Novel approach to the study of the antigenicities and receptor functions of carbohydrate chains of glycoproteins

Glycan Microarrays as Chemical Tools for Identifying Glycan Recognition by Immune Proteins

Glycans and glycan binding proteins (GBPs or lectins) are essential components in almost every aspect of immunology. Investigations of the interactions between glycans and GBPs have greatly advanced our understanding of the molecular basis of these fundamental immunological processes. In order to better study the glycan-GBP interactions, microscope glass slide-based glycan microarrays were conceived and proved to be an incredibly useful and successful tool. A variety of methods have been developed to better present the glycans so that they mimic natural presentations. Breakthroughs in chemical biology approaches have also made available glycans with sophisticated structures that were considered practically impossible just a few decade ago. Glycan microarrays provide a wealth of valuable information in immunological studies. They allow for discovery of detailed glycan binding preferences or novel binding epitopes of known endogenous immune receptors, which can potentially lead to the discovery of natural ligands that carry the glycans. Glycan microarrays also serve as a platform to discover new GBPs that are vital to the process of infection and invasion by microorganisms. This review summarizes the construction strategies and the immunological applications of glycan microarrays, particularly focused on those with the most comprehensive sets of glycan structures. We also review new methods and technologies that have evolved. We believe that glycan microarrays will continue to benefit the growing research community with various interests in the field of immunology.

Nanolithography of biointerfaces

This article is based on the Concluding remarks made at the Faraday Discussion meeting on Nanolithography of Biointerfaces, held in London, UK, 3-5th July 2019.

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.

Glycomics and Proteomics Approaches to Investigate Early Adenovirus-Host Cell Interactions

Adenoviruses as most viruses rely on glycan and protein interactions to attach to and enter susceptible host cells. The Adenoviridae family comprises more than 80 human types and they differ in their attachment factor and receptor usage, which likely contributes to the diverse tropism of the different types. In the past years, methods to systematically identify glycan and protein interactions have advanced. In particular sensitivity, speed and coverage of mass spectrometric analyses allow for high-throughput identification of glycans and peptides separated by liquid chromatography. Also, developments in glycan microarray technologies have led to targeted, high-throughput screening and identification of glycan-based receptors. The mapping of cell surface interactions of the diverse adenovirus types has implications for cell, tissue, and species tropism as well as drug development. Here we review known adenovirus interactions with glycan- and protein-based receptors, as well as glycomics and proteomics strategies to identify yet elusive virus receptors and attachment factors. We finally discuss challenges, bottlenecks, and future research directions in the field of non-enveloped virus entry into host cells.

O-Glycome Beam Search Arrays for Carbohydrate Ligand Discovery

O-glycosylation is a post-translational modification of proteins crucial to molecular mechanisms in health and disease. O-glycans are typically highly heterogeneous. The involvement of specific O-glycan sequences in many bio-recognition systems is yet to be determined because of a lack of efficient methodologies. We describe here a targeted microarray approach: O-glycome beam search that is both robust and efficient for O-glycan ligand-discovery. Substantial simplification of the complex O-glycome profile and facile chromatographic resolution is achieved by arraying O-glycans as branches, monitoring by mass spectrometry, focusing on promising fractions, and on-array immuno-sequencing. This is orders of magnitude more sensitive than traditional methods. We have applied beam search approach to porcine stomach mucin and identified extremely minor components previously undetected within the O-glycome of this mucin that are ligands for the adhesive proteins of two rotaviruses. The approach is applicable to O-glycome recognition studies in a wide range of biological settings to give insights into glycan recognition structures in natural microenvironments.

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.

Carbohydrate Microarrays

Carbohydrates, like nucleic acids and proteins, are essential biological molecules. Owing to their intrinsic physicochemical properties, carbohydrates are capable of generating structural diversity in a multitude of ways and are prominently displayed on the surfaces of cell membranes or on the exposed regions of macromolecules. Recent studies highlight that carbohydrate moieties are critical for molecular recognition, cell-cell interactions, and cell signaling in many physiological and pathological processes, and for biocommunication between microbes and host species. Modern carbohydrate microarrays emerged in 2002 and brought in new high-throughput tools for “glyco code” exploration. In this section, some basic concepts of sugar chain diversity, glyco-epitope recognition, and the evolving area of glyco-epitomics and biomarker discovery are discussed. Two complementary technologies, carbohydrate antigen arrays and photogenerated glyco-chips, serve as models to illustrate how to apply carbohydrate microarrays to address biomedical questions.

