Cooperation between proteolytic systems in cardiomyocyte recycling.

Cardiomyocytes are terminally differentiated cells and thus do not have the ability to dilute damaged proteins and organelles by cell division. Thus, proteolytic and recycling systems within the cardiomyocyte are essential to maintain cardiac function. The major proteolytic systems in the cell are: the ubiquitin-proteasome system, autophagy, and calpain. The ubiquitin-proteasome system degrades specific proteins by labelling them with ubiquitin. Autophagy degrades cytosolic proteins and organelles; this is generally believed to be a non-specific type of degradation. Calpain is a Ca(2+)-sensitive cysteine protease that degrades intracellular substrates including cytoskeletal proteins, and participates in Ca(2+)-mediated intracellular processes. All three systems exist in the cardiomyocyte and play pivotal roles in maintaining cardiac function. However, there is still controversy regarding the role of each protein-degradation system in the heart. Our recent reports using cardiac-specific knockout mice have revealed the cardioprotective roles of autophagy and calpain in the development of heart failure. While these proteolytic systems exhibit distinct molecular mechanisms, they work cooperatively (one process can regulate another).