Nonreductive release of O-linked oligosaccharides from mucin glycoproteins for structure/function assignments as neoglycolipids: application in the detection of novel ligands for E-selectin

Mass Spectrometry Imaging for Glycome in the Brain

Glycans are diverse structured biomolecules that play crucial roles in various biological processes. Glycosylation, an enzymatic system through which various glycans are bound to proteins and lipids, is the most common and functionally crucial post-translational modification process. It is known to be associated with brain development, signal transduction, molecular trafficking, neurodegenerative disorders, psychopathologies, and brain cancers. Glycans in glycoproteins and glycolipids expressed in brain cells are involved in neuronal development, biological processes, and central nervous system maintenance. The composition and expression of glycans are known to change during those physiological processes. Therefore, imaging of glycans and the glycoconjugates in the brain regions has become a “hot” topic nowadays. Imaging techniques using lectins, antibodies, and chemical reporters are traditionally used for glycan detection. However, those techniques offer limited glycome detection. Mass spectrometry imaging (MSI) is an evolving field that combines mass spectrometry with histology allowing spatial and label-free visualization of molecules in the brain. In the last decades, several studies have employed MSI for glycome imaging in brain tissues. The current state of MSI uses on-tissue enzymatic digestion or chemical reaction to facilitate successful glycome imaging. Here, we reviewed the available literature that applied MSI techniques for glycome visualization and characterization in the brain. We also described the general methodologies for glycome MSI and discussed its potential use in the three-dimensional MSI in the brain.

A semi-automated, high throughput approach for O-glycosylation profiling of in vitro established cancer cell lines by MALDI-FT-ICR MS

The study of protein O-glycosylation is important in biological research as O-glycans have been reported to regulate a multitude of molecular and cell biology processes occurring in cancer. It is known that alterations in O-glycosylation are involved in the development and progression of cancer. Their easy accessibility makes in vitro established cell lines suitable and useful models for studying biological mechanisms in disease. However, the O-glycosylation analysis of large numbers of samples, as required in systems biology and biomarker discovery studies, is often challenging. In the present study, O-glycans from three human colorectal cancer cell lines and two human pancreatic cancer cell lines were released by semi-automated, high throughput reductive β-elimination and analysed using ultrahigh resolution MALDI-FT-ICR MS. Automated data integration and processing was performed using MassyTools, where the analyte was automatically included for relative quantitation based on a range of selection criteria including signal-to-noise ratio, mass error and isotopic pattern quality scores. A total of 126 O-glycan compositions, ranging from a single monosaccharide to large oligosaccharides exhibiting complex glycan motifs, were detected. The use of ultrahigh resolution MALDI-FTICR MS enabled glycan identification and quantitation in the matrix region of the spectrum. This approach has the potential to be used for O-glycosylation analysis of large numbers of samples, such as patient sample cohorts.

Characterization of glycosylation in monoclonal antibodies and its importance in therapeutic antibody development

Glycosylation is one of the structurally diverse and complex forms of post translational modifications observed in proteins which influence the effector functions of IgG-Fc. Although the glycosylation constitutes 2-3% of the total mass of the IgG antibody, a thorough assessment of glycoform distribution present on the antibody is a critical quality attribute (cQA) for the majority of novel and biosimilar monoclonal antibody (mAb) development. This review paper will highlight the impact of different glycoforms such as galactose, fucose, high mannose, NANA (N-acetylneuraminic acid), and NGNA (N-glycoylneuraminic acid) on the safety/immunogeneicity, efficacy/biological activity and clearance (pharmacodynamics/pharmacokinetic property (PD/PK)) of biological molecules. In addition, this paper will summarize routinely employed reliable analytical techniques such as hydrophilic interaction chromatography (HILIC), high performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and mass spectrometry (MS) for characterizing and monitoring glycosylation in monoclonal antibodies (mAbs). The advantages and disadvantages of each of the methods are addressed. The scope of this review paper is limited to only N-linked and O-linked glycosylation.

Preparation of Complex Glycans From Natural Sources for Functional Study

One major barrier in glycoscience is the lack of diverse and biomedically relevant complex glycans in sufficient quantities for functional study. Complex glycans from natural sources serve as an important source of these glycans and an alternative to challenging chemoenzymatic synthesis. This review discusses preparation of complex glycans from several classes of glycoconjugates using both enzymatic and chemical release approaches. Novel technologies have been developed to advance the large-scale preparation of complex glycans from natural sources. We also highlight recent approaches and methods developed in functional and fluorescent tagging and high-performance liquid chromatography (HPLC) isolation of released glycans.

Mass spectrometry for protein sialoglycosylation

Sialic acids are a family of structurally unique and negatively charged nine-carbon sugars, normally found at the terminal positions of glycan chains on glycoproteins and glycolipids. The glycosylation of proteins is a universal post-translational modification in eukaryotic species and regulates essential biological functions, in which the most common sialic acid is N-acetyl-neuraminic acid (2-keto-5-acetamido-3,5-dideoxy-D-glycero-D-galactononulopyranos-1-onic acid) (Neu5NAc). Because of the properties of sialic acids under general mass spectrometry (MS) conditions, such as instability, ionization discrimination, and mixed adducts, the use of MS in the analysis of protein sialoglycosylation is still challenging. The present review is focused on the application of MS related methodologies to the study of both N- and O-linked sialoglycans. We reviewed MS-based strategies for characterizing sialylation by analyzing intact glycoproteins, proteolytic digested glycopeptides, and released glycans. The review concludes with future perspectives in the field.

