Dysregulation of the nutrient/stress sensor O-GlcNAcylation is involved in the etiology of cardiovascular disorders, type-2 diabetes and Alzheimer's disease

Post-translational modifications of vertebrate striated muscle myosin heavy chains

Post-translational modifications (PTMs) play a crucial role in regulating the function of many sarcomeric proteins, including myosin. Myosins comprise a family of motor proteins that play fundamental roles in cell motility in general and muscle contraction in particular. A myosin molecule consists of two myosin heavy chains (MyHCs) and two pairs of myosin light chains (MLCs); two MLCs are associated with the neck region of each MyHC's N-terminal head domain, while the two MyHC C-terminal tails form a coiled-coil that polymerizes with other MyHCs to form the thick filament backbone. Myosin undergoes extensive PTMs, and dysregulation of these PTMs may lead to abnormal muscle function and contribute to the development of myopathies and cardiovascular disorders. Recent studies have uncovered the significance of PTMs in regulating MyHC function and showed how these PTMs may provide additional modulation of contractile processes. Here, we discuss MyHC PTMs that have been biochemically and/or functionally studied in mammals' and rodents' striated muscle. We have identified hotspots or specific regions in three isoforms of myosin (MYH2, MYH6, and MYH7) where the prevalence of PTMs is more frequent and could potentially play a significant role in fine-tuning the activity of these proteins.

Evaluating the impact of the hexosamine biosynthesis pathway and O-GlcNAcylation on glucose metabolism in bovine granulosa cells

Granulosa cells (GCs) of ovarian follicles prefer glucose as a metabolic substrate for growth and maturation. Disruption of glucose utilization via the hexosamine biosynthesis pathway (HBP) impairs O-linked N-acetylglucosaminylation (O-GlcNAcylation) and inhibits proliferation of bovine GCs of both small (3-5 mm) and large (>8.5 mm) antral follicles. Knowing that 2-5% of all glucose in cells is utilized via the HBP, the aim of this study was to characterize glucose metabolism in bovine GCs and determine the impact of the HBP and O-GlcNAcylation on metabolic activity. The GCs were initially cultured in serum-containing medium to confluency and then sub-cultured in serum-free medium in 96 well plates (n = 10 ovary pairs). The cells were exposed to vehicle and inhibitors of the HBP and O-GlcNAcylation for 24 h. Extracellular acidification rate (ECAR; an indicator of glycolysis) and oxygen consumption rate (OCR; an indicator of oxidative phosphorylation) of the GCs were measured using a Seahorse xFe96 Analyzer, including the implementation of glycolytic and mitochondrial stress tests. GCs from small antral follicles exhibited overall greater metabolic activity than GCs from large antral follicles as evidenced by increased ECAR and OCR. Inhibition of the HBP and O-GlcNAcylation had no effect on the metabolic activity of GCs from either type of follicle. The glycolytic stress test indicated that GCs from both types of follicles possessed additional glycolytic capacity; but again, inhibition of the HBP and O-GlcNAcylation did not affect this. Interestingly, inhibition of cellular respiration by 2-Deoxy-D-glucose impaired OCR only in GCs from small antral follicles, but exposure to the mitochondrial stress test had no effect. Conversely, in GCs from large antral follicles, oxidative metabolism was impaired by the mitochondrial stress test and was accompanied by a concomitant increase in glycolytic metabolism. Immunodetection of glycolytic enzymes revealed that phosphofructokinase expression is increased in GCs of small antral follicles compared to large follicles. Inhibition of O-GlcNAcylation impaired the expression of hexokinase only in GCs of small antral follicles. Inhibition of O-GlcNAcylation also impaired the expression of phosphofructokinase, pyruvate kinase and pyruvate dehydrogenase in GCs of both types of follicles, but had no effect on the expression of lactate dehydrogenase. The results indicate that GCs of small antral follicles possess greater aerobic glycolytic capacity than GCs from large antral follicles; but disruption of the HBP and O-GlcNAcylation has little to no impact on metabolic activity.

Diverse pathways to neuronal necroptosis in Alzheimer's disease

Necroptosis, or programmed necrosis, involves the kinase activity of receptor interacting kinases 1 and 3, the activation of the pseudokinase mixed lineage kinase domain-like and formation of a complex called the necrosome. It is one of the non-apoptotic cell death pathways that has gained interest in the recent years, especially as a neuronal cell death pathway occurring in Alzheimer's disease. In this review, we focus our discussion on the various molecular mechanisms that could trigger neuronal death through necroptosis and have been shown to play a role in Alzheimer's disease pathogenesis and neuroinflammation. We describe how each of these pathways, such as tumour necrosis factor signalling, reactive oxygen species, endosomal sorting complex, post-translational modifications and certain individual molecules, is dysregulated or activated in Alzheimer's disease, and how this dysregulation/activation could trigger necroptosis. At the cellular level, many of these molecular mechanisms and pathways may act in parallel to synergize with each other or inhibit one another, and changes in the balance between them may determine different cellular vulnerabilities at different disease stages. However, from a therapeutic standpoint, it remains unclear how best to target one or more of these pathways, given that such diverse pathways could all contribute to necroptotic cell death in Alzheimer's disease.

Inhibition of O-GlcNAcylation Decreases the Cytotoxic Function of Natural Killer Cells

Natural killer (NK) cells mediate killing of malignant and virus-infected cells, a property that is explored as a cell therapy approach in the clinic. Various cell intrinsic and extrinsic factors affect NK cell cytotoxic function, and an improved understanding of the mechanism regulating NK cell function is necessary to accomplish better success with NK cell therapeutics. Here, we explored the role of O-GlcNAcylation, a previously unexplored molecular mechanism regulating NK cell function. O-GlcNAcylation is a post-translational modification mediated by O-GlcNAc transferase (OGT) that adds the monosaccharide N-acetylglucosamine to serine and threonine residues on intracellular proteins and O-GlcNAcase (OGA) that removes the sugar. We found that stimulation of NK cells with the cytokines interleukin-2 (IL-2) and IL-15 results in enhanced O-GlcNAcylation of several cellular proteins. Chemical inhibition of O-GlcNAcylation using OSMI-1 was associated with a decreased expression of NK cell receptors (NKG2D, NKG2A, NKp44), cytokines [tumor necrosis factor (TNF)-α, interferon (IFN-γ)], granulysin, soluble Fas ligand, perforin, and granzyme B in NK cells. Importantly, inhibition of O-GlcNAcylation inhibited NK cell cytotoxicity against cancer cells. However, increases in O-GlcNAcylation following OGA inhibition using an OGA inhibitor or shRNA-mediated suppression did not alter NK cell cytotoxicity. Finally, we found that NK cells pretreated with OSMI-1 to inhibit O-GlcNAcylation showed compromised cytotoxic activity against tumor cells in vivo in a lymphoma xenograft mouse model. Overall, this study provides the seminal insight into the role of O-GlcNAcylation in regulating NK cell cytotoxic function.

