Recombinant O-GlcNAc transferase isoforms: identification of O-GlcNAcase, yes tyrosine kinase, and tau as isoform-specific substrates

Targeting O-GlcNAcylation in cancer therapeutic resistance: The sugar Saga continues

O-linked-N-acetylglucosaminylation (O-GlcNAcylation), a dynamic post-translational modification (PTM), holds profound implications in controlling various cellular processes such as cell signaling, metabolism, and epigenetic regulation that influence cancer progression and therapeutic resistance. From the therapeutic perspective, O-GlcNAc modulates drug efflux, targeting and metabolism. By integrating signals from glucose, lipid, amino acid, and nucleotide metabolic pathways, O-GlcNAc acts as a nutrient sensor and transmits signals to exerts its function on genome stability, epithelial-mesenchymal transition (EMT), cell stemness, cell apoptosis, autophagy, cell cycle. O-GlcNAc also attends to tumor microenvironment (TME) and the immune response. At present, several strategies aiming at targeting O-GlcNAcylation are under mostly preclinical evaluation, where the newly developed O-GlcNAcylation inhibitors markedly enhance therapeutic efficacy. Here we systematically outline the mechanisms through which O-GlcNAcylation influences therapy resistance and deliberate on the prospects and challenges associated with targeting O-GlcNAcylation in future cancer treatments.

O-GlcNAc transferase regulates collagen deposition and fibrosis resolution in idiopathic pulmonary fibrosis

Background

Idiopathic pulmonary fibrosis (IPF) is a chronic pulmonary disease that is characterized by an excessive accumulation of extracellular matrix (ECM) proteins (e.g. collagens) in the parenchyma, which ultimately leads to respiratory failure and death. While current therapies exist to slow the progression, no therapies are available to resolve fibrosis.

Methods

We characterized the O-linked N-Acetylglucosamine (O-GlcNAc) transferase (OGT)/O-GlcNAc axis in IPF using single-cell RNA-sequencing (scRNA-seq) data and human lung sections and isolated fibroblasts from IPF and non-IPF donors. The underlying mechanism(s) of IPF were further investigated using multiple experimental models to modulate collagen expression and accumulation by genetically and pharmacologically targeting OGT. Furthermore, we hone in on the transforming growth factor-beta (TGF-β) effector molecule, Smad3, by co-expressing it with OGT to determine if it is modified and its subsequent effect on Smad3 activation.

Results

We found that OGT and O-GlcNAc levels are upregulated in patients with IPF compared to non-IPF. We report that the OGT regulates collagen deposition and fibrosis resolution, which is an evolutionarily conserved process demonstrated across multiple species. Co-expression of OGT and Smad3 showed that Smad3 is O-GlcNAc modified. Blocking OGT activity resulted in decreased phosphorylation at Ser-423/425 of Smad3 attenuating the effects of TGF-β1 induced collagen expression/deposition.

Conclusion

OGT inhibition or knockdown successfully blocked and reversed collagen expression and accumulation, respectively. Smad3 is discovered to be a substrate of OGT and its O-GlcNAc modification(s) directly affects its phosphorylation state. These data identify OGT as a potential target in pulmonary fibrosis resolution, as well as other diseases that might have aberrant ECM/collagen accumulation.

Increased O-GlcNAcylation by Upregulation of Mitochondrial O-GlcNAc Transferase (mOGT) Inhibits the Activity of Respiratory Chain Complexes and Controls Cellular Bioenergetics

O-linked β-N-acetylglucosamine (O-GlcNAc) is a reversible post-translational modification involved in the regulation of cytosolic, nuclear, and mitochondrial proteins. The interplay between O-GlcNAcylation and phosphorylation is critical to control signaling pathways and maintain cellular homeostasis. The addition of O-GlcNAc moieties to target proteins is catalyzed by O-linked N-acetylglucosamine transferase (OGT). Of the three splice variants of OGT described, one is destined for the mitochondria (mOGT). Although the effects of O-GlcNAcylation on the biology of normal and cancer cells are well documented, the role of mOGT remains poorly understood. In this manuscript, the effects of mOGT on mitochondrial protein phosphorylation, electron transport chain (ETC) complex activity, and the expression of VDAC porins were investigated. We performed studies using normal and breast cancer cells with upregulated mOGT or its catalytically inactive mutant. Proteomic approaches included the isolation of O-GlcNAc-modified proteins of the electron transport chain, followed by their analysis using mass spectrometry. We found that mitochondrial OGT regulates the activity of complexes I-V of the respiratory chain and identified a group of 19 ETC components as mOGT substrates in mammary cells. Furthermore, we observed that the upregulation of mOGT inhibited the interaction of VDAC1 with hexokinase II. Our results suggest that the deregulation of mOGT reprograms cellular energy metabolism via interaction with and O-GlcNAcylation of proteins involved in ATP production in mitochondria and its exchange between mitochondria and the cytosol.

Cryo-EM structure of human O-GlcNAcylation enzyme pair OGT-OGA complex

O-GlcNAcylation is a conserved post-translational modification that attaches N-acetyl glucosamine (GlcNAc) to myriad cellular proteins. In response to nutritional and hormonal signals, O-GlcNAcylation regulates diverse cellular processes by modulating the stability, structure, and function of target proteins. Dysregulation of O-GlcNAcylation has been implicated in the pathogenesis of cancer, diabetes, and neurodegeneration. A single pair of enzymes, the O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), catalyzes the addition and removal of O-GlcNAc on over 3,000 proteins in the human proteome. However, how OGT selects its native substrates and maintains the homeostatic control of O-GlcNAcylation of so many substrates against OGA is not fully understood. Here, we present the cryo-electron microscopy (cryo-EM) structures of human OGT and the OGT-OGA complex. Our studies reveal that OGT forms a functionally important scissor-shaped dimer. Within the OGT-OGA complex structure, a long flexible OGA segment occupies the extended substrate-binding groove of OGT and positions a serine for O-GlcNAcylation, thus preventing OGT from modifying other substrates. Conversely, OGT disrupts the functional dimerization of OGA and occludes its active site, resulting in the blocking of access by other substrates. This mutual inhibition between OGT and OGA may limit the futile O-GlcNAcylation cycles and help to maintain O-GlcNAc homeostasis.

