Increasing O-GlcNAc levels: An overview of small-molecule inhibitors of O-GlcNAcase

O-GlcNAcylation reduces proteome solubility and regulates the formation of biomolecular condensates in human cells

O-GlcNAcylation plays critical roles in the regulation of protein functions and cellular activities, including protein interactions with other macromolecules. While the formation of biomolecular condensates (or biocondensates) regulated by O-GlcNAcylation in a few individual proteins has been reported, systematic investigation of O-GlcNAcylation on the regulation of biocondensate formation remains to be explored. Here we systematically study the roles of O-GlcNAcylation in regulating protein solubility and its impacts on RNA-protein condensates using mass spectrometry-based chemoproteomics. Unexpectedly, we observe a system-wide decrease in the solubility of proteins modified by O-GlcNAcylation, with glycoproteins involved in focal adhesion and actin binding exhibiting the most significant decrease. Furthermore, O-GlcNAcylation sites located in disordered regions and with fewer acidic and aromatic residues nearby are related to a greater drop in protein solubility. Additionally, we discover that a specific group of O-GlcNAcylation events promotes the dissociation of RNA-protein condensates under heat stress, while some enhance the formation of RNA-protein condensates during the recovery phase. Using site mutagenesis, inhibition of O-GlcNAc transferase, and fluorescence microscopy, we validate that O-GlcNAcylation regulates the formation of biocondensates for YTHDF3 and NUFIP2. This work advances our understanding of the functions of protein O-GlcNAcylation and its roles in the formation of biomolecular condensates.

O-GlcNAcylation in tumorigenesis and its implications for cancer therapy

O-linked N-acetylglucosaminylation (O-GlcNAcylation) is a dynamic and reversible posttranslational modification that targets serine and threonine residues in a variety of proteins. Uridine diphospho-N-acetylglucosamine, which is synthesized from glucose via the hexosamine biosynthesis pathway, is the major donor of this modification. O-GlcNAc transferase is the sole enzyme that transfers GlcNAc onto protein substrates, while O-GlcNAcase is responsible for removing this modification. O-GlcNAcylation plays an important role in tumorigenesis and progression through the modification of specific protein substrates. In this review, we discuss the tumor-related biological functions of O-GlcNAcylation and summarize the recent progress in the development of pharmaceutical options to manipulate the O-GlcNAcylation of specific proteins as potential anticancer therapies.

Role of O-GlcNAcylation in Central Nervous System Development and Injuries: A Systematic Review

The development of central nervous system (CNS) can form perceptual, memory, and cognitive functions, while injuries to CNS often lead to severe neurological dysfunction and even death. As one of the prevalent post-translational modifications (PTMs), O-GlcNAcylation has recently attracted great attentions due to its functions in regulating the activity, subcellular localization, and stability of target proteins. It has been indicated that O-GlcNAcylation could interact with phosphorylation, ubiquitination, and methylation to jointly regulate the function and activity of proteins. Furthermore, a growing number of studies have suggested that O-GlcNAcylation played an important role in the CNS. During development, O-GlcNAcylation participated in the neurogenesis, neuronal development, and neuronal function. In addition, O-GlcNAcylation was involved in the progress of CNS injuries including ischemic stroke, subarachnoid hemorrhage (SAH), and intracerebral hemorrhage (ICH) and played a crucial role in the improvement of brain damage such as attenuating cognitive impairment, inhibiting neuroinflammation, suppressing endoplasmic reticulum (ER) stress, and maintaining blood-brain barrier (BBB) integrity. Therefore, O-GlcNAcylation showed great promise as a potential target in CNS development and injuries. In this article, we presented a review highlighting the role of O-GlcNAcylation in CNS development and injuries. Hence, on the basis of these properties and effects, intervention with O-GlcNAcylation may be developed as therapeutic agents for CNS diseases.

A protein O-GlcNAc glycosyltransferase regulates the antioxidative response in Yersinia pestis

Post-translational addition of O-linked N-acetylglucosamine (O-GlcNAc) to proteins is commonly associated with a variety of stress responses and cellular processes in eukaryotes, but its potential roles in bacteria are unclear. Here, we show that protein HmwC acts as an O-GlcNAc transferase (OGT) responsible for O-GlcNAcylation of multiple proteins in Yersinia pestis, a flea-borne pathogen responsible for plague. We identify 64 O-GlcNAcylated proteins (comprising 65 sites) with differential abundance under conditions mimicking the mammalian host (Mh) and flea vector (Fv) environments. Deletion of hmwC, encoding a putative OGT, structurally distinct from any existing member of the GT41 family, results in reduced O-GlcNAcylation, reduced growth, and alterations in virulence properties and survival under stress. Purified HmwC can modify target proteins in vitro using UDP-GlcNAc as sugar donor. One of the target proteins, OsdY, promotes Y. pestis survival under oxidative stress conditions. Thus, our results support that regulation of antioxidative responses through O-GlcNAcylation may be a conserved process shared by prokaryotes and eukaryotes.

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.

Transport of N-acetylchitooligosaccharides and fluorescent N-acetylchitooligosaccharide analogs into rat liver lysosomes

Free polymannose-type oligosaccharides (fOS) are processed by cytosolic enzymes to generate Man5GlcNAc which is transferred to lysosomes and degraded. Lysosomal fOS import was demonstrated in vitro but is poorly characterized in part due to lack of convenient substrates. As chitooligosaccharides (COS, oligomers β1,4-linked GlcNAc) block [3H]Man5GlcNAc transport into lysosomes, we asked if COS are themselves transported and if so, can they be chemically modified to generate fluorescent substrates. We show that COS are degraded by lysosomal hydrolases to generate GlcNAc, and robust ATP-dependent transport of [3H]COS2/4 di and tetrasaccharides into intact rat liver lysosomes was observed only after blocking lysosomal [3H]GlcNAc efflux with cytochalasin B. As oligosaccharides with unmodified reducing termini are the most efficient inhibitors of [3H]COS2/4 and [3H]Man5GlcNAc transport, the non-reducing GlcNAc residue of COS2-4 was de-N-acetylated using Sinorhizobium meliloti NodB, and the resulting amine substituted with rhodamine B (RB) to yield RB-COS2-4. The fluorescent compounds inhibit [3H]Man5GlcNAc transport and display temperature-sensitive, ATP-dependent transport into a sedimentable compartment that is ruptured with the lysosomotropic agent L-methyl methionine ester. Once in this compartment, RB-COS3 is converted to RB-COS2 further identifying it as the lysosomal compartment. RB-COS2/3 and [3H]Man5GlcNAc transports are blocked similarly by competing sugars, and are partially inhibited by the vacuolar ATPase inhibitor bafilomycin and high concentrations of the P-type ATPase inhibitor orthovanadate. These data show that Man5GlcNAc, COS2/4 and RB-COS2/3 are transported into lysosomes by the same or closely related mechanism and demonstrate the utility of COS modified at their non-reducing terminus to study lysosomal oligosaccharide transport.

