Identification of a tissue- and differentiation stage-specific enhancer of the VpreB1 gene

The CD53 and CEACAM-1 genes are genetic targets for early B cell factor

Early B cell factor (EBF)-1 is a transcription factor known to be of critical importance for early B lymphocyte development. EBF-1 has been shown to directly interact with and regulate expression of a set of genes involved in the functional formation of the pre-B cell receptor, but the dramatic phenotype observed in the EBF-1-deficient mice suggests that several additional genes are activated by this protein. In order to identify additional target genes for EBF-1, we transduced a hematopoietic progenitor cell line, BaF/3, with an EBF-1-encoding retrovirus and investigated the induced gene expression pattern by micro-arrays. This analysis suggested that among others, the CD53 and the carcinoembryonic antigen-related cell adhesion molecule (CEACAM)-1 genes both were induced by ectopic expression of EBF-1. Identification of the 5' end of the cDNA enabled the identification of promoter elements with functional binding sites for EBF-1 and ability to respond to EBF-1 expression in transient transfection assays. These data suggest that CD53 and CEACAM-1 are direct genetic targets for EBF-1, providing additional information concerning the activity of this crucial transcription factor in hematopoiesis.

The HSS3/4 enhancer of Crlz1-IgJ locus is another target of EBF in the pre-B cell stage of B cell development

The HSS3/4 enhancer of Crlz1-IgJ locus was first characterized with regard to the activity of HSS1 IgJ promoter in the plasma cells, where both of HSS3/4 enhancer and HSS1 IgJ promoter were found to be opened simultaneously to drive the IgJ gene expression. Unexpectedly, the HSS3/4 enhancer was also found to be opened in the pre-B cells. However, this opening of HSS3/4 enhancer in the pre-B cells could not be related to the IgJ gene expression, because neither the IgJ promoter was opened nor its gene was expressed at the pre-B cell stage of B cell development. Instead, it was postulated that the opened HSS3/4 enhancer might act on some other nearby promoter in pre-B cells, which is now guessed to be the Crlz1 promoter located at 22.5 kb from it. In consistence with this pre-B cell-specific opening of the HSS3/4 enhancer, a pre-B cell-specific in vivo footprint on a sequence similar to the EBF-binding consensus was detected within the enhancer. In this paper, we show that the protein causing the pre-B cell-specific in vivo footprint on a sequence similar to the EBF-binding consensus is truly EBF as judged by EMSA using various oligo-DNA competitors and anti-EBF antibodies. Also, as expected from other previous reports, EBF was shown to be expressed highly in pre-B cells, but very little or not in immature B, mature B and plasma cells using both the cell lines and FACS-sorted normal primary cells. Convincingly, mutations within the EBF site of HSS3/4 enhancer were shown to significantly impair the HSS3/4 enhancer activity in the pre-B cells, but not in the plasma cells.

The pre-B-cell receptor induces silencing of VpreB and lambda5 transcription

The pre-B-cell receptor (pre-BCR), composed of Ig heavy and surrogate light chain (SLC), signals pre-BII-cell proliferative expansion. We have investigated whether the pre-BCR also signals downregulation of the SLC genes (VpreB and lambda5), thereby limiting this expansion. We demonstrate that, as BM cells progress from the pre-BI to large pre-BII-cell stage, there is a shift from bi- to mono-allelic lambda5 transcription, while the second allele is silenced in small pre-BII cells. A VpreB1-promoter-driven transgene shows the same pattern, therefore suggesting that VpreB1 is similarly regulated and thereby defines the promoter as a target for transcriptional silencing. Analyses of pre-BCR-deficient mice show a temporal delay in lambda5 downregulation, thereby demonstrating that the pre-BCR is essential for monoallelic silencing at the large pre-BII-cell stage. Our data also suggest that SLP-65 is one of the signaling components important for this process. Furthermore, the VpreB1/lambda5 alleles undergo dynamic changes with respect to nuclear positioning and heterochromatin association, thereby providing a possible mechanism for their transcriptional silencing.

Early B-cell factor ‘pioneers’ the way for B-cell development

Early B-cell factor (EBF) is a DNA-binding protein required for B-cell lymphopoiesis. The lack of EBF results in an early developmental blockade, including the lack of functional B cells and Igs. Recent studies have elucidated a central role for EBF in the specification of B-lineage cells. EBF directs progenitor cells to undergo B lymphopoiesis and activates transcription of B cell-specific genes in the absence of upstream regulators. How EBF mediates these effects has yet to be thoroughly explored, however, it initiates epigenetic modifications necessary for gene activation and the function of other transcriptional regulators, including Pax5. Together, these observations suggest a molecular basis for the role of EBF in the hierarchical network of factors that control B lymphopoiesis.

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.

