Morphological signs of cell aging. Cellular senescence (in vitro)

Today, the science called cytogerontology studies the reproduction and aging of cells. Scientists have been able to establish that there is not only a simple process of natural aging of cells with their subsequent death, but it can also be programmed at the genetic level.

This process is also called “apoptosis,” as described above. This program was laid down at the genetic level of each person and its main goal is to rid the body of excess cellular material that is no longer needed.

To date, scientists have been able to simultaneously present several theories about exactly how the aging process of cells in the body can occur.

Today, scientists are quite actively conducting various studies, during which they study the aging process of connective tissue cells of the human body. These cells are located throughout the body, and they take a direct and quite active part in almost all metabolic processes of the body that are important for human life.

There is a group of scientists who are almost one hundred percent sure that connective tissue cells or fibroblasts literally “force” all other systems, as well as the cells of the human body, to age. That is, simultaneous aging of the entire organism occurs at once.

Thanks to ongoing research, scientists were able to determine that mutations also play a certain role in the aging process of body cells. After all, as is known, mitochondria gradually accumulate in DNA, appearing precisely as a result of certain mutations.

In the process of cell destruction, a specific protein such as the p53 protein also plays an important role. For example, if, due to certain circumstances, tumor cells appear in the human body, apoptosis is instantly activated, that is, the process of their natural destruction.

It was also possible to establish exactly what role this process plays in cell aging, but to date scientists have not been able to fully establish this theory, so it is purely theoretical. Various studies are carried out on muscles, and each time scientists get completely contradictory results.

There is another important factor that has a direct impact on the aging process of cells in the human body - the degradation of lymphocyte cells, which occurs gradually. The studies were conducted on primates, and the results showed that if the daily diet contains a minimum amount of calories, then the aging of immune cells will naturally slow down.

At the same time, a substance that is part of the blood and manifests itself at the onset of inflammation can also slow down the process of their aging - this is a unique C reactive protein. Infection with dangerous oncoviruses can result in the onset of a rapid process of natural withering of the immune system; therefore, the body’s defense system begins to suffer.

The youth of cells, as well as the entire human body, is directly related to such a special substance as telomerase. This substance is one of the special enzymes that have the ability to add unique sections of DNA molecules that can be repeated. As a result of this process, cells can acquire such a feature as literally endless reproduction, after which they will not die, but will continue dividing further.

For example, a fairly high activity of this unique substance (telomerase) is observed in an embryonic cell. It is as a result of the gradual extinction of this enzyme that cellular aging will occur.

The last two or three years have literally been breakthrough years for gerontology. First, scientists found a way to extend the life of yeast, worms and even mice through fasting, then they found several genes that can extend active longevity. It was even possible to discover a connection between the development of the reproductive system, adipose tissue and life expectancy.

But the approach to the molecular and cellular basis of aging has not changed for several decades: the accumulation of mutations that inevitably arise during division, the gradual destruction of proteins and the depletion of “reserve” systems.

Martin Hetzer from the Salk Institute for Biological Research and his colleagues were able to significantly refine this understanding:

Behind the general words about wear and tear, there is at least a disruption in the functioning of nuclear pores, which ensure selective exchange of contents between the nucleus and the cell.

Despite the microscopic dimensions - from five to hundreds of micrometers, the cell itself includes several dozen more organelles, the main one among which is the nucleus, which ensures the regulation of all intracellular and even extracellular processes. Inside the nucleus, which can occupy up to 80% of the volume (in sperm), the most valuable thing is located - genetic information encrypted in the DNA sequence.

If it were not for the nuclear membrane, then the number of mutations and failures in reading the code would simply not allow the cell to live. But despite the double membrane surrounding the chromosomes,

The genetic apparatus is not in isolation: various types of RNA constantly leave the nucleus, regulating protein synthesis, while signals activating transcription factors penetrate inside.

As is the case with “larger” barriers that operate at the level of the entire organism, the nuclear one also has selective permeability: for example, fat-soluble molecules, be it steroid hormones or certain drugs, easily penetrate the membrane itself, which is more like a thin oil film with interspersed.

But nucleic acids, proteins and other hydrophilic compounds are doomed to pass through special channels - nuclear pores. Despite the variety of molecules that pass through, the pores themselves are quite conservatively structured in most organisms and consist of an internal channel and symmetrical outer parts, similar to protein molecules located at the vertices of an octagon.

