Targeted Protein O-GlcNAcylation Using Bifunctional Small Molecules

Protein O-linked β-N-acetylglucosamine modification (O-GlcNAcylation) plays a crucial role in regulating essential cellular processes. The disruption of the homeostasis of O-GlcNAcylation has been linked to various human diseases, including cancer, diabetes, and neurodegeneration. However, there are limited chemical tools for protein- and site-specific O-GlcNAc modification, rendering the precise study of the O-GlcNAcylation challenging. To address this, we have developed heterobifunctional small molecules, named O-GlcNAcylation TArgeting Chimeras (OGTACs), which enable protein-specific O-GlcNAcylation in living cells. OGTACs promote O-GlcNAcylation of proteins such as BRD4, CK2α, and EZH2 in cellulo by recruiting FKBP12F36V-fused O-GlcNAc transferase (OGT), with temporal, magnitude, and reversible control. Overall, the OGTACs represent a promising approach for inducing protein-specific O-GlcNAcylation, thus enabling functional dissection and offering new directions for O-GlcNAc-targeting therapeutic development.

Spindly is a nucleocytosolic O-fucosyltransferase in Dictyostelium and related proteins are widespread in protists and bacteria

O-GlcNAcylation is a prominent modification of nuclear and cytoplasmic proteins in animals and plants and is mediated by a single O-GlcNAc transferase (OGT). Spindly (Spy), a paralog of OGT first discovered in higher plants, has an ortholog in the apicomplexan parasite Toxoplasma gondii, and both enzymes are now recognized as O-fucosyltransferases (OFTs). Here we investigate the evolution of spy-like genes and experimentally confirm OFT activity in the social amoeba Dictyostelium-a protist that is more related to fungi and metazoa. Immunofluorescence probing with the fucose-specific Aleuria aurantia lectin (AAL) and biochemical cell fractionation combined with western blotting suggested the occurrence of nucleocytoplasmic fucosylation. The absence of reactivity in mutants deleted in spy or gmd (unable to synthesize GDP-Fuc) suggested monofucosylation mediated by Spy. Genetic ablation of the modE locus, previously predicted to encode a GDP-fucose transporter, confirmed its necessity for fucosylation in the secretory pathway but not for the nucleocytoplasmic proteins. Affinity capture of these proteins combined with mass spectrometry confirmed monofucosylation of Ser and Thr residues of several known nucleocytoplasmic proteins. As in Toxoplasma, the Spy OFT was required for optimal proliferation of Dictyostelium under laboratory conditions. These findings support a new phylogenetic analysis of OGT and OFT evolution that indicates their occurrence in the last eukaryotic common ancestor but mostly complementary presence in its eukaryotic descendants with the notable exception that both occur in red algae and plants. Their generally exclusive expression, high degree of conservation, and shared monoglycosylation targets suggest overlapping roles in physiological regulation.

Combining Selective Enrichment and a Boosting Approach to Globally and Site-Specifically Characterize Protein Co-translational O-GlcNAcylation

Protein O-GlcNAcylation plays extremely important roles in mammalian cells, regulating signal transduction and gene expression. This modification can happen during protein translation, and systematic and site-specific analysis of protein co-translational O-GlcNAcylation can advance our understanding of this important modification. However, it is extraordinarily challenging because normally O-GlcNAcylated proteins are very low abundant and the abundances of co-translational ones are even much lower. Here, we developed a method integrating selective enrichment, a boosting approach, and multiplexed proteomics to globally and site-specifically characterize protein co-translational O-GlcNAcylation. The boosting approach using the TMT labeling dramatically enhances the detection of co-translational glycopeptides with low abundance when enriched O-GlcNAcylated peptides from cells with a much longer labeling time was used as a boosting sample. More than 180 co-translational O-GlcNAcylated proteins were site-specifically identified. Further analyses revealed that among co-translational glycoproteins, those related to DNA binding and transcription are highly overrepresented using the total identified O-GlcNAcylated proteins in the same cells as the background. Compared with the glycosylation sites on all glycoproteins, co-translational sites have different local structures and adjacent amino acid residues. Overall, an integrative method was developed to identify protein co-translational O-GlcNAcylation, which is very useful to advance our understanding of this important modification.

