Aberrant activation of AMP-activated protein kinase remodels metabolic network in favor of cardiac glycogen storage

Exercise training adaptations in liver glycogen and glycerolipids require hepatic AMP-activated protein kinase in mice

Regular exercise elicits adaptations in glucose and lipid metabolism that allow the body to meet energy demands of subsequent exercise bouts more effectively and mitigate metabolic diseases including fatty liver. Energy discharged during the acute exercise bouts that comprise exercise training may be a catalyst for liver adaptations. During acute exercise, liver glycogenolysis and gluconeogenesis are accelerated to supply glucose to working muscle. Lower liver energy state imposed by gluconeogenesis and related pathways activates AMP-activated protein kinase (AMPK), which conserves ATP partly by promoting lipid oxidation. This study tested the hypothesis that AMPK is necessary for liver glucose and lipid adaptations to training. Liver-specific AMPKα1α2 knockout (AMPKα1α2fl/fl+AlbCre) mice and littermate controls (AMPKα1α2fl/fl) completed sedentary and exercise training protocols. Liver nutrient fluxes were quantified at rest or during acute exercise following training. Liver metabolites and molecular regulators of metabolism were assessed. Training increased liver glycogen in AMPKα1α2fl/fl mice, but not in AMPKα1α2fl/fl+AlbCre mice. The inability to increase glycogen led to lower glycogenolysis, glucose production, and circulating glucose during acute exercise in trained AMPKα1α2fl/fl+AlbCre mice. Deletion of AMPKα1α2 attenuated training-induced declines in liver diacylglycerides. In particular, training lowered the concentration of unsaturated and elongated fatty acids comprising diacylglycerides in AMPKα1α2fl/fl mice, but not in AMPKα1α2fl/fl+AlbCre mice. Training increased liver triacylglycerides and the desaturation and elongation of fatty acids in triacylglycerides of AMPKα1α2fl/fl+AlbCre mice. These lipid responses were independent of differences in tricarboxylic acid cycle fluxes. In conclusion, AMPK is required for liver training adaptations that are critical to glucose and lipid metabolism.NEW & NOTEWORTHY This study shows that the energy sensor and transducer, AMP-activated protein kinase (AMPK), is necessary for an exercise training-induced: 1) increase in liver glycogen that is necessary for accelerated glycogenolysis during exercise, 2) decrease in liver glycerolipids independent of tricarboxylic acid (TCA) cycle flux, and 3) decline in the desaturation and elongation of fatty acids comprising liver diacylglycerides. The mechanisms defined in these studies have implications for use of regular exercise or AMPK-activators in patients with fatty liver.

Animal Models of Dysregulated Cardiac Metabolism

As a muscular pump that contracts incessantly throughout life, the heart must constantly generate cellular energy to support contractile function and fuel ionic pumps to maintain electrical homeostasis. Thus, mitochondrial metabolism of multiple metabolic substrates such as fatty acids, glucose, ketones, and lactate is essential to ensuring an uninterrupted supply of ATP. Multiple metabolic pathways converge to maintain myocardial energy homeostasis. The regulation of these cardiac metabolic pathways has been intensely studied for many decades. Rapid adaptation of these pathways is essential for mediating the myocardial adaptation to stress, and dysregulation of these pathways contributes to myocardial pathophysiology as occurs in heart failure and in metabolic disorders such as diabetes. The regulation of these pathways reflects the complex interactions of cell-specific regulatory pathways, neurohumoral signals, and changes in substrate availability in the circulation. Significant advances have been made in the ability to study metabolic regulation in the heart, and animal models have played a central role in contributing to this knowledge. This review will summarize metabolic pathways in the heart and describe their contribution to maintaining myocardial contractile function in health and disease. The review will summarize lessons learned from animal models with altered systemic metabolism and those in which specific metabolic regulatory pathways have been genetically altered within the heart. The relationship between intrinsic and extrinsic regulators of cardiac metabolism and the pathophysiology of heart failure and how these have been informed by animal models will be discussed.

A Novel Small Molecule Troponin Activator Increases Cardiac Contractile Function Without Negative Impact on Energetics

Background: Current heart failure (HF) therapies unload the failing heart without targeting the underlying problem of reduced cardiac contractility. Traditional inotropes (i.e. calcitropes) stimulate contractility via energetically costly augmentation of calcium cycling and worsen patient survival. A new class of agents - myotropes - activate the sarcomere directly, independent of calcium. We hypothesize that a novel myotrope TA1 increases contractility without the deleterious myocardial energetic impact of a calcitrope dobutamine. Methods: We determined the effect of TA1 in bovine cardiac myofibrils and human cardiac microtissues, ex vivo in mouse cardiac fibers and in vivo in anesthetized normal rats. Effects of increasing concentrations of TA1 or dobutamine on contractile function, phosphocreatine (PCr) and ATP concentrations and ATP production were assessed by ³¹P NMR spectroscopy on isolated perfused rat hearts. Results: TA1 increased the rate of myosin ATPase activity in isolated bovine myofibrils and calcium sensitivity in intact mouse papillary fibers. Contractility increased dose dependently in human cardiac microtissues and in vivo in rats as assessed by echocardiography. In isolated rat hearts, TA1 and dobutamine similarly increased rate pressure product (RPP). Dobutamine increased both developed pressure (DevP) and heart rate (HR) accompanied by decreased PCr to ATP ratio and decreased free energy of ATP hydrolysis (ΔG~_ATP) and elevated left ventricular end-diastolic pressure (LVEDP). In contrast, the TA1 increased DevP without any effect on HR, LVEDP, PCr/ATP ratio or ΔG~_ATP. Conclusions: Novel myotrope, TA1, increased myocardial contractility by sensitizing the sarcomere to calcium without impairing diastolic function or depleting the cardiac energy reserve. Since energetic depletion negatively correlates with long term survival, myotropes may represent a superior alternative to traditional inotropes in heart failure management.

LAMP2 Cardiomyopathy: Consequences of Impaired Autophagy in the Heart

Background

Human mutations in the X‐linked lysosome‐associated membrane protein‐2 (LAMP2) gene can cause a multisystem Danon disease or a primary cardiomyopathy characterized by massive hypertrophy, conduction system abnormalities, and malignant ventricular arrhythmias. We introduced an in‐frame LAMP2 gene exon 6 deletion mutation (denoted L2^Δ6) causing human cardiomyopathy, into mouse LAMP2 gene, to elucidate its consequences on cardiomyocyte biology. This mutation results in in‐frame deletion of 41 amino acids, compatible with presence of some defective LAMP2 protein.

Methods and Results

Left ventricular tissues from L2^Δ6 and wild‐type mice had equivalent amounts of LAMP2 RNA, but a significantly lower level of LAMP2 protein. By 20 weeks of age male mutant mice developed left ventricular hypertrophy which was followed by left ventricular dilatation and reduced systolic function. Cardiac electrophysiology and isolated cardiomyocyte studies demonstrated ventricular arrhythmia, conduction disturbances, abnormal calcium transients and increased sensitivity to catecholamines. Myocardial fibrosis was strikingly increased in 40‐week‐old L2^Δ6 mice, recapitulating findings of human LAMP2 cardiomyopathy. Immunofluorescence and transmission electron microscopy identified mislocalization of lysosomes and accumulation of autophagosomes between sarcomeres, causing profound morphological changes disrupting the cellular ultrastructure. Transcription profile and protein expression analyses of L2^Δ6 hearts showed significantly increased expression of genes encoding activators and protein components of autophagy, hypertrophy, and apoptosis.

