A lipid-droplet-targeted O-GlcNAcase isoform is a key regulator of the proteasome

O-GlcNAcylation and neurodegeneration

O-GlcNAcylation is a dynamic form of protein glycosylation which involves the addition of β-d-N-acetylglucosamine (GlcNAc) via an O-linkage to serine or threonine residues of nuclear, cytoplasmic, mitochondrial and transmembrane proteins. The two enzymes responsible for O-GlcNAc cycling are O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA); their expression and activities in brain are age dependent. More than 1000 O-GlcNAc protein targets have been identified which play critical roles in many cellular processes. In mammalian brain, O-GlcNAc modification of Tau decreases its phosphorylation and toxicity, suggesting a neuroprotective role of pharmacological elevation of brain O-GlcNAc for Alzheimer's disease treatment. Other observations suggest that elevating O-GlcNAc levels may decrease protein clearance or induce apoptosis. This review highlights some of the key findings regarding O-GlcNAcylation in models of neurodegenerative diseases.

Role of O-linked N-acetylglucosamine modification in diabetic nephropathy

Increased O-linked β-N-acetylglucosamine glycosylation (O-GlcNAcylation) is a known contributor to diabetes; however, its relevance in diabetic nephropathy (DN) is poorly elucidated. Here, we studied the process and enzymes of O-GlcNAcylation with a special emphasis on Akt-endothelial nitric oxide synthase (eNOS) and heat shock protein (HSP)72 signaling. Since tubular injury is the prominent site of DN, the effect of hyperglycemia was first measured in proximal tubular (HK2) cells cultured in high glucose. In vivo O-GlcNAcylation and protein levels of O-GlcNAc transferase (OGT), O-GlcNAcase (OGA), phosphorylated (p)Akt/Akt, peNOS/eNOS, and HSP72 were assessed in the kidney cortex of streptozotocin-induced diabetic rats. The effects of various renin-angiotensin-aldosterone system (RAAS) inhibitors were also evaluated. In proximal tubular cells, hyperglycemia-induced OGT expression led to increased O-GlcNAcylation, which was followed by a compensatory increase of OGA. In parallel, peNOS and pAkt levels decreased, whereas HSP72 increased. In diabetic rats, elevated O-GlcNAcylation was accompanied by decreased OGT and OGA. RAAS inhibitors ameliorated diabetes-induced kidney damage and prevented the elevation of O-GlcNAcylation and the decrement of pAkt, peNOS, and HSP72. In conclusion, hyperglycemia-induced elevation of O-GlcNAcylation contributes to the progression of DN via inhibition of Akt/eNOS phosphorylation and HSP72 induction. RAAS blockers successfully inhibit this process, suggesting a novel pathomechanism of their renoprotective action in the treatment of DN.

Histone demethylase LSD2 acts as an E3 ubiquitin ligase and inhibits cancer cell growth through promoting proteasomal degradation of OGT

Histone demethylases play important roles in various biological processes in a manner dependent on their demethylase activities. However, little is known about their demethylase-independent activities. Here, we report that LSD2, a well-known histone H3K4me1/me2 demethylase, possesses an unexpected E3 ubiquitin ligase activity. LSD2 directly ubiquitylates and promotes proteasome-dependent degradation of O-GlcNAc transferase (OGT), and inhibits A549 lung cancer cell growth in a manner dependent on its E3 ligase activity, but not demethylase activity. The depletion of LSD2 stabilizes OGT and promotes colony formation of 293T cells. LSD2 regulates distinct groups of target genes through histone demethylase and E3 ligase activities, respectively. Such regulation suggests a mechanism through which LSD2 suppresses tumorigenesis by promoting the degradation of OGT and other substrates yet to be discovered. Our study reveals an antigrowth function of LSD2 dependent on its E3 ligase activity and establishes a connection between histone demethylase and ubiquitin-dependent pathway.

