How cells age
Cells: between life and death
"Counterclockwise. What is aging and how to deal with it."
A chapter from Polina Loseva's book has been published
on the website "Elements".
The longer a cell lives and the more chemical reactions are bubbling in it, the more it is filled with molecular debris and the faster it ages. But the process of cellular aging is not limited to the accumulation of waste, it is a separate phase of life, just as a person's old age is not only a thick medical record, but also an experience, a specific type of thinking and a changing character.
Cells, it would seem, are quite independent organisms in miniature. But every time I say that a cell "decides to multiply" or "suffers from stress," from the point of view of biology, of course, I'm lying: there are no feelings, and even more so there is no free will in the cell. It rather resembles a matchbox boat, which a spring stream carries down the street downhill. And the only way to resist the flow is to cling to some twig or bury your nose in the curb of the sidewalk. Such a boat will stay firmly in place, but, alas, it will not sail far away.
Aging can be seen as such a safe haven, where the cell hides from the flow of debris that carries it towards turning into a tumor or death. But the more ships stranded in the middle of the stream, the smaller the fleet becomes and the more difficult it is for it to maneuver among many obstacles. And most often, old cells turn out to be an unprofitable acquisition for tissue, complicating the lives of all their neighbors.
Old or infirm
Before moving on, let's define the terms. If you open some aggregator of scientific literature (for example, Google Scholar or Pubmed) and look for the phrase "aged cell" (literally "old cell"), you will get surprisingly few results. But this does not mean that no one studies the aging of cells, it's just that in scientific circles it is customary to use another term – "senescent cell". There is no special Russian analogue for it, but the words "decrepit" or "infirm" would be most suitable. This makes it possible to separate the chronological age of cells and their real old age.
There are a lot of cells in our body that are as old as the rest of the body. Most often, nerve cells are cited as an example, which practically do not multiply after a person is born. And recently it turned out that some cells of the liver, pancreas and vascular walls also accompany the body (at least in mice) from birth. In an elderly person, these cells are also many years old, and they could be called "aged cells". However, this fact only says that they stopped sharing a long time ago, but does not characterize their performance in any way. And the term "senescent cell" means only such a cell whose functions are disrupted and which cannot be considered a full-fledged member of society, regardless of how old it is.
In the Russian-language scientific literature, the term "senescent cell" is sometimes found, a complete tracing paper from English. However, more often they talk about "old" or "aging" cells, meaning, in general, the same thing. For the sake of euphony, I will use all these designations as synonyms, but I will be talking about decrepit, defective cells. And it's not about their chronological age: a cell can lose its working capacity at any time – whether it's 100 years old or just a few months.
Old and senescent cells: not to be confused!
Senescent cells also have little to do with the age of the organism as a whole. Despite the fact that there are more and more of them with age (for example, 4% of such cells were found in the adipose tissue of people at the age of 31, and more than 10% at the age of 71), it is not yet possible to draw unambiguous conclusions about the old age of the body by the number of old cells. This is due to the fact that sometimes tissues age at different rates: in the same person, you can find many senescent cells in the skin, and none among the immune system. This may be due to specific diseases: it is known that senescent cells accumulate in obesity, osteoarthritis, dementia and many other diseases. Therefore, their number may indicate the general health of the body rather than its chronological age.
Cellular destinies
The life of a cell, like a human, represents the stages of growing up, which smoothly flow into each other, and old age is only one of them.
The earliest stage – immediately after separation from the mother cell – can be conditionally considered childhood (although there is no such separate term in cell biology). In this phase, the cell grows, increasing in size and accumulating proteins: without enough energy and building materials, it will not be able to reproduce further or choose a specialization. Childhood lasts until the cell receives a signal that it's time to "learn". It can come from neighboring cells, intercellular matter, or come from outside (the messenger may be, for example, some hormone from the blood).
