Description: Cardiac rupture is a fatal complication after myocardial infarction (MI). However, the detailed mechanism underlying cardiac rupture after MI remains to be fully elucidated. In this study, we investigated the role of mitochondrial DNA (mtDNA) and mitochondria in the pathophysiology of cardiac rupture by analyzing Twinkle helicase overexpression mice (TW mice). Twinkle overexpression increased mtDNA copy number approximately twofold and ameliorated ischemic cardiomyopathy at day 28 after MI. Notably, Twinkle overexpression markedly prevented cardiac rupture and improved post-MI survival, accompanied by the suppression of MMP-2 and MMP-9 in the MI border area at day 5 after MI when cardiac rupture frequently occurs. Additionally, these cardioprotective effects of Twinkle overexpression were abolished in transgenic mice overexpressing mutant Twinkle with an in-frame duplication of amino acids 353–365, which resulted in no increases in mtDNA copy number. Furthermore, although apoptosis and oxidative stress were induced and mitochondria were damaged in the border area, these injuries were improved in TW mice. Further analysis revealed that mitochondrial biogenesis, including mtDNA copy number, transcription, and translation, was severely impaired in the border area at day 5 . In contrast, Twinkle overexpression maintained mtDNA copy number and restored the impaired transcription and translation of mtDNA in the border area. These results demonstrated that Twinkle overexpression alleviated impaired mitochondrial biogenesis in the border area through maintained mtDNA copy number and thereby prevented cardiac rupture accompanied by the reduction of apoptosis and oxidative stress, and suppression of MMP activity.

Description: Dehydrogenase/reductase member 7C (DHRS7C) is a newly identified NAD/NADH-dependent dehydrogenase that is expressed in cardiac and skeletal muscle and localized in the endoplasmic/sarcoplasmic reticulum (ER/SR). However, its functional role in muscle cells remains to be fully elucidated. Here, we investigated the role of DHRS7C by analyzing mouse C2C12 myoblasts deficient in DHRS7C (DHRS7C-KO cells), overexpressing wild-type DHRS7C (DHRS7C-WT cells), or expressing mutant DHRS7C [DHRS7C-Y191F or DHRS7C-K195Q cells, harboring point mutations in the NAD/NADH-dependent dehydrogenase catalytic core domain (YXXXK)]. DHRS7C expression was induced as C2C12 myoblasts differentiated into mature myotubes, whereas DHRS7C-KO myotubes exhibited enlarged cellular morphology after differentiation. Notably, both DHRS7C-Y191F and DHRS7C-K195Q cells also showed similar enlarged cellular morphology, suggesting that the NAD/NADH-dependent dehydrogenase catalytic core domain is pivotal for DHRS7C function. In DHRS7C-KO, DHRS7C-Y191F, and DHRS7C-K195Q cells, the resting level of cytosolic Ca 2+ and total amount of Ca 2+ storage in the ER/SR were significantly higher than those in control C2C12 and DHRS7C-WT cells after differentiation. Additionally, Ca 2+ release from the ER/SR induced by thapsigargin and 4-chloro-m-cresol was augmented in these cells and calpain, a calcium-dependent protease, was significantly activated in DHRS7C-KO, DHRS7C-Y191F, and DHRS7C-K195Q myotubes, consistent with the higher resting level of cytosolic Ca 2+ concentration and enlarged morphology after differentiation. Furthermore, treatment with a calpain inhibitor abolished the enlarged cellular morphology. Taken together, our findings suggested that DHRS7C maintains intracellular Ca 2+ homeostasis involving the ER/SR and that functional loss of DHRS7C leads to Ca 2+ overload in the cytosol and ER/SR, resulting in enlarged cellular morphology via calpain activation.