BSAP can repress enhancer activity by targeting PU.1 function

Pax5 Negatively Regulates Osteoclastogenesis through Downregulation of Blimp1

Paired box protein 5 (Pax5) is a crucial transcription factor responsible for B-cell lineage specification and commitment. In this study, we identified a negative role of Pax5 in osteoclastogenesis. The expression of Pax5 was time-dependently downregulated by receptor activator of nuclear factor kappa B (RANK) ligand (RANKL) stimulation in osteoclastogenesis. Osteoclast (OC) differentiation and bone resorption were inhibited (68.9% and 48% reductions, respectively) by forced expression of Pax5 in OC lineage cells. Pax5 led to the induction of antiosteoclastogenic factors through downregulation of B lymphocyte-induced maturation protein 1 (Blimp1). To examine the negative role of Pax5 in vivo, we generated Pax5 transgenic (Pax5^Tg) mice expressing the human Pax5 transgene under the control of the tartrate-resistant acid phosphatase (TRAP) promoter, which is expressed mainly in OC lineage cells. OC differentiation and bone resorption were inhibited (54.2–76.9% and 24.0–26.2% reductions, respectively) in Pax5^Tg mice, thereby contributing to the osteopetrotic-like bone phenotype characterized by increased bone mineral density (13.0–13.6% higher), trabecular bone volume fraction (32.5–38.1% higher), trabecular thickness (8.4–9.0% higher), and trabecular number (25.5–26.7% higher) and decreased trabecular spacing (9.3–10.4% lower) compared to wild-type control mice. Furthermore, the number of OCs was decreased (48.8–65.3% reduction) in Pax5^Tg mice. These findings indicate that Pax5 plays a negative role in OC lineage specification and commitment through Blimp1 downregulation. Thus, our data suggest that the Pax5–Blimp1 axis is crucial for the regulation of RANKL-induced osteoclastogenesis.

RAG-Mediated DNA Breaks Attenuate PU.1 Activity in Early B Cells through Activation of a SPIC-BCLAF1 Complex

Early B cell development is regulated by stage-specific transcription factors. PU.1, an ETS-family transcription factor, is essential for coordination of early B cell maturation and immunoglobulin gene (Ig) rearrangement. Here we show that RAG DNA double-strand breaks (DSBs) generated during Ig light chain gene (Igl) rearrangement in pre-B cells induce global changes in PU.1 chromatin binding. RAG DSBs activate a SPIC/BCLAF1 transcription factor complex that displaces PU.1 throughout the genome and regulates broad transcriptional changes. SPIC recruits BCLAF1 to gene-regulatory elements that control expression of key B cell developmental genes. The SPIC/BCLAF1 complex suppresses expression of the SYK tyrosine kinase and enforces the transition from large to small pre-B cells. These studies reveal that RAG DSBs direct genome-wide changes in ETS transcription factor activity to promote early B cell development.

ETS-family transcription factors are key regulators of early B cell development. Soodgupta et al. show that RAG-induced DNA breaks generated during antigen receptor gene recombination activate a SPIC/BCLAF1 transcription factor complex that counters PU.1 activity and regulates gene expression changes to promote transition from large to small pre-B cells.

Role of mir-15a/16-1 in early B cell development in a mouse model of chronic lymphocytic leukemia

In both human chronic lymphocytic leukemia (CLL) and the New Zealand Black (NZB) murine model of CLL, decreased levels of microRNAs miR-15a/16 play an important role in the disease. Here we investigate the effects of this microRNA on early steps of B cell development and the capacity of miR-15a-deficient hematopoietic stem cells (HSC) and B1 progenitor cells (B1P) to reproduce CLL-like phenotype both in vitro and in vivo. Our results demonstrate that both miR-15a deficient HSC and B1P cells are capable of repopulating irradiated recipients and produce higher numbers of B1 cells than sources with normal miR-15a/16 levels. Furthermore, induced pluripotent stem (iPS) cells derived for the first time from NZB mice, provided insights into the B cell differentiation roadblock inherent in this strain. In addition, exogenously delivered miR-15a into the NZB derived B cell line provided valuable clues into novel targets such as Mmp10 and Mt2. Our data supports the hypothesis that miR-15a/16 deficient stem cells and B1Ps experience a maturation blockage, which contributes to B1 cells bias in development. This work will help understand the role of miR-15a in early events of CLL and points to B1P cells as potential cells of origin for this incurable disease.

Role of the glucocorticoid-induced leucine zipper gene in dexamethasone-induced inhibition of mouse neutrophil migration via control of annexin A1 expression

The glucocorticoid-induced leucine zipper (GILZ) gene is a pivotal mediator of the anti-inflammatory effects of glucocorticoids (GCs) that are known to regulate the function of both adaptive and innate immunity cells. Our aim was to investigate the role of GILZ in GC-induced inhibition of neutrophil migration, as this role has not been investigated before. We found that GILZ expression was induced by dexamethasone (DEX), a synthetic GC, in neutrophils, and that it regulated migration of these cells into inflamed tissues under DEX treatment. Of note, inhibition of neutrophil migration was not observed in GILZ-knockout mice with peritonitis that were treated by DEX. This was because DEX was unable to up-regulate annexin A1 (Anxa1) expression in the absence of GILZ. Furthermore, we showed that GILZ mediates Anxa1 induction by GCs by transactivating Anxa1 expression at the promoter level via binding with the transcription factor, PU.1. The present findings shed light on the role of GILZ in the mechanism of induction of Anxa1 by GCs. As Anxa1 is an important protein for the resolution of inflammatory response, GILZ may represent a new pharmacologic target for treatment of inflammatory diseases.-Ricci, E., Ronchetti, S., Pericolini, E., Gabrielli, E., Cari, L., Gentili, M., Roselletti, E., Migliorati, G., Vecchiarelli, A., Riccardi, C. Role of the glucocorticoid-induced leucine zipper gene in dexamethasone-induced inhibition of mouse neutrophil migration via control of annexin A1 expression.

E2A Antagonizes PU.1 Activity through Inhibition of DNA Binding

Antagonistic interactions between transcription factors contribute to cell fate decisions made by multipotent hematopoietic progenitor cells. Concentration of the transcription factor PU.1 affects myeloid/lymphoid development with high levels of PU.1 directing myeloid cell fate acquisition at the expense of B cell differentiation. High levels of PU.1 may be required for myelopoiesis in order to overcome inhibition of its activity by transcription factors that promote B cell development. The B cell transcription factors, E2A and EBF, are necessary for commitment of multipotential progenitors and lymphoid primed multipotential progenitors to lymphocytes. In this report we hypothesized that factors required for early B cell commitment would bind to PU.1 and antagonize its ability to induce myeloid differentiation. We investigated whether E2A and/or EBF associate with PU.1. We observed that the E2A component, E47, but not EBF, directly binds to PU.1. Additionally E47 represses PU.1-dependent transactivation of the MCSFR promoter through antagonizing PU.1's ability to bind to DNA. Exogenous E47 expression in hematopoietic cells inhibits myeloid differentiation. Our data suggest that E2A antagonism of PU.1 activity contributes to its ability to commit multipotential hematopoietic progenitors to the lymphoid lineages.

