Sirtuins are Not Conserved Longevity Genes

Abstract

It is central to biology that sequence conservation suggests functional conservation. Animal longevity is an emergent property of selected traits that integrates capacities to perform physical and mental functions after reproductive maturity. Though the yeast SIR2 gene was nominated as a longevity gene based on extended replicative longevity of old mother cells, this is not a selected trait: SIR2 is selected against in chronological aging and the direct targets of SIR2 in replicative lifespan are not conserved. Though it would be difficult to imagine how a gene that advantages 1 in 5 million yeast cells could have anticipated causes of aging in animals, overexpression of SIR2 homologs was tested in invertebrates for longevity. Because artifactual positive results were reported years before they were sorted out and because it was not known that SIR2 functions as a pro-aging gene in yeast chronological aging and in flies subject to amino acid deprivation, a global pursuit of longevity phenotypes was driven by a mixture of framing bias, confirmation bias and hype. Review articles that propagate these biases are so rampant that few investigators have considered how weak the case ever was for sirtuins as longevity genes. Acknowledging that a few positive associations between sirtuins and longevity have been identified after thousands of person-years and billions of dollars of effort, we review the data and suggest rejection of the notions that sirtuins 1) have any specific connection to lifespan in animals and 2) are primary mediators of the beneficial effects of NAD repletion.

Figure 1

Sir2 enzymes are gene silencers in fungi and are conserved as NAD⁺-dependent protein lysine deacylases. (a) In Saccharomyces cerevisiae, the SIR2 gene is required to silence genes in particular chromosomal locations. This function is to be conserved across divergent yeasts. (b) The biochemical function of sirtuins is largely conserved. These enzymes deacylate protein lysine substrates in a manner that depends on NAD⁺, which produces the deacylated protein lysine substrate, nicotinamide, and acylated ADPribose.

Figure 2

Sir2 favors a nonselected and disfavors a selected type of aging in budding yeast, both of which are extended by calorie restriction. (a) In replicative aging, cells are arrayed on a petri dish for a 2-week experiment in which daughter cells are removed every time the mother cell has replicated. The longevity benefit of CR—and the effect of SIR2—emerges after about 21.2 cell divisions [16]. (b) In a yeast culture mothers who have divided 21.2 times constitute 1 in 4.8 million cells. Because a yeast culture can be regenerated from any cell, this is not a selected trait. (c) Yeast have been recultured from the bottom of flasks, bottles, and pottery for millenia: the ability to regrow a culture over an extended period of time after the original culture exhausted resources is a selected trait termed chronological lifespan. CR extends chronological lifespan in yeast and does so better with sir2 deletion [15].

Figure 3

The fluorophore in peptide substrates used to screen for sirtuin activators interacts with resveratrol and other proposed activators. Sir2 was screened for activators using an acetylated peptide-methylcoumarinamide substrate in which the deacetylated product was incubated with trypsin to release aminomethylcoumarin, which is fluorigenic [58]. Resveratrol and other activators interact with the reporter group [64–66].