A One-Step Chemoenzymatic Labeling Strategy for Probing Sialylated Thomsen-Friedenreich Antigen

Efficient Enzymatic Glycan Engineering of Extracellular Vesicles Using Nanomaterial-Interfaced Microfluidics

Extracellular vesicles (EVs) present a promising modality for numerous biological and medical applications, including therapeutics. Developing facile methods to engineer EVs is essential to meeting the rapidly expanding demand for various functionalized EVs in these applications. Herein, we developed a technology that integrates enzymatic glycoengineering and microfluidics for effective EV functionalization. This method builds on a 3D nanostructured microfluidic device to streamline a multiple-step EV engineering process, which involves a step of enzymatic reaction to install azido-sialic acid residues to glycans on EVs using a sialyltransferase and an azide-tagged sialyl donor followed by the attachment of various functionalities, such as biotin and fluorescent labels, to the resulting azido-glycans on EVs through a biocompatible click reaction. Compared to traditional EV engineering methods, we show that our technology improves the efficiency of EV glycoengineering while simplifying and expediting the workflow. Furthermore, we demonstrated the applicability of this technology to EVs derived from the cell lines of different cancer types, including A549, PC3, and COLO-1 cells. Overall, this EV engineering technology could provide a potentially useful tool for broad applications.

Enzymatic Synthesis of Disialyllacto-N-Tetraose (DSLNT) and Related Human Milk Oligosaccharides Reveals Broad Siglec Recognition of the Atypical Neu5Acα2-6GlcNAc Motif

Sialic acids (Sias) are ubiquitously expressed on all types of glycans, typically as terminating residues. They usually link to galactose, N-acetylgalactosamine, or other Sia residues, forming ligands of many glycan-binding proteins. An atypical linkage to the C6 of N-acetylglucosamine (GlcNAc) has been identified in human milk oligosaccharides (HMOs, e.g., DSLNT) and tumor-associated glycoconjugates. Herein, describe the systematic synthesis of these HMOs in an enzymatic modular manner. The synthetic strategy relies on a novel activity of ST6GalNAc6 for efficient construction of the Neu5Acα2-6GlcNAc linkage, and another 12 specific enzyme modules for sequential HMO assembly. The structures enabled comprehensive exploration of their structure-function relationships using glycan microarrays, revealing broad yet distinct recognition by Siglecs of the atypical Neu5Acα2-6GlcNAc motif. The work provides tools and new insight for the functional study and potential applications of Siglecs and HMOs.

Enzyme-Sialylation-Controlled Chemical Sulfation of Glycan Epitopes for Decoding the Binding of Siglec Ligands

Widely distributed in nature, sulfated glycan epitopes play important roles in diverse pathophysiological processes. However, due to their structural complexity, the preparation of glycan epitopes with structurally defined sulfation patterns is challenging, which significantly hampers the detailed elucidation of their biological functions at the molecular level. Here, we introduce a strategy for site-specific chemical sulfation of glycan epitopes, leveraging enzymatic sialylation and desialylation processes to precisely control the regio-specificity of sulfation of disaccharide or trisaccharide glycan backbones. Using this method, a sulfated glycan library covering the most common sialylated glycan epitopes was prepared in high yield and efficiency. By screening a microarray prepared with this glycan library, we systematically probed their binding specificity with human Siglecs (sialic acid-binding immunoglobulin-type lectins), many of which function as glyco-immune checkpoints to suppress immune system activation. Our investigation revealed that sulfation and sialylation patterns serve as important determinants of Siglec binding affinity and specificity. Thus, these findings offer new insights for the development of research tools and potential therapeutic agents targeting glyco-immune checkpoints by modulating the Siglec signaling pathway.

Recent progress in chemoenzymatic synthesis of human glycans

Glycan is an essential cell component that usually exists in either a free form or a glycoconjugated form. Glycosylation affects the regulatory function of glycoconjugates in health and disease development, indicating the key role of glycan in organisms. Because of the complexity and diversity of glycan structures, it is challenging to prepare structurally well-defined glycans, which hinders the investigation of biological functions at the molecular level. Chemoenzymatic synthesis is an attractive approach for preparing complex glycans, because it avoids tedious protecting group manipulations in chemical synthesis and ensures high regio- and stereo-selectivity of glucosides during glycan assembly. Herein, enzymes, such as glycosyltransferases (GTs) and glycosidases (GHs), and sugar donors involved in the chemoenzymatic synthesis of human glycans are initially discussed. Many state-of-the-art chemoenzymatic methodologies are subsequently displayed and summarized to illustrate the development of synthetic human glycans, for example, N- and O-linked glycans, human milk oligosaccharides, and glycosaminoglycans. Thus, we provide an overview of recent chemoenzymatic synthetic designs and applications for synthesizing complex human glycans, along with insights into the limitations and perspectives of the current methods.

A Versatile One-Step Enzymatic Strategy for Efficient Imaging and Mapping of Tumor-Associated Tn Antigen

Tn antigen (CD175), recognized as the precursor monosaccharide (α-GalNAc) of mucin O-glycan, is a well-known tumor-associated carbohydrate antigen (TACA). It has emerged as a potential biomarker for cancer diagnosis and prognosis. However, the role it plays in cancer biology remains elusive due to the absence of a sensitive and selective detection method. In this study, we synthesized two new probes based on a unique uridine-5'-diphospho-α-d-galactose (UDP-Gal) derivative, each functionalized with either a fluorescence or a cleavable biotin tag, to develop an innovative one-step enzymatic labeling strategy, enabling the visualization, enrichment, and site-specific mapping of the Tn antigen with unparalleled sensitivity and specificity. Our versatile strategy has been successfully applied to detect and image Tn antigen across various samples, including the complex cell lysates, live cells, serum, and tissue samples. Compared to the traditional lectin method, this one-step enzymatic method is simpler and more efficient (>10/100-fold in sensitivity). Furthermore, it allowed us to map 454 Tn-glycoproteins and 624 Tn-glycosylation sites from HEK293FTn+ and Jurkat cells. Therefore, our strategy provides an exceptionally promising tool for revealing the biological functions of the Tn antigen and advancing cancer diagnostics.

