Detection of O-linked N-acetylglucosamine (O-GlcNAc) on cytoplasmic and nuclear proteins

Recent development of analytical methods for disease-specific protein O-GlcNAcylation

The enzymatic modification of protein serine or threonine residues by N-acetylglucosamine, namely O-GlcNAcylation, is a ubiquitous post-translational modification that frequently occurs in the nucleus and cytoplasm. O-GlcNAcylation is dynamically regulated by two enzymes, O-GlcNAc transferase and O-GlcNAcase, and regulates nearly all cellular processes in epigenetics, transcription, translation, cell division, metabolism, signal transduction and stress. Aberrant O-GlcNAcylation has been shown in a variety of diseases, including diabetes, neurodegenerative diseases and cancers. Deciphering O-GlcNAcylation remains a challenge due to its low abundance, low stoichiometry and extreme lability in most tandem mass spectrometry. Separation or enrichment of O-GlcNAc proteins or peptides from complex mixtures has been of great interest because quantitative analysis of protein O-GlcNAcylation can elucidate their functions and regulatory mechanisms in disease. However, valid and specific analytical methods are still lacking, and efforts are needed to further advance this direction. Here, we provide an overview of recent advances in various analytical methods, focusing on chemical oxidation, affinity of antibodies and lectins, hydrophilic interaction, and enzymatic addition of monosaccharides in conjugation with these methods. O-GlcNAcylation quantification has been described in detail using mass-spectrometric or non-mass-spectrometric techniques. We briefly summarized dysregulated changes in O-GlcNAcylation in disease.

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.

Advances in chemical probing of protein O-GlcNAc glycosylation: structural role and molecular mechanisms

The addition of O-linked-β-D-N-acetylglucosamine (O-GlcNAc) onto serine and threonine residues of nuclear and cytoplasmic proteins is an abundant, unique post-translational modification governing important biological processes. O-GlcNAc dysregulation underlies several metabolic disorders leading to human diseases, including cancer, neurodegeneration and diabetes. This review provides an extensive summary of the recent progress in probing O-GlcNAcylation using mainly chemical methods, with a special focus on discussing mechanistic insights and the structural role of O-GlcNAc at the molecular level. We highlight key aspects of the O-GlcNAc enzymes, including development of OGT and OGA small-molecule inhibitors, and describe a variety of chemoenzymatic and chemical biology approaches for the study of O-GlcNAcylation. Special emphasis is placed on the power of chemistry in the form of synthetic glycopeptide and glycoprotein tools for investigating the site-specific functional consequences of the modification. Finally, we discuss in detail the conformational effects of O-GlcNAc glycosylation on protein structure and stability, relevant O-GlcNAc-mediated protein interactions and its molecular recognition features by biological receptors. Future research in this field will provide novel, more effective chemical strategies and probes for the molecular interrogation of O-GlcNAcylation, elucidating new mechanisms and functional roles of O-GlcNAc with potential therapeutic applications in human health.

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.

Methods for the Detection, Study, and Dynamic Profiling of O-GlcNAc Glycosylation

The addition of O-linked β-N-acetylglucosamine (O-GlcNAc) to serine/threonine residues of proteins is a ubiquitous posttranslational modification found in all multicellular organisms. Like phosphorylation, O-GlcNAc glycosylation (O-GlcNAcylation) is inducible and regulates a myriad of physiological and pathological processes. However, understanding the diverse functions of O-GlcNAcylation is often challenging due to the difficulty of detecting and quantifying the modification. Thus, robust methods to study O-GlcNAcylation are essential to elucidate its key roles in the regulation of individual proteins, complex cellular processes, and disease. In this chapter, we describe a set of chemoenzymatic labeling methods to (1) detect O-GlcNAcylation on proteins of interest, (2) monitor changes in both the total levels of O-GlcNAcylation and its stoichiometry on proteins of interest, and (3) enable mapping of O-GlcNAc to specific serine/threonine residues within proteins to facilitate functional studies. First, we outline a procedure for the expression and purification of a multiuse mutant galactosyltransferase enzyme (Y289L GalT). We then describe the use of Y289L GalT to modify O-GlcNAc residues with a functional handle, N-azidoacetylgalactosamine (GalNAz). Finally, we discuss several applications of the copper-catalyzed azide-alkyne cycloaddition "click" reaction to attach various alkyne-containing chemical probes to GalNAz and demonstrate how this functionalization of O-GlcNAc-modified proteins can be used to realize (1)-(3) above. Overall, these methods, which utilize commercially available reagents and standard protein analytical tools, will serve to advance our understanding of the diverse and important functions of O-GlcNAcylation.

