THE LOST CHAPTER FROM THE LONG TOMORROW
This chapter was left out of MRR's recent book, The Long Tomorrow; How Advances in Evolutionary Biology Can Help Us Postpone Aging, Oxford University Press, 2005. Here it is for you to judge if this was the right decision.
"A Worm is at Work"
An Insider's View of The Genetics of Aging, circa 2004.
A bit after the evolutionary revolution in aging research came another: the genetic revolution. The odd thing was that it hadn't happened sooner. Raymond Pearl, one of the most eminent American biologists before WWII, studied Drosophila mutants for their effects on aging in the 1920's. In The Biology of Death, the book in which he discussed Carrel's cell research, Pearl published studies of the aging of well-known genetic mutants, which mostly had early deaths.
In traditional genetics, mutants are organisms that have undergone a change in the DNA coding for a single protein. Usually the term mutant refers to genetic changes that produce a large, easily seen change. Some fruit fly genetic mutants have their eye color change from red to white. In humans, dwarfism results from a genetic mutation, as do a variety of other conditions, from cystic fibrosis to Huntington's Disease. In aging research, genetic mutants are animals that have had their lifespan substantially changed by alterations to their DNA. In 1920, Pearl had fruit fly mutants that suffered large reductions in longevity. If such mutants were the highway to understanding aging, then we would have figured out aging by 1960.
But it was not to be. Fruit fly mutants that die early do not necessarily accelerate aging processes. Instead they may die of novel causes unrelated to aging. For example, the Drosophila mutant vestigial doesn't have functional wings. This leaves it vulnerable to getting stuck in fly food. It probably has other problems too -- but we can be sure that it is inferior at getting around. Therefore some of the early deaths that this mutant suffers arise for a novel reason: defective wings. Thus "aging in vestigial" just isn't the same thing as aging in normal flies.
There are mutants like vestigial in humans. In Tay-Sachs Disease, infants are not able to break down some fatty acids due to a genetic mutation. This leads to the accumulation of fat in the brain. This process starts early in life, noticeably impairing the development of toddlers. The brain is progressively damaged, undermining cognition and social development. Blindness and deafness set in, followed by death. These children suffer from grueling pain. Tay-Sachs is a disease that causes progressive and disabling pathologies, followed by death. But it is obviously not aging, not even accelerated aging.
Early deaths can be caused by genetic problems unrelated to normal aging. This makes mutants with early deaths unreliable in the study of aging. This was also a factor in the excitement about my postponement of aging using natural selection. My tiny Methuselahs weren't subject to this criticism.
But the status of aging mutants would change radically thanks to Tom Johnson, and his research on worms. Tom first phoned me in 1982, talking up his research. I had never had an experience like it. Among British and Canadian scientists, at that time, the usual pattern was to refrain from mentioning your latest publication, so that one's peers would discover it for themselves. But Tom was an American of the aggressively friendly variety: Hi. Here's my latest paper. Did you read the last one? Never mind, my new one is even better.
For all his self-promotion, Tom Johnson did have the goods. I went to his talk at the Evolution 83 meeting in St. Louis. Or rather, I went to his advertisement. He stood up there with his wild red hair and thick forearms, blowing us away with one of the most effective presentations of experimental science I have ever seen. His organism was the soil nematode, Caenorhabditis elegans. This worm is the size of a barely visible dot. Millions of them live in your backyard. Almost all of these worms are self-fertilizing hermaphrodites, producing their children with their own eggs and their own sperm. There are some rare males. The hermaphrodites always know whom they are dating on a Saturday night -- themselves. The adult worm is made up of 2,000 to 3,000 cells, including the cells of the body and the reproductive tissues. The development of each of these cells has been worked out in detail. These tiny worms conveniently age and die within a month. In animal genetics, there are two premier research organisms: the fruit fly and the nematode.
After Tom's performance, he and I walked around the St. Louis Botanical Gardens in the evening with a few thousand of our nearest and dearest colleagues. While the day had been oppressively hot, the night was beautiful. It was a good research meeting, with the nematode the highlight, as far as I was concerned.
Tom had more than a fabulous organism for aging research in general. He also had a mutant gene he called age-1. Age-1 mutants lived 40-60% longer than normal worms, and genetic analysis showed that just a single gene was involved. Was the many-headed monster of aging to be cut back to a single fanged mouth? Tom was open to the possibility, but not dogmatic about it. He seemed to understand evolutionary arguments quite well, though he did not always use them in his reasoning.
