Mastermind recruits CycC:CDK8 to phosphorylate the Notch ICD and coordinate activation with turnover

Notch regulation of early thymocyte development

Notch signaling plays multiple roles in T cell development. Following thymic entry, Notch signals are required to specify the T cell fate from a multipotent hematopoietic progenitor. At subsequent steps in early T cell development, Notch provides important differentiation, survival, proliferation and metabolic signals. This review focuses on the multiple functions of Notch in early T cell development, from T cell specification in the thymus through beta selection.

The ubiquitous nature of cancer: the role of the SCF(Fbw7) complex in development and transformation

The ubiquitin-proteasome system (UPS) is a multi-subunit pathway that allows for ubiquitin modification of proteins and leads to either degradation or other non-proteolytic processes such as trafficking or transcriptional activation. Given its role as a regulator of cellular homeostasis it is not surprising that members of the UPS are frequently aberrantly expressed in a number of disease states including cancer. This review will focus on one member of the UPS, the F-box protein, Fbw7 (also known as Sel-10, Ago, hCDC4) and mechanisms by which Fbw7 interacts with its substrates in the context of development and tumorigenesis will be discussed. In addition, antagonists of this pathway as well as current and future therapeutics for the UPS will be examined.

Regulation of skeletal myogenesis by Notch

Notch signaling has emerged as a key player in skeletal muscle development and regeneration. Simply stated, Notch signaling inhibits differentiation. Accordingly, fine-tuning the pathway is essential for proper muscle homeostasis. This review will address various aspects of Notch signaling, including our current views of the core pathway, its effects in muscle, its interactions with other signaling pathways, and its relationship with ageing.

Control of cyclin C levels during development of Dictyostelium

Background

Cdk8 and its partner cyclin C form part of the mediator complex which links the basal transcription machinery to regulatory proteins. The pair are required for correct regulation of a subset of genes and have been implicated in control of development in a number of organisms including the social amoeba Dictyostelium discoideum. When feeding, Dictyostelium amoebae are unicellular but upon starvation they aggregate to form a multicellular structure which develops into a fruiting body containing spores. Cells in which the gene encoding Cdk8 has been deleted fail to enter aggregates due to a failure of early gene expression.

Principal Findings

We have monitored the expression levels of cyclin C protein during development and find levels decrease after the multicellular mound is formed. This decrease is triggered by extracellular cAMP that, in turn, is working in part through an increase in intracellular cAMP. The loss of cyclin C is coincident with a reduction in the association of Cdk8 with a high molecular weight complex in the nucleus. Overexpression of cyclin C and Cdk8 lead to an increased rate of early development, consistent with the levels being rate limiting.

Conclusions

Overall these results show that both cyclin C and Cdk8 are regulated during development in response to extracellular signals and the levels of these proteins are important in controlling the timing of developmental processes. These findings have important implications for the role of these proteins in controlling development, suggesting that they are targets for developmental signals to regulate gene expression.

CDK8 expression in 470 colorectal cancers in relation to beta-catenin activation, other molecular alterations and patient survival

Alterations in the Wnt/beta-catenin pathway define a key event in the pathogenesis of colon cancer. We have recently shown that CDK8, the gene encoding a cyclin-dependent kinase (CDK) component of the Mediator complex, acts as a colon cancer oncogene that is necessary for beta-catenin activity. Here, we tested the hypothesis that colorectal cancers with CDK8 expression have distinct clinical, prognostic and molecular attributes. Among 470 colorectal cancers identified in 2 prospective cohort studies, CDK8 expression was detected in 329 (70%) tumors by immunohistochemistry. Cox proportional hazards model and backward stepwise elimination were used to compute hazard ratio (HR) of deaths according to CDK8 status, initially adjusted for various patient and molecular features, including beta-catenin, p53, p21, p27 (CDK inhibitors), cyclin D1, fatty acid synthase (FASN), cyclooxygenase-2 (COX-2), microsatellite instability (MSI), CpG island methylator phenotype (CIMP), LINE-1 methylation, and mutations in KRAS, BRAF and PIK3CA. CDK8 expression in colorectal cancer was independently associated with beta-catenin activation (p = 0.0002), female gender (p < 0.0001) and FASN overexpression (p = 0.0003). Among colon cancer patients, CDK8 expression significantly increased colon cancer-specific mortality in both univariate analysis [HR 1.70; 95% confidence interval (CI), 1.03-2.83; p = 0.039] and multivariate analysis (adjusted HR 2.05; 95% CI, 1.18-3.56; p = 0.011) that was adjusted for potential confounders including beta-catenin, COX-2, FASN, LINE-1 hypomethylation, CIMP and MSI. CDK8 expression was unrelated with clinical outcome among rectal cancer patients. These data support a potential link between CDK8 and beta-catenin, and suggest that CDK8 may identify a subset of colon cancer patients with a poor prognosis.

The human Mediator complex: a versatile, genome-wide regulator of transcription

The Mediator complex interacts extensively with the RNA polymerase II enzyme and regulates its ability to express protein-coding genes. The mechanisms by which Mediator regulates gene expression remain poorly understood, in part because the structure of Mediator and even its composition can change, depending upon the promoter context. Combined with the sheer size of the human Mediator complex (26 subunits, 1.2 MDa), this structural adaptability bestows seemingly unlimited regulatory potential within the complex. Recent efforts to understand Mediator structure and function have identified expanded roles that include control of both pre- and post-initiation events; it is also evident that Mediator performs both general and gene-specific roles to regulate gene expression.

