O-linked N-acetylglucosamine is present on the extracellular domain of notch receptors

Characterization of galactosyltransferase and sialyltransferase genes mediating the elongation of the extracellular O-GlcNAc glycans

O-GlcNAc is a unique post-translational modification found in cytoplasmic, nuclear, and mitochondrial proteins. In a limited number of extracellular proteins, O-GlcNAc modifications occur through the action of EOGT, which specifically modifies subsets of epidermal growth factor-like (EGF) domain-containing proteins such as Notch receptors. The abnormalities due to EOGT mutations in mice and humans and the increased EOGT expression in several cancers signify the importance of EOGT pathophysiology and extracellular O-GlcNAc. Unlike intracellular O-GlcNAc monosaccharides, extracellular O-GlcNAc extends to form elongated glycan structures. However, the enzymes involved in the O-GlcNAc glycan extension have not yet been reported. In our study, we comprehensively screened potential galactosyltransferase and sialyltransferase genes related to the canonical O-GlcNAc glycan pathway and revealed the essential roles of B4GALT1 and ST3GAL4 in O-GlcNAc glycan elongation in human HEK293 cells. These findings were confirmed by sequential glycosylation of Drosophila EGF20 in vitro by EOGT, β4GalT-1, and ST3Gal-IV. Thus, the findings from our study throw light on the specific glycosyltransferases that mediate O-GlcNAc glycan elongation in human HEK293 cells.

Protein glycosylation in cardiovascular health and disease

Protein glycosylation, which involves the attachment of carbohydrates to proteins, is one of the most abundant protein co-translational and post-translational modifications. Advances in technology have substantially increased our knowledge of the biosynthetic pathways involved in protein glycosylation, as well as how changes in glycosylation can affect cell function. In addition, our understanding of the role of protein glycosylation in disease processes is growing, particularly in the context of immune system function, infectious diseases, neurodegeneration and cancer. Several decades ago, cell surface glycoproteins were found to have an important role in regulating ion transport across the cardiac sarcolemma. However, with very few exceptions, our understanding of how changes in protein glycosylation influence cardiovascular (patho)physiology remains remarkably limited. Therefore, in this Review, we aim to provide an overview of N-linked and O-linked protein glycosylation, including intracellular O-linked N-acetylglucosamine protein modification. We discuss our current understanding of how all forms of protein glycosylation contribute to normal cardiovascular function and their roles in cardiovascular disease. Finally, we highlight potential gaps in our knowledge about the effects of protein glycosylation on the heart and vascular system, highlighting areas for future research.

Aberrant fucosylation sustains the NOTCH and EGFR/NF-κB pathways and has a prognostic value in human intrahepatic cholangiocarcinoma

BACKGROUND AND AIMS

Intrahepatic cholangiocarcinoma (iCCA) is a lethal malignancy, with increasing incidence worldwide and limited therapeutic options. Aberrant protein glycosylation is a hallmark of cancer. Here, we thoroughly investigated the possible involvement of fucosylation in cholangiocarcinogenesis.

APPROACH AND RESULTS

We discovered that the levels of global fucosylation and members of the fucosylation pathway are ubiquitously upregulated in human iCCA tissues compared to nontumorous surrounding livers and normal biliary cells. In addition, total fucosylation levels correlate with poor patients' prognosis. Furthermore, fucosylation inhibition following 6-alkynylfucose (6AF) administration triggered a dose-dependent decrease in the proliferation and migration of iCCA cell lines. Notably, adding fucose to the cell medium annulled these effects. At the molecular level, 6AF administration or small interfering RNA-mediated silencing of GDP-L-fucose synthetase (FX) and the GDP-fucose transmembrane transporter (SLC35C1), both pivotal players of cellular fucosylation, decreased NOTCH activity, NOTCH1/Jagged1 interaction, NOTCH receptors, and related target genes in iCCA cell lines. In the same cells, EGFR, nuclear factor kappa-light-chain-enhancer of activated B cells p65, and Bcl-xL protein levels diminished, whereas IκBα (a critical cellular NF-κB inhibitor) increased after FX/SLC35C1 knockdown or 6AF administration. In the chick chorioallantoic membrane assay, 6AF treatment profoundly suppresses the growth of iCCA cells.

CONCLUSIONS

Elevated global fucosylation characterizes human iCCA, contributing to cell growth and migration through the upregulation of the NOTCH and EGFR/NF-κB pathways. Thus, aberrant fucosylation is a novel pathogenetic player and a potential therapeutic target for human iCCA.

The O-GlcNAc dichotomy: when does adaptation become pathological?

O-Linked attachment of β-N-acetylglucosamine (O-GlcNAc) on serine and threonine residues of nuclear, cytoplasmic, and mitochondrial proteins is a highly dynamic and ubiquitous post-translational modification that impacts the function, activity, subcellular localization, and stability of target proteins. Physiologically, acute O-GlcNAcylation serves primarily to modulate cellular signaling and transcription regulatory pathways in response to nutrients and stress. To date, thousands of proteins have been revealed to be O-GlcNAcylated and this number continues to grow as the technology for the detection of O-GlcNAc improves. The attachment of a single O-GlcNAc is catalyzed by the enzyme O-GlcNAc transferase (OGT), and their removal is catalyzed by O-GlcNAcase (OGA). O-GlcNAcylation is regulated by the metabolism of glucose via the hexosamine biosynthesis pathway, and the metabolic abnormalities associated with pathophysiological conditions are all associated with increased flux through this pathway and elevate O-GlcNAc levels. While chronic O-GlcNAcylation is well associated with cardiovascular dysfunction, only until recently, and with genetically modified animals, has O-GlcNAcylation as a contributing mechanism of cardiovascular disease emerged. This review will address and critically evaluate the current literature on the role of O-GlcNAcylation in vascular physiology, with a view that this pathway can offer novel targets for the treatment and prevention of cardiovascular diseases.

Sulfated oligosaccharide activates endothelial Notch for inducing macrophage-associated arteriogenesis to treat ischemic diseases

Ischemic diseases lead to considerable morbidity and mortality, yet conventional clinical treatment strategies for therapeutic angiogenesis fall short of being impactful. Despite the potential of biomaterials to deliver pro-angiogenic molecules at the infarct site to induce angiogenesis, their efficacy has been impeded by aberrant vascular activation and off-target circulation. Here, we present a semisynthetic low-molecular sulfated chitosan oligosaccharide (SCOS) that efficiently induces therapeutic arteriogenesis with a spontaneous generation of collateral circulation and blood reperfusion in rodent models of hind limb ischemia and myocardial infarction. SCOS elicits anti-inflammatory macrophages' (Mφs') differentiation into perivascular Mφs, which in turn directs artery formation via a cell-to-cell communication rather than secretory factor regulation. SCOS-mediated arteriogenesis requires a canonical Notch signaling pathway in Mφs via the glycosylation of protein O-glucosyltransferases 2, which results in promoting arterial differentiation and tissue repair in ischemia. Thus, this highly bioactive oligosaccharide can be harnessed to direct efficiently therapeutic arteriogenesis and perfusion for the treatment of ischemic diseases.

Proteome-wide mendelian randomization study implicates therapeutic targets in common cancers

BACKGROUND

The interest in targeted cancer therapies has been growing rapidly. While numerous cancer biomarkers and targeted treatment strategies have been developed and employed, there are still significant limitations and challenges in the early diagnosis and targeted treatment of cancers. Accordingly, there is an urgent need to identify novel targets and develop new targeted drugs.

