Establishment of a real-time analytical method for free oligosaccharide transport from the ER to the cytosol

Rat hepatocytes secrete free oligosaccharides

We recently established a method for the isolation of serum-free oligosaccharides, and characterized various features of their structures. However, the precise mechanism for how these glycans are formed still remains unclarified. To further investigate the mechanism responsible for these serum glycans, here, we utilized rat primary hepatocytes to examine whether they are able to secrete free glycans. Our findings indicated that a diverse array of free oligosaccharides such as sialyl/neutral free N-glycans (FNGs), as well as sialyl lactose/LacNAc-type glycans, were secreted into the culture medium by primary hepatocytes. The structural features of these free glycans in the medium were similar to those isolated from the sera of the same rat. Further evidence suggested that an oligosaccharyltransferase is involved in the release of the serum-free N-glycans. Our results indicate that the liver is indeed secreting various types of free glycans directly into the serum.

Hydrolytic activity of yeast oligosaccharyltransferase is enhanced when misfolded proteins accumulate in the endoplasmic reticulum

It is known that oligosaccharyltransferase (OST) has hydrolytic activity toward dolichol-linked oligosaccharides (DLO), which results in the formation of free N-glycans (FNGs), i.e. unconjugated oligosaccharides with structural features similar to N-glycans. The functional importance of this hydrolytic reaction, however, remains unknown. In this study, the hydrolytic activity of OST was characterized in yeast. It was shown that the hydrolytic activity of OST is enhanced in ubiquitin ligase mutants that are involved in endoplasmic reticulum-associated degradation. Interestingly, this enhanced hydrolysis activity is completely suppressed in asparagine-linked glycosylation (alg) mutants, bearing mutations related to the biosynthesis of DLO, indicating that the effect of ubiquitin ligase on OST-mediated hydrolysis is context-dependent. The enhanced hydrolysis activity in ubiquitin ligase mutants was also found to be canceled upon treatment of the cells with dithiothreitol, a reagent that potently induces protein unfolding in the endoplasmic reticulum (ER). Our results clearly suggest that the hydrolytic activity of OST is enhanced under conditions in which the formation of unfolded proteins is promoted in the ER in yeast. The possible role of FNGs on protein folding is discussed.

A bioactive mammalian disaccharide associated with autoimmunity activates STING-TBK1-dependent immune response

Glycans from microbial pathogens are well known pathogen-associated molecular patterns that are recognized by the host immunity; however, little is known about whether and how mammalian self-glycans activate the host immune response, especially in the context of autoimmune disease. Using biochemical fractionation and two-dimensional HPLC, we identify an abundant and bioactive free glycan, the Manβ1-4GlcNAc disaccharide in TREX1-associated autoimmune diseases. We report that both monosaccharide residues and the β1-4 linkage are critical for bioactivity of this disaccharide. We also show that Manβ1-4GlcNAc is produced by oligosaccharyltransferase hydrolysis of lipid-linked oligosaccharides in the ER lumen, followed by ENGase and mannosidase processing in the cytosol and lysosomes. Furthermore, synthetic Manβ1-4GlcNAc disaccharide stimulates a broad immune response in vitro, which is in part dependent on the STING-TBK1 pathway, and enhances antibody response in vivo. Together, our data identify Manβ1-4GlcNAc as a novel innate immune modulator associated with chronic autoimmune diseases.

