Apoptosis and Necrosis

1. Provide a few examples of why cells undergo apoptosis.

Cells undergo apoptosis in development to help determine limb size and shape, especially in finger and toe formation. Cells also undergo apoptosis if they are infected eg with a virus or if damaged/old.

2. Overview the mechanism of apoptosis and compare it with necrosis.

Apoptosis is a controlled process carried out due to self-generated signals in a cell. The targeted cell undergoes chromatin condensation and the membrane blebs (blister-like bulge). The cell then undergoes cellular fragmentation, producing apoptotic bodies. The apoptotic cell and its fragments undergo phagocytosis therefore no inflammation is caused as all cell components are internalised and used by phagocytes. In comparison, necrosis is not controlled by the cell and is caused by external factors such as bacterial/fungal infection, mycobacterial infection, denatured proteins, pancreatitis or by antibody and antigen deposits. Instead of breaking into apoptotic bodies, in necrosis the cell membrane breaks, releasing the cell contents leading to inflammation and damage such as tissue death and decreased blood flow. Apoptosis requires no treatment but necrosis can prove to be fatal when left untreated. Apoptosis is a passive process whereas necrosis requires energy.

3. Describe the role and overview structure of caspases.

Caspases are specialised intracellular proteases which cleave specific sequences in many proteins in the cell, bringing about the changes leading to cell death and phagocytosis. They have a cysteine at their active site and cleave their target proteins at specific aspartic acids. They’re synthesised as inactive precursors and are activated only in apoptosis. There are two major classes: initiator and executioner.

4. Explain extrinsic and intrinsic apoptosis pathways from a molecular standpoint

Extrinsic apoptotic pathway triggered by the binding of extracellular proteins eg Fas ligands on killer lymphocytes to cell surface death receptors such as Fas receptors on target cell surfaces. Death receptors are homotrimers with a ‘death domain’.

Binding of Fas ligands with Fas receptors leads to clustering of several ligand-bound receptor trimers which activates death domains on the receptor tails which interact with similar domains on the adaptor protein FADD. Each FADD recruits an initiator caspase via death effector domain on both FADD and the caspase creating a death inducing signalling complex. Two adjacent initiator caspases in the DISC interact and cleave each other forming an active protease dimer which cleaves itself in the area linking the protease and death effector domain → active caspase dimer stabilised and released into cytosol to activate executioner caspases by cleaving.

The intrinsic pathway is the activation of apoptosis from within the cell in response to a signal eg DNA damage, cellular stress or a developmental cue. The signal causes mitochondria to release cytochrome C which interacts with Apaf1, causing Apaf1 to unfold partly, exposing a CARD which interacts with the same domain in other activated Apaf1s. Seven activated Apaf1 form a large ring complex, the apoptosome. The CARDs cluster above the central apoptosome hub and bind similar domains in multiple Cas9s which are recruited into the apoptosome and activated. Active caspase 9s cleave and activate downstream executioner caspases,