O-linked N-acetylglucosaminylation is involved in the Ca2+ activation properties of rat skeletal muscle

Tools for investigating O-GlcNAc in signaling and other fundamental biological pathways

Cells continuously fine-tune signaling pathway proteins to match nutrient and stress levels in their local environment by modifying intracellular proteins with O-linked N-acetylglucosamine (O-GlcNAc) sugars, an essential process for cell survival and growth. The small size of these monosaccharide modifications poses a challenge for functional determination, but the chemistry and biology communities have together created a collection of precision tools to study these dynamic sugars. This review presents the major themes by which O-GlcNAc influences signaling pathway proteins, including G-protein coupled receptors, growth factor signaling, mitogen-activated protein kinase (MAPK) pathways, lipid sensing, and cytokine signaling pathways. Along the way, we describe in detail key chemical biology tools that have been developed and applied to determine specific O-GlcNAc roles in these pathways. These tools include metabolic labeling, O-GlcNAc-enhancing RNA aptamers, fluorescent biosensors, proximity labeling tools, nanobody targeting tools, O-GlcNAc cycling inhibitors, light-activated systems, chemoenzymatic labeling, and nutrient reporter assays. An emergent feature of this signaling pathway meta-analysis is the intricate interplay between O-GlcNAc modifications across different signaling systems, underscoring the importance of O-GlcNAc in regulating cellular processes. We highlight the significance of O-GlcNAc in signaling and the role of chemical and biochemical tools in unraveling distinct glycobiological regulatory mechanisms. Collectively, our field has determined effective strategies to probe O-GlcNAc roles in biology. At the same time, this survey of what we do not yet know presents a clear roadmap for the field to use these powerful chemical tools to explore cross-pathway O-GlcNAc interactions in signaling and other major biological pathways.

The O-GlcNAc dichotomy: when does adaptation become pathological?

O-Linked attachment of β-N-acetylglucosamine (O-GlcNAc) on serine and threonine residues of nuclear, cytoplasmic, and mitochondrial proteins is a highly dynamic and ubiquitous post-translational modification that impacts the function, activity, subcellular localization, and stability of target proteins. Physiologically, acute O-GlcNAcylation serves primarily to modulate cellular signaling and transcription regulatory pathways in response to nutrients and stress. To date, thousands of proteins have been revealed to be O-GlcNAcylated and this number continues to grow as the technology for the detection of O-GlcNAc improves. The attachment of a single O-GlcNAc is catalyzed by the enzyme O-GlcNAc transferase (OGT), and their removal is catalyzed by O-GlcNAcase (OGA). O-GlcNAcylation is regulated by the metabolism of glucose via the hexosamine biosynthesis pathway, and the metabolic abnormalities associated with pathophysiological conditions are all associated with increased flux through this pathway and elevate O-GlcNAc levels. While chronic O-GlcNAcylation is well associated with cardiovascular dysfunction, only until recently, and with genetically modified animals, has O-GlcNAcylation as a contributing mechanism of cardiovascular disease emerged. This review will address and critically evaluate the current literature on the role of O-GlcNAcylation in vascular physiology, with a view that this pathway can offer novel targets for the treatment and prevention of cardiovascular diseases.

Functional glycoproteomics by integrated network assembly and partitioning

The post-translational modification (PTM) of proteins by O-linked β-N-acetyl-D-glucosamine (O-GlcNAcylation) is widespread across the proteome during the lifespan of all multicellular organisms. However, nearly all functional studies have focused on individual protein modifications, overlooking the multitude of simultaneous O-GlcNAcylation events that work together to coordinate cellular activities. Here, we describe Networking of Interactors and SubstratEs (NISE), a novel, systems-level approach to rapidly and comprehensively monitor O-GlcNAcylation across the proteome. Our method integrates affinity purification-mass spectrometry (AP-MS) and site-specific chemoproteomic technologies with network generation and unsupervised partitioning to connect potential upstream regulators with downstream targets of O-GlcNAcylation. The resulting network provides a data-rich framework that reveals both conserved activities of O-GlcNAcylation such as epigenetic regulation as well as tissue-specific functions like synaptic morphology. Beyond O-GlcNAc, this holistic and unbiased systems-level approach provides a broadly applicable framework to study PTMs and discover their diverse roles in specific cell types and biological states.

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.

Global O-GlcNAcylation changes impact desmin phosphorylation and its partition toward cytoskeleton in C2C12 skeletal muscle cells differentiated into myotubes

Desmin is the guardian of striated muscle integrity, permitting the maintenance of muscle shape and the efficiency of contractile activity. It is also a key mediator of cell homeostasis and survival. To ensure the fine regulation of skeletal muscle processes, desmin is regulated by post-translational modifications (PTMs). It is more precisely phosphorylated by several kinases connecting desmin to intracellular processes. Desmin is also modified by O-GlcNAcylation, an atypical glycosylation. However, the functional consequence of O-GlcNAcylation on desmin is still unknown, nor its impact on desmin phosphorylation. In a model of C2C12 myotubes, we modulated the global O-GlcNAcylation level, and we determined whether the expression, the PTMs and the partition of desmin toward insoluble material or cytoskeleton were impacted or not. We have demonstrated in the herein paper that O-GlcNAcylation variations led to changes in desmin behaviour. In particular, our data clearly showed that O-GlcNAcylation increase led to a decrease of phosphorylation level on desmin that seems to involve CamKII correlated to a decrease of its partition toward cytoskeleton. Our data showed that phosphorylation/O-GlcNAcylation interplay is highly complex on desmin, supporting that a PTMs signature could occur on desmin to finely regulate its partition (i.e. distribution) with a spatio-temporal regulation.

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.

FHL2 anchors mitochondria to actin and adapts mitochondrial dynamics to glucose supply

Mitochondrial movement and distribution are fundamental to their function. Here we report a mechanism that regulates mitochondrial movement by anchoring mitochondria to the F-actin cytoskeleton. This mechanism is activated by an increase in glucose influx and the consequent O-GlcNAcylation of TRAK (Milton), a component of the mitochondrial motor-adaptor complex. The protein four and a half LIM domains protein 2 (FHL2) serves as the anchor. FHL2 associates with O-GlcNAcylated TRAK and is both necessary and sufficient to drive the accumulation of F-actin around mitochondria and to arrest mitochondrial movement by anchoring to F-actin. Disruption of F-actin restores mitochondrial movement that had been arrested by either TRAK O-GlcNAcylation or forced direction of FHL2 to mitochondria. This pathway for mitochondrial immobilization is present in both neurons and non-neuronal cells and can thereby adapt mitochondrial dynamics to changes in glucose availability.

