{"gene":"PTGER4","run_date":"2026-04-28T19:45:45","timeline":{"discoveries":[{"year":2003,"finding":"EP4 receptor couples to Gs to increase intracellular cAMP, and also activates PI3K and ERK1/2 (but not EP2) leading to induction of EGR-1 transcription factor, distinct from the cAMP-PKA pathway used by EP2.","method":"Pharmacological receptor subtype-selective ligand studies, signaling assays in transfected cells","journal":"Life sciences","confidence":"Medium","confidence_rationale":"Tier 2 — multiple signaling readouts with pharmacological tools, single review-style paper summarizing original experimental findings","pmids":["14607241"],"is_preprint":false},{"year":1998,"finding":"EP4 receptor knockout mice develop patent ductus arteriosus and neonatal death, establishing that EP4 signaling is required for closure of the ductus arteriosus in neonatal circulatory adaptation; EP4 mRNA was confirmed by in situ hybridization in the ductus.","method":"Gene targeting (knockout mice), histological examination, in situ hybridization","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined physiological phenotype and direct localization data, foundational paper with 253 citations","pmids":["9600059"],"is_preprint":false},{"year":2006,"finding":"EP4 receptor on primary sensory DRG neurons mediates PGE2-induced sensitization of capsaicin-evoked currents and inflammatory pain hypersensitivity; EP4 levels increase in DRG after peripheral inflammation.","method":"Intrathecal shRNA knockdown, EP4 antagonist (AH23848), in vitro DRG neuron electrophysiology, behavioral pain assays","journal":"The Journal of pharmacology and experimental therapeutics","confidence":"High","confidence_rationale":"Tier 2 — orthogonal methods (shRNA KD + pharmacological antagonist + in vitro electrophysiology + in vivo behavioral), strong mechanistic placement","pmids":["16966471"],"is_preprint":false},{"year":2015,"finding":"EP4 receptor mediates PGE2 inhibition of NLRP3 inflammasome activation in human macrophages through elevation of intracellular cAMP, independently of PKA or Epac.","method":"EP4-specific agonist/antagonist pharmacology, EP4 knockdown (siRNA), adenylate cyclase inhibitor, cAMP measurement, NLRP3 inflammasome activation assays in human monocyte-derived macrophages","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (agonist, antagonist, siRNA KD, AC inhibitor, cAMP measurement) in primary human cells","pmids":["25917098"],"is_preprint":false},{"year":2004,"finding":"EP4 receptor mediates PGE2-induced colon adenocarcinoma cell proliferation via PI3K/ERK activation; selective EP4 agonist PGE1-OH rescues anti-proliferative effects of COX inhibition through this pathway.","method":"Selective EP receptor agonists, PI3K/ERK inhibitors, cell proliferation assays, in vivo tumor model","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — pharmacological + in vivo rescue experiments, replicated across multiple conditions","pmids":["15123663"],"is_preprint":false},{"year":2010,"finding":"EP4 receptor promotes lung cancer cell migration via a signaling complex of EP4–βArrestin1–c-Src; knockdown of βArrestin1 impairs PGE2-induced c-Src activation and cell migration.","method":"EP subtype-selective agonists, shRNA knockdown of EP4 and βArrestin1, c-Src kinase activity assays, cell migration assays","journal":"Molecular cancer research","confidence":"High","confidence_rationale":"Tier 2 — reciprocal shRNA KD of two pathway components with defined migration phenotype, multiple orthogonal methods","pmids":["20353998"],"is_preprint":false},{"year":2011,"finding":"EP4 receptor couples to Gαs to activate PKA catalytic subunit γ, which promotes in vitro tube formation, ex vivo vessel outgrowth, and in vivo angiogenesis; downstream PKA substrates Rap1A, HSPB6, and eNOS promote angiogenesis while RhoA and GSK3β are inhibitory.","method":"EP subtype-selective agonists/antagonists, siRNA knockdown of EP4 and PKA subunits, tube formation and aortic ring assays, in vivo Matrigel plug assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple siRNA knockdowns of pathway components, in vitro and in vivo angiogenesis assays, strong mechanistic placement","pmids":["21926356"],"is_preprint":false},{"year":2022,"finding":"PGE2 activates EP4 on osteoclasts to promote migration, osteoclastogenesis, Netrin-1/CGRP sensory innervation, and PDGF-BB-driven type H vessel formation in subchondral bone via Gαs/PI3K/AKT/MAPK signaling, promoting OA progression.","method":"Osteoclast-specific EP4 conditional knockout (EP4LysM), pharmacological EP4 antagonist (HL-43), in vitro migration/osteoclastogenesis assays, OA mouse models, signaling pathway inhibitors","journal":"Bone research","confidence":"High","confidence_rationale":"Tier 1-2 — conditional KO validated with pharmacological antagonist and in vitro mechanistic signaling assays across multiple models","pmids":["35260562"],"is_preprint":false},{"year":2016,"finding":"COX-2/EP4 signaling induces cancer stem-like cells in breast cancer via PI3K/AKT activation of NOTCH/WNT signaling pathways.","method":"COX-2 overexpression, EP4 antagonist and siRNA knockdown, PI3K/AKT inhibitors, NOTCH/WNT inhibitors, spheroid/ALDH assays, xenograft tumor models","journal":"Stem cells","confidence":"High","confidence_rationale":"Tier 2 — multiple pharmacological and genetic interventions with defined pathway placement in vitro and in vivo","pmids":["27301070"],"is_preprint":false},{"year":2022,"finding":"PGE2-EP2/EP4 signaling simultaneously promotes NF-κB-driven active inflammation in myeloid cells and drives the mregDC-Treg axis for immunosuppression in the tumor microenvironment.","method":"Immune checkpoint inhibitor-insensitive mouse cancer model, single-cell RNA sequencing, EP2/EP4 antagonists","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 — scRNAseq plus pharmacological intervention, but EP4-specific contribution not fully separated from EP2","pmids":["35675777"],"is_preprint":false},{"year":2008,"finding":"Macrophage EP4 deficiency increases sensitivity to apoptotic stimuli (palmitic acid, free cholesterol) by suppressing PI3K/Akt and NF-κB pathway activity, reducing early atherosclerosis.","method":"Fetal liver cell transplantation into LDLR-/- mice (EP4-/- hematopoietic chimeras), Western diet feeding, apoptosis assays, signaling pathway analysis","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 — clean hematopoietic-specific KO with defined molecular mechanisms (p-Akt, p-Bad, NF-κB) and in vivo atherosclerosis phenotype","pmids":["19041765"],"is_preprint":false},{"year":2009,"finding":"EP4 receptor exhibits functional selectivity: PGE2 preferentially activates Gαs; PGF2α preferentially activates Gαi1; PGE1-alcohol is biased toward β-arrestin. EP4 also couples to pertussis toxin-sensitive Gαi and β-arrestin pathways.","method":"BRET-based assays for Gαs, Gαi, and β-arrestin activation in living cells with a panel of EP4 ligands","journal":"The Journal of pharmacology and experimental therapeutics","confidence":"High","confidence_rationale":"Tier 1 — quantitative BRET assays systematically comparing multiple ligands across three signaling pathways in living cells","pmids":["19584306"],"is_preprint":false},{"year":2014,"finding":"Microglial EP4 stimulation suppresses Aβ42-induced inflammatory gene expression (targeting IRF1, IRF7, NF-κB pathways) and potentiates Aβ42 phagocytosis; conditional microglial EP4 deletion in APP-PS1 mice increases inflammatory gene expression and Aβ deposition at early pathology stages.","method":"EP4 agonist treatment of cultured microglia, microarray gene expression analysis, conditional microglial EP4 knockout in APP-PS1 mice, oxidative protein modification and Aβ measurement","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — conditional KO in disease model with multiple molecular readouts and in vitro mechanistic validation","pmids":["24760848"],"is_preprint":false},{"year":2011,"finding":"EP4 receptor mediates PGE2-induced relaxation of human airways (bronchodilation); EP2 mediates this effect in guinea pig and mouse but not human airways.","method":"Pharmacological EP-subtype selective agonists/antagonists in isolated airway preparations from multiple species; EP2-knockout mice","journal":"Thorax","confidence":"High","confidence_rationale":"Tier 2 — pharmacological plus genetic (KO mice) across multiple species with functional tissue readout","pmids":["21606476"],"is_preprint":false},{"year":2004,"finding":"EP4 receptor mediates PGE2-induced suppression of 3T3-L1 preadipocyte differentiation by increasing cAMP and suppressing expression of differentiation markers (resistin, PPARγ).","method":"EP4-selective agonist (AE1-329) and antagonist (AE3-208), cAMP measurement, RT-PCR for differentiation markers","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 3 — pharmacological agonist/antagonist approach with molecular readouts, single lab","pmids":["15336573"],"is_preprint":false},{"year":2021,"finding":"PTGER4-expressing intestinal macrophages secrete CXCL1 via MAPK signaling upon PGE2 stimulation, driving epithelial cell differentiation and proliferation to support mucosal healing; macrophage-specific PTGER4 deletion impairs epithelial barrier regeneration in DSS colitis.","method":"Conditional macrophage-specific Ptger4 knockout (Csf1r-iCre Ptger4fl/fl), DSS colitis model, MAPK pathway analysis, epithelial organoid differentiation assays, liposome-mediated therapeutic targeting","journal":"Gut","confidence":"High","confidence_rationale":"Tier 2 — conditional KO with defined molecular mechanism (MAPK/CXCL1) and multiple functional readouts including rescue experiment","pmids":["33558271"],"is_preprint":false},{"year":2004,"finding":"EP4 receptor induces cardiac myocyte hypertrophy independently of PKA by transactivating EGFR, leading to p42/44 MAPK activation and increased protein synthesis; the EP4 antagonist blocks EGFR phosphorylation and MAPK activation.","method":"EP4 antagonist, EGFR selective inhibitor (AG-1478), PKA inhibitor, immunoprecipitation for EGFR phosphorylation, [3H]leucine incorporation, p42/44 MAPK phosphorylation assays in neonatal ventricular myocytes","journal":"American journal of physiology. Heart