Dynamic O-linked glycosylation of nuclear and cytoskeletal proteins

Site specific O-GlcNAcylation of progesterone receptor (PR) supports PR attenuation of interferon stimulated genes (ISGs) and tumor growth in breast cancer

Hormone receptor (HR) positive breast cancer, defined by expression of estrogen (ER) and/or progesterone (PR) receptor expression, is the most commonly diagnosed type of breast cancer. PR alters the transcriptional landscape to support tumor growth in concert with or independent of ER. Thus, understanding the mechanisms regulating PR function are critical to developing new strategies to treat HR+ breast cancer. O-GlcNAc is a post-translational modification responsible for nutrient sensing that modulates protein function. Although PR is heavily post-translationally modified, through phosphorylation and O-GlcNAcylation, specific sites of O-GlcNAcylation on PR and how they regulate PR action, have not been investigated. Using established PR-expressing breast cancer cell lines, we mapped several sites of O-GlcNAcylation on PR. RNA-sequencing after PR O-GlcNAc site mutagenesis revealed site-specific O-GlcNAcylation of PR is critical for ligand-independent suppression of interferon signaling, a regulatory function of PR in breast cancer. Furthermore, O-GlcNAcylation of PR enhances PR-driven tumor growth in vivo. We have delineated one contributing mechanism to PR function in breast cancer that impacts tumor growth, and provided additional insight into the mechanism through which PR attenuates interferon signaling.

Modulation of extrinsic apoptotic pathway by intracellular glycosylation

The importance of post-translational modifications (PTMs), particularly O-GlcNAcylation, of cytoplasmic proteins in apoptosis has been neglected for quite a while. Modification of cytoplasmic proteins by a single N-acetylglucosamine sugar is a dynamic and reversible PTM exhibiting properties more like phosphorylation than classical O- and N-linked glycosylation. Due to the sparse information existing, we have only limited understanding of how GlcNAcylation affects cell death. Deciphering the role of GlcNAcylation in cell fate may provide further understanding of cell fate decisions. This review focus on the modulation of extrinsic apoptotic pathway via GlcNAcylation carried out by O-GlcNAc transferase (OGT) or by other bacterial effector proteins.

Regulation of Glycogen Synthase Kinase-3β by Phosphorylation and O-β-Linked N-Acetylglucosaminylation: Implications on Tau Protein Phosphorylation

Glycogen synthase kinase 3 (GSK3) plays a pivotal role in signaling pathways involved in insulin metabolism and the pathogenesis of neurodegenerative disorders. In particular, the GSK3β isoform is implicated in Alzheimer's disease (AD) as one of the key kinases involved in the hyperphosphorylation of tau protein, one of the neuropathological hallmarks of AD. As a constitutively active serine/threonine kinase, GSK3 is inactivated by Akt/PKB-mediated phosphorylation of Ser9 in the N-terminal disordered domain, and for most of its substrates, requires priming (prephosphorylation) by another kinase that targets the substrate to a phosphate-specific pocket near the active site. GSK3 has also been shown to be post-translationally modified by O-linked β-N-acetylglucosaminylation (O-GlcNAcylation), with still unknown functions. Here, we have found that binding of Akt inhibits GSK3β kinase activity on both primed and unprimed tau substrates. Akt-mediated Ser9 phosphorylation restores the GSK3β kinase activity only on primed tau, thereby selectively inactivating GSK3β toward unprimed tau protein. Additionally, we have shown that GSK3β is highly O-GlcNAcylated at multiple sites within the kinase domain and the disordered N- and C-terminal domains, including Ser9. In contrast to Akt-mediated regulation, neither the O-GlcNAc transferase nor O-GlcNAcylation significantly alters GSK3β kinase activity, but high O-GlcNAc levels reduce Ser9 phosphorylation by Akt. Reciprocally, Akt phosphorylation downregulates the overall O-GlcNAcylation of GSK3β, indicating a crosstalk between both post-translational modifications. Our results indicate that specific O-GlcNAc profiles may be involved in the phosphorylation-dependent Akt-mediated regulation of GSK3β kinase activity.

O-GlcNAcylation in ischemic diseases

Protein glycosylation is an extensively studied field, with the most studied forms being oxygen or nitrogen-linked N-acetylglucosamine (O-GlcNAc or N-GlcNAc) glycosylation. Particular residues on proteins are targeted by O-GlcNAcylation, which is among the most intricate post-translational modifications. Significantly contributing to an organism's proteome, it influences numerous factors affecting protein stability, function, and subcellular localization. It also modifies the cellular function of target proteins that have crucial responsibilities in controlling pathways related to the central nervous system, cardiovascular homeostasis, and other organ functions. Under conditions of acute stress, changes in the levels of O-GlcNAcylation of these proteins may have a defensive function. Nevertheless, deviant O-GlcNAcylation nullifies this safeguard and stimulates the advancement of several ailments, the prognosis of which relies on the cellular milieu. Hence, this review provides a concise overview of the function and comprehension of O-GlcNAcylation in ischemia diseases, aiming to facilitate the discovery of new therapeutic targets for efficient treatment, particularly in patients with diabetes.

Tools for investigating O-GlcNAc in signaling and other fundamental biological pathways

Cells continuously fine-tune signaling pathway proteins to match nutrient and stress levels in their local environment by modifying intracellular proteins with O-linked N-acetylglucosamine (O-GlcNAc) sugars, an essential process for cell survival and growth. The small size of these monosaccharide modifications poses a challenge for functional determination, but the chemistry and biology communities have together created a collection of precision tools to study these dynamic sugars. This review presents the major themes by which O-GlcNAc influences signaling pathway proteins, including G-protein coupled receptors, growth factor signaling, mitogen-activated protein kinase (MAPK) pathways, lipid sensing, and cytokine signaling pathways. Along the way, we describe in detail key chemical biology tools that have been developed and applied to determine specific O-GlcNAc roles in these pathways. These tools include metabolic labeling, O-GlcNAc-enhancing RNA aptamers, fluorescent biosensors, proximity labeling tools, nanobody targeting tools, O-GlcNAc cycling inhibitors, light-activated systems, chemoenzymatic labeling, and nutrient reporter assays. An emergent feature of this signaling pathway meta-analysis is the intricate interplay between O-GlcNAc modifications across different signaling systems, underscoring the importance of O-GlcNAc in regulating cellular processes. We highlight the significance of O-GlcNAc in signaling and the role of chemical and biochemical tools in unraveling distinct glycobiological regulatory mechanisms. Collectively, our field has determined effective strategies to probe O-GlcNAc roles in biology. At the same time, this survey of what we do not yet know presents a clear roadmap for the field to use these powerful chemical tools to explore cross-pathway O-GlcNAc interactions in signaling and other major biological pathways.

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

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

Progress of fluorescent probes for protein phosphorylation and glycosylation in atherosclerosis

Exploring the post-translational modification (PTM) of proteins in the course of atherosclerotic disease has important guiding significance for the early warning of atherosclerotic plaque, the development of targeted drugs and the treatment of disease. The advancement advanced detection and imaging methods for phosphorylated and glycosylated proteins is an important tool to further reveal the levels of protein phosphorylation and glycosylation during atherosclerotic plaque formation. We present research strategies for detecting protein phosphorylation and glycosylation from the perspective of fluorescent probes, and discuss the feasibility and future direction of the development of these methods for detecting and imaging phosphorylated and glycosylated proteins in atherosclerotic disease.