As Hetzer and co-authors of the publication in Cell showed, over time, these pores begin to “leak,” which causes more “noticeable” consequences - the deposition of amyloid plaques along the blood vessels of the brain, the destruction of cartilage in the joints, and the “decrepation” of the heart.

Using the example of muscle cells, and then the entire body of the nematode C. elegans, scientists demonstrated that the peripheral part of the nuclear channel is regularly renewed, while the central part is rebuilt only during cell division, during which the nuclear membrane is first destroyed and then formed again. Accordingly, nuclear pores gradually “wear out”, but, unlike other intracellular systems, they are not renewed, which leads to “leaking”. As a result, not only mutagens, but also other molecules that disrupt the functioning of the genetic apparatus enter the nucleus.

If it comes to constantly renewing skin cells or intestinal epithelium, then such a problem does not arise, but what about nerve or muscle cells that practically do not divide throughout life? It is not surprising that their metabolism is tied not only to “signals” from the nucleus, but also to established cascades of reactions that do not require rapid intervention of the genetic apparatus.

Hetzer's discovery did not become another “self-sufficient” hypothesis in the theory of aging. Scientists have demonstrated how reactive oxygen species, which have long become the main enemy of gerontologists, can accelerate the wear and tear of nuclear pores, and with it the aging of the entire cell. We can only hope that a system that compensates for these “leaks” still exists, and if it can be discovered, this will be a new milestone in the study of active longevity.

Despite its universality, the aging process is difficult to define clearly. With age, physiological and structural changes occur in almost all organ systems. When aging, genetic and social factors, nutritional patterns, as well as age-related diseases - atherosclerosis, diabetes, osteoarthritis - are of great importance. Age-related cell damage is also an important component of the aging body.

With age, a number of cellular functions progressively suffer. The activity of oxidative phosphorylation in mitochondria, the synthesis of enzymes and cell receptors decreases. Aging cells have a reduced ability to absorb nutrients and repair chromosomal damage. Morphological changes in aging cells include irregular and lobulated nuclei, polymorphic vacuolated mitochondria, a decrease in the endoplasmic reticulum, and deformation of the lamellar complex. At the same time, lipofuscin pigment accumulates.

Cell aging is a multifactorial process. It includes endogenous molecular programs of cellular aging as well as exogenous influences leading to progressive interference with cell survival processes.

The phenomenon of cellular aging is being intensively studied in in vitro experiments. It has been shown that in aging cells, genes specific to aging are activated, genes that regulate growth are damaged, growth inhibitors are stimulated, and other genetic mechanisms are also activated.

It is assumed that gene defects may be caused by telomeric shortening of chromosomes. Telomeres play an important role in stabilizing the terminal portions of chromosomes and attaching them to the nuclear matrix. For example, the length of telomeres decreases in the last passages of cell culture and in cell culture of elderly people. A relationship has been found between telomere length and telomerase activity.

Acquired cell damage during aging occurs under the influence of free radicals. The causes of these damages may be exposure to ionizing radiation or a progressive decrease in the activity of antioxidant defense mechanisms, such as vitamin E, glutathione peroxidase. Cell damage from free radicals is accompanied by the accumulation of lipofuscin, but the pigment itself is not toxic to the cell. In addition, SPOL and free radicals cause damage to nucleic acids in both the nucleus and mitochondria. The mutations and destruction of mitochondrial DNA become dramatic as we age. Free oxygen radicals also catalyze the formation of modifications of proteins, including enzymes, making them sensitive to the damaging effects of neutral and alkaline proteases contained in the cytosol, which leads to further disruption of cell functions.

Post-translational changes in intracellular and extracellular proteins also occur with age. One of the types of such changes is non-enzymatic glycosylation of proteins. For example, age-related glycosylation of lens proteins underlies senile cataracts.

Finally, there is evidence of impaired formation of stress proteins in vitro in experimental animals during aging. The formation of stress proteins is the most important mechanism of protection against various stresses.

Cell aging is a multifactorial process and has been of concern to scientists since time immemorial. This process involves endogenous molecular programs of cellular aging as well as exogenous influences leading to progressive interference with cell survival processes.

