Cell cycle and apoptosis

Cell cycle and programmed cell death

1. Cell Cycle and Apoptosis R. C. Gupta M.D. (Biochemistry) Jaipur, India

2. Cell cycle RCG

3. RCG The time period between one mitotic event and the next constitutes the ‘ cell cycle’

4. RCG The cell cycle can be divided into four phases: Mitotic or M phase Gap 1 or G 1 phase Synthetic or S phase Gap 2 or G 2 phase

5. G 1 S G 2 M RCG

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7. Hartwell , Hunt and Nurse showed how the cell cycle progresses They discovered the key regulators of cell cycle RCG Hartwell Hunt Nurse

8. RCG The regulators of cell cycle are some proteins and some enzymes Cyclin levels keep on changing during the cell cycle The cyclins act through some enzymes, called cyclin-dependent kinases (CDKs ) The regulatory proteins are cyclins

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10. G 2 M Cyclin A CDK 1 ( inactive) Cyclin A CDK 1 ( inactive) Active CDK 1 phosphorylates some proteins that results in: Breaking of nuclear membrane Reorganization of cytoskeleton Chromosome condensation RCG

11. G 1 S G 2 M Cyclin D- CDK 4 & 6 Cyclin E- CDK2 Cyclin A- CDK 1 & 2 Cyclin B- CDK1 Cyclin-cdk complexes regulate the cell cycle

12. Some c checkpoints are designed to monitor progression of the cell cycle Certain events are essential before the cell cycle proceeds from one phase to the next Checkpoints RCG

13. The cell cycle cannot go to the next phase until checkpoint requirements have been met The checkpoints verify that all the events required for progression to the next phase have occurred RCG

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15. Check points in cell cycle Entry into M phase is blocked if replication is not complete Entry into S phase is blocked if genome is damaged G 2 /M checkpoint G 1 /S checkpoint

16. G 1 /S checkpoint monitors whether all the requirements for DNA synthesis have been met G 1 /S transition is the rate-limiting step in cell cycle This is also known as  the restriction point G 1 /S checkpoint RCG

17. p53 gene plays a key role at G 1 /S restriction point p53 protein halts the cell cycle in G 1 phase if it detects any damage to DNA This is known as G 1 arrest The product of p53 gene is p53 protein RCG

18. Normally, cyclin D level rises in G 1 phase which activates CDK4 and CDK6 This pushes the cell cycle into S phase In G 1 arrest, CDK4 and CDK6 remain inactive RCG

19. G 1 arrest gives more time to DNA repair systems to repair the damage If the damage is repaired, the cell cycle is allowed to proceed to the S phase If the damage cannot be repaired, the cell is pushed into apoptosis RCG

20. G 2 /M checkpoint monitors whether DNA has been replicated completely and accurately If the replication is found to be defective, the cell is pushed into apoptosis G 2 /M checkpoint RCG

21. Apoptosis

22. Apoptosis is a Greek word that means “dropping off” It is d ropping off of leaves from a tree or petals from a flower In biology, apoptosis is a form of cell death The other form of cell death is necrosis RCG

23. Necrosis Apoptosis Results in disease, if excessive Generally confers some advantage Unnatural cell death Natural cell death Cell is programmed to die RCG Acute injury results in cell death

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28. RCG Glu ‒ Ala ‒ Val ‒ Asp ‒ Leu ‒ Arg ‒ Phe ‒ Ser Caspase

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30. RCG Caspases play a crucial role in: Apoptosis Inflammation Accordingly, the caspases can be: Apoptotic Pro-inflammatory

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32. RCG The apoptotic caspases can be divided into: Initiator caspases Effector caspases Initiator caspases start the apoptotic cascade Effector caspases execute actual cell death

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35. Caspase Procaspase Adaptor protein Adaptor protein induces dimerization of procaspases RCG

36. The active initiator caspases activate the effector caspases Activation occurs by proteolytic cleavage of effector procaspases RCG

37. Effector procaspase Effector caspase Initiator caspase Activation of an effector procaspase RCG

38. The activated effector caspase activates some downstream effector caspases The active effector caspases degrade a host of intracellular proteins This results in cell death RCG

39. Cell death by apoptosis

40. RCG Activation of i nitiator caspases requires some signals The signals may be: Extrinsic (extracellular) Intrinsic (intracellular)

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43. RCG Extrinsic (extracellular) pathway

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50. Apoptosis Procaspase 3 Caspase 3 Caspase 9 Procaspase 9 FADD protein Fas FasL Extrinsic pathway of apoptosis Caspase cascade RCG

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52. Mitochondria are essential for multicellular organisms A cell will cease to respire aerobically in the absence of mitochondria Apoptotic signals acting through mito-chondria either increase or decrease the permeability of mitochondrial membrane RCG Intrinsic (mitochondrial) pathway

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54. When membrane permeability increases, some mitochondrial proteins leak into cytosol These include Small Mitochondria-derived Activators of Caspases (SMACs) and cytochrome c RCG

55. SMAC Cytochrome c RCG

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57. SMACs IAP Θ Θ Procaspase C aspase Release of SMACs inhibits IAP and allows activation of procaspases RCG

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59. ATP/dATP Oligomerization C aspase 9 (dimer ) Procaspase 9 (monomer) Apaf-1 (rigid conformation) Apaf-1 (open) Apaf-1 (closed) Cytochrome c Release of cytochrome c leads to activation of procaspase 9 RCG

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61. Morphology of apoptotic cells RCG

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64. Cell undergoing apoptosis Blebs formed Apoptotic bodies formed Phagocytosis of remains

65. Thank you