Towards chemoenzymatic labeling strategies for profiling protein glycosylation

Protein glycosylation is one of the most common and important post-translational modifications of proteins involved in regulating glycoprotein functions. The chemoenzymatic glycan labeling strategy allows rapid, efficient, and selective interrogation of glycoproteins. Glycoproteomics identifies protein glycosylation events at a large scale, providing information such as peptide sequences, glycan structures, and glycosylated sites. This review discusses the recent development of chemoenzymatic labeling strategies for glycoprotein analysis, mainly including glycoprotein and glycosite profiling. Furthermore, we highlight the chemoenzymatic enrichment approaches in mass spectrometry analysis for three classes of glycan modifications, including N-glycosylation, O-GlcNAcylation, and mucin-type O-glycosylation. Finally, we highlight the emerging trends in new tools and cutting-edge technologies available for glycoproteomic research.

A "One-Step" Strategy for the Global Characterization of Core-Fucosylated Glycoproteome

Core fucosylation, a special type of N-linked glycosylation, is important in tumor proliferation, invasion, metastatic potential, and therapy resistance. However, the core-fucosylated glycoproteome has not been extensively profiled due to the low abundance and poor ionization efficiency of glycosylated peptides. Here, a "one-step" strategy has been described for protein core-fucosylation characterization in biological samples. Core-fucosylated peptides can be selectively labeled with a glycosylated probe, which is linked with a temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) polymer, by mutant endoglycosidase (EndoF3-D165A). The labeled probe can be further removed by wild-type endoglycosidase (EndoF3) in a traceless manner for mass spectrometry (MS) analysis. The feasibility and effectiveness of the "one-step" strategy are evaluated in bovine serum albumin (BSA) spiked with standard core-fucosylated peptides, H1299, and Jurkat cell lines. The "one-step" strategy is then employed to characterize core-fucosylated sites in human lung adenocarcinoma, resulting in the identification of 2494 core-fucosylated sites distributed on 1176 glycoproteins. Further data analysis reveals that 196 core-fucosylated sites are significantly upregulated in tumors, which may serve as potential drug development targets or diagnostic biomarkers. Together, this "one-step" strategy has great potential for use in global and in-depth analysis of the core-fucosylated glycoproteome to promote its mechanism research.

O-Glycome Profiling of Breast Cancer Cell Lines to Understand Breast Cancer Brain Metastasis

Breast cancer is the second leading cause of cancer-related death among women and a major source of brain metastases. Despite the increasing incidence of brain metastasis from breast cancer, the underlying mechanisms remain poorly understood. Altered glycosylation is known to play a role in various diseases including cancer metastasis. However, profiling studies of O-glycans and their isomers in breast cancer brain metastasis (BCBM) are scarce. This study analyzed the expression of O-glycans and their isomers in human breast cancer cell lines (MDA-MB-231, MDA-MB-361, HTB131, and HTB22), a brain cancer cell line (CRL-1620), and a brain metastatic breast cancer cell line (MDA-MB-231BR) using nanoLC-MS/MS, identifying 27 O-glycan compositions. We observed significant upregulation in the expression of HexNAc1Hex1NeuAc2 and HexNAc2Hex3, whereas the expression of HexNAc1Hex1NeuAc1 was downregulated in MDA-MB-231BR compared to other cell lines. In our isomeric analysis, we observed notable alterations in the isomeric forms of the O-glycan structure HexNAc1Hex1NeuAc1 in a comparison of different cell lines. Our analysis of O-glycans and their isomers in cancer cells demonstrated that changes in their distribution can be related to the metastatic process. We believe that our investigation will contribute to an enhanced comprehension of the significance of O-glycans and their isomers in BCBM.

In Situ Glycan Analysis and Editing in Living Systems

Besides proteins and nucleic acids, carbohydrates are also ubiquitous building blocks of living systems. Approximately 70% of mammalian proteins are glycosylated. Glycans not only provide structural support for living systems but also act as crucial regulators of cellular functions. As a result, they are considered essential pieces of the life science puzzle. However, research on glycans has lagged far behind that on proteins and nucleic acids. The main reason is that glycans are not direct products of gene coding, and their synthesis is nontemplated. In addition, the diversity of monosaccharide species and their linkage patterns contribute to the complexity of the glycan structures, which is the molecular basis for their diverse functions. Research in glycobiology is extremely challenging, especially for the in situ elucidation of glycan structures and functions. There is an urgent need to develop highly specific glycan labeling tools and imaging methods and devise glycan editing strategies. This Perspective focuses on the challenges of in situ analysis of glycans in living systems at three spatial levels (i.e., cell, tissue, and in vivo) and highlights recent advances and directions in glycan labeling, imaging, and editing tools. We believe that examining the current development landscape and the existing bottlenecks can drive the evolution of in situ glycan analysis and intervention strategies and provide glycan-based insights for clinical diagnosis and therapeutics.

