Structural diversity of cytosolic free oligosaccharides in the human hepatoma cell line, HepG2

Transport of N-acetylchitooligosaccharides and fluorescent N-acetylchitooligosaccharide analogs into rat liver lysosomes

Free polymannose-type oligosaccharides (fOS) are processed by cytosolic enzymes to generate Man5GlcNAc which is transferred to lysosomes and degraded. Lysosomal fOS import was demonstrated in vitro but is poorly characterized in part due to lack of convenient substrates. As chitooligosaccharides (COS, oligomers β1,4-linked GlcNAc) block [3H]Man5GlcNAc transport into lysosomes, we asked if COS are themselves transported and if so, can they be chemically modified to generate fluorescent substrates. We show that COS are degraded by lysosomal hydrolases to generate GlcNAc, and robust ATP-dependent transport of [3H]COS2/4 di and tetrasaccharides into intact rat liver lysosomes was observed only after blocking lysosomal [3H]GlcNAc efflux with cytochalasin B. As oligosaccharides with unmodified reducing termini are the most efficient inhibitors of [3H]COS2/4 and [3H]Man5GlcNAc transport, the non-reducing GlcNAc residue of COS2-4 was de-N-acetylated using Sinorhizobium meliloti NodB, and the resulting amine substituted with rhodamine B (RB) to yield RB-COS2-4. The fluorescent compounds inhibit [3H]Man5GlcNAc transport and display temperature-sensitive, ATP-dependent transport into a sedimentable compartment that is ruptured with the lysosomotropic agent L-methyl methionine ester. Once in this compartment, RB-COS3 is converted to RB-COS2 further identifying it as the lysosomal compartment. RB-COS2/3 and [3H]Man5GlcNAc transports are blocked similarly by competing sugars, and are partially inhibited by the vacuolar ATPase inhibitor bafilomycin and high concentrations of the P-type ATPase inhibitor orthovanadate. These data show that Man5GlcNAc, COS2/4 and RB-COS2/3 are transported into lysosomes by the same or closely related mechanism and demonstrate the utility of COS modified at their non-reducing terminus to study lysosomal oligosaccharide transport.

Evaluation of the context of downstream N- and free N-glycomic alterations induced by swainsonine in HepG2 cells

Swainsonine (SWA), a potent inhibitor of class II α-mannosidases, is present in a number of plant species worldwide and causes severe toxicosis in livestock grazing these plants. The mechanisms underlying SWA-induced animal poisoning are not fully understood. In this study, we analyzed the alterations that occur in N- and free N-glycomic upon addition of SWA to HepG2 cells to understand better SWA-induced glycomic alterations. After SWA addition, we observed the appearance of SWA-specific glycomic alterations, such as unique fucosylated hybrid-type and fucosylated M5 (M5F) N-glycans, and a remarkable increase in all classes of Gn1 FNGs. Further analysis of the context of these glycomic alterations showed that (fucosylated) hybrid type N-glycans were not the precursors of these Gn1 FNGs and vice versa. Time course analysis revealed the dynamic nature of glycomic alterations upon exposure of SWA and suggested that accumulation of free N-glycans occurred earlier than that of hybrid-type N-glycans. Hybrid-type N-glycans, of which most were uniquely core fucosylated, tended to increase slowly over time, as was observed for M5F N-glycans. Inhibition of swainsonine-induced unique fucosylation of hybrid N-glycans and M5 by coaddition of 2-fluorofucose caused significant increases in paucimannose- and fucosylated paucimannose-type N-glycans, as well as paucimannose-type free N-glycans. The results not only revealed the gross glycomic alterations in HepG2 cells induced by swainsonine, but also provide information on the global interrelationships between glycomic alterations.