Conclusions

We suggest that impaired autophagy results in cardiac hypertrophy and profound transcriptional reactions that impacted metabolism, calcium homeostasis, and cell survival. These responses define the molecular pathways that underlie the pathology and aberrant electrophysiology in cardiomyopathy of Danon disease.

FOSL2 Is Involved in the Regulation of Glycogen Content in Chicken Breast Muscle Tissue

The glycogen content in muscle of livestock and poultry animals affects the homeostasis of their body, growth performance, and meat quality after slaughter. FOS-like 2, AP-1 transcription factor subunit (FOSL2) was identified as a candidate gene related to muscle glycogen (MG) content in chicken in our previous study, but the role of FOSL2 in the regulation of MG content remains to be elucidated. Differential gene expression analysis and weighted gene coexpression network analysis (WGCNA) were performed on differentially expressed genes (DEGs) in breast muscle tissues from the high-MG-content (HMG) group and low-MG-content (LMG) group of Jingxing yellow chickens. Analysis of the 1,171 DEGs (LMG vs. HMG) identified, besides FOSL2, some additional genes related to MG metabolism pathway, namely PRKAG3, CEBPB, FOXO1, AMPK, and PIK3CB. Additionally, WGCNA revealed that FOSL2, CEBPB, MAP3K14, SLC2A14, PPP2CA, SLC38A2, PPP2R5E, and other genes related to the classical glycogen metabolism in the same coexpressed module are associated with MG content. Also, besides finding that FOSL2 expression is negatively correlated with MG content, a possible interaction between FOSL2 and CEBPB was predicted using the STRING (Search Tool for the Retrieval of Interacting Genes) database. Furthermore, we investigated the effects of lentiviral overexpression of FOSL2 on the regulation of the glycogen content in vitro, and the result indicated that FOSL2 decreases the glycogen content in DF1 cells. Collectively, our results confirm that FOSL2 has a key role in the regulation of the MG content in chicken. This finding is helpful to understand the mechanism of MG metabolism regulation in chicken and provides a new perspective for the production of high-quality broiler and the development of a comprehensive nutritional control strategy.

A Two-Stage Study Identifies Two Novel Polymorphisms in PRKAG2 Affecting Metformin Response in Chinese Type 2 Diabetes Patients

Objective

Individual differences in glycemic response to metformin in antidiabetic treatment exist widely. Although some associated genetic variations have been discovered, they still cannot accurately predict metformin response. In the current study, we set out to investigate novel genetic variants affecting metformin response in Chinese type 2 diabetes (T2D) patients.

Methods

A two-stage study enrolled 500 T2D patients who received metformin, glibenclamide or a combination of both were recruited from 2009 to 2012 in China. Change of HbA_1c, adjusted by clinical covariates, was used to evaluate glycemic response to metformin. Selected single nucleotide polymorphisms (SNPs) were genotyped using the Infinium iSelect and/or Illumina GoldenGate genotyping platform. A linear regression model was used to evaluate the association between SNPs and response.

Results

A total of 3739 SNPs were screened in Stage 1, of which 50 were associated with drug response. Except for one genetic variant preferred to affect glibenclamide, the remaining SNPs were subsequently verified in Stage 2, and two SNPs were successfully validated. These were PRKAG2 rs2727528 (discovery group: β=−0.212, P=0.046; validation group: β=−0.269, P=0.028) and PRKAG2 rs1105842 (discovery group: β=0.205, P=0.048; validation group: β=0.273, P=0.025). C allele carriers of rs2727528 and C allele carriers of rs1105842 would have a larger difference of HbA_1c level when using metformin.

Conclusion

Two variants rs2727528 and rs1105842 in PRKAG2, encoding γ2 subunit of AMP-activated protein kinase (AMPK), were found to be associated with metformin response in Chinese T2D patients. These findings may provide some novel information for personalized pharmacotherapy of metformin in China.

Cardiometabolism as an Interlocking Puzzle between the Healthy and Diseased Heart: New Frontiers in Therapeutic Applications

Cardiac metabolism represents a crucial and essential connecting bridge between the healthy and diseased heart. The cardiac muscle, which may be considered an omnivore organ with regard to the energy substrate utilization, under physiological conditions mainly draws energy by fatty acids oxidation. Within cardiomyocytes and their mitochondria, through well-concerted enzymatic reactions, substrates converge on the production of ATP, the basic chemical energy that cardiac muscle converts into mechanical energy, i.e., contraction. When a perturbation of homeostasis occurs, such as an ischemic event, the heart is forced to switch its fatty acid-based metabolism to the carbohydrate utilization as a protective mechanism that allows the maintenance of its key role within the whole organism. Consequently, the flexibility of the cardiac metabolic networks deeply influences the ability of the heart to respond, by adapting to pathophysiological changes. The aim of the present review is to summarize the main metabolic changes detectable in the heart under acute and chronic cardiac pathologies, analyzing possible therapeutic targets to be used. On this basis, cardiometabolism can be described as a crucial mechanism in keeping the physiological structure and function of the heart; furthermore, it can be considered a promising goal for future pharmacological agents able to appropriately modulate the rate-limiting steps of heart metabolic pathways.

Cardiac sodium-dependent glucose cotransporter 1 is a novel mediator of ischaemia/reperfusion injury

AIMS

We previously reported that sodium-dependent glucose cotransporter 1 (SGLT1) is highly expressed in cardiomyocytes and is further up-regulated in ischaemia. This study aimed to determine the mechanisms by which SGLT1 contributes to ischaemia/reperfusion (I/R) injury.

METHODS AND RESULTS

Mice with cardiomyocyte-specific knockdown of SGLT1 (TGSGLT1-DOWN) and wild-type controls were studied. In vivo, the left anterior descending coronary artery was ligated for 30 min and reperfused for 48 h. Ex vivo, isolated perfused hearts were exposed to 20 min no-flow and up to 2 h reperfusion. In vitro, HL-1 cells and isolated adult murine ventricular cardiomyocytes were exposed to 1 h hypoxia and 24 h reoxygenation (H/R). We found that TGSGLT1-DOWN hearts were protected from I/R injury in vivo and ex vivo, with decreased infarct size, necrosis, dysfunction, and oxidative stress. 5'-AMP-activated protein kinase (AMPK) activation increased SGLT1 expression, which was abolished by extracellular signal-related kinase (ERK) inhibition. Co-immunoprecipitation studies showed that ERK, but not AMPK, interacts directly with SGLT1. AMPK activation increased binding of the hepatocyte nuclear factor 1 and specificity protein 1 transcription factors to the SGLT1 gene, and HuR to SGLT1 mRNA. In cells, up-regulation of SGLT1 during H/R was abrogated by AMPK inhibition. Co-immunoprecipitation studies showed that SGLT1 interacts with epidermal growth factor receptor (EGFR), and EGFR interacts with protein kinase C (PKC). SGLT1 overexpression activated PKC and NADPH oxidase 2 (Nox2), which was attenuated by PKC inhibition, EGFR inhibition, and/or disruption of the interaction between EGFR and SGLT1.

