Notch ligand endocytosis: mechanistic basis of signaling activity

The Jagged-1/Notch1 Signaling Pathway Promotes the Construction of Tissue-Engineered Heart Valves

Background: Tissue-engineered heart valves (TEHVs) are promising new heart valve substitutes for valvular heart disease. The Notch signaling pathway plays a critical role in the development of congenital heart valves. Objective: To investigate the role of the Notch signaling pathway in the construction of TEHVs. Methods: The induced endothelial cells, which act as seed cells, were differentiated from adipose-derived stem cells and were treated with Jagged-1 (JAG-1) protein and γ-secretase inhibitor (DAPT, N-[N-(3,5-difluorophenacetyl)-l-alanyl]-s-phenylglycine t-butyl ester), respectively. Cell phenotypic changes, the expression of proteins relating to the epithelial-mesenchymal transition (EMT), and changes in paxillin expression were detected. Decellularized valve scaffolds were produced from decellularized porcine aortic valves. The seed cells were them inoculated into Matrigel-coated flap scaffolds for complex culture and characterization. Results: JAG-1 significantly reduced apoptosis and promoted the seeded cells' proliferation and migration ability, in contrast to the treatment of DAPT. In addition, the expression of EMT-related proteins, E-cadherin and N-cadherin, was significantly increased after treatment with JAG-1 and was reduced after the application of DAPT. Meanwhile, the adhesive-related expression of paxillin and fibronectin proteins was increased after the activation of Notch1 signaling and vice versa. Of interest, activation of the Notch1 signaling pathway resulted in more closely arranged cells on the valve surface after recellularization. Conclusion: Activation of the JAG-1/Notch1 signaling pathway increased seeded cells' proliferation and migratory ability and promoted the EMT and adhesion of seed cells, which was conducive to binding to the matrix, facilitating accelerated endothelialization of TEHVs.

Structural Requirements for Activity of Mind bomb1 in Notch Signaling

Mind bomb 1 (MIB1) is a RING E3 ligase that ubiquitinates Notch ligands, a necessary step for induction of Notch signaling. The structural basis for binding of the JAG1 ligand by the N-terminal region of MIB1 is known, yet how the ankyrin (ANK) and RING domains of MIB1 cooperate to catalyze ubiquitin transfer from E2~Ub to Notch ligands remains unclear. Here, we show that the third RING domain and adjacent coiled coil region of MIB1 (ccRING3) drives MIB1 dimerization and that ubiquitin transfer activity of MIB1 relies solely on RING3. We report x-ray crystal structures of a UbcH5B-ccRING3 complex as a fusion protein and of the ANK region. Directly tethering the N-terminal region to ccRING3 forms a minimal MIB1 protein, which is sufficient to induce a Notch response in receiver cells. Together, these studies define the functional elements of an E3 ligase needed for ligands to induce a Notch signaling response.

BioE3 identifies specific substrates of ubiquitin E3 ligases

Hundreds of E3 ligases play a critical role in recognizing specific substrates for modification by ubiquitin (Ub). Separating genuine targets of E3s from E3-interactors remains a challenge. We present BioE3, a powerful approach for matching substrates to Ub E3 ligases of interest. Using BirA-E3 ligase fusions and bioUb, site-specific biotinylation of Ub-modified substrates of particular E3s facilitates proteomic identification. We show that BioE3 identifies both known and new targets of two RING-type E3 ligases: RNF4 (DNA damage response, PML bodies), and MIB1 (endocytosis, autophagy, centrosome dynamics). Versatile BioE3 identifies targets of an organelle-specific E3 (MARCH5) and a relatively uncharacterized E3 (RNF214). Furthermore, BioE3 works with NEDD4, a HECT-type E3, identifying new targets linked to vesicular trafficking. BioE3 detects altered specificity in response to chemicals, opening avenues for targeted protein degradation, and may be applicable for other Ub-likes (UbLs, e.g., SUMO) and E3 types. BioE3 applications shed light on cellular regulation by the complex UbL network.

SNORA73B promotes endometrial cancer progression through targeting MIB1 and regulating host gene RCC1 alternative splicing

Endometrial cancer (EC) is a common gynaecological malignant tumour with unclear pathogenesis. Small nucleolar RNA (snoRNA) is involved in many biological processes, including those of cancers. Using the Cancer Genome Atlas (TCGA) database, the expression pattern of a snoRNA, SNORA73B, was analysed. The biological functions of SNORA73B were assessed by in vitro proliferation, apoptosis, migration, and invasion assays and in vivo by the xenograft model. RNA sequencing (RNA-seq) and RNA immunoprecipitation assays were performed to determine the relationship between SNORA73B and its target genes. High-performance liquid chromatography (HPLC) was performed to detect the pseudouridine content of the mindbomb E3 ubiquitin protein ligase 1 gene (MIB1). The stability of MIB1 mRNA was evaluated using a transcription inhibitor, actinomycin D. By performing co-immunoprecipitation assays, the change in the ubiquitin levels of the Jagged canonical Notch ligand 1 (Jag 1), caused by SNORA73B and MIB1, was identified. RNA-seq and qRT-PCR were performed to detect the alternative splicing of the regulator of the chromosome condensation 1 gene (RCC1). The TCGA database analysis showed that SNORA73B was highly expressed in EC. SNORA73B promoted cell proliferation, migration, and invasion and inhibited apoptosis. SNORA73B modified the pseudouridine content in MIB1 and increased the stability of MIB1 mRNA and protein; thus, it affected Jag 1 ubiquitination and further activated the Notch pathway. SNORA73B also affected the alternative splicing of RCC1, increasing the number of transcripts, RCC1-T2 and RCC1-T3, which promoted cell proliferation, migration, and invasion. SNORA73B can be a potential target for EC.

Novel Association of the NOTCH Pathway Regulator MIB1 Gene With the Development of Bicuspid Aortic Valve

Importance

Nonsyndromic bicuspid aortic valve (nsBAV) is the most common congenital heart valve malformation. BAV has a heritable component, yet only a few causative genes have been identified; understanding BAV genetics is a key point in developing personalized medicine.

Objective

To identify a new gene for nsBAV.

Design, Setting, and Participants

This was a comprehensive, multicenter, genetic association study based on candidate gene prioritization in a familial cohort followed by rare and common association studies in replication cohorts. Further validation was done using in vivo mice models. Study data were analyzed from October 2019 to October 2022. Three cohorts of patients with BAV were included in the study: (1) the discovery cohort was a large cohort of inherited cases from 29 pedigrees of French and Israeli origin; (2) the replication cohort 1 for rare variants included unrelated sporadic cases from various European ancestries; and (3) replication cohort 2 was a second validation cohort for common variants in unrelated sporadic cases from Europe and the US.

Main Outcomes and Measures

To identify a candidate gene for nsBAV through analysis of familial cases exome sequencing and gene prioritization tools. Replication cohort 1 was searched for rare and predicted deleterious variants and genetic association. Replication cohort 2 was used to investigate the association of common variants with BAV.

Results

A total of 938 patients with BAV were included in this study: 69 (7.4%) in the discovery cohort, 417 (44.5%) in replication cohort 1, and 452 (48.2%) in replication cohort 2. A novel human nsBAV gene, MINDBOMB1 homologue MIB1, was identified. MINDBOMB1 homologue (MIB1) is an E3-ubiquitin ligase essential for NOTCH-signal activation during heart development. In approximately 2% of nsBAV index cases from the discovery and replication 1 cohorts, rare MIB1 variants were detected, predicted to be damaging, and were significantly enriched compared with population-based controls (2% cases vs 0.9% controls; P = .03). In replication cohort 2, MIB1 risk haplotypes significantly associated with nsBAV were identified (permutation test, 1000 repeats; P = .02). Two genetically modified mice models carrying Mib1 variants identified in our cohort showed BAV on a NOTCH1-sensitized genetic background.

