Insights into O-linked N-acetylglucosamine ([0-9]O-GlcNAc) processing and dynamics through kinetic analysis of O-GlcNAc transferase and O-GlcNAcase activity on protein substrates

Opportunities for Therapeutic Modulation of O-GlcNAc

O-Linked β-N-acetylglucosamine (O-GlcNAc) is an essential, dynamic monosaccharide post-translational modification (PTM) found on serine and threonine residues of thousands of nucleocytoplasmic proteins. The installation and removal of O-GlcNAc is controlled by a single pair of enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery four decades ago, O-GlcNAc has been found on diverse classes of proteins, playing important functional roles in many cellular processes. Dysregulation of O-GlcNAc homeostasis has been implicated in the pathogenesis of disease, including neurodegeneration, X-linked intellectual disability (XLID), cancer, diabetes, and immunological disorders. These foundational studies of O-GlcNAc in disease biology have motivated efforts to target O-GlcNAc therapeutically, with multiple clinical candidates under evaluation. In this review, we describe the characterization and biochemistry of OGT and OGA, cellular O-GlcNAc regulation, development of OGT and OGA inhibitors, O-GlcNAc in pathophysiology, clinical progress of O-GlcNAc modulators, and emerging opportunities for targeting O-GlcNAc. This comprehensive resource should motivate further study into O-GlcNAc function and inspire strategies for therapeutic modulation of O-GlcNAc.

Structure and Inhibition of Insect UDP-N-acetylglucosamine Pyrophosphorylase: A Key Enzyme in the Hexosamine Biosynthesis Pathway

UDP-N-acetylglucosamine pyrophosphorylase (UAP) catalyzes the last step in the hexosamine biosynthesis pathway to directly produce UDP-N-acetylglucosamine (UDP-GlcNAc). Because UAPs play important physiological and pathological roles in organisms, they are considered potential targets for drug and pesticide development. However, the lack of efficient and selective inhibitors is a bottleneck that must be overcome. This study reports the first crystal structure of the insect UAP from Spodoptera frugiperda (SfUAP) in complex with UDP-GlcNAc. SfUAP has two insect-specific structural characteristics in the active pocket, namely, a free Cys (Cys334) and a Mg2+ binding site, which differentiate it from human UAP (HsAGX1) and fungal UAP (AfUAP) in terms of substrate and inhibitor binding. N-(4-Nitrophenyl)maleimide (pNPMI) and myricetin are discovered as potent covalent and noncovalent inhibitors of SfUAP, respectively. Moreover, myricetin can significantly reduce the level of cellular O-GlcNAcylation by inhibiting both UAP and O-GlcNAc transferase. These findings provide novel insights into the development of UAP-based drugs and pesticides.

OGT and OGA: Sweet guardians of the genome

The past 4 decades have witnessed tremendous efforts in deciphering the role of O-GlcNAcylation in a plethora of biological processes. Chemists and biologists have joined hand in hand in the sweet adventure to unravel this unique and universal yet uncharted post-translational modification, and the recent advent of cutting-edge chemical biology and mass spectrometry tools has greatly facilitated the process. Compared with O-GlcNAc, DNA damage response (DDR) is a relatively intensively studied area that could be traced to before the elucidation of the structure of DNA. Unexpectedly, yet somewhat expectedly, O-GlcNAc has been found to regulate various DDR pathways: homologous recombination, nonhomologous end joining, base excision repair, and translesion DNA synthesis. In this review, we first cover the recent structural studies of the O-GlcNAc transferase and O-GlcNAcase, the elegant duo that "writes" and "erases" O-GlcNAc modification. Then we delineate the intricate roles of O-GlcNAc transferase and O-GlcNAcase in DDR. We envision that this is only the beginning of our full appreciation of how O-GlcNAc regulates the blueprint of life-DNA.

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.

Exploiting O-GlcNAc transferase promiscuity to dissect site-specific O-GlcNAcylation

Protein O-GlcNAcylation is an evolutionary conserved post-translational modification catalysed by the nucleocytoplasmic O-GlcNAc transferase (OGT) and reversed by O-GlcNAcase (OGA). How site-specific O-GlcNAcylation modulates a diverse range of cellular processes is largely unknown. A limiting factor in studying this is the lack of accessible techniques capable of producing homogeneously O-GlcNAcylated proteins, in high yield, for in vitro studies. Here, we exploit the tolerance of OGT for cysteine instead of serine, combined with a co-expressed OGA to achieve site-specific, highly homogeneous mono-glycosylation. Applying this to DDX3X, TAB1, and CK2α, we demonstrate that near-homogeneous mono-S-GlcNAcylation of these proteins promotes DDX3X and CK2α solubility and enables production of mono-S-GlcNAcylated TAB1 crystals, albeit with limited diffraction. Taken together, this work provides a new approach for functional dissection of protein O-GlcNAcylation.