The Glycobiology of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive pulmonary vascular disease of complex etiology. Cases of PAH that do not receive therapy after diagnosis have a low survival rate. Multiple reports have shown that idiopathic PAH, or IPAH, is associated with metabolic dysregulation including altered bioavailability of nitric oxide (NO) and dysregulated glucose metabolism. Multiple processes such as increased proliferation of pulmonary vascular cells, angiogenesis, apoptotic resistance, and vasoconstriction may be regulated by the metabolic changes demonstrated in PAH. Recent reports have underscored similarities between metabolic abnormalities in cancer and IPAH. In particular, increased glucose uptake and altered glucose utilization have been documented and have been linked to the aforementioned processes. We were the first to report a link between altered glucose metabolism and changes in glycosylation. Subsequent reports have highlighted similar findings, including a potential role for altered metabolism and aberrant glycosylation in IPAH pathogenesis. This review will detail research findings that demonstrate metabolic dysregulation in PAH with an emphasis on glycobiology. Furthermore, this report will illustrate the similarities in the pathobiology of PAH and cancer and highlight the novel findings that researchers have explored in the field.

An Overview of Glycosylation and its Impact on Cardiovascular Health and Disease

The cardiovascular system is a complex and well-organized system in which glycosylation plays a vital role. The heart and vascular wall cells are constituted by an array of specific receptors; most of them are N- glycosylated and mucin-type O-glycosylated. There are also intracellular signaling pathways regulated by different post-translational modifications, including O-GlcNAcylation, which promote adequate responses to extracellular stimuli and signaling transduction. Herein, we provide an overview of N-glycosylation and O-glycosylation, including O-GlcNAcylation, and their role at different levels such as reception of signal, signal transduction, and exogenous molecules or agonists, which stimulate the heart and vascular wall cells with effects in different conditions, like the physiological status, ischemia/reperfusion, exercise, or during low-grade inflammation in diabetes and aging. Furthermore, mutations of glycosyltransferases and receptors are associated with development of cardiovascular diseases. The knowledge on glycosylation and its effects could be considered biochemical markers and might be useful as a therapeutic tool to control cardiovascular diseases.

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

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

Fluctuation in O-GlcNAcylation inactivates STIM1 to reduce store-operated calcium ion entry via down-regulation of Ser621 phosphorylation

Stromal interaction molecule 1 (STIM1) plays a pivotal role in store-operated Ca2+ entry (SOCE), an essential mechanism in cellular calcium signaling and in maintaining cellular calcium balance. Because O-GlcNAcylation plays pivotal roles in various cellular function, we examined the effect of fluctuation in STIM1 O-GlcNAcylation on SOCE activity. We found that both increase and decrease in STIM1 O-GlcNAcylation impaired SOCE activity. To determine the molecular basis, we established STIM1-knockout HEK293 (STIM1-KO-HEK) cells using the CRISPR/Cas9 system and transfected STIM1 WT (STIM1-KO-WT-HEK), S621A (STIM1-KO-S621A-HEK), or T626A (STIM1-KO-T626A-HEK) cells. Using these cells, we examined the possible O-GlcNAcylation sites of STIM1 to determine whether the sites were O-GlcNAcylated. Co-immunoprecipitation analysis revealed that Ser621 and Thr626 were O-GlcNAcylated and that Thr626 was O-GlcNAcylated in the steady state but Ser621 was not. The SOCE activity in STIM1-KO-S621A-HEK and STIM1-KO-T626A-HEK cells was lower than that in STIM1-KO-WT-HEK cells because of reduced phosphorylation at Ser621 Treatment with the O-GlcNAcase inhibitor Thiamet G or O-GlcNAc transferase (OGT) transfection, which increases O-GlcNAcylation, reduced SOCE activity, whereas treatment with the OGT inhibitor ST045849 or siOGT transfection, which decreases O-GlcNAcylation, also reduced SOCE activity. Decrease in SOCE activity due to increase and decrease in O-GlcNAcylation was attributable to reduced phosphorylation at Ser621 These data suggest that both decrease in O-GlcNAcylation at Thr626 and increase in O-GlcNAcylation at Ser621 in STIM1 lead to impairment of SOCE activity through decrease in Ser621 phosphorylation. Targeting STIM1 O-GlcNAcylation could provide a promising treatment option for the related diseases, such as neurodegenerative diseases.

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.

MK-8719, a Novel and Selective O-GlcNAcase Inhibitor That Reduces the Formation of Pathological Tau and Ameliorates Neurodegeneration in a Mouse Model of Tauopathy

Deposition of hyperphosphorylated and aggregated tau protein in the central nervous system is characteristic of Alzheimer disease and other tauopathies. Tau is subject to O-linked N-acetylglucosamine (O-GlcNAc) modification, and O-GlcNAcylation of tau has been shown to influence tau phosphorylation and aggregation. Inhibition of O-GlcNAcase (OGA), the enzyme that removes O-GlcNAc moieties, is a novel strategy to attenuate the formation of pathologic tau. Here we described the in vitro and in vivo pharmacological properties of a novel and selective OGA inhibitor, MK-8719. In vitro, this compound is a potent inhibitor of the human OGA enzyme with comparable activity against the corresponding enzymes from mouse, rat, and dog. In vivo, oral administration of MK-8719 elevates brain and peripheral blood mononuclear cell O-GlcNAc levels in a dose-dependent manner. In addition, positron emission tomography imaging studies demonstrate robust target engagement of MK-8719 in the brains of rats and rTg4510 mice. In the rTg4510 mouse model of human tauopathy, MK-8719 significantly increases brain O-GlcNAc levels and reduces pathologic tau. The reduction in tau pathology in rTg4510 mice is accompanied by attenuation of brain atrophy, including reduction of forebrain volume loss as revealed by volumetric magnetic resonance imaging analysis. These findings suggest that OGA inhibition may reduce tau pathology in tauopathies. However, since hundreds of O-GlcNAcylated proteins may be influenced by OGA inhibition, it will be critical to understand the physiologic and toxicological consequences of chronic O-GlcNAc elevation in vivo. SIGNIFICANCE STATEMENT: MK-8719 is a novel, selective, and potent O-linked N-acetylglucosamine (O-GlcNAc)-ase (OGA) inhibitor that inhibits OGA enzyme activity across multiple species with comparable in vitro potency. In vivo, MK-8719 elevates brain O-GlcNAc levels, reduces pathological tau, and ameliorates brain atrophy in the rTg4510 mouse model of tauopathy. These findings indicate that OGA inhibition may be a promising therapeutic strategy for the treatment of Alzheimer disease and other tauopathies.

