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

Transcriptional function of E2A, Ebf1, Pax5, Ikaros and Aiolos analyzed by in vivo acute protein degradation in early B cell development

Early B cell lymphopoiesis depends on E2A, Ebf1, Pax5 and Ikaros family members. In the present study, we used acute protein degradation in mice to identify direct target genes of these transcription factors in pro-B, small pre-B and immature B cells. E2A, Ebf1 and Pax5 predominantly function as transcriptional activators by inducing open chromatin at their target genes, have largely unique functions and are essential for early B cell maintenance. Ikaros and Aiolos act as dedicated repressors to cooperatively control early B cell development. The surrogate light-chain genes Igll1 and Vpreb1 are directly activated by Ebf1 and Pax5 in pro-B cells and directly repressed by Ikaros and Aiolos in small pre-B cells. Pax5 and E2A contribute to V(D)J recombination by activating Rag1, Rag2, Dntt, Irf4 and Irf8. Similar to Pax5, Ebf1 also represses the cohesin-release factor gene Wapl to mediate prolonged loop extrusion across the Igh locus. In summary, in vivo protein degradation has provided unprecedented insight into the control of early B cell lymphopoiesis by five transcription factors.

Early B-Cell Factor 1: An Archetype for a Lineage-Restricted Transcription Factor Linking Development to Disease

The development of highly specialized blood cells from hematopoietic stem cells (HSCs) in the bone marrow (BM) is dependent upon a stringently orchestrated network of stage- and lineage-restricted transcription factors (TFs). Thus, the same stem cell can give rise to various types of differentiated blood cells. One of the key regulators of B-lymphocyte development is early B-cell factor 1 (EBF1). This TF belongs to a small, but evolutionary conserved, family of proteins that harbor a Zn-coordinating motif and an IPT/TIG (immunoglobulin-like, plexins, transcription factors/transcription factor immunoglobulin) domain, creating a unique DNA-binding domain (DBD). EBF proteins play critical roles in diverse developmental processes, including body segmentation in the Drosophila melanogaster embryo, and retina formation in mice. While several EBF family members are expressed in neuronal cells, adipocytes, and BM stroma cells, only B-lymphoid cells express EBF1. In the absence of EBF1, hematopoietic progenitor cells (HPCs) fail to activate the B-lineage program. This has been attributed to the ability of EBF1 to act as a pioneering factor with the ability to remodel chromatin, thereby creating a B-lymphoid-specific epigenetic landscape. Conditional inactivation of the Ebf1 gene in B-lineage cells has revealed additional functions of this protein in relation to the control of proliferation and apoptosis. This may explain why EBF1 is frequently targeted by mutations in human leukemia cases. This chapter provides an overview of the biochemical and functional properties of the EBF family proteins, with a focus on the roles of EBF1 in normal and malignant B-lymphocyte development.

Genome-wide interference of ZNF423 with B-lineage transcriptional circuitries in acute lymphoblastic leukemia

Aberrant expression of the transcriptional modulator and early B-cell factor 1 (EBF1) antagonist ZNF423 has been implicated in B-cell leukemogenesis, but its impact on transcriptional circuitries in lymphopoiesis has not been elucidated in a comprehensive manner. Herein, in silico analyses of multiple expression data sets on 1354 acute leukemia samples revealed a widespread presence of ZNF423 in various subtypes of acute lymphoblastic leukemia (ALL). Average expression of ZNF423 was highest in ETV6-RUNX1, B-other, and TCF3-PBX1 ALL followed by BCR-ABL, hyperdiploid ALL, and KMT2A-rearranged ALL. In a KMT2A-AFF1 pro-B ALL model, a CRISPR-Cas9-mediated genetic ablation of ZNF423 decreased cell viability and significantly prolonged survival of mice upon xenotransplantation. For the first time, we characterized the genome-wide binding pattern of ZNF423, its impact on the chromatin landscape, and differential gene activities in a B-lineage context. In general, chromatin-bound ZNF423 was associated with a depletion of activating histone marks. At the transcriptional level, EBF1-dependent transactivation was disrupted by ZNF423, whereas repressive and pioneering activities of EBF1 were not discernibly impeded. Unexpectedly, we identified an enrichment of ZNF423 at canonical EBF1-binding sites also in the absence of EBF1, which was indicative of intrinsic EBF1-independent ZNF423 activities. A genome-wide motif search at EBF1 target gene loci revealed that EBF1 and ZNF423 co-regulated genes often contain SMAD1/SMAD4-binding motifs as exemplified by the TGFB1 promoter, which was repressed by ZNF423 outcompeting EBF1 by depending on its ability to bind EBF1 consensus sites and to interact with EBF1 or SMADs. Overall, these findings underscore the wide scope of ZNF423 activities that interfere with B-cell lymphopoiesis and contribute to leukemogenesis.

Dynamic EBF1 occupancy directs sequential epigenetic and transcriptional events in B-cell programming

B-cell fate determination requires the action of transcription factors that operate in a regulatory network to activate B-lineage genes and repress lineage-inappropriate genes. However, the dynamics and hierarchy of events in B-cell programming remain obscure. To uncouple the dynamics of transcription factor expression from functional consequences, we generated induction systems in developmentally arrested Ebf1-/- pre-pro-B cells to allow precise experimental control of EBF1 expression in the genomic context of progenitor cells. Consistent with the described role of EBF1 as a pioneer transcription factor, we show in a time-resolved analysis that EBF1 occupancy coincides with EBF1 expression and precedes the formation of chromatin accessibility. We observed dynamic patterns of EBF1 target gene expression and sequential up-regulation of transcription factors that expand the regulatory network at the pro-B-cell stage. A continuous EBF1 function was found to be required for Cd79a promoter activity and for the maintenance of an accessible chromatin domain that is permissive for binding of other transcription factors. Notably, transient EBF1 occupancy was detected at lineage-inappropriate genes prior to their silencing in pro-B cells. Thus, persistent and transient functions of EBF1 allow for an ordered sequence of epigenetic and transcriptional events in B-cell programming.

