{"gene":"PECAM1","run_date":"2026-04-29T11:37:58","timeline":{"discoveries":[{"year":1990,"finding":"PECAM-1 (CD31) was cloned and identified as a 130 kDa integral membrane glycoprotein with six extracellular immunoglobulin-like domains, a transmembrane domain, and a cytoplasmic tail; it is a member of the CAM subgroup of the Ig superfamily expressed on platelets, leukocytes, and enriched at endothelial cell intercellular junctions, consistent with a role in cellular recognition and adhesion.","method":"cDNA cloning from endothelial cell library, sequence analysis, immunostaining","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — original cloning paper with sequence and domain characterization, foundational and highly cited","pmids":["1690453"],"is_preprint":false},{"year":1990,"finding":"CD31 is a member of the Ig superfamily most closely related to carcinoembryonic antigen, comprising four contiguous C2 domains, and is concentrated at cell-cell contacts in endothelial cells, consistent with function as an intercellular adhesion molecule.","method":"cDNA cloning by transient expression, sequence analysis, immunofluorescence localization","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 1 — independent cloning and structural characterization, corroborated by PMID 1690453","pmids":["2351935"],"is_preprint":false},{"year":1992,"finding":"Full-length PECAM-1 transfected into NIH/3T3 null cells localizes to sites of cell-cell contact at the cell periphery, promotes cell-cell adhesion, and decreases the rate of cell migration, demonstrating that the intact molecule (requiring its cytoplasmic domain) is necessary for junction localization and migration inhibition.","method":"Transfection of full-length vs. extracellular-domain-only constructs into NIH/3T3 cells, quantitative migration assays, immunofluorescence","journal":"Journal of cellular physiology","confidence":"High","confidence_rationale":"Tier 2 — domain deletion experiments with functional readouts in null cells","pmids":["1429859"],"is_preprint":false},{"year":1994,"finding":"PECAM-1 becomes highly phosphorylated upon cellular activation and, coincident with phosphorylation, associates with the cytoskeleton of activated platelets; engagement of PECAM-1 upregulates integrin function on leukocytes, implicating it as a trigger for leukocyte trafficking.","method":"Biochemical fractionation, phosphorylation assays, functional leukocyte adhesion assays","journal":"Annals of the New York Academy of Sciences","confidence":"Medium","confidence_rationale":"Tier 3 — mechanistic description in review, based on primary biochemical data cited therein","pmids":["8017765"],"is_preprint":false},{"year":1995,"finding":"PECAM-1 mediates a specific homophilic interaction required for the transendothelial migration (diapedesis) phase of neutrophil and monocyte emigration; anti-PECAM-1 antibodies or soluble recombinant PECAM-1 directed against either leukocyte or endothelial PECAM-1 blocked passage across endothelial monolayers by 70–90% without affecting apical adhesion, and a monoclonal antibody against mouse PECAM-1 blocked leukocyte emigration into the peritoneum in vivo.","method":"Quantitative in vitro transendothelial migration assay with antibody/recombinant protein blocking, in vivo thioglycollate peritonitis model","journal":"Journal of leukocyte biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal in vitro and in vivo blocking approaches, replicated across labs","pmids":["7722409"],"is_preprint":false},{"year":1995,"finding":"TNF-α and IFN-γ induce redistribution of PECAM-1 away from endothelial cell intercellular junctions without altering total surface expression or transcription; this redistribution is associated with changes in PECAM-1 cytoskeletal association, providing a mechanism by which inflammatory cytokines regulate leukocyte transmigration.","method":"Flow cytometry, immunofluorescence, subcellular fractionation, RT-PCR, cytoskeletal association assays on HUVECs","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods in a single study","pmids":["7759892"],"is_preprint":false},{"year":1995,"finding":"PECAM-1 contributes to vascular barrier function; antibody blocking of murine vascular PECAM-1 in vivo caused detectable leakage from hepatic and renal blood vessels, and monolayer permeability assays on native or PECAM-1-transfected cells confirmed a barrier role.","method":"Macromolecule permeability assays on endothelial monolayers, in vivo antibody injection with vascular leakage readout","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro and in vivo functional data in single study","pmids":["7589563"],"is_preprint":false},{"year":1997,"finding":"PECAM-1 engagement induces a slow but sustained increase in intracellular Ca²⁺ in endothelial cells and PECAM-1-transfected cell lines, requiring the cytoplasmic domain, extracellular Ca²⁺, and tyrosine phosphorylation but not receptor cross-linking; this Ca²⁺ signal is accompanied by prostacyclin release.","method":"Single-cell fluorometry with anti-PECAM-1 antibodies and Fab fragments, prostacyclin ELISA, cytoplasmic-domain deletion constructs","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 1 — reconstitution in transfected cells with domain mutants and multiple functional readouts","pmids":["9421484"],"is_preprint":false},{"year":1997,"finding":"PECAM-1 is required for endothelial tube formation in vitro; anti-CD31 antibodies caused endothelial cells to become rounded or form attenuated tube-like structures with large single vacuoles rather than normal tubes, distinct from the VE-cadherin antibody phenotype, and CD31 associates with β-catenin via immunoprecipitation, linking it to the actin cytoskeleton.","method":"3D collagen gel tube formation assay with blocking antibodies, immunoprecipitation for β-catenin association","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — functional blocking with defined morphological readout plus biochemical Co-IP","pmids":["9120301"],"is_preprint":false},{"year":1997,"finding":"SHP-2, but not SHP-1, associates with tyrosine-phosphorylated PECAM-1 via its SH2 domains; this association correlates with the extent of PECAM-1 tyrosine phosphorylation, and immune precipitate phosphatase assays show SHP-2 dephosphorylates PECAM-1, identifying a signaling pathway downstream of integrin and immune receptor activation.","method":"Co-immunoprecipitation, SH2-domain fusion protein pull-down, in vitro phosphatase assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro phosphatase assay plus reciprocal pull-down with multiple controls","pmids":["9388260"],"is_preprint":false},{"year":1997,"finding":"Plasmodium falciparum-infected erythrocytes bind directly to PECAM-1/CD31 on the vascular endothelium; binding occurs to PECAM-1-transfected cells and to recombinant PECAM-1 absorbed onto plastic, is blocked by soluble PECAM-1 or antibodies to domains 1–4, and is augmented by IFN-γ.","method":"Binding assays to PECAM-1 transfected cells and recombinant PECAM-1, blocking with soluble PECAM-1 and monoclonal antibodies","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with recombinant protein and transfected cells, domain mapping","pmids":["9396614"],"is_preprint":false},{"year":1999,"finding":"CD31 exists as a dimer both in solution and on cell membranes (confirmed by ultracentrifugation and chemical cross-linking) and is heavily N-glycosylated (~21% carbohydrate content with 19 neutral and 13 sialylated glycans), suggesting dimerization plays a role in CD31 adhesion and signaling.","method":"Analytical ultracentrifugation, chemical cross-linking, mass spectrometry glycan analysis","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 — biophysical and biochemical characterization with multiple methods","pmids":["10425179"],"is_preprint":false},{"year":2000,"finding":"The cytoplasmic tail of CD31 activates the small GTPase Rap1 (selectively, not Ras, R-Ras, or Rap2) to promote T cell adhesion via β1 (VLA-4) and β2 (LFA-1) integrins; dominant-negative Rap1 or its GAP blocked CD31-dependent adhesion, while activated Rap1 or C3G exchange factor stimulated adhesion.","method":"Transfection of cytoplasmic-tail deletion constructs, dominant-negative and activated Rap1 mutants, adhesion assays to ICAM and VCAM","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with multiple gain- and loss-of-function constructs, specific GTPase selectivity demonstrated","pmids":["10725328"],"is_preprint":false},{"year":2000,"finding":"PECAM-1 expressed on endothelial cells (but not hematopoietic cells) modulates in vivo bleeding time; PECAM-1-deficient mice showed prolonged bleeding times that were not corrected by wild-type hematopoietic engraftment but were present when PECAM-1-deficient marrow was transplanted into wild-type mice.","method":"PECAM-1 knockout mice, bone marrow transplantation experiments, tail bleeding time assay","journal":"The American journal of pathology","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout with reciprocal bone marrow transplant controls clearly dissecting endothelial vs. hematopoietic contribution","pmids":["10880378"],"is_preprint":false},{"year":2002,"finding":"CD31-mediated homophilic ligation on viable leukocytes promotes their active, temperature-dependent detachment from macrophages under low shear; apoptosis disables this CD31 detachment signaling, converting CD31 into a tethering molecule that promotes macrophage engulfment of dying cells.","method":"Flow-based binding assay of viable vs. apoptotic