Hexosamine biosynthetic pathway and O-GlcNAc cycling of glucose metabolism in brain function and disease

Impaired brain glucose metabolism is considered a hallmark of brain dysfunction and neurodegeneration. Disruption of the hexosamine biosynthetic pathway (HBP) and subsequent O-linked N-acetylglucosamine (O-GlcNAc) cycling has been identified as an emerging link between altered glucose metabolism and defects in the brain. Myriads of cytosolic and nuclear proteins in the nervous system are modified at serine or threonine residues with a single N-acetylglucosamine (O-GlcNAc) molecule by O-GlcNAc transferase (OGT), which can be removed by β-N-acetylglucosaminidase (O-GlcNAcase, OGA). Homeostatic regulation of O-GlcNAc cycling is important for the maintenance of normal brain activity. Although significant evidence linking dysregulated HBP metabolism and aberrant O-GlcNAc cycling to induction or progression of neuronal diseases has been obtained, the issue of whether altered O-GlcNAcylation is causal in brain pathogenesis remains uncertain. Elucidation of the specific functions and regulatory mechanisms of individual O-GlcNAcylated neuronal proteins in both normal and diseased states may facilitate the identification of novel therapeutic targets for various neuronal disorders. The information presented in this review highlights the importance of HBP/O-GlcNAcylation in the neuronal system and summarizes the roles and potential mechanisms of O-GlcNAcylated neuronal proteins in maintaining normal brain function and initiation and progression of neurological diseases.

SPINDLY mediates O-fucosylation of hundreds of proteins and sugar-dependent growth in Arabidopsis

The recent discovery of SPINDLY (SPY)-catalyzed protein O-fucosylation revealed a novel mechanism for regulating nucleocytoplasmic protein functions in plants. Genetic evidence indicates the important roles of SPY in diverse developmental and physiological processes. However, the upstream signal controlling SPY activity and the downstream substrate proteins O-fucosylated by SPY remain largely unknown. Here, we demonstrated that SPY mediates sugar-dependent growth in Arabidopsis (Arabidopsis thaliana). We further identified hundreds of O-fucosylated proteins using lectin affinity chromatography followed by mass spectrometry. All the O-fucosylation events quantified in our proteomic analyses were undetectable or dramatically decreased in the spy mutants, and thus likely catalyzed by SPY. The O-fucosylome includes mostly nuclear and cytosolic proteins. Many O-fucosylated proteins function in essential cellular processes, phytohormone signaling, and developmental programs, consistent with the genetic functions of SPY. The O-fucosylome also includes many proteins modified by O-linked N-acetylglucosamine (O-GlcNAc) and by phosphorylation downstream of the target of rapamycin (TOR) kinase, revealing the convergence of these nutrient signaling pathways on key regulatory functions such as post-transcriptional/translational regulation and phytohormone responses. Our study identified numerous targets of SPY/O-fucosylation and potential nodes of crosstalk among sugar/nutrient signaling pathways, enabling future dissection of the signaling network that mediates sugar regulation of plant growth and development.

The Emerging Roles of Protein Interactions with O-GlcNAc Cycling Enzymes in Cancer

The dynamic O-GlcNAc modification of intracellular proteins is an important nutrient sensor for integrating metabolic signals into vast networks of highly coordinated cellular activities. Dysregulation of the sole enzymes responsible for O-GlcNAc cycling, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), and the associated cellular O-GlcNAc profile is a common feature across nearly every cancer type. Many studies have investigated the effects of aberrant OGT/OGA expression on global O-GlcNAcylation activity in cancer cells. However, recent studies have begun to elucidate the roles of protein-protein interactions (PPIs), potentially through regions outside of the immediate catalytic site of OGT/OGA, that regulate greater protein networks to facilitate substrate-specific modification, protein translocalization, and the assembly of larger biomolecular complexes. Perturbation of OGT/OGA PPI networks makes profound changes in the cell and may directly contribute to cancer malignancies. Herein, we highlight recent studies on the structural features of OGT and OGA, as well as the emerging roles and molecular mechanisms of their aberrant PPIs in rewiring cancer networks. By integrating complementary approaches, the research in this area will aid in the identification of key protein contacts and functional modules derived from OGT/OGA that drive oncogenesis and will illuminate new directions for anti-cancer drug development.

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.

Applying transcriptomics to studyglycosylation at the cell type level

The complex multi-step process of glycosylation occurs in a single cell, yet current analytics generally cannot measure the output (the glycome) of a single cell. Here, we addressed this discordance by investigating how single cell RNA-seq data can be used to characterize the state of the glycosylation machinery and metabolic network in a single cell. The metabolic network involves 214 glycosylation and modification enzymes outlined in our previously built atlas of cellular glycosylation pathways. We studied differential mRNA regulation of enzymes at the organ and single cell level, finding that most of the general protein and lipid oligosaccharide scaffolds are produced by enzymes exhibiting limited transcriptional regulation among cells. We predict key enzymes within different glycosylation pathways to be highly transcriptionally regulated as regulatable hotspots of the cellular glycome. We designed the Glycopacity software that enables investigators to extract and interpret glycosylation information from transcriptome data and define hotspots of regulation.

•

RNA-seq can provide information on the glycosylation metabolic network state

•

It is possible to readout glycosylation capacity from single cell RNA-seq data

•

Genes regulating the biosynthesis of common glycan scaffolds show little regulation

•

Key enzymes in the glycosylation network are predicted to be regulatable hotspots

The intra-mitochondrial O-GlcNAcylation system rapidly modulates OXPHOS function and ROS release in the heart

Protein O-GlcNAcylation is increasingly recognized as an important cellular regulatory mechanism, in multiple organs including the heart. However, the mechanisms leading to O-GlcNAcylation in mitochondria and the consequences on their function remain poorly understood. In this study, we use an in vitro reconstitution assay to characterize the intra-mitochondrial O-GlcNAc system without potential cytoplasmic confounding effects. We compare the O-GlcNAcylome of isolated cardiac mitochondria with that of mitochondria acutely exposed to NButGT, a specific inhibitor of glycoside hydrolase. Amongst the 409 O-GlcNAcylated mitochondrial proteins identified, 191 display increased O-GlcNAcylation in response to NButGT. This is associated with enhanced Complex I (CI) activity, increased maximal respiration in presence of pyruvate-malate, and a striking reduction of mitochondrial ROS release, which could be related to O-GlcNAcylation of specific subunits of ETC complexes (CI, CIII) and TCA cycle enzymes. In conclusion, our work underlines the existence of a dynamic mitochondrial O-GlcNAcylation system capable of rapidly modifying mitochondrial function.

