Bio202 Lecture 15: Cell Death PDF

Lecture 15: Cell Death Today’s agenda: Apoptosis (programmed cell death): ▪ When apoptosis is used ▪ Apoptosis vs. necrosis ▪ The apoptotic cell death pathway ▪ Intrinsic vs. extrinsic apoptosis ▪ Survival factors Exam 3 info  Apoptosis = prog...

Lecture 15: Cell Death Today’s agenda: Apoptosis (programmed cell death): ▪ When apoptosis is used ▪ Apoptosis vs. necrosis ▪ The apoptotic cell death pathway ▪ Intrinsic vs. extrinsic apoptosis ▪ Survival factors Exam 3 info  Apoptosis = programmed cell death “Apoptosis” named from the Greek word for “falling off” ▪ Apoptosis is a process by which a cell is removed without damaging its neighbors or the rest of the tissue ▪ Controlled and regulated ▪ Cell receives cue, then proceeds through apoptosis in an orderly manner Why would an organism actively initiate the process of cell death?  Cell death is a routine and essential part of life 1. Death of diseased or damaged cells: Ensures that these cells are removed before they threaten the health of the organism Examples: ▪ Virus-infected cells ▪ Cells with severely damaged DNA that cannot be repaired (better to get rid of the cell, rather than propagate deleterious mutations)  Cell death is a routine and essential part of life 1. Death of diseased or damaged cells: Ensures that these cells are removed before they threaten the health of the organism 2. Death during development: Carefully orchestrated patterns of cell death help determine the size and shape of limbs and other tissues Examples: ▪ Cells die to remove structures that are no longer needed ▪ Cells between digits die during embryonic development ▪ Excess neurons die during development Example: Cells die to remove structures that are no longer needed ▪ Apoptosis is required for frog morphogenesis ▪ Apoptosis is induced in the cells of the tadpole’s tail, a structure that is not needed in the adult frog Figure 18-39 Example: Cells die to remove structures that are no longer needed ▪ Tails are present as the frog develops, but are gone in the adult frog  Example: Cells between digits die during embryonic development ▪ Apoptosis in the developing mouse paw sculpts the digits ▪ Images of a developing mouse paw, taken 1 day apart ▪ Removal of cells by apoptosis creates the digits Figure 18-38 apoptotic cells are labeled bright green (indicated with arrows) Example: Cells between digits die during embryonic development ▪ Apoptosis in the developing human hand sculpts the digits Syndactyly - apoptosis of the interdigital tissue is incomplete or absent  Example: Excess neurons die during development Figure 18-43 ▪ During nervous system development, each nerve cell needs to contact a target cell, but initially there are more nerve cells than target cells ▪ Target cells secrete “survival factors” (will discuss these in more detail later in the lecture) ▪ Since there are too many nerve cells, there are not enough survival factors for all of the nerve cells to survive ▪ The excess nerve cells undergo apoptosis, adjusting the number of nerve cells to match the number of target cells  Cell death is a routine and essential part of life 1. Death of diseased or damaged cells: Ensures that these cells are removed before they threaten the health of the organism 2. Death during development: Carefully orchestrated patterns of cell death help determine the size and shape of limbs and other tissues 3. Death to maintain tissue/organ size: Adult tissues that are neither growing nor shrinking maintain a constant size through a balanced rate of cell division and cell death Example: ▪ If you treat an animal with a drug that stimulates liver cell division, the liver enlarges ▪ Once the drug is removed, the liver goes back to its normal size through apoptosis Cancer can arise from an imbalance between cell division and apoptosis Figure from Alberts, Molecular Biology of the Cell Important to maintain a proper balance between cell division and cell death!  