Cells tend to respond to serious DNA damage by committing suicide—apoptosis—and this provides a way of preventing the damaged cell becoming cancerous. This occurs much more often in aged tissues in which the background accumulation of damage is greater, and the resulting loss of cells may itself accelerate ageing. Long-lived organisms probably invest in better DNA maintenance. The benefit of this is seen both in slower ageing and delayed incidence of cancer, since genome instability contributes to both these processes. Humans are less likely to get cancer than mice, as they have invested more in DNA repair. While long-lived organisms make greater investments in cellular maintenance and repair than short-lived organisms, with age the repair mechanisms fade.
Nature and evolution seem to have a fine sense of irony when they made our lives so dependent on oxygen, which is essential for energy production but may also be a major cause of ageing and our eventual death. One possible cause of the damage to DNA and other molecules that leads to ageing places much of the blame on small modified oxygen molecules. Oxygen is required by the mitochondria in cells to produce energy from the molecules derived from food. The production of ATP, the key energy source in cells, by the mitochondria results in the production of these reactive oxygen molecules. Free radicals like reactive oxygen are formed due to loss of an electron which they steal from another molecule, and which makes them unstable and able to damage other molecules. Severe reduction of mitochondrial function in worms shortens lifespan significantly, and a prime candidate has been damage caused by reactive oxygen. Certain dwarf mice live almost twice as long and this is due to reduction in damage to the mitochondria in their brains. Many long-lived mutants are resistant to oxidative stress, and species of mammals that live longer tend to have cells that, when tested in culture, are more oxidative stress resistant. Large animals produce reactive oxygen at a slower rate.
Even single-cell organisms like bacteria and yeast age. The critical requirement for ageing in unicellular organisms is that a parent cell, when it divides, provides a smaller, and essentially younger, offspring cell. This occurs in yeast and the simple bacterium E. coli, and they share having a visibly asymmetric division and an identifiable juvenile phase. E. coli divides down the middle, giving each daughter cell one newly regenerated tip. But the cell’s other tip is passed down from its mother, or grandmother, or some older ancestor. The cell that inherits the old tip exhibits a diminished growth rate, decreased offspring production, and an increased incidence of death. Thus the two apparently identical cells produced during cell division are in fact functionally asymmetric; the old cell should be considered an ageing parent repeatedly producing rejuvenated offspring. Asymmetric division may be a way for the cells to get rid of damage by dumping it into the older cell at division.
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.