Lectin-array blotting: profiling protein glycosylation in complex mixtures

Boltzmann Model Predicts Glycan Structures from Lectin Binding

Glycans are complex oligosaccharides that are involved in many diseases and biological processes. Unfortunately, current methods for determining glycan composition and structure (glycan sequencing) are laborious and require a high level of expertise. Here, we assess the feasibility of sequencing glycans based on their lectin binding fingerprints. By training a Boltzmann model on lectin binding data, we predict the approximate structures of 88 ± 7% of N-glycans and 87 ± 13% of O-glycans in our test set. We show that our model generalizes well to the pharmaceutically relevant case of Chinese hamster ovary (CHO) cell glycans. We also analyze the motif specificity of a wide array of lectins and identify the most and least predictive lectins and glycan features. These results could help streamline glycoprotein research and be of use to anyone using lectins for glycobiology.

A Boltzmann model predicts glycan structures from lectin binding

Glycans are complex oligosaccharides involved in many diseases and biological processes. Unfortunately, current methods for determining glycan composition and structure (glycan sequencing) are laborious and require a high level of expertise. Here, we assess the feasibility of sequencing glycans based on their lectin binding fingerprints. By training a Boltzmann model on lectin binding data, we predict the approximate structures of 88 ± 7% of N-glycans and 87 ± 13% of O-glycans in our test set. We show that our model generalizes well to the pharmaceutically relevant case of Chinese Hamster Ovary (CHO) cell glycans. We also analyze the motif specificity of a wide array of lectins and identify the most and least predictive lectins and glycan features. These results could help streamline glycoprotein research and be of use to anyone using lectins for glycobiology.

N-glycoproteins in Plant Cell Walls: A Survey

Cell walls are an extracellular compartment specific to plant cells, which are not found in animal cells. Their composition varies between cell types, plant species, and physiological states. They are composed of a great diversity of polymers, i.e., polysaccharides, proteins, and lignins. Cell wall proteins (CWPs) are major players involved in the plasticity of cell walls which support cell growth and differentiation, as well as adaptation to environmental changes. In order to reach the extracellular space, CWPs are transported through the secretory pathway where they may undergo post-translational modifications, including N-glycosylations on the Asn residues in specific motifs (Asn-X-Ser/Thr-X, with X≠Pro). This review aims at providing a survey of the present knowledge related to cell wall N-glycoproteins with (i) an overview of the experimental workflows, (ii) a selection of relevant articles dedicated to N-glycoproteomics, (iii) a description of the diversity of N-glycans, and (iv) a focus on the importance of N-glycans for CWP structure and/or function.

Quantification-Promoted Discovery of Glycosylated Exosomal PD-L1 as a Potential Tumor Biomarker

Exosomal programmed cell death ligand 1 (exoPD-L1) has emerged as a promising biomarker for cancer diagnosis and immunotherapy outcome prediction. However, the existing quantitation methods are incapable of addressing the heterogeneity of exoPD-L1 glycosylation, which has been demonstrated to be the institutional basis for PD-L1/PD-1 interaction and the crucial participant in inhibiting the activity of CD8⁺ T cells. Herein, an aptamer- and lectin-induced proximity ligation assay combined with quantitative real-time polymerase chain reaction for precise quantitation of glycosylated exoPD-L1 is developed. Leveraging the metabolism-free lectin labeling of glycosylation, the glycosylation-independent aptamer tagging of PD-L1, and excellent selectivity of dual-recognition, this method enables glycosylated exoPD-L1 quantitation with high sensitivity and selectivity in a wash-free manner. As a result, this method is able to distinguish the levels of glycosylated exoPD-L1 between healthy donors and cancer patients with sensitivity and specificity of 100%. Compared with the total circulating exoPD-L1 level, glycosylated exoPD-L1 is for the first time identified to be a more reliable biomarker for tumor diagnosis. Overall, this strategy holds a great potential for revealing the significance of exoPD-L1 glycosylation and converting glycosylated exoPD-L1 into a reliable clinical indicator.

