Cardiac O-GlcNAc signaling is increased in hypertrophy and heart failure

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.

Emerging roles of protein O-GlcNAcylation in cardiovascular diseases: Insights and novel therapeutic targets

Cardiovascular diseases (CVDs) are the leading cause of death globally. While the major focus of pharmacological and non-pharmacological interventions has been on targeting disease pathophysiology and limiting predisposing factors, our understanding of the cellular and molecular mechanisms underlying the pathogenesis of CVDs remains incomplete. One mechanism that has recently emerged is protein O-GlcNAcylation. This is a dynamic, site-specific reversible post-translational modification of serine and threonine residues on target proteins and is controlled by two enzymes: O-linked β-N-acetylglucosamine transferase (OGT) and O-linked β-N-acetylglucosaminidase (OGA). Protein O-GlcNAcylation alters the cellular functions of these target proteins which play vital roles in pathways that modulate vascular homeostasis and cardiac function. Through this review, we aim to give insights on the role of protein O-GlcNAcylation in cardiovascular diseases and identify potential therapeutic targets in this pathway for development of more effective medicines to improve patient outcomes.

Exercise Training Stabilizes RyR2-Dependent Ca2+ Release in Post-infarction Heart Failure

Aim: Dysfunction of the cardiac ryanodine receptor (RyR2) is an almost ubiquitous finding in animal models of heart failure (HF) and results in abnormal Ca²⁺ release in cardiomyocytes that contributes to contractile impairment and arrhythmias. We tested whether exercise training (ET), as recommended by current guidelines, had the potential to stabilize RyR2-dependent Ca²⁺ release in rats with post-myocardial infarction HF.

Materials and Methods: We subjected male Wistar rats to left coronary artery ligation or sham operations. After 1 week, animals were characterized by echocardiography and randomized to high-intensity interval ET on treadmills or to sedentary behavior (SED). Running speed was adjusted based on a weekly VO_2max test. We repeated echocardiography after 5 weeks of ET and harvested left ventricular cardiomyocytes for analysis of RyR2-dependent systolic and spontaneous Ca²⁺ release. Phosphoproteins were analyzed by Western blotting, and beta-adrenoceptor density was quantified by radioligand binding.

Results: ET increased VO_2max in HF-ET rats to 127% of HF-SED (P < 0.05). This coincided with attenuated spontaneous SR Ca²⁺ release in left ventricular cardiomyocytes from HF-ET but also reduced Ca²⁺ transient amplitude and slowed Ca²⁺ reuptake during adrenoceptor activation. However, ventricular diameter and fractional shortening were unaffected by ET. Analysis of Ca²⁺ homeostasis and major proteins involved in the regulation of SR Ca²⁺ release and reuptake could not explain the attenuated spontaneous SR Ca²⁺ release or reduced Ca²⁺ transient amplitude. Importantly, measurements of beta-adrenoceptors showed a normalization of beta₁-adrenoceptor density and beta₁:beta₂-adrenoceptor ratio in HF-ET.

Conclusion: ET increased aerobic capacity in post-myocardial infarction HF rats and stabilized RyR2-dependent Ca²⁺ release. Our data show that these effects of ET can be gained without major alterations in SR Ca²⁺ regulatory proteins and indicate that future studies should include upstream parts of the sympathetic signaling pathway.

Cardiomyocyte Oga haploinsufficiency increases O-GlcNAcylation but hastens ventricular dysfunction following myocardial infarction

Rationale

The beta-O-linkage of N-acetylglucosamine (i.e., O-GlcNAc) to proteins is a pro-adaptive response to cellular insults. To this end, increased protein O-GlcNAcylation improves short-term survival of cardiomyocytes subjected to acute injury. This observation has been repeated by multiple groups and in multiple models; however, whether increased protein O-GlcNAcylation plays a beneficial role in more chronic settings remains an open question.

Objective

Here, we queried whether increasing levels of cardiac protein O-GlcNAcylation would be beneficial during infarct-induced heart failure.

Methods and results

To achieve increased protein O-GlcNAcylation, we targeted Oga, the gene responsible for removing O-GlcNAc from proteins. Here, we generated mice with cardiomyocyte-restricted, tamoxifen-inducible haploinsufficient Oga gene. In the absence of infarction, we observed a slight reduction in ejection fraction in Oga deficient mice. Overall, Oga reduction had no major impact on ventricular function. In additional cohorts, mice of both sexes and both genotypes were subjected to infarct-induced heart failure and followed for up to four weeks, during which time cardiac function was assessed via echocardiography. Contrary to our prediction, the Oga deficient mice exhibited exacerbated—not improved—cardiac function at one week following infarction. When the observation was extended to 4 wk post-MI, this acute exacerbation was lost.

Conclusions

The present findings, coupled with our previous work, suggest that altering the ability of cardiomyocytes to either add or remove O-GlcNAc modifications to proteins exacerbates early infarct-induced heart failure. We speculate that more nuanced approaches to regulating O-GlcNAcylation are needed to understand its role—and, in particular, the possibility of cycling, in the pathophysiology of the failing heart.

O-GlcNAc Transferase Promotes Compensated Cardiac Function and Protein Kinase A O-GlcNAcylation During Early and Established Pathological Hypertrophy From Pressure Overload

Background

Protein posttranslational modifications by O‐linked β‐N‐acetylglucosamine (O‐GlcNAc) increase with cardiac hypertrophy, yet the functional effects of these changes are incompletely understood. In other organs, O‐GlcNAc promotes adaptation to acute physiological stressors; however, prolonged O‐GlcNAc elevations are believed to be detrimental. We hypothesize that early O‐GlcNAcylation improves cardiac function during initial response to pressure overload hypertrophy, but that sustained elevations during established pathological hypertrophy negatively impact cardiac function by adversely affecting calcium handling proteins.

Methods and Results

Transverse aortic constriction or sham surgeries were performed on littermate controls or cardiac‐specific, inducible O‐GlcNAc transferase knockout (OGTKO) mice to reduce O‐GlcNAc levels. O‐GlcNAc transferase deficiency was induced at different times. To evaluate the initial response to pressure overload, OGTKO was completed preoperatively and mice were followed for 2 weeks post‐surgery. To assess prolonged O‐GlcNAcylation during established hypertrophy, OGTKO was performed starting 18 days after surgery and mice were followed until 6 weeks post‐surgery. In both groups, OGTKO with transverse aortic constriction caused significant left ventricular dysfunction. OGTKO did not affect levels of the calcium handling protein SERCA2a. OGTKO reduced phosphorylation of phospholamban and cardiac troponin I, which would negatively impact cardiac function. O‐GlcNAcylation of protein kinase A catalytic subunit, a kinase for phospholamban, decreased with OGTKO.

Conclusions

O‐GlcNAcylation promotes compensated cardiac function in both early and established pathological hypertrophy. We identified a novel O‐GlcNAcylation of protein kinase A catalytic subunit, which may regulate calcium handling and cardiac function.

Glucose transporters in cardiovascular system in health and disease

Glucose transporters are essential for the heart to sustain its function. Due to its nature as a high energy-consuming organ, the heart needs to catabolize a huge quantity of metabolic substrates. For optimized energy production, the healthy heart constantly switches between various metabolites in accordance with substrate availability and hormonal status. This metabolic flexibility is essential for the maintenance of cardiac function. Glucose is part of the main substrates catabolized by the heart and its use is fine-tuned via complex molecular mechanisms that include the regulation of the glucose transporters GLUTs, mainly GLUT4 and GLUT1. Besides GLUTs, glucose can also be transported by cotransporters of the sodium-glucose cotransporter (SGLT) (SLC5 gene) family, in which SGLT1 and SMIT1 were shown to be expressed in the heart. This SGLT-mediated uptake does not seem to be directly linked to energy production but is rather associated with intracellular signalling triggering important processes such as the production of reactive oxygen species. Glucose transport is markedly affected in cardiac diseases such as cardiac hypertrophy, diabetic cardiomyopathy and heart failure. These alterations are not only fingerprints of these diseases but are involved in their onset and progression. The present review will depict the importance of glucose transport in healthy and diseased heart, as well as proposed therapies targeting glucose transporters.