Cyclin-dependent kinase 8 positively cooperates with Mediator to promote thyroid hormone receptor-dependent transcriptional activation

Mediator is a multisubunit assemblage of proteins originally identified in humans as a coactivator bound to thyroid hormone receptors (TRs) and essential for thyroid hormone (T3)-dependent transcription. Cyclin-dependent kinase 8 (CDK8), cyclin C, MED12, and MED13 form a variably associated Mediator subcomplex (termed the CDK8 module) whose functional role in TR-dependent transcription remains unclear. Using in vitro and cellular approaches, we show here that Mediator complexes containing the CDK8 module are specifically recruited into preinitiation complexes at the TR target gene type I deiodinase (DioI) together with RNA polymerase II (Pol II) in a TR- and T3-dependent manner. We found that CDK8 is essential for robust T3-dependent Dio1 transcription and that CDK8 knockdown via RNA interference decreased Pol II occupancy, and also the recruitment of the Pol II kinase CDK9, at the DioI promoter. Chromatin immunoprecipitation revealed CDK8 occupancy at the DioI promoter concurrent with active transcription, thus suggesting CDK8 involvement in transcriptional reinitiation. Mutagenesis assays showed that CDK8 kinase activity is necessary for full T3-dependent DioI activation, whereas in vitro kinase studies indicated that CDK8 may contribute to Pol II phosphorylation. Collectively, our data suggest CDK8 plays an important coactivator role in TR-dependent transcription by promoting Pol II recruitment and activation at TR target gene promoters.

Targeting Notch signaling in pancreatic cancer patients--rationale for new therapy

Pancreatic cancer is one of the most aggressive and devastating human malignancies. Despite new knowledge in the molecular profile of pancreatic cancer and its precursor lesions, survival rates have changed very little over the last 40 years. Therefore, a better understanding of the detailed mechanisms underlying the pathogenesis of this disease is critical if we expect to develop new and effective strategies for prevention, early diagnosis and treatment of pancreatic cancer. The review herein focuses on a distinctive signaling pathway, the Notch pathway, which has recently been associated with carcinogenesis, including pancreatic cancer. It is aimed at summarizing key results which support a role for this pathway in the initiation, progression and maintenance of pancreatic cancer as a rationale for targeting and inhibiting this pathway in pancreatic cancer patients.

Therapeutic targeting of NOTCH1 signaling in T-cell acute lymphoblastic leukemia

The recent identification of activating mutations in NOTCH1 in the majority of T-cell acute lymphoblastic leukemias (T-ALLs) has brought major interest toward targeting the NOTCH signaling pathway in this disease. Small-molecule gamma-secretase inhibitors (GSIs), which block a critical proteolytic step required for NOTCH1 activation, can effectively block the activity of NOTCH1 mutant alleles. However, the clinical development of GSIs has been hampered by their low cytotoxicity against human T-ALL and the development of significant gastrointestinal toxicity derived from the inhibition of NOTCH signaling in the gut. Improved understanding of the oncogenic mechanisms of NOTCH1 and the effects of NOTCH inhibition in leukemic cells and the intestinal epithelium are required for the design of effective anti-NOTCH1 therapies in T-ALL.

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.

Mechanistic insights into Notch receptor signaling from structural and biochemical studies

Notch proteins are the receptors in a highly conserved signal transduction system used to communicate signals between cells that contact each other. Studies investigating structure-function relationships in Notch signaling have gained substantial momentum in recent years. Here, we summarize the current understanding of the molecular logic of Notch signal transduction, emphasizing structural and biochemical studies of Notch receptors, their ligands, and complexes of intracellular Notch proteins with their target transcription factors. Recent advances in the structure-based modulation of Notch-signaling activity are also discussed.

Thermodynamic analysis of the CSL x Notch interaction: distribution of binding energy of the Notch RAM region to the CSL beta-trefoil domain and the mode of competition with the viral transactivator EBNA2

The Notch signaling pathway is a cell-cell communication network giving rise to cell differentiation during metazoan development. Activation of the pathway releases the intracellular portion of the Notch receptor to translocate to the nucleus, where it is able to interact with the effector transcription factor CSL, converting CSL from a transcriptional repressor to an activator. This conversion is dependent upon the high affinity binding of the RAM region of the Notch receptor to the beta-trefoil domain (BTD) of CSL. Here we probe the energetics of binding to BTD of each conserved residue of RAM through the use of isothermal titration calorimetry and single residue substitution. We find that although the highly conserved PhiW PhiP motif is the largest determinant of binding, energetically significant interactions are contributed by N-terminal residues, including a conserved Arg/Lys-rich region. Additionally, we present a thermodynamic analysis of the interaction between the Epstein-Barr virus protein EBNA2 with BTD and explore the extent to which the EBNA2- and RAM-binding sites on BTD are nonoverlapping, as proposed by Fuchs et al. (Fuchs, K. P., Bommer, G., Dumont, E., Christoph, B., Vidal, M., Kremmer, E., and Kempkes, B. (2001) Eur. J. Biochem. 268, 4639-4646). Combining these results with displacement isothermal titration calorimetry, we propose a mechanism by which the PhiW PhiP motif of RAM and EBNA2 compete with one another for binding at the hydrophobic pocket of BTD using overlapping but specific interactions that are unique to each BTD ligand.

Nuclear CDKs drive Smad transcriptional activation and turnover in BMP and TGF-beta pathways

TGF-beta and BMP receptor kinases activate Smad transcription factors by C-terminal phosphorylation. We have identified a subsequent agonist-induced phosphorylation that plays a central dual role in Smad transcriptional activation and turnover. As receptor-activated Smads form transcriptional complexes, they are phosphorylated at an interdomain linker region by CDK8 and CDK9, which are components of transcriptional mediator and elongation complexes. These phosphorylations promote Smad transcriptional action, which in the case of Smad1 is mediated by the recruitment of YAP to the phosphorylated linker sites. An effector of the highly conserved Hippo organ size control pathway, YAP supports Smad1-dependent transcription and is required for BMP suppression of neural differentiation of mouse embryonic stem cells. The phosphorylated linker is ultimately recognized by specific ubiquitin ligases, leading to proteasome-mediated turnover of activated Smad proteins. Thus, nuclear CDK8/9 drive a cycle of Smad utilization and disposal that is an integral part of canonical BMP and TGF-beta pathways.

