched" (CD11b^Low) subpopulation of cells in the bistable/bimodal regime was isolated and cultured, these cells were found to differ from undifferentiated precursor cells in that they were "primed" to differentiate.

Conclusion

These findings indicate that differentiation of human HL60 cells into neutrophils does not result from a simple state transition of a bistable switch as traditionally modeled. Instead, mammalian differentiation appears to be a multi-step process in a high-dimensional system, a result which is consistent with the high connectivity of the cells' complex underlying gene regulatory network.

C/EBPalpha and the pathophysiology of acute myeloid leukemia

PURPOSE OF REVIEW

The transcription factor C/EBPalpha controls differentiation and proliferation in normal granulopoiesis in a stage-specific manner. Loss of C/EBPalpha function in myeloid cells in vitro and in vivo leads to a block to myeloid differentiation similar to that which is observed in malignant cells from patients with acute myeloid leukemia. The finding of C/EBPalpha alterations in subgroups of acute myeloid leukemia patients suggests a direct link between critically decreased C/EBPalpha function and the development of the disorder.

RECENT FINDINGS

Conditional mouse models provide direct evidence that loss of C/EBPalpha function leads to the accumulation of myeloid blasts in the bone marrow. Targeted disruption of the wild type C/EBPalpha protein, while conserving the dominant-negative 30 kDa isoform of C/EBPalpha, induces an AML-like disease in mice. In hematopoietic stem cells C/EBPalpha serves to limit cell self-renewal. Finally, C/EBPalpha function is disrupted at different levels in specific subgroups of acute myeloid leukemia patients.

SUMMARY

There is evidence that impaired C/EBPalpha function contributes directly to the development of acute myeloid leukemia. Normal myeloid development and acute myeloid leukemia are now thought to reflect opposite sides of the same hematopoietic coin. Restoring C/EBPalpha function represents a promising target for novel therapeutic strategies in acute myeloid leukemia.

The molecular mechanisms that control thrombopoiesis

Our understanding of thrombopoiesis--the formation of blood platelets--has improved greatly in the last decade, with the cloning and characterization of thrombopoietin, the primary regulator of this process. Thrombopoietin affects nearly all aspects of platelet production, from self-renewal and expansion of HSCs, through stimulation of the proliferation of megakaryocyte progenitor cells, to support of the maturation of these cells into platelet-producing cells. The molecular and cellular mechanisms through which thrombopoietin affects platelet production provide new insights into the interplay between intrinsic and extrinsic influences on hematopoiesis and highlight new opportunities to translate basic biology into clinical advances.

Antagonistic effect of CCAAT enhancer-binding protein-alpha and Pax5 in myeloid or lymphoid lineage choice in common lymphoid progenitors

Lymphoid lineage-committed progenitors, such as common lymphoid progenitors (CLPs), maintain a latent myeloid differentiation potential, which can be initiated by stimulation through exogenously expressed cytokine receptors, including IL-2 receptors. Here we show that the transcription factor CCAAT enhancer-binding protein-alpha (C/EBPalpha) is promptly up-regulated in CLPs upon ectopic IL-2 stimulation. Enforced C/EBPalpha expression is sufficient to initiate myeloid differentiation from CLPs, as well as from proT and proB cells, even though proB cells do not give rise to myeloid cells after ectopic IL-2 stimulation. Expression of Pax5, a B lymphoid-affiliated transcription factor, is completely suppressed by enforced C/EBPalpha but not by ectopic IL-2 stimulation in proB cells. Introduction of Pax5 blocks ectopic IL-2 receptor-mediated myeloid lineage conversion in CLPs. These data suggest that C/EBPalpha is a proximal target of cytokine-induced lineage conversion in lymphoid progenitors. Furthermore, complete loss of Pax5 expression triggered by up-regulation of C/EBPalpha is a critical event for lineage conversion from lymphoid to myeloid lineage in CLPs and proB cells.