Bladder Cancer-Derived Small Extracellular Vesicles Promote Tumor Angiogenesis by Inducing HBP-Related Metabolic Reprogramming and SerRS O-GlcNAcylation in Endothelial Cells

A malformed tumour vascular network provokes the nutrient-deprived tumour microenvironment (TME), which conversely activates endothelial cell (EC) functions and stimulates neovascularization. Emerging evidence suggests that the flexible metabolic adaptability of tumour cells helps to establish a metabolic symbiosis among various cell subpopulations in the fluctuating TME. In this study, the authors propose a novel metabolic link between bladder cancer (BCa) cells and ECs in the nutrient-scarce TME, in which BCa-secreted glutamine-fructose-6-phosphate aminotransferase 1 (GFAT1) via small extracellular vesicles (sEVs) reprograms glucose metabolism by increasing hexosamine biosynthesis pathway flux in ECs and thus enhances O-GlcNAcylation. Moreover, seryl-tRNA synthetase (SerRS) O-GlcNAcylation at serine 101 in ECs promotes its degradation by ubiquitination and impeded importin α5-mediated nuclear translocation. Intranuclear SerRS attenuates vascular endothelial growth factor transcription by competitively binding to the GC-rich region of the proximal promotor. Additionally, GFAT1 knockout in tumour cells blocks SerRS O-GlcNAcylation in ECs and attenuates angiogenesis both in vitro and in vivo. However, administration of GFAT1-overexpressing BCa cells-derived sEVs increase the angiogenetic activity in the ECs of GFAT1-knockout mice. In summary, this study suggests that inhibiting sEV-mediated GFAT1 secretion from BCa cells and targeting SerRS O-GlcNAcylation in ECs may serve as novel strategies for BCa antiangiogenetic therapy.

Nutrient sensing pathways regulating adult reproductive diapause in C. elegans

Genetic and environmental manipulations, such as dietary restriction, can improve both health span and lifespan in a wide range of organisms, including humans. Changes in nutrient intake trigger often overlapping metabolic pathways that can generate distinct or even opposite outputs depending on several factors, such as when dietary restriction occurs in the lifecycle of the organism or the nature of the changes in nutrients. Due to the complexity of metabolic pathways and the diversity in outputs, the underlying mechanisms regulating diet-associated pro-longevity are not yet well understood. Adult reproductive diapause (ARD) in the model organism Caenorhabditis elegans is a dietary restriction model that is associated with lengthened lifespan and reproductive potential. To explore the metabolic pathways regulating ARD in greater depth, we performed a candidate-based genetic screen analyzing select nutrient-sensing pathways to determine their contribution to the regulation of ARD. Focusing on the three phases of ARD (initiation, maintenance, and recovery), we found that ARD initiation is regulated by fatty acid metabolism, sirtuins, AMPK, and the O-linked N-acetyl glucosamine (O-GlcNAc) pathway. Although ARD maintenance was not significantly influenced by the nutrient sensors in our screen, we found that ARD recovery was modulated by energy sensing, stress response, insulin-like signaling, and the TOR pathway. Further investigation of downstream targets of NHR-49 suggest the transcription factor influences ARD initiation through the fatty acid β-oxidation pathway. Consistent with these findings, our analysis revealed a change in levels of neutral lipids associated with ARD entry defects. Our findings identify conserved genetic pathways required for ARD entry and recovery and uncover genetic interactions that provide insight into the role of OGT and OGA.

Brain Cell Type-Specific Nuclear Proteomics Is Imperative to Resolve Neurodegenerative Disease Mechanisms

Neurodegenerative diseases (NDs) involve complex cellular mechanisms that are incompletely understood. Emerging findings have revealed that disruption of nuclear processes play key roles in ND pathogenesis. The nucleus is a nexus for gene regulation and cellular processes that together, may underlie pathomechanisms of NDs. Furthermore, many genetic risk factors for NDs encode proteins that are either present in the nucleus or are involved in nuclear processes (for example, RNA binding proteins, epigenetic regulators, or nuclear-cytoplasmic transport proteins). While recent advances in nuclear transcriptomics have been significant, studies of the nuclear proteome in brain have been relatively limited. We propose that a comprehensive analysis of nuclear proteomic alterations of various brain cell types in NDs may provide novel biological and therapeutic insights. This may be feasible because emerging technical advances allow isolation and investigation of intact nuclei from post-mortem frozen human brain tissue with cell type-specific and single-cell resolution. Accordingly, nuclei of various brain cell types harbor unique protein markers which can be used to isolate cell-type specific nuclei followed by down-stream proteomics by mass spectrometry. Here we review the literature providing a rationale for investigating proteomic changes occurring in nuclei in NDs and then highlight the potential for brain cell type-specific nuclear proteomics to enhance our understanding of distinct cellular mechanisms that drive ND pathogenesis.

