Anti-aging Pathway Questioned

In 1995, a potential drug target emerged. When Kennedy, MIT molecular biologist Leonard Guarente, and colleagues targeted a gene which silenced sirtuins, a class of proteins that plays a role in several metabolic pathways, they increased the number of times yeast cells divide by 30 percent. In 2001, Guarente and his colleagues showed that transgenic worms that over-express a specific sirtuin called sir2 lived longer, and others showed that dietary restriction increased life span in fruit flies by way of a sirtuin pathway.

Taken together, the studies created a compelling story: cutting calories activates a sirtuin pathway, which extends lifespan in many animal species. That raised hopes that scientists could one day create a human anti-aging pill by stimulating sirtuin production directly, bypassing the need to skimp on calories. A company called Sirtris Pharmaceuticals even formed to develop anti-aging compounds that could do just that.

Doubts emerge

While some labs saw dramatic life extension with sirtuins, other groups had trouble replicating those findings.  After three years without success, Gems and his colleagues began to suspect the genetic background was confounding their studies. In other words, the method for activating the sir2 gene in worms and fruit flies may have inadvertently introduced other genetic mutations that increasing longevity.

To weed out this genetic background, he and his colleagues created transgenic worms that overexpressed sir2 using the same method for gene insertion that Guarente originally used in his long-lived worms. The researchers mated the transgenic worms with wild-type animals and selected for offspring that still expressed high levels of sir2. Those worms were mated again with wild-type worms. After six generations, the resultant worms still made more sir2, but were genetically almost identical to wild type.

C. elegans

WIKIMEDIA COMMONS, NATIONAL HUMAN GENOME RESEARCH INSTITUTE

The outcrossed sir2 worms did not live longer than the wild-type controls, suggesting that some other gene must account for life extension in the original worms. Sure enough, a little more digging revealed another mutation in a neurodegeneration pathway that was responsible for the longevity effect. When they took a second look at flies, they found the same story: sirtuins didn’t seem to extend lifespan compared to genetically similar controls.

To see whether dietary restriction’s increased longevity depends on the sirtuin pathway, the group knocked out sir2 expression in flies and put them on a meager diet. The flies still lived longer, suggesting that sirtuins were not responsible.

Gems thinks the new findings should lay to rest the notion that sirtuins play a major role in extending lifespan across species. “Because of what a huge paradigm sirtuins created, it’s quite extraordinary that it’s sort of vanishing so suddenly,” he said. “It’s like a bubble popping.”

Continued Controversy

But not everyone in the anti-aging community is ready to abandon sirtuins. In a Brief Communications Arising published in the same issue of Nature, Guarente, who published the first studies showing dramatic life extension in worms, said they also later recognized the problem with their transgenic method.  However, when they corrected for it in further experiments, they found a smaller, but still significant, 10 to 14 percent increase in lifespan in worms.  The effect has since been confirmed by another lab in a study published this year in PLoS Genetics, he said.

“I do not agree with the notion that in worms, sirtuins do not regulate aging. I think that’s wrong,” Guarente said.

Stephen Helfand, a molecular biologist at Brown University who did some of the original work showing life extension in fruit flies, also took issue with the conclusions of the Gems paper. In 2009, his group confirmed that sirtuins extended life in fruit flies, using a careful method that eliminates the problem of genetic background, he said. He also faults the current paper for not replicating all the conditions used in his original experiments.

But the modest effect found in the more recent studies isn’t particularly exciting, Kennedy said. “In Lenny’s paper the effects are 10 to 14 percent,” but hundreds of genes in worms can extend lifespan by that much, he added.

Furthermore, the findings in flies and worms may not be central to the question of sirtuins and aging, Kennedy said. “Really what we care about is humans, and the best way to understand that is to have better data in mice.”

While the jury is still out on sirtuins and longevity in mammals, there’s no question that they extend health. “There’s a huge body of work that implicates sirtuins in suppression of many age-related disorders in mammals,” such as cardiac fibrosis, diabetes, obesity, neurodegeneration, and even age-related hearing loss, said David Lombard, a biogerontologist at the University of Michigan, Ann Arbor, who wrote an accompanying News and Views piece in Nature.

And while the debate over sirtuins’ role in aging rages on, biologists will continue to investigate other potential targets, such as the Target of Rapamycin (TOR) pathway. “It’s been shown that other pathways have very dramatic effects on aging which are not controversial and have not been challenged,” Gems said. “There’s lots of activity elsewhere in the field moving towards drug treatments for aging.”

C. Burnett et al., “Absence of effects of Sir2 overexpression on lifespan in C. elegans and Drosophila,” Nature, doi:10.1038/nature10296, 2011.