Metabolic cross-talk allows labeling of O-linked beta-N-acetylglucosamine-modified proteins via the N-acetylgalactosamine salvage pathway

Investigation of relationships between metabolic chemical reporter structures and S-glyco-modification effects

The approach of metabolic chemical reporters (MCRs) for labeling proteins has been widely used in the past several decades. Nevertheless, artificial side reaction generated with fully protected MCRs, termed S-glyco-modification, occurs with cysteine residues through base-promoted β-elimination and Michael addition, leading to false positives in the proteomic identification. Therefore, next generation of MCRs, including partially protected strategy and modifications on the backbone of monosaccharides, have emerged to improve the labeling efficiency. In this paper, we prepared fifteen kinds of unnatural monosaccharides to investigate the relationships of structures and S-glyco-modification labeling. Our results demonstrated that Ac4GlcNAz and Ac4GalNAz exhibited the most remarkable labeling effects among the detected compounds. Of note, Ac4ManNAz, Ac46AzGlucose and Ac46AzGalactose containing similar structures but did not show similar robust signals as them. Moreover, other modifications on the 1-, 2-, 3-, 4- and 6-site indicated minimal side reactions of S-glyco-modification, raising a possibility that subtle modifications of monosaccharide substrate may alter its role in the process of biosynthesis, for example, by change of electronegativity or enhancement of steric hindrance effects. In conclusion, our discoveries provide a new avenue to choose appropriate probe for selective label proteins in vitro and in vivo without undesired S-glyco-modification.

Glycoengineering with neuraminic acid analogs to label lipooligosaccharides and detect native sialyltransferase activity in gram-negative bacteria

Lipooligosaccharides are the most abundant cell surface glycoconjugates on the outer membrane of Gram-negative bacteria. They play important roles in host-microbe interactions. Certain Gram-negative pathogenic bacteria cap their lipooligosaccharides with the sialic acid, N-acetylneuraminic acid (Neu5Ac), to mimic host glycans that among others protects these bacteria from recognition by the hosts immune system. This process of molecular mimicry is not fully understood and remains under investigated. To explore the functional role of sialic acid-capped lipooligosaccharides at the molecular level, it is important to have tools readily available for the detection and manipulation of both Neu5Ac on glycoconjugates and the involved sialyltransferases, preferably in live bacteria. We and others have shown that the native sialyltransferases of some Gram-negative bacteria can incorporate extracellular unnatural sialic acid nucleotides onto their lipooligosaccharides. We here report on the expanded use of native bacterial sialyltransferases to incorporate neuraminic acids analogs with a reporter group into the lipooligosaccharides of a variety of Gram-negative bacteria. We show that this approach offers a quick strategy to screen bacteria for the expression of functional sialyltransferases and the ability to use exogenous CMP-Neu5Ac to decorate their glycoconjugates. For selected bacteria we also show this strategy complements two other glycoengineering techniques, Metabolic Oligosaccharide Engineering and Selective Exo-Enzymatic Labeling, and that together they provide tools to modify, label, detect and visualize sialylation of bacterial lipooligosaccharides.

Click-free imaging of carbohydrate trafficking in live cells using an azido photothermal probe

Real-time tracking of intracellular carbohydrates remains challenging. While click chemistry allows bio-orthogonal tagging with fluorescent probes, the reaction permanently alters the target molecule and only allows a single snapshot. Here, we demonstrate click-free mid-infrared photothermal (MIP) imaging of azide-tagged carbohydrates in live cells. Leveraging the micromolar detection sensitivity for 6-azido-trehalose (TreAz) and the 300-nm spatial resolution of MIP imaging, the trehalose recycling pathway in single mycobacteria, from cytoplasmic uptake to membrane localization, is directly visualized. A peak shift of azide in MIP spectrum further uncovers interactions between TreAz and intracellular protein. MIP mapping of unreacted azide after click reaction reveals click chemistry heterogeneity within a bacterium. Broader applications of azido photothermal probes to visualize the initial steps of the Leloir pathway in yeasts and the newly synthesized glycans in mammalian cells are demonstrated.

Expanding the repertoire of GalNAc analogues for cell-specific bioorthogonal tagging of glycoproteins

Glycosylation is a ubiquitous modification of proteins, necessitating approaches for its visualization and characterization. Bioorthogonally tagged monosaccharides have been instrumental to this end, offering a chemical view into the cell biology of glycans. Understanding the use of such monosaccharides by cellular biosynthetic pathways has expanded their applicability in cell biology, for instance through the strategy named Bio-Orthogonal Cell-specific TAgging of Glycoproteins (BOCTAG). Here, we show that the cellular use of two azide-tagged analogues of the monosaccharide N-acetylgalactosamine (GalNAzMe and GalNPrAz) can be promoted through expression of two biosynthetic enzymes. More precisely, cellular expression of the bacterial kinase NahK and the engineered human pyrophosphorylase AGX1F383A led to biosynthesis of the corresponding activated nucleotide-sugars and subsequent bioorthogonal tagging of the cellular glycoproteome. We explore the use of both sugars for BOCTAG, demonstrating the visualization of cell surface glycosylation tagged with GalNPrAz in a specific cell line in a co-culture system. Our work adds to the toolbox of glycoprotein analysis in biomedicine.

OGA mutant aberrantly hydrolyzes O-GlcNAc modification from PDLIM7 to modulate p53 and cytoskeleton in promoting cancer cell malignancy

O-GlcNAcase (OGA) is the only human enzyme that catalyzes the hydrolysis (deglycosylation) of O-linked beta-N-acetylglucosaminylation (O-GlcNAcylation) from numerous protein substrates. OGA has broad implications in many challenging diseases including cancer. However, its role in cell malignancy remains mostly unclear. Here, we report that a cancer-derived point mutation on the OGA's noncatalytic stalk domain aberrantly modulates OGA interactome and substrate deglycosylation toward a specific set of proteins. Interestingly, our quantitative proteomic studies uncovered that the OGA stalk domain mutant preferentially deglycosylated protein substrates with +2 proline in the sequence relative to the O-GlcNAcylation site. One of the most dysregulated substrates is PDZ and LIM domain protein 7 (PDLIM7), which is associated with the tumor suppressor p53. We found that the aberrantly deglycosylated PDLIM7 suppressed p53 gene expression and accelerated p53 protein degradation by promoting the complex formation with E3 ubiquitin ligase MDM2. Moreover, deglycosylated PDLIM7 significantly up-regulated the actin-rich membrane protrusions on the cell surface, augmenting the cancer cell motility and aggressiveness. These findings revealed an important but previously unappreciated role of OGA's stalk domain in protein substrate recognition and functional modulation during malignant cell progression.

