Rapamycin and short telomeres
Rapamycin does not prolong, but shortens the life of mice with short telomeres
Polina Loseva, "Elements"
Rapamycin is one of the most promising candidates for "old age pills". Spanish biologists have tested how it will act on one of the model objects for the study of accelerated aging – mice with short telomeres – and found that it does not prolong their lives, as expected, but on the contrary, shortens. This is another story that the causes of aging are closely interrelated, and by acting on one of them, you can inadvertently strengthen the position of the other.
Fig. 1. (from the press release of Centro Nacional de Investigaciones Oncológicas The CNIO finds that rapamycin has harmful effects when telomeres are short) – WM.
Aging is a set of processes that act on the body simultaneously and gradually bring its death closer. Among them, for example, destruction of macromolecules, excessive activation of immunity, accumulation of improperly folded proteins, rearrangement of intercellular matter and many others. It is impossible to single out the main one among them – neither speculatively nor experimentally. Despite the fact that each research group designates some cause of aging as a key one (otherwise the angle from which the authors look at the problem is unclear), hardly any of the modern gerontologists consider "their" cause to be the only one.
But if there are several reasons, then there should be several ways to deal with them – at least one for each. Moreover, if the causes work together, then the therapy against aging should be composite in order to cut down each of the roots of the problem. Or will it still be possible to find one approach that will affect all causes at once? The latter option is supported by the fact that some causes of aging are still interrelated with each other.
An example is one of the most famous aging processes that occurs in most cells of the body – shortening of telomeres. These are the end sections of chromosomes, which consist of "meaningless" repeats and perform mainly a mechanical function. They serve as a kind of seal on DNA, which it is not a pity to demolish over time. Before each cell division, DNA doubles, and chromosomes shorten – part of telomeric repeats is lost. Therefore, if nothing is done (and some cells are able to build up the ends of chromosomes back), telomeres gradually wear out. When there is little left of them, and they are close to disappearing altogether, the cell stops dividing. For many cells and tissues in which they are located, this is a heavy loss – if the neighbors of this cell die, it will not be able to produce descendants to fill the void.
At the same time, telomeres can be shortened for other reasons, outside of cell division. One of these reasons may be another culprit of aging – oxidative stress. When mitochondria fail to do their job for one reason or another (for example, there are too few of them, or they lack oxygen), free radicals accumulate in them – chemically active molecules that react with different cellular polymers. They can disrupt the mitochondria, and if they leak from there into the cytoplasm of the cell, they damage proteins and lipids, which accelerates cell aging. If there are a lot of them, then some get to the core. There they oxidize DNA molecules, and telomeres get the most (see W. Qian et al., 2019. Chemoptogenetic damage to mitochondria causes rapid telomere dysfunction). Under severe oxidative stress, DNA repair systems cannot cope with repairing telomeres and cut off damaged areas from them (see E. Fouqerel et al., 2019. Targeted and persistent 8-oxoguanine base damage at telomeres promotes telomere loss and crisis). Thus, under the influence of oxidative stress, telomeres become shorter.
If such relationships are established for other causes of aging, then one can imagine a situation where one drug will be enough to stop them all at once. A group of scientists from the Spanish National Cancer Research Center (Spanish National Cancer Center) suggested that rapamycin could be such a drug. This substance is known in medicine as an antibiotic and an immunosuppressor, but gerontologists know it as an mTOR blocker. It is a protein complex that spurs cell growth and development – storage of substances, protein synthesis and active energy absorption – thereby accelerating cell wear. Rapamycin has repeatedly proven its ability to slow down cell aging and prolong the life of model organisms, so who knows, maybe it could solve the problem of telomere shortening?
As a model object that seriously suffers from the problem of telomere shortening, the researchers chose mice with a telomerase defect – this is the very enzyme that cells can use to build up the ends of chromosomes. Despite the fact that mice have telomeres several times longer than ours, in the absence of telomerase, they quickly shorten. This becomes especially noticeable in subsequent generations, because descendants inherit shorter and shorter telomeres from their parents. The second generation of such mutant mice lives for about a year instead of the required 2-3 years. The authors of the work suggested that rapamycin could cope with this problem of accelerated aging.
However, the results of the first experiment turned out to be strictly opposite (Fig. 2). The researchers began observing normal and telomerase-deprived mice at the age of 3 months. At the same time, within each group, some of the animals were fed with ordinary food, and rapamycin was added to it for others. And if in ordinary mice rapamycin, as in all previous studies, shifted the survival curve to the right (that is, extended life), then in mice deprived of telomerase the effect was the opposite: under the influence of rapamycin they began to live less.
Fig. 2. Survival curves of mice in the experiment. Dark gray – control group, light gray – wild-type mice under the action of rapamycin, dark green – mice without telomerase, light green - mice without telomerase under the action of rapamycin. The change in average life expectancy is indicated next to the arrows. Image from the discussed article in Nature Communications.
The concentration of the drug in the blood plasma of the mice was approximately the same, that is, it was not how much they ate it or how it was absorbed into the body. Then the authors of the work suggested that the increased mortality of mice with short telomeres under the action of rapamycin may be associated with an increase in the number of tumors. Usually, short telomeres prevent the tumor transformation of a cell – the less it has to divide, the more difficult it is to form a tumor. But rapamycin works as an immunosuppressor, that is, it suppresses the body's response to uncontrolled cell growth. It could turn out that rapamycin counteracts the effect of short telomeres and increases tumor growth. But this is not so: after the death of none of the mice with short telomeres, scientists found no traces of carcinogenesis.
