Combined Antibody/Lectin Enrichment Identifies Extensive Changes in the O-GlcNAc Sub-proteome upon Oxidative Stress

Antioxidant Potential of Exosomes in Animal Nutrition

This review delves into the advantages of exosomes as novel antioxidants in animal nutrition and their potential for regulating oxidative stress. Although traditional nutritional approaches promote oxidative stress defense systems in mammalian animals, several issues remain to be solved, such as low bioavailability, targeted tissue efficiency, and high-dose by-effect. As an important candidate offering regulation opportunities concerned with cellular communication, disease prevention, and physiology regulation in multiple biological systems, the potential of exosomes in mediating redox status in biological systems has not been well described. A previously reported relationship between redox system regulation and circulating exosomes suggested exosomes as a fundamental candidate for both a regulator and biomarker for a redox system. Herein, we review the effects of oxidative stress on exosomes in animals and the potential application of exosomes as antioxidants in animal nutrition. Then, we highlight the advantages of exosomes as redox regulators due to their higher bioavailability and physiological heterogeneity-targeted properties, providing a theoretical foundation and feed industry application. Therefore, exosomes have shown great potential as novel antioxidants in the field of animal nutrition. They can overcome the limitations of traditional antioxidants in terms of dosage and side effects, which will provide unprecedented opportunities in nutritional management and disease prevention, and may become a major breakthrough in the field of animal nutrition.

O-GlcNAc impacts mitophagy via the PINK1-dependent pathway

Background

The accumulation of dysfunctional mitochondria is an early feature of Alzheimer's disease (AD). The impaired turnover of damaged mitochondria increases reactive oxygen species production and lowers ATP generation, leading to cellular toxicity and neurodegeneration. Interestingly, AD exhibits a disruption in the global post-translational modification β-N-acetylglucosamine (O-GlcNAc). O-GlcNAc is a ubiquitous single sugar modification found in the nuclear, cytoplasmic, and mitochondrial proteins. Cells maintain a homeostatic level of O-GlcNAc by cycling the addition and removal of the sugar by O-GlcNAc transferase (OGT) or O-GlcNAcase (OGA), respectively.

Methods

We used patient-derived induced pluripotent stem cells, a transgenic mouse model of AD, SH-SY5Y neuroblastoma cell lines to examine the effect of sustained O-GlcNAcase inhibition by Thiamet-G (TMG) or OGT deficiency on mitophagy using biochemical analyses.

Results

Here, we established an essential role for O-GlcNAc in regulating mitophagy (mitochondria-selective autophagy). Stimulating mitophagy using urolithin A (UA) decreases cellular O-GlcNAc and elevates mitochondrial O-GlcNAc. Sustained elevation in O-GlcNAcylation via pharmacologically inhibiting OGA using Thiamet-G (TMG) increases the mitochondrial level of mitophagy protein PTEN-induced kinase 1 (PINK1) and autophagy-related protein light chain 3 (LC3). Moreover, we detected O-GlcNAc on PINK1 and TMG increases its O-GlcNAcylation level. Conversely, decreasing cellular O-GlcNAcylation by knocking down OGT decreases both PINK1 protein expression and LC3 protein expression. Mitochondria isolated from CAMKII-OGT-KO mice also had decreased PINK1 and LC3. Moreover, human brain organoids treated with TMG showed significant elevation in LC3 compared to control. However, TMG-treated AD organoids showed no changes in LC3 expression.

Conclusion

Collectively, these data demonstrate that O-GlcNAc plays a crucial role in the activation and progression of mitophagy, and this activation is disrupted in AD.

The multifaceted role of intracellular glycosylation in cytoprotection and heart disease

The modification of nuclear, cytoplasmic, and mitochondrial proteins by O-linked β-N-actylglucosamine (O-GlcNAc) is an essential posttranslational modification that is common in metozoans. O-GlcNAc is cycled on and off proteins in response to environmental and physiological stimuli impacting protein function, which, in turn, tunes pathways that include transcription, translation, proteostasis, signal transduction, and metabolism. One class of stimulus that induces rapid and dynamic changes to O-GlcNAc is cellular injury, resulting from environmental stress (for instance, heat shock), hypoxia/reoxygenation injury, ischemia reperfusion injury (heart attack, stroke, trauma hemorrhage), and sepsis. Acute elevation of O-GlcNAc before or after injury reduces apoptosis and necrosis, suggesting that injury-induced changes in O-GlcNAcylation regulate cell fate decisions. However, prolonged elevation or reduction in O-GlcNAc leads to a maladaptive response and is associated with pathologies such as hypertrophy and heart failure. In this review, we discuss the impact of O-GlcNAc in both acute and prolonged models of injury with a focus on the heart and biological mechanisms that underpin cell survival.

O-GlcNAcylation mediates H2O2-induced apoptosis through regulation of STAT3 and FOXO1

The O-linked-β-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) is a critical post-translational modification that couples the external stimuli to intracellular signal transduction networks. However, the critical protein targets of O-GlcNAcylation in oxidative stress-induced apoptosis remain to be elucidated. Here, we show that treatment with H2O2 inhibited O-GlcNAcylation, impaired cell viability, increased the cleaved caspase 3 and accelerated apoptosis of neuroblastoma N2a cells. The O-GlcNAc transferase (OGT) inhibitor OSMI-1 or the O-GlcNAcase (OGA) inhibitor Thiamet-G enhanced or inhibited H2O2-induced apoptosis, respectively. The total and phosphorylated protein levels, as well as the promoter activities of signal transducer and activator of transcription factor 3 (STAT3) and Forkhead box protein O 1 (FOXO1) were suppressed by OSMI-1. In contrast, overexpressing OGT or treating with Thiamet-G increased the total protein levels of STAT3 and FOXO1. Overexpression of STAT3 or FOXO1 abolished OSMI-1-induced apoptosis. Whereas the anti-apoptotic effect of OGT and Thiamet-G in H2O2-treated cells was abolished by either downregulating the expression or activity of endogenous STAT3 or FOXO1. These results suggest that STAT3 or FOXO1 are the potential targets of O-GlcNAcylation involved in the H2O2-induced apoptosis of N2a cells.