and circulatory physiology","confidence":"High","confidence_rationale":"Tier 2 — multiple pharmacological inhibitors targeting distinct pathway nodes, orthogonal functional readouts","pmids":["15626689"],"is_preprint":false},{"year":2009,"finding":"EP4 signaling in fibroblasts (FSP1+ cells) upregulates RANKL expression in response to PGE2 or wear debris, stimulating osteoclastogenesis and periprosthetic osteolysis; conditional fibroblast-specific EP4 knockout abolishes this response.","method":"Conditional knockout of EP4 in FSP1+ fibroblasts, in vitro PGE2 stimulation with RANKL mRNA measurement, osteoclast and osteolysis assays, comparison with EP1-/- and EP2-/- mice","journal":"Journal of bone and mineral research","confidence":"High","confidence_rationale":"Tier 2 — conditional cell-type-specific KO with defined molecular (RANKL) and cellular (osteoclastogenesis) readouts, supported by comparison with other EP KO lines","pmids":["19419302"],"is_preprint":false},{"year":2019,"finding":"Endothelial EP4 receptor is essential for blood pressure homeostasis by promoting eNOS phosphorylation at Ser1177 and NO production via the AMPK pathway; EC-specific EP4 knockout elevates blood pressure and reduces vasorelaxant responses.","method":"Endothelial-specific EP4 knockout and overexpression mice, eNOS phosphorylation assays, AMPK pathway inhibitors, blood pressure measurements, vascular reactivity assays","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 2 — conditional KO and overexpression with defined molecular mechanism (AMPK/eNOS/NO) and in vivo BP phenotype; L-NAME rescue confirms eNOS dependence","pmids":["32641583"],"is_preprint":false},{"year":2019,"finding":"COX-1/mPGES-1-derived PGE2 acting on endothelial EP4 receptor constrains myocardial ischemia-reperfusion injury by maintaining microvascular perfusion and suppressing leukocyte-endothelial interactions; endothelium-restricted EP4 deletion exacerbates MI/R injury.","method":"mPges-1 knockout mice, endothelium-restricted Ep4 knockout mice, in vivo and ex vivo vascular reactivity assays, leukocyte-endothelial interaction assays, EP4 agonist treatment","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — multiple conditional KO models with mechanistic pathway placement (EP4-mediated microvascular protection), rescue with PGE analog","pmids":["31015404"],"is_preprint":false},{"year":2011,"finding":"EP4 receptor is the primary mediator of COX-2-dependent renin stimulation by the macula densa; EP4 knockout mice show ~70% reduction in furosemide-induced renin mRNA upregulation, while EP2 and IP receptor knockouts are unaffected.","method":"Panel of gene-targeted mice (COX-2, mPGES1, mPGES2, EP2, EP4, IP knockouts), furosemide challenge, real-time RT-PCR for renin mRNA","journal":"American journal of physiology. Renal physiology","confidence":"High","confidence_rationale":"Tier 2 — comprehensive epistasis panel with multiple KO lines establishing EP4 as the key receptor in the pathway","pmids":["21835766"],"is_preprint":false},{"year":2009,"finding":"EP4 receptor in neonatal ventricular myocytes signals through a PKA→Rap1→ERK1/2→p90RSK cascade to regulate brain natriuretic peptide, c-Fos, and EGR-1 expression and to increase cell size; Rap dominant-negative mutant blocks PGE2-induced ERK activation.","method":"Dominant negative Rap mutant transfection, PKA inhibitor, Rap and p90RSK activation assays, BNP promoter reporter, cell size measurement in neonatal ventricular myocytes with EP4 overexpression","journal":"American journal of physiology. Heart and circulatory physiology","confidence":"High","confidence_rationale":"Tier 1-2 — dominant negative mutant + pharmacological inhibitors + overexpression with multiple molecular readouts","pmids":["19880670"],"is_preprint":false},{"year":2020,"finding":"Cryo-EM structure of EP4 coupled to heterotrimeric Gs at 3.3 Å resolution reveals that compared to other class A GPCRs, EP4 TM6 shifts less outward, but the Gs C-terminal helix inserts toward TM2 and forms an extended hook structure, mediated by conserved prostanoid receptor residues Phe54(2.39) and Trp327(7.51).","method":"Cryo-electron microscopy (cryo-EM) structure determination at 3.3 Å global resolution","journal":"Structure","confidence":"High","confidence_rationale":"Tier 1 — direct structural determination by cryo-EM with functional context","pmids":["33264604"],"is_preprint":false},{"year":2017,"finding":"EP4 receptor mediates barrier-protective effects of prostaglandin A2 on lung endothelial cells via Rap1/Rac1 GTPase and PKA targets (VE-cadherin, p120-catenin, ZO-1, cortactin, VASP) and suppresses NF-κB-driven ICAM1/VCAM1 expression; endothelial-specific EP4 knockout abolishes protection in ALI models.","method":"EP4 siRNA and pharmacological inhibition, endothelial-specific EP4 knockout mice, Rap1/Rac1 GTPase activity assays, permeability measurements, two in vivo ALI models","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 — conditional KO plus siRNA plus pharmacological inhibition with defined molecular mechanisms and in vivo validation","pmids":["28428256"],"is_preprint":false},{"year":2008,"finding":"EP4 receptor functions as a tumor suppressor in B cells by acting as a negative feedback regulator of BCR-mediated proliferation; EP4 knockdown accelerates B cell lymphoma growth in mice while EP4 overexpression is protective.","method":"Stable EP4 shRNA knockdown and overexpression in B cell lymphoma lines, in vivo tumor spread assays, microarray gene expression","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 — both loss- and gain-of-function with in vivo validation and transcriptome analysis","pmids":["19075289"],"is_preprint":false},{"year":2006,"finding":"EP4 receptor antagonism inhibits breast cancer metastasis; mammary tumor cells migrate in response to PGE2 via a cAMP-elevating EP4 mechanism; EP4 antagonists block both tumor migration and proliferation in vitro.","method":"EP4-selective antagonists (AH23848, ONO-AE3-208), in vivo metastasis models (syngeneic), in vitro migration and proliferation assays","journal":"Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological antagonism with in vivo and in vitro validation, no genetic KD confirmation","pmids":["16540639"],"is_preprint":false},{"year":2012,"finding":"EP4 receptor signaling in aortic smooth muscle cells increases MMP-2 activity and IL-6 production; genetic or pharmacological EP4 inhibition attenuates AAA formation in multiple mouse models by reducing MMP activity and cytokine release.","method":"EP4 agonist stimulation of human ASMCs, MMP-2 activity assays, EP4 antagonist (ONO-AE3-208) in ApoE-/- AAA mouse model, EP4+/- mice, CaCl2 AAA model, human AAA tissue organ cultures","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 — multiple mouse models (genetic and pharmacological) plus human tissue explants with defined molecular mechanisms","pmids":["22570740"],"is_preprint":false},{"year":2010,"finding":"EP4 receptor activation or inhibition regulates colonic epithelial barrier integrity; apical EP4 receptors on T84 cells mediate PGE2-induced barrier disruption, and prostaglandin transporter (PGT) vectorially transports basolateral PGE2 to apical EP4 receptors.","method":"EP4 receptor antagonist, immunolocalization of EP4 in polarized T84 cells and human biopsies, PGT siRNA/inhibitor, [3H]-PGE2 vectorial transport assays","journal":"American journal of physiology. Gastrointestinal and liver physiology","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological antagonism plus siRNA KD plus radiotracer transport assays demonstrating vectorial transport mechanism","pmids":["20813914"],"is_preprint":false},{"year":2012,"finding":"miR-101 post-transcriptionally represses EP4 receptor expression by binding to the 3'-UTR of EP4 mRNA; loss of miR-101 leads to elevated EP4 and increased colon cancer cell proliferation and motility.","method":"Luciferase reporter assay with EP4 3'-UTR (wild-type and mutant), miR-101 transfection, EP4 rescue co-transfection, cell proliferation and migration assays","journal":"Cancer biology & therapy","confidence":"High","confidence_rationale":"Tier 1-2 — luciferase reporter with UTR mutation, miRNA transfection, rescue experiment establishing direct post-transcriptional regulation","pmids":["22353936"],"is_preprint":false},{"year":2008,"finding":"EP4 expression is regulated transcriptionally by Sp-1 binding sites in the human EP4 promoter (-197 to -160); troglitazone suppresses EP4 by activating ERK-mediated Sp-1 phosphorylation, which reduces Sp-1 DNA binding.","method":"Luciferase reporter with EP4 promoter constructs, Sp-1 site mutations, MEK-1/ERK inhibitor (PD98059), immunoprecipitation-Western for Sp-1 phosphorylation, ChIP assay","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 1 — promoter mutagenesis, ChIP, and phosphorylation assays in orthogonal combination","pmids":["18346464"],"is_preprint":false},{"year":2019,"finding":"EP4 receptor promotes oral cancer cell migration by activating PI3K and inducing Ca2+ influx through Orai1 channel (independently of STIM1 store depletion), leading to ERK phosphorylation and MMP-2/9 activation; EP4 forms a complex with Orai1 and TRPC1.","method":"EP4 agonist/antagonist, siRNA knockdown of EP4 and Orai1, co-immunoprecipitation of EP4-Orai1-TRPC1, Ca2+ imaging, ERK phosphorylation, MMP activity assays, in vivo lung metastasis model","journal":"Cancer science","confidence":"High","confidence_rationale":"Tier 2 — co-IP demonstrating complex formation, siRNA KD of both components with defined signaling cascade, in vivo validation","pmids":["31755615"],"is_preprint":false},{"year":2020,"finding":"EP4/AC/cAMP/PKA signaling mediates GRK2 translocation to the plasma membrane in endothelial cells, decreasing GRK2-ERK1/2 interaction and allowing ERK1/2 activation to promote PGE2-induced angiogenesis; specific GRK2 residues Lys220 and Ser685 regulate this translocation.","method":"GRK2 siRNA, GRK2 site-directed mutants (Lys220, Ser685), EP4 inhibition, PKA inhibition, HUVEC migration and tube formation assays, in vivo Matrigel angiogenesis with GRK2-deficient aortic segments","journal":"Clinical science","confidence":"High","confidence_rationale":"Tier 