{"gene":"F2RL2","run_date":"2026-04-28T17:46:03","timeline":{"discoveries":[{"year":2007,"finding":"PAR3 forms heterodimers with PAR1 via receptor dimerization, and this PAR1/PAR3 heterodimer shows increased Gα13 coupling compared to PAR1/PAR1 homodimers, potentiating PAR1 responsiveness to thrombin and regulating endothelial permeability.","method":"Bioluminescent resonance energy transfer-2 (BRET-2), siRNA knockdown, endothelial permeability assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — reciprocal BRET dimerization assay, G-protein coupling measurement, functional permeability assay with KD, multiple orthogonal methods in one study","pmids":["17376866"],"is_preprint":false},{"year":2002,"finding":"PAR3 functions as a cofactor on mouse platelets that binds thrombin and promotes productive cleavage of PAR4 at low thrombin concentrations; Par3-knockout mice are protected against thrombosis and show prolonged bleeding times.","method":"Par3 knockout mice, ferric chloride thrombosis model, pulmonary embolism model, tail bleeding time","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — clean genetic KO with defined in vivo phenotypic readouts, replicated across two thrombosis models","pmids":["12384423"],"is_preprint":false},{"year":2007,"finding":"Crystal structures of murine thrombin bound to the extracellular fragment of murine PAR3 show that the cleaved form of PAR3 (residues 47-56, FEEFPLSDIE) makes extensive contacts with thrombin exosite I, allosterically changing the 60-loop conformation and widening access to the active site, thus acting as a cofactor that promotes PAR4 substrate diffusion into the active site.","method":"X-ray crystallography at 2.0 Å resolution","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structure with mechanistic interpretation, direct structural evidence for allosteric cofactor mechanism","pmids":["17606903"],"is_preprint":false},{"year":2004,"finding":"Tethered-ligand-derived peptides based on human or murine PAR3 sequences (TFRGAP-NH2 or SFNGGP-NH2) do not activate PAR3 itself but instead activate PAR1 and PAR2 in Jurkat T cells and other cell types.","method":"PAR-specific agonist peptide stimulation in Jurkat T cells and PAR-expressing cells, calcium signaling assays","journal":"Immunology","confidence":"Medium","confidence_rationale":"Tier 2 — functional pharmacological characterization with multiple peptides and cell lines; single lab","pmids":["15147561"],"is_preprint":false},{"year":2001,"finding":"Neutrophil proteases cathepsin G and elastase inactivate human PAR3 signaling through a mechanism other than amputation of the tethered ligand domain; cathepsin G also prevents murine PAR3 from serving its cofactor role for PAR4 activation at low thrombin concentrations.","method":"COS-7 cell transfection, murine platelet aggregation assays, phospholipase C activation assay, monoclonal antibody epitope mapping","journal":"Thrombosis and haemostasis","confidence":"Medium","confidence_rationale":"Tier 2 — functional assays in transfected cells and native platelets with mechanistic epitope mapping; single lab with orthogonal approaches","pmids":["11307827"],"is_preprint":false},{"year":2008,"finding":"In human vascular smooth muscle cells, PAR-3 is functionally active: thrombin or the synthetic PAR-3 agonist peptide TFRGAP triggers intracellular calcium mobilization, ERK1/2 activation, and increased DNA synthesis; prior thrombin challenge desensitizes the calcium response to TFRGAP, indicating PAR-3 cleavage and desensitization.","method":"RT-PCR, calcium mobilization assay, ERK1/2 phosphorylation assay, [3H]-thymidine incorporation, PAR-3 agonist peptide stimulation","journal":"Thrombosis and haemostasis","confidence":"Medium","confidence_rationale":"Tier 2 — multiple functional readouts in native human SMC; single lab","pmids":["14515192"],"is_preprint":false},{"year":2008,"finding":"PAR-3 can signal autonomously in HEK-293 cells stably transfected with human PAR3: thrombin induces calcium signaling, ERK1/2 activation, IL-8 gene expression, and IL-8 release independently of PAR-1 (confirmed by PAR-1 siRNA silencing); endogenous PAR-3 in lung epithelial and astrocytoma cells shows similar responses.","method":"Stable PAR3 transfection in HEK-293 cells, siRNA PAR-1 silencing, calcium signaling, ERK1/2 phosphorylation, ELISA for IL-8","journal":"Cellular and molecular life sciences : CMLS","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA control for PAR-1 exclusion, multiple functional readouts, confirmed in endogenous expressing cells; single lab","pmids":["18264801"],"is_preprint":false},{"year":2014,"finding":"Factor Xa (FXa) cleaves PAR3 at a noncanonical site (Arg41, similar to APC rather than thrombin), and this noncanonical PAR3 activation leads to prolonged Tie2 activation, ZO-1 upregulation, and tight junction stabilization in endothelial cells; this requires PAR3 and EPCR.","method":"PAR3 tethered-ligand peptide stimulation (canonical vs. noncanonical), Tie2 phosphorylation assay, ZO-1 immunofluorescence, siRNA knockdown of PAR3","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 — functional peptide pharmacology with siRNA validation and multiple cellular readouts; single lab","pmids":["25320242"],"is_preprint":false},{"year":2013,"finding":"Activated protein C (APC) stimulates neuronal differentiation from human neural progenitor cells via a PAR1-PAR3-S1PR1-Akt signaling axis; siRNA silencing of PAR1, PAR3, or S1PR1 each abolishes the neurogenic effect of APC and prevents Akt activation.","method":"siRNA knockdown of PARs 1-4 and S1PRs 1-5, Akt phosphorylation assay, neuronal differentiation quantification, adenoviral kinase-defective Akt transduction","journal":"The Journal of neuroscience : the official journal of the Society for Neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 — systematic siRNA screen with epistasis, pathway validation by dominant-negative Akt; single lab","pmids":["23554499"],"is_preprint":false},{"year":2017,"finding":"Activated protein C (aPC) signals via PAR2/PAR3 heterodimer on regulatory T-cells (CD4+FOXP3+) to restrict allogenic T-cell activation and expand Tregs, protecting from graft-versus-host disease.","method":"PAR2/PAR3 receptor biology in Treg cells, aPC pretreatment of T-cells, humanized mouse GvHD model, Treg frequency quantification","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo mouse model with mechanistic receptor identification; single lab with functional confirmation","pmids":["28827518"],"is_preprint":false}],"current_model":"F2RL2 (PAR3) is a protease-activated receptor that functions primarily as a cofactor/coreceptor: its cleaved extracellular domain docks onto thrombin exosite I (structurally characterized at atomic resolution) to allosterically promote PAR4 cleavage on platelets, while it can also heterodimerize with PAR1 to shift G-protein coupling toward Gα13 and modulate endothelial permeability; additionally, noncanonical cleavage of PAR3 by activated protein C or factor Xa activates barrier-protective Tie2 signaling, and PAR3 can signal autonomously through ERK1/2 to induce IL-8 release."},"narrative":{"teleology":[{"year":2001,"claim":"Before PAR3's susceptibility to neutrophil proteases was examined, it was unclear whether inflammatory enzymes could modulate PAR3 cofactor function; cathepsin G and elastase were shown to inactivate human PAR3 signaling and prevent murine PAR3 from facilitating PAR4 activation, establishing that neutrophil proteases can disable PAR3-dependent thrombin signaling.","evidence":"Transfected COS-7 cells, murine platelet aggregation, antibody epitope mapping","pmids":["11307827"],"confidence":"Medium","gaps":["Precise cleavage sites by cathepsin G and elastase on PAR3 not mapped","In vivo relevance of neutrophil-mediated PAR3 inactivation not tested"]},{"year":2002,"claim":"The in vivo function of PAR3 in hemostasis was undefined; Par3-knockout mice demonstrated that PAR3 serves as a thrombin cofactor enabling PAR4 activation at low thrombin concentrations, with knockout animals showing prolonged bleeding and protection from thrombosis.","evidence":"Par3-knockout mice, ferric chloride carotid thrombosis and pulmonary embolism models, tail bleeding time","pmids":["12384423"],"confidence":"High","gaps":["Mechanism by which PAR3 facilitates PAR4 cleavage not structurally resolved at this point","Whether human PAR3 serves the same cofactor role on human platelets (which express PAR1) remained open"]},{"year":2004,"claim":"Whether PAR3 tethered-ligand peptides activate PAR3 itself was unknown; synthetic PAR3-derived peptides were found to activate PAR1 and PAR2 rather than PAR3, suggesting PAR3 tethered ligands can act in trans on other PARs.","evidence":"PAR-specific agonist peptide stimulation, calcium signaling in Jurkat T cells and PAR-expressing cell lines","pmids":["15147561"],"confidence":"Medium","gaps":["Cross-reactivity