A novel pax5-binding regulatory element in the igκ locus

The Igκ locus undergoes a variety of different molecular processes during B cell development, including V(D)J rearrangement and somatic hypermutations (SHM), which are influenced by cis regulatory regions (RRs) within the locus. The Igκ locus includes three characterized RRs termed the intronic (iEκ), 3′Eκ, and Ed enhancers. We had previously noted that a region of DNA upstream of the iEκ and matrix attachment region (MAR) was necessary for demethylation of the locus in cell culture. In this study, we further characterized this region, which we have termed Dm, for demethylation element. Pre-rearranged Igκ transgenes containing a deletion of the entire Dm region, or of a Pax5-binding site within the region, fail to undergo efficient CpG demethylation in mature B cells in vivo. Furthermore, we generated mice with a deletion of the full Dm region at the endogenous Igκ locus. The most prominent phenotype of these mice is reduced SHM in germinal center B cells in Peyer’s patches. In conclusion, we propose the Dm element as a novel Pax5-binding cis regulatory element, which works in concert with the known enhancers, and plays a role in Igκ demethylation and SHM.

B-lineage transcription factors and cooperating gene lesions required for leukemia development

Differentiation of hematopoietic stem cells into B lymphocytes requires the concerted action of specific transcription factors, such as RUNX1, IKZF1, E2A, EBF1 and PAX5. As key determinants of normal B-cell development, B-lineage transcription factors are frequently deregulated in hematological malignancies, such as B-cell precursor acute lymphoblastic leukemia (BCP-ALL), and affected by either chromosomal translocations, gene deletions or point mutations. However, genetic aberrations in this developmental pathway are generally insufficient to induce BCP-ALL, and often complemented by genetic defects in cytokine receptors and tyrosine kinases (IL-7Rα, CRLF2, JAK2 and c-ABL1), transcriptional cofactors (TBL1XR1, CBP and BTG1), as well as the regulatory pathways that mediate cell-cycle control (pRB and INK4A/B). Here we provide a detailed overview of the genetic pathways that interact with these B-lineage specification factors, and describe how mutations affecting these master regulators together with cooperating lesions drive leukemia development.

A developmentally controlled competitive STAT5-PU.1 DNA binding mechanism regulates activity of the Ig κ E3' enhancer

Stage-specific rearrangement of Ig H and L chain genes poses an enigma because both processes use the same recombinatorial machinery, but the H chain locus is accessible at the pro-B cell stage, whereas the L chain loci become accessible at the pre-B cell stage. Transcription factor STAT5 is a positive-acting factor for rearrangement of distal V(H) genes, but attenuation of IL-7 signaling and loss of activated STAT5 at the pre-B cell stage corresponds with Igκ locus accessibility and rearrangement, suggesting that STAT5 plays an inhibitory role at this locus. Indeed, loss of IL-7 signaling correlates with increased activity at the Igκ intron enhancer. However, the κE3' enhancer must also be regulated as this enhancer plays a role in Igκ rearrangement. We show in this study that STAT5 can repress κE3' enhancer activity. We find that STAT5 binds to a site that overlaps the κE3' PU.1 binding site. We observed reciprocal binding by STAT5 and PU.1 to the κE3' enhancer in primary bone marrow cells, STAT5 and PU.1 retrovirally transduced pro-B cell lines, or embryonic stem cells induced to differentiate into B lineage cells. Binding by STAT5 corresponded with low occupancy of other enhancer binding proteins, whereas PU.1 binding corresponded with recruitment of IRF4 and E2A to the κE3' enhancer. We also find that IRF4 expression can override the repressive activity of STAT5. We propose a novel PU.1/STAT5 displacement model during B cell development, and this, coupled with increased IRF4 and E2A activity, regulates κE3' enhancer function.

YY1 controls immunoglobulin class switch recombination and nuclear activation-induced deaminase levels

Activation-induced deaminase (AID) is an enzyme required for class switch recombination (CSR) and somatic hypermutation (SHM), processes that ensure antibody maturation and expression of different immunoglobulin isotypes. AID function is tightly regulated by tissue- and stage-specific expression, nuclear localization, and protein stability. Transcription factor YY1 is crucial for early B cell development, but its function at late B cell stages is unknown. Here, we show that YY1 conditional knockout in activated splenic B cells interferes with CSR. Knockout of YY1 did not affect B cell proliferation, transcription of the AID and IgM genes, or levels of various switch region germ line transcripts. However, we show that YY1 physically interacts with AID and controls the accumulation of nuclear AID, at least in part, by increasing nuclear AID stability. We show for the first time that YY1 plays a novel role in CSR and controls nuclear AID protein levels.

The IgH locus 3' regulatory region: pulling the strings from behind

Antigen receptor gene loci are among the most complex in mammals. The IgH locus, encoding the immunoglobulin heavy chain (IgH) in B-lineage cells, undergoes major transcription-dependent DNA remodeling events, namely V(D)J recombination, Ig class-switch recombination (CSR), and somatic hypermutation (SHM). Various cis-regulatory elements (encompassing promoters, enhancers, and chromatin insulators) recruit multiple nuclear factors in order to ensure IgH locus regulation by tightly orchestrated physical and/or functional interactions. Among major IgH cis-acting regions, the large 3' regulatory region (3'RR) located at the 3' boundary of the locus includes several enhancers and harbors an intriguing quasi-palindromic structure. In this review, we report progress insights made over the past decade in order to describe in more details the structure and functions of IgH 3'RRs in mouse and human. Generation of multiple cellular, transgenic and knock-out models helped out to decipher the function of the IgH 3' regulatory elements in the context of normal and pathologic B cells. Beside its interest in physiology, the challenge of elucidating the locus-wide cross talk between distant cis-regulatory elements might provide useful insights into the mechanisms that mediate oncogene deregulation after chromosomal translocations onto the IgH locus.

The long winding road toward understanding the molecular mechanisms for B-cell suppression by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Suppression of humoral immune responses by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was first reported in the mid-1970s. Since this initial observation, much effort has been devoted by many laboratories toward elucidation of the cellular and molecular mechanisms responsible for the profound impairment of humoral immune responses by TCDD, which is characterized by decreased B cell to plasma cell differentiation and suppression of immunoglobulin production. These efforts have led to a significant body of research demonstrating a direct effect of TCDD on B-cell maturation and function as well as a requisite but as yet undefined role of the aryl hydrocarbon receptor (AhR) in these effects. Likewise, a number of molecular targets putatively involved in mediating B-cell dysfunction by TCDD, and other AhR ligands, have been identified. However, our current understanding has primarily relied on findings from mouse models, and the translation of this knowledge to effects on human B cells and humoral immunity in humans is less clear. Therefore, a current challenge is to determine how TCDD and the AhR affect human B cells. Efforts have been made in this direction but continued progress in developing adequate human models is needed. An in-depth discussion of these advances and limitations in elucidating the cellular and molecular mechanisms putatively involved in the suppression of B-cell function by TCDD as well as the implications on human diseases associated in epidemiological studies with exposure to TCDD and dioxin-like compounds is the primary focus of this review.

