O-Fucosylation of Notch Occurs in the Endoplasmic Reticulum

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.

In vivo evidence for GDP-fucose transport in the absence of transporter SLC35C1 and putative transporter SLC35C2

Slc35c1 encodes an antiporter that transports GDP-fucose into the Golgi and returns GMP to the cytoplasm. The closely related gene Slc35c2 encodes a putative GDP-fucose transporter and promotes Notch fucosylation and Notch signaling in cultured cells. Here, we show that HEK293T cells lacking SLC35C1 transferred reduced amounts of O-fucose to secreted epidermal growth factor-like repeats from NOTCH1 or secreted thrombospondin type I repeats from thrombospondin 1. However, cells lacking SLC35C2 did not exhibit reduced fucosylation of these epidermal growth factor-like repeats or thrombospondin type I repeats. To investigate SLC35C2 functions in vivo, WW6 embryonic stem cells were targeted for Slc35c2. Slc35c2[-/-] mice were viable and fertile and exhibited no evidence of defective Notch signaling during skeletal or T cell development. By contrast, mice with inactivated Slc35c1 exhibited perinatal lethality and marked skeletal defects in late embryogenesis, typical of defective Notch signaling. Compound Slc35c1[-/-]Slc35c2[-/-] mutants were indistinguishable in skeletal phenotype from Slc35c1[-/-] embryos and neonates. Double mutants did not exhibit the exacerbated skeletal defects predicted if SLC35C2 was functionally important for Notch signaling in vivo. In addition, NOTCH1 immunoprecipitated from Slc35c1[-/-]Slc35c2[-/-] neonatal lung carried fucose detected by binding of Aleuria aurantia lectin. Given that the absence of both SLC35C1, a known GDP-fucose transporter, and SLC35C2, a putative GDP-fucose transporter, did not lead to afucosylated NOTCH1 nor to the severe Notch signaling defects and embryonic lethality expected if all GDP-fucose transport were abrogated, at least one more mechanism of GDP-fucose transport into the secretory pathway must exist in mammals.

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.

Altered Glycosylation in Progression and Management of Bladder Cancer

Bladder cancer (BC) is the 10th most common malignancy worldwide, with an estimated 573,000 new cases and 213,000 deaths in 2020. Available therapeutic approaches are still unable to reduce the incidence of BC metastasis and the high mortality rates of BC patients. Therefore, there is a need to deepen our understanding of the molecular mechanisms underlying BC progression to develop new diagnostic and therapeutic tools. One such mechanism is protein glycosylation. Numerous studies reported changes in glycan biosynthesis during neoplastic transformation, resulting in the appearance of the so-called tumor-associated carbohydrate antigens (TACAs) on the cell surface. TACAs affect a wide range of key biological processes, including tumor cell survival and proliferation, invasion and metastasis, induction of chronic inflammation, angiogenesis, immune evasion, and insensitivity to apoptosis. The purpose of this review is to summarize the current information on how altered glycosylation of bladder cancer cells promotes disease progression and to present the potential use of glycans for diagnostic and therapeutic purposes.

Metabolic utilization and remodeling of glycan biosynthesis using fucose analogs

BACKGROUND

Fucose (Fuc), a monosaccharide present at the core or the termini of glycans, critically regulates various biological phenomena and is associated with various diseases. Specifically detecting Fuc residues or inhibiting the fucosylation pathway is pivotal in understanding the mechanisms of how fucosylated glycans are related to biological processes and diseases and in developing novel therapeutic agents.

SCOPE OF REVIEW

This review focuses on chemical biology approaches using Fuc analogs developed for metabolically labeling fucosylated glycans or inhibiting the biosynthesis of fucosylated glycans.

MAJOR CONCLUSIONS

Developed Fuc analogs have different potency, specificity and effects on protein and cellular functions. Developing highly enzyme-specific probes and inhibitors is desirable for future investigations.

GENERAL SIGNIFICANCE

Chemical glycobiology approaches using sugar analogs are useful for revealing novel mechanisms of inter-relationships among sugar metabolism pathways and manipulating glycan expression to develop new glycan-targeted therapies.

Delivery of Nucleotide Sugars to the Mammalian Golgi: A Very Well (un)Explained Story

Nucleotide sugars (NSs) serve as substrates for glycosylation reactions. The majority of these compounds are synthesized in the cytoplasm, whereas glycosylation occurs in the endoplasmic reticulum (ER) and Golgi lumens, where catalytic domains of glycosyltransferases (GTs) are located. Therefore, translocation of NS across the organelle membranes is a prerequisite. This process is thought to be mediated by a group of multi-transmembrane proteins from the SLC35 family, i.e., nucleotide sugar transporters (NSTs). Despite many years of research, some uncertainties/inconsistencies related with the mechanisms of NS transport and the substrate specificities of NSTs remain. Here we present a comprehensive review of the NS import into the mammalian Golgi, which consists of three major parts. In the first part, we provide a historical view of the experimental approaches used to study NS transport and evaluate the most important achievements. The second part summarizes various aspects of knowledge concerning NSTs, ranging from subcellular localization up to the pathologies related with their defective function. In the third part, we present the outcomes of our research performed using mammalian cell-based models and discuss its relevance in relation to the general context.