Lipid in the midst of metabolic remodeling - Therapeutic implications for the failing heart

A healthy heart relies on an intact cardiac lipid metabolism. Fatty acids represent the major source for ATP production in the heart. Not less importantly, lipids are directly involved in critical processes such as cell growth, proliferation, and cell death by functioning as building blocks or signaling molecules. In the development of heart failure, perturbations in fatty acid utilization impair cardiac energetics. Furthermore, they may affect glucose and amino acid metabolism and induce the synthesis of several lipid intermediates, whose biological functions are still poorly understood. This work outlines the pivotal role of lipid metabolism in the heart and provides a lipocentric view of metabolic remodeling in heart failure. We will also critically revisit therapeutic attempts targeting cardiac lipid metabolism in heart failure and propose specific strategies for future investigations in this regard.

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 rationale for repurposing funny current inhibition for management of ventricular arrhythmia

Management of ventricular arrhythmia in structural heart disease is complicated by the toxicity of the limited antiarrhythmic options available. In others, proarrhythmia and deleterious hemodynamic and noncardiac effects prevent practical use. This necessitates new thinking in therapeutic agents for ventricular arrhythmia in structural heart disease. Ivabradine, a funny current (I_f) inhibitor, has proven safety in heart failure, angina, and inappropriate sinus tachycardia. Although it is commonly known that funny channels are primarily expressed in the sinoatrial node, atrioventricular node, and conducting system of the ventricle, ivabradine is known to exert effects on metabolism, ion homeostasis, and membrane electrophysiology of remodeled ventricular myocardium. This review considers novel concepts and evidence from clinical and experimental studies regarding this paradigm, with a potential role of ivabradine in ventricular arrhythmia.

In vivo [U-13C]glucose labeling to assess heart metabolism in murine models of pressure and volume overload

Alterations in the metabolism of substrates such as glucose are integrally linked to the structural and functional changes that occur in the remodeling heart. Assessment of such metabolic changes under in vivo conditions would provide important insights into this interrelationship. We aimed to investigate glucose carbon metabolism in pressure-overload and volume-overload cardiac hypertrophy by using an in vivo [U-¹³C]glucose labeling strategy to enable analyses of the metabolic fates of glucose carbons in the mouse heart. Therefore, [U-¹³C]glucose was administered in anesthetized mice by tail vein infusion, and the optimal duration of infusion was established. Hearts were then excised for ¹³C metabolite isotopomer analysis by NMR spectroscopy. [U-¹³C]glucose infusions were performed in mice 2 wk following transverse aortic constriction (TAC) or aortocaval fistula (Shunt) surgery. At this time point, there were similar increases in left ventricular (LV) mass in both groups, but TAC resulted in concentric hypertrophy with impaired LV function, whereas Shunt caused eccentric hypertrophy with preserved LV function. TAC was accompanied by significant changes in glycolysis, mitochondrial oxidative metabolism, glucose metabolism to anaplerotic substrates, and de novo glutamine synthesis. In contrast to TAC, hardly any metabolic changes could be observed in the Shunt group. Taken together, in vivo [U-¹³C]glucose labeling is a valuable method to investigate the fate of nutrients such as glucose in the remodeling heart. We find that concentric and eccentric cardiac remodeling are accompanied by distinct differences in glucose carbon metabolism.

NEW & NOTEWORTHY This study implemented a method for assessing the fate of glucose carbons in the heart in vivo and used this to demonstrate that pressure and volume overload are associated with distinct changes. In contrast to volume overload, pressure overload-induced changes affect the tricarboxylic acid cycle, glycolytic pathways, and glutamine synthesis. A better understanding of cardiac glucose metabolism under pathological conditions in vivo may provide new therapeutic strategies specific for different types of hemodynamic overload.

Listen to this article’s corresponding podcast at: https://ajpheart.podbean.com/e/u-13c-glucose-and-in-vivo-heart-metabolism/.

O-GlcNAc transferase affects the signal transduction of β1 adrenoceptor in adult rat cardiomyocytes by increasing the O-GlcNAcylation of β1 adrenoceptor

O-GlcNAcylation was first found by Torres and Hart in monocytes. It is a dynamic and reversible post-translational modification catalyzed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). O-GlcNAcylation is increased in diabetic cardiomyopathy (DCM) patients and it has been reported that OGT plays an important role in the regulation of cardiac gene transcription, cell cycle and calcium homeostasis. The purpose of this study is to investigate the effects of OGT on signal transduction and function of β1-adrenoceptor (β1AR) in adult rat cardiomyocytes. We found that after overexpressing OGT by adenovirus vector in adult rat cardiomyocytes, cAMP formation and phosphorylation of phospholamban (PLB) at Ser16 (p16-PLB) were decreased under isoprenaline (ISO) stimulation. Over expression of OGT increased the intracellular [Ca²⁺]i and deteriorated the death of cardiomyocytes induced by prolonged stimulation with ISO. β1-adrenoceptor was overexpressed using a plasmid vector and then co-immunoprecipitation (co-IP) followed by Western blot was employed to define the O-GlcNAcylation of β1-adrenoceptor. The results showed that O-GlcNAcylation of β1-adrenoceptor was increased in OGT overexpressed cells, and there was no significant change in the formation of cAMP and phosphorylation of PLB after β1-adrenoceptor was blocked by CGP20712A. Given that OGT affects the signal transduction of β1-adrenoceptor in adult rat cardiomyocytes by increasing the O-GlcNAcylation of β1-adrenoceptor, the mechanism revealed in this study indicates that OGT and β1AR may be therapeutic targets in patients undergoing diabetic cardiomyopathy.

PET ligands [18F]LSN3316612 and [11C]LSN3316612 quantify O-linked-β-N-acetyl-glucosamine hydrolase in the brain

We aimed to develop effective radioligands for quantifying brain O-linked-β-N-acetyl-glucosamine (O-GlcNAc) hydrolase (OGA) using positron emission tomography in living subjects as tools for evaluating drug target engagement. Posttranslational modifications of tau, a biomarker of Alzheimer's disease, by O-GlcNAc through the enzyme pair OGA and O-GlcNAc transferase (OGT) are inversely related to the amounts of its insoluble hyperphosphorylated form. Increase in tau O-GlcNAcylation by OGA inhibition is believed to reduce tau aggregation. LSN3316612, a highly selective and potent OGA ligand [half-maximal inhibitory concentration (IC50) = 1.9 nM], emerged as a lead ligand after in silico analysis and in vitro evaluations. [3H]LSN3316612 imaged and quantified OGA in postmortem brains of rat, monkey, and human. The presence of fluorine and carbonyl functionality in LSN3316612 enabled labeling with positron-emitting fluorine-18 or carbon-11. Both [18F]LSN3316612 and [11C]LSN3316612 bound reversibly to OGA in vivo, and such binding was blocked by pharmacological doses of thiamet G, an OGA inhibitor of different chemotype, in monkeys. [18F]LSN3316612 entered healthy human brain avidly (~4 SUV) without radiodefluorination or adverse effect from other radiometabolites, as evidenced by stable brain total volume of distribution (VT) values by 110 min of scanning. Overall, [18F]LSN3316612 is preferred over [11C]LSN3316612 for future human studies, whereas either may be an effective positron emission tomography radioligand for quantifying brain OGA in rodent and monkey.