A little sugar goes a long way: the cell biology of O-GlcNAc

Unlike the complex glycans decorating the cell surface, the O-linked β-N-acetyl glucosamine (O-GlcNAc) modification is a simple intracellular Ser/Thr-linked monosaccharide that is important for disease-relevant signaling and enzyme regulation. O-GlcNAcylation requires uridine diphosphate-GlcNAc, a precursor responsive to nutrient status and other environmental cues. Alternative splicing of the genes encoding the O-GlcNAc cycling enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) yields isoforms targeted to discrete sites in the nucleus, cytoplasm, and mitochondria. OGT and OGA also partner with cellular effectors and act in tandem with other posttranslational modifications. The enzymes of O-GlcNAc cycling act preferentially on intrinsically disordered domains of target proteins impacting transcription, metabolism, apoptosis, organelle biogenesis, and transport.

The Early Metazoan Trichoplax adhaerens Possesses a Functional O-GlcNAc System

Protein O-GlcNAcylation is a reversible post-translational signaling modification of nucleocytoplasmic proteins that is essential for embryonic development in bilateria. In a search for a reductionist model to study O-GlcNAc signaling, we discovered the presence of functional O-GlcNAc transferase (OGT), O-GlcNAcase (OGA), and nucleocytoplasmic protein O-GlcNAcylation in the most basal extant animal, the placozoan Trichoplax adhaerens. We show via enzymatic characterization of Trichoplax OGT/OGA and genetic rescue experiments in Drosophila melanogaster that these proteins possess activities/functions similar to their bilaterian counterparts. The acquisition of O-GlcNAc signaling by metazoa may have facilitated the rapid and complex signaling mechanisms required for the evolution of multicellular organisms.

Conditional knock-out reveals a requirement for O-linked N-Acetylglucosaminase (O-GlcNAcase) in metabolic homeostasis

O-GlcNAc cycling is maintained by the reciprocal activities of the O-GlcNAc transferase and the O-GlcNAcase (OGA) enzymes. O-GlcNAc transferase is responsible for O-GlcNAc addition to serine and threonine (Ser/Thr) residues and OGA for its removal. Although the Oga gene (MGEA5) is a documented human diabetes susceptibility locus, its role in maintaining insulin-glucose homeostasis is unclear. Here, we report a conditional disruption of the Oga gene in the mouse. The resulting homozygous Oga null (KO) animals lack OGA enzymatic activity and exhibit elevated levels of the O-GlcNAc modification. The Oga KO animals showed nearly complete perinatal lethality associated with low circulating glucose and low liver glycogen stores. Defective insulin-responsive GSK3β phosphorylation was observed in both heterozygous (HET) and KO Oga animals. Although Oga HET animals were viable, they exhibited alterations in both transcription and metabolism. Transcriptome analysis using mouse embryonic fibroblasts revealed deregulation in the transcripts of both HET and KO animals specifically in genes associated with metabolism and growth. Additionally, metabolic profiling showed increased fat accumulation in HET and KO animals compared with WT, which was increased by a high fat diet. Reduced insulin sensitivity, glucose tolerance, and hyperleptinemia were also observed in HET and KO female mice. Notably, the respiratory exchange ratio of the HET animals was higher than that observed in WT animals, indicating the preferential utilization of glucose as an energy source. These results suggest that the loss of mouse OGA leads to defects in metabolic homeostasis culminating in obesity and insulin resistance.

In vivo and in vitro evidence that chronic activation of the hexosamine biosynthetic pathway interferes with leptin-dependent STAT3 phosphorylation

