Apoptosis Flashcards
What is apoptosis characterised by?
A series of dramatic perturbations to cellular architecture
What do these dramatic disturbances to cellular architecture contribute to?
Cell death
Preparation of cells for removal by phagocytes
Prevent unwanted immune response
Which protein family orchestrate the demolition phase of apoptosis?
Caspase family of cysteine proteases
What is a caspase?
Family of proteases with an essential cysteine residue in their active site
What are the two types of caspase?
Initiator and Effector
When are initiator caspases typically activated?
In response to particular stimuli
What are effector caspases important for?
Ordered dismantling of vital cellular structures
Why is apoptosis controlled?
To minimise damage and disruption to neighbouring cells.
How quickly are cells that undergo apoptosis replaced?
Within a few hours
How does the cell undergoing apoptosis initially appear from outside?
Becomes rounded and retracts from neighbouring cells
What follows cell rounding and retraction?
Plasma membrane blebbing. Blebs pinch off to form apoptotic bodies.
Why is phagocytic engulfment of cells undergoing apoptosis remarkable?
Phagocytes normally recognise and remove foreign/non-self entities. The phagocytes recognise a difference between apoptotic cells and their viable counterparts.
What do the many alterations in internal architecture do?
- Minimise activation of the immune system
- Increase the efficiency of disposal process
What is the most noticeable feature of apoptosis within the cell?
The condensation and fragmentation of the nucleus
What else undergoes fragmentation in apoptosis?
- Golgi apparatus
- ER
- Mitochondrial networks
What is released from the inter-membrane space of mitochondria?
Numerous proteins, including Cytochrome C
What does Cytochrome C do?
Triggers the assembly of caspase-activating complex (the apoptosome) on release into the cytosol.
What is the apoptosome?
A large protein complex that recruits pro-caspase-9 and allosterically activates caspase-9. Consists of cytochrome c and APAF1 (apoptotic protease-activating factor 1)
What are most proteolysis and cellular events in apoptosis due to?
Caspase-mediated cleavage of particular substrate proteins.
What is necrosis accompanied by?
Rapid loss of membrane integrity and release of cellular contents into the ECF
How does the immune system respond to necrotic and apoptotic cells?
Differently
How do necrotic cells trigger inflammation?
By releasing DAMPs (danger associated molecular patterns) stimulating receptors on neutrophils, macrophages, and other innate immune system elements.
How are caspases normally present (i.e. in a healthy cell)?
As zymogens (inactive precursor enzymes)
How many main routes to apoptosis associated caspase activation are there?
3
What are the major effector caspases?
Caspase-3, caspase-6, and caspase 7
What cytoskeleton components are substrates for caspases in apoptosis?
- Actin
- Actin-associated proteins - myosin, spectrins, alpha-actinin, gelsolin, filamin
- Tubulins, microtubule-associated proteins
What is a consequence of weakening the cytoskeleton?
Blebbing - this requires areas of the plasma membrane to be unsupported so blebs can form in these areas. Also requires some areas of intact filaments.
How do surrounding cells fill the gap left by dead cells?
Dying cell signal for the formation of actin cables within it’s neighbours. These cables contract and extrude the dying cell.
What does BCL-2 stand for?
B-cell lymphoma-2
What is the BCL-2 family?
Family of proteins crucial in regulation of apoptosis
How do BCL-2 proteins regulate apoptosis?
Regulate mitochondrial cytochrome c release
How many BCL-2 subfamilies are there?
3
What are the 3 BCL-2 subfamilies?
- Anti-apoptotic subfamily
- Pro-apoptotic BAX-like subfamily
- BH3-only proteins
What do BH3-ony proteins do?
Promote apoptosis when overexpressed
How many BH3-only proteins are transcriptionally upregulated byp53?
3 (NOXA, PUMA and BID)
What happens to some BH3-only proteins during cytockeleton disruption?
They are released from the cytoskeleton, where they are tethered, to promote apoptosis
What do anti-apoptotic BCL-2 family members do?
Block apoptosis by preventing BH3-only proteins from inducing the pro-apoptotic family members.
How do caspases act functionally?
As heterotetramers
Do all mammalian caspases participate in apoptosis?
No
Why does the nucleus fragment in apoptosis?
Dunno, no-one’s worked it out yet. Possibly to do with contributing to the irreversiblility of the death process.
What can result in delay to the onset of DNA fragmentation?
Mutant forms of lamins in the nuclear lamina that resist caspase-mediated proteolysis
How do cells undergoing apoptosis retract from neighbouring cells?
Caspase-dependant dismantling of cell-matrix focal adhesion points, and cell-cell adhesion complexes
What is a cadherin?
A transmembrane protein that plays a role in cell adhesion.
What do caspases do to cadherin?
Degrade them by proteolysis
What else do caspases target?
Proteins that function in transcription and translation, and rRNA.
What degrades DNA during apoptosis?
Caspase-activaed DNase (CAD)
How is CAD normally found?
In healthy cells, CAD is found with its inhibitor in a complex.
What happens to the CAD/ICAD complex in apoptosis?
Caspases cleave the ICAD to liberate CAD
What is the last act of the dying cell?
The generation of binding sites for phagocytes and the release of chemoattractant molecules
What are BCL-2 antagonists?
Cancer therapy that blocks the action of BCL-2 in cancer to promote apoptosis
What have BCL-2 antagonists proved effective in treating?
Chronic lymphocytic leukaemia
What are SMAC mimetics?
New targeted drug therapy that induce apoptotic death of cancer cells
What is IAP?
Inhibitor of Apoptosis protein - normally inhibit caspases to protect against inadvertent cell death. Expressed in excess in many human cancers.
What do SMAC mimetics do?
Induce proteasomal degradation of IAPs
