Sugar chains of serum transferrin from patients with carbohydrate deficient glycoprotein syndrome. Evidence of asparagine-N-linked oligosaccharide transfer deficiency

Chemical Synthesis of an Erythropoietin Glycoform Having a Triantennary N-Glycan: Significant Change of Biological Activity of Glycoprotein by Addition of a Small Molecular Weight Trisaccharide

The glycosylation of proteins contributes to the modulation of the structure and biological activity of glycoproteins. Asparagine-linked glycans (N-glycans) of glycoproteins naturally exhibit diverse antennary patterns, such as bi-, tri-, and tetra-antennary forms. However, there are no chemical or biological methods to obtain homogeneous glycoproteins via the intentional alteration of the antennary form of N-glycans. Thus, the functions of the individual antennary form of N-glycan at a molecular level remain unclear. Herein, we report the chemical synthesis of an erythropoietin (EPO) glycoform having a triantennary sialylglycan at position 83, as well as two biantennary sialylglycans at both positions 24 and 38. We demonstrated efficient liquid-phase condensation reactions to prepare a sialylglycopeptide having a triantennary N-glycan prepared by the addition of a Neu5Ac-α-2,6-Gal-β-1,4-GlcNAc element to the biantennary glycan under semisynthetic conditions. The molecular weight of the newly added antennary element was ∼3% of the EPO glycoform, and the introduced position was the most distant from the bioactive protein. However, in vivo assays using mice revealed that the additional antennary element at position 83 dramatically increased the hematopoietic activity compared to a commercially available native EPO. These unprecedented data clearly indicate that the antennary pattern of N-glycans inherently plays a critical role in the modulation of protein functions.

Glycosylation in health and disease

The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of 'glycomedicine'.

Identification of aberrantly expressed glycans in gastric cancer by integrated lectin microarray and mass spectrometric analyses

Cancer progression is usually associated with alterations of glycan expression patterns. Little is known regarding global glycomics in gastric cancer, the most common type of epithelial cancer. We integrated lectin microarray and mass spectrometry (MS) methods to profile glycan expression in three gastric cancer cell lines (SGC-7901, HGC-27, and MGC-803) and one normal gastric epithelial cell line (GES-1). Significantly altered glycans were confirmed by lectin staining and MALDI-TOF/TOF-MS. The three cancer cell lines showed increased levels of core-fucosylated N-glycans, GalNAcα-Ser/Thr (Tn antigen), and Sia2-6Galβ1-4GlcNAc N-glycans, but reduced levels of biantennary N-glycans, Galβ1-3GalNAcα-Ser/Thr (T antigen), and (GlcNAc)_n N-glycans. Lectin histochemistry was used to validate aberrant expression of four representative glycans (core-fucosylation, Sia2-6Galβ1-4GlcNAc, biantennary N-glycans, T antigen, recognized respectively by lectins LCA, SNA, PHA-E+L, and ACA) in clinical gastric cancer samples. Lower binding capacity for ACA was correlated with significantly poorer patient prognosis. Our findings indicate for the first time that glycans recognized by LCA, ACA, and PHA-E+L are aberrantly expressed in gastric cancer, and suggest that ACA is a potential prognostic factor for gastric cancer.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Biomolecules and Biomarkers Used in Diagnosis of Alcohol Drinking and in Monitoring Therapeutic Interventions

BACKGROUND

The quantitative, measurable detection of drinking is important for the successful treatment of alcohol misuse in transplantation of patients with alcohol disorders, people living with human immunodeficiency virus that need to adhere to medication, and special occupational hazard offenders, many of whom continually deny drinking. Their initial misconduct usually leads to medical problems associated with drinking, impulsive social behavior, and drunk driving. The accurate identification of alcohol consumption via biochemical tests contributes significantly to the monitoring of drinking behavior.

METHODS

A systematic review of the current methods used to measure biomarkers of alcohol consumption was conducted using PubMed and Google Scholar databases (2010-2015). The names of the tests have been identified. The methods and publications that correlate between the social instruments and the biochemical tests were further investigated. There is a clear need for assays standardization to ensure the use of these biochemical tests as routine biomarkers.

FINDINGS

Alcohol ingestion can be measured using a breath test. Because alcohol is rapidly eliminated from the circulation, the time for detection by this analysis is in the range of hours. Alcohol consumption can alternatively be detected by direct measurement of ethanol concentration in blood or urine. Several markers have been proposed to extend the interval and sensitivities of detection, including ethyl glucuronide and ethyl sulfate in urine, phosphatidylethanol in blood, and ethyl glucuronide and fatty acid ethyl esters in hair, among others. Moreover, there is a need to correlate the indirect biomarker carbohydrate deficient transferrin, which reflects longer lasting consumption of higher amounts of alcohol, with serum γ-glutamyl transpeptidase, another long term indirect biomarker that is routinely used and standardized in laboratory medicine.

Confident assignment of site-specific glycosylation in complex glycoproteins in a single step

A glycoprotein may contain several sites of glycosylation, each of which is heterogeneous. As a consequence of glycoform diversity and signal suppression from nonglycosylated peptides that ionize more efficiently, typical reversed-phase LC-MS and bottom-up proteomics database searching workflows do not perform well for identification of site-specific glycosylation for complex glycoproteins. We present an LC-MS system for enrichment, separation, and analysis of glycopeptides from complex glycoproteins (>4 N-glycosylation sequons) in a single step. This system uses an online HILIC enrichment trap prior to reversed-phase C18-MS analysis. We demonstrated the effectiveness of the system using a set of glycoproteins including human transferrin (2 sequons), human alpha-1-acid glycoprotein (5 sequons), and influenza A virus hemagglutinin (9 sequons). The online enrichment renders glycopeptides the most abundant ions detected, thereby facilitating the generation of high-quality data-dependent tandem mass spectra. The tandem mass spectra exhibited product ions from both glycan and peptide backbone dissociation for a majority of the glycopeptides tested using collisionally activated dissociation that served to confidently assign site-specific glycosylation. We demonstrated the value of our system to define site-specific glycosylation using a hemagglutinin containing 9 N-glycosylation sequons from a single HILIC-C18-MS acquisition.

