Nuclear localization of CBF1 is regulated by interactions with the SMRT corepressor complex

Novel Genome-Engineered H Alleles Differentially Affect Lateral Inhibition and Cell Dichotomy Processes during Bristle Organ Development

Hairless (H) encodes the major antagonist in the Notch signaling pathway, which governs cellular differentiation of various tissues in Drosophila. By binding to the Notch signal transducer Suppressor of Hairless (Su(H)), H assembles repressor complexes onto Notch target genes. Using genome engineering, three new H alleles, HFA, HLLAA and HWA were generated and a phenotypic series was established by several parameters, reflecting the residual H-Su(H) binding capacity. Occasionally, homozygous HWA flies develop to adulthood. They were compared with the likewise semi-viable HNN allele affecting H-Su(H) nuclear entry. The H homozygotes were short-lived, sterile and flightless, yet showed largely normal expression of several mitochondrial genes. Typical for H mutants, both HWA and HNN homozygous alleles displayed strong defects in wing venation and mechano-sensory bristle development. Strikingly, however, HWA displayed only a loss of bristles, whereas bristle organs of HNN flies showed a complete shaft-to-socket transformation. Apparently, the impact of HWA is restricted to lateral inhibition, whereas that of HNN also affects the respective cell type specification. Notably, reduction in Su(H) gene dosage only suppressed the HNN bristle phenotype, but amplified that of HWA. We interpret these differences as to the role of H regarding Su(H) stability and availability.

MicroRNAs as the pivotal regulators of cisplatin resistance in head and neck cancers

Although, there is a high rate of good prognosis in early stage head and neck tumors, about half of these tumors are detected in advanced stages with poor prognosis. A combination of chemotherapy, radiotherapy, and surgery is the treatment option in head and neck cancer (HNC) patients. Although, cisplatin (CDDP) as the first-line drug has a significant role in the treatment of HNC patients, CDDP resistance can be observed in a large number of these patients. Therefore, identification of the molecular mechanisms involved in CDDP resistance can help to reduce the side effects and also provides a better therapeutic management. MicroRNAs (miRNAs) as the post-transcriptional regulators play an important role in drug resistance. Therefore, in the present review we investigated the role of miRNAs in CDDP response of head and neck tumors. It has been reported that the miRNAs exerted their roles in CDDP response by regulation of signaling pathways such as WNT, NOTCH, PI3K/AKT, TGF-β, and NF-kB as well as apoptosis, autophagy, and EMT process. The present review paves the way to suggest a non-invasive miRNA based panel marker for the prediction of CDDP response among HNC patients. Therefore, such diagnostic miRNA based panel marker reduces the CDDP side effects and improves the clinical outcomes of these patients following an efficient therapeutic management.

OptIC-Notch reveals mechanism that regulates receptor interactions with CSL

Active Notch signalling is elicited through receptor-ligand interactions that result in release of the Notch intracellular domain (NICD), which translocates into the nucleus. NICD activates transcription at target genes, forming a complex with the DNA-binding transcription factor CSL [CBF1/Su(H)/LAG-1] and co-activator Mastermind. However, CSL lacks its own nuclear localisation sequence, and it remains unclear where the tripartite complex is formed. To probe the mechanisms involved, we designed an optogenetic approach to control NICD release (OptIC-Notch) and monitored the subsequent complex formation and target gene activation. Strikingly, we observed that, when uncleaved, OptIC-Notch sequestered CSL in the cytoplasm. Hypothesising that exposure of a juxta membrane ΦWΦP motif is key to sequestration, we masked this motif with a second light-sensitive domain (OptIC-Notch{ω}), which was sufficient to prevent CSL sequestration. Furthermore, NICD produced by light-induced cleavage of OptIC-Notch or OptIC-Notch{ω} chaperoned CSL into the nucleus and induced target gene expression, showing efficient light-controlled activation. Our results demonstrate that exposure of the ΦWΦP motif leads to CSL recruitment and suggest this can occur in the cytoplasm prior to nuclear entry.

Epstein-Barr Virus B Cell Growth Transformation: The Nuclear Events

Epstein-Barr virus (EBV) is the first human DNA tumor virus identified from African Burkitt's lymphoma cells. EBV causes ~200,000 various cancers world-wide each year. EBV-associated cancers express latent EBV proteins, EBV nuclear antigens (EBNAs), and latent membrane proteins (LMPs). EBNA1 tethers EBV episomes to the chromosome during mitosis to ensure episomes are divided evenly between daughter cells. EBNA2 is the major EBV latency transcription activator. It activates the expression of other EBNAs and LMPs. It also activates MYC through enhancers 400-500 kb upstream to provide proliferation signals. EBNALP co-activates with EBNA2. EBNA3A/C represses CDKN2A to prevent senescence. LMP1 activates NF-κB to prevent apoptosis. The coordinated activity of EBV proteins in the nucleus allows efficient transformation of primary resting B lymphocytes into immortalized lymphoblastoid cell lines in vitro.

The Binding of CSL Proteins to Either Co-Activators or Co-Repressors Protects from Proteasomal Degradation Induced by MAPK-Dependent Phosphorylation

The primary role of Notch is to specify cellular identities, whereby the cells respond to amazingly small changes in Notch signalling activity. Hence, dosage of Notch components is crucial to regulation. Central to Notch signal transduction are CSL proteins: together with respective cofactors, they mediate the activation or the silencing of Notch target genes. CSL proteins are extremely similar amongst species regarding sequence and structure. We noticed that the fly homologue suppressor of hairless (Su(H)) is stabilised in transcription complexes. Using specific transgenic fly lines and HeLa RBPJ^KO cells we provide evidence that Su(H) is subjected to proteasomal degradation with a half-life of about two hours if not protected by binding to co-repressor hairless or co-activator Notch. Moreover, Su(H) stability is controlled by MAPK-dependent phosphorylation, matching earlier data for RBPJ in human cells. The homologous murine and human RBPJ proteins, however, are largely resistant to degradation in our system. Mutating presumptive protein contact sites, however, sensitised RBPJ for proteolysis. Overall, our data highlight the similarities in the regulation of CSL protein stability across species and imply that turnover of CSL proteins may be a conserved means of regulating Notch signalling output directly at the level of transcription.

Chromatin Regulator SPEN/SHARP in X Inactivation and Disease

Simple Summary

Carcinogenesis is a multistep process involving not only the activation of oncogenes and disabling tumor suppressor genes, but also epigenetic modulation of gene expression. X chromosome inactivation (XCI) is a paradigm to study heterochromatin formation and maintenance. The double dosage of X chromosomal genes in female mammals is incompatible with early development. XCI is an excellent model system for understanding the establishment of facultative heterochromatin initiated by the expression of a 17,000 nt long non-coding RNA, known as Xinactivespecifictranscript (Xist), on the X chromosome. This review focuses on the molecular mechanisms of how epigenetic modulators act in a step-wise manner to establish facultative heterochromatin, and we put these in the context of cancer biology and disease. An in depth understanding of XCI will allow a better characterization of particular types of cancer and hopefully facilitate the development of novel epigenetic therapies.

Abstract

Enzymes, such as histone methyltransferases and demethylases, histone acetyltransferases and deacetylases, and DNA methyltransferases are known as epigenetic modifiers that are often implicated in tumorigenesis and disease. One of the best-studied chromatin-based mechanism is X chromosome inactivation (XCI), a process that establishes facultative heterochromatin on only one X chromosome in females and establishes the right dosage of gene expression. The specificity factor for this process is the long non-coding RNA Xinactivespecifictranscript (Xist), which is upregulated from one X chromosome in female cells. Subsequently, Xist is bound by the corepressor SHARP/SPEN, recruiting and/or activating histone deacetylases (HDACs), leading to the loss of active chromatin marks such as H3K27ac. In addition, polycomb complexes PRC1 and PRC2 establish wide-spread accumulation of H3K27me3 and H2AK119ub1 chromatin marks. The lack of active marks and establishment of repressive marks set the stage for DNA methyltransferases (DNMTs) to stably silence the X chromosome. Here, we will review the recent advances in understanding the molecular mechanisms of how heterochromatin formation is established and put this into the context of carcinogenesis and disease.

