Glycan microarrays for screening sialyltransferase specificities

Glycan microarrays from construction to applications

Through their specific interactions with proteins, cellular glycans play key roles in a wide range of physiological and pathological processes. One of the main goals of research in the areas of glycobiology and glycomedicine is to understand glycan-protein interactions at the molecular level. Over the past two decades, glycan microarrays have become powerful tools for the rapid evaluation of interactions between glycans and proteins. In this review, we briefly describe methods used for the preparation of glycan probes and the construction of glycan microarrays. Next, we highlight applications of glycan microarrays to rapid profiling of glycan-binding patterns of plant, animal and pathogenic lectins, as well as other proteins. Finally, we discuss other important uses of glycan microarrays, including the rapid analysis of substrate specificities of carbohydrate-active enzymes, the quantitative determination of glycan-protein interactions, discovering high-affinity or selective ligands for lectins, and identifying functional glycans within cells. We anticipate that this review will encourage researchers to employ glycan microarrays in diverse glycan-related studies.

Elucidating Human Milk Oligosaccharide biosynthetic genes through network-based multi-omics integration

Human Milk Oligosaccharides (HMOs) are abundant carbohydrates fundamental to infant health and development. Although these oligosaccharides were discovered more than half a century ago, their biosynthesis in the mammary gland remains largely uncharacterized. Here, we use a systems biology framework that integrates glycan and RNA expression data to construct an HMO biosynthetic network and predict glycosyltransferases involved. To accomplish this, we construct models describing the most likely pathways for the synthesis of the oligosaccharides accounting for >95% of the HMO content in human milk. Through our models, we propose candidate genes for elongation, branching, fucosylation, and sialylation of HMOs. Our model aggregation approach recovers 2 of 2 previously known gene-enzyme relations and 2 of 3 empirically confirmed gene-enzyme relations. The top genes we propose for the remaining 5 linkage reactions are consistent with previously published literature. These results provide the molecular basis of HMO biosynthesis necessary to guide progress in HMO research and application with the goal of understanding and improving infant health and development.

Cellular and Molecular Engineering of Glycan Sialylation in Heterologous Systems

Glycans have been shown to play a key role in many biological processes, such as signal transduction, immunogenicity, and disease progression. Among the various glycosylation modifications found on cell surfaces and in biomolecules, sialylation is especially important, because sialic acids are typically found at the terminus of glycans and have unique negatively charged moieties associated with cellular and molecular interactions. Sialic acids are also crucial for glycosylated biopharmaceutics, where they promote stability and activity. In this regard, heterogenous sialylation may produce variability in efficacy and limit therapeutic applications. Homogenous sialylation may be achieved through cellular and molecular engineering, both of which have gained traction in recent years. In this paper, we describe the engineering of intracellular glycosylation pathways through targeted disruption and the introduction of carbohydrate active enzyme genes. The focus of this review is on sialic acid-related genes and efforts to achieve homogenous, humanlike sialylation in model hosts. We also discuss the molecular engineering of sialyltransferases and their application in chemoenzymatic sialylation and sialic acid visualization on cell surfaces. The integration of these complementary engineering strategies will be useful for glycoscience to explore the biological significance of sialic acids on cell surfaces as well as the future development of advanced biopharmaceuticals.

Cell-Free Synthetic Glycobiology: Designing and Engineering Glycomolecules Outside of Living Cells

Glycans and glycosylated biomolecules are directly involved in almost every biological process as well as the etiology of most major diseases. Hence, glycoscience knowledge is essential to efforts aimed at addressing fundamental challenges in understanding and improving human health, protecting the environment and enhancing energy security, and developing renewable and sustainable resources that can serve as the source of next-generation materials. While much progress has been made, there remains an urgent need for new tools that can overexpress structurally uniform glycans and glycoconjugates in the quantities needed for characterization and that can be used to mechanistically dissect the enzymatic reactions and multi-enzyme assembly lines that promote their construction. To address this technology gap, cell-free synthetic glycobiology has emerged as a simplified and highly modular framework to investigate, prototype, and engineer pathways for glycan biosynthesis and biomolecule glycosylation outside the confines of living cells. From nucleotide sugars to complex glycoproteins, we summarize here recent efforts that harness the power of cell-free approaches to design, build, test, and utilize glyco-enzyme reaction networks that produce desired glycomolecules in a predictable and controllable manner. We also highlight novel cell-free methods for shedding light on poorly understood aspects of diverse glycosylation processes and engineering these processes toward desired outcomes. Taken together, cell-free synthetic glycobiology represents a promising set of tools and techniques for accelerating basic glycoscience research (e.g., deciphering the "glycan code") and its application (e.g., biomanufacturing high-value glycomolecules on demand).