2 — mutagenesis of GRK2 residues plus siRNA plus pharmacological inhibition of EP4/PKA, in vitro and in vivo validation","pmids":["31967309"],"is_preprint":false},{"year":2016,"finding":"EP4 receptor activation promotes Th17 cell development in ankylosing spondylitis by upregulating IL-23R expression, suppressing the RORγt inhibitor FoxO1, and enhancing STAT3 phosphorylation, in a positive feedback loop that further increases EP4 expression.","method":"EP4-specific agonists, flow cytometry for EP4 expression in Th17 cells, Western blot, quantitative RT-PCR, ex vivo functional analysis of Th17 differentiation from AS patients","journal":"Arthritis research & therapy","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological agonist with molecular pathway analysis in primary patient cells, single lab","pmids":["31253169"],"is_preprint":false},{"year":2021,"finding":"TNF-α impairs EP4 signaling in fibroblast-like synoviocytes by recruiting TRAF2 to the membrane, which brings GRK2 to the membrane; GRK2 then separates and phosphorylates/internalizes EP4, reducing cAMP production.","method":"cAMP FRET biosensor (PM-ICUE3), TRAF2 siRNA, co-immunoprecipitation of TRAF2-GRK2, GRK2 inhibitor (paroxetine), EP4 membrane distribution assays, CIA rat model","journal":"Acta pharmacologica Sinica","confidence":"High","confidence_rationale":"Tier 2 — FRET-based live cell signaling, co-IP, TRAF2 siRNA rescue, and in vivo validation in CIA model","pmids":["33859345"],"is_preprint":false},{"year":2021,"finding":"EP4 signaling in dendritic cells (cDC2s) drives upregulation of activation markers (CD80, CD86, CD83, MHC-II) and IL-10/IL-23 production, promotes CCR7-based migration, and drives expansion of suppressive (rather than pro-inflammatory) T-cell populations.","method":"EP2 and EP4 selective antagonists on human cDC2s, flow cytometry for activation markers, cytokine measurement, T-cell co-culture assays","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 3 — pharmacological only (no genetic KD), multiple functional readouts in primary human cells","pmids":["38088451"],"is_preprint":false},{"year":2022,"finding":"EP4 receptor antagonism in cartilage promotes chondrogenesis and cartilage anabolism through the cAMP/PKA/CREB/Sox9 signaling pathway; cartilage-specific EP4 knockout or EP4 antagonist (HL-43) enhances articular cartilage regeneration and reduces fibrocartilage formation.","method":"Cartilage-specific EP4 conditional knockout, EP4 antagonist HL-43, multiple OA/cartilage defect mouse and rat models, cAMP/PKA/CREB/Sox9 pathway analysis, human cartilage explant assays","journal":"Cell discovery","confidence":"High","confidence_rationale":"Tier 2 — conditional KO plus pharmacological antagonist with defined molecular pathway (cAMP/PKA/CREB/Sox9), multiple pre-clinical models and human tissue","pmids":["35256606"],"is_preprint":false},{"year":2016,"finding":"EP4 receptor activation increases expression of oncogenic miR-526b via downstream PI3K/AKT, cAMP/PKA, and NF-κB signaling pathways in breast cancer cells.","method":"EP4 agonist/antagonist, COX-2 overexpression, PI3K-AKT inhibitors, NF-κB inhibitor, miR-526b stable overexpression/knockdown, in vivo lung colonization model","journal":"Molecular cancer research","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological + genetic interventions targeting multiple pathway nodes, single lab","pmids":["25733698"],"is_preprint":false},{"year":2017,"finding":"EP4 receptor in myeloid cells promotes colorectal tumorigenesis by driving an anti-tumorigenic M1-to-M2 phenotype switch and activating mTOR and ERK signaling; myeloid-specific EP4 deletion or pharmacological EP4 inhibition markedly reduces adenoma number and size in ApcMin/+ mice.","method":"Myeloid-specific EP4 conditional knockout crossed to ApcMin/+ mice, EP4 antagonist treatment, mTOR/ERK signaling analysis, macrophage/DC phenotyping","journal":"Oncotarget","confidence":"High","confidence_rationale":"Tier 2 — conditional myeloid-specific KO with defined molecular (mTOR, ERK) and cellular (M1/M2 phenotype) mechanisms validated pharmacologically","pmids":["26378024"],"is_preprint":false},{"year":2023,"finding":"Microglial EP4 signaling promotes diet-induced obesity by maintaining a phagocytic microglial state (elevated CD68) that reduces POMC neurite contacts with the paraventricular nucleus; microglial-specific EP4 deletion reduces weight gain, food intake, and improves insulin sensitivity in HFD-fed mice.","method":"Microglia-specific EP4 knockout mice (MG-EP4 KO), high-fat diet feeding, metabolic phenotyping, microglial CD68/phagocytosis assays, POMC neurite density analysis","journal":"Diabetes","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific conditional KO with defined cellular mechanism (microglial phagocytosis/POMC cytoarchitecture) and in vivo metabolic phenotype","pmids":["36318114"],"is_preprint":false},{"year":2001,"finding":"Canine EP4 receptor (90% amino acid identity to human EP4) couples to adenylate cyclase to increase cAMP upon PGE2 binding; short-term PGE2-induced desensitization does not require the intracellular C-terminal tail, in contrast to the reported mechanism for the human receptor.","method":"cDNA library screening, CHO-K1 transfection, Scatchard binding analysis, cAMP accumulation assays, truncated C-terminus receptor construct comparison","journal":"Prostaglandins & other lipid mediators","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro reconstitution with truncation mutant, characterizing desensitization mechanism","pmids":["11444589"],"is_preprint":false},{"year":2016,"finding":"PTGER4 promoter demethylation in AI-resistant breast cancer cells leads to EP4 upregulation, and EP4 signaling promotes estrogen-independent growth likely via ligand-independent activation of ERα cofactor CARM1.","method":"Genome-wide DNA methylation and expression analysis, EP4 knockdown, EP4 inhibitor studies, downstream CARM1 signaling assay","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 — epigenomic plus functional KD studies, but CARM1 mechanism characterized as 'exploratory'","pmids":["27869171"],"is_preprint":false},{"year":2002,"finding":"EP4 receptor mediates PGE2-stimulated osteoclastogenesis and bone resorption by inducing RANKL mRNA in osteoblasts and increasing cAMP; selective EP4 antagonism also reduces 1,25D- and PTH-stimulated osteoclast formation, suggesting EP4 plays a general role in osteoclastic bone resorption.","method":"EP4-selective antagonist (EP4RA) in murine marrow cultures and fetal rat long bone organ cultures, TRAP+ multinucleated cell counting, cAMP measurement, RANKL mRNA quantification, 45Ca release assay","journal":"Bone","confidence":"Medium","confidence_rationale":"Tier 2 — pharmacological antagonism with multiple functional endpoints in primary bone cell cultures","pmids":["11792579"],"is_preprint":false},{"year":2021,"finding":"EP4 and EP2 signaling show distinct cAMP kinetics in dendritic cells: EP4 induces transient cAMP and EP2 induces sustained cAMP; simultaneous EP2+EP4 stimulation attenuates cAMP production (crosstalk); EP4 primarily mediates podosome dissolution in DCs; both receptors require intact microtubule networks for efficient signaling.","method":"Quantitative live-cell cAMP imaging (FRET), podosome dissolution assays, microtubule disruption, EP2/EP4 selective antagonists","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 — quantitative live-cell imaging with pharmacological tools revealing distinct kinetics and crosstalk mechanisms","pmids":["33643295"],"is_preprint":false}],"current_model":"PTGER4 encodes EP4, a class A Gαs-coupled GPCR for prostaglandin E2 that primarily signals through cAMP/PKA but also engages Gαi, β-arrestin, PI3K, EGFR transactivation, and Rap/ERK cascades in a ligand- and cell-type-dependent (functionally selective) manner; its cryo-EM structure bound to Gs reveals a distinct TM6 displacement and Gs C-terminal hook engagement mediated by conserved prostanoid receptor residues, and its cell-type-specific roles—established through conditional knockouts and in vitro reconstitution—include regulation of ductus arteriosus closure, renin secretion, blood pressure via endothelial eNOS/AMPK, bone homeostasis via RANKL/osteoclastogenesis, myocardial protection, microglial phagocytic tone, innate immune suppression (NLRP3, NF-κB, mregDC-Treg axis), and cancer cell migration/invasion via EP4–βArrestin1–c-Src and EP4–PI3K–Orai1–ERK signalomes."},"narrative":{"teleology":[{"year":1998,"claim":"The first loss-of-function study established that EP4 is non-redundant in vivo by showing it is required for neonatal ductus arteriosus closure, answering whether individual EP receptors have essential physiological roles.","evidence":"EP4 knockout mice exhibiting patent ductus arteriosus and neonatal lethality, confirmed by in situ hybridization","pmids":["9600059"],"confidence":"High","gaps":["Downstream signaling pathway mediating ductus closure not identified","Whether EP4 acts on smooth muscle or endothelium of the ductus not resolved"]},{"year":2001,"claim":"Cloning and reconstitution of canine EP4 confirmed canonical Gαs/cAMP coupling and revealed that short-term desensitization does not require the intracellular C-terminal tail, raising questions about species-specific desensitization mechanisms.","evidence":"CHO-K1 transfection of full-length and C-terminally truncated EP4, cAMP accumulation and Scatchard binding","pmids":["11444589"],"confidence":"Medium","gaps":["Contradiction with human EP4 C-tail desensitization reports not resolved","GRK/β-arrestin involvement not tested"]},{"year":2003,"claim":"Demonstration that EP4 activates PI3K and ERK1/2 independently of its canonical cAMP/PKA pathway established the principle of EP4 signaling bifurcation, distinguishing it from EP2.","evidence":"Pharmacological EP-subtype selective ligands and signaling assays in transfected cells measuring cAMP, PI3K, ERK, and EGR-1","pmids":["14607241"],"confidence":"Medium","gaps":["Adaptor proteins linking EP4 to PI3K not identified","Cell-type generalizability not established"]},{"year":2004,"claim":"Multiple groups showed EP4 drives pathologically significant outcomes—colon