Identification of Ewing's sarcoma gene product as a glycoprotein using a monoclonal antibody that recognizes an immunodeterminant containing O-linked N-acetylglucosamine moiety

An overview of tools to decipher O-GlcNAcylation from historical approaches to new insights

O-GlcNAcylation is a post-translational modification which affects approximately 5000 human proteins. Its involvement has been shown in many if not all biological processes. Variations in O-GlcNAcylation levels can be associated with the development of diseases. Deciphering the role of O-GlcNAcylation is an important issue to (i) understand its involvement in pathophysiological development and (ii) develop new therapeutic strategies to modulate O-GlcNAc levels. Over the past 30 years, despite the development of several approaches, knowledge of its role and regulation have remained limited. This review proposes an overview of the currently available tools to study O-GlcNAcylation and identify O-GlcNAcylated proteins. Briefly, we discuss pharmacological modulators, methods to study O-GlcNAcylation levels and approaches for O-GlcNAcylomic profiling. This review aims to contribute to a better understanding of the methods used to study O-GlcNAcylation and to promote efforts in the development of new strategies to explore this promising modification.

Detection and Analysis of Proteins Modified by O-Linked N-Acetylglucosamine

O-GlcNAc is a common post-translational modification of nuclear, mitochondrial, and cytoplasmic proteins that regulates normal physiology and the cell stress response. Dysregulation of O-GlcNAc cycling is implicated in the etiology of type II diabetes, heart failure, hypertension, and Alzheimer's disease, as well as cardioprotection. These protocols cover simple and comprehensive techniques for detecting proteins modified by O-GlcNAc and studying the enzymes that add or remove O-GlcNAc. © 2021 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Increasing the stoichiometry of O-GlcNAc on proteins before analysis Basic Protocol 2: Detection of proteins modified by O-GlcNAc using antibodies Basic Protocol 3: Detection of proteins modified by O-GlcNAc using the lectin sWGA Support Protocol 1: Control for O-linked glycosylation Basic Protocol 4: Detection and enrichment of proteins using WGA-agarose Support Protocol 2: Digestion of proteins with hexosaminidase Alternate Protocol: Detection of proteins modified by O-GlcNAc using galactosyltransferase Support Protocol 3: Autogalactosylation of galactosyltransferase Support Protocol 4: Assay of galactosyltransferase activity Basic Protocol 5: Characterization of labeled glycans by β-elimination and chromatography Basic Protocol 6: Detection of O-GlcNAc in 96-well plates Basic Protocol 7: Assay for OGT activity Support Protocol 5: Desalting of O-GlcNAc transferase Basic Protocol 8: Assay for O-GlcNAcase activity.

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 glycosylation stoichiometry of the FET protein family: only EWS is glycosylated with a high stoichiometry

Of the FET (fused in sarcoma [FUS]/Ewing sarcoma protein [EWS]/TATA binding protein-associated factor 15 [TAF15]) family of heterogeneous nuclear ribonucleoprotein particle proteins, FUS and TAF15 are consistently and EWS variably found in inclusion bodies in neurodegenerative diseases such as frontotemporal lobar degeneration associated with FUS. It is speculated that dysregulation of FET proteins at the post-translational level is involved in their cytoplasmic deposition. Here, the O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation stoichiometry of the FET proteins was chemoenzymatically analyzed, and it was found that only EWS is dynamically glycosylated with a high stoichiometry in the neural cell lines tested and in mouse brain. It was also confirmed that EWS, but not FUS and TAF15, is glycosylated with a high stoichiometry not only in the neural cells but also in the non-neural cell lines tested. These results indicate that O-GlcNAc glycosylation imparts a physicochemical property on EWS that is distinct from that of the other FET proteins in most of cell lineages or tissues.