A Glycan Array-Based Assay for the Identification and Characterization of Plant Glycosyltransferases

Growing plants with modified cell wall compositions is a promising strategy to improve resistance to pathogens, increase biomass digestibility, and tune other important properties. In order to alter biomass architecture, a detailed knowledge of cell wall structure and biosynthesis is a prerequisite. We report here a glycan array-based assay for the high-throughput identification and characterization of plant cell wall biosynthetic glycosyltransferases (GTs). We demonstrate that different heterologously expressed galactosyl-, fucosyl-, and xylosyltransferases can transfer azido-functionalized sugar nucleotide donors to selected synthetic plant cell wall oligosaccharides on the array and that the transferred monosaccharides can be visualized "on chip" by a 1,3-dipolar cycloaddition reaction with an alkynyl-modified dye. The opportunity to simultaneously screen thousands of combinations of putative GTs, nucleotide sugar donors, and oligosaccharide acceptors will dramatically accelerate plant cell wall biosynthesis research.

Golgi compartments enable controlled biomolecular assembly using promiscuous enzymes

The synthesis of eukaryotic glycans - branched sugar oligomers attached to cell-surface proteins and lipids - is organized like a factory assembly line. Specific enzymes within successive compartments of the Golgi apparatus determine where new monomer building blocks are linked to the growing oligomer. These enzymes act promiscuously and stochastically, causing microheterogeneity (molecule-to-molecule variability) in the final oligomer products. However, this variability is tightly controlled: a given eukaryotic protein type is typically associated with a narrow, specific glycan oligomer profile. Here, we use ideas from the mathematical theory of self-assembly to enumerate the enzymatic causes of oligomer variability and show how to eliminate each cause. We rigorously demonstrate that cells can specifically synthesize a larger repertoire of glycan oligomers by partitioning promiscuous enzymes across multiple Golgi compartments. This places limits on biomolecular assembly: glycan microheterogeneity becomes unavoidable when the number of compartments is limited, or enzymes are excessively promiscuous.

Multivalent glycan arrays

Glycan microarrays have become a powerful technology to study biological processes, such as cell-cell interaction, inflammation, and infections. Yet, several challenges, especially in multivalent display, remain. In this introductory lecture we discuss the state-of-the-art glycan microarray technology, with emphasis on novel approaches to access collections of pure glycans and their immobilization on surfaces. Future directions to mimic the natural glycan presentation on an array format, as well as in situ generation of combinatorial glycan collections, are discussed.

Glycan Array Technology

Glycan (or carbohydrate) arrays have become an essential tool in glycomics, providing fast and high-throughput data on protein-carbohydrate interactions with small amounts of carbohydrate ligands. The general concepts of glycan arrays have been adopted from other microarray technologies such as those used for nucleic acid and proteins. However, carbohydrates have presented their own challenges, in particular in terms of access to glycan probes, linker attachment chemistries and analysis, which will be reviewed in this chapter. As more and more glycan probes have become available through chemical and enzymatic synthesis and robust linker chemistries have been developed, the applications of glycan arrays have dramatically increased over the past 10 years, which will be illustrated with recent examples.

On-chip biosynthesis of GM1 pentasaccharide-related complex glycans

A novel strategy for on-chip enzymatic glycosylation of complex glycans and direct analysis of glycan-related interactions is reported.

