SPI-C, a PU-box binding ETS protein expressed temporarily during B-cell development and in macrophages, contains an acidic transactivation domain located to the N-terminus

Spic regulates one-carbon metabolism and histone methylation in ground-state pluripotency

Understanding mechanisms of epigenetic regulation in embryonic stem cells (ESCs) is of fundamental importance for stem cell and developmental biology. Here, we identify Spic, a member of the ETS family of transcription factors (TFs), as a marker of ground state pluripotency. We show that Spic is rapidly induced in ground state ESCs and in response to extracellular signal-regulated kinase (ERK) inhibition. We find that SPIC binds to enhancer elements and stabilizes NANOG binding to chromatin, particularly at genes involved in choline/one-carbon (1C) metabolism such as Bhmt, Bhmt2, and Dmgdh. Gain-of-function and loss-of-function experiments revealed that Spic controls 1C metabolism and the flux of S-adenosyl methionine to S-adenosyl-L-homocysteine (SAM-to-SAH), thereby, modulating the levels of H3R17me2 and H3K4me3 histone marks in ESCs. Our findings highlight betaine-dependent 1C metabolism as a hallmark of ground state pluripotency primarily activated by SPIC. These findings underscore the role of uncharacterized auxiliary TFs in linking cellular metabolism to epigenetic regulation in ESCs.

Lineage-instructive functions of the E26-transformation-specific-family transcription factor Spi-C in immune cell development and disease

Cell fate decisions during hematopoiesis are the consequence of a complex mixture of inputs from cell-intrinsic and cell-extrinsic factors. In rare cases, expression of a single transcription factor, or a few key factors, may be sufficient to dictate lineage differentiation in a precursor cell. The E26-transformation-specific-family transcription factor Spi-C has emerged as an example of a lineage-instructive factor involved in the generation of mature, specialized subsets of both myeloid and lymphoid cells. Spi-C can instruct differentiation of splenic precursors into red pulp macrophages responsible for phagocytosing senescent red blood cells. In the B cell compartment, Spi-C acts as a key regulator of cell fate decisions at the pro-B to pre-B cell stage and for plasma cell differentiation. Spi-C regulates key genes including Nfkb1, Bach2, Syk, and Blnk to regulate cell cycle entry and B cell differentiation. Here, we review the biology of the lineage-instructive transcription factor Spi-C and its contribution to mechanisms of disease in macrophages and B cells. This article is categorized under: Cancer > Molecular and Cellular Physiology Immune System Diseases > Molecular and Cellular Physiology Infectious Diseases > Genetics/Genomics/Epigenetics.

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

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

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

Spi-C positively regulates RANKL-mediated osteoclast differentiation and function

Spi-C is an SPI-group erythroblast transformation-specific domain transcription factor expressed during B-cell development. Here, we report that Spi-C is a novel receptor activator of nuclear factor-κB ligand (RANKL)-inducible protein that positively regulates RANKL-mediated osteoclast differentiation and function. Knockdown of Spi-C decreased the expression of RANKL-induced nuclear factor of activated T-cells, cytoplasmic 1, receptor activator of nuclear factor-κB (RANK), and tartrate-resistant acid phosphatase (TRAP), resulting in a marked decrease in the number of TRAP-positive multinucleated cells. Spi-C-transduced bone marrow-derived monocytes/macrophages (BMMs) displayed a significant increase in osteoclast formation in the presence of RANKL. In addition, Spi-C-depleted cells failed to show actin ring formation or bone resorption owing to a marked reduction in the expression of RANKL-mediated dendritic cell-specific transmembrane protein and the d2 isoform of vacuolar (H+) ATPase V0 domain, which are known osteoclast fusion-related genes. Interestingly, RANKL stimulation induced the translocation of Spi-C from the cytoplasm into the nucleus during osteoclastogenesis, which was specifically blocked by inhibitors of p38 mitogen-activated protein kinase (MAPK) or PI3 kinase. Moreover, Spi-C depletion prevented RANKL-induced MAPK activation and the degradation of inhibitor of κB-α (IκBα) in BMMs. Collectively, these results suggest that Spi-C is a novel positive regulator that promotes both osteoclast differentiation and function.