Notch and the skeleton

Notch receptors are transmembrane receptors that regulate cell fate decisions. There are four Notch receptors in mammals. Upon binding to members of the Delta and Jagged family of transmembrane proteins, Notch is cleaved and the Notch intracellular domain (NICD) is released. NICD then translocates to the nucleus, where it associates with the CBF-1, Suppressor of Hairless, and Lag-2 (CSL) and Mastermind-Like (MAML) proteins. This complex activates the transcription of Notch target genes, such as Hairy Enhancer of Split (Hes) and Hes-related with YRPF motif (Hey). Notch signaling is critical for the regulation of mesenchymal stem cell differentiation. Misexpression of Notch in skeletal tissue indicates a role as an inhibitor of skeletal development and postnatal bone formation. Overexpression of Notch inhibits endochondral bone formation and osteoblastic differentiation, causing severe osteopenia. Conditional inactivation of Notch in the skeleton causes an increase in cancellous bone volume and enhanced osteoblastic differentiation. Notch ligands are expressed in the hematopoietic stem cell niche and are critical for the regulation of hematopoietic stem cell self-renewal. Dysregulation of Notch signaling is the underlying cause of diseases affecting the skeletal tissue, including Alagille syndrome, spondylocostal dysostosis, and possibly, osteosarcoma.

GSK3beta is a negative regulator of the transcriptional coactivator MAML1

Glycogen synthase kinase 3β (GSK3β) is involved in several cellular signaling systems through regulation of the activity of diverse transcription factors such as Notch, p53 and β-catenin. Mastermind-like 1 (MAML1) was originally identified as a Notch coactivator, but has also been reported to function as a transcriptional coregulator of p53, β-catenin and MEF2C. In this report, we show that active GSK3β directly interacts with the MAML1 N-terminus and decreases MAML1 transcriptional activity, suggesting that GSK3β might target a coactivator in its regulation of gene expression. We have previously shown that MAML1 increases global acetylation of histones, and here we show that the GSK3 inhibitor SB41, further enhances MAML1-dependent histone acetylation in cells. Finally, MAML1 translocates GSK3β to nuclear bodies; this function requires full-length MAML1 protein.

Notch signaling: the core pathway and its posttranslational regulation

Notch signaling controls numerous cell-fate specification events in multicellular organisms, and dysregulated Notch signaling causes several diseases with underlying developmental defects. A key step in Notch receptor activation is its intramembrane proteolysis, which releases an intracellular fragment that participates directly in transcriptional regulation of nuclear target genes. Despite the apparent simplicity of this mechanism, a host of posttranslational processes regulate Notch activity during its synthesis and secretion, ligand-dependent activation at the surface, endocytic trafficking, and degradation. This review describes the core developmental logic of Notch signaling and how regulatory mechanisms tailor Notch pathway outputs to specific developmental scenarios.

Cell- and stage-specific chromatin structure across the Complement receptor 2 (CR2/CD21) promoter coincide with CBF1 and C/EBP-beta binding in B cells

Stringent developmental transcription requires multiple transcription factor (TF) binding sites, cell-specific expression of signaling molecules, TFs and co-regulators and appropriate chromatin structure. During B-lymphopoiesis, human Complement receptor 2 (CR2/CD21) is detected on immature and mature B cells but not on B cell precursors and plasma cells. We examined cell- and stage-specific human CR2 gene regulation using cell lines modeling B-lymphopoiesis. Chromatin accessibility assays revealed a region between -409 and -262 with enhanced accessibility in mature B cells and pre-B cells, compared to either non-lymphoid or plasma cell-types, however, accessibility near the transcription start site (TSS) was elevated only in CR2-expressing B cells. A correlation between histone acetylation and CR2 expression was observed, while histone H3K4 dimethylation was enriched near the TSS in both CR2-expressing B cells and non-expressing pre-B cells. Candidate sites within the CR2 promoter were identified which could regulate chromatin, including a matrix attachment region associated with CDP, SATB1/BRIGHT and CEBP-beta sites as well as two CBF1 sites. ChIP assays verified that both CBF1 and C/EBP-beta bind the CR2 promoter in B cells raising the possibility that these factors facilitate or respond to alterations in chromatin structure to control the timing and/or level of CR2 transcription.

Hedgehog-stimulated stem cells depend on non-canonical activity of the Notch co-activator Mastermind

Normal self-renewal of follicle stem cells (FSCs) in the Drosophila ovary requires Hedgehog (Hh) signaling. Excess Hh signaling, induced by loss of patched (ptc), causes cell-autonomous duplication of FSCs. We have used a genetic screen to identify Mastermind (Mam), the Notch pathway transcriptional co-activator, as a rare dose-dependent modifier of aberrant FSC expansion induced by excess Hh. Complete loss of Mam activity severely compromises the persistence of both normal and ptc mutant FSCs, but does not affect the maintenance of ovarian germline stem cells. Thus, Mam, like Hh, is a crucial stem cell factor that acts selectively on FSCs in the ovary. Surprisingly, other Notch pathway components, including Notch itself, are not similarly required for FSC maintenance. Furthermore, excess Notch pathway activity alone accelerates FSC loss and cannot ameliorate the more severe defects of mam mutant FSCs. This suggests an unconventional role for Mam in FSCs that is independent of Notch signaling. Loss of Mam reduces the expression of a Hh pathway reporter in FSCs but not in wing discs, suggesting that Mam might enhance Hh signaling specifically in stem cells of the Drosophila ovary.

Attenuation of Notch signalling by the Down-syndrome-associated kinase DYRK1A

Notch signalling is used throughout the animal kingdom to spatially and temporally regulate cell fate, proliferation and differentiation. Its importance is reflected in the dramatic effects produced on both development and health by small variations in the strength of the Notch signal. The Down-syndrome-associated kinase DYRK1A is coexpressed with Notch in various tissues during embryonic development. Here we show that DYRK1A moves to the nuclear transcription compartment where it interacts with the intracellular domain of Notch promoting its phosphorylation in the ankyrin domain and reducing its capacity to sustain transcription. DYRK1A attenuates Notch signalling in neural cells both in culture and in vivo, constituting a novel mechanism capable of modulating different developmental processes that can also contribute to the alterations observed during brain development in animal models of Down syndrome.

