Lecture 18. Programmed Cell Death Flashcards
What are the features of classical necrosis?
External trigger (non-specific)
Moderate chromatin condensation
Cytoplasmic, mitochondrial and ER swelling
Rupture of the plasma membrane
Collateral damage to surrounding tissues
What is necrosis?
Cells die because of a response to trauma: they release their contents into the extracellular space – stimulating inflammatory responses. Uncontrolled, passive. Collateral damage
What is apoptosis?
A sequence of genetically programmed responses. Cell contents not released, no immune sequelae. Cells absorbed by neighbours. Controlled, active.
Apoptosis also occurs in plant cells where no inflammation takes places
Are apoptosis and necrosis deemed different?
Nom, no longer deemed different
What does programmed cell death lead to in unisexual flowers?
PCD lads to abortion or developmental arrest: of carpel primordia in male flowers and stamen abortion in female flowers
What is the role of programmed cell death (PCD) during sporogenesis?
PCD occurs in supporting sporophytic tissues including the anther tapetum (supportive tissue for microspores) and the nucellus in the developing ovules. After meiosis, non-functional megaspores degenerate
What are the five genetically programmed stages in trichome development?
Rapidly dividing protodermal cells (cells on leaf)
Trichome cell selcetion
Mitosis replaced by endoreduplication
Genetically controlled branching
Death programme activated (stinging trichome), stops spread of pathogen
How is PCD used for developmental changes in animals?
Mouse hand formation
Almost all larval structures are destroyed during Drosophila metamorphosis e.g. Salivary glands (SG), Muscles (M), midgut (MG) and hindgut (HG)
Almost all larval cells die, with larval tissue being replaced by adult tissue
How is PCD used to control cell numbers in mammals?
Deletion of cells that fail to partner controls cell number
Elimination of dangerous and abnormal cells such as autoreactive lymphocytes
Neural deletion adjusts number of nerve cells to target size
What is the overview of how apoptotic signals trigger apoptosis?
- Apoptotic signal (can have multiple signals)
- Signals assembly of an activating complex with initiator procaspases (there are multiple activating complexes)
- Activation of initiator caspases by proximity, autocleavage and rearrangement into fully active form (multiple caspases)
- Activation of executioner caspses by initiator caspase cleavage and rearrangement (links with development and inflammation)
What is intrinsic apoptosis?
Regulated cell death initiated by multiple signals and marked by mitochondrial outer membrane permeabilisation
Release the contents of the inner membrane space into the cytosol
What are examples of the multiple signals that can cause intrinsic apoptosis?
DNA damage, ER stress, ROS burden, replication stress, microtubular changes, mitotic abnormalities and more
How does initiation of intrinsic apoptosis occur?
- Apoptotic stimulus triggers the release of cytochrome c from the mitochondrial IMS by a membrane channel formed by Bax
- cyt c ais captured by and activates apoptotic protease activating factor-1 (Apaf-1)
- Apad-1 forms a heptameric apoptosome
- The heptameric apoptosome recuits procaspase 9 via caspase activation and recruitment domain (CARD) interactions: CARDs are members of the death domain (DD) family
- Caspase cascade leads to apoptosis
How is the intrinsic apoptotic pathway initiation regulated?
BCL-2 gene which inhibits the release of cytochrome c following an apoptotic stimulus
BCL-2 inhibits the pro-apoptotic gene (Bax like proteins) which from the channels in the outer membrane of the mitochondria and stimulates the release of cyt c
BH3-only proteins inhibit BCL-2
How is the intrinsic apoptotic pathway regulated in healthy cells?
Pro-apoptotic Bax family proteins circulate between the cytosol (TM domain hidden) and the mt OM (TM unmasked), inhibited when interact with the membrane
Anti-apoptotic Bcl-2 family members heterodimerise with membrane bound Bax family members and inhibit them
How is the intrinsic apoptotic pathway regulated in apoptotic cells?
Pro-apoptotic BH3-only proteins inhibit Bcl-2 family proteins, allowing membrane oligomerisation of Bax family proteins
Bax family proteins form a channel that releases cytochrome c into the cytosol: mitochondrial outer membrane permeability (MOMP)
How do BH3-only proteins inhibit Bcl-2 family members?
It is displaced by the BH3 domain of the BH3-only family proteins, blocking the hydrophobic groove: Bcl-2 family members can no longer heterodimerise with membrane-bound Bax family members. Thus, Bad proteins permit Bax protein family channel formation
How are BH3-only proteins tightly controlled?
BH3-only proteins are regulated post-transcriptionally by both survival stimuli (trophic signals) and apoptotic stimuli
Survival stimuli (trophic signals): Phosphorylation on S₇₅ and S₉₉ which promotes phosphorylation of S₁₁₈ (and S₁₃₄). S₁₁₈ phosphorylation disrupts the BH3 domain, and so releases Bcl-XL
Apoptotic stimuli: Methylation of R₉₄ and R₉₆ inhibits phosphorylation of S₉₉
How is PUMA (p53 upregulated modulator of apoptosis) expression controlled?
By transcriptional regulation as well as by phosphorylation
What happens when caspase 9 is activate by procaspase 9?
Activated by proximity
Caspase 9 activates an executioner called procspase 3, activating to caspase 3
Caspase 3 activates procasapse 6 and 7
These target thousands of substrates
What is the role of p53 in intrinsic apoptosis and development?
p53 activates apoptosis program - guardian of the genome - responds to genomic stress by signalling apoptotic signals to that cell
How does p53 function?
If p53 responds to much stress, it will trigger the transcription of PUMA
PUMA is closely related to Bad which inhibits Bcl-2, promoting the release of cytochrome c, promoting apoptosis
What can p53 create if it’s stressed in the right way?
PIDDOsome (p53 induce death domain), which activates CASp2 that deletes webbing between fingers, carves tissues into organs, organ genesis and development
What are the important parts of the PIDDOsome?
There is a p53 induced death domain protein (PIDD-CC)
There is an adaptor that has a death domain that heterodimerises with PID that has a CARD domain (RAIDD)
And that can recruit the initiator CARD bearing procaspase-2