of PAR3 peptides on PAR1/PAR2 not confirmed by receptor-knockout controls","Does not address whether PAR3 can be autonomously activated by thrombin"]},{"year":2007,"claim":"The structural basis for PAR3's cofactor mechanism and its role in receptor heterodimerization were simultaneously resolved: crystal structures revealed that cleaved PAR3 peptide binds thrombin exosite I to allosterically widen active-site access for PAR4, while BRET studies showed PAR1–PAR3 heterodimerization enhances Gα13 coupling and regulates endothelial permeability.","evidence":"X-ray crystallography at 2.0 Å (murine thrombin–PAR3 complex); BRET-2 dimerization assay, siRNA knockdown, endothelial permeability assay","pmids":["17606903","17376866"],"confidence":"High","gaps":["Whether the allosteric mechanism operates identically with human thrombin–human PAR3 not structurally confirmed","Downstream signaling pathways from PAR1/PAR3-Gα13 coupling not fully delineated"]},{"year":2008,"claim":"Whether PAR3 can signal autonomously was debated; stable PAR3 expression in HEK-293 cells with PAR1 siRNA silencing demonstrated that thrombin activates calcium, ERK1/2, and IL-8 release through PAR3 alone, and similar responses were seen in native vascular smooth muscle cells.","evidence":"Stable PAR3-transfected HEK-293 cells, PAR-1 siRNA, calcium/ERK1/2/IL-8 assays; native human vascular SMC stimulation with TFRGAP peptide","pmids":["18264801","14515192"],"confidence":"Medium","gaps":["G-protein coupling specificity for autonomous PAR3 signaling not identified","Whether autonomous PAR3 signaling occurs at physiological thrombin concentrations in vivo is unclear"]},{"year":2013,"claim":"PAR3's role in activated protein C (APC) signaling beyond coagulation was unknown; APC was shown to drive neuronal differentiation from human neural progenitor cells via a PAR1–PAR3–S1PR1–Akt axis, with siRNA against PAR3 abolishing the neurogenic effect.","evidence":"Systematic siRNA knockdown of PARs 1–4 and S1PRs 1–5, Akt phosphorylation, neuronal differentiation quantification","pmids":["23554499"],"confidence":"Medium","gaps":["Physical interaction between PAR3 and S1PR1 not demonstrated","Whether PAR3 is cleaved by APC at canonical or noncanonical site in neural progenitors not determined"]},{"year":2014,"claim":"Noncanonical PAR3 cleavage and its functional consequences were uncharacterized; factor Xa was shown to cleave PAR3 at Arg41 (similar to APC), triggering sustained Tie2 activation, ZO-1 upregulation, and tight junction stabilization in endothelial cells in an EPCR-dependent manner.","evidence":"Canonical vs. noncanonical PAR3 tethered-ligand peptide stimulation, Tie2 phosphorylation, ZO-1 immunofluorescence, PAR3 siRNA","pmids":["25320242"],"confidence":"Medium","gaps":["Signaling intermediates between noncanonical PAR3 cleavage and Tie2 activation not identified","Structural basis for EPCR requirement not resolved"]},{"year":2017,"claim":"PAR3's function in adaptive immunity was unexplored; APC signaling through a PAR2/PAR3 heterodimer on regulatory T cells was shown to expand Tregs and suppress alloreactive T-cell activation, protecting against graft-versus-host disease.","evidence":"PAR2/PAR3 receptor studies in Tregs, APC pretreatment, humanized mouse GvHD model","pmids":["28827518"],"confidence":"Medium","gaps":["Direct physical evidence for PAR2/PAR3 heterodimer formation (e.g., BRET or co-IP) not provided","Downstream signaling pathway from PAR2/PAR3 in Tregs not characterized"]},{"year":null,"claim":"Key unresolved questions include the identity of PAR3's G-protein coupling partners during autonomous signaling, the signaling intermediates linking noncanonical PAR3 cleavage to Tie2 activation, and whether human PAR3 functions as a thrombin cofactor for PAR4 on human platelets analogously to its murine role.","evidence":"","pmids":[],"confidence":"Low","gaps":["G-protein specificity for autonomous PAR3 signaling undetermined","Mechanism connecting noncanonical PAR3 cleavage to Tie2 phosphorylation unknown","Human platelet cofactor role of PAR3 not directly demonstrated in vivo"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,1,5,6]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,2]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,2,5,6,7]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,5,6,7,8]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[1,2,4]}],"complexes":["PAR1/PAR3 