Occurrence of free sialyl oligosaccharides related to N-glycans (sialyl free N-glycans) in animal sera

Free oligosaccharides that are structurally related to N-glycans [free N-glycans (FNGs)] are widely distributed in the cytosol of animal cells. The diverse molecular mechanisms responsible for the formation of these FNGs have been well clarified. In this study we demonstrate the wide occurrence of sialylated FNGs in sera of various animals. The features of these extracellular FNGs are quite distinct from the cytosolic FNGs, as they are Gn2-type glycans, bearing an N,N'-diacetylchitobiose unit at their reducing termini, while the cytosolic FNGs are predominantly Gn1-type, with a single GlcNAc at their reducing termini. The major structures observed varied from species to species, and the structures of the FNGs appear to be correlated with the major sialyl N-glycans on serum glycoproteins, suggesting that the serum FNGs are produced by hepatocytes. Interestingly, glycan-profiles of the FNGs indicated that they are altered in a developmental stage-dependent manner. Sialyl FNGs in the sera may not only be of biological relevance, in that they might reflect the functionality of the liver, but also can be attractive sources for obtaining uniform sialyl FNGs in the chemoenzymatic synthesis of glycoproteins.

Non-lysosomal Degradation of Singly Phosphorylated Oligosaccharides Initiated by the Action of a Cytosolic Endo-β-N-acetylglucosaminidase

Phosphorylated oligosaccharides (POSs) are produced by the degradation of dolichol-linked oligosaccharides (DLOs) by an unclarified mechanism in mammalian cells. Although POSs are exclusively found in the cytosol, their intracellular fates remain unclear. Our findings indicate that POSs are catabolized via a non-lysosomal glycan degradation pathway that involves a cytosolic endo-β-N-acetylglucosaminidase (ENGase). Quantitative and structural analyses of POSs revealed that ablation of the ENGase results in the significant accumulation of POSs with a hexasaccharide structure composed of Manα1,2Manα1,3(Manα1,6)Manβ1,4GlcNAcβ1,4GlcNAc.In vitroENGase assays revealed that the presence of an α1,2-linked mannose residue facilitates the hydrolysis of POSs by the ENGase. Liquid chromatography-mass spectrometric analyses and fluorescent labeling experiments show that such POSs contain one phosphate group at the reducing end. These results indicate that ENGase efficiently hydrolyzes POSs that are larger than Man4GlcNAc2-P, generating GlcNAc-1-P and neutral Gn1-type free oligosaccharides. These results provide insight into important aspects of the generation and degradation of POSs.

Generation and degradation of free asparagine-linked glycans

Asparagine (N)-linked protein glycosylation, which takes place in the eukaryotic endoplasmic reticulum (ER), is important for protein folding, quality control and the intracellular trafficking of secretory and membrane proteins. It is known that, during N-glycosylation, considerable amounts of lipid-linked oligosaccharides (LLOs), the glycan donor substrates for N-glycosylation, are hydrolyzed to form free N-glycans (FNGs) by unidentified mechanisms. FNGs are also generated in the cytosol by the enzymatic deglycosylation of misfolded glycoproteins during ER-associated degradation. FNGs derived from LLOs and misfolded glycoproteins are eventually merged into one pool in the cytosol and the various glycan structures are processed to a near homogenous glycoform. This article summarizes the current state of our knowledge concerning the formation and catabolism of FNGs.

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.

Non-lysosomal degradation pathway for N-linked glycans and dolichol-linked oligosaccharides

There is growing evidence that asparagine (N)-linked glycans play pivotal roles in protein folding and intra- or intercellular trafficking of N-glycosylated proteins. During the N-glycosylation of proteins, significant amounts of free oligosaccharides (fOSs) and phosphorylated oligosaccharides (POSs) are generated at the endoplasmic reticulum (ER) membrane by unclarified mechanisms. fOSs are also formed in the cytosol by the enzymatic deglycosylation of misfolded glycoproteins destined for proteasomal degradation. This article summarizes the current knowledge of the molecular and regulatory mechanisms underlying the formation of fOSs and POSs in mammalian cells and Saccharomyces cerevisiae.