Regulation of cardiac O-GlcNAcylation: More than just nutrient availability

The post-translational modification of serine and threonine residues of nuclear, cytosolic, and mitochondrial proteins by O-linked β-N-acetyl glucosamine (O-GlcNAc) has long been seen as an important regulatory mechanism in the cardiovascular system. O-GlcNAcylation of cardiac proteins has been shown to contribute to the regulation of transcription, metabolism, mitochondrial function, protein quality control and turnover, autophagy, and calcium handling. In the heart, acute increases in O-GlcNAc have been associated with cardioprotection, such as those observed during ischemia/reperfusion. Conversely, chronic increases in O-GlcNAc, often associated with diabetes and nutrient excess, have been shown to contribute to cardiac dysfunction. Traditionally, many studies have linked changes in O-GlcNAc with nutrient availability and as such O-GlcNAcylation is often seen as a nutrient driven process. However, emerging evidence suggests that O-GlcNAcylation may also be regulated by non-nutrient dependent mechanisms, such as transcriptional and post-translational regulation. Therefore, the goals of this review are to provide an overview of the impact of O-GlcNAcylation in the cardiovascular system, how this is regulated and to discuss the emergence of regulatory mechanisms other than nutrient availability.

Chronic activation of hexosamine biosynthesis in the heart triggers pathological cardiac remodeling

The hexosamine biosynthetic pathway (HBP) plays critical roles in nutrient sensing, stress response, and cell growth. However, its contribution to cardiac hypertrophic growth and heart failure remains incompletely understood. Here, we show that the HBP is induced in cardiomyocytes during hypertrophic growth. Overexpression of Gfat1 (glutamine:fructose-6-phosphate amidotransferase 1), the rate-limiting enzyme of HBP, promotes cardiomyocyte growth. On the other hand, Gfat1 inhibition significantly blunts phenylephrine-induced hypertrophic growth in cultured cardiomyocytes. Moreover, cardiac-specific overexpression of Gfat1 exacerbates pressure overload-induced cardiac hypertrophy, fibrosis, and cardiac dysfunction. Conversely, deletion of Gfat1 in cardiomyocytes attenuates pathological cardiac remodeling in response to pressure overload. Mechanistically, persistent upregulation of the HBP triggers decompensated hypertrophy through activation of mTOR while Gfat1 deficiency shows cardioprotection and a concomitant decrease in mTOR activity. Taken together, our results reveal that chronic upregulation of the HBP under hemodynamic stress induces pathological cardiac hypertrophy and heart failure through persistent activation of mTOR.

Metabolic remodeling plays an important role in pathological cardiac hypertrophy. Here, the authors show that hexosamine biosynthetic pathway is elevated in the heart by pressure overload, which contributes to heart failure by persistent activation of mTOR.

O-linked β-N-acetylglucosamine transferase plays an essential role in heart development through regulating angiopoietin-1

O-linked N-acetylglucosamine (GlcNAc) transferase (OGT) is the only enzyme catalyzing O-GlcNAcylation. Although it has been shown that OGT plays an essential role in maintaining postnatal heart function, its role in heart development remains unknown. Here we showed that loss of OGT in early fetal cardiomyocytes led to multiple heart developmental defects including hypertrabeculation, biventricular dilation, atrial septal defects, ventricular septal defects, and defects in coronary vessel development. In addition, RNA sequencing revealed that Angiopoietin-1, required within cardiomyocytes for both myocardial and coronary vessel development, was dramatically downregulated in cardiomyocyte-specific OGT knockout mouse hearts. In conclusion, our data demonstrated that OGT plays an essential role in regulating heart development through activating expression of cardiomyocyte Angiopoietin-1.

Regulatory Effects of O-GlcNAcylation in Vascular Smooth Muscle Cells on Diabetic Vasculopathy

Vascular complications from uncontrolled hyperglycemia are the leading cause of death in patients with diabetes mellitus. Previous reports have shown a strong correlation between hyperglycemia and vascular calcification, which increases mortality and morbidity in individuals with diabetes. However, the precise underlying molecular mechanisms of hyperglycemia-induced vascular calcification remain largely unknown. Transdifferentiation of vascular smooth muscle cells (VSMC) into osteoblast-like cells is a known culprit underlying the development of vascular calcification in the diabetic vasculature. Pathological conditions such as high glucose levels and oxidative stress are linked to enhanced osteogenic differentiation of VSMC both in vivo and in vitro. It has been demonstrated that increased expression of runt-related transcription factor 2 (Runx2), a bone-related transcription factor, in VSMC is necessary and sufficient for the induction of VSMC calcification. Addition of a single O-linked β-N-acetylglucosamine (O-GlcNAc) moiety to the serine/threonine residues of target proteins (O-GlcNAcylation) has been observed in the arteries of diabetic patients, as well as in animal models in association with the enhanced expression of Runx2 and aggravated vascular calcification. O-GlcNAcylation is a dynamic and tightly regulated process, that is mediated by 2 enzymes, O-GlcNAc transferase and O-GlcNAcase. Glucose is metabolized into UDP-β-D-N-acetylglucosamine, an active sugar donor of O-GlcNAcylation via the hexosamine biosynthetic pathway. Overall increases in the O-GlcNAcylation of cellular proteins have been closely associated with cardiovascular complications of diabetes. In this review, the authors provide molecular insights into cardiovascular complications, including diabetic vasculopathy, that feature increased O-GlcNAcylation in people with diabetes.

First characterization of glucose flux through the hexosamine biosynthesis pathway (HBP) in ex vivo mouse heart

The hexosamine biosynthesis pathway (HBP) branches from glycolysis and forms UDP-GlcNAc, the moiety for O-linked β-GlcNAc (O-GlcNAc) post-translational modifications. An inability to directly measure HBP flux has hindered our understanding of the factors regulating protein O-GlcNAcylation. Our goals in this study were to (i) validate a LC-MS method that assesses HBP flux as UDP-GlcNAc (¹³C)-molar percent enrichment (MPE) and concentration and (ii) determine whether glucose availability or workload regulate cardiac HBP flux. For (i), we perfused isolated murine working hearts with [U-¹³C₆]glucosamine (1, 10, 50, or 100 μm), which bypasses the rate-limiting HBP enzyme. We observed a concentration-dependent increase in UDP-GlcNAc levels and MPE, with the latter reaching a plateau of 56.3 ± 2.9%. For (ii), we perfused isolated working hearts with [U-¹³C₆]glucose (5.5 or 25 mm). Glycolytic efflux doubled with 25 mm [U-¹³C₆]glucose; however, the calculated HBP flux was similar among the glucose concentrations at ∼2.5 nmol/g of heart protein/min, representing ∼0.003–0.006% of glycolysis. Reducing cardiac workload in beating and nonbeating Langendorff perfusions had no effect on the calculated HBP flux at ∼2.3 and 2.5 nmol/g of heart protein/min, respectively. To the best of our knowledge, this is the first direct measurement of glucose flux through the HBP in any organ. We anticipate that these methods will enable foundational analyses of the regulation of HBP flux and protein O-GlcNAcylation. Our results suggest that in the healthy ex vivo perfused heart, HBP flux does not respond to acute changes in glucose availability or cardiac workload.