O-GlcNAcylation: The Underestimated Emerging Regulators of Skeletal Muscle Physiology

O-GlcNAcylation is a highly dynamic, reversible and atypical glycosylation that regulates the activity, biological function, stability, sublocation and interaction of target proteins. O-GlcNAcylation receives and coordinates different signal inputs as an intracellular integrator similar to the nutrient sensor and stress receptor, which target multiple substrates with spatio-temporal analysis specifically to maintain cellular homeostasis and normal physiological functions. Our review gives a brief description of O-GlcNAcylation and its only two processing enzymes and HBP flux, which will help to better understand its physiological characteristics of sensing nutrition and environmental cues. This nutritional and stress-sensitive properties of O-GlcNAcylation allow it to participate in the precise regulation of skeletal muscle metabolism. This review discusses the mechanism of O-GlcNAcylation to alleviate metabolic disorders and the controversy about the insulin resistance of skeletal muscle. The level of global O-GlcNAcylation is precisely controlled and maintained in the “optimal zone”, and its abnormal changes is a potential factor in the pathogenesis of cancer, neurodegeneration, diabetes and diabetic complications. Although the essential role of O-GlcNAcylation in skeletal muscle physiology has been widely studied and recognized, it still is underestimated and overlooked. This review highlights the latest progress and potential mechanisms of O-GlcNAcylation in the regulation of skeletal muscle contraction and structural properties.

Targeted Inhibition of O-Linked β-N-Acetylglucosamine Transferase as a Promising Therapeutic Strategy to Restore Chemosensitivity and Attenuate Aggressive Tumor Traits in Chemoresistant Urothelial Carcinoma of the Bladder

Acquisition of acquired chemoresistance during treatment cycles in urothelial carcinoma of the bladder (UCB) is the major cause of death through enhancing the risk of cancer progression and metastasis. Elevated glucose flux through the abnormal upregulation of O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) controls key signaling and metabolic pathways regulating diverse cancer cell phenotypes. This study showed that OGT expression levels in two human UCB cell models with acquired resistance to gemcitabine and paclitaxel were significantly upregulated compared with those in parental cells. Reducing hyper-O-GlcNAcylation by OGT knockdown (KD) markedly facilitated chemosensitivity to the corresponding chemotherapeutics in both cells, and combination treatment with OGT-KD showed more severe growth defects in chemoresistant sublines. We subsequently verified the suppressive effects of OGT-KD monotherapy on cell migration/invasion in vitro and xenograft tumor growth in vivo in chemoresistant UCB cells. Transcriptome analysis of these cells revealed 97 upregulated genes, which were enriched in multiple oncogenic pathways. Our final choice of suspected OGT glycosylation substrate was VCAN, S1PR3, PDGFRB, and PRKCG, the knockdown of which induced cell growth defects. These findings demonstrate the vital role of dysregulated OGT activity and hyper-O-GlcNAcylation in modulating treatment failure and tumor aggression in chemoresistant UCB.

Nucleolin: a cell portal for viruses, bacteria, and toxins

The main localization of nucleolin is the nucleolus, but this protein is present in multiple subcellular sites, and it is unconventionally secreted. On the cell surface, nucleolin acts as a receptor for various viruses, some bacteria, and some toxins. Aim of this review is to discuss the characteristics that make nucleolin able to act as receptor or co-receptor of so many and different pathogens. The important features that emerge are its multivalence, and its role as a bridge between the cell surface and the nucleus. Multiple domains, short linear motifs and post-translational modifications confer and modulate nucleolin ability to interact with nucleic acids, with proteins, but also with carbohydrates and lipids. This modular multivalence allows nucleolin to participate in different types of biomolecular condensates and to move to various subcellular locations, where it can act as a kind of molecular glue. It moves from the nucleus to the cell surface and can accompany particles in the reverse direction, from the cell surface into the nucleus, which is the destination of several pathogens to manipulate the cell in their favour.

