Abstract
Mitochondria are dynamic organelles that undergo fusion and fission. This study aims to clarify the role of mitochondrial dynamics on energetic stress resistance in cardiomyocytes (CMs). Dynamin-related protein 1 (Drp-1) which mediates mitochondrial fission was localized primarily in the cytosol in CMs. Glucose deprivation (GD) induced modest mitochondrial accumulation of Drp1 in CMs within 4 hours. Four-hour GD increased the proportion of CMs with fused mitochondria (23.5±6.9 to 35.0±10.8%, p<0.05), but also increased that of CMs with fission (2.1±0.3 to 15.5±4.8%, p<0.05) in Ad-shScr-transduced CMs, whereas mitochondria with fission did not increase after GD in Ad-shDrp1-transduced CMs (0.3±0.6 to 0.8±0.5%). These results suggest that GD induces both mitochondrial fusion and fission, and Drp1 plays an essential role in mitochondrial fission in response to GD. Transduction with Ad-shDrp1 significantly increased TUNEL-positive CMs and reduced cell viability after 4 hours of GD compared to transduction with Ad-shScr, suggesting that endogenous Drp1 protects CMs against cell death during GD. We evaluated the role of Drp1 in mediating mitophagy using mitochondria-targeted Keima fluorescence. Keima has a bimodal excitation spectrum peaking at 440 and 560 nm, corresponding to the neutral and acidic pH states, respectively. Puncta with high 560/440, indicating the presence of mitochondria in lysosomes, were significantly increased after 4 hours of GD in CMs transduced with Ad-shScr, but not in CMs transduced with Ad-shDrp, suggesting that Drp1 is necessary for stimulating autophagic mitochondrial degradation. Next, we examined the role of fission on energy stress in vivo using cardiac specific Drp1 hetero knockout (Drp1-hetCKO) mouse. Translocation of Drp1 from cytosol to mitochondria significantly increased in response to 48-hour fasting in control mice but not in Drp1-hetCKO mice. Left ventricular ejection fraction did not change after fasting in control mice (79.8±6.4 to 78.6±8.3%). However that in Drp1-hetCKO mice decreased after GD (79.4±9.7 to 48.3±11.6%, p<0.05), suggesting that Drp1 acts to preserve cardiac function during fasting. In conclusion, mitochondrial fission is crucial to adapt CMs to energy stress through mitophagy.
Published Version
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