Transcript Extrinsic Pathway of Apoptosis Activation

Lecture 17
Regulation of the Cell Cycle
and Cell Death
The Cell Division Cycle
A cell contains all the information necessary for
making a copy of itself during cell division
Cells reproduce by carrying out a highly ordered
sequence of events called the CELL CYCLE
– Typically Involves:
Cell Growth
DNA Replication
Distribution of Duplicated Chromosomes to Daughter Cells
Cell Division
Phases of the
Eukaryotic Cell Cycle
Different Cell Types have Variations in Timing,
Control and Progression Through Cell Cycle
In the Adult Human Body
-Most Cells Are Not Actively Dividing
-Some Cells Are Continuously Dividing
- Example: Bone Marrow Cells, Epithelial Cells
Some Cells Do Not Divide After Differentiation
Example: Nerve Cells
Some Cells can Be Triggered to Divide to Replace Cells that have
Died
Example: Cells of Liver Tissue
Control of Cell Cycle Involves Monitoring
Intracellular And Extracellular Conditions
The Control System Needs to Regulate Progression through the
Cell Cycle to Ensure:
1)Events Associated with Each Phase are Carried Out at the
Appropriate Time and in the Appropriate Order
2) Each phase is complete Before Next Phase is Initiated
3) Must be Able to Respond to External Conditions Required for
Cell Growth and Division.
A Series of Control Points called Checkpoints
Accomplish These Objectives
Cell Cycle Checkpoints Regulate
Progression Through the Cell Cycle
Cell Fusion Experiments Provide Evidence for
Control Molecules in the Cell Cycle
S
S
.
G1
S
S Phase is Activated
In Original G1 Nucleus
M
M
G1
M
M phase is Activated
In Original Non-M Nucleus
The Cell Cycle Is Controlled by Cyclin
Dependent Kinases (CDK)
CDK forms a Complex with a Protein
called a Cyclin to form a CDK Complex
Different Cyclin-CDK Complexes Trigger
Different Steps in the Cell Cycle
4 Major Types of Cyclin-CDK Complexes:
G1-CDK
G1/S-CDK
S-CDK
M-CDK
Regulation of the Activity
of CDK Complexes
1) Proteolysis of Cyclin Component
2) Phosphorylation/Dephosphorylation
3) Binding of Inhibitory Proteins
4) Subcellular Localization
Cyclin Protein Concentration and Cyclin-CDK
Activity Oscillate during the Cell Cycle
Post Transcritional Control of Cyclins:
M-Cyclin is Ubiquitinated and Subsequently
Degraded by the Proteasome
Cyclin Destruction Occurs at Specific
Points during the Cell Cycle
Checkpoints in the Cell Cycle
Progression through the G1 Checkpoint
Cells Need to Check for :
Cell Size
Nutrients
Mitogens and Growth Factors
DNA Damage
Called the Restriction Point in Mammals
Commits Cell to the Process of Cell Division
Cells Can Delay Cell Division by
Entering Specialized Nondividing State, GO
Most cells in
our body are
In Go State:
G0
G1 checkpoint
G1
Green Light to Proceed:
Environment Favorable
G1
Red Light – Don’t Proceed
Environment Unfavorable
(Absence of Mitogenic Signals)
In the Absence of Mitogenic Signals,
The protein Rb Inhibits Cell Cycle Progression
Rb- Retinoblastoma Protein
Binds Transcription Factor
E2F and Prevents Function
E2F Required for:
Activating Transcription
Of Genes Encoding Proteins
Required for G1/S transition
G1/S Cyclins, S Cyclins and
Components of DNA
Replication Machinery
The G1-CDK Complex Controls the G1
Checkpoint by Phosphorylating the Rb Protein
G1 DNA Damage Checkpoint
DNA Damage Needs to be Repaired before DNA
Synthesis Occurs
Activation of Transcription Factor p53 leads to
synthesis of CDK inhibitor (CKI), p21
p21 binds and inhibits G1/S CDK and S-CDK
Complexes
Cells Arrest Until Damage is Repaired and Then
Progress
Or
Undergo Apoptosis if Unable to Repair
p53 Mediates the G1 DNA Damage Checkpoint
Through Inactivation of G1/S- and S-CDK Complexes
Checkpoints in the Cell Cycle
S-CDK Complexes Are Required for
DNA Replication During S-phase
S-CDK Controls:
1) Initiation of Replication
2) Prevents Re-replication
from a Particular Origin
Progression Through the G2
Checkpoint to Enter Mitosis
Check for:
Cell Size
DNA Replication Complete
Passage through G2 Checkpoint Requires Active M-CDK:
Functions to Phosphorylate Proteins Involved in Early Stages of Mitosis
1)
2)
3)
4)
Nuclear Envelope Breakdown
Chromosome Condensation
Mitotic Spindle Formation
Targeted Protein Degradation
Unreplicated DNA Blocks
Activation of M-CDK Complex
Unreplicated DNA Sensed
Blocks Activating
Phosphatase
T
Checkpoints in the Cell Cycle
M-CDK Controls The Spindle Assembly Checkpoint by
Activating the Anaphase Promoting Complex (APC)
Check for:
Proper Chromosome Attachment to Spindle
Phosphorylation of APC- now Activated
Sister Chromatids Can Separate
If Chromosomes not Properly Attached
Metaphase Arrest
Will not Separate
Exit From Mitosis
Now Need to Reverse Events:
Nuclear Envelope Breakdown
Chromosome Condensation
Mitotic Spindle Formation
Destroy M-Cyclin
M-CDK Activates APC
APC Targets M-Cyclin for Destruction by the
Proteasome
Cancer Involves Defective Cell
Cycle Control Mechanisms
Cancer Inducing Mutations
Inherited or Introduced by Viruses
Two Key Types of Mutated Genes That Can
Lead to Cancer:
Oncogenes - Gene whose presence can trigger
development of cancer.