The O-GlcNAc Modification of Recombinant Tau Protein and Characterization of the O-GlcNAc Pattern for Functional Study

The neuronal microtubule-associated tau protein is characterized in vivo by a large number of post-translational modifications along the entire primary sequence that modulates its function. The primary modification of tau is phosphorylation of serine/threonine or tyrosine residues that is involved in the regulation of microtubule binding and polymerization. In neurodegenerative disorders referred to as tauopathies including Alzheimer's disease, tau is abnormally hyperphosphorylated and forms fibrillar inclusions in neurons progressing throughout different brain area during the course of the disease. The O-β-linked N-acetylglucosamine (O-GlcNAc) is another reversible post-translational modification of serine/threonine residues that is installed and removed by the unique O-GlcNAc transferase (OGT) and O-GlcNAc hydrolase (OGA), respectively. This modification was described as a potential modulator of tau phosphorylation and functions in the physiopathology. Moreover, reducing protein O-GlcNAc levels in the brain upon treatment of tauopathy mouse models with an OGA inhibitor reveals a beneficial effect on tau pathology and neurodegeneration. However, whether the role of tau O-GlcNAcylation is responsible of the protective effect against tau toxicity remains to be determined. The production of O-GlcNAc modified recombinant tau protein is a valuable tool for the investigations of the impact of O-GlcNAcylation on tau functions, modulation of interactions with partners and crosstalk with other post-translational modifications, including but not restricted to phosphorylation. We describe here the in vitro O-GlcNAcylation of tau with recombinant OGT for which we provide an expression and purification protocol. The use of the O-GlcNAc tau protein in functional studies requires the analytical characterization of the O-GlcNAc pattern. Here, we describe a method for the O-GlcNAc modification of tau protein with recombinant OGT and the analytical characterization of the resulting O-GlcNAc pattern by a combination of methods for the overall characterization of tau O-GlcNAcylation by chemoenzymatic labeling and mass spectrometry, as well as the quantitative, site-specific pattern by NMR spectroscopy.

Affinity-Directed Site-Specific Protein Labeling and Its Application to Antibody-Drug Conjugates

Chemically modified proteins have diverse applications; however, conventional chemo-selective methods often yield heterogeneously labeled products. To address this limitation, site-specific protein labeling holds significant potential, driving extensive research in this area. Nevertheless, site-specific modification of native proteins remains challenging owing to the complexity of their functional groups. Therefore, a method for site-selective labeling of intact proteins is aimed to design. In this study, a novel approach to traceless affinity-directed intact protein labeling is established, which leverages small binding proteins and genetic code expansion technology. By applying this method, a site-specific antibody labeling with a drug, which leads to the production of highly effective antibody-drug conjugates specifically targeting breast cancer cell lines is achieved. This approach enables traceless conjugation of intact target proteins, which is a critical advantage in pharmaceutical applications. Furthermore, small helical binding proteins can be easily engineered for various target proteins, thereby expanding their potential applications in diverse fields. This innovative approach represents a significant advancement in site-specific modification of native proteins, including antibodies. It also bears immense potential for facilitating the development of therapeutic agents for various diseases.

Characterization of Posttranslationally Modified PHF-1 Tau Peptides Using Gaussian Accelerated Molecular Dynamics Simulation

The microtubule-associated protein, Tau, is an intrinsically disordered protein that plays a crucial role in neurodegenerative diseases like Alzheimer's disease. The posttranslational modifications across the Tau protein domains are involved in regulating Tau protein's function and disease onset. Of the various posttranslational modifications at Ser, Thr, and Tyr sites, O-GlcNAcylation and phosphorylation are the most critical ones, playing a vital role in Tau aggregation and tauopathies. To understand the function, it is essential to characterize the structural changes associated with Tau modification. Previous experimental studies have focused on high-resolution nuclear magnetic resonance techniques to structurally characterize the effect of phosphorylation, O-GlcNAcylation, and combination of both PTMs on Tau conformation in small peptides centered on the PHF-1 epitope from amino acid 392 to 411. The structural characterization using atomistic molecular dynamics simulation of such disordered peptides requires long simulation time, proper sampling method, and utilization of appropriate force fields for accurate determination of conformational ensembles, resembling the experimental data. This chapter details the protocol for the structural characterization of modified Tau peptides using the CHARMM36m force field and enhanced sampling methods like Gaussian accelerated molecular dynamics (GaMD) simulation. We have focused on a detailed explanation of the GaMD method and analyses of molecular dynamics trajectories to explain the relationship between two modifications, phospho- and glyco-, at C-terminus of Tau protein and its stable conformation over the longer simulation timeframes. The analyses involve energetics reweighting, clustering of simulation trajectories, and characterization of secondary structure using circular dichroism data from the simulation. The reader can utilize this protocol to investigate the structures of complex proteins, especially the disordered ones.

Diabetic Neuropathy: An Overview of Molecular Pathways and Protective Mechanisms of Phytobioactives

Diabetic neuropathy (DN) is a common and debilitating complication of diabetes mellitus that affects the peripheral nerves and causes pain, numbness, and impaired function. The pathogenesis of DN involves multiple molecular mechanisms, such as oxidative stress, inflammation, and pathways of advanced glycation end products, polyol, hexosamine, and protein kinase C. Phytochemicals are natural compounds derived from plants that have various biological activities and therapeutic potential. Flavonoids, terpenes, alkaloids, stilbenes, and tannins are some of the phytochemicals that have been identified as having protective potential for diabetic neuropathy. These compounds can modulate various cellular pathways involved in the development and progression of neuropathy, including reducing oxidative stress and inflammation and promoting nerve growth and repair. In this review, the current evidence on the effects of phytochemicals on DN by focusing on five major classes, flavonoids, terpenes, alkaloids, stilbenes, and tannins, are summarized. This compilation also discusses the possible molecular targets of numerous pathways of DN that these phytochemicals modulate. These phytochemicals may offer a promising alternative or complementary approach to conventional drugs for DN management by modulating multiple pathological pathways and restoring nerve function.

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.

Abnormal glycosylation in glioma: related changes in biology, biomarkers and targeted therapy

Glioma is a rapidly growing and aggressive primary malignant tumor of the central nervous system that can diffusely invade the brain tissue around, and the prognosis of patients is not significantly improved by traditional treatments. One of the most general posttranslational modifications of proteins is glycosylation, and the abnormal distribution of this modification in gliomas may shed light on how it affects biological behaviors of glioma cells, including proliferation, migration, and invasion, which may be produced by regulating protein function, cell-matrix and cell‒cell interactions, and affecting receptor downstream pathways. In this paper, from the perspective of regulating protein glycosylation changes and abnormal expression of glycosylation-related proteins (such as glycosyltransferases in gliomas), we summarize how glycosylation may play a crucial role in the discovery of novel biomarkers and new targeted treatment options for gliomas. Overall, the mechanistic basis of abnormal glycosylation affecting glioma progression remains to be more widely and deeply explored, which not only helps to inspire researchers to further explore related diagnostic and prognostic markers but also provides ideas for discovering effective treatment strategies and improving glioma patient survival and prognosis.

The role of protein glycosylation in the occurrence and outcome of acute ischemic stroke

Acute ischemic stroke (AIS) is a serious and life-threatening disease worldwide. Despite thrombolysis or endovascular thrombectomy, a sizeable fraction of patients with AIS have adverse clinical outcomes. In addition, existing secondary prevention strategies with antiplatelet and anticoagulant drugs therapy are not able to adequately decrease the risk of ischemic stroke recurrence. Thus, exploring novel mechanisms for doing so represents an urgent need for the prevention and treatment of AIS. Recent studies have discovered that protein glycosylation plays a critical role in the occurrence and outcome of AIS. As a common co- and post-translational modification, protein glycosylation participates in a wide variety of physiological and pathological processes by regulating the activity and function of proteins or enzymes. Protein glycosylation is involved in two causes of cerebral emboli in ischemic stroke: atherosclerosis and atrial fibrillation. Following ischemic stroke, the level of brain protein glycosylation becomes dynamically regulated, which significantly affects stroke outcome through influencing inflammatory response, excitotoxicity, neuronal apoptosis, and blood-brain barrier disruption. Drugs targeting glycosylation in the occurrence and progression of stroke may represent a novel therapeutic idea. In this review, we focus on possible perspectives about how glycosylation affects the occurrence and outcome of AIS. We then propose the potential of glycosylation as a therapeutic drug target and prognostic marker for AIS patients in the future.