Mgat2 ablation in the myeloid lineage leads to defective glycoantigen T cell responses

N-linked glycosylation is a central regulatory factor that influences the immune system in varied and profound ways, including leukocyte homing, T cell receptor signaling and others. Moreover, N-glycan branching has been demonstrated to change as a function of infection and inflammation. Our previous findings suggest that complex-type N-glycans on the class II major histocompatibility complex play an important role in antigen selection within antigen presenting cells (APCs) such that highly branched N-glycans promote polysaccharide (glycoantigen, GlyAg) presentation following Toll-like receptor 2 (TLR2)-dependent antigen processing. In order to explore the impact of N-glycan branching on the myeloid-derived APC population without the confounding problems of altering the branching of lymphocytes and non-hematopoietic cells, we created a novel myeloid-specific knockout of the β-1,2-N-acetylglucosaminyltransferase II (Mgat2) enzyme. Using this novel mouse, we found that the reduction in multi-antennary N-glycans characteristic of Mgat2 ablation had no impact on GlyAg-mediated TLR2 signaling. Likewise, no deficits in antigen uptake or cellular homing to lymph nodes were found. However, we discovered that Mgat2 ablation prevented GlyAg presentation and T cell activation in vitro and in vivo without apparent alterations in protein antigen response or myeloid-mediated protection from infection. These findings demonstrate that GlyAg presentation can be regulated by the N-glycan branching pattern of APCs, thereby establishing an in vivo model where the T cell-dependent activity of GlyAgs can be experimentally distinguished from GlyAg-mediated stimulation of the innate response through TLR2.

N-glycosylation engineering of plants for the biosynthesis of glycoproteins with bisected and branched complex N-glycans

Glycoengineering is increasingly being recognized as a powerful tool to generate recombinant glycoproteins with a customized N-glycosylation pattern. Here, we demonstrate the modulation of the plant glycosylation pathway toward the formation of human-type bisected and branched complex N-glycans. Glycoengineered Nicotiana benthamiana lacking plant-specific N-glycosylation (i.e. β1,2-xylose and core α1,3-fucose) was used to transiently express human erythropoietin (hEPO) and human transferrin (hTF) together with modified versions of human β1,4-mannosyl-β1,4-N-acetylglucosaminyltransferase (GnTIII), α1,3-mannosyl-β1,4-N-acetylglucosaminyltransferase (GnTIV) and α1,6-mannosyl-β1,6-N-acetylglucosaminyltransferase (GnTV). hEPO was expressed as a fusion to the IgG-Fc domain (EPO-Fc) and purified via protein A affinity chromatography. Recombinant hTF was isolated from the intracellular fluid of infiltrated plant leaves. Mass spectrometry-based N-glycan analysis of hEPO and hTF revealed the quantitative formation of bisected (GnGnbi) and tri- as well as tetraantennary complex N-glycans (Gn[GnGn], [GnGn]Gn and [GnGn][GnGn]). Co-expression of GnTIII together with GnTIV and GnTV resulted in the efficient generation of bisected tetraantennary complex N-glycans. Our results show the generation of recombinant proteins with human-type N-glycosylation at great uniformity. The strategy described here provides a robust and straightforward method for producing mammalian-type N-linked glycans of defined structures on recombinant glycoproteins, which can advance glycoprotein research and accelerate the development of protein-based therapeutics.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

Mass spectrometry in the characterization of human genetic N-glycosylation defects

Human genetic diseases that affect N-glycosylation result from the defective synthesis of the N-linked sugar moiety (glycan) of glycoproteins. The role of glycans for proper protein folding and biological functions is illustrated in the variety and severity of clinical manifestations shared by congenital disorders of glycosylation (CDG). This family of inherited metabolic disorders includes defects in the assembly of the oligosaccharide precursor that lead to an under-occupancy of N-glycosylation sites (CDG-I), and defects of glycan remodeling (CDG-II). Mass spectrometry constitutes a key tool for characterization of CDG-I defects by mass resolution of native protein glycoforms that differ for glycosylation-site occupancy. Glycan MS analyses in CDG-II is mandatory to detect whenever possible a repertoire of structures to pinpoint candidate enzymes and genes responsible for the abnormal N-glycan synthesis. In this manuscript, we review the MS applications in the area of CDG and related disorders with a special emphasis on those techniques that have been already applied or might become functional for diagnosis, characterization, and treatment monitoring in some specific conditions.

Sialic acids in human health and disease

The surfaces of all vertebrate cells are decorated with a dense and complex array of sugar chains, which are mostly attached to proteins and lipids. Most soluble secreted proteins are also similarly decorated with such glycans. Sialic acids are a diverse family of sugar units with a nine-carbon backbone that are typically found attached to the outermost ends of these chains. Given their location and ubiquitous distribution, sialic acids can mediate or modulate a wide variety of physiological and pathological processes. This review considers some examples of their established and newly emerging roles in aspects of human physiology and disease.

Alteration of N-glycans related to articular cartilage deterioration after anterior cruciate ligament transection in rabbits

OBJECTIVE

Osteoarthritis (OA) is the most common of all joint diseases, but the molecular basis of its onset and progression is controversial. Several studies have shown that modifications of N-glycans contribute to pathogenesis. However, little attention has been paid to N-glycan modifications seen in articular cartilage. The goal of this study was to identify disease specific N-glycan expression profiles in degenerated cartilage in a rabbit OA model induced by anterior cruciate ligament transection (ACLT).

