O-GlcNAcomic Profiling Identifies Widespread O-Linked β-N-Acetylglucosamine Modification (O-GlcNAcylation) in Oxidative Phosphorylation System Regulating Cardiac Mitochondrial Function

Roles of O-GlcNAcylation in Mitochondrial Homeostasis and Cardiovascular Diseases

Protein posttranslational modifications are important factors that mediate the fine regulation of signaling molecules. O-linked β-N-acetylglucosamine-modification (O-GlcNAcylation) is a monosaccharide modification on N-acetylglucosamine linked to the hydroxyl terminus of serine and threonine of proteins. O-GlcNAcylation is responsive to cellular stress as a reversible and posttranslational modification of nuclear, mitochondrial and cytoplasmic proteins. Mitochondrial proteins are the main targets of O-GlcNAcylation and O-GlcNAcylation is a key regulator of mitochondrial homeostasis by directly regulating the mitochondrial proteome or protein activity and function. Disruption of O-GlcNAcylation is closely related to mitochondrial dysfunction. More importantly, the O-GlcNAcylation of cardiac proteins has been proven to be protective or harmful to cardiac function. Mitochondrial homeostasis is crucial for cardiac contractile function and myocardial cell metabolism, and the imbalance of mitochondrial homeostasis plays a crucial role in the pathogenesis of cardiovascular diseases (CVDs). In this review, we will focus on the interactions between protein O-GlcNAcylation and mitochondrial homeostasis and provide insights on the role of mitochondrial protein O-GlcNAcylation in CVDs.

Subcellular imaging of lipids and sugars using genetically encoded proximity sensors

Live cell imaging of lipids and other metabolites is a long-standing challenge in cell biology. Bioorthogonal labeling tools allow for the conjugation of fluorophores to several phospholipid classes, but cannot discern their trafficking between adjacent organelles or asymmetry across individual membrane leaflets. Here we present fluorogen-activating coincidence sensing (FACES), a chemogenetic tool capable of quantitatively imaging subcellular lipid pools and reporting their transbilayer orientation in living cells. FACES combines bioorthogonal chemistry with genetically encoded fluorogen-activating proteins (FAPs) for reversible proximity sensing of conjugated molecules. We first validate this approach for quantifying discrete phosphatidylcholine pools in the ER and mitochondria that are trafficked by lipid transfer proteins. We then show that transmembrane domain-containing FAPs can be used to reveal the membrane asymmetry of multiple lipid classes that are generated in the trans-Golgi network. Lastly, we demonstrate that FACES is a generalizable tool for subcellular bioorthogonal imaging by measuring changes in mitochondrial N -acetylhexosamine levels. These results demonstrate the use of fluorogenic tags for spatially-defined molecular imaging.

O-GlcNAcylation mediates H2O2-induced apoptosis through regulation of STAT3 and FOXO1

The O-linked-β-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) is a critical post-translational modification that couples the external stimuli to intracellular signal transduction networks. However, the critical protein targets of O-GlcNAcylation in oxidative stress-induced apoptosis remain to be elucidated. Here, we show that treatment with H2O2 inhibited O-GlcNAcylation, impaired cell viability, increased the cleaved caspase 3 and accelerated apoptosis of neuroblastoma N2a cells. The O-GlcNAc transferase (OGT) inhibitor OSMI-1 or the O-GlcNAcase (OGA) inhibitor Thiamet-G enhanced or inhibited H2O2-induced apoptosis, respectively. The total and phosphorylated protein levels, as well as the promoter activities of signal transducer and activator of transcription factor 3 (STAT3) and Forkhead box protein O 1 (FOXO1) were suppressed by OSMI-1. In contrast, overexpressing OGT or treating with Thiamet-G increased the total protein levels of STAT3 and FOXO1. Overexpression of STAT3 or FOXO1 abolished OSMI-1-induced apoptosis. Whereas the anti-apoptotic effect of OGT and Thiamet-G in H2O2-treated cells was abolished by either downregulating the expression or activity of endogenous STAT3 or FOXO1. These results suggest that STAT3 or FOXO1 are the potential targets of O-GlcNAcylation involved in the H2O2-induced apoptosis of N2a cells.

Post-translational modifications in diabetic cardiomyopathy

The increasing attention towards diabetic cardiomyopathy as a distinctive complication of diabetes mellitus has highlighted the need for standardized diagnostic criteria and targeted treatment approaches in clinical practice. Ongoing research is gradually unravelling the pathogenesis of diabetic cardiomyopathy, with a particular emphasis on investigating various post-translational modifications. These modifications dynamically regulate protein function in response to changes in the internal and external environment, and their disturbance of homeostasis holds significant relevance for the development of chronic ailments. This review provides a comprehensive overview of the common post-translational modifications involved in the initiation and progression of diabetic cardiomyopathy, including O-GlcNAcylation, phosphorylation, methylation, acetylation and ubiquitination. Additionally, the review discusses drug development strategies for targeting key post-translational modification targets, such as agonists, inhibitors and PROTAC (proteolysis targeting chimaera) technology that targets E3 ubiquitin ligases.

O-GlcNAcylation: a pro-survival response to acute stress in the cardiovascular and central nervous systems

O-GlcNAcylation is a unique monosaccharide modification that is ubiquitously present in numerous nucleoplasmic and mitochondrial proteins. The hexosamine biosynthesis pathway (HBP), which is a key branch of glycolysis, provides the unique sugar donor UDP-GlcNAc for the O-GlcNAc modification. Thus, HBP/O-GlcNAcylation can act as a nutrient sensor to perceive changes in nutrient levels and trigger O-GlcNAc modifications of functional proteins in cellular (patho-)physiology, thereby regulating diverse metabolic processes. An imbalance in O-GlcNAcylation has been shown to be a pathogenic contributor to dysfunction in metabolic diseases, including type 2 diabetes, cancer, and neurodegeneration. However, under acute stress conditions, protein O-GlcNAc modification exhibits rapid and transient upregulation, which is strongly correlated with stress tolerance and cell survival. In this context, we discuss the metabolic, pharmacological and genetic modulation of HBP/O-GlcNAc modification in the biological system, the beneficial role of O-GlcNAcylation in regulating stress tolerance for cardioprotection, and neuroprotection, which is a novel and rapidly growing field. Current evidence suggests that transient activation of the O-GlcNAc modification represents a potent pro-survival signalling pathway and may provide a promising strategy for stress-related disorder therapy.

Increased O-GlcNAcylation by Upregulation of Mitochondrial O-GlcNAc Transferase (mOGT) Inhibits the Activity of Respiratory Chain Complexes and Controls Cellular Bioenergetics

O-linked β-N-acetylglucosamine (O-GlcNAc) is a reversible post-translational modification involved in the regulation of cytosolic, nuclear, and mitochondrial proteins. The interplay between O-GlcNAcylation and phosphorylation is critical to control signaling pathways and maintain cellular homeostasis. The addition of O-GlcNAc moieties to target proteins is catalyzed by O-linked N-acetylglucosamine transferase (OGT). Of the three splice variants of OGT described, one is destined for the mitochondria (mOGT). Although the effects of O-GlcNAcylation on the biology of normal and cancer cells are well documented, the role of mOGT remains poorly understood. In this manuscript, the effects of mOGT on mitochondrial protein phosphorylation, electron transport chain (ETC) complex activity, and the expression of VDAC porins were investigated. We performed studies using normal and breast cancer cells with upregulated mOGT or its catalytically inactive mutant. Proteomic approaches included the isolation of O-GlcNAc-modified proteins of the electron transport chain, followed by their analysis using mass spectrometry. We found that mitochondrial OGT regulates the activity of complexes I-V of the respiratory chain and identified a group of 19 ETC components as mOGT substrates in mammary cells. Furthermore, we observed that the upregulation of mOGT inhibited the interaction of VDAC1 with hexokinase II. Our results suggest that the deregulation of mOGT reprograms cellular energy metabolism via interaction with and O-GlcNAcylation of proteins involved in ATP production in mitochondria and its exchange between mitochondria and the cytosol.

