Senescence or biological aging

Senescence or biological aging:

Senescence or biological aging By- S wapnarani N ayak M . Sc 2 nd semester Biotechnology Dept. Fakir Mohan University

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Senescence is the biological process of age-related deterioration in function It manifests as dozens of changes in cells, tissues, and organs during aging. Human life is supported by a complex network of biochemical substances and reactions which affect the physical state and vitality of the body and mind. Senescent changes can be seen in the rate and outcome of many of these reactions. Such changes range from those affecting its cells and their function to that of the whole organism Definitions : life span--- Longest time that species is capable of living ( 110 years for humans ) life expectancy--- Average time that species lives (72-76 years for humans ) senescence--- The process of aging at the cell and organismal levels gerontology--- The study of aging

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Cellular senescence Cellular senescence is the phenomenon by which normal diploid cells lose the ability to divide, normally after about 50 cell divisions in vitro. Some cells become senescent after fewer replications cycles as a result of DNA double strand breaks, toxins, etc. This phenomenon is also known as " replicative senescence", the " Hayflick phenomenon", in 1965. In response to DNA damage (including shortened telomeres), cells either age or self-destruct (apoptosis, programmed cell death) if the damage cannot be repaired. In this 'cellular suicide', the death of one cell, or more, may benefit the organism as a whole. For example, in plants the death of the water-conducting xylem cells ( tracheidsand vessel elements) allows the cells to function more efficiently and so deliver water to the upper parts of a plant

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Cellular senescence (upper) Primary mouse embryonic fibroblast cells (MEFs) before senescence. Spindle-shaped. (lower) MEFs became senescent after passages. Cells grow larger, flatten shape and expressed senescence-associated β- galactosidase (SABG, blue areas), a marker of cellular senescence.

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Theories of aging Senescence is not universal, and scientific evidence suggests that cellular senescence evolved in certain species because it prevents the onset of cancer. In a few simple species, such as Hydra, senescence is negligible and cannot be detected. such species have no "post-mitotic" cells; they reduce the effect of damaging free radicals by cell division and dilution. In general, theories that explain senescence have been divided between the programmed and stochastic theories of aging . Programmed theories imply that aging is regulated by biological clocks operating throughout the lifespan. This regulation would depend on changes in gene expression that affect the systems responsible for maintenance, repair, and defense responses. Stochastic theories blame environmental impacts on living organisms that induce cumulative damage at various levels as the cause of aging, examples of which ranging from damage to DNA, damage to tissues and cells by oxygen radicals (widely known as free radicals countered by the even more well-known antioxidants), and cross-linking.

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Disposable Soma Theory for the Evolution of Senescence When multi cellular organisms develop, some of the cells (germ-line cells) are destined to become sperm or egg for the next generation. Other cells make up the body (soma cells), but will never become part of the next generation. In humans, some types of soma cells never divide again after reaching maturity (post mitotic or non mitotic cells). The brain, skeletal muscles, and heart contain large numbers of post mitotic cells. Our single-celled ancestors never lived as post mitotic cells, so they never had to evolve mechanisms for post mitotic immortality. Because multi cellular organisms are able to reproduce with germ-line cells before senescence of their soma incapacitates them, there was never any evolutionary impetus to develop biochemical mechanisms of preventing senescence in their post mitotic cells. The germ-line lives on in the next generation while the bodies of the former generations senesce and die. This scenario is called the Disposable Soma Theory for the Evolution of Senescence.

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Free-radical theory The free-radical theory of aging (FRTA) states that organisms age because cells accumulate free radical damage over time. A free radical is any atom or molecule that has a single unpaired electron in an outer shell. While a few free radicals such as melanin are not chemically reactive, most biologically-relevant free radicals are highly reactive. For most biological structures, free radical damage is closely associated with oxidative damage. Antioxidants are reducing agents, and limit oxidative damage to biological structures by passivating free radicals. T he free radical theory is only concerned with free radicals such as superoxide ( O 2 -) , but it has since been expanded to encompass oxidative damage from reactive oxygen species such as H 2 O 2 , or OH - . Denham Harman first proposed the free radical theory of aging in the 1950s,and in the 1970s extended the idea to implicate mitochondrial production of reactive oxygen species.

