Abstract
The mitochondrion is an essential organelle that supplies ATP in cardiomyocytes (CMs). However, damaged mitochondria are harmful via the production of reactive oxygen species and induction of apoptosis in pathological conditions. Therefore, quality of mitochondria should be controlled tightly through various mitochondrial quality control mechanisms. Mitochondrial autophagy (mitophagy) is considered an integral part of this mechanism, and recent investigations uncovered the role of PINK1 and Parkin in mitophagy. However, these observations were made under artificial conditions, such as over-expression of Parkin or treatment with CCCP, and thus the precise mechanism has not been fully elucidated in more pathophysiologically relevant conditions. Recent evidence suggests that mitophagy can take place independently of ATG7, a molecule essential for the conventional form of autophagy, and that this form of autophagy is ULK1-dependent. We investigated the role of ULK1 and ATG7 in mediating mitophagy using mitochondria-targeted Keima (Mito-Keima) in cultured rat neonatal CMs. Keima has a bimodal excitation spectrum peaking at 440 and 560 nm, corresponding to the neutral and acidic pH, respectively. In CMs transfected with Mito-Keima, the fluorescent dots with a high 560nm/440nm ratio represent the mitochondria incorporated into autolysosomes which indicate mitophagy. Here we report that ULK1 plays a more predominant role in glucose deprivation (GD) -induced mitophagy than ATG7. Control CMs exhibited 8.7±1.0 % of the area of high-ratio dots per cells after GD. Knockdown of ULK1 significantly reduced the area to 2.3±0.9 % in CMs after GD (p<0.01, vs sh-Control). The reduction was significantly greater in CMs with knockdown of ULK1 than that of ATG7 (7.0±1.6 %, p<0.05, sh-ULK1 vs sh-ATG7). In addition, knockdown of Beclin1 and Drp1 also significantly decreased the area of high-ratio dots (about 1.0 % and 0.5 %, respectively). Overexpression of ULK1 was sufficient to induce mitophagy without starvation, whereas that of ATG7 was not. These results suggest that ULK1, Beclin1 and Drp1 play an essential role in mediating GD-induced mitophagy in CMs.
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