Helios, a T cell-restricted Ikaros family member that quantitatively associates with Ikaros at centromeric heterochromatin

The nsdC gene encoding a putative C2H2-type transcription factor is a key activator of sexual development in Aspergillus nidulans

The formation of the Aspergillus nidulans fruiting body is affected by a number of genetic and environmental factors. Here, the nsdC (never in sexual development) gene-encoding a putative transcription factor carrying a novel type of zinc-finger DNA-binding domain consisting of two C(2)H(2)'s and a C(2)HC motif that are highly conserved in most fungi but not in plants or animals-was investigated. Two distinct transcripts of 2.6 and 3.0 kb were generated from nsdC. The 2.6-kb mRNA accumulated differentially in various stages of growth and development, while the level of the 3.0-kb mRNA remained relatively constant throughout the life cycle. While the deletion of nsdC resulted in the complete loss of fruiting body formation under all conditions favoring sexual development, overexpression of nsdC not only enhanced formation of fruiting bodies (cleistothecia) but also overcame inhibitory effects of certain stresses on cleistothecial development, implying that NsdC is a key positive regulator of sexual development. Deletion of nsdC also retarded vegetative growth and hyperactive asexual sporulation, suggesting that NsdC is necessary not only for sexual development but also for regulating asexual sporulation negatively. Overexpression of veA or nsdD does not rescue the failure of fruiting body formation caused by nsdC deletion. Furthermore, nsdC expression is not affected by either VeA or NsdD, and vice versa, indicating that NsdC regulates sexual development independently of VeA or NsdD.

A central role for induced regulatory T cells in tolerance induction in experimental colitis

In addition to thymus-derived or natural T regulatory (nT(reg)) cells, a second subset of induced T regulatory (iT(reg)) cells arises de novo from conventional CD4(+) T cells in the periphery. The function of iT(reg) cells in tolerance was examined in a CD45RB(high)CD4(+) T cell transfer model of colitis. In situ-generated iT(reg) cells were similar to nT(reg) cells in their capacity to suppress T cell proliferation in vitro and their absence in vivo accelerated bowel disease. Treatment with nT(reg) cells resolved the colitis, but only when iT(reg) cells were also present. Although iT(reg) cells required Foxp3 for suppressive activity and phenotypic stability, their gene expression profile was distinct from the established nT(reg) "genetic signature," indicative of developmental and possibly mechanistic differences. These results identified a functional role for iT(reg) cells in vivo and demonstrated that both iT(reg) and nT(reg) cells can act in concert to maintain tolerance.

Ikaros stability and pericentromeric localization are regulated by protein phosphatase 1

Ikaros encodes a zinc finger protein that is involved in gene regulation and chromatin remodeling. The majority of Ikaros localizes at pericentromeric heterochromatin (PC-HC) where it regulates expression of target genes. Ikaros function is controlled by posttranslational modification. Phosphorylation of Ikaros by CK2 kinase determines its ability to bind DNA and exert cell cycle control as well as its subcellular localization. We report that Ikaros interacts with protein phosphatase 1 (PP1) via a conserved PP1 binding motif, RVXF, in the C-terminal end of the Ikaros protein. Point mutations of the RVXF motif abolish Ikaros-PP1 interaction and result in decreased DNA binding, an inability to localize to PC-HC, and rapid degradation of the Ikaros protein. The introduction of alanine mutations at CK2-phosphorylated residues increases the half-life of the PP1-nonbinding Ikaros mutant. This suggests that dephosphorylation of these sites by PP1 stabilizes the Ikaros protein and prevents its degradation. In the nucleus, Ikaros forms complexes with ubiquitin, providing evidence that Ikaros degradation involves the ubiquitin/proteasome pathway. In vivo, Ikaros can target PP1 to the nucleus, and a fraction of PP1 colocalizes with Ikaros at PC-HC. These data suggest a novel function for the Ikaros protein; that is, the targeting of PP1 to PC-HC and other chromatin structures. We propose a model whereby the function of Ikaros is controlled by the CK2 and PP1 pathways and that a balance between these two signal transduction pathways is essential for normal cellular function and for the prevention of malignant transformation.

High-resolution analysis of chromosome copy number alterations in angioimmunoblastic T-cell lymphoma and peripheral T-cell lymphoma, unspecified, with single nucleotide polymorphism-typing microarrays

Angioimmunoblastic T-cell lymphoma (AILT) and peripheral T-cell lymphoma, unspecified (PTCL-u) are relatively frequent subtypes of T- or natural killer cell lymphoma. To characterize the structural anomalies of chromosomes associated with these disorders, we here determined chromosome copy number alterations (CNAs) and loss of heterozygosity (LOH) at >55,000 single nucleotide polymorphism loci for clinical specimens of AILT (n=40) or PTCL-u (n=33). Recurrent copy number gain common to both conditions was detected on chromosomes 8, 9 and 19, whereas common LOH was most frequent for a region of chromosome 2. AILT- or PTCL-u-specific CNAs or LOH were also identified at 21 regions, some spanning only a few hundred base pairs. We also identified prognosis-related CNAs or LOH by several approaches, including Cox's proportional hazard analysis. Among the genes that mapped to such loci, a poor prognosis was linked to overexpression of CARMA1 at 7p22 and of MYCBP2 at 13q22, with both genes being localized within regions of frequent copy number gain. For a frequent LOH region at 2q34, we also identified IKAROS family zinc-finger 2 cDNAs encoding truncated proteins. Our data indicate that AILT and PTCL-u consist of heterogeneous subgroups with distinct transforming genetic alterations.

Gene expression analysis of embryonic stem cells expressing VE-cadherin (CD144) during endothelial differentiation

Background

Endothelial differentiation occurs during normal vascular development in the developing embryo. This process is recapitulated in the adult when endothelial progenitor cells are generated in the bone marrow and can contribute to vascular repair or angiogenesis at sites of vascular injury or ischemia. The molecular mechanisms of endothelial differentiation remain incompletely understood. Novel approaches are needed to identify the factors that regulate endothelial differentiation.

Methods

Mouse embryonic stem (ES) cells were used to further define the molecular mechanisms of endothelial differentiation. By flow cytometry a population of VEGF-R2 positive cells was identified as early as 2.5 days after differentiation of ES cells, and a subset of VEGF-R2⁺ cells, that were CD41 positive at 3.5 days. A separate population of VEGF-R2⁺ stem cells expressing the endothelial-specific marker CD144 (VE-cadherin) was also identified at this same time point. Channels lined by VE-cadherin positive cells developed within the embryoid bodies (EBs) formed by differentiating ES cells. VE-cadherin and CD41 expressing cells differentiate in close proximity to each other within the EBs, supporting the concept of a common origin for cells of hematopoietic and endothelial lineages.

Results

Microarray analysis of >45,000 transcripts was performed on RNA obtained from cells expressing VEGF-R2⁺, CD41⁺, and CD144⁺ and VEGF-R2^-, CD41^-, and CD144^-. All microarray experiments were performed in duplicate using RNA obtained from independent experiments, for each subset of cells. Expression profiling confirmed the role of several genes involved in hematopoiesis, and identified several putative genes involved in endothelial differentiation.