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Mechanisms of Senescence Oxidation, glycation , cross-linking, and other chemical modifications all act to impair the molecular functioning of multiple vital components, including DNA, membranes, the extracellular matrix (ECM), enzymes, and structural proteins. Modifications which accumulate faster than they are repaired or recycled will cause progressive deterioration over time. Junk molecules and aggregates accumulate inside and outside of cells. The redox potential poise of some cells changes in response to these chemical modifications. This results in altered gene expression , altered enzyme activity, and altered signaling pathways .

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Repair and recycling mechanisms slow down. A minority of deteriorating cells release chemicals which harm other, healthy cells. The neuro -endocrine and immune systems seem to follow a developmental program of decline, which may cause them to send chemical signals of differentiation and death to various tissues. Cells are lost by apoptosis and necrosis, especially among nondividing cells of the heart, skeletal muscle, and brain substantia nigra . Organs and tissues deteriorate over time when cells are lost faster than they are replaced. Stem cells stop dividing and no longer replace essential cells or regenerate tissues. Cont……

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Lifestyle and Environment Proper nutrition and exercise can have profoundly beneficial effects upon human health. However, while unhealthy lifestyle and environmental stresses can contribute to senescence and other diseases, they are not solely responsible for senescence. On the other hand, animal research has shown significant lifespan extension and improved health in old age can be attained through calorie restriction, provided that good levels of micronutrients, protein, and antioxidants are maintained. The reasons are still under study, but may be related to lower blood sugar levels which reduce glycation , altered mitochondrial function which reduces production of free-radicals , or altered insulin receptor signaling.

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Oxidative Stress and Free-radicals (ROS) There is strong evidence for the hypothesis that reactive chemicals and radiation can impair health and shorten lifespan through oxidative modifications and cross-linking which eventually drags the physiological machinery to a halt . Several species of highly reactive chemicals, called free radicals or reactive oxygen species ( ROS ) are by-products of normal metabolism. They are produced primarily in the mitochondria and the lysosomes . In defense, cells produce enzymes which prevent and repair some of this oxidative damage. Among these are superoxide dismutase and catalase , which detoxify some of the reactive chemicals. Dietary antioxidants, such as vitamin C, vitamin E, N-acetyl- cysteine , and alpha- Lipoic Acid, might also be important protective factors. However, many dietary supplement do not ever travel to the subcellular mitochondria and lysosomes where most free radicals exist. Furthermore, homeostatic mechanisms may downregulate antioxidants . In cell culture experiments, antioxidants can protect cells somewhat from oxidizing chemicals and radiation.

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With respect to specific types of chemical damage caused by metabolism, it is suggested that damage to long-lived biopolymers such as structural proteins or DNA, caused by oxygen and sugars, are in part responsible for aging. The damage can include breakage of biopolymer chains, cross-linking of biopolymers, or chemical attachment of unnatural substituents . Under normal aerobic conditions, approximately 4% of the oxygen metabolized by mitochondria is converted to superoxide ion, which can subsequently be converted to hydrogen peroxide , hydroxyl radical and eventually other reactive species including other peroxides and singlet oxygen which can, in turn, generate free radicals capable of damaging structural proteins and DNA. Certain metal ions found in the body, such as copper and iron, may participate in the process. These processes are termed oxidative damage and are linked to the benefits of nutritionally derived polyphenol antioxidants. Cont………

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Redox Potential Poise Another consequence of increased ROS concentration in a cell, or increased numbers of oxidatively modified lipids and proteins, is that the chemical equilibrium or poise of several redox couples is altered. This alters signaling pathways and enzyme activities, which in turn can alter chromatin conformation, alter gene expression, change the cell's differentiation state, send signals to other cells, or even intiate cell death.

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Lysosomes and Lipofuscin The recycling centers inside of cells are lysosomes . Tiny bags of digestive enzymes sealed in a membrane skin, these organelles take apart damaged or unneeded proteins, lipids, and mitochondria. However, occasionally, the lysosome will take in an abnormal chemical structure which resists degradation by lysosomal enzymes. This just stays inside the lysosome , eventually joined by other abnormal structures. Iron atoms in the lysosome produce streams of ROS, which chemically crosslink the abnormal structures together into larger masses. During the lifespan of an animal, lysosomes in postmitotic cells accumulate a lot of this highly cross-linked organic substance called " lipofuscin " or " ceroid ".

