Human autophagy. A choice between life and death: apoptosis or autophagy? Autophagy and cancer

Prokaryotes as multicellular organisms

Today, enough data has been collected that in nature prokaryotes exist not in the form of isolated cells, but in the form of complex microbial communities. This bold idea was first put forward in the 80s of the twentieth century, and today it is supported by a solid experimental base. Natural colonies of prokaryotes have an analogue of endocrine signaling within the community (e.g. quorum sense), differentiation of cells into specialized subspecies, as well as complex patterns of collective behavior (joint hunting, collective digestion of prey, collective resistance to antibiotics, etc.). Autophagy, as a characteristic of differentiated communities, may well be another item on this list.

If a bacterial colony is a single biosystem, then its element will be a single bacterium. Similar to the eukaryotic organelle, the prokaryotic cell can be considered the simplest element of the bacterial community, surrounded by a membrane (and cell wall). This assumption leads to an interesting conclusion: autophagy should be looked for not inside the bacterial cell, but inside the bacterial colony. Indeed, “autophagic” processes are well known in prokaryotic colonies, although under other names - bacterial cannibalism, bacterial altruism, autolysis or programmed cell death. Bacterial cannibalism was first described as a bacterial colony's response to nutrient deprivation (see sidebar). The biological mechanism that triggers autophagy in this case is found in many species of bacteria - this is the so-called toxin-antitoxin system. Its essence is that during starvation, the colony lyses (“digests”) part of its cells so that the remaining bacteria receive enough food to survive. Thus, the colony experiences a lack of resources or external unfavorable conditions.

"Autophagy" in bacteria

Typical autophagic patterns have been described at the molecular level in many bacteria. For example, when there is a lack of food, some of the bacteria in the colony release a toxin into the environment. However, only some of them are capable of producing the molecule antitoxin- a protein that neutralizes the toxin when it enters the cell. Such cells survive and absorb the rest, killed and lysed by the toxin. This gives the survivors the energy needed for sporulation. Similar processes have been found in many species of bacteria.

For ease of description we will introduce the term protophagy as a collective synonym for the processes of bacterial cannibalism, altruism, autolysis and programmed cell death. The prokaryotic community is an integral biosystem that, if necessary, processes part of itself to maintain stability. In protophagy, the autophagosome (membrane vesicle with degradation products) is the prokaryotic cell itself. Protophagy is in many ways similar to autophagy in eukaryotes (Fig. 1):

both processes operate on similarly sized “vesicles” (the size of a bacterium is approximately equal to the size of a mitochondrion or peroxisome);
both pro- and autophagy are activated by similar signals (fasting or stress);
both processes are carried out according to the same principle (regulated consumption of its part by the biosystem);
both processes serve a common goal (the survival of the biosystem under stress and maintaining its homeostasis).

Figure 1. Fundamental similarity between protophagy and autophagy.

Like eukaryotic autophagy, protophagy is used for more than just food production. For example, protophagy serves pathogenic bacteria to invade the host organism (Fig. 2). It is known that the host microflora (symbionts) can effectively inhibit the growth of pathogenic microorganisms. In order to suppress competition, some pathogenic bacteria activate the antibacterial immune response of the host organism through protophagy. To do this, part of the pathogenic population inductively self-lyses, releasing toxins, which causes local inflammation. As a result, the body's immune system destroys most at This is part of the symbiont bacteria, while pathogenic bacteria avoid detection and, after the end of the inflammatory reaction, multiply unhindered in the host tissues. Interestingly, in the absence of symbiont microflora (for example, during experimental infection of special lines of sterile mice), such pathogenic bacteria colonize the intestine without inducing inflammation. This suggests that protophagy here is a specific survival mechanism of pathogenic organisms, which is activated only under unfavorable conditions.

Figure 2. Similar roles of protophagy and autophagy in the activation of the immune response.

What does the concept of protophagy give us?

The introduced concept of protophagy is interesting not only as a bare theory, but can also be useful in practice. For example, bacteria are widely used in biotechnology today, and manipulation of protophagy processes may provide a way to maintain bacterial culture stability on an industrial scale. Thus, protophagy activators should improve the quality of crops by activating natural mechanisms for eliminating weakened and damaged microorganisms.

Another important area of application of protophagy may be medicine. Today, bacterial resistance to antibiotics is one of the key pharmacological problems. Instead of killing individual bacterial cells (as is done today with antibiotics), we can concentrate on disrupting bacterial communities as a whole. Such methods are already being developed - these are, for example, blockers of bacterial “quorum sensing”, which are aimed specifically at disrupting intercellular signaling in bacterial colonies in order to make them vulnerable to the human immune system. And although this topic is just developing, and there are still more questions than answers, the general vector of work shows that disruption of communication between individual bacteria has every chance of becoming the therapy of tomorrow. In this context, protophagy activators will help to destroy the protective barriers of the bacterial colony and make it vulnerable to the host immune system.

Afterword

The main question that may arise after reading this article is whether it is necessary to introduce a new term - protophagy- to describe well-known facts? In our opinion, expanding the concept of autophagy and introducing the term “protophagy” is necessary and useful.

The biosphere in a certain sense resembles a fractal, where each subsequent level repeats the previous one. Similar processes are similar to each other not only externally - they all have similar causes and principles of regulation. The concept of protophagy, which unites disparate prokaryotic processes together, allows us to generalize and better understand the deep mechanisms that regulate the life of prokaryotic colonies. This provides undeniable benefits for the biotechnology and medicine of tomorrow.

Whether the term “protophagy” will catch on and whether other scientists will find it useful, time will tell. We outlined what we thought was important in an article published in the magazine Autophagy. If microbiologists accept these generalizations and find them useful, we will be very pleased. If the citation rate of our article does not break records, it means that we have fallen into medieval scholasticism and overestimated the significance of our own inventions. In any case, it was worth presenting this work to the esteemed public - after all, protophagy is a special case of autophagy in the bacterial world and follows the same laws as its other manifestations - be it autophagy in a eukaryotic cell, trophic chains in the biosphere, or fasting according to the fashionable method before the beach season, which, by the way, is already around the corner.

Based on an original essay in Autophagy .

Literature

Daniel J. Klionsky, Fabio C. Abdalla, Hagai Abeliovich, Robert T. Abraham, Abraham Acevedo-Arozena, et. al.. (2012). Guidelines for the use and interpretation of assays for monitoring autophagy. ";
K. Lewis. (2000). Programmed Death in Bacteria. Microbiology and Molecular Biology Reviews. 64 , 503-514;
Bärbel Stecher, Riccardo Robbiani, Alan W Walker, Astrid M Westendorf, Manja Barthel, et. al.. (2007). Salmonella enterica Serovar Typhimurium Exploits Inflammation to Compete with the Intestinal Microbiota. PLoS Biol. 5 , e244;
Morten Hentzer, Michael Givskov. (2003). Pharmacological inhibition of quorum sensing for the treatment of chronic bacterial infections. J. Clin. Invest.. 112 , 1300-1307;
Markina N. (2010). "Biologists have learned to command bacteria." INFOX.ru;
Petro Starokadomskyy, Kostyantyn V. Dmytruk. (2013). A bird’s-eye view of autophagy. Autophagy. 9 , 1121-1126.

While there are many different ways to help your body rid itself of accumulated toxins, ranging from detoxifying foods and chemical and/or natural sauna detox agents, a biological process known as autophagy plays a key role. The term autophagy means “self-eating” and refers to the processes by which your body cleanses itself of various debris, including toxins, and regenerates damaged cellular components.

If you try to explain it in a language understandable to non-specialists: “ Your cells create membranes that hunt for pieces of dead, diseased, or worn-out cells; devour them; clean them out; and use the resulting molecules for their energy or the production of new cellular parts .”

Dr. Colin Champion, radiation oncologist and assistant professor at the University of Pittsburgh, explains it this way: “ Just think, our bodies have an innate recycling program. Autophagy makes us more efficient machines to get rid of defective parts, stop cancerous growths and stop metabolic disorders such as obesity and diabetes. .”

By enhancing your body's autophagy process, you reduce inflammation, slow the aging process, and optimize biological function. “ More autophagy occurring in tissues should mean fewer damaged and weakened cells at any given time, which in turn should lead to a longer lifespan for the organism ».

SCHEMATIC MODEL OF AUTOPHAGY

Stimulating autophagy through exercise
Autophagy occurs in response to stress. And in fact, exercise is one of the ways that you will increase your autophagy levels. As you probably know, exercise creates mild damage to muscles and tissues, which forces your body to then repair itself, thereby making your body stronger. Exercise also helps eliminate toxins through sweating, which is beneficial for any detoxification program. In fact, many researchers consider exercise to be a fundamental aspect of effective detoxification.

Dr. George U., for example, who has been involved in clinical trials to help former US Army soldiers recover from post-Gulf War syndrome, recommends using a combination of exercise, sauna and niacin supplements to increase the removal of toxins through the skin. .

Exercise is an important component because it also causes the blood vessels to dilate and increase blood flow. In addition, as one article notes: “ The team studied autophagosomes, structures that form around pieces of cells that the body decides to dispose of. After studying specially bred mice that had glowing green autophagosomes... scientists discovered that the rate at which the mice were able to destroy their own cells increased dramatically after they ran for more than 30 minutes on a treadmill. And this efficiency of destruction continued to increase until they ran for about 80 minutes. ”.

How much exercise should you do to optimize autophagy?
The amount of exercise required to stimulate autophagy in the human body is still unknown, however it is believed that intense exercise is more effective than light exercise , which are certainly also useful.

However, some studies have shown that the ideal zone in which exercise shows the greatest benefit for increasing longevity ranges from 150 to 450 minutes of moderate exercise per week, which reduces the risk of early death by 31% and 39%, respectively. Including at least 30% of your workout at a high-intensity pace also showed an increase in longevity of approximately 13% more than exercise that was performed at a consistently moderate pace throughout the workout.

How can you inhibit autophagy?
One of the fastest ways to inhibit autophagy is to eat large amounts of protein. This will stimulate production insulin-like growth factor IGF-1 and activates mTOR pathway, which are potent inhibitors of autophagy.That is why It is better to limit protein intake to about 40-70 grams per day, depending on your lean body mass. The best formula is one gram of protein for every kilogram of lean body mass (not total body mass).

Significant amounts of protein can be found in meat, fish, eggs[, dairy products, legumes, nuts and seeds. Some vegetables are also high in protein, such as broccoli. Forty grams of protein is not a large amount of food, which is approximately 170 grams. chicken breast.To determine if you're getting too many protein foods, simply measure the weight of muscle in your body (there are bathroom scales that do this) and write down everything you eat over the course of a few days. Then calculate the amount of daily protein you consume from all sources relative to your pound of muscle mass.

The following table briefly shows how much protein is found in various foods..

PROTEIN CONTENT IN SOME FOODS

Importance of Mitochondrial Biogenesis
Healthy mitochondria are the basis for maintaining your health and preventing diseases. Mitochondrial damage can cause genetic mutations, that contribute to the development of cancer Therefore, optimizing the health of your mitochondria is a key component of cancer prevention.

Autophagy is one way of removing damaged mitochondria, and biogenesis is the process by which new healthy mitochondria can be duplicated.
Interestingly, exercise plays a dual role because it not only stimulates autophagy, but is also one of the most potent stimulators of mitochondrial biogenesis. It does this by increasing a signal in your body called AMPK, which in turn activates Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) .