Emerging roles of O-GlcNAcylation in protein trafficking and secretion

The emerging roles of O-GlcNAcylation, a distinctive post-translational modification, are increasingly recognized for their involvement in the intricate processes of protein trafficking and secretion. This modification exerts its influence on both conventional and unconventional secretory pathways. Under healthy and stress conditions, such as during diseases, it orchestrates the transport of proteins within cells, ensuring timely delivery to their intended destinations. O-GlcNAcylation occurs on key factors, like coat protein complexes (COPI and COPII), clathrin, SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), and GRASP55 (Golgi reassembly stacking protein of 55 kDa) that control vesicle budding and fusion in anterograde and retrograde trafficking and unconventional secretion. The understanding of O-GlcNAcylation offers valuable insights into its critical functions in cellular physiology and the progression of diseases, including neurodegeneration, cancer, and metabolic disorders. In this review, we summarize and discuss the latest findings elucidating the involvement of O-GlcNAc in protein trafficking and its significance in various human disorders.

Current Technologies Unraveling the Significance of Post-Translational Modifications (PTMs) as Crucial Players in Neurodegeneration

Neurodegenerative disorders, such as Parkinson's disease, Alzheimer's disease, and Huntington's disease, are identified and characterized by the progressive loss of neurons and neuronal dysfunction, resulting in cognitive and motor impairment. Recent research has shown the importance of PTMs, such as phosphorylation, acetylation, methylation, ubiquitination, sumoylation, nitration, truncation, O-GlcNAcylation, and hydroxylation, in the progression of neurodegenerative disorders. PTMs can alter protein structure and function, affecting protein stability, localization, interactions, and enzymatic activity. Aberrant PTMs can lead to protein misfolding and aggregation, impaired degradation, and clearance, and ultimately, to neuronal dysfunction and death. The main objective of this review is to provide an overview of the PTMs involved in neurodegeneration, their underlying mechanisms, methods to isolate PTMs, and the potential therapeutic targets for these disorders. The PTMs discussed in this article include tau phosphorylation, α-synuclein and Huntingtin ubiquitination, histone acetylation and methylation, and RNA modifications. Understanding the role of PTMs in neurodegenerative diseases may provide new therapeutic strategies for these devastating disorders.

O-GlcNAc regulates the mitochondrial integrated stress response by regulating ATF4

Background

Accumulation of mitochondrial dysfunctional is a hallmark of age-related neurodegeneration including Alzheimer's disease (AD). Impairment of mitochondrial quality control mechanisms leading to the accumulation of damaged mitochondria and increasing neuronal stress. Therefore, investigating the basic mechanisms of how mitochondrial homeostasis is regulated is essential. Herein, we investigate the role of O-GlcNAcylation, a single sugar post-translational modification, in controlling mitochondrial stress-induced transcription factor Activating Transcription Factor 4 (ATF4). Mitochondrial dysfunction triggers the integrated stress response (ISRmt), in which the phosphorylation of eukaryotic translation initiation factor 2α results in the translation of ATF4.

Methods

We used patient-derived induced pluripotent stem cells, a transgenic mouse model of AD, SH-SY5Y neuroblastoma and HeLa cell-lines to examine the effect of sustained O-GlcNAcase inhibition by Thiamet-G (TMG) on ISRmt using biochemical analyses.

Results

We show that TMG elevates ATF4 protein levels upon mitochondrial stress in SH-SY5Y neuroblastoma and HeLa cell-lines. An indirect downstream target of ATF4 mitochondrial chaperone glucose-regulated protein 75 (GRP75) is significantly elevated. Interestingly, knock-down of O-GlcNAc transferase (OGT), the enzyme that adds O-GlcNAc, in SH-SY5Y increases ATF4 protein and mRNA expression. Additionally, ATF4 target gene Activating Transcription Factor 5 (ATF5) is significantly elevated at both the protein and mRNA level. Brains isolated from TMG treated mice show elevated levels of ATF4 and GRP75. Importantly, ATF4 occupancy increases at the ATF5 promoter site in brains isolated from TMG treated mice suggesting that O-GlcNAc is regulating ATF4 targeted gene expression. Interestingly, ATF4 and GRP75 are not induced in TMG treated familial Alzheimer's Disease mice model. The same results are seen in a human in vitro model of AD.

Conclusion

Together, these results indicate that in healthy conditions, O-GlcNAc regulates the ISRmt through regulating ATF4, while manipulating O-GlcNAc in AD has no effect on ISRmt.

Cardioprotective O-GlcNAc signaling is elevated in murine female hearts via enhanced O-GlcNAc transferase activity

The post-translational modification of intracellular proteins by O-linked β-GlcNAc (O-GlcNAc) has emerged as a critical regulator of cardiac function. Enhanced O-GlcNAcylation activates cytoprotective pathways in cardiac models of ischemia-reperfusion (I/R) injury; however, the mechanisms underpinning O-GlcNAc cycling in response to I/R injury have not been comprehensively assessed. The cycling of O-GlcNAc is regulated by the collective efforts of two enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), which catalyze the addition and hydrolysis of O-GlcNAc, respectively. It has previously been shown that baseline heart physiology and pathophysiology are impacted by sex. Here, we hypothesized that sex differences in molecular signaling may target protein O-GlcNAcylation both basally and in ischemic hearts. To address this question, we subjected male and female WT murine hearts to ex vivo ischemia or I/R injury. We assessed hearts for protein O-GlcNAcylation, abundance of OGT, OGA, and glutamine:fructose-6-phosphate aminotransferase (GFAT2), activity of OGT and OGA, and UDP-GlcNAc levels. Our data demonstrate elevated O-GlcNAcylation in female hearts both basally and during ischemia. We show that OGT activity was enhanced in female hearts in all treatments, suggesting a mechanism for these observations. Furthermore, we found that ischemia led to reduced O-GlcNAcylation and OGT-specific activity. Our findings provide a foundation for understanding molecular mechanisms that regulate O-GlcNAcylation in the heart and highlight the importance of sex as a significant factor when assessing key regulatory events that control O-GlcNAc cycling. These data suggest the intriguing possibility that elevated O-GlcNAcylation in females contributes to reduced ischemic susceptibility.

