Size doesn't matter
Telomere length poorly predicted the life span of vertebrates
Therefore, instead of it, Spanish scientists suggest using another indicator – the rate of telomere loss
Polina Loseva, "The Attic"
Long telomeres – the end sections of DNA – are considered the key to a long human life. But not all animals have this: the telomeres of mice are 3-5 times longer than human ones, but they rarely manage to live longer than 2-3 years. Scientists from Madrid and Barcelona measured telomeres in the blood cells of different inhabitants of the Madrid Zoo and finally confirmed that their absolute length correlates very poorly with life expectancy. Apparently, longevity is determined not by the initial length of telomeres, but by the speed of their shortening.
The telomeric theory of aging is one of the simplest ways to explain what happens to the body over time. At the ends of each chromosome there are "meaningless" sections, repeats of telomeric DNA, which become shorter with each cell division. Accordingly, the longer an organism lives, the more its cells have to multiply, and the shorter their telomeric "lifeline" becomes. When the terminal "plugs" on chromosomes reach a critical length, it becomes dangerous to multiply – with the next divisions, the ends will continue to shorten, but not "meaningless" telomeres, but "meaningful" genes will suffer. Therefore, short telomeres become a target for the DNA repair system, and it, in turn, inhibits cell division. Replicative aging occurs – the cell passes into the category of "old", and gradually ceases to perform most of its functions in the tissue.
Therefore, telomere length is often used to predict life expectancy, and in humans it sometimes really allows you to do this. But if we compare different animal species, then telomeres no longer allow us to predict anything. In their article in the Proceedings of the National Academy of Sciences (Whittemore et al., Telomere shortening rate predicts species life span), Spanish scientists present the results of measurements of telomeres in different vertebrate species from the Madrid Zoo: dolphin, goat, deer, flamingo, vulture, seagull, elephant, as well as a laboratory mouse (she was not from the zoo).
The variation in length turned out to be significant: from 10.4 kb (that is, thousands of nucleotide pairs) in a goat to 90.7 kb in a bottlenose dolphin. But the researchers could not fit these figures into a model that would link the length of telomeres with life expectancy: a goat on average lives the same number of years as a dolphin, and a person with his 15 kb on average lives 40 times longer than a mouse with 50 kb.
Statistics on the length of telomeres, the rate of their shortening and the lifespan of various vertebrates (Whittemore et al.).
Probably, the lack of a direct connection is caused by the fact that the length of telomeres varies according to complex laws and under the influence of many factors. On the one hand, telomeric DNA suffers from oxidative stress, which can cause additional loss of its sections. On the other hand, in many cells of the body – as a rule, in stem and dividing cells – the enzyme telomerase works, superimposing disappearing ends. Therefore, the final length of telomeres depends on the ratio of these two factors: loss of sites and completion.
The researchers also tracked how the length of telomeres changes over time. To do this, they measured it in animals of different ages. It turned out that the rate of telomere loss is not related to the absolute length: here the record holders were a mouse (7000 kb per year) and a person (the authors cite the figure 71 kb / year, in other works you can find the figures 20-45 kb / year). And this value was used as a good predictor for life expectancy.
Interestingly, the rate of DNA shortening, which is cited by Spanish scientists, does not lead to a complete loss of telomeres. If death occurred at the moment when cells finally lose the ability to divide, people, according to the authors of the article, would live up to 211 years, and elephants – up to 333. Nevertheless, on average, the studied species live to lose 25% of their length, and the maximum lifespan corresponds to a shortening of telomeres by about half.
At the same time, cells stop dividing when the telomere length is about 2 kb (at least in humans) – that is, 13% of the original length. In their work, scientists do not discuss this contradiction, but several explanations can be offered for it: probably, the rate of telomere shortening is not constant throughout life, and it becomes higher with age (it is known, for example, that it can change in humans). In addition, one can imagine that short telomeres "attract" the attention of the repair system, and more serious DNA damage remains without repair, which causes premature aging of cells and death of the body.
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