O-GlcNAc processing enzymes: catalytic mechanisms, substrate specificity, and enzyme regulation

Downregulation of O-GlcNAcylation enhances etoposide-induced p53-mediated apoptosis in HepG2 human liver cancer cells

Etoposide, an anticancer drug that inhibits topoisomerase II, is commonly used in combination chemotherapy. However, the impact of O-GlcNAcylation regulation on etoposide's anticancer effects has rarely been investigated. This study evaluated the effect of etoposide on cellular O-GlcNAcylation and whether modulating this process enhances etoposide-induced apoptosis. O-GlcNAc expression was measured after 24 h of etoposide treatment, and the effect of O-GlcNAc transferase (OGT) inhibition by OSMI-1 on etoposide's anticancer activity in HepG2 human liver cancer cells was quantitatively analyzed. Additionally, molecular analyses were used to confirm that the observed effects were mediated by p53-induced apoptosis. Etoposide reduced O-GlcNAcylation in a dose-dependent manner without directly interacting with OGT. Cotreatment with 20 μm of OSMI-1 lowered the IC50 value for cell viability by approximately 1.64-fold to 60.68 μm and increased the EC50 value for cytotoxicity by around 4.07-fold to 43.41 μm. Furthermore, this synergistic effect was linked to the activation of the p53/caspase-3/PARP1 pathway. These findings suggest that downregulating O-GlcNAcylation may enhance the efficacy of etoposide-based chemotherapy and help overcome tumor resistance.

Mannose metabolism reshapes T cell differentiation to enhance anti-tumor immunity

Cellular metabolic status profoundly influences T cell differentiation, persistence, and anti-tumor efficacy. Our single-cell metabolic analyses of T cells reveal that diminished mannose metabolism is a prominent feature of T cell dysfunction. Conversely, experimental augmentation/restoration of mannose metabolism in adoptively transferred T cells via D-mannose supplementation enhances anti-tumor activity and restricts exhaustion differentiation both in vitro and in vivo. Mechanistically, D-mannose treatment induces intracellular metabolic programming and increases the O-GlcNAc transferase (OGT)-mediated O-GlcNAcylation of β-catenin, which preserves Tcf7 expression and epigenetic stemness, thereby promoting stem-like programs in T cells. Furthermore, in vitro expansion with D-mannose supplementation yields T cell products for adoptive therapy with stemness characteristics, even after extensive long-term expansion, that exhibits enhanced anti-tumor efficacy. These findings reveal cell-intrinsic mannose metabolism as a physiological regulator of CD8+ T cell fate, decoupling proliferation/expansion from differentiation, and underscoring the therapeutic potential of mannose modulation in cancer immunotherapy.

Glucose influences endometrial receptivity to embryo implantation through O-GlcNAcylation-mediated regulation of the cytoskeleton

Phenotypic changes to endometrial epithelial cells underpin receptivity to embryo implantation at the onset of pregnancy but the effect of hyperglycemia on these processes remains poorly understood. Here, we show that physiological levels of glucose (5 mM) abolished receptivity in the endometrial epithelial cell line, Ishikawa. However, embryo attachment was supported by 17 mM glucose as a result of glucose flux through the hexosamine biosynthetic pathway (HBP) and modulation of cell function via protein O-GlcNAcylation. Pharmacological inhibition of HBP or protein O-GlcNAcylation reduced embryo attachment in cocultures at 17 mM glucose. Mass spectrometry analysis of the O-GlcNAcylated proteome in Ishikawa cells revealed that myosin phosphatase target subunit 1 (MYPT1) is more highly O-GlcNAcylated in 17 mM glucose, correlating with loss of its target protein, phospho-myosin light chain 2, from apical cell junctions of polarized epithelium. Two-dimensional (2-D) and three-dimensional (3-D) morphologic analysis demonstrated that the higher glucose level attenuates epithelial polarity through O-GlcNAcylation. Inhibition of Rho (ras homologous)A-associated kinase (ROCK) or myosin II led to reduced polarity and enhanced receptivity in cells cultured in 5 mM glucose, consistent with data showing that MYPT1 acts downstream of ROCK signaling. These data implicate regulation of endometrial epithelial polarity through RhoA signaling upstream of actomyosin contractility in the acquisition of endometrial receptivity. Glucose levels impinge on this pathway through O-GlcNAcylation of MYPT1, which may impact endometrial receptivity to an implanting embryo in women with diabetes.NEW & NOTEWORTHY Understanding how glucose regulates endometrial function will support preconception guidance and/or the development of targeted interventions for individuals living with diabetes wishing to embark on pregnancy. We found that glucose can influence endometrial epithelial cell receptivity to embryo implantation by regulating posttranslational modification of proteins involved in the maintenance of cell polarity. Impaired or inappropriate endometrial receptivity could contribute to fertility and/or early pregnancy complications caused by poor glucose control.

Discovery of 4-(Arylethynyl)piperidine Derivatives as Potent Nonsaccharide O-GlcNAcase Inhibitors for the Treatment of Alzheimer's Disease

Inhibiting O-GlcNAcase and thereby up-regulation of the O-GlcNAc levels of tau was a potential approach for discovering AD treatments. Herein, a series of novel highly potent OGA inhibitors embracing a 4-(arylethynyl)piperidine moiety was achieved by capitalizing on the substrate recognition domain. Extensive structure-activity relationships resulted in compound 81 with significant enzymatic inhibition (IC50 = 4.93 ± 2.05 nM) and cellular potency (EC50 = 7.47 ± 3.96 nM in PC12 cells). It markedly increased the protein O-GlcNAcylation levels and reduced the phosphorylation on Ser199, Thr205, and Ser396 of tau in the OA-injured SH-SY5Y cell model, suggesting its potential role for AD treatment. In fact, an in vivo efficacy of ameliorating cognitive impairment was observed following treatment of APP/PS1 mice with compound 81 (100 mg/kg). Additionally, the appropriate plasma PK and beneficial BBB penetration properties were also observed. Compound 81 deserves to be further explored as an anti-AD agent.