The neoglycolipid (NGL) technology-based microarrays and future prospects

The neoglycolipid (NGL) technology is the basis of a state-of-the-art oligosaccharide microarray system, which we offer for screening analyses to the broad scientific community. We review here the sequential development of the technology and its power in pinpointing and isolating naturally occurring ligands for glycan-binding proteins (GBPs) within glycan populations. We highlight our Designer Array approach and Beam Search Array approach for generating natural glycome arrays to identify novel ligands of biological relevance. These two microarray approaches have been applied for assignments of ligands or antigens on glucan polysaccharides for effector proteins of the immune system (Dectin-1, DC-SIGN and DC-SIGNR) and carbohydrate-binding modules (CBMs) on bacterial hydrolases. We also discuss here the more recent applications to elucidate the structure of a prostate cancer- associated antigen F77 and identify ligands for adhesins of two rotaviruses, P[10] and P[19], expressed on an epithelial mucin glycoprotein.

Novel monoclonal antibody L2A5 specifically targeting sialyl-Tn and short glycans terminated by alpha-2-6 sialic acids

Incomplete O-glycosylation is a feature associated with malignancy resulting in the expression of truncated glycans such as the sialyl-Tn (STn) antigen. Despite all the progress in the development of potential anti-cancer antibodies, their application is frequently hindered by low specificities and cross-reactivity. In this study, a novel anti-STn monoclonal antibody named L2A5 was developed by hybridoma technology. Flow cytometry analysis showed that L2A5 specifically binds to sialylated structures on the cell surface of STn-expressing breast and bladder cancer cell lines. Moreover, immunoblotting assays demonstrated reactivity to tumour-associated O-glycosylated proteins, such as MUC1. Tumour recognition was further observed using immunohistochemistry assays, which demonstrated a high sensitivity and specificity of L2A5 mAb towards cancer tissue, using bladder and colorectal cancer tissues. L2A5 staining was exclusively tumoural, with a remarkable reactivity in invasive and metastasis sites, not detectable by other anti-STn mAbs. Additionally, it stained 20% of cases of triple-negative breast cancers, suggesting application in diseases with unmet clinical needs. Finally, the fine specificity was assessed using glycan microarrays, demonstrating a highly specific binding of L2A5 to core STn antigens and additional ability to bind 2-6-linked sialyl core-1 probes. In conclusion, this study describes a novel anti-STn antibody with a unique binding specificity that can be applied for cancer diagnostic and future development of new antibody-based therapeutic applications.

Decoding of O-Linked Glycosylation by Mass Spectrometry

Protein glycosylation (N- and O-linked) plays an important role in many biological processes, including protein structure and function. However, the structural elucidation of glycans, specifically O-linked glycans, remains a major challenge and is often overlooked during protein analysis. Recently, mass spectrometry (MS) has matured as a powerful technology for high-quality analytical characterization of O-linked glycans. This review summarizes the recent developments and insights of MS-based glycomics technologies, with a focus on mucin-type O-glycan analysis. Three main MS-based approaches are outlined: O-glycan profiling (structural analysis of released O-glycan), a "bottom-up" approach (analysis of an O-glycan covalently attached to a glycopeptide), and a "top-down" approach (analysis of a glycan attached to an intact glycoprotein). In addition, the most widely used MS ionization techniques, i.e., matrix-assisted laser desorption ionization and electrospray ionization, as well as ion activation techniques like collision-induced dissociation, electron capture dissociation, and electron transfer dissociation during O-glycan analysis are discussed. The MS technical approaches mentioned above are already major improvements for studying O-linked glycosylation and appear to be valuable for in-depth analysis of the type of O-glycan attached, branching patterns, and the occupancy of O-glycosylation sites.

Glycan microarrays of fluorescently-tagged natural glycans

This review discusses the challenges facing research in 'functional glycomics' and the novel technologies that are being developed to advance the field. The structural complexity of glycans and glycoconjugates makes studies of both their structures and recognition difficult. However, these intricate structures can be captured from their natural sources, isolated and fluorescently-tagged for detailed structural analysis and for presentation on glycan microarrays for functional recognition by glycan-binding proteins. These advances in glycan preparation and manipulation enable the streamlining of functional glycomics studies and will help to propel the field forward in studying natural, biologically relevant glycans.

Clinical Glycomics Employing Graphitized Carbon Liquid Chromatography-Mass Spectrometry

Glycoconjugates and free glycan are involved in a variety of biological processes such as cell-cell interaction and cell trafficking. Alterations in the complex glycosylation machinery have been correlated with various pathological processes including cancer progression and metastasis. Mass Spectrometry (MS) has evolved as one of the most powerful tools in glycomics and glycoproteomics and in combination with porous graphitized carbon-liquid chromatography (PGC-LC) it is a versatile and sensitive technique for the analysis of glycans and to some extent also glycopeptides. PGC-LC-ESI-MS analysis is characterized by a high isomer separation power enabling a specific glycan compound analysis on the level of individual structures. This allows the investigation of the biological relevance of particular glycan structures and glycan features. Consequently, this strategy is a very powerful technique suitable for clinical research, such as cancer biomarker discovery, as well as in-depth analysis of recombinant glycoproteins. In this review, we will focus on how PGC in conjunction with MS detection can deliver specific structural information for clinical research on protein-bound N-glycans and mucin-type O-glycans. In addition, we will briefly review PGC analysis approaches for glycopeptides, glycosaminoglycans (GAGs) and human milk oligosaccharides (HMOs). The presented applications cover systems that vary vastly with regard to complexity such as purified glycoproteins, cells, tissue or body fluids revealing specific glycosylation changes associated with various biological processes including cancer and inflammation.