By stimulating your mitochondria, the organelles found in almost every cell that produces ATP, you allow your mitochondria to begin creating reactive oxygen species (ROS), which act as signaling molecules. One of the functions of this signal is to stimulate the production of more mitochondria. Essentially, the key to preventing disease, virtually eliminating the risk of cancer, heart disease, diabetes, many other diseases and slowing down the aging process, is optimizing mitochondrial function and increasing the number of those mitochondria. Luckily, exercise can help you do these two beneficial things.

MITOCHONDRIA

Intermittent fasting is another way to increase autophagy levels
Dietary restriction is another biological stressor that produces many beneficial effects, including increased autophagy. In fact, there are some known benefits associated with dietary restriction: a reduced risk of diabetes and heart disease.

While there are many different types of fasting schedules, if you already have insulin resistance (your cells' resistance to insulin in absorbing sugar), Dr. Mercola (USA) recommends scheduling your meals each day within a window of time of about 8 hours or less. For example, you can limit your eating from 11 a.m. to 7 p.m. This amounts to approximately 16 hours without food.

Eating between 8 a.m. and 4 p.m. may be a much better schedule for some people, and this schedule has the added benefit of allowing you to fast for several hours before bed. Dr. Mercola believes that the best choice for most people is to not eat three hours before bed, since the last thing you want to do is produce energy when you don't need it.

There is compelling evidence showing that supplying fuel to your mitochondria at a time when they don't need it causes large amounts of electrons to leak out, which release reactive oxygen species, acting as free radicals. These free radicals damage mitochondrial and ultimately nuclear DNA. You should aim to fast for six hours before bed, but as a minimum, you shouldn't eat for at least three hours before bed.

To increase autophagy levels, you need to eat foods high in healthy fats and low in carbohydrates.
Nutritional ketogenesis This is the third strategy that will help increase your autophagy levels, and to achieve this, you must reduce the amount of carbohydrates that do not contain healthy dietary fiber and increase the amount of healthy fats in your diet, along with moderate amounts of protein. Many Russians tend to eat much more protein than they need, which will counteract your efforts to get into nutritional ketogenesis.

Most city dwellers consume unhealthy fats in the form of processed vegetable oils, which will invariably worsen your health. This is not only due to the very high content of Omega-6 fatty acids, but also because the excess Omega-6 will be integrated into the inner mitochondrial membrane and the mitochondria will become extremely susceptible to oxidative damage, as a result of which your mitochondria may die much earlier than it's supposed to.
It's best to keep your omega-6 fatty acid intake to 4 to 5 percent of your total daily calories, and replace the rest of the omega-6 fatty acids with healthier fats, such as natural, unprocessed fats in seeds, nuts, olive oils, avocado oil or coconut oil.

It is also important to distinguish between carbohydrates, so when we talk about low-carb foods, we are talking about all foods, including vegetables. However, carbohydrates from vegetable fiber won't push your metabolism in the wrong direction. It follows that the restriction includes easily digestible carbohydrates from sugar, sweet drinks, processed cereals (cereals), pasta, bread and cookies.
More importantly, fiber is not broken down into sugars, but passes down through your digestive system and is then consumed by the bacteria in your gut and converted into short-chain fats, which actually improve your health. Remember, you need carbohydrates found in vegetables, which also contain high amounts of fiber.

By restoring autophagy function, you help muscle stem cells
It has long been known that mesenchymal stem cells (MSCs), located in skeletal muscle, are an important part of the muscle repair process. Previous research has shown that exercise affects the behavior of your muscle stem cells, and may help prevent or even reverse age-related muscle loss. MSCs in muscle are highly responsive to mechanical stress, and these stem cells accumulate in muscle after exercise.

In the meantime, MSCs indirectly help create new muscle fibers by increasing the production of growth factors that stimulate other cells to create new muscle. It is also known that in people with age, the number of MSCs in the muscles decreases, and that the efficiency of autophagy decreases. As a result, toxic substances begin to accumulate in cells and tissues.

A recent Spanish study reports that satellite cells for MSCs are responsible for tissue regeneration and rely on autophagy to prevent cell cycle arrest known as cellular senescence; a process in which the activity of stem cells is significantly reduced. In short, improved muscle tissue regeneration can be achieved through increased efficiency and autophany levels. As autophagy becomes more efficient, your body improves its internal self-cleaning mechanism, where stem cells retain the ability to maintain and repair their tissues.

Your lifestyle determines your future destiny in terms of how long you will live and, ultimately, how many healthy years you will have. For optimal health and disease prevention, you need healthy and efficient mitochondria to achieve three key lifestyle factors:
1. What you eat: A diet high in quality fats, moderate in protein, and low in carbohydrates without dietary fiber. Eating organic, plant-based foods is also important, as commonly used pesticides such as glyphosate cause mitochondrial damage.
2. When do you eat: Daily intermittent fasting is generally the easiest to stick to, but you can schedule any other fast.
3. Physical exercise with a 30% time interval of high intensity - most effective in terms of health and longevity

Potapnev M.P.

AUTOPHAGY, APOPTOSIS, CELL NECROSIS AND IMMUNE RECOGNITION

one's own and someone else's

Belarusian State Medical University of the Ministry of Health of the Republic of Belarus, 220116, Minsk

The literature review presents data on the role of the main types of cell death in the formation of an immune response to pathogens and self-antigens. The basic mechanisms of autophagy, apoptosis and necrosis of cells, the significance of the resulting cellular products for the induction of an immune response are considered. The role of autophagy as a cell autonomous defense system against pathogens and cellular stress has been noted. The leading role of apoptosis and apoptosis-associated molecular images (patterns) in the induction of immunological tolerance has been determined. The crucial importance of necrosis and products of damage to one's own cells in the induction of the inflammatory response of the macroorganism and an effective immune response to one's own antigens, pathogens and molecular patterns of pathogens is emphasized. The interaction of different types of cell death in pathological conditions is discussed.

Key words: autophagy; apoptosis; necrosis; cell death; pathogens; inflammation; immune response. Potapnev M.P.

AUTOPHAGY, APOPTOSIS, NECROSIS AND IMMUNE RECOGNITION OF SELF AND NONSELF

Belarusian State Medical University, Ministry of Public Health, 220116, Minsk, Belarus

The review of literature discusses the role of the most essential types of cell death (autophagy, apoptosis, necrosis) for induction of immune response to pathogens and self antigens. The main mechanisms of cell death and biological characteristics of cellular products, released during autophagy, apoptosis, necrosis were reported. The role of autophagy as cellular self-defense system against pathogens and cellular stress was underlined. The receptor-ligand interaction for induction of immune tolerance by apoptotic cells and the role of apoptotic cell-associated molecular patterns (ACAMPs) and dendritic cells were described. Brief description of mechanisms of necrotic cell-induced inflammation and immune response as well as leading role of damage-associated molecular patterns/ DAMPs were done. Interaction of DAMPs and pathogen-associated molecular patterns/PAMPs in induction of host defense against pathogens was described. It was concluded that differential type of cell death may be occurred depending on the strength of danger signal affecting cells and their function.

Key words: autophagy; apoptosis; necrosis; cell death; pathogens; inflammation; immune response

It is believed that the main principle of action of the immune system is to recognize someone else’s or a modified one and its subsequent removal. A classic example of immune recognition of a stranger is the reactions of innate and acquired immunity against microorganisms (bacteria, viruses). Immune recognition of altered self is associated with autoimmune diseases. With the development of ideas about (pro)programmed cell death (PCD), it has become important to assess the connection between immunity and the maintenance of cellular homeostasis in the macroorganism. Any changes in cells during growth and differentiation, aging, natural death, metabolic dysfunction, stress, exposure to a pathological process (infection, sterile inflammation) should be considered by the immune system as a violation of cellular homeostasis. This review is devoted to assessing the role of PKC in triggering immune reactions.

Based on morphological and biochemical criteria, three main types of PKC are distinguished: apoptosis (PKC type I), autophagy (PKC type II) and necrosis (PKC type III). ACL types I and II have certain genetic mechanisms

Potapnev Michael Petrovich, email: [email protected]

We are implementations, which is why we are called active. Type III ACL (primary necrosis due to external damage) is uncontrollable and is therefore called passive. Additionally, secondary necrosis is distinguished as the end result of apoptosis, controlled necrosis (necroptosis) and other ways of cell death. The list of known (13) types of cell death is regulated by the Nomenclature Committee. Characteristics of the three main types of ACL are presented in the table.

The attention of immunologists to cell death is determined by the fact that not only infectious antigens and molecular patterns (patterns) of pathogens (pathogen-associated molecular patterns - PAMPs), which distinguish it from a macroorganism, but also products of damage to its own cells (damage-associated molecular patterns - DAMPs) cause inflammation and immune response. P. Matzinger emphasized that it is important for the immune system to recognize and respond to danger signals resulting from tissue (cell) damage, and not to distinguish between self and non-self.

Autophagy

Autophagy is the process of intravital utilization (degradation with the help of lysosomes) of cytoplasmic contents modified by metabolites to maintain cellular and energy homeostasis. Autophagy is considered

IMMUNOLOGY No. 2, 2014

Main types of cell death

Characters - Type of cell death

stick autophagy apoptosis necrosis

Purpose Degradation and intracellular recycling of damaged organelles and proteins without harm to the cell. In case of excessive degradation - cell death Degradation of dying cells without an inflammatory and immune response of the body Limitation of the focus of non-viable tissue through inflammation and an immune response to toxic and body-threatening influences

Cell morphology Vacuolization of the cell cytoplasm Condensation and compaction of the cell, chromatin condensation, nuclear fragmentation, formation of apoptotic bodies Swelling of organelles followed by rupture of internal and external membranes. Swelling and subsequent cell lysis

Mechanism of action Sequential formation in the cytoplasm of phagophore, autophagosome, autolysosome or chaperone-mediated fusion with lysosomes Caspase-dependent (receptor) or mitochondria-dependent pathways of DNA degradation Uncontrolled cell damage or receptor-dependent (RAGE, TLRs, CD91, etc.) pathway of cell destruction

Library LC3-II, ULK 1, ATG12, ATG4, GABARAP DNA fragments 50 kbp, outer membrane PS, FAS, CASP 3, APAF1 LDH, HBGH1, S100 proteins, ATP, HSP90

Involvement of phagocytosis Absent Present Present

as predominantly "programmed cell survival". Stress induces autophagy, and excess autophagy activity leads to cell death. Insufficiency of autophagy provokes the accumulation of metabolites associated with aging, degenerative processes in the nervous tissue and liver, autoimmune and pulmonary diseases (especially due to smoking). The connection between autophagy and Crohn's disease, cystic fibrosis, obesity, and sepsis has been shown.