O-GlcNAcylation Facilitates the Interaction between Keratin 18 and Isocitrate Dehydrogenases and Potentially Influencing Cholangiocarcinoma Progression

Glycosylation plays a pivotal role in the intricate landscape of human cholangiocarcinoma (CCA), actively participating in key pathophysiological processes driving tumor progression. Among the various glycosylation modifications, O-linked β-N-acetyl-glucosamine modification (O-GlcNAcylation) emerges as a dynamic regulator influencing diverse tumor-associated biological activities. In this study, we employed a state-of-the-art chemical proteomic approach to analyze intact glycopeptides, unveiling the critical role of O-GlcNAcylation in orchestrating Keratin 18 (K18) and its interplay with tricarboxylic acid (TCA) cycle enzymes, specifically isocitrate dehydrogenases (IDHs), to propel CCA progression. Our findings shed light on the mechanistic intricacies of O-GlcNAcylation, revealing that site-specific modification of K18 on Ser 30 serves as a stabilizing factor, amplifying the expression of cell cycle checkpoints. This molecular event intricately fosters cell cycle progression and augments cellular growth in CCA. Notably, the interaction between O-GlcNAcylated K18 and IDHs orchestrates metabolic reprogramming by down-regulating citrate and isocitrate levels while elevating α-ketoglutarate (α-KG). These metabolic shifts further contribute to the overall tumorigenic potential of CCA. Our study thus expands the current understanding of protein O-GlcNAcylation and introduces a new layer of complexity to post-translational control over metabolism and tumorigenesis.

Subcellular imaging of lipids and sugars using genetically encoded proximity sensors

Live cell imaging of lipids and other metabolites is a long-standing challenge in cell biology. Bioorthogonal labeling tools allow for the conjugation of fluorophores to several phospholipid classes, but cannot discern their trafficking between adjacent organelles or asymmetry across individual membrane leaflets. Here we present fluorogen-activating coincidence sensing (FACES), a chemogenetic tool capable of quantitatively imaging subcellular lipid pools and reporting their transbilayer orientation in living cells. FACES combines bioorthogonal chemistry with genetically encoded fluorogen-activating proteins (FAPs) for reversible proximity sensing of conjugated molecules. We first validate this approach for quantifying discrete phosphatidylcholine pools in the ER and mitochondria that are trafficked by lipid transfer proteins. We then show that transmembrane domain-containing FAPs can be used to reveal the membrane asymmetry of multiple lipid classes that are generated in the trans-Golgi network. Lastly, we demonstrate that FACES is a generalizable tool for subcellular bioorthogonal imaging by measuring changes in mitochondrial N -acetylhexosamine levels. These results demonstrate the use of fluorogenic tags for spatially-defined molecular imaging.

GlycoID Proximity Labeling to Identify O-GlcNAcylated Protein Interactomes in Live Cells

Cells continuously remodel their intracellular proteins with the monosaccharide O-linked N-acetylglucosamine (O-GlcNAc) to regulate metabolism, signaling, and stress. This protocol describes the use of GlycoID tools to capture O-GlcNAc dynamics in live cells. GlycoID constructs contain an O-GlcNAc binding domain linked to a proximity labeling domain and a subcellular localization sequence. When expressed in mammalian cells, GlycoID tracks changes in O-GlcNAc-modified proteins and their interactomes in response to chemical induction with biotin over time. Pairing the subcellular localization of GlycoID with the chemical induction of activity enables spatiotemporal studies of O-GlcNAc biology during cellular events such as insulin signaling. However, optimizing intracellular labeling experiments requires attention to several variables. Here, we describe two protocols to adapt GlycoID methods to a cell line and biological process of interest. Next, we describe how to conduct a semiquantitative proteomic analysis of O-GlcNAcylated proteins and their interactomes using insulin versus glucagon signaling as a sample application. This articles aims to establish baseline GlycoID protocols for new users and set the stage for widespread use over diverse cellular applications for the functional study of O-GlcNAc glycobiology. © 2024 Wiley Periodicals LLC. Basic Protocol 1: Expression of targeted GlycoID constructs to verify subcellular location and labeling activity in mammalian cells Basic Protocol 2: GlycoID labeling in live HeLa cells for O-GlcNAc proteomic comparisons.

Glyco-Engineering Cell Surfaces by Exo-Enzymatic Installation of GlcNAz and LacNAz Motifs

Exo-enzymatic glyco-engineering of cell-surface glycoconjugates enables the selective display of well-defined glyco-motifs bearing bioorthogonal functional groups, which can be used to study glycans and their interactions with glycan-binding proteins. In recent years, strategies to edit cellular glycans by installing monosaccharides and their derivatives using glycosyltransferase enzymes have rapidly expanded. However, analogous methods to introduce chemical reporter-functionalized type 2 LacNAc motifs have not been reported. Herein, we report the chemo-enzymatic synthesis of unnatural UDP-GlcNAc and UDP-GalNAc nucleotide-sugars bearing azide, alkyne, and diazirine functionalities on the C2-acetamido group using the mutant uridylyltransferase AGX1F383A. The unnatural UDP-GlcNAc derivatives were examined as substrates for the human GlcNAc-transferase B3GNT2, where it was found that modified donors were tolerated for transfer, albeit to a lesser extent than the natural UDP-GlcNAc substrate. When the GlcNAc derivatives were examined as acceptor substrates for the human Gal-transferase B4GalT1, all derivatives were well tolerated and the enzyme could successfully form derivatized LacNAcs. B3GNT2 was also used to exo-enzymatically install GlcNAc and unnatural GlcNAc derivatives on cell-surface glycans. GlcNAc- or GlcNAz-engineered cells were further extended by B4GalT1 and UDP-Gal, producing LacNAc- or LacNAz-engineered cells. Our proof-of-concept glyco-engineering labeling strategy is amenable to different cell types and our work expands the exo-enzymatic glycan editing toolbox to selectively introduce unnatural type 2 LacNAc motifs.

