Boosting Mitophagy Clears Toxic Protein Clumps and Rescues the Failing Heart

Heart muscle cells must survive for decades, and they depend on sophisticated protein quality control (PQC) systems to keep their structural machinery intact. Mutations in PQC proteins — most notably CRYAB-R120G (arginine-to-glycine at position 120 of the small heat shock protein crystallin alpha-B) and BAG3-P209L — cause accumulation of misfolded proteins into toxic aggregates, disrupting sarcomere architecture and driving inherited cardiomyopathy and heart failure. Existing strategies targeting the ubiquitin-proteasome system or bulk autophagy provide only partial relief, and the question of whether an alternative cellular pathway could be exploited remained open. This study tackles that question directly, building on a yeast-derived discovery that mitochondria can physically import aggregated cytosolic proteins — a pathway dubbed MAGIC (Mitochondria As Guardians In Cytosol).

The research team used a multi-platform experimental design spanning mouse genetics, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), and AAV9-mediated cardiac gene therapy. Mice with inducible, cardiomyocyte-specific ablation of TRAF2 (TRAF2-icKO), generated by tamoxifen-driven Cre activation in Traf2-floxed/Myh6-MerCreMer animals, were studied alongside transgenic mice (R120G-TG) expressing the human CRYAB-R120G mutant driven by the Myh6 promoter. Biochemical fractionation, super-resolution microscopy, and in-vitro mitochondrial uptake assays were the primary investigative tools.

TRAF2-icKO mice accumulated poly-ubiquitinated proteins and the aggregate scaffold protein p62 in both cytosolic and mitochondria-enriched biochemical fractions, despite intact macro-autophagy and normal proteasome activity (assessed by fluorogenic substrate assay). Super-resolution imaging confirmed co-localization of ubiquitinated proteins and p62 within COXIV-delimited inner mitochondrial membranes — a pattern absent in controls. Human TRAF2-null hiPSC-CMs phenocopied these findings, showing disrupted sarcomeres and elevated polyubiquitinated proteins; both TRAF2 and PARKIN lentiviral transduction rescued these defects. Critically, isolated cardiac mitochondria from wild-type mice actively took up recombinant R120G-CRYAB and P209L-BAG3 proteins in a cell-free uptake assay, and R120G-CRYAB protein increasingly localized to mitochondria in both human and mouse cardiomyocytes, validating the MAGIC paradigm in a mammalian cardiac context.

In the therapeutic arm, R120G-TG mice, which naturally upregulate TRAF2 and show increased mitophagy as a compensatory response, were subjected to adult-onset inducible haploinsufficiency of Traf2 (heterozygous deletion). This accelerated mortality and worsened left ventricular (LV) systolic dysfunction while increasing myocardial protein aggregates — confirming that endogenous mitophagy upregulation is a protective adaptation. By contrast, AAV9-cardiac troponin T promoter-driven TRAF2 overexpression in R120G-TG mice stimulated mitophagy, significantly reduced cytosolic protein aggregates, attenuated LV systolic dysfunction, restored DESMIN localization from aggregates back to Z-discs and intercalated discs, and reduced mortality. AAV9-PARKIN gene delivery produced similar salutary effects, demonstrating that both physiologic (TRAF2-dependent) and stress-induced (PARKIN-dependent) mitophagy pathways converge on clearing cytosolic protein aggregates.

The mechanistic implications are substantial. Mitophagy not only removes damaged mitochondria but also functions as an active cytosolic housekeeping mechanism — mitochondria engulf aggregate-prone proteins, and their subsequent lysosomal degradation via mitophagy resolves the aggregate pathology. For longevity science, this reframes mitophagy as a systemic proteostasis enforcer, not merely an organelle-quality system. The findings suggest that interventions known to stimulate mitophagy more broadly — including exercise, caloric restriction, NAD+ precursors, and urolithin A — may partially exert their cardioprotective effects by facilitating cytosolic aggregate clearance, a hypothesis now supported by mechanistic evidence in a mammalian heart model.