With age, a number of cellular functions progressively suffer. Aging cells have a reduced ability to absorb nutrients and repair chromosomal damage. Morphological changes in aging cells, in which the activity of oxidative phosphorylation in mitochondria and their vacuolization decreases, the endoplasmic reticulum decreases, and the activity of the synthesis of enzymes and cell receptors decreases. What transformations occur in the cell nucleus during the aging process.

CELL NUCLEUS is the place of storage and implementation of genetic material - plans for the construction and regulation of protein synthesis processes in the cell.

MAIN KERNEL COMPONENTS ARE:

nuclear membrane;
chromatin;
nucleolus;
nuclear matrix.

The nucleus is always localized in a specific place in the cell. The main functions performed by the cell nucleus are the storage, use and transmission of genetic information. In addition, the nucleus is responsible for the formation of ribosomal subunits.

The nucleus in a cell can be in two states: mitotic (during division) and interphase (between divisions). During interphase, under a microscope, only the nucleolus is visible in the nucleus of a living cell, and it appears optically empty. The structures of the nucleus in the form of threads and grains can only be observed when the cell is exposed to damaging factors, when it passes into the borderline state between life and death. From this state, the cell can return to normal life or die.

NUCLEAR MEMBRANE - main function is barrier. It is responsible for separating the contents of the nucleus from the cytoplasm, limiting the free transport of macromolecules between the nucleus and the cytoplasm, as well as creating intranuclear order - fixing chromosomal material. The nuclear envelope consists of an outer and an inner nuclear membrane.

The outer membrane of the nuclear envelope, which is in direct contact with the cytoplasm of the cell, has a number of structural features that allow it to be classified as part of the membrane system of the endoplasmic reticulum itself. First of all, such features include the presence of numerous polyribosomes on the hyaloplasm side, and the outer nuclear membrane itself can directly transform into the membranes of the granular endoplasmic reticulum.

The inner nuclear membrane is associated with the chromosomal material of the nucleus. On the side of the karyoplasm, the so-called fibrillar layer, consisting of fibrils, is adjacent to the inner nuclear membrane, but it is not characteristic of all cells.

The nuclear envelope is not continuous. It contains nuclear pores, which are formed as a result of the fusion of two nuclear membranes. In this case, round through holes are formed. These holes in the nuclear envelope are filled with complex globular and fibrillar structures. The number of nuclear pores depends on the metabolic activity of the cells: the more intense the synthetic processes in the cell, the more pores per unit surface of the cell nucleus.

CHROMATIN(from the Greek chroma - color, paint) is the main structure of the interphase nucleus. Chemically, it is a complex of proteins and DNA of varying degrees of helicity (twisting). DNA is a sequence of nucleotides, individual and unique for everyone. It is a unique cipher or code that determines the characteristics of the structure, functioning and development (aging) of the body. That is, the characteristics of aging are inherited.

Morphologically, two types of chromatin are distinguished: heterochromatin and euchromatin. Heterochromatin corresponds to chromosome regions partially condensed in interphase and is functionally inactive. Euchromatin- these are sections of chromosomes decondensed in interphase, functionally active chromatin.

During mitosis, all euchromatin is maximally condensed and becomes part of the chromosomes. During this period, chromosomes do not perform any synthetic functions. Sometimes, in some cases, an entire chromosome may remain in a condensed state during interphase, and it has the appearance of smooth heterochromatin. For example, one of the X chromosomes of the somatic cells of the female body is subject to heterochromatization at the initial stages of embryogenesis (during fragmentation) and does not function. This chromatin is called sex chromatin or Barr bodies.

Chromatin proteins make up 60-70% of its dry mass and are represented by two groups:

histone proteins;
non-histone proteins.

Histone proteins(histones) are alkaline proteins containing basic amino acids (mainly lysine, arginine). They are arranged unevenly in the form of blocks along the length of the DNA molecule. One block contains 8 histone molecules that form a nucleosome. The nucleosome is formed by compaction and supercoiling of DNA.

Non-histone proteins make up 20% of the amount of histones and in interphase nuclei form a structural network inside the nucleus, which is called the nuclear protein matrix. This matrix represents the scaffold that determines the morphology and metabolism of the nucleus.