leukocytes with macrophages, temperature dependence, antibody blocking","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — direct flow assay with rigorous controls; published in Nature with mechanistic dissection","pmids":["12110892"],"is_preprint":false},{"year":2002,"finding":"PECAM-1 transduces endothelial cell responses to H₂O₂ by activating a calcium-permeant non-selective cation current; this requires the PECAM-1 cytoplasmic tyrosine-containing domain and Src family kinase activity, as shown by tyrosine-domain mutants and dialysis of anti-cytoplasmic-domain antibodies.","method":"Patch-clamp electrophysiology, Ca²⁺ imaging, stable transfection of PECAM-1 and cytoplasmic-domain mutants into PECAM-1-negative REN cells","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1 — reconstitution in null cells with domain mutagenesis and electrophysiological readout","pmids":["11927609"],"is_preprint":false},{"year":2003,"finding":"Clustering of PECAM-1 on endothelial cells triggers a novel endocytic pathway that is distinct from clathrin- and caveolae-mediated endocytosis; internalization requires multimeric conjugates, is inhibited by amiloride and PKC inhibitors (macropinocytosis inhibitors), and depends on actin rearrangements requiring Src kinase and Rho kinase (ROCK).","method":"Internalization assays with anti-PECAM-1 conjugates, inhibitor studies, dominant-negative dynamin-2 transfection, actin cytoskeleton imaging","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — multiple pharmacological and genetic tools defining novel pathway","pmids":["12640043"],"is_preprint":false},{"year":2006,"finding":"PECAM-1 regulates β-catenin stability at endothelial adherens junctions by modulating GSK-3β activity: PECAM-1 tyrosine phosphorylation upon stimulation recruits SHP-2 and tyrosine-phosphorylated β-catenin, enabling β-catenin dephosphorylation and junction re-annealing; in PECAM-1-null ECs, β-catenin remains tyrosine-phosphorylated, GSK-3β serine phosphorylation (inactivation) is blunted, and β-catenin undergoes increased serine phosphorylation and proteasomal degradation.","method":"PECAM-1 knockout endothelial cells, co-immunoprecipitation, phospho-specific Western blotting, shear stress and histamine stimulation assays","journal":"The American journal of pathology","confidence":"High","confidence_rationale":"Tier 2 — genetic KO model plus multiple orthogonal biochemical methods","pmids":["16816383"],"is_preprint":false},{"year":2006,"finding":"The Kaposi sarcoma herpesvirus ubiquitin ligase K5 ubiquitinates pre-existing CD31 at the cell surface, triggering its endocytosis and lysosomal degradation, while newly synthesized CD31 is degraded by proteasomes in the ER via a mechanism requiring PACS-2 binding to acidic residues in the K5 cytoplasmic tail.","method":"Ubiquitination assays, endocytosis/degradation experiments with lysosomal and proteasomal inhibitors, PACS-2 binding mutants, cell migration assays","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — mechanistic dissection of dual degradation pathway with domain mutants and pharmacological inhibitors","pmids":["16601245"],"is_preprint":false},{"year":2010,"finding":"TCR stimulation drives ectodomain cleavage and shedding of CD31 from human T cells, releasing a soluble truncated form detectable in plasma; shedding abolishes CD31 inhibitory function because cis-homo-oligomerization (triggered by trans-homophilic engagement of domain 1) cannot occur on CD31-shed cells; a juxtamembrane peptide (aa 551–574) can restore ITIM phosphorylation and SHP-2 activation on CD31-shed cells.","method":"Flow cytometry, Western blot detection of soluble CD31 in plasma, T cell activation assays, phosphorylation assays, in vivo peptide administration in mice","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — mechanistic dissection with multiple methods including in vivo rescue","pmids":["20400708"],"is_preprint":false},{"year":2014,"finding":"NEU1 sialidase desialylates CD31 (specifically its α2,6-linked sialic acids) when recruited to CD31 in postconfluent endothelial cells; NEU1-mediated desialylation of CD31 inhibits in vitro capillary tube formation, and prior CD31 silencing or use of CD31-null endothelial cells abrogates NEU1's inhibitory effect, placing CD31 sialylation downstream of NEU1 in the angiogenic pathway.","method":"Lectin blotting, siRNA knockdown of NEU1 and CD31, adenoviral overexpression of catalytically active vs. dead NEU1, tube formation assays on Matrigel, CD31-null endothelial cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — catalytic dead mutant controls, genetic epistasis via CD31 KO, multiple orthogonal approaches","pmids":["24550400"],"is_preprint":false},{"year":2014,"finding":"CD31 signaling via SHP-1 phosphatase in dendritic cells reduces NF-κB nuclear translocation, expression of costimulatory molecules, and production of IL-12 and IL-6, while increasing TGF-β and IL-10, driving DCs toward tolerogenic function and T-cell tolerance; CD31-conditioned DCs promoted regulatory T cells and suppressed experimental autoimmune encephalomyelitis in vivo.","method":"DC maturation assays with CD31 gain/loss of function, NF-κB reporter assay, cytokine ELISA, adoptive transfer of CD31-conditioned antigen-loaded DCs into EAE model","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 — multiple in vitro and in vivo experiments with mechanistic signaling readouts","pmids":["24616502"],"is_preprint":false},{"year":2015,"finding":"PECAM1 specifically mediates flow-induced (but not HGF-induced) Gab1 tyrosine phosphorylation, membrane translocation of Gab1, and downstream Akt and eNOS activation in endothelial cells; PECAM1 siRNA abolished these flow responses, and PECAM1 knockout mice showed reduced flow-mediated Gab1 and eNOS phosphorylation in vivo.","method":"siRNA knockdown, PECAM1 knockout mice, phospho-Western blotting, flow stimulation apparatus, in vivo voluntary wheel running","journal":"Cellular signalling","confidence":"High","confidence_rationale":"Tier 2 — in vitro siRNA plus in vivo KO corroborating results with mechanistic specificity demonstrated","pmids":["26706435"],"is_preprint":false},{"year":2020,"finding":"CD31 is identified as the specific endothelial cell receptor for Clostridium perfringens β-toxin; CD31 expression correlates with cell-type susceptibility, ectopic CD31 expression renders resistant cells and liposomes susceptible to CPB-induced membrane damage, and the extracellular Ig6 domain is essential for the interaction with CPB.","method":"Ectopic expression of CD31 in resistant cell lines, liposome reconstitution, domain deletion mapping of Ig6, in vivo mouse experiments","journal":"Cell host & microbe","confidence":"High","confidence_rationale":"Tier 1 — reconstitution in null cells and liposomes, domain mutants, in vivo validation","pmids":["32497498"],"is_preprint":false},{"year":2020,"finding":"CD31 receptor-induced signaling preserves endothelial barrier function by triggering a glycolytic response via src-homology phosphatase activation leading to Akt-mediated nuclear exclusion of FoxO1 and β-catenin nuclear translocation, driving cMyc transcription; CD31-deficient mice show pathological microvascular leakage correctable by pharmacological Akt or AMPK activation.","method":"CD31-deficient mice, glycolytic flux measurements, pharmacological Akt/AMPK activation, transendothelial resistance assays, signaling pathway analysis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with mechanistic rescue experiments and multiple signaling readouts","pmids":["32681081"],"is_preprint":false},{"year":2021,"finding":"Leukocyte-induced stimulation of endothelial PECAM-1 causes dissociation of SHP2 phosphatase from PECAM-1, which then directly dephosphorylates VE-cadherin-Y731, enabling VE-cadherin endocytosis and junction opening for diapedesis; this mechanism requires Ca²⁺ signaling, non-muscle myosin II activation, and endothelial cell tension generated by leukocyte docking force on the VE-cadherin-catenin complex.","method":"SHP2 binding-site mutants of PECAM-1, VE-cadherin Y731 phospho-mutant mice, in vitro and in vivo leukocyte diapedesis assays, pharmacological inhibitors of myosin II and Ca²⁺ signaling, microscopy","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — domain mutants, phospho-specific readouts, mechanotransduction mechanism with in vivo confirmation","pmids":["33604918"],"is_preprint":false},{"year":2023,"finding":"PECAM1 physically interacts with the mechanosensitive PIEZO1 channel at endothelial cell-cell junctions; PECAM1 extracellular N-terminus is critical for this interaction and directs PIEZO1 to junctions; PIEZO1 is required for Ca²⁺-dependent formation of adherens junctions and associated cytoskeleton, supporting a cooperative role in force-dependent junctional remodeling.","method":"Endogenous tagging of PIEZO1 in mice, reconstitution studies, high-resolution microscopy (co-localization), shear stress experiments, CDH5 and VEGFR2 interaction comparisons","journal":"Communications biology","confidence":"High","confidence_rationale":"Tier 1 — in situ tagging, reconstitution, and domain mapping with functional Ca²⁺ and junctional readouts","pmids":["37005489"],"is_preprint":false},{"year":2023,"finding":"Physical traction on endothelial PECAM-1 during leukocyte transendothelial migration (TEM) initiates the endothelial signaling pathway; endothelial PECAM-1 acts as part of a mechanotransduction complex with VE-cadherin and VEGFR2; TEM requires VEGFR2 