Negative Regulator Of Notch Signaling Research Articles

BM-MSCs overexpressing the Numb enhance the therapeutic effect on cholestatic liver fibrosis by inhibiting the ductular reaction

BackgroundCholestatic liver fibrosis (CLF) is caused by inflammatory destruction of the intrahepatic bile duct and abnormal proliferation of the small bile duct after cholestasis. Activation of the Notch signaling pathway is required for hepatic stem cells to differentiate into cholangiocytes during the pathogenesis of CLF. Our previous research found that the expression of the Numb protein, a negative regulator of Notch signaling, was significantly reduced in the livers of patients with primary biliary cholangitis and CLF rats. However, the relationship between the Numb gene and CLF is largely unclear. In this study, we investigated the role of the Numb gene in the treatment of bile duct ligation (BDL)-induced CLF.MethodsIn vivo, bone marrow-derived mesenchymal stem cells (BM-MSCs) with Numb gene overexpression or knockdown obtained using lentivirus transfection were transplanted into the livers of rats with BDL-induced CLF. The effects of the Numb gene on stem cell differentiation and CLF were evaluated by performing histology, tests of liver function, and measurements of liver hydroxyproline, cytokine gene and protein levels. In vitro, the Numb gene was overexpressed or knocked down in the WB-F344 cell line by lentivirus transfection, Then, cells were subjected immunofluorescence staining and the detection of mRNA levels of related factors, which provided further evidence supporting the results from in vivo experiments.ResultsBM-MSCs overexpressing the Numb gene differentiated into hepatocytes, thereby inhibiting CLF progression. Conversely, BM-MSCs with Numb knockdown differentiated into biliary epithelial cells (BECs), thereby promoting the ductular reaction (DR) and the progression of CLF. In addition, we confirmed that knockdown of Numb in sodium butyrate-treated WB-F344 cells aggravated WB-F344 cell differentiation into BECs, while overexpression of Numb inhibited this process.ConclusionsThe transplantation of BM-MSCs overexpressing Numb may be a useful new treatment strategy for CLF.

Abstract 2896: DLL3-targeted T cell engager therapy (HPN328) for neuroendocrine prostate cancer

Abstract Background: Neuroendocrine Prostate Cancer (NEPC) is an aggressive variant of prostate cancer that can arise de novo or emerge during prostate cancer progression as a mechanism of therapy resistance. The development of effective therapies for patients with NEPC is a clinical unmet need. Delta-Like Ligand 3 (DLL3) is a negative regulator of Notch signaling and aberrantly expressed on the cell surface in the majority of NEPC and other small cell carcinomas. HPN328 is a DLL3-targeted Tri-specific T cell Activating Construct (TriTAC) that binds to DLL3 on tumor cells, CD3 on T cells, and human serum albumin. Here, we evaluated the specificity and anti-tumor activity of HPN328 in NEPC preclinical models. Methods: Immunohistochemistry (IHC) was used to evaluate DLL3 expression in prostate cancer preclinical models. We co-cultured DLL3-positive and DLL3-negative tumor cells in vitro with either human PBMCs or T cells and used CellTiter-Glo and live-cell imaging to examine tumor cell viability after the exposure to HPN328. We conducted flow cytometry to examine T cell activation. Patient-derived organoid xenografts were used to determine the anti-tumor activity of HPN328 in vivo and we applied digital spatial profiling (Nanostring) to examine effects on immune cell infiltration. Results: HPN328 suppressed cell growth and induced cell death in DLL3-positive prostate cancer organoid cell line models at a range of 0.7nM - 10nM with the presence of human T cells. Flow cytometry analysis illustrated that T cell activation markers, such as CD25, were upregulated in a dose- and time-dependent manner. In vivo studies demonstrated robust anti-tumor activity of HPN328 in causing tumor regression and stable disease after stopping the treatment in DLL3-positive models, but not in DLL3-negative models. Sustained anti-tumor effects were observed without adverse events in mice. Conclusions: The DLL3-targeted TriTAC HPN328 demonstrates robust and specific anti-tumor activity in DLL3-positive NEPC preclinical models in vitro and in vivo. T cell engagers targeting DLL3 represent a promising therapeutic strategy for NEPC. Acknowledgements: This research is funded by a Helen Trailblazers Award from the Helen Gurley Brown Foundation/DFCI Presidential Initiative (H.B.). Citation Format: Sheng-Yu Ku, Yasutaka Yamada, Patrick Ng, Laura Sun, Himisha Beltran. DLL3-targeted T cell engager therapy (HPN328) for neuroendocrine prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2896.