Cooperative interactions enable singular olfactory receptor expression in mouse olfactory neurons

The monogenic and monoallelic expression of only one out of >1000 mouse olfactory receptor (ORs) genes requires the formation of large heterochromatic chromatin domains that sequester the OR gene clusters. Within these domains, intergenic transcriptional enhancers evade heterochromatic silencing and converge into interchromosomal hubs that assemble over the transcriptionally active OR. The significance of this nuclear organization in OR choice remains elusive. Here, we show that transcription factors Lhx2 and Ebf specify OR enhancers by binding in a functionally cooperative fashion to stereotypically spaced motifs that defy heterochromatin. Specific displacement of Lhx2 and Ebf from OR enhancers resulted in pervasive, long-range, and trans downregulation of OR transcription, whereas pre-assembly of a multi-enhancer hub increased the frequency of OR choice in cis. Our data provide genetic support for the requirement and sufficiency of interchromosomal interactions in singular OR choice and generate general regulatory principles for stochastic, mutually exclusive gene expression programs.

ZNF423 and ZNF521: EBF1 Antagonists of Potential Relevance in B-Lymphoid Malignancies

The development of the B-lymphoid cell lineage is tightly controlled by the concerted action of a network of transcriptional and epigenetic regulators. EBF1, a central component of this network, is essential for B-lymphoid specification and commitment as well as for the maintenance of the B-cell identity. Genetic alterations causing loss of function of these B-lymphopoiesis regulators have been implicated in the pathogenesis of B-lymphoid malignancies, with particular regard to B-cell acute lymphoblastic leukaemias (B-ALLs), where their presence is frequently detected. The activity of the B-cell regulatory network may also be disrupted by the aberrant expression of inhibitory molecules. In particular, two multi-zinc finger transcription cofactors named ZNF423 and ZNF521 have been characterised as potent inhibitors of EBF1 and are emerging as potentially relevant contributors to the development of B-cell leukaemias. Here we will briefly review the current knowledge of these factors and discuss the importance of their functional cross talk with EBF1 in the development of B-cell malignancies.

Genome-Wide Mapping of Collier In Vivo Binding Sites Highlights Its Hierarchical Position in Different Transcription Regulatory Networks

Collier, the single Drosophila COE (Collier/EBF/Olf-1) transcription factor, is required in several developmental processes, including head patterning and specification of muscle and neuron identity during embryogenesis. To identify direct Collier (Col) targets in different cell types, we used ChIP-seq to map Col binding sites throughout the genome, at mid-embryogenesis. In vivo Col binding peaks were associated to 415 potential direct target genes. Gene Ontology analysis revealed a strong enrichment in proteins with DNA binding and/or transcription-regulatory properties. Characterization of a selection of candidates, using transgenic CRM-reporter assays, identified direct Col targets in dorso-lateral somatic muscles and specific neuron types in the central nervous system. These data brought new evidence that Col direct control of the expression of the transcription regulators apterous and eyes-absent (eya) is critical to specifying neuronal identities. They also showed that cross-regulation between col and eya in muscle progenitor cells is required for specification of muscle identity, revealing a new parallel between the myogenic regulatory networks operating in Drosophila and vertebrates. Col regulation of eya, both in specific muscle and neuronal lineages, may illustrate one mechanism behind the evolutionary diversification of Col biological roles.

The regulatory network of B-cell differentiation: a focused view of early B-cell factor 1 function

During the last decades, many studies have investigated the transcriptional and epigenetic regulation of lineage decision in the hematopoietic system. These efforts led to a model in which extrinsic signals and intrinsic cues establish a permissive chromatin context upon which a regulatory network of transcription factors and epigenetic modifiers act to guide the differentiation of hematopoietic lineages. These networks include lineage-specific factors that further modify the epigenetic landscape and promote the generation of specific cell types. The process of B lymphopoiesis requires a set of transcription factors, including Ikaros, PU.1, E2A, and FoxO1 to 'prime' cis-regulatory regions for subsequent activation by the B-lineage-specific transcription factors EBF1 and Pax-5. The expression of EBF1 is initiated by the combined action of E2A and FoxO1, and it is further enhanced and maintained by several positive feedback loops that include Pax-5 and IL-7 signaling. EBF1 acts in concert with Ikaros, PU.1, Runx1, E2A, FoxO1, and Pax-5 to establish the B cell-specific transcription profile. EBF1 and Pax-5 also collaborate to repress alternative cell fates and lock cells into the B-lineage fate. In addition to the functions of EBF1 in establishing and maintaining B-cell identity, EBF1 is required to coordinate differentiation with cell proliferation and survival.

Ebf factors and MyoD cooperate to regulate muscle relaxation via Atp2a1

Myogenic regulatory factors such as MyoD and Myf5 lie at the core of vertebrate muscle differentiation. However, E-boxes, the cognate binding sites for these transcription factors, are not restricted to the promoters/enhancers of muscle cell-specific genes. Thus, the specificity in myogenic transcription is poorly defined. Here we describe the transcription factor Ebf3 as a new determinant of muscle cell-specific transcription. In the absence of Ebf3 the lung does not unfold at birth, resulting in respiratory failure and perinatal death. This is due to a hypercontractile diaphragm with impaired Ca(2+) efflux-related muscle functions. Expression of the Ca(2+) pump Serca1 (Atp2a1) is downregulated in the absence of Ebf3, and its transgenic expression rescues this phenotype. Ebf3 binds directly to the promoter of Atp2a1 and synergises with MyoD in the induction of Atp2a1. In skeletal muscle, the homologous family member Ebf1 is strongly expressed and together with MyoD induces Atp2a1. Thus, Ebf3 is a new regulator of terminal muscle differentiation in the diaphragm, and Ebf factors cooperate with MyoD in the induction of muscle-specific genes.

