Notch signaling: the core pathway and its posttranslational regulation

NUMB-ing down cancer by more than just a NOTCH

The protein Numb does not live up to its name. This passive-sounding protein is anything but spent. Originally identified as a cell-fate determinant in Drosophila development, Numb received a good deal of attention as an inhibitor of the Notch receptor signaling pathway. It turns out, however, that Numb does a lot more than simply regulate Notch. It has been implicated in a variety of biochemical pathways connected with signaling (it regulates Notch-, Hedgehog- and TP53-activated pathways), endocytosis (it is involved in cargo internalization and recycling), determination of polarity (it interacts with the PAR complex, and regulates adherens and tight junctions), and ubiquitination (it exploits this mechanism to regulate protein function and stability). This complex biochemical network lies at the heart of Numb's involvement in diverse cellular phenotypes, including cell fate developmental decisions, maintenance of stem cell compartments, regulation of cell polarity and adhesion, and migration. Considering its multifaceted role in cellular homeostasis, it is not surprising that Numb has been implicated in cancer as a tumor suppressor. Our major goal here is to explain the cancer-related role of Numb based on our understanding of its role in cell physiology. We will attempt to do this by reviewing the present knowledge of Numb at the biochemical and functional level, and by integrating its apparently heterogeneous functions into a unifying scenario, based on our recently proposed concept of the "endocytic matrix". Finally, we will discuss the role of Numb in the maintenance of the normal stem cell compartment, as a starting point to interpret the tumor suppressor function of Numb in the context of the cancer stem cell hypothesis.

Notch signaling in the vasculature

Notch signaling is an evolutionarily conserved, intercellular signaling mechanism that plays myriad roles during vascular development and physiology in vertebrates. These roles include the regulation of arteriovenous specification and differentiation in both endothelial cells and vascular smooth muscle cells, regulation of blood vessel sprouting and branching during normal and pathological angiogenesis, and the physiological responses of vascular smooth muscle cells. Defects in Notch signaling also cause inherited vascular diseases, such as the degenerative vascular disorder cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy. This review summarizes recent studies that highlight the multiple roles the Notch signaling pathway plays during vascular development and physiology.

Prox1 regulates the notch1-mediated inhibition of neurogenesis

During development of the spinal cord, Prox1 controls the balance between proliferation and differentiation of neural progenitor cells via suppression of Notch1 gene expression.

Activation of Notch1 signaling in neural progenitor cells (NPCs) induces self-renewal and inhibits neurogenesis. Upon neuronal differentiation, NPCs overcome this inhibition, express proneural genes to induce Notch ligands, and activate Notch1 in neighboring NPCs. The molecular mechanism that coordinates Notch1 inactivation with initiation of neurogenesis remains elusive. Here, we provide evidence that Prox1, a transcription repressor and downstream target of proneural genes, counteracts Notch1 signaling via direct suppression of Notch1 gene expression. By expression studies in the developing spinal cord of chick and mouse embryo, we showed that Prox1 is limited to neuronal precursors residing between the Notch1+ NPCs and post-mitotic neurons. Physiological levels of Prox1 in this tissue are sufficient to allow binding at Notch1 promoter and they are critical for proper Notch1 transcriptional regulation in vivo. Gain-of-function studies in the chick neural tube and mouse NPCs suggest that Prox1-mediated suppression of Notch1 relieves its inhibition on neurogenesis and allows NPCs to exit the cell cycle and differentiate. Moreover, loss-of-function in the chick neural tube shows that Prox1 is necessary for suppression of Notch1 outside the ventricular zone, inhibition of active Notch signaling, down-regulation of NPC markers, and completion of neuronal differentiation program. Together these data suggest that Prox1 inhibits Notch1 gene expression to control the balance between NPC self-renewal and neuronal differentiation.

Early during development, neural progenitor cells (NPCs) can either proliferate or differentiate into neurons. Thus, generation of the correct number of neurons is governed by a tightly regulated balance between proliferation and differentiation, and disruption of this balance can result in severe developmental deficits, malformations, or cancers. Notch1 is a member of the Notch family of receptors, which make up a highly conserved cell signaling system. Notch1 signaling has been shown to inhibit NPC differentiation and to promote self-renewal, thereby allowing NPCs to divide and progressively generate the enormous number of neurons present in the central nervous system. The molecular mechanism by which NPCs overcome Notch1-mediated inhibition in order to differentiate into neurons, however, is not completely understood. In this study, we show that Prox1, a homeobox transcriptional repressor, plays a fundamental role in the switch to differentiation by suppressing the expression of Notch1 receptor, thereby preventing newly produced neuronal precursors from receiving inhibitory signals from Notch ligands present in neighboring cells. This transcriptional repression may regulate cell cycle exit and differentiation of NPCs as they migrate towards different regions and adopt their final cell fates. We suggest that Prox1 may exert its known influence on embryonic development, organ morphogenesis, and cancer through its ability to counteract Notch1 signaling.

Cyclic stretch stimulates vascular smooth muscle cell alignment by redox-dependent activation of Notch3