cancer proliferation via PI3K/ERK, cardiac hypertrophy via EGFR transactivation, and bone resorption via RANKL induction—establishing EP4 as a pleiotropic effector across tissues.","evidence":"EP4-selective agonists/antagonists in colon adenocarcinoma proliferation assays, neonatal myocyte EGFR phosphorylation/MAPK assays, and osteoblast RANKL mRNA quantification","pmids":["15123663","15626689","11792579"],"confidence":"High","gaps":["Direct structural basis for EP4 coupling to distinct G proteins unknown","Whether EGFR transactivation requires β-arrestin scaffold not tested"]},{"year":2006,"claim":"EP4 was placed in both the pain sensitization circuit (DRG neurons) and the metastatic cascade (breast cancer), broadening its pathological relevance to sensory neurobiology and oncology.","evidence":"Intrathecal EP4 shRNA knockdown plus antagonist in DRG electrophysiology and behavioral pain; EP4 antagonism in syngeneic breast cancer metastasis models","pmids":["16966471","16540639"],"confidence":"High","gaps":["EP4 interaction partners in DRG not mapped","Breast cancer metastasis study lacked genetic confirmation"]},{"year":2008,"claim":"Hematopoietic EP4 deletion and transcriptional regulation studies established that EP4 sustains macrophage survival via PI3K/Akt/NF-κB in atherosclerosis and that PTGER4 transcription is controlled by Sp-1 binding sites subject to ERK-mediated phosphorylation.","evidence":"EP4−/− fetal liver chimeras in LDLR−/− mice on Western diet; EP4 promoter luciferase reporters with Sp-1 site mutations and ChIP","pmids":["19041765","18346464"],"confidence":"High","gaps":["Whether Sp-1 regulation is tissue-specific not addressed","Atherosclerosis findings limited to early lesion stage"]},{"year":2009,"claim":"Quantitative BRET assays revealed that EP4 exhibits ligand-dependent functional selectivity across Gαs, Gαi, and β-arrestin pathways, and conditional KO identified fibroblast EP4 as the driver of RANKL-dependent osteoclastogenesis.","evidence":"BRET assays with panel of ligands in living cells; FSP1-Cre EP4 conditional KO with RANKL mRNA and osteolysis assays","pmids":["19584306","19419302","19880670"],"confidence":"High","gaps":["Structural basis for biased agonism at EP4 not determined","Whether β-arrestin bias translates to differential in vivo outcomes untested"]},{"year":2010,"claim":"The EP4–β-arrestin1–c-Src signaling complex was identified as the mechanism driving PGE2-induced cancer cell migration, establishing a G protein–independent oncogenic signalome for EP4.","evidence":"shRNA knockdown of EP4 and β-arrestin1 with c-Src kinase activity and cell migration assays in lung cancer cells","pmids":["20353998"],"confidence":"High","gaps":["Whether EP4–β-arrestin1–c-Src complex forms constitutively or only upon ligand binding not resolved","Stoichiometry of the complex unknown"]},{"year":2011,"claim":"EP4 was established as the key receptor mediating COX-2–dependent renin secretion by macula densa and PGE2-driven angiogenesis via PKA/Rap1/eNOS, broadening its cardiovascular roles.","evidence":"Epistasis panel of COX-2, mPGES1/2, EP2, EP4, IP KO mice with furosemide challenge; siRNA knockdown of EP4/PKA subunits in tube formation, aortic ring, and Matrigel plug assays","pmids":["21835766","21926356"],"confidence":"High","gaps":["Whether EP4 directly signals in juxtaglomerular cells or acts indirectly via macula densa not fully dissected","Relative contribution of eNOS vs Rap1A in angiogenesis not quantified"]},{"year":2012,"claim":"Post-transcriptional regulation of EP4 by miR-101 was demonstrated, and EP4 signaling in vascular smooth muscle was linked to MMP-2/IL-6 and abdominal aortic aneurysm progression.","evidence":"Luciferase reporter with wild-type and mutant EP4 3′-UTR for miR-101 binding; EP4 antagonist and EP4+/− mice in multiple AAA models","pmids":["22353936","22570740"],"confidence":"High","gaps":["Whether miR-101 regulation of EP4 is tissue-restricted not determined","Direct EP4 target cells in the AAA wall not identified by cell-type-specific KO"]},{"year":2014,"claim":"Conditional microglial EP4 deletion in an Alzheimer's disease model revealed EP4 as an anti-inflammatory and pro-phagocytic receptor in microglia, suppressing IRF1/IRF7/NF-κB while enhancing Aβ clearance.","evidence":"Microglial EP4 conditional KO in APP-PS1 mice with microarray profiling and Aβ deposition measurement; EP4 agonist in cultured microglia","pmids":["24760848"],"confidence":"High","gaps":["Signaling intermediary between EP4 and IRF1/IRF7 not identified","Long-term disease progression effects not reported"]},{"year":2016,"claim":"EP4 was linked to cancer stemness via PI3K/AKT–NOTCH/WNT in breast cancer, to pro-tumorigenic myeloid M1-to-M2 switching via mTOR/ERK in colorectal cancer, and to Th17 differentiation in ankylosing spondylitis, expanding its roles in adaptive immunity and tumor immunology.","evidence":"EP4 antagonist/siRNA with PI3K/NOTCH/WNT inhibitors in breast CSC assays and xenografts; myeloid-specific EP4 cKO in ApcMin/+ mice; EP4 agonist with IL-23R/STAT3/FoxO1 analysis in patient Th17 cells","pmids":["27301070","26378024","31253169"],"confidence":"High","gaps":["Whether EP4-driven CSC properties require cell-autonomous signaling or microenvironmental cues not resolved","Th17 study based on pharmacological agonist without genetic validation"]},{"year":2017,"claim":"Endothelial-specific EP4 deletion demonstrated that EP4 maintains vascular barrier integrity through Rap1/Rac1 and suppresses NF-κB-driven adhesion molecule expression, providing a mechanistic basis for EP4's protective role in acute lung injury.","evidence":"Endothelial EP4 cKO mice in ALI models, Rap1/Rac1 GTPase activity assays, EP4 siRNA and pharmacological inhibition","pmids":["28428256"],"confidence":"High","gaps":["Whether EP4 barrier protection involves junctional complex remodeling beyond VE-cadherin not fully explored","Role of β-arrestin in endothelial EP4 signaling not addressed"]},{"year":2019,"claim":"Endothelial EP4 was shown to regulate systemic blood pressure via AMPK-dependent eNOS phosphorylation and to protect against myocardial ischemia-reperfusion injury by maintaining microvascular perfusion, establishing EP4 as a cardioprotective endothelial receptor.","evidence":"EC-specific EP4 KO and overexpression mice with BP measurement and eNOS/AMPK pathway analysis; mPges-1 KO and EC-EP4 KO mice in MI/R models with vascular reactivity and leukocyte adhesion assays","pmids":["32641583","31015404"],"confidence":"High","gaps":["Whether AMPK activation by EP4 is cAMP-dependent or involves alternative upstream signals not fully resolved","Specific prostanoid source (COX-1 vs COX-2) feeding endothelial EP4 in different vascular beds not systematically compared"]},{"year":2019,"claim":"EP4 was found to form a complex with Orai1/TRPC1 channels to drive PI3K-dependent calcium influx and ERK-mediated MMP activation in oral cancer migration, revealing a non-canonical ion channel partnership.","evidence":"Co-immunoprecipitation of EP4–Orai1–TRPC1, siRNA knockdown, calcium imaging, in vivo lung metastasis model","pmids":["31755615"],"confidence":"High","gaps":["Whether EP4–Orai1 interaction is direct or mediated by adaptor proteins not determined","STIM1-independent Orai1 activation mechanism not structurally characterized"]},{"year":2020,"claim":"The cryo-EM structure of EP4 bound to Gs at 3.3 Å provided the first atomic-level view of prostanoid receptor–G protein coupling, revealing a restrained TM6 outward displacement and a Gs C-terminal hook structure mediated by conserved residues Phe54 and Trp327.","evidence":"Cryo-EM structure determination at 3.3 Å global resolution","pmids":["33264604"],"confidence":"High","gaps":["No structure of EP4 bound to Gαi or β-arrestin available","Ligand-bound inactive state structure not determined","Structural basis for functional selectivity not addressed"]},{"year":2021,"claim":"Macrophage-specific PTGER4 deletion revealed EP4/MAPK/CXCL1 as a paracrine axis driving epithelial regeneration in colitis, and TNF-α was shown to impair EP4 signaling via TRAF2-mediated GRK2 recruitment and receptor internalization.","evidence":"Csf1r-iCre Ptger4fl/fl conditional KO in DSS colitis with organoid assays and CXCL1 pathway analysis; cAMP FRET biosensor, TRAF2 siRNA, and co-IP of TRAF2-GRK2 in FLS with CIA model validation","pmids":["33558271","33859345"],"confidence":"High","gaps":["Whether TRAF2-GRK2 desensitization mechanism operates in cell types beyond synoviocytes not tested","Epithelial CXCL1 receptor identity not confirmed"]},{"year":2022,"claim":"Cell-type-specific EP4 functions in bone were dissected: osteoclast EP4 promotes OA via Gαs/PI3K/AKT/MAPK-driven osteoclastogenesis and sensory innervation, while cartilage EP4 antagonism promotes chondrogenesis via cAMP/PKA/CREB/Sox9, and tumor EP4 drives dual pro-inflammatory and immunosuppressive programs.","evidence":"Osteoclast-specific EP4 cKO and cartilage-specific EP4 cKO in OA models with pathway analysis; EP2/EP4 antagonists with scRNAseq in tumor models","pmids":["35260562","35256606","35675777"],"confidence":"High","gaps":["How EP4 activation produces opposite cAMP/PKA outcomes in osteoclasts vs chondrocytes not mechanistically explained","EP4 vs EP2 contribution to mregDC-Treg axis not fully separated"]},{"year":2023,"claim":"Microglial EP4 was identified as a metabolic regulator: it maintains a phagocytic microglial state in the hypothalamus that reduces POMC neurite density and promotes diet-induced obesity, revealing a neuroimmune metabolic role.","evidence":"Microglia-specific EP4 knockout mice on high-fat diet with metabolic phenotyping, CD68/phagocytosis assays, and POMC neurite analysis","pmids":["36318114"],"confidence":"High","gaps":["Whether EP4-driven microglial phagocytosis targets POMC neurites directly or acts indirectly not determined","Downstream signaling pathway (cAMP vs PI3K) mediating phagocytic phenotype not dissected"]},{"year":null,"claim":"Key unresolved questions include the structural basis for EP4 functional selectivity (no Gαi- or β-arrestin-bound structures), the molecular identity of adaptors linking EP4 to PI3K, and how identical cAMP signaling produces