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Programmed Cell Death
A genetically controlled cell suicide pathway
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The Morphology of Apoptosis
Cytoplasm shrinks
Chromosomes condense and fragment
Nuclear membrane breaks down
Apoptotic body formation
Engulfment of the cell corpse
Difference Between Apoptosis and
Necrosis
• Necrosis (pathological cell death): dying cells
swell and lyse; toxic contents leak out and
result in inflammatory response.
• Apoptosis (physiological or programmed cell
death): dying cells shrink, are engulfed and
degraded by other cells, leave no trace, and
don’t result in harmful outcomes
Functions of apoptosis
Sculpt body structures, e.g. hand digit
Serve some function but no longer needed
e.g. tadpole tail of frog.
Needed in one sex but not another
e.g. Mullerian duct important for female
is eliminated in males by apoptosis.
Produced in excess, e.g. extra neurons are
removed by apoptosis during neurogenesis.
Serve in immune system as a defense
mechanism to get rid of harmful or
damaged cells.
The Nematode C. elegans As a Model
Organism in the Study of PCD
• A great genetic system
• Completely defined cell lineage
• Study of cell death at a single cell
resolution in living animals
The C. elegans Cell Lineage
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Cell Death Can Be Studied at a Single Cell Resolution
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Adapted from Sulston and Horvitz,
Dev. Bology 56, 110-150, 1977
The First Cell Death Mutants
Identified in C. elegans
In 1976, J. Sulston first described programmed cell
death in nematodes and reported the first cell death
mutant (nuc-1), in which DNA in the death cells fail
to be degraded.
Wild-type
nuc-1
In 1980, E. Hedgecock isolated two cell death mutants
(ced-1 and ced-2) which are pivotal for identification
of the other cell death genes.
Wild-type
ced-1
What went wrong with the ced-1 mutant?
What could go wrong with the ced-1 mutant?
a) Apoptotic cells fail to die
b) Normal cells die ectopically
c) Apoptotic cells fail to be engulfed
d) Normal cells are ectopically engulfed
e) Cells undergo necrosis
Wild-type
A)a and b
B) b and c
C)c and d
D)d and e
E) e and a
ced-1
Phenotypic analysis of ced-1 and ced-2 mutants
1) More cell deaths?
2) Dying cells cannot be removed or engulfed
How to distinguish these two possibilities?
Follow the cell lineage in the mutant animals
What is next?
Suppressor screens: ced-3 and ced-4
What are the functions of ced-3 and ced-4?
H. Ellis and R.H. Horvitz
What could be the functions of ced-3 and ced-4?
1) ced-3 and ced-4 promote cell corpse engulfment
2) Inhibitors of cell corpse engulfment
3) ced-3 and ced-4 could promote cell deaths
4) ced-3 and ced-4 could inhibit cell death
5) ced-3 and ced-4 could promote necrosis
A)
B)
C)
D)
E)
1 and 2
2 and 3
3 and 4
4 and 5
5 and 1
What are the functions of ced-3 and ced-4?
1) ced-3 and ced-4 promote cell corpse engulfment?
Then the mutations must be increase-of-function
2) Inhibitors of cell corpse engulfment?
Then the mutations should be loss-of-function
3) ced-3 and ced-4 could promote cell deaths
Then the mutations should be loss-of-function
How to distinguish 2) and 3)?
Lineage analysis suggest:
Many cells that normally die now survive
ced-3 and ced-4 are involved in cell killing
How do ced-3 and ced-4 kill the cells?
1)
2)
3)
4)
5)
Cells die by murder?
Cells die by suicide?
cells die by aging?
Cells die because of injuries?
Cells die by sickness?
Cells Die by Suicide Rather Than Murder
• Yuan and Horvitz demonstrated by
mosaic analysis that ced-3 and ced-4
function in the dying cells to kill.
• ced-3 encodes a protein with homology
with IL-1b converting enzyme (ICE), a
cysteine protease.
• ced-4 encodes a protein similar to
apoptotic protease-activating factor
(Apaf-1).
THE GENETIC PATHWAY FOR PROGRAMMED CELL DEATH IN C. elegans
All dying cells
Specific cells
engulfing cell
healthy cell
committed to die
dead cell
healthy cell
DECISION
TO DIE
NSM sister cells:
ces-2
CEM cells
ces-1
egl-1
EXECUTION
OF DEATH
ced-9
ced-3
ced-4
ced-11
ced-8
ceh-30
DEGRADATION
ENGULFMENT
ces-1: zinc-finger transcription factor
ces-2: bZip transcription factor
egl-1: Bid-like protein
ced-9: Bcl-2-like protein
ced-1
ced-6
ced-7
ced-2
psr-1 ced-5
ced-10
ced-12
nuc-1
cps-6
wah-1
ced-2: Crk II
ced-5: DOCK 180
ced-6: PTB protein
ced-7: ABC transporter
ced-10: Rac 1
ced-3: ICE-like protease
ced-4: Apaf-1-like protein