Carbohydrate recognition in the immune system: contributions of neoglycolipid-based microarrays to carbohydrate ligand discovery

Oligosaccharide sequences in glycomes of eukaryotes and prokaryotes are enormously diverse. The reasons are not fully understood, but there is an increasing number of examples of the involvement of specific oligosaccharide sequences as ligands in protein-carbohydrate interactions in health and, directly or indirectly, in every major disease, be it infectious or noninfectious. The pinpointing and characterizing of oligosaccharide ligands within glycomes has been one of the most challenging aspects of molecular cell biology, as oligosaccharides cannot be cloned and are generally available in limited amounts. This overview recounts the background to the development of a microarray system that is poised for surveying proteomes for carbohydrate-binding activities and glycomes for assigning the oligosaccharide ligands. Examples are selected by way of illustrating the potential of "designer" microarrays for ligand discovery at the interface of infection, immunity, and glycobiology. Particularly highlighted are sulfo-oligosaccharide and gluco-oligosaccharide recognition systems elucidated using microarrays.

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.

Stage-specific expression and antigenicity of glycoprotein glycans isolated from the human liver fluke, Opisthorchis viverrini

Infection by Opisthorchis viverrini (liver fluke) is a major public health problem in southeastern Asia, resulting in hepatobiliary disease and cholangiocarcinoma. Fluke surface glycoconjugates are prominently presented to the host, thereby constituting a crucial immunological interface that can determine the parasite's success in establishing infection. Therefore, N- and O-linked glycoprotein glycan profiles of the infective metacercarial stage and of the mature adult were investigated by nanospray ionisation-linear ion trap mass spectrometry (NSI-MS(n)). Glycan immunogenicity was investigated by immunoblotting with serum from infected humans. Metacercariae and adult parasites exhibit similar glycan diversity, although the prevalence of individual glycans and glycan classes varies by stage. The N-glycans of the metacercaria are mostly high mannose and monofucosylated, truncated-type oligosaccharides (62.7%), with the remainder processed to complex and hybrid type glycans (37.3%). The N-linked glycan profile of the adult is also dominated by high mannose and monofucosylated, truncated-type oligosaccharides (80.0%), with a smaller contribution from complex and hybrid type glycans (20.0%). At both stages, complex and hybrid type glycans are detected as mono-, bi-, tri-, or tetra-antennary structures. In metacercariae and adults, O-linked glycans are detected as mono- to pentasaccharides. The mucin type core 1 structure, Galβ1-3GalNAc, predominates in both stages but is less prevalent in the adult than in the metacercaria. Immunogenic recognition of liver fluke glycoproteins is reduced after deglycosylation but infected human serum was unable to recognise glycans released from peptides. Therefore, the most potent liver fluke antigenic epitopes are mixed determinants, comprised of glycan and polypeptide elements.

Neoglycolipid-based oligosaccharide microarray system: preparation of NGLs and their noncovalent immobilization on nitrocellulose-coated glass slides for microarray analyses

Carbohydrate microarrays, since their advent in 2002, are revolutionizing studies of the molecular basis of protein-carbohydrate interactions both in endogenous recognition systems and pathogen-host interactions. We have developed a unique carbohydrate microarray system based on the neoglycolipid (NGL) technology, a well-validated microscale approach for generating lipid-tagged oligosaccharide probes for use in carbohydrate recognition studies. This chapter provides an overview of the principles and key features of the NGL-based oligosaccharide microarrays, and describes in detail the basic techniques - from the preparation of NGL probes to the generation of microarrays using robotic arraying hardware, as well as a general protocol for probing the microarrays with carbohydrate-binding proteins.

Neoglycolipid (NGL)-based oligosaccharide microarrays and highlights of their recent applications in studies of the molecular basis of pathogen–host interactions

Carbohydrate microarray technologies are new developments at the frontier of glycomics that are showing great promise as tools for high-throughput analysis of carbohydrate-mediated interactions and the elucidation of carbohydrate ligands involved not only in endogenous receptor systems, but also pathogen–host interactions. The main advantage of microarray analysis is that a broad range of glycan sequences can be immobilized on solid matrices as minute spots and simultaneously interrogated. Different methodologies have emerged for constructing carbohydrate microarrays. The NGL (neoglycolipid)-based oligosaccharide microarray platform is among the relatively few systems that are beyond proof-of-concept and have provided new biological information. In the present article, I dwell, in some detail, on the NGL-based microarray. Highlights are the recent applications of NGL-based microarrays that have contributed to knowledge on the molecular basis of pathogen–host interactions, namely the assignments of the carbohydrate-binding specificities of several key surface-adhesive proteins of Toxoplasma gondii and other apicomplexan parasites, and the elucidation of receptor-binding specificities of the pandemic influenza A (H1N1) 2009 (H1N1pdm) virus compared with seasonal H1N1 virus.

