Rubicon-regulated beta-1 adrenergic receptor recycling protects the heart from pressure overload.

Heart failure has high morbidity and mortality in the developed countries. Autophagy is important for the quality control of proteins and organelles in the heart. Rubicon (Run domain Beclin-1-interacting and cysteine-rich domain-containing protein) has been identified as a potent negative regulator of autophagy and endolysosomal trafficking. The aim of this study was to investigate the in vivo role of Rubicon-mediated autophagy and endosomal trafficking in the heart. We generated cardiomyocyte-specific Rubicon-deficient mice and subjected the mice to pressure overload by means of transverse aortic constriction. Rubicon-deficient mice showed heart failure with left ventricular dilatation, systolic dysfunction and lung congestion one week after pressure overload. While autophagic activity was unchanged, the protein amount of beta-1 adrenergic receptor was decreased in the pressure-overloaded Rubicon-deficient hearts. The increases in heart rate and systolic function by beta-1 adrenergic stimulation were significantly attenuated in pressure-overloaded Rubicon-deficient hearts. In isolated rat neonatal cardiomyocytes, the downregulation of the receptor by beta-1 adrenergic agonist was accelerated by knockdown of Rubicon through the inhibition of recycling of the receptor. Taken together, Rubicon protects the heart from pressure overload. Rubicon maintains the intracellular recycling of beta-1 adrenergic receptor, which might contribute to its cardioprotective effect.

Iron derived from autophagy-mediated ferritin degradation induces cardiomyocyte death and heart failure in mice.

Heart failure is a major public health problem, and abnormal iron metabolism is common in patients with heart failure. Although iron is necessary for metabolic homeostasis, it induces a programmed necrosis. Iron release from ferritin storage is through nuclear receptor coactivator 4 (NCOA4)-mediated autophagic degradation, known as ferritinophagy. However, the role of ferritinophagy in the stressed heart remains unclear. Deletion of Ncoa4 in mouse hearts reduced left ventricular chamber size and improved cardiac function along with the attenuation of the upregulation of ferritinophagy-mediated ferritin degradation 4 weeks after pressure overload. Free ferrous iron overload and increased lipid peroxidation were suppressed in NCOA4-deficient hearts. A potent inhibitor of lipid peroxidation, ferrostatin-1, significantly mitigated the development of pressure overload-induced dilated cardiomyopathy in wild-type mice. Thus, the activation of ferritinophagy results in the development of heart failure, whereas inhibition of this process protects the heart against hemodynamic stress.

Cytokine mRNA Degradation in Cardiomyocytes Restrains Sterile Inflammation in Pressure-Overloaded Hearts.

BACKGROUND: Proinflammatory cytokines play an important role in the pathogenesis of heart failure. The mechanisms responsible for maintaining sterile inflammation within failing hearts remain poorly defined. Although transcriptional control is important for proinflammatory cytokine gene expression, the stability of mRNA also contributes to the kinetics of immune responses. Regnase-1 is an RNase involved in the degradation of a set of proinflammatory cytokine mRNAs in immune cells. The role of Regnase-1 in nonimmune cells such as cardiomyocytes remains to be elucidated. METHODS: To examine the role of proinflammatory cytokine degradation by Regnase-1 in cardiomyocytes, cardiomyocyte-specific Regnase-1-deficient mice were generated. The mice were subjected to pressure overload by means of transverse aortic constriction to induce heart failure. Cardiac remodeling was assessed by echocardiography as well as histological and molecular analyses 4 weeks after operation. Inflammatory cell infiltration was examined by immunostaining. Interleukin-6 signaling was inhibited by administration with its receptor antibody. Overexpression of Regnase-1 in the heart was performed by adeno-associated viral vector-mediated gene transfer. RESULTS: Cardiomyocyte-specific Regnase-1-deficient mice showed no cardiac phenotypes under baseline conditions, but exhibited severe inflammation and dilated cardiomyopathy after 4 weeks of pressure overload compared with control littermates. Four weeks after transverse aortic constriction, the Il6 mRNA level was upregulated, but not other cytokine mRNAs, including tumor necrosis factor-α, in Regnase-1-deficient hearts. Although the Il6 mRNA level increased 1 week after operation in both Regnase-1-deficient and control hearts, it showed no increase in control hearts 4 weeks after operation. Administration of anti-interleukin-6 receptor antibody attenuated the development of inflammation and cardiomyopathy in cardiomyocyte-specific Regnase-1-deficient mice. In severe pressure overloaded wild-type mouse hearts, sustained induction of Il6 mRNA was observed, even though the protein level of Regnase-1 increased. Adeno-associated virus 9-mediated cardiomyocyte-targeted gene delivery of Regnase-1 or administration of anti-interleukin-6 receptor antibody attenuated the development of cardiomyopathy induced by severe pressure overload in wild-type mice. CONCLUSIONS: The degradation of cytokine mRNA by Regnase-1 in cardiomyocytes plays an important role in restraining sterile inflammation in failing hearts and the Regnase-1-mediated pathway might be a therapeutic target to treat patients with heart failure.