Increased levels of acidic free-N-glycans, including multi-antennary and fucosylated structures, in the urine of cancer patients

We recently reported increased levels of urinary free-glycans in some cancer patients. Here, we focused on cancer related alterations in the levels of high molecular weight free-glycans. The rationale for this study was that branching, elongation, fucosylation and sialylation, which lead to increases in the molecular weight of glycans, are known to be up-regulated in cancer. Urine samples from patients with gastric cancer, pancreatic cancer, cholangiocarcinoma and colorectal cancer and normal controls were analyzed. The extracted free-glycans were fluorescently labeled with 2-aminopyridine and analyzed by multi-step liquid chromatography. Comparison of the glycan profiles revealed increased levels of glycans in some cancer patients. Structural analysis of the glycans was carried out by performing chromatography and mass spectrometry together with enzymatic or chemical treatments. To compare glycan levels between samples with high sensitivity and selectivity, simultaneous measurements by reversed-phase liquid chromatography-selected ion monitoring of mass spectrometry were also performed. As a result, three lactose-core glycans and 78 free-N-glycans (one phosphorylated oligomannose-type, four sialylated hybrid-type and 73 bi-, tri- and tetra-antennary complex-type structures) were identified. Among them, glycans with α1,3-fucosylation ((+/− sialyl) Lewis X), triply α2,6-sialylated tri-antennary structures and/or a (Man3)GlcNAc1-core displayed elevated levels in cancer patients. However, simple α2,3-sialylation and α1,6-core-fucosylation did not appear to contribute to the observed increase in the level of glycans. Interestingly, one tri-antennary free-N-glycan that showed remarkable elevation in some cancer patients contained a unique Glcβ1-4GlcNAc-core instead of the common GlcNAc2-core at the reducing end. This study provides further insights into free-glycans as potential tumor markers and their processing pathways in cancer.

Structural analysis of N-glycans in medaka gut exposed to silver and titanium dioxide nanoparticles

Nanomaterials are in general use in a broad range of industries. However, there are concerns that their intense use leads to heavy damage to the aquatic environment, and their discharge harms many aquatic organisms. N-Glycans are widely distributed in eukaryotic organisms and are intimately involved in most life phenomena. However, little is known about N-glycans in aquatic organisms exposed to nanomaterials. In this study, we investigated how nanomaterials affect N-glycans in the gut of adult female medaka. We found that silver nanoparticles exposure had little effect on gut N-glycans, whereas titanium dioxide nanoparticles (TiO₂NPs) exposure increased the relative levels of several N-glycans in comparison with control. Structural analysis showed high levels of N-glycans of the high-mannose type, of which five N-glycans were free N-glycans with one β-N-acetylglucosamine residue on the reducing end. The levels of free N-glycans are closely related to protein quality control in the endoplasmic reticulum and cytosol. Our results suggest that TiO₂NPs exposure increases the levels of misfolded glycoproteins, resulting in generation of considerable amounts of free N-glycans. Our findings also suggest that TiO₂NPs exposure suppresses cytosolic α-mannosidase trimming. This study provides new evidence for the effect of TiO₂NPs on medaka gut from the aspect of environmental glycobiology.

Nrf2 activation attenuates genetic endoplasmic reticulum stress induced by a mutation in the phosphomannomutase 2 gene in zebrafish

