Identification of structural and functional O-linked N-acetylglucosamine-bearing proteins in Xenopus laevis oocyte

Genetically engineered human embryonic kidney cells as a novel vehicle for dual patch clamp study of human gap junction channels

Mutations in more than half of human connexin genes encoding gap junction (GJ) subunits have been linked to inherited human diseases. Functional studies of human GJ channels are essential for revealing mechanistic insights into the etiology of disease-linked connexin mutants. However, the commonly used Xenopus oocytes, N2A, HeLa, and other model cells for recombinant expression of human connexins have different and significant limitations. Here we developed a human cell line (HEK293) with each of the endogenous connexins (Cx43 and Cx45) knocked out using the CRISPR-Cas9 system. Double knockout HEK293 cells showed no background GJ coupling, were easily transfected with several human connexin genes (such as those encoding Cx46, Cx50, Cx37, Cx45, Cx26, and Cx36) which successfully formed functional GJs and were readily accessible for dual patch clamp analysis. Single knockout Cx43 or Cx45 HEK cell lines could also be used to characterize human GJ channels formed by Cx45 or Cx43, respectively, with an expression level suitable for studying macroscopic and single channel GJ channel properties. A cardiac arrhythmia linked Cx45 mutant R184G failed to form functional GJs in DKO HEK293 cells with impaired localizations. These genetically engineered HEK293 cells are well suited for patch clamp study of human GJ channels.

Targeting O-GlcNAcylation in cancer therapeutic resistance: The sugar Saga continues

O-linked-N-acetylglucosaminylation (O-GlcNAcylation), a dynamic post-translational modification (PTM), holds profound implications in controlling various cellular processes such as cell signaling, metabolism, and epigenetic regulation that influence cancer progression and therapeutic resistance. From the therapeutic perspective, O-GlcNAc modulates drug efflux, targeting and metabolism. By integrating signals from glucose, lipid, amino acid, and nucleotide metabolic pathways, O-GlcNAc acts as a nutrient sensor and transmits signals to exerts its function on genome stability, epithelial-mesenchymal transition (EMT), cell stemness, cell apoptosis, autophagy, cell cycle. O-GlcNAc also attends to tumor microenvironment (TME) and the immune response. At present, several strategies aiming at targeting O-GlcNAcylation are under mostly preclinical evaluation, where the newly developed O-GlcNAcylation inhibitors markedly enhance therapeutic efficacy. Here we systematically outline the mechanisms through which O-GlcNAcylation influences therapy resistance and deliberate on the prospects and challenges associated with targeting O-GlcNAcylation in future cancer treatments.

Growing and dividing: how O-GlcNAcylation leads the way

Cell cycle errors can lead to mutations, chromosomal instability, or death; thus, the precise control of cell cycle progression is essential for viability. The nutrient-sensing posttranslational modification, O-GlcNAc, regulates the cell cycle allowing one central control point directing progression of the cell cycle. O-GlcNAc is a single N-acetylglucosamine sugar modification to intracellular proteins that is dynamically added and removed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. These enzymes act as a rheostat to fine-tune protein function in response to a plethora of stimuli from nutrients to hormones. O-GlcNAc modulates mitogenic growth signaling, senses nutrient flux through the hexosamine biosynthetic pathway, and coordinates with other nutrient-sensing enzymes to progress cells through Gap phase 1 (G1). At the G1/S transition, O-GlcNAc modulates checkpoint control, while in S Phase, O-GlcNAcylation coordinates the replication fork. DNA replication errors activate O-GlcNAcylation to control the function of the tumor-suppressor p53 at Gap Phase 2 (G2). Finally, in mitosis (M phase), O-GlcNAc controls M phase progression and the organization of the mitotic spindle and midbody. Critical for M phase control is the interplay between OGT and OGA with mitotic kinases. Importantly, disruptions in OGT and OGA activity induce M phase defects and aneuploidy. These data point to an essential role for the O-GlcNAc rheostat in regulating cell division. In this review, we highlight O-GlcNAc nutrient sensing regulating G1, O-GlcNAc control of DNA replication and repair, and finally, O-GlcNAc organization of mitotic progression and spindle dynamics.

Follicular cells protect Xenopus oocyte from abnormal maturation via integrin signaling downregulation and O-GlcNAcylation control

Xenopus oocytes are encompassed by a layer of follicular cells that contribute to oocyte growth and meiosis in relation to oocyte maturation. However, the effects of the interaction between follicular cells and the oocyte surface on meiotic processes are unclear. Here, we investigated Xenopus follicular cell function using oocyte signaling and heterologous-expressing capabilities. We found that oocytes deprotected from their surrounding layer of follicular cells and expressing the epidermal growth factor (EGF) receptor (EGFR) and the Grb7 adaptor undergo accelerated prophase I to metaphase II meiosis progression upon stimulation by EGF. This unusual maturation unravels atypical spindle formation but is rescued by inhibiting integrin β1 or Grb7 binding to the EGFR. In addition, we determined that oocytes surrounded by their follicular cells expressing EGFR-Grb7 exhibit normal meiotic resumption. These oocytes are protected from abnormal meiotic spindle formation through the recruitment of O-GlcNAcylated Grb7, and OGT (O-GlcNAc transferase), the enzyme responsible for O-GlcNAcylation processes, in the integrin β1-EGFR complex. Folliculated oocytes can be forced to adopt an abnormal phenotype and exclusive Grb7 Y338 and Y188 phosphorylation instead of O-GlcNAcylation under integrin activation. Furthermore, an O-GlcNAcylation increase (by inhibition of O-GlcNAcase), the glycosidase that removes O-GlcNAc moieties, or decrease (by inhibition of OGT) amplifies oocyte spindle defects when follicular cells are absent highlighting a control of the meiotic spindle by the OGT-O-GlcNAcase duo. In summary, our study provides further insight into the role of the follicular cell layer in oocyte meiosis progression.

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

Introduction

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

Methods

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

Results

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

Conclusion

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

Sugar-responsive inhibition of Myc-dependent ribosome biogenesis by Clockwork orange

The ability to feed on a sugar-containing diet depends on a gene regulatory network controlled by the intracellular sugar sensor Mondo/ChREBP-Mlx, which remains insufficiently characterized. Here, we present a genome-wide temporal clustering of sugar-responsive gene expression in Drosophila larvae. We identify gene expression programs responding to sugar feeding, including downregulation of ribosome biogenesis genes, known targets of Myc. Clockwork orange (CWO), a component of the circadian clock, is found to be a mediator of this repressive response and to be necessary for survival on a high-sugar diet. CWO expression is directly activated by Mondo-Mlx, and it counteracts Myc through repression of its gene expression and through binding to overlapping genomic regions. CWO mouse ortholog BHLHE41 has a conserved role in repressing ribosome biogenesis genes in primary hepatocytes. Collectively, our data uncover a cross-talk between conserved gene regulatory circuits balancing the activities of anabolic pathways to maintain homeostasis during sugar feeding.