Conclusions and Relevance

This genetic association study identified the MIB1 gene as associated with nsBAV. This underscores the crucial role of the NOTCH pathway in the pathophysiology of BAV and its potential as a target for future diagnostic and therapeutic intervention.

Notch signaling as a master regulator of adult neurogenesis

Neurogenesis ceases in most regions of the mammalian brain before or shortly after birth, however, in a few restricted brain regions, the production of new neurons proceeds into adulthood. Neural stem cells (NSCs) in these neurogenic zones are integrated into niches that control their activity and fate. Most stem cells in the adult brain are mitotically inactive and these cells can remain quiescent for months or even years. One of the key questions is what are the molecular mechanisms that regulate NSC maintenance and differentiation. Notch signaling has been shown to be a critical regulator of stem cell activity and maintenance in many tissues including in the nervous system. In this mini-review we discuss the roles of Notch signaling and the functions of the different Notch receptors and ligands in regulating neurogenesis in the adult murine brain. We review the functions of Notch signaling components in controlling NSC quiescence and entry into cell cycle and neurogenesis.

Mechanical compression creates a quiescent muscle stem cell niche

Tissue stem cell niches are regulated by their mechanical environment, notably the extracellular matrix (ECM). Skeletal muscles consist of bundled myofibers for force transmission. Within this macroscopic architecture, quiescent Pax7-expressing (Pax7⁺) muscle stem cells (MuSCs) are compressed between ECM basally and myofiber apically. Muscle injury causes MuSCs to lose apical compression from the myofiber and re-enter the cell cycle for regeneration. While ECM elasticities have been shown to affect MuSC's renewal, the significance of apical compression remains unknown. To investigate the role of apical compression, we simulate the MuSCs' in vivo mechanical environment by applying physical compression to MuSCs' apical surface. We demonstrate that compression drives activated MuSCs back to a quiescent stem cell state, regardless of basal elasticities and chemistries. By mathematical modeling and cell tension manipulation, we conclude that low overall tension combined with high axial tension generated by compression leads to MuSCs' stemness and quiescence. Unexpectedly, we discovered that apical compression results in up-regulation of Notch downstream genes, accompanied by the increased levels of nuclear Notch1&3 in a Delta ligand (Dll) and ADAM10/17 independent manner. Our results fill a knowledge gap on the role of apical compression for MuSC fate and have implications to stem cells in other tissues.

The role of Notch signaling pathway in metabolic bone diseases

Metabolic bone diseases is the third most common endocrine diseases after diabetes and thyroid diseases. More than 500 million people worldwide suffer from metabolic bone diseases. The generation and development of bone metabolic diseases is a complex process regulated by multiple signaling pathways, among which the Notch signaling pathway is one of the most important pathways. The Notch signaling pathway regulates the differentiation and function of osteoblasts and osteoclasts, and affects the process of cartilage formation, bone formation and bone resorption. Genetic mutations in upstream and downstream of Notch signaling genes can lead to a series of metabolic bone diseases, such as Alagille syndrome, Adams-Oliver syndrome and spondylocostal dysostosis. In this review, we analyzed the mechanisms of Notch ligands, Notch receptors and signaling molecules in the process of signal transduction, and summarized the progress on the pathogenesis and clinical manifestations of bone metabolic diseases caused by Notch gene mutation. We hope to draw attention to the role of the Notch signaling pathway in metabolic bone diseases and provide new ideas and approaches for the diagnosis and treatment of metabolic bone diseases.

The structure, binding and function of a Notch transcription complex involving RBPJ and the epigenetic reader protein L3MBTL3

The Notch pathway transmits signals between neighboring cells to elicit downstream transcriptional programs. Notch is a major regulator of cell fate specification, proliferation, and apoptosis, such that aberrant signaling leads to a pleiotropy of human diseases, including developmental disorders and cancers. The pathway signals through the transcription factor CSL (RBPJ in mammals), which forms an activation complex with the intracellular domain of the Notch receptor and the coactivator Mastermind. CSL can also function as a transcriptional repressor by forming complexes with one of several different corepressor proteins, such as FHL1 or SHARP in mammals and Hairless in Drosophila. Recently, we identified L3MBTL3 as a bona fide RBPJ-binding corepressor that recruits the repressive lysine demethylase LSD1/KDM1A to Notch target genes. Here, we define the RBPJ-interacting domain of L3MBTL3 and report the 2.06 Å crystal structure of the RBPJ-L3MBTL3-DNA complex. The structure reveals that L3MBTL3 interacts with RBPJ via an unusual binding motif compared to other RBPJ binding partners, which we comprehensively analyze with a series of structure-based mutants. We also show that these disruptive mutations affect RBPJ and L3MBTL3 function in cells, providing further insights into Notch mediated transcriptional regulation.

Functional Comparison between Endogenous and Synthetic Notch Systems

The Notch pathway converts receptor-ligand interactions at the cell surface into a transcriptional response in the receiver cell. In recent years, synthetic Notch systems (synNotch) that respond to different inputs and transduce different transcriptional responses have been engineered. One class of synNotch systems uses antibody-antigen interactions at the cell surface to induce the proteolytic cleavage cascade of the endogenous Notch autoregulatory core and the consequent release of a synNotch intracellular domain (ICD), converting surface antigen detection into a cellular response. While the activation of endogenous Notch requires ubiquitylation and subsequent endocytosis of the ligand ICD, these synNotch systems do not seem to have such a requirement because the synNotch ligands completely lack an ICD. This observation raises questions about existing models for the synNotch activation mechanism. Here, we test how different structural and biochemical factors affect the dependence of endogenous and synthetic Notch activation on ligand ICD. We compare the behavior of antibody-antigen synNotch (aa-synNotch) to that of endogenous Notch, and to a synNotch system that uses rapamycin induced dimerization of FK506 binding protein (FKBP) and FKBP rapamycin binding (FRB) domaindimerization domains (ff-synNotch), which still requires a ligand ICD. We found that differences in receptor-ligand affinity, in the identity of the transmembrane domain, or in the presence or absence of extracellular epidermal growth factor repeats cannot explain the differences in ligand ICD requirement that distinguishes aa-synNotch from endogenous Notch or ff-synNotch. We also found that unlike endogenous Notch and ff-synNotch, the aa-synNotch system does not exhibit trans-endocytosis of the receptor extracellular domain into the sender cell. These findings suggest that the aa-synNotch systems bypass the ligand ICD requirement because antigen-antibody pairs are able to promote other adhesive cell-cell interactions that provide the mechanical tension needed for ligand activation.

Intrinsically disordered protein regions are required for cell wall homeostasis in Bacillus subtilis

Intrinsically disordered protein regions (IDRs) have been implicated in diverse nuclear and cytoplasmic functions in eukaryotes, but their roles in bacteria are less clear. Here, we report that extracytoplasmic IDRs in Bacillus subtilis are required for cell wall homeostasis. The B. subtilis σ^I transcription factor is activated in response to envelope stress through regulated intramembrane proteolysis (RIP) of its membrane-anchored anti-σ factor, RsgI. Unlike canonical RIP pathways, we show that ectodomain (site-1) cleavage of RsgI is constitutive, but the two cleavage products remain stably associated, preventing intramembrane (site-2) proteolysis. The regulated step in this pathway is their dissociation, which is triggered by impaired cell wall synthesis and requires RsgI's extracytoplasmic IDR. Intriguingly, the major peptidoglycan polymerase PBP1 also contains an extracytoplasmic IDR, and we show that this region is important for its function. Disparate IDRs can replace the native IDRs on both RsgI and PBP1, arguing that these unstructured regions function similarly. Our data support a model in which the RsgI-σ^I signaling system and PBP1 represent complementary pathways to repair gaps in the PG meshwork. The IDR on RsgI senses these gaps and activates σ^I, while the IDR on PBP1 directs the synthase to these sites to fortify them.