Structure, Maintenance, and Regulation of Nuclear Pore Complexes: The Gatekeepers of the Eukaryotic Genome

In eukaryotic cells, the genetic material is segregated inside the nucleus. This compartmentalization of the genome requires a transport system that allows cells to move molecules across the nuclear envelope, the membrane-based barrier that surrounds the chromosomes. Nuclear pore complexes (NPCs) are the central component of the nuclear transport machinery. These large protein channels penetrate the nuclear envelope, creating a passage between the nucleus and the cytoplasm through which nucleocytoplasmic molecule exchange occurs. NPCs are one of the largest protein assemblies of eukaryotic cells and, in addition to their critical function in nuclear transport, these structures also play key roles in many cellular processes in a transport-independent manner. Here we will review the current knowledge of the NPC structure, the cellular mechanisms that regulate their formation and maintenance, and we will provide a brief description of a variety of processes that NPCs regulate.

Analysis of Ubiquitylation and SUMOylation of Yeast Nuclear Pore Complex Proteins

Posttranslational modifications and in particular ubiquitylation and SUMOylation of the nuclear pore complex (NPC), have been shown to regulate some of its functions, particularly in response to diverse stress signals.Although proteomic approaches are extremely powerful to identify substrates and modification sites, dissecting specific mechanisms and regulation functions of ubiquitylation and SUMOylation of the diverse NPC proteins, in different genetic backgrounds or cell environmental conditions, requires specific biochemical assays based on purification and precise analysis of 6His-tagged ubiquitylated or SUMOylated protein of interest. Here we describe an approach that can be easily employed without specific equipment. It allowed to successfully analyze yeast NPC proteins but can easily be adapted to the study of the mammalian NPC.

Tools, tactics and objectives to interrogate cellular roles of O-GlcNAc in disease

The vast array of cell types of multicellular organisms must individually fine-tune their internal metabolism. One important metabolic and stress regulatory mechanism is the dynamic attachment/removal of glucose-derived sugar N-acetylglucosamine on proteins (O-GlcNAcylation). The number of proteins modified by O-GlcNAc is bewildering, with at least 7,000 sites in human cells. The outstanding challenge is determining how key O-GlcNAc sites regulate a target pathway amidst thousands of potential global sites. Innovative solutions are required to address this challenge in cell models and disease therapy. This Perspective shares critical suggestions for the O-GlcNAc field gleaned from the international O-GlcNAc community. Further, we summarize critical tools and tactics to enable newcomers to O-GlcNAc biology to drive innovation at the interface of metabolism and disease. The growing pace of O-GlcNAc research makes this a timely juncture to involve a wide array of scientists and new toolmakers to selectively approach the regulatory roles of O-GlcNAc in disease.

O-GlcNAcylation Quantification of Certain Protein by the Proximity Ligation Assay and Clostridium perfringen OGAD298N(CpOGAD298N)

O-GlcNAcylation is an O-linked β-N-acetyl-glucosamine (O-GlcNAc)-monosaccharide modification of serine or threonine in proteins that plays a vital role in many critical cellular processes. Owing to its low molecular weight, uncharged property, and difficulty in distinguishing from β-N-acetyl-galactosamine (GalNAc), the lack of high specificity and avidity tools and sophisticated quantification methods have always been the bottleneck in analyzing O-GlcNAc functions. Here, we compared glycan array data of the mutant of Clostridium perfringen OGA (CpOGA^D298N), O-GlcNAc antibody CTD110.6, and several lectins. We found that CpOGA^D298N can effectively distinguish GlcNAc from GalNAc. Glycan array analysis and isothermal titration calorimetry (ITC) show that CpOGA^D298N has a GlcNAc specific binding characteristic. CpOGA^D298N could be used in far-western, flow cytometry analysis, and confocal imaging to demonstrate the existence of O-GlcNAc proteins. Using the CpOGA^D298N affinity column, we identified 84 highly confident O-GlcNAc modified peptides from 82 proteins in the MCF-7 cell line and 33 highly confident peptides in 33 proteins from mouse liver tissue; most of them are novel O-GlcNAc proteins and could not bind with wheat germ agglutinin (WGA). Besides being used as a facile enrichment tool, a combination of CpOGA^D298N with the proximity ligation assay (PLA) is successfully used to quantify O-GlcNAc modified histone H2B, which is as low as femtomoles in MCF-7 cell lysate. These results suggest that CpOGA^D298N is a specific tool for detection (far-western, flow cytometry analysis, and confocal imaging) and enrichment of O-GlcNAcylated proteins and peptides, and the CpOGA^D298N-PLA method is useful for quantifying certain O-GlcNAc protein.