The influence of estrogen on myocardial post-translational modifications and cardiac function in women

The lifetime risk of heart failure (HF) is comparable in men and women; nevertheless, disparities exist in our understanding of how HF differs between sexes. Several differences in cardiac physiology exist between men and women including the propensity to develop specific HF phenotypes. Men are more likely to be diagnosed with HF failure with reduced ejection fraction, while women have a greater propensity to develop HF with preserved ejection fraction. The mechanisms responsible for these differences remain unclear. Post-translational modifications (PTMs) of myofilament proteins likely contribute to these sex-specific propensities. The role of PTMs in heart disease is an expanding field with immense potential therapeutic targets. However, numerous PTMs remain underexplored, particularly in the context of the female heart. Estrogen, a key gonadal hormone, cardioprotective in pre-menopausal women and its loss with menopause likely contributes to disease in aging women. However, how estrogen regulates PTMs to contribute to HF development is not fully clear. This review outlines key sex differences in HF along with characterizing the contributions of novel myocardial PTMs in cardiac physiology and their regulation by estrogen. Collectively, we highlight the necessity for further investigation into women's heart health and the distinctive mechanisms distinguishing women from men.

Oxidative Post-translational Protein Modifications upon Ischemia/Reperfusion Injury

While reperfusion, or restoration of coronary blood flow in acute myocardial infarction, is a requisite for myocardial salvage, it can paradoxically induce a specific damage known as ischemia/reperfusion (I/R) injury. Our understanding of the precise pathophysiological molecular alterations leading to I/R remains limited. In this study, we conducted a comprehensive and unbiased time-course analysis of post-translational modifications (PTMs) in the post-reperfused myocardium of two different animal models (pig and mouse) and evaluated the effect of two different cardioprotective therapies (ischemic preconditioning and neutrophil depletion). In pigs, a first wave of irreversible oxidative damage was observed at the earliest reperfusion time (20 min), impacting proteins essential for cardiac contraction. A second wave, characterized by irreversible oxidation on different residues and reversible Cys oxidation, occurred at late stages (6-12 h), affecting mitochondrial, sarcomere, and inflammation-related proteins. Ischemic preconditioning mitigated the I/R damage caused by the late oxidative wave. In the mouse model, the two-phase pattern of oxidative damage was replicated, and neutrophil depletion mitigated the late wave of I/R-related damage by preventing both Cys reversible oxidation and irreversible oxidation. Altogether, these data identify protein PTMs occurring late after reperfusion as an actionable therapeutic target to reduce the impact of I/R injury.

Discovery of a novel OGT inhibitor through high-throughput screening based on Homogeneous Time-Resolved Fluorescence (HTRF)

O-GlcNAcylation is a specific type of post-translational glycosylation modification, which is regulated by two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Aberrant overexpression of OGT is associated with the development of many solid tumors. In this study, we have developed and optimized a sensitive Homogeneous Time-Resolved Fluorescence (HTRF) assay then identified a novel OGT inhibitor CDDO (also called Bardoxolone) through a high-throughput screening (HTS) based on HTRF assay. Further characterization suggested that CDDO is an effective OGT inhibitor with an IC50 value of 6.56 ± 1.69 μM. CPMG-NMR analysis confirmed that CDDO is a direct binder of OGT with a binding affinity (Kd) of approximately 1.7 μM determined by the MST analysis. Moreover, HDX-MS analysis indicated that CDDO binds to the TPR domain and N-Terminal domain of OGT, which was further confirmed by the enzymatic competition experiments as the binding of CDDO to OGT was not affected by the catalytic site binding inhibitor OSMI-4. Our docking modeling analysis further predicted the possible interactions between CDDO and OGT, providing informative molecular basis for further optimization of the inhibitor in the future. Together, our results suggested CDDO is a new inhibitor of OGT with a distinct binding pocket from the reported OGT inhibitors. Our work paved a new direction for developing OGT inhibitors driven by novel mechanisms.

Foetal recapitulation of nutrient surplus signalling by O-GlcNAcylation and the failing heart

The development of the foetal heart is driven by increased glucose uptake and activation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1α (HIF-1α), which drives glycolysis. In contrast, the healthy adult heart is governed by sirtuin-1 (SIRT1) and adenosine monophosphate-activated protein kinase (AMPK), which promote fatty-acid oxidation and the substantial mitochondrial ATP production required for survival in a high-workload normoxic environment. During cardiac injury, the heart recapitulates the foetal signalling programme, which (although adaptive in the short term) is highly deleterious if sustained for long periods of time. Prolonged increases in glucose uptake in cardiomyocytes under stress leads to increased flux through the hexosamine biosynthesis pathway; its endproduct - uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) - functions as a critical nutrient surplus sensor. UDP-GlcNAc drives the post-translational protein modification known as O-GlcNAcylation, which rapidly and reversibly modifies thousands of intracellular proteins. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated by only two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), which adds or removes GlcNAc (N-acetylglucosamine), respectively, from target proteins. Recapitulation of foetal programming in heart failure (regardless of diabetes) is accompanied by marked increases in O-GlcNAcylation, both experimentally and clinically. Heightened O-GlcNAcylation in the heart leads to impaired calcium kinetics and contractile derangements, arrhythmias related to activation of voltage-gated sodium channels and Ca2+ /calmodulin-dependent protein kinase II, mitochondrial dysfunction, and maladaptive hypertrophy, microvascular dysfunction, fibrosis and cardiomyopathy. These deleterious effects can be prevented by suppression of O-GlcNAcylation, which can be achieved experimentally by upregulation of AMPK and SIRT1 or by pharmacological inhibition of OGT or stimulation of OGA. The effects of sodium-glucose cotransporter 2 (SGLT2) inhibitors on the heart are accompanied by reduced O-GlcNAcylation, and their cytoprotective effects are reportedly abrogated if their action to suppress O-GlcNAcylation is blocked. Such an action may represent one of the many mechanisms by which enhanced AMPK and SIRT1 signalling following SGLT2 inhibition leads to cardiovascular benefits. These observations, taken collectively, suggest that UDP-GlcNAc functions as a critical nutrient surplus sensor (which acting in concert with mTOR and HIF-1α) can promote the development of cardiomyopathy.

Relationships between gene expression and behavior in mice in response to systemic modulation of the O-GlcNAcylation pathway

Enhancing protein O-GlcNAcylation by pharmacological inhibition of the enzyme O-GlcNAcase (OGA), which removes the O-GlcNAc modification from proteins, has been explored in mouse models of amyloid-beta and tau pathology. However, the O-GlcNAcylation-dependent link between gene expression and neurological behavior remains to be explored. Using chronic administration of Thiamet G (TG, an OGA inhibitor) in vivo, we used a protocol designed to relate behavior with the transcriptome and selected biochemical parameters from the cortex of individual animals. TG-treated mice showed improved working memory as measured using a Y-maze test. RNA sequencing analysis revealed 151 top differentially expressed genes with a Log2fold change >0.33 and adjusted p-value <0.05. Top TG-dependent upregulated genes were related to learning, cognition and behavior, while top downregulated genes were related to IL-17 signaling, inflammatory response and chemotaxis. Additional pathway analysis uncovered 3 pathways, involving gene expression including 14 cytochrome c oxidase subunits/regulatory components, chaperones or assembly factors, and 5 mTOR (mechanistic target of rapamycin) signaling factors. Multivariate Kendall correlation analyses of behavioral tests and the top TG-dependent differentially expressed genes revealed 91 statistically significant correlations in saline-treated mice and 70 statistically significant correlations in TG-treated mice. These analyses provide a network regulation landscape that is important in relating the transcriptome to behavior and the potential impact of the O-GlcNAC pathway.

Orchestration of Mitochondrial Function and Remodeling by Post-Translational Modifications Provide Insight into Mechanisms of Viral Infection

The regulation of mitochondria structure and function is at the core of numerous viral infections. Acting in support of the host or of virus replication, mitochondria regulation facilitates control of energy metabolism, apoptosis, and immune signaling. Accumulating studies have pointed to post-translational modification (PTM) of mitochondrial proteins as a critical component of such regulatory mechanisms. Mitochondrial PTMs have been implicated in the pathology of several diseases and emerging evidence is starting to highlight essential roles in the context of viral infections. Here, we provide an overview of the growing arsenal of PTMs decorating mitochondrial proteins and their possible contribution to the infection-induced modulation of bioenergetics, apoptosis, and immune responses. We further consider links between PTM changes and mitochondrial structure remodeling, as well as the enzymatic and non-enzymatic mechanisms underlying mitochondrial PTM regulation. Finally, we highlight some of the methods, including mass spectrometry-based analyses, available for the identification, prioritization, and mechanistic interrogation of PTMs.