Protein acetylation regulates both PU.1 transactivation and Ig kappa 3' enhancer activity

Igkappa gene expression and chromatin structure change during B cell development. At the pre-B cell stage, the locus is relatively hypoacetylated on histone H3, whereas it is hyperacetylated at the plasma cell stage. We find in this study that the histone deacetylase inhibitor, trichostatin A (TSA) stimulated 3' enhancer activity through the PU.1 binding site. TSA also stimulated PU.1 transactivation potential. PU.1 activity was increased by the coactivator acetyltransferase protein, p300, and p300 physically interacted with PU.1 residues 7-30. PU.1 served as a substrate for p300 and was acetylated on lysine residues 170, 171, 206, and 208. Mutation of PU.1 lysines 170 and 171 did not affect PU.1 DNA binding, but did lower the ability of PU.1 to activate transcription in association with p300. Lysine 170 was acetylated in pre-B cells and plasmacytoma cells, but TSA treatment did not stimulate PU.1 acetylation at this residue arguing that a second mechanism can stimulate 3' enhancer activity. Using chromatin immunoprecipitation assays we found that TSA caused preferential acetylation of histone H3 at the 3' enhancer. The relevance of these studies for PU.1 function in transcription and hemopoietic development is discussed.

The molecular pathogenesis of acute myeloid leukemia

The description of the molecular pathogenesis of acute myeloid leukemias (AML) has seen dramatic progress over the last years. Two major types of genetic events have been described that are crucial for leukemic transformation: alterations in myeloid transcription factors governing hematopoietic differentiation and activating mutations of signal transduction intermediates. These processes are highly interdependent, since the molecular events changing the transcriptional control in hematopoietic progenitor cells modify the composition of signal transduction molecules available for growth factor receptors, while the activating mutations in signal transduction molecules induce alterations in the activity and expression of several transcription factors that are crucial for normal myeloid differentiation. The purpose of this article is to review the current literature describing these genetic events, their biological consequences and their clinical implications. As the article will show, the recent description of several critical transforming mutations in AML may soon give rise to more efficient and less toxic molecularly targeted therapies of this deadly disease.

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

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

Lineage promiscuous expression of transcription factors in normal hematopoiesis

Hematopoiesis has provided a valuable model for examining how genetic programs are established and executed in terms of cell fate decision. Identification of common myeloid and lymphoid progenitors allows us to directly assess the regulatory mechanisms of lineage commitment. Multiple markers of hematopoietic lineages are coexpressed in hematopoietic stem cells and progenitors, a phenomenon referred to as lineage priming. Promiscuous expression of several lineage-affiliated genes precedes lineage commitment but does not alter the biological potential of hematopoietic stem cells and multipotent progenitors. Promiscuous accessibility of multiple programs allows flexibility in cell fate commitment at the multipotent stages, indicating that transcriptional promiscuity can operate in stem cells and progenitors to control their transition from multipotency to single-lineage commitment.

PU.1 inhibits the erythroid program by binding to GATA-1 on DNA and creating a repressive chromatin structure

Transcriptional repression mechanisms are important during differentiation of multipotential hematopoietic progenitors, where they are thought to regulate lineage commitment and to extinguish alternative differentiation programs. PU.1 and GATA-1 are two critical hematopoietic transcription factors that physically interact and mutually antagonize each other's transcriptional activity and ability to promote myeloid and erythroid differentiation, respectively. We find that PU.1 inhibits the erythroid program by binding to GATA-1 on its target genes and organizing a complex of proteins that creates a repressive chromatin structure containing lysine-9 methylated H3 histones and heterochromatin protein 1. Although these features are thought to be stable aspects of repressed chromatin, we find that silencing of PU.1 expression leads to removal of the repression complex, loss of the repressive chromatin marks and reactivation of the erythroid program. This process involves incorporation of the replacement histone variant H3.3 into nucleosomes. Repression of one transcription factor bound to DNA by another transcription factor not on the DNA represents a new mechanism for downregulating an alternative gene expression program during lineage commitment of multipotential hematopoietic progenitors.