Nucleolus- the densest structure of the nucleus, it is a derivative of the chromosome, one of its loci with the highest concentration and active synthesis of RNA in interphase, but is not an independent structure or organelle.

The nucleolus has a heterogeneous structure and consists of two main components - granular and fibrillar. The granular component is represented by granules (maturing ribosomal subunits) and is localized along the periphery. The fibrillar component is the ribonucleoprotein strands of ribosome precursors, which are concentrated in the central part of the nucleolus.

The ultrastructure of nucleoli depends on the activity of RNA synthesis: at a high level of synthesis, a large number of granules are detected in the nucleolus; when synthesis stops, the number of granules decreases, and the nucleoli turn into dense fibrillar strands of basophilic nature.

NUCLEAR MATRIX (karyoplasm) is the liquid part of the nucleus that fills the space between the chromatin and the nucleoli.

Karyoplasm consists mainly of proteins, metabolites, and ions. Part of the nuclear matrix is the fibrous lamina of the nuclear envelope. The nuclear skeleton probably contributes to the formation of a protein backbone to which DNA loops are attached.

PHYSIOLOGICAL AGING OF CELLS is a state of irreversible cessation of growth. The basis of the aging of the body is cellular aging. Which, in turn, is caused by genome reorganization that occurs as a result of telomere shortening and defects in DNA repair systems.

It has been proven that in aging cells, specific genes are activated, regulators are damaged and growth inhibitors are stimulated, and other genetic mechanisms are also activated. Scientists suggest that gene defects may be caused by telomeric shortening of chromosomes.

Telomeres play an important role in stabilizing the terminal portions of chromosomes and attaching them to the nuclear matrix. Telomere length decreases in cells of senile people. A connection has been discovered between the length of the telomere and the activity of telomerase, as a result of which the length of the telomeric regions of the cell chromosomes increases or remains at a constant level.

Aged cells do not look like young ones, and this is manifested in the accumulation of excess DNA fragments and incorrect cellular proteins, as well as the appearance of abnormal structures in the nucleolus (a cluster of proteins and nucleic acids located in the cell nucleus). Also, these cells are subject to replicative aging, that is, they are able to divide a certain number of times (about 50).

Scientists also found that in In some cases it is possible to reset the counter. At the same time, it was possible to find out how the special gene NDT80 is activated. To verify the intended role of this gene, the researchers activated it in an old and non-reproducing cell. The inclusion of NDT80 brought a double effect - the cell lived twice as long as usual, and age-related defects in the nucleolus were corrected. This indicates that nucleolar abnormalities hold one of the keys to cellular aging (the second key, telomeric, is much better studied). Unfortunately, it is not yet clear how the mechanism for resetting the cellular clock works. It is only known that the protein encoded by the NDT80 gene is a transcription factor, that is, it serves as a trigger - it activates other genes in the cell.

Followers of the free radical theory of aging believe that acquired cell damage during aging occurs under the influence of free radicals. The causes of this damage may be exposure to ionizing radiation or a progressive decline in the activity of antioxidant defense mechanisms, such as vitamin E. In addition, free radicals cause damage to nucleic acids in both the nucleus and mitochondria. Mutations and destruction of mitochondrial DNA become simply “dramatic” with age. Free oxygen radicals also catalyze the formation of modifications of proteins, including enzymes, making them sensitive to the damaging effects of neutral and alkaline proteases contained in the cytosol, which leads to further disruption of cell functions.

Thus, the process of cellular aging is diverse. It is triggered by various factors and goes through different signaling pathways. The aging process is different in different cells, occurs at different points in time, but in any case leads to dysfunction and death of the cell. The discussion of the causes of cellular aging and its influence on the overall aging of the body has not yet been reached, and scientists have yet to find answers to numerous questions that are important for the development of means to combat aging.

