The role of ligand endocytosis in notch signalling

NOTCH3 and Pulmonary Arterial Hypertension

NOTCH3 receptor signaling has been linked to the regulation of smooth muscle cell proliferation and the maintenance of smooth muscle cells in an undifferentiated state. Pulmonary arterial hypertension (World Health Organization Group 1 idiopathic disease: PAH) is a fatal disease characterized clinically by elevated pulmonary vascular resistance caused by extensive vascular smooth muscle cell proliferation, perivascular inflammation, and asymmetric neointimal hyperplasia in precapillary pulmonary arteries. In this review, a detailed overview of the specific role of NOTCH3 signaling in PAH, including its mechanisms of activation by a select ligand, downstream signaling effectors, and physiologic effects within the pulmonary vascular tree, is provided. Animal models showing the importance of the NOTCH3 pathway in clinical PAH will be discussed. New drugs and biologics that inhibit NOTCH3 signaling and reverse this deadly disease are highlighted.

Identifications of three novel alleles of Serrate in Drosophila

The Notch signaling pathway, an evolutionarily highly conserved pathway, participates in various essential physiological processes in organisms. Activation of Notch signaling in the canonical manner requires the combination of ligand and receptor. There are two ligands of Notch in Drosophila: Delta (Dl) and Serrate (Ser). A mutation mf157 is identified for causing nicks of fly wings in genetic analysis from a mutant library (unpublished) that was established previously. Immunofluorescent staining illustrates that mf157 represses the expression of Cut and Wingless (Wg), the targets of Notch signaling. MARCM cloning analysis reveals that mf157 functions at the same level or the upstream of ligands of Notch in signaling sending cells. Sequencing demonstrates that mf157 is a novel allele of the Ser gene. Subsequently, mf553 and mf167 are also identified as new alleles of Ser from our library. Furthermore, the complementary assays and the examination of transcripts confirm the sequencing results. Besides, the repressed phenotypes of Notch signaling were reverted by transposon excision experiments of mf157. In conclusion, we identify three fresh alleles of Ser. Our works supply additional genetic resources for further study of functions of Ser and Notch signaling regulation.

Separable Roles for Neur and Ubiquitin in Delta Signalling in the Drosophila CNS Lineages

The execution of a Notch signal at the plasma membrane relies on the mechanical force exerted onto Notch by its ligand. It has been appreciated that the DSL ligands need to collaborate with a ubiquitin (Ub) ligase, either Neuralized or Mindbomb1, in order to exert this pulling force, but the role of ubiquitylation per se is uncertain. Regarding the Delta-Neur pair, it is documented that neither the Neur catalytic domain nor the Delta intracellular lysines (putative Ub acceptors) are needed for activity. Here, we present a dissection of the Delta activity using the Delta-Notch-dependent expression of Hey in newborn Drosophila neurons as a sensitive in vivo assay. We show that the Delta-Neur interaction per se, rather than ubiquitylation, is needed for activity, pointing to the existence of a Delta-Neur signaling complex. The Neur catalytic domain, although not strictly needed, greatly improves Delta-Neur complex functionality when the Delta lysines are mutated, suggesting that the ubiquitylation of some component of the complex, other than Delta, can enhance signaling. Since Hey expression is sensitive to the perturbation of endocytosis, we propose that the Delta-Neur complex triggers a force-generating endocytosis event that activates Notch in the adjacent cell.

Delta of Exopalaemon carinicauda: molecular characterization, expression in different tissues and developmental stages, and its SNPs association analysis with development

BACKGROUND

The Notch signaling pathway plays a significant role in the gene regulatory network of development of vertebrate and invertebrate. However, as a ligand for the Notch signaling pathway, the mechanism of Delta in the development of Exopalaemon carinicauda is still unclear.

METHODS AND RESULTS

The Delta's molecular characteristics, tissue distribution and their association with development in E. carinicauda were studied by RACE (rapid amplification of cDNA end), qRT-PCR (quantitative Real-time PCR) and SNP (single nucleotide polymorphism), respectively. The delta in E. carinicauda had a full-length cDNA of 2807 bp and its Delta of 808 amino-acid residue had the highest identity with the Delta of Homarus americanus (identity = 76.63%). Delta had the highest expression in the ovary, and its expression varied with different stages of embryonic, larval, and ovarian development. After delta RNA interference (with a highest interference efficiency of 66% at 24 h), the expression of Notch signaling pathway genes and developmental related genes was significantly reduced, and the ovarian development was significantly delayed. Further study found that there were 4 SNPs (ds1-4) in delta cDNA, of which two (ds2 T1521G caused a mutation Asn422Lys and ds3 G1674A caused a mutation Tyr473Cys in the EGF-like domain) were associated with the development of E. carinicauda. The Gonadosomatic Index (GSI) of the ds2 TT genotypes was 37.28% and 134.60% higher than E. carinicauda of GT and GG genotype respectively (P < 0.05).