Nrf2 plays critical roles in animals' defense against electrophiles and oxidative stress by orchestrating the induction of cytoprotective genes. We previously isolated the zebrafish mutant it768, which displays up-regulated expression of Nrf2 target genes in an uninduced state. In this paper, we determine that the gene responsible for it768 was the zebrafish homolog of phosphomannomutase 2 (Pmm2), which is a key enzyme in the initial steps of N-glycosylation, and its mutation in humans leads to PMM2-CDG (congenital disorders of glycosylation), the most frequent type of CDG. The pmm2^it768 larvae exhibited mild defects in N-glycosylation, indicating that the pmm2^it768 mutation is a hypomorph, as in human PMM2-CDG patients. A gene expression analysis showed that pmm2^it768 larvae display up-regulation of endoplasmic reticulum (ER) stress, suggesting that the activation of Nrf2 was induced by the ER stress. Indeed, the treatment with the ER stress-inducing compounds up-regulated the gstp1 expression in an Nrf2-dependent manner. Furthermore, the up-regulation of gstp1 by the pmm2 inactivation was diminished by knocking down or out double-stranded RNA-activated protein kinase (PKR)-like ER kinase (PERK), one of the main ER stress sensors, suggesting that Nrf2 was activated in response to the ER stress via the PERK pathway. ER stress-induced activation of Nrf2 was reported previously, but the results have been controversial. Our present study clearly demonstrated that ER stress can indeed activate Nrf2 and this regulation is evolutionarily conserved among vertebrates. Moreover, ER stress induced in pmm2^it768 mutants was ameliorated by the treatment of the Nrf2-activator sulforaphane, indicating that Nrf2 plays significant roles in the reduction of ER stress.

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.

Accumulation of free oligosaccharides and tissue damage in cytosolic α-mannosidase (Man2c1)-deficient mice

Free Man(7-9)GlcNAc2 is released during the biosynthesis pathway of N-linked glycans or from misfolded glycoproteins during the endoplasmic reticulum-associated degradation process and are reduced to Man5GlcNAc in the cytosol. In this form, free oligosaccharides can be transferred into the lysosomes to be degraded completely. α-Mannosidase (MAN2C1) is the enzyme responsible for the partial demannosylation occurring in the cytosol. It has been demonstrated that the inhibition of MAN2C1 expression induces accumulation of Man(8-9)GlcNAc oligosaccharides and apoptosis in vitro. We investigated the consequences caused by the lack of cytosolic α-mannosidase activity in vivo by the generation of Man2c1-deficient mice. Increased amounts of Man(8-9)GlcNAc oligosaccharides were recognized in all analyzed KO tissues. Histological analysis of the CNS revealed neuronal and glial degeneration with formation of multiple vacuoles in deep neocortical layers and major telencephalic white matter tracts. Enterocytes of the small intestine accumulate mannose-containing saccharides and glycogen particles in their apical cytoplasm as well as large clear vacuoles in retronuclear position. Liver tissue is characterized by groups of hepatocytes with increased content of mannosyl compounds and glycogen, some of them undergoing degeneration by hydropic swelling. In addition, lectin screening showed the presence of mannose-containing saccharides in the epithelium of proximal kidney tubules, whereas scattered glomeruli appeared collapsed or featured signs of fibrosis along Bowman's capsule. Except for a moderate enrichment of mannosyl compounds and glycogen, heterozygous mice were normal, arguing against possible toxic effects of truncated Man2c1. These findings confirm the key role played by Man2c1 in the catabolism of free oligosaccharides.

Free glycans derived from glycoproteins present in human sera

During the course of studies on the analysis of O-glycans in biological samples, we found that significant amount of free glycans are present in normal human serum samples. The most abundant free glycan was disialo-biantennary glycan typically observed in transferrin which is one of the abundant glycoproteins found in sera. Minor glycans were also considered to be mainly due to transferrin, but some glycans were derived from mucin-type O-glycans, although the amount was quite minute. However, high mannose-type glycans could not be detected at all. Although there have been many reports on the presence of intracellular "free" N-glycans (mainly derived from high mannose-type glycans) generated either from lipid-linked oligosaccharides or from misfolded glycoproteins through endoplasmic-reticulum associated protein degradation pathway, there is little information on the presence of free glycans in extracellular matrix and biological fluids such as serum. This report is the first one which demonstrates the presence of free glycans due to glycoproteins in sera.

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.