A Drosophila Su(H) model of Adams-Oliver Syndrome reveals cofactor titration as a mechanism underlying developmental defects

Notch signaling is a conserved pathway that converts extracellular receptor-ligand interactions into changes in gene expression via a single transcription factor (CBF1/RBPJ in mammals; Su(H) in Drosophila). In humans, RBPJ variants have been linked to Adams-Oliver syndrome (AOS), a rare autosomal dominant disorder characterized by scalp, cranium, and limb defects. Here, we found that a previously described Drosophila Su(H) allele encodes a missense mutation that alters an analogous residue found in an AOS-associated RBPJ variant. Importantly, genetic studies support a model that heterozygous Drosophila with the AOS-like Su(H) allele behave in an opposing manner to heterozygous flies with a Su(H) null allele, due to a dominant activity of sequestering either the Notch co-activator or the antagonistic Hairless co-repressor. Consistent with this model, AOS-like Su(H) and Rbpj variants have decreased DNA binding activity compared to wild type proteins, but these variants do not significantly alter protein binding to the Notch co-activator or the fly and mammalian co-repressors, respectively. Taken together, these data suggest a cofactor sequestration mechanism underlies AOS phenotypes associated with RBPJ variants, whereby the AOS-associated RBPJ allele encodes a protein with compromised DNA binding activity that retains cofactor binding, resulting in Notch target gene dysregulation.

Adams-Oliver Syndrome (AOS) is a rare disease defined by missing skin/skull tissue, limb malformations, and cardiovascular abnormalities. Human genetic studies have revealed that ~40% of AOS patients inherit dominant mutations within specific genes in the Notch signaling pathway. Notch signaling is a highly conserved cell-to-cell communication pathway found in all metazoans and plays crucial roles during embryogenesis and tissue homeostasis in organisms from Drosophila (fruit-flies) to mammals. The Notch receptor converts cell-to-cell interactions into a Notch signal that enters the nucleus and activates target genes by binding to a highly conserved transcription factor. Here, we took advantage of the unexpected finding that a previously described dominant allele in the Drosophila Notch pathway transcription factor contains a missense variant in an analogous residue found in a family with AOS. Using this novel animal model of AOS along with biochemical DNA binding, protein-protein interaction, and transcriptional reporter assays, we found that this transcription factor variant selectively compromises DNA binding but not binding to the Notch signal nor binding to other proteins in the Notch pathway. Taken together with prior human genetic studies, these data suggest AOS phenotypes associated with variants in the Notch pathway transcription factor are caused by a dominant mechanism that sequesters the Notch signal, leading to Notch target gene dysregulation.

Assemblies of JAG1 and JAG2 determine tracheobronchial cell fate in mucosecretory lung disease

Mucosecretory lung disease compromises airway epithelial function and is characterized by goblet cell hyperplasia and ciliated cell hypoplasia. Goblet and ciliated cell types are derived from tracheobronchial stem/progenitor cells via a Notch-dependent mechanism. Although specific arrays of Notch receptors regulate cell fate determination, the function of the ligands Jagged1 (JAG1) and JAG2 is unclear. This study examined JAG1 and JAG2 function using human air-liquid-interface cultures that were treated with γ-secretase complex (GSC) inhibitors, neutralizing peptides/antibodies, or WNT/β-catenin pathway antagonists/agonists. These experiments revealed that JAG1 and JAG2 regulated cell fate determination in the tracheobronchial epithelium; however, their roles did not adhere to simple necessity and sufficiency rules. Biochemical studies indicated that JAG1 and JAG2 underwent posttranslational modifications that resulted in generation of a JAG1 C-terminal peptide and regulated the abundance of full-length JAG2 on the cell surface. GSC and glycogen synthase kinase 3 were implicated in these posttranslational events, but WNT agonist/antagonist studies and RNA-Seq indicated a WNT-independent mechanism. Collectively, these data suggest that posttranslational modifications create distinct assemblies of JAG1 and JAG2, which regulate Notch signal strength and determine the fate of tracheobronchial stem/progenitor cells.

Notch ankyrin domain: evolutionary rise of a thermodynamic sensor

Notch signalling pathway plays a key role in metazoan biology by contributing to resolution of binary decisions in the life cycle of cells during development. Outcomes such as proliferation/differentiation dichotomy are resolved by transcriptional remodelling that follows a switch from Notch^on to Notch^off state, characterised by dissociation of Notch intracellular domain (NICD) from DNA-bound RBPJ. Here we provide evidence that transitioning to the Notch^off state is regulated by heat flux, a phenomenon that aligns resolution of fate dichotomies to mitochondrial activity. A combination of phylogenetic analysis and computational biochemistry was utilised to disclose structural adaptations of Notch1 ankyrin domain that enabled function as a sensor of heat flux. We then employed DNA-based micro-thermography to measure heat flux during brain development, followed by analysis in vitro of the temperature-dependent behaviour of Notch1 in mouse neural progenitor cells. The structural capacity of NICD to operate as a thermodynamic sensor in metazoans stems from characteristic enrichment of charged acidic amino acids in β-hairpins of the ankyrin domain that amplify destabilising inter-residue electrostatic interactions and render the domain thermolabile. The instability emerges upon mitochondrial activity which raises the perinuclear and nuclear temperatures to 50 °C and 39 °C, respectively, leading to destabilization of Notch1 transcriptional complex and transitioning to the Notch^off state. Notch1 functions a metazoan thermodynamic sensor that is switched on by intercellular contacts, inputs heat flux as a proxy for mitochondrial activity in the Notch^on state via the ankyrin domain and is eventually switched off in a temperature-dependent manner.

Immobilized DLL4-induced Notch signaling is mediated by dynamics of the actin cytoskeleton

Notch signaling, which is essential for tissue development and homeostasis, has received attention as an attractive target for cancer therapy, tissue engineering and regenerative medicine. For signal activation, the Notch receptor undergoes proteolysis after binding to its ligand. This process is mediated by a mechanical pulling force, and receptor trans-endocytosis is known to play a central role in supplying the force. On the other hand, Notch ligands immobilized on carrier materials also induce artificial Notch activation. However, the mechanism of signal activation by immobilized ligand proteins is not fully understood. Here, we found that the actin cytoskeleton in Notch1-expressing cells contributes to signal activation induced by immobilized DLL4 (Delta-like ligand 4), and the results showed that pharmacological inhibition of actin dynamics impaired Notch signaling induced by DLL4-coated beads. Moreover, inhibition of actin dynamics remarkably impaired cell migration and was correlated with Notch signaling activity. We also investigated the contribution of Notch cis-endocytosis (the endocytosis of Notch receptor into signal-receiving cells) as an actin-mediated cell biological process to further explore the mechanism of Notch activation by DLL4-coated beads. Compromising the receptor cis-endocytosis pathway with the dynamin inhibitor did not alter DLL4-coated bead-induced Notch signaling, indicating that signal activation is not mediated by dynamin-dependent receptor cis-endocytosis. These findings suggest that Notch activation by immobilized ligands is primarily driven by actin-based cell movement, which might supply a sufficient mechanical force for receptor cleavage, but not by receptor cis-endocytosis.

Anti-Jagged-1 immunotherapy in cancer

Notch signaling is a highly conserved pathway and it plays an essential role in regulating cellular proliferation, differentiation, and apoptosis. The human Notch family includes four receptors, Notch 1-4, and five ligands, delta-like ligand 1 (DLL1), delta-like ligand 3 (DLL3), delta-like ligand 4 (DLL4), Jagged-1 (JAG1), and Jagged-2 (JAG2). It is widely known, that Notch signaling components are often mutated and have deregulated expression in many types of cancer and other diseases. Thus, various therapeutic approaches targeting receptors and ligands of the Notch pathway are being investigated. Human JAG1 is closely related to tumor biology among the Notch ligands, and recent studies have shown potential for monoclonal antibodies targeting JAG1 in cancer therapy. Therefore, this review focuses on current reports on the significance of JAG1 directed cancer treatment, emphasizing immunotherapy.