The canonical Notch signaling pathway: unfolding the activation mechanism

Notch signaling regulates many aspects of metazoan development and tissue renewal. Accordingly, the misregulation or loss of Notch signaling underlies a wide range of human disorders, from developmental syndromes to adult-onset diseases and cancer. Notch signaling is remarkably robust in most tissues even though each Notch molecule is irreversibly activated by proteolysis and signals only once without amplification by secondary messenger cascades. In this Review, we highlight recent studies in Notch signaling that reveal new molecular details about the regulation of ligand-mediated receptor activation, receptor proteolysis, and target selection.

The transcriptional coactivator MAML1 regulates p300 autoacetylation and HAT activity

MAML1 is a transcriptional coregulator originally identified as a Notch coactivator. MAML1 is also reported to interact with other coregulator proteins, such as CDK8 and p300, to modulate the activity of Notch. We, and others, previously showed that MAML1 recruits p300 to Notch-regulated genes through direct interactions with the DNA–CSL–Notch complex and p300. MAML1 interacts with the C/H3 domain of p300, and the p300–MAML1 complex specifically acetylates lysines of histone H3 and H4 tails in chromatin in vitro. In this report, we show that MAML1 potentiates p300 autoacetylation and p300 transcriptional activation. MAML1 directly enhances p300 HAT activity, and this coincides with the translocation of MAML1, p300 and acetylated histones to nuclear bodies.

The human CDK8 subcomplex is a molecular switch that controls Mediator coactivator function

The human CDK8 subcomplex (CDK8, cyclin C, Med12, and Med13) negatively regulates transcription in ways not completely defined; past studies suggested CDK8 kinase activity was required for its repressive function. Using a reconstituted transcription system together with recombinant or endogenous CDK8 subcomplexes, we demonstrate that, in fact, Med12 and Med13 are critical for subcomplex-dependent repression, whereas CDK8 kinase activity is not. A hallmark of activated transcription is efficient reinitiation from promoter-bound scaffold complexes that recruit a series of pol II enzymes to the gene. Notably, the CDK8 submodule strongly represses even reinitiation events, suggesting a means to fine tune transcript levels. Structural and biochemical studies confirm the CDK8 submodule binds the Mediator leg/tail domain via the Med13 subunit, and this submodule-Mediator association precludes pol II recruitment. Collectively, these results reveal the CDK8 subcomplex functions as a simple switch that controls the Mediator-pol II interaction to help regulate transcription initiation and reinitiation events. As Mediator is generally required for expression of protein-coding genes, this may reflect a common mechanism by which activated transcription is shut down in human cells.

FBXW7 and NOTCH1 mutations in childhood T cell acute lymphoblastic leukaemia and T cell non-Hodgkin lymphoma

Mutation analysis of FBXW7 and NOTCH1 genes was performed in 55 T cell acute lymphoblastic leukaemia (T-ALL) and 14 T cell non-Hodgkin lymphoma (T-NHL) patients who were treated on the Japan Association of Childhood Leukaemia Study (JACLS) protocols ALL-97 and NHL-98. FBXW7 and/or NOTCH1 mutations were found in 22 (40.0%) of 55 T-ALL and 7 (50.0%) of 14 T-NHL patients. FBXW7 mutations were found in 8 (14.6%) of 55 T-ALL and 3 (21.4%) of 14 T-NHL patients, and NOTCH1 mutations in 17 (30.9%) of 55 T-ALL and 6 (42.9%) of 14 T-NHL patients. Three (5.4%) T-ALL and two (1.4%) T-NHL patients had mutations in both FBXW7 and NOTCH1. FBXW7 mutations included one insertion, one deletion, one deletion/insertion and nine missense mutations. NOTCH1 mutations were detected in the heterodimerization domain (HD) in 15 cases, in the PEST domain in seven cases, and in both the HD and PEST domains in one case. Five-year event-free survival and overall survival for patients with FBXW7 and/or NOTCH1 mutations were 95.5% (95% CI, 71.9-99.4%) and 100% respectively, suggesting that T-ALL patients with FBXW7 and/or NOTCH1 mutation represent a good prognosis compared to those without FBXW7 and/or NOTCH1 mutations (63.6%, P = 0.007 and 78.8%, P = 0.023, respectively).

The prolyl-isomerase Pin1 is a Notch1 target that enhances Notch1 activation in cancer

Signalling through Notch receptors requires ligand-induced cleavage to release the intracellular domain, which acts as a transcriptional activator in the nucleus. Deregulated Notch1 signalling has been implicated in mammary tumorigenesis; however the mechanisms underlying Notch activation in breast cancer remain unclear. Here, we demonstrate that the prolyl-isomerase Pin1 interacts with Notch1 and affects Notch1 activation. Pin1 potentiates Notch1 cleavage by gamma-secretase, leading to an increased release of the active intracellular domain and ultimately enhancing Notch1 transcriptional and tumorigenic activity. We found that Notch1 directly induces transcription of Pin1, thereby generating a positive loop. In human breast cancers, we observed a strong correlation between Pin1 overexpression and high levels of activated Notch1. Thus, the molecular circuitry established by Notch1 and Pin1 may have a key role in cancer.

The HLH protein Extramacrochaetae is required for R7 cell and cone cell fates in the Drosophila eye

Notch signaling is one of the most important pathways in development and homeostasis, and is altered in multiple pathologies. Study of Drosophila eye development shows that Notch signaling depends on the HLH protein Extramacrochaetae. Null mutant clones show that extramacrochaetae is required for multiple aspects of eye development that depend on Notch signaling, including morphogenetic furrow progression, differentiation of R4, R7 and cone cell types, and rotation of ommatidial clusters. Detailed analysis of R7 and cone cell specification reveals that extramacrochaetae acts cell autonomously and epistatically to Notch, and is required for normal expression of bHLH genes encoded by the E(spl)-C which are effectors of most Notch signaling. A model is proposed in which Extramacrochaetae acts in parallel to or as a feed-forward regulator of the E(spl)-Complex to promote Notch signaling in particular cellular contexts.

