id: MODULE:trail_apoptotic_signaling title: 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 description: >- 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. status: DRAFT scope: CONCRETE references: - id: PMID:10894161 title: Apo2L/TRAIL-dependent recruitment of endogenous FADD and caspase-8 to death receptors 4 and 5. findings: - statement: >- Endogenous TRAIL DISC analysis: TRAIL induces DR4/DR5 homomeric and heteromeric complexes that recruit FADD and caspase-8, whereas TRADD and RIP do not bind DR4 or DR5. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Abstract-only cache; the abstract explicitly states the endogenous recruitment results and the negative TRADD/RIP result used to exclude TRADD from the DISC part. - id: PMID:10894160 title: FADD/MORT1 and caspase-8 are recruited to TRAIL receptors 1 and 2 and are essential for apoptosis mediated by TRAIL receptor 2. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Native TRAIL DISC analysis plus FADD- and caspase-8-deficient Jurkat cells. - id: PMID:9430228 title: TRAIL receptors 1 (DR4) and 2 (DR5) signal FADD-dependent apoptosis and activate NF-kappaB. reference_review: relevance: HIGH correctness: VERIFIED review_notes: >- Overexpression study reporting that both TRAIL receptors bind FADD and TRADD; the TRADD result is the basis of the TRADD step in the human TRAIL GO-CAMs but was not reproduced for endogenous DISCs (PMID:10894161). - id: PMID:10862756 title: 'FADD is required for DR4- and DR5-mediated apoptosis: lack of trail-induced apoptosis in FADD-deficient mouse embryonic fibroblasts.' reference_review: relevance: HIGH correctness: VERIFIED - id: PMID:26457518 title: The structure of the death receptor 4-TNF-related apoptosis-inducing ligand (DR4-TRAIL) complex. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Crystal structure of the TRAIL trimer bound to three DR4 ectodomains. - id: PMID:12198154 title: Caspase-10 is recruited to and activated at the native TRAIL and CD95 death-inducing signalling complexes in a FADD-dependent manner but can not functionally substitute caspase-8. reference_review: relevance: HIGH correctness: VERIFIED - id: PMID:11717445 title: Caspase-10 is an initiator caspase in death receptor signaling. reference_review: relevance: HIGH correctness: DISPUTED review_notes: >- Reports caspase-8-independent initiation by caspase-10 in Fas and TRAIL signalling; contradicted for TRAIL by PMID:12198154, which found that caspase-10 cannot rescue caspase-8 deficiency. Recorded as a knowledge gap. - id: PMID:9217161 title: Inhibition of death receptor signals by cellular FLIP. reference_review: relevance: HIGH correctness: VERIFIED - id: PMID:9242610 title: An antagonist decoy receptor and a death domain-containing receptor for TRAIL. reference_review: relevance: MEDIUM correctness: LOW_QUALITY review_notes: >- Correctly cited for the TRAIL-R3 (TRID/DcR1) decoy function, but the protective effect rests on ectopic expression, the caveat RGD's own record text raises for the decoy receptors. - id: PMID:9430226 title: The novel receptor TRAIL-R4 induces NF-kappaB and protects against TRAIL-mediated apoptosis, yet retains an incomplete death domain. reference_review: relevance: MEDIUM correctness: LOW_QUALITY review_notes: >- Correctly cited for TRAIL-R4 protection, but the evidence is transient overexpression. - id: PMID:9765224 title: Pro-caspase-3 is a major physiologic target of caspase-8. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Purified-system kinetics of direct pro-caspase-3 activation by caspase-8 and caspase-10. - id: PMID:9727492 title: Cleavage of BID by caspase 8 mediates the mitochondrial damage in the Fas pathway of apoptosis. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Established in the Fas pathway; the same caspase-8 to BID step operates in TRAIL type II cells (PMID:12407450). - id: PMID:9501089 title: Two CD95 (APO-1/Fas) signaling pathways. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: Original type I/type II definition, made for CD95; used for the variant-set axis. - id: PMID:12407450 title: Defining characteristics of Types I and II apoptotic cells in response to TRAIL. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Full text cached; extends the type I/II distinction to TRAIL and identifies BID cleavage as the limiting step. evidence: - source_id: PW:0000682 title: trail mediated signaling pathway statement: >- The module boundary and its core gene products follow the RGD interactive diagram for this Pathway Ontology term. The diagram draws Tnfsf10 associated with TNFRSF10A and Tnfrsf10b; both receptors associated with Fadd; Fadd associated with Casp8 and with Cflar; Cflar negatively affecting Casp8; and Casp8 positively affecting Bid, Casp3 and Casp7, with Casp3/Casp7 acting on caspase targets and Bid leading into the intrinsic apoptotic pathway (PW:0000104). The record text adds Casp10 at the DISC "when present", two decoy receptors lacking a functional death domain, and osteoprotegerin as a low-affinity fifth receptor of undetermined role, and distinguishes type I from type II cells. Derived through projects/RGD_PATHWAY_ONTOLOGY (pathways/PW_0000682.yaml). - source_id: GO:0036462 title: TRAIL-activated apoptotic signaling pathway statement: >- The module is grounded in GO TRAIL-activated apoptotic signaling pathway, the relatedMatch GO mapping recorded for PW:0000682 in the project's module seeds. GO:0036462 is_a GO:0008625 extrinsic apoptotic signaling pathway via death domain receptors, the process of the generic modules/death_receptor_apoptotic_signaling.yaml. - source_id: PMID:10894161 title: Apo2L/TRAIL-dependent recruitment of endogenous FADD and caspase-8 to death receptors 4 and 5. statement: >- Endogenous TRAIL DISCs contain DR4/DR5, FADD and caspase-8, the core of the module. supporting_text: >- Apo2L/TRAIL induced homomeric and heteromeric complexes of DR4 and DR5 and stimulated recruitment of FADD and caspase-8 and caspase-8 activation in nontransfected cells. - source_id: PMID:12407450 title: Defining characteristics of Types I and II apoptotic cells in response to TRAIL. statement: >- TRAIL signalling shows the type I (mitochondria-independent) versus type II (mitochondria-dependent) split, with BID cleavage the limiting step for mitochondrial involvement. supporting_text: >- These results suggest BID cleavage as a possible limiting factor in the involvement of mitochondria in TRAIL-induced cell death. notes: >- 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. knowledge_gaps: - gap_statement: >- Whether caspase-10 can initiate TRAIL-induced apoptosis independently of caspase-8 in human cells is unresolved. boundary: >- CASP10 is modelled as an optional DISC component recruited and activated at the native TRAIL DISC, not as a substitute for CASP8 in executioner caspase activation. gap_kind: - BIOLOGY status: OPEN significance: >- PMID:11717445 reports caspase-8-independent initiation by caspase-10, while PMID:12198154 found that caspase-10 is not required for apoptosis and cannot rescue caspase-8-deficient cells. The two results imply different module topologies (parallel initiators versus one initiator with an accessory protease). resolution: >- Compare TRAIL responses in isogenic CASP8-null, CASP10-null and double-null human cells with native DISC composition measured in parallel, and define caspase-10's distinct substrate set. - gap_statement: >- What determines whether a cell uses the type I or the type II route downstream of the TRAIL DISC is only partly known. boundary: >- The two routes are modelled as an explicit variant set; the module does not assert a mechanism that selects between them. gap_kind: - BIOLOGY status: OPEN significance: >- PMID:12407450 found that neither endogenous protein levels nor the efficiency of procaspase-8 processing at the TRAIL DISC predicted the kinetics of BID and executioner caspase cleavage, pointing to BID cleavage itself as the limiting step without identifying what limits it. resolution: >- Quantitative measurement of BID cleavage rate, tBID membrane translocation and executioner caspase activation in isogenic cells titrated for BID, BCL2-family and XIAP levels. - gap_statement: >- Whether TRAIL-R3 and TRAIL-R4 act as decoy receptors at physiological expression levels is not established. boundary: >- The decoy receptors form an optional regulatory part that negatively regulates receptor ligation; the core pathway does not depend on them. gap_kind: - BIOLOGY status: OPEN significance: >- Both the RGD record text and the primary reports (PMID:9242610, PMID:9430226) rest the protective effect on ectopic or transient overexpression, so the endogenous contribution to TRAIL resistance in normal tissue is unquantified. resolution: >- Endogenous-level loss-of-function of TNFRSF10C and TNFRSF10D in TRAIL-resistant primary cells, with measurement of ligand capture versus receptor-complex poisoning. module: id: trail_apoptotic_signaling label: TRAIL-activated apoptotic signaling module_type: SIGNALING_PATHWAY concepts: - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway description: >- An extrinsic apoptotic signaling pathway initiated by binding of the trimeric ligand TRAIL to the death-domain receptors TRAIL-R1 and TRAIL-R2, proceeding through FADD/caspase-8 DISC assembly to executioner caspase activation, directly or through mitochondrial amplification. context: taxa: - preferred_term: Homo sapiens term: id: NCBITaxon:9606 label: Homo sapiens description: >- Grounded on the human proteins. The diagram is RGD's rat curation; rodents express only the Tnfrsf10b receptor ortholog and no CASP10 ortholog, so the human instance has alternatives the rat lacks. cellular_components: - preferred_term: plasma membrane term: id: GO:0005886 label: plasma membrane description: Ligand-receptor clustering and DISC assembly occur at the plasma membrane. - preferred_term: cytosol term: id: GO:0005829 label: cytosol description: Caspase-8, c-FLIP and the executioner caspases act in the cytosol. parts: - order: 1 role: TRAIL trimer ligation of a death-domain TRAIL receptor node: id: trail_receptor_ligation label: TRAIL binds TRAIL-R1 or TRAIL-R2 module_type: REGULATORY_STEP description: >- Membrane-bound or shed soluble TRAIL homotrimers engage the cysteine-rich ectodomains of TRAIL-R1 (DR4) and TRAIL-R2 (DR5), clustering receptor death domains on the cytoplasmic face. A TRAIL trimer binds three receptor monomers, and TRAIL induces both homomeric and heteromeric DR4/DR5 complexes. annotons: - id: tnfsf10_trail_ligand label: TRAIL (TNFSF10) death receptor ligand participant: selector_type: GENE_PRODUCT gene_product: preferred_term: TNFSF10 (TRAIL, Apo2L) term: id: UniProtKB:P50591 label: Tumor necrosis factor ligand superfamily member 10 function: preferred_term: cytokine activity term: id: GO:0005125 label: cytokine activity targets: - preferred_term: TNFRSF10A (TRAIL-R1/DR4) term: id: UniProtKB:O00220 label: Tumor necrosis factor receptor superfamily member 10A - preferred_term: TNFRSF10B (TRAIL-R2/DR5) term: id: UniProtKB:O14763 label: Tumor necrosis factor receptor superfamily member 10B processes: - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway locations: - preferred_term: plasma membrane term: id: GO:0005886 label: plasma membrane - preferred_term: extracellular region term: id: GO:0005576 label: extracellular region role_description: >- A type II transmembrane TNF-family ligand that homotrimerises and is also active as a protease-shed soluble trimer; each trimer binds three receptor ectodomains. evidence: - source_id: PMID:26457518 statement: >- TRAIL is a trimeric type II transmembrane ligand, also active as a shed soluble trimer, and binds three DR4 monomers. supporting_text: >- A total of three DR4 molecules are bound to the TRAIL trimer, with each DR4 binding to two adjacent TRAIL monomers in the trimer gocam_associations: - model: gomodel:662af8fa00002877 activity: gomodel:662af8fa00002877/663d668500001433 title: TRAIL/TRAILR1 interaction leading to apoptosis (human) description: >- Same gene and molecular function (GO:0005125) in GO:0036462; gomodel:663d668500001492/663d668500001502 is the equivalent node of the TRAILR2 model. variant_sets: - id: trail_death_receptors label: Apoptosis-inducing TRAIL receptors axis: receptor paralog selection: ONE_OR_MORE notes: >- Of the five TRAIL-binding receptors, only TRAIL-R1 and TRAIL-R2 carry a functional death domain and can assemble a DISC. Either suffices, they are recruited independently of each other, and TRAIL induces heteromeric DR4/DR5 complexes, so ONE_OR_MORE rather than EXACTLY_ONE. Rodents express only the TRAIL-R2 ortholog. variants: - id: trailr1_variant label: TRAIL-R1 / DR4 (TNFRSF10A) module_type: REGULATORY_STEP annotons: - id: tnfrsf10a_receptor label: TNFRSF10A TRAIL receptor activity participant: selector_type: GENE_PRODUCT gene_product: preferred_term: TNFRSF10A (TRAIL-R1/DR4) term: id: UniProtKB:O00220 label: Tumor necrosis factor receptor superfamily member 10A function: preferred_term: TRAIL receptor activity term: id: GO:0036463 label: TRAIL receptor activity processes: - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway locations: - preferred_term: plasma membrane raft term: id: GO:0044853 label: plasma membrane raft role_description: >- Death-domain TRAIL receptor; transmits the TRAIL signal across the membrane and nucleates FADD