Regulation of pancreatic cancer TRAIL resistance by protein O-GlcNAcylation

TRAIL-activating therapy is promising in treating various cancers, including pancreatic cancer, a highly malignant neoplasm with poor prognosis. However, many pancreatic cancer cells are resistant to TRAIL-induced apoptosis despite their expression of intact death receptors (DRs). Protein O-GlcNAcylation is a versatile posttranslational modification that regulates various biological processes. Elevated protein O-GlcNAcylation has been recently linked to cancer cell growth and survival. In this study, we evaluated the role of protein O-GlcNAcylation in pancreatic cancer TRAIL resistance, and identified higher levels of O-GlcNAcylation in TRAIL-resistant pancreatic cancer cells. With gain- and loss-of-function of the O-GlcNAc-adding enzyme, O-GlcNActransferase (OGT), we determined that increasing O-GlcNAcylation rendered TRAIL-sensitive cells more resistant to TRA-8-induced apoptosis, while inhibiting O-GlcNAcylation promoted TRA-8-induced apoptosis in TRAIL-resistance cells. Furthermore, we demonstrated that OGT knockdown sensitized TRAIL-resistant cells to TRA-8 therapy in a mouse model in vivo. Mechanistic studies revealed direct O-GlcNAc modifications of DR5, which regulated TRA-8-induced DR5 oligomerization. We further defined that DR5 O-GlcNAcylation was independent of FADD, the adapter protein for the downstream death-inducing signaling. These studies have demonstrated an important role of protein O-GlcNAcylation in regulating TRAIL resistance of pancreatic cancer cells; and uncovered the contribution of O-GlcNAcylation to DR5 oligomerization and thus mediating DR-inducing signaling.

Succinylated Wheat Germ Agglutinin Colocalizes with the Toxoplasma gondii Cyst Wall Glycoprotein CST1

The glycosylated mucin domain of the Toxoplasma gondii cyst wall glycoprotein CST1 is heavily stained by Dolichos biflorus agglutinin, a lectin that binds to N-acetylgalactosamine. The cyst wall is also heavily stained by the chitin binding lectin succinylated wheat germ agglutinin (s-WGA), which selectively binds to N-acetylglucosamine-decorated structures. Here, we tracked the localization of N-acetylglucosamine-decorated structures that bind to s-WGA in immature and mature in vitro cysts. s-WGA localization was observed at the cyst periphery 6 h after the differentiation of the tachyzoite-stage parasitophorous vacuole. By day 1 and at all later times after differentiation, s-WGA was localized in a continuous staining pattern at the cyst wall. Coinciding with the maturation of the cyst matrix by day 3 of cyst development, s-WGA also localized in a continuous matrix pattern inside the cyst. s-WGA localized in both the outer and inner layer regions of the cyst wall and in a continuous matrix pattern inside mature 7- and 10-day-old cysts. In addition, s-WGA colocalized in the cyst wall with CST1, suggesting that N-acetylglucosamine- and N-acetylgalactosamine-decorated molecules colocalized in the cyst wall. In contrast to CST1, GRA4, and GRA6, the relative accumulation of the molecules that bind s-WGA in the cyst wall was not dependent on the expression of GRA2. Our results suggest that GRA2-dependent and GRA2-independent mechanisms regulate the trafficking and accumulation of glycosylated molecules that colocalize in the cyst wall.IMPORTANCE Chronic Toxoplasma gondii infection is maintained in the central nervous system by thick-walled cysts. If host immunity wanes, cysts recrudesce and cause severe and often lethal toxoplasmic encephalitis. Currently, there are no therapies to eliminate cysts, and little biological information is available regarding cyst structure(s). Here, we investigated cyst wall molecules recognized by succinylated wheat germ agglutinin (s-WGA), a lectin that specifically binds to N-acetylglucosamine-decorated structures. N-Acetylglucosamine regulates cell signaling and plays structural roles at the cell surface in many organisms. The cyst wall and cyst matrix were heavily stained by s-WGA in mature cysts and were differentially stained during cyst development. The relative accumulation of molecules that bind to s-WGA in the cyst wall was not dependent on the expression of GRA2. Our findings suggest that glycosylated cyst wall molecules gain access to the cyst wall via GRA2-dependent and GRA2-independent mechanisms and colocalize in the cyst wall.

Title: Human Serum/Plasma Glycoprotein Analysis by 1H-NMR, an Emerging Method of Inflammatory Assessment

Several studies suggest that variations in the concentration of plasma glycoproteins can influence cellular changes in a large number of diseases. In recent years, proton nuclear magnetic resonance (1H-NMR) has played a major role as an analytical tool for serum and plasma samples. In recent years, there is an increasing interest in the characterization of glycoproteins through 1H-NMR in order to search for reliable and robust biomarkers of disease. The objective of this review was to examine the existing studies in the literature related to the study of glycoproteins from an analytical and clinical point of view. There are currently several techniques to characterize circulating glycoproteins in serum or plasma, but in this review, we focus on 1H-NMR due to its great robustness and recent interest in its translation to the clinical setting. In fact, there is already a marker in H-NMR representing the acetyl groups of the glycoproteins, GlycA, which has been increasingly studied in clinical studies. A broad search of the literature was performed showing a general consensus that GlycA is a robust marker of systemic inflammation. The results also suggested that GlycA better captures systemic inflammation even more than C-reactive protein (CRP), a widely used classical inflammatory marker. The applications reviewed here demonstrated that GlycA was potentially a key biomarker in a wide range of diseases such as cancer, metabolic diseases, cardiovascular risk, and chronic inflammatory diseases among others. The profiling of glycoproteins through 1H-NMR launches an encouraging new paradigm for its future incorporation in clinical diagnosis.

O-GlcNAc Transferase Regulates Angiogenesis in Idiopathic Pulmonary Arterial Hypertension