Heart Failure in Type 2 Diabetes Mellitus

Patients with diabetes mellitus have >2× the risk for developing heart failure (HF; HF with reduced ejection fraction and HF with preserved ejection fraction). Cardiovascular outcomes, hospitalization, and prognosis are worse for patients with diabetes mellitus relative to those without. Beyond the structural and functional changes that characterize diabetic cardiomyopathy, a complex underlying, and interrelated pathophysiology exists. Despite the success of many commonly used antihyperglycemic therapies to lower hyperglycemia in type 2 diabetes mellitus the high prevalence of HF persists. This, therefore, raises the possibility that additional factors beyond glycemia might contribute to the increased HF risk in diabetes mellitus. This review summarizes the state of knowledge about the impact of existing antihyperglycemic therapies on HF and discusses potential mechanisms for beneficial or deleterious effects. Second, we review currently approved pharmacological therapies for HF and review evidence that addresses their efficacy in the context of diabetes mellitus. Dysregulation of many cellular mechanisms in multiple models of diabetic cardiomyopathy and in human hearts have been described. These include oxidative stress, inflammation, endoplasmic reticulum stress, aberrant insulin signaling, accumulation of advanced glycated end-products, altered autophagy, changes in myocardial substrate metabolism and mitochondrial bioenergetics, lipotoxicity, and altered signal transduction such as GRK (g-protein receptor kinase) signaling, renin angiotensin aldosterone signaling and β-2 adrenergic receptor signaling. These pathophysiological pathways might be amenable to pharmacological therapy to reduce the risk of HF in the context of type 2 diabetes mellitus. Successful targeting of these pathways could alter the prognosis and risk of HF beyond what is currently achieved using existing antihyperglycemic and HF therapeutics.

DNA methylation reprograms cardiac metabolic gene expression in end-stage human heart failure

Heart failure (HF) is a leading cause of morbidity and mortality in the United States and worldwide. As a multifactorial syndrome with unpredictable clinical outcomes, identifying the common molecular underpinnings that drive HF pathogenesis remains a major focus of investigation. Disruption of cardiac gene expression has been shown to mediate a common final cascade of pathological hallmarks wherein the heart reactivates numerous developmental pathways. Although the central regulatory mechanisms that drive this cardiac transcriptional reprogramming remain unknown, epigenetic contributions are likely. In the current study, we examined whether the epigenome, specifically DNA methylation, is reprogrammed in HF to potentiate a pathological shift in cardiac gene expression. To accomplish this, we used paired-end whole genome bisulfite sequencing and next-generation RNA sequencing of left ventricle tissue obtained from seven patients with end-stage HF and three nonfailing donor hearts. We found that differential methylation was localized to promoter-associated cytosine-phosphate-guanine islands, which are established regulatory regions of downstream genes. Hypermethylated promoters were associated with genes involved in oxidative metabolism, whereas promoter hypomethylation enriched glycolytic pathways. Overexpression of plasmid-derived DNA methyltransferase 3A in vitro was sufficient to lower the expression of numerous oxidative metabolic genes in H9c2 rat cardiomyoblasts, further supporting the importance of epigenetic factors in the regulation of cardiac metabolism. Last, we identified binding-site competition via hypermethylation of the nuclear respiratory factor 1 (NRF1) motif, an established upstream regulator of mitochondrial biogenesis. These preliminary observations are the first to uncover an etiology-independent shift in cardiac DNA methylation that corresponds with altered metabolic gene expression in HF.NEW & NOTEWORTHY The failing heart undergoes profound metabolic changes because of alterations in cardiac gene expression, reactivating glycolytic genes and suppressing oxidative metabolic genes. In the current study, we discover that alterations to cardiac DNA methylation encode this fetal-like metabolic gene reprogramming. We also identify novel epigenetic interference of nuclear respiratory factor 1 via hypermethylation of its downstream promoter targets, further supporting a novel contribution of DNA methylation in the metabolic remodeling of heart failure.

Metabolic Stress and Cardiovascular Disease in Diabetes Mellitus: The Role of Protein O-GlcNAc Modification

Mammalian cells metabolize glucose primarily for energy production, biomass synthesis, and posttranslational glycosylation; and maintaining glucose metabolic homeostasis is essential for normal physiology of cells. Impaired glucose homeostasis leads to hyperglycemia, a hallmark of diabetes mellitus. Chronically increased glucose in diabetes mellitus promotes pathological changes accompanied by impaired cellular function and tissue damage, which facilitates the development of cardiovascular complications, the major cause of morbidity and mortality of patients with diabetes mellitus. Emerging roles of glucose metabolism via the hexosamine biosynthesis pathway (HBP) and increased protein modification via O-linked β-N-acetylglucosamine (O-GlcNAcylation) have been demonstrated in diabetes mellitus and implicated in the development of diabetic cardiovascular complications. This review will discuss the biological outcomes of the glucose metabolism via the hexosamine biogenesis pathway and protein O-GlcNAcylation in regulating cellular homeostasis, and highlight the regulations and contributions of elevated O-GlcNAcylation to the pathogenesis of diabetic cardiovascular disease.

Augmented O-GlcNAcylation attenuates intermittent hypoxia-induced cardiac remodeling through the suppression of NFAT and NF-κB activities in mice

Type 2 diabetes mellitus (T₂DM) has been reported to be associated with cardiac remodeling. Although O-GlcNAcylation is known to be elevated in diabetic and ischemic hearts, the effects of O-GlcNAcylation on cardiac remodeling induced by intermittent hypoxia (IH), such as sleep apnea syndrome (SAS), remain unknown. To evaluate the effects, we induced IH in wild-type (WT) and transgenic O-GlcNAc transferase (Ogt-Tg) mice. Two weeks of IH increased O-GlcNAcylation in the heart tissues of both strains of mice, whereas O-GlcNAcylation in Ogt-Tg mice was significantly higher than that in WT mice under both normoxic and IH conditions. WT mice exhibited cardiac remodeling after IH, whereas cardiac remodeling was significantly attenuated in Ogt-Tg mice. Oxidative stress and apoptosis increased after IH in both strains of mice, whereas the rate of increase in these processes in Ogt-Tg mice was significantly lower than that in WT mice. To examine the mechanism of cardiac remodeling attenuation in Ogt-Tg mice after IH, the effects of O-GlcNAcylation on the activities of the master regulators nuclear factor of activated T cells (NFAT) and NF-κB were determined. The O-GlcNAcylation of GSK-3β, a negative regulator of NFAT, was significantly increased in Ogt-Tg mice, whereas the phosphorylation of GSK-3β was reciprocally reduced. The same result was observed for NF-κB p65. An in vitro reporter assay showed that the augmentation of O-GlcNAcylation by an O-GlcNAcase inhibitor suppressed NFAT and NF-κB promoter activity. These data suggest that augmented O-GlcNAcylation mitigates IH-induced cardiac remodeling by suppressing NFAT and NF-κB activities through the O-GlcNAcylation of GSK-3β and NF-κB p65.

The role of post-translational modifications in cardiac hypertrophy

Pathological cardiac hypertrophy involves excessive protein synthesis, increased cardiac myocyte size and ultimately the development of heart failure. Thus, pathological cardiac hypertrophy is a major risk factor for many cardiovascular diseases and death in humans. Extensive research in the last decade has revealed that post‐translational modifications (PTMs), including phosphorylation, ubiquitination, SUMOylation, O‐GlcNAcylation, methylation and acetylation, play important roles in pathological cardiac hypertrophy pathways. These PTMs potently mediate myocardial hypertrophy responses via the interaction, stability, degradation, cellular translocation and activation of receptors, adaptors and signal transduction events. These changes occur in response to pathological hypertrophy stimuli. In this review, we summarize the roles of PTMs in regulating the development of pathological cardiac hypertrophy. Furthermore, PTMs are discussed as potential targets for treating or preventing cardiac hypertrophy.