A gene-controlling protein called Spi-C promotes the development of bone-processing cells called osteoclasts; details of the molecular mechanisms involved will aid understanding of Spi-C’s role in bone health and disease. Osteoclasts degrade bone during the normal process of bone remodeling, balanced by the activity of osteoblast cells that form new bone. Excessive osteoclast activity can cause the bone loss associated with various bone diseases including early-onset osteoporosis. Researchers in South Korea led by Soo Young Lee at Ewha Womans University and Na Kyung Lee at Soonchunhyang University, Asan, found that Spi-C promotes osteoclast development by activating genes that code for key proteins of a signaling pathway known to be crucial for bone health. Drugs that interfere with Spi-C activity may therefore offer a new approach for treating bone disease.

At the intersection of DNA damage and immune responses

DNA damage occurs on exposure to genotoxic agents and during physiological DNA transactions. DNA double-strand breaks (DSBs) are particularly dangerous lesions that activate DNA damage response (DDR) kinases, leading to initiation of a canonical DDR (cDDR). This response includes activation of cell cycle checkpoints and engagement of pathways that repair the DNA DSBs to maintain genomic integrity. In adaptive immune cells, programmed DNA DSBs are generated at precise genomic locations during the assembly and diversification of lymphocyte antigen receptor genes. In innate immune cells, the production of genotoxic agents, such as reactive nitrogen molecules, in response to pathogens can also cause genomic DNA DSBs. These DSBs in adaptive and innate immune cells activate the cDDR. However, recent studies have demonstrated that they also activate non-canonical DDRs (ncDDRs) that regulate cell type-specific processes that are important for innate and adaptive immune responses. Here, we review these ncDDRs and discuss how they integrate with other signals during immune system development and function.

RAG-mediated DNA double-strand breaks activate a cell type-specific checkpoint to inhibit pre-B cell receptor signals

DNA double-strand breaks (DSBs) activate a canonical DNA damage response, including highly conserved cell cycle checkpoint pathways that prevent cells with DSBs from progressing through the cell cycle. In developing B cells, pre-B cell receptor (pre-BCR) signals initiate immunoglobulin light (Igl) chain gene assembly, leading to RAG-mediated DNA DSBs. The pre-BCR also promotes cell cycle entry, which could cause aberrant DSB repair and genome instability in pre-B cells. Here, we show that RAG DSBs inhibit pre-BCR signals through the ATM- and NF-κB2-dependent induction of SPIC, a hematopoietic-specific transcriptional repressor. SPIC inhibits expression of the SYK tyrosine kinase and BLNK adaptor, resulting in suppression of pre-BCR signaling. This regulatory circuit prevents the pre-BCR from inducing additional Igl chain gene rearrangements and driving pre-B cells with RAG DSBs into cycle. We propose that pre-B cells toggle between pre-BCR signals and a RAG DSB-dependent checkpoint to maintain genome stability while iteratively assembling Igl chain genes.

Identification of a negative regulatory role for spi-C in the murine B cell lineage

Spi-C is an E26 transformation-specific family transcription factor that is highly related to PU.1 and Spi-B. Spi-C is expressed in developing B cells, but its function in B cell development and function is not well characterized. To determine whether Spi-C functions as a negative regulator of Spi-B (encoded by Spib), mice were generated that were germline knockout for Spib and heterozygous for Spic (Spib(-/-)Spic(+/-)). Interestingly, loss of one Spic allele substantially rescued B cell frequencies and absolute numbers in Spib(-/-) mouse spleens. Spib(-/-)Spic(+/-) B cells had restored proliferation compared with Spib(-/-) B cells in response to anti-IgM or LPS stimulation. Investigation of a potential mechanism for the Spib(-/-)Spic(+/-) phenotype revealed that steady-state levels of Nfkb1, encoding p50, were elevated in Spib(-/-)Spic(+/-) B cells compared with Spib(-/-) B cells. Spi-B was shown to directly activate the Nfkb1 gene, whereas Spi-C was shown to repress this gene. These results indicate a novel role for Spi-C as a negative regulator of B cell development and function.

Activation of counter-regulatory mechanisms in a rat renal acute rejection model

BACKGROUND

Microarray analysis provides a powerful approach to identify gene expression alterations following transplantation. In patients the heterogeneity of graft specimens, co-morbidity, co-medications and the challenges in sample collection and preparation complicate conclusions regarding the underlying mechanisms of graft injury, rejection and immune regulation.

