O-GlcNAc on NOTCH1 EGF repeats regulates ligand-induced Notch signaling and vascular development in mammals

Novel insights into the roles of migrasome in cancer

Cell migration, a hallmark of cancer malignancy, plays a critical role in cancers. Improperly initiated or misdirected cell migration can lead to invasive metastatic cancer. Migrasomes are newly discovered vesicular cellular organelles produced by migrating cells and depending on cell migration. Four marker proteins [NDST1 (bifunctionalheparan sulfate N-deacetylase/N-sulfotransferase 1), EOGT (Epidermal growth factor domains pecific O-linked N-acetylglucosaminetransferase), CPQ (carboxypeptidase Q), and PIGK (phosphatidylinositol glycan anchor biosynthesis, class K)] of migrasomes were successfully identified. There are three marker proteins (NDST1, PIGK, and EOGT) of migrasome expressed in cancer. In this review, we will discuss the process of migrasome discovery, the formation of migrasome, the possible functions of migrasome, and the differences between migrasomes and exosomes, especially, the biological functions of migrasome marker proteins in cancer, and discuss some possible roles of migrasomes in cancer. We speculate that migrasomes and migracytosis can play key roles in regulating the development of cancer.

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.

Efficient Escorting Strategy for Aggregation-Prone Notch EGF Repeats with Sparcl1

Epidermal growth factor (EGF) repeats are present in various proteins and form well-defined structures with three disulfide bonds. One representative protein is the Notch receptor. Each EGF repeat contains unique atypical O-linked glycans, such as O-linked N-acetylglucosamine (O-GlcNAc). To generate a monoclonal antibody against the O-GlcNAc moiety in mouse Notch1, we expressed the recombinant C-terminal His6-tagged Notch1 EGF14-15 protein in HEK293T cells to prepare the immunogen. Most of the proteins were not secreted and showed higher molecular weight ladders in the cell lysate, suggesting protein aggregation. To overcome this issue, we fused Sparcl1 as an extracellular escorting tag to the N-terminus of Notch1 EGF14-15. The fusion protein was efficiently secreted extracellularly without protein aggregates in the lysates. Following PreScission protease treatment, Notch1 EGF14-15 was efficiently released from the escorting tag. Notch1 EGF14-15 prepared using this method was indeed O-GlcNAcylated. The optimal length of the escorting tag was determined by generating deletion mutants to improve the extracellular secretion of EGF14-15. Hence, a large amount of EGF14-15 was successfully prepared from the culture supernatant of HEK293T cells, which were otherwise prone to aggregation.

The Notch signaling-regulated angiogenesis in rheumatoid arthritis: pathogenic mechanisms and therapeutic potentials

Angiogenesis plays a key role in the pathological process of inflammation and invasion of the synovium, and primarily drives the progression of rheumatoid arthritis (RA). Recent studies have demonstrated that the Notch signaling may represent a new therapeutic target of RA. Although the Notch signaling has been implicated in the M1 polarization of macrophages and the differentiation of lymphocytes, little is known about its role in angiogenesis in RA. In this review, we discourse the unique roles of stromal cells and adipokines in the angiogenic progression of RA, and investigate how epigenetic regulation of the Notch signaling influences angiogenesis in RA. We also discuss the interaction of the Notch-HIF signaling in RA's angiogenesis and the potential strategies targeting the Notch signaling to improve the treatment outcomes of RA. Taken together, we further suggest new insights into future research regarding the challenges in the therapeutic strategies of RA.

EOGT enables residual Notch signaling in mouse intestinal cells lacking POFUT1

Notch signaling determines cell fates in mouse intestine. Notch receptors contain multiple epidermal growth factor-like (EGF) repeats modified by O-glycans that regulate Notch signaling. Conditional deletion of protein O-fucosyltransferase 1 (Pofut1) substantially reduces Notch signaling and markedly perturbs lineage development in mouse intestine. However, mice with inactivated Pofut1 are viable, whereas complete elimination of Notch signaling in intestine is lethal. Here we investigate whether residual Notch signaling enabled by EGF-domain-specific O-linked N-acetylglucosamine transferase (Eogt) permits mice conditionally lacking Pofut1 in intestine to survive. Mice globally lacking Eogt alone were grossly unaffected in intestinal development. In contrast, mice lacking both Eogt and Pofut1 died at ~ 28 days after birth with greater loss of body weight, a greater increase in the number of goblet and Paneth cells, and greater downregulation of the Notch target gene Hes1, compared to Pofut1 deletion alone. These data reveal that both O-fucose and O-GlcNAc glycans are fundamental to Notch signaling in the intestine and provide new insights into roles for O-glycans in regulating Notch ligand binding. Finally, EOGT and O-GlcNAc glycans provide residual Notch signaling and support viability in mice lacking Pofut1 in the intestine.

