Implications of the O-GlcNAc modification in the regulation of nuclear apoptosis in T cells

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

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

Naegleria fowleri Induces Jurkat T Cell Death via O-deGlcNAcylation

The pathogenic free-living amoeba Naegleria fowleri causes primary amoebic meningoencephalitis, a fatal infection, by penetrating the nasal mucosa and migrating to the brain via the olfactory nerves. N. fowleri can induce host cell death via lytic necrosis. Similar to phosphorylation, O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) is involved in various cell-signaling processes, including apoptosis and proliferation, with O-GlcNAc addition and removal regulated by O-GlcNAc transferase and O-GlcNAcase (OGA), respectively. However, the detailed mechanism of host cell death induced by N. fowleri is unknown. In this study, we investigated whether N. fowleri can induce the modulation of O-GlcNAcylated proteins during cell death in Jurkat T cells. Co-incubation with live N. fowleri trophozoites increased DNA fragmentation. In addition, incubation with N. fowleri induced a dramatic reduction in O-GlcNAcylated protein levels in 30 min. Moreover, pretreatment of Jurkat T cells with the OGA inhibitor PUGNAc prevented N. fowleri-induced O-deGlcNAcylation and DNA fragmentation. These results suggest that O-deGlcNAcylation is an important signaling process that occurs during Jurkat T cell death induced by N. fowleri.

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.

DNA fragmentation factor 40 expression in T cells confers sensibility to tributyltin-induced apoptosis

DNA fragmentation factor 40 (DFF40), an endonuclease, mediates the final and irreversible step of apoptosis by conducting oligonucleosomal DNA fragmentation. New emerging studies have proposed a role of DFF40 in genomic stability, besides its nuclease activity. Overexpression of DFF40 in tumoral cells increases their sensitivity to chemotherapeutic drugs. In this study, we sought to determine if DFF40 expression influences the toxicity of tributyltin (TBT), a well-known immunotoxic and apoptosis-inducing compound. The strategy used was to knockout DFF40 expression by CRISPR-cas9 method in Jurkat T cells and to determine the toxicity of TBT in DFF40 KO cells and DFF40 WT Jurkat cells. DFF40 KO Jurkat cells show an increase of cell viability following a 24-h TBT exposure (p < 0.05). There is a resistance to TBT-induced apoptosis determined by annexin V/PI am labeling (p < 0.05). Interestingly, the basal level of ROS rises in DFF40 KO Jurkat cells, but ROS production levels after TBT exposure remains at the same basal level. Other apoptosis or DNA damage makers (procaspase-3, caspase-6, and PARP cleavage) are significantly delayed and decreased. DFF40 deficient cells do not present histone H2AX phosphorylation, whereas wild-type cells present a phosphorylation following a 6-h exposure to TBT (p < 0.001). The re-expression of DFF40 in DFF40 KO cells restores the cytotoxic effects of TBT. Overall, these data suggest a role of DFF40 in cells sensitivity to TBT and possibly in DNA stability.

Effects of Acute Cold Stress on Liver O-GlcNAcylation and Glycometabolism in Mice

Protein O-linked β-N-acetylglucosamine glycosylation (O-GlcNAcylation) regulates many biological processes. Studies have shown that O-GlcNAc modification levels can increase during acute stress and suggested that this may contribute to the survival of the cell. This study investigated the possible effects of O-GlcNAcylation that regulate glucose metabolism, apoptosis, and autophagy in the liver after acute cold stress. Male C57BL/6 mice were exposed to cold conditions (4 °C) for 0, 2, 4, and 6 h, then their livers were extracted and the expression of proteins involved in glucose metabolism, apoptosis, and autophagy was determined. It was found that acute cold stress increased global O-GlcNAcylation and protein kinase B (AKT) phosphorylation levels. This was accompanied by significantly increased activation levels of the glucose metabolism regulators 160 kDa AKT substrate (AS160), 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 2 (PFKFB2), and glycogen synthase kinase-3β (GSK3β). The levels of glycolytic intermediates, fructose-1,6-diphosphate (FDP) and pyruvic acid (PA), were found to show a brief increase followed by a sharp decrease. Additionally, adenosine triphosphate (ATP), as the main cellular energy source, had a sharp increase. Furthermore, the B-cell lymphoma 2(Bcl-2)/Bcl-2-associated X (Bax) ratio was found to increase, whereas cysteine-aspartic acid protease 3 (caspase-3) and light chain 3-II (LC3-II) levels were reduced after acute cold stress. Therefore, acute cold stress was found to increase O-GlcNAc modification levels, which may have resulted in the decrease of the essential processes of apoptosis and autophagy, promoting cell survival, while altering glycose transport, glycogen synthesis, and glycolysis in the liver.

