Apoptotic Signaling Flashcards
Forms of cell death
- Necrosis (passive play in cell murder)
- Apoptosis
- Autophagic Cell Death
- Cornficatoin (epidermis)
- all modes of cell death exhibit distinct morphologic features
Apoptosis
tightly regulated cell suicide process; cell numbers controlled at level of cell growth and by managing rate of cell death destroy cells that are no longer needed or are a threat to the organism
2 types: extrinsic and intrinsic apoptosis
Apoptosis main concepts
- fundamental cellular process essential in normal development and physiology
- highly regulated by specialized signaling pathways initiated form within or outside cells
- Excessive apoptosis or too little apoptosis contributes to range dx pathologies
- Novel clinical treatments in development for selected dx based on controlling apoptosis
Why do cells undergo apoptosis
cells die throughout the life cycle of animal in order to:
- Allow for proper development
- formation of fingers and toes
- proper nerve connects
- resorption tadpole tail at time of metamorphosis - Allow for tissue growth and regression in adult tissue
- remodels lactating gland after weaning via apoptosis milk secreting alveolar cells
- phenobarbital -> liver division and enlargement apoptosis increases when treatment stops apoptosis remains increased until liver returns to normal size - Remove cells that threaten organism survival
- cells with DNA damage
- cells infected with viruses (cytotoxic T cells kill infected cells by apoptosis)
- cells of immune system (regulate number of immune cells to prevent them from eliciting immune response against normal body constituents)
Morphologic changes of cells associated with apoptosis
Early events:
- cell shrinkage
- aggregation of chromatin at nuclear membrane
- bubbling and blebbing from cell surface but no loss cell integrity
Later events:
- Break up of nuclear envelope
- Separation of cell into smaller membrane bound bodies
- engulfment by resident phagocytes
- ** no inflammatory response during apoptosis, cells are eaten and digested by macrophage or neighboring cell; usually few dead cells in apoptotic tissue
Biochemical changes of cells associated with apoptosis
- Microscopy:
- Gel electrophoresis based assay
- Enzymatic assay
biochemical changes of cells associated with apoptosis Microscopy
- Detect relocalization of cytochrome C from mitochondria to cytoplasm (mirochondria becomes leaky during apoptosis bc pores formed in mitochondria membrane)
- Detect externalization fo phosphatidylserine; normally exclusively located in inner leaflet of PM in apoptotic cells PS flips to outer leaflets serving as “eat-me” signal -> rapid engulfment by macrophage (rapid clearance apoptotic cells prevent inflammatory response) (measure externalization with antibody annexin V)
biochemical changes of cells associated with apoptosis gel electrophoresis based assay
detect DNA ladder of 180 bp intervals (characteric of apoptosis) caused by internucleosomal DNA cleavage
biochemical changes of cells associated with apoptosis enzymatic assays
Measure caspase protease activity (detect proteolytic cleavage products of caspase activity)
Signals for apoptosis
can be intracellular or extracellular
Extracellular signals apoptosis
Changes in levels of extracelluar molecules
- increase in bone morphogenic proteins leads to cell death of cells btwn developing digits
- Binding of ligands called “death activators” to cell surface receptors including TNF-alpha and FAS ligand (FASL); T cell clearance stimulated by presence of death ligands
- withdraw of survival factors initiates apoptosis in nerve cells (produced in excess in development and compete for limited survival factors, insufficient amounts -> apoptosis)
Intracellular signals apoptosis
usually DNA damge
- increase levels of oxidants within cell
- increase in P53 levels resulting from DNA damage b/c UV light or chemo drugs
Two main apoptotic pathways
Extrinsic and intrinsic pathways
- have diff triggers and regulators
- both result in activation of caspase (protease cascade)
- is some “cross talk” cascade (synergy to amplify signal)
Extrinsic pathway
aka death receptor pathway (responds to extracellular signals)
2 main players= death receptors (link btwn signal and stress) and caspases (aka effectors execute final stages of apoptosis)
- signal
- death receptor activation
-caspase activaiton
- cell death
Death receptors
transmembrane proteins present on cell-surface; contain extra-celluar ligand-binding domain and intracellular death domain; receptors belong to tumor necrosis receptor family (TNF)
bind death ligands (pro-apoptotic signals) (ligand examples TNF and FasL, form trimers recruit FADD protein links to initiator caspase forming disc
DISC
death inducing signal complex; includes Death receptor, FADD protein (= adaptor, form glue that binds complex), initiator caspace
Caspase
family or proteases have a cystine active site and cleave their targets at aspartic acid; synthesized in cell as inactive precursors (procaspases or zymogens) activated upon cleavage that removes prodomain
How does caspase work
Synthesized as inactive precurosr -> cleavage that removes pro domain -> activation -> cleave and activate other procaspases -> amplifying proteolytic caspase cascade within cells (at critical point cascade becomes irreversible)
* caspases form cascade of multiple caspases, initially activated caspases = initiator caspases later executioner caspases
initiator caspases
initially activated caspases are most closely linked to apoptotic signal upon activation they cleave and activate executioner capspaces
executioner caspases
activated by initiator caspases; cleave target proteins, targets include cytoskeleton and nuclear proteins and cell-cell adhesion proteins their cleavage -> loss cell structure -> cell destruction and dissolution