Nutrient-Driven O-GlcNAcylation Controls DNA Damage Repair Signaling and Stem/Progenitor Cell Homeostasis

Stem/progenitor cells exhibit high proliferation rates, elevated nutrient uptake, altered metabolic flux, and stress-induced genome instability. O-GlcNAcylation is an essential post-translational modification mediated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), which act in a nutrient- and stress-responsive manner. The precise role of O-GlcNAc in adult stem cells and the relationship between O-GlcNAc and the DNA damage response (DDR) is poorly understood. Here, we show that hyper-O-GlcNacylation leads to elevated insulin signaling, hyperproliferation, and DDR activation that mimic the glucose- and oxidative-stress-induced response. We discover a feedback mechanism involving key downstream effectors of DDR, ATM, ATR, and CHK1/2 that regulates OGT stability to promote O-GlcNAcylation and elevate DDR. This O-GlcNAc-dependent regulatory pathway is critical for maintaining gut homeostasis in Drosophila and the DDR in mouse embryonic stem cells (ESCs) and mouse embryonic fibroblasts (MEFs). Our findings reveal a conserved mechanistic link among O-GlcNAc cycling, stem cell self-renewal, and DDR with profound implications for stem-cell-derived diseases including cancer.

Generation of an Interactome for the Tetratricopeptide Repeat Domain of O-GlcNAc Transferase Indicates a Role for the Enzyme in Intellectual Disability

The O-GlcNAc transferase (OGT) modifies nuclear and cytoplasmic proteins with β-N-acetyl-glucosamine (O-GlcNAc). With thousands of O-GlcNAc-modified proteins but only one OGT encoded in the mammalian genome, a prevailing question is how OGT selects its substrates. Prior work has indicated that the tetratricopeptide repeat (TPR) domain of OGT is involved in substrate selection. Furthermore, several variants of OGT causal for X-linked intellectual disability (XLID) occur in the TPR domain. Therefore, we adapted the BioID labeling method to identify interactors of a TPR-BirA* fusion protein in HeLa cells. We identified 115 interactors representing known and novel O-GlcNAc-modified proteins and OGT interactors (raw data deposited in MassIVE, Dataset ID MSV000085626). The interactors are enriched in known OGT processes (e.g., chromatin remodeling) as well as processes in which OGT has yet to be implicated (e.g., pre-mRNA processing). Importantly, the identified TPR interactors are linked to several disease states but most notably are enriched in pathologies featuring intellectual disability that may underlie the mechanism by which mutations in OGT lead to XLID. This interactome for the TPR domain of OGT serves as a jumping-off point for future research exploring the role of OGT, the TPR domain, and its protein interactors in multiple cellular processes and disease mechanisms, including intellectual disability.

Stem cell fate determination through protein O-GlcNAcylation

Embryonic and adult stem cells possess the capability of self-renewal and lineage-specific differentiation. The intricate balance between self-renewal and differentiation is governed by developmental signals and cell-type-specific gene regulatory mechanisms. A perturbed intra/extracellular environment during lineage specification could affect stem cell fate decisions resulting in pathology. Growing evidence demonstrates that metabolic pathways govern epigenetic regulation of gene expression during stem cell fate commitment through the utilization of metabolic intermediates or end products of metabolic pathways as substrates for enzymatic histone/DNA modifications. UDP-GlcNAc is one such metabolite that acts as a substrate for enzymatic mono-glycosylation of various nuclear, cytosolic, and mitochondrial proteins on serine/threonine amino acid residues, a process termed protein O-GlcNAcylation. The levels of GlcNAc inside the cells depend on the nutrient availability, especially glucose. Thus, this metabolic sensor could modulate gene expression through O-GlcNAc modification of histones or other proteins in response to metabolic fluctuations. Herein, we review evidence demonstrating how stem cells couple metabolic inputs to gene regulatory pathways through O-GlcNAc-mediated epigenetic/transcriptional regulatory mechanisms to govern self-renewal and lineage-specific differentiation programs. This review will serve as a primer for researchers seeking to better understand how O-GlcNAc influences stemness and may catalyze the discovery of new stem-cell-based therapeutic approaches.

