Abstract 13441: Two Forms of Autophagy in the Heart - Constitutive Autophagy via an LC3-dependent Pathway and Starvation-Induced Autophagy via a Rab9-Dependent Pathway
Autophagy is a bulk degradation process in which proteins/organelles are sequestered into autophagosomes and degraded at lysosomes. Besides conventional autophagy (CA), an alternative autophagy (AA), utilizing a distinct mechanism of autophagosome formation, may exist. Whether AA exists in the heart is unknown. Cardiac-specific Atg7-KO (Atg7-CKO) and systemic Ulk1-KO mice were subjected to starvation for 48 hours. LC3-II expression and p62 degradation were increased in wild-type (WT) mice after starvation (ST), indicating increased autophagy. In Atg7-CKO mice, LC3-II was reduced and p62 was accumulated at baseline and after ST, indicating inhibition of CA. In Ulk1-KO mice, neither LC3-II nor p62 was affected at baseline or after ST. However, expression of Lamp2/Rab9, indicators of AA, was abolished. Furthermore, polyubiquitinated protein accumulated after ST. Left ventricular fractional shortening (FS) was maintained after ST in WT mice (38%). In Atg7-CKO mice, FS was decreased at baseline (22%) but was not further affected by ST (20%). In Ulk1-KO mice, FS was maintained at baseline (32%) but remarkably deteriorated during ST (19%). After glucose deprivation (GD), LC3 puncta were co-localized with calnexin (ER marker) in Ad-sh-Control- and Ad-sh-Ulk1-transduced, but not in Ad-sh-Atg7-transduced, cardiomyocytes (CMs) in vitro. In contrast, Lamp2 was co-localized with TGN46 (Golgi marker) in Ad-sh-Control- and Ad-sh-Atg7-transduced, but not in Ad-sh-Ulk1-transduced, CMs after GD. These results suggest that autophagosomes/autolysosomes are derived from ER in CA and Golgi in Ulk1-dependent autophagy in CMs. LC3-II expression and p62 degradation were increased in Ad-sh-Ulk1- or Ad-sh-Ulk2-transduced CMs after ST, indicating increased CA. In contrast, LC3-II expression was reduced and p62 was increased at baseline and after ST in Ad-sh-Ulk1/2-transduced CMs, indicating inhibition of CA. These results suggest that Ulk2 compensates for Ulk1 in the regulation of CA. Atg7 is required for CA, thereby maintaining cardiac function at baseline, whereas Ulk1 is required for preserving cardiac function during ST, possibly through stimulation of Rab9-dependent AA. These results support the presence of Ulk1-dependent AA in the heart during ST.
- © 2011 by American Heart Association, Inc.