Sci. Aging Knowl. Environ., 26 February 2003
Single-gene mutations can dramatically alter aging of laboratory animals. Scientists are beginning to address whether those genes are relevant to aging in more natural settings
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/sageke;2003/8/ns2
Abstract: Numerous genetic alterations can profoundly extend the life-span of model organisms. But some researchers question whether the life-stretching effect is just a peculiarity of the lab and whether the same mutations influence survival in a natural environment. Scientists are beginning to test how long-lived animals fare in natural settings and whether life-extending mutations have a fighting chance in nature. The results suggest that the advantage depends on environment and other genes, and they could help guide the design of therapies for humans.
Lab animals lead a cushy life: meals served daily by diligent servants, free workout equipment, and a climate-controlled, pathogen-free home. Meanwhile, their wild relatives must scour for food, contend with floods and droughts, and repel predators and diseases. Scientists have debated whether mutations that slow aging of animals in the lab also extend life in the wild, but data on the subject have been scarce. Researchers have recently started to address the issue experimentally, and those studies are beginning to bear fruit.
In the lab, investigators have pinpointed numerous genetic alterations that lengthen the life-span of model organisms (see Genes/Interventions Database, Genetically Altered Mice, and Hekimi Review). For instance, nematodes and fruit flies live twice as long as normal when they harbor mutations that short-circuit a protein-signaling cascade that resembles the mammalian insulin and insulin-like growth factor-1 (IGF-1) pathways (see "Growing Old Together" and Tatar Review). Fruit flies also live longer when they make unusually large amounts of a protein that disarms superoxide, a free radical generated during metabolism that can damage DNA, proteins, and membranes (see SOD2). Yeast that produce bonus SIR2--a molecule that helps control how tightly DNA coils in chromosomes--divide more times than normal yeast do before pooping out (see Kaeberlein Perspective).
The profound changes in life-span wrought by mutations in single genes have raised hopes that manipulating the same pathways could extend human life-span--or at least improve health in old age. But skeptics wonder whether the longevity effect is just a lab peculiarity and whether the same mutations influence survival in nature. For instance, the physiology of organisms changes after only a few generations of the laboratory good life. "The strains we use are really messed up," says evolutionary biologist Marc Tatar of Brown University in Providence, Rhode Island. "[The animals] reproduce fast and die young, so if we make them live longer, are we just restoring normal function?" Steven Austad, an evolutionary biologist at the University of Idaho in Moscow, agrees. "It's a real issue," he says. "Are we simply fixing up something that the wild animals have already fixed up?"
Lab Heroes, Wild Zeroes
Many long-lived lab animals wouldn't survive in the wild. Mice on a calorie-restricted diet--a regime that extends the life-span of several species (see "Dietary Drawbacks" and Masoro Review)--don't endure a drop in temperature as well as normal animals do, and "the biggest challenge to a [wild] mouse--aside from a hawk--is cold," says Austad. Other animals perish even in the lab when they aren't coddled. Long-lived Snell dwarf mice do fine when housed alone, but normal littermates "beat the hell out of the dwarfs" if they are raised together, Austad says. He views such lab models as "instructive but crippled genotypes" that inform researchers about mechanisms that underlie aging but don't represent anything found in nature. Long-lived mutants obtained in the lab aren't the result of natural selection, says James Carey, a biodemographer at the University of California (UC), Davis: "Longevity is a trait; it has to be brought into evolutionary balance with other traits." A single-gene mutant is like a mast that's too big for the ship, he says: "It's out of balance with the rest of the boat. Once you leave the safe harbor of the lab, you're doomed."
Evolutionary theory predicts that long-lived creatures have weaknesses. Life-extending mutations should cost the organism early in life; they might stunt growth or delay reproduction, for instance, and undermine the ability to compete in the wild. If such mutations didn't exact a cost, they would quickly become the predominant form of the gene--assuming that they'd give an organism more time for reproduction.
But the identification of so many mutations that extend life-span in the lab initially suggested to some molecular biologists that an organism can gain time without paying a price. This issue has become entangled in the debate over whether aging is genetically programmed, an idea at odds with evolutionary theory. "Coming into aging research, I found it very difficult to accept that the genes I was studying were doing things other than causing aging," says molecular biologist Gordon Lithgow of the Buck Institute in Novato, California. "If we see a mutation that extends life-span, [the normal gene] must be there to cause aging" is the molecular biologist's way of thinking, he says.