Practical Three-Component Regioselective Synthesis of Drug-Like 3-Aryl(or heteroaryl)-5,6-dihydrobenzo[h]cinnolines as Potential Non-Covalent Multi-Targeting Inhibitors To Combat Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are one of the prominent health challenges facing contemporary society, and many efforts have been made to overcome and (or) control it. In this research paper, we described a practical one-pot two-step three-component reaction between 3,4-dihydronaphthalen-1(2H)-one (1), aryl(or heteroaryl)glyoxal monohydrates (2a-h), and hydrazine monohydrate (NH2NH2•H2O) for the regioselective preparation of some 3-aryl(or heteroaryl)-5,6-dihydrobenzo[h]cinnoline derivatives (3a-h). After synthesis and characterization of the mentioned cinnolines (3a-h), the in silico multi-targeting inhibitory properties of these heterocyclic scaffolds have been investigated upon various Homo sapiens-type enzymes, including hMAO-A, hMAO-B, hAChE, hBChE, hBACE-1, hBACE-2, hNQO-1, hNQO-2, hnNOS, hiNOS, hPARP-1, hPARP-2, hLRRK-2(G2019S), hGSK-3β, hp38α MAPK, hJNK-3, hOGA, hNMDA receptor, hnSMase-2, hIDO-1, hCOMT, hLIMK-1, hLIMK-2, hRIPK-1, hUCH-L1, hPARK-7, and hDHODH, which have confirmed their functions and roles in the neurodegenerative diseases (NDs), based on molecular docking studies, and the obtained results were compared with a wide range of approved drugs and well-known (with IC50, EC50, etc.) compounds. In addition, in silico ADMET prediction analysis was performed to examine the prospective drug properties of the synthesized heterocyclic compounds (3a-h). The obtained results from the molecular docking studies and ADMET-related data demonstrated that these series of 3-aryl(or heteroaryl)-5,6-dihydrobenzo[h]cinnolines (3a-h), especially hit ones, can really be turned into the potent core of new drugs for the treatment of neurodegenerative diseases (NDs), and/or due to the having some reactionable locations, they are able to have further organic reactions (such as cross-coupling reactions), and expansion of these compounds (for example, with using other types of aryl(or heteroaryl)glyoxal monohydrates) makes a new avenue for designing novel and efficient drugs for this purpose.

Tissue-specific O-GlcNAcylation profiling identifies substrates in translational machinery in Drosophila mushroom body contributing to olfactory learning

O-GlcNAcylation is a dynamic post-translational modification that diversifies the proteome. Its dysregulation is associated with neurological disorders that impair cognitive function, and yet identification of phenotype-relevant candidate substrates in a brain-region specific manner remains unfeasible. By combining an O-GlcNAc binding activity derived from Clostridium perfringens OGA (CpOGA) with TurboID proximity labeling in Drosophila, we developed an O-GlcNAcylation profiling tool that translates O-GlcNAc modification into biotin conjugation for tissue-specific candidate substrates enrichment. We mapped the O-GlcNAc interactome in major brain regions of Drosophila and found that components of the translational machinery, particularly ribosomal subunits, were abundantly O-GlcNAcylated in the mushroom body of Drosophila brain. Hypo-O-GlcNAcylation induced by ectopic expression of active CpOGA in the mushroom body decreased local translational activity, leading to olfactory learning deficits that could be rescued by dMyc overexpression-induced increase of protein synthesis. Our study provides a useful tool for future dissection of tissue-specific functions of O-GlcNAcylation in Drosophila, and suggests a possibility that O-GlcNAcylation impacts cognitive function via regulating regional translational activity in the brain.

Developing hydrogels for gene therapy and tissue engineering

Hydrogels are a class of highly absorbent and easily modified polymer materials suitable for use as slow-release carriers for drugs. Gene therapy is highly specific and can overcome the limitations of traditional tissue engineering techniques and has significant advantages in tissue repair. However, therapeutic genes are often affected by cellular barriers and enzyme sensitivity, and carrier loading of therapeutic genes is essential. Therapeutic gene hydrogels can well overcome these difficulties. Moreover, gene-therapeutic hydrogels have made considerable progress. This review summarizes the recent research on carrier gene hydrogels for the treatment of tissue damage through a summary of the most current research frontiers. We initially introduce the classification of hydrogels and their cross-linking methods, followed by a detailed overview of the types and modifications of therapeutic genes, a detailed discussion on the loading of therapeutic genes in hydrogels and their characterization features, a summary of the design of hydrogels for therapeutic gene release, and an overview of their applications in tissue engineering. Finally, we provide comments and look forward to the shortcomings and future directions of hydrogels for gene therapy. We hope that this article will provide researchers in related fields with more comprehensive and systematic strategies for tissue engineering repair and further promote the development of the field of hydrogels for gene therapy.

O-GlcNAc regulates anti-fibrotic genes in lung fibroblasts through EZH2

Epigenetic modifications are involved in fibrotic diseases, such as idiopathic pulmonary fibrosis (IPF), and contribute to the silencing of anti-fibrotic genes. H3K27me3, a key repressive histone mark, is catalysed by the methyltransferase enhancer of Zeste homologue 2 (EZH2), which is regulated by the post-translational modification, O-linked N-Acetylglucosamine (O-GlcNAc). In this study, we explored the effects of O-GlcNAc and EZH2 on the expression of antifibrotic genes, cyclooxygenase-2 (Cox2) and Heme Oxygenase (Homx1). The expression of Cox2 and Hmox1 was examined in primary IPF or non-IPF lung fibroblasts with or without EZH2 inhibitor EZP6438, O-GlcNAc transferase (OGT) inhibitor (OSMI-1) or O-GlcNAcase (OGA) inhibitor (thiamet G). Non-IPF cells were also subjected to TGF-β1 with or without OGT inhibition. The reduced expression of Cox2 and Hmox1 in IPF lung fibroblasts is restored by OGT inhibition. In non-IPF fibroblasts, TGF-β1 treatment reduces Cox2 and Hmox1 expression, which was restored by OGT inhibition. ChIP assays demonstrated that the association of H3K27me3 is reduced at the Cox2 and Hmox1 promoter regions following OGT or EZH2 inhibition. EZH2 levels and stability were decreased by reducing O-GlcNAc. Our study provided a novel mechanism of O-GlcNAc modification in regulating anti-fibrotic genes in lung fibroblasts and in the pathogenesis of IPF.

