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Single
Gene Alteration Points To Longevity, Thinness
When
the function of this single gene is knocked down, the animal can extend
its lifespan."
July 2, 2004 - Imagine
that by altering the function of a single gene, you could live longer,
be thinner and have lower cholesterol and fat levels in your blood.
Medical College of Georgia researchers are using a tiny worm called C.
elegans to transform that vision into reality.
Researchers You-Jun Fei
and Vadivel Ganapathy have found the Indy gene is critical in providing
cells with energy, producing a transporter that helps deliver key
ingredients of the fuel that drives cells. Indy delivers metabolic
substrates such as citrate and succinate to cells where they enter the
powerhouse called the mitochondria. Inside the powerhouse, oxygen also
is critical to the biochemical reaction that occurs to produce ATP, the
fuel for cells, says Dr. Fei, molecular biologist.
An unfortunate byproduct
of this oxygen metabolism is reactive oxygen species, a sort of cellular
trash that ages cells and may contribute to diseases from Parkinson's to
Alzheimer's. "This is why people think we age; these byproducts of
oxygen metabolism cause cells to degenerate," says Dr. Ganapathy,
biochemist who becomes chair of the MCG Department of Biochemistry and
Molecular Biology July 1.
That also is why
decreased activity of the Indy transporter seems to make animal models
at least live longer, healthier lives.
The MCG researchers have
identified this longevity gene in humans, mice, rats and zebrafish as
well as C. elegans.
Armed with a new
$605,000, three-year grant from the National Institutes of Health's
Institute on Aging, the researchers want to know the activity level that
optimizes longevity and find compounds to control that level.
"The human lifespan is a
phenotype determined by multiple genes," says Dr. Fei, principal
investigator on the grant. "Our Indy gene is only one of the
life-determinant genes. But I can say that when the function of this
single gene is knocked down, the animal can extend its lifespan."
University of
Connecticut researchers were the first to recognize the relationship
between Indy short for 'I'm not dead yet' and longevity when they
found spontaneous mutations of the gene in the adult fruit fly that
nearly doubled its lifespan. Their research, published in the journal
Science in December 2000, says the mutations may create a metabolic
state that mimics caloric restriction, which has been shown to extend
lifespan. They were uncertain of the gene's function, but suspected it
was a transporter.
"When you look at the
protein coded by this gene you can guess what the gene does because
transporters have certain structural features and the protein made by
this gene has the same kind of structural features of the transport
system," Dr. Ganapathy says. The structure looked a lot like two
dicarboxylate transporters Drs. Fei and Ganapathy had been studying for
years. So they cloned the Indy gene from the fruit fly but found it
didn't quite match either transporter. "We knew there had to be
something else," says Dr. Ganapathy.
That something else
turned out to be a third transporter of dicarboxylates and
tricarboxylates, which include citrate, succinate and other components
of the citric acid cycle, the primary pathway for energy production in
cells. "Now there are three transporters with a similar function. How do
we prove that the third one is actually Indy? We need an animal model
that enables us to study the effect on lifespan," he says.
So the researchers
wouldn't grow too old trying to clarify that this was indeed Indy, they
chose C-elegans as their animal model, a worm that goes from embryo to
adult in about three days and has a maximal lifespan of about four
weeks.
Dr. Fei cloned all three
of the acid transporters in the C. elegans, knocked down the activity of
each and found that the newest transporter Indy increased the lifespan
of the worm and decreased body size and fat content without apparent ill
effects. They published their initial cloning work in the Journal of
Biochemistry in 2003 and the work on the biological function of Indy in
the Biochemical Journal this year.
They were able to mimic
the spontaneous genetic mutation Connecticut researchers found in the
fruit fly by feeding C. elegans specially engineered bacteria that knock
down the activity of Indy. Their model netted a 15-20 percent increase
in lifespan in addition to the other benefits. Unlike true genetic
knockouts, with scientists completely removing both copies of a gene so
100 percent of function is gone or taking out one copy so the gene
functions at half capacity, the MCG scientists cannot determine the
exact gene activity level in their animal model. "These worms reflect
what happens with reduced activity in the transporter," Dr. Ganapathy
says. "But we don't yet have a stable mutant line. That is one of the
aims for the NIH grant."
Oddly, the maximum
benefit, at least in the fruit fly, doesn't come from zero activity.
Rather flies live the longest with about half the normal gene activity.
Dr. Fei wants to find the optimal degree of activity. He and his
co-investigator, Dr. Ganapathy, already are working on a knockout mouse
that has half the normal Indy activity so they can look at the impact on
longevity in mice that usually live two years instead of a few weeks.
To confirm that the gene
functions similarly in worms and humans, they also plan to take the Indy
gene out of the C. elegans and replace it with the human gene to see if
that reverses the effect. "We call it humanizing the worm," Dr.
Ganapathy says.
He noted an interesting
difference between worm and human genes is that the human Indy gene is
more adept at transporting tricarboxylates or citrates, a primary
precursor for fat and cholesterol. "If you find a drug which can block
the function of this transporter, it might interfere with the use of
citrate for fat and cholesterol synthesis which should help people lose
weight and reduce their cholesterol," Dr. Ganapathy says.
Drs. Fei and Ganapathy
also are working to identify compounds that can control gene activity.
They may have to look no further than store shelves to find a good
starting point: hydroxycitrate, an analogue of citrate found in the skin
of the Indian fruit garcinia, already is being touted for its
weight-loss and cholesterol-reducing properties. "We think the mechanism
for how this compound works is at least partly by manipulating this
transport system," Dr. Ganapathy says, adding that studies of
hydroxycitrate might point toward more specific, potent compounds.
The potential benefit
derived from manipulating the activity of Indy has prompted the MCG
Office of Technology Transfer and Economic Development to seek national
and international patents on the transporter technology.
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