Altered Actin Dynamics in Cell Migration of GNE Mutant Cells

Cell migration is an essential cellular process that requires coordination of cytoskeletal dynamics, reorganization, and signal transduction. The actin cytoskeleton is central in maintaining the cellular structure as well as regulating the mechanisms of cell motility. Glycosylation, particularly sialylation of cell surface proteins like integrins, regulates signal transduction from the extracellular matrix to the cytoskeletal network. The activation of integrin by extracellular cues leads to recruitment of different focal adhesion complex proteins (Src, FAK, paxillin, etc.) and activates the signal including Rho GTPases for the regulation of actin assembly and disassembly. During cell migration, the assembly and disassembly of actin filament provides the essential force for the cell to move. Abnormal sialylation can lead to actin signaling dysfunction leading to aberrant cell migration, one of the main characteristics of cancer and myopathies. In the present study, we have reported altered F-actin to G-actin ratios in GNE mutated cells. These cells exhibit pathologically relevant mutations of GNE (UDP N-acetylneuraminic 2-epimerase/N-acetylmannosamine kinase), a key sialic acid biosynthetic enzyme. It was found that GNE neither affects the actin polymerization nor binds directly to actin. However, mutation in GNE resulted in increased binding of α-actinin to actin filaments. Further, through confocal imaging, GNE was found to be localized in focal adhesion complex along with paxillin. We further elucidated that mutation in GNE resulted in upregulation of RhoA protein and Cofilin activity is downregulated, which could be rescued with Rhosin and chlorogenic acid, respectively. Lastly, mutant in GNE reduced cell migration as implicated from wound healing assay. Our study indicates that molecules altering Cofilin function could significantly revert the cell migration defect due to GNE mutation in sialic acid-deficient cells. We propose cytoskeletal proteins to be alternate drug targets for disorders associated with GNE such as GNE myopathy.

Reprint of: Role of O-linked N-acetylglucosamine (O-GlcNAc) modification of proteins in diabetic cardiovascular complications

The post-translational modification of serine and threonine residues of proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates diverse cellular processes in the cardiovascular system. UDP-GlcNAc is a substrate for O-GlcNAc transferase, which catalyzes the attachment of O-GlcNAc to proteins. O-GlcNAcase catalyzes the removal of O-GlcNAc from proteins. UDP-GlcNAc is the end product of the hexosamine biosynthesis pathway, which is regulated primarily by glucose-6-phosphate-Glutamine:fructose-6-phosphate amidotransferase (GFAT). GFAT catalyzes the formation of glucosamine-6-phosphate from fructose-6-phosphate and glutamine. Whereas O-GlcNAc is essential for cell viability, sustained increases in O-GlcNAc levels have been implicated in the etiology of many chronic diseases and is associated with glucose toxicity and diabetic complications in various organs including the cardiovascular system. This review provides an overview of the regulation of protein O-GlcNAcylation followed by a discussion of potential mechanisms by which dysregulation in O-GlcNAc cycling contributes to the adverse effects of diabetes on the cardiovascular system.

MYPT1 O-GlcNAc modification regulates sphingosine-1-phosphate mediated contraction

Many intracellular proteins are modified by N-acetylglucosamine, a posttranslational modification termed O-GlcNAc. This modification is found on serine and threonine side-chains and has the potential to regulate signaling pathways through interplay with phosphorylation. Here, we discover and characterize one such example. We find that O-GlcNAc levels control the sensitivity of fibroblasts to actin contraction induced by the signaling lipid sphingosine-1-phosphate (S1P), culminating in the phosphorylation of myosin light chain (MLC) and cellular contraction. Specifically, O-GlcNAc modification of the phosphatase subunit MYPT1 inhibits this pathway by blocking MYPT1 phosphorylation, maintaining its activity and causing the dephosphorylation of MLC. Finally, we demonstrate that O-GlcNAc levels alter the sensitivity of primary human dermal fibroblasts in a collagen-matrix model of wound healing. Our findings have important implications for the role of O-GlcNAc in fibroblast motility and differentiation, particularly in diabetic wound healing.

Role of O-linked N-acetylglucosamine (O-GlcNAc) modification of proteins in diabetic cardiovascular complications

The post-translational modification of serine and threonine residues of proteins by O-linked N-acetylglucosamine (O-GlcNAc) regulates diverse cellular processes in the cardiovascular system. UDP-GlcNAc is a substrate for O-GlcNAc transferase, which catalyzes the attachment of O-GlcNAc to proteins. O-GlcNAcase catalyzes the removal of O-GlcNAc from proteins. UDP-GlcNAc is the end product of the hexosamine biosynthesis pathway, which is regulated primarily by glucose-6-phosphate-Glutamine:fructose-6-phosphate amidotransferase (GFAT). GFAT catalyzes the formation of glucosamine-6-phosphate from fructose-6-phosphate and glutamine. Whereas O-GlcNAc is essential for cell viability, sustained increases in O-GlcNAc levels have been implicated in the etiology of many chronic diseases and is associated with glucose toxicity and diabetic complications in various organs including the cardiovascular system. This review provides an overview of the regulation of protein O-GlcNAcylation followed by a discussion of potential mechanisms by which dysregulation in O-GlcNAc cycling contributes to the adverse effects of diabetes on the cardiovascular system.

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.

Improved Alignment and Quantification of Protein Signals in Two-Dimensional Western Blotting

Western blotting is widely used for protein identification and quantification in research applications, but different protein species, resulting from alternative splicing and post-translational modifications, can often only be detected individually by two-dimensional gel electrophoresis and immunodetection by Western blotting (2D-WB). The additional separation by isoelectric focusing enables the detection of different protein species with the same specific antibody. Reliable assignment of signals from antibody-based detection to the total protein spot pattern of the original gel image is a challenge in 2D-WB, often resulting in ambiguous results. We therefore propose a reliable strategy for assignment of antibody signals from 2D-WB to the total protein spot pattern, using an imaging workflow in combination with a straightforward and easily reproducible image alignment strategy. The strategy employs vector-based alignment of protein spots and image contours in a stepwise manner. Our workflow is compatible with various protein visualization techniques, including prelabeling of proteins and poststaining of gels and membranes, as well as with chemiluminescent and fluorescent detection of bound antibody. Here, we provide a detailed description of potential applications and benefits of our workflow. We use experimental test settings with gold-standard stressors in combination with multiple staining and detection methods, as well as spike-in recombinant proteins. Our results demonstrate reliable attribution of signals to very similar heat shock proteins, phosphorylation patterns, and global analysis of proteins modified with O-linked N-acetylglucosamine (O-GlcNAc).