Y1175 phosphorylation but not VEGF or VEGFR2 kinase activity; endothelial-specific VEGFR2-deficient mice show ≥75% reduction in neutrophil extravasation.","method":"Fluorescence lifetime imaging microscopy (FLIM) for force sensing, inducible endothelial-specific VEGFR2-knockout mice, VEGFR2 phospho-mutants, three mouse inflammation models","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 1 — FLIM mechanotransduction assay, genetic KO with multiple in vivo models, phospho-mutant dissection","pmids":["37643615"],"is_preprint":false},{"year":2009,"finding":"PECAM-1 expression negatively regulates multiple platelet signaling pathways; PECAM-1 clustering inhibits platelet responses to CRP-XL (collagen receptor pathway), ADP, and thrombin, resulting in a marked combined reduction in thrombus formation.","method":"Flow cytometry correlation of PECAM-1 surface expression with platelet reactivity, PECAM-1 clustering assays, thrombus formation assays","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — clustering experiments with multiple agonist readouts in single study","pmids":["19850043"],"is_preprint":false},{"year":2007,"finding":"PECAM-1 modulates thrombin-induced tissue factor (TF) expression in endothelial cells via a signaling pathway involving PAR-1, Gαi/o, Rho kinase, p38 MAPK, and Egr-1 nuclear translocation; PECAM-1 promotes PI3K-Akt activation that inversely regulates p38 MAPK and thereby suppresses TF expression; PECAM-1-null mice show increased TF expression and fibrin deposition after renal ischemia-reperfusion.","method":"Antisense knockdown of PECAM-1 in HUVECs, PECAM-1 KO mice, pharmacological inhibitors (PAR-1 antagonist, pertussis toxin, Rho kinase, p38 inhibitor, PI3K inhibitor), TF mRNA and protein assays","journal":"Journal of cellular physiology","confidence":"High","confidence_rationale":"Tier 2 — combined in vitro knockdown plus in vivo KO with pathway dissection using multiple inhibitors","pmids":["17111362"],"is_preprint":false}],"current_model":"PECAM-1 (CD31) is a homophilically-adhesive, ITIM-bearing Ig superfamily receptor concentrated at endothelial junctions that functions as a mechanotransducer (partnering with VE-cadherin, VEGFR2, and PIEZO1) to initiate leukocyte diapedesis via SHP2-mediated dephosphorylation of VE-cadherin-Y731, maintains endothelial barrier integrity through glycolysis-coupled β-catenin/Akt/cMyc signaling, inhibits platelet and T-cell activation via ITIM-recruited SHP-1/SHP-2, activates Rap1 to upregulate integrin adhesion on leukocytes, and regulates intracellular Ca²⁺ through a cytoplasmic-tyrosine-dependent, Src kinase-activated cation channel downstream of H₂O₂ stimulation."},"narrative":{"teleology":[{"year":1990,"claim":"Molecular cloning revealed PECAM1 as a six-Ig-domain transmembrane glycoprotein of the Ig superfamily concentrated at endothelial junctions, establishing its identity as a potential adhesion molecule.","evidence":"cDNA cloning from endothelial libraries with sequence analysis and immunostaining by two independent groups","pmids":["1690453","2351935"],"confidence":"High","gaps":["No functional adhesion data yet","Ligand/counter-receptor unknown","Signaling capacity not addressed"]},{"year":1992,"claim":"Reconstitution in null cells demonstrated that PECAM1 localizes to cell-cell contacts, promotes adhesion, and inhibits migration in a cytoplasmic-domain-dependent manner, establishing it as a functional adhesion molecule with signaling requirements.","evidence":"Transfection of full-length vs. extracellular-only constructs into NIH/3T3 cells with migration assays and immunofluorescence","pmids":["1429859"],"confidence":"High","gaps":["Cytoplasmic domain signaling mechanism unresolved","Nature of adhesive interaction (homophilic vs. heterophilic) not determined"]},{"year":1995,"claim":"Blocking experiments in vitro and in vivo established that PECAM1 homophilic interaction is specifically required for the diapedesis step of leukocyte transendothelial migration, defining its central role in inflammation.","evidence":"Antibody/recombinant protein blocking in transmigration assays and thioglycollate peritonitis model","pmids":["7722409"],"confidence":"High","gaps":["Molecular mechanism linking homophilic engagement to junction opening unknown","Endothelial vs. leukocyte PECAM1 relative contributions not genetically dissected"]},{"year":1995,"claim":"TNF-α/IFN-γ were shown to redistribute junctional PECAM1 without changing surface levels, and antibody blocking in vivo caused vascular leakage, linking PECAM1 junctional localization to barrier function.","evidence":"Flow cytometry, subcellular fractionation, RT-PCR on HUVECs; in vivo antibody injection with permeability readout","pmids":["7759892","7589563"],"confidence":"Medium","gaps":["Mechanism of redistribution unclear","Whether barrier loss is direct or secondary to junction disassembly not resolved"]},{"year":1997,"claim":"A burst of discoveries defined PECAM1's signaling repertoire: engagement triggers sustained Ca²⁺ influx requiring the cytoplasmic domain; tyrosine-phosphorylated PECAM1 recruits SHP-2 phosphatase; and PECAM1 associates with β-catenin, linking it to adherens junction regulation and tube formation.","evidence":"Single-cell fluorometry with domain mutants; co-IP and in vitro phosphatase assays for SHP-2; β-catenin co-IP and collagen gel tube assays with blocking antibodies","pmids":["9421484","9388260","9120301"],"confidence":"High","gaps":["Downstream targets of SHP-2 recruited to PECAM1 not identified","Nature of the Ca²⁺ channel unknown","β-catenin association site on PECAM1 not mapped"]},{"year":2000,"claim":"PECAM1 cytoplasmic signaling was shown to selectively activate Rap1 (not Ras) to upregulate integrin function on T cells, and knockout mice revealed that endothelial (not hematopoietic) PECAM1 controls hemostasis.","evidence":"Dominant-negative/activated Rap1 constructs with adhesion assays; PECAM1 KO mice with reciprocal bone marrow transplants and bleeding time","pmids":["10725328","10880378"],"confidence":"High","gaps":["GEF linking PECAM1 to Rap1 not fully confirmed in endothelial cells","Mechanism of endothelial PECAM1 control of hemostasis undefined"]},{"year":2002,"claim":"Two distinct functions were delineated: PECAM1 homophilic engagement on viable leukocytes drives active detachment from macrophages (disabled by apoptosis, converting CD31 to a tether for efferocytosis), and H₂O₂-activated PECAM1 opens a Src-dependent non-selective cation channel requiring its cytoplasmic tyrosines.","evidence":"Flow-based macrophage binding assay comparing viable vs. apoptotic cells; patch-clamp electrophysiology in PECAM1-reconstituted REN cells with domain mutants","pmids":["12110892","11927609"],"confidence":"High","gaps":["Molecular identity of the cation channel not determined","Signaling switch between viable detachment and apoptotic tethering not molecularly defined"]},{"year":2006,"claim":"Genetic knockout studies showed PECAM1 stabilizes β-catenin at adherens junctions by recruiting SHP-2 to dephosphorylate β-catenin tyrosines and modulating GSK-3β, explaining barrier fragility in PECAM1-null endothelium.","evidence":"PECAM1 KO endothelial cells with phospho-specific Western blotting, co-IP, shear stress and histamine stimulation","pmids":["16816383"],"confidence":"High","gaps":["Direct GSK-3β regulation mechanism by PECAM1 not fully dissected","Whether β-catenin stabilization is the sole barrier-maintenance mechanism unclear"]},{"year":2010,"claim":"TCR-driven ectodomain shedding of CD31 from T cells was identified as a mechanism that disables CD31 inhibitory signaling by preventing cis-homo-oligomerization; a juxtamembrane peptide could restore ITIM phosphorylation and SHP-2 activation.","evidence":"Flow cytometry, soluble CD31 detection in plasma, T cell activation assays, in vivo peptide rescue in mice","pmids":["20400708"],"confidence":"High","gaps":["Protease responsible for ectodomain shedding not identified","Therapeutic applicability of juxtamembrane peptide not established"]},{"year":2014,"claim":"CD31 inhibitory signaling was extended to dendritic cells: SHP-1-dependent suppression of NF-κB drives tolerogenic DC function and regulatory T cell induction; separately, NEU1 sialidase-mediated desialylation of CD31 was shown to inhibit angiogenesis.","evidence":"DC maturation assays with NF-κB reporters, adoptive DC transfer in EAE model; lectin blotting, CD31-null epistasis, tube formation assays","pmids":["24616502","24550400"],"confidence":"High","gaps":["Structural basis for how sialylation state modulates CD31 adhesion/signaling unknown","Whether SHP-1 vs. SHP-2 usage in DCs is context-dependent not resolved"]},{"year":2020,"claim":"CD31 was identified as the receptor for Clostridium perfringens β-toxin via its Ig6 domain, and its barrier-maintenance function was linked to a glycolytic pathway involving SHP-Akt-FoxO1 nuclear exclusion and β-catenin/cMyc-driven transcription.","evidence":"Ectopic expression in resistant cells and liposomes with domain mapping for CPB; CD31 KO mice with glycolytic flux measurements and pharmacological Akt/AMPK rescue","pmids":["32497498","32681081"],"confidence":"High","gaps":["How Ig6 domain engagement by CPB leads to membrane damage not structurally resolved","Whether glycolytic barrier maintenance operates in all vascular beds unknown"]},{"year":2021,"claim":"The diapedesis mechanism was resolved: leukocyte traction on PECAM1 causes SHP-2 to dissociate from PECAM1 and dephosphorylate VE-cadherin