Metabolic manipulation of glycosylation disorders in humans and animal models

Influence of glycosylation on the immunogenicity and antigenicity of viral immunogens

A key aspect of successful viral vaccine design is the elicitation of neutralizing antibodies targeting viral attachment and fusion glycoproteins that embellish viral particles. This observation has catalyzed the development of numerous viral glycoprotein mimetics as vaccines. Glycans can dominate the surface of viral glycoproteins and as such, the viral glycome can influence the antigenicity and immunogenicity of a candidate vaccine. In one extreme, glycans can form an integral part of epitopes targeted by neutralizing antibodies and are therefore considered to be an important feature of key immunogens within an immunization regimen. In the other extreme, the existence of peptide and bacterially expressed protein vaccines shows that viral glycosylation can be dispensable in some cases. However, native-like glycosylation can indicate native-like protein folding and the presence of conformational epitopes. Furthermore, going beyond native glycan mimicry, in either occupancy of glycosylation sites or the glycan processing state, may offer opportunities for enhancing the immunogenicity and associated protection elicited by an immunogen. Here, we review key determinants of viral glycosylation and how recombinant immunogens can recapitulate these signatures across a range of enveloped viruses, including HIV-1, Ebola virus, SARS-CoV-2, Influenza and Lassa virus. The emerging understanding of immunogen glycosylation and its control will help guide the development of future vaccines in both recombinant protein- and nucleic acid-based vaccine technologies.

A roadmap for translational cancer glycoimmunology at single cell resolution

Cancer cells can evade immune responses by exploiting inhibitory immune checkpoints. Immune checkpoint inhibitor (ICI) therapies based on anti-CTLA-4 and anti-PD-1/PD-L1 antibodies have been extensively explored over the recent years to unleash otherwise compromised anti-cancer immune responses. However, it is also well established that immune suppression is a multifactorial process involving an intricate crosstalk between cancer cells and the immune systems. The cancer glycome is emerging as a relevant source of immune checkpoints governing immunosuppressive behaviour in immune cells, paving an avenue for novel immunotherapeutic options. This review addresses the current state-of-the-art concerning the role played by glycans controlling innate and adaptive immune responses, while shedding light on available experimental models for glycoimmunology. We also emphasize the tremendous progress observed in the development of humanized models for immunology, the paramount contribution of advances in high-throughput single-cell analysis in this context, and the importance of including predictive machine learning algorithms in translational research. This may constitute an important roadmap for glycoimmunology, supporting careful adoption of models foreseeing clinical translation of fundamental glycobiology knowledge towards next generation immunotherapies.

Mannose: A Sweet Option in the Treatment of Cancer and Inflammation

As a natural sugar, mannose is a type of hexose that is abundant in many different types of fruits. Since mannose is rarely used for glycolysis in mammals, studies on the role of mannose have not attracted much attention. Glycosylation of specific proteins was thought to be the major function of mannose. Surprisingly, during the past few years, mannose was found to be effective in promoting immune tolerance and suppressing inflammatory diseases related to autoimmunity and allergy. Moreover importantly, mannose was also found to be efficient in suppressing tumors by suppressing glycolysis and enhancing chemotherapeutic agents. In this review, we summarize the recent studies of mannose on antitumor properties and anti-inflammatory characteristics. We emphasize that mannose could play a beneficial role in the treatment of a variety of diseases, including cancers and inflammatory diseases, and could be a novel therapeutic strategy that deserves continued evaluation.

The role of the cell surface glycocalyx in drug delivery to and through the endothelium

Cell membranes are key interfaces where materials engineering meets biology. Traditionally regarded as just the location of receptors regulating the uptake of molecules, we now know that all mammalian cell membranes are 'sugar coated'. These sugars, or glycans, form a matrix bound at the cell membrane via proteins and lipids, referred to as the glycocalyx, which modulate access to cell membrane receptors crucial for interactions with drug delivery systems (DDS). Focusing on the key blood-tissue barrier faced by most DDS to enable transport from the place of administration to target sites via the circulation, we critically assess the design of carriers for interactions at the endothelial cell surface. We also discuss the current challenges for this area and provide opportunities for future research efforts to more fully engineer DDS for controlled, efficient, and targeted interactions with the endothelium for therapeutic application.

