Notch signaling: a different sort makes the cut

Mammalian NOTCH Receptor Activation and Signaling Protocols

The NOTCH signaling pathway is one of the highly conserved key pathways involved in most cell differentiation and proliferation processes during both developmental and adult stages in most animals. The NOTCH signaling pathway appears to be very simple but the existence of several receptors and ligands, their posttranslational modifications, their activation in the cell surface and its migration to the cell nucleus, as well as their interaction with multiple signaling pathways in the cytoplasm and the nucleus of cells, make the study of its function very complex.To determine the activation of NOTCH signaling in animal cells, several complementary approaches can be performed. One of them is the analysis of the transcription of NOTCH receptor target genes HES/HEY by qRT-PCR and Western blot. This approach would give us an idea of the global NOTCH activation and signaling. We can also analyze the NOTCH transcriptional activity by luciferase assays to determine the global or specific activation of NOTCH receptors under a given treatment or in response to the modification of gene expression. On the other hand, we can determine the specific activation of each NOTCH receptor by Western blot with antibodies that recognize the active forms of each NOTCH receptor. For this assay will be very important to collect the cells to be analyzed under the appropriate conditions. Finally, we can detect the intracellular domain of each NOTCH receptor into the cell nucleus by confocal microscopy using the appropriate antibodies that recognize the intracellular domain of the receptors.

The Role of Intracellular Trafficking of Notch Receptors in Ligand-Independent Notch Activation

Aberrant Notch signaling has been found in a broad range of human malignancies. Consequently, small molecule inhibitors and antibodies targeting Notch signaling in human cancers have been developed and tested; however, these have failed due to limited anti-tumor efficacy because of dose-limiting toxicities in normal tissues. Therefore, there is an unmet need to discover novel regulators of malignant Notch signaling, which do not affect Notch signaling in healthy tissues. This review provides a comprehensive overview of the current knowledge on the role of intracellular trafficking in ligand-independent Notch receptor activation, the possible mechanisms involved, and possible therapeutic opportunities for inhibitors of intracellular trafficking in Notch targeting.

Ubiquitination and Deubiquitination in Oral Disease

Oral health is an integral part of the general health and well-being of individuals. The presence of oral disease is potentially indicative of a number of systemic diseases and may contribute to their early diagnosis and treatment. The ubiquitin (Ub) system has been shown to play a role in cellular immune response, cellular development, and programmed cell death. Ubiquitination is a post-translational modification that occurs in eukaryotes. Its mechanism involves a number of factors, including Ub-activating enzymes, Ub-conjugating enzymes, and Ub protein ligases. Deubiquitinating enzymes, which are proteases that reversely modify proteins by removing Ub or Ub-like molecules or remodeling Ub chains on target proteins, have recently been regarded as crucial regulators of ubiquitination-mediated degradation and are known to significantly affect cellular pathways, a number of biological processes, DNA damage response, and DNA repair pathways. Research has increasingly shown evidence of the relationship between ubiquitination, deubiquitination, and oral disease. This review investigates recent progress in discoveries in diseased oral sites and discusses the roles of ubiquitination and deubiquitination in oral disease.

Targeting Notch Trafficking and Processing in Cancers

The Notch family comprises a group of four ligand-dependent receptors that control evolutionarily conserved developmental and homeostatic processes and transmit signals to the microenvironment. NOTCH undergoes remodeling, maturation, and trafficking in a series of post-translational events, including glycosylation, ubiquitination, and endocytosis. The regulatory modifications occurring in the endoplasmic reticulum/Golgi precede the intramembrane γ-secretase proteolysis and the transfer of active NOTCH to the nucleus. Hence, NOTCH proteins coexist in different subcellular compartments and undergo continuous relocation. Various factors, including ion concentration, enzymatic activity, and co-regulatory elements control Notch trafficking. Interfering with these regulatory mechanisms represents an innovative therapeutic way to bar oncogenic Notch signaling. In this review, we briefly summarize the role of Notch signaling in cancer and describe the protein modifications required for NOTCH to relocate across different subcellular compartments. We focus on the functional relationship between these modifications and the corresponding therapeutic options, and our findings could support the development of trafficking modulators as a potential alternative to the well-known γ-secretase inhibitors.