PDGF regulates guanylate cyclase expression and cGMP signaling in vascular smooth muscle

The nitric oxide-cGMP (NO-cGMP) pathway is of outstanding importance for vascular homeostasis and has multiple beneficial effects in vascular disease. Neointimal hyperplasia after vascular injury is caused by increased proliferation and migration of vascular smooth muscle cells (VSMCs). However, the role of NO-cGMP signaling in human VSMCs in this process is still not fully understood. Here, we investigate the interaction between platelet derived growth factor (PDGF)-signaling, one of the major contributors to neointimal hyperplasia, and the cGMP pathway in vascular smooth muscle, focusing on NO-sensitive soluble guanylyl cyclase (sGC). We show that PDGF reduces sGC expression by activating PI3K and Rac1, which in turn alters Notch ligand signaling. These data are corroborated by gene expression analysis in human atheromas, as well as immunohistological analysis of diseased and injured arteries. Collectively, our data identify the crosstalk between PDGF and NO/sGC signaling pathway in human VSMCs as a potential target to tackle neointimal hyperplasia.

PDGF reduces expression of nitric oxide-sensitive soluble guanylyl cyclase (NO-sGC) through PI3K-P-Rex1-Rac1 signaling in vascular smooth muscle cells. These insights provide possible avenues to prevent dysregulation of NO/cGMP signaling in vascular disease.

Artificial Notch Signaling Activation Method Using Immobilized Ligand Beads

Activation of Notch signaling requires physical interaction between ligand- and receptor-expressing cells and pulling force to release the Notch intracellular domain. Therefore, the soluble recombinant ligand protein is not suitable for the activation of Notch signaling in a cell culture system. Here, we describe an efficient method for transient activation of Notch signaling using immobilized ligand beads. Using this method, the timing of Notch signaling can be efficiently controlled.

Novel Therapeutical Approaches to Managing Atherosclerotic Risk

Atherosclerosis is a multifactorial vascular disease that leads to inflammation and stiffening of the arteries and decreases their elasticity due to the accumulation of calcium, small dense Low Density Lipoproteins (sdLDL), inflammatory cells, and fibrotic material. A review of studies pertaining to cardiometabolic risk factors, lipids alterations, hypolipidemic agents, nutraceuticals, hypoglycaemic drugs, atherosclerosis, endothelial dysfunction, and inflammation was performed. There are several therapeutic strategies including Proprotein Convertase Subtilisin/Kexin 9 (PCSK9) inhibitors, inclisiran, bempedoic acid, Glucagon-Like Peptide-1 Receptor agonists (GLP-1 RAs), and nutraceuticals that promise improvement in the atheromatous plaque from a molecular point of view, because have actions on the exposure of the LDL-Receptor (LDL-R), on endothelial dysfunction, activation of macrophages, on lipid oxidation, formations on foam cells, and deposition extracellular lipids. Atheroma plaque reduction both as a result of LDL-Cholesterol (LDL-C) intensive lowering and reducing inflammation and other residual risk factors is an integral part of the management of atherosclerotic disease, and the use of valid therapeutic alternatives appear to be appealing avenues to solving the problem.

Notch Signaling Function in the Angiocrine Regulation of Tumor Development

The concept of tumor growth being angiogenesis dependent had its origin in the observations of Judah Folkman in 1969 of a retinoblastoma in a child. Tumor angiogenesis is initiated when endothelial cells (ECs) respond to local stimuli and migrate towards the growing mass, which results in the formation of tubular structures surrounded by perivascular support cells that transport blood to the inner tumor. In turn, the neo-vasculature supports tumor development and eventual metastasis. This process is highly regulated by several signaling pathways. Central to this process is the Notch signaling pathway. Beyond the role of Notch signaling in tumor angiogenesis, a major hallmark of cancer development, it has also been implicated in the regulation of tumor cell proliferation and survival, in epithelial-to-mesenchymal transition, invasion and metastasis and in the regulation of cancer stem cells, in a variety of hematologic and solid malignancies. There is increasing evidence for the tumor vasculature being important in roles other than those linked to blood perfusion. Namely, endothelial cells act on and influence neighboring tumor cells by use of angiocrine factors to generate a unique cellular microenvironment, thereby regulating tumor stem-like cells’ homeostasis, modulating tumor progression, invasiveness, trafficking and metastasis. This review will focus on Notch signaling components that play a part in angiocrine signaling in a tumor setting.

The Role of DLLs in Cancer: A Novel Therapeutic Target

Delta-like ligands (DLLs) control Notch signaling. DLL1, DLL3 and DLL4 are frequently deregulated in cancer and influence tumor growth, the tumor vasculature and tumor immunity, which play different roles in cancer progression. DLLs have attracted intense research interest as anti-cancer therapeutics. In this review, we discuss the role of DLLs in cancer and summarize the emerging DLL-relevant targeting methods to aid future studies.

Delta-like 1 and Delta-like 4 differently require their extracellular domains for triggering Notch signaling in mice

Delta-like (Dll) 1 and Dll4 differently function as Notch ligands in a context-dependent manner. As these ligands share structural properties, the molecular basis for their functional difference is poorly understood. Here, we investigated the superiority of Dll4 over Dll1 with respect to induction of T cell development using a domain-swapping approach in mice. The DOS motif, shared by Notch ligands-except Dll4-contributes to enhancing the activity of Dll for signal transduction. The module at the N-terminus of Notch ligand (MNNL) of Dll4 is inherently advantageous over Dll1. Molecular dynamic simulation revealed that the loop structure in MNNL domain of Dll1 contains unique proline residues with limited range of motion. The Dll4 mutant with Dll1-derived proline residues showed reduced activity. These results suggest that the loop structure-present within the MNNL domain-with a wide range of motion ensures the superiority of Dll4 and uniquely contributes to the triggering of Notch signaling.

Activation of Notch Signaling by Oocytes and Jag1 in Mouse Ovarian Granulosa Cells

The Notch pathway plays diverse and complex roles in cell signaling during development. In the mammalian ovary, Notch is important for the initial formation and growth of follicles, and for regulating the proliferation and differentiation of follicular granulosa cells during the periovulatory period. This study seeks to determine the contribution of female germ cells toward the initial activation and subsequent maintenance of Notch signaling within somatic granulosa cells of the ovary. To address this issue, transgenic Notch reporter (TNR) mice were crossed with Sohlh1-mCherry (S1CF) transgenic mice to visualize Notch-active cells (EGFP) and germ cells (mCherry) simultaneously in the neonatal ovary. To test the involvement of oocytes in activation of Notch signaling in ovarian somatic cells, we ablated germ cells using busulfan, a chemotherapeutic alkylating agent, or investigated KitWv/Wv (viable dominant white-spotting) mice that lack most germ cells. The data reveal that Notch pathway activation in granulosa cells is significantly suppressed when germ cells are reduced. We further demonstrate that disruption of the gene for the Notch ligand Jag1 in oocytes similarly impacts Notch activation and that recombinant JAG1 enhances Notch target gene expression in granulosa cells. These data are consistent with the hypothesis that germ cells provide a ligand, such as Jag1, that is necessary for activation of Notch signaling in the developing ovary.