The main type of autophagy is macroautophagy, which includes the stages of initiation, nucleation, elongation and fusion (with the lysosome). Altered cytoplasmic proteins (as a result of stress, lack of energy supply), damaged mitochondria, excess endoplasmic reticulum (ER), peroxisomes are translocated to organelle membranes due to complexation with proteins ULK 1/2, Atg13, Atg101, fIp-200. On the membranes of organelles (ER, mitochondria, Golgi apparatus), these proteins form complex I, which additionally includes the proteins Vps34, Beclin

I, Vps15, Atg14L. The inner membrane of the phagophore is formed around complex I. Formation of an autophagosome (0.3-1 µm in diameter) with a double membrane requires the participation of LC3

II, formed as a result of lipolization of the cytosolic protein LC3 and the Atg5-Atg12/Atg16L1 protein complex with phosphatidylethanolamine. Subsequent maturation of the autophagosome into an autophagolysosome is carried out by fusion with lysosomes using protein complex II, including Vps34, Beclin 1, UVRAG. In the autophagolysosome, the degradation of altered proteins occurs under the action of hydrolases and the release of nutritional and energy-intensive substances into the cytoplasm. In addition to macroautophagy, microautophagy is distinguished (when the capture of cytoplasmic contents is carried out by invagination of the lysosome membrane) and chaperone-mediated autophagy (when the delivery of cytoplasmic material to lysosomes is carried out using chaperone proteins).

Due to the presence of altered self- and foreign macromolecules in the cytoplasm of the cell, the autophagy process, being metabolic, also acts as a mechanism for recognition and utilization of intracellular microorganisms (viruses, bacteria, protozoa) carrying PAMPs. Penetration of microorganisms and their products into the cytoplasm triggers autophagy mechanisms as a cell-autonomous defense system. The division of the cell cytoplasm into separate areas and organelles bounded by (endo)membranes (i.e., compartmentalization) assumes the presence in each of them of its own set of receptors that recognize foreign PAMPs and altered self-DAMPs. This creates a multi-stage system of protection against pathogens that penetrate

moved inside the cell. At each stage of the pathogen’s advancement in the cell, recognition of DNA, aggregated self-proteins, a complex of microbes and serum proteins occurs. The pathogen encounters various enzymes; NO and H2O2; presence or lack of nutrients. Microbes activate receptors on the endomembranes of the cytoplasm, which leads to the formation of an inflammasome and the production of interleukin (IL)-1β and IL-18. The entry of a pathogen into autophagolysosomes dramatically changes the conditions of its existence due to the action of pH, hydrolases, and superoxide anions. In this case, persistence of the pathogen (long for M. tuberculosis, short for other bacteria) in autophagosomes or destruction of the pathogen in autophagolysosomes is possible. Toll-like receptors (TLRs) recognize bacterial lipopolysaccharide (LPS), viral single-stranded ribonucleic acid (ssRNA), and other polymeric nucleic acids that have entered the cytoplasm of macrophages. During autophagy, TLRs, RLRs (retinoid acid inducible gene I-like receptors), NLRs (nucleotide oligomerization domain-like receptors) participate in the recognition of intracellular pathogens (Str. pyogenes, M. tuberculosis, BCG, Salmonella, viruses). TLR3, which recognizes RNA viruses, is localized in cell endosomes; TLR7, TLR8, TLR9, which recognize RNA and DNA of viruses and bacteria, CpG motifs of nucleic acids of microbial origin, are found in endolysosomes. RLRs that recognize viral RNA, and NLRs that recognize PAMPs (muramyl dipeptide, toxins, salt crystals, other components) of bacteria, viruses, cellular products of chemical exposure and UV irradiation, are located in the cytoplasm. An important function of TLRs is to provide tight control over the normal (commensal) intestinal microflora.

PAMPs, recognized by TLR1, TLR2, TLR4, TLR5, TLR6, induce the formation of inflammatory cytokines IL-f and IL-18 in the inflammasome. PAMPs, recognized by TLR7, TLR9, stimulate the production of interferon-a (IFNa) and IFNr, which contributes to the formation of a Th1 immune response. The production of IL-1R and IL-18 protects cells from influenza virus and Shigella bacteria, respectively. And pyroptosis caused as a result of activation of inflammasomes (cell death with signs of apoptosis and necrosis) is destructive for salmonella, legionella and other bacteria. Activation of TLR4 disrupts the binding of Bcl-2 to the Beclin 1 protein, which leads to the formation of a phagosome from the phagophore. Activation of TLRs induces a rapid transition of Lc3 from the cytoplasm to the phagosome, cell activation, promotes the maturation of the phagosome and its fusion with the lysosome. L. monocytogenesis in the cell cytoplasm recognizes NLRs and TLR2, and S. flexneri recognizes NLRs, which leads to the degradation of microbes by autophagy mechanisms involving inflammasomes. When captured

of living bacteria (as opposed to dead ones), microbial mRNA enters the infected cell, which creates an additional danger signal (vita-PAMPs), activating NLRP3-type inflammasomes and TRIF-dependent production of IFNr. Thus, autophagy acts as a mechanism for the degradation of microorganisms when they enter the cell cytoplasm and are recognized by pathogen-associated receptors.

Autophagy is involved in the presentation of antigens to T cells. The formation of ER-associated proteasomes or autophagosomes creates favorable conditions for the contact of membrane-bound MHC class I or II molecules with peptides and the subsequent transfer of their complexes to the outer membrane of antigen-presenting cells for the induction of CD8- or CD4-dependent T-cell responses, respectively. . Autophagy proteins LC3 and GABARAP in autophagosomes increase the affinity of self and foreign peptides for MHC class II molecules by 20 times. Blocking the autophagy gene Atg5 suppresses the generation of CD4+ T-cell (Th1) responses to herpes simplex virus or HIV-1, and also prevents the recognition of Epstein-Barr virus-infected B cells.

Autophagy in the thymic epithelium is the basis of negative selection of autoreactive T cells. Blockage of the autophagy gene Atg5 leads to autoimmune CD4+ T cell proliferative disease in mice and accumulation of apoptotic CD4+ and CD8+ T cells. Autophagy deficiency in peripheral T cells causes accelerated cell death of naive, but not memory T cells, which is associated with the production of superoxide anions upon activation of naive T cells. An important function of autophagy is the isolation of damaged mitochondria that generate superoxide anions as a source of stress and damage (even death) to the cell itself.

The autoimmune response in diabetes mellitus and autoimmune hepatitis is caused by the autoantigens GAD65 (glutamate decarboxylase 65) and SMA (mutant immunoglobulin K-light chain), which undergo chaperone-mediated autophagy in the cytoplasm with the participation of HSC70 and the lysosome-associated membrane protein LAMP-2A, respectively. After degradation in lysosomes, they, together with MHC class II molecules, are presented to autoreactive cD4+ T cells. The formation of citrulated peptides in autophagolysosomes under the action of peptidylarginine deaminases and the formation of their complexes with class II MHc molecules is the basis of the autoimmune cD4+ T-cell response in rheumatoid arthritis - RA. In T cells of MRL mice with lymphoproliferative syndrome, an analogue of human systemic lupus erythematosus (SLE), a significant number of autophagosomes are detected in T cells, which is explained by their long survival.

The production of superoxide anions by macrophage mitochondria promotes bacterial digestion through the process of autophagy. Bacteria recognized by NLRs stimulate autophagy in fibroblasts. In dendritic cells (DCs), this results in the presentation of bacterial peptides along with MHC class II molecules to CD4+ T cells. An important protective function of autophagy is the ability to reduce the level of its own DAMPs in the cytoplasm and restrain the secretion of IL-α and IL-18 in response to exogenous sources of DAMPs. Autophagy mechanisms ensure the degradation of inflammasomes - a complex of proteins that convert procaspase-1 into caspase-1, which converts pro-IL-f and pro-IL-18 into secreted active cytokines. Blocking the autophagy gene Atg16L1 in mice leads to increased production of IL-f and IL-18, inflammation, and increased mortality during antigenic stimulation with dextran sulfate.

Extracellular cytokines affect the processes of bacterial autophagy and their digestion in phagolysosomes. Cytokines TH-dependent response IFNa and tumor necrosis factor α (TNFα) stimulate autophagy. Cytokines No. 2-dependent

IL-4 and IL-13 responses, on the contrary, reduce the formation of phago-lysosomes and increase the intracellular survival of M. tuberculosis. Differentiation of T cells into Th1 and Th2 in vitro is characterized by greater and lesser formation of autophagosomes, respectively. Intracellular infectious agents (cytomegalovirus, HIV, herpes simplex virus I, influenza A virus, Yersinia, Listeria, Shigella, Salmonella, E. coli, etc.) evade the immune response by weakening the process of autophagy.

Autophagy is a physiological process of cell self-renewal, which, under stress, can lead to cell death. At the same time, natural cell death (in humans, from 50 to 500 billion cells daily) occurs primarily through apoptosis.

Apoptosis. Apoptosis ensures the removal of dying cells through phagocytosis without inflammation, which is detrimental to the macroorganism, or accompanies the focus of inflammation to limit it and ultimately heal. The formation of the immune system and the maturation of antigen-specific T and B lymphocytes is also accompanied by massive cell apoptosis. Apoptosis ensures the maintenance of cellular homeostasis, stimulation of cellular regeneration, and wound healing. Apoptotic cells (AC) are utilized by neighboring epithelial and endothelial cells, fibroblasts, macrophages, and DCs. In case of diseases and transfusion of stored donor blood, apoptotic bodies with a diameter of 0.2 μm, formed from AKs, are detected in peripheral blood, lymph nodes, and bone marrow. Lipid mediators released by AA (lysophosphatidylcholine, sphingosine-1-phosphate), ribosomal dRP S19, EMAP II of endothelial cells, TyrRS synthetase, thrombospondin 1, soluble receptor for IL-6, fractalkine (CX3-CR1L), nucleotides ATP and UTP attract phagocytes. In this case, lactoferrin, released by mucosal cells and neutrophils during apoptosis, selectively suppresses the chemotaxis of neutrophils, but not macrophages. Surface expression of phosphatidylserine (PS), other oxidized lipids and calreticulin is a sign of early AKs recognized by macrophage receptors (stabilin-2, CR3, scavenger receptors, CD91, CD31, TIM4, CD36, steroid receptor activator 1; TAM- receptors (Ty-ro2, Ax1, Mer); LRP-1). Molecular markers of AKs are collectively called apoptotic cell-associated molecular patterns (ACAMPs). Macrophages recognize apoptotic cells through multiple apoptosis-associated receptors simultaneously to rapidly remove cells during the early stages of apoptosis. Expression of surface CD31 (and/or CD47) on AKs prevents their uptake by macrophages. It is important that the macrophage receptors that recognize AKs and apoptotic bodies differ from the receptors that recognize PAMPs and DAMPs. Moreover, activation of receptors that distinguish between AKs and apoptotic bodies helps to suppress the recognition of infectious agents by PAM-Ps macrophages through TLRs.