O-GlcNAcylation: a pro-survival response to acute stress in the cardiovascular and central nervous systems

O-GlcNAcylation is a unique monosaccharide modification that is ubiquitously present in numerous nucleoplasmic and mitochondrial proteins. The hexosamine biosynthesis pathway (HBP), which is a key branch of glycolysis, provides the unique sugar donor UDP-GlcNAc for the O-GlcNAc modification. Thus, HBP/O-GlcNAcylation can act as a nutrient sensor to perceive changes in nutrient levels and trigger O-GlcNAc modifications of functional proteins in cellular (patho-)physiology, thereby regulating diverse metabolic processes. An imbalance in O-GlcNAcylation has been shown to be a pathogenic contributor to dysfunction in metabolic diseases, including type 2 diabetes, cancer, and neurodegeneration. However, under acute stress conditions, protein O-GlcNAc modification exhibits rapid and transient upregulation, which is strongly correlated with stress tolerance and cell survival. In this context, we discuss the metabolic, pharmacological and genetic modulation of HBP/O-GlcNAc modification in the biological system, the beneficial role of O-GlcNAcylation in regulating stress tolerance for cardioprotection, and neuroprotection, which is a novel and rapidly growing field. Current evidence suggests that transient activation of the O-GlcNAc modification represents a potent pro-survival signalling pathway and may provide a promising strategy for stress-related disorder therapy.

The O-GlcNAc dichotomy: when does adaptation become pathological?

O-Linked attachment of β-N-acetylglucosamine (O-GlcNAc) on serine and threonine residues of nuclear, cytoplasmic, and mitochondrial proteins is a highly dynamic and ubiquitous post-translational modification that impacts the function, activity, subcellular localization, and stability of target proteins. Physiologically, acute O-GlcNAcylation serves primarily to modulate cellular signaling and transcription regulatory pathways in response to nutrients and stress. To date, thousands of proteins have been revealed to be O-GlcNAcylated and this number continues to grow as the technology for the detection of O-GlcNAc improves. The attachment of a single O-GlcNAc is catalyzed by the enzyme O-GlcNAc transferase (OGT), and their removal is catalyzed by O-GlcNAcase (OGA). O-GlcNAcylation is regulated by the metabolism of glucose via the hexosamine biosynthesis pathway, and the metabolic abnormalities associated with pathophysiological conditions are all associated with increased flux through this pathway and elevate O-GlcNAc levels. While chronic O-GlcNAcylation is well associated with cardiovascular dysfunction, only until recently, and with genetically modified animals, has O-GlcNAcylation as a contributing mechanism of cardiovascular disease emerged. This review will address and critically evaluate the current literature on the role of O-GlcNAcylation in vascular physiology, with a view that this pathway can offer novel targets for the treatment and prevention of cardiovascular diseases.

Trans-cyclosulfamidate mannose-configured cyclitol allows isoform-dependent inhibition of GH47 α-d-mannosidases through a bump-hole strategy

Class I inverting exo-acting α-1,2-mannosidases (CAZY family GH47) display an unusual catalytic itinerary featuring ring-flipped mannosides, 3S1 → 3H4‡ → 1C4. Conformationally locked 1C4 compounds, such as kifunensine, display nanomolar inhibition but large multigene GH47 mannosidase families render specific "isoform-dependent" inhibition impossible. Here we develop a bump-and-hole strategy in which a new mannose-configured 1,6-trans-cyclic sulfamidate inhibits α-d-mannosidases by virtue of its 1C4 conformation. This compound does not inhibit the wild-type GH47 model enzyme by virtue of a steric clash, a "bump", in the active site. An L310S (a conserved residue amongst human GH47 enzymes) mutant of the model Caulobacter GH47 awoke 574 nM inhibition of the previously dormant inhibitor, confirmed by structural analysis of a 0.97 Å structure. Considering that L310 is a conserved residue amongst human GH47 enzymes, this work provides a unique framework for future biotechnological studies on N-glycan maturation and ER associated degradation by isoform-specific GH47 α-d-mannosidase inhibition through a bump-and-hole approach.

Targeted protein modification as a paradigm shift in drug discovery

Targeted Protein Modification (TPM) is an umbrella term encompassing numerous tools and approaches that use bifunctional agents to induce a desired modification over the POI. The most well-known TPM mechanism is PROTAC-directed protein ubiquitination. PROTAC-based targeted degradation offers several advantages over conventional small-molecule inhibitors, has shifted the drug discovery paradigm, and is acquiring increasing interest as over ten PROTACs have entered clinical trials in the past few years. Targeting the protein of interest for proteasomal degradation by PROTACS was the pioneer of various toolboxes for selective protein degradation. Nowadays, the ever-increasing number of tools and strategies for modulating and modifying the POI has expanded far beyond protein degradation, which phosphorylation and de-phosphorylation of the protein of interest, targeted acetylation, and selective modification of protein O-GlcNAcylation are among them. These novel strategies have opened new avenues for achieving more precise outcomes while remaining feasible and minimizing side effects. This field, however, is still in its infancy and has a long way to precede widespread use and translation into clinical practice. Herein, we investigate the pros and cons of these novel strategies by exploring the latest advancements in this field. Ultimately, we briefly discuss the emerging potential applications of these innovations in cancer therapy, neurodegeneration, viral infections, and autoimmune and inflammatory diseases.

O-GlcNAcylation regulates extracellular signal-regulated kinase (ERK) activation in Alzheimer's disease

Introduction

Aberrant activation of Extracellular Signal-Regulated Kinase (ERK) signaling is associated with Alzheimer's disease (AD) pathogenesis. For example, enhanced ERK signal activation mediated by Apolipoprotein E4 (APOE4), which is a critical genetic risk factor for AD, increases the transcription of amyloid precursor protein (APP). We hypothesize that O-linked N-acetylglucosamine (O-GlcNAc) regulates the phosphorylation and activation of ERK. O-GlcNAc is a single sugar post-translational modification that dynamically cycles on and off proteins in response to nutrient changes by the action of the enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. However, O-GlcNAc quickly returns to a baseline level after stimulus removal (called O-GlcNAc homeostasis).

Methods

We did a serum reactivation time-course followed by western blot in SH-SY5Y neuroblastoma cells after long-term O-GlcNAcase (OGA) inhibition by Thiamet-G (TMG) treatment, O-GlcNAc transferase (OGT) knock-down (KD) and OGA KD. Brain tissues of C57BL6/J mice and 5XFAD Alzheimer's disease mice intra-peritoneally injected with TMG for 1 month and C57BL6/J mice intra-peritoneally injected with TMG for 6 months were also used for western blot.