IDH2, a novel target of OGT, facilitates glucose uptake and cellular bioenergy production via NF-κB signaling to promote colorectal cancer progression

BACKGROUND

Although isocitrate dehydrogenase 2 (IDH2) mutations have been the hotspots in recent anticancer studies, the impact of wild-type IDH2 on cancer cell growth and metabolic alterations is still elusive.

METHODS

IDH2 expression in CRC tissues was evaluated by immunohistochemistry, and the correlation between the expression level and the patient's survival rate was analyzed. Cell functional assays included CCK8 and colony formation for cell proliferation in vitro and ectopic xenograft as in vivo experimental model for tumor progression. A targeted metabolomic procedure was performed by liquid chromatography/tandem mass spectrometry to profile the metabolites from glycolysis and tricarboxylic acid (TCA) cycle. Mitochondrial function was assessed by measuring cellular oxygen consumption (OCR) and mitochondrial membrane potential (ΔΨ). Confocal microscope analysis and Western blotting were applied to detect the expression of GLUT1 and NF-κB signaling. O-GlcNAcylation and the interaction of IDH2 with OGT were confirmed by co-immunoprecipitation, followed by Western blotting analysis.

RESULTS

IDH2 protein was highly expressed in CRC tissues, and correlated with poor survival of CRC patients. Wild-type IDH2 promoted CRC cell growth in vitro and tumor progression in xenograft mice. Overexpression of wild-type IDH2 significantly increased glycolysis and TCA cycle metabolites, the ratios of NADH/NAD⁺ and ATP/ADP, OCR and mitochondrial membrane potential (ΔΨ) in CRC cells. Furthermore, α-KG activated NF-κB signaling to promote glucose uptake by upregulating GLUT1. Interesting, O-GlcNAcylation enhanced the protein half-time of IDH2 by inhibiting ubiquitin-mediated proteasome degradation. The O-GlcNAc transferase (OGT)-IDH2 axis promoted CRC progression.

CONCLUSION

Wild-type IDH2 reprogrammed glucose metabolism and bioenergetic production via the NF-κB signaling pathway to promote CRC development and progression. O-GlcNAcylation of IDH2 elevated the stability of IDH2 protein. And the axis of OGT-IDH2 played an essential promotive role in tumor progression, suggesting a novel potential therapeutic strategy in CRC treatment.

OGT controls mammalian cell viability by regulating the proteasome/mTOR/ mitochondrial axis

O-GlcNAc transferase (OGT) modifies serine and threonine residues on nuclear and cytosolic proteins with O-linked N-acetylglucosamine (GlcNAc). OGT is essential for mammalian cell viability, but the underlying mechanisms are still enigmatic. We performed a genome-wide CRISPR-Cas9 screen in mouse embryonic stem cells (mESCs) to identify candidates whose depletion rescued the block in cell proliferation induced by OGT deficiency. We show that the block in cell proliferation in OGT-deficient cells stems from mitochondrial dysfunction secondary to mTOR (mechanistic target of rapamycin) hyperactivation. In normal cells, OGT maintains low mTOR activity and mitochondrial fitness through suppression of proteasome activity; in the absence of OGT, increased proteasome activity results in increased steady-state amino acid levels, which in turn promote mTOR lysosomal translocation and activation, and increased oxidative phosphorylation. mTOR activation in OGT-deficient mESCs was confirmed by an independent phospho-proteomic screen. Our study highlights a unique series of events whereby OGT regulates the proteasome/ mTOR/ mitochondrial axis in a manner that maintains homeostasis of intracellular amino acid levels, mitochondrial fitness, and cell viability. A similar mechanism operates in CD8⁺ T cells, indicating its generality across mammalian cell types. Manipulating OGT activity may have therapeutic potential in diseases in which this signaling pathway is impaired.

Integration of O-GlcNAc into Stress Response Pathways

The modification of nuclear, mitochondrial, and cytosolic proteins by O-linked βN-acetylglucosamine (O-GlcNAc) has emerged as a dynamic and essential post-translational modification of mammalian proteins. O-GlcNAc is cycled on and off over 5000 proteins in response to diverse stimuli impacting protein function and, in turn, epigenetics and transcription, translation and proteostasis, metabolism, cell structure, and signal transduction. Environmental and physiological injury lead to complex changes in O-GlcNAcylation that impact cell and tissue survival in models of heat shock, osmotic stress, oxidative stress, and hypoxia/reoxygenation injury, as well as ischemic reperfusion injury. Numerous mechanisms that appear to underpin O-GlcNAc-mediated survival include changes in chaperone levels, impacts on the unfolded protein response and integrated stress response, improvements in mitochondrial function, and reduced protein aggregation. Here, we discuss the points at which O-GlcNAc is integrated into the cellular stress response, focusing on the roles it plays in the cardiovascular system and in neurodegeneration.

Protein O-GlcNAcylation in Metabolic Modulation of Skeletal Muscle: A Bright but Long Way to Go

O-GlcNAcylation is an atypical, dynamic and reversible O-glycosylation that is critical and abundant in metazoan. O-GlcNAcylation coordinates and receives various signaling inputs such as nutrients and stresses, thus spatiotemporally regulating the activity, stability, localization and interaction of target proteins to participate in cellular physiological functions. Our review discusses in depth the involvement of O-GlcNAcylation in the precise regulation of skeletal muscle metabolism, such as glucose homeostasis, insulin sensitivity, tricarboxylic acid cycle and mitochondrial biogenesis. The complex interaction and precise modulation of O-GlcNAcylation in these nutritional pathways of skeletal muscle also provide emerging mechanical information on how nutrients affect health, exercise and disease. Meanwhile, we explored the potential role of O-GlcNAcylation in skeletal muscle pathology and focused on its benefits in maintaining proteostasis under atrophy. In general, these understandings of O-GlcNAcylation are conducive to providing new insights into skeletal muscle (patho) physiology.

Nutrient-sensitive protein O-GlcNAcylation shapes daily biological rhythms

O-linked-N-acetylglucosaminylation (O-GlcNAcylation) is a nutrient-sensitive protein modification that alters the structure and function of a wide range of proteins involved in diverse cellular processes. Similar to phosphorylation, another protein modification that targets serine and threonine residues, O-GlcNAcylation occupancy on cellular proteins exhibits daily rhythmicity and has been shown to play critical roles in regulating daily rhythms in biology by modifying circadian clock proteins and downstream effectors. We recently reported that daily rhythm in global O-GlcNAcylation observed in Drosophila tissues is regulated via the integration of circadian and metabolic signals. Significantly, mistimed feeding, which disrupts coordination of these signals, is sufficient to dampen daily O-GlcNAcylation rhythm and is predicted to negatively impact animal biological rhythms and health span. In this review, we provide an overview of published and potential mechanisms by which metabolic and circadian signals regulate hexosamine biosynthetic pathway metabolites and enzymes, as well as O-GlcNAc processing enzymes to shape daily O-GlcNAcylation rhythms. We also discuss the significance of functional interactions between O-GlcNAcylation and other post-translational modifications in regulating biological rhythms. Finally, we highlight organ/tissue-specific cellular processes and molecular pathways that could be modulated by rhythmic O-GlcNAcylation to regulate time-of-day-specific biology.

Short O-GlcNAcase Is Targeted to the Mitochondria and Regulates Mitochondrial Reactive Oxygen Species Level

O-GlcNAcylation is a reversible post-translational modification involved in the regulation of cytosolic, nuclear, and mitochondrial proteins. Only two enzymes, OGT (O-GlcNAc transferase) and OGA (O-GlcNAcase), control the attachment and removal of O-GlcNAc on proteins, respectively. Whereas a variant OGT (mOGT) has been proposed as the main isoform that O-GlcNAcylates proteins in mitochondria, identification of a mitochondrial OGA has not been performed yet. Two splice variants of OGA (short and long isoforms) have been described previously. In this work, using cell fractionation experiments, we show that short-OGA is preferentially recovered in mitochondria-enriched fractions from HEK-293T cells and RAW 264.7 cells, as well as mouse embryonic fibroblasts. Moreover, fluorescent microscopy imaging confirmed that GFP-tagged short-OGA is addressed to mitochondria. In addition, using a Bioluminescence Resonance Energy Transfer (BRET)-based mitochondrial O-GlcNAcylation biosensor, we show that co-transfection of short-OGA markedly reduced O-GlcNAcylation of the biosensor, whereas long-OGA had no significant effect. Finally, using genetically encoded or chemical fluorescent mitochondrial probes, we show that short-OGA overexpression increases mitochondrial ROS levels, whereas long-OGA has no significant effect. Together, our work reveals that the short-OGA isoform is targeted to the mitochondria where it regulates ROS homoeostasis.