METHODS

The study was conducted using combined cis-Mendelian randomization (cis-MR) and colocalization analysis. We analyzed data from 732 plasma proteins to identify potential drug targets associated with eight site-specific cancers. These findings were further validated using the UK Biobank dataset. Then, a protein-protein interaction network was also constructed to examine the interplay between the identified proteins and the targets of existing cancer medications.

RESULTS

This MR analysis revealed associations between five plasma proteins and prostate cancer, five with breast cancer, and three with lung cancer. Subsequently, these proteins were classified into four distinct target groups, with a focus on tier 1 and 2 targets due to their higher potential to become drug targets. Our study indicatied that genetically predicted KDELC2 (OR: 0.89, 95% CI 0.86-0.93) and TNFRSF10B (OR: 0.74, 95% CI 0.65-0.83) are inversely associated with prostate cancer. Furthermore, we observed an inverse association between CPNE1 (OR: 0.96, 95% CI 0.94-0.98) and breast cancer, while PDIA3 (OR: 1.19, 95% CI 1.10-1.30) were found to be associated with the risk of breast cancer. In addition, we also propose that SPINT2 (OR: 1.05, 95% CI 1.03-1.06), GSTP1 (OR: 0.82, 95% CI 0.74-0.90), and CTSS (OR: 0.91, 95% CI 0.88-0.95) may serve as potential therapeutic targets in prostate cancer. Similarly, GDI2 (OR: 0.85, 95% CI 0.80-0.91), ISLR2 (OR: 0.87, 95% CI 0.82-0.93), and CTSF (OR: 1.14, 95% CI 1.08-1.21) could potentially be targets for breast cancer. Additionally, we identified SFTPB (OR: 0.93, 95% CI 0.91-0.95), ICAM5 (OR: 0.95, 95% CI 0.93-0.97), and FLRT3 (OR: 1.10, 95% CI 1.05-1.15) as potential targets for lung cancer. Notably, TNFRSF10B, GSTP1, and PDIA3 were found to interact with the target proteins of current medications used in prostate or breast cancer treatment.

CONCLUSIONS

This comprehensive analysis has highlighted thirteen plasma proteins with potential roles in three site-specific cancers. Continued research in this area may reveal their therapeutic potential, particularly KDELC2, TNFRSF10B, CPNE1, and PDIA3, paving the way for more effective cancer treatments.

Synergistic regulation of Notch signaling by different O-glycans promotes hematopoiesis

Glycosylation of Notch receptors by O-fucose glycans regulates Notch ligand binding and Notch signaling during hematopoiesis. However, roles in hematopoiesis for other O-glycans that modify Notch receptors have not been determined. Here we show that the EGF domain specific GlcNAc transferase EOGT is required in mice for the optimal production of lymphoid and myeloid cells. The phenotype of Eogt null mice was largely cell-autonomous, and Notch target gene expression was reduced in T cell progenitors. Moreover, EOGT supported residual Notch signaling following conditional deletion of Pofut1 in hematopoietic stem cells (HSC). Eogt : Pofut1 double mutant HSC had more severe defects in bone marrow and in T and B cell development in thymus and spleen, compared to deletion of Pofut1 alone. The combined results show that EOGT and O-GlcNAc glycans are required for optimal hematopoiesis and T and B cell development, and that they act synergistically with POFUT1 and O-fucose glycans to promote Notch signaling in lymphoid and myeloid differentiation.

1H, 15N, 13C backbone and sidechain resonance assignments and secondary structure of mouse NOTCH1 EGF27

NOTCH1 is a transmembrane receptor in metazoans that is linked to a variety of disorders. The receptor contains an extracellular domain (ECD) with 36 tandem epidermal growth factor-like (EGF) repeats. The ECD is responsible for intercellular signaling via protein-ligand interactions with neighboring cells. Each EGF repeat consists of approximately 40 amino acids and 3 conserved disulfide bonds. The Abruptex region (EGF24-29) is critical for NOTCH1 signaling and is known for its missense mutations. Certain EGF repeats are modified with the addition of O-linked glycans and many have calcium binding sites, which give each EGF repeat a unique function. It has been shown that the loss of the O-fucose site of EGF27 alters NOTCH1 activity. To investigate the role of glycosylation in the NOTCH1 signaling pathway, nuclear magnetic resonance spectroscopy has been employed to study the structures of EGF27 and its glycoforms. Here, we report the backbone and sidechain 1H, 15N, and 13C-resonance assignments of the unmodified EGF27 protein and the predicted secondary structure derived from the assigned chemical shifts.

Glycosylation and behavioral symptoms in neurological disorders

Glycosylation, the addition of glycans or carbohydrates to proteins, lipids, or other glycans, is a complex post-translational modification that plays a crucial role in cellular function. It is estimated that at least half of all mammalian proteins undergo glycosylation, underscoring its importance in the functioning of cells. This is reflected in the fact that a significant portion of the human genome, around 2%, is devoted to encoding enzymes involved in glycosylation. Changes in glycosylation have been linked to various neurological disorders, including Alzheimer's disease, Parkinson's disease, autism spectrum disorder, and schizophrenia. Despite its widespread occurrence, the role of glycosylation in the central nervous system remains largely unknown, particularly with regard to its impact on behavioral abnormalities in brain diseases. This review focuses on examining the role of three types of glycosylation: N-glycosylation, O-glycosylation, and O-GlcNAcylation, in the manifestation of behavioral and neurological symptoms in neurodevelopmental, neurodegenerative, and neuropsychiatric disorders.

Notch1 and Notch4 core binding domain peptibodies exhibit distinct ligand-binding and anti-angiogenic properties

The Notch signaling pathway is an important therapeutic target for the treatment of inflammatory diseases and cancer. We previously created ligand-specific inhibitors of Notch signaling comprised of Fc fusions to specific EGF-like repeats of the Notch1 extracellular domain, called Notch decoys, which bound ligands, blocked Notch signaling, and showed anti-tumor activity with low toxicity. However, the study of their function depended on virally mediated expression, which precluded dosage control and limited clinical applicability. We have refined the decoy design to create peptibody-based Notch inhibitors comprising the core binding domains, EGF-like repeats 10-14, of either Notch1 or Notch4. These Notch peptibodies showed high secretion properties and production yields that were improved by nearly 100-fold compared to previous Notch decoys. Using surface plasmon resonance spectroscopy coupled with co-immunoprecipitation assays, we observed that Notch1 and Notch4 peptibodies demonstrate strong but distinct binding properties to Notch ligands DLL4 and JAG1. Both Notch1 and Notch4 peptibodies interfere with Notch signaling in endothelial cells and reduce expression of canonical Notch targets after treatment. While prior DLL4 inhibitors cause hyper-sprouting, the Notch1 peptibody reduced angiogenesis in a 3-dimensional in vitro sprouting assay. Administration of Notch1 peptibodies to neonate mice resulted in reduced radial outgrowth of retinal vasculature, confirming anti-angiogenic properties. We conclude that purified Notch peptibodies comprising EGF-like repeats 10-14 bind to both DLL4 and JAG1 ligands and exhibit anti-angiogenic properties. Based on their secretion profile, unique Notch inhibitory activities, and anti-angiogenic properties, Notch peptibodies present new opportunities for therapeutic Notch inhibition.