Results

We found that ERK1/2 phosphorylation at Thr 202/Tyr204 and Thr183/Tyr185 (p-ERK) are amplified and hence ERK1/2 are activated after long-term OGA inhibition in SH-SY5Y cells. In addition to pharmacological treatment, genetic disruption of O-GlcNAc by OGT KD and OGA KD also increased p-ERK in SH-SY5Y cells suggesting O-GlcNAc homeostasis controls ERK signaling. To determine how O-GlcNAc regulates p-ERK, we probed the expression of phosphorylated mitogen-activated protein kinase-kinase (p-MEK) which phosphorylates and activates ERK and Dual specificity phosphatase-4 (DUSP4) which dephosphorylates and inactivates ERK in SH-SY5Y cells. p-MEK increases in TMG treated and OGT KD cells whereas total DUSP4 decreases in OGT KD and OGA KD cells with serum reactivation time course. Next, we probed the role of OGA inhibition in regulating ERK activation using mice brain-tissue samples. Interestingly, 6-month intra-peritoneal TMG injection in C57BL/6J mice showed an increase in amplitude of p-ERK and APP protein levels, indicating long-term OGA inhibition potentially contributes to AD progression. Furthermore, 1-month TMG injection was sufficient to increase the amplitude of p-ERK in 5XFAD AD mice brains suggesting AD phenotype contributes to the acceleration of ERK activation mediated by OGA inhibition.

Conclusion

Together, these results indicate that disruptions to O-GlcNAc homeostasis amplify ERK signal activation in AD.

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

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

O-GlcNAcylation: The Underestimated Emerging Regulators of Skeletal Muscle Physiology

O-GlcNAcylation is a highly dynamic, reversible and atypical glycosylation that regulates the activity, biological function, stability, sublocation and interaction of target proteins. O-GlcNAcylation receives and coordinates different signal inputs as an intracellular integrator similar to the nutrient sensor and stress receptor, which target multiple substrates with spatio-temporal analysis specifically to maintain cellular homeostasis and normal physiological functions. Our review gives a brief description of O-GlcNAcylation and its only two processing enzymes and HBP flux, which will help to better understand its physiological characteristics of sensing nutrition and environmental cues. This nutritional and stress-sensitive properties of O-GlcNAcylation allow it to participate in the precise regulation of skeletal muscle metabolism. This review discusses the mechanism of O-GlcNAcylation to alleviate metabolic disorders and the controversy about the insulin resistance of skeletal muscle. The level of global O-GlcNAcylation is precisely controlled and maintained in the "optimal zone", and its abnormal changes is a potential factor in the pathogenesis of cancer, neurodegeneration, diabetes and diabetic complications. Although the essential role of O-GlcNAcylation in skeletal muscle physiology has been widely studied and recognized, it still is underestimated and overlooked. This review highlights the latest progress and potential mechanisms of O-GlcNAcylation in the regulation of skeletal muscle contraction and structural properties.

Deciphering the Structure and Formation of Amyloids in Neurodegenerative Diseases With Chemical Biology Tools

Protein aggregation into highly ordered, regularly repeated cross-β sheet structures called amyloid fibrils is closely associated to human disorders such as neurodegenerative diseases including Alzheimer's and Parkinson's diseases, or systemic diseases like type II diabetes. Yet, in some cases, such as the HET-s prion, amyloids have biological functions. High-resolution structures of amyloids fibrils from cryo-electron microscopy have very recently highlighted their ultrastructural organization and polymorphisms. However, the molecular mechanisms and the role of co-factors (posttranslational modifications, non-proteinaceous components and other proteins) acting on the fibril formation are still poorly understood. Whether amyloid fibrils play a toxic or protective role in the pathogenesis of neurodegenerative diseases remains to be elucidated. Furthermore, such aberrant protein-protein interactions challenge the search of small-molecule drugs or immunotherapy approaches targeting amyloid formation. In this review, we describe how chemical biology tools contribute to new insights on the mode of action of amyloidogenic proteins and peptides, defining their structural signature and aggregation pathways by capturing their molecular details and conformational heterogeneity. Challenging the imagination of scientists, this constantly expanding field provides crucial tools to unravel mechanistic detail of amyloid formation such as semisynthetic proteins and small-molecule sensors of conformational changes and/or aggregation. Protein engineering methods and bioorthogonal chemistry for the introduction of protein chemical modifications are additional fruitful strategies to tackle the challenge of understanding amyloid formation.

O-GlcNAcylation of MORC2 at threonine 556 by OGT couples TGF-β signaling to breast cancer progression

MORC family CW-type zinc finger 2 (MORC2) is a newly identified chromatin-remodeling enzyme involved in DNA damage response and gene transcription, and its dysregulation has been linked with Charcot-Marie-Tooth disease, neurodevelopmental disorder, and cancer. Despite its functional importance, how MORC2 is regulated remains enigmatic. Here, we report that MORC2 is O-GlcNAcylated by O-GlcNAc transferase (OGT) at threonine 556. Mutation of this site or pharmacological inhibition of OGT impairs MORC2-mediated breast cancer cell migration and invasion in vitro and lung colonization in vivo. Moreover, transforming growth factor-β1 (TGF-β1) induces MORC2 O-GlcNAcylation through enhancing the stability of glutamine-fructose-6-phosphate aminotransferase (GFAT), the rate-limiting enzyme for producing the sugar donor for OGT. O-GlcNAcylated MORC2 is required for transcriptional activation of TGF-β1 target genes connective tissue growth factor (CTGF) and snail family transcriptional repressor 1 (SNAIL). In support of these observations, knockdown of GFAT, SNAIL or CTGF compromises TGF-β1-induced, MORC2 O-GlcNAcylation-mediated breast cancer cell migration and invasion. Clinically, high expression of OGT, MORC2, SNAIL, and CTGF in breast tumors is associated with poor patient prognosis. Collectively, these findings uncover a previously unrecognized mechanistic role for MORC2 O-GlcNAcylation in breast cancer progression and provide evidence for targeting MORC2-dependent breast cancer through blocking its O-GlcNAcylation.

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.