N-glycosylation, a leading role in viral infection and immunity development

N-linked protein glycosylation is an essential co-and posttranslational protein modification that occurs in all three domains of life; the assembly of N-glycans follows a complex sequence of events spanning the (Endoplasmic Reticulum) ER and the Golgi apparatus. It has a significant impact on both physicochemical properties and biological functions. It plays a significant role in protein folding and quality control, glycoprotein interaction, signal transduction, viral attachment, and immune response to infection. Glycoengineering of protein employed for improving protein properties and plays a vital role in the production of recombinant glycoproteins and struggles to humanize recombinant therapeutic proteins. It considers an alternative platform for biopharmaceuticals production. Many immune proteins and antibodies are glycosylated. Pathogen’s glycoproteins play vital roles during the infection cycle and their expression of specific oligosaccharides via the N-glycosylation pathway to evade detection by the host immune system. This review focuses on the aspects of N-glycosylation processing, glycoengineering approaches, their role in viral attachment, and immune responses to infection.

Mannose Treatment: A Promising Novel Strategy to Suppress Inflammation

High glucose and fructose intake have been proven to display pro-inflammatory roles during the progression of inflammatory diseases. However, mannose has been shown to be a special type of hexose that has immune regulatory functions. In this review, we trace the discovery process of the regulatory functions of mannose and summarize some past and recent studies showing the therapeutic functions of mannose in inflammatory diseases. We conclude that treatment with mannose can suppress inflammation by inducing regulatory T cells, suppressing effector T cells and inflammatory macrophages, and increasing anti-inflammatory gut microbiome. By summarizing all the important findings, we highlight that mannose treatment is a safe and promising novel strategy to suppress inflammatory diseases, including autoimmune disease and allergic disease.

Recent advances in improving tumor-targeted delivery of imaging nanoprobes

Tumor-targeted delivery of imaging nanoprobes provides a promising approach for the precision imaging diagnosis of cancers. Nanoprobes with desired bio-nano interface properties can preferably enter tumor tissues through the vascular endothelium, penetrate into deep tissues, and detect target lesions. Surface engineering of nanoparticles offers a critical strategy to improve tumor-targeting capacities of nanoprobes. Improvements to the efficacy of targeted nanoprobes have been intensively explored and much of this work centers on the selection of suitable targeting ligands. Herein, in this review, various recent strategies based on different targeting ligands to improve tumor-targeting of imaging nanoprobes have been developed, ranging from small molecule ligands to biomimetic coatings, with highlights on emerging coating techniques using cell membranes and dual-targeting ligands. In particular, construction and surface modification methods, targeting capacities, and imaging/theranostic performance with key issues and potential questions have been described and discussed together with considerations for future development and innovations.

Preparation of Boronic Acid-Functionalized Silica Nanocomposites for Selective Enrichment of Glycoproteins

A facile method was developed for synthesis of boronic acid-functionalized silica nanocomposites (SiO₂ -BA) by 'thiol-ene' click reaction, where silica nanoparticles were synthesized by using tetraethoxysilane (TEOS) and γ-mercaptopropyl trimethoxysilane (γ-MPTS) as precursors. The morphology and structure properties of the resultant SiO₂ -BA were characterized by transmission electronic microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and Brunner-Emmet-Teller measurements (BET). The adsorption behavior of the SiO₂ -BA for glycoproteins was evaluated. Under the optimized conditions, the SiO₂ -BA exhibited higher adsorption capacity towards glycoproteins (ovalbumin, OVA, 7.64 μmol/g) than non-glycoproteins (bovine serum albumin, BSA, 0.83 μmol/g). In addition, the practicality of the SiO₂ -BA was further assessed by selective enrichment of glycoproteins from egg white samples.

Unveiling the metabolic fate of monosaccharides in cell membranes with glycomic and glycoproteomic analyses

Cell membrane protein glycosylation is dependent on the metabolic state of the cell as well as exogenous nutrients available. Although the metabolism and interconversion of monosaccharides have been well-studied, their incorporation into cell surface glycans and their corresponding glycoproteins remains relatively unknown. In this study, we developed a method to investigate quantitatively the incorporation pathways of dietary saccharides into specific glycans and glycoproteins on the cell membrane by treating intestinal Caco-2 and hepatic KKU-M213 cells with ¹³C-labeled monosaccharides and characterizing the resulting cell surface glycans and glycopeptides by LC-MS/MS. Time-course studies using uniformly labeled glucose revealed that the rate of incorporation was both glycan-specific and protein-dependent. Comparative studies using different dietary saccharides and multiple cell lines revealed the variance of monosaccharide utilization and interconversion in different tissues and organisms. The robust isotope-labeling and glycan profiling methods can provide a useful tool for differentiating glycosylation pathways and enhance the understanding of how dietary sugar intake affects health.

