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Elucidating the protein substrate recognition of O-GlcNAc transferase (OGT) toward O-GlcNAcase (OGA) using a GlcNAc electrophilic probe.

ABSTRACT: The essential human O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is the sole enzyme responsible for modifying thousands of intracellular proteins with the monosaccharide O-GlcNAc. This unique modification plays crucial roles in human health and disease, but the substrate recognition of OGT remains poorly understood. Intriguingly, the only human enzyme reported to remove this modification, O-GlcNAcase (OGA), is O-GlcNAc modified. Here, we exploited a GlcNAc electrophilic probe (GEP1A) to rapidly screen OGT mutants in a fluorescence assay that can discriminate between altered OGT-sugar and -protein substrate binding to help elucidate the binding mode of OGT toward OGA protein substrate. Since OGT tetratricopeptide repeat (TPR) domain plays a key role in OGT-OGA binding, we screened 30 OGT TPR mutants, which revealed 15 "ladder like" asparagine or aspartate residues spanning TPRs 3-7 and 10-13.5 that affect OGA O-GlcNAcylation. By applying a truncated OGA construct, we found that OGA's N-terminal region or pseudo histone acetyltransferase domain is not required for its O-GlcNAcylation, suggesting OGT functionally interacts with OGA through its catalytic and/or stalk domains. This work represents the first effort to systemically investigate each OGT TPR and our findings will facilitate the development of new strategies to investigate the role of substrate-specific O-GlcNAcylation.

SUBMITTER: Kositzke A

PROVIDER: S-EPMC7856287 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Electrophilic probes for deciphering substrate recognition by O-GlcNAc transferase.

Project description:O-linked ?-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential human glycosyltransferase that adds O-GlcNAc modifications to numerous proteins. However, little is known about the mechanism with which OGT recognizes various protein substrates. Here we report on GlcNAc electrophilic probes (GEPs) to expedite the characterization of OGT-substrate recognition. Data from mass spectrometry, X-ray crystallization, and biochemical and radiolabeled kinetic assays support the application of GEPs to rapidly report the impacts of OGT mutations on protein substrate or sugar binding and to discover OGT residues crucial for protein recognition. Interestingly, we found that the same residues on the inner surface of the N-terminal domain contribute to OGT interactions with different protein substrates. By tuning reaction conditions, a GEP enables crosslinking of OGT with acceptor substrates in situ, affording a unique method to discover genuine substrates that weakly or transiently interact with OGT. Hence, GEPs provide new strategies to dissect OGT-substrate binding and recognition.

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Regulating the Regulators: Mechanisms of Substrate Selection of the O-GlcNAc Cycling Enzymes OGT and OGA.

Project description:Thousands of nuclear and cytosolic proteins are modified with a single β-N-acetylglucosamine on serine and threonine residues in mammals, a modification termed O-GlcNAc. This modification is essential for normal development and plays important roles in virtually all intracellular processes. Additionally, O-GlcNAc is involved in many disease states, including cancer, diabetes, and X-linked intellectual disability. Given the myriad of functions of the O-GlcNAc modification, it is therefore somewhat surprising that O-GlcNAc cycling is mediated by only two enzymes: the O-GlcNAc transferase (OGT), which adds O-GlcNAc, and the O-GlcNAcase (OGA), which removes it. A significant outstanding question in the O-GlcNAc field is how do only two enzymes mediate such an abundant and dynamic modification. In this review, we explore the current understanding of mechanisms for substrate selection for the O-GlcNAc cycling enzymes. These mechanisms include direct substrate interaction with specific domains of OGT or OGA, selection of interactors via partner proteins, posttranslational modification of OGT or OGA, nutrient sensing, and localization alteration. Altogether, current research paints a picture of an exquisitely regulated and complex system by which OGT and OGA select substrates. We also make recommendations for future work, toward the goal of identifying interaction mechanisms for specific substrates that may be able to be exploited for various research and medical treatment goals.