Differential Detection of O-GlcNAcylated proteins in the heart using antibodies

Thousands of mammalian intracellular proteins are dynamically modified by O-linked β-N-acetylglucosamine (O-GlcNAc). Global changes in O-GlcNAcylation have been associated with the development of cardiomyopathy, heart failure, hypertension, and neurodegenerative disease. Levels of O-GlcNAc in cells and tissues can be detected using numerous approaches; however, immunoblotting using GlcNAc-specific antibodies and lectins is commonplace. The goal of this study was to optimize the detection of O-GlcNAc in heart lysates by immunoblotting. Using a combination of tissue fractionation, immunoblotting, and galactosyltransferase labeling, as well as hearts from wild-type and O-GlcNAc transferase transgenic mice, we demonstrate that contractile proteins in the heart are differentially detected by two commercially available antibodies (CTD110.6 and RL2). As CTD110.6 displays poor reactivity toward contractile proteins, and as these proteins represent a major fraction of the heart proteome, a better assessment of cardiac O-GlcNAcylation is obtained in total tissue lysates with RL2. The data presented highlight tissue lysis approaches that should aid the assessment of the cardiac O-GlcNAcylation by immunoblotting.

Emerging field: O-GlcNAcylation in ferroptosis

In 2012, researchers proposed a non-apoptotic, iron-dependent form of cell death caused by lipid peroxidation called ferroptosis. During the past decade, a comprehensive understanding of ferroptosis has emerged. Ferroptosis is closely associated with the tumor microenvironment, cancer, immunity, aging, and tissue damage. Its mechanism is precisely regulated at the epigenetic, transcriptional, and post-translational levels. O-GlcNAc modification (O-GlcNAcylation) is one of the post-translational modifications of proteins. Cells can modulate cell survival in response to stress stimuli, including apoptosis, necrosis, and autophagy, through adaptive regulation by O-GlcNAcylation. However, the function and mechanism of these modifications in regulating ferroptosis are only beginning to be understood. Here, we review the relevant literature within the last 5 years and present the current understanding of the regulatory function of O-GlcNAcylation in ferroptosis and the potential mechanisms that may be involved, including antioxidant defense system-controlled reactive oxygen species biology, iron metabolism, and membrane lipid peroxidation metabolism. In addition to these three areas of ferroptosis research, we examine how changes in the morphology and function of subcellular organelles (e.g., mitochondria and endoplasmic reticulum) involved in O-GlcNAcylation may trigger and amplify ferroptosis. We have dissected the role of O-GlcNAcylation in regulating ferroptosis and hope that our introduction will provide a general framework for those interested in this field.

Efficient TurboID-based proximity labelling method for identifying terminal sialic acid glycosylation in living cells

TurboID, a proximity labelling method based on mutant biotin ligase, is an efficient new technique for recognizing protein-protein interactions and has been successfully applied to living cells. Sialic acid is typically the terminal monosaccharide attached to many glycoproteins and plays many important roles in many biological processes. However, the lack of enrichment methods for terminal sialic acid glycosylation in vivo hinders the identification and analysis of this glycosylation. Here, we introduce TurboID to identify terminal sialic acid glycosylation in living cells. SpCBM, the carbohydrate-binding domain of sialidase from Streptococcus pneumoniae, is fused with TurboID and overexpressed in HeLa cells. After streptavidin-based purification and detection by mass spectrometry, 31 terminal sialic acid N-glycosylated sites and 1359 putative terminal sialic acid glycosylated proteins are identified, many of which are located in the cytoplasm and nucleus.

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.

Integration of O-GlcNAc into Stress Response Pathways

The modification of nuclear, mitochondrial, and cytosolic proteins by O-linked βN-acetylglucosamine (O-GlcNAc) has emerged as a dynamic and essential post-translational modification of mammalian proteins. O-GlcNAc is cycled on and off over 5000 proteins in response to diverse stimuli impacting protein function and, in turn, epigenetics and transcription, translation and proteostasis, metabolism, cell structure, and signal transduction. Environmental and physiological injury lead to complex changes in O-GlcNAcylation that impact cell and tissue survival in models of heat shock, osmotic stress, oxidative stress, and hypoxia/reoxygenation injury, as well as ischemic reperfusion injury. Numerous mechanisms that appear to underpin O-GlcNAc-mediated survival include changes in chaperone levels, impacts on the unfolded protein response and integrated stress response, improvements in mitochondrial function, and reduced protein aggregation. Here, we discuss the points at which O-GlcNAc is integrated into the cellular stress response, focusing on the roles it plays in the cardiovascular system and in neurodegeneration.

RNA binding motif protein 3 (RBM3) promotes protein kinase B (AKT) activation to enhance glucose metabolism and reduce apoptosis in skeletal muscle of mice under acute cold exposure

The main danger of cold stress to animals in cold regions is systemic metabolic changes and protein synthesis inhibition. RBM3, an exceptional cold shock protein, is rapidly upregulated in response to hypothermia to resist the adverse effects of cold stress. However, the mechanism of the protective effect and the rapid upregulation of RBM3 remains unclear. O-GlcNAcylation, an atypical O-glycosylation, is precisely regulated only by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) and participates in the signal transduction of multiple cellular stress responses as a "stress and nutrition receptor." Therefore, our study aimed to explore the mechanism of RBM3 regulating glucose metabolism and promoting survival in skeletal muscle under acute cold exposure. Meanwhile, our study verifies whether O-GlcNAcylation mediated by OGT rapidly upregulates RBM3. The blood and skeletal muscle of mice were collected at the end of cold exposure treatment for 0, 2, and 4 h. Changes in levels of RBM3, AKT, glycolysis apoptosis, and OGT were measured. The results show that acute cold exposure upregulated RBM3, OGT, and AKT phosphorylation and increased energy consumption, which enhanced glycolysis and prevent apoptosis. In the 32 °C mild hypothermia model in vitro, overexpression of RBM3 enhanced AKT phosphorylation. Meanwhile, inactivation of AKT by wortmannin resulted in increased apoptosis and decreased glucose metabolism in skeletal muscle under acute cold exposure. In addition, OGT-mediated O-GlcNAcylation of p65 was confirmed in mouse myoblast cell line (C2C12) cells at mild hypothermia. O-GlcNAcylation level affected p65 activity and nuclear translocation. Compared with wild type (WT) mice, RBM3 and p65 phosphorylation were decreased in specific skeletal muscle Ogt (KO) mice, whereas AKT phosphorylation, glycolysis, and apoptosis were increased. Taken together, O-GlcNAcylation of p65 upregulates RBM3 to promote AKT phosphorylation, enhance glucose metabolism, and reduce apoptosis in skeletal muscle of mice under acute cold exposure.