Click-free imaging of carbohydrate trafficking in live cells using an azido photothermal probe

Real-time tracking of intracellular carbohydrates remains challenging. While click chemistry allows bio-orthogonal tagging with fluorescent probes, the reaction permanently alters the target molecule and only allows a single snapshot. Here, we demonstrate click-free mid-infrared photothermal (MIP) imaging of azide-tagged carbohydrates in live cells. Leveraging the micromolar detection sensitivity for 6-azido-trehalose (TreAz) and the 300-nm spatial resolution of MIP imaging, the trehalose recycling pathway in single mycobacteria, from cytoplasmic uptake to membrane localization, is directly visualized. A peak shift of azide in MIP spectrum further uncovers interactions between TreAz and intracellular protein. MIP mapping of unreacted azide after click reaction reveals click chemistry heterogeneity within a bacterium. Broader applications of azido photothermal probes to visualize the initial steps of the Leloir pathway in yeasts and the newly synthesized glycans in mammalian cells are demonstrated.

Tools for investigating O-GlcNAc in signaling and other fundamental biological pathways

Cells continuously fine-tune signaling pathway proteins to match nutrient and stress levels in their local environment by modifying intracellular proteins with O-linked N-acetylglucosamine (O-GlcNAc) sugars, an essential process for cell survival and growth. The small size of these monosaccharide modifications poses a challenge for functional determination, but the chemistry and biology communities have together created a collection of precision tools to study these dynamic sugars. This review presents the major themes by which O-GlcNAc influences signaling pathway proteins, including G-protein coupled receptors, growth factor signaling, mitogen-activated protein kinase (MAPK) pathways, lipid sensing, and cytokine signaling pathways. Along the way, we describe in detail key chemical biology tools that have been developed and applied to determine specific O-GlcNAc roles in these pathways. These tools include metabolic labeling, O-GlcNAc-enhancing RNA aptamers, fluorescent biosensors, proximity labeling tools, nanobody targeting tools, O-GlcNAc cycling inhibitors, light-activated systems, chemoenzymatic labeling, and nutrient reporter assays. An emergent feature of this signaling pathway meta-analysis is the intricate interplay between O-GlcNAc modifications across different signaling systems, underscoring the importance of O-GlcNAc in regulating cellular processes. We highlight the significance of O-GlcNAc in signaling and the role of chemical and biochemical tools in unraveling distinct glycobiological regulatory mechanisms. Collectively, our field has determined effective strategies to probe O-GlcNAc roles in biology. At the same time, this survey of what we do not yet know presents a clear roadmap for the field to use these powerful chemical tools to explore cross-pathway O-GlcNAc interactions in signaling and other major biological pathways.

Siglec-15/sialic acid axis as a central glyco-immune checkpoint in breast cancer bone metastasis

Immunotherapy is a promising approach for treating metastatic breast cancer (MBC), offering new possibilities for therapy. While checkpoint inhibitors have shown great progress in the treatment of metastatic breast cancer, their effectiveness in patients with bone metastases has been disappointing. This lack of efficacy seems to be specific to the bone environment, which exhibits immunosuppressive features. In this study, we elucidate the multiple roles of the sialic acid-binding Ig-like lectin (Siglec)-15/sialic acid glyco-immune checkpoint axis in the bone metastatic niche and explore potential therapeutic strategies targeting this glyco-immune checkpoint. Our research reveals that elevated levels of Siglec-15 in the bone metastatic niche can promote tumor-induced osteoclastogenesis as well as suppress antigen-specific T cell responses. Next, we demonstrate that antibody blockade of the Siglec-15/sialic acid glyco-immune checkpoint axis can act as a potential treatment for breast cancer bone metastasis. By targeting this pathway, we not only aim to treat bone metastasis but also inhibit the spread of metastatic cancer cells from bone lesions to other organs.

Metabolic Usage and Glycan Destinations of GlcNAz in E. coli

Bacteria use a diverse range of carbohydrates to generate a profusion of glycans, with amino sugars, such as N-acetylglucosamine (GlcNAc), being prevalent in the cell wall and in many exopolysaccharides. The primary substrate for GlcNAc-containing glycans, UDP-GlcNAc, is the product of the bacterial hexosamine pathway and a key target for bacterial metabolic glycan engineering. Using the strategy of expressing NahK, to circumvent the hexosamine pathway, it is possible to directly feed the analogue of GlcNAc, N-azidoacetylglucosamine (GlcNAz), for metabolic labeling in Escherichia coli. The cytosolic production of UDP-GlcNAz was confirmed by using fluorescence-assisted polyacrylamide gel electrophoresis. The key question of where GlcNAz is incorporated was interrogated by analyzing potential sites including peptidoglycan (PGN), the biofilm-related exopolysaccharide poly-β-1,6-N-acetylglucosamine (PNAG), lipopolysaccharide (LPS), and the enterobacterial common antigen (ECA). The highest levels of incorporation were observed in PGN with lower levels in PNAG and no observable incorporation in LPS or ECA. The promiscuity of the PNAG synthase (PgaCD) toward UDP-GlcNAz in vitro and the lack of undecaprenyl-pyrophosphoryl-GlcNAz intermediates generated in vivo confirmed the incorporation preferences. The results of this work will guide the future development of carbohydrate-based probes and metabolic engineering strategies.

SiaNAl can be efficiently incorporated in glycoproteins of human mesenchymal stromal cells by metabolic glycoengineering

Metabolic glycoengineering involves the stimulation of cells with functionalized monosaccharides. Glucosamine, galactosamine, and mannosamine derivatives are commercially available, but their application may lead to undirected (i.e., chemical) incorporation into proteins. However, sialic acids are attached to the ends of complex sugar chains of glycoproteins, which might be beneficial for cell surface modification via click chemistry. Thus, we studied the incorporation of chemically synthesized unnatural alkyne modified sialic acid (SiaNAl) into glycoproteins of human telomerase-immortalized mesenchymal stromal cells (hMSC-TERT) and we show that SiaNAl can be efficiently incorporated in glycoproteins involved in signal transduction and cell junction.

Selenium-based metabolic oligosaccharide engineering strategy for quantitative glycan detection

Metabolic oligosaccharide engineering (MOE) is a classical chemical approach to perturb, profile and perceive glycans in physiological systems, but probes upon bioorthogonal reaction require accessibility and the background signal readout makes it challenging to achieve glycan quantification. Here we develop SeMOE, a selenium-based metabolic oligosaccharide engineering strategy that concisely combines elemental analysis and MOE,enabling the mass spectrometric imaging of glycome. We also demonstrate that the new-to-nature SeMOE probes allow for detection, quantitative measurement and visualization of glycans in diverse biological contexts. We also show that chemical reporters on conventional MOE can be integrated into a bifunctional SeMOE probe to provide multimodality signal readouts. SeMOE thus provides a convenient and simplified method to explore the glyco-world.

