A Caenorhabditis elegans model of insulin resistance: altered macronutrient storage and dauer formation in an OGT-1 knockout

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

Mutations in N-acetylglucosamine (O-GlcNAc) transferase in patients with X-linked intellectual disability

N-Acetylglucosamine (O-GlcNAc) transferase (OGT) regulates protein O-GlcNAcylation, an essential and dynamic post-translational modification. The O-GlcNAc modification is present on numerous nuclear and cytosolic proteins and has been implicated in essential cellular functions such as signaling and gene expression. Accordingly, altered levels of protein O-GlcNAcylation have been associated with developmental defects and neurodegeneration. However, mutations in the OGT gene have not yet been functionally confirmed in humans. Here, we report on two hemizygous mutations in OGT in individuals with X-linked intellectual disability (XLID) and dysmorphic features: one missense mutation (p.Arg284Pro) and one mutation leading to a splicing defect (c.463–6T>G). Both mutations reside in the tetratricopeptide repeats of OGT that are essential for substrate recognition. We observed slightly reduced levels of OGT protein and reduced levels of its opposing enzyme O-GlcNAcase in both patient-derived fibroblasts, but global O-GlcNAc levels appeared to be unaffected. Our data suggest that mutant cells attempt to maintain global O-GlcNAcylation by down-regulating O-GlcNAcase expression. We also found that the c.463–6T>G mutation leads to aberrant mRNA splicing, but no stable truncated protein was detected in the corresponding patient-derived fibroblasts. Recombinant OGT bearing the p.Arg284Pro mutation was prone to unfolding and exhibited reduced glycosylation activity against a complex array of glycosylation substrates and proteolytic processing of the transcription factor host cell factor 1, which is also encoded by an XLID-associated gene. We conclude that defects in O-GlcNAc homeostasis and host cell factor 1 proteolysis may play roles in mediation of XLID in individuals with OGT mutations.

The Biochemistry of O-GlcNAc Transferase: Which Functions Make It Essential in Mammalian Cells?

O-linked N-acetylglucosamine transferase (OGT) is found in all metazoans and plays an important role in development but at the single-cell level is only essential in dividing mammalian cells. Postmitotic mammalian cells and cells of invertebrates such as Caenorhabditis elegans and Drosophila can survive without copies of OGT. Why OGT is required in dividing mammalian cells but not in other cells remains unknown. OGT has multiple biochemical activities. Beyond its well-known role in adding β-O-GlcNAc to serine and threonine residues of nuclear and cytoplasmic proteins, OGT also acts as a protease in the maturation of the cell cycle regulator host cell factor 1 (HCF-1) and serves as an integral member of several protein complexes, many of them linked to gene expression. In this review, we summarize current understanding of the mechanisms underlying OGT's biochemical activities and address whether known functions of OGT could be related to its essential role in dividing mammalian cells.

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

Protein O-GlcNAcylation: emerging mechanisms and functions

O-GlcNAcylation - the attachment of O-linked N-acetylglucosamine (O-GlcNAc) moieties to cytoplasmic, nuclear and mitochondrial proteins - is a post-translational modification that regulates fundamental cellular processes in metazoans. A single pair of enzymes - O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) - controls the dynamic cycling of this protein modification in a nutrient- and stress-responsive manner. Recent years have seen remarkable advances in our understanding of O-GlcNAcylation at levels that range from structural and molecular biology to cell signalling and gene regulation to physiology and disease. New mechanisms and functions of O-GlcNAcylation that are emerging from these recent developments enable us to begin constructing a unified conceptual framework through which the significance of this modification in cellular and organismal physiology can be understood.

O-GlcNAc cycling and the regulation of nucleocytoplasmic dynamics

The dynamic carbohydrate post-translational modification (PTM) O-linked β-N-acetyl glucosamine (O-GlcNAc) is found on thousands of proteins throughout the nucleus and cytoplasm, and rivals phosphorylation in terms of the number of substrates and pathways influenced. O-GlcNAc is highly conserved and essential in most organisms, with disruption of O-GlcNAc cycling linked to diseases ranging from cancer to neurodegeneration. Nuclear pore proteins were the first identified O-GlcNAc-modified substrates, generating intense and ongoing interest in understanding the role of O-GlcNAc cycling in nuclear pore complex structure and function. Recent advances in detecting and altering O-GlcNAcylation levels have provided insights into many mechanisms by which O-GlcNAcylation influences the nucleocytoplasmic localization and stability of protein targets. The emerging view is that the multifunctional enzymes of O-GlcNAc cycling are critical nutrient-sensing components of a complex network of signaling cascades involving multiple PTMs. Furthermore, O-GlcNAc plays a role in maintaining the structural integrity of the nuclear pore and regulating its function as the gatekeeper of nucleocytoplasmic trafficking.

O-GlcNAcylation of SKN-1 modulates the lifespan and oxidative stress resistance in Caenorhabditis elegans

In C. elegans, the transcription factor skinhead-1 (SKN-1), the ortholog of human NF-E2-related factor 2 (Nrf-2), plays important roles in oxidative stress defense and aging processes. It has been documented that the activity of SKN-1 is regulated by its phosphorylation modification. However, whether other posttranslational modifications of SKN-1 affect its function remains unclear to date. Here we report, for the first time, that SKN-1 is O-GlcNAcylated at Ser470 and Thr493 by O-GlcNActransferase OGT-1. By generating the double mutations of Ser470/Thr493 in the wild type and skn-1(zu67) worms, respectively, we found that disruption of O-GlcNAc modification on SKN-1 repressed the accumulation of SKN-1 in the intestinal nuclei, and decreased the activities of SKN-1 in modulating lifespan and oxidative stress resistance. Moreover, under oxidative stress, SKN-1 was highly O-GlcNAcylated, resulting in the decrease of GSK-3-mediated phosphorylation at Ser483 adjacent to the O-GlcNAcylated residues (Ser470 and Thr493). These data suggest that O-GlcNAcylation of SKN-1 is crucial for regulating lifespan and oxidative stress resistance via the crosstalk with its phosphorylation in C. elegans. These findings have important implications for studying the functions of O-GlcNAcylation on Nrf-2 in human aging-related diseases.

Dual functionality of O-GlcNAc transferase is required for Drosophila development

Post-translational modification of intracellular proteins with O-linked N-acetylglucosamine (O-GlcNAc) catalysed by O-GlcNAc transferase (OGT) has been linked to regulation of diverse cellular functions. OGT possesses a C-terminal glycosyltransferase catalytic domain and N-terminal tetratricopeptide repeats that are implicated in protein-protein interactions. Drosophila OGT (DmOGT) is encoded by super sex combs (sxc), mutants of which are pupal lethal. However, it is not clear if this phenotype is caused by reduction of O-GlcNAcylation. Here we use a genetic approach to demonstrate that post-pupal Drosophila development can proceed with negligible OGT catalysis, while early embryonic development is OGT activity-dependent. Structural and enzymatic comparison between human OGT (hOGT) and DmOGT informed the rational design of DmOGT point mutants with a range of reduced catalytic activities. Strikingly, a severely hypomorphic OGT mutant complements sxc pupal lethality. However, the hypomorphic OGT mutant-rescued progeny do not produce F2 adults, because a set of Hox genes is de-repressed in F2 embryos, resulting in homeotic phenotypes. Thus, OGT catalytic activity is required up to late pupal stages, while further development proceeds with severely reduced OGT activity.