Carbohydrate sequence of the prostate cancer-associated antigen F77 assigned by a mucin O-glycome designer array

Monoclonal antibody F77 was previously raised against human prostate cancer cells and has been shown to recognize a carbohydrate antigen, but the carbohydrate sequence of the antigen was elusive. Here, we make multifaceted approaches to characterize F77 antigen, including binding analyses with the glycolipid extract of the prostate cancer cell line PC3, microarrays with sequence-defined glycan probes, and designer arrays from the O-glycome of an antigen-positive mucin, in conjunction with mass spectrometry. Our results reveal F77 antigen to be expressed on blood group H on a 6-linked branch of a poly-N-acetyllactosamine backbone. We show that mAb F77 can also bind to blood group A and B analogs but with lower intensities. We propose that the close association of F77 antigen with prostate cancers is a consequence of increased blood group H expression together with up-regulated branching enzymes. This is in contrast to other epithelial cancers that have up-regulated branching enzymes but diminished expression of H antigen. With knowledge of the structure and prevalence of F77 antigen in prostate cancer, the way is open to explore rationally its application as a biomarker to detect F77-positive circulating prostate cancer-derived glycoproteins and tumor cells.

Improved nonreductive O-glycan release by hydrazinolysis with ethylenediaminetetraacetic acid addition

The study of protein O-glycosylation is receiving increasing attention in biological, medical, and biopharmaceutical research. Improved techniques are required to allow reproducible and quantitative analysis of O-glycans. An established approach for O-glycan analysis relies on their chemical release in high yield by hydrazinolysis, followed by fluorescent labeling at the reducing terminus and high-performance liquid chromatography (HPLC) profiling. However, an unwanted degradation known as "peeling" often compromises hydrazinolysis for O-glycan analysis. Here we addressed this problem using low-molarity solutions of ethylenediaminetetraacetic acid (EDTA) in hydrazine for O-glycan release. O-linked glycans from a range of different glycoproteins were analyzed, including bovine fetuin, bovine submaxillary gland mucin, and serum immunoglobulin A (IgA). The data for the O-glycans released by hydrazine with anhydrous EDTA or disodium salt dihydrate EDTA show high yields of the native O-glycans compared with the peeled product, resulting in a markedly increased robustness of the O-glycan profiling method. The presented method for O-glycan release demonstrates significant reduction in peeling and reduces the number of sample handling steps prior to release.

In-gel β-elimination and aqueous-organic partition for improved O- and sulfoglycomics

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) is a widely used technique for protein separation, and in-gel tryptic digestion of resolved protein bands has enhanced the resolution of protoeomic analysis. To augment this technology and expand its usefulness for glycoproteomics, we have developed and improved methods to release and recover O-linked glycans from proteins resolved in SDS-PAGE gels for subsequent analysis by mass spectrometry (MS). Gel pieces containing target proteins are washed to remove contaminants. O-linked glycans are released through reductive β-elimination by hydrating gel pieces in base and adding reductant. Following straightforward sample cleanup, this simple treatment of glycoprotein gel pieces produces material suitable for MS analysis. We have applied this method to the analysis of mucin-type glycoproteins that are expected to carry high densities of sialylated and sulfated O-linked glycans. However, the strongly acidic nature of the sulfate moiety suppresses MS signal intensities, hampering detection and quantitative analysis. To enhance detection, we present an improved method for sulfoglycomics. A mixture of sulflo-, sialo-, and neutral glycans were permethylated and partitioned into a water-dichloromethane (DCM) solvent mixture. Sulfated glycans were selectively recovered from the aqueous phase, while neutral and sialylated glycans remained in the DCM phase. When applied to the analysis of human mucin salivary glycans, this partition method generated material of sufficient quality to identify more than 60 glycan structures by NSI-MS (LTQ-Orbitrap) in positive and negative ion modes. Also, nearly 100% of the sulfated O-linked glycans were recovered in the aqueous phase, demonstrating the feasibility of in-depth sulfoglycomic analysis using SDS-PAGE resolved proteins.

Typing of blood-group antigens on neutral oligosaccharides by negative-ion electrospray ionization tandem mass spectrometry

Blood-group antigens, such as those containing fucose and bearing the ABO(H)- and Lewis-type determinants expressed on the carbohydrate chains of glycoproteins and glycolipids, and also on unconjugated free oligosaccharides in human milk and other secretions, are associated with various biological functions. We have previously shown the utility of negative-ion electrospay ionization tandem mass spectrometry with collision-induced dissociation (ESI-CID-MS/MS) for typing of Lewis (Le) determinants, for example, Le(a), Le(x), Le(b), and Le(y) on neutral and sialylated oligosaccharide chains. In the present report, we extended the strategy to characterization of blood-group A-, B-, and H-determinants on type 1 and type 2 and also on type 4 globoside chains to provide a high sensitivity method for typing of all the major blood-group antigens, including the A, B, H, Le(a), Le(x), Le(b), and Le(y) determinants, present in oligosaccharides. Using the principles established, we identified two minor unknown oligosaccharide components present in the products of enzymatic synthesis by bacterial fermentation. We also demonstrated that the unique fragmentations derived from the D- and (0,2)A-type cleavages observed in ESI-CID-MS/MS, which are important for assigning blood-group and chain types, only occur under the negative-ion conditions for reducing sugars but not for reduced alditols or under positive-ion conditions.

