Lecture 24: Apoptosis

Apoptosis

• distinctive and important mode of programmed cell death

• Orderly, energy-dependent, and removes unwanted or damaged cells without harming neighbours

• important for:

  • development

    • sculpting digit tissues

    • joint formation

  • homeostasis

    • eliminated damaged, excess, infected cells

• depends on a proteolytic cascade mediated by Caspases

C. Elegans Apoptosis

• Nobel Prize 2002: for their discoveries concerning genetic regulation of organ development and programmed cell death”

• the lineage of the known cells of C. Elegans were followed in-vivo so they could be seen dying

  • mutants of C. elegans showed impaired/failed clearance of apoptotic cells

  • from this, the apoptotic pathway was built

Apoptotic Pathway

• specification

• killing

• execution

Necrotic vs. Apoptotic Cells

• Necrotic cells

  • spill their contents into their neighbours

• Apoptotic cells

  • die neatly, without damaging it neighbours

Caspases

• cysteine asparate proteases

  • apoptosis depends on a proteolytic cascade mediated by caspases

• cysteine proteases which use the sulfur atom in cysteine to preform the cleavage reaction

  • activated by cleavage at aspartic acids by other caspases

  • once activated, they cut strategic proteins in the cell next to asparate amino acids

• may be either:

  • initiator

  • executioner

Initiator Caspases

• Exist as inactive monomers

  • Activated by dimerization or self-cleavage

• begin the apoptotic program by activating the executioner caspases

• caspase 8 and 9

Executioner Caspases

• Have short prodomains

• Activated by initiator caspases

• Cleave hundreds of cellular targets

• Caspase 3,6, and 7

Irreversibility of Apoptosis

• Caspases activate other caspases

• Creates an amplifying cascade

• Once started → cell is committed

CAD

• caspase activated DNAse

• catalyses the hydrolytic cleavage of DNA by releasing the brake on DNAses to degrade DNA

Extrinsic Pathway

• 1 of 2 main activation pathways

• signalled from outside the cell

  • often via killer immune cells

• mechanism:

1. Fas ligand binds Fas receptor

2. Fas receptors cluster

3. Death domains exposed

4. Recruit adaptor protein FADD

5. FADD recruits inactive initiator caspases

6. Large complex forms: DISC

7. Initiator caspases activate → apoptosis begins

Intrinsic Pathway

• 1 of 2 main activation pathways

• signalled from the mitochondria inside of the cell

  • often in response to developmental signals or to injury (ex. DNA damage)

  • initiated by the release of cytochrome c from the mitochondria into the cytoplasm

• Mechanism

  • Cytochrome c binds Apaf-1

  • Apaf-1 oligomerizes

  • Forms large complex: apoptosome

  • Recruits inactive caspase-9

  • Caspase-9 dimerizes → activated

  • Executioner caspases activated

BcI-2

• main regulators of the intrinsic pathway

  • pro-apoptotic or anti-apoptotic

    • balance between them determines cell fate

• Control mitochondrial membrane permeability

Pro-apoptotic BcI-2

• BcI2 proteins make holes into the mitochondrial membrane

Anti-apoptotic BcI-2

• BcI2 proteins inactivate the pro-apoptotic BcI2

Inhibitors of Apoptosis

• IAP

• line of defense against inappropriate caspase activation

• bind and prevent activation of some procaspases

Post-Apoptosis Cleanup

• healthy neighbours phagocytose and digest apoptotic cells

• cell competition is a fitness control mechanism in which less fit cells are eliminated from a tissue

• PtdSer

Phosphatidyl Serine (PtdSer)

• Normally located on inner membrane leaflet

• Caspases inactivate flippases, therefore allowing PtdSer to appear on the cell’s outer surface

• as a result, neighbouring cells recognize apoptotic cells

  • Apoptotic cells are then phagocytosed and digested

Ferroptosis

• intracellular iron dependent form of cell death

  • distinct from apoptosis and necrosis

• characterised by the accumulation of oxidatively damaged phospholipids