AMPK Negatively Regulates Mitophagy in the Heart

Maintaining mitochondrial homeostasis and energy metabolism is essential for normal cardiac function. Adenosine-monophosphate activated protein kinase (AMPK) is an energy sensor in the cell that detects and reacts to fluctuations in intracellular AMP: ATP ratio and it is activated by increased AMP levels. Activated AMPK promotes ATP production by inhibiting anabolic consuming pathways and enhancing catabolic pathways. AMPK has been shown to protect the heart under several cardiac conditions including ischemia and starvation. The cardioprotective effects have been attributed partially to its ability to induce autophagy, a cellular degradation pathway that eliminates protein aggregates and damaged organelles through the lysosome. When mitochondria are targeted for degradation through autophagy, it is termed mitophagy. Both autophagy and mitophagy may occur under the same conditions but they do not always go in the same direction, suggesting that they may be regulated by distinct pathways. Despite its ability to positively regulate autophagy, AMPK has not been shown to directly regulate mitophagy. In the present study, we investigated if AMPK is required for mitophagy in the heart using mice that lack AMPK alpha 2 gene (knockout, KO) and express a novel mitophagy reporter known as mito-Rosella, a dual-emission biosensor composed of a mitochondrial targeting sequence and a RFP-GFP fusion protein. Mitophagy events were seen as red puncta on merged confocal microscopic images. These red puncta represent mitochondrial fragments that are being degraded in lysosomes where the pH sensitive GFP is quenched. To assess mitophagy flux, these mice were also treated with lysosomal protease inhibitors pepstatin A and E64-d, which lead to an accumulation of red puncta in mitolysosomes. To our surprise, AMPK alpha 2 KO mouse hearts had markedly increased amount of red puncta than wild type hearts, the number of which was further increased by the lysosomal protease inhibitors, suggesting an enhanced mitophagy flux in the absence of AMPK alpha 2 gene. Consistently, western blot analysis showed significantly increased autophagy marker protein LC3-II in AMPK alpha 2 KO mouse hearts. These findings were confirmed in H9c2 cardiac myoblast cells treated with siRNAs targeting both AMPK alpha 1 and alpha 2 genes. In addition, the protein expression levels of FUN14 domain containing 1 (FUNDC1), a positive regulator of mitophagy, were up-regulated in the AMPK alpha 2 KO hearts, which may contribute to the enhanced mitophagy. Collectively, these results suggest that AMPK is a negative regulator of mitophagy, contrary to the widely held hypothesis that AMPK signalling is necessary for mitophagy. Future studies are warranted to investigate if AMPK over expression can inhibit mitophagy either at basal level or in response to various cardiac stresses.