Apoptosis and Aging

APOPTOSIS AND AGING APOPTOSIS Apoptosis is a pathway of cell death in which cells activate enzymes that degrade the cells’ own nuclear DNA and nuclear and cytoplasmic proteins.  The plasma membrane of the apoptotic cell remains intact, but the membrane is altered in such a way that the cell and its fragments become avid targets for phagocytes.   The dead cell and its fragments are rapidly cleared before cellular contents have leaked out, so apoptotic cell death does not elicit an inflammatory reaction in the host.  Apoptosis differs in this respect from necrosis, which is characterized by loss of membrane integrity, enzymatic digestion of cells, leakage of cellular contents ,and frequently a host reaction. CAUSES OF APOPTOSIS Apoptosis in Physiologic Situations:  Death by apoptosis is a normal phenomenon that serves to eliminate cells that are no longer needed and to maintain a constant number of cells of various types in tissues.  The programmed destruction of cells during embryogenesis.  Involution of hormone-dependent tissues upon hormone deprivation, such as endometrial cell breakdown during the menstrual cycle, and regression of the lactating breast after weaning.  Cell loss in proliferating cell populations, such as intestinal crypt epithelia, in order to maintain a constant number.  Elimination of cells that have served their useful purpose, such as neutrophils in an acute inflammatory response.  Elimination of potentially harmful self-reactive lymphocytes, either before or after they have completed their maturation,I n order to prevent reactions against the body’s own tissues (   Cell death induced by cytotoxic T lymphocytes, a defense mechanism against viruses and tumors that serves to kill virus-infected and neoplastic cells . APOPTOSIS IN PATHOLOGIC CONDITIONS DNA damage. Radiation, cytotoxic anticancer drugs, extremes of temperature, and even hypoxia can damage DNA, either directly or through production of free radicals.  Accumulation of misfolded proteins.   Improperly folded proteins may arise because of mutations in the genes encoding these proteins or because of extrinsic factors, such as damage caused by free radicals. Excessive accumulation of these proteins in the ER leads to a condition called ER stress, which culminates in apoptotic death ofcells  Cell injury in certain infections, particularly viral infections, in which loss of infected cells is largely due to apoptotic death that may be induced by the virus (as in adenovirus and human immunodeficiency virus infections)or by the host immune response.  Pathologic atrophy in parenchymal organs after duct obstruction,such as occurs in the pancreas, parotid gland, and kidney. MORPHOLOGY In H&E-stained tissue sections, the nuclei of apoptotic cells show various stages of chromatin condensation and aggregation and, ultimately, karyorrhexis.  The cells rapidly shrink, form cytoplasmic buds, and fragment into apoptotic bodies composed of membrane-bound vesicles of cytosol and organelles.  MECHANISMS OF APOPTOSIS  Apoptosis results from the activation of enzymes called caspases (so named because they are cysteine proteases that cleave proteins after aspartic residues). The activation of caspases depends on a finely tuned balance between production of pro- and anti-apoptotic proteins. THE MITOCHONDRIAL (INTRINSIC) PATHWAY OF APOPTOSIS Mitochondria contain several proteins that are capable of inducing apoptosis; these proteins include cytochrome c and other proteins that neutralize endogenous inhibitors of apoptosis.  The choice between cell survival and death is determined by the permeability of mitochondria, which is controlled by a family of more than 20 proteins, the prototype of which is Bcl-2.   When cells are deprived of growth factors and other survival signals, or are exposed to agents that damage DNA, or accumulate unacceptable amounts of misfolded proteins, a number of sensors are activated. These sensors are members of the Bcl-2 family called “BH3 proteins”.  They in turn activate two pro-apoptotic members of the family called Bax and Bak, which dimerize, insert into the mitochondrial membrane, and form channels through which cytochrome c and other mitochondrial proteins escape into the cytosol. These sensors also inhibit the anti-apoptotic molecules Bcl-2 and Bcl-xL. THE DEATH RECEPTOR (EXTRINSIC) PATHWAY OF APOPTOSIS  Many cells express surface molecules, called death receptors, that trigger apoptosis. Most of these are members of the tumor necrosis factor (TNF) receptor family, which contain in their cytoplasmic regions a conserved “death domain,” The prototypic death receptors are the type I TNF receptor and Fas (CD95). Fas ligand (FasL) is a membrane protein expressed mainly on activated T lymphocytes.  When these T cells recognizeFas-expressing targets, Fas molecules are cross-linked byFasL and bind adaptor proteins via the death domain.These in turn recruit and activate caspase-8.   In many cell types caspase-8 may cleave and activate a pro-apoptotic member of the Bcl-2 family called Bid, thus feeding into the mitochondrial pathway. A VIABLE CELL Survival signal (e.g.. gro•a"t.1h factor) Production of anti-ap optotic proteins (e.g .. Bcl-2. Bcl-xi_J B APOPTOSIS Irradiation DNA damage I CELL SURVIVAL Misfolded prot.eins �ivation of sensors -���o� �nism of Bci-2 No leakage of cytochrome c •• u Activation of Bax/Bak channel l.8akage of cytoc-hrome c. other proteins Activation of caspa.ses J APOPTOSIS, ACTIVATION AND FUNCTION OF CASPASES The mitochondrial and death receptor pathways lead to the activation of the initiator caspases, caspase-9 and -8, respectively.  