Idiopathic pulmonary arterial hypertension (IPAH) is considered a vasculopathy characterized by elevated pulmonary vascular resistance due to vasoconstriction and/or lung remodeling such as plexiform lesions, the hallmark of the PAH, as well as cell proliferation and vascular and angiogenic dysfunction. The serine/threonine hydroxyl-linked N-Acetylglucosamine (O-GlcNAc) transferase (OGT) has been shown to drive pulmonary arterial smooth muscle cell (PASMC) proliferation in IPAH. OGT is a cellular nutrient sensor that is essential in maintaining proper cell function through the regulation of cell signaling, proliferation, and metabolism. The aim of this study was to determine the role of OGT and O-GlcNAc in vascular and angiogenic dysfunction in IPAH. Primary isolated human control and IPAH patient PASMCs and pulmonary arterial endothelial cells (PAECs) were grown in the presence or absence of OGT inhibitors and subjected to biochemical assessments in monolayer cultures and tube formation assays, in vitro vascular sprouting 3D spheroid co-culture models, and de novo vascularization models in NODSCID mice. We showed that knockdown of OGT resulted in reduced vascular endothelial growth factor (VEGF) expression in IPAH primary isolated vascular cells. In addition, specificity protein 1 (SP1), a known stimulator of VEGF expression, was shown to have higher O-GlcNAc levels in IPAH compared to control at physiological (5 mM) and high (25 mM) glucose concentrations, and knockdown resulted in decreased VEGF protein levels. Furthermore, human IPAH PAECs demonstrated a significantly higher degree of capillary tube-like structures and increased length compared to control PAECs. Addition of an OGT inhibitor, OSMI-1, significantly reduced the number of tube-like structures and tube length similar to control levels. Assessment of vascular sprouting from an in vitro 3D spheroid co-culture model using IPAH and control PAEC/PASMCs and an in vivo vascularization model using control and PAEC-embedded collagen implants demonstrated higher vascularization in IPAH compared to control. Blocking OGT activity in these experiments, however, altered the vascular sprouting and de novo vascularization in IPAH similar to control levels when compared to controls. Our findings in this report are the first to describe a role for the OGT/O-GlcNAc axis in modulating VEGF expression and vascularization in IPAH. These findings provide greater insight into the potential role that altered glucose uptake and metabolism may have on the angiogenic process and the development of plexiform lesions. Therefore, we believe that the OGT/O-GlcNAc axis may be a potential therapeutic target for treating the angiogenic dysregulation that is present in IPAH.

Mitochondria as central hub of the immune system

Nearly 130 years after the first insights into the existence of mitochondria, new rolesassociated with these organelles continue to emerge. As essential hubs that dictate cell fate, mitochondria integrate cell physiology, signaling pathways and metabolism. Thus, recent research has focused on understanding how these multifaceted functions can be used to improve inflammatory responses and prevent cellular dysfunction. Here, we describe the role of mitochondria on the development and function of immune cells, highlighting metabolic aspects and pointing out some metabolic- independent features of mitochondria that sustain cell function.

The Many Ways by Which O-GlcNAcylation May Orchestrate the Diversity of Complex Glycosylations

Unlike complex glycosylations, O-GlcNAcylation consists of the addition of a single N-acetylglucosamine unit to serine and threonine residues of target proteins, and is confined within the nucleocytoplasmic and mitochondrial compartments. Nevertheless, a number of clues tend to show that O-GlcNAcylation is a pivotal regulatory element of its complex counterparts. In this perspective, we gather the evidence reported to date regarding this connection. We propose different levels of regulation that encompass the competition for the nucleotide sugar UDP-GlcNAc, and that control the wide class of glycosylation enzymes via their expression, catalytic activity, and trafficking. We sought to better envision that nutrient fluxes control the elaboration of glycans, not only at the level of their structure composition, but also through sweet regulating actors.

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.

Characterization of glycopeptides using a stepped higher-energy C-trap dissociation approach on a hybrid quadrupole orbitrap

RATIONALE

Accurate characterization of glycopeptides without a prior glycan cleavage could provide valuable information on site-specific glycosylation, which is critical to reveal the biological functions of protein glycosylation. However, due to the distinct nature of oligosaccharides and ploypeptides, it is usually difficult to effectively fragment glycopeptides in mass spectrometry analysis.

METHODS

Here we applied a stepped normalized collisional energy (NCE) approach, which is able to combine fragment ions from three different collision energies, in a hybrid quadrupole orbitrap (Q Exactive Plus) to characterize glycopeptides. A systematic evaluation was firstly performed to find optimal NCE values for the fragmentation of glycan chains and peptide backbones from glycopeptides. Guided by the results of the systematic evaluation, the stepped NCE method was optimized and employed to analyze glycopeptides enriched from human serum.

RESULTS

The stepped NCE approach was found to effectively fragment both the glycan chains and peptide backbones from glycopeptides and record these fragments in a single MS/MS spectrum. In comparison with the regular HCD methods, the stepped NCE method identified more glycopeptides with higher scores from human serum samples.

CONCLUSIONS

Our studies demonstrate the capability of stepped NCE for the effective characterization of glycopeptides on a large scale.

'O-GlcNAc Code' Mediated Biological Functions of Downstream Proteins

As one of the post-translational modifications, O-linked β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) often occurs on serine (Ser) and threonine (Thr) residues of specific substrate cellular proteins via the addition of O-GlcNAc group by O-GlcNAc transferase (OGT). Maintenance of normal intracellular levels of O-GlcNAcylation is controlled by OGT and glycoside hydrolase O-GlcNAcase (OGA). Unbalanced O-GlcNAcylation levels have been involved in many diseases, including diabetes, cancer, and neurodegenerative disease. Recent research data reveal that O-GlcNAcylation at histones or non-histone proteins may provide recognition platforms for subsequent protein recruitment and further initiate intracellular biological processes. Here, we review the current understanding of the 'O-GlcNAc code' mediated intracellular biological functions of downstream proteins.

O-Glycosylation with O-linked β-N-acetylglucosamine increases vascular contraction: Possible modulatory role on Interleukin-10 signaling pathway

AIMS

The interleukin-10 (IL-10) is an immuno-regulatory cytokine that plays a protective effect in the vasculature. IL-10 binding to its receptor, activating the IL-10/JAK1/STAT3 cascade to exert its effects. Therefore, STAT3 phosphorylation is essential for IL-10 actions. O-Glycosylation with linked β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification able to regulate many proteins by interfering with protein on a phosphorylation level. Our aim was to determine whether O-GlcNAc promotes the inhibition of IL-10-pathway (JAK1/STAT3/IL-10), inactivationg its action in the vasculature.

MAIN METHODS

Mice (C57BL/6) aortic segments were incubated with vehicle or Thiamet G (0.1 mM, for 24 h) to increase global O-GlcNAc levels. Aortas from knockout mice for IL-10 were also used. Vascular reactivity and western blot tests were performed to evaluate protein expression.

KEY FINDINGS

High levels of O-GlcNAc, induced by Thiamet G incubation, increased vascular expression of JAK1, but decreased expression and activity of STAT3. In addition, IL-10 levels were diminished in arteries treated with Thiamet G. Absence of IL-10, as well as augmented O-GlcNAcylation, increased vascular reactivity to constrictor stimuli, an effect that was abolished by ERK 1/2 inhibitor. High levels of O-GlcNAc and the absence of IL-10 also leads to increased vascular expression of ERK1/2.

SIGNIFICANCE

Our data suggest that O-GlcNAc modification seems to (dys)regulate IL-10 signaling pathway and consequently, compromise the protective effect of this cytokine in vasculature. It is possible that there is a promising relationship in pathophysiological conditions where changes in O-GlcNAcylation and IL-10 levels are observed, such as hypertension and diabetes.

