Lecture 7 (apoptosis) Flashcards
Types of cell death mechanisms
-Apoptosis
-Autophagy
-Pyroptosis
-Necroptoisis
-Necrosis
Key morphological changes that apoptotic cells undergo:
-cell shrinkage
-blebbing
-nuclear fragmentation
-formation of apoptotic bodies
Morphological steps of apoptosis:
- Apoptotic signal
- Cell shrinkage and chromatin condensation
- Nuclear fragmentation and membrane blebbing
- Apoptotic body formation
Triggers of apoptosis
-Radiation
-Cytotoxic agents
-Toxins
-Growth factor/nutrient/oxygen deprivation
Caspases involvement in apoptosis:
Acts as a enzymatic mediator of apoptosis.
-Expressed as larger inactive pro-enzymes (pro-caspases).
Function and structure of caspase
-It is a cysteine protease that cleaves after aspartic acid residues.
-It is composed of a large and small subunit.
Two types of Caspases
- Initiator caspase
- Executioner caspase
Types of initiator caspases and their domains
-Caspase 8, 9, and 10
-Prodomain has DED in procaspase 8 and 10 and CARD in procaspase 9.
-These domains allow for homophilic interactions with adaptor proteins
Types of executioner caspases and how they are activated:
-Important in both intrinsic and extrinsic pathways
-Caspase 3 and 7
-Activation by proteolytic cleavage, mediated by initiator caspases
-Procaspase cleavage leads to tetramer of small and large heterodimers
3 ways executioner caspases kill cells?
-It disassembles the cell structures by degradation of nuclear lamina.
-It inactivates apoptotic inhibitors e.g i-CAD
-It deregulates other protein activity e.g gelsolin to actin depolymerisation
The Intrinsic Pathway of Apoptosis:
-Also known as mitochondrial pathway
-Activated by stresses signals being relayed to the mitochondria. Leading to mitochondrial outer membrane permeabilization (MOMP), which releases apoptotic proteins into the cytoplasm
Pro-apoptotic proteins released in the intrinsic pathway:
- Cytochrome C which associates with other proteins such as APaf-1 to allow for the formation of apoptosome.
- Smac (Diablo) inactivates a group of anti-apoptotic proteins called IAPs (inhibitors of apoptosis)
Cytochrome c fuction:
-Cytochome c + Apaf-1 + procaspase 9 = activation of caspase 9
-Caspase9 leads to activation of
effector caspases
What is APAF-1:
-It is a scaffold protein and stands for apoptosis protease activating factor 1.
-Together with cytochrome c and procaspase 9 it activates caspase 9.
-It also oligomerizes in response to cytochrome c release and forms a large complex known as apoptosome.
Apoptotic proteins:
-Proapoptotic (BH3 only): BIM, BAD, NOXA, PUMA, BID which activate Bax/Bak
-Proapoptotic (BAX subfamily): BAX, BOK and BAK which are essential for MOMP
-Anti-apoptotic: Bcl-2, Bcl-XL, MCL-1, Bcl-w are necessary for cell survival
BH3 proteins:
-Activate BAX/BAK either through direct binding and/or indirectly by binding to their repressors, prosurvival BCL2 protein
How are BH3 proteins regulated and activated and give examples of some:
-BH3 proteins are regulated at the transcriptional and post-translational level
and activated in response to given stimuli:
Examples:
-UV activates Bim
-Genotoxic damage activates Noxa, PUMA.
-Cytokine deprivation activates Bad
-Death receptors activates tBid
What determines cytochrome c release
-The relative levels of pro and anti-apoptotic proteins within each channel determine whether cytochrome c will be released from the mitochondrion.
Inhibitors of apoptosis (IAPs):
-Family of 8 proteins including c-IAP1, c-IAP2, XIAP.
-X-IAP inhibits caspase-3 and -9
-cIAP1 ubiquitinates caspase-3 and -7
Apoptosis in cancer cells:
-Upregulation of anti-apoptotic proteins is common in cancer (e.g. Bcl-2 and Mcl-1).
-Inactivation of pro-apoptotic proteins (e.g. Bax is inactivated in the majority of colon cancers)
Signaling regulation of the intrinsic pathway is controlled by:
-AKT
-P53
AKT role in intrinsic pathway:
AKT is a kinase activated by numerous growth factor receptor signals.
-AKT is anti apoptosis and does this by:
-Inactivating Bad and indirectly downregulating BIM and PUMA expression through the transcription factor, FOXO3a.
-It also prevents cleavage of Bid from extrinsic pathway.
P53 role in intrinsic pathway:
-P53 is proapoptotic and induces apoptosis by inducing expression of BAX, PUMA and NOXA (pro-apoptotic) and inactivating Bcl-2 (anti-apoptotic).
Extrinsic pathway of apoptosis:
-Also known as death receptor pathway
-Induced by external signalling caused by ligand binding to a specific membrane receptor