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.

NAT10 Maintains OGA mRNA Stability Through ac4C Modification in Regulating Oocyte Maturation

In vitro maturation (IVM) refers to the process of developing immature oocytes into the mature in vitro under the microenvironment analogous to follicle fluid. It is an important technique for patients with polycystic ovary syndrome and, especially, those young patients with the need of fertility preservation. However, as the mechanisms of oocyte maturation have not been fully understood yet, the cultivation efficiency of IVM is not satisfactory. It was confirmed in our previous study that oocyte maturation was impaired after N-acetyltransferase 10 (NAT10) knockdown (KD). In the present study, we further explored the transcriptome alteration of NAT10-depleted oocytes and found that O-GlcNAcase(OGA) was an important target gene for NAT10-mediated ac4C modification in oocyte maturation. NAT10 might regulate OGA stability and expression by suppressing its degradation. To find out whether the influence of NAT10-mediated ac4C on oocyte maturation was mediated by OGA, we further explored the role of OGA in IVM. After knocking down OGA of oocytes, oocyte maturation was inhibited. In addition, as oocytes matured, OGA expression increased and, conversely, O-linked N-acetylglucosamine (O-GlcNAc) level decreased. On the basis of NAT10 KD transcriptome and OGA KD transcriptome data, NAT10-mediated ac4C modification of OGA might play a role through G protein-coupled receptors, molecular transduction, nucleosome DNA binding, and other mechanisms in oocyte maturation. Rsph6a, Gm7788, Gm41780, Trpc7, Gm29036, and Gm47144 were potential downstream genes. In conclusion, NAT10 maintained the stability of OGA transcript by ac4C modification on it, thus positively regulating IVM. Moreover, our study revealed the regulation mechanisms of oocytes maturation and provided reference for improving IVM outcomes. At the same time, the interaction between mRNA ac4C modification and protein O-GlcNAc modification was found for the first time, which enriched the regulation network of oocyte maturation.

O-GlcNAcylation Is Essential for Rapid Pomc Expression and Cell Proliferation in Corticotropic Tumor Cells

Pituitary adenomas have a staggering 16.7% lifetime prevalence and can be devastating in many patients because of profound endocrine and neurologic dysfunction. To date, no clear genomic or epigenomic markers correlate with their onset or severity. Herein, we investigate the impact of the O-GlcNAc posttranslational modification in their etiology. Found in more than 7000 human proteins to date, O-GlcNAcylation dynamically regulates proteins in critical signaling pathways, and its deregulation is involved in cancer progression and endocrine diseases such as diabetes. In this study, we demonstrated that O-GlcNAc enzymes were upregulated, particularly in aggressive adrenocorticotropin (ACTH)-secreting tumors, suggesting a role for O-GlcNAcylation in pituitary adenoma etiology. In addition to the demonstration that O-GlcNAcylation was essential for their proliferation, we showed that the endocrine function of pituitary adenoma is also dependent on O-GlcNAcylation. In corticotropic tumors, hypersecretion of the proopiomelanocortin (POMC)-derived hormone ACTH leads to Cushing disease, materialized by severe endocrine disruption and increased mortality. We demonstrated that Pomc messenger RNA is stabilized in an O-GlcNAc-dependent manner in response to corticotrophin-releasing hormone (CRH). By affecting Pomc mRNA splicing and stability, O-GlcNAcylation contributes to this new mechanism of fast hormonal response in corticotropes. Thus, this study stresses the essential role of O-GlcNAcylation in ACTH-secreting adenomas' pathophysiology, including cellular proliferation and hypersecretion.

Putting the pieces together: mapping the O-glycoproteome

Protein glycosylation is the most diverse and omnipresent protein modification. Glycosylation provides glycoproteins with important structural and functional properties to facilitate critical biological processes. Despite the significance of protein glycosylation, the investigation of glycoproteome, especially O-linked glycoproteome, remains elusive due to the lack of a comprehensive methodology to conform with the diversity of O-linked glycoforms of O-linked glycoproteins. In recent years, mass spectrometry has become an indispensable tool for the characterization of O-linked glycosylated proteins across biological systems. We herein highlight the recent developments in MS-based O-linked glycoproteomic technologies, quantitative data acquisition strategy and bioinformatic tools, with a special focus on mucin-type O-linked glycosylation.

Dual regulation of fatty acid synthase (FASN) expression by O-GlcNAc transferase (OGT) and mTOR pathway in proliferating liver cancer cells

Fatty acid synthase (FASN) participates in many fundamental biological processes, including energy storage and signal transduction, and is overexpressed in many cancer cells. We previously showed in a context of lipogenesis that FASN is protected from degradation by its interaction with O-GlcNAc transferase (OGT) in a nutrient-dependent manner. We and others also reported that OGT and O-GlcNAcylation up-regulate the PI3K/AKT/mTOR pathway that senses mitogenic signals and nutrient availability to drive cell cycle. Using biochemical and microscopy approaches, we show here that FASN co-localizes with OGT in the cytoplasm and, to a lesser extent, in the membrane fraction. This interaction occurs in a cell cycle-dependent manner, following the pattern of FASN expression. Moreover, we show that FASN expression depends on OGT upon serum stimulation. The level of FASN also correlates with the activation of the PI3K/AKT/mTOR pathway in hepatic cell lines, and in livers of obese mice and in a chronically activated insulin and mTOR signaling mouse model (PTEN-null mice). These results indicate that FASN is under a dual control of O-GlcNAcylation and mTOR pathways. In turn, blocking FASN with the small-molecule inhibitor C75 reduces both OGT and O-GlcNAcylation levels, and mTOR activation, highlighting a novel reciprocal regulation between these actors. In addition to the role of O-GlcNAcylation in tumorigenesis, our findings shed new light on how aberrant activity of FASN and mTOR signaling may promote the emergence of hepatic tumors.

Nutrient regulation of the flow of genetic information by O-GlcNAcylation

O-linked-β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification (PTM) that is actively added to and removed from thousands of intracellular proteins. As a PTM, O-GlcNAcylation tunes the functions of a protein in various ways, such as enzymatic activity, transcriptional activity, subcellular localization, intermolecular interactions, and degradation. Its regulatory roles often interplay with the phosphorylation of the same protein. Governed by 'the Central Dogma', the flow of genetic information is central to all cellular activities. Many proteins regulating this flow are O-GlcNAc modified, and their functions are tuned by the cycling sugar. Herein, we review the regulatory roles of O-GlcNAcylation on the epigenome, in DNA replication and repair, in transcription and in RNA processing, in protein translation and in protein turnover.