Fringe GlcNAc-transferases differentially extend O-fucose on endogenous NOTCH1 in mouse activated T cells

NOTCH1 is a transmembrane receptor that initiates a cell-cell signaling pathway controlling various cell fate specifications in metazoans. The addition of O-fucose by protein O-fucosyltransferase 1 (POFUT1) to epidermal growth factor-like (EGF) repeats in the NOTCH1 extracellular domain is essential for NOTCH1 function, and modification of O-fucose with GlcNAc by the Fringe family of glycosyltransferases modulates Notch activity. Prior cell-based studies showed that POFUT1 modifies EGF repeats containing the appropriate consensus sequence at high stoichiometry, while Fringe GlcNAc-transferases (LFNG, MFNG, and RFNG) modify O-fucose on only a subset of NOTCH1 EGF repeats. Previous in vivo studies showed that each FNG affects naïve T cell development. To examine Fringe modifications of NOTCH1 at a physiological level, we used mass spectral glycoproteomic methods to analyze O-fucose glycans of endogenous NOTCH1 from activated T cells obtained from mice lacking all Fringe enzymes or expressing only a single FNG. While most O-fucose sites were modified at high stoichiometry, only EGF6, EGF16, EGF26, and EGF27 were extended in WT T cells. Additionally, cell-based assays of NOTCH1 lacking fucose at each of those O-fucose sites revealed small but significant effects of LFNG on Notch-Delta binding in the EGF16 and EGF27 mutants. Finally, in activated T cells expressing only LFNG, MFNG, or RFNG alone, the extension of O-fucose with GlcNAc in the same EGF repeats was diminished, consistent with cooperative interactions when all three Fringes were present. The combined data open the door for the analysis of O-glycans on endogenous NOTCH1 derived from different cell types.

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.

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.

Structural Insights in Mammalian Sialyltransferases and Fucosyltransferases: We Have Come a Long Way, but It Is Still a Long Way Down

Mammalian cell surfaces are modified with complex arrays of glycans that play major roles in health and disease. Abnormal glycosylation is a hallmark of cancer; terminal sialic acid and fucose in particular have high levels in tumor cells, with positive implications for malignancy. Increased sialylation and fucosylation are due to the upregulation of a set of sialyltransferases (STs) and fucosyltransferases (FUTs), which are potential drug targets in cancer. In the past, several advances in glycostructural biology have been made with the determination of crystal structures of several important STs and FUTs in mammals. Additionally, how the independent evolution of STs and FUTs occurred with a limited set of global folds and the diverse modular ability of catalytic domains toward substrates has been elucidated. This review highlights advances in the understanding of the structural architecture, substrate binding interactions, and catalysis of STs and FUTs in mammals. While this general understanding is emerging, use of this information to design inhibitors of STs and FUTs will be helpful in providing further insights into their role in the manifestation of cancer and developing targeted therapeutics in cancer.

A Review of Advances in Hematopoietic Stem Cell Mobilization and the Potential Role of Notch2 Blockade

Hematopoietic stem cell (HSC) transplantation can be a potential cure for hematological malignancies and some nonhematologic diseases. Hematopoietic stem and progenitor cells (HSPCs) collected from peripheral blood after mobilization are the primary source to provide HSC transplantation. In most of the cases, mobilization by the cytokine granulocyte colony-stimulating factor with chemotherapy, and in some settings, with the CXC chemokine receptor type 4 antagonist plerixafor, can achieve high yield of hematopoietic progenitor cells (HPCs). However, adequate mobilization is not always successful in a significant portion of donors. Research is going on to find new agents or strategies to increase HSC mobilization. Here, we briefly review the history of HSC transplantation, current mobilization regimens, some of the novel agents that are under investigation for clinical practice, and our recent findings from animal studies regarding Notch and ligand interaction as potential targets for HSPC mobilization.

POFUT1 acts as a tumor promoter in glioblastoma by enhancing the activation of Notch signaling

Dysregulation of protein O-fucosyl transferase 1 (POFUT1) contributes to the occurrence and progression of multiple cancers. However, whether POFUT1 has a relationship with the pathogenesis of glioblastoma (GBM) is unknown. This work was aimed at evaluating the detailed relevance of POFUT1 in GBM. Here, we demonstrated high levels of POFUT1 in GBM tissue and elucidated that GBM patients with high levels of POFUT1 had a shorter survival rate than those with low levels of POFUT1. POFUT1 knockdown in GBM cells markedly downregulated the ability to proliferate and invade, while overexpression of POFUT1 potentiated the proliferative and invasive ability of GBM cells. Further mechanistic studies indicated that silencing POFUT1 prohibited the activation of Notch signaling, leading to a reduction in the expression of HES1 and HEY1. On the contrary, overexpression of POFUT1 enhanced the activation of Notch signaling. Notably, inhibition of Notch signaling markedly reversed POFUT1-overexpression-induced tumor promotion effects in GBM cells. In addition, POFUT1 silencing markedly repressed the potential of GBM cells to form tumors in vivo. In conclusion, the data of this work indicates that POFUT1 serves a tumor promotion role in GBM by enhancing the activation of Notch signaling. This study underlines the potential role of the POFUT1/Notch axis in GBM progression and proposes POFUT1 as a promising anticancer target for GBM.

