Abstract 283: Parkin Fails to Rescue Age-Dependent Cardiomyopathy in Mitochondrial DNA Mutator Mice

Abstract

Temporal decline in mitochondrial function is widely considered to be a driver of cardiomyocyte aging, which in turn contributes to the prevalence of cardiovascular disease in the aging population. The accumulation of dysfunctional mitochondria appears to be a direct consequence of reduced autophagy and mitochondrial quality control in the aging heart. Parkin, an E3 ubiquitin ligase, plays a critical role in this process, marking dysfunctional mitochondria for degradation by autophagosomes; however, whether Parkin functions to prevent cardiac aging by maintaining a healthy population of mitochondria is still unclear. To examine the role of Parkin in the context of mtDNA damage and myocardial aging, we used a mouse model carrying a proofreading defective mitochondrial DNA polymerase gamma (POLG). We observed a significant decrease in Parkin protein levels in the hearts of aged (6 months) POLG mice, in spite of elevated Parkin mRNA. Seahorse analysis revealed a decrease in cardiac mitochondrial respiration in 6-month POLG mice. While cardiac structure and function were similar in both genotypes, POLG mice displayed modest but significant cardiac hypertrophy at this age. Next, we generated mice with concomitant Parkin deletion or cardiac specific Parkin overexpression with the mutant POLG. However, loss of Parkin did not exacerbate the accelerated cardiac aging phenotype observed in the POLG mice, and enhancing cardiac Parkin protein levels did not rescue the mitochondrial dysfunction or the cardiac hypertrophy observed in POLG mice up to 12 months of age. Surprisingly, we found that Parkin levels were reduced in POLG hearts, even with cardiac specific overexpression of Parkin. Translation of Parkin was unperturbed in 12-month POLGxParkin TG hearts, suggesting instead that Parkin protein stability is compromised in aged POLG hearts, a hypothesis we are currently testing. We also found both diminished protein ubiquitination and reduced degradation of Mfn1, a known Parkin substrate, in POLGxParkin TG hearts compared to Parkin TG hearts. These results provide new insights into mitophagy and aging, and suggest that Parkin plays a minor role in baseline mitochondrial maintenance and that overexpression of Parkin fails to prevent the cardiac aging process.