Identification of roles for peptide: N-glycanase and endo-beta-N-acetylglucosaminidase (Engase1p) during protein N-glycosylation in human HepG2 cells

Background

During mammalian protein N-glycosylation, 20% of all dolichol-linked oligosaccharides (LLO) appear as free oligosaccharides (fOS) bearing the di-N-acetylchitobiose (fOSGN2), or a single N-acetylglucosamine (fOSGN), moiety at their reducing termini. After sequential trimming by cytosolic endo β-N-acetylglucosaminidase (ENGase) and Man2c1 mannosidase, cytosolic fOS are transported into lysosomes. Why mammalian cells generate such large quantities of fOS remains unexplored, but fOSGN2 could be liberated from LLO by oligosaccharyltransferase, or from glycoproteins by NGLY1-encoded Peptide-N-Glycanase (PNGase). Also, in addition to converting fOSGN2 to fOSGN, the ENGASE-encoded cytosolic ENGase of poorly defined function could potentially deglycosylate glycoproteins. Here, the roles of Ngly1p and Engase1p during fOS metabolism were investigated in HepG2 cells.

Methods/Principal Findings

During metabolic radiolabeling and chase incubations, RNAi-mediated Engase1p down regulation delays fOSGN2-to-fOSGN conversion, and it is shown that Engase1p and Man2c1p are necessary for efficient clearance of cytosolic fOS into lysosomes. Saccharomyces cerevisiae does not possess ENGase activity and expression of human Engase1p in the png1Δ deletion mutant, in which fOS are reduced by over 98%, partially restored fOS generation. In metabolically radiolabeled HepG2 cells evidence was obtained for a small but significant Engase1p-mediated generation of fOS in 1 h chase but not 30 min pulse incubations. Ngly1p down regulation revealed an Ngly1p-independent fOSGN2 pool comprising mainly Man₈GlcNAc₂, corresponding to ∼70% of total fOS, and an Ngly1p-dependent fOSGN2 pool enriched in Glc₁Man₉GlcNAc₂ and Man₉GlcNAc₂ that corresponds to ∼30% of total fOS.

Conclusions/Significance

As the generation of the bulk of fOS is unaffected by co-down regulation of Ngly1p and Engase1p, alternative quantitatively important mechanisms must underlie the liberation of these fOS from either LLO or glycoproteins during protein N-glycosylation. The fully mannosylated structures that occur in the Ngly1p-dependent fOSGN2 pool indicate an ERAD process that does not require N-glycan trimming.

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.

The role of MRH domain-containing lectins in ERAD

The endoplasmic reticulum (ER) quality control system ensures that newly synthesized proteins in the early secretory pathway are in the correct conformation. Polypeptides that have failed to fold into native conformers are subsequently retrotranslocated and degraded by the cytosolic ubiquitin-proteasome system, a process known as endoplasmic reticulum-associated degradation (ERAD). Most of the polypeptides that enter the ER are modified by the addition of N-linked oligosaccharides, and quality control of these glycoproteins is assisted by lectins that recognize specific sugar moieties and molecular chaperones that recognize unfolded proteins, resulting in proper protein folding and ERAD substrate selection. In Saccharomyces cerevisiae, Yos9p, a lectin that contains a mannose 6-phosphate receptor homology (MRH) domain, was identified as an important component of ERAD. Yos9p was shown to associate with the membrane-embedded ubiquitin ligase complex, Hrd1p-Hrd3p, and provide a proofreading mechanism for ERAD. Meanwhile, the function of the mammalian homologues of Yos9p, OS-9 and XTP3-B remained elusive until recently. Recent studies have determined that both OS-9 and XTP3-B are ER resident proteins that associate with the HRD1-SEL1L ubiquitin ligase complex and are important for the regulation of ERAD. Moreover, recent studies have identified the N-glycan species with which both yeast Yos9p and mammalian OS-9 associate as M7A, a Man(7)GlcNAc(2) isomer that lacks the alpha1,2-linked terminal mannose from both the B and C branches. M7A has since been demonstrated to be a degradation signal in both yeast and mammals.