Protein O-Fucosyltransferase 1 Undergoes Interdomain Flexibility in Solution

Protein O-fucosyltransferase 1 (PoFUT1) is a GT-B fold enzyme that fucosylates proteins containing EGF-like repeats. GT-B glycosyltransferases have shown a remarkable grade of plasticity adopting closed and open conformations as a way of tuning their catalytic cycle, a feature that has not been observed for PoFUT1. Here, we analyzed Caenorhabditis elegans PoFUT1 (CePoFUT1) conformational behavior in solution by atomic force microscopy (AFM) and single-molecule fluorescence resonance energy transfer (SMF-FRET). Our results show that this enzyme is very flexible and adopts mainly compact conformations and to a lesser extend a highly dynamic population that oscillates between compact and highly extended conformations. Overall, our experiments illustrate the inherent complexity of CePoFUT1 dynamics, which might play a role during its catalytic cycle.

Notch Signaling Regulation in HCC: From Hepatitis Virus to Non-Coding RNAs

The Notch family includes evolutionary conserved genes that encode for single-pass transmembrane receptors involved in stem cell maintenance, development and cell fate determination of many cell lineages. Upon activation by different ligands, and depending on the cell type, Notch signaling plays pleomorphic roles in hepatocellular carcinoma (HCC) affecting neoplastic growth, invasion capability and stem like properties. A specific knowledge of the deregulated expression of each Notch receptor and ligand, coupled with resultant phenotypic changes, is still lacking in HCC. Therefore, while interfering with Notch signaling might represent a promising therapeutic approach, the complexity of Notch/ligands interactions and the variable consequences of their modulations raises concerns when performed in undefined molecular background. The gamma-secretase inhibitors (GSIs), representing the most utilized approach for Notch inhibition in clinical trials, are characterized by important adverse effects due to the non-specific nature of GSIs themselves and to the lack of molecular criteria guiding patient selection. In this review, we briefly summarize the mechanisms involved in Notch pathway activation in HCC supporting the development of alternatives to the γ-secretase pan-inhibitor for HCC therapy.

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.

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.

Targeting Notch Trafficking and Processing in Cancers

The Notch family comprises a group of four ligand-dependent receptors that control evolutionarily conserved developmental and homeostatic processes and transmit signals to the microenvironment. NOTCH undergoes remodeling, maturation, and trafficking in a series of post-translational events, including glycosylation, ubiquitination, and endocytosis. The regulatory modifications occurring in the endoplasmic reticulum/Golgi precede the intramembrane γ-secretase proteolysis and the transfer of active NOTCH to the nucleus. Hence, NOTCH proteins coexist in different subcellular compartments and undergo continuous relocation. Various factors, including ion concentration, enzymatic activity, and co-regulatory elements control Notch trafficking. Interfering with these regulatory mechanisms represents an innovative therapeutic way to bar oncogenic Notch signaling. In this review, we briefly summarize the role of Notch signaling in cancer and describe the protein modifications required for NOTCH to relocate across different subcellular compartments. We focus on the functional relationship between these modifications and the corresponding therapeutic options, and our findings could support the development of trafficking modulators as a potential alternative to the well-known γ-secretase inhibitors.

Differential Labeling of Glycoproteins with Alkynyl Fucose Analogs

Fucosylated glycans critically regulate the physiological functions of proteins and cells. Alterations in levels of fucosylated glycans are associated with various diseases. For detection and functional modulation of fucosylated glycans, chemical biology approaches using fucose (Fuc) analogs are useful. However, little is known about how efficiently each unnatural Fuc analog is utilized by enzymes in the biosynthetic pathway of fucosylated glycans. We show here that three clickable Fuc analogs with similar but distinct structures labeled cellular glycans with different efficiency and protein specificity. For instance, 6-alkynyl (Alk)-Fuc modified O-Fuc glycans much more efficiently than 7-Alk-Fuc. The level of GDP-6-Alk-Fuc produced in cells was also higher than that of GDP-7-Alk-Fuc. Comprehensive in vitro fucosyltransferase assays revealed that 7-Alk-Fuc is commonly tolerated by most fucosyltransferases. Surprisingly, both protein O-fucosyltransferases (POFUTs) could transfer all Fuc analogs in vitro, likely because POFUT structures have a larger space around their Fuc binding sites. These findings demonstrate that labeling and detection of fucosylated glycans with Fuc analogs depend on multiple cellular steps, including conversion to GDP form, transport into the ER or Golgi, and utilization by each fucosyltransferase, providing insights into design of novel sugar analogs for specific detection of target glycans or inhibition of their functions.