We previously reported that a 2-day peripheral infusion of glucosamine caused leptin resistance in rats, suggesting a role for the hexosamine biosynthetic pathway (HBP) in the development of leptin resistance. Here we tested leptin responsiveness in mice in which HBP activity was stimulated by offering 30% sucrose solution in addition to chow and water or by infusing glucosamine. Mice were leptin resistant after 33 days of access to sucrose. Resistance was associated with increased activity of the HBP and with phosphorylation of transcription factor signal transducer and activator of transcription-3 Tyr705 [pSTAT3(Y705)] but inhibition of suppressor of cytokine signaling 3 in the liver and hypothalamus. Intravenous infusion of glucosamine for 3 h stimulated pSTAT3(Y705) but prevented leptin-induced phosphorylation of STAT3(S727). In an in vitro system, glucose, glucosamine, and leptin each dose dependently increased O-linked β-N-acetylglucosamine (O-GlcNAc) protein and pSTAT3(Y705) in HepG2 cells. To test the effect of glucose on leptin responsiveness cells were incubated in 5.5 mM (LG) or 20 mM (HG) glucose for 18 h and were treated with 0 or 50 ng/ml leptin for 15 min. HG alone and LG + leptin produced similar increases in O-GlcNAc protein, glutamine fructose-6-phosphate amidotransferase (GFAT), and pSTAT3(Y705) compared with LG media. Leptin did not stimulate these proteins in HG cells, suggesting leptin resistance. Leptin-induced pSTAT3(S727) was prevented by HG media. Inhibition of GFAT with azaserine prevented LG + leptin and HG stimulation of pSTAT3. These data demonstrate development of leptin resistance in sucrose-drinking mice and provide new evidence of leptin-induced stimulation of the HBP.

Intracellular protein O-GlcNAc modification integrates nutrient status with transcriptional and metabolic regulation

The inducible, nutrient-sensitive posttranslational modification of protein Ser/Thr residues with O-linked β-N-acetylglucosamine (O-GlcNAc) occurs on histones, transcriptional regulators, metabolic enzymes, oncogenes, tumor suppressors, and many critical intermediates of growth factor signaling. Cycling of O-GlcNAc modification on and off of protein substrates is catalyzed by the actions of O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. To date, there are less than 150 publications addressing the role of O-GlcNAc modification in cancer and over half were published in the last 2 years. These studies have clearly established that increased expression of OGT and hyper-O-GlcNAcylation is common to human cancers of breast, prostate, colon, lung, and pancreas. Furthermore, attenuating OGT activity reduces tumor growth in vitro and metastasis in vivo. This chapter discusses the structure and function of the O-GlcNAc cycling enzymes, mechanisms by which protein O-GlcNAc modification sense changes in nutrient status, the influence of O-GlcNAc cycling enzymes on glucose metabolism, and provides an overview of recent observations regarding the role of O-GlcNAcylation in cancer.

O-GlcNAc and the epigenetic regulation of gene expression

O-GlcNAcylation is an abundant nutrient-driven modification linked to cellular signaling and regulation of gene expression. Utilizing precursors derived from metabolic flux, O-GlcNAc functions as a homeostatic regulator. The enzymes of O-GlcNAc cycling, OGT and O-GlcNAcase, act in mitochondria, the cytoplasm, and the nucleus in association with epigenetic "writers" and "erasers" of the histone code. Both O-GlcNAc and O-phosphate modify repeats within the RNA polymerase II C-terminal domain (CTD). By communicating with the histone and CTD codes, O-GlcNAc cycling provides a link between cellular metabolic status and the epigenetic machinery. Thus, O-GlcNAcylation is poised to influence trans-generational epigenetic inheritance.

O-GlcNAcase: promiscuous hexosaminidase or key regulator of O-GlcNAc signaling?

O-GlcNAc signaling is regulated by an opposing pair of enzymes: O-GlcNAc transferase installs and O-GlcNAcase (OGA) removes the modification from proteins. The dynamics and regulation of this process are only beginning to be understood as the physiological functions of both enzymes are being probed using genetic and pharmacological approaches. This minireview charts the discovery and functional and structural analysis of OGA and summarizes the insights gained from recent studies using OGA inhibition, gene knock-out, and overexpression. We identify several areas of “known unknowns” that would benefit from future research, such as the enigmatic C-terminal domain of OGA.