CONCLUSION

During ischaemia, AMPK up-regulates SGLT1 through ERK, and SGLT1 interacts with EGFR, which in turn increases PKC and Nox2 activity and oxidative stress. SGLT1 may represent a novel therapeutic target for mitigating I/R injury.

Metabolic Alterations in Inherited Cardiomyopathies

The normal function of the heart relies on a series of complex metabolic processes orchestrating the proper generation and use of energy. In this context, mitochondria serve a crucial role as a platform for energy transduction by supplying ATP to the varying demand of cardiomyocytes, involving an intricate network of pathways regulating the metabolic flux of substrates. The failure of these processes results in structural and functional deficiencies of the cardiac muscle, including inherited cardiomyopathies. These genetic diseases are characterized by cardiac structural and functional anomalies in the absence of abnormal conditions that can explain the observed myocardial abnormality, and are frequently associated with heart failure. Since their original description, major advances have been achieved in the genetic and phenotype knowledge, highlighting the involvement of metabolic abnormalities in their pathogenesis. This review provides a brief overview of the role of mitochondria in the energy metabolism in the heart and focuses on metabolic abnormalities, mitochondrial dysfunction, and storage diseases associated with inherited cardiomyopathies.

Molecular Pathogenesis of Familial Wolff-Parkinson-White Syndrome

Familial Wolff-Parkinson-White (WPW) syndrome is an autosomal dominant inherited disease and consists of a small percentage of WPW syndrome which exhibits ventricular pre-excitation by development of accessory atrioventricular pathway. A series of mutations in PRKAG2 gene encoding gamma2 subunit of 5'AMP-activated protein kinase (AMPK) has been identified as the cause of familial WPW syndrome. AMPK is one of the most important metabolic regulators of carbohydrates and lipids in many types of tissues including cardiac and skeletal muscles. Patients and animals with the mutation in PRKAG2 gene exhibit aberrant atrioventricular conduction associated with cardiac glycogen overload. Recent studies have revealed "novel" significance of canonical pathways leading to glycogen synthesis and provided us profound insights into molecular mechanism of the regulation of glycogen metabolism by AMPK. This review focuses on the molecular basis of the pathogenesis of cardiac abnormality due to PRKAG2 mutation and will provide current overviews of the mechanism of glycogen regulation by AMPK. J. Med. Invest. 65:1-8, February, 2018.

Biochemical Measurement of Glycogen: Method to Investigate the AMPK-Glycogen Relationship

Glycogen is a main carbohydrate energy storage primarily found in fungi and animals. It is a glucose polymer that comprises α(1-4) glycosidic linkages attaching UDP-glucose molecules linearly and α(1-6) linkages branching glucose chains every 8-10 molecules to the main backbone chain. Glycogen synthase, branching enzyme, and glycogen phosphorylase are key enzymes involved in glycogen synthesis and degradation. These enzymes are tightly regulated by upstream kinases and phosphatases that respond to hormonal cues in order to coordinate storage and degradation and meet the cellular and organismal metabolic needs. The 5'AMP-activated protein kinase (AMPK) is one of the main regulators of glycogen metabolism. Despite extensive research, the role of AMPK in glycogen synthesis and degradation remains controversial. Specifically, the level and duration of AMPK activity highly influence the outcome on glycogen reserves. Here, we describe a rapid and robust protocol to efficiently measure the levels of glycogen in vitro. We use the commercially available glycogen determination kit to hydrolyze glycogen into glucose, which is oxidized to form D-gluconic acid and hydrogen peroxide that react with the OxiRed/Amplex Red probe generating a product that could be detected either in a colorimetric or fluorimetric plate format. This method is quantitative and could be used to address the role of AMPK in glycogen metabolism in cells and tissues. Summary This chapter provides a quick and reliable biochemical quantitative method to measure glycogen in cells and tissues. Briefly, this method is based on the degradation of glycogen to glucose, which is then specifically oxidized to generate a product that reacts with the OxiRed probe with maximum absorbance at 570 nm. This method is very accurate and highly sensitive. In the notes of this chapter, we shed the light on important actions that should be followed to get reliable results. We also state advantages and disadvantages of this method in comparison to other glycogen measurement techniques.

Human γ2-AMPK Mutations

In humans, dominant mutations in the gene encoding the regulatory γ2-subunit of AMP-activated protein kinase (PRKAG2) result in a highly penetrant phenotype dominated by cardiac features: left ventricular hypertrophy, ventricular pre-excitation, atrial tachyarrhythmia, cardiac conduction disease, and myocardial glycogen storage. The discovery of a link between the cell's fundamental energy sensor, AMPK, and inherited cardiac disease catalyzed intense interest into the biological role of AMPK in the heart. In this chapter, we provide an introduction to the spectrum of human disease resulting from pathogenic variants in PRKAG2, outlining its discovery, clinical genetics, and current perspectives on its pathogenesis and highlighting mechanistic insights derived through the evaluation of disease models. We also present a clinical perspective on the major components of the cardiomyopathy associated with mutations in PRKAG2, together with less commonly described extracardiac features, its prognosis, and principles of management.

Loss of hepatic AMP-activated protein kinase impedes the rate of glycogenolysis but not gluconeogenic fluxes in exercising mice

Pathologies including diabetes and conditions such as exercise place an unusual demand on liver energy metabolism, and this demand induces a state of energy discharge. Hepatic AMP-activated protein kinase (AMPK) has been proposed to inhibit anabolic processes such as gluconeogenesis in response to cellular energy stress. However, both AMPK activation and glucose release from the liver are increased during exercise. Here, we sought to test the role of hepatic AMPK in the regulation of in vivo glucose-producing and citric acid cycle-related fluxes during an acute bout of muscular work. We used ²H/¹³C metabolic flux analysis to quantify intermediary metabolism fluxes in both sedentary and treadmill-running mice. Additionally, liver-specific AMPK α1 and α2 subunit KO and WT mice were utilized. Exercise caused an increase in endogenous glucose production, glycogenolysis, and gluconeogenesis from phosphoenolpyruvate. Citric acid cycle fluxes, pyruvate cycling, anaplerosis, and cataplerosis were also elevated during this exercise. Sedentary nutrient fluxes in the postabsorptive state were comparable for the WT and KO mice. However, the increment in the endogenous rate of glucose appearance during exercise was blunted in the KO mice because of a diminished glycogenolytic flux. This lower rate of glycogenolysis was associated with lower hepatic glycogen content before the onset of exercise and prompted a reduction in arterial glucose during exercise. These results indicate that liver AMPKα1α2 is required for maintaining glucose homeostasis during an acute bout of exercise.

Activation of γ2-AMPK Suppresses Ribosome Biogenesis and Protects Against Myocardial Ischemia/Reperfusion Injury

RATIONALE

AMPK (AMP-activated protein kinase) is a heterotrimeric protein that plays an important role in energy homeostasis and cardioprotection. Two isoforms of each subunit are expressed in the heart, but the isoform-specific function of AMPK remains unclear.

OBJECTIVE

We sought to determine the role of γ2-AMPK in cardiac stress response using bioengineered cell lines and mouse models containing either isoform of the γ-subunit in the heart.