Highly cooperative recruitment of Ets-1 and release of autoinhibition by Pax5

Pax5 (paired box binding factor 5) is a critical regulator of transcription and lineage commitment in B lymphocytes. In B cells, mb-1 (Ig-alpha/immunoglobulin-associated alpha) promoter transcription is activated by Pax5 through its recruitment of E74-like transforming sequence (Ets) family proteins to a composite site, the P5-EBS (Pax5-Ets binding site). Previously, X-ray crystallographic analysis revealed a network of contacts between the DNA-binding domains of Pax5 and Ets-1 while bound to the P5-EBS. Here, we report that Pax5 assembles these ternary complexes via highly cooperative interactions that overcome the autoinhibition of Ets-1. Using recombinant proteins, we calculated K(d(app)) values for the binding of Pax5, Ets-1, and GA-binding proteins, separately or together, to the P5-EBS. By itself, Pax5 binds the P5-EBS with high affinity (K(d) approximately equal 2 nM). Ets-1(331-440) bound the P5-EBS by itself with low affinity (K(d)=136 nM). However, autoinhibited Ets-1(280-440) alone does not bind detectably to the suboptimal sequences of the P5-EBS. Recruitment of Ets-1(331-440) or Ets-1(280-440) resulted in highly efficient ternary complex assembly with Pax5. Pax5 counteracts autoinhibition and increases binding of Ets-1 of the mb-1 promoter by >1000-fold. Mutation of Pax5 Gln22 to alanine (Q22A) enhances promoter binding by Pax5; however, Q22A greatly reduces recruitment of Ets-1(331-440) and Ets-1(280-440) by Pax5 (8.9- or >300-fold, respectively). Thus, Gln22 of Pax5 is essential for overcoming Ets-1 autoinhibition. Pax5 wild type and Q22A each recruited GA-binding protein alpha/beta1 to the mb-1 promoter with similar affinities, but recruitment was less efficient than that of Ets-1 (reduced by approximately 8-fold). Our results suggest a mechanism that allows Pax5 to overcome autoinhibition of Ets-1 DNA binding. In summary, these data illustrate requirements for partnerships between Ets proteins and Pax5.

Predicting combinatorial binding of transcription factors to regulatory elements in the human genome by association rule mining

Background

Cis-acting transcriptional regulatory elements in mammalian genomes typically contain specific combinations of binding sites for various transcription factors. Although some cis-regulatory elements have been well studied, the combinations of transcription factors that regulate normal expression levels for the vast majority of the 20,000 genes in the human genome are unknown. We hypothesized that it should be possible to discover transcription factor combinations that regulate gene expression in concert by identifying over-represented combinations of sequence motifs that occur together in the genome. In order to detect combinations of transcription factor binding motifs, we developed a data mining approach based on the use of association rules, which are typically used in market basket analysis. We scored each segment of the genome for the presence or absence of each of 83 transcription factor binding motifs, then used association rule mining algorithms to mine this dataset, thus identifying frequently occurring pairs of distinct motifs within a segment.

Results

Support for most pairs of transcription factor binding motifs was highly correlated across different chromosomes although pair significance varied. Known true positive motif pairs showed higher association rule support, confidence, and significance than background. Our subsets of high-confidence, high-significance mined pairs of transcription factors showed enrichment for co-citation in PubMed abstracts relative to all pairs, and the predicted associations were often readily verifiable in the literature.

Conclusion

Functional elements in the genome where transcription factors bind to regulate expression in a combinatorial manner are more likely to be predicted by identifying statistically and biologically significant combinations of transcription factor binding motifs than by simply scanning the genome for the occurrence of binding sites for a single transcription factor.

The regulation of the B-cell gene expression programme by Pax5

The activity of the transcription factor paired box gene 5 (Pax5) is essential for many aspects of B lymphopoiesis including the initial commitment to the lineage, immunoglobulin rearrangement, pre-B cell receptor signalling and maintaining cell identity in mature B cells. Deregulated or reduced Pax5 activity has also been implicated in B-cell malignancies both in human disease and mouse models. Candidate gene approaches and biochemical analysis have revealed that Pax5 regulates B lymphopoiesis by concurrently activating B cell-specific gene expression as well as repressing the expression of genes, many of which are associated with non-B cell lineages. These studies have been recently complemented with more exhaustive microarray studies, which have identified and validated a large panel of Pax5 target genes. These target genes reveal a gene regulatory network, with Pax5 at its centre that controls the B-cell gene expression programme.

B cell activator PAX5 promotes lymphomagenesis through stimulation of B cell receptor signaling

The presumed involvement of paired box gene 5 (PAX5) in B-lymphomagenesis is based largely on the discovery of Pax5-specific translocations and somatic hypermutations in non-Hodgkin lymphomas. Yet mechanistically, the contribution of Pax5 to neoplastic growth remains undeciphered. Here we used 2 Myc-induced mouse B lymphoma cell lines, Myc5-M5 and Myc5-M12, which spontaneously silence Pax5. Reconstitution of these cells with Pax5-tamoxifen receptor fusion protein (Pax5ER(TAM)) increased neoplastic growth in a hormone-dependent manner. Conversely, expression of dominant-negative Pax5 in murine lymphomas and Pax5 knockdown in human lymphomas negatively affected cell expansion. Expression profiling revealed that Pax5 was required to maintain mRNA levels of several crucial components of B cell receptor (BCR) signaling, including CD79a, a protein with the immunoreceptor tyrosine-based activation motif (ITAM). In contrast, expression of 2 known ITAM antagonists, CD22 and PIR-B, was suppressed. The key role of BCR/ITAM signaling in Pax5-dependent lymphomagenesis was corroborated in Syk, an ITAM-associated tyrosine kinase. Moreover, we observed consistent expression of phosphorylated BLNK, an activated BCR adaptor protein, in human B cell lymphomas. Thus, stimulation of neoplastic growth by Pax5 occurs through BCR and is sensitive to genetic and pharmacological inhibitors of this pathway.