Glycosyltransferases as versatile tools to study the biology of glycans

All cells are decorated with complex carbohydrate structures called glycans that serve as ligands for glycan-binding proteins (GBPs) to mediate a wide range of biological processes. Understanding the specific functions of glycans is key to advancing an understanding of human health and disease. However, the lack of convenient and accessible tools to study glycan-based interactions has been a defining challenge in glycobiology. Thus, the development of chemical and biochemical strategies to address these limitations has been a rapidly growing area of research. In this review, we describe the use of glycosyltransferases (GTs) as versatile tools to facilitate a greater understanding of the biological roles of glycans. We highlight key examples of how GTs have streamlined the preparation of well-defined complex glycan structures through chemoenzymatic synthesis, with an emphasis on synthetic strategies allowing for site- and branch-specific display of glyco-epitopes. We also describe how GTs have facilitated expansion of glyco-engineering strategies, on both glycoproteins and cell surfaces. Coupled with advancements in bioorthogonal chemistry, GTs have enabled selective glyco-epitope editing of glycoproteins and cells, selective glycan subclass labeling, and the introduction of novel biomolecule functionalities onto cells, including defined oligosaccharides, antibodies, and other proteins. Collectively, these approaches have contributed great insight into the fundamental biological roles of glycans and are enabling their application in drug development and cellular therapies, leaving the field poised for rapid expansion.

One-Step Selective Labeling of Native Cell Surface Sialoglycans by Exogenous α2,8-Sialylation

Exo-enzymatic glycan labeling strategies have emerged as versatile tools for efficient and selective installation of terminal glyco-motifs onto live cell surfaces. Through employing specific enzymes and nucleotide-sugar probes, cells can be equipped with defined glyco-epitopes for modulating cell function or selective visualization and enrichment of glycoconjugates. Here, we identifyCampylobacter jejunisialyltransferase Cst-II I53S as a tool for cell surface glycan modification, expanding the exo-enzymatic labeling toolkit to include installation of α2,8-disialyl epitopes. Labeling with Cst-II was achieved with biotin- and azide-tagged CMP-Neu5Ac derivatives on a model glycoprotein and native sialylated cell surface glycans across a panel of cell lines. The introduction of modified Neu5Ac derivatives onto cells by Cst-II was also retained on the surface for 6 h. By examining the specificity of Cst-II on cell surfaces, it was revealed that the α2,8-sialyltransferase primarily labeled N-glycans, with O-glycans labeled to a lesser extent, and there was an apparent preference for α2,3-linked sialosides on cells. This approach thus broadens the scope of tools for selective exo-enzymatic labeling of native sialylated glycans and is highly amenable for the construction of cell-based arrays.

Site-Specific and Multiple Fluorogenic Metabolic Glycan Labeling and Glycoproteomic Profiling in Live Cells

Multicolor labeling for monitoring the intracellular localization of the same target type in the native environment using chemical fluorescent dyes is a challenging task. This approach requires both bioorthogonal and biocompatible ligations with an excellent fluorescence signal-to-noise ratio. Here, we present a metabolic glycan labeling technique that uses homemade fluorogenic dyes to investigate glycosylation patterns in live cells. These dyes allowed us to demonstrate rapid and efficient simultaneous multilabeling of glycoconjugates with minimum fluorescence noise. Our results demonstrate that this approach is capable of not only probing sialylation and GlcNAcylation in cells but also specifically labeling the cell-surface and intracellular sialylated glycoconjugates in live cells. In particular, we performed site-specific dual-channel fluorescence imaging of extra and intracellular sialylated glycans in HeLa and PC9 cancer cells as well as identified fluorescently labeled sialylated glycoproteins and glycans by a direct enrichment approach combined with an MS-based proteomic analysis in the same experiment. In conclusion, this study provides multilabeling tools in cellular systems for simultaneous site-specific glycan imaging and glycoproteomic analysis to study potential cancer- and disease-associated glycoconjugates.

Chemoenzymatic Tagging of Tn/TF/STF Antigens in Living Systems

Truncated mucin-type O-glycans, such as Tn-associated antigens, are aberrantly expressed biomarkers of cancer, but remain challenging to target. Reactive antibodies to these antigens either lack high-affinity or are prone to antigen escape. Here, we have developed a robust chemoenzymatic strategy for the global labeling of Tn-associated antigens, i.e. Tn (GalNAcα-O-Ser/Thr), Thomsen-Friedenreich (Galβ1-3GalNAcα-O-Ser/Thr, TF) and STF (Neu5Acα2-3Galβ1-3GalNAcα-O-Ser/Thr, STF) antigens, in human whole blood with high efficiency and selectivity. This method relies on the use of the O-glycan sialyltransferase ST6GalNAc1 to transfer a sialic acid-functionalized adaptor to the GalNAc residue of these antigens. By tagging, the adaptor functionalized antigens can be easily targeted by customized strategies such as, but not limited to, chimeric antigen receptor T-Cells (CAR-T). We expect this tagging system to find broad applications in cancer diagnostics and targeting in combination with established strategies.

Transforms of Cell Surface Glycoproteins Charge Influences Tumor Cell Metastasis via Atypically Inhibiting Epithelial-Mesenchymal Transition Including Matrix Metalloproteinases and Cell Junctions

Cell communication and signal transduction rely heavily on the charge on the cell surface. The cell surface is negatively charged, with glycoproteins on the cell membrane providing a large percentage of the charge. Sialic acid is found on the outermost side of glycan chains and contributes to glycoprotein's negative charge. Sialic acid is highly expressed in tumor cells and plays an important role in tumor metastasis and immune escape by interacting with extracellular ligands. However, the specific effect of negative charge changes on glycoproteins is still poorly understood. In this study, we used 9-azido sialic acid (9Az-Sia) to create artificial epitopes on glycoproteins via metabolic glycan labeling, and we attached charged groups such as amino and carboxyl to 9Az-Sia via a click reaction with dibenzocyclooctyne (DBCO). The charge of glycoproteins was changed by metabolic glycan labeling and click modification. The results showed that the migration and invasion ability of the MDA-MB-231 cell labeled with 9Az-Sia was significantly reduced after the modification with amino groups rather than carboxyl groups. Epithelial-mesenchymal transition (EMT) is the biological process of metastatic tumor cells, with an increasing ability of tumor cells to migrate and invade. In particular, the expression of adhesion molecules increased in the amine-linked group, whereas the expression of matrix metalloproteinases (MMPs) increased significantly, which is not identical to EMT characteristics. In vivo experiments have demonstrated that the loss of negative charge on glycoproteins has an inhibitory effect on tumors. In conclusion, modifying the positive charge on the surface of glycoproteins can inhibit tumor cell metastasis and has great potential for tumor therapy.