O-GlcNAcylation as a regulator of the functional and structural properties of the sarcomere in skeletal muscle: An update review

Although the O-GlcNAcylation process was discovered in 1984, its potential role in the physiology and physiopathology of skeletal muscle only emerged 20 years later. An increasing number of publications strongly support a key role of O-GlcNAcylation in the modulation of important cellular processes which are essential for skeletal muscle functions. Indeed, over a thousand of O-GlcNAcylated proteins have been identified within skeletal muscle since 2004, which belong to various classes of proteins, including sarcomeric proteins. In this review, we focused on these myofibrillar proteins, including contractile and structural proteins. Because of the modification of motor and regulatory proteins, the regulatory myosin light chain (MLC2) is related to several reports that support a key role of O-GlcNAcylation in the fine modulation of calcium activation parameters of skeletal muscle fibres, depending on muscle phenotype and muscle work. In addition, another key function of O-GlcNAcylation has recently emerged in the regulation of organization and reorganization of the sarcomere. Altogether, this data support a key role of O-GlcNAcylation in the homeostasis of sarcomeric cytoskeleton, known to be disturbed in many related muscle disorders.

O-GlcNAcylation and phosphorylation of β-actin Ser199 in diabetic nephropathy

The function of actin is regulated by various posttranslational modifications. We have previously shown that in the kidneys of nonobese type 2 diabetes model Goto-Kakizaki rats, increased O-GlcNAcylation of β-actin protein is observed. It has also been reported that both O-GlcNAcylation and phosphorylation occur on Ser199 of β-actin. However, their roles are not known. To elucidate their roles in diabetic nephropathy, we examined the rat kidney for changes in O-GlcNAcylation of Ser199 (gS199)-actin and in the phosphorylation of Ser199 (pS199)-actin. Both gS199- and pS199-actin molecules had an apparent molecular weight of 40 kDa and were localized as nonfilamentous actin in both the cytoplasm and nucleus. Compared with the normal kidney, the immunostaining intensity of gS199-actin increased in podocytes of the glomeruli and in proximal tubules of the diabetic kidney, whereas that of pS199-actin did not change in podocytes but decreased in proximal tubules. We confirmed that the same results could be observed in the glomeruli of the human diabetic kidney. In podocytes of glomeruli cultured in the presence of the O-GlcNAcase inhibitor Thiamet G, increased O-GlcNAcylation was accompanied by a concomitant decrease in the amount of filamentous actin and in morphological changes. Our present results demonstrate that dysregulation of O-GlcNAcylation and phosphorylation of Ser199 occurred in diabetes, which may contribute partially to the causes of the morphological changes in the glomeruli and tubules. gS199- and pS199-actin will thus be useful for the pathological evaluation of diabetic nephropathy.

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.

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

Involvement of O-GlcNAcylation in the Skeletal Muscle Physiology and Physiopathology: Focus on Muscle Metabolism

Skeletal muscle represents around 40% of whole body mass. The principal function of skeletal muscle is the conversion of chemical energy toward mechanic energy to ensure the development of force, provide movement and locomotion, and maintain posture. This crucial energy dependence is maintained by the faculty of the skeletal muscle for being a central place as a "reservoir" of amino acids and carbohydrates in the whole body. A fundamental post-translational modification, named O-GlcNAcylation, depends, inter alia, on these nutrients; it consists to the transfer or the removal of a unique monosaccharide (N-acetyl-D-glucosamine) to a serine or threonine hydroxyl group of nucleocytoplasmic and mitochondrial proteins in a dynamic process by the O-GlcNAc Transferase (OGT) and the O-GlcNAcase (OGA), respectively. O-GlcNAcylation has been shown to be strongly involved in crucial intracellular mechanisms through the modulation of signaling pathways, gene expression, or cytoskeletal functions in various organs and tissues, such as the brain, liver, kidney or pancreas, and linked to the etiology of associated diseases. In recent years, several studies were also focused on the role of O-GlcNAcylation in the physiology and the physiopathology of skeletal muscle. These studies were mostly interested in O-GlcNAcylation during muscle exercise or muscle-wasting conditions. Major findings pointed out a different "O-GlcNAc signature" depending on muscle type metabolism at resting, wasting and exercise conditions, as well as depending on acute or long-term exhausting exercise protocol. First insights showed some differential OGT/OGA expression and/or activity associated with some differential stress cellular responses through Reactive Oxygen Species and/or Heat-Shock Proteins. Robust data displayed that these O-GlcNAc changes could lead to (i) a differential modulation of the carbohydrates metabolism, since the majority of enzymes are known to be O-GlcNAcylated, and to (ii) a differential modulation of the protein synthesis/degradation balance since O-GlcNAcylation regulates some key signaling pathways such as Akt/GSK3β, Akt/mTOR, Myogenin/Atrogin-1, Myogenin/Mef2D, Mrf4 and PGC-1α in the skeletal muscle. Finally, such involvement of O-GlcNAcylation in some metabolic processes of the skeletal muscle might be linked to some associated diseases such as type 2 diabetes or neuromuscular diseases showing a critical increase of the global O-GlcNAcylation level.

Insights into the role of maladaptive hexosamine biosynthesis and O-GlcNAcylation in development of diabetic cardiac complications

Diabetes mellitus significantly increases the risk of heart failure, independent of coronary artery disease. The mechanisms implicated in the development of diabetic heart disease, commonly termed diabetic cardiomyopathy, are complex, but much of the impact of diabetes on the heart can be attributed to impaired glucose handling. It has been shown that the maladaptive nutrient-sensing hexosamine biosynthesis pathway (HBP) contributes to diabetic complications in many non-cardiac tissues. Glucose metabolism by the HBP leads to enzymatically-regulated, O-linked attachment of a sugar moiety molecule, β-N-acetylglucosamine (O-GlcNAc), to proteins, affecting their biological activity (similar to phosphorylation). In normal physiology, transient activation of HBP/O-GlcNAc mechanisms is an adaptive, protective means to enhance cell survival; interventions that acutely suppress this pathway decrease tolerance to stress. Conversely, chronic dysregulation of HBP/O-GlcNAc mechanisms has been shown to be detrimental in certain pathological settings, including diabetes and cancer. Most of our understanding of the impact of sustained maladaptive HBP and O-GlcNAc protein modifications has been derived from adipose tissue, skeletal muscle and other non-cardiac tissues, as a contributing mechanism to insulin resistance and progression of diabetic complications. However, the long-term consequences of persistent activation of cardiac HBP and O-GlcNAc are not well-understood; therefore, the goal of this timely review is to highlight current understanding of the role of the HBP pathway in development of diabetic cardiomyopathy.