Suppression of peeling during the release of O-glycans by hydrazinolysis

The analysis of O-glycans is essential for better understanding their functions in biological processes. Although many techniques for O-glycan release have been developed, the hydrazinolysis release method is the best for producing O-glycans with free reducing termini in high yield. This release technique allows the glycans to be labeled with a fluorophore and analyzed by fluorescence detection. Under the hydrazinolysis release conditions, a side reaction is observed and causes the loss of monosaccharides from the reducing terminus of the glycans (known as peeling). Using bovine fetuin (because it contains the sialylated O-glycans most commonly found on biopharmaceuticals) and bovine submaxillary gland mucin (BSM), here we demonstrate that peeling can be greatly reduced when the sample is buffer exchanged prior to hydrazinolysis with solutions of either 0.1% trifluoroacetic acid (TFA) or low-molarity (100, 50, 20, and 5 mM) ethylenediaminetetraacetic acid (EDTA). The addition of calcium chloride to fetuin resulted in an increase in peeling, whereas subsequent washing with EDTA abolished this effect, suggesting a role of calcium and possibly other cations in causing peeling. The presented technique for sample preparation prior to hydrazinolysis greatly reduces the level of undesirable cleavage products in O-glycan analysis and increases the robustness of the method.

Analysis of glycans derived from glycoconjugates by capillary electrophoresis-mass spectrometry

The high structural variation of glycan derived from glycoconjugates, which substantially increases with the molecular size of a protein, contributes to the complexity of glycosylation patterns commonly associated with glycoconjugates. In the case of glycoproteins, such variation originates from the multiple glycosylation sites of proteins and the number of glycan structures associated with each site (microheterogeneity). The ability to comprehensively characterize highly complex mixture of glycans has been analytically stimulating and challenging. Although the most powerful MS and MS/MS techniques are capable of providing a wealth of structural information, they are still not able to readily identify isomeric glycan structures without high-order MS/MS (MS(n) ). The analysis of isomeric glycan structures has been attained using several separation methods, including high-pH anion-exchange chromatography, hydrophilic interaction chromatography and GC. However, CE and microfluidics CE (MCE) offer high separation efficiency and resolutions, allowing the separation of closely related glycan structures. Therefore, interfacing CE and MCE to MS is a powerful analytical approach, allowing potentially comprehensive and sensitive analysis of complex glycan samples. This review describes and discusses the utility of different CE and MCE approaches in the structural characterization of glycoproteins and the feasibility of interfacing these approaches to MS.

Specificities of Ricinus communis agglutinin 120 interaction with sulfated galactose

Lectins are used extensively as research tools to detect and target specific oligosaccharide sequences. Ricinus communis agglutinin I (RCA(120)) recognizes non-reducing terminal β-D-galactose (Galβ) and its specificities of interactions with neutral and sialylated oligosaccharides have been well documented. Here we use carbohydrate arrays of sulfated Galβ-containing oligosaccharide probes, prepared from marine-derived galactans, to investigate their interactions with RCA(120). Our results showed that RCA(120) binding to Galβ1-4 was enhanced by 2-O- or 6-O-sulfation but abolished by 4-O-sulfation. The results were corroborated with competition experiments. Erythrina cristagalli lectin is also a Galβ-binding protein but it cannot accommodate any sulfation on Galβ.

One-pot nonreductive O-glycan release and labeling with 1-phenyl-3-methyl-5-pyrazolone followed by ESI-MS analysis

A novel one-pot procedure for the nonreductive release of O-linked glycans from glycoproteins and the simultaneous derivatization of released glycans with 1-phenyl-3-methyl-5-pyrazolone (PMP) is described. Unlike the traditional reductive β-elimination, which produces alditols, this new method employs PMP/ammonia aqueous solution as the reaction medium. The O-glycans are released from glycoproteins and derivatized with PMP nonreductively, specifically, and quantitatively. Samples can be easily purified from ammonia, excess PMP, and peptide residues by evaporation, chloroform extraction, and solid-phase extraction (SPE) column fractionation for HPLC, CE, or MS analysis. The procedure has been elaborated with two purified glycoproteins, porcine stomach mucin and bovine fetuin, and successfully applied to O-glycan profiling of a challenging biological specimen, healthy human plasma. This new procedure has shown methodological significance in O-glycan analysis.

Characterization and scope of S-layer protein O-glycosylation in Tannerella forsythia

Cell surface glycosylation is an important element in defining the life of pathogenic bacteria. Tannerella forsythia is a Gram-negative, anaerobic periodontal pathogen inhabiting the subgingival plaque biofilms. It is completely covered by a two-dimensional crystalline surface layer (S-layer) composed of two glycoproteins. Although the S-layer has previously been shown to delay the bacterium's recognition by the innate immune system, we characterize here the S-layer protein O-glycosylation as a potential virulence factor. The T. forsythia S-layer glycan was elucidated by a combination of electrospray ionization-tandem mass spectrometry and nuclear magnetic resonance spectroscopy as an oligosaccharide with the structure 4-Me-β-ManpNAcCONH(2)-(1→3)-[Pse5Am7Gc-(2→4)-]-β-ManpNAcA-(1→4)-[4-Me-α-Galp-(1→2)-]-α-Fucp-(1→4)-[-α-Xylp-(1→3)-]-β-GlcpA-(1→3)-[-β-Digp-(1→2)-]-α-Galp, which is O-glycosidically linked to distinct serine and threonine residues within the three-amino acid motif (D)(S/T)(A/I/L/M/T/V) on either S-layer protein. This S-layer glycan obviously impacts the life style of T. forsythia because increased biofilm formation of an UDP-N-acetylmannosaminuronic acid dehydrogenase mutant can be correlated with the presence of truncated S-layer glycans. We found that several other proteins of T. forsythia are modified with that specific oligosaccharide. Proteomics identified two of them as being among previously classified antigenic outer membrane proteins that are up-regulated under biofilm conditions, in addition to two predicted antigenic lipoproteins. Theoretical analysis of the S-layer O-glycosylation of T. forsythia indicates the involvement of a 6.8-kb gene locus that is conserved among different bacteria from the Bacteroidetes phylum. Together, these findings reveal the presence of a protein O-glycosylation system in T. forsythia that is essential for creating a rich glycoproteome pinpointing a possible relevance for the virulence of this bacterium.