Advances on the compositional analysis of glycosphingolipids combining thin-layer chromatography with mass spectrometry

Glycosphingolipids (GSLs), composed of a hydrophilic carbohydrate chain and a lipophilic ceramide anchor, play pivotal roles in countless biological processes, including infectious diseases and the development of cancer. Knowledge of the number and sequence of monosaccharides and their anomeric configuration and linkage type, which make up the principal items of the glyco code of biologically active carbohydrate chains, is essential for exploring the function of GSLs. As part of the investigation of the vertebrate glycome, GSL analysis is undergoing rapid expansion owing to the application of novel biochemical and biophysical technologies. Mass spectrometry (MS) takes part in the network of collaborations to further unravel structural and functional aspects within the fascinating world of GSLs with the ultimate aim to better define their role in human health and disease. However, a single-method analytical MS technique without supporting tools is limited yielding only partial structural information. Because of its superior resolving power, robustness, and easy handling, high-performance thin-layer chromatography (TLC) is widely used as an invaluable tool in GSL analysis. The intention of this review is to give an insight into current advances obtained by coupling supplementary techniques such as TLC and mass spectrometry. A retrospective view of the development of this concept and the recent improvements by merging (1) TLC separation of GSLs, (2) their detection with oligosaccharide-specific proteins, and (3) in situ MS analysis of protein-detected GSLs directly on the TLC plate, are provided. The procedure works on a nanogram scale and was successfully applied to the identification of cancer-associated GSLs in several types of human tumors. The combination of these two supplementary techniques opens new doors by delivering specific structural information of trace quantities of GSLs with only limited investment in sample preparation.

Preparation of homogenous oligosaccharide chains from glycosphingolipids

After the discovery of glycosphingolipid (GSL) glycan detaching enzymes, Rhodococcal endoglycoceramidase (EGCase) and leech ceramide glycanase (CGase), the method for enzymatically releasing glycans from GSLs has become the method of choice for preparing intact ceramide-free oligosaccharide chains from GSLs. This paper describes (1) the preparation of the intact oligosaccharides from GM1 (II(3)NeuAcGgOse(4)Cer) and GbOse(4)Cer as examples to show the use of CGase to prepare intact glycan chains from GSLs, and (2) the specificity and detergent requirements of Rhodococcal EGCases for the release of glycan chains from different GSLs.

Glycan array: a powerful tool for glycomics studies

Glycan arrays have become a powerful tool for the high-throughput elucidation of interactions of different carbohydrate structures with a wide variety of biological targets, including antibodies, proteins, viruses and cells. This technique is especially suitable for glycomics studies, because arrays present carbohydrate ligands in a manner that mimics interactions at cell-cell interfaces. This review assesses the recent advances involving glycan arrays, including new methods for glycan-array fabrication, new platforms for novel biological information, and new perceptions of glycomics for improving the understanding of disease-related glycobiology. Furthermore, this review attempts to forecast trends in the development of glycan arrays and possible solutions for some remaining challenges to improve this new technology.

Glycan arrays: recent advances and future challenges

Carbohydrate arrays, also referred to as glycan arrays, are composed of various oligosaccharides and/or polysaccharides immobilized on a solid support in a spatially defined arrangement. This technology provides a powerful, high-throughput approach to examining carbohydrate-macromolecule interactions, and glycan arrays have had a significant impact on the field of glycobiology. This review focuses on recent advances in glycan array technology, limitations, and opportunities for improvement. In particular, new methods for the production of natural glycan arrays and chemoenzymatic approaches are greatly expanding the diversity of structures on arrays. Since multivalent complex formation is generally required to achieve tight binding, methods to evaluate and modulate presentation are vital for enhancing the capabilities of this technology.

Carbohydrate microarrays: key developments in glycobiology

Carbohydrate chains of glycoproteins, glycolipids, proteoglycans, and polysaccharides mediate processes of biological and medical importance through their interactions with complementary proteins. The unraveling of these interactions is therefore a priority in biomedical sciences. Carbohydrate microarray technology is a new development at the frontier of glycomics that is revolutionizing the study of carbohydrate-protein interactions and the elucidation of their specificities in endogenous biological processes, microbe-host interactions, and immune defense mechanisms. In this review, we briefly refer to the principles of numerous platforms since the introduction of carbohydrate microarrays in 2002, and we highlight platforms that are beyond proof-of-concept and have provided new biological information.

Glycoarrays--tools for determining protein-carbohydrate interactions and glycoenzyme specificity

Carbohydrate arrays (glycoarrays) have recently emerged as a high-throughput tool for studying carbohydrate-binding proteins and carbohydrate-processing enzymes. A number of sophisticated array platforms that allow for qualitative and quantitative analysis of carbohydrate binding and modification on the array surface have been developed, including analysis by fluorescence spectroscopy, mass spectrometry and surface plasmon resonance spectroscopy. These platforms, together with examples of biologically-relevant applications are reviewed in this Feature Article.