An in vitro assay in isolated heart mitochondria reveals that O-GlcNAcase inhibitor NButGT rapidly increases protein O-GlcNAcylation leading to increased respiratory capacity and complex I activity and decreased ROS release.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

Targeting Protein O-GlcNAcylation, a Link between Type 2 Diabetes Mellitus and Inflammatory Disease

Unresolved hyperglycaemia, a hallmark of type 2 diabetes mellitus (T2DM), is a well characterised manifestation of altered fuel homeostasis and our understanding of its role in the pathologic activation of the inflammatory system continues to grow. Metabolic disorders like T2DM trigger changes in the regulation of key cellular processes such as cell trafficking and proliferation, and manifest as chronic inflammatory disorders with severe long-term consequences. Activation of inflammatory pathways has recently emerged as a critical link between T2DM and inflammation. A substantial body of evidence has suggested that this is due in part to increased flux through the hexosamine biosynthetic pathway (HBP). The HBP, a unique nutrient-sensing metabolic pathway, produces the activated amino sugar UDP-GlcNAc which is a critical substrate for protein O-GlcNAcylation, a dynamic, reversible post-translational glycosylation of serine and threonine residues in target proteins. Protein O-GlcNAcylation impacts a range of cellular processes, including inflammation, metabolism, trafficking, and cytoskeletal organisation. As increased HBP flux culminates in increased protein O-GlcNAcylation, we propose that targeting O-GlcNAcylation may be a viable therapeutic strategy for the prevention and management of glucose-dependent pathologies with inflammatory components.

Truncation of the TPR domain of OGT alters substrate and glycosite selection

O-GlcNAc transferase (OGT) is an essential enzyme that installs O-linked N-acetylglucosamine (O-GlcNAc) to thousands of protein substrates. OGT and its isoforms select from these substrates through the tetratricopeptide repeat (TPR) domain, yet the impact of truncations to the TPR domain on substrate and glycosite selection is unresolved. Here, we report the effects of iterative truncations to the TPR domain of OGT on substrate and glycosite selection with the model protein GFP-JunB and the surrounding O-GlcNAc proteome in U2OS cells. Iterative truncation of the TPR domain of OGT maintains glycosyltransferase activity but alters subcellular localization of OGT in cells. The glycoproteome and glycosites modified by four OGT TPR isoforms were examined on the whole proteome and a single target protein, GFP-JunB. We found the greatest changes in O-GlcNAc on proteins associated with mRNA splicing processes and that the first four TPRs of the canonical nucleocytoplasmic OGT had the broadest substrate scope. Subsequent glycosite analysis revealed that alteration to the last four TPRs corresponded to the greatest shift in the resulting O-GlcNAc consensus sequence. This dataset provides a foundation to analyze how perturbations to the TPR domain and expression of OGT isoforms affect the glycosylation of substrates, which will be critical for future efforts in protein engineering of OGT, the biology of OGT isoforms, and diseases associated with the TPR domain of OGT.

Role and Function of O-GlcNAcylation in Cancer

Simple Summary

Despite the rapid advancement in immunotherapy and targeted agents, many patients diagnosed with cancer have poor prognosis with dismal overall survival. One of the key hallmarks of cancer is the ability of cancer cells to reprogram their energy metabolism. O-GlcNAcylation is an emerging potential mechanism for cancer cells to induce proliferation and progression of tumor cells and resistance to chemotherapy. This review summarizes the mechanism behind O-GlcNAcylation and discusses the role of O-GlcNAcylation, including its function with receptor tyrosine kinase and chemo-resistance in cancer, and immune response to cancer and as a prognostic factor. Further pre-clinical studies on O-GlcNAcylation are warranted to assess the clinical efficacy of agents targeting O-GlcNAcylation.

Abstract

Cancer cells are able to reprogram their glucose metabolism and retain energy via glycolysis even under aerobic conditions. They activate the hexosamine biosynthetic pathway (HBP), and the complex interplay of O-linked N-acetylglucosaminylation (O-GlcNAcylation) via deprivation of nutrients or increase in cellular stress results in the proliferation, progression, and metastasis of cancer cells. Notably, cancer is one of the emerging diseases associated with O-GlcNAcylation. In this review, we summarize studies that delineate the role of O-GlcNAcylation in cancer, including its modulation in metastasis, function with receptor tyrosine kinases, and resistance to chemotherapeutic agents, such as cisplatin. In addition, we discuss the function of O-GlcNAcylation in eliciting immune responses associated with immune surveillance in the tumor microenvironment. O-GlcNAcylation is increasingly accepted as one of the key players involved in the activation and differentiation of T cells and macrophages. Finally, we discuss the prognostic role of O-GlcNAcylation and potential therapeutic agents such as O-linked β-N-acetylglucosamine-transferase inhibitors, which may help overcome the resistance mechanism associated with the reprogramming of glucose metabolism.

Tau Protein Interaction Partners and Their Roles in Alzheimer's Disease and Other Tauopathies

Tau protein plays a critical role in the assembly, stabilization, and modulation of microtubules, which are important for the normal function of neurons and the brain. In diseased conditions, several pathological modifications of tau protein manifest. These changes lead to tau protein aggregation and the formation of paired helical filaments (PHF) and neurofibrillary tangles (NFT), which are common hallmarks of Alzheimer's disease and other tauopathies. The accumulation of PHFs and NFTs results in impairment of physiological functions, apoptosis, and neuronal loss, which is reflected as cognitive impairment, and in the late stages of the disease, leads to death. The causes of this pathological transformation of tau protein haven't been fully understood yet. In both physiological and pathological conditions, tau interacts with several proteins which maintain their proper function or can participate in their pathological modifications. Interaction partners of tau protein and associated molecular pathways can either initiate and drive the tau pathology or can act neuroprotective, by reducing pathological tau proteins or inflammation. In this review, we focus on the tau as a multifunctional protein and its known interacting partners active in regulations of different processes and the roles of these proteins in Alzheimer's disease and tauopathies.

