Titin is a Target of MMP-2: Implications in Myocardial Ischemia/ Reperfusion Injury

Background—Titin is the largest mammalian (∼3000-4000 kDa) and myofilament protein which acts as a molecular spring in the cardiac sarcomere and determines systolic and diastolic function. Loss of titin in ischemic hearts has been reported, but the mechanism of titin degradation is not well understood. Matrix metalloproteinase-2 (MMP-2) is localized to the cardiac sarcomere and upon activation in ischemia/reperfusion injury proteolyzes specific myofilament proteins. Here we determine whether titin is an intracellular substrate for MMP-2 and if its degradation during ischemia/reperfusion contributes to cardiac contractile dysfunction. Methods and Results—Immunohistochemistry and confocal microscopy in rat and human hearts showed discrete co-localization between MMP-2 and titin in the Z-disc region of titin and that MMP-2 is mainly localized to titin near the Z-disc of the cardiac sarcomere. Both purified titin or titin in skinned cardiomyocytes were proteolyzed when incubated with MMP-2 in a concentration-dependent manner and this was prevented by MMP inhibitors. Isolated rat hearts subjected to ischemia/reperfusion injury showed cleavage of titin in ventricular extracts by gel electrophoresis which was confirmed by reduced titin immunostaining in tissue sections