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.

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.

Biallelic variants in SLC35C1 as a cause of isolated short stature with intellectual disability

Variants in SLC35C1 underlie leucocyte adhesion deficiency (LADII) or congenital disorder of glycosylation type 2c (CDGIIc), an autosomal recessive disorder of fucosylation. This immunodeficiency syndrome is generally characterized by severe recurrent infections, Bombay blood group, reduced growth and intellectual disability (ID). Features are all caused by an inability to generate key fucosylated molecules due to a defective transport of GDP-fucose into the Golgi. Here we report the use of exome sequencing to identify biallelic variants in SLC35C1 (c.501_503delCTT, p.(Phe168del) and c.891T > G, p.(Asn297Lys)) in an individual with short stature and ID. Retrospective clinical examination based on the genetic findings revealed increased otitis media as the only immunological feature present in this child. Biochemical analysis of patient serum identified a clear but mild decrease in protein fucosylation. Modelling all described missense mutations on a SLC35C1 protein model showed pathogenic substitutions localise to close to the dimer interface, providing insight into the possible pathophysiology of non-synonymous causative variants identified in patients. Our evidence confirms this is the second family presenting with only a subset of features and broadens the clinical presentation of this syndrome. Of note, both families segregated a common allele (p.Phe168del), suggesting there could be an associated genotype-phenotype relationship for specific variants. Based on two out of 14 reported families not presenting with the characteristic features of SLC35C1-CDG, we suggest there is clinical utility in considering this gene in patients with short stature and ID.

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.

Downregulation of POFUT1 Impairs Secondary Myogenic Fusion Through a Reduced NFATc2/IL-4 Signaling Pathway

Past work has shown that the protein O-fucosyltransferase 1 (POFUT1) is involved in mammal myogenic differentiation program. Pofut1 knockdown (Po –) in murine C2C12 cells leads to numerous elongated and thin myotubes, suggesting significant defects in secondary fusion. Among the few pathways involved in this process, NFATc2/IL-4 is described as the major one. To unravel the impact of POFUT1 on secondary fusion, we used wild-type (WT) C2C12 and Po – cell lines to follow Myf6, Nfatc2, Il-4 and Il-4rα expressions during a 120 h myogenic differentiation time course. Secreted IL-4 was quantified by ELISA. IL-4Rα expression and its labeling on myogenic cell types were investigated by Western blot and immunofluorescence, respectively. Phenotypic observations of cells treated with IL-4Rα blocking antibody were performed. In Po –, we found a decrease in nuclei number per myotube and a downexpression of Myf6. The observed downregulation of Nfatc2 is correlated to a diminution of secreted IL-4 and to the low level of IL-4Rα for reserve cells. Neutralization of IL-4Rα on WT C2C12 promotes myonuclear accretion defects, similarly to those identified in Po –. Thus, POFUT1 could be a new controller of myotube growth during myogenesis, especially through NFATc2/IL-4 signaling pathway.

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.

Caveolin-1 promotes invasion and metastasis by upregulating Pofut1 expression in mouse hepatocellular carcinoma

Caveolin-1 (Cav-1) is an important structural protein of caveolae and plays an oncogene-like role by influencing protein glycosylation in hepatocellular carcinoma (HCC) cells. However, the mechanism by which Cav-1 promotes invasion and metastasis capacity has not been completely clarified. In this study, we demonstrate that Pofut1 is a fucosyltransferase induced by Cav-1. Mouse Hepa1-6 HCC cells lacking Cav-1 expression exhibited low transcription levels of Pofut1, whereas strong Pofut1 expression was found in high-metastasis-potential Hca-F cells with high levels of Cav-1. Cav-1 activated MAPK signaling and promoted phosphorylation of the transcription factors CREB, Sp1, HNF4A and c-Myc, which bound to the Pofut1 promoter region to induce its transcription. As Notch signaling receptors can be modified with O-fucose by Pofut1, we further showed that Cav-1-induced upregulation of Pofut1 expression activated the Notch pathway and thus enhanced invasion and metastasis by mouse HCC cells in vitro and in vivo. Collectively, our findings reveal a novel mechanism by which Cav-1 promotes tumor metastasis by upregulating expression of Pofut1, suggesting that Cav-1 may function as a new biomarker for HCC.