opposing outcomes in different cell types (e.g., osteoclasts vs chondrocytes).","evidence":"","pmids":[],"confidence":"High","gaps":["No EP4–Gαi or EP4–β-arrestin structure available","Adaptor linking EP4 to PI3K unknown","Cell-type-specific signal interpretation mechanisms not characterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,11,22]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3,12,24]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[22,27,33]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,6,11,21,22]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[3,9,12,34,37]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[1]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[4,5,8,30,37]}],"complexes":[],"partners":["ARRB1","SRC","ORAI1","TRPC1","GRK2","TRAF2","GNAS"],"other_free_text":[]},"mechanistic_narrative":"PTGER4 encodes the prostaglandin E2 receptor EP4, a class A GPCR that serves as a central transducer of PGE2 signaling across cardiovascular, immune, skeletal, neural, and epithelial systems. EP4 primarily couples to Gαs to elevate cAMP but also engages Gαi, β-arrestin, PI3K/AKT, Rap1/ERK, and EGFR transactivation pathways in a ligand- and cell-type-dependent manner, as demonstrated by BRET-based functional selectivity assays and cryo-EM structural determination of the EP4–Gs complex revealing a distinctive TM6 displacement and Gs C-terminal hook engagement [PMID:19584306, PMID:33264604]. Cell-type-specific conditional knockouts have established essential roles for EP4 in neonatal ductus arteriosus closure, macula densa–mediated renin secretion, endothelial eNOS/AMPK-dependent blood pressure regulation, macrophage-driven mucosal healing via CXCL1/MAPK, microglial phagocytic tone affecting both neuroinflammation and hypothalamic energy balance, fibroblast RANKL-driven osteoclastogenesis, and NLRP3 inflammasome suppression in macrophages [PMID:9600059, PMID:21835766, PMID:32641583, PMID:33558271, PMID:36318114, PMID:19419302, PMID:25917098]. In cancer, EP4 promotes tumor cell migration and stemness through β-arrestin1–c-Src, PI3K–Orai1–ERK, and PI3K/AKT–NOTCH/WNT signalomes, while in myeloid cells it drives a pro-tumorigenic M2 polarization via mTOR/ERK signaling [PMID:20353998, PMID:31755615, PMID:27301070, PMID:26378024]."},"prefetch_data":{"uniprot":{"accession":"P35408","full_name":"Prostaglandin E2 receptor EP4 subtype","aliases":["Prostanoid EP4 receptor"],"length_aa":488,"mass_kda":53.1,"function":"Receptor for prostaglandin E2 (PGE2). The activity of this receptor is mediated by G(s) proteins that stimulate adenylate cyclase. Has a relaxing effect on smooth muscle. May play an important role in regulating renal hemodynamics, intestinal epithelial transport, adrenal aldosterone secretion, and uterine function","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/P35408/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PTGER4","classification":"Not Classified","n_dependent_lines":5,"n_total_lines":1208,"dependency_fraction":0.0041390728476821195},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PTGER4","total_profiled":1310},"omim":[{"mim_id":"615770","title":"ATRIAL FIBRILLATION, FAMILIAL, 15; ATFB15","url":"https://www.omim.org/entry/615770"},{"mim_id":"612381","title":"INFLAMMATORY BOWEL DISEASE 23; IBD23","url":"https://www.omim.org/entry/612381"},{"mim_id":"612380","title":"INFLAMMATORY BOWEL DISEASE 22; IBD22","url":"https://www.omim.org/entry/612380"},{"mim_id":"612354","title":"INFLAMMATORY BOWEL DISEASE 21; IBD21","url":"https://www.omim.org/entry/612354"},{"mim_id":"612288","title":"INFLAMMATORY BOWEL DISEASE 20; IBD20","url":"https://www.omim.org/entry/612288"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"bone marrow","ntpm":81.0},{"tissue":"pancreas","ntpm":51.1}],"url":"https://www.proteinatlas.org/search/PTGER4"},"hgnc":{"alias_symbol":["EP4"],"prev_symbol":[]},"alphafold":{"accession":"P35408","domains":[{"cath_id":"1.20.1070.10","chopping":"22-232_258-350","consensus_level":"high","plddt":87.515,"start":22,"end":350}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P35408","model_url":"https://alphafold.ebi.ac.uk/files/AF-P35408-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P35408-F1-predicted_aligned_error_v6.png","plddt_mean":70.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PTGER4","jax_strain_url":"https://www.jax.org/strain/search?query=PTGER4"},"sequence":{"accession":"P35408","fasta_url":"https://rest.uniprot.org/uniprotkb/P35408.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P35408/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P35408"}},"corpus_meta":[{"pmid":"14607241","id":"PMC_14607241","title":"EP2 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\"confidence_rationale\": \"Tier 2 — multiple signaling readouts with pharmacological tools, single review-style paper summarizing original experimental findings\",\n      \"pmids\": [\"14607241\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"EP4 receptor knockout mice develop patent ductus arteriosus and neonatal death, establishing that EP4 signaling is required for closure of the ductus arteriosus in neonatal circulatory adaptation; EP4 mRNA was confirmed by in situ hybridization in the ductus.\",\n      \"method\": \"Gene targeting (knockout mice), histological examination, in situ hybridization\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined physiological phenotype and direct localization data, foundational paper with 253 citations\",\n      \"pmids\": [\"9600059\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"EP4 receptor on primary sensory DRG neurons mediates PGE2-induced sensitization of capsaicin-evoked currents and inflammatory pain hypersensitivity; EP4 levels increase in DRG after peripheral inflammation.\",\n      \"method\": \"Intrathecal shRNA knockdown, EP4 antagonist (AH23848), in vitro DRG neuron electrophysiology, behavioral pain assays\",\n      \"journal\": \"The Journal of pharmacology and experimental therapeutics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — orthogonal methods (shRNA KD + pharmacological antagonist + in vitro electrophysiology + in vivo behavioral), strong mechanistic placement\",\n      \"pmids\": [\"16966471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"EP4 receptor mediates PGE2 inhibition of NLRP3 inflammasome activation in human macrophages through elevation of intracellular cAMP, independently of PKA or Epac.\",\n      \"method\": \"EP4-specific agonist/antagonist pharmacology, EP4 knockdown (siRNA), adenylate cyclase inhibitor, cAMP measurement, NLRP3 inflammasome activation assays in human monocyte-derived macrophages\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (agonist, antagonist, siRNA KD, AC inhibitor, cAMP measurement) in primary human cells\",\n      \"pmids\": [\"25917098\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"EP4 receptor mediates PGE2-induced colon adenocarcinoma cell proliferation via PI3K/ERK activation; selective EP4 agonist PGE1-OH rescues anti-proliferative effects of COX inhibition through this pathway.\",\n      \"method\": \"Selective EP receptor agonists, PI3K/ERK inhibitors, cell proliferation assays, in vivo tumor model\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological + in vivo rescue experiments, replicated across multiple conditions\",\n      \"pmids\": [\"15123663\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"EP4 receptor promotes lung cancer cell migration via a signaling complex of EP4–βArrestin1–c-Src; knockdown of βArrestin1 impairs PGE2-induced c-Src activation and cell migration.\",\n      \"method\": \"EP subtype-selective agonists, shRNA knockdown of EP4 and βArrestin1, c-Src kinase activity assays, cell migration assays\",\n      \"journal\": \"Molecular cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal shRNA KD of two pathway components with defined migration phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"20353998\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"EP4 receptor couples to Gαs to activate PKA catalytic subunit γ, which promotes in vitro tube formation, ex vivo vessel outgrowth, and in vivo angiogenesis; downstream PKA substrates Rap1A, HSPB6, and eNOS promote angiogenesis while RhoA and GSK3β are inhibitory.\",\n      \"method\": \"EP subtype-selective agonists/antagonists, siRNA knockdown of EP4 and PKA subunits, tube formation and aortic ring assays, in vivo Matrigel plug assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple siRNA knockdowns of pathway components, in vitro and in vivo angiogenesis assays, strong mechanistic placement\",\n      \"pmids\": [\"21926356\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PGE2 activates EP4 on osteoclasts to promote migration, osteoclastogenesis, Netrin-1/CGRP sensory innervation, and PDGF-BB-driven type H vessel formation in subchondral bone via Gαs/PI3K/AKT/MAPK signaling, promoting OA progression.\",\n      \"method\": \"Osteoclast-specific EP4 conditional knockout (EP4LysM), pharmacological EP4 antagonist (HL-43), in vitro migration/osteoclastogenesis assays, OA mouse models, signaling pathway inhibitors\",\n      \"journal\": \"Bone research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — conditional KO validated with pharmacological antagonist and in vitro mechanistic signaling assays across multiple models\",\n      \"pmids\": [\"35260562\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"COX-2/EP4 signaling induces cancer stem-like cells in breast cancer via PI3K/AKT activation of NOTCH/WNT signaling pathways.\",\n      \"method\": \"COX-2 overexpression, EP4 antagonist and siRNA knockdown, PI3K/AKT inhibitors, NOTCH/WNT inhibitors, spheroid/ALDH assays, xenograft tumor models\",\n      \"journal\": \"Stem cells\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple pharmacological and genetic interventions with defined pathway placement in vitro and in vivo\",\n      \"pmids\": [\"27301070\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PGE2-EP2/EP4 signaling simultaneously promotes NF-κB-driven active inflammation in myeloid cells and drives the mregDC-Treg axis for immunosuppression in the tumor microenvironment.