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

Protein glycosylation in cardiovascular health and disease

Protein glycosylation, which involves the attachment of carbohydrates to proteins, is one of the most abundant protein co-translational and post-translational modifications. Advances in technology have substantially increased our knowledge of the biosynthetic pathways involved in protein glycosylation, as well as how changes in glycosylation can affect cell function. In addition, our understanding of the role of protein glycosylation in disease processes is growing, particularly in the context of immune system function, infectious diseases, neurodegeneration and cancer. Several decades ago, cell surface glycoproteins were found to have an important role in regulating ion transport across the cardiac sarcolemma. However, with very few exceptions, our understanding of how changes in protein glycosylation influence cardiovascular (patho)physiology remains remarkably limited. Therefore, in this Review, we aim to provide an overview of N-linked and O-linked protein glycosylation, including intracellular O-linked N-acetylglucosamine protein modification. We discuss our current understanding of how all forms of protein glycosylation contribute to normal cardiovascular function and their roles in cardiovascular disease. Finally, we highlight potential gaps in our knowledge about the effects of protein glycosylation on the heart and vascular system, highlighting areas for future research.

O-GlcNAcylation regulates β1,4-GlcNAc-branched N-glycan biosynthesis via the OGT/SLC35A3/GnT-IV axis

N-Linked glycosylation and O-linked N-acetylglucosamine (O-GlcNAc) are important protein post-translational modifications that are orchestrated by a diverse set of gene products. Thus far, the relationship between these two types of glycosylation has remained elusive, and it is unclear whether one influences the other via UDP-GlcNAc, which is a common donor substrate. Theoretically, a decrease in O-GlcNAcylation may increase the products of GlcNAc-branched N-glycans. In this study, via examination by lectin blotting, HPLC, and mass spectrometry analysis, however, we found that the amounts of GlcNAc-branched tri-antennary N-glycans catalyzed by N-acetylglucosaminyltransferase IV (GnT-IV) and tetra-antennary N-glycans were significantly decreased in O-GlcNAc transferase knockdown cells (OGT-KD) compared with those in wild type cells. We examined this specific alteration by focusing on SLC35A3, which is the main UDP-GlcNAc transporter in mammals that is believed to modulate GnT-IV activation. It is interesting that a deficiency of SLC35A3 specifically leads to a decrease in the amounts of GlcNAc-branched tri- and tetra-antennary N-glycans. Furthermore, co-immunoprecipitation experiments have shown that SLC35A3 interacts with GnT-IV, but not with N-acetylglucosaminyltransferase V. Western blot and chemoenzymatic labeling assay have confirmed that OGT modifies SLC35A3 and that O-GlcNAcylation contributes to its stability. Furthermore, we found that SLC35A3-KO enhances cell spreading and suppresses both cell migration and cell proliferation, which is similar to the phenomena observed in the OGT-KD cells. Taken together, these data are the first to demonstrate that O-GlcNAcylation specifically governs the biosynthesis of tri- and tetra-antennary N-glycans via the OGT-SLC35A3-GnT-IV axis.

Global Transcriptome Changes That Accompany Alterations in Serotonin Levels in Caenorhabditis elegans

