Ch. 17&18 Cell Cycle and Apoptosis

4 major subdivisions/phases of cell cycle

M: Mitosis = Metaphase-Anaphase Checkpoint → are chromosomes attached

G₁: Gap phase = G₁ Checkpoint

S: Synthesis → DNA replication

G₂: Gap phase = G₂/M Checkpoint → pushes to prophase

What is G_o?

• growth arrest = exit cell cycle in G₁

• quiescence: normal growth arrest

  • neurons in brain

  • liver cells enter if chunk is taken out

• senescence: induced growth arrest

What happens to Cyclin levels during the cell cycle?

Cdk: cyclin dependent kinase → only active when cyclin is bound

• G₁ Cyclin

  • peak concentration at G₁ checkpoint

  • allosterically activiating G₁Cdk → phosphorylated

  • degraded at S phase-Ubiquination in proteosome

• M Cyclin

  • mitotic cyclin

  • rises during G₂ and peaks at G₂/M checkpoint

  • M-Cdk complex

  • important for M phase/Meta-Anaphase Checkpoint

Mammalian Cell Cycle Regulation

prophase?

Targets Mitotic Cyclin Cdk

• MAP phosphorylation: mitotic spindle fiber

• Condensin phosphorylation: chromosome condensation

• Lamin phosphorylation: nuclear envelope breakdown

Breakdown and Reassembly of Nuclear Envelope During Mitosis

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Importance of Cell Death → PCD/Apoptosis

• if no PCD in brain = DEATH

• brakes for cancer cells

• 50% of neurons during development of the brain dies

• severe development defects and cancer occur if apoptosis is prevented

• Ex: Caspase-9 knockout mice has abnormal cells survive causing developmental defects and possibly cancer, while the wild-type embryo removed extra or damaged cells during development

Where was PCD 1^st discovered?

• C. elegans Nematode Worm → model organism

  • animal model

  • transparent embryos

  • 959 somatic cells

• Mutant worm → extra 131 cells

  • led to current understanding of PCD

  • Cell Death genes discovered = caspase genes

Apoptosis vs. Necrosis

• Apop: no swelling/inflammation, neat and tidy cell death

• Necrosis: cell swelling → lysis → inflammation

Hallmarks of Apoptosis

• shrinkage and detachment from neighbors

• phosphotidylserine translocation (inner → outer PM)

• membrane blebbing

• chromatin condensation

• DNA fragmentation between nucleosomes

• cell degrades into apoptic bodies → phagocytosed by neighboring cells or macrophages

Induction of Apoptosis

• extra cells during development

• DNA damage (unrepairable)

• cell cycle checkpoint abnormality

What triggers the intrinsic pathway of apoptosis?

Intracellular stress or damage causes mitochondria to release cytochrome c.

What molecule is released from mitochondria during intrinsic apoptosis?

Cytochrome c

What does cytochrome c bind to in the cytosol?

Apaf1 (Apoptotic Protease Activating Factor 1)

What happens to Apaf1 after cytochrome c binds?

Apaf1 changes shape and becomes activated.

What does activated Apaf1 expose?

• dATP binding site

• Oligomerization domain

• CARD domain

What is the role of dATP in apoptosis?

dATP binds Apaf1 and helps assemble the apoptosome.

What is formed when Apaf1 molecules oligomerize?

A wheel-like heptamer called the apoptosome.

What does CARD stand for?

Caspase Recruitment Domain

What is recruited to the apoptosome?

Inactive procaspase-9 monomers

How is caspase-9 activated?

Procaspase-9 monomers dimerize inside the apoptosome.

What does activated caspase-9 do?

It activates executioner caspases.

What is the role of executioner caspases?

They break down cellular components and cause apoptosis.

What is the apoptosome?

A large protein complex made of Apaf1 and caspase-9 that activates apoptosis.

What is the final result of the intrinsic apoptosis pathway?

Programmed cell death (apoptosis)

What does Apaf stand for?

Apoptotic Protease Activating Factor

What does MOMP stand for?

Mitochondrial Outer Membrane Permeabilization

• allows cytochrome c to leak out of mitochondria intermembrane space (e^- carrier)

• Bak and Bak Oligomerization

• Regulated by Bcl-2 (B cell lymphoma) family members

What proteins promote MOMP?

Bak and Bax (pro-apoptotic Bcl2 family proteins)

Where is Bak normally located?

Attached to the outer mitochondrial membrane

What activates Bak?

An apoptotic stimulus → extra cell, DNA damaged

What happens to Bak after activation?

Bak changes shape and exposes its BH3 domain and BH3-binding groove.

What allows Bak proteins to oligomerize?

Interaction between exposed BH3 domains and BH3-binding grooves

What is oligomerization?

Multiple proteins joining together into a larger complex

What do Bak oligomers do to mitochondria?

They create openings in the outer mitochondrial membrane.

What is released through the openings caused by MOMP?

Cytochrome c and other proteins from the intermembrane space

What happens after cytochrome c enters the cytosol?

It helps form apoptosomes and activates apoptosis.

How are Bak and Bax thought to form membrane openings?

They likely form large ring-like structures that disrupt the membrane.

What is the role of BclxL?

BclxL is an anti-apoptotic protein that blocks MOMP.

How does BclxL stop apoptosis?

It binds activated Bak and prevents Bak oligomerization.

What happens when Bak oligomerization is blocked?

MOMP and apoptosis are prevented.

Are BclxL proteins pro-apoptotic or anti-apoptotic?

Anti-apoptotic

What are BH3-only proteins like Bad thought to do?

They promote apoptosis by inhibiting anti-apoptotic proteins like BclxL.

How do BH3-only proteins indirectly induce apoptosis?

They block anti-apoptotic proteins, allowing Bak/Bax to cause MOMP.

What is the overall purpose of MOMP in apoptosis?

To release cytochrome c and trigger the intrinsic apoptosis pathway.

Anti apoptotic proteins

keep cell alive

• Bcl-2

• Bcl-xL

Pro apoptotic proteins

kill cell

• Bak: Bcl-2 antagonist/kiler

• Bax: Bcl-2 associated x

What does CASPASE stand for?

cysteine-containing aspartate-specific protease