Y731, enabling cadherin endocytosis and junction opening, dependent on Ca²⁺ and non-muscle myosin II.","evidence":"SHP-2 binding-site mutants of PECAM1, VE-cadherin Y731 phospho-mutant mice, in vitro and in vivo diapedesis assays","pmids":["33604918"],"confidence":"High","gaps":["How PECAM1 senses mechanical force at the molecular level not resolved","Whether SHP-2 acts catalytically on VE-cadherin in cis or after release not fully distinguished"]},{"year":2023,"claim":"PECAM1 was placed in a mechanotransduction complex with PIEZO1, VE-cadherin, and VEGFR2: PECAM1 directs PIEZO1 to junctions via its N-terminal extracellular domain, and leukocyte traction activates VEGFR2 Y1175 phosphorylation independently of VEGF or VEGFR2 kinase activity, required for neutrophil extravasation.","evidence":"Endogenous PIEZO1 tagging in mice with co-localization and reconstitution; FLIM force-sensing, endothelial-specific VEGFR2 KO mice, three inflammation models","pmids":["37005489","37643615"],"confidence":"High","gaps":["How VEGFR2 Y1175 is phosphorylated without its own kinase activity is unclear—trans-phosphorylation source not identified","Stoichiometry and structure of the PECAM1-PIEZO1-VEGFR2-VE-cadherin complex unknown"]},{"year":null,"claim":"The structural basis of force sensing by PECAM1 and the full stoichiometry of its mechanosensory complex remain undefined, as does the molecular identity of the PECAM1-dependent cation channel.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure of PECAM1 in complex with junctional partners","Molecular identity of H₂O₂-activated cation channel downstream of PECAM1 unknown","Protease(s) mediating CD31 ectodomain shedding on T cells not identified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[0,1,2,4]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[7,15,22,25,27]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[9,17,21,28,29]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,2,4,26]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3,9]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[4,5,14,21,25,27]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[7,9,12,15,22,24,29]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[2,6,17,26]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[13,28,29]}],"complexes":["PECAM1-VE-cadherin-VEGFR2 mechanosensory complex","PECAM1-PIEZO1 junctional complex"],"partners":["CDH5","KDR","PIEZO1","PTPN11","PTPN6","CTNNB1","RAP1A"],"other_free_text":[]},"mechanistic_narrative":"PECAM1 (CD31) is an immunoglobulin superfamily transmembrane glycoprotein that functions as a homophilic adhesion receptor and mechanotransducer at endothelial cell-cell junctions, integrating cell adhesion, vascular barrier maintenance, leukocyte transendothelial migration, and inhibitory signaling on platelets and immune cells. At endothelial junctions, PECAM1 forms a mechanosensory complex with VE-cadherin, VEGFR2, and PIEZO1; physical traction during leukocyte diapedesis triggers SHP-2 dissociation from PECAM1's ITIM-containing cytoplasmic tail, enabling SHP-2-mediated dephosphorylation of VE-cadherin Y731 and junction opening, a process requiring Ca²⁺ signaling and non-muscle myosin II [PMID:33604918, PMID:37643615, PMID:37005489]. PECAM1 maintains endothelial barrier integrity through β-catenin stabilization via GSK-3β modulation and a glycolysis-coupled Akt/FoxO1/cMyc pathway, and transduces H₂O₂-induced activation of a Src-dependent non-selective cation channel [PMID:16816383, PMID:32681081, PMID:11927609]. On leukocytes and platelets, PECAM1 cytoplasmic signaling activates Rap1 to upregulate integrin adhesion, recruits SHP-1/SHP-2 phosphatases to inhibit platelet activation and promote dendritic cell tolerogenesis, and undergoes TCR-driven ectodomain shedding that disables its inhibitory function [PMID:10725328, PMID:19850043, PMID:24616502, PMID:20400708]."},"prefetch_data":{"uniprot":{"accession":"P16284","full_name":"Platelet endothelial cell adhesion molecule","aliases":["EndoCAM","GPIIA'","PECA1"],"length_aa":738,"mass_kda":82.5,"function":"Cell adhesion molecule which is required for leukocyte transendothelial migration (TEM) under most inflammatory conditions (PubMed:17580308, PubMed:19342684). Tyr-690 plays a critical role in TEM and is required for efficient trafficking of PECAM1 to and from the lateral border recycling compartment (LBRC) and is also essential for the LBRC membrane to be targeted around migrating leukocytes (PubMed:19342684). Trans-homophilic interaction may play a role in endothelial cell-cell adhesion via cell junctions (PubMed:27958302). Heterophilic interaction with CD177 plays a role in transendothelial migration of neutrophils (PubMed:17580308). Homophilic ligation of PECAM1 prevents macrophage-mediated phagocytosis of neighboring viable leukocytes by transmitting a detachment signal (PubMed:12110892). Promotes macrophage-mediated phagocytosis of apoptotic leukocytes by tethering them to the phagocytic cells; PECAM1-mediated detachment signal appears to be disabled in apoptotic leukocytes (PubMed:12110892). Modulates bradykinin receptor BDKRB2 activation (PubMed:18672896). Regulates bradykinin- and hyperosmotic shock-induced ERK1/2 activation in endothelial cells (PubMed:18672896). 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neurology","url":"https://pubmed.ncbi.nlm.nih.gov/11487054","citation_count":33,"is_preprint":false},{"pmid":"19897387","id":"PMC_19897387","title":"CD31+ T cells represent a functionally distinct vascular T cell phenotype.","date":"2009","source":"Blood cells, molecules & diseases","url":"https://pubmed.ncbi.nlm.nih.gov/19897387","citation_count":33,"is_preprint":false},{"pmid":"10233418","id":"PMC_10233418","title":"Human myeloma cells express the CD38 ligand CD31.","date":"1999","source":"British journal of haematology","url":"https://pubmed.ncbi.nlm.nih.gov/10233418","citation_count":33,"is_preprint":false},{"pmid":"19850043","id":"PMC_19850043","title":"PECAM-1 expression and activity negatively regulate multiple platelet signaling pathways.","date":"2009","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/19850043","citation_count":31,"is_preprint":false},{"pmid":"7858258","id":"PMC_7858258","title":"Involvement of CD31 in lymphocyte-mediated immune responses: importance of the membrane-proximal immunoglobulin domain and identification of an inhibiting CD31 peptide.","date":"1995","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/7858258","citation_count":31,"is_preprint":false},{"pmid":"17510564","id":"PMC_17510564","title":"Evaluation of CD31 (PECAM-1) expression using tissue microarray in patients with renal cell carcinoma.","date":"2007","source":"Tumour biology : the journal of the International Society for Oncodevelopmental Biology and Medicine","url":"https://pubmed.ncbi.nlm.nih.gov/17510564","citation_count":30,"is_preprint":false},{"pmid":"11259373","id":"PMC_11259373","title":"Human CD38 and its ligand CD31 define a unique lamina propria T lymphocyte signaling pathway.","date":"2001","source":"FASEB journal : official publication of the Federation of American Societies for Experimental Biology","url":"https://pubmed.ncbi.nlm.nih.gov/11259373","citation_count":30,"is_preprint":false},{"pmid":"17875702","id":"PMC_17875702","title":"The role of platelet/endothelial cell adhesion molecule 1 (CD31) and CD38 antigens in marrow microenvironmental retention of acute myelogenous leukemia cells.","date":"2007","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/17875702","citation_count":29,"is_preprint":false},{"pmid":"34455700","id":"PMC_34455700","title":"Role of aneuploid circulating tumor cells and CD31+ circulating tumor endothelial cells in predicting and monitoring anti-angiogenic therapy efficacy in advanced NSCLC.","date":"2021","source":"Molecular oncology","url":"https://pubmed.ncbi.nlm.nih.gov/34455700","citation_count":28,"is_preprint":false},{"pmid":"28013181","id":"PMC_28013181","title":"Platelets modulate endothelial cell response to dynamic shear stress through PECAM-1.","date":"2016","source":"Thrombosis research","url":"https://pubmed.ncbi.nlm.nih.gov/28013181","citation_count":28,"is_preprint":false},{"pmid":"24158612","id":"PMC_24158612","title":"Proangiogenic TIE2(+)/CD31 (+) macrophages are the predominant population of tumor-associated macrophages infiltrating metastatic lymph nodes.","date":"2013","source":"Molecules and cells","url":"https://pubmed.ncbi.nlm.nih.gov/24158612","citation_count":28,"is_preprint":false},{"pmid":"17111362","id":"PMC_17111362","title":"PECAM-1 modulates thrombin-induced tissue factor expression on endothelial cells.","date":"2007","source":"Journal of cellular physiology","url":"https://pubmed.ncbi.nlm.nih.gov/17111362","citation_count":27,"is_preprint":false},{"pmid":"26238489","id":"PMC_26238489","title":"Endothelial LSP1 Modulates Extravascular Neutrophil Chemotaxis by Regulating Nonhematopoietic Vascular PECAM-1 Expression.","date":"2015","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/26238489","citation_count":27,"is_preprint":false},{"pmid":"15550034","id":"PMC_15550034","title":"PECAM-1 (CD31) is required for interactions of platelets with endothelial cells after irradiation.","date":"2004","source":"Journal of thrombosis and haemostasis : JTH","url":"https://pubmed.ncbi.nlm.nih.gov/15550034","citation_count":25,"is_preprint":false},{"pmid":"21734076","id":"PMC_21734076","title":"CD31 is required on CD4+ T cells to promote T cell survival during Salmonella