Cell cycle and regulation

cellcycle,cell cycle regulation,phases of cell cycle,cell injury,etiology of cell injury,mechanism of cell injury,apoptosisand necrosis,autophagy,cell death

1. CELL CYCLE 1

2. CONTENT INTRODUCTION PHASES REGULATIONS CELL INJURY NECROSIS AND APOPTOSIS AUTOPHAGY 2

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4. INTRODUCTION OF CELL CYCLE Multiplication of the somatic (mitosis) and germ (meiosis)cell. Cells is the most complex of all cell functions. Mitosis is controlled by genes which encode for release of specific protein molecules that promotes or inhibit the process of mitosis at different steps. mitosis promoting protein molecules are cyclin A,B and E. These cyclins activate Cyclin Depended Kinase(CDK).which are conjuction with cyclins. After the mitosis complete cyclins,CDK will degrade. 4

5. PHASES OF CELL CYCLE G 1 (pre - mitotic gap) S phase G 2 (pre – mitotic phase) M phase Prophase Metaphase Anaphase Telophase G phase 5

6. REGULATION 6

7. 7 CHECKPOINTS OF CELL CYCLE

8. 8 G1 Checkpoint: This checkpoint is present at the end of the G 1 phase and before S phase. This checkpoint helps in taking the decision of whether the cell should divide, delay division, or enter a resting phase (Go phase). If there are unfavourable conditions for the cell division, then this restriction point restrict the progression to the next phase by passing the cell to Go phase for an extended period of time. This restriction point is mainly controlled by the action of the CKI-p16 (CDK inhibitor p16).The inhibited CDK not bind with cyclin D1, hence there is no cell progression.   Active cyclin D- cdk complexes phosphorylate retinoblastoma protein ( pRb ) in the nucleus.   Un- phosphorylated pRb acts as an inhibitor of G 1 by preventing E2F-mediated transcription.   Once PRb get phosporylated activates the transcription of cyclins E and A, Which then intract withCDK2 to allow for G1-S phase transition.   This brings the cell to the end of the first checkpoint ( unphosphorylated Rb inhibits the E2F).

9. 9 G2 Checkpoint This restriction point is located at the end of the G 2 phase. This checks the number of factors which   are essential for the cell division.   Maturation-promoting factor or mitosis promoting factor or M-phase promoting factor-(MPF) is a protein composed of cyclin -B and CDK-1. This protein promotes the G 2 phase into   the entrance of M-phase. MPF is activated at the end of G 2 by a phosphatase ( Chk ) which removes an inhibitory phosphate group added earlier. The main functions of MPF in this restriction point are: a. Triggers the formation of mitotic spindle.   b. Promotes chromosome condensation.   c. Causes nuclear envelop breakdown.   If there are any damages are noticed in this restriction point, then the phasphatase not activate the MPF, resulting in the arrest of cell cycle in G 2 phase till the repair of the damaged DNA. This prevents the transfer of defected DNA into the daughter cells.

10. 10 M-Checkpoint:   This occurs at metaphase. Anaphase-promoting complex (APC) regulates this checkpoint.This is also called spindle checkpoint.   This checks whether all chromosomes are properly attached to the spindle or not.This also governs the alignment of the chromosomes and integrity of the spindles. If there are mistakes then it delays the cell in entering into anaphase from metaphase.

11. 11 CELL CYCLE REGULATORS   The cell cycle is regulated by cycles.   1.Cyclins   2.cyclin-dependent kinases (CDKs)   3.cyclin-dependent kinase inhibitors (CDKIs).   CYLCLINS:   Their concentration varies during the cell cycle. Cyclins are the family of proteins which regulates the cell cycle.   There are several types of cyclins that are active in different parts of the cell cycle and causes phosphorylation of CDK.   There are also several “orphan” cyclins for which no CDK partner has beenidentified .   For example, cyclin F is an orphan cyclin that is essential for G2/M transition.   There are two main groups of cyclins .

12. 12 2.Cyclin-Dependent kinases (CDKs)   These are a family of protein kinases that regulates the cell cycle. They are present in all known eukaryotic cells.   Inactive on their own but becomes active when attached to cyclins .