Overexpression of a hematopoietic transcriptional regulator EDAG induces myelopoiesis and suppresses lymphopoiesis in transgenic mice

Erythroid differentiation-associated gene (EDAG) is a hematopoietic tissue-specific gene that is highly expressed in the earliest CD34+ lin- bone marrow (BM) cells and involved in the proliferation and differentiation of hematopoietic cells. To investigate the role of EDAG in hematopoiesis, we established an EDAG transgenic mouse model driven by human CD11a promoter. The transgenic mice showed increased mortality with severe organ infiltration by neutrophils, and the homeostasis of hematopoiesis was broken. The myelopoiesis was enhanced with expansion of myeloid cells in BM, increased peripheral granulocytes and extramedullary myelopoiesis in spleen. In contrast to myeloid cells, the lymphoid commitment was severely impaired with the B lymphopoiesis blocked at the transition from pro/pre-B I to pre-B II stage in BM and T thymocytes development blocked at the most immature stage (DN I). Moreover, we showed that EDAG was a transcriptional regulator which had transactivation activity and regulated the expression of several key transcription factors such as PU.1 and Pax5 in transgenic hematopoietic stem cells. These data suggested that EDAG was a key transcriptional regulator in maintaining the homeostasis of hematopoietic lineage commitment.

The transcriptional repressor GFI-1 antagonizes PU.1 activity through protein-protein interaction

Mice lacking the zinc finger transcriptional repressor protein GFI-1 are neutropenic. These mice generate abnormal immature myeloid cells exhibiting characteristics of both macrophages and granulocytes. Furthermore, Gfi-1(-/-) mice are highly susceptible to bacterial infection. Interestingly, Gfi-1(-/-) myeloid cells overexpress target genes of the PU.1 transcription factor such as the macrophage colony-stimulating factor receptor and PU.1 itself. We therefore determined whether GFI-1 modulates the transcriptional activity of PU.1. Our data demonstrate that GFI-1 physically interacts with PU.1, repressing PU.1-dependent transcription. This repression is functionally significant, as GFI-1 blocked PU.1-induced macrophage differentiation of a multipotential hematopoietic progenitor cell line. Retroviral expression of GFI-1 in primary murine hematopoietic progenitors increased granulocyte differentiation at the expense of macrophage differentiation. We interbred Gfi-1(+/-) and PU.1(+/-) mice and observed that heterozygosity at the PU.1 locus partially rescued the Gfi-1(-/-) mixed myeloid lineage phenotype, but failed to restore granulocyte differentiation. Our data demonstrate that GFI-1 represses PU.1 activity and that lack of this repression in Gfi-1(-/-) myeloid cells contributes to the observed mixed lineage phenotype.

Epigenetic histone modifications do not control Igkappa locus contraction and intranuclear localization in cells with dual B cell-macrophage potential

Somatic rearrangement of the Ig genes during B cell development is believed to be controlled, at least in part, by accessibility of the loci to the recombinational machinery. Accessibility is poorly understood, but appears to be controlled by a combination of histone posttranslational modifications, large scale Ig locus contractions, and changes in intranuclear localization of the loci. These changes are regulated by developmental stage-specific as well as tissue-specific mechanisms. We previously isolated a murine B cell lymphoma line, Myc5, that can oscillate between the B cell and macrophage lineages depending upon growth conditions. This line provides an opportunity to study tissue-specific regulation of epigenetic mechanisms operating on the Ig loci. We found that when Myc5 cells are induced to differentiate from B cells into macrophages, expression of macrophage-specific transcripts was induced (M-CSFR, F4/80, and CD14), whereas B cell-specific transcripts decreased dramatically (mb-1, E47, IRF4, Pax5, and Igkappa). Loss of Igkappa transcription was associated with reduced Igkappa locus contraction, as well as increased association with heterochromatin protein-1 and association of the Igkappa locus with the nuclear periphery. Surprisingly, however, we found that histone modifications at the Igkappa locus remained largely unchanged whether the cells were grown in vivo as B cells, or in vitro as macrophages. These results mechanistically uncouple histone modifications at the Igkappa locus from changes in locus contraction and intranuclear localization.

Epstein-Barr virus EBNA-3C is targeted to and regulates expression from the bidirectional LMP-1/2B promoter

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA-3C) is essential for EBV-mediated immortalization of human B lymphocytes and regulates both the cell cycle and transcription. Transient reporter gene assays have implicated a pivotal role for EBNA-3C in the regulation of transcription of the majority of latency-associated genes expressed during the EBV growth program, including the viral oncoprotein LMP-1. To examine the regulation of latency gene expression by EBNA-3C, we generated an EBV-positive cell line that inducibly expresses EBNA-3C. This cell line allowed us to examine expression from the endogenous latency gene promoters in the context of an actual latent infection and the presence of other EBNA proteins, in particular EBNA-2, which is presumed to coregulate transcription with EBNA-3C. EBNA-3C induced the expression of both LMP-1 and LMP-2B mRNAs from the bidirectional LMP-1/LMP-2B promoter. In contrast, no effect was seen on expression from the common EBNA promoter Cp, which is responsive to EBNA-3C in reporter assays. Activation of LMP expression was not the consequence of increases in EBNA-2, PU.1 or Spi-B transcription factors, all of which are believed to be critical for activation of LMP-1. Chromatin immunoprecipitation assays furthermore indicated that EBNA-3C is present at the bidirectional LMP-1/LMP-2B promoter. These results indicate that EBNA-3C directly activates the expression of LMP-1 and LMP-2B but is unlikely to significantly regulate EBNA expression via Cp under normal growth conditions.

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.

Maturation stage-specific regulation of megakaryopoiesis by pointed-domain Ets proteins

Numerous megakaryocyte-specific genes contain signature Ets-binding sites in their regulatory regions. Fli-1 (friend leukemia integration 1), an Ets transcription factor, is required for the normal maturation of megakaryocytes and controls the expression of multiple megakaryocyte-specific genes. However, in Fli-1-/- mice, early megakaryopoiesis persists, and the expression of the early megakaryocyte-specific genes, alphaIIb and cMpl, is maintained, consistent with functional compensation by a related Ets factor(s). Here we identify the Ets protein GABPalpha (GA-binding protein alpha) as a regulator of early megakaryocyte-specific genes. Notably, GABPalpha preferentially occupies Ets elements of early megakaryocyte-specific genes in vitro and in vivo, whereas Fli-1 binds both early and late megakaryocyte-specific genes. Moreover, the ratio of GABPalpha/Fli-1 expression declines throughout megakaryocyte maturation. Consistent with this expression pattern, primary fetal liver-derived megakaryocytes from Fli-1-deficient murine embryos exhibit reduced expression of genes associated with late stages of maturation (glycoprotein [GP] Ibalpha, GPIX, and platelet factor 4 [PF4]), whereas GABPalpha-deficient megakaryocytes were mostly impaired in the expression of early megakaryocyte-specific genes (alphaIIb and cMpl). Finally, mechanistic experiments revealed that GABPalpha, like Fli-1, can impart transcriptional synergy between the hematopoietic transcription factor GATA-1 and its cofactor FOG-1 (friend of GATA-1). In concert, these data reveal disparate, but overlapping, functions of Ets transcription factors at distinct stages of megakaryocyte maturation.

