Cell Death - Foundations of Medical Science PDF

Dr Stuart Knight Foundations of Medical Science Cell Biology & Signalling Block Department of Biochemistry Cell Death Teaching Objectives To appreciate when cell death takes place To understand the differences between...

Dr Stuart Knight Foundations of Medical Science Cell Biology & Signalling Block Department of Biochemistry Cell Death Teaching Objectives To appreciate when cell death takes place To understand the differences between necrosis and apoptosis To define the intracellular pathways in apoptosis To define the intracellular pathway in necrosis Be aware of the clinical importance of apoptosis Dr Stuart Knight Cell Death Apoptosis versus Necrosis Apoptosis regulated cell death (RCD) / Programmed Cell Death (PCD) : cell dies in a controlled way Cells are quickly removed Triggers are either extrinsic or intrinsic Contents of cells are not released No inflammatory response Necrosis Accidental cell death (ACD) Cells die due to lack of ATP (hypoxia or ischemia) Cell death due to physical damage – injury Contents of cells are released into extracellular space Cell contents trigger inflammatory response Dr Stuart Knight Cell Death 3 Role of apoptosis in tissue re-modelling/ development During development many cells are removed to Click icon to add picture form the mature tissues Metamorphosis Limb development Establishment of adaptive immune system Bruce, Alberts, et al. Molecular Biology of the Cell, Taylor & Francis Group, 2014. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/kcl/detail.action?do cID=5320520. Figure 18-2 Dr Stuart Knight Cell Death 4 Role of apoptosis to remove damaged cells Excessive DNA damage Exposure to UVB (290-320nm) induces chemical bonds between adjacent thymines (forming thymine dimers) Thymine dimers disrupt DNA structure and interfere with accurate replication leading to mutations Thymine dimers usually repaired by Nucleotide Excision Repair (NER) pathway If DNA damage is excessive apoptosis removes cell to prevent the possible development of cancer Skin keratinocyctes form “sunburn cells” and https://www2.samford.edu/~djohnso2/ undergo apoptosis in response to excessive jlb/333/(11)mutation2.html exposure to sunlight Dr Stuart Knight Cell Death 5 Morphological changes during apoptosis Cells become smaller Cytoskeleton collapses Nuclear envelope disassembles Chromatin condense and is fragmented Cell surface bulges (blebs) Cells breaks up into membrane bound “apoptotic bodies” Apoptotic bodies are engulfed and destroyed by phagocytosis by macrophages Rapid process – few dead cells can be observed in tissue https://www.researchgate.net/publication/ 274013152_Regulation_of_Ceramide_Channel_Formation _and_Disassembly_Insights_on_the_Initiation_of_Apoptos is Dr Stuart Knight Cell Death 6 Caspases – intracellular mediators of apoptosis Sequence of intracellular events triggered by action of family of proteases called caspases Caspases synthesised as inactive precursors that are activated by cleavage during apoptosis Initiator caspases Inactive monomers in cytosol, apoptotic trigger causes dimerization, activation and cleavage Executor caspases Initiator caspases cleave and activate executor caspases Activated executor caspases activate by cleavage all the changes in cell biology observed in apoptosis Bruce, Alberts, et al. Molecular Biology of the Cell, Taylor & Francis Group, 2014. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/kcl/detail.action? Dr Stuart Knight Cell Death docID=5320520 7. Figure 18-3 DNA fragmentation – activation of endonuclease by caspase 1000s of substrates for executioner caspases have been identified – the function of most is unclear DNA fragmentation of into discrete chunks due to activation of CAD - a specific endonuclease Executioner caspases degrades iCAD – a specific inhibitor of CAD DNA is cleaved in the regions between nucleosomes that results in a ladder of fragments Bruce, Alberts, et al. Molecular Biology of the Cell, Taylor & Francis Group, 2014. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/kcl/detail.action? docID=5320520. Figure 18-4 Dr Stuart Knight Cell Death 8 Extrinsic pathway to trigger apoptosis Apoptosis triggered from outside the cell Activation of death receptors on the cell surface For cytotoxic T-cells binding of Fas ligand to the Fas receptor activates the caspase cascade Death receptors are part of the tumour necrosis factor (TNF) family of receptors Binding of Fas ligand to Fas receptor activates initiator caspases and forms the death-inducing signalling complex (DISC) Bruce, Alberts, et al. Molecular Biology of the Cell, Taylor & Francis Group, 2014. ProQuest Ebook Central, Dr Stuart Knight Cell Death https://ebookcentral.proquest.com/lib/kcl/detail.a 9 ction?docID=5320520 Intrinsic pathway (mitochondrial) to trigger apoptosis Activated in response to DNA damage DNA damage detected via p53 pathway p53 simulates the aggregation of Bax and Bak in mitochondrial membrane Bax/Bak aggregates allows for mitochondrial outer membrane permeabilization (MOMP) and cytochrome c (part of electron transport chain) to be released by mitochondria Cytochrome c binds to Apaf1 (apoptotic protease activating factor 1) to form apoptosome complex Apoptosome recruits initiator caspases and Bruce, Alberts, et al. Molecular Biology of the triggers caspase cascade Cell, Taylor & Francis Group, 2014. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/kcl/detail.a ction?docID=5320520. Figure 18-7 Dr Stuart Knight Cell Death 10  Removal of apoptotic cell by macrophage Cells undergoing apoptosis are detected and removed by phagocytosis Composition of outer leaf of plasma membrane is altered Phosphatidylserine (PS) is flipped to the outer leaf of membrane This is detected and triggers phagocytosis Caspases presumably simulate this PS flipping outer leaf apoptotic signal phagocytosis inner leaf Dr Stuart Knight Cell Death 11 Survival factors block apoptosis Some proteins interfere with Bax/Bak aggregation and block apoptosis – this includes Bcl2 Most cells require continuous signalling by survival factors produced by neighbouring cells that block apoptosis Survival factors bind to cell surface receptors that leads to the synthesis of Bcl2 The situation is particular relevant in the adjusting the number of nerve cells in development Bruce, Alberts, et al. Molecular Biology of the Cell, Taylor & Francis Group, 2014. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/kcl/detail.a ction?docID=5320520. Figure 18-11 Dr Stuart Knight Cell Death 12 Necrosis – cell death with collateral damage Cell death in response to a dramatic event: hypoxia Ischemia (leading to hypoxia) Very high temperature Chemical (strong acid/base) High radiation dose Circumstances occur too suddenly for apoptosis to occur Cells swell (oncosis) Membrane become leaky and breaks Contents released Inflammatory response is triggered Cell Biology International, Volume: 43, Issue: 6, Pages: 582-592, First published: 08 April 2019, Dr Stuart Knight Cell Death DOI: (10.1002/cbin.11137) 13 Other mechanisms of cell death : necroptosis Necroptosis (programmed necrosis) Binding of death ligand to death receptor triggers pathway via serine/threonine kinases receptor interacting protein kinase 1 (RIPK1) and RIPK3 RIPK1/RIPK2 phosphorylates MLKL Phosphorylated MLKL oligomerises and forms ion channel in plasma membrane Cell then swells and breaks Dhuriya, Y.K., Sharma, D. Necroptosis: a regulated Tang, D., Kang, R., Berghe, T.V. et al. The molecular machinery inflammatory mode of cell death. J of regulated cell death. Cell Res 29, 347–364 (2019). Neuroinflammation 15, 199 (2018). https://doi.org/10.1038/s41422-019-0164-5 https://doi.org/10.1186/s12974-018-1235-0 Dr Stuart Knight Cell Death 14 Apoptosis and cancer Cancers caused by the production of excessive number of cells Cancers are triggered by mutations and subsequent development by further mutations Increase in cell division or decrease in apoptosis can contribute to their development At the centre of solid tumours often necrotic core due to lack of blood supply Bruce, Alberts, et al. Molecular Biology of the Cell, Taylor & Francis Group, 2014. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/kcl/detail.a ction?docID=5320520. Figure 20-13 Dr Stuart Knight Cell Death 15 A Case based discussion will be presented in the lecture Dr Stuart Knight Cell Death 16 Summary Highlight the differences between necrosis and apoptosis Role of apoptosis in biology Extrinsic pathway Intrinsic pathway Importance of survival factors Dr Stuart Knight Cell Death 17