Nutrient-sensitive protein O-GlcNAcylation shapes daily biological rhythms

O-linked-N-acetylglucosaminylation (O-GlcNAcylation) is a nutrient-sensitive protein modification that alters the structure and function of a wide range of proteins involved in diverse cellular processes. Similar to phosphorylation, another protein modification that targets serine and threonine residues, O-GlcNAcylation occupancy on cellular proteins exhibits daily rhythmicity and has been shown to play critical roles in regulating daily rhythms in biology by modifying circadian clock proteins and downstream effectors. We recently reported that daily rhythm in global O-GlcNAcylation observed in Drosophila tissues is regulated via the integration of circadian and metabolic signals. Significantly, mistimed feeding, which disrupts coordination of these signals, is sufficient to dampen daily O-GlcNAcylation rhythm and is predicted to negatively impact animal biological rhythms and health span. In this review, we provide an overview of published and potential mechanisms by which metabolic and circadian signals regulate hexosamine biosynthetic pathway metabolites and enzymes, as well as O-GlcNAc processing enzymes to shape daily O-GlcNAcylation rhythms. We also discuss the significance of functional interactions between O-GlcNAcylation and other post-translational modifications in regulating biological rhythms. Finally, we highlight organ/tissue-specific cellular processes and molecular pathways that could be modulated by rhythmic O-GlcNAcylation to regulate time-of-day-specific biology.

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.

O-GlcNAcylation and Regulation of Galectin-3 in Extraembryonic Endoderm Differentiation

The regulation of proteins through the addition and removal of O-linked β-N-acetylglucosamine (O-GlcNAc) plays a role in many signaling events, specifically in stem cell pluripotency and the regulation of differentiation. However, these post-translational modifications have not been explored in extraembryonic endoderm (XEN) differentiation. Of the plethora of proteins regulated through O-GlcNAc, we explored galectin-3 as a candidate protein known to have various intracellular and extracellular functions. Based on other studies, we predicted a reduction in global O-GlcNAcylation levels and a distinct galectin expression profile in XEN cells relative to embryonic stem (ES) cells. By conducting dot blot analysis, XEN cells had decreased levels of global O-GlcNAc than ES cells, which reflected a disbalance in the expression of genes encoding O-GlcNAc cycle enzymes. Immunoassays (Western blot and ELISA) revealed that although XEN cells (low O-GlcNAc) had lower concentrations of both intracellular and extracellular galectin-3 than ES cells (high O-GlcNAc), the relative secretion of galectin-3 was significantly increased by XEN cells. Inducing ES cells toward XEN in the presence of an O-GlcNAcase inhibitor was not sufficient to inhibit XEN differentiation. However, global O-GlcNAcylation was found to decrease in differentiated cells and the extracellular localization of galectin-3 accompanies these changes. Inhibiting global O-GlcNAcylation status does not, however, impact pluripotency and the ability of ES cells to differentiate to the XEN lineage.

Evidence for nutrient-dependent regulation of the COPII coat by O-GlcNAcylation

O-linked β-N-acetylglucosamine (O-GlcNAc) is a dynamic form of intracellular glycosylation common in animals, plants and other organisms. O-GlcNAcylation is essential in mammalian cells and is dysregulated in myriad human diseases, such as cancer, neurodegeneration and metabolic syndrome. Despite this pathophysiological significance, key aspects of O-GlcNAc signaling remain incompletely understood, including its impact on fundamental cell biological processes. Here, we investigate the role of O-GlcNAcylation in the coat protein II complex (COPII), a system universally conserved in eukaryotes that mediates anterograde vesicle trafficking from the endoplasmic reticulum. We identify new O-GlcNAcylation sites on Sec24C, Sec24D and Sec31A, core components of the COPII system, and provide evidence for potential nutrient-sensitive pathway regulation through site-specific glycosylation. Our work suggests a new connection between metabolism and trafficking through the conduit of COPII protein O-GlcNAcylation.

The O-Linked N-Acetylglucosamine Containing Epitope H (O-GlcNAcH) is Upregulated in the Trophoblastic and Downregulated in the Fibroblastic Cells in Missed Miscarriage Human Chorionic Villi With Simple Hydropic Degeneration

Epitope H contains an O-linked N-acetylglucosamine (O-GlcNAcH) residue in a specific conformation and/or environment recognized by the mouse monoclonal antibody H. O-GlcNAcH is present in several types of cells and in several polypeptides, including cytokeratin 8 and vimentin, on the latter in cells under stress. In the present work, we examined the expression of the O-GlcNAcH in 60 cases of endometrial curettings from missed miscarriage cases containing normal and simple hydropic degenerated chorionic villi in each case, using monoclonal antibody H and indirect immunoperoxidase and Western blot immunoblot. In all cases examined the expression of the O-GlcNAcH was cytoplasmic as follows: (1) syncytiotrophoblastic cells showed very low expression in chorionic villi (CV) with nonhydropic degeneration (NHD) and high expression in hydropic degenerated (HD) CV; (2) cytotrophoblastic cells showed low expression in CV with NHD and high expression in HD CV; (3) fibroblastic cells showed high expression in CV with NHD and very low expression in HD CV; (4) histiocytes showed very low expression in both types of CV; (5) endothelial cells showed high expression in both types of CV. An immunoblot of CV from one case of a legal abortion from a normal first-trimester pregnancy showed 5 polypeptides with 118.5, 106.3, 85, 53, and 36.7 kD bearing the epitope H and the 53 kD corresponded to cytokeratin 8. The expression of the O-GlcNAcH is upregulated in the trophoblastic cells and downregulated in the fibroblastic cells in the HD CV in comparison to the NHD CV.

Detection and Analysis of Proteins Modified by O-Linked N-Acetylglucosamine

O-GlcNAc is a common post-translational modification of nuclear, mitochondrial, and cytoplasmic proteins that regulates normal physiology and the cell stress response. Dysregulation of O-GlcNAc cycling is implicated in the etiology of type II diabetes, heart failure, hypertension, and Alzheimer's disease, as well as cardioprotection. These protocols cover simple and comprehensive techniques for detecting proteins modified by O-GlcNAc and studying the enzymes that add or remove O-GlcNAc. © 2021 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Increasing the stoichiometry of O-GlcNAc on proteins before analysis Basic Protocol 2: Detection of proteins modified by O-GlcNAc using antibodies Basic Protocol 3: Detection of proteins modified by O-GlcNAc using the lectin sWGA Support Protocol 1: Control for O-linked glycosylation Basic Protocol 4: Detection and enrichment of proteins using WGA-agarose Support Protocol 2: Digestion of proteins with hexosaminidase Alternate Protocol: Detection of proteins modified by O-GlcNAc using galactosyltransferase Support Protocol 3: Autogalactosylation of galactosyltransferase Support Protocol 4: Assay of galactosyltransferase activity Basic Protocol 5: Characterization of labeled glycans by β-elimination and chromatography Basic Protocol 6: Detection of O-GlcNAc in 96-well plates Basic Protocol 7: Assay for OGT activity Support Protocol 5: Desalting of O-GlcNAc transferase Basic Protocol 8: Assay for O-GlcNAcase activity.