L-Fucose ameliorates DSS-induced acute colitis via inhibiting macrophage M1 polarization and inhibiting NLRP3 inflammasome and NF-kB activation

Previous studies reported that L-fucose had anti-inflammatory effects in respiratory and cutaneous system. However, the effect of L-fucose on colitis and the underlying mechanism is poorly understood. We studied the anti-inflammatory effects of L-fucose on Dextran sulfate sodium (DSS)-induced acute colitis in vivo and on LPS/ATP-induced bone marrow derived macrophages (BMDMs) damage in vitro. Our results show that L-fucose significantly alleviated weight loss and disease activity index (DAI) scores in colitis and reduced the infiltration of macrophages and neutrophils. In addition, L-fucose can inhibit macrophage M1 polarization, inactivate the NLRP3 inflammasome and reduce the release of TNFα, IL1β, IL6 pro-inflammatory cytokines. In vitro studies showed that L-fucose ameliorated cell damage resulting from the administration of LPS with ATP in BMDMs, inhibited NLRP3 inflammasome activation and reduced the release of corresponding pro-inflammatory cytokines. Finally, L-fucose can inhibit the expression of p-NF-kB in vivo and in vitro. Overall, our results show that L-fucose can attenuate colitis by inhibiting macrophage M1 polarization, inhibiting NLRP3 inflammasome and NF-kB activation, and down-regulation of pro-inflammatory cytokines.

Health effects of kiwi wine on rats: an untargeted metabolic fingerprint study based on GC-MS/TOF

Kiwi wine is a popular fermentation product of kiwi fruit in Asian countries. To better understand the potential health effects of kiwi wine, an untargeted gas chromatography-mass spectrometer (GC-MS) approach was taken to assess the metabolic fingerprint of rats after dietary ingestion of kiwi wine. 7 differentially expressed endogenous metabolites from serum and 8 from urine were enriched in carbohydrate metabolism, amino acid metabolism pathway, fat metabolism and other metabolisms and selected from the KEGG. The above results showed that kiwi wine mainly led to a pronounced perturbation of energy metabolism (especially carbohydrate metabolism) during the consumption period. After stopping the supply of kiwi wine 30 days later, 6 and 3 endogenous metabolites from serum and urine respectively were screened and involved in a small part of carbohydrate related amino acid metabolism and fat metabolism, which indicated that the effect of kiwi wine sustained a lasting effect on energy metabolism, amino acid metabolism and lipid metabolism after stopping the supply. Thus, kiwi wine might have a positive function on health associated with the metabolism of its constituents. To the best of our knowledge, this study provides a nutrition field view for the development of the kiwi wine agricultural industry via an untargeted GC-MS metabolomic approach.

Studies of Arabinose- and Mannose-Related Anionic Species and Comparison to Ribose and Fructose

Gas phase, isolated monosaccharides arabinose- and mannose-related anionic species generated through the matrix-assisted laser desorption ionization (MALDI) method are investigated via negative ion photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The vertical detachment energies (VDEs) of the observed anionic species are experimentally determined: the corresponding structures are assigned based on good agreement between experimental and theoretical VDEs. Arabinose^- parent anion is found to exist as open chain structures in the gas phase, while mannose^- parent anionic species are not observed. Both monosaccharides undergo dissociation through loss of H and loss of H₂O. (saccharide-H)^- anions evidence coexisting positional and conformational isomers. (saccharide-H₂O)^- species have only two positional isomers, each with conformational differences. The present results for arabinose and mannose are further compared to those previously reported for ribose and fructose. This comparison is based on the anions observed and identified through the same PES/DFT techniques for the four saccharides (arabinose, mannose, ribose, and fructose). The issue of natural selection of ribose as the sugar backbone constituent of RNA is thereby explored from the point of view of anionic electronic structure and stability of the four species. Saccharide phosphates are also discussed in the present work with regard to addressing the unique natural selection of ribose for the backbone support of RNA and DNA.