Sialyltransferase-Based Chemoenzymatic Histology for the Detection of N- and O-Glycans

Profiling specific glycans in histopathological samples is hampered by the lack of selective and sensitive tools for their detection. Here, we report on the development of chemoenzymatic histology of membrane polysaccharide (CHoMP)-based methods for the detection of O- and N-linked glycans on tissue sections via the use of sialyltransferases ST3Gal1 and ST6Gal1, respectively. Combining these two methods, we developed tandem labeling and double labeling strategies that permit the detection of unsialylated and sialylated glycans or the detection of O- and N-linked glycans on the same tissue section, respectively. We applied these methods to screen murine tissue specimens, human multiple-organ cancer arrays, and lymphoma and prostate cancer arrays. Using tandem labeling with ST6Gal1 to analyze N-glycans in a prostate cancer array, we found striking differences in expression patterns of both sialylated and unsialylated N-glycans between cancerous and healthy samples. Such differences were also observed between normal tissue from healthy donors and healthy tissue adjacent to tumors. Our double labeling technique identified significant differences in unsialylated O-glycans between B-cell and T-cell lymphomas and between B-cell lymphomas and normal adjacent lymph nodes. Remarkable differences were also detected between adjacent lymph nodes and spleen tissue samples. These new chemoenzymatic histology methods therefore provide valuable tools for the analysis of glycans in clinically relevant tissue samples.

Analysis of binding properties of pathogens and toxins using multivalent glycan microarrays

Pathogens infect hosts often through initial binding of their cell surface lectins to glycans expressed on the exterior of host cells. Thus, methods to evaluate the glycan-binding properties of pathogens are of great importance. Because of the multivalent nature of interactions of pathogens with glycans, the ability to assess the glycan density-dependent binding of pathogens is particularly important. In this study, we developed a facile technique to construct multivalent carbohydrate microarrays through immobilization of unmodified glycans on multivalent hydrazide-derivatized glass surfaces. This immobilization strategy does not require the use of multivalent glycoconjugates, which are typically prepared by using multistep sequences. The results of analysis of microarray images, obtained after incubation of multivalent glycan microarrays with cholera toxin B and pathogens such as uropathogenic E. coli and H. pylori, show that the binding affinities of toxins and pathogens for glycans are highly glycan density-dependent. Specifically, toxins and pathogens bind to glycans more strongly as the valency of the glycans on the microarrays is increased from 1 to 4. It is anticipated that the newly developed immobilization method will be applicable to the preparation of multivalent carbohydrate microarrays that are employed to evaluate multivalent glycan binding properties of a variety of pathogens and toxins.

Microarrays constructed by immobilizing free glycans on multivalent hydrazide-coated surfaces were applied to evaluate multivalent glycan binding properties of pathogens.

Design starch: stochastic modeling of starch granule biogenesis

Starch is the most widespread and abundant storage carbohydrate in plants and the main source of carbohydrate in the human diet. Owing to its remarkable properties and commercial applications, starch is still of growing interest. Its unique granular structure made of intercalated layers of amylopectin and amylose has been unraveled thanks to recent progress in microscopic imaging, but the origin of such periodicity is still under debate. Both amylose and amylopectin are made of linear chains of α-1,4-bound glucose residues, with branch points formed by α-1,6 linkages. The net difference in the distribution of chain lengths and the branching pattern of amylose (mainly linear), compared with amylopectin (racemose structure), leads to different physico-chemical properties. Amylose is an amorphous and soluble polysaccharide, whereas amylopectin is insoluble and exhibits a highly organized structure of densely packed double helices formed between neighboring linear chains. Contrarily to starch degradation that has been investigated since the early 20th century, starch production is still poorly understood. Most enzymes involved in starch growth (elongation, branching, debranching, and partial hydrolysis) are now identified. However, their specific action, their interplay (cooperative or competitive), and their kinetic properties are still largely unknown. After reviewing recent results on starch structure and starch growth and degradation enzymatic activity, we discuss recent results and current challenges for growing polysaccharides on granular surface. Finally, we highlight the importance of novel stochastic models to support the analysis of recent and complex experimental results, and to address how macroscopic properties emerge from enzymatic activity and structural rearrangements.

