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Enzymatic proteolysis by calpains, Ca2+-dependent intracellular cysteine proteases, has been implicated in pathological processes such as cellular degeneration or death. Here, we investigated the role of calpain activation in the hearts subjected to myocardial infarction. We produced myocardial infarction in Cast-/- mice deficient for calpastatin, the specific endogenous inhibitory protein for calpains, and Cast+/+ mice. The activity of cardiac calpains in Cast+/+ mice was not elevated within 1 day but showed a gradual elevation after 7 days following myocardial infarction, which was further pronounced in Cast-/- mice. Although the prevalence of cardiomyocyte death was indistinguishable between Cast-/- and Cast+/+ mice, Cast-/- mice exhibited profound contractile dysfunction and chamber dilatation and showed a significant reduction in survival rate after myocardial infarction as compared with Cast+/+ mice. Notably, immunofluorescence revealed that at 28 days after myocardial infarction, calpains were activated in cardiomyocytes exclusively at the border zone and that Cast-/- mice showed higher intensity and a broader extent of calpain activation at the border zone than Cast+/+ mice. In the border zone of Cast-/- mice, pronounced activation of calpains was associated with a decrease in N-cadherin expression and up-regulation of molecular markers for cardiac hypertrophy and fibrosis. In cultured rat neonatal cardiomyocytes, calpain activation by treatment with ionomycin induced cleavage of N-cadherin and decreased expression levels of β-catenin and connexin 43, which was attenuated by calpain inhibitor. These results thus demonstrate that activation of calpains disassembles cell-cell adhesion at intercalated discs by degrading N-cadherin and thereby promotes left ventricular remodeling after myocardial infarction. [ABSTRACT FROM AUTHOR]