Cell death: necrosis versus apoptosis NECROSIS APOPTOSIS Cell death in response to an Programmed cell death acute insult (e.g. trauma) ▪ Cells shrink, the cytoskeleton ▪ Cells swell, burst, and collapses, the nuclear envelope spill their contents disassembles, DNA fragments, the cell surface becomes ▪ Leakage of enzymes chemically altered, and the can damage nearby dying cell breaks into cells membrane-enclosed fragments ▪ Often triggers an ▪ Quickly disposed of by inflammatory response phagocytosis VERY MESSY!! NEAT AND TIDY!!  Necrosis versus apoptosis NECROSIS APOPTOSIS Contents Cell spilling membrane out intact Figure 18-40  Reminder from lecture 8: A macrophage phagocytosing two damaged red blood cells ▪ Phagocytic cells play an important role in scavenging dead and damaged cells and cell debris Phagocytic cells also remove the membrane-enclosed fragments of cells undergoing apoptosis Figure 15-32  Cell death: Apoptosis What is the apoptotic cell death program? How is it initiated?  Caspases: a family of proteases that are required for apoptosis There are two major types Figure 18-41 of apoptotic caspases: ▪ Initiator caspases (initiate apoptosis) ▪ Executioner caspases (cleave substrates to dismantle the cell) Terminology: Initiator ▪ Proteases caspases cleave proteins activate the executioner Let’s walk through caspases each of these steps….  Caspase activation during apoptosis ▪ Caspases are always present in cells, but are made as inactive precursors (“procaspases”) that are only activated during apoptosis ▪ Initiator procaspases exist as inactive monomers ▪ Adaptor proteins facilitate procaspase dimerization, allowing them to cleave a site in the protease domain of the other caspase in the dimer Adaptor proteins Figure 18-41 mediate dimerization Large subunit These domains Small subunit are cleaved after dimerization (“cross-cleavage” – one Rearrangement of large and small subunits - domain cleaves the other) this is the active complex!  Caspase activation during apoptosis Note: Once activated, the initiator caspases cleave the ▪ Initiator executioner procaspases procaspases are (which exist as inactive dimers) inactive monomers – this activates them ▪ Executioner Active procaspases are initiator inactive dimers caspase Executioner caspases These domains are now active and can are cleaved by the initiator caspases cleave substrates, and then rearranged dismantling cellular components Figure 18-41  Summary: Caspase activation during apoptosis Apoptotic signal Cleavage/activation of initiator caspases Cleavage/activation of executioner caspases Figure 18-41  Over a thousand different proteins are cleaved by caspases A few of the proteins cleaved by caspases: Nuclear lamins – breakdown of the nuclear envelope Components of the cytoskeleton – collapse of cellular structure Cell-cell adhesion proteins – detachment from neighboring cells Figure 17-1, 17-7  Over a thousand different proteins are cleaved by caspases A few of the proteins cleaved by caspases: An inhibitor of the CAD nuclease – this triggers DNA fragmentation by CAD Terminology: ▪ Nucleases cleave DNA/RNA (nucleic acids) ▪ Proteases cleave proteins (Figure from Alberts, Molecular Biology of the Cell)  The CAD nuclease cleaves DNA during apoptosis ▪ CAD is a nuclease that can cut up DNA ▪ CAD is bound by an inhibitor called iCAD ▪ An executioner caspase cleaves iCAD ▪ Activation of CAD triggers DNA fragmentation during apoptosis (Figure from Alberts, Molecular Biology of the Cell)  Extrinsic versus intrinsic apoptosis Major difference: what type of signal initiates the apoptotic program (external vs. internal) Extrinsic: Intrinsic: External signals tell the cell to The cell responds to damage or die, and the cell politely obliges absence of critical survival factors by inducing a death program by inducing the death program The “mitochondrial pathway”  The extrinsic pathway of apoptosis ▪ Damaged cells induce the expression of death receptors (Fas receptor) ▪ Fas receptors activated by Fas ligand (on killer lymphocytes) ▪ This induces the formation of DISC (death induced signaling complex) in the damaged cell (recruitment of adaptor proteins and initiator procaspases) ▪ DISC enables dimerization of an initiator procaspase (caspase-8), which facilitates its cross- cleavage