Tools and tactics to define specificity of metabolic chemical reporters

Metabolic chemical reporters (MCRs) provide easily accessible means to study glycans in their native environments. However, because monosaccharide precursors are shared by many glycosylation pathways, selective incorporation has been difficult to attain. Here, a strategy for defining the selectivity and enzymatic incorporation of an MCR is presented. Performing β-elimination to interrogate O-linked sugars and using commercially available glycosidases and glycosyltransferase inhibitors, we probed the specificity of widely used azide (Ac4GalNAz) and alkyne (Ac4GalNAlk and Ac4GlcNAlk) sugar derivatives. Following the outlined strategy, we provide a semiquantitative assessment of the specific and non-specific incorporation of this bioorthogonal sugar (Ac4GalNAz) into numerous N- and O-linked glycosylation pathways. This approach should be generally applicable to other MCRs to define the extent of incorporation into the various glycan species.

Site-selected in situ polymerization for living cell surface engineering

The construction of polymer-based mimicry on cell surface to manipulate cell behaviors and functions offers promising prospects in the field of biotechnology and cell therapy. However, precise control of polymer grafting sites is essential to successful implementation of biomimicry and functional modulation, which has been overlooked by most current research. Herein, we report a biological site-selected, in situ controlled radical polymerization platform for living cell surface engineering. The method utilizes metabolic labeling techniques to confine the growth sites of polymers and designs a Fenton-RAFT polymerization technique with cytocompatibility. Polymers grown at different sites (glycans, proteins, lipids) have different membrane retention time and exhibit differential effects on the recognition behaviors of cellular glycans. Of particular importance is the achievement of in situ copolymerization of glycomonomers on the outermost natural glycan sites of cell membrane, building a biomimetic glycocalyx with distinct recognition properties.

O-GlcNAcylation determines the translational regulation and phase separation of YTHDF proteins

N6-methyladenosine (m6A) is the most abundant internal mRNA nucleotide modification in mammals, regulating critical aspects of cell physiology and differentiation. The YTHDF proteins are the primary readers of m6A modifications and exert physiological functions of m6A in the cytosol. Elucidating the regulatory mechanisms of YTHDF proteins is critical to understanding m6A biology. Here we report a mechanism that protein post-translational modifications control the biological functions of the YTHDF proteins. We find that YTHDF1 and YTHDF3, but not YTHDF2, carry high levels of nutrient-sensing O-GlcNAc modifications. O-GlcNAcylation attenuates the translation-promoting function of YTHDF1 and YTHDF3 by blocking their interactions with proteins associated with mRNA translation. We further demonstrate that O-GlcNAc modifications on YTHDF1 and YTHDF3 regulate the assembly, stability and disassembly of stress granules to enable better recovery from stress. Therefore, our results discover an important regulatory pathway of YTHDF functions, adding an additional layer of complexity to the post-transcriptional regulation function of mRNA m6A.

Click-iG: Simultaneous Enrichment and Profiling of Intact N-linked, O-GalNAc, and O-GlcNAcylated Glycopeptides

Proteins are ubiquitously modified with glycans of varied chemical structures through distinct glycosidic linkages, making the landscape of protein glycosylation challenging to map. Profiling of intact glycopeptides with mass spectrometry (MS) has recently emerged as a powerful tool for revealing matched information of the glycosylation sites and attached glycans (i.e., intact glycosites), but is largely limited to individual glycosylation types. Herein, we describe Click-iG, which integrates metabolic labeling of glycans with clickable unnatural sugars, an optimized MS method, and a tailored version of pGlyco3 software to enable simultaneous enrichment and profiling of three types of intact glycopeptides: N-linked, mucin-type O-linked, and O-GlcNAcylated glycopeptides. We demonstrate the utility of Click-iG by the identification of thousands of intact glycosites in cell lines and living mice. From the mouse lung, heart, and spleen, a total of 2053 intact N-glycosites, 262 intact O-GalNAc glycosites, and 1947 O-GlcNAcylation sites were identified. Click-iG-enabled comprehensive coverage of the protein glycosylation landscape lays the foundation for interrogating crosstalk between different glycosylation pathways.

The Metabolic Usage and Glycan Destinations of GlcNAz in E. coli

Bacteria use a diverse range of carbohydrates to generate a profusion of glycans, with amino sugars such as N-acetylglucosamine (GlcNAc) being prevalent in the cell wall and in many exopolysaccharides. The primary substrate for GlcNAc-containing glycans, UDP-GlcNAc, is the product of the bacterial hexosamine pathway, and a key target for bacterial metabolic glycan engineering. Using the strategy of expressing NahK, to circumvent the hexosamine pathway, it is possible to directly feed the analogue of GlcNAc, N-azidoacetylglucosamine (GlcNAz), for metabolic labelling in E. coli. The cytosolic production of UDP-GlcNAz was confirmed using fluorescence assisted polyacrylamide gel electrophoresis. The key question of where GlcNAz is incorporated, was interrogated by analyzing potential sites including: peptidoglycan (PGN), the biofilm-related exopolysaccharide poly-β-1,6-N-acetylglucosamine (PNAG), lipopolysaccharide (LPS) and the enterobacterial common antigen (ECA). The highest levels of incorporation were observed in PGN with lower levels in PNAG and no observable incorporation in LPS or ECA. The promiscuity of the PNAG synthase (PgaCD) towards UDP-GlcNAz in vitro and lack of undecaprenyl-pyrophosphoryl-GlcNAz intermediates generated in vivo confirmed the incorporation preferences. The results of this work will guide the future development of carbohydrate-based probes and metabolic engineering strategies.

Selective bioorthogonal probe for N-glycan hybrid structures

Metabolic incorporation of chemically tagged monosaccharides is a facile means of labelling cellular glycoprotein and glycolipids. Yet, since the monosaccharide precursors are often shared by several pathways, selectivity has been difficult to attain. For example, N-linked glycosylation is a chemically complex, and ubiquitous post translational modification with three distinct classes of GlcNAc-containing N-glycan structures: oligomannose, hybrid, and complex. Here we describe synthesis of 1,3-Pr2-6-OTs GlcNAlk as a next generation metabolic chemical reporter (MCR) for the specific labeling of hybrid N-glycan structures. We first developed a general strategy for defining the selectivity of labelling with chemically tagged monosaccharides. We then applied this approach to establish that 1,3-Pr2-6-OTs GlcNAlk is specifically incorporated into hybrid N-glycans. Using this MCR as a detection tool, we carried out imaging experiments to define the intracellular localization and trafficking of target proteins bearing hybrid N-glycan structures.