heterodimer","PAR2/PAR3 heterodimer"],"partners":["F2R","F2RL1","F2RL3","F2"],"other_free_text":[]},"mechanistic_narrative":"F2RL2 (PAR3) is a protease-activated receptor that serves as a multifunctional cofactor, coreceptor, and autonomous signaling module in hemostasis, vascular biology, and immune regulation. On mouse platelets, PAR3 binds thrombin via its cleaved extracellular domain docking onto thrombin exosite I, allosterically remodeling the 60-loop to widen active-site access and thereby promoting PAR4 cleavage at low thrombin concentrations; Par3-knockout mice are consequently protected from thrombosis [PMID:12384423, PMID:17606903]. PAR3 also heterodimerizes with PAR1 to shift G-protein coupling toward Gα13 and regulate endothelial permeability, and with PAR2 to mediate activated protein C signaling in regulatory T cells [PMID:17376866, PMID:28827518]. Independent of other PARs, PAR3 signals autonomously through ERK1/2 to drive IL-8 release, and noncanonical cleavage by activated protein C or factor Xa at Arg41 activates a barrier-protective Tie2–ZO-1 pathway in endothelial cells [PMID:18264801, PMID:25320242]."},"prefetch_data":{"uniprot":{"accession":"O00254","full_name":"Proteinase-activated receptor 3","aliases":["Coagulation factor II receptor-like 2","Thrombin receptor-like 2"],"length_aa":374,"mass_kda":42.5,"function":"Receptor for activated thrombin coupled to G proteins that stimulate phosphoinositide hydrolysis","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/O00254/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/F2RL2","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/F2RL2","total_profiled":1310},"omim":[{"mim_id":"601919","title":"COAGULATION FACTOR II RECEPTOR-LIKE 2; F2RL2","url":"https://www.omim.org/entry/601919"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"gallbladder","ntpm":6.4}],"url":"https://www.proteinatlas.org/search/F2RL2"},"hgnc":{"alias_symbol":["PAR3"],"prev_symbol":[]},"alphafold":{"accession":"O00254","domains":[{"cath_id":"1.20.1070.10","chopping":"81-373","consensus_level":"high","plddt":86.2718,"start":81,"end":373}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O00254","model_url":"https://alphafold.ebi.ac.uk/files/AF-O00254-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O00254-F1-predicted_aligned_error_v6.png","plddt_mean":76.31},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=F2RL2","jax_strain_url":"https://www.jax.org/strain/search?query=F2RL2"},"sequence":{"accession":"O00254","fasta_url":"https://rest.uniprot.org/uniprotkb/O00254.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O00254/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O00254"}},"corpus_meta":[{"pmid":"10934474","id":"PMC_10934474","title":"The 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Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal BRET dimerization assay, G-protein coupling measurement, functional permeability assay with KD, multiple orthogonal methods in one study\",\n      \"pmids\": [\"17376866\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PAR3 functions as a cofactor on mouse platelets that binds thrombin and promotes productive cleavage of PAR4 at low thrombin concentrations; Par3-knockout mice are protected against thrombosis and show prolonged bleeding times.\",\n      \"method\": \"Par3 knockout mice, ferric chloride thrombosis model, pulmonary embolism model, tail bleeding time\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic KO with defined in vivo phenotypic readouts, replicated across two thrombosis models\",\n      \"pmids\": [\"12384423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Crystal structures of murine thrombin bound to the extracellular fragment of murine PAR3 show that the cleaved form of PAR3 (residues 47-56, FEEFPLSDIE) makes extensive contacts with thrombin exosite I, allosterically changing the 60-loop conformation and widening access to the active site, thus acting as a cofactor that promotes PAR4 substrate diffusion into the active site.\",\n      \"method\": \"X-ray crystallography at 2.0 Å resolution\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structure with mechanistic interpretation, direct structural evidence for allosteric cofactor mechanism\",\n      \"pmids\": [\"17606903\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Tethered-ligand-derived peptides based on human or murine PAR3 sequences (TFRGAP-NH2 or SFNGGP-NH2) do not activate PAR3 itself but instead activate PAR1 and PAR2 in Jurkat T cells and other cell types.