How do planarians ensure their immortality
Happiness is not in telomerase
Kirill Stasevich, Compulenta
Planarians are known for their fantastic regeneration abilities, but apparently not all of them ensure the immortality of their stem cells by maintaining a constant length of telomeric sections of chromosomes.
The restoration of damaged tissues occurs, as is known, at the expense of stem cells. Some of them turn into specialized cells and take the place of the dead, and some continue to remain immortal stem cells, which are ready to make up for losses if necessary. But immortality is rather conditional: even stem cells age and fail over time. And then there is no one to restore damaged, aged tissues. The most obvious example is age—related skin changes, which in many ways occur precisely because of the depletion of the reserves of epithelial stem cells.
If we could find a way to keep stem cells young, it would mean prolonging the life of the whole organism. Therefore, researchers do not get tired of looking for a solution to this problem and sometimes turn to various immortal organisms found in the wild for clues. A special place among them is occupied by planarians, flatworms with incredible regeneration abilities: they can rebuild any tissue and organ from scratch. It is also known that planarians use both sexual reproduction and asexual reproduction; in the latter case, the worm simply divides in two. Researchers have long suspected that the solution to the regenerative abilities of planarians should be sought in their special use of the enzyme telomerase or its analogue. It is believed that aging and cell death are associated with shortening of the end sections of chromosomes — telomeres, which decrease with each cell division. But there is an enzyme telomerase that completes shortened telomeres. The problem is that usually this enzyme is active only in the early stages of development: if it had been working for us all our lives, the problem of aging might not have been so acute.
Researchers from the University of Nottingham (UK) have suggested that planarians somehow manage to maintain telomerase activity in stem cells. Scientists have identified the corresponding gene encoding the human telomerase analog in planarians: if this gene was turned off, the telomeres of chromosomes began to shorten. But the main result of the experiments was that it was possible to detect a sharp increase in the activity of this gene during asexual reproduction of worms. When the planaria was divided into two, the enzyme tried to keep the stem cells in a working, "immortal" state as much as possible — so that after division they would finish building tissues and organs. At the same time, nothing like this was observed in sexually reproducing worms.
The results of the experiments are published by scientists in the journal PNAS (Tan et al., Telomere maintenance and telomerase activity are differently regulated in asexual and sexual worms).
This means that a new planaria formed as a result of asexual reproduction receives updated stem cells, an opportunity, so to speak, to start all over again. One could say that planarians fit into the well-known theory that only organisms with asexual reproduction can afford full-fledged immortality. But it should be remembered that those worms that reproduce sexually have the same ability to regenerate indefinitely. At the same time, no additional activity of telomerase genes could be detected in them.
That is why these results turn out to be highly ambivalent. On the one hand, they confirm the hypothesis that immortality can really be obtained by constantly lengthening the telomeres of chromosomes, and asexual planarians have convincingly shown this. On the other hand, it is clear that this is not the only way to immortality, since worms with sexual reproduction do without hyperactive telomerase. And, obviously, researchers have a lot of work to do to unravel this alternative method.
Prepared based on the materials of the University of Nottingham: Immortal worms defy aging.
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28.02.2012