and activation ▪ Once caspase-8 is activated, it cleaves and activates executioner caspases, triggering apoptosis (Figure from Alberts, Molecular Biology of the Cell)  Intrinsic apoptosis ▪ Cell triggers its own destruction in intrinsic apoptosis ▪ Mitochondria play a key role by releasing cytochrome C (a component of the respiratory energy regeneration system) Images show control cells compared to cells treated with UV (which induces DNA damage and can trigger apoptosis): (Figure from Alberts, Molecular Biology of the Cell) Cytochrome-C in mitochondria Cytochrome-C released  Cytosolic cytochrome C activates an adaptor protein that leads to apoptosome assembly Recruitment/ Adaptor proteins activation of activated initiator pro-caspases Bax/Bak mediate cytochrome C release Apoptosome = wheel-like heptamer of adaptor proteins Figure 18-42  Cytosolic cytochrome C activates an adaptor protein that leads to apoptosome assembly Apoptosome = “Death star” Figure 18-42  APOPTOSIS Intrinsic pathway Extrinsic pathway Cytochrome C Death receptors Caspases Caspases  Survival factors can inhibit apoptosis ▪ Apoptosis is highly controlled ▪ There are signals that can suppress apoptosis (“Survival factors”), to ensure that cells only die when they need to Figure 18-43  Survival factors can inhibit apoptosis ▪ A “survival factor” binds a cell surface receptor ▪ This triggers a signaling cascade that leads to activation of a transcription factor Figure 18-44 Reminder: Genes are transcribed to RNA and then translated to proteins TF ▪ Proteins called “transcription factors” bind to DNA and stimulate TF the production of particular RNAs (and thus the corresponding proteins) ▪ The cell often keeps these transcription factors inactive until they are needed ▪ This allows the cell to limit the transcription of particular genes to particular times Figure 1-2  Survival factors can inhibit apoptosis ▪ A “survival factor” binds a cell surface receptor ▪ This triggers a signaling cascade that leads to activation of a transcription factor ▪ The transcription factor goes into the nucleus and stimulates the production of Bcl2 ▪ Bcl2 is an anti-apoptotic protein Figure 18-44  Survival factors can inhibit apoptosis Survive, you will Figure 18-44 The balance between Pro- and Anti-apoptotic factors determines whether a cell lives or dies Pro- and Anti-apoptotic proteins belong to the same family (the Bcl2 family) The Bcl2 family The Skywalker family Anti-apoptotic Bcl2 family proteins (Bcl2 itself and others) Pro-apoptotic Bcl2 family proteins (e.g. Bax, Bak) (Don’t memorize the protein domain structures – just know that they are related proteins)  Bcl2 inhibits Bax/Bak, preventing cytochrome C release ▪ Bax/Bak must oligomerize to form channels that can release cytochrome C ▪ Bcl2 antagonizes the formation of these channels Bcl2 Bax/Bak Bcl2 Figure from Alberts, Molecular Biology of the Cell Bcl2 is inhibited following an apoptotic stimulus, triggering apoptosis ▪ Following an apoptotic stimulus, the anti-apoptotic protein Bcl2 is inhibited ▪ Bax/Bak aggregate on the mitochondrial membrane and release cytochrome C Bcl2 Bax/Bak Bcl2 Figure from Alberts, Molecular Biology of the Cell  Too much or too little apoptosis can be a bad thing Too much: Excessive numbers of cells undergo apoptosis and thus cause tissue damage Examples: ▪ Heart attacks (lack of oxygen triggers apoptosis in heart tissue) ▪ Strokes (lack of oxygen triggers apoptosis in brain tissue) Too little: Prevention of normal apoptotic death Examples: ▪ Prevention of apoptosis contributes to many cancers ▪ Some cancer cells overproduce anti-apoptotic protein Bcl2 ▪ This suppresses apoptosis and promotes over-proliferation of cells, leading to tumor formation  The beauty of apoptosis Human cells undergoing intrinsic apoptosis Exam 3 Covers: The cytoskeleton (3 lectures) Cell division Apoptosis ▪ Same format as Exams 1&2 (30 questions, 80 minutes) ▪ Check the Canvas site and syllabus for the times of office hours and drop-in peer tutoring ▪ Canvas discussion board will be monitored Next class: In-class review session Lecture 16: The cell cycle