CONCLUSION

Our research indicated that delta was involved in the development of E. carinicauda and provided new insights for molecular breeding with SNP markers in E. carinicauda.

Mechanical regulation of the Notch signaling pathway

The mechanical regulation of Notch signaling is an emerging area of interest in cell biology. Notch is essential in many physiological processes in which mechanical stress plays an important role. This review provides an overview of the mechanoregulation of Notch signaling in multiple steps of the pathway. First, we discuss the current knowledge on the direct mechanoregulation of Notch receptor maturation and localization to the membrane and the effect of mechanical stress on the Notch components. Next, we explore how ligand-receptor interactions and membrane dynamics are possible subjects to mechano-regulation, emphasizing the role of cytoskeletal interactions, membrane stiffness, and endocytic complex formation. We further delve into the necessity of tension generation for negative regulatory region (NRR) domain unfolding, facilitated by ligand endocytosis and other microforces. Additionally, we examine the indirect mechano-regulation of S2 and S3 cleavages. Finally, we discuss the mechanoregulation of the Notch intracellular domain (NICD) trafficking and nuclear entry and the impact of mechanical stress on heterochromatin dynamics and nuclear NICD interactions. This review aims to draw attention to the intricate interplay between mechanical cues and Notch signaling regulation, offering novel insights into the multifaceted nature of cellular mechanobiology.

The meaning of ubiquitylation of the DSL ligand Delta for the development of Drosophila

BACKGROUND

Ubiquitylation (ubi) of the intracellular domain of the Notch ligand Delta (Dl) by the E3 ligases Neuralized (Neur) and Mindbomb1 (Mib1) on lysines (Ks) is thought to be essential for the its signalling activity. Nevertheless, we have previously shown that DlK2R-HA, a Dl variant where all Ks in its intracellular domain (ICD) are replaced by the structurally similar arginine (R), still possess weak activity if over-expressed. This suggests that ubi is not absolutely required for Dl signalling. However, it is not known whether the residual activity of DlK2R-HA is an effect of over-expression and, if not, whether DlK2R can provide sufficient activity for the whole development of Drosophila.

RESULTS

To clarify these issues, we generated and analysed DlattP-DlK2R-HA, a knock-in allele into the Dl locus. Our analysis of this allele reveals that the sole presence of one copy of DlattP-DlK2R-HA can provide sufficient activity for completion of development. It further indicates that while ubi is required for the full activity of Dl in Mib1-dependent processes, it is not essential for Neur-controlled neural development. We identify three modes of Dl signalling that are either dependent or independent of ubi. Importantly, all modes depend on the presence of the endocytic adapter Epsin. During activation of Dl, direct binding of Epsin appears not to be an essential requirement. In addition, our analysis further reveals that the Ks are required to tune down the cis-inhibitory interaction of Dl with Notch.

CONCLUSIONS

Our results indicate that Dl can activate the Notch pathway without ubi of its ICD. It signals via three modes. Ubi is specifically required for the Mib1-dependent processes and the adjustment of cis-inhibition. In contrast to Mib1, Neur can efficiently activate Dl without ubi. Neur probably acts as an endocytic co-adapter in addition to its role as E3 ligase. Endocytosis, regulated in a ubi-dependent or ubi-independent manner is required for signalling and also suppression of cis-inhibition. The findings clarify the role of ubi of the ligands during Notch signalling.

Expression of Epsin3 and its interaction with Notch signalling in oral epithelial dysplasia and oral squamous cell carcinoma

BACKGROUND

Most oral squamous cell carcinoma patients present with late-stage disease. Early detection of the disease is considered to be the most effective way of improving patient outcomes. Several biomarkers have been identified as indicators of oral cancer development and progression; however, none have been translated into clinical practice. In this study, we have investigated the role of Epsin3, an endocytic adaptor protein, and Notch1, a transmembrane signalling protein, in oral carcinogenesis with a view to explore their potential as biomarkers.