The intra-mitochondrial O-GlcNAcylation system rapidly modulates OXPHOS function and ROS release in the heart

Protein O-GlcNAcylation is increasingly recognized as an important cellular regulatory mechanism, in multiple organs including the heart. However, the mechanisms leading to O-GlcNAcylation in mitochondria and the consequences on their function remain poorly understood. In this study, we use an in vitro reconstitution assay to characterize the intra-mitochondrial O-GlcNAc system without potential cytoplasmic confounding effects. We compare the O-GlcNAcylome of isolated cardiac mitochondria with that of mitochondria acutely exposed to NButGT, a specific inhibitor of glycoside hydrolase. Amongst the 409 O-GlcNAcylated mitochondrial proteins identified, 191 display increased O-GlcNAcylation in response to NButGT. This is associated with enhanced Complex I (CI) activity, increased maximal respiration in presence of pyruvate-malate, and a striking reduction of mitochondrial ROS release, which could be related to O-GlcNAcylation of specific subunits of ETC complexes (CI, CIII) and TCA cycle enzymes. In conclusion, our work underlines the existence of a dynamic mitochondrial O-GlcNAcylation system capable of rapidly modifying mitochondrial function.

An in vitro assay in isolated heart mitochondria reveals that O-GlcNAcase inhibitor NButGT rapidly increases protein O-GlcNAcylation leading to increased respiratory capacity and complex I activity and decreased ROS release.

The Glycobiology of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive pulmonary vascular disease of complex etiology. Cases of PAH that do not receive therapy after diagnosis have a low survival rate. Multiple reports have shown that idiopathic PAH, or IPAH, is associated with metabolic dysregulation including altered bioavailability of nitric oxide (NO) and dysregulated glucose metabolism. Multiple processes such as increased proliferation of pulmonary vascular cells, angiogenesis, apoptotic resistance, and vasoconstriction may be regulated by the metabolic changes demonstrated in PAH. Recent reports have underscored similarities between metabolic abnormalities in cancer and IPAH. In particular, increased glucose uptake and altered glucose utilization have been documented and have been linked to the aforementioned processes. We were the first to report a link between altered glucose metabolism and changes in glycosylation. Subsequent reports have highlighted similar findings, including a potential role for altered metabolism and aberrant glycosylation in IPAH pathogenesis. This review will detail research findings that demonstrate metabolic dysregulation in PAH with an emphasis on glycobiology. Furthermore, this report will illustrate the similarities in the pathobiology of PAH and cancer and highlight the novel findings that researchers have explored in the field.

Demystifying the O-GlcNAc Code: A Systems View

Post-translational modification with O-linked β-N-acetylglucosamine (O-GlcNAc), a process referred to as O-GlcNAcylation, occurs on a vast variety of proteins. Mounting evidence in the past several decades has clearly demonstrated that O-GlcNAcylation is a unique and ubiquitous modification. Reminiscent of a code, protein O-GlcNAcylation functions as a crucial regulator of nearly all cellular processes studied. The primary aim of this review is to summarize the developments in our understanding of myriad protein substrates modified by O-GlcNAcylation from a systems perspective. Specifically, we provide a comprehensive survey of O-GlcNAcylation in multiple species studied, including eukaryotes (e.g., protists, fungi, plants, Caenorhabditis elegans, Drosophila melanogaster, murine, and human), prokaryotes, and some viruses. We evaluate features (e.g., structural properties and sequence motifs) of O-GlcNAc modification on proteins across species. Given that O-GlcNAcylation functions in a species-, tissue-/cell-, protein-, and site-specific manner, we discuss the functional roles of O-GlcNAcylation on human proteins. We focus particularly on several classes of relatively well-characterized human proteins (including transcription factors, protein kinases, protein phosphatases, and E3 ubiquitin-ligases), with representative O-GlcNAc site-specific functions presented. We hope the systems view of the great endeavor in the past 35 years will help demystify the O-GlcNAc code and lead to more fascinating studies in the years to come.

Acute inhibition of OGA sex-dependently alters the networks associated with bioenergetics, autophagy, and neurodegeneration

The accumulation of neurotoxic proteins characteristic of age-related neurodegenerative pathologies such as Alzheimer’s and Parkinson’s diseases is associated with the perturbation of metabolism, bioenergetics, and mitochondrial quality control. One approach to exploit these interactions therapeutically is to target the pathways that regulate metabolism. In this respect, the nutrient-sensing hexosamine biosynthesis pathway is of particular interest since it introduces a protein post-translational modification known as O-GlcNAcylation, which modifies different proteins in control versus neurodegenerative disease postmortem brains. A potent inhibitor of the O-GlcNAcase enzyme that removes the modification from proteins, Thiamet G (TG), has been proposed to have potential benefits in Alzheimer’s disease. We tested whether key factors in the O-GlcNAcylation are correlated with mitochondrial electron transport and proteins related to the autophagy/lysosomal pathways in the cortex of male and female mice with and without exposure to TG (10 mg/kg i.p.). Mitochondrial complex activities were measured in the protein homogenates, and a panel of metabolic, autophagy/lysosomal proteins and O-GlcNAcylation enzymes were assessed by either enzyme activity assay or by western blot analysis. We found that the networks associated with O-GlcNAcylation enzymes and activities with mitochondrial parameters, autophagy-related proteins as well as neurodegenerative disease-related proteins exhibited sex and TG dependent differences. Taken together, these studies provide a framework of interconnectivity for multiple O-GlcNAc-dependent pathways in mouse brain of relevance to aging and sex/age-dependent neurodegenerative pathogenesis and response to potential therapies.

The dual role of the hexosamine biosynthetic pathway in cardiac physiology and pathophysiology

The heart is a highly metabolic organ with extensive energy demands and hence relies on numerous fuel substrates including fatty acids and glucose. However, oxidative stress is a natural by-product of metabolism that, in excess, can contribute towards DNA damage and poly-ADP-ribose polymerase activation. This activation inhibits key glycolytic enzymes, subsequently shunting glycolytic intermediates into non-oxidative glucose pathways such as the hexosamine biosynthetic pathway (HBP). In this review we provide evidence supporting the dual role of the HBP, i.e. playing a unique role in cardiac physiology and pathophysiology where acute upregulation confers cardioprotection while chronic activation contributes to the onset and progression of cardio-metabolic diseases such as diabetes, hypertrophy, ischemic heart disease, and heart failure. Thus although the HBP has emerged as a novel therapeutic target for such conditions, proposed interventions need to be applied in a context- and pathology-specific manner to avoid any potential drawbacks of relatively low cardiac HBP activity.

With No Lysine Kinase 1 Promotes Metabolic Derangements and RV Dysfunction in Pulmonary Arterial Hypertension

Small molecule inhibition of with no lysine kinase 1 (WNK1) (WNK463) signaling activates adenosine monophosphate-activated protein kinase signaling and mitigates membrane enrichment of glucose transporters 1 and 4, which decreases protein O-GlcNAcylation and glycation. Quantitative proteomics of right ventricular (RV) mitochondrial enrichments shows WNK463 prevents down-regulation of several mitochondrial metabolic enzymes. and metabolomics analysis suggests multiple metabolic processes are corrected. Physiologically, WNK463 augments RV systolic and diastolic function independent of pulmonary arterial hypertension severity. Hypochloremia, a condition of predicted WNK1 activation in patients with pulmonary arterial hypertension, is associated with more severe RV dysfunction. These results suggest WNK1 may be a druggable target to combat metabolic dysregulation and may improve RV function and survival in pulmonary arterial hypertension.