Caffeine-induced protein kinase A activation restores cognitive deficits induced by sleep deprivation by regulating O-GlcNAc cycling in adult zebrafish

Sleep deprivation (SD) is widely acknowledged as a significant risk factor for cognitive impairment. In this study, intraperitoneal caffeine administration significantly ameliorated the learning and memory (L/M) deficits induced by SD and reduced aggressive behaviors in adult zebrafish. SD led to a reduction in protein kinase A (PKA) phosphorylation, phosphorylated-cAMP response element-binding protein (p-CREB), and c-Fos expression in zebrafish brain. Notably, these alterations were effectively reversed by caffeine. In addition, caffeine mitigated neuroinflammation induced by SD, as evident from suppression of the SD-mediated increase in glial fibrillary acidic protein (GFAP) and nuclear factor-κB (NF-κB) activation. Caffeine restored normal O-GlcNAcylation and O-GlcNAc transferase (OGT) levels while reversing the increased expression of O-GlcNAcase (OGA) in zebrafish brain after SD. Intriguingly, rolipram, a selective phosphodiesterase 4 (PDE4) inhibitor, effectively mitigated cognitive deficits, restored p-CREB and c-Fos levels, and attenuated the increase in GFAP in brain induced by SD. In addition, rolipram reversed the decrease in O-GlcNAcylation and OGT expression as well as elevation of OGA expression following SD. Treatment with H89, a PKA inhibitor, significantly impaired the L/M functions of zebrafish compared with the control group, inducing a decrease in O-GlcNAcylation and OGT expression and, conversely, an increase in OGA expression. The H89-induced changes in O-GlcNAc cycling and L/M dysfunction were effectively reversed by glucosamine treatment. H89 suppressed, whereas caffeine and rolipram promoted O-GlcNAc cycling in Neuro2a cells. Our collective findings underscore the interplay between PKA signaling and O-GlcNAc cycling in the regulation of cognitive function in the brain, offering potential therapeutic targets for cognitive deficits associated with SD.NEW & NOTEWORTHY Our observation highlights the intricate interplay between cAMP/PKA signaling and O-GlcNAc cycling, unveiling a novel mechanism that potentially governs the regulation of learning and memory functions. The dynamic interplay between these two pathways provides a novel and nuanced perspective on the molecular foundation of learning and memory regulation. These insights open avenues for the development of targeted interventions to treat conditions that impact cognitive function, including SD.

Regulation of protein O-GlcNAcylation by circadian, metabolic, and cellular signals

O-linked β-N-acetylglucosamine (O-GlcNAcylation) is a dynamic post-translational modification that regulates thousands of proteins and almost all cellular processes. Aberrant O-GlcNAcylation has been associated with numerous diseases, including cancer, neurodegenerative diseases, cardiovascular diseases, and type 2 diabetes. O-GlcNAcylation is highly nutrient-sensitive since it is dependent on UDP-GlcNAc, the end product of the hexosamine biosynthetic pathway (HBP). We previously observed daily rhythmicity of protein O-GlcNAcylation in a Drosophila model that is sensitive to the timing of food consumption. We showed that the circadian clock is pivotal in regulating daily O-GlcNAcylation rhythms given its control of the feeding-fasting cycle and hence nutrient availability. Interestingly, we reported that the circadian clock also modulates daily O-GlcNAcylation rhythm by regulating molecular mechanisms beyond the regulation of food consumption time. A large body of work now indicates that O-GlcNAcylation is likely a generalized cellular status effector as it responds to various cellular signals and conditions, such as ER stress, apoptosis, and infection. In this review, we summarize the metabolic regulation of protein O-GlcNAcylation through nutrient availability, HBP enzymes, and O-GlcNAc processing enzymes. We discuss the emerging roles of circadian clocks in regulating daily O-GlcNAcylation rhythm. Finally, we provide an overview of other cellular signals or conditions that impact O-GlcNAcylation. Many of these cellular pathways are themselves regulated by the clock and/or metabolism. Our review highlights the importance of maintaining optimal O-GlcNAc rhythm by restricting eating activity to the active period under physiological conditions and provides insights into potential therapeutic targets of O-GlcNAc homeostasis under pathological conditions.

Protein O-GlcNAcylation: The sweet hub in liver metabolic flexibility from a (patho)physiological perspective

O-GlcNAcylation is a dynamic, reversible and atypical O-glycosylation that regulates various cellular physiological processes via conformation, stabilisation, localisation, chaperone interaction or activity of target proteins. The O-GlcNAcylation cycle is precisely controlled by collaboration between O-GlcNAc transferase and O-GlcNAcase. Uridine-diphosphate-N-acetylglucosamine, the sole donor of O-GlcNAcylation produced by the hexosamine biosynthesis pathway, is controlled by the input of glucose, glutamine, acetyl coenzyme A and uridine triphosphate, making it a sensor of the fluctuation of molecules, making O-GlcNAcylation a pivotal nutrient sensor for the metabolism of carbohydrates, amino acids, lipids and nucleotides. O-GlcNAcylation, particularly prevalent in liver, is the core hub for controlling systemic glucose homeostasis due to its nutritional sensitivity and precise spatiotemporal regulation of insulin signal transduction. The pathology of various liver diseases has highlighted hepatic metabolic disorder and dysfunction, and abnormal O-GlcNAcylation also plays a specific pathological role in these processes. Therefore, this review describes the unique features of O-GlcNAcylation and its dynamic homeostasis maintenance. Additionally, it explains the underlying nutritional sensitivity of O-GlcNAcylation and discusses its mechanism of spatiotemporal modulation of insulin signal transduction and liver metabolic homeostasis during the fasting and feeding cycle. This review emphasises the pathophysiological implications of O-GlcNAcylation in nonalcoholic fatty liver disease, nonalcoholic steatohepatitis and hepatic fibrosis, and focuses on the adverse effects of hyper O-GlcNAcylation on liver cancer progression and metabolic reprogramming.

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.