Serotonin (5-hydroxytryptamine, 5-HT), is a phylogenetically ancient molecule best characterized as a neurotransmitter that modulates multiple aspects of mood and social cognition. The roles that 5-HT plays in normal and abnormal behavior are not fully understood but have been posited to be due to its common function as a 'defense signal'. However, 5-HT levels also systemically impact cell physiology, modulating cell division, migration, apoptosis, mitochondrial biogenesis, cellular metabolism and differentiation. Whether these diverse cellular effects of 5-HT also share a common basis is unclear. C. elegans provides an ideal system to interrogate the systemic effects of 5-HT, since lacking a blood-brain barrier, 5-HT synthesized and released by neurons permeates the organism to modulate neuronal as well as non-neuronal cells throughout the body. Here we used RNA-Seq to characterize the systemic changes in gene expression that occur in C. elegans upon altering 5-HT levels, and compared the transcriptomes to published datasets. We find that an acute increase in 5-HT is accompanied by a global decrease in gene expression levels, upregulation of genes involved in stress pathways, changes that significantly correlate with the published transcriptomes of animals that have activated defense and immune responses, and an increase in levels of phosphorylated eukaryotic initiation factor, eIF2α. In 5-HT deficient animals lacking tryptophan hydroxylase (tph-1(mg280)II) there is a net increase in gene expression, with an overrepresentation of genes related to development and chromatin. Surprisingly, the transcriptomes of animals with acute increases in 5-HT levels, and 5-HT deficiency do not overlap with transcriptomes of mutants with whom they share striking physiological resemblance. These studies are the first to catalog systemic transcriptome changes that occur upon alterations in 5-HT levels. They further show that in C. elegans changes in gene expression upon altering 5-HT levels, and changes in physiology, are not directly correlated.

OGT Controls the Expression and the Glycosylation of E-cadherin, and Affects Glycosphingolipid Structures in Human Colon Cell Lines

Colorectal cancer (CRC) affects both women and men living in societies with a high sedentary lifestyle. Amongst the phenotypic changes exhibited by tumor cells, a wide range of glycosylation has been reported for colon cancer-derived cell lines and CRC tissues. These aberrant modifications affect different aspects of glycosylation, including an increase in core fucosylation and GlcNAc branching on N-glycans, alteration of O-glycans, upregulated sialylation, and O-GlcNAcylation. Although O-GlcNAcylation and complex glycosylations differ in many aspects, sparse evidences report on the interference of O-GlcNAcylation with complex glycosylation. Nevertheless, this relationship is still a matter of debate. Combining different approaches on three human colon cell lines (HT29, HCT116 and CCD841CoN), it is herein reported that silencing O-GlcNAc transferase (OGT, the sole enzyme driving O-GlcNAcylation), only slightly affects overall N- and O-glycosylation patterns. Interestingly, silencing of OGT in HT29 cells upregulates E-cadherin (a major actor of epithelial-to-mesenchymal transition) and changes its glycosylation. On the other hand, OGT silencing perturbs biosynthesis of glycosphingolipids resulting in a decrease in gangliosides and an increase in globosides. Together, these results provide novel insights regarding the selective regulation of complex glycosylations by O-GlcNAcylation in colon cancer cells.

Correlations Between LC-MS/MS-Detected Glycomics and NMR-Detected Metabolomics in Caenorhabditis elegans Development

This study examined the relationship between glycans, metabolites, and development in C. elegans. Samples of N2 animals were synchronized and grown to five different time points ranging from L1 to a mixed population of adults, gravid adults, and offspring. Each time point was replicated seven times. The samples were each assayed by a large particle flow cytometer (Biosorter) for size distribution data, LC-MS/MS for targeted N- and O-linked glycans, and NMR for metabolites. The same samples were utilized for all measurements, which allowed for statistical correlations between the data. A new protocol was developed to correlate Biosorter developmental data with LC-MS/MS data to obtain stage-specific information of glycans. From the five time points, four distinct sizes of worms were observed from the Biosorter distributions, ranging from the smallest corresponding to L1 to adult animals. A network model was constructed using the four binned sizes of worms as starting nodes and adding glycans and metabolites that had correlations with r ≥ 0.5 to those nodes. The emerging structure of the network showed distinct patterns of N- and O-linked glycans that were consistent with previous studies. Furthermore, some metabolites that were correlated to these glycans and worm sizes showed interesting interactions. Of note, UDP-GlcNAc had strong positive correlations with many O-glycans that were expressed in the largest animals. Similarly, phosphorylcholine correlated with many N-glycans that were expressed in L1 animals.