Dissecting molecular mechanisms of resistance to NOTCH1-targeted therapy in T-cell acute lymphoblastic leukemia xenografts

Despite substantial progress in treatment of T-cell acute lymphoblastic leukemia (T-ALL), mortality remains relatively high, mainly due to primary or acquired resistance to chemotherapy. Further improvements in survival demand better understanding of T-ALL biology and development of new therapeutic strategies. The Notch pathway has been involved in the pathogenesis of this disease and various therapeutic strategies are currently under development, including selective targeting of NOTCH receptors by inhibitory antibodies. We previously demonstrated that the NOTCH1-specific neutralizing antibody OMP52M51 prolongs survival in TALL patient-derived xenografts bearing NOTCH1/FBW7 mutations. However, acquired resistance to OMP52M51 eventually developed and we used patient-derived xenografts models to investigate this phenomenon. Multi-level molecular characterization of T-ALL cells resistant to NOTCH1 blockade and serial transplantation experiments uncovered heterogeneous types of resistance, not previously reported with other Notch inhibitors. In one model, resistance appeared after 156 days of treatment, it was stable and associated with loss of Notch inhibition, reduced mutational load and acquired NOTCH1 mutations potentially affecting the stability of the heterodimerization domain. Conversely, in another model resistance developed after only 43 days of treatment despite persistent down-regulation of Notch signaling and it was accompanied by modulation of lipid metabolism and reduced surface expression of NOTCH1. Our findings shed light on heterogeneous mechanisms adopted by the tumor to evade NOTCH1 blockade and support clinical implementation of antibody-based target therapy for Notch-addicted tumors.

Histone deacetylase 6 controls Notch3 trafficking and degradation in T-cell acute lymphoblastic leukemia cells

Several studies have revealed that endosomal sorting controls the steady-state levels of Notch at the cell surface in normal cells and prevents its inappropriate activation in the absence of ligands. However, whether this highly dynamic physiologic process can be exploited to counteract dysregulated Notch signaling in cancer cells remains unknown. T-ALL is a malignancy characterized by aberrant Notch signaling, sustained by activating mutations in Notch1 as well as overexpression of Notch3, a Notch paralog physiologically subjected to lysosome-dependent degradation in human cancer cells. Here we show that treatment with the pan-HDAC inhibitor Trichostatin A (TSA) strongly decreases Notch3 full-length protein levels in T-ALL cell lines and primary human T-ALL cells xenografted in mice without substantially reducing NOTCH3 mRNA levels. Moreover, TSA markedly reduced the levels of Notch target genes, including pTα, CR2, and DTX-1, and induced apoptosis of T-ALL cells. We further observed that Notch3 was post-translationally regulated following TSA treatment, with reduced Notch3 surface levels and increased accumulation of Notch3 protein in the lysosomal compartment. Surface Notch3 levels were rescued by inhibition of dynein with ciliobrevin D. Pharmacologic studies with HDAC1, 6, and 8-specific inhibitors disclosed that these effects were largely due to inhibition of HDAC6 in T-ALL cells. HDAC6 silencing by specific shRNA was followed by reduced Notch3 expression and increased apoptosis of T-ALL cells. Finally, HDAC6 silencing impaired leukemia outgrowth in mice, associated with reduction of Notch3 full-length protein in vivo. These results connect HDAC6 activity to regulation of total and surface Notch3 levels and suggest HDAC6 as a potential novel therapeutic target to lower Notch signaling in T-ALL and other Notch3-addicted tumors.