METHODS AND RESULTS

We found that γ2 but not γ1 or γ3 subunit translocated into nucleus on AMPK activation. Nuclear accumulation of AMPK complexes containing γ2-subunit phosphorylated and inactivated RNA Pol I (polymerase I)-associated transcription factor TIF-IA at Ser-635, precluding the assembly of transcription initiation complexes for rDNA. The subsequent downregulation of pre-rRNA level led to attenuated endoplasmic reticulum (ER) stress and cell death. Deleting γ2-AMPK led to increases in pre-rRNA level, ER stress markers, and cell death during glucose deprivation, which could be rescued by inhibition of rRNA processing or ER stress. To study the function of γ2-AMPK in the heart, we generated a mouse model with cardiac-specific deletion of γ2-AMPK (cardiac knockout [cKO]). Although the total AMPK activity was unaltered in cKO hearts because of upregulation of γ1-AMPK, the lack of γ2-AMPK sensitizes the heart to myocardial ischemia/reperfusion injury. The cKO failed to suppress pre-rRNA level during ischemia/reperfusion and showed a greater infarct size. Conversely, cardiac-specific overexpression of γ2-AMPK decreased ribosome biosynthesis and ER stress during ischemia/reperfusion insult, and the infarct size was reduced.

CONCLUSIONS

The γ2-AMPK translocates into the nucleus to suppress pre-rRNA transcription and ribosome biosynthesis during stress, thus ameliorating ER stress and cell death. Increased γ2-AMPK activity is required to protect against ischemia/reperfusion injury. Our study reveals an isoform-specific function of γ2-AMPK in modulating ribosome biosynthesis, cell survival, and cardioprotection.

A novel, de novo mutation in the PRKAG2 gene: infantile-onset phenotype and the signaling pathway involved

PRKAG2 encodes the γ2-subunit isoform of 5'-AMP-activated protein kinase (AMPK), a heterotrimeric enzyme with major roles in the regulation of energy metabolism in response to cellular stress. Mutations in PRKAG2 have been implicated in a unique hypertrophic cardiomyopathy (HCM) characterized by cardiac glycogen overload, ventricular preexcitation, and hypertrophy. We identified a novel, de novo PRKAG2 mutation (K475E) in a neonate with prenatal onset of HCM. We aimed to investigate the cellular impact, signaling pathways involved, and therapeutic options for K475E mutation using cells stably expressing human wild-type (WT) or the K475E mutant. In human embryonic kidney-293 cells, the K475E mutation induced a marked increase in the basal phosphorylation of T172 and AMPK activity, reduced sensitivity to AMP in allosteric activation, and a loss of response to phenformin. In H9c2 cardiomyocytes, the K475E mutation induced inhibition of AMPK and reduced the response to phenformin and increases in the phosphorylation of p70S6 kinase (p70S6K) and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1). Primary fibroblasts from the patient with the K475E mutation also showed marked increases in the phosphorylation of p70S6K and 4E-BP1 compared with those from age-matched, nondiseased controls. Moreover, overexpression of K475E induced hypertrophy in H9c2 cells, which was effectively reversed by treatment with rapamycin. Taken together, we have identified a novel, de novo infantile-onset PRKAG2 mutation causing HCM. Our study suggests the K475E mutation induces alteration in basal AMPK activity and results in a hypertrophy phenotype involving the mechanistic target of rapamycin signaling pathway, which can be reversed with rapamycin.NEW & NOTEWORTHY We identified a novel, de novo PRKAG2 mutation (K475E) in the cystathionine β-synthase 3 repeat, a region critical for AMP binding but with no previous reported mutation. Our data suggest the mutation affects AMP-activated protein kinase activity, activates cell growth pathways, and results in cardiac hypertrophy, which can be reversed with rapamycin.

AMP-Activated Protein Kinase: An Ubiquitous Signaling Pathway With Key Roles in the Cardiovascular System

The AMP-activated protein kinase (AMPK) is a key regulator of cellular and whole-body energy homeostasis, which acts to restore energy homoeostasis whenever cellular energy charge is depleted. Over the last 2 decades, it has become apparent that AMPK regulates several other cellular functions and has specific roles in cardiovascular tissues, acting to regulate cardiac metabolism and contractile function, as well as promoting anticontractile, anti-inflammatory, and antiatherogenic actions in blood vessels. In this review, we discuss the role of AMPK in the cardiovascular system, including the molecular basis of mutations in AMPK that alter cardiac physiology and the proposed mechanisms by which AMPK regulates vascular function under physiological and pathophysiological conditions.

Pharmacological Strategies to Retard Cardiovascular Aging

Aging is the major risk factor for cardiovascular diseases, which are the leading cause of death in the United States. Traditionally, the effort to prevent cardiovascular disease has been focused on addressing the conventional risk factors, including hypertension, hyperglycemia, hypercholesterolemia, and high circulating levels of triglycerides. However, recent preclinical studies have identified new approaches to combat cardiovascular disease. Calorie restriction has been reproducibly shown to prolong lifespan in various experimental model animals. This has led to the development of calorie restriction mimetics and other pharmacological interventions capable to delay age-related diseases. In this review, we will address the mechanistic effects of aging per se on the cardiovascular system and focus on the prolongevity benefits of various therapeutic strategies that support cardiovascular health.

Defective Branched-Chain Amino Acid Catabolism Disrupts Glucose Metabolism and Sensitizes the Heart to Ischemia-Reperfusion Injury

Elevated levels of branched-chain amino acids (BCAAs) have recently been implicated in the development of cardiovascular and metabolic diseases, but the molecular mechanisms are unknown. In a mouse model of impaired BCAA catabolism (knockout [KO]), we found that chronic accumulation of BCAAs suppressed glucose metabolism and sensitized the heart to ischemic injury. High levels of BCAAs selectively disrupted mitochondrial pyruvate utilization through inhibition of pyruvate dehydrogenase complex (PDH) activity. Furthermore, downregulation of the hexosamine biosynthetic pathway in KO hearts decreased protein O-linked N-acetylglucosamine (O-GlcNAc) modification and inactivated PDH, resulting in significant decreases in glucose oxidation. Although the metabolic remodeling in KO did not affect baseline cardiac energetics or function, it rendered the heart vulnerable to ischemia-reperfusion injury. Promoting BCAA catabolism or normalizing glucose utilization by overexpressing GLUT1 in the KO heart rescued the metabolic and functional outcome. These observations revealed a novel role of BCAA catabolism in regulating cardiac metabolism and stress response.