The glycosylation stoichiometry of EWS species in neuronal cells

Although Ewing sarcoma protein (EWS) is known to be glycosylated by O-linked β-N-acetylglucosamine (O-GlcNAc), the dynamics and stoichiometry of its glycosylation remain obscure. Here, we report a dynamic change in the glycosylation stoichiometry of EWS species during neuronal differentiation of embryonic carcinoma P19 cells. Our findings suggest that O-GlcNAc glycosylation participates in the regulation of EWS functions in neuronal cells.

O-GlcNAc profiling: from proteins to proteomes

O-linked β-D-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) onto serine and threonine residues of proteins is an important post-translational modification (PTM), which is involved in many crucial biological processes including transcription, translation, proteasomal degradation, and signal transduction. Aberrant protein O-GlcNAcylation is directly linked to the pathological progression of chronic diseases including diabetes, cancer, and neurodegenerative disorders. Identification, site mapping, and quantification of O-GlcNAc proteins are a prerequisite to decipher their functions. In this review, we mainly focus on technological developments regarding O-GlcNAc protein profiling. Specifically, on one hand, we show how these techniques are being used for the comprehensive characterization of certain targeted proteins in which biologists are most interested. On the other hand, we present several newly developed approaches for O-GlcNAcomic profiling as well as how they provide us with a systems perspective to crosstalk amongst different PTMs and complicated biological events. Promising technical trends are also highlighted to evoke more efforts by diverse laboratories, which would further expand our understanding of the physiological and pathological roles of protein O-GlcNAcylation in chronic diseases.

Detection and analysis of proteins modified by O-linked N-acetylglucosamine

O-GlcNAc is a common post-translational modification of nuclear, mitochondrial, and cytoplasmic proteins that is implicated in the etiology of type II diabetes and Alzheimer's disease, as well as cardioprotection. This unit covers simple and comprehensive techniques for identifying proteins modified by O-GlcNAc, studying the enzymes that add and remove O-GlcNAc, and mapping O-GlcNAc modification sites.

Detection and analysis of proteins modified by O-linked N-acetylglucosamine

O-GlcNAc is a common post-translational modification of nuclear, mitochondrial, and cytoplasmic proteins that is implicated in the etiology of type II diabetes and Alzheimer's disease, as well as cardioprotection. This unit covers simple and comprehensive techniques for identifying proteins modified by O-GlcNAc, studying the enzymes that add and remove O-GlcNAc, and mapping O-GlcNAc modification sites.

Detection and analysis of (O-linked beta-N-acetylglucosamine)-modified proteins

Glycosylation is one of the most common and complex forms of posttranslational modifications of proteins in eukaryotes. Seven different protein-carbohydrate linkages have been characterized on nuclear and cytoplasmic glycoproteins, the most widespread of which is the modification of Ser/Thr residues with monosaccharides of O-linked beta-N-acetylglucosamine (O-GlcNAc). O-GlcNAc modification is concentrated in nuclear proteins. O-GlcNAc is thought to regulate protein function in a manner analogous to phosphorylation; and is implicated in the regulation of transcription, the proteasome, insulin and MAP kinase signaling, the cell cycle, and the cellular stress response. In this chapter we focus on methods for the detection of O-GlcNAc-modified proteins and discuss general techniques for the detection and subsequent analysis of other protein-carbohydrate conjugates.

Detecting the "O-GlcNAc-ome"; detection, purification, and analysis of O-GlcNAc modified proteins

The modification of Ser and Thr residues of cytoplasmic and nuclear proteins with a monosaccharide of O-linked beta-N-acetylglucosamine is an essential and dynamic post-translational modification of metazoans. Deletion of the O-GlcNAc transferase (OGT), the enzyme that adds O-GlcNAc, is lethal in mammalian cells highlighting the importance of this post-translational modification in regulating cellular function. O-GlcNAc is believed to modulate protein function in a manner analogous to protein phosphorylation. Notably, on some proteins O-GlcNAc and O-phosphate modify the same Ser/Thr residue, suggesting that a reciprocal relationship exists between these two post-translational modifications. In this chapter we describe the most robust techniques for the detection and purification of O-GlcNAc modified proteins, and discuss some more specialized techniques for site-mapping and detection of O-GlcNAc during mass spectrometry.