Dual regulation of fatty acid synthase (FASN) expression by O-GlcNAc transferase (OGT) and mTOR pathway in proliferating liver cancer cells

Fatty acid synthase (FASN) participates in many fundamental biological processes, including energy storage and signal transduction, and is overexpressed in many cancer cells. We previously showed in a context of lipogenesis that FASN is protected from degradation by its interaction with O-GlcNAc transferase (OGT) in a nutrient-dependent manner. We and others also reported that OGT and O-GlcNAcylation up-regulate the PI3K/AKT/mTOR pathway that senses mitogenic signals and nutrient availability to drive cell cycle. Using biochemical and microscopy approaches, we show here that FASN co-localizes with OGT in the cytoplasm and, to a lesser extent, in the membrane fraction. This interaction occurs in a cell cycle-dependent manner, following the pattern of FASN expression. Moreover, we show that FASN expression depends on OGT upon serum stimulation. The level of FASN also correlates with the activation of the PI3K/AKT/mTOR pathway in hepatic cell lines, and in livers of obese mice and in a chronically activated insulin and mTOR signaling mouse model (PTEN-null mice). These results indicate that FASN is under a dual control of O-GlcNAcylation and mTOR pathways. In turn, blocking FASN with the small-molecule inhibitor C75 reduces both OGT and O-GlcNAcylation levels, and mTOR activation, highlighting a novel reciprocal regulation between these actors. In addition to the role of O-GlcNAcylation in tumorigenesis, our findings shed new light on how aberrant activity of FASN and mTOR signaling may promote the emergence of hepatic tumors.

Mitochondrial O-GlcNAc Transferase Interacts with and Modifies Many Proteins and Its Up-Regulation Affects Mitochondrial Function and Cellular Energy Homeostasis

O-GlcNAcylation is a cell glucose sensor. The addition of O-GlcNAc moieties to target protein is catalyzed by the O-Linked N-acetylglucosamine transferase (OGT). OGT is encoded by a single gene that yields differentially spliced OGT isoforms. One of them is targeted to mitochondria (mOGT). Although the impact of O-GlcNAcylation on cancer cells biology is well documented, mOGT's role remains poorly investigated. We performed studies using breast cancer cells with up-regulated mOGT or its catalytic inactive mutant to identify proteins specifically modified by mOGT. Proteomic approaches included isolation of mOGT protein partners and O-GlcNAcylated proteins from mitochondria-enriched fraction followed by their analysis by mass spectrometry. Moreover, we analyzed the impact of mOGT dysregulation on mitochondrial activity and cellular metabolism using a variety of biochemical assays. We found that mitochondrial OGT expression is glucose-dependent. Elevated mOGT expression affected the mitochondrial transmembrane potential and increased intramitochondrial ROS generation. Moreover, mOGT up-regulation caused a decrease in cellular ATP level. We identified many mitochondrial proteins as mOGT substrates. Most of these proteins are localized in the mitochondrial matrix and the inner mitochondrial membrane and participate in mitochondrial respiration, fatty acid metabolism, transport, translation, apoptosis, and mtDNA processes. Our findings suggest that mOGT interacts with and modifies many mitochondrial proteins, and its dysregulation affects cellular bioenergetics and mitochondria function.

Revascularisation of type 2 diabetics with coronary artery disease: Insights and therapeutic targeting of O-GlcNAcylation

AIM

Coronary artery bypass graft (CABG) using autologous saphenous vein continues to be a gold standard procedure to restore the supply of oxygen-rich blood to the heart muscles in coronary artery disease (CAD) patients with or without type 2 diabetes mellitus (T2DM). However, CAD patients with T2DM are at higher risk of graft failure. While failure rates have been reduced through improvements in procedure-related factors, much less is known about the molecular and cellular mechanisms by which T2DM initiates vein graft failure. This review gives novel insights into these cellular and molecular mechanisms and identifies potential therapeutic targets for development of new medicines to improve vein graft patency.

DATA SYNTHESIS

One important cellular process that has been implicated in the pathogenesis of T2DM is protein O-GlcNAcylation, a dynamic, reversible post-translational modification of serine and threonine residues on target proteins that is controlled by two enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Protein O-GlcNAcylation impacts a range of cellular processes, including trafficking, metabolism, inflammation and cytoskeletal organisation. Altered O-GlcNAcylation homeostasis have, therefore, been linked to a range of human pathologies with a metabolic component, including T2DM.

CONCLUSION

We propose that protein O-GlcNAcylation alters vascular smooth muscle and endothelial cell function through modification of specific protein targets which contribute to the vascular re-modelling responsible for saphenous vein graft failure in T2DM.

O-GlcNAcylation: the "stress and nutrition receptor" in cell stress response

O-GlcNAcylation is an atypical, reversible, and dynamic glycosylation that plays a critical role in maintaining the normal physiological functions of cells by regulating various biological processes such as signal transduction, proteasome activity, apoptosis, autophagy, transcription, and translation. It can also respond to environmental changes and physiological signals to play the role of "stress receptor" and "nutrition sensor" in a variety of stress responses and biological processes. Even, a homeostatic disorder of O-GlcNAcylation may cause many diseases. Therefore, O-GlcNAcylation and its regulatory role in stress response are reviewed in this paper.

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.

ELT-2 promotes O-GlcNAc transferase OGT-1 expression to modulate Caenorhabditis elegans lifespan

O-GlcNAc transferase (OGT) is the enzyme catalyzing protein O-GlcNAcylation by addition of a single O-linked-β-N-acetylglucosamine molecule (O-GlcNAc) to nuclear and cytoplasmic targets, and it uses uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) as a donor. As UDP-GlcNAc is the final product of the nutrient-sensing hexosamine signaling pathway, overexpression or knockout of ogt in mammals or invertebrate models influences cellular nutrient-response signals and increases susceptibility to chronic diseases of aging. Evidence shows that OGT expression levels decrease in tissues of older mice and rats. However, how OGT expression is modulated in the aging process remains poorly understood. In Caenorhabditis elegans, the exclusive mammalian OGT ortholog OGT-1 is crucial for lifespan control. Here, we observe that worm OGT-1 expression gradually reduces during aging. By combining prediction via the "MATCH" algorithm and luciferase reporter assays, GATA factor ELT-2, the homolog of human GATA4, is identified as a transcriptional factor driving OGT-1 expression. Chromatin immunoprecipitation-quantitative polymerase chain reaction and electrophoretic mobility shift assays show ELT-2 directly binds to and activates the ogt-1 promoter. Knockdown of elt-2 decreases the global O-GlcNAc modification level and reduces the lifespan of wild-type worms. The reduction in lifespan caused by elt-2 RNA interference is abrogated by the loss of ogt-1. These results imply that GATA factors are able to activate OGT expression, which could be beneficial for longevity and the development of therapeutic treatment for aging-related diseases.

