By David N. Leff

In the year 1513, a boatload of Spanish conquistadors landed on the west coast of Florida. The expedition's leader, 53-year-old Juan Ponce de Leon, had heard from Indians in Puerto Rico of a miraculous fountain of youth, which rejuvenated those who drank its waters.

Fast forward to modern times. Florida today bulges with millions of aging American settlers in search of rejuvenation from the state's smiling sunshine, seashores and lifestyle.

Nine years ago, molecular biologist Leonard Guarente began looking for an anti-aging factor in test tubes and electron microscopes, at the Massachusetts Institute of Technology in Cambridge. He has found in yeast cells, round worms and mice the first signs of a longevity gene - Sir2.

"The interesting thing about Sir2," Guarente observed, "is that it's a conserved gene. You and I have it in our genome. The way we got into this research nine years ago," he recalled, "we were trying to determine what caused the genome instability in the ribosomal DNA of yeast mother cells. And that led us two years ago to this Sir2, the protein expressed by Sir2, which regulates the structure of the chromatin in the rDNA.

"In fact," he pointed out, "Sir stands for 'Silent Information Regulator.' It causes silencing in the chromosome's chromatin structure, which represses recombination, and stabilizes that region. So that's how we got to Sir2, which was originally discovered because it silences information on the genome of yeast."

Guarente explained: "What we think happens is that Sir2 deacetylates the histones in the rDNA of chromatin - which is nucleic acid plus its supporting proteins. The DNA of chromosomes, and the histone proteins, form a particular structure that the DNA wraps around.

"The crux," he went on, "is the difference between active chromatin and silenced chromatin. It's the DNA plus the histone proteins that configure a particular structure in eukaryotic cells. And it turns out that these histones are marked with acetyl groups, which you can view as tags on active chromatin. To render the chromatin silent, Sir2 removes those tags."

Gene Silencing Works Two Ways

Silencing a chromosome's chromatin puts its gene into a state of suspended animation. The process is reversible - like corking and uncorking a bottle. "The way to think about it," Guarente offered, "is that these tags keep the chromatin pried open. And if you remove them, it closes shut. But they can be added back again by opening, so the silencing is totally reversible.

"The surprising thing we found about this reaction," Guarente recounted, "is that removal of the tags is driven by NAD - nicotinamide adenine dinucleotide. NADs are co-enzymes that help transfer electrons and hydrogen in some oxidation-reduction reactions. Meaning," he pointed out, "that this silencing by Sir2 is controlled by the metabolic rate of the cell.

"Sir2 probably connects three basic things," he suggested: "the structure of the chromatin, the metabolic rate in cells, and longevity. So what we think happens is that the amount of NAD that is available to SIR2 varies in proportion to the rate of metabolism. Our still-speculative idea is that under a slow metabolic rate - for example, when calories are restricted - there's more NAD available to Sir2. That renders chromatin silencing stronger, which promotes longevity."

Guarente is senior author of a paper in today's Nature, dated Feb. 17, 2000, which reports these findings and hypotheses. Its title: "Transcriptional silencing and longevity protein Sir2 is an NAD-dependent histone deacetylase."

"In the paper," he told BioWorld Today, "we show that a mouse version of Sir2 has this same biochemical activity. That makes the million-dollar question: Will its role in aging of yeast cells be conserved in higher life forms?

"We're testing that now in mice and worms," he continued, "the same way we did it in yeast, which was to put in an extra copy of the Sir2 gene - and cells lived longer. The question is, if we do that in animals, will they live longer?

"We began a few months ago," Guarente recounted, "but these are not real fast experiments. We'll know about the worms in a few months; a few years for the mice."

The co-authors, he related, "created transgenic mice with an extra copy of the same Sir2 gene. We found in yeast that the more Sir2 you have, the stronger the silencing and the greater the longevity. So if it works in animals, at least we'll know we're on the right track for aging - and we'll fill in the details."

Guarente noted, "Several Sir2 genes have been identified already in humans. We know the genes are there, we know the proteins they make, and we believe that they're going to have this activity, because we showed in vitro that mouse Sir2 can catalyze this reaction. The protein in humans is almost identical to the mouse one, so if it's true of the mouse, it's going to be true in humans. The acid test now is, if we vary the number of these genes in an animal, can we vary their life span?"

Small Molecules To Slow Debility Of Aging

Looking ahead - perhaps for the next nine years - Guarente sees his long-range goal as "trying to work out the underlying causes, and starting to work toward high-throughput screening to identify small molecules that can intervene in this process, and result in slowing down aging.

"Advising people to exercise more and eat less is definitely good advice," he said, "but our idea would also be to develop therapeutics, which would function, for example, if we know we have more Sir2, and that it promotes longevity. So if we could develop a drug that could make Sir2 more active, or keep it active longer, that might promote longer life.

"Not to make people live forever," he added, "but to mitigate all these diseases intimately associated with longevity, such as osteoporosis and similar debilitating disorders. And if you could slow down aging, I think you would compress morbidity, rather than prevent mortality. It's going to be hard to have a huge impact on life span, Guarente pointed out, "but I think it would be very possible to slow the age-related degenerative processes.