Apoptosis Signaling Pathway

Upon binding to their cognate ligand, death receptors such as Fas and TRAILR can activate initiator Caspases (Pro-caspase 8 and Pro-caspase 10) through dimerization mediated by adaptor proteins such as FADD and TRADD. Active Caspase 8 and Caspase 10 then cleave and activate the effector Caspase 3, 6 and 7, leading to apoptosis. ROS/DNA damage and ER stress trigger Caspase 2 activation. Active Caspase 2 cleaves and activates Caspase 3 and initiates apoptosis directly. Caspase 2, 8 and 10 can also cleave Bid, stimulate mitochondrial outer membrane permeabilization (MOMP) and initiate the intrinsic apoptotic pathway. Following MOMP, mitochondrial intermembrane space proteins such as Smac and Cytochrome C are released into the cytosol. Cytochrome C interacts with Apaf-1, triggering apoptosome assembly, which activates Caspase 9. Active Caspase 9, in turn, activates Caspase 3, 6 and 7, leading to apoptosis. Mitochondrial release of Smac facilitates apoptosis by blocking the inhibitor of apoptosis (IAP) proteins. 

Following the binding of TNF to TNFR1, TNFR1 binds to TRADD, which recruits RIPK1, TRAF2/5 and cIAP1/2 to form TNFR1 signaling complex I. Formation of the complex IIa and complex IIb is initiated either by RIPK1 deubiquitylation mediated by CYLD or by RIPK1 non-ubiquitylation due to depletion of cIAPs. The Pro-caspase 8 homodimer in complex IIa and complex IIb generates active Caspase 8. This active Caspase 8 in the cytosol then carries out cleavage reactions to activate downstream executioner caspases and thus induce classical apoptosis^[1][2]. 

Reference:

[1]. Thomas C, et al. Caspases in retinal ganglion cell death and axon regeneration. Cell Death Discovery volume 3, Article number: 17032 (2017).
[2]. Brenner D, et al. Regulation of tumour necrosis factor signalling: live or let die. Nat Rev Immunol. 2015 Jun;15(6):362-74.