METHODS

Cartilage samples were harvested at 7, 10, 14, and 28 days after ACLT and assessed for cartilage degeneration and alteration in N-glycans. N-Glycans from cartilage were analyzed by high performance liquid chromatography and mass spectrometry.

RESULTS

Histological analysis showed that osteoarthritic changes in cartilage occurred 10 days after ACLT. Apparent alterations in the N-glycan peak pattern in cartilage samples were observed 7 days after ACLT, and overall N-glycan changes in OA reflected alterations in both sialylation and fucosylation. These changes apparently preceded histological changes in cartilage.

CONCLUSION

These results indicate that changes in the expression of N-glycans are correlated with OA in an animal model. Understanding mechanisms underlying changes in N-glycans seen in OA may be of therapeutic value in treating cartilage deterioration.

A rapid mass spectrometric strategy for the characterization of N- and O-glycan chains in the diagnosis of defects in glycan biosynthesis

Glycosylation of proteins is a very complex process which involves numerous factors such as enzymes or transporters. A defect in one of these factors in glycan biosynthetic pathways leads to dramatic disorders named congenital disorders of glycosylation (CDG). CDG can affect the biosynthesis of not only protein N-glycans but also O-glycans. The structural analysis of glycans on serum glycoproteins is essential to solving the defect. For this reason, we propose in this paper a strategy for the simultaneous characterization of both N- and O-glycan chains isolated from the serum glycoproteins. The serum (20 microL) is used for the characterization of N-glycans which are released by enzymatic digestion with PNGase F. O-glycans are chemically released by reductive elimination from whole serum glycoproteins using 10 microL of the serum. Using strategies based on mass spectrometric analysis, the structures of N- and O-glycan chains are defined. These strategies were applied on the sera from one patient with CDG type IIa, and one patient with a mild form of congenital disorder of glycosylation type II (CDG-II) that is caused by a deficiency in the Cog1 subunit of the complex.

Quantitative glycomics of human whole serum glycoproteins based on the standardized protocol for liberating N-glycans

Global glycomics of human whole serum glycoproteins appears to be an innovative and comprehensive approach to identify surrogate non-invasive biomarkers for various diseases. Despite the fact that quantitative glycomics is premised on highly efficient and reproducible oligosaccharide liberation from human serum glycoproteins, it should be noted that there is no validated protocol for which deglycosylation efficiency is proven to be quantitative. To establish a standard procedure to evaluate N-glycan release from whole human serum glycoproteins by peptide-N-glycosidase F (PNGase F) treatment, we determined the efficiencies of major N-glycan liberation from serum glycoproteins in the presence of reducing agents, surfactants, protease treatment, or combinations of pretreatments prior to PNGase F digestion. We show that de-N-glycosylation efficiency differed significantly depending on the condition used, indicative of the importance of a standardized protocol for the accumulation and comparison of glycomics data. Maximal de-N-glycosylation was achieved when serum was subjected to reductive alkylation in the presence of 2-hydroxyl-3-sulfopropyl dodecanoate, a surfactant used for solubilizing proteins, or related analogues, followed by tryptic digestion prior to PNGase F treatment. An optimized de-N-glycosylation protocol permitted relative and absolute quantitation of up to 34 major N-glycans present in serum glycoproteins of normal subjects for the first time. Moreover PNGase F-catalyzed de-N-glycosylation of whole serum glycoproteins was characterized kinetically, allowing accurate simulation of PNGase F-catalyzed de-N-glycosylation required for clinical glycomics using human serum samples. The results of the current study may provide a firm basis to identify new diagnostic markers based on serum glycomics analysis.

Construction of recombinant hemagglutinin derived from the gingipain-encoding gene of Porphyromonas gingivalis, identification of its target protein on erythrocytes, and inhibition of hemagglutination by an interdomain regional peptide

Porphyromonas gingivalis, an anaerobic gram-negative bacterium associated with chronic periodontitis, can agglutinate human erythrocytes. In general, hemagglutination can be considered the ability to adhere to host cells; however, P. gingivalis-mediated hemagglutination has special significance because heme markedly accelerates growth of this bacterium. Although a number of studies have indicated that a major hemagglutinin of P. gingivalis is intragenically encoded by rgpA, kgp, and hagA, direct evidence has not been obtained. We demonstrated in this study that recombinant HGP44(720-1081), a fully processed HGP44 domain protein, had hemagglutinating activity but that an unprocessed form, HGP44(720-1138), did not. A peptide corresponding to residues 1083 to 1102, which was included in HGP44(720-1138) but not in HGP44(720-1081), could bind HGP44(720-1081) in a dose-dependent manner and effectively inhibited HGP44(720-1081)-mediated hemagglutination, indicating that the interdomain regional amino acid sequence may function as an intramolecular suppressor of hemagglutinating activity. Analyses by solid-phase binding and chemical cross-linking suggested that HGP44 interacted with glycophorin A on the erythrocyte membrane. Glycophorin A and, more effectively, asialoglycophorin, which were added exogenously, inhibited HGP44(720-1081)-mediated hemagglutination. Treatment of erythrocytes with RgpB proteinase resulted in degradation of glycophorin A on the membrane and a decrease in HGP44(720-1081)-mediated hemagglutination. Surface plasmon resonance detection analysis revealed that HGP44(720-1081) could bind to asialoglycophorin with a dissociation constant of 3.0 x 10(-7) M. These results indicate that the target of HGP44 on the erythrocyte membrane appears to be glycophorin A.