\",\n      \"method\": \"Immune checkpoint inhibitor-insensitive mouse cancer model, single-cell RNA sequencing, EP2/EP4 antagonists\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — scRNAseq plus pharmacological intervention, but EP4-specific contribution not fully separated from EP2\",\n      \"pmids\": [\"35675777\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Macrophage EP4 deficiency increases sensitivity to apoptotic stimuli (palmitic acid, free cholesterol) by suppressing PI3K/Akt and NF-κB pathway activity, reducing early atherosclerosis.\",\n      \"method\": \"Fetal liver cell transplantation into LDLR-/- mice (EP4-/- hematopoietic chimeras), Western diet feeding, apoptosis assays, signaling pathway analysis\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean hematopoietic-specific KO with defined molecular mechanisms (p-Akt, p-Bad, NF-κB) and in vivo atherosclerosis phenotype\",\n      \"pmids\": [\"19041765\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"EP4 receptor exhibits functional selectivity: PGE2 preferentially activates Gαs; PGF2α preferentially activates Gαi1; PGE1-alcohol is biased toward β-arrestin. EP4 also couples to pertussis toxin-sensitive Gαi and β-arrestin pathways.\",\n      \"method\": \"BRET-based assays for Gαs, Gαi, and β-arrestin activation in living cells with a panel of EP4 ligands\",\n      \"journal\": \"The Journal of pharmacology and experimental therapeutics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — quantitative BRET assays systematically comparing multiple ligands across three signaling pathways in living cells\",\n      \"pmids\": [\"19584306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Microglial EP4 stimulation suppresses Aβ42-induced inflammatory gene expression (targeting IRF1, IRF7, NF-κB pathways) and potentiates Aβ42 phagocytosis; conditional microglial EP4 deletion in APP-PS1 mice increases inflammatory gene expression and Aβ deposition at early pathology stages.\",\n      \"method\": \"EP4 agonist treatment of cultured microglia, microarray gene expression analysis, conditional microglial EP4 knockout in APP-PS1 mice, oxidative protein modification and Aβ measurement\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO in disease model with multiple molecular readouts and in vitro mechanistic validation\",\n      \"pmids\": [\"24760848\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"EP4 receptor mediates PGE2-induced relaxation of human airways (bronchodilation); EP2 mediates this effect in guinea pig and mouse but not human airways.\",\n      \"method\": \"Pharmacological EP-subtype selective agonists/antagonists in isolated airway preparations from multiple species; EP2-knockout mice\",\n      \"journal\": \"Thorax\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological plus genetic (KO mice) across multiple species with functional tissue readout\",\n      \"pmids\": [\"21606476\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"EP4 receptor mediates PGE2-induced suppression of 3T3-L1 preadipocyte differentiation by increasing cAMP and suppressing expression of differentiation markers (resistin, PPARγ).\",\n      \"method\": \"EP4-selective agonist (AE1-329) and antagonist (AE3-208), cAMP measurement, RT-PCR for differentiation markers\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — pharmacological agonist/antagonist approach with molecular readouts, single lab\",\n      \"pmids\": [\"15336573\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PTGER4-expressing intestinal macrophages secrete CXCL1 via MAPK signaling upon PGE2 stimulation, driving epithelial cell differentiation and proliferation to support mucosal healing; macrophage-specific PTGER4 deletion impairs epithelial barrier regeneration in DSS colitis.\",\n      \"method\": \"Conditional macrophage-specific Ptger4 knockout (Csf1r-iCre Ptger4fl/fl), DSS colitis model, MAPK pathway analysis, epithelial organoid differentiation assays, liposome-mediated therapeutic targeting\",\n      \"journal\": \"Gut\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with defined molecular mechanism (MAPK/CXCL1) and multiple functional readouts including rescue experiment\",\n      \"pmids\": [\"33558271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"EP4 receptor induces cardiac myocyte hypertrophy independently of PKA by transactivating EGFR, leading to p42/44 MAPK activation and increased protein synthesis; the EP4 antagonist blocks EGFR phosphorylation and MAPK activation.\",\n      \"method\": \"EP4 antagonist, EGFR selective inhibitor (AG-1478), PKA inhibitor, immunoprecipitation for EGFR phosphorylation, [3H]leucine incorporation, p42/44 MAPK phosphorylation assays in neonatal ventricular myocytes\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple pharmacological inhibitors targeting distinct pathway nodes, orthogonal functional readouts\",\n      \"pmids\": [\"15626689\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"EP4 signaling in fibroblasts (FSP1+ cells) upregulates RANKL expression in response to PGE2 or wear debris, stimulating osteoclastogenesis and periprosthetic osteolysis; conditional fibroblast-specific EP4 knockout abolishes this response.\",\n      \"method\": \"Conditional knockout of EP4 in FSP1+ fibroblasts, in vitro PGE2 stimulation with RANKL mRNA measurement, osteoclast and osteolysis assays, comparison with EP1-/- and EP2-/- mice\",\n      \"journal\": \"Journal of bone and mineral research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional cell-type-specific KO with defined molecular (RANKL) and cellular (osteoclastogenesis) readouts, supported by comparison with other EP KO lines\",\n      \"pmids\": [\"19419302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Endothelial EP4 receptor is essential for blood pressure homeostasis by promoting eNOS phosphorylation at Ser1177 and NO production via the AMPK pathway; EC-specific EP4 knockout elevates blood pressure and reduces vasorelaxant responses.\",\n      \"method\": \"Endothelial-specific EP4 knockout and overexpression mice, eNOS phosphorylation assays, AMPK pathway inhibitors, blood pressure measurements, vascular reactivity assays\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO and overexpression with defined molecular mechanism (AMPK/eNOS/NO) and in vivo BP phenotype; L-NAME rescue confirms eNOS dependence\",\n      \"pmids\": [\"32641583\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"COX-1/mPGES-1-derived PGE2 acting on endothelial EP4 receptor constrains myocardial ischemia-reperfusion injury by maintaining microvascular perfusion and suppressing leukocyte-endothelial interactions; endothelium-restricted EP4 deletion exacerbates MI/R injury.\",\n      \"method\": \"mPges-1 knockout mice, endothelium-restricted Ep4 knockout mice, in vivo and ex vivo vascular reactivity assays, leukocyte-endothelial interaction assays, EP4 agonist treatment\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple conditional KO models with mechanistic pathway placement (EP4-mediated microvascular protection), rescue with PGE analog\",\n      \"pmids\": [\"31015404\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"EP4 receptor is the primary mediator of COX-2-dependent renin stimulation by the macula densa; EP4 knockout mice show ~70% reduction in furosemide-induced renin mRNA upregulation, while EP2 and IP receptor knockouts are unaffected.\",\n      \"method\": \"Panel of gene-targeted mice (COX-2, mPGES1, mPGES2, EP2, EP4, IP knockouts), furosemide challenge, real-time RT-PCR for renin mRNA\",\n      \"journal\": \"American journal of physiology. Renal physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — comprehensive epistasis panel with multiple KO lines establishing EP4 as the key receptor in the pathway\",\n      \"pmids\": [\"21835766\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"EP4 receptor in neonatal ventricular myocytes signals through a PKA→Rap1→ERK1/2→p90RSK cascade to regulate brain natriuretic peptide, c-Fos, and EGR-1 expression and to increase cell size; Rap dominant-negative mutant blocks PGE2-induced ERK activation.\",\n      \"method\": \"Dominant negative Rap mutant transfection, PKA inhibitor, Rap and p90RSK activation assays, BNP promoter reporter, cell size measurement in neonatal ventricular myocytes with EP4 overexpression\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — dominant negative mutant + pharmacological inhibitors + overexpression with multiple molecular readouts\",\n      \"pmids\": [\"19880670\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cryo-EM structure of EP4 coupled to heterotrimeric Gs at 3.3 Å resolution reveals that compared to other class A GPCRs, EP4 TM6 shifts less outward, but the Gs C-terminal helix inserts toward TM2 and forms an extended hook structure, mediated by conserved prostanoid receptor residues Phe54(2.39) and Trp327(7.51).\",\n      \"method\": \"Cryo-electron microscopy (cryo-EM) structure determination at 3.3 Å global resolution\",\n      \"journal\": \"Structure\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct structural determination by cryo-EM with functional context\",\n      \"pmids\": [\"33264604\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"EP4 receptor mediates barrier-protective effects of prostaglandin A2 on lung endothelial cells via Rap1/Rac1 GTPase and PKA targets (VE-cadherin, p120-catenin, ZO-1, cortactin, VASP) and suppresses NF-κB-driven ICAM1/VCAM1 expression; endothelial-specific EP4 knockout abolishes protection in ALI models.\",\n      \"method\": \"EP4 siRNA and pharmacological inhibition, endothelial-specific EP4 knockout mice, Rap1/Rac1 GTPase activity assays, permeability measurements, two in vivo ALI models\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO plus siRNA plus pharmacological inhibition with defined molecular mechanisms and in vivo validation\",\n      \"pmids\": [\"28428256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"EP4 receptor functions as a tumor suppressor in B cells by acting as a negative feedback regulator of BCR-mediated proliferation; EP4 knockdown accelerates B cell lymphoma growth in mice while EP4 overexpression is protective.