C-Terminal Tag Location Hampers in Vitro Profiling of OGT Peptide Substrates by mRNA Display

O-GlcNAc transferase (OGT) is the only enzyme that catalyzes the post-translational modification of proteins at Ser/Thr with a single β-N-acetylglucosamine (O-GlcNAcylation). Its activity has been associated with chronic diseases such as cancer, diabetes and neurodegenerative disease. Although numerous OGT substrates have been identified, its accepted substrate scope can still be refined. We report here an attempt to better define the peptide-recognition requirements of the OGT active site by using mRNA display, taking advantage of its extremely high throughput to assess the substrate potential of a library of all possible nonamer peptides. An antibody-based selection process is described here that is able to enrich an OGT substrate peptide from such a library, but with poor absolute recovery. Following four rounds of selection for O-GlcNAcylated peptides, sequencing revealed 14 peptides containing Ser/Thr, but these were shown by luminescence-coupled assays and peptide microarray not to be OGT substrates. By contrast, subsequent testing of an N-terminal tag approach showed exemplary recovery. Our approach demonstrates the power of genetically encoded libraries for selection of peptide substrates, even from a very low initial starting abundance and under suboptimal conditions, and emphasizes the need to consider the binding biases of antibodies and both C- and N-terminal tags in profiling peptide substrates by high-throughput display.

Tumour Microenvironment: Roles of the Aryl Hydrocarbon Receptor, O-GlcNAcylation, Acetyl-CoA and Melatonergic Pathway in Regulating Dynamic Metabolic Interactions across Cell Types-Tumour Microenvironment and Metabolism

This article reviews the dynamic interactions of the tumour microenvironment, highlighting the roles of acetyl-CoA and melatonergic pathway regulation in determining the interactions between oxidative phosphorylation (OXPHOS) and glycolysis across the array of cells forming the tumour microenvironment. Many of the factors associated with tumour progression and immune resistance, such as yin yang (YY)1 and glycogen synthase kinase (GSK)3β, regulate acetyl-CoA and the melatonergic pathway, thereby having significant impacts on the dynamic interactions of the different types of cells present in the tumour microenvironment. The association of the aryl hydrocarbon receptor (AhR) with immune suppression in the tumour microenvironment may be mediated by the AhR-induced cytochrome P450 (CYP)1b1-driven 'backward' conversion of melatonin to its immediate precursor N-acetylserotonin (NAS). NAS within tumours and released from tumour microenvironment cells activates the brain-derived neurotrophic factor (BDNF) receptor, TrkB, thereby increasing the survival and proliferation of cancer stem-like cells. Acetyl-CoA is a crucial co-substrate for initiation of the melatonergic pathway, as well as co-ordinating the interactions of OXPHOS and glycolysis in all cells of the tumour microenvironment. This provides a model of the tumour microenvironment that emphasises the roles of acetyl-CoA and the melatonergic pathway in shaping the dynamic intercellular metabolic interactions of the various cells within the tumour microenvironment. The potentiation of YY1 and GSK3β by O-GlcNAcylation will drive changes in metabolism in tumours and tumour microenvironment cells in association with their regulation of the melatonergic pathway. The emphasis on metabolic interactions across cell types in the tumour microenvironment provides novel future research and treatment directions.

A Pragmatic Guide to Enrichment Strategies for Mass Spectrometry-Based Glycoproteomics

Glycosylation is a prevalent, yet heterogeneous modification with a broad range of implications in molecular biology. This heterogeneity precludes enrichment strategies that can be universally beneficial for all glycan classes. Thus, choice of enrichment strategy has profound implications on experimental outcomes. Here we review common enrichment strategies used in modern mass spectrometry-based glycoproteomic experiments, including lectins and other affinity chromatographies, hydrophilic interaction chromatography and its derivatives, porous graphitic carbon, reversible and irreversible chemical coupling strategies, and chemical biology tools that often leverage bioorthogonal handles. Interest in glycoproteomics continues to surge as mass spectrometry instrumentation and software improve, so this review aims to help equip researchers with the necessary information to choose appropriate enrichment strategies that best complement these efforts.

Global and site-specific analysis of protein glycosylation in complex biological systems with Mass Spectrometry

Protein glycosylation is ubiquitous in biological systems and plays essential roles in many cellular events. Global and site-specific analysis of glycoproteins in complex biological samples can advance our understanding of glycoprotein functions and cellular activities. However, it is extraordinarily challenging because of the low abundance of many glycoproteins and the heterogeneity of glycan structures. The emergence of mass spectrometry (MS)-based proteomics has provided us an excellent opportunity to comprehensively study proteins and their modifications, including glycosylation. In this review, we first summarize major methods for glycopeptide/glycoprotein enrichment, followed by the chemical and enzymatic methods to generate a mass tag for glycosylation site identification. We next discuss the systematic and quantitative analysis of glycoprotein dynamics. Reversible protein glycosylation is dynamic, and systematic study of glycoprotein dynamics helps us gain insight into glycoprotein functions. The last part of this review focuses on the applications of MS-based proteomics to study glycoproteins in different biological systems, including yeasts, plants, mice, human cells, and clinical samples. Intact glycopeptide analysis is also included in this section. Because of the importance of glycoproteins in complex biological systems, the field of glycoproteomics will continue to grow in the next decade. Innovative and effective MS-based methods will exponentially advance glycoscience, and enable us to identify glycoproteins as effective biomarkers for disease detection and drug targets for disease treatment. © 2019 Wiley Periodicals, Inc. Mass Spec Rev 9999: XX-XX, 2019.

Mechanical stress alters protein O-GlcNAc in human periodontal ligament cells

Protein O-linked N-acetylglucosamine (O-GlcNAc) is a post-translational modification of intracellular proteins that regulates several physiological and pathophysiological process, including response to various stressors. However, O-GlcNAc's response to mechanical stress has not been investigated yet. As human periodontal ligament (PDL) cells are stimulated by compression force during orthodontic tooth movement that results in structural remodelling, in this study we investigated whether mechanical stress induces any alteration in protein O-GlcNAc in PDL cells. In this study, PDL cells isolated from premolars extracted for orthodontic indications were exposed to 0, 1.5, 3, 7 and 14 g/cm² compression forces for 12 hours. Cell viability was measured by flow cytometry, and protein O-GlcNAc was analysed by Western blot. Cellular structure and intracellular distribution of O-GlcNAc was studied by immunofluorescence microscopy. We found that between 1.5 and 3 g/cm² mechanical compression, O-GlcNAc significantly elevated; however, at higher forces O-GlcNAc level was not increased. We also found that intracellular localization of O-GlcNAc proteins became more centralized under 2 g/cm² compression force. Our results suggest that structural changes stimulated by compression forces have a significant effect on the regulation of O-GlcNAc; thus, it might play a role in the mechanical stress adaptation of PDL cells.

