Oxidation of a single amino acid in ATG5 turns out to control fasting-induced cellular cleanup, and without it the normally beneficial fast rapidly kills mice
Oxidation of a single amino acid in ATG5 turns out to control fasting-induced cellular cleanup, and without it the normally beneficial fast rapidly kills mice
Biologists at the Dana-Farber Cancer Institute and Harvard mapped protein oxidation across mouse tissues during dietary restriction and identified a single switch without which fasting fails to activate recycling of cellular waste. When mice carrying a disabled version of this switch were subjected to fasting, instead of benefiting they suffered severe tissue damage, and some did not survive the experiment.
Dietary restriction, a reduction in calorie intake short of malnutrition, remains one of the most reliable ways to extend lifespan in yeast, worms, flies, mice, and primates. Part of this benefit depends on autophagy, a process in which the cell packages worn-out proteins and damaged mitochondria into vesicles and digests them for reuse. But which molecular sensor translates the signal "food supply has dropped" into the command "activate cleanup" had remained unknown.
In a preprint dated September 24, the team of Edward Chouchani (who in 2020 had built a similar atlas of protein oxidation during aging) and William Mair mapped protein oxidation across six tissues in young and old mice under normal feeding and under caloric restriction, and found that fasting acts with precision, altering only a few percent of protein sites. Among them was amino acid number 19 in ATG5, one of the key components of the cellular cleanup machinery. The authors narrowed the list of altered sites to those on genes with a proven role in lifespan that are conserved from humans to Drosophila. Only three sites passed this filter, and one of them fell on ATG5, a gene whose overexpression was linked in 2013 to a 17% lifespan extension in mice, while its knockout causes death in newborn pups.
Oxidation of this amino acid (a cysteine) causes it to form a transient bond with another protein, ATG10. This was confirmed both by structural modeling (the sulfur atoms come within 2.6 angstroms of each other) and by experiment. Without this bond, ATG10 cannot conjugate the partner protein ATG12 onto ATG5, and without the completed ATG5-ATG12 complex the final step does not proceed: attachment of LC3B to the membrane of the forming recycling vesicle. Mutant cells in which the cysteine was replaced with a non-reactive amino acid assembled half as many of these vesicles, and the unprotected ATG5 itself was rapidly degraded by the proteasome, the cell's machinery for disposing of unneeded proteins.
Mice carrying the disabled switch were born and lived normally on a regular diet. But during a standard two-day fast, the kind routinely used in the laboratory to elicit the beneficial effects of dietary restriction, these mice lost weight rapidly and weakened; some had to be removed from the experiment on humane grounds. Even 24 hours after refeeding, their livers remained smaller than normal and showed no recovery, while elevated markers of DNA damage, cell death, and cellular debris pointed to accelerated tissue aging. The same liver senescence in aged mice is triggered by actin-mediated blockade of autophagy through the protein ASAP3.
One of the best-validated interventions against aging turns out to have a precise safeguard: without oxidation of a single amino acid, fasting shifts from a beneficial stress to uncontrolled tissue damage.