My reaction to age-1 was somewhat equivocal. I had my doubts. Back in 1983, we didn't know what age-1 did. But it seemed reasonable then to suppose that other longevity mutants would be found in the nematode. The existence of age-1, by itself, didn't reveal how many genes controlled aging in nematodes.
Tom got down to work on his worm mutant. I went back to my flies. My graduate students pricked up their ears when I raved about the advantages of studying the little nematode. Ted Hutchinson, my ex-hippie graduate student, went on to do postdoctoral research with nematodes under Tom's supervision.
Tom and I frequently sparred at meetings. We liked to ask each other tough questions at the end of the other's talks. Tom had been trained to be sharp at MIT, where he was a student, and I was used to Brian Charlesworth's "no quarter" version of academic etiquette. Tom was at the University of California, Irvine, in the 1980's. When I was invited to apply there for a job by a third party, Tom was the biggest attraction. Once I came to Irvine, we taught a genetics course together.
I had a peculiar experience in that class. I attended Tom's lectures, as he attended mine. One day, before Tom showed up for his lecture, the students started to chant in a mysterious low pitch: "Oompa-Loompa didgery-doo, Oompa-Loompa didgery-doh." Or at least that was what I heard. I asked the student next to me what was going on. He told me that they were singing a song from "Willy Wonka and the Chocolate Factory," because Dr. Johnson looked like an Oompaloompa. As soon as Tom appeared, they fell silent. I wonder if all faculty have songs sung by the students, but never hear them?
One of Tom's first papers on age-1 reflected my influence. He published an article in the journal Genetics showing that age-1 had reduced fertility, along with its extended lifespan. This was apparently a coup, because it was the clearest demonstration yet of a trade-off between reproduction and longevity in a single gene. Unfortunately, it turned out that another gene, near age-1 but distinct from it, was responsible for the effect on reproduction. Tom had to retract the finding. It was still true that age-1 postponed aging, but it was not clear if it had an effect on fertility. For my part, I think that age-1 has a small effect on fertility, though I am only speculating. I await the day when Tom's original conclusion will be vindicated.
Tom had worse headaches to come. In the world of late 20th Century genetics, you lived or died by your success in cloning, sequencing, and manipulating the gene that you studied. Tom spent years trying to clone age-1. It was a painful thing to go to his talks hoping to see him present the cloning of the gene, year after year. It was finally cloned and sequenced, but by then events had undermined the centrality of age-1.
Everything changed for nematode aging in the middle of the 1990's. I experienced the transition rather abruptly, because I don't go to as many aging meetings as some of my colleagues. My wake-up call came at a National Institute on Aging Workshop in Montecito, California, near Santa Barbara. I drove from Irvine to the Montecito hotel, along the coast of southern California. Montecito is a fair imitation of paradise, with a perfect climate and a spectacular view down rolling hills toward the Pacific. I think that the meeting was primarily scheduled for California so that National Institute on Aging staff could get in some nice weather. The next morning I found myself in a small meeting room with fly and nematode aging geneticists.
One of the first speakers was from the lab of Wayne Van Voorhies. They had found a nematode mutant which reduced male fertility, but increased male lifespan. This work suited me down to the ground. It was the analog of fruit fly mutants that lost their ovaries. Reducing fertility gave increased nematode lifespan, in males. It was a vindication of the idea of a genetic trade-off between early life and late life.
A bit later, Seigfried Hekimi of McGill University would publish on "clk," or clock, nematode mutants. These mutants have reductions in activity and reproduction. The sluggard worms live much longer than normal, but they do less living. In a sense, these mutants stretch out their lives. Chronological life was increased, but the amount of life lived per day was much less in these mutants. These mutants were comparable to those found by Van Voorhies. While lifespan was clearly increased, it was equally clear that natural selection would select against genes that made organisms slower in getting about their business, because in nature that business is reproducing, not living a long time.
These new nematode mutants had the genetic trade-offs between fitness and longevity that Tom Johnson had originally inferred for age-1. In addition, the profusion of new nematode aging mutants supported the many-headed monster model. Nematode aging was no longer just age-1. Other genes that control aging had been discovered, and no doubt more would be soon. That was all easy for me to accept.