Early B-cell factor 1 is an essential transcription factor for postnatal glomerular maturation

The coordination of multiple cytokines and transcription factors with their downstream signaling pathways have been shown to be integral to nephron maturation. Here we present a completely novel role for the helix-loop-helix transcription factor Early B cell Factor 1 (Ebf1), originally identified for B cell maturation, for the proper maturation of glomerular cells from mesenchymal progenitors. The expression of Ebf1 was both spatially and temporally regulated within the developing cortex and glomeruli. Using Ebf1-null mice we then identified biochemical, metabolic, and histological abnormalities in renal development that arose in the absence of this transcription factor. In the Ebf1 knockout mice the developed kidneys show thinned cortices and reduced glomerular maturation. The glomeruli showed abnormal vascularization and severely effaced podocytes. The mice exhibited early albuminuria and elevated blood urea nitrogen levels. Moreover, the GFR was reduced over 66 percent and the expression of podocyte-derived VEGF-A was decreased compared to wild type control mice. Thus, Ebf1 has a significant and novel role in glomerular development, podocyte maturation, and the maintenance of kidney integrity and function.

Coe genes are expressed in differentiating neurons in the central nervous system of protostomes

Genes of the coe (collier/olfactory/early B-cell factor) family encode Helix-Loop-Helix transcription factors that are widely conserved in metazoans and involved in many developmental processes, neurogenesis in particular. Whereas their functions during vertebrate neural tube formation have been well documented, very little is known about their expression and role during central nervous system (CNS) development in protostomes. Here we characterized the CNS expression of coe genes in the insect Drosophila melanogaster and the polychaete annelid Platynereis dumerilii, which belong to different subgroups of protostomes and show strikingly different modes of development. In the Drosophila ventral nerve cord, we found that the Collier-expressing cells form a subpopulation of interneurons with diverse molecular identities and neurotransmitter phenotypes. We also demonstrate that collier is required for the proper differentiation of some interneurons belonging to the Eve-Lateral cluster. In Platynereis dumerilii, we cloned a single coe gene, Pdu-coe, and found that it is exclusively expressed in post mitotic neural cells. Using an original technique of in silico 3D registration, we show that Pdu-coe is co-expressed with many different neuronal markers and therefore that, like in Drosophila, its expression defines a heterogeneous population of neurons with diverse molecular identities. Our detailed characterization and comparison of coe gene expression in the CNS of two distantly-related protostomes suggest conserved roles of coe genes in neuronal differentiation in this clade. As similar roles have also been observed in vertebrates, this function was probably already established in the last common ancestor of all bilaterians.

B lymphocyte lineage specification, commitment and epigenetic control of transcription by early B cell factor 1

Early B cell factor 1 (EBF1) is a transcription factor that is critical for both B lymphopoiesis and B cell function. EBF1 is a requisite component of the B lymphocyte transcriptional network and is essential for B lineage specification. Recent studies revealed roles for EBF1 in B cell commitment. EBF1 binds its target genes via a DNA-binding domain including a unique 'zinc knuckle', which mediates a novel mode of DNA recognition. Chromatin immunoprecipitation of EBF1 in pro-B cells defined hundreds of new, as well as previously identified, target genes. Notably, expression of the pre-B cell receptor (pre-BCR), BCR and PI3K/Akt/mTOR signaling pathways is controlled by EBF1. In this review, we highlight these current developments and explore how EBF1 functions as a tissue-specific regulator of chromatin structure at B cell-specific genes.

Developmental expression of COE across the Metazoa supports a conserved role in neuronal cell-type specification and mesodermal development

The transcription factor COE (collier/olfactory-1/early B cell factor) is an unusual basic helix-loop-helix transcription factor as it lacks a basic domain and is maintained as a single copy gene in the genomes of all currently analysed non-vertebrate Metazoan genomes. Given the unique features of the COE gene, its proposed ancestral role in the specification of chemosensory neurons and the wealth of functional data from vertebrates and Drosophila, the evolutionary history of the COE gene can be readily investigated. We have examined the ways in which COE expression has diversified among the Metazoa by analysing its expression from representatives of four disparate invertebrate phyla: Ctenophora (Mnemiopsis leidyi); Mollusca (Haliotis asinina); Annelida (Capitella teleta and Chaetopterus) and Echinodermata (Strongylocentrotus purpuratus). In addition, we have studied COE function with knockdown experiments in S. purpuratus, which indicate that COE is likely to be involved in repressing serotonergic cell fate in the apical ganglion of dipleurula larvae. These analyses suggest that COE has played an important role in the evolution of ectodermally derived tissues (likely primarily nervous tissues) and mesodermally derived tissues. Our results provide a broad evolutionary foundation from which further studies aimed at the functional characterisation and evolution of COE can be investigated.

Early B-cell factor regulates the expression of Hemokinin-1 in the olfactory epithelium and differentiating B lymphocytes

Hemokinin-1, encoded by the TAC4 gene, is a tachykinin most closely related to substance P. Previous studies have shown that TAC4 distinguishes itself from other tachykinins by its predominantly non-neuronal expression profile, particularly in cells of the immune system. Here we report for the first time that the highest levels of TAC4 expression are found in the olfactory epithelium. Furthermore, we identify olfactory neuron-specific transcription factor (Olf-1), also known as early B-cell factor (EBF), as a novel regulator of TAC4 expression. EBF present in the olfactory epithelium and in B cells binds to two sites in the TAC4 promoter and modulates expression in developing B cells. Our findings suggest a role for TAC4 in cell differentiation, and represent a regulatory bridge between the nervous system and the immune system.