Example: ras, bcl-2
Tumor Suppressors- Gene whose absence or
inactivation can lead to cancer
Example: p53, Rb
Mitogens and Growth Factors Activate the Ras
Pathway Leads to Passage through the G1 Checkpoint
p53 Can Induce Apoptosis in
DNA Damaged Cells
Can’t Repair
DNA DAMAGE
Apoptosis
Apoptosis:
Regulated Cell Death
Role in Killing of Unneeded, Damaged, or Potentially Deleterious
Cells
Occurs in Embryonic and Adult Tissues
Proteins Involved are Always Present in Cells- Needs to Be
Activated by Stimuli
Can Result From:
Developmental Cues
Withdrawl of Essential Growth Factors
DNA Damage
Various Cell Stresses
Programmed Cell Death
• Cell Death Occurring at a Defined Point in
Development
• Usually proceeds by Apoptosis
Mouse Paws
Not All Cell Death is Apoptotic
Oncosis and Necrosis:
Apoptosis:
Unregulated Cell Death Due to Injury
An Active Regulated Process
Cell Swells (Oncosis)
DNA Fragmentation
Chromatin Condensation
Nucleus Swells
Disruption of Organelles and
Rupture/Release of Contents
Contents Released into
Extracellular Space
Fragmentation of Nucleus
Cell Shrinks
Formation of Membrane Enclosed
Fragments called Apoptotic Bodies
Recognition and Engulfment
by Phagocytic Cells
or Neighboring Cells
Necrosis and Apoptosis are
Morphologically Distinct
Necrosis
Apoptosis
The Morphological Changes of Apoptosis Are
Orchestrated by Caspases
Cysteine Proteases that cleave at Aspartic Acid Residues
Activate Apoptosis by Cleaving Specific Substrates
Present but inactive in cells
Two Main Types of Caspases
1) Initiators- Need to dimerize to become active “induced proximity”
2) Executioners- Need to be proteolytically cleaved to become active
- Cleavage is usually Mediated by Initiator Caspases
Once Executioners are Activated their Key Targets of Proteolysis Include:
1)An Inhibitor of a DNAse- Fragmentation of DNA
2)Nuclear Lamins- Fragmentation of Nucleus
3)Other Cytoskeletal Associated Proteins- Disruption of Cytoskeleton and Cell
Fragmentation
4)Activation of Additional Caspases
Caspases are Present
but Inactive in Cells
Zymogens: Proteins initially synthesized as inactive
precursors- undergo proteolytic cleavage to become active
Caspase Activation Amplification Cascade
So How Are Initiator Caspases Activated to get the Process Going??
Main Pathways Regulating Caspase
Activation During Apoptosis
Intrinsic Pathway- Mitochondrial Mediated
Major Pathway in Mammalian Cells
– Outer Mitochondrial Membrane Permeabilization (MOMP)
– Release of Cytochrome C from Mitochondrial Intermembrane
Space into Cytosol
– Apoptosome Formation- Activation of Initiator Caspase
– Effector Caspases Activated
Extrinsic Pathway- Signaling through Death Receptors
–
–
–
–
Ligand Bound Death Receptors
Adaptor Protein Association
Initiator Caspase Recruitment and Activation
Effector Caspases Activated
Intrinsic Pathway of Apoptosis Activation
MOMPs
cytochrome c Release
Apoptosome Formation:
Adaptor (Apaf1), dATP
cytochrome c and
procaspase complex
Association of Adaptor
with Procaspase allows
Procaspase self cleavage
Active Initiator Caspase
Cleaves Effector
Caspases
Which now Cleave
Targets
Critical Regulators of Cell Death
Bcl-2 Family – Regulate whether MOMPs Occurs
Anti-Apoptotic Factors - Death Inhibitors
A) Function to Inhibit Cytochrome C release
Pro-Apoptotic Factors- Death Activators
A) Bind and inhibit Death Inhibitors
B) Directly cause Permeabilization of Stimulate Release of
Cytochrome C ( BAX AND BAK)
IAP Family (Inhibitor of Apoptosis)
Bind Procaspases prevent activation
Bind Caspases and inhibit Activity
Survival Factor Signaling is
Required to Prevent Apoptosis
Programmed Cell Death in Neuronal
Development
Survival Factors Signaling Keeps
Death Inhibitor Bcl-2 Active
No Survival Signal
Bcl-2 Complexes with
Bad
Can’t prevent
BAK and BAX
Mediated
MOMPs
Extrinsic Pathway of Apoptosis Activation:
Signaling through the Death Receptors
Ligand Bound Death Receptors
Target cells :
Adaptor Protein and
Procaspase Recruitment
Viral Infected Cells or
Cancer Cells
Initiator Caspase Activation
Removal of Excess
Lymphocytes after
Infection
Effector Caspases Activated