METHODS

Oral cancer cell lines and a normal oral keratinocyte cell line were used together with tissue samples of normal oral mucosa (n = 21), oral epithelial dysplasia (n = 74) and early stage (Stages I and II) oral squamous cell carcinoma (n = 31). Immunocytochemical staining, immunoblotting and real-time quantitative polymerase chain reaction (PCR) were performed to assess protein as well as gene expression levels.

RESULTS

The expression levels of Epsin3 and Notch1 mRNA and protein are variable across different oral squamous cell carcinoma derived cell lines. Epsin3 was upregulated in oral epithelial dysplasia and oral squamous cell carcinoma tissues compared with normal epithelium. Overexpression of Epsin3 resulted in a significant reduction of Notch1 expression in oral squamous cell carcinoma. Notch1 was generally downregulated in the dysplasia and oral squamous cell carcinoma samples.

CONCLUSION

Epsin3 is upregulated in oral epithelial dysplasia and oral squamous cell carcinoma and has the potential to be used as a biomarker for oral epithelial dysplasia. Notch signalling is downregulated in oral squamous cell carcinoma, possibly through an Epsin3-induced de-activation pathway.

Tension-tuned receptors for synthetic mechanotransduction and intercellular force detection

Cells interpret mechanical stimuli from their environments and neighbors, but the ability to engineer customized mechanosensing capabilities has remained a synthetic and mechanobiology challenge. Here we introduce tension-tuned synthetic Notch (SynNotch) receptors to convert extracellular and intercellular forces into specifiable gene expression changes. By elevating the tension requirements of SynNotch activation, in combination with structure-guided mutagenesis, we designed a set of receptors with mechanical sensitivities spanning the physiologically relevant picoNewton range. Cells expressing these receptors can distinguish between varying tensile forces and respond by enacting customizable transcriptional programs. We applied these tools to design a decision-making circuit, through which fibroblasts differentiate into myoblasts upon stimulation with distinct tension magnitudes. We also characterize cell-generated forces transmitted between cells during Notch signaling. Overall, this work provides insight into how mechanically induced changes in protein structure can be used to transduce physical forces into biochemical signals. The system should facilitate the further programming and dissection of force-related phenomena in biological systems.

Notch signaling in development and homeostasis

Notch signaling is a highly conserved signaling pathway that coordinates cellular differentiation during the development and homeostasis in numerous organs and tissues across metazoans. Activation of Notch signaling relies on direct contact between neighboring cells and mechanical pulling of the Notch receptors by the Notch ligands. Notch signaling is commonly used in developmental processes to coordinate the differentiation into distinct cell fates of neighboring cells. In this Development at a Glance article, we describe the current understanding of the Notch pathway activation and the different regulatory levels that control the pathway. We then describe several developmental processes where Notch is crucial for coordinating differentiation. These examples include processes that are largely based on lateral inhibition mechanisms giving rise to alternating patterns (e.g. SOP selection, hair cell in the inner ear and neural stem cell maintenance), as well as processes where Notch activity is oscillatory (e.g. somitogenesis and neurogenesis in mammals).

Affinity-matured DLL4 ligands as broad-spectrum modulators of Notch signaling

The Notch pathway regulates cell fate decisions and is an emerging target for regenerative and cancer therapies. Recombinant Notch ligands are attractive candidates for modulating Notch signaling; however, their intrinsically low receptor-binding affinity restricts their utility in biomedical applications. To overcome this limitation, we evolved variants of the ligand Delta-like 4 with enhanced affinity and cross-reactivity. A consensus variant with maximized binding affinity, DeltaMAX, binds human and murine Notch receptors with 500- to 1,000-fold increased affinity compared with wild-type human Delta-like 4. DeltaMAX also potently activates Notch in plate-bound, bead-bound and cellular formats. When administered as a soluble decoy, DeltaMAX inhibits Notch in reporter and neuronal differentiation assays, highlighting its dual utility as an agonist or antagonist. Finally, we demonstrate that DeltaMAX stimulates increased proliferation and expression of effector mediators in T cells. Taken together, our data define DeltaMAX as a versatile tool for broad-spectrum activation or inhibition of Notch signaling.