Cyclin D1 Stability Is Partly Controlled by O-GlcNAcylation

Cyclin D1 is the regulatory partner of the cyclin-dependent kinases (CDKs) CDK4 or CDK6. Once associated and activated, the cyclin D1/CDK complexes drive the cell cycle entry and G1 phase progression in response to extracellular signals. To ensure their timely and accurate activation during cell cycle progression, cyclin D1 turnover is finely controlled by phosphorylation and ubiquitination. Here we show that the dynamic and reversible O-linked β-N-Acetyl-glucosaminylation (O-GlcNAcylation) regulates also cyclin D1 half-life. High O-GlcNAc levels increase the stability of cyclin D1, while reduction of O-GlcNAcylation strongly decreases it. Moreover, elevation of O-GlcNAc levels through O-GlcNAcase (OGA) inhibition significantly slows down the ubiquitination of cyclin D1. Finally, biochemical and cell imaging experiments in human cancer cells reveal that the O-GlcNAc transferase (OGT) binds to and glycosylates cyclin D1. We conclude that O-GlcNAcylation promotes the stability of cyclin D1 through modulating its ubiquitination.

O-GlcNAc transferase associates with the MCM2-7 complex and its silencing destabilizes MCM-MCM interactions

O-GlcNAcylation of proteins is governed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). The homeostasis of O-GlcNAc cycling is regulated during cell cycle progression and is essential for proper cellular division. We previously reported the O-GlcNAcylation of the minichromosome maintenance proteins MCM2, MCM3, MCM6 and MCM7. These proteins belong to the MCM2-7 complex which is crucial for the initiation of DNA replication through its DNA helicase activity. Here we show that the six subunits of MCM2-7 are O-GlcNAcylated and that O-GlcNAcylation of MCM proteins mainly occurs in the chromatin-bound fraction of synchronized human cells. Moreover, we identify stable interaction between OGT and several MCM subunits. We also show that down-regulation of OGT decreases the chromatin binding of MCM2, MCM6 and MCM7 without affecting their steady-state level. Finally, OGT silencing or OGA inhibition destabilizes MCM2/6 and MCM4/7 interactions in the chromatin-enriched fraction. In conclusion, OGT is a new partner of the MCM2-7 complex and O-GlcNAcylation homeostasis might regulate MCM2-7 complex by regulating the chromatin loading of MCM6 and MCM7 and stabilizing MCM/MCM interactions.

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-Linked N-Acetylglucosamine Transiently Elevates in HeLa Cells during Mitosis

O-linked N-acetylglucosamine (O-GlcNAc) is a dynamic post-translational modification of serine and threonine residues on nuclear and cytoplasmic proteins. O-GlcNAc modification influences many cellular mechanisms, including carbohydrate metabolism, signal transduction and protein degradation. Multiple studies also showed that cell cycle might be modulated by O-GlcNAc. Although the role of O-GlcNAc in the regulation of some cell cycle processes such as mitotic spindle organization or histone phosphorylation is well established, the general behaviour of O-GlcNAc regulation during cell cycle is still controversial. In this study, we analysed the dynamic changes of overall O-GlcNAc levels in HeLa cells using double thymidine block. O-GlcNAc levels in G₁, S, G₂ and M phase were measured. We observed that O-GlcNAc levels are significantly increased during mitosis in comparison to the other cell cycle phases. However, this change could only be detected when mitotic cells were enriched by harvesting round shaped cells from the G₂/M fraction of the synchronized cells. Our data verify that O-GlcNAc is elevated during mitosis, but also emphasize that O-GlcNAc levels can significantly change in a short period of time. Thus, selection and collection of cells at specific cell-cycle checkpoints is a challenging, but necessary requirement for O-GlcNAc studies.

Cross regulation between mTOR signaling and O-GlcNAcylation

The hexosamine biosynthetic pathway (HBP) integrates glucose, amino acids, fatty acids and nucleotides metabolisms for uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) synthesis. UDP-GlcNAc is the nucleotide sugar donor for O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) processes. O-GlcNAc transferase (OGT) is the enzyme which transfers the N-acetylglucosamine (O-GlcNAc) residue onto target proteins. Several studies previously showed that glucose metabolism dysregulations associated with obesity, diabetes or cancer correlated with an increase of OGT expression and global O-GlcNAcylation levels. Moreover, these diseases present an increased activation of the nutrient sensing mammalian target of rapamycin (mTOR) pathway. Other works demonstrate that mTOR regulates protein O-GlcNAcylation in cancer cells through stabilization of OGT. In this context, we studied the cross-talk between these two metabolic sensors in vivo in obese mice predisposed to diabetes and in vitro in normal and colon cancer cells. We report that levels of OGT and O-GlcNAcylation are increased in obese mice colon tissues and colon cancer cells and are associated with a higher activation of mTOR signaling. In parallel, treatments with mTOR regulators modulate OGT and O-GlcNAcylation levels in both normal and colon cancer cells. However, deregulation of O-GlcNAcylation affects mTOR signaling activation only in cancer cells. Thus, a crosstalk exists between O-GlcNAcylation and mTOR signaling in contexts of metabolism dysregulation associated to obesity or cancer.

Involvement of O-GlcNAcylation in the Skeletal Muscle Physiology and Physiopathology: Focus on Muscle Metabolism

Skeletal muscle represents around 40% of whole body mass. The principal function of skeletal muscle is the conversion of chemical energy toward mechanic energy to ensure the development of force, provide movement and locomotion, and maintain posture. This crucial energy dependence is maintained by the faculty of the skeletal muscle for being a central place as a "reservoir" of amino acids and carbohydrates in the whole body. A fundamental post-translational modification, named O-GlcNAcylation, depends, inter alia, on these nutrients; it consists to the transfer or the removal of a unique monosaccharide (N-acetyl-D-glucosamine) to a serine or threonine hydroxyl group of nucleocytoplasmic and mitochondrial proteins in a dynamic process by the O-GlcNAc Transferase (OGT) and the O-GlcNAcase (OGA), respectively. O-GlcNAcylation has been shown to be strongly involved in crucial intracellular mechanisms through the modulation of signaling pathways, gene expression, or cytoskeletal functions in various organs and tissues, such as the brain, liver, kidney or pancreas, and linked to the etiology of associated diseases. In recent years, several studies were also focused on the role of O-GlcNAcylation in the physiology and the physiopathology of skeletal muscle. These studies were mostly interested in O-GlcNAcylation during muscle exercise or muscle-wasting conditions. Major findings pointed out a different "O-GlcNAc signature" depending on muscle type metabolism at resting, wasting and exercise conditions, as well as depending on acute or long-term exhausting exercise protocol. First insights showed some differential OGT/OGA expression and/or activity associated with some differential stress cellular responses through Reactive Oxygen Species and/or Heat-Shock Proteins. Robust data displayed that these O-GlcNAc changes could lead to (i) a differential modulation of the carbohydrates metabolism, since the majority of enzymes are known to be O-GlcNAcylated, and to (ii) a differential modulation of the protein synthesis/degradation balance since O-GlcNAcylation regulates some key signaling pathways such as Akt/GSK3β, Akt/mTOR, Myogenin/Atrogin-1, Myogenin/Mef2D, Mrf4 and PGC-1α in the skeletal muscle. Finally, such involvement of O-GlcNAcylation in some metabolic processes of the skeletal muscle might be linked to some associated diseases such as type 2 diabetes or neuromuscular diseases showing a critical increase of the global O-GlcNAcylation level.