Implication of alpha-Klotho as the predictive factor of stress.

Stress is known as a risk factor for both mental and physical health problems. While stress is known as one of the major health problems in modern society, a biomarker of stress has not yet been well established. In the present study, we focused on the serum levels of α-Klotho (αKl) as a possible objective biomarker of stress. Subjects included apparently healthy individuals who underwent a health examination in the Osaka University Health and Counseling Center. Physical and biochemical parameters were obtained from all subjects. Information regarding the lifestyle of each individual was obtained via questionnaires. Among male subjects, serum levels of soluble αKl (sαKl) were significantly elevated in subjects who had poor stress management and unsatisfactory sleep, suggesting that stress management and sleeping conditions influenced the serum levels of sαKl. The total Kessler Screening Scale for Psychological Distress (K6) score was significantly increased in subjects who reported experiencing considerable stress, had poor stress management and unsatisfactory sleep. Since serum levels of sαKl showed the same tendency as the K6 score in terms of the relationship between stress management and sleeping conditions in male subjects, increased sαKl levels could be associated with considerable psychological stress in healthy men.

Administration of a TLR9 Inhibitor Attenuates the Development and Progression of Heart Failure in Mice.

Mitochondrial deoxyribonucleic acid, containing the unmethylated cytidine-phosphate-guanosine motif, stimulates Toll-like receptor 9 to induce inflammation and heart failure. A small chemical, E6446 [(6-[3-(pyrrolidin-1-yl)propoxy)-2-(4-(3-(pyrrolidin-1-yl)propoxy)phenyl]benzo[d]oxazole)], is a specific Toll-like receptor 9 inhibitor in cardiomyocytes. In this study, we showed that E6446 exerts beneficial effects for the prevention and treatment of pressure overload-induced heart failure in mice. When administered before the operation and chronically thereafter, E6446 prevented the development of left ventricular dilatation as well as cardiac dysfunction, fibrosis, and inflammation. Furthermore, when administered after the manifestation of cardiac dysfunction, E6446 slowed progression of cardiac remodeling. Thus, the inhibitor may be a novel therapeutic agent for treating patients with heart failure.

A Mammalian Mitophagy Receptor, Bcl2-L-13, Recruits the ULK1 Complex to Induce Mitophagy.

Degradation of mitochondria by selective autophagy, termed mitophagy, contributes to the control of mitochondrial quality. Bcl2-L-13 is a mammalian homolog of Atg32, which is an essential mitophagy receptor in yeast. However, the molecular machinery involved in Bcl2-L-13-mediated mitophagy remains to be elucidated. Here, we show that the ULK1 (unc-51-like kinase) complex is required for Bcl2-L-13 to process mitophagy. Screening of a series of yeast Atg mutants revealed that a different set of ATG genes is used for Bcl2-L-13- and Atg32-mediated mitophagy in yeast. The components of the Atg1 complex essential for starvation-induced autophagy were indispensable in Bcl2-L-13-, but not Atg32-mediated, mitophagy. The ULK1 complex, a counterpart of the Atg1 complex, is necessary for Bcl2-L-13-mediated mitophagy in mammalian cells. We propose a model where, upon mitophagy induction, Bcl2-L-13 recruits the ULK1 complex to process mitophagy and the interaction of LC3B with ULK1, as well as Bcl2-L-13, is important for the mitophagy.

mTOR Hyperactivation by Ablation of Tuberous Sclerosis Complex 2 in the Mouse Heart Induces Cardiac Dysfunction with the Increased Number of Small Mitochondria Mediated through the Down-Regulation of Autophagy.

