What is the Hayflick limit? Hayflick limit and cellular basis of aging Elevation theory of aging

Introduction

The problem of aging of the body and prolongation of human life is one of the most important topics that interested almost any human civilization. The study of the mechanisms of aging of the human body remains an extremely urgent problem at the present time. Let us point out only one demographic indicator: by the beginning of the 21st century, in developed countries, the share of the population aged 65 years and over is 10-14%. According to available forecasts, this figure will double in 20 years. Population aging poses many unsolved problems for modern medicine, including the task of extending life in a state of active old age for a significant period of time. It is impossible to solve this grandiose problem without having an idea about the mechanisms of aging of the organism. We will focus only on the discussion of the mechanisms of cell aging, and those of them that are genetically determined, that is, inherent in the human body from birth to death.

Hayflick limit

In 1961, the American cytologist Leonard Hayflick, together with another scientist P. Moorhead, conducted experiments on the cultivation of human embryonic fibroblasts. These researchers placed individual cells in a nutrient medium (before incubation, the tissue was treated with trypsin, due to which the tissue dissociated into individual cells). In addition, L. Hayflick and P. Moorhead used a solution of amino acids, salts, and some other low molecular weight components as a nutrient medium.

Fibroblast division began in the tissue culture, and when the cell layer reached a certain size, it was divided in half, again treated with trypsin, and transferred to a new vessel. These passages continued until cell division ceased. Regularly this phenomenon occurred after 50 divisions. Cells that stopped dividing died after a while. The experiments of L. Hayflick and P. Moorhead were repeated many times in various laboratories in many countries of the world. In all cases, the result was the same: dividing cells (and not only fibroblasts, but also other somatic cells) stopped dividing after 50-60 passages. The critical number of somatic cell divisions is called the Hayflick limit. Interestingly, for somatic cells of different vertebrate species, the Hayflick limit turned out to be different and correlated with the lifespan of these organisms.

How to go beyond the Hayflick limit, or all the ways to extend life

Text: Nadezhda Markina

WHEN IT IS BEST IN ROUND NEMATODE WORMS. SCIENTISTS HAVE INCREASED THEIR LIFETIME TEN TIMES.

Demographic studies convincingly show that a person's life expectancy depends mainly on social factors - the standard of living and the state of medicine in the country where he lives. In Japan, for example, the average life expectancy over the past 20 years has increased to 82.15 years, and in the Kingdom of Swaziland it has also increased to 32.3. Therefore, it is difficult to calculate the biological “exploitation life” of a person, especially since it hurts

Most older people die of disease, not old age. Most, but not all. In the 19th century, scientists discovered a law that bears the names of Gompertz and Makeham and describes the dependence of mortality on age. At first, with increasing age, mortality increases exponentially. It seems clear that more 70-year-olds die than 60-year-olds, and more 80-year-olds die than 70-year-olds. But there is one mystery in the curve describing the law - after the turn of 90 years, it reaches a plateau. This means that if a person steps over

(A girl born today can live an average of 71 years. At the beginning of the 21st century, this figure was 68 years. Men still live less than women - by an average of 5 years. Highest Duration Rates life in Japan: 86 years for women and 79 years for men.)

this age, then the probability of death - at 90, at 100 or more years for him is approximately the same. Scientists cannot explain this phenomenon of centenarians. Most likely, the lucky ones who managed to avoid senile diseases reach the plateau. And we can also assume that the aging process at this advanced age, as it were, stops. However, aging asks researchers even more mysteries than longevity. This is evidenced primarily by the sheer number of theories of aging.

Aging is... ...a program

This postulate underlies the theory one of the main experts on aging in Russia, Vladimir Skulachev. He introduced the concept of "phenoptosis" - the programmed death of an organism, by analogy with apoptosis, the programmed death of a cell. It would seem, why do we need a program for death? Then, that it is beneficial to the population and species. According to Skulachev, the "samurai law of biology" operates in nature, which says: "It is better to die than to make a mistake." This means that an organism that is no longer needed. But since aging is a program, Vladimir Skulachev believes, it means “it can be canceled.” In support of his theory, he gives examples of non-aging organisms in nature, in which death occurs without aging.

Other Evolutionary Scientists The theory of aging emphasizes that the body makes a choice between repair and reproduction. Repairing cells and tissues requires a lot of energy - it's cheaper to multiply quickly.