Identification of novel O-GlcNAc transferase substrates using yeast cells expressing OGT

O-GlcNAc modification mediated by O-GlcNAc transferase (OGT) is a reversible protein modification in which O-GlcNAc moieties are attached to target proteins in the cytosol, nucleus, and mitochondria. O-GlcNAc moieties attached to proteins can be removed by O-GlcNAcase (OGA). The addition of an O-GlcNAc moiety can influence several aspects of protein function, and aberrant O-GlcNAc modification is linked to a number of diseases. While OGT and OGA are conserved across eukaryotic cells, yeasts lack these enzymes. Previously, we reported that protein O-GlcNAc modification occurred in the budding yeast Saccharomyces cerevisiae when OGT was ectopically expressed. Because yeast cells lack OGA, O-GlcNAc moieties are stably attached to target proteins. Thus, the yeast system may be useful for finding novel OST substrates. By proteomic analysis, we identified 468 O-GlcNAcylated proteins in yeast cells expressing human OGT. Among these proteins, 13 have human orthologues that show more than 30% identity to their corresponding yeast orthologue, and possible glycosylation residues are conserved in these human orthologues. In addition, the orthologues have not been reported as substrates of OGT. We verified that some of these human orthologues are O-GlcNAcylated in cultured human cells. These proteins include an ubiquitin-conjugating enzyme, UBE2D1, and an eRF3-similar protein, HBS1L. Thus, the yeast system would be useful to find previously unknown O-GlcNAcylated proteins and regulatory mechanisms.

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.

The Diabetic Cardiac Fibroblast: Mechanisms Underlying Phenotype and Function

Diabetic cardiomyopathy involves remodeling of the heart in response to diabetes that includes microvascular damage, cardiomyocyte hypertrophy, and cardiac fibrosis. Cardiac fibrosis is a major contributor to diastolic dysfunction that can ultimately result in heart failure with preserved ejection fraction. Cardiac fibroblasts are the final effector cell in the process of cardiac fibrosis. This review article aims to describe the cardiac fibroblast phenotype in response to high-glucose conditions that mimic the diabetic state, as well as to explain the pathways underlying this phenotype. As such, this review focuses on studies conducted on isolated cardiac fibroblasts. We also describe molecules that appear to oppose the pro-fibrotic actions of high glucose on cardiac fibroblasts. This represents a major gap in knowledge in the field that needs to be addressed.

Neuronal O-GlcNAcylation Improves Cognitive Function in the Aged Mouse Brain

Mounting evidence in animal models indicates potential for rejuvenation of cellular and cognitive functions in the aging brain. However, the ability to utilize this potential is predicated on identifying molecular targets that reverse the effects of aging in vulnerable regions of the brain, such as the hippocampus. The dynamic post-translational modification O-linked N-Acetylglucosamine (O-GlcNAc) has emerged as an attractive target for regulating aging-specific synaptic alterations as well as neurodegeneration. While speculation exists about the role of O-GlcNAc in neurodegenerative conditions, such as Alzheimer’s disease, its role in physiological brain aging remains largely unexplored. Here, we report that countering age-related decreased O-GlcNAc transferase (OGT) expression and O-GlcNAcylation ameliorates cognitive impairments in aged mice. Mimicking an aged condition in young adults by abrogating OGT, using a temporally controlled neuron-specific conditional knockout mouse model, recapitulated cellular and cognitive features of brain aging. Conversely, overexpressing OGT in mature hippocampal neurons using a viral-mediated approach enhanced associative fear memory in young adult mice. Excitingly, in aged mice overexpressing neuronal OGT in the aged hippocampus rescued in part age-related impairments in spatial learning and memory as well as associative fear memory. Our data identify O-GlcNAcylaton as a key molecular mediator promoting cognitive rejuvenation.

Wheatley et al. identify O-GlcNAcylation as a key posttranslational modification promoting cognitive rejuvenation. Mimicking age-related decreased neuronal OGT and O-GlcNAc levels in the young hippocampus impaired cognition, while restoring neuronal OGT and O-GlcNAc in the aged hippocampus rejuvenated cognition.

O-GlcNAcylation of Thr12/Ser56 in short-form O-GlcNAc transferase (sOGT) regulates its substrate selectivity

O-GlcNAcylation is a ubiquitous protein glycosylation playing different roles on variant proteins. O-GlcNAc transferase (OGT) is the unique enzyme responsible for the sugar addition to nucleocytoplasmic proteins. Recently, multiple O-GlcNAc sites have been observed on short-form OGT (sOGT) and nucleocytoplasmic OGT (ncOGT), both of which locate in the nucleus and cytoplasm in cell. Moreover, O-GlcNAcylation of Ser³⁸⁹ in ncOGT (1036 amino acids) affects its nuclear translocation in HeLa cells. To date, the major O-GlcNAcylation sites and their roles in sOGT remain unknown. Here, we performed LC-MS/MS and mutational analyses to seek the major O-GlcNAcylation site on sOGT. We identified six O-GlcNAc sites in the tetratricopeptide repeat domain in sOGT, with Thr¹² and Ser⁵⁶ being two "key" sites. Thr¹² is a dominant O-GlcNAcylation site, whereas the modification of Ser⁵⁶ plays a role in regulating sOGT O-GlcNAcylation, partly through Thr¹² In vitro activity and pulldown assays demonstrated that O-GlcNAcylation does not affect sOGT activity but does affect sOGT-interacting proteins. In HEK293T cells, S56A bound to and hence glycosylated more proteins in contrast to T12A and WT sOGT. By proteomic and bioinformatics analyses, we found that T12A and S56A differed in substrate proteins (e.g. HNRNPU and PDCD6IP), which eventually affected cell cycle progression and/or cell proliferation. These findings demonstrate that O-GlcNAcylation modulates sOGT substrate selectivity and affects its role in the cell. The data also highlight the regulatory role of O-GlcNAcylation at Thr¹² and Ser⁵⁶.

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.

Chemical and Biochemical Strategies To Explore the Substrate Recognition of O-GlcNAc-Cycling Enzymes

The O-linked N-acetylglucosamine (O-GlcNAc) modification is an essential component in cell regulation. A single pair of human enzymes conducts this modification dynamically on a broad variety of proteins: O-GlcNAc transferase (OGT) adds the GlcNAc residue and O-GlcNAcase (OGA) hydrolyzes it. This modification is dysregulated in many diseases, but its exact effect on particular substrates remains unclear. In addition, no apparent sequence motif has been found in the modified proteins, and the factors controlling the substrate specificity of OGT and OGA are largely unknown. In this minireview, we will discuss recent developments in chemical and biochemical methods toward addressing the challenge of OGT and OGA substrate recognition. We hope that the new concepts and knowledge from these studies will promote research in this area to advance understanding of O-GlcNAc regulation in health and disease.