Protein O-GlcNAcylation homeostasis regulates facultative heterochromatin to fine-tune sog-Dpp signaling during Drosophila early embryogenesis

Protein O-GlcNAcylation is a monosaccharide post-translational modification maintained by two evolutionarily conserved enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Mutations in human OGT have recently been associated with neurodevelopmental disorders, although the mechanisms linking O-GlcNAc homeostasis to neurodevelopment are not understood. Here, we investigate the effects of perturbing protein O-GlcNAcylation using transgenic Drosophila lines that overexpress a highly active OGA. We reveal that temporal reduction of protein O-GlcNAcylation in early embryos leads to reduced brain size and olfactory learning in adult Drosophila. Downregulation of O-GlcNAcylation induced by the exogenous OGA activity promotes nuclear foci formation of Polycomb-group protein Polyhomeotic and the accumulation of excess K27 trimethylation of histone H3 (H3K27me3) at the mid-blastula transition. These changes interfere with the zygotic expression of several neurodevelopmental genes, particularly shortgastrulation (sog), a component of an evolutionarily conserved sog-Decapentaplegic (Dpp) signaling system required for neuroectoderm specification. Our findings highlight the importance of early embryonic O-GlcNAcylation homeostasis for the fidelity of facultative heterochromatin redeployment and initial cell fate commitment of neuronal lineages, suggesting a possible mechanism underpinning OGT-associated intellectual disability.

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.

O-GlcNAcylation: an important post-translational modification and a potential therapeutic target for cancer therapy

O-linked β-d-N-acetylglucosamine (O-GlcNAc) is an important post-translational modification of serine or threonine residues on thousands of proteins in the nucleus and cytoplasm of all animals and plants. In eukaryotes, only two conserved enzymes are involved in this process. O-GlcNAc transferase is responsible for adding O-GlcNAc to proteins, while O-GlcNAcase is responsible for removing it. Aberrant O-GlcNAcylation is associated with a variety of human diseases, such as diabetes, cancer, neurodegenerative diseases, and cardiovascular diseases. Numerous studies have confirmed that O-GlcNAcylation is involved in the occurrence and progression of cancers in multiple systems throughout the body. It is also involved in regulating multiple cancer hallmarks, such as metabolic reprogramming, proliferation, invasion, metastasis, and angiogenesis. In this review, we first describe the process of O-GlcNAcylation and the structure and function of O-GlcNAc cycling enzymes. In addition, we detail the occurrence of O-GlcNAc in various cancers and the role it plays. Finally, we discuss the potential of O-GlcNAc as a promising biomarker and novel therapeutic target for cancer diagnosis, treatment, and prognosis.

Bioinformatic prediction of putative conveyers of O-GlcNAc transferase intellectual disability

Protein O-GlcNAcylation is a dynamic posttranslational modification that is catalyzed by the enzyme O-GlcNAc transferase (OGT) and is essential for neurodevelopment and postnatal neuronal function. Missense mutations in OGT segregate with a novel X-linked intellectual disability syndrome, the OGT congenital disorder of glycosylation (OGT-CDG). One hypothesis for the etiology of OGT-CDG is that loss of OGT activity leads to hypo-O-GlcNAcylation of as yet unidentified, specific neuronal proteins, affecting essential embryonic, and postnatal neurodevelopmental processes; however, the identity of these O-GlcNAcylated proteins is not known. Here, we used bioinformatic techniques to integrate sequence conservation, structural data, clinical data, and the available literature to identify 22 candidate proteins that convey OGT-CDG. We found using gene ontology and PANTHER database data that these candidate proteins are involved in diverse processes including Ras/MAPK signaling, translational repression, cytoskeletal dynamics, and chromatin remodeling. We also identify pathogenic missense variants at O-GlcNAcylation sites that segregate with intellectual disability. This work establishes a preliminary platform for the mechanistic dissection of the links between protein O-GlcNAcylation and neurodevelopment in OGT-CDG.

Structure-based discovery and development of novel O-GlcNAcase inhibitors for the treatment of Alzheimer's disease

The neurofibrillary tangles (NFTs) formed from hyperphosphorylation of tau protein are closely associated with Alzheimer's disease (AD). O-GlcNAcylation of tau can negatively regulate hyperphosphorylation and the O-GlcNAcase (OGA) catalyzes the removal of O-linked β-N-acetylglucosamine (O-GlcNAc) from tau protein. Therefore, preventing tau hyperphosphorylation by increasing the levels of tau O-GlcNAcylation via OGA inhibitors could be a promising approach. Based on Thiamet-G, a potent OGA inhibitor, and its binding mode to OGA, a novel OGA inhibitor scaffold bearing three parts was designed and hit compound 7j was successfully identified via extensive exploring. Further chemical optimization and diversification of the 7j structure resulted in compound 39 which possesses excellent OGA inhibition, no cytotoxicity, and has good pharmacokinetic properties. In acute AD model mice, 39 was more effective than Thiamet-G in inhibiting OGA activity attributable to its better blood-brain barrier permeability. In addition, 39 restored the cognitive function in mice and reduced amyloid-β (Aβ) concentrations to a greater extent than Thiamet-G. Molecular docking studies demonstrated that 39 was well associated with OGA through H-bonds and hydrophobic interaction. Together, these findings suggest that 39 was promising as a potent OGA inhibitor in the treatment of AD.

Computer Simulation to Rationalize “Rational” Engineering of Glycoside Hydrolases and Glycosyltransferases

Glycoside hydrolases and glycosyltransferases are the main classes of enzymes that synthesize and degrade carbohydrates, molecules essential to life that are a challenge for classical chemistry. As such, considerable efforts have been made to engineer these enzymes and make them pliable to human needs, ranging from directed evolution to rational design, including mechanism engineering. Such endeavors fall short and are unreported in numerous cases, while even success is a necessary but not sufficient proof that the chemical rationale behind the design is correct. Here we review some of the recent work in CAZyme mechanism engineering, showing that computational simulations are instrumental to rationalize experimental data, providing mechanistic insight into how native and engineered CAZymes catalyze chemical reactions. We illustrate this with two recent studies in which (i) a glycoside hydrolase is converted into a glycoside phosphorylase and (ii) substrate specificity of a glycosyltransferase is engineered toward forming O-, N-, or S-glycosidic bonds.