Structurally conserved binding motifs of transcriptional regulators to notch nuclear effector CSL

This mini review discusses the protein complexes comprised of the universal Notch signaling transcription factor, CSL (CBF1/Su(H)/Lag-1), and its activating or repressing transcriptional coregulation partners. Many of these complex structures have been solved crystallographically as well as undergoing extensive binding studies with wild-type and mutant variants. Notch signaling is critically important in a large variety of basic biological processes: cell proliferation, differentiation, cell cycle control to name a few. Aberrant Notch thus remains a coveted target for pharmaceutical intervention. To that end, we provide a molecular-level summary of the similarities and differences in the Notch coregulator complexes that ultimately govern these processes. We highlight a conserved binding motif that multiple superficially unrelated proteins have adopted to become involved in Notch target gene regulation. As CSL-interacting small molecules begin to be characterized, this review will provide insight to potential binding sites and differential complex disruption.

Impact statement

Proper Notch signaling regulation is informed by many distinct protein complexes involving a single nuclear effector. A decade of research into these protein complexes yields multiple crystal structures and a wealth of binding data to guide drug development for Notch-related diseases – cancer, cardiovascular, development disorders.

Notch and interacting signalling pathways in cardiac development, disease, and regeneration

Cardiogenesis is a complex developmental process involving multiple overlapping stages of cell fate specification, proliferation, differentiation, and morphogenesis. Precise spatiotemporal coordination between the different cardiogenic processes is ensured by intercellular signalling crosstalk and tissue-tissue interactions. Notch is an intercellular signalling pathway crucial for cell fate decisions during multicellular organismal development and is aptly positioned to coordinate the complex signalling crosstalk required for progressive cell lineage restriction during cardiogenesis. In this Review, we describe the role of Notch signalling and the crosstalk with other signalling pathways during the differentiation and patterning of the different cardiac tissues and in cardiac valve and ventricular chamber development. We examine how perturbation of Notch signalling activity is linked to congenital heart diseases affecting the neonate and adult, and discuss studies that shed light on the role of Notch signalling in heart regeneration and repair after injury.

Proximity interactions of the ubiquitin ligase Mind bomb 1 reveal a role in regulation of epithelial polarity complex proteins

MIB1 belongs to the RING domain containing family of E3 ubiquitin ligases. In vertebrates, MIB1 plays an essential role in activation of Notch signaling during development, through the ubiquitination and endocytosis of Notch ligands. More recently, Notch independent functions for MIB1 have been described in centriole homeostasis, dendritic spine outgrowth and directional cell migration. Here we use proximity-dependent biotin identification (BioID) to define the MIB1 interactome that included 163 high confidence interactions with polypeptides linked to centrosomes and cilia, endosomal trafficking, RNA and DNA processing, the ubiquitin system, and cell adhesion. Biochemical analysis identified several proteins within these groups including CCDC14 and EPS15 that were ubiquitinated but not degraded when co-expressed with MIB1. The MIB1 interactome included the epithelial cell polarity protein, EPB41L5. MIB1 binds to and ubiquitinates EPB41L5 resulting in its degradation. Furthermore, MIB1 ubiquitinates the EPB41L5-associated polarity protein CRB1, an important determinant of the apical membrane. In polarized cells, MIB1 localized to the lateral membrane with EPB41L5 and to the tight junction with CRB1, CRB3 and ZO1. Furthermore, over expression of MIB1 resulted in altered epithelial cell morphology and apical membrane expansion. These results support a role for MIB1 in regulation of polarized epithelial cell morphology.

The Use of Nutraceuticals to Counteract Atherosclerosis: The Role of the Notch Pathway

Despite the currently available pharmacotherapies, today, thirty percent of worldwide deaths are due to cardiovascular diseases (CVDs), whose primary cause is atherosclerosis, an inflammatory disorder characterized by the buildup of lipid deposits on the inside of arteries. Multiple cellular signaling pathways have been shown to be involved in the processes underlying atherosclerosis, and evidence has been accumulating for the crucial role of Notch receptors in regulating the functions of the diverse cell types involved in atherosclerosis onset and progression. Several classes of nutraceuticals have potential benefits for the prevention and treatment of atherosclerosis and CVDs, some of which could in part be due to their ability to modulate the Notch pathway. In this review, we summarize the current state of knowledge on the role of Notch in vascular health and its modulation by nutraceuticals for the prevention of atherosclerosis and/or treatment of related CVDs.

A role for actomyosin contractility in Notch signaling

Background

Notch-Delta signaling functions across a wide array of animal systems to break symmetry in a sheet of undifferentiated cells and generate cells with different fates, a process known as lateral inhibition. Unlike many other signaling systems, however, since both the ligand and receptor are transmembrane proteins, the activation of Notch by Delta depends strictly on cell-cell contact. Furthermore, the binding of the ligand to the receptor may not be sufficient to induce signaling, since recent work in cell culture suggests that ligand-induced Notch signaling also requires a mechanical pulling force. This tension exposes a cleavage site in Notch that, when cut, activates signaling. Although it is not known if mechanical tension contributes to signaling in vivo, others have suggested that this is how endocytosis of the receptor-ligand complex contributes to the cleavage and activation of Notch. In a similar way, since Notch-mediated lateral inhibition at a distance in the dorsal thorax of the pupal fly is mediated via actin-rich protrusions, it is possible that cytoskeletal forces generated by networks of filamentous actin and non-muscle myosin during cycles of protrusion extension and retraction also contribute to Notch signaling.

Results

To test this hypothesis, we carried out a detailed analysis of the role of myosin II-dependent tension in Notch signaling in the developing fly and in cell culture. Using dynamic fluorescence-based reporters of Notch, we found that myosin II is important for signaling in signal sending and receiving cells in both systems—as expected if myosin II-dependent tension across the Notch-Delta complex contributes to Notch activation. While myosin II was found to contribute most to signaling at a distance, it was also required for maximal signaling between adjacent cells that share lateral contacts and for signaling between cells in culture.

Conclusions

Together these results reveal a previously unappreciated role for non-muscle myosin II contractility in Notch signaling, providing further support for the idea that force contributes to the cleavage and activation of Notch in the context of ligand-dependent signaling, and a new paradigm for actomyosin-based mechanosensation.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0625-9) contains supplementary material, which is available to authorized users.

Multiple roles for O-glycans in Notch signalling

Notch signalling regulates a plethora of developmental processes and is also essential for the maintenance of tissue homeostasis in adults. Therefore, fine-tuning of Notch signalling strength needs to be tightly regulated. Of key importance for the regulation of Notch signalling are O-fucose, O-GlcNAc and O-glucose glycans attached to the extracellular domain of Notch receptors. The EGF repeats of the Notch receptor extracellular domain harbour consensus sites for addition of the different types of O-glycan to Ser or Thr, which takes place in the endoplasmic reticulum. Studies from Drosophila to mammals have demonstrated the multifaceted roles of O-glycosylation in regulating Notch signalling. O-glycosylation modulates different aspects of Notch signalling including recognition by Notch ligands, the strength of ligand binding, Notch receptor trafficking, stability and activation at the cell surface. Defects in O-glycosylation of Notch receptors give rise to pathologies in humans. This Review summarizes the nature of the O-glycans on Notch receptors and their differential effects on Notch signalling.