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Substrate specificity provides insights into the sugar donor recognition mechanism of O-GlcNAc transferase (OGT).

Project description:O-Linked β-N-acetylglucosaminyl transferase (OGT) plays an important role in the glycosylation of proteins, which is involved in various cellular events. In human, three isoforms of OGT (short OGT [sOGT]; mitochondrial OGT [mOGT]; and nucleocytoplasmic OGT [ncOGT]) share the same catalytic domain, implying that they might adopt a similar catalytic mechanism, including sugar donor recognition. In this work, the sugar-nucleotide tolerance of sOGT was investigated. Among a series of uridine 5'-diphosphate-N-acetylglucosamine (UDP-GlcNAc) analogs tested using the casein kinase II (CKII) peptide as the sugar acceptor, four compounds could be used by sOGT, including UDP-6-deoxy-GlcNAc, UDP-GlcNPr, UDP-6-deoxy-GalNAc and UDP-4-deoxy-GlcNAc. Determined values of Km showed that the substitution of the N-acyl group, deoxy modification of C6/C4-OH or epimerization of C4-OH of the GlcNAc in UDP-GlcNAc decreased its affinity to sOGT. A molecular docking study combined with site-directed mutagenesis indicated that the backbone carbonyl oxygen of Leu653 and the hydroxyl group of Thr560 in sOGT contributed to the recognition of the sugar moiety via hydrogen bonds. The close vicinity between Met501 and the N-acyl group of GlcNPr, as well as the hydrophobic environment near Met501, were responsible for the selective binding of UDP-GlcNPr. These findings illustrate the interaction of OGT and sugar nucleotide donor, providing insights into the OGT catalytic mechanism.

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Inhibition of O-GlcNAc transferase (OGT) by peptidic hybrids.

Project description:O-GlcNAc transferase (OGT) attaches a GlcNAc moiety on specific substrate proteins using UDP-GlcNAc as the sugar donor. This modification can alter protein function by regulating cellular signaling and transcription pathways in response to altered nutrient availability and stress. Specific inhibitors of OGT would be valuable tools for biological studies and lead structures for therapeutics. The existing OGT inhibitors are mainly derived from the sugar donor substrate, but poor cell permeability and off-target effects limit their use. Here, we describe our progress on OGT inhibition based on substrate peptides identified by array screening. Subsequently, bisubstrate inhibitors were prepared by conjugating these peptides to uridine in various ways. In parallel, an in silico fragment screening was conducted to obtain small molecules targeting the UDP binding pocket. After evaluation of the initial hits, one of these small molecules was elaborated into a novel OGT hybrid inhibitor, as the replacement of uridine. The novel compounds inhibit OGT activity with IC50 values in the micromolar range.

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OGT (O-GlcNAc Transferase) Selectively Modifies Multiple Residues Unique to Lamin A.

Project description:The LMNA gene encodes lamins A and C with key roles in nuclear structure, signaling, gene regulation, and genome integrity. Mutations in LMNA cause over 12 diseases ('laminopathies'). Lamins A and C are identical for their first 566 residues. However, they form separate filaments in vivo, with apparently distinct roles. We report that lamin A is ?-O-linked N-acetylglucosamine-(O-GlcNAc)-modified in human hepatoma (Huh7) cells and in mouse liver. In vitro assays with purified O-GlcNAc transferase (OGT) enzyme showed robust O-GlcNAcylation of recombinant mature lamin A tails (residues 385?646), with no detectable modification of lamin B1, lamin C, or 'progerin' (?50) tails. Using mass spectrometry, we identified 11 O-GlcNAc sites in a 'sweet spot' unique to lamin A, with up to seven sugars per peptide. Most sites were unpredicted by current algorithms. Double-mutant (S612A/T643A) lamin A tails were still robustly O-GlcNAc-modified at seven sites. By contrast, O-GlcNAcylation was undetectable on tails bearing deletion ?50, which causes Hutchinson?Gilford progeria syndrome, and greatly reduced by deletion ?35. We conclude that residues deleted in progeria are required for substrate recognition and/or modification by OGT in vitro. Interestingly, deletion ?35, which does not remove the majority of identified O-GlcNAc sites, does remove potential OGT-association motifs (lamin A residues 622?625 and 639?645) homologous to that in mouse Tet1. These biochemical results are significant because they identify a novel molecular pathway that may profoundly influence lamin A function. The hypothesis that lamin A is selectively regulated by OGT warrants future testing in vivo, along with two predictions: genetic variants may contribute to disease by perturbing OGT-dependent regulation, and nutrient or other stresses might cause OGT to misregulate wildtype lamin A.