Inhibition of the hexosamine biosynthesis pathway potentiates cisplatin cytotoxicity by decreasing BiP expression in non-small-cell lung cancer cells

Platinum anticancer agents are essential components in chemotherapeutic regimens for non-small-cell lung cancer (NSCLC) patients ineligible for targeted therapy. However, platinum-based regimens have reached a plateau of therapeutic efficacy; therefore, it is critical to implement novel approaches for improvement. The hexosamine biosynthesis pathway (HBP), which produces amino-sugar N-acetyl-glucosamine for protein glycosylation, is important for protein function and cell survival. Here we show a beneficial effect by the combination of cisplatin with HBP inhibition. Expression of glutamine:fructose-6-phosphate amidotransferase (GFAT), the rate-limiting enzyme of HBP, was increased in NSCLC cell lines and tissues. Pharmacological inhibition of GFAT activity or knockdown of GFATimpaired cell proliferation and exerted synergistic or additive cytotoxicity to the cells treated with cisplatin. Mechanistically, GFAT positively regulated the expression of binding immunoglobulin protein (BiP; also known as glucose-regulated protein 78, GRP78), an endoplasmic reticulum chaperone involved in unfolded protein response (UPR). Suppressing GFAT activity resulted in downregulation of BiP that activated inositol-requiring enzyme 1α, a sensor protein of UPR, and exacerbated cisplatin-induced cell apoptosis. These data identify GFAT-mediated HBP as a target for improving platinum-based chemotherapy for NSCLC.

The Role of O-GlcNAcylation for Protection against Ischemia-Reperfusion Injury

Ischemia reperfusion injury (IR injury) associated with ischemic heart disease contributes significantly to morbidity and mortality. O-linked β-N-acetylglucosamine (O-GlcNAc) is a dynamic posttranslational modification that plays an important role in numerous biological processes, both in normal cell functions and disease. O-GlcNAc increases in response to stress. This increase mediates stress tolerance and cell survival, and is protective. Increasing O-GlcNAc is protective against IR injury. Experimental cellular and animal models, and also human studies, have demonstrated that protection against IR injury by ischemic preconditioning, and the more clinically applicable remote ischemic preconditioning, is associated with increases in O-GlcNAc levels. In this review we discuss how the principal mechanisms underlying tissue protection against IR injury and the associated immediate elevation of O-GlcNAc may involve attenuation of calcium overload, attenuation of mitochondrial permeability transition pore opening, reduction of endoplasmic reticulum stress, modification of inflammatory and heat shock responses, and interference with established cardioprotective pathways. O-GlcNAcylation seems to be an inherent adaptive cytoprotective response to IR injury that is activated by mechanical conditioning strategies.

Ryanodine receptor dispersion disrupts Ca2+ release in failing cardiac myocytes

Reduced cardiac contractility during heart failure (HF) is linked to impaired Ca²⁺ release from Ryanodine Receptors (RyRs). We investigated whether this deficit can be traced to nanoscale RyR reorganization. Using super-resolution imaging, we observed dispersion of RyR clusters in cardiomyocytes from post-infarction HF rats, resulting in more numerous, smaller clusters. Functional groupings of RyR clusters which produce Ca²⁺ sparks (Ca²⁺ release units, CRUs) also became less solid. An increased fraction of small CRUs in HF was linked to augmented ‘silent’ Ca²⁺ leak, not visible as sparks. Larger multi-cluster CRUs common in HF also exhibited low fidelity spark generation. When successfully triggered, sparks in failing cells displayed slow kinetics as Ca²⁺ spread across dispersed CRUs. During the action potential, these slow sparks protracted and desynchronized the overall Ca²⁺ transient. Thus, nanoscale RyR reorganization during HF augments Ca²⁺ leak and slows Ca²⁺ release kinetics, leading to weakened contraction in this disease.

The muscle cells of the heart coordinate how they contract and relax in order to produce the heartbeat. During heart failure, these cells become less able to contract. As a result the heart becomes inefficient, pumping less blood around the body.

For the cardiac muscle cells to contract, the levels of calcium ions in the cells needs to rapidly increase. In failing hearts, these increases in calcium ion levels are smaller, slower and less well coordinated. It was not known what causes these changes, making it difficult to treat heart failure.

Calcium ions are released in cardiac muscle cells through protein channels called ryanodine receptors. These receptors form clusters that allow them to synchronize when they open and close. Could the reorganization of ryanodine receptors account for the problems seen in failing hearts? To investigate, Kolstad et al. examined rat hearts using a technique called super-resolution microscopy. This showed that the clusters of ryanodine receptors break apart during heart failure to form smaller clusters. Further experiments showed that calcium ions ‘leak’ from these smaller clusters, reducing the amount of calcium that can be released into cardiac muscle cells during each heartbeat. Released calcium also spreads between the dispersed clusters, resulting in a slower rise of the calcium levels in the cells. Both changes contribute to weakened contractions of cells in failing hearts.

Therefore, heart failure can be traced back to very small rearrangements of the ryanodine receptors. This understanding will help researchers as they investigate new ways to treat heart failure.

O-GlcNAcylation reduces proximal tubule protein reabsorption and promotes proteinuria in spontaneously hypertensive rats

Hypertensive individuals are at greater risk for developing chronic kidney disease (CKD). Reducing proteinuria has been suggested as a possible therapeutic approach to treat CKD. However, the mechanisms underlying the development of proteinuria in hypertensive conditions are incompletely understood. Cardiac and vascular dysfunction is associated with changes in the O-GlcNAcylation pathway in hypertensive models. We hypothesized that O-GlcNAcylation is also involved in renal damage, especially development of proteinuria, associated with hypertension. Using the spontaneously hypertensive rat (SHR) model, we observed higher renal cortex O-GlcNAcylation, glutamine-fructose aminotransferase (GFAT), and O-GlcNAc transferase (OGT) protein expression, which positively correlated with proteinuria. Interestingly, this was observed in hypertensive, but not pre-hypertensive, rats. Pharmacological inhibition of GFAT decreased renal cortex O-GlcNAcylation, proteinuria, and albuminuria in SHR. Using a proximal tubule cell line, we observed that increased O-GlcNAcylation reduced megalin surface expression and albumin endocytosis in vitro, and the effects were correlated in vivo Moreover, megalin is O-GlcNAcylated both in vitro and in vivo In conclusion, our results demonstrate a new mechanism involved in hypertension-associated proteinuria.

Acute and Chronic Effects of Protein Kinase-D Signaling on Cardiac Energy Metabolism

Protein kinase-D (PKD) is increasingly recognized as a key regulatory signaling hub in cardiac glucose uptake and also a major player in the development of hypertrophy. Glucose is one of the predominant energy substrates for the heart to support contraction. However, a cardiac substrate switch toward glucose over-usage is associated with the development of cardiac hypertrophy. Hence, regulation of PKD activity must be strictly coordinated. This review provides mechanistic insights into the acute and chronic regulatory functions of PKD signaling in the healthy and hypertrophied heart. First an overview of the activation pathways of PKD1, the most abundant isoform in the heart, is provided. Then the various regulatory roles of the PKD isoforms in the heart in relation to cardiac glucose and fatty acid metabolism, contraction, morphology, function, and the development of cardiac hypertrophy are described. Finally, these findings are integrated and the possibility of targeting this kinase as a novel strategy to combat cardiac diseases is discussed.

The Glycoproteomics-MS for Studying Glycosylation in Cardiac Hypertrophy and Heart Failure

With recent advancements of analytical techniques and mass spectrometric instrumentations, proteomics has been widely exploited to study the regulation of protein expression associated with disease states. Many proteins may undergo abnormal change in response to the stimulants, leading to regulation of posttranslationally modified proteins. In this review, the physiological and pathological roles of protein glycosylation in cardiac hypertrophy is discussed, and how the signal pathways regulate heart function and leading to heart failure. The analytical methods for analysis of protein glycosylation, including glycans, glycosite, occupancy, and heterogeneity is emphasized. The rationale on glycoproteins as disease biomarkers is also discussed. The authors also propose potential research in this field and challenges in the diagnosis and treatment of this disease.