Quantification of d-mannose in plasma: Development and validation of a reliable and accurate HPLC-MS-MS method

The present paper describes the development and the validation process - in compliance with the EMA guidelines - of a method based on tandem mass spectrometry coupled to liquid chromatography for the accurate quantification of mannose in human plasma samples. The quick sample preparation procedure, simplified by the absence of any derivatization step, makes the assay suitable for routine use in a clinical chemistry laboratory. The method validation yielded satisfactory selectivity, with a good separation of mannose from its epimers (glucose and galactose), linearity over the whole concentration range of interest (0.31-40 μg/mL), reproducibility with RSD <10%, and accuracy in the range 96-104%. Instrumental LLOD (0.31 μg/mL) and LLOQ (1.25 μg/mL) were good enough to detect endogenous plasma mannose levels and in agreement with recent data from the literature. Sensitivity was affected by a 5-fold dilution factor, which, if necessary, can be reduced. The method robustness was proven in >600 injections, most of them being of plasma samples, used also to assess the reference ranges in healthy subjects (9.93 ± 3.37 μg/mL) and type 2 diabetic patients (23.47 ± 6.19 μg/mL).

The role of post-translational modifications in hearing and deafness

Post-translational modifications (PTMs) are key molecular events that modify proteins after their synthesis and modulate their ultimate functional properties by affecting their stability, localisation, interaction potential or activity. These chemical changes expand the size of the proteome adding diversity to the molecular pathways governing the biological outcome of cells. PTMs are, thus, crucial in regulating a variety of cellular processes such as apoptosis, proliferation and differentiation and have been shown to be instrumental during embryonic development. In addition, alterations in protein PTMs have been implicated in the pathogenesis of many human diseases, including deafness. In this review, we summarize the recent progress made in understanding the roles of PTMs during cochlear development, with particular emphasis on the enzymes driving protein phosphorylation, acetylation, methylation, glycosylation, ubiquitination and SUMOylation. We also discuss how these enzymes may contribute to hearing impairment and deafness.

Picolinic Acid Promoted Permanganate Oxidation of D-Mannitol in Micellar Medium

Kinetics of permanganate oxidation of D-mannitol have been investigated spectrophotometrically under pseudo-first-order conditions in aqueous acidic media at 30 °C. The spectral analysis of hydrazone derivative of the product indicates the product to be an aldehyde. The observed rate constant value was found to be relatively slow in the uncatalyzed path, which increases by the presence of four isomeric promoters: 2-picolinic acid (2-PA), 4-picolinic acid (4-PA), 2,3-dipicolinic acid (2,3-diPA) and 2,6-dipicolinic acid (2,6-diPA). The catalytic effect of sodium dodecylbenzene sulfonate (SDBS) surfactant on the permanganate oxidation of D-mannitol has been also studied in the presence of the promoters. The critical micelle concentration (CMC) of SDBS alone and in presence of D-mannitol was determined by conductometry and spectrophotometry. The aggregation and morphological changes during reaction were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The variation of the reaction rates for the different promoters in the presence and absence of SDBS micellar catalyst is discussed qualitatively in the terms of partitioning nature of substrate, charge of surfactant and reactants. 2,3-diPA in association with SDBS as micellar catalyst accelerated the reaction velocity compared to the uncatalyzed path.

Mannose metabolism: more than meets the eye

Mannose is a simple sugar with a complex life. It is a welcome therapy for genetic and acquired human diseases, but it kills honeybees and blinds baby mice. It could cause diabetic complications. Mannose chemistry, metabolism, and metabolomics in cells, tissues and mammals can help explain these multiple systemic effects. Mannose has good, bad or ugly outcomes depending on its steady state levels and metabolic flux. This review describes the role of mannose at cellular level and its impact on organisms.

Solving glycosylation disorders: fundamental approaches reveal complicated pathways

Over 100 human genetic disorders result from mutations in glycosylation-related genes. In 2013, a new glycosylation disorder was reported every 17 days. This trend will probably continue given that at least 2% of the human genome encodes glycan-biosynthesis and -recognition proteins. Established biosynthetic pathways provide many candidate genes, but finding unanticipated mutated genes will offer new insights into glycosylation. Simple glycobiomarkers can be used in narrowing the candidates identified by exome and genome sequencing, and those can be validated by glycosylation analysis of serum or cells from affected individuals. Model organisms will expand the understanding of these mutations' impact on glycosylation and pathology. Here, we highlight some recently discovered glycosylation disorders and the barriers, breakthroughs, and surprises they presented. We predict that some glycosylation disorders might occur with greater frequency than current estimates of their prevalence. Moreover, the prevalence of some disorders differs substantially between European and African Americans.