Differentiation, or specialization, is a complex process in which a cell, under the influence of external stimuli, chooses its future fate: whether it will become, say, a bone, liver or eye cell. To get a profession at the cellular level means to produce a sufficient amount of the necessary proteins. And already under the influence of these proteins, the cell takes on a new form, grows flagella or learns to conduct impulses, that is, acquires professional skills. When the cell is fully specialized, it becomes terminally differentiated. And maturity comes.
Maturity is the time during which a cell performs its basic duties, for example, contracts and relaxes, accumulates fats or secretes hormones in accordance with its profession. Mature cells, as a rule, do not divide, almost do not grow (except fat and other storage cells) and are not able to change their vocation. For most cells in the body, this is how most of life goes. At any given time, except perhaps the embryonic period, there are most of these cells in the body.
But there are others who don't spend their days at work. These are, for example, stem cells, they are engaged in reproduction.
Division is a difficult decision for a cell, which it takes under the influence of many factors. Growth factors come from blood or tissue fluid – signaling substances that cause it to divide. They are especially convincing if there is enough energy and food in the cell – then it begins to produce division-stimulating proteins. Neighboring cells, on the contrary, prohibit division if there is not enough space in the tissue – under their influence, blocking proteins are formed in the cell. The cell itself can forbid itself to reproduce if it has accumulated a lot of protein debris or mutations. If, taking into account all these factors, there are still more stimulants than blockers, the cell begins to double its DNA and organelles. After splitting in half, the concentration of stimulant proteins drops, but as soon as a new cell is fully formed, they can accumulate again and start another cycle of reproduction.
There are periods of rest in the life of a stem cell – a resting state. If the tissue does not need constant replenishment of cellular resources (such as an eye or a muscle), then the stem cells freeze at the beginning of the division cycle and wait for a signal from the outside. When there is a need for new cells, for example, when tissue is damaged, stem cells come out of a dormant state and begin to multiply.
The different phases of existence in cells, as in humans, almost do not overlap with each other. So, reproduction is poorly compatible with work, and if the cell is fully differentiated, it is usually not able to divide. Therefore, the choice: to remain stem or to go into differentiation is an important fork in the life of each cell. If a stem cell can still one day "get educated" and stop reproducing, then there is no turning back for a professional. Most differentiated cells are unable to return back to a resting or stem state. That is why regeneration is so bad for a person: the cellular reserve is gradually being depleted, and specialists have already forgotten how to multiply and are unable to replenish it.
The finale of cellular life is death, which can come in different guises. The modern classification of cell deaths includes a couple of dozen scenarios that Dante would envy. Among them there is melting, absorption, and slow eating, but in most cases the natural death of the cell occurs through apoptosis. It is also called programmed cell death or cell suicide. This is the reaction of a cell to an external signal (for example, from an immune cell) or to internal malfunctions: mutations in DNA, protein aggregates, leaky organelles. The suicide weapon is a group of caspase enzymes: they break down intracellular molecules, as a result of which the cell breaks up into many membrane bubbles. Apoptosis can be prevented in the early stages, but if caspases have already taken over, then the cell is doomed.
Old age is another phase of cellular life, from which there is no way back. But, unlike a human, a cell can grow old at almost any stage of its life, whether it is an exemplary family man (stem), a successful professional (differentiated), a notorious bandit (cancer) or a teenager (recently separated from the mother). This requires severe stress: it can be internal oxidative stress (accumulation of reactive oxygen species) or other circumstances – pressure and temperature drops or toxic substances (we will talk more about how they are related to each other in the chapter "Stress is to blame"). Under the influence of stress, the cell ceases to perform its former functions and benefit the cellular community.
Old age, like death, can overtake a cell at any stage of life.
It is quite difficult to catch the moment when a cell suddenly turns into an old one. This is a gradual process. But it is much easier to distinguish an old cell from a young one than to find a definition for an old person. Perhaps the fact is that scientists are not faced with the task of identifying every single old cell in the body. For each experiment, a general understanding of the picture is sufficient: are there many or few such cells and where they are located. Therefore, the criteria of old age can be purely functional: what can, what can't, how it looks, what it does.