Two NOTCH1 O-fucose sites have opposing functions in mouse retinal angiogenesis

Previous in vitro studies demonstrated that Fringe glycosylation of the NOTCH1 extracellular domain at O-fucose residues in Epidermal Growth Factor-like Repeats (EGFs) 6 and 8 is a significant contributor to suppression of NOTCH1 activation by JAG1 or enhancement of NOTCH1 activation by DLL1, respectively. In this study, we sought to evaluate the significance of these glycosylation sites in a mammalian model by generating 2 C57BL/6J mouse lines carrying NOTCH1 point mutations, which eliminate O-fucosylation and Fringe activity at EGFs 6 (T232V) or 8 (T311V). We assessed changes to morphology during retinal angiogenesis, a process in which expression of Notch1, Jag1, Dll4, Lfng, Mfng, and Rfng genes coordinate cell-fate decisions to grow vessel networks. In the EGF6 O-fucose mutant (6f/6f) retinas, we observed reduced vessel density and branching, suggesting that this mutant is a Notch1 hypermorph. This finding agrees with prior cell-based studies showing that the 6f mutation increased JAG1 activation of NOTCH1 during co-expression with inhibitory Fringes. Although we predicted that the EGF8 O-fucose mutant (8f/8f) would not complete embryonic development due to the direct involvement of the O-fucose in engaging ligand, the 8f/8f mice were viable and fertile. In the 8f/8f retina, we measured increased vessel density consistent with established Notch1 hypomorphs. Overall, our data support the importance of NOTCH1 O-fucose residues for pathway function and confirms that single O-glycan sites are rich in signaling instructions for mammalian development.

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.

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.

Secretory expression of mammalian NOTCH tandem epidermal growth factor-like repeats based on increased O-glycosylation

The Notch pathway represents evolutionarily conserved intercellular signaling essential for cell-to-cell communication during development. Dysregulation of Notch signaling has been implicated in various diseases, and its control represents a potential cancer treatment strategy. Notch signaling is initiated by the interaction of NOTCH receptors with their ligands on neighboring cells. Therefore, the truncated NOTCH ectodomain, composed mainly of tandem repeats of epidermal growth factor-like (EGF) domains, serves as a decoy molecule that competes for ligand binding and thus inhibits ligand-dependent Notch signaling. Although full-length NOTCH EGF repeats exhibited potent Notch inhibitory activity, they were poorly produced in the transfected cells. This study evaluated the effect of EGF domain-modifying glycosyltransferases on the secretion of NOTCH EGF repeats. Our results in HEK293T cells revealed that, unlike the effect on endogenous NOTCH receptors, overexpressed EGF domain-specific O-GlcNAc transferase (EOGT) markedly enhanced the secretion of NOTCH1 EGF repeats in an enzyme activity-dependent manner. The co-expression of protein O-glucosyltransferase 1 further manifested the effect of EOGT. The resultant changes in O-glycosylation of NOTCH3 were evaluated by label-free glycopeptide quantification. This study provides an experimental strategy to efficiently generate NOTCH EGF repeats by manipulating the expression of glycosyltransferases that alter the O-glycosylation of EGF domains.

Bladder Cancer-Derived Small Extracellular Vesicles Promote Tumor Angiogenesis by Inducing HBP-Related Metabolic Reprogramming and SerRS O-GlcNAcylation in Endothelial Cells

A malformed tumour vascular network provokes the nutrient-deprived tumour microenvironment (TME), which conversely activates endothelial cell (EC) functions and stimulates neovascularization. Emerging evidence suggests that the flexible metabolic adaptability of tumour cells helps to establish a metabolic symbiosis among various cell subpopulations in the fluctuating TME. In this study, the authors propose a novel metabolic link between bladder cancer (BCa) cells and ECs in the nutrient-scarce TME, in which BCa-secreted glutamine-fructose-6-phosphate aminotransferase 1 (GFAT1) via small extracellular vesicles (sEVs) reprograms glucose metabolism by increasing hexosamine biosynthesis pathway flux in ECs and thus enhances O-GlcNAcylation. Moreover, seryl-tRNA synthetase (SerRS) O-GlcNAcylation at serine 101 in ECs promotes its degradation by ubiquitination and impeded importin α5-mediated nuclear translocation. Intranuclear SerRS attenuates vascular endothelial growth factor transcription by competitively binding to the GC-rich region of the proximal promotor. Additionally, GFAT1 knockout in tumour cells blocks SerRS O-GlcNAcylation in ECs and attenuates angiogenesis both in vitro and in vivo. However, administration of GFAT1-overexpressing BCa cells-derived sEVs increase the angiogenetic activity in the ECs of GFAT1-knockout mice. In summary, this study suggests that inhibiting sEV-mediated GFAT1 secretion from BCa cells and targeting SerRS O-GlcNAcylation in ECs may serve as novel strategies for BCa antiangiogenetic therapy.