infection.","date":"2011","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/21734076","citation_count":25,"is_preprint":false},{"pmid":"25344833","id":"PMC_25344833","title":"Altered CD31 expression and activity in helper T cells of acute coronary syndrome patients.","date":"2014","source":"Basic research in cardiology","url":"https://pubmed.ncbi.nlm.nih.gov/25344833","citation_count":24,"is_preprint":false},{"pmid":"32323830","id":"PMC_32323830","title":"Low shear stress induces endothelial cell apoptosis and monocyte adhesion by upregulating PECAM‑1 expression.","date":"2020","source":"Molecular medicine reports","url":"https://pubmed.ncbi.nlm.nih.gov/32323830","citation_count":24,"is_preprint":false},{"pmid":"35440765","id":"PMC_35440765","title":"Co-expression of ERG and CD31 in a subset of CIC-rearranged sarcoma: a potential diagnostic pitfall.","date":"2022","source":"Modern pathology : an official journal of the United States and Canadian Academy of Pathology, Inc","url":"https://pubmed.ncbi.nlm.nih.gov/35440765","citation_count":23,"is_preprint":false},{"pmid":"26706435","id":"PMC_26706435","title":"PECAM1 regulates flow-mediated Gab1 tyrosine phosphorylation and signaling.","date":"2015","source":"Cellular signalling","url":"https://pubmed.ncbi.nlm.nih.gov/26706435","citation_count":22,"is_preprint":false},{"pmid":"33249476","id":"PMC_33249476","title":"Nestin+/CD31+ cells in the hypoxic perivascular niche regulate glioblastoma chemoresistance by upregulating JAG1 and DLL4.","date":"2021","source":"Neuro-oncology","url":"https://pubmed.ncbi.nlm.nih.gov/33249476","citation_count":22,"is_preprint":false},{"pmid":"34987111","id":"PMC_34987111","title":"Comprehensive Immune Profiling Reveals CD56+ Monocytes and CD31+ Endothelial Cells Are Increased in Severe COVID-19 Disease.","date":"2022","source":"Journal of immunology (Baltimore, Md. : 1950)","url":"https://pubmed.ncbi.nlm.nih.gov/34987111","citation_count":21,"is_preprint":false},{"pmid":"23737650","id":"PMC_23737650","title":"Increased expression of VEGF and CD31 in postradiation rectal tissue: implications for radiation proctitis.","date":"2013","source":"Mediators of inflammation","url":"https://pubmed.ncbi.nlm.nih.gov/23737650","citation_count":21,"is_preprint":false},{"pmid":"26607202","id":"PMC_26607202","title":"Platelet endothelial cell adhesion molecule-1 (PECAM1) plays a critical role in the maintenance of human vascular endothelial barrier function.","date":"2015","source":"Cell biochemistry and function","url":"https://pubmed.ncbi.nlm.nih.gov/26607202","citation_count":20,"is_preprint":false},{"pmid":"10592470","id":"PMC_10592470","title":"Vascular-endothelial cadherin (CD144)- but not PECAM-1 (CD31)-based cell-to-cell contacts convey the maintenance of a quiescent endothelial monolayer.","date":"1999","source":"International archives of allergy and immunology","url":"https://pubmed.ncbi.nlm.nih.gov/10592470","citation_count":20,"is_preprint":false},{"pmid":"30063264","id":"PMC_30063264","title":"Frontline Science: PECAM-1 (CD31) expression in naïve and memory, but not acutely activated, CD8+ T cells.","date":"2018","source":"Journal of leukocyte biology","url":"https://pubmed.ncbi.nlm.nih.gov/30063264","citation_count":19,"is_preprint":false},{"pmid":"12616538","id":"PMC_12616538","title":"Modulation of PECAM-1 expression and alternative splicing during differentiation and activation of hematopoietic cells.","date":"2003","source":"Journal of cellular biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12616538","citation_count":19,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":49630,"output_tokens":7411,"usd":0.130027},"stage2":{"model":"claude-opus-4-6","input_tokens":11216,"output_tokens":3824,"usd":0.22752},"total_usd":0.357547,"stage1_batch_id":"msgbatch_012ZiiCUBfB62NCTQd4Bk1Jo","stage2_batch_id":"msgbatch_01Xuqq9aUHoQouS7UVGZNStZ","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1990,\n      \"finding\": \"PECAM-1 (CD31) was cloned and identified as a 130 kDa integral membrane glycoprotein with six extracellular immunoglobulin-like domains, a transmembrane domain, and a cytoplasmic tail; it is a member of the CAM subgroup of the Ig superfamily expressed on platelets, leukocytes, and enriched at endothelial cell intercellular junctions, consistent with a role in cellular recognition and adhesion.\",\n      \"method\": \"cDNA cloning from endothelial cell library, sequence analysis, immunostaining\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — original cloning paper with sequence and domain characterization, foundational and highly cited\",\n      \"pmids\": [\"1690453\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1990,\n      \"finding\": \"CD31 is a member of the Ig superfamily most closely related to carcinoembryonic antigen, comprising four contiguous C2 domains, and is concentrated at cell-cell contacts in endothelial cells, consistent with function as an intercellular adhesion molecule.\",\n      \"method\": \"cDNA cloning by transient expression, sequence analysis, immunofluorescence localization\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — independent cloning and structural characterization, corroborated by PMID 1690453\",\n      \"pmids\": [\"2351935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"Full-length PECAM-1 transfected into NIH/3T3 null cells localizes to sites of cell-cell contact at the cell periphery, promotes cell-cell adhesion, and decreases the rate of cell migration, demonstrating that the intact molecule (requiring its cytoplasmic domain) is necessary for junction localization and migration inhibition.\",\n      \"method\": \"Transfection of full-length vs. extracellular-domain-only constructs into NIH/3T3 cells, quantitative migration assays, immunofluorescence\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain deletion experiments with functional readouts in null cells\",\n      \"pmids\": [\"1429859\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"PECAM-1 becomes highly phosphorylated upon cellular activation and, coincident with phosphorylation, associates with the cytoskeleton of activated platelets; engagement of PECAM-1 upregulates integrin function on leukocytes, implicating it as a trigger for leukocyte trafficking.\",\n      \"method\": \"Biochemical fractionation, phosphorylation assays, functional leukocyte adhesion assays\",\n      \"journal\": \"Annals of the New York Academy of Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — mechanistic description in review, based on primary biochemical data cited therein\",\n      \"pmids\": [\"8017765\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"PECAM-1 mediates a specific homophilic interaction required for the transendothelial migration (diapedesis) phase of neutrophil and monocyte emigration; anti-PECAM-1 antibodies or soluble recombinant PECAM-1 directed against either leukocyte or endothelial PECAM-1 blocked passage across endothelial monolayers by 70–90% without affecting apical adhesion, and a monoclonal antibody against mouse PECAM-1 blocked leukocyte emigration into the peritoneum in vivo.\",\n      \"method\": \"Quantitative in vitro transendothelial migration assay with antibody/recombinant protein blocking, in vivo thioglycollate peritonitis model\",\n      \"journal\": \"Journal of leukocyte biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal in vitro and in vivo blocking approaches, replicated across labs\",\n      \"pmids\": [\"7722409\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"TNF-α and IFN-γ induce redistribution of PECAM-1 away from endothelial cell intercellular junctions without altering total surface expression or transcription; this redistribution is associated with changes in PECAM-1 cytoskeletal association, providing a mechanism by which inflammatory cytokines regulate leukocyte transmigration.\",\n      \"method\": \"Flow cytometry, immunofluorescence, subcellular fractionation, RT-PCR, cytoskeletal association assays on HUVECs\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in a single study\",\n      \"pmids\": [\"7759892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"PECAM-1 contributes to vascular barrier function; antibody blocking of murine vascular PECAM-1 in vivo caused detectable leakage from hepatic and renal blood vessels, and monolayer permeability assays on native or PECAM-1-transfected cells confirmed a barrier role.\",\n      \"method\": \"Macromolecule permeability assays on endothelial monolayers, in vivo antibody injection with vascular leakage readout\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro and in vivo functional data in single study\",\n      \"pmids\": [\"7589563\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"PECAM-1 engagement induces a slow but sustained increase in intracellular Ca²⁺ in endothelial cells and PECAM-1-transfected cell lines, requiring the cytoplasmic domain, extracellular Ca²⁺, and tyrosine phosphorylation but not receptor cross-linking; this Ca²⁺ signal is accompanied by prostacyclin release.\",\n      \"method\": \"Single-cell fluorometry with anti-PECAM-1 antibodies and Fab fragments, prostacyclin ELISA, cytoplasmic-domain deletion constructs\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in transfected cells with domain mutants and multiple functional readouts\",\n      \"pmids\": [\"9421484\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"PECAM-1 is required for endothelial tube formation in vitro; anti-CD31 antibodies caused endothelial cells to become rounded or form attenuated tube-like structures with large single vacuoles rather than normal tubes, distinct from the VE-cadherin antibody phenotype, and CD31 associates with β-catenin via immunoprecipitation, linking it to the actin cytoskeleton.