How To Defy Death - Stay Thin
By Nell Boyce

Opera singers are supposed to be fat. But James Gifford, a 25-year-old opera student at Simon Fraser University in Burnaby, British Columbia, is so thin his costumes don't fit. When he pretends to die in a swordfight, he has to fall down gently so he doesn't bruise. "I don't have as much padding as other people," says Gifford, who stands 1·8 metres (six feet) tall yet weighs under 63 kilograms (138 pounds).
Gifford risks injury during fake deaths in an attempt to postpone his real demise. Six years ago, he saw a television programme about how slashing the calorie intake of animals dramatically extends the lifespan of every creature tested, from water fleas to worms to rodents. Mice that normally lived for 40 months at most survived for up to 56 months when they were fed 60 per cent as much as usual.
Nowadays, Gifford subsists on a mere 1500 kilocalories (6280 kilojoules) a day--half what people his size usually eat. If such a diet has the same effect on humans as it does in rodents, Gifford could end up living for 150 years or more, making Jeanne Louise Calment, the longest-lived person on record when she died aged 122 in 1997, seem a spring chicken in comparison.
Ever since the life extending power of calorie restriction was discovered in rats in 1935, researchers have been trying to work out exactly how it works. Now Leonard Guarente and his colleagues at the Massachusetts Institute of Technology in Boston believe that they may have uncovered the key in the form of a gene called SIR2. This gene has the power to make yeast cells live longer, and Guarente's team has discovered tantalising clues that connect the gene to calorie intake.
"The link to energy and metabolism is extremely intriguing," says Guarente, who last month reported his results in Nature (vol 403, p 795). If SIR2 does turn out to be the missing link, it will be the perfect target for anyone hoping to develop a drug that tricks the human body into living longer without having to skip lunch.
Still, while we know that SIR2 has equivalents in many other organisms, including people, and that restricting food intake extends the lives of animals, no one knows for sure that calorie restriction also works in people. Three studies in primates should help answer that question, says Barbara Hansen of the University of Maryland in Baltimore. Hansen's 29-year-old rhesus monkeys have already lived six years longer than their average life expectancy. It will take another 12 years to see if any of them live longer than their normal maximum lifespan--one extraordinary rhesus monkey, though not on a restricted diet, is still alive and kicking at 40. But it's looking good. The monkeys are in rude health and show metabolic changes similar to long-lived rodents, such as lower blood glucose, insulin levels, blood pressure, blood lipids like cholesterol and body temperature.
An accidental--and, it has to be said, far from ideal--human experiment in Biosphere 2, the sealed environment covering 1·2 hectares in the Arizona desert also looks hopeful for any one planning to live longer by counting calories (see "The bubble people", p 22). When Biosphere's inmates were forced onto calorie restricted diets their blood lipids, glucose and insulin reacted in the same way they do in rodents (Toxicological Sciences, vol 52, p 61).
The Biosphere 2 inhabitants had the advantage of being locked up in the desert, which is what it would take to keep most people from indulging. Eating is a sensual pleasure, and the majority would prefer a short satiated life to an eternity of deprivation. Once researchers have a better grasp of how calorie restriction works, however, it might just be possible for us to have our cake and live a long life, too. But working out the mechanisms is proving tough. "There are so many changes in the body induced by caloric restriction. Any constellation of them could be involved," says Hansen.
Leading theories include the idea that eating less slows the progressive damage caused by free radicals that are released when oxygen is used to break down fats and carbohydrates. Another theory is that calorie restriction does something critical to the insulin-signalling pathway that helps regulate how glucose is used by the body. Cynthia Kenyon of the University of California at San Francisco and her colleagues have found a gene called daf-2 in roundworms that, when mutated, can make the worms live two to three times as long as normal. Daf-2, it turns out, encodes the worm equivalent of the human insulin receptor. As animals on calorie-restricted diets are far less likely to get diabetes and related disorders, this suggests that genes involved in glucose metabolism might be linked to the genes involved in ageing.