Aberrant expression of neutrophil and macrophage-related genes in a murine model for human neutrophil-specific granule deficiency

Neutrophil-specific granule deficiency involves inheritance of germline mutations in the CCAAT/enhancer-binding protein epsilon (C/EBPE) gene. Humans and mice lacking active C/EBPepsilon suffer frequent bacterial infections as a result of functionally defective neutrophils and macrophages. We hypothesized that these defects reflected dysregulation of important immune response genes. To test this, gene expression differences of peritoneally derived neutrophils and macrophages from C/EBPepsilon-/- and wild-type mice were determined with DNA microarrays. Of 283 genes, 146 known genes and 21 expressed sequence tags (ESTs) were down-regulated, and 85 known genes and 31 ESTs were up-regulated in the C/EBP-/- mice. These included genes involved in cell adhesion/chemotaxis, cytoskeletal organization, signal transduction, and immune/inflammatory responses. The cytokines CC chemokine ligand 4, CXC chemokine ligand 2, and interleukin (IL)-6, as well as cytokine receptors IL-8RB and granulocyte-colony stimulating factor, were down-regulated. Chromatin immunoprecipitation analysis identified binding of C/EBPepsilon to their promoter regions. Increased expression for lipid metabolism genes apolipoprotein E (APOE), scavenger receptor class B-1, sorting protein-related receptor containing low-density lipoprotein receptor class A repeat 1, and APOC2 in the C/EBPepsilon-/- mice correlated with reduced total cholesterol levels in these mice before and after maintenance on a high-fat diet. Also, C/EBPepsilon-deficient macrophages showed a reduced capacity to accumulate lipids. In summary, dysregulation of numerous, novel C/EBPepsilon target genes impairs innate immune response and possibly other important biological processes mediated by neutrophils and macrophages.

GATA1-mediated megakaryocyte differentiation and growth control can be uncoupled and mapped to different domains in GATA1

The DNA-binding hemopoietic zinc finger transcription factor GATA1 promotes terminal megakaryocyte differentiation and restrains abnormal immature megakaryocyte expansion. How GATA1 coordinates these fundamental processes is unclear. Previous studies of synthetic and naturally occurring mutant GATA1 molecules demonstrate that DNA-binding and interaction with the essential GATA1 cofactor FOG-1 (via the N-terminal finger) are required for gene expression in terminally differentiating megakaryocytes and for platelet production. Moreover, acquired mutations deleting the N-terminal 84 amino acids are specifically detected in megakaryocytic leukemia in human Down syndrome patients. In this study, we have systematically dissected GATA1 domains required for platelet release and control of megakaryocyte growth by ectopically expressing modified GATA1 molecules in primary GATA1-deficient fetal megakaryocyte progenitors. In addition to DNA binding, distinct N-terminal regions, including residues in the first 84 amino acids, promote platelet release and restrict megakaryocyte growth. In contrast, abrogation of GATA1-FOG-1 interaction leads to loss of differentiation, but growth of blocked immature megakaryocytes is controlled. Thus, distinct GATA1 domains regulate terminal megakaryocyte gene expression leading to platelet release and restrain megakaryocyte growth, and these processes can be uncoupled.

Transcriptional regulation of CD1D1 by Ets family transcription factors

CD1 molecules are MHC class I-like glycoproteins specialized in presenting lipid/glycolipid Ags to T cells. The distinct cell-type specific expression of CD1D1 plays an important role in the development and function of NKT cells, a unique subset of immunoregulatory T cells. However, the mechanisms regulating CD1D1 expression are largely unknown. In this study, we have characterized the upstream region of the CD1D1 gene and identified a minimal promoter region within 200 bp from the translational start site of CD1D1 that exhibits cell-type specific promoter activity. Analysis of this region revealed an Ets binding site critical for CD1D1 promoter activity. Gel shift assays and chromatin immunoprecipitation experiments showed that Elf-1 and PU.1 bind to the CD1D1 promoter. Furthermore, we found that gene disruption of Elf-1 resulted in decreased CD1D1 expression on B cells but not other cell types, whereas conditional activation of PU.1 negatively regulated CD1D1 expression in PU.1-deficient myeloid cells. These findings are the first to demonstrate that Ets proteins are involved in the transcriptional regulation of CD1D1 and that they may function uniquely in different cell types.