recruitment. evidence: - source_id: PMID:9430228 statement: TRAIL-R1 (DR4) signals FADD-dependent apoptosis. supporting_text: >- TRAIL induces apoptosis through two closely related receptors, TRAIL-R1 (DR4) and TRAIL-R2 (DR5). gocam_associations: - model: gomodel:662af8fa00002877 activity: gomodel:662af8fa00002877/663d668500001440 title: TRAIL/TRAILR1 interaction leading to apoptosis (human) description: >- Same gene, same process and location; the model types the receptor with the generic GO:0038023 signaling receptor activity where this module uses the specific GO:0036463 TRAIL receptor activity. - id: trailr2_variant label: TRAIL-R2 / DR5 (TNFRSF10B) module_type: REGULATORY_STEP annotons: - id: tnfrsf10b_receptor label: TNFRSF10B TRAIL receptor activity participant: selector_type: GENE_PRODUCT gene_product: preferred_term: TNFRSF10B (TRAIL-R2/DR5) term: id: UniProtKB:O14763 label: Tumor necrosis factor receptor superfamily member 10B function: preferred_term: TRAIL receptor activity term: id: GO:0036463 label: TRAIL receptor activity processes: - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway locations: - preferred_term: plasma membrane term: id: GO:0005886 label: plasma membrane role_description: >- Death-domain TRAIL receptor, the only apoptosis-inducing TRAIL receptor in rodents; FADD and caspase-8 are essential for its apoptotic output. evidence: - source_id: PMID:10894160 statement: >- FADD and caspase-8 are required for TRAIL-R2-mediated apoptosis. supporting_text: >- FADD/MORT1- and caspase-8-deficient Jurkat cells expressing only TRAIL-R2 were resistant to TRAIL-induced apoptosis. gocam_associations: - model: gomodel:663d668500001492 activity: gomodel:663d668500001492/663d668500001509 title: TRAIL/TRAILR2 interaction leading to apoptosis (human) description: >- Same gene, process and location; the model uses the generic GO:0038023 where this module uses GO:0036463. - order: 2 role: TRAIL DISC assembly and caspase-8 activation node: id: trail_disc_assembly label: TRAIL DISC assembly and initiator caspase activation module_type: REGULATORY_STEP description: >- The ligated receptor death domains recruit FADD, whose death effector domain then recruits procaspase-8, forming the TRAIL death-inducing signaling complex. Proximity-induced dimerisation activates caspase-8, which is released as the active protease. concepts: - preferred_term: TRAIL death-inducing signaling complex term: id: GO:0031266 label: TRAIL death-inducing signaling complex - preferred_term: TRAIL death-inducing signaling complex assembly term: id: GO:1903074 label: TRAIL death-inducing signaling complex assembly annotons: - id: fadd_disc_adaptor label: FADD adaptor bridging receptor to procaspase-8 participant: selector_type: GENE_PRODUCT gene_product: preferred_term: FADD term: id: UniProtKB:Q13158 label: FAS-associated death domain protein function: preferred_term: protein-macromolecule adaptor activity term: id: GO:0030674 label: protein-macromolecule adaptor activity targets: - preferred_term: CASP8 (procaspase-8) term: id: UniProtKB:Q14790 label: Caspase-8 processes: - preferred_term: TRAIL death-inducing signaling complex assembly term: id: GO:1903074 label: TRAIL death-inducing signaling complex assembly - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway locations: - preferred_term: cytoplasmic side of plasma membrane term: id: GO:0009898 label: cytoplasmic side of plasma membrane role_description: >- The obligatory adaptor: its death domain binds the receptor and its death effector domain binds procaspase-8. Loss of FADD abolishes DR4- and DR5-mediated apoptosis. evidence: - source_id: PMID:10894161 statement: >- Endogenous FADD and caspase-8 are recruited to ligated DR4 and DR5, whereas TRADD and RIP are not. supporting_text: >- TRADD and RIP, which bound TNFR1, did not bind DR4 and DR5. - source_id: PMID:10862756 statement: FADD is required for DR4- and DR5-mediated apoptosis. supporting_text: >- We conclude that FADD is required for DR4- and DR5-mediated apoptosis. gocam_associations: - model: gomodel:662af8fa00002877 activity: gomodel:662af8fa00002877/663d668500001472 title: TRAIL/TRAILR1 interaction leading to apoptosis (human) description: >- Same gene and molecular function (GO:0030674) in GO:0036462; the equivalent TRAILR2 node is gomodel:663d668500001492/663d668500001530. In both models FADD is downstream of a TRADD activity, which this module omits. - id: casp8_initiator_protease label: Caspase-8 initiator protease