The Glycan Microarray Story from Construction to Applications

Not only are glycan-mediated binding processes in cells and organisms essential for a wide range of physiological processes, but they are also implicated in various pathological processes. As a result, elucidation of glycan-associated biomolecular interactions and their consequences is of great importance in basic biological research and biomedical applications. In 2002, we and others were the first to utilize glycan microarrays in efforts aimed at the rapid analysis of glycan-associated recognition events. Because they contain a number of glycans immobilized in a dense and orderly manner on a solid surface, glycan microarrays enable multiple parallel analyses of glycan-protein binding events while utilizing only small amounts of glycan samples. Therefore, this microarray technology has become a leading edge tool in studies aimed at elucidating roles played by glycans and glycan binding proteins in biological systems. In this Account, we summarize our efforts on the construction of glycan microarrays and their applications in studies of glycan-associated interactions. Immobilization strategies of functionalized and unmodified glycans on derivatized glass surfaces are described. Although others have developed immobilization techniques, our efforts have focused on improving the efficiencies and operational simplicity of microarray construction. The microarray-based technology has been most extensively used for rapid analysis of the glycan binding properties of proteins. In addition, glycan microarrays have been employed to determine glycan-protein interactions quantitatively, detect pathogens, and rapidly assess substrate specificities of carbohydrate-processing enzymes. More recently, the microarrays have been employed to identify functional glycans that elicit cell surface lectin-mediated cellular responses. Owing to these efforts, it is now possible to use glycan microarrays to expand the understanding of roles played by glycans and glycan binding proteins in biological systems.

The minimum information required for a glycomics experiment (MIRAGE) project: improving the standards for reporting glycan microarray-based data

MIRAGE (Minimum Information Required for AGlycomics Experiment) is an initiative that was created by experts in the fields of glycobiology, glycoanalytics and glycoinformatics to produce guidelines for reporting results from the diverse types of experiments and analyses used in structural and functional studies of glycans in the scientific literature. As a sequel to the guidelines for sample preparation (Struwe et al. 2016, Glycobiology, 26:907–910) and mass spectrometry  data (Kolarich et al. 2013, Mol. Cell Proteomics, 12:991–995), here we present the first version of guidelines intended to improve the standards for reporting data from glycan microarray analyses. For each of eight areas in the workflow of a glycan microarray experiment, we provide guidelines for the minimal information that should be provided in reporting results. We hope that the MIRAGE glycan microarray guidelines proposed here will gain broad acceptance by the community, and will facilitate interpretation and reproducibility of the glycan microarray results with implications in comparison of data from different laboratories and eventual deposition of glycan microarray data in international databases.

Multi-level regulation of cellular glycosylation: from genes to transcript to enzyme to structure

Glycosylation is a ubiquitous mammalian post-translational modification that both decorates a majority of expressed proteins and regulates their function. Cellular glycan biosynthesis is facilitated by a few hundred enzymes that are collectively termed 'glycoenzymes'. The expression and activity of these enzymes is controlled at the transcription, translation and post-translation levels. New wet-lab advances are providing analytical methods to collect large-scale data at these multiple levels, relational databases are starting to collate these results, and computer models are beginning to integrate this information across scales in order to gain new knowledge. These activities are likely to enable the qualitative and quantitative mapping of pathways regulating glycan production and function in proteins, cells and tissue.

One-Step Selective Exoenzymatic Labeling (SEEL) Strategy for the Biotinylation and Identification of Glycoproteins of Living Cells

Technologies that can visualize, capture, and identify subsets of biomolecules that are not encoded by the genome in the context of healthy and diseased cells will offer unique opportunities to uncover the molecular mechanism of a multitude of physiological and disease processes. We describe here a chemical reporter strategy for labeling of cell surface glycoconjugates that takes advantage of recombinant glycosyltransferases and a corresponding sugar nucleotide functionalized by biotin. The exceptional efficiency of this method, termed one-step selective exoenzymatic labeling, or SEEL, greatly improved the ability to enrich and identify large numbers of tagged glycoproteins by LC-MS/MS. We further demonstrated that this labeling method resulted in far superior enrichment and detection of glycoproteins at the plasma membrane compared to a sulfo-NHS-activated biotinylation or two-step SEEL. This new methodology will make it possible to profile cell surface glycoproteomes with unprecedented sensitivity in the context of physiological and disease states.