Identification of a candidate regulatory element within the 5' flanking region of the mouse Igh locus defined by pro-B cell-specific hypersensitivity associated with binding of PU.1, Pax5, and E2A

The Igh locus is controlled by cis-acting elements, including Emu and the 3' IgH regulatory region which flank the C region genes within the well-studied 3' part of the locus. Although the presence of additional control elements has been postulated to regulate rearrangements of the VH gene array that extends to the 5' end of the locus, the 5' border of Igh and its flanking region have not been characterized. To facilitate the analysis of this unexplored region and to identify potential novel control elements, we physically mapped the most D-distal VH segments and scanned 46 kb of the immediate 5' flanking region for DNase I hypersensitive sites. Our studies revealed a cluster of hypersensitive sites 30 kb upstream of the most 5' VH gene. Detection of one site, HS1, is restricted to pro-B cell lines and HS1 is accessible to restriction enzyme digestion exclusively in normal pro-B cells, the stage defined by actively rearranging Igh-V loci. Sequence motifs within HS1 for PU.1, Pax5, and E2A bind these proteins in vitro and these factors are recruited to HS1 sequence only in pro-B cells. Transient transfection assays indicate that the Pax5 binding site is required for the repression of transcriptional activity of HS1-containing constructs. Thus, our characterization of the region 5' of the VH gene cluster demonstrated the presence of a single cluster of DNase I hypersensitive sites within the 5' flanking region, and identified a candidate Igh regulatory region defined by pro-B cell-specific hypersensitivity and interaction with factors implicated in regulating VDJ recombination.

The mechanism of repression of the myeloid-specific c-fms gene by Pax5 during B lineage restriction

The transcription factor Pax5 (BSAP) is required for the expression of a B-cell-specific genetic program and for B-cell differentiation, and also to suppress genes of alternative lineages. The molecular mechanism by which repression of myeloid genes occurs during early B-lineage restriction is unknown and in this study we addressed this question. One of the genes repressed by Pax5 in B cells is the colony-stimulating factor receptor 1 gene (csf1r or c-fms). We examined the changes in chromatin caused by Pax5 activity, and we show that Pax5 is directly recruited to c-fms resulting in the rapid loss of RNA polymerase II binding, followed by loss of transcription factor binding and DNaseI hypersensitivity at all cis-regulatory elements. We also show that Pax5 targets the basal transcription machinery of c-fms by interacting with a binding site within the major transcription start sites. Our results support a model by which Pax5 does not lead to major alterations in chromatin modification, but inhibits transcription by interfering with the action of myeloid transcription factors.

Transcription factors drive B cell development

Transcription factors including PU.1, E2A and early B cell factor (EBF) are essential for the earliest stages of B lymphocyte development. Recent advances suggest that, although PU.1 initiates events leading to B lymphopoiesis, it might be dispensable at later stages of development. E2A proteins are also crucial for B cell lineage determination, as shown by the pluripotency of E2A-deficient progenitors. Both PU.1 and E2A are required for expression of EBF. EBF activates the early program of genes unique to B cells, including the lineage commitment factor Pax5. EBF also facilitates the function of Pax5 by mediating epigenetic changes necessary for the function of Pax5 at gene targets. Together, these proteins function in a hierarchy of factors that orchestrates B cell development.

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.

Long-range interactions between three transcriptional enhancers, active Vkappa gene promoters, and a 3' boundary sequence spanning 46 kilobases

The mouse immunoglobulin kappa (Igkappa) gene contains an intronic enhancer and two enhancers downstream of its transcription unit. Using chromosome conformation capture technology, we demonstrate that rearranged and actively transcribed Igkappa alleles in MPC-11 plasmacytoma cells exhibit mutual interactions over 22 kb between these three enhancers and Vkappa gene promoters. In addition, the 5' region of the active transcription unit exhibits a continuum of interactions with downstream chromatin segments. We also observe interactions between Ei and E3' with 3' boundary sequences 24 kb downstream of Ed, adjacent to a neighboring housekeeping gene. Very similar interactions between the enhancers are also exhibited by normal B cells isolated from mouse splenic tissue but not by germ line transcriptionally inactive alleles of T cells or P815 mastocytoma cells, which exhibit a seemingly linear chromatin organization. These results fit a looping mechanism for enhancer function like in the beta-globin locus and suggest a dynamic modulation of the spatial organization of the active Igkappa locus. Chromatin immunoprecipitation experiments reveal that the interacting Igkappa gene cis-acting sequences are associated with AP-4, E47, and p65NF-kappaB, potential protein candidates that may be responsible for initiating and/or maintaining the formation of these higher-order complexes. However, S107 plasmacytoma cells that lack NF-kappaB still exhibit mutual interactions between the Igkappa gene enhancers.

The third helix of the homeodomain of paired class homeodomain proteins acts as a recognition helix both for DNA and protein interactions

The transcription factor Pax6 is essential for the development of the eyes and the central nervous system of vertebrates and invertebrates. Pax6 contains two DNA-binding domains; an N-terminal paired domain and a centrally located homeodomain. We have previously shown that the vertebrate paired-less isoform of Pax6 (Pax6ΔPD), and several other homeodomain proteins, interact with the full-length isoform of Pax6 enhancing Pax6-mediated transactivation from paired domain-DNA binding sites. By mutation analyses and molecular modeling we now demonstrate that, surprisingly, the recognition helix for specific DNA binding of the homeodomains of Pax6 and Chx10 interacts with the C-terminal RED subdomain of the paired domain of Pax6. Basic residues in the recognition helix and the N-terminal arm of the homeodomain form an interaction surface that binds to an acidic patch involving residues in helices 1 and 2 of the RED subdomain. We used fluorescence resonance energy transfer assays to demonstrate such interactions between Pax6 molecules in the nuclei of living cells. Interestingly, two mutations in the homeodomain recognition helix, R57A and R58A, reduced protein–protein interactions, but not DNA binding of Pax6ΔPD. These findings suggest a critical role for the recognition helix and N-terminal arm of the paired class homeodomain in protein–protein interactions.

The Ig kappa 3' enhancer is activated by gradients of chromatin accessibility and protein association

The Igkappa locus is recombined following initiation of a signaling cascade during the early pre-B stage of B cell development. The Ig kappa3' enhancer plays an important role in normal B cell development by regulating kappa locus activation. Quantitative analyses of kappa3' enhancer chromatin structure by restriction endonuclease accessibility and protein association by chromatin immunoprecipitation in a developmental series of primary murine B cells and murine B cell lines demonstrate that the enhancer is activated progressively through multiple steps as cells mature. Moderate kappa3' chromatin accessibility and low levels of protein association in pro-B cells are increased substantially as the cells progress from pro- to pre-B, then eventually mature B cell stages. Chromatin immunoprecipitation assays suggest transcriptional regulators of the kappa3' enhancer, specifically PU.1 and IFN regulatory factor-4, exploit enhanced accessibility by increasing association as cells mature. Characterization of histone acetylation patterns at the kappa3' enhancer and experimental inhibition of histone deacetylation suggest changes therein may determine changes in enzyme and transcription factor accessibility. This analysis demonstrates kappa activation is a multistep process initiated in early B cell precursors before Igmu recombination and finalized only after the pre-B cell stage.