...damage accumulation

Since with age the body begins work worse, it means that something is spoiling in it. The question is what exactly. Some experts consider the most important thing is that proteins deteriorate. For example, in collagen molecules, which is about a third of all structural proteins in the body, transverse “bridges” are formed between long spiral threads, which sew the threads together, as a result, the tissues lose their elasticity. Mitochondria deteriorate at the cell level

– cellular energy substations. This can lead to the fact that the cell embarks on the path of programmed death. Telomeres are DNA regions at the ends of chromosomes. They consist of a series of repeating sequences of nucleotides, and in all vertebrates these repeats have the same structure (TTA GGG). Telomeres shorten with each cell division and thus serve as a counter to the number cell divisions. The counter works because the DNA enzyme - polymerase, which duplicates DNA during cell division, cannot read information from its end, so each

the next copy of DNA becomes shorter than the previous one. According to Harvard's David Sinclair, sirtuin proteins play a key role in the mechanisms of gene regulation. These are enzymes involved in the process of packaging the DNA molecule into a protein shell in the cell nucleus in the form of chromatin. In this form, the genes are inactive. In order for genetic information to be considered from them, they must be unpacked. Sirtuins prevent the unpacking of genes that should not work in this place and at the moment. Sirtuins play the role of guards: they make sure that the silent genes are silent and do not take it into their heads to appear where they should not. But in addition to regulation, they are also involved in the repair of damaged DNA. The combination of two positions - a traffic controller and a repairman - is not good for the cage. With age, DNA damage accumulates, sirtuins are overloaded with repairs and can no longer cope with gene regulation. As the body ages, DNA damage increases, and sirtuins have to rush to repair more often. If the traffic cop is constantly absent from his post to fix cars instead of directing traffic, it will not end well. Gene regulation goes wrong. Genes unpacked without supervision can no longer be packed and silenced.

Giant tortoises (Megalochelys gigantea).

Live up to 150 years, retain the ability

to reproduction. They die because they

the shell becomes too heavy.

Atlantic salmon (Salmo salar).

Usually rapidly aging "according to the program

me" - immediately after spawning, and its decomposition

the remaining remains attract crustaceans, which

rye serve as food for salmon fry.

He "sacrifices himself".

Wandering albatrosses (Diomedea

exulans). Live an average of 50 years

as they age, they lay eggs. And then

die, suddenly, by unknown

reason.

During the work of mitochondria, deadly compounds are formed in them - reactive forms of nitrogen and oxygen. These are free radicals that have an unpaired electron. They are highly reactive and attack the first molecule that comes across indiscriminately, be it DNA or not. lok. Of course, after such violence, the molecules become inadequate and do not work properly.

...damage to the genes

Finally, genetic damage appears in old age. Once an organism has stopped reproducing, it accumulates harmful mutations. There is no longer a risk of passing them on to offspring, which means that you can “spoil” as much as you like. Harmful mutations can lead to both impaired protein synthesis and cancer, for example. To the genetic factors of aging, many refer to still mysterious ways. Violent elements are short sequences that move along the DNA molecule and affect the work of genes. There are more of them with age. And there are mutations that directly cause premature aging- progeria or, conversely, "eternal youth" .... razregulation

About ten years ago, American scientists found out why yeast is aging - their gene regulation mechanism breaks down. A new study has shown: in mammals, everything is exactly the same. This reason is universal, scientists say. This means that the causes of aging may not be genetic, but epigenetic, that is, lying next to the genes.

... damage to the "packaging" of DNA

In the cell nucleus, the DNA molecule is wound around histone proteins. These proteins can change, which determines the packing density. With age, the chromatin in the nucleus becomes looser, and this leads to the fact that unnecessary and harmful genes begin to work. The packaging is tight - the genes do not work, the packaging

loose - genes work.

...oxidation by free radicals

One of the most popular theories of aging is free radical theory. Its author Danchen Harman suggested in 1956 that we age because our molecules are exposed to the action of a powerful antioxidant defense system acting out of the mitochondria. But with age, it weakens, due to which the damage caused by free radicals becomes more numerous.

The roots of the evolutionary approach to aging lie in the work of a German biologist

August Weismann.

He was the first to suggest that aging occurs according to evolutionary

a program that removes old and unnecessary individuals from the population.

Weissmann considered the limited ability of cells to be the key to this.

to division.

They die after about 50 divisions and show signs of aging as they approach this limit.

This boundary has been found in cultures of all fully differentiated cells, both in humans and in other multicellular organisms. The maximum number of divisions varies depending on the type of cell and varies even more depending on the organism. For most human cells, the Hayflick limit is 52 divisions.

