Abstract 11640: Conditional Myocardial Calpain-Inhibition Prevents Proteolysis of Calcineurin
Background: Calcineurin is crucial in the development of myocardial hypertrophy. We recently demonstrated that calpain truncated calcineurin at the C-terminus causing a constitutive active and nuclear calcineurin. We hypothesized that in vivo inhibition of the calcineurin truncation would be beneficial in the recovery after myocardial challenges.
Methods: We generated transgenic mice with conditional overexpression of calpastatin, a calpain inhibitor. Mice were challenged with Ang II for 4 weeks, and then the stimulus was removed. We investigated development and regression of hypertrophy and calcineurin activation/translocation.
Results: Discontinuing doxycycline resulted in 3.7-fold overexpression of calpastatin in the heart causing a 68% reduction in myocardial calpain activity. Overexpression of calpastatin (3 months) did not affect cardiac function or hypertrophy at baseline. 4 weeks of Ang II stimulation resulted in myocardial hypertrophy in both non-induced mice and mice with calpastatin overexpression (and subsequent calpain inhibition). Heart-body weight ratio, cross sectional area of cardiomyocytes, LV wall diameter (as assessed in MRI) and NFAT activity increased significantly. 3 weeks after explantation of the Ang II minipumps, there was regression of myocardial hypertrophy in the animals with calpastatin overexpression whereas non-induced animals with intact calpain function still had signs of hypertrophy. In the non-induced animals (with intact calpain function) we still found nuclear calcineurin 3 weeks after cessation of Ang II stimulation. Further experiments revealed that this was the truncated calcineurin isoform which was constitutively active and remained nuclear. We also demonstrated that the truncated and activated calcineurin isoform escaped further degradation by the ubiquitin-proteasome system, as this active calcineurin isoform (lacking the C-terminus) could not be ubiquitinated.
Conclusions: Employing a transgene mouse model with conditional calpain inhibition we demonstrate that constitutive active and nuclear calcineurin escapes further degradation by the UPS and sustains an ongoing hypertrophic response even after withdrawal of the hypertrophic stimulus.
- © 2011 by American Heart Association, Inc.