MiRNA-34c Suppresses Osteosarcoma Progression In Vivo by Targeting Notch and E2F.

ABSTRACTThe expression of microRNAs (miRNAs) is dysregulated in many types of cancers including osteosarcoma (OS) due to genetic and epigenetic alterations. Among these, miR‐34c, an effector of tumor suppressor P53 and an upstream negative regulator of Notch signaling in osteoblast differentiation, is dysregulated in OS. Here, we demonstrated a tumor suppressive role of miR‐34c in OS progression using in vitro assays and in vivo genetic mouse models. We found that miR‐34c inhibits the proliferation and the invasion of metastatic OS cells, which resulted in reduction of the tumor burden and increased overall survival in an orthotopic xenograft model. Moreover, the osteoblast‐specific overexpression of miR‐34c increased survival in the osteoblast specific p53 mutant OS mouse model. We found that miR‐34c regulates the transcription of several genes in Notch signaling (NOTCH1, JAG1, and HEY2) and in p53‐mediated cell cycle and apoptosis (CCNE2, E2F5, E2F2, and HDAC1). More interestingly, we found that the metastatic‐free survival probability was increased among a patient cohort from Therapeutically Applicable Research to Generate Effective Treatments (TARGET) OS, which has lower expression of direct targets of miR‐34c that was identified in our transcriptome analysis, such as E2F5 and NOTCH1. In conclusion, we demonstrate that miR‐34c is a tumor suppressive miRNA in OS progression in vivo. In addition, we highlight the therapeutic potential of targeting miR‐34c in OS. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

NRARP displays either pro- or anti-tumoral roles in T-cell acute lymphoblastic leukemia depending on Notch and Wnt signaling

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy with a dismal prognosis in patients with resistant or relapsed disease. Although NOTCH is a known driver in T-ALL, its clinical inhibition has significant limitations. Our previous studies suggested that NRARP, a negative regulator of Notch signaling, could have a suppressive role in T-ALL. Here, we report that NRARP levels are significantly increased in primary T-ALL cells suggesting that NRARP is not sufficient to block NOTCH oncogenic signals. Interestingly, although NRARP overexpression blocks NOTCH1 signaling and delays the proliferation of T-ALL cells that display high levels of Notch1 signaling, it promotes the expansion of T-ALL cells with lower levels of Notch1 activity. We found that NRARP interacts with lymphoid enhancer-binding factor 1 (LEF1) and potentiates Wnt signaling in T-ALL cells with low levels of Notch. Together these results indicate that NRARP plays a dual role in T-ALL pathogenesis, regulating both Notch and Wnt pathways, with opposite functional effects depending on Notch activity. Consistent with this hypothesis, mice transplanted with T-cells co-expressing NOTCH1 and NRARP develop leukemia later than mice transplanted with T-NOTCH1 cells. Importantly, mice transplanted with T-cells overexpressing NRARP alone developed leukemia with similar kinetics to those transplanted with T-NOTCH1 cells. Our findings uncover a role for NRARP in T-ALL pathogenesis and indicate that Notch inhibition may be detrimental for patients with low levels of Notch signaling, which would likely benefit from the use of Wnt signaling inhibitors. Importantly, our findings may extend to other cancers where Notch and Wnt play a role.

Ubiquitin ligase RNF8 suppresses Notch signaling to regulate mammary development and tumorigenesis.