2-Acetamido-2-deoxy-d-glucono-1,5-lactone Sulfonylhydrazones: Synthesis and Evaluation as Inhibitors of Human OGA and HexB Enzymes

Inhibition of the human O-linked β-N-acetylglucosaminidase (hOGA, GH84) enzyme is pharmacologically relevant in several diseases such as neurodegenerative and cardiovascular disorders, type 2 diabetes, and cancer. Human lysosomal hexosaminidases (hHexA and hHexB, GH20) are mechanistically related enzymes; therefore, selective inhibition of these enzymes is crucial in terms of potential applications. In order to extend the structure–activity relationships of OGA inhibitors, a series of 2-acetamido-2-deoxy-d-glucono-1,5-lactone sulfonylhydrazones was prepared from d-glucosamine. The synthetic sequence involved condensation of N-acetyl-3,4,6-tri-O-acetyl-d-glucosamine with arenesulfonylhydrazines, followed by MnO₂ oxidation to the corresponding glucono-1,5-lactone sulfonylhydrazones. Removal of the O-acetyl protecting groups by NH₃/MeOH furnished the test compounds. Evaluation of these compounds by enzyme kinetic methods against hOGA and hHexB revealed potent nanomolar competitive inhibition of both enzymes, with no significant selectivity towards either. The most efficient inhibitor of hOGA was 2-acetamido-2-deoxy-d-glucono-1,5-lactone 1-naphthalenesulfonylhydrazone (5f, K_i = 27 nM). This compound had a K_i of 6.8 nM towards hHexB. To assess the binding mode of these inhibitors to hOGA, computational studies (Prime protein–ligand refinement and QM/MM optimizations) were performed, which suggested the binding preference of the glucono-1,5-lactone sulfonylhydrazones in an s-cis conformation for all test compounds.

Structural variation of the 3-acetamido-4,5,6-trihydroxyazepane iminosugar through epimerization and C-alkylation leads to low micromolar HexAB and NagZ inhibitors

We report the synthesis of seven-membered iminosugars derived from a 3S-acetamido-4R,5R,6S-trihydroxyazepane scaffold and their evaluation as inhibitors of functionally related exo-N-acetylhexosaminidases including human O-GlcNAcase (OGA), human lysosomal β-hexosaminidase (HexAB), and Escherichia coli NagZ. Capitalizing on the flexibility of azepanes and the active site tolerances of hexosaminidases, we explore the effects of epimerization of stereocenters at C-3, C-5 and C-6 and C-alkylation at the C-2 or C-7 positions. Accordingly, epimerization at C-6 (L-ido) and at C-5 (D-galacto) led to selective HexAB inhibitors whereas introduction of a propyl group at C-7 on the C-3 epimer furnished a potent NagZ inhibitor.

Synthesis, conformational analysis and glycosidase inhibition of bicyclic nojirimycin C-glycosides based on an octahydrofuro[3,2-b]pyridine motif

A set of bicyclic iminosugar C-glycosides, based on an octahydrofuro[3,2-b]pyridine motif, has been synthesized from a C-allyl iminosugar exploiting a debenzylative iodocycloetherification and an iodine nucleophilic displacement as the key steps. The halogen allowed the introduction of a range of aglycon moieties of different sizes bearing several functionalities such as alcohol, amine, amide and triazole. In these carbohydrate mimics the fused THF ring forces the piperidine to adopt a flattened ⁴C₁ conformation according to NMR and DFT calculations studies. In their deprotected form, these bicycles were assayed on a panel of 23 glycosidases. The iminosugars displaying hydrophobic aglycon moieties proved to be superior glycosidase inhibitors, leading to a low micromolar inhibition of human lysosome β-glucosidase (compound 11; IC₅₀ = 2.7 μM) and rice α-glucosidase (compound 10; IC₅₀ = 7.7 μM). Finally, the loose structural analogy of these derivatives with Thiamet G, a potent OGA bicyclic inhibitor, was illustrated by the weak OGA inhibitory activity (Ki = 140 μM) of iminosugar 5.

Writing and erasing O-GlcNAc from target proteins in cells

O-linked N-acetylglucosamine (O-GlcNAc) is a widespread reversible modification on nucleocytoplasmic proteins that plays an important role in many biochemical processes and is highly relevant to numerous human diseases. The O-GlcNAc modification has diverse functional impacts on individual proteins and glycosites, and methods for editing this modification on substrates are essential to decipher these functions. Herein, we review recent progress in developing methods for O-GlcNAc regulation, with a focus on methods for editing O-GlcNAc with protein- and site-selectivity in cells. The applications, advantages, and limitations of currently available strategies for writing and erasing O-GlcNAc and future directions are also discussed. These emerging approaches to manipulate O-GlcNAc on a target protein in cells will greatly accelerate the development of functional studies and enable therapeutic interventions in the O-GlcNAc field.

Advances in chemical probing of protein O-GlcNAc glycosylation: structural role and molecular mechanisms

The addition of O-linked-β-D-N-acetylglucosamine (O-GlcNAc) onto serine and threonine residues of nuclear and cytoplasmic proteins is an abundant, unique post-translational modification governing important biological processes. O-GlcNAc dysregulation underlies several metabolic disorders leading to human diseases, including cancer, neurodegeneration and diabetes. This review provides an extensive summary of the recent progress in probing O-GlcNAcylation using mainly chemical methods, with a special focus on discussing mechanistic insights and the structural role of O-GlcNAc at the molecular level. We highlight key aspects of the O-GlcNAc enzymes, including development of OGT and OGA small-molecule inhibitors, and describe a variety of chemoenzymatic and chemical biology approaches for the study of O-GlcNAcylation. Special emphasis is placed on the power of chemistry in the form of synthetic glycopeptide and glycoprotein tools for investigating the site-specific functional consequences of the modification. Finally, we discuss in detail the conformational effects of O-GlcNAc glycosylation on protein structure and stability, relevant O-GlcNAc-mediated protein interactions and its molecular recognition features by biological receptors. Future research in this field will provide novel, more effective chemical strategies and probes for the molecular interrogation of O-GlcNAcylation, elucidating new mechanisms and functional roles of O-GlcNAc with potential therapeutic applications in human health.

A systematic review of carbohydrate-based bioactive molecules for Alzheimer's disease

The abundance, low cost, high density of functional groups and ease of purification of carbohydrates are among the most important features that make them a prime candidate for designing therapeutics. Several carbohydrate-based molecules, of both natural and synthetic origin, are known for their wide range of therapeutic activities. The incorporation of a carbohydrate moiety not only retains the pharmacological characteristics of a molecule but also improves its activity. Several sugar conjugates have been designed and reported to inhibit acetylcholinesterase, β-amyloid and tau aggregation. This systematic review provides a brief overview of carbohydrate-based bioactive molecules having anti-Alzheimer's activity along with improved therapeutic potential. Most importantly, several reported carbohydrate-based molecules for Alzheimer's disease act on β-amyloid aggregation, tau protein, cholinesterase and oxidative stress, with enhanced pharmacokinetic and mechanistic properties. The prospect of designing carbohydrate-based molecules for Alzheimer's disease will definitely provide potential opportunities to discover novel carbohydrate-based drugs.

Region- and voxel-based quantification in human brain of [18F]LSN3316612, a radioligand for O-GlcNAcase

Background

Previous studies found that the positron emission tomography (PET) radioligand [¹⁸F]LSN3316612 accurately quantified O-GlcNAcase in human brain using a two-tissue compartment model (2TCM). This study sought to assess kinetic model(s) as an alternative to 2TCM for quantifying [¹⁸F]LSN3316612 binding, particularly in order to generate good-quality parametric images.