Different Biophysical Properties of Cell Surface α2,3- and α2,6-Sialoglycans Revealed by Electron Paramagnetic Resonance Spectroscopic Studies

Sialoglycans on HeLa cells were labeled with a nitroxide spin radical through enzymatic glycoengineering (EGE)-mediated installation of azide-modified sialic acid (Neu5Ac9N₃) and then click reaction-based attachment of a nitroxide spin radical. α2,6-Sialyltransferase (ST) Pd2,6ST and α2,3-ST CSTII were used for EGE to install α2,6- and α2,3-linked Neu5Ac9N₃, respectively. The spin-labeled cells were analyzed by X-band continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy to gain insights into the dynamics and organizations of cell surface α2,6- and α2,3-sialoglycans. Simulations of the EPR spectra revealed average fast- and intermediate-motion components for the spin radicals in both sialoglycans. However, α2,6- and α2,3-sialoglycans in HeLa cells possess different distributions of the two components, e.g., a higher average population of the intermediate-motion component for α2,6-sialoglycans (78%) than that for α2,3-sialoglycans (53%). Thus, the average mobility of spin radicals in α2,3-sialoglycans was higher than that in α2,6-sialoglycans. Given the fact that a spin-labeled sialic acid residue attached to the 6-O-position of galactose/N-acetyl-galactosamine would experience less steric hindrance and show more flexibility than that attached to the 3-O-position, these results may reflect the differences in local crowding/packing that restrict the spin-label and sialic acid motion for α2,6-linked sialoglycans. The studies further suggest that Pd2,6ST and CSTII may have different preferences for glycan substrates in the complex environment of the extracellular matrix. The discoveries of this work are biologically important as they are useful for interpreting the different functions of α2,6- and α2,3-sialoglycans and indicate the possibility of using Pd2,6ST and CSTII to target different glycoconjugates on cells.

Systematic characterization of extracellular glycoproteins using mass spectrometry

Surface and secreted glycoproteins are essential to cells and regulate many extracellular events. Because of the diversity of glycans, the low abundance of many glycoproteins, and the complexity of biological samples, a system-wide investigation of extracellular glycoproteins is a daunting task. With the development of modern mass spectrometry (MS)-based proteomics, comprehensive analysis of different protein modifications including glycosylation has advanced dramatically. This review focuses on the investigation of extracellular glycoproteins using MS-based proteomics. We first discuss the methods for selectively enriching surface glycoproteins and investigating protein interactions on the cell surface, followed by the application of MS-based proteomics for surface glycoprotein dynamics analysis and biomarker discovery. We then summarize the methods to comprehensively study secreted glycoproteins by integrating various enrichment approaches with MS-based proteomics and their applications for global analysis of secreted glycoproteins in different biological samples. Collectively, MS significantly expands our knowledge of extracellular glycoproteins and enables us to identify extracellular glycoproteins as potential biomarkers for disease detection and drug targets for disease treatment.

Convergent chemoenzymatic synthesis of O-GalNAc rare cores 5, 7, 8 and their sialylated forms

All O-GalNAc glycans are derived from 8 cores with 2 or 3 monosaccharides linked via α- or β-glycosidic bonds. While chemical and chemoenzymatic syntheses of β-linked cores 1-4 and 6 and derived glycans have been well developed, the preparation of α-linked rare cores 5, 7, and 8 is challenging due to the presence of this 1,2-cis linkage. Meanwhile, the biosynthesis and functional roles of these structures are poorly understood. Herein, we synthesize 3 α-linked rare cores with exclusive α-configuration from a versatile precursor through multifaceted chemical modulations. Efficient regioselective α2-6sialylion of the rare cores was then achieved by Photobacterium damselae α2-6sialyltransferase-catalyzed reactions. These structures, together with β-linked cores 1-4 and 6, and their sialylated forms, were fabricated into a comprehensive O-GalNAc core microarray to profile the binding of clinically important GalNAc-specific lectins. It is found that only Tn, (sialyl-)core 5, and core 7 are the binders of WFL, VVL, and SBA, while DBA only recognized (sialyl-)core 5, and Jacalin is the only lectin that binds core 8. In addition, activity assays of human α-N-acetylgalactosaminide α2-6sialyltransferases (ST6GalNAcTs) towards the cores suggested that ST6GalNAc1 may be involved in the biosynthesis of previously identified sialyl-core 5 and sialyl-core 8 glycans. In conclusion, we provide efficient routes to access α-linked O-GalNAc rare cores and derived structures, which are valuable tools for functional glycomics studies of mucin O-glycans.

Resveratrol-loaded macrophage exosomes alleviate multiple sclerosis through targeting microglia

Despite exosome promise as endogenous drug delivery vehicles, the current understanding of exosome may be insufficient to develop their various applications. Here we synthesized five sialic acid analogues with different length N-acyl side chains and screened out the optimal metabolic precursor for exosome labeling via bio-orthogonal click chemistry. In proof-of-principle labeling experiments, exosomes derived from macrophages (RAW-Exo) strongly co-localized with central nervous system (CNS) microglia. Inspired by this discovery, we developed a resveratrol-loaded RAW-Exo formulation (RSV&Exo) for multiple sclerosis (MS) treatment. Intranasal administration of RSV&Exo significantly inhibited inflammatory responses in the CNS and peripheral system in a mouse model of MS and effectively improved the clinical evolution of MS in vivo. These findings suggested the feasibility and efficacy of engineered RSV&Exo administration for MS, providing a potential therapeutic strategy for CNS diseases.

