Gene alteration points to longevity, thinness
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.
Toni Baker | EurekAlert!