MCB 2210 001 4 29 2015 Cell Death Cells age old cells have different characteristics than young ones Cells die o Immortalized cell lines can die too o But cell death is not always bad In fact cell death plays a crucial role in animal development in the function of the immune system and in the prevention of cancer o Cells can die by several mechanisms Necrosis refers to premature death due to infection or injury Apoptosis is a form of programmed cell death cell suicide This process is sometimes influenced by an intracellular degradation mechanism termed autophagy self eating Phagocytosis involves one cell a phagocyte like a macrophage eating a dying cell such as an apoptotic or infected cell Phagocytes function to kill freeliving bacteria as well Telomeres the physical ends of linear chromosomes that consist of tandem arrays of a short DNA sequence TTAGGG o Telomerase is a reverse transcriptase enzyme that adds these telomere sequences to the ends of chromosomes and maintains 3 20kb of DNA repeats in humans o Germline cells and stem cells produce high levels of telomerase but most somatic cells produce only low levels of telomerase As a result telomeres in somatic cells shorten with each cell cycle a sign of cell aging o After many cell generations chromosomes will not have normal ends and cells will withdraw from the cell cycle and stop dividing a process called cell senescence o Telomere repeats are proposed to provide each cell with a counting mechanism to prevent unlimited proliferation In fact depletion of telomerase in HeLa cells using RNAi causes them to stop growing within a month And some mice that lack telomerase show signs of premature aging o Telomere shortening over many cell generations could provide a safeguard against uncontrolled proliferation of abnormal cells cancer cells Extensive telomere shortening can be recognized by the cell as a form of DNA damage Under these circumstances p53 becomes activated and can trigger apoptosis However mice that lack telomerase have also been shown to develop tumors because of genetic instability complete loss of telomeres can lead to end toend chromosome fusions Importantly many tumor cells overcome telomere shortening by re expressing high levels of telomerase In fact the lifespan of cultured human cells that lack telomerase and have a limited replication potential in vitro can be increased by 20 doublings by reintroducing telomerase into these cells Apoptosis crucial for normal development and tissue homeostasis o 1 Chromosome condensation at the nuclear periphery o 2 Shrinking of the cell body o 3 Blebbing of the cell membrane not well depicted in this figure o 4 Endonuclease cleavage of nuclear DNA can be visualized upon agarose gel electrophoresis o 5 Flipping of phosphatidylserine PS a phospholipid from the inner leaflet of the plasma membrane to the outer leaflet this can serve as an eat me signal for macrophages o Some benefits of apoptosis include Eliminating abnormal cells during development in a quality control process Sculpting the embryo during embryonic morphogenesis below Elimination of cells that could be dangerous to the organism such as selfreactive immune cells Elimination of cells with the poten3al to cause cancer including those with damaged DNA The killing of virus infected cells by cytotoxic T lymphocytes CTLs Maintenance of organs at a constant proper size e g maintaining normal liver size or adjusting the number of developing neurons to the amount of target tissue o The developmental fate of every single somatic cell of C elegans has been mapped out These patterns of cell lineage are largely invariant between individuals In hermaphrodites 131 cells are eliminated by programmed cell death Many of the genes that control apoptosis in worms are conserved in humans o Driven by signaling cascades catalyzed by proteases called caspases The intracellular machinery responsible for apoptosis is comprised of proteases called caspases that have a cysteine residue at their active site and cleave their target proteins at specific aspartate residues Caspases are synthesized as inactive precursors called procaspases that are then activated upon proteolytic cleavage by active caspases This intracellular caspase cascade drives apoptosis o Intrinsic pathways to apoptosis are triggered from within the cell usually in response to stresses such as DNA damage hypoxia low nutrients or a lack of proper extracellular survival signals This pathway depends on the release of mitochondrial proteins from the intermembrane space into the cytosol A particularly crucial protein that is released into the cytosol is cytochrome c a component of the electron transport chain Cytosolic cytochrome c binds to proteins that activate procaspases Intrinsic pathways are regulated by the Bcl2 family of proteins o Extrinsic pathways to apoptosis are triggered by the binding of extracellular signaling proteins to cell surface death receptors An example of how this pathway operates can be illustrated by the Fas Ligand pathway o Cytotoxic T lymphocytes CTLs kill virusinfected cells by triggering apoptosis The CTL releases perforin and proteolytic enzymes onto the surface of an infected target cell by exocytosis These factors ultimately cause release of cytochrome c from mitochondria to initiate a caspase cascade leading to apoptosis Fas ligand on the surface of the CTL binds to and activates the Fas protein on the surface of a target cell The cytosolic tail of Fas contains a death domain which when activated binds to an adaptor protein which in turn recruits a specific procaspase which initiates a proteolytic caspase cascade leading to apoptosis Autophagy is an intracellular degradation process that influences cell survival and death o Autophagy self eating is a conserved intracellular pathway that controls protein and organelle degradation and has key roles in development survival and homeostasis This process involves the assembly of double membrane vacuoles termed autophagosomes and the degradation of cytoplasmic components after fusion with lysosomes This breakdown and recycling of material can aid in cell survival during starvation by maintaining cellular energy levels o Autophagy is integral to human health and is involved in physiology development lifespan and many diseases including cancer neurodegeneration and infections o Autophagy can tip the balance between cell survival and death Autophagy can be a survival mechanism or a cell death