Richard Weindruch at the University of Wisconsin has made the most comprehensive attempt to get at the genes that are altered by extreme dieting. He and his colleagues recently measured the activity of no fewer than 6347 genes in the muscle cells of 5-month-old and 30-month-old mice. The activity of 58 genes more than doubled in the elderly mice, while another 55 genes were only half as active (Science, vol 285, p 1392). Putting mice on a calorie-restricted diet from early adulthood suppressed the vast majority of those age- related changes in gene activity, suggesting that a draconian diet leads to a metabolic overhaul that mimics many of the characteristics of youth.
The sheer number of genes involved could discourage any drug company hoping to craft a pill or injection that produces the same effects as calorie restriction. But Roy Walford, a biologist at the University of California, Los Angeles, and the most famous advocate of calorie restriction, remains optimistic. "Probably there's some few changes underlying these multiple changes," he says. He envisions metabolism as a pyramid with a few key changes at the top flowing down to a larger base that transforms lifespan. Those high-level changes, Walford says, are likely to be conserved throughout evolution.
This is where Guarente and his long-lived yeast come in. Guarente believes he may have stumbled across one of these high-level changes while he was studying how SIR2 "silences" other genes.
To picture how gene silencing works, you have to know a little about how DNA is stored in a cell's nucleus. The long strands of DNA that make up our chromosomes don't just float around any old how: instead, they wrap round discs of proteins called histones, like cotton around a reel. These discs may be strung out loosely along the DNA strand or packed together very closely. And when chromatin, as wrapped-up DNA is known, is tightly packed, genes can't get turned on, possibly because the proteins that control gene activity can't get close enough to the DNA.
Exactly what SIR2 does to silence genes has been a mystery, though scientists had some theories. "Histone tails" are known to stick out of the discs around which DNA wraps. In loosely packed chromatin, large numbers of acetyl groups are stuck to these tails, whereas tightly packed chromatin is less acetylated. That suggests that removing acetyl groups somehow tightens up the chromatin, perhaps by allowing the positively charged tails to stick to the negatively charged DNA.
The de-acetylation theory of gene silencing was bolstered by the discovery in 1996 of enzymes that add acetyl groups to histones or remove them. But although many research teams suspected that the SIR2 protein might also be capable of removing acetyl groups, experiment after experiment failed to find the evidence.
Then just last year, two researchers--Roy Frye at the University of Pittsburgh and Danesh Moazed of Harvard Medical School--suggested that SIR2 might silence genes not by removing acetyl groups, but by sticking on a chemical group called ADP-ribose. In the body, ADP-ribose comes from the breakdown of NAD--a molecule that plays a crucial role in metabolism. NAD is an oxidising agent that traps energy-rich electrons obtained when glucose is broken down and helps convert the energy into a form that the cell can use.
So last year, Shin-ichiro Imai, a postdoc in Guarente's lab, mixed SIR2, NAD and part of a histone tail in a test tube, to see if SIR2 adds ADP-ribose to the tails. "Something weird happened," he recalls.
Instead of the histones getting heavier, as they would if weighed down by an extra ADP-ribose group, many of them got lighter--lighter by 42 atomic masses, which is just the amount that would disappear if an acetyl group had been ripped away. "Lenny [Guarente] was with me in front of the mass spectrometry machine," Imai says. "He just shouted, 'That might be de-acetylation!'" Rather than simply supplying the ADP-ribose groups, NAD was also catalysing the reaction by which SIR2 removes acetyl groups.
Imai says that he was extremely anxious about the results, as NAD had never been known to act as a catalyst before. But a series of further investigations, such as examining the histone to make absolutely sure that it had been de-acetylated, set his mind at rest. SIR2, it turns out, has at least two jobs--adding ADP-ribose groups to molecules and removing acetyl groups. What's more, studies of yeast with mutations that knock out SIR2's ability to do one task or the other suggest that it's the de-acetylation that is most important for silencing genes. But what excites Guarente is the possibility that SIR2 and NAD are the answer to the riddle of how semistarvation prolongs life. "Clearly, there's some link between energy, SIR2 and ageing," he says.