Stepwise activation of enhancer and promoter regions of the B cell commitment gene Pax5 in early lymphopoiesis

Pax5 is an essential regulator of B cell identity and function. Here, we used transgenesis and deletion mapping to identify a potent enhancer in intron 5 of the Pax5 locus. This enhancer in combination with the promoter region was sufficient to recapitulate the B lymphoid expression of Pax5. The enhancer was silenced by DNA methylation in embryonic stem cells, but became activated in multipotent hematopoietic progenitors. It contained functional binding sites for the transcription factors PU.1, IRF4, IRF8, and NF-kappaB, suggesting that these regulators contribute to sequential enhancer activation in hematopoietic progenitors and during B cell development. In contrast, the promoter region was repressed by Polycomb group proteins in non-B cells and was activated only at the onset of pro-B cell development through induction of chromatin remodeling by the transcription factor EBF1. These experiments demonstrate a stepwise activation of Pax5 in early lymphopoiesis and provide mechanistic insights into the process of B cell commitment.

Early B cell factor: Regulator of B lineage specification and commitment

B lymphocytes are generated from hematopoietic stem cells in a series of steps controlled by transcription factors. One of the most important regulators of this process is early B cell factor (EBF). Multiple lines of evidence indicate that expression of EBF is a principle determinant of the B cell fate. In the absence of EBF, progenitor cells fail to express classical markers of B cells, including immunoglobulins. EBF drives B cell differentiation by activating the Pax5 gene and other genes required for the pre-B and B cell receptors. New evidence suggests that expression of EBF in common lymphoid progenitors directs B cell fate decisions. Specification and commitment of cells to the B cell lineage are further established by Pax5, which increases expression of EBF. Recently, it was demonstrated that both EBF and Pax5 contribute to the commitment of cells to the B lineage. Together, these studies confirm that EBF is a keystone in the regulatory network that coordinates B cell lineage specification and commitment.

The 'zinc knuckle' motif of Early B cell Factor is required for transcriptional activation of B cell-specific genes

Early B cell factor (EBF) is a critical regulator of B lymphocyte-specific gene transcription. EBF functions, in part, by binding to regulatory sites of genes required for the pre-B- and mature B cell receptors. These DNA targets include the promoters of the mb-1 and Vpreb1 genes that encode Ig-alpha and one of the components of surrogate light chain, respectively. The biochemical basis of DNA binding and gene activation by EBF is poorly understood. The DNA-binding domain (DBD) of EBF includes a putative zinc-binding motif (HX(3)CX(2)CX(5)C), which we have designated the 'Zn-knuckle'. The Zn-knuckle is required for binding of the mb-1 promoter site in EMSA, but it has not been demonstrated to be important for functional activities of EBF in B cells. Therefore, we expressed EBF with mutations in the Zn-knuckle motif or flanking sequences in plasmacytoma cells in which activation of endogenous mb-1 and Vpreb1 genes is dependent on EBF. EBF with mutations that prevent zinc coordination by the Zn-knuckle did not activate transcription of either target gene. Other mutations affected the sequence preference of DNA binding and differentially inhibited activation of these genes. Our results demonstrate the importance of the Zn-knuckle motif in EBF. These experiments also confirm that EBF can re-activate multiple genes of the early B cell program in plasmacytoma cells, which provide a useful cell-based assay for dissecting mechanisms involving EBF.

The metazoan history of the COE transcription factors. Selection of a variant HLH motif by mandatory inclusion of a duplicated exon in vertebrates

Background

The increasing number of available genomic sequences makes it now possible to study the evolutionary history of specific genes or gene families. Transcription factors (TFs) involved in regulation of gene-specific expression are key players in the evolution of metazoan development. The low complexity COE (Collier/Olfactory-1/Early B-Cell Factor) family of transcription factors constitutes a well-suited paradigm for studying evolution of TF structure and function, including the specific question of protein modularity. Here, we compare the structure of coe genes within the metazoan kingdom and report on the mechanism behind a vertebrate-specific exon duplication.

Results

COE proteins display a modular organisation, with three highly conserved domains : a COE-specific DNA-binding domain (DBD), an Immunoglobulin/Plexin/transcription (IPT) domain and an atypical Helix-Loop-Helix (HLH) motif. Comparison of the splice structure of coe genes between cnidariae and bilateriae shows that the ancestral COE DBD was built from 7 separate exons, with no evidence for exon shuffling with other metazoan gene families. It also confirms the presence of an ancestral H1LH2 motif present in all COE proteins which partly overlaps the repeated H2d-H2a motif first identified in rodent EBF. Electrophoretic Mobility Shift Assays show that formation of COE dimers is mediated by this ancestral motif. The H2d-H2a α-helical repetition appears to be a vertebrate characteristic that originated from a tandem exon duplication having taken place prior to the splitting between gnathostomes and cyclostomes. We put-forward a two-step model for the inclusion of this exon in the vertebrate transcripts.

Conclusion

Three main features in the history of the coe gene family can be inferred from these analyses: (i) each conserved domain of the ancestral coe gene was built from multiple exons and the same scattered structure has been maintained throughout metazoan evolution. (ii) There exists a single coe gene copy per metazoan genome except in vertebrates. The H2a-H2d duplication that is specific to vertebrate proteins provides an example of a novel vertebrate characteristic, which may have been fixed early in the gnathostome lineage. (iii) This duplication provides an interesting example of counter-selection of alternative splicing.