Natural glycan microarrays

Glycan microarrays are emerging as increasingly used screening tools with a high potential for unraveling protein-carbohydrate interactions: probing hundreds or even thousands of glycans in parallel, they provide the researcher with a vast amount of data in a short time-frame, while using relatively small amounts of analytes. Natural glycan microarrays focus on the glycans' repertoire of natural sources, including both well-defined structures as well as still-unknown ones. This article compares different natural glycan microarray strategies. Glycan probes may comprise oligosaccharides from glycoproteins as well as glycolipids and polysaccharides. Oligosaccharides may be purified from scarce biological samples that are of particular relevance for the carbohydrate-binding protein to be studied. We give an overview of strategies for glycan isolation, derivatization, fractionation, immobilization and structural characterization. Detection methods such as fluorescence analysis and surface plasmon resonance are summarized. The importance of glycan density and multivalency is discussed. Furthermore, some applications of natural glycan microarrays for studying lectin and antibody binding are presented.

Discovery and structural characterization of fucosylated oligomannosidic N-glycans in mushrooms

L-fucose is a common constituent of Asn-linked glycans in vertebrates, invertebrates, and plants, but in fungal glycoproteins, fucose has not been found so far. However, by mass spectrometry we detected N-glycans and O-glycans containing one to six deoxyhexose residues in fruit bodies of several basidiomycetes. The N-glycans of chanterelles (Cantharellus cibarius) contained a deoxyhexose chromatographically identical to fucose and sensitive to α-L-fucosidase. Analysis of individual glycan species by tandem MS, glycosidase digestion, and finally (1)H NMR revealed the presence of L-fucose in α1,6-linkage to an α1,6-mannose of oligomannosidic N-glycans. The substitution by α1,6-mannose of α1,2-mannosyl residues of the canonical precursor structure was yet another hitherto unknown modification. No indication for the occurrence of yet other modifications, e.g. bisecting N-acetylglucosamine, was seen. Besides fucosylated N-glycans, short O-linked mannan chains substituted with fucose were present on chanterelle proteins. Although undiscovered so far, L-fucose appears to represent a prominent feature of protein-linked glycans in the fungal kingdom.

Glycan labeling strategies and their use in identification and quantification

Most methods for the analysis of oligosaccharides from biological sources require a glycan derivatization step: glycans may be derivatized to introduce a chromophore or fluorophore, facilitating detection after chromatographic or electrophoretic separation. Derivatization can also be applied to link charged or hydrophobic groups at the reducing end to enhance glycan separation and mass-spectrometric detection. Moreover, derivatization steps such as permethylation aim at stabilizing sialic acid residues, enhancing mass-spectrometric sensitivity, and supporting detailed structural characterization by (tandem) mass spectrometry. Finally, many glycan labels serve as a linker for oligosaccharide attachment to surfaces or carrier proteins, thereby allowing interaction studies with carbohydrate-binding proteins. In this review, various aspects of glycan labeling, separation, and detection strategies are discussed.

Rapid de-O-glycosylation concomitant with peptide labeling using microwave radiation and an alkyl amine base

Procedures are detailed for a quantitative release of O-linked glycans from peptides that now provide a shorter reaction time, a possible identification of O-linked sites, and a quantification of all reaction products. The release was initiated by a mild base, dimethylamine, and accelerated by microwave radiation. Differential analysis using standard glycoproteins has shown improved release efficiency concurrent with facile incorporation of dimethylamine into the former O-linked sites. In situ glycan reduction insures protection against peeling and is synchronous with subsequent studies by high performance MS(n) sequencing. The protocols were established with a synthetic O-GlcNAc peptide that would mimic the linkage chemistry and applied to a well characterized glycoprotein bovine fetuin with both N- and O-linked glycans and a highly glycosylated swine mucin.

Multifaceted approaches including neoglycolipid oligosaccharide microarrays to ligand discovery for malectin

In this chapter, we describe the key procedures for isolation of the oligosaccharides and the preparation of neoglycolipid probes together with expression of malectin that have enabled the discovery of the highly selective binding of this newly described protein in the endoplasmic reticulum (ER) to a diglucosyl high-mannose N-glycan. This is the first indication of a bioactivity for a diglucosyl high-mannose N-glycan of the type that occurs in the ER of eukaryotic cells and which is an intermediate in the early steps of the N-glycosylation pathway of nascent proteins. The malectin story is an example of a powerful convergence of disciplines in biological sciences: (i) developmental biology, (ii) bioinformatics, (iii) recombinant protein expression, (iv) protein structural studies, (v) glucan biochemistry, and (vi) drug-assisted engineering of oligosaccharide biosynthesis, culminating in (vii) oligosaccharide "designer" microarrays, to clinch the remarkable selectivity of the binding of this newly discovered ER protein. Thus, the way is open to the identification of the role of malectin in the N-glycosylation pathway.