Apparently, something inside the cells of animals with short telomeres prevented rapamycin from working. This is supported by another observation made by the researchers: mice with telomerase deficiency did not lose weight under the action of rapamycin. At the same time, in ordinary animals, this happens invariably, because rapamycin blocks the growth of adipose tissue.
One of the immediate effects that mTOR has on cells is an increase in protein synthesis. Therefore, its activity can be assessed by the level of phosphorylation of ribosomal protein S6: the higher it is, the more intense the synthesis. In normal cells, rapamycin reduces the phosphorylation of S6, inhibiting the work of ribosomes. The researchers measured the concentration of phosphorylated S6 in mice that had been on a regular or rapamycin diet for two months. It turned out that, unlike ordinary animals, rapamycin did not work in the liver of mice with short telomeres: the level of S6 phosphorylation, despite the introduction of the drug, remained the same. The same thing happened with other potential effects of rapamycin: in mutant mice, it did not increase the level of autophagy (self–digestion) in cells and did not reduce the number of mitochondria - that is, it did not affect the intensity of metabolism in cells. This means that rapamycin has not fulfilled its main function – it has not blocked the mTOR signaling pathway.
To find out whether the drug really does not work, the authors of the work checked what happens in the liver cells of mice with short telomeres two hours after its administration. It turned out that in two hours rapamycin reduces the amount of phosphorylated S6 – similar to what happens in ordinary animals. Thus, the problem was not in rapamycin itself. Since he can do his job in cages, it may be that his work is simply not enough.
The researchers suggested that the activity of mTOR in the cells of mice with short telomeres is itself so high that rapamycin cannot reduce it. Indeed, when they compared the amount of phosphorylated S6 in normal and mutant animals, they noticed that it was consistently higher in mice with short telomeres. Then they sequenced the RNA in liver cells and found that animals with short telomeres have higher expression of genes that are associated with different metabolic processes – glucose breakdown, division and growth, protein and fat synthesis – and all of them are under the control of mTOR.
But if the mTOR pathway is so active in the cells of mice with short telomeres, and its blocker rapamycin shortens their lives, then mTOR can serve as a compensatory mechanism and mitigate the effects of a lack of telomerase. To test this hypothesis, the authors of the work created double knockout animals in which not only telomerase, but also S6 kinase (S6K), a protein responsible for the phosphorylation of S6, did not work. After measuring their life expectancy, the researchers noticed the following pattern (Fig. 3). In animals with working telomerase, the S6K knockout prolongs life, since it acts similarly to rapamycin, blocking the effects of the mTOR pathway. In the first two generations of animals without telomerase, there is practically no difference in life length. But in the third generation of mice with short telomeres, the S6K knockout, on the contrary, shortens life. Thus, with long telomeres, the mTOR pathway can be dispensed with, and its blockade works to prolong life. But with short telomeres, it becomes critical for survival.
Fig. 3. Survival graph of mice with different sets of mutations. In each pair, a darker shade indicates an animal with a working S6 kinase, a light one indicates a knockout by S6K. Gray – mice with working telomerase, red – the first generation of mice without telomerase, blue – the second, green – the third. The dotted line indicates the median life expectancy: the time to which half of the population lives. The red arrow indicates a sharp decrease in life expectancy in the third generation of mice without telomerase during S6K knockout. Image from the discussed article in Nature Communications.
Thus, the idea of using one weapon against several causes of aging at the same time was defeated – at least, the weapon chosen by the researchers is not suitable for it. By blocking mTOR, rapamycin thereby prevents cells with short telomeres from surviving.
In the article under discussion, the researchers worked only with mice, however, people also have diseases that are associated with a strong shortening of telomeres. This is, for example, congenital dyskeratosis (dyskeratosis congenita) – telomerase deficiency, which initially affects skin pigmentation, and then disrupts the work of the bone marrow, which leads to the death of patients. Rapamycin and its analogues probably won't be able to help such people, as well as mice with telomerase knockout.
At the same time, it is still unclear to what extent mTOR blockers will be applicable for the elderly. It is known that with age, the average length of a person's telomeres decreases, but will this become an obstacle to prolonging life with the help of rapamycin and similar drugs? Or do you have to somehow act in turn with rapamycin and telomerase activators to achieve the desired effect? Anyway, it is already clear that there will be no simple answer to this question.
The story of rapamycin and telomeres is an illustrative example of the problem that modern science of aging is facing. Every time gerontologists discover some process that aggravates the aging of the body, and come up with a way to stop this process, it invariably reveals a reverse "positive" side. For example, the reduction of telomeres can be considered a protection against cancer. Oxidative stress is a stimulus that prompts the cell to mobilize internal reserves to combat adverse conditions. And mTOR, in turn, saves cells in conditions of too short telomeres. Therefore, once and for all, we will hardly be able to declare any of the causes of aging as the main enemy and unleash a war against it. Instead of a decisive offensive, dodgy diplomacy will be required – weighing risks, alternating medications, searching for compromises that could prolong the life of the body without making it vulnerable to another enemy.
Source: Ferrara-Romeo et al., The mTOR pathway is necessary for survival of mice with short telomeres // Nature Communications. 2020.
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