In silico analysis of the possible crosstalk between O-linked β-GlcNAcylation and phosphorylation sites of Disabled 1 adaptor protein in vertebrates

Disabled 1 (Dab1) is an adaptor protein with essential functions regulated by reelin signaling and affects many biological processes in the nervous system, including cell motility, adhesion, cortical development, maturation, and synaptic plasticity. Posttranslational modifications directly guide the fates of cytoplasmic proteins to complete their functions correctly. Reciprocal crosstalk between O-GlcNAcylation and phosphorylation is a dynamic modification in cytoplasmic proteins. It modulates the functions of the proteins by regulating their interactions with other molecules in response to the continuously changeable cell microenvironment. Although Dab1 contains conserved recognition sites for phosphorylation in their N-terminal protein interaction domain, the O-β-GlcNAcylation and phosphorylation sites of human Dab1 sequence, their reciprocal crosstalk, and potential kinases catalyzing the phosphorylation remain unknown. In this study, we determined potential thirty-seven O-β-GlcNAcylation and sixty-seven phosphorylation sites. Conserved twenty-one residues of these glycosylated sites were also phosphorylated with various kinases, including ATM, CKI, DNAPK, GSK3, PKC, PKG, RSK, cdc2, cdk5, and p38MAPK. In addition, we analyzed these conserved sites at our constructed two- and three-dimensional structures of human Dab1 protein. Dab1 protein models were frequently composed of coil structures as well as α-helix and β-strands. Many of these conserved crosstalk sites between O-β-GlcNAcylation and phosphorylation were localized at the coil region of the protein model. These findings may guide biochemical, genetic, and glyco-biology based on further experiments about the Dab1 signaling process. Understanding these modifications might change the point of view of the Dab1 signaling process and treatment for pathological conditions in neurodegenerative diseases such as Alzheimer's disease.

Recent development of analytical methods for disease-specific protein O-GlcNAcylation

The enzymatic modification of protein serine or threonine residues by N-acetylglucosamine, namely O-GlcNAcylation, is a ubiquitous post-translational modification that frequently occurs in the nucleus and cytoplasm. O-GlcNAcylation is dynamically regulated by two enzymes, O-GlcNAc transferase and O-GlcNAcase, and regulates nearly all cellular processes in epigenetics, transcription, translation, cell division, metabolism, signal transduction and stress. Aberrant O-GlcNAcylation has been shown in a variety of diseases, including diabetes, neurodegenerative diseases and cancers. Deciphering O-GlcNAcylation remains a challenge due to its low abundance, low stoichiometry and extreme lability in most tandem mass spectrometry. Separation or enrichment of O-GlcNAc proteins or peptides from complex mixtures has been of great interest because quantitative analysis of protein O-GlcNAcylation can elucidate their functions and regulatory mechanisms in disease. However, valid and specific analytical methods are still lacking, and efforts are needed to further advance this direction. Here, we provide an overview of recent advances in various analytical methods, focusing on chemical oxidation, affinity of antibodies and lectins, hydrophilic interaction, and enzymatic addition of monosaccharides in conjugation with these methods. O-GlcNAcylation quantification has been described in detail using mass-spectrometric or non-mass-spectrometric techniques. We briefly summarized dysregulated changes in O-GlcNAcylation in disease.

An introduction to dynamic nucleoporins in Leishmania species: Novel targets for tropical-therapeutics

As an ailment, leishmaniasis is still an incessant challenge in neglected tropical diseases and neglected infections of poverty worldwide. At present, the diagnosis and treatment to combat Leishmania tropical infections are not substantial remedies and require advanced & specific research. Therefore, there is a need for a potential novel target to overcome established medicament modalities' limitations in pathogenicity. In this review, we proposed a few ab initio findings in nucleoporins of nuclear pore complex in Leishmania sp. concerning other infectious protists. So, through structural analysis and dynamics studies, we hypothesize the nuclear pore molecular machinery & functionality. The gatekeepers Nups, export of mRNA, mitotic spindle formation are salient features in cellular mechanics and this is regulated by dynamic nucleoporins. Here, diverse studies suggest that Nup93/NIC96, Nup155/Nup144, Mlp1/Mlp2/Tpr of Leishmania Species can be a picked out marker for diagnostic, immune-modulation, and novel drug targets. In silico prediction of nucleoporin-functional interactors such as NUP54/57, RNA helicase, Ubiquitin-protein ligase, Exportin 1, putative T-lymphocyte triggering factor, and 9 uncharacterized proteins suggest few more noble targets. The novel drug targeting to importins/exportins of Leishmania sp. and defining mechanism of Leptomycin-B, SINE compounds, Curcumins, Selinexor can be an arc-light in therapeutics. The essence of the review in Leishmania's nucleoporins is to refocus our research on noble molecular targets for tropical therapeutics.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12639-022-01515-0.

O-GlycNacylation Remission Retards the Progression of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a metabolic disease spectrum associated with insulin resistance (IR), from non-alcoholic fatty liver (NAFL) to non-alcoholic steatohepatitis (NASH), cirrhosis, and hepatocellular carcinoma (HCC). O-GlcNAcylation is a posttranslational modification, regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Abnormal O-GlcNAcylation plays a key role in IR, fat deposition, inflammatory injury, fibrosis, and tumorigenesis. However, the specific mechanisms and clinical treatments of O-GlcNAcylation and NAFLD are yet to be elucidated. The modification contributes to understanding the pathogenesis and development of NAFLD, thus clarifying the protective effect of O-GlcNAcylation inhibition on liver injury. In this review, the crucial role of O-GlcNAcylation in NAFLD (from NAFL to HCC) is discussed, and the effect of therapeutics on O-GlcNAcylation and its potential mechanisms on NAFLD have been highlighted. These inferences present novel insights into the pathogenesis and treatments of NAFLD.

Hexosamine pathway activation improves memory but does not extend lifespan in mice

Glucosamine feeding and genetic activation of the hexosamine biosynthetic pathway (HBP) have been linked to improved protein quality control and lifespan extension. However, as an energy sensor, the HBP has been implicated in tumor progression and diabetes. Given these opposing outcomes, it is imperative to explore the long-term effects of chronic HBP activation in mammals. Thus, we asked if HBP activation affects metabolism, coordination, memory, and survival in mice. N-acetyl-D-glucosamine (GlcNAc) supplementation in the drinking water had no adverse effect on weight in males but increased weight in young females. Glucose or insulin tolerance was not affected up to 20 months of age. Of note, we observed improved memory in young male mice supplemented with GlcNAc. Survival was not changed by GlcNAc treatment. To assess the effects of genetic HBP activation, we overexpressed the pathway's key enzyme GFAT1 and a constitutively activated mutant form in all mouse tissues. We detected elevated levels of the HBP product UDP-GlcNAc in mouse brains, but did not find any effects on behavior, memory, or survival. Together, while dietary GlcNAc supplementation did not extend survival in mice, it positively affected memory and is generally well tolerated.