Mammalian target of rapamycin complex 1 (mTORC1) is a key regulator of cell growth, proliferation and metabolism. mTORC1 regulates protein synthesis positively and autophagy negatively. Autophagy is a major system to manage bulk degradation and recycling of cytoplasmic components and organelles. Tuberous sclerosis complex (TSC) 1 and 2 form a heterodimeric complex and inactivate Ras homolog enriched in brain, resulting in inhibition of mTORC1. Here, we investigated the effects of hyperactivation of mTORC1 on cardiac function and structure using cardiac-specific TSC2-deficient (TSC2-/-) mice. TSC2-/- mice were born normally at the expected Mendelian ratio. However, the median life span of TSC2-/- mice was approximately 10 months and significantly shorter than that of control mice. TSC2-/- mice showed cardiac dysfunction and cardiomyocyte hypertrophy without considerable fibrosis, cell infiltration or apoptotic cardiomyocyte death. Ultrastructural analysis of TSC2-/- hearts revealed misalignment, aggregation and a decrease in the size and an increase in the number of mitochondria, but the mitochondrial function was maintained. Autophagic flux was inhibited, while the phosphorylation level of S6 or eukaryotic initiation factor 4E -binding protein 1, downstream of mTORC1, was increased. The upregulation of autophagic flux by trehalose treatment attenuated the cardiac phenotypes such as cardiac dysfunction and structural abnormalities of mitochondria in TSC2-/- hearts. The results suggest that autophagy via the TSC2-mTORC1 signaling pathway plays an important role in maintenance of cardiac function and mitochondrial quantity and size in the heart and could be a therapeutic target to maintain mitochondrial homeostasis in failing hearts.

Bcl-2-like protein 13 is a mammalian Atg32 homologue that mediates mitophagy and mitochondrial fragmentation.

Damaged mitochondria are removed by mitophagy. Although Atg32 is essential for mitophagy in yeast, no Atg32 homologue has been identified in mammalian cells. Here, we show that Bcl-2-like protein 13 (Bcl2-L-13) induces mitochondrial fragmentation and mitophagy in mammalian cells. First, we hypothesized that unidentified mammalian mitophagy receptors would share molecular features of Atg32. By screening the public protein database for Atg32 homologues, we identify Bcl2-L-13. Bcl2-L-13 binds to LC3 through the WXXI motif and induces mitochondrial fragmentation and mitophagy in HEK293 cells. In Bcl2-L-13, the BH domains are important for the fragmentation, while the WXXI motif facilitates mitophagy. Bcl2-L-13 induces mitochondrial fragmentation in the absence of Drp1, while it induces mitophagy in Parkin-deficient cells. Knockdown of Bcl2-L-13 attenuates mitochondrial damage-induced fragmentation and mitophagy. Bcl2-L-13 induces mitophagy in Atg32-deficient yeast cells. Induction and/or phosphorylation of Bcl2-L-13 may regulate its activity. Our findings offer insights into mitochondrial quality control in mammalian cells.

Cooperation between proteolytic systems in cardiomyocyte recycling.