Mice deficient in Notch3 have defects in arterial vascular smooth muscle cell (VSMC) mechanosensitivity, including impaired myogenic responses and autoregulation, and inappropriate VMSC orientation. Experiments were performed to determine if Notch3 is activated by mechanical stimulation and contributes to mechanosensitive responses of VSMCs, including cell realignment. Cyclic, uniaxial stretch (10%, 1 Hz) of human VSMCs caused Notch3 activation, demonstrated by a stretch-induced increase in hairy and enhancer of split 1/hairy-related transcription factor-1 expression, translocation of Notch3 to the nucleus, and a decrease in the Notch3 extracellular domain. These effects were prevented by inhibiting the expression [small interfering (si)RNA] or proteolytic activation of Notch3 {N-(R)-[2-(hydroxyaminocarbonyl)methyl]-4-methylpentanoyl-l-naphthylalanyl-l-alanine-2-aminoethyl amide (TAPI-1; 50 μmol/l) to inhibit TNF-α-converting enzyme (TACE) or N-[N-(3,5-difluorophenacetyl-l-alanyl)]-S-phenylglycine t-butyl ester (DAPT; 20 μmol/l) to inhibit γ-secretase}. Stretch increased the activity of ROS within VSMCs, determined using dichlorodihydrofluorescein fluorescence. Catalase (1,200 U/ml), which degrades H₂O₂, inhibited the stretch-induced activation of Notch3, whereas in nonstretched cells, increasing H₂O₂ activity [H₂O₂ or manganese(III) tetrakis(1-methyl-4-pyridyl)porphyrin] caused activation of Notch3. Stretch increased the activity of TACE, which was prevented by catalase. Stretch-induced activation of p38 MAPK in VSMCs was inhibited either by catalase or by inhibiting Notch3 expression (siRNA). Stretch caused VSMCs to realign perpendicular to the direction of the mechanical stimulus, which was significantly inhibited by catalase or by inhibiting the expression (siRNA) or activation of Notch3 (TAPI-1 or DAPT). Therefore, cyclic uniaxial stretch activates Notch3 signaling through a ROS-mediated mechanism, and the presence of Notch3 is necessary for proper stretch-induced cell alignment in VSMCs. This mechanism may contribute to the physiological role of Notch3 in mediating developmental maturation of VSMCs.

Reciprocal regulation of Notch and nuclear factor of activated T-cells (NFAT) c1 transactivation in osteoblasts

Notch are transmembrane receptors involved in the determination of cell fate. Nuclear factor of activated T-cells (NFAT)c are transcription factors that control cell differentiation and function. We tested whether Notch and NFAT signaling pathways interacted in osteoblastic cells. Notch signaling was induced in ST-2 cells using vectors expressing Notch1 intracellular domain (NICD), and in Rosa(Notch) osteoblastic cells by Cre recombinase-mediated excision of a loxP-flanked STOP cassette cloned between the Rosa26 promoter and NICD. NFATc1 was induced in Rosa(Notch) osteoblastic cells by transducing an adenoviral vector expressing constitutively active NFATc1. Notch inhibited NFAT transactivation and NFATc1 transcription. In ST-2 cells, suppression of NFAT transactivation by Notch was reversed by constitutively active cGMP-dependent protein kinase type II. NFATc1 inhibited the transactivation of Notch target genes, and competed for binding to DNA with the Notch interacting protein Epstein-Barr virus latency C promoter binding factor-1, suppressor of hairless, Lag-1 (CSL). Co-immunoprecipitation and confocal microscopy demonstrated that NFATc1 and CSL interacted. Studies on the effects of NICD and NFATc1 on the differentiation and function of osteoblastic cells demonstrated that NICD and NFATc1 inhibited expression of osteoblast gene markers in Rosa(Notch) osteoblasts, but only NICD suppressed the commitment of bone marrow stromal cells to the osteoblastic lineage. In conclusion, NICD and NFATc1 reciprocally inhibit their signaling pathways, and form a regulatory network to control their activity in osteoblasts.

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.

A novel interaction between hedgehog and Notch promotes proliferation at the anterior-posterior organizer of the Drosophila wing

Notch has multiple roles in the development of the Drosophila melanogaster wing imaginal disc. It helps specify the dorsal-ventral compartment border, and it is needed for the wing margin, veins, and sensory organs. Here we present evidence for a new role: stimulating growth in response to Hedgehog. We show that Notch signaling is activated in the cells of the anterior-posterior organizer that produce the region between wing veins 3 and 4, and we describe strong genetic interactions between the gene that encodes the Hedgehog pathway activator Smoothened and the Notch pathway genes Notch, presenilin, and Suppressor of Hairless and the Enhancer of split complex. This work thus reveals a novel collaboration by the Hedgehog and Notch pathways that regulates proliferation in the 3-4 intervein region independently of Decapentaplegic.

Processing of the synaptic cell adhesion molecule neurexin-3beta by Alzheimer disease alpha- and gamma-secretases

Neurexins (NRXNs) are synaptic cell adhesion molecules having essential roles in the assembly and maturation of synapses into fully functional units. Immunocytochemical and electrophysiological studies have shown that specific binding across the synaptic cleft of the ectodomains of presynaptic NRXNs and postsynaptic neuroligins have the potential to bidirectionally coordinate and trigger synapse formation. Moreover, in vivo studies as well as genome-wide association studies pointed out implication of NRXNs in the pathogenesis of cognitive disorders including autism spectrum disorders and different types of addictions including opioid and alcohol dependences, suggesting an important role in synaptic function. Despite extensive investigations, the mechanisms by which NRXNs modulate the properties of synapses remain largely unknown. We report here that α- and γ-secretases can sequentially process NRXN3β, leading to the formation of two final products, an ∼80-kDa N-terminal extracellular domain of Neurexin-3β (sNRXN3β) and an ∼12-kDa C-terminal intracellular NRXN3β domain (NRXN3β-ICD), both of them being potentially implicated in the regulation of NRXNs and neuroligins functions at the synapses or in yet unidentified signal transduction pathways. We further report that this processing is altered by several PS1 mutations in the catalytic subunit of the γ-secretase that cause early-onset familial Alzheimer disease.

Anthrax toxins--roadblocks for exocytic trafficking

Anthrax toxins cause vascular dysfunction, in part by perturbing the endothelial cell barrier. Reporting in Nature, Guichard et al. shed new light on the mechanism by which this occurs and show that anthrax toxins interfere with exocytic delivery of cadherins to endothelial cell junctions by antagonizing the exocyst complex.

