Intercellular mitochondrial transfer and trans-mitophagy in response to protein import dysfunction.

Mitochondrial protein import is critical for organelle biogenesis, maintenance, and regeneration-essential for cellular homeostasis. Import dysfunction compromises cellular energy supplies, which is damaging to cells, particularly those with high energetic demands like neurons. Previously, we have shown that import failure is rescued by intercellular mitochondrial transfer (IMT) via tunnelling nanotubes (TNTs) however, the fate of the transferred mitochondria and the mechanistic basis for rescue were unresolved. Here, we show that bidirectional mitochondrial trafficking between cells harboring import-defective and import-competent mitochondria is distinct in terms of their regulation and ensuing consequences. Transferred import-defective mitochondria are highly fragmented and destined for canonical lysosomal degradation. In contrast, reactive oxygen species (ROS)-producing mitochondria at the periphery of cells with import-competent mitochondria are transferred into neighboring cells undergoing import failure. These new arrivals then accumulate within previously uncharacterized "mitochondrial degradation bodies" (MDBs). We speculate that the cooperation of these distinct cases of TNT-mediated conventional and noncanonical "trans-mitophagy" instigates mitochondrial regeneration, and thereby rescues mitochondrial function.