Regulation of activation and recombination of the murine Igkappa locus

The murine immunoglobulin (Ig) kappa locus has been intensively studied in an attempt to understand its developmentally regulated activation for both transcription and V(D)J recombination. A variety of signaling proteins, cis-acting DNA elements, and trans-acting DNA-binding proteins have been discovered and shown to be involved in the regulated changes in chromatin structure, which are associated with recombinase accessibility. In addition, key roles have been suggested for DNA methylation and replication in kappa-locus expression and rearrangement. This review summarizes data in this area and considers what studies of the murine kappa locus have revealed about the lineage specificity, order, and allelic exclusion of lymphoid V(D)J recombination.

Transcriptional control of B cell development and function

The generation, development, maturation and selection of mammalian B lymphocytes is a complex process that is initiated in the embryo and proceeds throughout life to provide the organism an essential part of the immune system it requires to cope with pathogens. Transcriptional regulation of this highly complex series of events is a major control mechanism, although control is also exerted on all other layers, including splicing, translation and protein stability. This review summarizes our current understanding of transcriptional control of the well-studied murine B cell development, which bears strong similarity to its human counterpart. Animal and cell models with loss of function (gene "knock outs") or gain of function (often transgenes) have significantly contributed to our knowledge about the role of specific transcription factors during B lymphopoiesis. In particular, a large number of different transcriptional regulators have been linked to distinct stages of the life of B lymphocytes such as: differentiation in the bone marrow, migration to the peripheral organs and antigen-induced activation.

Corecruitment of the Grg4 repressor by PU.1 is critical for Pax5-mediated repression of B-cell-specific genes

PU.1 and Pax5 are important regulators of immunoglobulin heavy-chain (IgH) gene expression in B lineage cells. We have previously shown that PU.1 can potentiate the transcription of an IgH HS1,2 enhancer-linked reporter gene, and that Pax5 represses the same enhancer in transient transfection assays. Here we report that PU.1, like Pax5, can recruit and physically interact with a member of the Groucho family of co-repressors, Grg4. As a consequence, PU.1 in conjunction with Pax5 represses enhancer function in a position-dependent manner when Grg4 is recruited. Interestingly, Grg4 levels decrease following B-cell activation, suggesting temporal regulation of Grg4. Moreover, the joining-chain promoter, with an activity pattern and architecture resembling HS1,2 can also be repressed by the combinatorial action of Pax5/PU.1/Grg4. These data indicate that Pax5 depends on PU.1, acting in cis, for stable recruitment of Grg co-repressors to B-cell-specific genes.

Identification and characterization of a PU.1/Spi-B binding site in the bovine leukemia virus long terminal repeat

Bovine leukemia virus (BLV) is a B-lymphotropic oncogenic retrovirus whose transcriptional promoter is located in the viral 5' long terminal repeat (LTR). To date, no B-lymphocyte-specific cis-regulatory element has been identified in this region. Since ETS proteins are known to regulate transcription of numerous retroviruses, we searched for the presence in the BLV promoter region of binding sites for PU.1/Spi-1, a B-cell- and macrophage-specific ETS family member. In this report, nucleotide sequence analysis of the viral LTR identified a PUbox located at -95/-84 bp. We demonstrated by gel shift and supershift assays that PU.1 and the related Ets transcription factor Spi-B interacted specifically with this PUbox. A 2-bp mutation (GGAA-->CCAA) within this motif abrogated PU.1/Spi-B binding. This mutation caused a marked decrease in LTR-driven basal gene expression in transient transfection assays of B-lymphoid cell lines, but did not impair the responsiveness of the BLV promoter to the virus-encoded transactivator Tax(BLV). Moreover, ectopically expressed PU.1 and Spi-B proteins transactivated the BLV promoter in a PUbox-dependent manner. Taken together, our results provide the first demonstration of regulation of the BLV promoter by two B-cell-specific Ets transcription factors, PU.1 and Spi-B. The PU.1/Spi-B binding site identified here could play an important role in BLV replication and B-lymphoid tropism.

Oscillation between B-lymphoid and myeloid lineages in Myc-induced hematopoietic tumors following spontaneous silencing/reactivation of the EBF/Pax5 pathway

B lymphomagenesis is an uncontrolled expansion of immature precursors that fail to complete their differentiation program. This failure could be at least partly explained by inappropriate expression of several oncogenic transcription factors, such as Pax5 and Myc. Both Pax5 and c-Myc are implicated in the pathogenesis of non-Hodgkin lymphomas. To address their role in lymphomagenesis, we analyzed B-cell lymphomas derived from p53-null bone marrow progenitors infected in vivo by a Myc-encoding retrovirus. All Myc-induced lymphomas invariably maintained expression of Pax5, which is thought to be incompatible with terminal differentiation. However, upon culturing in vitro, several cell lines spontaneously down-regulated Pax5 and its target genes CD19, N-Myc, and MB1. Unexpectedly, other B-cell markers (eg, CD45R) were also down-regulated, and markers of myeloid lineage (CD11b and F4/80 antigen) were acquired instead. Moreover, cells assumed the morphology reminiscent of myeloid cells. A pool of F4/80-positive cells as well as several single-cell clones were obtained and reinjected into syngeneic mice. Remarkably, pooled cells rapidly re-expressed Pax5 and formed tumors of relatively mature lymphoid phenotype, with surface immunoglobulins being abundantly expressed. Approximately half of tumorigenic single-cell clones also abandoned myeloid differentiation and gave rise to B lymphomas. However, when secondary lymphoma cells were returned to in vitro conditions, they once again switched to myeloid differentiation. This process could be curbed via enforced expression of retrovirally encoded Pax5. Our data demonstrate that some Myc target cells are bipotent B-lymphoid/myeloid progenitors with the astonishing capacity to undergo successive rounds of lineage switching.

T-lineage specification and commitment: a gene regulation perspective

T lymphocytes originate from pluripotent precursors and undergo lasting commitment to the T cell developmental fate during their processing in the thymus. Commitment includes both the acquisition of essential T cell characteristics and the foreclosing of other developmental options. Gain of T cell characteristics is probably mediated by separate mechanisms, at least in detail, from loss of alternative developmental potentials. Programmed shifts in survival requirements make changes irreversible. Here we review the current evidence identifying the regulatory components of this commitment pathway, and the first hints of how they work together. Roles for PU.1, GATA-3, and their target genes are highlighted.