Protein O-GlcNAcylation Promotes Trophoblast Differentiation at Implantation

Embryo implantation begins with blastocyst trophectoderm (TE) attachment to the endometrial epithelium, followed by the breaching of this barrier by TE-derived trophoblast. Dynamic protein modification with O-linked β-N-acetylglucosamine (O-GlcNAcylation) is mediated by O-GlcNAc transferase and O-GlcNAcase (OGA), and couples cellular metabolism to stress adaptation. O-GlcNAcylation is essential for blastocyst formation, but whether there is a role for this system at implantation remains unexplored. Here, we used OGA inhibitor thiamet g (TMG) to induce raised levels of O-GlcNAcylation in mouse blastocysts and human trophoblast cells. In an in vitro embryo implantation model, TMG promoted mouse blastocyst breaching of the endometrial epithelium. TMG reduced expression of TE transcription factors Cdx2, Gata2 and Gata3, suggesting that O-GlcNAcylation stimulated TE differentiation to invasive trophoblast. TMG upregulated transcription factors OVOL1 and GCM1, and cell fusion gene ERVFRD1, in a cell line model of syncytiotrophoblast differentiation from human TE at implantation. Therefore O-GlcNAcylation is a conserved pathway capable of driving trophoblast differentiation. TE and trophoblast are sensitive to physical, chemical and nutritive stress, which can occur as a consequence of maternal pathophysiology or during assisted reproduction, and may lead to adverse neonatal outcomes and associated adult health risks. Further investigation of how O-GlcNAcylation regulates trophoblast populations arising at implantation is required to understand how peri-implantation stress affects reproductive outcomes.

Role of O-Linked N-Acetylglucosamine Protein Modification in Cellular (Patho)Physiology

In the mid-1980s, the identification of serine and threonine residues on nuclear and cytoplasmic proteins modified by a N-acetylglucosamine moiety (O-GlcNAc) via an O-linkage overturned the widely held assumption that glycosylation only occurred in the endoplasmic reticulum, Golgi apparatus, and secretory pathways. In contrast to traditional glycosylation, the O-GlcNAc modification does not lead to complex, branched glycan structures and is rapidly cycled on and off proteins by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. Since its discovery, O-GlcNAcylation has been shown to contribute to numerous cellular functions, including signaling, protein localization and stability, transcription, chromatin remodeling, mitochondrial function, and cell survival. Dysregulation in O-GlcNAc cycling has been implicated in the progression of a wide range of diseases, such as diabetes, diabetic complications, cancer, cardiovascular, and neurodegenerative diseases. This review will outline our current understanding of the processes involved in regulating O-GlcNAc turnover, the role of O-GlcNAcylation in regulating cellular physiology, and how dysregulation in O-GlcNAc cycling contributes to pathophysiological processes.

O-GlcNAc modification of Sox2 regulates self-renewal in pancreatic cancer by promoting its stability

Pancreatic adenocarcinoma (PDAC) claims more than 90% of the patients diagnosed with the disease owing to its aggressive biology that is manifested by high rate of tumor recurrence. Aberrant upregulation in the transcriptional activity of proteins involved in self-renewal like Sox2, Oct4 and Nanog is instrumental in these recurrence phenomena. In cancer, Sox2 is aberrantly "turned-on" leading to activation of downstream genes those results in relapse of the tumor. Molecular mechanisms that regulate the activity of Sox2 in PDAC are not known. In the current study, we have studied the how glycosylation of Sox2 by O-GlcNAc transferase (OGT) can affect its transcriptional activity and thus regulate self-renewal in cancer. Methods: RNA-Seq analysis of CRISPR-OGTi PDAC cells indicated a deregulation of differentiation and self-renewal pathways in PDAC. Pancreatic tumor burden following inhibition of OGT in vivo was done by using small molecule inhibitor, OSMI, on subcutaneous implantation of PDAC cells. Sox2 activity assay was performed by Dual Luciferase Reporter Assay kit. Results: Our study shows for the first time that in PDAC, glycosylation of Sox2 by OGT stabilizes it in the nucleus. Site directed mutagenesis of this site (S246A) prevents this modification. We further show that inhibition of OGT delayed initiation of pancreatic tumors by inhibition of Sox2. We also show that targeting OGT in vivo with a small molecule-inhibitor OSMI, results in decreased tumor burden in PDAC. Conclusion: Understanding this mechanism of SOX2 regulation by its glycosylation is expected to pave the way for development of novel therapy that has the potential to eradicate the cells responsible for tumor-recurrence.