Synthetic Ligands of Cannabinoid Receptors Affect Dauer Formation in the Nematode Caenorhabditis elegans

Under adverse environmental conditions the nematode Caenorhabditis elegans can enter an alternate developmental stage called the dauer larva. To identify lipophilic signaling molecules that influence this process, we screened a library of bioactive lipids and found that AM251, an antagonist of the human cannabinoid (CB) receptor, suppresses dauer entry in daf-2 insulin receptor mutants. AM251 acted synergistically with glucose supplementation indicating that the metabolic status of the animal influenced the activity of this compound. Similarly, loss of function mutations in the energy-sensing AMP-activated kinase subunit, aak-2, enhanced the dauer-suppressing effects of AM251, while constitutive activation of aak-2 in neurons was sufficient to inhibit AM251 activity. Chemical epistasis experiments indicated that AM251 acts via G-protein signaling and requires the TGF-β ligand DAF-7, the insulin peptides DAF-28 and INS-6, and a functional ASI neuron to promote reproductive growth. AM251 also required the presence of the SER-5 serotonin receptor, but in vitro experiments suggest that this may not be via a direct interaction. Interestingly, we found that other antagonists of mammalian CB receptors also suppress dauer entry, while the nonselective CB receptor agonist, O-2545, not only inhibited the activity of AM251, but also was able to promote dauer entry when administered alone. Since worms do not have obvious orthologs of CB receptors, the effects of synthetic CBs on neuroendocrine signaling in C. elegans are likely to be mediated via another, as yet unknown, receptor mechanism. However, we cannot exclude the existence of a noncanonical CB receptor in C. elegans.

Dissecting PUGNAc-mediated inhibition of the pro-survival action of insulin

Previous studies utilizing PUGNAc, the most widely used β-N-acetylglucosaminidase (OGA) inhibitor to increase global O-N-acetylglucosamine (GlcNAc) levels, have reported a variety of effects including insulin resistance as a direct result of elevated O-GlcNAc levels. The notion of OGA inhibition causing insulin resistance was not replicated in studies in which elevated global O-GlcNAc levels were achieved using two other OGA inhibitors. Related to insulin action, work by others has suggested that O-GlcNAc elevation may inhibit the anti-apoptotic action of insulin. Thus, we examined the pro-survival action of insulin upon serum deprivation in the presence of PUGNAc as well as two selective OGA inhibitors (GlcNAcstatin-g and Thiamet-G), and a selective lysosomal hexosaminidase inhibitor (INJ2). We established that PUGNAc inhibits the pro-survival action of insulin but this effect is not recapitulated by the selective OGA inhibitors suggesting that elevation in O-GlcNAc levels alone is not responsible for PUGNAc's effect on the anti-apoptotic action of insulin. Further, we demonstrate that a selective hexosaminidase A/B (HexA/B) inhibitor does not impact insulin action suggesting that PUGNAc's effect is not due to inhibition of lysosomal hexosaminidase. Finally, we tested a combination of selective OGA and lysosomal hexosaminidase inhibitors but were not able to recapitulate the inhibition of insulin action generated by PUGNAc alone. These results strongly suggest that the defect in insulin action upon PUGNAc treatment does not derive from its inhibition of OGA or HexA/B, and that there is an unknown target of PUGNAc that is the likely culprit in inhibiting the protective effect of insulin from apoptosis.

Proteolysis of HCF-1 by Ser/Thr glycosylation-incompetent O-GlcNAc transferase:UDP-GlcNAc complexes

In this study, Kapuria et al. investigate the dual glycosyltransferase–protease activity (which occurs in the same active site) of OGT. They show that glycosylation and proteolysis occur through separable mechanisms and present a model for the evolution of HCF-1 proteolysis by OGT.

In complex with the cosubstrate UDP-N-acetylglucosamine (UDP-GlcNAc), O-linked-GlcNAc transferase (OGT) catalyzes Ser/Thr O-GlcNAcylation of many cellular proteins and proteolysis of the transcriptional coregulator HCF-1. Such a dual glycosyltransferase–protease activity, which occurs in the same active site, is unprecedented and integrates both reversible and irreversible forms of protein post-translational modification within one enzyme. Although occurring within the same active site, we show here that glycosylation and proteolysis occur through separable mechanisms. OGT consists of tetratricopeptide repeat (TPR) and catalytic domains, which, together with UDP-GlcNAc, are required for both glycosylation and proteolysis. Nevertheless, a specific TPR domain contact with the HCF-1 substrate is critical for proteolysis but not Ser/Thr glycosylation. In contrast, key catalytic domain residues and even a UDP-GlcNAc oxygen important for Ser/Thr glycosylation are irrelevant for proteolysis. Thus, from a dual glycosyltransferase–protease, essentially single-activity enzymes can be engineered both in vitro and in vivo. Curiously, whereas OGT-mediated HCF-1 proteolysis is limited to vertebrate species, invertebrate OGTs can cleave human HCF-1. We present a model for the evolution of HCF-1 proteolysis by OGT.

Damaging effects of hyperglycemia on cardiovascular function: spotlight on glucose metabolic pathways

The incidence of cardiovascular complications associated with hyperglycemia is a growing global health problem. This review discusses the link between hyperglycemia and cardiovascular diseases onset, focusing on the role of recently emerging downstream mediators, namely, oxidative stress and glucose metabolic pathway perturbations. The role of hyperglycemia-mediated activation of nonoxidative glucose pathways (NOGPs) [i.e., the polyol pathway, hexosamine biosynthetic pathway, advanced glycation end products (AGEs), and protein kinase C] in this process is extensively reviewed. The proposal is made that there is a unique interplay between NOGPs and a downstream convergence of detrimental effects that especially affect cardiac endothelial cells, thereby contributing to contractile dysfunction. In this process the AGE pathway emerges as a crucial mediator of hyperglycemia-mediated detrimental effects. In addition, a vicious metabolic cycle is established whereby hyperglycemia-induced NOGPs further fuel their own activation by generating even more oxidative stress, thereby exacerbating damaging effects on cardiac function. Thus NOGP inhibition, and particularly that of the AGE pathway, emerges as a novel therapeutic intervention for the treatment of cardiovascular complications such as acute myocardial infarction in the presence hyperglycemia.