One, two, many: Strategies to alter the number of carbohydrate binding sites of lectins

Carbohydrates are more than an energy-storage. They are ubiquitously found on cells and most proteins, where they encode biological information. Lectins bind these carbohydrates and are essential for translating the encoded information into biological functions and processes. Hundreds of lectins are known, and they are found in all domains of life. For half a century, researchers have been preparing variants of lectins in which the binding sites are varied. In this way, the traits of the lectins such as the affinity, avidity and specificity towards their ligands as well as their biological efficacy were changed. These efforts helped to unravel the biological importance of lectins and resulted in improved variants for biotechnological exploitation and potential medical applications. This review gives an overview on the methods for the preparation of artificial lectins and complexes thereof and how reducing or increasing the number of binding sites affects their function.

Role and therapeutic implications of protein glycosylation in neuroinflammation

The importance of glycosylation (post-translational attachment of glycan residues to proteins) in the context of neuroinflammation is only now beginning to be understood. Although the glycome is challenging to investigate due to its complexity, this field is gaining interest because of the emergence of novel analytical methods. These investigations offer the possibility of further understanding the molecular signature of disorders with underlying neuroinflammatory cascades. In this review, we portray the clinically relevant trends in glyconeurobiology and suggest glyco-related paths that could be targeted therapeutically to decrease neuroinflammation. A combinatorial insight from glycobiology and neurology can be harnessed to better understand neuroinflammatory-related conditions to identify relevant molecular targets.

Coupling Aptamer-based Protein Tagging with Metabolic Glycan Labeling for In Situ Visualization and Biological Function Study of Exosomal Protein-Specific Glycosylation

Exosomal glycoproteins play important roles in many physiological and pathological functions. Herein, we developed a dual labeling strategy based on a protein-specific aptamer tagging and metabolic glycan labeling for visualizing glycosylation of specific proteins on exosomes. The glycosylation of exosomal PD-L1 (exoPD-L1) was imaged in situ using intramolecular fluorescence resonance energy transfer (FRET) between fluorescent PD-L1 aptamers bound on exoPD-L1 and fluorescent tags on glycans introduced via metabolic glycan labeling. This method enables in situ visualization and biological function study of exosomal protein glycosylation. Exosomal PD-L1 glycosylation was confirmed to be required in interaction with PD-1 and participated in inhibiting of CD8⁺ T cell proliferation. This is an efficient and non-destructive method to study the presence and function of exosomal protein-specific glycosylation in situ, which provides a powerful tool for exosomal glycoproteomics research.

Proximity-Induced Hybridization Chain Reaction-Based Photoacoustic Imaging System for Amplified Visualization Protein-Specific Glycosylation in Mice

Glycosylation is a key cellular mechanism that regulates several physiological and pathological functions. Therefore, identification and characterization of specific-protein glycosylation in vivo are highly desirable for studying glycosylation-related pathology and developing personalized theranostic modalities. Herein, we demonstrated a photoacoustic (PA) nanoprobe based on the proximity-induced hybridization chain reaction (HCR) for amplified visual detection of protein-specific glycosylation in vivo. Two kinds of functional DNA probes were designed. A glycan probe (DBCO-GP) was attached to glycans through metabolic oligosaccharide engineering (MOE) and protein probe (PP)-targeted proteins by aptamer recognition. Proximity-induced hybridization of the complementary domain between the two kinds of probes promoted conformational changes in the protein probes and in situ release of the HCR initiator domain. Gold nanoparticles (AuNPs) modified by complementary sequences (Au-H1 and Au-H2) self-assembled into Au aggregates via the HCR, thereby converting DNA signals to photoacoustic signals. Due to the high contrast and deep penetration of photoacoustic imaging, this strategy enabled in situ detection of Mucin 1 (MUC1)-specific glycosylation in mice with breast cancer and successfully monitored its dynamic states during tunicamycin treatment. This imaging technique provides a powerful platform for studying the effects of glycosylation on the protein structure and function, which helps to elucidate its role in disease processes.

