Immunoglobulin enhancer and promoter motifs 5' of the B29 B-cell-specific gene

Tetrabromobisphenol A effects on differentiating mouse embryonic stem cells reveals unexpected impact on immune system

Tetrabromobisphenol A (TBBPA) is a potent flame retardant used in numerous appliances and a major pollutant in households and ecosystems. In vertebrates, it was shown to affect neurodevelopment, the hypothalamic-pituitary-gonadal axis and thyroid signaling, but its toxicity and modes of actions are still a matter of debate. The molecular phenotype resulting from exposure to TBBPA is only poorly described, especially at the level of transcriptome reprogramming, which further limits our understanding of its molecular toxicity. In this work, we combined functional genomics and system biology to provide a system-wide description of the transcriptomic alterations induced by TBBPA acting on differentiating mESCs, and provide potential new toxicity markers. We found that TBBPA-induced transcriptome reprogramming affect a large collection of genes loosely connected within the network of biological pathways, indicating widespread interferences on biological processes. We also found two hotspots of action: at the level of neuronal differentiation markers, and surprisingly, at the level of immune system functions, which has been largely overlooked until now. This effect is particularly strong, as terminal differentiation markers of both myeloid and lymphoid lineages are strongly reduced: the membrane T cell receptor (Cd79a, Cd79b), interleukin seven receptor (Il7r), macrophages cytokine receptor (Csf1r), monocyte chemokine receptor (Ccr2). Also, the high affinity IgE receptor (Fcer1g), a key mediator of allergic reactions, is strongly induced. Thus, the molecular imbalance induce by TBBPA may be stronger than initially realized.

Downregulation of the B-cell receptor signaling component CD79b in plasma cell myeloma: a possible post transcriptional regulation

The CD79 molecule, encoded by the CD79a and CD79b genes, is a signaling unit of the B-cell receptor complex, which transmits signals of B-cell activation, growth, and differentiation. They are B-cell-specific and expressed at most stages of B-cell development. Although plasma cells have been believed to lack these gene products, the regulation of CD79 expression in plasma cells is still controversial. In particular, the regulation of CD79b expression remains unclear. We sought to examine CD79b expression in normal and neoplastic plasma cells by immunohistochemical analysis. Out of the 23 clinical samples and 11 cell lines of plasma cell myeloma (PCM), none of the clinical samples and only 1 of 11 cell lines expressed CD79b immunohistologically, whereas non-neoplastic plasma cells in reactive hyperplastic lymph nodes exhibited loss of CD79b protein expression. This finding is quite different from our previous report on CD79a. Not only immunocytochemistry, but also RT-PCR and Western blot analysis of PCM cell lines gave identical results. Interestingly, we detected mRNA transcripts of CD79b in PCM cell lines, although protein translation was lacking. These findings suggest that expression of CD79b is downregulated in both plasma cells and plasma cell myeloma, and this process is possibly under post transcriptional regulation.

Molecular characterisation of the CD79a and CD79b subunits of the B cell receptor complex in the gray short-tailed opossum (Monodelphis domestica) and tammar wallaby (Macropus eugenii): Delayed B cell immunocompetence in marsupial neonates

The B cell receptor (BCR) is a multiprotein complex that is pivotal to antigen recognition and signal transduction in B cells. It consists of an antigen binding component, membrane Ig (mIg), non-covalently associated with the signaling component, a disulphide-linked heterodimer of CD79a and CD79b. In this study, the gene and corresponding cDNA for CD79a and CD79b in the gray short-tailed opossum, as well as the cDNA sequences for CD79a and CD79b in the tammar wallaby, are described. Many of the structural and functional features of CD79a and CD79b were conserved in both marsupials, including the ITAM regulatory motif in the cytoplasmic tails of both subunits. The marsupial CD79 sequences shared a high degree of amino acid identities of 76% (CD79a) and 72% (CD79b) to each other, as well as 60-61% (CD79a) and 58-59% (CD79b) with their eutherian counterparts. RT-PCR analysis of CD79a and CD79b transcripts in the immune tissues of tammar pouch young revealed CD79a transcripts in the bone marrow, cervical thymus and spleen at day 10 postpartum. CD79b transcripts were detected in the bone marrow and cervical thymus at day 10 but were not detected in the spleen until day 21 postpartum. These results suggest that a functional BCR may not be assembled until day 21 postpartum and the tammar neonate may not be capable of mounting an effective adaptive immune response until this time. The molecular information presented here will allow further investigation of the role of the CD79 subunits in marsupial B cell signaling, especially during ontogeny and disease.

Genomic suppression of murine B29/Ig-β promoter-driven transgenes

Immunoglobulin beta (Ig-beta) is a critical signal transducer of precursor B cell and B cell receptors. B29, the gene coding for Ig-beta, is switched on in progenitor B cells and expressed until the terminal stage of antibody-producing plasma cells. Although several cis-acting elements and transcription factors required for B29 expression have been characterized in cell lines, the in vivo significance of individual motifs located in the 1.2-kb promoter region remained unclear. To address whether this region drives B lineage-specific expression in mice as efficiently as in transfected cell lines, we established transgenic animals carrying the B29 promoter fused to either enhanced green fluorescent protein (EGFP) or the precursor B cell receptor component lambda5. Surprisingly, only minimal levels of B29-derived transcripts were produced in B lymphoid tissues of several independent transgenic lines, and the respective proteins were below the detection limit. In addition, transgenic transcripts were found in testis, kidney and brain. Hence, the 1.2-kb-sized B29 promoter does not define a strong, B lineage-restricted expression unit when randomly integrated into the genome and passed through the murine germ line. Therefore, yet unidentified genomic locus control elements are required to efficiently drive B29 expression in B lymphocytes.