Protein O-fucosylation: structure and function

Fucose is a common terminal modification on protein and lipid glycans. Fucose can also be directly linked to protein via an O-linkage to Serine or Threonine residues located within consensus sequences contained in Epidermal Growth Factor-like (EGF) repeats and Thrombospondin Type 1 Repeats (TSRs). In this context, fucose is added exclusively to properly folded EGF repeats and TSRs by Protein O-fucosyltransferases 1 and 2, respectively. In both cases, the O-linked fucose can also be elongated with other sugars. Here, we describe the biological importance of these O-fucose glycans and molecular mechanisms by which they affect the function of the proteins they modify. O-Fucosylation of EGF repeats modulates the Notch signaling pathway, while O-fucosylation of TSRs is predicted to influence secretion of targets including several extracellular proteases. Recent data show O-fucose glycans mediate their effects by participating in both intermolecular and intramolecular interactions.

In vitro acellular method to reveal O-fucosylation on EGF-like domains

A hundred of human proteins have one or more EGF-like domains (EGF-LD) bearing the O-fucosylation consensus motif C2X4(S/T)C3 but to date, only a few of them have been shown to be O-fucosylated. The protein O-fucosyltransferase (POFUT1) specifically recognizes correctly folded EGF-LD of the human EGF (hEGF) type and transfers fucose on serine or threonine residue within the O-fucosylation motif. Here, we propose a strategy for a rapid screening for ability of any EGF-LD to be O-fucosylated, using copper-catalyzed azide-alkyne cycloaddition (CuAAC). By an oligonucleotide hybridization approach, double-stranded fragments encoding any EGF-LD can be first rapidly cloned into the prokaryotic vector pET-25b to promote its targeting to periplasm and formation of the three conserved disulfide bonds. After protein production and purification, an in vitro POFUT1-mediated O-fucosylation can be performed with azido GDP-fucose. Successful transfer of O-fucose is finally revealed by blotting technique after CuAAC. In this study, we specially focused on mouse NOTCH1 EGF12 and EGF26, which are both known to be O-fucosylated although having different binding affinities towards POFUT1. Indeed, we clearly showed here that addition of O-fucose by POFUT1 was much more efficient for EGF26 than for EGF12. This experimental approach is rapid and sufficiently sensitive to reveal propensity of any EGF-LD to be O-fucosylated; it is thus useful prior to perform structure-function studies on target proteins containing one or several EGF-LD.

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.

Lysine at position 329 within a C-terminal dilysine motif is crucial for the ER localization of human SLC35B4

SLC35B4 belongs to the solute carrier 35 (SLC35) family whose best-characterized members display a nucleotide sugar transporting activity. Using an experimental model of HepG2 cells and indirect immunofluorescent staining, we verified that SLC35B4 was localized to the endoplasmic reticulum (ER). We demonstrated that dilysine motif, especially lysine at position 329, is crucial for the ER localization of this protein in human cells and therefore one should use protein C-tagging with caution. To verify the importance of the protein in glycoconjugates synthesis, we generated SLC35B4-deficient HepG2 cell line using CRISPR-Cas9 approach. Our data showed that knock-out of the SLC35B4 gene does not affect major UDP-Xyl- and UDP-GlcNAc-dependent glycosylation pathways.

POFUT1 as a Promising Novel Biomarker of Colorectal Cancer

Background: While protein O-fucosyltransferase 1 (POFUT1) overexpression has been recently proposed as a potential biomarker for different cancer types, no study was carried out on POFUT1 implication in colorectal cancer (CRC). Methods: Data from 626 tumors and 51 non-tumor adjacent tissues available in FireBrowse had been used in this study. Statistical analyses on POFUT1 expression and gene copy number, NOTCH receptors (main targets of POFUT1 enzymatic activity) expression and association of POFUT1 and NOTCH1 expressions with clinical parameters were investigated. Data were completed by POFUT1 histological labeling on six tumor tissues from patients with CRC. Results: We found that POFUT1 is overexpressed from the stage I (p < 0.001) and 76.02% of tumors have a 20q11.21 amplification, associated in 90.13% of cases with a POFUT1 overexpression, compared to non-tumor adjacent tissues. The POFUT1 copy number in tumors is mainly between 2 and 3. POFUT1 is positively correlated with NOTCH1 (r_s = 0.34, p < 0.001), NOTCH3 (r_s = 0.087, p = 0.0297), and NOTCH4 (r_s = 0.097, p = 0.0148) expressions, while negatively correlated with NOTCH2 expression (r_s = −0.098, p = 0.0142). POFUT1 overexpression is markedly associated with rectal location, non-mucinous adenocarcinoma and cancer stages IV and M1. NOTCH1 overexpression is only associated with rectal location and non-mucinous adenocarcinoma. Conclusion: We conclude that POFUT1 is overexpressed in CRC from stage I, and its high expression is associated with metastatic process, probably through NOTCH pathway activation. Then, POFUT1 could represent a potential novel biomarker for CRC diagnosis.