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.

Identification of O-GlcNAcylated proteins in Plasmodium falciparum

BACKGROUND

Post-translational modifications (PTMs) constitute a huge group of chemical modifications increasing the complexity of the proteomes of living beings. PTMs have been discussed as potential anti-malarial drug targets due to their involvement in many cell processes. O-GlcNAcylation is a widespread PTM found in different organisms including Plasmodium falciparum. The aim of this study was to identify O-GlcNAcylated proteins of P. falciparum, to learn more about the modification process and to understand its eventual functions in the Apicomplexans.

METHODS

The P. falciparum strain 3D7 was amplified in erythrocytes and purified. The proteome was checked for O-GlcNAcylation using different methods. The level of UDP-GlcNAc, the donor of the sugar moiety for O-GlcNAcylation processes, was measured using high-pH anion exchange chromatography. O-GlcNAcylated proteins were enriched and purified utilizing either click chemistry labelling or adsorption on succinyl-wheat germ agglutinin beads. Proteins were then identified by mass-spectrometry (nano-LC MS/MS).

RESULTS

While low when compared to MRC5 control cells, P. falciparum disposes of its own pool of UDP-GlcNAc. By using proteomics methods, 13 O-GlcNAcylated proteins were unambiguously identified (11 by click-chemistry and 6 by sWGA-beads enrichment; 4 being identified by the 2 approaches) in late trophozoites. These proteins are all part of pathways, functions and structures important for the parasite survival. By probing clicked-proteins with specific antibodies, Hsp70 and α-tubulin were identified as P. falciparum O-GlcNAc-bearing proteins.

CONCLUSIONS

This study is the first report on the identity of P. falciparum O-GlcNAcylated proteins. While the parasite O-GlcNAcome seems close to those of other species, the structural differences exhibited by the proteomes provides a glimpse of innovative therapeutic paths to fight malaria. Blocking biosynthesis of UDP-GlcNAc in the parasites is another promising option to reduce Plasmodium life cycle.

High-Sensitivity and Low-Toxicity Fucose Probe for Glycan Imaging and Biomarker Discovery

Fucose, a terminal sugar in glycoconjugates, critically regulates various physiological and pathological phenomena, including cancer development and inflammation. However, there are currently no probes for efficient labeling and detection of this sugar. We chemically synthesized a novel series of alkynyl-fucose analogs as probe candidates and found that 7-alkynyl-fucose gave the highest labeling efficiency and low cytotoxicity. Among the fucose analogs, 7-alkynyl-fucose was the best substrate against all five fucosyltransferases examined. We confirmed its conversion to the corresponding guanosine diphosphate derivative in cells and found that cellular glycoproteins were labeled much more efficiently with 7-alkynyl-fucose than with an existing probe. 7-Alkynyl-fucose was detected in the N-glycan core by mass spectrometry, and 7-alkynyl-fucose-modified proteins mostly disappeared in core-fucose-deficient mouse embryonic fibroblasts, suggesting that this analog mainly labeled core fucose in these cells. These results indicate that 7-alkynyl-fucose is a highly sensitive and powerful tool for basic glycobiology research and clinical application for biomarker discovery.

Reduced O-GlcNAcase expression promotes mitotic errors and spindle defects

Alterations in O-GlcNAc cycling, the addition and removal of O-GlcNAc, lead to mitotic defects and increased aneuploidy. Herein, we generated stable O-GlcNAcase (OGA, the enzyme that removes O-GlcNAc) knockdown HeLa cell lines and characterized the effect of the reduction in OGA activity on cell cycle progression. After release from G1/S, the OGA knockdown cells progressed normally through S phase but demonstrated mitotic exit defects. Cyclin A was increased in the knockdown cells while Cyclin B and D expression was reduced. Retinoblastoma protein (RB) phosphorylation was also increased in the knockdown compared to control. At M phase, the knockdown cells showed more compact spindle chromatids than control cells and had a greater percentage of cells with multipolar spindles. Furthermore, the timing of the inhibitory tyrosine phosphorylation of Cyclin Dependent Kinase 1 (CDK1) was altered in the OGA knockdown cells. Although expression and localization of the chromosomal passenger protein complex (CPC) was unchanged, histone H3 threonine 3 phosphorylation was decreased in one of the OGA knockdown cell lines. The Ewing Sarcoma Breakpoint Region 1 Protein (EWS) participates in organizing the CPC at the spindle and is a known substrate for O-GlcNAc transferase (OGT, the enzyme that adds O-GlcNAc). EWS O-GlcNAcylation was significantly increased in the OGA knockdown cells promoting uneven localization of the mitotic midzone. Our data suggests that O-GlcNAc cycling is an essential mechanism for proper mitotic signaling and spindle formation, and alterations in the rate of O-GlcNAc cycling produces aberrant spindles and promotes aneuploidy.

What has passed is prolog: new cellular and physiological roles of G6PD

G6PD deficiency has been the most pervasive inherited disorder in the world since having been discovered. G6PD has an antioxidant role by functioning as a major nicotinamide adenine dinucleotide phosphate (NADPH) provider to reduce excessive oxidative stress. NADPH can produce reactive oxygen species (ROS) and reactive nitrogen species (RNS) mediated by NADPH oxidase (NOX) and nitric oxide synthase (NOS), respectively. Hence, G6PD also has a pro-oxidant role. Research in the past has focused on the enhanced susceptibility of G6PD-deficient cells or individuals to oxidative challenge. The cytoregulatory role of G6PD has largely been overlooked. By using a metabolomic approach, it is noted that upon oxidant challenge, G6PD-deficient cells will reprogram the GSH metabolism from regeneration to synthesis with exhaustive energy consumption. Recently, new cellular/physiologic roles of G6PD have been discovered. By using a proteomic approach, it has been found that G6PD plays a regulatory role in xenobiotic metabolism possibly via NOX and the redox-sensitive Nrf2-signaling pathway to modulate the expression of xenobiotic-metabolizing enzymes. Since G6PD is a key regulator responsible for intracellular redox homeostasis, G6PD deficiency can alter redox balance leading to many abnormal cellular effects such as the cellular inflammatory and immune response against viral infection. G6PD may play an important role in embryogenesis as G6PD-knockdown mouse cannot produce offspring and G6PD-deficient C. elegans with defective egg production and hatching. This array of findings indicates that the cellular and physiologic roles of G6PD, other than the classical role as an antioxidant enzyme, deserve further attention.