HexNAcQuest: A Tool to Distinguish O-GlcNAc and O-GalNAc

Protein glycosylation plays crucial roles in the regulation of diverse biological processes. As a critical step, mass spectrometry-based site-specific analysis of protein glycosylation is important to better understand these events. Despite the great progress, characterization of structural isomers of glycans and glycopeptides remains challenging. In typical glycoproteomic analysis, collision-induced dissociation (CID) or higher-energy collisional dissociation (HCD) fragmentation produces abundant saccharide oxonium ions containing N-acetylhexosamine (HexNAc) residues. However, it has been difficult to distinguish isobaric GalNAc and GlcNAc modifications by using mass spectrometry only. By using intensities of oxonium ions of standard O-GlcNAc/O-GalNAc peptides, we systematically investigated the fragmentation patterns of different ions. Then a binary logistic regression model was established by training comprehensive data sets from glycoproteomics studies reported. The model was then tested with independent O-glycoproteomics data sets, with reliable classification achieved (>87% accuracy). In comparison to empirical observations and criteria used previously, our model is accurate and generalized. Based on this model, a corresponding Web server HexNAcQuest has been constructed, which is freely accessible to users. The model can also be easily integrated in MS-based glycoproteomics workflows to distinguish the isobaric HexNAc modifications.

Current Opportunities for Targeting Dysregulated Neurodevelopmental Signaling Pathways in Glioblastoma

Glioblastoma (GBM) is the most common and highly lethal type of brain tumor, with poor survival despite advances in understanding its complexity. After current standard therapeutic treatment, including tumor resection, radiotherapy and concomitant chemotherapy with temozolomide, the median overall survival of patients with this type of tumor is less than 15 months. Thus, there is an urgent need for new insights into GBM molecular characteristics and progress in targeted therapy in order to improve clinical outcomes. The literature data revealed that a number of different signaling pathways are dysregulated in GBM. In this review, we intended to summarize and discuss current literature data and therapeutic modalities focused on targeting dysregulated signaling pathways in GBM. A better understanding of opportunities for targeting signaling pathways that influences malignant behavior of GBM cells might open the way for the development of novel GBM-targeted therapies.

Proteome analysis of high affinity mouse saliva proteins to hydroxyapatite

Caries sensitivity varies between the two strains of inbred mice, BALB/cA has high sensitivity and C3H/HeN has low sensitivity. One potential reason seems to be a difference in pellicle-forming saliva protein composition. Here, we performed a proteomic analysis in order to identify differences of hydroxyapatite (HAP) adsorbed saliva proteins between these two mouse strains. HAP column chromatography revealed twice the quantity of high-affinity saliva proteins in C3H/HeN compared to BALB/cA. One- and two-dimensional electrophoresis showed 2 bands/spots with deviating migration. They were identified as murine carbonic anhydrase VI (CAVI) by peptide mass fingerprinting and confirmed with western blotting using a specific polyclonal antibody. Total RNA from the salivary glands of both mouse strains, PCR amplification of cDNA with a CAVI specific primer, and sequence analysis revealed one different base in codon 96, resulting in one different amino acid. Glyco-chains of CAVI deviate in one N-glycan, confirmed by mass analysis. CAVI activity was estimated from distinct circular dichroism spectra of the molecules and found higher in C3H/HeN mice. In summary, the CAVI composition of BALB/cA and C3H/HeN differs in one amino acid and a glyco-chain modification. Further, saliva from caries resistant C3H/HeN mice displayed higher CAVI activity and also overall hydroxyapatite adsorption, suggesting a relationship with caries susceptibility.

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CAVI was the salivary protein with high affinity for hydroxyapatite in two mice strains with different caries susceptibility.

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CAVI of the two strains showed differences in molecular weight, amino acids and genes, glyco-chain modification and enzyme activity.

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Differences in CAVI activity might contribute to caries susceptibility.

Fringe family genes and their modulation of Notch signaling in cancer

Fringes are glycosyltransferases that transfer N-acetylglucosamine to the O-linked fucose of Notch receptors. They regulate the Notch signaling activity that drives tumor formation and progression, resulting in poor prognosis. However, the specific tumor-promoting role of Fringes differs depending on the type of cancer. Although a particular Fringe member could act as a tumor suppressor in one cancer type, it may act as an oncogene in another. This review discusses the tumorigenic role of the Fringe family (lunatic fringe, manic fringe, and radical fringe) in modulating Notch signaling in various cancers. Although the crucial functions of Fringes continue to emerge as more mechanistic studies are being pursued, further translational research is needed to explore their roles and therapeutic benefits in various malignancies.

Mapping the O-GlcNAc Modified Proteome: Applications for Health and Disease

O-GlcNAc is a pleotropic, enigmatic post-translational modification (PTM). This PTM modifies thousands of proteins differentially across tissue types and regulates diverse cellular signaling processes. O-GlcNAc is implicated in numerous diseases, and the advent of O-GlcNAc perturbation as a novel class of therapeutic underscores the importance of identifying and quantifying the O-GlcNAc modified proteome. Here, we review recent advances in mass spectrometry-based proteomics that will be critical in elucidating the role of this unique glycosylation system in health and disease.

Shrimp thrombospondin (TSP): presence of O-β1,4 N-acetylglucosamine polymers and its function in TSP chain association in egg extracellular matrix

We characterized the existence of O-β(1,4)-GlcNAc polymers (β1,4GNP) that were anchored on the O-linked glycosylation sites of shrimp thrombospondin (pmTSP-II). There were five putative β1,4GNP linkages on the epithelial growth factor-like domain of pmTSP-II. Antibody against O-β-GlcNAc (CTD110.6) was used to prove the existence of linear and complex β1,4GNP. The antibody well reacted with linear chito-triose, -tetraose and -pentaose conjugated with phosphatidylethanolamine lipid. The immunoreactivity could also be detected with a complex β1,4GNP within pmTSP-II (at MW > 250 kDa). Upon denaturing the protein with SDS-PAGE buffer, the size of pmTSP-II was shifted to be 250 kDa, approximately 2.5 folds larger than the deduced molecular mass of pmTSP-II (110 kDa), suggesting additional association of pmTSP-II apart from its known disulfide bridging. This was confirmed by chitinase digestion on pmTSP-II protein leading to the subsequent smaller protein bands at 110–170 kDa in time- and concentration-dependent manners. These bands well reacted with CTD110.6 antibody and disappeared after extensive chitinase hydrolysis. Together, we believe that β1,4GNP on pmTSP-II serve the function in an inter-chain association to provide structural architecture of egg extracellular matrix, a novel function of pmTSP-II in reproductive biology.