The E3 ubiquitin ligase RNF8 plays critical roles in maintaining genomic stability by promoting the repair of DNA double-strand breaks (DSBs) through ubiquitin signaling. Abnormal activation of Notch signaling and defective repair of DSBs promote breast cancer risk. Here, we found that low expression of the full-length RNF8 correlated with poor prognosis for breast cancer patients. Our data revealed that in addition to its role in the repair of DSBs, RNF8 regulated Notch1 signaling and cell-fate determination of mammary luminal progenitors. Mechanistically, RNF8 acted as a negative regulator of Notch signaling by ubiquitylating the active NOTCH1 protein (N1ICD), leading to its degradation. Consistent with abnormal activation of Notch signaling and impaired repair of DSBs in Rnf8-mutant mammary epithelial cells, we observed increased risk of mammary tumorigenesis in mouse models for RNF8 deficiency. Notably, deficiency of RNF8 sensitized breast cancer cells to combination of pharmacological inhibitors of Notch signaling and poly(ADP-ribose) polymerase (PARP), suggesting implications for treatment of breast cancer associated with impaired RNF8 expression or function.

Abstract 2863: Dissecting the structural basis for inhibitors of RNA-binding proteins

Abstract RNA-binding proteins (RBPs) are key regulators of cellular functions, especially in post-transcriptional regulations. Dysregulation of RBPs is implicated in many diseases including cancer. One of the RBPs that is overexpressed in a variety of human cancer is Musashi-2 (MSI2). Elevated MSI2 expression is associated with ectopic oncogenic pathways, including but not limited to NUMB/Notch, PTEN/mTOR, TGF-β/SMAD3, making MSI2 a promising therapeutic target for cancer. In the case of NUMB, MSI2 binds to and negatively regulates translation of NUMB, a negative regulator of Notch signaling. Protein structure is critical for drug discovery and structure-based rational design. However, so far there is no structure available for MSI2 protein. Recently, using nuclear magnetic resonance (NMR) we solved the first solution structure of MSI2 RNA-recognition Motif 1 (MSI2-RRM1) that forms the major long narrow RNA-binding pocket. Using a fluorescence polarization (FP) chemical libraries screening, we identified several groups of hit compounds that disrupt the binding of MSI2-NUMB RNA potently. These compounds induced apoptosis, inhibited cancer cell proliferation, invasion and metastasis in multiple tumor models tested in vitro and in vivo. They also interfered with cancer stem cell functions with reduced tumorsphere formation. Several lead compounds showed promising efficacy in animal tumor models of human breast, prostate and colon cancer. To investigate the specificity of the compounds towards other RBPs, the compounds were tested in biophysical binding assays such as FP, time resolved Fluorescence Resonance Energy Transfer assay (TR-FRET), NMR spectroscopy and cell-based assays. We compared the affinity of these MSI2 inhibitors towards other RBPs such as MSI1 and HuR. Using docking and molecular dynamics simulation, we analyzed the structure-activity relationship of these inhibitors towards different RBP targets. This work adds significant information to the structure of MSI2-RRM1 and the structural basis for designing more potent and specific inhibitors of RBPs. Citation Format: Lan Lan, Minli Xing, Xiaoqing Wu, Philip Gao, Justin T. Douglas, Yu Zhan, Gulhumay Gardashova, Jiajun Liu, Robert P. Hanzlik, Jeffrey Aubé, Kristi L. Neufeld, Berl R. Oakley, Roberto N. De Guzman, Liang Xu. Dissecting the structural basis for inhibitors of RNA-binding proteins [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2863.

Stereotypical architecture of the stem cell niche is spatiotemporally established by miR-125-dependent coordination of Notch and steroid signaling.

ABSTRACTStem cell niches act as signaling platforms that regulate stem cell self-renewal and sustain stem cells throughout life; however, the specific developmental events controlling their assembly are not well understood. Here, we show that during Drosophila ovarian germline stem cell niche formation, the status of Notch signaling in the cell can be reprogrammed. This is controlled via steroid-induced miR-125, which targets a negative regulator of Notch signaling, Tom. Thus, miR-125 acts as a spatiotemporal coordinator between paracrine Notch and endocrine steroid signaling. Moreover, a dual security mechanism for Notch signaling activation exists to ensure the robustness of niche assembly. Particularly, stem cell niche cells can be specified either via lateral inhibition, in which a niche cell precursor acquires Notch signal-sending status randomly, or via peripheral induction, whereby Delta is produced by a specific cell. When one mechanism is perturbed due to mutations, developmental defects or environmental stress, the remaining mechanism ensures that the niche is formed, perhaps abnormally, but still functional. This guarantees that the germline stem cells will have their residence, thereby securing progressive oogenesis and, thus, organism reproduction.

