Intercellular mitochondrial transfer and transmitophagy in response to protein import dysfunction

Mitochondrial protein import is critical for organelle biogenesis and maintenance, essential processes 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 via tunnelling nanotubes (TNTs) however, the fate of the transferred mitochondria and the mechanistic basis for rescue was unresolved. Here, we show that bi-directional mitochondrial trafficking between cells harbouring import-defective and import-competent mitochondria has distinct regulation and consequences. Transferred import-defective mitochondria are highly fragmented and destined for canonical lysosomal degradation. In contrast, ROS-producing mitochondria at the periphery of cells with import-competent mitochondria are transferred into neighbouring cells undergoing import failure; these new arrivals then accumulate within previously uncharacterised ‘mitochondrial degradation bodies’ (MBDs). We speculate that the co-operation of these distinct examples of TNT- mediated conventional and non-canonical ‘transmitophagy’ instigate mitochondrial regeneration, and thereby rescue.