\",\n      \"method\": \"Stable EP4 shRNA knockdown and overexpression in B cell lymphoma lines, in vivo tumor spread assays, microarray gene expression\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — both loss- and gain-of-function with in vivo validation and transcriptome analysis\",\n      \"pmids\": [\"19075289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"EP4 receptor antagonism inhibits breast cancer metastasis; mammary tumor cells migrate in response to PGE2 via a cAMP-elevating EP4 mechanism; EP4 antagonists block both tumor migration and proliferation in vitro.\",\n      \"method\": \"EP4-selective antagonists (AH23848, ONO-AE3-208), in vivo metastasis models (syngeneic), in vitro migration and proliferation assays\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological antagonism with in vivo and in vitro validation, no genetic KD confirmation\",\n      \"pmids\": [\"16540639\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"EP4 receptor signaling in aortic smooth muscle cells increases MMP-2 activity and IL-6 production; genetic or pharmacological EP4 inhibition attenuates AAA formation in multiple mouse models by reducing MMP activity and cytokine release.\",\n      \"method\": \"EP4 agonist stimulation of human ASMCs, MMP-2 activity assays, EP4 antagonist (ONO-AE3-208) in ApoE-/- AAA mouse model, EP4+/- mice, CaCl2 AAA model, human AAA tissue organ cultures\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple mouse models (genetic and pharmacological) plus human tissue explants with defined molecular mechanisms\",\n      \"pmids\": [\"22570740\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"EP4 receptor activation or inhibition regulates colonic epithelial barrier integrity; apical EP4 receptors on T84 cells mediate PGE2-induced barrier disruption, and prostaglandin transporter (PGT) vectorially transports basolateral PGE2 to apical EP4 receptors.\",\n      \"method\": \"EP4 receptor antagonist, immunolocalization of EP4 in polarized T84 cells and human biopsies, PGT siRNA/inhibitor, [3H]-PGE2 vectorial transport assays\",\n      \"journal\": \"American journal of physiology. Gastrointestinal and liver physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological antagonism plus siRNA KD plus radiotracer transport assays demonstrating vectorial transport mechanism\",\n      \"pmids\": [\"20813914\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"miR-101 post-transcriptionally represses EP4 receptor expression by binding to the 3'-UTR of EP4 mRNA; loss of miR-101 leads to elevated EP4 and increased colon cancer cell proliferation and motility.\",\n      \"method\": \"Luciferase reporter assay with EP4 3'-UTR (wild-type and mutant), miR-101 transfection, EP4 rescue co-transfection, cell proliferation and migration assays\",\n      \"journal\": \"Cancer biology & therapy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — luciferase reporter with UTR mutation, miRNA transfection, rescue experiment establishing direct post-transcriptional regulation\",\n      \"pmids\": [\"22353936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"EP4 expression is regulated transcriptionally by Sp-1 binding sites in the human EP4 promoter (-197 to -160); troglitazone suppresses EP4 by activating ERK-mediated Sp-1 phosphorylation, which reduces Sp-1 DNA binding.\",\n      \"method\": \"Luciferase reporter with EP4 promoter constructs, Sp-1 site mutations, MEK-1/ERK inhibitor (PD98059), immunoprecipitation-Western for Sp-1 phosphorylation, ChIP assay\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — promoter mutagenesis, ChIP, and phosphorylation assays in orthogonal combination\",\n      \"pmids\": [\"18346464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"EP4 receptor promotes oral cancer cell migration by activating PI3K and inducing Ca2+ influx through Orai1 channel (independently of STIM1 store depletion), leading to ERK phosphorylation and MMP-2/9 activation; EP4 forms a complex with Orai1 and TRPC1.\",\n      \"method\": \"EP4 agonist/antagonist, siRNA knockdown of EP4 and Orai1, co-immunoprecipitation of EP4-Orai1-TRPC1, Ca2+ imaging, ERK phosphorylation, MMP activity assays, in vivo lung metastasis model\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — co-IP demonstrating complex formation, siRNA KD of both components with defined signaling cascade, in vivo validation\",\n      \"pmids\": [\"31755615\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"EP4/AC/cAMP/PKA signaling mediates GRK2 translocation to the plasma membrane in endothelial cells, decreasing GRK2-ERK1/2 interaction and allowing ERK1/2 activation to promote PGE2-induced angiogenesis; specific GRK2 residues Lys220 and Ser685 regulate this translocation.\",\n      \"method\": \"GRK2 siRNA, GRK2 site-directed mutants (Lys220, Ser685), EP4 inhibition, PKA inhibition, HUVEC migration and tube formation assays, in vivo Matrigel angiogenesis with GRK2-deficient aortic segments\",\n      \"journal\": \"Clinical science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis of GRK2 residues plus siRNA plus pharmacological inhibition of EP4/PKA, in vitro and in vivo validation\",\n      \"pmids\": [\"31967309\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"EP4 receptor activation promotes Th17 cell development in ankylosing spondylitis by upregulating IL-23R expression, suppressing the RORγt inhibitor FoxO1, and enhancing STAT3 phosphorylation, in a positive feedback loop that further increases EP4 expression.\",\n      \"method\": \"EP4-specific agonists, flow cytometry for EP4 expression in Th17 cells, Western blot, quantitative RT-PCR, ex vivo functional analysis of Th17 differentiation from AS patients\",\n      \"journal\": \"Arthritis research & therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological agonist with molecular pathway analysis in primary patient cells, single lab\",\n      \"pmids\": [\"31253169\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"TNF-α impairs EP4 signaling in fibroblast-like synoviocytes by recruiting TRAF2 to the membrane, which brings GRK2 to the membrane; GRK2 then separates and phosphorylates/internalizes EP4, reducing cAMP production.\",\n      \"method\": \"cAMP FRET biosensor (PM-ICUE3), TRAF2 siRNA, co-immunoprecipitation of TRAF2-GRK2, GRK2 inhibitor (paroxetine), EP4 membrane distribution assays, CIA rat model\",\n      \"journal\": \"Acta pharmacologica Sinica\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — FRET-based live cell signaling, co-IP, TRAF2 siRNA rescue, and in vivo validation in CIA model\",\n      \"pmids\": [\"33859345\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"EP4 signaling in dendritic cells (cDC2s) drives upregulation of activation markers (CD80, CD86, CD83, MHC-II) and IL-10/IL-23 production, promotes CCR7-based migration, and drives expansion of suppressive (rather than pro-inflammatory) T-cell populations.\",\n      \"method\": \"EP2 and EP4 selective antagonists on human cDC2s, flow cytometry for activation markers, cytokine measurement, T-cell co-culture assays\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — pharmacological only (no genetic KD), multiple functional readouts in primary human cells\",\n      \"pmids\": [\"38088451\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"EP4 receptor antagonism in cartilage promotes chondrogenesis and cartilage anabolism through the cAMP/PKA/CREB/Sox9 signaling pathway; cartilage-specific EP4 knockout or EP4 antagonist (HL-43) enhances articular cartilage regeneration and reduces fibrocartilage formation.\",\n      \"method\": \"Cartilage-specific EP4 conditional knockout, EP4 antagonist HL-43, multiple OA/cartilage defect mouse and rat models, cAMP/PKA/CREB/Sox9 pathway analysis, human cartilage explant assays\",\n      \"journal\": \"Cell discovery\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO plus pharmacological antagonist with defined molecular pathway (cAMP/PKA/CREB/Sox9), multiple pre-clinical models and human tissue\",\n      \"pmids\": [\"35256606\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"EP4 receptor activation increases expression of oncogenic miR-526b via downstream PI3K/AKT, cAMP/PKA, and NF-κB signaling pathways in breast cancer cells.\",\n      \"method\": \"EP4 agonist/antagonist, COX-2 overexpression, PI3K-AKT inhibitors, NF-κB inhibitor, miR-526b stable overexpression/knockdown, in vivo lung colonization model\",\n      \"journal\": \"Molecular cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological + genetic interventions targeting multiple pathway nodes, single lab\",\n      \"pmids\": [\"25733698\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"EP4 receptor in myeloid cells promotes colorectal tumorigenesis by driving an anti-tumorigenic M1-to-M2 phenotype switch and activating mTOR and ERK signaling; myeloid-specific EP4 deletion or pharmacological EP4 inhibition markedly reduces adenoma number and size in ApcMin/+ mice.\",\n      \"method\": \"Myeloid-specific EP4 conditional knockout crossed to ApcMin/+ mice, EP4 antagonist treatment, mTOR/ERK signaling analysis, macrophage/DC phenotyping\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional myeloid-specific KO with defined molecular (mTOR, ERK) and cellular (M1/M2 phenotype) mechanisms validated pharmacologically\",\n      \"pmids\": [\"26378024\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Microglial EP4 signaling promotes diet-induced obesity by maintaining a phagocytic microglial state (elevated CD68) that reduces POMC neurite contacts with the paraventricular nucleus; microglial-specific EP4 deletion reduces weight gain, food intake, and improves insulin sensitivity in HFD-fed mice.\",\n      \"method\": \"Microglia-specific EP4 knockout mice (MG-EP4 KO), high-fat diet feeding, metabolic phenotyping, microglial CD68/phagocytosis assays, POMC neurite density analysis\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific conditional KO with defined cellular mechanism (microglial phagocytosis/POMC cytoarchitecture) and in vivo metabolic phenotype\",\n      \"pmids\": [\"36318114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Canine EP4 receptor (90% amino acid identity to human EP4) couples to adenylate cyclase to increase cAMP upon PGE2 binding; short-term PGE2-induced desensitization does not require the intracellular C-terminal tail, in contrast to the reported mechanism for the human receptor.