activate another series of caspasesthat are called the executioner caspases. These activated caspases cleave numerous targets, culminating in activation of nucleases that degrade DNA and nucleoproteins. Caspases also degrade components of the nuclear matrix and cytoskeleton,leading to fragmentation of cells.  CLEARANCE OF APOPTOTIC CELLS  In normal cells phosphatidylserine is present on the inner leaflet of the plasma membrane, but in apoptotic cells this phospholipid “flips” to the outer leaflet, where it is recognized by tissue macrophages and leads to phagocytosis of the apoptotic cells. EXAMPLES OF APOPTOSIS  Growth Factor Deprivation: apoptosis is triggered bythe mitochondrial pathway and is attributable to activationof proapoptotic members of the Bcl-2 family and decreased synthesis of Bcl-2 and Bcl-xl DNA DAMAGE When DNA is damaged, the p53 protein accumulates in cells. It first arrests the cell cycle (at the G1 phase)to allow the DNA to be repaired before it is replicated.  p53 triggers apoptosis, mainly by stimulating sensors that ultimately activate Bax and Bak, and by increasing the synthesis of pro-apoptotic members of the Bcl-2 family. When p53 is mutated or absent , cells with damaged DNA that would otherwise undergo apoptosis survive.  ACCUMULATION OF MISFOLDED PROTEINS: ER STRESS  unfolded or misfolded proteins accumulate in the ER because of inherited mutations or environmental perturbations, they induce a protective cellular response that is called the unfolded protein response.  This response activates signaling pathways thatincrease the production of chaperones and retard protein translation, thus reducing the levels of misfolded proteinsin the cell. In circumstances in which the accumulation ofmisfolded proteins overwhelms these adaptations, theresult is ER stress, which leads to the activation of caspasesand ultimately apoptosis.   Apoptosis of Self-Reactive Lymphocytes: these lymphocytes encounter self antigens, the cells die by apoptosis. Both the mitochondrial pathway and the Fas death receptor pathway have been implicated in this process. Cytotoxic T Lymphocyte–Mediated Apoptosis: On activation, CTL granule proteasescalled granzymes enter the target cells. Granzymescleave proteins at aspartate residues and are able to activatecellular caspases. In this way, the CTL kills target cellsby directly inducing the effector phase of apoptosis,without engaging mitochondria or death receptors AUTOPHAGY  Autophagy (“self-eating”) refers to lysosomal digestion of the cell’s own components. It is a survival mechanism in times of nutrient deprivation, such that the starved cell subsists by eating its own contents and recycling these contents to provide nutrients and energy.  Autophagy is also involved in the clearance of misfolded proteins, for instance, in neurons and hepatocytes. CELLULAR AGING the result of a progressive decline in the life span and functional capacity of cells.  DNA damage: Although most DNA damage is repaired by DNA repair enzymes, some persists and accumulates as cells age.  A role of free radicals in DNA damage leading to aging has been postulated but remains controversial.  Decreased cellular replication  Aging is associated with progressive replicative senescence of cells.  Werner syndrome: rare disease characterized by premature aging, have a markedly reduced in vitro life span.   TElomeres are short repeated sequences of DNA present at the ends of linear chromosomes that are important for ensuring the complete replication of chromosome ends and for protecting the ends from fusion and degradation.  shortened. As the telomeres become shorter, the ends of chromosomes cannot be protected and are seen as broken DNA, which signals cell cycle arrest. Telomere length is maintained by nucleotide addition mediated by an enzyme called telomerase.  Telomerase is a specialized RNA-protein complex that uses its own RNA as a template for adding nucleotides to the ends of chromosomes. Telomerase activity is expressed in germ cells and is present at low levels in stem cells, but it is absent in most somatic tissues.  Conversely, in immortalized cancer cells, telomerase is usually reactivated and telomere length is stabilized, allowing the cells to proliferate indefinitely. Defective protein homeostasis.  Abnormal protein homeostasis(chaperons and protoesome) can have many effects on cell survival, replication, and functions. In addition, it may lead to accumulation of misfolded proteins, which can trigger pathways of apoptosis.  BIOCHEMICAL AGANTS  playing a role in counteracting the aging process are reduced signaling by insulin-like growth factor receptors, reduced activation of kinases (notably the “target of rapamycin,” [TOR], and the AKT kinase), and altered transcriptional activity.  Environmental stresses may also activate proteins of the Sirtuinfamily, such as Sir2, which function as protein deacetylases .These proteins may deacetylate and thereby activate DNA repair enzymes, thus stabilizing the DNA; in the absence of these proteins, DNA is more prone to damage.