Notch and Vascular Smooth Muscle Cell Phenotype

The Notch signaling pathway is critical for cell fate determination during embryonic development, including many aspects of vascular development. An emerging paradigm suggests that the Notch gene regulatory network is often recapitulated in the context of phenotypic modulation of vascular smooth muscle cells (VSMC), vascular remodeling, and repair in adult vascular disease following injury. Notch ligand receptor interactions lead to cleavage of receptor, translocation of the intracellular receptor (Notch IC), activation of transcriptional CBF-1/RBP-Jκ–dependent and –independent pathways, and transduction of downstream Notch target gene expression. Hereditary mutations of Notch components are associated with congenital defects of the cardiovascular system in humans such as Alagille syndrome and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Recent loss- or gain-of-function studies have provided insight into novel Notch-mediated CBF-1/RBP-Jκ–dependent and –independent signaling and cross-regulation to other molecules that may play a critical role in VSMC phenotypic switching. Notch receptors are critical for controlling VSMC differentiation and dictating the phenotypic response following vascular injury through interaction with a triad of transcription factors that act synergistically to regulate VSMC differentiation. This review focuses on the role of Notch receptor ligand interactions in dictating VSMC behavior and phenotype and presents recent findings on the molecular interactions between the Notch components and VSMC-specific genes to further understand the function of Notch signaling in vascular tissue and disease.

The human CDK8 subcomplex is a histone kinase that requires Med12 for activity and can function independently of mediator

The four proteins CDK8, cyclin C, Med12, and Med13 can associate with Mediator and are presumed to form a stable "CDK8 subcomplex" in cells. We describe here the isolation and enzymatic activity of the 600-kDa CDK8 subcomplex purified directly from human cells and also via recombinant expression in insect cells. Biochemical analysis of the recombinant CDK8 subcomplex identifies predicted (TFIIH and RNA polymerase II C-terminal domain [Pol II CTD]) and novel (histone H3, Med13, and CDK8 itself) substrates for the CDK8 kinase. Notably, these novel substrates appear to be metazoan-specific. Such diverse targets imply strict regulation of CDK8 kinase activity. Along these lines, we observe that Mediator itself enables CDK8 kinase activity on chromatin, and we identify Med12--but not Med13--to be essential for activating the CDK8 kinase. Moreover, mass spectrometry analysis of the endogenous CDK8 subcomplex reveals several associated factors, including GCN1L1 and the TRiC chaperonin, that may help control its biological function. In support of this, electron microscopy analysis suggests TRiC sequesters the CDK8 subcomplex and kinase assays reveal the endogenous CDK8 subcomplex--unlike the recombinant submodule--is unable to phosphorylate the Pol II CTD.

Antagonistic function of Lmd and Zfh1 fine tunes cell fate decisions in the Twi and Tin positive mesoderm of Drosophila melanogaster

In this study we show that cell fate decisions in the dorsal and lateral mesoderm of Drosophila melanogaster depend on the antagonistic action of the Gli-like transcription factor Lame duck (Lmd) and the zinc finger homeodomain factor Zfh1. Lmd expression leads to the reduction of Zfh1 positive cell types, thereby restricting the number of Odd-skipped (Odd) positive and Tinman (Tin) positive pericardial cells in the dorsal mesoderm. In more lateral regions, ectopic activation of Zfh1 or loss of Lmd leads to an excess of adult muscle precursor (AMP) like cells. We also observed that Lmd is co-expressed with Tin in the early dorsal mesoderm and leads to a reduction of Tin expression in cells destined to become dorsal fusion competent myoblasts (FCMs). In the absence of Lmd function, these cells remain Tin positive and develop as Tin positive pericardial cells although they do not express Zfh1. We show further that Tin repression and pericardial restriction in the dorsal mesoderm facilitated by Lmd is instructed by a late Decapentaplegic (Dpp) signal that is abolished in embryos carrying the disk region mutation dpp(d6).

Varicella-zoster virus IE62 protein utilizes the human mediator complex in promoter activation

The varicella-zoster virus (VZV) major transactivator, IE62, is involved in the expression of all kinetic classes of VZV genes and can also activate cellular promoters, promoters from heterologous viruses, and artificial promoters containing only TATA elements. A key component of the mechanism of IE62 transactivation is an acidic activation domain comprising the N-terminal 86 amino acids of IE62. However, the cellular target of this N-terminal acidic activation is unknown. In the work presented here, we show that the IE62 activation domain targets the human Mediator complex via the Med25 (ARC92) subunit and that this interaction appears to be fundamental for transactivation by the IE62 activation domain. In contrast, the Med23 subunit (Sur2/TRAP150beta/DRIP130/CRSP130) of the Mediator complex is not essential for IE62-mediated activation. Further, the IE62 activation domain appears to selectively interact with a form of the Mediator complex lacking CDK8. Chromatin immunoprecipitation experiments showed that IE62 stimulates recruitment of Mediator to an IE62-responsive model promoter. Finally, immunofluorescence microscopy of VZV-infected cells demonstrated intranuclear translocation of the Mediator complex to viral replication compartments. These studies suggest that Mediator is an essential component for efficient VZV gene expression.