Gain of function AMP-activated protein kinase γ3 mutation (AMPKγ3R200Q) in pig muscle increases glycogen storage regardless of AMPK activation

Chronic activation of AMP‐activated protein kinase (AMPK) increases glycogen content in skeletal muscle. Previously, we demonstrated that a mutation in the ryanodine receptor (RyR1^R615C) blunts AMPK phosphorylation in longissimus muscle of pigs with a gain of function mutation in the AMPKγ3 subunit (AMPKγ3^R200Q); this may decrease the glycogen storage capacity of AMPKγ3^R200Q + RyR1^R615C muscle. Therefore, our aim in this study was to utilize our pig model to understand how AMPKγ3^R200Q and AMPK activation contribute to glycogen storage and metabolism in muscle. We selected and bred pigs in order to generate offspring with naturally occurring AMPKγ3^R200Q, RyR1^R615C, and AMPKγ3^R200Q + RyR1^R615C mutations, and also retained wild‐type littermates (control). We assessed glycogen content and parameters of glycogen metabolism in longissimus muscle. Regardless of RyR1^R615C, AMPKγ3^R200Q increased the glycogen content by approximately 70%. Activity of glycogen synthase (GS) without the allosteric activator glucose 6‐phosphate (G6P) was decreased in AMPKγ3^R200Q relative to all other genotypes, whereas both AMPKγ3^R200Q and AMPKγ3^R200Q + RyR1^R615C muscle exhibited increased GS activity with G6P. Increased activity of GS with G6P was not associated with increased abundance of GS or hexokinase 2. However, AMPKγ3^R200Q enhanced UDP‐glucose pyrophosphorylase 2 (UGP2) expression approximately threefold. Although UGP2 is not generally considered a rate‐limiting enzyme for glycogen synthesis, our model suggests that UGP2 plays an important role in increasing flux to glycogen synthase. Moreover, we have shown that the capacity for glycogen storage is more closely related to the AMPKγ3^R200Q mutation than activity.

Genome editing with CRISPR/Cas9 in postnatal mice corrects PRKAG2 cardiac syndrome

PRKAG2 cardiac syndrome is an autosomal dominant inherited disease resulted from mutations in the PRKAG2 gene that encodes γ2 regulatory subunit of AMP-activated protein kinase. Affected patients usually develop ventricular tachyarrhythmia and experience progressive heart failure that is refractory to medical treatment and requires cardiac transplantation. In this study, we identify a H530R mutation in PRKAG2 from patients with familial Wolff-Parkinson-White syndrome. By generating H530R PRKAG2 transgenic and knock-in mice, we show that both models recapitulate human symptoms including cardiac hypertrophy and glycogen storage, confirming that the H530R mutation is causally related to PRKAG2 cardiac syndrome. We further combine adeno-associated virus-9 (AAV9) and the CRISPR/Cas9 gene-editing system to disrupt the mutant PRKAG2 allele encoding H530R while leaving the wild-type allele intact. A single systemic injection of AAV9-Cas9/sgRNA at postnatal day 4 or day 42 substantially restores the morphology and function of the heart in H530R PRKAG2 transgenic and knock-in mice. Together, our work suggests that in vivo CRISPR/Cas9 genome editing is an effective tool in the treatment of PRKAG2 cardiac syndrome and other dominant inherited cardiac diseases by selectively disrupting disease-causing mutations.

ORM Promotes Skeletal Muscle Glycogen Accumulation via CCR5-Activated AMPK Pathway in Mice

We found previously that acute phase protein orosomucoid reacts to fatigue and activates C-C chemokine receptor type 5 to increase muscle glycogen storage and enhance muscle endurance (Lei et al., 2016). To explore the underlying molecular mechanisms, we investigated the role of AMP-activated protein kinase, a critical fuel sensor in skeletal muscle, in C-C chemokine receptor type 5-mediated orosomucoid action. It was found orosomucoid increased skeletal muscle AMP-activated protein kinase activation in a time- and dose- dependent manner, which was largely prevented by pharmacological blocking or knockout of C-C chemokine receptor type 5. Administration of orosomucoid also significantly increased the de-phosphorylation and activity of muscle glycogen synthase, the rate-limiting enzyme for glycogen synthesis. The effect was largely absent in mice deficient in C-C chemokine receptor type 5^−/− or AMP-activated protein kinase α2^−/−, the predominant isoform in skeletal muscle. Moreover, deletion of AMP-activated protein kinase α2 abolished the effect of orosomucoid on fatigue and muscle glycogen. These findings indicate that orosomucoid may promote glycogen storage and enhance muscle function through C-C chemokine receptor type 5-mdiated activation of AMP-activated protein kinase, which in turn activates glycogen synthase and increases muscle glycogen.

Physiological Expression of AMPKγ2RG Mutation Causes Wolff-Parkinson-White Syndrome and Induces Kidney Injury in Mice

Mutations of the AMP-activated kinase gamma 2 subunit (AMPKγ2), N488I (AMPKγ2^NI) and R531G (AMPKγ2^RG), are associated with Wolff-Parkinson-White (WPW) syndrome, a cardiac disorder characterized by ventricular pre-excitation in humans. Cardiac-specific transgenic overexpression of human AMPKγ2^NI or AMPKγ2^RG leads to constitutive AMPK activation and the WPW phenotype in mice. However, overexpression of these mutant proteins also caused profound, non-physiological increase in cardiac glycogen, which might abnormally alter the true phenotype. To investigate whether physiological levels of AMPKγ2^NI or AMPKγ2^RG mutation cause WPW syndrome and metabolic changes in other organs, we generated two knock-in mouse lines on the C57BL/6N background harboring mutations of human AMPKγ2^NI and AMPKγ2^RG, respectively. Similar to the reported phenotypes of mice overexpressing AMPKγ2^NI or AMPKγ2^RG in the heart, both lines developed WPW syndrome and cardiac hypertrophy; however, these effects were independent of cardiac glycogen accumulation. Compared with AMPKγ2^WT mice, AMPKγ2^NI and AMPKγ2^RG mice exhibited reduced body weight, fat mass, and liver steatosis when fed with a high fat diet (HFD). Surprisingly, AMPKγ2^RG but not AMPKγ2^NI mice fed with an HFD exhibited severe kidney injury characterized by glycogen accumulation, inflammation, apoptosis, cyst formation, and impaired renal function. These results demonstrate that expression of AMPKγ2^NI and AMPKγ2^RG mutations at physiological levels can induce beneficial metabolic effects but that this is accompanied by WPW syndrome. Our data also reveal an unexpected effect of AMPKγ2^RG in the kidney, linking lifelong constitutive activation of AMPK to a potential risk for kidney dysfunction in the context of an HFD.

mTORC2 and AMPK differentially regulate muscle triglyceride content via Perilipin 3

Objective

We have recently shown that acute inhibition of both mTOR complexes (mTORC1 and mTORC2) increases whole-body lipid utilization, while mTORC1 inhibition had no effect. Therefore, we tested the hypothesis that mTORC2 regulates lipid metabolism in skeletal muscle.

Methods

Body composition, substrate utilization and muscle lipid storage were measured in mice lacking mTORC2 activity in skeletal muscle (specific knockout of RICTOR (Ric mKO)). We further examined the RICTOR/mTORC2-controlled muscle metabolome and proteome; and performed follow-up studies in other genetic mouse models and in cell culture.

Results

Ric mKO mice exhibited a greater reliance on fat as an energy substrate, a re-partitioning of lean to fat mass and an increase in intramyocellular triglyceride (IMTG) content, along with increases in several lipid metabolites in muscle. Unbiased proteomics revealed an increase in the expression of the lipid droplet binding protein Perilipin 3 (PLIN3) in muscle from Ric mKO mice. This was associated with increased AMPK activity in Ric mKO muscle. Reducing AMPK kinase activity decreased muscle PLIN3 expression and IMTG content. AMPK agonism, in turn, increased PLIN3 expression in a FoxO1 dependent manner. PLIN3 overexpression was sufficient to increase triglyceride content in muscle cells.

