Selective Manipulation of Discrete Mannosidase Activities in the Endoplasmic Reticulum by Using Reciprocally Selective Inhibitors

Multisite Partial Glycosylation Approach for Preparation of Biologically Relevant Oligomannosyl Branches Contribute to Lectin Affinity Analysis

High-mannose-type glycans play essential biological roles, e.g., immune response and glycoprotein quality control, and preparing a series of oligomannosyl branches of high-mannose-type glycans is critical for biological studies. However, obtaining sufficient amounts of the various oligomannosyl branches is challenging. In this study, we demonstrated a partial glycosylation strategy for the single-step synthesis of various biologically relevant oligomannosyl-branched structures. First, Manα1-6(Manα1-3)Man-type oligomannosyl branch was synthesized via double glycosylation from a 3,6-di-OH mannosyl acceptor and fluorinated mannosyl donor with perfect α-selectivity. Subsequent partial glycosylation by reducing the equivalent of the mannosyl donor enabled to obtain biologically relevant Manα1-2Manα1-6(Manα1-2Manα1-3)Man, Manα1-6(Manα1-2Manα1-3)Man, Manα1-2Manα1-6(Manα1-3)Man, and Manα1-6(Manα1-3)Man in one-pot. Each oligomannosyl branch could be easily purified by liquid chromatography. The resulting structural isomers were identified by 2D-HMBC NMR. A systematic lectin affinity assay using the prepared oligomannosyl branches showed different specificities for the Galanthus nivalis lectin between structural isomers of the oligomannosyl branches with the same number of mannose residues..

Harnessing natural-product-inspired combinatorial chemistry and computation-guided synthesis to develop N-glycan modulators as anticancer agents

Modulation of N-glycosylation using human Golgi α-mannosidase II (α-hGMII) inhibitors is a potential anticancer approach, but the clinical utility of current α-hGMII inhibitors is limited by their co-inhibition of human lysosomal α-mannosidase (α-hLM), resulting in abnormal storage of oligomannoses. We describe the synthesis and screening of a small library of novel bicyclic iminosugar-based scaffolds, prepared via natural product-inspired combinatorial chemistry (NPICC), which resulted in the identification of a primary α-hGMII inhibitor with 13.5-fold selectivity over α-hLM. Derivatization of this primary inhibitor using computation-guided synthesis (CGS) yielded an advanced α-hGMII inhibitor with nanomolar potency and 106-fold selectivity over α-hLM. In vitro studies demonstrated its N-glycan modulation and inhibitory effect on hepatocellular carcinoma (HCC) cells. In vivo studies confirmed its encouraging anti-HCC activity, without evidence of oligomannose accumulation.

An integrated strategy of Natural-Product-Inspired Combinatorial Chemistry (NPICC) and Computation-Guided Synthesis is used to develop an α-hGMII inhibitor with 106-fold selectivity over α-hLM, with inhibitory effect on hepatocellular carcinoma.

Oligomannose-Type Glycan Processing in the Endoplasmic Reticulum and Its Importance in Misfolding Diseases

Simple Summary

Glycans play many roles in biological processes. For instance, they mediate cell–cell interaction, viral infection, and protein folding of glycoproteins. Glycoprotein folding in the endoplasmic reticulum (ER) is closely related to the onset of diseases such as misfolding diseases caused by accumulation of misfolded proteins in the ER. In this review, we focused on oligomannose-type glycan processing in the ER, which has central roles in glycoprotein folding in the ER, and we summarise relationship between oligomannose-type glycan processing and misfolding diseases arising from the disruption of ER homeostasis.

Abstract

Glycoprotein folding plays a critical role in sorting glycoprotein secretion and degradation in the endoplasmic reticulum (ER). Furthermore, relationships between glycoprotein folding and several diseases, such as type 2 diabetes and various neurodegenerative disorders, are indicated. Patients’ cells with type 2 diabetes, and various neurodegenerative disorders induce ER stress, against which the cells utilize the unfolded protein response for protection. However, in some cases, chronic and/or massive ER stress causes critical damage to cells, leading to the onset of ER stress-related diseases, which are categorized into misfolding diseases. Accumulation of misfolded proteins may be a cause of ER stress, in this respect, perturbation of oligomannose-type glycan processing in the ER may occur. A great number of studies indicate the relationships between ER stress and misfolding diseases, while little evidence has been reported on the connection between oligomannose-type glycan processing and misfolding diseases. In this review, we summarize alteration of oligomannose-type glycan processing in several ER stress-related diseases, especially misfolding diseases and show the possibility of these alteration of oligomannose-type glycan processing as indicators of diseases.

