1. Chapter 18
Cell Death

2. Cell Death
Apoptosis
Necrosis
Necroptosis
Autophagy
Cornification

3. Apoptosis
Cell death: billions of cells die every hours in a human adult body, important for development
Apoptosis (one form of programmed cell death): cell suicide, proposed in 1970s  acceptance (20 years later)  Nobel prize in 2002
Occur from unicellular organisms to multicellular organisms (molecular mechanisms are distinct)
Astonishing number of cell death: never cells, thymus (T cells), bone marrow (B cells and neutrophils), eye (photoreceptors), spleen

4. How many apoptotic cells?

5. Apoptosis
Quality control process: eliminating abnormal, superfluous, nonfunctional and potentially dangerous cells (cancerous cells)
Development (sculpt) and tissue homeostasis (change of aged, nonfunctional cells with new cells)
Morphological and biochemical changes

6. Programmed cell death eliminates unwanted cells
Programmed cell death eliminates unwanted cells, as seen in the development of mouse paw. Apoptosis is a form of programmed cell death, and it plays important role in normal development (e.g., tissue sculpturing)

7. Programmed cell death eliminates unwanted cells
Apoptosis during the metamorphosis of a tadpole into a frog

8. Apoptosis Morphological and biochemical changes shrinkage and condense
cytoskeleton collapse
Disassembly of the nuclear envelope, nuclear chromatin condensation, DNA fragmentation
blebs
apoptotic bodies
Phosphatidylserine exposure(eat me signal)
Cytochrome C
Loss of Mt membrane potential

9. Apoptosis

10. Apoptotic cells are morphologically and biochemically recognizable
DNA Fragmentation

11. Molecular Mechanisms of Apoptosis

12. Apoptosis: intracellular proteolytic cascade mediated by caspases
Intracellular machinery for apoptosis is similar in all animal cells
Caspases: cysteine protease, Cleave after aspartate
Procaspase: inactive caspase, activated by proteolytic cleavage
Inflammation caspases: 1, 4, 5
Initiator caspases: 2, 8, 9, 10 (CARD in prodomain-assembled with adaptor proteins in activation complex for activation)

13. Apoptosis: intracellular proteolytic cascade mediated by caspases
Executioner caspases: 3, 6, 7 (targets: nuclear lamins, DNA (endonucleases), cytoskeleton, cell-cell adhesion molecules. Activated by initiator caspases)
Extrinsic and intrinsic pathway can activate a caspase cascade

14. Cell-surface death receptors activate the extrinsic pathway of apoptosis
Ligand binding region, transmembrane region, death domain
TNF receptor family
Death domain
Fas-Fas ligand (formation of DISC (Death inducing signal complex): signal  receptor  adaptor protein (FADD-FAS associated protein with death domain)  Caspase 8 or 10  activation of downstream proteins)

15. Cell-surface death receptors activate the extrinsic pathway of apoptosis

16. Cell-surface death receptors activate the extrinsic pathway of apoptosis
Inhibitors of extrinsic pathway
Decoy receptor: ligand binding domain, but not death domain
FLIP: resembles initiator caspases, but lacks proteolytic domain, compete with caspase 8 and caspase 10 for binding site in the DISC

17. The intrinsic pathway of apoptosis depends on mitochondria
Apoptosis: also activated from inside the cell
Caused by injury, DNA damage, lack of oxygen, nutrients, or survival signals
Release of mitochondrial intermembrane space proteins
Cytochrome C: binding to Apaf1  form apoptosome  recruitment of procasepase (9)  activation of downstream proteins
Extrinsic pathway  harness intrinsic pathway to amplify apoptotic signal

18. The intrinsic pathway of apoptosis depends on mitochondria

19. The intrinsic pathway of apoptosis depends on mitochondria
Release of cytochrome C during apoptosis

20. Bcl2 proteins regulates the intrinsic pathway of apoptosis
Bcl2 family proteins
Tightly regulate intrinsic pathway
Evolutionary conserved from worms to humans
Controlling the release of cytochrome C and other intermembrane space proteins
Pro-apoptotic VS anti-apoptotic proteins: bind each other, balance regulates determines live or death
Contain Bcl2 homology domain (BH)
Anti: Bcl2, Bcl2-XL (contains BH 1~4 domain), Pro: Bax, Bak, Bad, Bim, Puma, Noxa (two subfamily-BH123 or BH3 only)