activated at the DISC participant: selector_type: GENE_PRODUCT gene_product: preferred_term: CASP8 term: id: UniProtKB:Q14790 label: Caspase-8 function: preferred_term: cysteine-type endopeptidase activity term: id: GO:0004197 label: cysteine-type endopeptidase activity processes: - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- The initiator protease of the pathway: recruited to FADD as a zymogen, activated by proximity-induced dimerisation at the DISC, and the source of both the type I and type II outputs. evidence: - source_id: PMID:10894161 statement: >- TRAIL stimulates recruitment and activation of caspase-8 at endogenous DR4/DR5 complexes. supporting_text: >- stimulated recruitment of FADD and caspase-8 and caspase-8 activation in nontransfected cells gocam_associations: - model: gomodel:662af8fa00002877 activity: gomodel:662af8fa00002877/663d668500001482 title: TRAIL/TRAILR1 interaction leading to apoptosis (human) description: >- Same gene and molecular function (GO:0004197) in GO:0036462; the equivalent TRAILR2 node is gomodel:663d668500001492/663d668500001540. connections: - source: fadd_disc_adaptor target: casp8_initiator_protease connection_type: POSITIVELY_REGULATES description: >- FADD's death effector domain recruits procaspase-8 to the DISC, where dimerisation activates it. - order: 3 role: caspase-10 recruitment to the TRAIL DISC optional: true node: id: trail_disc_casp10 label: Caspase-10 at the TRAIL DISC module_type: REGULATORY_STEP description: >- Caspase-10, the second death-effector-domain caspase, is recruited to and activated at the native TRAIL DISC in a FADD-dependent manner. It is present in human but has no rodent ortholog, and its contribution to apoptosis induction is disputed, so it is an optional part rather than an alternative initiator. annotons: - id: casp10_disc_protease label: Caspase-10 recruited to FADD at the TRAIL DISC participant: selector_type: GENE_PRODUCT gene_product: preferred_term: CASP10 term: id: UniProtKB:Q92851 label: Caspase-10 function: preferred_term: death effector domain binding term: id: GO:0035877 label: death effector domain binding targets: - preferred_term: FADD term: id: UniProtKB:Q13158 label: FAS-associated death domain protein processes: - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- Binds the FADD death effector domain and is activated at the native TRAIL DISC, but cannot substitute for caspase-8 and has a partly distinct substrate set. evidence: - source_id: PMID:12198154 statement: >- Caspase-10 is recruited to and activated at the native TRAIL DISC in a FADD-dependent manner but cannot replace caspase-8. supporting_text: >- Here we show that caspase-10 is recruited not only to the native TRAIL DISC but also to the native CD95 DISC, and that FADD is necessary for its recruitment to and activation at these two protein complexes. - order: 4 role: c-FLIP restraint of caspase-8 activation at the DISC node: id: trail_disc_cflip_restraint label: c-FLIP (CFLAR) restrains DISC caspase-8 activation module_type: REGULATORY_STEP description: >- c-FLIP is a caspase-8 paralog whose caspase-like domain lacks the catalytic cysteine. Its death effector domains bind FADD and procaspase-8 at the same platform, competing for recruitment and blocking caspase-8 activation; it sets the threshold for TRAIL-induced death and its level is a determinant of TRAIL resistance. annotons: - id: cflar_disc_inhibitor label: c-FLIP caspase-8 inhibition at the DISC participant: selector_type: GENE_PRODUCT gene_product: preferred_term: CFLAR (c-FLIP) term: id: UniProtKB:O15519 label: CASP8 and FADD-like apoptosis regulator function: preferred_term: cysteine-type endopeptidase inhibitor activity term: id: GO:0004869 label: cysteine-type endopeptidase inhibitor activity targets: - preferred_term: CASP8 term: id: UniProtKB:Q14790 label: Caspase-8 processes: - preferred_term: negative regulation of TRAIL-activated apoptotic signaling pathway term: id: GO:1903122 label: negative regulation of TRAIL-activated apoptotic signaling pathway locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- Recruited to FADD through death effector domain contacts and inhibits death-receptor-induced apoptosis; the RGD diagram draws it associating with Fadd and negatively affecting Casp8. evidence: - source_id: PMID:9217161 statement: >- c-FLIP binds FADD and FLICE/caspase-8 and inhibits apoptosis induced by all known human death receptors; its caspase-like domain lacks the