Understanding and exploiting the roles of O-GlcNAc in neurodegenerative diseases

O-GlcNAc is a common modification found on nuclear and cytoplasmic proteins. Determining the catalytic mechanism of the enzyme O-GlcNAcase (OGA), which removes O-GlcNAc from proteins, enabled the creation of potent and selective inhibitors of this regulatory enzyme. Such inhibitors have served as important tools in helping to uncover the cellular and organismal physiological roles of this modification. In addition, OGA inhibitors have been important for defining the augmentation of O-GlcNAc as a promising disease-modifying approach to combat several neurodegenerative diseases including both Alzheimer's disease and Parkinson's disease. These studies have led to development and optimization of OGA inhibitors for clinical application. These compounds have been shown to be well tolerated in early clinical studies and are steadily advancing into the clinic. Despite these advances, the mechanisms by which O-GlcNAc protects against these various types of neurodegeneration are a topic of continuing interest since improved insight may enable the creation of more targeted strategies to modulate O-GlcNAc for therapeutic benefit. Relevant pathways on which O-GlcNAc has been found to exert beneficial effects include autophagy, necroptosis, and processing of the amyloid precursor protein. More recently, the development and application of chemical methods enabling the synthesis of homogenous proteins have clarified the biochemical effects of O-GlcNAc on protein aggregation and uncovered new roles for O-GlcNAc in heat shock response. Here, we discuss the features of O-GlcNAc in neurodegenerative diseases, the application of inhibitors to identify the roles of this modification, and the biochemical effects of O-GlcNAc on proteins and pathways associated with neurodegeneration.

O-GlcNAcylation: cellular physiology and therapeutic target for human diseases

O-linked-β-N-acetylglucosamine (O-GlcNAcylation) is a distinctive posttranslational protein modification involving the coordinated action of O-GlcNAc transferase and O-GlcNAcase, primarily targeting serine or threonine residues in various proteins. This modification impacts protein functionality, influencing stability, protein-protein interactions, and localization. Its interaction with other modifications such as phosphorylation and ubiquitination is becoming increasingly evident. Dysregulation of O-GlcNAcylation is associated with numerous human diseases, including diabetes, nervous system degeneration, and cancers. This review extensively explores the regulatory mechanisms of O-GlcNAcylation, its effects on cellular physiology, and its role in the pathogenesis of diseases. It examines the implications of aberrant O-GlcNAcylation in diabetes and tumorigenesis, highlighting novel insights into its potential role in cardiovascular diseases. The review also discusses the interplay of O-GlcNAcylation with other protein modifications and its impact on cell growth and metabolism. By synthesizing current research, this review elucidates the multifaceted roles of O-GlcNAcylation, providing a comprehensive reference for future studies. It underscores the potential of targeting the O-GlcNAcylation cycle in developing novel therapeutic strategies for various pathologies.

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.

OGT Binding Peptide-Tagged Strategy Increases Protein O-GlcNAcylation Level in E. coli

O-GlcNAcylation is a single glycosylation of GlcNAc mediated by OGT, which regulates the function of substrate proteins and is closely related to many diseases. However, a large number of O-GlcNAc-modified target proteins are costly, inefficient, and complicated to prepare. In this study, an OGT binding peptide (OBP)-tagged strategy for improving the proportion of O-GlcNAc modification was established successfully in E. coli. OBP (P1, P2, or P3) was fused with target protein Tau as tagged Tau. Tau or tagged Tau was co-constructed with OGT into a vector expressed in E. coli. Compared with Tau, the O-GlcNAc level of P1Tau and TauP1 increased 4~6-fold. Moreover, the P1Tau and TauP1 increased the O-GlcNAc-modified homogeneity. The high O-GlcNAcylation on P1Tau resulted in a significantly slower aggregation rate than Tau in vitro. This strategy was also used successfully to increase the O-GlcNAc level of c-Myc and H2B. These results indicated that the OBP-tagged strategy was a successful approach to improve the O-GlcNAcylation of a target protein for further functional research.

Investigation of in vitro histone H3 glycosylation using H3 tail peptides

Posttranslational modifications (PTMs) on histone tails regulate eukaryotic gene expression by impacting the chromatin structure and by modulating interactions with other cellular proteins. One such PTM has been identified as serine and threonine glycosylation, the introduction of the ß-N-acetylglucosamine (GlcNAc) moiety on histone H3 tail at position Ser10 and Thr32. The addition of the ß-O-GlcNAc moiety on serine or threonine residues is facilitated by the O-GlcNAc transferase (OGT), and can be removed by the action of O-GlcNAcase (OGA). Conflicting reports on histone tail GlcNAc modification in vivo prompted us to investigate whether synthetic histone H3 tail peptides in conjunction with other PTMs are substrates for OGT and OGA in vitro. Our enzymatic assays with recombinantly expressed human OGT revealed that the unmodified and PTM-modified histone H3 tails are not substrates for OGT at both sites, Ser10 and Thr32. In addition, full length histone H3 was not a substrate for OGT. Conversely, our work demonstrates that synthetic peptides containing the GlcNAc functionality at Ser10 are substrates for recombinantly expressed human OGA, yielding deglycosylated histone H3 peptides. We also show that the catalytic domains of human histone lysine methyltransferases G9a, GLP and SETD7 and histone lysine acetyltransferases PCAF and GCN5 do somewhat tolerate glycosylated H3Ser10 close to lysine residues that undergo methylation and acetylation reactions, respectively. Overall, this work indicates that GlcNAcylation of histone H3 tail peptide in the presence of OGT does not occur in vitro.