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

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

O-GlcNAcylation confers protection against Staphylococcus aureus infection in Caenorhabditis elegans through ubiquitination

Glycosylation is one of the most prevalent post-translational modifications in biological systems. In Caenorhabditis elegans, O-GlcNAcylation has been shown to be actively involved in the regulation of dauer formation and detoxification of toxins secreted by invading pathogens. On this backdrop, the present study is focused on understanding the role of O-GlcNAcylation in C. elegans during Staphylococcus aureus infection using a gel based proteomic approach. Results of time course killing assays with wild-type and mutants of glycosylation and comparison of results revealed an increase in the survival of the C. elegans oga-1 mutant when compared to wild-type N2 and the ogt-1 mutant. Increased survival of C. elegans N2 upon S. aureus infection in the presence of O-(2-acetamido-2-deoxy-d-glucopyranosylidenamino) N-phenylcarbamate (PUGNAc-an OGA inhibitor) further confirmed the involvement of O-GlcNAcylation in protecting C. elegans from infection. The two-dimensional gel-based proteomic analysis of the control and S. aureus infected C. elegans oga-1 mutant followed by mass spectrometric identification of differentially expressed proteins has been carried out. The results revealed that key proteins involved in ubiquitination such as Cullin-1 (CUL-1), Cullin-3 (CUL-3), BTB and MATH domain-containing protein 15 (BATH-15), ubiquitin-conjugating enzyme E2 variant 3 (UEV-3) and probable ubiquitin-conjugating enzyme E2 7 (UBC-7) are upregulated. Real-time PCR analysis further confirms the upregulation of genes encoding the above-mentioned proteins which are involved in the ubiquitin-mediated pathways in C. elegans. In addition, treatment of C. elegans wild-type N2 and the oga-1 mutant with PUGNAc + suramin and suramin (an ubiquitination inhibitor), respectively has resulted in increased sensitivity to S. aureus infection. Hence, it is presumed that upregulation of proteins involved in the ubiquitination pathway could be the key regulatory mechanism responsible for the enhanced survival of the oga-1 mutant during S. aureus infection.

Inhibition of poly-LacNAc biosynthesis with release of CMP-Neu5Ac feedback inhibition increases the sialylation of recombinant EPO produced in CHO cells

Sialylation of recombinant therapeutic glycoproteins modulates their pharmacokinetic properties by affecting their in vivo half-life. N-glycan branching on glycoproteins increases the number of potential attachment sites for sialic acid. Here, we introduce a new approach for increasing the sialylation of recombinant human erythropoietin (rhEPO) produced in CHO cells by modulating poly-N-acetyllactosamine (poly-LacNAc) biosynthesis. We did not observe an increase in rhEPO sialylation, however, until the feedback inhibition by intracellular cytidine monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), which is a limiting factor for sialylation, was released. Thus, we found that a combined approach inhibiting poly-LacNAc biosynthesis and releasing CMP-Neu5Ac feedback inhibition produces the most significant increase in rhEPO sialylation in metabolically engineered CHO cells. Furthermore, a detailed analysis of the resulting N-glycan structures using LC/MS revealed increased tri- and tetra- sialylated N-glycan structures accompanied by a reduction of di-sialylated N-glycan structures. These results validate our new approach for glycosylation engineering, and we expect this approach will be useful in future efforts to enhance the efficacy of other therapeutic glycoproteins.