\",\n      \"method\": \"3D collagen gel tube formation assay with blocking antibodies, immunoprecipitation for β-catenin association\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional blocking with defined morphological readout plus biochemical Co-IP\",\n      \"pmids\": [\"9120301\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"SHP-2, but not SHP-1, associates with tyrosine-phosphorylated PECAM-1 via its SH2 domains; this association correlates with the extent of PECAM-1 tyrosine phosphorylation, and immune precipitate phosphatase assays show SHP-2 dephosphorylates PECAM-1, identifying a signaling pathway downstream of integrin and immune receptor activation.\",\n      \"method\": \"Co-immunoprecipitation, SH2-domain fusion protein pull-down, in vitro phosphatase assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro phosphatase assay plus reciprocal pull-down with multiple controls\",\n      \"pmids\": [\"9388260\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Plasmodium falciparum-infected erythrocytes bind directly to PECAM-1/CD31 on the vascular endothelium; binding occurs to PECAM-1-transfected cells and to recombinant PECAM-1 absorbed onto plastic, is blocked by soluble PECAM-1 or antibodies to domains 1–4, and is augmented by IFN-γ.\",\n      \"method\": \"Binding assays to PECAM-1 transfected cells and recombinant PECAM-1, blocking with soluble PECAM-1 and monoclonal antibodies\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with recombinant protein and transfected cells, domain mapping\",\n      \"pmids\": [\"9396614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"CD31 exists as a dimer both in solution and on cell membranes (confirmed by ultracentrifugation and chemical cross-linking) and is heavily N-glycosylated (~21% carbohydrate content with 19 neutral and 13 sialylated glycans), suggesting dimerization plays a role in CD31 adhesion and signaling.\",\n      \"method\": \"Analytical ultracentrifugation, chemical cross-linking, mass spectrometry glycan analysis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biophysical and biochemical characterization with multiple methods\",\n      \"pmids\": [\"10425179\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The cytoplasmic tail of CD31 activates the small GTPase Rap1 (selectively, not Ras, R-Ras, or Rap2) to promote T cell adhesion via β1 (VLA-4) and β2 (LFA-1) integrins; dominant-negative Rap1 or its GAP blocked CD31-dependent adhesion, while activated Rap1 or C3G exchange factor stimulated adhesion.\",\n      \"method\": \"Transfection of cytoplasmic-tail deletion constructs, dominant-negative and activated Rap1 mutants, adhesion assays to ICAM and VCAM\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with multiple gain- and loss-of-function constructs, specific GTPase selectivity demonstrated\",\n      \"pmids\": [\"10725328\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PECAM-1 expressed on endothelial cells (but not hematopoietic cells) modulates in vivo bleeding time; PECAM-1-deficient mice showed prolonged bleeding times that were not corrected by wild-type hematopoietic engraftment but were present when PECAM-1-deficient marrow was transplanted into wild-type mice.\",\n      \"method\": \"PECAM-1 knockout mice, bone marrow transplantation experiments, tail bleeding time assay\",\n      \"journal\": \"The American journal of pathology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout with reciprocal bone marrow transplant controls clearly dissecting endothelial vs. hematopoietic contribution\",\n      \"pmids\": [\"10880378\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CD31-mediated homophilic ligation on viable leukocytes promotes their active, temperature-dependent detachment from macrophages under low shear; apoptosis disables this CD31 detachment signaling, converting CD31 into a tethering molecule that promotes macrophage engulfment of dying cells.\",\n      \"method\": \"Flow-based binding assay of viable vs. apoptotic leukocytes with macrophages, temperature dependence, antibody blocking\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct flow assay with rigorous controls; published in Nature with mechanistic dissection\",\n      \"pmids\": [\"12110892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PECAM-1 transduces endothelial cell responses to H₂O₂ by activating a calcium-permeant non-selective cation current; this requires the PECAM-1 cytoplasmic tyrosine-containing domain and Src family kinase activity, as shown by tyrosine-domain mutants and dialysis of anti-cytoplasmic-domain antibodies.\",\n      \"method\": \"Patch-clamp electrophysiology, Ca²⁺ imaging, stable transfection of PECAM-1 and cytoplasmic-domain mutants into PECAM-1-negative REN cells\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in null cells with domain mutagenesis and electrophysiological readout\",\n      \"pmids\": [\"11927609\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Clustering of PECAM-1 on endothelial cells triggers a novel endocytic pathway that is distinct from clathrin- and caveolae-mediated endocytosis; internalization requires multimeric conjugates, is inhibited by amiloride and PKC inhibitors (macropinocytosis inhibitors), and depends on actin rearrangements requiring Src kinase and Rho kinase (ROCK).\",\n      \"method\": \"Internalization assays with anti-PECAM-1 conjugates, inhibitor studies, dominant-negative dynamin-2 transfection, actin cytoskeleton imaging\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple pharmacological and genetic tools defining novel pathway\",\n      \"pmids\": [\"12640043\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"PECAM-1 regulates β-catenin stability at endothelial adherens junctions by modulating GSK-3β activity: PECAM-1 tyrosine phosphorylation upon stimulation recruits SHP-2 and tyrosine-phosphorylated β-catenin, enabling β-catenin dephosphorylation and junction re-annealing; in PECAM-1-null ECs, β-catenin remains tyrosine-phosphorylated, GSK-3β serine phosphorylation (inactivation) is blunted, and β-catenin undergoes increased serine phosphorylation and proteasomal degradation.\",\n      \"method\": \"PECAM-1 knockout endothelial cells, co-immunoprecipitation, phospho-specific Western blotting, shear stress and histamine stimulation assays\",\n      \"journal\": \"The American journal of pathology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO model plus multiple orthogonal biochemical methods\",\n      \"pmids\": [\"16816383\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The Kaposi sarcoma herpesvirus ubiquitin ligase K5 ubiquitinates pre-existing CD31 at the cell surface, triggering its endocytosis and lysosomal degradation, while newly synthesized CD31 is degraded by proteasomes in the ER via a mechanism requiring PACS-2 binding to acidic residues in the K5 cytoplasmic tail.\",\n      \"method\": \"Ubiquitination assays, endocytosis/degradation experiments with lysosomal and proteasomal inhibitors, PACS-2 binding mutants, cell migration assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mechanistic dissection of dual degradation pathway with domain mutants and pharmacological inhibitors\",\n      \"pmids\": [\"16601245\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"TCR stimulation drives ectodomain cleavage and shedding of CD31 from human T cells, releasing a soluble truncated form detectable in plasma; shedding abolishes CD31 inhibitory function because cis-homo-oligomerization (triggered by trans-homophilic engagement of domain 1) cannot occur on CD31-shed cells; a juxtamembrane peptide (aa 551–574) can restore ITIM phosphorylation and SHP-2 activation on CD31-shed cells.\",\n      \"method\": \"Flow cytometry, Western blot detection of soluble CD31 in plasma, T cell activation assays, phosphorylation assays, in vivo peptide administration in mice\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic dissection with multiple methods including in vivo rescue\",\n      \"pmids\": [\"20400708\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"NEU1 sialidase desialylates CD31 (specifically its α2,6-linked sialic acids) when recruited to CD31 in postconfluent endothelial cells; NEU1-mediated desialylation of CD31 inhibits in vitro capillary tube formation, and prior CD31 silencing or use of CD31-null endothelial cells abrogates NEU1's inhibitory effect, placing CD31 sialylation downstream of NEU1 in the angiogenic pathway.\",\n      \"method\": \"Lectin blotting, siRNA knockdown of NEU1 and CD31, adenoviral overexpression of catalytically active vs. dead NEU1, tube formation assays on Matrigel, CD31-null endothelial cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — catalytic dead mutant controls, genetic epistasis via CD31 KO, multiple orthogonal approaches\",\n      \"pmids\": [\"24550400\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CD31 signaling via SHP-1 phosphatase in dendritic cells reduces NF-κB nuclear translocation, expression of costimulatory molecules, and production of IL-12 and IL-6, while increasing TGF-β and IL-10, driving DCs toward tolerogenic function and T-cell tolerance; CD31-conditioned DCs promoted regulatory T cells and suppressed experimental autoimmune encephalomyelitis in vivo.