Sci. Aging Knowl. Environ., 14 May 2003
Hungering for Simplicity
Calorie restriction activates stress-based longevity pathway
The revitalizing powers of calorie restriction (CR) have made headlines in the popular press and captured the interest of biogerontologists, yet the molecular events that connect food deprivation with long life remain mysterious. New work on yeast indicates that this regimen goads a protein without previously known antiaging talents to activate a longevity pathway. The protein, Pnc1p, acts through an enzyme called Sir2p that slows aging in several organisms, and the new results suggest an unexpected means of controlling Sir2p. These observations support the notion that CR rejuvenates yeast by inducing stress and bolster the disputed idea that diverse stimuli funnel into a single system that stalls aging.
"It's very exciting when you've got a gene that's up-regulated by all sorts of activities that extend life span and when manipulation of that gene by itself controls life span," says Gordon Lithgow, a molecular gerontologist at the Buck Institute for Age Research in Novato, California. "Plus, the current paper challenges a high-profile idea about the mechanism of a CR-like phenomenon in yeast. Previous research suggested that one particular metabolite was what was controlling life span through Sir2, and this publication highlights the importance of a different one."
To model CR in Saccharomyces cerevisiae, researchers drop the growth medium's glucose concentration. The treatment boosts the number of times a mother can bud--a common measure of aging in yeast--and this pathway to long life requires Sir2p (see "High-Octane Endurance--Yeast in the Metabolic Fast Lane Live Longer"). The enzyme removes acetyl groups from the histone spools around which DNA winds in cells; as a result, the DNA packs more tightly, which promotes a so-called silencing process that prevents DNA from reshuffling. The DNA circles that otherwise arise from such rearrangements cause yeast to age. Three years ago, researchers discovered that Sir2p is an unusual deacetylase: It requires the molecule NAD+, famous for its role in turning food into cellular energy.
Although Sir2p plays an essential role in the glucose-restriction longevity pathway, quantities of the protein don't change when nourishment is scarce. Presumably, researchers reasoned, this condition boosts the enzyme's activity rather than its production. Sir2p's predilection for NAD+ suggested that extra dollops of this molecule prod Sir2p to work overtime. However, when David Sinclair, a molecular geneticist at Harvard Medical School in Boston, and colleagues added NAD+ precursors to yeast, they got a surprise. Nicotinamide--a molecule that cells transform into NAD+ through a series of chemical reactions--abolished silencing rather than activating it.
Nicotinamide is also a product of the Sir2p-catalyzed reaction, but the small molecule did not foil Sir2p just by pushing the reaction backward, Sinclair says. It inhibited silencing at miniscule concentrations, and the kinetics of the reaction revealed that "there was more than just simple competition for binding to Sir2 between NAD+ [whose chemical structure contains nicotinamide] and nicotinamide." The reaction had apparently "evolved to be very sensitive to nicotinamide levels--more than it would have by normal product inhibition," says Sinclair.
These results raised the possibility that nicotinamide itself governs life span. To test this idea, the researchers engineered yeast to overproduce an enzyme--Pnc1p--that breaks down nicotinamide. Draining nicotinamide stores dramatically slowed aging, but only in cells that contained intact Sir2p; the result indicates that Pnc1p operates through Sir2p. Furthermore, heating or starving cells relies on Pnc1p to prolong life, and its quantities rise under these conditions, as well as under others that increase life span, such as amino acid deprivation and high salt concentrations. Additional experiments suggest that Pnc1p enlivens Sir2p by consuming nicotinamide rather than by amplifying NAD+ concentrations. The observations "tie together multiple environmental [triggers] of life extension into a single genetic pathway." Lithgow agrees, but he points out that the researchers have not yet measured nicotinamide directly.
Sinclair's group has "uncovered a new life span regulator that responds to stress by activating Sir2," says Leonard Guarente, a molecular biologist at the Massachusetts Institute of Technology in Cambridge. "It's a very nice story because [the team] shows that [Pnc1p] responds to so many signals. ... Where we probably have different opinions is, what's the most important signal in glucose-deprived cells?" Unpublished results from Guarente's lab suggest that the ratio of NAD+ to the related molecule NADH, rather than nicotinamide quantities, is what alters Sir2 activity in CR, he says. Perhaps, he suggests, Pnc1p controls the response to stress, and the ratio of NAD+ to NADH controls the response to CR.
However, if CR kick-starts the same life span-extension pathway as do other stressors, the current work supports the notion that a single system retards aging. In this view, glucose deprivation is just another stress inducer, and CR plugs into a process called hormesis, in which moderate amounts of destructive agents protect an organism against more robust assaults, including those assumed to promote aging, says Edward Masoro, an emeritus professor at the University of Texas Health Science Center in San Antonio, who pioneered the study of CR (see "Stress for Success"). Masoro has suggested that hormesis underlies the benefits of CR in rodents. "This is an important piece of work because it supports the hormesis view of CR--at least in yeast," says Masoro. Aging in yeast is likely to be very different from aging in mammals, he says, citing the lack of evidence that DNA reshuffling sends mammals to their end. So perhaps a "general [antiaging] mechanism diverges someplace after CR induces hormesis." By serving up evidence that eating less sparks a stress-response pathway and by offering new ideas about Sir2p regulation, the work gives further food for thought to scientists who want to know how CR delivers its beneficial effects.
May 14, 2003
Evelyn Strauss is senior news editor of SAGE KE. She still prefers stress reduction and hot fudge sundaes.
Science of Aging Knowledge Environment. ISSN 1539-6150