Glycan Arrays: From Basic Biochemical Research to Bioanalytical and Biomedical Applications

A major branch of glycobiology and glycan-focused biomedicine studies the interaction between carbohydrates and other biopolymers, most importantly, glycan-binding proteins. Today, this research into glycan-biopolymer interaction is unthinkable without glycan arrays, tools that enable high-throughput analysis of carbohydrate interaction partners. Glycan arrays offer many applications in basic biochemical research, for example, defining the specificity of glycosyltransferases and lectins such as immune receptors. Biomedical applications include the characterization and surveillance of influenza strains, identification of biomarkers for cancer and infection, and profiling of immune responses to vaccines. Here, we review major applications of glycan arrays both in basic and applied research. Given the dynamic nature of this rapidly developing field, we focus on recent findings.

Bacteria microarrays as sensitive tools for exploring pathogen surface epitopes and recognition by host receptors

We have developed a readily adaptable microarray technology for high-throughput screening of pathogen-binding biomolecules and inhibitors of pathogen–counter-receptor interactions, based on the generation of bacteria microarrays.

Sialic acids in the brain: gangliosides and polysialic acid in nervous system development, stability, disease, and regeneration

Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

Enzymatic reactions on immobilised substrates

This review gives an overview of enzymatic reactions that have been conducted on substrates attached to solid surfaces. Such biochemical reactions have become more important with the drive to miniaturisation and automation in chemistry, biology and medicine. Technical aspects such as choice of solid surface and analytical methods are discussed and examples of enzyme reactions that have been successful on these surfaces are provided.

Exploration of sialic acid diversity and biology using sialoglycan microarrays

Sialic acids (Sias) are a group of α-keto acids with a nine-carbon backbone, which display many types of modifications in nature. The diversity of natural Sia presentations is magnified by a variety of glycosidic linkages to underlying glycans, the sequences and classes of such glycans, as well as the spatial organization of Sias with their surroundings. This diversity is closely linked to the numerous and varied biological functions of Sias. Relatively large libraries of natural and unnatural Sias have recently been chemically/chemoenzymatically synthesized and/or isolated from natural sources. The resulting sialoglycan microarrays have proved to be valuable tools for the exploration of diversity and biology of Sias. Here we provide an overview of Sia diversity in nature, the approaches used to generate sialoglycan microarrays, and the achievements and challenges arising.

Array-assisted characterization of a fucosyltransferase required for the biosynthesis of complex core modifications of nematode N-glycans

Fucose is a common monosaccharide component of cell surfaces and is involved in many biological recognition events. Therefore, definition and exploitation of the specificity of the enzymes (fucosyltransferases) involved in fucosylation is a recurrent theme in modern glycosciences. Despite various studies, the specificities of many fucosyltransferases are still unknown, so new approaches are required to study these. The model nematode Caenorhabditis elegans expresses a wide range of fucosylated glycans, including N-linked oligosaccharides with unusual complex core modifications. Up to three fucose residues can be present on the standard N,N'-diacetylchitobiose unit of these N-glycans, but only the fucosyltransferases responsible for transfer of two of these (the core α1,3-fucosyltransferase FUT-1 and the core α1,6-fucosyltransferase FUT-8) were previously characterized. By use of a glycan library in both array and solution formats, we were able to reveal that FUT-6, another C. elegans α1,3-fucosyltransferase, modifies nematode glycan cores, specifically the distal N-acetylglucosamine residue; this result is in accordance with glycomic analysis of fut-6 mutant worms. This core-modifying activity of FUT-6 in vitro and in vivo is in addition to its previously determined ability to synthesize Lewis X epitopes in vitro. A larger scale synthesis of a nematode N-glycan core in vitro using all three fucosyltransferases was performed, and the nature of the glycosidic linkages was determined by NMR. FUT-6 is probably the first eukaryotic glycosyltransferase whose specificity has been redefined with the aid of glycan microarrays and so is a paradigm for the study of other unusual glycosidic linkages in model and parasitic organisms.

