The Role of Autophagy in Aged Cardiomyocyte Arrhythmogenesis
Authors: Murphy Kevin R. , Lu Yi Chun , O‐Uchi Jin , Terentyev Dmitry , Koren Gideon 
Published: November 2023

Aging is associated with a 10–20 fold increase in the incidence of sudden cardiac death. We previously reported that post‐translational modification of ryanodine receptors (RyR2) (e.g. thiol oxidation) by mitochondria‐derived reactive oxygen species (ROS) contributes to aberrant Ca2+ handling in cardiomyocytes from aging rabbit hearts (Cooper et al. J Physiol. 2013). Autophagy, a recycling process capable of degrading cellular components, such as mitochondria, is downregulated in aging hearts. Indeed, abnormal mitochondrial morphology possibly regulated by unbalanced fission and fusion is frequently reported to change with age. However, the precise mechanism of how mitochondrial morphology, ROS, and dysfunction initiates arrhythmogenesis in the aging heart remains elusive. Here we report that autophagy signaling is a critical modulator of mitochondrial function, as well as cellular calcium handling in the aging cardiomyocyte. First, we confirmed the machinery for mitochondrial autophagy is significantly down‐regulated in the aging rabbit heart as assessed by the altered protein expression levels including p62, the LC3II/I ratio, p53, and Pink1. We also found that the mitochondrial fission protein, dynamin‐related protein 1, increased in the aging rabbit heart, whereas the fusion protein, mitofusin 2, was unchanged. Analysis of the ultra‐structure revealed mitochondria with a greater variance in mitochondrion cross‐sectional area. To investigate the involvement of autophagy, we next applied the autophagy blocker chloroquine (1 μM) in HL‐1 cardiomyocytes for 3 hours. The results showed an increase in mitochondria ROS production, depolarization of mitochondrial membrane potential, and upregulation of fission proteins and downregulation of autophagy proteins. The application chloroquine significantly increased the oxidation levels of RyR2 compared to vehicle as assessed by using fluorescent C2‐Maleimide. Importantly, we observed a decreased latency to spontaneous Ca2+ wave formation after 20 seconds of 1.0‐hz field electrical stimulation under β‐adrenergic stimulation (100 nM isoproterenol for 5 minutes) and a larger proportion of cells exhibiting waves after chloroquine treatment compared to vehicle. The mitochondrial ROS scavenger, MitoTEMPO (25 μM), reduced mitochondrial ROS, and normalized cytosolic calcium homeostasis. Lastly, treating aged primary rabbit cardiomyocytes with the autophagy enhancer, Torin1 (250 nM), for 6 hours resulted in decreased mitochondrial ROS, hyperpolarization of mitochondrial membrane potential, and half the instance of pro‐arrhythmic spontaneous Ca2+ wave formation. Our data demonstrate that decreased activity of autophagy signaling induced abnormal mitochondrial morphology, raised mitochondria derived ROS, followed by RyR2 oxidation, which resulted in increased formation of pro‐arrhythmic Ca2+ waves. Conversely, activation of autophagy in aged primary rabbit cardiomyocytes reduced pro‐arrhythmic Ca2+ waves. We conclude that decreased autophagy contributes to pro‐arrhythmic changes in the aging cardiomyocyte and promoting autophagy may reduce lethal arrhythmogenesis.