AH Biology: Unit 1 Apoptosis

Apoptosis Los 1 The destruction of cells must be carefully controlled in a multicellular organism. Programmed cell death (apoptosis) is triggered by cell death signals that activate inactive forms of DNAase and proteinases (collectively known as capsases) that destroy the cell. cell death signals may originate out with the cell (for examples from lymphocytes) and bind to a surface receptor protein to activate a protein cascade that produces active capsases.

Apoptosis Los 2 Death signals may also originate within the cell, for example as a result of DNA damage, the presence of p53 protein can activate a caspase cascade. In the absence of cell growth factors cells may also initiate apoptosis.

What do falling leaves, the development of a mouse’s paw and a tadpole losing its tail all have in common?

What do falling leaves, the development of a mouse’s paw and a tadpole losing its tail all have in common? Answer: They all depend on programmed cell death.

Apoptosis: Programmed Cell Death (Greek for ‘falling off’) A matter of life or death for cells!

Control of apoptosis: contents Purpose of apoptosis Comparison to necrosis Mechanism of apoptosis Initiation of apoptosis: intracellular death signals extracellular death signals Malfunction of apoptosis: causes and consequences

Why do multicellular organisms need to regulate the death of their cells?

Why do multicellular organisms need to regulate the death of their cells? To allow body structures to grow and develop correctly.

http://jmg.bmj.com/content/41/1/60.full. Development of mouse limb depends on apoptosis. D–G indicate sequential passage of time.

Why do multicellular organisms need to regulate the death of their cells? To remove body structures that are no longer required.

http://upload. wikimedia http://upload.wikimedia.org/wikipedia/commons/a/ae/Metamorphosis_frog_Meyers.png a: tadpole; b: tadpole with hind limbs developing; c: tadpole with four limbs and tail; d: froglet with tail reabsorbed into body; e: froglet

Why do multicellular organisms need to regulate the death of their cells? To regulate the sizes of cell populations in adult bodies cell proliferation must be balanced with cell death. This allows organisms to precisely control the sizes of their tissues and organs.

Death can be good for the body… Normal tissue Tumour Tumour Cell division (proliferation) Cell death (by apoptosis)

Liver removed during autosopy with tumours present. File:Secondary_tumor_deposits_in_the_liver_from_a_primary_cancer_of_the_pancreas

Why do multicellular organisms need to regulate the death of their cells? To prevent cells from surviving in environments where they should not be present.

Wikicommons

Why do multicellular organisms need to regulate the death of their cells? To kill cells that have been infected by pathogens. To prevent the survival of cells with damaged DNA (these have the potential to become cancerous). To eliminate white blood cells that would produce an immune response to the body’s own cells.

Cytotoxic (killer) T cell on left. Cell infected with virus on right. http://media-1.web.britannica.com/eb-media/35/21135-004-6CF60E1F.jpg

The two forms of cell death 1. Apoptosis: programmed cell death 2. Necrosis: uncontrolled cell death Animated comparison Link to view of apoptosis in cell culture. Can liken apoptosis to a controlled demolition.

Apoptosis compared to necrosis: What happens to the cell when it dies? Feature Apoptosis Necrosis Regulated by organism Yes: involves a series of enzyme-controlled reactions No: can occur as a result of injury DNA broken down Yes Only after cell has lysed Cell membrane disintegration No Nuclear membrane broken down Number of cells affected May be single cells Usually sheets of cells Energy requirement ATP dependent (active process) Energy input not required (passive process) Fate of dead cells Ingested by neighbouring cells or phagocytes Ingested by phagocytes Leakage of cell contents End point Cell fragments into smaller bodies Lysis of whole cell

The mechanism of apoptosis 1. Activation of procaspase proteins Active caspase A Procaspases are examples of proenzymes. Spot the paradox (how is caspase A activated?). Active caspase B Inactive procaspase Prodomain

The mechanism of apoptosis 2. Caspase activation cascade Amplification of response. Still a paradox (what activates caspase A?). Active caspase A Active caspase B Active caspase C

The mechanism of apoptosis 3. Caspases are proteinases: they cleave key cell proteins Caspase activity Inactive DNAse Active DNAse Nuclear lamins Students predict the effects of caspase activity on the cell. Fragmented nuclear lamins Gelsolin Actin-digesting enzyme

Cleaved proteins include nuclear lamins. http://www.sciencemag.org/content/318/5855/1408/F1.large.jpg

The mechanism of apoptosis 4. Caspase activity results in the controlled destruction of the cell Breakdown of nuclear DNA Active DNAse Fragmented nuclear lamins Disassembly of nucleus Actin-digesting enzyme Disassembly of cytoskeleton

Initiation of apoptosis: the death signal Adaptor proteins cause initiator procaspases to cluster together. Clustering induces a conformational change that activates the procaspases. Caspase activation Caspase cascade Next question: what activates the adaptor proteins?