Diaminocyclopentane-derived O-GlcNAcase inhibitors for combating tau hyperphosphorylation in Alzheimer's disease

We developed potent and selective aminocyclopentane-derived inhibitors of human O-N-acetyl-β-D-glucosaminidase (OGA) implicated in Alzheimer's disease. For example compound 13 was a nanomolar OGA inhibitor with 92 000-fold selectivity over human HexB. It was non-toxic and increased protein O-GlcNAcylation in the culture of murine neural cells, showing new alternatives in the treatment of tauopathies.

Structure-based design of UDP-GlcNAc analogs as candidate GnT-V inhibitors

BACKGROUND

N-Glycan branching regulates various functions of glycoproteins. N-Acetylglucosaminyltransferase V (GnT-V) is a GlcNAc transferase that acts on N-glycans and the GnT-V-producing branch is highly related to cancer progression. This indicates that specific GnT-V inhibitors may be drug candidates for cancer treatment. To design novel GnT-V inhibitors, we focused on the unique and weak recognition of the donor substrate UDP-GlcNAc by GnT-V. On the basis of the catalytic pocket structure, we hypothesized that UDP-GlcNAc analogs with increasing hydrophobicity may be GnT-V inhibitors.

METHODS

We chemically synthesized 10 UDP-GlcNAc analogs in which one or two phosphate groups were replaced with hydrophobic groups. To test these compounds, we set up an HPLC-based enzyme assay system for all N-glycan-branching GlcNAc transferases in which GnT-I-V activity was measured using purified truncated enzymes. Using this system, we assessed the inhibitory effects of the synthesized compounds on GnT-V and their specificity.

RESULTS

Several UDP-GlcNAc analogs inhibited GnT-V activity, although the inhibition potency was modest. Compared with other GnTs, these compounds showed a preference for GnT-V, which suggested that GnT-V was relatively tolerant of hydrophobicity in the donor substrate. Docking models of the inhibitory compounds with GnT-V suggested the mechanisms of how these compounds interacted with GnT-V and inhibited its action.

CONCLUSIONS

Chemical modification of the donor substrate may be a promising strategy to develop selective inhibitors of GnT-V.

GENERAL SIGNIFICANCE

Our findings provide new insights into the design of GnT inhibitors and how GnTs recognize the donor substrate.

Thermal Proteome Profiling Reveals the O-GlcNAc-Dependent Meltome

Posttranslational modifications alter the biophysical properties of proteins and thereby influence cellular physiology. One emerging manner by which such modifications regulate protein functions is through their ability to perturb protein stability. Despite the increasing interest in this phenomenon, there are few methods that enable global interrogation of the biophysical effects of posttranslational modifications on the proteome. Here, we describe an unbiased proteome-wide approach to explore the influence of protein modifications on the thermodynamic stability of thousands of proteins in parallel. We apply this profiling strategy to study the effects of O-linked N-acetylglucosamine (O-GlcNAc), an abundant modification found on hundreds of proteins in mammals that has been shown in select cases to stabilize proteins. Using this thermal proteomic profiling strategy, we identify a set of 72 proteins displaying O-GlcNAc-dependent thermostability and validate this approach using orthogonal methods targeting specific proteins. These collective observations reveal that the majority of proteins influenced by O-GlcNAc are, surprisingly, destabilized by O-GlcNAc and cluster into distinct macromolecular complexes. These results establish O-GlcNAc as a bidirectional regulator of protein stability and provide a blueprint for exploring the impact of any protein modification on the meltome of, in principle, any organism.

Tools, tactics and objectives to interrogate cellular roles of O-GlcNAc in disease

The vast array of cell types of multicellular organisms must individually fine-tune their internal metabolism. One important metabolic and stress regulatory mechanism is the dynamic attachment/removal of glucose-derived sugar N-acetylglucosamine on proteins (O-GlcNAcylation). The number of proteins modified by O-GlcNAc is bewildering, with at least 7,000 sites in human cells. The outstanding challenge is determining how key O-GlcNAc sites regulate a target pathway amidst thousands of potential global sites. Innovative solutions are required to address this challenge in cell models and disease therapy. This Perspective shares critical suggestions for the O-GlcNAc field gleaned from the international O-GlcNAc community. Further, we summarize critical tools and tactics to enable newcomers to O-GlcNAc biology to drive innovation at the interface of metabolism and disease. The growing pace of O-GlcNAc research makes this a timely juncture to involve a wide array of scientists and new toolmakers to selectively approach the regulatory roles of O-GlcNAc in disease.

Altered protein O-GlcNAcylation in placentas from mothers with diabetes causes aberrant endocytosis in placental trophoblast cells

Women with pre-existing diabetes have an increased risk of poor pregnancy outcomes, including disordered fetal growth, caused by changes to placental function. Here we investigate the possibility that the hexosamine biosynthetic pathway, which utilises cellular nutrients to regulate protein function via post-translationally modification with O-linked N-acetylglucosamine (GlcNAc), mediates the placental response to the maternal metabolic milieu. Mass spectrometry analysis revealed that the placental O-GlcNAcome is altered in women with type 1 (n = 6) or type 2 (n = 6) diabetes T2D (≥ twofold change in abundance in 162 and 165 GlcNAcylated proteins respectively compared to BMI-matched controls n = 11). Ingenuity pathway analysis indicated changes to clathrin-mediated endocytosis (CME) and CME-associated proteins, clathrin, Transferrin (TF), TF receptor and multiple Rabs, were identified as O-GlcNAcylation targets. Stimulating protein O-GlcNAcylation using glucosamine (2.5 mM) increased the rate of TF endocytosis by human placental cells (p = 0.02) and explants (p = 0.04). Differential GlcNAcylation of CME proteins suggests altered transfer of cargo by placentas of women with pre-gestational diabetes, which may contribute to alterations in fetal growth. The human placental O-GlcNAcome provides a resource to aid further investigation of molecular mechanisms governing placental nutrient sensing.