Eukaryotic oligosaccharyltransferase generates free oligosaccharides during N-glycosylation

Asparagine (N)-linked glycosylation regulates numerous cellular activities, such as glycoprotein quality control, intracellular trafficking, and cell-cell communications. In eukaryotes, the glycosylation reaction is catalyzed by oligosaccharyltransferase (OST), a multimembrane protein complex that is localized in the endoplasmic reticulum (ER). During N-glycosylation in the ER, the protein-unbound form of oligosaccharides (free oligosaccharides; fOSs), which is structurally related to N-glycan, is released into the ER lumen. However, the enzyme responsible for this process remains unidentified. Here, we demonstrate that eukaryotic OST generates fOSs. Biochemical and genetic analyses using mutant strains of Saccharomyces cerevisiae revealed that the generation of fOSs is tightly correlated with the N-glycosylation activity of OST. Furthermore, we present evidence that the purified OST complex can generate fOSs by hydrolyzing dolichol-linked oligosaccharide, the glycan donor substrate for N-glycosylation. The heterologous expression of a single subunit of OST from the protozoan Leishmania major in S. cerevisiae demonstrated that this enzyme functions both in N-glycosylation and generation of fOSs. This study provides insight into the mechanism of PNGase-independent formation of fOSs.

Identification and characterization of endo-β-N-acetylglucosaminidase from methylotrophic yeast Ogataea minuta

In four yeast strains, Ogataea minuta, Candida parapolymorpha, Pichia anomala and Zygosaccharomyces rouxii, we identified endo-β-N-acetylglucosaminidase (ENGase) homologous sequences by database searches; in each of the four species, a corresponding enzyme activity was also confirmed in crude cell extract obtained from each strain. The O. minuta ENGase (Endo-Om)-encoding gene was directly amplified from O. minuta genomic DNA and sequenced. The Endo-Om-encoding gene contained a 2319-bp open-reading frame; the deduced amino acid sequence indicated that the putative protein belonged to glycoside hydrolase family 85. The gene was introduced into O. minuta, and the recombinant Endo-Om was overexpressed and purified. When the enzyme assay was performed using an agalacto-biantennary oligosaccharide as a substrate, Endo-Om exhibited both hydrolysis and transglycosylation activities. Endo-Om exhibited hydrolytic activity for high-mannose, hybrid, biantennary and (2,6)-branched triantennary N-linked oligosaccharides, but not for tetraantennary, (2,4)-branched triantennary, bisecting N-acetylglucosamine structure and core-fucosylated biantennary N-linked oligosaccharides. Endo-Om also was able to hydrolyze N-glycans attached to RNase B and human transferrin under both denaturing and nondenaturing conditions. Thus, the present study reports the detection and characterization of a novel yeast ENGase.

Demonstration that endoplasmic reticulum-associated degradation of glycoproteins can occur downstream of processing by endomannosidase

During quality control in the ER (endoplasmic reticulum), nascent glycoproteins are deglucosylated by ER glucosidases I and II. In the post-ER compartments, glycoprotein endo-α-mannosidase provides an alternative route for deglucosylation. Previous evidence suggests that endomannosidase non-selectively deglucosylates glycoproteins that escape quality control in the ER, facilitating secretion of aberrantly folded as well as normal glycoproteins. In the present study, we employed FOS (free oligosaccharides) released from degrading glycoproteins as biomarkers of ERAD (ER-associated degradation), allowing us to gain a global rather than single protein-centred view of ERAD. Glucosidase inhibition was used to discriminate between glucosidase- and endomannosidase-mediated ERAD pathways. Endomannosidase expression was manipulated in CHO (Chinese-hamster ovary)-K1 cells, naturally lacking a functional version of the enzyme, and HEK (human embryonic kidney)-293T cells. Endomannosidase was shown to decrease the levels of total FOS, suggesting decreased rates of ERAD. However, following pharmacological inhibition of ER glucosidases I and II, endomannosidase expression resulted in a partial switch between glucosylated FOS, released from ER-confined glycoproteins, to deglucosylated FOS, released from endomannosidase-processed glycoproteins transported from the Golgi/ERGIC (ER/Golgi intermediate compartment) to the ER. Using this approach, we have identified a previously unknown pathway of glycoprotein flow, undetectable by the commonly employed methods, in which secretory cargo is targeted back to the ER after being processed by endomannosidase.