The glyco-redox interplay: Principles and consequences on the role of reactive oxygen species during protein glycosylation

Reactive oxygen species (ROS) carry out prime physiological roles as intracellular signaling agents, yet pathologically high concentrations of ROS cause irreversible damage to biomolecules, alter cellular programs and contribute to various diseases. While decades of intensive research have identified redox-related patterns and signaling pathways, very few addressed how the glycosylation machinery senses and responds to oxidative stress. A common trait among ROS and glycans residing on glycoconjugates is that they are both highly dynamic, as they are quickly fine-tuned in response to stressors such as inflammation, cancer and infectious diseases. On this account, the delicate balance of the redox potential, which is tightly regulated by dozens of enzymes including NOXs, and the mitochondrial electron transport chain as well as the fluidity of glycan biosynthesis resulting from the cooperation of glycosyltransferases, glycosidases, and nucleotide sugar transporters, is paramount to cell survival. Here, we review the broad spectrum of the interplay between redox changes and glycosylation with respect to their principle consequences on human physiology.

Interaction hot spots for phase separation revealed by NMR studies of a CAPRIN1 condensed phase

The role of biomolecular condensates in regulating biological function and the importance of dynamic interactions involving intrinsically disordered protein regions (IDRs) in their assembly are increasingly appreciated. While computational and theoretical approaches have provided significant insights into IDR phase behavior, establishing the critical interactions that govern condensation with atomic resolution through experiment is more difficult, given the lack of applicability of standard structural biological tools to study these highly dynamic large-scale associated states. NMR can be a valuable method, but the dynamic and viscous nature of condensed IDRs presents challenges. Using the C-terminal IDR (607 to 709) of CAPRIN1, an RNA-binding protein found in stress granules, P bodies, and messenger RNA transport granules, we have developed and applied a variety of NMR methods for studies of condensed IDR states to provide insights into interactions driving and modulating phase separation. We identify ATP interactions with CAPRIN1 that can enhance or reduce phase separation. We also quantify specific side-chain and backbone interactions within condensed CAPRIN1 that define critical sequences for phase separation and that are reduced by O-GlcNAcylation known to occur during cell cycle and stress. This expanded NMR toolkit that has been developed for characterizing IDR condensates has generated detailed interaction information relevant for understanding CAPRIN1 biology and informing general models of phase separation, with significant potential future applications to illuminate dynamic structure-function relationships in other biological condensates.

Quantitative Proteomics Reveals that the OGT Interactome Is Remodeled in Response to Oxidative Stress

The dynamic modification of specific serine and threonine residues of intracellular proteins by O-linked N-acetyl-β-D-glucosamine (O-GlcNAc) mitigates injury and promotes cytoprotection in a variety of stress models. The O-GlcNAc transferase (OGT) and the O-GlcNAcase are the sole enzymes that add and remove O-GlcNAc, respectively, from thousands of substrates. It remains unclear how just two enzymes can be specifically controlled to affect glycosylation of target proteins and signaling pathways both basally and in response to stress. Several lines of evidence suggest that protein interactors regulate these responses by affecting OGT and O-GlcNAcase activity, localization, and substrate specificity. To provide insight into the mechanisms by which OGT function is controlled, we have used quantitative proteomics to define OGT's basal and stress-induced interactomes. OGT and its interaction partners were immunoprecipitated from OGT WT, null, and hydrogen peroxide-treated cell lysates that had been isotopically labeled with light, medium, and heavy lysine and arginine (stable isotopic labeling of amino acids in cell culture). In total, more than 130 proteins were found to interact with OGT, many of which change their association upon hydrogen peroxide stress. These proteins include the major OGT cleavage and glycosylation substrate, host cell factor 1, which demonstrated a time-dependent dissociation after stress. To validate less well-characterized interactors, such as glyceraldehyde 3-phosphate dehydrogenase and histone deacetylase 1, we turned to parallel reaction monitoring, which recapitulated our discovery-based stable isotopic labeling of amino acids in cell culture approach. Although the majority of proteins identified are novel OGT interactors, 64% of them are previously characterized glycosylation targets that contain varied domain architecture and function. Together these data demonstrate that OGT interacts with unique and specific interactors in a stress-responsive manner.

Elucidating the protein substrate recognition of O-GlcNAc transferase (OGT) toward O-GlcNAcase (OGA) using a GlcNAc electrophilic probe

The essential human O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is the sole enzyme responsible for modifying thousands of intracellular proteins with the monosaccharide O-GlcNAc. This unique modification plays crucial roles in human health and disease, but the substrate recognition of OGT remains poorly understood. Intriguingly, the only human enzyme reported to remove this modification, O-GlcNAcase (OGA), is O-GlcNAc modified. Here, we exploited a GlcNAc electrophilic probe (GEP1A) to rapidly screen OGT mutants in a fluorescence assay that can discriminate between altered OGT-sugar and -protein substrate binding to help elucidate the binding mode of OGT toward OGA protein substrate. Since OGT tetratricopeptide repeat (TPR) domain plays a key role in OGT-OGA binding, we screened 30 OGT TPR mutants, which revealed 15 "ladder like" asparagine or aspartate residues spanning TPRs 3-7 and 10-13.5 that affect OGA O-GlcNAcylation. By applying a truncated OGA construct, we found that OGA's N-terminal region or pseudo histone acetyltransferase domain is not required for its O-GlcNAcylation, suggesting OGT functionally interacts with OGA through its catalytic and/or stalk domains. This work represents the first effort to systemically investigate each OGT TPR and our findings will facilitate the development of new strategies to investigate the role of substrate-specific O-GlcNAcylation.