Hexosaminidase A (HEXA) regulates hepatic sphingolipid and lipoprotein metabolism in mice

Hexosaminidase A (HexA), a heterodimer consisting of HEXA and HEXB, converts the ganglioside sphingolipid GM2 to GM3 by removing a terminal N-acetyl-d-galactosamine. HexA enzyme deficiency in humans leads to GM2 accumulation in cells, particularly in neurons, and is associated with neurodegeneration. While HexA and sphingolipid metabolism have been extensively investigated in the context of neuronal lipid metabolism, little is known about the metabolic impact of HexA and ganglioside degradation in other tissues. Here, we focussed on the role of HexA in the liver, which is a major regulator of systemic lipid metabolism. We find that hepatic Hexa expression is induced by lipid availability and increased in the presence of hepatic steatosis, which is associated with increased hepatic GM3 content. To assess the impact of HEXA on hepatic lipid metabolism, we used an adeno-associated virus to overexpress HEXA in the livers of high-fat diet fed mice. HEXA overexpression was associated with increased hepatic GM3 content and increased expression of enzymes involved in the degradation of glycated sphingolipids, ultimately driving sphingomyelin accumulation in the liver. In addition, HEXA overexpression led to substantial proteome remodeling in cell surface lipid rafts, which was associated with increased VLDL processing and secretion, hypertriglyceridemia and ectopic lipid accumulation in peripheral tissues. This study established an important role of HEXA in modulating hepatic sphingolipid and lipoprotein metabolism.

O-GlcNAcylation drives calcium signaling toward osteoblast differentiation: A bioinformatics-oriented study

This study aimed to reveal the possible mechanisms by which O-linked-N-acetylglucosaminylation (O-GlcNAcylation) regulates osteoblast differentiation using a series of bioinformatics-oriented experiments. To examine the influence of O-GlcNAcylation levels on osteoblast differentiation, osteoblastic MC3T3-E1 cells were treated with O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) inhibitors. Correlations between the levels of O-GlcNAcylation and the expression of osteogenic markers as well as OGT were evaluated by qPCR and western blotting. The O-GlcNAcylated proteins assumed to correlate with Runx2 expression were retrieved from several public databases and used for further bioinformatics analysis. Following the findings of the bioinformatics analysis, intracellular calcium ([Ca²⁺ ]_i ) was monitored in the cells treated with OGT and OGA inhibitors using a confocal laser-scanning microscope (CLS). The interaction effect between O-GlcNAcylation and [Ca²⁺ ]_i on osteogenic marker expression was determined using stable OGT knockdown MC3T3-E1 cells. O-GlcNAcylation was positively associated with osteoblast differentiation. The time-course profile of global O-GlcNAcylated proteins showed a distinctive pattern with different molecular weights during osteoblast differentiation. The expression pattern of several O-GlcNAcylated proteins was significantly similar to that of Runx2 expression. Bioinformatic analysis of the retrieved Runx2-related-O-GlcNAcylated-proteins revealed the importance of [Ca²⁺ ]_i . CLS showed that alteration of O-GlcNAcylation rapidly changed [Ca²⁺ ]_i in MC3T3-E1 cells. O-GlcNAcylation and [Ca²⁺ ]_i showed an interaction effect on the expression of osteogenic markers. OGT knockdown disrupted the [Ca²⁺ ]_i -induced expression changes of osteogenic markers. O-GlcNAcylation interacts with [Ca²⁺ ]_i and elicits osteoblast differentiation by regulating the expression of osteogenic markers.

Enhancement of O-GlcNAcylation on Mitochondrial Proteins with 2-(4-Methoxyphenyl)ethyl-2-acetamido-2-deoxy-β-d-pyranoside, Contributes to the Mitochondrial Network, Cellular Bioenergetics and Stress Response in Neuronal Cells under Ischemic-like Conditions

O-GlcNAcylation is a nutrient-driven post-translational modification known as a metabolic sensor that links metabolism to cellular function. Recent evidences indicate that the activation of O-GlcNAc pathway is a potential pro-survival pathway and that acute enhancement of this response is conducive to the survival of cells and tissues. 2-(4-Methoxyphenyl)ethyl-2-acetamido-2-deoxy-β-d-pyranoside (SalA-4g), is a salidroside analogue synthesized in our laboratory by chemical structure-modification, with a phenyl ring containing a para-methoxy group and a sugar ring consisting of N-acetylglucosamine. We have previously shown that SalA-4g elevates levels of protein O-GlcNAc and improves neuronal tolerance to ischemia. However, the specific target of SalA-4g regulating O-GlcNAcylation remains unknown. To address these questions, in this study, we have focused on mitochondrial network homeostasis mediated by O-GlcNAcylation in SalA-4g’s neuroprotection in primary cortical neurons under ischemic-like conditions. O-GlcNAc-modified mitochondria induced by SalA-4g demonstrated stronger neuroprotection under oxygen glucose deprivation and reoxygenation stress, including the improvement of mitochondrial homeostasis and bioenergy, and inhibition of mitochondrial apoptosis pathway. Blocking mitochondrial protein O-GlcNAcylation with OSMI-1 disrupted mitochondrial network homeostasis and antagonized the protective effects of SalA-4g. Collectively, these data demonstrate that mitochondrial homeostasis mediated by mitochondrial protein O-GlcNAcylation is critically involved in SalA-4g neuroprotection.

OGT Protein Interaction Network (OGT-PIN): A Curated Database of Experimentally Identified Interaction Proteins of OGT

Interactions between proteins are essential to any cellular process and constitute the basis for molecular networks that determine the functional state of a cell. With the technical advances in recent years, an astonishingly high number of protein–protein interactions has been revealed. However, the interactome of O-linked N-acetylglucosamine transferase (OGT), the sole enzyme adding the O-linked β-N-acetylglucosamine (O-GlcNAc) onto its target proteins, has been largely undefined. To that end, we collated OGT interaction proteins experimentally identified in the past several decades. Rigorous curation of datasets from public repositories and O-GlcNAc-focused publications led to the identification of up to 929 high-stringency OGT interactors from multiple species studied (including Homo sapiens, Mus musculus, Rattus norvegicus, Drosophila melanogaster, Arabidopsis thaliana, and others). Among them, 784 human proteins were found to be interactors of human OGT. Moreover, these proteins spanned a very diverse range of functional classes (e.g., DNA repair, RNA metabolism, translational regulation, and cell cycle), with significant enrichment in regulating transcription and (co)translation. Our dataset demonstrates that OGT is likely a hub protein in cells. A webserver OGT-Protein Interaction Network (OGT-PIN) has also been created, which is freely accessible.

Protein O-GlcNAcylation in the heart

O-GlcNAcylation is a ubiquitous post-translational modification that is extremely labile and plays a significant role in physiology, including the heart. Sustained activation of cardiac O-GlcNAcylation is frequently associated with alterations in cellular metabolism, leading to detrimental effects on cardiovascular function. This is particularly true during conditions such as diabetes, hypertension, cardiac remodelling, heart failure and arrhythmogenesis. Paradoxically, transient elevation of cardiac protein O-GlcNAcylation can also exert beneficial effects in the heart. There is compelling evidence to suggest that a complex interaction between O-GlcNAcylation and phosphorylation also exists in the heart. Beyond direct functional consequences on cardiomyocytes, O-GlcNAcylation also acts indirectly by altering the function of transcription factors that affect downstream signalling. This review focuses on the potential cardioprotective role of protein O-GlcNAcylation during ischaemia-reperfusion injury, the deleterious consequences of chronically elevated O-GlcNAc levels, the interplay between O-GlcNAcylation and phosphorylation in the cardiomyocytes and the effects of O-GlcNAcylation on other major non-myocyte cell types in the heart.

Male and Female Rats Have Different Physiological Response to High-Fat High-Sucrose Diet but Similar Myocardial Sensitivity to Ischemia-Reperfusion Injury

Prediabetes is a strong predictor of type 2 diabetes and its associated cardiovascular complications, but few studies explore sexual dimorphism in this context. Here, we aim to determine whether sex influences physiological response to high-fat high-sucrose diet (HFS) and myocardial tolerance to ischemia-reperfusion injury. Male and female Wistar rats were subjected to standard (CTRL) or HFS diet for 5 months. Then, ex-vivo experiments on isolated perfused heart model were performed to evaluate tolerance to ischemia-reperfusion injury. HFS diet induced fasting hyperglycemia and increased body fat percent to a similar level in both sexes. However, glucose intolerance was more pronounced in female HFS. Cholesterol was increased only in female while male displayed higher level of plasmatic leptin. We observed increased heart weight to tibia length ratio only in males, but we showed a similar decrease in tolerance to ischemia-reperfusion injury in female and male HFS compared with respective controls, characterized by impaired cardiac function, energy metabolism and coronary flow during reperfusion. In conclusion, as soon as glucose intolerance and hyperglycemia develop, we observe higher sensitivity of hearts to ischemia-reperfusion injury without difference between males and females.