A living cell-based fluorescent reporter for high-throughput screening of anti-tumor drugs

Suppression of cellular O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) can repress proliferation and migration of various cancer cells, which opens a new avenue for cancer therapy. Based on the regulation of insulin gene transcription, we designed a cell-based fluorescent reporter capable of sensing cellular O-GlcNAcylation in HEK293T cells. The fluorescent reporter mainly consists of a reporter (green fluorescent protein (GFP)), an internal reference (red fluorescent protein), and an operator (neuronal differentiation 1), which serves as a “sweet switch” to control GFP expression in response to cellular O-GlcNAcylation changes. The fluorescent reporter can efficiently sense reduced levels of cellular O-GlcNAcylation in several cell lines. Using the fluorescent reporter, we screened 120 natural products and obtained one compound, sesamin, which could markedly inhibit protein O-GlcNAcylation in HeLa and human colorectal carcinoma-116 cells and repress their migration in vitro. Altogether, the present study demonstrated the development of a novel strategy for anti-tumor drug screening, as well as for conducting gene transcription studies.

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The reporter developed in this study is living cell-based with convenient utility.

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The method can be used for high-throughput screening.

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The reporter is versatile with potential applicability in the discovery of OGT/GFAT inhibitors and antitumor drugs.

Reprint of: Advanced glycosidases as ingenious biosynthetic instruments

Until recently, glycosidases, naturally hydrolyzing carbohydrate-active enzymes, have found few synthetic applications in industry, being primarily used for cleaving unwanted carbohydrates. With the establishment of glycosynthase and transglycosidase technology by genetic engineering, the view of glycosidases as industrial biotechnology tools has started to change. Their easy production, affordability, robustness, and substrate versatility, added to the possibility of controlling undesired side hydrolysis by enzyme engineering, have made glycosidases competitive synthetic tools. Current promising applications of engineered glycosidases include the production of well-defined chitooligomers, precious galactooligosaccharides or specialty chemicals such as glycosylated flavonoids. Other synthetic pathways leading to human milk oligosaccharides or remodeled antibodies are on the horizon. This work provides an overview of the synthetic achievements to date for glycosidases, emphasizing the latest trends and outlining possible developments in the field.

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

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

REM Sleep Deprivation Impairs Learning and Memory by Decreasing Brain O-GlcNAc Cycling in Mouse

Rapid eye movement (REM) sleep is implicated learning and memory (L/M) functions and hippocampal long-term potentiation (LTP). Here, we demonstrate that REM sleep deprivation (REMSD)-induced impairment of contextual fear memory in mouse is linked to a reduction in hexosamine biosynthetic pathway (HBP)/O-GlcNAc flux in mouse brain. In mice exposed to REMSD, O-GlcNAcylation, and O-GlcNAc transferase (OGT) were downregulated while O-GlcNAcase was upregulated compared to control mouse brain. Foot shock fear conditioning (FC) induced activation of protein kinase A (PKA) and cAMP response element binding protein (CREB), which were significantly inhibited in brains of the REMSD group. Intriguingly, REMSD-induced defects in L/M functions and FC-induced PKA/CREB activation were restored upon increasing O-GlcNAc cycling with glucosamine (GlcN) or Thiamet G. Furthermore, Thiamet G restored the REMSD-induced decrease in dendritic spine density. Suppression of O-GlcNAcylation by the glutamine fructose-6-phosphate amidotransferase (GFAT) inhibitor, 6-diazo-5-oxo-L-norleucine (DON), or OGT inhibitor, OSMI-1, impaired memory function, and inhibited FC-induced PKA/CREB activation. DON additionally reduced the amplitude of baseline field excitatory postsynaptic potential (fEPSP) and magnitude of long-term potentiation (LTP) in normal mouse hippocampal slices. To our knowledge, this is the first study to provide comprehensive evidence of dynamic O-GlcNAcylation changes during the L/M process in mice and defects in this pathway in the brain of REM sleep-deprived mice. Our collective results highlight HBP/O-GlcNAc cycling as a novel molecular link between sleep and cognitive function.

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

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

O-GlcNAcase inhibitors as potential therapeutics for the treatment of Alzheimer's disease and related tauopathies: analysis of the patent literature

Introduction: O-GlcNAcylation is a highly abundant post-translational modification of multiple proteins, including the microtubule-binding protein tau, governed by just two enzymes' concerted action O-GlcNAc transferase OGT and the hydrolase OGA. It is an approach to reduce abnormal tau hyperphosphorylation and aggregation in Alzheimer's disease (AD) and related tauopathies based on the ability of O-GlcNAcylation competing with tau phosphorylation, thus minimizing aggregation. The preclinical validation confirmed OGA inhibitors' efficacy in different transgenic tau mice models. Only three other OGA inhibitors have advanced into clinical trials thus far.Areas covered: 2008-2020 patent literature on OGA inhibitors.Expert opinion: Neurodegenerative disorders and AD specifically represent an enormous challenge since no effective treatments are available. Promising preclinical data has prompted considerable interest in searching for OGA inhibitors as a potential treatment for neurodegenerative disorders. Efforts from different companies have yielded a diverse set of chemotypes. OGA is a highly ubiquitous enzyme with many client proteins, generated data confirms a promising benign profile for OGA inhibition in healthy volunteers. Additionally, OGA PET tracers' existence will be critical for proper dose selection for future PoC Phase II studies, which will proof the true potential of OGA inhibition for the treatment of AD and other tauopathies.

OGA is associated with deglycosylation of NONO and the KU complex during DNA damage repair

Accumulated evidence shows that OGT-mediated O-GlcNAcylation plays an important role in response to DNA damage repair. However, it is unclear if the “eraser” O-GlcNAcase (OGA) participates in this cellular process. Here, we examined the molecular mechanisms and biological functions of OGA in DNA damage repair, and found that OGA was recruited to the sites of DNA damage and mediated deglycosylation following DNA damage. The recruitment of OGA to DNA lesions is mediated by O-GlcNAcylation events. Moreover, we have dissected OGA using deletion mutants and found that C-terminal truncated OGA including the pseudo HAT domain was required for the recruitment of OGA to DNA lesions. Using unbiased protein affinity purification, we found that the pseudo HAT domain was associated with DNA repair factors including NONO and the Ku70/80 complex. Following DNA damage, both NONO and the Ku70/80 complex were O-GlcNAcylated by OGT. The pseudo HAT domain was required to recognize NONO and the Ku70/80 complex for their deglycosylation. Suppression of the deglycosylation prolonged the retention of NONO at DNA lesions and delayed NONO degradation on the chromatin, which impaired non-homologus end joining (NHEJ). Collectively, our study reveals that OGA-mediated deglycosylation plays a key role in DNA damage repair.