Carbohydrate analysis throughout the development of a protein therapeutic

This review discusses the challenges involved in the characterization of the glycosylation of therapeutic glycoproteins. The focus is on methods that are most commonly used in regulatory filings and lot release testing of therapeutic glycoproteins. The different types of assays for carbohydrate analysis are reviewed, including the distinction between assays appropriate for lot release or better suited to testing during early drug development or in-depth characterization of the glycosylation. Characteristics of the glycoprotein and production process that should be considered when determining the amount of testing, the number of different methods to employ and when the testing should be performed during development of protein therapeutics is also discussed.

Generation of a natural glycan microarray using 9-fluorenylmethyl chloroformate (FmocCl) as a cleavable fluorescent tag

Glycan microarray technology has become a successful tool for studying protein-carbohydrate interactions, but a limitation has been the laborious synthesis of glycan structures by enzymatic and chemical methods. Here we describe a new method to generate quantifiable glycan libraries from natural sources by combining widely used protease digestion of glycoproteins and Fmoc chemistry. Glycoproteins including chicken ovalbumin, bovine fetuin, and horseradish peroxidase (HRP) were digested by Pronase, protected by FmocCl, and efficiently separated by 2D-HPLC. We show that glycans from HRP glycopeptides separated by HPLC and fluorescence monitoring retained their natural reducing end structures, mostly core alpha1,3-fucose and core alpha1,2-xylose. After simple Fmoc deprotection, the glycans were printed on NHS-activated glass slides. The glycans were interrogated using plant lectins and antibodies in sera from mice infected with Schistosoma mansoni, which revealed the presence of both IgM and IgG antibody responses to HRP glycopeptides. This simple approach to glycopeptide purification and conjugation allows for the development of natural glycopeptide microarrays without the need to remove and derivatize glycans and potentially compromise their reducing end determinants.

Analysis of N- and O-linked glycans from glycoproteins using MALDI-TOF mass spectrometry

Glycosylation represents the most common of all known protein post-translational modifications. Carbohydrates can modulate the biological functions of a glycoprotein, protect a protein against hydrolysis via protease activity, and reduce or prevent aggregation of a protein. The determination of the carbohydrate structure and function in glycoproteins remains one of the most challenging tasks given to biochemists, as these molecules can exhibit complex branched structures that can differ in linkage and in the level of branching. In this review, we will present the approach followed in our laboratory for the elucidation of N- and O-glycan chains of glycoproteins. First, reduced/carboxamidomethylated glycoproteins are digested with a protease or a chemical reagent. N-Glycans are then released from the resulting peptides/glycopeptides via digestion with peptide N-glycosidase F (PNGase F). Oligosaccharides released by PNGase F are separated from peptides and glycopeptides using a C18 Sep-Pak, and their methylated derivatives are characterized by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS). O-Glycans are released by reductive elimination, which are permethylated, purified on a Sep-Pak C18 cartridge, and analyzed with MALDI-TOF-MS. Finally, to confirm the structures N-glycans released by PNGase F are characterized using MALDI-TOF-MS following on-plate sequential exoglycosidase digestions. The clean-up procedures of native and permethylated oligosaccharides for an efficient MALDI-TOF-MS analysis will also be described. This strategy was applied to calf fetuin and glycoproteins present in human serum.

Investigations with O-linked protein glycosylations by matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry

Posttranslational modifications such as glycosylation can play a fundamental role in signaling pathways that transform an ordinary cell into a malignant one. The development of a protocol to detect these changes in the preliminary stages of disease can lead to a sensitive and specific diagnostic for the early detection of malignancies such as ovarian cancer in which differential glycan patterns are linked to etiology and progression. Small variations in instrument parameters and sample preparation techniques are known to have significant influence on the outcome of an experiment. For an experiment to be effective and reproducible, these parameters must be optimized for the analyte(s) under study. We present a detailed examination of sample preparation and matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FT-ICR-MS) analysis of O-linked glycans globally cleaved from mucin glycoproteins. Experiments with stable isotope-labeled biomolecules allowed for the establishment of appropriate acquisition times and excitation voltages for MALDI-FT-ICR-MS of oligosaccharides. Quadrupole ion guide optimization studies with mucin glycans identified conditions for the comprehensive analysis of the entire mass range of O-linked carbohydrates in this glycoprotein. Separately optimized experimental parameters were integrated in a method that allowed for the effective study of O-linked glycans.

Resistance to deglycosylation by ammonia of IgA1 O-glycopeptides: implications for the beta-elimination of O-glycans linked to serine and threonine

Pools of O-glycopeptides (and their deglycosylated analogues) derived from trypsin-digested normal human serum IgA1 have been treated with ammonia under conditions reported to result in complete liberation of O-glycans linked to serine and threonine residues in glycopeptides and glycoproteins. MALDI-TOF MS analysis has revealed that only one of the six glycosylated sites is susceptible to beta-elimination under these conditions. It is likely that resistance to beta-elimination is due to very close proximity of proline to the glycosylated serine or threonine residues. Preliminary results using 0.1M NaOH (instead of ammonia) to perform beta-elimination indicated that there was also selective de-O-glycosylation with this reagent, however, these results were complicated by the concomitant hydrolysis of the peptide bonds. These findings may have implications for similarly O-glycosylated peptides and proteins and possibly for other chemical methods that are used to carry out beta-eliminations of O-glycans.