Analysis of the promoter region of human placenta-specific DSCR4 gene

The gene DSCR4 locates in the band q22.2 of human chromosome 21 and encodes a protein of 118 amino acids. Expression of DSCR4 is restricted to human placenta and placental choriocarcinoma cell lines BeWo and JEG3. The 5'-RACE method using RNA from human placenta indicated the major transcription start site at 93 nt upstream (nt -93) of the initiation codon. Transfection assay using a series of deletion constructs of the 5'-flanking region fused to the luciferase reporter gene identified three positive regions nt -2200 to -2088, nt -2064 to -1924, nt -810 to -632 and two negative regions nt -1923 to -1740, nt -631 to -425. The computer analysis predicted the presence of several cis-elements in these regions and the promoter assay using various mutants of consensus sequence identified two distinct cis-elements for OLF-1 and E47. The electrophoretic mobility shift assay (EMSA) using the extracts of DSCR4-expressing cells confirmed the binding of certain protein factors to these cis-elements. In fact, OLF-1-like transcription factor, EBF-3 and EBF-4 were detected in the DSCR4-expressing cell lines and human placenta. Based on these data, we postulated that transcription of DSCR4 gene is regulated positively by binding of OLF-1-like transcription factor and negatively by binding of E47-like transcription factor.

Collier transcription in a single Drosophila muscle lineage: the combinatorial control of muscle identity

Specification of muscle identity in Drosophila is a multistep process: early positional information defines competence groups termed promuscular clusters, from which muscle progenitors are selected, followed by asymmetric division of progenitors into muscle founder cells (FCs). Each FC seeds the formation of an individual muscle with morphological and functional properties that have been proposed to reflect the combination of transcription factors expressed by its founder. However, it is still unclear how early patterning and muscle-specific differentiation are linked. We addressed this question, using Collier (Col; also known as Knot) expression as both a determinant and read-out of DA3 muscle identity. Characterization of the col upstream region driving DA3 muscle specific expression revealed the existence of three separate phases of cis-regulation, correlating with conserved binding sites for different mesodermal transcription factors. Examination of col transcription in col and nautilus (nau) loss-of-function and gain-of-function conditions showed that both factors are required for col activation in the ;naïve' myoblasts that fuse with the DA3 FC, thereby ensuring that all DA3 myofibre nuclei express the same identity programme. Together, these results indicate that separate sets of cis-regulatory elements control the expression of identity factors in muscle progenitors and myofibre nuclei and directly support the concept of combinatorial control of muscle identity.

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.

Role of transcription factors in commitment and differentiation of early B lymphoid cells

B lymphopoiesis is a differentiation process in which hematopoietic stem cells are converted into antibody-producing plasma cells. B cell differentiation involves multiple steps, including cell specification, commitment to the B cell lineage, immunoglobulin rearrangements, maturation of B cells and terminal differentiation into plasma cells. Each of these steps is controlled by signaling pathways and transcription factors that act in synergy, feedback-loops or cross-antagonism to generate complex regulatory networks that allow for plasticity and stability of B cell differentiation.

EBF2 Regulates Osteoblast-Dependent Differentiation of Osteoclasts

Communication between bone-depositing osteoblasts and bone-resorbing osteoclasts is required for bone development and homeostasis. Here, we identify EBF2, a member of the early B cell factor (EBF) family of transcription factors that is expressed in osteoblast progenitors, as a regulator of osteoclast differentiation. We find that mice homozygous for a targeted inactivation of Ebf2 show reduced bone mass and an increase in the number of osteoclasts. These defects are accompanied by a marked downregulation of the osteoprotegerin (Opg) gene, encoding a RANK decoy receptor. EBF2 binds to sequences in the Opg promoter and transactivates the Opg promoter in synergy with the Wnt-responsive LEF1/TCF:beta-catenin pathway. Taken together, these data identify EBF2 as a regulator of RANK-RANKL signaling and osteoblast-dependent differentiation of osteoclasts.

The λ5–VpreB1 locus—a model system for studying gene regulation during early B cell development

The lambda5 and VpreB genes encode the components of the surrogate light-chain, which forms part of the pre-B cell receptor. In mouse, the lambda5 and VpreB1 genes of mouse are closely linked and coordinately regulated by a locus control region (LCR). Activation of the genes in pro-B cells depends on the combined effects of early B cell factor (EBF) and the E2A factors E12 and E47. Silencing of lambda5 expression in mature B cells occurs through the action of Ikaros on the gene promoter where it may compete for binding of EBF and initiate the formation of a silent chromatin structure.

Genetic disruptions of O/E2 and O/E3 genes reveal involvement in olfactory receptor neuron projection

The mammalian Olf1/EBF (O/E) family of repeated helix-loop-helix (rHLH)transcription factors has been implicated in olfactory system gene regulation,nervous system development and B-cell differentiation. Ebf(O/E1) mutant animals showed defects in B-cell lineage and brain regions where it is the only O/E family member expressed, but the olfactory epithelium appeared unaffected and olfactory marker expression was grossly normal in these animals. In order to further study the mammalian O/E proteins,we disrupted O/E2 and O/E3 genes in mouse and placed tau-lacZ and tau-GFP reporter genes under the control of the respective endogenous O/E promoters. Mice mutant for each of these genes display reduced viability and other gene-specific phenotypes. Interestingly, both O/E2 and O/E3 knockout mice as well as O/E2/O/E3 double heterozygous animals share a common phenotype:olfactory neurons (ORN) fail to project to dorsal olfactory bulb. We suggest that a decreased dose of O/E protein may alter expression of O/E target genes and underlie the ORN projection defect.