Conclusions

We identified a novel link between mTORC2 and PLIN3, which regulates lipid storage in muscle. While mTORC2 is a negative regulator, we further identified AMPK as a positive regulator of PLIN3, which impacts whole-body substrate utilization and nutrient partitioning.

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Lack of mTORC2 activity in muscle alters overall body composition, substrate utilization and the muscle metabolite profile.

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mTORC2 and AMPK regulate IMTG and PLIN3 in opposite fashion in muscle.

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AMPK agonism increases PLIN3 expression in a FoxO1 dependent manner.

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PLIN3 overexpression increases triglyceride levels in muscle cells.

Oxidative Stress and Metabolic Perturbations in Wooden Breast Disorder in Chickens

This study was conducted to characterize metabolic features of the breast muscle (pectoralis major) in chickens affected with the Wooden Breast myopathy. Live birds from two purebred chicken lines and one crossbred commercial broiler population were clinically examined by manual palpation of the breast muscle (pectoralis major) at 47–48 days of age. Metabolite abundance was determined by gas chromatography/mass spectrometry (GC/MS) and liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) using breast muscle tissue samples from 16 affected and 16 unaffected chickens. Muscle glycogen content was also quantified in breast muscle tissue samples from affected and unaffected chickens. In total, levels of 140 biochemicals were significantly different (FDR < 0.1 and fold-change A/U > 1.3 or < 0.77) between affected and unaffected chickens. Glycogen content measurements were considerably lower (1.7-fold) in samples taken from Wooden Breast affected birds when compared with samples from unaffected birds. Affected tissues exhibited biomarkers related to increased oxidative stress, elevated protein levels, muscle degradation, and altered glucose utilization. Affected muscle also showed elevated levels of hypoxanthine, xanthine, and urate molecules, the generation of which can contribute to altered redox homeostasis. In conclusion, our findings show that Wooden Breast affected tissues possess a unique metabolic signature. This unique profile may identify candidate biomarkers for diagnostic utilization and provide mechanistic insight into altered biochemical processes contributing to tissue hardening associated with the Wooden Breast myopathy in commercial chickens.

Glycogen: A must have storage to survive stressful emergencies

Mechanisms of adaptation to acute changes in osmolarity are fundamental for life. When exposed to hyperosmotic stress, cells and organisms utilize conserved strategies to prevent water loss and maintain cellular integrity and viability. The production of glycerol is a common strategy utilized by the nematode Caenorhabditis elegans (C. elegans) and many other organisms to survive hyperosmotic stress. Specifically, the transcriptional upregulation of glycerol-3-phosphate dehydrogenase, a rate-limiting enzyme in the production of glycerol, has been previously implicated in many model organisms. However, what fuels this massive and rapid production of glycerol upon hyperosmotic stress has not been clearly elucidated. We have recently discovered an AMPK-dependent pathway that mediates hyperosmotic stress resistance in C. elegans. Specifically, we demonstrated that the chronic activation of AMPK leads to glycogen accumulation, which under hyperosmotic stress exposure, is rapidly degraded to mediate glycerol production. Importantly, we demonstrate that this strategy is utilized by flcn-1 mutant C. elegans nematodes in an AMPK-dependent manner. FLCN-1 is the worm homolog of the human renal tumor suppressor Folliculin (FLCN) responsible for the Birt-Hogg-Dubé neoplastic syndrome. Here, we comment on the dual role for glycogen in stress resistance: it serves as an energy store and a fuel for osmolyte production. We further discuss the potential utilization of this mechanism by organisms in general and by human cancer cells in order to survive harsh environmental conditions and notably hyperosmotic stress.

Rapamycin transiently induces mitochondrial remodeling to reprogram energy metabolism in old hearts

Rapamycin, an inhibitor of mTOR signaling, has been shown to reverse diastolic dysfunction in old mice in 10 weeks, highlighting its therapeutic potential for a poorly treatable condition. However, the mechanisms and temporal regulation of its cardiac benefits remain unclear. We show that improved diastolic function in old mice begins at 2-4 weeks, progressing over the course of 10-week treatment. While TORC1-mediated S6 phosphorylation and TORC2 mediated AKT and PKCα phosphorylation are inhibited throughout the course of treatment, rapamycin inhibits ULK phosphorylation and induces autophagy during just the first week of treatment, returning to baseline at two weeks and after. Concordantly, markers of mitochondrial biogenesis increase over the first two weeks of treatment and return to control levels thereafter. This transient induction of autophagy and mitochondrial biogenesis suggests that damaged mitochondria are replaced by newly synthesized ones to rejuvenate mitochondrial homeostasis. This remodeling is shown to rapidly reverse the age-related reduction in fatty acid oxidation to restore a more youthful substrate utilization and energetic profile in old isolated perfused hearts, and modulates the myocardial metabolomein vivo. This study demonstrates the differential and dynamic mechanisms following rapamycin treatment and highlights the importance of understanding the temporal regulation of rapamycin effects.

Animal Models to Study AMPK

AMPK is an evolutionary conserved energy sensor involved in the regulation of energy metabolism. Based on biochemical studies, AMPK has brought much of interest in recent years due to its potential impact on metabolic disorders. Suitable animal models are therefore essential to promote our understanding of the molecular and functional roles of AMPK but also to bring novel information for the development of novel therapeutic strategies. The organism systems include pig (Sus scrofa), mouse (Mus musculus), fly (Drosophila melanogaster), worm (Caenorhabditis elegans), and fish (Danio rerio) models. These animal models have provided reliable experimental evidence demonstrating the crucial role of AMPK in the regulation of metabolism but also of cell polarity, autophagy, and oxidative stress. In this chapter, we update the new development in the generation and application of animal models for the study of AMPK biology. We also discuss recent breakthroughs from studies in mice, flies, and worms showing how AMPK has a primary role in initiating or promoting pathological or beneficial impact on health.

FLCN and AMPK Confer Resistance to Hyperosmotic Stress via Remodeling of Glycogen Stores

Mechanisms of adaptation to environmental changes in osmolarity are fundamental for cellular and organismal survival. Here we identify a novel osmotic stress resistance pathway in Caenorhabditis elegans (C. elegans), which is dependent on the metabolic master regulator 5'-AMP-activated protein kinase (AMPK) and its negative regulator Folliculin (FLCN). FLCN-1 is the nematode ortholog of the tumor suppressor FLCN, responsible for the Birt-Hogg-Dubé (BHD) tumor syndrome. We show that flcn-1 mutants exhibit increased resistance to hyperosmotic stress via constitutive AMPK-dependent accumulation of glycogen reserves. Upon hyperosmotic stress exposure, glycogen stores are rapidly degraded, leading to a significant accumulation of the organic osmolyte glycerol through transcriptional upregulation of glycerol-3-phosphate dehydrogenase enzymes (gpdh-1 and gpdh-2). Importantly, the hyperosmotic stress resistance in flcn-1 mutant and wild-type animals is strongly suppressed by loss of AMPK, glycogen synthase, glycogen phosphorylase, or simultaneous loss of gpdh-1 and gpdh-2 enzymes. Our studies show for the first time that animals normally exhibit AMPK-dependent glycogen stores, which can be utilized for rapid adaptation to either energy stress or hyperosmotic stress. Importantly, we show that glycogen accumulates in kidneys from mice lacking FLCN and in renal tumors from a BHD patient. Our findings suggest a dual role for glycogen, acting as a reservoir for energy supply and osmolyte production, and both processes might be supporting tumorigenesis.