Recognition of AKs and apoptotic bodies is facilitated by the participation of serum opsonins Gas6, MFG-E8, P2GP1, annexin I, C-reactive protein (CRP), pentraxin PTX-3, collectins, dq-component of complement, surfactants SP-A and SP-D (in lung tissue), etc. At the same time, opsonin MFG-E8, which is involved in the uptake of AKs by macrophages, simultaneously suppresses the phagocytosis of necrotic cells (NC) and their immunogenicity for DCs. C1q interacts with the PS of early AKs, and collectin mannose-binding lectin (MBL) interacts with late AKs. Calreticulin (in combination with CD91), pentraxins CRP, SAP (component of serum amyloid P); phi-colins interact with late AKs. Assessing the role of the complement system and natural antibodies in AK clearance. A number of authors have determined that lysophosphatidylcholine, which appears (and is partially secreted) on the surface of AK, is the target of natural antibodies - IgM, as well as mannose-binding proteins and other collectins. Their interaction in turn leads to binding

IMMUNOLOGY No. 2, 2014

with C1q, C3b/bi. As a result, AKs are phagocytosed without activating the release of proinflammatory cytokines by macrophages. Autoimmune reactions involving class G anticardiolipin antibodies, on the contrary, occur with the participation of complement and autoantibodies to membrane phospholipids of late AKs. It is important that apoptotic bodies at the early stages of apoptosis are covered with elements of the PS-containing outer cell membrane, and at later stages - with elements of endoplasmic membranes. And if the antigenic presentation of early apoptotic bodies causes the formation of immunoregulatory T cells (Treg), then the contact of late apoptotic bodies with DCs causes the formation of Th7 cells. Apoptotic neutrophils (and the outer membranes of lysed neutrophils) cause the production of transforming growth factor B (TGF) by macrophages, and the internal contents of lysed neutrophils cause the formation of IL-8, TNFa, and the chemokine MIP-2. At the site of inflammation, neutrophils themselves exhibit “cannibalism” by phagocytizing apoptotic neutrophils (for example, those induced by UV irradiation). This is facilitated by additional activation of TLRs of effector neutrophils and cytokines TNFa and granulocyte-macrophage colony-stimulating factor (GM-CSF), but not IL-1-β, IL-6, IL-8, IL-12, IL-17. At the site of inflammation, macrophages are the main phagocytes of the AK. This does not lead to the production of pro-inflammatory cytokines (IL-1β, TNFa, IL-6, IL-12), but causes the formation of immunosuppressive IL-10, TRF, prostaglandin E2 (PGE2). Immune tolerance to AK antigens and simultaneously to other antigens, including PAMPs of microorganisms, is formed, which is mediated by CD8a + DC. AA-stimulated DCs present antigen(s) only to CD8+ T cells, while NK-stimulated DCs present antigen(s) to CD4+ and CD8+ T cells. Immunosuppression, which develops as a result of the massive formation of AKs and their capture by macrophages, underlies the therapeutic effect of extracorporeal photopheresis in patients with chronic inflammatory diseases.

A long-term process of apoptosis in the site of inflammation can lead to the formation of fibrosis, which is associated with the ability of macrophages that have phagocytosed AK to secrete TGF and other growth factors. At the same time, suppression of inflammation and enhancement of reparative processes during phagocytosis of AKs lead to autoimmune diseases (SLE, chronic obstructive pulmonary disease) in the presence of a genetic predisposition. Normally, B1-like cells with the CD43+CD27-IgM+ or cD24++cD38++cD27-IgM+ phenotype are the main source of natural antibodies to surface AA molecules. A significant amount of AKs in the germinal centers of lymph nodes in patients with SLE ensures long-term survival and costimulation of autoreactive B cells activated by single-stranded DNA, nucleosomes, and other cellular antigens. This is associated with an Oq-dependent genetic defect in the rapid clearance of early AKs and the accumulation of late AKs with signs of secondary necrosis. The resulting low-affinity antibodies of the IgM class interact with cells in the early stages of apoptosis, and high-affinity antibodies of the IgG class interact with cells in the later stages of apoptosis. Plasmacytoid DCs and activation of DNA-binding TLR9 B cells mediate T-independent autoantibody production. AA-induced production of the immunosuppressive IL-10 is significantly reduced when B cells are stimulated by immune complexes including chromatin or by apoptotic bodies formed during late stages of apoptosis.

Elimination of AK occurs mainly in the early stages of apoptosis, when the expression of PS and calreticulin on the outer membrane signals that it has been “changed.” The early stages of apoptosis are reversible; their prolongation ensures the phagocytosis of most AKs and the formation of tolerance of the immune system. Transition of cells to later stages

apoptosis is characterized by a decrease in the level of glycosylation of surface molecules, fragmentation of nuclear DNA and signs of secondary necrosis, causing inflammation and an immune response.

The main pathways for triggering cell apoptosis are receptor (extrinsic), caused by external influences, or stress-induced (intrinsic), associated with internal influences. The receptor pathway for triggering cell apoptosis is mediated by death receptors, including Fas, TNFR (type I TNF receptor), TRAIL, Apo2/Apo3. Activation of caspases is key for apoptosis and the sequence of their activation is well described in the literature. The stress-induced (mitochondrial) pathway of apoptosis is associated with the release of cytochrome C from mitochondria and is regulated by proteins of the Bcl2 family. Caspase-dependent activation and an increase in the level of superoxide anions (mainly due to mitochondrial damage) determine the immunosuppressive effect of AA. The tolerogenic effect of AA is believed to be mediated by Heg cells, causing TRAIL-induced death of CD4+ T helper cells [52]. Both pathways of apoptosis lead to surface expression of PS, fragmentation of nuclear DNA, formation of apoptotic bodies and their rapid phagocytosis. This prevents the immune response to the dying cell, the production of inflammatory cytokines by macrophages, and the presentation of cellular antigens by DCs.

When infected, cells show signs of early apoptosis (expression of PS on cell membranes, the beginning of DNA fragmentation) and an NF-κB-dependent pathway of cellular activation. At the same time, the cells inhibit the replication of pathogens without the formation of DAMPs characteristic of necrotic cells. Defects in the links of apoptosis (mainly the mitochondria-dependent activation pathway), or a delayed start of apoptosis lead to the spread of infection (caused by Legionella pneumonia, Pseudomonas aeroginosa, Helicobacter pylori), sepsis. Many viruses contain caspase inhibitors, and Chlamydiae and Coxiella burnetii block the release of cytochrome c from mitochondria and cell apoptosis, which ensures the pathogen's life cycle early in infection. The capture of AKs containing bacteria causes DC maturation, inflammation, and a full-fledged (Th17) immune response; when uninfected AKs are captured, there are no signs of DC maturation and inflammation, and immunosuppression is formed. The strategy of limited replication of the pathogen in the AC is advantageous in the absence of a strong immune response to cell necrosis and massive release of bacteria into the extracellular space.

Necrosis. Cells that die as a result of injury, degenerative processes, or exposure to a pathogen are effectively disposed of through necrosis. Necrosis demarcates non-viable tissue, subject to destruction and subsequent restoration. Cell necrosis is always accompanied by inflammation and leads to a pronounced immune response and subsequent tissue repair. NK are characterized by the destruction of the outer cell membrane and the entry of hidden intracellular molecules into the extracellular space (see table), which causes a toxic reaction of surrounding healthy cells and an immune response. Primary cell necrosis does not depend on the action of caspases and is a direct result of external traumatic injury or genetically programmed events associated with damage to the mitochondrial matrix protein cyclophilin D; effects on death receptors or TLR3/TLR4 and receptor-independent DNA damage. Oxidative stress of cells, reactive oxygen species are inducers of (controlled) necrosis. Secondary necrosis is the end result of late apoptosis; it often underlies autoimmune pathology (SLE and others).

NKs are phagocytosed by macropinocytosis after the disappearance of surface CD31 and CD47 molecules that block phagocytosis. NK, unlike AK, induce DC maturation

and (Th1) immune response. NKs secrete intracellular molecules that provoke inflammation and an immune response, which is why they are called alarmins or DAMPs. They attract neutrophils to the site of necrosis. NKs secrete heat shock proteins (HSP70, HSP90, gp96), calgranulins, cytokines (IL-1a, IL-6), mitochondrial formyl peptides, RNA, double-stranded (genomic) DNA, and other molecules. The release of nuclear protein HMGB1 (high-mobility group box 1), normally associated with chromatin, is the main marker of (primary) cell necrosis. During apoptosis and secondary necrosis, HMGB1 is retained in the nucleus or located in the cytoplasm or extracellularly in an inactive (oxidized) state as a result of the action of superoxide anions. HMGB1 itself is a mitogen and a chemoattractant, but the complexes it forms with single-stranded DNA, bacterial LPS, and nucleosomes cause macrophages to secrete the inflammatory cytokines TNFa, IL-1β, IL-6, and the chemokines IL-8, MIP-1a, and MIP-ip. High levels of HMGB1 in the blood are associated with massive necrosis of body cells and are a marker of systemic inflammation. HMGB1 is a powerful adjuvant for the formation of high-affinity antibodies and DC maturation. Unoxidized (active) HMGB1 circulating in the bloodstream interacts with TLR2, TLR4, TLR9 and RAGE (receptor for advanced glycation end-products) of phagocytes, causing an inflammatory response. Simultaneously, HMGB1 (as well as HSPs) interacts with CD24 and Siglec-10 on the surface of phagocytes, which limits inflammation caused by DAMPs, but not PAMPs. The distinction between the immune response to pathogen-associated PAMPs and self-cell damage-associated DAMPs occurs at the level of cell receptors. A typical receptor for DAMPs is RAGE on cells of the immune and nervous systems, endothelial cells, and cardiomyocytes. RAGE recognizes proteins and lipids modified by non-enzymatic glycosylation and appearing in chronic inflammatory diseases as a result of oxidative stress. RAGE recognizes NK products such as HMGB1 and calgranulins (S 100 family proteins).

NCs secrete nucleic acids. In this case, the RNA becomes double-stranded, interacts with TLR3 on DCs, and double-stranded DNA interacts with TLR9 of phagocytes, which leads to the production of IFN, CXCL10 (IP-10), IL-1R, and the expression of costimulatory molecules (cD40, cD54, cD69, MHc class II) on the surface of macrophages and DCs. In order not to cause inflammation, DNA molecules undergo enzymatic cleavage, such as caspases in apoptosis. A defect in DNases that cut double-stranded DNA causes autoimmune diseases (SLE, polyarthritis) in mice. The nucleotides ATP and UTP, normally located in the cytoplasm, are released into the extracellular space during cell necrosis. Acting on the purinergic receptors of DCs, they induce chemotaxis of immature DCs, the formation of NALP3 inflammasomes and the secretion of IL-1β, a Th2 immune response. The effect of ATP on allergen-activated myeloid DCs provokes the development of pulmonary allergies and the maintenance of bronchial asthma. Nuclear ribonucleoproteins (their short fragments) are released during the destruction of NK and act as DAMPs, stimulating the formation of cytokines and α-chemokines. Urate salts, formed from uric acid during the destruction of endogenous nuclear or microbial DNA and sodium ions in the extracellular space in the cytoplasm, stimulate the formation of inflammasomes in macrophages and DCs, the synthesis of cytokines IL-1R, IL-18, IL-33, neutrophil infiltration, DC maturation, enhancement of antigen-specific T-cell response.

Stress-induced cytoplasmic chaperone proteins HSP70 and HSP90 enter the intercellular space during cell necrosis (but not apoptosis). Extracellular HSP70, HSP90 stimulate the formation of inflammatory cytokines (TNFa, IL-1R, IL-6, IL-12). The antigen-specific immune response to the peptide-HSP complex is significantly increased. Cellular receptors of HSPs are cD91,

CD40, TLR2/TLR4/CD14, scavenger receptors, LOX-1. NK secrete calgranulins (S100 proteins), which are recognized by RAGE receptors of endothelial cells, microglia, monocytes and become markers of inflammation (for pneumonia, polyarthritis, etc.). The release of cytokines (IL-1, IL-6, IL-33) can also be the result of stress on cells and their necrotic death. Proteases and biologically active molecules released from NK act on surrounding tissues and cleave off low molecular weight fragments from them (hyaluronic acid, fibrillar protein, collagen, heparan sulfate), which also cause inflammation.