Olf/EBF proteins are expressed in neuroblastoma cells: Potential regulators of theChromogranin A andSCG10 promoters

The childhood malignancy neuroblastoma is derived from developmentally arrested sympathetic nervous system precursor cells. To obtain further insight into the molecular processes involved in the formation of these tumors, we decided to investigate the functional role of Olf/EBF (O/E) transcription factors in human neuroblastoma cells. We here report that O/E-1 and O/E-2 are expressed at variable levels in neuroblastoma cell lines and that O/E proteins could be identified by electrophoretic mobility shift assays. To identify potential neuronal target genes for O/E proteins in neuroblastoma cells we investigated the ability of a set of neuronal promoters to interact with O/E-1 in electrophoretic mobility shift assays. This analysis suggested that the Chromogranin A (CgA) and SCG10 promoters both contained binding sites for O/E-1. O/E-1 was able to activate the CgA promoter in vivo and mutation of the O/E-1 binding site in the CgA promoter reduced the functional activity of the element to about 60% of the wild-type in neuroblastoma cells, supporting the idea that O/E proteins may be involved in the control of the CgA promoter. Furthermore, overexpression of O/E-1 in hippocampal progenitor cells led to neurite outgrowth, indicative of a role for O/E proteins in neuronal differentiation.

CD40 stimulation induces Pax5/BSAP and EBF activation through a APE/Ref-1-dependent redox mechanism

CD40 is a member of the growing tumor necrosis factor receptor family that has been shown to play important roles in T cell-mediated B lymphocyte activation. Ligation of B cell CD40 by CD154, mainly expressed on activated T cells, stimulates B cell proliferation, differentiation, isotype switching, up-regulation of surface molecules contributing to antigen presentation, development of the germinal center, and the humoral memory response. In this study we demonstrate that the redox factor APE/Ref-1 acts as a key signaling intermediate in response to CD40-mediated B cell activation. The transcription factors Pax5a or BSAP (B cell lineage-specific activator protein) and EBF (early B cell factor) are constitutively expressed in spleen B cells and CD40 cross-linking induces increases in Pax5a and EBF binding activity compared with nonstimulated B cells. We show that upon CD40 antibody-mediated cross-linking, APE/Ref-1 translocates from the cytoplasm to the nucleus of activated B cells, where it modulates the DNA binding activity of both Pax5a and EBF. Moreover, we show that the repression of APE/Ref-1 protein production is able to block CD40-mediated Pax5a activation. We also provide evidence that APE/Ref-1 can modulate the cooperative activation of the blk promoter operated by Pax5a and EBF and that APE/Ref-1 might directly regulate EBF functional activity. Finally, we show that the interaction between Pax5a and EBF enhances EBF binding activity to its consensus sequence, suggesting that Pax5a can physically interact with EBF and modulate its DNA binding activity.

Ebf gene function is required for coupling neuronal differentiation and cell cycle exit

Helix-loop-helix transcription factors of the Ebf/Olf1 family have previously been implicated in the control of neurogenesis in the central nervous system in both Xenopus laevis and the mouse, but their precise roles have remained unclear. We have characterised two family members in the chick, and have performed a functional analysis by gain- and loss-of-function experiments. This study revealed several specific roles for Ebf genes in the spinal cord and hindbrain regions of higher vertebrates, and enabled their precise positioning along the neurogenic cascade. During neurogenesis, cell cycle exit appears to be tightly coupled to migration to the mantle layer and to neuronal differentiation. We show that antagonizing Ebf gene activity allows the uncoupling of these processes. Ebf gene function is necessary to initiate neuronal differentiation and migration toward the mantle layer in neuroepithelial progenitors, but it is not required for cell cycle exit. Ebf genes therefore appear to be master controllers of neuronal differentiation and migration, coupling them to cell cycle exit and earlier steps of neurogenesis. Mutual activation between proneural and Ebf genes suggests that besides their involvement in the engagement of differentiation, Ebf genes may also participate in the stabilisation of the committed state. Finally, gain-of-function data raise the possibility that, in addition to these general roles, Ebf genes may be involved in neuronal subtype specification in particular regions of the CNS.

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.

Cloning and characterization of a promoter flanking the early B cell factor (EBF) gene indicates roles for E-proteins and autoregulation in the control of EBF expression

The early B cell factor (EBF) is a transcription factor shown crucial for the development of B lymphocytes. The protein is expressed from the earliest stages of B cell development until the mature B cell stage, but the control elements responsible for the regulation of the gene are unknown. In this study, we report of the identification of a promoter region flanking the EBF gene. Several transcription start sites were identified by primer extension analysis in a region approximately 3.1 kb from the predicted ATG. Transient transfections revealed that this region was able to stimulate transcription of a reporter gene in B lymphoid and to a lesser extent, myeloid cells, but not in a pre-T cell line. The promoter was also able to functionally interact with E47, suggesting that the EBF gene may be a direct target for activation by E-proteins. In addition, functional binding of EBF to its own promoter was confirmed by EMSA and transfection assays indicating that the EBF protein may be involved in an autoregulatory loop. Finally, a tissue-restricted factor was able to bind an upstream regulatory region in B-lineage cells, further supporting the idea that the cloned promoter participates in the regulation of stage and lineage specific expression of the EBF gene.

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.

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.