\",\n      \"method\": \"DC maturation assays with CD31 gain/loss of function, NF-κB reporter assay, cytokine ELISA, adoptive transfer of CD31-conditioned antigen-loaded DCs into EAE model\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple in vitro and in vivo experiments with mechanistic signaling readouts\",\n      \"pmids\": [\"24616502\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"PECAM1 specifically mediates flow-induced (but not HGF-induced) Gab1 tyrosine phosphorylation, membrane translocation of Gab1, and downstream Akt and eNOS activation in endothelial cells; PECAM1 siRNA abolished these flow responses, and PECAM1 knockout mice showed reduced flow-mediated Gab1 and eNOS phosphorylation in vivo.\",\n      \"method\": \"siRNA knockdown, PECAM1 knockout mice, phospho-Western blotting, flow stimulation apparatus, in vivo voluntary wheel running\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro siRNA plus in vivo KO corroborating results with mechanistic specificity demonstrated\",\n      \"pmids\": [\"26706435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CD31 is identified as the specific endothelial cell receptor for Clostridium perfringens β-toxin; CD31 expression correlates with cell-type susceptibility, ectopic CD31 expression renders resistant cells and liposomes susceptible to CPB-induced membrane damage, and the extracellular Ig6 domain is essential for the interaction with CPB.\",\n      \"method\": \"Ectopic expression of CD31 in resistant cell lines, liposome reconstitution, domain deletion mapping of Ig6, in vivo mouse experiments\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in null cells and liposomes, domain mutants, in vivo validation\",\n      \"pmids\": [\"32497498\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CD31 receptor-induced signaling preserves endothelial barrier function by triggering a glycolytic response via src-homology phosphatase activation leading to Akt-mediated nuclear exclusion of FoxO1 and β-catenin nuclear translocation, driving cMyc transcription; CD31-deficient mice show pathological microvascular leakage correctable by pharmacological Akt or AMPK activation.\",\n      \"method\": \"CD31-deficient mice, glycolytic flux measurements, pharmacological Akt/AMPK activation, transendothelial resistance assays, signaling pathway analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with mechanistic rescue experiments and multiple signaling readouts\",\n      \"pmids\": [\"32681081\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Leukocyte-induced stimulation of endothelial PECAM-1 causes dissociation of SHP2 phosphatase from PECAM-1, which then directly dephosphorylates VE-cadherin-Y731, enabling VE-cadherin endocytosis and junction opening for diapedesis; this mechanism requires Ca²⁺ signaling, non-muscle myosin II activation, and endothelial cell tension generated by leukocyte docking force on the VE-cadherin-catenin complex.\",\n      \"method\": \"SHP2 binding-site mutants of PECAM-1, VE-cadherin Y731 phospho-mutant mice, in vitro and in vivo leukocyte diapedesis assays, pharmacological inhibitors of myosin II and Ca²⁺ signaling, microscopy\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — domain mutants, phospho-specific readouts, mechanotransduction mechanism with in vivo confirmation\",\n      \"pmids\": [\"33604918\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"PECAM1 physically interacts with the mechanosensitive PIEZO1 channel at endothelial cell-cell junctions; PECAM1 extracellular N-terminus is critical for this interaction and directs PIEZO1 to junctions; PIEZO1 is required for Ca²⁺-dependent formation of adherens junctions and associated cytoskeleton, supporting a cooperative role in force-dependent junctional remodeling.\",\n      \"method\": \"Endogenous tagging of PIEZO1 in mice, reconstitution studies, high-resolution microscopy (co-localization), shear stress experiments, CDH5 and VEGFR2 interaction comparisons\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in situ tagging, reconstitution, and domain mapping with functional Ca²⁺ and junctional readouts\",\n      \"pmids\": [\"37005489\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Physical traction on endothelial PECAM-1 during leukocyte transendothelial migration (TEM) initiates the endothelial signaling pathway; endothelial PECAM-1 acts as part of a mechanotransduction complex with VE-cadherin and VEGFR2; TEM requires VEGFR2 Y1175 phosphorylation but not VEGF or VEGFR2 kinase activity; endothelial-specific VEGFR2-deficient mice show ≥75% reduction in neutrophil extravasation.\",\n      \"method\": \"Fluorescence lifetime imaging microscopy (FLIM) for force sensing, inducible endothelial-specific VEGFR2-knockout mice, VEGFR2 phospho-mutants, three mouse inflammation models\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — FLIM mechanotransduction assay, genetic KO with multiple in vivo models, phospho-mutant dissection\",\n      \"pmids\": [\"37643615\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"PECAM-1 expression negatively regulates multiple platelet signaling pathways; PECAM-1 clustering inhibits platelet responses to CRP-XL (collagen receptor pathway), ADP, and thrombin, resulting in a marked combined reduction in thrombus formation.\",\n      \"method\": \"Flow cytometry correlation of PECAM-1 surface expression with platelet reactivity, PECAM-1 clustering assays, thrombus formation assays\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clustering experiments with multiple agonist readouts in single study\",\n      \"pmids\": [\"19850043\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"PECAM-1 modulates thrombin-induced tissue factor (TF) expression in endothelial cells via a signaling pathway involving PAR-1, Gαi/o, Rho kinase, p38 MAPK, and Egr-1 nuclear translocation; PECAM-1 promotes PI3K-Akt activation that inversely regulates p38 MAPK and thereby suppresses TF expression; PECAM-1-null mice show increased TF expression and fibrin deposition after renal ischemia-reperfusion.\",\n      \"method\": \"Antisense knockdown of PECAM-1 in HUVECs, PECAM-1 KO mice, pharmacological inhibitors (PAR-1 antagonist, pertussis toxin, Rho kinase, p38 inhibitor, PI3K inhibitor), TF mRNA and protein assays\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — combined in vitro knockdown plus in vivo KO with pathway dissection using multiple inhibitors\",\n      \"pmids\": [\"17111362\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PECAM-1 (CD31) is a homophilically-adhesive, ITIM-bearing Ig superfamily receptor concentrated at endothelial junctions that functions as a mechanotransducer (partnering with VE-cadherin, VEGFR2, and PIEZO1) to initiate leukocyte diapedesis via SHP2-mediated dephosphorylation of VE-cadherin-Y731, maintains endothelial barrier integrity through glycolysis-coupled β-catenin/Akt/cMyc signaling, inhibits platelet and T-cell activation via ITIM-recruited SHP-1/SHP-2, activates Rap1 to upregulate integrin adhesion on leukocytes, and regulates intracellular Ca²⁺ through a cytoplasmic-tyrosine-dependent, Src kinase-activated cation channel downstream of H₂O₂ stimulation.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PECAM1 (CD31) is an immunoglobulin superfamily transmembrane glycoprotein that functions as a homophilic adhesion receptor and mechanotransducer at endothelial cell-cell junctions, integrating cell adhesion, vascular barrier maintenance, leukocyte transendothelial migration, and inhibitory signaling on platelets and immune cells. At endothelial junctions, PECAM1 forms a mechanosensory complex with VE-cadherin, VEGFR2, and PIEZO1; physical traction during leukocyte diapedesis triggers SHP-2 dissociation from PECAM1's ITIM-containing cytoplasmic tail, enabling SHP-2-mediated dephosphorylation of VE-cadherin Y731 and junction opening, a process requiring Ca²⁺ signaling and non-muscle myosin II [PMID:33604918, PMID:37643615, PMID:37005489]. PECAM1 maintains endothelial barrier integrity through β-catenin stabilization via GSK-3β modulation and a glycolysis-coupled Akt/FoxO1/cMyc pathway, and transduces H₂O₂-induced activation of a Src-dependent non-selective cation channel [PMID:16816383, PMID:32681081, PMID:11927609]. On leukocytes and platelets, PECAM1 cytoplasmic signaling activates Rap1 to upregulate integrin adhesion, recruits SHP-1/SHP-2 phosphatases to inhibit platelet activation and promote dendritic cell tolerogenesis, and undergoes TCR-driven ectodomain shedding that disables its inhibitory function [PMID:10725328, PMID:19850043, PMID:24616502, PMID:20400708].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"Molecular cloning revealed PECAM1 as a six-Ig-domain transmembrane glycoprotein of the Ig superfamily concentrated at endothelial junctions, establishing its identity as a potential adhesion molecule.\",\n      \"evidence\": \"cDNA cloning from endothelial libraries with sequence analysis and immunostaining by two independent groups\",\n      \"pmids\": [\"1690453\", \"2351935\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No functional adhesion data yet\", \"Ligand/counter-receptor unknown\", \"Signaling capacity not addressed\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Reconstitution in null cells demonstrated that PECAM1 localizes to cell-cell contacts, promotes adhesion, and inhibits migration in a cytoplasmic-domain-dependent manner, establishing it as a functional adhesion molecule with signaling requirements.