The real shock at the Montecito meeting was the work of Cynthia Kenyon. Cynthia has an endowed professorial chair at the University of California, San Francisco. Good-looking, at that time she resembled the young Joni Mitchell. Like everybody in the burgeoning nematode aging field, she had her own mutant that increased lifespan. Her mutant was involved in the worm's resting stage, the "dauer."
Dauers are immature worms that have gone through a metabolic arrest, somewhat like hibernation, only more extreme. Under difficult environmental conditions, such as starvation or cold weather, immature worms stop growing and enter the dauer state. The alternative would probably be death. In the dauer state, nematodes can survive stressful conditions. Indeed, nematode dauers can survive for months, long after their unarrested kin have died of aging. The formation of dauers, and their eventual transition back to normal development, is one of the nematode's preeminent adaptations for surviving difficult conditions.
The genes that control dauer formation are well-known. It turns out that age-1 is a dauer gene. Once that was understood, it was open season on aging genetics in the nematode. The formula was simple. Take a dauer gene, create a collection of mutants of that gene, then test these mutants for effects on lifespan. This is what Cynthia Kenyon did. She made nematode aging genetics virtually a production line. Worm aging mutants have since been discovered in abundance. The mutant genes have been combined with each other, to make double-mutant worms. Some of these double mutants live longer than single mutants, three or four times the normal lifespan.
The playing out of nematode aging genetics has generated a major scientific controversy. The problem is the nature of the increased lifespan in nematode mutants. Do they have a stretched life, instead of more life? Or at least not much more life. Some nematode geneticists, such as Cynthia Kenyon, have claimed repeatedly that their nematode mutants are perfectly normal, just robustly longer-lived.
The first clue that all might not be right in the worm garden of eternal youth was the discovery that the long-lived mutants have increased resistance to stress when young. This was too much of a good thing. Not only do the worm mutants live longer, young mutant worms also have improved resistance to acute stress. This paralleled our results with Drosophila, so there was no basic implausibility about the nematode stress results. But our flies with increased stress resistance often paid a price in reduced early fertility. Cynthia was claiming that no such price was ever paid in worms. Yet if that were so, why wouldn't natural selection favor these always wonderful mutants, increasing their frequency in natural populations? Why aren't all nematodes mutants with increased lifespan and increased stress resistance?
Gordon Lithgow's lab supplied the answer. Gordon is a Scot with a real Scottish accent and the looks of Robert Carlyle. He had been a student of Tom Johnson's, and had been exposed to evolutionary thinking about aging. The experiment was an ingenious combination of evolutionary and genetic methods. They mixed longer-lived mutant worms with normal worms in experimental populations. Then he took those mixed populations through multiple, short generations in which they were moderately starved. If the longer-lived mutants had actually been generally superior, they should have increased in numbers. But they didn't. The normal worms increased in numbers instead. Longer-lived mutants were impaired relative to normal worms under semi-natural conditions. There was indeed a cost to their increased longevity. This result was very similar to our work with fruit flies.
Twenty years of nematode research on aging appear to have led back to the very beginning of Drosophila aging research. This is good for the field, because it connects nematode research with fruit fly research. The nematode also helps us to understand aging in general, and our aging in particular.
The proliferation of mutants affecting nematode longevity inspired fruit fly geneticists to intensify their search for genes that increase longevity. Up until the 1980's, large screens of fruit fly mutations hadn't given any useful anti-aging genes. The latter part of the 1980's saw fly aging research turning toward new genetic technology, "genetic transformation," the insertion of additional genes into genomes. Genetic transformation is also sometimes called "genetic engineering." Aging researchers hoped to identify anti-aging genes in flies by sticking extra copies of candidate anti-aging genes into fly genomes.
The early days of this work were not entirely auspicious. In 1989, John Shepherd and Walter Gehring of the Biozentrum at the University of Basel published a paper on aging and transformation in the Proceedings of the National Academy of Sciences, USA. They made fruit flies with additional copies of an "ELF" gene. This gene helps the manufacture of proteins. If aging was caused in part by less protein synthesis, then increasing the amount of this ELF protein might give a big extension in lifespan. Shepherd and Gehring produced a figure that seemed to show just that -- their genetically engineered flies lived a lot longer than normal flies. This result seemed to show that fruit flies could have their aging substantially postponed by changing just one gene, like nematodes.