Olf-1/early B cell factor is a regulator of glut4 gene expression in 3T3-L1 adipocytes

A negative regulatory element in the 5'-flanking region of the murine glut4 gene mediates chronic insulin- and cAMP-induced repression in 3T3-L1 adipocytes. Previous work demonstrated that members of the nuclear factor 1 (NF1) family of transcription factors and an unidentified factor bind to and mediate repression from this regulatory element. By using a yeast one-hybrid screen, Olf-1/Early B cell factor (O/E-1) was isolated as a candidate for this unidentified factor. A protein complex from 3T3-L1 adipocyte nuclear extract that bound the negative regulatory element was recognized by O/E-specific antiserum, and binding activity was competed effectively by distinct O/E-binding sequences. O/E binding activity was also detected in nuclear extracts from insulin-responsive, GLUT4-expressing tissues including adipose, skeletal muscle, and heart. Mutations within the negative regulatory element that abolish binding of O/E proteins concomitantly blocked insulin-induced repression in reporter gene assays. These results suggest that one or more members of the O/E transcription factor family function as important regulators of glut4 gene expression and therefore may play a heretofore unanticipated role in glucose homeostasis and insulin signaling.

The COE--Collier/Olf1/EBF--transcription factors: structural conservation and diversity of developmental functions

One major conclusion of studies in Developmental Biology during the last two decades is that, despite profound anatomical differences, the building of vertebrate and arthropod bodies relies on the same fundamental molecular networks, including conserved cell signalling and transcription-regulatory cascades. Rodent Early B-Cell Factor/Olfactory-1 and Drosophila Collier belong to a recently defined, novel family of transcription factors, the Collier/Olf1/EBF (COE) proteins which have a unique DNA-binding domain. Early investigations revealed that, despite their high degree of sequence identity, the different vertebrate and invertebrate COE proteins play a variety of developmental roles. We review here the current evidence for this diversity of COE functions, including in the specification and differentiation of various neuronal populations. We also discuss the existence of an evolutionarily conserved pathway linking Notch signalling and COE regulatory functions in various developmental decisions.

Xebf3 is a regulator of neuronal differentiation during primary neurogenesis in Xenopus

During primary neurogenesis in Xenopus, a cascade of helix--loop--helix (HLH) transcription factors regulates neuronal determination and differentiation. While XNeuroD functions at a late step in this cascade to regulate neuronal differentiation, the factors that carry out terminal differentiation are still unknown. We have isolated a new Xenopus member of the Ebf/Olf-1 family of HLH transcription factors, Xebf3, and provide evidence that, during primary neurogenesis, it regulates neuronal differentiation downstream of XNeuroD. In developing Xenopus embryos, Xebf3 is turned on in the three stripes of primary neurons at stage 15.5, after XNeuroD. In vitro, XEBF3 binds the EBF/OLF-1 binding site and functions as a transcriptional activator. When overexpressed, Xebf3 is able to induce ectopic neurons at neural plate stages and directly convert ectodermal cells into neurons in animal cap explants. Expression of Xebf3 can be activated by XNeuroD both in whole embryos and in animal caps, indicating that this new HLH factor might be regulated by XNeuroD. Furthermore, in animal caps, XNeuroD can activate Xebf3 in the absence of protein synthesis, suggesting that, in vitro, this regulation is direct. Similar to XNeuroD, but unlike Xebf2/Xcoe2, Xebf3 expression and function are insensitive to Delta/Notch-mediated lateral inhibition. In summary, we conclude that Xebf3 functions downstream of XNeuroD and is a regulator of neuronal differentiation in Xenopus.

Transcription factor regulation of B lineage commitment

The past year has provided insight into the mechanisms by which multipotent progenitors commit to differentiation through the B lymphocyte lineage. Mice lacking the Pax5 gene develop pro-B lymphocytes but these cells are not uniquely committed to the B lineage as they can give rise to all hematopoietic cell types if cultured under appropriate conditions. Regulators of lymphocyte proliferation and survival have also been identified that may allow lymphocytes to respond to information provided by the external environment.

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.

Differentiation-Specific, Octamer-Dependent Costimulation of κ Transcription

By mutational analysis of the octamer-TATA box intervening region in the mouse SP6 κ promoter, we have mapped two octamer-dependent, costimulatory regions, A and B. The A region was active in late B cells only, while the B region was active throughout B cell differentiation. The B region was TATA proximal and contained a heptamer and an E box of the E2A type that is common in Vκ promoters. Mutation of the heptamer element did not decrease transcriptional stimulation from this region, but mutations in, or immediately 5′ of, the E box core sequence did. A protein binding to this region could be detected in nuclear extracts. The complex could only partially be competed with a μE5 binding site and could not be supershifted with Abs raised to E2A gene products, indicating that it may represent a novel E-box binding complex. The A region was located proximal to the octamer and contained a CCCT element that is conserved both with regard to position and sequence in human VκII promoters. By mutational analysis, the transcriptional stimulatory activity was mapped to the CCCT element that also is part of an early B cell factor (EBF) binding site. In late B cells, a novel protein (FA), which did not bind to the EBF binding site in the mb1 promoter, interacted with the A region. This protein was found to be expressed at lower levels in early B cells as well as in HeLa cells. Thus, the octamer-flanking sequence contains positive control elements that may act independently but that differ in the stage of B cell differentiation at which they are active. One of these factors is an example of an ubiquitously expressed transcription factor that participate in differentiation-specific transcriptional activation.