| S-EPMC5981268 | biostudies-literature

OGA inhibition by GlcNAc-selenazoline.

Project description:The title compound, which differs from the powerful O-GlcNAcase (OGA) inhibitor GlcNAc-thiazoline only at the chalcogen atom (Se for S), is a much weaker inhibitor in a direct OGA assay. In human cells, however, the selenazoline shows comparable ability to induce hyper-O-GlcNAc-ylation, and the two show similar reduction of insulin-stimulated translocation of glucose transporter 4 in differentiated 3T3 adipocytes.

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Caenorhabditis elegans ortholog of a diabetes susceptibility locus: oga-1 (O-GlcNAcase) knockout impacts O-GlcNAc cycling, metabolism, and dauer.

Project description:A dynamic cycle of O-linked N-acetylglucosamine (O-GlcNAc) addition and removal acts on nuclear pore proteins, transcription factors, and kinases to modulate cellular signaling cascades. Two highly conserved enzymes (O-GlcNAc transferase and O-GlcNAcase) catalyze the final steps in this nutrient-driven "hexosamine-signaling pathway." A single nucleotide polymorphism in the human O-GlcNAcase gene is linked to type 2 diabetes. Here, we show that Caenorhabditis elegans oga-1 encodes an active O-GlcNAcase. We also describe a knockout allele, oga-1(ok1207), that is viable and fertile yet accumulates O-GlcNAc on nuclear pores and other cellular proteins. Interfering with O-GlcNAc cycling with either oga-1(ok1207) or the O-GlcNAc transferase-null ogt-1(ok430) altered Ser- and Thr-phosphoprotein profiles and increased glycogen synthase kinase 3beta (GSK-3beta) levels. Both the oga-1(ok1207) and ogt-1(ok430) strains showed elevated stores of glycogen and trehalose, and decreased lipid storage. These striking metabolic changes prompted us to examine the insulin-like signaling pathway controlling nutrient storage, longevity, and dauer formation in the C. elegans O-GlcNAc cycling mutants. Indeed, we found that the oga-1(ok1207) knockout augmented dauer formation induced by a temperature sensitive insulin-like receptor (daf-2) mutant under conditions in which the ogt-1(ok430)-null diminished dauer formation. Our findings suggest that the enzymes of O-GlcNAc cycling "fine-tune" insulin-like signaling in response to nutrient flux. The knockout of O-GlcNAcase (oga-1) in C. elegans mimics many of the metabolic and signaling changes associated with human insulin resistance and provides a genetically amenable model of non-insulin-dependent diabetes.