Congenital disorders of N-glycosylation including diseases associated with O- as well as N-glycosylation defects

The congenital disorders of N-glycosylation (CDG), a steadily increasing group of multi-systemic disorders, have severe clinical implications in infancy and early childhood. The various inborn errors responsible adversely affect N-glycosylation of lysosomal proteins because of either failing assembly of lipid-linked (LL) oligosaccharides (OS) in the endoplasmic reticulum, CDG Type I, or faulty processing of the asparagines (N)-linked OS in the ER and in the Golgi, CDG Type II. The overlap of phenotypes precludes specific clinical delineation. Isoelectric focusing (IEF) of plasma transferrin remains a valuable, albeit imperfect, screening tool. IEF of plasma ApoC-III protein, introduced O-glycosylation defects that delineated some new CDGs due to mutations of both N- and O-glycosylation. Only CDG-Ib is amenable to treatment with free mannose supplementation. Hence, early specific diagnosis of any one entity is crucial for genetic counseling and elective preventive measures.

Binding properties of Clostridium botulinum type C progenitor toxin to mucins

It has been reported that Clostridium botulinum type C 16S progenitor toxin (C16S toxin) first binds to the sialic acid on the cell surface of mucin before invading cells [A. Nishikawa, N. Uotsu, H. Arimitsu, J.C. Lee, Y. Miura, Y. Fujinaga, H. Nakada, T. Watanabe, T. Ohyama, Y. Sakano, K. Oguma, The receptor and transporter for internalization of Clostridium botulinum type C progenitor toxin into HT-29 cells, Biochem. Biophys. Res. Commun. 319 (2004) 327-333]. In this study we investigated the binding properties of the C16S toxin to glycoproteins. Although the toxin bound to membrane blotted mucin derived from the bovine submaxillary gland (BSM), which contains a lot of sialyl oligosaccharides, it did not bind to neuraminidase-treated BSM. The binding of the toxin to BSM was inhibited by N-acetylneuraminic acid, N-glycolylneuraminic acid, and sialyl oligosaccharides strongly, but was not inhibited by neutral oligosaccharides. Both sialyl alpha2-3 lactose and sialyl alpha2-6 lactose prevented binding similarly. On the other hand, the toxin also bound well to porcine gastric mucin. In this case, neutral oligosaccharides might play an important role as ligand, since galactose and lactose inhibited binding. These results suggest that the toxin is capable of recognizing a wide variety of oligosaccharide structures.

From glycomics to functional glycomics of sugar chains: Identification of target proteins with functional changes using gene targeting mice and knock down cells of FUT8 as examples

Comprehensive analyses of proteins from cells and tissues are the most effective means of elucidating the expression patterns of individual disease-related proteins. On the other hand, the simultaneous separation and characterization of proteins by 1-DE or 2-DE followed by MS analysis are one of the fundamental approaches to proteomic analysis. However, these analyses do not permit the complete structural identification of glycans in glycoproteins or their structural characterization. Over half of all known proteins are glycosylated and glycan analyses of glycoproteins are requisite for fundamental proteomics studies. The analysis of glycan structural alterations in glycoproteins is becoming increasingly important in terms of biomarkers, quality control of glycoprotein drugs, and the development of new drugs. However, usual approach such as proteoglycomics, glycoproteomics and glycomics which characterizes and/or identifies sugar chains, provides some structural information, but it does not provide any information of functionality of sugar chains. Therefore, in order to elucidate the function of glycans, functional glycomics which identifies the target glycoproteins and characterizes functional roles of sugar chains represents a promising approach. In this review, we show examples of functional glycomics technique using alpha 1,6 fucosyltransferase gene (Fut8) in order to identify the target glycoprotein(s). This approach is based on glycan profiling by CE/MS and LC/MS followed by proteomic approaches, including 2-DE/1-DE and lectin blot techniques and identification of functional changes of sugar chains.

The normal phenotype of Pmm1-deficient mice suggests that Pmm1 is not essential for normal mouse development

Phosphomannomutases (PMMs) are crucial for the glycosylation of glycoproteins. In humans, two highly conserved PMMs exist: PMM1 and PMM2. In vitro both enzymes are able to convert mannose-6-phosphate (mannose-6-P) into mannose-1-P, the key starting compound for glycan biosynthesis. However, only mutations causing a deficiency in PMM2 cause hypoglycosylation, leading to the most frequent type of the congenital disorders of glycosylation (CDG): CDG-Ia. PMM1 is as yet not associated with any disease, and its physiological role has remained unclear. We generated a mouse deficient in Pmm1 activity and documented the expression pattern of murine Pmm1 to unravel its biological role. The expression pattern suggested an involvement of Pmm1 in (neural) development and endocrine regulation. Surprisingly, Pmm1 knockout mice were viable, developed normally, and did not reveal any obvious phenotypic alteration up to adulthood. The macroscopic and microscopic anatomy of all major organs, as well as animal behavior, appeared to be normal. Likewise, lectin histochemistry did not demonstrate an altered glycosylation pattern in tissues. It is especially striking that Pmm1, despite an almost complete overlap of its expression with Pmm2, e.g., in the developing brain, is apparently unable to compensate for deficient Pmm2 activity in CDG-Ia patients. Together, these data point to a (developmental) function independent of mannose-1-P synthesis, whereby the normal knockout phenotype, despite the stringent conservation in phylogeny, could be explained by a critical function under as-yet-unidentified challenge conditions.

Conserved oligomeric Golgi complex subunit 1 deficiency reveals a previously uncharacterized congenital disorder of glycosylation type II

The conserved oligomeric Golgi (COG) complex is a heterooctameric complex that regulates intraGolgi trafficking and the integrity of the Golgi compartment in eukaryotic cells. Here, we describe a patient with a mild form of congenital disorder of glycosylation type II (CDG-II) that is caused by a deficiency in the Cog1 subunit of the complex. This patient has a defect in both N- and O-glycosylation. Mass spectrometric analysis of the structures of the N-linked glycans released from glycoproteins from the patient's serum revealed a reduction in sialic acid and galactose residues. Peanut agglutinin (PNA) lectin staining revealed a decrease in sialic acids on core 1 mucin type O-glycans, indicating a combined defect in N- and O-glycosylation. Sequence analysis of the COG1 cDNA and gene identified a homozygous insertion of a single nucleotide (2659-2660insC), which is predicted to lead to a premature translation stop and truncation of the C terminus of the Cog1 protein by 80 amino acids. This mutation destabilizes several other COG subunits and alters their subcellular localization and hence the overall integrity of the COG complex. This results in reduced levels and/or altered Golgi localization of alpha-mannosidase II and beta-1,4 galactosyltransferase I, which links it to the glycosylation deficiency. Transfection of primary fibroblasts of this patient with the full length hemagglutinin-tagged Cog1 indeed restored beta-1,4 galactosyltransferase Golgi localization. We propose naming this disorder CDG-II/Cog1, or CDG-II caused by Cog1 deficiency.