Pearson correlation analysis of microarray data allows for the identification of genetic targets for early B-cell factor

B lymphocyte development is a complex biological process critically dependent on the transcription factor early B cell factor (EBF). To deepen understanding of the roles for EBF in this process, we have used Pearson correlation analysis to evaluate microarray data from a set of mouse B lymphoid cell lines representing different stages of development. Comparing the expression pattern of EBF to that of the other genes in the data set revealed that VpreB1, mb-1, and lambda5, all known target genes, presented high correlation values to EBF. High correlations were also seen for the VpreB3 and CD19 genes and biochemical as well as functional data supported that they are target genes for EBF even though the expression of CD19 was critically dependent of Pax-5. We also obtained evidence for extensive collaborative actions of EBF and E47 even though microarray analysis of hematopoetic progenitor cells ectopically expressing these proteins suggested that they activated only a subset of pre-B cell restricted genes.

Roles of EBF and Pax-5 in B lineage commitment and development

B lymphocyte development is regulated by the nuclear proteins Early B cell factor (EBF) and Pax-5. EBF and Pax-5 work separately and in concert to activate genes required for B cell differentiation. Recent studies have defined mechanisms by which these two factors control transcription, including chromatin remodeling activities and recruitment of partner proteins. This review addresses the structures, functions, and roles of these proteins in early B cell commitment and development, as well as in later stages of B cell differentiation.

The human V-preB promoter is a target for coordinated activation by early B cell factor and E47

The development of mature B lymphoid cells involves a highly orchestrated regulation of stage- and lineage-specific genes. In this study, we report an analysis of the human surrogate L chain VpreB promoter. The promoter has an overall homology of 56% to the mouse counterpart and displays a preB cell-restricted activity in transient transfections in cell lines. The promoter harbors three independent binding sites for early B cell factor (EBF) as defined by EMSA and supershift experiments. These sites were important for the full function of the promoter in a preB cell line, and chromatin immunoprecipitation experiments indicate that EBF interacts with the promoter in vivo. In addition to this, ectopic expression of EBF induces the activity of a reporter gene under control of the VpreB promoter in epithelioid HeLa cells, an effect augmented by coexpression of the basic-helix-loop helix transcription factor E47. The ability to interact directly with E47 was shared by the promoters controlling the human mb-1 and B29 genes. These data indicate that the human VpreB promoter is a direct target for activation by EBF and E47 and that functional collaboration between these proteins may be of great importance in human B cell development.

Differential surrogate light chain expression governs B-cell differentiation

Surrogate light chain expression during B lineage differentiation was examined by using indicator fluorochrome-filled liposomes in an enhanced immunofluorescence assay. Pro-B cells bearing surrogate light chain components were found in mice, but not in humans. A limited subpopulation of relatively large pre-B cells in both species expressed pre-B cell receptors. These cells had reduced expression of the recombinase activating genes, RAG-1 and RAG-2. Their receptor-negative pre-B cell progeny were relatively small, expressed RAG-1 and RAG-2, and exhibited selective down-regulation of VpreB and λ5expression. Comparative analysis of the 2 pre-B cell subpopulations indicated that loss of the pre-B cell receptors from surrogate light chain gene silencing was linked with exit from the cell cycle and light chain gene rearrangement to achieve B-cell differentiation.

Analysis of VpreB expression during B lineage differentiation in lambda5-deficient mice

The VpreB/lambda5 surrogate L chain complex is an essential component of the pre-B cell receptor, the expression of which serves as an important checkpoint in B cell development. Surrogate L chains also may serve as components of murine pro-B cell receptors whose function is unknown. We have produced two new mAbs, R3 and R5, that recognize a different VpreB epitope than the one recognized by the previously described VP245 anti-mouse VpreB Ab. These Abs were used to confirm the expression of surrogate L chains on wild-type pro-B and pre-B cell lines. Although undetectable on the cell surface, VpreB was found to be normally expressed within B lineage cells of lambda5-deficient mice. Nevertheless, VpreB expression was extinguished at the B cell stage of differentiation in these mice. The normal pattern of VpreB expression in lambda5-deficient mice excludes an essential role for pro-B and pre-B cell receptors in VpreB regulation.

Cloning of human early B-cell factor and identification of target genes suggest a conserved role in B-cell development in man and mouse

Early B-cell factor (EBF) is a helix–loop–helix transcription factor suggested to be essential for B-cell development in the mouse. Several genetic targets for EBF have been identified in mice, among these the surrogate light chain λ5 and the signal-transducing molecules Igα (mb-1) and Igβ (B29). This article reports cloning of the human homologue of EBF, hEBF. This protein has 93% sequence and 98.8% amino acid homology with mouse EBF. The encoded protein binds DNA and is expressed in cells of the B lineage, but not in cell populations representing T lymphocytes or myeloid cells. It is also shown that EBF-binding sites are functionally conserved in the humanmb-1 and B29 promoters because hEBF interacts with these in the electrophoretic mobility shift assay (EMSA) and have the ability to increase the activity of reporter constructs under the control of these promoters in nonlymphoid HeLa cells. A third genetic target for hEBF is the promoter of the human surrogate light chain14.1. This promoter contains 5 independent binding sites capable of interacting with hEBF in the EMSA, and the activity of the promoter was induced 24-fold in co-transfection experiments. These findings suggest that the human homologue of mouse EBF displays conserved biochemical features as well as genetic targets, indicating that this protein also has an important role in human B-cell development.