Methods

The current study reanalyzed data from a previous study of 10 healthy volunteers who underwent both test and retest PET scans with [¹⁸F]LSN3316612. Kinetic analysis was performed at the region level with 2TCM using 120-min PET data and arterial input function, which was considered as the gold standard. Quantification was then obtained at both the region and voxel levels using Logan plot, Ichise's multilinear analysis-1 (MA1), standard spectral analysis (SA), and impulse response function at 120 min (IRF₁₂₀). To avoid arterial sampling, a noninvasive relative quantification (standardized uptake value ratio (SUVR)) was also tested using the corpus callosum as a pseudo-reference region. Venous samples were also assessed to see whether they could substitute for arterial ones.

Results

Logan and MA1 generated parametric images of good visual quality and their total distribution volume (V_T) values at both the region and voxel levels were strongly correlated with 2TCM-derived V_T (r = 0.96–0.99) and showed little bias (up to − 8%). SA was more weakly correlated to 2TCM-derived V_T (r = 0.93–0.98) and was more biased (~ 16%). IRF₁₂₀ showed a strong correlation with 2TCM-derived V_T (r = 0.96) but generated noisier parametric images. All techniques were comparable to 2TCM in terms of test–retest variability and reliability except IRF₁₂₀, which gave significantly worse results. Noninvasive SUVR values were not correlated with 2TCM-derived V_T, and arteriovenous equilibrium was never reached.

Conclusions

Compared to SA and IRF, Logan and MA1 are more suitable alternatives to 2TCM for quantifying [¹⁸F]LSN3316612 and generating good-quality parametric images.

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.

Pharmacological inhibition and knockdown of O-GlcNAcase reduces cellular internalization of α-synuclein preformed fibrils

The pathological hallmark of Parkinson's disease (PD) is Lewy bodies that form within the brain from aggregated forms of α-synuclein (α-syn). These toxic α-syn aggregates are transferred from cell to cell by release of fibrils from dying neurons into the extracellular environment, followed by their subsequent uptake by neighboring cells. This process leads to spreading of the pathology throughout the brain in a prion-like manner. Identifying new pathways that hinder the internalization of such α-syn fibrils is of high interest for their downstream potential exploitation as a way to create disease-modifying therapeutics for PD. Here, we show that Thiamet-G, a highly selective pharmacological agent that inhibits the glycoside hydrolase O-GlcNAcase (OGA), blunts the cellular uptake of α-syn fibrils. This effect correlates with increased nucleocytoplasmic levels of O-linked N-acetylglucosamine (O-GlcNAc)-modified proteins, and genetic knockdown of OGA expression closely phenocopies both these effects. These reductions in the uptake of α-syn fibrils caused by inhibition of OGA are both concentration- and time-dependent and are observed in multiple cell lines including mouse primary cortical neurons. Moreover, treatment of cells with the OGT inhibitor, 5SGlcNHex, increases the level of uptake of α-syn PFFs, further supporting O-GlcNAcylation of proteins driving these effects. Notably, this effect is mediated through an unknown mechanism that does not involve well-characterized endocytotic pathways. These data suggest one mechanism by which OGA inhibitors might exert their protective effects in prion-like neuropathologies and support exploration of OGA inhibitors as a potential disease-modifying approach to treat PD.

An intellectual disability syndrome with single-nucleotide variants in O-GlcNAc transferase

Intellectual disability (ID) is a neurodevelopmental condition that affects ~1% of the world population. In total 5-10% of ID cases are due to variants in genes located on the X chromosome. Recently, variants in OGT have been shown to co-segregate with X-linked intellectual disability (XLID) in multiple families. OGT encodes O-GlcNAc transferase (OGT), an essential enzyme that catalyses O-linked glycosylation with β-N-acetylglucosamine (O-GlcNAc) on serine/threonine residues of thousands of nuclear and cytosolic proteins. In this review, we compile the work from the last few years that clearly delineates a new syndromic form of ID, which we propose to classify as a novel Congenital Disorder of Glycosylation (OGT-CDG). We discuss potential hypotheses for the underpinning molecular mechanism(s) that provide impetus for future research studies geared towards informed interventions.

O-GlcNAc transferase affects the signal transduction of β1 adrenoceptor in adult rat cardiomyocytes by increasing the O-GlcNAcylation of β1 adrenoceptor

O-GlcNAcylation was first found by Torres and Hart in monocytes. It is a dynamic and reversible post-translational modification catalyzed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). O-GlcNAcylation is increased in diabetic cardiomyopathy (DCM) patients and it has been reported that OGT plays an important role in the regulation of cardiac gene transcription, cell cycle and calcium homeostasis. The purpose of this study is to investigate the effects of OGT on signal transduction and function of β1-adrenoceptor (β1AR) in adult rat cardiomyocytes. We found that after overexpressing OGT by adenovirus vector in adult rat cardiomyocytes, cAMP formation and phosphorylation of phospholamban (PLB) at Ser16 (p16-PLB) were decreased under isoprenaline (ISO) stimulation. Over expression of OGT increased the intracellular [Ca²⁺]i and deteriorated the death of cardiomyocytes induced by prolonged stimulation with ISO. β1-adrenoceptor was overexpressed using a plasmid vector and then co-immunoprecipitation (co-IP) followed by Western blot was employed to define the O-GlcNAcylation of β1-adrenoceptor. The results showed that O-GlcNAcylation of β1-adrenoceptor was increased in OGT overexpressed cells, and there was no significant change in the formation of cAMP and phosphorylation of PLB after β1-adrenoceptor was blocked by CGP20712A. Given that OGT affects the signal transduction of β1-adrenoceptor in adult rat cardiomyocytes by increasing the O-GlcNAcylation of β1-adrenoceptor, the mechanism revealed in this study indicates that OGT and β1AR may be therapeutic targets in patients undergoing diabetic cardiomyopathy.

O-GlcNAc stimulation: A new metabolic approach to treat septic shock

Septic shock is a systemic inflammation associated with cell metabolism disorders and cardiovascular dysfunction. Increases in O-GlcNAcylation have shown beneficial cardiovascular effects in acute pathologies. We used two different rat models to evaluate the beneficial effects of O-GlcNAc stimulation at the early phase of septic shock. Rats received lipopolysaccharide (LPS) to induce endotoxemic shock or saline (control) and fluid resuscitation (R) with or without O-GlcNAc stimulation (NButGT-10 mg/kg) 1 hour after shock induction. For the second model, rats received cecal ligature and puncture (CLP) surgery and fluid therapy with or without NButGT. Cardiovascular function was evaluated and heart and blood samples were collected and analysed. NButGT treatment efficiently increased total O-GlcNAc without modification of HBP enzyme expression.Treatment improved circulating parameters and cardiovascular function in both models, and restored SERCA2a expression levels. NButGT treatment also reduced animal mortality. In this study, we demonstrate that in septic shock O-GlcNAc stimulation improves global animal and cardiovascular function outcomes associated with a restoration of SERCA2a levels. This pre-clinical study opens avenues for a potential therapy of early-stage septic shock.