The Hayflick boundary is associated with a reduction in the size of telomeres, the stretches of DNA at the ends of chromosomes. If the cell does not have active telomerase, as most somatic cells do, the size of the telomeres decreases with each cell division because DNA polymerase is unable to replicate the ends of the DNA molecule. Nevertheless, due to this phenomenon, telomeres should shorten very slowly - by several (3-6) nucleotides per cell cycle, that is, for the number of divisions corresponding to the Hayflick limit, they will be shortened by only 150-300 nucleotides. Currently, an epigenetic theory of aging has been proposed, which explains telomere erosion primarily by the activity of cellular recombinases that are activated in response to DNA damage, mainly caused by age-related derepression of mobile genome elements. When, after a certain number of divisions, telomeres disappear completely, the cell freezes at a certain stage cell cycle or launches a program of apoptosis - a phenomenon of gradual cell destruction discovered in the second half of the 20th century, manifested in a decrease in cell size and minimization of the amount of a substance entering the intercellular space after its destruction.

Notes

Now we take this as a very common thing, but it is important to remember that just two centuries ago, human life expectancy was much less. It is widely believed that global life expectancy in 1900 was only 31 years. Thanks to the rapid development of medical knowledge in the 20th century, as well as the globalization of such knowledge across vast areas of the world, life expectancy worldwide has increased to about 72 years in 2014.

This means that during the hundreds of thousands of years it evolved as a species, it probably had a lifespan of no more than 25-30 years. You can compare this to chimps, which average 40-50 years in the wild and 50-60 years in captivity, or gorillas, which live about 40 years.

Considering how closely related we are to the great apes—sharing roughly 99% of the same as chimpanzees and gorillas—one can understand our rather impressive modern lifespan.

Although the average life expectancy around the globe has steadily increased over the past century, there is the question of whether there is a limit to human life, or whether, due to the constant progress in medicine, the average life expectancy will increase from 72 to 100 years.

Why do humans live so long compared to most other species?
As mentioned above, the exact mechanism by which a creature's lifespan is determined is hotly debated, but some of the strongest contenders for an explanation include total energy expenditure and an upper limit on the number of cell division cycles.

Energy consumption
Compared to most other species, humans and apes take a long time to reach maturity. For example, newborn antelopes can run 90 minutes after birth, while humans often do not walk until they are 1 year old.

Some species of shrew, such as mammals and humans, live less than a year and often die within a few weeks of giving birth to their only offspring. On the other hand, people do not reach puberty for at least the first decade, and the average age of women who have given birth to their first child in countries around the world varies from 18 to 31 years.

All this suggests that other species develop, mature and reproduce much faster, and therefore require a much higher energy intake, because their energy expenditure is much higher. The shrews mentioned above eat almost their own weight of insects every day because their metabolism is incredibly fast and their heart beats over 600 times a minute!

That is, other species develop and reproduce faster, reaching maturity within 1-2 years and breeding as often as possible during their viable breeding season.

Humans and other primates are the exact opposite of this, and their metabolic rate is relatively lower—about half that of other mammals. Cellular respiration and energy expenditure lead to faster depletion of the body and its systems, and a lower metabolic rate can extend life by decades.

Cell divisions
Another potential explanation is a built-in limit on the number of times a cell population can divide before becoming senescent, i.e. unable to divide further.

This limit is called the Hayflick limit, and for human cells it is approximately 52 cycles of division. This cell division expiration seems to hint at a natural cut-off point for human life, and holds true for other animals.

Species with notoriously short lifespans, such as mice (2-3 years), have a Hayflick limit of 15 divisions, while animals with even longer lifespans than humans have a higher Hayflick limit (e.g., sea turtles, with a life expectancy of more than two centuries) have a Hayflick limit of approximately 110.

As cells age, their telomeres, the stretches of DNA at the ends of chromosomes, decrease in length, which ultimately makes it impossible for further precise cell division. humans show signs of aging as they approach this limit and die after approximately 52 divisions.

In a number of other simple species, a gene has been found that effectively limits lifespan by activating other genes that control everything from transcription and protein production to reproductive triggers. It was found that when this single gene mutated in certain earthworms, their lifespan could double.

Hayflick limit. The average cell divides about 50-70 times before it dies. As the cell divides, the telomeres at the end of the chromosome get smaller.
© CC BY-SA 4.0, Azmistowski17

This gene appears to be an early precursor of a gene that controls insulin production in humans, which may also work as a control mechanism to inhibit and activate other genes. These discoveries are exciting because they may hint at the underlying genetic blueprint for an organism's life. For researchers seeking the "fountain of youth" or "immortality", these frontiers of research are of particular interest.

Exceptions to the rule
While humans have the potential to live for a century or more, we are by no means the longest-lived organism on the planet. Giant tortoises found in the Galapagos Islands are known to live for over 150 years, while the oldest specimen of the Greenland shark is over 400 years old. As for invertebrates, there are some types of molluscs that can actually live for more than five centuries!