Carbohydrate microarrays

In the last decade, carbohydrate microarrays have been core technologies for analyzing carbohydrate-mediated recognition events in a high-throughput fashion. A number of methods have been exploited for immobilizing glycans on the solid surface in a microarray format. This microarray-based technology has been widely employed for rapid analysis of the glycan binding properties of lectins and antibodies, the quantitative measurements of glycan-protein interactions, detection of cells and pathogens, identification of disease-related anti-glycan antibodies for diagnosis, and fast assessment of substrate specificities of glycosyltransferases. This review covers the construction of carbohydrate microarrays, detection methods of carbohydrate microarrays and their applications in biological and biomedical research.

Microarray Glycoprofiling of CA125 improves differential diagnosis of ovarian cancer

The CA125 biomarker assay plays an important role in the diagnosis and management of primary invasive epithelial ovarian/tubal cancer (iEOC). However, a fundamental problem with CA125 is that it is not cancer-specific and may be elevated in benign gynecological conditions such as benign ovarian neoplasms and endometriosis. Aberrant O-glycosylation is an inherent and specific property of cancer cells and could potentially aid in differentiating cancer from these benign conditions, thereby improving specificity of the assay. We report on the development of a novel microarray-based platform for profiling specific aberrant glycoforms, such as Neu5Acα2,6GalNAc (STn) and GalNAc (Tn), present on CA125 (MUC16) and CA15-3 (MUC1). In a blinded cohort study of patients with an elevated CA125 levels (30-500 kU/L) and a pelvic mass from the UK Ovarian Cancer Population Study (UKOPS), we measured STn-CA125, ST-CA125 and STn-CA15-3. The combined glycoform profile was able to distinguish benign ovarian neoplasms from invasive epithelial ovarian/tubule cancer (iEOCs) with a specificity of 61.1% at 90% sensitivity. The findings suggest that microarray glycoprofiling could improve differential diagnosis and significantly reduce the number of patients elected for further testing. The approach warrants further investigation in other cancers.

Synthesis of O-glycopeptides and construction of glycopeptide microarrays

O-glycosylation of proteins is an important modification which affects biological function and immunity. In this chapter, we provide protocols for efficient solid-phase O-glycopeptide synthesis (SPGPS) and protocols for the construction of glycopeptide microarray chips for screening applications. This will be exemplified for mucin-type glycopeptides and the construction of glycopeptide microarrays. To this end, the protocols provided are particularly suited for small-scale robotic parallel synthesis. N-Terminal amine capping of deletion peptides during synthesis stands out as vital to this strategy. It allows for direct on-slide enrichment of the full-length target product and thereby bypasses tedious isolation and purification procedures.

Glycan microarray screening assay for glycosyltransferase specificities

Glycan microarrays represent a high-throughput approach to determining the specificity of glycan-binding proteins against a large set of glycans in a single format. This chapter describes the use of a glycan microarray platform for evaluating the activity and substrate specificity of glycosyltransferases (GTs). The methodology allows simultaneous screening of hundreds of immobilized glycan acceptor substrates by in situ incubation of a GT and its appropriate donor substrate on the microarray surface. Using biotin-conjugated donor substrate enables direct detection of the incorporated sugar residues on acceptor substrates on the array. In addition, the feasibility of the method has been validated using label-free donor substrate combined with lectin-based detection of product to assess enzyme activity. Here, we describe the application of both procedures to assess the specificity of a recombinant human α2-6 sialyltransferase. This technique is readily adaptable to studying other glycosyltransferases.

Discovery of glycosyltransferases using carbohydrate arrays and mass spectrometry

Glycosyltransferases (GTs) catalyze the reaction between an activated sugar donor and an acceptor to form a new glycosidic linkage. GTs are responsible for the assembly of oligosaccharides in vivo and are also important for the in vitro synthesis of these biomolecules. However, the functional identification and characterization of new GTs are both difficult and tedious. This paper describes an approach that combines arrays of reactions on an immobilized array of acceptors with analysis by mass spectrometry to screen putative GTs. A total of 14,280 combinations of GT, acceptor and donor in four buffer conditions were screened and led to the identification and characterization of four new GTs. This work is significant because it provides a label-free method for the rapid functional annotation of putative enzymes.