Exercise training increases protein O-GlcNAcylation in rat skeletal muscle

Protein O-GlcNAcylation has emerged as an important intracellular signaling system with both physiological and pathophysiological functions, but the role of protein O-GlcNAcylation in skeletal muscle remains elusive. In this study, we tested the hypothesis that protein O-GlcNAcylation is a dynamic signaling system in skeletal muscle in exercise and disease. Immunoblotting showed different protein O-GlcNAcylation pattern in the prototypical slow twitch soleus muscle compared to fast twitch EDL from rats, with greater O-GlcNAcylation level in soleus associated with higher expression of the modulating enzymes O-GlcNAc transferase (OGT), O-GlcNAcase (OGA), and glutamine fructose-6-phosphate amidotransferase isoforms 1 and 2 (GFAT1, GFAT2). Six weeks of exercise training by treadmill running, but not an acute exercise bout, increased protein O-GlcNAcylation in rat soleus and EDL There was a striking increase in O-GlcNAcylation of cytoplasmic proteins ~50 kDa in size that judged from mass spectrometry analysis could represent O-GlcNAcylation of one or more key metabolic enzymes. This suggests that cytoplasmic O-GlcNAc signaling is part of the training response. In contrast to exercise training, postinfarction heart failure (HF) in rats and humans did not affect skeletal muscle O-GlcNAcylation level, indicating that aberrant O-GlcNAcylation cannot explain the skeletal muscle dysfunction in HF Human skeletal muscle displayed extensive protein O-GlcNAcylation that by large mirrored the fiber-type-related O-GlcNAcylation pattern in rats, suggesting O-GlcNAcylation as an important signaling system also in human skeletal muscle.

O-GlcNAcylation is a key modulator of skeletal muscle sarcomeric morphometry associated to modulation of protein-protein interactions

BACKGROUND

The sarcomere structure of skeletal muscle is determined through multiple protein-protein interactions within an intricate sarcomeric cytoskeleton network. The molecular mechanisms involved in the regulation of this sarcomeric organization, essential to muscle function, remain unclear. O-GlcNAcylation, a post-translational modification modifying several key structural proteins and previously described as a modulator of the contractile activity, was never considered to date in the sarcomeric organization.

METHODS

C2C12 skeletal myotubes were treated with Thiamet-G (OGA inhibitor) in order to increase the global O-GlcNAcylation level.

RESULTS

Our data clearly showed a modulation of the O-GlcNAc level more sensitive and dynamic in the myofilament-enriched fraction than total proteome. This fine O-GlcNAc level modulation was closely related to changes of the sarcomeric morphometry. Indeed, the dark-band and M-line widths increased, while the I-band width and the sarcomere length decreased according to the myofilament O-GlcNAc level. Some structural proteins of the sarcomere such as desmin, αB-crystallin, α-actinin, moesin and filamin-C have been identified within modulated protein complexes through O-GlcNAc level variations. Their interactions seemed to be changed, especially for desmin and αB-crystallin.

CONCLUSIONS

For the first time, our findings clearly demonstrate that O-GlcNAcylation, through dynamic regulations of the structural interactome, could be an important modulator of the sarcomeric structure and may provide new insights in the understanding of molecular mechanisms of neuromuscular diseases characterized by a disorganization of the sarcomeric structure.

GENERAL SIGNIFICANCE

In the present study, we demonstrated a role of O-GlcNAcylation in the sarcomeric structure modulation.

Roles of O-GlcNAc in chronic diseases of aging

O-GlcNAcylation, a dynamic nutrient and stress sensitive post-translational modification, occurs on myriad proteins in the cell nucleus, cytoplasm and mitochondria. O-GlcNAcylation serves as a nutrient sensor to regulate signaling, transcription, translation, cell division, metabolism, and stress sensitivity in all cells. Aberrant protein O-GlcNAcylation plays a critical role both in the development, as well as in the progression of a variety of age related diseases. O-GlcNAcylation underlies the etiology of diabetes, and changes in specific protein O-GlcNAc levels and sites are responsible for insulin expression and sensitivity and glucose toxicity. Abnormal O-GlcNAcylation contributes directly to diabetes related dysfunction of the heart, kidney and eyes and affects progression of cardiomyopathy, nephropathy and retinopathy. O-GlcNAcylation is a critical modification in the brain and plays a role in both plaque and tangle formation, thus making its study important in neurodegenerative disorders. O-GlcNAcylation also affects cellular growth and metabolism during the development and metastasis of cancer. Finally, alterations in O-GlcNAcylation of transcription factors in macrophages and lymphocytes affect inflammation and cytokine production. Thus, O-GlcNAcylation plays key roles in many of the major diseases associated with aging. Elucidation of its specific functions in both normal and diseased tissues is likely to uncover totally novel avenues for therapeutic intervention.

TNNI1, TNNI2 and TNNI3: Evolution, regulation, and protein structure-function relationships

Troponin I (TnI) is the inhibitory subunit of the troponin complex in the sarcomeric thin filament of striated muscle and plays a central role in the calcium regulation of contraction and relaxation. Vertebrate TnI has evolved into three isoforms encoded by three homologous genes: TNNI1 for slow skeletal muscle TnI, TNNI2 for fast skeletal muscle TnI and TNNI3 for cardiac TnI, which are expressed under muscle type-specific and developmental regulations. To summarize the current knowledge on the TnI isoform genes and products, this review focuses on the evolution, gene regulation, posttranslational modifications, and structure-function relationship of TnI isoform proteins. Their physiological and medical significances are also discussed.

Removal of Abnormal Myofilament O-GlcNAcylation Restores Ca2+ Sensitivity in Diabetic Cardiac Muscle

Contractile dysfunction and increased deposition of O-linked β-N-acetyl-d-glucosamine (O-GlcNAc) in cardiac proteins are a hallmark of the diabetic heart. However, whether and how this posttranslational alteration contributes to lower cardiac function remains unclear. Using a refined β-elimination/Michael addition with tandem mass tags (TMT)-labeling proteomic technique, we show that CpOGA, a bacterial analog of O-GlcNAcase (OGA) that cleaves O-GlcNAc in vivo, removes site-specific O-GlcNAcylation from myofilaments, restoring Ca(2+) sensitivity in streptozotocin (STZ) diabetic cardiac muscles. We report that in control rat hearts, O-GlcNAc and O-GlcNAc transferase (OGT) are mainly localized at the Z-line, whereas OGA is at the A-band. Conversely, in diabetic hearts O-GlcNAc levels are increased and OGT and OGA delocalized. Consistent changes were found in human diabetic hearts. STZ diabetic hearts display increased physical interactions of OGA with α-actin, tropomyosin, and myosin light chain 1, along with reduced OGT and increased OGA activities. Our study is the first to reveal that specific removal of O-GlcNAcylation restores myofilament response to Ca(2+) in diabetic hearts and that altered O-GlcNAcylation is due to the subcellular redistribution of OGT and OGA rather than to changes in their overall activities. Thus, preventing sarcomeric OGT and OGA displacement represents a new possible strategy for treating diabetic cardiomyopathy.