As with the utilization of AA, serum factors (collectin MBL) bind to NA, enhancing their recognition and binding to calreticulin on the surface of macrophages. Macrophages recognize necrotic cells through TLRs, C-type lectin receptors Clec9A, RAGE; CD14, CD91, CD40, Mincle (interacting with SAP-130) and others. It is important that phagocyte receptors that recognize NK do not recognize AA and (partially) recognize molecules (PAMPs) of pathogens (mycobacteria, fungi, etc.).

Regulated necrosis (necroptosis) of cells is associated with the activity of RIPK1 and RIPK3 kinases, manifested by a rapid increase in the permeability of cell membranes and the release of intracellular DAMPs into the extracellular space. Necroptosis of skin cells, mucous membranes, and leukocytes during ischemic reperfusion causes a strong inflammatory response. At the same time, it acts as a protective mechanism during viral infection (in the presence of viral caspase 8 inhibitors), and also participates in maintaining T-lymphocyte homeostasis. Necroptosis of an infected cell means a sharp change in the habitat of intracellular pathogens, which is detrimental to them. Pyroptosis of cells, having features of apoptosis and necrosis, is characterized by the formation of inflammasomes as a complex of activated caspases and producers of inflammatory cytokines IL-1R and IL-18. Pyroptosis effectively protects cells from S. aureus, S. typhimurium, P. aeruginosa, L. pneumophila, F. tularensis, B. anthracis. In this case, different types of specialized inflammasomes are formed in response to living bacteria, their toxins, LPS, spores, flagellin, DNA, RNA of viruses and bacteria. Cell necrosis characterizes the advanced (not early) stages of the infectious process, when pathogens (Shigella, Salmonella, Yersinia, M. tuberculosis) move from the tactics of survival in apoptotic cells to the tactics of cell destruction and intercellular spread.

Secondary necrosis as an outcome of cell apoptosis is characterized by the release of nucleosome DAMPs (180 base pair genomic DNA fragments), HMGB1. Immunostimu-

Induction of various types of cell death by “danger signals”. Solid lines - the main effect, the dotted line - an additional effect (with a weak effect), -I means suppression of cell death. Other symbols are in the text.

IMMUNOLOGY No. 2, 2014

The lytic effect of such DAMPs is associated with the formation of nucleosome complexes with HMGB1, which are characteristic of patients with SLE. Secondary necrosis is accompanied by a massive release of modified (as a result of enzymatic treatment, oxidation) autoantigens, which, in combination with HSPs (and other DAMPs), cause an antigen-specific immune response. But only the presence of a genetic predisposition leads to the formation of autoimmune pathology.

Interactions between cell death pathways.

Autophagy and cell apoptosis are considered as mechanisms for maintaining the viability of a multicellular organism, and the formation of inflammasomes and necroinduced inflammation are considered mechanisms of limited tissue death to preserve the macroorganism. Recognition of DAMPs during autophagy creates additional insurance for the cells of the macroorganism in protection against pathogens with unknown PAMPs. As a result of infection of macrophages with L. pneumophila, activation of inflammasomes causes pyroptosis and autophagy, which protects the cell from pyroptosis and the pathogen. But insufficiency of autophagy to counteract the pathogen leads the infected cell to pyroptosis. The triggering of the PIRK1-3-dependent mechanism of necroptosis involves an initially high level of autophagy of damaged mitochondria and, if it is ineffective, subsequent cell degradation. Autophagy acts as a mechanism for the disposal of phagocytosed apoptotic bodies by macrophages and DCs. During cell necrosis, an increase in the level of HMGBT in the cytoplasm stimulates, together with HSP27, autophagy (mitophagy) of mitochondria and suppresses apoptosis. Other DAMPs (ATP, S100 proteins/calgranulins, double-stranded DNA), interacting with TLRs, also stimulate autophagy in foci of apoptosis. It is known that the main Beclin 1-dependent autophagy pathway (macroautophagy) can be suppressed by anti-apoptotic proteins of the Bcl-2 family and the formation of NLRP3 inflammasomes, i.e., increasing cell resistance to apoptotic death increases its resistance to excessive autophagy leading to death cells During phagocytosis of cells that have died by autophagy or apoptosis, there is no inflammation. Blocking autophagy in the cell leads to the accumulation of damaged mitochondria, superoxide anions, activation of the NALP3 inflammasome, and inflammation in the cytoplasm. The interaction of DAMPs with RAGE receptors stimulates autophagy and suppresses cell apoptosis. When DAMPs are insufficiently released from NK at the site of injury, apoptotic cells induce a state of tolerance and a decrease in inflammation. DC maturation is caused by DAMPs from NK, but not by ACAMPs from AK. Macrophages that have phagocytosed AK release TGF, which causes the formation of Teg cells. During phagocytosis of AKs infected with E. coli, macrophages release TGF and IL-6, which leads to the formation of Th7 cells, and during phagocytosis of AKs, a Th1 immune response. When PAMPs and DAMPs act together, the latter act as an adjuvant. It is known that, depending on the dose of exposure (for example, TNF), the cell dies by apoptosis (at low concentrations) or necrosis (at high concentrations). The connection between apoptosis and cell necrosis is also determined by the presence of intermediate subtypes of cell death - necroptosis and others.

Different types of cell death as a result of cell response to external (including microorganisms) and internal influences can occur simultaneously and regulate each other (see diagram). The mechanisms that determine the choice of the path of cell death are not completely clear, but the stronger the impact, the stronger the response in the form of cell necrosis, a powerful inflammatory and immune reaction of the macroorganism. Weak effects (due to autologous apoptotic cell-associated molecular patterns (AcAMPs) or DAMPs, PAMPs of normal microflora) cause an intensification of autophagy and cell apoptosis without obvious inflammatory and immune reactions.

Conclusion. Death of cells of a macroorganism (human,

animals), due to external or internal reasons, causes an immune response to damage. At the same time, microbial effects are always dosed by the concentration and viability of the pathogen, its soluble products, and the localization of the source of damage. The combined action of PAMPs and DAMPs, which is most often encountered in real conditions, as well as the influence of tolerogenic apoptotic cells on their interaction require further study and assessment of the immunological consequences.

literature

1. Yarilin A.A. Apoptosis. The nature of the phenomenon and its role in the integrity of the organism. Pathological physiology. 1998; 2: 38-48.

3. Bra M., Queenan B., Suzin S.A. Mitochondria in programmed cell death: various mechanisms of death. Biochemistry. 2005; 70 (2): 284-93.

4. Chernikov V.P., Belousova T.A., Kaktursky L.V. Morphological and biochemical criteria for cell death. Pathology archive. 2010; 72 (3): 48-54.

5. Galluzzi L., Vitale I., Abrams J.M., Alnemri E.S., Baehrecke E.H., Blagosklonny M.V et al. Molecular definition of cellular death subroutines: recommendations of the Nomenclature Committee on Cell Death 2012. Cell Death Different. 2012; 19 (1): 107-20.

9. Manskikh V.N. Pathways of cell death and their biological significance. Cytology. 2007; 49 (11): 909-15.

11. Khaitov R.M., Pashchenkov M.V., Pinegin B.V. The role of pattern recognition receptors in innate and adaptive immunity. Immunology. 2009; 1: 66-76.

15. Romao S., Gannage M., Munz C. Checking the garbage bin for problems in the house, or how autophagy assists in antigen presentation to the immune system. Semin. Cancer Biol. 2013; 23 (5): 391-6.

16. Rubinsztein D.C., Marino G., Kroemer G. Autophagy and aging. Cell. 2011; 146 (5): 682-95.

19. Walsh C.M., Edinger A.L. The complex interplay between autophagy, apoptosis and necrotic signals promotes T-cell homeostasis. Immunol. Rev. 2010; 236(1):95-109.

20. Amre D.K., Mack D.R., Morgan K., Krupoves A., Costea I., Lambrette P. et al. Autophagy gene ATG16L1 but not IRGM is associated with Crohn’s disease in Canadian children. Inflamm. BowelDis. 2009; 15 (4): 501-7.

21. Salminen A., Kaarniranta K., Kauppinen A. Beclin 1 interactome controls the crosstalk apoptosis, autophagy and inflammasome activation: impact on the aging process. Aging Res. Rev 2012; 12 (2): 520-34.

24. Mostowy S., Cossart P. Bacterial autophagy: restriction or promotion of bacterial replication? Trends Cell Biol. 2012; 22 (6): 283-91.

25. Randow F., MacMicking J.D., James L.C. Cellular self-defense:

how cell-autonomous immunity protects against pathogens. Science. 2013; 340 (6133): 701-6.

26. Lamkanfi M., Dixit v.M. Manipulation of host cell death pathways during microbial infections. Cell Host Microbe. 2010; 8(l): 44-54.

30. Bonarenko V.M., Likhoded V.G. Recognition of commensal microflora by pattern recognition receptors in human physiology and pathology. Journal of Microbiology, Epidemiology and Immunology. 2012; 3:82-9.

31. Paul-Clark M.J., George P.M., Gatheral T., Parzych K., Wright W.R., Crawford D. et al. Pharmacology and therapeutic potential of pattern recognition receptors. Pharmacol. Ther 2012; 135 (2): 200-15.

40. Byrne B.G., Dubuisson J.-F., Joshi A.D., Persson J.J., Swanson M.S. Inflammasome components coordinate autophage and pyroptosis as macrophage response to infection. mBio.2013; 4(1):e00620-

12. Available at http://mbio.asm.org/content/4/1/e00620-12.full. pdf+html

41. Kleinnijenhuis J., Oosting M., Platinga T. S., van der Meer J. W. M., Joosten L. A. B., Crevel R. V. et al. Autophagy modulates the Mycobacterium tuberculosis-induced cytokine response. Immunology. 2011; 134 (3): 341-8.

42. Garib F.Yu., Rizopoulu A.P. Interaction of pathogenic bacteria with host innate immune responses. Infection and immunity. 2012; 2 (3): 581-96.

47. Saas P., Angelot F., Bardiaux L., Seilles E., Garnache-Ottou F., Per-ruche S. Phosphatidylserine-expressing cell by-products in transfusion: a pro-inflammatory or an anti-inflammatory effects? Transfus. Clin. Biol. 2012; 19 (3): 90-7.

54. Miles K., Heaney J., Sibinska Z., Salter D., Savill J., Gray D. et al. A tolerogenic role for Toll-like receptor 9 is revealed by B-cell interaction with DNA complexes expressed on apoptotic cells. Proc. Natl Acad. Sci. USA. 2012; 109 (3): 887-92.

59. Proskuryakov S.Ya., Gabai V.L., Konoplyannikov A.G. Necrosis is a controlled form of programmed cell death. Biochemistry. 2002; 67 (4): 467-91.

63. Blander J.M., Sander L.E. Beyond pattern recognition: immune checkpoints for scaling the microbial threat. Nature Rev. Immunol. 2012; 12 (3): 215-25.

1. Yarilin A.A. Apoptosis. Nature of the phenomenon and its role in the whole organism. Pathologicheskaya fiziologiya. 1998; 2: 38-48 (in Russian).