Regulating the Regulators: Mechanisms of Substrate Selection of the O-GlcNAc Cycling Enzymes OGT and OGA

Thousands of nuclear and cytosolic proteins are modified with a single β-N-acetylglucosamine on serine and threonine residues in mammals, a modification termed O-GlcNAc. This modification is essential for normal development and plays important roles in virtually all intracellular processes. Additionally, O-GlcNAc is involved in many disease states, including cancer, diabetes, and X-linked intellectual disability. Given the myriad of functions of the O-GlcNAc modification, it is therefore somewhat surprising that O-GlcNAc cycling is mediated by only two enzymes: the O-GlcNAc transferase (OGT), which adds O-GlcNAc, and the O-GlcNAcase (OGA), which removes it. A significant outstanding question in the O-GlcNAc field is how do only two enzymes mediate such an abundant and dynamic modification. In this review, we explore the current understanding of mechanisms for substrate selection for the O-GlcNAc cycling enzymes. These mechanisms include direct substrate interaction with specific domains of OGT or OGA, selection of interactors via partner proteins, posttranslational modification of OGT or OGA, nutrient sensing, and localization alteration. Altogether, current research paints a picture of an exquisitely regulated and complex system by which OGT and OGA select substrates. We also make recommendations for future work, toward the goal of identifying interaction mechanisms for specific substrates that may be able to be exploited for various research and medical treatment goals.

Mechanistic roles for altered O-GlcNAcylation in neurodegenerative disorders

Neurodegenerative diseases such as Alzheimer's and Parkinson's remain highly prevalent and incurable disorders. A major challenge in fully understanding and combating the progression of these diseases is the complexity of the network of processes that lead to progressive neuronal dysfunction and death. An ideal therapeutic avenue is conceivably one that could address many if not all of these multiple misregulated mechanisms. Over the years, chemical intervention for the up-regulation of the endogenous posttranslational modification (PTM) O-GlcNAc has been proposed as a potential strategy to slow down the progression of neurodegeneration. Through the development and application of tools that allow dissection of the mechanistic roles of this PTM, there is now a growing body of evidence that O-GlcNAc influences a variety of important neurodegeneration-pertinent mechanisms, with an overall protective effect. As a PTM that is appended onto numerous proteins that participate in protein quality control and homeostasis, metabolism, bioenergetics, neuronal communication, inflammation, and programmed death, O-GlcNAc has demonstrated beneficence in animal models of neurodegenerative diseases, and its up-regulation is now being pursued in multiple clinical studies.

O-GlcNAcylated peptides and proteins for structural and functional studies

O-GlcNAcylation is an enzymatic post-translational modification occurring in hundreds of protein substrates. This modification occurs through the addition of the monosaccharide N-acetylglucosamine to serine and threonine residues on intracellular proteins in the cytosol, nucleus, and mitochondria. As a highly dynamic form of modification, changes in O-GlcNAc levels coincide with alterations in metabolic state, the presence of stressors, and cellular health. At the protein level, the consequences of the sugar modification can vary, thus necessitating biochemical investigations on protein-specific and site-specific effects. To this end, enzymatic and chemical methods to 'encode' the modification have been developed and the utilization of these synthetic glycopeptides and glycoproteins has since been instrumental in the discovery of the mechanisms by which O-GlcNAcylation can affect a diverse array of biological processes.

Posttranscriptional regulation of de novo lipogenesis by glucose-induced O-GlcNAcylation

O-linked β-N-acetyl glucosamine (O-GlcNAc) is attached to proteins under glucose-replete conditions; this posttranslational modification results in molecular and physiological changes that affect cell fate. Here we show that posttranslational modification of serine/arginine-rich protein kinase 2 (SRPK2) by O-GlcNAc regulates de novo lipogenesis by regulating pre-mRNA splicing. We found that O-GlcNAc transferase O-GlcNAcylated SRPK2 at a nuclear localization signal (NLS), which triggers binding of SRPK2 to importin α. Consequently, O-GlcNAcylated SRPK2 was imported into the nucleus, where it phosphorylated serine/arginine-rich proteins and promoted splicing of lipogenic pre-mRNAs. We determined that protein nuclear import by O-GlcNAcylation-dependent binding of cargo protein to importin α might be a general mechanism in cells. This work reveals a role of O-GlcNAc in posttranscriptional regulation of de novo lipogenesis, and our findings indicate that importin α is a "reader" of an O-GlcNAcylated NLS.