Conclusion

The isolation of CD144⁺ cells during ES cell differentiation from embryoid bodies provides an excellent model system and method for identifying genes that are expressed during endothelial differentiation and that are distinct from hematopoiesis.

ZNF397, a new class of interphase to early prophase-specific, SCAN-zinc-finger, mammalian centromere protein

The centromere is a complex structure required for equal segregation of newly synthesised sister chromatids at mitosis. One of the significant objectives in centromere research is to determine the complete repertoire of protein components that constitute the kinetochore. Here, we identify a novel centromere protein using a centromere-positive autoimmune serum from a patient with watermelon stomach disease. Western blot and screening of a lambda phage expression library revealed a 60-kDa protein, ZNF397. This protein belongs to the classical Cys(2)His(2) group of the zinc-finger protein superfamily and contains two conserved domains: a leucine-rich SCAN domain and nine Cys(2)His(2) zinc fingers. Bioinformatic analysis shows that ZNF397 is conserved in placental mammals. Stable GFP:ZNF397-expressing human cells show co-localisation of ZNF397 with the constitutive centromere protein CENP-A during interphase and early prophase. Deletion and domain-swap constructs indicate that the SCAN domain is necessary but not sufficient for centromere localisation. Gene-knockout studies in mice using the mouse orthologue (Zfp397) reveal that ZNF397 is a non-essential protein. These properties define ZNF397 as a member of a new class of interphase to early prophase-specific and SCAN domain-containing mammalian centromere protein. The possible role of this protein in transcription at the centromere is discussed.

Keep your fingers off my DNA: protein-protein interactions mediated by C2H2 zinc finger domains

Cys2-His2 (C2H2) zinc finger domains (ZFs) were originally identified as DNA-binding domains, and uncharacterized domains are typically assumed to function in DNA binding. However, a growing body of evidence suggests an important and widespread role for these domains in protein binding. There are even examples of zinc fingers that support both DNA and protein interactions, which can be found in well-known DNA-binding proteins such as Sp1, Zif268, and Ying Yang 1 (YY1). C2H2 protein-protein interactions (PPIs) are proving to be more abundant than previously appreciated, more plastic than their DNA-binding counterparts, and more variable and complex in their interactions surfaces. Here we review the current knowledge of over 100 C2H2 zinc finger-mediated PPIs, focusing on what is known about the binding surface, contributions of individual fingers to the interaction, and function. An accurate understanding of zinc finger biology will likely require greater insights into the potential protein interaction capabilities of C2H2 ZFs.

Recruitment of ikaros to pericentromeric heterochromatin is regulated by phosphorylation

Ikaros encodes a zinc finger protein that is involved in heritable gene silencing. In hematopoietic cells, Ikaros localizes to pericentromeric heterochromatin (PC-HC) where it recruits its target genes, resulting in their activation or repression via chromatin remodeling. The function of Ikaros is controlled by post-translational modifications. CK2 kinase has been shown to phosphorylate Ikaros at its C terminus, affecting cell cycle progression. Using in vivo labeling of murine thymocytes followed by phosphopeptide mapping, we identified four novel Ikaros phosphorylation sites. Functional analysis of phosphomimetic mutants showed that the phosphorylation of individual amino acids determines the affinity of Ikaros toward probes derived from PC-HC. In vivo experiments demonstrated that targeting of Ikaros to PC-HC is regulated by phosphorylation. The ability of Ikaros to bind the upstream regulatory elements of its known target gene terminal deoxynucleotidyltransferase (TdT) was decreased by phosphorylation of two amino acids. In thymocytes, Ikaros acts as a repressor of the TdT gene. Induction of differentiation of thymocytes with phorbol 12-myristate 13-acetate plus ionomycin results in transcriptional repression of TdT expression. This process has been associated with increased binding of Ikaros to the upstream regulatory element of TdT. Phosphopeptide analysis of in vivo-labeled thymocytes revealed that Ikaros undergoes dephosphorylation during induction of thymocyte differentiation and that dephosphorylation is responsible for increased DNA binding affinity of Ikaros toward the TdT promoter. We propose a model whereby reversible phosphorylation of Ikaros at specific amino acids controls the subcellular localization of Ikaros as well as its ability to regulate TdT expression during thymocyte differentiation.

Combinatorial effects of splice variants modulate function of Aiolos

The transcription factor Aiolos (also known as IKZF3), a member of the Ikaros family of zinc-finger proteins, plays an important role in the control of B lymphocyte differentiation and proliferation. Previously, multiple isoforms of Ikaros family members arising from differential splicing have been described and we now report a number of novel isoforms of Aiolos. It has been demonstrated that full-length Ikaros family isoforms localize to heterochromatin and that they can associate with complexes containing histone deacetylase (HDAC). In this study, for the first time we directly investigate the cellular localization of various Aiolos isoforms, their ability to heterodimerize with Ikaros and associate with HDAC-containing complexes, and the effects on histone modification and binding to putative targets. Our work demonstrates that the cellular activities of Aiolos isoforms are dependent on combinations of various functional domains arising from the differential splicing of mRNA transcripts. These data support the general principle that the function of an individual protein is modulated through alternative splicing, and highlight a number of potential implications for Aiolos in normal and aberrant lymphocyte function.

Antagonistic interactions between Ikaros and the chromatin remodeler Mi-2beta determine silencer activity and Cd4 gene expression

Lineage commitment is induced by changes in gene expression dictated by the intimate interaction between transcription factors and chromatin regulators. Here, we revealed the antagonistic interplay between Ikaros and its associate the chromatin remodeler Mi-2beta during T cell development, as exemplified by the regulation of Cd4 expression. Loss of Ikaros or Mi-2beta led to activation or repression, respectively, of the Cd4 locus at inappropriate stages of development. Their combined mutation reverted to normal CD4 expression. In double-negative thymocytes, Ikaros binding to the Cd4 silencer contributed to its repressive activity. In double-positive thymocytes, concomitant binding of Mi-2beta with Ikaros to the Cd4 silencer caused silencer inactivation, thereby allowing for CD4 expression. Mi-2beta facilitated recruitment of histone acetyl transferases to the silencer. This recruitment possibly antagonized Ikaros and associated repressive activities. Thus, concomitant interactions between functionally opposing chromatin-regulating machineries are an important mode of gene regulation during lineage determination.