catalytic cysteine. supporting_text: >- FLIPs and FLIP(L) interact with the adaptor protein FADD and the protease FLICE, and potently inhibit apoptosis induced by all known human death receptors. gocam_associations: - model: gomodel:685de18700006529 activity: gomodel:685de18700006529/685de18700006542 title: >- GMEB1 interacts with the deubiquitinase USP40 to stabilize CFLARL and inhibit apoptosis (human) description: >- Same gene and the same negative regulation of CASP8, but the model types CFLAR with GO:0004197 cysteine-type endopeptidase activity; c-FLIP has no catalytic cysteine and the CFLAR review removes that term, so this module uses GO:0004869. - order: 5 role: decoy TRAIL receptors optional: true node: id: trail_decoy_receptors label: Decoy TRAIL receptors sequester ligand module_type: REGULATORY_STEP description: >- TRAIL-R3 (TNFRSF10C) has no intracellular domain and TRAIL-R4 (TNFRSF10D) retains only a fragment of a death domain; both bind TRAIL without assembling a DISC and protect cells from TRAIL-induced apoptosis when expressed ectopically. RGD's record text names them while cautioning that the decoy function has only been shown in overexpression studies. annotons: - id: tnfrsf10c_decoy label: TNFRSF10C (TRAIL-R3/DcR1) receptor decoy participant: selector_type: GENE_PRODUCT gene_product: preferred_term: TNFRSF10C (TRAIL-R3/DcR1) term: id: UniProtKB:O14798 label: Tumor necrosis factor receptor superfamily member 10C function: preferred_term: receptor decoy activity term: id: GO:0140319 label: receptor decoy activity targets: - preferred_term: TNFSF10 (TRAIL) term: id: UniProtKB:P50591 label: Tumor necrosis factor ligand superfamily member 10 processes: - preferred_term: negative regulation of TRAIL-activated apoptotic signaling pathway term: id: GO:1903122 label: negative regulation of TRAIL-activated apoptotic signaling pathway locations: - preferred_term: plasma membrane term: id: GO:0005886 label: plasma membrane role_description: >- A TRAIL-binding receptor with an extracellular and transmembrane domain but no intracellular signalling domain; proposed to explain the TRAIL resistance of normal tissue. evidence: - source_id: PMID:9242610 statement: >- TRID/TRAIL-R3 lacks an intracellular signalling domain and ectopic expression protects cells from TRAIL-induced apoptosis. supporting_text: >- Ectopic expression of TRID protected mammalian cells from TRAIL-induced apoptosis, which is consistent with a protective role. - id: tnfrsf10d_decoy label: TNFRSF10D (TRAIL-R4/DcR2) receptor decoy participant: selector_type: GENE_PRODUCT gene_product: preferred_term: TNFRSF10D (TRAIL-R4/DcR2) term: id: UniProtKB:Q9UBN6 label: Tumor necrosis factor receptor superfamily member 10D function: preferred_term: receptor decoy activity term: id: GO:0140319 label: receptor decoy activity targets: - preferred_term: TNFSF10 (TRAIL) term: id: UniProtKB:P50591 label: Tumor necrosis factor ligand superfamily member 10 processes: - preferred_term: negative regulation of TRAIL-activated apoptotic signaling pathway term: id: GO:1903122 label: negative regulation of TRAIL-activated apoptotic signaling pathway locations: - preferred_term: plasma membrane term: id: GO:0005886 label: plasma membrane role_description: >- Binds TRAIL but retains only one third of a consensus death domain and cannot induce apoptosis; overexpression confers complete protection. Its NF-kappaB output is outside this module. evidence: - source_id: PMID:9430226 statement: >- TRAIL-R4 cannot induce apoptosis and its overexpression protects TRAIL-sensitive cells. supporting_text: >- Transient overexpression of TRAIL-R4 in cells normally sensitive to TRAIL-mediated killing confers complete protection, suggesting that one function of TRAIL-R4 may be inhibition of TRAIL cytotoxicity. - order: 6 role: caspase-8 output to executioner caspases (type I) or via mitochondria (type II) node: id: trail_caspase_amplification label: Type I and type II routes from caspase-8 to executioner caspases module_type: REGULATORY_STEP description: >- Active caspase-8 reaches the executioner caspases by one of two routes, which is the type I / type II distinction. The two variants are alternative implementations of the same step, not successive steps. variant_sets: - id: trail_type_i_ii_routes label: Type I (direct) versus type II (mitochondrial) amplification axis: cell-type-dependent signal amplification route selection: ONE_OR_MORE notes: >- The axis is the cell's dependence on