13. 13 3.Cyclin-Dependent Kinase Inhibitors (CDKIs)   CDKI is a protein which inhibits cyclin -dependent kinase (CDK). Cell cycle progression is negatively controlled by cyclin -dependent kinase inhibitors (called CDIs, CKIs or CDKIs). These are involved in cell cycle arrest at the G 1 phase. CDKI p 16 CDK 4, CDK6 p 15 CDK4 p 18 CDK4, CDK6 p 19 CDK4, CDK6 p 21 CYCLIN E1, CDK2 CDK3, p 27 CDK4, CDK2,CYCLIN E1 p 57 CYCLIN E1, CDK2

14. 14 POSITIVE REGULATORS Are those which control the changes necessary for cell division. They include:- Cyclins Cyclin -dependent kinases ( cdks ) Polo-like kinases NEGATIVE REGULATORS Are those which control the positive regulators. Ink family (Inhibitors of kinases ) P 19 ,P 15 CIP family ( cdks inhibitory proteins) P 21 , P 57 They include :- Rb proteins P 53 gene Inhibitors of cdks - which are of 2 types

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16. ETIOLOGY OF CELL INJURY Cell injury can be acquired by the two ways By genetic causes By acquired causes ACQUIRED CAUSES Hypoxia a and ischemia Physical agents Chemical agents and drugs Microbial agents Immunological agents Nutritional derangements Aging Psychogenic diseases Iatrogenic factors Idiopathic diseases 16 Cell injury is defined as the effect of a variety of stresses due to etiologic agents a cell encounters resulting in changes in its internal and external environment. CEL INJURY

17. PATHOGENISIS OF CELL INJURY Injury to the cell by one or more etiological agents may results cause reversible and irreversible cell injury. Some of the principles may apply for pathogenesis of cell injury by various agents . 1.Type, duration and severity of injurious agents The extent of cell injury depend upon the type, duration and severity of the stimulus. 2.Type, status and adaptability of target cell the type of cell as regards its susceptibility to injury , its nutritional and metabolic Status ,and adaptation of the cell to hostile to environment. 3.underlying intracellular phenomena mitochondrial damage causing ATP depletion cell membrane damage release of toxic free radicals. 4.morphological consequences based on the morphological changes of the cell 17

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19. PATHOGENISIS OF ISCHAMIC AND HYPOXIC INJURY Reversible cell injury Irreversible cell injury Reversible cell injury MECHANISM EFFECT 1. Decreased generation of cellular ATP 1. Damage by ischemia versus hypoxia from other causes 2. Intracellular lactic acidosis 2. Nuclear clumping 3.Damage to plasma membrane pumps (Na-K, Ca) 3. Hydropic swelling and other membrane 4. Reduced protein synthesis 4. Dispersed ribosomes 19

20. Irreversible cell injury Persistence of ischemia or hypoxia results in irreversible damage to the structure And function of the cell(cell death). MECHANISM EFFECT 1. Calcium influx 1. Mitochondrial damage 2. Activated phospholipases 2. Membrane damage 3. Intracellular proteases 3. Cytoskeletal damage 4. Activated end nucleases 4. Nuclear damage 5. Lysosomal hydrolytic enzymes 5. Lysosomal damage, cell death, phagocytosis 20

21. FREE RADICAL MEDIATED CELL INJURY Mechanism of reperfusion injury and free radical mediated injury is complex, but following three main components are involved in it. Calcium overload Generation of reactive oxygen free radicals (mechanism of action of free radical generation) 3. Subsequent inflammatory reaction. 21

22. Mechanism of cell death by hydroxyl radical 22

23. Pathogenesis of chemical cell injury Chemically induce cell injury by one of the following by direct cytotoxicity and chemical into reactive metabolites. Direct cytotoxic effects:   e.g. chemotherapeutic agents in cancer, toxic heavy metals (cyanide, mercury, lead, iron) Conversion to reactive toxic metabolites:   e.g. CCl 4 , acetaminophen, bromobenzene 23

24. PATHOGENSIS OF PHYSICAL INJURY Mechanical injury Thermal trauma   Electricity Rapid changes in atmospheric pressure   Radiation injury (UV, ionization) 24

25. REACTION OF APOPTOSIS AND NECROSIS 25

26. APOPTOSIS AND NECROSIS NECROSIS Necrosis is defined as a localised area of death of tissue followed later by degradation of tissue by hydrolytic enzymes liberated from dead cells; it is invariably accompanied by inflammatory reaction. Necrosis can be caused by various agents such as hypoxia, chemical and physical agents, microbial agents, immunological injury, etc. Based on etiology and morphologic appearance, there are 5 types of necrosis: COAGULATIVE NECROSIS LIQUEFACTION (COLLIQUATIVE) NECROSIS CASEOUS NECROSIS FAT NECROSIS FIBRINOID NECROSIS 26