Chicken ovalbumin upstream promoter-transcription factor interacts with estrogen receptor, binds to estrogen response elements and half-sites, and inhibits estrogen-induced gene expression

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF) was identified as a low abundance protein in bovine uterus that co-purified with estrogen receptor (ER) in a ligand-independent manner and was separated from the ER by its lower retention on estrogen response element (ERE)-Sepharose. In gel mobility shift assays, COUP-TF bound as an apparent dimer to ERE and ERE half-sites. COUP-TF bound to an ERE half-site with high affinity, Kd = 1.24 nM. In contrast, ER did not bind a single ERE half-site. None of the class II nuclear receptors analyzed, i.e. retinoic acid receptor, retinoid X receptor, thyroid receptor, peroxisome proliferator-activated receptor, or vitamin D receptor, were constituents of the COUP-TF.DNA binding complex detected in gel mobility shift assays. Direct interaction of COUP-TF with ER was indicated by GST "pull-down" and co-immunoprecipitation assays. The nature of the ER ligand influenced COUP-TF-ERE half-site binding. When ER was liganded by the antiestrogen 4-hydroxytamoxifen (4-OHT), COUP-TF-half-site interaction decreased. Conversely, COUP-TF transcribed and translated in vitro enhanced the ERE binding of purified estradiol (E2)-liganded ER but not 4-OHT-liganded ER. Co-transfection of ER-expressing MCF-7 human breast cancer cells with an expression vector for COUP-TFI resulted in a dose-dependent inhibition of E2-induced expression of a luciferase reporter gene under the control of three tandem copies of EREc38. The ability of COUP-TF to bind specifically to EREs and half-sites, to interact with ER, and to inhibit E2-induced gene expression suggests COUP-TF regulates ER action by both direct DNA binding competition and through protein-protein interactions.

Family of Ebf/Olf-1-related genes potentially involved in neuronal differentiation and regional specification in the central nervous system

Two novel mouse genes, Ebf2 and Ebf3, have been identified which show high similarity to the rodent Ebf/Olf-1 and the Drosophila collier genes. The strong conservation of the protein regions corresponding to the DNA binding and dimerisation domains previously defined in Ebf strongly suggests that Ebf2 and Ebf3 also constitute DNA sequence-specific transcription factors. Determination of the chromosomal locations of the two genes indicated that the different members of this novel mouse multigene family are not clustered. A detailed analysis of the expression of each of the three Ebf genes in the developing central nervous system revealed partially overlapping patterns with two salient features: 1) In the region extending from the midbrain to the spinal cord, the expression of the three genes correlated with neuronal maturation, with a general activation in early post-mitotic cells, followed by specific patterns of extinction also consistent with the neurogenic gradient. 2) In the forebrain area, although the patterns of expression of the Ebf genes also reflected neuronal maturation, they appeared in addition to be region specific. These data suggest that Ebf genes may be involved in the control of neuronal differentiation in the CNS and in enforcing regional diversity in populations of post-mitotic forebrain neurons.

EBF and E47 collaborate to induce expression of the endogenous immunoglobulin surrogate light chain genes

Early B cell factor (EBF) and E47 participate in the transcriptional control of early B lymphocyte differentiation. With the aim of identifying genetic targets for these transcription factors, we stably transfected cDNAs encoding EBF or a covalent homodimer of E47, individually or together, into immature hematopoietic Ba/F3 cells, which lack both factors. In combination, EBF and E47 induce efficient expression of the endogenous immunoglobulin surrogate light chain genes, lambda5 and VpreB, whereas other pre-B cell-specific genes remain silent. Multiple functionally important EBF and E47 binding sites were identified in the lambda5 promoter/enhancer region, indicating that lambda5 is a direct genetic target for these transcription factors. Taken together, these data suggest that EBF and E47 synergize to activate expression of a subset of genes that define an early stage of the B cell lineage.

Regulatory elements of the mb-1 gene encoding the Ig-alpha component of the human B-cell antigen receptor

The mb-1 gene encodes the Ig-alpha component of the B-cell antigen receptor. It is specifically expressed in pre-B and mature B cells but not in plasma cells losing membrane Ig (mIg) expression. We looked for transcriptional regulatory elements within a 12 kb genomic fragment. A strong promoter activity was found in a 591 bp fragment harboring consensus binding sites for known transcription factors including Ets, EBF/BlyF, LyF1/micro B and Spl. It was able to drive transcription of a reporter gene in the absence of any additional enhancer and was mostly active in B lymphocytes not in plasma cells or T cells. Although no fragment from the mb-1 gene displayed enhancer activity in combination with either the SV40, a Ig VH or a Ig VL promoter, a 1078 bp fragment corresponding to the 5' part of the gene behaved as a strong enhancer in either orientation in constructs driven by the mb-1 promoter itself. Deletions within this fragment allowed to delineate shorter sequences with enhancer activity upstream the first exon. The tissue-restricted, promoter-restricted and stage-specific activity of this 5' flanking region suggests that it is the main regulatory element of the mb-1 gene.

LacZ and OMP are co-expressed during ontogeny and regeneration in olfactory receptor neurons of OMP promoter-lacZ transgenic mice

The ontogeny and cellular specificity of expression of beta-galactosidase activity and olfactory marker protein (OMP) are compared in olfactory tissue of the H-OMP-lacZ-3 line of transgenic mice. In this line the expression of lacZ is driven by a 0.3 kb fragment of the rat OMP promoter. During fetal development, lacZ expression is detectable in olfactory receptor neurons (ORNs) shortly after the initial appearance of endogenous OMP. The beta-galactosidase marker was observed only in mature olfactory receptor neurons where it co-localized with endogenous OMP. It was absent from immature neurons that express the growth associated phosphoprotein B50/GAP43. Lesion of the peripheral olfactory pathway by intranasal irrigation with Triton X-100 eliminated expression of both OMP and lacZ in the olfactory neuroepithelium. Subsequent regeneration of the full complement of olfactory receptor neurons was associated with co-expression of both OMP and beta-galactosidase activity. Neither OMP nor beta-galactosidase activity was induced in any other cell type of the regenerating olfactory mucosa. Thus, as little as 0.3 kb of the OMP promoter has the ability to target lacZ expression to olfactory receptor neurons in a temporally and spatially defined manner. We discuss the potential utility of this transgenic line for future studies of the olfactory system.