In the fall of 1990, I managed to put together a dream trip to Europe. It began with a meeting of the Royal Society in London. Unfortunately, I was jet-lagged from California, and didn't get much from the experience. A few days later I participated in a select CIBA Foundation symposium on aging, at CIBA's small building in London. My greatest thrill at this symposium was meeting Alex Comfort. Comfort had written two landmark books, one about human sexuality and one about aging. The former was The Joy of Sex. Brian Charlesworth loved to point out that The Joy of Sex was written by A. Comfort. I read a lot of Comfort's aging stuff during my doctoral stint. When Brian left for a sabbatical in North Carolina, I felt as if Comfort's aging book was my only company in the study of aging.
Comfort turned out to be quite short, with unimaginably dry hands. His eyes moved about uneasily, and his voice was raspy. But his words were a stream of commentary and reference. I have met a number of great scientists, from James Watson to Sewall Wright, but Comfort was the most articulate person I have ever encountered.
It was my good fortune on this trip that Stephen Jay Gould had cancelled out of an endowed lecture at the Max Planck Institute in Frankfurt, Germany. The Institute paid me to fly to the continent to give the lecture, and I chose Basel, Switzerland, as my first stop.
My host in Basel was Stephen Stearns, one of the founding fathers of modern life-history research. Steve is a bear-like man with a grizzled beard. I have always felt indebted to him, because he invited me to give my first symposium lecture back in 1981, when I was a wet-behind-the-ears assistant professor. Steve came to Basel as a full professor in the 1980's, managing to learn enough German to function as a professor. He confessed to me that it had been humiliating to arrive in Basel with the German of a three year-old.
But it was to Europe's benefit. Arriving in Switzerland, he had been shocked by how little the scientists of different European countries communicated with each other. He proceeded to set up the European Society for Evolutionary Biology and founded the Journal of Evolutionary Biology. Of course a few European scientists helped.
As if that hadn't been enough of a contribution, Steve Stearns had taken on the task of figuring out the ELF result of Shepherd and Gehring. This was a challenge. While Shepherd is a quiet Englishman, Walter Gehring is a formidable Drosophila molecular geneticist. But Steve just had to know. He started with the most basic question in experimental science: was the original result reproducible?
Right away, there was a problem. Shepherd and Gehring had more data that didn't find its way into their paper. And that data wasn't as strong as the result that they published in full in their article. This was no big issue in molecular biology, where scientists usually publish only their prettiest results. But for evolutionary biologists like Steve and myself, this wasn't acceptable.
That was when I appeared on the scene at Basel. It was one of the most interesting visits in my career as a scientist. I stayed in a University of Basel building that had housed both Paracelsus and Neitzche, at different times of course. I gave my usual visitor's seminar, speaking on aging. Gehring came to my talk, and demanded a definition of aging. I had one ready, having just finished my first book on aging. I defined aging as a sustained decline in adult survival and fertility in the absence of disease, stress, etc. This definition wasn't good enough for Gehring, and we went a few rounds. Like other big-time scientists, in my experience, he seemed to expect that I would back down in the face of tough questioning. But I never do.
Gehring wasn't too angry, however, and invited me to visit his lab. There I met Shepherd too. They seemed to be interested in sorting things out with help from Steve Stearns. I went off to Frankfurt by train to impersonate Gould.
But things were not to remain smooth in Basel. The effect of the Shepherd genetic transformation turned out to be like the Cheshire Cat, appearing and disappearing as a function of which sex was studied, the environmental conditions, and the site at which the gene was inserted. The Basel fly people were discovering that genetic engineering doesn't work reliably in fruit flies. Other labs would find the same thing with other genes. It's hard to get the molecular genetics of aging to behave themselves, impressive molecular technology notwithstanding.
But things would change with the arrival of John Tower on the aging scene. John bears a startling resemblance to John Malkovich, though he has never shown any discernable desire to be John Malkovich. John, Tower not Malkovich, had done a post-doc with Alan Spradling, one of the inventors of fly genetic engineering. I met John at a 1992 aging meeting at Cold Spring Harbor Laboratory, in Long Island, New York. The Cold Spring Harbor lab is one of the holy shrines of molecular genetics -- the place that produced many of the breakthroughs of molecular biology. James Watson of double-helix fame was Director of the Laboratory at that time. He gave a little pep talk to the aging meeting -- innocuous, but good theater nonetheless.