Pyridylamination as a means of analyzing complex sugar chains

Herein, I describe pyridylamination for versatile analysis of sugar chains. The reducing ends of the sugar chains are tagged with 2-aminopyridine and the resultant chemically stable fluorescent derivatives are used for structural/functional analysis. Pyridylamination is an effective "operating system" for increasing sensitivity and simplifying the analytical procedures including mass spectrometry and NMR. Excellent separation of isomers is achieved by reversed-phase HPLC. However, separation is further improved by two-dimensional HPLC, which involves a combination of reversed-phase HPLC and size-fractionation HPLC. Moreover, a two-dimensional HPLC map is also useful for structural analysis. I describe a simple procedure for preparing homogeneous pyridylamino sugar chains that is less laborious than existing techniques and can be used for functional analysis (e.g., sugar-protein interaction). This novel approach was applied and some of the results are described: i) a glucosyl-serine type sugar chain found in blood coagulation factors; ii) discovery of endo-beta-mannosidase (EC 3.2.1.152) and a new type plant alpha1,2-L-fucosidase; and iii) novel substrate specificity of a cytosolic alpha-mannosidase. Moreover, using homogeneous sugar chains of a size similar to in vivo substrates we were able to analyze interactions between sugar chains and proteins such as enzymes and lectins in detail. Interestingly, our studies reveal that some enzymes recognize a wider region of the substrate than anticipated.

Glycobiology in the cytosol: the bitter side of a sweet world

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of "glycobiology in the cytosol" which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.

Dual-gradient high-performance liquid chromatography for identification of cytosolic high-mannose-type free glycans

It has been shown that free oligosaccharides derived from N-linked glycans accumulate in the cytosol of animal cells. Most of the glycans have only a single GlcNAc at their reducing termini (Gn1 glycans), whereas the original N-glycans retain N,N'-diacetylchitobiose at their reducing termini (Gn2 glycans). Under the conditions of high-performance liquid chromatography (HPLC) mapping established for pyridylamine (PA)-labeled Gn2 N-glycans, Gn1 glycans are not well retained on reversed-phase HPLC, making simultaneous analysis of Gn1 and Gn2 glycans problematic. We introduced a dual gradient (i.e., pH and butanol gradient) for the separation of Gn1 and Gn2 glycans in a single reversed-phase HPLC. Determination of elution time for various standard Gn2 high-mannose-type glycans, as well as Gn1 glycans found in the cytosol of animal cells, showed that elution of Gn1 and Gn2 glycans could be separated. Sufficient separation for most of the structural isomers could be achieved for Gn1 and Gn2 glycans. This HPLC, therefore, is a powerful method for identification of the structures of PA-labeled glycans, especially Gn1-type glycans, isolated from the cytosol of animal cells.

Characterization and subcellular localization of human neutral class IIα-mannosidase cytosolic enzymes/free oligosaccharides/glycosidehydrolase family 38/M2C1/N-glycosylation

A glycosyl hydrolase family 38 enzyme, neutral alpha-mannosidase, has been proposed to be involved in hydrolysis of cytosolic free oligosaccharides originating either from ER-misfolded glycoproteins or the N-glycosylation process. Although this enzyme has been isolated from the cytosol, it has also been linked to the ER by subcellular fractionations. We have studied the subcellular localization of neutral alpha-mannosidase by immunofluorescence microscopy and characterized the human recombinant enzyme with natural substrates to elucidate the biological function of this enzyme. Immunofluorescence microscopy showed neutral alpha-mannosidase to be absent from the ER, lysosomes, and autophagosomes, and being granularly distributed in the cytosol. In experiments with fluorescent recovery after photo bleaching, neutral alpha-mannosidase had slower than expected two-phased diffusion in the cytosol. This result together with the granular appearance in immunostaining suggests that portion of the neutral alpha-mannosidase pool is somehow complexed. The purified recombinant enzyme is a tetramer and has a neutral pH optimum for activity. It hydrolyzed Man(9)GlcNAc to Man(5)GlcNAc in the presence of Fe(2+), Co(2+), and Mn(2+), and uniquely to neutral alpha-mannosidases from other organisms, the human enzyme was more activated by Fe(2+) than Co(2+). Without activating cations the main reaction product was Man(8)GlcNAc, and Cu(2+) completely inhibited neutral alpha-mannosidase. Our findings from enzyme-substrate characterizations and subcellular localization studies support the suggested role for neutral alpha-mannosidase in hydrolysis of soluble cytosolic oligomannosides.