The Role of Notch1 Signaling in Anaplastic Thyroid Carcinoma

PURPOSE

The Notch signaling pathway is widely expressed in normal, reactive, and neoplastic tissues; however, its role in thyroid tissues has not been fully elucidated. Therefore, this study was conducted to characterize the expression of the Notch signaling pathway in papillary thyroid cancer (PTC) cells and anaplastic thyroid cancer (ATC) cells.

MATERIALS AND METHODS

Expression of activated Notch1 in ATC and PTC paraffin-embedded tissues was determined by immunohistochemistry. The small interfering RNA techniquewas employed to knock down Notch1 expression in ATC and PTC cell lines.

RESULTS

The expression of activated Notch1 was higher in ATC cases than in PTC cases. Inhibition of Notch1 significantly reduced proliferation and migration of ATC cells, but not PTC cells. In addition, inhibition of Notch1 in ATC cells significantly reduced the expression of key markers of epithelial-mesenchymal transition and cancer stem cells. Conversely, changes in the expression of these proteins were not observed in PTC cells.

CONCLUSION

The results of this study suggest that Notch1 expression plays different roles in tumor progression in ATC and PTC cells. We also found that Notch1 expression was significantly related to the highly invasive or proliferative activity of ATC cells.

Identification of novel cell-adhesion molecules in peripheral nerves using a signal-sequence trap

The development and maintenance of myelinated nerves in the PNS requires constant and reciprocal communication between Schwann cells and their associated axons. However, little is known about the nature of the cell-surface molecules that mediate axon-glial interactions at the onset of myelination and during maintenance of the myelin sheath in the adult. Based on the rationale that such molecules contain a signal sequence in order to be presented on the cell surface, we have employed a eukaryotic-based, signal-sequence-trap approach to identify novel secreted and membrane-bound molecules that are expressed in myelinating and non-myelinating Schwann cells. Using cDNA libraries derived from dbcAMP-stimulated primary Schwann cells and 3-day-old rat sciatic nerve mRNAs, we generated an extensive list of novel molecules expressed in myelinating nerves in the PNS. Many of the identified proteins are cell-adhesion molecules (CAMs) and extracellular matrix (ECM) components, most of which have not been described previously in Schwann cells. In addition, we have identified several signaling receptors, growth and differentiation factors, ecto-enzymes and proteins that are associated with the endoplasmic reticulum and the Golgi network. We further examined the expression of several of the novel molecules in Schwann cells in culture and in rat sciatic nerve by primer-specific, real-time PCR and in situ hybridization. Our results indicate that myelinating Schwann cells express a battery of novel CAMs that might mediate their interactions with the underlying axons.

Delta-like 1-Lysine613 regulates notch signaling

Delta ligands are important for regulating Notch signaling through transcellular stimulation of Notch receptors. The cytoplasmic tails of Delta ligands have multiple potential regulatory sites including several lysine residues that are putative targets for ubiquitination by the E3 ubiquitin ligases, Mind Bomb and Neuralized. To identify possible roles for specific lysine residues in the cytoplasmic tail of the Notch ligand Dll1 a mutational and functional analysis was performed. Examination of a panel of individual or clustered lysine mutants demonstrated that lysine 613 (K613) in the cytoplasmic tail of Dll1 is a key residue necessary for transcellular activation of Notch signaling. Multi-ubiquitination of the Dll1 mutant Dll1-K613R was altered compared to wild type Dll1, and the K613R mutation blocked the ability of Dll1 to interact with Notch1. Finally, mutation of K613 did not affect the stability of Dll1 or its ability to traffic to recycle to the plasma membrane, but did enhance the fraction associated with lipid rafts. Collectively these results suggest that the transcellular defect in Notch signaling attributed to residue K613 in cytoplasmic tail of Dll1 may result from altering its multi-ubiquitination and increasing its retention in lipid rafts.