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Late in life, many of these lysosomes have so much lipofuscin that they are less able to recycle the proteins, lipids, and mitochondria. Consequently, damaged mitochondria accumulate in these cells, lowering ATP production and increasing ROS production. Furthermore, oxidatively damaged enzymes accumulate in the cytosol , reducing the rate of essential cellular functions. Lipofuscin may also cause lysosomes to leak degradative enzymes and ROS into the cytoplasm, causing further damage to cellular components. Cont….

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Glycation and Cross-Linking Sugar molecules in the blood and in the cells chemically bond to proteins and to DNA. (This bonding is called " glycation ", "the Maillard reaction", "the browning reaction", or " nonenzymatic glycosylation "). The process happens gradually, so that glycation accumulates over the years on the longest-lived proteins which do not get recycled very often. The clearest evidence of this is found in the extracellular collagen and elastin . Over time, in the presence of reactive oxygen species (ROS), glycation promotes covalent crosslinking between adjacent protein strands. Crosslinking reduces the flexibility, elasticity, and functionality of the proteins. Furthermore, the chemical modifications of glycation and crosslinking can initiate harmful inflammatory and autoimmune responses. Glycation and crosslinking contribute strongly to many progressive diseases of aging, including cardiovascular diseases, kidney disease, stiffness of joints and skin, impaired wound healing, complications of diabetes.

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sugars such as glucose and fructose can react with certain amino acids such as lysine and arginine and certain DNA bases such as guanine to produce sugar adducts, in a process called glycation . These adducts can further rearrange to form reactive species, which can then cross-link the structural proteins or DNA to similar biopolymers or other biomolecules such as non-structural proteins. It can damage proteins, lipids or DNA. Glycation mainly damages proteins. Damaged proteins and lipids accumulate in lysosomes as lipofuscin . Chemical damage to structural proteins can lead to loss of function; for example, damage to collagen of blood vessel walls can lead to vessel-wall stiffness and, thus, hypertension, and vessel wall thickening and reactive tissue formation (atherosclerosis); similar processes in the kidney can lead to renal failure. Damage to enzymes reduces cellular functionality. Lipid peroxidation of the inner mitochondrial membrane reduces the electric potential and the ability to generate energy. It is probably no accident that nearly all of the so-called "accelerated aging diseases" are due to defective DNA repair enzymes. Cont….

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Cellular Senescence, Telomeres, and Telomerase The telomerase gene is turned off in many adult human cells. As a result, these cells lose a bit off the ends (telomeres) of their chromosomes each time they divide. This appears to be related to their finite replicative lifespan in cell culture (The Hayflick Limit). One hypothesis suggests that as telomeres in some cells become very short, normally suppressed genes would be activated causing these cells to become "senescent". As a result, this small population of senescent cells would stop dividing and might export harmful signaling molecules (perhaps interleukins) to other cells in the body, resulting in some of the deleterious effects of human aging.

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Stem cell stasis If all stem cells were prevented from dividing, then tissue turnover and replacement of dying cells would be prevented. This could result in declines in functionality of many vital systems in the body. It has been hypothesized that stem cell division might be halted by either telomere shortening, chromatin modification, nuclear mutation, or altered hormonal signaling. This field is in need of further study. Stem cell stasis probably contributes to the deterioration of human aging. Progress in stem cell therapy could improve wound healing, tissue repair, and the treatment of many diseases.

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Insulin receptor signaling Side-effects of intracellular molecular signals from the insulin receptors affect the lifespan of the nematode worm, C. elegans . It is hypothesized that they may affect human health and lifespan as well. It has been suggested that lower insulin levels in calorie restricted animals might be partly responsible for their improved health and longer lives. This remains to be proven, as calorie restricted animals also have lower blood sugar levels, less glycation , and perhaps reduced ROS production.

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Chromatin changes and gene expression Changes in chromatin structure can alter access to regions of DNA by enzymes and ROS. Consequently, the expression of certain genes, and the probability of DNA damage and repair in these regions are altered when the chromatin structure changes. Chromatin changes are mediated by histone acetylation and DNA methylation , which are a cause and a result of gene expression and differentiation. It has been hypothesized that some of these changes might accumulate with age, perhaps in a small population of senescent cells which may spread damage throughout the body by malfunctioning or dying or by secreting damaging chemicals. Chromatin changes may also make the nuclear DNA more accessible to mutation events or to activation of cancer-promoting genes.