Detecting and Imaging O-GlcNAc Sites Using Glycosyltransferases: A Systematic Approach to Study O-GlcNAc

O-GlcNAcylation is a reversible serine/threonine glycosylation for regulating protein activity and availability inside cells. In a given protein, O-GlcNAcylated and unoccupied O-linked β-N-acetylglucosamine (O-GlcNAc) sites are referred to as closed and open sites, respectively. The balance between open and closed sites is believed to be dynamically regulated. In this report, closed sites are detected using in vitro incorporation of GalNAz by B3GALNT2, and open sites are detected by in vitro incorporation of GlcNAz by O-GlcNAc transferase (OGT), via click chemistry. For assessing total O-GlcNAc sites, a sample is O-GlcNAcylated in vitro by OGT before detecting by B3GALNT2. The methods are demonstrated on purified recombinant proteins including CK2, AKT1, and PFKFB3, and cellular extracts of HEK cells. Through O-GlcNAc imaging, the modification degree of O-GlcNAc in nuclei of Chinese hamster ovary cells was estimated. The detection and imaging of both open and closed O-GlcNAc sites provide a systematic approach to study this important post-translational modification.

Too sweet to resist: Control of immune cell function by O-GlcNAcylation

O-linked β-N-acetyl glucosamine modification (O-GlcNAcylation) is a dynamic, reversible posttranslational modification of cytoplasmic and nuclear proteins. O-GlcNAcylation depends on nutrient availability and the hexosamine biosynthetic pathway (HBP), which produces the donor substrate UDP-GlcNAc. O-GlcNAcylation is mediated by a single enzyme, O-GlcNAc transferase (OGT), which adds GlcNAc and another enzyme, O-GlcNAcase (OGA), which removes O-GlcNAc from proteins. O-GlcNAcylation controls vital cellular processes including transcription, translation, the cell cycle, metabolism, and cellular stress. Aberrant O-GlcNAcylation has been implicated in various pathologies including Alzheimer's disease, diabetes, obesity, and cancer. Growing evidences indicate that O-GlcNAcylation plays crucial roles in regulating immunity and inflammatory responses, especially under hyperglycemic conditions. This review will highlight the emerging functions of O-GlcNAcylation in mammalian immunity under physiological and various pathological conditions.

O-GlcNAc transferase missense mutations linked to X-linked intellectual disability deregulate genes involved in cell fate determination and signaling

It is estimated that ∼1% of the world's population has intellectual disability, with males affected more often than females. OGT is an X-linked gene encoding for the enzyme O-GlcNAc transferase (OGT), which carries out the reversible addition of N-acetylglucosamine (GlcNAc) to Ser/Thr residues of its intracellular substrates. Three missense mutations in the tetratricopeptide (TPR) repeats of OGT have recently been reported to cause X-linked intellectual disability (XLID). Here, we report the discovery of two additional novel missense mutations (c.775 G>A, p.A259T, and c.1016 A>G, p.E339G) in the TPR domain of OGT that segregate with XLID in affected families. Characterization of all five of these XLID missense variants of OGT demonstrates modest declines in thermodynamic stability and/or activities of the variants. We engineered each of the mutations into a male human embryonic stem cell line using CRISPR/Cas9. Investigation of the global O-GlcNAc profile as well as OGT and O-GlcNAc hydrolase levels by Western blotting showed no gross changes in steady-state levels in the engineered lines. However, analyses of the differential transcriptomes of the OGT variant-expressing stem cells revealed shared deregulation of genes involved in cell fate determination and liver X receptor/retinoid X receptor signaling, which has been implicated in neuronal development. Thus, here we reveal two additional mutations encoding residues in the TPR regions of OGT that appear causal for XLID and provide evidence that the relatively stable and active TPR variants may share a common, unelucidated mechanism of altering gene expression profiles in human embryonic stem cells.

O-GlcNAc in cancer: An Oncometabolism-fueled vicious cycle

Cancer cells exhibit unregulated growth, altered metabolism, enhanced metastatic potential and altered cell surface glycans. Fueled by oncometabolism and elevated uptake of glucose and glutamine, the hexosamine biosynthetic pathway (HBP) sustains glycosylation in the endomembrane system. In addition, the elevated pools of UDP-GlcNAc drives the O-GlcNAc modification of key targets in the cytoplasm, nucleus and mitochondrion. These targets include transcription factors, kinases, key cytoplasmic enzymes of intermediary metabolism, and electron transport chain complexes. O-GlcNAcylation can thereby alter epigenetics, transcription, signaling, proteostasis, and bioenergetics, key 'hallmarks of cancer'. In this review, we summarize accumulating evidence that many cancer hallmarks are linked to dysregulation of O-GlcNAc cycling on cancer-relevant targets. We argue that onconutrient and oncometabolite-fueled elevation increases HBP flux and triggers O-GlcNAcylation of key regulatory enzymes in glycolysis, Kreb's cycle, pentose-phosphate pathway, and the HBP itself. The resulting rerouting of glucose metabolites leads to elevated O-GlcNAcylation of oncogenes and tumor suppressors further escalating elevation in HBP flux creating a 'vicious cycle'. Downstream, elevated O-GlcNAcylation alters DNA repair and cellular stress pathways which influence oncogenesis. The elevated steady-state levels of O-GlcNAcylated targets found in many cancers may also provide these cells with a selective advantage for sustained growth, enhanced metastatic potential, and immune evasion in the tumor microenvironment.

O-Linked β-N-acetylglucosamine (O-GlcNAc) modification: a new pathway to decode pathogenesis of diabetic retinopathy

The incidence of diabetes continues to rise among all ages and ethnic groups worldwide. Diabetic retinopathy (DR) is a complication of diabetes that affects the retinal neurovasculature causing serious vision problems, including blindness. Its pathogenesis and severity is directly linked to the chronic exposure to high glucose conditions. No treatments are currently available to stop the development and progression of DR. To develop new and effective therapeutic approaches, it is critical to better understand how hyperglycemia contributes to the pathogenesis of DR at the cellular and molecular levels. We propose alterations in O-GlcNAc modification of target proteins during diabetes contribute to the development and progression of DR. The O-GlcNAc modification is regulated through hexosamine biosynthetic pathway. We showed this pathway is differentially activated in various retinal vascular cells under high glucose conditions perhaps due to their selective metabolic activity. O-GlcNAc modification can alter protein stability, activity, interactions, and localization. By targeting the same amino acid residues (serine and threonine) as phosphorylation, O-GlcNAc modification can either compete or cooperate with phosphorylation. Here we will summarize the effects of hyperglycemia-induced O-GlcNAc modification on the retinal neurovasculature in a cell-specific manner, providing new insight into the role of O-GlcNAc modification in early loss of retinal pericytes and the pathogenesis of DR.