Combined Antibody/Lectin Enrichment Identifies Extensive Changes in the O-GlcNAc Sub-proteome upon Oxidative Stress

O-Linked N-acetyl-β-d-glucosamine (O-GlcNAc) is a dynamic post-translational modification that modifies and regulates over 3000 nuclear, cytoplasmic, and mitochondrial proteins. Upon exposure to stress and injury, cells and tissues increase the O-GlcNAc modification, or O-GlcNAcylation, of numerous proteins promoting the cellular stress response and thus survival. The aim of this study was to identify proteins that are differentially O-GlcNAcylated upon acute oxidative stress (H₂O₂) to provide insight into the mechanisms by which O-GlcNAc promotes survival. We achieved this goal by employing Stable Isotope Labeling of Amino Acids in Cell Culture (SILAC) and a novel "G5-lectibody" immunoprecipitation strategy that combines four O-GlcNAc-specific antibodies (CTD110.6, RL2, HGAC39, and HGAC85) and the lectin WGA. Using the G5-lectibody column in combination with basic reversed phase chromatography and C₁₈ RPLC-MS/MS, 990 proteins were identified and quantified. Hundreds of proteins that were identified demonstrated increased (>250) or decreased (>110) association with the G5-lectibody column upon oxidative stress, of which we validated the O-GlcNAcylation status of 24 proteins. Analysis of proteins with altered glycosylation suggests that stress-induced changes in O-GlcNAcylation cluster into pathways known to regulate the cell's response to injury and include protein folding, transcriptional regulation, epigenetics, and proteins involved in RNA biogenesis. Together, these data suggest that stress-induced O-GlcNAcylation regulates numerous and diverse cellular pathways to promote cell and tissue survival.

Glyco-seek: Ultrasensitive Detection of Protein-Specific Glycosylation by Proximity Ligation Polymerase Chain Reaction

We report a non-destructive biochemical technique, termed "Glyco-seek", for analysis of O-GlcNAcylated proteins. Glyco-seek combines chemoenzymatic labeling, proximity ligation, and quantitative polymerase chain reaction to detect O-GlcNAcylated proteins with ultrahigh sensitivity. Our glycan-specific assay can be paired with traditional proximity ligation assays to simultaneously determine the change in total protein levels. We show that Glyco-seek detects attomoles of glycoproteins of interest from cell lysates, with sensitivity several orders of magnitude higher than that of current techniques. We used the method to directly assay the O-GlcNAcylation status of a low-abundance transcription factor from cell lysates without need for isolation or enrichment.

Glucokinase expression is regulated by glucose through O-GlcNAc glycosylation

Blood glucose fluctuates with the fasting-feeding cycle. One of the liver's functions is to maintain blood glucose concentrations within a physiological range. Glucokinase (GCK) or hexokinase IV, is the main enzyme that regulates the flux and the use of glucose in the liver leading to a compensation of hyperglycemia. In hepatocytes, GCK catalyzes the phosphorylation of glucose into glucose-6-phosphate. This critical enzymatic reaction is determinant for the metabolism of glucose in the liver which includes glycogen synthesis, glycolysis, lipogenesis and gluconeogenesis. In liver, simultaneous increase of glucose and insulin enhances GCK activity and gene expression, changes its subcellular location and interaction with regulatory proteins. The post-translational O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) acts as a glucose-sensitive modification and is believed to take part in hepatic glucose sensing by modifying key regulatory proteins. Therefore, we aimed to determine whether GCK is modified by O-GlcNAcylation in the liver of mice and investigated the role that this modification plays in regulating GCK protein expression. We demonstrated that endogenous GCK expression correlated with O-GlcNAc levels in the pathophysiological model ob/ob mice. More specifically, in response to the pharmacological inhibition of O-GlcNAcase (OGA) contents of GCK increased. Using the GlcNAc specific lectin succinylated-WGA and click chemistry labeling approaches, we demonstrated that GCK is modified by O-GlcNAcylation. Further, we demonstrated that siRNA-mediated Ogt knock-down not only decreases O-GlcNAc content but also GCK protein level. Altogether, our in vivo and in vitro results demonstrate that GCK expression is regulated by nutrient-sensing O-GlcNAc cycling in liver.

Two-way communication between the metabolic and cell cycle machineries: the molecular basis

The relationship between cellular metabolism and the cell cycle machinery is by no means unidirectional. The ability of a cell to enter the cell cycle critically depends on the availability of metabolites. Conversely, the cell cycle machinery commits to regulating metabolic networks in order to support cell survival and proliferation. In this review, we will give an account of how the cell cycle machinery and metabolism are interconnected. Acquiring information on how communication takes place among metabolic signaling networks and the cell cycle controllers is crucial to increase our understanding of the deregulation thereof in disease, including cancer.

An ultrasensitive colorimetric strategy for protein O-GlcNAcylation detection via copper deposition-enabled nonenzymatic signal amplification

An ultrasensitive colorimetric method for protein O-GlcNAcylation analysis is developed using AuNP-catalyzed copper deposition as a nonenzymatic signal amplification strategy.

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.

Novel Reporter for Faithful Monitoring of ERK2 Dynamics in Living Cells and Model Organisms

Uncoupling of ERK1/2 phosphorylation from subcellular localization is essential towards the understanding of molecular mechanisms that control ERK1/2-mediated cell-fate decision. ERK1/2 non-catalytic functions and discoveries of new specific anchors responsible of the subcellular compartmentalization of ERK1/2 signaling pathway have been proposed as regulation mechanisms for which dynamic monitoring of ERK1/2 localization is necessary. However, studying the spatiotemporal features of ERK2, for instance, in different cellular processes in living cells and tissues requires a tool that can faithfully report on its subcellular distribution. We developed a novel molecular tool, ERK2-LOC, based on the T2A-mediated coexpression of strictly equimolar levels of eGFP-ERK2 and MEK1, to faithfully visualize ERK2 localization patterns. MEK1 and eGFP-ERK2 were expressed reliably and functionally both in vitro and in single living cells. We then assessed the subcellular distribution and mobility of ERK2-LOC using fluorescence microscopy in non-stimulated conditions and after activation/inhibition of the MAPK/ERK1/2 signaling pathway. Finally, we used our coexpression system in Xenopus laevis embryos during the early stages of development. This is the first report on MEK1/ERK2 T2A-mediated coexpression in living embryos, and we show that there is a strong correlation between the spatiotemporal subcellular distribution of ERK2-LOC and the phosphorylation patterns of ERK1/2. Our approach can be used to study the spatiotemporal localization of ERK2 and its dynamics in a variety of processes in living cells and embryonic tissues.

