TRAIL-activated apoptotic signaling — TNFSF10 ligation of TRAIL-R1/R2, FADD/caspase-8 DISC assembly restrained by c-FLIP and decoy receptors, and type I (direct executioner caspase) versus type II (BID/mitochondrial) caspase amplification

TRAIL (TNFSF10, Apo2L) is a TNF-family ligand that exists as a type II transmembrane protein and as a shed soluble form; both act as homotrimers. A TRAIL trimer binds the two death-domain receptors TRAIL-R1 (DR4, TNFRSF10A) and TRAIL-R2 (DR5, TNFRSF10B), which form homomeric and heteromeric ligand-receptor clusters at the plasma membrane. The receptor death domains recruit the adaptor FADD through death-domain contacts, and FADD in turn recruits procaspase-8 (and, where expressed, procaspase-10) through death-effector-domain contacts, forming the TRAIL death-inducing signaling complex (DISC). Proximity-induced dimerisation activates caspase-8 at the DISC. The catalytically inactive caspase-8 paralog c-FLIP (CFLAR) is recruited to the same FADD platform and restrains caspase-8 activation, and the decoy receptors TRAIL-R3 (TNFRSF10C, which lacks an intracellular domain) and TRAIL-R4 (TNFRSF10D, which carries a truncated death domain) can bind TRAIL without assembling a DISC and protect cells when overexpressed. Active caspase-8 then propagates the signal by one of two routes. In type I cells enough caspase-8 is generated at the DISC to cleave and activate the executioner caspases-3 and -7 directly. In type II cells caspase-8 output is limiting and must be amplified through the mitochondrial pathway: caspase-8 cleaves the BH3-only protein BID to truncated tBID, which moves to mitochondria and activates BAX/BAK-dependent outer membrane permeabilisation, cytochrome c release, apoptosome assembly and caspase-9 activation, which then feeds the same executioner caspases.

Derived from the RGD Pathway Ontology diagram PW:0000682 (trail mediated signaling pathway) through the RGD_PATHWAY_ONTOLOGY project. RGD's human gene list is BID, CASP3, CASP7, CASP8, CFLAR, FADD, TNFRSF10A, TNFRSF10B and TNFSF10. The diagram supplied the receptor-FADD-caspase-8 core, CFLAR as the DISC inhibitor, and the two caspase-8 outputs (CASP3/CASP7 directly; BID into the intrinsic pathway PW:0000104). Molecular functions were taken from the completed human gene reviews and QuickGO experimental annotations and checked against go-basic (releases/2026-07-26). Relation to other modules. modules/death_receptor_apoptotic_signaling.yaml is the generic, FAS-centred death-receptor module and records TRAIL as an open expansion gap; this module is the concrete TRAIL instance of that pattern and does not modify the generic module. The mitochondrial arm is not re-modelled: the type II variant includes the TRAIL-specific step (caspase-8 cleavage of BID and tBID as the activator) and then references modules/intrinsic_apoptotic_signaling.yaml by module_ref (BAX/BAK, MOMP, apoptosome, caspase-9; tBID's BAX/BAK-activating role is modelled in more detail in modules/bcl2_family_momp_control.yaml#bh3_only_activation). Executioner caspases CASP3 and CASP7 are on the RGD list but are included by module_ref to modules/apoptotic_execution_phase.yaml#executioner_caspase_proteolysis rather than duplicated; the type I variant carries only the caspase-8 activity that cleaves them. Receptors. Human TRAIL-R1 and TRAIL-R2 are modelled as a ONE_OR_MORE variant set: either can transduce the signal and they form heteromeric complexes (PMID:10894161). RGD notes that rodents express only the Tnfrsf10b ortholog, so the concrete human module has two alternatives where rat has one. Both carry TRAIL receptor activity (GO:0036463), the is_a child of death receptor activity used as core function in the TNFRSF10A and TNFRSF10B reviews. Decoy receptors (decision: include TNFRSF10C/TNFRSF10D as an optional regulatory part; exclude TNFRSF11B). They are not on RGD's gene list but are described in the RGD record text. TNFRSF10C and TNFRSF10D carry receptor decoy activity (GO:0140319) by IDA in GOA (PMID:9242610, PMID:9430226), so they are modelled as an optional part that negatively regulates receptor ligation. RGD's caveat that the decoy function has only been shown in overexpression studies is carried in the reference reviews and part notes. TNFRSF10D also signals NF-kappaB (PMID:9430226); that arm is out of scope. Osteoprotegerin (TNFRSF11B) is excluded: RGD states that the role of its low-affinity TRAIL interaction, if any, remains to be determined, and its experimental GO annotations concern RANKL decoy biology rather than TRAIL. CASP10 (decision: optional DISC part). CASP10 is not on the