O-GlcNAcylation: An Emerging Protein Modification Regulating the Hippo Pathway

Simple Summary

The contact point between the Hippo pathway, which serves as a central hub for various external environments, and O-GlcNAcylation, which is a non-canonical glycosylation process acting as a dynamic regulator in various signal transduction pathways, has recently been identified. This review aims to summarize the function of O-GlcNAcylation as an intrinsic and extrinsic regulator of the Hippo pathway.

Abstract

The balance between cellular proliferation and apoptosis and the regulation of cell differentiation must be established to maintain tissue homeostasis. These cellular responses involve the kinase cascade-mediated Hippo pathway as a crucial regulator. Hence, Hippo pathway dysregulation is implicated in diverse diseases, including cancer. O-GlcNAcylation is a non-canonical glycosylation that affects multiple signaling pathways through its interplay with phosphorylation in the nucleus and cytoplasm. An abnormal increase in the O-GlcNAcylation levels in various cancer cells is a potent factor in Hippo pathway dysregulation. Intriguingly, Hippo pathway dysregulation also disrupts O-GlcNAc homeostasis, leading to a persistent elevation of O-GlcNAcylation levels, which is potentially pathogenic in several diseases. Therefore, O-GlcNAcylation is gaining attention as a protein modification that regulates the Hippo pathway. This review presents a framework on how O-GlcNAcylation regulates the Hippo pathway and forms a self-perpetuating cycle with it. The pathological significance of this self-perpetuating cycle and clinical strategies for targeting O-GlcNAcylation that causes Hippo pathway dysregulation are also discussed.

Optogenetic control of NOTCH1 signaling

The Notch signaling pathway is a crucial regulator of cell differentiation as well as tissue organization, whose deregulation is linked to the pathogenesis of different diseases. NOTCH1 plays a key role in breast cancer progression by increasing proliferation, maintenance of cancer stem cells, and impairment of cell death. NOTCH1 is a mechanosensitive receptor, where mechanical force is required to activate the proteolytic cleavage and release of the Notch intracellular domain (NICD). We circumvent this limitation by regulating Notch activity by light. To achieve this, we have engineered an optogenetic NOTCH1 receptor (optoNotch) to control the activation of NOTCH1 intracellular domain (N1ICD) and its downstream transcriptional activities. Using optoNotch we confirm that NOTCH1 activation increases cell proliferation in MCF7 and MDA-MB-468 breast cancer cells in 2D and spheroid 3D cultures, although causing distinct cell-type specific migratory phenotypes. Additionally, optoNotch activation induced chemoresistance on the same cell lines. OptoNotch allows the fine-tuning, ligand-independent, regulation of N1ICD activity and thus a better understanding of the spatiotemporal complexity of Notch signaling. Video Abstract.

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.

EOGT Correlated With Immune Infiltration: A Candidate Prognostic Biomarker for Hepatocellular Carcinoma

Background

A preliminary study by our group revealed that the deficiency of EGF domain-specific O-linked N-acetylglucosamine transferase (EOGT) impaired regulatory T-cell differentiation in autoimmune hepatitis. Nevertheless, the prognostic value of EOGT in advanced hepatocellular carcinoma (HCC) and its relationship with immune infiltration remain obscured.

Methods

Initially, EOGT expression was evaluated by Oncomine, TIMER, GEO, and UALCAN databases. Besides, the prognostic potential of EOGT expression was analyzed using GEPIA, Kaplan-Meier plotter, CPTAC, Cox regression, and nomogram in HCC samples. Furthermore, we investigated the association between EOGT expression and tumor mutation burden, DNA methylation, and immune infiltration in addition to its possible mechanism via cBioPortal, TIMER, GEPIA, ESTIMATE, CIBERSORT, GSEA, STRING, and Cytoscape.

