Abstract 16086: Sustained PKA Phosphorylation of Small Heat Shock Protein 20 (Hsp20) Results in Contractile Dysfunction and Cardiac Remodeling Associated with the Induction of Autophagy
Background: The levels of small heat shock 20 (Hsp20) and its phosphorylation at Serine 16, a conserved PKA phosphorylation site, are adaptively elevated upon various stresses. We previously reported that acute overexpression of constitutively phosphorylated Hsp20 (Hsp20-S16D) in adult cardiomyocytes resulted in enhanced contractility. However, the in vivo role of phosphorylated Hsp20 remains to be clarified.
Methods and Results: We generated a cardiac-specific transgenic mouse model with 12-fold overexpression of Hsp20-S16D. Unexpectedly, contractile parameters were significantly depressed in isolated transgenic (TG) cardiomyocytes (maximal rates of cell shortening (+dL/dt): 53.0±5.34 vs. 83.6±6.2 micrometer/s; minimum rates of cell shortening (-dL/dt): 37.5±3.7 vs. 68.2±4.2 micrometer/s; and fractional shortening (FS): 5.0±0.3% vs. 7.4±0.3%, P<0.01, n=45), compared with non-TGs. In vivo contractile function was also significantly impaired in TGs, compared with their non-TG littermates (ejection fraction (EF): 79.13±1.99% vs. 91.29±0.43% fractional shortening (FS): 42.95±2.27% vs. 57.91±0.91%, P<0.01, n=6). However, the expression levels of Ca2+ cycling proteins,such as phospholamban (PLN), SERCA2a, RyR2, and phosphorylation of PLN and RyR2 were unaltered in TG vs. WT hearts. In addition, TG mice developed left ventricular (LV) fibrosis at 8-weeks of age, without evidence of LV hypertrophy or dilation, and their life span was markedly shorter (mean age at death: 9 months), relative to non-TGs (P<0.01, n=20). Interestingly, cardiac autophagy was activated in TGs, evidenced by: 1) 2-fold increasing of autophagosome formation examined by electron microscopy; 2) increased conversion of the autophagic marker LC3-I to LC3-II by Western blot. The observed autophagosome accumulation was not due to diminished activity of distal lysosomal pathways since immunostaining for lysosomal-associated membrane protein-1(LAMP-1) revealed increased lysosomal activity.
Conclusions: Our data indicate that long-term augmentation of cardiac Hsp20 phosphorylation results in increased cardiac autophagic activity, which may contribute to depressed myocardial function and pathological remodeling, leading to premature death.
- © 2010 by American Heart Association, Inc.