mitochondria downstream of the DISC, defined for CD95 in PMID:9501089 and shown for TRAIL in PMID:12407450, where BID cleavage is the limiting step. The two routes are not mutually exclusive in a given cell, so ONE_OR_MORE; which dominates is a knowledge gap. variants: - id: trail_type_i_direct label: Type I - caspase-8 activates executioner caspases directly module_type: REGULATORY_STEP description: >- Enough active caspase-8 is generated at the DISC to process pro-caspase-3 and pro-caspase-7 directly, without a mitochondrial step. parts: - order: 1 role: direct processing of executioner caspase zymogens node: id: trail_casp8_direct_processing label: Caspase-8 processes pro-caspase-3 and pro-caspase-7 module_type: REACTION description: >- Caspase-8 cleaves the executioner caspase zymogens at rates sufficient for direct processing in vivo. annotons: - id: casp8_executioner_maturation label: Caspase-8 maturation of executioner caspases participant: selector_type: GENE_PRODUCT gene_product: preferred_term: CASP8 term: id: UniProtKB:Q14790 label: Caspase-8 function: preferred_term: cysteine-type endopeptidase activity term: id: GO:0004197 label: cysteine-type endopeptidase activity targets: - preferred_term: CASP3 (pro-caspase-3) term: id: UniProtKB:P42574 label: Caspase-3 - preferred_term: CASP7 (pro-caspase-7) term: id: UniProtKB:P55210 label: Caspase-7 processes: - preferred_term: protein maturation term: id: GO:0051604 label: protein maturation - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- The same caspase-8 activity as at the DISC, here acting on the executioner zymogens; this is the step the RGD diagram draws as Casp8 --+> Casp3 and Casp8 --+> Casp7. evidence: - source_id: PMID:9765224 statement: >- Caspase-8 (and caspase-10) process pro-caspase-3 directly at rates indicating direct processing in vivo, without pro-caspase-9 as intermediary. supporting_text: >- Taken together these data demonstrate that the initiator caspase-8 can directly activate pro-caspase-3 without the requirement for an accelerator. gocam_associations: - model: gomodel:62b4ffe300000784 activity: gomodel:62b4ffe300000784/62b4ffe300000799 title: >- Involvement of CASP7 in apoptosis, its activation by initiator caspases (CASP8, CASP9 and CASP10) and regulation by XIAP and PAK2 (human) description: >- Same gene and molecular function (GO:0004197) in protein maturation (GO:0051604), activating CASP7; the model is death-receptor-agnostic. - order: 2 role: executioner caspase proteolysis node: id: trail_type_i_execution label: Executioner caspase proteolysis (shared) description: >- Included by reference to the executioner-caspase step of the apoptotic execution phase module; CASP3 and CASP7 are on the RGD gene list and the diagram sends them to "caspase targets", which that module models. module_ref: apoptotic_execution_phase#executioner_caspase_proteolysis connections: - source: trail_casp8_direct_processing target: trail_type_i_execution connection_type: CAUSES description: >- Direct zymogen processing yields active executioner caspases, which carry out the execution phase. - id: trail_type_ii_mitochondrial label: Type II - BID cleavage and mitochondrial amplification module_type: REGULATORY_STEP description: >- DISC caspase-8 output is limiting, and the signal must be amplified through the mitochondria: caspase-8 cleaves BID, and tBID activates BAX/BAK-dependent permeabilisation, cytochrome c release, apoptosome assembly and caspase-9, which then processes the executioner caspases. parts: - order: 1 role: caspase-8 cleavage of BID to tBID node: id: trail_bid_cleavage label: Caspase-8 cleaves BID module_type: REACTION description: >- Caspase-8 cleaves full-length cytosolic BID to the truncated form tBID, the rate-limiting step for mitochondrial involvement in TRAIL-induced death. annotons: - id: casp8_bid_cleavage label: Caspase-8 proteolysis of BID participant: selector_type: GENE_PRODUCT gene_product: preferred_term: CASP8 term: id: UniProtKB:Q14790 label: Caspase-8 function: preferred_term: cysteine-type endopeptidase activity term: id: GO:0004197 label: cysteine-type endopeptidase activity targets: - preferred_term: BID term: id: UniProtKB:P55957 label: BH3-interacting domain death agonist processes: - preferred_term: TRAIL-activated apoptotic signaling pathway term: id: GO:0036462 label: TRAIL-activated apoptotic signaling pathway locations: - preferred_term: cytosol term: id: GO:0005829 label: cytosol role_description: >- The