Chondroprotective effects of 4-methylumbelliferone and hyaluronan synthase-2 overexpression involve changes in chondrocyte energy metabolism

Hyaluronan is a critical component of articular cartilage and partially helps retain aggrecan within the extracellular matrix of this tissue. During osteoarthritis, hyaluronan and aggrecan loss are an early sign of tissue damage. However, our recent attempts to mimic hyaluronan loss with the hyaluronan inhibitor 4-methylumbelliferone (4MU) did not exacerbate arthritis-like features of in vitro models of arthritis, but surprisingly, caused the reverse (i.e. provided potent chondroprotection). Moreover, the protective effects of 4MU did not depend on its role as a hyaluronan inhibitor. To understand the molecular mechanism in 4MU-mediated chondroprotection, we considered recent studies suggesting that shifts in intracellular UDP-hexose pools promote changes in metabolism. To determine whether such metabolic shifts are associated with the mechanism of 4MU-mediated pro-catabolic inhibition, using molecular and metabolomics approaches, we examined whether bovine and human chondrocytes exhibit changes in the contribution of glycolysis and mitochondrial respiration to ATP production rates as well as in other factors that respond to or might drive these changes. Overexpression of either HA synthase-2 or 4MU effectively reduced dependence on glycolysis in chondrocytes, especially enhancing glycolysis use by interleukin-1β (IL1β)-activated chondrocytes. The reduction in glycolysis secondarily enhanced mitochondrial respiration in chondrocytes, which, in turn, rescued phospho-AMP-activated protein kinase (AMPK) levels in the activated chondrocytes. Other glycolysis inhibitors, unrelated to hyaluronan biosynthesis, namely 2-deoxyglucose and dichloroacetate, caused metabolic changes in chondrocytes equivalent to those elicited by 4MU and similarly protected both chondrocytes and cartilage explants. These results suggest that fluxes in UDP-hexoses alter metabolic energy pathways in cartilage.

O-GlcNAcylation, a sweet link to the pathology of diseases

O-linked N-acetylglucosamine (O-GlcNAc) is a dynamic post-translational modification occurring on myriad proteins in the cell nucleus, cytoplasm, and mitochondria. The donor sugar for O-GlcNAcylation, uridine-diphosphate N-acetylglucosamine (UDP-GlcNAc), is synthesized from glucose through the hexosamine biosynthetic pathway (HBP). The recycling of O-GlcNAc on proteins is mediated by two enzymes in cells-O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), which catalyze the addition and removal of O-GlcNAc, respectively. O-GlcNAcylation is involved in a number of important cell processes including transcription, translation, metabolism, signal transduction, and apoptosis. Deregulation of O-GlcNAcylation has been reported to be associated with various human diseases such as cancer, diabetes, neurodegenerative diseases, and cardiovascular diseases. A better understanding of the roles of O-GlcNAcylation in physiopathological processes would help to uncover novel avenues for therapeutic intervention. The aim of this review is to discuss the recent updates on the mechanisms and impacts of O-GlcNAcylation on these diseases, and its potential as a new clinical target.

O-GlcNAcylation and neuronal energy status: Implications for Alzheimer's disease

Since the first clinical case reported more than 100 years ago, it has been a long and winding road to demystify the initial pathological events underling the onset of Alzheimer's disease (AD). Fortunately, advanced imaging techniques extended the knowledge regarding AD origin, being well accepted that a decline in brain glucose metabolism occurs during the prodromal phases of AD and is aggravated with the progression of the disease. In this sense, in the last decades, the post-translational modification O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) has emerged as a potential causative link between hampered brain glucose metabolism and AD pathology. This is not surprising taking into account that this dynamic post-translational modification acts as a metabolic sensor that links glucose metabolism to normal neuronal functioning. Within this scenario, the present review aims to summarize the current understanding on the role of O-GlcNAcylation in neuronal physiology and AD pathology, emphasizing the close association of this post-translational modification with the emergence of AD-related hallmarks and its potential as a therapeutic target.

A genetic model to study O-GlcNAc cycling in immortalized mouse embryonic fibroblasts

O-GlcNAcylation is an abundant posttranslational protein modification in which the monosaccharide O-GlcNAc is added to Ser/Thr residues by O-GlcNAc transferase and removed by O-GlcNAcase. Analyses of O-GlcNAc-mediated signaling and metabolic phenomena are complicated by factors including unsatisfactory inhibitors and loss-of-function cell lines lacking identical genetic backgrounds. In this work, we generated immortalized WT, Oga knockout, and Ogt floxed allele (Ogt floxed) mouse embryonic fibroblast (MEF) cell lines with similar genetic backgrounds. These lines will facilitate experiments and serve as a platform to study O-GlcNAc cycling in mammals. As a test paradigm, we used the immortalized MEF lines to investigate how changes in O-GlcNAcylation affected pathological phosphorylation of the tau protein. The activity of glycogen synthase kinase 3β (GSK3β), a kinase that phosphorylates tau, decreases when expressed in Oga knockout MEFs compared with WT cells. Phosphorylation at Thr²³¹ in recombinant, tauopathy-associated tau with a proline-to-leucine mutation at position 301 (P301L) was altered when expressed in MEFs with altered O-GlcNAc cycling. In aggregate, our data support that O-GlcNAc cycling indirectly affects tau phosphorylation at Thr²³¹, but tau phosphorylation was highly variable, even in genetically stable, immortalized MEF cells. The variable nature of tau phosphorylation observed here supports the need to use cells akin to those generated here with genetically defined lesions and similar backgrounds to study complex biological processes.