Systematic analysis of the impact of phosphorylation and O-GlcNAcylation on protein subcellular localization

The subcellular localization of proteins is critical for their functions in eukaryotic cells and is tightly correlated with protein modifications. Here, we comprehensively investigate the nuclear-cytoplasmic distributions of the phosphorylated, O-GlcNAcylated, and non-modified forms of proteins to dissect the correlation between protein distribution and modifications. Phosphorylated and O-GlcNAcylated proteins have overall higher nuclear distributions than non-modified ones. Different distributions among the phosphorylated, O-GlcNAcylated, and non-modified forms of proteins are associated with protein size, structure, and function, as well as local environment and adjacent residues around modification sites. Moreover, we perform site-mutagenesis experiments using phosphomimetic and phospho-null mutants of two proteins to validate the proteomic results. Additionally, the effects of the OGT/OGA inhibition on glycoprotein distribution are systematically investigated, and the distribution changes of glycoproteins are related to their abundance changes under the inhibitions. Systematic investigation of the relationship between protein modification and localization advances our understanding of protein functions.

Newly synthesized glycoprotein profiling to identify molecular signatures of warm ischemic injury in donor lungs

Despite recent technological advances such as ex vivo lung perfusion (EVLP), the outcome of lung transplantation remains unsatisfactory with ischemic injury being a common cause for primary graft dysfunction. New therapeutic developments are hampered by limited understanding of pathogenic mediators of ischemic injury to donor lung grafts. Here, to identify novel proteomic effectors underlying the development of lung graft dysfunction, using bioorthogonal protein engineering, we selectively captured and identified newly synthesized glycoproteins (NewS-glycoproteins) produced during EVLP with unprecedented temporal resolution of 4 h. Comparing the NewS-glycoproteomes in lungs with and without warm ischemic injury, we discovered highly specific proteomic signatures with altered synthesis in ischemic lungs, which exhibited close association to hypoxia response pathways. Inspired by the discovered protein signatures, pharmacological modulation of the calcineurin pathway during EVLP of ischemic lungs offered graft protection and improved posttransplantation outcome. In summary, the described EVLP-NewS-glycoproteomics strategy delivers an effective new means to reveal molecular mediators of donor lung pathophysiology and offers the potential to guide future therapeutic development.NEW & NOTEWORTHY This study developed and implemented a bioorthogonal strategy to chemoselectively label, enrich, and characterize newly synthesized (NewS-)glycoproteins during 4-h ex vivo lung perfusion (EVLP). Through this approach, the investigators uncovered specific proteomic signatures associated with warm ischemic injury in donor lung grafts. These signatures exhibit high biological relevance to ischemia-reperfusion injury, validating the robustness of the presented approach.

O-GalNAc glycosylation determines intracellular trafficking of APP and Aβ production

A primary pathology of Alzheimer's disease (AD) is Aβ deposition in brain parenchyma and blood vessels, the latter being called cerebral amyloid angiopathy (CAA). Parenchymal amyloid plaques presumably originate from neuronal Aβ precursor protein (APP). Although vascular amyloid deposits' origins remain unclear, endothelial APP expression in APP-knock-in mice was recently shown to expand CAA pathology, highlighting endothelial APP's importance. Furthermore, two types of endothelial APP-highly O-glycosylated APP and hypo-O-glycosylated APP-have been biochemically identified, but only the former is cleaved for Aβ production, indicating the critical relationship between APP O-glycosylation and processing. Here, we analyzed APP glycosylation and its intracellular trafficking in neurons and endothelial cells. Although protein glycosylation is generally believed to precede cell surface trafficking, which was true for neuronal APP, we unexpectedly observed that hypo-O-glycosylated APP is externalized to the endothelial cell surface and transported back to the Golgi apparatus, where it then acquires additional O-glycans. Knockdown of genes encoding enzymes initiating APP O-glycosylation significantly reduced Αβ production, suggesting this non-classical glycosylation pathway contributes to CAA pathology and is a novel therapeutic target.

Hyperglycemia and O-GlcNAc transferase activity drive a cancer stem cell pathway in triple-negative breast cancer

BACKGROUND

Enhanced glucose metabolism is a feature of most tumors, but downstream functional effects of aberrant glucose flux are difficult to mechanistically determine. Metabolic diseases including obesity and diabetes have a hyperglycemia component and are correlated with elevated pre-menopausal cancer risk for triple-negative breast cancer (TNBC). However, determining pathways for hyperglycemic disease-coupled cancer risk remains a major unmet need. One aspect of cellular sugar utilization is the addition of the glucose-derived protein modification O-GlcNAc (O-linked N-acetylglucosamine) via the single human enzyme that catalyzes this process, O-GlcNAc transferase (OGT). The data in this report implicate roles of OGT and O-GlcNAc within a pathway leading to cancer stem-like cell (CSC) expansion. CSCs are the minor fraction of tumor cells recognized as a source of tumors as well as fueling metastatic recurrence. The objective of this study was to identify a novel pathway for glucose-driven expansion of CSC as a potential molecular link between hyperglycemic conditions and CSC tumor risk factors.

METHODS

We used chemical biology tools to track how a metabolite of glucose, GlcNAc, became linked to the transcriptional regulatory protein tet-methylcytosine dioxygenase 1 (TET1) as an O-GlcNAc post-translational modification in three TNBC cell lines. Using biochemical approaches, genetic models, diet-induced obese animals, and chemical biology labeling, we evaluated the impact of hyperglycemia on CSC pathways driven by OGT in TNBC model systems.

RESULTS

We showed that OGT levels were higher in TNBC cell lines compared to non-tumor breast cells, matching patient data. Our data identified that hyperglycemia drove O-GlcNAcylation of the protein TET1 via OGT-catalyzed activity. Suppression of pathway proteins by inhibition, RNA silencing, and overexpression confirmed a mechanism for glucose-driven CSC expansion via TET1-O-GlcNAc. Furthermore, activation of the pathway led to higher levels of OGT production via feed-forward regulation in hyperglycemic conditions. We showed that diet-induced obesity led to elevated tumor OGT expression and O-GlcNAc levels in mice compared to lean littermates, suggesting relevance of this pathway in an animal model of the hyperglycemic TNBC microenvironment.

