3. LEARNING OBJECTIVES
Define Apoptosis
Enlist clinical examples of apoptosis in physiological
conditions
Enlist clinical examples of apoptosis in pathological
conditions
Describe mechanisms of apoptosis
Tabulate differences between necrosis and apoptosis
4. Definition
Apoptosis is a well controlled programmed individual
cell death that is caspase mediated and leads to
fragmentation of the cell and organelles into
numerous small buds, which are then engulfed by
macrophages without surrounding inflammation.
5.
6. Why should a cell commit suicide?
Programmed cell death is
as needed for proper
normal development as
mitosis is.
7. 1-The resorption of the tadpole
tail in frog .
2- The formation of the fingers
and toes of the fetus requires
the removal, by apoptosis.
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Apoptosis in Physiological condition:
8. 3- Programmed destruction during embryogenesis
4- Involution of hormone dependent tissues upon
hormone withdrawal such as
Endometrial cell breakdown during the menstrual
cycle
Ovarian follicular atresia in menopause
Regression of lactating breast after weaning.
9. 5- Cell loss in proliferating cell populations such as
Immature lymphocytes
Thymus that fail to express useful antigen receptors.
6- Elimination of harmful self- reactive lymphocytes
either before or after they have completed their
maturation.
7- Death of host cells that have served their useful
purpose, neutrophils and lymphocytes .
10. Apoptosis in bud
formation during
which many
interdigital cells
die. They are stained
black by a TUNEL
method
Incomplete differentiation
in two toes due to lack of
apoptosis
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11. Apoptosis in pathological conditions
1- DNA damage:
Radiation, cytotoxic anticancer drugs, hypoxia can
damage DNA via free radicals.
2- Accumulation of misfolded proteins
3- Cell death in certain infections
HIV infections, viral hepatitis.
4- Pathological atrophy in parenchymal organs
Duct obstruction(pancreas, parotid gland)
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14. What are death receptor activators ?
Tumor necrosis factor alpha (TNF-)
Lymphotoxin (TNF-β)
Fas ligand (FasL)
15. What is a Caspase?
A specific feature of apoptosis is the activation of several
members of a family of cysteine proteases named “caspases.”
The caspase family has more than 10 members and is divided
functionally into two groups- initiator and executioner.
Initiator caspases are caspase 8 and 9.
Executioner caspases are caspase 3 and caspase 6.
Caspases exist as inactive pro-enzymes called zymogens, and
must undergo an enzymatic cleavage to become active .
16. Other Important Proteins
Bcl-2 family helps regulate the activation of procaspases.
Inhibitors - Bcl-2, Bcl-X, Mcl-1(antiapoptotic)
Promoters – Bad, Bax, Bak (pro-apoptotic)
17. Phases of Apoptosis
The process of apoptosis is divided into
1) Initiation phase: during which some caspases
become catalytically active.
2) Execution phase: during which other caspases
trigger the degradation of critical cellular components.
18. 1- Initiation phase
Signals from two distinct pathways:
1) The intrinsic, or mitochondrial pathway
2) The extrinsic or death receptor-initiated pathway
Both pathways are induced by distinct stimuli and
involve different sets of proteins.
Both pathways converge to activate caspases which are
the actual mediators of cell death.
20. Mechanism (Extrinsic)
Death Receptors- cell surface receptor that transmit
apoptotic signals, initiated by ligands.
Ligands: Fas ligand, Tumor Necrosis Factor (TNF)
alpha, and Tumor necrosis (TNF)-related apoptosis-
inducing ligand TRAIL.
Lead to generation of ceramide
Result in large number of clustering of death receptors.
This helps strengthen the apoptotic signaling.
21. Death domain
Conformation change in the intracellular domain of the
receptor.
Allows recruitment of various apoptotic proteins to the
receptor.
Final step- recruitment of caspase 8
Resulting the activation of caspase 8 and the initiation
of apoptosis.
23. The extrinsic (death receptor-initiated) pathway of apoptosis, illustrated by
the events following Fas engagement. FADD= Fas associated death domain.
27. Mechanism (Intrinsic)
Mitochondria contains proteins like Apoptosis
Inducing Factor (AIF), Smac/ DIABLO and
Cytochrome c that regulate cell death.
These proteins are release through a pore called
Permeability Transition (PT) pore
Form by pro-apoptotic members of Bcl-2 family
(activated by apoptotic signals)
28. The release of Cytochrome leads to recruitment of pro-
caspase 9 and Apoptotic protease activating factor 1
(Apaf-1).
This forms the apoptosome
Followed by the activation of caspase 9 which in turn
results in the induction of apoptosis.
29.
30. The intrinsic (mitochondrial) pathway of apoptosis. Death agonists cause changes in the inner
mitochondrial membrane, resulting in the mitochondrial permeability transition (MPT) and
release of cytochrome c and other pro-apoptotic proteins into the cytosol, which activate
caspases. AIF= Apoptosis inhibitory factor; IAPs= Inhibitors of apoptosis proteins; Apaf-1=
apoptosis protease activating factor
31. Execution phase
•Mediated by caspase 3 and 6
•Caspase 8 and 9 in initiation phase activate
caspases 3 and 6
•They break down cytoskeleton protein
that results in bleb formation and endonuclease
activation ( break down of nucleus)
32. Loss of membrane integrity
Begins with swelling of
cytoplasm and mitochondria
Ends with total cell lysis, no
vesicle formation, complete
lysis
Disintegration (swelling) of
organelles
Membrane blebbing, but no
loss of integrity
Begins with shrinking of
cytoplasm and condensation
of nucleus
Ends with fragmentation of
cell into smaller bodies
Mitochondria become leaky
due to pore formation .
33. Loss of regulation of ion
homeostasis
No energy requirement
Random digestion of
DNA.DNA fragmentation
(late event of death)
Tightly regulated process
Energy (ATP)-dependent
Non-random mono- and
oligonucleosomal length
fragmentation of DNA(ladder
type pattern)
Release of various factors into
cytoplasm by mitochondria
Activation of caspase cascade
34. Affects groups of contiguous
cells
Evoked by non-physiological
stimuli(complement attack,
lytic viruses, hypothermia,
hypoxia, ischemia, metabolic
poisons)
Phagocytosis by
macrophages
Significant inflammatory
response
Affects individual cells
Induced by physiological
stimuli (lack of growth
factors, changes in hormonal
environment.
Phagocytosis by adjacent
cells or macrophages
No inflammatory response
35. SUMMARY
Four stages of apoptosis have been defined:
i. Commitment to death by extracellular or intracellular
triggers/signals
ii. Cell killing (execution) by activation of intracellular
proteases (caspases)
iii. Engulfment of cell corpse by other cells
iv. Degradation of the cell corpse within the lysosomes of
phagocytic cells