Gene expression analysis of myeloid and lymphoid lineage markers during mouse haematopoiesis

Expression profiling of haematopoietic cells is hampered by the heterogeneous nature of haematopoietic tissues and the absolute rarity of early unrestricted progenitors. To overcome this, the expression profile of lymphoid and myeloid-associated genes (LEF1, EBF, CD19, Sox-4, B29, CD45, C-fms, lysozyme, PU.1 and CD5) were investigated in 40 mouse myeloid haematopoietic precursors covering the entire haematopoietic hierarchy from multipotential to committed single lineages. The lineage-specific expression seen in single-cell studies was confirmed by examining fractionated bone marrow, whole tissues and differentiation of the multipotent cell line FDCP (Factor Dependent Cell Paterson) mix. Analysis of the 40 single myeloid precursors failed to detect expression of lymphoid-associated genes, LEF1, EBF, CD19 and CD5, despite detection in lymphoid cell controls. Surprisingly, the lymphoid-associated genes, Sox-4 and B29 were detected in the single myeloid precursors, which was confirmed in bone marrow and a multipotential myeloid cell line. The pattern of Sox-4 and B29, is consistent with a potential role in the commitment of bipotential granulocytic/macrophage precursors towards the granulocyte or macrophage lineage. In addition to providing baseline values for myeloid and lymphoid lineage markers during mouse haematopoiesis, these results highlight the importance of single-cell analysis in the study of complex tissues.

Btk expression is controlled by Oct and BOB.1/OBF.1

BOB.1/OBF.1 is a lymphocyte-restricted transcriptional coactivator. It binds together with the Oct1 and Oct2 transcription factors to DNA and enhances their transactivation potential. Mice deficient for the transcriptional coactivator BOB.1/OBF.1 show several defects in differentiation, function and signaling of B cells. In search of BOB.1/OBF.1 regulated genes we identified Btk—a cytoplasmic tyrosine kinase—as a direct target of BOB.1/OBF.1. Analyses of the human as well as murine Btk promoters revealed a non-consensus octamer site close to the start site of transcription. Here we show that Oct proteins together with BOB.1/OBF.1 are able to form ternary complexes on these sites in vitro and in vivo. This in turn leads to the induction of Btk promoter activity in synergism with the transcription factor PU.1. Btk, like BOB.1/OBF.1, plays a critical role in B cell development and B cell receptor signalling. Therefore the down-regulation of Btk expression in BOB.1/OBF.1-deficient B cells could be related to the functional and developmental defects observed in BOB.1/OBF.1-deficient mice.

DNase I hypersensitive sites and histone acetylation status in the chicken Ig-beta locus

DNase I hypersensitive sites (DHSs) and histone acetylation status were examined in the Ig-beta locus of chicken B lymphocyte-derived DT40 cells and liver-derived LMH cells. Twelve DT40-specific DHSs were identified: one in the Ig-beta promoter, one in the first intron of the Ig-beta gene, three in the sodium channel gene located upstream of the Ig-beta gene, two between the sodium channel gene and the Ig-beta gene, four between the Ig-beta gene and a downstream growth hormone (GH) gene, and one in the downstream region of the GH gene. Transient transfection studies show that the DHS in the intron of Ig-beta gene enhances the activity of the Ig-beta promoter fourfold. A 1.6 kb DNA fragment, which includes two DHSs, from the sodium channel gene enhanced promoter activity threefold. The transcription enhancing ability of the intron DHS was dependent on orientation, but was not promoter specific. Electrophoretic mobility shift assays (EMSA) demonstrated that an Ets protein family member binds to the intron DHS. In DT40 cells, a distinguished acetylation of H3 and H4 histones was found at the Ig-beta promoter, in addition to the enhanced acetylation of both histones at DT40-specific DHSs.

A conserved sequence upstream of the B29 (Ig beta, CD79b) gene interacts with YY1

The human, murine, and rat B29 (Ig beta, CD79b) genes are highly conserved in sequence and organization and exhibit strict B cell-specific expression. In the human and rat genomes, the B29 gene is located between the skeletal muscle-specific Na-channel alpha subunit (SCN4A) gene and the pituitary-specific growth hormone (GH-N) gene. The human pituitary-specific GH-N gene is controlled by a tissue-specific locus control region (LCR) located just upstream of the B29 promoter that mediates tissue-specific enhancement, histone acetylation, and an open chromatin conformation across the B29 gene in growth hormone (GH)-expressing pituitary cells. Here we show that B29 mRNA is not detected in a GH-expressing pituitary cell line and that GH-N mRNA is not detected in B cells. This differential expression suggests that the B29 gene is insulated or otherwise protected from the regulatory influences of the closely proximal GH LCR. We searched available sequences upstream of the human, mouse, and rat B29 genes and found a highly conserved sequence that fulfills the criteria recently established for non-coding DNA elements potentially involved in gene control. This B29 conserved sequence (BCS) bound ubiquitously expressed nuclear protein complexes. DNase I protection analysis of the BCS revealed a central 'footprinted' core which was confirmed to bind the multifunctional transcription factor, YY1. However, neither the BCS nor the YY1-binding core motif exhibited silencer or enhancer activity in transient transfections or position-independent insulator activity in enhancer-blocking assays. Thus, the BCS may function as a tissue-specific LCR or position-dependent insulator specifically countering the influences of the 5' GH LCR and controlling B29 gene expression.

Impact of deletions within the Bam HI-L fragment of attenuated Marek's disease virus on vIL-8 expression and the newly identified transcript of open reading frame LORF4

Marek's disease (MD) in chickens is caused by MD herpesvirus (MDV), which induces T cell lymphomas. The early pathogenesis of MDV infection is characterized by a primary infection in B lymphocytes followed by infection of activated T lymphocytes. It has been speculated that a MDV-encoded homologue of interleukin-8 (vIL-8) may be important to attract activated T lymphocytes to infected B lymphocytes. Recently, more virulent strains of MDV have emerged, named very virulent plus (vv+)MDV, that cause earlier and more prolonged cytolytic infections compared to less virulent strains. In this report, it was found that vIL-8 mRNA expression in vivo was increased in very virulent (vv) and vv+MDV strains compared to mild (m) and virulent (v) strains, and could not be detected in two attenuated MDV strains examined using very sensitive real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assays. In order to identify potential mechanisms for the increased vIL-8 mRNA expression in more virulent strains, and lack thereof in attenuated strains, the vIL-8 gene and putative promoter sequences upstream of the vIL-8 gene were compared from 10 different MDV strains, including attenuated derivatives. Only the JM-16 strain (both non-attenuated and attenuated) and attenuated 584A (584Ap80C) encoded a predicted vIL-8 gene sequence different from all other strains examined. Within the putative vIL-8 gene promoter sequence, there was little difference among the non-attenuated strains; however significant deletions were identified in the attenuated JM-16/p71, Md11 (R2/23), and 584Ap80C strains. Additionally, these deletions were located within a previously hypothetical open reading frame (ORF) named LORF4. Rapid amplification of cDNA ends identified a full-length transcript of LORF4 in the MDV-transformed lymphoblastoid cell line MSB-1, and deletions within this ORF caused truncated predicted proteins in 4 out of 6 attenuated MDV strains examined.