O-GlcNAcylation of G6PD promotes the pentose phosphate pathway and tumor growth

The pentose phosphate pathway (PPP) plays a critical role in macromolecule biosynthesis and maintaining cellular redox homoeostasis in rapidly proliferating cells. Upregulation of the PPP has been shown in several types of cancer. However, how the PPP is regulated to confer a selective growth advantage on cancer cells is not well understood. Here we show that glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the PPP, is dynamically modified with an O-linked β-N-acetylglucosamine sugar in response to hypoxia. Glycosylation activates G6PD activity and increases glucose flux through the PPP, thereby providing precursors for nucleotide and lipid biosynthesis, and reducing equivalents for antioxidant defense. Blocking glycosylation of G6PD reduces cancer cell proliferation in vitro and impairs tumor growth in vivo. Importantly, G6PD glycosylation is increased in human lung cancers. Our findings reveal a mechanistic understanding of how O-glycosylation directly regulates the PPP to confer a selective growth advantage to tumours.

The pentose phosphate pathway is aberrantly activated in cancer cells but the mechanism is unclear. Here, the authors show that G6PD, the rate-limiting enzyme in the pathway, is post-translationally modified with a sugar moiety under hypoxic conditions leading to increased production of precursors for macromolecular synthesis and antioxidants.

The Utilities of Chemical Reactions and Molecular Tools for O-GlcNAc Proteomic Studies

The post-translational modification of nuclear and cytoplasmic proteins with O-linked β-N-acetylglucosamine (O-GlcNAc) is involved in a wide variety of cellular processes and is associated with the pathological progression of chronic diseases. Considering its emerging biological significance, systematic identification, site mapping, and quantification of O-GlcNAc proteins are essential and have led to the development of several approaches for O-GlcNAc protein profiling. This minireview mainly focuses on the various useful chemical reactions and molecular tools with detailed reaction mechanisms widely adopted for O-GlcNAc protein/peptide enrichment and its quantification for comprehensive O-GlcNAc protein profiling.

Enrichment of O-GlcNAc-modified peptides using novel thiol-alkyne and thiol-disulfide exchange

We have developed a selective method for the enrichment of O-linked β-N-acetylglucosamine (O-GlcNAc)-modified peptides, which uses a newly synthesized thiol-alkyne and a thiol-disulfide exchange. First, O-GlcNAc-modified peptides were enzymatically labeled with an azide-containing GalNAc analog. Then, the azide moiety was reacted with thiol-alkyne through a copper(I)-catalyzed azide-alkyne cycloaddition. The thiol-modified peptides were enriched with thiol-reactive resin through a thiol-disulfide exchange. At least 500fmol of O-GlcNAc-modified peptides was selectively isolated from α-crystallin tryptic peptides and detected by mass spectrometry. This novel enrichment strategy could be used for O-GlcNAcome analysis of biological samples.

O-GlcNAc modification of the runt-related transcription factor 2 (Runx2) links osteogenesis and nutrient metabolism in bone marrow mesenchymal stem cells

Runx2 is the master switch controlling osteoblast differentiation and formation of the mineralized skeleton. The post-translational modification of Runx2 by phosphorylation, ubiquitinylation, and acetylation modulates its activity, stability, and interactions with transcriptional co-regulators and chromatin remodeling proteins downstream of osteogenic signals. Characterization of Runx2 by electron transfer dissociation tandem mass spectrometry revealed sites of O-linked N-acetylglucosamine (O-GlcNAc) modification, a nutrient-responsive post-translational modification that modulates the action of numerous transcriptional effectors. O-GlcNAc modification occurs in close proximity to phosphorylated residues and novel sites of arginine methylation within regions known to regulate Runx2 transactivation. An interaction between Runx2 and the O-GlcNAcylated, O-GlcNAc transferase enzyme was also detected. Pharmacological inhibition of O-GlcNAcase (OGA), the enzyme responsible for the removal of O-GlcNAc from Ser/Thr residues, enhanced basal (39.9%) and BMP2/7-induced (43.3%) Runx2 transcriptional activity in MC3T3-E1 pre-osteoblasts. In bone marrow-derived mesenchymal stem cells differentiated for 6 days in osteogenic media, inhibition of OGA resulted in elevated expression (24.3%) and activity (65.8%) of alkaline phosphatase (ALP) an early marker of bone formation and a transcriptional target of Runx2. Osteogenic differentiation of bone marrow-derived mesenchymal stem cells in the presence of BMP2/7 for 8 days culminated in decreased OGA activity (39.0%) and an increase in the abundance of O-GlcNAcylated Runx2, as compared with unstimulated cells. Furthermore, BMP2/7-induced ALP activity was enhanced by 35.6% in bone marrow-derived mesenchymal stem cells differentiated in the presence of the OGA inhibitor, demonstrating that direct or BMP2/7-induced inhibition of OGA is associated with increased ALP activity. Altogether, these findings link O-GlcNAc cycling to the Runx2-dependent regulation of the early ALP marker under osteoblast differentiation conditions.

O-GlcNAc transferase and O-GlcNAcase: achieving target substrate specificity

O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) catalyze the dynamic cycling of intracellular, post-translational O-GlcNAc modification on thousands of Ser/Thr residues of cytosolic, nuclear, and mitochondrial signaling proteins. The identification of O-GlcNAc modified substrates has revealed a functionally diverse set of proteins, and the extent of O-GlcNAcylation fluctuates in response to nutrients and cellular stress. As a result, OGT and OGA are implicated in widespread, nutrient-responsive regulation of numerous signaling pathways and transcriptional programs. These enzymes are required for normal embryonic development and are dysregulated in metabolic and age-related disease states. While a recent surge of interest in the field has contributed to understanding the functional impacts of protein O-GlcNAcylation, little is known about the upstream mechanisms which modulate OGT and OGA substrate targeting. This review focuses on elements of enzyme structure among splice variants, post-translational modification, localization, and regulatory protein interactions which drive the specificity of OGT and OGA toward different subsets of the cellular proteome. Ongoing efforts in this rapidly advancing field are aimed at revealing mechanisms of OGT and OGA regulation to harness the potential therapeutic benefit of manipulating these enzymes' activities.