Matrine blocks AGEs- induced HCSMCs phenotypic conversion via suppressing Dll4-Notch pathway

Vascular smooth muscle cells (VSMCs) contractile- synthetic phenotypic conversion takes responsibility in the atherosclerotic plaque formation by abnormal synthesis, secretion and deposition of extracellular matrix (ECM). Matrine exerts therapeutic effects on both cardiovascular diseases and organ fibrosis. In this study, we investigated matrine's inhibitory effect and mechanisms on AGEs- induced VSMC contractile- synthetic phenotypic conversion. Cultured human coronary smooth muscle cells (HCSMCs) were exposed to AGEs. Matrine at serially diluted concentrations were used to treat the cells. HCSMCs phenotype was identified by immunofluorescent staining of contractile phenotypic markers including mooth muscle myosin heavy chain (MYH11) and smooth muscle α-actin (ACTA2). Sircol collagen assay was used to assess the collagen secretion level. Notch signaling activation was determined by luciferase assay. Western blotting was used to evaluate expression levels of collagen I, collagen VIII, Delta-like (Dll)1, Dll3, Dll4, Jagged1, Jagged2, Notch intracellular domain (NICD)1 and Hes family basic helix-loop-helix (bHLH) transcription factor1 (HES1). Matrine pre-treatment recovered the AGEs- induced contractile- synthetic phenotypic conversion by increasing MYH11 and ACTA2 in HCSMCs. Matrine reduced AGEs- mediated activation of Notch signaling, down-regulated expression levels of NICD1, HES1, collagen I and collagen VIII and collagen secretion contents in HCSMCs. Matrine inhibited expression level of Dll4 without affecting other Notch ligands including Dll1, Dll3, Jagged1 and Jagged2 in HCSMCs exposed to AGEs. These results suggested that AGEs exposure facilitated the contractile- synthetic phenotypic conversion of HCSMCs. Matrine blocked this phenotypic conversion by suppressing Dll4- Notch signaling pathway activation.

Inhibition of HER2 Increases JAGGED1-dependent Breast Cancer Stem Cells: Role for Membrane JAGGED1

Purpose: HER2-positive breast cancer is driven by cells possessing stem-like properties of self-renewal and differentiation, referred to as cancer stem cells (CSC). CSCs are implicated in radiotherapy, chemotherapy resistance, and tumor recurrence. NOTCH promotes breast CSC survival and self-renewal, and overexpression of NOTCH1 and the NOTCH ligand JAGGED1 predict poor outcome. Resistance to anti-HER2 therapy in HER2⁺ breast cancer requires NOTCH1, and that combination of trastuzumab and a gamma secretase inhibitor (GSI) prevents tumor relapse in xenograft models.Experimental Design: The current study investigates mechanisms by which HER2 tyrosine kinase activity regulates NOTCH-dependent CSC survival and tumor initiation.Results: Lapatinib-mediated HER2 inhibition shifts the population of HER2⁺ breast cancer cells from low membrane JAGGED1 expression to higher levels, independent of sensitivity to anti-HER2 treatment within the bulk cell population. This increase in membrane JAGGED1 is associated with higher NOTCH receptor expression, activation, and enrichment of CSCs in vitro and in vivo Importantly, lapatinib treatment results in growth arrest and cell death of JAGGED1 low-expressing cells while the JAGGED1 high-expressing cells continue to cycle. High membrane JAGGED1 protein expression predicts poor overall cumulative survival in women with HER2⁺ breast cancer.Conclusions: These results indicate that higher membrane JAGGED1 expression may be used to either predict response to anti-HER2 therapy or for detection of NOTCH-sensitive CSCs posttherapy. Sequential blockade of HER2 followed by JAGGED1 or NOTCH could be more effective than simultaneous blockade to prevent drug resistance and tumor progression. Clin Cancer Res; 24(18); 4566-78. ©2018 AACR.

Notch Signaling Regulates Differentiation and Steroidogenesis in Female Mouse Ovarian Granulosa Cells

The Notch pathway is a highly conserved juxtacrine signaling mechanism that is important for many cellular processes during development, including differentiation and proliferation. Although Notch is important during ovarian follicle formation and early development, its functions during the gonadotropin-dependent stages of follicle development are largely unexplored. We observed positive regulation of Notch activity and expression of Notch ligands and receptors following activation of the luteinizing hormone-receptor in prepubertal mouse ovary. JAG1, the most abundantly expressed Notch ligand in mouse ovary, revealed a striking shift in localization from oocytes to somatic cells following hormone stimulation. Using primary cultures of granulosa cells, we investigated the functions of Jag1 using small interfering RNA knockdown. The loss of JAG1 led to suppression of granulosa cell differentiation as marked by reduced expression of enzymes and factors involved in steroid biosynthesis, and in steroid secretion. Jag1 knockdown also resulted in enhanced cell proliferation. These phenotypes were replicated, although less robustly, following knockdown of the obligate canonical Notch transcription factor RBPJ. Intracellular signaling analysis revealed increased activation of the mitogenic phosphatidylinositol 3-kinase/protein kinase B and mitogen-activated protein kinase/extracellular signal-regulated kinase pathways following Notch knockdown, with a mitogen-activated protein kinase kinase inhibitor blocking the enhanced proliferation observed in Jag1 knockdown granulosa cells. Activation of YB-1, a known regulator of granulosa cell differentiation genes, was suppressed by Jag1 knockdown. Overall, this study reveals a role of Notch signaling in promoting the differentiation of preovulatory granulosa cells, adding to the diverse functions of Notch in the mammalian ovary.

The Notch Interactome: Complexity in Signaling Circuitry

The Notch pathway controls a very broad spectrum of cell fates in metazoans during development, influencing proliferation, differentiation and cell death. Given its central role in normal development and homeostasis, misregulation of Notch signals can lead to various disorders including cancer. How the Notch pathway mediates such pleiotropic and differential effects is of fundamental importance. It is becoming increasingly clear through a number of large-scale genetic and proteomic studies that Notch interacts with a staggeringly large number of other genes and pathways in a context-dependent, complex, and highly regulated network, which determines the ultimate biological outcome. How best to interpret and analyze the continuously increasing wealth of data on Notch interactors remains a challenge. Here we review the current state of genetic and proteomic data related to the Notch interactome.

The Molecular Mechanism of Notch Activation

Research in the last several years has shown that Notch proteolysis, and thus Notch activation, is conformationally controlled by the extracellular juxtamembrane NRR of Notch, which sterically occludes the S2 protease site until ligand binds. The question of how conformational exposure of the protease site is achieved during physiologic activation, and thus how normal activation is bypassed in disease pathogenesis, has been the subject of intense study in the last several years, and is the subject of this chapter. Here, we summarize the structural features of the NRR domains of Notch receptors that establish the autoinhibited state and then review a number of recent studies aimed at testing the mechanotransduction model for Notch signaling using force spectroscopy and molecular tension sensors.

Epsin-Dependent Ligand Endocytosis Activates Notch by Force

DSL ligands activate Notch by inducing proteolytic cleavage of the receptor ectodomain, an event that requires ligand to be endocytosed in signal-sending cells by the adaptor protein Epsin. Two classes of explanation for this unusual requirement are (1) recycling models, in which the ligand must be endocytosed to be modified or repositioned before it binds Notch and (2) pulling models, in which the ligand must be endocytosed after it binds Notch to exert force that exposes an otherwise buried site for cleavage. We demonstrate in vivo that ligands that cannot enter the Epsin pathway nevertheless bind Notch but fail to activate the receptor because they cannot exert sufficient force. This argues against recycling models and in favor of pulling models. Our results also suggest that once ligand binds receptor, activation depends on a competition between Epsin-mediated ligand endocytosis, which induces cleavage, and transendocytosis of the ligand by the receptor, which aborts the incipient signal.

Culturing human intestinal stem cells for regenerative applications in the treatment of inflammatory bowel disease

Both the incidence and prevalence of inflammatory bowel disease (IBD) is increasing globally; in the industrialized world up to 0.5% of the population are affected and around 4.2 million individuals suffer from IBD in Europe and North America combined. Successful engraftment in experimental colitis models suggests that intestinal stem cell transplantation could constitute a novel treatment strategy to re-establish mucosal barrier function in patients with severe disease. Intestinal stem cells can be grown in vitro in organoid structures, though only a fraction of the cells contained are stem cells with regenerative capabilities. Hence, techniques to enrich stem cell populations are being pursued through the development of multiple two-dimensional and three-dimensional culture protocols, as well as co-culture techniques and multiple growth medium compositions. Moreover, research in support matrices allowing for efficient clinical application is in progress. In vitro culture is accomplished by modulating the signaling pathways fundamental for the stem cell niche with a suitable culture matrix to provide additional contact-dependent stimuli and structural support. The aim of this review was to discuss medium compositions and support matrices for optimal intestinal stem cell culture, as well as potential modifications to advance clinical use in IBD.