Gene and protein expression of O-GlcNAc-cycling enzymes in human laryngeal cancer

Aberrant protein O-GlcNAcylation may contribute to the development and malignant behavior of many cancers. This modification is controlled by O-linked β-N-acetylglucosamine transferase (OGT) and O-GlcNAcase (OGA). The aim of this study was to determine the expression of O-GlcNAc cycling enzymes mRNA/protein and to investigate their relationship with clinicopathological parameters in laryngeal cancer. The mRNA levels of OGT and MGEA5 genes were determined in 106 squamous cell laryngeal cancer (SCLC) cases and 73 non-cancerous adjacent laryngeal mucosa (NCLM) controls using quantitative real-time PCR. The level of OGT and OGA proteins was analyzed by Western blot. A positive expression of OGT and MGEA5 transcripts and OGT and OGA proteins was confirmed in 75.5 and 68.9 % and in 43.7 and 59.4 % samples of SCLC, respectively. Higher levels of mRNA/protein for both OGT and OGA as well as significant increases of 60 % in total protein O-GlcNAcylation levels were noted in SCLC compared with NCLM (p < 0.05). As a result, an increased level of OGT and MGEA5 mRNA was related to larger tumor size, nodal metastases, higher grade and tumor behavior according to TFG scale, as well as incidence of disease recurrence (p < 0.05). An inverse association between OGT and MGEA5 transcripts was determined with regard to prognosis (p < 0.05). In addition, the highest OGT and OGA protein levels were observed in poorly differentiated tumors (p < 0.05). No correlations with other parameters were noted, but the results showed a trend of more advanced tumors to be more frequently OGT and OGA positive. The results suggest that increased O-GlcNAcylation may have an effect on tumor aggressiveness and prognosis in laryngeal cancer.

[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.

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.

Increased O-GlcNAc levels correlate with decreased O-GlcNAcase levels in Alzheimer disease brain

The potential role of the posttranslational modification of proteins with O-linked N-acetyl-β-d-glucosamine (O-GlcNAc) in the pathogenesis of Alzheimer disease (AD) has been studied extensively, yet the exact function of O-GlcNAc in AD remains elusive. O-GlcNAc cycling is facilitated by only two highly conserved enzymes: O-GlcNAc transferase (OGT) catalyzes the addition, while O-GlcNAcase (OGA) catalyzes the removal of GlcNAc from proteins. Studies analyzing global O-GlcNAc levels in AD brain have produced inconsistent results and the reasons for altered O-GlcNAcylation in AD are still poorly understood. In this study, we show a 1.2-fold increase in cytosolic protein O-GlcNAc modification in AD brain when compared to age-matched controls. Interestingly, O-GlcNAc changes seem to be attributable to differential modification of a few individual proteins. While our finding of augmented O-GlcNAcylation concurs with some reports, it is contrary to others demonstrating decreased O-GlcNAc levels in AD brain. These conflicting results emphasize the need for further studies providing conclusive evidence on the subject of O-GlcNAcylation in AD. We further demonstrate that, while OGT protein levels are unaffected in AD, OGA protein levels are significantly decreased to 75% of those in control samples. In addition, augmented protein O-GlcNAc modification correlates to decreased OGA protein levels in AD subjects. While OGA inhibitors are already being tested for AD treatment, our results provide a strong indication that the general subject of O-GlcNAcylation and specifically its regulation by OGA and OGT in AD need further investigation to conclusively elucidate its potential role in AD pathogenesis and treatment.

Autoubiquitination of the 26S proteasome on Rpn13 regulates breakdown of ubiquitin conjugates