Results

The expression of EOGT in HCC was significantly higher than that in normal tissues. Additionally, elevated EOGT expression was correlated with advanced tumor staging and linked to poor overall survival and relapse-free survival, serving as a significant unfavorable prognostic indicator in HCC patients. Remarkably, our results revealed that high-EOGT expression subgroups with elevated TP53 or low CTNNB1 mutations have worse clinical outcomes than the others. Regarding immune infiltration, immunofluorescent staining showed that immune cells in HCC were positive for EOGT. Besides, elevated EOGT expression was linked to exhausted T cells and immune suppressor cells in HCC samples. More importantly, the proportion of CD8⁺ T cells was reduced in HCC samples with a high level of EOGT expression, but EOGT did not exhibit prognostic potential in HCC samples with increased CD8⁺ T cells.

Conclusions

EOGT may hold great potential as a novel biomarker to distinguish prognosis and immune profiles of HCC patients.

A Cyclic Peptide Epitope of an Under-Explored VEGF-B Loop 1 Demonstrated In Vivo Anti-Angiogenic and Anti-Tumor Activities

Pathological angiogenesis is mainly initiated by the binding of abnormal expressed vascular endothelial growth factors (VEGFs) to their receptors (VEGFRs). Blocking the VEGF/VEGFR interaction is a clinically proven treatment in cancer. Our previous work by epitope scan had identified cyclic peptides, mimicking the loop 1 of VEGF-A, VEGF-B and placental growth factor (PlGF), inhibited effectively the VEGF/VEGFR interaction in ELISA. We described here the docking study of these peptides on VEGFR1 to identify their binding sites. The cellular anti-angiogenic activities were examined by inhibition of VEGF-A induced cell proliferation, migration and tube formation in human umbilical vein endothelial cells (HUVECs). The ability of these peptides to inhibit MAPK/ERK1/2 signaling pathway was examined as well. On chick embryo chorioallantoic membrane (CAM) model, a cyclic peptide named B-cL1 with most potent in vitro activity showed important in vivo anti-angiogenic effect. Finally, B-cL1 inhibited VEGF induced human gastric cancer SGC-7901 cells proliferation. It showed anti-tumoral effect on SGC-7901 xenografted BALB/c nude mouse model. The cyclic peptides B-cL1 constitutes an anti-angiogenic peptide drug lead for the design of new and more potent VEGFR antagonists in the treatment of angiogenesis related diseases.

Other Types of Glycosylation

O-Linked glycosylation such as O-fucose, O-glucose, and O-N-acetylglucosamine are considered to be unusual. As suggested by the high levels of evolutional conservation, these O-glycans are fundamentally important for life. In the last two decades, our understanding of the importance of these glycans has greatly advanced. In particular, identification of the glycosyltransferases responsible for the biosynthesis of these glycans has accelerated basic research on the functional significance and molecular mechanisms by which these O-glycans regulate protein functions as well as clinical research on human diseases due to changes in these types of O-glycosylation. Notably, Notch receptor signaling is modified with and regulated by these types of O-glycans. Here, we summarize the current view of the structures and the significance of these O-glycans mainly in the context of Notch signaling regulation and human diseases.

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.

Detection of horizontal gene transfer in the genome of the choanoflagellate Salpingoeca rosetta

Horizontal gene transfer (HGT), the movement of heritable materials between distantly related organisms, is crucial in eukaryotic evolution. However, the scale of HGT in choanoflagellates, the closest unicellular relatives of metazoans, and its possible roles in the evolution of animal multicellularity remains unexplored. We identified at least 175 candidate HGTs in the genome of the colonial choanoflagellate Salpingoeca rosetta using sequence-based tests. The majority of these were orthologous to genes in bacterial and microalgal lineages, yet displayed genomic features consistent with the rest of the S. rosetta genome-evidence of ancient acquisition events. Putative functions include enzymes involved in amino acid and carbohydrate metabolism, cell signaling, and the synthesis of extracellular matrix components. Functions of candidate HGTs may have contributed to the ability of choanoflagellates to assimilate novel metabolites, thereby supporting adaptation, survival in diverse ecological niches, and response to external cues that are possibly critical in the evolution of multicellularity in choanoflagellates.

Current Views on the Roles of O-Glycosylation in Controlling Notch-Ligand Interactions

The 100th anniversary of Notch discovery in Drosophila has recently passed. The Notch is evolutionarily conserved from Drosophila to humans. The discovery of human-specific Notch genes has led to a better understanding of Notch signaling in development and diseases and will continue to stimulate further research in the future. Notch receptors are responsible for cell-to-cell signaling. They are activated by cell-surface ligands located on adjacent cells. Notch activation plays an important role in determining the fate of cells, and dysregulation of Notch signaling results in numerous human diseases. Notch receptors are primarily activated by ligand binding. Many studies in various fields including genetics, developmental biology, biochemistry, and structural biology conducted over the past two decades have revealed that the activation of the Notch receptor is regulated by unique glycan modifications. Such modifications include O-fucose, O-glucose, and O-N-acetylglucosamine (GlcNAc) on epidermal growth factor-like (EGF) repeats located consecutively in the extracellular domain of Notch receptors. Being fine-tuned by glycans is an important property of Notch receptors. In this review article, we summarize the latest findings on the regulation of Notch activation by glycosylation and discuss future challenges.