\",\n      \"method\": \"PAR-specific agonist peptide stimulation in Jurkat T cells and PAR-expressing cells, calcium signaling assays\",\n      \"journal\": \"Immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional pharmacological characterization with multiple peptides and cell lines; single lab\",\n      \"pmids\": [\"15147561\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Neutrophil proteases cathepsin G and elastase inactivate human PAR3 signaling through a mechanism other than amputation of the tethered ligand domain; cathepsin G also prevents murine PAR3 from serving its cofactor role for PAR4 activation at low thrombin concentrations.\",\n      \"method\": \"COS-7 cell transfection, murine platelet aggregation assays, phospholipase C activation assay, monoclonal antibody epitope mapping\",\n      \"journal\": \"Thrombosis and haemostasis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional assays in transfected cells and native platelets with mechanistic epitope mapping; single lab with orthogonal approaches\",\n      \"pmids\": [\"11307827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"In human vascular smooth muscle cells, PAR-3 is functionally active: thrombin or the synthetic PAR-3 agonist peptide TFRGAP triggers intracellular calcium mobilization, ERK1/2 activation, and increased DNA synthesis; prior thrombin challenge desensitizes the calcium response to TFRGAP, indicating PAR-3 cleavage and desensitization.\",\n      \"method\": \"RT-PCR, calcium mobilization assay, ERK1/2 phosphorylation assay, [3H]-thymidine incorporation, PAR-3 agonist peptide stimulation\",\n      \"journal\": \"Thrombosis and haemostasis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple functional readouts in native human SMC; single lab\",\n      \"pmids\": [\"14515192\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PAR-3 can signal autonomously in HEK-293 cells stably transfected with human PAR3: thrombin induces calcium signaling, ERK1/2 activation, IL-8 gene expression, and IL-8 release independently of PAR-1 (confirmed by PAR-1 siRNA silencing); endogenous PAR-3 in lung epithelial and astrocytoma cells shows similar responses.\",\n      \"method\": \"Stable PAR3 transfection in HEK-293 cells, siRNA PAR-1 silencing, calcium signaling, ERK1/2 phosphorylation, ELISA for IL-8\",\n      \"journal\": \"Cellular and molecular life sciences : CMLS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA control for PAR-1 exclusion, multiple functional readouts, confirmed in endogenous expressing cells; single lab\",\n      \"pmids\": [\"18264801\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Factor Xa (FXa) cleaves PAR3 at a noncanonical site (Arg41, similar to APC rather than thrombin), and this noncanonical PAR3 activation leads to prolonged Tie2 activation, ZO-1 upregulation, and tight junction stabilization in endothelial cells; this requires PAR3 and EPCR.\",\n      \"method\": \"PAR3 tethered-ligand peptide stimulation (canonical vs. noncanonical), Tie2 phosphorylation assay, ZO-1 immunofluorescence, siRNA knockdown of PAR3\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional peptide pharmacology with siRNA validation and multiple cellular readouts; single lab\",\n      \"pmids\": [\"25320242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Activated protein C (APC) stimulates neuronal differentiation from human neural progenitor cells via a PAR1-PAR3-S1PR1-Akt signaling axis; siRNA silencing of PAR1, PAR3, or S1PR1 each abolishes the neurogenic effect of APC and prevents Akt activation.\",\n      \"method\": \"siRNA knockdown of PARs 1-4 and S1PRs 1-5, Akt phosphorylation assay, neuronal differentiation quantification, adenoviral kinase-defective Akt transduction\",\n      \"journal\": \"The Journal of neuroscience : the official journal of the Society for Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic siRNA screen with epistasis, pathway validation by dominant-negative Akt; single lab\",\n      \"pmids\": [\"23554499\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Activated protein C (aPC) signals via PAR2/PAR3 heterodimer on regulatory T-cells (CD4+FOXP3+) to restrict allogenic T-cell activation and expand Tregs, protecting from graft-versus-host disease.