branch point drawn by RGD as Casp8 --+> Bid; BID is a specific proximal caspase-8 substrate. evidence: - source_id: PMID:9727492 statement: >- BID is a specific proximal caspase-8 substrate and truncated BID translocates to mitochondria. supporting_text: >- While full-length BID is localized in cytosol, truncated BID (tBID) translocates to mitochondria and thus transduces apoptotic signals from cytoplasmic membrane to mitochondria. - id: tbid_momp_activator label: tBID activation of mitochondrial permeabilisation participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BID (tBID) term: id: UniProtKB:P55957 label: BH3-interacting domain death agonist function: preferred_term: pro-apoptotic BCL2 family effector activator activity description: >- Proposed molecular function, as in the BID gene review and modules/bcl2_family_momp_control.yaml: transient BH3-dependent activation of BAX or BAK. No GO term exists, so no id is asserted. processes: - preferred_term: >- positive regulation of mitochondrial outer membrane permeabilization involved in apoptotic signaling pathway term: id: GO:1901030 label: >- positive regulation of mitochondrial outer membrane permeabilization involved in apoptotic signaling pathway - preferred_term: positive regulation of release of cytochrome c from mitochondria term: id: GO:0090200 label: positive regulation of release of cytochrome c from mitochondria locations: - preferred_term: mitochondrial outer membrane term: id: GO:0005741 label: mitochondrial outer membrane role_description: >- Cleaved tBID is the entity that performs the next step, engaging BAX/BAK at the outer mitochondrial membrane; this is why BID is a participant and not merely a substrate. evidence: - source_id: PMID:12407450 statement: >- In TRAIL-treated type II cells tBID accumulates in the mitochondrial membrane fraction and BID cleavage limits mitochondrial involvement. supporting_text: >- The truncated form of BID accumulated in the heavy membrane fraction after TRAIL treatment but was not detectable in the cytosolic fraction. connections: - source: casp8_bid_cleavage target: tbid_momp_activator connection_type: CAUSES description: >- Cleavage generates tBID, the active mitochondrial activator. - order: 2 role: mitochondrial amplification loop node: id: trail_type_ii_intrinsic label: Intrinsic apoptotic signaling (shared) description: >- Included by reference to the intrinsic apoptotic signaling module (BCL2-family MOMP decision, cytochrome c / APAF1 apoptosome, caspase-9 activation); RGD draws this as the subpathway link to PW:0000104. tBID's BAX/BAK-activating role is modelled in detail in modules/bcl2_family_momp_control.yaml#bh3_only_activation. module_ref: intrinsic_apoptotic_signaling - order: 3 role: executioner caspase proteolysis node: id: trail_type_ii_execution label: Executioner caspase proteolysis (shared) description: >- Included by reference to the executioner-caspase step of the apoptotic execution phase module, reached here through caspase-9 rather than directly from caspase-8. module_ref: apoptotic_execution_phase#executioner_caspase_proteolysis connections: - source: trail_bid_cleavage target: trail_type_ii_intrinsic connection_type: CAUSES description: >- tBID triggers the mitochondrial arm, which amplifies the death-receptor signal. - source: trail_type_ii_intrinsic target: trail_type_ii_execution connection_type: CAUSES description: >- Apoptosome-activated caspase-9 processes the executioner caspases. connections: - source: trail_receptor_ligation target: trail_disc_assembly connection_type: CAUSES description: >- Ligand-induced receptor clustering nucleates FADD recruitment and DISC assembly. - source: trail_disc_assembly target: trail_disc_casp10 connection_type: CAUSES description: >- FADD at the assembled DISC also recruits and activates caspase-10 where it is expressed. - source: trail_disc_cflip_restraint target: trail_disc_assembly connection_type: NEGATIVELY_REGULATES description: >- c-FLIP competes for FADD and blocks caspase-8 activation at the DISC. - source: trail_decoy_receptors target: trail_receptor_ligation connection_type: NEGATIVELY_REGULATES description: >- Decoy receptors bind TRAIL without signalling, reducing ligation of the death-domain receptors. - source: trail_disc_assembly target: trail_caspase_amplification connection_type: CAUSES description: >- Active caspase-8 released from the DISC drives the executioner caspases directly or through the mitochondrial loop.