Reproductive sunset
Unlike humans, senescent cells meet the reproductive criterion – they really do not reproduce. In the earliest works on cellular aging, the authors estimated the age of cells as follows: they planted them singly in Petri dishes, and after a while they calculated the size of colonies that grew out of each cell. And the number of descendants served as a measure of cellular youth.
This criterion is not true for all cells, nor for all people. Most cells in an adult organism are mature, terminally differentiated, and reproduction is the prerogative of stem cells and their immediate descendants. And many specialized cells will not divide even in culture, regardless of age and working capacity. But for stem cells, this criterion works and is widely used, since their main function in the body is to ensure replenishment of the cellular reserve at any time.
However, in some cases, differentiated cells acquire the ability to divide. It is in this way that amphibians regrow their lost limbs. But in mammals, the reproduction of a specialist cell can be the beginning of trouble: this most often means that the cell "abandons" its profession and turns into a tumor. To prevent this from happening, in aging cells – which accumulate mutations and risk becoming tumor – two proteins, p16 and p21, begin their work, which tightly block the division cycle. It is these proteins that are considered the most reliable distinguishing feature of senescent cells today. And it is they who do not allow cells not only to turn into a tumor, but also to participate in the regeneration process (I will tell you more about this in the chapter "Cancer is to blame").
Between life and death
Unlike human life, the aging of a cell does not entail its death. A cell can grow old early and spend the rest of its life in this state until the whole organism dies. Old age in this sense is also a career, it does not allow the cell to go into other states: to reproduce, change profession and even die. Old cells are resistant to the main mechanisms of cell death. In a sense, they are stuck on the border of two worlds: they are no longer fully alive, since they cannot reproduce, but they cannot die either.
Usually, cells use an apoptosis program to commit suicide. It is triggered in response to intracellular breakdowns: damage to the mitochondrial membrane or mutations in DNA. The trigger, that is, the main engine of apoptosis, is the protein p53. It is activated by proteins that "fall out" of leaky mitochondria, and it also receives a chemical "alarm signal" from the DNA repair system when it detects mutations. In turn, p53 stops cell division and starts the work of destructive caspase enzymes.
The cellular suicide system has many useful functions in the body. In addition to clearing space for new cells, it also allows you to destroy potentially dangerous cells that can then turn into tumor cells.
But if we have a senescent cell in front of us, then it is pointless to kill it, since it is still not capable of reproducing, and therefore cannot grow into a tumor. In addition, it is unprofitable for economic reasons. Apoptosis is an expensive process, each caspase molecule spends cellular energy on each reaction of destruction of another molecule. But if the cell has aged, then its mitochondria are probably damaged, and then there are not enough meager energy "stashes" even for household repairs, not to mention suicide.
Finally, it is simply dangerous to get rid of cells in an old organism: cell stocks are getting smaller, and if you also get rid of old cells, then an acute personnel crisis will arise in the tissues. Therefore, senescent cells produce a large number of anti–apoptotic proteins - blockers of apoptosis, and can break down as much as they like without risking death. Only immune cells can cope with them if they are mistaken for an alien enemy or cellular garbage.
Senile conservatism
The next sign of an old cell is the inability to learn new things. In other words, she cannot use her own genetic information.
All cells of the body contain identical (except for point differences) DNA molecules, which means that each of them has a complete set of genes. But after differentiation, the professional cell will need only a small part of the information. The bone cell does not need to produce visual pigments at all, and the neuron will never need insulin or hemoglobin. And just as a schoolboy, growing up, throws textbooks on the mezzanine, the cell gradually twists part of its DNA, forcing the genes it does not need to be "silent".