Cancer glycomics offers potential biomarkers and therapeutic targets in the framework of 3P medicine

Glycosylation is one of the most important post-translational modifications (PTMs) in a protein, and is the most abundant and diverse biopolymer in nature. Glycans are involved in multiple biological processes of cancer initiation and progression, including cell-cell interactions, cell-extracellular matrix interactions, tumor invasion and metastasis, tumor angiogenesis, and immune regulation. As an important biomarker, tumor-associated glycosylation changes have been extensively studied. This article reviews recent advances in glycosylation-based biomarker research, which is useful for cancer diagnosis and prognostic assessment. Truncated O-glycans, sialylation, fucosylation, and complex branched structures have been found to be the most common structural patterns in malignant tumors. In recent years, immunochemical methods, lectin recognition-based methods, mass spectrometry (MS)-related methods, and fluorescence imaging-based in situ methods have greatly promoted the discovery and application potentials of glycomic and glycoprotein biomarkers in various cancers. In particular, MS-based proteomics has significantly facilitated the comprehensive research of extracellular glycoproteins, increasing our understanding of their critical roles in regulating cellular activities. Predictive, preventive and personalized medicine (PPPM; 3P medicine) is an effective approach of early prediction, prevention and personalized treatment for different patients, and it is known as the new direction of medical development in the 21st century and represents the ultimate goal and highest stage of medical development. Glycosylation has been revealed to have new diagnostic, prognostic, and even therapeutic potentials. The purpose of glycosylation analysis and utilization of biology is to make a fundamental change in health care and medical practice, so as to lead medical research and practice into a new era of 3P medicine.

Facile Synthesis of Sugar Nucleotides from Common Sugars by the Cascade Conversion Strategy

Sugar nucleotides are essential glycosylation donors in the carbohydrate metabolism. Naturally, most sugar nucleotides are derived from a limited number of common sugar nucleotides by de novo biosynthetic pathways, undergoing single or multiple reactions such as dehydration, epimerization, isomerization, oxidation, reduction, amination, and acetylation reactions. However, it is widely believed that such complex bioconversions are not practical for synthetic use due to the high preparation cost and great difficulties in product isolation. Therefore, most of the discovered sugar nucleotides are not readily available. Here, based on de novo biosynthesis mainly, 13 difficult-to-access sugar nucleotides were successfully prepared from two common sugars D-Man and sucrose in high yields, at a multigram scale, and without the need for tedious purification manipulations. This work demonstrated that de novo biosynthesis, although undergoing complex reactions, is also practical and cost-effective for synthetic use by employing a cascade conversion strategy.

Cofactor-Driven Cascade Reactions Enable the Efficient Preparation of Sugar Nucleotides

Glycosylation is catalyzed by glycosyltransferases using sugar nucleotides or occasionally lipid-linked phosphosugars as donors. However, only very few common sugar nucleotides that occur in humans can be obtained readily, while the majority of sugar nucleotides that exist in bacteria, plants, archaea, or viruses cannot be synthesized in sufficient quantities by either enzymatic or chemical synthesis. The limited availability of such rare sugar nucleotides is one of the major obstacles that has greatly hampered progress in glycoscience. Herein we describe a general cofactor-driven cascade conversion strategy for the efficient synthesis of sugar nucleotides. The described strategy allows the large-scale preparation of rare sugar nucleotides from common sugars in high yields and without the need for tedious purification processes.

Enzymatic glycoengineering-based spin labelling of cell surface sialoglycans to enable their analysis by electron paramagnetic resonance (EPR) spectroscopy

A novel method for spin labelling of sialoglycans on the cell surface is described. C9-Azido sialic acid was linked to glycans on live cells via CSTII-catalysed α2,3-sialylation utilizing azido-sialic acid nucleotide as a sialyl donor, which was followed by attachment of a spin label to the azide via click reaction. It enables the study of cell surface sialoglycans by EPR spectroscopy.

Putting the pieces together: mapping the O-glycoproteome

Protein glycosylation is the most diverse and omnipresent protein modification. Glycosylation provides glycoproteins with important structural and functional properties to facilitate critical biological processes. Despite the significance of protein glycosylation, the investigation of glycoproteome, especially O-linked glycoproteome, remains elusive due to the lack of a comprehensive methodology to conform with the diversity of O-linked glycoforms of O-linked glycoproteins. In recent years, mass spectrometry has become an indispensable tool for the characterization of O-linked glycosylated proteins across biological systems. We herein highlight the recent developments in MS-based O-linked glycoproteomic technologies, quantitative data acquisition strategy and bioinformatic tools, with a special focus on mucin-type O-linked glycosylation.

One-Step Enzymatic Labeling Reveals a Critical Role of O-GlcNAcylation in Cell-Cycle Progression and DNA Damage Response

O-linked N-acetylglucosamine (O-GlcNAcylation) is a ubiquitous post-translational modification of proteins that is essential for cell function. Perturbation of O-GlcNAcylation leads to altered cell-cycle progression and DNA damage response. However, the underlying mechanisms are poorly understood. Here, we develop a highly sensitive one-step enzymatic strategy for capture and profiling O-GlcNAcylated proteins in cells. Using this strategy, we discover that flap endonuclease 1 (FEN1), an essential enzyme in DNA synthesis, is a novel substrate for O-GlcNAcylation. FEN1 O-GlcNAcylation is dynamically regulated during the cell cycle. O-GlcNAcylation at the serine 352 of FEN1 disrupts its interaction with Proliferating Cell Nuclear Antigen (PCNA) at the replication foci, and leads to altered cell cycle, defects in DNA replication, accumulation of DNA damage, and enhanced sensitivity to DNA damage agents. Thus, our study provides a sensitive method for profiling O-GlcNAcylated proteins, and reveals an unknown mechanism of O-GlcNAcylation in regulating cell cycle progression and DNA damage response.

