dental bio: apoptosis

apoptosis importance

• programmed cell death

• key aspect of cellular homeostasis

• major role from embryonic development to senescence

apoptosis occurance

• evolutionary conserved

• occurs in all multicellular animals studies (and plants)

• stages and genes conserved from nematodes (worms) and flies to mice and humans

necrosis

• results from damage by an external agent

• causes rupturing of cells and leakage of their contents into surrounding tissues

• non specific immune response occurs leading to inflammation

• loss of functional tissue

• impaired organ function, transient or permanent

apoptosis

• continuous, controlled process

• maintains cell numbers, balancing production of new cells by mitosis and death of older cells

• no leakage of the cell contents into surrounding tissues

• no inflammation

• removal of damaged or unnecessary cells

necrosis pathway

• cellular swelling → cell lyses causing inflammation

• membranes are broken

• ATP is depleted

• DNA fragmentation is random, or smeared

• in vivo, whole areas of tissues are affected

apoptosis pathway

• cellular condensation

• membranes remain intact

• requires ATP

• cell is phagocytosed → no tissue reaction

• ladder-like DNA fragmentation

• in vivo, individual cells affected NOT whole tissue

apoptosis destroy

• cells infected with virus

• cells of the immune system

• cells with DNA damage

• cancer cells

cell death mechanism involvement

• arrangement of cells during morphogeneis (human finger webs or tadpole tail dissapearing)

• regulation of cell number during homeostasis

• elimination of abnormal cells during pathogenesis

embryological development apoptosis

• in the 5th week: our hands and feet develop as paddle-like structures; ridges develop on our hands and feet which becomes our fingers and toes, apoptosis between the ridges allow our fingers and toes to separate

childhood apoptosis

between ages 8-14 the average child loses between 20-30 billion cells each day as a result of apoptosis

adulthood apoptosis

the average adult loses between 50-70 billion cells each day as a result of apoptosis

embryological apoptosis importance

• morphogenesis (eliminates excess cells)

• selection (eliminates non-functional cells)

• immunity (eliminates dangerous cells)

• organ size (eliminates excess cells)

adult apoptosis importance

• tissue remodeling (eliminates cells no longer needed)

• maintains organ size and function

too little apopotosis (hyperplasia)

cancerous cells have mutations that stop them from reacting to chemical signals that would normally start apoptosis, so they over-proliferate

too much apoptosis (tissue atrophy)

in neurodegenerative disease, like Alzheimer’s, apoptosis causes the progressive loss of neurons

apoptosis in pathologic conditions

• DNA damage due to radiation, chemotherapy

• accumulation of misfolded proteins leads to ER stress which ends with apoptosis

• cell death in viral infections that induce apoptosis such as HIV and adenovirus or by the host immune response such as hepatitis

• organ atrophy after duct obstruction

neurodegeneration

• neurons are post-mitotic (cannot replace themselves/replacement is inefficient)

• neuronal death caused by loss of proper connections, loss of proper growth factors, and/or damage (especially oxidative damage)

• neuronal dysfunction or damage results in loss of synapses or loss of cell bodies (synaptosis can be reversible; apoptosis = irreversible)

aging

• too much and too little apoptosis

• too much = tissue degeneration

• too little = dysfunctional cells accumulate, hyperplasia

cancer

• tumor supressor p53 controls senescence and apoptosis responses to damage

• most cells are defectice in apoptotic response (damages or mutant cells survive)

• can be caused by high levels or anti-apoptotic proteins OR low levels of pro-apoptotic proteins

apoptosis trigger

a range of internal and external circumstances

• normal development

• hormone-dependent changes in tissues

• cell loss in proliferating populations

• response to radiation or chemotherapeutic drugs

• elimination of abnormal cells

• elimination of lymphocytes

apoptosis stages

healthy cells → death signal → commitment to die (reversible) → execution (irreversible) → dead cell → engulfment → degradation

cell death by injury

• mechanical damage

• exposure to toxic chemicals

cell death by suicide

• internal signals

• external signals

withdrawal of positive signals (suicide)

• growth factors for neurons

• IL 2

receipt of negative signals (suicide)