Mechanism for Decreased Gene Expression of β4-Galactosyltransferase 5 upon Differentiation of 3T3-L1 Mouse Preadipocytes to Adipocytes

Upon differentiation of cells, remarkable changes in the structures of glycans linked to lipids on cell surface have been observed. Lactosylceramide (Lac-Cer) serves as a common precursor for a series of glycosphingolipids with diverse structures. In the present study, we examined the underlying mechanism for the biosynthesis of Lac-Cer upon differentiation of 3T3-L1 mouse preadipocytes to adipocytes. TLC analysis showed that the amounts of Lac-Cer decrease in 3T3-L1 adipocytes compared to 3T3-L1 preadipocytes. In accordance with this change, the gene expression level of β4-galactosyltransferase (β4GalT) 5, which was identified as Lac-Cer synthase, decreased drastically upon differentiation of 3T3-L1 preadipocytes. The analysis of the transcriptional mechanism of the β4GalT5 gene demonstrated that the core promoter region is identified between nucleotides -299 and -1 relative to the translational start site. During adipocyte differentiation, the expression levels and promoter activities of the β4GalT5 gene decreased dramatically. Since the Specificity protein 1 (Sp1)-binding sites in the promoter region were critical for the promoter activity, it is suggested that Sp1 plays an important role for the expression of the β4GalT5 gene in 3T3-L1 cells. The gene and protein expression of Sp1 decreased significantly upon differentiation of 3T3-L1 preadipocytes. Taken together, the present study suggest that the expression of the β4GalT5 gene decreases through reduced expression of the Sp1 gene and protein upon differentiation of 3T3-L1 peradipocytes to adipocytes, which may lead to the decreased amounts of Lac-Cer in 3T3-L1 adipocytes.

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.

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

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

Skeletal muscle O-GlcNAc transferase is important for muscle energy homeostasis and whole-body insulin sensitivity

Objective

Given that cellular O-GlcNAcylation levels are thought to be real-time measures of cellular nutrient status and dysregulated O-GlcNAc signaling is associated with insulin resistance, we evaluated the role of O-GlcNAc transferase (OGT), the enzyme that mediates O-GlcNAcylation, in skeletal muscle.

Methods

We assessed O-GlcNAcylation levels in skeletal muscle from obese, type 2 diabetic people, and we characterized muscle-specific OGT knockout (mKO) mice in metabolic cages and measured energy expenditure and substrate utilization pattern using indirect calorimetry. Whole body insulin sensitivity was assessed using the hyperinsulinemic euglycemic clamp technique and tissue-specific glucose uptake was subsequently evaluated. Tissues were used for histology, qPCR, Western blot, co-immunoprecipitation, and chromatin immunoprecipitation analyses.

Results

We found elevated levels of O-GlcNAc-modified proteins in obese, type 2 diabetic people compared with well-matched obese and lean controls. Muscle-specific OGT knockout mice were lean, and whole body energy expenditure and insulin sensitivity were increased in these mice, consistent with enhanced glucose uptake and elevated glycolytic enzyme activities in skeletal muscle. Moreover, enhanced glucose uptake was also observed in white adipose tissue that was browner than that of WT mice. Interestingly, mKO mice had elevated mRNA levels of Il15 in skeletal muscle and increased circulating IL-15 levels. We found that OGT in muscle mediates transcriptional repression of Il15 by O-GlcNAcylating Enhancer of Zeste Homolog 2 (EZH2).

Conclusions

Elevated muscle O-GlcNAc levels paralleled insulin resistance and type 2 diabetes in humans. Moreover, OGT-mediated signaling is necessary for proper skeletal muscle metabolism and whole-body energy homeostasis, and our data highlight O-GlcNAcylation as a potential target for ameliorating metabolic disorders.

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Type 2 diabetic humans have elevated O-GlcNAc levels in skeletal muscle.

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Knockout of OGT in muscle elevates whole body insulin sensitivity.

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Knockout of OGT in muscle increases resistance to diet-induced obesity.

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Muscle-specific OGT knockout mice have elevated plasma IL-15 levels.

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OGT in muscle controls Il15 expression by O-GlcNAcylation and inhibition of EZH2.

Apart From Rhoptries, Identification of Toxoplasma gondii's O-GlcNAcylated Proteins Reinforces the Universality of the O-GlcNAcome

O-linked β-N-acetylglucosaminylation or O-GlcNAcylation is a widespread post-translational modification that belongs to the large and heterogeneous group of glycosylations. The functions managed by O-GlcNAcylation are diverse and include regulation of transcription, replication, protein's fate, trafficking, and signaling. More and more evidences tend to show that deregulations in the homeostasis of O-GlcNAcylation are involved in the etiology of metabolic diseases, cancers and neuropathologies. O-GlcNAc transferase or OGT is the enzyme that transfers the N-acetylglucosamine residue onto target proteins confined within the cytosolic and nuclear compartments. A form of OGT was predicted for Toxoplasma and recently we were the first to show evidence of O-GlcNAcylation in the apicomplexans Toxoplasma gondii and Plasmodium falciparum. Numerous studies have explored the O-GlcNAcome in a wide variety of biological models but very few focus on protists. In the present work, we used enrichment on sWGA-beads and immunopurification to identify putative O-GlcNAcylated proteins in Toxoplasma gondii. Many of the proteins found to be O-GlcNAcylated were originally described in higher eukaryotes and participate in cell shape organization, response to stress, protein synthesis and metabolism. In a more original way, our proteomic analyses, confirmed by sWGA-enrichment and click-chemistry, revealed that rhoptries, proteins necessary for invasion, are glycosylated. Together, these data show that regardless of proteins strictly specific to organisms, O-GlcNAcylated proteins are rather similar among living beings.

Intracellular Imaging of Protein-Specific Glycosylation

Posttranslational protein glycosylation is conserved in all kingdoms of life and implicated in the regulation of protein structure, function, and localization. The visualization of glycosylation states of designated proteins within living cells is of great importance for unraveling the biological roles of intracellular protein glycosylation. Our generally applicable approach is based on the incorporation of a glucosamine analog, Ac₄GlcNCyoc, into the cellular glycome via metabolic engineering. Ac₄GlcNCyoc can be labeled in a second step via inverse-electron-demand Diels-Alder chemistry with fluorophores inside living cells. Additionally, target proteins can be expressed as enhanced green fluorescent protein (EGFP)-fusion proteins. To assess the proximity of the donor EGFP and the glycan-anchored acceptor fluorophore, Förster resonance energy transfer (FRET) is employed and read out with high contrast by fluorescence lifetime imaging (FLIM) microscopy. In this chapter, we present a detailed description of methods required to perform protein-specific imaging of glycosylation inside living cells. These include the complete synthesis of Ac₄GlcNCyoc, immunoprecipitation of EGFP-fusion proteins to examine the Ac₄GlcNCyoc modification state, and a complete section on basics, performance, as well as data analysis for FLIM-FRET microscopy. We also provide useful notes necessary for reproducibility and point out strengths and limitations of the approach.