Leukemia-associated NOTCH1 alleles are weak tumor initiators but accelerate K-ras-initiated leukemia

Gain-of-function NOTCH1 mutations are found in 50%-70% of human T cell acute lymphoblastic leukemia/lymphoma (T-ALL) cases. Gain-of-function NOTCH1 alleles that initiate strong downstream signals induce leukemia in mice, but it is unknown whether the gain-of-function NOTCH1 mutations most commonly found in individuals with T-ALL generate downstream signals of sufficient strength to induce leukemia. We addressed this question by expressing human gain-of-function NOTCH1 alleles of varying strength in mouse hematopoietic precursors. Uncommon gain-of-function NOTCH1 alleles that initiated strong downstream signals drove ectopic T cell development and induced leukemia efficiently. In contrast, although gain-of-function alleles that initiated only weak downstream signals also induced ectopic T cell development, these more common alleles failed to efficiently initiate leukemia development. However, weak gain-of-function NOTCH1 alleles accelerated the onset of leukemia initiated by constitutively active K-ras and gave rise to tumors that were sensitive to Notch signaling pathway inhibition. These data show that induction of leukemia requires doses of Notch1 greater than those needed for T cell development and that most NOTCH1 mutations found in T-ALL cells do not generate signals of sufficient strength to initiate leukemia development. Furthermore, low, nonleukemogenic levels of Notch1 can complement other leukemogenic events, such as activation of K-ras. Even when Notch1 participates secondarily, the resulting tumors show "addiction" to Notch, providing a further rationale for evaluating Notch signaling pathway inhibitors in leukemia.

Oncogenic NOTCH1 control of MYC and PI3K: challenges and opportunities for anti-NOTCH1 therapy in T-cell acute lymphoblastic leukemias and lymphomas

The identification of activating mutations in NOTCH1 in the majority of T-cell acute lymphoblastic leukemias and lymphomas (T-ALL) has brought much interest in inhibiting NOTCH1 signaling as therapeutic target in this disease. Small-molecule inhibitors of the gamma-secretase complex, which mediates a critical proteolytic cleavage required for NOTCH1 activation, hold the promise of becoming an effective molecular therapy against relapsed and refractory T-ALL. Recent progress in the elucidation of the transcriptional regulatory networks downstream of oncogenic NOTCH1 has uncovered a central role of NOTCH1 signaling in promoting leukemic cell growth and revealed an intricate circuitry that connects NOTCH1 signaling with MYC and the PI3K-AKT signaling pathway. The identification of the downstream effector pathways controlled by NOTCH1 should pave the way for the rational design of anti-NOTCH1 therapies for the treatment of T-ALL.

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.

Mastermind-like transcriptional co-activators: emerging roles in regulating cross talk among multiple signaling pathways

A family of Mastermind-like (MAML) genes encodes critical transcriptional co-activators for Notch signaling, an evolutionarily conserved pathway with numerous roles in both development and human diseases. Notch receptors are cleaved upon ligand engagement and the intracellular domain of Notch shuttles to the nucleus. MAMLs form a functional DNA-binding complex with the cleaved Notch receptor and the transcription factor CSL, thereby regulating transcriptional events that are specific to the Notch pathway. Here, we review recent studies that have utilized molecular, cellular and physiological model system strategies to reveal the pivotal roles of the MAML proteins in Notch signaling. Unexpectedly, however, emerging evidence implicate MAML proteins as exciting key transcriptional co-activators in other signal transduction pathways including: muscle differentiation and myopathies (MEF2C), tumor suppressor pathway (p53) and colon carcinoma survival (beta-catenin). Thus, the MAML family appears to function in transcriptional co-activation in a multitude of cellular processes. It is hypothesized that MAML proteins mediate cross-talk among the various signaling pathways and the diverse activities of the MAML proteins converge to impact normal biological processes and human diseases, including cancers.

CDK8 is a colorectal cancer oncogene that regulates beta-catenin activity

Aberrant activation of the canonical WNT/beta-catenin pathway occurs in almost all colorectal cancers and contributes to their growth, invasion and survival. Although dysregulated beta-catenin activity drives colon tumorigenesis, further genetic perturbations are required to elaborate full malignant transformation. To identify genes that both modulate beta-catenin activity and are essential for colon cancer cell proliferation, we conducted two loss-of-function screens in human colon cancer cells and compared genes identified in these screens with an analysis of copy number alterations in colon cancer specimens. One of these genes, CDK8, which encodes a member of the mediator complex, is located at 13q12.13, a region of recurrent copy number gain in a substantial fraction of colon cancers. Here we show that the suppression of CDK8 expression inhibits proliferation in colon cancer cells characterized by high levels of CDK8 and beta-catenin hyperactivity. CDK8 kinase activity was necessary for beta-catenin-driven transformation and for expression of several beta-catenin transcriptional targets. Together these observations suggest that therapeutic interventions targeting CDK8 may confer a clinical benefit in beta-catenin-driven malignancies.

Mediator links epigenetic silencing of neuronal gene expression with x-linked mental retardation

Mediator occupies a central role in RNA polymerase II transcription as a sensor, integrator, and processor of regulatory signals that converge on protein-coding gene promoters. Compared to its role in gene activation, little is known regarding the molecular mechanisms and biological implications of Mediator as a transducer of repressive signals. Here we describe a protein interaction network required for extraneuronal gene silencing comprising Mediator, G9a histone methyltransferase, and the RE1 silencing transcription factor (REST; also known as neuron restrictive silencer factor, NRSF). We show that the MED12 interface in Mediator links REST with G9a-dependent histone H3K9 dimethylation to suppress neuronal genes in nonneuronal cells. Notably, missense mutations in MED12 causing the X-linked mental retardation (XLMR) disorders FG syndrome and Lujan syndrome disrupt its REST corepressor function. These findings implicate Mediator in epigenetic restriction of neuronal gene expression to the nervous system and suggest a pathologic basis for MED12-associated XLMR involving impaired REST-dependent neuronal gene regulation.

Message in a nucleus: signaling to the transcriptional machinery

Tissue differentiation and signal transduction involve dramatic changes in gene expression. These changes can be brought about by the expression or activation of sequence-specific transcription factors. In order to regulate their target genes, such factors must navigate the intricate chromatin environment and engage the complex basal transcriptional machinery. We discuss three mechanisms through which signaling pathways can interact with complexes that alter chromatin structure or recruit RNA polymerase II. Signals that promote differentiation may alter the properties of such transcriptional regulatory complexes by incorporating tissue-specific subunits. Alternatively, adaptor subunits specialized to interact with specific transcription factors may allow a single complex to respond to multiple signals. Finally, individual regulatory proteins may integrate a variety of signals, allowing crosstalk between pathways.

