Lecture 9 - Introduction to Apoptosis (1) Flashcards
Why is apoptosis important?
Maintain homeostasis in organisms
Removal of
- damaged (damage to tissues)
- superfluous cells (development)
What role does apoptosis play in vertebrates?
Shaping the embryo
Ensuring homeostasis within adult tissues.
During apoptosis, cells shrink, fragment their DNA, bleb and break up into ‘apoptotic bodies’.
- Engulfed by phagocytes
- No cell lysis - no inflammatory responses
How is apoptosis different from other types of cell death?
Different from necrosis
+ unregulated cell death mostly from trauma to cells. Trauma including oxygen deprivation or hypoxia, and extreme environmental conditions such as heat, radiation, or exposure to ultraviolet irradiation.
Different from pyroptosis
+ caspase 1-dependent cell death, is inherently inflammatory, is triggered by various pathological stimuli, such as stroke, heart attack or cancer, and is crucial for controlling microbial infections.
New:
Ferroptosis - iron ions
Necroptosis
Autophagy
How is apoptosis regulated?
Energy-dependent
Regulated process - different pathways involved:
+ Extrinsic pathway is triggered by the engagement of cell surface ‘death receptors’ of the tumour necrosis factor (TNF) receptor family.
+ Intrinsic pathway is initiated by diverse cellular stresses.
Our focus is on the intrinsic pathway as an example to illustrate the apoptotic process
How is the intrinsic pathway of apoptosis mediated?
Mitochondria mediated
BCL-2 family of proteins:
+ pro-apoptotic
+ anti-apoptotic / pro-survival
Balance between these = life or death
What are the characteristics of BCL-2 proteins?
BCL-2 proteins are the key regulators of the intrinsic apoptosis pathway.
Each member of this family contains one or more BCL-2 homology (BH) domains.
+ So the proteins might carry BH1, BH2, BH3 or BH4 domains.
Pro-apoptotic or pro-survival proteins.
What is the intrinsic pathway?
Stimuli: DNA damage (p53) / oncogene activation / cytokine deprivation
BH3-only (pro-apoptotic initiators)
BCL-2, inhibited by BH3-only (pro-survival guardians)
BAX, inhibited by BCL-2 (pro-apoptotic effectors)
Mitochondrial outer membrane permeabilisation
Cytochrome C release
Capsases
Apoptosis
What are the kinds of stimuli that can cause the activation of the intrinsic pathway?
Apoptotic stimuli
* deprivation
* Growth factor deprivation
* Ionizing radiation
* Reactive oxygen species
* Chemotherapy (cytotoxic or targeted)
* Excessive mitogenic stimulation
Upstream signalling
* p53
* ER stress
* MYC
* Kinase inhibition
Regulation of BH-3 only proteins
How does regulation of BH3-only activators occur?
- BH3-only proteins are regulated by diverse transcriptional and post-translational mechanisms.
- E.g., BIM - transcriptionally activated, also splicing to give rise to splice variants BIMs, BIML, BIMEL.
- Also, post-translationally modified by phosphorylation.
E.g., tBID - cleavage needed for activation
E.g., PUMA - transcriptional targets of p53
What are the functions of the different types of BH3-only proteins?
BH3-only activators
- These pro-apoptotic activator proteins can be bound and sequestered by pro-survival proteins such as BCL-2.
- When the pro-survival proteins are saturated or absent, can activate BCL-2-associated X protein (BAX) and/or BCL-2 antagonist/ killer (BAK).
- Activated BAX or BAK oligomerises and forms pores to cause mitochondrial outer membrane permeabilisation
BH3-only sensitisers
- proteins that do not efficiently activate BAX and BAK but inhibit the activity of pro-survival BCL-2 family proteins to release any sequestered BH3-only activators, which trigger MOMP.
What are the downstream effects of the release of apoptogenic molecules?
Including second mitochondria-derived activator of caspases initiator (SMAC), serine protease OMI and cytochrome c from the caspase mitochondrial intermembrane space.
- Cytochrome c binds apoptotic protease-activating factor 1 (APAF1) in the cytosol to form the apoptosome, which serves as a platform for the activation of caspase 9, which then activates the effector caspases 3 and 7 to dismantle the cell and prepare it for phagocytosis.
- Caspase activation can be blocked by the X-linked inhibitor of apoptosis protein (XIAP), which in turn is inhibited by the released SMAC and OMI protein from the mitochondria.
What are the downstream effects of apoptosis in cells?
a. Apoptotic cells actively secrete ‘find me’ signals to attract monocytes and macrophages.
These signals induce a chemotactic response in monocytes and macrophages by binding to receptors of these phagocytic cells.
Apoptotic cells send out ‘keep out’ signals to neutrophils.
b. The phagocyte such as the macrophage recognizes its apoptotic target via exposed ‘eat me’ signals, such as externalized phosphatidylserine, which is bound directly by specific phosphatidylserine receptors or indirectly via bridging proteins.
- Activation of signaling molecules downstream of receptors that induce rearrangement of the actin cytoskeleton and internalization of the apoptotic cell by the phagocyte.
c. Upon ingestion of the apoptotic cell, the macrophages actively produce anti-inflammatory mediators as shown.
This actively creates an anti-inflammatory milieu at the sites of apoptotic cell death. ‘Tolerate me’
What are some markers of apoptosis?
Cytochrome c
DNA integrity
Activation of caspase
