Loss of autophagy protein ATG5 impairs cardiac capacity in mice and humans through diminishing mitochondrial abundance and disrupting Ca(2+) cycling

AIMS: Autophagy protects against the development of cardiac hypertrophy and failure. While aberrant Ca(2+) handling promotes myocardial remodelling and contributes to contractile dysfunction, the role of autophagy in maintaining Ca(2+) homeostasis remains elusive. Here, we examined whether Atg5 deficiency-mediated autophagy promotes early changes in subcellular Ca(2+) handling in ventricular cardiomyocytes, and whether those alterations associate with compromised cardiac reserve capacity, which commonly precedes the onset of heart failure. METHODS AND RESULTS: RT–qPCR and immunoblotting demonstrated reduced Atg5 gene and protein expression and decreased abundancy of autophagy markers in hypertrophied and failing human hearts. The function of ATG5 was examined using cardiomyocyte-specific Atg5-knockout mice (Atg5(−/−)). Before manifesting cardiac dysfunction, Atg5(−/−) mice showed compromised cardiac reserve in response to β-adrenergic stimulation. Consequently, effort intolerance and maximal oxygen consumption were reduced during treadmill-based exercise tolerance testing. Mechanistically, cellular imaging revealed that Atg5 deprivation did not alter spatial and functional organization of intracellular Ca(2+) stores or affect Ca(2+) cycling in response to slow pacing or upon acute isoprenaline administration. However, high-frequency stimulation exposed stunted amplitude of Ca(2+) transients, augmented nucleoplasmic Ca(2+) load, and increased CaMKII activity, especially in the nuclear region of hypertrophied Atg5(−/−) cardiomyocytes. These changes in Ca(2+) cycling were recapitulated in hypertrophied human cardiomyocytes. Finally, ultrastructural analysis revealed accumulation of mitochondria with reduced volume and size distribution, meanwhile functional measurements showed impaired redox balance in Atg5(−/−) cardiomyocytes, implying energetic unsustainability due to overcompensation of single mitochondria, particularly under increased workload. CONCLUSION: Loss of cardiac Atg5-dependent autophagy reduces mitochondrial abundance and causes subtle alterations in subcellular Ca(2+) cycling upon increased workload in mice. Autophagy-related impairment of Ca(2+) handling is progressively worsened by β-adrenergic signalling in ventricular cardiomyocytes, thereby leading to energetic exhaustion and compromised cardiac reserve.