Tumor suppressor properties of the splicing regulatory factor RBM10

ABSTRACTRBM10 is an RNA binding protein and alternative splicing regulator frequently mutated in lung adenocarcinomas. Recent results indicate that RBM10 inhibits proliferation of lung cancer cells by promoting skipping of exon 9 of the gene NUMB, a frequent alternative splicing change in lung cancer generating a negative regulator of Notch signaling. Complementing these observations, we show that knock down of RBM10 in human cancer cells enhances growth of mouse tumor xenografts, confirming that RBM10 acts as a tumor suppressor, while knock down of an oncogenic mutant version of RBM10 reduces xenograft tumor growth. A RBM10 mutation found in lung cancer cells, V354E, disrupts RBM10-mediated regulation of NUMB alternative splicing, inducing the cell proliferation-promoting isoform. We now show that 2 natural RBM10 isoforms that differ by the presence or absence of V354 in the second RNA Recognition Motif (RRM2), display similar regulatory effects on NUMB alternative splicing, suggesting that V354E actively disrupts RBM10 activity. Structural modeling localizes V354 in the outside surface of one α-helix opposite to the RNA binding surface of RBM10, and we show that the mutation does not compromise binding of the RRM2 domain to NUMB RNA regulatory sequences. We further show that other RBM10 mutations found in lung adenocarcinomas also compromise regulation of NUMB exon 9. Collectively, our previous and current results reveal that RBM10 is a tumor suppressor that represses Notch signaling and cell proliferation through the regulation of NUMB alternative splicing.

Aberrant expression of Notch1/numb/snail signaling, an epithelial mesenchymal transition related pathway, in adenomyosis.

BackgroundEpithelial mesenchymal transition (EMT) is involved in the pathogenesis of adenomyosis, and Notch signaling is crucial to EMT. The objective of this study was to explore Notch1/Numb/Snail signaling in adenomyosis.MethodsThe expression levels of the members of the Notch1/Numb/Snail signaling cascade in normal endometria (proliferative phase: n = 15; secretory phase: n = 15; postmenopausal phase: n = 15) and adenomyotic endometria (proliferative phase: n = 15; secretory phase: n = 15) were determined by immunohistochemistry analysis.ResultsWe found that the expressions of Notch1 and the EMT-related proteins N-cadherin, Snail and Slug were upregulated in the ectopic endometrium of adenomyosis compared with normal endometrium. Numb, a negative regulator of Notch signaling, was significantly decreased in adenomyosis. In addition, reduced immunoexpression of E-cadherin was observed in adenomyosis.ConclusionsWe conclude that Notch1/Numb/Snail signaling plays an important role in the pathogenesis and development of adenomyosis.

A Novel Role of Numb as A Regulator of Pro-inflammatory Cytokine Production in Macrophages in Response to Toll-like Receptor 4.

Activation of macrophages triggers the release of pro-inflammatory cytokines leading to inflammation. Numb is a negative regulator of Notch signaling, but the role of Numb in macrophages is not fully understood. In this study, the role of Numb as a regulator of inflammatory responses in macrophages was investigated. Murine bone marrow-derived macrophages, in which expression of Numb was silenced, secreted significantly less TNFα, IL-6 and IL-12 and more IL-10 upon activation by lipopolysaccharide (LPS), a ligand for Toll-like receptor 4 (TLR4), despite increased Notch signaling. The Tnfα mRNA levels both in Numb-deficient and wild-type macrophages were not significantly different, unlike those of Il6 and Il12-p40. In Numb-deficient macrophages, the Tnfα mRNAs were degraded at faster rate, compared to those in control macrophages. Activation of p38 MAPK and NF-κΒ p65 were compromised in activated Numb deficient macrophages. Numb was found to interact with the E3 ubiquitin ligase, Itch, which reportedly regulates p38 MAPK. In addition, blocking the Notch signaling pathway in activated, Numb-deficient macrophages did not further reduce TNFα levels, suggesting a Notch-independent role for Numb. A proteomics approach revealed a novel funciton for Numb in regulating complex signaling cascades downstream of TLRs, partially involving Akt/NF-κB p65/p38 MAPK in macrophages.