Cardiomyocytes are terminally differentiated cells and thus do not have the ability to dilute damaged proteins and organelles by cell division. Thus, proteolytic and recycling systems within the cardiomyocyte are essential to maintain cardiac function. The major proteolytic systems in the cell are: the ubiquitin-proteasome system, autophagy, and calpain. The ubiquitin-proteasome system degrades specific proteins by labelling them with ubiquitin. Autophagy degrades cytosolic proteins and organelles; this is generally believed to be a non-specific type of degradation. Calpain is a Ca(2+)-sensitive cysteine protease that degrades intracellular substrates including cytoskeletal proteins, and participates in Ca(2+)-mediated intracellular processes. All three systems exist in the cardiomyocyte and play pivotal roles in maintaining cardiac function. However, there is still controversy regarding the role of each protein-degradation system in the heart. Our recent reports using cardiac-specific knockout mice have revealed the cardioprotective roles of autophagy and calpain in the development of heart failure. While these proteolytic systems exhibit distinct molecular mechanisms, they work cooperatively (one process can regulate another).

Calpain protects the heart from hemodynamic stress.

Calpains make up a family of Ca(2+)-dependent intracellular cysteine proteases that include ubiquitously expressed µ- and m-calpains. Both are heterodimers consisting of a distinct large catalytic subunit (calpain 1 for µ-calpain and calpain 2 for m-calpain) and a common regulatory subunit (calpain 4). The physiological roles of calpain remain unclear in the organs, including the heart, but it has been suggested that calpain is activated by Ca(2+) overload in diseased hearts, resulting in cardiac dysfunction. In this study, cardiac-specific calpain 4-deficient mice were generated to elucidate the role of calpain in the heart in response to hemodynamic stress. Cardiac-specific deletion of calpain 4 resulted in decreased protein levels of calpains 1 and 2 and showed no cardiac phenotypes under base-line conditions but caused left ventricle dilatation, contractile dysfunction, and heart failure with interstitial fibrosis 1 week after pressure overload. Pressure-overloaded calpain 4-deficient hearts took up a membrane-impermeant dye, Evans blue, indicating plasma membrane disruption. Membrane repair assays using a two-photon laser-scanning microscope revealed that calpain 4-deficient cardiomyocytes failed to reseal a plasma membrane that had been disrupted by laser irradiation. Thus, the data indicate that calpain protects the heart from hemodynamic stresses, such as pressure overload.

Downregulation of ferritin heavy chain increases labile iron pool, oxidative stress and cell death in cardiomyocytes.

Ferritin heavy chain (FHC) protein was significantly reduced in murine failing hearts following left coronary ligation or thoracic transverse aortic constriction. The mRNA expression of FHC was not significantly altered in failing hearts, compared to that in control sham-operated hearts. Prussian blue staining revealed spotty iron depositions in myocardial infarct failing hearts. Oxidative stress was enhanced in the myocardial infarct failing hearts, as evidenced by increases in 4-hydroxy-2-nonenal and 8-hydroxy-2'-deoxyguanosine immunoreactivity. To clarify the functional significance of FHC downregulation in hearts, we infected rat neonatal cardiomyocytes with adenoviral vector expressing short hairpin RNA targeted to FHC (Ad-FHC-RNAi). The downregulation of FHC induced a reduction in the viability of cardiomyocytes. The relative number of iron deposition-, 4-hydroxy-2-nonenal- or 8-hydroxy-2'-deoxyguanosine-positive cardiomyocytes was significantly higher in Ad-FHC-RNAi-infected cardiomyocytes than in control vector-infected cardiomyocytes. Treatment of Ad-FHC-RNAi-infected cardiomyocytes with desferrioxamine, an iron chelator, significantly reduced the number of iron, 4-hydroxy-2-nonenal or 8-hydroxy-2'-deoxyguanosine-positive cells, and increased viability. In addition, treatment with N-acetyl cysteine, an antioxidant, significantly reduced the number of 4-hydroxy-2-nonenal- or 8-hydroxy-2'-deoxyguanosine-positive cells. Reduced viability in Ad-FHC-RNAi-infected cardiomyocytes was significantly improved with N-acetyl cysteine treatment. These findings indicate that excessive free iron and the resultant enhanced oxidative stress caused by downregulation of FHC lead to cardiomyocyte death. The decrease in FHC expression in failing hearts may play an important role in the pathogenesis of heart failure.