Glycosylation Biomarkers Associated with Age-Related Diseases and Current Methods for Glycan Analysis

Ageing is a complex process which implies the accumulation of molecular, cellular and organ damage, leading to an increased vulnerability to disease. In Western societies, the increase in the elderly population, which is accompanied by ageing-associated pathologies such as cardiovascular and mental diseases, is becoming an increasing economic and social burden for governments. In order to prevent, treat and determine which subjects are more likely to develop these age-related diseases, predictive biomarkers are required. In this sense, some studies suggest that glycans have a potential role as disease biomarkers, as they modify the functions of proteins and take part in intra- and intercellular biological processes. As the glycome reflects the real-time status of these interactions, its characterisation can provide potential diagnostic and prognostic biomarkers for multifactorial diseases. This review gathers the alterations in protein glycosylation profiles that are associated with ageing and age-related diseases, such as cancer, type 2 diabetes mellitus, metabolic syndrome and several chronic inflammatory diseases. Furthermore, the review includes the available techniques for the determination and characterisation of glycans, such as liquid chromatography, electrophoresis, nuclear magnetic resonance and mass spectrometry.

130 years of Plant Lectin Research

Lectins are proteins with diverse molecular structures that share the ability to recognize and bind specifically and reversibly to carbohydrate structures without changing the carbohydrate moiety. The history of lectins started with the discovery of ricin about 130 years ago but since then our understanding of lectins has dramatically changed. Over the years the research focus was shifted from 'the characterization of carbohydrate-binding proteins' to 'understanding the biological function of lectins'. Nowadays plant lectins attract a lot of attention especially because of their potential for crop improvement and biomedical research, as well as their application as tools in glycobiology. The present review aims to give an overview of plant lectins and their applications, and how the field evolved in the last decades.

Application of Lectin Microarrays for Biomarker Discovery

Many proteins in living organisms are glycosylated. As their glycan patterns exhibit protein-, cell-, and tissue-specific heterogeneity, changes in the glycosylation levels could serve as useful indicators of various pathological and physiological states. Thus, the identification of glycoprotein biomarkers from specific changes in the glycan profiles of glycoproteins is a trending field. Lectin microarrays provide a new glycan analysis platform, which enables rapid and sensitive analysis of complex glycans without requiring the release of glycans from the protein. Recent developments in lectin microarray technology enable high-throughput analysis of glycans in complex biological samples. In this review, we will discuss the basic concepts and recent progress in lectin microarray technology, the application of lectin microarrays in biomarker discovery, and the challenges and future development of this technology. Given the tremendous technical advancements that have been made, lectin microarrays will become an indispensable tool for the discovery of glycoprotein biomarkers.

O-Benzylhydroxylamine (BHA) as a Cleavable Tag for Isolation and Purification of Reducing Glycans

Glycoscience has been recognized as an important area in biomedical research. Currently, a major obstacle for glycoscience study is the lack of diverse, biomedically relevant, and complex glycans in quantities sufficient for exploring their structural and functional aspects. Complementary to chemoenzymatic synthesis, natural glycans could serve as a great source of biomedically relevant glycans if they are available in sufficient quantities. We have recently developed oxidative release of natural glycans (ORNG) for large-scale release of N-glycans as free reducing glycans. While free reducing glycans can be readily derivatized with ultraviolet or fluorescent tags for high-performance liquid chromatography (HPLC) and mass spectrometry (MS) analysis, it is difficult to remove tags for the regeneration of free reducing glycans without affecting the structural integrity of glycans. To address this inconvenience, we explored the use of a cleavable tag, O-benzylhydroxylamine (BHA). Free reducing glycans are easily and efficiently labeled with BHA under mild conditions, enabling UV detection during HPLC purification. Individual glycan-BHA conjugates can then be separated using multidimensional HPLC and characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and MS/MS. The BHA tag can then be easily removed by palladium-on-carbon (Pd/C)-catalyzed hydrogenation to efficiently regenerate free reducing glycans with little effect on glycan structures. This procedure provides a simple and straightforward way to tag free reducing glycans for purification at a preparative scale using multidimensional HPLC and subsequently recover purified free reducing glycans.