Downregulating Notch counteracts KrasG12D-induced ERK activation and oxidative phosphorylation in myeloproliferative neoplasm

The Notch signaling pathway contributes to the pathogenesis of a wide spectrum of human cancers, including hematopoietic malignancies. Its functions are highly dependent on the specific cellular context. Gain-of-function NOTCH1 mutations are prevalent in human T cell leukemia, while loss of Notch signaling is reported in myeloid leukemias. Here, we report a novel oncogenic function of Notch signaling in oncogenic Kras-induced myeloproliferative neoplasm (MPN). We find that downregulation of Notch signaling in hematopoietic cells via DNMAML expression or Pofut1 deletion significantly blocks MPN development in Kras^G12D mice in a cell-autonomous manner. Further mechanistic studies indicate that inhibition of Notch signaling significantly upregulates Dusp1, a dual phosphatase that inactivates p-ERK, and downregulates cytokine-evoked ERK activation in Kras^G12D cells. Moreover, mitochondrial metabolism is greatly enhanced in Kras^G12D cells but significantly reprogrammed by DNMAML close to that in control cells. Consequently, cell proliferation and expanded myeloid compartment in Kras^G12D mice are significantly reduced. Consistent with these findings, combined inhibition of the MEK/ERK pathway and mitochondrial oxidative phosphorylation effectively inhibited the growth of human and mouse leukemia cells in vitro. Our study provides a strong rational to target both ERK signaling and aberrant metabolism in oncogenic Ras-driven myeloid leukemia.

Two novel protein O-glucosyltransferases that modify sites distinct from POGLUT1 and affect Notch trafficking and signaling

The Notch-signaling pathway is normally activated by Notch-ligand interactions. A recent structural analysis suggested that a novel O-linked hexose modification on serine 435 of the mammalian NOTCH1 core ligand-binding domain lies at the interface with its ligands. This serine occurs between conserved cysteines 3 and 4 of Epidermal Growth Factor-like (EGF) repeat 11 of NOTCH1, a site distinct from those modified by protein O-glucosyltransferase 1 (POGLUT1), suggesting that a different enzyme is responsible. Here, we identify two novel protein O-glucosyltransferases, POGLUT2 and POGLUT3 (formerly KDELC1 and KDELC2, respectively), which transfer O-glucose (O-Glc) from UDP-Glc to serine 435. Mass spectrometric analysis of NOTCH1 produced in HEK293T cells lacking POGLUT2, POGLUT3, or both genes showed that either POGLUT2 or POGLUT3 can add this novel O-Glc modification. EGF11 of NOTCH2 does not have a serine residue in the same location for this O-glucosylation, but EGF10 of NOTCH3 (homologous to EGF11 in NOTCH1 and -2) is also modified at the same position. Comparison of the sites suggests a consensus sequence for modification. In vitro assays with POGLUT2 and POGLUT3 showed that both enzymes modified only properly folded EGF repeats and displayed distinct acceptor specificities toward NOTCH1 EGF11 and NOTCH3 EGF10. Mutation of the O-Glc modification site on EGF11 (serine 435) in combination with sensitizing O-fucose mutations in EGF8 or EGF12 affected cell-surface presentation of NOTCH1 or reduced activation of NOTCH1 by Delta-like1, respectively. This study identifies a previously undescribed mechanism for fine-tuning the Notch-signaling pathway in mammals.

Biological functions of fucose in mammals

Fucose is a 6-deoxy hexose in the l-configuration found in a large variety of different organisms. In mammals, fucose is incorporated into N-glycans, O-glycans and glycolipids by 13 fucosyltransferases, all of which utilize the nucleotide-charged form, GDP-fucose, to modify targets. Three of the fucosyltransferases, FUT8, FUT12/POFUT1 and FUT13/POFUT2, are essential for proper development in mice. Fucose modifications have also been implicated in many other biological functions including immunity and cancer. Congenital mutations of a Golgi apparatus localized GDP-fucose transporter causes leukocyte adhesion deficiency type II, which results in severe developmental and immune deficiencies, highlighting the important role fucose plays in these processes. Additionally, changes in levels of fucosylated proteins have proven as useful tools for determining cancer diagnosis and prognosis. Chemically modified fucose analogs can be used to alter many of these fucose dependent processes or as tools to better understand them. In this review, we summarize the known roles of fucose in mammalian physiology and pathophysiology. Additionally, we discuss recent therapeutic advances for cancer and other diseases that are a direct result of our improved understanding of the role that fucose plays in these systems.

Flagellin glycosylation with pseudaminic acid in Campylobacter and Helicobacter: prospects for development of novel therapeutics

Many pathogenic bacteria require flagella-mediated motility to colonise and persist in their hosts. Helicobacter pylori and Campylobacter jejuni are flagellated epsilonproteobacteria associated with several human pathologies, including gastritis, acute diarrhea, gastric carcinoma and neurological disorders. In both species, glycosylation of flagellin with an unusual sugar pseudaminic acid (Pse) plays a crucial role in the biosynthesis of functional flagella, and thereby in bacterial motility and pathogenesis. Pse is found only in pathogenic bacteria. Its biosynthesis via six consecutive enzymatic steps has been extensively studied in H. pylori and C. jejuni. This review highlights the importance of flagella glycosylation and details structural insights into the enzymes in the Pse pathway obtained via a combination of biochemical, crystallographic, and mutagenesis studies of the enzyme-substrate and -inhibitor complexes. It is anticipated that understanding the underlying structural and molecular basis of the catalytic mechanisms of the Pse-synthesising enzymes will pave the way for the development of novel antimicrobials.

