Lorena Garcia-Menendez

Mitochondrial Complex I Deficiency Increases Protein Acetylation and Accelerates Heart Failure

Mitochondrial respiratory dysfunction is linked to the pathogenesis of multiple diseases, including heart failure, but the specific mechanisms for this link remain largely elusive. We modeled the impairment of mitochondrial respiration by the inactivation of the Ndufs4 gene, a protein critical for complex I assembly, in the mouse heart (cKO). Although complex I-supported respiration decreased by >40%, the cKO mice maintained normal cardiac function in vivo and high-energy phosphate content in isolated perfused hearts. However, the cKO mice developed accelerated heart failure after pressure overload or repeated pregnancy. Decreased NAD(+)/NADH ratio by complex I deficiency inhibited Sirt3 activity, leading to an increase in protein acetylation and sensitization of the permeability transition in mitochondria (mPTP). NAD(+) precursor supplementation to cKO mice partially normalized the NAD(+)/NADH ratio, protein acetylation, and mPTP sensitivity. These findings describe a mechanism connecting mitochondrial dysfunction to the susceptibility to diseases and propose a potential therapeutic target.

Cardiac-Specific Deletion of Acetyl CoA Carboxylase 2 Prevents Metabolic Remodeling During Pressure-Overload Hypertrophy

Rationale: Decreased fatty acid oxidation (FAO) with increased reliance on glucose are hallmarks of metabolic remodeling that occurs in pathological cardiac hypertrophy and is associated with decreased myocardial energetics and impaired cardiac function. To date, it has not been tested whether prevention of the metabolic switch that occurs during the development of cardiac hypertrophy has unequivocal benefits on cardiac function and energetics. Objective: Because malonyl CoA production via acetyl CoA carboxylase 2 (ACC2) inhibits the entry of long chain fatty acids into the mitochondria, we hypothesized that mice with a cardiac-specific deletion of ACC2 (ACC2H−/−) would maintain cardiac FAO and improve function and energetics during the development of pressure-overload hypertrophy. Methods and Results: ACC2 deletion led to a significant reduction in cardiac malonyl CoA levels. In isolated perfused heart experiments, left ventricular function and oxygen consumption were similar in ACC2H−/− mice despite an ≈60% increase in FAO compared with controls (CON). After 8 weeks of pressure overload via transverse aortic constriction (TAC), ACC2H−/− mice exhibited a substrate utilization profile similar to sham animals, whereas CON-TAC hearts had decreased FAO with increased glycolysis and anaplerosis. Myocardial energetics, assessed by 31P nuclear magnetic resonance spectroscopy, and cardiac function were maintained in ACC2H−/− after 8 weeks of TAC. Furthermore, ACC2H−/−-TAC demonstrated an attenuation of cardiac hypertrophy with a significant reduction in fibrosis relative to CON-TAC. Conclusions: These data suggest that reversion to the fetal metabolic profile in chronic pathological hypertrophy is associated with impaired myocardial function and energetics and maintenance of the inherent cardiac metabolic profile and mitochondrial oxidative capacity is a viable therapeutic strategy.

Normalization of NAD+ Redox Balance as a Therapy for Heart Failure.

Background: Impairments of mitochondrial function in the heart are linked intricately to the development of heart failure, but there is no therapy for mitochondrial dysfunction. Methods: We assessed the reduced/oxidized ratio of nicotinamide adenine dinucleotide (NADH/NAD+ ratio) and protein acetylation in the failing heart. Proteome and acetylome analyses were followed by docking calculation, mutagenesis, and mitochondrial calcium uptake assays to determine the functional role of specific acetylation sites. The therapeutic effects of normalizing mitochondrial protein acetylation by expanding the NAD+ pool also were tested. Results: Increased NADH/NAD+ and protein hyperacetylation, previously observed in genetic models of defective mitochondrial function, also are present in human failing hearts as well as in mouse hearts with pathologic hypertrophy. Elevation of NAD+ levels by stimulating the NAD+ salvage pathway suppressed mitochondrial protein hyperacetylation and cardiac hypertrophy, and improved cardiac function in responses to stresses. Acetylome analysis identified a subpopulation of mitochondrial proteins that was sensitive to changes in the NADH/NAD+ ratio. Hyperacetylation of mitochondrial malate-aspartate shuttle proteins impaired the transport and oxidation of cytosolic NADH in the mitochondria, resulting in altered cytosolic redox state and energy deficiency. Furthermore, acetylation of oligomycin-sensitive conferring protein at lysine-70 in adenosine triphosphate synthase complex promoted its interaction with cyclophilin D, and sensitized the opening of mitochondrial permeability transition pore. Both could be alleviated by normalizing the NAD+ redox balance either genetically or pharmacologically. Conclusions: We show that mitochondrial protein hyperacetylation due to NAD+ redox imbalance contributes to the pathologic remodeling of the heart via 2 distinct mechanisms. Our preclinical data demonstrate a clear benefit of normalizing NADH/NAD+ imbalance in the failing hearts. These findings have a high translational potential as the pharmacologic strategy of increasing NAD+ precursors are feasible in humans.