Metabolism of free oligosaccharides is facilitated in the och1Δ mutant of Saccharomyces cerevisiae

In eukaryotic cells, it is known that N-glycans play a pivotal role in quality control of carrier proteins. Although "free" forms of oligosaccharides (fOSs) are known to be accumulated in the cytosol, the precise mechanism of their formation, degradation and biological relevance remains poorly understood. It has been shown that, in budding yeast, almost all fOSs are formed from misfolded glycoproteins. Precise structural analysis of fOSs revealed that several yeast fOSs bear a yeast-specific modification by Golgi-resident α-1,6-mannosyltransferase, Och1. In this study, structural diversity of fOSs in och1Δ cells was analyzed. To our surprise, several fOSs in och1Δ cells have unusual α-1,3-linked mannose residues at their non-reducing termini. These mannose residues were not observed in wild-type cells, suggesting that the addition of these unique mannoses occurred as a compensation of Och1 defect. A significant increase in the amount of fOSs modified by Golgi-localized mannosyltransferases was also observed in och1Δ cells. Moreover, the amount of processed fOSs and intracellular α-mannosidase (Ams1) both increased in this mutant. Up-regulation of Ams1 activity was also apparent for cells treated with cell wall perturbation reagent. These results provide an insight into a possible link between catabolism of fOSs and cell wall stress.

Calystegine B3 as a specific inhibitor for cytoplasmic alpha-mannosidase, Man2C1

Cytoplasmic α-mannosidase (Man2C1) has been implicated in non-lysosomal catabolism of free oligosaccharides derived from N-linked glycans accumulated in the cytosol. Suppression of Man2C1 expression reportedly induces apoptosis in various cell lines, but its molecular mechanism remains unclear. Development of a specific inhibitor for Man2C1 is critical to understanding its biological significance. In this study, we identified a plant-derived alkaloid, calystegine B(3), as a potent specific inhibitor for Man2C1 activity. Biochemical enzyme assay revealed that calystegine B(3) was a highly specific inhibitor for Man2C1 among various α-mannosidases prepared from rat liver. Consistent with this in vitro result, an in vivo experiment also showed that treatment of mammalian-derived cultured cells with this compound resulted in drastic change in both structure and quantity of free oligosaccharides in the cytosol, whereas no apparent change was seen in cell-surface oligosaccharides. Calystegine B(3) could thus serve as a potent tool for the development of a highly specific in vivo inhibitor for Man2C1.

Free oligosaccharides to monitor glycoprotein endoplasmic reticulum-associated degradation in Saccharomyces cerevisiae

In eukaryotic cells, N-glycosylation has been recognized as one of the most common and functionally important co- or post-translational modifications of proteins. "Free" forms of N-glycans accumulate in the cytosol of mammalian cells, but the precise mechanism for their formation and degradation remains unknown. Here, we report a method for the isolation of yeast free oligosaccharides (fOSs) using endo-beta-1,6-glucanase digestion. fOSs were undetectable in cells lacking PNG1, coding the cytoplasmic peptide:N-glycanase gene, suggesting that almost all fOSs were formed from misfolded glycoproteins by Png1p. Structural studies revealed that the most abundant fOS was M8B, which is not recognized well by the endoplasmic reticulum-associated degradation (ERAD)-related lectin, Yos9p. In addition, we provide evidence that some of the ERAD substrates reached the Golgi apparatus prior to retrotranslocation to the cytosol. N-Glycan structures on misfolded glycoproteins in cells lacking the cytosol/vacuole alpha-mannosidase, Ams1p, was still quite diverse, indicating that processing of N-glycans on misfolded glycoproteins was more complex than currently envisaged. Under ER stress, an increase in fOSs was observed, whereas levels of M7C, a key glycan structure recognized by Yos9p, were unchanged. Our method can thus provide valuable information on the molecular mechanism of glycoprotein ERAD in Saccharomyces cerevisiae.