Regulation of Notch Function by O-Glycosylation

The Notch receptor initiates a unique intercellular signaling pathway that is evolutionarily conserved across all metazoans and contributes to the development and maintenance of numerous tissues. Consequently, many diseases result from aberrant Notch signaling. Emerging roles for Notch in disease are being uncovered as studies reveal new information regarding various components of this signaling pathway. Notch activity is regulated at several levels, but O-linked glycosylation of Epidermal Growth Factor (EGF) repeats in the Notch extracellular domain has emerged as a major regulator that, depending on context, can increase or decrease Notch activity. Three types of O-linked glycosylation occur at consensus sequences found within the EGF repeats of Notch: O-fucosylation, O-glucosylation, and O-GlcNAcylation. Recent studies have investigated the site occupancy of these types of glycosylation and also defined specific roles for these glycans on Notch structure and function. Nevertheless, there are many functional aspects to each type of O-glycosylation that remain unclear. Here, we will discuss molecular mechanisms of how O-glycosylation regulates Notch signaling and describe disorders associated with defects in Notch O-glycosylation.

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

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

Predicting the Retention Behavior of Specific O-Linked Glycopeptides

O-Linked glycosylation is a common post-translational modification that can alter the overall structure, polarity, and function of proteins. Reverse-phase (RP) chromatography is the most common chromatographic approach to analyze O-glycosylated peptides and their unmodified counterparts, even though this approach often does not provide adequate separation of these two species. Hydrophilic interaction liquid chromatography (HILIC) can be a solution to this problem, as the polar glycan interacts with the polar stationary phase and potentially offers the ability to resolve the peptide from its modified form(s). In this paper, HILIC is used to separate peptides with O-N-acetylgalactosamine (O-GalNAc), O-N-acetylglucosamine (O-GlcNAc), and O-fucose additions from their native forms, and coefficients representing the extent of hydrophilicity were derived using linear regression analysis as a means to predict the retention times of peptides with these modifications.

O-Glycosylation modulates the stability of epidermal growth factor-like repeats and thereby regulates Notch trafficking

Glycosylation in the endoplasmic reticulum (ER) is closely associated with protein folding and quality control. We recently described a non-canonical ER quality control mechanism for folding of thrombospondin type 1 repeats by protein O-fucosyltransferase 2 (POFUT2). Epidermal growth factor-like (EGF) repeats are also small cysteine-rich protein motifs that can be O-glycosylated by several ER-localized enzymes, including protein O-glucosyltransferase 1 (POGLUT1) and POFUT1. Both POGLUT1 and POFUT1 modify the Notch receptor on multiple EGF repeats and are essential for full Notch function. The fact that POGLUT1 and POFUT1 can distinguish between folded and unfolded EGF repeats raised the possibility that they participate in a quality control pathway for folding of EGF repeats in proteins such as Notch. Here, we demonstrate that cell-surface expression of endogenous Notch1 in HEK293T cells is dependent on the presence of POGLUT1 and POFUT1 in an additive manner. In vitro unfolding assays reveal that addition of O-glucose or O-fucose stabilizes a single EGF repeat and that addition of both O-glucose and O-fucose enhances stability in an additive manner. Finally, we solved the crystal structure of a single EGF repeat covalently modified by a full O-glucose trisaccharide at 2.2 Å resolution. The structure reveals that the glycan fills up a surface groove of the EGF with multiple contacts with the protein, providing a chemical basis for the stabilizing effects of the glycans. Taken together, this work suggests that O-fucose and O-glucose glycans cooperatively stabilize individual EGF repeats through intramolecular interactions, thereby regulating Notch trafficking in cells.

[Role of Fucosylation in Cancer]

岩藻糖基化是重要的糖基化修饰方式，在哺乳动物中发挥重要作用，其参与ABO血型H抗原、Lewis血型抗原形成、选择素介导的白细胞外渗或归巢、宿主病原相互作用及信号通路修饰。在多种肿瘤中存在岩藻糖基化异常，其在肿瘤生长、侵袭、转移、免疫逃逸以及药物敏感性方面发挥重要作用，与肺癌的发生发展及预后密切相关。因此，靶向肿瘤中异常岩藻糖基化可能成为治疗肿瘤的新策略。本文将对岩藻糖基化在肿瘤发生发展中的作用进行综述。

Deciphering the Fringe-Mediated Notch Code: Identification of Activating and Inhibiting Sites Allowing Discrimination between Ligands