MPI depletion enhances O-GlcNAcylation of p53 and suppresses the Warburg effect

Rapid cellular proliferation in early development and cancer depends on glucose metabolism to fuel macromolecule biosynthesis. Metabolic enzymes are presumed regulators of this glycolysis-driven metabolic program, known as the Warburg effect; however, few have been identified. We uncover a previously unappreciated role for Mannose phosphate isomerase (MPI) as a metabolic enzyme required to maintain Warburg metabolism in zebrafish embryos and in both primary and malignant mammalian cells. The functional consequences of MPI loss are striking: glycolysis is blocked and cells die. These phenotypes are caused by induction of p53 and accumulation of the glycolytic intermediate fructose 6-phosphate, leading to engagement of the hexosamine biosynthetic pathway (HBP), increased O-GlcNAcylation, and p53 stabilization. Inhibiting the HBP through genetic and chemical methods reverses p53 stabilization and rescues the Mpi-deficient phenotype. This work provides mechanistic evidence by which MPI loss induces p53, and identifies MPI as a novel regulator of p53 and Warburg metabolism.

DOI:http://dx.doi.org/10.7554/eLife.22477.001

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

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

Cyclopropenes: a new tool for the study of biological systems

Cyclopropenes have become an important mini-tag tool in chemical biology, participating in fast inverse electron demand Diels–Alder and photoclick reactions in biological settings.

The Inhibitory Role of α2,6-Sialylation in Adipogenesis

Adipose tissue plays critical roles in obesity and related diseases such as diabetes and cardiovascular diseases. Previous reports suggest that glycans, the most common posttranslational modifications, are involved in obesity-related diseases, but what type of glycan regulates adipogenesis during obesity remains unclear. In this study, we first quantified the mRNA levels of 167 genes (encoding 144 glycosyltransferases and 23 related enzymes) in visceral adipose tissues (VATs) from control mice and high-fat diet (HFD)-induced obese mice. We found that a gene encoding β-galactoside α2,6-sialyltransferase-1 (St6gal1), a key enzyme responsible for the biosynthesis of α2,6-linked sialic acid in N-linked glycans, was most down-regulated in VATs from obese mice. We confirmed the reduction in α2,6-sialic acid in VATs from obese mice and differentiated adipocyte model 3T3-L1 cells. Using proteomic analysis, integrin-β1 was identified as one of the target α2,6-sialylated proteins in adipose tissues, and phosphorylation of its downstream molecule focal adhesion kinase was found to be decreased after HFD feeding. St6gal1 overexpression in differentiating 3T3-L1 cells inhibited adipogenesis with increased phosphorylation of focal adhesion kinase. Furthermore, St6gal1 knockout mice exhibited increased bodyweight and VAT weight after HFD feeding. The down-regulation of St6gal1 during adipogenesis was canceled by treatment with a DNA methyltransferase inhibitor, suggesting an involvement of epigenetic DNA methylation in St6gal1 silencing. Our findings suggest that ST6GAL1 has an inhibitory role in adipogenesis through integrin-β1 activation, providing new insights into the roles and regulation mechanisms of glycans in adipocytes during obesity.

O-GlcNAcylation of the human epidermal growth factor receptor

The reversible O-linked attachment of single β-D-N-acetylglucosamine (GlcNAc) moieties to serine/threonine residues in target proteins is a frequently occurring post-translational modification affecting the functionality of many cellular systems. In this report we present experimental evidence suggesting that the epidermal growth factor receptor (EGFR) is subjected to O-GlcNAcylation in human carcinoma epidermoid A431 cells and human lung carcinoma A549 cells. However, no signal was detected in human cervix adenocarcinoma HeLa cells or in mouse EGFR-T17 fibroblasts ectopically expressing the human EGFR. We detected a positive O-GlcNAcylation signal in the immunoprecipitated EGFR by Western blotting using two distinct specific anti-O-GlcNAc antibodies even after N-deglycosylation of the receptor using peptide-N-glycosidase F (PNGase F). Conversely, the presence of EGFR was detected by Western blotting using an anti-EGFR antibody in the immunocomplex of O-GlcNAcylated proteins immunoprecipitated with an anti-O-GlcNAc antibody. These signals were enhanced when the O-linked β-N-acetylglucosaminidase (OGA) inhibitor Thiamet G was added to prevent the deglycosylation of the GlcNAc moiety(ies). Moreover, we also detected a positive signal in the immunoprecipitated and N-deglycosylated EGFR using PNGase F, and tunicamycin when the cells were metabolically labeled with azido-GlcNAc (GlcNAz), biotinylated and probed with a streptavidin-labeled peroxidase. Finally, EGFR and O-linked β-N-acetylglucosamine transferase (OGT) co-immunoprecipitate, and incubation of the immunoprecipitated EGFR with the immunoprecipitated OGT in the presence of uridine 5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) resulted in a significant enhancement of the EGFR O-GlcNAcylation signal as detected by Western blotting using an anti-O-GlcNAc antibody. We conclude that the human EGFR is subjected to O-GlcNAcylation in the A431 and A549 tumor cell lines.

O-GlcNAcylation and the Metabolic Shift in High-Proliferating Cells: All the Evidence Suggests that Sugars Dictate the Flux of Lipid Biogenesis in Tumor Processes

Cancer cells are characterized by their high capability to proliferate. This imposes an accelerated biosynthesis of membrane compounds to respond to the need for increasing the membrane surface of dividing cells and remodeling the structure of lipid microdomains. Recently, attention has been paid to the upregulation of O-GlcNAcylation processes observed in cancer cells. Although O-GlcNAcylation of lipogenic transcriptional regulators is described in the literature (e.g., FXR, LXR, ChREBP), little is known about the regulation of the enzymes that drive lipogenesis: acetyl co-enzyme A carboxylase and fatty acid synthase (FAS). The expression and catalytic activity of both FAS and O-GlcNAc transferase (OGT) are high in cancer cells but the reciprocal regulation of the two enzymes remains unexplored. In this perspective, we collected data linking FAS and OGT and, in so doing, pave the way for the exploration of the intricate functions of these two actors that play a central role in tumor growth.

