Abstract 1279: Intracellular Protein Aggregation is a Proximal Trigger of Cardiomyocyte Autophagy
Background: Autophagy is a tightly regulated biological process that plays a central role in tissue homeostasis, development, and disease; under certain circumstances, autophagy can lead to cell death. Cardiomyocyte autophagy can be induced by multiple forms of stress, including pressure overload, chronic ischemia, and diphtheria toxin-induced injury. However, little is known regarding the proximal events that trigger this pathway. Given that autophagy is a highly conserved process of bulk protein degradation, we hypothesized that stress-induced protein aggregation is sufficient to induce cardiomyocyte autophagy.
Methods and Results: Here, we report that severe pressure overload in mice triggers protein aggregation (protein accumulation in an insoluble lysate fraction) and development of aggresome-like structures (vimentin-ensheathed aggregates localized within the cell at the MTOC, Microtubule Organizing Center). Using a novel line of αMHC-GFP-LC3-expressing “autophagy reporter” mice, we found that these changes were associated with robust induction of autophagic activity in cardiomy-ocytes. To test for causal links, we used neonatal rat ventricular myocytes (NRVM) in culture. 16 hours of MG-132-mediated proteasome inhibition was sufficient to induce protein aggregation. Also, over-expression of a mutatant form of αB-crystallin that causes desmin-related cardiomyopathy in humans was similarly capable of inducing protein aggregation. In these 2 paradigms - where protein aggregation was induced by distinct mechanisms - we observed a similar 2-fold increase in autophagic activity as measured by LC3 (microtubule-associated light chain 3) processing. To define the role of protein aggregation-induced autophagy in NRVM, we attenuated autophagic activity using 3-methyladenine. Consistent with a role of autophagy in bulk protein degradation in cardiomyocytes, attenuation of autophagy dramatically enhanced aggresome size and abundance.
Conclusions: Protein aggregation is a proximal trigger of cardiomyocyte autophagy. Further, autophagic activity functions to attenuate protein aggregate accumulation and aggresome formation. These are the first data to elucidate a non-metabolic trigger of cardiomyocyte autophagy.