Short note on apoptosis

1. Apoptosis
Dr. Rohan

2. Apoptosis is a type of cell death that
is induced by tightly regulated suicide
program in which cells destined to die
activate intrinsic enzymes that
degrade the cell’s genomic DNA and
nuclear & cytoplasmic proteins.
It was first recognized in 1972 by
distinctive morphologic appearance
of membrane bound fragments
derived from cells.

3. Causes of Apoptosis
Occurs in
two broad
context:
In
physiologic
situation
In
pathologic
conditions

4. Apoptosis in physiologic situation:
The removal of
supernumerary cells
during development.
Involution of
hormone dependent
tissue on hormone
withdrawal.
Cell turnover in
proliferating cell
populations.
Elimination of
potentially harmful
self reactive
lymphocytes.

5. Apoptosis in pathologic conditions:
DNA damage: by
radiation or cytotoxic
anticancer drugs
Accumulation of
misfolded proteins
Infections (mainly viral):
as a result of virus itself
in HIV or adenovirus
infection or host
immune response in
viral hepatitis

6. Morphologic
changes
Cell shrinkage: cell size is reduced; cytoplasm is
eosinophilic, and organelles are tightly packed.
Chromatin condensation: It aggregates peripherally
into dense masses of various shapes and sizes.
Formation of cytoplasmic blebs and apoptoic bodies:
Cell first show extensive surface membrane blebing
– followed by fragmentation of the dead cells into
membrane bound apoptotic bodies.
Phagocytosis of apoptotic cells and cell bodies,
usually by macrophages.

7. On H&E- stained tissue, apoptotic cell appears
as a round to oval mass of intensely eosinophilic
cytoplasm with fragmentations of dense nuclear
chromatin.

8. Mechanism of apoptosis
Enzymes
required for
apoptosis:
Endonucleases:
Produces
internucleosomal
cleavage of DNA
Caspases:
cleaves protein
near aspartic
acid residue
Initiator: 8,9,10
Executioner:
3,6,7

9. Factors involved in apoptosis
Pro- apoptotic:
BAX, BAK,
P53
Anti-apoptotic:
BCL-2, BCL-
XL, MCL-1
Arbiters of apoptosis:
regulate balance
between the anti and
pro apototic factor:
BIM, BID,
BAD, PUMA,
NOXA

10. Caspases exist as inactive proenzymes and undergo enzymatic cleavage to become active.
Presence of caspase is thus a marker for cell undergoing apoptosis.
The process in divided into
• initiation phase: some caspases become catalytically active and unleash cascade of other
caspases.
• Execution phase: terminal caspases trigger cellular fragmentation.
Regulation of these enzymes depends on finely tuned balance between pro-apoptotic and anti
apoptotic proteins.

11. There are two pathways to
apoptosis:
• Mitochondrial pathway
• Death receptor pathway
Although the two pathways
differ in their induction and
regulation, they both culminate
in the activation of caspases.

12. Mitochondrial (Intrinsic) pathway
bcl2, bcl-xl,
mcl1 in the
outer
membrane of
mitochondria
and
endoplasmic
reticulum
Normal cell
have:
maintains
membrane
permeability
no apoptosis
no leakage
of
cytochrome
C

13. DNA damage,
Radiation or
Infection
Activates BH3 only
sensors/ arbiters of
apoptosis
• And shift the balance
towards pro-apoptotic
Formation of bax-
bak channel
between inner &
outer mitochondrial
membrane
Leakage of
cytochrome C
Cytochrome C +
Apoptosis activating
factor-I
Apoptosome
Activate caspase 9 Caspase 3,6,7 Apoptosis

14. Death receptor mediated (extrinsic) pathway
This pathway is initiated by engagement of plasma membrane death receptors.
Death receptors are members of tumor necrosis factor receptor family that contain a cytoplasmic
domain involved in protein-protein interaction.
This domain is essential for delivering apoptotic signals.
The best know death receptor are type 1 TNF receptor (TNFR1) and a related protein called Fas
(CD95).
The ligand for fas is fas ligand (fasL) which is expressed on T cells that recognize self antigen.

15. FasL bind to
Fas, three or
more molecule
of fas are
brought
together
Their
cytoplasmic
death domain
form a binding
site for an
adaptor protein
called FADD
(Fas-
associated
death domain)
Once attached
to this
complex,
FADD binds
inactive
caspase-8
Bring together
multiple
caspase
molecule
Autocatalytic
cleavage
Generate
active caspase
8
Caspase 8
initiates the
same
executioner
pathway as in
mitochondrial
pathway
This pathway can be inhibited by a protein called FLIP, which binds to pro caspase 8, blocking binding to FADD

17. THANK YOU