RGD gene list but the record text names it "when present", and it has no rodent ortholog. It is recruited to and activated at the native TRAIL DISC in a FADD-dependent way (PMID:12198154), but whether it can initiate apoptosis without caspase-8 is disputed (PMID:11717445 versus PMID:12198154), so it is an optional part and the dispute is a knowledge gap. CFLAR. c-FLIP is modelled with cysteine-type endopeptidase inhibitor activity (GO:0004869), the CFLAR review's core inhibitory function (IDA, PMID:9217161), acting on caspase-8 at the DISC. The module supplies the apoptotic context, so the precomposed "involved in apoptotic process" child GO:0043027 is not used (projects/INVOLVED_IN_PRECOMPOSITION.md). The review also records FLIP-L's caspase-8 activator activity in the necroptosis/ripoptosome context, which is outside this module. Excluded upstream and parallel biology. TRADD is excluded from the DISC part: the endogenous TRAIL DISC analyses found that TRADD does not bind DR4 or DR5 (PMID:10894161) and that FADD and caspase-8 are recruited directly (PMID:10894160), and RGD draws FADD binding the receptors directly. ZDHHC3 palmitoylation of DR4 (raft localisation) is a regulatory input shown in the human GO-CAM but not drawn by RGD and is left out of scope. Non-apoptotic TRAIL signalling (NF-kappaB, MAPK, necroptosis via RIPK1/RIPK3) is not drawn by RGD and is out of scope (see modules/nfkb_canonical_signaling.yaml and modules/necroptosis.yaml). DISC inhibitors outside the RGD diagram (e.g. PARK7/DJ-1, PMID:21785459) are not modelled. BID as a participant, not a mere substrate. BID is a caspase-8 substrate, but the cleavage product tBID does the work of the next step (it activates BAX/BAK to permeabilise the outer mitochondrial membrane), so it is a functional participant. No GO molecular-function term exists for this activator activity; the BID review proposes one ("pro-apoptotic BCL2 family effector activator activity") and the annoton uses that free-text name with no GO id, as in modules/bcl2_family_momp_control.yaml. GO-CAM alignment and disagreements. The human production models gomodel:662af8fa00002877 (TRAIL/TRAILR1) and gomodel:663d668500001492 (TRAIL/TRAILR2) contain TNFSF10, the receptor, TRADD, FADD and CASP8 in a linear directly-positively-regulates chain, plus (TRAILR1 model only) ZDHHC3 palmitoyltransferase. Agreement: FADD protein-macromolecule adaptor activity (GO:0030674) and CASP8 cysteine-type endopeptidase activity (GO:0004197) are used here unchanged; TNFSF10 is typed cytokine activity (GO:0005125) as in both models (also IBA and IDA in GOA, accepted in the TNFSF10 review). Disagreements: (1) the models type TNFRSF10A and TNFRSF10B with the generic signaling receptor activity (GO:0038023); this module uses the specific TRAIL receptor activity (GO:0036463), which TNFRSF10B carries by IDA (PMID:28362429) and which both receptor reviews adopt as core. (2) The models interpose a TRADD adaptor activity between receptor and FADD, based on the overexpression study PMID:9430228; the endogenous DISC studies (PMID:10894161, PMID:10894160) and the RGD diagram do not support TRADD in the apoptotic TRAIL DISC, so it is excluded. (3) The TNFSF10 review's core function is tumor necrosis factor receptor binding (GO:0005164); the module follows the GO-CAM ligand-activity convention instead, and death receptor agonist activity (GO:0038177) would be a more specific ligand term that no GOA row yet carries. (4) The models stop at CASP8; the type I output is matched by gomodel:62b4ffe300000784 (CASP8 and CASP10 activating CASP7). (5) gomodel:685de18700006529 types CFLAR with cysteine-type endopeptidase activity (GO:0004197) negatively regulating CASP8; c-FLIP lacks the catalytic cysteine (PMID:9217161 describes the caspase-like domain in which the active-centre cysteine is replaced by tyrosine), and the CFLAR review removes GO:0004197, so this looks like an upstream GO-CAM typing error; the module uses GO:0004869 instead. Validator warnings (all explained, none blocking). Two "feedback loop" notices fire for the c-FLIP and decoy-receptor parts because they only inhibit and have no incoming activating edge; that is correct here, since neither is induced by the pathway in the biology RGD draws (c-FLIP level is set by transcription and turnover outside this module). One function-conformance notice reports that TNFSF10's reviewed core function is GO:0005164 while the module uses GO:0005125, which is the deliberate GO-CAM alignment described above. One notice reports that tBID's activator function has no GO id, which is intentional pending the proposed term.