• increased levels of oxidants within the cell

• damage to DNA by oxidants

• Death activators: TNF-a, TNF-b, Fas ligand (FasL)

mitogens (extracellular signaling molecule)

stimulate cell division, primarily by relieving intracellular negative controls that otherwise block progress through the cell cycle

growth factors (extracellular signaling molecule)

stimulate cell growth by promoting the synthesis of proteins and other macromolecules and by inhibiting their degradation

survival factors (extracellular signaling molecule)

promote cell survival by suppressing apoptosis

execution phase

• the intrinsic and extrinsic pathways converge to a caspase activation cascade

• caspases (cysteine-aspartic acid proteases) are conserved across species

• synthesized as inactive precursors; activated by proteolytic cleavage

• at least 12 proteases (a few involved in inflammation, and many in apoptosis)

removal of dead cells

dying cells secrete factors that recruit phagocytes. This facilitates prompt clearance before they undergo secondary necrosis. Dead cells disappear without a trace and do not produce inflammation.

mechanisms of apoptosis

• triggered by cell-death signals

• death receptor (extrinsic) pathway

• mitochondrial (intrinsic) pathway

• execution phase

• removal of dead cells

• carefully controlled by caspases

major players in apoptosis

• caspases

• adaptor proteins

• TNF and TNFR family

• Bcl-2 family

extrinsic pathway

death ligands → death receptors (TNF and Fas CD95)→ initiator caspase 8 → effector caspase 3 → apoptosis

• cells which originate from outside the cell

• can be triggered by NK cell that has detected abnormal behavior

• these ligands bind to surface receptor, causing a conformational change inside of the cell. this acts as a signal transduction and activates a protein cascade that produces caspases

intrinsic pathway

dna damage & p53 → mitochondria / cytochrome C → initiator caspase 9 → effector caspase 3 → apoptosis

• signals originate from inside the cell

• can be triggered by cytochrome C, DNA damage, or the presence of an abnormal protein like p53

• mitochondrial membranes become more permeable, and cytochrome c leaks into the cytosol. This activates a caspase cascade

• cells may also initiate apoptosis if certain factors required for growth are not present

cancer extrinsic apoptosis evasion

• extrinsic pathway is mediated by the death receptors: TNFR1, CD95, FasR, APO-1, DR4, and DR5

• death receptors are activated by regulatory ligands such as TRAIL, which induce DISC formation and initiate the apoptotic pathway

• in cancer cells, TRAIL resistance may be caused by various genetic mutations or over-expression of anti-apoptotic Bcl-2 proteins

ligand and receptor repectively

Ligands: FasL, TNF, TRAIL

receptor: FAS (CD95), TNF-R, Dr4 (TRAIL-R)

intrinsic pathway stress factors

• radiation

• heat

• nutrient deprivation

• viral infection

• an increase in the calcium ion concentration of the cytoplasm

• binding of glucocorticoids to receptors on the nuclear envelope

caspase roles

• activate more procaspase to caspase

• hydrolyse structural proteins in the cytoplasm and nucleus (breaks up the nucleus and cell shrinks and casue extensions called blebs)

• activate dna-hydrolysing enzymes

• small membrane-bound fragments bud off from the blebs and are engulfed by phagocytic white blood cells

intrinsic capsase activaiton

• intracellular signal in response to stress

• mitochondria release substances into the cytosol of the cell

• one of these is cytochrom c, one component of the electron transport chain of aerobic respiration

• in the cytosol, cytochrome c causes hydrolysis of inactive procaspase to activate caspase

extrinsic caspase activation

• a signal from a different cell, often a killer t cell

• each cell has so-called death receptors on its cell surface membrane

• when bound to a complementary protein on the cell-surface membrane of another cell, the death receptor recruits adaptor proteins inside the cytoplasm that attract procaspase molecules and hydrolyse them to activate caspase

p53

upregulated pro-apoptotic BH3 only proteins such as PUMA and NOXA and is inhibited by amplified expression of MDM2

Bcl-2

regulated by metabolite stresses, p53 inhibits glycolysis and activation of oncogenes drive aerobic glycolysis in cancer cells and promote a glycolytic phenotype which subverts micronutrient driven apoptotic activation.