CELLULAR COSMETICS CELLCOSMET & CELLMEN (SWITZERLAND)
COSMETICS "DOCTOR SPIELLER BIOCOSMETIC" ( DR.SPILLER)
BEAUTY INJECTIONS

Article for the “bio/mol/text” competition: More than 50 years have passed since the phenomenon of cell aging was proven in fibroblast cultures, but the existence of old cells in organism has long been questioned. There was no evidence that aging individual cells plays an important role in the aging of everything body. In recent years, the molecular mechanisms of cell aging and their connection with cancer and inflammation have been discovered. According to modern concepts, inflammation plays a leading role in the genesis of almost all age-related diseases, which ultimately lead to death in the body. It turned out that old cells, on the one hand, act as tumor suppressors (since they irreversibly stop dividing themselves and reduce the risk of transformation of surrounding cells), and on the other hand, the specific metabolism of old cells can cause inflammation and degeneration of neighboring precancerous cells into malignant ones. Clinical trials of drugs that selectively eliminate old cells in organs and tissues are currently underway, thereby preventing degenerative changes in organs and cancer.

There are approximately 300 types of cells in the human body, and they are all divided into two large groups: some can divide and multiply (that is, they mitotically competent), and others - postmitotic- do not divide: these are neurons that have reached the extreme stage of differentiation, cardiomyocytes, granular leukocytes and others.

In our body there are renewing tissues, in which there is a pool of constantly dividing cells that replace spent or dying cells. Such cells are found in the intestinal crypts, in the basal layer of the skin epithelium, and in the bone marrow (hematopoietic cells). Cell renewal can occur quite intensively: for example, connective tissue cells in the pancreas are replaced every 24 hours, cells of the gastric mucosa - every three days, leukocytes - every 10 days, skin cells - every six weeks, approximately 70 g of proliferating cells of the small intestine are removed from body every day.

Stem cells, which exist in almost all organs and tissues, are capable of dividing unlimitedly. Tissue regeneration occurs due to the proliferation of stem cells, which can not only divide, but also differentiate into cells of the tissue whose regeneration occurs. Stem cells are found in the myocardium, in the brain (in the hippocampus and olfactory bulbs) and in other tissues. This holds great promise for the treatment of neurodegenerative diseases and myocardial infarction.

Constantly renewing tissues help increase life expectancy. When cells divide, tissue rejuvenation occurs: new cells come to replace damaged ones, while repair (elimination of DNA damage) occurs more intensively and regeneration is possible in case of tissue damage. It is not surprising that vertebrates have a significantly longer lifespan than invertebrates - the same insects whose cells do not divide as adults.

But at the same time, renewing tissues are subject to hyperproliferation, which leads to the formation of tumors, including malignant ones. This occurs due to dysregulation of cell division and increased rates of mutagenesis in actively dividing cells. According to modern concepts, for a cell to acquire the property of malignancy, it needs 4–6 mutations. Mutations occur rarely, and for a cell to become cancerous - this is calculated for human fibroblasts - about 100 divisions must occur (this number of divisions usually occurs in a person around the age of 40).

It is worth remembering, however, that mutations are different mutations, and according to the latest genomic research, in each generation a person acquires about 60 new mutations (which were not in the DNA of his parents). Obviously, most of them are quite neutral (see “Passed over a thousand: the third phase of human genomics”). - Ed.

In order to protect itself from itself, special cellular mechanisms have formed in the body tumor suppression. One of them is replicative cell aging ( senescence), which consists in the irreversible stop of cell division at the G1 stage of the cell cycle. With aging, the cell stops dividing: it does not respond to growth factors and becomes resistant to apoptosis.

Hayflick limit

The phenomenon of cell aging was first discovered in 1961 by Leonard Hayflick and colleagues using fibroblast culture. It turned out that cells in a culture of human fibroblasts, under good conditions, live for a limited time and are capable of doubling approximately 50 ± 10 times - and this number began to be called the Hayflick limit. Before Hayflick's discovery, the prevailing point of view was that cells are immortal, and aging and death are a property of the organism as a whole.

This concept was considered irrefutable largely due to the experiments of Carrel, who maintained a culture of chicken heart cells for 34 years (it was discarded only after his death). However, as it turned out later, the immortality of Carrel’s culture was an artifact, since along with the fetal serum, which was added to the culture medium for cell growth, the embryonic cells themselves got there (and, most likely, Carrel’s culture was no longer what it was in beginning).

Cancer cells are truly immortal. Thus, HeLa cells, isolated in 1951 from a cervical tumor of Henrietta Lacks, are still used by cytologists (in particular, a vaccine against polio was developed using HeLa cells). These cells have even been to space.