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.

Protein O-GlcNAc Modification Increases in White Blood Cells After a Single Bout of Physical Exercise

Background

Protein O-linked N-acetylglucosamine (O-GlcNAc) is a dynamic posttranslational modification influencing the function of many intracellular proteins. Recently it was revealed that O-GlcNAc regulation is modified under various stress states, including ischemia and oxidative stress. Aside from a few contradictory studies based on animal models, the effect of exercise on O-GlcNAc is unexplored.

Purpose

To evaluate O-GlcNAc levels in white blood cells (WBC) of human volunteers following physical exercise.

Methods

Young (age 30 ± 5.2), healthy male volunteers (n = 6) were enlisted for the study. Blood parameters including metabolites, ions, “necro”-enzymes, and cell counts were measured before and after a single bout of exercise (2-mile run). From WBC samples, we performed western blots to detect O-GlcNAc modified proteins. The distribution of O-GlcNAc in WBC subpopulations was assessed by flow cytometry.

Results

Elevation of serum lactic acid (increased from 1.3 ± 0.4 to 6.9 ± 1.7 mM), creatinine (from 77.5 ± 6.3 U/L to 102.2 ± 7.0 μM), and lactate dehydrogenase (from 318.5 ± 26.2 to 380.5 ± 33.2 U/L) confirmed the effect of exercise. WBC count also significantly increased (from 6.6 ± 1.0 to 8.4 ± 1.4 G/L). The level of O-GlcNAc modified proteins in WBCs showed significant elevation after exercise (85 ± 51%, p < 0.05). Flow cytometry revealed that most of this change could be attributed to lymphocytes and monocytes.

Conclusion

Our results indicate that short-term exercise impacts the O-GlcNAc status of WBCs. O-GlcNAc modification could be a natural process by which physical activity modulates the immune system. Further research could elucidate the role of O-GlcNAc during exercise and validate O-GlcNAc as a biomarker for fitness assessment.

MicroRNA-15b Suppresses Th17 Differentiation and Is Associated with Pathogenesis of Multiple Sclerosis by Targeting O-GlcNAc Transferase

IL-17-producing Th17 cells have gradually become considered as key factors in the pathogenesis of many autoimmune diseases, including multiple sclerosis (MS). Although the involvement of certain microRNAs in the development of MS has been reported, their role in Th17-driven autoimmunity is still poorly understood. In this study, we identified microRNA (miR)-15b as an important factor in Th17-associated effects and determined that the expression of miR-15b is significantly downregulated in MS patients and in mice with experimental autoimmune encephalomyelitis. Overexpression of miR-15b alleviated experimental autoimmune encephalomyelitis, whereas knockdown of miR-15b aggravated it. We demonstrated that miR-15b suppressed Th17 differentiation both in vivo and in vitro. We also found that O-linked N-acetylglucosamine transferase is a potential target of miR-15b, enabling it to affect the transcriptional regulation of retinoic acid-related orphan receptor γT through O-linked N-acetylglucosamine glycosylation of NF-κB. These results contribute to the importance of miR-15b in Th17 differentiation and the pathogenesis of MS.

Transcriptome dynamics in the asexual cycle of the chordate Botryllus schlosseri

Background

We performed an analysis of the transcriptome during the blastogenesis of the chordate Botryllus schlosseri, focusing in particular on genes involved in cell death by apoptosis. The tunicate B. schlosseri is an ascidian forming colonies characterized by the coexistence of three blastogenetic generations: filter-feeding adults, buds on adults, and budlets on buds. Cyclically, adult tissues undergo apoptosis and are progressively resorbed and replaced by their buds originated by asexual reproduction. This is a feature of colonial tunicates, the only known chordates that can reproduce asexually.

Results

Thanks to a newly developed web-based platform (http://botryllus.cribi.unipd.it), we compared the transcriptomes of the mid-cycle, the pre-take-over, and the take-over phases of the colonial blastogenetic cycle. The platform is equipped with programs for comparative analysis and allows to select the statistical stringency. We enriched the genome annotation with 11,337 new genes; 581 transcripts were resolved as complete open reading frames, translated in silico into amino acid sequences and then aligned onto the non-redundant sequence database. Significant differentially expressed genes were classified within the gene ontology categories. Among them, we recognized genes involved in apoptosis activation, de-activation, and regulation.