Once the subject of explosive arguments, the conflict has settled to a simmer. Most molecular biologists and geneticists now agree, at least to some extent, with evolutionary biologists: Although genes influence aging, the process is not genetically programmed in the same way that, say, embryonic development is.
The two camps, however, still clash over whether longevity can come for free. Some mutants seem to gain life-span without paying a price (see Longo and Finch Review). Mice with a half-dose of the protein that triggers the cellular response to IGF-1 live long and are fully fertile (see "One for All"). Longevity must be beneficial in some instances, says molecular geneticist Cynthia Kenyon of UC San Francisco: "[Mice, flies, and nematodes] all came from a common precursor with a shorter life-span, so there must have been many times that mutations arose that lengthen life-span, and the [resulting] animals didn't have a selective disadvantage."
It is true that long life can benefit an individual, says evolutionary biologist Daniel Promislow of the University of Georgia, Athens, but animals give up early, rapid reproduction to achieve added life-span. "Our ancestors were clearly selected to delay reproduction a long, long time," he says. "Otherwise, someone who came along and started reproducing at six would quickly take over the entire population." Lithgow embraces both the evolutionary and the molecular viewpoints. "Some people say longevity can be separated from everything else. That's right under certain environmental conditions," he says. "Evolutionary biologists say the long-lived mutants are really sick animals, and I say yes, they are--under certain environmental conditions. I think everybody's right."
Testing Methuselah's Might
Lithgow has merged those ideas in interpreting results obtained from his own studies on long-lived nematodes that carry mutations in the age-1 gene. The mutants seem to have it all: They live longer, resist stress better, and are as fertile as normal nematodes. Lithgow wondered whether the mutant animals could compete with regular ones. He and his research team investigated the question by playing a laboratory game of survival of the fittest. They reared normal and age-1 mutant worms on the same petri plate and monitored which animals persisted. To ensure that the two genetic lines remained distinct, the researchers used hermaphrodites--individuals with both male and female sex organs--which reproduce by self-fertilization when males are absent. After several generations, the two strains made up an equal percentage of the total population, suggesting that the life-extending mutation didn't hinder age-1 mutants' ability to compete.
But in nature, food isn't constantly available, as it was in the experiment--and under conditions that mimic those in the wild, the outcome was different. The researchers repeated the test, but this time they subjected the worms to cycles of feast and famine. This time, age-1 mutants didn't fare as well. After only a few generations, normal worms far outnumbered mutants, suggesting that the long-lived mutants were at a competitive disadvantage when food wasn't always abundant. Under all-you-can-eat conditions, "these animals hold their own," says Austad, "but in a more challenging environment, they're just hopeless."
Kenyon disputes the notion that long-lived mutants are always weak, however. "That one experiment is held up as an example of how long-lived mutants will always have a disadvantage," she says. "But we know that if you put the worms [at high temperature], wild-type worms die, while age-1 mutants live. It's not always the mutant that dies."
Into the Wild--Strains, That Is
Kenyon says she believes that under the right conditions, longevity-spurring mutations identified in the lab could persist in nature. "I'd like to go to the Central Valley [of California] in summer, when it's deadly hot, and look for Caenorhabditis elegans," she says. "I wonder if you might find ones that would be more thermotolerant that might have lower levels of [the worm insulin/IGF-1 receptor] daf-2." Other researchers are addressing a related question by looking for genetic variations that arise in natural populations of flies and influence life-span. Geneticist Trudy Mackay of North Carolina State University in Raleigh and her colleagues caught wild flies and then introduced their chromosomes into lab strains. Tatar and co-worker Michael Palmer have used these animals to look for naturally occurring genetic variations in known longevity genes. According to unpublished results, Tatar and Palmer found changes in the fly insulin receptor gene that are associated with long life. However, the increase in life-span due to the natural variants isn't as large as that conferred by some insulin-receptor gene mutations that have arisen in lab strains, says Tatar, and it varies from strain to strain. "Yes, we can find natural [genetic variations] that affect aging," he says, "but how much they contribute depends on interactions with other genes."