N-glycosylation deficiency reduces ICAM-1 induction and impairs inflammatory response

Congenital disorders of glycosylation (CDGs) result from mutations in various N-glycosylation genes. The most common type, phosphomannomutase-2 (PMM2)-CDG (CDG-Ia), is due to deficient PMM2 (Man-6-P → Man-1-P). Many patients die from recurrent infections, but the mechanism is unknown. We found that glycosylation-deficient patient fibroblasts have less intercellular adhesion molecule-1 (ICAM-1), and because of its role in innate immune response, we hypothesized that its reduction might help explain recurrent infections in CDG patients. We, therefore, studied mice with mutations in Mpi encoding phosphomannose isomerase (Fru-6-P → Man-6-P), the cause of human MPI-CDG. We challenged MPI-deficient mice with an intraperitoneal injection of zymosan to induce an inflammatory response and found decreased neutrophil extravasation compared with control mice. Immunohistochemistry of mesenteries showed attenuated neutrophil egress, presumably due to poor ICAM-1 response to acute peritonitis. Since phosphomannose isomerase (MPI)-CDG patients and their cells improve glycosylation when given mannose, we provided MPI-deficient mice with mannose-supplemented water for 7 days. This restored ICAM-1 expression on mesenteric endothelial cells and enhanced transendothelial migration of neutrophils during acute inflammation. Attenuated inflammatory response in glycosylation-deficient mice may result from a failure to increase ICAM-1 on the vascular endothelial surface and may help explain recurrent infections in patients.

A novel congenital disorder of glycosylation type without central nervous system involvement caused by mutations in the phosphoglucomutase 1 gene

Recent years have seen great advances in our knowledge of congenital disorders of glycosylation (CDG), a clinically and biochemically heterogeneous group of genetic diseases caused by defects in the synthesis (CDG-I) or processing (CDG-II) of glycans that form glycoconjugates. This paper reports a new subtype of non-neurological CDG involving the impaired cytoplasmic biosynthesis of nucleotide sugars needed for glycan biosynthesis. A patient presented with muscle fatigue, elevated creatine kinase, growth hormone deficiency, and first branchial arch syndrome. These findings, together with the abnormal type II plasma transferrin isoform profile detected, was compatible with a CDG. Functional testing and clinical analyses suggested a deficiency in the interconversion of glucose-1-phosphate and glucose-6-phosphate catalyzed by phosphoglucomutase (PGM1), a defect previously described as glycogenosis type XIV (GSDXIV, MIM 612934). PGM1 activity in patient-derived fibroblasts was significantly reduced, as was the quantity of immunoreactive PGM1 protein (Western blot assays). Mutation analysis of PGM1 and subsequent functional analysis investigating transient expression of PGM1 in immortalized patient fibroblasts, followed by ex vivo splicing assays using minigenes, allowed the characterization of two novel pathogenic mutations: c.871G>A (p.Gly291Arg) and c.1144 + 3A>T. The latter represents a severe splicing mutation leading to the out-of-frame skipping of exon 7 and the formation of a truncated protein (p.Arg343fs). MALDI mass spectra of permethylated protein N-glycans from the patient's serum suggested a marked hypoglycosylation defect. The present findings confirm that, in addition to a rare muscular glycolytic defect, PGM1 deficiency causes a non-neurological disorder of glycosylation.

New horizons for congenital myasthenic syndromes

During the past five years an increasing number of patients have been diagnosed with congenital myasthenic syndromes (CMS) and a number of novel syndromes have been recognized and investigated. This presentation focuses on the CMS caused by defects in choline acetyltransferase, novel fast-channel syndromes that hinder isomerization of the acetylcholine receptor from the closed to the open state, the consequences of deleterious mutations in the intermediate filament linker plectin, altered neuromuscular transmission in a centronuclear myopathy, and two recently identified CMS caused by congenital defects in glycosylation.