All's quiet . . .
Guarente's hypothesis--and he admits it's still speculative--goes like this: the amount of SIR2 protein affects life expectancy in yeast. SIR2 needs NAD to operate, and because NAD is used up as cells break down glucose, the amount of NAD should increase as food intake falls and metabolism slows. That, in turn, should help SIR2 silence genes more efficiently. And it may be by silencing genes that SIR2 extends the lifespan of yeast. There's even a smattering of evidence to support Guarente's hunch that gene silencing has a role to play in ageing: in female mammals, one of the two X chromosomes is silenced in most cells in the body, and sometimes parts of the spare X become reactivated in old mice.
"The model is very plausible, absolutely," says Kenyon. "What Lenny's done is raise a very, very nice possibility. You could imagine silencing having a specific effect on genes related to ageing." In other words, SIR2 might be responsible for some of the changes in gene activity with age that Weindruch's survey revealed.
Still, Guarente, Kenyon and others also point out there's reason to be cautious before embracing gene silencing as a mechanism for staying young. For starters, yeast and higher animals age in very different ways. Guarente gauges a yeast's age by how many times it can divide and produce daughter cells--20 times is the average for normal yeast, whereas yeast with a double dose of SIR2 genes can divide more than 30 times.
What's more, gene silencing clearly plays a role in ageing in these creatures--as a yeast ages, genes get pinched off from the chromosome, and form extraneous loops of DNA in the nucleus. These loops replicate exponentially as the cell divides, eventually killing the yeast. SIR2's gene silencing slows this process in yeast but it may not play such a vital role in mammals.
For Kenyon, however, the best thing about the theory is that it provides a new way of thinking about ageing that is imminently testable. Guarente's lab is already genetically engineering worms and mice so that they make too much or too little SIR2 protein. If an abundance of SIR2 extends the lifespan of these animals, as it does in yeast, Guarente will go on to measure insulin, body temperature and so on to get a sense of whether SIR2 produces the same kinds of changes as calorie restriction. An experiment to see whether yeast with defective SIR2 can respond in the same way as healthy yeast to a reduction in calorie intake has been finished, but Guarente is coy about revealing what he found. "That study is a few months away from being submitted," he says.
All these studies will go a long way towards revealing whether SIR2 is the missing link in the quest to understand the anti-ageing benefits of calorie restriction. In the meanwhile, people like Gifford and Phil Harris, one of the few people in Britain who has decided to give the semi-starvation diet a go, see no reason to wait for the results. "Feeling a bit cold or a little hungry some of the time seems a small price to pay," says Harris, "for having a future."
The bubble people
Did the eight men and women who in 1993 spent 24 months in Biosphere 2, designed as a miniature version of Biosphere 1--the Earth--enter a metabolic state similar to hibernation? Roy Walford, a University of California at Los Angeles researcher and one of the Biosphere volunteers, thinks so.
Biosphere 2, a sealed glass-and-steel structure set on 1·2 hectares in the Arizona desert, had problems maintaining an Earth-like atmosphere, so the volunteers ended up breathing air with less oxygen than normal. Usually, in oxygen-poor conditions such as high altitudes, haemoglobin becomes more efficient at carrying oxygen. But the biospherans' blood haemoglobin became even less efficient than usual--much like that of hibernating animals.
Walford suggests that the key difference was that he and his colleagues were also severely calorie deprived--again, much like a hibernating animal. The Biosphere 2 crops had failed, and Walford persuaded everyone to keep to the frugal diet that he has routinely followed for the past two decades in an effort to stave off death.
Further reading...
* "Gene expression profile of ageing and its retardation by caloric restriction" by Cheol-Koo Lee and others, Science, vol 285, p 1390 (1999)*
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