1. Apoptosis:
Extrinsic Pathway
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

2. Introduction
 Extrinsic pathway also known as the “death receptor
pathway”.
 The extrinsic pathway begins outside the cell through
activation of pro-apoptotic receptors on the cell surface.
 The pro-apoptotic receptors are activated by molecules
known as pro-apoptotic ligands.
 Ligand binding causes receptors to cluster and ultimately
form a death-inducing signaling complex (DISC).
 Upon DISC activation, the extrinsic pathway has been
seen to adopt the same effector caspase machinery as the
intrinsic pathway
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

3. Types of Extrinsic Pathways
 Based on the triggering stimulus and nature of the
components involved two Apoptotic pathways can be
differentiated:
 One involving Receptor Systems
 Second triggered by Cytotoxic Stress
 Receptor mediated pathways include those activated by
Death ligands.
 Stress effects that can induce Apoptosis are Gamma- and
UV radiation, treatment with cytotoxic drugs such as
Actinomycin D.
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

4. Death Receptors
• Death Receptors (DRs) are Cell surface Receptors.
• They transmit Apoptotic signals initiated by specific
ligands.
• Play a central role in instructive Apoptosis.
• Death receptors (DRs) activate Death Caspases within
seconds of ligand binding, causing an Apoptotic
demise of the cell within hours.
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

5. Death Receptors
• DRs (Death Receptors) belong to the superfamily of
TNFR (Tumor Necrosis Factor Receptor).
• TNFR (Tumor Necrosis Factor Receptor) are
characterized by a Cys-rich Extracellular Domain.
• These have a homologous Intracellular Domain
known as the Death Domain.
• The Fas receptor binds the Fas ligand (FasL), a transmembrane
protein part of the TNF family. The interaction between Fas
and FasL results in the formation of the death-inducing
signaling complex (DISC), which contains the FADD, caspase-8
and caspase-10.
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

6. Adapter Molecules
• FADD (Fas-Associated via Death Domain)
• TRADD (Tumor Necrosis Factor Receptor-1-Associated
Death Domain) or Daxx
• The Adapter-molecules contain Death Domains so that
they can interact with the DRs and transmit the Apoptotic
signal to the death-machinery. The best-characterized
Death Receptors are Fas and TNFR1 (Tumor Necrosis
Factor Receptor-1).
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

7. Death Inducing Signaling Complex
 Death Inducing Signaling Complex (DISC) formation requires
following steps:
a. FasL (homotrimeric protein) acts as ligand for Fas causing
oligomerization of its Receptor on binding.
b. Clustering of the Death Domains and binding of cofactor
FADD.
c. Binding of FADD protein via its DED (Death Effector
Domain) motif to a homologous motif in Procaspase8.
Complex of Fas, FADD and ProCaspase8 is called the DISC
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

8. Caspases
• Caspase play the central role in the transduction of
DR apoptotic signals.
• Caspases are proteins that are highly conserved,
cysteine-dependent aspartate-specific proteases.
• There are two types of caspases:
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initiator caspases, caspase 8,10,9,2, and
effector caspases, caspase 3,7,6.
• The activation of initiator caspases requires binding
to specific oligomeric adaptor protein.
• Effector caspases are then activated by these active
initiator caspases.
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

9. Cell Apoptosis
• Disc formation is followed by blockage of FADD cofactor
function by interaction with the regulator FLIP (FLICE
Inhibitory Protein).
• Recruitment by FADD
• Procaspase8 oligomerization
• FADD activation through self-cleavage of oligomerized
Procaspase8.
• Activation of downstream Caspases (Caspase3 and 7) by
activated Caspase8 committing the cell to Apoptosis.
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

10. Animation
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

11. Execution
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After a cell receives stimulus, it undergoes organized degradation of
cellular organelles by activated caspases. A cell undergoing apoptosis
shows a characteristic morphology:
Cell shrinkage and rounding are shown because of the breakdown of the
proteinaceous cytoskeleton by caspases.
The cytoplasm appears dense, and the organelles appear tightly packed.
Chromatin undergoes condensation into compact patches against
the nuclear envelope (also known as the perinuclear envelope) in a
process known as pyknosis, a hallmark of apoptosis.
The nuclear envelope becomes discontinuous and the DNA inside it is
fragmented in a process referred to as karyorrhexis. The nucleus breaks
into several discrete chromatin bodies ornucleosomal units due to the
degradation of DNA.
The cell membrane shows irregular buds known as blebs.
The cell breaks apart into several vesicles called apoptotic bodies, which
are then phagocytosed.
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

12. Removal of dead cells
• The removal of dead cells by neighboring phagocytic cells has been
termed efferocytosis.
• Dying cells that undergo the final stages of apoptosis display
phagocytotic molecules, such as phosphatidylserine, on their cell
surface.
• Phosphatidylserine is normally found on the cytosolic surface of
the plasma membrane, but is redistributed during apoptosis to the
extracellular surface by a protein known as scramblase.
• These molecules mark the cell for phagocytosis by cells possessing
the appropriate receptors, such as macrophages.
• Upon recognition, the phagocyte reorganizes its cytoskeleton for
engulfment of the cell.
• The removal of dying cells by phagocytes occurs in an orderly
manner without eliciting an inflammatory response.
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956