Modeling type 2 diabetes-like hyperglycemia in C. elegans on a microdevice

Caenorhabditis elegans (C. elegans) has been widely used as a model organism for biomedical research due to its sufficient homology with mammals at the molecular and genomic levels. In this work, we describe a microfluidic assay to model type 2 diabetes-like hyperglycemia in C. elegans to examine several aspects of this disease on a microdevice. The microdevice is characterized by the integration of long-term worm culture, worm immobilization, and precise chemical stimuli in a single device, thus enabling the multi-parameter analysis of individual worms at a single-animal resolution. With this device, the lifespan, oxidative stress responses, and lipid metabolism of individual worms in response to different glucose concentrations were characterized. It was found that the mean lifespan of worms was significantly reduced by as much as 29.0% and 30.8% in worms that were subjected to 100 mM and 200 mM glucose, respectively. The expression of oxidative stress protein gst-4 was increased, and the expression of hsp-70 (heat shock protein) and skn-1 (redox sensitive transcription factor) genes was down-regulated in worms treated with a high level of glucose. Moreover, fat storage was markedly increased in the bodies of VS29 worms (vha-6p::GFP::dgat-2) that were exposed to the high-glucose condition. The established approach is not only suitable for further elucidation of the mechanism of metabolic disorders involved in diabetes and its complications, but also facilitates the evaluation of anti-diabetic drugs in a high-throughput manner.

Quantitative phosphoproteomics reveals crosstalk between phosphorylation and O-GlcNAc in the DNA damage response pathway

The modification of intracellular proteins by monosaccharides of O-linked β-N-acetylglucosamine (O-GlcNAc) is an essential and dynamic PTM of metazoans. The addition and removal of O-GlcNAc is catalyzed by the O-GlcNAc transferase (OGT) and O-GlcNAcase, respectively. One mechanism by which O-GlcNAc is thought to mediate proteins is by regulating phosphorylation. To provide insight into the pathways regulated by O-GlcNAc, we have utilized SILAC-based quantitative proteomics to carry out comparisons of site-specific phosphorylation in OGT wild-type and Null cells. Quantitation of the phosphoproteome demonstrated that of 5529 phosphoserine, phosphothreonine, and phosphotyrosine sites, 232 phosphosites were upregulated and 133 downregulated in the absence of O-GlcNAc. Collectively, these data suggest that deletion of OGT has a profound effect on the phosphorylation of cell cycle and DNA damage response proteins. Key events were confirmed by biochemical analyses and demonstrate an increase in the activating autophosphorylation event on ATM (Ser1987) and on ATM's downstream targets p53, H2AX, and Chk2. Together, these data support widespread changes in the phosphoproteome upon removal of O-GlcNAc, suggesting that O-GlcNAc regulates processes such as the cell cycle, genomic stability, and lysosomal biogenesis. All MS data have been deposited in the ProteomeXchange with identifier PXD001153 (http://proteomecentral.proteomexchange.org/dataset/PXD001153).

Loss of C. elegans GON-1, an ADAMTS9 Homolog, Decreases Secretion Resulting in Altered Lifespan and Dauer Formation

ADAMTS9 is a metalloprotease that cleaves components of the extracellular matrix and is also implicated in transport from the endoplasmic reticulum (ER) to the Golgi. It has been reported that an ADAMTS9 gene variant is associated with type 2 diabetes. The underlying pathology of type 2 diabetes is insulin resistance and beta-cell dysfunction. However, the molecular mechanisms underlying ADAMTS9 function in beta cells and peripheral tissues are unknown. We show that loss of C. elegans GON-1, an ADAMTS9 homolog, alters lifespan and dauer formation. GON-1 loss impairs secretion of proteins such as insulin orthologs and TGF-beta, and additionally impacts insulin/IGF-1 signaling in peripheral tissues. The function of the GON domain, but not the protease domain, is essential for normal lifespan and dauer formation in these scenarios. We conclude that the GON domain is critical for ADAMTS9/GON-1 function across species, which should help the understanding of type 2 diabetes in humans.

Glycogen Fuels Survival During Hyposmotic-Anoxic Stress in Caenorhabditis elegans

Oxygen is an absolute requirement for multicellular life. Animals that are deprived of oxygen for sufficient periods of time eventually become injured and die. This is largely due to the fact that, without oxygen, animals are unable to generate sufficient quantities of energy. In human diseases triggered by oxygen deprivation, such as heart attack and stroke, hyposmotic stress and cell swelling (edema) arise in affected tissues as a direct result of energetic failure. Edema independently enhances tissue injury in these diseases by incompletely understood mechanisms, resulting in poor clinical outcomes. Here, we present investigations into the effects of osmotic stress during complete oxygen deprivation (anoxia) in the genetically tractable nematode Caenorhabditis elegans. Our findings demonstrate that nematode survival of a hyposmotic environment during anoxia (hyposmotic anoxia) depends on the nematode’s ability to engage in glycogen metabolism. We also present results of a genome-wide screen for genes affecting glycogen content and localization in the nematode, showing that nematode survival of hyposmotic anoxia depends on a large number of these genes. Finally, we show that an inability to engage in glycogen synthesis results in suppression of the enhanced survival phenotype observed in daf-2 insulin-like pathway mutants, suggesting that alterations in glycogen metabolism may serve as a basis for these mutants’ resistance to hyposmotic anoxia.

Post-translational O-GlcNAcylation is essential for nuclear pore integrity and maintenance of the pore selectivity filter

O-glycosylation of the nuclear pore complex (NPC) by O-linked N-acetylglucosamine (O-GlcNAc) is conserved within metazoans. Many nucleoporins (Nups) comprising the NPC are constitutively O-GlcNAcylated, but the functional role of this modification remains enigmatic. We show that loss of O-GlcNAc, induced by either inhibition of O-GlcNAc transferase (OGT) or deletion of the gene encoding OGT, leads to decreased cellular levels of a number of natively O-GlcNAcylated Nups. Loss of O-GlcNAc enables increased ubiquitination of these Nups and their increased proteasomal degradation. The decreased half-life of these deglycosylated Nups manifests in their gradual loss from the NPC and a downstream malfunction of the nuclear pore selective permeability barrier in both dividing and post-mitotic cells. These findings define a critical role of O-GlcNAc modification of the NPC in maintaining its composition and the function of the selectivity filter. The results implicate NPC glycosylation as a regulator of NPC function and reveal the role of conserved glycosylation of the NPC among metazoans.