Nucleo-cytoplasmic shuttling of murine RBPJ by Hairless protein matches that of Su(H) protein in the model system Drosophila melanogaster

Abstract

CSL transcription factors are central to signal transduction in the highly conserved Notch signaling pathway. CSL acts as a molecular switch: depending on the cofactors recruited, CSL induces either activation or repression of Notch target genes. Unexpectedly, CSL depends on its cofactors for nuclear entry, despite its role as gene regulator. In Drosophila, the CSL homologue Suppressor of Hairless (Su(H)), recruits Hairless (H) for repressor complex assembly, and eventually for nuclear import. We recently found that Su(H) is subjected to a dynamic nucleo-cytoplasmic shuttling, thereby strictly following H subcellular distribution. Hence, regulation of nuclear availability of Su(H) by H may represent a new layer of control of Notch signaling activity. Here we extended this work on the murine CSL homologue RBPJ. Using a ‘murinized’ fly model bearing RBPJ^wt in place of Su(H) at the endogenous locus we demonstrate that RBPJ protein likewise follows H subcellular distribution. For example, overexpression of a H^*NLS3 protein variant defective of nuclear import resulted in a cytosolic localization of RBPJ protein, whereas the overexpression of a H^*NES protein variant defective in the nuclear export signal caused the accumulation of RBPJ protein in the nucleus. Evidently, RBPJ is exported from the nucleus as well. Overall these data demonstrate that in our fly model, RBPJ is subjected to H-mediated nucleo-cytoplasmic shuttling as is Su(H). These data raise the possibility that nuclear availability of mammalian CSL proteins is likewise restricted by cofactors, and may hence present a more general mode of regulating Notch signaling activity.

Graphical abstract

An RBPJ-Drosophila Model Reveals Dependence of RBPJ Protein Stability on the Formation of Transcription-Regulator Complexes

Notch signaling activity governs widespread cellular differentiation in higher animals, including humans, and is involved in several congenital diseases and different forms of cancer. Notch signals are mediated by the transcriptional regulator RBPJ in a complex with activated Notch (NICD). Analysis of Notch pathway regulation in humans is hampered by a partial redundancy of the four Notch receptor copies, yet RBPJ is solitary, allowing its study in model systems. In Drosophila melanogaster, the RBPJ orthologue is encoded by Suppressor of Hairless [Su(H)]. Using genome engineering, we replaced Su(H) by murine RBPJ in order to study its function in the fly. In fact, RBPJ largely substitutes for Su(H)’s function, yet subtle phenotypes reflect increased Notch signaling activity. Accordingly, the binding of RBPJ to Hairless (H) protein, the general Notch antagonist in Drosophila, was considerably reduced compared to that of Su(H). An H-binding defective RBPJ^LLL mutant matched the respective Su(H)^LLL allele: homozygotes were lethal due to extensive Notch hyperactivity. Moreover, RBPJ^LLL protein accumulated at lower levels than wild type RBPJ, except in the presence of NICD. Apparently, RBPJ protein stability depends on protein complex formation with either H or NICD, similar to Su(H), demonstrating that the murine homologue underlies the same regulatory mechanisms as Su(H) in Drosophila. These results underscore the importance of regulating the availability of RBPJ protein to correctly mediate Notch signaling activity in the fly.

Nucleo-cytoplasmic shuttling of Drosophila Hairless/Su(H) heterodimer as a means of regulating Notch dependent transcription

Activation and repression of Notch target genes is mediated by transcription factor CSL, known as Suppressor of Hairless (Su(H)) in Drosophila and CBF1 or RBPJ in human. CSL associates either with co-activator Notch or with co-repressors such as Drosophila Hairless. The nuclear translocation of transcription factor CSL relies on co-factor association, both in mammals and in Drosophila. The Drosophila CSL orthologue Su(H) requires Hairless for repressor complex formation. Based on its role in transcriptional silencing, H protein would be expected to be strictly nuclear. However, H protein is also cytosolic, which may relate to its role in the stabilization and nuclear translocation of Su(H) protein. Here, we investigate the function of the predicted nuclear localization signals (NLS 1-3) and single nuclear export signal (NES) of co-repressor Hairless using GFP-fusion proteins, reporter assays and in vivo analyses using Hairless wild type and shuttling-defective Hairless mutants. We identify NLS3 and NES to be critical for Hairless function. In fact, H^⁎NLS3 mutant flies match H null mutants, whereas H^{⁎NLS3⁎NES} double mutants display weaker phenotypes in agreement with a crucial role for NES in H export. As expected for a transcriptional repressor, Notch target genes are deregulated in H^⁎NLS3 mutant cells, demonstrating nuclear requirement for its activity. Importantly, we reveal that Su(H) protein strictly follows Hairless protein localization. Together, we propose that shuttling between the nucleo-cytoplasmic compartments provides the possibility to fine tune the regulation of Notch target gene expression by balancing of Su(H) protein availability for Notch activation.

Long non-coding RNA AFAP1-AS1/miR-320a/RBPJ axis regulates laryngeal carcinoma cell stemness and chemoresistance

AFAP1-AS1 is a long non-coding RNA that is associated with tumorigenesis and poor prognosis in a variety of cancers. We have been suggested that AFAP1-AS1 increases tumorigenesis in laryngeal carcinoma specifically by enhancing stemness and chemoresistance. We assessed AFAP1-AS1 expression in human laryngeal specimens, paired adjacent normal tissues and human HEp-2 cells. Indeed, we found not only that AFAP1-AS1 was up-regulated in laryngeal carcinoma specimens and cells, but also that stemness-associated genes were overexpressed. Silencing of AFAP1-AS1 promoted HEp-2 cell chemoresistance under cisplatin treatment. Expression of AFAP1-AS1 was increased in drug-resistant Hep-2 cells. We then probed the mechanism of AFAP1-AS1 activity and determined that miR-320a was a potential molecular target of AFAP1-AS1. Luciferase reporter and qRT-PCR assays of AFAP1-AS1 and miR-320a levels in human specimens and cell cultures indicated that AFAP1-AS1 negatively regulates miR-320a. To discover the molecular mechanism of miR-320a, we again used the DIANA Tools algorithm to predict its genetic target, RBPJ. After cloning the 3'-untranslated regions (3'-UTR) of RBPJ into a luciferase reporter, we determined that miR-320a did in fact reduce RBPJ mRNA and protein levels. Ultimately, we determined that AFAP1-AS1 increases RBPJ expression by negatively regulating miR-320a and RBPJ overexpression rescues stemness and chemoresistance inhibited by AFAP1-AS1 silencing. Taken together, these results suggest that AFAP1-AS1 can serve as a prognostic biomarker in laryngeal carcinoma and that miR-320a has the potential to improve standard therapeutic approaches to the disease, especially for cases in which cancer cell stemness and drug resistance present significant barriers to effective treatment.

Notch1 pathway-mediated microRNA-151-5p promotes gastric cancer progression

Gastric carcinoma is the third leading cause of lethal cancer worldwide. Previous studies showed that Notch1 receptor intracellular domain (N1IC), the activated form of Notch1 receptor, promotes gastric cancer progression. It has been demonstrated that a significant cross-talk interplays between Notch pathways and microRNAs (miRNAs) in controlling tumorigenesis. This study identified an intronic microRNA-151 (miR-151), which consists of two mature miRNAs, miR-151-3p and miR-151-5p, as a Notch1 receptor-induced miRNA in gastric cancer cells. Activation of Notch1 pathway enhanced expressions of miR-151 and its host gene, focal adhesion kinase (FAK), in gastric cancer cells. The levels of miR-151 in gastric cancer samples were higher than those of adjacent non-tumor samples. Activated Notch1 pathway induced CBF1-dependent FAK promoter activity. The ectopic expression of miR-151 promoted growth and progression of SC-M1 gastric cancer cells including cell viability and colony formation, migration, and invasion abilities. Activated Notch1 pathway could augment progression of gastric cancer cells through miR-151-5p and FAK. The mRNA levels of pluripotency genes, Nanog and SOX-2, tumorsphere formation ability, tumor growth, and lung metastasis of SC-M1 cells were elevated by activated Notch1 pathway through miR-151-5p. Furthermore, miR-151-5p could target 3′-untranslated region (3′-UTR) of p53 mRNA and down-regulate p53 level in SC-M1 cells. Mechanistically, Notch1/miR-151-5p axis contributed to progression of SC-M1 cells through down-regulation of p53 which in turn repressed FAK promoter activity. Taken together, these results suggest that Notch1 pathway and miR-151-5p interplay with p53 in a reciprocal regulation loop in controlling gastric carcinogenesis.