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Apoptosis is the process of programmed cell death (PCD) that may occur in multicellular organisms. These changes include blebbing , loss of cell membrane asymmetry and attachment, cell shrinkage, fragmentation, chromatin condensation, and chromosomal DNA fragmentation. Unlike necrosis, apoptosis produces cell fragments called apoptotic bodies that surrounding cells are able to engulf and quickly remove before the contents of the cell can spill out onto surrounding cells and cause damage Apoptosis

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Apoptosis Apoptosis increasing from normal cells (top) to apoptotic ones (bottom).

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The process of apoptosis is controlled by a diverse range of cell signals, which may originate either extracellularly ( extrinsic inducers ) or intracellularly ( intrinsic inducers ). Extracellular signals may include toxins ,hormones, growth factors , nitric oxide or cytokines, that must either cross the plasma membrane or transduce to effect a response. These signals may positively (i.e., trigger) or negatively (i.e., repress, inhibit, or dampen) affect apoptosis. (Binding and subsequent initiation of apoptosis by a molecule is termed positive induction , whereas the active repression or inhibition of apoptosis by a molecule is termed negative inductio n.) A cell initiates intracellular apoptotic signalling in response to a stress, which may bring about cell suicide. The binding of nuclear receptors by glucocorticoids , heat, radiation, nutrient deprivation, viral infection, and increased intracellular calcium concentration for example, by damage to the membrane, can all trigger the release of intracellular apoptotic signals by a damaged cell. Cont…….

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Before the actual process of cell death is precipitated by enzymes, apoptotic signals must cause regulatory proteins to initiate the apoptosis pathway. This step allows apoptotic signals to cause cell death, or the process to be stopped, should the cell no longer need to die. Several proteins are involved, but two main methods of regulation have been identified: targeting mitochondria functionality, or directly transducing the signal via adaptor proteins to the apoptotic mechanisms. Another extrinsic pathway for initiation identified in several toxin studies is an increase in calcium concentration within a cell caused by drug activity, which also can cause apoptosis via a calcium binding protease calpain . Cont…….

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Mitochondrial Deterioration Several diseases associated with senescence appear to be the direct result of cells containing dysfunctional mitochondria. The typical human cell has several hundred mitochondria. Mitochondria are the sites of three important processes which are involved in the progression of senescence: 1- Energy conversion (ATP) 2- Production of reactive oxygen species (ROS) 3- Initiation of cell death

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Mitochondrial regulation The mitochondria are essential to multicellular life. Without them, a cell ceases to respire aerobically and quickly dies, a fact exploited by some apoptotic pathways. Apoptotic proteins that target mitochondria affect them in different ways. They may cause mitochondrial swelling through the formation of membrane pores, or they may increase the permeability of the mitochondrial membrane and cause apoptotic effectors to leak out. There is also a growing body of evidence indicating that nitric oxide is able to induce apoptosis by helping to dissipate the membrane potential of mitochondria and therefore make it more permeable.

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Mitochondrial proteins known as SMACs (second mitochondria-derived activator of caspases ) are released into the cytosol following an increase in permeability. SMAC binds to inhibitor of apoptosis proteins (IAPs) and deactivates them, preventing the IAPs from arresting the apoptotic process and therefore allowing apoptosis to proceed. IAP also normally suppresses the activity of a group of cysteine proteases called caspases ,which carry out the degradation of the cell, therefore the actual degradation enzymes can be seen to be indirectly regulated by mitochondrial permeability. Cytochrome c is also released from mitochondria due to formation of a channel, MAC, in the outer mitochondrial membrane and serves a regulatory function as it precedes morphological change associated with apoptosis. Cont…….