Control of megakaryocyte-specific gene expression by GATA-1 and FOG-1: role of Ets transcription factors

The transcription factor GATA-1 and its cofactor FOG-1 are essential for the normal development of erythroid cells and megakaryocytes. FOG-1 can stimulate or inhibit GATA-1 activity depending on cell and promoter context. How the GATA-1-FOG-1 complex controls the expression of distinct sets of gene in megakaryocytes and erythroid cells is not understood. Here, we examine the molecular basis for the megakaryocyte-restricted activation of the alphaIIb gene. FOG-1 stimulates GATA-1-dependent alphaIIb gene expression in a manner that requires their direct physical interaction. Transcriptional output by the GATA-1-FOG-1 complex is determined by the hematopoietic Ets protein Fli-1 that binds to an adjacent Ets element. Chromatin immunoprecipitation experiments show that GATA-1, FOG-1 and Fli-1 co-occupy the alphaIIb promoter in vivo. Expression of several additional megakaryocyte-specific genes that bear tandem GATA and Ets elements in their promoters also depends on the physical interaction between GATA-1 and FOG-1. Our studies define a molecular context for transcriptional activation by GATA-1 and FOG-1, and may explain the occurrence of tandem GATA and Ets elements in the promoters of numerous megakaryocyte-expressed genes.

Loss of B cell identity correlates with loss of B cell-specific transcription factors in Hodgkin/Reed-Sternberg cells of classical Hodgkin lymphoma

In classical Hodgkin lymphoma the malignant Hodgkin/Reed-Sternberg (HRS) cells characteristically constitute only a small minority of the tumour load. Their origin has been debated for decades, but on the basis of rearrangement and somatic hypermutations of their immunoglubulin (Ig) genes, HRS cells are now ascribed to the B-cell lineage. Nevertheless, phenotypically HRS cells have lost their B cell identity: they usually lack common B cell-specific surface markers such as CD19 and CD79a as well as Ig gene transcripts. Here we demonstrate that Ig promoters as well as both intronic and 3' enhancer sequences are transcriptionally inactive in HRS cell lines. This inactivity correlates with either reduced levels or even a complete lack of several B cell-specific transcription factors required for their expression: Oct-2, OBF-1, PU.1, E47/E12, PAX-5 and EBF. Moreover, we demonstrate that PU.1 and PAX-5 are significantly down-regulated in HRS cells in pathological specimens from primary tumour tissues. However, forced expression of these transcription factors can activate regulatory sequences of silenced B cell marker genes, and in one instance also transcription from a silenced endogenous locus. Thus, HRS cells are dedifferentiated B cells with global down-regulation of B cell-specific genes.

Elements of transcription factor network design for T-lineage specification

The complex spectrum of cell types produced in mammalian hematopoiesis can be understood as the output of highly combinatorial transcription factor action. The generation of multiple diverse combinations of transcription factors from the common starting state of the hematopoietic stem cell must be explained through the cross-regulatory interactions of these transcription factors at several levels. Here, the operation of such a network is addressed through a focus on murine T cell development, where we have recently established regulatory linkages between GATA-3 and PU.1 and multiple other factors essential to this differentiation pathway. The action of both essential/rate-limiting factors and factors with effects that shift qualitatively with dose and time of action can be traced through the regulatory interaction network. Hypothetical models are proposed to indicate the network nodes that are differentially activated in normal T cell lineage progression and in cells diverted to other potential fates.

Hematopoietic cytokines, transcription factors and lineage commitment

The past two decades have witnessed significant advances in our understanding of the cellular physiology and molecular regulation of hematopoiesis. At the heart of stem cell self-renewal and lineage commitment decisions lies the relative expression levels of lineage-specific transcription factors. The expression of these transcription factors in early stem cells may be promiscuous and fluctuate, but ultimately comes under the influence of extracellular regulatory signals in the form of hematopoietic cytokines. In this review, we first summarize our current understanding of the phenotypic characterization of hematopoietic stem cells. Next, we describe key known transcription factors which govern stem cell self-renewal and lineage commitment decisions. Finally, we review data concerning the role of specific cytokines in influencing these decisions. From this review, a picture emerges in which stem cell fate decisions are governed by the integrated effects of intrinsic transcription factors and external signaling pathways initiated by regulatory cytokines.

The interaction of Pax5 (BSAP) with Daxx can result in transcriptional activation in B cells

Pax5 (BSAP) is essential for B cell development and acts both as a transcriptional activator and a repressor. Using a yeast two-hybrid assay to identify potential coregulators of Pax5, we identified Daxx, a protein that is highly conserved, ubiquitously expressed, and essential for embryonic mouse development. The interaction between Pax5 and Daxx involves the partial homeodomain of Pax5 and the C-terminal fragment of Daxx. A component of promyelocytic leukemia protein nuclear bodies, Daxx has been implicated in apoptosis and characterized as a transcriptional corepressor. Upon transient transfection assay of Daxx in B cells expressing endogenous Daxx and Pax5, we observed not only transcriptional corepression but also, unexpectedly, coactivation in M12.4.1 and A20 mouse B cell lines. Pax5 domains required for coactivation were identified using 293T cells. Coactivation apparently involves recruitment of the CREB binding protein (CBP), because we precipitated complexes containing Pax5, Daxx, and CBP in B cell lines. These data suggest that Daxx can affect Pax5's roles as an activator or repressor in B cells and describe a role for Daxx as a transcriptional coactivator.

Constitutive expression of PU.1 in fetal hematopoietic progenitors blocks T cell development at the pro-T cell stage

The essential hematopoietic transcription factor PU.1 is expressed in multipotent thymic precursors but downregulated during T lineage commitment. The significance of PU.1 downregulation was tested using retroviral vectors to force hematopoietic precursors to maintain PU.1 expression during differentiation in fetal thymic organ culture. PU.1 reduced thymocyte expansion and blocked development at the pro-T cell stage. PU.1-expressing cells could be rescued by switching to conditions permissive for macrophage development; thus, the inhibition depends on both lineage and developmental stage. An intact DNA binding domain was required for these effects. PU.1 expression can downregulate pre-Talpha, Rag-1, and Rag-2 in a dose-dependent manner, and higher PU.1 levels induce Mac-1 and Id-2. Thus, downregulation of PU.1 is specifically required for progression in the T cell lineage.

PU.1 is involved in the regulation of B lineage-associated and developmental stage-dependent expression of the germinal center-associated DNA primase GANP

Germinal center-associated DNA primase (GANP) associated with MCM3 of the DNA replication complex is up-regulated selectively in germinal center B cells. We studied promoter activity of the 5' region involved in the developmental stage-dependent expression in B lineage cells by luciferase reporter assay. Selective regulation of ganp expression was observed in the -737-bp promoter region in B and plasma cell lines but was significantly low in pre-B and T cell lines. The deletion constructs displayed a gap decrease after shortening the region from -134 to -108 bp. Further narrowing suggested the involvement of the PU.1 consensus sequence at -126 bp by electrophoretic mobility shift assay. The protein component PU.1 complex is not inhibited with mutated probes at the consensus site but is inhibited with the known PU.1 probe of CD72 and with anti-PU.1 antibody. Moreover, introduction of PU.1 cDNA enhanced the reporter gene activity in a dose-dependent manner in B cells, whereas the reporter construct with the mutated PU.1 site did not respond. Anti-CD40 stimulation induced the reporter activity with a 100% increase, which is not observed with the PU.1-mutated reporter construct. These results demonstrate that the germinal center-associated DNA primase expression is partly regulated by the transcription factor PU.1 expressed in B lineage cells.