Nucleocytoplasmic O-glycosylation in protists

O-Glycosylation is an increasingly recognized modification of intracellular proteins in all kingdoms of life, and its occurrence in protists has been investigated to understand its evolution and its roles in the virulence of unicellular pathogens. We focus here on two kinds of glycoregulation found in unicellular eukaryotes: one is a simple O-fucose modification of dozens if not hundreds of Ser/Thr-rich proteins, and the other a complex pentasaccharide devoted to a single protein associated with oxygen sensing and the assembly of polyubiquitin chains. These modifications are not required for life but contingently modulate biological processes in the social amoeba Dictyostelium and the human pathogen Toxoplasma gondii, and likely occur in diverse unicellular protists. O-Glycosylation that is co-localized in the cytoplasm allows for glycoregulation over the entire life of the protein, contrary to the secretory pathway where glycosylation usually occurs before its delivery to its site of function. Here, we interpret cellular roles of nucleocytoplasmic glycans in terms of current evidence for their effects on the conformation and dynamics of protist proteins, to serve as a guide for future studies to examine their broader significance.

The Role of Insulin Resistance and Protein O-GlcNAcylation in Neurodegeneration

Metabolic syndrome including obesity and type 2 diabetes is increasing at an alarming rate worldwide. Similarly, there has been an increase in the cases of neurodegenerative diseases such as Alzheimer’s disease (AD) possibility due to increase in elderly population in the past few decades. Both, metabolic diseases and AD have one common feature that is insulin resistance. Recent studies suggest a link between the regulatory functions of insulin in the brain and AD. Hypoglycemia, a characteristic feature of AD may be a result of impaired insulin signaling in the affected regions of the brain. O-GlcNAcylation is a post-translational protein modification, the levels of which are dependent on the availability of glucose inside the cells. Hyperphosphorylation of Tau is a major molecular feature, which leads to its aggregation and neurotoxicity in AD. In addition, impaired processing of Amyloid precursor protein (APP) leading to toxic amyloid β (Aβ) aggregation is also implicated in the pathogenesis of AD. Both APP and Tau are also found to be O-GlcNAcylated. Reduced O-GlcNAcylation of APP and Tau due to hypoglycemia is found to be associated with their pathological features in AD brain. Recent studies have also identified perturbed O-GlcNAcylation/phosphorylation of several other proteins important for normal neuronal function, which may be contributing to the neuropathological development in AD. Herein, we discuss about the uptake and distribution of insulin inside the brain, brain insulin signaling and insulin resistance as well as its relation to neurodegenerative diseases with a special focus on protein O-GlcNAcylation and its potential role in the treatment of AD.

Disruption of O-GlcNAc homeostasis during mammalian oocyte meiotic maturation impacts fertilization

Meiotic maturation and fertilization are metabolically demanding processes, and thus the mammalian oocyte is highly susceptible to changes in nutrient availability. O-GlcNAcylation-the addition of a single sugar residue (O-linked β-N-acetylglucosamine) on proteins-is a posttranslational modification that acts as a cellular nutrient sensor and likely modulates the function of oocyte proteins. O-GlcNAcylation is mediated by O-GlcNAc transferase (OGT), which adds O-GlcNAc onto proteins, and O-GlcNAcase (OGA), which removes it. Here we investigated O-GlcNAcylation dynamics in bovine and human oocytes during meiosis and determined the developmental sequelae of its perturbation. OGA, OGT, and multiple O-GlcNAcylated proteins were expressed in bovine cumulus oocyte complexes (COCs), and they were localized throughout the gamete but were also enriched at specific subcellular sites. O-GlcNAcylated proteins were concentrated at the nuclear envelope at prophase I, OGA at the cortex throughout meiosis, and OGT at the meiotic spindles. These expression patterns were evolutionarily conserved in human oocytes. To examine O-GlcNAc function, we disrupted O-GlcNAc cycling during meiotic maturation in bovine COCs using Thiamet-G (TMG), a highly selective OGA inhibitor. Although TMG resulted in a dramatic increase in O-GlcNAcylated substrates in both cumulus cells and the oocyte, there was no effect on cumulus expansion or meiotic progression. However, zygote development was significantly compromised following in vitro fertilization of COCs matured in TMG due to the effects on sperm penetration, sperm head decondensation, and pronuclear formation. Thus, proper O-GlcNAc homeostasis during meiotic maturation is important for fertilization and pronuclear stage development.