A broadly active fucosyltransferase LmjFUT1 whose mitochondrial localization and activity are essential in parasitic Leishmania

Glycoconjugates play major roles in the infectious cycle of the trypanosomatid parasite Leishmania While GDP-Fucose synthesis is essential, fucosylated glycoconjugates have not been reported in Leishmania major [H. Guo et al., J. Biol. Chem. 292, 10696-10708 (2017)]. Four predicted fucosyltransferases appear conventionally targeted to the secretory pathway; SCA1/2 play a role in side-chain modifications of lipophosphoglycan, while gene deletion studies here showed that FUT2 and SCAL were not essential. Unlike most eukaryotic glycosyltransferases, the predicted α 1-2 fucosyltransferase encoded by FUT1 localized to the mitochondrion. A quantitative "plasmid segregation" assay, expressing FUT1 from the multicopy episomal pXNG vector in a chromosomal null ∆fut1- background, established that FUT1 is essential. Similarly, "plasmid shuffling" confirmed that both enzymatic activity and mitochondrial localization were required for viability, comparing import-blocked or catalytically inactive enzymes, respectively. Enzymatic assays of tagged proteins expressed in vivo or of purified recombinant FUT1 showed it had a broad fucosyltransferase activity including glycan and peptide substrates. Unexpectedly, a single rare ∆fut1- segregant (∆fut1s ) was obtained in rich media, which showed severe growth defects accompanied by mitochondrial dysfunction and loss, all of which were restored upon FUT1 reexpression. Thus, FUT1 along with the similar Trypanosoma brucei enzyme TbFUT1 [G. Bandini et al., bioRxiv, https://www.biorxiv.org/content/10.1101/726117v2 (2021)] joins the eukaryotic O-GlcNAc transferase isoform as one of the few glycosyltransferases acting within the mitochondrion. Trypanosomatid mitochondrial FUT1s may offer a facile system for probing mitochondrial glycosylation in a simple setting, and their essentiality for normal growth and mitochondrial function renders it an attractive target for chemotherapy of these serious human pathogens.

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

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

Mitochondrial O-GlcNAc Transferase Interacts with and Modifies Many Proteins and Its Up-Regulation Affects Mitochondrial Function and Cellular Energy Homeostasis

O-GlcNAcylation is a cell glucose sensor. The addition of O-GlcNAc moieties to target protein is catalyzed by the O-Linked N-acetylglucosamine transferase (OGT). OGT is encoded by a single gene that yields differentially spliced OGT isoforms. One of them is targeted to mitochondria (mOGT). Although the impact of O-GlcNAcylation on cancer cells biology is well documented, mOGT's role remains poorly investigated. We performed studies using breast cancer cells with up-regulated mOGT or its catalytic inactive mutant to identify proteins specifically modified by mOGT. Proteomic approaches included isolation of mOGT protein partners and O-GlcNAcylated proteins from mitochondria-enriched fraction followed by their analysis by mass spectrometry. Moreover, we analyzed the impact of mOGT dysregulation on mitochondrial activity and cellular metabolism using a variety of biochemical assays. We found that mitochondrial OGT expression is glucose-dependent. Elevated mOGT expression affected the mitochondrial transmembrane potential and increased intramitochondrial ROS generation. Moreover, mOGT up-regulation caused a decrease in cellular ATP level. We identified many mitochondrial proteins as mOGT substrates. Most of these proteins are localized in the mitochondrial matrix and the inner mitochondrial membrane and participate in mitochondrial respiration, fatty acid metabolism, transport, translation, apoptosis, and mtDNA processes. Our findings suggest that mOGT interacts with and modifies many mitochondrial proteins, and its dysregulation affects cellular bioenergetics and mitochondria function.

O-GlcNAcylation is a key regulator of multiple cellular metabolic pathways

O-GlcNAcylation modifies proteins in serine or threonine residues in the nucleus, cytoplasm, and mitochondria. It regulates a variety of cellular biological processes and abnormal O-GlcNAcylation is associated with diabetes, cancer, cardiovascular disease, and neurodegenerative diseases. Recent evidence has suggested that O-GlcNAcylation acts as a nutrient sensor and signal integrator to regulate metabolic signaling, and that dysregulation of its metabolism may be an important indicator of pathogenesis in disease. Here, we review the literature focusing on O-GlcNAcylation regulation in major metabolic processes, such as glucose metabolism, mitochondrial oxidation, lipid metabolism, and amino acid metabolism. We discuss its role in physiological processes, such as cellular nutrient sensing and homeostasis maintenance. O-GlcNAcylation acts as a key regulator in multiple metabolic processes and pathways. Our review will provide a better understanding of how O-GlcNAcylation coordinates metabolism and integrates molecular networks.

Excessive O-GlcNAcylation Causes Heart Failure and Sudden Death

BACKGROUND

Heart failure is a leading cause of death worldwide and is associated with the rising prevalence of obesity, hypertension, and diabetes. O-GlcNAcylation (the attachment of O-linked β-N-acetylglucosamine [O-GlcNAc] moieties to cytoplasmic, nuclear, and mitochondrial proteins) is a posttranslational modification of intracellular proteins and serves as a metabolic rheostat for cellular stress. Total levels of O-GlcNAcylation are determined by nutrient and metabolic flux, in addition to the net activity of 2 enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Failing myocardium is marked by increased O-GlcNAcylation, but whether excessive O-GlcNAcylation contributes to cardiomyopathy and heart failure is unknown.

METHODS

We developed 2 new transgenic mouse models with myocardial overexpression of OGT and OGA to control O-GlcNAcylation independent of pathologic stress.

RESULTS

We found that OGT transgenic hearts showed increased O-GlcNAcylation and developed severe dilated cardiomyopathy, ventricular arrhythmias, and premature death. In contrast, OGA transgenic hearts had lower O-GlcNAcylation but identical cardiac function to wild-type littermate controls. OGA transgenic hearts were resistant to pathologic stress induced by pressure overload with attenuated myocardial O-GlcNAcylation levels after stress and decreased pathologic hypertrophy compared with wild-type controls. Interbreeding OGT with OGA transgenic mice rescued cardiomyopathy and premature death, despite persistent elevation of myocardial OGT. Transcriptomic and functional studies revealed disrupted mitochondrial energetics with impairment of complex I activity in hearts from OGT transgenic mice. Complex I activity was rescued by OGA transgenic interbreeding, suggesting an important role for mitochondrial complex I in O-GlcNAc-mediated cardiac pathology.

CONCLUSIONS

Our data provide evidence that excessive O-GlcNAcylation causes cardiomyopathy, at least in part, attributable to defective energetics. Enhanced OGA activity is well tolerated and attenuation of O-GlcNAcylation is beneficial against pressure overload-induced pathologic remodeling and heart failure. These findings suggest that attenuation of excessive O-GlcNAcylation may represent a novel therapeutic approach for cardiomyopathy.

