Abstract 20062: The Anti-Apoptotic Protein Mcl-1 is an Important Regulator of Mitochondrial Turnover
Mcl-1 is an anti-apoptotic Bcl-2 protein which is expressed at high levels in the heart compared to other anti-apoptotic Bcl-2 proteins. Surprisingly, very little is known with respect to how Mcl-1 regulates cell survival in myocytes. We have generated a heart specific inducible knockout of Mcl-1 and discovered that loss of Mcl-1 in cardiac myocytes led to rapid heart failure. Echocardiography demonstrated severe reduction in fractional shortening (WT=45.5%±6.6 vs. KO=7.9%±3.4, n=6, p<0.05) and ejection fraction (WT=83.4%±3.0 vs. KO=20.7%±8.5, n=6, p<0.05) in Mcl-1−/− mice. Surprisingly, Mcl-1−/− myocytes did not undergo apoptotic cell death as would be expected. Mcl-1−/− myocytes were TUNEL negative and had no active caspase-3. Instead, ultrastructural analysis revealed signs of necrotic cell death characterized by permeability transition pore opening, swelling of mitochondria, cellular rupture and release of intracellular contents into the extracellular space. Moreover, mitochondria isolated from Mcl-1−/− hearts exhibited reduced oxidative respiration and showed pronounced swelling at baseline. This suggests that besides its anti-apoptotic role, Mcl-1 is essential for maintaining normal mitochondrial function. Mitochondrial fission and autophagy play important roles in the normal turnover of mitochondria. Dysfunctional mitochondria are removed by autophagy via Drp-1-mediated mitochondrial fission and recruitment of the ubiquitin ligase Parkin to the mitochondria. We found that Mcl-1 associated with Drp-1 in WT hearts and overexpression of Mcl-1 induced Drp-1-mediated mitochondrial fission in neonatal myocytes. Drp-1 and Parkin were present in the mitochondrial fraction of WT hearts suggesting continuous turnover of mitochondria even in the absence of stress. In contrast, Mcl-1−/− hearts lacked both Drp-1 and Parkin at the mitochondria and there was no evidence of mitochondrial autophagy. In addition, Parkin was found as a stable mitochondrial 110 kDa complex in WT hearts, but existed as an inactive cytosolic 52 kDa monomer in Mcl-1−/− hearts. Thus, our data suggest that Mcl-1 is important for mitochondrial turnover in myocytes and defects in this process leads to accumulation of dysfunctional mitochondria and cell death.
- © 2010 by American Heart Association, Inc.