Modeling polarity buildup and cell fate decision in the fly eye: insight into the connection between the PCP and Notch pathways

Metazoan development critically depends on a surprisingly short list of conserved pathways. How can such ubiquitous systems regulate a variety of cell-biological events at various developmental stages in different tissues and in different organisms? In the fruit fly, the planar cell polarity (PCP) pathway regulates widely different processes. It is known to be involved in the correct alignment of hairs on the wing and in the determination of R3/R4 photoreceptor cell fates in the eye. In the wing, PCP regulates the spatial structure of cells sharing the same transcriptional fate, while in the eye the Notch signaling pathway has been recruited to additionally transduce the PCP signal to the nuclei in the two differentiating members of a photoreceptor pair. We have recently proposed a computational model for PCP in the wing; this model posited, on the basis of all known data, that planar polarity buildup is driven by asymmetric molecular complexes constructed around the cadherin Flamingo and spanning the space between two cells. In this paper, we show that the same model, combined with a novel Notch module, equally applies in the eye. The model provides insight into the crosstalk between the PCP and Notch modules in development and illustrates the ability of signaling modules to robustly maintain vital phenotypes in a noisy environment.

Notch and presenilin regulate cellular expansion and cytokine secretion but cannot instruct Th1/Th2 fate acquisition

Recent reports suggested that Delta1, 4 and Jagged1, 2 possessed the ability to instruct CD4⁺ T cell into selection of Th1 or Th2 fates, respectively, although the underlying mechanism endowing the cleaved Notch receptor with memory of ligand involved in its activation remains elusive. To examine this, we prepared artificial antigen-presenting cells expressing either DLL1 or Jag1. Although both ligands were efficient in inducing Notch2 cleavage and activation in CD4⁺ T or reporter cells, the presence of Lunatic Fringe in CD4⁺ T cells inhibited Jag1 activation of Notch1 receptor. Neither ligand could induce Th1 or Th2 fate choice independently of cytokines or redirect cytokine-driven Th1 or Th2 development. Instead, we find that Notch ligands only augment cytokine production during T cell differentiation in the presence of polarizing IL-12 and IL-4. Moreover, the differentiation choices of naïve CD4⁺ T cells lacking γ-secretase, RBP-J, or both in response to polarizing cytokines revealed that neither presenilin proteins nor RBP-J were required for cytokine-induced Th1/Th2 fate selection. However, presenilins facilitate cellular proliferation and cytokine secretion in an RBP-J (and thus, Notch) independent manner. The controversies surrounding the role of Notch and presenilins in Th1/Th2 polarization may reflect their role as genetic modifiers of T-helper cells differentiation.

Notch-dependent cell cycle arrest and apoptosis in mouse embryonic fibroblasts lacking Fbxw7

The F-box protein Fbxw7 mediates the ubiquitylation and consequent degradation of proteins that regulate cell cycle progression, including cyclin E, c-Myc, c-Jun and Notch. Moreover, certain human cancer cell lines harbor loss-of-function mutations in FBXW7 that result in excessive accumulation of Fbxw7 substrates, implicating Fbxw7 in tumor suppression. To elucidate the physiological function of Fbxw7, we conditionally ablated Fbxw7 in mouse embryonic fibroblasts (MEFs). Unexpectedly, loss of Fbxw7 induced cell cycle arrest and apoptosis that were accompanied by abnormal accumulation of the intracellular domain of Notch1 (NICD1). Forced expression of NICD1 in wild-type MEFs recapitulated the phenotype of the Fbxw7-deficient (Fbxw7(Delta/Delta)) MEFs. Conversely, deletion of Rbpj normalized the phenotype of Fbxw7(Delta/Delta) MEFs, indicating that this phenotype is dependent on the Notch1-RBP-J signaling pathway. Deletion of the p53 gene prevented cell cycle arrest but not the induction of apoptosis in Fbxw7(Delta/Delta) cells. These observations suggest that Fbxw7 does not function as an oncosuppressor in MEFs. Instead, it promotes cell cycle progression and cell survival through degradation of Notch1, with loss of Fbxw7 resulting in NICD1 accumulation, cell cycle arrest and apoptosis.

Human mediator kinase subunit CDK11 plays a negative role in viral activator VP16-dependent transcriptional regulation

Mediator is an essential transcriptional cofactor of RNA polymerase II (Pol II) in eukaryotes. This cofactor is a large complex containing up to 30 subunits and consisting of four modules: head, middle, tail, and CDK/Cyclin. Generally, Mediator connects transcriptional regulators, cofactors, chromatin regulators, and chromatin remodellers, with the pre-initiation complex to provide a platform for the assembly of these factors. Many previous studies have revealed that CDK8, a subunit of the CDK/Cyclin module, is one of the key subunits mediating the pivotal roles of Mediator in transcriptional regulation. In addition to CDK8, CDK11 is conserved among vertebrates as a Mediator subunit and closely resembles CDK8. While the role of CDK8 has been studied extensively, little is known of the role of CDK11 in Mediator. We purified human CDK11 (hCDK11)-containing protein complexes from an epitope-tagged hCDK11-expressing HeLa cell line and found that hCDK11 could independently form Mediator complexes devoid of human CDK8 (hCDK8). To investigate the in vivo transcriptional activity of the complex, we employed a luciferase assay. Although hCDK11 has nearly 80% amino acid sequence identity to hCDK8, siRNA-knockdown study revealed that hCDK8 and hCDK11 possess opposing functions in viral activator VP16-dependent transcriptional regulation.