Glycan analysis of Lamin A/C protein at G2/M and S phases of the cell cycle

During mitosis, phosphorylation and dephosphorylation of lamins triggers the nuclear envelope disassembly/assembly. However, it hasn't been known whether lamin proteins undergo any modification other than phosphorylation during the cell cycle. Glycosylation of lamin proteins is one of the less studied post-translational modification. Glycosylation and phosphorylation compete for the same positions and interplay between two modifications generate a post-translational code in the cell. Based on this, we hypothesized that glycosylation of lamin A/C protein may be important in the regulation of the structural organization of the nuclear lamina during interphase and mitosis. We analysed the glycan units of lamin A/C protein in lung carcinoma cells synchronized at G2/M and S phases via CapLC-ESI-MS/MS. Besides, the outermost glycan units were determined using lectin blotting and gold-conjugated antibody and lectin staining. TEM studies also allowed us to observe the localization of glycosylated lamin A/C protein. With this study, we determined that lamin A/C protein shows O-glycosylation at G2/M and S phases of the cell cycle. In addition to O-GlcNAcylation and O-GalNAcylation, lamin A/C is found to be contain Gal, Fuc, Man, and Sia sugars at G2/M and S phases for the first time. Having found the glycan units of the lamin A/C protein suggests that glycosylation might have a role in the nuclear organization during the cell cycle.

Delivery of Nucleotide Sugars to the Mammalian Golgi: A Very Well (un)Explained Story

Nucleotide sugars (NSs) serve as substrates for glycosylation reactions. The majority of these compounds are synthesized in the cytoplasm, whereas glycosylation occurs in the endoplasmic reticulum (ER) and Golgi lumens, where catalytic domains of glycosyltransferases (GTs) are located. Therefore, translocation of NS across the organelle membranes is a prerequisite. This process is thought to be mediated by a group of multi-transmembrane proteins from the SLC35 family, i.e., nucleotide sugar transporters (NSTs). Despite many years of research, some uncertainties/inconsistencies related with the mechanisms of NS transport and the substrate specificities of NSTs remain. Here we present a comprehensive review of the NS import into the mammalian Golgi, which consists of three major parts. In the first part, we provide a historical view of the experimental approaches used to study NS transport and evaluate the most important achievements. The second part summarizes various aspects of knowledge concerning NSTs, ranging from subcellular localization up to the pathologies related with their defective function. In the third part, we present the outcomes of our research performed using mammalian cell-based models and discuss its relevance in relation to the general context.

Disaccharide-tag for highly sensitive identification of O-GlcNAc-modified proteins in mammalian cells

O-GlcNAcylation is the only sugar modification for proteins present in the cytoplasm and nucleus and is thought to be involved in the regulation of protein function and localization. Currently, several methods are known for detecting O-GlcNAcylated proteins using monoclonal antibodies or wheat germ agglutinin, but these methods have some limitations in their sensitivity and quantitative comparison. We developed a new disaccharide-tag method to overcome these problems. This is a method in which a soluble GalNAc transferase is expressed intracellularly, extended to a disaccharide of GalNAc-GlcNAc, and detected using a Wisteria japonica agglutinin specific to this disaccharide. We verified the method using human c-Rel protein and also highly sensitively compared the difference in O-GlcNAc modification of intracellular proteins associated with differentiation from embryonic stem cell (ESC) to epiblast-like cells (EpiLC). As one example of such a modification, a novel O-GlcNAc modification was found in the transcription factor Sox2 at residue Ser263, and the modification site could be identified by nano liquid chromatography-mass spectrometry.

Demystifying the O-GlcNAc Code: A Systems View

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

GFPT2/GFAT2 and AMDHD2 act in tandem to control the hexosamine pathway

The hexosamine biosynthetic pathway (HBP) produces the essential metabolite UDP-GlcNAc and plays a key role in metabolism, health, and aging. The HBP is controlled by its rate-limiting enzyme glutamine fructose-6-phosphate amidotransferase (GFPT/GFAT) that is directly inhibited by UDP-GlcNAc in a feedback loop. HBP regulation by GFPT is well studied but other HBP regulators have remained obscure. Elevated UDP-GlcNAc levels counteract the glycosylation toxin tunicamycin (TM), and thus we screened for TM resistance in haploid mouse embryonic stem cells (mESCs) using random chemical mutagenesis to determine alternative HBP regulation. We identified the N-acetylglucosamine deacetylase AMDHD2 that catalyzes a reverse reaction in the HBP and its loss strongly elevated UDP-GlcNAc. To better understand AMDHD2, we solved the crystal structure and found that loss-of-function (LOF) is caused by protein destabilization or interference with its catalytic activity. Finally, we show that mESCs express AMDHD2 together with GFPT2 instead of the more common paralog GFPT1. Compared with GFPT1, GFPT2 had a much lower sensitivity to UDP-GlcNAc inhibition, explaining how AMDHD2 LOF resulted in HBP activation. This HBP configuration in which AMDHD2 serves to balance GFPT2 activity was also observed in other mESCs and, consistently, the GFPT2:GFPT1 ratio decreased with differentiation of human embryonic stem cells. Taken together, our data reveal a critical function of AMDHD2 in limiting UDP-GlcNAc production in cells that use GFPT2 for metabolite entry into the HBP.

O-GlcNAcylation promotes fatty acid synthase activity under nutritional stress as a pro-survival mechanism in cancer cells

Protein O-GlcNAcylation is a specific form of protein glycosylation that targets a wide range of proteins with important functions. O-GlcNAcylation is known to be deregulated in cancer and has been linked to multiple aspects of cancer pathology. Despite its ubiquity and importance, the current understanding of the role of O-GlcNAcylation in the stress response remains limited. In this study, we performed a quantitative chemical proteomics-based open study of the O-GlcNAcome in HeLa cells, and identified 163 differentially-glycosylated proteins under starvation, involving multiple metabolic pathways. Among them, fatty acid metabolism was found to be targeted and subsequent analysis confirmed that fatty acid synthase (FASN) is O-GlcNAcylated. O-GlcNAcylation led to enhanced de novo fatty acid synthesis activity, and fatty acids contributed to the cytoprotective effects of O-GlcNAcylation under starvation. Moreover, dual inhibition of O-GlcNAcylation and FASN displayed a strong synergistic effect in vitro in inducing cell death in cancer cells. Together, the results from this study provide novel insights into the role of O-GlcNAcylation in the nutritional stress response and suggest the potential of combining inhibition of O-GlcNAcylation and fatty acid synthesis in cancer therapy. This article is protected by copyright. All rights reserved.

O-GlcNAcylation regulates β1,4-GlcNAc-branched N-glycan biosynthesis via the OGT/SLC35A3/GnT-IV axis

N-Linked glycosylation and O-linked N-acetylglucosamine (O-GlcNAc) are important protein post-translational modifications that are orchestrated by a diverse set of gene products. Thus far, the relationship between these two types of glycosylation has remained elusive, and it is unclear whether one influences the other via UDP-GlcNAc, which is a common donor substrate. Theoretically, a decrease in O-GlcNAcylation may increase the products of GlcNAc-branched N-glycans. In this study, via examination by lectin blotting, HPLC, and mass spectrometry analysis, however, we found that the amounts of GlcNAc-branched tri-antennary N-glycans catalyzed by N-acetylglucosaminyltransferase IV (GnT-IV) and tetra-antennary N-glycans were significantly decreased in O-GlcNAc transferase knockdown cells (OGT-KD) compared with those in wild type cells. We examined this specific alteration by focusing on SLC35A3, which is the main UDP-GlcNAc transporter in mammals that is believed to modulate GnT-IV activation. It is interesting that a deficiency of SLC35A3 specifically leads to a decrease in the amounts of GlcNAc-branched tri- and tetra-antennary N-glycans. Furthermore, co-immunoprecipitation experiments have shown that SLC35A3 interacts with GnT-IV, but not with N-acetylglucosaminyltransferase V. Western blot and chemoenzymatic labeling assay have confirmed that OGT modifies SLC35A3 and that O-GlcNAcylation contributes to its stability. Furthermore, we found that SLC35A3-KO enhances cell spreading and suppresses both cell migration and cell proliferation, which is similar to the phenomena observed in the OGT-KD cells. Taken together, these data are the first to demonstrate that O-GlcNAcylation specifically governs the biosynthesis of tri- and tetra-antennary N-glycans via the OGT-SLC35A3-GnT-IV axis.