Consequences of early life stress on genomic landscape of H3K4me3 in prefrontal cortex of adult mice

Background

Maternal separation models in rodents are widely used to establish molecular mechanisms underlying prolonged effects of early life adversity on neurobiological and behavioral outcomes in adulthood. However, global epigenetic signatures following early life stress in these models remain unclear.

Results

In this study, we carried out a ChIP-seq analysis of H3K4 trimethylation profile in the prefrontal cortex of adult male mice with a history of early life stress. Two types of stress were used: prolonged separation of pups from their mothers (for 3 h once a day, maternal separation, MS) and brief separation (for 15 min once a day, handling, HD). Adult offspring in the MS group demonstrated reduced locomotor activity in the open field test accompanied by reduced exploratory activity, while the HD group showed decreased anxiety-like behavior only. In a group of maternal separation, we have found a small number (45) of slightly up-regulated peaks, corresponding to promoters of 70 genes, while no changes were observed in a group of handling. Among the genes whose promoters have differential enrichment of H3K4me3, the most relevant ones participate in gene expression regulation, modulation of chromatin structure and mRNA processing. For two genes, Ddias and Pip4k2a, increased H3K4me3 levels were associated with the increased mRNA expression in MS group.

Conclusion

The distribution of H3K4me3 in prefrontal cortex showed relatively low variability across all individuals, and only some subtle changes were revealed in mice with a history of early life stress. It is possible that the observed long-lasting behavioral alterations induced by maternal separation are mediated by other epigenetic mechanisms, or other brain structures are responsible for these effects.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4479-2) contains supplementary material, which is available to authorized users.

CSL-Associated Corepressor and Coactivator Complexes

The highly conserved Notch signal transduction pathway orchestrates fundamental cellular processes including, differentiation, proliferation, and apoptosis during embryonic development and in the adult organism. Dysregulated Notch signaling underlies the etiology of a variety of human diseases, such as certain types of cancers, developmental disorders and cardiovascular disease. Ligand binding induces proteolytic cleavage of the Notch receptor and nuclear translocation of the Notch intracellular domain (NICD), which forms a ternary complex with the transcription factor CSL and the coactivator MAML to upregulate transcription of Notch target genes. The DNA-binding protein CSL is the centrepiece of transcriptional regulation in the Notch pathway, acting as a molecular hub for interactions with either corepressors or coactivators to repress or activate, respectively, transcription. Here we review previous structure-function studies of CSL-associated coregulator complexes and discuss the molecular insights gleaned from this research. We discuss the functional consequences of both activating and repressing binding partners using the same interaction platforms on CSL. We also emphasize that although there has been a significant uptick in structural information over the past decade, it is still under debate how the molecular switch from repression to activation mediated by CSL occurs at Notch target genes and whether it will be possible to manipulate these transcription complexes therapeutically in the future.

Hairless-binding deficient Suppressor of Hairless alleles reveal Su(H) protein levels are dependent on complex formation with Hairless

Cell fate choices during metazoan development are driven by the highly conserved Notch signalling pathway. Notch receptor activation results in release of the Notch intracellular domain (NICD) that acts as transcriptional co-activator of the DNA-binding protein CSL. In the absence of signal, a repressor complex consisting of CSL bound to co-repressors silences Notch target genes. The Drosophila repressor complex contains the fly CSL orthologue Suppressor of Hairless [Su(H)] and Hairless (H). The Su(H)-H crystal structure revealed a large conformational change within Su(H) upon H binding, precluding interactions with NICD. Based on the structure, several sites in Su(H) and H were determined to specifically engage in complex formation. In particular, three mutations in Su(H) were identified that affect interactions with the repressor H but not the activator NICD. To analyse the effects these mutants have on normal fly development, we introduced these mutations into the native Su(H) locus by genome engineering. We show that the three H-binding deficient Su(H) alleles behave similarly. As these mutants lack the ability to form the repressor complex, Notch signalling activity is strongly increased in homozygotes, comparable to a complete loss of H activity. Unexpectedly, we find that the abundance of the three mutant Su(H) protein variants is altered, as is that of wild type Su(H) protein in the absence of H protein. In the presence of NICD, however, Su(H) mutant protein persists. Apparently, Su(H) protein levels depend on the interactions with H as well as with NICD. Based on these results, we propose that in vivo levels of Su(H) protein are stabilised by interactions with transcription-regulator complexes.

Notch1-promoted TRPA1 expression in erythroleukemic cells suppresses erythroid but enhances megakaryocyte differentiation

The Notch1 pathway plays important roles in modulating erythroid and megakaryocyte differentiation. To screen the Notch1-related genes that regulate differentiation fate of K562 and HEL cells, the expression of transient receptor potential ankyrin 1 (TRPA1) was induced by Notch1 receptor intracellular domain (N1IC), the activated form of Notch1 receptor. N1IC and v-ets erythroblastosis virus E26 oncogene homolog 1 (Ets-1) bound to TRPA1 promoter region to regulate transcription in K562 cells. Transactivation of TRPA1 promoter by N1IC depended on the methylation status of TRPA1 promoter. N1IC and Ets-1 suppressed the DNA methyltransferase 3B (DNMT3B) level in K562 cells. Inhibition of TRPA1 expression after Notch1 knockdown could be attenuated by nanaomycin A, an inhibitor of DNMT3B, in K562 and HEL cells. Functionally, hemin-induced erythroid differentiation could be suppressed by TRPA1, and the reduction of erythroid differentiation of both cells by N1IC and Ets-1 occurred via TRPA1. However, PMA-induced megakaryocyte differentiation could be enhanced by TRPA1, and the surface markers of megakaryocytes could be elevated by nanaomycin A. Megakaryocyte differentiation could be reduced by Notch1 or Ets-1 knockdown and relieved by TRPA1 overexpression. The results suggest that Notch1 and TRPA1 might be critical modulators that control the fate of erythroid and megakaryocyte differentiation.

Transcription factors, transcriptional coregulators, and epigenetic modulation in the control of pulmonary vascular cell phenotype: therapeutic implications for pulmonary hypertension (2015 Grover Conference series)

Pulmonary hypertension (PH) is a complex and multifactorial disease involving genetic, epigenetic, and environmental factors. Numerous stimuli and pathological conditions facilitate severe vascular remodeling in PH by activation of a complex cascade of signaling pathways involving vascular cell proliferation, differentiation, and inflammation. Multiple signaling cascades modulate the activity of certain sequence-specific DNA-binding transcription factors (TFs) and coregulators that are critical for the transcriptional regulation of gene expression that facilitates PH-associated vascular cell phenotypes, as demonstrated by several studies summarized in this review. Past studies have largely focused on the role of the genetic component in the development of PH, while the presence of epigenetic alterations such as microRNAs, DNA methylation, histone levels, and histone deacetylases in PH is now also receiving increasing attention. Epigenetic regulation of chromatin structure is also recognized to influence gene expression in development or disease states. Therefore, a complete understanding of the mechanisms involved in altered gene expression in diseased cells is vital for the design of novel therapeutic strategies. Recent technological advances in DNA sequencing will provide a comprehensive improvement in our understanding of mechanisms involved in the development of PH. This review summarizes current concepts in TF and epigenetic control of cell phenotype in pulmonary vascular disease and discusses the current issues and possibilities in employing potential epigenetic or TF-based therapies for achieving complete reversal of PH.

The CSL proteins, versatile transcription factors and context dependent corepressors of the notch signaling pathway

The Notch signaling pathway is a reiteratively used cell to cell communication pathway that triggers pleiotropic effects. The correct regulation of the pathway permits the efficient regulation of genes involved in cell fate decision throughout development. This activity relies notably on the CSL proteins, (an acronym for CBF-1/RBPJ-κ in Homo sapiens/Mus musculus respectively, Suppressor of Hairless in Drosophila melanogaster, Lag-1 in Caenorhabditis elegans) which is the unique transcription factor and DNA binding protein involved in this pathway. The CSL proteins have the capacity to recruit activation or repression complexes according to the cellular context. The aim of this review is to describe the different co-repressor proteins that interact directly with CSL proteins to form repression complexes thereby regulating the Notch signaling pathway in animal cells to give insights into the paralogous evolution of these co-repressors in higher eumetazoans and their subsequent effects at developmental processes.

