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

Abstract

Titin-truncating variants (TTNtv) are the most common genetic cause of dilated cardiomyopathy (DCM).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 also was 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 (OCR) in cardiomyocytes, increased levels of reactive oxygen species (ROS), 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.

Haploinsufficiency of mechanistic target of rapamycin ameliorates bag3 cardiomyopathy in adult zebrafish

“…On the other hand, it is also possible that mTOR inhibition attenuates a common pathological event that is shared among different types of cardiomyopathy, as suggested by its cardioprotective effects on several subtypes of cardiomyopathies in both zebrafish and mouse (Ding et al, 2011; Sciarretta et al, 2018). Consistent with this hypothesis, impaired cardiomyocyte autophagy was reported in rat models of Titin-truncating variants (TTNtv), which can be rescued by rapamycin, a specific pharmacologic inhibitor of mTOR signaling (Zhou et al, 2019). Dysregulated proteostasis might be part of the common pathological event, and the accumulation of toxic proteins might further drive the pathogenesis into irreversible heart failure.…”

Metabolic Alterations in Inherited Cardiomyopathies

“…The increased levels of respiratory chain complexes could reflect an attempt to enhance ATP production to compensate for the sarcomeric defects related to TTN mutation. More recently, TTN-truncating variants leading to DCM were studied in rats and correlated with impaired autophagy, decreased O2 consumption rate, excessive ROS production, and increase of mitochondrial protein ubiquitination in cardiomyocytes [37]. Additionally, aberrant ERK1/2 signaling was associated with altered mitochondrial shape, distribution, fragmentation, and degeneration in a mouse model of DCM carrying the p.H222P mutation in lamin A/C (LMNA) gene [38].…”

Modifications of Titin Contribute to the Progression of Cardiomyopathy and Represent a Therapeutic Target for Treatment of Heart Failure

“…Identification of TTNtv in DCM is helpful for prognosis; however, as targeted therapies for TTNtv become available, this may also change clinical management ( Table 1 ).Given the association of TTNtv inducing upregulation of the mTOR complex, mTOR inhibitors such as rapamycin may be reasonable to consider for therapeutic trials of TTNtv DCM [ 52 ]. There have also been promising results in correcting the frameshift and early termination in TTNtv using antisense oligonucleotide (AON) mediated exon skipping.…”