Ligands for the β-Glucan Receptor, Dectin-1, Assigned Using “Designer” Microarrays of Oligosaccharide Probes (Neoglycolipids) Generated from Glucan Polysaccharides

Dectin-1 is a C-type lectin-like receptor on leukocytes that mediates phagocytosis and inflammatory mediator production in innate immunity to fungal pathogens. Dectin-1 lacks residues involved in calcium ligation that mediates carbohydrate-binding by classical C-type lectins; nevertheless, it binds zymosan, a particulate beta-glucan-rich extract of Saccharomyces cerevisiae, and binding is inhibited by polysaccharides rich in beta1,3- or both beta1,3- and beta1,6-linked glucose. The oligosaccharide ligands on glucans recognized by Dectin-1 have not yet been delineated precisely. It is also not known whether Dectin-1 can interact with other types of carbohydrates. We have investigated this, since Dectin-1 shows glucan-independent binding to a subset of T-lymphocytes and is involved in triggering their proliferation. Here we assign oligosaccharide ligands for Dectin-1 using the neoglycolipid-based oligosaccharide microarray technology, a unique approach for constructing microarrays of lipid-linked oligosaccharide probes from desired sources. We generate "designer" microarrays from three glucan polysaccharides, a neutral soluble glucan isolated from S. cerevisiae and two bacterial glucans, curdlan from Alcaligenes faecalis and pustulan from Umbilicaria papullosa, and use these in conjunction with 187 diverse, sequence-defined, predominantly mammalian-type, oligosaccharide probes. Among these, Dectin-1 binding is detected exclusively to 1,3-linked glucose oligomers, the minimum length required for detectable binding being a 10- or 11-mer. Thus, the ligands assigned so far are exogenous rather than endogenous. We further show that Dectin-1 ligands, 11-13 gluco-oligomers, in clustered form (displayed on liposomes), mimic the macromolecular beta-glucans and compete with zymosan binding and triggering of tumor necrosis factor-alpha secretion by a Dectin-1-expressing macrophage cell line.

Structural determination of novel lacto-N-decaose and its monofucosylated analogue from human milk by electrospray tandem mass spectrometry and 1H NMR spectroscopy

We have isolated and characterised two neutral oligosaccharides, one nonfucosylated and the other monofucosylated, from human milk that are based on the doubly branched lacto-N-decaose core. Their structures have been determined by a combined use of electrospray tandem mass spectrometry (ES-MS/MS) and NMR spectroscopy. The sequences of the three branches resulted from the double-branching, including the identity and location of the blood-group-related Lewis determinant and partial linkages, were elucidated by the unique method of high sensitivity negative-ion ES-MS/MS analysis. Their full structure assignment was completed by methylation analysis and 1H NMR. The monofucosylated lacto-N-decaose, Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-6(Galbeta1-3GlcNAcbeta1-3)Galbeta1-4GlcNAcbeta1-6(Galbeta1-3GlcNAcbeta1-3)Galbeta1-4Glc is a novel sequence, whereas the nonfucosylated lacto-N-decaose, Galbeta1-4GlcNAcbeta1-6(Galbeta1-3GlcNAcbeta1-3)Galbeta1-4GlcNAcbeta1-6(Galbeta1-3GlcNAcbeta1-3)Galbeta1-4Glc, has not been isolated and identified as an individual oligosaccharide.

Methods in enzymology: O-glycosylation of proteins

Cell surface and extracellular proteins are O-glycosylated, where the most abundant type of O-glycosylation in proteins is the GalNAc attachment to serine (Ser) or threonine (Thr) in the protein chain by an a-glycosidic linkage. Most eukaryotic nuclear and cytoplasmic proteins modified by a-linked O-GlcNAc to Ser or Thr exhibit reciprocal O-GlcNAc glycosylation and phosphorylation during the cell cycle, cell stimulation, and/or cell growth. Less-investigated types of O-glycosylation are O-fucosylation, O-mannosylation, and O-glucosylation, but they are functionally of high relevance for early stages of development and for vital physiological functions of proteins. Glycosaminoglycans are a-linked to proteoglycans via a xylose-containing tetrasaccharide, represented by linear chains of repetitive disaccharides modified by carboxylates and O- or/and N-linked sulfates. Analysis of O-glycosylation by mass spectrometry (MS) is a complex task due to the high structural diversity of glycan and protein factors. The parameters in structural analysis of O-glycans include determination of (i) O-glycosylation attachment sites in the protein sequence, (ii) the type of attached monosaccharide moiety, (iii) a core type in the case of GalNAc O-glycosylation, (iv) the type and size of the oligosaccharide portion, (v) carbohydrate branching patterns, (vi) the site of monosaccharide glycosidic linkages, (vii) the anomericity of glycosidic linkages, and (viii) covalent modifications of the sugar backbone chains by carbohydrate- and noncarbohydrate-type of substitutents. Classical and novel analytical strategies for identification and sequencing of O-glycans by MS are described. These include methods to analyze O-glycans after total or partial release from the parent protein by chemical or enzymatic approach or to analyze O-glycosylated peptides by mapping and sequencing from proteolytic mixtures. A recombination process of multiply charged glycopeptides with electrons by electron capture dissociation Fourier transform ion cyclotrone resonance (FTICR)-MS has been introduced and is instrumental for nonergodic polypeptide backbone cleavages without losses of labile glycan substituents. A method for O-glycoscreening under increased sensitivity and efficient sequencing as a combination of an on-line coupling of capillary electrophoresis separation, as well as an automated MS-tandem MS (MS/MS) switching under variable energy conditions collision-induced dissociation (CID) protocol, is beneficial for determination of O-acetylation and oversulfation (Bindila et al., 2004a; Zamfir et al., 2004a). O-glycomics by robotized chip-electrospray/ionization (ESI)-MS and MS/MS on the quadrupole time-of-flight (QTOF) and FTICR analyzers, accurate mass determination, and software for assignment of fragmentation spectra represent essentials for high-throughput (HTP) in serial screenings (Bindila et al., 2004b; Froesch et al., 2004; Vakhrushev et al., 2005). Dimerization of intact O-glycosylated proteins can be investigated by matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF)-MS after blotting.