The role of inflammation and neurodegeneration in diabetic macular edema

The pathogenesis of diabetic macular edema (DME) is complex. Persistently high blood glucose activates multiple cellular pathways and induces inflammation, oxidation stress, and vascular dysfunction. Retinal ganglion cells, macroglial and microglial cells, endothelial cells, pericytes, and retinal pigment epithelium cells are involved. Neurodegeneration, characterized by dysfunction or apoptotic loss of retinal neurons, occurs early and independently from the vascular alterations. Despite the increasing knowledge on the pathways involved in DME, only limited therapeutic strategies are available. Besides antiangiogenic drugs and intravitreal corticosteroids, alternative therapeutic options tackling inflammation, oxidative stress, and neurodegeneration have been considered, but none of them has been currently approved.

Inhibition of O-GlcNAc Transferase Alters the Differentiation and Maturation Process of Human Monocyte Derived Dendritic Cells

The O-GlcNAcylation is a posttranslational modification of proteins regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase. These enzymes regulate the development, proliferation and function of cells, including the immune cells. Herein, we focused on the role of O-GlcNAcylation in human monocyte derived dendritic cells (moDCs). Our study suggests that inhibition of OGT modulates AKT and MEK/ERK pathways in moDCs. Changes were also observed in the expression levels of relevant surface markers, where reduced expression of CD80 and DC-SIGN, and increased expression of CD14, CD86 and HLA-DR occurred. We also noticed decreased IL-10 and increased IL-6 production, along with diminished endocytotic capacity of the cells, indicating that inhibition of O-GlcNAcylation hampers the transition of monocytes into immature DCs. Furthermore, the inhibition of OGT altered the maturation process of immature moDCs, since a CD14^medDC-SIGN^lowHLA-DR^medCD80^lowCD86^high profile was noticed when OGT inhibitor, OSMI-1, was present. To evaluate DCs ability to influence T cell differentiation and polarization, we co-cultured these cells. Surprisingly, the observed phenotypic changes of mature moDCs generated in the presence of OSMI-1 led to an increased proliferation of allogeneic T cells, while their polarization was not affected. Taken together, we confirm that shifting the O-GlcNAcylation status due to OGT inhibition alters the differentiation and function of moDCs in in vitro conditions.

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.

Phosphorylation and O-GlcNAcylation of the PHF-1 Epitope of Tau Protein Induce Local Conformational Changes of the C-Terminus and Modulate Tau Self-Assembly Into Fibrillar Aggregates

Phosphorylation of the neuronal microtubule-associated Tau protein plays a critical role in the aggregation process leading to the formation of insoluble intraneuronal fibrils within Alzheimer's disease (AD) brains. In recent years, other posttranslational modifications (PTMs) have been highlighted in the regulation of Tau (dys)functions. Among these PTMs, the O-β-linked N-acetylglucosaminylation (O-GlcNAcylation) modulates Tau phosphorylation and aggregation. We here focus on the role of the PHF-1 phospho-epitope of Tau C-terminal domain that is hyperphosphorylated in AD (at pS396/pS404) and encompasses S400 as the major O-GlcNAc site of Tau while two additional O-GlcNAc sites were found in the extreme C-terminus at S412 and S413. Using high resolution NMR spectroscopy, we showed that the O-GlcNAc glycosylation reduces phosphorylation of PHF-1 epitope by GSK3β alone or after priming by CDK2/cyclin A. Furthermore, investigations of the impact of PTMs on local conformation performed in small peptides highlight the role of S404 phosphorylation in inducing helical propensity in the region downstream pS404 that is exacerbated by other phosphorylations of PHF-1 epitope at S396 and S400, or O-GlcNAcylation of S400. Finally, the role of phosphorylation and O-GlcNAcylation of PHF-1 epitope was probed in in-vitro fibrillization assays in which O-GlcNAcylation slows down the rate of fibrillar assembly while GSK3β phosphorylation stimulates aggregation counteracting the effect of glycosylation.

Consequences of Dicarbonyl Stress on Skeletal Muscle Proteins in Type 2 Diabetes

Skeletal muscle is the largest organ in the body and constitutes almost 40% of body mass. It is also the primary site of insulin-mediated glucose uptake, and skeletal muscle insulin resistance, that is, diminished response to insulin, is characteristic of Type 2 diabetes (T2DM). One of the foremost reasons posited to explain the etiology of T2DM involves the modification of proteins by dicarbonyl stress due to an unbalanced metabolism and accumulations of dicarbonyl metabolites. The elevated concentration of dicarbonyl metabolites (i.e., glyoxal, methylglyoxal, 3-deoxyglucosone) leads to DNA and protein modifications, causing cell/tissue dysfunctions in several metabolic diseases such as T2DM and other age-associated diseases. In this review, we recapitulated reported effects of dicarbonyl stress on skeletal muscle and associated extracellular proteins with emphasis on the impact of T2DM on skeletal muscle and provided a brief introduction to the prevention/inhibition of dicarbonyl stress.

Protein kinase A controls the hexosamine pathway by tuning the feedback inhibition of GFAT-1

The hexosamine pathway (HP) is a key anabolic pathway whose product uridine 5'-diphospho-N-acetyl-D-glucosamine (UDP-GlcNAc) is an essential precursor for glycosylation processes in mammals. It modulates the ER stress response and HP activation extends lifespan in Caenorhabditis elegans. The highly conserved glutamine fructose-6-phosphate amidotransferase 1 (GFAT-1) is the rate-limiting HP enzyme. GFAT-1 activity is modulated by UDP-GlcNAc feedback inhibition and via phosphorylation by protein kinase A (PKA). Molecular consequences of GFAT-1 phosphorylation, however, remain poorly understood. Here, we identify the GFAT-1 R203H substitution that elevates UDP-GlcNAc levels in C. elegans. In human GFAT-1, the R203H substitution interferes with UDP-GlcNAc inhibition and with PKA-mediated Ser205 phosphorylation. Our data indicate that phosphorylation affects the interactions of the two GFAT-1 domains to control catalytic activity. Notably, Ser205 phosphorylation has two discernible effects: it lowers baseline GFAT-1 activity and abolishes UDP-GlcNAc feedback inhibition. PKA controls the HP by uncoupling the metabolic feedback loop of GFAT-1.

Protein O-GlcNAcylation alleviates small intestinal injury induced by ischemia-reperfusion and oxygen-glucose deprivation

Protein O-GlcNAcylation is a dynamic post-translational protein modification that regulates fundamental cellular functions in both normal physiology and diseases. The levels of protein O-GlcNAcylation are determined by flux of the hexosamine biosynthetic pathway (HBP), which is a branch of glycolysis, and are directly controlled by a pair of enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). An increase in protein O-GlcNAcylation has been shown to have protective effects on ischemia-related insults in the heart and brain. To determine whether O-GlcNAcylation plays a beneficial role in ischemia-reperfusion (IR)-induced intestinal injury, we used pharmacological manipulation of O-GlcNAc to induce loss- and gain-of-function conditions and evaluated the viability and apoptosis of intestinal epithelioid cells in an in vitro oxygen-glucose deprivation (OGD) model and tissue injury grade in a small intestinal ischemia-reperfusion (SIIR) mouse model. We found that 1) Upregulation of O-GlcNAcylation induced by glucosamine (GlcN, increase in HBP flux) or thiamet G (an OGA inhibitor) enhanced intestinal cell survival in the OGD model. In contrast, downregulation of O-GlcNAcylation induced by DON (due to a reduction in HBP flux) or OMSI-1 (an OGT inhibitor) made the cells more susceptible to hypoxia injury. 2) Reducing the increase in O-GlcNAcylation levels with a combination of either GlcN with DON or thiamet G with OMSI-1 partly canceled its protective effect on OGD-induced cell injury. 3) In the in vivo SIIR mouse model, GlcN augmented intestinal protein O-GlcNAcylation and significantly alleviated intestinal injury by inhibiting cell apoptosis. These results indicate that acute increases in protein O-GlcNAcylation confer protection against intestinal ischemia insults, suggesting that O-GlcNAcylation, as an endogenous stress sensor, could be a universal protective mechanism and could be a potential therapeutic target for intestinal ischemic disease.