MS-based proteomics for comprehensive investigation of protein O-GlcNAcylation

Protein O-GlcNAcylation refers to the covalent binding of a single N-acetylglucosamine (GlcNAc) to the serine or threonine residue. This modification primarily occurs on proteins in the nucleus and the cytosol, and plays critical roles in many cellular events, including regulation of gene expression and signal transduction. Aberrant protein O-GlcNAcylation is directly related to human diseases such as cancers, diabetes and neurodegenerative diseases. In the past decades, considerable progress has been made for global and site-specific analysis of O-GlcNAcylation in complex biological samples using mass spectrometry (MS)-based proteomics. In this review, we summarized previous efforts on comprehensive investigation of protein O-GlcNAcylation by MS. Specifically, the review is focused on methods for enriching and site-specifically mapping O-GlcNAcylated peptides, and applications for quantifying protein O-GlcNAcylation in different biological systems. As O-GlcNAcylation is an important protein modification for cell survival, effective methods are essential for advancing our understanding of glycoprotein functions and cellular events.

Role of O-Linked N-Acetylglucosamine Protein Modification in Cellular (Patho)Physiology

In the mid-1980s, the identification of serine and threonine residues on nuclear and cytoplasmic proteins modified by a N-acetylglucosamine moiety (O-GlcNAc) via an O-linkage overturned the widely held assumption that glycosylation only occurred in the endoplasmic reticulum, Golgi apparatus, and secretory pathways. In contrast to traditional glycosylation, the O-GlcNAc modification does not lead to complex, branched glycan structures and is rapidly cycled on and off proteins by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery, O-GlcNAcylation has been shown to contribute to numerous cellular functions, including signaling, protein localization and stability, transcription, chromatin remodeling, mitochondrial function, and cell survival. Dysregulation in O-GlcNAc cycling has been implicated in the progression of a wide range of diseases, such as diabetes, diabetic complications, cancer, cardiovascular, and neurodegenerative diseases. This review will outline our current understanding of the processes involved in regulating O-GlcNAc turnover, the role of O-GlcNAcylation in regulating cellular physiology, and how dysregulation in O-GlcNAc cycling contributes to pathophysiological processes.

Loss of CRMP2 O-GlcNAcylation leads to reduced novel object recognition performance in mice

O-GlcNAcylation is an abundant post-translational modification in the nervous system, linked to both neurodevelopmental and neurodegenerative disease. However, the mechanistic links between these phenotypes and site-specific O-GlcNAcylation remain largely unexplored. Here, we show that Ser517 O-GlcNAcylation of the microtubule-binding protein Collapsin Response Mediator Protein-2 (CRMP2) increases with age. By generating and characterizing a Crmp2S517A knock-in mouse model, we demonstrate that loss of O-GlcNAcylation leads to a small decrease in body weight and mild memory impairment, suggesting that Ser517 O-GlcNAcylation has a small but detectable impact on mouse physiology and cognitive function.

O-GlcNAc Site Mapping by Using a Combination of Chemoenzymatic Labeling, Copper-Free Click Chemistry, Reductive Cleavage, and Electron-Transfer Dissociation Mass Spectrometry

As a dynamic post-translational modification, O-linked β- N-acetylglucosamine ( O-GlcNAc) modification (i.e., O-GlcNAcylation) of proteins regulates many biological processes involving cellular metabolism and signaling. However, O-GlcNAc site mapping, a prerequisite for site-specific functional characterization, has been a challenge since its discovery. Herein we present a novel method for O-GlcNAc enrichment and site mapping. In this method, the O-GlcNAc moiety on peptides was labeled with UDP-GalNAz followed by copper-free azide-alkyne cycloaddition with a multifunctional reagent bearing a terminal cyclooctyne, a disulfide bridge, and a biotin handle. The tagged peptides were then released from NeutrAvidin beads upon reductant treatment, alkylated with (3-acrylamidopropyl)trimethylammonium chloride, and subjected to electron-transfer dissociation mass spectrometry analysis. After validation by using standard synthetic peptide gCTD and model protein α-crystallin, such an approach was applied to the site mapping of overexpressed TGF-β-activated kinase 1/MAP3K7 binding protein 2 (TAB2), with four O-GlcNAc sites unambiguously identified. Our method provides a promising tool for the site-specific characterization of O-GlcNAcylation of important proteins.

Critical observations that shaped our understanding of the function(s) of intracellular glycosylation (O-GlcNAc)

Almost 100 years after the first descriptions of proteins conjugated to carbohydrates (mucins), several studies suggested that glycoproteins were not restricted to the serum, extracellular matrix, cell surface, or endomembrane system. In the 1980s, key data emerged demonstrating that intracellular proteins were modified by monosaccharides of O-linked β-N-acetylglucosamine (O-GlcNAc). Subsequently, this modification was identified on thousands of proteins that regulate cellular processes as diverse as protein aggregation, localization, post-translational modifications, activity, and interactions. In this Review, we will highlight critical discoveries that shaped our understanding of the molecular events underpinning the impact of O-GlcNAc on protein function, the role that O-GlcNAc plays in maintaining cellular homeostasis, and our understanding of the mechanisms that regulate O-GlcNAc-cycling.

Effects of Acute Cold Stress on Liver O-GlcNAcylation and Glycometabolism in Mice

Protein O-linked β-N-acetylglucosamine glycosylation (O-GlcNAcylation) regulates many biological processes. Studies have shown that O-GlcNAc modification levels can increase during acute stress and suggested that this may contribute to the survival of the cell. This study investigated the possible effects of O-GlcNAcylation that regulate glucose metabolism, apoptosis, and autophagy in the liver after acute cold stress. Male C57BL/6 mice were exposed to cold conditions (4 °C) for 0, 2, 4, and 6 h, then their livers were extracted and the expression of proteins involved in glucose metabolism, apoptosis, and autophagy was determined. It was found that acute cold stress increased global O-GlcNAcylation and protein kinase B (AKT) phosphorylation levels. This was accompanied by significantly increased activation levels of the glucose metabolism regulators 160 kDa AKT substrate (AS160), 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 2 (PFKFB2), and glycogen synthase kinase-3β (GSK3β). The levels of glycolytic intermediates, fructose-1,6-diphosphate (FDP) and pyruvic acid (PA), were found to show a brief increase followed by a sharp decrease. Additionally, adenosine triphosphate (ATP), as the main cellular energy source, had a sharp increase. Furthermore, the B-cell lymphoma 2(Bcl-2)/Bcl-2-associated X (Bax) ratio was found to increase, whereas cysteine-aspartic acid protease 3 (caspase-3) and light chain 3-II (LC3-II) levels were reduced after acute cold stress. Therefore, acute cold stress was found to increase O-GlcNAc modification levels, which may have resulted in the decrease of the essential processes of apoptosis and autophagy, promoting cell survival, while altering glycose transport, glycogen synthesis, and glycolysis in the liver.