Methyltransferase enzymes are responsible for DNA folding: they put labels (methyl groups) on certain parts of the genes, which make these areas more sticky, that is, they actually work like scotch tape, adhesive tape. The labeled regions of DNA firmly stick together, and the cell will no longer be able to read information from them. The transition of genes from "active" to "silent" and vice versa is called epigenetic changes (in contrast to hereditary, genetic changes in the DNA sequence itself), and labels on DNA are epigenetic markers.
The main rearrangements of genes occur when a cell gets its profession and chooses which ones to "close". Therefore, there is much more untwisted DNA in the nucleus of a stem cell than in a terminally differentiated one. But even a mature professional cell continues to fold DNA little by little throughout its life, and sometimes senescent cells can be calculated simply by the amount of twisted DNA.
Epigenetic markers determine the degree of DNA twisting.
At the same time, there is another process – the unwinding of old tangles of DNA. As other useful books are gradually sent to the mezzanine after school textbooks, long-forgotten junk crawls out of old closets, takes up space on the table and dusts the whole apartment. So, with age, sections of DNA with hidden retrotransposons are unwound, which we talked about in the previous chapter. The mad xerox goes free and begins to multiply, damaging those genes that are still available to the cell.
Thus, cellular life is accompanied by large repackages in the nucleus: some sections of DNA are folded, others unfold. This is due to the fact that there are several types of methyltransferases in the cell: some are responsible for maintaining old epigenetic markers, others for attaching new ones. With age, the ratio of their activity changes: the former relax, and the latter, on the contrary, strengthen their positions. Therefore, it is possible to identify an old cell not only by its abilities and infirmities, but also by which parts of its DNA are active and which, on the contrary, are "silent" – that is, by a set of epigenetic markers.
Memory Loss
With age, DNA not only rearranges itself, but also becomes shorter. Each of our 46 chromosomes is a separate thread, along the edges of which there are special areas – telomeres. With each cell division, a small piece of telomeres is lost. This is due to the fact that polymerases – proteins that copy DNA – cannot begin to build a second chain from the very edge, they must first gain a foothold and take overclocking. Therefore, they sit on the "seed" RNA molecule, which later disappears, and the very tip of the molecule on which the RNA was attached remains unbroken. Single DNA chains in a cell do not live long: the antiviral defense system recognizes them, taking them for a piece of an alien genome, and destroys them. So the chromosome becomes shorter.
There is no big trouble in this, precisely because telomeres are located at the ends of chromosomes – a set of meaningless repeats that do not carry genetic information. They are needed just to lose them. But after a certain number of divisions, telomeres end, and real, "meaningful" genes are in danger of disappearing when copied. At this point, the cell stops multiplying, so as not to lose valuable information and not to breed mutant descendants, and becomes senescent.
This phenomenon, not yet knowing anything about the role of telomeres, was first described by Leonard Hayflick. He found that even actively reproducing cells lose this ability after about 50 rounds of division. This threshold value is called the Hayflick limit . That is, in 50 cycles of reproduction, telomeres become critically short and the cell ages. Therefore, the cellular age can also be estimated by the length of telomeres. The faster a cell stops multiplying, the shorter its telomeres were initially and, consequently, the older it was at the start of the experiment.
Telomere shortening is an unavoidable defect of DNA copying.
However, the loss of telomeres is not just an age–related deterioration of DNA. The cell has ways to cope with the shortening of chromosomes (we'll talk about this in more detail in the chapter "Cancer is to blame"), but most often it does not use them. The fact is that stopping division is another defense mechanism. In this way, the cell is protected from the same thing – from cancer. After all, if you give cells the opportunity to multiply indefinitely, then mutant cells will also divide along with healthy cells, and you will have to deal not with one tumor cell, but with an entire colony.
Granny's closet
The next sign of aging was to be the accumulation of intracellular debris. However, it is quite difficult to use it as a marker of old age. The problem is that most of the breakdowns in the cells are individual. "All happy families are similar to each other, each unhappy family is unhappy in its own way" - we know what "healthy", untouched cellular molecules should look like, but it is almost impossible to predict in which place which of them will break.