Structure and Function of the Su(H)-Hairless Repressor Complex, the Major Antagonist of Notch Signaling in Drosophila melanogaster

Notch is a conserved signaling pathway that specifies cell fates in metazoans. Receptor-ligand interactions induce changes in gene expression, which is regulated by the transcription factor CBF1/Su(H)/Lag-1 (CSL). CSL interacts with coregulators to repress and activate transcription from Notch target genes. While the molecular details of the activator complex are relatively well understood, the structure-function of CSL-mediated repressor complexes is poorly defined. In Drosophila, the antagonist Hairless directly binds Su(H) (the fly CSL ortholog) to repress transcription from Notch targets. Here, we determine the X-ray structure of the Su(H)-Hairless complex bound to DNA. Hairless binding produces a large conformational change in Su(H) by interacting with residues in the hydrophobic core of Su(H), illustrating the structural plasticity of CSL molecules to interact with different binding partners. Based on the structure, we designed mutants in Hairless and Su(H) that affect binding, but do not affect formation of the activator complex. These mutants were validated in vitro by isothermal titration calorimetry and yeast two- and three-hybrid assays. Moreover, these mutants allowed us to solely characterize the repressor function of Su(H) in vivo.

The transcription factor CSL regulates gene expression in response to Notch pathway signaling. The X-ray structure of the complex between the fruit fly version of CSL, Su(H), and its antagonist, Hairless, reveals a novel binding mode and unanticipated structural plasticity.

Notch signaling is a form of cell-to-cell communication, in which extracellular receptor-ligand interactions ultimately result in changes in gene expression. The Notch pathway is highly conserved from the model organism Drosophila melanogaster to humans. When mutations occur within Notch pathway components, this often leads to human disease, such as certain types of cancers and birth defects. Transcription of Notch target genes is regulated by the transcription factor CSL (for CBF1/RBP-J in mammals, Su(H) in Drosophila, and Lag-1 in Caenorhabditis elegans). CSL functions as both a transcriptional activator and repressor by forming complexes with coactivator and corepressor proteins, respectively. Here we determine the high-resolution X-ray structure of Su(H) (the fly CSL ortholog) in complex with the corepressor Hairless, which is the major antagonist of Notch signaling in Drosophila. The structure unexpectedly reveals that Hairless binding results in a dramatic conformational change in Su(H). In parallel, we designed mutations in Su(H) and Hairless based on our structure and showed that these mutants are defective in complex formation in vitro and display functional deficiencies in in vivo assays. Taken together, our work provides significant molecular insights into how CSL functions as a transcriptional repressor in the Notch pathway.

Negative control of CSL gene transcription by stress/DNA damage response and p53

CSL is a key transcriptional repressor and mediator of Notch signaling. Despite wide interest in CSL, mechanisms responsible for its own regulation are little studied. CSL down-modulation in human dermal fibroblasts (HDFs) leads to conversion into cancer associated fibroblasts (CAF), promoting keratinocyte tumors. We show here that CSL transcript levels differ among HDF strains from different individuals, with negative correlation with genes involved in DNA damage/repair. CSL expression is negatively regulated by stress/DNA damage caused by UVA, Reactive Oxygen Species (ROS), smoke extract, and doxorubicin treatment. P53, a key effector of the DNA damage response, negatively controls CSL gene transcription, through suppression of CSL promoter activity and, indirectly, by increased p21 expression. CSL was previously shown to bind p53 suppressing its activity. The present findings indicate that p53, in turn, decreases CSL expression, which can serve to enhance p53 activity in acute DNA damage response of cells.

Analysis of the interaction between human RITA and Drosophila Suppressor of Hairless

Notch signalling mediates intercellular communication, which is effected by the transcription factor CSL, an acronym for vertebrate CBF1/RBP-J, Drosophila Suppressor of Hairless [Su(H)] and C. elegans Lag1. Nuclear import of CBF1/RBP-J depends on co-activators and co-repressors, whereas the export relies on RITA. RITA is a tubulin and CBF1/RBP-J binding protein acting as a negative regulator of Notch signalling in vertebrates. RITA protein is highly conserved in eumatazoa, but no Drosophila homologue was yet identified. In this work, the activity of human RITA in the fly was addressed. To this end, we generated transgenic flies that allow a tissue specific induction of human RITA, which was demonstrated by Western blotting and in fly tissues. Unexpectedly, overexpression of RITA during fly development had little phenotypic consequences, even when overexpressed simultaneously with either Su(H) or the Notch antagonist Hairless. We demonstrate the in vivo binding of human RITA to Su(H) and to tubulin by co-immune precipitation. Moreover, RITA and tubulin co-localized to some degree in several Drosophila tissues. Overall our data show that human RITA, albeit binding to Drosophila Su(H) and tubulin, cannot influence the Notch signalling pathway in the fly, suggesting that a nuclear export mechanism of Su(H), if existent in Drosophila, does not depend on RITA.

Loss of the Notch effector RBPJ promotes tumorigenesis

Kulic et al. show that RBPJ, a transcriptional repressor of Notch, is frequently deleted in human cancers and can function as a tumor suppressor. Loss of RBPJ acts to derepress target gene promoters, allowing Notch-independent activation by alternate transcription factors that promote tumor growth.

Aberrant Notch activity is oncogenic in several malignancies, but it is unclear how expression or function of downstream elements in the Notch pathway affects tumor growth. Transcriptional regulation by Notch is dependent on interaction with the DNA-binding transcriptional repressor, RBPJ, and consequent derepression or activation of associated gene promoters. We show here that RBPJ is frequently depleted in human tumors. Depletion of RBPJ in human cancer cell lines xenografted into immunodeficient mice resulted in activation of canonical Notch target genes, and accelerated tumor growth secondary to reduced cell death. Global analysis of activated regions of the genome, as defined by differential acetylation of histone H4 (H4ac), revealed that the cell death pathway was significantly dysregulated in RBPJ-depleted tumors. Analysis of transcription factor binding data identified several transcriptional activators that bind promoters with differential H4ac in RBPJ-depleted cells. Functional studies demonstrated that NF-κB and MYC were essential for survival of RBPJ-depleted cells. Thus, loss of RBPJ derepresses target gene promoters, allowing Notch-independent activation by alternate transcription factors that promote tumorigenesis.

Nuclear receptor corepressor complexes in cancer: mechanism, function and regulation

Nuclear receptor corepressor (NCoR) and silencing mediator for retinoid and thyroid hormone receptors (SMRT) function as corepressors for diverse transcription factors including nuclear receptors such as estrogen receptors and androgen receptors. Deregulated functions of NCoR and SMRT have been observed in many types of cancers and leukemias. NCoR and SMRT directly bind to transcription factors and nucleate the formation of stable complexes that include histone deacetylase 3, transducin b-like protein 1/TBL1-related protein 1, and G-protein pathway suppressor 2. These NCoR/SMRT-interacting proteins also show deregulated functions in cancers. In this review, we summarize the literature on the mechanism, regulation, and function of the core components of NCoR/SMRT complexes in the context of their involvement in cancers and leukemias. While the current studies support the view that the corepressors are promising targets for cancer treatment, elucidation of the mechanisms of corepressors involved in individual types of cancers is likely required for effective therapy.

Gain of function notch phenotypes associated with ectopic expression of the Su(H) C-terminal domain illustrate separability of Notch and hairless-mediated activities

The Notch signaling pathway is instrumental for cell fate decisions. Signals from the Notch receptor are transduced by CSL-type DNA-binding proteins. In Drosophila, this protein is named Suppressor of Hairless [Su(H)]. Together with the intracellular domain of the activated Notch receptor ICN, Su(H) assembles a transcriptional activator complex on Notch target genes. Hairless acts as the major antagonist of the Notch signaling pathway in Drosophila by means of the formation of a repressor complex together with Su(H) and several co-repressors. Su(H) is characterized by three domains, the N-terminal domain NTD, the beta-trefoil domain BTD and the C-terminal domain CTD. NTD and BTD bind to the DNA, whereas BTD and CTD bind to ICN. Hairless binds to the CTD, however, to sites different from ICN. In this work, we have addressed the question of competition and availability of Su(H) for ICN and Hairless binding in vivo. To this end, we overexpressed the CTD during fly development. We observed a strong activation of Notch signaling processes in various tissues, which may be explained by an interference of CTD with Hairless corepressor activity. Accordingly, a combined overexpression of CTD together with Hairless ameliorated the effects, unlike Su(H) which strongly enhances repression when overexpressed concomitantly with Hairless. Interestingly, in the combined overexpression CTD accumulated in the nucleus together with Hairless, whereas it is predominantly cytoplasmic on its own.