O-Linked N-acetylglucosamine transferase 1 regulates global histone H4 acetylation via stabilization of the nonspecific lethal protein NSL3

The human males absent on the first (MOF)-containing histone acetyltransferase nonspecific lethal (NSL) complex comprises nine subunits including the O-linked N-acetylglucosamine (O-GlcNAc) transferase, isoform 1 (OGT1). However, whether the O-GlcNAc transferase activity of OGT1 controls histone acetyltransferase activity of the NSL complex and whether OGT1 physically interacts with the other NSL complex subunits remain unclear. Here, we demonstrate that OGT1 regulates the activity of the NSL complex by mainly acetylating histone H4 Lys-16, Lys-5, and Lys-8 via O-GlcNAcylation and stabilization of the NSL complex subunit NSL3. Knocking down or overexpressing OGT1 in human cells remarkably affected the global acetylation of histone H4 residues Lys-16, Lys-5, and Lys-8. Because OGT1 is a subunit of the NSL complex, we also investigated the function of OGT1 in this complex. Co-transfection/co-immunoprecipitation experiments combined with in vitro O-GlcNAc transferase assays confirmed that OGT1 specifically binds to and O-GlcNAcylates NSL3. In addition, wheat germ agglutinin affinity purification verified the occurrence of O-GlcNAc modification on NSL3 in cells. Moreover, O-GlcNAcylation of NSL3 by wild-type OGT1 (OGT1-WT) stabilized NSL3. This stabilization was lost after co-transfection of NSL3 with an OGT1 mutant, OGT1^C964A, that lacks O-GlcNAc transferase activity. Furthermore, stabilization of NSL3 by OGT1-WT significantly increased the global acetylation levels of H4 Lys-5, Lys-8, and Lys-16 in cells. These results suggest that OGT1 regulates the activity of the NSL complex by stabilizing NSL3.

Chemical Glycoproteomics

Chemical tools have accelerated progress in glycoscience, reducing experimental barriers to studying protein glycosylation, the most widespread and complex form of posttranslational modification. For example, chemical glycoproteomics technologies have enabled the identification of specific glycosylation sites and glycan structures that modulate protein function in a number of biological processes. This field is now entering a stage of logarithmic growth, during which chemical innovations combined with mass spectrometry advances could make it possible to fully characterize the human glycoproteome. In this review, we describe the important role that chemical glycoproteomics methods are playing in such efforts. We summarize developments in four key areas: enrichment of glycoproteins and glycopeptides from complex mixtures, emphasizing methods that exploit unique chemical properties of glycans or introduce unnatural functional groups through metabolic labeling and chemoenzymatic tagging; identification of sites of protein glycosylation; targeted glycoproteomics; and functional glycoproteomics, with a focus on probing interactions between glycoproteins and glycan-binding proteins. Our goal with this survey is to provide a foundation on which continued technological advancements can be made to promote further explorations of protein glycosylation.

Use of a mutant OGA for detecting O-GlcNAc modified proteins

In the previous issue of Biochemical Journal Mariappa et al. [(2015) Biochem J. 470,: 255-262] demonstrate a new method for visualizing O-linked N-acetylglucosamine (O-GlcNAc) modified proteins by making use of a catalytically dead version of the enzyme that normally removes this modification. They show their approach has broader specificity than current antibody-based techniques and higher specificity than lectin and chemical biology-based labelling approaches. This commentary discusses methods for O-GlcNAc detection and the significance of this work for characterizing this common, but currently poorly understood regulatory modification.

A mutant O-GlcNAcase as a probe to reveal global dynamics of protein O-GlcNAcylation during Drosophila embryonic development

O-GlcNAcylation is a reversible type of serine/threonine glycosylation on nucleocytoplasmic proteins in metazoa. Various genetic approaches in several animal models have revealed that O-GlcNAcylation is essential for embryogenesis. However, the dynamic changes in global O-GlcNAcylation and the underlying mechanistic biology linking them to embryonic development is not understood. One of the limiting factors towards characterizing changes in O-GlcNAcylation has been the limited specificity of currently available tools to detect this modification. In the present study, harnessing the unusual properties of an O-GlcNAcase (OGA) mutant that binds O-GlcNAc (O-N-acetylglucosamine) sites with nanomolar affinity, we uncover changes in protein O-GlcNAcylation as a function of Drosophila development.

Detecting O-GlcNAc using in vitro sulfation

O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation, the covalent attachment of N-acetylglucosamine to serine and threonine residues of proteins, is a post-translational modification that shares many features with protein phosphorylation. O-GlcNAc is essential for cell survival and plays important role in many biological processes (e.g. transcription, translation, cell division) and human diseases (e.g. diabetes, Alzheimer's disease, cancer). However, detection of O-GlcNAc is challenging. Here, a method for O-GlcNAc detection using in vitro sulfation with two N-acetylglucosamine (GlcNAc)-specific sulfotransferases, carbohydrate sulfotransferase 2 and carbohydrate sulfotransferase 4, and the radioisotope (35)S is described. Sulfation on free GlcNAc is first demonstrated, and then on O-GlcNAc residues of peptides as well as nuclear and cytoplasmic proteins. It is also demonstrated that the sulfation on O-GlcNAc is sensitive to OGT and O-β-N-acetylglucosaminidase treatment. The labeled samples are separated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and visualized by autoradiography. Overall, the method is sensitive, specific and convenient.

Glycosylation of the nuclear pore

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

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

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

Multiple TSC22D4 iso-/phospho-glycoforms display idiosyncratic subcellular localizations and interacting protein partners

Proteins of the TSC22 domain (TSC22D) family, including TSC22D1 and TSC22D4, play pivotal roles in cell proliferation, differentiation and apoptosis, interacting with other factors in a still largely unknown manner. This study explores this issue by biochemically characterizing various TSC22D4 forms (both iso- and glyco-phospho-, namely the splice variants 42 and 55 kDa and the post-translationally modified 67 and 72 kDa forms) and their subcellular localization and protein partners during cerebellar granule neuron (CGN) differentiation. The TSC22D4-42 form is mostly cytosolic, and is the only TSC22D4 form that associates with TSC22D1.2 in undifferentiated but not differentiated CGNs. In contrast, TSC22D4-55 is prominently associated with the nuclear matrix in differentiated but not undifferentiated CGNs. As for TSC22D4-67, it is localized in the cytosol and nuclei of undifferentiated CGNs and enters mitochondria of differentiated CGNs, associating with apoptosis-inducing factor. TSC22D4-72 is modified by O-linked beta-N-acetylglucosamine (O-GlcNAcylated) and phosphorylated and is always associated with chromatin irrespective of CGN differentiation. The various subcellular localization patterns and interacting protein partners of TSC22D4 forms during CGN differentiation suggest the existence of form-specific function(s) and provide a novel framework to further investigate the biological functions of TSC22D proteins.