O-GlcNAcylation is involved in the transcriptional activity of EWS-FLI1 in Ewing's sarcoma

The oncogene EWS-FLI1 encodes a chimeric transcription factor expressed in Ewing's sarcoma family tumors (ESFTs). EWS-FLI1 target gene expression is thought to drive ESFT pathogenesis and, therefore, inhibition of EWS-FLI1 activity holds high therapeutic promise. As the activity of many transcription factors is regulated by post-translational modifications, we studied the presence of modifications on EWS-FLI1. The immuno-purified fusion-protein was recognized by an antibody specific for O-linked beta-N-acetylglucosaminylation, and bound readily to a phosphoprotein-specific dye. Inhibition of Ser/Thr-specific phophatases increased EWS-FLI1 molecular weight and reduced its O-GlcNAc content, suggesting that phosphorylation and O-GlcNAcylation of EWS-FLI1 interact dynamically. By mutation analysis, O-GlcNAcylation was delineated to Ser/Thr residues of the amino-terminal EWS transcriptional-activation domain. Metabolic inhibition of the hexosamine biosynthetic pathway abrogated O-GlcNAcylation of EWS-FLI1 and interfered specifically with transcriptional activation of the EWS-FLI1 target Id2. These results suggest that drugs modulating glycosylation of EWS-FLI1 interfere functionally with its activity and might, therefore, constitute promising additions to the current ESFT chemotherapy.

Identification of cell surface markers to differentiate rat endothelial and fibroblast cells using lectin arrays and LC-ESI-MS/MS

Vascular endothelial cells located at the inner surface of blood vessels are a key component in angiogenesis and are employed as a primary cell type in the study of angiogenesis. These endothelial cells are, however, easily contaminated with fibroblast cells, which are located in proximity to the endothelial cells, during their isolation from tissue. It is thus important to find markers to distinguish the two cell types. In the present work, lectin arrays were prepared using aldehyde-terminated self-assembled monolayers (SAMs) and utilized to explore cell surface carbohydrate expression patterns on endothelial and fibroblast cells. It was found that the lectins Griffonia simplicifolia II (GS II) and Ulex europaeus agglutinin I (UEA I) selectively bind to rat fibroblast cells and not to rat endothelial cells. GS II-binding glycoproteins on fibroblast cells, which are potential cell surface markers to differentiate endothelial and fibroblast cells, were captured on a GS II lectin column and analyzed by LC-ESI-MS/MS. Six candidate cell surface glycoproteins were identified. Differential expression was confirmed by Western blot analysis for two of these proteins, lysosome-associated membrane glycoprotein-1 and transmembrane glycoprotein NMB.

Identification of O-GlcNAc sites on proteins

O-linked N-acetylglucosamine (O-GlcNAc) is a monosaccharide posttranslational modification that modifies serine/threonine residues of nucleocytoplasmic proteins in metazoans. O-GlcNAc, like phosphorylation, is dynamic and responsive to numerous stimuli in diverse regulatory pathways. O-GlcNAc may also be found adjacent to or at the same sites as phosphorylation, demonstrating the potential for a reciprocal function on some of these proteins. Like most posttranslational modifications, O-GlcNAc is substoichiometric and may be found at multiple sites with other posttranslational modifications present. Additionally, there is no consensus sequence defining the addition of O-GlcNAc to the peptide backbone, further complicating identification and site mapping. This chapter describes several strategies to confirm that proteins are O-GlcNAc modified and provide subsequent determination of O-GlcNAc attachment sites. We have listed the strengths and limitations of each protocol to allow readers to decide which suits their system and availability of resources. These protocols include galactosyltransferase labeling, immunoblotting, using mass spectrometry based on beta-elimination followed by Michael addition with dithiothreitol, and chemoenzymatic labeling, enrichment, and detection.