Understanding human glycosylation disorders: biochemistry leads the charge

Nearly 70 inherited human glycosylation disorders span a breathtaking clinical spectrum, impacting nearly every organ system and launching a family-driven diagnostic odyssey. Advances in genetics, especially next generation sequencing, propelled discovery of many glycosylation disorders in single and multiple pathways. Interpretation of whole exome sequencing results, insights into pathological mechanisms, and possible therapies will hinge on biochemical analysis of patient-derived materials and animal models. Biochemical diagnostic markers and readouts offer a physiological context to confirm candidate genes. Recent discoveries suggest novel perspectives for textbook biochemistry and novel research opportunities. Basic science and patients are the immediate beneficiaries of this bidirectional collaboration.

A compendium of inborn errors of metabolism mapped onto the human metabolic network

Inborn errors of metabolism (IEMs) are hereditary metabolic defects, which are encountered in almost all major metabolic pathways occurring in man. Many IEMs are screened for in neonates through metabolomic analysis of dried blood spot samples. To enable the mapping of these metabolomic data onto the published human metabolic reconstruction, we added missing reactions and pathways involved in acylcarnitine (AC) and fatty acid oxidation (FAO) metabolism. Using literary data, we reconstructed an AC/FAO module consisting of 352 reactions and 139 metabolites. When this module was combined with the human metabolic reconstruction, the synthesis of 39 acylcarnitines and 22 amino acids, which are routinely measured, was captured and 235 distinct IEMs could be mapped. We collected phenotypic and clinical features for each IEM enabling comprehensive classification. We found that carbohydrate, amino acid, and lipid metabolism were most affected by the IEMs, while the brain was the most commonly affected organ. Furthermore, we analyzed the IEMs in the context of metabolic network topology to gain insight into common features between metabolically connected IEMs. While many known examples were identified, we discovered some surprising IEM pairs that shared reactions as well as clinical features but not necessarily causal genes. Moreover, we could also re-confirm that acetyl-CoA acts as a central metabolite. This network based analysis leads to further insight of hot spots in human metabolism with respect to IEMs. The presented comprehensive knowledge base of IEMs will provide a valuable tool in studying metabolic changes involved in inherited metabolic diseases.

The Drosophila neurally altered carbohydrate mutant has a defective Golgi GDP-fucose transporter

Studying genetic disorders in model organisms can provide insights into heritable human diseases. The Drosophila neurally altered carbohydrate (nac) mutant is deficient for neural expression of the HRP epitope, which consists of N-glycans with core α1,3-linked fucose residues. Here, we show that a conserved serine residue in the Golgi GDP-fucose transporter (GFR) is substituted by leucine in nac(1) flies, which abolishes GDP-fucose transport in vivo and in vitro. This loss of function is due to a biochemical defect, not to destabilization or mistargeting of the mutant GFR protein. Mass spectrometry and HPLC analysis showed that nac(1) mutants lack not only core α1,3-linked, but also core α1,6-linked fucose residues on their N-glycans. Thus, the nac(1) Gfr mutation produces a previously unrecognized general defect in N-glycan core fucosylation. Transgenic expression of a wild-type Gfr gene restored the HRP epitope in neural tissues, directly demonstrating that the Gfr mutation is solely responsible for the neural HRP epitope deficiency in the nac(1) mutant. These results validate the Drosophila nac(1) mutant as a model for the human congenital disorder of glycosylation, CDG-IIc (also known as LAD-II), which is also the result of a GFR deficiency.

Posttranslational modifications in connexins and pannexins

Posttranslational modification is a common cellular process that is used by cells to ensure a particular protein function. This can happen in a variety of ways, e.g., from the addition of phosphates or sugar residues to a particular amino acid, ensuring proper protein life cycle and function. In this review, we assess the evidence for ubiquitination, glycosylation, phosphorylation, S-nitrosylation as well as other modifications in connexins and pannexin proteins. Based on the literature, we find that posttranslational modifications are an important component of connexin and pannexin regulation.

Identification of intercellular cell adhesion molecule 1 (ICAM-1) as a hypoglycosylation marker in congenital disorders of glycosylation cells