Glycoprotein oligosaccharides as recognition structures

A series of observations--the pronounced changes in the expression and distribution of oligosaccharide antigens during embryonic development, cell differentiation and oncogenesis, the prominence of these changing structures (oncodevelopmental antigens) on the receptor for epidermal growth factor, and the stimulation of receptor autophosphorylation following their perturbation with antibodies--has suggested that the oligosaccharides of growth factor receptors and complementary lectins may be intimately involved in molecular recognition events in growth and differentiation processes. For elucidating oligosaccharide recognition by diverse cellular and secreted proteins and microbial adhesins, a new technique has been developed which involves the overlay of immobilized oligosaccharide probes (neoglycolipids) derived from glycoproteins and other sources. New insights have been gained into carbohydrate recognition by several mammalian lectins, and a novel receptor system has been discovered in Escherichia coli isolated from patients with urinary tract infections. This new technique seems ideal for elucidating oligosaccharide recognition in diverse biological settings, and for 'quality control' of the sugar chains of recombinant glycoproteins engineered for the purpose of administration to man.

Sweet spots in functional glycomics

Information contained in the mammalian glycome is decoded by glycan-binding proteins (GBPs) that mediate diverse functions including host-pathogen interactions, cell trafficking and transmembrane signaling. Although information on the biological roles of GBPs is rapidly expanding, challenges remain in identifying the glycan ligands and their impact on GBP function. Protein-glycan interactions are typically low affinity, requiring multivalent interactions to achieve a biological effect. Though many glycoproteins can carry the glycan structure recognized by the GBP, other factors, such as recognition of protein epitopes and microdomain localization, may restrict which glycoproteins are functional ligands in situ. Recent advances in development of glycan arrays, synthesis of multivalent glycan ligands, bioengineering of cell-surface glycans and glycomics databases are providing new tools to identify the ligands of GBPs and to elucidate the mechanisms by which they participate in GBP function.

Preparation of Neoglycolipids with Ring‐Closed Cores via Chemoselective Oxime‐Ligation for Microarray Analysis of Carbohydrate–Protein Interactions

Affinities of most oligosaccharide-protein interactions are so low that multivalent forms of ligand and protein are required for detecting interactions. The neoglycolipid (NGL) technology was designed to address the need for microscale presentation of oligosaccharides in a multivalent form for studying carbohydrate-protein interactions, and this is now the basis of a state-of-the-art carbohydrate microarray system. NGL technology involves conjugating oligosaccharides by reductive amination to the aminolipid 1,2-dihexadecyl-sn-glycero-3-phosphoethanolamine (DHPE). Other than ring-opening of the monosaccharide residues at reducing ends, oligosaccharides remain intact, and the NGLs derived from trisaccharides or larger oligosaccharides have performed well for the majority of carbohydrate-recognition systems that have the peripheral or backbone regions of oligosaccharides as recognition motifs. However, ring-opening of reducing end monosaccharides limits applicability to very short oligosaccharides (di- and trisaccharides) and, potentially, to N-glycans recognized by proteins such as Pisum sativum agglutinin (pea lectin) that require both intact core and backbone regions for strong binding. This chapter describes a method for preparing NGLs (designated AO-NGLs) from reducing oligosaccharides by chemoselective oxime-ligation to a new lipid reagent, N-aminooxyacetyl-DHPE. Microarray analyses of the AO-NGL derived from Lewis x (Le(x)) trisaccharide probed with anti-Le(x) antibodies indicate that a significant proportion of the core monosaccharide linked to lipid is in ring-closed form. Thus, AO-NGLs have broadened the applicability of NGLs as probes in studies of carbohydrate-protein interactions.

Ceramide glycosylation and fatty acid hydroxylation influence serological reactivity in Trypanosoma cruzi glycosphingolipids

Ceramide mono (CMH) or dihexoside (CDH) fractions from Trypanosoma cruzi (Dm28c clone) were identified as glucosyl and lactosylceramides containing non-hydroxylated fatty acids. The di-glycosylated form was much more efficiently recognized by sera from T. cruzi-immunized rabbits, indicating that glycosylation influences antigenicity. Fatty acid hydroxylation was also a determinant of serological reactivity, since an alpha-hydroxylated CMH, only present at the Y clone, was recognized by the hyperimmune sera. In summary, these data indicate that T. cruzi CMHs with non-hydroxylated fatty acids are unable to induce antibody responses in animal hosts, which is reverted by the addition of a sugar residue or an alpha-hydroxyl group.

Novel approaches to map protein interactions

Although we now have the sequence of the human genome at hand, we face the challenge of assigning function to the identified genes. Genes usually ascribe their function through proteins, and the role of proteins is to interact with other molecules. Therefore, if we could map the interactions of proteins we would be able to understand protein function. The challenge of mapping protein interactions is vast and many novel approaches have recently been developed for this task using molecular biology, mass spectrometry and chemiproteomic techniques.