O-GlcNAc in cancer: An Oncometabolism-fueled vicious cycle

Cancer cells exhibit unregulated growth, altered metabolism, enhanced metastatic potential and altered cell surface glycans. Fueled by oncometabolism and elevated uptake of glucose and glutamine, the hexosamine biosynthetic pathway (HBP) sustains glycosylation in the endomembrane system. In addition, the elevated pools of UDP-GlcNAc drives the O-GlcNAc modification of key targets in the cytoplasm, nucleus and mitochondrion. These targets include transcription factors, kinases, key cytoplasmic enzymes of intermediary metabolism, and electron transport chain complexes. O-GlcNAcylation can thereby alter epigenetics, transcription, signaling, proteostasis, and bioenergetics, key 'hallmarks of cancer'. In this review, we summarize accumulating evidence that many cancer hallmarks are linked to dysregulation of O-GlcNAc cycling on cancer-relevant targets. We argue that onconutrient and oncometabolite-fueled elevation increases HBP flux and triggers O-GlcNAcylation of key regulatory enzymes in glycolysis, Kreb's cycle, pentose-phosphate pathway, and the HBP itself. The resulting rerouting of glucose metabolites leads to elevated O-GlcNAcylation of oncogenes and tumor suppressors further escalating elevation in HBP flux creating a 'vicious cycle'. Downstream, elevated O-GlcNAcylation alters DNA repair and cellular stress pathways which influence oncogenesis. The elevated steady-state levels of O-GlcNAcylated targets found in many cancers may also provide these cells with a selective advantage for sustained growth, enhanced metastatic potential, and immune evasion in the tumor microenvironment.

Characterization of tools to detect and enrich human and mouse O-GlcNAcase

O-linked β-N-acetylglucosamine (O-GlcNAc) is an essential regulatory post-translational modification of thousands of nuclear, cytoplasmic, and mitochondrial proteins. O-GlcNAc is dynamically added and removed from proteins by the O-GlcNAc transferase and the O-GlcNAcase (OGA), respectively. Dysregulation of O-GlcNAc-cycling is implicated in the etiology of numerous diseases including tumorigenesis, metabolic dysfunction, and neurodegeneration. To facilitate studies focused on the role of O-GlcNAc and OGA in disease, we sought to identify commercially available antibodies that enable the enrichment of full-length OGA from lysates of mouse and human origin. Here, we report that antibodies from Abcam and Bethyl Laboratories can be used to immunoprecipitate OGA to near-saturation from human and mouse cell lysates. However, Western blotting analysis indicates that both antibodies, as well as three non-commercially available antibodies (345, 346, 352), detect full-length OGA and numerous cross-reacting proteins. These non-specific signals migrate similarly to full-length OGA and are detected robustly, suggesting that the use of appropriate controls is essential to avoid the misidentification of OGA.

Metabolic Reprogramming by Hexosamine Biosynthetic and Golgi N-Glycan Branching Pathways

De novo uridine-diphosphate-N-acetylglucosamine (UDP-GlcNAc) biosynthesis requires glucose, glutamine, acetyl-CoA and uridine, however GlcNAc salvaged from glycoconjugate turnover and dietary sources also makes a significant contribution to the intracellular pool. Herein we ask whether dietary GlcNAc regulates nutrient transport and intermediate metabolism in C57BL/6 mice by increasing UDP-GlcNAc and in turn Golgi N-glycan branching. GlcNAc added to the drinking water showed a dose-dependent increase in growth of young mice, while in mature adult mice fat and body-weight increased without affecting calorie-intake, activity, energy expenditure, or the microbiome. Oral GlcNAc increased hepatic UDP-GlcNAc and N-glycan branching on hepatic glycoproteins. Glucose homeostasis, hepatic glycogen, lipid metabolism and response to fasting were altered with GlcNAc treatment. In cultured cells GlcNAc enhanced uptake of glucose, glutamine and fatty-acids, and enhanced lipid synthesis, while inhibition of Golgi N-glycan branching blocked GlcNAc-dependent lipid accumulation. The N-acetylglucosaminyltransferase enzymes of the N-glycan branching pathway (Mgat1,2,4,5) display multistep ultrasensitivity to UDP-GlcNAc, as well as branching-dependent compensation. Indeed, oral GlcNAc rescued fat accumulation in lean Mgat5^−/− mice and in cultured Mgat5^−/− hepatocytes, consistent with N-glycan branching compensation. Our results suggest GlcNAc reprograms cellular metabolism by enhancing nutrient uptake and lipid storage through the UDP-GlcNAc supply to N-glycan branching pathway.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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.