Fringe proteins are β3-N-acetylglucosaminyltransferases that modulate Notch activity by modifying O-fucose residues on epidermal growth factor-like (EGF) repeats of Notch. Mammals have three Fringes: Lunatic, Manic, and Radical. While Lunatic and Manic Fringe inhibit Notch1 activation from Jagged1 and enhance activation from Delta-like 1, Radical Fringe enhances signaling from both. We used a mass spectrometry approach to determine whether the variable effects of Fringes on Notch1 result from generation of unique glycosylation patterns on Notch1. We found that Lunatic and Manic Fringe modified similar sites on Notch1, while Radical Fringe modified a subset. Fringe modifications at EGF8 and EGF12 enhanced Notch1 binding to and activation from Delta-like 1, while modifications at EGF6 and EGF36 (added by Manic and Lunatic but not Radical) inhibited Notch1 activation from Jagged1. Combined, these results suggest that Fringe modifications "mark" different regions in the Notch1 extracellular domain for activation or inhibition.

Identification of glycans on plasma-derived ADAMTS13

Patients suffering from acquired thrombotic thrombocytopenic purpura develop autoantibodies directed toward the plasma glycoprotein ADAMTS13. Here, we studied the glycan composition of plasma-derived ADAMTS13. Purified ADAMTS13 was reduced, alkylated, and processed into peptides with either trypsin or chymotrypsin. Glycopeptides were enriched using zwitterionic HILIC zip-tips and analyzed by tandem mass spectrometry employing higher-energy collision dissociation fragmentation. Upon detection of a diagnostic ion of a glycan fragment, electron transfer dissociation fragmentation was performed on the same precursor ion. The majority of N-linked glycans were of the complex type containing terminal sialic acids and fucose residues. A high mannose-containing glycan was attached to Asn614 in the spacer domain. Six O-linked glycans mostly terminating in sialic acid were found dispersed over ADAMTS13. Five O-linked glycans were attached to a Ser and one to Thr. All 6 O-linked glycans contained a terminal sialic acid. O-fucosylation is a common posttranslational modification of thrombospondin type 1 repeats. We identified 7 O-fucosylation sites in the thrombospondin (TSP) type 1 repeats. Unexpectedly, one additional O-fucosylation site was found in the disintegrin domain. This O-fucosylation site did not meet the proposed consensus sequence CSX(S/T)CG. C-mannosylation sites were identified in TSP1, linker TSP4-TSP5, and TSP8. Overall, our findings highlight the complexity of glycan modifications on ADAMTS13, which may have implications for its interaction with immune- or clearance receptors containing carbohydrate recognition domains.

Glycosylated Notch and Cancer

Glycosylation is one of the key components influencing several signaling pathways implicated in cell survival and growth. The Notch signaling pathway plays a pivotal role in numerous cell fate specifications during metazoan development. Both Notch and its ligands are repeatedly glycosylated by the addition of sugar moieties, such as O-fucose, O-glucose, or O-xylose, to bring about structural and functional changes. Disruption to glycosylation processes of Notch proteins result in developmental disorders and disease, including cancer. This review summarizes the importance and recent updates on the role of glycosylated Notch proteins in tumorigenesis and tumor metastasis.

Inactivation of GDP-fucose transporter gene (Slc35c1) in CHO cells by ZFNs, TALENs and CRISPR-Cas9 for production of fucose-free antibodies

Removal of core fucose from N-glycans attached to human IgG1 significantly enhances its affinity for the receptor FcγRIII and thereby dramatically improves its antibody-dependent cellular cytotoxicity activity. While previous works have shown that inactivation of fucosyltransferase 8 results in mutants capable of producing fucose-free antibodies, we report here the use of genome editing techniques, namely ZFNs, TALENs and the CRISPR-Cas9, to inactivate the GDP-fucose transporter (SLC35C1) in Chinese hamster ovary (CHO) cells. A FACS approach coupled with a fucose-specific lectin was developed to rapidly isolate SLC35C1-deficient cells. Mass spectrometry analysis showed that both EPO-Fc produced in mutants arising from CHO-K1 and anti-Her2 antibody produced in mutants arising from a pre-existing antibody-producing CHO-HER line lacked core fucose. Lack of functional SLC35C1 in these cells does not affect cell growth or antibody productivity. Our data demonstrate that inactivating Slc35c1 gene represents an alternative approach to generate CHO cells for production of fucose-free antibodies.

O-Fucosylation of CCN1 is required for its secretion

The matricellular protein CCN1, also known as Cyr61, is a secreted ligand and has numerous functions. Human CCN1 contains one predicted O-fucosylation site in the thrombospondin type-1 repeat (TSR1) domain at Thr(242). In this report, we demonstrated that CCN1 is O-fucosylated at Thr(242) using mass spectrometry. Deficiency of O-fucosylation resulted in the decrement of the cell surface localization and the secretion of CCN1. Furthermore, knockdown of protein O-fucosyltransferase 2, which modifies a specific Ser/Thr residue in the TSR1 domain, decreased secreted levels of CCN1. These results demonstrated that O-fucosylation of CCN1 at Thr(242) regulates its secretion.