Trafficking and proteolytic processing of RNF13, a model PA-TM-RING family endosomal membrane ubiquitin ligase

RING finger protein 13 (RNF13) is a ubiquitously expressed, highly regulated ubiquitin ligase anchored in endosome membranes. A RING domain located in the cytoplasmic half of this type 1 membrane protein mediates ubiquitination in vitro but physiological substrates have not yet been identified. The protein localized in endosomal membranes undergoes extensive proteolysis in a proteasome-dependent manner, but the mRNA level can be increased and the encoded protein stabilized under specific physiological conditions. The cytoplasmic half of RNF13 is released from the membrane by regulatory proteases and therefore has the potential to mediate ubiquitination at distant sites independent of the full-length protein. In response to protein kinase C activation, the full-length protein is stabilized and moves to recycling endosomes and to the inner nuclear membrane, which exposes the RING domain to the nucleoplasm. Thus RNF13 is a ubiquitin ligase that can potentially mediate ubiquitination in endosomes, on the plasma membrane, in the cytoplasm, in the nucleoplasm or on the inner nuclear membrane, with the site(s) regulated by signaling events that modulate protein targeting and proteolysis.

Neurogenetics: advancing the "next-generation" of brain research

There can be little doubt that genetics has transformed our understanding of mechanisms mediating brain disorders. The last two decades have brought tremendous progress in terms of accurate molecular diagnoses and knowledge of the genes and pathways that are involved in a large number of neurological and psychiatric disorders. Likewise, new methods and analytical approaches, including genome array studies and "next-generation" sequencing technologies, are bringing us deeper insights into the subtle complexities of the genetic architecture that determines our risks for these disorders. As we now seek to translate these discoveries back to clinical applications, a major challenge for the field will be in bridging the gap between genes and biology. In this Overview of Neuron's special review issue on neurogenetics, we reflect on progress made over the last two decades and highlight the challenges as well as the exciting opportunities for the future.

Notch activation differentially regulates renal progenitors proliferation and differentiation toward the podocyte lineage in glomerular disorders

Glomerular diseases account for 90% of end-stage kidney disease. Podocyte loss is a common determining factor for the progression toward glomerulosclerosis. Mature podocytes cannot proliferate, but recent evidence suggests that they can be replaced by renal progenitors localized within the Bowman's capsule. Here, we demonstrate that Notch activation in human renal progenitors stimulates entry into the S-phase of the cell cycle and cell division, whereas its downregulation is required for differentiation toward the podocyte lineage. Indeed, a persistent activation of the Notch pathway induced podocytes to cross the G(2)/M checkpoint, resulting in cytoskeleton disruption and death by mitotic catastrophe. Notch expression was virtually absent in the glomeruli of healthy adult kidneys, while a strong upregulation was observed in renal progenitors and podocytes in patients affected by glomerular disorders. Accordingly, inhibition of the Notch pathway in mouse models of focal segmental glomerulosclerosis ameliorated proteinuria and reduced podocyte loss during the initial phases of glomerular injury, while inducing reduction of progenitor proliferation during the regenerative phases of glomerular injury with worsening of proteinuria and glomerulosclerosis. Taken altogether, these results suggest that the severity of glomerular disorders depends on the Notch-regulated balance between podocyte death and regeneration provided by renal progenitors.

Drug discovery approaches to target Wnt signaling in cancer stem cells

Cancer stem cells (CSCs) represent a unique subset of cells within a tumor that possess self-renewal capacity and pluripotency, and can drive tumor initiation and maintenance. First identified in hematological malignancies, CSCs are now thought to play an important role in a wide variety of solid tumors such as NSCLC, breast and colorectal cancer. The role of CSCs in driving tumor formation illustrates the dysregulation of differentiation in tumorigenesis. The Wnt, Notch and Hedgehog (HH) pathways are developmental pathways that are commonly activated in many types of cancer. While substantial progress has been made in developing therapeutics targeting Notch and HH, the Wnt pathway has remained an elusive therapeutic target. This review will focus on the clinical relevance of the Wnt pathway in CSCs and tumor cell biology, as well as points of therapeutic intervention and recent advances in targeting Wnt/β-catenin signaling.

Hepatic progenitor cells: an update

Liver progenitor cells are activated in most human liver diseases. The dynamics, and therefore subpopulations, of progenitor cells are, however, different in acute versus chronic hepatocytic diseases and in biliary diseases. The role of Wnt and Notch signaling pathways in activation and differentiation of human hepatic progenitor cells holds great promise because they can be manipulated by drugs. Hepatocytic differentiation requires inhibition of Notch (numb switched on), whereas cholangiocytic differentiation requires Notch activation. In this way, the patients' own regenerative response could be supported, which could eventually even avoid the need for transplantation in several patients.

Delivering the lateral inhibition punchline: it's all about the timing

Experimental and theoretical biologists have long been fascinated with the emergence of self-organizing patterns in developing organisms, and much attention has focused on Notch-mediated lateral inhibition. Within sheets of cells that may adopt either of two possible cell fates, lateral inhibition establishes patterns through the activity of a negative intercellular feedback loop involving the receptor, Notch, and its ligands Delta or Serrate. Despite a long history of intensive study in Drosophila, where the mechanism was first described, as well as in other organisms, new work continues to yield important insights. Mathematical modeling, combined with biological analyses, has now shed light on two features of the process: how antagonistic and activating ligand-receptor interactions work together to accelerate inhibition and ensure fidelity, and how filopodial dynamics contribute to the observed pattern refinement and spacing.

Delta-like ligand 4-Notch signaling regulates bone marrow-derived pericyte/vascular smooth muscle cell formation

Delta-like ligand 4 (DLL4) is essential for the formation of mature vasculature. However, the role of DLL4-Notch signaling in pericyte/vascular smooth muscle cell (vSMC) development is poorly understood. We sought to determine whether DLL4-Notch signaling is involved in pericyte/vSMC formation in vitro and during vasculogenesis in vivo using 2 Ewing sarcoma mouse models. Inhibition of DLL4 with the antibody YW152F inhibited pericyte/vSMC marker expression by bone marrow (BM) cells in vitro. Conversely, transfection of 10T1/2 cells with the active domains of Notch receptors led to increased expression of pericyte/vSMC markers. Furthermore, the blood vessels of Ewing sarcoma tumors from mice treated with YW152F had reduced numbers of BM-derived pericytes/vSMCs, fewer open lumens, and were less functional than the vessels in tumors of control-treated mice. Tumor growth was also inhibited. These data demonstrate a specific role for DLL4 in the formation of BM-derived pericytes/vSMCs and indicate that DLL4 may be a novel therapeutic target for the inhibition of vasculogenesis.

