Apoptosis Flashcards
Necrosis
Premature death of cells/cell injury caused by external factors like trauma or infection
Apoptosis
Programmed and controlled cell death in response to DNA or cell damage
Used during organism development, cell damage, or T cell death
Experimental Work
2002 Nobel Prize
- Bob Horvitz, Sydney Brenner, John Sulsoton for their work on C Elegans
- 14 essential apoptotic genes (ced mutants)
- complementation to determine proteins
- caspases, adaptors, regulators
Caspases
- cysteine aspartate proteases
- cleave target proteins C terminal to an aspartate residue
- 14 in humans, some with domains and some without
- initiators = activated first and have N terminal extentions
- executioners = activated later and don’t have N terminal extentions/more common
- caspase cascades lead to signal amplification that commits the cell completely and quickly to the apoptotic process
- caspases proteolyse much of the cellular proteome and activate caspase activated DNAse
Caspase Activation
- inactive procaspase has a cleavage sit
- the prodomain is removed and the caspase is cleaved into a large and small subunit in its active form
- procaspases retain 2% of activity so in pairs they can randomly cleave each other
Adaptor Proteins
- bridge between caspases and proaptotic input signal
Regulator Proteins
- pro and anti apoptosis versions
- all have TM sequence rich in hydrophobic residues that embed in the membranes (specifically mitochondrial membranes)
Intrinsic Pathway
- triggered by the release of cytochrome c from the inner mitochondrial membranes
- Bak/Bax are pro apoptosis regulator proteins that oligerimise and embed in the membrane to allow leakage
- Bcl2 is an anti apoptotic protein that prevents oligerimerisation
- cytochrome c binds to Apaf 1 adaptor protein
- Apaf 1 aggregates to form a complex apoptosome that ungergoes conformational changes and binds to procaspases
- activation of procaspase 9 (bringing many procaspases together increases the chances of cleavage)
Extrinsic Pathway
- Fas death receptor protein is found in the cell membrane containing a cytoplasmic death domain
- Fas ligand = TM protein whose expression is induced upon killer lymphocyte activation
- killer lymphocyte expresses Fas ligand that binds to Fas on the target cell membrane
- aggregation of Fas protein + adaptor proteins + procaspases that leads to caspase 8 activation
Apoptotic Bodies
- spherical membrane bound blebs containing cellular contents
- segments cells
- packages degraded components of cell
- blebs are phagocytozed and digested by lysosomal enzymes
- phosphatidylserine exposure to membrane surfaces serves as a ‘eat me’ signal to phagosomes
Morphological Changes in Apoptosis
- cell shrinkage/condensation
- cytoskeleton collapse
- nuclear envelope disassembly
- nuclear chromatin condenses/fragments
- apoptotic bodies form
BH3 Proteins
Link apoptotic stimuli and intrinsic paths via Bid protein activation (signal amplification)
IAP
Inhibitors of Apoptosis (IAP) bind to caspases or polyubiquilate them to set an inhibitory threshold caspases must overcome.
p53 gene and apoptosis
- induces gene expression of death receptors and pro apoptotic Bcl 2 proteins
- activates PUMA protein transcription
- releases apoptotic proteins from sequestration in the cytoplasm
p53 > PUMA transcription > p53 release from cytoplasmic Bcl-XL > Bax activation by p53
Cancer
- Attempt to restore apoptotic activity via caspase activation
- Cancer cells are closer to apoptosis than normal cells
- chemotherapy depends on cancer cells being able to undergo apoptosis but this isn’t always the case
- lack of apoptosis means DNA damage accumulates and carcinogenesis increases, potentially leading to therapy related leukemia