Functional crosstalk among oxidative stress and O-GlcNAc signaling pathways

In metazoans, thousands of intracellular proteins are modified with O-linked β-N-acetylglucosamine (O-GlcNAc) in response to a wide range of stimuli and stresses. In particular, a complex and evolutionarily conserved interplay between O-GlcNAcylation and oxidative stress has emerged in recent years. Here, we review the current literature on the connections between O-GlcNAc and oxidative stress, with a particular emphasis on major signaling pathways, such as KEAP1/NRF2, FOXO, NFκB, p53 and cell metabolism. Taken together, this work sheds important light on the signaling functions of protein glycosylation and the mechanisms of stress responses alike and illuminates how the two are integrated in animal cell physiology.

A Novel Glycoproteomics Workflow Reveals Dynamic O-GlcNAcylation of COPγ1 as a Candidate Regulator of Protein Trafficking

O-linked β-N-acetylglucosamine (O-GlcNAc) is an abundant and essential intracellular form of protein glycosylation in animals and plants. In humans, dysregulation of O-GlcNAcylation occurs in a wide range of diseases, including cancer, diabetes, and neurodegeneration. Since its discovery more than 30 years ago, great strides have been made in understanding central aspects of O-GlcNAc signaling, including identifying thousands of its substrates and characterizing the enzymes that govern it. However, while many O-GlcNAcylated proteins have been reported, only a small subset of these change their glycosylation status in response to a typical stimulus or stress. Identifying the functionally important O-GlcNAcylation changes in any given signaling context remains a significant challenge in the field. To address this need, we leveraged chemical biology and quantitative mass spectrometry methods to create a new glycoproteomics workflow for profiling stimulus-dependent changes in O-GlcNAcylated proteins. In proof-of-principle experiments, we used this new workflow to interrogate changes in O-GlcNAc substrates in mammalian protein trafficking pathways. Interestingly, our results revealed dynamic O-GlcNAcylation of COPγ1, an essential component of the coat protein I (COPI) complex that mediates Golgi protein trafficking. Moreover, we detected 11 O-GlcNAc moieties on COPγ1 and found that this modification is reduced by a model secretory stress that halts COPI trafficking. Our results suggest that O-GlcNAcylation may regulate the mammalian COPI system, analogous to its previously reported roles in other protein trafficking pathways. More broadly, our glycoproteomics workflow is applicable to myriad systems and stimuli, empowering future studies of O-GlcNAc in a host of biological contexts.

Dynamic Glycosylation Governs the Vertebrate COPII Protein Trafficking Pathway

The COPII coat complex, which mediates secretory cargo trafficking from the endoplasmic reticulum, is a key control point for subcellular protein targeting. Because misdirected proteins cannot function, protein sorting by COPII is critical for establishing and maintaining normal cell and tissue homeostasis. Indeed, mutations in COPII genes cause a range of human pathologies, including cranio-lenticulo-sutural dysplasia (CLSD), which is characterized by collagen trafficking defects, craniofacial abnormalities, and skeletal dysmorphology. Detailed knowledge of the COPII pathway is required to understand its role in normal cell physiology and to devise new treatments for disorders in which it is disrupted. However, little is known about how vertebrates dynamically regulate COPII activity in response to developmental, metabolic, or pathological cues. Several COPII proteins are modified by O-linked β-N-acetylglucosamine (O-GlcNAc), a dynamic form of intracellular protein glycosylation, but the biochemical and functional effects of these modifications remain unclear. Here, we use a combination of chemical, biochemical, cellular, and genetic approaches to demonstrate that site-specific O-GlcNAcylation of COPII proteins mediates their protein-protein interactions and modulates cargo secretion. In particular, we show that individual O-GlcNAcylation sites of SEC23A, an essential COPII component, are required for its function in human cells and vertebrate development, because mutation of these sites impairs SEC23A-dependent in vivo collagen trafficking and skeletogenesis in a zebrafish model of CLSD. Our results indicate that O-GlcNAc is a conserved and critical regulatory modification in the vertebrate COPII-dependent trafficking pathway.

A Sweet Embrace: Control of Protein-Protein Interactions by O-Linked β-N-Acetylglucosamine

O-Linked β-N-acetylglucosamine (O-GlcNAc) is a critical post-translational modification (PTM) of thousands of intracellular proteins. Reversible O-GlcNAcylation governs many aspects of cell physiology and is dysregulated in numerous human diseases. Despite this broad pathophysiological significance, major aspects of O-GlcNAc signaling remain poorly understood, including the biochemical mechanisms through which O-GlcNAc transduces information. Recent work from many laboratories, including our own, has revealed that O-GlcNAc, like other intracellular PTMs, can control its substrates' functions by inhibiting or inducing protein-protein interactions. This dynamic regulation of multiprotein complexes exerts diverse downstream signaling effects in a range of processes, cell types, and organisms. Here, we review the literature about O-GlcNAc-regulated protein-protein interactions and suggest important questions for future studies in the field.

Electrophilic probes for deciphering substrate recognition by O-GlcNAc transferase

O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential human glycosyltransferase that adds O-GlcNAc modifications to numerous proteins. However, little is known about the mechanism with which OGT recognizes various protein substrates. Here we report on GlcNAc electrophilic probes (GEPs) to expedite the characterization of OGT-substrate recognition. Data from mass spectrometry, X-ray crystallization, and biochemical and radiolabeled kinetic assays support the application of GEPs to rapidly report the impacts of OGT mutations on protein substrate or sugar binding and to discover OGT residues crucial for protein recognition. Interestingly, we found that the same residues on the inner surface of the N-terminal domain contribute to OGT interactions with different protein substrates. By tuning reaction conditions, a GEP enables crosslinking of OGT with acceptor substrates in situ, affording a unique method to discover genuine substrates that weakly or transiently interact with OGT. Hence, GEPs provide new strategies to dissect OGT-substrate binding and recognition.