Stable Isotope Sequential Derivatization for Linkage-Specific Analysis of Sialylated N-Glycan Isomers by MS

Sialylated N-glycans play pivotal role in several important biological and pathological processes. Their sialyl-linkage isomers, mostly α-2,3- and α-2,6-linked, act differently during the cellular events and several diseases. While mass spectrometry (MS) technology is a powerful tool in N-glycome analysis, it still suffers from an inability to distinguish linkage isomers of native N-glycans. Herein, we described a sequential selective derivatization method, by which α-2,6- and α-2,3-linked sialic acids are sequentially labeled with methylamide incorporated with a different stable isotope. Isobaric labeling avoids inducing bias in ionization efficiency and chromatographic behavior. In optimized reaction conditions, high derivatization selectivity (∼99%) was achieved for both α-2,3- and α-2,6-linked sialic acid. High accuracy of quantitation within a dynamic range of 2 orders of magnitude and high reproducibility (CV < 20%, n = 3) were demonstrated using standard glycans and multisialylated N-glycans. Finally, this method was applied in profiling the N-glycome of serum from CRC patients, where a level of six sialyl-linkage isomers were found to be altered significantly compared with that from healthy individuals.

N-Glycosylation of mollusk hemocyanins contributes to their structural stability and immunomodulatory properties in mammals

Hemocyanins are widely used as carriers, adjuvants, and nonspecific immunostimulants in cancer because they promote Th1 immunity in mammals. Hemocyanins also interact with glycan-recognizing innate immune receptors on antigen-presenting cells, such as the C-type lectin immune receptors mannose receptor (MR), macrophage galactose lectin (MGL), and the Toll-like receptors (TLRs), stimulating proinflammatory cytokine secretion. However, the role of N-linked oligosaccharides on the structural and immunological properties of hemocyanin is unclear. Mollusk hemocyanins, such as Concholepas concholepas (CCH), Fissurella latimarginata (FLH), and Megathura crenulata (KLH), are oligomeric glycoproteins with complex dodecameric quaternary structures and heterogeneous glycosylation patterns, primarily consisting of mannose-rich N-glycans. Here, we report that enzyme-catalyzed N-deglycosylation of CCH, FLH, and KLH disrupts their quaternary structure and impairs their immunogenic effects. Biochemical analyses revealed that the deglycosylation does not change hemocyanin secondary structure but alters their refolding mechanism and dodecameric structure. Immunochemical analyses indicated decreased binding of N-deglycosylated hemocyanins to the MR and MGL receptors and TLR4 and reduced endocytosis concomitant with an impaired production of tumor necrosis factor α, and interleukins 6 and 12 (IL-6 and IL-12p40, respectively) in macrophages. Evaluating the function of N-deglycosylated hemocyanins in the humoral immune response and their nonspecific antitumor effects in the B16F10 melanoma model, we found that compared with native hemocyanins N-deglycosylated hemocyanins elicited reduced antibody titers, as well as partially diminished antitumor effects and altered carrier activities. In conclusion, the glycan content of hemocyanins is, among other structural characteristics, critically required for their immunological activities and should be considered in biomedical applications.

Lectin biosensors in cancer glycan biomarker detection

Cancer has high incidence and it will continue to increase over the next decades. Detection and quantification of cancer-associated biomarkers is frequently carried out for diagnosis, prognosis and treatment monitoring at various disease stages. It is well-known that glycosylation profiles change significantly during oncogenesis. Aberrant glycans produced during tumorigenesis are, therefore, valuable molecules for detection and characterization of cancer, and for therapeutic design and monitoring. Although glycoproteomics has benefited from the development of analytical tools such as high performance liquid chromatography, two-dimensional gel and capillary electrophoresis and mass spectrometry, these approaches are not well suited for rapid point-of-care (POC) testing easily performed by medical staff. Lectins are biomolecules found in nature with specific affinities toward particular glycan structures and bind them thus forming a relatively strong complex. Because of this characteristic, lectins have been used in analytical techniques for the selective capture or separation of certain glycans in complex samples, namely, in lectin affinity chromatography, or to characterize glycosylation profiles in diverse clinical situations, using lectin microarrays. Lectin-based biosensors have been developed for the detection of specific aberrant and cancer-associated glycostructures to aid diagnosis, prognosis and treatment assessment of these patients. The attractive features of biosensors, such as portability and simple use make them highly suitable for POC testing. Recent developments in lectin biosensors, as well as their potential and pitfalls in cancer glycan biomarker detection, are presented in this chapter.