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.

SHCBP1 interacting with EOGT enhances O-GlcNAcylation of NOTCH1 and promotes the development of pancreatic cancer

O-GlcNAcylation is important in the development and progression of pancreatic ductal adenocarcinoma (PDAC). The glycosyltransferase EGF domain-specific O-linked GlcNAc transferase (EOGT) acts as a key participant in glycosylating NOTCH1. High-throughput sequencing of specimens from 30 advanced PDAC patients identified SHCBP1 and EOGT as factors of poor prognosis. We hypothesized that they could mediate PDAC progression by influencing NOTCH1 O-GlcNAcylation. Thus, 186 PDAC tissue specimens were immunostained for EOGT and SHCBP1. Pancreatic cancer cell lines and nude mouse models were used for in vitro and in vivo experiments. Respectively, The protein expression of EOGT and SHCBP1 was significantly elevated and correlated with worse prognosis in PDAC patients. In vitro, SHCBP1 overexpression promoted pancreatic cancer cell proliferation, migration and invasion, while knocking down SHCBP1 and EOGT inhibited these malignant processes. In vivo data showed that SHCBP1 overexpression promoted xenograft growth and lung metastasis and shortened survival in mice, whereas knocking down either EOGT or SHCBP1 expression suppressed xenograft growth and metastasis and prolonged survival. We further clarified the molecular mechanisms by which EOGT and SHCBP1 enhance the O-GlcNAcylation of NOTCH1, Subsequently promoting the nuclear localization of the Notch intracellular domain (NICD) and inhibiting the transcription of E-cadherin and P21 in pancreatic cancer cells.

The Skeleton of Lateral Meningocele Syndrome

Notch (Notch1 through 4) are transmembrane receptors that determine cell differentiation and function, and are activated following interactions with ligands of the Jagged and Delta-like families. Notch has been established as a signaling pathway that plays a critical role in the differentiation and function of cells of the osteoblast and osteoclast lineages as well as in skeletal development and bone remodeling. Pathogenic variants of Notch receptors and their ligands are associated with a variety of genetic disorders presenting with significant craniofacial and skeletal manifestations. Lateral Meningocele Syndrome (LMS) is a rare genetic disorder characterized by neurological manifestations, meningoceles, skeletal developmental abnormalities and bone loss. LMS is associated with NOTCH3 gain-of-function pathogenic variants. Experimental mouse models of LMS revealed that the bone loss is secondary to increased osteoclastogenesis due to enhanced expression of receptor activator of nuclear factor kappa B ligand by cells of the osteoblast lineage. There are no effective therapies for LMS. Antisense oligonucleotides targeting Notch3 and antibodies that prevent the activation of NOTCH3 are being tested in preclinical models of the disease. In conclusion, LMS is a serious genetic disorder associated with NOTCH3 pathogenic variants. Novel experimental models have offered insight on mechanisms responsible and ways to correct the disease.

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.

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.

Notch and the regulation of osteoclast differentiation and function

Notch 1 through 4 are transmembrane receptors that play a pivotal role in cell differentiation and function; this review addresses the role of Notch signaling in osteoclastogenesis and bone resorption. Notch receptors are activated following interactions with their ligands of the Jagged and Delta-like families. In the skeleton, Notch signaling controls osteoclast differentiation and bone-resorbing activity either directly acting on osteoclast precursors, or indirectly acting on cells of the osteoblast lineage and cells of the immune system. NOTCH1 inhibits osteoclastogenesis, whereas NOTCH2 enhances osteoclast differentiation and function by direct and indirect mechanisms. NOTCH3 induces the expression of RANKL in osteoblasts and osteocytes and as a result induces osteoclast differentiation. There is limited expression of NOTCH4 in skeletal cells. Selected congenital disorders and skeletal malignancies are associated with dysregulated Notch signaling and enhanced bone resorption. In conclusion, Notch signaling is a critical pathway that controls osteoblast and osteoclast differentiation and function and regulates skeletal homeostasis in health and disease.

