Apoptosis is a programmed cell death process that is essential for maintaining homeostasis. It involves the activation of caspases that trigger cell dismantling. Central regulators include Bcl-2 family proteins like Bax and Bak that induce mitochondrial membrane permeabilization and cytochrome c release upon activation. Cytochrome c then activates caspase-9 through formation of the apoptosome complex, triggering a caspase cascade that leads to apoptosis. Proteins like IAPs inhibit caspase activity and must be neutralized by other proteins like SMAC/Diablo to allow apoptosis to proceed. Together, these events ensure damaged or unnecessary cells are removed in a controlled manner.

1. APOPTOSIS

2. APOPTOSIS
In Greek apoptosis means dropping off/
falling off .
Apoptosis is a programed cell death
responsible for balancing cell proliferation
& maintaining constant cell number in
tissue .
5×10¹¹ RBC cells are eliminated daily in
humans by apoptosis balancing their
continual production in bone marrow .

3. Importance :-
It is a defense mechanism in which damaged &
dangerous cells can be eliminated .
Neurons are produced in excess & up to 50% of
developing neurons are eliminated by programed
cell death .
Virus infected cells undergo programed cell death .
DNA damaged cells also induce programed cell
death .
Events of apoptosis :-
During apoptosis chromosomal DNA is usually
fragmented as a result of cleavage b/w
nucleosomes chromatin condenses & nucleus
breaks in to small pieces .

5. Finally the cell it self shrinks &
Breaks up in to membrane enclosed
Fragments called apoptotic bodies .
Apoptotic cells are usually
Recognized by both macrophages &
Neighboring cells so they removed
From tissues .

6. The removal of apoptotic
Cell is mediated by signals
Induce phosphatidyl serine
Which is restricted to inner
Plasma membrane .
During apoptosis phospha-
tidyl serine expressed on cell
Surface where it is Recog. by
Receptor expressed on
phagocytic cells .

7. Proteins participate in apoptosis
2 genes ced3 and ced4 were required for apoptosis.
If either ced3 or was inactivated by mutations . The
normal programmed cell death did not take place.
Ced9 functioned as -ve regulator of apoptosis if
Ced9 was inactivated by mutations the cells cannot
survive .
If ced9 was expressed by higher levels apoptosis can
not occur .

8. Caspases amplify the initial apoptotic signal :-
 Initiator Caspases - cas 9
Effector Caspases - cas 3 and cas7
The Caspases are named because they contain
cysteine residue in the catalytic site & cleaves
proteins at C terminal to aspartic residues .
Caspases are the ultimate effectors of the apoptosis
cleaving nearly 100 cell target proteins .
Key targets of caspases include
1.fragments of nuclear DNA
2.cleavage of nuclear lamins
3.cytoskletal proteins

9. Ced 3 is only caspases in c . Elegans.
All caspases are synthesized as inactive precursor
(procaspases) that can be converted to active forms
by proteolytic cleavage catalyzed by caspases .
Caspases 9 activated as complex with apaf -1 & cyt
c in the apoptosome .
Caspases – 9 that cleaves & activates effector
caspases , such as caspases 3 & 7 .
The effector caspases cleave variety of cell , cell
protein , including nuclear lamins , cytoskeletal
proteins & an inhibitor of DNAse , leading to cell
death .

11. Central regulators in apoptosis :-
Ced 9 gene in c.elegans was closely related to
a mammalian gene Bcl – 2 .
Bcl – 2 was found to inhibit apoptosis .
Bcl – 2 & ced9 was thus similar in function .
In addition both Bcl – 2 & Ced 9 contain a
single trans membrane domain in the outer
mitochondrial membrane .

12. Mammals encode approximately 20 related apoptotic
proteins which were divided into 3 functional groups
1. Antiapoptotic family :-
Functions as an inhibitor of apoptosis .
Eg:- Bcl – 2 & Bcl Xl .
This family contains 4 bcl-2 homology domains
2. Proapoptotic multi domain :-
Induce apoptosis .
Eg:- BAX , BAK .
Contains 3 homologous domains
3. Pro apoptotic BH – 3 only :-
Induce apoptosis.
Eg:-Bid , Bad , Noxa , PUMA , Bin .
Contains only one bcl –3homology domain .

13. FIGURE 17.7 Regulatory interactions between Bcl-2 family members In normal
cells, the BH3-only proapoptotic proteins are inactive, and the multidomain
proapoptotic proteins are inhibited by interaction with antiapoptotic proteins. Cell
death signals activate the BH3-only proteins, which then interact with the antiapoptotic
proteins, leading to activation of the multidomain proapoptotic proteins
and cell death.

14. Role of mitochondrion in regulation of
apoptosis
BAX and BAK is required for the induction of
apoptosis.
These residues in the outer mitochondrial
membrane normally tightly bound to Bcl – 2 .
When released from Bcl – 2 these residues form
oligomers that generate pores in the outer
mitochondrial membrane .
These generates the release of mitochondrial
protein cyt c .

16. Cyt – c bound to apaf – 1 forming a complex called
apoptosome .
Apoptosome results in the activation of caspases 9.
Caspases are also regulated by a family of proteins
called IAP (inhibitor of apoptosis proteins) .
IAP has zinc binding domain that directly binds to
caspases inhibit the protease activity .
SMAC/diablo & omi/htr2 released by mitochondria
which inhibits IAPs .
Assembly of BAX/BAK channels leads to release of
these SMAC/diablo from mitochondria .
these binds to IAPs in cytosol their by blocking the
IAPs from binding to caspases .