Yeast cells reproduce by the daughter cell budding off from the mother cell. After budding off some twenty daughter cells, the mother cell dies from what can be considered to be old age. Initially the mother cell buds every hour or so, but then the time between buds increases to three to four hours. Different strains of yeast age to different extents and the genes involved have been identified—the sirtuin (Silent Information Regulatory Two) genes. These genes are involved in life extension in a number of model organisms. In the nematode increased dosage of a sirtuin increases the mean lifespan by up to 50 per cent and involves the insulin signalling pathway. In flies, a sirtuin has also been reported to extend lifespan.
Model animal organisms have been invaluable in investigating what determines ageing and lifespan. These organisms include the nematode worm C. Elegans, which has about half our number of genes, a fixed small number of cells—959—and normally only lives for about 25 days; the fruit fly Drosophila, with an average lifespan of 30 days, which is a key model for genetic studies; and mice, which live for several years. The reason the nematode worm begins to die after a couple of weeks is due to the degeneration of its muscle after 15 days. Just why this occurs so soon is not understood, but the worm does not build its muscle nearly as robustly as mammalian muscle, and it contains no satellite cells that can replace damaged muscle cells.
Recent landmark molecular genetic studies have identified an evolutionarily conserved insulin-like growth-factor pathway that regulates lifespan in the nematode, fruit fly, rodents, and probably in humans. Reduction of the activity of this pathway appears to increase lifespan and enhance resistance to environmental stress. Genetic variation within the FOXO3A gene (the names given to genes can be quite weird), which can reduce this pathway’s activity, is strongly associated with human longevity.
A dramatic example of an increase in lifespan came from the nematode worm. If the worms are placed under conditions where there is a limited food supply and many other worms are present, then instead of developing into adult worms through a series of larval stages, they develop into an alternative larval form known as a dauer larva. These dauer larvae neither feed nor reproduce, but if conditions improve they moult into adulthood and can then reproduce. But the dauer larvae, with their very dull lives, can live for up to 60 days, more than twice as long as normal worms. This is due to interference with the insulin pathway. Insulin plays a major role in the ageing process. A major discovery was a mutation in a single gene that caused the worms to live twice as long and remain healthy. This gene codes for a receptor for an insulin-like growth factor. The mechanism by which this increases longevity is not clear, but involves many other proteins. When sirtuins are over-expressed there is an increase in lifespan, and they were shown to interact with proteins of the insulin signalling cascade.
Reduced signalling by chemicals similar to our insulin also extends the lifespan of the fly Drosophila. It has recently been shown that in mice, less insulin receptor signalling throughout the body, or just in the brain, extends lifespan up to 18 per cent. Taken collectively, these genetic models indicate that diminished insulin-like growth-factor signalling may play a central role in the determination of mammalian lifespan by conferring resistance to internal and external stressors. The effects of eating less—calorie restriction—which can increase lifespan, also operate via the insulin effect. Fasting does reduce insulin secretion, but one must be cautious in trying too hard to reduce insulin secretion, as this can lead to diabetes.
There are genes that can extend lifespan or reduce it. The AGE-1 gene, for example, encodes part of a cellular signalling pathway that regulates dauer formation in the nematode worm via insulin-like growth-factor signalling. Mutations in genes encoding constituents of this pathway can extend lifespan not only in the nematode, but also in the fruit fly and the mouse. Single-gene mutations that affect longevity act via their interaction with multiple target genes. The increased lifespan in age-1 and related mutants in the nematode is associated with reduced reproductive fitness. The age of first reproduction is sometimes delayed or even prevented by the inappropriate formation of a dauer larva.
Sirtuins are also involved in mammalian ageing. A protein in the cell nucleus of mammals, NF-kappaB, is not only the master regulator of immune system responses, but can also regulate ageing. Activation of NF-kappaB signalling has the capacity to induce ageing in cells. Several longevity genes, such as the sirtuins, can suppress NF-kappaB signalling, and in this way delay the ageing process and extend lifespan. The protein SIRT1—the mammalian equivalent of sirtuins—manages the packaging of DNA into chromosomes, and this role controls gene activity. When DNA damage occurs, SIRT1 abandons this critical task in favour of assisting with DNA repair. Mice that were bred for increased SIRT1 activity demonstrated an improved capacity to repair DNA and to help prevent undesirable changes in gene expression with ageing. It is involved in life extension that comes from calorie restriction.
There are other ways in which cells can age. A limit to the number of times some cells can divide in culture was discovered by Leonard Hayflick in 1965, when he demonstrated that normal human body cells in a cell culture divide about 52 times, but the number is less when the cells are taken from older individuals. There is no such limit for germ cells or cancer cells or embryonic stem cells. The explanation for the decline in cellular division of body cells in culture with age appears to be linked to the fact that the telomeres, from the Greek word for ‘end part’, which protect the ends of chromosomes, get progressively shorter as cells divide. This is due to the absence of the enzyme telomerase, which makes the telomere grow back to its normal length after each division. This enzyme is normally expressed only in germ cells, in the testis and ovary, and in certain adult stem cells such as those that replace cells in the skin and gut, as these cells have to be prevented from ageing. If the telomeres get very short, the cell is no longer able to divide and this means it cannot become a cancer cell. It may be that the telomeres can count how many divisions the cell has gone through, as they get a little shorter at each division. This could function to protect the cell against runaway cell divisions as happens in cancer, and ageing of the cells so that they have a limited number of divisions could be the price we have to pay for this protection.
