Mitochondrial protein import stress causes lysosomal damage and progressive tissue atrophy.

Mitochondrial and lysosomal abnormalities co-occur in aging-related diseases with progressive tissue atrophy. It remains unclear whether these two pathogenic pathways affect tissue homeostasis independently, convergently or epistatically. We show that mitochondrial protein import stress causes vacuolar damage in yeast, manifested by V-ATPase disassembly, and vacuolar deacidification and fragmentation. In a mouse model of mitochondrial protein import stress induced by overloading of the nuclear-encoded ANT1 protein, we observe progressive muscle atrophy independent of bioenergetic defects. Like in yeast mutants with severe vacuolar damage, genes involved in amino acid uptake/biosynthesis, one-carbon metabolism, lysosomal biogenesis and iron homeostasis are activated in the skeletal muscle of Ant1-transgenic mice. The affected muscles accumulate glycogen, lipofuscin and poorly processed multivesicular bodies. Despite activation of lysosomal repair and lysophagic pathways, autophagic flux is severely stalled. During aging, various proteolytic cathepsins are increasingly released from the lysosomal lumen into the cytosol. Together with proteasomal activation, this may contribute to unbalanced proteostasis, reduced myofiber size and skeletal muscle atrophy. Our study therefore discovered an evolutionarily conserved mitochondria-to-lysosome proteotoxic axis that affects tissue mass homeostasis during aging.