Somatic hypermutation of the B cell receptor genes B29 (Igbeta, CD79b) and mb1 (Igalpha, CD79a)

Somatic hypermutation (SHM), coupled to selection by antigen, generates high-affinity antibodies during germinal center (GC) B cell maturation. SHM is known to affect Bcl6, four additional oncogenes in diffuse large B cell lymphoma, and the CD95Fas gene and is regarded as a major mechanism of B cell tumorigenesis. We find that mutations in the genes encoding the B cell receptor (BCR) accessory proteins B29 (Igbeta, CD79b) and mb1 (Igalpha, CD79a) occur as often as Ig genes in a broad spectrum of GC- and post-GC-derived malignant B cell lines, as well as in normal peripheral B cells. These B29 and mb1 mutations are typical SHM consisting largely of single nucleotide substitutions targeted to hotspots. The B29 and mb1 mutations appear at frequencies similar to those of other non-Ig genes but lower than Ig genes. The distribution of mb1 mutations followed the characteristic pattern found in Ig and most non-Ig genes. In contrast, B29 mutations displayed a bimodal distribution resembling the CD95Fas gene, in which promoter distal mutations conferred resistance to apoptosis. Distal B29 mutations in the cytoplasmic domain may contribute to B cell survival by limiting BCR signaling. B29 and mb1 are mutated in a much broader spectrum of GC-derived B cells than any other known somatically hypermutated non-Ig gene. This may be caused by the common cis-acting regulatory sequences that control the requisite coexpression of the B29, mb1, and Ig chains in the BCR.

Chicken HDAC2 down-regulates IgM light chain gene promoter activity

In a chicken B cell line, DT40, the disruption of HDAC2 (chHDAC2) gene causes an alteration of several gene expressions including chicken IgM light chain (chIgM-L) gene by 2D-PAGE analysis. To investigate the transcriptional function of chHDAC2, we employed the chIgM-L promoter reporter plasmid. We found that chHDAC2 represses activated chIgM-L promoter activity. In transient expression experiments in NIH 3T3 cell, the specific histone deacetylase inhibitor tricostatin A (TSA) increased transactivation of chIgM-L promoter activity mediated by chicken Oct-1 and OBF-1 proteins. In transient coexpression of the three class I chicken histone deacetylases (chHDAC1-3) tested, only chHDAC2 repressed the activated chIgM-L promoter activity. These findings suggest that chHDAC2 might be recruited to the chIgM-L promoter and specifically repress chIgM-L transcription.

Cloning and characterization of the chick Oct binding factor OBF-1

We have cloned the chicken homolog of OBF-1, chOBF-1, which comprises 256 amino acids, and exhibits only 65% overall identity to the human and mouse OBF-1 proteins. Amino acid sequence alignment revealed the putative Oct-binding sequence, RPYQGVRVKEPVKELL(K/R)RKRG, which is conserved among chicken, mouse and human. chOBF-1 protein was demonstrated to bind chicken Oct-1 protein by the in vitro immunoprecipitation experiment, and chOBF-1 was shown to functionally activate the chicken immunoglobulin (Ig) light chain promoter in the NIH 3T3 cell. Taken together, these data indicate that the Ig gene transcription machinery, including Oct-1 and OBF-1, has been highly conserved in vertebrate evolution.

Identification of potential regulatory elements in the human immunoglobulin loci

In this post-genomic era, it is necessary to formulate specific questions and develop bioinformatics tools to understand the vast amounts of information stored in DNA sequence. Using the combinatorial pattern discovery algorithm called Teiresias developed by the Bioinformatics and Pattern Discovery Group at IBM, we have identified novel conserved motifs present in the immunoglobulin loci. In the human VH promoter regions, two new putative regulatory elements have been implicated in basal transcriptional regulation of immunoglobulin. In the intergenic regions of the immunoglobulin constant region genes segments, elements were identified that are absent in similar regions of the Igbeta gene. Since the expression patterns of Igbeta and the Ig genes are similar such elements may have functions in activities specific to the Ig genes such as somatic hypermutation. These elements represent V gene-specific motifs identified through the use of a pattern discovery algorithm.

Bob1 (OCA-B/OBF-1) differential transactivation of the B cell-specific B29 (Ig beta) and mb-1 (Ig alpha) promoters

The B29 (Igbeta) and mb-1 (Igalpha) gene products are B cell-specific essential components of the B cell receptor that are coexpressed at all stages of B cell differentiation, with the exception of plasma cells, which lack mb-1 expression. Transcription of both genes is governed by a similar cassette of interactive transcription factor-binding elements, including octamer motifs, in TATA-less promoters. In this study, we show the B cell-specific B29 gene promoter is transactivated in B and non-B cells by cotransfection with the B cell-specific octamer cofactor gene, Bob1 (OCA-B/OBF-1). The expression of Bob1 is also sufficient to override the silencing effects of the B29 silencer. This indicates that Bob1 plays a critical role in B cell-specific B29 promoter expression. In contrast, coexpression of Bob1 had no effect on mb-1 promoter activity. Bob1 transactivation only occurs with select octamer sequences that have an adenosine at position 5 (ATGCAAAT). The B29 promoter conforms to this consensus octamer motif, while the mb-1 promoter octamer motif does not. Octamer motif swapping between B29 and mb-1 promoters renders B29 unresponsive to Bob1 transactivation and makes mb-1 competent for Bob1 transactivation, thereby indicating that the B29 octamer motif is solely responsible for Bob1 interaction. Additionally, the mb-1 construct containing the B29 octamer motif is expressed in a plasmacytoma cell line, while the wild-type mb-1 promoter is not. Bob1 transactivation of B29 and the lack of this transactivation of mb-1 account for the differential expression of B29 and mb-1 in terminally differentiated plasma cells.

