Down-regulation of OPA1 alters mouse mitochondrial morphology, PTP function, and cardiac adaptation to pressure overload

The optic atrophy 1 (OPA1) protein is an essential protein involved in the fusion of the mitochondrial inner membrane. Despite its high level of expression, the role of OPA1 in the heart is largely unknown. We investigated the role of this protein in Opa1(/) mice, having a 50 reduction in OPA1 protein expression in cardiac tissue. In mutant mice, cardiac function assessed by echocardiography was not significantly different from that of the Opa1(/). Electron and fluorescence microscopy revealed altered morphology of the Opa1(/) mice mitochondrial network; unexpectedly, mitochondria were larger with the presence of clusters of fused mitochondria and altered cristae. In permeabilized mutant ventricular fibres, mitochondrial functional properties were maintained, but direct energy channelling between mitochondria and myofilaments was weakened. Importantly, the mitochondrial permeability transition pore (PTP) opening in isolated permeabilized cardiomyocytes and in isolated mitochondria was significantly less sensitive to mitochondrial calcium accumulation. Finally, 6 weeks after transversal aortic constriction, Opa1(/) hearts demonstrated hypertrophy almost two-fold higher (P 0.01) than in wild-type mice with altered ejection fraction (decrease in 43 vs. 22 in Opa1(/) mice, P 0.05). These results suggest that, in adult cardiomyocytes, OPA1 plays an important role in mitochondrial morphology and PTP functioning. These properties may be critical for cardiac function under conditions of chronic pressure overload.