Detection and identification of O-GlcNAc-modified proteins using 6-azido-6-deoxy-N-acetyl-galactosamine

In Situ Imaging of O-Linked β-N-Acetylglucosamine Using On-Tissue Hydrolysis and MALDI Mass Spectrometry

Post-translational O-glycosylation of proteins via the addition of N-acetylglucosamine (O-GlcNAc) is a regulator of many aspects of cellular physiology. Processes driven by perturbed dynamics of O-GlcNAcylation modification have been implicated in cancer development. Variability in O-GlcNAcylation is emerging as a metabolic biomarker of many cancers. Here, we evaluate the use of MALDI-mass spectrometry imaging (MSI) to visualize the location of O-GlcNAcylated proteins in tissue sections by mapping GlcNAc that has been released by the enzymatic hydrolysis of glycoproteins using an O-GlcNAc hydrolase. We use this strategy to monitor O-GlcNAc within hepatic VX2 tumor tissue. We show that increased O-GlcNAc is found within both viable tumor and tumor margin regions, implicating GlcNAc in tumor progression.

Ac34FGlcNAz is an effective metabolic chemical reporter for O-GlcNAcylated proteins with decreased S-glyco-modification

O-GlcNAcylation is a ubiquitous post-translational modification governing vital biological processes in cancer, diabetes and neurodegeneration. Metabolic chemical reporters (MCRs) containing bio-orthogonal groups such as azido or alkyne, are widely used for labeling of interested proteins. However, most MCRs developed for O-GlcNAc modification are not specific and always lead to unexpected side reactions termed S-glyco-modification. Here, we attempt to develop a new MCR of Ac₃4FGlcNAz that replacing the 4-OH of Ac₄GlcNAz with fluorine, which is supposed to abolish the epimerization of GALE and enhance the selectivity. The discoveries demonstrate that Ac₃4FGlcNAz is a powerful MCR for O-GlcNAcylation with high efficiency and the process of this labeling is conducted by the two enzymes of OGT and OGA. Most importantly, Ac₃4FGlcNAz is predominantly incorporated intracellular proteins in the form of O-linkage and leads to negligible S-glyco-modification, indicating it is a selective MCR for O-GlcNAcylation. Therefore, we reason that Ac₃4FGlcNAz developed here is a well characterized MCR of O-GlcNAcylation, which provides more choice for label and enrichment of O-GlcNAc associated proteins.

Metabolic glycoengineering - exploring glycosylation with bioorthogonal chemistry

Glycans are involved in numerous biological recognition events. Being secondary gene products, their labeling by genetic methods - comparable to GFP labeling of proteins - is not possible. To overcome this limitation, metabolic glycoengineering (MGE, also known as metabolic oligosaccharide engineering, MOE) has been developed. In this approach, cells or organisms are treated with synthetic carbohydrate derivatives that are modified with a chemical reporter group. In the cytosol, the compounds are metabolized and incorporated into newly synthesized glycoconjugates. Subsequently, the reporter groups can be further derivatized in a bioorthogonal ligation reaction. In this way, glycans can be visualized or isolated. Furthermore, diverse targeting strategies have been developed to direct drugs, nanoparticles, or whole cells to a desired location. This review summarizes research in the field of MGE carried out in recent years. After an introduction to the bioorthogonal ligation reactions that have been used in in connection with MGE, an overview on carbohydrate derivatives for MGE is given. The last part of the review focuses on the many applications of MGE starting from mammalian cells to experiments with animals and other organisms.

Mass Spectrometry for O-GlcNAcylation

O-linked β-N-acetylglucosamine modification (O-GlcNAcylation) at proteins with low-abundance expression level and species diversity, shows important roles in plenty of biological processes. O-GlcNAcylations with abnormal expression levels are associated with many diseases. Systematically profiling of O-GlcNAcylation at qualitative or quantitative level is vital for their function understanding. Recently, the combination of affinity enrichment, metabolic labeling or chemical tagging with mass spectrometry (MS) have made significant contributions to structure-function mechanism elucidating of O-GlcNAcylations in organisms. Herein, this review provides a comprehensive update of MS-based methodologies for quali-quantitative characterization of O-GlcNAcylation.

The Metabolic Chemical Reporter Ac46AzGal Could Incorporate Intracellular Protein Modification in the Form of UDP-6AzGlc Mediated by OGT and Enzymes in the Leloir Pathway

Galactose is a naturally occurring monosaccharide used to build complex glycans that has not been targeted for labeling as a metabolic reporter. Here, we characterize the cellular modification of proteins by using Ac₄6AzGal in a dose- and time-dependent manner. It is noted that a vast majority of this labeling of Ac₄6AzGal occurs intracellularly in a range of mammalian cells. We also provided evidence that this labeling is dependent on not only the enzymes of OGT responsible for O-GlcNAcylation but also the enzymes of GALT and GALE in the Leloir pathway. Notably, we discover that Ac₄6AzGal is not the direct substrate of OGT, and the labeling results may attribute to UDP-6AzGlc after epimerization of UDP-6AzGal via GALE. Together, these discoveries support the conclusion that Ac₄6AzGal as an analogue of galactose could metabolically label intracellular O-glycosylation modification, raising the possibility of characterization with impaired functions of the galactose metabolism in the Leloir pathway under certain conditions, such as galactosemias.

Synthesis of Na2S2O4 mediated cleavable affinity tag for labeling of O-GlcNAc modified proteins via azide-alkyne cycloaddition

A facile and convergent procedure for the synthesis of azobenzene-based probe was reported, which could selectively release interested proteins conducted with sodium dithionite. Besides, the cleavage efficiency is closely associated with the structural features, in which an ortho-hydroxyl substituent is necessary for reactivity. In addition, the azobenzene tag applied in the Ac₄GlcNAz-labled proteins demonstrated high efficiency and selectivity in comparison with Biotin-PEG₄-Alkyne, which provides a useful platform for enrichment of any desired bioorthogonal proteomics.

Precision Mapping of O-Linked N-Acetylglucosamine Sites in Proteins Using Ultraviolet Photodissociation Mass Spectrometry

Despite its central importance as a regulator of cellular physiology, identification and precise mapping of O-linked N-acetylglucosamine (O-GlcNAc) post-translational modification (PTM) sites in proteins by mass spectrometry (MS) remains a considerable technical challenge. This is due in part to cleavage of the glycosidic bond occurring prior to the peptide backbone during collisionally activated dissociation (CAD), which leads to generation of characteristic oxocarbenium ions and impairs glycosite localization. Herein, we leverage CAD-induced oxocarbenium ion generation to trigger ultraviolet photodissociation (UVPD), an alternate high-energy deposition method that offers extensive fragmentation of peptides while leaving the glycosite intact. Upon activation using UV laser pulses, efficient photodissociation of glycopeptides is achieved with production of multiple sequence ions that enable robust and precise localization of O-GlcNAc sites. Application of this method to tryptic peptides originating from O-GlcNAcylated proteins TAB1 and Polyhomeotic confirmed previously reported O-GlcNAc sites in TAB1 (S395 and S396) and uncovered new sites within both proteins. We expect this strategy will complement existing MS/MS methods and be broadly useful for mapping O-GlcNAcylated residues of both proteins and proteomes.