Free oligosaccharides in the cytosol of Caenorhabditis elegans are generated through endoplasmic reticulum-golgi trafficking

Free oligosaccharides (FOSs) in the cytosol of eukaryotic cells are mainly generated during endoplasmic reticulum (ER)-associated degradation (ERAD) of misfolded glycoproteins. We analyzed FOS of the nematode Caenorhabditis elegans to elucidate its detailed degradation pathway. The major FOSs were high mannose-type ones bearing 3-9 Man residues. About 94% of the total FOSs had one GlcNAc at their reducing end (FOS-GN1), and the remaining 6% had two GlcNAc (FOS-GN2). A cytosolic endo-beta-N-acetylglucosaminidase mutant (tm1208) accumulated FOS-GN2, indicating involvement of the enzyme in conversion of FOS-GN2 into FOS-GN1. The most abundant FOS in the wild type was Man(5)GlcNAc(1), the M5A' isomer (Manalpha1-3(Manalpha1-6)Manalpha1-6(Manalpha1-3)Manbeta1-4GlcNAc), which is different from the corresponding M5B' (Manalpha1-2Manalpha1-2Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAc) in mammals. Analyses of FOS in worms treated with Golgi alpha-mannosidase I inhibitors revealed decreases in Man(5)GlcNAc(1) and increases in Man(7)GlcNAc(1). These results suggested that Golgi alpha-mannosidase I-like enzyme is involved in the production of Man(5-6)-GlcNAc(1), which is unlike in mammals, in which cytosolic alpha-mannosidase is involved. Thus, we assumed that major FOSs in C. elegans were generated through Golgi trafficking. Analysis of FOSs from a Golgi alpha-mannosidase II mutant (tm1078) supported this idea, because GlcNAc(1)Man(5)GlcNAc(1), which is formed by the Golgi-resident GlcNAc-transferase I, was found as a FOS in the mutant. We concluded that significant amounts of misfolded glycoproteins in C. elegans are trafficked to the Golgi and are directly or indirectly retro-translocated into the cytosol to be degraded.

Man2C1, an alpha-mannosidase, is involved in the trimming of free oligosaccharides in the cytosol

The endoplasmic-reticulum-associated degradation of misfolded (glyco)proteins ensures that only functional, correctly folded proteins exit from the endoplasmic reticulum and that misfolded ones are degraded by the ubiquitin-proteasome system. During the degradation of misfolded glycoproteins, they are deglycosylated by the PNGase (peptide:N-glycanase). The free oligosaccharides released by PNGase are known to be further catabolized by a cytosolic alpha-mannosidase, although the gene encoding this enzyme has not been identified unequivocally. The findings in the present study demonstrate that an alpha-mannosidase, Man2C1, is involved in the processing of free oligosaccharides that are formed in the cytosol. When the human Man2C1 orthologue was expressed in HEK-293 cells, most of the enzyme was localized in the cytosol. Its activity was enhanced by Co2+, typical of other known cytosolic alpha-mannosidases so far characterized from animal cells. The down-regulation of Man2C1 activity by a small interfering RNA drastically changed the amount and structure of oligosaccharides accumulating in the cytosol, demonstrating that Man2C1 indeed is involved in free oligosaccharide processing in the cytosol. The oligosaccharide processing in the cytosol by PNGase, endo-beta-N-acetylglucosaminidase and alpha-mannosidase may represent the common 'non-lysosomal' catabolic pathway for N-glycans in animal cells, although the molecular mechanism as well as the functional importance of such processes remains to be determined.