Conclusions

With the current work, we contributed to the improvement of the first released B. schlosseri genome assembly and offer an overview of the transcriptome changes during the blastogenetic cycle, showing up- and down-regulated genes. These results are important for the comprehension of the events underlying colony growth and regression, cell proliferation, colony homeostasis, and competition among different generations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2598-1) contains supplementary material, which is available to authorized users.

The augmentation of O-GlcNAcylation reduces glyoxal-induced cell injury by attenuating oxidative stress in human retinal microvascular endothelial cells

It has recently been reported that O-linked β-N-acetyl glucosamine (O-GlcNAc) modification (a simple intracellular serine (Ser)/threonine (Thr)-linked monosaccharide) in human retinal microvascular endothelial cells (HRECs) is related to diabetic retinopathy (DR). During O-GlcNAcylation, O-GlcNAc is added to Ser and Thr residues. As the generation of reactive oxygen species (ROS) is one of the characteristics of advanced glycation end product (AGE) injury, and the most important key pathogenic factor of DR, in the present study, we aimed to investigate the association between O-GlcNAcylation and ROS generation in order to ascertain whether O-GlcNAcylation mitigates cellular injury through the generation of ROS. For this purpose, HRECs were divided into 4 groups as follows: HRECs treated with normal glucose (5 mM), HRECs treated with glyoxal (500 µM), glyoxal-treated HRECs also treated with 200 µM PUGNAc, and glyoxal-treated HRECs infected with O-GlcNAc transferase (OGT) siRNA. We detected increased O-GlcNAc levels and increased ROS production in the glyoxal-treated HRECs. The cellular redox status was determined by cellular ROS staining and by measuring the expression levels of the antioxidant genes, superoxide dismutase (SOD) and glutathione peroxidase (GPX). While the augmentation of O-GlcNAcylation following treatment with PUGNAc significantly attenuated the production of ROS (p<0.01) and increased the expression levels of SOD and GPX, the reduction of O-GlcNAcylation following infection with OGT siRNA, exacerbated the production of ROS (p<0.01) and decreased the expression of antioxidant genes. The effects of O-GlcNAcylation on the viability of HRECs were significant (p<0.01), particularly in the hydrogen peroxide (H2O2)-treated HRECs. Treatment with PUGNAc reduced glyoxal-induced cell apoptosis and transfection with OGT siRNA increased HREC apoptosis; these results were confirmed by flow cytometry and by the assessment of mitochondrial membrane potential. The augmentation of O-GlcNAcylation exerted cytoprotective effects on the HRECs by reducing the generation of ROS, increasing the expression of antioxidant genes, preventing the dissipation of mitochondrial membrane potential and preventing HREC apoptosis. Therefore, it can be concluded that O-GlcNAcylation plays a role in the early developmental process of DR.

The mitochondrial O-linked N-acetylglucosamine transferase (mOGT) in the diabetic patient could be the initial trigger to develop Alzheimer disease

Diabetes mellitus (DM) is considered a risk factor for the development of Alzheimer disease (AD); however, how DM favors evolution of AD is still insufficiently understood. Hyperglycemia in DM is associated to an increase in mitochondrial reactive oxygen species (ROS) generation, as well as damage of hippocampal cells, reflected by changes in morphological and mitochondrial functionality. Similar mitochondrial damage has been observed when amyloid beta (Aβ) accumulates in the brain of AD patients. In DM, the excess of glucose in the brain induces higher activity of the hexosamine biosynthesis pathway (HBP), it synthesizes UDP-N-acetylglucosamine (UDP-GlcNAc), which is used by O-linked N-acetylglucosamine transferase (OGT) to catalyze O-GlcNAcylation of numerous proteins. Although O-GlcNAcylation plays an important role in maintaining structure and cellular functionality, chronic activity of this pathway has been associated with insulin resistance and hyperglycemia-induced glucose toxicity. Three different forms of OGT are known: nucleocytoplasmic (ncOGT), short (sOGT), and mitochondrial (mOGT). Previous reports showed that overexpression of ncOGT is not toxic to the cell; in contrast, overexpression of mOGT is associated with cellular apoptosis. In this work, we suggest that hyperglycemia in the diabetic patient could induce greater expression and activity of mOGT, modifying the structure and functionality of mitochondria in hippocampal cells, accelerating neuronal damage, and favoring the start of AD. In consequence, mOGT activity could be a key point for AD development in patients with DM.