John Tower was interested in collaborating with me on the molecular genetics of my Methuselah flies. But his main strength was engineering fly genes. During the ten years that followed, John created the best system for manipulating the molecular biology of aging in fruit flies. It was all based on a trick. He would stick new genes into flies, but leave them turned off. The new genes wouldn't do anything unless he gave them an environmental trigger -- usually a brief pulse of heat. This meant that he could test the effect of an inserted gene on aging with a perfectly matched control. The control flies had the gene, but didn't express it. The experimental flies had the inserted gene turned on by a pulse of heat.
John has used his technology to test a variety of ideas in the genetics of aging. Some of them did not fare well, one of mine included. John showed that several enzymes weren't important factors in fly aging. That upset some people, including me, because one of those enzymes was a favorite of mine. But John showed that superoxide dismutase, the most powerful of all antioxidant enzymes, was a key player in aging. John Tower's work on antioxidant enzymes supports the idea that aging involves the oxidation of protein and DNA.
A later meeting at Cold Spring Harbor led me to Seymour Benzer, the Drosophila geneticist recently portrayed in Jonathan Weiner's book Time, Love, Memory. Seymour Benzer has long been a leading scientist. He was best known to me for his work on the genetics of memory and learning in fruit flies, although he has contributed to several fields. His work on memory supplied genetic foundations for learning itself. But in the late 1990's he turned to aging research. Perhaps that was why he invited me to dinner in the Cold Spring Harbor Lab's cafeteria. It was a nice meal, but he revealed nothing of his plans or results with fruit fly aging.
Then his lab's paper appeared in Science in 1998. Benzer had searched for aging mutants using their effects on stress resistance. He had taken the results associating stress resistance and fruit fly aging, first obtained by Phil Service in my lab, and inverted them. He reasoned, quite astutely, that if stress resistance was associated with increased longevity, then stress could be used right from the start, as an alternative to dealing with longevity. Benzer's lab used this strategy to find a fruit fly aging gene much like Tom Johnson's original age-1. Benzer called it methuselah. The gene gives a lifespan increase of about 50%. The full story of how this gene works has yet to be published, however. I have learned how close to his chest Benzer keeps his cards, so I am expecting great things.
The molecular genetics of aging are now up and running. There are some controversies. The genes involved in dauer formation in nematodes certainly can be mutated to increase lifespan, but is there a cost to the worm from living longer? Fruit fly genes that seemed promising at one time haven't worked out as aging genes, but a few have.
There are results that are virtually certain, like John Tower's studies of the superoxide dismutase gene in fruit flies. Together with all the less certain findings, we now have our first look at the deepest foundations of aging. The salient question is, what can we learn from these findings about the postponement of human aging?
The diversity of genes that have been implicated by molecular methods supports the many-headed monster model of aging. Lots of genes are involved. The monster won't go away. We have to find a way to live with it.
Yet individual pathways can have big effects. The strongest evidence for this comes from the gene for superoxide dismutase, which can be engineered to increase fruit fly lifespan by tens of percent. Many genes can postpone aging, and some of them are part of pathways that can have large effects on lifespan. In principle, if one were really interested in the aging of a particular animal, it might be possible to find many pathways that control aging, one mutation at a time. Then those pathways could be used to postpone aging by stages, culminating in radical increases in lifespan, by 50%, two-fold, and five-fold. Worm geneticists are already at this point. In other animals, in principle, we will definitely be able to do this in the future.
In humans, the prospects for using lifespan-increasing genes or their products depend in part on the interpretation of what these genes do. The good thing about aging genetics in the 21st Century is that it has made so much progress that we can now consider the general features of the genes involved in aging. There is some very good news in recent aging genetics, but also some important caveats.
Aging is not exclusively the province of animals and plants. Some tiny microorganisms also age. The best known microorganism that ages is yeast, the same species with which we brew beer and bake bread. Even though yeast is a single-celled fungus, it reproduces by budding. The budded-off cells are young, while the mother cell progressively deteriorates with each bud, her cell surface forming a new scar with each offspring that she produces.
I first learned about yeast aging from Michal Jazwinski, a Polish-American of great charm and acumen. Many of my favorite colleagues are East European, and Mike was no exception. In the late 1980's, Mike started using a genetic approach to yeast aging, and had no difficulty producing mutants with increased lifespan. This was good for the field, because Tom Johnson's age-1 was the only solid anti-aging gene at the time. Scientists don't like to generalize from a single case, however convincing.