O-GlcNAc occurs cotranslationally to stabilize nascent polypeptide chains

Nucleocytoplasmic glycosylation of proteins with O-linked N-acetylglucosamine residues (O-GlcNAc) is recognized as a conserved post-translational modification found in all metazoans. O-GlcNAc has been proposed to regulate diverse cellular processes. Impaired cellular O-GlcNAcylation has been found to lead to decreases in the levels of various proteins, which is one mechanism by which O-GlcNAc seems to exert its varied physiological effects. Here we show that O-GlcNAcylation also occurs cotranslationally. This process protects nascent polypeptide chains from premature degradation by decreasing cotranslational ubiquitylation. Given that hundreds of proteins are O-GlcNAcylated within cells, our findings suggest that cotranslational O-GlcNAcylation may be a phenomenon regulating proteostasis of an array of nucleocytoplasmic proteins. These findings set the stage to assess whether O-GlcNAcylation has a role in protein quality control in a manner that bears similarity with the role played by N-glycosylation within the secretory pathway.

A critical perspective of the diverse roles of O-GlcNAc transferase in chromatin

O-linked β-N-Acetylglucosamine (O-GlcNAc) is a posttranslational modification that is catalyzed by O-GlcNAc transferase (Ogt) and found on a plethora of nuclear and cytosolic proteins in animals and plants. Studies in different model organisms revealed that while O-GlcNAc is required for selected processes in Caenorhabditis elegans and Drosophila, it has evolved to become required for cell viability in mice, and this has challenged investigations to identify cellular functions that critically require this modification in mammals. Nevertheless, a principal cellular process that engages O-GlcNAcylation in all of these species is the regulation of gene transcription. Here, we revisit several of the primary experimental observations that led to current models of how O-GlcNAcylation affects gene expression. In particular, we discuss the role of the stable association of Ogt with the transcription factors Hcf1 and Tet, the two main Ogt-interacting proteins in nuclei of mammalian cells. We also critically evaluate the evidence that specific residues on core histones, including serine 112 of histone 2B (H2B-S112), are O-GlcNAcylated in vivo and discuss possible physiological effects of these modifications. Finally, we review our understanding of the role of O-GlcNAcylation in Drosophila, where recent studies suggest that the developmental defects in Ogt mutants are all caused by lack of O-GlcNAcylation of a single transcriptional regulator, the Polycomb repressor protein Polyhomeotic (Ph). Collectively, this reexamination of the experimental evidence suggests that a number of recently propagated models about the role of O-GlcNAcylation in transcriptional control should be treated cautiously.

Nucleotide substitutions revealing specific functions of Polycomb group genes

POLYCOMB group (PCG) proteins belong to the family of epigenetic regulators of genes playing important roles in differentiation and development. Mutants of PcG genes were isolated first in the fruit fly, Drosophila melanogaster, resulting in spectacular segmental transformations due to the ectopic expression of homeotic genes. Homologs of Drosophila PcG genes were also identified in plants and in vertebrates and subsequent experiments revealed the general role of PCG proteins in the maintenance of the repressed state of chromatin through cell divisions. The past decades of gene targeting experiments have allowed us to make significant strides towards understanding how the network of PCG proteins influences multiple aspects of cellular fate determination during development. Being involved in the transmission of specific expression profiles of different cell lineages, PCG proteins were found to control wide spectra of unrelated epigenetic processes in vertebrates, such as stem cell plasticity and renewal, genomic imprinting and inactivation of X-chromosome. PCG proteins also affect regulation of metabolic genes being important for switching programs between pluripotency and differentiation. Insight into the precise roles of PCG proteins in normal physiological processes has emerged from studies employing cell culture-based systems and genetically modified animals. Here we summarize the findings obtained from PcG mutant fruit flies and mice generated to date with a focus on PRC1 and PRC2 members altered by nucleotide substitutions resulting in specific alleles. We also include a compilation of lessons learned from these models about the in vivo functions of this complex protein family. With multiple knockout lines, sophisticated approaches to study the consequences of peculiar missense point mutations, and insights from complementary gain-of-function systems in hand, we are now in a unique position to significantly advance our understanding of the molecular basis of in vivo functions of PcG proteins.

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.

Chemical tools to explore nutrient-driven O-GlcNAc cycling

Posttranslational modifications (PTM) including glycosylation, phosphorylation, acetylation, methylation and ubiquitination dynamically alter the proteome. The evolutionarily conserved enzymes O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) and O-GlcNAcase are responsible for the addition and removal, respectively, of the nutrient-sensitive PTM of protein serine and threonine residues with O-GlcNAc. Indeed, the O-GlcNAc modification acts at every step in the "central dogma" of molecular biology and alters signaling pathways leading to amplified or blunted biological responses. The cellular roles of OGT and the dynamic PTM O-GlcNAc have been clarified with recently developed chemical tools including high-throughput assays, structural and mechanistic studies and potent enzyme inhibitors. These evolving chemical tools complement genetic and biochemical approaches for exposing the underlying biological information conferred by O-GlcNAc cycling.

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.

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.

Evidence of a MOF histone acetyltransferase-containing NSL complex in C. elegans

Regulation of chromatin is a key process in the developmental control of gene expression. Many multi-subunit protein complexes have been found to regulate chromatin through the modification of histone residues. One such complex is the MOF histone acetyltransferase-containing NSL complex. While the composition of the human and Drosophila NSL complexes has been determined and the functions of these complexes investigated, the existence of an equivalent complex in nematodes such as Caenorhabditis elegans has not yet been explored. Here we summarise evidence, from our own work and that of others, that homologues of NSL complex components are found in C. elegans. We review data suggesting that nematode proteins SUMV-1 and SUMV-2 are homologous to NSL2 and NSL3, respectively, and that SUMV-1 and SUMV-2 may form a complex with MYS-2, the worm homolog of MOF. We propose that these interactions suggest the existence of a nematode NSL-like complex and discuss the roles of this putative NSL complex in worms as well as exploring the possibility of crosstalk between NSL and COMPASS complexes via components that are common to both. We present the groundwork from which a full characterization of a nematode NSL complex may begin.

O-GlcNAcylation prevents aggregation of the Polycomb group repressor polyhomeotic

The glycosyltransferase Ogt adds O-linked N-Acetylglucosamine (O-GlcNAc) moieties to nuclear and cytosolic proteins. Drosophila embryos lacking Ogt protein arrest development with a remarkably specific Polycomb phenotype, arising from the failure to repress Polycomb target genes. The Polycomb protein Polyhomeotic (Ph), an Ogt substrate, forms large aggregates in the absence of O-GlcNAcylation both in vivo and in vitro. O-GlcNAcylation of a serine/threonine (S/T) stretch in Ph is critical to prevent nonproductive aggregation of both Drosophila and human Ph via their C-terminal sterile alpha motif (SAM) domains in vitro. Full Ph repressor activity in vivo requires both the SAM domain and O-GlcNAcylation of the S/T stretch. We demonstrate that Ph mutants lacking the S/T stretch reproduce the phenotype of ogt mutants, suggesting that the S/T stretch in Ph is the key Ogt substrate in Drosophila. We propose that O-GlcNAcylation is needed for Ph to form functional, ordered assemblies via its SAM domain.

