Abstract 19501: Ulk1 Induces Mitochondrial Autophagy by Regulating Mitochondrial Fission and Alternative Autophagy in the Heart
Degradation of damaged mitochondria by mitochondrial autophagy (MA) is essential for maintaining the healthy function of mitochondria. Although Pink1-Parkin-mediated MA has been extensively investigated in mammalian cells, its importance in the heart remains to be established, particularly in response to stress. Glucose deprivation (GD) and hypoxia (HO) induced MA in cardiomyocytes (CMs), as evaluated with electron microscopy (EM) and mitochondria-targeted (Mito)-Keima. Although downregulation of Atg7 did not affect GD- or HO-induced MA, downregulation of Ulk1 markedly attenuated MA in CMs. Similarly, although induction of MA in the heart during 48-hour fasting (FT) was not attenuated in cardiac-specific atg7 knockout (KO) (atg7cKO) mice, it was suppressed in cardiac-specific ulk1KO (ulk1cKO) mice. The size of myocardial infarction/area at risk was significantly greater in ulk1cKO mice (34.9%, p<0.05) than in wild-type mice (19.7 %) and atg7cKO mice (21.5 %) during 2-hour ischemia. These results suggest that Ulk1-dependent, rather than Atg7-dependent, autophagy is the primary form of MA in the heart and the CMs therein during energy stress. Immune-gold EM revealed that the autophagosomes containing mitochondria observed in CMs subjected to GD or FT associated with Rab9 in an Ulk1-dependent manner. GD-induced increases in MA were attenuated in the presence of short hairpin RNA (sh)-rab9 in CMs. These results suggest that MA induced by energy stress is alternative autophagy, originally reported by Nishida et al. Ulk1 interacted with Rab9 in a manner dependent on phosphorylation of Ulk1 at Ser555. MA induced by overexpression of Ulk1 was suppressed by sh-rab9 or sh-drp1, suggesting that Ulk1-induced MA is mediated through Rab9- and Drp1-dependent mechanisms. GD induced phosphorylation of Drp1 at Ser616, which was inhibited by sh-ulk1 in CMs. FT-induced Ser616 phosphorylation of Drp1 and mitochondrial fragmentation (MF) were also impaired in ulk1cKO mice. These results suggest that Ulk1 mediates mitochondrial fission and alternative autophagy through coordinated actions of Rab9 and Drp1, thereby playing a central role in mediating MA in response to energy stress in CMs.
Author Disclosures: T. Saito: None. J. Sadoshima: None.
- © 2016 by American Heart Association, Inc.