Protein kinase networks regulating glucocorticoid-induced apoptosis of hematopoietic cancer cells: fundamental aspects and practical considerations

Glucocorticoids (GCs) are integral components in the treatment protocols of acute lymphoblastic leukemia, multiple myeloma, and non-Hodgkin lymphoma owing to their ability to induce apoptosis of these malignant cells. Resistance to GC therapy is associated with poor prognosis. Although they have been used in clinics for decades, the signal transduction pathways involved in GC-induced apoptosis have only partly been resolved. Accumulating evidence shows that this cell death process is mediated by a communication between nuclear GR affecting gene transcription of pro-apoptotic genes such as Bim, mitochondrial GR affecting the physiology of the mitochondria, and the protein kinase glycogen synthase kinase-3 (GSK3), which interacts with Bim following exposure to GCs. Prevention of Bim up-regulation, mitochondrial GR translocation, and/or GSK3 activation are common causes leading to GC therapy failure. Various protein kinases positively regulating the pro-survival Src-PI3K-Akt-mTOR and Raf-Ras-MEK-ERK signal cascades have been shown to be activated in malignant leukemic cells and antagonize GC-induced apoptosis by inhibiting GSK3 activation and Bim expression. Targeting these protein kinases has proven effective in sensitizing GR-positive malignant lymphoid cells to GC-induced apoptosis. Thus, intervening with the pro-survival kinase network in GC-resistant cells should be a good means of improving GC therapy of hematopoietic malignancies.

The Notch2-Jagged1 interaction mediates stem cell factor signaling in erythropoiesis

Stem cell factor (SCF), the ligand for the c-kit receptor, is essential for the production of red blood cells during development and stress erythropoiesis. SCF promotes erythroblast proliferation and survival, while delaying erythroid differentiation through mechanisms that are largely unknown. In cultures of primary human differentiating erythroblasts, we found that SCF induces an increase in the expression of Notch2, a member of the Notch family implicated in the control of cell growth and differentiation. Functional inhibition of either Notch or its ligand Jagged1 inhibited the effects of SCF on erythroid cell expansion. SCF also induced the expression of Hes-1 and GATA-2, which may contribute to transduce Notch2 signals in response to SCF. Transduction of primary erythroid precursors with a dominant-negative Notch2 mutant inhibited both basal and SCF-mediated erythroblast expansion, and counteracted the effects of SCF on erythroblast differentiation. These findings provide a clue to understand the effects of increased proliferation and delayed differentiation elicited by SCF on the erythroid compartment and indicate Notch2 as a new player in the regulation of red cell differentiation.

Rabconnectin-3 is a functional regulator of mammalian Notch signaling

The Notch signaling pathway is important for cell fate decisions in embryonic development and adult life. Defining the functional importance of the Notch pathway in these contexts requires the elucidation of essential signal transduction components that have not been fully characterized. Here, we show that Rabconnectin-3B is required for the Notch pathway in mammalian cells. siRNA-mediated silencing of Rabconnectin-3B in mammalian cells attenuated Notch signaling and disrupted the activation and nuclear accumulation of the Notch target Hes1. Rabconnectin-3B knockdown also disrupted V-ATPase activity in mammalian cells, consistent with previous observations in Drosophila. Pharmacological inhibition of the V-ATPase complex significantly reduced Notch signaling in mammalian cells. Finally, Rabconnectin-3B knockdown phenocopied functional disruption of Notch signaling during osteoclast differentiation. Collectively, these findings define an important role for Rabconnectin-3 and V-ATPase activity in the Notch signaling pathway in mammalian cells.

Keeping in touch with contact inhibition of locomotion

Contact inhibition of locomotion (CIL) is the process by which cells in vitro change their direction of migration upon contact with another cell. Here, we revisit the concept that CIL plays a central role in the migration of single cells and in collective migration, during both health and disease. Importantly, malignant cells exhibit a diminished CIL behaviour which allows them to invade healthy tissues. Accumulating evidence indicates that CIL occurs in vivo and that regulation of small Rho GTPases is important in the collapse of cell protrusions upon cell contact, the first step of CIL. Finally, we propose possible cell surface proteins that could be involved in the initial contact that regulates Rho GTPases during CIL.

Convergence of Kaposi's sarcoma-associated herpesvirus reactivation with Epstein-Barr virus latency and cellular growth mediated by the notch signaling pathway in coinfected cells

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent of primary effusion lymphoma (PEL). All PEL cell lines are infected with KSHV, and 70% are coinfected with Epstein-Barr virus (EBV). KSHV reactivation from latency requires promoter-specific transactivation by the KSHV Rta protein through interactions with RBP-Jk (CSL), the cellular DNA-binding component of the Notch signal transduction pathway. EBV transformation of primary B cells requires EBV nuclear antigen 2 (EBNA-2) to interact with RBP-Jk to direct the latent viral and cellular gene expression program. Although KSHV Rta and EBV EBNA-2 both require RBP-Jk for transactivation, previous studies have suggested that RBP-Jk-dependent transactivators do not function identically. We have found that the EBV latent protein LMP-1 is expressed in less than 5% of KSHV(+)/EBV(+) PEL cells but is induced in an Rta-dependent fashion when KSHV reactivates. KSHV Rta transactivates the EBV latency promoters in an RBP-Jk-dependent fashion and forms a ternary complex with RBP-Jk on the promoters. In B cells that are conditionally transformed by EBV alone, we show that KSHV Rta complements a short-term EBNA-2 growth deficiency in an autocrine/paracrine manner. Complementation of EBNA-2 deficiency by Rta depends on RBP-Jk and LMP-1, and Rta transactivation is required for optimal growth of KSHV(+)/EBV(+) PEL lines. Our data suggest that Rta can contribute to EBV-driven cellular growth by transactivating RBP-Jk-dependent EBV latency genes. However, our data also suggest that EBNA-2 and Rta induce distinct alterations in the cellular proteomes that contribute to the growth of infected cells.