Multiple tissue-specific roles for the O-GlcNAc post-translational modification in the induction of and complications arising from type II diabetes

In this minireview, we will highlight work in the last 30 years that has clearly demonstrated that the O-GlcNAc modification is nutrient-responsive and plays multiple roles in metabolic regulation of signaling and gene expression. Further, we will examine recent studies that have investigated the impact of O-GlcNAc in a variety of glucose- and insulin-responsive tissues and the roles attributed to O-GlcNAc in the induction of insulin resistance and glucose toxicity, the hallmarks of type II diabetes mellitus. We will also summarize potential causal roles for the O-GlcNAc modification in complications associated with diabetes.

Role of O-GlcNAcylation in nutritional sensing, insulin resistance and in mediating the benefits of exercise

The purpose of this review is to highlight the role of O-linked β-N-acetylglucosamine (O-GlcNAc) protein modification in metabolic disease states and to summarize current knowledge of how exercise affects this important post-translational signalling pathway. O-GlcNAc modification is an intracellular tool capable of integrating energy supply with demand. The accumulation of excess energy associated with obesity and insulin resistance is mediated, in part, by the hexosamine biosynthetic pathway (HBP), which results in the O-GlcNAcylation of a myriad of proteins, thereby affecting their respective function, stability, and localization. Insulin resistance is related to the excessive O-GlcNAcylation of key metabolic proteins causing a chronic blunting of insulin signalling pathways and precipitating the accompanying pathologies, such as heart and kidney disease. Lifestyle modifications such as diet and exercise also modify the pathway. Exercise is a front-line and cost-effective therapeutic approach for insulin resistance, and recent work shows that the intervention can alter O-GlcNAc gene expression, signalling, and protein modification. However, there is currently no consensus on the effect of frequency, intensity, type, and duration of exercise on O-GlcNAc modification, the HBP, and its related enzymes. On one end of the spectrum, mild, prolonged swim training reduces O-GlcNAcylation, while on the other end, higher intensity treadmill running increases cardiac protein O-GlcNAc modification. Clearly, a balance between acute and chronic stress of exercise is needed to reap the benefits of the intervention on O-GlcNAc signalling.

The hexosamine biosynthesis pathway and O-GlcNAcylation maintain insulin-stimulated PI3K-PKB phosphorylation and tumour cell growth after short-term glucose deprivation

Glucose provides an essential nutrient source that supports glycolysis and the hexosamine biosynthesis pathway (HBP) to maintain tumour cell growth and survival. Here we investigated if short-term glucose deprivation specifically modulates the phosphatidylinositol 3-kinase/protein kinase B (PI3K/PKB) cell survival pathway. Insulin-stimulated PKB activation was strongly abrogated in the absence of extracellular glucose as a consequence of the loss of insulin-stimulated PI3K activation and short-term glucose deprivation inhibited subsequent tumour cell growth. Loss of insulin-stimulated PKB signalling and cell growth was rescued by extracellular glucosamine and increased flux through the HBP. Disruption of O-GlcNAc transferase activity, a terminal step in the HBP, implicated O-GlcNAcylation in PKB signalling and cell growth. Glycogenolysis is known to support cell survival during glucose deprivation, and in A549 lung cancer cells its inhibition attenuates PKB activation which is rescued by increased flux through the HBP. Our studies show that rerouting of glycolytic metabolites to the HBP under glucose-restricted conditions maintains PI3K/PKB signalling enabling cell survival and proliferation.

MORPHIN: a web tool for human disease research by projecting model organism biology onto a human integrated gene network

Despite recent advances in human genetics, model organisms are indispensable for human disease research. Most human disease pathways are evolutionally conserved among other species, where they may phenocopy the human condition or be associated with seemingly unrelated phenotypes. Much of the known gene-to-phenotype association information is distributed across diverse databases, growing rapidly due to new experimental techniques. Accessible bioinformatics tools will therefore facilitate translation of discoveries from model organisms into human disease biology. Here, we present a web-based discovery tool for human disease studies, MORPHIN (model organisms projected on a human integrated gene network), which prioritizes the most relevant human diseases for a given set of model organism genes, potentially highlighting new model systems for human diseases and providing context to model organism studies. Conceptually, MORPHIN investigates human diseases by an orthology-based projection of a set of model organism genes onto a genome-scale human gene network. MORPHIN then prioritizes human diseases by relevance to the projected model organism genes using two distinct methods: a conventional overlap-based gene set enrichment analysis and a network-based measure of closeness between the query and disease gene sets capable of detecting associations undetectable by the conventional overlap-based methods. MORPHIN is freely accessible at http://www.inetbio.org/morphin.

LC-MS proteomics analysis of the insulin/IGF-1-deficient Caenorhabditis elegans daf-2(e1370) mutant reveals extensive restructuring of intermediary metabolism

The insulin/IGF-1 receptor is a major known determinant of dauer formation, stress resistance, longevity, and metabolism in Caenorhabditis elegans. In the past, whole-genome transcript profiling was used extensively to study differential gene expression in response to reduced insulin/IGF-1 signaling, including the expression levels of metabolism-associated genes. Taking advantage of the recent developments in quantitative liquid chromatography mass spectrometry (LC-MS)-based proteomics, we profiled the proteomic changes that occur in response to activation of the DAF-16 transcription factor in the germline-less glp-4(bn2);daf-2(e1370) receptor mutant. Strikingly, the daf-2 profile suggests extensive reorganization of intermediary metabolism, characterized by the upregulation of many core intermediary metabolic pathways. These include glycolysis/gluconeogenesis, glycogenesis, pentose phosphate cycle, citric acid cycle, glyoxylate shunt, fatty acid β-oxidation, one-carbon metabolism, propionate and tyrosine catabolism, and complexes I, II, III, and V of the electron transport chain. Interestingly, we found simultaneous activation of reciprocally regulated metabolic pathways, which is indicative of spatiotemporal coordination of energy metabolism and/or extensive post-translational regulation of these enzymes. This restructuring of daf-2 metabolism is reminiscent to that of hypometabolic dauers, allowing the efficient and economical utilization of internal nutrient reserves and possibly also shunting metabolites through alternative energy-generating pathways to sustain longevity.