GATA-1 mediates auto-regulation of Gfi-1B transcription in K562 cells

Gfi-1B (growth factor independence-1B) gene is an erythroid-specific transcription factor, whose expression plays an essential role in erythropoiesis. Our laboratory has previously defined the human Gfi-1B promoter region and shown that GATA-1 mediates erythroid-specific Gfi-1B transcription. By further investigating the regulation of the Gfi-1B promoter, here we report that (i) Gfi-1B transcription is negatively regulated by its own gene product, (ii) GATA-1, instead of Gfi-1B, binds directly to the Gfi-1-like sites in the Gfi-1B promoter and (iii) Gfi-1B suppresses GATA-1-mediated stimulation of Gfi-1B promoter through their protein interaction. These results not only demonstrate that interaction of GATA-1 and Gfi-1B participates in a feedback regulatory pathway in controlling the expression of the Gfi-1B gene, but also provide the first evidence that Gfi-1B can exert its repression function by acting on GATA-1-mediated transcription without direct binding to the Gfi-1 site of the target genes. Based on these data, we propose that this negative auto-regulatory feedback loop is important in restricting the expression level of Gfi-1B, thus optimizing its function in erythroid cells.

Impaired repressor activity and biological functions of PU.1 in MEL cells induced by mutations in the acetylation motifs within the ETS domain

PU.1, a hematopoietic Ets transcription factor, is required for development of the lymphoid and myeloid lineages. We have previously shown that PU.1 functions as both a transcriptional activator and repressor through complex formation with CBP/p300 and HDAC1/mSin3A/MeCP2, respectively. To determine whether modification of PU.1 is responsible for switching its association between co-activators and co-repressors, we examined whether acetylation regulates the physical and functional activities of PU.1. PU.1 was acetylated in vivo and its repressor activity was reduced when the putative acetylation motifs in the Ets domain were mutated. The mutant cooperated with CBP similar to wild type PU.1, but insufficiently with GATA-1 and mSin3A. Whereas overexpression of wild type PU.1 induced differentiation block, growth inhibition, and apoptotic cell death in MEL erythroleukemia cells as we reported previously, overexpression of the mutant-acetylation motif PU.1 did not. Taken together, our data suggest that acetylation might regulate the biological functions of PU.1 in erythroid cells.

GATA-1: friends, brothers, and coworkers

GATA-1 is the founding member of the GATA family of transcription factors. GATA-1 and GATA family member GATA-2 are expressed in erythroid and megakaryocytic lineages, in which they play a crucial role in cell maturation and differentiation. GATA-1 regulates the transcription of many specific and nonspecific erythroid genes by binding to DNA at the consensus sequence WGATAR, which is recognized by all of the GATA family of transcription factors. However, it was identified in eosinophilic cells and also in Sertoli cells in testis. Its activity depends on close cooperation with a functional network of cofactors, among them Friend of GATA, PU.1, and CBP/p300. The GATA-1 protein structure has been well described and includes two zinc fingers that are directly involved in the interaction with DNA and other proteins in vivo. GATA-1 mutations in the zinc fingers can cause deregulation of required interactions and lead to severe dysfunction in the hematopoietic system.