Detection of O-GlcNAc modifications on cardiac myofilament proteins

In this chapter it is described a general method that has been used successfully by more than one laboratory interested in detecting O-GlcNAc in myofilament proteins. Alternative reagents for chemo-enzymatic or metabolic labeling will be indicated, as well as references for more details in alternative methods. The outline is divided into (1) Enrichment of O-GlcNAc Stoichiometry, (2) Cardiac Myofilament Protein Isolation, (3) SDS-PAGE, (4) "Reduction and Alkylation," (5) In-Gel Protein Digestion, (6) Chemo-enzymatic Labeling of O-GlcNAc Moieties (Click Chemistry), (7) Biotin Alkyne Tagging, (8) Strong Cation Exchange (SCX) and Streptavidin, and (9) β-Elimination and Michael Addition (BEMAD) for O-GlcNAc Site-Mapping.

Enhanced cardiac protein glycosylation (O-GlcNAc) of selected mitochondrial proteins in rats artificially selected for low running capacity

O-linked β-N-acetyl glucosamine (O-GlcNAc) is a posttranslational modification consisting of a single N-acetylglucosamine moiety attached by an O-β-glycosidic linkage to serine and threonine residues of both nuclear and cytosolic proteins. Analogous to phosphorylation, the modification is reversible and dynamic, changing in response to stress, nutrients, hormones, and exercise. Aims of this study were to examine differences in O-GlcNAc protein modification in the cardiac tissue of rats artificially selected for low (LCR) or high (HCR) running capacity. Hyperinsulinemic-euglycemic clamps in conscious animals assessed insulin sensitivity while 2-[(14)C] deoxyglucose tracked both whole body and tissue-specific glucose disposal. Immunoblots of cardiac muscle examined global O-GlcNAc modification, enzymes that control its regulation (OGT, OGA), and specific proteins involved in mitochondrial oxidative phosphorylation. LCR rats were insulin resistant disposing of 65% less glucose than HCR. Global tissue O-GlcNAc, OGT, OGA, and citrate synthase were similar between groups. Analysis of cardiac proteins revealed enhanced O-GlcNAcylation of mitochondrial Complex I, Complex IV, VDAC, and SERCA in LCR compared with HCR. These results are the first to establish an increase in specific protein O-GlcNAcylation in LCR animals that may contribute to progressive mitochondrial dysfunction and the pathogenesis of insulin resistance observed in the LCR phenotype.

O-GlcNAcylation: a novel post-translational mechanism to alter vascular cellular signaling in health and disease: focus on hypertension

O-Linked attachment of beta-N-acetyl-glucosamine (O-GlcNAc) on serine and threonine residues of nuclear and cytoplasmic proteins is a highly dynamic posttranslational modification that plays a key role in signal transduction pathways. Preliminary data show that O-GlcNAcylation may represent a key regulatory mechanism in the vasculature, modulating contractile and relaxant responses. Proteins with an important role in vascular function, such as endothelial nitric oxide synthase, sarcoplasmic reticulum Ca(2+)-ATPase, protein kinase C, mitogen-activated protein kinases, and proteins involved in cytoskeleton regulation and microtubule assembly are targets for O-GlcNAcylation, indicating that this posttranslational modification may play an important role in vascular reactivity. Here, we will focus on a few specific pathways that contribute to vascular function and cardiovascular disease-associated vascular dysfunction, and the implications of their modification by O-GlcNAc. New chemical tools have been developed to detect and study O-GlcNAcylation, including inhibitors of O-GlcNAc enzymes, chemoenzymatic tagging methods, and quantitative proteomics strategies; these will also be briefly addressed. An exciting challenge in the future will be to better understand the cellular dynamics of this posttranslational modification, as well as the signaling pathways and mechanisms by which O-GlcNAc is regulated on specific proteins in the vasculature in health and disease.

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.

Cross talk between O-GlcNAcylation and phosphorylation: roles in signaling, transcription, and chronic disease

O-GlcNAcylation is the addition of β-D-N-acetylglucosamine to serine or threonine residues of nuclear and cytoplasmic proteins. O-linked N-acetylglucosamine (O-GlcNAc) was not discovered until the early 1980s and still remains difficult to detect and quantify. Nonetheless, O-GlcNAc is highly abundant and cycles on proteins with a timescale similar to protein phosphorylation. O-GlcNAc occurs in organisms ranging from some bacteria to protozoans and metazoans, including plants and nematodes up the evolutionary tree to man. O-GlcNAcylation is mostly on nuclear proteins, but it occurs in all intracellular compartments, including mitochondria. Recent glycomic analyses have shown that O-GlcNAcylation has surprisingly extensive cross talk with phosphorylation, where it serves as a nutrient/stress sensor to modulate signaling, transcription, and cytoskeletal functions. Abnormal amounts of O-GlcNAcylation underlie the etiology of insulin resistance and glucose toxicity in diabetes, and this type of modification plays a direct role in neurodegenerative disease. Many oncogenic proteins and tumor suppressor proteins are also regulated by O-GlcNAcylation. Current data justify extensive efforts toward a better understanding of this invisible, yet abundant, modification. As tools for the study of O-GlcNAc become more facile and available, exponential growth in this area of research will eventually take place.