Protein Substrates Engage the Lumen of O-GlcNAc Transferase's Tetratricopeptide Repeat Domain in Different Ways

Glycosylation of nuclear and cytoplasmic proteins is an essential post-translational modification in mammals. O-GlcNAc transferase (OGT), the sole enzyme responsible for this modification, glycosylates more than 1000 unique nuclear and cytoplasmic substrates. How OGT selects its substrates is a fundamental question that must be answered to understand OGT's unusual biology. OGT contains a long tetratricopeptide repeat (TPR) domain that has been implicated in substrate selection, but there is almost no information about how changes to this domain affect glycosylation of individual substrates. By profiling O-GlcNAc in cell extracts and probing glycosylation of purified substrates, we show here that ladders of asparagines and aspartates that extend the full length of OGT's TPR lumen control substrate glycosylation. Different substrates are sensitive to changes in different regions of OGT's TPR lumen. We also found that substrates with glycosylation sites close to the C-terminus bypass lumenal binding. Our findings demonstrate that substrates can engage OGT in a variety of different ways for glycosylation.

Chemical Synthesis and Biological Applications of O-GlcNAcylated Peptides and Proteins

All human cells use O-GlcNAc protein modifications (O-linked N-acetylglucosamine) to rapidly adapt to changing nutrient and stress conditions through signaling, epigenetic, and proteostasis mechanisms. A key challenge for biologists in defining precise roles for specific O-GlcNAc sites is synthetic access to homogenous isoforms of O-GlcNAc proteins, a result of the non-genetically templated, transient, and heterogeneous nature of O-GlcNAc modifications. Toward a solution, this review details the state of the art of two strategies for O-GlcNAc protein modification: advances in "bottom-up" O-GlcNAc peptide synthesis and direct "top-down" installation of O-GlcNAc on full proteins. We also describe key applications of synthetic O-GlcNAc peptide and protein tools as therapeutics, biophysical structure-function studies, biomarkers, and as disease mechanistic probes to advance translational O-GlcNAc biology.

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.

Novel bisubstrate uridine-peptide analogues bearing a pyrophosphate bioisostere as inhibitors of human O-GlcNAc transferase

Protein O-linked β-D-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation), an essential post-translational as well as cotranslational modification, is the attachment of β-D-N-acetylglucosamine to serine and threonine residues of nucleocytoplasmic proteins. An aberrant O-GlcNAc profile on certain proteins has been implicated in metabolic diseases such as diabetes and cancer. Inhibitors of O-GlcNAc transferase (OGT) are valuable tools to study the cell biology of protein O-GlcNAc modification. In this study we report novel uridine-peptide conjugate molecules composed of an acceptor peptide covalently linked to a catalytically inactive donor substrate analogue that bears a pyrophosphate bioisostere and explore their inhibitory activities against OGT by a radioactive hOGT assay. Further, we investigate the structural basis of their activities via molecular modelling, explaining their lack of potency towards OGT inhibition.

MYPT1 O-GlcNAc modification regulates sphingosine-1-phosphate mediated contraction

Many intracellular proteins are modified by N-acetylglucosamine, a posttranslational modification termed O-GlcNAc. This modification is found on serine and threonine side-chains and has the potential to regulate signaling pathways through interplay with phosphorylation. Here, we discover and characterize one such example. We find that O-GlcNAc levels control the sensitivity of fibroblasts to actin contraction induced by the signaling lipid sphingosine-1-phosphate (S1P), culminating in the phosphorylation of myosin light chain (MLC) and cellular contraction. Specifically, O-GlcNAc modification of the phosphatase subunit MYPT1 inhibits this pathway by blocking MYPT1 phosphorylation, maintaining its activity and causing the dephosphorylation of MLC. Finally, we demonstrate that O-GlcNAc levels alter the sensitivity of primary human dermal fibroblasts in a collagen-matrix model of wound healing. Our findings have important implications for the role of O-GlcNAc in fibroblast motility and differentiation, particularly in diabetic wound healing.

Biological evaluation and molecular modeling of peptidomimetic compounds as inhibitors for O-GlcNAc transferase (OGT)

The vital enzyme O-linked β-N-acetylglucosamine transferase (OGT) catalyzes the O-GlcNAcylation of intracellular proteins coupling the metabolic status to cellular signaling and transcription pathways. Aberrant levels of O-GlcNAc and OGT have been linked to metabolic diseases as cancer and diabetes. Here, a new series of peptidomimetic OGT inhibitors was identified highlighting the compound LQMed 330, which presented better IC₅₀ compared to the most potent inhibitors found in the literature. Molecular modeling study of selected inhibitors into the OGT binding site provided insight into the behavior by which these compounds interact with the enzyme. The results obtained in this study provided new perspectives on the design and synthesis of highly specific OGT inhibitors.

Tools for functional dissection of site-specific O-GlcNAcylation

Protein O-GlcNAcylation is an abundant post-translational modification of intracellular proteins with the monosaccharide N-acetylglucosamine covalently tethered to serines and threonines. Modification of proteins with O-GlcNAc is required for metazoan embryo development and maintains cellular homeostasis through effects on transcription, signalling and stress response. While disruption of O-GlcNAc homeostasis can have detrimental impact on cell physiology and cause various diseases, little is known about the functions of individual O-GlcNAc sites. Most of the sites are modified sub-stoichiometrically which is a major challenge to the dissection of O-GlcNAc function. Here, we discuss the application, advantages and limitations of the currently available tools and technologies utilised to dissect the function of O-GlcNAc on individual proteins and sites in vitro and in vivo. Additionally, we provide a perspective on future developments required to decipher the protein- and site-specific roles of this essential sugar modification.