O-GlcNAc reports ambient temperature and confers heat resistance on ectotherm development

Effects of temperature on biological processes are complex. Diffusion is less affected than the diverse enzymatic reactions that have distinct individual temperature profiles. Hence thermal fluctuations pose a formidable challenge to ectothermic organisms in which body temperature is largely dictated by the ambient temperature. How cells in ectotherms cope with the myriad disruptive effects of temperature variation is poorly understood at the molecular level. Here we show that nucleocytoplasmic posttranslational modification of proteins with O-linked GlcNAc (O-GlcNAc) is closely correlated with ambient temperature during development of distantly related ectotherms ranging from the insect Drosophila melanogaster to the nematode Caenorhabditis elegans to the fish Danio rerio. Regulation seems to occur at the level of activity of the only two enzymes, O-GlcNAc transferase and O-GlcNAcase, that add and remove, respectively, this posttranslational modification in nucleus and cytoplasm. With genetic approaches in D. melanogaster and C. elegans, we demonstrate the importance of high levels of this posttranslational modification for successful development at elevated temperatures. Because many cytoplasmic and nuclear proteins in diverse pathways are O-GlcNAc targets, temperature-dependent regulation of this modification might contribute to an efficient coordinate adjustment of cellular processes in response to thermal change.

Protein O-GlcNAcylation in diabetes and diabetic complications

The post-translational modification of serine and threonine residues of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) is highly ubiquitous, dynamic and inducible. Protein O-GlcNAcylation serves as a key regulator of critical biological processes including transcription, translation, proteasomal degradation, signal transduction and apoptosis. Increased O-GlcNAcylation is directly linked to insulin resistance and to hyperglycemia-induced glucose toxicity, two hallmarks of diabetes and diabetic complications. In this review, we briefly summarize what is known about protein O-GlcNAcylation and nutrient metabolism, as well as discuss the commonly used tools to probe changes of O-GlcNAcylation in cultured cells and in animal models. We then focus on some key proteins modified by O-GlcNAc, which play crucial roles in the etiology and progression of diabetes and diabetic complications. Proteomic approaches are also highlighted to provide a system view of protein O-GlcNAcylation. Finally, we discuss how aberrant O-GlcNAcylation on certain proteins may be exploited to develop methods for the early diagnosis of pre-diabetes and/or diabetes.

Glycosylation of the nuclear pore

The O-linked β-N-acetylglucosamine (O-GlcNAc) posttranslational modification was first discovered 30 years ago and is highly concentrated in the nuclear pore. In the years since the discovery of this single sugar modification, substantial progress has been made in understanding the biochemistry of O-GlcNAc and its regulation. Nonetheless, O-GlcNAc modification of proteins continues to be overlooked, due in large part to the lack of reliable methods available for its detection. Recently, a new crop of immunological and chemical detection reagents has changed the research landscape. Using these tools, approximately 1000 O-GlcNAc-modified proteins have been identified. While other forms of glycosylation are typically associated with extracellular proteins, O-GlcNAc is abundant on nuclear and cytoplasmic proteins. In particular, phenylalanine-glycine nucleoporins are heavily O-GlcNAc-modified. Recent experiments are beginning to provide insight into the functional implications of O-GlcNAc modification on certain proteins, but its role in the nuclear pore has remained enigmatic. However, tantalizing new results suggest that O-GlcNAc may play roles in regulating nucleocytoplasmic transport.

Disruption of O-GlcNAc Cycling in C. elegans Perturbs Nucleotide Sugar Pools and Complex Glycans

The carbohydrate modification of serine and threonine residues with O-linked beta- N-acetylglucosamine (O-GlcNAc) is ubiquitous and governs cellular processes ranging from cell signaling to apoptosis. The O-GlcNAc modification along with other carbohydrate modifications, including N-linked and O-linked glycans, glycolipids, and sugar polymers, all require the use of the nucleotide sugar UDP-GlcNAc, the end product of the hexosamine biosynthetic pathway (HBP). In this paper, we describe the biochemical consequences resulting from perturbation of the O-GlcNAc pathway in C. elegans lacking O-GlcNAc transferase and O-GlcNAcase activities. In ogt-1 null animals, steady-state levels of UDP-GlcNAc/UDP-GalNAc and UDP-glucose were substantially elevated. Transcripts of genes encoding for key members in the HBP (gfat-2, gna-2, C36A4.4) and trehalose metabolism (tre-1, tre-2, tps-2) were elevated in ogt-1 null animals. While there is no evidence to suggest changes in the profile of N-linked glycans in the ogt-1 and oga-1 mutants, glycans insensitive to PNGase digestion (including O-linked glycans, glycolipids, and glycopolymers) were altered in these strains. Our data support that changes in O-GlcNAcylation alters nucleotide sugar production, overall glycan composition, and transcription of genes encoding glycan processing enzymes. These data along with our previous findings that disruption in O-GlcNAc cycling alters macronutrient storage underscores the noteworthy influence this posttranslational modification plays in nutrient sensing.

O-GlcNAc cycling shows neuroprotective potential in C. elegans models of neurodegenerative disease

C. elegans has proven to be an excellent organism in which to model human neurodegenerative disease.(1) (-) (7) The worm's simple nervous system, lineage, and neural maps, easily scored movement phenotypes, and robust forward and reverse genetics make it optimal for studying age-dependent processes on a reasonable time scale. A popular approach has been the introduction of transgenes expressing GFP-tagged proteotoxic human proteins into neurons leading to visible aggregation or movement phenotypes.(2) (,) (4) (,) (6) (,) (8) (-) (13) In addition, the maintenance of proteostasis networks has been extensively studied using the power of worm genetics.(8) (-) (13) These networks include genes involved in insulin-like signaling, the heat shock response, the response to hypoxia, and mTOR and AMPK pathways linked to aging.(14) Another pathway with suggestive links to neurodegeneration is the O-GlcNAc cycling pathway, a nutrient-dependent post-translational modification known to be altered in brains from patients with Alzheimer disease.(15) (-) (19) In this commentary, we summarize our recent findings showing that viable mutants of O-GlcNAc cycling in C. elegans dramatically alter the neurotoxicity of four distinct C. elegans models of neurodegenerative disease.(7) Mutants in O-GlcNAc cycling alter the toxicity of mutant tau, polyglutamine expansion reporters, and amyloid β-peptide. The findings further suggest that O-GlcNAc cycling acts at many steps in the lifecycle of aggregation-prone targets. The C. elegans system is likely to continue to provide insights into this complex problem. The involvement of O-GlcNAc cycling in the maintenance of proteostasis raises the possibility of targeting the enzymes catalyzing this critical post-translational modification for therapeutic intervention.

O-GlcNAcylation at promoters, nutrient sensors, and transcriptional regulation

Post-translational modifications play important roles in transcriptional regulation. Among the less understood PTMs is O-GlcNAcylation. Nevertheless, O-GlcNAcylation in the nucleus is found on hundreds of transcription factors and coactivators and is often found in a mutually exclusive ying-yang relationship with phosphorylation. O-GlcNAcylation also links cellular metabolism directly to the proteome, serving as a conduit of metabolic information to the nucleus. This review serves as a brief introduction to O-GlcNAcylation, emphasizing its important thematic roles in transcriptional regulation, and highlights several recent and important additions to the literature that illustrate the connections between O-GlcNAc and transcription.