Oligosaccharide microarrays for high-throughput detection and specificity assignments of carbohydrate-protein interactions

We describe microarrays of oligosaccharides as neoglycolipids and their robust display on nitrocellulose. The arrays are obtained from glycoproteins, glycolipids, proteoglycans, polysaccharides, whole organs, or from chemically synthesized oligosaccharides. We show that carbohydrate-recognizing proteins single out their ligands not only in arrays of homogeneous oligosaccharides but also in arrays of heterogeneous oligosaccharides. Initial applications have revealed new findings, including: (i) among O-glycans in brain, a relative abundance of the Lewis(x) sequence based on N-acetyllactosamine recognized by anti-L5, and a paucity of the Lewis(x) sequence based on poly-N-acetyllactosamine recognized by anti-SSEA-1; (ii) insights into chondroitin sulfate oligosaccharides recognized by an antiserum and an antibody (CS-56) to chondroitin sulfates; and (iii) binding of the cytokine interferon-gamma (IFN-gamma) and the chemokine RANTES to sulfated sequences such as HNK-1, sulfo-Lewis(x), and sulfo-Lewis(a), in addition to glycosaminoglycans. The approach opens the way for discovering new carbohydrate-recognizing proteins in the proteome and for mapping the repertoire of carbohydrate recognition structures in the glycome.

Ganglioside GD3 and its mimetics induce cytochrome c release from mitochondria

Ganglioside GD3 induced the release of cytochrome c from isolated rat liver mitochondria. This process was completely prevented by cyclosporin A and partially prevented by a cysteine protease inhibitor, n-acetyl-leu-leu-norleucinal. Cyclosporin A is a potent inhibitor of the permeability transition pore, whereas n-acetyl-leu-leu-norleucinal has no effect on this pore. These results indicate that the release of cytochrome c from mitochondria requires both the opening of the permeability transition pore and a cysteine protease inhibitor-sensitive mechanism. Gangliosides GD1a, GD1b, GT1b, and GQ1b along with the synthetic GD3 mimetics TMS-42 and CI-22, which are glycerophospholipids carrying a disialo residue, also induced cytochrome c release. In contrast, gangliosides GM1, GM2, and GM3 did not induce cytochrome c release. These results indicate that two sialo residues must play an important role in the induction of cytochrome c release by gangliosides.

Progress in deciphering the information content of the 'glycome'--a crescendo in the closing years of the millennium

The closing years of the second millennium have been uplifting for carbohydrate biology. Optimism that oligosaccharide sequences are bearers of crucial biological information has been borne out by the constellation of efforts of carbohydrate chemists, biochemists, immunochemists, and cell- and molecular biologists. The direct involvement of specific oligosaccharide sequences in protein targeting and folding, and in mechanisms of infection, inflammation and immunity is now unquestioned. With the emergence of families of proteins with carbohydrate-binding activities, assignments of information content for defined oligosaccharide sequences will become more common, but the pinpointing and elucidation of the bioactive domains on oligosaccharides will continue to pose challenges even to the most experienced carbohydrate biologists. The neoglycolipid technology incorporates some of the key requirements for this challenge: namely the resolution of complex glycan mixtures, and ligand binding coupled with sequence determination by mass spectrometry.

Fluorescent neoglycolipids. Improved probes for oligosaccharide ligand discovery

A second generation of lipid-linked oligosaccharide probes, fluorescent neoglycolipids, has been designed and synthesized for ligand discovery within highly complex mixtures of oligosaccharides. The aminolipid 1,2-dihexadecyl-sn-glycero-3-phosphoethanolamine (DHPE), which has been used extensively to generate neoglycolipids for biological and structural studies, has been modified to incorporate a fluorescent label, anthracene. This new lipid reagent, N-aminoacetyl-N-(9-anthracenylmethyl)-1, 2-dihexadecyl-sn-glycero-3-phosphoethanolamine (ADHP), synthesized from anthracenaldehyde and DHPE gives an intense fluorescence under UV light. Fluorescent neoglycolipids derived from a variety of neutral and acidic oligosaccharides by conjugation to ADHP, by reductive amination, can be detected and quantified by spectrophotometry and scanning densitometry, and resolved by TLC and HPLC with subpicomole detection. Antigenicities of the ADHP-neoglycolipids are well retained, and picomole levels can be detected using monoclonal carbohydrate sequence-specific antibodies. Among O-glycans from an ovarian cystadenoma mucin, isomeric oligosaccharide sequences, sialyl-Lea- and sialyl-Lex-active, could be resolved by HPLC as fluorescent neoglycolipids, and sequenced by liquid secondary-ion mass spectrometry. Thus the neoglycolipid technology now uniquely combines high sensitivity of immuno-detection with a comparable sensitivity of chemical detection. Principles are thus established for a streamlined technology whereby an oligosaccharide population is carried through ligand detection and ligand isolation steps, and sequence determination by mass spectrometry, enzymatic sequencing and other state-of-the-art technologies for carbohydrate analysis.

