Lecture 6- Apoptosis Flashcards
give an example of cell death being important in normal processes
- apoptosis of cells joining digits (fingers and toes)- no webbed hands/feet
- tubes of cells formed by central cells undergoing apoptosis
what do dying cells release?
signals into their surroundings
what are the 3 main ways cells can die?
- necrosis
- apoptosis
- autophagy
what happens during necrosis?
- cell swells
- plasma membrane loses integrity
- components of cell released into surrounding tissue (detected as DAMPs)
what happens during apoptosis?
- cell shrinks
- plasma membrane retains integrity
- cell phagocytosed by macrophages
what happens during autophagy?
- plasma membranes maintain integrity
- organelles are broken down and used as nutrients- meaning cell doesn’t always die from autophagy
- cell is basically eating itself from inside out using lysosomes
what is apoptosis a response to?
cellular stress. e.g. infection or damage
what does apoptosis minimise (as opposed to necrosis)?
collateral damage
give the 6 main steps of how apoptosis occurs
- cell shrinkage
- cytoskeleton collapse
- loss of nuclear membrane
- chromatin condenses and DNA cleaved into fragments
- membrane blebs and breaks off into apoptotic bodies
- cell surface alters to attract phagocytes
give a flow diagram showing a general (non-specific) cell pathway showing controlled cell response to stimuli
signal –> activates receptor –> series of enzymes –> biological effects
during a normal signaling pathway, what enzyme is usually used?
protein kinases
what enzymes are used in the apoptotic signaling pathway?
proteinases
which proteinases are used in apoptosis?
caspases
what type of peptidase are caspases?
endopeptidases
what does a endopeptidase do?
cleaves specifically within a protein - DOES NOT DEGRADE CELL. Done to turn protein substrate on
what does the active site of caspase contain?
cysteine amino acid
where do caspase cleave substrate?
after aspartate residue
are all caspases involved in apoptosis?
no
generally, in what 2 ways do caspases kill cells?
- cuts up DNA
- allows apoptotic cell to be recognised by phagocytes
how do caspases cut up DNA?
CAD (Caspase Activated DNAse) is usually bound to an inhibitor (ICAD)
ICAD is cleaved at one site by caspase to remove the Inhibitor
the DNAse then dimerises, enters nucleus and cuts DNA between histones
how do caspases signal an apoptotic cell for phagocyte recognition?
- lipids in plasma membrane aren’t evenly distributed between inner and outer leaflet - phosphatidylcholine and phosphatidylserine are normally on inner leaflet
- iPLA2 is cleaved by caspase, activating it to cut phosphatidylcholine to release lysophosphatidylcholine that diffuses as a signal to phagocytes to attract to sell
- one type of flippase is turned off by caspase, catalysing phosphatidylserine to switch to be on the outerleaflet to signal to macrophages
what are the 2 main groups of caspase?
- initiator caspase
- executioner caspase
do initiator caspase have large or small pro-domain?
large
do executioner caspase have large or small pro-domain?
small
how are executioner caspases activated?
they’re proteolytically cleaved by initiator caspases
what is a caspase in an inactive form called?
zymogen
draw a flow diagram showing the caspase flow from signal to biological substrate
signal –> receptor –> initiator caspase (casp.9)–> executioner caspase (casp. 7 + 3) –> biological substrate
how must executioner zymogers become active caspases?
start as dimers and be cleaved to monomers . active monomer caspase.
how must initation zymogens become active caspases?
start as monomers- induced dimerisation. active dimer caspase
how specifically is caspase 9 activated?
- it’s activated by the apoptosome
- apoptosome is formed when APAF-1 is oligermerised by cytochrome C to a heptamer
the heptameric APAF-1 complex exposes its CARD domain which binds to the CARD domain of caspase 9 - this recruits multiple molecules of caspase 9 to the apoptosome- giving an active dimer
- the active caspase 9 cleaves itself from the apoptosome and can go on to activate caspase 3 and 7
how is apoptosome formation controlled?
- by the mitochondria which sequesters cytochrome C
how is cytochrome C involved in life and death of cell?
life- has an important role in the electron transport chain in respiration
death- allows caspase activation for apoptosis
what controls cytochrome release from mitochondria?
BCL-2 protein family
what does BAX do?
promotes cell death by antagonising BCL-2 expression
What does BCL-2 alone do for cell?
keeps it alive
what domain structure do all members of the BCL-2 family contain?
BH3
What does BCL-2 set for apoptosis to occur?
a threshold- only once enough BCL-2 is released from cell can BAX be activated in order to activate cytochrome c etc.
what are BH3 only proteins regulated by? (2)
GF signalling
DNA damage
What does DNA damage regulate?
PUMA via p53 (tumour suppressor)
what does chemotherapy try to do?
activate BH3 to cause cell death
what can suppress apoptosis (2 gene types)?
- oncogenes
- tumour supressor genes