Splicing and beyond: the many faces of the Prp19 complex

The conserved Prp19 complex (Prp19C) - also known as NineTeen Complex (NTC) - functions in several processes of paramount importance for cellular homeostasis. NTC/Prp19C was discovered as a complex that functions in splicing and more specifically during the catalytic activation of the spliceosome. More recent work revealed that NTC/Prp19C plays a role in transcription elongation in Saccharomyces cerevisiae and in genome maintenance in higher eukaryotes. In addition, mouse PRP19 might ubiquity late proteins targeted for degradation and guide them to the proteasome. Furthermore, NTC/Prp19C has been implicated in lipid droplet biogenesis. In the future, the molecular function of NTC/Prp19C in all of these processes needs to be refined or elucidated. Most of NTC/Prp19C's functions have been shown in only one or few organisms. However, since this complex is highly conserved it is likely that it has the same functions across all species. Moreover, one NTC/Prp19C or different subcomplexes could function in the above-mentioned processes. Intriguingly, NTC/Prp19C might link these different processes to ensure an optimal coordination of cellular processes. Thus, many important questions about the functions of this interesting complex remain to be investigated. In this review we discuss the different functions of NTC/Prp19C focusing on the novel and emerging ones as well as open questions.

Notch signaling at a glance

Cell-cell interactions define a quintessential aspect of multicellular development. Metazoan morphogenesis depends on a handful of fundamental, conserved cellular interaction mechanisms, one of which is defined by the Notch signaling pathway. Signals transmitted through the Notch surface receptor have a unique developmental role: Notch signaling links the fate of one cell with that of a cellular neighbor through physical interactions between the Notch receptor and the membrane-bound ligands that are expressed in an apposing cell. The developmental outcome of Notch signals is strictly dependent on the cellular context and can influence differentiation, proliferation and apoptotic cell fates. The Notch pathway is conserved across species (Artavanis-Tsakonas et al., 1999; Bray, 2006; Kopan and Ilagan, 2009). In humans, Notch malfunction has been associated with a diverse range of diseases linked to changes in cell fate and cell proliferation including cancer (Louvi and Artavanis-Tsakonas, 2012). In this Cell Science at a Glance article and the accompanying poster we summarize the molecular biology of Notch signaling, its role in development and its relevance to disease.

Immunohistochemical analysis of medullary breast carcinoma autoantigens in different histological types of breast carcinomas

BACKGROUND

On the past decade a plethora of investigations were directed on identification of molecules involved in breast tumorogenesis, which could represent a powerful tool for monitoring, diagnostics and treatment of this disease. In current study we analyzed six previously identified medullary breast carcinoma autoantigens including LGALS3BP, RAD50, FAM50A, RBPJ, PABPC4, LRRFIP1 with cancer restricted serological profile in different histological types of breast cancer.

METHODS

Semi-quantitative immunohistochemical analysis of 20 tissue samples including medullary breast carcinoma, invasive ductal carcinoma, invasive lobular carcinoma and non-cancerous tissues obtained from patients with fibrocystic disease (each of five) was performed using specifically generated polyclonal antibodies. Differences in expression patterns were evaluated considering percent of positively stained cells, insensitivity of staining and subcellular localization in cells of all tissue samples.

RESULTS

All 6 antigens predominantly expressed in the most cells of all histological types of breast tumors and non-cancerous tissues with slight differences in intensity of staining and subcellular localization. The most significant differences in expression pattern were revealed for RAD50 and LGALS3BP in different histological types of breast cancer and for PABPC4 and FAM50A antigens in immune cells infiltrating breast tumors.

CONCLUSIONS

This pilot study made possible to select 4 antigens LGALS3BP, RAD50, PABPC4, and FAM50A as promising candidates for more comprehensive research as potential molecular markers for breast cancer diagnostics and therapy.

VIRTUAL SLIDES

The virtual slides' for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1860649350796892.

Nuclear factor of activated T-cells (NFAT)C2 inhibits Notch receptor signaling in osteoblasts

Notch receptors regulate osteoblastogenesis, and Notch activation induces cleavage and nuclear translocation of the Notch intracellular domain (NICD), which associates with Epstein-Barr virus latency C-promoter binding factor-1/suppressor of hairless/lag-1 (CSL) and induces transcription of Notch target genes, such as hairy enhancer of split-related with YRPW motif (Hey)1 and Hey2. Nuclear factors of activated T-cells (NFAT) are transcription factors that regulate osteoclastogenesis, but their function in osteoblasts is not clear. Notch inhibits NFATc1 transcription, but interactions between Notch and NFAT are understood poorly. To determine the regulation of NFAT expression by Notch, osteoblasts from Rosa(Notch) mice, where NICD is transcribed following excision of a loxP flanked STOP cassette, were used. Alternatively, wild-type C57BL/6 osteoblasts were exposed to the Notch ligand Delta-like (Dll)1 to induce Notch signaling or to bovine serum albumin as control. In Rosa(Notch) osteoblasts, Notch suppressed NFATc1 expression, increased Nfatc2 mRNA by post-transcriptional mechanisms, and had no effect on NFATc3 and NFATc4 transcripts. Induction of Nfatc2 transcripts by Notch was confirmed in C57BL/6 osteoblasts exposed to Dll1. To investigate NFATc2 function in osteoblasts, constitutively active NFATc2 was overexpressed in Rosa(Notch) osteoblasts. NFATc2 suppressed Notch transactivation and expression of Hey genes. Electrophoretic mobility shift assays revealed that NFATc2 and CSL bind to similar DNA sequences, and chromatin immunoprecipitation indicated that NFATc2 displaced CSL from the Hey2 promoter. The effects of NICD and NFATc2 in Rosa(Notch) osteoblasts were assessed, and both proteins inhibited osteoblast function. In conclusion, Notch stabilizes Nfatc2 transcripts, NFATc2 suppresses Notch signaling, and both proteins inhibit osteoblast function.

Cohesins repress Kaposi's sarcoma-associated herpesvirus immediate early gene transcription during latency

Chromatin-organizing factors such as CTCF and cohesins have been implicated in the control of complex viral regulatory programs. We investigated the role of CTCF and cohesins in the control of the switch from latency to the lytic cycle for Kaposi's sarcoma-associated herpesvirus (KSHV). We found that cohesin subunits but not CTCF are required for the repression of KSHV immediate early gene transcription. Depletion of the cohesin subunits Rad21, SMC1, and SMC3 resulted in lytic cycle gene transcription and viral DNA replication. In contrast, depletion of CTCF failed to induce lytic transcription or DNA replication. Chromatin immunoprecipitation with high-throughput sequencing (ChIP-Seq) revealed that cohesins and CTCF bound to several sites within the immediate early control region for ORF50 and to more distal 5' sites that also regulate the divergently transcribed ORF45-ORF46-ORF47 gene cluster. Rad21 depletion led to a robust increase in ORF45, ORF46, ORF47, and ORF50 transcripts, with similar kinetics to that observed with chemical induction by sodium butyrate. During latency, the chromatin between the ORF45 and ORF50 transcription start sites was enriched in histone H3K4me3, with elevated H3K9ac at the ORF45 promoter and elevated H3K27me3 at the ORF50 promoter. A paused form of RNA polymerase II (Pol II) was loosely associated with the ORF45 promoter region during latency but was converted to an active elongating form upon reactivation induced by Rad21 depletion. Butyrate treatment caused a rapid dissociation of cohesins and loss of CTCF binding at the immediate early gene locus, suggesting that cohesins may be a direct target of butyrate-mediated lytic induction. Our findings implicate cohesins as a major repressor of KSHV lytic gene activation and show that they function coordinately with CTCF to regulate the switch between latent and lytic gene activity.