Scaling down the size and increasing the throughput of glycosyltransferase assays: activity changes on stem cell differentiation

Glycosyltransferases (glycoTs) catalyze the transfer of monosaccharides from nucleotide-sugars to carbohydrate-, lipid-, and protein-based acceptors. We examined strategies to scale down and increase the throughput of glycoT enzymatic assays because traditional methods require large reaction volumes and complex chromatography. Approaches tested used (i) microarray pin printing, an appropriate method when glycoT activity was high; (ii) microwells and microcentrifuge tubes, a suitable method for studies with cell lysates when enzyme activity was moderate; and (iii) C(18) pipette tips and solvent extraction, a method that enriched reaction product when the extent of reaction was low. In all cases, reverse-phase thin layer chromatography (RP-TLC) coupled with phosphorimaging quantified the reaction rate. Studies with mouse embryonic stem cells (mESCs) demonstrated an increase in overall β(1,3)galactosyltransferase and α(2,3)sialyltransferase activity and a decrease in α(1,3)fucosyltransferases when these cells differentiate toward cardiomyocytes. Enzymatic and lectin binding data suggest a transition from Lewis(x)-type structures in mESCs to sialylated Galβ1,3GalNAc-type glycans on differentiation, with more prominent changes in enzyme activity occurring at later stages when embryoid bodies differentiated toward cardiomyocytes. Overall, simple, rapid, quantitative, and scalable glycoT activity analysis methods are presented. These use a range of natural and synthetic acceptors for the analysis of complex biological specimens that have limited availability.

Poly(acrylic acid)-poly(ethylene glycol) layers on positively charged surface coatings: molecular structure, protein resistance, and application to single protein deposition

A new copolymer (PAA-PEG2000) has been designed, consisting of a negatively charged poly(acrylic acid) (PAA) backbone to which poly(ethylene glycol) (PEG) side chains with a molecular weight of about 2 kDa were grafted in a molecular ratio of 3:10. It readily adsorbs to positively charged surfaces and may be considered to be the anionic counterpart of PEG-grafted poly(l-lysine) (PLL-PEG), which was first described by Kenausis et al. and is widely used to render negatively charged surfaces protein-resistant. The synthesis of PAA-PEG2000 can be carried out in aqueous solution at room temperature and does not require any sophisticated techniques such as handling in an inert gas atmosphere. Using ellipsometry and infrared reflection absorption spectroscopy (IRRAS), the film structure has been carefully analyzed for copolymer adsorption onto three different positively charged surfaces, namely, thin layers of poly(allylamine) (PAH), poly(ethyleneimine) (PEI) and (3-aminopropyl)triethoxysilane (APTES). Besides the film thickness, the conformation of the PEG chains and their orientation with respect to the surface normal appear to be important parameters for the protein resistance of the films. Although PAA-PEG2000 adsorbed to PAH and PEI renders the surfaces inert, only partial protein resistance has been observed if the copolymer is deposited on APTES. In a model application, we have generated heterogeneous surfaces composed of isolated small Au nanoparticles (AuNP's) embedded in a protein-resistant layer of PAA-PEG2000 and demonstrated that the AuNP's can serve as adsorption sites for single protein species. In the future, these nanopatterned surfaces may be used for the investigation of isolated proteins.

Chemoenzymatic probes for detecting and imaging fucose-α(1-2)-galactose glycan biomarkers

The disaccharide motif fucose-α(1-2)-galactose (Fucα(1-2)Gal) is involved in many important physiological processes, such as learning and memory, inflammation, asthma, and tumorigenesis. However, the size and structural complexity of Fucα(1-2)Gal-containing glycans have posed a significant challenge to their detection. We report a new chemoenzymatic strategy for the rapid, sensitive detection of Fucα(1-2)Gal glycans. We demonstrate that the approach is highly selective for the Fucα(1-2)Gal motif, detects a variety of complex glycans and glycoproteins, and can be used to profile the relative abundance of the motif on live cells, discriminating malignant from normal cells. This approach represents a new potential strategy for biomarker detection and expands the technologies available for understanding the roles of this important class of carbohydrates in physiology and disease.