Binding affinity of aluminium to human serum transferrin and effects of carbohydrate chain modification as studied by HPLC/high-resolution ICP-MS--speciation of aluminium in human serum

Aluminium (Al) in the blood is bound to transferrin (Tf), a glycoprotein of about 80kDa that is characterized by its need for a synergistic anion. In this focused review, the binding affinity of Al to Tf is surveyed in the context of our recent studies using on-line high-performance liquid chromatography/high-resolution inductively coupled plasma mass spectrometry (HPLC/HR-ICP-MS). Al in human serum without any in vitro Al-spikes was present in a form bound to the N-lobe site of Tf. The influences of sialic acid in the carbohydrate chain of human serum Tf (hTf) were studied using asialo-hTf, obtained by treatment with sialidase. The binding affinity of Fe was similar between asialo-hTf and native-hTf, while that of Al for asialo-hTf was larger than that for native-hTf, especially in the presence of oxalate, a synergistic anion. The above findings are discussed in relation to diseases in which the serum concentrations of carbohydrate-deficient Tf and oxalate are augmented.

Glycoproteomics ofN-glycosylation by in-gel deglycosylation and matrix-assisted laser desorption/ionisation-time of flight mass spectrometry mapping: Application to congenital disorders of glycosylation

A general strategy for the structural evaluation of N-glycosylation, a common post-translational protein modification, is presented. The methods for the release of N-linked glycans from the gel-separated proteins, their isolation, purification and matrix-assisted laser desorption/ionisation-mass spectrometry (MALDI-MS) analysis of their mixtures were optimised. Since many glycoproteins are available only at low quantities from sodium dodecyl sulphate-polyacrylamide gel electrophoresis or two-dimensional gels, high attention was paid to obtain N-glycan mixtures representing their actual composition in human plasma by in-gel deglycosylation. The relative sensitivity of solid MALDI matrices for MS analysis of acidic N-glycans was compared. The most favourable results for native acidic N-glycans were obtained with 2,4,6-trihydroxyacetophenone monohydrate/diammoniumcitrate as a matrix. This matrix provided good results for both neutral and acidic mixtures as well as for methylated N-glycans. In the second part of this paper the potential of such an optimised MS strategy alone or in combination with high pH anion-exchange chromatography profiling for the clinical diagnosis of congenital disorders of glycosylation is presented.

Should we use carbohydrate deficient transferrin as a marker for alcohol abusers?

Carbohydrate deficient transferrin (CDT) is one of the conventional markers for chronic alcohol consumption, is used by researchers and clinicians. A number of enzymes are affected by ethanol intake. The induction or inhibition of sialyl transferase and plasma sialidase may be involved in the CDT level elevation. An alteration of protein transport during post-translational modification could be a primary mechanism in the impairment of protein metabolism associated with chronic alcohol abuse. Transferrin being a steroid responsive protein, sex-based hormonal variations might contribute to the lower sensitivity of CDT. Varying hormonal statuses such as pregnancy, use of contraceptives, menopause/ menstrual cycle can alter iron homeostasis in women. CDT levels are markedly affected by iron homeostasis. Several CDT assay methods appeared promising, but it is not readily apparent which technique is the most accurate. Moreover, false-positive results of CDT have been reported in non-alcohol related hepatic failure and in rare conditions. Therefore clinical interpretation of CDT needs careful assessment in patients with alcohol-related or non-alcohol-related health disorders.

A genetic approach to Mammalian glycan function

The four essential building blocks of cells are proteins, nucleic acids, lipids, and glycans. Also referred to as carbohydrates, glycans are composed of saccharides that are typically linked to lipids and proteins in the secretory pathway. Glycans are highly abundant and diverse biopolymers, yet their functions have remained relatively obscure. This is changing with the advent of genetic reagents and techniques that in the past decade have uncovered many essential roles of specific glycan linkages in living organisms. Glycans appear to modulate biological processes in the development and function of multiple physiologic systems, in part by regulating protein-protein and cell-cell interactions. Moreover, dysregulation of glycan synthesis represents the etiology for a growing number of human genetic diseases. The study of glycans, known as glycobiology, has entered an era of renaissance that coincides with the acquisition of complete genome sequences for multiple organisms and an increased focus upon how posttranslational modifications to protein contribute to the complexity of events mediating normal and disease physiology. Glycan production and modification comprise an estimated 1% of genes in the mammalian genome. Many of these genes encode enzymes termed glycosyltransferases and glycosidases that reside in the Golgi apparatus where they play the major role in constructing the glycan repertoire that is found at the cell surface and among extracellular compartments. We present a review of the recently established functions of glycan structures in the context of mammalian genetic studies focused upon the mouse and human species. Nothing tends so much to the advancement of knowledge as the application of a new instrument. The native intellectual powers of men in different times are not so much the causes of the different success of their labours, as the peculiar nature of the means and artificial resources in their possession. T. Hager: Force of Nature (1)

Detailed glycan analysis of serum glycoproteins of patients with congenital disorders of glycosylation indicates the specific defective glycan processing step and provides an insight into pathogenesis