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

HS1,2 enhancer regulation of germline epsilon and gamma2b promoters in murine B lymphocytes: evidence for specific promoter-enhancer interactions

During an immune response, activated B cells develop into high rate Ig-secreting plasma cells. They also switch from production of IgM to IgG, IgA, or IgE. This process requires a DNA recombination event, which is regulated at the transcriptional level by the production of isotype-specific, sterile germline (GL) transcripts. Induction of these transcripts is controlled by GL promoters and, possibly, by IgH 3' enhancers. We investigated the interaction of the GL epsilon and gamma2b promoters with the HS1,2 enhancer using transiently transfected mouse primary B cells and cell lines. The constructs used for the transfections contained a GL promoter upstream and HS1,2 downstream of a luciferase reporter gene. Both GL epsilon and gamma2b promoters synergized strongly with the HS1,2 enhancer in activated primary B cells, a mature B cell line, and a plasma cell line. We show that the major activity of HS1,2 in activated primary B cells occurs within a 310-bp fragment that includes NF-kappaB, OCT, and NF of activated B cells (Ets/AP-1) sites. By mutating the consensus sequences for various transcription factors, we have determined which sites in HS1,2 are important for synergy with the GL epsilon and gamma2b promoters. Our findings indicate that different sites in HS1,2 might selectively interact with the GL epsilon and gamma2b promoters. We also provide evidence that B cell-specific activator protein is not an absolute suppressor of HS1,2 activity.

Role for transcription Pax5A factor in maintaining commitment to the B cell lineage by selective inhibition of granulocyte-macrophage colony-stimulating factor receptor expression

During early B lymphopoiesis, developing B cells maintain lineage commitment despite the local presence of myeloid lineage-promoting cytokines such as GM-CSF and IL-3. Previous observations suggest that the B cell-specific transcription factor Pax5A (paired box 5A transcription factor) plays a role in maintaining B cell lineage commitment by limiting expansion and survival of early IL-3/GM-CSF-dependent myeloid lineage cells. To define a mechanism by which Pax5A can exert these inhibitory effects on myeloid lineage differentiation, an inducible form of the Pax5A protein was expressed in the myeloid cell line FDC-P1. This cell line models myeloid progenitors in that it responds to the survival and growth-potentiating effects of IL-3 and GM-CSF. We observed that enforced expression of Pax5A selectively suppressed proliferation in response to GM-CSF, but not IL-3. This effect was associated with specific down-regulation of GM-CSFR alpha-chain, but not beta-chain expression. These data provide a molecular mechanism to enforce commitment to the B cell lineage despite the presence of GM-CSF, a factor that has been shown to convert early developing B cells to the myeloid lineage. Furthermore, they indicate a role for B cell Pax5A expression in maintaining rather than directing commitment to the B cell lineage.

Superactivation of Pax6-mediated transactivation from paired domain-binding sites by dna-independent recruitment of different homeodomain proteins

The Pax6 genes encode evolutionary conserved transcription factors that act high up in the regulatory hierarchy controlling development of central organs such as the eyes and the central nervous system. These proteins contain two DNA-binding domains. The N-terminal paired domain is separated from a paired-type homeodomain by a linker region, and a transactivation domain is located C-terminal to the homeodomain. Vertebrate Pax6 genes express a paired-less isoform of Pax6 (Pax6DeltaPD) from an internal start codon in the coding region between the paired domain and homeodomain. We now provide evidence for an interaction between the full-length isoform and Pax6DeltaPD, which enhances the transactivation activity of Pax6 from paired domain-binding sites. The paired-like homeodomain protein Rax behaved similarly to Pax6DeltaPD. Both Pax6DeltaPD and Rax bound to the homeodomain of Pax6 in vitro in the absence of specific DNA binding. Coimmunoprecipitation experiments following cotransfection confirmed the existence of complexes between Pax6 and Pax6DeltaPD, Pax6 and Rax, and Pax6DeltaPD and Rax in vivo. Interestingly, the C-terminal subdomain of the paired domain and the homeodomain can interact with each other. The paired domain can also interact with itself. Surprisingly, GST pull-down assays revealed that the homeodomains of such diverse proteins as Chx10, Six3, Lhx2, En-1, Prep1, Prox1, and HoxB1 could all bind to Pax6, and several of these enhanced Pax6-mediated transactivation upon coexpression. Since many homeodomain proteins are coexpressed with Pax6 in several tissues during development, our results indicate the existence of novel regulatory interactions that may be important for fine tuning of gene regulation.

Pax5 determines the identity of B cells from the beginning to the end of B-lymphopoiesis

Despite being one of the most intensively studied cell types, the molecular basis of B cell specification is largely unknown. The Pax5 gene encoding the transcription factor BSAP is required for progression of B-lymphopoiesis beyond the pro-B cell stage. Pax5-deficient pro-B cells are, however, not yet committed to the B-lymphoid lineage, but instead have a broad lymphomyeloid developmental potential. Pax5 appears to mediate B-lineage commitment by repressing the transcription of non-B-lymphoid genes and by simultaneously activating the expression of B-lineage-specific genes. Pax5 thus functions both as a transcriptional repressor and activator, depending on its interactions with corepressors of the Groucho protein family or with positive regulators such as the TATA-binding protein. Once committed to the B-lineage, B cells require Pax5 function to maintain their B-lymphoid identity throughout B cell development.

Evolution of hematopoiesis: Three members of the PU.1 transcription factor family in a cartilaginous fish, Raja eglanteria

T lymphocytes and B lymphocytes are present in jawed vertebrates, including cartilaginous fishes, but not in jawless vertebrates or invertebrates. The origins of these lineages may be understood in terms of evolutionary changes in the structure and regulation of transcription factors that control lymphocyte development, such as PU.1. The identification and characterization of three members of the PU.1 family of transcription factors in a cartilaginous fish, Raja eglanteria, are described here. Two of these genes are orthologs of mammalian PU.1 and Spi-C, respectively, whereas the third gene, Spi-D, is a different family member. In addition, a PU.1-like gene has been identified in a jawless vertebrate, Petromyzon marinus (sea lamprey). Both DNA-binding and transactivation domains are highly conserved between mammalian and skate PU.1, in marked contrast to lamprey Spi, in which similarity is evident only in the DNA-binding domain. Phylogenetic analysis of sequence data suggests that the appearance of Spi-C may predate the divergence of the jawed and jawless vertebrates and that Spi-D arose before the divergence of the cartilaginous fish from the lineage leading to the mammals. The tissue-specific expression patterns of skate PU.1 and Spi-C suggest that these genes share regulatory as well as structural properties with their mammalian orthologs.