Glycan Labeling and Analysis in Cells and In Vivo

As one of the major types of biomacromolecules in the cell, glycans play essential functional roles in various biological processes. Compared with proteins and nucleic acids, the analysis of glycans in situ has been more challenging. Herein we review recent advances in the development of methods and strategies for labeling, imaging, and profiling of glycans in cells and in vivo. Cellular glycans can be labeled by affinity-based probes, including lectin and antibody conjugates, direct chemical modification, metabolic glycan labeling, and chemoenzymatic labeling. These methods have been applied to label glycans with fluorophores, which enables the visualization and tracking of glycans in cells, tissues, and living organisms. Alternatively, labeling glycans with affinity tags has enabled the enrichment of glycoproteins for glycoproteomic profiling. Built on the glycan labeling methods, strategies enabling cell-selective and tissue-specific glycan labeling and protein-specific glycan imaging have been developed. With these methods and strategies, researchers are now better poised than ever to dissect the biological function of glycans in physiological or pathological contexts.

Chemoenzymatic modular assembly of O-GalNAc glycans for functional glycomics

O-GalNAc glycans (or mucin O-glycans) play pivotal roles in diverse biological and pathological processes, including tumor growth and progression. Structurally defined O-GalNAc glycans are essential for functional studies but synthetic challenges and their inherent structural diversity and complexity have limited access to these compounds. Herein, we report an efficient and robust chemoenzymatic modular assembly (CEMA) strategy to construct structurally diverse O-GalNAc glycans. The key to this strategy is the convergent assembly of O-GalNAc cores 1-4 and 6 from three chemical building blocks, followed by enzymatic diversification of the cores by 13 well-tailored enzyme modules. A total of 83 O-GalNAc glycans presenting various natural glycan epitopes are obtained and used to generate a unique synthetic mucin O-glycan microarray. Binding specificities of glycan-binding proteins (GBPs) including plant lectins and selected anti-glycan antibodies towards these O-GalNAc glycans are revealed by this microarray, promoting their applicability in functional O-glycomics. Serum samples from colorectal cancer patients and healthy controls are assayed using the array reveal higher bindings towards less common cores 3, 4, and 6 than abundant cores 1 and 2, providing insights into O-GalNAc glycan structure-activity relationships.

Towards structure-focused glycoproteomics

Facilitated by advances in the separation sciences, mass spectrometry and informatics, glycoproteomics, the analysis of intact glycopeptides at scale, has recently matured enabling new insights into the complex glycoproteome. While diverse quantitative glycoproteomics strategies capable of mapping monosaccharide compositions of N- and O-linked glycans to discrete sites of proteins within complex biological mixtures with considerable sensitivity, quantitative accuracy and coverage have become available, developments supporting the advancement of structure-focused glycoproteomics, a recognised frontier in the field, have emerged. Technologies capable of providing site-specific information of the glycan fine structures in a glycoproteome-wide context are indeed necessary to address many pending questions in glycobiology. In this review, we firstly survey the latest glycoproteomics studies published in 2018–2020, their approaches and their findings, and then summarise important technological innovations in structure-focused glycoproteomics. Our review illustrates that while the O-glycoproteome remains comparably under-explored despite the emergence of new O-glycan-selective mucinases and other innovative tools aiding O-glycoproteome profiling, quantitative glycoproteomics is increasingly used to profile the N-glycoproteome to tackle diverse biological questions. Excitingly, new strategies compatible with structure-focused glycoproteomics including novel chemoenzymatic labelling, enrichment, separation, and mass spectrometry-based detection methods are rapidly emerging revealing glycan fine structural details including bisecting GlcNAcylation, core and antenna fucosylation, and sialyl-linkage information with protein site resolution. Glycoproteomics has clearly become a mainstay within the glycosciences that continues to reach a broader community. It transpires that structure-focused glycoproteomics holds a considerable potential to aid our understanding of systems glycobiology and unlock secrets of the glycoproteome in the immediate future.

A Pragmatic Guide to Enrichment Strategies for Mass Spectrometry-Based Glycoproteomics

Glycosylation is a prevalent, yet heterogeneous modification with a broad range of implications in molecular biology. This heterogeneity precludes enrichment strategies that can be universally beneficial for all glycan classes. Thus, choice of enrichment strategy has profound implications on experimental outcomes. Here we review common enrichment strategies used in modern mass spectrometry-based glycoproteomic experiments, including lectins and other affinity chromatographies, hydrophilic interaction chromatography and its derivatives, porous graphitic carbon, reversible and irreversible chemical coupling strategies, and chemical biology tools that often leverage bioorthogonal handles. Interest in glycoproteomics continues to surge as mass spectrometry instrumentation and software improve, so this review aims to help equip researchers with the necessary information to choose appropriate enrichment strategies that best complement these efforts.