CONCLUSIONS

Taken together, our data revealed a mechanism whereby hyperglycemic conditions activated a CSC pathway in TNBC models. This pathway can be potentially targeted to reduce hyperglycemia-driven breast cancer risk, for instance in metabolic diseases. Because pre-menopausal TNBC risk and mortality are correlated with metabolic diseases, our results could lead to new directions including OGT inhibition for mitigating hyperglycemia as a risk factor for TNBC tumorigenesis and progression.

Identification and functional characterisation of N-acetylglucosamine kinase from Helicoverpa armigera divulge its potential role in growth and development via UDP-GlcNAc salvage pathway

N-acetylglucosamine kinase (NAGK), a major enzyme of sugar-kinase/Hsp70/actin superfamily, catalyses the conversion of N-acetylglucosamine to GlcNAc-6-phosphate, the first step leading to the salvage synthesis of uridine diphosphate N-acetylglucosamine. Here, we present the first report on identification, cloning, recombinant expression and functional characterisation of NAGK from Helicoverpa armigera (HaNAGK). The purified soluble HaNAGK exhibited a molecular mass of ~39 kDa with monomeric conformation. It catalysed the sequential transformation of GlcNAc into UDP-GlcNAc, indicating its role as the initiator of UDP-GlcNAc salvage pathway. HaNAGK exhibited ubiquitous expressions across all the developmental stages and major tissues of H. armigera. The gene was significantly upregulated (80 %; p < 0.01) by the moulting hormone 20-hydroxyecdysone and significantly downregulated (89 %; p < 0.001) by the chitin synthesis inhibitor novaluron, indicating its involvement in ecdysis and chitin metabolism. Furthermore, RNAi of HaNAGK caused poor weight gain, deformed insect bodies, aberrant metamorphosis and pronounced wing abnormalities in >55 % of surviving adults, while recording 7.79 ± 1.52 % and 24.25 ± 7.21 % mortality during larval and pupal stages, respectively. Altogether, the present findings suggest that HaNAGK plays a crucial role in the growth and development of H. armigera and thus, could be considered as a compelling gene of interest while formulating novel pest management strategies.

Spatially Segregated MOF Bioreactor Enables Versatile Modular Glycoenzyme Assembly for Hierarchical Glycan Library Construction

The multienzyme cascade has received growing attention to obtain structurally defined glycans in vitro. However, due to poor enzyme stability and low compatibility between glycoenzymes, artificially designed multienzyme pathways to access glycans are often inefficient. Herein, based on the strategy "Modular-Enzymes Assembly by Spatial Segregation" (MASS), we developed a universal immobilization platform to assemble multiple glycoenzymes in compartmentalized MOF particles, inside and outside, significantly reducing the undesired interference and cross-inhibitions. By changing the enzyme modules, a series of glycosyl donor, disaccharides, oligosaccharides, and polysaccharides bearing cofactor regeneration were efficiently prepared. This bioreactor was further successfully applied to the reaction system with high substrate concentration to demonstrate its industrial potential. This robust multienzyme immobilization platform should serve to promote the enzymatic synthesis of more complex glycans.

Growing impact of sialic acid-containing glycans in future drug discovery

In nature, almost all cells are covered with a complex array of glycan chain namely sialic acids or nuraminic acids, a negatively charged nine carbon sugars which is considered for their great therapeutic importance since long back. Owing to its presence at the terminal end of lipid bilayer (commonly known as terminal sugars), the well-defined sialosides or sialoconjugates have served pivotal role on the cell surfaces and thus, the sialic acid-containing glycans can modulate and mediate a number of imperative cellular interactions. Understanding of the sialo-protein interaction and their roles in vertebrates in regard of normal physiology, pathological variance, and evolution has indeed a noteworthy journey in medicine. In this tutorial review, we present a concise overview about the structure, linkages in chemical diversity, biological significance followed by chemical and enzymatic modification/synthesis of sialic acid containing glycans. A more focus is attempted about the recent advances, opportunity, and more over growing impact of sialosides and sialoconjugates in future drug discovery and development.

Cancer-derived mutation in the OGA stalk domain promotes cell malignancy through dysregulating PDLIM7 and p53

O-GlcNAcase (OGA) is the sole enzyme that hydrolyzes O-GlcNAcylation from thousands of proteins and is dysregulated in many diseases including cancer. However, the substrate recognition and pathogenic mechanisms of OGA remain largely unknown. Here we report the first discovery of a cancer-derived point mutation on the OGA's non-catalytic stalk domain that aberrantly regulated a small set of OGA-protein interactions and O-GlcNAc hydrolysis in critical cellular processes. We uncovered a novel cancer-promoting mechanism in which the OGA mutant preferentially hydrolyzed the O-GlcNAcylation from modified PDLIM7 and promoted cell malignancy by down-regulating p53 tumor suppressor in different types of cells through transcription inhibition and MDM2-mediated ubiquitination. Our study revealed the OGA deglycosylated PDLIM7 as a novel regulator of p53-MDM2 pathway, offered the first set of direct evidence on OGA substrate recognition beyond its catalytic site, and illuminated new directions to interrogate OGA's precise role without perturbing global O-GlcNAc homeostasis for biomedical applications.

Combining Selective Enrichment and a Boosting Approach to Globally and Site-Specifically Characterize Protein Co-translational O-GlcNAcylation

Protein O-GlcNAcylation plays extremely important roles in mammalian cells, regulating signal transduction and gene expression. This modification can happen during protein translation, and systematic and site-specific analysis of protein co-translational O-GlcNAcylation can advance our understanding of this important modification. However, it is extraordinarily challenging because normally O-GlcNAcylated proteins are very low abundant and the abundances of co-translational ones are even much lower. Here, we developed a method integrating selective enrichment, a boosting approach, and multiplexed proteomics to globally and site-specifically characterize protein co-translational O-GlcNAcylation. The boosting approach using the TMT labeling dramatically enhances the detection of co-translational glycopeptides with low abundance when enriched O-GlcNAcylated peptides from cells with a much longer labeling time was used as a boosting sample. More than 180 co-translational O-GlcNAcylated proteins were site-specifically identified. Further analyses revealed that among co-translational glycoproteins, those related to DNA binding and transcription are highly overrepresented using the total identified O-GlcNAcylated proteins in the same cells as the background. Compared with the glycosylation sites on all glycoproteins, co-translational sites have different local structures and adjacent amino acid residues. Overall, an integrative method was developed to identify protein co-translational O-GlcNAcylation, which is very useful to advance our understanding of this important modification.