Mutation in the γ2-Subunit of AMP-activated protein kinase stimulates cardiomyocyte proliferation and hypertrophy independent of glycogen storage

Rationale: AMP-activated protein kinase is a master regulator of cell metabolism and an attractive drug target for cancer and metabolic and cardiovascular diseases. Point mutations in the regulatory &ggr;2-subunit of AMP-activated protein kinase (encoded by Prkag2 gene) caused a unique form of human cardiomyopathy characterized by cardiac hypertrophy, ventricular preexcitation, and glycogen storage. Understanding the disease mechanisms of Prkag2 cardiomyopathy is not only beneficial for the patients but also critical to the use of AMP-activated protein kinase as a drug target. Objective: We sought to identify the pro–growth-signaling pathway(s) triggered by Prkag2 mutation and to distinguish it from the secondary response to glycogen storage. Methods and Results: In a mouse model of N488I mutation of the Prkag2 gene (R2M), we rescued the glycogen storage phenotype by genetic inhibition of glucose-6-phosphate–stimulated glycogen synthase activity. Ablation of glycogen storage eliminated the ventricular preexcitation but did not affect the excessive cardiac growth in R2M mice. The progrowth effect in R2M hearts was mediated via increased insulin sensitivity and hyperactivity of Akt, resulting in activation of mammalian target of rapamycin and inactivation of forkhead box O transcription factor–signaling pathways. Consequently, cardiac myocyte proliferation during the postnatal period was enhanced in R2M hearts followed by hypertrophic growth in adult hearts. Inhibition of mammalian target of rapamycin activity by rapamycin or restoration of forkhead box O transcription factor activity by overexpressing forkhead box O transcription factor 1 rescued the abnormal cardiac growth. Conclusions: Our study reveals a novel mechanism for Prkag2 cardiomyopathy, independent of glycogen storage. The role of &ggr;2-AMP-activated protein kinase in cell growth also has broad implications in cardiac development, growth, and regeneration.

Substrain specific response to cardiac pressure overload in C57BL/6 mice

The C57BL/6 mouse strain is one of the most commonly used in experimental research. It is known to differ from other strains in baseline cardiovascular phenotypes as well as in response to pressure overload induced by aortic constriction. Since the generation of the C57BL/6 mouse line over a century ago, multiple substrains have been generated from the original. To identify potential substrain specific differences in response to pressure overload, we evaluated the effects of transverse aortic constriction (TAC) on survival, cardiac function, and expression of hypertrophic markers in three commonly used C57BL/6 substrains: C57BL/6J (JL), C57BL/6NCrl (CL), and C57BL/6NTac (TF). Survival and cardiac function were significantly lower in the CL and TF substrains compared with JL mice after TAC. Furthermore, the heart weight and lung weight as well as the expression of the hypertrophic marker Bnp were significantly greater in the CL mice compared with the JL. Histological assessment revealed marked left ventricular dilatation of CL and TF hearts while JL hearts showed increased wall thickness without dilatation. Our data demonstrate that cardiac response to pressure overload is distinct among the three commonly used C57BL/6 substrains of mice, which raises a cautionary note in study design and data interpretation.

Mutation in the γ2-subunit of AMPK Stimulates Cardiomyocyte Proliferation and Hypertrophy Independent of Glycogen Storage

Rationale: AMP-activated protein kinase is a master regulator of cell metabolism and an attractive drug target for cancer and metabolic and cardiovascular diseases. Point mutations in the regulatory &ggr;2-subunit of AMP-activated protein kinase (encoded by Prkag2 gene) caused a unique form of human cardiomyopathy characterized by cardiac hypertrophy, ventricular preexcitation, and glycogen storage. Understanding the disease mechanisms of Prkag2 cardiomyopathy is not only beneficial for the patients but also critical to the use of AMP-activated protein kinase as a drug target. Objective: We sought to identify the pro–growth-signaling pathway(s) triggered by Prkag2 mutation and to distinguish it from the secondary response to glycogen storage. Methods and Results: In a mouse model of N488I mutation of the Prkag2 gene (R2M), we rescued the glycogen storage phenotype by genetic inhibition of glucose-6-phosphate–stimulated glycogen synthase activity. Ablation of glycogen storage eliminated the ventricular preexcitation but did not affect the excessive cardiac growth in R2M mice. The progrowth effect in R2M hearts was mediated via increased insulin sensitivity and hyperactivity of Akt, resulting in activation of mammalian target of rapamycin and inactivation of forkhead box O transcription factor–signaling pathways. Consequently, cardiac myocyte proliferation during the postnatal period was enhanced in R2M hearts followed by hypertrophic growth in adult hearts. Inhibition of mammalian target of rapamycin activity by rapamycin or restoration of forkhead box O transcription factor activity by overexpressing forkhead box O transcription factor 1 rescued the abnormal cardiac growth. Conclusions: Our study reveals a novel mechanism for Prkag2 cardiomyopathy, independent of glycogen storage. The role of &ggr;2-AMP-activated protein kinase in cell growth also has broad implications in cardiac development, growth, and regeneration.