Synthetic trisaccharides reveal discrimination of endo-glycosidic linkages by exo-acting α-1,2-mannosidases in the endoplasmic reticulum

A tri-antennary Man9GlcNAc2 glycan on the surface of endoplasmic reticulum (ER) glycoproteins functions as a glycoprotein secretion or degradation signal after regioselective cleavage of the terminal α-1,2-mannose residue of each branch. Four α-1,2-mannosidases-ER mannosidase I, ER degradation-enhancing α-mannosidase-like protein 1 (EDEM1), EDEM2, and EDEM3-are involved in the production of these signal glycans. Although selective production of signal glycans is important in determining the fate of glycoproteins, the branch-discrimination abilities of the α-1,2-mannosidases are not well understood. A structural feature of the Man9GlcNAc2 glycan is that all terminal glycosidic linkages of the three branches are of the α-1,2 type, while the adjacent inner glycosidic linkages are different. In this study, we examined whether the α-1,2-mannosidases showed branch specificity by discriminating between different inner glycosides. Four trisaccharides with different glycosidic linkages [Manα1-2Manα1-2Man (natural A-branch), Manα1-2Manα1-3Man (natural B-branch), Manα1-2Manα1-6Man (natural C-branch), and Manα1-2Manα1-4Man (unnatural D-branch)] were synthesized and used to evaluate the hypothesis. When synthesizing these oligosaccharides, highly stereoselective glycosylation was achieved with a high yield in each case by adding a weak base or tuning the polarity of the mixed solvent. Enzymatic hydrolysis of the synthetic trisaccharides by a mouse liver ER fraction containing the target enzymes showed that the ER α-1,2-mannosidases had clear specificity for the trisaccharides in the order of A-branch > B-branch > C-branch ≈ D-branch. Various competitive experiments have revealed for the first time that α-1,2-mannosidase with inner glycoside specificity is present in the ER. Our findings suggest that exo-acting ER α-1,2-mannosidases can discriminate between endo-glycosidic linkages.

Glycan structure-based perspectives on the entry and release of glycoproteins in the calnexin/calreticulin cycle

N-glycans are attached to newly synthesised polypeptides and are involved in the folding, secretion, and degradation of N-linked glycoproteins. In particular, the calnexin/calreticulin cycle, which is the central mechanism of the entry and release of N-linked glycoproteins depending on the folding sates, has been well studied. In addition to biological studies on the calnexin/calreticulin cycle, several studies have revealed complementary roles of in vitro chemistry-based research in the structure-based understanding of the cycle. In this mini-review, we summarise chemistry-based results and highlight their importance for further understanding of the cycle.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

Structural insights into N-linked glycan-mediated protein folding from chemical and biological perspectives

About half of all newly synthesized proteins have N-linked glycans. These glycans play pivotal roles in controlling the folding, sorting, and degradation of glycoproteins via several glycan-related proteins. The glycan-mediated protein quality control system is important for cellular homeostasis. In this review, we summarize recent advances in our understanding of the system and discuss structural insights from chemical and biological perspectives. In particular, we focus on the mechanisms by which these mediators respond to several folding states of glycoproteins.

α-glucosidase inhibitors as host-directed antiviral agents with potential for the treatment of COVID-19

The ongoing COVID-19 pandemic, caused by SARS-CoV-2, has pushed the health systems of many countries to breaking point and precipitated social distancing measures that have crippled economic activities across the globe. A return to normality is unlikely until effective therapeutics and a vaccine are available. The immediacy of this problem suggests that drug strategies should focus on repurposing approved drugs or late-stage clinical candidates, as these have the shortest path to use in the clinic. Here, we review and discuss the role of host cell N-glycosylation pathways to virus replication and the drugs available to disrupt these pathways. In particular, we make a case for evaluation of the well-tolerated drugs miglitol, celgosivir and especially miglustat for the treatment of COVID-19.

Mannosidase activity of EDEM1 and EDEM2 depends on an unfolded state of their glycoprotein substrates

Extensive mannose trimming of nascent glycoprotein N-glycans signals their targeting to endoplasmic reticulum-associated degradation (ERAD). ER mannosidase I (ERManI) and the EDEM protein family participate in this process. However, whether the EDEMs are truly mannosidases can be addressed only by measuring mannosidase activity in vitro. Here, we reveal EDEM1 and EDEM2 mannosidase activities in vitro. Whereas ERManI significantly trims free N-glycans, activity of the EDEMs is modest on free oligosaccharides and on glycoproteins. However, mannosidase activity of ERManI and the EDEMs is significantly higher on a denatured glycoprotein. The EDEMs associate with oxidoreductases, protein disulfide isomerase, and especially TXNDC11, enhancing mannosidase activity on glycoproteins but not on free N-glycans. The finding that substrate unfolded status increases mannosidase activity solves an important conundrum, as current models suggest general slow mannose trimming. As we show, misfolded or unfolded glycoproteins are subject to differentially faster trimming (and targeting to ERAD) than well-folded ones.