Losing two mitochondrial sirtuins extends worm lifespan by 25%, but only on the right diet

Sirtuins are NAD+-dependent enzymes that regulate metabolism, and the mitochondrial family members (SIRT3, SIRT4 and SIRT5 in mammals) are being studied as targets for cancer, diabetes and neurodegeneration. Their physiological roles in aging are still poorly defined. This Brief Investigation in Genetics asks what the two C. elegans mitochondrial sirtuins, sir-2.2 and sir-2.3, do in lifespan regulation.

The team used genetic alleles with deletions that destroy catalytic activity in either gene, then measured lifespan under several dietary and stress conditions. Controls were fed the normal lab diet of live E. coli OP50. They also compared the alternative E. coli strain HT115 (commonly used for RNAi), food bacteria unable to grow, and a virulent pathogenic strain (Pseudomonas, per the keywords). They also assessed pharyngeal pumping, a proxy for food intake, and the animals' responses to oxidative stress.

Both sir-2.2 and sir-2.3 mutants lived on average about 25% longer than controls on live OP50. Reduced food intake is a well-known route to longer life, but the authors found no evidence of reduced pharyngeal pumping, so simple dietary restriction does not seem to explain the result.

The lifespan benefit depended on the diet. On HT115, the extension in sir-2.2 mutants was eliminated and the extension in sir-2.3 mutants was attenuated. sir-2.3 deletion extended lifespan in all conditions tested, suggesting it acts more robustly or independently of the food source. Removing the ability of the food bacteria to grow, however, eliminated the sir-2-mediated effect. This suggests the benefit depends on live, proliferating bacteria and so may involve host-microbe interactions or bacterial metabolism. The oxidative stress experiments suggest that a hormetic response, in which mild stress induces protective adaptation, contributes to lifespan extension in both mutants. Together, the data indicate that sir-2.2 and sir-2.3 regulate lifespan through overlapping yet distinct mechanisms.

The work shows that the effects of mitochondrial sirtuin loss depend on the environment, especially diet and bacterial food source. That matters for interpreting sirtuin biology and for any attempt to modulate these enzymes pharmacologically. The results are in a short-lived nematode and show that loss of sirtuin activity can be beneficial, which complicates the assumption that more sirtuin activity is always better for aging.

Caveats: this summary draws on the abstract and metadata, since the full methods and data were not available in the supplied text. The molecular mechanisms linking sirtuin loss, bacterial growth and hormesis remain to be defined, and the findings may not translate directly to mammals.