Glycosylation with O-linked β-N-acetylglucosamine induces vascular dysfunction via production of superoxide anion/reactive oxygen species

Overproduction of superoxide anion (•O₂^-) and O-linked β-N-acetylglucosamine (O-GlcNAc) modification in the vascular system are contributors to endothelial dysfunction. This study tested the hypothesis that increased levels of O-GlcNAc-modified proteins contribute to •O₂^- production via activation of NADPH oxidase, resulting in impaired vasodilation. Rat aortic segments and vascular smooth muscle cells (VSMCs) were incubated with vehicle (methanol) or O-(2-acetamido-2-deoxy-d-glucopyranosylidenamino) N-phenylcarbamate (PUGNAc) (100 μM). PUGNAc produced a time-dependent increase in O-GlcNAc levels in VSMC and decreased endothelium-dependent relaxation, which was prevented by apocynin and tiron, suggesting that •O₂^- contributes to endothelial dysfunction under augmented O-GlcNAc levels. Aortic segments incubated with PUGNAc also exhibited increased levels of reactive oxygen species, assessed by dihydroethidium fluorescence, and augmented •O₂^- production, determined by lucigenin-enhanced chemiluminescence. Additionally, PUGNAc treatment increased Nox-1 and Nox-4 protein expression in aortas and VSMCs. Translocation of the p47^phox subunit from the cytosol to the membrane was greater in aortas incubated with PUGNAc. VSMCs displayed increased p22^phox protein expression after PUGNAc incubation, suggesting that NADPH oxidase is activated in conditions where O-GlcNAc protein levels are increased. In conclusion, O-GlcNAc levels reduce endothelium-dependent relaxation by overproduction of •O₂^- via activation of NADPH oxidase. This may represent an additional mechanism by which augmented O-GlcNAc levels impair vascular function.

Glycosylation of KEAP1 links nutrient sensing to redox stress signaling

O-GlcNAcylation is an essential, nutrient-sensitive post-translational modification, but its biochemical and phenotypic effects remain incompletely understood. To address this question, we investigated the global transcriptional response to perturbations in O-GlcNAcylation. Unexpectedly, many transcriptional effects of O-GlcNAc transferase (OGT) inhibition were due to the activation of NRF2, the master regulator of redox stress tolerance. Moreover, we found that a signature of low OGT activity strongly correlates with NRF2 activation in multiple tumor expression datasets. Guided by this information, we identified KEAP1 (also known as KLHL19), the primary negative regulator of NRF2, as a direct substrate of OGT We show that O-GlcNAcylation of KEAP1 at serine 104 is required for the efficient ubiquitination and degradation of NRF2. Interestingly, O-GlcNAc levels and NRF2 activation co-vary in response to glucose fluctuations, indicating that KEAP1 O-GlcNAcylation links nutrient sensing to downstream stress resistance. Our results reveal a novel regulatory connection between nutrient-sensitive glycosylation and NRF2 signaling and provide a blueprint for future approaches to discover functionally important O-GlcNAcylation events on other KLHL family proteins in various experimental and disease contexts.

Stress-induced O-GlcNAcylation: an adaptive process of injured cells

In the 30 years, since the discovery of nucleocytoplasmic glycosylation, O-GlcNAc has been implicated in regulating cellular processes as diverse as protein folding, localization, degradation, activity, post-translational modifications, and interactions. The cell co-ordinates these molecular events, on thousands of cellular proteins, in concert with environmental and physiological cues to fine-tune epigenetics, transcription, translation, signal transduction, cell cycle, and metabolism. The cellular stress response is no exception: diverse forms of injury result in dynamic changes to the O-GlcNAc subproteome that promote survival. In this review, we discuss the biosynthesis of O-GlcNAc, the mechanisms by which O-GlcNAc promotes cytoprotection, and the clinical significance of these data.

Global and Site-Specific Analysis Revealing Unexpected and Extensive Protein S-GlcNAcylation in Human Cells

Protein glycosylation is highly diverse and essential for mammalian cell survival. Heterogeneous glycans may be bound to different amino acid residues, forming multiple types of protein glycosylation. In this work, unexpected protein S-GlcNAcylation on cysteine residues was observed to extensively exist in human cells through global and site-specific analysis of protein GlcNAcylation by mass spectrometry. Three independent experiments produced similar results of many cysteine residues bound to N-acetylglucosamine (GlcNAc). Among well-localized S-GlcNAcylation sites, several motifs with an acidic amino acid around the sites were identified, which strongly suggests that a particular type of enzyme is responsible for this modification. Clustering results show that glycoproteins modified with S-GlcNAc are mainly involved in cell-cell adhesion and gene expression. For the first time, we found that proteins were extensively bound to GlcNAc through the side chains of cysteine residues in human cells, and the current discovery further advances our understanding of protein glycosylation.

Fatty acid synthase inhibits the O-GlcNAcase during oxidative stress

The dynamic post-translational modification O-linked β-N-acetylglucosamine (O-GlcNAc) regulates thousands of nuclear, cytoplasmic, and mitochondrial proteins. Cellular stress, including oxidative stress, results in increased O-GlcNAcylation of numerous proteins, and this increase is thought to promote cell survival. The mechanisms by which the O-GlcNAc transferase (OGT) and the O-GlcNAcase (OGA), the enzymes that add and remove O-GlcNAc, respectively, are regulated during oxidative stress to alter O-GlcNAcylation are not fully characterized. Here, we demonstrate that oxidative stress leads to elevated O-GlcNAc levels in U2OS cells but has little impact on the activity of OGT. In contrast, the expression and activity of OGA are enhanced. We hypothesized that this seeming paradox could be explained by proteins that bind to and control the local activity or substrate targeting of OGA, thereby resulting in the observed stress-induced elevations of O-GlcNAc. To identify potential protein partners, we utilized BioID proximity biotinylation in combination with stable isotopic labeling of amino acids in cell culture (SILAC). This analysis revealed 90 OGA-interacting partners, many of which exhibited increased binding to OGA upon stress. The associations of OGA with fatty acid synthase (FAS), filamin-A, heat shock cognate 70-kDa protein, and OGT were confirmed by co-immunoprecipitation. The pool of OGA bound to FAS demonstrated a substantial (∼85%) reduction in specific activity, suggesting that FAS inhibits OGA. Consistent with this observation, FAS overexpression augmented stress-induced O-GlcNAcylation. Although the mechanism by which FAS sequesters OGA remains unknown, these data suggest that FAS fine-tunes the cell's response to stress and injury by remodeling cellular O-GlcNAcylation.