For the first time, scientists experimenting on lab mice have actually reversed some effects of aging. After receiving a genetic tweak, the rodents went from the equivalent of enfeebled, near-death 80-year-olds to "the physiological equivalent of young adults," says Ronald DePinho at Boston's Dana-Farber Cancer Institute, the co-author of the study, published in the journal Nature. What happened with these mice, and when might humans reap the benefits?

How did researchers rejuvenate the mice?
DePinho and his team genetically engineered the mice to shrink protective DNA caps (known as telomeres) that keep chromosomes from fraying, like plastic caps on shoelaces. This made the mice age prematurely and left them with atrophied spleens, small testes, a poor sense of smell, and shrunken brains. After the researchers used an estrogen-based drug to switch on the telomerase enzyme that generate telomeres, those physical ailments quickly reversed and the animals lived to a normal age.

Why did it work?
Telomeres naturally erode with age, leaving chromosomes exposed and their working cells inactive, and this contributes to greying hair, organ decline, the death of brain cells, and other signs of aging. The researchers wanted to see if keeping telomeres intact would stop the decline, and their success with mice justifies "exploration of telomere rejuvenation strategies for age-associated diseases," their paper says.

Will this work on humans?
"We're not in the business of genetically engineering humans," DePinho says, but his team is hoping their research leads to drugs that will help prevent aging or reverse some of its effects, possibly including Alzheimer's disease. There are obstacles, though. Telomerase is linked to cancer (though none of the mice developed tumors), nobody has tried it on mice that haven't been made prematurely old, and as Jennifer Welsh notes in Discover, while "mice are often used to develop and test pharmaceuticals, many of those are revealed to be ineffective or unsafe in human trials."

So are we any closer to living forever?
"Anyone hoping for the fountain of youth will have to hang around a little longer," says Welsh, while researchers figure out, among other things, how big a role telomerase plays in human aging. "That there is as yet no drug for immortality," adds Tom Junod in Esquire, "does not mean that immortality is theoretically or even technically impossible," but it is unlikely. Despite our faith in science, and scientists' belief that we can cure aging, "we are not going to have our life spans scientifically amplified to biblical lengths."

Sources: Wall Street Journal, CNN, Discover, USA Today, Esquire