Effect of transcription-factor concentrations on leukemic stem cells

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

Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their differentiation

The PU.1 transcription factor is a key regulator of hematopoietic development, but its role at each hematopoietic stage remains unclear. In particular, the expression of PU.1 in hematopoietic stem cells (HSCs) could simply represent "priming" of genes related to downstream myelolymphoid lineages. By using a conditional PU.1 knock-out model, we here show that HSCs express PU.1, and its constitutive expression is necessary for maintenance of the HSC pool in the bone marrow. Bone marrow HSCs disrupted with PU.1 in situ could not maintain hematopoiesis and were outcompeted by normal HSCs. PU.1-deficient HSCs also failed to generate the earliest myeloid and lymphoid progenitors. PU.1 disruption in granulocyte/monocyte-committed progenitors blocked their maturation but not proliferation, resulting in myeloblast colony formation. PU.1 disruption in common lymphoid progenitors, however, did not prevent their B-cell maturation. In vivo disruption of PU.1 in mature B cells by the CD19-Cre locus did not affect B-cell maturation, and PU.1-deficient mature B cells displayed normal proliferation in response to mitogenic signals including the cross-linking of surface immunoglobulin M (IgM). Thus, PU.1 plays indispensable and distinct roles in hematopoietic development through supporting HSC self-renewal as well as commitment and maturation of myeloid and lymphoid lineages.

Predicting the effect of transcription therapy in hematologic malignancies

Molecular lesions of genes encoding for transcriptional regulatory proteins are common oncogenic events in hematologic malignancies. Transcriptional activation and repression both occur by virtue of the choreographed recruitment of multisubunit cofactor complexes to target gene loci. As a consequence, the three-dimensional structure of the target gene is altered and its potential to support transcription is increased or decreased. The complexity of the transcriptional process offers a rich substrate for designing therapeutic agents. The objective of such 'transcription therapy' is to regain control over cohorts of target genes and restore the normal genetic and epigenetic programming of the cancer cell. The success of all-trans retinoic acid in the treatment of acute promyelocytic leukemia indicates that transcription therapy can be highly effective and safe. A classification scheme of these therapeutic strategies is proposed herein, which allows predictions to be made regarding specificity, efficacy, disease spectrum and side effects. This framework could help facilitate discussion of the mechanisms of action of transcription therapy drugs as well as the design of preclinical and clinical trials in the future.

Potential autoregulation of transcription factor PU.1 by an upstream regulatory element

Regulation of the hematopoietic transcription factor PU.1 (Spi-1) plays a critical role in the development of white cells, and abnormal expression of PU.1 can lead to leukemia. We previously reported that the PU.1 promoter cannot induce expression of a reporter gene in vivo, and cell-type-specific expression of PU.1 in stable lines was conferred by a 3.4-kb DNA fragment including a DNase I hypersensitive site located 14 kb upstream of the transcription start site. Here we demonstrate that this kb -14 site confers lineage-specific reporter gene expression in vivo. This kb -14 upstream regulatory element contains two 300-bp regions which are highly conserved in five mammalian species. In Friend virus-induced erythroleukemia, the spleen focus-forming virus integrates into the PU.1 locus between these two conserved regions. DNA binding experiments demonstrated that PU.1 itself and Elf-1 bind to a highly conserved site within the proximal homologous region in vivo. A mutation of this site abolishing binding of PU.1 and Elf-1 led to a marked decrease in the ability of this upstream element to direct activity of reporter gene in myelomonocytic cell lines. These data suggest that a potential positive autoregulatory loop mediated through an upstream regulatory element is essential for proper PU.1 gene expression.