Dynamic O-GlcNAcylation and its roles in the cellular stress response and homeostasis

O-linked N-acetyl-β-D-glucosamine (O-GlcNAc) is a ubiquitous and dynamic post-translational modification known to modify over 3,000 nuclear, cytoplasmic, and mitochondrial eukaryotic proteins. Addition of O-GlcNAc to proteins is catalyzed by the O-GlcNAc transferase and is removed by a neutral-N-acetyl-β-glucosaminidase (O-GlcNAcase). O-GlcNAc is thought to regulate proteins in a manner analogous to protein phosphorylation, and the cycling of this carbohydrate modification regulates many cellular functions such as the cellular stress response. Diverse forms of cellular stress and tissue injury result in enhanced O-GlcNAc modification, or O-GlcNAcylation, of numerous intracellular proteins. Stress-induced O-GlcNAcylation appears to promote cell/tissue survival by regulating a multitude of biological processes including: the phosphoinositide 3-kinase/Akt pathway, heat shock protein expression, calcium homeostasis, levels of reactive oxygen species, ER stress, protein stability, mitochondrial dynamics, and inflammation. Here, we will discuss the regulation of these processes by O-GlcNAc and the impact of such regulation on survival in models of ischemia reperfusion injury and trauma hemorrhage. We will also discuss the misregulation of O-GlcNAc in diseases commonly associated with the stress response, namely Alzheimer's and Parkinson's diseases. Finally, we will highlight recent advancements in the tools and technologies used to study the O-GlcNAc modification.

O-GlcNAc modification of the coat protein of the potyvirus Plum pox virus enhances viral infection

O-GlcNAcylation is a dynamic protein modification which has been studied mainly in metazoans. We reported previously that an Arabidopsis thaliana O-GlcNAc transferase modifies at least two threonine residues of the Plum pox virus (PPV) capsid protein (CP). Now, six additional residues were shown to be involved in O-GlcNAc modification of PPV CP. CP O-GlcNAcylation was abolished in the PPV CP7-T/A mutant, in which seven threonines were mutated. PPV CP7-T/A infected Nicotiana clevelandii, Nicotiana benthamiana, and Prunus persica without noticeable defects. However, defects in infection of A. thaliana were readily apparent. In mixed infections of wild-type arabidopsis, the CP7-T/A mutant was outcompeted by wild-type virus. These results indicate that CP O-GlcNAcylation has a major role in the infection process. O-GlcNAc modification may have a role in virion assembly and/or stability as the CP of PPV CP7-T/A was more sensitive to protease digestion than that of the wild-type virus.

Shifts in the distribution of mass densities is a signature of caloric restriction in Caenorhabditis elegans

Although the starvation response of the model multicellular organism Caenorhabditis elegans is a subject of much research, there is no convenient phenotypic readout of caloric restriction that can be applicable to large numbers of worms. This paper describes the distribution of mass densities of populations of C. elegans, from larval stages up to day one of adulthood, using isopycnic centrifugation, and finds that density is a convenient, if complex, phenotypic readout in C. elegans. The density of worms in synchronized populations of wildtype N2 C. elegans grown under standard solid-phase culture conditions was normally distributed, with distributions peaked sharply at a mean of 1.091 g/cm³ for L1, L2 and L3 larvae, 1.087 g/cm³ for L4 larvae, 1.081 g/cm³ for newly molted adults, and 1.074 g/cm³ at 24 hours of adulthood. The density of adult worms under starvation stress fell well outside this range, falling to a mean value of 1.054 g/cm³ after eight hours of starvation. This decrease in density correlated with the consumption of stored glycogen in the food-deprived worms. The density of the worms increased when deprived of food for longer durations, corresponding to a shift in the response of the worms: worms sacrifice their bodies by retaining larvae, which consume the adults from within. Density-based screens with the drug Ivermectin on worms cultured on single plates resulted in a clear bimodal (double-peaked) distribution of densities corresponding to drug exposed and non-exposed worms. Thus, measurements of changes in density could be used to conduct screens on the effects of drugs on several populations of worms cultured on single plates.

Cracking the O-GlcNAc code in metabolism

Nuclear, cytoplasmic, and mitochondrial proteins are extensively modified by O-linked β-N-acetylglucosamine (O-GlcNAc) moieties. This sugar modification regulates fundamental cellular processes in response to diverse nutritional and hormonal cues. The enzymes O-GlcNAc transferase (OGT) and O-linked β-N-acetylglucosaminase (O-GlcNAcase) mediate the addition and removal of O-GlcNAc, respectively. Aberrant O-GlcNAcylation has been implicated in a plethora of human diseases, including diabetes, cancer, aging, cardiovascular disease, and neurodegenerative disease. Because metabolic dysregulation is a vital component of these diseases, unraveling the roles of O-GlcNAc in metabolism is of emerging importance. Here, we review the current understanding of the functions of O-GlcNAc in cell signaling and gene transcription involved in metabolism, and focus on its relevance to diabetes, cancer, circadian rhythm, and mitochondrial function.

O-GlcNAc cycling: a link between metabolism and chronic disease

To maintain homeostasis under variable nutrient conditions, cells rapidly and robustly respond to fluctuations through adaptable signaling networks. Evidence suggests that the O-linked N-acetylglucosamine (O-GlcNAc) posttranslational modification of serine and threonine residues functions as a critical regulator of intracellular signaling cascades in response to nutrient changes. O-GlcNAc is a highly regulated, reversible modification poised to integrate metabolic signals and acts to influence many cellular processes, including cellular signaling, protein stability, and transcription. This review describes the role O-GlcNAc plays in governing both integrated cellular processes and the activity of individual proteins in response to nutrient levels. Moreover, we discuss the ways in which cellular changes in O-GlcNAc status may be linked to chronic diseases such as type 2 diabetes, neurodegeneration, and cancers, providing a unique window through which to identify and treat disease conditions.

Identification of O-linked N-acetylglucosamine (O-GlcNAc)-modified osteoblast proteins by electron transfer dissociation tandem mass spectrometry reveals proteins critical for bone formation

The nutrient-responsive β-O-linked N-acetylglucosamine (O-GlcNAc) modification of critical effector proteins modulates signaling and transcriptional pathways contributing to cellular development and survival. An elevation in global protein O-GlcNAc modification occurs during the early stages of osteoblast differentiation and correlates with enhanced transcriptional activity of RUNX2, a key regulator of osteogenesis. To identify other substrates of O-GlcNAc transferase in differentiating MC3T3E1 osteoblasts, O-GlcNAc-modified peptides were enriched by wheat germ agglutinin lectin weak affinity chromatography and identified by tandem mass spectrometry using electron transfer dissociation. This peptide fragmentation approach leaves the labile O-linkage intact permitting direct identification of O-GlcNAc-modified peptides. O-GlcNAc modification was observed on enzymes involved in post-translational regulation, including MAST4 and WNK1 kinases, a ubiquitin-associated protein (UBAP2l), and the histone acetyltransferase CREB-binding protein. CREB-binding protein, a transcriptional co-activator that associates with CREB and RUNX2, is O-GlcNAcylated at Ser-147 and Ser-2360, the latter of which is a known site of phosphorylation. Additionally, O-GlcNAcylation of components of the TGFβ-activated kinase 1 (TAK1) signaling complex, TAB1 and TAB2, occurred in close proximity to known sites of Ser/Thr phosphorylation and a putative nuclear localization sequence within TAB2. These findings demonstrate the presence of O-GlcNAc modification on proteins critical to bone formation, remodeling, and fracture healing and will enable evaluation of this modification on protein function and regulation.

