The effort to map the aging process led Laura Hoopes to one of Nature's humbler creatures.
The Yeast
of Aging
Half a millennium after Juan Ponce de León abandoned his search for the Fountain of Youth, Laura L. Mays Hoopes is helping to assemble a map that, gene by gene, may eventually point the way.
She did not originally set out to explore the mysteries of aging. As a young biochemist, she was working on transfer RNA modification when her lab director asked her to examine its role in the aging process. While researching the literature she became intrigued by the error-catastrophe theory, which held that as an organism grows older, it begins to make more and more mistakes in putting together its molecules, until ultimately too many are tainted, and the organism dies.
"I thought that was fascinating, and I thought transfer RNA could easily be a part of it. Having something go wrong with transfer RNA could result in its being unable to be as accurate as it should," says Hoopes. She spent about a decade studying this aspect of the larger concept of accumulative molecular errors that lead to catastrophe.
Unfortunately, the theory was not correct.
"We found that if anything, it was better, not worse, in old animals," Hoopes says of the process she studied. "I was only one of many people who couldn't substantiate aspects of this theory, but it was a theory that attracted a lot of people to the molecular biology of aging." By then, she was hooked by a question that has persisted as long as humans' ability to contemplate it: What causes us to grow old? In particular, she wondered, why do cellular defense mechanisms that work when we are young not work later in life? (In some organisms, it appears they do: Bristlecone pine and some species of fish exhibit what is called negligible senescence.)
Hoopes' research was sidetracked when she arrived at Pomona in 1993 as vice president and dean of the college. She had been chair of the biology department at Occidental College. In 1998, she decided to return to teaching and research. To help her "reconnect," she spent a sabbatical year at the California Institute of Technology. There, she switched her research from mice and other rodents to Saccharomyces cerevisiae--baker's and brewer's yeast--whose entire genome had been sequenced, making the genetic tools she would need readily accessible.
Moreover, a team at the Massachusetts Institute of Technology had recently discovered that a gene called SGS1 plays a crucial role in the life span of yeast. That gene's DNA code corresponds structurally to a human gene that when mutated results in Werner's syndrome, an early aging syndrome, or progeria.
"So it gave me some hope that there could be some genes you could find in yeast that might have some applicability to human aging," says Hoopes.
The error-catastrophe theory that helped attract her to the field has since yielded to a concept of "life maintenance processes," that suggests that organisms age at different rates because some are better at limiting or repairing the damage that seems to occur inevitably in older molecules.
Of paramount significance, says Hoopes, is that the process for putting together DNA be extremely accurate.
At Caltech, working in the lab of Judith L. Campbell, she began studying a protein, DNA2, that serves as a repair and replication helicase, meaning it is able to cause the unwinding of supercoiled DNA strands so that copying can occur and any genetic coding errors can be corrected. This protein was known to be important because its deletion results in the death of a yeast cell.
With SGS1, which has been studied in some detail, the MIT researchers had demonstrated that experimental mutations of the gene produced early symptoms of aging in yeast cells. One symptom was that the cells became sterile. Another was that the nucleolus--a region within the cell nucleus that acts as a sort of protein synthesis factory--became fragmented much sooner than normal in the cell's life span. A third symptom, and the apparent basis for the fragmentation, was an accumulation of ribosomal DNA that popped out of a chromosome during replication. This material, called extrachromosomal ribosomal DNA circles, or ERCs, may be a critical component of a cell's aging and death.
"Those are not in and of themselves dangerous to the cells," Hoopes says of the ERCs. "But if many, many of them accumulate, they could choke up the cellular processes, mop up proteins that are needed to control gene activity, and maybe even prevent DNA replication."
Hoopes and collaborator Martin Budd, a senior researcher at Caltech, began investigating the possible role in aging of DNA2, which belongs to a different "superfamily" of helicases than the SGS1 gene examined by the MIT scientists.
What they found is that it, too, is required for the normal life span of yeast. When DNA2 was experimentally mutated, the resulting cells had life spans only about one-third as long as the unaltered versions.
"And not only do they approach the end of their life span quickly, but they accelerate processes that take place in normal aging," just like the SGS1 helicase was shown to do, Hoopes says. The DNA2 strains became sterile early, had fragmented nucleoli, and accumulated ERCs.
Hoopes and Budd also found, though, that DNA2 appears to act through a different pathway than SGS1. When a double mutant was created, incorporating both SGS1 and DNA2, cell life spans were shorter than with either mutant helicase alone.
"It means that, although they're both affecting life span, they're not doing exactly the same thing,'' Hoopes says. The researchers found that DNA2 appears to belong to a grouping of genes involved in post-replication DNA repair, which Hoopes describes as a genetic strategy of "skip the error while you're replicating and fill in the gap later as best you can." Other genes that had been found to affect DNA repair and cause a short life span were all involved in what is called double-strand break repair.
Hoopes and Budd believe the two yeast helicases, SGS1 and DNA2, act independently to protect ribosomal DNA from processes that produce ERCs, whose formation may signify the start of aging and portend the eventual death of the cell.
The likelihood that scientists will one day be able to manipulate and perhaps forestall or even prevent these processes in human cells--to genetically engineer the Fountain of Youth that eluded Ponce de León--is of philosophical and ethical concern to Hoopes.
"I grew up at a time when it was all about the quest," she says of her initiation into the realm of scientific inquiry. "You find a problem and it's sort of your lodestar, and you work on it and try to solve it in your career. But I have not been unaffected by this whole move to asking,'What are the human effects? Everyone that I meet wants a pill to cure aging."
Alleviating the infirmities that elderly people suffer would be a laudable application of any techniques developed to influence aging, she says, but "what will happen to humanity if we stop dying is kind of mind-boggling. I have spent a great deal of time talking about that with students and collaborators, and I wouldn't be really happy to contribute to it because it would cause a great societal mess. On the other hand, the problem that I'm working on is likely to do that, and so I'm very ambivalent about it."
Hoopes believes society has given too little attention to finding some means to control the application of research that may have undesirable consequences without resorting to stopping the research itself.
The continuation of her own research may be imperiled not by its potential implications but by the difficulty of securing funding at small colleges like Pomona. Funding agencies often emphasize grants that help young faculty members get started at such schools, while senior faculty members go begging, she says. "They haven't assimilated the idea that it's the deadwood that's holding up the tree," she says.
Students also benefit from participating in advanced experimentation, says Hoopes, a former president of the Council on Undergraduate Research, an organization that provides opportunities for students and faculty to enlarge the role of research in undergraduate science education.
"I like to think about teaching at a place like this as producing not just science but producing students," she says.
Only in real research, Hoopes says, do students experience the ultimate reward of scientific inquiry: the "ahas!"
"When you figure out what could be happening--and there are times when it fits together so beautifully you just know you're right--that's a great feeling," she says. "It's so exciting. You feel like you're present at the dawn of the universe, and secrets are being revealed to you."
--Michael Balchunas