Modulation of O-GlcNAc Levels in the Liver Impacts Acetaminophen-Induced Liver Injury by Affecting Protein Adduct Formation and Glutathione Synthesis

Overdose of acetaminophen (APAP) results in acute liver failure. We have investigated the role of a posttranslational modification of proteins called O-GlcNAcylation, where the O-GlcNAc transferase (OGT) adds and O-GlcNAcase (OGA) removes a single β-D-N-acetylglucosamine (O-GlcNAc) moiety, in the pathogenesis of APAP-induced liver injury. Hepatocyte-specific OGT knockout mice (OGT KO), which have reduced O-GlcNAcylation, and wild-type (WT) controls were treated with 300 mg/kg APAP and the development of injury was studied over a time course from 0 to 24 h. OGT KO mice developed significantly lower liver injury as compared with WT mice. Hepatic CYP2E1 activity and glutathione (GSH) depletion following APAP treatment were not different between WT and OGT KO mice. However, replenishment of GSH and induction of GSH biosynthesis genes were significantly faster in the OGT KO mice. Next, male C57BL/6 J mice were treated Thiamet-G (TMG), a specific inhibitor of OGA to induce O-GlcNAcylation, 1.5 h after APAP administration and the development of liver injury was studied over a time course of 0-24 h. TMG-treated mice exhibited significantly higher APAP-induced liver injury. Treatment with TMG did not affect hepatic CYP2E1 levels, GSH depletion, APAP-protein adducts, and APAP-induced mitochondrial damage. However, GSH replenishment and GSH biosynthesis genes were lower in TMG-treated mice after APAP overdose. Taken together, these data indicate that induction in cellular O-GlcNAcylation exacerbates APAP-induced liver injury via dysregulation of hepatic GSH replenishment response.

The O-β-linked N-acetylglucosaminylation of the Lamin B receptor and its impact on DNA binding and phosphorylation

Lamin B Receptor (LBR) is an integral protein of the interphase inner nuclear membrane that is implicated in chromatin anchorage to the nuclear envelope. Phosphorylation of a stretch of arginine-serine (RS) dipeptides in the amino-terminal nucleoplasmic domain of LBR regulates the interactions of the receptor with other nuclear proteins, DNA and RNA and thus modulates tethering of heterochromatin to the nuclear envelope. While phosphorylation has been extensively studied, very little is known about other post-translational modifications of the protein. There is only one report on the O-β-linked N-acetyl-glucosaminylation (O-GlcNAcylation) of a serine residue downstream of the RS domain of rat LBR. In the present study we identify additional O-GlcNAcylation sites by using as substrates of O-β-N-acetylglucosaminyltransferase (OGT) a set of peptides containing the entire LBR RS domain or parts of it as well as flanking sequences. The in vitro activity of OGT was assessed by tandem mass spectrometry and NMR spectroscopy. Furthermore, we provide evidence that O-GlcNAcylation hampers DNA binding while it marginally affects RS domain phosphorylation mediated by SRPK1, Akt2 and cdk1 kinases.

GENERAL SIGNIFICANCE

Our methodology providing a quantitative description of O-GlcNAc patterns based on a combination of mass spectrometry and high resolution NMR spectroscopy on short peptide substrates allows subsequent functional analyses. Hence, our approach is of general interest to a wide audience of biologists aiming at deciphering the functional role of O-GlcNAc glycosylation and its crosstalk with phosphorylation.

The Modulatory Roles of N-glycans in T-Cell-Mediated Autoimmune Diseases

Glycosylation is a ubiquitous posttranslational modification of proteins that occurs in the endoplasmic reticulum/Golgi. N-glycans and mucin-type O-glycans are achieved via a series of glycohydrolase- and glycosyltransferase-mediated reactions. Glycosylation modulates immune responses by regulating thymocyte development and T helper cell differentiation. Autoimmune diseases result from an abnormal immune response by self-antigens and subsequently lead to the destruction of the target tissues. The modification of N-glycans has been studied in several animal models of T-cell-mediated autoimmune diseases. This review summarizes and highlights the modulatory effects of N-glycosylation in several autoimmune diseases, including multiple sclerosis, systemic lupus erythematosus, inflammatory bowel disease, and type 1 diabetes mellitus.

Tau protein aggregation in Alzheimer's disease: An attractive target for the development of novel therapeutic agents

Alzheimer's Disease (AD) is a neurodegenerative brain disorder in which many biological dysfunctions are involved. Among them, two main types of lesions were discovered and widely studied: the amyloid plaques and the neurofibrillary tangles (NFTs). These two lesions are caused by the dysfunction and the accumulation of two proteins which are, respectively, the beta-amyloid peptide and the tau protein. The process that leads these two proteins to aggregate is complex and is the subject of current studies. After a brief description of the aggregation mechanisms, we will provide an overview of new therapeutic agents targeting the different dysfunctions and toxic species found during aggregation.

PUGNAc treatment provokes globotetraosylceramide accumulation in human umbilical vein endothelial cells

PUGNAc is a well-investigated inhibitor for protein-O-GlcNAcase, whereas recent investigations showed that PUGNAc had a broad range as inhibitor for cellular β-hexosaminidases. Here we report that PUGNAc treatment provokes globotetraosylceramide (Gb4Cer) accumulation in human umbilical vein endothelial cells (HUVEC). HPLC analysis and a quantitative ELISA using newly developed anti-Gb4Cer monoclonal antibody revealed that PUGNAc treatment specifically increased the expression of Gb4Cer among glycosphingolipids expressed in HUVEC. Although the effect was weaker than PUGNAc, an O-GlcNAcase selective inhibitor (Thiamet-G) treatment also increased Gb4Cer levels in HUVEC. Furthermore, both of PUGNAc and Thiamet-G treatment up-regulated the expression levels of α-1,4-galactosyltransferase/Gb3Cer synthase gene which encodes a key enzyme in Gb4Cer synthesis. These results indicate that protein-O-GlcNAcylation can regulate the expression levels of cellular Gb4Cer.

Structures of human O-GlcNAcase and its complexes reveal a new substrate recognition mode

Human O-GlcNAcase (hOGA) is the unique enzyme responsible for the hydrolysis of the O-linked β-N-acetyl glucosamine (O-GlcNAc) modification, an essential protein glycosylation event that modulates the function of numerous cellular proteins in response to nutrients and stress. Here we report crystal structures of a truncated hOGA, which comprises the catalytic and stalk domains, in apo form, in complex with an inhibitor, and in complex with a glycopeptide substrate. We found that hOGA forms an unusual arm-in-arm homodimer in which the catalytic domain of one monomer is covered by the stalk domain of the sister monomer to create a substrate-binding cleft. Notably, the residues on the cleft surface afford extensive interactions with the peptide substrate in a recognition mode that is distinct from that of its bacterial homologs. These structures represent the first model of eukaryotic enzymes in the glycoside hydrolase 84 (GH84) family and provide a crucial starting point for understanding the substrate specificity of hOGA, which regulates a broad range of biological and pathological processes.

Deciphering the Functions of Protein O-GlcNAcylation with Chemistry

O-GlcNAcylation is the modification of serine and threonine residues with β-N-acetylglucosamine (O-GlcNAc) on intracellular proteins. This dynamic modification is attached by O-GlcNAc transferase (OGT) and removed by O-GlcNAcase (OGA) and is a critical regulator of various cellular processes. Furthermore, O-GlcNAcylation is dysregulated in many diseases, such as diabetes, cancer, and Alzheimer's disease. However, the precise role of this modification and its cycling enzymes (OGT and OGA) in normal and disease states remains elusive. This is partially due to the difficulty in studying O-GlcNAcylation with traditional genetic and biochemical techniques. In this review, we will summarize recent progress in chemical approaches to overcome these obstacles. We will cover new inhibitors of OGT and OGA, advances in metabolic labeling and cellular imaging, synthetic approaches to access homogeneous O-GlcNAcylated proteins, and cross-linking methods to identify O-GlcNAc-protein interactions. We will also discuss remaining gaps in our toolbox for studying O-GlcNAcylation and questions of high interest that are yet to be answered.