AMPK activation counteracts cardiac hypertrophy by reducing O-GlcNAcylation

AMP-activated protein kinase (AMPK) has been shown to inhibit cardiac hypertrophy. Here, we show that submaximal AMPK activation blocks cardiomyocyte hypertrophy without affecting downstream targets previously suggested to be involved, such as p70 ribosomal S6 protein kinase, calcineurin/nuclear factor of activated T cells (NFAT) and extracellular signal-regulated kinases. Instead, cardiomyocyte hypertrophy is accompanied by increased protein O-GlcNAcylation, which is reversed by AMPK activation. Decreasing O-GlcNAcylation by inhibitors of the glutamine:fructose-6-phosphate aminotransferase (GFAT), blocks cardiomyocyte hypertrophy, mimicking AMPK activation. Conversely, O-GlcNAcylation-inducing agents counteract the anti-hypertrophic effect of AMPK. In vivo, AMPK activation prevents myocardial hypertrophy and the concomitant rise of O-GlcNAcylation in wild-type but not in AMPKα2-deficient mice. Treatment of wild-type mice with O-GlcNAcylation-inducing agents reverses AMPK action. Finally, we demonstrate that AMPK inhibits O-GlcNAcylation by mainly controlling GFAT phosphorylation, thereby reducing O-GlcNAcylation of proteins such as troponin T. We conclude that AMPK activation prevents cardiac hypertrophy predominantly by inhibiting O-GlcNAcylation.

Protein O-GlcNAcylation in Cardiac Pathologies: Past, Present, Future

O-GlcNAcylation is a ubiquitous and reversible post-translational protein modification that has recently gained renewed interest due to the rapid development of analytical tools and new molecules designed to specifically increase the level of protein O-GlcNAcylation. The level of O-GlcNAc modification appears to have either deleterious or beneficial effects, depending on the context (exposure time, pathophysiological context). While high O-GlcNAcylation levels are mostly reported in chronic diseases, the increase in O-GlcNAc level in acute stresses such as during ischemia reperfusion or hemorrhagic shock is reported to be beneficial in vitro, ex vivo, or in vivo. In this context, an increase in O-GlcNAc levels could be a potential new cardioprotective therapy, but the ambivalent effects of protein O-GlcNAcylation augmentation remains as a key problem to be solved prior to their transfer to the clinic. The emergence of new analytical tools has opened new avenues to decipher the mechanisms underlying the beneficial effects associated with an O-GlcNAc level increase. A better understanding of the exact roles of O-GlcNAc on protein function, targeting or stability will help to develop more targeted approaches. The aim of this review is to discuss the mechanisms and potential beneficial impact of O-GlcNAc modulation, and its potential as a new clinical target in cardiology.

Chronic O-GlcNAcylation and Diabetic Cardiomyopathy: The Bitterness of Glucose

Type 2 diabetes (T2D) is a major risk factor for heart failure. Diabetic cardiomyopathy (DC) is characterized by diastolic dysfunction and left ventricular hypertrophy. Epidemiological data suggest that hyperglycaemia contributes to the development of DC. Several cellular pathways have been implicated in the deleterious effects of high glucose concentrations in the heart: oxidative stress, accumulation of advanced glycation end products (AGE), and chronic hexosamine biosynthetic pathway (HBP) activation. In the present review, we focus on the effect of chronic activation of the HBP on diabetic heart function. The HBP supplies N-acetylglucosamine moiety (O-GlcNAc) that is O-linked by O-GlcNAc transferase (OGT) to proteins on serine or threonine residues. This post-translational protein modification modulates the activity of the targeted proteins. In the heart, acute activation of the HBP in response to ischaemia-reperfusion injury appears to be protective. Conversely, chronic activation of the HBP in the diabetic heart affects Ca²⁺ handling, contractile properties, and mitochondrial function and promotes stress signaling, such as left ventricular hypertrophy and endoplasmic reticulum stress. Many studies have shown that O-GlcNAc impairs the function of key protein targets involved in these pathways, such as phospholamban, calmodulin kinase II, troponin I, and FOXO1. The data show that excessive O-GlcNAcylation is a major trigger of the glucotoxic events that affect heart function under chronic hyperglycaemia. Supporting this finding, pharmacological or genetic inhibition of the HBP in the diabetic heart improves heart function. In addition, the SGLT2 inhibitor dapagliflozin, a glucose lowering agent, has recently been shown to lower cardiac HBP in a lipodystophic T2D mice model and to concomitantly improve the diastolic dysfunction of these mice. Therefore, targeting cardiac-excessive O-GlcNAcylation or specific target proteins represents a potential therapeutic option to treat glucotoxicity in the diabetic heart.

AMP-Activated Protein Kinase and O-GlcNAcylation, Two Partners Tightly Connected to Regulate Key Cellular Processes

The AMP-activated protein kinase (AMPK) is an important cellular energy sensor. Its activation under energetic stress is known to activate energy-producing pathways and to inactivate energy-consuming pathways, promoting ATP preservation and cell survival. AMPK has been shown to play protective role in many pathophysiological processes including cardiovascular diseases, diabetes, and cancer. Its action is multi-faceted and comprises short-term regulation of enzymes by direct phosphorylation as well as long-term adaptation via control of transcription factors and cellular events such as autophagy. During the last decade, several studies underline the particular importance of the interaction between AMPK and the post-translational modification called O-GlcNAcylation. O-GlcNAcylation means the O-linked attachment of a single N-acetylglucosamine moiety on serine or threonine residues. O-GlcNAcylation plays a role in multiple physiological cellular processes but is also associated with the development of various diseases. The first goal of the present review is to present the tight molecular relationship between AMPK and enzymes regulating O-GlcNAcylation. We then draw the attention of the reader on the putative importance of this interaction in different pathophysiological events.

Nox4 reprograms cardiac substrate metabolism via protein O-GlcNAcylation to enhance stress adaptation

Cardiac hypertrophic remodeling during chronic hemodynamic stress is associated with a switch in preferred energy substrate from fatty acids to glucose, usually considered to be energetically favorable. The mechanistic interrelationship between altered energy metabolism, remodeling, and function remains unclear. The ROS-generating NADPH oxidase-4 (Nox4) is upregulated in the overloaded heart, where it ameliorates adverse remodeling. Here, we show that Nox4 redirects glucose metabolism away from oxidation but increases fatty acid oxidation, thereby maintaining cardiac energetics during acute or chronic stresses. The changes in glucose and fatty acid metabolism are interlinked via a Nox4-ATF4–dependent increase in the hexosamine biosynthetic pathway, which mediates the attachment of O-linked N-acetylglucosamine (O-GlcNAcylation) to the fatty acid transporter CD36 and enhances fatty acid utilization. These data uncover a potentially novel redox pathway that regulates protein O-GlcNAcylation and reprograms cardiac substrate metabolism to favorably modify adaptation to chronic stress. Our results also suggest that increased fatty acid oxidation in the chronically stressed heart may be beneficial.

Nox4 reprograms intermediary metabolism in the heart through an ATF4-mediated enhancement of protein O-GlcNAcylation, and the resulting switch to increased fatty acid oxidation protects the overloaded heart.

The continuing evolution of cardiac troponin I biomarker analysis: from protein to proteoform

INTRODUCTION

The troponin complex consists of three proteins that fundamentally couple excitation with contraction. Circulating cardiac-specific Troponin I (cTnI) serves as diagnostic biomarker tools for risk stratification of acute coronary syndromes and acute myocardial infarction (MI). Within the heart, cTnI oscillates between inactive and active conformations to either block or disinhibit actinomyosin formation. This molecular mechanism is fine-tuned through extensive protein modifications whose profiles are maladaptively altered with co-morbidities including hypertrophic cardiomyopathy, diabetes, and heart failure. Technological advances in analytical platforms over the last decade enable routine baseline cTnI analysis in patients without cardiovascular complications, and hold potential to expand cTnI readouts that include modified cTnI proteoforms. Areas covered: This review covers the current state, advances, and prospects of analytical platforms that now enable routine baseline cTnI analysis in patients. In parallel, improved mass spectrometry instrumentation and workflows already reveal an array of modified cTnI proteoforms with promising diagnostic implications. Expert commentary: New analytical capabilities provide clinicians and researchers with an opportunity to address important questions surrounding circulating cTnI in the improved diagnosis of specific patient cohorts. These techniques also hold considerable promise for new predictive and prescriptive applications for individualized profiling and improve patient care.