Yes, it's quite remarkable that human life expectancy has more than doubled in just one century, but based on what we know so far, there's an average limit to how long we can live if we don't find a way to genetically extend life. .

As cells and tissues age and accumulate more errors in their genetic code, the body begins to break down, disease becomes more likely, and the ability to heal becomes more difficult. You need to take it easy, because as we all know, life is beautiful and unpredictable, so it's best to live while we have such an opportunity!

For a year, Hayflick observed how human cells dividing in cell culture die after about 50 divisions and show signs of aging as they approach this limit.

This boundary was found in cultures of all fully differentiated cells of both humans and other multicellular organisms. The maximum number of cell divisions differs depending on its type and differs even more depending on the organism to which this cell belongs. For most human cells, the Hayflick limit is 52 divisions.

The Hayflick boundary is associated with a reduction in the size of telomeres, the stretches of DNA at the ends of chromosomes. As you know, the DNA molecule is capable of replication before each cell division. At the same time, the telomeres at the ends of it are shortened after each cell division. Telomeres shorten very slowly - by several (3-6) nucleotides per cell cycle, that is, for the number of divisions corresponding to the Hayflick limit, they will shorten by only 150-300 nucleotides. Thus, the shorter the “telomeric tail” of DNA, the more divisions it has gone through, which means that the older the cell.

In the cell there is an enzyme telomerase, the activity of which can ensure the elongation of telomeres, while lengthening the life of the cell. Cells in which telomerase functions (sex, cancer cells) are immortal. In ordinary (somatic) cells, of which the body mainly consists, telomerase "does not work", therefore, telomeres are shortened with each cell division, which ultimately leads to its death within the Hayflick limit, because another enzyme is DNA polymerase - unable to replicate the ends of the DNA molecule.

Currently, an epigenetic theory of aging has been proposed, which explains telomere erosion primarily by the activity of cellular recombinases that are activated in response to DNA damage, caused mainly by age-related depression of mobile genome elements. When, after a certain number of divisions, telomeres disappear completely, the cell freezes at a certain stage of the cell cycle or launches a program of apoptosis, a phenomenon of smooth cell destruction discovered in the second half of the 20th century, which manifests itself in a decrease in cell size and minimization of the amount of substance entering the intercellular space after its destruction.

Principle of experiment

In principle, the experiment conducted by Leonard Hayflick in collaboration with Paul Moorehead was quite simple: equal parts of normal male and female fibroblasts were mixed, differing in the number of cell divisions passed (male - 40 divisions, female - 10 divisions) so that fibroblasts could be distinguished from each other in the future. In parallel, a control was placed with 40-day-old male fibroblasts. When the control unmixed population of male cells stopped dividing, the mixed experimental culture contained only female cells, because all male cells had already died. Based on this, Hayflick concluded that normal cells have a limited ability to divide, unlike cancer cells, which are immortal. So it was suggested that the so-called "mitotic clock" is inside every cell, based on the following observations:

Normal human fetal fibroblasts in culture are only capable of doubling the population a limited number of times;
Cells that have undergone cryogenic treatment "remember" how many times they divided before freezing.

The biological meaning of the phenomenon

At present, the point of view that links the Hayflick limit with the manifestation of the mechanism of suppression of tumor formation that has arisen in multicellular organisms dominates. In other words, tumor suppressor mechanisms, such as replicative senescence and apoptosis, are undeniably useful in early ontogeny and maturity, but they also cause aging by the way - they limit lifespan as a result of the accumulation of dysfunctional senescent cells or excessive death of functional ones.

Notes

Hayflick L., Moorhead P.S. //Exp. Cell Res., 1961, v. 253, p. 585-621.
Galitsky V.A. (2009). “The epigenetic nature of aging” (PDF). Cytology. 51 : 388-397.
L. Hayflick, P. S. Moorhead. The serial cultivation of human diploid cell strains // Experimental Cell Research. - 1961-12-01. - T. 25. - pp. 585–621. - ISSN 0014-4827.
J. W. Shay, W. E. Wright. Hayflick, his limit, and cellular ageing // Nature Reviews. Molecular Cell Biology. - 2000-10-01. - Vol. 1, no. 1 . - pp. 72–76. -

What is the Hayflick limit? Hayflick limit and cellular basis of aging Elevation theory of aging

Introduction

Hayflick limit

Notes

see also

See what the "Hayflick Limit" is in other dictionaries:

Principle of experiment

The biological meaning of the phenomenon

see also

Notes