Abstract 863: Suppression of Autophagy and Accumulation of Protein Aggregates in a Mouse Model of Dilated Cardiomyopathy
Damaged proteins and organelles accumulate during heart failure (HF) and impaired clearance of them causes formation of protein aggregates and cellular malfunction, respectively. Autophagy is an important mechanism of protein degradation and the only mechanism for degrading damaged organelles, and, thus, speculated to mediate protein quality control during HF. We examined whether autophagy is activated in a mouse model of dilated cardiomyopathy induced by cardiac specific overexpression of mammalian STE20 like-kinase 1 (Tg-Mst1), which develops HF within 3 months of age. In order to examine the extent of autophagosome formation in the heart, Tg-Mst1 mice were crossed with GFP-LC3 mice (Tg-GFP-LC3) and the number of GFP-LC3 dots, an indicator of autophagosomes, in the myocardium was counted. The number of GFP-LC3 dots was significantly smaller in Tg-Mst1-GFP-LC3 (0.36 ± 0.13 fold, p<0.01) than in Tg-GFP-LC3 mice, suggesting that Tg-Mst1 have a smaller number of autophagosomes than non-transgenic (NTg) mice. Consistently, LC3-II/LC3-I, another indicator of autophagy, was significantly smaller in Tg-Mst1 (0.58±0.01 fold, p<0.01) than in NTg mice. Accumulation of p62, a protein subjected to degradation through autophagy, was greater in Tg-Mst1 (3.40±0.09 fold, p<0.01) than in NTg, suggesting that lysosomal degradation is impaired in Tg-Mst1. The amount of poly-ubiquitinated proteins was greater in Tg-Mst1 than in NTg, and the number of vimentin aggregates was also significantly greater in Tg-Mst1 than in NTg mice (4.8±0.4 fold, 1.6±0.3 fold, p<0.01), suggesting that protein degradation through the ubiquitin proteasome pathway was impaired in Tg-Mst1. Adenovirus mediated overexpression of Mst1 significantly increased accumulation of p62 (1.70±0.01 fold, p<0.01) in cultured cardiac myocytes. Mst1 also inhibited autophagy-mediated cleavage of GFP from GFP-LC3 in response to amino acid starvation, suggesting that Mst1 directly suppresses autophagy in cardiac myocytes. These results suggest that autophagy may be suppressed by Mst1 and that suppression of autophagy is accompanied by accumulation of protein aggregates in Tg-Mst1. The impairment of autophagy may contribute to the reduced level of protein quality control during HF.