Nongenetic Bioconjugation Strategies for Modifying Cell Membranes and Membrane Proteins: A Review

The cell membrane possesses an extensive library of proteins, carbohydrates, and lipids that control a significant portion of inter- and intracellular functions, including signaling, proliferation, migration, and adhesion, among others. Augmenting the cell surface composition would open possibilities for advances in therapy, tissue engineering, and probing fundamental cell processes. While genetic engineering has proven effective for many in vitro applications, these techniques result in irreversible changes to cells and are difficult to apply in vivo. Another approach is to instead attach exogenous functional groups to the cell membrane without changing the genetic nature of the cell. This review focuses on more recent approaches of nongenetic methods of cell surface modification through metabolic pathways, anchorage by hydrophobic interactions, and chemical conjugation. Benefits and drawbacks of each approach are considered, followed by a discussion of potential applications for nongenetic cell surface modification and an outlook on the future of the field.

The Potential Action of Thomsen-Friedenreich Monoclonal Antibody (A78-G/A7) in Thyroid Cancer

Background

Thomsen–Friedenreich antibody (TF-Ab) is a specific antibody against the Thomsen–Friedenreich antigen (TF-Ag). At present, studies on a number of other tumors have shown that TF-Ab can effectively inhibit metastasis and induce apoptosis in tumor cells. However, the role of TF-Ab in thyroid cancer (TC) remains unclear.

Materials and Methods

Normal subjects and patients with primary papillary TC with or without lymph node metastasis were tested for TF-Ab expression by enzyme-linked immunosorbent assays (ELISAs). Immunofluorescence was used to assess the expression of TF-Ag in thyroid papillary carcinoma with or without lymph node metastasis and undifferentiated cancer tissues. To evaluate the role of TF-Ab in TC, the effects of TF monoclonal antibody (mAb A78-G/A7) on cell biological function were investigated by MTT assays, flow cytometry, adhesion assays and transwell experiments.

Results

Compared with normal individuals, TF-Ab levels in patients with TC were decreased, but no changes were observed with respect to lymph node metastasis. The expression of TF-Ag in TC tissues was relatively higher than that detected in adjacent tissues, but it was not affected by the presence or absence of lymph node metastasis. Upon treatment mAb A78-G/A7 treating, TC cell cycles were affected, meanwhile the abilities to adhere, invade and migrate were also significantly reduced.

Conclusion

The results of the present study showed that mAb A78-G/A7 could affect the invasion and migration of all assayed TC cell lines. The effects of mAb A78-G/A7 on the cell cycle, adhesion, invasion and migration of TC cells were more significant than those observed for proliferation and apoptosis.

Improved efficacy of doxorubicin delivery by a novel dual-ligand-modified liposome in hepatocellular carcinoma

Liposomes have been widely used as drug carriers in both biomedical research and for clinical applications, allowing the stabilisation of therapeutic compounds and overcoming obstacles to cellular and tissue uptake. However, liposomes still have low targeting efficiency, resulting in insufficient killing of tumour cells and unnecessary damage to normal cells. In this study, glycyrrhetinic acid (GA) and peanut agglutinin (PNA) were used as ligands to prepare dual-ligand-modified doxorubicin-loaded liposomes (DOX-GA/PNA-Lips) to enhance the targeting accuracy and efficacy of drug delivery against malignant liver cancer. PNA and GA modification enhanced the binding ability of liposomes to liver cancer cells, leading to excellent tissue and cell targeting of DOX-GA/PNA-Lips. DOX-GA/PNA-Lips showed an effective anti-tumour effect in vivo and in vitro, with its targeted delivery facilitating attenuation of the toxic side effects of DOX. These results demonstrated that dual-ligand-modified liposomes may provide an effective strategy for the treatment of hepatocellular carcinoma.

Identifying Sialylation Linkages at the Glycopeptide Level by Glycosyltransferase Labeling Assisted Mass Spectrometry (GLAMS)

Precise assignment of sialylation linkages at the glycopeptide level is of importance in bottom-up glycoproteomics and an indispensable step to understand the function of glycoproteins in pathogen-host interactions and cancer progression. Even though some efforts have been dedicated to the discrimination of α2,3/α2,6-sialylated isomers, unambiguous identification of sialoglycopeptide isomers is still needed. Herein, we developed an innovative glycosyltransferase labeling assisted mass spectrometry (GLAMS) strategy. After specific enzymatic labeling, oxonium ions from higher-energy C-trap dissociation (HCD) fragmentation of α2,3-sailoglycopeptides then generate unique reporters to distinctly differentiate those of α2,6-sailoglycopeptide isomers. With this strategy, a total of 1236 linkage-specific sialoglycopeptides were successfully identified from 161 glycoproteins in human serum.

Direct fluorescent glycan labeling with recombinant sialyltransferases

Glycosylation is a common modification found on numerous proteins and lipids. However, direct detection of glycans on these intact biomolecules has been challenge. Here, utilizing enzymatic incorporation of fluorophore-conjugated sialic acids, dubbed as direct fluorescent glycan labeling, we report the labeling and detection of N- and O-glycans on glycoproteins. The method allows detection of specific glycans without the laborious gel blotting and chemiluminescence reactions used in Western blotting. The method can also be used with a variety of fluorescent dyes.

Direct Visualization of Live Zebrafish Glycans via Single-Step Metabolic Labeling with Fluorophore-Tagged Nucleotide Sugars

Dynamic turnover of cell-surface glycans is involved in a myriad of biological events, making this process an attractive target for in vivo molecular imaging. Metabolic glycan labeling coupled with bioorthogonal chemistry has paved the way for visualizing glycans in living organisms. However, a two-step labeling sequence is required, which suffers from the tissue-penetration difficulties of the imaging probes. Here, by exploring the substrate promiscuity of endogenous glycosyltransferases, we developed a single-step fluorescent glycan labeling strategy by using fluorophore-tagged analogues of the nucleotide sugars. Injecting fluorophore-tagged sialic acid and fucose into the yolk of zebrafish embryos at the one-cell stage enables systematic imaging of sialylation and fucosylation in live zebrafish embryos at distinct developmental stages. From these studies, we obtained insights into the role of sialylated and fucosylated glycans in zebrafish hematopoiesis.