Lenny Guarente was a later recruit to yeast aging research, but he has certainly made up for lost time by selling his results in a bumptious manner. A professor at MIT and a native of Massachusetts, he has an indefinable self-confidence. I will never forget the moment when I heard him announce he had discovered that "the cause of aging," was circles of DNA that loop out of the genome. Of course he had to retract this model within months, but he soon had another candidate for the universal control of aging.
One of the most interesting things about Lenny Guarente's research on yeast aging is that his lab performed the same inadvertent stress experiment that Anne Coyle accomplished in my lab. They tried to maintain their yeast strains on food that had been left in the refrigerator for months. Only long-lived strains survived. They soon switched to such stressful environments as screens for increased longevity. This happened in the 1990's, but it is not clear that the Guarente lab had any idea that the connection between stress and the genetics of aging had been discovered years earlier in my lab. No shame in that. Few of the people who re-discovered the importance of stress for aging acknowledged our work, two notable exceptions being Seymour Benzer and Michal Jazwinski. In any case, the use of stress resistance as a screen soon led Guarente to a mutation that significantly postponed yeast aging: Sir2.
The Sir2 gene acts to stop gene expression when yeast cells experience a loss of chemical energy. What Lenny claimed was that this is the key to the effect of caloric restriction in all organisms: reduced food intake led Sir2 to shut down much of the organism's activity, so that it could survive to reproduce later. In his book Ageless Quest, he said that "SIR2 was just too pretty not to be used by nature to promote survival in contexts other than yeast."
To test whether Sir2 was indeed as important as he thought, he used C. elegans as a test case. His lab surveyed the worm genome for genes that were similar to yeast's Sir2. Four were found. One of these genes was used by the Guarente lab to produce a much longer worm lifespan. Even more notable was the fact that no other worm gene gave such a big increase in lifespan in their experiment. It also turned out that the anti-aging effect of Guarente's worm gene required the dauer modification pathway worked out by Cynthia Kenyon and other worm researchers, the pathway which affects both development and aging.
Lenny found even more evidence to support his theory about Sir2. When yeast are forced to undergo semi-starvation, like rodents suffering caloric restriction, they live longer. The Sir2 gene is indispensable for the increase in yeast longevity with caloric restriction.
All these points combined together to form a plausible argument in favor of the idea that animal species might share universal molecular controls regulating lifespan, metabolism, and development. And if these controls worked in humans, we might be one or two drugs away from large increases in our lifespan.
Circumstantial evidence in favor of this hope is that the mammalian equivalent of Sir2 regulates the action of an enzyme that controls cell turnover in our tissues. Sir genes keep appearing in critical pathways related to aging.
Together with the amazing achievements of worm genetics, especially the discovery and manipulation of dauer genes, the Sir2 research makes it seem as if we will soon have pharmaceuticals that give remarkable increases in human longevity.
This is about the right point in time to take a deep breath and consider these possibilities rationally. What is happening when Guarente's or Kenyon's genes increase longevity? What kind of longer life is given to yeast, worms, and flies in genetics labs?
Cynthia Kenyon is very defensive about this point. Every time I hear her speak, she insists that her worms can live three, four, or six times longer with no loss of function or happiness. [She does in fact use words like happiness.] She has little Powerpoint films of her longer-lived worms, cavorting in bacteria long after normal worms have died. I don't wish to deny Cynthia's great achievements as a geneticist. Her worms are indeed the best worms in the land. But there should be a more objective way of settling the issue.
Lenny Guarente's results reveal a switch that shuts down metabolism in order to ensure survival. The effects of such de-tuning of metabolism has been known for a long time. Cold-blooded animals have their metabolism turned down when they are chilled, and usually live longer as a result. This was shown in fruit flies in the early 1900's. Rodents on caloric restriction have the same metabolic rate, relative to their body weight, as normal rodents, but they reproduce much less, or not at all, as described . These are two different ways of having less of a life over a longer period of time, but either way life is "strectched".
What do long-lived nematodes do? There is some controversy about this, because people measure metabolic rates of nematodes under different conditions. Some scientists study them suspended in liquid, others let them crawl over solid surfaces. Sometimes the worms are fed, and sometimes they are starved. Even the chemical reactions that are used to measure metabolism vary.