2. Green D.R. The end and after: how dying cells impact the living organism. Immunity. 2011; 35 (4): 441-5.

3. Bras M., Queenan B., Susin S.A. Programmed cell death via mitochondria: Different modes of dying. Biokhimiya. 2005; 70 (2): 231-9 (in Russian).

4. Chernikov V.P., Belousova T.A., Kaktursky L.V. Morphological and biochemical criteria for cell death. Arkhiv patholoii. 2010; 72 (3): 48-54 (in Russian).

5. Galluzzi L., Vitale I., Abrams J.M., Alnemri E.S., Baehrecke E.H., Blagosklonny M.V. et al. Molecular definition of cellular death subroutines: recommendations of the Nomenclature Committee on Cell Death 2012. Cell Death Different. 2012; 19 (1): 107-20.

6. Peter C., Wesselborg S., Herrman M., Lauber K. Dangerous attraction: phagocyte recruitment and danger signals of apoptotic and necrotic cells. Apoptosis. 2010; 15 (9): 1007-28.

7. Kaczmarek A., Vandenabeele P., Krysko D.V. Necroptosis: the release of damage-associated molecular patterns and its physiological relevance. Immunity. 2013; 38 (2): 209-23.

8. Rock K.L., Lai J.-J., Kono H. Innate and adaptive immune responses to cell death. Immunol. Rev. 2011; 243 (1): 191-205.

9. Manskikh V.N. Pathways of cell death and their biological importance. Tsitologiya. 2007; 49 (11): 909-15 (in Russian).

10. Janeway C.A. Jr., Medzhitov R. Innate immune recognition. Ann. Rev. Immunol. 2002; 20 (1): 197-216.

11. Khaitov R.M., Pashchenkov M.V., Pinegin B.V. The role of pattern-recognizing receptors in congenital and active immunity. Immunology. 2009; 1: 66-76 (in Russian).

12. Seong S.Y., Matzinger P. Hydrophobicity: an ancient damage-associated molecular pattern that initiates innate immune responses. Nature Rev. Immunol. 2004; 4 (6): 469-78.

13. Chen G.Y., Nunez G. Sterile inflammation: sensing and reacting to damage. Nature Rev. Immunol. 2010; 10 (12): 826-37.

14. Kuballa P., Nolte W.M., Castoreno A.B., Xavier R.J. Autophagy and the immune system. Ann. Rev. Immunol. 2012; 30: 611-46.

15. Romao S., Gannage M., Munz C. Checking the garbage bin for problems in the house, or how autophagy assists in antigen

IMMUNOLOGY No. 2, 2014

presentation to the immune system. Semin. Cancer Biol. 2013; 23 (5): 391-6.

16. Rubinsztein D.c., Marino G., Kroemer G. Autophagy and aging. Cell. 2011; 146 (5): 682-95.

17. Tang D., Kang R., Coyne C.B., Zeh H.J., Lotze M.T. PAMPs and DAMPS: signal Os that spur autophagy and immunity. Immunol. Rev. 2012; 249 (1): 158-75.

18. Zelenay S., Reis e Sousa C. Adaptive immunity after cell death. Trends Immunol. 2013; 34 (7): 329-35.

19. Walsh C.M., Edinger A.L. The complex interplay between autophagy, apoptosis and necrotic signals promotes T-cell homeostasis. Immunol. Rev. 2010; 236(1):95-109.

22. Levine B., Mizushima N., Virgin H.W. Autophagy in immunity and inflammation. Nature. 2011; 469 (7330): 323-35.

23. Liu G., Bi Y., Wang R., Wang X. Self-eating and self-defense: autophagy controls innate immunity and adaptive immunity. J. Leukoc. Biol. 2013; 93 (4): 511-9.

24. Mostowy S., Cossart P. Bacterial autophagy: restriction or promotion of bacterial replication? Trends Cell Biol. 2012; 22 (6): 283-91.

25. Randow F., MacMicking J.D., James L.C. Cellular self-defense: how cell-autonomous immunity protects against pathogens. Science. 2013; 340 (6133): 701-6.

26. Lamkanfi M., Dixit V.M. Manipulation of host cell death pathways during microbial infections. Cell Host Microbe. 2010; 8 (1): 44-54.

27. Mintern J.D., Villadangos J.A. Autophagy and mechanisms of effective immunity. Front. Immunol. 2012; 3:60.

28. Travassos L.H., Carneiro L.A.M., Ramjeet M., Hussey S., Kim Y.-G., Magalhaes J.G. et al. Nod1 and Nod2 direct autophagy by recruiting ATG16L1 to the plasma membrane at the site of bacterial entry. Nature Immunol. 2010; 11 (1): 55-62.

29. Kumar H., Kawai T., Akira S. Pathogen recognition by the innate immune system. Int. Rev. Immunol. 2011; 30 (1): 16-34.

30. Bondarenko V.M., Likhoded V.G. Recognition of commensal microflora by pattern recognition receptors in human physiology and pathology. Zhurnal Mikrobiologii, epidemiologii i immunologii. 2012; 3: 82-9 (in Russian).

32. Strowig T., Henao-Mejia J., Elinav E., Flavell R. Inflammasomes in health and disease. Nature. 2012; 481 (7381): 278-86.

33. Underhill D.M., Goodridge H.S. Information processing during phagocytosis. Nature Rev. Immunol. 2012; 12 (7): 492-502.

34. Sander L.E., Davis M.J., Boekschoten M.V., Amsen D., Dascher C.C., Ryffel B. et al. Detection of prokaryotic mRNA signifies microbial viability and promotes immunity. Nature. 2011; 474 (7351): 385-9.

35. Schmid D., Pypaert M., Munz C. Antigen-loading compartments for major histocompatibility complex class II molecules continuously receive input from autophagosomes. Immunity. 2007; 26 (1): 79-92.

36. Paludan C., Schmid D., Landthaler M., Vockerodt M., Kube D., Tuschl T. et al. Endogenous MHC class II processing of a viral nuclear antigen after autophagy. Science. 2005; 307(5709):593-6.

37. Pua H.H., Guo J., Komatsu M., He Y.W. Autophagy is essential for mitochondrial clearance in mature T lymphocytes. J. Immunol. 2009; 182 (7): 4046-55.

38. Lu J.V., Walsh C.M. Programmed necrosis and autophagy in immune function. Immunol. Rev. 2012; 249 (1): 205-17.

39. Gros F., Arnold J., Page N., Decossas M., Korganow A.-S., Martin T. et al. Macroautophagy is deregulated in murine and human lupus T lymphocytes. Autophagy. 2012; 8 (7): 1113-23.

40. Byrne B.G., Dubuisson J.-F., Joshi A.D., Persson J.J., Swanson M.S. Inflammasome components coordinate autophage and pyroptosis as

macrophage response to infection. mBio. 2013; 4(1):e00620-12. Available at http://mbio.asm.org/content/4/1/e00620-12.full.pdf+html

41. Kleinnijenhuis J., Oosting M., Platinga T.S. , van der Meer J.W.M., Joosten L.A.B., Crevel R.V. et al. Autophagy modulates the Mycobacterium tuberculosis-induced cytokine response. Immunology. 2011; 134 (3): 341-8.

42. Garib F.Yu., Rizopulu A.P. Interaction of pathogenic bacteria with innate immune reactions of host. Infection and immunity. 2012; 2 (3): 581-96 (in Russian).

43. Majai G., Petrovski G., Fesus L. Inflammation and the apopto-phagocytic system. Immunol. Lett. 2006; 104 (1-2): 94-101.

44. Janssen W.J., Henson P.M. Cellular regulation of the inflammatory response. Toxicol. Pathol. 2012; 40 (2): 166-73.

45. Zitvogel L., Kepp O., Kroemer G. Decoding cell death signals in inflammation and immunity. Cell. 2010; 140(6):798-804.

46. Bekeredjian-Ding I. B cell encounters with apoptotic cells. Autoimmunity. 2013; 46 (5): 307-11.

47. Saas P., Angelot F., Bardiaux L., Seilles E., Garnache-Ottou F., Perruche S. Phosphatidylserine-expressing cell by-products in transfusion: a pro-inflammatory or an anti-inflammatory effects? Transfus. Clin. Biol. 2012; 19 (3): 90-7.

48. Jeannin P., Jaillon S., Delneste Y. Pattern recognition receptors in the immune response against dying cells. Curr. Opin. Immunol. 2008; 20 (5): 530-7.

49. Lauber K., Blumenthal S.B., Waibel M., Wesselborg S. Clearance of apoptotic cells: getting rid of the corpses. Mol. Cell. 2004; 14 (3): 277-87.

50. Fadok V.A., Bratton D.L., Guthrie L., Henson P.M. Differential effects of apoptotic versus lysed cells on macrophage production of cytokines: role of proteases. J. Immunol. 2001; 166 (11): 6847-54.

51. Hellberg L., Fuchs S., Gericke C., Sarkar A., Behhen M., Solbach W. et al. Proinflammatory stimuli enhance phagocytosis of apoptotic cells by neutrophil granulocytes. Scient. World J. 2011; 11: 2230-6.

52. Ferguson T.A., Choi J., Green D.R. Armed response: how dying cells influence T-cell functions. Immunol. Rev. 2011; 241 (1): 77-88.

53. Douglas I. S., Diaz del Valle F., Winn R. A., Voelkel N. F. P-catenin in the fibroproliferative response to acute lung injury. Am. J. Respira. Cell Mol. Biol. 2006; 34 (3): 274-85.

55. Ashida H., Mimuro H., Ogawa M., Kobayashi T., Sanada T., Kim M. et al. Cell death and infection: a double-edged sword for host and pathogen survival. J Cell Biol. 2011; 195 (6): 931-42.

56. Manfredi A.A., Capobianco A., Bianchi M.E., Rovere-Querini P. Regulation of dendritic- and T-cell fate by injury-associated endogenous signals. Crit. Rev. Immunol. 2009; 29 (1): 69-86.

57. Torchinsky M.B., Garaude J., Martin A.P., Blander J.M. Innate immune recognition of infected apoptotic cells directs T(H)17 cell differentiation. Nature. 2009; 458 (7234): 78-82.

58. Bianchi M.E. HMGB1 loves company. J. Leukoc. Biol. 2009; 86 (3): 573-6.

59. Proskuryakov S.Ya., Gabai V.L., Konoplyannikov A.G. Necrosis - an active, regulated form of programmed cell death (review). Biokhimiya. 2002; 67 (4): 467-91 (in Russian).

60. Idzko M., Hammad H., van Nimwegen M., Kool M., Willart M.A.M., Muskens F. et al. Extracellular ATP triggers and maintains asthmatic airway inflammation by activating dendritric cells. Nature Med. 2007; 13 (8): 913-9.

61. Kono H., Rock K.L. How dying cells alert the immune system to danger. Nature Rev. Immunol. 2008; 8 (4): 279-89.

62. Eigenbrod T., Park J.-H., Harder J., Iwakura Y., Nunez G. Cutting edge: Critical role for mesothelial cells in necrosis-induced inflammation through the recognition of IL-1a released from dying cells. J. Immunol. 2008; 181(2):8194-8.

