BIOL2021: Chapter 18 - Cell Death

What is Necrosis a response to?

Cell damage

What are the 2 types of cell death?

1. Apoptosis; Programed cell death

2. a)necrosis; unregulated cell death

2. b)necropoptosis; Programmed cell death(more complicated)

What is apoptosis important for?

• Creation and maintenance of limbs and organs

• Quality control in immunity

• To eliminate damaged/abnormal cells in adults (irreparable damage, infection, cancer)

What are the biochemical markers of apoptosis?

• DNA cleavage

• PtdSer exposed on cell surface (“eat me” signal)

• Inactivation of “don’t eat me” signals

• Loss of mitochondrial membrane potential; release of cytochrome C and other proteins into cytoplasm

What is apoptosis triggered by?

Intracellular proteases called caspases

What 2 characteristic amino acid residues do caspases have

• Cysteine; In the active site

• Aspartate; Cleaves proteins

What are the 2 major classes of caspases?

• Initiator Caspases; Begin apoptosis by activating

• Executioner Caspases; Act on target molecules (>1000)

Both exist initially as inactive Procaspases

What is a procaspase?

Inactive form of caspase

What do Initiator caspases do in the Intracellular Proteolytic Cascade?

1. Start off as inactive soluble monomers in the cytosol

2. An apoptic signal triggers assembly of adaptors which complex with initiator caspases

3. Caspase dimers form, cleave, and activate protease activity

What do executioner caspases do in the Intracellular Proteolytic Cascade?

1. Start off as inactive dimers

2. Gets cleaved by initiator caspase

3. Activated by conformational rearrangement

True/False; one molecule of active initiator caspase can only act on one molecule of executioner caspases.

False; One initiator caspase can act on multiple executioner caspases

What 2 pathways can apoptosis be activated by?

• Extrinsic pathway; Outside signals

• Intrinsic/mitochondrial pathway; Injury, stress, DNA damage, loss of survival signals

How is the extrinsic pathway activated?

• By signal proteins binding to the cell surface death receptors

What are TNFs and TNF receptors?

• TNF: Tumor necrosis factor (ex FAS)

• TNF receptors; Death receptors

What are the 3 main components of death receptors?

• Single transmembrane domains; form homotrimers

• Extracellular ligand binding domain

• Intracellular death domain

What family are TNF receptors and Fas receptors apart of?

Tumor necrosis factor (TNF) receptor family

How does The Fas death receptor extrinsic pathway apoptosis activation work?

1. Fas ligan on cytotoxic killer lymphocytes binds to Fas death receptor on target cell, activating Fas death receptors

2. Activated Fas death receptors lead to activation of intracellular death domains

3. Fas death receptor death domain binds to death domain on adaptor protein, FADD (Fas-associated death domain)

4. FADD death effector domain binds to the death effector domain on initiator caspase (primarily caspase-8)

5. FADD-caspase complex is called the death-inducing signaling complex (DISC)

6. Adjacent initiator caspases in the DISC interact and form an activated dimer that cleaves itself from death effector domain

7. Activated initiator caspase dimer cleaves executioner caspases by proteolytic cleavage

What is the key protein for the intrinsic apoptosis pathway?

Cytochrome c

Which organelle does the Intrinsic pathway primarily involve?

Mitochondria

How does the apoptosis Intrinsic/Mitochondrial pathway work?

1. Cytochrome c is released into the cytosol binds to the adaptor protein, Apaf1 (apoptotic protease activating factor 1)

2. Apaf1 oligomerizes into a wheel-like heptamer - apoptosome

3. Apaf1 has a caspase recruitment domain (CARD) cluster in a central hub of the apoptosome

4. Apad1 CARD bind to similar CARD of caspase-9

5. Apaf1 CARD binding to CARD of caspase-9 activates caspase

6. Activated caspase-9 activates downstream executioner caspases

What does Bcl2 (B-cell lymphoma 2) do?

Controls the release of cytochrome c and other mitochondrial proteins into the cytosol

What are the 3 main types of Bcl2 proteins?

• Anti-apoptotic Bcl2 family protein

• Pro-apoptotic effector Bcl2 family protein

• Pro-apoptotic BH3-only protein

Where are Pro-apoptotic Bcl2 proteins found?

In the outer mitochondrial membrane

What triggers oligomerization and activation of pro-apoptotic Bcl2 proteins?

Apoptotic stimulus

What do anti-apoptotic Bcl2s do?

Bind Pro-apoptotic effector proteins and inhibit aggregation

What do BH3 only proteins do?

Inactivate anti-apoptotic proteins

How does DNA damage lead to apoptosis?

DNA damage leads to phosphorylation and activation of p53, which stimulates BH3-only genes

How can the extrinsic apoptosis pathway stimulate the intrinsic apoptosis pathway?

• Activation of caspases in the extrinsic pathway cleaves BH3-only proteins to activate them

• Activated BH3-only translocases to the outer mitochondrial membrane to inhibit anti-apoptotic Bcl2 family proteins

What extracellular survival factors can inhibit apoptosis

• Increased production of anti-apoptotic Bcl2 family proteins

• Inactivation of pro-apoptotic BH3-only protein

• Inactivation of anti-IAPs (inhibitors of apoptosis)

How does increased production of anti-apoptotic Bcl2 family proteins inhibit apoptosis?

Survival factor binds to cell surface receptor, leading ti activation of a transcription regulator to increase synthesis of an anti-apoptotic Bcl2 family protein

How does inactivation of pro-apoptotic BH3-only protein inhibit apoptosis?

Survival factor binds to cell surface receptor leading to activation of a cell signaling cascade which phosphorylates and inhibits pro-apoptotic BH3-only family protein

How does inactivation of anti-IAPs inhibit apoptosis?

• IAPs (inhibitors of apoptosis) bind to and inhibit activated caspases. Some IAPs can polyubiquitylate caspases for destruction by proteosomes

• Anti-IAPs are released from the mitochondria intermembrane space into the cytosol when the intrinsic pathway is activated, blocking IAPs

• Survival factors can bind to cell surface receptors and activate cell signaling cascade leading to phosphorylation and inhibition of anti-IAP(ex Hid)

How is the target amount of nerve cells regulated?

By using apoptosis to adjust the number of cells

What impact does increased cell division or decreased apoptosis lead to?

Tumor formation

What diseases can result from too much apoptosis?

Heart attacks, strokes

What diseases can result from too little apoptosis?

Lymphocytes, autoimmune disorders