Predominant interaction of both Ikaros and Helios with the NuRD complex in immature thymocytes

Ikaros is the founding member of a small family of C2H2 zinc-finger DNA-binding proteins that carry out critical functions during lymphocyte development. Although interactions between Ikaros and various proteins have been reported, Ikaros-containing complexes have not been purified to determine their composition and identify the predominant interacting partners. In this study, a tandem affinity purification-mass spectrometry strategy was developed for the isolation of complexes formed by Ikaros and by Helios, a T-cell-restricted member of the Ikaros family that remains largely uncharacterized. This strategy, which appears to be well suited for general use in mammalian cells, relies on an N-terminal polypeptide containing a double FLAG epitope, followed by a tobacco etch virus protease cleavage site and calmodulin binding peptide. In extracts from a murine thymocyte line, Ikaros and Helios associated under moderate stringency conditions only with other members of the Ikaros family. However, under low stringency conditions, both tagged proteins assembled into higher molecular weight complexes. Mass spectrometry revealed that both proteins associated predominantly with subunits of NuRD, an ATP-dependent nucleosome remodeling complex implicated in transcriptional repression and activation and previously reported to associate with Ikaros. Further analysis of the affinity-purified Ikaros revealed that several serines and threonines are phosphorylated in the thymocyte line, with apparent changes upon thymocyte maturation. These results support the hypothesis that the NuRD complex makes major contributions to the functions of both Ikaros and Helios and that the activities of these proteins may be regulated in part by changes in phosphorylation.

Ikaros is expressed in developing striatal neurons and involved in enkephalinergic differentiation

The Ikaros (Ik) gene encodes alternatively spliced zinc-finger proteins originally identified in developing hematopoietic organs and acts as master regulator of lymphoid development. During our search for transcription factors that control the developmental expression of the enkephalin (ENK) gene we found that Ik-1 and Ik-2 isoforms are specifically expressed in the embryonic striatum and bind the Ik-like cis-regulatory DNA element present on the ENK gene. Ik proteins are expressed by both proliferating (BrdU+/nestin+) and by post-mitotic differentiating (MAP2+) cells in the developing striatum between embryonic day 12 and post-natal day 2 and mRNAs encoding for the Ik and ENK genes are co-expressed by a subset of differentiating striatal neurons. Blocking the DNA binding of Ik proteins in differentiating embryonic striatal neuronal cultures resulted in decreased ENK expression and mutant animals lacking the DNA-binding domain of Ik had a deficit in the number of ENK but not in dynorphin or substance P mRNA+ cells. Animals lacking the protein interaction domain of Ik showed no deficit. These results demonstrate that Ik-1 and Ik-2 proteins through their DNA binding act as positive regulators of ENK gene expression in the developing striatum and participate in regulating enkephalinergic differentiation.

Aiolos: an ungrateful member of the Ikaros family

Ikaros family members play an important role in hematopoietic development. Thompson et al. (2007) show that pre-BCR signaling induces Aiolos expression, which in turn suppresses the expression of the pre-BCR component lambda5 and eliminates the very structure that signaled its appearance.

Eos, MITF, and PU.1 recruit corepressors to osteoclast-specific genes in committed myeloid progenitors

Transcription factors MITF and PU.1 collaborate to increase expression of target genes like cathepsin K (Ctsk) and acid phosphatase 5 (Acp5) during osteoclast differentiation. We show that these factors can also repress transcription of target genes in committed myeloid precursors capable of forming either macrophages or osteoclasts. The direct interaction of MITF and PU.1 with the zinc finger protein Eos, an Ikaros family member, was necessary for repression of Ctsk and Acp5. Eos formed a complex with MITF and PU.1 at target gene promoters and suppressed transcription through recruitment of corepressors CtBP (C-terminal binding protein) and Sin3A, but during osteoclast differentiation, Eos association with Ctsk and Acp5 promoters was significantly decreased. Subsequently, MITF and PU.1 recruited coactivators to these target genes, resulting in robust expression of target genes. Overexpression of Eos in bone marrow-derived precursors disrupted osteoclast differentiation and selectively repressed transcription of MITF/PU.1 targets, while small interfering RNA knockdown of Eos resulted in increased basal expression of Ctsk and Acp5. This work provides a mechanism to account for the modulation of MITF and PU.1 activity in committed myeloid progenitors prior to the initiation of osteoclast differentiation in response to the appropriate extracellular signals.

Characterization of the short isoform of Helios overexpressed in patients with T-cell malignancies

In an earlier report, we demonstrated overexpression of a short isoform of Helios, Hel-5, which lacks three of four N-terminal zinc fingers, in patients with adult T-cell leukemia/lymphoma. Here, we characterized Hel-5 using immunoprecipitation, and gel shift and luciferase promoter assays, and found that Hel-5 lacks the repressor function observed with a full-length isoform of Helios. Moreover, Hel-5 associates with the full-length isoforms of the Ikaros gene family, Ikaros, Aiolos and Helios, and inhibits their DNA binding activity when present in excess, leading to dominant-negative effects on the full-length isoforms of the Ikaros gene family. Our results suggest a critical role for Helios in the mechanism of leukemogenesis.

Human Ikaros Function in Activated T Cells Is Regulated by Coordinated Expression of Its Largest Isoforms

The Ikaros gene is alternately spliced to generate multiple zinc finger proteins involved in gene regulation and chromatin remodeling. Whereas murine studies have provided important information regarding the role of Ikaros in the mouse, little is known of Ikaros function in human. We report functional analyses of the two largest human Ikaros (hIK) isoforms, hIK-VI and hIK-H, in T cells. Abundant expression of hIK-H, the largest described isoform, is restricted to human hematopoietic cells. We find that the DNA binding affinity of hIK-H differs from that of hIK-VI. Co-expression of hIk-H with hIk-VI alters the ability of Ikaros complexes to bind DNA motifs found in pericentromeric heterochromatin (PC-HC). In the nucleus, hIK-VI is localized solely in PC-HC, whereas the hIK-H protein exhibits dual centromeric and non-centromeric localization. Mutational analysis defined the amino acids responsible for the distinct DNA binding ability of hIK-H, as well as the sequence required for the specific subcellular localization of this isoform. In proliferating cells, the binding of hIK-H to the upstream regulatory region of known Ikaros target genes correlates with their positive regulation by Ikaros. Results suggest that expression of hIK-H protein restricts affinity of Ikaros protein complexes toward specific PC-HC repeats. We propose a model, whereby the binding of hIK-H-deficient Ikaros complexes to the regulatory sequence of target genes would recruit these genes to the restrictive pericentromeric compartment, resulting in their repression. The presence of hIK-H in the Ikaros complex would alter its affinity for PC-HC, leading to chromatin remodeling and activation of target genes.

Expression of a non-DNA-binding isoform of Helios induces T-cell lymphoma in mice

Helios is a zinc-finger protein belonging to the Ikaros family of transcriptional regulators. It is expressed, along with Ikaros, throughout early stages of thymocyte development where it quantitatively associates with Ikaros through C-terminal zinc-finger domains that mediate heterodimerization between Ikaros family members. To understand the role of Helios in T-cell development, we used a retroviral vector to express full-length Helios or a Helios isoform that lacked the N-terminal DNA-binding domain in hematopoietic progenitor cells of reconstituted mice. Constitutive expression of full-length Helios resulted in an inhibition of T-cell development at the double-negative stage within the thymus. Although expression of the DNA-binding mutant of Helios did not contribute to developmental abnormalities at early times after transplantation, 60% of animals that expressed the Helios DNA-binding mutant developed an aggressive and transplantable T-cell lymphoma 4 to 10 months after transplantation. These results demonstrate a vital function for Helios in maintaining normal homeostasis of developing T cells and formally show that non-DNA-binding isoforms of Helios are lymphomagenic if aberrantly expressed within the T-cell lineage.