Neoglycolipids derived from phosphatidylethanolamine serve as probes in cell culture studies on glycolipid metabolism

The neoglycolipid (NeoGL) N-acetyl-1-deoxy-1-phosphatidylethanolamino lacto-N-tetraositol [Lc4Ose-PtdEtn(NAc)] and the radioactivly labeled analog [Lc4Ose-PtdEtn(N[14C]Ac)] were synthesized by coupling the corresponding oligosaccharide to phosphatidylethanolamine (dihexadecyl) via reductive amination and subsequent N-acetylation with unlabeled and [14C]acetic acid anhydride, respectively. Lc4Ose-PtdEtn(N[14C]Ac) was then incubated with homogenates of rat small intestine epithelial cells (IEC-6) at pH 4. The reaction products were shown to be the degradation products formed by glycosidases by fast atom bombardment mass spectrometry (FAB MS). On the other hand, incubation of Lc4Ose-PtdEtn(NAc) with IEC-6 cell homogenates in sialyltransferase assays yielded the corresponding sialylated product. When Lc4Ose-PtdEtn(N[14C]Ac) was fed to IEC-6 cells as BSA complex, up to 5% of the NeoGL administered were taken up by the cells. After extraction of the NeoGL and separation by thin layer chromatography (TLC) the catabolic products Lc3Ose-PtdEtn(N[14C]Ac), Lac-PtdEtn(N[14C]Ac), and Glc-PtdEtn(N[14C]Ac), as well as the main anabolic product NeuGc-Lc4Ose-PtdEtn(N[14C]Ac) could be identified by FAB MS. These results demonstrate that PtdEtn-derived NeoGL can be used as probes for studies on the metabolism of specific oligosaccharide structures in cell culture.

L-selectin interactions with novel mono- and multisulfated Lewisx sequences in comparison with the potent ligand 3'-sulfated Lewisa

The cell adhesion molecule L-selectin binds to 3'-sialyl-Lewis (Le)x and -Lea and to 3'-sulfo-Lex and -Lea sequences. The binding to 3'-sialyl-Lex is strongly affected by the presence of 6-O-sulfate as found on oligosaccharides of the counter receptor, GlyCAM-1; 6-O-sulfate on the N-acetylglucosamine (6-sulfation) enhances, whereas 6-O-sulfate on the galactose (6'-sulfation) virtually abolishes binding. To extend knowledge on the specificity of L-selectin, we have investigated interactions with novel sulfo-oligosaccharides based on the Lex pentasaccharide sequence. We observe that, also with 3'-sulfo-Lex, the 6-sulfation enhances and 6'-sulfation suppresses L-selectin binding. The 6'-sulfation without 3'-sialyl or 3'-sulfate gives no binding signal with L-selectin. Where the 6-sulfo,3'-sialyl-Lex is on an extended di-N-acetyllactosamine backbone, additional 6-O-sulfates on the inner galactose and inner N-acetylglucosamine do not influence the binding. Although binding to the 6,3'-sulfo-Lex and 6-sulfo, 3'-sialyl-Lex sequences is comparable, the former is a more effective inhibitor of L-selectin binding. This difference is most apparent when L-selectin is in paucivalent form (predominantly di- and tetramer) rather than multivalent. Indeed, as inhibitors of the paucivalent L-selectin, the 3'-sulfo-Lex series are more potent than the corresponding 3'-sialyl-Lex series. Thus, for synthetic strategies to design therapeutic oligosaccharide analogs as antagonists of L-selectin binding, those based on the simpler 3'-sulfo-Lex (and also the 3'-sulfo-Lea) would seem most appropriate.