\",\n      \"evidence\": \"Transfection of full-length vs. extracellular-only constructs into NIH/3T3 cells with migration assays and immunofluorescence\",\n      \"pmids\": [\"1429859\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cytoplasmic domain signaling mechanism unresolved\", \"Nature of adhesive interaction (homophilic vs. heterophilic) not determined\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Blocking experiments in vitro and in vivo established that PECAM1 homophilic interaction is specifically required for the diapedesis step of leukocyte transendothelial migration, defining its central role in inflammation.\",\n      \"evidence\": \"Antibody/recombinant protein blocking in transmigration assays and thioglycollate peritonitis model\",\n      \"pmids\": [\"7722409\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism linking homophilic engagement to junction opening unknown\", \"Endothelial vs. leukocyte PECAM1 relative contributions not genetically dissected\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"TNF-α/IFN-γ were shown to redistribute junctional PECAM1 without changing surface levels, and antibody blocking in vivo caused vascular leakage, linking PECAM1 junctional localization to barrier function.\",\n      \"evidence\": \"Flow cytometry, subcellular fractionation, RT-PCR on HUVECs; in vivo antibody injection with permeability readout\",\n      \"pmids\": [\"7759892\", \"7589563\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of redistribution unclear\", \"Whether barrier loss is direct or secondary to junction disassembly not resolved\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"A burst of discoveries defined PECAM1's signaling repertoire: engagement triggers sustained Ca²⁺ influx requiring the cytoplasmic domain; tyrosine-phosphorylated PECAM1 recruits SHP-2 phosphatase; and PECAM1 associates with β-catenin, linking it to adherens junction regulation and tube formation.\",\n      \"evidence\": \"Single-cell fluorometry with domain mutants; co-IP and in vitro phosphatase assays for SHP-2; β-catenin co-IP and collagen gel tube assays with blocking antibodies\",\n      \"pmids\": [\"9421484\", \"9388260\", \"9120301\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream targets of SHP-2 recruited to PECAM1 not identified\", \"Nature of the Ca²⁺ channel unknown\", \"β-catenin association site on PECAM1 not mapped\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"PECAM1 cytoplasmic signaling was shown to selectively activate Rap1 (not Ras) to upregulate integrin function on T cells, and knockout mice revealed that endothelial (not hematopoietic) PECAM1 controls hemostasis.\",\n      \"evidence\": \"Dominant-negative/activated Rap1 constructs with adhesion assays; PECAM1 KO mice with reciprocal bone marrow transplants and bleeding time\",\n      \"pmids\": [\"10725328\", \"10880378\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"GEF linking PECAM1 to Rap1 not fully confirmed in endothelial cells\", \"Mechanism of endothelial PECAM1 control of hemostasis undefined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Two distinct functions were delineated: PECAM1 homophilic engagement on viable leukocytes drives active detachment from macrophages (disabled by apoptosis, converting CD31 to a tether for efferocytosis), and H₂O₂-activated PECAM1 opens a Src-dependent non-selective cation channel requiring its cytoplasmic tyrosines.\",\n      \"evidence\": \"Flow-based macrophage binding assay comparing viable vs. apoptotic cells; patch-clamp electrophysiology in PECAM1-reconstituted REN cells with domain mutants\",\n      \"pmids\": [\"12110892\", \"11927609\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular identity of the cation channel not determined\", \"Signaling switch between viable detachment and apoptotic tethering not molecularly defined\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Genetic knockout studies showed PECAM1 stabilizes β-catenin at adherens junctions by recruiting SHP-2 to dephosphorylate β-catenin tyrosines and modulating GSK-3β, explaining barrier fragility in PECAM1-null endothelium.\",\n      \"evidence\": \"PECAM1 KO endothelial cells with phospho-specific Western blotting, co-IP, shear stress and histamine stimulation\",\n      \"pmids\": [\"16816383\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct GSK-3β regulation mechanism by PECAM1 not fully dissected\", \"Whether β-catenin stabilization is the sole barrier-maintenance mechanism unclear\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"TCR-driven ectodomain shedding of CD31 from T cells was identified as a mechanism that disables CD31 inhibitory signaling by preventing cis-homo-oligomerization; a juxtamembrane peptide could restore ITIM phosphorylation and SHP-2 activation.\",\n      \"evidence\": \"Flow cytometry, soluble CD31 detection in plasma, T cell activation assays, in vivo peptide rescue in mice\",\n      \"pmids\": [\"20400708\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Protease responsible for ectodomain shedding not identified\", \"Therapeutic applicability of juxtamembrane peptide not established\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"CD31 inhibitory signaling was extended to dendritic cells: SHP-1-dependent suppression of NF-κB drives tolerogenic DC function and regulatory T cell induction; separately, NEU1 sialidase-mediated desialylation of CD31 was shown to inhibit angiogenesis.\",\n      \"evidence\": \"DC maturation assays with NF-κB reporters, adoptive DC transfer in EAE model; lectin blotting, CD31-null epistasis, tube formation assays\",\n      \"pmids\": [\"24616502\", \"24550400\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for how sialylation state modulates CD31 adhesion/signaling unknown\", \"Whether SHP-1 vs. SHP-2 usage in DCs is context-dependent not resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"CD31 was identified as the receptor for Clostridium perfringens β-toxin via its Ig6 domain, and its barrier-maintenance function was linked to a glycolytic pathway involving SHP-Akt-FoxO1 nuclear exclusion and β-catenin/cMyc-driven transcription.\",\n      \"evidence\": \"Ectopic expression in resistant cells and liposomes with domain mapping for CPB; CD31 KO mice with glycolytic flux measurements and pharmacological Akt/AMPK rescue\",\n      \"pmids\": [\"32497498\", \"32681081\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How Ig6 domain engagement by CPB leads to membrane damage not structurally resolved\", \"Whether glycolytic barrier maintenance operates in all vascular beds unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"The diapedesis mechanism was resolved: leukocyte traction on PECAM1 causes SHP-2 to dissociate from PECAM1 and dephosphorylate VE-cadherin Y731, enabling cadherin endocytosis and junction opening, dependent on Ca²⁺ and non-muscle myosin II.\",\n      \"evidence\": \"SHP-2 binding-site mutants of PECAM1, VE-cadherin Y731 phospho-mutant mice, in vitro and in vivo diapedesis assays\",\n      \"pmids\": [\"33604918\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How PECAM1 senses mechanical force at the molecular level not resolved\", \"Whether SHP-2 acts catalytically on VE-cadherin in cis or after release not fully distinguished\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"PECAM1 was placed in a mechanotransduction complex with PIEZO1, VE-cadherin, and VEGFR2: PECAM1 directs PIEZO1 to junctions via its N-terminal extracellular domain, and leukocyte traction activates VEGFR2 Y1175 phosphorylation independently of VEGF or VEGFR2 kinase activity, required for neutrophil extravasation.\",\n      \"evidence\": \"Endogenous PIEZO1 tagging in mice with co-localization and reconstitution; FLIM force-sensing, endothelial-specific VEGFR2 KO mice, three inflammation models\",\n      \"pmids\": [\"37005489\", \"37643615\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How VEGFR2 Y1175 is phosphorylated without its own kinase activity is unclear—trans-phosphorylation source not identified\", \"Stoichiometry and structure of the PECAM1-PIEZO1-VEGFR2-VE-cadherin complex unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of force sensing by PECAM1 and the full stoichiometry of its mechanosensory complex remain undefined, as does the molecular identity of the PECAM1-dependent cation channel.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of PECAM1 in complex with junctional partners\", \"Molecular identity of H₂O₂-activated cation channel downstream of PECAM1 unknown\", \"Protease(s) mediating CD31 ectodomain shedding on T cells not identified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [0, 1, 2, 4]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [7, 15, 22, 25, 27]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [9, 17, 21, 28, 29]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 2, 4, 26]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [4, 5, 14, 21, 25, 27]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [7, 9, 12, 15, 22, 24, 29]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [2, 6, 17, 26]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [13, 28, 29]}\n    ],\n    \"complexes\": [\n      \"PECAM1-VE-cadherin-VEGFR2 mechanosensory complex\",\n      \"PECAM1-PIEZO1 junctional complex\"\n    ],\n    \"partners\": [\n      \"CDH5\",\n      \"KDR\",\n      \"PIEZO1\",\n      \"PTPN11\",\n      \"PTPN6\",\n      \"CTNNB1\",\n      \"RAP1A\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}