Apoptosis Flashcards
What is apoptosis?
programmed cell death
most common type of cell death
what are the two types of cell deaths?
necrosis - accidental death; dirty way of dying; in necrosis, contents spill out of cell
apoptosis - clean way; programmed; in apoptosis, cell shrinks and condenses and contents never leak out
what does apoptosis help minimize?
apoptosis is a routine controlled cell death that minimizes spread of damage and/ or inflammation
how is apoptosis critical for development?
helps in development of mature forms.
cell death sculpts hands and feet during embryonic development
what are some importances of apoptosis?
its important for certain cells: abnormal, non-functional, potentially dangerous (cancer) cells
eliminate lymphocytes after destroying and ingesting microbes
organs can be kept the correct size (liver)
DNA damaged cells - destroyed if damage is irreparable
what are some of the phenotypes of apoptosis?
overall shrinkage in volume of cell and its nucleus
loss of adhesion to neighboring cells
formation of blebs on surface
DNA fragmentation
cytoskeleton collapses
nuclear envelope disassembles
Rapid engulfment of dying cell by phagocytosis (macrophages)
what are some characteristics of biochemistry of apoptotic cells?
- an endonuclease cleaves DNA not ladder of fragments in distinctive sizes
- cleavages occur in linker regions of nucleosomes; agarose gel shows pattern
what is the significance of Cytochrome C?
it is a marker of apoptosis
it is released from mitochondria
what is the significance of caspase?
apoptosis is an intracellular proteolytic cascade mediated by the proteases called caspases
activation of caspases is a key event in apoptosis
what does caspase stand for and what does it target? what amino acid is in the active site?
caspase = cysteine aspartyl specific protease
it targets proteins and cleaves them in their sequence where an aspartic amino acid residue occurs
cysteine is in the active site
where does caspase come from?
it is synthesized first as an inactive precursor - procaspase
becomes activated by protease cleavage
procaspases cleaved at specific sites to form a large and small subunit which form a heterodimer AND caspases activates procaspases
what are the two major classes of caspases? what do they do?
initiator caspase (initiates caspase)
executioner caspase: destroys actual targets - executes apoptosis
*does so by: cleaving downstream proteins; cleaves inactive endonuclease; targets cytoskeleton; attacks cell adhesion proteins
what are some characteristics of the caspase cascade?
- its irreversible
- there are specific caspases
- machinery for apoptosis (caspases) are always in place
- initiator caspase auto-activates itself (may create problem)
- executioner caspases cleaves cellular targets
what are the two apoptotic pathways? what are some characteristics about them?
internal pathway and external pathway; they depend on factors that INDUCE apoptosis (eg: internal, DNA abnormalities; external, removal of survival factors or proteins of TNF family)
internal: mitochondrial dependent
external: mitochondrial independent
describe the extrinsic pathway
-extracellular signals (Fas ligand) binds to cell surface death receptors (homotrimer) and triggers extrinsic pathway
then adaptor proteins are recruited: FADD adaptor (fas associated death domain) & procaspase-8 with death effector domain
- this forms DISC: death inducing signal complex
- activates caspase-8 or 10, which activates downstream executioner caspase-3
whats one method of controlling or restraining extrinsic pathway?
decoy receptors: FLIP (a protein resembling initiator procaspase with no proteolytic domain; competitive inhibitor against procaspase-8/10
they have ligand binding domain but no death domain; so it does not activate apoptosis
what is the intrinsic pathway?
apoptosis that is activated from inside the cell
what triggers intrinsic pathway?
in response to injury, DNA damage, lack of oxygen, nutrients or lack of extracellular survival signals
what is the key event in intrinsic pathway?
the translocation of cytochrome c from the intermediate space of mitochondrial
explain intrinsic pathway
the translocation of cytochrome c from the intermediate space of mitochondria to cytosol leads to it binding to adaptor protein to activate procaspase.
it binds to procaspase-activating adaptor protein: Apaf1
Apaf1 forms apotosome which activates caspase-9; caspase-9 activates executioner caspase-3 (common to both pathways)
what does Bcl2 family proteins do?
they regulate intrinsic pathways
what does Bcl2 protein itself do?
controls release of cytochrome c into cytosol (Bcl = B cell lymphoma)
what are the two types of Bcl2 proteins?
anti-apoptotic = blocks release of cytochrome c (Bcl2); Bcl2 protein has 4 distinctive domains called Bcl homology domains or BH domains
pro-apoptotic = promotes release of cytochrome c; (BH123 protein and BH-3 only protein)
what are the 3 classes of Bcl2 proteins?
Bcl2 protein (anti-apoptotic)
BH123 protein (pro-apoptotic)
BH3-only protein (pro-apoptotic)
how does the BH123 protein work? is it pro-apoptotic or anti-apoptotic?
it’s pro-apoptotic
- an apoptotic stimulus triggers intrinsic pathway
- BH123 protein becomes activated, forming an aggregation in the mitochondrial outer membrane and induces the release of cytochrome c - (which then goes on to form the apoptosome by binding to Apaf1)
what inhibits BH123?
Bcl2 proteins (Bcl2 and Bcl-XL) (both BH1234)
- mainly located on cytosolic surface of outer mito membrane
- these proteins prevent apoptosis by binding to pro-apoptotic proteins (BH123) and prevent aggregation into active form
What protein inhibits Bcl2 activity?
BH3-only protein; it’s pro-apoptotic
an active BH3-only protein is cytosolic
translocates to mito after apoptotic signal activates it
*inhibits Bcl2 protein FROM INHIBITING aggregation to release cytochrome c
what regulates caspases?
IAPs: Inhibitors of APotosis
they bind and inhibit caspases
some IAPs add ubiquitin to caspases to mark them for destruction by proteasome
others block apoptosis by binding to caspases
*this solves auto-activating caspases
What happens when you do need apoptosis to occur though? how do you get around the IAPs?
if there is an apoptotic stimuli or signal, then this causes the release of ANTI-IAPs from the mitochondria. these block activity of IAPs; the executioner caspases can therefore be activated with the IAPs blocked
speak on insufficient apoptosis leading to disease in regards to Bcl2
Excess Bcl2 promotes development of cancer by inhibiting apoptosis: DNA-damaged cells will not be programmed for cell death but can go on to cause cancer
speak on p53 mutation leading to disease
mutated p53 can no longer cause cell cycle arrest; insufficient and no longer promotes apoptosis. cells with DNA damage stick around and cancer can be generated.
what can excessive apoptosis lead to?
heart attacks and strokes