Facile Synthesis of Clickable Unnatural Sugars in the Unprotected and 1,6-Di-O-Acylated Forms for Metabolic Glycan Labeling

Clickable unnatural sugars have been widely used in studying glycosylation in living systems via the metabolic glycan labelling (MGL) strategy. Partial protection of unnatural sugars by 1,6-di-O-acylation increases the labelling efficiency while avoiding the non-specific S-glyco-modification. Herein, we report the facile synthesis of a series of clickable unnatural sugars in both the unprotected and 1,6-di-O-acylated forms at the ten-gram scale. By evaluation of the labelling specificity, efficiency, and biocompatibility of various 1,6-di-O-acylated sugars for MGL in cell lines and living mice, we demonstrate that 1,6-di-O-propionylated unnatural sugars are optimal chemical reporters for glycan labelling. The synthetic routes developed in this work should facilitate the widespread use of MGL with no artificial S-glyco-modification for investigating the functional roles of glycans.

Recombinant mucin biotechnology and engineering

Mucins represent a largely untapped class of polymeric building block for biomaterials, therapeutics, and other biotechnology. Because the mucin polymer backbone is genetically encoded, sequence-specific mucins with defined physical and biochemical properties can be fabricated using recombinant technologies. The pendent O-glycans of mucins are increasingly implicated in immunomodulation, suppression of pathogen virulence, and other biochemical activities. Recent advances in engineered cell production systems are enabling the scalable synthesis of recombinant mucins with precisely tuned glycan side chains, offering exciting possibilities to tune the biological functionality of mucin-based products. New metabolic and chemoenzymatic strategies enable further tuning and functionalization of mucin O-glycans, opening new possibilities to expand the chemical diversity and functionality of mucin building blocks. In this review, we discuss these advances, and the opportunities for engineered mucins in biomedical applications ranging from in vitro models to therapeutics.

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.

Chemical biology tools to interrogate the roles of O-GlcNAc in immunity

The O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation of proteins is an essential and dynamic post-translational modification in mammalian cells that is regulated by the action of two enzymes. O-GlcNAc transferase (OGT) incorporates this monosaccharide on serine/threonine residues, whereas O-GlcNAcase (OGA) removes it. This modification is found on thousands of intracellular proteins involved in vital cellular processes, both under physiological and pathological conditions. Aberrant expression of O-GlcNAc has been implicated in diseases such as Alzheimer, diabetes, and cancer, and growing evidence over the last decade has also revealed key implications of O-GlcNAcylation in immunity. While some key signaling pathways involving O-GlcNAcylation in immune cells have been discovered, a complete mechanistic understanding of how O-GlcNAcylated proteins function in the immune system remains elusive, partly because of the difficulties in mapping and quantifying O-GlcNAc sites. In this minireview, we discuss recent progress on chemical biology tools and approaches to investigate the role of O-GlcNAcylation in immune cells, with the intention of encouraging further research and developments in chemical glycoimmunology that can advance our understanding of O-GlcNAc in immunity.

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.

Anticancer Properties of Hexosamine Analogs Designed to Attenuate Metabolic Flux through the Hexosamine Biosynthetic Pathway

Altered cellular metabolism is a hallmark of cancer pathogenesis and progression; for example, a near-universal feature of cancer is increased metabolic flux through the hexosamine biosynthetic pathway (HBP). This pathway produces uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), a potent oncometabolite that drives multiple facets of cancer progression. In this study, we synthesized and evaluated peracetylated hexosamine analogs designed to reduce flux through the HBP. By screening a panel of analogs in pancreatic cancer and glioblastoma multiform (GBM) cells, we identified Ac₄Glc2Bz─a benzyl-modified GlcNAc mimetic─as an antiproliferative cancer drug candidate that down-regulated oncogenic metabolites and reduced GBM cell motility at concentrations non-toxic to non-neoplastic cells. More specifically, the growth inhibitory effects of Ac₄Glc2Bz were linked to reduced levels of UDP-GlcNAc and concomitant decreases in protein O-GlcNAc modification in both pancreatic cancer and GBM cells. Targeted metabolomics analysis in GBM cells showed that Ac₄Glc2Bz disturbed glucose metabolism, amino acid pools, and nucleotide precursor biosynthesis, consistent with reduced proliferation and other anti-oncogenic properties of this analog. Furthermore, Ac₄Glc2Bz reduced the invasion, migration, and stemness of GBM cells. Importantly, normal metabolic functions mediated by UDP-GlcNAc were not disrupted in non-neoplastic cells, including maintenance of endogenous levels of O-GlcNAcylation with no global disruption of N-glycan production. Finally, a pilot in vivo study showed that a potential therapeutic window exists where animals tolerated 5- to 10-fold higher levels of Ac₄Glc2Bz than projected for in vivo efficacy. Together, these results establish GlcNAc analogs targeting the HBP through salvage mechanisms as a new therapeutic approach to safely normalize an important facet of aberrant glucose metabolism associated with cancer.

Recent development of analytical methods for disease-specific protein O-GlcNAcylation

The enzymatic modification of protein serine or threonine residues by N-acetylglucosamine, namely O-GlcNAcylation, is a ubiquitous post-translational modification that frequently occurs in the nucleus and cytoplasm. O-GlcNAcylation is dynamically regulated by two enzymes, O-GlcNAc transferase and O-GlcNAcase, and regulates nearly all cellular processes in epigenetics, transcription, translation, cell division, metabolism, signal transduction and stress. Aberrant O-GlcNAcylation has been shown in a variety of diseases, including diabetes, neurodegenerative diseases and cancers. Deciphering O-GlcNAcylation remains a challenge due to its low abundance, low stoichiometry and extreme lability in most tandem mass spectrometry. Separation or enrichment of O-GlcNAc proteins or peptides from complex mixtures has been of great interest because quantitative analysis of protein O-GlcNAcylation can elucidate their functions and regulatory mechanisms in disease. However, valid and specific analytical methods are still lacking, and efforts are needed to further advance this direction. Here, we provide an overview of recent advances in various analytical methods, focusing on chemical oxidation, affinity of antibodies and lectins, hydrophilic interaction, and enzymatic addition of monosaccharides in conjugation with these methods. O-GlcNAcylation quantification has been described in detail using mass-spectrometric or non-mass-spectrometric techniques. We briefly summarized dysregulated changes in O-GlcNAcylation in disease.