I have some experience with metabolic measurements in fruit flies. I haven't made them myself, but my immediate collaborators have made them, particularly Phil Service and Tim Bradley. A general rule of thumb is that you should measure metabolic rate under normal, low-stress conditions. When we do that in our fruit flies, Methuselah flies do not have a reduction in their metabolic rates. They do have a reduction in their early reproduction, but their total reproductive output is increased. By no measure are the Methuselah flies living a reduced life in total. They do everything normal flies do, if not more.
Wayne Van Voorhies has upset a lot of worm researchers with his studies of worm metabolic rates. Under low stress conditions, with food, he finds that the metabolic rates of long-lived worms are significantly reduced. I think that these results are fundamentally correct. Long-lived worms seem to be living less, though living longer. Furthermore, the longer a mutant worm lives, the greater the reduction in its metabolic rate. Wayne interprets the effect of many, but perhaps not all, nematode aging mutants as reducing the amount of physiological activity of the worm, just like lower temperatures in fruit flies. This conclusion has been controversial among nematode researchers. But the genes in the pathways that modulate nematode aging are well-known to regulate metabolism in a variety of animals, so even if Van Voorhies's precise interpretations will need qualification later, there is little question that nematode aging mutants are also, as a general class, also mutants affecting metabolism and, often, reproduction. In particular, they tend to slow metabolism or reproduction.
How are we to interpret this, and what are its practical implications for altering the human lifespan? If we were nematodes, and nematode genetics were to continue along its present lines, then we would have a clear choice between a stretched-out life, or a short life. I think that the Sir2 findings are even more explicit, because they clearly connect shutting down genes with increased lifespan. Even the rodent caloric restriction work is in the same vein: eat less and reproduce less, but live longer. Less happens in such a life, especially by rodent standards, but it lasts.
The one animal that doesn't fit this pattern is the lab-evolved Methuselah fly. It gets as much or more of everything that life has to offer, along with a greater lifespan, except the ability to reproduce as a "teenager" under "trailer park" conditions. If we were fruit flies, we have a choice between a short life of limited robustness and a much longer life of greater robustness and productivity.
Considered from the standpoint of brutal practicality, notwithstanding the fine work of many of my colleagues, I don't think that there is a great future in the Sir2/dauer/CR models of increased lifespan. They don't offer postponed aging. They offer the prolonged survival of an adult that is eking out a limited existence. In nature, animals do this when they can't find enough food and are trying to survive until better times. The Okinawa story suggests that humans can do this also, to a very limited extent. But when animals are given the chance to resume feeding, and thus reproduction, they do so with gusto. Humans are similar, as the Okinawan elders are discovering to their horror. Even if we could pharmaceutically emulate the physiology of the Biosphereans, I doubt that most people would choose their chronic lack of energy and low-grade depression. A hardy few probably would, but I doubt that most of us are that stoic.
The future, it seems to me, lies with research that attempts to give to us the beneficent life of the evolved Methuselah flies. They live even longer when they are calorically deprived, but such deprivation is not necessary to their postponed aging. If we could eat our normal diets, keep our normal activity levels, take care of our children, AND do all of these things for longer, then we would have true amelioration of aging. The creations of the gene jockeys and the diet manipulators can be admired for their brilliance, their scientific acuity. But they aren't supplying a future for humankind that most of us are likely to want.
Annotated Bibliography
Tom Johnson's many worm aging publications include: "Genetic analysis of life-span in Caenorhabditis elegans" (T.E. Johnson & W.B. Wood; 1982; Proceedings of the National Academy, USA 79: 6603-07), "Aging can be genetically dissected into component processes using long-lived lines of Caenorhabditis elegans" (T.E. Johnson; 1987; Proceedings of the National Academy, USA 84: 3777-81), and "A mutation in the age-1 gene in Caenorhabditis elegans lengthens life and reduces hermaphrodite fertility" (D.B. Friedman & T.E. Johnson; 1988; Genetics 118: 75-86).
Wayne Van Voorhies's nematode aging research can be sampled in "Production of sperm reduces nematode lifespan" (1992; Nature 360: 456-58) and "Genetic and environmental conditions that increase longevity in Caenorhabditis elegans decrease metabolic rate" (W.A. van Voorhies & S. Ward; 1999; Proceedings of the National Academy of Sciences, USA 96: 11399-11403).