Degradation rates of most proteins in eukaryotic cells are determined by their rates of ubiquitination. However, possible regulation of the proteasome's capacity to degrade ubiquitinated proteins has received little attention, although proteasome inhibitors are widely used in research and cancer treatment. We show here that mammalian 26S proteasomes have five associated ubiquitin ligases and that multiple proteasome subunits are ubiquitinated in cells, especially the ubiquitin receptor subunit, Rpn13. When proteolysis is even partially inhibited in cells or purified 26S proteasomes with various inhibitors, Rpn13 becomes extensively and selectively poly-ubiquitinated by the proteasome-associated ubiquitin ligase, Ube3c/Hul5. This modification also occurs in cells during heat-shock or arsenite treatment, when poly-ubiquitinated proteins accumulate. Rpn13 ubiquitination strongly decreases the proteasome's ability to bind and degrade ubiquitin-conjugated proteins, but not its activity against peptide substrates. This autoinhibitory mechanism presumably evolved to prevent binding of ubiquitin conjugates to defective or stalled proteasomes, but this modification may also be useful as a biomarker indicating the presence of proteotoxic stress and reduced proteasomal capacity in cells or patients.

Nutrient-driven O-GlcNAc cycling - think globally but act locally

Proper cellular functioning requires that cellular machinery behave in a spatiotemporally regulated manner in response to global changes in nutrient availability. Mounting evidence suggests that one way this is achieved is through the establishment of physically defined gradients of O-GlcNAcylation (O-linked addition of N-acetylglucosamine to serine and threonine residues) and O-GlcNAc turnover. Because O-GlcNAcylation levels are dependent on the nutrient-responsive hexosamine signaling pathway, this modification is uniquely poised to inform upon the nutritive state of an organism. The enzymes responsible for O-GlcNAc addition and removal are encoded by a single pair of genes: both the O-GlcNAc transferase (OGT) and the O-GlcNAcase (OGA, also known as MGEA5) genes are alternatively spliced, producing protein variants that are targeted to discrete cellular locations where they must selectively recognize hundreds of protein substrates. Recent reports suggest that in addition to their catalytic functions, OGT and OGA use their multifunctional domains to anchor O-GlcNAc cycling to discrete intracellular sites, thus allowing them to establish gradients of deacetylase, kinase and phosphatase signaling activities. The localized signaling gradients established by targeted O-GlcNAc cycling influence many important cellular processes, including lipid droplet remodeling, mitochondrial functioning, epigenetic control of gene expression and proteostasis. As such, the tethering of the enzymes of O-GlcNAc cycling appears to play a role in ensuring proper spatiotemporal responses to global alterations in nutrient supply.

Hepatic glucose sensing and integrative pathways in the liver

The hepatic glucose-sensing system is a functional network of enzymes and transcription factors that is critical for the maintenance of energy homeostasis and systemic glycemia. Here we review the recent literature on its components and metabolic actions. Glucokinase (GCK) is generally considered as the initial postprandial glucose-sensing component, which acts as the gatekeeper for hepatic glucose metabolism and provides metabolites that activate the transcription factor carbohydrate response element binding protein (ChREBP). Recently, liver receptor homolog 1 (LRH-1) has emerged as an upstream regulator of the central GCK-ChREBP axis, with a critical role in the integration of hepatic intermediary metabolism in response to glucose. Evidence is also accumulating that O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) and acetylation can act as glucose-sensitive modifications that may contribute to hepatic glucose sensing by targeting regulatory proteins and the epigenome. Further elucidation of the components and functional roles of the hepatic glucose-sensing system may contribute to the future treatment of liver diseases associated with deregulated glucose sensors.

The role of O-GlcNAc signaling in the pathogenesis of diabetic retinopathy

Diabetic retinopathy is a leading cause of blindness worldwide. Despite laser and surgical treatments, antiangiogenic and other therapies, and strict metabolic control, many patients progress to visual impairment and blindness. New insights are needed into the pathophysiology of diabetic retinopathy in order to develop new methods to improve the detection and treatment of disease and the prevention of blindness. Hyperglycemia and diabetes result in increased flux through the hexosamine biosynthetic pathway, which, in turn, results in increased PTM of Ser/Thr residues of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc). O-GlcNAcylation is involved in regulation of many nuclear and cytoplasmic proteins in a manner similar to protein phosphorylation. Altered O-GlcNAc signaling has been implicated in the pathogenesis of diabetes and may play an important role in the pathogenesis of diabetic retinopathy. The goal of this review is to summarize the biology of the hexosamine biosynthesis pathway and O-GlcNAc signaling, to present the current evidence for the role of O-GlcNAc signaling in diabetes and diabetic retinopathy, and to discuss future directions for research on O-GlcNAc in the pathogenesis of diabetic retinopathy.