RESULTS

We used a rat kidney transplantation model with strict transplant and sample preparation procedures to analyze genome wide changes in gene expression four days after syngeneic and allogeneic transplantation. Both interventions were associated with substantial changes in gene expression. After allogeneic transplantation, genes and pathways related to transport and metabolism were predominantly down-regulated consistent with rejection-mediated graft injury and dysfunction. Up-regulated genes were primarily related to the acute immune response including antigen presentation, T-cell receptor signaling, apoptosis, interferon signaling and complement cascades. We observed a cytokine and chemokine expression profile consistent with activation of a Th1-cell response. A novel finding was up-regulation of several regulatory and protective genes after allogeneic transplantation, specifically IL10, Bcl2a1, C4bpa, Ctla4, HO-1 and the SOCS family.

CONCLUSION

Our data indicate that in parallel with the predicted activation of immune response and tissue injury pathways, there is simultaneous activation of pathways for counter regulatory and protective mechanisms that would balance and limit the ongoing inflammatory/immune responses. The pathophysiological mechanisms behind and the clinical consequences of alterations in expression of these gene classes in acute rejection, injury and dysfunction vs. protection and immunoregulation, prompt further analyses and open new aspects for therapeutic approaches.

B-1a cells are imprinted by the microenvironment in spleen and peritoneum

B-1a cells are found mainly in the peritoneal cavity of mice but are also present in the spleen. Gene expression profiling defined many genes differentially expressed in B-1a cells from these two sites. To see whether this gene expression pattern was imprinted by the particular microenvironment, peritoneal or spleen cells from recombinant L2 mice mainly consisting of B-1a cells were adoptively transferred into Rag1-/- mice. Re-isolated peritoneal and splenic B-1a cells were analyzed for expression of three indicator genes--vcam-1, adamdec1 and spi-c. The expression of these genes was up-regulated in splenic and down-regulated in peritoneal cells. This particular pattern was observed for peritoneal or splenic donor cells transferred either intraperitoneally or intravenously. Similar results were obtained when levels of surface IgM or frequencies of Mac-1+ B-1 cells were compared after transfer. This suggests that the environment induces the particular genetic program of B-1a cells and argues against an independent ontogeny.

Spi-C has opposing effects to PU.1 on gene expression in progenitor B cells

The Ets transcription factor Spi-C, expressed in B cells and macrophages, is closely related to PU.1 and has the ability to recognize the same DNA consensus sequence. However, the function of Spi-C has yet to be determined. The purpose of this study is to further examine Spi-C activity in B cell development. First, using retroviral vectors to infect PU.1(-/-) fetal liver progenitors, Spi-C was found to be inefficient at inducing cytokine-dependent proliferation and differentiation of progenitor B (pro-B) cells or macrophages relative to PU.1 or Spi-B. Next, Spi-C was ectopically expressed in fetal liver-derived, IL-7-dependent pro-B cell lines. Wild-type (WT) pro-B cells ectopically expressing Spi-C (WT-Spi-C) have several phenotypic characteristics of pre-B cells such as increased CD25 and decreased c-Kit surface expression. In addition, WT-Spi-C pro-B cells express increased levels of IgH sterile transcripts and reduced levels of expression and transcription of the FcgammaRIIb gene. Gel-shift analysis suggests that Spi-C, ectopically expressed in pro-B cells, can bind PU.1 consensus sites in the IgH intronic enhancer and FcgammaRIIb promoter. Transient transfection analysis demonstrated that PU.1 functions to repress the IgH intronic enhancer and activate the FcgammaRIIb promoter, while Spi-C opposes these activities. WT-Spi-C pro-B cells have reduced levels of dimethylation on lysine 9 of histone H3 within the IgH 3' regulatory region, indicating that Spi-C can contribute to removal of repressive features in the IgH locus. Overall, these studies suggest that Spi-C may promote B cell differentiation by modulating the activity of PU.1-dependent genes.

SPI-C and STAT6 can cooperate to stimulate IgE germline transcription

SPI-C is a novel ETS protein that is expressed in B lymphocytes. No target gene for SPI-C has so far been defined. We have performed a yeast two-hybrid screen using SPI-C as bait in order to further analyze the functional role of this orphan transcription factor. We found that SPI-C interacted specifically with the C-terminus of STAT6 in yeast. By co-immunoprecipitation in transfected COS7 cells the physical interaction between SPI-C and STAT6 was confirmed. Furthermore, this protein-protein interaction is functional since we could demonstrate that SPI-C and STAT6 stimulated IL4 induced Iepsilon transcription synergistically but only when both proteins bound to DNA. Thus, a protein interaction between SPI-C and STAT6 is the basis for a novel mechanism for regulation of IL4 induced gene expression.

