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

Abstract

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.

Fig. 1. The structure of Early B cell factor 1 (EBF1)

The domains of EBF are labeled and colored consistently throughout. a A schematic representation of the domains in EBF1_1–591 includes the DNA-binding domain’s (DBD) unique zinc knuckle (gold). The DBD (blue), TIG/IPT (teal) and HLHLH (magenta) domains all participate in EBF dimerization. The carboxyl terminus includes the Ser/Thr/Pro-rich activation domain. The amino acids demarcating each domain are numbered. b An expanded view of the zinc knuckle highlights the histidine and three cystine residues that coordinate the zinc ion required for DNA binding. c The structure of an EBF1_26–422 dimer bound to DNA (grey). The perspective is parallel to the helical axis of the DNA molecule. The visible portion of the HLHLH domain, the TIG/IPT domain, the DBD domains and the zinc knuckle motifs are indicated. The zinc ions are depicted as purple spheres. The structure was generated using PDB file ID 3MLP (Treiber et al. 2010a) and was modeled using Discovery Studio Visualizer 3.0, Accelrys Inc., San Diego, CA

Fig. 2. EBF1 expression in the context of hematopoiesis

Beginning with HSCs (hematopoietic stem cells), progressive steps of cell-fate restriction result in the designated cell types and lineages. MPPs (multi-potent progenitors) give rise to CMPs (common-myeloid progenitors), which possess myeloid and lymphoid potential. Additionally, MPPs yield LMPPs (lymphoid-primed multi-potent progenitors), which possess myeloid, lymphoid and NK cell potential. Common-lymphoid progenitors (CLPs), can be trisected into a Rag1^loλ5⁻ cells which are confined to the NK lineage, b Rag1^hi λ5⁻ cells which can develop into NK cells or T lymphocytes and c Rag1^hi λ5⁺ cells which retain NK, T and B cell potential. Pre-B cells express the pre-B cell receptor (pre-BCR), which is composed of Ig heavy chains and the surrogate light chain proteins VpreB1 and λ5. Immature and mature B cells express mature BCRs comprised of Ig heavy and light chains. The relative levels of EBF1 expression in CLPs through mature B cells are represented by the size of the green shaded block. Developmental steps that occur in the bone marrow versus the periphery are indicated

Fig. 3. A model of stepwise epigenetic modifications of Cd79a (mb-1) promoters during early B cell development

Transcriptional activation of the Cd79a gene is necessary for progression to the pre-B cell stage. Inactive Cd79a promoters in non-hematopoietic cells (right) are characterized by histone H3K27 trimethylation, a mark of heterochromatin and repression. Chromatin at these promoters is likely maintained in an inactive state by repressive Mi-2/NuRD chromatin remodeling complexes (CRCs). During pre-activation of Cd79a genes in common lymphoid progenitors (CLPs) and pre-pro-B cells (top left), H3K4 mono- and di-methylation is correlated with binding of the transcription factor E2A to Cd79a promoters. Poised promoters generally display low levels of mono-, di- and tri-methylated H3K4 and do not display significant levels of H3K27 trimethylation. Binding of EBF1 initiates demethylation of promoter DNA and recruits activating SWI/SNF CRCs, which result in nucleosome displacement (middle left). Activation of Cd79a expression in pro-B cells involves the subsequent recruitment of other transcription factors including Pax5 and Ets proteins and correlates with a displacement of repressive Mi-2/NuRD complexes, b complete demethylation of promoter DNA, c increased H3K4 di- and trimethylation with loss of monomethylation and d high levels of H3K9 acetylation (bottom left). This model is based, in part, on data from Gao et al. (2009), Lin et al. (2010), Treiber et al. (2010b) and unpublished data (J.R. and J.H.)