A Drosophila model of high sugar diet-induced cardiomyopathy

Diets high in carbohydrates have long been linked to progressive heart dysfunction, yet the mechanisms by which chronic high sugar leads to heart failure remain poorly understood. Here we combine diet, genetics, and physiology to establish an adult Drosophila melanogaster model of chronic high sugar-induced heart disease. We demonstrate deterioration of heart function accompanied by fibrosis-like collagen accumulation, insulin signaling defects, and fat accumulation. The result was a shorter life span that was more severe in the presence of reduced insulin and P38 signaling. We provide evidence of a role for hexosamine flux, a metabolic pathway accessed by glucose. Increased hexosamine flux led to heart function defects and structural damage; conversely, cardiac-specific reduction of pathway activity prevented sugar-induced heart dysfunction. Our data establish Drosophila as a useful system for exploring specific aspects of diet-induced heart dysfunction and emphasize enzymes within the hexosamine biosynthetic pathway as candidate therapeutic targets.

Metabolic labeling of Caenorhabditis elegans primary embryonic cells with azido-sugars as a tool for glycoprotein discovery

Glycobiology research with Caenorhabditis elegans (C. elegans) has benefitted from the numerous genetic and cell biology tools available in this system. However, the lack of a cell line and the relative inaccessibility of C. elegans somatic cells in vivo have limited the biochemical approaches available in this model. Here we report that C. elegans primary embryonic cells in culture incorporate azido-sugar analogs of N-acetylgalactosamine (GalNAc) and N-acetylglucosamine (GlcNAc), and that the labeled glycoproteins can be analyzed by mass spectrometry. By using this metabolic labeling approach, we have identified a set of novel C. elegans glycoprotein candidates, which include several mitochondrially-annotated proteins. This observation was unexpected given that mitochondrial glycoproteins have only rarely been reported, and it suggests that glycosylation of mitochondrially-annotated proteins might occur more frequently than previously thought. Using independent experimental strategies, we validated a subset of our glycoprotein candidates. These include a mitochondrial, atypical glycoprotein (ATP synthase α-subunit), a predicted glycoprotein (aspartyl protease, ASP-4), and a protein family with established glycosylation in other species (actin). Additionally, we observed a glycosylated isoform of ATP synthase α-subunit in bovine heart tissue and a primate cell line (COS-7). Overall, our finding that C. elegans primary embryonic cells are amenable to metabolic labeling demonstrates that biochemical studies in C. elegans are feasible, which opens the door to labeling C. elegans cells with other radioactive or azido-substrates and should enable the identification of additional post-translationally modified targets and analysis of the genes required for their modification using C. elegans mutant libraries.

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.

Sex differences in carbohydrate metabolism are linked to gene expression in Caenorhabditis elegans

The male and the hermaphrodite forms of the nematode Caenorhabditis elegans (C. elegans) differ markedly in anatomy, nervous system and behavior at adulthood. Using the male mutants fog-2, him-5, and him-8, we compared body proportions and composition, and aspects of carbohydrate metabolism and gene expression between the C. elegans sexes in three adult stages. In all experiments, both sexes were grown on the same plate and separated using flow cytometry. The fat to fat-free mass ratio and the body volume-adjusted fat mass is similar between the sexes, although the body size is more than 50% smaller in adult males than in age-matched hermaphrodites. The volume-adjusted total RNA content is approximately 2-fold lower in males. Biochemical and NMR-based analyses reveal higher trehalose levels and much lower glucose levels in males than in hermaphrodites. The resulting trehalose-to-glucose ratio is 5.4-fold higher in males. These sex differences are reflected in gene expression data because the genes encoding key enzymes of the glycolysis and trehalose synthesis pathways are more highly expressed in males than in hermaphrodites. Notably, expression of the phosphofructokinase gene (C50F4.2) is 29-fold higher in males. Comparative analysis of gene expression data identifies 285 male-specific and 160 hermaphrodite-specific genes. These include transcription factor and C-type lectin-encoding genes. More than 35% of all C-type lectin genes are more highly expressed in males. The expression of many C-type lectin genes differs by a factor of >100 between the sexes. In conclusion, we found sex differences in carbohydrate metabolism that are linked to gene expression and identified certain lectin genes that are differentially expressed by the C. elegans sexes.

O-GlcNAc modification affects the ATM-mediated DNA damage response

BACKGROUND

O-Linked β-N-acetylglucosamine (O-GlcNAc) is a reversible, post-translational, and regulatory modification of nuclear, mitochondrial, and cytoplasmic proteins that is responsive to cellular stress. The role of O-GlcNAcylation in the ataxia-telangiectasia mutated (ATM)-mediated DNA damage response is unknown. It is unclear whether ATM, which is an early acting and central component of the signal transduction system activated by DNA double strand breaks, is an O-GlcNAc-modified protein.

METHODS

The effect of O-GlcNAc modification on ATM activation was examined using two inhibitors, PUGNAc and DON that increase and decrease, respectively, levels of protein O-GlcNAcylation. To assess O-GlcNAcylation of ATM, immunoprecipitation and immunoblot analyses using anti-ATM or anti-O-GlcNAc antibody were performed in HeLa cells and primary cultured neurons. Interaction of ATM with O-GlcNAc transferase (OGT), the enzyme that adds O-GlcNAc to target proteins, was examined by immunoprecipitation and immunoblot analyses using anti-ATM.

RESULTS

Enhancement of protein O-GlcNAcylation increased levels of X-irradiation-induced ATM activation. However, decreases in protein O-GlcNAcylation did not affect levels of ATM activation, but these decreases did delay ATM activation and ATM recovery processes based on assessment of de-phosphorylation of phospho-ATM. Thus, activation and recovery of ATM were affected by O-GlcNAcylation. ATM was subjected to O-GlcNAcylation, and ATM interacted with OGT. The steady-state O-GlcNAc level of ATM was not significantly responsive to X-irradiation or oxidative stress.

GENERAL SIGNIFICANCE

ATM is an O-GlcNAc modified protein, and dynamic O-GlcNAc modification affects the ATM-mediated DNA damage response.