Novel regulatory regions found downstream of the rat B29/Ig-beta gene

To search for novel regulatory regions, we examined the features of chromatin structure in the rat B29/Ig-beta gene and its flanking regions by determining DNase I hypersensitive sites (DHS) in plasmacytoma-derived Y3 cells. Six Y3 cell-specific DHS were detected at -8.6, promoter, +0.7, +4.4, +6.0, and +8.7 kb. The DHS at +4.4, +6.0, and +8.7 kb were present in the intergenic region between B29/Ig-beta and growth hormone (GH) genes and were mapped inside conserved sequences in rat and humans. In transient transfection into Y3 cells, 2.9-kb DNA containing the +4.4 and +6.0-kb DHS demonstrated six times more enhancing activity than B29/Ig-beta promoter alone. Three intergenic DHS each possessed enhancing activity that was highest in the +4.4-kb region. In the electrophoretic mobility shift assay, a major band shift was demonstrated with Y3 nuclear extract and 0.3-kb DNA containing the +4.4-kb region with a conserved 0.22-kb sequence. By footprint analysis, 20 bases in the middle of the 0.3-kb DNA were protected by Y3 nuclear extract in which the consensus binding site for the OCT family was present. Deletion of the footprinted region reduced enhancing activity to that of the B29/Ig-beta promoter alone. The sequence responsible for the major band shift and transcriptional enhancing activity in the conserved +4.4-kb region thus coincided with the 20-bp footprinted region.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

B cell development and immunoglobulin gene transcription in the absence of Oct-2 and OBF-1

Oct-2 and OBF-1 (also called OCA-B or Bob-1) are B cell-specific transcription factors that bind to the conserved octamer site of immunoglobulin promoters, yet their role in immunoglobulin transcription has remained unclear. We generated mice in which the lymphoid compartment was reconstituted with cells that lack both Oct-2 and OBF-1. Even in the absence of these two transcription factors, B cells develop normally to the membrane immunoglobulin M-positive (IgM+) stage and immunoglobulin gene transcription is essentially unaffected. These observations imply that the ubiquitous factor Oct-1 plays a previously unrecognized role in the control of immunoglobulin gene transcription and suggest the existence of another, as yet unidentified, cofactor. In addition, both factors are essential for germinal center formation, although OBF-1 is more important than Oct-2 for IgG production after immunization.

An upstream Oct-1- and Oct-2-binding silencer governs B29 (Ig beta) gene expression

The B cell-specific B29 (Igbeta) gene is activated in the earliest B cell precursors and is expressed throughout B cell development. Tissue-specific expression of the murine B29 gene is controlled by a B cell-specific promoter whose activity is governed by a cassette of upstream transcriptional silencers. This study describes a potent new silencer that is located 5' of the previously identified B29 silencer elements, FROG and TOAD. Like these known elements, the new B29 silencer is not restricted to the B29 promoter. Nuclear proteins from all cell lines tested interacted with this A+T-rich sequence, which closely resembled a noncanonical octamer binding motif and also conformed to the consensus sequence for nuclear matrix attachment regions. Interaction of Oct-1 and Oct-2 with the B29 A+T-rich sequence was confirmed using octamer-specific Abs. Oct-1/Oct-2 binding was required for the inhibitory activity of this sequence because mutations that blocked Oct-1/Oct-2 binding also eliminated inhibition of the B29 promoter. This B29 A+T-rich sequence specifically interacted with isolated nuclear matrix proteins in vitro, suggesting that it may also function as a matrix attachment region element. Maintenance of the level of B29 gene expression through the interaction of the minimal promoter and the upstream silencer elements FROG, TOAD, and the A+T-rich Oct-1/Oct-2 binding motif may be essential for normal B cell development and/or function.

Distinct factors regulate the murine RAG-2 promoter in B- and T-cell lines

The recombination activating genes RAG-1 and RAG-2 are expressed in a lymphoid-cell-specific and developmentally regulated fashion. To understand the transcriptional basis for this regulation, we have cloned and characterized the murine RAG-2 promoter. The promoter was lymphoid cell specific, showing activity in various B- and T-cell lines but little activity in nonlymphoid cells. To our surprise, however, the promoter was regulated differently in B and T cells. Using nuclear extracts from B-cell lines, we found that the B-cell-specific transcription factor BSAP (Pax-5) could bind to a conserved sequence critical for promoter activity. BSAP activated the promoter in transfected cells, and the BSAP site was occupied in a tissue-specific manner in vivo. An overlapping DNA sequence binding to a distinct factor was necessary for promoter activity in T cells. Full promoter activity in T cells was also dependent on a more distal DNA sequence whose disruption had no effect on B-cell activity. The unexpected finding that a B-cell-specific factor regulates the RAG-2 promoter may explain some of the recently observed differences in the regulation of RAG transcription between B and T cells.

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.

The gene structure and promoter analysis of mouse lymphocyte signal transduction molecule alpha 4 that is related to the yeast TAP42 involved in a rapamycin-sensitive pathway

The mouse alpha 4 phosphoprotein encoding a component associated with the B cell antigen receptor (BCR)-mediated signal transduction is suggested to be involved in a unique rapamycin-sensitive pathway. We studied the structure and the molecular mechanism of the expression of alpha 4 gene by isolating two phage clones, named #10 and #23, covering entire exons of the mouse alpha 4 gene. The alpha 4 gene is located within about 25 kb and composed of six exons. To analyze the regulation of alpha 4 gene expression, we determined the nucleotide sequence toward 2 kb upstream of the translation start site of the alpha 4 gene. The 5'-flanking region does not contain a typical TATA box or the initiation consensus sequence, but it contains a CCAAT box, E-boxes, and several DNA binding motifs such as c-Myc, c-Myb, and c-Ets. Transcription of the alpha 4 gene starts at four different sites, determined by primer extension analysis, that were surrounded by Y-rich sequences. We further characterized the functional promoter of the alpha 4 gene at the region between -263 and the transcription start site of alpha 4 gene by luciferase assay system and suggested that the 5' upstream region of alpha 4 gene contains the silencer element of MT repetitive sequence.

Physical linkage of the B29/Ig-beta (CD79B) gene to the skeletal muscle, sodium-channel, and growth hormone genes in rat and human

In the region between the polyadenylation site of the rat skeletal muscle (SkM) Na-channel gene and the 5' end of the growth hormone (GH) gene, a gene coding for B-cell-specific membrane protein B29/Ig-beta was found and noted to have the same orientation as the Na-channel and GH genes. Rat B29/Ig-beta gene was 3.1 kb in length with six exons and was separated by 3.3 and 9.3 kb from Na-channel and GH genes, respectively. Rat B29/Ig-beta protein comprised 228 amino acids, and its amino acid sequence was 85 and 69% identical with the mouse and human counterparts, respectively. With the long-area PCR method, genomic DNA connecting human SkM Na-channel (SCN4A) and B29/Ig-beta (CD79B) genes and CD79B and GH (GH1) genes was amplified, and the physical linkage of SCN4A/CD79B/ GH1 genes in the human genome was established. The human CD79B gene was separated by 6.3 and 10.5 kb from the SCN4A and GH1 genes, respectively.