Endothelial Yes-Associated Protein 1 Promotes Astrocyte Proliferation and Maturation via Cytoplasmic Leukemia Inhibitory Factor Secretion in Oxygen-Induced Retinopathy

Purpose

Purpose The role of endothelial Yes-associated protein 1 (YAP) in the pathogenesis of retinal angiogenesis and the astrocyte network in the mouse oxygen-induced retinopathy (OIR) model is unknown.

Methods

For in vivo studies, OIR was induced in conditional endothelial YAP knockout mice and their wild-type littermates. Retinal vascularization and the astrocyte network were evaluated by whole-mount fluorescence and Western blotting. In vitro experiments were performed in astrocytes cultured with human microvascular endothelial cell-1–conditioned medium to analyze the mechanisms underlying the effect of endothelial YAP on astrocytes.

Results

Endothelial YAP deletion not only impaired retinal blood vessels, but also caused a sparse and disrupted astrocyte network in response to OIR. Levels of the immature astrocyte marker (platelet-derived growth factor A) in the retina were substantially increased owing to YAP deficiency, suggesting a possible failure in astrocyte maturation, whereas retinal expression of leukemia inhibitory factor (LIF) was decreased. In vitro studies suggested that loss or overexpression of YAP resulted in elevated or decreased LIF secretion by human microvascular endothelial cell-1, respectively. Increased LIF levels in the culture medium promoted astrocyte maturation and proliferation and rescued YAP inhibition-induced astrocyte loss. Finally, activating YAP could protect against the pathology of the astrocyte network and even suppress pathologic retinal vascularization in control OIR mice, but not in endothelial YAP-deficient OIR mice.

Conclusions

Endothelial YAP regulation of LIF secretion is required for normalized astrocyte network formation in OIR, thereby providing a novel target for protecting the astrocyte network and thus benefiting retinal blood vessels.

Pleiotropic Role of Notch Signaling in Human Skin Diseases

Notch signaling orchestrates the regulation of cell proliferation, differentiation, migration and apoptosis of epidermal cells by strictly interacting with other cellular pathways. Any disruption of Notch signaling, either due to direct mutations or to an aberrant regulation of genes involved in the signaling route, might lead to both hyper- or hypo-activation of Notch signaling molecules and of target genes, ultimately inducing the onset of skin diseases. The mechanisms through which Notch contributes to the pathogenesis of skin diseases are multiple and still not fully understood. So far, Notch signaling alterations have been reported for five human skin diseases, suggesting the involvement of Notch in their pathogenesis: Hidradenitis Suppurativa, Dowling Degos Disease, Adams-Oliver Syndrome, Psoriasis and Atopic Dermatitis. In this review, we aim at describing the role of Notch signaling in the skin, particularly focusing on the principal consequences associated with its alterations in these five human skin diseases, in order to reorganize the current knowledge and to identify potential cellular mechanisms in common between these pathologies.

The single EGF-like domain of mouse PAMR1 is modified by O-Glucose, O-Fucose and O-GlcNAc

Epidermal growth factor-like domains (EGF-LDs) of membrane and secreted proteins can be modified by N-glycans and/or potentially elongated O-linked monosaccharides such as O-glucose (O-Glc) found at two positions (O-Glc 1 and O-Glc2), O-fucose (O-Fuc) and O-N-acetylglucosamine (O-GlcNAc). The presence of three O-linked sugars within the same EGF-LD, such as in EGF-LD 20 of NOTCH1, has rarely been evidenced. We searched in KEGG GENES database to list mouse and human proteins with an EGF-LD sequence including one, two, three or four potential O-glycosylation consensus sites. Among the 129 murine retrieved proteins, most had predicted O-fucosylation and/or O-GlcNAcylation sites. Around 68% of EGF-LDs were subjected to only one O-linked sugar modification and near 5% to three modifications. Among these latter, we focused on the peptidase domain-containing protein associated with muscle regeneration 1 (PAMR1), having only one EGF-LD. To test the ability of this domain to be glycosylated, a correctly folded EGF-LD was produced in Escherichia coli periplasm, purified and subjected to in vitro incubations with the recombinant O-glycosyltransferases POGLUT1, POFUT1 and EOGT, adding O-Glc1, O-Fuc and O-GlcNAc, respectively. Using click chemistry and mass spectrometry, isolated PAMR1 EGF-LD was demonstrated to be modified by the three O-linked sugars. Their presence was individually confirmed on EGF-LD of full-length mouse recombinant PAMR1, with at least some molecules modified by both O-Glc1 and O-Fuc. Overall, these results are consistent with the presence of a triple O-glycosylated EGF-LD in mouse PAMR1.

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.