\",\n      \"method\": \"PAR2/PAR3 receptor biology in Treg cells, aPC pretreatment of T-cells, humanized mouse GvHD model, Treg frequency quantification\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo mouse model with mechanistic receptor identification; single lab with functional confirmation\",\n      \"pmids\": [\"28827518\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"F2RL2 (PAR3) is a protease-activated receptor that functions primarily as a cofactor/coreceptor: its cleaved extracellular domain docks onto thrombin exosite I (structurally characterized at atomic resolution) to allosterically promote PAR4 cleavage on platelets, while it can also heterodimerize with PAR1 to shift G-protein coupling toward Gα13 and modulate endothelial permeability; additionally, noncanonical cleavage of PAR3 by activated protein C or factor Xa activates barrier-protective Tie2 signaling, and PAR3 can signal autonomously through ERK1/2 to induce IL-8 release.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"F2RL2 (PAR3) is a protease-activated receptor that serves as a multifunctional cofactor, coreceptor, and autonomous signaling module in hemostasis, vascular biology, and immune regulation. On mouse platelets, PAR3 binds thrombin via its cleaved extracellular domain docking onto thrombin exosite I, allosterically remodeling the 60-loop to widen active-site access and thereby promoting PAR4 cleavage at low thrombin concentrations; Par3-knockout mice are consequently protected from thrombosis [PMID:12384423, PMID:17606903]. PAR3 also heterodimerizes with PAR1 to shift G-protein coupling toward Gα13 and regulate endothelial permeability, and with PAR2 to mediate activated protein C signaling in regulatory T cells [PMID:17376866, PMID:28827518]. Independent of other PARs, PAR3 signals autonomously through ERK1/2 to drive IL-8 release, and noncanonical cleavage by activated protein C or factor Xa at Arg41 activates a barrier-protective Tie2–ZO-1 pathway in endothelial cells [PMID:18264801, PMID:25320242].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Before PAR3's susceptibility to neutrophil proteases was examined, it was unclear whether inflammatory enzymes could modulate PAR3 cofactor function; cathepsin G and elastase were shown to inactivate human PAR3 signaling and prevent murine PAR3 from facilitating PAR4 activation, establishing that neutrophil proteases can disable PAR3-dependent thrombin signaling.\",\n      \"evidence\": \"Transfected COS-7 cells, murine platelet aggregation, antibody epitope mapping\",\n      \"pmids\": [\"11307827\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Precise cleavage sites by cathepsin G and elastase on PAR3 not mapped\", \"In vivo relevance of neutrophil-mediated PAR3 inactivation not tested\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"The in vivo function of PAR3 in hemostasis was undefined; Par3-knockout mice demonstrated that PAR3 serves as a thrombin cofactor enabling PAR4 activation at low thrombin concentrations, with knockout animals showing prolonged bleeding and protection from thrombosis.\",\n      \"evidence\": \"Par3-knockout mice, ferric chloride carotid thrombosis and pulmonary embolism models, tail bleeding time\",\n      \"pmids\": [\"12384423\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which PAR3 facilitates PAR4 cleavage not structurally resolved at this point\", \"Whether human PAR3 serves the same cofactor role on human platelets (which express PAR1) remained open\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Whether PAR3 tethered-ligand peptides activate PAR3 itself was unknown; synthetic PAR3-derived peptides were found to activate PAR1 and PAR2 rather than PAR3, suggesting PAR3 tethered ligands can act in trans on other PARs.\",\n      \"evidence\": \"PAR-specific agonist peptide stimulation, calcium signaling in Jurkat T cells and PAR-expressing cell lines\",\n      \"pmids\": [\"15147561\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cross-reactivity of PAR3 peptides on PAR1/PAR2 not confirmed by receptor-knockout controls\", \"Does not address whether PAR3 can be autonomously activated by thrombin\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"The structural basis for PAR3's cofactor mechanism and its role in receptor heterodimerization were simultaneously resolved: crystal structures revealed that cleaved PAR3 peptide binds thrombin exosite I to allosterically widen active-site access for PAR4, while BRET studies showed PAR1–PAR3 heterodimerization enhances Gα13 coupling and regulates endothelial permeability.