How Nanotechniques Could Vitalize the O-GlcNAcylation-Targeting Approach for Cancer Therapy

Accumulated data indicated that many types of cancers have increased protein O-GlcNAcylation at cell surface and inside cells. The aberrant O-GlcNAcylation is considered a potential therapeutic target. Although several types of compounds capable of inhibiting O-GlcNAcylation have been developed, their low solubility, poor permeability and delivery efficiency have impeded the application for in vivo and pre-clinical studies. Nanocarriers have the advantages of controllable drug release and active cancer-targeting capability. Moreover, nanoparticles can improve drug delivery efficiency and reduce the non-specific distribution in normal tissues by the enhanced permeability and retention (EPR) effect in cancer. Taking the advantage of O-GlcNAc-specific antibodies or lectins, nanoparticles could further improve their cancer-targeting capability. Although nanocarriers targeting the canonical N- and O-linked glycosylation have been extensively investigated for cancer detection and therapy, application of nanotechniques for the specific targeting of O-GlcNAcylation has not been actively pursued. This review summarizes the general features of GlcNAcylation and its alterations in cancers. Analyses are focused on the following areas: How the nanocarriers may improve the solubility and/or cell permeability of O-GlcNAc transferase (OGT) inhibitors; The modification of nanocarriers with lectins or antibodies for active targeting of O-GlcNAc; The nanocarriers-mediated co-delivery of OGT inhibitors and conventional drugs, which may lead to synergistic effects. Unsolved issues impeding the research progression on O-GlcNAcylation-targeting scheme are also discussed.

Identification, function, and biological relevance of POGLUT2 and POGLUT3

O-glycosylation of Epidermal Growth Factor-like (EGF) repeats plays crucial roles in protein folding, trafficking and function. The Notch extracellular domain has been used as a model to study these mechanisms due to its many O-glycosylated EGF repeats. Three enzymes were previously known to O-glycosylate Notch EGF repeats: Protein O-Glucosyltransferase 1 (POGLUT1), Protein O-Fucosyltransferase 1 (POFUT1), and EGF Domain Specific O-Linked N-Acetylglucosamine Transferase (EOGT). All of these modifications affect Notch activity. Recently, POGLUT2 and POGLUT3 were identified as two novel O-glucosyltransferases that modify a few Notch EGF repeats at sites distinct from those modified by POGLUT1. Comparison of these modification sites revealed a putative consensus sequence which predicted modification of many extracellular matrix proteins including fibrillins (FBNs) and Latent TGFβ-binding proteins (LTBPs). Glycoproteomic analysis revealed that approximately half of the 47 EGF repeats in FBN1 and FBN2, and half of the 18 EGF repeats in LTBP1, are modified by POGLUT2 and/or POGLUT3. Cellular assays showed that loss of modifications by POGLUT2 and/or POGLUT3 significantly reduces FBN1 secretion. There is precedent for EGF modifications to affect protein-protein interactions, as has been demonstrated by research of POGLUT1 and POFUT1 modifications on Notch. Here we discuss the identification and characterization of POGLUT2 and POGLUT3 and the ongoing research that continues to elucidate the biological significance of these novel enzymes.

Exploring the Candidate Terminal Glycan Profile in Neural Regeneration of the Sea Urchin Paracentrotus lividus, Using Lectin Blotting and Mass Spectrometry

Glycans are expressed as conjugates of glycoproteins, glycolipids, and proteoglycans. The huge diversity of glycans on glycoconjugates contributes to many biological processes, from glycan-based molecular recognition to developmental events, such as regeneration in the nervous system. Echinoderms, which have a close phylogenetic relationship with chordates, are an important group of marine invertebrates for body regeneration. Although many major roles of glycans on glycoconjugates are known, their role in the glycosylation profile of the nervous system in sea urchins is poorly understood. In this study, we aimed to determine the terminal glycan profile by lectin blotting and to quantify sialic acids by the capillary liquid chromatography electrospray ionization tandem mass spectrometry system in the nervous tissue of the sea urchin Paracentrotus lividus. We determined the N-acetyl-D-glucosamine, mannose, and sialic acids (mainly α2,3 linked) by lectin blotting and five types of sialic acids (N-glycolylneuraminic acid, N-acetylneuraminic acid, 9-O-acetyl-N-alycolylneuraminic acid, 5-N-acetyl-9-O-acetyl-N-acetylneuraminic acid, and di-O-acetylated-N-alycolylneuraminic acid) by capillary liquid chromatography electrospray ionization tandem mass spectrometry. This potential first description of the terminal glycan profile in the nervous system of the sea urchin is expected to help us understand its role in nervous system development and regeneration.

Glycans that regulate Notch signaling in the intestine

Intestinal homeostasis is key to the maintenance of good health. The small intestine plays important roles in absorption, digestion, hormonal and immune functions. Crypt base columnar (CBC) stem cells residing at the bottom of crypts are nurtured by Paneth cells, and together create the stem cell niche, the foundation of intestinal homeostasis. CBC stem cells replicate to replenish their number, or differentiate into a variety of epithelial cells with specialized functions. Notch signaling is a cell-cell signaling pathway that regulates both the proliferation and differentiation of CBC stem cells. NOTCH1 and NOTCH2 stimulated by canonical Notch ligands DLL1 and DLL4 mediate Notch signaling in the intestine that, in concert with other signaling pathways including the WNT and BMP pathways, determines cell fates. Importantly, interactions between Notch receptors and canonical Notch ligands are regulated by O-glycans linked to Ser/Thr in epidermal growth factor-like (EGF) repeats of the Notch receptor extracellular domain (NECD). The O-glycans attached to NECD are key regulators of the strength of Notch signaling. Imbalances in Notch signaling result in altered cell fate decisions and may lead to cancer in the intestine. In this review, we summarize the impacts of mutations in Notch pathway members on intestinal development and homeostasis, with a focus on the glycosyltransferases that transfer O-glycans to EGF repeats of NOTCH1, NOTCH2, DLL1 and DLL4.

Significant Roles of Notch O-Glycosylation in Cancer

Notch signaling, which was initially identified in Drosophila wing morphogenesis, plays pivotal roles in cell development and differentiation. Optimal Notch pathway activity is essential for normal development and dysregulation of Notch signaling leads to various human diseases, including many types of cancers. In hematopoietic cancers, such as T-cell acute lymphoblastic leukemia, Notch plays an oncogenic role, while in acute myeloid leukemia, it has a tumor-suppressive role. In solid tumors, such as hepatocellular carcinoma and medulloblastoma, Notch may have either an oncogenic or tumor-suppressive role, depending on the context. Aberrant expression of Notch receptors or ligands can alter the ligand-dependent Notch signaling and changes in trafficking can lead to ligand-independent signaling. Defects in any of the two signaling pathways can lead to tumorigenesis and tumor progression. Strikingly, O-glycosylation is one such process that modulates ligand–receptor binding and trafficking. Three types of O-linked modifications on the extracellular epidermal growth factor-like (EGF) repeats of Notch receptors are observed, namely O-glucosylation, O-fucosylation, and O-N-acetylglucosamine (GlcNAc) modifications. In addition, O-GalNAc mucin-type O-glycosylation outside the EGF repeats also appears to occur in Notch receptors. In this review, we first briefly summarize the basics of Notch signaling, describe the latest information on O-glycosylation of Notch receptors classified on a structural basis, and finally describe the regulation of Notch signaling by O-glycosylation in cancer.

