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Lecture 16: Programmed Cell Death

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Review Concepts of Apoptosis

When is apoptosis important?

Cell is damaged beyond repair

Cell is infected with a virus - can prevent viral spread

Removal of ineffective or potentially damaging immature B or T cells (remember back to T cell tolerance)

Control of malignant cells

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Two Distinct Apoptotic Pathways (Back in Lecture 7 notes)

Programmed Cell Death

Two distinct Pathways

Signaling & activation of Caspases

Intrinsic major pathway in all cells regulation of mito integrity release of key apoptotic factors Cytochrome C

Extrinsic death receptors (Fas) trimeric cell surface proteins bind to specific ligands (FasL) pro-apoptotic - mito leakage

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Apoptosis and Necrosis Paradigm

Typically cell death is discussed either as Apoptosis or Necrosis.

Apoptosis Active programmed process that is non-inflammatory Can be beneficial

Necrosis Passive, accidental cell death resulting from environmental perturbations with uncontrolled release of inflammatory cellular contents. Basically describing the end result of dead tissue. Almost always detrimental

In the end, only two terms to describe cell death is not sufficient.

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More Terms to Describe Dying Cells

Two Basic categories: Non-inflammatory Pro-inflammatory

6 Cell Death Pathways

Apoptosis Pyroptosis

Removal of unwanted and harmful cells

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New Pathway for Proinflammatory Cell Death

Apoptosis Word derivation - greek - falling off (such as leaves from a tree) Idea: controlled removal of individual components (cells) without destruction to organism. Pyroptosis

Word derivation - greek - roots "pyro," relating to fire or fever Idea: programmed cell death that results in inflammation (fire).

Pyroptosis Morphologicaly and mechanistically different from other cell death pathways Defining features: caspase-1 dependent

proinflammatory cytokines

8 New Pathway for Proinflammatory Cell Death

(recent! ~ 2001)

Caspase-1 is activated by cleavage - various stimuli Rapid formation of plasma membrane pores

Water influx osmotic swelling and lysis IL-1beta and IL-18 are processed and released

Nuclear integrity maintained

Possible mechanisms for cytokine release

Proinflammatory cytokine release

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Bacteria that Induce Pyroptosis

Cellular Exit

Shigella flexneri - T3SS factor IpaB Salmonella - T3SS factor flagellin

Rapid pyroptosis ~45 min

Terms: do not worry about the different inflammasome names

, SipB, IpaF

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Induces Apoptosis Mechanism

Cellular Exit

Sort of a centric view by these authors - they study Francisella!

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Mitochondrial Protein Import - Model Yeast

TOM translocase of

outer membrane

TIM translocase of

inner membrane

SAM sorting and assembly machinery

Import of proteins w/ N term targeting sequence Import of proteins w/ internal targeting sequence

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Import of Bacterial Factors into Mitochondria

Enteropathogenic E. coli EspF & Map T3SS factors:

N term targeting signals

Neisseria and Acinetobacter PorB & Omp38

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Bacterial Factors Targeting Host Mitochondria

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Back to Apoptosis Mechanism

Layers of Regulation via complex signaling events

1. Activation of caspase cascade FLIP - Flice-like inhibitory protein IAP - Inhibitor of apoptosis proteins

2. Anti-apoptotic proteins Bcl-2 like proteins

3. Expression of pro-survival factors NFκβ - transcription factor

4. Additional survival pathways AKT signaling pathway ERK signaling pathway

Pro-apoptotic

Pro-survival

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Bacteria Inhibit Apoptosis Three Main Ways

3 Main Inhibitory Routes

1. Interaction with caspases

2. Prevent cytochrome C release

3. Activation of survival pathways These also prevent cyt C release.

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16 Prevention of CytC

Release Cyt C:

A.  Chlamydia proteasome-like activity factor CPAF: Targets BH3 proteins for degradation Outcome Bcl-2 proteins stabilized

B.  Activation of AKT (phosphorylation) Prevents CytC release

C.  Neisseria outer membrane protein PorB is target to mito

Inhibits CytC release

17 Prevention of CytC

Release Cyt C:

A.  Chlamydia proteasome-like activity factor CPAF: Targets BH3 proteins for degradation Outcome Bcl-2 proteins stabilized

B.  Activation of AKT (phosphorylation) Prevents CytC release

18 Activation of

Survival Pathways NFκβ: major pro-survival Transcription

factor - central role Exploit the cell survival pathways

A.  Upregulation of pro-survival protein with accompanying downregulation of pro-apoptotic proteins

B.  SopB - possibile activator of AKT Inhibits CytC release

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19 Activation of

Survival Pathways

IAPs: Inhibitor of apoptosis proteins

Some evidence for direct binding to caspases to inhibit the cascade

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Overall Summary