High-Fat Diet Leads to Reduced Protein O-GlcNAcylation and Mitochondrial Defects Promoting the Development of Alzheimer's Disease Signatures

The disturbance of protein O-GlcNAcylation is emerging as a possible link between altered brain metabolism and the progression of neurodegeneration. As observed in brains with Alzheimer's disease (AD), flaws of the cerebral glucose uptake translate into reduced protein O-GlcNAcylation, which promote the formation of pathological hallmarks. A high-fat diet (HFD) is known to foster metabolic dysregulation and insulin resistance in the brain and such effects have been associated with the reduction of cognitive performances. Remarkably, a significant role in HFD-related cognitive decline might be played by aberrant protein O-GlcNAcylation by triggering the development of AD signature and mitochondrial impairment. Our data support the impairment of total protein O-GlcNAcylation profile both in the brain of mice subjected to a 6-week high-fat-diet (HFD) and in our in vitro transposition on SH-SY5Y cells. The reduction of protein O-GlcNAcylation was associated with the development of insulin resistance, induced by overfeeding (i.e., defective insulin signaling and reduced mitochondrial activity), which promoted the dysregulation of the hexosamine biosynthetic pathway (HBP) flux, through the AMPK-driven reduction of GFAT1 activation. Further, we observed that a HFD induced the selective impairment of O-GlcNAcylated-tau and of O-GlcNAcylated-Complex I subunit NDUFB8, thus resulting in tau toxicity and reduced respiratory chain functionality respectively, highlighting the involvement of this posttranslational modification in the neurodegenerative process.

mtDNA Heteroplasmy at the Core of Aging-Associated Heart Failure. An Integrative View of OXPHOS and Mitochondrial Life Cycle in Cardiac Mitochondrial Physiology

The most common aging-associated diseases are cardiovascular diseases which affect 40% of elderly people. Elderly people are prone to suffer aging-associated diseases which are not only related to health and medical cost but also to labor, household productivity and mortality cost. Aging is becoming a world problem and it is estimated that 21.8% of global population will be older than 65 years old in 2050; and for the first time in human history, there will be more elderly people than children. It is well accepted that the origin of aging-associated cardiovascular diseases is mitochondrial dysfunction. Mitochondria have their own genome (mtDNA) that is circular, double-stranded, and 16,569 bp long in humans. There are between 500 to 6000 mtDNA copies per cell which are tissue-specific. As a by-product of ATP production, reactive oxygen species (ROS) are generated which damage proteins, lipids, and mtDNA. ROS-mutated mtDNA co-existing with wild type mtDNA is called mtDNA heteroplasmy. The progressive increase in mtDNA heteroplasmy causes progressive mitochondrial dysfunction leading to a loss in their bioenergetic capacity, disruption in the balance of mitochondrial fusion and fission events (mitochondrial dynamics, MtDy) and decreased mitophagy. This failure in mitochondrial physiology leads to the accumulation of depolarized and ROS-generating mitochondria. Thus, besides attenuated ATP production, dysfunctional mitochondria interfere with proper cellular metabolism and signaling pathways in cardiac cells, contributing to the development of aging-associated cardiovascular diseases. In this context, there is a growing interest to enhance mitochondrial function by decreasing mtDNA heteroplasmy. Reduction in mtDNA heteroplasmy is associated with increased mitophagy, proper MtDy balance and mitochondrial biogenesis; and those processes can delay the onset or progression of cardiovascular diseases. This has led to the development of mitochondrial therapies based on the application of nutritional, pharmacological and genetic treatments. Those seeking to have a positive impact on mtDNA integrity, mitochondrial biogenesis, dynamics and mitophagy in old and sick hearts. This review covers the current knowledge of mitochondrial physiopathology in aging, how disruption of OXPHOS or mitochondrial life cycle alter mtDNA and cardiac cell function; and novel mitochondrial therapies to protect and rescue our heart from cardiovascular diseases.

Fine-tuning the cardiac O-GlcNAcylation regulatory enzymes governs the functional and structural phenotype of the diabetic heart

AIMS

The glucose-driven enzymatic modification of myocardial proteins by the sugar moiety, β-N-acetylglucosamine (O-GlcNAc), is increased in pre-clinical models of diabetes, implicating protein O-GlcNAc modification in diabetes-induced heart failure. Our aim was to specifically examine cardiac manipulation of the two regulatory enzymes of this process on the cardiac phenotype, in the presence and absence of diabetes, utilising cardiac-targeted recombinant-adeno-associated viral-vector-6 (rAAV6)-mediated gene delivery.

METHODS AND RESULTS

In human myocardium, total protein O-GlcNAc modification was elevated in diabetic relative to non-diabetic patients, and correlated with left ventricular (LV) dysfunction. The impact of rAAV6-delivered O-GlcNAc transferase (rAAV6-OGT, facilitating protein O-GlcNAcylation), O-GlcNAcase (rAAV6-OGA, facilitating de-O-GlcNAcylation) and empty vector (null) were determined in non-diabetic and diabetic mice. In non-diabetic mice, rAAV6-OGT was sufficient to impair LV diastolic function and induce maladaptive cardiac remodelling, including cardiac fibrosis and increased Myh-7 and Nppa pro-hypertrophic gene expression, recapitulating characteristics of diabetic cardiomyopathy. In contrast, rAAV6-OGA (but not rAAV6-OGT) rescued LV diastolic function and adverse cardiac remodelling in diabetic mice. Molecular insights implicated impaired cardiac PI3K(p110α)-Akt signalling as a potential contributing mechanism to the detrimental consequences of rAAV6-OGT in vivo. In contrast, rAAV6-OGA preserved PI3K(p110α)-Akt signalling in diabetic mouse myocardium in vivo and prevented high glucose-induced impairments in mitochondrial respiration in human cardiomyocytes in vitro.

CONCLUSION

Maladaptive protein O-GlcNAc modification is evident in human diabetic myocardium, and is a critical regulator of the diabetic heart phenotype. Selective targeting of cardiac protein O-GlcNAcylation to restore physiological O-GlcNAc balance may represent a novel therapeutic approach for diabetes-induced heart failure.

TRANSLATIONAL PERSPECTIVE

There remains a lack of effective clinical management of diabetes-induced cardiac dysfunction, even via conventional intensive glucose-lowering approaches. Here we reveal that the modification of myocardial proteins by O-GlcNAc, a glucose-driven process, is not only increased in human diabetic myocardium, but correlates with reduced cardiac function in affected patients. Moreover, manipulation of the two regulatory enzymes of this process exert opposing influences on the heart, whereby increasing O-GlcNAc transferase (OGT) is sufficient to replicate the cardiac phenotype of diabetes (in the absence of this disease), while increasing O-GlcNAc-ase (OGA) rescues diabetes-induced impairments in both cardiac dysfunction and remodelling. Cardiac O-GlcNAcylation thus represents a novel therapeutic target for diabetes-induced heart failure.

O-GlcNAcAtlas: A database of experimentally identified O-GlcNAc sites and proteins

O-linked β-N-acetylglucosamine (O-GlcNAc) is a post-translational modification (i.e., O-GlcNAcylation) on the serine/threonine residues of proteins. As a unique intracellular monosaccharide modification, protein O-GlcNAcylation plays important roles in almost all biochemical processes examined. Aberrant O-GlcNAcylation underlies the etiologies of a number of chronic diseases. With the tremendous improvement of techniques, thousands of proteins along with their O-GlcNAc sites have been reported. However, until now, there are few databases dedicated to accommodate the rapid accumulation of such information. Thus, O-GlcNAcAtlas is created to integrate all experimentally identified O-GlcNAc sites and proteins. O-GlcNAcAtlas consists of two datasets (Dataset-I and Dataset-II, for unambiguously identified sites and ambiguously identified sites, respectively), representing a total number of 4571 O-GlcNAc modified proteins from all species studied from 1984 to 31 Dec 2019. For each protein, comprehensive information (including species, sample type, gene symbol, modified peptides and/or modification sites, site mapping methods and literature references) is provided. To solve the heterogeneity among the data collected from different sources, the sequence identity of these reported O-GlcNAc peptides are mapped to the UniProtKB protein entries. To our knowledge, O-GlcNAcAtlas is a highly comprehensive and rigorously curated database encapsulating all O-GlcNAc sites and proteins identified in the past 35 years. We expect that O-GlcNAcAtlas will be a useful resource to facilitate O-GlcNAc studies and computational analyses of protein O-GlcNAcylation. The public version of the web interface to the O-GlcNAcAtlas can be found at http://oglcnac.org/.