Silencing the Nucleocytoplasmic O-GlcNAc Transferase Reduces Proliferation, Adhesion, and Migration of Cancer and Fetal Human Colon Cell Lines

The post-translational modification of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) is regulated by a unique couple of enzymes. O-GlcNAc transferase (OGT) transfers the GlcNAc residue from UDP-GlcNAc, the final product of the hexosamine biosynthetic pathway (HBP), whereas O-GlcNAcase (OGA) removes it. This study and others show that OGT and O-GlcNAcylation levels are increased in cancer cell lines. In that context, we studied the effect of OGT silencing in the colon cancer cell lines HT29 and HCT116 and the primary colon cell line CCD841CoN. Herein, we report that OGT silencing diminished proliferation, in vitro cell survival and adhesion of primary and cancer cell lines. SiOGT dramatically decreased HT29 and CCD841CoN migration, CCD841CoN harboring high capabilities of migration in Boyden chamber system when compared to HT29 and HCT116. The expression levels of actin and tubulin were unaffected by OGT knockdown but siOGT seemed to disorganize microfilament, microtubule, and vinculin networks in CCD841CoN. While cancer cell lines harbor higher levels of OGT and O-GlcNAcylation to fulfill their proliferative and migratory properties, in agreement with their higher consumption of HBP main substrates glucose and glutamine, our data demonstrate that OGT expression is not only necessary for the biological properties of cancer cell lines but also for normal cells.

PUGNAc induces protein ubiquitination in C2C12 myotube cells

O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) regulates many cellular processes including the cell cycle, cell signaling, and protein trafficking. Dysregulation of O-GlcNAcylation may be involved in the development of insulin resistance and type 2 diabetes. Therefore, it is necessary to identify cellular proteins that are induced by elevated O-GlcNAcylation. Here, using adenosine 5'-triphosphate affinity chromatography, we employed a proteomic approach in order to identify differentially expressed proteins in response to treatment with the O-GlcNAcase inhibitor, O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc), in mouse C2C12 myotube cells. Among 205 selected genes, we identified 68 nucleotide-binding proteins, 14 proteins that have adenosinetriphosphatase activity, and 10 proteins with ligase activity. Upregulation of proteins, including ubiquitin-activating enzyme E1, proteasome subunit 20S, cullin-associated NEDD8-dissociated protein 1, ezrin, and downregulation of the protein nucleoside diphosphate kinase B, were confirmed by western blot analysis. In particular, we found that the protein ubiquitination level in C2C12 cells was increased by PUGNAc treatment. This is the first report of quantitative proteomic profiles of myotube cells after treatment with PUGNAc, and our results demonstrate the potential to enhance understanding of the relationship between insulin resistance, O-GlcNAc, and PUGNAc in the future.

OGT-mediated O-GlcNAcylation promotes NF-κB activation and inflammation in acute pancreatitis

OBJECTIVE

Activation of the transcription factor κB (NF-κB) and secretion of pro-inflammatory mediators are major events in acute pancreatitis (AP). Recently, O-linked-N-acetylglucosamine (O-GlcNAc) modification, one type of posttranslational modifications, reportedly attunes NF-κB function. However, the expression of O-GlcNAc transferase (OGT), the enzyme responsible for O-GlcNAcylation of proteins, in AP, and the possible contribution of OGT-mediated O-GlcNAcylation to the NF-κB inflammatory activation in pancreatic acinar cells and to the AP progression have not been understood. This study focused on the effects and mechanisms of OGT-mediated O-GlcNAcylation during AP.

METHODS

An AP cell model was established with the caerulein-stimulated AR42 J rat pancreatic acinar cells. The secretion of pro-inflammatory cytokines TNF-α was detected by ELISA kits, and the production of NO was determined using the colorimetric Griess reaction. Expression of OGT was measured by RT-PCR and Western blot. Expression levels of RL2, phosphorylation of p65, total p65, IKKα were detected by Western blot. The NF-κB activity was evaluated by luciferase reporter gene assay. To determine the biological functions of OGT in caerulein-induced inflammatory response, RNA interference and PUGNAc, the inhibitor of O-GlcNAcase (OGA) was employed to regulate OGT expression in AR42 J cells.

RESULTS

Caerulein significantly up-regulated the expression of OGT, and increased the global protein O-GlcNAcylation level in AR42 J cells. Reduction of OGT by small interfering RNA (siRNA) inhibited caerulein-triggered inflammation, assessed by the production of pro-inflammatory mediators (TNF-α and NO). We also demonstrated that O-GlcNAcylation directly modified the NF-κB p65 subunit and its upstream activating kinases IKKα in AR42 J cells. Lowering O-GlcNAcylation by OGT knockdown attenuated p65 activating phosphorylation, nuclear translocation, NF-κB transcriptional activity and levels of NF-κB transcriptional targets TNF-α and NO; on the contrary, elevating O-GlcNAc through PUGNAc increased IKKα and p65 O-GlcNAcylation accompanied by increased p65 phosphorylation, activity and levels of TNF-α and NO in caerulein-treated cells.

CONCLUSIONS

Our results demonstrate for the first time that OGT-mediated O-GlcNAcylation promotes NF-κB signaling activation and inflammation in pancreatic acinar cells, which might promote the progression of AP.

You are what you eat: O-linked N-acetylglucosamine in disease, development and epigenetics

PURPOSE OF REVIEW

The O-linked N-acetylglucosamine (O-GlcNAc) modification is both responsive to nutrient availability and capable of altering intracellular cellular signalling. We summarize data defining a role for O-GlcNAcylation in metabolic homeostasis and epigenetic regulation of development in the intrauterine environment.

RECENT FINDINGS

O-GlcNAc transferase (OGT) catalyzes nutrient-driven O-GlcNAc addition and is subject to random X-inactivation. OGT plays key roles in growth factor signalling, stem cell biology, epigenetics and possibly imprinting. The O-GlcNAcase, which removes O-GlcNAc, is subject to tight regulation by higher order chromatin structure. O-GlcNAc cycling plays an important role in the intrauterine environment wherein OGT expression is an important biomarker of placental stress.

SUMMARY

Regulation of O-GlcNAc cycling by X-inactivation, epigenetic regulation and nutrient-driven processes makes it an ideal candidate for a nutrient-dependent epigenetic regulator of human disease. In addition, O-GlcNAc cycling influences chromatin modifiers critical to the regulation and timing of normal development including the polycomb repression complex and the ten-eleven translocation proteins mediating DNA methyl cytosine demethylation. The pathway also impacts the hypothalamic-pituitary-adrenal axis critical to intrauterine programming influencing disease susceptibility in later life.