Generation of Properly Folded Epidermal Growth Factor-Like (EGF) Repeats and Glycosyltransferases Enables In Vitro O-Glycosylation

The epidermal growth factor-like (EGF) domain is an evolutionarily conserved motif found widely distributed among numerous secreted and membrane-anchored proteins, including the Notch receptors. Notch receptors include numerous EGF repeats tandemly connected in the extracellular domain. These EGF repeats must be properly folded in order for them to undergo the three different types of O-glycosylation associated with these extracellular proteins: O-fucose, O-glucose, and O-N-acetylglucosamine via glycosyltransferases POFUT1, POGLUT1, and EOGT. The O-glycosylation of the EGF repeats in the Notch receptors regulates the activation of Notch signaling and mutations in POFUT1, POGLUT1, and EOGT have been linked to specific human diseases. A large amount of EGF repeat and glycosyltransferase protein is required to construct an in vitro O-glycosylation system. Here, we describe how we prepared properly folded EGF repeats using two different bacterial expression vectors, generated recombinant glycosyltransferases, and performed in vitro O-glycosylation and subsequent product analysis by mass spectrometry. The methods described here are useful for investigating the enzymatic activities of mutated glycosyltransferases, revealing the structural basis of the O-glycosylation mechanism by co-crystallization of the glycosyltransferase-EGF repeat complexes, or identifying potential inhibitors of these glycosyltransferases.

Advances in chemical probing of protein O-GlcNAc glycosylation: structural role and molecular mechanisms

The addition of O-linked-β-D-N-acetylglucosamine (O-GlcNAc) onto serine and threonine residues of nuclear and cytoplasmic proteins is an abundant, unique post-translational modification governing important biological processes. O-GlcNAc dysregulation underlies several metabolic disorders leading to human diseases, including cancer, neurodegeneration and diabetes. This review provides an extensive summary of the recent progress in probing O-GlcNAcylation using mainly chemical methods, with a special focus on discussing mechanistic insights and the structural role of O-GlcNAc at the molecular level. We highlight key aspects of the O-GlcNAc enzymes, including development of OGT and OGA small-molecule inhibitors, and describe a variety of chemoenzymatic and chemical biology approaches for the study of O-GlcNAcylation. Special emphasis is placed on the power of chemistry in the form of synthetic glycopeptide and glycoprotein tools for investigating the site-specific functional consequences of the modification. Finally, we discuss in detail the conformational effects of O-GlcNAc glycosylation on protein structure and stability, relevant O-GlcNAc-mediated protein interactions and its molecular recognition features by biological receptors. Future research in this field will provide novel, more effective chemical strategies and probes for the molecular interrogation of O-GlcNAcylation, elucidating new mechanisms and functional roles of O-GlcNAc with potential therapeutic applications in human health.

The developmental origins of Notch-driven intrahepatic bile duct disorders

The Notch signaling pathway is an evolutionarily conserved mechanism of cell-cell communication that mediates cellular proliferation, cell fate specification, and maintenance of stem and progenitor cell populations. In the vertebrate liver, an absence of Notch signaling results in failure to form bile ducts, a complex tubular network that radiates throughout the liver, which, in healthy individuals, transports bile from the liver into the bowel. Loss of a functional biliary network through congenital malformations during development results in cholestasis and necessitates liver transplantation. Here, we examine to what extent Notch signaling is necessary throughout embryonic life to initiate the proliferation and specification of biliary cells and concentrate on the animal and human models that have been used to define how perturbations in this signaling pathway result in developmental liver disorders.

POGLUT2 and POGLUT3 O-glucosylate multiple EGF repeats in fibrillin-1, -2, and LTBP1 and promote secretion of fibrillin-1

Fibrillin-1 (FBN1) is the major component of extracellular matrix microfibrils, which are required for proper development of elastic tissues, including the heart and lungs. Through protein-protein interactions with latent transforming growth factor (TGF) β-binding protein 1 (LTBP1), microfibrils regulate TGF-β signaling. Mutations within the 47 epidermal growth factor-like (EGF) repeats of FBN1 cause autosomal dominant disorders including Marfan Syndrome, which is characterized by disrupted TGF-β signaling. We recently identified two novel protein O-glucosyltransferases, Protein O-glucosyltransferase 2 (POGLUT2) and 3 (POGLUT3), that modify a small fraction of EGF repeats on Notch. Here, using mass spectral analysis, we show that POGLUT2 and POGLUT3 also modify over half of the EGF repeats on FBN1, fibrillin-2 (FBN2), and LTBP1. While most sites are modified by both enzymes, some sites show a preference for either POGLUT2 or POGLUT3. POGLUT2 and POGLUT3 are homologs of POGLUT1, which stabilizes Notch proteins by addition of O-glucose to Notch EGF repeats. Like POGLUT1, POGLUT2 and 3 can discern a folded versus unfolded EGF repeat, suggesting POGLUT2 and 3 are involved in a protein folding pathway. In vitro secretion assays using the N-terminal portion of recombinant FBN1 revealed reduced FBN1 secretion in POGLUT2 knockout, POGLUT3 knockout, and POGLUT2 and 3 double-knockout HEK293T cells compared with wild type. These results illustrate that POGLUT2 and 3 function together to O-glucosylate protein substrates and that these modifications play a role in the secretion of substrate proteins. It will be interesting to see how disease variants in these proteins affect their O-glucosylation.

Bioinformatics and Functional Analyses Implicate Potential Roles for EOGT and L-fringe in Pancreatic Cancers

Notch signaling receptors, ligands, and their downstream target genes are dysregulated in pancreatic ductal adenocarcinoma (PDAC), suggesting a role of Notch signaling in pancreatic tumor development and progression. However, dysregulation of Notch signaling by post-translational modification of Notch receptors remains poorly understood. Here, we analyzed the Notch-modifying glycosyltransferase involved in the regulation of the ligand-dependent Notch signaling pathway. Bioinformatic analysis revealed that the expression of epidermal growth factor (EGF) domain-specific O-linked N-acetylglucosamine (EOGT) and Lunatic fringe (LFNG) positively correlates with a subset of Notch signaling genes in PDAC. The lack of EOGT or LFNG expression inhibited the proliferation and migration of Panc-1 cells, as observed by the inhibition of Notch activation. EOGT expression is significantly increased in the basal subtype, and low expression of both EOGT and LFNG predicts better overall survival in PDAC patients. These results imply potential roles for EOGT- and LFNG-dependent Notch signaling in PDAC.

Structure, function, and pathology of protein O-glucosyltransferases

Protein O-glucosylation is a crucial form of O-glycosylation, which involves glucose (Glc) addition to a serine residue within a consensus sequence of epidermal growth factor epidermal growth factor (EGF)-like repeats found in several proteins, including Notch. Glc provides stability to EGF-like repeats, is required for S2 cleavage of Notch, and serves to regulate the trafficking of Notch, crumbs2, and Eyes shut proteins to the cell surface. Genetic and biochemical studies have shown a link between aberrant protein O-glucosylation and human diseases. The main players of protein O-glucosylation, protein O-glucosyltransferases (POGLUTs), use uridine diphosphate (UDP)-Glc as a substrate to modify EGF repeats and reside in the endoplasmic reticulum via C-terminal KDEL-like signals. In addition to O-glucosylation activity, POGLUTs can also perform protein O-xylosylation function, i.e., adding xylose (Xyl) from UDP-Xyl; however, both activities rely on residues of EGF repeats, active-site conformations of POGLUTs and sugar substrate concentrations in the ER. Impaired expression of POGLUTs has been associated with initiation and progression of human diseases such as limb-girdle muscular dystrophy, Dowling-Degos disease 4, acute myeloid leukemia, and hepatocytes and pancreatic dysfunction. POGLUTs have been found to alter the expression of cyclin-dependent kinase inhibitors (CDKIs), by affecting Notch or transforming growth factor-β1 signaling, and cause cell proliferation inhibition or induction depending on the particular cell types, which characterizes POGLUT's cell-dependent dual role. Except for a few downstream elements, the precise mechanisms whereby aberrant protein O-glucosylation causes diseases are largely unknown, leaving behind many questions that need to be addressed. This systemic review comprehensively covers literature to understand the O-glucosyltransferases with a focus on POGLUT1 structure and function, and their role in health and diseases. Moreover, this study also raises unanswered issues for future research in cancer biology, cell communications, muscular diseases, etc.