AMPK and autophagy in glucose/glycogen metabolism

Glucose/glycogen metabolism is a primary metabolic pathway acting on a variety of cellular needs, such as proliferation, growth, and survival against stresses. The multiple regulatory mechanisms underlying a specific metabolic fate have been documented and explained the molecular basis of various pathophysiological conditions, including metabolic disorders and cancers. AMP-activated protein kinase (AMPK) has been appreciated for many years as a central metabolic regulator to inhibit energy-consuming pathways as well as to activate the compensating energy-producing programs. In fact, glucose starvation is a potent physiological AMPK activating condition, in which AMPK triggers various subsequent metabolic events depending on cells or tissues. Of note, the recent studies show bidirectional interplay between AMPK and glycogen. A growing number of studies have proposed additional level of metabolic regulation by a lysosome-dependent catabolic program, autophagy. Autophagy is a critical degradative pathway not only for maintenance of cellular homeostasis to remove potentially dangerous constituents, such as protein aggregates and dysfunctional subcellular organelles, but also for adaptive responses to metabolic stress, such as nutrient starvation. Importantly, many lines of evidence indicate that autophagy is closely connected with nutrient signaling modules, including AMPK, to fine-tune the metabolic pathways in response to many different cellular cues. In this review, we introduce the studies demonstrating the role of AMPK and autophagy in glucose/glycogen metabolism. Also, we describe the recent advances on their contributions to the metabolic disorders.

AMP-activated protein kinase: a key regulator of energy balance with many roles in human disease

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that regulates cellular and whole-body energy balance. A recently reported crystal structure has illuminated the complex regulatory mechanisms by which AMP and ADP cause activation of AMPK, involving phosphorylation by the upstream kinase LKB1. Once activated by falling cellular energy status, AMPK activates catabolic pathways that generate ATP whilst inhibiting anabolic pathways and other cellular processes that consume ATP. A role of AMPK is implicated in many human diseases. Mutations in the γ2 subunit cause heart disease due to excessive glycogen storage in cardiac myocytes, leading to ventricular pre-excitation. AMPK-activating drugs reverse many of the metabolic defects associated with insulin resistance, and recent findings suggest that the insulin-sensitizing effects of the widely used antidiabetic drug metformin are mediated by AMPK. The upstream kinase LKB1 is a tumour suppressor, and AMPK may exert many of its antitumour effects. AMPK activation promotes the oxidative metabolism typical of quiescent cells, rather than the aerobic glycolysis observed in tumour cells and cells involved in inflammation, explaining in part why AMPK activators have both antitumour and anti-inflammatory effects. Salicylate (the major in vivo metabolite of aspirin) activates AMPK, and this could be responsible for at least some of the anticancer and anti-inflammatory effects of aspirin. In addition to metformin and salicylates, novel drugs that modulate AMPK are likely to enter clinical trials soon. Finally, AMPK may be involved in viral infection: downregulation of AMPK during hepatitis C virus infection appears to be essential for efficient viral replication.

Transgenic knockdown of cardiac sodium/glucose cotransporter 1 (SGLT1) attenuates PRKAG2 cardiomyopathy, whereas transgenic overexpression of cardiac SGLT1 causes pathologic hypertrophy and dysfunction in mice

Background

The expression of a novel cardiac glucose transporter, SGLT1, is increased in glycogen storage cardiomyopathy secondary to mutations in PRKAG2. We sought to determine the role of SGLT1 in the pathogenesis of PRKAG2 cardiomyopathy and its role in cardiac structure and function.

Methods and Results

Transgenic mice with cardiomyocyte‐specific overexpression of human T400N mutant PRKAG2 cDNA (TG^T400N) and transgenic mice with cardiomyocyte‐specific RNA interference knockdown of SGLT1 (TG^SGLT1‐DOWN) were crossed to produce double‐transgenic mice (TG^T400N/TG^SGLT1‐DOWN). Tet‐off transgenic mice conditionally overexpressing cardiac SGLT1 in the absence of doxycycline were also constructed (TG^SGLT‐ON). Relative to TG^T400N mice, TG^T400N/TG^SGLT1‐DOWN mice exhibited decreases in cardiac SGLT1 expression (63% decrease, P<0.05), heart/body weight ratio, markers of cardiac hypertrophy, and cardiac glycogen content. TG^T400N/TG^SGLT1‐DOWN mice had less left ventricular dilation at age 12 weeks compared to TG^T400N mice. Relative to wildtype (WT) mice, TG^SGLT1‐ON mice exhibited increases in heart/body weight ratio, glycogen content, and markers of cardiac hypertrophy at ages 10 and 20 weeks. TG^SGLT1‐ON mice had increased myocyte size and interstitial fibrosis, and progressive left ventricular dysfunction. When SGLT1 was suppressed after 10 weeks of overexpression (TG^{SGLT1‐ON/OFF}), there was a reduction in cardiac hypertrophy and improvement in left ventricular failure.

Conclusions

Cardiac knockdown of SGLT1 in a murine model of PRKAG2 cardiomyopathy attenuates the disease phenotype, implicating SGLT1 in the pathogenesis. Overexpression of SGLT1 causes pathologic cardiac hypertrophy and left ventricular failure that is reversible. This is the first report of cardiomyocyte‐specific transgenic knockdown of a target gene.

DAF-16/FoxO directly regulates an atypical AMP-activated protein kinase gamma isoform to mediate the effects of insulin/IGF-1 signaling on aging in Caenorhabditis elegans

The DAF-16/FoxO transcription factor controls growth, metabolism and aging in Caenorhabditis elegans. The large number of genes that it regulates has been an obstacle to understanding its function. However, recent analysis of transcript and chromatin profiling implies that DAF-16 regulates relatively few genes directly, and that many of these encode other regulatory proteins. We have investigated the regulation by DAF-16 of genes encoding the AMP-activated protein kinase (AMPK), which has α, β and γ subunits. C. elegans has 5 genes encoding putative AMP-binding regulatory γ subunits, aakg-1-5. aakg-4 and aakg-5 are closely related, atypical isoforms, with orthologs throughout the Chromadorea class of nematodes. We report that ∼75% of total γ subunit mRNA encodes these 2 divergent isoforms, which lack consensus AMP-binding residues, suggesting AMP-independent kinase activity. DAF-16 directly activates expression of aakg-4, reduction of which suppresses longevity in daf-2 insulin/IGF-1 receptor mutants. This implies that an increase in the activity of AMPK containing the AAKG-4 γ subunit caused by direct activation by DAF-16 slows aging in daf-2 mutants. Knock down of aakg-4 expression caused a transient decrease in activation of expression in multiple DAF-16 target genes. This, taken together with previous evidence that AMPK promotes DAF-16 activity, implies the action of these two metabolic regulators in a positive feedback loop that accelerates the induction of DAF-16 target gene expression. The AMPK β subunit, aakb-1, also proved to be up-regulated by DAF-16, but had no effect on lifespan. These findings reveal key features of the architecture of the gene-regulatory network centered on DAF-16, and raise the possibility that activation of AMP-independent AMPK in nutritionally replete daf-2 mutant adults slows aging in C. elegans. Evidence of activation of AMPK subunits in mammals suggests that such FoxO-AMPK interactions may be evolutionarily conserved.