BMPs and FGFs target Notch signalling via jagged 2 to regulate tooth morphogenesis and cytodifferentiation

The Notch signalling pathway is an evolutionarily conserved intercellular signalling mechanism that is essential for cell fate specification and proper embryonic development. We have analysed the expression, regulation and function of the jagged 2 (Jag2) gene, which encodes a ligand for the Notch family of receptors, in developing mouse teeth. Jag2 is expressed in epithelial cells that give rise to the enamel-producing ameloblasts from the earliest stages of tooth development. Tissue recombination experiments showed that its expression in epithelium is regulated by mesenchyme-derived signals. In dental explants cultured in vitro, the local application of fibroblast growth factors upregulated Jag2 expression, whereas bone morphogenetic proteins provoked the opposite effect. Mice homozygous for a deletion in the Notch-interaction domain of Jag2 presented a variety of severe dental abnormalities. In molars, the crown morphology was misshapen, with additional cusps being formed. This was due to alterations in the enamel knot, an epithelial signalling structure involved in molar crown morphogenesis, in which Bmp4 expression and apoptosis were altered. In incisors, cytodifferentiation and enamel matrix deposition were inhibited. The expression of Tbx1 in ameloblast progenitors, which is a hallmark for ameloblast differentiation and enamel formation, was dramatically reduced in Jag2(-/-) teeth. Together, these results demonstrate that Notch signalling mediated by Jag2 is indispensable for normal tooth development.

Linking model systems to cancer therapeutics: the case of Mastermind

Genetics, and more recently genomics, reveal striking conservation in the fundamental signaling pathways that underlie normal and aberrant cell processes. Consequently, various genetic model organisms are now attracting the interest of biomedical scientists who are focused on therapeutic approaches to human disease. There are now several examples of studies in which Drosophila seems likely to facilitate advances in potential therapies, and a recent report has demonstrated the utility of the fly model for understanding and treating human disease. Basic developmental genetic information first obtained in Drosophila was used to design a therapeutic block to oncogenic Notch signaling that was associated with leukemia in mice. The story of Notch signaling in Drosophila demonstrates the potential for standard Drosophila molecular genetics in developing therapeutic strategies that are relevant to human disease.

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.

A combined ex vivo and in vivo RNAi screen for notch regulators in Drosophila reveals an extensive notch interaction network

Notch signaling plays a fundamental role in cellular differentiation and has been linked to human diseases, including cancer. We report the use of comprehensive RNAi analyses to dissect Notch regulation and its connections to cellular pathways. A cell-based RNAi screen identified 900 candidate Notch regulators on a genome-wide scale. The subsequent use of a library of transgenic Drosophila expressing RNAi constructs enabled large-scale in vivo validation and confirmed 333 of 501 tested genes as Notch regulators. Mapping the phenotypic attributes of our data on an interaction network identified another 68 relevant genes and revealed several modules of unexpected Notch regulatory activity. In particular, we note an intriguing relationship to pyruvate metabolism, which may be relevant to cancer. Our study reveals a hitherto unappreciated diversity of tissue-specific modulators impinging on Notch and opens new avenues for studying Notch regulation and function in development and disease.

Synthetic lethality through combined Notch-epidermal growth factor receptor pathway inhibition in basal-like breast cancer

Basal-like breast cancers (BLBC) are highly aggressive, yet selective therapies targeting the specific oncoproteins driving these tumors have not been developed. These cancers frequently express epidermal growth factor receptor (EGFR), with resistance to its inhibition being well documented, albeit poorly understood. Notch pathway activation is also common in this breast cancer subtype and can be suppressed by gamma-secretase inhibitors, which effectively block receptor cleavage and activation. Herein, we show that although inhibition of either EGFR or Notch signaling alone is insufficient to suppress basal-like breast tumor cell survival and proliferation, simultaneous inhibition uncovers a synthetic lethal relationship between these two oncogenic pathways. This lethality is due in part to significant decreases in AKT activation caused by combined EGFR and Notch inhibition. Expression of the activated form of Notch1 restores AKT activity and enables cells to overcome cell death after dual-pathway blockade. Combined pathway inhibition is also dramatically more effective at suppressing tumor growth in mice than blocking EGFR or Notch signaling alone. Thus, we show that Notch pathway activation contributes to resistance to EGFR inhibition, and provide a novel treatment strategy for BLBCs.

Targeting Notch signaling pathway to overcome drug resistance for cancer therapy

Chemotherapy is an important therapeutic strategy for cancer treatment and remains the mainstay for the management of human malignancies; however, chemotherapy fails to eliminate all tumor cells because of intrinsic or acquired drug resistance, which is the most common cause of tumor recurrence. Recently, emerging evidences suggest that Notch signaling pathway is one of the most important signaling pathways in drug-resistant tumor cells. Moreover, down-regulation of Notch pathway could induce drug sensitivity, leading to increased inhibition of cancer cell growth, invasion, and metastasis. This article will provide a brief overview of the published evidences in support of the roles of Notch in drug resistance and will further summarize how targeting Notch by "natural agents" could become a novel and safer approach for the improvement of tumor treatment by overcoming drug resistance.