Core-branching pattern and sequence analysis of mannitol-terminating oligosaccharides by neoglycolipid technology

The occurrence of mannitol-terminating oligosaccharides (2-substituted or 2,6-disubstituted) among the O-glycans released by alkaline borohydride treatment from glycoproteins of the nervous system has prompted the development of a microscale method to analyze the core-branching pattern and sequence by the neoglycolipid (NGL) technology, analogous to a method previously described for GalNAcol-terminating oligosaccharides (M. S. Stoll, E. F. Hounsell, A. M. Lawson, W. Chai, and T. Feizi, Eur. J. Biochem. 189, 499-507, 1990). The approach involves the selective cleavage at the core mannitol by mild periodate treatment and analysis of the reaction products as NGLs by in situ TLC/liquid secondary ion mass spectrometry. Oxidation conditions have been optimized using as reference compounds 2-, 3-, 4-, or 6-monosubstituted mannobi-itols, 3,6-disubstituted mannitol-terminating pentasaccharides, and 2-mono- and 2,6-disubstituted mannitol-terminating neutral and sialylated oligosaccharides isolated from brain glycopeptides. When a 2:1 molar ratio of periodate to alditol is used, the core mannitol is cleaved at the C3-C4 threo-diol bond and in the absence of a threo-diol cleavage occurs to a lesser extent at erythro-diols. Saccharide ring diols are not cleaved under these conditions, and it is also shown that the side chain of sialic acid on the oligosaccharide is largely unaffected. Substituents at 2- and 6-positions of the core mannitol can be identified, and the method is applicable to neutral and sialylated oligosaccharide alditols. Typically, the starting material is 5 nmol of oligosaccharide and 0.5-1 nmol of derivatives is applied for analysis. By this strategy, the core-branching pattern and position of sialic acid of two branched monosialylated mannitol-terminating oligosaccharide isomers have been determined.

Structural analysis of O-linked oligosaccharide-alditols by electrospray-tandem mass spectrometry after mild periodate oxidation and derivatization with 2-aminopyridine

O-linked oligosaccharide-alditols were analyzed by a combination of high-performance liquid chromatography (HPLC) and electrospray-tandem mass spectrometry (ESI-MS/MS). First, oligosaccharide-alditols were treated with sodium meta-periodate under conditions where core N-acetylgalactosaminitol is specifically degraded. The resulting fragments were labeled with 2-aminopyridine and purified on a reversed-phase column. Pyridylamino oligosaccharides yielded protonated molecular ions in positive-ion ES-MS and gave Y-series sequence ions, arising from glycosidic cleavages, by ESI-tandem mass spectrometry. Information on sugar sequence and branching of oligosaccharides linked at C6 and C3 to the N-acetylgalactosaminitol can be obtained. A systematic study of various oligosaccharide-alditols demonstrated that this approach constitutes a powerful tool for the structural characterization of O-glycans available only in limited quantities.

Interaction of GM2 activator protein with glycosphingolipids

GM2 activator protein is a protein cofactor that has been shown to stimulate the enzymatic hydrolysis of both GalNAc and NeuAc from GM2 (Wu, Y. Y., Lockyer, J. M., Sugiyama, E., Pavlova, N.V., Li, Y.-T., and Li, S.-C. (1994) J. Biol. Chem. 269, 16276-16283). To understand the mechanism by which GM2 activator stimulates the hydrolysis of GM2, we examined the interaction of this activator protein with GM2 as well as with other glycosphingolipids by TLC overlay and Sephacryl S-200 gel filtration. The TLC overlay analysis unveiled the binding specificity of GM2 activator, which was not previously revealed. Under the conditions optimal for the activator protein to stimulate the hydrolysis of GM2 by beta-hexosaminidase A, GM2 activator was found to bind avidly to acidic glycosphingolipids, including gangliosides and sulfated glycosphingolipids, but not to neutral glycosphingolipids. The gangliosides devoid of sialic acids, such as asialo-GM1 and asialo-GM2, and the GM2 derivatives whose carboxyl function in the NeuAc had been modified by methyl esterification or reduction, were only very weakly bound to GM2 activator. These results indicate that the negatively charged sugar residue or sulfate group in gangliosides is one of the important sites recognized by GM2 activator. For comparison, we also studied in parallel the complex formation between glycosphingolipids and saposin B, a separate activator protein with broad specificity to stimulate the hydrolysis of various glycosphingolipids. We found that saposin B bound to neutral glycosphingolipids and gangliosides equally well, and there was an exceptionally strong binding to sulfatide. In contrast to previous reports, we found that GM2 activator formed complexes with GM2 and other gangliosides in different proportions depending on the ratio between the activator protein and the ganglioside in the incubation mixture prior to gel filtration. We were not able to detect the specific binding of GM2 activator to GM2 when GM2 was mixed with GM1 or GM3. Thus, the specificity or the mode of action of GM2 activator cannot be simply explained by its interaction with glycosphingolipids based on complex formation. The binding of GM2 activator to a wide variety of negatively charged glycosphingolipids may indicate that this activator protein has functions other than assisting the enzymatic hydrolysis of GM2.

