1Universitätsklinikum Brandenburg an der Havel GmbH Zentrum für Innere Medizin I Brandenburg an der Havel, Deutschland; 2Medizinische Hochschule Brandenburg Theodor Fontane Zentrum für Translationale Medizin Brandenburg an der Havel, Deutschland; 3Universitätsklinikum Ruppin-Brandenburg Med. Klinik A Schwerpunkt Kardiologie Neuruppin, Deutschland; 4Universitätsklinikum Würzburg Medizinische Klinik und Poliklinik I Würzburg, Deutschland; 5Universitätsklinikum Düsseldorf Institut für Pharmakologie Düsseldorf, Deutschland; 6Universitätsklinikum Brandenburg Innere Medizin 1 Brandenburg, Deutschland
Background:
It was recently demonstrated that calcineurin is subject to proteolytic cleavage by calpain, resulting in a truncated calcineurin fragment that is a strong inductor of myocardial hypertrophy. We hypothesized that inhibition of calpain function and thereby prevention of calcineurin truncation attenuates development of myocardial hypertrophy.
Methods and Results:
We generated a transgenic mouse model with conditional cardiac calpastatin overexpression (CSTN mice). Induction of calpastatin overexpression lead to calpain inhibition and suppressed calcineurin truncation. In CSTN mice, basal phenotype and Angiotensin-II-induced myocardial hypertrophy were comparable to non-induced controls. However, CSTN mice demonstrated fast regression of hypertrophy when Angiotensin‑II was removed, whereas hypertrophy persisted in non-induced controls. Persisting hypertrophy in controls was accompanied by nuclear accumulation of truncated calcineurin, caused by exposure of an internal nuclear localization sequence and simultaneous loss of the nuclear export sequence during truncation. Furthermore, we found that truncated calcineurin was insufficiently ubiquitinylated compared to its full length form and thus escaped degradation over several weeks in our in vivo experiments.
Conclusion: Our data demonstrate that calpain-mediated cleavage resulted in nuclear accumulation of a truncated, constitutively active calcineurin isoform that sustained a myocardial hypertrophic response to Angiotensin-II even after withdrawal of the stimulus. Transgenic calpain inhibition prevented the ongoing myocardial hypertrophic response.