Epigenome alterations in aortic valve stenosis and its related left ventricular hypertrophy

Aortic valve stenosis is the most common cardiac valve disease, and with current trends in the population demographics, its prevalence is likely to rise, thus posing a major health and economic burden facing the worldwide societies. Over the past decade, it has become more than clear that our traditional genetic views do not sufficiently explain the well-known link between AS, proatherogenic risk factors, flow-induced mechanical forces, and disease-prone environmental influences. Recent breakthroughs in the field of epigenetics offer us a new perspective on gene regulation, which has broadened our perspective on etiology of aortic stenosis and other aortic valve diseases. Since all known epigenetic marks are potentially reversible this perspective is especially exciting given the potential for development of successful and non-invasive therapeutic intervention and reprogramming of cells at the epigenetic level even in the early stages of disease progression. This review will examine the known relationships between four major epigenetic mechanisms: DNA methylation, posttranslational histone modification, ATP-dependent chromatin remodeling, and non-coding regulatory RNAs, and initiation and progression of AS. Numerous profiling and functional studies indicate that they could contribute to endothelial dysfunctions, disease-prone activation of monocyte-macrophage and circulatory osteoprogenitor cells and activation and osteogenic transdifferentiation of aortic valve interstitial cells, thus leading to valvular inflammation, fibrosis, and calcification, and to pressure overload-induced maladaptive myocardial remodeling and left ventricular hypertrophy. This is especcialy the case for small non-coding microRNAs but was also, although in a smaller scale, convincingly demonstrated for other members of cellular epigenome landscape. Equally important, and clinically most relevant, the reported data indicate that epigenetic marks, particularly certain microRNA signatures, could represent useful non-invasive biomarkers that reflect the disease progression and patients prognosis for recovery after the valve replacement surgery.

The emerging link between O-GlcNAcylation and neurological disorders

O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is involved in the regulation of many cellular cascades and neurological diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and stroke. In the brain, the expression of O-GlcNAcylation is notably heightened, as is that of O-linked N-acetylglucosaminyltransferase (OGT) and β-N-acetylglucosaminidase (OGA), the presence of which is prominent in many regions of neurological importance. Most importantly, O-GlcNAcylation is believed to contribute to the normal functioning of neurons; conversely, its dysregulation participates in the pathogenesis of neurological disorders. In neurodegenerative diseases, O-GlcNAcylation of the brain's key proteins, such as tau and amyloid-β, interacts with their phosphorylation, thereby triggering the formation of neurofibrillary tangles and amyloid plaques. An increase of O-GlcNAcylation by pharmacological intervention prevents neuronal loss. Additionally, O-GlcNAcylation is stress sensitive, and its elevation is cytoprotective. Increased O-GlcNAcylation ameliorated brain damage in victims of both trauma-hemorrhage and stroke. In this review, we summarize the current understanding of O-GlcNAcylation's physiological and pathological roles in the nervous system and provide a foundation for development of a therapeutic strategy for neurological disorders.

Integration of flux measurements to resolve changes in anabolic and catabolic metabolism in cardiac myocytes

Although ancillary pathways of glucose metabolism are critical for synthesizing cellular building blocks and modulating stress responses, how they are regulated remains unclear. In the present study, we used radiometric glycolysis assays, [¹³C₆]-glucose isotope tracing, and extracellular flux analysis to understand how phosphofructokinase (PFK)-mediated changes in glycolysis regulate glucose carbon partitioning into catabolic and anabolic pathways. Expression of kinase-deficient or phosphatase-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in rat neonatal cardiomyocytes co-ordinately regulated glycolytic rate and lactate production. Nevertheless, in all groups, >40% of glucose consumed by the cells was unaccounted for via catabolism to pyruvate, which suggests entry of glucose carbons into ancillary pathways branching from metabolites formed in the preparatory phase of glycolysis. Analysis of ¹³C fractional enrichment patterns suggests that PFK activity regulates glucose carbon incorporation directly into the ribose and the glycerol moieties of purines and phospholipids, respectively. Pyrimidines, UDP-N-acetylhexosamine, and the fatty acyl chains of phosphatidylinositol and triglycerides showed lower ¹³C incorporation under conditions of high PFK activity; the isotopologue ¹³C enrichment pattern of each metabolite indicated limitations in mitochondria-engendered aspartate, acetyl CoA and fatty acids. Consistent with this notion, high glycolytic rate diminished mitochondrial activity and the coupling of glycolysis to glucose oxidation. These findings suggest that a major portion of intracellular glucose in cardiac myocytes is apportioned for ancillary biosynthetic reactions and that PFK co-ordinates the activities of the pentose phosphate, hexosamine biosynthetic, and glycerolipid synthesis pathways by directly modulating glycolytic intermediate entry into auxiliary glucose metabolism pathways and by indirectly regulating mitochondrial cataplerosis.

Thiamet G mediates neuroprotection in experimental stroke by modulating microglia/macrophage polarization and inhibiting NF-κB p65 signaling

Inflammatory responses are accountable for secondary injury induced by acute ischemic stroke (AIS). Previous studies indicated that O-GlcNAc modification (O-GlcNAcylation) is involved in the pathology of AIS, and increase of O-GlcNAcylation by glucosamine attenuated the brain damage after ischemia/reperfusion. Inhibition of β-N-acetylglucosaminidase (OGA) with thiamet G (TMG) is an alternative option for accumulating O-GlcNAcylated proteins. In this study, we investigate the neuroprotective effect of TMG in a mouse model of experimental stroke. Our results indicate that TMG administration either before or after middle cerebral artery occlusion (MCAO) surgery dramatically reduced infarct volume compared with that in untreated controls. TMG treatment ameliorated the neurological deficits and improved clinical outcomes in neurobehavioral tests by modulating the expression of pro-inflammatory and anti-inflammatory cytokines. Additionally, TMG administration reduced the number of Iba1+ cells in MCAO mice, decreased expression of the M1 markers, and increased expression of the M2 markers in vivo. In vitro, M1 polarization of BV2 cells was inhibited by TMG treatment. Moreover, TMG decreased the expression of iNOS and COX2 mainly by suppressing NF-κB p65 signaling. These results suggest that TMG exerts a neuroprotective effect and could be useful as an anti-inflammatory agent for ischemic stroke therapy.

Stress-induced O-GlcNAcylation: an adaptive process of injured cells

In the 30 years, since the discovery of nucleocytoplasmic glycosylation, O-GlcNAc has been implicated in regulating cellular processes as diverse as protein folding, localization, degradation, activity, post-translational modifications, and interactions. The cell co-ordinates these molecular events, on thousands of cellular proteins, in concert with environmental and physiological cues to fine-tune epigenetics, transcription, translation, signal transduction, cell cycle, and metabolism. The cellular stress response is no exception: diverse forms of injury result in dynamic changes to the O-GlcNAc subproteome that promote survival. In this review, we discuss the biosynthesis of O-GlcNAc, the mechanisms by which O-GlcNAc promotes cytoprotection, and the clinical significance of these data.