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.

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

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

Increased O-GlcNAcylation of c-Myc Promotes Pre-B Cell Proliferation

O-linked β-N-acetylglucosamine (O-GlcNAc) modification regulates the activity of hundreds of nucleocytoplasmic proteins involved in a wide variety of cellular processes, such as gene expression, signaling, and cell growth; however, the mechanism underlying the regulation of B cell development and function by O-GlcNAcylation remains largely unknown. Here, we demonstrate that changes in cellular O-GlcNAc levels significantly affected the growth of pre-B cells, which rapidly proliferate to allow expansion of functional clones that express successfully rearranged heavy chains at the pro-B stage during early B cell development. In our study, the overall O-GlcNAc levels in these proliferative pre-B cells, which are linked to the glucose uptake rate, were highly induced when compared with those in pro-B cells. Thus, pharmacologically, genetically, or nutritionally, inhibition of O-GlcNAcylation in pre-B cells markedly downregulated c-Myc expression, resulting in cell cycle arrest via blockade of cyclin expression. Importantly, the population of B cells after the pro-B cell stage in mouse bone marrow was severely impaired by the administration of an O-GlcNAc inhibitor. These results strongly suggest that O-GlcNAcylation-dependent expression of c-Myc represents a new regulatory component of pre-B cell proliferation, as well as a potential therapeutic target for the treatment of pre-B cell-derived leukemia.

Human UDP-galactose 4'-epimerase (GALE) is required for cell-surface glycome structure and function

Glycan biosynthesis relies on nucleotide sugars (NSs), abundant metabolites that serve as monosaccharide donors for glycosyltransferases. In vivo, signal-dependent fluctuations in NS levels are required to maintain normal cell physiology and are dysregulated in disease. However, how mammalian cells regulate NS levels and pathway flux remains largely uncharacterized. To address this knowledge gap, here we examined UDP-galactose 4'-epimerase (GALE), which interconverts two pairs of essential NSs. Using immunoblotting, flow cytometry, and LC-MS-based glycolipid and glycan profiling, we found that CRISPR/Cas9-mediated GALE deletion in human cells triggers major imbalances in NSs and dramatic changes in glycolipids and glycoproteins, including a subset of integrins and the cell-surface death receptor FS-7-associated surface antigen. In particular, we observed substantial decreases in total sialic acid, galactose, and GalNAc levels in glycans. These changes also directly impacted cell signaling, as GALE ^-/- cells exhibited FS-7-associated surface antigen ligand-induced apoptosis. Our results reveal a role of GALE-mediated NS regulation in death receptor signaling and may have implications for the molecular etiology of illnesses characterized by NS imbalances, including galactosemia and metabolic syndrome.

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.

Molecular mechanisms regulating O-linked N-acetylglucosamine (O-GlcNAc)-processing enzymes

The post-translational modification of proteins by O-linked N-acetylglucosamine (O-GlcNAc) dynamically programmes cellular physiology to maintain homoeostasis and tailor biochemical pathways to meet context-dependent cellular needs. Despite diverse roles of played by O-GlcNAc, only two enzymes act antagonistically to govern its cycling; O-GlcNAc transferase installs the monosaccharide on target proteins, and O-GlcNAc hydrolase removes it. The recent literature has exposed a network of mechanisms regulating these two enzymes to choreograph global, and target-specific, O-GlcNAc cycling in response to cellular stress and nutrient availability. Herein, we amalgamate these emerging mechanisms from a structural and molecular perspective to explore how the cell exerts fine control to regulate O-GlcNAcylation of diverse proteins in a selective fashion.

HCF-1 Regulates De Novo Lipogenesis through a Nutrient-Sensitive Complex with ChREBP

Carbohydrate response element binding protein (ChREBP) is a key transcriptional regulator of de novo lipogenesis (DNL) in response to carbohydrates and in hepatic steatosis. Mechanisms underlying nutrient modulation of ChREBP are under active investigation. Here we identify host cell factor 1 (HCF-1) as a previously unknown ChREBP-interacting protein that is enriched in liver biopsies of nonalcoholic steatohepatitis (NASH) patients. Biochemical and genetic studies show that HCF-1 is O-GlcNAcylated in response to glucose as a prerequisite for its binding to ChREBP and subsequent recruitment of OGT, ChREBP O-GlcNAcylation, and activation. The HCF-1:ChREBP complex resides at lipogenic gene promoters, where HCF-1 regulates H3K4 trimethylation to prime recruitment of the Jumonji C domain-containing histone demethylase PHF2 for epigenetic activation of these promoters. Overall, these findings define HCF-1's interaction with ChREBP as a previously unappreciated mechanism whereby glucose signals are both relayed to ChREBP and transmitted for epigenetic regulation of lipogenic genes.