Glycan microarrays for decoding the glycome

In the last decade, glycan microarrays have revolutionized the analysis of the specificity of glycan-binding proteins (GBPs), providing information that simultaneously illuminates the biology mediated by them and decodes the informational content of the glycome. Numerous methods have emerged for arraying glycans in a "chip" format, and glycan libraries have been assembled that address the diversity of the human glycome. Such arrays have been successfully used for analysis of GBPs, which mediate mammalian biology, host-pathogen interactions, and immune recognition of glycans relevant to vaccine production and cancer antigens. This review covers the development of glycan microarrays and applications that have provided insights into the roles of mammalian and microbial GBPs.

The engagement of selectins and their ligands in colorectal cancer liver metastases

The colonization of the liver by colorectal cancer (CRC) cells is a complicated process which includes many stages, until macrometastases occur. The entrapment of malignant cells within the hepatic sinusoids and their interactions with resident non-parenchymal cells are considered very important for the whole metastatic sequence. In the sinusoids, cell connection and signalling is mediated by multiple cell adhesion molecules, such as the selectins. The three members of the selectin family, E-, P- and L-selectin, in conjunction with sialylated Lewis ligands and CD44 variants, regulate colorectal cell communication and adhesion with platelets, leucocytes, sinusoidal endothelial cells and stellate cells. Their role in CRC liver metastases has been investigated in animal models and human tissue, in vivo and in vitro, in static and shear flow conditions, and their key-function in several molecular pathways has been displayed. Therefore, trials have already commenced aiming to exploit selectins and their ligands in the treatment of benign and malignant diseases. Multiple pharmacological agents have been developed that are being tested for potential therapeutic applications.

A high-throughput O-glycopeptide discovery platform for seromic profiling

Biomarker microarrays are becoming valuable tools for serological screening of disease-associated autoantibodies. Post-translational modifications (PTMs) such as glycosylation extend the range of protein function, and a variety of glycosylated proteins are known to be altered in disease progression. Here, we have developed a synthetic screening microarray platform for facile display of O-glycosylated peptides (O-PTMs). By introduction of a capping step during chemical solid-phase glycopeptide synthesis, selective enrichment of N-terminal glycopeptide end products was achieved on an amine-reactive hydrogel-coated microarray glass surface, allowing high-throughput display of large numbers of glycopeptides. Utilizing a repertoire of recombinant glycosyltransferases enabled further diversification of the array libraries in situ and display of a new level of potential biomarker candidates for serological screening. As proof-of-concept, we have demonstrated that MUC1 glycopeptides could be assembled and used to detect autoantibodies in vaccine-induced disease-free breast cancer patients and in patients with confirmed disease at time of diagnosis.

alpha2,3-sialyltransferase ST3Gal III modulates pancreatic cancer cell motility and adhesion in vitro and enhances its metastatic potential in vivo

Background

Cell surface sialylation is emerging as an important feature of cancer cell metastasis. Sialyltransferase expression has been reported to be altered in tumours and may account for the formation of sialylated tumour antigens. We have focused on the influence of alpha-2,3-sialyltransferase ST3Gal III in key steps of the pancreatic tumorigenic process.

Methodology/Principal Findings

ST3Gal III overexpressing pancreatic adenocarcinoma cell lines Capan-1 and MDAPanc-28 were generated. They showed an increase of the tumour associated antigen sialyl-Lewis^x. The transfectants' E-selectin binding capacity was proportional to cell surface sialyl-Lewis^x levels. Cellular migration positively correlated with ST3Gal III and sialyl-Lewis^x levels. Moreover, intrasplenic injection of the ST3Gal III transfected cells into athymic nude mice showed a decrease in survival and higher metastasis formation when compared to the mock cells.

Conclusion

In summary, the overexpression of ST3Gal III in these pancreatic adenocarcinoma cell lines underlines the role of this enzyme and its product in key steps of tumour progression such as adhesion, migration and metastasis formation.