Transcription factors drive B cell development

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

Origins of lymphocyte developmental programs: transcription factor evidence

This review explores the evolutionary origins of lymphocyte development by focusing on the transcription factors that direct mammalian lymphocyte development today. Gene expression data suggest that the programs to make lymphocytes involve the same transcription factor ensembles in all animals with lymphocytes. Most of these factors, GATA, Runx, PU.1/Spi, EBF/Olf, Ikaros, and Pax-2/5/8 family members, are also encoded in the genomes of animals without lymphocytes. We consider the functions of these factors in animals without lymphocytes in terms of discrete program components, which could have been assembled in a new way to create the lymphocyte developmental program approximately 500 My ago.

Isoforms of the Ets transcription factor NERF/ELF-2 physically interact with AML1 and mediate opposing effects on AML1-mediated transcription of the B cell-specific blk gene

We previously isolated different isoforms of a new Ets transcription factor family member, NERF/ELF-2, NERF-2, NERF-1a, and NERF-1b. In contrast to the inhibitory isoforms NERF-1a and NERF-1b, NERF-2 acts as a transactivator of the B cell-specific blk promoter. We now report that NERF-2 and NERF-1 physically interact with AML1 (RUNX1), a frequent target for chromosomal translocations in leukemia. NERF-2 bound to AML1 via an interaction site located in a basic region upstream of the Ets domain. This is in contrast to most other Ets factors such as Ets-1 that bind to AML1 via the Ets domain, suggesting that different Ets factors utilize different domains for interaction with AML1. The interaction between AML1 and NERF-2 led to cooperative transactivation of the blk promoter, whereas the interaction between AML1 and NERF-1a led to repression of AML1-mediated transactivation. To delineate the differences in function of the different NERF isoforms, we determined that the transactivation domain of NERF-2 is encoded by the N-terminal 100 amino acids, which have been replaced in NERF-1a by a 19-amino acid transcriptionally inactive sequence. Furthermore, acidic domains A and B, which are conserved in NERF-2 and the related proteins ELF-1 and MEF/ELF-4, but not in NERF-1a, are largely responsible for NERF-2-mediated transactivation. Because translocation of the Ets factor Tel to AML1 is a frequent event in childhood pre-B leukemia, understanding the interaction of Ets factors with AML1 in the context of a B cell-specific promoter might help to determine the function of Ets factors and AML1 in leukemia.

Analysis of Gene Expression and Ig Transcription in PU.1/Spi-B-Deficient Progenitor B Cell Lines

A number of presumptive target genes for the Ets-family transcription factor PU.1 have been identified in the B cell lineage. However, the precise function of PU.1 in B cells has not been studied because targeted null mutation of the PU.1 gene results in a block to lymphomyeloid development at an early developmental stage. In this study, we take advantage of recently developed PU.1(-/-)Spi-B(-/-) IL-7 and stromal cell-dependent progenitor B (pro-B) cell lines to analyze the function of PU.1 and Spi-B in B cell development. We show that contrary to previously published expectations, PU.1 and/or Spi-B are not required for Ig H chain (IgH) gene transcription in pro-B cells. In fact, PU.1(-/-)Spi-B(-/-) pro-B cells have increased levels of IgH transcription compared with wild-type pro-B cells. In addition, high levels of Igkappa transcription are induced after IL-7 withdrawal of wild-type or PU.1(-/-)Spi-B(-/-) pro-B cells. In contrast, we found that Iglambda transcription is reduced in PU.1(-/-)Spi-B(-/-) pro-B cells relative to wild-type pro-B cells after IL-7 withdrawal. These results suggest that Iglambda, but not IgH or Igkappa, transcription, is dependent on PU.1 and/or Spi-B. The PU.1(-/-)Spi-B(-/-) pro-B cells have other phenotypic changes relative to wild-type pro-B cells including increased proliferation, increased CD25 expression, decreased c-Kit expression, and decreased RAG-1 expression. Taken together, our observations suggest that reduction of PU.1 and/or Spi-B activity in pro-B cells promotes their differentiation to a stage intermediate between late pro-B cells and large pre-B cells.