Exhausting treadmill running causes dephosphorylation of sMLC2 and reduced level of myofilament MLCK2 in slow twitch rat soleus muscle

Myosin light chain 2 (MLC2) is a small protein in the myosin complex, regulating muscle contractile function by modulating Ca²⁺ sensitivity of myofilaments. MLC2 can be modified by phosphorylation and O-GlcNAcylation, two reversible and dynamic posttranslational modifications. The slow isoform of MLC2 (sMLC2) is dephosphorylated in soleus muscle during in situ loaded shortening contractions, which correlates with reduction in shortening capacity. Here, we hypothesize that exhausting in vivo treadmill running induces dephosphorylation of MLC2 in slow twitch soleus, but not in fast twitch EDL muscle, and that there are reciprocal changes in MLC2 O-GlcNAcylation. At rest, both phosphorylation and O-GlcNAcylation of MLC2 were lower in slow than fast twitch muscles. One bout of exhausting treadmill running induced dephosphorylation of sMLC2 in soleus, paralleled by reduced levels of the kinase MLCK2 associated to myofilaments, suggesting that the acute reduction in phosphorylation is mediated by dissociation of MLCK2 from myofilaments. O-GlcNAcylation of MLC2 did not change significantly, and seems of limited importance in the regulation of MLC2 phosphorylation during in vivo running. After 6 weeks of treadmill running, the dephosphorylation of sMLC2 persisted in soleus along with reduction in MLCK2 both in myofilament- and total protein fraction. In EDL on the contrary, phosphorylation of MLC2 was not altered after one exercise bout or after 6 weeks of treadmill running. Thus, in contrast to fast twitch muscle, MLC2 dephosphorylation occurs in slow twitch muscle during in vivo exercise and may be linked to reduced myofilament-associated MLCK2 and reduced shortening capacity.

Glutathione depletion and acute exercise increase O-GlcNAc protein modification in rat skeletal muscle

Post-translational modification of intracellular proteins with O-linked β-N-acetylglucosamine (O-GlcNAc) profoundly affects protein structure, function, and metabolism. Although many skeletal muscle proteins are O-GlcNAcylated, the modification has not been extensively studied in this tissue, especially in the context of exercise. This study investigated the effects of glutathione depletion and acute exercise on O-GlcNAc protein modification in rat skeletal muscle. Diethyl maleate (DEM) was used to deplete intracellular glutathione and rats were subjected to a treadmill run. White gastrocnemius and soleus muscles were analyzed for glutathione status, O-GlcNAc and O-GlcNAc transferase (OGT) protein levels, and mRNA expression of OGT, O-GlcNAcase and glutamine:fructose-6-phosphate amidotransferase. DEM and exercise both reduced intracellular glutathione and increased O-GlcNAc. DEM upregulated OGT protein expression. The effects of the interventions were significant 4 h after exercise (P < 0.05). The changes in the mRNA levels of O-GlcNAc enzymes were different in the two muscles, potentially resulting from different rates of oxidative stress and metabolic demands between the muscle types. These findings indicate that oxidative environment promotes O-GlcNAcylation in skeletal muscle and suggest an interrelationship between cellular redox state and O-GlcNAc protein modification. This could represent one mechanism underlying cellular adaptation to oxidative stress and health benefits of exercise.

O-GlcNAcylation, contractile protein modifications and calcium affinity in skeletal muscle

O-GlcNAcylation, a generally undermined atypical protein glycosylation process, is involved in a dynamic and highly regulated interplay with phosphorylation. Akin to phosphorylation, O-GlcNAcylation is also involved in the physiopathology of several acquired diseases, such as muscle insulin resistance or muscle atrophy. Recent data underline that the interplay between phosphorylation and O-GlcNAcylation acts as a modulator of skeletal muscle contractile activity. In particular, the O-GlcNAcylation level of the phosphoprotein myosin light chain 2 seems to be crucial in the modulation of the calcium activation properties, and should be responsible for changes in calcium properties observed in functional atrophy. Moreover, since several key structural proteins are O-GlcNAc-modified, and because of the localization of the enzymes involved in the O-GlcNAcylation/de-O-GlcNAcylation process to the nodal Z disk, a role of O-GlcNAcylation in the modulation of the sarcomeric structure should be considered.

Vascular O-GlcNAcylation augments reactivity to constrictor stimuli by prolonging phosphorylated levels of the myosin light chain

O-GlcNAcylation is a modification that alters the function of numerous proteins. We hypothesized that augmented O-GlcNAcylation levels enhance myosin light chain kinase (MLCK) and reduce myosin light chain phosphatase (MLCP) activity, leading to increased vascular contractile responsiveness. The vascular responses were measured by isometric force displacement. Thoracic aorta and vascular smooth muscle cells (VSMCs) from rats were incubated with vehicle or with PugNAc, which increases O-GlcNAcylation. In addition, we determined whether proteins that play an important role in the regulation of MLCK and MLCP activity are directly affected by O-GlcNAcylation. PugNAc enhanced phenylephrine (PE) responses in rat aortas (maximal effect, 14.2 ± 2 vs 7.9 ± 1 mN for vehicle, n=7). Treatment with an MLCP inhibitor (calyculin A) augmented vascular responses to PE (13.4 ± 2 mN) and abolished the differences in PE-response between the groups. The effect of PugNAc was not observed when vessels were preincubated with ML-9, an MLCK inhibitor (7.3 ± 2 vs 7.5 ± 2 mN for vehicle, n=5). Furthermore, our data showed that differences in the PE-induced contractile response between the groups were abolished by the activator of AMP-activated protein kinase (AICAR; 6.1 ± 2 vs 7.4 ± 2 mN for vehicle, n=5). PugNAc increased phosphorylation of myosin phosphatase target subunit 1 (MYPT-1) and protein kinase C-potentiated inhibitor protein of 17 kDa (CPI-17), which are involved in RhoA/Rho-kinase-mediated inhibition of myosin phosphatase activity. PugNAc incubation produced a time-dependent increase in vascular phosphorylation of myosin light chain and decreased phosphorylation levels of AMP-activated protein kinase, which decreased the affinity of MLCK for Ca(2+)/calmodulin. Our data suggest that proteins that play an important role in the regulation of MLCK and MLCP activity are directly affected by O-GlcNAcylation, favoring vascular contraction.