Mass spectrometry-based protein-protein interaction networks for the study of human diseases

A better understanding of the molecular mechanisms underlying disease is key for expediting the development of novel therapeutic interventions. Disease mechanisms are often mediated by interactions between proteins. Insights into the physical rewiring of protein-protein interactions in response to mutations, pathological conditions, or pathogen infection can advance our understanding of disease etiology, progression, and pathogenesis and can lead to the identification of potential druggable targets. Advances in quantitative mass spectrometry (MS)-based approaches have allowed unbiased mapping of these disease-mediated changes in protein-protein interactions on a global scale. Here, we review MS techniques that have been instrumental for the identification of protein-protein interactions at a system-level, and we discuss the challenges associated with these methodologies as well as novel MS advancements that aim to address these challenges. An overview of examples from diverse disease contexts illustrates the potential of MS-based protein-protein interaction mapping approaches for revealing disease mechanisms, pinpointing new therapeutic targets, and eventually moving toward personalized applications.

Diseases related to Notch glycosylation

The Notch receptors are a family of transmembrane proteins that mediate direct cell-cell interactions and control numerous cell-fate specifications in humans. The extracellular domains of mammalian Notch proteins contain 29-36 tandem epidermal growth factor-like (EGF) repeats, most of which have O-linked glycan modifications: O-glucose added by POGLUT1, O-fucose added by POFUT1 and elongated by Fringe enzymes, and O-GlcNAc added by EOGT. The extracellular domain is also N-glycosylated. Mutations in the glycosyltransferases modifying Notch have been identified in several diseases, including Dowling-Degos Disease (haploinsufficiency of POFUT1 or POGLUT1), a form of limb-girdle muscular dystrophy (autosomal recessive mutations in POGLUT1), Spondylocostal Dysostosis 3 (autosomal recessive mutations in LFNG), Adams-Oliver syndrome (autosomal recessive mutations in EOGT), and some cancers (amplification, gain or loss-of-function of POFUT1, Fringe enzymes, POGLUT1, MGAT3). Here we review the characteristics of these diseases and potential molecular mechanisms.

Overproduction of inter-α-trypsin inhibitor heavy chain 1 after loss of Gα13 in liver exacerbates systemic insulin resistance in mice

The impact of liver disease on whole-body glucose homeostasis is largely attributed to dysregulated release of secretory proteins in response to metabolic stress. The molecular cues linking liver to whole-body glucose metabolism remain elusive. We found that expression of G protein α-13 (Gα₁₃) was decreased in the liver of mice and humans with diabetes. Liver-specific deletion of the Gna13 gene in mice resulted in systemic glucose intolerance. Comparative secretome analysis identified inter-α-trypsin inhibitor heavy chain 1 (ITIH1) as a protein secreted by liver that was responsible for systemic insulin resistance in Gna13-deficient mice. Liver expression of ITIH1 positively correlated with surrogate markers for diabetes in patients with impaired glucose tolerance or overt diabetes. Mechanistically, a decrease in hepatic Gα₁₃ caused ITIH1 oversecretion by liver through induction of O-GlcNAc transferase expression, facilitating ITIH1 deposition on the hyaluronan surrounding mouse adipose tissue and skeletal muscle. Neutralization of secreted ITIH1 ameliorated glucose intolerance in obese mice. Our findings demonstrate systemic insulin resistance in mice resulting from liver-secreted ITIH1 downstream of Gα₁₃ and its reversal by ITIH1 neutralization.

Molecular Mechanisms of KDELC2 on Glioblastoma Tumorigenesis and Temozolomide Resistance

The activation of the Notch pathway induces glioblastoma (GBM) development. Since KDEL (Lys-Asp-Glu-Leu) containing 2 (KDELC2) is involved in the Notch pathway, the detailed mechanism is still undetermined. The Cancer Genome Atlas (TCGA) and Chinese Glioma Genome Atlas (CGGA) databases revealed that KDELC2 mRNA was associated with oncologic factors of GBM. U87, LN229, LNZ308, U118MG, and GBM8401 cells showed higher KDELC2 expression than normal brain tissues. The results of MTT, wound healing, and invasion assays proved that KDELC2 knockdown suppressed GBM-aggressive behaviors. The inhibitory properties of GBM stemness and angiogenesis under KDELC2 knockdown were evaluated by tumor spheroid and tube formation assays. Suppression of KDELC2 downregulated Notch factors’ expressions, including KDELC1, pofut1, Notch receptors 1–3, and HES-1. Immunoblot assay showed that KDELC2 knockdown promoted tumor apoptosis by downregulating PI3k/mTOR/Akt, MAPK/ERK, and NF-kB pathways. The combination of KDELC2 knockdown and temozolomide (TMZ) treatment had an optimal therapeutic effect by suppressing MGMT expression. Results of an orthotopic xenograft animal model and human tissue confirmed that KDELC2 correlated with glioma proliferation, advanced grades, and poor prognosis. Therefore, KDELC2 might be a potential pharmacological target to inhibit tumorigenesis, epithelial–mesenchymal transition, angiogenesis, and chemo-resistance of GBM.

Effect of Site-Specific O-Glycosylation on the Structural Behavior of NOTCH1 Receptor Extracellular EGF-like Domains 11 and 10

Human NOTCH1 receptor contains 36 epidermal growth factor (EGF)-like repeating domains, in which O-glycosylation status of EGF12 domain regulates the interaction with Notch ligands. Our interest is focused on the effect of specific O-glycosylation states on the structural behavior of EGF11 and EGF10, because they appeared to affect molecular mechanism in receptor-ligand interactions by inducing some conformational alterations in these domains and/or the regions connecting two domains. To understand the structural impact of various O-glycosylation patterns on the pivotal EGF-like repeats 10, 11, and 12, we performed chemical synthesis and NMR studies of site-specifically O-glycosylated EGF11 and EGF10. Our strategy enabled us to synthesize four EGF11 and five EGF10 modules. The specific O-glycosylation states affected in vitro folding of EGF10 more than EGF11, while calcium ion had a larger effect on EGF11 folding. Comprehensive NMR studies shed light on the new type "sugar bridges" crosslinking Thr-O-GlcNAc in the consensus sequence C5-X-X-G-X-(T/S)-G-X-X-C6 and an amino acid in the hinge region between the domains, 445Thr-O-GlcNAc-IIe451 in domain 11 and 405Thr-O-GlcNAc-Gln411 in domain 10, respectively.