The only exception is that "aging pigments", for example, lipofuscin – a lump of cross-linked proteins. It looks like yellow-brown granules that are easy to see through a microscope. However, not all human cells can detect it. But its analogues are also found in animals quite far from us, for example, the roundworm Caenorhabditis elegans, which allows us to study aging at the cellular level in him.
In other cases, looking for specific types of breakdowns in cells is a thankless task. And researchers usually focus on indirect signs: the work of cellular systems for garbage collection. This can be, for example, autophagy, that is, self-eating. To digest large accumulations of proteins or whole organelles, the cell surrounds them with a membrane bubble – a vacuole – and injects digestive enzymes inside. So the cells are filled with transparent membrane bubbles with age. However, their presence only indicates that the cage was cleaning inside itself, and does not say anything about whether it is winning over the garbage or is ready to give up.
Another sign of cleaning is digestive enzymes. In many modern studies, cellular aging is determined by the activity of beta-galactosidase, an enzyme for the breakdown of carbohydrates that works in all cells and which becomes more and more with age. However, the beta-galactosidase test does not guarantee our success. Sometimes it gives a positive answer in cases where cells are not considered old by other signs: for example, in resting stem cells or in immortal (immortalized) laboratory cultures. Therefore, to be sure that it is a senescent cell in front of us, it is better to use several criteria at the same time: for example, a beta-galactosidase test and the accumulation of proteins p16 and p21, which stop division.
Harmful advice
Finally, like many elderly people, senescent cells like to give advice – that is, to secrete substances that affect others. Strictly speaking, this is typical for any cells of our body, but in senescent cells, the set of these substances is very special. It was called SASP 67 – an abbreviation that has no Russian analogues (senescence-associated secretory phenotype, literally "secretory phenotype associated with senescence").
This set includes several groups of proteins: growth factors (they signal the need for cells to multiply), metalloproteases (trigger the destruction of intercellular matter) and pro-inflammatory proteins (attract immune cells and spur their work). This set of tips seriously harms neighboring cells and tissue in general. The intercellular substance is destroyed, holes form in the tissue, cells begin to divide – and turn into tumors, and immune cells unleash a real war. They crawl into the tissue to destroy old cells, but in the process of fighting them, they release a lot of toxic substances. This leads to the fact that neighboring cells begin to feel uncomfortable (experience stress) and also age. An epidemic of old age is rolling through the fabric.
Senescent cell: dossier.
In the summer of 2018, an article was published in the journal Nature, the authors of which conducted an experiment on the transplantation of old age. They took ordinary mice, took their cells, aged them with radiation and planted these cells in other young mice. Just a week later, the condition of the recipient mice seriously deteriorated: their health became worse in many indicators, they began to run slower, and their muscles weakened. At the same time, their own senescent cells began to form rapidly in the body of mice – that is, the example of "decoy old men" literally turned out to be contagious. This effect occurred when only half a million senescent cells were transplanted – this is about 0.01% of all cells in the mouse body. Therefore, do not underestimate the danger of senescent cells: they quickly replenish their ranks.
Based on this, it seems that selective shooting of senescent cells could be the right way to combat aging. Such methods do exist, I will tell you more about them in detail in the third part of the book. The only problem is that the old cells in the tissue – like the elderly in human society – play their important role.
In small doses, their advice is quite useful to the body: as soon as some cells have ceased to perform their functions, then it is necessary to rebuild the tissue (destroy the intercellular substance), give way to the young (that is, start the division of new cells) and call on immune cells to clear the tissue of debris. Senescent cells are found even in embryos – in growing tissues they help to mark parts of the future organ, and then die under the onslaught of immune cells, as, for example, in the webs between the fingers of mammals. Therefore, it will not work to ban them once and for all, which means that we will have to look for some other ways to come to terms with their existence.
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