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.

Primer preactivation of peptidoglycan polymerases

Peptidoglycan glycosyltransferases are highly conserved bacterial enzymes that catalyze glycan strand polymerization to build the cell wall. Because the cell wall is essential for bacterial cell survival, these glycosyltransferases are potential antibiotic targets, but a detailed understanding of their mechanisms is lacking. Here we show that a synthetic peptidoglycan fragment that mimics the elongating polymer chain activates peptidoglycan glycosyltransferases by bypassing the rate-limiting initiation step.

Chemical arsenal for the study of O-GlcNAc

The concepts of both protein glycosylation and cellular signaling have been influenced by O-linked-β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) on the hydroxyl group of serine or threonine residues. Unlike conventional protein glycosylation, O-GlcNAcylation is localized in the nucleocytoplasm and its cycling is a dynamic process that operates in a highly regulated manner in response to various cellular stimuli. These characteristics render O-GlcNAcylation similar to phosphorylation, which has long been considered a major regulatory mechanism in cellular processes. Various efficient chemical approaches and novel mass spectrometric (MS) techniques have uncovered numerous O-GlcNAcylated proteins that are involved in the regulation of many important cellular events. These discoveries imply that O-GlcNAcylation is another major regulator of cellular signaling. However, in contrast to phosphorylation, which is regulated by hundreds of kinases and phosphatases, dynamic O-GlcNAc cycling is catalyzed by only two enzymes: uridine diphospho-N-acetyl-glucosamine:polypeptide β-N-acetylglucosaminyl transferase (OGT) and β-D-N-acetylglucosaminidase (OGA). Many useful chemical tools have recently been used to greatly expand our understanding of the extensive crosstalk between O-GlcNAcylation and phosphorylation and hence of cellular signaling. This review article describes the various useful chemical tools that have been developed and discusses the considerable advances made in the O-GlcNAc field.

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.

The E2F-1 associated retinoblastoma-susceptibility gene product is modified by O-GlcNAc

The retinoblastoma-susceptibility gene product (pRB) is a classical tumor suppressor. pRB regulates a number of cellular processes including proliferation, differentiation, and apoptosis. One of the essential mechanisms by which pRB, and the related p107 and p130 family members, act is through its interactions with the E2F class of transcription factors. E2F-1 transcription is necessary for entry into S-phase during the cell-cycle. pRB binds E2F-1 and represses transcription via recruitment of a histone deacetylase complex and by preventing co-activator complexes from binding E2F-1. Current dogma suggests that phosphorylation of pRB during mid- to late-G1 leads to release of E2F-1 and E2F-1 dependent transcriptional activation of essential S-phase genes. Here we show that pRB, and the related p107 protein, are modified by O-linked β-N-acetylglucosamine (O-GlcNAc) in an in vitro transcription/translation system. Furthermore, we show in vivo that pRB is more heavily glycosylated in G1 of the cell-cycle when pRB is known to be in an active, hypophosphorylated state. Finally, we demonstrate that E2F-1 associated pRB is modified by O-GlcNAc. These studies suggest that regulation of pRB function(s) may be controlled by dynamic O-GlcNAc modification, as well as phosphorylation.

O-GlcNAcylation of the Plum pox virus capsid protein catalyzed by SECRET AGENT: characterization of O-GlcNAc sites by electron transfer dissociation mass spectrometry

The capsid protein of Plum pox virus (PPV-CP) is modified with O-linked β-N-acetylglucosamine (O-GlcNAc). In Arabidopsis thaliana this modification is made by an O-GlcNAc transferase named SECRET AGENT (SEC). Modification of PPV-CP by SEC is hypothesized to have a direct role in the infection process, because virus titer and rate of spread are reduced in SEC mutants. Previous studies used deletion mapping and site-directed mutagenesis to identify four O-GlcNAc sites on the capsid protein that are modified by Escherichia coli-expressed SEC. The infection process was not affected when two of these sites were mutated suggesting that O-GlcNAcylation of these sites does not have a significant role in the infection process or that a subset of the modifications is sufficient. Since it is possible that the mutational mapping approach missed or incorrectly identified O-GlcNAc sites, the modifications produced by E. coli-expressed SEC were characterized using mass spectrometry. O-GlcNAcylated peptides were enzymatically tagged with galactose, the products were enriched on immobilized Ricinus communis agglutinin I and sequenced by electron transfer dissociation (ETD) mass spectrometry. Five O-GlcNAc sites on PPV-CP were identified. Two of these sites were not identified in by the previous mutational mapping. In addition, one site previously predicted by mutation mapping was not detected, but modification of this site was not supported when the mutation mapping was repeated. This study suggests that mapping modification sites by ETD mass spectrometry is more comprehensive and accurate than mutational mapping.