Global and site-specific analysis of protein glycosylation in complex biological systems with Mass Spectrometry

Protein glycosylation is ubiquitous in biological systems and plays essential roles in many cellular events. Global and site-specific analysis of glycoproteins in complex biological samples can advance our understanding of glycoprotein functions and cellular activities. However, it is extraordinarily challenging because of the low abundance of many glycoproteins and the heterogeneity of glycan structures. The emergence of mass spectrometry (MS)-based proteomics has provided us an excellent opportunity to comprehensively study proteins and their modifications, including glycosylation. In this review, we first summarize major methods for glycopeptide/glycoprotein enrichment, followed by the chemical and enzymatic methods to generate a mass tag for glycosylation site identification. We next discuss the systematic and quantitative analysis of glycoprotein dynamics. Reversible protein glycosylation is dynamic, and systematic study of glycoprotein dynamics helps us gain insight into glycoprotein functions. The last part of this review focuses on the applications of MS-based proteomics to study glycoproteins in different biological systems, including yeasts, plants, mice, human cells, and clinical samples. Intact glycopeptide analysis is also included in this section. Because of the importance of glycoproteins in complex biological systems, the field of glycoproteomics will continue to grow in the next decade. Innovative and effective MS-based methods will exponentially advance glycoscience, and enable us to identify glycoproteins as effective biomarkers for disease detection and drug targets for disease treatment. © 2019 Wiley Periodicals, Inc. Mass Spec Rev 9999: XX-XX, 2019.

Efficient chemoenzymatic synthesis of UDP-α-6-N3-glucose

A novel chemo-enzymatic synthetic method for UDP-α-6-N₃-glucose was developed by combining the versatility of chemical synthesis and natural enzyme stereo-selectivity of Bifidobacterium longum (BLUSP). This flexible and efficient platform expanded the substrate scope for UDP-sugars on an improved scale, particularly for UDP-sugar substrates containing bioorthogonal functional groups.

Machine-Driven Enzymatic Oligosaccharide Synthesis by Using a Peptide Synthesizer

For decades, researchers have endeavored to develop a general automated system to synthesize oligosaccharides that is comparable to the preparation of oligonucleotides and oligopeptides by commercially available machines. Inspired by the success of automated oligosaccharide synthesis through chemical glycosylation, a fully automated system is reported for oligosaccharides synthesis through enzymatic glycosylation in aqueous solution. The designed system is based on the use of a thermosensitive polymer and a commercially available peptide synthesizer. This study represents a proof-of-concept demonstration that the enzymatic synthesis of oligosaccharides can be achieved in an automated manner using a commercially available peptide synthesizer.

Mass Spectrometry-Based Chemical and Enzymatic Methods for Global Analysis of Protein Glycosylation

Glycosylation is one of the most common protein modifications, and it is essential for mammalian cell survival. It often determines protein folding and trafficking, and regulates nearly every extracellular activity, including cell-cell communication and cell-matrix interactions. Aberrant protein glycosylation events are hallmarks of human diseases such as cancer and infectious diseases. Therefore, glycoproteins can serve as effective biomarkers for disease detection and targets for drug and vaccine development. Despite the importance of glycoproteins, global analysis of protein glycosylation (either glycoproteins or glycans) in complex biological samples has been a daunting task, and here we mainly focus on glycoprotein analysis using mass spectrometry (MS)-based bottom-up proteomics. Although the emergence of MS-based proteomics has provided a great opportunity to analyze glycoproteins globally, the low abundance of many glycoproteins and the heterogeneity of glycans dramatically increase the technical difficulties. In order to overcome these obstacles, considerable progress has been made in recent years, which has contributed to comprehensive analysis of glycoproteins. In our lab, we developed effective MS-based chemical and enzymatic methods to (1) globally analyze glycoproteins in complex biological samples, (2) target glycoproteins specifically on the surface of human cells, (3) systematically quantify glycoprotein and surface glycoprotein dynamics (the abundance changes of glycoproteins as a function of time), and (4) selectively characterize glycoproteins with a particular and important glycan. In this Account, we first briefly describe the glycopeptide/protein enrichment methods in the literature and then discuss the developments of boronic acid-based methods to enrich glycopeptides for large-scale analysis of protein glycosylation. Boronic acids can form reversible covalent interactions with sugars, but the low binding affinity of normal boronic acid-based methods prevents us from capturing glycoproteins with low abundance, which often contain more valuable information. We enhanced the boronic acid-glycan interactions by using a boronic acid derivative (benzoboroxole) and conjugating it onto a dendrimer to allow synergistic interactions between the boronic acid derivative and sugars. The new method is capable of globally analyzing protein glycosylation with site and glycan structure information, especially for those with low abundance. In the next part, we discuss the combination of metabolic labeling, click chemistry and enzymatic reactions, and MS-based proteomics as a very powerful approach for surface glycoproteome analysis in human cells. The methods enable us to specifically identify surface glycoproteins and to quantify their abundance changes and dynamics together with quantitative proteomics. The last section of this Account focuses on chemical and enzymatic methods to study glycoproteins containing a particular and important glycan (the Tn antigen, i.e., O-GalNAc). Although not comprehensive, this Account provides an overview of chemical and enzymatic methods to characterize protein glycosylation in combination with MS-based proteomics. These methods will have extensive applications in the fields of biology and biomedicine, which will lead to a better understanding of glycoprotein functions and the molecular mechanisms of diseases. Eventually, glycoproteins will be identified as effective biomarkers for disease detection and drug targets for disease treatment.