A Global Profile of Reversible and Irreversible Cysteine Redox Post-Translational Modifications During Myocardial Ischemia/Reperfusion Injury and Antioxidant Intervention

Aims: Cysteine (Cys) is a major target for redox post-translational modifications (PTMs) that occur in response to changes in the cellular redox environment. We describe multiplexed, peptide-based enrichment and quantitative mass spectrometry (MS) applied to globally profile reversible redox Cys PTM in rat hearts during ischemia/reperfusion (I/R) in the presence or absence of an aminothiol antioxidant, N-2-mercaptopropionylglycine (MPG). Parallel fractionation also allowed identification of irreversibly oxidized Cys peptides (Cys-SO₂H/SO₃H). Results: We identified 4505 reversibly oxidized Cys peptides of which 1372 were significantly regulated by ischemia and/or I/R. An additional 219 peptides (247 sites) contained Cys-SO₂H/Cys-SO₃H modifications, and these were predominantly identified from hearts subjected to I/R (n = 168 peptides). Parallel reaction monitoring MS (PRM-MS) enabled relative quantitation of 34 irreversibly oxidized Cys peptides. MPG attenuated a large cluster of I/R-associated reversibly oxidized Cys peptides and irreversible Cys oxidation to less than nonischemic controls (n = 24 and 34 peptides, respectively). PRM-MS showed that Cys sites oxidized during ischemia and/or I/R and "protected" by MPG were largely mitochondrial, and were associated with antioxidant functions (peroxiredoxins 5 and 6) and metabolic processes, including glycolysis. Metabolomics revealed I/R induced changes in glycolytic intermediates that were reversed in the presence of MPG, which were consistent with irreversible PTM of triose phosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), altered GAPDH enzyme activity, and reduced I/R glycolytic payoff as evidenced by adenosine triphosphate and NADH levels. Innovation: Novel enrichment and PRM-MS approaches developed here enabled large-scale relative quantitation of Cys redox sites modified by reversible and irreversible PTM during I/R and antioxidant remediation. Conclusions: Cys sites identified here are targets of reactive oxygen species that can contribute to protein dysfunction and the pathogenesis of I/R.

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.

Excess Protein O-GlcNAcylation Links Metabolic Derangements to Right Ventricular Dysfunction in Pulmonary Arterial Hypertension

The hexosamine biosynthetic pathway (HBP) converts glucose to uridine-diphosphate-N-acetylglucosamine, which, when added to serines or threonines, modulates protein function through protein O-GlcNAcylation. Glutamine-fructose-6-phosphate amidotransferase (GFAT) regulates HBP flux, and AMP-kinase phosphorylation of GFAT blunts GFAT activity and O-GlcNAcylation. While numerous studies demonstrate increased right ventricle (RV) glucose uptake in pulmonary arterial hypertension (PAH), the relationship between O-GlcNAcylation and RV function in PAH is unexplored. Therefore, we examined how colchicine-mediated AMP-kinase activation altered HBP intermediates, O-GlcNAcylation, mitochondrial function, and RV function in pulmonary artery-banded (PAB) and monocrotaline (MCT) rats. AMPK activation induced GFAT phosphorylation and reduced HBP intermediates and O-GlcNAcylation in MCT but not PAB rats. Reduced O-GlcNAcylation partially restored the RV metabolic signature and improved RV function in MCT rats. Proteomics revealed elevated expression of O-GlcNAcylated mitochondrial proteins in MCT RVs, which fractionation studies corroborated. Seahorse micropolarimetry analysis of H9c2 cardiomyocytes demonstrated colchicine improved mitochondrial function and reduced O-GlcNAcylation. Presence of diabetes in PAH, a condition of excess O-GlcNAcylation, reduced RV contractility when compared to nondiabetics. Furthermore, there was an inverse relationship between RV contractility and HgbA1C. Finally, RV biopsy specimens from PAH patients displayed increased O-GlcNAcylation. Thus, excess O-GlcNAcylation may contribute to metabolic derangements and RV dysfunction in PAH.

Glutaredoxin-2 and Sirtuin-3 deficiencies impair cardiac mitochondrial energetics but their effects are not additive

Altered redox biology and oxidative stress have been implicated in the progression of heart failure. Glutaredoxin-2 (GRX2) is a glutathione-dependent oxidoreductase and catalyzes the reversible deglutathionylation of mitochondrial proteins. Sirtuin-3 (SIRT3) is a class III histone deacetylase and regulates lysine acetylation in mitochondria. Both GRX2 and SIRT3 are considered as key in the protection against oxidative damage in the myocardium. Knockout of either contributes to adverse heart pathologies including hypertrophy, hypertension, and cardiac dysfunction. Here, we created and characterized a GRX2 and SIRT3 double-knockout mouse model, hypothesizing that their deletions would have an additive effect on oxidative stress, and exacerbate mitochondrial function and myocardial structural remodeling. Wildtype, single-gene knockout (Sirt3^-/-, Grx2^-/-), and double-knockout mice (Grx2^-/-/Sirt3^-/-) were compared in heart weight, histology, mitochondrial respiration and H₂O₂ production. Overall, the hearts from Grx2^-/-/Sirt3^-/- mice displayed increased fibrosis and hypertrophy versus wildtype. In the Grx2^-/- and the Sirt3^-/- we observed changes in mitochondrial oxidative capacity, however this was associated with elevated H₂O₂ emission only in the Sirt3^-/-. Similar changes were observed but not worsened in hearts from Grx2^-/-/Sirt3^-/- mice, suggesting that these changes were not additive. In human myocardium, using genetic and histopathological data from the human Genotype-Tissue Expression consortium, we confirmed that SIRT3 expression correlates inversely with heart pathology. Altogether, GRX2 and SIRT3 are important in the control of cardiac mitochondrial redox and oxidative processes, but their combined absence does not exacerbate effects, consistent with the overall conclusion that they function together in the complex redox and antioxidant systems in the heart.

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.

The O-GlcNAc Modification on Kinases

O-Linked N-acetyl glucosamine (O-GlcNAc) is a protein modification found on thousands of nuclear, cytosolic, and mitochondrial proteins. Many O-GlcNAc sites occur in proximity to protein sites that are likewise modified by phosphorylation. While several studies have uncovered crosstalk between these two signaling modifications on individual proteins and pathways, an understanding of the role of O-GlcNAc in regulating kinases, the enzymes that install the phosphate modification, is still emerging. Here we review recent methods to profile the O-GlcNAc modification on a global scale that have revealed more than 100 kinases are modified by O-GlcNAc and highlight existing studies about regulation of these kinases by O-GlcNAc. Continuing efforts to profile the O-GlcNAc proteome and understand the role of O-GlcNAc on kinases will reveal new mechanisms of regulation and potential avenues for manipulation of the signaling mechanisms at the intersection of O-GlcNAc and phosphorylation.

Angiotensin-(1-7) Attenuates Protein O-GlcNAcylation in the Retina by EPAC/Rap1-Dependent Inhibition of O-GlcNAc Transferase

Purpose

O-GlcNAcylation of cellular proteins contributes to the pathophysiology of diabetes and evidence supports a role for augmented O-GlcNAcylation in diabetic retinopathy. The aim of this study was to investigate the impact of the renin-angiotensin system on retinal protein O-GlcNAcylation.

Methods

Mice fed a high-fat diet were treated chronically with the angiotensin-converting enzyme inhibitor captopril or captopril plus the angiotensin-(1-7) Mas receptor antagonist A779. Western blotting and quantitative polymerase chain reaction were used to analyze retinal homogenates. Similar analyses were performed on lysates from human MIO-M1 retinal Müller cell cultures exposed to media supplemented with angiotensin-(1-7). Culture conditions were manipulated to influence the hexosamine biosynthetic pathway and/or signaling downstream of the Mas receptor.

Results

In the retina of mice fed a high-fat diet, captopril attenuated protein O-GlcNAcylation in a manner dependent on Mas receptor activation. In MIO-M1 cells, angiotensin-(1-7) or adenylate cyclase activation were sufficient to enhance cyclic AMP (cAMP) levels and inhibit O-GlcNAcylation. The repressive effect of cAMP on O-GlcNAcylation was dependent on exchange protein activated by cAMP (EPAC), but not protein kinase A, and was recapitulated by a constitutively active variant of the small GTPase Rap1. We provide evidence that cAMP and angiotensin-(1-7) act to suppress O-GlcNAcylation by inhibition of O-GlcNAc transferase (OGT) activity. In cells exposed to an O-GlcNAcase inhibitor or hyperglycemic culture conditions, mitochondrial superoxide levels were elevated; however, angiotensin-(1-7) signaling prevented the effect.

Conclusions

Angiotensin-(1-7) inhibits retinal protein O-GlcNAcylation via an EPAC/Rap1/OGT signaling axis.