Demystifying O-GlcNAcylation: hints from peptide substrates

O-GlcNAcylation, analogous to phosphorylation, is an essential post-translational modification of proteins at Ser/Thr residues with a single β-N-acetylglucosamine moiety. This dynamic protein modification regulates many fundamental cellular processes and its deregulation has been linked to chronic diseases such as cancer, diabetes and neurodegenerative disorders. Reversible attachment and removal of O-GlcNAc is governed only by O-GlcNAc transferase and O-GlcNAcase, respectively. Peptide substrates, derived from natural O-GlcNAcylation targets, function in the catalytic cores of these two enzymes by maintaining interactions between enzyme and substrate, which makes them ideal models for the study of O-GlcNAcylation and deglycosylation. These peptides provide valuable tools for a deeper understanding of O-GlcNAc processing enzymes. By taking advantage of peptide chemistry, recent progress in the study of activity and regulatory mechanisms of these two enzymes has advanced our understanding of their fundamental specificities as well as their potential as therapeutic targets. Hence, this review summarizes the recent achievements on this modification studied at the peptide level, focusing on enzyme activity, enzyme specificity, direct function, site-specific antibodies and peptide substrate-inspired inhibitors.

A widely compatible expression system for the production of highly O-GlcNAcylated recombinant protein in Escherichia coli

O-GlcNAcylation is a ubiquitous and dynamic post-translational modification on serine/threonine residues of nucleocytoplasmic proteins in metazoa, which plays a critical role in numerous physiological and pathological processes. But the O-GlcNAcylation on most proteins is often substoichiometric, which hinders the functional study of the O-GlcNAcylation. This study aimed to improve the production of highly O-GlcNAcylated recombinant proteins in Escherichia coli (E. coli). To achieve this goal, we constructed a bacterial artificial chromosome-based chloramphenicol-resistant expression vector co-expressing O-GlcNAc transferase (OGT) and key enzymes (phosphoglucose mutase, GlmM and N-acetylglucosamine-1-phosphate uridyltransferase, GlmU) of the uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) synthesis pathway in E. coli, which can effectively increase the O-GlcNAcylation of the OGT target protein expressed by another vector. The results revealed that the expression of GlmM and GlmU increases the cellular concentration of UDP-GlcNAc in E. coli, which markedly enhanced the activity of the co-expressed OGT to its target proteins, such as H2B, p53 and TAB1. Altogether, we established a widely compatible E. coli expression system for producing highly O-GlcNAcylated protein, which could be used for modifying OGT target proteins expressed by almost any commercial expression vectors in E. coli. This new expression system provides possibility for investigating the roles of O-GlcNAcylation in the enzymatic activity, protein-protein interaction and structure of OGT target proteins.

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.

Analysis of UDP-Sugars from Cultured Cells and Small Tissue Samples

UDP-sugars are important substrates for the synthesis of various cellular glycans and glycoconjugates, many of which play essential roles in the pathobiology of diseases associated with deranged glucose metabolism, such as cancer and type 2 diabetes. Hence, their analysis from cultured cells and especially from tissue samples can give valuable information. This chapter describes a method for UDP-sugar isolation from various sources utilizing ion-pair solid-phase extraction with graphitized carbon cartridges, and their analysis using anion-exchange high-performance liquid chromatography.

Structural studies of a surface-entropy reduction mutant of O-GlcNAcase

The enzyme O-GlcNAcase catalyses the removal of the O-GlcNAc co/post-translational modification in multicellular eukaryotes. The enzyme has become of acute interest given the intimate role of O-GlcNAcylation in tau modification and stability; small-molecular inhibitors of human O-GlcNAcase are under clinical assessment for the treatment of tauopathies. Given the importance of structure-based and mechanism-based inhibitor design for O-GlcNAcase, it was sought to test whether different crystal forms of the human enzyme could be achieved by surface mutagenesis. Guided by surface-entropy reduction, a Glu602Ala/Glu605Ala variant [on the Gly11-Gln396/Lys535-Tyr715 construct; Roth et al. (2017), Nature Chem. Biol. 13, 610-612] was obtained which led to a new crystal form of the human enzyme. An increase in crystal contacts stabilized disordered regions of the protein, enabling 88% of the structure to be modelled; only 83% was possible for the wild-type construct. Although the binding of the C-terminus was consistent with the wild type, Lys713 in monomer A was bound in the -1 subsite of the symmetry-related monomer A and the active sites of the B monomers were vacant. The new crystal form presents an opportunity for enhanced soaking experiments that are essential to understanding the binding mechanism and substrate specificity of O-GlcNAcase.

Wheat Germ Agglutinin (WGA)-SDS-PAGE: A Novel Method for the Detection of O-GlcNAc-modified Proteins by Lectin Affinity Gel Electrophoresis

Diverse cytoplasmic and nuclear proteins dynamically change their molecular functions by O-linked β-N-acetylglucosamine (O-GlcNAc) modification on serine and/or threonine residues. Evaluation of the O-GlcNAcylation level of a specific protein, however, needs multiple and time-consuming steps if using conventional methods (e.g., immune-purification, mass spectrometric analysis). To overcome this drawback, we developed the following easy and rapid method for detection of O-GlcNAcylated proteins of interest. An O-GlcNAc affinity gel layer containing wheat germ agglutinin (WGA), a GlcNAc-specific lectin, selectively induces retardation of the mobility of O-GlcNAcylated proteins during electrophoresis. This WGA-layer thereby separates O-GlcNAcylated and non-modified forms of proteins, allowing the detection and quantification of the O-GlcNAcylation level of these proteins. This new method therefore provides qualitative and quantitative analysis of O-GlcNAcylated proteins in a relatively shorter time compared to conventional methods.

