Apoptosis Flashcards
What are the distinct forms of programmed cell death?
- Apoptosis
- Autophagy (self eating): degradation of cytoplasmic material via lysosomes
- Necrosis
What are the differences between what happens during apoptosis and necrosis?
Apoptosis:
- cell shrinks, chromatin condenses
- budding occurs
- cell breaks apart, membrane is kept in tact
- the apoptotic bodies are phagozytosed, engulfed by neighbouring cells
Necrosis:
- cell swells
- cell (membrane) becomes leaky, blebbing occurs
- cellular and nuclear lysis causes inflammation
Why does apoptosis occur?
developmentally programmed apoptosis
- patterning/ morphogenesis
- e.g seen in between fingers in embryonic development to separate them
- acridine orange is an apoptosis marker
stress-induced apoptosis
- eliminate damaged cells
- environmental stresses can cause cell death straight away, or quiescence and then recovery
What mediates apoptosis?
Caspases
What are caspases?
a family of cysteine proteases that are synthesised as inactive precursor enzymes
How do you activate caspases?
- activated by cleavage events and formation of a heterotetramer of 2 large subunits (p20) and 2 small subunits (p10)
- cysteine residue is important for the cleavage
What are the two groups of apoptotic caspases?
Initiator caspases and executioner caspases
How can you differentiate between the 2 groups of caspases?
by looking at their pro-domains and function
- initiator caspases have long pro-domains whereas executioner caspases have short pro-domains
- function of initiator caspases is to cleave and activate executioner caspases
Name the key processes in apoptosis.
(a) release of mitochondrial cytochrome c and other pro-apoptotic proteins (IAP antagonists)
(b) activation of caspases
(c) intrinsic and extrinsic pathways
(d) effector caspases working on different proteins
How can caspase independent apoptosis occur?
With the release of AIF (apoptosis inducing factor) form the mitochondria
What happens during the release of mitochondria during apoptosis?
Release of mitochondrial cytochrome c and other pro-apoptotic proteins (IAP antagonists)
- Outer membrane of mitochondria becomes permeable and releases certain molecules in responses to apoptosis stimuli
- leads to caspase-dependent apoptosis
What happens during the activation of caspases during apoptosis?
Activation of caspases
- the cytochrome c released from the mitochondria binds to a scaffold protein (APAF1)
- forming a apoptosome (7 molecules of cytochrome c bind to 7 molecules of APAF1)
- caspase 9 binds to apoptosome and becomes activated
- further cleaves caspases 3/7, further cleaves cell and destroys it
- IAP antagonists released inhibit the act of caspase inhibitors
What happens during the intrinsic pathway during apoptosis?
- DNA damage signals
- Activation of p53 pathway
- Activates Bcl-2 family proteins
- Can be either pro or against apoptosis - Cyt c release from the mitochondria
- Caspase 3/7 activation
- Apoptosis
What happens during the extrinsic pathway during apoptosis?
- Surface cell death signals
- Binds and forms a complex
- death-inducing signalling complex (DISC) - Leads to activation of caspase 8
- can lead straight to Caspase 3/7 activation
- can activate death promoting protein BID to Cyt C release
What can effector caspases act on during apoptosis?
- inactivation of DNA repair enzymes such as PARP if the cell is beyond repair
- in response to stress, either a quiescence/ recovery path is taken or apoptosis - inactivation of anti-apoptotic proteins
- breakdown of structural nuclear proteins
- e.g. Caspase 6
- when cells shrink and undergo DNA fragmentation, nuclear structure needs to be broken down - endonuclease activation and chromosomal DNA fragmentation
- DNAse chops DNA - phosphatidylserine exposure
- sending out “eat me” signals to phagocytes/ neighbouring cells so that they can be engulfed
- Caspase cleaves flippases so that the PS is exposed on the outside of the cell by floppases
What are the methods for detection of apoptosis?
Morphological changes
- apoptotic bodies
- chromatin condensation using staining
Cellular markers
- dyes (acridine orange)
- labelling antibodies and seeing the nuclear breakdown - loss of lamin B from nucleus
- labelling the “eat me” signal on the outer membrane of the cell
- recognise what is left after caspases cleave using cleavage reporters
How can apoptosis promote cancer?
- apoptotic cell is not silent
- if a cancer can inhibit apoptosis or resist it
- proliferation occurs around the cells that were meant to be killed but have not
- leading to tumour growth
- macrophages cumulated in the area can help give out signals for hyperplastic over growth
- apoptosis-induced compensatory proliferation
- if caspases are left in tact after radiation of chemo, it can lead to tumor repopulation
What are some non-apopototic functions of caspases?
- differentiation
- enucleation
- dendritic pruning
- learning/ memory
- sperm individualisation
- compensatory proliferation
- immunity