Nutrient-driven O-GlcNAc in proteostasis and neurodegeneration

Proteostasis is essential in the mammalian brain where post-mitotic cells must function for decades to maintain synaptic contacts and memory. The brain is dependent on glucose and other metabolites for proper function and is spared from metabolic deficits even during starvation. In this review, we outline how the nutrient-sensitive nucleocytoplasmic post-translational modification O-linked N-acetylglucosamine (O-GlcNAc) regulates protein homeostasis. The O-GlcNAc modification is highly abundant in the mammalian brain and has been linked to proteopathies, including neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. C. elegans, Drosophila, and mouse models harboring O-GlcNAc transferase- and O-GlcNAcase-knockout alleles have helped define the role O-GlcNAc plays in development as well as age-associated neurodegenerative disease. These enzymes add and remove the single monosaccharide from protein serine and threonine residues, respectively. Blocking O-GlcNAc cycling is detrimental to mammalian brain development and interferes with neurogenesis, neural migration, and proteostasis. Findings in C. elegans and Drosophila model systems indicate that the dynamic turnover of O-GlcNAc is critical for maintaining levels of key transcriptional regulators responsible for neurodevelopment cell fate decisions. In addition, pathways of autophagy and proteasomal degradation depend on a transcriptional network that is also reliant on O-GlcNAc cycling. Like the quality control system in the endoplasmic reticulum which uses a 'mannose timer' to monitor protein folding, we propose that cytoplasmic proteostasis relies on an 'O-GlcNAc timer' to help regulate the lifetime and fate of nuclear and cytoplasmic proteins. O-GlcNAc-dependent developmental alterations impact metabolism and growth of the developing mouse embryo and persist into adulthood. Brain-selective knockout mouse models will be an important tool for understanding the role of O-GlcNAc in the physiology of the brain and its susceptibility to neurodegenerative injury.

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.

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.

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

In Vitro Biochemical Assays for O-GlcNAc-Processing Enzymes

O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) are the only enzymes that regulate the dynamics of protein O-GlcNAcylation. Protein O-GlcNAcylation is an important post-translational modification (PTM) of nuclear and cytoplasmic proteins with O-linked β-N-acetyl-glucosamine (O-GlcNAc). O-GlcNAc and its enzymes are involved in a wide variety of cellular processes and are linked to the pathological progression of chronic diseases. Considering their emerging biological significance, systematic and rapid methods to determine the activities of OGT and OGA have become essential, and several chemical/biochemical methods for measuring the activities of these enzymes have been developed. This minireview mainly focuses on the various biochemical assay methods developed to date, while also providing a description of the fundamental principles underlying the monitoring of O-GlcNAc enzyme activities.

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

Potential coordination role between O-GlcNAcylation and epigenetics

Dynamic changes of the post-translational O-GlcNAc modification (O-GlcNAcylation) are controlled by O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) and the glycoside hydrolase O-GlcNAcase (OGA) in cells. O-GlcNAcylation often occurs on serine (Ser) and threonine (Thr) residues of the specific substrate proteins via the addition of O-GlcNAc group by OGT. It has been known that O-GlcNAcylation is not only involved in many fundamental cellular processes, but also plays an important role in cancer development through various mechanisms. Recently, accumulating data reveal that O-GlcNAcylation at histones or non-histone proteins can lead to the start of the subsequent biological processes, suggesting that O-GlcNAcylation as ‘protein code’ or ‘histone code’ may provide recognition platforms or executive instructions for subsequent recruitment of proteins to carry out the specific functions. In this review, we summarize the interaction of O-GlcNAcylation and epigenetic changes, introduce recent research findings that link crosstalk between O-GlcNAcylation and epigenetic changes, and speculate on the potential coordination role of O-GlcNAcylation with epigenetic changes in intracellular biological processes.

Mitochondrial O-GlcNAc Transferase (mOGT) Regulates Mitochondrial Structure, Function, and Survival in HeLa Cells

O-Linked N-acetylglucosamine transferase (OGT) catalyzes O-GlcNAcylation of target proteins and regulates numerous biological processes. OGT is encoded by a single gene that yields nucleocytosolic and mitochondrial isoforms. To date, the role of the mitochondrial isoform of OGT (mOGT) remains largely unknown. Using high throughput proteomics, we identified 84 candidate mitochondrial glycoproteins, of which 44 are novel. Notably, two of the candidate glycoproteins identified (cytochrome oxidase 2 (COX2) and NADH:ubiquinone oxidoreductase core subunit 4 (MT-ND4)) are encoded by mitochondrial DNA. Using siRNA in HeLa cells, we found that reducing endogenous mOGT expression leads to alterations in mitochondrial structure and function, including Drp1-dependent mitochondrial fragmentation, reduction in mitochondrial membrane potential, and a significant loss of mitochondrial content in the absence of mitochondrial ROS. These defects are associated with a compensatory increase in oxidative phosphorylation per mitochondrion. mOGT is also critical for cell survival; siRNA-mediated knockdown of endogenous mOGT protected cells against toxicity mediated by rotenone, a complex I inhibitor. Conversely, reduced expression of both nucleocytoplasmic (ncOGT) and mitochondrial (mOGT) OGT isoforms is associated with increased mitochondrial respiration and elevated glycolysis, suggesting that ncOGT is a negative regulator of cellular bioenergetics. Last, we determined that mOGT is probably involved in the glycosylation of a restricted set of mitochondrial targets. We identified four proteins implicated in mitochondrial biogenesis and metabolism regulation as candidate substrates of mOGT, including leucine-rich PPR-containing protein and mitochondrial aconitate hydratase. Our findings suggest that mOGT is catalytically active in vivo and supports mitochondrial structure, health, and survival, whereas ncOGT predominantly regulates cellular bioenergetics.