Ikaros is required for plasmacytoid dendritic cell differentiation

Plasmacytoid dendritic cells (pDCs) are specialized DCs that produce high levels of type I IFN upon viral infection. Despite their key immunoregulatory role, little is known about pDC ontogeny or how developmental events regulate their function. We show that mice expressing low levels of the transcription factor Ikaros (Ik(L/L)) lack peripheral pDCs, but not other DC subsets. Loss of pDCs is associated with an inability to produce type I IFN after challenge with Toll-like receptor-7 and -9 ligands, or murine cytomegalovirus (MCMV) infection. In contrast, conventional DCs are present in normal numbers and exhibit normal responses in vivo after challenge with MCMV or inactivated toxoplasma antigen. Interestingly, Ik(L/L) bone marrow (BM) cells contain a pDC population that appears blocked at the Ly-49Q- stage of differentiation and fails to terminally differentiate in response to Flt-3L, a cytokine required for pDC differentiation. This differentiation block is strictly dependent on a cell-intrinsic requirement for Ikaros in pDC-committed precursors. Global gene expression profiling of Ik(L/L) BM pDCs reveals an up-regulation of genes not normally expressed, or expressed at low levels, in WT pDCs. These studies suggest that Ikaros controls pDC differentiation by silencing a large array of genes.

Ikaros has a crucial role in regulation of B cell receptor signaling

The transcription factor Ikaros, a key regulator of hematopoiesis, has an essential role in lymphocyte development. In mice, fetal lymphoid differentiation is blocked in the absence of Ikaros, and whereas T cells develop postnatally, B cells are totally absent. The significance of Ikaros in the B cell development is evident, but how Ikaros regulates B cell function has neither been established nor previously been studied with B cells that lack Ikaros expression. Here we show that disruption of Ikaros in the chicken B cell line DT40 induces a B cell receptor (BCR) signaling defect with reduced phospholipase Cgamma2 phosphorylation and impaired intracellular calcium mobilization, which is restored by Ikaros reintroduction. Furthermore, we show that lack of Ikaros induces hyperphosphorylation of Casitas B lymphoma protein subsequent to BCR activation. These results indicate that the absolute need of Ikaros for development, cell fate decisions and maintenance of B cells is due to the enhancement of BCR signaling.

Synthetic protein-protein interaction domains created by shuffling Cys2His2 zinc-fingers

Cys₂His₂ zinc-fingers (C2H2 ZFs) mediate a wide variety of protein–DNA and protein–protein interactions. DNA-binding C2H2 ZFs can be shuffled to yield artificial proteins with different DNA-binding specificities. Here we demonstrate that shuffling of C2H2 ZFs from transcription factor dimerization zinc-finger (DZF) domains can also yield two-finger DZFs with novel protein–protein interaction specificities. We show that these synthetic protein–protein interaction domains can be used to mediate activation of a single-copy reporter gene in bacterial cells and of an endogenous gene in human cells. In addition, the synthetic two-finger domains we constructed can also be linked together to create more extended, four-finger interfaces. Our results demonstrate that shuffling of C2H2 ZFs can yield artificial protein-interaction components that should be useful for applications in synthetic biology.

Ikaros is expressed in human extravillous trophoblasts and involved in their migration and invasion

The transcriptional factor Ikaros was originally found to function as a key regulator of lymphocyte differentiation. In addition, we have reported that Ikaros regulates the human placental leucine aminopeptidase (P-LAP)/insulin-regulated aminopeptidase (IRAP) gene in choriocarcinoma trophoblastic cells, suggesting that Ikaros might be involved in placental development, while even its presence in human placenta remains undetermined. We therefore sought to clarify the location and roles of Ikaros in human placenta. Immunohistochemical analysis showed modest Ikaris expression in syncytium, and intense expression in extravillous trophoblasts (EVTs) in first trimester placenta. Western blot analysis showed that villous trophoblasts principally expressed Ikaros-2/3, while Ikaros-x (Ikx) was predominantly expressed in cultured EVTs. Furthermore, to investigate the functional role of Ikx in EVTs, the EVT cell line HTR-8/SVneo was infected with a retrovirus vector expressing the hemagglutinin (HA)-tagged dominant negative isoform Ikaros-6 (Ik6), which prevents the DNA-binding activity of Ikx. Antibody against HA showed successful transduction of Ik6 in HTR-8/SVneo cells on immunocytochemistry and Western blotting. Transduction of Ik6 significantly reduced the migratory and invasive abilities of HTR-8/SVneo cells. These results suggest that Ikx is involved in migration and invasion of EVTs in early placentation.

The nuclear oncoprotein TLX1/HOX11 associates with pericentromeric satellite 2 DNA in leukemic T-cells

TLX1/HOX11, a DNA-binding homeodomain protein, was originally identified by virtue of its aberrant expression in T-cell leukemia and subsequently found to be crucial for normal spleen development. The precise mechanism of TLX1 function remains poorly understood, although it is known that it can act as both a transcriptional activator and repressor and can downregulate the Aldh1a1 gene in embryonic mouse spleen. Using a whole-genome PCR approach, we show here that TLX1 protein directly interacts with pericentromeric human satellite 2 DNA sequences. Such DNA is known to localize to heterochromatin, which among other roles has been implicated in gene silencing. The interaction was confirmed in vitro and in vivo by gel retardation and chromatin immunoprecipitation assays involving satellite 2 DNA, which contained sequences resembling TLX1 binding sites. Using immunofluorescence microscopy, TLX1 demonstrated a punctate pattern of staining in the nuclei of leukemic T-cells (ALL-SIL). Double labelling indicated that TLX1 colocalized with the centromeric protein CENP-B, demonstrating that the TLX1 foci corresponded to clusters of centromeric DNA. The novel interaction of TLX1 with constitutive heterochromatin adds an additional level of complexity to the intracellular functions of this transcriptional regulator and may have relevance to its roles in transcriptional repression and T-cell immortalization.

Transgenic expression of Helios in B lineage cells alters B cell properties and promotes lymphomagenesis

Helios, a member of the Ikaros family of DNA-binding proteins, is expressed in multipotential lymphoid progenitors and throughout the T lineage. However, in most B lineage cells, Helios is not expressed, suggesting that its absence may be critical for B cell development and function. To test this possibility, transgenic mice were generated that express Helios under the control of an Ig mu enhancer. Commitment to the B cell lineage was unaltered in Helios transgenic mice, and numbers of surface IgM(+) B cells were normal in the bone marrow and spleen. However, both bone marrow and splenic B cells exhibited prolonged survival and enhanced proliferation. B cells in Helios transgenic mice were also hyperresponsive to Ag stimulation. These alterations were observed even though the concentration of ectopic Helios in B lineage cells, like that of endogenous Helios in thymocytes, was well below the concentration of Ikaros. Further evidence that ectopic Helios expression contributes to B cell abnormalities was provided by the observation that Helios transgenic mice developed metastatic lymphoma as they aged. Taken together, these results demonstrate that silencing of Helios is critical for normal B cell function.