For the fascinating story of Henrietta Lacks' immortality, see the article “The Immortal Cells of Henrietta Lacks”, as well as “The Heirs of HeLa Cells”. - Ed.

As it turned out, the Hayflick limit depends on age: the older a person is, the fewer times his cells double in culture. Interestingly, frozen cells, when thawed and subsequently cultured, seem to remember the number of divisions before freezing. In fact, there is a “division counter” inside the cell, and upon reaching a certain limit (the Hayflick limit), the cell stops dividing and becomes senescent. Senescent (old) cells have a specific morphology - they are large, flattened, with large nuclei, highly vacuolated, and their gene expression profile changes. In most cases they are resistant to apoptosis.

However, the aging of the body cannot be reduced only to the aging of cells. This is a much more complex process. There are old cells in a young body, but there are few of them! When, with age, senescent cells accumulate in tissues, degenerative processes begin, which lead to age-related diseases. One of the factors of these diseases is the so-called senile "sterile" inflammation, which is associated with the expression of proinflammatory cytokines by senescent cells.

Another important factor in biological aging is the structure of chromosomes and their tips - telomeres.

Telomere theory of aging

Figure 1. Telomeres are the ends of chromosomes. Since humans have 23 pairs of chromosomes (that is, 46 pieces), there are 92 telomeres.

In 1971, our compatriot Alexey Matveevich Olovnikov suggested that the Hayflick limit is associated with “underreplication” of the terminal sections of linear chromosomes (they have a special name - telomeres). The fact is that in each cycle of cell division, telomeres are shortened due to the inability of DNA polymerase to synthesize a copy of DNA from the very tip. In addition, Olovnikov predicted the existence telomerase(an enzyme that adds repeating DNA sequences to the ends of chromosomes), based on the fact that otherwise in actively dividing cells the DNA would quickly be “eaten up” and the genetic material would be lost. (The problem is that telomerase activity fades in most differentiated cells.)

Telomeres (Fig. 1) play an important role: they stabilize the ends of chromosomes, which otherwise, as cytogeneticists say, would become “sticky”, i.e. susceptible to various chromosomal aberrations, which leads to degradation of genetic material. Telomeres consist of repeated (1000–2000 times) sequences (5′-TTAGGG-3′), giving a total of 10–15 thousand nucleotide pairs at each chromosomal tip. At the 3′ end, telomeres have a rather long single-stranded DNA region (150–200 nucleotides), which is involved in the formation of a lasso-type loop (Fig. 2). Several proteins are associated with telomeres, forming a protective “cap” - this complex is called shelterin(Fig. 3). Shelterin protects telomeres from the action of nucleases and adhesion and, apparently, it is precisely it that preserves the integrity of the chromosome.

Figure 2. Telomere composition and structure. Repeated cell division in the absence of telomerase activity leads to shortening of telomeres and replicative senescence.

Figure 3. Structure of the telomeric complex ( shelterina). Telomeres are found at the ends of chromosomes and consist of tandem TTAGGG repeats that end in a 32-mer single-strand overhang. Associated with telomeric DNA shelterin- a complex of six proteins: TRF1, TRF2, RAP1, TIN2, TPP1 and POT1.

Unprotected ends of chromosomes are perceived by the cell as damage to the genetic material, which activates DNA repair. The telomeric complex, together with shelterin, “stabilizes” the chromosome tips, protecting the entire chromosome from destruction. In senescent cells, critical shortening of telomeres disrupts this protective function, and therefore chromosomal aberrations begin to form, which often lead to malignancy. To prevent this from happening, special molecular mechanisms block cell division, and the cell goes into a state senescence- irreversible arrest of the cell cycle. In this case, the cell is guaranteed not to be able to reproduce, which means it will not be able to form a tumor. In cells with an impaired ability to senescence (which reproduce despite telomere dysfunction), chromosomal aberrations are formed.

The length of telomeres and the rate of their shortening depend on age. In humans, telomere length varies from 15 thousand nucleotide pairs (kb) at birth to 5 kb. for chronic diseases. Telomere length is maximum at 18 months of age and then rapidly decreases to 12 kb. by the age of five. After this, the speed of shortening decreases.