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PROCESS Many pathways and signals lead to apoptosis, but there is only one mechanism that actually causes the death of a cell. After a cell receives stimulus, it undergoes organized degradation of cellular organelles by activated proteolytic caspases . A cell undergoing apoptosis shows a characteristic morphology: Cell shrinkage and rounding are shown because of the breakdown of the proteinaceous cytoskeleton by caspases . The cytoplasm appears dense, and the organelles appear tightly packed. Chromatin undergoes condensation into compact patches against the nuclear envelope(also known as the perinuclear envelope) in a process known as pyknosis , a hallmark of apoptosis. The nuclear envelope becomes discontinuous and the DNA inside it is fragmented in a process referred to as karyorrhexis . The nucleus breaks into several discrete chromatin bodies or nucleosomal units due to the degradation of DNA. The cell membrane shows irregular buds known as blebs .

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The cell breaks apart into several vesicles called apoptotic bodies , which are then phagocytosed . Apoptosis progresses quickly and its products are quickly removed, making it difficult to detect or visualize. During karyorrhexis , endonuclease activation leaves short DNA fragments, regularly spaced in size. These give a characteristic "laddered" appearance on agar gel after electrophoresis. Tests for DNA laddering differentiate apoptosis from toxic cell death. Cont…….

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Process

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Removal of dead cells The removal of dead cells by neighboring phagocytic cells has been termed efferocytosis . Dying cells that undergo the final stages of apoptosis display phagocytotic molecules, such as phosphatidylserine , on their cell surface. Phosphatidylserine is normally found on the cytosolic surface of the plasma membrane, but is redistributed during apoptosis to the extracellular surface by a hypothetical protein known as scramblase . These molecules mark the cell for phagocytosis by cells possessing the appropriate receptors, such as macrophages. Upon recognition, the phagocyte reorganizes its cytoskeleton for engulfment of the cell. The removal of dying cells by phagocytes occurs in an orderly manner without eliciting an inflammatory response .

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A section of mouse liver stained to show cells undergoing apoptosis (orange)

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Dysregulation of p53 The tumor-suppressor protein p53 accumulates when DNA is damaged due to a chain of biochemical factors. Part of this pathway includes alpha-interferon and beta-interferon, which induce transcription of the p53 gene and result in the increase of p53 protein level and enhancement of cancer cell-apoptosis. p53 prevents the cell from replicating by stopping the cell cycle at G1, or interphase , to give the cell time to repair, however it will induce apoptosis if damage is extensive and repair efforts fail. Any disruption to the regulation of the p53 or interferon genes will result in impaired apoptosis and the possible formation of tumors.

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Antioxidant An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions that damage cells. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions. They do this by being oxidized themselves, so antioxidants are often reducing agents such as thiols , ascorbic acid or polyphenols .

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Cont……. Although oxidation reactions are crucial for life, they can also be damaging; hence, plants and animals maintain complex systems of multiple types of antioxidants, such as glutathione, vitamin C, and vitamin E as well as enzymes such as catalase , superoxide dismutase and various peroxidases . Low levels of antioxidants, or inhibition of the antioxidant enzymes, cause oxidative stress and may damage or kill cells. Oxidative stress might be an important part of many human diseases, the use of antioxidants in pharmacology is intensively studied, particularly as treatments for stroke and neurodegenerative diseases. However, it is unknown whether oxidative stress is the cause or the consequence of disease.

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Study of aging is a multi-discipline process All aging organisms are able to undergo differentiation. A cell that does not differentiate does not age. Cells that do not differentiate only take in food, grow and divide. Can change food requirements by changing active genes and enzymes Conclusion:

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Bacteria and cancer cells do not age (they are immortal. as long as there is a food supply and wastes are taken away they will continue to grow and divide) As cells differentiate they need to be able to respond to external stimuli. Do this by receptors. Need to remove waste products and toxic substances and these processes require a lot of energy. The use of energy for these processes takes away energy for reproduction. Sexual reproduction takes a lot of energy from individual cells Cont…….

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Aging is species specific (mice live 20 X shorter than humans ). It is not the failure of individual cells that causes aging, but a breakdown in communication and response processes between cells so that the system is no longer controlled . Mice and people have different metabolic rates. Does this create the difference in life span ? Many studies needed and many things need to be found out to fully understand aging and how to delay the overall effects of aging in humans Cont…….

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all characteristics of living organisms are the result of natural selection Aging and its logical outcome, death, are ubiquitous and have survived therefore=implication is that aging and death confer success and are characteristics selected for during evolution. Remove all disease and still die at 100+ Cont…….