Generation of an Interactome for the Tetratricopeptide Repeat Domain of O-GlcNAc Transferase Indicates a Role for the Enzyme in Intellectual Disability

The O-GlcNAc transferase (OGT) modifies nuclear and cytoplasmic proteins with β-N-acetyl-glucosamine (O-GlcNAc). With thousands of O-GlcNAc-modified proteins but only one OGT encoded in the mammalian genome, a prevailing question is how OGT selects its substrates. Prior work has indicated that the tetratricopeptide repeat (TPR) domain of OGT is involved in substrate selection. Furthermore, several variants of OGT causal for X-linked intellectual disability (XLID) occur in the TPR domain. Therefore, we adapted the BioID labeling method to identify interactors of a TPR-BirA* fusion protein in HeLa cells. We identified 115 interactors representing known and novel O-GlcNAc-modified proteins and OGT interactors (raw data deposited in MassIVE, Dataset ID MSV000085626). The interactors are enriched in known OGT processes (e.g., chromatin remodeling) as well as processes in which OGT has yet to be implicated (e.g., pre-mRNA processing). Importantly, the identified TPR interactors are linked to several disease states but most notably are enriched in pathologies featuring intellectual disability that may underlie the mechanism by which mutations in OGT lead to XLID. This interactome for the TPR domain of OGT serves as a jumping-off point for future research exploring the role of OGT, the TPR domain, and its protein interactors in multiple cellular processes and disease mechanisms, including intellectual disability.

Reprint of: Role of O-linked N-acetylglucosamine (O-GlcNAc) modification of proteins in diabetic cardiovascular complications

The post-translational modification of serine and threonine residues of proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates diverse cellular processes in the cardiovascular system. UDP-GlcNAc is a substrate for O-GlcNAc transferase, which catalyzes the attachment of O-GlcNAc to proteins. O-GlcNAcase catalyzes the removal of O-GlcNAc from proteins. UDP-GlcNAc is the end product of the hexosamine biosynthesis pathway, which is regulated primarily by glucose-6-phosphate-Glutamine:fructose-6-phosphate amidotransferase (GFAT). GFAT catalyzes the formation of glucosamine-6-phosphate from fructose-6-phosphate and glutamine. Whereas O-GlcNAc is essential for cell viability, sustained increases in O-GlcNAc levels have been implicated in the etiology of many chronic diseases and is associated with glucose toxicity and diabetic complications in various organs including the cardiovascular system. This review provides an overview of the regulation of protein O-GlcNAcylation followed by a discussion of potential mechanisms by which dysregulation in O-GlcNAc cycling contributes to the adverse effects of diabetes on the cardiovascular system.

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

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

Increased O-GlcNAcylation induces myocardial hypertrophy

Myocardial hypertrophy is a common precursor of many diseases, and it can lead to myocardial ischemia and weaken cardiac contractility. High-sugar diets and diabetes are high risk factors for cardiac hypertrophy. O-GlcNAcylation, a dynamic and ubiquitous post-translational glycosylation of proteins on serine/threonine residues, has been usually considered as a nutrient sensor. Hyperglycemia, hyperlipidemia, and hyperinsulinemia lead to an enhancement of protein O-GlcNAcylation; however, whether excessive O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation of proteins in cardiomyocytes causes cardiac hypertrophy remains unclear. In this study, we treated cultured primary cardiomyocytes or mice with streptozotocin (STZ) or PUGNAc, two inhibitors of O-GlcNAcase (OGA) to elevate cellular O-GlcNAcylation. We found that increased O-GlcNAcylation induced hypertrophy-like changes by detecting cardiomyocyte morphology or measuring the thickness of mice left ventricular wall with HE staining. The mRNA levels of cardiac hypertrophy-related genes, atrial natriuretic peptide (ANP) and β-myosin heavy chain (β-MHC), are increased in drug treatment groups. We further found that the increase of O-GlcNAcylation upregulated the activity of cAMP response element-binding protein (CREB) in cultured primary cells and in vivo by detecting the phosphorylation level of CREB by Western blot and the mRNA levels of CREB downstream targets C-fos and C-jun by RT-qPCR. These results suggest that the increased O-GlcNAcylation in cardiomyocytes is associated with cardiac hypertrophy both in cultured cells and in vivo, which provides possible intervention targets and approaches for the clinical treatment of myocardial hypertrophy triggered by high carbohydrate diets.

Role of O-linked N-acetylglucosamine (O-GlcNAc) modification of proteins in diabetic cardiovascular complications

The post-translational modification of serine and threonine residues of proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates diverse cellular processes in the cardiovascular system. UDP-GlcNAc is a substrate for O-GlcNAc transferase, which catalyzes the attachment of O-GlcNAc to proteins. O-GlcNAcase catalyzes the removal of O-GlcNAc from proteins. UDP-GlcNAc is the end product of the hexosamine biosynthesis pathway, which is regulated primarily by glucose-6-phosphate-Glutamine:fructose-6-phosphate amidotransferase (GFAT). GFAT catalyzes the formation of glucosamine-6-phosphate from fructose-6-phosphate and glutamine. Whereas O-GlcNAc is essential for cell viability, sustained increases in O-GlcNAc levels have been implicated in the etiology of many chronic diseases and is associated with glucose toxicity and diabetic complications in various organs including the cardiovascular system. This review provides an overview of the regulation of protein O-GlcNAcylation followed by a discussion of potential mechanisms by which dysregulation in O-GlcNAc cycling contributes to the adverse effects of diabetes on the cardiovascular system.

A Chemoenzymatic Method Based on Easily Accessible Enzymes for Profiling Protein O-GlcNAcylation

O-GlcNAcylation has gradually been recognized as a critically important protein post-translational modification in mammalian cells. Besides regulation of gene expression, its crosstalk with protein phosphorylation is vital for cell signaling. Despite its importance, comprehensive analysis of O-GlcNAcylation is extraordinarily challenging due to the low abundances of many O-GlcNAcylated proteins and the complexity of biological samples. Here, we developed a novel chemoenzymatic method based on a wild-type galactosyltransferase and uridine diphosphate galactose (UDP-Gal) for global and site-specific analysis of protein O-GlcNAcylation. This method integrates enzymatic reactions and hydrazide chemistry to enrich O-GlcNAcylated peptides. All reagents used are more easily accessible and cost-effective as compared to the engineered enzyme and click chemistry reagents. Biological triplicate experiments were performed to validate the effectiveness and the reproducibility of this method, and the results are comparable with the previous chemoenzymatic method using the engineered enzyme and click chemistry. Moreover, because of the promiscuity of the galactosyltransferase, 18 unique O-glucosylated peptides were identified on the EGF domain from nine proteins. Considering that effective and approachable methods are critical to advance glycoscience research, the current method without any sample restrictions can be widely applied for global analysis of protein O-GlcNAcylation in different samples.

Targeting Glycoproteins as a therapeutic strategy for diabetes mellitus and its complications

OBJECTIVES

Glycoproteins are organic compounds formed from proteins and carbohydrates, which are found in many parts of the living systems including the cell membranes. Furthermore, impaired metabolism of glycoprotein components plays the main role in the pathogenesis of diabetes mellitus. The aim of this study is to investigate the influence of glycoprotein levels in the treatment of diabetes mellitus.