Metabolic control of hyaluronan synthases

Hyaluronan (HA) is a glycosaminoglycan composed by repeating units of D-glucuronic acid (GlcUA) and N-acetylglucosamine (GlcNAc) that is ubiquitously present in the extracellular matrix (ECM) where it has a critical role in the physiology and pathology of several mammalian tissues. HA represents a perfect environment in which cells can migrate and proliferate. Moreover, several receptors can interact with HA at cellular level triggering multiple signal transduction responses. The control of the HA synthesis is therefore critical in ECM assembly and cell biology; in this review we address the metabolic regulation of HA synthesis. In contrast with other glycosaminoglycans, which are synthesized in the Golgi apparatus, HA is produced at the plasma membrane by HA synthases (HAS1-3), which use cytoplasmic UDP-glucuronic acid and UDP-N-acetylglucosamine as substrates. UDP-GlcUA and UDP-hexosamine availability is critical for the synthesis of GAGs, which is an energy consuming process. AMP activated protein kinase (AMPK), which is considered a sensor of the energy status of the cell and is activated by low ATP:AMP ratio, leads to the inhibition of HA secretion by HAS2 phosphorylation at threonine 110. However, the most general sensor of cellular nutritional status is the hexosamine biosynthetic pathway that brings to the formation of UDP-GlcNAc and intracellular protein glycosylation by O-linked attachment of the monosaccharide β-N-acetylglucosamine (O-GlcNAcylation) to specific aminoacid residues. Such highly dynamic and ubiquitous protein modification affects serine 221 residue of HAS2 that lead to a dramatic stabilization of the enzyme in the membranes.

Regulation of protein degradation by O-GlcNAcylation: crosstalk with ubiquitination

The post-translational modification of intracellular proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates essential cellular processes such as signal transduction, transcription, translation, and protein degradation. Misfolded, damaged, and unwanted proteins are tagged with a chain of ubiquitin moieties for degradation by the proteasome, which is critical for cellular homeostasis. In this review, we summarize the current knowledge of the interplay between O-GlcNAcylation and ubiquitination in the control of protein degradation. Understanding the mechanisms of action of O-GlcNAcylation in the ubiquitin-proteosome system shall facilitate the development of therapeutics for human diseases such as cancer, metabolic syndrome, and neurodegenerative diseases.

O-GlcNAc processing enzymes: catalytic mechanisms, substrate specificity, and enzyme regulation

The addition of N-acetylglucosamine (GlcNAc) O-linked to serine and threonine residues of proteins is known as O-GlcNAc. This post-translational modification is found within multicellular eukaryotes on hundreds of nuclear and cytoplasmic proteins. O-GlcNAc transferase (OGT) installs O-GlcNAc onto target proteins and O-GlcNAcase (OGA) removes O-GlcNAc. Their combined action makes O-GlcNAc reversible and serves to regulate cellular O-GlcNAc levels. Here I review select recent literature on the catalytic mechanism of these enzymes and studies on the molecular basis by which these enzymes identify and process their substrates. Molecular level understanding of how these enzymes work, and the basis for their specificity, should aid understanding how O-GlcNAc contributes to diverse cellular processes ranging from cellular signaling through to transcriptional regulation.