Targeting Notch to target cancer stem cells

The cellular heterogeneity of neoplasms has been at the center of considerable interest since the "cancer stem cell hypothesis", originally formulated for hematologic malignancies, was extended to solid tumors. The origins of cancer "stem" cells (CSC) or tumor-initiating cells (TIC; henceforth referred to as CSCs) and the methods to identify them are hotly debated topics. Nevertheless, the existence of subpopulations of tumor cells with stem-like characteristics has significant therapeutic implications. The stem-like phenotype includes indefinite self-replication, pluripotency, and, importantly, resistance to chemotherapeutics. Thus, it is plausible that CSCs, regardless of their origin, may escape standard therapies and cause disease recurrences and/or metastasis after apparently complete remissions. Consequently, the idea of selectively targeting CSCs with novel therapeutics is gaining considerable interest. The Notch pathway is one of the most intensively studied putative therapeutic targets in CSC, and several investigational Notch inhibitors are being developed. However, successful targeting of Notch signaling in CSC will require a thorough understanding of Notch regulation and the context-dependent interactions between Notch and other therapeutically relevant pathways. Understanding these interactions will increase our ability to design rational combination regimens that are more likely to prove safe and effective. Additionally, to determine which patients are most likely to benefit from treatment with Notch-targeting therapeutics, reliable biomarkers to measure pathway activity in CSC from specific tumors will have to be identified and validated. This article summarizes the most recent developments in the field of Notch-targeted cancer therapeutics, with emphasis on CSC.

Analysis of the subcellular phosphoproteome using a novel phosphoproteomic reactor

Protein phosphorylation is an important post-translational modification involved in the regulation of many cellular processes. Mass spectrometry has been successfully used to identify protein phosphorylation in specific pathways and for global phosphoproteomic analysis. However, phosphoproteomics approaches do not evaluate the subcellular localization of the phosphorylated forms of proteins, which is an important factor for understanding the roles of protein phosphorylation on a global scale. The in-depth mapping of protein phosphorylation at the subcellular level necessitates the development of new methods capable of specifically and efficiently enriching phosphopeptides from highly complex samples. Here, we report a novel microfluidic device called the phosphoproteomic reactor that combines efficient processing of proteins followed by phosphopeptide enrichment by Ti-IMAC. To illustrate the potential of this novel technology, we mapped the phosphoproteins in subcellular organelles of liver cells. Fifteen subcellular fractions from liver cell cultures were processed on the phosphoproteomic reactor in combination with nano-LC-MS/MS analysis. We identified thousands of phosphorylation sites in over 600 phosphoproteins in different organelles using minute amounts of starting material. Overall, this approach provides a new avenue for studying the phosphoproteome of the subcellular organelles.

NOTCH2 in breast cancer: association of SNP rs11249433 with gene expression in ER-positive breast tumors without TP53 mutations

Background

A recent genome-wide association study (GWAS) has identified a single nucleotide polymorphism (SNP) rs11249433 in the 1p11.2 region as a novel genetic risk factor for breast cancer, and this association was stronger in patients with estrogen receptor (ER)⁺ versus ER^- cancer.

Results

We found association between SNP rs11249433 and expression of the NOTCH2 gene located in the 1p11.2 region. Examined in 180 breast tumors, the expression of NOTCH2 was found to be lowest in tumors with TP53 mutations and highest in TP53 wild-type/ER⁺ tumors (p = 0.0059). In the latter group, the NOTCH2 expression was particularly increased in carriers of the risk genotypes (AG/GG) of rs11249433 when compared to the non-risk AA genotype (p = 0.0062). Similar association between NOTCH2 expression and rs11249433 was observed in 60 samples of purified monocytes from healthy controls (p = 0.015), but not in total blood samples from 302 breast cancer patients and 76 normal breast tissue samples. We also identified the first possible dominant-negative form of NOTCH2, a truncated version of NOTCH2 consisting of only the extracellular domain.

Conclusion

This is the first study to show that the expression of NOTCH2 differs in subgroups of breast tumors and by genotypes of the breast cancer-associated SNP rs11249433. The NOTCH pathway has key functions in stem cell differentiation of ER⁺ luminal cells in the breast. Therefore, increased expression of NOTCH2 in carriers of rs11249433 may promote development of ER⁺ luminal tumors. Further studies are needed to investigate possible mechanisms of regulation of NOTCH2 expression by rs11249433 and the role of NOTCH2 splicing forms in breast cancer development.

Intestinal stem cells lacking the Math1 tumour suppressor are refractory to Notch inhibitors

Intestinal cells are constantly produced from a stem cell reservoir that gives rise to proliferating transient amplifying cells, which subsequently differentiate into one of the four principal cell types. Signalling pathways, including the Notch signalling pathway, coordinate these differentiation processes and their deregulation may cause cancer. Pharmacological inhibition through γ-secretase inhibitors or genetic inactivation of the Notch signalling pathway results in the complete loss of proliferating crypt progenitors due to their conversion into post-mitotic goblet cells. The basic helix-loop-helix transcription factor Math1 is essential for intestinal secretory cell differentiation. Because of the critical roles of both Math1 and Notch signalling in intestinal homeostasis and neoplastic transformation, we sought to determine the genetic hierarchy regulating the differentiation of intestinal stem cells into secretory cells. In this paper, we demonstrate that the conversion of intestinal stem cells into goblet cells upon inhibition of the Notch signalling pathway requires Math1.

Molecular structure and dimeric organization of the Notch extracellular domain as revealed by electron microscopy

BACKGROUND

The Notch receptor links cell fate decisions of one cell to that of the immediate cellular neighbor. In humans, malfunction of Notch signaling results in diseases and congenital disorders. Structural information is essential for gaining insight into the mechanism of the receptor as well as for potentially interfering with its function for therapeutic purposes.

METHODOLOGY/PRINCIPAL FINDINGS

We used the Affinity Grid approach to prepare specimens of the Notch extracellular domain (NECD) of the Drosophila Notch and human Notch1 receptors suitable for analysis by electron microscopy and three-dimensional (3D) image reconstruction. The resulting 3D density maps reveal that the NECD structure is conserved across species. We show that the NECD forms a dimer and adopts different yet defined conformations, and we identify the membrane-proximal region of the receptor and its ligand-binding site.

CONCLUSIONS/SIGNIFICANCE

Our results provide direct and unambiguous evidence that the NECD forms a dimer. Our studies further show that the NECD adopts at least three distinct conformations that are likely related to different functional states of the receptor. These findings open the way to now correlate mutations in the NECD with its oligomeric state and conformation.