An oncogenic enhancer enemy (N-Me) in T-ALL

Constitutive activation of NOTCH signaling plays a central role in the pathogenesis of over 60% of T-cell acute lymphoblastic leukemias (T-ALL) harboring activating mutations in NOTCH1 or disruption of FBXW7, a critical negative regulator of NOTCH signaling.1 Expression of oncogenic NOTCH1 drives T-cell transformation activating a transcriptional program that includes multiple genes promoting leukemia cell growth and proliferation. Among these, the MYC oncogene has been highlighted as an important mediator of NOTCH1 induced transformation as NOTCH1 signaling induces transcriptional upregulation of MYC expression and NOTCH1 and MYC share multiple common target genes involved in leukemia cell growth.2 Still, demonstrating direct regulation of MYC by NOTCH1 has remained elusive as NOTCH1 fails to activate MYC promoter activity in reporter assays. This enigmatic link between NOTCH1 and MYC in T-ALL has been now clarified with the identification of N-Me (for NOTCH MYC enhancer), a long-range distal enhancer controlling MYC expression and T-cell transformation downstream of NOTCH1 (Fig. 1).3 The N-Me enhancer is located +1,427 kb from MYC in a large gene desert region extending telomeric from the MYC locus and represents the strongest peak of NOTCH1 binding in ChIP-seq analysis of T-ALL cells. Notably, and despite its distal genomic location, this regulatory element directly interacts with and controls MYC promoter activity as demonstrated by chromatin configuration 3C analysis and reporter assays. The functional importance of N-Me in the control of MYC expression is further supported by multispecies DNA sequence homology analyses which revealed remarkable conservation of this region in mammals, birds and reptiles. In addition, analysis of functional chromatin marks showed bona fide active enhancer features including P300 occupancy, high levels of H3K4me1 and low levels of H3K4me3 associated with N-Me. Moreover, detailed epigenetic profiling across 64 haematopoietic and non-haematopoietic cell lines and tissues revealed that this regulatory element is located within a major super-enhancer specifically active in T-cells. Consistently with these results, analysis of N-Me knockout and conditional knockout mice demonstrated a critical role for this long-range enhancer in T-cell development and NOTCH1-induced T-ALL.3 Thus, N-Me deficient mice showed marked thymic atrophy with depletion of CD4 CD8 double positive thymocytes. This phenotype was cell autonomous and was associated with decreased Myc expression in DN3, DN4 and ISP T-cell progenitors. Importantly, these T-cell developmental defects recapitulate those observed in T-cell specific Myc knockout mice,4 and were rescued by retroviral expression of MYC, formally demonstrating a strict requirement for N-Me mediated Myc expression in T-cell development. In addition, the NOTCH-N-Me-Myc regulatory axis showed to be essential for NOTCH1 induced T-ALL. Thus, N-Me knockout bone marrow progenitor cells failed to generate leukemia upon retroviral expression of oncogenic NOTCH1, and secondary deletion of N-Me in fully established NOTCH1-induced T-ALL resulted in profound antileukemic effects and almost complete suppression of leukemia initiating cell activity. Strikingly, loss of one copy of N-Me showed intermediate phenotypes including partial reduction of thymocyte numbers; reduced penetrance of T-ALL upon oncogenic NOTCH1 activation; and delayed tumor progression of fully established NOTCH1-driven tumors. These results highlight the exact dose-dependent function of the NOTCH1-NMe-Myc regulatory circuitry. Finally, genomic profiling of human leukemias identified recurrent focal duplications encompassing the N-Me enhancer in T-ALL supporting that increased N-Me dosage may contribute to T-cell transformation by strengthening NOTCH1 driven MYC expression. Figure 1. Schematic representation of N-Me activity during thymocyte development and in NOTCH1-induced T-ALL. Overall our results highlight the importance of long-range enhancers involved in the control of MYC expression in the pathogenesis of human cancer and are in line with previous results in myeloid leukemia and colon cancer.5,6 Still, the detailed characterization of the N-Me enhancer opens new outstanding questions that remain to be addressed. Most intriguingly, NOTCH1 and MYC play important developmental roles in multiple tissues other than T-cells and NOTCH signaling regulation of MYC expression has been implicated in the pathogenesis of breast cancer and other solid tumors;7,8 however N-Me seems to be strictly T-cell specific, suggesting that other as yet unidentified NOTCH-controlled MYC enhancers may be responsible for MYC expression outside T-cells. In addition, it is unclear what factors drive the T-cell specific activity of the N-Me enhancer. Finally, it has not escaped our attention that our identification of N-Me duplications in T-ALL support a potentially important and broader role for chromosomal copy number alterations and mutations occurring in intergenic regulatory elements in the pathogenesis of human cancer.