The fundamental importance of correct protein glycosylation is abundantly clear in a group of diseases known as congenital disorders of glycosylation (CDGs). In these diseases, many biological functions are compromised, giving rise to a wide range of severe clinical conditions. By performing detailed analyses of the total serum glycoproteins as well as isolated transferrin and IgG, we have directly correlated aberrant glycosylation with a faulty glycosylation processing step. In one patient the complete absence of complex type sugars was consistent with ablation of GlcNAcTase II activity. In another CDG type II patient, the identification of specific hybrid sugars suggested that the defective processing step was cell type-specific and involved the mannosidase III pathway. In each case, complementary serum proteome analyses revealed significant changes in some 31 glycoproteins, including components of the complement system. This biochemical approach to charting diseases that involve alterations in glycan processing provides a rapid indicator of the nature, severity, and cell type specificity of the suboptimal glycan processing steps; allows links to genetic mutations; indicates the expression levels of proteins; and gives insight into the pathways affected in the disease process.

Abnormal glycosylation of red cell membrane band 3 in the congenital disorder of glycosylation Ig

A description is provided of the clinical presentation in an infant of the recently described congenital disorder of glycosylation type Ig, and the changes affecting glycosylation of red cell membrane band 3, the anion exchanger. It has been shown that the condition stems from a homozygous mutation within the human ortholog of yeast ALG12 gene, which encodes a dolichol-P-mannose:Man7GlcNAc2-PP-dolichol alpha,1-6 mannosyltransferase of the endoplasmic reticulum. The clinical phenotype included prominent central and peripheral manifestations in the CNS. Although the infant studied had no anemia, band 3 abnormally separated into two fractions upon electrophoresis. The chemical composition of the glycans of both fractions was analyzed in detail. The fraction with low electrophoretic mobility was moderately hypoglycosylated (by 27%) and its mannose content was normal. The fraction with high electrophoretic mobility was deeply carbohydrate deficient (by 64%) and had 1 mol mannose in excess but only three residues of N-acetylglucosamine. Glycophorin A was hypoglycosylated with respect to O-linked glycans. Glycosphingolipids of red cells were normal. We suggest that the incomplete biosynthesis of the N-linked glycan of band 3 was largely caused by the persistence of the 3-linked mannose residue on the 6-mannose arm of the trimannosyl moiety of the glycoprotein. It is remarkable that the changes recorded in band 3 have no clinical consequences. Band 3 alteration might serve as an additional indicator (some serum N-glycoproteins of hepatic origin are also indicative) of the congenital disorder of glycosylation type Ig.

[Analysis of the growth system of Candida albicans in a host and the development of new antifungal material]

Hyphal cells of Candida albicans bind to human hemoglobin, but not yeast cells. The amount of hemoglobin receptor is significantly higher in hyphal cells than on yeast cells. Only the hyphal cells of C. albicans use hemoglobin as a source of iron. The culture supernatant of C. albicans promoted the disruption of human red blood cells (RBC). Hemolytic activity was detected in a sugar-rich fraction (about 200 kDa) purified by Sephacryl S-100 chromatography. As the hemolytic activity was adsorbed by concanavalin A (Con A)-Sepharose, the hemolytic factor might be a mannoprotein. The activity was inactivated by periodate oxidation, indicating that the sugar moiety of the mannoprotein plays an important role in hemolysis. The structure of the sugar moiety of the mannoprotein was identified as a cell wall mannan by 1H-NMR analysis, and purified C. albicans mannan promoted the disruption of RBC. The binding of mannan to RBC was demonstrated by flow cytometric analysis and was inhibited by the addition of the band 3 protein inhibitor, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). The hemolysis caused by mannan is inhibited by DIDS, 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid, and Bis (sulfosuccinimidyl) suberate, but not by pyridoxal-5'-phosphate. A new platinum derivative of the form H[Pt(IV) (Hdigly)Cl2(OH)2] (Hdigly = glycylglycine) has candidacidal activity 10-fold lower than that of cisplatin.

Phosphomannomutase activity in congenital disorders of glycosylation type Ia determined by direct analysis of the interconversion of mannose-1-phosphate to mannose-6-phosphate by high-pH anion-exchange chromatography with pulsed amperometric detection

Congenital disorders of glycosylation (CDG) are a group of multisystemic disorders resulting from defects in the synthesis and processing of N-linked oligosaccharides. The most common form, CDG type Ia (CDG-Ia), results from a deficiency of the enzyme phosphomannomutase (PMM). PMM converts mannose 6-phosphate (man-6-P) to mannose-1-phosphate (man-1-P), which is required for the synthesis of GDP-mannose, a substrate for dolichol-linked oligosaccharide synthesis. The traditional assay for PMM, a coupled enzyme system based on the reduction of NADP(+) to NADPH using man-1-P as a substrate, has limitations in accuracy and reproducibility. Therefore, a more sensitive, direct test for PMM activity, based on the detection of the conversion of man-1-P to man-6-P by high-pH anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD), was developed. Using this assay, the activity of PMM was markedly deficient in fibroblasts and lymphoblasts from 23 patients with CDG-Ia (range 0-15.3% of control, average 4.9+/-4.7%) and also decreased in seven obligate heterozygotes (range 33.0-72.0% of control, average 52.2+/-14.7%). Unlike the spectrophotometric method, there was no overlap in PMM activity among patients, obligate heterozygotes, or controls. Thus, the PMM assay based on HPAEC-PAD has increased utility in the clinical setting, and can be used, together with transferrin isoelectric focusing, to diagnose patients with CDG-Ia and to identify heterozygotes when clinically indicated.