Regulatory O-GlcNAcylation sites on FoxO1 are yet to be identified

O-GlcNAcylation is a reversible post-translational modification that regulates cytosolic and nuclear proteins. We and others previously demonstrated that FoxO1 is O-GlcNAcylated in different cell types, resulting in an increase in its transcriptional activity. Four O-GlcNAcylation sites were identified in human FOXO1 but directed mutagenesis of each site individually had modest (T317) or no effect (S550, T648, S654) on its O-GlcNAcylation status and transcriptional activity. Moreover, the consequences of mutating all four sites had not been investigated. In the present work, we mutated these sites in the mouse Foxo1 and found that mutation of all four sites did not decrease Foxo1 O-GlcNAcylation status and transcriptional activity, and would even tend to increase them. In an attempt to identify other O-GlcNAcylation sites, we immunoprecipitated wild-type O-GlcNAcylated Foxo1 and analysed the tryptic digest peptides by mass spectrometry using High-energy Collisional Dissociation. We identified T646 as a new O-GlcNAcylation site on Foxo1. However, site directed mutagenesis of this site individually or together with all four previously identified residues did not impair Foxo1 O-GlcNAcylation and transcriptional activity. These results suggest that residues important for the control of Foxo1 activity by O-GlcNAcylation still remain to be identified.

N-glycomic changes in serum proteins in type 2 diabetes mellitus correlate with complications and with metabolic syndrome parameters

Background

Glycosylation, i.e the enzymatic addition of oligosaccharides (or glycans) to proteins and lipids, known as glycosylation, is one of the most common co-/posttranslational modifications of proteins. Many important biological roles of glycoproteins are modulated by N-linked oligosaccharides. As glucose levels can affect the pathways leading to glycosylation of proteins, we investigated whether metabolic syndrome (MS) and type 2 diabetes mellitus (T2DM), pathological conditions characterized by altered glucose levels, are associated with specific modifications in serum N-glycome.

Methods

We enrolled in the study 562 patients with Type 2 Diabetes Mellitus (T2DM) (mean age 65.6±8.2 years) and 599 healthy control subjects (CTRs) (mean age, 58.5±12.4 years). N-glycome was evaluated in serum glycoproteins.

Results

We found significant changes in N-glycan composition in the sera of T2DM patients. In particular, α(1,6)-linked arm monogalactosylated, core-fucosylated diantennary N-glycans (NG1(6)A2F) were significantly reduced in T2DM compared with CTR subjects. Importantly, they were equally reduced in diabetic patients with and without complications (P<0.001) compared with CTRs. Macro vascular-complications were found to be related with decreased levels of NG1(6)A2F. In addition, NG1(6)A2F and NG1(3)A2F, identifying, respectively, monogalactosylated N-glycans with α(1,6)- and α(1,3)-antennary galactosylation, resulted strongly correlated with most MS parameters. The plasmatic levels of these two glycans were lower in T2DM as compared to healthy controls, and even lower in patients with complications and MS, that is the extreme “unhealthy” phenotype (T2DM+ with MS).

Conclusions

Imbalance of glycosyltransferases, glycosidases and sugar nucleotide donor levels is able to cause the structural changes evidenced by our findings. Serum N-glycan profiles are thus sensitive to the presence of diabetes and MS. Serum N-glycan levels could therefore provide a non-invasive alternative marker for T2DM and MS.

O-GlcNAcylation, contractile protein modifications and calcium affinity in skeletal muscle

O-GlcNAcylation, a generally undermined atypical protein glycosylation process, is involved in a dynamic and highly regulated interplay with phosphorylation. Akin to phosphorylation, O-GlcNAcylation is also involved in the physiopathology of several acquired diseases, such as muscle insulin resistance or muscle atrophy. Recent data underline that the interplay between phosphorylation and O-GlcNAcylation acts as a modulator of skeletal muscle contractile activity. In particular, the O-GlcNAcylation level of the phosphoprotein myosin light chain 2 seems to be crucial in the modulation of the calcium activation properties, and should be responsible for changes in calcium properties observed in functional atrophy. Moreover, since several key structural proteins are O-GlcNAc-modified, and because of the localization of the enzymes involved in the O-GlcNAcylation/de-O-GlcNAcylation process to the nodal Z disk, a role of O-GlcNAcylation in the modulation of the sarcomeric structure should be considered.

Deficient hippocampal insulin signaling and augmented Tau phosphorylation is related to obesity- and age-induced peripheral insulin resistance: a study in Zucker rats

Background

Insulin signaling and Tau protein phosphorylation in the hippocampi of young and old obese Zucker fa/fa rats and their lean controls were assessed to determine whether obesity-induced peripheral insulin resistance and aging are risk factors for central insulin resistance and whether central insulin resistance is related to the pathologic phosphorylation of the Tau protein.

Results

Aging and obesity significantly attenuated the phosphorylation of the insulin cascade kinases Akt (protein kinase B, PKB) and GSK-3β (glycogen synthase kinase 3β) in the hippocampi of the fa/fa rats. Furthermore, the hyperphosphorylation of Tau Ser396 alone and both Tau Ser396 and Thr231 was significantly augmented by aging and obesity, respectively, in the hippocampi of these rats.

Conclusions

Both age-induced and obesity-induced peripheral insulin resistance are associated with central insulin resistance that is linked to hyperTau phosphorylation. Peripheral hyperinsulinemia, rather than hyperglycemia, appears to promote central insulin resistance and the Tau pathology in fa/fa rats.

[Protein O-GlcNAcylation and regulation of cell signalling: involvement in pathophysiology]

O-GlcNAcylation corresponds to the addition of N-acetyl glucosamine (GlcNAc) on serine or threonine residues of cytosolic and nuclear proteins. This reversible post-translational modification regulates protein phosphorylation, sub-cellular localisation, stability and activity. Only two enzymes, OGT (O-linked N-acetyl-glucosaminyltransferase) and OGA (O-linked N-acetyl-β-D glucosaminidase), control the addition and removal of GlcNAc from more than a thousand of proteins. Alternative splicing generates different isoforms of OGT and OGA, and address these enzymes to different sub-cellular compartments (mitochondria, cytosol...), restraining their action to specific subsets of substrates. Moreover, interaction with adaptor proteins may also help address these enzymes to specific substrates. Alterations in protein O-GlcNAcylation have been observed in a number of important human diseases, such as Alzheimer, cancer and diabetes. A reciprocal relationship between Tau protein phosphorylation and O-GlcNAcylation has been observed, and decreased O-GlcNAcylation in the brain of patients with Alzheimer diseases may favour Tau aggregation, destabilisation of microtubules and neuronal alterations. Alterations in OGT/OGA expression levels, and in protein O-GlcNAcylation, have been described in different types of cancer, and much evidence indicates that O-GlcNAcylation may participate in abnormal proliferation and migration of cancer cells. O-GlcNAcylation of transcription factors and signalling effectors may also participate in defects observed in diabetes. Indeed, in situation of chronic hyperglycaemia, abnormal O-GlcNAcylation may have deleterious effect on insulin secretion and action, resulting in further impairment of glucose homeostasis. Therefore, O-GlcNAcylation appears to be a major regulator of cellular activities and may play an important part in different human diseases. However, because of the large spectrum of OGT and OGA substrates, targeting O-GlcNAc for treatment of these diseases will be a highly challenging task.