METHODS

All relevant papers in the English language were compiled by searching electronic databases, including Scopus, PubMed and Cochrane library. The keywords of glycoprotein, diabetes mellitus, glycan, glycosylation, and inhibitor were searched until January 2019.

RESULTS

Glycoproteins are pivotal elements in the regulation of cell proliferation, growth, maturation and signaling pathways. Moreover, they are involved in drug binding, drug transportation, efflux of chemicals and stability of therapeutic proteins. These functions, structure, composition, linkages, biosynthesis, significance and biological effects are discussed as related to their use as a therapeutic strategy for the treatment of diabetes mellitus and its complications.

CONCLUSIONS

The findings revealed several chemical and natural compounds have significant beneficial effects on glycoprotein metabolism. The comprehension of glycoprotein structure and functions are very essential and inevitable to enhance the knowledge of glycoengineering for glycoprotein-based therapeutics as may be required for the treatment of diabetes mellitus and its associated complications. Graphical abstract.

Spatiotemporal gating of SIRT1 functions by O-GlcNAcylation is essential for liver metabolic switching and prevents hyperglycemia

Inefficient physiological transitions are known to cause metabolic disorders. Therefore, investigating mechanisms that constitute molecular switches in a central metabolic organ like the liver becomes crucial. Specifically, upstream mechanisms that control temporal engagement of transcription factors, which are essential to mediate physiological fed-fast-refed transitions are less understood. SIRT1, a NAD⁺-dependent deacetylase, is pivotal in regulating hepatic gene expression and has emerged as a key therapeutic target. Despite this, if/how nutrient inputs regulate SIRT1 interactions, stability, and therefore downstream functions are still unknown. Here, we establish nutrient-dependent O-GlcNAcylation of SIRT1, within its N-terminal domain, as a crucial determinant of hepatic functions. Our findings demonstrate that during a fasted-to-refed transition, glycosylation of SIRT1 modulates its interactions with various transcription factors and a nodal cytosolic kinase involved in insulin signaling. Moreover, sustained glycosylation in the fed state causes nuclear exclusion and cytosolic ubiquitin-mediated degradation of SIRT1. This mechanism exerts spatiotemporal control over SIRT1 functions by constituting a previously unknown molecular relay. Of note, loss of SIRT1 glycosylation discomposed these interactions resulting in aberrant gene expression, mitochondrial dysfunctions, and enhanced hepatic gluconeogenesis. Expression of nonglycosylatable SIRT1 in the liver abrogated metabolic flexibility, resulting in systemic insulin resistance, hyperglycemia, and hepatic inflammation, highlighting the physiological costs associated with its overactivation. Conversely, our study also reveals that hyperglycosylation of SIRT1 is associated with aging and high-fat-induced obesity. Thus, we establish that nutrient-dependent glycosylation of SIRT1 is essential to gate its functions and maintain physiological fitness.

Effect of Phosphorylation and O-GlcNAcylation on Proline-Rich Domains of Tau

The microtubule-associated protein Tau (MAPT) is a phosphoprotein in neurons of the brain. Aggregation of Tau is the leading cause of tauopathies such as Alzheimer's disease. Tau undergoes several post-translational modifications of which phosphorylation and O-GlcNAcylation are key chemical modifications. Tau aggregates into paired helical filaments and neurofibrillary tangles upon hyperphosphorylation, whereas O-GlcNAcylation stabilizes the soluble form of Tau. How specific phosphorylation and/or O-GlcNAcylation events influence Tau conformations remains largely unknown due to the disordered nature of Tau. In this study, we have investigated the phosphorylation- and O-GlcNAcylation-induced conformational effects on a Tau segment (Tau_225-246) from the proline-rich domain (P2), by performing metadynamics simulations. We study two different phosphorylation patterns: Tau_225-246, phosphorylated at T231 and S235, and Tau_225-246, phosphorylated at T231, S235, S237, and S238. We also study O-GlcNAcylation at T231 and S235. We find that phosphorylation leads to the formation of strong salt-bridge contacts with adjacent lysine and arginine residues, which disrupts the native β-sheet structure observed in Tau_225-246. We also observe the formation of a transient α-helix (²³⁸SAKSRLQ²⁴⁴) when Tau_225-246 is phosphorylated at four sites. In contrast, O-GlcNAcylation shows only modest structural effects, and the resultant structure resembles the native form of the peptide. Our studies suggest the opposing structural effects of both protein post-translational modifications (PTMs) and the importance of salt bridges in governing the conformational preferences upon phosphorylation, highlighting the role of proximal arginine and lysine upon hyperphosphorylation.

Loss of GFAT-1 feedback regulation activates the hexosamine pathway that modulates protein homeostasis

Glutamine fructose-6-phosphate amidotransferase (GFAT) is the key enzyme in the hexosamine pathway (HP) that produces uridine 5′-diphospho-N-acetyl-d-glucosamine (UDP-GlcNAc), linking energy metabolism with posttranslational protein glycosylation. In Caenorhabditis elegans, we previously identified gfat-1 gain-of-function mutations that elevate UDP-GlcNAc levels, improve protein homeostasis, and extend lifespan. GFAT is highly conserved, but the gain-of-function mechanism and its relevance in mammalian cells remained unclear. Here, we present the full-length crystal structure of human GFAT-1 in complex with various ligands and with important mutations. UDP-GlcNAc directly interacts with GFAT-1, inhibiting catalytic activity. The longevity-associated G451E variant shows drastically reduced sensitivity to UDP-GlcNAc inhibition in enzyme activity assays. Our structural and functional data point to a critical role of the interdomain linker in UDP-GlcNAc inhibition. In mammalian cells, the G451E variant potently activates the HP. Therefore, GFAT-1 gain-of-function through loss of feedback inhibition constitutes a potential target for the treatment of age-related proteinopathies.

Mutations in the hexosamine pathway key enzyme glutamine fructose-6-phosphate amidotransferase (GFAT-1) improve protein quality control and extend C. elegans lifespan. Here the authors present the crystal structures of full-length human GFAT-1 alone and with bound ligands and perform activity assays, which show that gain-of-function in the longevity-associated G451E variant is caused by a loss of feedback regulation.

A study on enhanced O-glycosylation strategy for improved production of recombinant human chorionic gonadotropin in Chinese hamster ovary cells

Human chorionic gonadotropin (hCG) is a glycoprotein hormone that exists as a heterodimer comprised of an α subunit and β subunit linked with disulfide bridges. The β subunit contains four O-glycosylation sites. Previous studies have found that the translation of mRNA to polypeptides of the β subunit was a severely limiting step for the expression of recombinant hCG protein in Chinese hamster ovary (CHO) cells. The effects of O-glycosylation on recombinant hCG protein expression were assessed by adding O-glycan precursors and overexpressing and knocking down key regulatory genes of O-glycan precursor synthesis and O-glycan sugar chain synthesis or hydrolases. The results indicated that O-glycosylation was indeed limiting in the expression of recombinant hCG protein, and N-acetylgalactosamine (GalNAc) was the major limiting precursor. Glutamine-fructose-6-phosphate transaminase 2 (Gfat2) and Uridine diphosphate-glucose pyrophosphorylase 2 (Ugp2), key regulatory genes of O-glycan precursor synthesis, were overexpressed. Ugp2 overexpression significantly increased the recombinant hCG protein level by 1.92 times compared to that of the control. The LC-MS/MS analysis and Phaseolus vulgaris leucoagglutinin (PHA-L) lectin blot analysis showed that Ugp2 overexpression significantly increased the total galactosylation levels of intracellular proteins and the O-glycosylation of recombinant hCG protein. The stability of the hCG protein to trypsin digestion was also enhanced. Ugp2 is the major limiting enzyme of the O-glycan precursor synthesis in recombinant hCG protein production. Furthermore, the effects and mechanisms of the key genes of O-glycan sugar chain synthesis and hydrolases such as polypeptide N-acetylgalactosaminyltransferase1 (Galnt1), Core 1 synthase, glycoprotein-N-acetylgalactosamine 3-beta-galactosyltransferase (C1galt1), O-linked N-acetylglucosamine transferase (Ogt) and Hexosaminidase (Hex), were evaluated. The results indicated that Galnt1 overexpression increased the recombinant hCG protein level by 1.57 times and improved the total galactosylation of intracellular proteins, O-glycosylation and the stability of recombinant hCG protein. Galnt1 is the major limiting enzyme of O-glycan sugar chain synthesis. Overexpression of Ugp2 and Galnt1 simultaneously improved the recombinant hCG protein level by 2.44 times, and both had synergistic effects. Based on the results of overexpression of Galnt1, the major limiting gene of O-Glycan chain synthesis, the precursors GalNAc and Gal were added and increased the recombinant hCG protein level by 3.68 times. This study revealed the major limiting factors of O-glycosylation of recombinant hCG protein in CHO cells and proposed an effective expression regulation strategy.