O-GlcNAcylation Prediction: An Unattained Objective

BACKGROUND

O-GlcNAcylation is an essential post-translational modification (PTM) in mammalian cells. It consists in the addition of a N-acetylglucosamine (GlcNAc) residue onto serines or threonines by an O-GlcNAc transferase (OGT). Inhibition of OGT is lethal, and misregulation of this PTM can lead to diverse pathologies including diabetes, Alzheimer's disease and cancers. Knowing the location of O-GlcNAcylation sites and the ability to accurately predict them is therefore of prime importance to a better understanding of this process and its related pathologies.

PURPOSE

Here, we present an evaluation of the current predictors of O-GlcNAcylation sites based on a newly built dataset and an investigation to improve predictions.

METHODS

Several datasets of experimentally proven O-GlcNAcylated sites were combined, and the resulting meta-dataset was used to evaluate three prediction tools. We further defined a set of new features following the analysis of the primary to tertiary structures of experimentally proven O-GlcNAcylated sites in order to improve predictions by the use of different types of machine learning techniques.

RESULTS

Our results show the failure of currently available algorithms to predict O-GlcNAcylated sites with a precision exceeding 9%. Our efforts to improve the precision with new features using machine learning techniques do succeed for equal proportions of O-GlcNAcylated and non-O-GlcNAcylated sites but fail like the other tools for real-life proportions where ~1.4% of S/T are O-GlcNAcylated.

CONCLUSION

Present-day algorithms for O-GlcNAcylation prediction narrowly outperform random prediction. The inclusion of additional features, in combination with machine learning algorithms, does not enhance these predictions, emphasizing a pressing need for further development. We hypothesize that the improvement of prediction algorithms requires characterization of OGT's partners.

O-GlcNAcylation is a key regulator of multiple cellular metabolic pathways

O-GlcNAcylation modifies proteins in serine or threonine residues in the nucleus, cytoplasm, and mitochondria. It regulates a variety of cellular biological processes and abnormal O-GlcNAcylation is associated with diabetes, cancer, cardiovascular disease, and neurodegenerative diseases. Recent evidence has suggested that O-GlcNAcylation acts as a nutrient sensor and signal integrator to regulate metabolic signaling, and that dysregulation of its metabolism may be an important indicator of pathogenesis in disease. Here, we review the literature focusing on O-GlcNAcylation regulation in major metabolic processes, such as glucose metabolism, mitochondrial oxidation, lipid metabolism, and amino acid metabolism. We discuss its role in physiological processes, such as cellular nutrient sensing and homeostasis maintenance. O-GlcNAcylation acts as a key regulator in multiple metabolic processes and pathways. Our review will provide a better understanding of how O-GlcNAcylation coordinates metabolism and integrates molecular networks.

Advanced glycosidases as ingenious biosynthetic instruments

Until recently, glycosidases, naturally hydrolyzing carbohydrate-active enzymes, have found few synthetic applications in industry, being primarily used for cleaving unwanted carbohydrates. With the establishment of glycosynthase and transglycosidase technology by genetic engineering, the view of glycosidases as industrial biotechnology tools has started to change. Their easy production, affordability, robustness, and substrate versatility, added to the possibility of controlling undesired side hydrolysis by enzyme engineering, have made glycosidases competitive synthetic tools. Current promising applications of engineered glycosidases include the production of well-defined chitooligomers, precious galactooligosaccharides or specialty chemicals such as glycosylated flavonoids. Other synthetic pathways leading to human milk oligosaccharides or remodeled antibodies are on the horizon. This work provides an overview of the synthetic achievements to date for glycosidases, emphasizing the latest trends and outlining possible developments in the field.

NleB/SseKs ortholog effectors as a general bacterial monoglycosyltransferase for eukaryotic proteins

Protein glycosylation is the most common post-translational modification as more than 50% of all human proteins are glycosylated. Pathogenic bacteria glycosylation allows adhesion to host cells and manipulates eukaryotic functions. A variety of acceptor proteins in bacterial glycosylation was recently discovered. Especially NleB/SseKs type III effectors unexpectedly glycosylate a poor nucleophile arginine. Other pathogenic toxins modify the unusual tyrosine, as well as canonical serine/threonine residues. And a huge diversity is found in target proteins; Rho/Ras families, death domains and moreover themselves for autoglycosylation. However, in spite of this acceptor diversity, all their sugar donors are only UDP-Glc/-GlcNAc and structural alignments as liganded show their catalytic cores are geometrically conserved, where DRY and DXD motives and W residues equally position to hold the sugar donors and to π-π bind with a uridine ring, respectively. Therefore, bacterial glycosyltransferases have a key for carbohydrate research problems concerning the sugar donors and target proteins recognition.

Protein O-GlcNAc Modification Links Dietary and Gut Microbial Cues to the Differentiation of Enteroendocrine L Cells

Intestinal L cells regulate a wide range of metabolic processes, and L-cell dysfunction has been implicated in the pathogenesis of obesity and diabetes. However, it is incompletely understood how luminal signals are integrated to control the development of L cells. Here we show that food availability and gut microbiota-produced short-chain fatty acids control the posttranslational modification on intracellular proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) in intestinal epithelial cells. Via FOXO1 O-GlcNAcylation, O-GlcNAc transferase (OGT) suppresses expression of the lineage-specifying transcription factor Neurogenin 3 and, thus, L cell differentiation from enteroendocrine progenitors. Intestinal epithelial ablation of OGT in mice not only causes L cell hyperplasia and increased secretion of glucagon-like peptide 1 (GLP-1) but also disrupts gut microbial compositions, which notably contributes to decreased weight gain and improved glycemic control. Our results identify intestinal epithelial O-GlcNAc signaling as a brake on L cell development and function in response to nutritional and microbial cues.

Zhao et al. identify OGT in intestinal epithelial cells as a “molecular brake” on L cell development and function in response to nutritional and microbial cues. OGT inhibits Ngn3 gene transcription and enteroendocrine differentiation via FOXO1 O-GlcNAcylation. Microbiota-derived SCFAs drive epithelial O-GlcNAcylation, which further influences gut microbiota to control systemic metabolism.

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.

