Download APOPTOSIS: An overview

Apoptosis Signaling
Live??? Or Die???
Life of cell
• Mitosis checkpoints
• Apoptosis will be
triggered to prevent
cells from
becoming cancers
and harming the
body
Cell death by injury
-Mechanical damage
-Exposure to toxic chemicals
Cell death by suicide
-Internal signals
-External signals
• Definition
Apo: apart
Ptosis: fallen
– Shedding of leaves from trees
• During embriogenesis ------ occurs as
PCD
• Post-embrional life------- as apoptosis
apoptosis
• Apoptosis is used as a synonymous for
PCD but PCD is physiological death,
occurs only during embriogenesis.
• It is a functional death and it is a good
mechanism to eliminate wasted, useless,
unwanted, or crippled cells!
Necrosis vs. Apoptosis
Necrosis
•
•
•
•
Cellular swelling
Membranes are broken
ATP is depleted
Cell lyses, eliciting an
inflammatory reaction
• DNA fragmentation is
random, or smeared
• In vivo, whole areas of
the tissue are affected
Apoptosis
•
•
•
•
Cellular condensation
Membranes remain intact
Requires ATP
Cell is phagocytosed, no
tissue reaction
• Ladder-like DNA
fragmentation
• In vivo, individual cells
appear affected
NECROSIS Vs APOPTOSIS
Wilde, 1999
Why have we developed such a
self-destructive system?
• A. PCD allows a constant selection for the
fittest cell in a colony
• Every cell carries the molecular machinery
to do PCD!
• Cells that are sensitive to extracellular
signals will survive, cell that cannot
compete with their more vital sisters will
undergo apoptosis.
• PCD machinery is silent until signals arrive
to start PCD:
• Signals:
• damage to DNA
• Activation of membrane receptors.
Ligands are: peptides, cytokines, ATP,
ROS etc
• Fas receptor?
Receptors for growth factors, cytokines
and hormones
• Membrane alterations cause apoptosis.
What kind of membrane alterations ??
Phospholipid redistributions, changes in
membrane charge, carbohydrate and
surface markers.
Proteins involved in apoptosis
•
Fas ligand (FasL or CD95L) is a type-II transmembrane protein that belongs to
the tumor necrosis factor (TNF) family
•
Fas-Associated protein with Death Domain (FADD) is an adaptor molecule that
bridges the Fas-receptor, and other death receptors,
•
Apoptotic protease activating factor 1, also known as APAF1
•
Bcl-2 (B-cell lymphoma 2) is the founding member of the Bcl-2
family of apoptosis regulator proteins encoded by the BCL2gene
Caspases
• Inflammatory Caspases: -1, -4, and -5
• Initiator Caspases: -2, -8, -9, and -10
–
–
•
Effector Caspases: -3, -6, and -7
–
–
•
•
•
•
•
Long N-terminal domain
Interact with effector caspases
Little to no N-terminal domain
Initiate cell death
The Mitochondrial Apoptosis-Induced Channel (or MAC),
BAK: Bcl-2 homologous antagonist killer
BAX: Bcl-2 associated x protein
BID: BH3 interacting domain death agonist, a pro-apoptotic protein
BAD: The Bcl-2-associated death promoter (BAD) protein is a proapoptotic member of the Bcl-2 gene family which is involved in initiating apoptosis.
BAD is a member of the BH3-only family
STAGES OF APOPTOSIS
Induction of apoptosis related genes, signal transduction
Sherman et al., 1997
APOPTOSIS: Morphology
organelle
membrane
blebbing &
changes
reduction
cell
mitochondrial
leakage
shrinkage
nuclear
fragmentation
chromatin
condensation
Hacker., 2000
APOPTOSIS: Morphological events
cell shrinkage
organelle reduction
mitochondrial leakage
chromatin condensation
nuclear fragmentation
membrane blebbing & changes
Blebbing & Apoptotic bodies
Bleb
The control retained over the cell
membrane & cytoskeleton allows intact
pieces of the cell to separate for
recognition & phagocytosis by MFs
Apoptotic body
MF
MF
Apoptosis: Pathways
“Extrinsic Pathway”
Death
Ligands
Death
Receptors
“Intrinsic Pathway”
DNA
damage
& p53
Mitochondria/
Cytochrome C
Initiator
Caspase 8
Effector
Caspase 3
Initiator
Caspase 9
Cell
death
Extrinsic or
Death Receptor
Pathway
• Binding of Fas by FasL
induces recruitment of FADD to
the cytoplasmic tail of Fas
• The opposite end of FADD
contains a death effector
domain (hatched boxes);
recruitment of either
procaspase-8 or c-FLIP
• Caspase-8 can cleave Bid
• truncated Bid (tBid) can
inactivate Bcl-2 in the
mitochondrial membrane.
• This allows the escape of
cytochrome c, which clusters
with Apaf-1 and caspase-9 in
the presence of dATP to
activate caspase-9.
• Smac/DIABLO is also released
from the mitochondria and
inactivates inhibitors of
apoptosis (IAPs).
• breakdown of several
cytoskeletal proteins and
degradation of the inhibitor of
caspase-activated DNase
(ICAD).
MAJOR PLAYERS IN
APOPTOSIS
• Caspases
• Adaptor proteins
• Bcl-2 family
Modulation of apoptosis
• Apoptotic cell death can be switched to
necrosis during oxidative stress by 2
mechanisms:
Inactivation of caspases due to oxidation
of their active site thiol group by oxidants
Decrease in ATP due to failure of
mitochondrial energy production by
oxidants
Importance of Apoptosis
• Important in normal physiology / development
– Development: Immune systems maturation,
Morphogenesis, Neural development
– Adult: Immune privilege, DNA Damage and wound
repair.
• Excess apoptosis
– Neurodegenerative diseases
• Deficient apoptosis
– Cancer
– Autoimmunity
The bcl-2 family
N
BH4
BH3
BH1
Receptor domain
Ligand
Pore
domain
formation
phosphorylation
Raf-1
calcineurin
BH2
TM
C
Membrane
anchor
Group I
Bcl-2
Group II
bax
Group III
Bad
bid
bik
Back
P53 & Apoptosis
p53 first arrests cell growth between G1  S
This allows for DNA repair during delay
If the damage is too extensive then p53
induces gene activation leading to
apoptosis (programmed cell death)