Abstract 270: The Core Function of Calcineurin

Background: Recent evidence demonstrates that not only NFAT, but also calcineurin is translocated into the nucleus upon hypertrophic stimulation. Previously it was also demonstrated that calpain-mediated degradation caused a constitutive active calcineurin. We hypothesised that nuclear calcineurin is an intranuclear Ca 2+ sensor hypertrophied myocardium and that inhibition of nuclear translocation of calcineurin is a therapeutic strategy to prevent hypertrophy. Methods: Employing a transgene mouse model with conditional calpastatin overexpression (”tet-off”, resulting in calpain inhibition), different adenoviral calcineurin mutants and confocal microscopy in isolated adult cardiac myocytes we investigated calcineurin translocation and nuclear Ca 2+ transients. Assessment of cardiac function if transgenic animals was performed by 7T MRI. Results: We could demonstrate that chronic Ang II stimulation of mice caused calpain-mediated degradation of calcineurin resulting in a constitutive active calcineurin with nuclear translocation. The constitutive active calcineurin in the nucleus escaped further degradation by the UPS and sustained an ongoing hypertrophic response, even after removal of Ang II. Inhibition of nuclear translocation of activated calcineurin by a small inhibitory peptide prevented myocardial hypertrophy in vivo. Transgenic inhibition of calpain activity by calpastatin overexpression prevented proteolysis of calcineurin and allowed for relocation of calcineurin from the nucleus back to the cytosol and regression hypertrophy after removal of Ang II. We were also able to demonstrate that Ang II increases nuclear Ca 2+ transients via InsP3 receptors and that calcineurin is able to act as nuclear Ca 2+ sensor detecting local Ca 2+ release from the nuclear envelope via InsP3R. Nuclear calcineurin mutants that are defective for Ca 2+ activation failed to activate NFAT dependent transcription. Conclusion: This provides an explanation how Ca 2+ and calcineurin can regulate transcription in cardiomyocytes in response to neurohumoral signals apart from Ca 2+ changes in contraction regulation.