Direct One-Step Fluorescent Labeling of O-GlcNAc-Modified Proteins in Live Cells Using Metabolic Intermediates

The modification of proteins with O-linked N-acetylglucosamine ( O-GlcNAc) by the enzyme O-GlcNAc transferase (OGT) has emerged as an important regulator of cellular physiology. Metabolic labeling strategies to monitor O-GlcNAcylation in cells have proven of great value for uncovering the molecular roles of O-GlcNAc. These strategies rely on two-step labeling procedures, which limits the scope of experiments that can be performed. Here, we report on the creation of fluorescent uridine 5'-diphospho- N-acetylglucosamine (UDP-GlcNAc) analogues in which the N-acyl group of glucosamine is modified with a suitable linker and fluorophore. Using human OGT, we show these donor sugar substrates permit direct monitoring of OGT activity on protein substrates in vitro. We show that feeding cells with a corresponding fluorescent metabolic precursor for the last step of the hexosamine biosynthetic pathway (HBP) leads to its metabolic assimilation and labeling of O-GlcNAcylated proteins within live cells. This one-step metabolic feeding strategy permits labeling of O-GlcNAcylated proteins with a fluorescent glucosamine-nitrobenzoxadiazole (GlcN-NBD) conjugate that accumulates in a time- and dose-dependent manner. Because no genetic engineering of cells is required, we anticipate this strategy should be generally amenable to studying the roles of O-GlcNAc in cellular physiology as well as to gain an improved understanding of the regulation of OGT within cells. The further expansion of this one-step in-cell labeling strategy should enable performing a range of experiments including two-color pulse chase experiments and monitoring OGT activity on specific protein substrates in live cells.

O-GlcNAc as an Integrator of Signaling Pathways

O-GlcNAcylation is an important posttranslational modification governed by a single pair of enzymes-O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). These two enzymes mediate the dynamic cycling of O-GlcNAcylation on a wide variety of cytosolic, nuclear and mitochondrial proteins in a nutrient- and stress-responsive fashion. While cellular functions of O-GlcNAcylation have been emerging, little is known regarding the precise mechanisms how the enzyme pair senses the environmental cues to elicit molecular and physiological changes. In this review, we discuss how the OGT/OGA pair acts as a metabolic sensor that integrates signaling pathways, given their capability of receiving signaling inputs from various partners, targeting multiple substrates with spatiotemporal specificity and translocating to different parts of the cell. We also discuss how the pair maintains homeostatic signaling within the cell and its physiological relevance. A better understanding of the mechanisms of OGT/OGA action would enable us to derive therapeutic benefits of resetting cellular O-GlcNAc levels within an optimal range.

Quantitative Profiling of Protein O-GlcNAcylation Sites by an Isotope-Tagged Cleavable Linker

Large-scale quantification of protein O-linked β- N-acetylglucosamine (O-GlcNAc) modification in a site-specific manner remains a key challenge in studying O-GlcNAc biology. Herein, we developed an isotope-tagged cleavable linker (isoTCL) strategy, which enabled isotopic labeling of O-GlcNAc through bioorthogonal conjugation of affinity tags. We demonstrated the application of the isoTCL in mapping and quantification of O-GlcNAcylation sites in HeLa cells. Furthermore, we investigated the O-GlcNAcylation sensitivity to the sugar donor by quantifying the levels of modification under different concentrations of the O-GlcNAc labeling probe in a site-specific manner. In addition, we applied isoTCL to compare the O-GlcNAcylation stoichiometry levels of more than 100 modification sites between placenta samples from male and female mice and confirmed site-specifically that female placenta has a higher O-GlcNAcylation than its male counterpart. The isoTCL platform provides a powerful tool for quantitative profiling of O-GlcNAc modification.

The Metabolic Chemical Reporter 6-Azido-6-deoxy-glucose Further Reveals the Substrate Promiscuity of O-GlcNAc Transferase and Catalyzes the Discovery of Intracellular Protein Modification by O-Glucose

Metabolic chemical reporters of glycosylation in combination with bioorthogonal reactions have been known for two decades and have been used by many different research laboratories for the identification and visualization of glycoconjugates. More recently, however, they have begun to see utility for the investigation of cellular metabolism and the tolerance of biosynthetic enzymes and glycosyltransferases to different sugars. Here, we take this concept one step further by using the metabolic chemical reporter 6-azido-6-deoxy-glucose (6AzGlc). We show that treatment of mammalian cells with the per- O-acetylated version of 6AzGlc results in robust labeling of a variety of proteins. Notably, the pattern of this labeling was consistent with O-GlcNAc modifications, suggesting that the enzyme O-GlcNAc transferase is quite promiscuous for its donor sugar substrates. To confirm this possibility, we show that 6AzGlc-treatment results in the labeling of known O-GlcNAcylated proteins, that the UDP-6AzGlc donor sugar is indeed produced in living cells, and that recombinant OGT will accept UDP-6AzGlc as a substrate in vitro. Finally, we use proteomics to first identify several bona fide 6AzGlc-modifications in mammalian cells and then an endogenous O-glucose modification on host cell factor. These results support the conclusion that OGT can endogenously modify proteins with both N-acetyl-glucosamine and glucose, raising the possibility that intracellular O-glucose modification may be a widespread modification under certain conditions or in particular tissues.