Individuals can have their own telomere profile. In addition to the common profile, it is found that each person has specific characteristics, which are also conserved throughout life. Studies on both twins and families indicate that these individual characteristics are at least partly inherited. The length of individual telomeres might occasionally play a role in the heritability of lifespan. In diseases that result in premature ageing there is accelerated telomere shortening, and this may be partly responsible for the condition. There is new evidence that telomere shortening affects ageing in the general population, and is also likely to affect the way a person ages facially. A mutation in the so-called Peter Pan gene speeds up ageing due to telomere shortening. Up to 7 per cent of the population have two copies of this mutation, and they look up to eight years older than other people of the same age. About one third of the population has one copy, ageing them by three to four years. A fortunate, and fresh-faced, 55 per cent do not have the mutation and they remain youthful-looking for longer. Previous research has linked long telomeres with good health and shorter ones with age-related ills such as heart disease and some cancers. Shorter telomeres may thus be associated with shorter lives. One study found that among people older than 60, those with shorter telomeres were three times more likely to die from heart disease and eight times more likely to die from infectious disease. A study of centenarians, Ashkenazi Jews, found that their offspring have longer telomeres, and these are associated with protection from ageing diseases and better cognitive function, and can confer exceptional longevity.
There is increasing evidence that the nervous system may act as a central regulator of ageing by coordinating the physiology of body tissues. In worms, a number of different mutatio
ns that disrupt the function of sensory neurons extend lifespan. Furthermore, killing of specific neurons can increase lifespan in worms and flies. An intriguing question is whether functional disconnection in the brain leads to disruption of brain-systemic feedback loops involving crucial hormonal and autonomic systems. Such a loss of integrated function may contribute to age-related physiological changes, such as hypertension and insulin resistance, and predispose individuals to age-related pathological changes in the brain. It will be exciting to explore the extent of these functional connections in future studies.
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It is an amazing fact that our skin cells are replaced about every 5 weeks, so by the time you are 20 years old you would have replaced your skin cells about 200 times. Do the cells that give rise to skin—skin stem cells—not age? Stem cells are cells that divide and then one daughter cell remains a stem cell to divide again, whereas the other can become specialised cell such as a skin cell. Using mouse skin cells as a model system, researchers compared several properties of young and old adult skin stem cells. They found that, over an average mouse’s lifetime, there was no measurable loss in their functional capacity. It seems that skin stem cells resist cellular ageing. There is no evidence that the lifespan of any species is determined by a limited supply or limited functionality of its stem-cell populations. An analysis of changes in gene activity as a mouse ages found that some tissues displayed large differences: in old mice, for example, there were genes in the brain which were more active, while other genes were less so, compared to a younger mouse.
Just less than one third of the variation in human lifespan is due to genetic differences that are important for survival after 60. Research on Danish twins born since 1870 found no evidence for an innate maximum lifespan shared by identical twins. Only about 25 per cent of the variation in adult lifespans could be attributed to genetic variation among individuals. The search for the genes that positively affect human ageing has been intense, but it has been very difficult. One example is the Peter Pan gene that extends human lifespan and acts via the insulin pathway which is so central in animal studies. Most of the long-lived men—those who eventually reached an average age of 98 years—had the same version of a gene which regulates the insulin pathway.
There are several illnesses related to old age which have a clear genetic basis and result in premature ageing. A genetic disease that causes premature ageing is Werner syndrome, which is a mutation in a gene that codes for a protein that unwinds DNA. Those with the illness typically grow and develop normally until they reach puberty, but usually do not have a growth spurt, resulting in short stature. The characteristic aged appearance of individuals with Werner syndrome typically begins to develop when they are in their 20s and includes greying and loss of hair; a hoarse voice; and thin, hardened skin. Affected individuals may then develop disorders such as cataracts, skin ulcers, type 2 diabetes, diminished fertility, severe hardening of the arteries, thinning of the bones and some types of cancer. People with Werner syndrome usually live into their late 40s or early 50s. The most common causes of death are cancer and atherosclerosis.
Premature ageing is known as progeria. Hutchinson-Gilford Progeria Syndrome—a very rare, genetic disease: only about 50 cases are currently identified worldwide—is due to a mutation in the LMNA gene that codes for a protein involved in the structure of the cell nucleus. Children with this mutation have small, fragile bodies, like those of elderly people, and typically live only about 13 years, and die from atherosclerosis and cardiovascular problems, although some have been known to live into their late teens and early 20s. The condition almost always occurs in people with no history of the disorder in their family. Whether the illness is similar to normal ageing is not known. Other genes are involved in age-related illnesses like Alzheimer’s.