Cloning of human early B-cell factor and identification of target genes suggest a conserved role in B-cell development in man and mouse

Early B-cell factor (EBF) is a helix–loop–helix transcription factor suggested to be essential for B-cell development in the mouse. Several genetic targets for EBF have been identified in mice, among these the surrogate light chain λ5 and the signal-transducing molecules Igα (mb-1) and Igβ (B29). This article reports cloning of the human homologue of EBF, hEBF. This protein has 93% sequence and 98.8% amino acid homology with mouse EBF. The encoded protein binds DNA and is expressed in cells of the B lineage, but not in cell populations representing T lymphocytes or myeloid cells. It is also shown that EBF-binding sites are functionally conserved in the humanmb-1 and B29 promoters because hEBF interacts with these in the electrophoretic mobility shift assay (EMSA) and have the ability to increase the activity of reporter constructs under the control of these promoters in nonlymphoid HeLa cells. A third genetic target for hEBF is the promoter of the human surrogate light chain14.1. This promoter contains 5 independent binding sites capable of interacting with hEBF in the EMSA, and the activity of the promoter was induced 24-fold in co-transfection experiments. These findings suggest that the human homologue of mouse EBF displays conserved biochemical features as well as genetic targets, indicating that this protein also has an important role in human B-cell development.

Early B Cell Factor Is an Activator of the B Lymphoid Kinase Promoter in Early B Cell Development

Early B cell factor (EBF) is a transcription factor suggested to be involved in the transcriptional control of several B cell restricted genes. EBF is also essential for B lymphocyte development because mice carrying a homologous disruption of the EBF gene lack mature B lymphocytes. This makes the identification of genetic targets for EBF important for the understanding of early B cell development. Examination of the nucleotide sequence of the B lymphoid kinase (Blk) promoter suggested the presence of an EBF binding site, and in vivo footprinting analysis showed that the site was protected from methylation in a pre-B cell line. EMSA indicated that recombinant and cellular EBF interact physically with this site; furthermore, transient transfections indicated that ectopic expression of EBF in nonlymphoid HeLa cells activate a Blk promoter-controlled reporter construct 9-fold. The defined EBF binding site was also important for the function of the Blk promoter in pre-B cells, because transient transfections of a reporter construct under the control of an EBF site-mutated Blk promoter displayed only 20–30% of the activity of the wild-type promoter. Furthermore, transient transfections in HeLa cells proposed that EBF and B cell-specific activator protein were able to cooperate in the activation of a Blk promoter-controlled reporter construct. These data indicate that EBF plays an important role in the regulation of the Blk promoter in early B cell development and that EBF and BSAP are capable to act in cooperation to induce a target gene.

A human early B-cell factor-like protein participates in the regulation of the human CD19 promoter

CD19 is a functional component of the B-cell receptor complex where it acts as a modulator of the cellular response to surface immunoglobulin signaling. The gene is expressed from early B-cell developmental stages until the mature B-cell stage, but not in the plasma cell. The human CD19 promoter has been suggested to be regulated by the B-cell specific activator protein. BSAP, the Erg transcription factor and unidentified factors interacting with a GC rich binding site denoted PyG. In this report we present data suggesting that one of the PyG interacting factors is related to mouse early B-cell factor (EBF). Recombinant mouse EBF binds to the PyG site with an affinity about 3-fold lower than to the EBF binding site from the mouse mb-1 promoter in electrophoretic mobility shift assays. Furthermore, the PyG box binds to a factor in nuclear extracts from human B-cell lines that also interact with the mouse mb-1 promoter EBF binding site. Mutation of the PyG box impaired binding of the factor and the function of a minimal CD19 promoter in human cells of the B lineage, but not in Jurkat T or non-lymphoid HeLa cells. In addition to this, murine EBF was able to activate a wild type but not a PyG mutant human CD19 promoter 7-fold upon transient co-transfection in HeLa cells. Thus, we suggest that a human homologue of mouse EBF participate in transcriptional regulation of the human CD19 promoter.

The B29 (immunoglobulin beta-chain) gene is a genetic target for early B-cell factor

Early B-cell factor (EBF) is a transcription factor suggested as essential for early B-lymphocyte development by findings in mice where the coding gene has been inactivated by homologous disruption. This makes the identification of genetic targets for this transcription factor pertinent for the understanding of early B-cell development. The lack of B29 transcripts, coding for the beta subunit of the B-cell receptor complex, in pro-B cells from EBF-deficient mice suggested that B29 might be a genetic target for EBF. We here present data suggesting that EBF interacts with three independent sites within the mouse B29 promoter. Furthermore, ectopic expression of EBF in HeLa cells activated a B29 promoter-controlled reporter construct 13-fold and induced a low level of expression from the endogenous B29 gene. Finally, mutations in the EBF binding sites diminished B29 promoter activity in pre-B cells while the same mutations did not have as striking an effect on the promoter function in B-cell lines of later differentiation stages. These data suggest that the B29 gene is a genetic target for EBF in early B-cell development.