Rab 5 and Alsin regulate stress-activated cytoprotective signaling on mitochondria 1 2

20 Mitochondrial stress response is essential for cell survival, and damaged mitochondria are a 21 hallmark of neurodegenerative diseases. Thus, it is fundamental to understand how mitochondria relay 22 information within the cell. Here, by investigating mitochondrial-endosom al contact sites we made the 23 surprising observation that the small GTPase Rab5 translocates from early endosomes to mitochondria 24 upon oxidative stress. This process is reversible and accompanied by an increase in Rab5-positive 25 endosomes in contact with mitochondria. Interestingly, activation of Rab5 on mitochondria depends on 26 the Rab5-GEF ALS2/Alsin, encoded by a gene mutated in amyotrophic lateral sclerosis (ALS). Alsin27 deficient human induced pluripotent stem cell-derived spinal motor neurons are defective in relocating 28 Rab5 to mitochondria and display increased susceptibility to oxidative stress. These findings define a 29 novel pathway whereby Alsin catalyzes the assembly of the Rab5 endocytic machinery on 30 mitochondria. Defects in stress-sensing by endosomes could be crucial for mitochondrial quality 31 control during the onset of ALS. 32