O-GlcNAc profiling: from proteins to proteomes

O-linked β-D-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) onto serine and threonine residues of proteins is an important post-translational modification (PTM), which is involved in many crucial biological processes including transcription, translation, proteasomal degradation, and signal transduction. Aberrant protein O-GlcNAcylation is directly linked to the pathological progression of chronic diseases including diabetes, cancer, and neurodegenerative disorders. Identification, site mapping, and quantification of O-GlcNAc proteins are a prerequisite to decipher their functions. In this review, we mainly focus on technological developments regarding O-GlcNAc protein profiling. Specifically, on one hand, we show how these techniques are being used for the comprehensive characterization of certain targeted proteins in which biologists are most interested. On the other hand, we present several newly developed approaches for O-GlcNAcomic profiling as well as how they provide us with a systems perspective to crosstalk amongst different PTMs and complicated biological events. Promising technical trends are also highlighted to evoke more efforts by diverse laboratories, which would further expand our understanding of the physiological and pathological roles of protein O-GlcNAcylation in chronic diseases.

Antibodies and activity measurements for the detection of O-GlcNAc transferase and assay of its substrate, UDP-GlcNAc

Since the discovery of O-GlcNAc modification (O-GlcNAcylation) 20 years ago, much attention has been given to OGT (O-GlcNAc transferase), the unique enzyme responsible for the nuclear and cytosolic O-GlcNAcylation processes. This review focuses on protocols that are routinely used to analyze OGT expression and activity. First are detailed techniques using rabbit polyclonal anti-OGT antibodies, namely, Western blot, (co-)immunoprecipitation, and immunofluorescence. We also describe the measurement of OGT activity by using synthetic peptides as acceptors and radiolabeled UDP-GlcNAc. Finally, a sensitive HPAEC-based technique to measure the cellular content of UDP-GlcNAc, the donor substrate of OGT, is described in detail.

Aberrant O-GlcNAcylated Proteins: New Perspectives in Breast and Colorectal Cancer

Increasing glucose consumption is thought to provide an evolutionary advantage to cancer cells. Alteration of glucose metabolism in cancer influences various important metabolic pathways including the hexosamine biosynthesis pathway (HBP), a relatively minor branch of glycolysis. Uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), an end product of HBP, is a sugar substrate used for classical glycosylation and O-GlcNAcylation, a post-translational protein modification implicated in a wide range of effects on cellular functions. Emerging evidence reveals that certain cellular proteins are abnormally O-GlcNAc modified in many kinds of cancers, indicating O-GlcNAcylation is associated with malignancy. Since O-GlcNAc rapidly on and off modifies in a similar time scale as in phosphorylation and these modifications may occur on proteins at either on the same or adjacent sites, it suggests that both modifications can work to regulate the cellular signaling pathways. This review describes the metabolic shifts related to the HBP, which are commonly found in most cancers. It also describes O-GlcNAc modified proteins identified in primary breast and colorectal cancer, as well as in the related cancer cell lines. Moreover, we also discuss the potential use of aberrant O-GlcNAcylated proteins as novel biomarkers of cancer.

O-GlcNAcylation and Metabolic Reprograming in Cancer

Although cancer metabolism has received considerable attention over the past decade, our knowledge on its specifics is still fragmentary. Altered cellular metabolism is one of the most important hallmarks of cancer. Cancer cells exhibit aberrant glucose metabolism characterized by aerobic glycolysis, a phenomenon known as Warburg effect. Accelerated glucose uptake and glycolysis are main characteristics of cancer cells that allow them for intensive growth and proliferation. Accumulating evidence suggests that O-GlcNAc transferase (OGT), an enzyme responsible for modification of proteins with N-acetylglucosamine, may act as a nutrient sensor that links hexosamine biosynthesis pathway to oncogenic signaling and regulation of factors involved in glucose and lipid metabolism. Recent studies suggest that metabolic reprograming in cancer is connected to changes at the epigenetic level. O-GlcNAcylation seems to play an important role in the regulation of the epigenome in response to cellular metabolic status. Through histone modifications and assembly of gene transcription complexes, OGT can impact on expression of genes important for cellular metabolism. This paper reviews recent findings related to O-GlcNAc-dependent regulation of signaling pathways, transcription factors, enzymes, and epigenetic changes involved in metabolic reprograming of cancer.

Regulation of cancer metabolism by O-GlcNAcylation

Cancer cells exhibit increased uptake of glucose and glutamine, and rewire the metabolic flux toward anabolic pathways important for cell growth and proliferation. Understanding how this altered metabolism is regulated has recently emerged as an intense research focus in cancer biology. O-linked β-N-acetylglucosamine (O-GlcNAc) is a reversible posttranslational modification of serine and/or threonine residues of nuclear and cytosolic proteins. O-GlcNAcylation has been identified in numerous proteins that are involved in many important cellular functions, including transcription, translation, signal transduction, and stress responses. More recently, increasing evidence indicates that O-GlcNAcylation plays important roles in regulating cancer metabolic reprogramming by modifying key transcription factors, metabolic enzymes and major oncogenic signaling pathways. Thus, O-GlcNAcylation emerges as a novel regulatory mechanism linking altered metabolism to cancer pathogenesis.

Proteomic analysis and abrogated expression of O-GlcNAcylated proteins associated with primary breast cancer

O-GlcNAcylation is a dynamic PTM of nuclear and cytoplasmic proteins, regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase, which catalyze the addition and removal of O-GlcNAc, respectively. This modification is associated with glucose metabolism, which plays important roles in many diseases including cancer. Although emerging evidence reveals that some tumor-associated proteins are O-GlcNAc modified, the total O-GlcNAcylation in cancer is still largely unexplored. Here, we demonstrate that O-GlcNAcylation was increased in primary breast malignant tumors, not in benign tumors and that this augmentation was associated with increased expression of OGT level. Using 2D O-GlcNAc immnoblotting and LC-MS/MS analysis, we successfully identified 29 proteins, with seven being uniquely O-GlcNAcylated or associated with O-GlcNAcylation in cancer. Of these identified proteins, some were related to the Warburg effect, including metabolic enzymes, proteins involved in stress responses and biosynthesis. In addition, proteins associated with RNA metabolism, gene expression, and cytoskeleton were highly O-GlcNAcylated or associated with O-GlcNAcylation. Moreover, OGT knockdown showed that decreasing O-GlcNAcylation was related to inhibition of the anchorage-independent growth in vitro. These data indicate that aberrant protein O-GlcNAcylation is associated with breast cancer. Abnormal modification of these O-GlcNAc-modified proteins might be one of the vital malignant characteristics of cancer.