Global Analysis of O-GlcNAc Glycoproteins in Activated Human T Cells

T cell activation in response to Ag is largely regulated by protein posttranslational modifications. Although phosphorylation has been extensively characterized in T cells, much less is known about the glycosylation of serine/threonine residues by O-linked N-acetylglucosamine (O-GlcNAc). Given that O-GlcNAc appears to regulate cell signaling pathways and protein activity similarly to phosphorylation, we performed a comprehensive analysis of O-GlcNAc during T cell activation to address the functional importance of this modification and to identify the modified proteins. Activation of T cells through the TCR resulted in a global elevation of O-GlcNAc levels and in the absence of O-GlcNAc, IL-2 production and proliferation were compromised. T cell activation also led to changes in the relative expression of O-GlcNAc transferase (OGT) isoforms and accumulation of OGT at the immunological synapse of murine T cells. Using a glycoproteomics approach, we identified >200 O-GlcNAc proteins in human T cells. Many of the identified proteins had a functional relationship to RNA metabolism, and consistent with a connection between O-GlcNAc and RNA, inhibition of OGT impaired nascent RNA synthesis upon T cell activation. Overall, our studies provide a global analysis of O-GlcNAc dynamics during T cell activation and the first characterization, to our knowledge, of the O-GlcNAc glycoproteome in human T cells.

Dual functionality of O-GlcNAc transferase is required for Drosophila development

Post-translational modification of intracellular proteins with O-linked N-acetylglucosamine (O-GlcNAc) catalysed by O-GlcNAc transferase (OGT) has been linked to regulation of diverse cellular functions. OGT possesses a C-terminal glycosyltransferase catalytic domain and N-terminal tetratricopeptide repeats that are implicated in protein-protein interactions. Drosophila OGT (DmOGT) is encoded by super sex combs (sxc), mutants of which are pupal lethal. However, it is not clear if this phenotype is caused by reduction of O-GlcNAcylation. Here we use a genetic approach to demonstrate that post-pupal Drosophila development can proceed with negligible OGT catalysis, while early embryonic development is OGT activity-dependent. Structural and enzymatic comparison between human OGT (hOGT) and DmOGT informed the rational design of DmOGT point mutants with a range of reduced catalytic activities. Strikingly, a severely hypomorphic OGT mutant complements sxc pupal lethality. However, the hypomorphic OGT mutant-rescued progeny do not produce F2 adults, because a set of Hox genes is de-repressed in F2 embryos, resulting in homeotic phenotypes. Thus, OGT catalytic activity is required up to late pupal stages, while further development proceeds with severely reduced OGT activity.

Comprehensive mapping of O-GlcNAc modification sites using a chemically cleavable tag

The post-translational modification of serine or threonine residues of proteins with a single N-acetylglucosamine monosaccharide (O-GlcNAcylation) is essential for cell survival and function. However, relatively few O-GlcNAc modification sites have been mapped due to the difficulty of enriching and detecting O-GlcNAcylated peptides from complex samples. Here we describe an improved approach to quantitatively label and enrich O-GlcNAcylated proteins for site identification. Chemoenzymatic labelling followed by copper(i)-catalysed azide-alkyne cycloaddition (CuAAC) installs a new mass spectrometry (MS)-compatible linker designed for facile purification of O-GlcNAcylated proteins from cell lysates. The linker also allows subsequent quantitative release of O-GlcNAcylated proteins for downstream MS analysis. We validate the approach by unambiguously identifying several established O-GlcNAc sites on the proteins α-crystallin and O-GlcNAc transferase (OGT), as well as discovering new, previously unreported sites on OGT. Notably, these novel sites on OGT lie in key functional domains of the protein, underscoring how this site identification method may reveal important biological insights into protein activity and regulation.

Proteolysis of HCF-1 by Ser/Thr glycosylation-incompetent O-GlcNAc transferase:UDP-GlcNAc complexes

In this study, Kapuria et al. investigate the dual glycosyltransferase–protease activity (which occurs in the same active site) of OGT. They show that glycosylation and proteolysis occur through separable mechanisms and present a model for the evolution of HCF-1 proteolysis by OGT.

In complex with the cosubstrate UDP-N-acetylglucosamine (UDP-GlcNAc), O-linked-GlcNAc transferase (OGT) catalyzes Ser/Thr O-GlcNAcylation of many cellular proteins and proteolysis of the transcriptional coregulator HCF-1. Such a dual glycosyltransferase–protease activity, which occurs in the same active site, is unprecedented and integrates both reversible and irreversible forms of protein post-translational modification within one enzyme. Although occurring within the same active site, we show here that glycosylation and proteolysis occur through separable mechanisms. OGT consists of tetratricopeptide repeat (TPR) and catalytic domains, which, together with UDP-GlcNAc, are required for both glycosylation and proteolysis. Nevertheless, a specific TPR domain contact with the HCF-1 substrate is critical for proteolysis but not Ser/Thr glycosylation. In contrast, key catalytic domain residues and even a UDP-GlcNAc oxygen important for Ser/Thr glycosylation are irrelevant for proteolysis. Thus, from a dual glycosyltransferase–protease, essentially single-activity enzymes can be engineered both in vitro and in vivo. Curiously, whereas OGT-mediated HCF-1 proteolysis is limited to vertebrate species, invertebrate OGTs can cleave human HCF-1. We present a model for the evolution of HCF-1 proteolysis by OGT.

Nucleocytoplasmic human O-GlcNAc transferase is sufficient for O-GlcNAcylation of mitochondrial proteins

O-linked N-acetylglucosamine modification (O-GlcNAcylation) is a nutrient-dependent protein post-translational modification (PTM), dynamically and reversibly driven by two enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) that catalyse the addition and the removal of the O-GlcNAc moieties to/from serine and threonine residues of target proteins respectively. Increasing evidence suggests involvement of O-GlcNAcylation in many biological processes, including transcription, signalling, neuronal development and mitochondrial function. The presence of a mitochondrial O-GlcNAc proteome and a mitochondrial OGT (mOGT) isoform has been reported. We explored the presence of mOGT in human cell lines and mouse tissues. Surprisingly, analysis of genomic sequences indicates that this isoform cannot be expressed in most of the species analysed, except some primates. In addition, we were not able to detect endogenous mOGT in a range of human cell lines. Knockdown experiments and Western blot analysis of all the predicted OGT isoforms suggested the expression of only a single OGT isoform. In agreement with this, we demonstrate that overexpression of the nucleocytoplasmic OGT (ncOGT) isoform leads to increased O-GlcNAcylation of mitochondrial proteins, suggesting that ncOGT is necessary and sufficient for the generation of the O-GlcNAc mitochondrial proteome.