Enrichment of O-GlcNAc modified proteins by the periodate oxidation-hydrazide resin capture approach

A chemical derivatization approach has been developed for the enrichment of O-GlcNAc modified proteins. The procedure is based on the isolation technique used for N-glycoproteins with appropriate modifications because of the differences in the two types of glycosylation: a prolonged periodate oxidation is followed by hydrazide resin capture, on-resin proteolytic digestion, and release of the modified peptides by hydroxylamine. This enrichment strategy offers a fringe benefit in mass spectrometry analysis. Upon collisional activation, the presence of the open carbohydrate ring leads to characteristic fragmentation facilitating both glycopeptide identification and site assignment. The enrichment protocol was applied to the Drosophila proteasome complex previously described as O-GlcNAc modified. The O-GlcNAc modification was located on proteasome interacting proteins, deubiquitinating enzyme Faf (CG1945) and a ubiquitin-like domain containing protein (CG7546). Three other proteins were also found GlcNAc modified, a HSP70 homologue (CG2918), scribbled (CG5462) and the 205 kDa microtubule-associated protein (CG1483). Interestingly, in the HSP70 homologue the GlcNAc modification is attached to an asparagine residue of a N-glycosylation motif.

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.

Phenotypical transitions and Ca2+activation properties in human muscle fibers: effects of a 60-day bed rest and countermeasures

Muscle biopsies were taken from soleus and vastus lateralis before and after a 60-day bed rest (BR) to examine expression changes in the regulatory proteins of the thin filament and in contractile function. Twenty-four women separated in three groups were submitted to BR or a combined protocol of resistance and aerobic exercises during BR or received a supplementation of amino acids during BR. Ca2+-tension relationships were established in single skinned fibers identified by their myosin heavy chain and troponin C isoform expressions. Expression patterns of regulatory proteins were analyzed on muscle pieces. For both muscles, BR produced similar decreases in slow and fast fiber diameters but larger decreases in P0maximal forces in slow than in fast fibers. Specific forces were decreased in slow soleus and vastus fibers, which displayed a reduction in Ca2+affinity. These changes were accompanied by slow-to-fast transitions in regulatory proteins, with troponins C and T appearing as sensitive markers of unloading. Exercises prevented the changes in fiber diameters and forces and counteracted most of the slow-to-fast transitions. The nutrition program had a morphological beneficial effect on slow fibers. However, these fibers still presented decreases in specific P0after BR. Phenotypical transitions due to BR were not prevented by amino acids. Finally, in vastus lateralis muscle, BR induced a decrease in O-glycosylation level that was prevented by exercise and attenuated by nutrition. In conclusion, this study has addressed for the first time in women the respective efficiencies of two countermeasures associated with BR on muscle properties and regulatory protein expression.

O-glycosylation of protein subpopulations in alcohol-extracted rice proteins

Mucin-type O-glycosylation has been well characterized in mammalian systems but not in plants. In this study, the purified alcohol-soluble, non-reduced protein (prolamin) fraction from rice seed was investigated for the occurrence of O-linked oligosaccharides. As storage prolamins are unlikely to be O-glycosylated, any O-glycosylation found was likely to belong to co-extracted proteins, whether because of association with the protein body or solubility. SDS-PAGE and MS analyses revealed 14 and 16kDa protein families in fractions that bound to the lectins peanut agglutinin (PNA), Vicia villosa lectin (VVL) and Jacalin, indicative of the presence of O-linked saccharides. Enzymatic cleavage, fluorescent labeling and high-performance liquid chromatography (HPLC) analysis demonstrated a peak consistent with Gal-beta-(1-->3)-GalNAc, with similar MS/MS fragmentation. Additionally, upon chemical analysis, a GlcNAc-containing O-linked carbohydrate moiety was discovered. Protein blotting with anti-O-GlcNAc antibody (clone CTD110.6) was positive in a subpopulation of the 14kDa alcohol-soluble protein fraction, but a hot capping experiment was negative. Therefore, the GlcNAc residue in this case is unlikely to be terminal. Additionally, a positive reaction with CTD110.6mAb cannot be taken as absolute proof of O-GlcNAc modification and further confirmatory experiments should be employed. We hypothesize that O-glycosylation may contribute to protein functionality or regulation. Further investigation is required to identify the specific proteins with these modifications. This 'reverse' approach could lead to the identification of proteins involved in mRNA targeting, signaling, translation, anchoring or maintenance of translational quiescence and may be applied to germinating rice seed extracts for further elucidation of protein function and regulation.

Increased enzymatic O-GlcNAcylation of mitochondrial proteins impairs mitochondrial function in cardiac myocytes exposed to high glucose

Increased nuclear protein O-linked beta-N-acetylglucosamine glycosylation (O-GlcNAcylation) mediated by high glucose treatment or the hyperglycemia of diabetes mellitus contributes to cardiac myocyte dysfunction. However, whether mitochondrial proteins in cardiac myocytes are also submitted to O-GlcNAcylation or excessive O-GlcNAcylation alters mitochondrial function is unknown. In this study, we determined if mitochondrial proteins are O-GlcNAcylated and explored if increased O-GlcNAcylation is linked to high glucose-induced mitochondrial dysfunction in neonatal rat cardiomyocytes. By immunoprecipitation, we found that several mitochondrial proteins, which are members of complexes of the respiratory chain, like subunit NDUFA9 of complex I, subunits core 1 and core 2 of complex III, and the mitochondrial DNA-encoded subunit I of complex IV (COX I) are O-GlcNAcylated. By mass spectrometry, we identified that serine 156 on NDUFA9 is O-GlcNAcylated. High glucose treatment (30 mm glucose) increases mitochondrial protein O-GlcNAcylation, including those of COX I and NDUFA9 which are reduced by expression of O-GlcNAcase (GCA). Increased mitochondrial O-GlcNAcylation is associated with impaired activity of complex I, III, and IV in addition to lower mitochondrial calcium and cellular ATP content. When the excessive O-GlcNAc modification is reduced by GCA expression, mitochondrial function improves; the activity of complex I, III, and IV increases to normal and mitochondrial calcium and cellular ATP content are returned to control levels. From these results we conclude that specific mitochondrial proteins of cardiac myocytes are O-GlcNAcylated and that exposure to high glucose increases mitochondrial protein O-GlcNAcylation, which in turn contributes to impaired mitochondrial function.