The potential for molecular therapeutic targets in Ewing's sarcoma

Ewing's sarcoma is an uncompromising tumor of children and young adults. Before the introduction of chemotherapy for Ewing's sarcoma, nearly all patients succumbed to their disease, even with highly aggressive approaches to local control. The realization that most patients have micrometastatic disease at presentation, and the identification of active chemotherapeutic agents for this tumor, have resulted in significant improvements in patient survival. Modern therapy for Ewing's sarcoma combines high-dose chemotherapy for systemic control of disease, with advanced surgical and/or radiation therapeutic approaches for local control. Current therapy remains imperfect. Despite optimal management, the cure rate for localized disease is only approximately 70%, whereas the cure rate for metastatic disease at presentation is less than 30%. Patients who experience long-term disease-free survival are at risk for significant side effects of therapy, including infertility, limb dysfunction, and an increased risk for second malignancies. More effective and less toxic therapies are needed. This report presents an overview of dysregulated molecular pathways in Ewing's sarcoma and highlights the possibility that they may serve as therapeutic targets for the disease. Although a great deal of additional investigation is required before most of these approaches can be assessed in the clinic, we think that these potential new targets offer a great deal of hope for patients with Ewing's sarcoma.

Ewing tumor biology: perspectives for innovative treatment approaches

The Ewing's sarcoma family of tumors (EFT) is a group of malignancies affecting bone and soft tissue in adolescents. It is characterized by a unique gene rearrangement between the EWS gene and an ets transcription factor gene. EFT can be cured with conventional multi modal treatment, however, about 40% of patients still succumb to the disease. Relapses can be observed more than 5 years after the end of primary treatment suggesting persistence of minimal residual disease (MRD). Due to the still enigmatic nature of EFT histogenesis the phenotype of EFT stem cells and of dormant tumor cells remains unknown. The most frequent fusion product associated with EFT, EWS-FLI1, is the founding member of a whole class of similarly structured chimeric proteins associated with a variety of human sarcomas and also specific leukemias. The corresponding gene rearrangement constitutes a rate limiting step in oncogenesis as implied by the high association of EFT with EWS-ETS fusions, strong selective pressure for maintenance of a correct reading frame in the tumors, and by experimental data confirming the transforming and tumorigenic potential of EWS-FLI1. Understanding the biology of EWS-ETS gene fusions and its interplay with essential cellular pathways regulating cell growth, apoptosis, differentiation, genomic integrity, and treatment resistance may unravel specifically vulnerable sites for therapeutic targeting. This review summarizes the current knowledge about the EWS-FLI1 pathway in EFT and provides some ideas as to how this knowledge may be translated into innovative treatment approaches.

Proteomic approaches to analyze the dynamic relationships between nucleocytoplasmic protein glycosylation and phosphorylation

O-linked beta-N-acetylglucosamine (O-GlcNAc) is both an abundant and dynamic posttranslational modification similar to phosphorylation that occurs on serine and threonine residues of cytosolic and nuclear proteins in all metazoans and cell types examined, including cardiovascular tissue. Since the discovery of O-GlcNAc more than 20 years ago, the elucidation of O-GlcNAc as a posttranslational modification has been slow, albeit similar to the rate of acceptance of phosphorylation, because of the lack of tools available for its study. Identifying O-GlcNAc posttranslational modifications on proteins is a major challenge to proteomics. The recent development of mild beta-elimination followed by Michael addition with dithiothreitol has significantly improved the site mapping of both O-GlcNAc and O-phosphate in functional proteomics. beta-Elimination followed by Michael addition with dithiothreitol facilitates the study of the labile O-GlcNAc modification in the etiology of disease states. We discuss how recent technological innovations will expand our present understanding of O-GlcNAc and what the implications are for diabetes and cardiovascular complications.