Xylosyl Extension of O-Glucose Glycans on the Extracellular Domain of NOTCH1 and NOTCH2 Regulates Notch Cell Surface Trafficking

Biochemical and genetic studies have indicated that O-linked glycosylation such as O-glucose (Glc), fucose (Fuc), and N-acetylglucosamine (GlcNAc) is critical for Notch signaling; however, it is not fully understood how O-glycans regulate the Notch receptor function. Notch receptors are type-I transmembrane proteins with large extracellular domains (ECD), containing 29–36 epidermal growth factor-like (EGF) repeats. Here, we analyzed O-Glc glycans on NOTCH1 and NOTCH2 expressed in HEK293T cells using an Orbitrap Fusion mass spectrometer and successfully revealed the structures and stoichiometries of all 17 EGF repeats of NOTCH1 with the O-Glc consensus sequence (C1-X-S-X-(P/A)-C2), and 16 out of 17 EGF repeats of NOTCH2 with the same consensus sequence. High levels of O-Glc attachment and xylosyl elongation were detected on most NOTCH1 and NOTCH2 EGF repeats. When both glucoside xylosyltransferases, GXYLT1 and GXYLT2, responsible for the xylosyl elongation of O-glucose, were genetically deleted, the expression of endogenous NOTCH1 and NOTCH2 on the surface of HEK293T cells did not change, but the cell surface expression of overexpressed NOTCH1 and NOTCH2 decreased compared with that in the wild type cells. In vitro secretion assays consistently showed a reduced secretion of both the NOTCH1 and NOTCH2 ECDs in GXYLT1 and GXYLT2 double knockout cells compared with the wild type cells, suggesting a significant role of the elongation of O-Glc glycans on the Notch ECDs in the quality control of Notch receptors.

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.

Neuroprotective effects of irisin against cerebral ischemia/ reperfusion injury via Notch signaling pathway

Irisin, a 112-amino acid peptide induced with exercise in mice and human is thought to have correlation with the short-term outcomes of patients in ischemic stroke. In the present study, the neuroprotective effects of irisin were evaluated in vivo and in vitro and its underlying mechanism was also explored. The global cerebral ischemia/reperfusion (I/R) model was established by bilateral common carotid artery occlusion for 20 min and reperfusion for 24 h in mice and oxygen-glucose deprivation/reperfusion in HT22 cells. Neurological function was scored and then the mice were sacrificed. The brains were harvested for HE staining and detection of brain water content (BWC). The percentage of neuronal apoptosis was evaluated by TUNEL and flow cytometry analysis. The mRNA expression of TNF-α and IL-1β was detected by RT-PCR analysis. The Notch intracellular domain (NICD) was detected by double immunofluorescence staining and western blot, and the protein expression of Notch1 and Hes 1 was detected by western blot. It was observed that irisin could alleviate morphological damage and improve neurological function after global cerebral I/R injury in mice. The apoptosis of hippocampal neurons reduced in the presence of irisin in vivo and in vitro. Additionally irisin could downregulate the expression of IL-1β and TNF-α and upregulate the expression of NICD, Notch1 and Hes 1 in vitro and in vivo. After the application of γ-secretase inhibitor DAPT, all the morphological, neurological and biochemical changes were reversed. Taken together, these results suggest that irisin could regulate the Notch signaling pathway that leads to the alleviation of transient global cerebral I/R injury.

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.

Life is sweet: the cell biology of glycoconjugates

Cells are dazzling in their diversity, both within and across organisms. And yet, throughout this variety runs at least one common thread: sugars. All cells on Earth, in all domains of life, are literally covered in glycans, a term referring to the carbohydrate portion of glycoproteins and glycolipids. In spite of (or, perhaps, because of) their tremendous structural and functional complexity, glycans have historically been underexplored compared with other areas of cell biology. Recently, however, advances in experimental systems and analytical methods have ushered in a renaissance in glycobiology, the study of the biosynthesis, structures, interactions, functions, and evolution of glycans. Today, glycobiology is poised to make major new contributions to cell biology and become more fully integrated into our understanding of cell and organismal physiology.

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.

Meeting Between Rumi and Shams in Notch Signaling; Implications for Pain Management: A Narrative Review

The meeting between Rumi and Shams, in the 13th century, was a turning point in the life of Rumi leading to a revolutionary effect in his thoughts, ideas, and poems. This was an ever-inspiring meeting with many results throughout the centuries. This meeting has created some footprints in cellular and molecular medicine: The discovery of two distinct genes in Drosophila, i.e. Rumi and Shams and their role in controlling Notch signaling, which has a critical role in cell biology. This nomination and the interactions between the two genes has led us to a number of novel studies during the last years. This article reviews the interactions between Rumi and Shams and their effects on Notch signaling in order to find potential novel drugs for pain control through drug development studies in the future.