Other researchers are also finding that genetic background influences how much time a mutation adds to life. Promislow and co-worker Christine Spencer have captured wild flies, manipulated them so that they carry mutations that confer longevity in lab flies, and measured their life-span in the lab. Their unpublished results reveal that the mutations extend longevity--but only in some strains.
Even in the lab, studies of genetically diverse populations that originated in the wild will reveal the intricacies of how genes influence life-span. "Scientists have been focusing on individual genes," says Promislow. "The next step is to start thinking about how genes interact with each other and with the environment to determine life-span."
Although longevity mutations discovered in the lab might be relevant in nature, lab studies might miss important genes that influence life-span in the wild. Mackay's group has looked for genetic variations in wild chromosomes that appear more frequently in animals that live longer than average. The researchers have found that certain variations in a gene that encodes dopa decarboxylase--an enzyme necessary for making the neurotransmitters dopamine and serotonin--correlate with extra life-span. How changes in such a gene could alter longevity remains unclear, Mackay says. But the result is exciting because no one had previously suggested that dopa decarboxylase was involved in aging and because some age-related neurodegenerative diseases, such as Parkinson's, involve the death of dopamine- and serotonin-producing neurons. Although abundant neurotransmitters are beneficial early in life, they might harm an organism later by overloading neurons with noxious oxygen radicals or by altering the brain's control of metabolism, she speculates.
Mackay cautions that her approach has limitations. Although the researchers are studying naturally occurring genetic variations, they are measuring their effects on lab flies. The question, says Mackay, is "How do you carry this outside into nature?" Ideally, such a pursuit would involve capturing flies, identifying genetic variations, and--crucially--assessing the animals' ages. "I'm not sure you can do that," she says.
But Carey thinks he has come up with a way. "When you pluck a fly out of the air, you have no idea what age it is," he says. By capturing a population of wild flies and then monitoring the insects in the lab for the remainder of their lives, it is possible to statistically reconstruct the mortality curves for that natural population, he says. He is currently seeking funding for such a project, which he hopes will reveal more about how life-span evolves in the wild.
Are Humans Wild Beasts or Lab Rats?
Such endeavors will likely uncover how aging in the lab relates to aging in the wild. Knowledge gained from studying animals in both environments might inform researchers about how to intervene in human aging--particularly because people don't fit neatly into either category. We're not wild in the sense that a field mouse is wild. Saber-toothed tigers no longer menace us, and we've conquered many of the infectious diseases that shortened human life-span even 100 years ago. Most humans get their meals at the local supermarket, rather than having to hunt and gather. Still, "we're not nearly as cosseted as lab animals, and we aren't fed optimum diets," says Austad.
Identifying the traits that animals relinquish for life-extension could be important for designing interventions that combat aging. "Should we be worried about these sorts of [trade-off] issues? Yes, absolutely," says Lithgow. "We should know the function of these genes under different environments so that we can say, 'Intervention in that pathway will have certain risks.' " But treatment for humans won't necessarily present such hazards, say others. Antiaging interventions might not be administered until late in life, long after side effects such as altered growth or impaired reproduction are of concern. For instance, blocking daf-2 in old worms extends life-span, according to Kenyon's work (see Sonntag and Ramsey Perspective). The gene "is acting to control aging in an animal where the reproductive die is already cast. ... I don't think there's any reason to think that there would be a problem [in targeting the pathway therapeutically]," she says.
Marrying conventional experiments with those that reach beyond laboratory walls poses an opportunity as well as a challenge for researchers who study aging. Ideally, findings made in the two settings will complement each other. Controlled situations will continue to allow scientists to uncover what single genes do and how they fit into physiological networks. And as researchers improve their methods for studying animals in the wild, information from laboratory studies will provide a handle for tackling aging in less restricted conditions and in more genetically mixed populations than those that typically inhabit cages and petri dishes. Together, these approaches will likely point toward ways to tame human aging.
February 26, 2003
R. John Davenport is an associate editor of SAGE KE and a former lab rat.
Science of Aging Knowledge Environment. ISSN 1539-6150