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

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

TRAF6 is a novel regulator of Notch signaling in Drosophila melanogaster

Notch signaling pathway unravels a fundamental cellular communication system that plays an elemental role in development. It is evident from different studies that the outcome of Notch signaling depends on signal strength, timing, cell type, and cellular context. Since Notch signaling affects a spectrum of cellular activity at various developmental stages by reorganizing itself in more than one way to produce different intensities in the signaling output, it is important to understand the context dependent complexity of Notch signaling and different routes of its regulation. We identified, TRAF6 (Drosophila homolog of mammalian TRAF6) as an interacting partner of Notch intracellular domain (Notch-ICD). TRAF6 genetically interacts with Notch pathway components in trans-heterozygous combinations. Immunocytochemical analysis shows that TRAF6 co-localizes with Notch in Drosophila third instar larval tissues. Our genetic interaction data suggests that the loss-of-function of TRAF6 leads to the rescue of previously identified Kurtz–Deltex mediated wing notching phenotype and enhances Notch protein survival. Co-expression of TRAF6 and Deltex results in depletion of Notch in the larval wing discs and down-regulates Notch targets, Wingless and Cut. Taken together, our results suggest that TRAF6 may function as a negative regulator of Notch signaling.

The WD40-repeat Proteins WDR-20 and WDR-48 Bind and Activate the Deubiquitinating Enzyme USP-46 to Promote the Abundance of the Glutamate Receptor GLR-1 in the Ventral Nerve Cord of Caenorhabditis elegans

Ubiquitin-mediated endocytosis and degradation of glutamate receptors controls their synaptic abundance and is implicated in modulating synaptic strength. The deubiquitinating enzymes (DUBs) that function in the nervous system are beginning to be defined, but the mechanisms that control DUB activity in vivo are understood poorly. We found previously that the DUB USP-46 deubiquitinates the Caenorhabditis elegans glutamate receptor GLR-1 and prevents its degradation in the lysosome. The WD40-repeat (WDR) proteins WDR20 and WDR48/UAF1 have been shown to bind to USP46 and stimulate its catalytic activity in other systems. Here we identify the C. elegans homologs of these WDR proteins and show that C. elegans WDR-20 and WDR-48 can bind and stimulate USP-46 catalytic activity in vitro. Overexpression of these activator proteins in vivo increases the abundance of GLR-1 in the ventral nerve cord, and this effect is further enhanced by coexpression of USP-46. Biochemical characterization indicates that this increase in GLR-1 abundance correlates with decreased levels of ubiquitin-GLR-1 conjugates, suggesting that WDR-20, WDR-48, and USP-46 function together to deubiquitinate and stabilize GLR-1 in neurons. Overexpression of WDR-20 and WDR-48 results in alterations in locomotion behavior consistent with increased glutamatergic signaling, and this effect is blocked in usp-46 loss-of-function mutants. Conversely, wdr-20 and wdr-48 loss-of-function mutants exhibit changes in locomotion behavior that are consistent with decreased glutamatergic signaling. We propose that WDR-20 and WDR-48 form a complex with USP-46 and stimulate the DUB to deubiquitinate and stabilize GLR-1 in vivo.