Activation of the Notch1/STAT3/Twist signaling axis promotes gastric cancer progression

Gastric carcinoma is one of the most common malignancies and a lethal cancer in the world. Notch signaling and transcription factors STAT3 (signal transducer and activator of transcription 3) and Twist regulate tumor development and are critical regulators of gastric cancer progression. Herein, the relationship among Notch, STAT3 and Twist pathways in the control of gastric cancer progression was studied. We found that Twist and phosphorylated STAT3 levels were promoted by the activated Notch1 receptor in human stomach adenocarcinoma SC-M1, embryonic kidney HEK293 and erythroleukemia K562 cells. Notch1 signaling dramatically induced Twist promoter activity through a C promoter binding factor-1-independent manner and STAT3 phosphorylation. Overexpression of Notch1 receptor intracellular domain (N1IC) enhanced the interaction between nuclear STAT3 and Twist promoter in cells. Gastric cancer progression of SC-M1 cells was promoted by N1IC through STAT3 phosphorylation and Twist expression including colony formation, migration and invasion. STAT3 regulated gastric cancer progression of SC-M1 cells via Twist. N1IC also elevated the progression of other gastric cancer cells such as AGS and KATO III cells through STAT3 and Twist. The N1IC-promoted tumor growth and lung metastasis of SC-M1 cells in mice were suppressed by the STAT3 inhibitor JSI-124 and Twist knockdown. Furthermore, Notch1 and Notch ligand Jagged1 expressions were significantly associated with phosphorylated STAT3 and Twist levels in gastric cancer tissues of patients. Taken together, these results suggest that Notch1/STAT3/Twist signaling axis is involved in progression of human gastric cancer and modulation of this cascade has potential for the targeted combination therapy.

The transcriptional corepressor SMRTER influences both Notch and ecdysone signaling during Drosophila development

SMRTER (SMRT-related and ecdysone receptor interacting factor) is the Drosophila homologue of the vertebrate proteins SMRT and N-CoR, and forms with them a well-conserved family of transcriptional corepressors. Molecular characterization of SMRT-family proteins in cultured cells has implicated them in a wide range of transcriptional regulatory pathways. However, little is currently known about how this conserved class of transcriptional corepressors regulates the development of particular tissues via specific pathways. In this study, through our characterization of multiple Smrter (Smr) mutant lines, mosaic analysis of a loss-of-function Smr allele, and studies of two independent Smr RNAi fly lines, we report that SMRTER is required for the development of both ovarian follicle cells and the wing. In these two tissues, SMRTER inhibits not only the ecdysone pathway, but also the Notch pathway. We differentiate SMRTER's influence on these two signaling pathways by showing that SMRTER inhibits the Notch pathway, but not the ecdysone pathway, in a spatiotemporally restricted manner. We further confirm the likely involvement of SMRTER in the Notch pathway by demonstrating a direct interaction between SMRTER and Suppressor of Hairless [Su(H)], a DNA-binding transcription factor pivotal in the Notch pathway, and the colocalization of both proteins at many chromosomal regions in salivary glands. Based on our results, we propose that SMRTER regulates the Notch pathway through its association with Su(H), and that overcoming a SMRTER-mediated transcriptional repression barrier may represent a key mechanism used by the Notch pathway to control the precise timing of events and the formation of sharp boundaries between cells in multiple tissues during development.

Mapping the Deltex-binding surface on the notch ankyrin domain using analytical ultracentrifugation

The Notch signal transduction pathway controls cell fate determination during metazoan development. The Notch gene encodes a transmembrane receptor that is cleaved upon activation, liberating the Notch intracellular domain, which enters the nucleus and assembles transcriptional activation complexes that drive expression of Notch-responsive genes. The most conserved region of the Notch intracellular domain is an ankyrin domain (Nank), which binds directly to the cytosolic effector protein Deltex (Dx), controlling intracellular Notch activity. However, the structural and energetic basis for this interaction remains unknown. Here, we analyze the thermodynamics and hydrodynamics of the Nank:Dx heteroassociation, as well as a weaker Nank self-association, using sedimentation velocity analytical ultracentrifugation. By comparing g(s*) and c(s) distributions, and by direct fitting of sedimentation boundaries with thermodynamic association models, we were able to characterize the Nank:Dx heterodimer, measure its affinity, and map the interaction on the surface on Nank. N- and C-terminal deletions of whole ankyrin units implicate repeats 3 and 4 as key for mediating heteroassociation. An alanine scan across the interaction loops of Nank identifies a conserved hot spot in repeats 3 and 4, centered at R127, as critical for Dx binding. In addition, we were able to detect weak but reproducible Nank homodimerization (K(d) in the millimolar range). This association is disrupted by substitution of a conserved arginine (R107) with alanine, a residue previously implicated in a functionally relevant mode of interaction within dimeric transcription complexes. The distinct binding surfaces on Nank for homotypic versus Dx interaction appear to be compatible with teterameric Notch(2):Dx(2) assembly.

Differentiation of the ductal epithelium and smooth muscle in the prostate gland are regulated by the Notch/PTEN-dependent mechanism

We have shown previously that during branching morphogenesis of the mouse prostate gland, Bone morphogenetic protein 7 functions to restrict Notch1-positive progenitor cells to the tips of the prostate buds. Here, we employed prostate-specific murine bi-genic systems to investigate the effects of gain and loss of Notch function during prostate development. We show that Nkx3.1(Cre) and Probasin(Cre) alleles drive expression of Cre recombinase to the prostate epithelium and periepithelial stroma. We investigated the effects of gain of Notch function using the Rosa(NI1C) conditional allele, which carries a constitutively active intracellular domain of Notch1 receptor. We carried out the analysis of loss of Notch function in Nkx3.1(Cre/+);RBP-J(flox/flox) prostates, where RBP-J is a ubiquitous transcriptional mediator of Notch signaling. We found that gain of Notch function resulted in inhibition of the tumor suppressor PTEN, and increase in cell proliferation and progenitor cells in the basal epithelium and smooth muscle compartments. In turn, loss of Notch/RBP-J function resulted in decreased cell proliferation and loss of epithelial and smooth muscle progenitors. Gain of Notch function resulted in an early onset of benign prostate hyperplasia by three months of age. Loss of Notch function also resulted in abnormal differentiation of the prostate epithelium and stroma. In particular, loss of Notch signaling and increase in PTEN promoted a switch from myoblast to fibroblast lineage, and a loss of smooth muscle. In summary, we show that Notch signaling is necessary for terminal differentiation of the prostate epithelium and smooth muscle, and that during normal prostate development Notch/PTEN pathway functions to maintain patterned progenitors in the epithelial and smooth muscle compartments. In addition, we found that both positive and negative modulation of Notch signaling results in abnormal organization of the prostate tissue, and can contribute to prostate disease in the adult organ.

Real-time imaging of notch activation with a luciferase complementation-based reporter

Notch signaling regulates many cellular processes during development and adult tissue renewal. Upon ligand binding, Notch receptors undergo ectodomain shedding followed by γ-secretase-mediated release of the Notch intracellular domain (NICD), which translocates to the nucleus and associates with the DNA binding protein CSL [CBF1/RBPjκ/Su(H)/Lag1] to activate gene expression. Mammalian cells contain four Notch receptors that can have both redundant and specific activities. To monitor activation of specific Notch paralogs in live cells and in real time, we developed luciferase complementation imaging (LCI) reporters for NICD-CSL association and validated them as a specific, robust, and sensitive assay system that enables structure-function and pharmacodynamic analyses. Detailed kinetic analyses of various mechanistic aspects of Notch signaling, including nuclear translocation and inhibition of the activities of γ-secretase and ADAM metalloproteases, as well as agonist- and ligand-dependent activation, were conducted in live cells. These experiments showed that Notch-LCI is an effective approach for characterizing modulators that target Notch signaling and for studying pathway dynamics in normal and disease contexts.

