25 - Apoptosis

History of cell death research and the importance of C. elegans as a genetic model

Researchers know exactly which cells in C. elegans will die and when. Studying mutant worms allowed scientists to identify candidate apoptosis genes

Caspases and their activation

Caspases are aspartate-directed cysteine proteases (cleave at the C terminal). Not all involved in apoptosis. They are synthesised as zymogens with low intrinsic activity and are processed to heterotetramers (2 large subunits and 2 small subunits)

Caspases contain their own caspase cleaving sites. Initiator caspases cleave effector caspases into their large and small subunits. This activates them. Initiator caspases can be activated by induced proximity

Apoptosis induced by cell surface death receptors

Death ligand binds to a death receptor. Death receptor binds to an adaptor protein. The adaptor protein consists of a death domain which it uses to bind to the receptor and a death effector domain used to activate initiator caspases. The death ligand, death receptor, adaptor protein and caspase form a death inducing signalling complex (DISC).

DISC can activate other caspases and also cleave Bid to tBid (BH3-only protein)

Mitochondrial apoptosis and cytochrome c

Cytochrome c released from the mitochondria by mitochondrial outer membrane permeabilisation (MOMP) can associate with Apaf-1, which recruits procaspase-9 forming an apoptosome. The apoptosome can activate caspases.

BCL-2 family proteins

Control the release of cytochrome c from mitochondria. Some are anti-apoptotic, others are pro-apoptotic. They all contain the BH3 domain which is pro-apoptotic

BH3-only proteins target Bax/Bak on the mitochondrial membrane to form pores (direct activation). The BH3-only protein does a domain swap with one of the domains on Bax/Bak and the displaced domain is responsible for creating the pore. BCL-2 and BCL-X inhibit this but these are inhibited by other BH3-only proteins (de-repression)

Caspase targets and apoptotic cellular remodelling

GRASP65 is cleaved resulting in apoptotic Golgi fragmentation

Cleavage of the inhibitor iCAD allows caspase-activated DNAse (CAD) to cut up the DNA

Cleavage of ROCK I activates it which activates myosin allowing cell contraction, membrane blebbing and apoptotic body formation