Lack of the evidence for the enzymatic catabolism of Man1GlcNAc2 in Saccharomyces cerevisiae

In the cytosol of Saccharomyces cerevisiae, most of the free N-glycans (FNGs) are generated from misfolded glycoproteins by the action of the cytoplasmic peptide: N-glycanase (Png1). A cytosol/vacuole α-mannosidase, Ams1, then trims the FNGs to eventually form a trisaccharide composed of Manβ1,4GlcNAc β1,4GlcNAc (Man1GlcNAc2). Whether or not the resulting Man1GlcNAc2 is enzymatically degraded further, however, is currently unknown. The objective of this study was to unveil the fate of Man1GlcNAc2 in S. cerevisiae. Quantitative analyses of the FNGs revealed a steady increase in the amount of Man1GlcNAc2 produced in the post-diauxic and stationary phases, suggesting that this trisaccharide is not catabolized during this period. Inoculation of the stationary phase cells into fresh medium resulted in a reduction in the levels of Man1GlcNAc2. However, this reduction was caused by its dilution due to cell division in the fresh medium. Our results thus indicate that Man1GlcNAc2 is not enzymatically catabolized in S. cerevisiae.

Co-Expression of NEU2 and GBA3 Causes a Drastic Reduction in Cytosolic Sialyl Free N-glycans in Human MKN45 Stomach Cancer Cells-Evidence for the Physical Interaction of NEU2 and GBA3

It is well known that the "free" form of glycans that are structurally related to asparagine (N)-linked glycans ("free N-glycans") are found in a wide variety of organisms. The mechanisms responsible for the formation/degradation of high mannose-type free N-glycans have been extensively studied in mammalian cells. Recent evidence, however, also suggests that sialylated, complex-type free N-glycans are also present in the cytosol of various mammalian-derived cultured cells/tissues. We report herein on an investigation of the mechanism responsible for the degradation of such sialyl free N-glycans. The findings show that the amount of glycans is dramatically reduced upon the co-expression of cytosolic sialidase NEU2 with cytosolic β-glycosidase GBA3 in human stomach cancer-derived MKN45 cells. The physical interaction between NEU2 and GBA3 was confirmed by co-precipitation analyses as well as gel filtration assays. The NEU2 protein was found to be stabilized in the presence of GBA3 both in cellulo and in vitro. Our results thus indicate that cytosolic GBA3 is likely involved in the catabolism of cytosolic sialyl free N-glycans, possibly by stabilizing the activity of the NEU2 protein.

Endo-β-N-acetylglucosaminidase forms N-GlcNAc protein aggregates during ER-associated degradation in Ngly1-defective cells

The cytoplasmic peptide:N-glycanase (PNGase; Ngly1 in mice) is a deglycosylating enzyme involved in the endoplasmic reticulum (ER)-associated degradation (ERAD) process. The precise role of Ngly1 in the ERAD process, however, remains unclear in mammals. The findings reported herein, using mouse embryonic fibroblast (MEF) cells, that the ablation of Ngly1 causes dysregulation of the ERAD process. Interestingly, not only delayed degradation but also the deglycosylation of a misfolded glycoprotein was observed in Ngly1(-/-) MEF cells. The unconventional deglycosylation reaction was found to be catalyzed by the cytosolic endo-β-N-acetylglucosaminidase (ENGase), generating aggregation-prone N-GlcNAc proteins. The ERAD dysregulation in cells lacking Ngly1 was restored by the additional knockout of ENGase gene. Thus, our study underscores the functional importance of Ngly1 in the ERAD process and provides a potential mechanism underlying the phenotypic consequences of a newly emerging genetic disorder caused by mutation of the human NGLY1 gene.

Autophagy regulates the stability of sialin, a lysosomal sialic acid transporter

Macroautophagy plays a critical role in catabolizing cytosolic components via lysosomal degradation. Recent findings from our studies indicate that basal autophagy is required for the efficient lysosomal catabolism of sialyloligosaccharides, and that the downregulation of sialin, a lysosomal transporter of sialic acids can cause a significant delay in the cytosolic accumulation of such glycans. The findings reported herein show that the sialin protein level was increased when the autophagy process was inhibited. This effect appears to be specific to sialin, since the amount of LAMP1, another lysosomal membrane protein, remains constant under the same conditions. Our results suggest that autophagy may regulate the stability of sialin, and it could lead to the cytosolic accumulation of sialyloligosaccharides in autophagy-defective cells.

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.

Basal autophagy is required for the efficient catabolism of sialyloligosaccharides

Macroautophagy is an essential, homeostatic process involving degradation of a cell's own components; it plays a role in catabolizing cellular components, such as protein or lipids, and damaged or excess organelles. Here, we show that in Atg5(-/-) cells, sialyloligosaccharides specifically accumulated in the cytosol. Accumulation of these glycans was observed under non-starved conditions, suggesting that non-induced, basal autophagy is essential for their catabolism. Interestingly, once accumulated in the cytosol, sialylglycans cannot be efficiently catabolized by resumption of the autophagic process, suggesting that functional autophagy is important for preventing sialyloligosaccharides from accumulating in the cytosol. Moreover, knockdown of sialin, a lysosomal transporter of sialic acids, resulted in a significant reduction of sialyloligosaccharides, implying that autophagy affects the substrate specificity of this transporter. This study thus provides a surprising link between basal autophagy and catabolism of N-linked glycans.

Dual functions for cytosolic α-mannosidase (Man2C1): its down-regulation causes mitochondria-dependent apoptosis independently of its α-mannosidase activity

Cytosolic α-mannosidase (Man2C1) trims free oligosaccharides in mammalian cells, and its down-regulation reportedly delays cancer growth by inducing mitotic arrest or apoptosis. However, the mechanism by which Man2C1 down-regulation induces apoptosis is unknown. Here, we demonstrated that silencing of Man2C1 via small hairpin RNAs induced mitochondria-dependent apoptosis in HeLa cells. Expression of CHOP (C/EBP homologous protein), a transcription factor critical to endoplasmic reticulum stress-induced apoptosis, was significantly up-regulated in Man2C1 knockdown cells. However, this enhanced CHOP expression was not caused by endoplasmic reticulum stress. Interestingly, Man2C1 catalytic activity was not required for this regulation of apoptosis; introduction of mutant, enzymatically inactive Man2C1 rescued apoptotic phenotypes of Man2C1 knockdown cells. These results show that Man2C1 has dual functions: one in glycan catabolism and another in apoptotic signaling.

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.