Aging is an important problem for human health and is regulated by complex gene regulatory networks. In a simple nematode worm (Caenorhabditis elegans) mutation of the insulin/IGF-1 receptor daf-2 dramatically extends lifespan. This is due to the increased activity of DAF-16, a FoxO transcription factor, leading to altered expression of genes, many encoding other regulatory proteins. We have focused on one such protein, AMP-activated protein kinase (AMPK), that is important for regulating cellular homeostasis under conditions of low energy availability (e.g. starvation). We find that DAF-16 binds to the promoter of aakg-4 (a gene encoding an atypical γ subunit of AMPK) and increases its expression. Inhibition of aakg-4 leads to down-regulation of multiple DAF-16 target genes and shortens the life of daf-2 mutants. Taken together with a previous report showing that AMPK activates DAF-16, this suggests that AAKG-4 and DAF-16 are involved in a positive feedback loop which accelerates effects of DAF-16 on gene expression, and could contribute to longevity. This study defines a new part of the complex gene regulatory network in which DAF-16 acts to control aging. FoxO-AMPK interactions are present in higher animals, where they could potentially also influence aging.

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 γ2-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 γ2-AMP-activated protein kinase in cell growth also has broad implications in cardiac development, growth, and regeneration.

Maternal cardiac metabolism in pregnancy

Pregnancy causes dramatic physiological changes in the expectant mother. The placenta, mostly foetal in origin, invades maternal uterine tissue early in pregnancy and unleashes a barrage of hormones and other factors. This foetal 'invasion' profoundly reprogrammes maternal physiology, affecting nearly every organ, including the heart and its metabolism. We briefly review here maternal systemic metabolic changes during pregnancy and cardiac metabolism in general. We then discuss changes in cardiac haemodynamic during pregnancy and review what is known about maternal cardiac metabolism during pregnancy. Lastly, we discuss cardiac diseases during pregnancy, including peripartum cardiomyopathy, and the potential contribution of aberrant cardiac metabolism to disease aetiology.

AMPK and Exercise: Glucose Uptake and Insulin Sensitivity

AMPK is an evolutionary conserved sensor of cellular energy status that is activated during exercise. Pharmacological activation of AMPK promotes glucose uptake, fatty acid oxidation, mitochondrial biogenesis, and insulin sensitivity; processes that are reduced in obesity and contribute to the development of insulin resistance. AMPK deficient mouse models have been used to provide direct genetic evidence either supporting or refuting a role for AMPK in regulating these processes. Exercise promotes glucose uptake by an insulin dependent mechanism involving AMPK. Exercise is important for improving insulin sensitivity; however, it is not known if AMPK is required for these improvements. Understanding how these metabolic processes are regulated is important for the development of new strategies that target obesity-induced insulin resistance. This review will discuss the involvement of AMPK in regulating skeletal muscle metabolism (glucose uptake, glycogen synthesis, and insulin sensitivity).

AMP-activated protein kinase regulation and biological actions in the heart

AMP-activated protein kinase (AMPK) is a stress-activated kinase that functions as a cellular fuel gauge and master metabolic regulator. Recent investigation has elucidated novel molecular mechanisms of AMPK regulation and important biological actions of the AMPK pathway that are highly relevant to cardiovascular disease. Activation of the intrinsic AMPK pathway plays an important role in the myocardial response to ischemia, pressure overload, and heart failure. Pharmacological activation of AMPK shows promise as a therapeutic strategy in the treatment of heart disease. The purpose of this review is to assess how recent discoveries have extended and in some cases challenged existing paradigms, providing new insights into the regulation of AMPK, its diverse biological actions, and therapeutic potential in the heart.

iPS cell modeling of cardiometabolic diseases

Cardiometabolic diseases encompass simple monogenic enzyme deficiencies with well-established pathogenesis and clinical outcomes to complex polygenic diseases such as the cardiometabolic syndrome. The limited availability of relevant human cell types such as cardiomyocytes has hampered our ability to adequately model and study pathways or drugs relevant to these diseases in the heart. The recent discovery of induced pluripotent stem (iPS) cell technology now offers a powerful opportunity to establish translational platforms for cardiac disease modeling, drug discovery, and pre-clinical testing. In this article, we discuss the excitement and challenges of modeling cardiometabolic diseases using iPS cell and their potential to revolutionize translational research.

The structure of cardiac glycogen in healthy mice

Transmission electron micrographs of glycogen extracted from healthy mouse hearts reveal aggregate structures around 133 nm in diameter. These structures are similar to, but on average somewhat smaller than, the α-particles of glycogen found in mammalian liver. Like the larger liver glycogens, these new particles in cardiac tissue appear to be aggregates of β-particles. Free β-particles are also present in liver, and are the only type of particle seen in skeletal muscle. They have diameters from 20 to 50 nm. We discuss the number distributions of glycogen particle diameters and the implications for the structure-function relationship of glycogens in these tissues. We point out the possible implications for the study of glycogen storage diseases, and of non-insulin dependent diabetes mellitus.

Regulation of NAD(P)H oxidases by AMPK in cardiovascular systems

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are ubiquitously produced in cardiovascular systems. Under physiological conditions, ROS/RNS function as signaling molecules that are essential in maintaining cardiovascular function. Aberrant concentrations of ROS/RNS have been demonstrated in cardiovascular diseases owing to increased production or decreased scavenging, which have been considered common pathways for the initiation and progression of cardiovascular diseases such as atherosclerosis, hypertension, (re)stenosis, and congestive heart failure. NAD(P)H oxidases are primary sources of ROS and can be induced or activated by all known cardiovascular risk factors. Stresses, hormones, vasoactive agents, and cytokines via different signaling cascades control the expression and activity of these enzymes and of their regulatory subunits. But the molecular mechanisms by which NAD(P)H oxidase is regulated in cardiovascular systems remain poorly characterized. Investigations by us and others suggest that adenosine monophosphate-activated protein kinase (AMPK), as an energy sensor and modulator, is highly sensitive to ROS/RNS. We have also obtained convincing evidence that AMPK is a physiological suppressor of NAD(P)H oxidase in multiple cardiovascular cell systems. In this review, we summarize our current understanding of how AMPK functions as a physiological repressor of NAD(P)H oxidase.

Actions and interactions of AMPK with insulin, the peroxisomal-proliferator activated receptors and sirtuins

AMP-activated protein kinase (AMPK) activity responds to a requirement to increase cellular ATP production and/or to conserve available ATP. AMPK is therefore central to the mechanisms of adjustment to fluctuating energy demand or metabolic substrate supply. AMPK has important actions in several insulin-responsive tissues, as well as in the pancreatic β cell, through which it can modulate glycemic control, insulin action and metabolic substrate selection and disposal. We review recent novel findings elucidating the mechanisms by which AMPK activation can correct impaired insulin action. However, we also emphasize not only the similarities, but also the differences in the actions of insulin and AMPK. We focus on metabolic interfaces between AMPK, peroxisomal proliferator-activated receptors, sirtuins and mTORC.