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.
