Apoptosis, or programmed cell death, is a vital process in the body that occurs through development, aging, and when cells are damaged. It is characterized by distinct morphological changes and biochemical mechanisms. The process involves caspase activation and a complex cascade of events. There are three main pathways that trigger apoptosis - the intrinsic pathway which involves mitochondria, the extrinsic or death receptor pathway activated by extracellular signals, and the perforin/granzyme pathway used by cytotoxic T cells. All pathways ultimately activate executioner caspases like caspase-3 to carry out the final stages of apoptosis.

1. Apoptosis
A review on Programmed Cell Death

2. Introduction
• Apoptosis is a vital part of various processes including :-
i)normal cell turnover,
ii)proper development and functioning of the immune system,
iii)embryonic development and
iv)chemical-induced cell death.
• The process of programmed cell death, or apoptosis, is generally
characterized by distinct morphological characteristics and energy-
dependent biochemical mechanisms.

3. Introduction
• Apoptosis is defined as a distinctive and important mode of
“programmed” cell death, which involves the genetically determined
elimination of cells.
• The term apoptosis (a-po-toe-sis) was first used to describe a
morphologically distinct form of cell death.
• Apoptosis is a coordinated and often energy-driven process that
involves the activation of a group of cysteine proteases called
“caspases” and a complex cascade of events that link the initiating
stimuli to the final demise of the cell.

4. Introduction
• Apoptosis occurs normally in the body through development and
ageing to maintain the cell population.
• And also when cell is damaged by disease or toxins.
• It is opposite to mitosis.
• Protein triggered cell death occurs when certain specific protein is
attacked by activated “caspases” which are activated by external
stimuli like cell insults: DNA damage.
• Some cell have Fas or TNF binding receptor on cell membrane that
lead to apoptosis by ligand binding and protein cross-linking.
• Default death pathway of apoptosis are blocked by survival factor.

5. Necrosis Vs Apoptosis
• A variety of injurious stimulus such as heat, toxic, radiation can induce
apoptosis at low intensity but at higher dosage of such trigger can cause
necrosis.
• Necrosis is not organized cell death, affected cells burst to die and affect
the neighboring cells are affected.
• This eruption triggers a potentially damaging inflammatory response.
• But a cell that undergoes apoptosis dies neatly, without inflammatory
response.
• Oncosis leads to necrosis with karyolysis and cell swelling whereas
apoptosis leads to cell death with cell shrinkage, pyknosis, and
karyorrhexis.

6. Necrosis
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7. Apoptosis
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8. Morphology Of Apoptosis
• Cell shrinkage and pyknosis(the irreversible condensation of
chromatin in the nucleus of a cell undergoing necrosis or apoptosis
which is followed by karyorrhexis: fragmentation of the nucleus.)
• After karyorrhexis, extensive plasma membrane protrude and
budding or formation of apoptotic bodies are formed, which consist
of cytoplasm with tightly packed cell organelles, with or without
nuclear fragment.
• These bodies are ultimately digested by macrophages, parenchymal
cells or neoplastic cells and degraded within phagolysosomes.

10. Necrosis and Apoptosis
• Necrosis and Apoptosis can occur simultaneously depending upon
intensity and duration of stimulus, Caspases availability and extent of
ATP depletion.
• Apoptosis changes to necrosis if less caspases and ATP are available.
• Necrosis include cell swelling, disrupted cell organelle membrane and
lysosome membrane which leads to disruption of cell membrane
releasing intracellular matter into surrounding interstitial tissues.

11. Mechanism of Apoptosis
• It is a highly complicated process.
• 3 pathways has been identified till date:
• i) Intrisic pathway ii) Extrinsic or death receptor pathway and iii)
Granzyme B pathways
• All the pathways join to execution pathway.

13. Extrinsic Pathway
• It involves transmembrane receptor which mediate signal for apoptosis.
• Transmembrane receptor or Death receptor transmit death signal from
extracellular matrix to intracellular signaling.
• Example of death ligand and receptor are Fas and TNF.
• When this ligand bind to the receptor protein, cytoplasmic adaptor
protein(FADD) bind with the death domain of the receptor protein.
• Adaptor protein consist of two domain, death domain and death effector
domain.
• On death effector domain, inactive procaspases-8 bind(disc formation) and
get cleaved for auto-catalytic activation.

14. Cell-surface death receptors - extrinsic
pathway of apoptosis

15. Extrinsic pathway
• Activated caspase-8 activates execution caspase-3.
• After this it enters execution pathway.
• Execution pathway is similar for all the pathways except granzyme A
which doesn’t need caspase -3 for DNA cleavage.
• This activation by death receptor can be inhibited by c-FLIP which
binds to FADD and caspase-8.

16. Intrinsic Pathway
• Involves non-receptor mediated cell signaling that are produced inside cell,
with the help of Bak and Bax protein of Bcl-2 family and cytochrome c.
• Bak and Bax protein are activated by death promoting member of the Bcl-2
family, which are activated by various insults to the cell like DNA damage.
• These Bak and Bax protein reside in the mitochondrial outer
transmembrane, when activated, facilitates release of cytochrome c to
cytosol.
• These cytochrome c promote assembly of large adaptor protein(Apaf-1)
with procaspase-9 into wheel like structure called apoptosome, this
apoptosome is activated with caspase-9 which finally activates caspase-3
for execution pathway.

17. Intrinsic Pathway

18. Perforin/granzyme Pathway
• This is T-cell mediated cytotoxicity where sensitized CD8+ cells kill
antigen bearing cells.
• Cytotoxic effect on tumor bearing cell and virus infected cells.
• Involves secretion of transmembrane pore-forming
molecule(perforin) and cytoplasmic granules(granzyme A and B).
• Granzyme B cleaves protein at aspartate residue and activates
procaspase-10. It can utilize mitochondrial pathway(cleave Bid) or
directly activate caspase-3 for execution pathway.
• Granzyme A induce caspase independent apoptosis. It cleaves SET
complex(inhibits gene that causes DNA nicking), gene is not inhibited
by SET complex and thus DNA nicking takes place.

19. Granzyme/perforin pathway

20. Execution Pathway
• All the pathway leads to activation of caspase-3.
• Caspase-3 activate cytoplasmic endonuclease, which degrades nuclear
material, and proteases that degrade the nuclear and cytoskeletal proteins.
• Caspase-3 also induces cytoskeletal reorganization and disintegration of
the cell into apoptotic bodies.
• Phagocytic uptake of apoptotic cells is the last component of apoptosis.
• Phosphotidylserine on the outer leaflet of apoptotic cells facilitates
noninflammatory phagocytic recognition, allowing for their early uptake
and disposal.

21. Thank you