Pofut1 point-mutations that disrupt O-fucosyltransferase activity destabilize the protein and abolish Notch1 signaling during mouse somitogenesis

The segmental pattern of the vertebrate body is established via the periodic formation of somites from the presomitic mesoderm (PSM). This periodical process is controlled by the cyclic and synchronized activation of Notch signaling in the PSM. Protein O-fucosyltransferase1 (Pofut1), which transfers O-fucose to the EGF domains of the Notch1 receptor, is indispensable for Notch signaling activation. The Drosophila homologue Ofut1 was reported to control Notch localization via two different mechanisms, working as a chaperone for Notch or as a regulator of Notch endocytosis. However, these were found to be independent of O-fucosyltransferase activity because the phenotypes were rescued by Ofut1 mutants lacking O-fucosyltransferase activity. Pofut1 may also be involved in the Notch receptor localization in mice. However, the contribution of enzymatic activity of Pofut1 to the Notch receptor dynamics remains to be elucidated. In order to clarify the importance of the O-fucosyltransferase activity of Pofut1 for Notch signaling activation and the protein localization in the PSM, we established mice carrying point mutations at the 245th a.a. or 370-372th a.a., highly conserved amino-acid sequences whose mutations disrupt the O-fucosyltransferase activity of both Drosophila Ofut1 and mammalian Pofut1, with the CRISPR/Cas9 mediated genome-engineering technique. Both mutants displayed the same severely perturbed somite formation and Notch1 subcellular localization defects as the Pofut1 null mutants. In the mutants, Pofut1 protein, but not RNA, became undetectable by E9.5. Furthermore, both wild-type and mutant Pofut1 proteins were degraded through lysosome dependent machinery. Pofut1 protein loss in the point mutant embryos caused the same phenotypes as those observed in Pofut1 null embryos.

Selective regulation of Notch ligands during angiogenesis is mediated by vimentin

Notch signaling is a key regulator of angiogenesis, in which sprouting is regulated by an equilibrium between inhibitory Dll4-Notch signaling and promoting Jagged-Notch signaling. Whereas Fringe proteins modify Notch receptors and strengthen their activation by Dll4 ligands, other mechanisms balancing Jagged and Dll4 signaling are yet to be described. The intermediate filament protein vimentin, which has been previously shown to affect vascular integrity and regenerative signaling, is here shown to regulate ligand-specific Notch signaling. Vimentin interacts with Jagged, impedes basal recycling endocytosis of ligands, but is required for efficient receptor ligand transendocytosis and Notch activation upon receptor binding. Analyses of Notch signal activation by using chimeric ligands with swapped intracellular domains (ICDs), demonstrated that the Jagged ICD binds to vimentin and contributes to signaling strength. Vimentin also suppresses expression of Fringe proteins, whereas depletion of vimentin enhances Fringe levels to promote Dll4 signaling. In line with these data, the vasculature in vimentin knockout (VimKO) embryos and placental tissue is underdeveloped with reduced branching. Disrupted angiogenesis in aortic rings from VimKO mice and in endothelial 3D sprouting assays can be rescued by reactivating Notch signaling by recombinant Jagged ligands. Taken together, we reveal a function of vimentin and demonstrate that vimentin regulates Notch ligand signaling activities during angiogenesis.

Delta-Notch signalling in segmentation

Modular body organization is found widely across multicellular organisms, and some of them form repetitive modular structures via the process of segmentation. It's vastly interesting to understand how these regularly repeated structures are robustly generated from the underlying noise in biomolecular interactions. Recent studies from arthropods reveal similarities in segmentation mechanisms with vertebrates, and raise the possibility that the three phylogenetic clades, annelids, arthropods and chordates, might share homology in this process from a bilaterian ancestor. Here, we discuss vertebrate segmentation with particular emphasis on the role of the Notch intercellular signalling pathway. We introduce vertebrate segmentation and Notch signalling, pointing out historical milestones, then describe existing models for the Notch pathway in the synchronization of noisy neighbouring oscillators, and a new role in the modulation of gene expression wave patterns. We ask what functions Notch signalling may have in arthropod segmentation and explore the relationship between Notch-mediated lateral inhibition and synchronization. Finally, we propose open questions and technical challenges to guide future investigations into Notch signalling in segmentation.

The Retinal Pigment Epithelium Is a Notch Signaling Niche in the Mouse Retina

Notch signaling in neural progenitor cell is triggered by ligands expressed in adjacent cells. To identify the sources of active Notch ligands in the mouse retina, we negatively regulated Notch ligand activity in various neighbors of retinal progenitor cells (RPCs) by eliminating mindbomb E3 ubiquitin protein ligase 1 (Mib1). Mib1-deficient retinal cells failed to induce Notch activation in intra-lineage RPCs, which prematurely differentiated into neurons; however, Mib1 in post-mitotic retinal ganglion cells was not important. Interestingly, Mib1 in the retinal pigment epithelium (RPE) also contributed to Notch activation in adjacent RPCs by supporting the localization of active Notch ligands at RPE-RPC contacts. Combining this RPE-driven Notch signaling and intra-retinal Notch signaling, we propose a model in which one RPC daughter receives extra Notch signals from the RPE to become an RPC, whereas its sister cell receives only a subthreshold level of intra-retinal Notch signal and differentiates into a neuron.

Identification of Domains for Efficient Notch Signaling Activity in Immobilized Notch Ligand Proteins

Notch is a critical signaling pathway that controls cell fate and tissue homeostasis, but the functional characterization of Notch ligand domains that activate Notch receptors remains incomplete. Here, we established a method for immobilizing Notch ligand proteins onto beads to measure time-dependent Notch activity after the addition of Notch ligand-coated beads. A comparison between activities by the Notch ligand found on the cell surface to that of the ligand immobilized on beads showed that immobilized Notch ligand protein produces comparable signal activity during the first 10 h. Follow-up truncation studies showed that the N-terminal epidermal growth factor (EGF) repeat three region of delta like canonical Notch ligand 4 (DLL4) or jagged 1 (JAG1) is the minimum region for activating Notch signaling, and the DLL4 EGF repeat three domain may have a role in activation through a mechanism other than by increasing binding affinity. In addition, we found that reconstruction of the DLL4 delta and OSM-11 (DOS) motif (N257P) resulted in an increase in both binding affinity and signaling activity, which suggests that the role of the DOS motif is conserved among Notch ligands. Furthermore, active DLL4 protein on beads promoted T cell differentiation or inhibited B cell differentiation in vitro, whereas JAG1 proteins on beads did not have any effect. Taken together, our findings provide unambiguous evidence for the role of different Notch ligands and their domains in Notch signal activation, and may be potential tools for controlling Notch signaling activation. J. Cell. Biochem. 118: 785-796, 2017. © 2016 Wiley Periodicals, Inc.

Growth factor and co-receptor release by structural regulation of substrate metalloprotease accessibility

Release of cytokines, growth factors and other life-essential molecules from precursors by a-disintegrin-and-metalloproteases (ADAMs) is regulated with high substrate-specificity. We hypothesized that this is achieved by cleavage-regulatory intracellular-domain (ICD)-modifications of the precursors. We show here that cleavage-stimuli-induced specific ICD-modifications cause structural substrate changes that enhance ectodomain sensitivity of neuregulin-1 (NRG1; epidermal-growth-factor) or CD44 (receptor-tyrosine-kinase (RTK) co-receptor) to chymotrypsin/trypsin or soluble ADAM. This inside-out signal transfer required substrate homodimerization and was prevented by cleavage-inhibitory ICD-mutations. In chimeras, regulation could be conferred to a foreign ectodomain, suggesting a common higher-order structure. We predict that substrate-specific protease-accessibility-regulation controls release of numerous ADAM substrates.

