Autophagy and Megamitochondria in Cardiac Aging and Heart Failure

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Project summary As average life expectancy continues to rise in the developed world, age associated pathologies are increasingly prevalent. Aging is a major risk factor for cardiovascular diseases and the hallmarks of cardiac aging include loss of myocytes, fibrosis and hypertrophy, all of which contribute to increased incidence of cardiac disease. At the molecular level, cellular aging is characterized by increased reactive oxygen species (ROS) production, mitochondrial dysfunction and accumulation of damaged proteins and organelles. Cardiac myocytes rely upon autophagy, a lysosome-mediated degradation pathway, to remove toxic protein aggregates and damaged organelles from the cellular milieu. Increasing lines of evidence point to an age- associated decrease in myocyte autophagy, with predictably negative consequences for cardiac function and health. However, it is still unclear why autophagy declines with age and whether specific proteins or pathways involved in regulating autophagy are altered with age. Mitochondrial dysfunction is also a key hallmark of aging and has been linked to a number of age-related pathologies, including heart failure. In addition, enlarged or megamitochondria are often present in aged tissues, but their potential contribution to the aging process and disease development remain unknown. We have confirmed that autophagic activity is reduced in aged mouse hearts which correlates with accumulation of ubiquitinated mitochondria. Our preliminary data also suggest that the decrease in autophagic activity in the aged heart is due to altered expression of Atg9b, a key regulator of autophagosome formation and elongation. We also found that the aged mouse heart contains a substantial number of enlarged mitochondria. Why these megamitochondria form with age in the heart and whether they contribute to the aging process are currently unknown. In this proposal, we will examine the hypothesis that a decline in autophagosome formation and mitochondrial clearance in the aging heart leads to increased fusion between dysfunctional and healthy mitochondria in an attempt to dilute damaged components. Over time, these megamitochondria accumulate increased levels of damage. They become less functional and generate increased ROS, which directly contribute to the cardiac aging process. This hypothesis will be tested with two specific aims. Specific aim 1 will dissect the mechanism underlying the age-associated decline in autophagy. Specific aim 2 will characterize the interplay between mitochondrial morphology and autophagy during aging. We will also investigate if restoring Atg9b will enhance baseline autophagy in the aged hearts and whether reduced mitochondrial ROS production will prevent or delay the age associated decline in autophagy and abrogate formation of harmful megamitochondria. Overall, these studies will further our understanding of the molecular mechanism underlying the aging process and help identify interventions to preserve mitochondrial homeostasis and prevent development of disease.