Post-translational modifications of EZH2 in cancer

Enhancer of zeste homolog 2 (EZH2), as a main component of Polycomb Repressive Complex 2, catalyzes histone H3K27me3 to silence its target gene expression. EZH2 upregulation results in cancer development and poor prognosis of cancer patients. Post-translational modifications (PTMs) are important biological events in cancer progression. PTMs regulate protein conformation and diversity functions. Recently, mounting studies have demonstrated that EZH2 stability, histone methyltransferase activity, localization, and binding partners can be regulated by PTMs, including phosphorylation, O-GlcNAcylation, acetylation, methylation and ubiquitination. However, the detailed molecular mechanisms of the EZH2-PTMs and whether other types of PTMs occur in EZH2 remain largely unclear. This review presents an overview of different roles of EZH2 modification and EZH2-PTMs crosstalk during tumorigenesis and cancer metastasis. We also discussed the therapeutic potential of targeting EZH2 modifications for cancer therapy.

Exosomal O-GlcNAc transferase from esophageal carcinoma stem cell promotes cancer immunosuppression through up-regulation of PD-1 in CD8+ T cells

Esophageal carcinoma stem cells (ECSCs) are responsible for the initiation and therapy-resistance of esophageal cancer. Nutrient sensor O-GlcNAc transferase (OGT) promoted the growth and metastasis of cancer cells. However, the contributions of OGT to the tumorigenesis of ECSCs remain largely uncover. In the present study, as compared to matched non-stem cancer cells, the expression of OGT was higher in ALDH⁺ ECSCs. Knock down of OGT by lentivirus system reduced the self-renewal capacities and tumorigenicity of ALDH⁺ ECSCs. In addition, OGT in exosome derived from ALDH⁺ ECSCs was taken up by neighboring CD8⁺ T cells and increased the expression of PD-1 in CD8⁺ T cells. Down-regulation of OGT increased the apoptosis of ALDH⁺ ECSCs induced by CD8⁺ T cells, which could be blocked by overexpression of PD-1 in CD8⁺ T cells. Together, OGT in exosome from ECSCs protects ECSCs from CD8⁺ T cells through up-regulation of PD-1.

Proteomic profiling and genome-wide mapping of O-GlcNAc chromatin-associated proteins reveal an O-GlcNAc-regulated genotoxic stress response

O-GlcNAc modification plays critical roles in regulating the stress response program and cellular homeostasis. However, systematic and multi-omics studies on the O-GlcNAc regulated mechanism have been limited. Here, comprehensive data are obtained by a chemical reporter-based method to survey O-GlcNAc function in human breast cancer cells stimulated with the genotoxic agent adriamycin. We identify 875 genotoxic stress-induced O-GlcNAc chromatin-associated proteins (OCPs), including 88 O-GlcNAc chromatin-associated transcription factors and cofactors (OCTFs), subsequently map their genomic loci, and construct a comprehensive transcriptional reprogramming network. Notably, genotoxicity-induced O-GlcNAc enhances the genome-wide interactions of OCPs with chromatin. The dynamic binding switch of hundreds of OCPs from enhancers to promoters is identified as a crucial feature in the specific transcriptional activation of genes involved in the adaptation of cancer cells to genotoxic stress. The OCTF nuclear respiratory factor 1 (NRF1) is found to be a key response regulator in O-GlcNAc-modulated cellular homeostasis. These results provide a valuable clue suggesting that OCPs act as stress sensors by regulating the expression of various genes to protect cancer cells from genotoxic stress.

Protein O-GlcNAcylation is involved in regulating gene expression and maintaining cellular homeostasis. Here, the authors develop a chemical reporter-based strategy for the proteomic profiling and genome-wide mapping of genotoxic stress-induced O-GlcNAcylated chromatin-associated proteins.

Post-translational modifications of PRC2: signals directing its activity

Polycomb repressive complex 2 (PRC2) is a chromatin-modifying enzyme that catalyses the methylation of histone H3 at lysine 27 (H3K27me1/2/3). This complex maintains gene transcriptional repression and plays an essential role in the maintenance of cellular identity as well as normal organismal development. The activity of PRC2, including its genomic targeting and catalytic activity, is controlled by various signals. Recent studies have revealed that these signals involve cis chromatin features, PRC2 facultative subunits and post-translational modifications (PTMs) of PRC2 subunits. Overall, these findings have provided insight into the biochemical signals directing PRC2 function, although many mysteries remain.

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.

Intracellular Hydrolysis of Small-Molecule O-Linked N-Acetylglucosamine Transferase Inhibitors Differs among Cells and Is Not Required for Its Inhibition

O-GlcNAcylation is an essential post-translational modification that occurs on nuclear and cytoplasmic proteins, regulating their function in response to cellular stress and altered nutrient availability. O-GlcNAc transferase (OGT) is the enzyme that catalyzes this reaction and represents a potential therapeutic target, whose biological role is still not fully understood. To support this research field, a series of cell-permeable, low-nanomolar OGT inhibitors were recently reported. In this study, we resynthesized the most potent OGT inhibitor of the library, OSMI-4, and we used it to investigate OGT inhibition in different human cell lines. The compound features an ethyl ester moiety that is supposed to be cleaved by carboxylesterases to generate its active metabolite. Our LC-HRMS analysis of the cell lysates shows that this is not always the case and that, even in the cell lines where hydrolysis does not occur, OGT activity is inhibited.

The O-GlcNAc modification promotes terminal differentiation of human corneal epithelial cells

Dynamic modification of nuclear and cytoplasmic proteins with O-linked β-N-acetylglucosamine (O-GlcNAc) plays an important role in orchestrating the transcriptional activity of eukaryotic cells. Here, we report that the O-GlcNAc modification contributes to maintaining ocular surface epithelial homeostasis by promoting mucin biosynthesis and barrier function. We found that induction of human corneal epithelial cell differentiation stimulated the global transfer of O-GlcNAc to both nuclear and cytosolic proteins. Inflammatory conditions, on the other hand, were associated with a reduction in the expression of O-GlcNAc transferase at the ocular surface epithelia. Loss- and gain-of-function studies using small interfering RNA targeting O-GlcNAc transferase, or Thiamet G, a selective inhibitor of O-GlcNAc hydrolase, respectively, revealed that the presence of O-GlcNAc was necessary to promote glycocalyx barrier function. Moreover, we found that Thiamet G triggered a correlative increase in both surface expression of MUC16 and apical epithelial cell area while reducing paracellular permeability. Collectively, these results identify intracellular protein O-glycosylation as a novel pathway responsible for promoting the terminal differentiation of human corneal epithelial cells.