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.

Structural basis of O-GlcNAc recognition by mammalian 14-3-3 proteins

O-GlcNAc is an intracellular posttranslational modification that governs myriad cell biological processes and is dysregulated in human diseases. Despite this broad pathophysiological significance, the biochemical effects of most O-GlcNAcylation events remain uncharacterized. One prevalent hypothesis is that O-GlcNAc moieties may be recognized by "reader" proteins to effect downstream signaling. However, no general O-GlcNAc readers have been identified, leaving a considerable gap in the field. To elucidate O-GlcNAc signaling mechanisms, we devised a biochemical screen for candidate O-GlcNAc reader proteins. We identified several human proteins, including 14-3-3 isoforms, that bind O-GlcNAc directly and selectively. We demonstrate that 14-3-3 proteins bind O-GlcNAc moieties in human cells, and we present the structures of 14-3-3β/α and γ bound to glycopeptides, providing biophysical insights into O-GlcNAc-mediated protein-protein interactions. Because 14-3-3 proteins also bind to phospho-serine and phospho-threonine, they may integrate information from O-GlcNAc and O-phosphate signaling pathways to regulate numerous physiological functions.

The Role of Stress-Induced O-GlcNAc Protein Modification in the Regulation of Membrane Transport

O-linked N-acetylglucosamine (O-GlcNAc) is a posttranslational modification that is increasingly recognized as a signal transduction mechanism. Unlike other glycans, O-GlcNAc is a highly dynamic and reversible process that involves the addition and removal of a single N-acetylglucosamine molecule to Ser/Thr residues of proteins. UDP-GlcNAc—the direct substrate for O-GlcNAc modification—is controlled by the rate of cellular metabolism, and thus O-GlcNAc is dependent on substrate availability. Serving as a feedback mechanism, O-GlcNAc influences the regulation of insulin signaling and glucose transport. Besides nutrient sensing, O-GlcNAc was also implicated in the regulation of various physiological and pathophysiological processes. Due to improvements of mass spectrometry techniques, more than one thousand proteins were detected to carry the O-GlcNAc moiety; many of them are known to participate in the regulation of metabolites, ions, or protein transport across biological membranes. Recent studies also indicated that O-GlcNAc is involved in stress adaptation; overwhelming evidences suggest that O-GlcNAc levels increase upon stress. O-GlcNAc elevation is generally considered to be beneficial during stress, although the exact nature of its protective effect is not understood. In this review, we summarize the current data regarding the oxidative stress-related changes of O-GlcNAc levels and discuss the implications related to membrane trafficking.

Cellular glycosylation senses metabolic changes and modulates cell plasticity during epithelial to mesenchymal transition

Epithelial to mesenchymal transition (EMT) is a developmental program reactivated by tumor cells that leads to the switch from epithelial to mesenchymal phenotype. During EMT, cells are transcriptionally regulated to decrease E-cadherin expression while expressing mesenchymal markers such as vimentin, fibronectin, and N-cadherin. Growing body of evidences suggest that cells engaged in EMT undergo a metabolic reprograming process, redirecting glucose flux toward hexosamine biosynthesis pathway (HBP), which fuels aberrant glycosylation patterns that are extensively observed in cancer cells. HBP depends on nutrient availability to produce its end product UDP-GlcNAc, and for this reason is considered a metabolic sensor pathway. UDP-GlcNAc is the substrate used for the synthesis of major types of glycosylation, including O-GlcNAc and cell surface glycans. In general, the rate limiting enzyme of HBP, GFAT, is overexpressed in many cancer types that present EMT features as well as aberrant glycosylation. Moreover, altered levels of O-GlcNAcylation can modulate cell morphology and favor EMT. In this review, we summarize some of the current knowledge that correlates glucose metabolism, aberrant glycosylation and hyper O-GlcNAcylation supported by HBP that leads to EMT activation. Developmental Dynamics 247:481-491, 2018. © 2017 Wiley Periodicals, Inc.