Telomeres shorten at different rates in different people. So, this speed is greatly influenced by stress. E. Blackburn (Nobel Prize winner in Physiology or Medicine 2009) found that women who are constantly under stress (for example, mothers of chronically ill children) have significantly shorter telomeres compared to their peers (by about ten years!). E. Blackburn's laboratory has developed a commercial test to determine the “biological age” of people based on telomere length.

Interestingly, mice have very long telomeres (50–40 kb, compared to 10–15 kb in humans). In some strains of laboratory mice, telomere length reaches 150 kb. Moreover, in mice, telomerase is always active, which prevents telomeres from shortening. However, as everyone knows, this does not make mice immortal. Not only that, but they develop tumors at a much higher rate than humans, suggesting that telomere shortening as a tumor defense mechanism does not work in mice.

When comparing telomere length and telomerase activity in different mammals, it turned out that species characterized by replicative cell aging have a longer lifespan and greater weight. These are, for example, whales, whose lifespan can reach 200 years. For such organisms, replicative aging is simply necessary, since too many divisions generate many mutations that must be somehow combated. Presumably, replicative aging is such a fighting mechanism, which is also accompanied by repression of telomerase.

Aging of differentiated cells occurs differently. Both neurons and cardiomyocytes age, but they do not divide! For example, lipofuscin accumulates in them, an senile pigment that disrupts cell functioning and triggers apoptosis. Fat accumulates in the cells of the liver and spleen as we age.

The connection between replicative cell aging and the aging of the body, strictly speaking, has not been proven, but age-related pathology is also accompanied by cell aging (Fig. 4). Malignant neoplasms of the elderly are mostly associated with renewed tissues. Cancer in developed countries is one of the main causes of morbidity and mortality, and an independent risk factor for cancer is simply... age. The number of deaths from tumor diseases increases exponentially with age, as does overall mortality. This tells us that there is a fundamental link between aging and carcinogenesis.

Figure 4. Human fibroblasts of the WI-38 line histochemically stained for the presence of β-galactosidase activity. A - young; B - old (senescent).

Telomerase is an enzyme that has been predicted

There must be a mechanism in the body that compensates for the shortening of telomeres, this assumption was made by A.M. Olovnikov. Indeed, in 1984 such an enzyme was discovered by Carol Greider and named telomerase. Telomerase (Fig. 5) is a reverse transcriptase that increases the length of telomeres, compensating for their underreplication. In 2009, E. Blackburn, K. Grader and D. Shostak were awarded the Nobel Prize for the discovery of this enzyme and a series of works on the study of telomeres and telomerase (see: "The 'ageless' Nobel Prize: 2009 honors work on telomeres and telomerase").

Figure 5. Telomerase contains a catalytic component (TERT reverse transcriptase), telomerase RNA (hTR or TERC), which contains two copies of the telomeric repeat and is a template for the synthesis of telomeres, and the protein dyskerin.

According to E. Blackburn, telomerase is involved in the regulation of the activity of approximately 70 genes. Telomerase is active in germinal and embryonic tissues, in stem and proliferating cells. It is found in 90% of cancer tumors, which ensures the uncontrollable proliferation of cancer cells. Currently, among the drugs that are used to treat cancer, there is a telomerase inhibitor. But in most somatic cells of an adult organism, telomerase is not active.

A cell can be brought into a state of senescence by many stimuli - telomere dysfunction, DNA damage, which can be caused by mutagenic environmental influences, endogenous processes, strong mitogenic signals (overexpression of oncogenes Ras, Raf, Mek, Mos, E2F-1, etc.), disorders chromatin, stress, etc. In fact, cells stop dividing - becoming senescent - in response to potentially cancer-causing events.

Genome Guardian

Telomere dysfunction, which occurs when they are shortened or shelterin is disrupted, activates the p53 protein. This transcription factor brings the cell into a state of senescence, or causes apoptosis. In the absence of p53, chromosome instability develops, characteristic of human carcinomas. Mutations in the p53 protein are found in 50% of breast adenocarcinomas and in 40–60% of colorectal adenocarcinomas. Therefore, p53 is often called the “guardian of the genome.”

Telomerase is reactivated in most tumors of epithelial origin that occur in the elderly. Telomerase reactivation is thought to be an important step in malignant processes because it allows cancer cells to “defy” the Hayflick limit. Telomere dysfunction promotes chromosomal fusions and aberrations, which in the absence of p53 most often leads to malignancy.