Assessing the role of surface glycans of extracellular vesicles on cellular uptake

Extracellular vesicles (EVs) are important mediators of cell-cell communication in a broad variety of physiological contexts. However, there is ambiguity around the fundamental mechanisms by which these effects are transduced, particularly in relation to their uptake by recipient cells. Multiple modes of cellular entry have been suggested and we have further explored the role of glycans as potential determinants of uptake, using EVs from the murine hepatic cell lines AML12 and MLP29 as independent yet comparable models. Lectin microarray technology was employed to define the surface glycosylation patterns of EVs. Glycosidases PNGase F and neuraminidase which cleave N-glycans and terminal sialic acids, respectively, were used to analyze the relevance of these modifications to EV surface glycans on the uptake of fluorescently labelled EVs by a panel of cells representing a variety of tissues. Flow cytometry revealed an increase in affinity for EVs modified by both glycosidase treatments. High-content screening exhibited a broader range of responses with different cell types preferring different vesicle glycosylation states. We also found differences in vesicle charge after treatment with glycosidases. We conclude that glycans are key players in the tuning of EV uptake, through charge-based effects, direct glycan recognition or both, supporting glycoengineering as a toolkit for therapy development.

Proteomic Analysis of Extracellular Vesicles for Cancer Diagnostics

Extracellular vesicles (EVs) including exosomes and microvesicles are lipid bilayer-encapsulated nanoparticles released by cells, ranging from 40 nm to several microns in diameter. Biological cargoes including proteins, RNAs, and DNAs can be ferried by EVs to neighboring and distant cells via biofluids, serving as a means of cell-to-cell communication under normal and pathological conditions, especially cancers. On the other hand, EVs have been investigated as a novel "information capsule" for early disease detection and monitoring via liquid biopsy. This review summarizes current advancements in EV subtype characterization, cancer EV capture, proteomic analysis technologies, as well as possible EV-based multiomics for cancer diagnostics.

Glycosylation of extracellular vesicles: current knowledge, tools and clinical perspectives

It is now acknowledged that extracellular vesicles (EVs) are important effectors in a vast number of biological processes through intercellular transfer of biomolecules. Increasing research efforts in the EV field have yielded an appreciation for the potential role of glycans in EV function. Indeed, recent reports show that the presence of glycoconjugates is involved in EV biogenesis, in cellular recognition and in the efficient uptake of EVs by recipient cells. It is clear that a full understanding of EV biology will require researchers to focus also on EV glycosylation through glycomics approaches. This review outlines the major glycomics techniques that have been applied to EVs in the context of the recent findings. Beyond understanding the mechanisms by which EVs mediate their physiological functions, glycosylation also provides opportunities by which to engineer EVs for therapeutic and diagnostic purposes. Studies characterising the glycan composition of EVs have highlighted glycome changes in various disease states, thus indicating potential for EV glycans as diagnostic markers. Meanwhile, glycans have been targeted as molecular handles for affinity-based isolation in both research and clinical contexts. An overview of current strategies to exploit EV glycosylation and a discussion of the implications of recent findings for the burgeoning EV industry follows the below review of glycomics and its application to EV biology.

Lectin-Array Blotting

Aberrant protein glycosylation is a hallmark of cancer, infectious diseases, and autoimmune or neurodegenerative disorders. Unlocking the potential of glycans as disease markers will require rapid and unbiased glycoproteomics methods for glycan biomarker discovery. The present method is a facile and rapid protocol for qualitative analysis of protein glycosylation in complex biological mixtures. While traditional lectin arrays only provide an average signal for the glycans in the mixture, which is usually dominated by the most abundant proteins, our method provides individual lectin binding profiles for all proteins separated in the gel electrophoresis step. Proteins do not have to be excised from the gel for subsequent analysis via the lectin array but are transferred by contact diffusion from the gel to a glass slide presenting multiple copies of printed lectin arrays. Fluorescently marked glycoproteins are trapped by the printed lectins via specific carbohydrate-lectin interactions and after a washing step their binding profile with up to 20 lectin probes is analyzed with a fluorescent scanner. The method produces the equivalent of 20 lectin blots in a single experiment, giving detailed insight into the binding epitopes present in the fractionated proteins. © 2017 by John Wiley & Sons, Inc.