Silencer elements controlling the B29 (Igbeta) promoter are neither promoter- nor cell-type-specific

The murine B29 (Igbeta) promoter is B cell specific and contains essential SP1, ETS, OCT, and Ikaros motifs. Flanking 5' DNA sequences inhibit B29 promoter activity, suggesting this region contains silencer elements. Two adjacent 5' DNA segments repress transcription by the murine B29 promoter in a position- and orientation-independent manner, analogous to known silencers. Both these 5' segments also inhibit transcription by several heterologous promoters in B cells, including mb-1, c-fos, and human B29. These 5' segments also inhibit transcription by the c-fos promoter in T cells suggesting they are not B cell-specific elements. DNase I footprint analyses show an approximately 70-bp protected region overlapping the boundary between the two negative regulatory DNA segments and corresponding to binding sites for at least two different DNA-binding proteins. Within this footprint, two unrelated 30-bp cis-acting DNA motifs (designated TOAD and FROG) function as position- and orientation-independent silencers when located directly 5' of the murine B29 promoter. These two silencer motifs act cooperatively to restrict the transcriptional activity of the B29 promoter. Neither of these motifs resembles any known silencers. Mutagenesis of the TOAD and FROG motifs in their respective 5' DNA segments eliminates the silencing activity of these upstream regions, indicating these two motifs as the principal B29 silencer elements within these regions.

The transcriptional promoter regulates hypermutation of the antibody heavy chain locus

A somatic process introduces mutations into antibody variable (V) region genes at a high rate in many vertebrates, and is a major source of antibody diversity. The mechanism of this hypermutation process remains enigmatic, although retrospective studies and transgenic experiments have recently suggested a role for transcriptional regulatory elements. Here, we demonstrate that mouse heavy (H) chain loci in which the natural VH promoter has been replaced by a heterologous promoter undergo hypermutation. However, while the distribution of mutation in such loci appears normal, the frequency of mutation does not. Conversely, moving the VH promoter 750 bp upstream of its normal location results in a commensurate change in the site specificity of hypermutation in H chain loci, and the foreign DNA inserted into the VH leader intron to produce this promoter displacement is hypermutated in a manner indistinguishable from natural Ig DNA. These data establish a direct mechanistic link between the IgH transcription and hypermutation processes.

Gene structure and characterization of the murine homologue of the B cell-specific transcriptional coactivator OBF-1

The B cell-specific activity of immunoglobulin (Ig) gene promoters is to a large extent mediated by the conserved octamer motif ATTTGCAT. This requires the DNA binding octamer factors Oct-1 and/or Oct-2, as well as an additional B cell-restricted non-DNA binding cofactor. We recently cloned such a coactivator specific for Oct-1 or Oct-2 from human B cells and called it OBF-1. Here we report the isolation and characterization of the murine homologue. Full-length cDNA clones as well as genomic clones were isolated and the gene structure was determined. The deduced protein sequence shows that the mouse protein has an identical length, is likewise proline rich and shows 89% overall identity to the human protein. The OBF-1 gene is expressed in a very highly B cell-specific manner and is transcribed in cells representative of all stages of B cell differentiation, including the earliest ones. We show that OBF-1 interacts in the absence of DNA with the POU domain of Oct-1 or Oct-2 and also with the general transcription factors TBP and TFIIB. Furthermore, we demonstrate that although OBF-1 efficiently activates promoter octamer sites, it does not activate enhancer octamer sites.

Functional characterization of the murine homolog of the B cell-specific coactivator BOB.1/OBF.1

B cell-specific transcriptional promoter activity mediated by the octamer motif requires the Oct1 or Oct2 protein and additional B cell-restricted cofactors. One such cofactor, BOB.1/OBF.1, was recently isolated from human B cells. Here, we describe the isolation and detailed characterization of the murine homolog. Full-length cDNAs and genomic clones were isolated, and the gene structure was determined. Comparison of the deduced amino acids shows 88% sequence identity between mouse and human BOB.1/OBF.1. The NH2-terminal 126 amino acids of BOB.1/OBF.1 are both essential and sufficient for interaction with the POU domains of either Oct1 or Oct2. This protein-protein interaction does not require the simultaneous binding of Oct proteins to DNA, and high resolution footprinting of the Oct-DNA interaction reveals that binding of BOB.1/OBF.1 to Oct1 or Oct2 does not alter the interaction with DNA. BOB.1/OBF.1 can efficiently activate octamer-dependent promoters in fibroblasts; however, it fails to stimulate octamer-dependent enhancer activity. Fusion of subdomains of BOB.1/OBF.1 with the GAL4 DNA binding domain reveals that both NH2- and COOH-terminal domains of BOB.1/OBF.1 contribute to full transactivation function, the COOH-terminal domain is more efficient in this transactivation assay. Consistent with the failure of full-length BOB.1/OBF.1 to stimulate octamer-dependent enhancer elements in non B cells, the GAL4 fusions likewise only stimulate from a promoter-proximal position.

Transcription of the blk gene in human B lymphocytes is controlled by two promoters

Genomic DNA containing the first exon and 5'-flanking region of the human protein tyrosine kinase, blk, was isolated. Sequence analysis identified a TG repeat element in this region with enhancer activity, but no TATA or CCAAT sequences were found. Two blk transcripts of 2.2 and 2.5 kilobases were identified in various B-cell lines by Northern blot analyses, and primer extension experiments demonstrated two clusters of multiple transcription start sites. Subsequent promoter analyses by transient transfection assays with a reporter gene identified two promoter elements in the human blk gene. Promoter P1 contains sequences that have been shown to regulate the expression of immunoglobulin genes and promoter P2 contains elements that are highly conserved in the promoter of major histocompatibility complex class II genes, as well as a B-cell-specific activator protein- (BSAP) binding site. Electrophoretic mobility shift assays demonstrated that the binding of a protein to the BSAP-binding site was correlated with the presence of the 2.5-kilobase blk transcript. These data suggest that the two human blk RNAs arise from the transcription of the blk gene by two distinct promoters and that these promoters may be subject to regulation by different trans-acting factors.