Types and mechanisms of autophagy

There are now three types of autophagy: microautophagy, macroautophagy and chaperone-dependent autophagy. During microautophagy, macromolecules and fragments of cell membranes are simply captured by the lysosome. In this way, the cell can digest proteins when there is a lack of energy or building material (for example, during starvation). But microautophagy processes also occur under normal conditions and are generally non-selective. Sometimes organelles are also digested during microautophagy; Thus, microautophagy of peroxisomes and partial microautophagy of nuclei, in which the cell remains viable, have been described in yeast.

In macroautophagy, a region of cytoplasm (often containing some kind of organelle) is surrounded by a membrane compartment similar to the endoplasmic reticulum tank. As a result, this area is separated from the rest of the cytoplasm by two membranes. Such double-membrane organelles surrounding the excised organelles and cytoplasm are called autophagosomes. Autophagosomes combine with lysosomes to form autophagolysosomes, in which the organelles and the rest of the contents of the autophagosome are digested.
Apparently, macroautophagy is also nonselective, although it is often emphasized that with its help the cell can get rid of “outdated” organelles (mitochondria, ribosomes, etc.).
The third type of autophagy is chaperone-mediated. With this method, the directed transport of partially denatured proteins from the cytoplasm occurs through the lysosome membrane into its cavity, where they are digested. This type of autophagy, described only in mammals, is induced by stress. It occurs with the participation of cytoplasmic chaperone proteins of the hsc-70 family, auxiliary proteins and LAMP-2, which serves as a membrane receptor for the complex of chaperone and protein to be transported into the lysosome.
In the autophagic type of cell death, all organelles of the cell are digested, leaving only cellular debris that is absorbed by macrophages.

Regulation of autophagy

Autophagy accompanies the life of any normal cell under normal conditions. The main stimuli for enhancing autophagy processes in cells can be

lack of nutrients
the presence of damaged organelles in the cytoplasm
the presence of partially denatured proteins and their aggregates in the cytoplasm

In addition to starvation, autophagy can be induced by oxidative or toxic stress.
The genetic mechanisms regulating autophagy are currently being studied in detail in yeast. Thus, the formation of autophagosomes requires the activity of numerous proteins of the Atg family (autophagosome-related proteins). Homologues of these proteins have been found in mammals (including humans) and plants.

The importance of autophagy in normal and pathological processes

Autophagy is one of the ways to rid cells of unnecessary organelles, as well as the body of unnecessary cells.
Autophagy is especially important during embryogenesis, during the so-called self-programmed cell death. Nowadays, this variant of autophagy is more often called caspase-independent apoptosis. If these processes are disrupted and the destroyed cells are not removed, then the embryo most often becomes non-viable.
Sometimes, thanks to autophagy, the cell can compensate for the lack of nutrients and energy and return to normal functioning. On the contrary, in the case of intensification of autophagy processes, cells are destroyed, and in many cases their place is taken by connective tissue. Such disorders are one of the causes of heart failure.
Disturbances in the autophagy process can lead to inflammatory processes if parts of dead cells are not removed.
Autophagy disorders play a particularly important (albeit not fully understood) role in the development of myopathies and neurodegenerative diseases. Thus, in Alzheimer's disease, in the processes of neurons in the affected areas of the brain, there is an accumulation of immature autophagosomes, which are not transported to the cell body and do not fuse with lysosomes. Mutant huntingtin and alpha-synuclein - proteins whose accumulation in neurons causes Huntington's disease and Parkinson's disease, respectively - are taken up and digested by chaperone-dependent autophagy, and activation of this process prevents the formation of their aggregates in neurons.

Literature

Huang J, Klionsky D.J. Autophagy and human disease. Cell Cycle. 2007 Aug 1;6(15):1837-1849
Takahiro Shintani and Daniel J. Klionsky/Review/ Autophagy in Health and Disease: A Double-Edged Sword/Science, 2004, Vol. 306, no. 5698, pp. 990-995

Links

Wikimedia Foundation. 2010.

See what “Autophagy” is in other dictionaries:

- (auto + Greek phagein is) the process of destruction of parts of cells or whole cells by lysosomes of data or other cells, for example. with involution of the uterus after childbirth... Large medical dictionary

Diagram showing the cytoplasm, along with its components (or organelles), in a typical animal cell. Organelles: (1) Nucleolus (2) Nucleus (3) ... Wikipedia

Lysosome (from the Greek λύσις dissolve and sōma body) cellular organelle 0.2–0.4 µm in size, one of the types of vesicles. These single-membrane organelles are part of the vacuome (endomembrane system of the cell). Different types of lysosomes can be considered as separate... ... Wikipedia

- (from the Greek lýsis decay, decomposition and soma body) structures in the cells of animal and plant organisms containing enzymes (about 40) capable of breaking down (lysing) proteins, nucleic acids, polysaccharides, lipids (hence the name).... ... Great Soviet Encyclopedia

- ... Wikipedia

Andrea Solario. Madonna with a Green Cushion (circa 1507, Louvre). Breastfeeding, or natural feeding, is a form of nutrition for a newborn... Wikipedia

One of the features of aging is the inability of cells to adapt to stress conditions.
During life, irreversible damage accumulates in cells and, as a result,
dividing cells of regenerating tissues resort to two main mechanisms to prevent division. They can either permanently stop the cell cycle (login to a state of rest, "senescence"), or trigger the mechanism of programmed death.
There are several types of cell death. (suicide) is the most thoroughly described form of planned cell death. There is, however, another form of cell death - autophagy (self-eating), which is carried out using lysosomal degradation, which is important for maintaining homeostasis.
Unlike mitotic (dividing) cells, postmitotic cells such as neurons or cardiomyocytes cannot enter a resting state because they are already terminally differentiated. The fate of these cells is thus entirely dependent on their ability to cope with stress.
Autophagy is one of the main mechanisms for the elimination of damaged organelles, long-lived and abnormal proteins and excess cytoplasm.

Functioning of the cell as a system

Unicellular and multicellular organisms live in constant adaptation to external and internal damaging stimuli. The inevitable accumulation of damage leads to deterioration of cell components, deterioration of cellular functions and changes in tissue homeostasis, which ultimately affects the entire body.

Thus, aging is now viewed as the natural deterioration of the body over time, the deterioration of its “fitness,” presumably as a result of the accumulation of irreparable damage.

Many age-related pathologies arise from poor functioning of DNA repair mechanisms or abnormalities in the antioxidant mechanisms that promote detoxification.
reactive oxygen species. Oxidative stress plays an important role in tumorigenesis and in the decline of brain function, which have been attributed to age-dependent lipid peroxidation, protein oxidation, and oxidative modification of the mitochondrial genome and DNA.
Despite the common origin of these diseases, there are some differences depending on the age at which they occur. The incidence of cancer increases sharply after age 50, while the incidence of neurodegenerative disorders increases after age 70. One important difference between these two pathologies is the type of cells they affect.
Cancer primarily affects mitotic cells, while neurodegenerative disorders primarily affect postmitotic (non-dividing) cells.
Thus, the question arises of how the reaction of these types of cells in response to stress is fundamentally different. According to the proliferative structure of tissues, multicellular organisms can be divided into simple And complex. After development and differentiation, simple organisms (eg Caenorhabditis elegans and Drosophila melanogaster) consist only of postmitotic cells that are terminally differentiated and no longer divide. Conversely, complex organisms (such as mammals) are composed of both postmitotic and mitotic cells that are present in regenerating tissues and support their ability to reproduce.
One important difference between simple and complex organisms is their lifespan: the nematodes C. elegans live only a few weeks, the fruit flies D. melanogaster live several months, while mice can live several years and humans many decades. It is likely that the presence of regenerating tissues in the body makes it possible to replace damaged cells, thereby increasing life expectancy.
However, the self-regenerative potential of renewable tissues poses a risk for cancer. Accumulation of damage increases the risk of mitotic cells to acquire modifications in genomic DNA and therefore the risk of becoming a cancer cell.
In order to preserve the organism, damaged cells rely on two different mechanisms to stop their growth: they can either enter a state of cell cycle arrest (a process known as “senescence”) or trigger genetic cell death programs to die “quietly.” without affecting neighboring cells (through apoptosis and possibly autophagy).
For postmitotic cells, however, the cell damage behavior scenario is radically different. Since they're already stopped in phase G0, they cannot enter into a state of rest, into senescence. Lacking the advantage of proliferative renewal, post-motic cells, such as neurons or cardiomyocytes, are forced to adapt to stress in order to provide vital functions of the entire body.
In neurodegenerative pathologies such as Parkinson's disease, Alzheimer's disease, and Huntington's disease, protein aggregation results from insufficient removal of oxidized, misformed, or abnormal proteins in the brain. In this context, autophagy is the main pathway to ensure normal function of damaged tissue.

Cellular senescence (senescence)

is essentially a stopover phase G1 cell cycle of constantly proliferating cells in response to stress, in order to avoid the danger of transformation into a malignant cell. Resting cells adopt a flattened shape and trigger the expression of specific molecular markers associated with senescence—beta-galactosidase, aging-associated heterochromatic loci, and accumulation of lipofuscin granules.
promoting the transition of the cell to a state of rest.
Among them, telomere shortening, DNA damage and oxidative stress are the most well studied. Despite the diversity of these signals, they converge on two major effector pathways: the pathway and the pRB pathway (Fig. 1).
Under normal conditions, the activity of the tumor suppressor protein p53 is regulated by the MDM2 protein. However, under mitogenic stress or DNA damage, MDM2 activity is suppressed and functional p53 is able to activate cyclin-dependent kinase inhibitor p21, which stops the cell cycle.
In the second pathway, the retinoblastoma protein pRB is activated by the p16 protein under conditions of stress or DNA damage, which in turn binds to members of the E2F transcription factors known to initiate the cell cycle.
These two pathways overlap in the control of cellular aging and may also coincide with the initiation of cell death programs. For example, ventricular cardiomyocytes activate mitochondrial apoptosis when E2F expression is increased in these cells.
Although senescence is a way for cells to adapt in response to stressful conditions, this mechanism can, however, have a negative impact on the survival of the organism.
With age, senescent cells accumulate in proliferative tissues and produce various degradative proteases, growth factors and cytokines, which affect the functions of neighboring non-quiescent cells.
After massive accumulation of senescent cells, the proliferative potential of regenerating tissues decreases due to a decrease in stem cells. Altogether, these consequences can create an unfavorable environment that influences the development of neoplastic cells in the tumor, which ultimately increases the risk of cancer.