Transcriptional regulation in myelopoiesis: Hematopoietic fate choice, myeloid differentiation, and leukemogenesis

Myeloid cells (granulocytes and monocytes) are derived from multipotent hematopoietic stem cells. Gene transcription plays a critical role in hematopoietic differentiation. However, there is no single transcription factor that is expressed exclusively by myeloid cells and that, alone, acts as a "master" regulator of myeloid fate choice. Rather, myeloid gene expression is controlled by the combinatorial effects of several key transcription factors. Hematopoiesis has traditionally been viewed as linear and hierarchical, but there is increasing evidence of plasticity during blood cell development. Transcription factors strongly influence cellular lineage during hematopoiesis and expression of some transcription factors can alter the fate of developing hematopoietic progenitor cells. PU.1 and CCAAT/enhancer-binding protein alpha (C/EBPalpha) regulate expression of numerous myeloid genes, and gene disruption studies have shown that they play essential, nonredundant roles in myeloid cell development. They function in cooperation with other transcription factors, co-activators, and co-repressors to regulate genes in the context of chromatin. Because of their essential roles in regulating myeloid genes and in myeloid cell development, it has been hypothesized that abnormal expression of PU.1 and C/EBPalpha would contribute to aberrant myeloid differentiation, i.e. acute leukemia. Such a direct link has been elusive until recently. However, there is now persuasive evidence that mutations in both PU.1 and C/EBPalpha contribute directly to development of acute myelogenous leukemia. Thus, normal myeloid development and acute leukemia are now understood to represent opposite sides of the same hematopoietic coin.

GATA Transcription Factors Inhibit Cytokine-dependent Growth and Survival of a Hematopoietic Cell Line through the Inhibition of STAT3 Activity

Although GATA-1 and GATA-2 were shown to be essential for the development of hematopoietic cells by gene targeting experiments, they were also reported to inhibit the growth of hematopoietic cells. Therefore, in this study, we examined the effects of GATA-1 and GATA-2 on cytokine signals. A tamoxifen-inducible form of GATA-1 (GATA-1/ERT) showed a minor inhibitory effect on interleukin-3 (IL-3)-dependent growth of an IL-3-dependent cell line Ba/F3. On the other hand, it drastically inhibited TPO-dependent growth and gp130-mediated growth/survival of Ba/F3. Similarly, an estradiol-inducible form of GATA-2 (GATA-2/ER) disrupted thrombopoietin (TPO)-dependent growth and gp130-mediated growth/survival of Ba/F3. As for this mechanism, we found that both GATA-1 and GATA-2 directly bound to STAT3 both in vitro and in vivo and inhibited its DNA-binding activity in gel shift assays and chromatin immunoprecipitation assays, whereas they hardly affected STAT5 activity. In addition, endogenous GATA-1 was found to interact with STAT3 in normal megakaryocytes, suggesting that GATA-1 may inhibit STAT3 activity in normal hematopoietic cells. Furthermore, we found that GATA-1 suppressed STAT3 activity through its N-zinc finger domain. Together, these results suggest that, besides the roles as transcription factors, GATA family proteins modulate cytokine signals through protein-protein interactions, thereby regulating the growth and survival of hematopoietic cells.

Stem Cell Fate Specification: Role of Master Regulatory Switch Transcription Factor PU.1 in Differential Hematopoiesis

PU.1 is a versatile hematopoietic cell-specific ETS-family transcriptional regulator required for the development of both the inborn and the adaptive immunity, owing to its potential ability to regulate the expression of multiple genes specific for different lineages during normal hematopoiesis. It functions in a cell-autonomous manner to control the proliferation and differentiation, predominantly of lymphomyeloid progenitors, by binding to the promoters of many myeloid genes including the macrophage colony-stimulating factor (M-CSF) receptor, granulocyte-macrophage (GM)-CSF receptor alpha, and CD11b. In B cells, it regulates the immunoglobulin lambda 2-4 and kappa 3' enhancers, and J chain promoters. Besides lineage development, PU.1 also directs homing and long-term engraftment of hematopoietic progenitors to the bone marrow. PU.1 gene disruption causes a cell-intrinsic defect in hematopoietic progenitor cells, recognized by an aberrant myeloid and B lymphoid development. It also immortalizes erythroblasts when overexpressed in many cell lines. Although a number of reviews have been published on its functional significance, in the following review we attempted to consolidate information about the differential participation and role of transcription factor PU.1 at various stages of hematopoietic development beginning from stem cell proliferation, lineage commitment and terminal differentiation into distinct blood cell types, and leukemogenesis.