Many human inherited disorders cause protein N-glycosylation defects, but there are few cellular markers to test gene complementation for such defects. Plasma membrane glycoproteins are potential biomarkers because they may be reduced or even absent in plasma membranes of glycosylation-deficient cells. We combined stable isotope labeling by amino acids in cell culture (SILAC) with linear ion trap mass spectrometry (LTQ Orbitrap(TM)) to identify and quantify membrane proteins from wild-type CHO and glycosylation-deficient CHO (Lec9) cells. We identified 165 underrepresented proteins from 1447 unique quantified proteins, including 18 N-glycosylated plasma membrane proteins. Using various methods, we found that intercellular cell adhesion molecule 1 (ICAM-1) was reduced in Lec9 cells and in fibroblasts from 31 congenital disorder of glycosylation (CDG) patients compared with normal controls. Mannose supplementation of phosphomannose isomerase-deficient CDG-Ib (MPI-CDG) cells and complementation with PMM2 in PMM2-deficient CDG-Ia (PMM2-CDG) cells partially corrected hypoglycosylation based on increased ICAM-1 presence on the plasma membrane. These data indicate that ICAM-1 could be a useful hypoglycosylation biomarker to assess gene complementation of CDG-I patient cells and to monitor improved glycosylation in response to therapeutic drugs.

Phosphomannose isomerase inhibitors improve N-glycosylation in selected phosphomannomutase-deficient fibroblasts

Congenital disorders of glycosylation (CDG) are rare genetic disorders due to impaired glycosylation. The patients with subtypes CDG-Ia and CDG-Ib have mutations in the genes encoding phosphomannomutase 2 (PMM2) and phosphomannose isomerase (MPI or PMI), respectively. PMM2 (mannose 6-phosphate → mannose 1-phosphate) and MPI (mannose 6-phosphate ⇔ fructose 6-phosphate) deficiencies reduce the metabolic flux of mannose 6-phosphate (Man-6-P) into glycosylation, resulting in unoccupied N-glycosylation sites. Both PMM2 and MPI compete for the same substrate, Man-6-P. Daily mannose doses reverse most of the symptoms of MPI-deficient CDG-Ib patients. However, CDG-Ia patients do not benefit from mannose supplementation because >95% Man-6-P is catabolized by MPI. We hypothesized that inhibiting MPI enzymatic activity would provide more Man-6-P for glycosylation and possibly benefit CDG-Ia patients with residual PMM2 activity. Here we show that MLS0315771, a potent MPI inhibitor from the benzoisothiazolone series, diverts Man-6-P toward glycosylation in various cell lines including fibroblasts from CDG-Ia patients and improves N-glycosylation. Finally, we show that MLS0315771 increases mannose metabolic flux toward glycosylation in zebrafish embryos.

Mouse models for congenital disorders of glycosylation

Glycoprotein biosynthesis describes the process of co- and posttranslational attachment of sugar chains to proteins, a process that has been found in nearly all known organisms. Human deficiencies evoked by mutations in the glycosylation pathway of glycoproteins lead to congenital disorders of glycosylation (CDG), a rapidly expanding group of autosomal recessive inherited metabolic diseases with multisystemic phenotypes that are mostly combined with severe neurological impairment. Although investigations on new types of CDG have proceeded rapidly in recent years, the correlation between inaccurate protein glycosylation and pathological loss of functionality of distinct organ systems remains widely unknown, and therapeutics for the patients are mostly not available. Therefore, mouse models provide an outstanding helpful tool for investigations on different aspects of glycosylation deficiencies that cannot be performed in patients or cell culture. This review focuses on existing mouse models generated for the types of CDG that affect the N-glycosylation pathway.

Potent, selective, and orally available benzoisothiazolone phosphomannose isomerase inhibitors as probes for congenital disorder of glycosylation Ia

We report the discovery and validation of a series of benzoisothiazolones as potent inhibitors of phosphomannose isomerase (PMI), an enzyme that converts mannose-6-phosphate (Man-6-P) into fructose-6-phosphate (Fru-6-P) and, more importantly, competes with phosphomannomutase 2 (PMM2) for Man-6-P, diverting this substrate from critical protein glycosylation events. In congenital disorder of glycosylation type Ia, PMM2 activity is compromised; thus, PMI inhibition is a potential strategy for the development of therapeutics. High-throughput screening (HTS) and subsequent chemical optimization led to the identification of a novel class of benzoisothiazolones as potent PMI inhibitors having little or no PMM2 inhibition. Two complementary synthetic routes were developed, enabling the critical structural requirements for activity to be determined, and the compounds were subsequently profiled in biochemical and cellular assays to assess efficacy. The most promising compounds were also profiled for bioavailability parameters, including metabolic stability, plasma stability, and permeability. The pharmacokinetic profile of a representative of this series (compound 19; ML089) was also assessed, demonstrating the potential of this series for in vivo efficacy when dosed orally in disease models.