Silence of the genes--mechanisms of long-term repression

A large fraction of genes in the mammalian genome is repressed in every cell throughout development. Here, we propose that this long-term silencing is carried out by distinct molecular mechanisms that operate in a global manner and, once established, can be maintained autonomously through DNA replication. Both individually and in combination these mechanisms bring about repression, mainly by lowering gene accessibility through closed chromatin structures.

Ikaros-family proteins: in search of molecular functions during lymphocyte development

The regulatory steps that lead to the differentiation of hematopoietic cells from a multipotential stem cell remain largely unknown. A beginning to the understanding of these steps has come from the study of DNA-binding proteins that are thought to regulate the expression of genes required for specific developmental events. Ikaros is the founding member of a small family of DNA-binding proteins required for lymphocyte development, but the members of this family differ from other key regulators of lymphopoiesis in that direct target genes have not been conclusively identified, and reasonable support has been presented for only a few potential targets. Therefore, the molecular mechanisms that Ikaros uses for regulating lymphocyte development remain largely unknown. Current data suggest that, in some instances, Ikaros may function as a typical transcription factor. However, recent results suggest that it may function more broadly, perhaps in the formation of silent and active chromatin structures. In this review, our current knowledge of the molecular functions of Ikaros will be discussed.

Assembly of silent chromatin during thymocyte development

Epigenetic events that contribute to the assembly and maintenance of silent chromatin structures have been defined through genetic, molecular, and cytological studies in a variety of eukaryotic model organisms. However, the precise cascade of events responsible for converting a developmentally regulated gene from an active euchromatic state to a heritably silent heterochromatic state remains to be elucidated. To establish a molecular framework for studying this cascade, we examined the temporal order of events associated with silencing of the murine terminal transferase (Dntt) gene during thymocyte maturation. This article describes our findings in the context of current knowledge of gene silencing mechanisms.

Delayed, asynchronous, and reversible T-lineage specification induced by Notch/Delta signaling

Using the OP9-DL1 system to deliver temporally controlled Notch/Delta signaling, we show that pluripotent hematolymphoid progenitors undergo T-lineage specification and B-lineage inhibition in response to Notch signaling in a delayed and asynchronous way. Highly enriched progenitors from fetal liver require > or =3 d to begin B- or T-lineage differentiation. Clonal switch-culture analysis shows that progeny of some single cells can still generate both B- and T-lineage cells, after 1 wk of continuous delivery or deprivation of Notch/Delta signaling. Notch signaling induces T-cell genes and represses B-cell genes, but kinetics of activation of lineage-specific transcription factors are significantly delayed after induction of Notch target genes and can be temporally uncoupled from the Notch response. In the cells that initiate T-cell differentiation and gene expression most slowly in response to Notch/Delta signaling, Notch target genes are induced to the same level as in the cells that respond most rapidly. Early lineage-specific gene expression is also rapidly reversible in switch cultures. Thus, while necessary to induce and sustain T-cell development, Notch/Delta signaling is not sufficient for T-lineage specification and commitment, but instead can be permissive for the maintenance and proliferation of uncommitted progenitors that are omitted in binary-choice models.

Ikaros SUMOylation: switching out of repression

Ikaros plays a key role in lymphocyte development and homeostasis by both potentiating and repressing gene expression. Here we show that Ikaros interacts with components of the SUMO pathway and is SUMOylated in vivo. Two SUMOylation sites are identified on Ikaros whose simultaneous modification results in a loss of Ikaros' repression function. Ikaros SUMOylation disrupts its participation in both histone deacetylase (HDAC)-dependent and HDAC-independent repression but does not influence its nuclear localization into pericentromeric heterochromatin. These studies reveal a new dynamic way by which Ikaros-mediated gene repression is controlled by SUMOylation.

Roles of USF, Ikaros and Sp proteins in the transcriptional regulation of the human reduced folate carrier B promoter

The hRFC (human reduced folate carrier) is ubiquitously but differentially expressed in human tissues and its levels are regulated by up to seven non-coding regions (A1, A2, A, B, C, D and E) and at least four promoters. For the hRFC-B basal promoter, regulation involves binding of Sp (specificity protein) transcription factors to a critical GC-box. By transiently transfecting HT1080 cells with 5'- and 3'-deletion constructs spanning 1057 bp of upstream sequence, a transcriptionally important region was localized to 158 bp flanking the transcriptional start sites. By gel shift and chromatin immunoprecipitation assays, USF (upstream stimulatory factor), Sp1 and Ikaros-related proteins were bound to consensus elements (one E-box, two GC-box and three Ikaros) within this region. The functional importance of these elements was confirmed by transient tranfections of HT1080 cells with hRFC-B reporter constructs in which they were mutated, and by co-transfections of Drosophila Mel-2 cells with wild-type hRFC-B promoter and expression constructs for USF1, USF2a, Sp1 and Ikaros 2 and 8. Both USF1 and Sp1 proteins transactivated the hRFC-B promoter. Sp1 combined with USF1 resulted in a synergistic transactivation. Identical results were obtained with USF2a. Ikaros 2 was a repressor of hRFC-B promoter activity whose effects were partly reversed by the dominant-negative Ikaros 8. In HT1080 cells, transfection with Ikaros 2 decreased endogenous hRFC-B transcripts, whereas USF1 and Sp1 increased transcript levels. Ikaros 2 also decreased reporter gene activity and levels of acetylated chromatin associated with the endogenous promoter. Collectively, these results identify transcriptionally important regions in the hRFC-B promoter that include multiple GC-box, Ikaros and E-box elements. Our results also suggest that co-operative interactions between transcription factors Sp1 and USF are essential for high-level hRFC-B transactivation and imply that these effects are modulated by the family of Ikaros proteins and by histone acetylation.

Histone hypomethylation is an indicator of epigenetic plasticity in quiescent lymphocytes

Post-translational modifications of histone amino termini are thought to convey epigenetic information that extends the coding potential of DNA. In particular, histone lysine methylation has been implicated in conveying transcriptional memory and maintaining lineage fidelity. Here an analysis of histone lysine methylation in quiescent (G(0)) and cycling lymphocytes showed that methylation of histone H3 at lysines 4 (H3K4), 9 (H3K9), 27 (H3K27) and histone H4 at lysine 20 is markedly reduced in resting B lymphocytes as compared with cycling cells. Quiescent B cells also lacked heterochromatin-associated HP1beta and Ikaros at pericentric chromatin and expressed low levels of Ezh2 and ESET histone methyl transferases (HMTases). Nuclei from resting B or T cells were approximately three times more efficiently reprogrammed in nuclear transfer assays than cells in which HMTase expression, histone methylation and HP1beta binding had been restored following mitotic stimulation. These results showing local and global changes in histone lysine methylation levels in vivo demonstrate that constitutive heterochromatin organization is modified in resting lymphocytes and suggest that histone hypomethylation is a useful indicator of epigenetic plasticity.