Functional differences between the Oct2 transactivation domains determine the transactivation potential of individual Oct2 isoforms

The lymphocyte specific transcription factor Oct2 is involved in mediating the B-cell specific transcriptional activity of the octamer motif. Mutational analyses in the context of the complete Oct2 protein had indicated that Oct2 contains two transactivation domains. These two domains appeared to be redundant for activation from a promoter proximal position, whereas stimulation from a remote enhancer position specifically required the C-terminal transactivation domain and an additional B-cell restricted activity. We have generated fusion proteins between the DNA binding domain of the yeast Gal4 transcription factor and individual Oct2 protein domains to analyze their transactivation potential separately. We show that both N- and C-terminal domains can stimulate transcription from a promoter proximal position independently. However, only the C-terminal transactivation domain activates from a distance and it can only do so in B-cells. The C-terminal transactivation domain represents a composite transactivation domain. Whereas removal of just 9 aminoacids from the extreme C-terminus lead to a complete inactivation of this domain deletions from the other side resulted in a gradual loss of activity. We also characterized the transactivation potential of different N-terminal regions of Oct2 generated by alternative splicing. We show that the N-terminus of one of the isoforms, Oct2.3, contains a negative regulatory domain (NRD), which can inactivate the neighbouring glutamine-rich transactivation in cis. The presence of this NRD affects the overall phosphorylation state of the Oct2 protein. This result suggests that the mechanism of inactivation might involve differential protein phosphorylation.

Chromosomal localization, genomic structure, and allelic polymorphism of the human CD79 alpha (Ig-alpha/mb-1) gene

The germline DNA sequence of the human CD79 alpha (Ig-alpha/mb-1) gene was determined by polymerase chain reaction sequencing of a cosmid clone derived from an arrayed human chromosome 19 library. The CD79 alpha gene was localized to chromosome 19q13.2; this localization places the gene within the CEA-like gene cluster with the following gene order: -CEA-CGM1-CD79 alpha-RPS11-ATP1A3-BGP-CGM9-. The genomic organization of the human CD79 alpha gene resembles the mouse counterpart with five exons interrupted by four introns. Computer analyses suggest the presence of transcription regulatory elements known to be important in the regulation of mouse CD79 alpha (AP-1, EBF, AP-2, MUF2, and SP-1 sites), as well as elements not found in the mouse gene (an NK-kappa B binding site and a series of E-box motifs). Similar to the mouse gene, the 5' flanking region of human CD79 alpha lacks a TATA box; however, unlike mouse CD79 alpha, a classical octamer motif could not be identified in the human gene. Finally, a new Rsa I restriction fragment length polymorphism was defined in the non-coding regions of the human gene.

The 5' boundary of somatic hypermutation in a V kappa gene is in the leader intron

The maturation of the immune response involves the hypermutation of antibody genes and the selection of B cells expressing receptors with improved antigen binding properties. Somatic hypermutation of antibody genes is targeted to a small region approximately 1 kb surrounding the rearranged V gene. The precise definition of the 5' limit is not yet clear since the data base of somatic mutations upstream of the V region is very restricted. The available data suggest that it lies close to the promoter region and this has been used to implicate transcription in the mechanism leading to hypermutation. Here we present an extensive analysis of mutations in the 5' region of a single kappa light chain gene. A large data base from highly mutated sequences was obtained from anti-oxazolone hybridomas expressing the V kappa Ox1-J kappa 5 light chain and from polymerase chain reaction-derived clones from splenic and Peyer's patches of transgenic mice expressing the same V kappa Ox1-J kappa 5 gene combination. Although mutations were found in the 5'-flanking segment, the rate of mutation in the V-J segment was about 20-fold higher. A sharp decline between those two mutation rates is evident but the boundary was found in the leader intron of the V kappa Ox1 gene, about 150 bases downstream of the initiation of transcription site.

The complete sequence of the human CD79b (Ig beta/B29) gene: identification of a conserved exon/intron organization, immunoglobulin-like regulatory regions, and allelic polymorphism

We determined the complete genomic sequence of the human CD79b (Ig beta/B29) gene. The CD79b gene product is associated with the membrane immunoglobulin signaling complex which is composed of immunoglobulin (Ig) itself, associated in a noncovalent fashion with CD79b and a second polypeptide chain, CD79a (Ig alpha/mb1). The sequence and exon/intron organization of the human and mouse CD79b genes are highly similar. The gene organization suggests that some variant forms of CD79b may arise by virtue of alternative splicing of mRNA. In addition, a number of conserved regulatory sequences commonly found in Ig genes are present in sequences which flank the human CD79b gene. Some of these sequences are distinct from those found in the CD79a promoter. These differences may explain why transcription of CD79b, but not CD79a, is observed in plasma cells. A new Taq 1 restriction fragment length polymorphism is described that is not associated with any structural polymorphisms of the expressed CD79b polypeptide.

Multiple motifs regulate the B-cell-specific promoter of the B29 gene

The B-cell-specific B29 and mb1 genes code for covalently linked proteins (B29 or Ig beta and mb1 or Ig alpha, respectively) associated with membrane immunoglobulins in the antigen receptor complex on B cells. We have functionally analyzed the upstream region of the B29 gene and have identified a 164-bp region which comprises the minimal promoter responsible for B-cell-specific transcription. Linker scanning mutagenesis of this minimal promoter has established that both the previously identified octamer motif and a DNA motif that binds an unknown protein factor are critical for B29 gene expression in a pre-B-cell and B-cell line. Further mutations showed that binding motifs for Ets, microB/LyF1, and Sp1 also significantly contributed to the overall activity of the minimal B29 promoter. However, the relative contribution of certain motifs to promoter activity was different in a pre-B versus a B-cell line. The microB/LyF1 motif was necessary for full promoter activity in the pre-B cells but was not required in the B cells.

Pi, a pre-B-cell-specific enhancer element in the immunoglobulin heavy-chain enhancer

We have identified a new immunoglobulin heavy-chain enhancer element, designated pi, between the microE2 and microE3 elements. The pi enhancer element is transcriptionally active primarily during early stages of B-cell development but becomes virtually inactive during B-cell maturation at about the stage of immunoglobulin kappa light-chain gene rearrangement. Mutational analysis suggests that the pi element is crucial for immunoglobulin heavy-chain enhancer activity at the pre-B-cell stage but is almost irrelevant for enhancer activity at the mature B-cell or plasma-cell stage. The activity of the pi enhancer element correlates with the presence of an apparently pre-B-cell-specific protein-DNA complex. The similarity of the pi site to recognition sequences for members of the ets gene family suggests that the protein(s) interacting with the pi site most likely are ets-related transcription factors.