Intriguing Roles for Endothelial ADAM10/Notch Signaling in the Development of Organ-Specific Vascular Beds

The vasculature is a remarkably interesting, complex, and interconnected organ. It provides a conduit for oxygen and nutrients, filtration of waste products, and rapid communication between organs. Much remains to be learned about the specialized vascular beds that fulfill these diverse, yet vital functions. This review was prompted by the discovery that Notch signaling in mouse endothelial cells is crucial for the development of specialized vascular beds found in the heart, kidneys, liver, intestines, and bone. We will address the intriguing questions raised by the role of Notch signaling and that of its regulator, the metalloprotease ADAM10, in the development of specialized vascular beds. We will cover fundamentals of ADAM10/Notch signaling, the concept of Notch-dependent cell fate decisions, and how these might govern the development of organ-specific vascular beds through angiogenesis or vasculogenesis. We will also consider common features of the affected vessels, including the presence of fenestra or sinusoids and their occurrence in portal systems with two consecutive capillary beds. We hope to stimulate further discussion and study of the role of ADAM10/Notch signaling in the development of specialized vascular structures, which might help uncover new targets for the repair of vascular beds damaged in conditions like coronary artery disease and glomerulonephritis.

Novel circular RNAs expressed in brain microvascular endothelial cells after oxygen-glucose deprivation/recovery

Circular RNAs (circRNAs) are generated by head-to-tail splicing and are ubiquitously expressed in all multicellular organisms. Their important biological functions are increasingly recognized. Cerebral ischemia reperfusion injury-induced brain microvascular endothelial cell dysfunction is an initial stage of blood-brain barrier disruption. The expression profile and potential function of circRNAs in brain microvascular endothelial cells is unknown. Rat brain microvascular endothelial cells were extracted and cultured in glucose-free medium for 4 hours with 5% CO₂ and 95% N₂, and the medium was then replaced with complete growth medium for 6 hours. The RNA in these cells was then extracted. The circRNA was identified by Find_circ and CIRI2 software. Functional and pathway enrichment analysis of genes that were common to differentially expressed mRNAs and circRNA host genes was performed by the Database for Annotation, Visualization and Integrated Discovery Functional Annotation Tool. Miranda software was used to predict microRNAs that were potentially sponged by circRNAs. Furthermore, cytoscape depicted the circR-NA-microRNA interaction network. The results showed that there were 1288 circRNAs in normal and oxygen-glucose deprived/recovered primary brain microvascular endothelial cells. There are 211 upregulated and 326 downregulated differentially expressed circRNAs. The host genes of these differentially expressed circRNAs overlapped with those of differentially expressed mRNAs. The shared genes were further studied by functional enrichment analyses, which revealed that circRNAs may contribute to calcium ion function and the cyclic guanosine 3′,5′-monophosphate (CAMP) dependent protein kinase (PKα) signaling pathway. Next, quantitative reverse transcription polymerase chain reaction assays were performed to detect circRNA levels transcribed from the overlapping host genes. Eight out of the ten circRNAs with the highest fold-change identified by sequencing were successfully verified. Subsequently, the circRNA-microRNA interaction networks of these eight circRNAs were explored by bioinformatic analysis. These results demonstrate that altered circRNAs may be important in the pathogenesis of cerebral ischemia reperfusion injury and consequently may also be potential therapeutic targets for cerebral ischemia diseases. All animal experiments were approved by the Chongqing Medical University Committee on Animal Research, China (approval No. CQMU20180086) on March 22, 2018.

Predicted glycosyltransferases promote development and prevent spurious cell clumping in the choanoflagellate S. rosetta

In a previous study we established forward genetics in the choanoflagellate Salpingoeca rosetta and found that a C-type lectin gene is required for rosette development (Levin et al., 2014). Here we report on critical improvements to genetic screens in S. rosetta while also investigating the genetic basis for rosette defect mutants in which single cells fail to develop into orderly rosettes and instead aggregate promiscuously into amorphous clumps of cells. Two of the mutants, Jumble and Couscous, mapped to lesions in genes encoding two different predicted glycosyltransferases and displayed aberrant glycosylation patterns in the basal extracellular matrix (ECM). In animals, glycosyltransferases sculpt the polysaccharide-rich ECM, regulate integrin and cadherin activity, and, when disrupted, contribute to tumorigenesis. The finding that predicted glycosyltransferases promote proper rosette development and prevent cell aggregation in S. rosetta suggests a pre-metazoan role for glycosyltransferases in regulating development and preventing abnormal tumor-like multicellularity.

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.