Regulating the balance between synthesis and proteasomal degradation of cellular
proteins is essential for tissue growth and maintenance, but the critical pathways regulating
protein ubiquitination and degradation are incompletely defined. Although participation of
calpain calcium-activated
proteases in post-necrotic myocardial
autolysis is well characterized, their importance in homeostatic turnover of normal cardiac tissue is controversial. Hence, we evaluated the consequences of physiologic
calpain (
calcium-activated protease) activity in cultured cardiomyocytes and unstressed mouse hearts. Comparison of in vitro proteolytic activities of cardiac-expressed calpains 1 and 2 revealed
calpain 1, but not
calpain 2, activity at physiological
calcium concentrations. Physiological
calpain 1 activation was evident in adenoviral transfected cultured cardiomyocytes as proteolysis of specific substrates, generally increased
protein ubiquitination, and accelerated
protein turnover, that were each inhibited by coexpression of the inhibitor
protein calpastatin. Conditional forced expression of
calpain 1, but not
calpain 2, in mouse hearts demonstrated substrate-specific proteolytic activity under basal conditions, with hyperubiquitination of cardiac
proteins and increased
26S proteasome activity. Loss of myocardial
calpain activity by forced expression of
calpastatin diminished ubiquitination of 1 or more specific myocardial
proteins, without affecting overall ubiquitination or
proteasome activity, and resulted in a progressive
dilated cardiomyopathy characterized by accumulation of intracellular
protein aggregates, formation of autophagosomes, and degeneration of sarcomeres. Thus,
calpain 1 is upstream of, and necessary for, ubiquitination and proteasomal degradation of a subset of myocardial
proteins whose abnormal accumulation produces autophagosomes and degeneration of cardiomyocytes with functional decompensation.