Revealing Pathway Dynamics in Heart Diseases by Analyzing Multiple Differential Networks

Development of heart diseases is driven by dynamic changes in both the activity and connectivity of gene pathways. Understanding these dynamic events is critical for understanding pathogenic mechanisms and development of effective treatment. Currently, there is a lack of computational methods that enable analysis of multiple gene networks, each of which exhibits differential activity compared to the network of the baseline/healthy condition. We describe the iMDM algorithm to identify both unique and shared gene modules across multiple differential co-expression networks, termed M-DMs (multiple differential modules). We applied iMDM to a time-course RNA-Seq dataset generated using a murine heart failure model generated on two genotypes. We showed that iMDM achieves higher accuracy in inferring gene modules compared to using single or multiple co-expression networks. We found that condition-specific M-DMs exhibit differential activities, mediate different biological processes, and are enriched for genes with known cardiovascular phenotypes. By analyzing M-DMs that are present in multiple conditions, we revealed dynamic changes in pathway activity and connectivity across heart failure conditions. We further showed that module dynamics were correlated with the dynamics of disease phenotypes during the development of heart failure. Thus, pathway dynamics is a powerful measure for understanding pathogenesis. iMDM provides a principled way to dissect the dynamics of gene pathways and its relationship to the dynamics of disease phenotype. With the exponential growth of omics data, our method can aid in generating systems-level insights into disease progression.

Promoting PGC-1α-driven mitochondrial biogenesis is detrimental in pressure-overloaded mouse hearts.

Mitochondrial dysfunction in animal models of heart failure is associated with downregulation of the peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α pathway. To test whether PGC-1α is an appropriate therapeutic target for increasing mitochondrial biogenesis and improving function in heart failure, we used a transgenic (TG) mouse model of moderate overexpression of PGC-1α (∼3-fold) in the heart. TG mice had small increases in citrate synthase activity and mitochondria size in the heart without alterations in myocardial energetics or cardiac function at baseline. In vivo dobutamine stress increased fractional shortening in wild-type mice, but this increase was attenuated in TG mice, whereas ex vivo isolated perfused TG hearts demonstrated normal functional and energetic response to high workload challenge. When subjected to pressure overload by transverse aortic constriction (TAC), TG mice displayed a significantly greater acute mortality for both male and female mice; however, long-term survival up to 8 wk was similar between the two groups. TG mice also showed a greater decrease in fractional shortening and a greater increase in left ventricular chamber dimension in response to TAC. Mitochondrial gene expression and citrate synthase activity were mildly increased in TG mice compared with wild-type mice, and this difference was also maintained after TAC. Our data suggest that a moderate level of PGC-1α overexpression in the heart compromises acute survival and does not improve cardiac function during chronic pressure overload in mice.

Mutation in the γ2-Subunit of AMP-Activated Protein Kinase Stimulates Cardiomyocyte Proliferation and Hypertrophy Independent of Glycogen StorageNovelty and Significance

Rationale: AMP-activated protein kinase is a master regulator of cell metabolism and an attractive drug target for cancer and metabolic and cardiovascular diseases. Point mutations in the regulatory &ggr;2-subunit of AMP-activated protein kinase (encoded by Prkag2 gene) caused a unique form of human cardiomyopathy characterized by cardiac hypertrophy, ventricular preexcitation, and glycogen storage. Understanding the disease mechanisms of Prkag2 cardiomyopathy is not only beneficial for the patients but also critical to the use of AMP-activated protein kinase as a drug target. Objective: We sought to identify the pro–growth-signaling pathway(s) triggered by Prkag2 mutation and to distinguish it from the secondary response to glycogen storage. Methods and Results: In a mouse model of N488I mutation of the Prkag2 gene (R2M), we rescued the glycogen storage phenotype by genetic inhibition of glucose-6-phosphate–stimulated glycogen synthase activity. Ablation of glycogen storage eliminated the ventricular preexcitation but did not affect the excessive cardiac growth in R2M mice. The progrowth effect in R2M hearts was mediated via increased insulin sensitivity and hyperactivity of Akt, resulting in activation of mammalian target of rapamycin and inactivation of forkhead box O transcription factor–signaling pathways. Consequently, cardiac myocyte proliferation during the postnatal period was enhanced in R2M hearts followed by hypertrophic growth in adult hearts. Inhibition of mammalian target of rapamycin activity by rapamycin or restoration of forkhead box O transcription factor activity by overexpressing forkhead box O transcription factor 1 rescued the abnormal cardiac growth. Conclusions: Our study reveals a novel mechanism for Prkag2 cardiomyopathy, independent of glycogen storage. The role of &ggr;2-AMP-activated protein kinase in cell growth also has broad implications in cardiac development, growth, and regeneration.