Notch signaling is not essential in sonic hedgehog-activated medulloblastoma

Dysregulated signal transduction through the notch pathway has been noted in human and mouse medulloblastoma studies. Gamma secretase inhibitors (GSIs) impair notch signaling by preventing the cleavage of transmembrane notch proteins into their active intracellular domain fragments. Previous studies have shown that GSI treatment caused apoptosis and impaired medulloblastoma cell engraftment in xenograft systems. In this study, we used in vivo genetic and pharmacologic approaches to quantify the contribution of notch signaling to sonic hedgehog (shh)-activated mouse medulloblastoma models. In contrast to prior in vitro studies, pharmacologic inhibition of notch pathways did not reduce the efficiency of medulloblastoma xenotransplantation nor did systemic therapy impact tumor size, proliferation, or apoptosis in genetically engineered mouse medulloblastoma models. The incidence and pathology of medulloblastomas driven by the SmoA1 transgene was unchanged by the bi-allelic absence of Notch1, Notch2, or Hes5 genes. These data show that notch signaling is not essential for the initiation, engraftment, or maintenance of sonic hedgehog pathway-driven medulloblastomas.

In vivo reconstitution of gamma-secretase in Drosophila results in substrate specificity

The intramembrane aspartyl protease gamma-secretase plays a fundamental role in several signaling pathways involved in cellular differentiation and has been linked with a variety of human diseases, including Alzheimer's disease. Here, we describe a transgenic Drosophila model for in vivo-reconstituted gamma-secretase, based on expression of epitope-tagged versions of the four core gamma-secretase components, Presenilin, Nicastrin, Aph-1, and Pen-2. In agreement with previous cell culture and yeast studies, coexpression of these four components promotes the efficient assembly of mature, proteolytically active gamma-secretase. We demonstrate that in vivo-reconstituted gamma-secretase has biochemical properties and a subcellular distribution resembling those of endogenous gamma-secretase. However, analysis of the cleavage of alternative substrates in transgenic-fly assays revealed unexpected functional differences in the activity of reconstituted gamma-secretase toward different substrates, including markedly reduced cleavage of some APP family members compared to cleavage of the Notch receptor. These findings indicate that in vivo under physiological conditions, additional factors differentially modulate the activity of gamma-secretase toward its substrates. Thus, our approach for the first time demonstrates the overall functionality of reconstituted gamma-secretase in a multicellular organism and the requirement for substrate-specific factors for efficient in vivo cleavage of certain substrates.

Role of glycans and glycosyltransferases in the regulation of Notch signaling

The evolutionarily conserved Notch signaling pathway plays broad and important roles during embryonic development and in adult tissue homeostasis. Unlike most other pathways used during animal development, Notch signaling does not rely on second messengers and intracellular signaling cascades. Instead, pathway activation results in the cleavage of the Notch intracellular domain and its translocation into the nucleus, where it functions as a transcriptional co-activator of the Notch target genes. To ensure tight spatial and temporal regulation of a pathway with such an unusually direct signaling transduction, animal cells have devised a variety of specialized modulatory mechanisms. One such mechanism takes advantage of decorating the Notch extracellular domain with rare types of O-linked glycans. In this review, we will discuss the genetic and biochemical data supporting the notion that carbohydrate modification is essential for Notch signaling and attempt to provide a brief historical overview of how we have learned what we know about the glycobiology of Notch. We will also summarize what is known about the contribution of specific nucleotide-sugar transporters to Notch biology and the roles-enzymatic and non-enzymatic-played by specific glycosyltransferases in the regulation of this pathway. Mutations in the Notch pathway components cause a variety of human diseases, and manipulation of Notch signaling is emerging as a powerful tool in regenerative medicine. Therefore, studying how sugar modification modulates Notch signaling provides a framework for better understanding the role of glycosylation in animal development and might offer new tools to manipulate Notch signaling for therapeutic purposes.

Role of miR-34c microRNA in the late steps of spermatogenesis

Spermatogenesis is a cyclic process in which diploid spermatogonia differentiate into haploid spermatozoa. This process is highly regulated, notably at the post-transcriptional level. MicroRNAs (miRNAs), single-stranded noncoding RNA molecules of about 20-25 nucleotides, are implicated in the regulation of many important biological pathways such as proliferation, apoptosis, and differentiation. We wondered whether miRNAs could play a role during spermatogenesis. The miRNA expression repertoire was tested in germ cells, and we present data showing that miR-34c was highly expressed only in these cells. Furthermore, our findings indicate that in male gonads, miR-34c expression is largely p53 independent in contrast to previous results showing a direct link in somatic cells between the miR-34 family and this tumor suppressor protein. In order to identify target genes involved in germinal lineage differentiation, we overexpressed miR-34c in HeLa cells, analyzed the transcriptome of these modified cells, and noticed a shift of the expression profile toward the germinal lineage. Recently, it has been shown that exogenous expression of Ddx4/Vasa in embryonic chicken stem cells (cESC) induces cESC reprogramming toward a germ cell fate. When we simultaneously expressed miR-34c in such cells, we could detect an up-regulation of germ cell-specific genes whereas the expression of other lineage specific markers remained unchanged. These data suggest that miR-34c could play a role by enhancing the germinal phenotype of cells already committed to this lineage.

Notch ligand activity is modulated by glycosphingolipid membrane composition in Drosophila melanogaster

Endocytosis of the transmembrane ligands Delta (Dl) and Serrate (Ser) is required for the proper activation of Notch receptors. The E3 ubiquitin ligases Mindbomb1 (Mib1) and Neuralized (Neur) regulate the ubiquitination of Dl and Ser and thereby promote both ligand endocytosis and Notch receptor activation. In this study, we identify the alpha1,4-N-acetylgalactosaminyltransferase-1 (alpha4GT1) gene as a gain of function suppressor of Mib1 inhibition. Expression of alpha4GT1 suppressed the signaling and endocytosis defects of Dl and Ser resulting from the inhibition of mib1 and/or neur activity. Genetic and biochemical evidence indicate that alpha4GT1 plays a regulatory but nonessential function in Notch signaling via the synthesis of a specific glycosphingolipid (GSL), N5, produced by alpha4GT1. Furthermore, we show that the extracellular domain of Ser interacts with GSLs in vitro via a conserved GSL-binding motif, raising the possibility that direct GSL-protein interactions modulate the endocytosis of Notch ligands. Together, our data indicate that specific GSLs modulate the signaling activity of Notch ligands.