Notch post-translationally regulates β-catenin protein in stem and progenitor cells

Cellular decisions of self-renewal or differentiation arise from integration and reciprocal titration of numerous regulatory networks. Notch and Wnt/β-catenin signalling often intersect in stem and progenitor cells and regulate each other transcriptionally. The biological outcome of signalling through each pathway often depends on the context and timing as cells progress through stages of differentiation. Here, we show that membrane-bound Notch physically associates with unphosphorylated (active) β-catenin in stem and colon cancer cells and negatively regulates post-translational accumulation of active β-catenin protein. Notch-dependent regulation of β-catenin protein did not require ligand-dependent membrane cleavage of Notch or the glycogen synthase kinase-3β-dependent activity of the β-catenin destruction complex. It did, however, require the endocytic adaptor protein Numb and lysosomal activity. This study reveals a previously unrecognized function of Notch in negatively titrating active β-catenin protein levels in stem and progenitor cells.

Endocytosis and intracellular trafficking of Notch and its ligands

Notch signaling occurs through direct interaction between Notch, the receptor, and its ligands, presented on the surface of neighboring cells. Endocytosis has been shown to be essential for Notch signal activation in both signal-sending and signal-receiving cells, and numerous genes involved in vesicle trafficking have recently been shown to act as key regulators of the pathway. Defects in vesicle trafficking can lead to gain- or loss-of-function defects in a context-dependent manner. Here, we discuss how endocytosis and vesicle trafficking regulate Notch signaling in both signal-sending and signal-receiving cells. We will introduce the key players in different trafficking steps, and further illustrate how they impact the signal outcome. Some of these players act as general factors and modulate Notch signaling in all contexts, whereas others modulate signaling in a context-specific fashion. We also discuss Notch signaling during mechanosensory organ development in the fly to exemplify how endocytosis and vesicle trafficking are effectively used to determine correct cell fates. In summary, endocytosis plays an essential role in Notch signaling, whereas intracellular vesicle trafficking often plays a context-dependent or regulatory role, leading to divergent outcomes in different developmental contexts.

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.

Regulation of cell proliferation and differentiation in the kidney

The mammalian cut proteins are a broadly expressed family of nuclear transcription factors related to the Drosophila protein cut. One member of the cut family, Cux1, has been shown to function as a cell cycle dependent transcription factor, regulating the expression of a number of cell cycle regulatory proteins. Cux1 expression is developmentally regulated in multiple tissues suggesting an important regulatory function. Cux1 exists as multiple isoforms that arise from proteolytic processing of a 200 kD protein or use of an alternate promoter. Several mouse models of Cux1 have been generated that suggest important roles for this gene in cell cycle regulation during hair growth, lung development and maturation, and genitourinary tract development. Moreover, the aberrant expression of Cux1 may contribute to diseases such as polycystic kidney disease and cancer. In this review, we will focus on the phenotypes observed in the five existing transgenic mouse models of Cux1, and discuss the role of Cux1 in kidney development and disease.

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.

Structure and Notch receptor binding of the tandem WWE domain of Deltex

Deltex is a cytosolic effector of Notch signaling thought to bind through its N-terminal domain to the Notch receptor. Here we report the structure of the Drosophila Deltex N-terminal domain, which contains two tandem WWE sequence repeats. The WWE repeats, which adopt a novel fold, are related by an approximate two-fold axis of rotation. Although the WWE repeats are structurally distinct, they interact extensively and form a deep cleft at their junction that appears well suited for ligand binding. The two repeats are thermodynamically coupled; this coupling is mediated in part by a conserved segment that is immediately C-terminal to the second WWE domain. We demonstrate that although the Deltex WWE tandem is monomeric in solution, it forms a heterodimer with the ankyrin domain of the Notch receptor. These results provide structural and functional insight into how Deltex modulates Notch signaling, and how WWE modules recognize targets for ubiquitination.