For a review of Seigfried Hekimi's work, see "Crossroads of aging in the nematode Caenorhabditis elegans" (2000; Pp. 81-112 in The Molecular Genetics of Aging, S. Hekimi, Ed.; Springer-Verlag). The reduction in fitness of clk mutations is illustrated in " Reproductive fitness and quinone content of Caenorhabditis elegans clk-1 mutants fed coenzyme Q isoforms of varying length" (T. Jonassen, D.E. Davis, P.L. Larsen, & C.F. Clarke; 2003; Journal of Biological Chemistry 278: 51735-51742).
The work of Cynthia Kenyon and Lenny Guarente is engagingly summarized in Lenny's Ageless Quest, mentioned at the outset of the Bibliography. A good review of the parallels in the molecular biology of postponed aging among model organisms is "Evolutionary medicine: From dwarf model systems to healthy cenenarians?" (V.D. Longo & C.E. Finch; 2003; Science 299: 1342-1346). The key weakness throughout this body of molecular work is the relative lack of good assays of reproduction, perhaps because of undernourished understanding among most geneticists of the centrality of reproduction for the evolutionary genetics of all populations.
The key paper about selection against life-prolonging mutants from Gordon Lithgow's lab is "Natural selection - Evolution of lifespan in C. elegans" (D.W. Walker, G. McColl, N.L. Jenkins, J. Harris, & G.J. Lithgow; 2000; Nature 405:296-297). Other evidence for a cost to the life-prolonging effects of dauer pathways is supplied in "daf-16 integrates developmental and environmental inputs to mediate aging in the nematode Caenorhabditis elegans" (S.T. Henderson & T.E. Johnson; 2001; Current Biology 11: 1975-1980).
The paper from the Gehring lab that started this miniature controversy was "Fruit flies with additional expression of the elongation factor EF-1a live longer" (J.C.W. Shepherd, U. Walldorf, P. Hug, & W.J. Gehring; 1989; Proceedings of the National Academy of Sciences, USA 86: 7520-21). A riposte from the Stearns lab was "The effects of enhanced expression of elongation factor EF-1a on lifespan in Drosophila melanogaster" (S.C. Stearns & M. Kaiser; 1993; Genetica 91: 167-182). There were other papers along these lines as well, as well as a paper showing the EF-1a transgene was not even expressed ("Protein synthesis elongation factor EF-1a expression and longevity in Drosophila melanogaster"; N. Shikama, R. Ackermann, & C. Brack; 1994; Proceedings of the National Academy of Science, USA 91: 4199-4203). It's a "buyer beware" situation in the molecular genetics of aging.
Some of the highlights of John Tower's work on the genetic engineering of aging in fruit flies include "FLP recombinase-mediated induction of Cu/Zn-superoxide dismutase transgene expression can extend the life span of adult Drosophila melanogaster flies" (J. Sun & J. Tower; 1999; Molecular and Cellular Biology 19: 216-228) and "Transgenic methods for increasing Drosophila life span" (2000; Mechanisms of Ageing and Development 118: 1-14).
The profile of Seymour Benzer is Time, Love, Memory : A Great Biologist and His Quest for the Origins of Behavior (J. Weiner; 1999; Knopf). His first foray into the aging field was "Extended life-span and stress resistance in the Drosophila mutant methuselah" (Y.J. Lin, L. Seroude, & S. Benzer; 1998; Science 282:943-946).
For an example of Mike Jazwinski's work on yeast aging, see "The genetics of aging in the yeast Saccharomyces cerevisiae" (S. M. Jazwinski; 1993; Genetica 91: 35-51).
There is now a burbling stream of publications on the molecular genetics of insulin/nutrient/energetic pathways in the control of aging. Two recent examples are "Longevity regulation by Drosophila Rpd3 Deacetylase and caloric restriction" (B. Rognia, S.L. Helfrand, & S. Frankel; 2002; Science 298: 1745) and "Regulation of lifespan in Drosophila by modulation of genes in the TOR signaling pathway" (P. Kaphai, B.M. Zid, T. Hrper, D. Koslover, V. Sapin, & S. Benzer; 2004; Current Biology 14: 885-890).
See Methuselah Flies (M.R. Rose, H.B. Passananti, M. Matos, 2004, World Scientific Press) for a variety of articles that feature tests of metabolic rate in long-lived fruit flies, as well as discussions of the general question of appropriate model organisms for the postponement of human aging.