Congenital disorders of glycosylation: review of their molecular bases, clinical presentations and specific therapies

Congenital disorders of glycosylation (CDG, formerly named carbohydrate-deficient glycoprotein syndromes) are a rapidly growing family of inherited disorders affecting the assembly or processing of glycans on glycoconjugates. The clinical spectrum of the different types of CDG discovered so far is variable, ranging from severe multisystemic disorders to disorders restricted to specific organs. This review deals with clinical, diagnostic, and biochemical aspects of all characterized CDGs, including a disorder affecting the N-glycosylation of erythrocytes, congenital dyserythropoietic anemia type II (CDA II/HEMPAS), and the first disorders affecting O-glycosylation. Since the clinical spectrum of symptoms in CDG is variable and may be unspecific, a generous selective screening for the presence of CDG is recommended.

Mice with a homozygous deletion of the Mgat2 gene encoding UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,2-N-acetylglucosaminyltransferase II: a model for congenital disorder of glycosylation type IIa

Mice homozygous for a deletion of the Mgat2 gene encoding UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,2-N-acetylglucosaminyltransferase II (GlcNAcT-II, EC 2.4.1.143) have been reported. GlcNAcT-II is essential for the synthesis of complex N-glycans. The Mgat2-null mice were studied in a comparison with the symptoms of congenital disorder of glycosylation type IIa (CDG-IIa) in humans. Mutant mouse tissues were shown to be deficient in GlcNAcT-II enzyme activity and complex N-glycan synthesis, resulting in severe gastrointestinal, hematologic and osteogenic abnormalities. All mutant mice died in early post-natal development. However, crossing the Mgat2 mutation into a distinct genetic background resulted in a low frequency of survivors exhibiting additional and novel disease signs of CDG-IIa. Analysis of N-glycan structures in the kidneys of Mgat2-null mice showed a novel bisected hybrid N-glycan structure in which the bisecting GlcNAc residue was substituted with a beta1,4-linked galactose or the Lewis(x) structure. These studies suggest that some of the functions of complex N-glycan branches are conserved in mammals and that human disease due to aberrant protein N-glycosylation may be modeled in the mouse, with the expectation in this case of gaining insights into CDG-IIa disease pathogenesis. Further analyses of the Mgat2-deficient phenotype in the mouse have been accomplished involving cells in which the Mgat2 gene is dispensable, as well as other cell lineages in which a severe defect is present. Pre-natal defects appear in a significant number of embryos, and likely reflect a limited window of time in which a future therapeutic approach might effectively operate.

Congenital disorders of glycosylation: a review

Congenital disorders of glycosylation (CDGs) are a rapidly growing group of inherited disorders caused by defects in the synthesis and processing of the asparagine(ASN)-linked oligosaccharides of glycoproteins. The first CDG patients were described in 1980. Fifteen years later, a phosphomannomutase deficiency was found as the basis of the most frequent type, CDG-Ia. In recent years several novel types have been identified. The N-glycosylation pathway is highly conserved from yeast to human, and the rapid progress in this field can largely be attributed to the systematic application of the knowledge of yeast mutants. Up to now, eight diseases have been characterized, resulting from enzyme or transport defects in the cytosol, endoplasmic reticulum, or Golgi compartment. CDGs affect all organs and particularly the CNS, except for CDG-Ib, which is mainly a hepatic-intestinal disease.

Modeling human congenital disorder of glycosylation type IIa in the mouse: conservation of asparagine-linked glycan-dependent functions in mammalian physiology and insights into disease pathogenesis

The congenital disorders of glycosylation (CDGs) are recent additions to the repertoire of inherited human genetic diseases. Frequency of CDGs is unknown since most cases are believed to be misdiagnosed or unrecognized. With few patients identified and heterogeneity in disease signs noted, studies of animal models may provide increased understanding of pathogenic mechanisms. However, features of mammalian glycan biosynthesis and species-specific variations in glycan repertoires have cast doubt on whether animal models of human genetic defects in protein glycosylation will reproduce pathogenic events and disease signs. We have introduced a mutation into the mouse germline that recapitulates the glycan biosynthetic defect responsible for human CDG type IIa (CDG-IIa). Mice lacking the Mgat2 gene were deficient in GlcNAcT-II glycosyltransferase activity and complex N-glycans, resulting in severe gastrointestinal, hematologic, and osteogenic abnormalities. With use of a lectin-based diagnostic screen for CDG-IIa, we found that all Mgat2-null mice died in early postnatal development. However, crossing the Mgat2 mutation into a distinct genetic background resulted in a low frequency of survivors. Mice deficient in complex N-glycans exhibited most CDG-IIa disease signs; however, some signs were unique to the aged mouse or are prognostic in human CDG-IIa. Unexpectedly, analyses of N-glycan structures in Mgat2-null mice revealed a novel oligosaccharide branch on the "bisecting" N-acetylglucosamine. These genetic, biochemical, and physiologic studies indicate conserved functions for N-glycan branches produced in the Golgi apparatus among two mammalian species and suggest possible therapeutic approaches to GlcNAcT-II deficiency. Our findings indicate that human genetic disease due to aberrant protein glycosylation can be modeled in the mouse to gain insights into N-glycan-dependent physiology and the pathogenesis of CDG-IIa.

Effects of sialic acid residues of transferrin on the binding with aluminum and iron studied by HPLC/high-resolution ICP-MS

Transferrins (Tfs) are glycoproteins with carbohydrate chains in the C-lobe. Carbohydrate-deficient Tfs (CDTs) with fewer sialic acids increased in several diseases. In this study, the affinity of metals (Al and Fe) to Tfs was compared between native- and asialo-Tf by on-line high-performance liquid chromatography/high-resolution inductively coupled plasma mass spectrometry, to clarify whether the presence of sialic acids influences the metal binding. Fe added as Fe-citrate in the presence of bicarbonate preferred the N-lobe site and the binding affinity was similar between native- and asialo-Tfs. Al-citrate added at Al/Tf = 1 also preferred the N-lobe site, while the binding affinity was higher to asialo-Tf than to native-Tf. In Al-oxalate addition, the affinity to the N-lobe site of both Tfs increased further. In the absence of bicarbonate, Al-oxalate showed a preference for the C-lobe site in native-Tf and comparable affinity to both lobes in asialo-Tf. In asialo-Tf, Al2-Tf was the largest peak even at Al/Tf = 1. Thus, the lack of sialic acid in glycans and the presence of oxalate enhanced the binding affinity of Al to Tf. Therefore, it was suggested that the binding affinity of Al in patients with CDTs may be enhanced.