Avian Helios and Evolution of the Ikaros Family

Abstract Helios (Znfn1a2) is an Ikaros-related lymphoid regulatory protein with possible involvement in T-cell development and function as well as in the early events of haematopoietic stem cell differentiation. To evaluate the role of Helios in avian haemato/lymphopoiesis, we have characterized the avian Helios gene. In contrast to studies in mouse and human, we have found that the highly conserved avian Helios encodes a novel exon and three isoforms. Furthermore, the avian Helios expression precedes Ikaros in the ontogeny, being present already on the first day of embryonic development. Additionally, expression in the bursa of Fabricius, germinal centres and B-cell lines suggests a role for Helios also in the B-cell lineage. Phylogenetic studies of the Ikaros family along with data on paralogous chromosome segments in the human genome connect the expansion of the Ikaros family and thus possibly the emergence of the adaptive immune system with the putative second round of genome duplications and indicate that the Ikaros gene family is linked with the Hox gene clusters.

Overexpression of the Ikaros 6 isoform is restricted to t(4;11) acute lymphoblastic leukaemia in children and infants and has a role in B-cell survival

The Ikaros transcription factor has been shown to play an important role in the differentiation of both myeloid and lymphoid lineages. Mice heterozygous for a dominant negative (DN) ikaros isoform develop T-cell leukaemia and lymphoma with 100% penetrance. Overexpression of DN Ikaros isoforms has been reported in some forms of leukaemia, such as childhood acute myelomonocytic and monocytic leukaemias, adult B-cell acute lymphoblastic leukaemias (B ALL) and in childhood and adult pre-B ALL. In this study, the expression of Ikaros isoforms in 49 infant and childhood leukaemia patients was analysed by reverse transcription polymerase chain reaction and Western blot analysis. We found overexpression of the DN Ikaros 6 (Ik6) isoform in a subset of leukaemia patients harbouring t(4;11) translocations. To further study the consequences of Ik6 overexpression in B ALL, we inducibly expressed Ik6 in BaF3 cells and found that Ik6 overexpression delayed cell death after interleukin-3 withdrawal, suggesting that overexpression of Ik6 found in t(4;11) B cells could contribute to leukaemogenesis by preventing the apoptosis of cells in an environment with reduced survival factors.

Ikaros family members from the agnathan Myxine glutinosa and the urochordate Oikopleura dioica: emergence of an essential transcription factor for adaptive immunity

The Ikaros multigene family encodes a number of zinc finger transcription factors that play key roles in vertebrate hemopoietic stem cell differentiation and the generation of B, T, and NK cell lineages. In this study, we describe the identification and characterization of an Ikaros family-like (IFL) protein from the agnathan hagfish Myxine glutinosa and the marine urochordate Oikopleura dioica, both of which lie on the evolutionary boundary between the vertebrates and invertebrates. The IFL molecules identified in these animals displayed high conservation in the zinc finger motifs critical for DNA binding and dimerization in comparison with those of jawed vertebrates. Expression of the IFL gene in hagfish was strongest in blood, intestine, and gills. In O. dioica, transcription from the IFL gene was initiated at or around the time of hatching and maintained throughout the life span of the animal. In situ hybridization localized O. dioica IFL expression to the Fol cells, which are responsible for generating the food filter of the house. Biochemical analysis of the DNA binding and dimerization domains from M. glutinosa and O. dioici IFLs showed that M. glutinosa behaves as a true Ikaros family member. Taken together, these results indicate that the properties associated with the Ikaros family preceded the emergence of the jawed vertebrates and thus adaptive immunity.

The roles of transcription factors in B lymphocyte commitment, development, and transformation

Studies of normal blood cell development and malignant transformation of hematopoietic cells have shown that the correctly regulated expression of stage- and lineage-specific genes is a key issue in hematopoiesis. Experiments in transgenic mice have defined a number of transcription factors such as SCL/Tal, core-binding factor/acute myeloid leukemia, and c-myb, all crucial for the establishment of definitive hematopoiesis and development of all blood cell lineages. Other regulators such as IKAROS, E47/E2A, early B cell factor, Sox-4, and B cell-specific activator protein (Pax-5) appear crucial, more or less selectively, for B lymphopoiesis, allowing for detailed analysis of the development of this lineage. In addition, several of these transcription factors are found translocated in human tumors, often resulting in aberrant gene expression or production of modified proteins. This article concerns the role of transcription factors in B lymphoid development with special focus on lineage initiation and commitment events but also to some extent on the roles of transcription factors in human B lymphoid malignancies.

Altered expression of Tfg and Dap3 in Ikaros-defective T-cell lymphomas induced by X-irradiation in B6C3F1 mice

Ikaros is a Kruppel-type zinc finger protein that is essential for normal lymphocyte development and differentiation. Recently, it has been demonstrated that Ikaros is frequently inactivated in both human and mouse leukaemias/lymphomas. Although this inactivation is thought to be involved in leukaemogenesis, little is known about the molecular mechanisms that lead to neoplastic transformation. To identify the genes that may be controlled by Ikaros, we performed differential display analysis of RNAs from mouse 3T3-L1 cells that had been transfected with the Ikaros gene. Two cDNAs, the Trk-fused gene (Tfg) and death-associated protein 3 gene (Dap3) were upregulated in Ikaros-transfected cells. Expression of Tfg and Dap3 was consistently downregulated in radiation-induced T-cell lymphomas that exhibited defective Ikaros expression. These results suggest that Tfg and Dap3 function downstream of Ikaros and may be involved in radiation-induced lymphomagenesis.

Upstream of Ikaros

Haematopoiesis - the process by which pluripotent haematopoietic stem cells choose to become lymphocytes, myeloid cells, red blood cells or platelets - has long been a useful model to study cellular commitment and differentiation. Each cell type is defined by lineage-specific patterns of gene expression. The DNA binding protein Ikaros is an important regulator of haematopoiesis and recent work promises to shed light on how Ikaros itself is regulated.

The C-terminal domain of Eos forms a high order complex in solution

Ikaros family transcription factors play important roles in the control of hematopoiesis. Family members are predicted to contain up to six classic zinc fingers that are arranged into N- and C-terminal domains. The N-terminal domain is responsible for site-specific DNA binding, whereas the C-terminal domain primarily mediates the homo- and hetero-oligomerization between family members. Although the mechanisms of action of these proteins are not completely understood, the zinc finger domains are known to play a central role. In the current study, we have sought to understand the physical and functional properties of these domains, in particular the C-terminal domain. We show that the N-terminal domain from Eos, and not its C-terminal region, is required to recognize GGGA consensus sequences. Surprisingly, in contrast to the behavior exhibited by Ikaros, the C-terminal domain of Eos inhibits the DNA-binding activity of the full-length protein. In addition, we have used a range of biophysical techniques to demonstrate that the C-terminal domain of Eos mediates the formation of complexes that consist of nine or ten molecules. These results constitute the first direct demonstration that Ikaros family proteins can form higher order complexes in solution, and we discuss this unexpected result in the context of what is currently known about the family members and their possible mechanism of action.

