Mitophagy Facilitates Cytosolic Proteostasis to Preserve Cardiac Function

Protein quality control is critical for maintaining sarcomere structure and function in cardiomyocytes. Mutations in protein quality control pathway proteins, namely, CRYAB-R120G (arginine to glycine at position 120) and BAG3-P209L (proline to lysine at position 209), induce protein aggregates and cardiomyopathy in humans. Novel observations in yeast demonstrate mitochondrial uptake of cytosolic protein aggregates. We hypothesized that mitochondrial uptake of cytosolic protein aggregates, and their removal by mitophagy, a lysosomal degradative pathway, facilitates cytosolic protein quality control in cardiomyocytes.

Mice with inducible cardiac myocyte-specific ablation of TRAF2 (TNF receptor-associated factor 2; TRAF2-icKO), which impairs mitophagy, were assessed for protein aggregates with biochemical fractionation and super-resolution imaging. Human-induced pluripotent stem cell-derived cardiomyocytes with TRAF2 ablation or R120G knock-in to the CRYAB locus were assessed for protein aggregates and effects of mitophagy stimulation. Transgenic mice expressing R120G-CRYAB protein (R120G-transgenic mice) were subjected to adeno-associated virus 9-cTnT (cardiac troponin T) promoter-driven TRAF2 or PARKIN gain-of-function and TRAF2 loss of function in cardiomyocytes to determine the effect of mitophagy modulation on cardiac structure, function, and protein aggregate pathology.

TRAF2-icKO mice demonstrate accumulation of mitochondrial and cytosolic protein aggregates and DESMIN mislocalization to protein aggregates. TRAF2 null human-induced pluripotent stem cell-derived cardiomyocytes demonstrate impaired mitophagy with accumulation of polyubiquitinated proteins and disrupted sarcomeres, which are rescued by both TRAF2 and PARKIN transduction. Isolated mitochondria take up cardiomyopathy-associated aggregate-prone cytosolic proteins, namely, R120G-CRYAB and P209L-BAG3. R120G-CRYAB mutant protein increasingly localizes to mitochondria in human and mouse cardiomyocytes. R120G-transgenic mice demonstrate upregulation of myocardial TRAF2 with increased mitophagy. Adult-onset inducible haplo-insufficiency of TRAF2 resulted in accelerated mortality, left ventricular systolic dysfunction, and increased protein aggregates in R120G-transgenic mice. Conversely, adeno-associated virus 9-TRAF2 transduction in R120G-transgenic mice stimulated mitophagy, reduced mortality, attenuated LV systolic dysfunction, reduced cytosolic protein aggregates, and restored DESMIN localization.

Stimulation of mitophagy in cardiomyocytes facilitates removal of cytosolic protein aggregates as a mechanism to ameliorate proteotoxic cardiomyopathy.