The RNA-Binding Protein Musashi-1 Regulates Proteasome Subunit Expression in Breast Cancer- and Glioma-Initiating Cells

Cancer stem cells (CSCs) or tumor-initiating cells, similar to normal tissue stem cells, rely on developmental pathways, such as the Notch pathway, to maintain their stem cell state. One of the regulators of the Notch pathway is Musashi-1, a mRNA-binding protein. Musashi-1 promotes Notch signaling by binding to the mRNA of Numb, the negative regulator of Notch signaling, thus preventing its translation. CSCs have also been shown to downregulate their 26S proteasome activity in several types of solid tumors, thus making them resistant to proteasome-inhibitors used as anticancer agents in the clinic. Interestingly, the Notch pathway can be inhibited by proteasomal degradation of the Notch intracellular domain (Notch-ICD); therefore, downregulation of the 26S proteasome activity can lead to stabilization of Notch-ICD. Here, we present evidence that the downregulation of the 26S proteasome in CSCs constitutes another level of control by which Musashi-1 promotes signaling through the Notch pathway and maintenance of the stem cell phenotype of this subpopulation of cancer cells. We demonstrate that Musashi-1 mediates the downregulation of the 26S proteasome by binding to the mRNA of NF-YA, the transcriptional factor regulating 26S proteasome subunit expression, thus providing an additional route by which the degradation of Notch-ICD is prevented, and Notch signaling is sustained.

Structure and Function of the CSL-KyoT2 Corepressor Complex: A Negative Regulator of Notch Signaling

Notch refers to a highly conserved cell-to-cell signaling pathway with essential roles in embryonic development and tissue maintenance. Dysfunctional signaling causes human disease, highlighting the importance of pathway regulation. Notch signaling ultimately results in the activation of target genes, which is regulated by the nuclear effector CSL (CBF-1/RBP-J, Su(H), Lag-1). CSL dually functions as an activator and a repressor of transcription through differential interactions with coactivator orcorepressor proteins, respectively. Although the structures of CSL-coactivator complexes have been determined, the structures of CSL-corepressor complexes are unknown. Here, using a combination of structural, biophysical, and cellular approaches, we characterize the structure and function of CSL in complex with the corepressor KyoT2. Collectively, our studies provide molecular insights into how KyoT2 binds CSL with high affinity and competes with coactivators, such as Notch, for binding CSL. These studies are important for understanding how CSL functions as both an activator and a repressor of transcription of Notch target genes.

The ascidian numb gene involves in the formation of neural tissues.

Notch signaling plays fundamental roles in various animal development. It has been suggested that Hr-Notch, a Notch homologue in the ascidian Halocynthia roretzi, is involved in the formation of peripheral neurons by suppressing the neural fates and promoting the epidermal differentiation. However, roles of Notch signaling remain controversial in the formation of nervous system in ascidian embryos. To precisely investigate functions of Notch signaling, we have isolated and characterized Hr-Numb, a Numb homologue which is a negative regulator of Notch signaling, in H. roretzi. Maternal expression of Hr-Numb mRNAs was detected in egg cytoplasm and the transcripts were inherited by the animal blastomeres. Its zygotic expression became evident by the early neurula stage and the transcripts were detected in dorsal neural precursor cells. Suppression of Hr-Numb function by an antisense morpholino oligonucleotide resulted in larvae with defect in brain vesicle and palps formation. Similar results have been obtained by overexpression of the constitutively activated Hr-Notch forms. Therefore, these results suggest that Hr-Numb is involved in Notch signaling during ascidian embryogenesis.

