Development of BODIPY labelled sialic acids as sialyltransferase substrates for direct detection of terminal galactose on N- and O-linked glycans

Detection Strategies for Sialic Acid and Sialoglycoconjugates

Glycoconjugates are a vast class of biomolecules implicated in biological processes important for human health and disease. The structural complexity of glycoconjugates remains a challenge to deciphering their precise biological roles and for their development as biomarkers and therapeutics. Human glycoconjugates on the outside of the cell are modified with sialic (neuraminic) acid residues at their termini. The enzymes that install sialic acids are sialyltransferases (SiaTs), a family of 20 different isoenzymes. The removal and degradation of sialic acids is mediated by neuraminidase (NEU; sialidase) enzymes, of which there are four isoenzymes. In this review, we discuss chemical and biochemical approaches for the detection and analysis of sialoglycoconjugate (SGC) structures and their enzymatic products. The most common methods include affinity probes and synthetic substrates. Fluorogenic and radiolabelled substrates are also important tools for many applications, including screening for enzyme inhibitors. Strategies that give insight into the native substrate-specificity of enzymes that regulate SGCs (SiaT & NEU) are necessary to improve our understanding of the role of sialic acid metabolism in health and disease.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2021-2022

The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates is a well-established technique and this review is the 12th update of the original article published in 1999 and brings coverage of the literature to the end of 2022. As with previous review, this review also includes a few papers that describe methods appropriate to analysis by MALDI, such as sample preparation, even though the ionization method is not MALDI. The review follows the same format as previous reviews. It is divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of computer software for structural identification. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other general areas such as medicine, industrial processes, natural products and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. MALDI is still an ideal technique for carbohydrate analysis, particularly in its ability to produce single ions from each analyte and advancements in the technique and range of applications show little sign of diminishing.

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.

Cell Surface Engineering Tools for Programming Living Assemblies

Breakthroughs in precision cell surface engineering tools are supporting the rapid development of programmable living assemblies with valuable features for tackling complex biological problems. Herein, the authors overview the most recent technological advances in chemically- and biologically-driven toolboxes for engineering mammalian cell surfaces and triggering their assembly into living architectures. A particular focus is given to surface engineering technologies for enabling biomimetic cell-cell social interactions and multicellular cell-sorting events. Further advancements in cell surface modification technologies may expand the currently available bioengineering toolset and unlock a new generation of personalized cell therapeutics with clinically relevant biofunctionalities. The combination of state-of-the-art cell surface modifications with advanced biofabrication technologies is envisioned to contribute toward generating living materials with increasing tissue/organ-mimetic bioactivities and therapeutic potential.

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.

Dual targeting pH responsive chitosan nanoparticles for enhanced active cellular internalization of gemcitabine in non-small cell lung cancer

Lung cancer (LC), related with the enhanced expression of epidermal growth factor receptor (EGFR) and sialic acid binding receptors (glycan) brought about the development of EGFR and glycan receptor specific anticancer therapeutics. The current study assessed the formulation, physiochemical characterization, in vitro and in vivo effects of sialic acid (SA) and cetuximab (Cxmab) decorated chitosan nanoparticles (CSN-NPs) loaded with gemcitabine (GMC) targeted to glycan and EGFR over-expressing non-small-cell lung-cancer (NSCLC) A-549 cells. Chitosan (CSN) was conjugated with sialic acid via EDC/NHS chemistry followed by gemcitabine loaded sialic acid conjugated chitosan nanoparticles (GMC-CSN-SA-NPs) were prepared by ionic gelation method decorated with Cxmab by electrostatic interaction. In vitro cytotoxicity of NPs quantified using cell based MTT, DAPI and Annexing-V/PI apoptosis assays showed superior antiproliferative activity of targeted nanoformulations (GMC-CSN-SA-Cxmab-NPs ≫ GMC-CSN-SA-NPs, GMC-CSN-Cxmab-NPs) over non-targeted nanoformulation (GMC-CSN-NPs) against A-549 cells. In vivopharmacokinetic study showed superior bioavailability and in vivo therapeutic efficacy investigation exhibited strongest anticancer activity of glycan and EGFR targeted NPs (GMC-CSN-SA-Cxmab-NPs). GMC-CSN-SA-Cxmab-NPs demonstrated enhanced cellular internalization and better therapeutic potential, by specifically targeting glycan and EGFR on NSCLC A-549 cells and B[a]P induced lung cancer mice model, hence it might be a good substitute for non-targeted, conventional chemotherapy.

XPS and quantum chemical analysis of 4Me-BODIPY derivatives

The results of a X-ray photoelectron spectroscopy (XPS) and steady-state absorption spectroscopy study of the electronic structure, and cationic and excited states of a series of 1,3,5,7-tetramethyl-substituted BODIPYs (4Me,2R-BODIPYs) are presented. The experimental data were interpreted using high-level ab initio quantum chemical computations, including the algebraic diagrammatic construction method for the polarization propagator of the second order (ADC(2)), the outer-valence Green's function (OVGF) method, the density functional (DFT) approach, and the time-dependent DFT (TD-DFT) approach. Substitution effects on the XPS and absorption spectra were determined for 2,6-positions of 4Me,2R-BODIPY pyrrole nuclei (R = H, Br, Bu, benzyl). A very satisfactory performance of the DFT Koopmans theorem analogue was demonstrated with respect to the energy intervals between the electronic levels of 4Me,2R-BODIPY above 13 eV (BHHLYP functional) and the values of the HOMO-LUMO energy gap (ωB97X functional).