Regulation of Notch1 signaling by the APP intracellular domain facilitates degradation of the Notch1 intracellular domain and RBP-Jk

The Notch1 receptor is a crucial controller of cell fate decisions, and is also a key regulator of cell growth and differentiation in a variety of contexts. In this study, we have demonstrated that the APP intracellular domain (AICD) attenuates Notch1 signaling by accelerated degradation of the Notch1 intracellular domain (Notch1-IC) and RBP-Jk, through different degradation pathways. AICD suppresses Notch1 transcriptional activity by the dissociation of the Notch1-IC-RBP-Jk complex after processing by γ-secretase. Notch1-IC is capable of forming a trimeric complex with Fbw7 and AICD, and AICD enhances the protein degradation of Notch1-IC through an Fbw7-dependent proteasomal pathway. AICD downregulates the levels of RBP-Jk protein through the lysosomal pathway. AICD-mediated degradation is involved in the preferential degradation of non-phosphorylated RBP-Jk. Collectively, our results demonstrate that AICD functions as a negative regulator in Notch1 signaling through the promotion of Notch1-IC and RBP-Jk protein degradation.

EBV nuclear antigen EBNALP dismisses transcription repressors NCoR and RBPJ from enhancers and EBNA2 increases NCoR-deficient RBPJ DNA binding

EBV nuclear antigen 2 (EBNA2) and EBV nuclear antigen LP (EBNALP) are critical for B-lymphocyte transformation to lymphoblastoid cell lines (LCLs). EBNA2 activates transcription through recombination signal-binding immunoglobulin κJ region (RBPJ), a transcription factor associated with NCoR repressive complexes, and EBNALP is implicated in repressor relocalization. EBNALP coactivation with EBNA2 was found to dominate over NCoR repression. EBNALP associated with NCoR and dismissed NCoR, NCoR and RBPJ, or NCoR, RBPJ, and EBNA2 from matrix-associated deacetylase (MAD) bodies. In non-EBV-infected BJAB B lymphoma cells that stably express EBNA2, EBNALP, or EBNA2 and EBNALP, EBNALP was associated with hairy and enhancer of split 1 (hes1), cd21, cd23, and arginine and glutamate-rich 1 (arglu1) enhancer or promoter DNA and was associated minimally with coding DNA. With the exception of RBPJ at the arglu1 enhancer, NCoR and RBPJ were significantly decreased at enhancer and promoter sites in EBNALP or EBNA2 and EBNALP BJAB cells. EBNA2 DNA association was unaffected by EBNALP, and EBNALP was unaffected by EBNA2. EBNA2 markedly increased RBPJ at enhancer sites without increasing NCoR. EBNALP further increased hes1 and arglu1 RNA levels with EBNA2 but did not further increase cd21 or cd23 RNA levels. EBNALP in which the 45 C-terminal residues critical for transformation and transcriptional activation were deleted associated with NCoR but was deficient in dismissing NCoR from MAD bodies and from enhancer and promoter sites. These data strongly support a model in which EBNA2 association with NCoR-deficient RBPJ enhances transcription and EBNALP dismisses NCoR and RBPJ repressive complexes from enhancers to coactivate hes1 and arglu1 but not cd21 or cd23.

Novel RBPJ transcripts identified in human amniotic fluid cells

The NOTCH signaling pathway plays important roles in stem cell maintenance, cell-fate determination and differentiation during development. Following ligand binding, the cleaved NOTCH intracellular domain (NICD) interacts directly with the recombinant signal binding protein for immunoglobulin kappa J region (RBPJ) transcription factor and the resulting complex targets gene expression in the nucleus. To date, four human RBPJ isoforms have been described in Entrez Gene, varying in the first 5'coding exons. Using an improved protocol, we were able to further identify all four known and five novel RBPJ transcript variants in human amniotic fluid (AF) cells, a cell type known for its stem cell characteristics. In addition, we used human embryonal carcinoma (EC) NTera2/D1 (NT2) cells and NT2-derived neuron and astrocytes to compare the expression pattern of RBPJ transcripts. Further examination of RBPJ transcripts showed that the novel splice variants contain open reading frames in-frame with the known isoforms, suggesting that they can putatively generate similar function proteins. All known and novel RBPJ transcripts contain the putative nuclear localization signal (NLS), an important component of RBPJ-mediated gene regulation.

Notch signalling in T-cell lymphoblastic leukaemia/lymphoma and other haematological malignancies

Notch receptors participate in a highly conserved signalling pathway that regulates normal development and tissue homeostasis in a context- and dose-dependent manner. Deregulated Notch signalling has been implicated in many diseases, but the clearest example of a pathogenic role is found in T-cell lymphoblastic leukaemia/lymphoma (T-LL), in which the majority of human and murine tumours have acquired mutations that lead to aberrant increases in Notch1 signalling. Remarkably, it appears that the selective pressure for Notch mutations is virtually unique among cancers to T-LL, presumably reflecting a special context-dependent role for Notch in normal T-cell progenitors. Nevertheless, there are some recent reports suggesting that Notch signalling has subtle, yet important roles in other forms of haematological malignancy as well. Here, we review the role of Notch signalling in various blood cancers, focusing on T-LL with an eye towards targeted therapeutics.

Notch-independent functions of CSL

Notch-dependent CSL transcription complexes control essential biological processes such as cell proliferation, differentiation, and cell-fate decisions in diverse developmental systems. The orthologous proteins CBF1/Rbpj (mammalian), Su(H) (Drosophila), and Lag-1 (Caenorhabditis elegans) compose the CSL family of sequence-specific DNA-binding transcription factors. The CSL proteins are best known for their role in canonical Notch signaling. However, CSL factors also form transcription complexes that can function independent of Notch signaling and include repression and activation of target gene transcription. Because the different complexes share CSL as a DNA-binding subunit, they can control overlapping sets of genes; but they can also control distinct sets when partnered with tissue-specific cofactors that restrict DNA-sequence recognition or stability of the DNA-bound complex. The Notch-independent functions of CSL and the processes they regulate will be reviewed here with a particular emphasis on the tissue-specific CSL-activator complex with the bHLH factor Ptf1a.

The Notch signaling pathway: molecular basis of cell context dependency

Notch receptor signaling controls cell-fate specification, self-renewal, differentiation, proliferation and apoptosis throughout development and regeneration in all animal species studied to date. Its dysfunction causes several developmental defects and diseases in the adult. A key feature of Notch signaling is its remarkable cell-context dependency. In this review, we summarize the influences of the cellular context that regulate Notch activity and propose a model how the interplay between the cell-intrinsically established chromatin state and the cell-extrinsic signals that modify chromatin may select for Notch target accessibility and activation in different cellular contexts.

RITA, a novel modulator of Notch signalling, acts via nuclear export of RBP-J

The evolutionarily conserved Notch signal transduction pathway regulates fundamental cellular processes during embryonic development and in the adult. Ligand binding induces presenilin-dependent cleavage of the receptor and a subsequent nuclear translocation of the Notch intracellular domain (NICD). In the nucleus, NICD binds to the recombination signal sequence-binding protein J (RBP-J)/CBF-1 transcription factor to induce expression of Notch target genes. Here, we report the identification and functional characterization of RBP-J interacting and tubulin associated (RITA) (C12ORF52) as a novel RBP-J/CBF-1-interacting protein. RITA is a highly conserved 36 kDa protein that, most interestingly, binds to tubulin in the cytoplasm and shuttles rapidly between cytoplasm and nucleus. This shuttling RITA exports RBP-J/CBF-1 from the nucleus. Functionally, we show that RITA can reverse a Notch-induced loss of primary neurogenesis in Xenopus laevis. Furthermore, RITA is able to downregulate Notch-mediated transcription. Thus, we propose that RITA acts as a negative modulator of the Notch signalling pathway, controlling the level of nuclear RBP-J/CBF-1, where its amounts are limiting.

Notch targets and their regulation

The proteolytic cleavages elicited by activation of the Notch receptor release an intracellular fragment, Notch intracellular domain, which enters the nucleus to activate the transcription of targets. Changes in transcription are therefore a major output of this pathway. However, the Notch outputs clearly differ from cell type to cell type. In this review we discuss current understanding of Notch targets, the mechanisms involved in their transcriptional regulation, and what might underlie the activation of different sets of targets in different cell types.