Label-Free Discovery Array Platform for the Characterization of Glycan Binding Proteins and Glycoproteins

The identification of carbohydrate-protein interactions is central to our understanding of the roles of cell-surface carbohydrates (the glycocalyx), fundamental for cell-recognition events. Therefore, there is a need for fast high-throughput biochemical tools to capture the complexity of these biological interactions. Here, we describe a rapid method for qualitative label-free detection of carbohydrate-protein interactions on arrays of simple synthetic glycans, more complex natural glycosaminoglycans (GAG), and lectins/carbohydrate binding proteins using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The platform can unequivocally identify proteins that are captured from either purified or complex sample mixtures, including biofluids. Identification of proteins bound to the functionalized array is achieved by analyzing either the intact protein mass or, after on-chip proteolytic digestion, the peptide mass fingerprint and/or tandem mass spectrometry of selected peptides, which can yield highly diagnostic sequence information. The platform described here should be a valuable addition to the limited analytical toolbox that is currently available for glycomics.

Novel interactions of complex carbohydrates with peanut (PNA), Ricinus communis (RCA-I), Sambucus nigra (SNA-I) and wheat germ (WGA) agglutinins as revealed by the binding specificities of these lectins towards mucin core-2 O-linked and N-linked glycans and related structures

Plant lectins through their multivalent quaternary structures bind intrinsically flexible oligosaccharides. They recognize fine structural differences in carbohydrates and interact with different sequences in mucin core 2 or complex-type N-glycan chain and also in healthy and malignant tissues. They are used in characterizing cellular and extracellular glycoconjugates modified in pathological processes. We study here, the complex carbohydrate-lectin interactions by determining the effects of substituents in mucin core 2 tetrasaccharide Galβ1-4GlcNAcβ1-6(Galβ1-3)GalNAcα-O-R and fetuin glycopeptides on their binding to agarose-immobilized lectins PNA, RCA-I, SNA-I and WGA. Briefly, in mucin core 2 tetrasaccharide (i) structures modified by α2-3/6-Sialyl LacNAc, LewisX and α1-3-Galactosyl LacNAc resulted in regular binding to PNA whereas compounds with 6-sulfo LacNAc displayed no-binding; (ii) strucures bearing α2-6-sialyl 6-sulfo LacNAc, or 6-sialyl LacdiNAc carbohydrates displayed strong binding to SNA-I; (iii) structures with α2-3/6-sialyl, α1-3Gal LacNAc or LewisX were non-binder to RCA-I and compounds with 6-sulfo LacNAc only displayed weak binding; (iv) structures containing LewisX, 6-Sulfo LewisX, α2-3/6-sialyl LacNAc, α2-3/6-sialyl 6-sulfo LacNAc and GalNAc Lewis-a were non-binding to WGA, those with α1-2Fucosyl, α1-3-Galactosyl LacNAc, α2-3-sialyl T-hapten plus 3'/6'sulfo LacNAc displayed weak binding, and compounds with α2-3-sialyl T-hapten, α2.6-Sialyl LacdiNAc, α2-3-sialyl D-Fucβ1-3 GalNAc and Fucα-1-2 D-Fucβ-1-3GalNAc displaying regular binding and GalNAc LewisX and LacdiNAc plus D-Fuc β-1-3 GalNAcα resulting in tight binding. RCA-I binds Fetuin triantennary asialoglycopeptide 100 % after α-2-3 and 25 % after α-2-6 sialylation, 30 % after α-1-2 and 100 % after α-1-3 fucosylation, and 50 % after α-1-3 galactosylation. WGA binds 3-but not 6-Fucosyl chitobiose core. Thus, information on the influence of complex carbohydrate chain constituents on lectin binding is apparently essential for the potential application of lectins in glycoconjugate research.