Protein O-GlcNAcylation in diabetes and diabetic complications

The post-translational modification of serine and threonine residues of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc) is highly ubiquitous, dynamic and inducible. Protein O-GlcNAcylation serves as a key regulator of critical biological processes including transcription, translation, proteasomal degradation, signal transduction and apoptosis. Increased O-GlcNAcylation is directly linked to insulin resistance and to hyperglycemia-induced glucose toxicity, two hallmarks of diabetes and diabetic complications. In this review, we briefly summarize what is known about protein O-GlcNAcylation and nutrient metabolism, as well as discuss the commonly used tools to probe changes of O-GlcNAcylation in cultured cells and in animal models. We then focus on some key proteins modified by O-GlcNAc, which play crucial roles in the etiology and progression of diabetes and diabetic complications. Proteomic approaches are also highlighted to provide a system view of protein O-GlcNAcylation. Finally, we discuss how aberrant O-GlcNAcylation on certain proteins may be exploited to develop methods for the early diagnosis of pre-diabetes and/or diabetes.

O-GlcNAcylation of AMPA receptor GluA2 is associated with a novel form of long-term depression at hippocampal synapses

Serine phosphorylation of AMPA receptor (AMPAR) subunits GluA1 and GluA2 modulates AMPAR trafficking during long-term changes in strength of hippocampal excitatory transmission required for normal learning and memory. The post-translational addition and removal of O-linked β-N-acetylglucosamine (O-GlcNAc) also occurs on serine residues. This, together with the high expression of the enzymes O-GlcNAc transferase (OGT) and β-N-acetylglucosamindase (O-GlcNAcase), suggests a potential role for O-GlcNAcylation in modifying synaptic efficacy and cognition. Furthermore, because key synaptic proteins are O-GlcNAcylated, this modification may be as important to brain function as phosphorylation, yet its physiological significance remains unknown. We report that acutely increasing O-GlcNAcylation in Sprague Dawley rat hippocampal slices induces an NMDA receptor and protein kinase C-independent long-term depression (LTD) at hippocampal CA3-CA1 synapses (O-GcNAc LTD). This LTD requires AMPAR GluA2 subunits, which we demonstrate are O-GlcNAcylated. Increasing O-GlcNAcylation interferes with long-term potentiation, and in hippocampal behavioral assays, it prevents novel object recognition and placement without affecting contextual fear conditioning. Our findings provide evidence that O-GlcNAcylation dynamically modulates hippocampal synaptic function and learning and memory, and suggest that altered O-GlcNAc levels could underlie cognitive dysfunction in neurological diseases.

Z-band alternatively spliced PDZ motif protein (ZASP) is the major O-linked β-N-acetylglucosamine-substituted protein in human heart myofibrils

We studied O-linked β-N-acetylglucosamine (O-GlcNAc) modification of contractile proteins in human heart using SDS-PAGE and three detection methods: specific enzymatic conjugation of O-GlcNAc with UDP-N-azidoacetylgalactosamine (UDP-GalNAz) that is then linked to a tetramethylrhodamine fluorescent tag and CTD110.6 and RL2 monoclonal antibodies to O-GlcNAc. All three methods showed that O-GlcNAc modification was predominantly in a group of bands ~90 kDa that did not correspond to any of the major myofibrillar proteins. MALDI-MS/MS identified the 90-kDa band as the protein ZASP (Z-band alternatively spliced PDZ motif protein), a minor component of the Z-disc (about 1 per 400 α-actinin) important for myofibrillar development and mechanotransduction. This was confirmed by the co-localization of O-GlcNAc and ZASP in Western blotting and by immunofluorescence microscopy. O-GlcNAcylation of ZASP increased in diseased heart, being 49 ± 5% of all O-GlcNAc in donor, 68 ± 9% in end-stage failing heart, and 76 ± 6% in myectomy muscle samples (donor versus myectomy p < 0.05). ZASP is only 22% of all O-GlcNAcylated proteins in mouse heart myofibrils.

Post-translational modification and regulation of actin

Many of the best-studied actin regulatory proteins use non-covalent means to modulate the properties of actin. Yet, actin is also susceptible to covalent modifications of its amino acids. Recent work is increasingly revealing that actin processing and its covalent modifications regulate important cellular events. In addition, numerous pathogens express enzymes that specifically use actin as a substrate to regulate their hosts' cells. Actin post-translational alterations have been linked to different normal and disease processes and the effects associated with metabolic and environmental stressors. Herein, we highlight specific co-translational and post-translational modifications of actin and discuss the current understanding of the role that these modifications play in regulating actin.

Increasing O-GlcNAcylation level on organ culture of soleus modulates the calcium activation parameters of muscle fibers

O-N-acetylglucosaminylation is a reversible post-translational modification which presents a dynamic and highly regulated interplay with phosphorylation. New insights suggest that O-GlcNAcylation might be involved in striated muscle physiology, in particular in contractile properties such as the calcium activation parameters. By the inhibition of O-GlcNAcase, we investigated the effect of the increase of soleus O-GlcNAcylation level on the contractile properties by establishing T/pCa relationships. We increased the O-GlcNAcylation level on soleus biopsies performing an organ culture of soleus treated or not with PUGNAc or Thiamet-G, two O-GlcNAcase inhibitors. The enhancement of O-GlcNAcylation pattern was associated with an increase of calcium affinity on slow soleus skinned fibers. Analysis of the glycoproteins pattern showed that this effect is solely due to O-GlcNAcylation of proteins extracted from skinned biopsies. We also characterized the O-GlcNAcylated contractile proteins using a proteomic approach, and identified among others troponin T and I as being O-GlcNAc modified. We quantified the variation of O-GlcNAc level on all these identified proteins, and showed that several regulatory contractile proteins, predominantly fast isoforms, presented a drastic increase in their O-GlcNAc level. Since the only slow isoform of contractile protein presenting an increase of O-GlcNAc level was MLC2, the effect of enhanced O-GlcNAcylation pattern on calcium activation parameters could involve the O-GlcNAcylation of sMLC2, without excluding that an unidentified O-GlcNAc proteins, such as TnC, could be potentially involved in this mechanism. All these data strongly linked O-GlcNAcylation to the modulation of contractile activity of skeletal muscle.

Cardiac myofilaments: from proteome to pathophysiology

This review addresses the functional consequences of altered post-translational modifications of cardiac myofilament proteins in cardiac diseases such as heart failure and ischemia. The modifications of thick and thin filament proteins as well as titin are addressed. Understanding the functional consequences of altered protein modifications is an essential step in the development of targeted therapies for common cardiac diseases.

