Apoptosis Flashcards
Morphological features of apoptotic cells
- cell shrinkage/condensation
- cytoskeleton collapse
- heterochromatisation of nucleus (darkening around nuclear envelope)
- nuclear fragmentation
- cell surface bulges (blebs, apoptotic bodies)
- no inflammation
Differences between necrosis and apoptosis
little inflammatory response
specific genes involved in apoptosis
distinct biochemical pathways for apop
- removes unwanted cells (forming digits, elimination of lymphocytes)
- activates suicide programme as a response to damage
Example of when apoptosis is needed in the immune system
post infection neutrophils and other lymphocytes are in high concentration
- apoptosis helps bring back to normal level
What is an key biochemical feature that is needed for phagocytosis of apoptotic cells
plasma membrane has phosphatidyl serine (a negatively charges phospholipid) usually in the inner layer of the PM
When undergoing apoptosis:
- phosphatidyl serines migrate to outer layer of PM which signals phagocytic cells to ingest the apoptotic cell
What are caspases
proteases
- cleaves specific target sequences in proteins
synthesised as procaspases which can be activated into caspases by other caspases
What are the classes of caspases and what are their functions
initiator caspases (begin apoptosis)
- 8 and 9
- exist as inactive, soluble monomers (procaspases) on cytosol
executioner caspases (activated by initiator caspses)
- 3, 6 and 7
- exist as an inactive dimer where their active site is rearranged when cleaves to the active conformation
How is the cell killed in apoptosis
cleavage of key proteins by caspases
What are the 2 signalling pathways which lead to apoptosis
- extrinsic pathway
- caspases activated by the binding of apoptotic signals to death receptors - intrinsic pathway
- caspases activated from within the cell
- used when DNA is damaged or there is a lack of oxygen
what is the Fas death receptor
tumour necrosis factor (TNF) receptor family
- transmembrane homotrimer protein
- CD95 best known type
Structure:
- extracellular ligand binding domain
- intracellular death domain
- ligands are also homotrimers
What responses is CD95 death receptor involved in?
- deletion of activated T cells at the end of immune response
- killing of virus-infected and cancer cells by T cells
Fas receptor pathway of apoptosis
- killer lymphocytes produce a Fas ligand which binds to the extracellular death receptor
- Fas receptor recruits intracellular adaptor proteins which bind to the death domain
- procaspse 8 binds to the death effector domain on the adaptor protein
- caspase 8 becomes activated and then activates other executioner caspases
- apoptosis is induced
Intrinsic pathway (full process)
- cytochrome c is released from mitochondria into cytosol
- cytochrome c activates Apaf-1 (apoptotic protease activating factor 1) which forms an apoptosome
- many of the Apaf-1 and cytochrome c molecules forms a spinning
wheel via CARD domains which allows the addition of procaspase
9 to bind via CARD domains
- many of the Apaf-1 and cytochrome c molecules forms a spinning
- apoptosome binds to and activates pro-caspase 9
- caspase 9 activation leads to activation of exectutioner caspases
- cleavage of cellular proteins which induces apoptosis
Which proteins help regulate the intrinsic pathway
Bcl2 proteins
What are Bcl2 proteins
conserved proteins
2 functional classes:
- pro-apoptotic (induces apoptosis)
- anti-apoptotic (stops apoptosis)
these proteins can bind to each. other forming heterodimer combinations which influence the release of cytochrome c
What are the classes distinct BH domains of Bcl2 proteins
- anti apoptotic
- BH1-4
- e.g. Bcl2, Bcl-XL
- pro-apoptotic BH123
- e.g Bax, Bak
- pro-apoptotic BH3 only
- e.g Bad, Bim, Bid, Puma, Nova
When the intrinsic pathway is inactive how do the Bcl2 proteins interact
Bax and Bak (effector Bcl2 proteins) found in mitochondrial membrane but inactive as anti-apoptotic Bcl2 inhibits effector Bcl2 proteins
Stopping the release of cytochrome c
When the intrinsic pathway is active how to Bcl2 proteins interact?
- apoptotic stimulus activates BH3 only Bcl2 effector proteins (Puma, Noxa)
- activated BH3 only inactivates anti-apoptotic Bcl2 proteins
- this allows effector Bcl2 proteins (Bax and Bak) on the mitochondrial membrane to bind together forming a heterodimer channel allowing cytochrome c to be released
How can the extrinsic and intrinsic apoptotic pathways be linked?
- activated caspase 8 from the extrinsic pathway can cleave Bid (BH3 only protein)
- This activates Bid which can then inactivate anti-apoptotic Bcl2 protein blocking bax and bak in the mitochondrial membrane
what is the function of Inhibitors of Apoptosis (IAP’s) and how do they do this
XIAP
ensures apoptosis is not accidentally triggered by binding and inhibiting activated caspases
- some ubiquitinate caspases for proteasome digestion
How are IAP’s stopped when apoptosis does need to occur and examples of them
anti-IAP’s which block IAP’s to promote apoptosis
examples:
- Smac/Diablo (released with cytochrome c from mitochondria
What is Syndactyly
incomplete separation of digits due to lack of apoptosis during embryonic development