Evidence Denies the Presence of O-GlcNAcylation on Mouse Glucocorticoid Receptor and Its Potential Involvement in Receptor Transcriptional Activity

O-linked N-acetylglucosamine (GlcNAc) glycosylation (O-GlcNAcylation) is commonly found on many cytoplasmic and nuclear proteins. It can play a role in protein trafficking, signal transduction, and in some nuclear proteins it is involved in the control of gene expression. The steroid receptor family consists of proteins that have similar domain architecture including individual DNA and hormone-binding domains that have closely related three-dimensional structures. The discovery of O-linked GlcNAc on both androgen and estrogen receptors and the realization that the GlcNAc plays a role in the transcriptional activity of these receptors raise the possibility that this glycosylation is a common mechanism involved in transcriptional modulation in all members of the steroid receptor family. To test this hypothesis, we affinity purified the mouse glucocorticoid receptor from cell lines engineered to overexpress the receptor and used GlcNAc-specific lectin chromatography, lectin-blotting analysis, and galactosylation assay to assess the presence of GlcNAc modification. All three techniques were found to be highly sensitive when used with proteins known to harbor GlcNAc yet they failed to show the presence of GlcNAcylation on the mouse GR. We also determined the effect of mutation at seven major potential glycosylation sites of the receptor on its transcriptional activity. We conclude that either the mouse GR is not modified by GlcNAc or that the amount of the modification is so low that it cannot be detected. Therefore, the O-GlcNAcylation appears not to be a common mechanism used to modify the activity of all steroid receptors.

Glycomic Approaches to Study GlcNAcylation: Protein Identification, Site-mapping, and Site-specific O-GlcNAc Quantitation

Background

O-Linked β-N-acetylglucosamine (O-GlcNAc) is an enzyme-catalyzed posttranslational modification of serine or threonine side chains of nuclear and cytoplasmic proteins. O-GlcNAc is present in all metazoans and in viruses that infect eukaryotic cells. GlcNAcylation is dynamic and has a high cycling rate on many proteins in response to cellular metabolism and various environmental stimuli. The rapid cycling of O-GlcNAc modulates many biological processes, including transcriptional regulation, stress responses, cell cycle regulation, and protein synthesis and turnover.

Rationale

Despite the importance of O-GlcNAc, progress during the past two decades in this field has been slow. One of the major obstacles is the lack of simple and sensitive tools for efficient O-GlcNAc detection and localization. Recently developed O-GlcNAc derivatization and enrichment approaches, together with new techniques in mass spectrometric instrumentation and methods, have provided breakthroughs in O-GlcNAc site localization and site-specific quantitation. In this review, we will discuss how the current techniques are expanding our knowledge about O-GlcNAc proteomics/glycomics and functions.

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

First, a protocol for increasing the stoichiometry of O-GlcNAc on proteins is given. This is followed by simple techniques for the detection/screening of O-GlcNAc-modified proteins either by immunoblotting or lectin affinity chromatography. Separate protocols verify that the glycan is O-linked GlcNAc. These methods are followed by protocols for more comprehensive analysis of O-GlcNAc modified proteins, including labeling of O-GlcNAc residues with [3H]Gal, and subsequent product analysis. The final two protocols assay for O-GlcNAc transferase and O-GlcNAcase activity, respectively.

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

The modification of Ser and Thr residues with O-linked b-N-acetyl glucosamine (O-GlcNAc) is a common and essential modification of nuclear and cytoplasmic proteins, and it is thought that O-GlcNAc performs a regulatory role in the cell. This unit concentrates on the techniques for the detection and analysis of proteins modified by O-GlcNAc as well as methods for the analysis of enzymes responsible for the addition and removal of this group.

Chemical approaches to understanding O-GlcNAc glycosylation in the brain

O-GlcNAc glycosylation is a unique, dynamic form of glycosylation found on intracellular proteins of all multicellular organisms. Studies suggest that O-GlcNAc represents a key regulatory modification in the brain, contributing to transcriptional regulation, neuronal communication and neurodegenerative disease. Recently, several new chemical tools have been developed to detect and study the modification, including chemoenzymatic tagging methods, quantitative proteomics strategies and small-molecule inhibitors of O-GlcNAc enzymes. Here we highlight some of the emerging roles for O-GlcNAc in the nervous system and describe how chemical tools have significantly advanced our understanding of the scope, functional significance and cellular dynamics of this modification.

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.