High-resolution thin-layer chromatography of gangliosides

In this review an updated overview of current improvements on thin-layer chromatography (TLC) of gangliosides over the past decade is provided. Basic general techniques and special advice is given for successful separation of glycosphingolipids. New approaches concerning continuous and multiple development, and several preparative TLC methods are also included. Emphasis is placed on TLC immunostaining and related techniques, i.e. practical applications of carbohydrate-specific antibodies, toxins and bacteria, viruses, lectins and eukaryotic cells. Thus, this review on ganglioside TLC summarizes its power as an analytical tool for a wide range of purposes.

n-alkylglucosides serve as acceptors for galactosyltransferases from rat liver Golgi vesicles

n-Alkyl alpha- and beta-D-glucopyranosides with different alkyl chain lengths (Glc-O-CxH2x+1) and n-octyl beta-D-thioglucopyranoside (Glc-S-C8H17) were synthesized, and used as acceptors for galactosyltransferases from rat liver Golgi vesicles. Only the beta-anomers were galactosylated and at constant substrate concentration, the reaction rates reached a maximum for medium alkyl chain lengths (C6, C8 and C10). Apparent Km and Vmax values decreased with increasing alkyl chain length. The reaction products were identified as n-alkyl beta-lactosides by means of thin layer chromatography, fast atom bombardment mass spectrometry and 1H-NMR spectroscopy. Competition experiments showed that UDP-Gal: N-acetylglucosamine beta 1-4-galactosyltransferase (EC 2.4.1.38) and not UDP-Gal: glucosylceramide beta 1-4-galactosyltransferase (lactosylceramide synthase, GalT-2) was responsible for the galactosylation of alkyl glucosides.

Synthesis of neoglycopeptides and analyses of their biodistribution in vivo to identify tissue specific uptake and novel putative membrane lectins

Complex type N-linked oligosaccharides derived from fetuin, fibrinogen and thyroglobulin were coupled to acetyltyrosine affording a series of neoglycopeptides with retention of terminal structures and the beta-anomeric configuration of their reducing end N-acetylglycosamine residue. The neoglycopeptides thus synthesized could be labelled to high specific activities with 125I in the aromatic side chain of tyrosine. Analysis of the fate of these neoglycopeptides in conjunction with inhibition with asialofetuin and oligosaccharides of defined structure in mice in vivo revealed the uptake of galactosylated biantennary compound by kidneys, in addition to the known itinerary of triantennary galactosylated complex oligosaccharide from fetuin to liver and the galactosylated biantennary chain with fucosylation in the core to bone marrows. On the other hand, the agalacto, aglucosamino biantennary chains with and without fucosylation in the core region are taken up by submaxillary glands while the conserved trimannosyl core with fucose is primarily concentrated in stomach tissue. These studies thus define new routes for the uptake of complex N-linked glycans and also subserve to identify lectins presumably involved in their recognition.

Synthesis of neoglycolipids containing a mucin-type core unit

The unnatural glycolipids O-beta-D-galactopyranosyl-(1-->4)-O-(2-acetamido- 2-deoxy-beta-D-glucopyranosyl)-(1-->3)-O-(2-acetamido-2-deoxy-alpha-D- galactopyranosyl)-(1-->1)-ceramide (1), O-beta-D-galactopyranosyl-(1-->4)-O-(2-acetamido-2-deoxy-beta-D-glucopyr anosyl) - (1-->6)-O-(2-acetamido-2-deoxy-alpha-D-galactopyranosyl)-(1-->1)-ceramid e (2), and O-beta-D-galactopyranosyl-(1-->4)-O-(2-acetamido-2-deoxy-beta-D- glucopyranosyl)-(1-->3)-O-[O-beta-D-galactopyranosyl-(1-->4)-(2-acetamid o-2- deoxy-beta-D-glucopyranosyl)-(1-->6)]-O-(2-acetamido-2-deoxy-alpha-D- galactopyranosyl)-(1-->1)-ceramide (3), and their beta-(1-->1)-linked isomers, were synthesized. The precursor oligosaccharides for 1, 2, and 3 were made by coupling O-2,3,4,6- tetra-O-acetyl-beta-D-galactopyranosyl)-(1-->4)-3,6-di-O-acetyl-2- deoxy-2-phthalimido-alpha,beta-D-glucopyranosyl trichloroacetimidate with tert-butyldiphenylsilyl 2-azido-4,6-O-benzylidene-2-deoxy-beta-D-galactopyranoside, tert-butyldiphenylsilyl 2-azido-3-O-benzoyl-2-deoxy-beta-D-galactopyranoside, and tert-butyl-diphenylsilyl O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-(1-->4)-O-(3,6-di-O-a cetyl-2 - deoxy-2-phthalimido-beta-D-glucopyranosyl)-(1-->3)-2-azido-2-deoxy-beta- D- galactopyranoside, respectively. These oligosaccharides were converted into their trichloroacetimidates, which were coupled with 3,2'-di-O-benzoyl ceramide. Deprotection of the coupling products gave the title compounds 1, 2, and 3.