Targeting O-GlcNAcylation to develop novel therapeutics

O-linked β-D-N-acetylglucosamine (O-GlcNAc) is an abundant post-translational modification (PTM) that modifies the serine or threonine residues of thousands of proteins in the nucleus, cytoplasm and mitochondria. Being a major "nutrient sensor" in cells, the O-GlcNAc pathway is sensitive to cellular metabolic states. Extensive crosstalk is observed between O-GlcNAcylation and protein phosphorylation. O-GlcNAc regulates protein functions at multiple levels, including enzymatic activity, transcriptional activity, subcellular localization, intermolecular interactions and degradation. Abnormal O-GlcNAcylation is associated with many human diseases including cancer, diabetes and neurodegenerative diseases. Though research on O-GlcNAc is still in its infantry, accumulating evidence suggest O-GlcNAcylation to be a promising therapeutic target. In this review, we briefly discuss the basic features of this PTM, the O-GlcNAc signaling pathway, its regulatory functions on different proteins, and its involvement in human diseases. We hope this review will provide insights to researchers who study human disease, as well as researchers who are interested in the fundamental roles of O-GlcNAcylation in all cells.

Expression, purification, and glycosylation of epidermal growth factor-like repeat 27 from mouse NOTCH1

Notch receptors have large extracellular domains containing up to 36 tandem epidermal growth factor-like (EGF) repeats, which facilitate cell signaling by binding ligands on neighboring cells. Notch receptors play major roles in a variety of developmental processes by controlling cell fate decisions. Each EGF repeat consists of about 40 amino acids with 3 conserved disulfide bonds. Many of the EGF repeats are modified by O-linked fucose glycans, and more than half have calcium-binding sites, but the sequences of the EGF repeats vary giving distinct roles to each repeat. EGF repeat 27 (EGF27) from mouse NOTCH1 is modified with O-fucose and is 1 of 7 repeats that is differentially modified by specific Fringe enzymes, which are known to regulate NOTCH1 activation and ligand binding. To better understand the role of EGF27 in NOTCH1 function and regulation, the 3-dimensional structures of EGF27 and its glycoforms are being pursued. E. coli cells were used to produce EGF27 in sufficient quantities for nuclear magnetic resonance analysis. Previous attempts to express the repeat alone and refold the repeat under a steady redox environment were unsuccessful due to low yields and extensive mixed-disulfide bond cross-linking. A new strategy using a cleavable maltose binding protein fusion tag increased the solubility and yield of EGF27. With the fusion tag, EGF27 was refolded to produce the correct disulfide bond arrangement, which was verified enzymatically with the glycosyltransferases, Protein O-fucosyltransferase 1 (POFUT1) and Lunatic Fringe (LFNG).

Multifaceted regulation of Notch signaling by glycosylation

To build a complex body composed of various cell types and tissues and to maintain tissue homeostasis in the postembryonic period, animals use a small number of highly conserved intercellular communication pathways. Among these is the Notch signaling pathway, which is mediated via the interaction of transmembrane Notch receptors and ligands usually expressed by neighboring cells. Maintaining optimal Notch pathway activity is essential for normal development, as evidenced by various human diseases caused by decreased and increased Notch signaling. It is therefore not surprising that multiple mechanisms are used to control the activation of this pathway in time and space. Over the last 20 years, protein glycosylation has been recognized as a major regulatory mechanism for Notch signaling. In this review, we will provide a summary of the various types of glycan that have been shown to modulate Notch signaling. Building on recent advances in the biochemistry, structural biology, cell biology and genetics of Notch receptors and the glycosyltransferases that modify them, we will provide a detailed discussion on how various steps during Notch activation are regulated by glycans. Our hope is that the current review article will stimulate additional research in the field of Notch glycobiology and will potentially be of benefit to investigators examining the contribution of glycosylation to other developmental processes.

N-Glycans on EGF domain-specific O-GlcNAc transferase (EOGT) facilitate EOGT maturation and peripheral endoplasmic reticulum localization

Epidermal growth factor (EGF) domain-specific O-GlcNAc transferase (EOGT) is an endoplasmic reticulum (ER)-resident protein that modifies EGF repeats of Notch receptors and thereby regulates Delta-like ligand-mediated Notch signaling. Several EOGT mutations that may affect putative N-glycosylation consensus sites are recorded in the cancer database, but the presence and function of N-glycans in EOGT have not yet been characterized. Here, we identified N-glycosylation sites in mouse EOGT and elucidated their molecular functions. Three predicted N-glycosylation consensus sequences on EOGT are highly conserved among mammalian species. Within these sites, we found that Asn-263 and Asn-354, but not Asn-493, are modified with N-glycans. Lectin blotting, endoglycosidase H digestion, and MS analysis revealed that both residues are modified with oligomannose N-glycans. Loss of an individual N-glycan on EOGT did not affect its endoplasmic reticulum (ER) localization, enzyme activity, and ability to O-GlcNAcylate Notch1 in HEK293T cells. However, simultaneous substitution of both N-glycosylation sites affected both EOGT maturation and expression levels without an apparent change in enzymatic activity, suggesting that N-glycosylation at a single site is sufficient for EOGT maturation and expression. Accordingly, a decrease in O-GlcNAc stoichiometry was observed in Notch1 co-expressed with an N263Q/N354Q variant compared with WT EOGT. Moreover, the N263Q/N354Q variant exhibited altered subcellular distribution within the ER in HEK293T cells, indicating that N-glycosylation of EOGT is required for its ER localization at the cell periphery. These results suggest critical roles of N-glycans in sustaining O-GlcNAc transferase function both by maintaining EOGT levels and by ensuring its proper subcellular localization in the ER.

Contribution of extracellular O-GlcNAc to the stability of folded epidermal growth factor-like domains and Notch1 trafficking

The Notch signaling pathway is highly conserved and essential in animal development and tissue homeostasis. Regulation of Notch signaling is a crucial process for human health. Ligands initiate a signal cascade by binding to Notch receptors expressed on the neighboring cell. Notch receptors interact with ligands through their epidermal growth factor-like repeats (EGF repeats). Most EGF repeats are modified by O-glycosylation with residues, such as O-linked N-acetylglucosamine (O-GlcNAc), O-fucose, and O-glucose. A recent study revealed the distinct roles of these O-glycans in ligand binding, processing, and trafficking of Notch receptors. In particular, O-GlcNAc glycans are essential for Delta-like (DLL) ligand-mediated Notch signaling. In this study, we showed that O-GlcNAc promotes Notch1 trafficking to the cell surfaces under the condition that O-fucose and O-glucose are removed from consecutive EGF repeats of Notch1. Through in vitro experiments, we showed that O-GlcNAc mediates the stability of EGF domains in the same manner as O-fucose and O-glucose. Thus, O-GlcNAc on EGF domains possesses a shared function in the stability of EGF domains and Notch1 trafficking.

Effects of Notch glycosylation on health and diseases

Notch signaling is an evolutionarily conserved signaling pathway and is essential for cell-fate specification in metazoans. Dysregulation of Notch signaling results in various human diseases, including cardiovascular defects and cancer. In 2000, Fringe, a known regulator of Notch signaling, was discovered as a Notch-modifying glycosyltransferase. Since then, glycosylation-a post-translational modification involving literal sugars-on the Notch extracellular domain has been noted as a critical mechanism for the regulation of Notch signaling. Additionally, the presence of diverse O-glycans decorating Notch receptors has been revealed in the extracellular domain epidermal growth factor-like (EGF) repeats. Here, we concisely summarize the recent studies in the human diseases associated with aberrant Notch glycosylation.