Characterization of the Adenovirus Fiber Protein

Entry of the adenovirus (Ad) capsids during the early stages of infection is a multistep process that includes initial attachment of the virus capsid to the cell surface followed by internalization of the virus into early endosomes. The Ad fiber protein, a complex of three apparently identical subunits, mediates the initial attachment step. In this chapter, methods for the purification and characterization of the Ad fiber protein are presented. Chromatographic methods for the isolation of the protein from infected cells can yield substantial quantities of protein for biochemical analysis. Protocols for characterization of the protein by Western blot and by indirect immunofluorescence of infected cells are also presented. The specificity of different monoclonal and polyclonal antibodies that recognize Ad fiber is also discussed. Ad fiber from a number of serotypes also contains a posttranslational modification, O-linked N-acetyl-glucosamine; methods for detection and characterization of this modification are also provided. With these tools and protocols, one can address important questions about this protein, which helps direct the tissue tropism of Ad.

SECRET AGENT, an Arabidopsis thaliana O-GlcNAc transferase, modifies the Plum pox virus capsid protein

The capsid protein of Plum pox virus (PPV-CP) is modified with O-linked GlcNAc (O-GlcNAc). While Arabidopsis has two O-GlcNAc transferases, SECRET AGENT (SEC) and SPINDLY (SPY), previous work suggests that SEC modifies PPV-CP and that the modification plays a role in the infection process. Here, we show that when co-expressed in Escherichia coli SEC modifies PPV-CP. Deletion mapping and site-directed mutagenesis identified three threonine and a serine located near the N-terminus of PPV-CP that are modified by SEC. Two of these threonines have recently been shown to be modified in virus from plants suggesting that SEC has the same specificity in plants and E. coli.

Targeted glycoproteomics: serial lectin affinity chromatography in the selection of O-glycosylation sites on proteins from the human blood proteome

Although lectin selection is gaining increasing acceptance as a tool for targeting glycosylation in glycoproteomics, most of the work has been directed at N-glycosylation. The work reported here focuses on the use of lectins in the study of O-glycosylation. The problem with using lectins for studying O-glycosylation is that they are not sufficiently specific. This paper reports that through the use of serial lectin affinity chromatography (SLAC) it is possible to select predominantly O-glycosylated peptides from tryptic digests of human serum. Jacalin is relatively specific for O-glycosylation but has the problem that it also selects high mannose N-type glycans. This problem was addressed by using a concanavalin A affinity column to first remove high mannose, hybrid-type and biantennary complex-type N-type glycans before application of the Jacalin columns. When used in a serial format, concanavalin A and Jacalin together provide essentially O-glycosylated peptides. The glycoprotein parents of glycopeptides were identified by deglycosylating the selected O-glycopeptides by oxidative elimination. These peptides were then separated by RPC and further analyzed using ESI-MS/MS and MALDI-MS/MS. Using this approach all the O-glycosylated sites in a model protein (fetuin) and over thirty glycoprotein parents from human serum were identified. It is concluded that a serial combination of Con A and Jacalin can be of utility in the study of O-glycosylation in glycoproteomics.

Akt1 is dynamically modified with O-GlcNAc following treatments with PUGNAc and insulin-like growth factor-1

The Ser/Thr kinase Akt1 is activated by growth factors subsequent to its phosphorylation on Thr308 and Ser473. In the present study, Akt1 was found to be constitutively modified with O-GlcNAc. Treatment of SH-SY5Y cells with O(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc), which inhibits the enzymatic removal of O-GlcNAc from proteins, increased cytosolic O-GlcNAc-Akt1 levels. Treatment of cells with insulin-like growth factor-1 (IGF-1) also increased O-GlcNAc-Akt1 levels and increased Akt1 phosphorylation. PUGNAc treatment did not attenuate IGF-1 induced Akt1 phosphorylation. These results indicate that Akt1 can be simultaneously modified with O-GlcNAc and phosphorylated. However, PUGNAc induced the nuclear accumulation of Akt1 suggesting that the O-GlcNAc-modification on Akt1 may play a role in Akt1 nuclear localization.

O-GlcNAc level variations are associated with the development of skeletal muscle atrophy

O-linked N-acetylglucosaminylation (O-GlcNAc) is a regulatory posttranslational modification of nucleocytoplasmic proteins, which consists of the attachment of N-acetylglucosamine to serine or threonine residues of a protein. This glycosylation is a ubiquitous posttranslational modification, which probably plays important roles in many aspects of protein function. Our laboratory has previously reported that, in skeletal muscle, proteins of the glycolytic pathway and energetic metabolism and contractile proteins were O-GlcNAc modified (Cieniewski-Bernard C, Bastide B, Lefebvre T, Lemoine J, Mounier Y, and Michalski JC. Mol Cell Proteomics 3: 577-585, 2004). O-GlcNAc has been recently demonstrated to play a role in modulating cellular function in response to nutrition and also in stress conditions. Therefore, we have investigated here the implication of the glycosylation/deglycosylation process in the development of atrophy in rat skeletal muscle after hindlimb unloading. The high O-GlcNAc level found in control soleus [compared with control extensor digitorum longus (EDL)] becomes lower in atrophied soleus. On the opposite side, the low rate of O-GlcNAc in control EDL reaches higher levels in EDL, not atrophied after hindlimb unloading. These variations in O-GlcNAc level are correlated with a variation of the O-GlcNAc process enzyme activities and could be associated with a differential expression of heat shock proteins. Our results suggest that O-GlcNAc variations could control the muscle protein homeostasis and be implicated in the regulation of muscular atrophy.

