Question

1. Be able to compare/contrast:
   • Intracellular vs. extracellular apoptotic cascade
   • Know the role of eachof the following: initiator caspases 8 and 9, executioner caspases 3,6, 7, adaptor binding domain, protease domain, CAD and iCAD, Fas and TNF ligands, death receptor, FADD adaptor protein, death effector domain, DISC

Answer 1

	Extrinsic Pathway	Intrinsic Pathway
Inducing Signal	Fas, TNF, etc	Intracellular stress, DNA damage, drugs, etc
Initiator caspases	Caspase-8 & 10	Caspase-9
Executioner caspases	Caspase-3 & 7	Caspase-3 & 7
Involvement of death receptors	Yes	No

Extracellular (extrinsic) apoptotic cascade:

The extrinsic pathway is activated by binding of death ligands like fibroblast associated antigen (Fas) ligand (FASL), TNFα or TNF-related apoptosis-inducing ligand (TRAIL) to their receptors. Fas interacts with FasL to form death-inducing signal complex (DISC) which contains Fas-associated death domain (FADD), caspase-8 and caspase-10. This caspase-8/10, in turn, activates caspase-3 to induce apoptosis.

Intracellular (intrinsic) apoptotic cascade:

The intrinsic pathway can be induced by oxidative stress, chemotherapeutic drugs, irradiation or removal of growth stimuli and involves various B-cell lymphoma 2 (BCL-2) homology 3 (BH3)-only BCL2 family proteins such as Bad, Bid, Bim, Bax, Bak, Bcl-XL, Bcl-2, etc. The intrinsic pathway is initiated upon disruption of mitochondrial outer membrane potential (MOMP) releasing cytochrome c and second mitochondrial activator of caspases (SMAC). MOMP is achieved by Bax/Bak dimers which form pores on the mitochondrial membrane. Cytochrome c and SMAC along with Apaf1 & pro-caspase-9 form apoptosome complex. This complex activates caspase-9 which in turn activates caspases-3 and -7 ultimately leading to apoptosis.

Initiator caspases 8 and 9:

Caspase-8 is activated in extrinsic apoptosis by death receptors, while caspase-9 is activated in intrinsic pathway by cytochrome-c-Apaf1 apoptosome complex.

Executioner caspases-3, -6, & -7:

These are activated by both intrinsic and extrinsic apoptosis pathways. These caspases-3, in turn, activates DNA fragmentation and membrane blebbing.

Adaptor binding domain:

Adaptor binding domains are present in the death receptors and initiator caspases-8 & 10. These domains connect the death receptor with initiator caspases-8 & 10.

CAD and iCAD:

Caspase-activated DNase (CAD), an endonuclease, is induced by executioner caspase-3 to promote DNA fragmentation. CAD is always bound with its inhibitor iCAD (DFF45). Upon proper stimuli, CAD is released from iCAD forming a large functional complex which the fragments DNA into small pieces.

Fas and TNF ligands:

These ligands act as death signals binding to their respective receptors thereby inducing extrinsic apoptotic pathway.

Death receptor:

Death receptors, upon binding to their ligands, activate initiator caspase-8/10 which in turn induce apoptosis.

FADD adaptor protein:

FADD is an adaptor protein that connects death receptors with initiator caspases-8/10.

Death effector domain:

These domains are present in the initiator caspase-8/10 which interacts with FADD and death receptors through these domains.

DISC:

It is a complex of death receptor, adaptor protein (FADD), and caspase-8. It sends death signal to executioner caspases.