Regulation of myeloid and lymphoid cell development by O-glycans on Notch

Notch signaling via NOTCH1 stimulated by Delta-like ligand 4 (DLL4) is required for the development of T cells in thymus, and NOTCH2 stimulated by Notch ligand DLL1 is required for the development of marginal zone (MZ) B cells in spleen. Notch signaling also regulates myeloid cell production in bone marrow and is an essential contributor to the generation of early hematopoietic stem cells (HSC). The differentiation program in each of these cellular contexts is optimized by the regulation of Notch signaling strength by O-glycans attached to epidermal growth factor-like (EGF) repeats in the extracellular domain of Notch receptors. There are three major types of O-glycan on NOTCH1 and NOTCH2 - O-fucose, O-glucose and O-GlcNAc. The initiating sugar of each O-glycan is added in the endoplasmic reticulum (ER) by glycosyltransferases POFUT1 (fucose), POGLUT1/2/3 (glucose) or EOGT (GlcNAc), respectively. Additional sugars are added in the Golgi compartment during passage through the secretory pathway to the plasma membrane. Of particular significance for Notch signaling is the addition of GlcNAc to O-fucose on an EGF repeat by the Fringe GlcNAc-transferases LFNG, MFNG or RFNG. Canonical Notch ligands (DLL1, DLL4, JAG1, JAG2) expressed in stromal cells bind to the extracellular domain of Notch receptors expressed in hematopoietic stem cells and myeloid and lymphoid progenitors to activate Notch signaling. Ligand-receptor binding is differentially regulated by the O-glycans on Notch. This review will summarize our understanding of the regulation of Notch signaling in myeloid and lymphoid cell development by specific O-glycans in mice with dysregulated expression of a particular glycosyltransferase and discuss how this may impact immune system development and malignancy in general, and in individuals with a congenital defect in the synthesis of the O-glycans attached to EGF repeats.

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.

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.

Ubiquitylation is required for the incorporation of the Notch receptor into intraluminal vesicles to prevent prolonged and ligand-independent activation of the pathway

BACKGROUND

Ubiquitylation of the ligands and the receptor plays an important part in the regulation of the activity of the evolutionary conserved Notch signalling pathway. However, its function for activation of Notch is not completely understood, despite the identification of several E3 ligases devoted to the receptor.

RESULTS

Here we analysed a variant of the Notch receptor where all lysines in its intracellular domain are replaced by arginines. Our analysis of this variant revealed that ubiquitylation of Notch is not essential for its endocytosis. We identified two functions for ubiquitylation of lysines in the Notch receptor. First, it is required for the degradation of free Notch intracellular domain (NICD) in the nucleus, which prevents a prolonged activation of the pathway. More importantly, it is also required for the incorporation of Notch into intraluminal vesicles of maturing endosomes to prevent ligand-independent activation of the pathway from late endosomal compartments.

CONCLUSIONS

The findings clarify the role of lysine-dependent ubiquitylation of the Notch receptor and indicate that Notch is endocytosed by several independent operating mechanisms.

The E3 Ubiquitin Ligase Mindbomb1 controls planar cell polarity-dependent convergent extension movements during zebrafish gastrulation

Vertebrate Delta/Notch signaling involves multiple ligands, receptors and transcription factors. Delta endocytosis - a critical event for Notch activation - is however essentially controlled by the E3 Ubiquitin ligase Mindbomb1 (Mib1). Mib1 inactivation is therefore often used to inhibit Notch signaling. However, recent findings indicate that Mib1 function extends beyond the Notch pathway. We report a novel Notch-independent role of Mib1 in zebrafish gastrulation. mib1 null mutants and morphants display impaired Convergence Extension (CE) movements. Comparison of different mib1 mutants and functional rescue experiments indicate that Mib1 controls CE independently of Notch. Mib1-dependent CE defects can be rescued using the Planar Cell Polarity (PCP) downstream mediator RhoA, or enhanced through knock-down of the PCP ligand Wnt5b. Mib1 regulates CE through its RING Finger domains that have been implicated in substrate ubiquitination, suggesting that Mib1 may control PCP protein trafficking. Accordingly, we show that Mib1 controls the endocytosis of the PCP component Ryk and that Ryk internalization is required for CE. Numerous morphogenetic processes involve both Notch and PCP signaling. Our observation that during zebrafish gastrulation Mib1 exerts a Notch-independent control of PCP-dependent CE movements suggest that Mib1 loss of function phenotypes should be cautiously interpreted depending on the biological context.