O-GlcNAc modification of the extracellular domain of Notch receptors

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

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

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.

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.

Novel roles for O-linked glycans in protein folding

N-Glycosylation has long been linked to protein folding and quality control in the endoplasmic reticulum (ER). Recent work has shown that O-linked glycosylation and the corresponding glycosyltransferases also participate in this important function. Notably, Protein O-fucosyltransferase 1 (Ofut1/Pofut1), a soluble, ER localized enzyme that fucosylates Epidermal Growth Factor-like (EGF) repeats, functions as a chaperone involved in the proper localization of the Notch receptor in certain contexts. Pofut2, a related enzyme that modifies Thrombospondin type I repeats (TSRs), has also been hypothesized to play a role in the folding and quality control of TSR-containing proteins. Both enzymes only modify fully folded substrates suggesting that they are able to distinguish between folded and unfolded structures. Pofuts have known physiological relevance and are conserved across metazoans. Though consensus sequences for O-fucosylation have been established and structures of both Pofuts have been studied, the mechanism of how they participate in protein folding is not known. This article discusses past and recent advances made in novel roles for these protein O-glycosyltransferases.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

N-acetylglucosamine modification in the lumen of the endoplasmic reticulum

BACKGROUND

O-linked β-N-acetylglucosamine (O-GlcNAc) modification of epidermal growth factor (EGF) domains catalyzed by EGF domain O-GlcNAc transferase (EOGT) is the first example of GlcNAc modification in the lumen of the endoplasmic reticulum (ER).

SCOPE OF REVIEW

This review summarizes current knowledge on the EOGT-catalyzed O-GlcNAc modification of EGF domains obtained through biochemical characterization, genetic analysis in Drosophila, and identification of human EOGT mutation. Additionally, this review discusses GTDC2-another ER protein homologous to EOGT that catalyzes the GlcNAc modification of O-mannosylated α-dystroglycan-and other components of the biosynthetic pathway involved in GlcNAc modification in the ER lumen.

MAJOR CONCLUSIONS

GlcNAc modification in the ER lumen has been identified as a novel type of protein modification that regulates specific protein function. Moreover, abnormal GlcNAc modification in the ER lumen is responsible for Adams-Oliver syndrome and Walker-Warburg syndrome.

GENERAL SIGNIFICANCE

Elucidation of the biological function of GlcNAc modification in the ER lumen will provide new insights into the unique roles of O-glycans, whose importance has been demonstrated in multifunctional glycoproteins such as Notch receptors and α-dystroglyan.

Impaired O-linked N-acetylglucosaminylation in the endoplasmic reticulum by mutated epidermal growth factor (EGF) domain-specific O-linked N-acetylglucosamine transferase found in Adams-Oliver syndrome

Epidermal growth factor (EGF) domain-specific O-linked N-acetylglucosamine (EOGT) is an endoplasmic reticulum (ER)-resident O-linked N-acetylglucosamine (O-GlcNAc) transferase that acts on EGF domain-containing proteins such as Notch receptors. Recently, mutations in EOGT have been reported in patients with Adams-Oliver syndrome (AOS). Here, we have characterized enzymatic properties of mouse EOGT and EOGT mutants associated with AOS. Simultaneous expression of EOGT with Notch1 EGF repeats in human embryonic kidney 293T (HEK293T) cells led to immunoreactivity with the CTD110.6 antibody in the ER. Consistent with the GlcNAc modification in the ER, the enzymatic properties of EOGT are distinct from those of Golgi-resident GlcNAc transferases; the pH optimum of EOGT ranges from 7.0 to 7.5, and the Km value for UDP N-acetylglucosamine (UDP-GlcNAc) is 25 μm. Despite the relatively low Km value for UDP-GlcNAc, EOGT-catalyzed GlcNAcylation depends on the hexosamine pathway, as revealed by the increased O-GlcNAcylation of Notch1 EGF repeats upon supplementation with hexosamine, suggesting differential regulation of the luminal UDP-GlcNAc concentration in the ER and Golgi. As compared with wild-type EOGT, O-GlcNAcylation in the ER is nearly abolished in HEK293T cells exogenously expressing EOGT variants associated with AOS. Introduction of the W207S mutation resulted in degradation of the protein via the ubiquitin-proteasome pathway, although the stability and ER localization of EOGT(R377Q) were not affected. Importantly, the interaction between UDP-GlcNAc and EOGT(R377Q) was impaired without adversely affecting the acceptor substrate interaction. These results suggest that impaired glycosyltransferase activity in mutant EOGT proteins and the consequent defective O-GlcNAcylation in the ER constitute the molecular basis for AOS.

Antibodies that detect O-linked β-D-N-acetylglucosamine on the extracellular domain of cell surface glycoproteins