Genetic identification of intracellular trafficking regulators involved in Notch-dependent binary cell fate acquisition following asymmetric cell division

Notch signalling is involved in numerous cellular processes during development and throughout adult life. Although ligands and receptors are largely expressed in the whole organism, activation of Notch receptors only takes place in a subset of cells and/or tissues and is accurately regulated in time and space. Previous studies have demonstrated that endocytosis and recycling of both ligands and/or receptors are essential for this regulation. However, the precise endocytic routes, compartments and regulators involved in the spatiotemporal regulation are largely unknown. In order to identify intracellular trafficking regulators of Notch signalling, we have undertaken a tissue-specific dsRNA genetic screen of candidates potentially involved in endocytosis and recycling within the endolysosomal pathway. dsRNA against 418 genes was induced in the Drosophila melanogaster sensory organ lineage in which Notch signalling regulates binary cell fate acquisition. Gain or loss of Notch signalling phenotypes were observed in adult sensory organs for 113 of them. Furthermore, 26 genes were found to regulate the steady state localisation of Notch, Sanpodo, a Notch co-factor, and/or Delta in the pupal lineage. In particular, we identified 20 genes with previously unknown function in D. melanogaster intracellular trafficking. Among them, we identified CG2747 and we show that it regulates the localisation of clathrin adaptor AP-1 complex, a negative regulator of Notch signalling. Together, our results further demonstrate the essential function of intracellular trafficking in regulating Notch-signalling-dependent binary cell fate acquisition and constitute an additional step toward the elucidation of the routes followed by Notch receptor and ligands during signalling.

The Ubiquitin-specific Protease 12 (USP12) Is a Negative Regulator of Notch Signaling Acting on Notch Receptor Trafficking toward Degradation

Notch signaling is critical for development and adult tissue physiology, controlling cell fate in a context-dependent manner. Upon ligand binding, the transmembrane Notch receptor undergoes two ordered proteolytic cleavages releasing Notch intracellular domain, which regulates the transcription of Notch target genes. The strength of Notch signaling is of crucial importance and depends notably on the quantity of Notch receptor at the cell surface. Using an shRNA library screen monitoring Notch trafficking and degradation in the absence of ligand, we identified mammalian USP12 and its Drosophila melanogaster homolog as novel negative regulators of Notch signaling. USP12 silencing specifically interrupts Notch trafficking to the lysosomes and, as a consequence, leads to an increased amount of receptor at the cell surface and to a higher Notch activity. At the biochemical level, USP12 with its activator UAF1 deubiquitinate the nonactivated form of Notch in cell culture and in vitro. These results characterize a new level of conserved regulation of Notch signaling by the ubiquitin system.

MiR-34a Represses Numbl in Murine Neural Progenitor Cells and Antagonizes Neuronal Differentiation

MicroRNA (miRNA) function is required for normal animal development, in particular in differentiation pathways from stem cell and precursor populations. In neurogenesis, it is becoming increasingly appreciated that miRNAs act at many stages to ensure proper progression. In this study we examined the role of miR-34a in neural progenitor cells (NPC) derived from murine embryonic cortex. We found that over-expression of miR-34a in NPC significantly reduced the neuron yield upon in vitro induction of differentiation. MiR-34a has several predicted targets in the Notch pathway, which operates to balance progenitor self-renewal and differentiation during cortical neurogenesis. We tested several Notch pathway players for regulation by miR-34a in undifferentiated NPC, and found that mRNA and protein levels of Numbl, a negative regulator of Notch signaling, as well as two downstream pro-neural genes usually blocked by Notch signaling, NeuroD1 and Mash1, were diminished, while Notch1 and Cbf1 transcripts were enhanced by miR-34a over-expression. Using a luciferase reporter assay, we verified the Numbl 3′-UTR as a direct miR-34a target. Correspondingly, knock-down of endogenous miR-34a resulted in increased Numbl, NeuroD1 and Mash1, and reduced Notch1 transcript levels. Together these results implicate Numbl as a physiologically relevant target of miR-34a in NPC, allowing for enhanced Notch signaling and inhibition of neuronal differentiation. This work extends our understanding of miR-34a-mediated control of cell differentiation from cancer to mammalian nervous system development.