Decoding the PTM-switchboard of Notch

The developmentally indispensable Notch pathway exhibits a high grade of pleiotropism in its biological output. Emerging evidence supports the notion of post-translational modifications (PTMs) as a modus operandi controlling dynamic fine-tuning of Notch activity. Although, the intricacy of Notch post-translational regulation, as well as how these modifications lead to multiples of divergent Notch phenotypes is still largely unknown, numerous studies show a correlation between the site of modification and the output. These include glycosylation of the extracellular domain of Notch modulating ligand binding, and phosphorylation of the PEST domain controlling half-life of the intracellular domain of Notch. Furthermore, several reports show that multiple PTMs can act in concert, or compete for the same sites to drive opposite outputs. However, further investigation of the complex PTM crosstalk is required for a complete understanding of the PTM-mediated Notch switchboard. In this review, we aim to provide a consistent and up-to-date summary of the currently known PTMs acting on the Notch signaling pathway, their functions in different contexts, as well as explore their implications in physiology and disease. Furthermore, we give an overview of the present state of PTM research methodology, and allude to a future with PTM-targeted Notch therapeutics.

Association of variants in HTRA1 and NOTCH3 with MRI-defined extremes of cerebral small vessel disease in older subjects

We report a composite extreme phenotype design using distribution of white matter hyperintensities and brain infarcts in a population-based cohort of older persons for gene-mapping of cerebral small vessel disease. We demonstrate its application in the 3C-Dijon whole exome sequencing (WES) study (n = 1924, nWESextremes = 512), with both single variant and gene-based association tests. We used other population-based cohort studies participating in the CHARGE consortium for replication, using whole exome sequencing (nWES = 2,868, nWESextremes = 956) and genome-wide genotypes (nGW = 9924, nGWextremes = 3308). We restricted our study to candidate genes known to harbour mutations for Mendelian small vessel disease: NOTCH3, HTRA1, COL4A1, COL4A2 and TREX1. We identified significant associations of a common intronic variant in HTRA1, rs2293871 using single variant association testing (Pdiscovery = 8.21 × 10-5, Preplication = 5.25 × 10-3, Pcombined = 4.72 × 10-5) and of NOTCH3 using gene-based tests (Pdiscovery = 1.61 × 10-2, Preplication = 3.99 × 10-2, Pcombined = 5.31 × 10-3). Follow-up analysis identified significant association of rs2293871 with small vessel ischaemic stroke, and two blood expression quantitative trait loci of HTRA1 in linkage disequilibrium. Additionally, we identified two participants in the 3C-Dijon cohort (0.4%) carrying heterozygote genotypes at known pathogenic variants for familial small vessel disease within NOTCH3 and HTRA1. In conclusion, our proof-of-concept study provides strong evidence that using a novel composite MRI-derived phenotype for extremes of small vessel disease can facilitate the identification of genetic variants underlying small vessel disease, both common variants and those with rare and low frequency. The findings demonstrate shared mechanisms and a continuum between genes underlying Mendelian small vessel disease and those contributing to the common, multifactorial form of the disease.

O-Linked glycosylation in Drosophila melanogaster

Glycosylation, or the addition of sugars to proteins, is a highly conserved protein modification defined by both the monosaccharide initially added as well as the amino acid to which it is attached. O-Linked glycosylation represents a diverse group of protein modifications occurring on the hydroxyl groups of serine and/or threonine residues. O-Glycosylation can have wide-ranging effects on protein stability and function, which translate into crucial consequences at the organismal level. This review will summarize structural and biological insights into the major O-glycans formed within the secretory apparatus (O-GalNAc, O-Man, O-Fuc, O-Glc and extracellular O-GlcNAc) from studies in the fruit fly Drosophila melanogaster. Drosophila has many advantages for investigating these complex modifications, boasting reduced functional redundancy within gene families, reduced length/complexity of glycan chains and sophisticated genetic tools. Gaining an understanding of the normal cellular and developmental roles of these conserved modifications in Drosophila will provide insight into how changes in O-glycans are involved in human disease and disease susceptibilities.

Structure and function of extracellular O-GlcNAc

Extracellular O-GlcNAc is a unique modification restricted to the epidermal growth factor (EGF) domain-containing glycoproteins. This O-GlcNAcylation is catalyzed by the EGF-domain specific O-GlcNAc transferase (EOGT), which is localized in the lumen of endoplasmic reticulum. In humans, EOGT is one of the causative genes of a congenital disease, Adams-Oliver syndrome. EOGT is highly expressed in endothelial cells and regulates vascular development and integrity by potentiating Delta-like ligand-mediated Notch signaling. In Drosophila, Eogt modifies Dumpy, an apical extracellular matrix glycoprotein, and affects Dumpy-dependent cell-matrix interaction. In this review, we summarize the current findings of the structure and functions of extracellular O-GlcNAc in animals.

Congenital diseases caused by defective O-glycosylation of Notch receptors

The Notch signaling pathway is highly conserved and essential for animal development. It is required for cell differentiation, survival, and proliferation. Regulation of Notch signaling is a crucial process for human health. Ligands initiate a signal cascade by binding to Notch receptors expressed on a neighboring cell. Notch receptors interact with ligands through their epidermal growth factor-like repeats (EGF repeats). Most EGF repeats are modified by O-glycosylation with residues such as O-linked N-acetylglucosamine (O-GlcNAc), O-fucose, and O-glucose. These O-glycan modifications are important for Notch function. Defects in O-glycosylation affect Notch-ligand interaction, trafficking of Notch receptors, and Notch stability on the cell surface. Although the roles of each modification are not fully understood, O-fucose is essential for binding of Notch receptors to their ligands. We reported an EGF domain-specific O-GlcNAc transferase (EOGT) localized in the endoplasmic reticulum. Mutations in genes encoding EOGT or NOTCH1 cause Adams-Oliver syndrome. Dysregulation of Notch signaling because of defects or mutations in Notch receptors or Notch signal-regulating proteins, such as glycosyltransferases, induce a variety of congenital disorders. In this review, we discuss O-glycosylation of Notch receptors and congenital human diseases caused by defects in O-glycans on Notch receptors.

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.

Glycans as Key Checkpoints of T Cell Activity and Function

The immune system is highly controlled and fine-tuned by glycosylation, through the addition of a diversity of carbohydrates structures (glycans) to virtually all immune cell receptors. Despite a relative backlog in understanding the importance of glycans in the immune system, due to its inherent complexity, remarkable findings have been highlighting the essential contributions of glycosylation in the regulation of both innate and adaptive immune responses with important implications in the pathogenesis of major diseases such as autoimmunity and cancer. Glycans are implicated in fundamental cellular and molecular processes that regulate both stimulatory and inhibitory immune pathways. Besides being actively involved in pathogen recognition through interaction with glycan-binding proteins (such as C-type lectins), glycans have been also shown to regulate key pathophysiological steps within T cell biology such as T cell development and thymocyte selection; T cell activity and signaling as well as T cell differentiation and proliferation. These effects of glycans in T cells functions highlight their importance as determinants of either self-tolerance or T cell hyper-responsiveness which ultimately might be implicated in the creation of tolerogenic pathways in cancer or loss of immunological tolerance in autoimmunity. This review discusses how specific glycans (with a focus on N-linked glycans) act as regulators of T cell biology and their implications in disease.