Pi, a pre-B-cell-specific enhancer element in the immunoglobulin heavy-chain enhancer

We have identified a new immunoglobulin heavy-chain enhancer element, designated pi, between the microE2 and microE3 elements. The pi enhancer element is transcriptionally active primarily during early stages of B-cell development but becomes virtually inactive during B-cell maturation at about the stage of immunoglobulin kappa light-chain gene rearrangement. Mutational analysis suggests that the pi element is crucial for immunoglobulin heavy-chain enhancer activity at the pre-B-cell stage but is almost irrelevant for enhancer activity at the mature B-cell or plasma-cell stage. The activity of the pi enhancer element correlates with the presence of an apparently pre-B-cell-specific protein-DNA complex. The similarity of the pi site to recognition sequences for members of the ets gene family suggests that the protein(s) interacting with the pi site most likely are ets-related transcription factors.

Purification of early-B-cell factor and characterization of its DNA-binding specificity

Early-B-cell factor (EBF) is a nuclear protein that recognizes a functionally important sequence in the promoter of the mb-1 gene. Like the mb-1 gene, which encodes an immunoglobulin-associated protein, EBF is specifically expressed in the early stages of B-lymphocyte differentiation. We purified EBF by sequence-specific DNA affinity chromatography and examined its biochemical properties and DNA-binding specificity. Crude nuclear extract and affinity-purified EBF generated protein-DNA complexes with the mb-1 promoter that were indistinguishable in electrophoretic mobility shift and DNase I footprint assays. Fractionation of affinity-purified EBF by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and renaturation of isolated polypeptides indicated that EBF DNA-binding activity could be reconstituted from polypeptides with molecular masses of 62 to 65 kDa. Gel filtration chromatography suggested that native EBF has a molecular mass of 140 kDa, if a globular shape of the protein is assumed. Thus, EBF appears to be a dimer with subunits of 62 to 65 kDa. To characterize the DNA-binding specificity of purified EBF, we performed two sets of experiments. First, we examined various mutant EBF-binding sites for interaction with purified EBF in an electrophoretic mobility shift assay. Second, we used oligonucleotides containing pairs of randomized bases in a binding-site selection and amplification experiments to determine a preferred sequence for DNA binding by EBF. Taken together, the results of these experiments indicated that EBF recognizes variations on the palindromic sequence 5'-ATTCCCNNGGGAAT, with an optimal spacer of 2 bp between the half-sites.

Purification of early-B-cell factor and characterization of its DNA-binding specificity

Early-B-cell factor (EBF) is a nuclear protein that recognizes a functionally important sequence in the promoter of the mb-1 gene. Like the mb-1 gene, which encodes an immunoglobulin-associated protein, EBF is specifically expressed in the early stages of B-lymphocyte differentiation. We purified EBF by sequence-specific DNA affinity chromatography and examined its biochemical properties and DNA-binding specificity. Crude nuclear extract and affinity-purified EBF generated protein-DNA complexes with the mb-1 promoter that were indistinguishable in electrophoretic mobility shift and DNase I footprint assays. Fractionation of affinity-purified EBF by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and renaturation of isolated polypeptides indicated that EBF DNA-binding activity could be reconstituted from polypeptides with molecular masses of 62 to 65 kDa. Gel filtration chromatography suggested that native EBF has a molecular mass of 140 kDa, if a globular shape of the protein is assumed. Thus, EBF appears to be a dimer with subunits of 62 to 65 kDa. To characterize the DNA-binding specificity of purified EBF, we performed two sets of experiments. First, we examined various mutant EBF-binding sites for interaction with purified EBF in an electrophoretic mobility shift assay. Second, we used oligonucleotides containing pairs of randomized bases in a binding-site selection and amplification experiments to determine a preferred sequence for DNA binding by EBF. Taken together, the results of these experiments indicated that EBF recognizes variations on the palindromic sequence 5'-ATTCCCNNGGGAAT, with an optimal spacer of 2 bp between the half-sites.

Oct-2, although not required for early B-cell development, is critical for later B-cell maturation and for postnatal survival

Oct-2, a POU homeo domain transcription factor, is believed to stimulate B-cell-restricted expression of immunoglobulin genes through binding sites in immunoglobulin gene promoters and enhancers. To determine whether Oct-2 is required for B-cell development or function, or has other developmental roles, the gene was disrupted by homologous recombination. Oct-2-l- mice develop normally but die within hours of birth for undetermined reasons. Mutants contain normal numbers of B-cell precursors but are somewhat deficient in IgM+ B cells. These B cells have a marked defect in their capacity to secrete immunoglobulin upon mitogenic stimulation in vitro. Thus, Oct-2 is not required for the generation of immunoglobulin-bearing B cells but is crucial for their maturation to immunoglobulin-secreting cells and for another undetermined organismal function.

Expression of the Ig-associated heterodimer (mb-1 and B29) in Hodgkin's disease

Eighty-three cases of Hodgkin's disease were studied immunocytochemically for the presence of the Ig associated heterodimer (mb-1 and B29) which is believed to be a specific pan B-cell marker. These results were compared with those achieved using other B-cell markers against CD19, CD20 and CD22. Although a small number of cases of nodular sclerosis and mixed cellularity subtype showed positivity for CD19, CD20 or CD22, no case showed any reactivity with antibodies against mb-1 or B29. This contrasted markedly with the cases of lymphocyte predominance where all seven cases expressed one or more of the B-cell antigens, with six cases being positive for mb-1. These results confirm previous studies that have suggested lymphocyte-predominance Hodgkin's disease is of B-cell origin and different from the other subtypes. However, they do not provide support for the thesis that these other subtypes may also have a B-cell origin, albeit with a different phenotype to lymphocyte predominance.

