Aims: We have shown that autophagy and mitophagy are required for preconditioning. While statin's cardioprotective effects are well known, the role of autophagy/mitophagy in statin-mediated cardioprotection is not. In this study, we used HL-1 cardiomyocytes and mice subjected to ischemia/reperfusion to elucidate the mechanism of statin-mediated cardioprotection. Results: HL-1 cardiomyocytes exposed to simvastatin for 24 h exhibited diminished protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling, increased activation of unc-51-like kinase 1, and upregulation of autophagy and mitophagy. Similar findings were obtained in hearts of mice given simvastatin. Mevalonate abolished simvastatin's effects on Akt/mTOR signaling and autophagy induction in HL-1 cells, indicating that the effects are mediated through inhibition of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Simvastatin-treated HL-1 cells exhibited mitochondrial translocation of Parkin and p62/SQSTM1, fission, and mitophagy. Because Parkin is required for mitophagy and is expressed in heart, we investigated the effect of simvastatin on infarct size in Parkin knockout mice. Simvastatin reduced infarct size in wild-type mice but showed no benefit in Parkin knockout mice. Inhibition of HMG-CoA reductase limits mevalonate availability for both cholesterol and coenzyme Q 10 (CoQ) biosynthesis. CoQ supplementation had no effect on statin-induced Akt/mTOR dephosphorylation or macroautophagy in HL-1 cells, but it potently blocked mitophagy. Importantly, CoQ supplementation abolished statin-mediated cardioprotection in vivo. Innovation and Conclusion: Acute simvastatin treatment suppresses mTOR signaling and triggers Parkindependent mitophagy, the latter which is required for cardioprotection. Coadministration of CoQ with simvastatin impairs mitophagy and cardioprotection. These results raise the concern that CoQ may interfere with anti-ischemic benefits of statins mediated through stimulation of mitophagy.

Fluorescent Timer, or DsRed1-E5, is a mutant of the red fluorescent protein, dsRed, in which fluorescence shifts over time from green to red as the protein matures. This molecular clock gives temporal and spatial information on protein turnover. To visualize mitochondrial turnover, we targeted Timer to the mitochondrial matrix with a mitochondrial-targeting sequence (coined “MitoTimer”) and cloned it into a tetracycline-inducible promoter construct to regulate its expression. Here we report characterization of this novel fluorescent reporter for mitochondrial dynamics. Tet-On HEK 293 cells were transfected with pTRE-tight-MitoTimer and production was induced with doxycycline (Dox). Mitochondrial distribution was demonstrated by fluorescence microscopy and verified by subcellular fractionation and western blot analysis. Dox addition for as little as 1 h was sufficient to induce MitoTimer expression within 4 h, with persistence in the mitochondrial fraction for up to 6 d. The color-specific conformation of MitoTimer was stable after fixation with 4% paraformaldehyde. Ratiometric analysis of MitoTimer revealed a time-dependent transition from green to red over 48 h and was amenable to analysis by fluorescence microscopy and flow cytometry of whole cells or isolated mitochondria. A second Dox administration 48 h after the initial induction resulted in a second round of expression of green MitoTimer. The extent of new protein incorporation during a second pulse was increased by administration of a mitochondrial uncoupler or simvastatin, both of which trigger mitophagy and biogenesis. MitoTimer is a novel fluorescent reporter protein that can reveal new insights into mitochondrial dynamics within cells. Coupled with organelle flow cytometry, it offers new opportunities to investigate mitochondrial subpopulations by biochemical or proteomic methods.

The cardioprotective effects of statins are well known yet the mechanism is unclear. Previously we showed that autophagy is required for cardioprotection from ischemia/reperfusion injury. More recently, we reported that ischemic preconditioning involves Parkin-mediated mitophagy. We hypothesized that the molecular basis of statin-mediated cardioprotection may involve mitochondrial quality control through mitophagy. HL-1 cardiomyocytes treated with simvastatin for 24hr exhibited diminished Akt/mTOR signaling, increased activation of ULK1, and upregulation of autophagy (n=3, p<0.05). Similar findings were obtained in cardiac tissue in mice 4hr after i.p. administration of simvastatin. Mevalonate addition abolished statin’s effects on Akt/mTOR signaling and autophagy induction in HL-1 cells, indicating that the effects are mediated through inhibition of HMG-CoA reductase. Statin treatment in HL-1 cells triggered mitochondrial fragmentation, translocation of Parkin and p62/SQSTM1 to the mitochondria followed by mitophagy. To establish the requirement for statin-mediated mitophagy in cardioprotection, we investigated the ability of statins to reduce infarct size in Parkin knockout (KO) mice. While statin treatment reduced infarct size from 55% of area at risk to 30% in wild type mice, it had no protective benefit in Parkin KO mice (n=4-6, p<0.05). These findings indicate that cardioprotection by HMG-CoA reductase inhibitors involves suppression of mTOR signaling and induction of Parkin-dependent mitophagy. Figure: Statin-induced cardioprotection against I/R injury: solid bars/diamonds = wild-type; open bars/diamonds = Parkin knockout mice.

Fluorescent Timer, or DsRed1-E5, is a mutant of the red fluorescent protein, dsRed, developed by Terskikh and colleagues. Its fluorescence shifts over time from green to red as the protein matures. This molecular clock gives temporal and spatial information on protein turnover. To visualize mitochondrial turnover, we targeted Timer to the mitochondrial matrix with a mitochondrial targeting sequence (coined “MitoTimer”) and cloned it into a tetracycline-inducible promoter construct to regulate its expression. Here we report characterization of this novel fluorescent reporter for mitochondrial dynamics. Tet-On HEK 293 cells were transfected with pTRE-tight-MitoTimer and induced production with doxycycline. Mitochondrial distribution was demonstrated by fluorescence microscopy and verified by subcellular fractionation and western blot analysis. Doxycycline addition for as little as 1hr was sufficient to label mitochondria. MitoTimer was detected as early as 4hr following doxycycline addition, and persisted in mitochondria for at least 72hr. The color-specific conformation of MitoTimer was stable after fixation with 4% paraformaldehyde. MitoTimer matured to red fluorescence within 48hr, at which time a second pulse of doxycycline induced expression of green (immature) MitoTimer which was selectively incorporated into a subset of mitochondria actively engaged in protein import. The extent of new protein incorporation during a second pulse was increased under conditions of mito-biogenesis and reduced if mitochondrial membrane potential was dissipated. We conclude that MitoTimer can be used to monitor mitophagy and biogenesis.