| S-EPMC1567679 | biostudies-literature

Effect of O-GlcNAc Transferase (OGT) siRNA in trophoblastic BeWo cells

Project description:Every year, about 18 million babies are born from mothers with gestational diabetes mellitus (GDM). While diabetic symptoms usually resolve after delivery, lasting complications can occur for both mother and child, including fetal overgrowth, type 2 diabetes (T2D), cardiovascular diseases, and obesity. The rapid progression of GDM is unique to pregnancies, and likely arises from placental dysfunction. Indeed, stress, nutrition and fetal gender are thought to trigger changes in placental endocrine function, including metabolic hormones and inflammatory cytokines, potentially causing GDM. Nutrient-sensing O-GlcNAcylation and its enzyme O-GlcNAc Transferase (OGT, X-linked) has been previously identified as a placental biomarker of maternal stress particularly deleterious for male offspring. Thus, we wondered whether O-GlcNAcylation participate in GDM development in a sex-dependent manner. After demonstrating that OGT is largely downregulated in male GDM compared to healthy placentas, we knocked down OGT in male trophoblastic cells. We observed changes in placental hormones, cytokines and nutrient-sensing pathways, correlating with previously demonstrated disruption in GDM placentas. Altogether, this study demonstrates that OGT and O-GlcNAcylation are partly responsible for changes observed in GDM placenta and might be the cause of sexual dimorphism observed in this pathology.

2022-04-21 | GSE200983 | GEO

E2F1 Transcription Factor Regulates O-linked N-acetylglucosamine (O-GlcNAc) Transferase and O-GlcNAcase Expression.

Project description:Protein O-GlcNAcylation, which is controlled by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), has emerged as an important posttranslational modification that may factor in multiple diseases. Until recently, it was assumed that OGT/OGA protein expression was relatively constant. Several groups, including ours, have shown that OGT and/or OGA expression changes in several pathologic contexts, yet the cis and trans elements that regulate the expression of these enzymes remain essentially unexplored. Here, we used a reporter-based assay to analyze minimal promoters and leveraged in silico modeling to nominate several candidate transcription factor binding sites in both Ogt (i.e. the gene for OGT protein) and Mgea5 (i.e. the gene for OGA protein). We noted multiple E2F binding site consensus sequences in both promoters. We performed chromatin immunoprecipitation in both human and mouse cells and found that E2F1 bound to candidate E2F binding sites in both promoters. In HEK293 cells, we overexpressed E2F1, which significantly reduced OGT and MGEA5 expression. Conversely, E2F1-deficient mouse fibroblasts had increased Ogt and Mgea5 expression. Of the known binding partners for E2F1, we queried whether retinoblastoma 1 (Rb1) might be involved. Rb1-deficient mouse embryonic fibroblasts showed increased levels of Ogt and Mgea5 expression, yet overexpression of E2F1 in the Rb1-deficient cells did not alter Ogt and Mgea5 expression, suggesting that Rb1 is required for E2F1-mediated suppression. In conclusion, this work identifies and validates some of the promoter elements for mouse Ogt and Mgea5 genes. Specifically, E2F1 negatively regulates both Ogt and Mgea5 expression in an Rb1 protein-dependent manner.

| S-EPMC4692227 | biostudies-literature

Disruption of O-GlcNAc cycling by deletion of O-GlcNAcase (Oga/Mgea5) changed gene expression pattern in mouse embryonic fibroblast (MEF) cells.

Project description:Adding a single O-GlcNAc moiety to a Ser/Thr molecule of a protein by O-GlcNAc transferase and transiently removing it by O-GlcNAcase is referred to as O-GlcNAc cycling (or O-GlcNAcylation). This O-GlcNAc modification is sensitive to nutrient availability and also shows cross talk with phosphorylation signaling, affecting downstream targets. A mouse model system was developed and evaluated to show genome wide transcriptional changes associated with disruption of O-GlcNAc cycling. Mouse embryonic fibroblast cells derived from O-GlcNAcase (Oga) knock out (KO), heterozygous (Het) and wild type (WT) embryos were used for an Affymetrix based microarray. Results are deposited in GEO dataset GSE52721. Data reveals that Oga KO MEFs had 2534 transcripts differentially expressed at 1.5 fold level while Oga heterozygous MEFs had 959 transcripts changed compared to WT MEFs. There were 1835 transcripts differentially expressed at 1.5 fold Het versus WT comparison group. Gene ontology analysis indicated differentially expressed genes enriched in metabolic, growth, and cell proliferation categories.

| S-EPMC4583686 | biostudies-literature

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