Ikaros isoforms in human pituitary tumors: distinct localization, histone acetylation, and activation of the 5' fibroblast growth factor receptor-4 promoter

Targeted expression of a human pituitary tumor derived-fibroblast growth factor receptor-4 (FGFR4) recapitulates pituitary tumorigenesis. We have shown that FGFR4 is a target for Ikaros, a zinc finger-containing transcription factor that localizes to heterochromatin regions and participates in higher order chromatin complexes and control of gene expression. We report here the expression of Ikaros and functional differences between its alternatively spliced variants in human pituitary tumors. Ik1 expression was detected in human pituitary tumors and we also identified a truncated isoform consistent with the non-DNA-binding Ik6 isoform in a subset of adenomas by reverse transcriptase-polymerase chain reaction, sequencing, and Western immunoblotting. Transfection of Ik6 in GH4 pituitary cells resulted in predominantly cytoplasmic expression as compared to Ik1, which resulted in exclusively nuclear expression as determined by immunofluorescence and immunoblotting of fractionated protein. Immunohistochemistry of primary human pituitary adenomas localized Ikaros expression to the nuclear compartment but also in the cytoplasm, the latter consistent with Ik6. Expression of Ikaros and truncated non-DNA-binding isoforms was also suggested by electromobility shift assays using nuclear proteins from primary human pituitary adenomas. Ik6 resulted in reversal of the effects of Ik1 on wild-type 5' FGFR4 promoter activity, histone acetylation, and regulation of the endogenous gene. We conclude that dominant-negative Ik6 isoforms with their distinct localization and effects on Ik1 action may contribute to the altered expression of FGFR4 and possibly other target genes in human pituitary tumors.

Widespread failure of hematolymphoid differentiation caused by a recessive niche-filling allele of the Ikaros transcription factor

A central issue in understanding the hematolymphoid system is the generation of appropriate mutant alleles in mice to reveal the function of regulatory genes. Here we describe a mouse strain, Plastic, with a point mutation in a zinc finger of Ikaros that disrupts DNA binding but preserves efficient assembly of the full-length protein into higher order complexes. Ikaros(Plastic) homozygosity is embryonically lethal with severe defects in terminal erythrocyte and granulocyte differentiation, excessive macrophage formation, and blocked lymphopoiesis, while heterozygotes display a partial block in lymphocyte differentiation. The contrast with more circumscribed effects of Ikaros alleles that ablate the full-length protein highlights the importance in mammals of generating recessive niche-filling alleles that inactivate function without creating a void in multimolecular assemblies.

Hepatocyte NF-1 and STAT6 cooperate with additional DNA-binding factors to activate transcription of the human polymeric Ig receptor gene in response to IL-4

Secretory IgA and IgM, which protect the mucosal surfaces, are generated by selective transport of locally produced polymeric (p)Igs through the epithelial barrier by the pIgR. The expression of this receptor, and hence the generation of secretory Igs, is modulated by numerous extracellular factors. We have previously identified a STAT6 site in intron 1 of the human pIgR gene that is required for the slow and de novo protein synthesis-dependent IL-4-mediated transcriptional activation of the gene. In this study, we show that this intronic IL-4-responsive enhancer is confined to a 250-bp region that is highly conserved in the murine pIgR gene. The enhancer was dependent on the cooperation between the STAT6 site and at least four additional DNA elements. EMSA experiments demonstrated binding by hepatocyte NF-1 to one of these DNA elements. Extensive overlap in the tissue distribution of hepatocyte NF-1 and pIgR suggests that this transcription factor contributes to tissue-specific pIgR expression. Changing the helical phase between the STAT6 site and downstream DNA elements greatly reduced the strength of the IL-4 response, suggesting that the precise organization of this enhancer is important for its proper function. Thus, several transcription factors cooperate in this enhanceosome to mediate IL-4 responsiveness in HT-29 epithelial cells.

Ikaros, Aiolos and Helios: transcription regulators and lymphoid malignancies

Ikaros, Aiolos and Helios encode zinc finger transcription factors that are important regulators of lymphoid development and differentiation. These proteins are involved in the control of gene expression and when associated with nuclear complexes, participate in nucleosome remodeling. Because differential splicing produces multiple protein isoforms with potentially different functions, the Ikaros protein family provides a useful model for the study of whether post-transcriptional modifications are involved in tumoral transformation. Several reports reinforce the hypothesis that Ikaros, Aiolos and Helios expression is deregulated in human leukaemias. The direct involvement of aberrant protein expression of Ikaros family members in human haematological malignancies is discussed.

Over-expression of the dominant-negative isoform of Ikaros confers resistance to dexamethasone-induced and anti-IgM-induced apoptosis

In previous studies, we demonstrated an over-expression of the dominant-negative isoform of the transcription factor Ikaros, Ik-6, in patients with B-cell malignancies, including blast crisis of chronic myelogenous leukaemia and acute lymphoblastic leukaemia. To investigate the consequence of over-expression of Ik-6 in B cells, we constructed Ik-6 transfectants of the FDH-1 and Ramos cell lines. FDH-1, which was established from a patient with early pre-B acute lymphoblastic leukaemia, undergoes apoptosis with dexamethasone treatment, whereas Ramos undergoes apoptosis following anti-IgM antibody treatment. Compared with the wild type, the over-expression of Ik-6 rendered the FDH-1 and Ramos transfectants resistant to dexamethasone-induced and anti-IgM-induced apoptosis respectively. An immunoblotting study demonstrated bcl-2 upregulation in anti-IgM-induced Ramos Ik-6 transfectants, but not in FDH-1 Ik-6 transfectants. Further investigations of the mechanism of leukaemogenesis associated with the over-expression of Ik-6 are warranted.

Over-expression of short isoforms of Helios in patients with adult T-cell leukaemia/lymphoma

In previous studies, we demonstrated an over-expression of the dominant-negative isoform of the transcription factor Ikaros in patients with blast crisis of both chronic myelogenous leukaemia and B-cell acute lymphoblastic leukaemia (ALL). Recently, we reported an over-expression of the short isoforms of Helios, which is one of the members of the Ikaros gene family, in a patient with T-cell ALL. In the present study, we found over-expression of short isoforms of Helios in human T lymphotropic virus-I (HTLV1)-infected patients who had developed chronic and acute forms of adult T-cell leukaemia/lymphoma. In contrast, we could not detect any over-expression of short isoforms of Helios in healthy HTLV1 carriers. By Southern blotting, we detected a small deletion in the Helios gene locus of adult T-cell leukaemia/lymphoma patients. The present results suggest that Helios gene abnormalities might be one of the important mechanisms in the disease progression of HTLV1 infection.