Role of glycosylation of Notch in development

The Notch pathway is one of the major signaling pathways required for proper development in metazoans. Notch activity is regulated at numerous levels, and increasing evidence reveals the importance of "protein glycosylation" (modification of Notch receptors with sugars) for its regulation. In this review we summarize the significance of the Notch pathway in development and the players responsible for its glycosylation, and then discuss the molecular mechanisms by which protein glycosylation may regulate Notch function.

The endocytic matrix

Endocytosis has long been thought of as simply a way for cells to internalize nutrients and membrane-associated molecules. But an explosive growth in knowledge has given a new dimension to our understanding of this process. It now seems that endocytosis is a master organizer of signalling circuits, with one of its main roles being the resolution of signals in space and time. Many of the functions of endocytosis that are emerging from recent research cannot yet be reconciled with the canonical view of intracellular trafficking but, instead, point to endocytosis being integrated at a deeper level in the cellular 'master plan' (the cellular network of signalling circuits that lie at the base of the cell's make-up). Deconvolution of this level, which we call the 'endocytic matrix', might uncover a fundamental aspect of how a cell is built.

Effects of gamma-secretase inhibition on the proliferation and vitamin D(3) induced osteogenesis in adipose derived stem cells

As a gamma-secretase inhibitor, DAPT has been widely used to evaluate the biological behaviors and Notch signaling pathway in various cells. This study was aimed to examine the effects of DAPT on the growth and vitamin D(3) induced osteogenesis in adipose derived stem cells (ASCs). The cells were treated with or without DAPT and induced to osteoblastic lineage in the presence of vitamin D(3). Alizarin red staining and real-time PCR results indicated that the addition of DAPT to vitamin D(3) treatments enhanced osteogenesis in ASCs. According to the fold increase and colony-forming unit assay results, the cells cultured in DAPT exhibited lower proliferation rate than those cultured in control medium. Hey1, expressed in the nucleus of ASCs to act as a transcriptional repressor, was downregulated when Notch signaling was inhibited by DAPT. Whereas the expression of Runx2 increased in the nucleus of osteogenic induced ASCs after DAPT treatment. This study demonstrated that DAPT reduced the proliferation and enhanced the osteogenesis in ASCs via regulation of Notch and Runx2 expression.

Activation of a latent nuclear localization signal in the NH2 terminus of γ-ENaC initiates feedback regulation of channel activity

Proteolytic enzymes cleave the epithelial Na(+) channel (ENaC) at several positions releasing, in part, the NH(2) terminus of the γ-subunit. Cleavage increases ENaC activity by increasing open probability; however, the role of polypeptides cleaved from the channel core remains unclear. We find that the cytosolic NH(2) terminus of γ-ENaC unexpectedly targets to the nucleus being particularly strong in nucleoli. In contrast, the cytosolic COOH terminus targets to the cytoplasm and plasma membrane in a manner similar to full-length subunits. Targeting of the cytosolic NH(2) terminus of γ-ENaC to the nucleus has functional consequences for coexpression of eGFP-fusion proteins containing this segment of the channel, but not the COOH terminus, decrease ENaC activity in a dose-dependent manner. The mechanism of this negative regulation is associated with a decrease in the functional half-life of ENaC at the plasma membrane. Inspection of the primary amino acid sequence of γ-ENaC reveals possible nuclear localization signals (NLS) conserved at the extreme NH(2) terminus and just preceding the first transmembrane domain. Disruption of the putative NLS preceding the first transmembrane domain in γ-ENaC but not that at the extreme NH(2) terminus abolishes both targeting to the nucleus and negative regulation of ENaC activity. These findings are consistent with the release of the NH(2) terminus of γ-ENaC following cleavage being functionally important for signaling to the nucleus in a manner similar to Notch signaling and release of the cytosolic COOH-terminal tail of polycystin-1.

Moonlighting proteins: an intriguing mode of multitasking

Proteins are macromolecules, which perform a large variety of functions. Most of them have only a single function, but an increasing number of proteins are being identified as multifunctional. Moonlighting proteins form a special class of multifunctional proteins. They perform multiple autonomous and often unrelated functions without partitioning these functions into different domains of the protein. Striking examples are enzymes, which in addition to their catalytic function are involved in fully unrelated processes such as autophagy, protein transport or DNA maintenance. In this contribution we present an overview of our current knowledge of moonlighting proteins and discuss the significant implications for biomedical and fundamental research.

Myeloid translocation gene 16 (MTG16) interacts with Notch transcription complex components to integrate Notch signaling in hematopoietic cell fate specification

The Notch signaling pathway regulates gene expression programs to influence the specification of cell fate in diverse tissues. In response to ligand binding, the intracellular domain of the Notch receptor is cleaved by the gamma-secretase complex and then translocates to the nucleus. There, it binds the transcriptional repressor CSL, triggering its conversion to an activator of Notch target gene expression. The events that control this conversion are poorly understood. We show that the transcriptional corepressor, MTG16, interacts with both CSL and the intracellular domains of Notch receptors, suggesting a pivotal role in regulation of the Notch transcription complex. The Notch1 intracellular domain disrupts the MTG16-CSL interaction. Ex vivo fate specification in response to Notch signal activation is impaired in Mtg16-/- hematopoietic progenitors, and restored by MTG16 expression. An MTG16 derivative lacking the binding site for the intracellular domain of Notch1 fails to restore Notch-dependent cell fate. These data suggest that MTG16 interfaces with critical components of the Notch transcription complex to affect Notch-dependent lineage allocation in hematopoiesis.