Boost in protein repair extends fly lives

John Travis

Here's news that may interest the many senior citizens in tropical Florida: In warmer-than-normal conditions, fruit flies that overproduce a protein-repair enzyme live about one-third longer than typical flies.

This finding lends support to the theory that the buildup of damaged proteins in cells limits lifespan. "It's been hypothesized for many years that one of the reasons we age is that important macromolecules [such as proteins] become damaged," explains Jonathan E. Visick of North Central College in Naperville, Ill.

Scientists have identified dozens of molecules involved in the repair of DNA, but they've made far less progress unraveling mechanisms by which cells fix damaged proteins. Indeed, many biologists have assumed that protein repair isn't important because cells can simply make replacements.

In 1997, however, Steven G. Clarke of the University of California, Los Angeles and his colleagues created mice lacking an enzyme that counters the natural degradation of two protein components—asparagine and aspartic acid (SN: 6/14/97, p. 365). At first, such mice seemed healthy, but they typically died from seizures after less than 2 months.

Clare M. O'Connor of Boston College in Chestnut Hill, Mass., and her colleagues have now given fruit flies extra copies of the gene for the repair enzyme, a so-called methyltransferase. In initial experiments, the researchers tied the activity of the extra genes to a heat-shock promoter, a piece of DNA that turns on genes only when the organism is stressed, such as when it's exposed to temperatures of 29°C (84°F) and higher.

When reared at 29°C, the flies made extra amounts of the methyltransferase and lived longer than similar flies grown at a lower temperature, O'Connor reported at the American Society of Cell Biology meeting in Washington, D.C., this week.

Using a different gene promoter, O'Connor's group also created flies that overproduce the repair enzyme at 25°C. Surprisingly, at that temperature, the insects didn't live longer than normal. When such flies grew at 29°C, however, they survived 32 to 39 percent longer than their cooler counterparts did.

"At 25°, the extra enzyme doesn't seem to make a difference, whereas at 29° it does," says Clarke.

O'Conner speculates that the methyltransferase becomes most beneficial to flies experiencing heat shock or other stresses, perhaps because proteins suffer extensive damage at such times. Another possibility is that the repair enzyme has a molecular partner that's only produced during stressful periods.

Visick notes that his research reinforces the fly findings. Bacteria lacking the same repair enzyme have shortened life spans under certain stresses, he says.

While biologists can extend the lives of several animal species by mutating genes, the only other example of prolonging life by overproducing a protein involves an enzyme that prevents damage to macromolecules, notes O'Connor.

Even though most tissues produce the methyltransferase studied by O'Connor, the enzyme seems particularly important in the brain. Clarke's team recently created mice that make the enzyme only in their nerve cells, or neurons. Unlike mice lacking the methyltransferase entirely, these new mice don't suffer fatal seizures, and many of them enjoy an almost normal life span.

"There's something about neurons that really depends upon this [repair] pathway," notes Dana Aswad of the University of California, Irvine.

His group has found that a crucial nerve cell protein called synapsin suffers the type of damage that the methyltransferase can fix. "We're going to find a specific neurological disease or even a major psychiatric illness related to a defect in this enzyme," predicts Aswad.

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References:

Chavous, D.A., F.R. Jackson, and C.M. O'Connor. 2001. Extension of the Drosophila lifespan by overexpression of a protein repair methyltransferase. Proceedings of the National Academy of Sciences 98(Dec. 18):14814–14818. Abstract available at http://dx.doi.org/10.1073/pnas.251446498.

Lowenson, J.D., … and S. Clarke. 2001. Limited accumulation of damaged proteins in L-isoaspartyl (D- aspartyl) O-methyltransferase-deficient mice. Journal of Biological Chemistry 276(June 8):20695–20702.

Further Readings:

Travis, J. 1997. Faulty protein repair spurs mouse seizures. Science News 151(June 14):365.

Sources:

Dana W. Aswad
University of California, Irvine
3205 Biological Sciences II
Irvine, CA 92697-3900

Steven G. Clarke
Department of Chemistry and Biology
Molecular Biology Institute
University of California, Los Angeles
611 Charles E. Young Drive East
Los Angeles, CA 90095-1570

From Science News, Volume 160, No. 24, December 15, 2001, p. 373.