About the molecular mechanisms of cell aging

Figure 6. Cell cycle diagram. The cell cycle is divided into four stages: 1. G1(pre-synthetic) - the period when the cell prepares for DNA replication. At this stage, cell cycle arrest may occur if DNA damage is detected (during repair). If errors are detected in DNA replication and they cannot be corrected by repair, the cell does not enter the S stage. 2.S(synthetic) - when DNA replication occurs. 3. G2(postsynthetic) - preparation of the cell for mitosis, when the accuracy of DNA replication is checked; if under-replicated fragments or other disturbances in synthesis are detected, the transition to the next stage (mitosis) does not occur. 4. M(mitosis) - the formation of a cell spindle, segregation (chromosome divergence) and the formation of two daughter cells (division itself).

In order to understand the molecular mechanisms of a cell’s transition to a state of senescence, I will remind you how cell division occurs.

The process of cell reproduction is called proliferation. The time a cell exists from division to division is called the cell cycle. The proliferation process is regulated both by the cell itself - autocrine growth factors - and its microenvironment - paracrine signals.

Activation of proliferation occurs through the cell membrane, which contains receptors that perceive mitogenic signals - these are mainly growth factors and intercellular contact signals. Growth factors are usually of a peptide nature (about 100 of them are known to date). These are, for example, platelet growth factor, which is involved in thrombus formation and wound healing, epithelial growth factor, various cytokines - interleukins, tumor necrosis factor, colony-stimulating factors, etc. After activation of proliferation, the cell exits the G0 resting phase and the cell cycle begins (Fig. 6).

The cell cycle is regulated by cyclin-dependent kinases, different for each stage of the cell cycle. They are activated by cyclins and inactivated by a number of inhibitors. The purpose of such complex regulation is to ensure DNA synthesis with as few errors as possible, so that daughter cells have absolutely identical hereditary material. Checking the correctness of DNA copying is carried out at four “checkpoints” of the cycle: if errors are detected, the cell cycle stops and DNA repair is activated. If damage to the DNA structure can be corrected, the cell cycle continues. If not, it is better for the cell to “commit suicide” (by apoptosis) to avoid the possibility of becoming cancerous.

The molecular mechanisms leading to irreversible cell cycle arrest are controlled by tumor suppressor genes, including p53 and pRB, associated with cyclin-dependent kinase inhibitors. Suppression of the cell cycle in the G1 phase is carried out by the p53 protein, acting through the inhibitor of cyclin-dependent kinase p21. The transcription factor p53 is activated by DNA damage, and its function is to remove from the pool of replicating cells those that are potentially oncogenic (hence the nickname p53 - “guardian of the genome”). This idea is supported by the fact that p53 mutations are found in ~50% of malignant tumors. Another manifestation of p53 activity is associated with apoptosis of the most damaged cells.

Cell senescence and age-related diseases

Figure 7. Relationship between cell aging and body aging.

Senescent cells accumulate with age and contribute to age-related diseases. They reduce the proliferative potential of the tissue and deplete the pool of stem cells, which leads to degenerative tissue disorders and reduces the ability to regenerate and renew.

Senescent cells are characterized by specific gene expression: they secrete inflammatory cytokines and metalloproteinases that destroy the intercellular matrix. It turns out that old cells provide sluggish senile inflammation, and the accumulation of old fibroblasts in the skin causes an age-related decrease in the ability to heal wounds (Fig. 7). Old cells also stimulate the proliferation and malignancy of nearby precancerous cells through the secretion of epithelial growth factor.

Senescent cells accumulate in many human tissues and are present in atherosclerotic plaques, skin ulcers, arthritic joints, and in benign and preneoplastic hyperproliferative lesions of the prostate and liver. When cancerous tumors are irradiated, some cells also enter a state of senescence, thereby ensuring relapses of the disease.

Thus, cellular aging demonstrates the effect of negative pleiotropy, the essence of which is that what is good for a young organism can become bad for an old one. The most striking example is the processes of inflammation. A pronounced inflammatory reaction contributes to the rapid recovery of the young body from infectious diseases. In old age, active inflammatory processes lead to age-related diseases. It is now generally accepted that inflammation plays a decisive role in almost all age-related diseases, starting with neurodegenerative ones.