Interaction between GATA and the C/EBP family of transcription factors is critical in GATA-mediated suppression of adipocyte differentiation

We have previously demonstrated that GATA-2 and GATA-3 are expressed in adipocyte precursors and control the preadipocyte-to-adipocyte transition. Constitutive expression of both GATA-2 and GATA-3 suppressed adipocyte differentiation, partially through direct binding to the peroxisome proliferator-activated receptor gamma (PPARgamma) promoter and suppression of its basal activity. In the present study, we demonstrate that both GATA-2 and GATA-3 form protein complexes with CCAAT/enhancer binding protein alpha (C/EBPalpha) and C/EBPbeta, members of a family of transcription factors that are integral to adipogenesis. We mapped this interaction to the basic leucine zipper domain of C/EBPalpha and a region adjacent to the carboxyl zinc finger of GATA-2. The interaction between GATA and C/EBP factors is critical for the ability of GATA to suppress adipocyte differentiation. Thus, these results show that in addition to its previously recognized function in suppressing PPARgamma transcriptional activity, interaction of GATA factors with C/EBP is necessary for their ability to negatively regulate adipogenesis.

Molecular genetics of T cell development

T cell development is guided by a complex set of transcription factors that act recursively, in different combinations, at each of the developmental choice points from T-lineage specification to peripheral T cell specialization. This review describes the modes of action of the major T-lineage-defining transcription factors and the signal pathways that activate them during intrathymic differentiation from pluripotent precursors. Roles of Notch and its effector RBPSuh (CSL), GATA-3, E2A/HEB and Id proteins, c-Myb, TCF-1, and members of the Runx, Ets, and Ikaros families are critical. Less known transcription factors that are newly recognized as being required for T cell development at particular checkpoints are also described. The transcriptional regulation of T cell development is contrasted with that of B cell development, in terms of their different degrees of overlap with the stem-cell program and the different roles of key transcription factors in gene regulatory networks leading to lineage commitment.

Dynamic regulation of PU.1 expression in multipotent hematopoietic progenitors

PU.1 is an Ets family transcription factor that is essential for fetal liver hematopoiesis. We have generated a PU.1^gfp reporter strain that allowed us to examine the expression of PU.1 in all hematopoietic cell lineages and their early progenitors. Within the bone marrow progenitor compartment, PU.1 is highly expressed in the hematopoietic stem cell, the common lymphoid progenitor, and a proportion of common myeloid progenitors (CMPs). Based on Flt3 and PU.1 expression, the CMP could be divided into three subpopulations, Flt3⁺ PU.1^hi, Flt3⁻ PU.1^hi, and Flt3⁻ PU.1^lo CMPs. Colony-forming assays and in vivo lineage reconstitution demonstrated that the Flt3⁺ PU.1^hi and Flt3⁻ PU.1^hi CMPs were efficient precursors for granulocyte/macrophage progenitors (GMPs), whereas the Flt3⁻ PU.1^lo CMPs were highly enriched for committed megakaryocyte/erythrocyte progenitors (MEPs). CMPs have been shown to rapidly differentiate into GMPs and MEPs in vitro. Interestingly, short-term culture revealed that the Flt3⁺ PU.1^hi and Flt3⁻ PU.1^hi CMPs rapidly became CD16/32^high (reminiscent of GMPs) in culture, whereas the Flt3⁻ PU.1^lo CMPs were the immediate precursors of the MEP. Thus, down-regulation of PU.1 expression in the CMP is the first molecularly identified event associated with the restriction of differentiation to erythroid and megakaryocyte lineages.