Quantifying Carbohydrate-Active Enzyme Activity with Glycoprotein Substrates Using Electrospray Ionization Mass Spectrometry and Center-of-Mass Monitoring

Carbohydrate-active enzymes (CAZymes) play critical roles in diverse physiological and pathophysiological processes and are important for a wide range of biotechnology applications. Kinetic measurements offer insight into the activity and substrate specificity of CAZymes, information that is of fundamental interest and supports diverse applications. However, robust and versatile kinetic assays for monitoring the kinetics of intact glycoprotein and glycolipid substrates are lacking. Here, we introduce a simple but quantitative electrospray ionization mass spectrometry (ESI-MS) method for measuring the kinetics of CAZyme reactions involving glycoprotein substrates. The assay, referred to as center-of-mass (CoM) monitoring (CoMMon), relies on continuous (real-time) monitoring of the CoM of an ensemble of glycoprotein substrates and their corresponding CAZyme products. Notably, there is no requirement for calibration curves, internal standards, labeling, or mass spectrum deconvolution. To demonstrate the reliability of CoMMon, we applied the method to the neuraminidase-catalyzed cleavage of N-acetylneuraminic acid (Neu5Ac) residues from a series of glycoproteins of varying molecular weights and degrees of glycosylation. Reaction progress curves and initial rates determined with CoMMon are in good agreement (initial rates within ≤5%) with results obtained, simultaneously, using an isotopically labeled Neu5Ac internal standard, which enabled the time-dependent concentration of released Neu5Ac to be precisely measured. To illustrate the applicability of CoMMon to glycosyltransferase reactions, the assay was used to measure the kinetics of sialylation of a series of asialo-glycoproteins by a human sialyltransferase. Finally, we show how combining CoMMon and the competitive universal proxy receptor assay enables the relative reactivity of glycoprotein substrates to be quantitatively established.

Investigation of sequon engineering for improved O-glycosylation by the human polypeptide N-acetylgalactosaminyl transferase T2 isozyme and two orthologues

We have been developing bacterial expression systems for human mucin-type O-glycosylation on therapeutic proteins, which is initiated by the addition of α-linked GalNAc to serine or threonine residues by enzymes in the GT-27 family of glycosyltransferases. Substrate preference across different isoforms of this enzyme is influenced by isoform-specific amino acid sequences at the site of glycosylation, which we have exploited to engineer production of Core 1 glycan structures in bacteria on human therapeutic proteins. Using RP-HPLC with a novel phenyl bonded phase to resolve intact protein glycoforms, the effect of sequon mutation on O-glycosylation initiation was examined through in vitro modification of the naturally O-glycosylated human interferon α-2b, and a sequon engineered human growth hormone. As part of the development of our glycan engineering in the bacterial expression system we are surveying various orthologues of critical enzymes to ensure complete glycosylation. Here we present an in vitro enzyme kinetic profile of three related GT-27 orthologues on natural and engineered sequons in recombinant human interferon α2b and human growth hormone where we show a significant change in kinetic properties with the amino acid changes. It was found that optimizing the protein substrate amino acid sequence using Isoform Specific O-Glycosylation Prediction (ISOGlyP, http://isoglyp.utep.edu/index.php) resulted in a measurable increase in kcat/KM, thus improving glycosylation efficiency. We showed that the Drosophila orthologue showed superior activity with our human growth hormone designed sequons compared with the human enzyme.

Cellular and Molecular Engineering of Glycan Sialylation in Heterologous Systems

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

Bringing Color to Sugars: The Chemical Assembly of Carbohydrates to BODIPY Dyes

The combination of carbohydrates with BODIPY fluorophores gives rise to a family of BODIPY-carbohydrate hybrids or glyco-BODIPYs, which mutually benefit from the encounter. Thus, from the carbohydrates standpoint, glyco-BODIPYs can be regarded as fluorescent glycoconjugate derivatives with application in imaging techniques, whereas from the fluorophore view the BODIPY-carbohydrate hybrids benefit from the biocompatibility, water-solubility, and reduced toxicity, among others, brought about by the sugar moiety. In this Account we have intended to present the collection of available methods for the synthesis of BODIPY-carbohydrate hybrids, with a focus on the chemical transformations on the BODIPY core.

Chemical reporters to study mammalian O-glycosylation

Glycans play essential roles in a range of cellular processes and have been shown to contribute to various pathologies. The diversity and dynamic nature of glycan structures and the complexities of glycan biosynthetic pathways make it challenging to study the roles of specific glycans in normal cellular function and disease. Chemical reporters have emerged as powerful tools to characterise glycan structures and monitor dynamic changes in glycan levels in a native context. A variety of tags can be introduced onto specific monosaccharides via the chemical modification of endogenous glycan structures or by metabolic or enzymatic incorporation of unnatural monosaccharides into cellular glycans. These chemical reporter strategies offer unique opportunities to study and manipulate glycan functions in living cells or whole organisms. In this review, we discuss recent advances in metabolic oligosaccharide engineering and chemoenzymatic glycan labelling, focusing on their application to the study of mammalian O-linked glycans. We describe current barriers to achieving glycan labelling specificity and highlight innovations that have started to pave the way to overcome these challenges.