杏吧原创

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鈥檛 fit. When he pretends to die in a swordfight, he has to fall down
gently so he doesn鈥檛 bruise. 鈥淚 don鈥檛 have as much padding as other people,鈥�
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鈥攈alf 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鈥檚 team has discovered tantalising clues that connect the gene to
calorie intake.

鈥淭he link to energy and metabolism is extremely intriguing,鈥� 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鈥檚 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鈥攐ne extraordinary rhesus monkey, though not on a
restricted diet, is still alive and kicking at 40. But it鈥檚 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鈥攁nd, it has to be said, far from ideal鈥攈uman
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 鈥淭he bubble people鈥�). When Biosphere鈥檚 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. 鈥淭here are so many changes in the body induced by
caloric restriction. Any constellation of them could be involved,鈥� 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. 鈥淧robably there鈥檚 some few changes underlying these multiple
changes,鈥� 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 鈥渟ilences鈥� other genes.

To picture how gene silencing works, you have to know a little about how DNA
is stored in a cell鈥檚 nucleus. The long strands of DNA that make up our
chromosomes don鈥檛 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鈥檛 get
turned on, possibly because the proteins that control gene activity can鈥檛 get
close enough to the DNA.

Exactly what SIR2 does to silence genes has been a mystery, though
scientists had some theories. 鈥淗istone tails鈥� 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鈥擱oy Frye at the University of
Pittsburgh and Danesh Moazed of Harvard Medical School鈥攕uggested 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鈥攁 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鈥檚 lab, mixed SIR2, NAD
and part of a histone tail in a test tube, to see if SIR2 adds ADP-ribose to the
tails. 鈥淪omething weird happened,鈥� 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鈥攍ighter by 42 atomic
masses, which is just the amount that would disappear if an acetyl group had
been ripped away. 鈥淟enny [Guarente] was with me in front of the mass
spectrometry machine,鈥� Imai says. 鈥淗e just shouted, `That might be
de-acetylation!'鈥� 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鈥攁dding ADP-ribose groups to molecules and removing acetyl groups.
What鈥檚 more, studies of yeast with mutations that knock out SIR2鈥檚 ability to do
one task or the other suggest that it鈥檚 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.
鈥淐learly, there鈥檚 some link between energy, SIR2 and ageing,鈥� he says.

All鈥檚 quiet . . .

Guarente鈥檚 hypothesis鈥攁nd he admits it鈥檚 still speculative鈥攇oes
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鈥檚 even a
smattering of evidence to support Guarente鈥檚 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.

鈥淭he model is very plausible, absolutely,鈥� says Kenyon. 鈥淲hat Lenny鈥檚 done is
raise a very, very nice possibility. You could imagine silencing having a
specific effect on genes related to ageing.鈥� In other words, SIR2 might
be responsible for some of the changes in gene activity with age that
Weindruch鈥檚 survey revealed.

Still, Guarente, Kenyon and others also point out there鈥檚 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鈥檚 age by how many times it can divide and produce daughter cells鈥�20
times is the average for normal yeast, whereas yeast with a double dose of
SIR2 genes can divide more than 30 times.

What鈥檚 more, gene silencing clearly plays a role in ageing in these
creatures鈥攁s 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鈥檚 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鈥檚 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. 鈥淭hat study is a few months away from being submitted,鈥�
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. 鈥淔eeling a bit cold or a little hungry some
of the time seems a small price to pay,鈥� says Harris, 鈥渇or having a future.鈥�

Did the eight men and women who in 1993 spent 24 months in Biosphere 2,
designed as a miniature version of Biosphere 1鈥攖he Earth鈥攅nter 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鈥� blood haemoglobin became even
less efficient than usual鈥攎uch like that of hibernating animals.

Walford suggests that the key difference was that he and his colleagues were
also severely calorie deprived鈥攁gain, 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.