Aging is Associated with a Decline in Atg9b‐mediated Autophagosome Formation and Appearance of Enlarged Mitochondria in the Heart

Abstract

Advancing age is a major risk factor for developing heart disease and the biological processes contributing to aging are currently under intense investigation. Autophagy is an important cellular quality control mechanism that is reduced in tissues with age but the molecular mechanisms underlying the age-associated defects in autophagy remain poorly characterized. Here, using young (4 months) and aged (24 months) male mice, we have investigated how the autophagic process is altered in aged mouse hearts. We found that autophagic activity is reduced in aged hearts due to a reduction in autophagosome formation. Gene expression profile analysis to evaluate changes in autophagy regulators uncovered a reduction in Atg9b transcript and protein levels. The Atg9 proteins are critical in delivering membrane to the growing autophagosome and we confirmed that siRNA knockdown of Atg9b in cells leads to a reduction in autophagosome formation. We also found that both Atg9a and Atg9b are expressed in the myocardium and that both are rapidly increased in response to myocardial infarction in the young heart. Atg9b-positive vesicles are localized to vesicles throughout the cytosol whereas Atg9a-positive vesicles are more closely associated with mitochondria, indicating that these proteins have distinct functions in autophagy. Autophagy is also the main pathway involved in eliminating dysfunctional mitochondria via a process known as mitophagy. The E3 ubiquitin ligase Parkin plays a key role in labeling mitochondria for mitophagy. We found increased levels of Parkin-positive, ubiquitinated mitochondria in the aged hearts, an indication that they have been labeled for mitophagy. Moreover, we found that the decreased mitochondrial clearance coincided with formation of megamitochondria in the aged heart. Overall, our findings suggest that aging leads to reduced formation of autophagosomes with subsequent accumulation of mitochondria that have been labeled for mitophagy. The decreased clearance of Parkin-positive, ubiquitinated mitochondria in cells leads to enhanced compensatory mitochondrial fusion to dilute damaged components.