Localization of O-GlcNAc-modified proteins in neuromuscular diseases

O-linked N-acetylglucosamine (O-GlcNAc) is a ubiquitous post-translational modification of nucleocytoplasmic proteins that induces the attachment of N-acetylglucosamine to serine or threonine residues of a protein. In contrast to other protein glycosylations, this modification is highly reversible and, similar to phosphorylation, it plays important roles in various cell signals. Here, we immunolocalized O-GlcNAc-modified proteins in muscle biopsy specimens from 40 patients with neuromuscular diseases and controls. In normal muscle fibers, O-GlcNAc was found along plasma membranes and in nuclei. Diffuse and increased cytoplasmic staining of O-GlcNAc was detected in (1) regenerating muscle fibers in muscular dystrophy, myositis, and rhabdomyolysis; (2) a proportion of atrophic fibers in myositis, such as those found in perifascicular regions in dermatomyositis; and (3) vacuolated fibers in sporadic inclusion body myositis (s-IBM) and distal myopathy with rimmed vacuoles (DMRV). Target formations in neurogenic muscular atrophy were O-GlcNAc positive. Increase of O-GlcNAc glycosylation could be associated with the stress response, as these lesions have been shown to be positive for several stress markers. Vacuolar rims in s-IBM and DMRV were sometimes sharply lined by O-GlcNAc-positive deposits, which reflects myonuclear breakdown occurring from the disease.

Altered calcium signaling in colonic smooth muscle of type 1 diabetic mice

Seventy-six percent of diabetic patients develop gastrointestinal symptoms, such as constipation. However, the direct effects of diabetes on intestinal smooth muscle are poorly described. This study aimed to identify the role played by smooth muscle in mediating diabetes-induced colonic dysmotility. To induce type 1 diabetes, mice were injected intraperitoneally with low-dose streptozotocin once a day for 5 days. Animals developed hyperglycemia (>200 mg/dl) 1 wk after the last injection and were euthanized 7-8 wk after the last treatment. Computed tomography demonstrated decreased overall gastrointestinal motility in the diabetic mice. In vitro contractility of colonic smooth muscle rings from diabetic mice was also decreased. Fura-2 ratiometric Ca(2+) imaging showed attenuated Ca(2+) increases in response to KCl stimulation that were associated with decreased light chain phosphorylation in diabetic mice. The diabetic mice also exhibited elevated basal Ca(2+) levels, increased myosin phosphatase targeting subunit 1 expression, and significant changes in expression of Ca(2+) handling proteins, as determined by quantitative RT-PCR and Western blotting. Mice that were hyperglycemic for <1 wk also showed decreased colonic contractile responses that were associated with decreased Ca(2+) increases in response to KCl stimulation, although without an elevation in basal Ca(2+) levels or a significant change in the expression of Ca(2+) signaling molecules. These data demonstrate that type 1 diabetes is associated with decreased depolarization-induced Ca(2+) influx in colonic smooth muscle that leads to attenuated myosin light chain phosphorylation and impaired colonic contractility.

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.

A striking common O-linked N-acetylglucosaminyl moiety between cruzipain and myosin

Single units of O-linked N-acetylglucosamine (GlcNAc), usually components of nuclear and cytoplasmatic proteins, are present at the C-terminal domain of cruzipain (Cz), a lysosomal major antigen from Trypanosoma cruzi. On the other hand, antibodies directed against some self-antigens like myosin are associated with Chagas heart disease. The participation of O-GlcNAc moieties in the molecular antigenicity of Cz was determined using GlcNAc linked to aprotinin by ELISA. The immune cross-reactivity between Cz and myosin is mainly focused in the C-T domain. ELISA inhibition assays using rabbit sera specific for Cz and C-T in conjunction with immune-gold electron microscopy analysis of heart tissues from mice immunized with C-T confronted with polyclonal rabbit sera specific for Cz and C-T prior and after myosin adsorption provided evidence which indicates that O-GlcNAc moieties constitute a common epitope between Cz and either myosin or other cardiac O-GlcNAc-containing proteins, showing a new insight into the molecular immune pathogenesis of Chagas heart disease.

Muscle-specific overexpression of NCOATGK, splice variant of O-GlcNAcase, induces skeletal muscle atrophy

The protein O-linked β-N-acetylglucosamine (O-GlcNAc) modification plays an important role in skeletal muscle development and physiological function. In this study, bitransgenic mice were generated that overexpressed NCOAT(GK), an O-GlcNAcase-inactive spliced variant of the O-GlcNAcase gene, specifically in skeletal muscle using the muscle creatine kinase promoter. Expression of the chimeric enhanced green fluorescent protein-NCOAT(GK) transgene caused an increase of cellular O-GlcNAc levels, along with the accumulation and activation of proapoptotic factors in muscles of bitransgenic mice. The consequence of overexpressing the transgene for a 2-wk period was muscle atrophy and, in some cases, resulted in the death of male mice. Muscle atrophy is a common complication of many diseases, some of which correlate markedly with high cellular O-GlcNAc levels, such as diabetes. Our study provides direct evidence linking muscle atrophy and the disruption of O-GlcNAcase activity.

N-acetylglucosamine transferase is an integral component of a kinesin-directed mitochondrial trafficking complex

Trafficking kinesin proteins (TRAKs) 1 and 2 are kinesin-associated proteins proposed to function in excitable tissues as adaptors in anterograde trafficking of cargoes including mitochondria. They are known to associate with N-acetylglucosamine transferase and the mitochondrial rho GTPase, Miro. We used confocal imaging, Förster resonance energy transfer and immunoprecipitations to investigate association between TRAKs1/2, N-acetylglucosamine transferase, the prototypic kinesin-1, KIF5C, and Miro. We demonstrate that in COS-7 cells, N-acetylglucosamine transferase, KIF5C and TRAKs1/2 co-distribute. Förster resonance energy transfer was observed between N-acetylglucosamine transferase and TRAKs1/2. Despite co-distributing with KIF5C and immunoprecipitations demonstrating a TRAK1/2, N-acetylglucosamine transferase and KIF5C ternary complex, no Förster resonance energy transfer was detected between N-acetylglucosamine transferase and KIF5C. KIF5C, N-acetylglucosamine transferase, TRAKs1/2 and Miro formed a quaternary complex. The presence of N-acteylglucosamine transferase partially prevented redistribution of mitochondria induced by trafficking proteins 1/2 and KIF5C. TRAK2 was a substrate for N-acetylglucosamine transferase with TRAK2 (S562) identified as a site of O-N-acetylglucosamine modification. These findings substantiate trafficking kinesin proteins as scaffolds for the formation of a multi-component complex involved in anterograde trafficking of mitochondria. They further suggest that O-glycosylation may regulate complex formation.

