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The failing heart is subject to elevated metabolic demands, adverse remodeling, chronic apoptosis, and ventricular dysfunction. The interplay among such pathologic changes is largely unknown. Several laboratories have identified a unique posttranslational modification that may have significant effects on cardiovascular function. The O-linked [beta]-N-acetylglucosamine (O-GlcNAc) posttranslational modification (O-GlcNAcylation) integrates glucose metabolism with intracellular protein activity and localization. Because O-GlcNAc is derived from glucose, we hypothesized that altered O-GlcNAcylation would occur during heart failure and figure prominently in its pathophysiology. After 5 d of coronary ligation in WT mice, cardiac O-GlcNAc transferase (OGT; which adds O-GlcNAc to proteins) and levels of OGlcNAcylation were significantly (P < 0.05) elevated in the surviving remote myocardium. We used inducible, cardiac myocyte-specific Cre recombinase transgenic mice crossed with loxP-flanked OGT mice to genetically delete cardiomyocyte OGT (cmOGT KO) and ascertain its role in the failing heart After tamoxifen induction, cardiac O-GlcNAcylation of proteins and OGT levels were significantly reduced compared with WT, but not in other tissues. WT and cardiomyocyte OGT KO mice underwent nonreperfused coronary ligation and were followed for 4 wk. Although OGT deletion caused no functional change in sham-operated mice, OGT deletion in infarcted mice significantly exacerbated cardiac dysfunction compared with WT. These data provide keen insights into the pathophysiology of the failing heart and illuminate a previously unrecognized point of integration between metabolism and cardiac function in the failing heart. heart failure | metabolism | O-GlcNAc | remodeling | infarct doi/ 10.1073/pnas.1001907107

We previously demonstrated that the O-linked [beta]-N-acetylglucosamine (O-GlcNAc) posttranslational modification confers cardioprotection at least partially through mitochondrial-dependent mechanisms, but it remained unclear if O-GlcNAc signaling interfered with other mechanisms of cell death. Because ischemia/hypoxia causes endoplasmic reticulum (ER) stress, we ascertained whether O-GlcNAc signaling could attenuate ER stress-induced cell death per se. Before induction of ER stress (with tunicamycin or brefeldin A), we adenovirally overexpressed O-GlcNAc transferase (AdOGT) or pharmacologically inhibited O-GlcNAcase [via O-(2-acetamido-2-deoxy-D-glucopyranosylidene) amino-N-phenylcarbamate] to augment O-GlcNAc levels or adenovirally overexpressed O-GlcNAcase to reduce O-GlcNAc levels. AdOGT significantly (P < 0.05) attenuated the activation of the maladaptive arm of the unfolded protein response [according to C/EBP homologous protein (CHOP) activation] and cardiomyocyte death (reflected by percent propidium iodide positivity). Moreover, pharmacological inhibition of O-GlcNAcase significantly (P < 0.05) mitigated ER stress-induced CHOP activation and cardiac myocyte death. Interestingly, overexpression of GCA did not alter ER stress markers but exacerbated brefeldin A-induced cardiomyocyte death. We conclude that enhanced O-GlcNAc signaling represents a partially proadaptive response to reduce ER stress-induced cell death. These results provide new insights into a possible interaction between O-GlcNAc signaling and ER stress and may partially explain a mechanism of O-GlcNAc-mediated cardioprotection. O-linked [beta]-N-acetylglucosamine; endoplasmic reticulum doi: 10.1152/ajpheart.00553.2009.

The regulation of cardiomyocyte hypertrophy is a complex interplay among many known and unknown processes. One specific pathway involves the phosphatase calcineurin, which regulates nuclear translocation of the essential cardiac hypertrophy transcription factor, nuclear factor of activated T-cells (NFAT). Although metabolic dysregulation is frequently described during cardiac hypertrophy, limited insights exist regarding various accessory pathways. One metabolically derived signal, beta-O-linked N-acetylglucosamine (O-GlcNAc), has emerged as a highly dynamic posttranslational modification of serine and threonine residues regulating physiological and stress processes. Given the metabolic dysregulation during hypertrophy, we hypothesized that NFAT activation is dependent on O-GlcNAc signaling. Pressure overload-induced hypertrophy (via transverse aortic constriction) in mice or treatment of neonatal rat cardiac myocytes with phenylephrine significantly enhanced global O-GlcNAc signaling. NFAT-luciferase reporter activity revealed O-GlcNAc-dependent NFAT activation during hypertrophy. Reversal of enhanced O-GlcNAc signaling blunted cardiomyocyte NFAT-induced changes during hypertrophy. Taken together, these results demonstrate a critical role of O-GlcNAc signaling in NFAT activation during hypertrophy and provide evidence that O-GlcNAc signaling is coordinated with the onset and progression of cardiac hypertrophy. This represents a potentially significant and novel mechanism of cardiac hypertrophy, which may be of particular interest in future in vivo studies of hypertrophy.

Molecular studies examining the impact of mitochondrial morphology on the mammalian heart have previously focused on dynamin related protein-1 (Drp-1) and mitofusin-2 (Mfn-2), while the role of the other mitofusin isoform, Mfn-1, has remained largely unexplored. In the present study, we report the generation and initial characterization of cardiomyocyte-specific Mfn-1 knockout (Mfn-1 KO) mice. Using electron microscopic analysis, we detect a greater prevalence of small, spherical mitochondria in Mfn-1 KO hearts, indicating that the absence of Mfn-1 causes a profound shift in the mitochondrial fusion/fission balance. Nevertheless, Mfn-1 KO mice exhibit normal left-ventricular function, and isolated Mfn-1 KO heart mitochondria display a normal respiratory repertoire. Mfn-1 KO myocytes are protected from mitochondrial depolarization and exhibit improved viability when challenged with reactive oxygen species (ROS) in the form of hydrogen peroxide (H(2)O(2)). Furthermore, in vitro studies detect a blunted response of KO mitochondria to undergo peroxide-induced mitochondrial permeability transition pore opening. These data suggest that Mfn-1 deletion confers protection against ROS-induced mitochondrial dysfunction. Collectively, we suggest that mitochondrial fragmentation in myocytes is not sufficient to induce heart dysfunction or trigger cardiomyocyte death. Additionally, our data suggest that endogenous levels of Mfn-1 can attenuate myocyte viability in the face of an imminent ROS overload, an effect that could be associated with the ability of Mfn-1 to remodel the outer mitochondrial membrane.