The transfer of N-acetylglucosamine (GlcNAc) to Ser or Thr in cytoplasmic and nuclear proteins is a well known post-translational modification that is catalyzed by the O-GlcNAc transferase OGT. A more recently identified O-GlcNAc transferase, EOGT, functions in the secretory pathway and transfers O-GlcNAc to proteins with epidermal growth factor-like (EGF) repeats. A number of antibodies that detect O-GlcNAc in cytosolic and nuclear extracts have been described previously. Here we compare seven of these antibodies (CTD110.6, 10D8, RL2, HGAC85, 18B10.C7(#3), 9D1.E4(#10), and 1F5.D6 (#14) for detection of the O-GlcNAc modification on extracellular domains of membrane or secreted glycoproteins that may also carry various N- and O-glycans. We found that CTD110.6 binds not only to O-GlcNAc on proteins but also to terminal β-GlcNAc on the complex N-glycans of Lec8 Chinese hamster ovary (CHO) cells that lack UDP-Gal transporter activity and express GlcNAc-terminating, complex N-glycans. We show that CTD110.6, #3, and #10 antibodies can be used to detect cell surface glycoproteins bearing O-GlcNAc. Cell surface glycoproteins recognized by CTD110.6 antibody included NOTCH1 that possesses many EGF repeats with a consensus site for EOGT. Knockdown of CHO Eogt reduced binding of CTD110.6 to Lec1 CHO cells, and expression of a human EOGT cDNA increased the O-GlcNAc signal on Lec1 cells and the extracellular domain of NOTCH1. Thus, with careful controls, antibodies CTD110.6 (IgM), #3 (IgG), and #10 (IgG) can be used to detect membrane and secreted proteins modified by O-GlcNAc on EGF repeats.

Control of mucin-type O-glycosylation: a classification of the polypeptide GalNAc-transferase gene family

Glycosylation of proteins is an essential process in all eukaryotes and a great diversity in types of protein glycosylation exists in animals, plants and microorganisms. Mucin-type O-glycosylation, consisting of glycans attached via O-linked N-acetylgalactosamine (GalNAc) to serine and threonine residues, is one of the most abundant forms of protein glycosylation in animals. Although most protein glycosylation is controlled by one or two genes encoding the enzymes responsible for the initiation of glycosylation, i.e. the step where the first glycan is attached to the relevant amino acid residue in the protein, mucin-type O-glycosylation is controlled by a large family of up to 20 homologous genes encoding UDP-GalNAc:polypeptide GalNAc-transferases (GalNAc-Ts) (EC 2.4.1.41). Therefore, mucin-type O-glycosylation has the greatest potential for differential regulation in cells and tissues. The GalNAc-T family is the largest glycosyltransferase enzyme family covering a single known glycosidic linkage and it is highly conserved throughout animal evolution, although absent in bacteria, yeast and plants. Emerging studies have shown that the large number of genes (GALNTs) in the GalNAc-T family do not provide full functional redundancy and single GalNAc-T genes have been shown to be important in both animals and human. Here, we present an overview of the GalNAc-T gene family in animals and propose a classification of the genes into subfamilies, which appear to be conserved in evolution structurally as well as functionally.

O-linked-N-acetylglucosamine modification of mammalian Notch receptors by an atypical O-GlcNAc transferase Eogt1

O-linked-β-N-acetylglucosamine (O-GlcNAc) modification is a unique cytoplasmic and nuclear protein modification that is common in nearly all eukaryotes, including filamentous fungi, plants, and animals. We had recently reported that epidermal growth factor (EGF) repeats of Notch and Dumpy are O-GlcNAcylated by an atypical O-GlcNAc transferase, EOGT, in Drosophila. However, no study has yet shown whether O-GlcNAcylation of extracellular proteins is limited to insects such as Drosophila or whether it occurs in other organisms, including mammals. Here, we report the characterization of A130022J15Rik, a mouse gene homolog of Drosophila Eogt (Eogt 1). Enzymatic analysis revealed that Eogt1 has a substrate specificity similar to that of Drosophila EOGT, wherein the Thr residue located between the fifth and sixth conserved cysteines of the folded EGF-like domains is modified. This observation is supported by the fact that the expression of Eogt1 in Drosophila rescued the cell-adhesion defect caused by Eogt downregulation. In HEK293T cells, Eogt1 expression promoted modification of Notch1 EGF repeats by O-GlcNAc, which was further modified, at least in part, by galactose to generate a novel O-linked-N-acetyllactosamine structure. These results suggest that Eogt1 encodes EGF domain O-GlcNAc transferase and that O-GlcNAcylation reaction in the secretory pathway is a fundamental biochemical process conserved through evolution.

O-linked-N-acetylglucosamine on extracellular protein domains mediates epithelial cell-matrix interactions

The O-linked-N-acetylglucosamine (O-GlcNAc) modification of cytoplasmic and nuclear proteins regulates basic cellular functions and is involved in the aetiology of diabetes and neurodegeneration. This intracellular O-GlcNAcylation is catalyzed by a single O-GlcNAc transferase, OGT. Here we report a novel OGT, EOGT, responsible for extracellular O-GlcNAcylation. Although both OGT and EOGT are regulated by hexosamine flux, EOGT localizes to the lumen of the endoplasmic reticulum and transfers GlcNAc to epidermal growth factor-like domains in an OGT-independent manner. Loss of Eogt gives phenotypes similar to those caused by defects in the apical extracellular matrix. Dumpy (Dp), a membrane-anchored extracellular protein, is O-GlcNAcylated, and EOGT is required for Dp-dependent epithelial cell-matrix interactions. Thus, O-GlcNAcylation of secreted and membrane glycoproteins is a novel mediator of cell-cell or cell-matrix interactions at the cell surface.

Fringe benefits: functional and structural impacts of O-glycosylation on the extracellular domain of Notch receptors

The Notch family of receptors plays essential roles in many phases of development, and dysregulation of Notch activity is increasingly recognized as a player in many diseases. O-Glycosylation of the Notch extracellular domain is essential for Notch activity, and tissue-specific alterations in the glycan structures are known to regulate activity. Here we review recent advances in identification and characterization of the enzymes responsible for glycosylating Notch and molecular mechanisms for how these O-glycans affect Notch activity.