Identification of a low affinity mannose 6-phosphate-binding site in domain 5 of the cation-independent mannose 6-phosphate receptor

The 300-kDa cation-independent mannose 6-phosphate receptor (CI-MPR) and the 46-kDa cation-dependent MPR (CD-MPR) are type I integral membrane glycoproteins that play a critical role in the intracellular delivery of newly synthesized mannose 6-phosphate (Man-6-P)-containing acid hydrolases to the lysosome. The extracytoplasmic region of the CI-MPR contains 15 contiguous domains, and the two high affinity ( approximately 1 nm) Man-6-P-binding sites have been mapped to domains 1-3 and 9, with essential residues localized to domains 3 and 9. Domain 5 of the CI-MPR exhibits significant sequence homology to domains 3 and 9 as well as to the CD-MPR. A structure-based sequence alignment was performed that predicts that domain 5 contains the four conserved key residues (Gln, Arg, Glu, and Tyr) identified as essential for carbohydrate recognition by the CD-MPR and domains 3 and 9 of the CI-MPR, but lacks two cysteine residues predicted to form a disulfide bond within the binding pocket. To determine whether domain 5 harbors a carbohydrate-binding site, a construct that encodes domain 5 alone (Dom5His) was expressed in Pichia pastoris. Microarray analysis using 30 different oligosaccharides demonstrated that Dom5His bound specifically to a Man-6-P-containing oligosaccharide (pentamannosyl 6-phosphate). Frontal affinity chromatography showed that the affinity of Dom5His for Man-6-P was approximately 300-fold lower (K(i) = 5.3 mm) than that observed for domains 1-3 and 9. The interaction affinity for the lysosomal enzyme beta-glucuronidase was also much lower (K(d) = 54 microm) as determined by surface plasmon resonance analysis. Taken together, these results demonstrate that the CI-MPR contains a third Man-6-P recognition site that is located in domain 5 and that exhibits lower affinity than the carbohydrate-binding sites present in domains 1-3 and 9.

Determination by electrospray mass spectrometry and 1H-NMR spectroscopy of primary structures of variously fucosylated neutral oligosaccharides based on the iso-lacto-N-octaose core

We have isolated a nonfucosylated and three variously fucosylated neutral oligosaccharides from human milk that are based on the iso-lacto-N-octaose core. Their structures were characterized by the combined use of electrospray mass spectrometry (ES-MS) and NMR spectroscopy. The branching pattern and blood group-related Lewis determinants, together with partial sequences and linkages of these oligosaccharides, were initially elucidated by high-sensitivity ES-MS/MS analysis, and then their full structure assignment was completed by methylation analysis and 1H-NMR. Three new structures were identified. The nonfucosylated iso-lacto-N-octaose, Galbeta1-3GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1-6[Galbeta1-3GlcNAcbeta1-3]Galbeta1-4Glc, has not previously been reported as an individual oligosaccharide. The monofucosylated and trifucosylated iso-lacto-N-octaose, Galbeta1-3GlcNAcbeta1-3Galbeta1-4(Fucalpha1-3) GlcNAcbeta1-6[Galbeta1-3GlcNAcbeta1-3]Galbeta1-4Glc and Galbeta1-3(Fucalpha1-4)GlcNAcbeta1-3Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-6[Galbeta1-3(Fucalpha1-4)GlcNAcbeta1-3]Galbeta1-4Glc, both containing an internal Lex epitope, are also novel structures.

On-line overpressure thin-layer chromatographic separation and electrospray mass spectrometric detection of glycolipids

On-line thin-layer chromatographic separation and electrospray mass spectrometry (TLC/ESI-MS) has been accomplished by direct linking of a commercial overpressure TLC instrument, OPLC 50, and a Q-TOF mass spectrometer. Mass spectrometric detection sensitivity and chromatographic resolution achieved by this configuration were assessed using acidic glycolipids as examples. Under the optimized conditions, a sensitivity of 5 pmol of glycosphingolipid was readily demonstrated for TLC/ESI-MS and 20 pmol for TLC/ESI-MS/MS production scanning to derive the saccharide sequence and long chain base/fatty acid composition of the ceramide. Initial preconditioning of TLC plates is necessary to achieve high sensitivity detection by reducing chemical background noise. Plates can be used repeatedly (at least 10 times) for analysis, although this may result in a minor reduction in TLC resolution. Following solvent development, separated components on the TLC plates can be detected in the conventional way by nondestructive staining or UV absorption or fluorescence and can be stored for on-line TLC/ESI-MS analysis at a later stage without reduction in mass spectrometric detection sensitivity and chromatographic resolution. Aspects for further improvement of OPLC instrumentation include use of narrower TLC plate dimensions and refined design of the eluate exit system.