27. APOPTOSIS Apoptosis is a form of ‘coordinated and internally programmed cell death’ having significance in a variety of physiologic and pathologic conditions (apoptosis=falling off or dropping off , as that of leaves or petals). Apoptosis is not accompanied by any inflammation and collateral tissue damage. MOLECULAR MECHANISMS OF APOPTOSIS Several physiologic and pathologic processes activate apoptosis in a variety of ways. However, in general the following molecular events sum up the sequence involved in apoptosis 27

28. Initiators of apoptosis i ) Withdrawal of normal cell survival signals ii) Agents of cell injury 2. Initial steps in apoptosis i ) Intrinsic (mitochondrial) pathway ii) Extrinsic (cell death receptor initiated) pathway 3. Final phase of apoptosis 4. Phagocytosis MOLECULAR MECHANISMS OF APOPTOSIS 28

29. INTRINSIC AND EXTRINSIC PATHWAY 29

30. MECHANISM OF CELL DEATH IN OXIDATIVE STRESS MODELS Hydrogen peroxide induced apoptosis Reactive nitrogen species and apoptosis Toll like receptor pathways and apoptosis Lipid metabolites and apoptosis Photodynamic therapy Ionizing radiation Cigarette smoke MECHANISM OF CELL DEATH IN OXIDATIVE LUNG INJURY AND ISCHEMIA/REPERFUSION INJURY Hyperoxia LPS induced lung injury Ischemia/perfusion(I/R) 30

31. AUTOPHAGY The term ‘ autophagy ’, derived from the Greek meaning ‘eating of self.’ It is one type of programmed cell death (PCD) distinct from the apoptosis and This is regulated pathway for internal organelle or protein degradation . Finally the delivered content become degrade and recycled. 31 TYPES OF AUTOPHAGY Macro- autophagy Micro- autophagy Chaperone-mediated autophagy (CMA)

32. 32 1.MACRO AUTOPHAGY  Macro- autophagy delivers cytoplasmic cargo to the lysosome through the intermediary of a double membrane-bound vesicle, referred to as an autophagosome , that fuses with the lysosome to form an autolysosome .

33. 33 2.MICRO AUTOPHAGY In micro- autophagy , by contrast, cytosolic components are directly taken up by the lysosome itself through invagination of the lysosomal membrane. Both macro-and micro- autophagy are able to engulf large structures through both selective and non-selective mechanisms. In chaperone-mediated autophagy (CMA), targeted proteins are translocated across the lysosomal membrane in a complex with chaperone proteins (such as Hsc-70) that are recognized by the lysosomal membrane receptor lysosomal -associated membrane protein 2A (LAMP-2A), resulting in their unfolding and degradation  3.CHAPERONE-MEDIATED AUTOPHAGY

34. 34 BASIC AUTOPHAGY MACHINERY

35. 35 control of phagophore formation by Beclin-1/VPS34 at the ER and other membranes in response to stress signalling pathways; Atg5–Atg12 conjugation, interaction with Atg16L and multimerization at the phagophore ; LC3 processing and insertion into the extending phagophore membrane; capture of random or selective targets for degradation, completion of the autophagosome accompanied by recycling of some LC3-II/ATG8 by ATG4, followed by; Autophagy is induced by hypoxia and low cytosolic ATP levels that feed through REDD1 and AMP- kinase to inhibit mTOR activity through reduced Rheb GTPase activity. Conversely, autophagy is inhibited by increased growth factor signalling through the insulin receptor and its adaptor, IRS1, as well as other growth factor receptors that activate the Class I group of PI3-kinases and Akt , to promote mTOR activity through inhibition of TSC1/TSC2 and increased Rheb GTPase activity

36. 36 REFERENCES 1 .Text book of pathology by Harsh Mohan 6 th ,7 th edition. 2. HM/CH-1/L-4,cell injury 3. Danielle Glick1,2, Sandra Barth1, and Kay F.Macleod1,2,* Autophagy : cellular and molecular mechanisms, NIH Public Access Author Manuscript,2010.

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