Intracellular death signals Mitochondrion-mediated pathway 3. Caspase cascade 2. Cytochrome C protein released 1. Damaged mitochondrion

Intracellular death signals DNA damage can trigger apoptosis via the p53 protein. http://upload.wikimedia.org/wikipedia/commons/b/bb/P53.png p53 protein is a gene regulatory protein.

Increased protein kinase activity Damaged DNA Caspase cascade Increased protein kinase activity Increased transcription of genes that code for proteins that activate procaspases Increased concentration of p53 protein in cell Genes code for proteins that belong to the Bcl-2 family (role = regulation of procaspases). The proteins produced as a result of p53 activity promote the release of cytochrome c from mitochondria.

Extracellular death signals: survival factors Animal cells undergo apoptosis if they are deprived of survival factors released by other cells. This ensures that cells only survive in locations where they are needed, when they are needed. The default program for most cells is suicide!

Extracellular death signals: inhibitory signal proteins Some signal proteins oppose the effects of growth factors and survival factors. They can inhibit the growth of organs by stimulating apoptosis. For example, mouse limb formation (see earlier images D, E, F and G).

Extracellular death signals Body cells infected with certain pathogens, eg some viruses, present fragments of antigenic proteins on surface receptor proteins on their cell membranes.

Proteasome = large proteolytic enzyme complex in cytosol Proteasome = large proteolytic enzyme complex in cytosol. Responsible for degrading proteins that have been labelled for destruction. http://ars.sciencedirect.com/content/image/1-s2.0-S1286457904002369-fx1.jpg

Extracellular death signals Activated killer T lymphocytes can recognise body cells displaying antigens. The lymphocytes bind surface receptor proteins on the target cell and trigger apoptosis.

Animation http://www.csa.com/discoveryguides/cancer/images/fig2.jpg

Putting it together http://www.ncbi.nlm.nih.gov/books/NBK26926/

Initiation of apoptosis by killer T lymphocytes Death receptor membrane proteins Killer lymphocytes produce Fas ligand protein. Fas ligand binds to death receptor protein Fas on the surface of the target cell. Fas ligand membrane proteins Target cell

Initiation of apoptosis by killer T lymphocytes Killer lymphocytes produce Fas ligand protein. Fas ligand binds to death receptor protein Fas on the surface of the target cell. Target cell Caspase cascade Animation

Which cellular events could cause the control of apoptosis to be impaired?

What could be the consequence of impaired apoptosis to: (a) the affected cell? (b) the organism containing the affected cell?

Cancer The development of some forms of leukemia is promoted by the loss of control of apoptosis. Mutations to the DNA of B lymphocytes can cause them to produce abnormally large quantities of a protein that inhibits apoptosis. p53 is a cancer-critical gene. What could the mechanism for DNA mutation in lymphocytes effect be? Cancer-critical gene = gene that when mutated promotes the development of cancer.

1. Chromosome translocation 2. Over-expression of a regulatory protein 5. Survival of B lymphocytes that would normally have died http://upload.wikimedia.org/wikipedia/commons/1/10/Large_b_cell_lymphoma_-_cytology_small.jpg 3. Inhibition of procaspase activation 4. Cell develops resistance to apoptosis

Further reading and viewing College of St Benedict/St John’s University website Protein Data Bank website (p53) Apoptosis animation

Apoptosis Los 1 The destruction of cells must be carefully controlled in a multicellular organism. Programmed cell death (_______) is triggered by cell death signals that activate inactive forms of _____ and ________ (collectively known as capsases) that destroy the cell. Cell death signals may originate ___ _____the cell (for examples from ______) and bind to a surface receptor protein to activate a protein cascade that produces _____ _______.

Apoptosis Los 2 Death signals may also originate within the cell, for example as a result of DNA damage, the presence of ___ protein can activate a ______ _______. In the absence of cell growth factors cells may also initiate _______.

Apoptosis Los 1 The destruction of cells must be carefully controlled in a multicellular organism. Programmed cell death (apoptosis) is triggered by cell death signals that activate inactive forms of DNAase and proteinases (collectively known as capsases) that destroy the cell. Cell death signals may originate out with the cell (for examples from lymphocytes) and bind to a surface receptor protein to activate a protein cascade that produces active capsases.

Apoptosis Los 2 Death signals may also originate within the cell, for example as a result of DNA damage, the presence of p53 protein can activate a caspase cascade. In the absence of cell growth factors cells may also initiate apoptosis.