Activation of Notch-mediated protective signaling in the myocardium

The Notch network regulates multiple cellular processes, including cell fate determination, development, differentiation, proliferation, apoptosis, and regeneration. These processes are regulated via Notch-mediated activity that involves hepatocyte growth factor (HGF)/c-Met receptor and phosphatidylinositol 3-kinase/Akt signaling cascades. The impact of HGF on Notch signaling was assessed following myocardial infarction as well as in cultured cardiomyocytes. Notch1 is activated in border zone cardiomyocytes coincident with nuclear c-Met following infarction. Intramyocardial injection of HGF enhances Notch1 and Akt activation in adult mouse myocardium. Corroborating evidence in cultured cardiomyocytes shows treatment with HGF or insulin increases levels of Notch effector Hes1 in immunoblots, whereas overexpression of activated Notch intracellular domain prompts a 3-fold increase in phosphorylated Akt. Infarcted hearts injected with adenoviral vector expressing Notch intracellular domain treatment exhibit improved hemodynamic function in comparison with control mice after 4 weeks, implicating Notch signaling in a cardioprotective role following cardiac injury. These results indicate Notch activation in cardiomyocytes is mediated through c-Met and Akt survival signaling pathways, and Notch1 signaling in turn enhances Akt activity. This mutually supportive crosstalk suggests a positive survival feedback mechanism between Notch and Akt signaling in adult myocardium following injury.

Tumor suppressor role of Notch-1 signaling in neuroendocrine tumors

A growing body of literature is demonstrating that Notch signaling is a more complex process than originally thought. Contradictory findings of notch-1 acting as an oncogene or a tumor suppressor revealed that its role is very specific to the cellular context. In this review we focus on the tumor suppressor role of Notch-1 signaling in neuroendocrine tumors (NETs) such as carcinoid and medullary thyroid cancers. NETs secrete various bioactive hormones that can cause debilitating symptoms. Surgery is the only potential curative treatment for the patients with NETs. Notch-1 signaling is absent in these tumors and activation of Notch-1 significantly reduces tumor growth in vitro. Therefore, identification of compound(s) that activate the Notch-1 pathway in NETs could be a potential strategy to treat patients with NETs.

Effect of presenilins in the apoptosis of thymocytes and homeostasis of CD8+ T cells

Many studies have positioned Notch signaling at various critical junctions during T-cell development. There is, however, debate regarding the role of Notch in the CD4 versus CD8 lineage commitment. Because there are 4 Notch receptors and RBP-Jkappa-independent Notch signaling has been reported, we decided to eliminate gamma-secretase activity once its activity is required for all forms of Notch signaling. T-cell-specific elimination of gamma-secretase was carried out by crossing presenilin-1 (PS1) floxed mice with CD4-Cre mice and PS2 KO mice, generating PS KO mice. Thymic CD4+CD8+ double-positive (DP) cells from these mice were strikingly resistant to apoptosis by anti-CD3 treatment in vivo and expressed more Bcl-X(L) than control thymocytes, and deletion of only one allele of Bcl-X(L) gene restored wild-type levels of sensitivity to apoptosis. In addition, these PS KO animals displayed a significant decrease in the number of CD8+ T cells in the periphery, and these cells had higher level of phosphorylated p38 than cells from control littermates. Our results show that ablation of presenilins results in deficiency of CD8 cells in the periphery and a dramatic change in the physiology of thymocytes, bringing to our attention the potential side effects of presenilin inhibitors in ongoing clinical trials.

Notch1 signaling regulates radial glia differentiation through multiple transcriptional mechanisms

Signaling by the Notch1 receptor is critical for the formation of radial glia in the developing nervous system. We have shown previously that Notch1 regulates the molecular and morphological differentiation of radial glia through the transcriptional activation of at least two genes, brain lipid binding protein (BLBP) and the erbB2 receptor tyrosine kinase. However, the mechanisms by which this occurs remained undefined. Here we demonstrate that Notch1 effects on radial glia gene expression are mediated by two downstream mechanisms, one that the depends on Suppressor of Hairless [Su(H)] and the other on Deltex1 (DTX1). These two Notch1-binding proteins contribute to the regulation of BLBP and erbB2 expression, respectively. Importantly, our results suggest that, although these events can occur simultaneously, a hierarchical relationship might exist between DTX1 and Su(H), because overexpression of DTX1 or a dominant-negative form of this protein inhibits Su(H)-mediated events but not vice versa. In contrast to the effects of DTX1 overexpression, interference RNA-mediated knock-down of DTX1 blocks Notch1-induced erbB2 promoter activation and radial glia formation selectively, without affecting Su(H)-dependent pathways, indicating that loss of DTX1 expression and expression of dominant-negative DTX1 result in different alterations in cell differentiation and gene expression. Together, these results show that Notch1 regulates radial glia formation through two distinct transcriptional mechanisms and that the outcomes of Notch1 signaling may depend on the relative expression levels of its coregulators.