Dissecting PUGNAc-mediated inhibition of the pro-survival action of insulin

Previous studies utilizing PUGNAc, the most widely used β-N-acetylglucosaminidase (OGA) inhibitor to increase global O-N-acetylglucosamine (GlcNAc) levels, have reported a variety of effects including insulin resistance as a direct result of elevated O-GlcNAc levels. The notion of OGA inhibition causing insulin resistance was not replicated in studies in which elevated global O-GlcNAc levels were achieved using two other OGA inhibitors. Related to insulin action, work by others has suggested that O-GlcNAc elevation may inhibit the anti-apoptotic action of insulin. Thus, we examined the pro-survival action of insulin upon serum deprivation in the presence of PUGNAc as well as two selective OGA inhibitors (GlcNAcstatin-g and Thiamet-G), and a selective lysosomal hexosaminidase inhibitor (INJ2). We established that PUGNAc inhibits the pro-survival action of insulin but this effect is not recapitulated by the selective OGA inhibitors suggesting that elevation in O-GlcNAc levels alone is not responsible for PUGNAc's effect on the anti-apoptotic action of insulin. Further, we demonstrate that a selective hexosaminidase A/B (HexA/B) inhibitor does not impact insulin action suggesting that PUGNAc's effect is not due to inhibition of lysosomal hexosaminidase. Finally, we tested a combination of selective OGA and lysosomal hexosaminidase inhibitors but were not able to recapitulate the inhibition of insulin action generated by PUGNAc alone. These results strongly suggest that the defect in insulin action upon PUGNAc treatment does not derive from its inhibition of OGA or HexA/B, and that there is an unknown target of PUGNAc that is the likely culprit in inhibiting the protective effect of insulin from apoptosis.

Temporal regulation of Lsp1 O-GlcNAcylation and phosphorylation during apoptosis of activated B cells

Crosslinking of B-cell receptor (BCR) sets off an apoptosis programme, but the underlying pathways remain obscure. Here we decipher the molecular mechanisms bridging B-cell activation and apoptosis mediated by post-translational modification (PTM). We find that O-GlcNAcase inhibition enhances B-cell activation and apoptosis induced by BCR crosslinking. This proteome-scale analysis of the functional interplay between protein O-GlcNAcylation and phosphorylation in stimulated mouse primary B cells identifies 313 O-GlcNAcylation-dependent phosphosites on 224 phosphoproteins. Among these phosphoproteins, temporal regulation of the O-GlcNAcylation and phosphorylation of lymphocyte-specific protein-1 (Lsp1) is a key switch that triggers apoptosis in activated B cells. O-GlcNAcylation at S209 of Lsp1 is a prerequisite for the recruitment of its kinase, PKC-β1, to induce S243 phosphorylation, leading to ERK activation and downregulation of BCL-2 and BCL-xL. Thus, we demonstrate the critical PTM interplay of Lsp1 that transmits signals for initiating apoptosis after BCR ligation.

B cell receptor (BCR) activation can trigger signalling causing apoptosis in order to eliminate auto-reactive B cells. Here the authors show that the O-GlcNAcylation and phosphorylation of lymphocyte-specific protein-1 are involved in a switch that regulates the initiation of apoptosis induced by BCR cross-linking.

OGT restrains the expansion of DNA damage signaling

O-linked N-acetylglucosamine linkage (O-GlcNAcylation) to serine or threonine residues regulates numerous biological processes; however, its role in DNA damage response remains elusive. Here, we found that O-GlcNAcylation is induced by DNA damage response. O-GlcNAc transferase (OGT), the solo enzyme for O-GlcNAcylation, relocates to the sites of DNA damage and induces the O-GlcNAcylation of histone H2AX and mediator of DNA damage checkpoint 1 (MDC1). The O-GlcNAcylation negatively regulates DNA double-strand break-induced phosphorylation of H2AX and MDC1 by restraining the expansion of these phosphorylation events from the sites of DNA damage. Therefore, our study reveals the molecular mechanism and biological function of OGT-dependent O-GlcNAcylation in response to DNA damage.

Human RNA Polymerase II Promoter Recruitment in Vitro Is Regulated by O-Linked N-Acetylglucosaminyltransferase (OGT)

Although the O-linked N-acetylglucosamine (O-GlcNAc) modification of the RNA polymerase II C-terminal domain was described 20 years ago, the function of this RNA polymerase II (pol II) species is not known. We show here that an O-GlcNAcylated pol II species (pol IIγ) exists on promoters in vitro Inhibition of O-GlcNAc-transferase activity and O-GlcNAcylation prevents pol II entry into the promoter, and O-GlcNAc removal from pol II is an ATP-dependent step during initiation. These data indicate that O-GlcNAc-transferase activity is essential for RNA pol II promoter recruitment and that pol II goes through a cycling of O-GlcNAcylation at the promoter. Mass spectrometry shows that serine residues 2 and 5 of the pol II C-terminal domain are O-GlcNAcylated, suggesting an overlap with the transcription factor IIH (TFIIH)-dependent serine 5 phosphorylation events during initiation and P-TEFb (positive transcriptional elongation factor b) events during elongation. These data provide unexpected and important insights into the role of a previously ill-defined species of RNA polymerase II in regulating transcription.

O-GlcNAcylation: A regulator of tau pathology and neurodegeneration

O-GlcNAcylation is the posttranslational modification of intracellular proteins by O-linked β-N-acetylglucosamine (O-GlcNAc). The discovery of O-GlcNAc modification of tau and its impact on tau phosphorylation has attracted recent research interest in O-GlcNAc studies in the Alzheimer's disease (AD) field. Modification of proteins by O-GlcNAc occurs extensively in the brain. The expressions and activities of the enzymes catalyzing O-GlcNAc cycling are several-fold higher in the brain than in the peripheral tissues. The O-GlcNAcylation levels of brain proteins including tau are decreased in AD brain, probably due to decreased brain glucose metabolism. The reduction of brain O-GlcNAcylation appears to mediate the molecular mechanism by which decreased brain glucose metabolism contributes to neurodegeneration. Studies on mouse models of tauopathies suggest a neuroprotective role of pharmacological elevation of brain O-GlcNAc, which could potentially be a promising approach for treating AD and other neurodegenerative diseases.