Identification of the major site of O-linked beta-N-acetylglucosamine modification in the C terminus of insulin receptor substrate-1

Signal transduction from the insulin receptor to downstream effectors is attenuated by phosphorylation at a number of Ser/Thr residues of insulin receptor substrate-1 (IRS-1) resulting in resistance to insulin action, the hallmark of type II diabetes. Ser/Thr residues can also be reversibly glycosylated by O-linked beta-N-acetylglucosamine (O-GlcNAc) monosaccharide, a dynamic posttranslational modification that offers an alternative means of protein regulation to phosphorylation. To identify sites of O-GlcNAc modification in IRS-1, recombinant rat IRS-1 isolated from HEK293 cells was analyzed by two complementary mass spectrometric methods. Using data-dependent neutral loss MS3 mass spectrometry, MS/MS data were scanned for peptides that exhibited a neutral loss corresponding to the mass of N-acetylglucosamine upon dissociation in an ion trap. This methodology provided sequence coverage of 84% of the protein, permitted identification of a novel site of phosphorylation at Thr-1045, and facilitated the detection of an O-GlcNAc-modified peptide of IRS-1 at residues 1027-1073. The level of O-GlcNAc modification of this peptide increased when cells were grown under conditions of high glucose with or without chronic insulin stimulation or in the presence of an inhibitor of the O-GlcNAcase enzyme. To map the exact site of O-GlcNAc modification, IRS-1 peptides were chemically derivatized with dithiothreitol following beta-elimination and Michael addition prior to LC-MS/MS. This approach revealed Ser-1036 as the site of O-GlcNAc modification. Site-directed mutagenesis and Western blotting with an anti-O-GlcNAc antibody suggested that Ser-1036 is the major site of O-GlcNAc modification of IRS-1. Identification of this site will facilitate exploring the biological significance of the O-GlcNAc modification.

Molecular forms and microheterogeneity of the oligosaccharide chains of pregnancy-associated CA125 antigen

BACKGROUND

The cancer antigen CA125 has a very complex molecular architecture in terms of both protein backbone and oligosaccharide chains. In this study, we examined the molecular forms and microheterogeneity of oligosaccharide chains of pregnancy-associated CA125, as a first step towards gaining an insight into its possible involvement as a ligand in carbohydrate-dependent interactions. The glycobiochemical properties of CA125 may be of diagnostic and biomedical importance as specific markers of physiological and pathological conditions of early pregnancy, as well as targets in different therapeutic procedures.

METHODS

Pregnancy-associated CA125 was characterized by gel filtration and ion-exchange chromatography, followed by lectin-affinity chromatography with a panel of plant lectins as ligands.

RESULTS

CA125 antigen isolated from first trimester placental extract was found to be heterogeneous in respect to molecular mass and the existence of different glyco-isoforms. Thus, elution profiles from the lectin-affinity columns demonstrated molecular subpopulations bound with low, intermediate and high affinity. Under the applied experimental conditions, CA125 bound most strongly to Triticum vulgaris agglutinin (WGA) and Ricinus communis agglutinin (RCA), but low affinity interactions occurred with the other lectins tested.

CONCLUSIONS

The assessment of the carbohydrate composition of N- and O-glycans of pregnancy-associated CA125 was in general agreement with available data on CA125 of cancer origin. The main difference was observed in reactivity to Canavalia ensiformis agglutinin (ConA) and Phaseolus vulgaris erythroagglutinin (PHA-E) binding.

Relationship between protein O-linked glycosylation and insulin-stimulated glucose transport in rat skeletal muscle following calorie restriction or exposure to O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate

AIMS AND BACKGROUND

Protein O-linked glycosylation is regulated in vivo by the concentration of hexosamine substrates. Calorie restriction (60% of ad libitum intake) for 20 days causes decreased UDP-N-acetylhexosamine levels and increased insulin-mediated glucose transport in rat skeletal muscle. Conversely, prolonged incubation (19 h) of muscle with O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenyl-carbamate (PUGNAc; an inhibitor of N-acetyl-beta-D-glucosaminidase) is characterized by increased O-linked glycosylation and insulin resistance. We aimed to determine the calorie restriction effect on O-linked glycosylation and characterize the temporal relationship between PUGNAc-induced O-linked glycosylation and insulin resistance.

HYPOTHESIS

A calorie restriction protocol characterized by decreased muscle hexosamine levels will result in a global reduction in O-linked glycosylated proteins in muscle, and PUGNAc-induced insulin resistance will coincide with increased O-linked glycosylation.

METHODS

Plantaris muscle and liver from rats (ad libitum or calorie restricted) were analysed for O-linked glycosylation using two antibodies against different O-linked N-acetylglucosamine epitopes. Also, rat epitrochlearis muscles were incubated for 8.5 h +/- 100 mum PUGNAc prior to measurement of [(3)H]-3-O-methylglucose transport and O-linked glycosylation.

RESULTS

Calorie restriction did not alter protein O-linked glycosylated levels in muscle or liver. Incubation with PUGNAc for 8.5 h resulted in increased in O-linked glycosylation but unaltered basal or insulin-stimulated glucose transport.

CONCLUSIONS

The delay between O-linked glycosylation and insulin resistance in muscle incubated with PUGNAc suggests an indirect, relatively slow mechanism for insulin resistance. The effect of calorie restriction on insulin action in muscle is unlikely to be the direct result of a global change in protein O-linked glycosylation.