Roles of platelet-derived growth factor in vascular calcification

Vascular calcification (VC) is prevalent in aging, and patients with hypertension, chronic kidney disease (CKD), or diabetes. VC is regarded as an active and complex process that involves multiple mechanisms responsible for calcium deposition in vessel wall. In light of the complicated pathogenesis of VC, effective therapy for ameliorating VC is limited. Thus, it is urgent to explore the potential mechanisms and find new targets for the therapy of VC. Platelet-derived growth factor (PDGF), a potent mitogen, and chemoattractant have been found to disturb the vascular homeostasis by inducing inflammation, oxidative stress, and phenotype transition, all of which accelerate the process of VC. The aim of current review is to present a review about the roles of PDGF in affecting VC and to establish a potential target for treating VC.

Metabolic Origins of Heart Failure

For more than half a century, metabolic perturbations have been explored in the failing myocardium, highlighting a reversion to a more fetal-like metabolic profile (characterized by depressed fatty acid oxidation and concomitant increased reliance on use of glucose). More recently, alterations in ketone body and amino acid/protein metabolism have been described during heart failure, as well as mitochondrial dysfunction and perturbed metabolic signaling (e.g., acetylation, O-GlcNAcylation). Although numerous mechanisms are likely involved, the current review provides recent advances regarding the metabolic origins of heart failure, and their potential contribution toward contractile dysfunction of the heart.

O-GlcNAcylation and cardiovascular disease

The post-translational modification of serine and threonine residues of proteins found in numerous subcellular locations by O-linked N-acetylglucosamine (O-GlcNAc) is emerging as a key mediator of many cardiovascular pathophysiological processes. Early studies implicated increased protein O-GlcNAcylation as contributing to the cardiovascular complications associated with diabetes, whereas subsequent studies demonstrated that acute increases in O-GlcNAc levels were protective against ischemia/reperfusion injury. There is now a growing understanding that O-GlcNAc modification of proteins influences numerous cellular functions, including transcription, protein turnover, calcium handling, and bioenergetics. As a result, a more nuanced view of the role of protein O-GlcNAcylation in the cardiovascular system is emerging along with the recognition that it is required for normal cellular function and homeostasis. Consequently, the impact of changes in O-GlcNAc cycling due to stress or disease on the heart is complex and highly dependent on the specific context of these events. The goal of this review is to provide an overview of some of the more recent advances in our understanding of the role O-GlcNAcylation plays in mediating cardiovascular function and disease.

Cardiomyocyte Ogt limits ventricular dysfunction in mice following pressure overload without affecting hypertrophy

The myocardial response to pressure overload involves coordination of multiple transcriptional, posttranscriptional, and metabolic cues. The previous studies show that one such metabolic cue, O-GlcNAc, is elevated in the pressure-overloaded heart, and the increase in O-GlcNAcylation is required for cardiomyocyte hypertrophy in vitro. Yet, it is not clear whether and how O-GlcNAcylation participates in the hypertrophic response in vivo. Here, we addressed this question using patient samples and a preclinical model of heart failure. Protein O-GlcNAcylation levels were increased in myocardial tissue from heart failure patients compared with normal patients. To test the role of OGT in the heart, we subjected cardiomyocyte-specific, inducibly deficient Ogt (i-cmOgt ^-/-) mice and Ogt competent littermate wild-type (WT) mice to transverse aortic constriction. Deletion of cardiomyocyte Ogt significantly decreased O-GlcNAcylation and exacerbated ventricular dysfunction, without producing widespread changes in metabolic transcripts. Although some changes in hypertrophic and fibrotic signaling were noted, there were no histological differences in hypertrophy or fibrosis. We next determined whether significant differences were present in i-cmOgt ^-/- cardiomyocytes from surgically naïve mice. Interestingly, markers of cardiomyocyte dedifferentiation were elevated in Ogt-deficient cardiomyocytes. Although no significant differences in cardiac dysfunction were apparent after recombination, it is possible that such changes in dedifferentiation markers could reflect a larger phenotypic shift within the Ogt-deficient cardiomyocytes. We conclude that cardiomyocyte Ogt is not required for cardiomyocyte hypertrophy in vivo; however, loss of Ogt may exert subtle phenotypic differences in cardiomyocytes that sensitize the heart to pressure overload-induced ventricular dysfunction.

Sex Differences in Metabolic Cardiomyopathy

In contrast to ischemic cardiomyopathies which are more common in men, women are over-represented in diabetic cardiomyopathies. Diabetes is a risk factor for cardiovascular disease; however, there is a sexual dimorphism in this risk factor: heart disease is five times more common in diabetic women but only two-times more common in diabetic men. Heart failure with preserved ejection fraction, which is associated with metabolic syndrome, is also more prevalent in women. This review will examine potential mechanisms for the sex differences in metabolic cardiomyopathies. Sex differences in metabolism, calcium handling, nitric oxide, and structural proteins will be evaluated. Nitric oxide synthase and PPARα exhibit sex differences and have also been proposed to mediate the development of hypertrophy and heart failure. We focused on a role for these signalling pathways in regulating sex differences in metabolic cardiomyopathies.

Noninvasive stratification of postinfarction rats based on the degree of cardiac dysfunction using magnetic resonance imaging and echocardiography

The myocardial infarction (MI) rat model plays a crucial role in modern cardiovascular research, but the inherent heterogeneity of this model represents a challenge. We sought to identify subgroups among the post-MI rats and establish simple noninvasive stratification protocols for such subgroups. Six weeks after induction of MI, 49 rats underwent noninvasive examinations using magnetic resonance imaging (MRI) and echocardiography. Twelve sham-operated rats served as controls. Increased end-diastolic left ventricular (LV) pressure and lung weight served as indicators for congestive heart failure (CHF). A clustering algorithm using 13 noninvasive and invasive parameters was used to identify distinct groups among the animals. The cluster analysis revealed four distinct post-MI phenotypes; two without congestion but with different degree of LV dilatation, and two with different degree of congestion and right ventricular (RV) affection. Among the MRI parameters, RV mass emerged as robust noninvasive marker of CHF with 100% specificity/sensitivity. Moreover, LV infarct size and RV ejection fraction further predicted subgroup among the non-CHF and CHF rats with excellent specificity/sensitivity. Of the echocardiography parameters, left atrial diameter predicted CHF. Moreover, LV end-diastolic diameter predicted the subgroups among the non-CHF rats. We propose two simple noninvasive schemes to stratify post-MI rats, based on the degree of heart failure; one for MRI and one for echocardiography.NEW & NOTEWORTHY In vivo phenotyping of rats is essential for robust and reliable data. Here, we present two simple noninvasive schemes for the stratification of postinfarction rats based on the degree of heart failure: one using magnetic resonance imaging and one based on echocardiography.

Exercise mediated protection of diabetic heart through modulation of microRNA mediated molecular pathways

Hyperglycaemia, hypertension, dyslipidemia and insulin resistance collectively impact on the myocardium of people with diabetes, triggering molecular, structural and myocardial abnormalities. These have been suggested to aggravate oxidative stress, systemic inflammation, myocardial lipotoxicity and impaired myocardial substrate utilization. As a consequence, this leads to the development of a spectrum of cardiovascular diseases, which may include but not limited to coronary endothelial dysfunction, and left ventricular remodelling and dysfunction. Diabetic heart disease (DHD) is the term used to describe the presence of heart disease specifically in diabetic patients. Despite significant advances in medical research and long clinical history of anti-diabetic medications, the risk of heart failure in people with diabetes never declines. Interestingly, sustainable and long-term exercise regimen has emerged as an effective synergistic therapy to combat the cardiovascular complications in people with diabetes, although the precise molecular mechanism(s) underlying this protection remain unclear. This review provides an overview of the underlying mechanisms of hyperglycaemia- and insulin resistance-mediated DHD with a detailed discussion on the role of different intensities of exercise in mitigating these molecular alterations in diabetic heart. In particular, we provide the possible role of exercise on microRNAs, the key molecular regulators of several pathophysiological processes.

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.