\",\n      \"evidence\": \"X-ray crystallography at 2.0 Å (murine thrombin–PAR3 complex); BRET-2 dimerization assay, siRNA knockdown, endothelial permeability assay\",\n      \"pmids\": [\"17606903\", \"17376866\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the allosteric mechanism operates identically with human thrombin–human PAR3 not structurally confirmed\", \"Downstream signaling pathways from PAR1/PAR3-Gα13 coupling not fully delineated\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Whether PAR3 can signal autonomously was debated; stable PAR3 expression in HEK-293 cells with PAR1 siRNA silencing demonstrated that thrombin activates calcium, ERK1/2, and IL-8 release through PAR3 alone, and similar responses were seen in native vascular smooth muscle cells.\",\n      \"evidence\": \"Stable PAR3-transfected HEK-293 cells, PAR-1 siRNA, calcium/ERK1/2/IL-8 assays; native human vascular SMC stimulation with TFRGAP peptide\",\n      \"pmids\": [\"18264801\", \"14515192\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"G-protein coupling specificity for autonomous PAR3 signaling not identified\", \"Whether autonomous PAR3 signaling occurs at physiological thrombin concentrations in vivo is unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"PAR3's role in activated protein C (APC) signaling beyond coagulation was unknown; APC was shown to drive neuronal differentiation from human neural progenitor cells via a PAR1–PAR3–S1PR1–Akt axis, with siRNA against PAR3 abolishing the neurogenic effect.\",\n      \"evidence\": \"Systematic siRNA knockdown of PARs 1–4 and S1PRs 1–5, Akt phosphorylation, neuronal differentiation quantification\",\n      \"pmids\": [\"23554499\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physical interaction between PAR3 and S1PR1 not demonstrated\", \"Whether PAR3 is cleaved by APC at canonical or noncanonical site in neural progenitors not determined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Noncanonical PAR3 cleavage and its functional consequences were uncharacterized; factor Xa was shown to cleave PAR3 at Arg41 (similar to APC), triggering sustained Tie2 activation, ZO-1 upregulation, and tight junction stabilization in endothelial cells in an EPCR-dependent manner.\",\n      \"evidence\": \"Canonical vs. noncanonical PAR3 tethered-ligand peptide stimulation, Tie2 phosphorylation, ZO-1 immunofluorescence, PAR3 siRNA\",\n      \"pmids\": [\"25320242\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Signaling intermediates between noncanonical PAR3 cleavage and Tie2 activation not identified\", \"Structural basis for EPCR requirement not resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"PAR3's function in adaptive immunity was unexplored; APC signaling through a PAR2/PAR3 heterodimer on regulatory T cells was shown to expand Tregs and suppress alloreactive T-cell activation, protecting against graft-versus-host disease.\",\n      \"evidence\": \"PAR2/PAR3 receptor studies in Tregs, APC pretreatment, humanized mouse GvHD model\",\n      \"pmids\": [\"28827518\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct physical evidence for PAR2/PAR3 heterodimer formation (e.g., BRET or co-IP) not provided\", \"Downstream signaling pathway from PAR2/PAR3 in Tregs not characterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the identity of PAR3's G-protein coupling partners during autonomous signaling, the signaling intermediates linking noncanonical PAR3 cleavage to Tie2 activation, and whether human PAR3 functions as a thrombin cofactor for PAR4 on human platelets analogously to its murine role.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"G-protein specificity for autonomous PAR3 signaling undetermined\", \"Mechanism connecting noncanonical PAR3 cleavage to Tie2 phosphorylation unknown\", \"Human platelet cofactor role of PAR3 not directly demonstrated in vivo\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 1, 5, 6]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 2, 5, 6, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 5, 6, 7, 8]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [1, 2, 4]}\n    ],\n    \"complexes\": [\n      \"PAR1/PAR3 heterodimer\",\n      \"PAR2/PAR3 heterodimer\"\n    ],\n    \"partners\": [\n      \"F2R\",\n      \"F2RL1\",\n      \"F2RL3\",\n      \"F2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}