Myocardial Ischemia‐Reperfusion Injury: Does Disruption of Mitochondrial Cristae Integrity Play a Requisite Mechanistic Role?

Abstract

Lethal myocardial ischemia‐reperfusion (IR) injury (i.e., cardiomyocyte death resulting in ‘heart attack’ or myocardial infarction, MI) is the leading global cause of death and disability. Despite improvements in the clinical management of MI, the molecular mechanisms responsible for lethal IR injury are incompletely understood. Data from our group and others have revealed that cardiomyocytes subjected to IR display a loss in integrity of optic atrophy protein (OPA1), the master‐regulator of fusion at the inner mitochondrial membrane and putative gate‐keeper of cristae junction integrity. Our goal in the current study was to establish whether disruption of OPA1 integrity plays a causal role in determining cardiomyocyte fate. To test this concept, HL‐1 cardiomyocytes were subjected to 2.5 hours of simulated ischemia + reperfusion. In Protocol 1, the cleavage and subsequent release of OPA1 from mitochondria into the cytosol was assessed by immunoblotting at 30 min post‐R. As expected, IR was associated with degradation of OPA1 (in particular, the long (L) forms of the protein critical for inner mitochondrial membrane architecture), and a >15‐fold increase in cytosolic expression of the lower molecular weight short (S) forms (data not shown). In Protocol 2, to demonstrate cause‐and‐effect, HL‐1 cells were transfected with either siRNA targeting OPA1 or scrambled peptide before undergoing 2.5 hours I + 24 hours R or a time‐matched normoxic period. Additional non‐transfected cells served as controls. Using 50 nM siRNA, near‐total knockdown (KD) of OPA1 (most notably, the critical L‐forms) was achieved (Figure 1). In the normoxic cohorts, cardiomyocyte viability (quantified using the MTT assay) was modestly but significantly reduced in siRNA‐transfected cells (Figure 2 – left: *p<0.05 versus normoxic non‐transfected cells). However, contrary to the premise of our study, OPA1 knockdown had no effect on viability in cells subjected to IR: i.e., did not exacerbate IR‐induced cardiomyocyte death (Figure 2 – right). These data suggest that maintenance of OPA1 integrity does not have a requisite, causal role in mediating lethal myocardial ischemia‐reperfusion injury.Support or Funding InformationDetroit Cardiovascular Training Grant T32 (NIH 2T32HL120822) through the National Heart, Lung, and Blood Institute ‐ Wayne State UniversityOPA1 Knockdown with siRNAFigure 1HL‐1 Cell viability with OPA1 KnockdownFigure 2