Binding of Ikaros to germline Ig heavy chain gamma1 and epsilon promoters

Immunoglobulin (Ig) class switching occurs in activated B cells and results in production of antigen-specific IgA, IgE or IgG. It involves a DNA recombination event and is partly regulated by germline (GL) immunoglobulin heavy chain promoters. Ikaros is an abundant nuclear protein expressed in hematopoietic cells. Many different functions have been ascribed to Ikaros, such as transcriptional activation or repression, cell cycle control and tumor suppression. A typical feature of Ikaros is its expression in large clusters in the nucleus of activated lymphocytes. We give evidence that Ikaros can bind to several sites in the germline gamma1 and epsilon immunoglobulin heavy chain promoters, in a cooperative manner. Using a promoter reporter assay, we found evidence that Ikaros can suppress germline gamma1 and epsilon promoter activity in a B cell line. When a mutated non-DNA-binding form of Ikaros was introduced into primary activated B cells by retrovirus transduction, the endogenous Ikaros clusters were disrupted. In spite of this, there was no effect on transcription or Ig class switching. The data are discussed in relation to the different hypotheses for the function of Ikaros.

Selective dimerization of a C2H2 zinc finger subfamily

The C2H2 zinc finger is the most prevalent protein motif in the mammalian proteome. Two C2H2 fingers in Ikaros are dedicated to homotypic interactions between family members. We show here that these fingers comprise a bona fide dimerization domain. Dimerization is highly selective, however, as homologous domains from the TRPS-1 and Drosophila Hunchback proteins support homodimerization, but not heterodimerization with Ikaros. Ikaros-Hunchback selectivity is determined by 11 residues concentrated within the alpha-helical regions typically involved in base recognition. Preferential homodimerization of one chimeric protein predicts a parallel dimer interface and establishes the feasibility of creating novel dimer specificities. These results demonstrate that the C2H2 motif provides a versatile platform for both sequence-specific protein-nucleic acid interactions and highly specific dimerization.

A PIT-1 homeodomain mutant blocks the intranuclear recruitment of the CCAAT/enhancer binding protein alpha required for prolactin gene transcription

The pituitary-specific homeodomain protein Pit-1 cooperates with other transcription factors, including CCAAT/enhancer binding protein alpha (C/EBPalpha), in the regulation of pituitary lactotrope gene transcription. Here, we correlate cooperative activation of prolactin (PRL) gene transcription by Pit-1 and C/EBPalpha with changes in the subnuclear localization of these factors in living pituitary cells. Transiently expressed C/EBPalpha induced PRL gene transcription in pituitary GHFT1-5 cells, whereas the coexpression of Pit-1 and C/EBPalpha in HeLa cells demonstrated their cooperativity at the PRL promoter. Individually expressed Pit-1 or C/EBPalpha, fused to color variants of fluorescent proteins, occupied different subnuclear compartments in living pituitary cells. When coexpressed, Pit-1 recruited C/EBPalpha from regions of transcriptionally quiescent centromeric heterochromatin to the nuclear regions occupied by Pit-1. The homeodomain region of Pit-1 was necessary for the recruitment of C/EBPalpha. A point mutation in the Pit-1 homeodomain associated with the syndrome of combined pituitary hormone deficiency in humans also failed to recruit C/EBPalpha. This Pit-1 mutant functioned as a dominant inhibitor of PRL gene transcription and, instead of recruiting C/EBPalpha, was itself recruited by C/EBPalpha to centromeric heterochromatin. Together our results suggest that the intranuclear positioning of these factors determines whether they activate or silence PRL promoter activity.

The Ikaros family protein Eos associates with C-terminal-binding protein corepressors

Eos is a zinc finger transcription factor of the Ikaros family. It binds typical GGGAA Ikaros recognition sites in DNA and functions as a transcriptional repressor. Here we show that Eos associates with the corepressor C-terminal-binding protein (CtBP). CtBP has previously been shown to bind Pro-X-Asp-Leu-Ser (PXDLS) motifs in several DNA-binding proteins. We note that Eos contains a related motif PEDLA, and we demonstrate that CtBP can bind this site weakly but that it also contacts additional regions of Eos. Consistent with this finding, mutation of the PEDLA motif does not negate CtBP binding or CtBP-mediated repression by Eos. CtBP has previously been shown to bind to a PXDLS-type motif in Ikaros, and we show that another Ikaros-related protein TRPS1 also contains a PXDLS CtBP contact motif within its repression domain. We conclude that several Ikaros family proteins utilize CtBP corepressors to inhibit gene expression.

COUP-TF (chicken ovalbumin upstream promoter transcription factor)-interacting protein 1 (CTIP1) is a sequence-specific DNA binding protein

Chicken ovalbumin upstream promoter transcription factor (COUP-TF)-interacting proteins 1 and 2 [CTIP1/Evi9/B cell leukaemia (Bcl) l1a and CTIP2/Bcl11b respectively] are highly related C(2)H(2) zinc finger proteins that are abundantly expressed in brain and the immune system, and are associated with immune system malignancies. A selection procedure was employed to isolate high-affinity DNA binding sites for CTIP1. The core binding site on DNA identified in these studies, 5'-GGCCGG-3' (upper strand), is highly related to the canonical GC box and was bound by a CTIP1 oligomeric complex(es) in vitro. Furthermore, both CTIP1 and CTIP2 repressed transcription of a reporter gene harbouring a multimerized CTIP binding site, and this repression was neither reversed by trichostatin A (an inhibitor of known class I and II histone deacetylases) nor stimulated by co-transfection of a COUP-TF family member. These results demonstrate that CTIP1 is a sequence-specific DNA binding protein and a bona fide transcriptional repressor that is capable of functioning independently of COUP-TF family members. These findings may be relevant to the physiological and/or pathological action(s) of CTIPs in cells that do not express COUP-TF family members, such as cells of the haematopoietic and immune systems.

An alternative internal splicing site defines new Ikaros isoforms in Pleurodeles waltl

The Ikaros gene encodes a family of transcription factors which plays a crucial role in hematopoiesis. To improve our knowledge about the immune system of Pleurodeles waltl, we sequenced the cDNA coding for the Ik-1 isoform of that salamander and analyzed its tissue expression by semi-quantitative RT-PCR. Ikaros transcripts are abundant in the thymus and the spleen, thereby confirming that these organs are, respectively, the primary and secondary lymphoid tissues of Pleurodeles. Analysis of the isoforms produced by this animal revealed two isoforms characteristic of amphibians in which an alternative internal splicing site deletes the 3' half of exon 3 which interestingly comprises the first Zn finger of Ikaros. Ikaros transcripts were found at the earliest stages of development of Pleurodeles indicating that Ikaros has a function at the very early lymphopoietic stages. Moreover, the presence of Ikaros transcripts in spermatozoa suggests that this protein could have another and yet unknown function.