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Background The mitochondrial specific deacetylase, Sirtuin-3 has been reported to regulate oxidative phosphorylation, the activity of Cyclophilin D (a key component of the mitochondrial permeability transition pore, MPTP), and the ROS scavenger MnSOD. We hypothesise Sirtuin-3 to be a potential therapeutic target for cardio-protection based its ability to prevent MPTP formation and inhibit ROS generation. Methods and Results In HL-1 cells (a murine cardiac cell line), Sirtuin-3 over-expression reduced cell death following simulated ischemia-reperfusion injury. Futhermore, Sirtuin-3 over-expression reduced MPTP formation, and induced mitochondrial fusion. To investigate the role of endogenous Sirtuin-3 in the adult heart, Sirtuin-3 (whole body) KO mice and WT littermates were subjected to in vivo cardiac ischemia (30 min) followed by 24 hrs reperfusion. Myocardial infarct (MI) size was determined as a percentage of area at risk. Interestingly, no differences in MI size were observed between WT and KO mice under fed conditions. However, overnight fasting (to induce Sirtuin-3 expression and activity) resulted in a smaller MI size in the Sirtuin-3 KO when compared to WT mice. Conclusions We report that the role of Sirtuin-3 in cardio-protection differs between the HL-1 cardiac cell line and the adult heart. In HL-1 cells, Sirtuin-3 over-expression protects against acute IRI, suggesting that activating Sirtuin-3 in this cell-line may be cardio-protective. In contrast, fasted mice deficient in Sirtuin-3 had smaller MI following IRI, suggesting that inhibiting Sirtuin-3 in the fasted adult heart may be cardio-protective. This finding may have clinical implications in patients who are fasted before surgery.

Rationale Ischaemic heart disease is the leading cause of death and disability worldwide. Modulating mitochondrial morphology by inhibiting mitochondrial fission during acute ischaemia-reperfusion injury (IRI) has been reported to protect the heart against cell death. Here, we investigate the newly discovered mitochondrial fission proteins, MiD49 and MiD51, as novel targets for cardioprotection. Methods and Results Genetic ablation of both MiD49 and MiD51 in HL-1 cardiac cell using shRNAs had the following effects: A significant increase in the proportion of cells expressing predominantly elongated mitochondria (81.5±2.9% MiD49+MiD51 knockdown vs. 49.4±4.9% vector control: n=120 cells/group; P A delay in time taken to induce mitochondrial permeability transition pore (MPTP) opening (485±61 seconds MiD49+MiD51 knockdown vs. 233±20 seconds vector control: n=120 cells/group; P Less cell death following simulated IRI (27.9±3.0% MiD49+MiD51 knockdown vs. 56.7±2.4% vector control: n=300 cells/group; P Ablation of MiD49 or MiD51 individually had no beneficial effects. Unexpectedly, the over-expression of either MiD49 or MiD51 also had beneficial effects: The formation of abnormally elongated (hyperfused) mitochondria with a peri-nuclear distribution; It delayed the induction of MPTP opening; and It reduced cell death following simulated IRI. The explanation for this apparent beneficial effect with MiD49 or MiD51 over-expression, may be due to Drp1 sequestration to mitochondria, leading to unopposed abnormal mitochondrial elongation (hyperfusion). Conclusions Here, we show that combined knockdown of the newly discovered mitochondrial fission proteins, MiD49 and MiD51, induced mitochondrial elongation, delayed MPTP opening, and protected cells against simulated acute IRI, positioning these proteins as novel targets for cardioprotection.

Background Cardiac-specific ablation of both Mfn1 and Mfn2 in the adult heart results in mitochondrial fragmentation and a lethal cardiomyopathy after about 6 weeks. The effect of combined Mfn1 and Mfn2 deletion on the susceptibility to acute ischemia-reperfusion injury (IRI) and subsequent calcium overload is not known, and is investigated in this study. Methods and Results Cardiac-specific ablation of both Mfn1 and Mfn2 (DKO) was initiated in mice aged 5 weeks using 5 days administration of tamoxifen (MerCreMer), resulting in total knockout of both these proteins at the age of 10 weeks. These mice were subjected to in vivo myocardial ischemia (30 mins) followed by 24 hrs reperfusion before myocardial infarct size was determined. The sustained MI size in the DKO mice was 50% smaller than that in the WT control mice. These findings were associated with decreased MPTP opening susceptibility (assessed by calcium-induced mitochondrial swelling), reduced mitochondrial calcium overload after simulated IR (assessed by Rhod2 staining) and impaired mitochondrial respiration in the DKO hearts when compared to WT control. Conclusions We have shown that the adult murine heart deficient in both Mfn1 and Mfn2 was protected against acute IRI, a finding which was associated with defects in mitochondrial function and reduced mitochondrial calcium overload. This data suggests that the Mitofusins may be therapeutic targets for cardioprotection.

Background: A mitochondrial specific deacetylase, Sirtuin-3 has been reported to regulate oxidative phosphorylation, the activity of Cyclophilin D (a key component of the mitochondrial permeability transition pore, MPTP), and the ROS scavenger MnSOD. We hypothesised that Sirtuin-3 could be a potential therapeutic target for cardio-protection based its ability to prevent MPTP formation and inhibit ROS generation. Methods and Results: In HL-1 cells (a murine cardiac cell line), over-expression of Sirtuin-3 reduced cell death following simulated ischemia-reperfusion injury (assessed by propidium iodide staining). Futhermore, Sirtuin-3 over-expression reduced MPTP formation (assessed by ROS-induced mitochondrial depolarization), and induced mitochondrial fusion (assessed by 3 blinded investigators and the PEG fusion assay). The catalytically inactive mutant form of Sirtuin-3 failed to mediate any of these beneficial effects. To investigate the role of endogenous Sirtuin-3 in the adult heart, Sirtuin-3 (whole body) KO mice and WT littermates were subjected to in vivo cardiac ischemia (30 min) followed by 24 hrs reperfusion. myocardial infarct (MI) size was determined as a percentage of area at risk. Interestingly, no differences in MI size were observed between WT and KO mice under fed conditions. However, overnight fasting (to induce Sirtuin-3 expression and activity) resulted in a smaller MI size in the Sirtuin-3 KO when compared to WT mice. Conclusions: We report that the role of Sirtuin-3 in cardio-protection differs between the HL-1 cardiac cell line and the adult heart. In the HL-1 cell, Sirtuin-3 over-expression had beneficial effects against acute IRI, suggesting that activating Sirtuin-3 in this cell-line may be cardio-protective. In contrast, fasted mice deficient in Sirtuin-3 had smaller MI following IRI, suggesting that inhibiting Sirtuin-3 in the fasted adult heart may be cardio-protective. This finding may have clinical implications in patients who are fasted before surgery.

This study aimed to investigate the role of the intrinsic cardiac nervous system in the mechanism of classical myocardial ischaemic preconditioning (IPC). Isolated perfused rat hearts were subjected to 35-min regional ischaemia and 60-min reperfusion. IPC was induced as three cycles of 5-min global ischaemia-reperfusion, and provided significant reduction in infarct size (IS/AAR = 14 ± 2% vs control IS/AAR = 48 ± 3%, p