Clostridium botulinum type A haemagglutinin-positive progenitor toxin (HA(+)-PTX) binds to oligosaccharides containing Gal beta1-4GlcNAc through one subcomponent of haemagglutinin (HA1)

Haemagglutinin (HA) activity of Clostridium botulinum type A 19S and 16S toxins (HA-positive progenitor toxin; HA(+)-PTX) was characterized. HA titres against human erythrocytes of HA(+)-PTX were inhibited by the addition of lactose, D-galactose, N-acetyl-D-galactosamine and D-fucose to the reaction mixtures. A direct glycolipid binding test demonstrated that type A HA(+)-PTX strongly bound to paragloboside and some neutral glycolipids, but did not bind to gangliosides. Type A HA(+)-PTX also bound to asialoglycoproteins (asialofetuin, neuraminidase-treated transferrin), but not to sialoglycoproteins (fetuin, transferrin). Although glycopeptidase F treatment of asialofetuin abolished the binding of HA(+)-PTX, endo-alpha-N-acetylgalactosaminidase treatment did not. Thus these results can be interpreted as indicating that type A HA(+)-PTX detects and binds to Gal beta 1-4GlcNAc in paragloboside and the N-linked oligosaccharides of glycoproteins. Regardless of neuraminidase treatment, type A HA(+)-PTX bound to glycophorin A which is a major sialoglycoprotein on the surface of erythrocytes. Both native glycophorin A and neuraminidase-treated glycophorin A inhibited the binding of erythrocytes to type A HA(+)-PTX. Since the N:-linked oligosaccharide of glycophorin A is di-branched and more than 50% of this sugar chain is monosialylated, type A HA(+)-PTX probably bound to the unsialylated branch of the N-linked oligosaccharide of glycophorin A and agglutinated erythrocytes. One subcomponent of HA, designated HA1, did not agglutinate native erythrocytes, although it did bind to erythrocytes, paragloboside and asialoglycoproteins in a manner quite similar to that of HA(+)-PTX. These results indicate that type A HA(+)-PTX binds to oligosaccharides through HA1.

Congenital disorders of glycosylation: glycosylation defects in man and biological models for their study

Several inherited disorders affecting the biosynthetic pathways of N-glycans have been discovered during the past years. This review summarizes the current knowledge in this rapidly expanding field and covers the molecular bases of these disorders as well as their phenotypical consequences.

Defect in N-glycosylation of proteins is tissue-dependent in congenital disorders of glycosylation Ia

The biochemical hallmark of Congenital Disorders of Glycosylation (CDG) including type Ia is a defective N-glycosylation of serum glycoproteins. Hypoglycosylated forms of alpha1-antitrypsin have been detected by Western blot in serum from CDG Ia patients. In contrast we were not able to detect hypoglycosylation in alpha1-antitrypsin synthesized by fibroblasts, keratinocytes, enterocytes, and leukocytes. Similarly no hypoglycosylation was detectable in a membrane-associated N-linked glycoprotein, the facilitative glucose transporter GLUT-1 and also in serum immunoglobulin G isolated from sera of CDG Ia patients. We conclude that the phenotypic expression of CDG Ia is tissue-dependent.

Mannose supplementation corrects GDP-mannose deficiency in cultured fibroblasts from some patients with Congenital Disorders of Glycosylation (CDG)

Congenital Disorders of Glycosylation (CDG) are human deficiencies in glycoprotein biosynthesis. Previous studies showed that 1 mM mannose corrects defective protein N-glycosylation in cultured fibroblasts from some CDG patients. We hypothesized that these CDG cells have limited GDP-mannose (GDP-Man) and that exogenous mannose increases the GDP-Man levels. Using a well established method to measure GDP-Man, we found that normal fibroblasts had an average of 23.5 pmol GDP-Man/10(6) cells, whereas phosphomannomutase (PMM)-deficient fibroblasts had only 2.3-2.7 pmol/10(6) cells. Adding 1 mM mannose to the culture medium increased the GDP-Man level in PMM-deficient cells to approximately 15.5 pmol/10(6) cells, but had no significant effect on GDP-Man levels in normal fibroblasts. Similarly, mannose supplementation increased GDP-Man from 4.6 pmol/10(6) cells to 24.6 pmol/10(6) cells in phosphomannose isomerase (PMI)-deficient fibroblasts. Based on the specific activity of the GDP-[(3)H]Man pool present in [2-(3)H]mannose labeled cells, mannose supplementation also partially corrected the impaired synthesis of mannosylphosphoryldolichol (Man-P-Dol) and Glc(0)(-)(3)Man(9)GlcNAc(2)-P-P-Dol. These results confirm directly that deficiencies in PMM and PMI result in lowered cellular GDP-Man levels that are corrected by the addition of mannose. In contrast to these results, GDP-Man levels in fibroblasts from a CDG-Ie patient, who is deficient in Man-P-Dol synthase, were normal and unaffected by mannose supplementation even though mannose addition was found to correct abnormal lipid intermediate synthesis in another study (Kim et al. [2000] J. Clin. Invest., 105, 191-198). The mechanism by which mannose supplementation corrects abnormal protein N-glycosylation in Man-P-Dol synthase deficient cells is unknown, but this observation suggests that the regulation of Man-P-Dol synthesis and utilization may be more complex than is currently understood.