All these results suggest that no life strategy is immune to the effects of ageing, and therefore immortality may be either too costly or mechanistically impossible in natural organisms. Yet there are exceptions. Germ cells are immortal and a few primitive organisms, including hydra, a primitive simple animal in the form of a tube with tentacles, exhibit very slow or negligible ageing. Individual hydra were observed over a period of four years and yet showed no age-related deterioration, either in terms of survival or reproduction rates. The reason is not clear, but may be related to the fact that hydra can reproduce by forming buds which will develop into mature hydra without sexual involvement, and are also capable of undergoing complete regeneration from almost any part of their body. Most of their body cells can contribute to regeneration, so if some age, they may die or be lost during growth or budding.
Amongst the environmental factors that are linked to ageing, nutrition plays a prominent role. The great increase of non-insulin-dependent diabetes—type 2—in industrialised nations as a consequence of eating too much is an expression of this environmental challenge that also affects ageing processes. The most consistent effects of the environmental factors that slow down ageing—from simple organisms to rodents and primates—have been observed for calorie restriction. In yeast, the fruit fly and the nematode, sirtuins have been observed to mediate as ‘molecular sensors’ in the effects of calorie restriction on ageing processes. Sirtuins are activated when cell energy status is low.
Exposure to a variety of mild stressors such as calorie restriction and heat can induce an adaptive response that increases lifespan. For example, long-lived nematode insulin-signalling mutants are more resistant to thermal and oxidative stress. The term hormesis describes such effects, which are beneficial at a low level but harmful at a higher level. If induction of stress resistance increases lifespan and hormesis induces stress resistance, can hormesis result in increased lifespan? Here the answer is definitively yes. For example, in nematodes, brief thermal stress sufficient to induce tolerance to heat also causes small but statistically significant increases in lifespan. One possibility raised by studies of hormesis is that the increase in lifespan in animals due to dietary restriction, or to insulin signalling mutants, results from hormesis.
Increased longevity can thus be associated with greater resistance to a range of stressors. This may result from the increased expression of genes contributing to cellular maintenance processes, thereby protecting against the molecular damage that causes ageing. Similarly, the physiological stress of exercise has an optimal point for developing muscle strength and improving cardiovascular health, beyond which detrimental effects can be experienced such as attrition of cartilage in joints, leading to arthritis. Another possible example here is alcohol consumption: relative to abstainers, moderate drinkers have reduced mortality risk, especially from coronary heart disease. However, it is not known whether this effect involves stress-response hormesis. The study of stress-response hormesis and the induction by stressors of biochemical processes that protect against stress is providing new insights into the mechanisms that protect against a range of pathological processes, including ageing.
There is a great deal of research into the cellular basis of ageing and the progress is impressive, but there is still a long way to go before we fully understand how cells get damaged with time and, more important, how they repair that damage. One area that may illuminate the repair mechanisms will be by understanding how germ cells are prevented from ageing.
Professor Tom Kirwood is a leading scientist in ageing who gave the Reith Lectures in 2001. I asked him how much do we understand about ageing?
We have a pretty good general understanding as to why ageing happens and a broad thrust of the mechanisms, but in terms of what there is still to be learned and in terms of any intervention we are only at the beginning. The number of scientists working on ageing is tiny compared, for example, to those working on cancer. Extending life expectancy is one of humanity’s greatest successes, as we have doubled it over the last two hundred years, and for the first 150 of those years it was done by preventing people dying young by getting rid of infections and advances in general sanitat
ion, vaccines and so on. Until about 25 years ago that was thought to be the end of the story. But it is a great surprise that the increase in life expectancy has not slowed one jot as people are getting old in better shape, and there is a decline in death rate in older people. After all this success, should we now be tampering with the ageing process itself?
This raises challenging questions. In my view it is perfectly OK to use science to increase lifespan provided the emphasis is on the quality of the years gained. Is immortality possible? At a theoretical level, yes. When I had, some years ago just finished my book on ageing, Time of Our Lives, I had an idea for a work of fiction, a short story, ‘Miranda’s Tale’, where science has managed to indefinitely postpone the ageing process. It has to be a possibility as the germ line does not age, they have better repair mechanisms, and there is also elimination of less good cells. It would not be by taking a drug, but would require changing our genetic constitution. There are animals like hydra which do not age. But I do not think it a practical objective. It is science fiction and should stay there.
What did he feel about his own ageing?
I think ageing is a challenging process—I am just coming up to 60 so not yet much affected. I enjoy being alive so I want to become older and I enjoy talking with older people. One has to accept reduction in mobility. Many young people do not want the problems of old age, but when they reach that age they may enjoy a very full and active life. Most people including scientists still think that we are programmed to age, although the evidence is totally against it—it is part of the need to see a purpose.
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