Role of lectin microarrays in cancer diagnosis

The majority of cell differentiation associated tumor markers reported to date are either glycoproteins or glycolipids. Despite there being a large number of glycoproteins reported as candidate markers for various cancers, only a handful are approved by the US Food and Drug Administration. Lectins, which bind to the glycan part of the glycoproteins, can be exploited to identify aberrant glycosylation patterns, which in turn would help in enhancing the specificity of cancer diagnosis. Although conventional techniques such as HPLC and MS have been instrumental in performing the glycomic analyses, these techniques lack multiplexity. Lectin microarrays have proved to be useful in studying multiple lectin-glycan interactions in a single experiment and, with the advances made in the field, hold a promise of enabling glycomic profiling of cancers in a fast and efficient manner.

Methods for the absolute quantification of N-glycan biomarkers

BACKGROUND

Many treatment options especially for cancer show a low efficacy for the majority of patients demanding improved biomarker panels for patient stratification. Changes in glycosylation are a hallmark of many cancers and inflammatory diseases and show great potential as clinical disease markers. The large inter-subject variability in glycosylation due to hereditary and environmental factors can complicate rapid transfer of glycan markers into the clinical practice but also presents an opportunity for personalized medicine.

SCOPE OF REVIEW

This review discusses opportunities of glycan biomarkers in personalized medicine and reviews the methodology for N-glycan analysis with a specific focus on methods for absolute quantification.

MAJOR CONCLUSIONS

The entry into the clinical practice of glycan markers is delayed in large part due to a lack of adequate methodology for the precise and robust quantification of protein glycosylation. Only absolute glycan quantification can provide a complete picture of the disease related changes and will provide the method robustness required by clinical applications.

GENERAL SIGNIFICANCE

Glycan biomarkers have a huge potential as disease markers for personalized medicine. The use of stable isotope labeled glycans as internal standards and heavy-isotope labeling methods will provide the necessary method precision and robustness acceptable for clinical use. This article is part of a Special Issue entitled "Glycans in personalized medicine" Guest Editor: Professor Gordan Lauc.

Lectin-based protein microarray analysis of differences in serum alpha-2-macroglobulin glycosylation between patients with colorectal cancer and persons without cancer

Glycosylation is co- and posttranslational modifications affecting proteins. The glycopattern changes are associated with changes in biological function and are involved in many diseases including cancer. We present the lectin-based protein microarray method enabling determination of differences in protein glycosylation. The method involves isolation of targeted protein from samples by immunoprecipitation, spotting of protein from multiple samples into arrays on a microarray slide, incubation with set of biotinylated lectins, the reaction with fluorescent conjugate of streptavidin, and detection of fluorescent intensities by microarray scanner. Lectin-based protein microarray was applied in investigation of differences in alpha-2-macroglobulin (α2M) glycosylation isolated from sera samples of healthy persons and patients with colorectal cancer (CC). From 14 lectins used in analysis, statistically significant differences (Student's t-test, P < 0.05) between two groups of samples (persons without cancer and CC patients) were found for 5 of them. α2M molecules isolated from sera of CC patients have higher content of α2,6 sialic acid, N-acetylglucosamine and mannose residues, and tri-/tetraantennary complex type high-mannose N-glycans. A novel lectin-based protein microarray developed and described can serve as a suitable analytical technique for sensitive, simple, fast, and high-throughput determination of differences in protein glycosylation isolated from serum or other samples.