HIV Nef- and Notch1-dependent Endocytosis of ADAM17 Induces Vesicular TNF Secretion in Chronic HIV Infection

Tumor necrosis factor (TNF) is a key cytokine in HIV replication and pathogenesis. For reasons that are not entirely clear, the cytokine remains upregulated despite anti-retroviral therapy (ART). Here we demonstrate that HIV Nef induces an alternative TNF secretion mechanism that remains active in chronic infection. Ingestion of Nef-containing plasma extracellular vesicles (pEV) from ART patients by primary immune cells, but also Nef expression, induced intracellular proTNF cleavage and secretion of vesicular TNF endosomes. Key event was the Nef-mediated routing of the TNF-converting enzyme ADAM17 into Rab4 + early endosomes and the Rab27 + secretory pathway. Analysis of lymph-node tissue by multi-epitope-ligand-cartography (MELC) confirmed a vesicular TNF secretion phenotype that co-localized with persistent Nef expression, and implicated Notch1 as an essential co-factor. Surprisingly Notch1 had no transcriptional effect but was required for the endosomal trafficking of ADAM17. We conclude that Nef expression and Nef-containing pEV mobilize TNF from endosomal compartments in acute and chronic infection.

•

Nef/ADAM17 containing extracellular vesicles induce an endosomal TNF secretion type in primary target cells.

•

The mechanism required the shuttling of ADAM17 into Rab4 + endosomal compartments in a Notch1-dependent manner.

•

The mechanism could be demonstrated in tissue by multi-epitope-ligand-cartography (MELC) technology.

Despite antiviral therapy, plasma levels of TNF remain upregulated and likely play a role in many comorbidities seen in chronic HIV infection. We found that this is due to high levels of HIV-induced plasma extracellular vesicles (pEV) containing the TNF processing ADAM17 protease. Interestingly these vesicles induced a different TNF secretion type. Whereas TNF is usually shed from the plasma membrane, pEV mobilized intracellular TNF storage compartments, secreting endosomal vesicles. We could confirm this mechanism analyzing lymph node tissue sections by a novel immunostaining technology. Our report supports our previous publication implying ongoing viral activity despite successful antiretroviral therapy.

Notch, lipids, and endothelial cells

PURPOSE OF REVIEW

Notch signaling is an evolutionary conserved pathway critical for cardiovascular development and angiogenesis. More recently, the contribution of Notch signaling to the homeostasis of the adult vasculature has emerged as an important novel paradigm, but much remains to be understood.

RECENT FINDINGS

Recent findings shed light on the impact of Notch in vascular and immune responses to microenvironmental signals as well as on the onset of atherosclerosis. In the past year, studies in human and mice explored the role of Notch in the maintenance of a nonactivated endothelium. Novel pieces of evidence suggest that this pathway is sensitive to environmental factors, including inflammatory mediators and diet-derived by-products.

SUMMARY

An emerging theme is the ability of Notch to respond to changes in the microenvironment, including glucose and lipid metabolites. In turn, alterations in Notch enable an important link between metabolism and transcriptional changes, thus this receptor appears to function as a metabolic sensor with direct implications to gene expression.

Notch signaling in oral squamous neoplasia

Notch signaling is involved in cell-cell communication. It is an evolutionarily ancient mechanism and plays a fundamental role in development. The typical function of Notch signaling is the regulation of cell fate segregation at asymmetric division; however, a role in tumorigenesis has also been suggested. Inactivating mutations of NOTCH1 are present in about 10 % of cases of squamous cell carcinoma of the skin, oral cavity, esophagus, and lung, rendering it one of the most frequently mutated genes in squamous cell carcinoma. Mouse knockout studies have demonstrated that Notch1 is imperative for early development but is dispensable for formation of the squamous epithelium. However, loss of Notch signaling predisposes the epidermis to hyperplasia and increases tumor incidence. This tumor-inducing effect resulting from the loss of Notch signaling is associated with non-cell-autonomous effects that are elicited by subtle alteration of epithelial cell features, generating a wound-like microenvironment in the underlying stroma. We found that Notch1 was expressed specifically in the basal cells of the oral squamous epithelium. In cancer and oral epithelial dysplasia, it was significantly downregulated, suggesting that reduced Notch activity plays a distinct role in oral neoplasia.

Memory of cell shape biases stochastic fate decision-making despite mitotic rounding

Cell shape influences function, and the current model suggests that such shape effect is transient. However, cells dynamically change their shapes, thus, the critical question is whether shape information remains influential on future cell function even after the original shape is lost. We address this question by integrating experimental and computational approaches. Quantitative live imaging of asymmetric cell-fate decision-making and their live shape manipulation demonstrates that cellular eccentricity of progenitor cell indeed biases stochastic fate decisions of daughter cells despite mitotic rounding. Modelling and simulation indicates that polarized localization of Delta protein instructs by the progenitor eccentricity is an origin of the bias. Simulation with varying parameters predicts that diffusion rate and abundance of Delta molecules quantitatively influence the bias. These predictions are experimentally validated by physical and genetic methods, showing that cells exploit a mechanism reported herein to influence their future fates based on their past shape despite dynamic shape changes.

Cell shape influences function but during mitotic cell rounding the original shape is lost. Here the authors show that the cellular eccentricity of progenitor cell biases stochastic fate-decisions using a combination of quantitative live imaging, genetic manipulations and computational simulations.

Defining Single Molecular Forces Required for Notch Activation Using Nano Yoyo

Notch signaling, involved in development and tissue homeostasis, is activated at the cell-cell interface through ligand-receptor interactions. Previous studies have implicated mechanical forces in the activation of Notch receptor upon binding to its ligand. Here we aimed to determine the single molecular force required for Notch activation by developing a novel low tension gauge tether (LTGT). LTGT utilizes the low unbinding force between single-stranded DNA (ssDNA) and Escherichia coli ssDNA binding protein (SSB) (∼4 pN dissociation force at 500 nm/s pulling rate). The ssDNA wraps around SSB and, upon application of force, unspools from SSB, much like the unspooling of a yoyo. One end of this nano yoyo is attached to the surface though SSB, while the other end presents a ligand. A Notch receptor, upon binding to its ligand, is believed to undergo force-induced conformational changes required for activating downstream signaling. If the required force for such activation is larger than 4 pN, ssDNA will unspool from SSB, and downstream signaling will not be activated. Using these LTGTs, in combination with the previously reported TGTs that rupture double-stranded DNA at defined forces, we demonstrate that Notch activation requires forces between 4 and 12 pN, assuming an in vivo loading rate of 60 pN/s. Taken together, our study provides a direct link between single-molecular forces and Notch activation.

Structure and function of the Mind bomb E3 ligase in the context of Notch signal transduction

The Notch signaling pathway has a critical role in cell fate determination and tissue homeostasis in a variety of different lineages. In the context of normal Notch signaling, the Notch receptor of the 'signal-receiving' cell is activated in trans by a Notch ligand from a neighboring 'signal-sending' cell. Genetic studies in several model organisms have established that ubiquitination of the Notch ligand, and its regulated endocytosis, is essential for transmission of this activation signal. In mammals, this ubiquitination step is dependent on the protein Mind bomb 1 (Mib1), a large multi-domain RING-type E3 ligase, and its direct interaction with the intracellular tails of Notch ligand molecules. Here, we discuss our current understanding of Mind bomb structure and mechanism in the context of Notch signaling and beyond.