Ski negatively regulates erythroid differentiation through its interaction with GATA1

The Ski oncoprotein dramatically affects cell growth, differentiation, and/or survival. Recently, Ski was shown to act in distinct signaling pathways including those involving nuclear receptors, transforming growth factor beta, and tumor suppressors. These divergent roles of Ski are probably dependent on Ski's capacity to bind multiple partners with disparate functions. In particular, Ski alters the growth and differentiation program of erythroid progenitor cells, leading to malignant leukemia. However, the mechanism underlying this important effect has remained elusive. Here we show that Ski interacts with GATA1, a transcription factor essential in erythropoiesis. Using a Ski mutant deficient in GATA1 binding, we show that this Ski-GATA1 interaction is critical for Ski's ability to repress GATA1-mediated transcription and block erythroid differentiation. Furthermore, the repression of GATA1-mediated transcription involves Ski's ability to block DNA binding of GATA1. This finding is in marked contrast to those in previous reports on the mechanism of repression by Ski, which have described a model involving the recruitment of corepressors into DNA-bound transcription complexes. We propose that Ski cooperates in the process of transformation in erythroid cells by interfering with GATA1 function, thereby contributing to erythroleukemia.

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.

High-dimensional switches and the modelling of cellular differentiation

Many genes have been identified as driving cellular differentiation, but because of their complex interactions, the understanding of their collective behaviour requires mathematical modelling. Intriguingly, it has been observed in numerous developmental contexts, and particularly haematopoiesis, that genes regulating differentiation are initially co-expressed in progenitors despite their antagonism, before one is upregulated and others downregulated. We characterise conditions under which three classes of generic "master regulatory networks", modelled at the molecular level after experimentally observed interactions (including bHLH protein dimerisation), and including an arbitrary number of antagonistic components, can behave as a "multi-switch", directing differentiation in an all-or-none fashion to a specific cell-type chosen among more than two possible outcomes. bHLH dimerisation networks can readily display coexistence of many antagonistic factors when competition is low (a simple characterisation is derived). Decision-making can be forced by a transient increase in competition, which could correspond to some unexplained experimental observations related to Id proteins; the speed of response varies with the initial conditions the network is subjected to, which could explain some aspects of cell behaviour upon reprogramming. The coexistence of antagonistic factors at low levels, early in the differentiation process or in pluripotent stem cells, could be an intrinsic property of the interaction between those factors, not requiring a specific regulatory system.

Clonogenicity, gene expression and phenotype during neutrophil versus erythroid differentiation of cytokine-stimulated CD34+human marrow cellsin vitro

With the objective to correlate clonogenicity, gene expression and phenotype during differentiation, human bone marrow CD34(+) cells were cultured in vitro to stimulate erythroid or neutrophil development, and sorted into five subpopulations according to their surface expression of CD15/CD33 and blood group antigen A/CD117 respectively. Sorted cells were cultured in methylcellulose and analysed by real-time reverse transcription polymerase chain reaction for expression of neutrophil and erythroid marker genes. Surface expression of CD15 coincided with restriction to neutrophil/monocyte differentiation and A antigen with restriction to erythroid differentiation. GATA-2 mRNA was down-regulated during both neutrophil and erythroid maturation, whereas GATA-1, SCL, ABO, erythropoietin receptor, Kell, glycophorin A, beta-globin and alpha-haemoglobin stabilizing protein were up-regulated during erythroid differentiation and silenced during neutrophil differentiation. CCAAT/enhancer-binding protein (C/EBP)-alpha, PU.1, granulocyte colony-stimulating factor receptor, PR3, C/EBP-epsilon and lactoferrin were sequentially expressed during neutrophil differentiation but rapidly down-regulated during the early erythroid stages. Nuclear factor erythroid-derived 2 (NF-E2) and glycophorin C were expressed both during neutrophil and erythroid differentiation. Our data support the notion of early expression of several lineage-associated genes prior to actual lineage commitment, defined by surface expression of CD15 and A antigen as markers for definitive neutrophil/monocyte and erythroid differentiation respectively. Previous findings, primarily from cell lines and mouse models, have been extended to adult human haematopoiesis.