21. Bcl2 proteins regulates the intrinsic pathway of apoptosis
The three classes of Bcl2 proteins

22. Bcl2 proteins regulates the intrinsic pathway of apoptosis
Action of BH123 protein during apoptosis
Binding to the outer membrane
Form oligomers
helping the release of the proteins

23. Bcl2 proteins regulates the intrinsic pathway of apoptosis
Action of anti-apoptotic proteins (Bcl2 and Bcl2-XL)
Mainly located on the outer membrane
Inhibit apoptosis by binding to pro-apoptotic proteins (preventing oligomerization of Bak)

24. Bcl2 proteins regulates the intrinsic pathway of apoptosis
Action of pro-apoptotic proteins (BH3 only proteins)
promotes apoptosis by binding to anti-apoptotic proteins
promotes apoptosis by directly binding to pro-apoptotic proteins (helping trigger and aggregate pro-apoptotic proteins)

25. IAPs inhibit caspases Inhibitors of apoptosis
Another intracellular regulators for apoptosis besides Bcl2
Identified first in certain insect viruses, found in most animal cells
BIR (baculovirus IAP) domain: binds to active capases  inhibits activation of caspases or polyubiquitylates caspases
Anti-IAP: IAP binding motif  binding to BIR domain of IAPs (balance between IAPs and anti-IAPs: determine live or death)
Controversial for mammalian homologes IAP and anti-IAP (Smac, Omi): depletion of the genes  no effects on apoptosis

26. IAPs inhibit caspases
Action of IAPs

27. IAPs inhibit caspases
Action of anti-IAPs

28. Extracellular survival factors inhibit apoptosis in various ways
Extracellular signals
Death or survival signals
Death signals: Fas ligands, thyroid hormone, BMP
Survival factors (inhibit apoptosis): bind to cell-surface receptors  suppress apoptotic program by regulating members of Bcl2 family proteins (increase of anti-apoptotic protein production, inhibiting the function of pro-apoptotic proteins, phosphorylation of anti-IAPs)

29. Extracellular survival factors inhibit apoptosis in various ways
Survival factor secretion during neuronal cell targeting

30. Extracellular survival factors inhibit apoptosis in various ways
Mechanisms of action for survival factors during apoptosis

31. Phagocytes remove the apoptotic cells
Who? - Professional and non-professional phagocytes
When? - Form birth to death, throughout life in multi- cellular organisms
Where? - essentially anywhere cell death occurs (thymus, eye, spleen, bone marrow)
Why? - Initially thought of simply garbage disposal, recent studies imply a critical role for clearance of apoptotic cell in development and tissue homeostasis and in preventing autoimmune diseases and inflammatory diseases

32. Either excess or insufficient apoptosis can contribute to disease
Excess apoptosis
Tissue damage: heart attacks, strokes
Insufficient apoptosis
Autoimmune diseases
Cancer: mutations in Bcl2, p53 and so on

33. Necrosis

34. Necrosis Accidental cell death by physical or chemical stress
A form of cell injury that results in the premature death of cells by autolysis
Passive processes
Morphological and biochemical features are distinctive from those of apoptosis

35. Necrosis Features of necrotic cells Loss of membrane integrity
Begins with swelling of cytoplasm and mitochondria
Ends with total cell lysis
No vesicle formation, complete lysis
Disintegration of organelles
No energy requirement
Random digestion of DNA

36. Programmed Necrosis (necroptosis)
Necrosis was regarded as an unregulated and uncontrollable process
Evidence now reveals that necrosis can also occur in a regulated manner
Physiological necrosis in vivo was also observed under various physiological settings including mammalian development and tissue homeostasis
Death receptor, RIP1 and RIP3 are involved in these processes
Necroptosis participates in the pathogenesis of diseases including ischaemic injury, neurodegeneration and viral infection
An attractive target for the avoidance of unwarranted cell death

37. Programmed Necrosis (necroptosis)

38. Autophagy

39. Autophagy Auto-’self’, phagein-’to eat’ in Greek
Processes to remove unnecessary or non-functional organelles through lysosomes
Inhibit accumulation of damaged or non-functional organelles by degrading them
Increase survival by maintaining energy levels at nutrient deficient conditions
Type II programmed cell death (death mechanism vs survival mechanism)

41. Figure Q18-1 Molecular Biology of the Cell (© Garland Science 2008)

42. Figure Q18-2 Molecular Biology of the Cell (© Garland Science 2008)