Ligand-Dependent Cleavage of the P75 Neurotrophin Receptor Is Necessary for NRIF Nuclear Translocation and Apoptosis in Sympathetic Neurons

The p75 neurotrophin receptor regulates neuronal survival, promoting it in some contexts yet activating apoptosis in others. The mechanism by which the receptor elicits these differential effects is poorly understood. Here, we demonstrate that p75 is cleaved by gamma-secretase in sympathetic neurons, specifically in response to proapoptotic ligands. This cleavage resulted in ubiquitination and subsequent nuclear translocation of NRIF, a DNA binding protein essential for p75-mediated apoptosis. Inhibition of gamma-secretase or expression of a mutant p75 resistant to this protease prevented receptor proteolysis, blocked NRIF nuclear entry, and prevented apoptosis. In contrast, overexpression of the p75 ICD resulted in NRIF nuclear accumulation and apoptosis. The receptor proteolysis and NRIF nuclear localization were also observed in vivo during naturally occurring cell death in the superior cervical ganglia. These results indicate that p75-mediated apoptosis requires gamma-secretase dependent release of its ICD, which facilitates nuclear translocation of NRIF.

The Drosophila tumor suppressor vps25 prevents nonautonomous overproliferation by regulating notch trafficking

Cell-cell signaling coordinates proliferation of metazoan tissues during development, and its alteration can induce malignant transformation. Endocytosis regulates signaling by controlling the levels and activity of transmembrane receptors, both prior to and following ligand engagement. Here, we identify Vps25, a component of the ESCRT machinery that regulates endocytic sorting of signaling receptors, as an unconventional type of Drosophila tumor suppressor. vps25 mutant cells undergo autonomous neoplastic-like transformation, but they also stimulate nonautonomous cell proliferation. Endocytic trafficking defects in vps25 cells cause endosomal accumulation of the signaling receptor Notch and enhanced Notch signaling. Increased Notch activity leads to ectopic production of the mitogenic JAK-STAT pathway ligand Unpaired, which is secreted from mutant cells to induce overproliferation of the surrounding epithelium. Our data show that defects in endocytic sorting can both transform cells and, through heterotypic signaling, alter the behavior of neighboring wild-type tissue.

Endocytic control of epithelial polarity and proliferation in Drosophila

Intracellular protein transport is a key factor in epithelial cell polarity. Here we report that mutations in two core components of the vesicle trafficking machinery - a syntaxin and a Rab protein - cause an expansion of the apical membrane domain of Drosophila melanogaster epithelia; this polarity defect is coupled with overproliferation to form neoplastic tumours. Surprisingly, these proteins are associated with the endocytic, and not the exocytic, pathway. The syntaxin Avalanche (Avl) localizes to early endosomes, and loss of avl results in the cellular accumulation of specific membrane proteins, including the Notch signalling receptor and the polarity determinant Crumbs (Crb). Protein accumulation results from a failure in endosomal entry and progression towards lysosomal degradation; these and other avl phenotypes are also detected in Rab5 null mutant cells. Overexpression of Crb alone is sufficient to induce overproliferation of wild-type imaginal tissue, suggesting that polarity alterations in avl and Rab5 mutants directly contribute to tumour formation. Our findings reveal a critical and specific role for endocytic traffic in the control of both apico-basal polarity and cell proliferation.