Detection and identification of O-GlcNAc-modified proteins using 6-azido-6-deoxy-N-acetyl-galactosamine

An unnatural monosaccharide with a C6-azide, Ac36AzGalNAc, has been developed as a potent and selective probe for O-GlcNAc-modified proteins. Combined with click chemistry, we demonstrate that Ac36AzGalNAc can robustly label O-GlcNAc glycosylation in a wide range of cell lines. Meanwhile, cell imaging and LC-MS/MS proteomics verify its selective activity on O-GlcNAc. More importantly, the protocol presented here provides a general methodology for tracking, capturing and identifying unnatural monosaccharide modified proteins in cells or cell lysates.

Curcumin Ameliorates Nonalcoholic Fatty Liver Disease through Inhibition of O-GlcNAcylation

The cause of progression to non-alcoholic fatty liver disease (NAFLD) is not fully understood. In the present study, we aimed to investigate how curcumin, a natural phytopolyphenol pigment, ameliorates NAFLD. Initially, we demonstrated that curcumin dramatically suppresses fat accumulation and hepatic injury induced in methionine and choline-deficient (MCD) diet mice. The severity of hepatic inflammation was alleviated by curcumin treatment. To identify the proteins involved in the pathogenesis of NAFLD, we also characterized the hepatic proteome in MCD diet mice. As a result of two-dimensional proteomic analysis, it was confirmed that thirteen proteins including antioxidant protein were differentially expressed in hepatic steatosis. However, the difference in expression was markedly improved by curcumin treatment. Interestingly, eight of the identified proteins are known to undergo O-GlcNAcylation modification. Thus, we further focused on elucidating how the regulation of O-linked β-N-acetylglucosamine (O-GlcNAc) modification is associated with the progression of hepatic steatosis leading to hepatitis in MCD diet mice. In parallel with lipid accumulation and inflammation, the MCD diet significantly up-regulated hexosamine biosynthetic pathway (HBP) and O-GlcNAc transferase (OGT) via ER stress. Curcumin treatment alleviates the severity of hepatic steatosis by relieving the dependence of O-GlcNAcylation on nuclear factor-κB (NF-κB) in inflammation signaling. Conversely, the expressions of superoxide dismutase 1 (SOD1) and SIRT1 were significantly upregulated by curcumin treatment. In conclusion, curcumin inhibits O-GlcNAcylation pathway, leading to antioxidant responses in non-alcoholic steatohepatitis (NASH) mice. Therefore, curcumin will be a promising therapeutic agent for diseases involving hyper-O-GlcNAcylation, including cancer.

Regulation of AβPP Glycosylation Modification and Roles of Glycosylation on AβPP Cleavage in Alzheimer's Disease

The presence of senile plaques in the gray matter of the brain is one of the major pathologic features of Alzheimer's disease (AD), and amyloid-β (Aβ) is the main component of extracellular deposits of the senile plaques. Aβ derives from amyloid-β precursor protein (AβPP) cleaved by β-secretase (BACE1) and γ-secretase, and the abnormal cleavage of AβPP is an important event leading to overproduction and aggregation of Aβ species. After translation, AβPP undergoes post-translational modifications (PTMs) including glycosylation and phosphorylation in the endoplasmic reticulum (ER) and Golgi apparatus, and these modifications play an important role in regulating the cleavage of this protein. In this Review, we summarize research progress on the modification of glycosylation, especially O-GlcNAcylation and mucin-type O-linked glycosylation (also known as O-GalNAcylation), on the regulation of AβPP cleavage and on the influence of AβPP's glycosylation in the pathogenesis of AD.

Diabetic retinopathy: a complex pathophysiology requiring novel therapeutic strategies

INTRODUCTION

Diabetic retinopathy (DR) is the leading cause of vision loss in the working age population of the developed world. DR encompasses a complex pathology, and one that is reflected in the variety of currently available treatments, which include laser photocoagulation, glucocorticoids, vitrectomy and agents which neutralize vascular endothelial growth factor (VEGF). Whilst these options demonstrate modest clinical benefits, none is yet to fully attenuate clinical progression or reverse damage to the retina. This has led to an interest in developing novel therapies for the condition, such as mediators of angiopoietin signaling axes, immunosuppressants, nonsteroidal anti-inflammatory drugs (NSAIDs), oxidative stress inhibitors and vitriol viscosity inhibitors. Further, preclinical research suggests that gene therapy treatment for DR could provide significant benefits over existing treatments options.

AREAS COVERED

Here we review the pathophysiology of DR and provide an overview of currently available treatments. We then outline recent advances made towards improved patient outcomes and highlight the potential of the gene therapy paradigm to revolutionize DR management.

EXPERT OPINION

Whilst significant progress has been made towards our understanding of DR, further research is required to enable the development of a detailed spatiotemporal model of the disease. In addition, we hope that improvements in our knowledge of the condition facilitate therapeutic innovations that continue to address unmet medical need and improve patient outcomes, with a focus on the development of targeted medicines.

Increased antitumor activities of glypican-3-specific chimeric antigen receptor-modified T cells by coexpression of a soluble PD1-CH3 fusion protein

Our recent clinical study demonstrated that glypican-3 (GPC3)-specific chimeric antigen receptor-modified T (CAR-T) cells are a promising treatment for hepatocellular carcinoma (HCC). However, the interaction of programmed cell death 1 (PD-1) and PD-L1-mediated T-cell inhibition is involved in immune evasion in a wide range of solid tumors, including HCC. To overcome this problem, we introduced a fusion protein composed of a PD-1 extracellular domain and CH3 from IgG4 into GPC3-specific CAR-T cells (GPC3-28Z) to block the PD-1/PD-L1 pathway. GPC3-specific CAR-T cells carrying the PD-1-CH3 fusion protein (sPD1) specifically recognized and lysed GPC3-positive HCC cells. The proliferation capacity of GPC3-28Z-sPD1 T cells after weekly stimulation with target cells was much higher than that of control GPC3-28Z T cells. Additionally, the coexpression of sPD1 could protect CAR-T cells from exhaustion when incubated with target cells, as phosphorylated AKT and Bcl-xL expression levels were higher in GPC3-28Z-sPD1 T cells than in GPC3-28Z cells. Importantly, in two HCC tumor xenograft models, GPC3-28Z-sPD1 T cells displayed a significantly higher tumor suppression capacity than GPC3-28Z T cells. In addition, an increased number of CD3+ T cells in the circulation and tumors and increased granzyme B levels and decreased Ki67 expression levels in the tumors were observed in the mice treated with GPC3-28Z-sPD1 T cells. Together, these data indicated that GPC3-specific CAR-T cells carrying sPD1 show promise as a treatment for patients with HCC.