Oct2 transactivation from a remote enhancer position requires a B-cell-restricted activity

Previous cotransfection experiments had demonstrated that ectopic expression of the lymphocyte-specific transcription factor Oct2 could efficiently activate a promoter containing an octamer motif. Oct2 expression was unable to stimulate a multimerized octamer enhancer element in HeLa cells, however. We have tested a variety of Oct2 isoforms generated by alternative splicing for the capability to activate an octamer enhancer in nonlymphoid cells and a B-cell line. Our analyses show that several Oct2 isoforms can stimulate from a remote position but that this stimulation is restricted to B cells. This result indicates the involvement of either a B-cell-specific cofactor or a specific modification of a cofactor or the Oct2 protein in Oct2-mediated enhancer activation. Mutational analyses indicate that the carboxy-terminal domain of Oct2 is critical for enhancer activation. Moreover, this domain conferred enhancing activity when fused to the Oct1 protein, which by itself was unable to stimulate from a remote position. The glutamine-rich activation domain present in the amino-terminal portion of Oct2 and the POU domain contribute only marginally to the transactivation function from a distal position.

Oct2 transactivation from a remote enhancer position requires a B-cell-restricted activity

Previous cotransfection experiments had demonstrated that ectopic expression of the lymphocyte-specific transcription factor Oct2 could efficiently activate a promoter containing an octamer motif. Oct2 expression was unable to stimulate a multimerized octamer enhancer element in HeLa cells, however. We have tested a variety of Oct2 isoforms generated by alternative splicing for the capability to activate an octamer enhancer in nonlymphoid cells and a B-cell line. Our analyses show that several Oct2 isoforms can stimulate from a remote position but that this stimulation is restricted to B cells. This result indicates the involvement of either a B-cell-specific cofactor or a specific modification of a cofactor or the Oct2 protein in Oct2-mediated enhancer activation. Mutational analyses indicate that the carboxy-terminal domain of Oct2 is critical for enhancer activation. Moreover, this domain conferred enhancing activity when fused to the Oct1 protein, which by itself was unable to stimulate from a remote position. The glutamine-rich activation domain present in the amino-terminal portion of Oct2 and the POU domain contribute only marginally to the transactivation function from a distal position.

The promoter of the CD19 gene is a target for the B-cell-specific transcription factor BSAP

The CD19 protein is expressed on the surface of all B-lymphoid cells with the exception of terminally differentiated plasma cells and has been implicated as a signal-transducing receptor in the control of proliferation and differentiation. Here we demonstrate complete correlation between the expression pattern of the CD19 gene and the B-cell-specific transcription factor BSAP in a large panel of B-lymphoid cell lines. The human CD19 gene has been cloned, and several BSAP-binding sites have been mapped by in vitro protein-DNA binding studies. In particular, a high-affinity BSAP-binding site instead of a TATA sequence is located in the -30 promoter region upstream of a cluster of heterogeneous transcription start sites. Moreover, this site is occupied by BSAP in vivo in a CD19-expressing B-cell line but not in plasma or HeLa cells. This high-affinity site has been conserved in the promoters of both human and mouse CD19 genes and was furthermore shown to confer B-cell specificity to a beta-globin reporter gene in transient transfection experiments. In addition, BSAP was found to be the only abundant DNA-binding activity of B-cell nuclear extracts that interacts with the CD19 promoter. Together, this evidence strongly implicates BSAP in the regulation of the CD19 gene.

The promoter of the CD19 gene is a target for the B-cell-specific transcription factor BSAP

The CD19 protein is expressed on the surface of all B-lymphoid cells with the exception of terminally differentiated plasma cells and has been implicated as a signal-transducing receptor in the control of proliferation and differentiation. Here we demonstrate complete correlation between the expression pattern of the CD19 gene and the B-cell-specific transcription factor BSAP in a large panel of B-lymphoid cell lines. The human CD19 gene has been cloned, and several BSAP-binding sites have been mapped by in vitro protein-DNA binding studies. In particular, a high-affinity BSAP-binding site instead of a TATA sequence is located in the -30 promoter region upstream of a cluster of heterogeneous transcription start sites. Moreover, this site is occupied by BSAP in vivo in a CD19-expressing B-cell line but not in plasma or HeLa cells. This high-affinity site has been conserved in the promoters of both human and mouse CD19 genes and was furthermore shown to confer B-cell specificity to a beta-globin reporter gene in transient transfection experiments. In addition, BSAP was found to be the only abundant DNA-binding activity of B-cell nuclear extracts that interacts with the CD19 promoter. Together, this evidence strongly implicates BSAP in the regulation of the CD19 gene.

Glycosyl-phosphatidylinositol linkage as a mechanism for cell-surface expression of immunoglobulin D

The B-cell antigen receptor of the IgM and IgD class is a multimeric complex consisting of the membrane-bound form of the immunoglobulin molecule and two other proteins, Ig-alpha and Ig-beta. The Ig-alpha and Ig-beta proteins form a disulphide-linked alpha/beta heterodimer and are encoded by the mb-1 (ref 9, 10) and B29 genes, respectively. Surface expression of the membrane-bound IgM molecule requires assembly with the alpha/beta heterodimer. The IgD molecule, however, can be expressed on the cell surface in an alpha/beta-dependent and -independent form. We show here that in the alpha/beta-independent form the IgD molecule is anchored in the plasma membrane through a glycosyl-phosphatidylinositol linker. In the presence of the alpha/beta heterodimer, most of the otherwise glycosyl-phosphatidylinositol-linked IgD molecule is expressed on the cell surface as transmembrane proteins.

Structure of the genes encoding the CD19 antigen of human and mouse B lymphocytes

CD19 is a B lymphocyte cell-surface marker that is expressed early during pre-B-cell differentiation with expression persisting until terminal differentiation into plasma cells. CD19 is a member of the Ig gene superfamily with two extracellular Ig-like domains separated by a non-Ig-like domain, and with an extensive approximately 240 amino acid cytoplasmic domain. In this study, Southern blot analysis revealed that the human and mouse CD19 genes were compact single copy genes. Both the human and mouse CD19 genes were isolated and the nucleotide sequences flanking each exon were determined. Both genes were composed of 15 exons and spanned approximately 8 kilobases (kb) of DNA in human and approximately 6 kb in mouse. The positions of exon-intron boundaries were identical between human and mouse and correlated with the putative functional domains of the CD19 protein. The 200 bp region 5' of the putative translation initiation AUG codon was well conserved in sequence between human and mouse and contained potential transcription regulatory elements. In addition, the 3' untranslated regions (UT) of the CD19 genes following the termination codon were conserved in sequence. The high level of conservation of nucleotide sequences between species in all exons and 5' and 3' UT suggests that expression of the CD19 gene may be regulated in a similar fashion in human and mouse.