Apoptosis

Apoptosis is the most extensively studied form of programmed cell death, which plays an important role in embryonic development and organismal aging. It involves the controlled activation of proteases and other hydrolases, which quickly destroy all cellular structures.
Unlike cell death through necrosis, in which the cell membrane is destroyed and an inflammatory reaction is triggered, apoptosis occurs within an intact membrane, without damage to neighboring cells.
At the morphological level, the classic features of apoptosis are chromatin condensation (pyknosis), nuclear fragmentation (karyorrhexis), cell shrinkage and membrane blebbing. There are two main pathways for the initiation of apoptosis: intracellular (or mitochondrial) and extrinsic (Fig. 2).
During the intracellular pathway, several sensors, including BH3 proteins and p53, respond in response to various stress signals or DNA damage and activate a signaling cascade that leads to mitochondrial outer membrane permebelization (MOMP).
Proteins released from the intermembrane space of permeabilized mitochondria form a characteristic structure, the apoptosome, a caspase activation complex consisting of the APAF-1 protein (apoptotic protease activating factor 1), caspase-9 and cytochrome C, which leads to the activation of effector caspases, which destroys important cellular structures. Apoptosis triggered
at the mitochondrial level it is tightly regulated by the Bcl-2 family of proteins, which are divided into 3 groups: (1) anti-apoptotic multi-domain members (Bcl-2, Bcl-X L and Mcl-1), which contain four Bcl-2 homologous domains (BH1, BH2, BH3 and BH4), (2) pro-apoptotic multidomain members (such as Bax and Bak) lacking BH4 domains and (3) pro-apoptotic BH3 proteins (eg Bid, Bim and Bad).
Internal and external stimuli can activate proteolytic degradation of the Bid protein and translocation of the truncated bid ( tBid) to the mitochondrial membrane, where it stimulates MOMP, presumably by activating Bax/Bak channels and through other mechanisms.
The many intracellular interactions between Bcl-2 family members involve the integration of signaling cascades that modulate the levels and activity of these proteins to promote or avoid the initiation of mitochondrial apoptosis.
The extrinsic pathway begins in the plasma membrane through activation of the TNFR family of death receptors (tumor necrosis factor receptors), which are activated by the ligands Fas/CD95 and TRAIL (TNF-related apoptosis-inducing ligand). Receptor trimerization leads to the recruitment and activation of caspase-8 through special adapter proteins such as FADD/TRADD (Fas-associated death domains/TNFR1-associated death domains) to form a signaling complex that further transmits signals in at least three directions: (1) by direct proteolysis and activation of effector caspases, (2) by proteolysis of the BH3 protein Bid, translocation of tBid into mitochondria and subsequent permeabilization of the outer mitochondrial membrane, or (3) by activation of RIP1 kinase and (C-Jun N-terminal kinases), which leads to translocation of tBid into lysosomes and permebelization of Bax-dependent lysosomal membranes, resulting in general proteolysis by cathepsin B/D and MOMP.

Apoptosis and senescence

Like cellular senescence, apoptosis is an extreme form of the cellular response to stress and represents an important mechanism of tumor suppression. It is not yet clear what determines the path a cell takes. Although most cells are capable of both of these processes, they are still mutually exclusive.
The cell type is decisive, as damaged epithelial cells and fibroblasts generally enter quiescence, while damaged lymphocytes undergo apoptosis. In addition, it has been reported that by manipulating the expression level of Bcl-2 or inhibiting caspases, it is possible to direct a cell that would normally die by apoptosis into a quiescent state. Also, attempts have been made to inhibit cellular aging by increasing the level of telomerase, which ultimately does not prevent cellular aging, but protects cells from apoptosis.
These studies clearly indicate an intersection between the processes of apoptosis and cellular senescence, for example at the level of the tumor suppressor protein p53.
In colon cancer cells, activation of p53 leads to the initiation of apoptosis rather than quiescence after oncogenic exposure through increased expression of c-myc. However, the details and mechanisms of cross-regulation between apoptosis and cellular senescence need to be studied in more detail.

Autophagy

Autophagy (from the Greek words "auto" meaning self and "phagein" meaning "to absorb") is the process by which a cell's own components are delivered to lysosomes for global degradation (Figure 3). This ubiquitous process is involved in as an important regulatory mechanism for the elimination of damaged organelles, intracellular pathogens and excess cytoplasm, as well as long-lived, abnormal or aggregated proteins.
It has been shown that short-lived proteins are eliminated primarily through proteasomes.
At least three different types of autophagy have been described, which differ in the way they deliver organelles to lysosomes. The most detailed type of macroautophagy is described, in which elements of the cytoplasm and entire organelles are absorbed by so-called autophagosomes, which have a double membrane structure, or primary autophagic vacuoles(AV-I). After fusion with lysosomes, autophagosomes form a single-membrane structure called autolysosome(autolysosome) or late autophagic vacuoles(AV-II), the contents of which are degraded and the resulting elements are returned to the cytoplasm for metabolic reactions.
A comprehensive review on the formation of autophagosomal complexes.
The main negative regulator of macro-autophagy is , which typically triggers basic autophagosome formation, but its inhibition (for example, by rapamycin in the absence of nutrients) triggers macro-autophagy. Suppression of mTOR activity promotes enzymatic activation of a multiprotein complex, which is formed from phosphatidylinositol 3-kinase III (PI3K), vacuolar sorting protein 34 (Vps34), Beclin 1, vacuolar sorting protein 15 (Vps15), UV resistance protein (UVRAG), endophilin B1 (Bif-1), Beclin-1-dependent autophagy activation molecules (Ambra 1) and possibly other proteins.
This complex is negatively regulated by Bcl-2/X L proteins. Vps34 produces phosphatidylinositol 3-phosphate, a molecular signal for the assembly of autophagy complexes to form vesicle elongation and closure.
The macro-autophagy process can be inhibited through the insulin/IGF-1 pathway, where PI3Ks produce phosphatidylinositol-3,4,5-trisphosphate, which stimulate mTOR function.
The next type of autophagy, microautophagy, is not so well studied, in which organelles are absorbed directly into lysosomal membranes. This mechanism
is also a pathway for the degradation of organelles and long-lived proteins, but, unlike macro-autophagy, it is not responsible for adaptation to nutrient deprivation.
One specific form of micro-autophagy is the highly selective degradation of peroxisomes (micropexophagy), described in yeast as a mechanism of adaptation to oxidative stress.
The third type of self-eating is chaperone-associated autophagy(CMA). Although this pathway is also sensitive to nutritional deficiencies. substances, there is no total absorption of organelles or selective recognition of the substrate. In CMA, cytoplasmic proteins that contain specific penta-peptide motifs recognized by lysosomes (consensus sequence KFERQ) are recognized by a complex of chaperone proteins (including heat shock protein 73 kDa, hsc73) and targeted to the lysosomal membrane, where they interact with proteins lysosomal membrane associated (LAMP) 2a. Substrate proteins are then unfolded and transported to the lysosome lumen for degradation.
The KFERQ motif is found in approximately 30% of cytoplasmic proteins, including RNase A and amyloid precursor proteins (APP). Interestingly, APPs can be bound by hsc73 (and therefore fed to SMA) when the main pathway of their degradation is inhibited and this interaction does not occur through the APP KFFEQ sequence. It is not yet clear how the KFERQ motif is recognized by the chaperone complex.
Certain post-translational changes in substrates (e.g., oxidation or denaturation) can make this motif more accessible to chaperones, increasing their level of lysosomal uptake into the CMA.

Autophagy and apoptosis during cellular aging

In most cases, autophagy promotes cell survival by adapting cells to stress conditions. In this context, it is paradoxical that the autophagy mechanism is also a non-apoptotic cell death program, which is called “autophagic” or “type-II” cell death.
This is based on the fact that some cases of cell death are accompanied by massive autophagic vacuolation. However, these morphological observations cannot indicate whether cell death is accompanied by the formation of autophagic vacuoles or whether cell death actually occurs through autophagy. Indeed, the relationship between autophagy and apoptosis is complex, and
exactly what determines whether a cell dies by apoptosis or another mechanism remains unclear. In some cellular systems, autophagy is the only death mechanism, acting as a backup death mechanism when apoptosis in the cell is simply inhibited. Conversely, if, during cellular starvation, the autophagy process is blocked (for example, using small interfering RNA), then the apoptosis program is initiated.
In tumor cells of cell lines, when exposed to cytotoxic substances, the cells prefer autophagy, avoiding apoptosis and cellular senescence. Again, the p53 protein has been identified as one of the main regulators of the direction a cell will take. In senescent and postmitotic cells, autophagy serves as a stress adaptation mechanism.
It has been shown that autophagosomes accumulate in aging fibroblasts in order to promote the renewal of cytoplasmic substances and its organelles. Similarly, in cardiomyocytes, optimal mitochondrial functioning depends on macro-autophagy.
The functioning of one type of autophagy, CMA, decreases with age, which increases the risk of neuronal degeneration associated with the accumulation of mutant proteins susceptible to aggregation. It is noteworthy that age-related neurodegenerative diseases share similar characteristics with pathologies caused by autophagy-related gene (ATG) knockouts in the brain, such as the accumulation of ubiquitinated proteins and inclusion bodies in the cytoplasm, increased apoptosis in neurons and gradual loss of neuronal cells.
Nutrient starvation is the most commonly used method to induce autophagy in cultured cells, and indeed autophagy is a mechanism by which single-celled organisms (such as yeast cells) as well as mammalian cells can adapt to depleted resources.
During the degradation of macromolecules, ATP is released, which makes it possible to compensate for the lack of external power sources. It is important to note that this ability of autophagy may be involved in prolonging the life of the organism due to caloric restriction. Fasting or dietary restriction is one of the strongest stimuli for triggering autophagy throughout the body in mice and the nematode C. elegans.
In an interesting study, it was shown that turning off the atg genes in C. elegans reversed the anti-aging effects that were observed in individuals during calorie restriction.
The precise mechanism by which autophagy reduces aging is far from clear. However, it can be assumed that regular renewal of cytoplasmic structures and molecules “cleanses” and thereby rejuvenates cells. In addition, autophagy plays an important role in maintaining genome stability through mechanisms that are not yet understood.
Thus, an overall increase in autophagy levels may help avoid the long-term effects of DNA damage, a hypothesis that requires further study.

Concluding remarks

Embryogenesis and the development of multicellular organisms are the result of a balance between cell proliferation and cell death.
After tissue differentiation, tissues with proliferating cells and tissues with non-proliferating cells accumulate damage that is essential for maintaining life and accelerating aging.
In proliferative tissues, there are two different mechanisms that allow cells to avoid the progression of damaged cells into cancer cells: division arrest (a process known as cellular senescence) or programmed cell death (apoptosis and possibly also massive autophagy). In addition, aging is associated with an increasing risk of developing various pathologies associated with cellular damage.
In particular, neurodegeneration can develop due to a decrease in cellular mechanisms that are aimed at removing damaged elements. The main pathway for degradation of cytoplasmic elements is autophagy, which is reported to decrease with age.
Stimulating autophagy through caloric restriction may serve as a strategy to avoid the development of age-related diseases, as has been shown in C. elegans. However, the question remains open about what can have a positive effect on age-related changes in humans: induction of autophagy (periodic or continuous) through caloric restriction (intermittent or constant) or exposure to pharmacological agents.

Hall of Fame

Craig B. Thompson
Chairman and Professor, Dept of Cancer Biology and Medicine
University of Pennsylvania.
Thompson's laboratory studies the regulation of leukocyte development, cell proliferation, adaptation to stressful conditions, and apoptosis. One of the directions is the study of evolutionary modification of multicellular organisms as a mechanism of strict control over the processes of cell death and aging.

Russell T. Hepple, PhD

Associate Professor, Faculty of Kinesiology, University of Calgary, Canada
Hepple's laboratory focuses on the decline of muscle tissue function in relation to the regulation of cellular aging and death.

Judy Campisi, Buck Institute for Age Research, Buck Institute
8001 Redwood Blvd.
Novato, CA 94945

Radiobiologist [b] works at the Institute of Biology of the Scientific Center of the Ural Branch of the Russian Academy of Sciences in Syktyvkar: he is engaged in environmental genetics.