Cell culture and adenoviral transduction
One day-old Sprague-Dawley neonatal rats were euthanasied by 2% isoflurane inhalation and cervical dislocation. The heart were then excised, enzymatically digested and the NRVMs (Neonatal Rat Ventricular Myocytes) were cultured as previously described[24]. Adenovirus vectors harboring PINK1 (Ad-Pink1) were designed and synthesized by GeneChem Co(Shanghai, China), adenovirus vectors harboring Mfn2(Ad-Mfn2) were designed and synthesized by Obio Technology Company (Shanghai, China). The viruses were added to cells per manufacturer's instructions. Cells were stimulated with 10 μM Iso for 48 h after adenoviral transduction for 72h, and subsequent experiments and analyses were then performed.
Chemicals and Reagents
Fetal bovine serum (FBS) and Dulbecco’s Modified Eagle’s Medium (DMEM) were purchased from Gibco (Oklahoma, USA). Trypsin and collagenase were purchased from Sigma-Aldrich Co. (Saint Louis, MO, USA). Iso and metformin was purchased from Sigma-Aldrich Co. The following primary antibodies were used in this study: anti-Mfn2 and anti-PINK1 (Abcam, Cambridge, United Kingdom), anti-Beclin1, anti-P62, anti-LC3B, anti-PGC 1α, anti-TFAM , anti-NRF1, anti-GAPDH, anti-b-actin (Proteintech ,USA).
Transmission Electron Microscopy (TEM)
Cardiomyocytes were fixed in 2.5% Glutaraldehyde in cacodylate buffer for 1h at 37°C and then fixed for 3h at 4°C. Subsequently,the cells were washed with PBS thrice, then post fixed in 1% osmium tetraoxide for 1-2 hours, dehydrated in a graded series of ethanol concentrations, and embedded in Sparr resin. Ultrathin sections (50-70 nm) were cut with an ultramicrotome (UC7, Leica,Germany), contrast stained with uranyl acetate and lead citrate.Samples were examined and visualized with an electron microscope (JEM-1400, Japan) operated at 80 kV.
Evaluation of Mitochondrial Morphology
The co-localization of mitochondria and lysosomes was performed to indirectly examine mitophagy. Cardiomyocytes were co-incubated with MitoTracker Green (100 nM) and LysoTracker Red (50 uM,Molecular Probes, Eugene, OR,United States) at 37°C for 30 min. Then, nucleus were stained with Hoechst for 10 min. Mitochondrial morphology in each group was captured with a confocal microscope (Leica TCS SP8, Germany) after washing three times with PBS. Bright red fluorescence was used to highlight lysosomes and blue fluorescence represents cell nuclei .Bright orange fluorescence is the area where mitochondria and lysosomes overlap, which indirectly represents mitophagy.
ROS Determination
The cellular ROS production was measured with 2,7-dichlorodihydrofluorescein di-acetate (DCFH,Beyotime,China) according to the manufacturer's protocol. Cells were incubated in a culture medium including 10 μM DCFH-DA for 30 min at 37°C and then washed three times in PBS for 10 min each time. A fluorescence microscope(Leica, Germany) was used to detect the ROS production. Bright green fluorescence represents ROS. Image-Pro Plus was used for analysis.
TUNEL Assay
A TdT-mediated dUTP nick-end labeling (TUNEL) assay (One Step TUNEL Apoptosis Assay Kit; Beyotime, Jiangsu, China), was used to examine the apoptosis rate of cardiomyocytes per manufacturer’s instructions. The adherent cardiomyocytes were fixed with 4% paraformaldehyde for 30 min and then incubated with 0.3% Triton X-100 PBS at room temperature for 5 min. Subsequently, 50μl TUNEL solution were added to the sample, followed by a 60 min incubation at 37 ° C and lastly washed three times with PBS. Apoptotic morphological features were imaged using a fluorescence microscope (Leica, Germany). Cy3 has an excitation wavelength of 550nm and an emission wavelength of 570nm (red fluorescence). Image-Pro Plus was used for analysis.
MMP Determination
The mitochondrial membrane potential (MMP) was measured using a JC-1 Mitochondrial Membrane Potential Assay Kit (Beyotime, Jiangsu, China) according to the manufacturer's protocol. Cardiomyocytes were incubated with JC-1 staining solution at 37◦C for 20 min and then washed three times in PBS for 5 min each time. The fluorescent images were captured using a confocal microscope(Leica, Germany) by red and green fluorescence. Image-Pro Plus was used for analysis.
ATP Assay
The ATP contents of cardiomyocytes were evaluated using a Firefly Luciferase ATP Assay Kit (Beyotime, China) according to the manufacturer's protocol. The level of ATP was detected using Multi-Mode Detection Luminometer.
Cell Viability Assay
The cell viability was performed using the Cell Counting Kit-8 (CCK-8) purchased from Beyotime following the manufacturer's protocol.
Evaluation of Mitochondrial Respiration
The cardiomyocytes were seeded in a 96-well culture plate. The oxygen consumption was measured using extracellular oxygen consumption assay kits (Abcam, Cambridge, UK), according to the manufacturer’s instructions. The oxygen consumption rate was calculated in order to assess mitochondrial respiration.
Immunoblot Analysis
Cardiomyocytes were washed with PBS and lysed with a radioimmunoprecipitation assay (RIPA) lysis buffer(Beyotime biotechnology, China ) containing protease and phosphatase inhibitors (Sigma, U.S.A ).Then the protein was quantified by BCA assay (Thermo fisher,USA). Primary antibodies used included Anti-PINK1 antibody (1:1000, Abcam, USA), Anti-PGC-1α antibody (1:1000, Abcam, U.S.A) and anti-β-actin antibody (1:5000, Proteintech, USA). Second antibody used was goal anti-rabbit IgG-HRP (1:5000,Proteintech, USA). The protein bands were examined by ECL Substrate (FDbio, Hangzhou, China) and visualized by Gene Gnome Imaging System (Syngene Bio Imaging) and quantified by densitometry with Image J software (NIH).
Statistical Analysis
Quantitative data were expressed as mean ± SD.Statistical analyses were performed using either Dunnett's test of one-way ANOVA or Student's t-tests. P-values<0.05 were considered statistically significant.