O-GlcNAcylation as a Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia and the number of elderly patients suffering from AD has been steadily increasing. Despite worldwide efforts to cope with this disease, little progress has been achieved with regard to identification of effective therapeutics. Thus, active research focusing on identification of new therapeutic targets of AD is ongoing. Among the new targets, post-translational modifications which modify the properties of mature proteins have gained attention. O-GlcNAcylation, a type of PTM that attaches O-linked β-N-acetylglucosamine (O-GlcNAc) to a protein, is being sought as a new target to treat AD pathologies. O-GlcNAcylation has been known to modify the two important components of AD pathological hallmarks, amyloid precursor protein, and tau protein. In addition, elevating O-GlcNAcylation levels in AD animal models has been shown to be effective in alleviating AD-associated pathology. Although studies investigating the precise mechanism of reversal of AD pathologies by targeting O-GlcNAcylation are not yet complete, it is clearly important to examine O-GlcNAcylation regulation as a target of AD therapeutics. This review highlights the mechanisms of O-GlcNAcylation and its role as a potential therapeutic target under physiological and pathological AD conditions.

Inhibiting the Hexosamine Biosynthetic Pathway Lowers O-GlcNAcylation Levels and Sensitizes Cancer to Environmental Stress

The amounts of the intracellular glycosylation, O-GlcNAc modification, are increased in essentially all tumors when compared to healthy tissue, and lowering O-GlcNAcylation levels results in reduced tumorigenesis and increased cancer cell death. Therefore, the pharmacological reduction of O-GlcNAc may represent a therapeutic vulnerability. The most direct approach to this goal is the inhibition of O-GlcNAc transferase (OGT), the enzyme that directly adds the modification to proteins. However, despite some recent success, this enzyme has proven difficult to inhibit. An alternative strategy involves starving OGT of its sugar substrate UDP-GlcNAc by targeting enzymes of the hexosamine biosynthetic pathway (HBP). Here, we explore the potential of the rate-determining enzyme of this pathway, glutamine fructose-6-phosphate amidotransferase (GFAT). We first show that CRISPR-mediated knockout of GFAT results in inhibition of cancer cell growth in vitro and a xenograft model that correlates with O-GlcNAcylation levels. We then demonstrate that pharmacological inhibition of GFAT sensitizes a small panel of cancer cells to undergo apoptosis in response to diamide-induced oxidative stress. Finally, we find that GFAT expression and O-GlcNAc levels are increased in a spontaneous mouse model of liver cancer. Together these experiments support the further development of inhibitors of the HBP as an indirect approach to lowering O-GlcNAcylation levels in cancer.

ULK1 O-GlcNAcylation Is Crucial for Activating VPS34 via ATG14L during Autophagy Initiation

Unc-51-like-kinase 1 (ULK1) is a target of both the mechanistic target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK), whose role is to facilitate the initiation of autophagy in response to starvation. Upon glucose starvation, dissociation of mTOR from ULK1 and phosphorylation by AMPK leads to the activation of ULK1 activity. Here, we provide evidence that ULK1 is the attachment of O-linked N-acetylglucosamine (O-GlcNAcylated) on the threonine 754 site by O-linked N-acetylglucosamine transferase (OGT) upon glucose starvation. ULK1 O-GlcNAcylation occurs after dephosphorylation of adjacent mTOR-dependent phosphorylation on the serine 757 site by protein phosphatase 1 (PP1) and phosphorylation by AMPK. ULK1 O-GlcNAcylation is crucial for binding and phosphorylation of ATG14L, allowing the activation of lipid kinase VPS34 and leading to the production of phosphatidylinositol-(3)-phosphate (PI(3)P), which is required for phagophore formation and initiation of autophagy. Our findings provide insights into the crosstalk between dephosphorylation and O-GlcNAcylation during autophagy and specify a molecular framework for potential therapeutic intervention in autophagy-related diseases.

Targeted covalent inhibition of O-GlcNAc transferase in cells

O-GlcNAc transferase (OGT) glycosylates numerous proteins and is implicated in many diseases. To date, most OGT inhibitors lack either sufficient potency or characterized specificity in cells. We report the first targeted covalent inhibitor that predominantly reacts with OGT but does not affect other functionally similar enzymes. This study provides a new strategy to interrogate cellular OGT functions and to investigate other glycosyltransferases.

Structural and Functional Insights into Host Death Domains Inactivation by the Bacterial Arginine GlcNAcyltransferase Effector

Enteropathogenic E. coli NleB and related type III effectors catalyze arginine GlcNAcylation of death domain (DD) proteins to block host defense, but the underlying mechanism is unknown. Here we solve crystal structures of NleB alone and in complex with FADD-DD, UDP, and Mn²⁺ as well as NleB-GlcNAcylated DDs of TRADD and RIPK1. NleB adopts a GT-A fold with a unique helix-pair insertion to hold FADD-DD; the interface contacts explain the selectivity of NleB for certain DDs. The acceptor arginine is fixed into a cleft, in which Glu253 serves as a base to activate the guanidinium. Analyses of the enzyme-substrate complex and the product structures reveal an inverting sugar-transfer reaction and a detailed catalytic mechanism. These structural insights are validated by mutagenesis analyses of NleB-mediated GlcNAcylation in vitro and its function in mouse infection. Our study builds a structural framework for understanding of NleB-catalyzed arginine GlcNAcylation of host death domain.

Identifying potentially O-GlcNAcylated proteins using metabolic labeling, bioorthogonal enrichment, and Western blotting

O-GlcNAcylation is a widespread posttranslational modification of intracellular proteins. Phenotypic and genetic experiments have established key roles for O-GlcNAc in development, mammalian cell survival, and several human diseases. However, the underlying mechanisms by which this modification alters biological pathways are still being discovered. An important part of this discovery process is the discovery of O-GlcNAcylated proteins, where chemical approaches have been particularly powerful. Here we describe how to combine one of these approaches, metabolic chemical reporters (MCRs), with bioorthogonal chemistry and Western blotting to identify potentially O-GlcNAcylated proteins.