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Titin truncations lead to impaired cardiomyocyte autophagy and mitochondrial function in vivo.

Titin-truncating variants (TTNtv) are the most common genetic cause of dilated cardiomyopathy. TTNtv occur in ~1% of the general population and causes subclinical cardiac remodeling in asymptomatic carriers. In rat models with either proximal or distal TTNtv, we previously showed altered cardiac metabolism at baseline and impaired cardiac function in response to stress. However, the molecular mechanism(s) underlying these effects remains unknown. In the current study, we used rat models of TTNtv to investigate the effect of TTNtv on autophagy and mitochondrial function, which are essential for maintaining cellular metabolic homeostasis and cardiac function. In both the proximal and distal TTNtv rat models, we found increased levels of LC3B-II and p62 proteins, indicative of diminished autophagic degradation. The accumulation of autophagosomes and p62 protein in cardiomyocytes was also demonstrated by electron microscopy and immunochemistry, respectively. Impaired autophagy in the TTNtv heart was associated with increased phosphorylation of mTOR and decreased protein levels of the lysosomal protease, cathepsin B. In addition, TTNtv hearts showed mitochondrial dysfunction, as evidenced by decreased oxygen consumption rate in cardiomyocytes, increased levels of reactive oxygen species and mitochondrial protein ubiquitination. We also observed increased acetylation of mitochondrial proteins associated with decreased NAD+/NADH ratio in the TTNtv hearts. mTORC1 inhibitor, rapamycin, was able to rescue the impaired autophagy in TTNtv hearts. In summary, TTNtv leads to impaired autophagy and mitochondrial function in the heart. These changes not only provide molecular mechanisms that underlie TTNtv-associated ventricular remodeling but also offer potential targets for its intervention.