O-GlcNAc cycling mutants modulate proteotoxicity in Caenorhabditis elegans models of human neurodegenerative diseases

O-GlcNAcylation is an abundant posttranslational modification in the brain implicated in human neurodegenerative diseases. We have exploited viable null alleles of the enzymes of O-GlcNAc cycling to examine the role of O-GlcNAcylation in well-characterized Caenorhabditis elegans models of neurodegenerative proteotoxicity. O-GlcNAc cycling dramatically modulated the severity of the phenotype in transgenic models of tauopathy, amyloid β-peptide, and polyglutamine expansion. Intriguingly, loss of function of O-GlcNAc transferase alleviated, whereas loss of O-GlcNAcase enhanced, the phenotype of multiple neurodegenerative disease models. The O-GlcNAc cycling mutants act in part by altering DAF-16-dependent transcription and modulating the protein degradation machinery. These findings suggest that O-GlcNAc levels may directly influence neurodegenerative disease progression, thus making the enzymes of O-GlcNAc cycling attractive targets for neurodegenerative disease therapies.

Role of UDP-N-acetylglucosamine (GlcNAc) and O-GlcNAcylation of hyaluronan synthase 2 in the control of chondroitin sulfate and hyaluronan synthesis

Hyaluronan (HA) is a glycosaminoglycan present in most tissue microenvironments that can modulate many cell behaviors, including proliferation, migration, and adhesive proprieties. In contrast with other glycosaminoglycans, which are synthesized in the Golgi, HA is synthesized at the plasma membrane by one or more of the three HA synthases (HAS1-3), which use cytoplasmic UDP-glucuronic acid and UDP-N-acetylglucosamine as substrates. Previous studies revealed the importance of UDP-sugars for regulating HA synthesis. Therefore, we analyzed the effect of UDP-GlcNAc availability and protein glycosylation with O-linked N-acetylglucosamine (O-GlcNAcylation) on HA and chondroitin sulfate synthesis in primary human aortic smooth muscle cells. Glucosamine treatment, which increases UDP-GlcNAc availability and protein O-GlcNAcylation, increased synthesis of both HA and chondroitin sulfate. However, increasing O-GlcNAcylation by stimulation with O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate without a concomitant increase of UDP-GlcNAc increased only HA synthesis. We found that HAS2, the main synthase in aortic smooth muscle cells, can be O-GlcNAcylated on serine 221, which strongly increased its activity and its stability (t(&frac12;) >5 h versus ∼17 min without O-GlcNAcylation). S221A mutation prevented HAS2 O-GlcNAcylation, which maintained the rapid turnover rate even in the presence of GlcN and increased UDP-GlcNAc. These findings could explain the elevated matrix HA observed in diabetic vessels that, in turn, could mediate cell dedifferentiation processes critical in vascular pathologies.

Mapping of O-GlcNAc sites of 20 S proteasome subunits and Hsp90 by a novel biotin-cystamine tag

The post-translational modification of proteins with O-GlcNAc is involved in various cellular processes including signal transduction, transcription, translation, and nuclear transport. This transient protein modification enables cells or tissues to adapt to nutrient conditions or stress. O-Glycosylation of the 26 S proteasome ATPase subunit Rpt2 is known to influence the stability of proteins by reducing their proteasome-dependent degradation. In contrast, knowledge of the sites of O-GlcNAcylation on the subunits of the catalytic core of the 26 S proteasome, the 20 S proteasome, and the impact on proteasome activity is very limited. This is predominantly because O-GlcNAc modifications are often substoichiometric and because 20 S proteasomes represent a complex protein mixture of different subtypes. Therefore, identification of O-GlcNAcylation sites on proteasome subunits essentially requires effective enrichment strategies. Here we describe an adapted β-elimination-based derivatization method of O-GlcNAc peptides using a novel biotin-cystamine tag. The specificity of the reaction was increased by differential isotopic labeling with either "light" biotin-cystamine or deuterated "heavy" biotin-cystamine. The enriched peptides were analyzed by LC-MALDI-TOF/TOF-MS and relatively quantified. The method was optimized using bovine α-crystallin and then applied to murine 20 S proteasomes isolated from spleen and brain and murine Hsp90 isolated from liver. Using this approach, we identified five novel and one known O-GlcNAc sites within the murine 20 S proteasome core complex that are located on five different subunits and in addition two novel O-GlcNAc sites on murine Hsp90β, of which one corresponds to a previously described phosphorylation site.