Lecture 12 - Apoptosis Flashcards
What is apoptosis? (very simply)
a distinctive and important mode of programmed cell death
What species was an important tool for studying programmed cell death?
c. elegans; can easily see all cells, researchers observed cells ‘dying’
How did researcher use c. elegans to study programmed cell death?
they gave worms toxic chemicals to mutate genes; isolated these mutants in which the cell remains (i.e., figured out the genes important for cell death)
- when genes that play role in cell death were removed, the cells that needed to die didn’t
What are the 3 groups of genes needed for apoptosis?
- specification: genes that tell cells they are in the lineage that needs to die
- killing: genes that coordinate the killing cascade
- execution: genes that actually do the killing
What are 2 examples of why apoptosis is important?
- sculpting of digits in the developing mouse paw: w/o apoptosis these would remain webbed
- apoptosis is required to clear the connecting tissue to make arthropod joints
Apoptotic cells vs Necrotic cells
apoptotic cells have an intact plasma membrane, they die neatly and do not damage their neighbours (inside -> out death)
necrotic cells spill their contents into their neighbours, damaging them
What does apoptosis depend on?
a proteolytic cascade mediated by Cysteine Aspartate Proteases = Caspases
Caspases
- once activated they chop p strategic proteins in the cell
- they are cysteine proteases that use the sulfur atom in cysteine to perform the cleavage reaction
- they cut proteins next to aspartate amino acids
What do initiator caspases do?
they begin the apoptotic program by activating the executioner cascades, which orchestrate the apoptosis program
ex: caspase 8 & 9
What are procaspases?
inactive precursor of caspases
- caspases are activated through cleavage at aspartic acids by other caspases
How do initiator caspases get activated?
by dimerization and self-cleavage
- an apoptotic signal triggers the assembly of an adaptor protein which then draws the 2 caspases together
- cleavage removes the pro domain where the adaptor protein binds and creates a small subunit and a large subunit
How are executioner caspases activated?
through cleavage by initiator caspases
- cleaves shorter pro domain and created the small and large subunits
Cascade of caspase activations makes apoptosis _______ and irreversible
amplifying; irreversible
Caspase activated DNAse (CAD) activation and function
activation: an active executioner caspase cleaves iCAD (inhibits CAD)
function: catalyzes the hydrolytic cleavage of DNA
- cleavage occurs between nucleosomes
Why does DNA need to be degraded?
its too long for other cells to deal with => must be made shorter
What are the two main activation pathways of apoptosis?
extrinsic pathway (signalled from outside of cell) and intrinsic pathway (signalled from within the cell => from mitochondria)
Activation of apoptosis through the extrinsic pathway
- A killer lymphocyte binds to an inactive Fas death receptor on the target cell with a Fas ligand
- death receptors cluster together and the death domains on the receptors recruit FADD adaptor protein
- FADD cluster recruit multiple copies of inactive initiator caspases, which oligermize
large structure is called the death-inducing signalling complex (DISC)
Activation of apoptosis through the intrinsic pathway
- mitochondria release cytochrome C into cytoplasm
- Cyt C should not be in cytoplasm, its presence there = bad - this release is the signal that activates the intrinsic pathway activation
- binding of Cyt C to Apaf1 exposes an oligomerization domain and a caspase recruitment domain on Apaf1
- oligomerized Apaf1 recruits an inactive caspase-9 monomer => forms apoptosome (large complex of Cyt C bound to Apaf1 and caspase-9 monomers)
- caspase-9 monomers are activated by dimerization
How does Apaf1 activate caspases?
through oligermerization into a huge complex called the apoptosome
What are the main regulators of the intrinsic pathway?
Bcl2 proteins
- pro-apoptotic Bcl2 proteins make holes into the mitochondrial membrane => Cyt C release
- proteins oligermize and form a bridge/hole in the membrane for Cyt C to leave
- anti-apoptotic Bcl2 proteins inactive pro-apoptotic Bcl2
- block oligomerization of pro-apoptotic Bcl2 proteins
- must happen before cascade starts
How do normal cells recognize apoptotic cells? What do they do to these cells?
- caspases degrade a flippase that keeps phosphatidyl serine (phospholipid) inside the cell
- in apoptotic cells PtdSer is outside
- normal cells recognize the presence of PtdSer on the outside of the cell
- normal cells then phagocytose and digest apoptotic cell
What happens in cancer cells?
- normally cells with reduced fitness will undergo apoptosis
- in cancer cells, the cells will kill other cells around them through apoptosis to make more room for cancerous cells to grow
Ferroptosis
an intracellular iron-dependent form of cell death distinct from apoptosis and necrosis
- characterized by the accumulation of oxidatively damaged phospholipids (lipid peroxidation)