Cell-specific bioorthogonal tagging of glycoproteins

Altered glycoprotein expression is an undisputed corollary of cancer development. Understanding these alterations is paramount but hampered by limitations underlying cellular model systems. For instance, the intricate interactions between tumour and host cannot be adequately recapitulated in monoculture of tumour-derived cell lines. More complex co-culture models usually rely on sorting procedures for proteome analyses and rarely capture the details of protein glycosylation. Here, we report a strategy termed Bio-Orthogonal Cell line-specific Tagging of Glycoproteins (BOCTAG). Cells are equipped by transfection with an artificial biosynthetic pathway that transforms bioorthogonally tagged sugars into the corresponding nucleotide-sugars. Only transfected cells incorporate bioorthogonal tags into glycoproteins in the presence of non-transfected cells. We employ BOCTAG as an imaging technique and to annotate cell-specific glycosylation sites in mass spectrometry-glycoproteomics. We demonstrate application in co-culture and mouse models, allowing for profiling of the glycoproteome as an important modulator of cellular function.

An overview of tools to decipher O-GlcNAcylation from historical approaches to new insights

O-GlcNAcylation is a post-translational modification which affects approximately 5000 human proteins. Its involvement has been shown in many if not all biological processes. Variations in O-GlcNAcylation levels can be associated with the development of diseases. Deciphering the role of O-GlcNAcylation is an important issue to (i) understand its involvement in pathophysiological development and (ii) develop new therapeutic strategies to modulate O-GlcNAc levels. Over the past 30 years, despite the development of several approaches, knowledge of its role and regulation have remained limited. This review proposes an overview of the currently available tools to study O-GlcNAcylation and identify O-GlcNAcylated proteins. Briefly, we discuss pharmacological modulators, methods to study O-GlcNAcylation levels and approaches for O-GlcNAcylomic profiling. This review aims to contribute to a better understanding of the methods used to study O-GlcNAcylation and to promote efforts in the development of new strategies to explore this promising modification.

Tools for mammalian glycoscience research

Cellular carbohydrates or glycans are critical mediators of biological function. Their remarkably diverse structures and varied activities present exciting opportunities for understanding many areas of biology. In this primer, we discuss key methods and recent breakthrough technologies for identifying, monitoring, and manipulating glycans in mammalian systems.

Synthesis and mammalian cell compatibility of light-released glycan precursors for controlled metabolic engineering

Sugar additions to biomolecules, or glycans, are some of the most abundant biomolecule modifications in biology because they enable cells to adapt to changing nutrient and stress conditions. An unmet challenge for the field of glycobiology is the study of glycan biosynthetic pathways with chemical control, especially in live cell settings. The objective of this study was to create biocompatible glycan precursors with controlled release properties. Here, we report eleven "caged" sugar probes that release glycan biosynthetic precursor molecules upon light exposure. The specific sugar pathways we target with our probes regulate the addition of the N-acetyl sugars GlcNAc, GalNAc, and sialic acid onto biomolecules in cells, each of which has the potential to alter glycan processes involved in cell morphology, signaling, and behavior. We hypothesized that our glycan precursor probes would remain biologically inert until light-initiated decaging conditions were met, avoiding biological activities including metabolism and cytotoxicity. The photocaged analogs of GlcNAc, GalNAc, and ManNAc (sialic acid precursor) sugars, which we call "photo-sugars," were released within minutes of light exposure at their optimal wavelengths. During the course of the study, we characterized the cell compatibility of these sugars under their respective decaging conditions, and found highly cell compatible GlcNAc, GalNAc, and ManNAc photocaged precursors. Release of GlcNAc-1-phosphate precursors led to altered ATP levels in cells, demonstrating preliminary metabolic engineering. We envision these probes as useful additions to the chemical glycobiology field that will enable spatiotemporal control over glycosylation pathways in living mammalian cells.

Spatial and temporal proteomics reveals the distinct distributions and dynamics of O-GlcNAcylated proteins

Protein O-GlcNAcylation plays critical roles in many cellular events, and its dysregulation is related to multiple diseases. Integrating bioorthogonal chemistry and multiplexed proteomics, we systematically and site specifically study the distributions and dynamics of protein O-GlcNAcylation in the nucleus and the cytoplasm of human cells. The results demonstrate that O-GlcNAcylated proteins with different functions have distinct distribution patterns. The distributions vary site specifically, indicating that different glycoforms of the same protein may have different distributions. Moreover, we comprehensively analyze the dynamics of O-GlcNAcylated and non-modified proteins in these two compartments, respectively, and the half-lives of glycoproteins in different compartments are markedly different, with the median half-life in the cytoplasm being much longer. In addition, glycoproteins in the nucleus are more dramatically stabilized than those in the cytoplasm under the O-GlcNAcase inhibition. The comprehensive spatial and temporal analyses of protein O-GlcNAcylation provide valuable information and advance our understanding of this important modification.

Xu et al. systematically and site specifically study the distribution and dynamics of O-GlcNAcylated proteins in the nucleus and the cytoplasm. O-GlcNAcylated proteins with different functions have distinct distribution patterns. The half-lives of glycoproteins in the two cellular compartments are markedly different, with the much longer median half-life in the cytoplasm.

Endo-M Mediated Chemoenzymatic Approach Enables Reversible Glycopeptide Labeling for O-GlcNAcylation Analysis

To selectively enrich O-linked β-N-acetylglucosamine (O-GlcNAc) peptides in their original form from complex samples, we report the first reversible chemoenzymatic labeling approach for proteomic analysis. In this strategy, the O-GlcNAc moieties are ligated with long N-glycans using an Endo-M mutant, which enables the enrichment of the labeled glycopeptides by hydrophilic interaction liquid chromatography (HILIC). The attached glycans on the enriched glycopeptides are removed by wild-type Endo-M/S to restore the O-GlcNAc moiety. Compared with classic chemoenzymatic labeling, this approach enables the tag-free identification, and eliminates the interference of bulky tags in glycopeptide detection. This approach presents a unique avenue for the proteome-wide analysis of protein O-GlcNAcylation to promote its mechanism research.

Mapping the O-GlcNAc Modified Proteome: Applications for Health and Disease

O-GlcNAc is a pleotropic, enigmatic post-translational modification (PTM). This PTM modifies thousands of proteins differentially across tissue types and regulates diverse cellular signaling processes. O-GlcNAc is implicated in numerous diseases, and the advent of O-GlcNAc perturbation as a novel class of therapeutic underscores the importance of identifying and quantifying the O-GlcNAc modified proteome. Here, we review recent advances in mass spectrometry-based proteomics that will be critical in elucidating the role of this unique glycosylation system in health and disease.