The mammalian target of rapamycin (mTOR) inhibitor rapamycin extended the lifespan of female but not male flies, through sex-specific effects on the enterocytes that line the gut.

Researchers at the Max Planck Institute for Biology of Ageing have reported those results in a preprint published in bioRxiv, and at the second Euro-Geroscience conference held in March in Toulouse.

Rapamycin has a storied history, and from the looks of it, its descendants – collectively, the rapalogs – have a bright future as well.

Rapamycin is approved as an immune suppressant to prevent transplant rejection. But its target, mTOR, sits at the intersection of nutrient sensing and cellular growth decisions, which gives it a central role in the life of cells.

This central role is the reason that, while it is approved at one dose as an immune suppressant, at low doses it improves the immune response of elderly people, as measured by their response to the flu vaccine. And in oncology, Afinitor (everolimus; Novartis) appears to work by altering glycolysis and, consequently, T-cell proliferation.

The paper, however, focuses on yet another claim to fame for rapamycin – the fact that it is "currently the only pharmacological intervention that extends lifespan in all major model organisms," the authors wrote.

Francesca Macchiarini is a program director and the chief of the Biological Resources Branch, Division of Aging Biology at the National Institute on Aging. She serves as the NIH scientific officer for NIA's Intervention Testing Program (ITP), a program that rigorously vets and tests treatments that have the potential to extend lifespan and healthspan.

"Rapamycin was discovered very, very early in the life of the ITP, and it is the poster child of what the ITP is for," Macchiarini told BioWorld Science. ITP researchers first reported in 2009 that rapamycin extended the lifespan and healthspan of mice, even though treatment did not begin until the animals were already well into middle age.

Sex matters

Unlike most research programs at the time, the ITP has been using male and female mice in its studies since its inception in 2000. Rapamycin extended lifespan in both male and female mice, but the effect was stronger in females.

Those results fit into a larger picture, which is that aging is surprisingly dichotomous. Genetic studies suggest that in fruit flies and mice, the gene sets that affect male and female longevity are mostly distinct.

And several of the successful interventions the ITP has identified – for example, aspirin – increase longevity only in one sex in mice.

In their new study, the team first demonstrated that in flies, rapamycin extended lifespan in females but not in males. The team tested three different concentrations of rapamycin added to the food; females lived longer than controls at all dose levels, and males, at none of them.

Testing the molecular mechanism of the effects, the team first showed that inhibition of mTORC1 complex did not appear to be responsible for the results. Because caloric restriction had improved the gut physiology of female flies in previous work, the team then tested whether changes in the physiology of the gut were responsible for the effects of rapamycin.

The team first showed that enterocytes, the epithelial cells that line gut tissue, differed in females and males, and were differently affected by rapamycin as well.

Enterocytes in female flies were larger, and had lower levels of autophagy, one of the processes whose decline is a cellular hallmark of aging. Rapamycin treatment reduced cell size and increased autophagy levels in the enterocytes of females. Due to a floor effect in size and a ceiling effect in autophagy, neither effect occurred in male enterocytes.

The researchers next tested whether knocking out the autophagy gene Atg5 affected rapamycin's abilities. "That is one of the advantages of using flies, because we can manipulate gene expression in different tissues easily," co-author Yu-Xuan Lu, a postdoctoral fellow at the Max Planck Institute for Biology of Ageing, told BioWorld Science.

In those experiments, he said, the team found that "rapamycin can change... chromatin only in the fly gut, not in other tissues."

The team plans to "see if we can develop a newer version of rapamycin... a tissue-specific targeted treatment," Lu said.

Such tissue specificity in one sense fails to take advantage of rapamycin's greatest strength – that it appears to have general antiaging properties. But as Lu noted, aging remains a thorny issue to tackle, due in part to the lack of a regulatory pathway for anti-aging drugs by either the FDA or the EMA.

At the recent Euro-Geroscience conference, Lu said, "we had a lot of debate on how we can convince FDA and EMA," neither of which currently view aging as an indication for therapeutics development.

Even if aging were accepted as an indication, though, it would not be a useful endpoint measurement compared with functional changes in specific tissues.

To see effects on aging overall, "whatever you do, you probably need to wait 10 years," Lu said. "Especially in industry, I don't think any company wants to invest the time or the money."