Abstract 959: Enhancement of Myocardial Function by Hsp20 is Associated with Decreased Type-1 Phosphatase Activity
Background The type 1 protein phosphatase (PP1), the major Ser/Thr protein phosphatase in cardiomyocytes, has been reported to be over-activated in human and experimental heart failure. More interestingly, microcystin-affinity chromatography indicated that the catalytic subunit of PP1 may interact with Hsp20 in rabbit skeletal muscle. We have previously shown that acute or chronic overexpression of Hsp20 can augment myocardial contractile function. However, the mechanism underlying the enhancement of cardiac contractility is not clear.
Methods and Results To address the functional role of Hsp20 in vivo and its underlying mechanisms, transgenic (TG) mice were developed with cardiac-restricted overexpression of Hsp20. Characterization of a TG line with10-fold overexpression of Hsp20 revealed marked augmentation of in vivo basal cardiac function [+dP/dt (mmHg/sec), TG: 13391±360 vs WT: 8442±382, n=6, p<0.05], as determined by pressure-volume relationships. Contractile function and Ca handling properties were also increased in isolated cardiomyocytes from Hsp20 TG mice (FS%, TG: 11.5±0.7 vs WT: 9.2±0.9; Peak Ca amplitude, TG: 0.45±0.01 vs WT: 0.39±0.02, p<0.05, n=6 hearts, 10 –12 cells/heart). However, there were no differences between TG and WT myocytes after β-agonist stimulation. Enhanced cellular Ca cycling by Hsp20 was associated with increased SR Ca content (Peak Ca amplitude, TG: 1.13±0.03 vs WT: 0.95±0.05, p<0.05, n=6), measured by caffeine-induced Ca release. Western-blots indicated no significant alterations in the levels of SERCA2a, phospholamban (PLN), Calsequestrin (CSQ) and ryanodine receptor (RyR). However, the phosphorylation levels of PLN were increased by 2 fold in TGs (n=4). Further in vitro phosphotase activity assay indicated that PP1 activity in transgenic hearts was decreased by 20±3% (n=6, p<0.05), compared to WT hearts, while there was no significant difference in PP2a activity.
Conclusions These results suggest that Hsp20 plays an important in vivo role in the regulation of cardiac function through decreases in PP1 activity,and its downstream SR phosphoprotein, PLN. Thus, inhibition of PP1 by Hsp20 overexpression may be beneficial in restoring the depressed function in heart failure.