Notch Signaling Regulates Platelet-Derived Growth Factor Receptor-β Expression in Vascular Smooth Muscle Cells

Notch signaling is critically important for proper architecture of the vascular system, and mutations in NOTCH3 are associated with CADASIL, a stroke and dementia syndrome with vascular smooth muscle cell (VSMC) dysfunction. In this report, we link Notch signaling to platelet-derived growth factor (PDGF) signaling, a key determinant of VSMC biology, and show that PDGF receptor ( PDGFR )-β is a novel immediate Notch target gene. PDGFR -β expression was upregulated by Notch ligand induction or by activated forms of the Notch receptor. Moreover, upregulation of PDGFR -β expression in response to Notch activation critically required the Notch signal integrator CSL. In primary VSMCs, PDGFR -β expression was robustly upregulated by Notch signaling, leading to an augmented intracellular response to PDGF stimulation. In newborn Notch3 -deficient mice, PDGFR-β expression was strongly reduced in the VSMCs that later develop an aberrant morphology. In keeping with this, PDGFR-β upregulation in response to Notch activation was reduced also in Notch3 -deficient embryonic stem cells. Finally, in VSMCs from a CADASIL patient carrying a NOTCH3 missense mutation, upregulation of PDGFR -β mRNA and protein in response to ligand-induced Notch activation was significantly reduced. In sum, these data reveal a hierarchy for 2 important signaling systems, Notch and PDGF, in the vasculature and provide insights into how dysregulated Notch signaling perturbs VSMC differentiation and function.

Zfp64 participates in Notch signaling and regulates differentiation in mesenchymal cells

Notch signaling is required for multiple aspects of tissue and cell differentiation. In this study, we identified zinc finger protein 64 (Zfp64) as a novel coactivator of Notch1. Zfp64 is associated with the intracellular domain of Notch1, recruited to the promoters of the Notch target genes Hes1 and Hey1, and transactivates them. Zfp64 expression is under the control of Runx2, and is upregulated by direct transactivation of its promoter. Zfp64 suppresses the myogenic differentiation of C2C12 cells and promotes their osteoblastic differentiation. Our data demonstrate two functions of Zfp64: (1) it is a downstream target of Runx2 and, (2) its cognate protein acts as a coactivator of Notch1, which suggests that Zfp64 mediates mesenchymal cell differentiation by modulating Notch signaling.

Notch signaling in leukemia

Recent discoveries indicate that gain-of-function mutations in the Notch1 receptor are very common in human T cell acute lymphoblastic leukemia/lymphoma. This review discusses what these mutations have taught us about normal and pathophysiologic Notch1 signaling, and how these insights may lead to new targeted therapies for patients with this aggressive form of cancer.

Notch-1 mutations are secondary events in some patients with T-cell acute lymphoblastic leukemia

PURPOSE

Activating Notch-1 mutations are frequent in T-cell acute lymphoblastic leukemia (T-ALL), occurring in >50% of patients. In murine models of T-ALL, Notch-1 activation can both directly initiate leukemia and cooperate secondarily to other primary events. Whether acquisition of Notch-1 mutations is an early initiating event or a secondary event in the pathogenesis of human T-ALL is unclear.

EXPERIMENTAL DESIGN

We used denaturing high-performance liquid chromatography, sequencing, and fragment analysis to analyze Notch-1 mutational status and mutant level in 62 patients at presentation as well as 16 matched presentation-relapse samples.

RESULTS

We detected Notch-1 mutations in 47 patients (76%). Seven of these were low-level mutations (quantified at < or =10%), despite high blast counts, suggesting that they were acquired as a secondary event in a subclone. Of 16 matched presentation-relapse samples studied, 7 were wild-type at both presentation and relapse. Five of nine mutant-positive patients at presentation relapsed with the same mutation(s) at the same high level. Four patients had evidence of a change in mutant at relapse. One lost a PEST mutation and became wild-type. Two others lost mutations at relapse but acquired different mutations, despite unchanged T-cell receptor rearrangements, suggesting that the latter event predated the acquisition of the Notch-1 mutation. One relapsed with a secondary T-cell leukemia and different Notch mutation.

CONCLUSIONS

These results suggest that Notch-1 mutations can sometimes be acquired as secondary events in leukemogenesis and must be used cautiously as solitary minimal residual disease markers.

Pygopus activates Wingless target gene transcription through the mediator complex subunits Med12 and Med13

Wnt target gene transcription is mediated by nuclear translocation of stabilized beta-catenin, which binds to TCF and recruits Pygopus, a cofactor with an unknown mechanism of action. The mediator complex is essential for the transcription of RNA polymerase II-dependent genes; it associates with an accessory subcomplex consisting of the Med12, Med13, Cdk8, and Cyclin C subunits. We show here that the Med12 and Med13 subunits of the Drosophila mediator complex, encoded by kohtalo and skuld, are essential for the transcription of Wingless target genes. kohtalo and skuld act downstream of beta-catenin stabilization both in vivo and in cell culture. They are required for transcriptional activation by the N-terminal domain of Pygopus, and their physical interaction with Pygopus depends on this domain. We propose that Pygopus promotes Wnt target gene transcription by recruiting the mediator complex through interactions with Med12 and Med13.

Ikaros directly represses the notch target gene Hes1 in a leukemia T cell line: implications for CD4 regulation

Ikaros and Notch1, two regulators of gene transcription, are critically important at many stages of T cell development. Deregulation of Ikaros and Notch activities cooperate to promote T cell leukemogenesis, providing evidence that they function in converging pathways in developing T cells. In this report, a mechanism for Ikaros:Notch cooperativity is described, revealing a non-redundant role for Ikaros in regulating expression of the Notch target gene Hes1 in a leukemia T cell line. We provide evidence that Ikaros directly represses Hes1 in concert with the transcriptional repressor, RBP-Jkappa, allowing for cross-talk between Notch and Ikaros that impacts regulation of CD4 expression. Taken together, these data describe a potential mechanism for Ikaros' function during T cell development and define Ikaros as an obligate repressor of Hes1.