O-GlcNAcylation contributes to the vascular effects of ET-1 via activation of the RhoA/Rho-kinase pathway

AIMS

Glycosylation with β-N-acetylglucosamine (O-GlcNAcylation) is one of the most complex post-translational modifications. The cycling of O-GlcNAc is controlled by two enzymes: UDP-NAc transferase (OGT) and O-GlcNAcase (OGA). We recently reported that endothelin-1 (ET-1) augments vascular levels of O-GlcNAcylated proteins. Here we tested the hypothesis that O-GlcNAcylation contributes to the vascular effects of ET-1 via activation of the RhoA/Rho-kinase pathway.

METHODS AND RESULTS

Incubation of vascular smooth muscle cells (VSMCs) with ET-1 (0.1 μM) produces a time-dependent increase in O-GlcNAc levels. ET-1-induced O-GlcNAcylation is not observed when VSMCs are previously transfected with OGT siRNA, treated with ST045849 (OGT inhibitor) or atrasentan (ET(A) antagonist). ET-1 as well as PugNAc (OGA inhibitor) augmented contractions to phenylephrine in endothelium-denuded rat aortas, an effect that was abolished by the Rho kinase inhibitor Y-27632. Incubation of VSMCs with ET-1 increased expression of the phosphorylated forms of myosin phosphatase target subunit 1 (MYPT-1), protein kinase C-potentiated protein phosphatase 1 inhibitor protein (protein kinase C-potentiated phosphatase inhibitor-17), and myosin light chain (MLC) and RhoA expression and activity, and this effect was abolished by both OGT siRNA transfection or OGT inhibition and atrasentan. ET-1 also augmented expression of PDZ-Rho GEF (guanine nucleotide exchange factor) and p115-Rho GEF in VSMCs and this was prevented by OGT siRNA, ST045849, and atrasentan.

CONCLUSION

We suggest that ET-1 augments O-GlcNAcylation and this modification contributes to increased vascular contractile responses via activation of the RhoA/Rho-kinase pathway.

A novel, in-solution separation of endogenous cardiac sarcomeric proteins and identification of distinct charged variants of regulatory light chain

The molecular conformation of the cardiac myosin motor is modulated by intermolecular interactions among the heavy chain, the light chains, myosin binding protein-C, and titin and is governed by post-translational modifications (PTMs). In-gel digestion followed by LC/MS/MS has classically been applied to identify cardiac sarcomeric PTMs; however, this approach is limited by protein size, pI, and difficulties in peptide extraction. We report a solution-based work flow for global separation of endogenous cardiac sarcomeric proteins with a focus on the regulatory light chain (RLC) in which specific sites of phosphorylation have been unclear. Subcellular fractionation followed by OFFGEL electrophoresis resulted in isolation of endogenous charge variants of sarcomeric proteins, including regulatory and essential light chains, myosin heavy chain, and myosin-binding protein-C of the thick filament. Further purification of RLC using reverse-phase HPLC separation and UV detection enriched for RLC PTMs at the intact protein level and provided a stoichiometric and quantitative assessment of endogenous RLC charge variants. Digestion and subsequent LC/MS/MS unequivocally identified that the endogenous charge variants of cardiac RLC focused in unique OFFGEL electrophoresis fractions were unphosphorylated (78.8%), singly phosphorylated (18.1%), and doubly phosphorylated (3.1%) RLC. The novel aspects of this study are that 1) milligram amounts of endogenous cardiac sarcomeric subproteome were focused with resolution comparable with two-dimensional electrophoresis, 2) separation and quantification of post-translationally modified variants were achieved at the intact protein level, 3) separation of intact high molecular weight thick filament proteins was achieved in solution, and 4) endogenous charge variants of RLC were separated; a novel doubly phosphorylated form was identified in mouse, and singly phosphorylated, singly deamidated, and deamidated/phosphorylated forms were identified and quantified in human non-failing and failing heart samples, thus demonstrating the clinical utility of the method.

Mapping of O-linked beta-N-acetylglucosamine modification sites in key contractile proteins of rat skeletal muscle

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a widespread modification of serine/threonine residues of nucleocytoplasmic proteins. Recently, several key contractile proteins in rat skeletal muscle (i.e., myosin heavy and light chains and actin) were identified as O-GlcNAc modified. Moreover, it was demonstrated that O-GlcNAc moieties involved in contractile protein interactions could modulate Ca(2+) activation parameters of contraction. In order to better understand how O-GlcNAc can modulate the contractile activity of muscle fibers, we decided to identify the sites of O-GlcNAc modification in purified contractile protein homogenates. Using an MS-based method that relies on mild beta-elimination followed by Michael addition of DTT (BEMAD), we determined the localization of one O-GlcNAc site in the subdomain four of actin and four O-GlcNAc sites in the light meromyosin region of myosin heavy chains (MHC). According to previous reports concerning the role of these regions, our data suggest that O-GlcNAc sites might modulate the actin-tropomyosin interaction, and be involved in MHC polymerization or interactions between MHC and other contractile proteins. Thus, the results suggest that this PTM might be involved in protein-protein interactions but could also modulate the contractile properties of skeletal muscle.

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.

Protein O-GlcNAcylation: a new signaling paradigm for the cardiovascular system

The posttranslational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic and ubiquitous protein modification. Protein O-GlcNAcylation is rapidly emerging as a key regulator of critical biological processes including nuclear transport, translation and transcription, signal transduction, cytoskeletal reorganization, proteasomal degradation, and apoptosis. Increased levels of O-GlcNAc have been implicated as a pathogenic contributor to glucose toxicity and insulin resistance, which are both major hallmarks of diabetes mellitus and diabetes-related cardiovascular complications. Conversely, there is a growing body of data demonstrating that the acute activation of O-GlcNAc levels is an endogenous stress response designed to enhance cell survival. Reports on the effect of altered O-GlcNAc levels on the heart and cardiovascular system have been growing rapidly over the past few years and have implicated a role for O-GlcNAc in contributing to the adverse effects of diabetes on cardiovascular function as well as mediating the response to ischemic injury. Here, we summarize our present understanding of protein O-GlcNAcylation and its effect on the regulation of cardiovascular function. We examine the pathways regulating protein O-GlcNAcylation and discuss, in more detail, our understanding of the role of O-GlcNAc in both mediating the adverse effects of diabetes as well as its role in mediating cellular protective mechanisms in the cardiovascular system. In addition, we also explore the parallels between O-GlcNAc signaling and redox signaling, as an alternative paradigm for understanding the role of O-GlcNAcylation in regulating cell function.