Fast epitope mapping for the anti-MUC1 monoclonal antibody by combining a one-bead-one-glycopeptide library and a microarray platform

Anti-MUC1 monoclonal antibodies (mAbs) are powerful tools that can be used to recognize cancer-related MUC1 molecules, the O-glycosylation status of which is believed to affect binding affinity. We demonstrate the feasibility of using a rapid screening methodology to elucidate those effects. The approach involves i) "one-bead-one-compound"-based preparation of bilayer resins carrying glycopeptides on the shell and mass-tag tripeptides coding O-glycan patterns in the core, ii) on-resin screening with an anti-MUC1 mAb, iii) separating positive resins by utilizing secondary antibody conjugation with magnetic beads, and (iv) decoding the mass-tag that is detached from the positive resins pool by using mass spectrometric analysis. We tested a small library consisting of 27 MUC1 glycopeptides with different O-glycosylations against anti-MUC1 mAb clone VU-3C6. Qualitative mass-tag analysis showed that increasing the number of glycans leads to an increase in the binding affinity. Six glycopeptides selected from the library were validated by using a microarray-based assay. Our screening provides valuable information on O-glycosylations of epitopes leading to high affinity with mAb.

Analysis of microarrays by MALDI-TOF MS

Ligand libraries can be printed onto a sandwich composed of activated lipids embedded in a hydrophobic layer conjugated to an indium-tin oxide (ITO) surface. Arrays produced this way can be analyzed by fluorescence spectroscopy and mass spectrometry. Applications include the assignment of enzyme specificity, the profiling of glycoforms and the identification of lectins.

Molecular recognition of complex-type biantennary N-glycans by protein receptors: a three-dimensional view on epitope selection by NMR

The current surge in defining glycobiomarkers by applying lectins rekindles interest in definition of the sugar-binding sites of lectins at high resolution. Natural complex-type N-glycans can present more than one potential binding motif, posing the question of the actual mode of interaction when interpreting, for example, lectin array data. By strategically combining N-glycan preparation with saturation-transfer difference NMR and modeling, we illustrate that epitope recognition depends on the structural context of both the sugar and the lectin (here, wheat germ agglutinin and a single hevein domain) and cannot always be predicted from simplified model systems studied in the solid state. We also monitor branch-end substitutions by this strategy and describe a three-dimensional structure that accounts for the accommodation of the α2,6-sialylated terminus of a biantennary N-glycan by viscumin. In addition, we provide a structural explanation for the role of terminal α2,6-sialylation in precluding the interaction of natural N-glycans with lectin from Maackia amurensis . The approach described is thus capable of pinpointing lectin-binding motifs in natural N-glycans and providing detailed structural explanations for lectin selectivity.

Binding sugars: from natural lectins to synthetic receptors and engineered neolectins

The large diversity and complexity of glycan structures together with their crucial role in many biological or pathological processes require the development of new high-throughput techniques for analyses. Lectins are classically used for characterising, imaging or targeting glycoconjugates and, when printed on microarrays, they are very useful tools for profiling glycomes. Development of recombinant lectins gives access to reliable and reproducible material, while engineering of new binding sites on existing scaffolds allows tuning of specificity. From the accumulated knowledge on protein-carbohydrate interactions, it is now possible to use nucleotide and peptide (bio)synthesis for producing new carbohydrate-binding molecules. Such a biomimetic approach can also be addressed by boron chemistry and supra-molecular chemistry for the design of fully artificial glycosensors.

Ultrasensitive impedimetric lectin based biosensor for glycoproteins containing sialic acid

We report on an ultrasensitive label-free lectin-based impedimetric biosensor for the determination of the sialylated glycoproteins fetuin and asialofetuin. A sialic acid binding agglutinin from Sambucus nigra I was covalently immobilised on a mixed self-assembled monolayer (SAM) consisting of 11-mercaptoundecanoic acid and 6-mercaptohexanol. Poly(vinyl alcohol) was used as a blocking agent. The sensor layer was characterised by atomic force microscopy, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The biosensor exhibits a linear range that spans 7 orders of magnitude for both glycoproteins, with a detection limit as low as 0.33 fM for fetuin and 0.54 fM for asialofetuin. We also show, by making control experiments with oxidised asialofetuin, that the biosensor is capable of quantitatively detecting changes in the fraction of sialic acid on glycoproteins. We conclude that this work lays a solid foundation for future applications of such a biosensor in terms of the diagnosis of diseases such as chronic inflammatory rheumatoid arthritis, genetic disorders and cancer, all of which are associated with aberrant glycosylation of protein biomarkers.