Two opposing roles of RBP-J in Notch signaling

RBP-J/Su(H)/Lag1, the main transcriptional mediator of Notch signaling, binds DNA with the consensus sequence YRTGDGAD. Notch target genes can be controlled by two opposing activities of RBP-J. The interaction of the Notch intracellular domain with RBP-J induces a weak transcriptional activation and requires an additional tissue-specific transcriptional activator such as bHLH proteins or GATA to mediate strong target gene expression. For example, during Drosophila sensory organ precursor (SOP) cell development, proneural bHLH interacts with Da, a Drosophila orthologue of E2A, to form a tissue-specific activator of Su(H), the Drosophila orthologue of RBP-J. This complex and Su(H) act synergistically to promote the epidermal cell fate. In contrast, a complex of Su(H) with Hairless, a Drosophila functional homologue of MINT, has transcriptional repression activity that promotes SOP differentiation to neurons. Recent conditional loss-of-function studies demonstrated that transcriptional networks involving RBP-J, MINT, and E2A are conserved in mammalian cell differentiation, including multiple steps of lymphocyte development, and probably also in neuronal maturation in adult neurogenesis. During neurogenesis, Notch-RBP-J signaling was thought historically to be involved mainly in the maintenance of undifferentiated neural progenitors. However, the identification of a tissue-specific transcriptional activator of RBP-J-Notch has revealed new roles of RBP-J in the promotion of neuronal maturation. Finally, the Notch-independent function of RBP-J was recently discovered and will be reviewed here.

Silencing mediator of retinoic acid and thyroid hormone receptor regulates enhanced activation of signal transducer and activator of transcription 3 by epstein-barr virus-derived epstein-barr nuclear antigen 2

The Epstein-Barr virus (EBV)-encoded latency protein Epstein-Barr nuclear antigen 2 (EBNA2) is a nuclear transcriptional activator that is essential for EBV-induced cellular transformation. In a previous study, we demonstrated that EBNA2 interacts with signal transducer and activator of transcription 3 (STAT3), a signal transducer for an interleukin (IL)-6 family cytokine, and enhances its transcriptional activity. Here, we show that overexpression of a corepressor, silencing mediator of retinoic acid and thyroid hormone receptor (SMRT), decreases the EBNA2-mediated enhanced STAT3 activation. Furthermore, small-interfering RNA-mediated reduction of endogenous SMRT expression augments the EBNA2-mediated enhanced STAT3 activation. Importantly, EBNA2 reduces interactions between STAT3 and SMRT. These data demonstrate that EBNA2 acts as a transcriptional coactivator of STAT3 by influencing the SMRT corepressor complex.

RBP-J promotes neuronal differentiation and inhibits oligodendroglial development in adult neurogenesis

Neurogenesis persists in restricted regions of the adult vertebrate brain. However, the molecular mechanisms supporting adult neurogenesis are not fully understood. Here we demonstrated that C cell-specific deletion of RBP-J in the adult subventricular zones (SVZs) caused reduction in numbers of mature granule cells in the olfactory bulbs (OBs) with concomitant increase in Olig2(+) oligodendroglial progenitors, although generation of immature neurons was enhanced in the SVZs. Adenovirus-mediated Cre introduction to the SVZs of RBP-J-floxed mice indicated that Olig2(+) cells in the OBs can be generated from RBP-J-deficient SVZs, although no oligodendroglial cells in the OBs are derived from the normal SVZs. This preferential differentiation to oligodendroglial progenitor cells and reduction in differentiation of mature neurons were also confirmed by in vitro culture of RBP-J-deficient SVZ-derived neural progenitor cells, in addition to defects in the maintenance of adult neural stem cell population. The defects in maturation of RBP-J-deficient neurons could be partly rescued by knockdown of Olig2 in vivo. Our findings suggest that RBP-J might regulate neuronal maturation at least in part through transcriptional repression of Olig2.

More complicated than it looks: assembly of Notch pathway transcription complexes

The Notch pathway is a short-range signaling mechanism between neighboring cells that results in changes in gene expression. Extracellular interactions between Notch receptors and ligands trigger proteolytic cleavage of the receptor Notch. Following cleavage, the freed intracellular domain of Notch (NotchIC) moves from the cytoplasm to the nucleus, engaging the DNA-binding transcription factor CBF-1, Su(H), Lag-1 (CSL)--the nuclear effector of the pathway. NotchIC, together with the transcriptional coactivator Mastermind, form a ternary complex with CSL that activates transcription from genes that are responsive to Notch signaling. Illuminating the molecular details that underlie formation of the transcriptionally active CSL-NotchIC-Mastermind ternary complex is key for understanding how genes are turned on in response to a Notch signal. Recently, several studies using biophysical and computational methods have scrutinized how the CSL-NotchIC-Mastermind ternary complex forms and the role individual domains play in this process. These detailed analyses have provided a wealth of molecular insights into the assembly of a Notch pathway active transcription complex but have also raised several intriguing, yet confounding questions. This review will focus on the findings of these recent biophysical studies and provide speculative models that address these unanswered questions.

Notch activation stimulates transient and selective binding of Su(H)/CSL to target enhancers

The CSL [CBF1/Su(H)/Lag2] proteins [Su(H) in Drosophila] are implicated in repression and activation of Notch target loci. Prevailing models imply a static association of these DNA-binding transcription factors with their target enhancers. Our analysis of Su(H) binding and chromatin-associated features at 11 E(spl) Notch target genes before and after Notch revealed large differences in Su(H) occupancy at target loci that correlated with the presence of polymerase II and other marks of transcriptional activity. Unexpectedly, Su(H) occupancy was significantly and transiently increased following Notch activation, suggesting a more dynamic interaction with targets than hitherto proposed.

The transcriptional repression activity of KyoT2 on the Notch/RBP-J pathway is regulated by PIAS1-catalyzed SUMOylation

The LIM domain protein KyoT2 negatively regulates the Notch signaling pathway through interaction with RBP-J, the core element of the Notch signaling pathway in the nucleus. Here we show that PIAS1 (the protein inhibitor of activated STAT1) interacts with KyoT2 directly and attenuates KyoT2-mediated transcriptional repression. We demonstrate that KyoT2 is modified by SUMOylation at two lysine residues, K144 and K171. SUMOylation of the transfected KyoT2 is enhanced by PIAS1 but not hPc2, another KyoT2-interacting protein with SUMO E3 ligase activity, and is repressed by a PIAS1 mutant that is deficient of E3 ligase activity. Using mutants disrupting either or both of the SUMO sites, we show that SUMOylation of KyoT2 does not influence its expression, intracellular localization, or interaction with known partners. However, disruption of the K171 SUMOylation site does reinforce the transcriptional repression activity of KyoT2, suggesting that SUMOylation of this site counters the repression activity of KyoT2. Finally, we show that PIAS1 fails to attenuate the repression activity of the K171R mutant of KyoT2, suggesting that PIAS1 may potentially antagonize the transcriptional repression activity of KyoT2 through catalyzing its SUMOylation at K171. These results suggest that KyoT2 is a substrate of SUMO modification catalyzed by PIAS1, and that SUMOylation may modulate the transcriptional repression effect of KyoT2 on the Notch/RBP-J signaling pathway.

Ex vivo expansion of umbilical cord blood stem cells for transplantation: growing knowledge from the hematopoietic niche

Umbilical cord blood transplantation (UCBT) in adults is limited by the small number of primitive hematopoietic stem cells (HSC) in each graft, resulting in delayed engraftment post transplant, and both short- and long-term infectious complications. Initial efforts to expand UCB progenitors ex vivo have resulted in expansion of mature rather than immature HSC, confounded by the inability to accurately and reliably measure long-term reconstituting cells. Ex vivo expansion of UCB HSC has failed to improve engraftment because of resulting defects that promote apoptosis, disrupt marrow homing and initiate cell cycling. Here we discuss the future of ex vivo expansion, which we suggest will include the isolation of immature hematopoietic progenitors on the basis of function rather than surface phenotype and will employ both cytokines and stroma to maintain and expand the stem cell niche. We suggest that ex vivo expansion could be enhanced by manipulating newly discovered signaling pathways (Notch, Wnt, bone morphogenetic protein 4 and Tie2/angiopoietin-1) and intracellular mediators (phosphatase and tensin homolog and glycogen synthase kinase-3) in a manner that promotes HSC expansion with less differentiation. Improved methods for ex vivo expansion will make UCBT available to more patients, decrease engraftment times and allow more rapid immune reconstitution post transplant.