\",\n      \"method\": \"cDNA library screening, CHO-K1 transfection, Scatchard binding analysis, cAMP accumulation assays, truncated C-terminus receptor construct comparison\",\n      \"journal\": \"Prostaglandins & other lipid mediators\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with truncation mutant, characterizing desensitization mechanism\",\n      \"pmids\": [\"11444589\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PTGER4 promoter demethylation in AI-resistant breast cancer cells leads to EP4 upregulation, and EP4 signaling promotes estrogen-independent growth likely via ligand-independent activation of ERα cofactor CARM1.\",\n      \"method\": \"Genome-wide DNA methylation and expression analysis, EP4 knockdown, EP4 inhibitor studies, downstream CARM1 signaling assay\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epigenomic plus functional KD studies, but CARM1 mechanism characterized as 'exploratory'\",\n      \"pmids\": [\"27869171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"EP4 receptor mediates PGE2-stimulated osteoclastogenesis and bone resorption by inducing RANKL mRNA in osteoblasts and increasing cAMP; selective EP4 antagonism also reduces 1,25D- and PTH-stimulated osteoclast formation, suggesting EP4 plays a general role in osteoclastic bone resorption.\",\n      \"method\": \"EP4-selective antagonist (EP4RA) in murine marrow cultures and fetal rat long bone organ cultures, TRAP+ multinucleated cell counting, cAMP measurement, RANKL mRNA quantification, 45Ca release assay\",\n      \"journal\": \"Bone\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological antagonism with multiple functional endpoints in primary bone cell cultures\",\n      \"pmids\": [\"11792579\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"EP4 and EP2 signaling show distinct cAMP kinetics in dendritic cells: EP4 induces transient cAMP and EP2 induces sustained cAMP; simultaneous EP2+EP4 stimulation attenuates cAMP production (crosstalk); EP4 primarily mediates podosome dissolution in DCs; both receptors require intact microtubule networks for efficient signaling.\",\n      \"method\": \"Quantitative live-cell cAMP imaging (FRET), podosome dissolution assays, microtubule disruption, EP2/EP4 selective antagonists\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — quantitative live-cell imaging with pharmacological tools revealing distinct kinetics and crosstalk mechanisms\",\n      \"pmids\": [\"33643295\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PTGER4 encodes EP4, a class A Gαs-coupled GPCR for prostaglandin E2 that primarily signals through cAMP/PKA but also engages Gαi, β-arrestin, PI3K, EGFR transactivation, and Rap/ERK cascades in a ligand- and cell-type-dependent (functionally selective) manner; its cryo-EM structure bound to Gs reveals a distinct TM6 displacement and Gs C-terminal hook engagement mediated by conserved prostanoid receptor residues, and its cell-type-specific roles—established through conditional knockouts and in vitro reconstitution—include regulation of ductus arteriosus closure, renin secretion, blood pressure via endothelial eNOS/AMPK, bone homeostasis via RANKL/osteoclastogenesis, myocardial protection, microglial phagocytic tone, innate immune suppression (NLRP3, NF-κB, mregDC-Treg axis), and cancer cell migration/invasion via EP4–βArrestin1–c-Src and EP4–PI3K–Orai1–ERK signalomes.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PTGER4 encodes the prostaglandin E2 receptor EP4, a class A GPCR that serves as a central transducer of PGE2 signaling across cardiovascular, immune, skeletal, neural, and epithelial systems. EP4 primarily couples to Gαs to elevate cAMP but also engages Gαi, β-arrestin, PI3K/AKT, Rap1/ERK, and EGFR transactivation pathways in a ligand- and cell-type-dependent manner, as demonstrated by BRET-based functional selectivity assays and cryo-EM structural determination of the EP4–Gs complex revealing a distinctive TM6 displacement and Gs C-terminal hook engagement [PMID:19584306, PMID:33264604]. Cell-type-specific conditional knockouts have established essential roles for EP4 in neonatal ductus arteriosus closure, macula densa–mediated renin secretion, endothelial eNOS/AMPK-dependent blood pressure regulation, macrophage-driven mucosal healing via CXCL1/MAPK, microglial phagocytic tone affecting both neuroinflammation and hypothalamic energy balance, fibroblast RANKL-driven osteoclastogenesis, and NLRP3 inflammasome suppression in macrophages [PMID:9600059, PMID:21835766, PMID:32641583, PMID:33558271, PMID:36318114, PMID:19419302, PMID:25917098]. In cancer, EP4 promotes tumor cell migration and stemness through β-arrestin1–c-Src, PI3K–Orai1–ERK, and PI3K/AKT–NOTCH/WNT signalomes, while in myeloid cells it drives a pro-tumorigenic M2 polarization via mTOR/ERK signaling [PMID:20353998, PMID:31755615, PMID:27301070, PMID:26378024].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"The first loss-of-function study established that EP4 is non-redundant in vivo by showing it is required for neonatal ductus arteriosus closure, answering whether individual EP receptors have essential physiological roles.\",\n      \"evidence\": \"EP4 knockout mice exhibiting patent ductus arteriosus and neonatal lethality, confirmed by in situ hybridization\",\n      \"pmids\": [\"9600059\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling pathway mediating ductus closure not identified\", \"Whether EP4 acts on smooth muscle or endothelium of the ductus not resolved\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Cloning and reconstitution of canine EP4 confirmed canonical Gαs/cAMP coupling and revealed that short-term desensitization does not require the intracellular C-terminal tail, raising questions about species-specific desensitization mechanisms.\",\n      \"evidence\": \"CHO-K1 transfection of full-length and C-terminally truncated EP4, cAMP accumulation and Scatchard binding\",\n      \"pmids\": [\"11444589\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Contradiction with human EP4 C-tail desensitization reports not resolved\", \"GRK/β-arrestin involvement not tested\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstration that EP4 activates PI3K and ERK1/2 independently of its canonical cAMP/PKA pathway established the principle of EP4 signaling bifurcation, distinguishing it from EP2.\",\n      \"evidence\": \"Pharmacological EP-subtype selective ligands and signaling assays in transfected cells measuring cAMP, PI3K, ERK, and EGR-1\",\n      \"pmids\": [\"14607241\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Adaptor proteins linking EP4 to PI3K not identified\", \"Cell-type generalizability not established\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Multiple groups showed EP4 drives pathologically significant outcomes—colon cancer proliferation via PI3K/ERK, cardiac hypertrophy via EGFR transactivation, and bone resorption via RANKL induction—establishing EP4 as a pleiotropic effector across tissues.\",\n      \"evidence\": \"EP4-selective agonists/antagonists in colon adenocarcinoma proliferation assays, neonatal myocyte EGFR phosphorylation/MAPK assays, and osteoblast RANKL mRNA quantification\",\n      \"pmids\": [\"15123663\", \"15626689\", \"11792579\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct structural basis for EP4 coupling to distinct G proteins unknown\", \"Whether EGFR transactivation requires β-arrestin scaffold not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"EP4 was placed in both the pain sensitization circuit (DRG neurons) and the metastatic cascade (breast cancer), broadening its pathological relevance to sensory neurobiology and oncology.\",\n      \"evidence\": \"Intrathecal EP4 shRNA knockdown plus antagonist in DRG electrophysiology and behavioral pain; EP4 antagonism in syngeneic breast cancer metastasis models\",\n      \"pmids\": [\"16966471\", \"16540639\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"EP4 interaction partners in DRG not mapped\", \"Breast cancer metastasis study lacked genetic confirmation\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Hematopoietic EP4 deletion and transcriptional regulation studies established that EP4 sustains macrophage survival via PI3K/Akt/NF-κB in atherosclerosis and that PTGER4 transcription is controlled by Sp-1 binding sites subject to ERK-mediated phosphorylation.\",\n      \"evidence\": \"EP4−/− fetal liver chimeras in LDLR−/− mice on Western diet; EP4 promoter luciferase reporters with Sp-1 site mutations and ChIP\",\n      \"pmids\": [\"19041765\", \"18346464\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Sp-1 regulation is tissue-specific not addressed\", \"Atherosclerosis findings limited to early lesion stage\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Quantitative BRET assays revealed that EP4 exhibits ligand-dependent functional selectivity across Gαs, Gαi, and β-arrestin pathways, and conditional KO identified fibroblast EP4 as the driver of RANKL-dependent osteoclastogenesis.\",\n      \"evidence\": \"BRET assays with panel of ligands in living cells; FSP1-Cre EP4 conditional KO with RANKL mRNA and osteolysis assays\",\n      \"pmids\": [\"19584306\", \"19419302\", \"19880670\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for biased agonism at EP4 not determined\", \"Whether β-arrestin bias translates to differential in vivo outcomes untested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The EP4–β-arrestin1–c-Src signaling complex was identified as the mechanism driving PGE2-induced cancer cell migration, establishing a G protein–independent oncogenic signalome for EP4.\",\n      \"evidence\": \"shRNA knockdown of EP4 and β-arrestin1 with c-Src kinase activity and cell migration assays in lung cancer cells\",\n      \"pmids\": [\"20353998\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether EP4–β-arrestin1–c-Src complex forms constitutively or only upon ligand binding not resolved\", \"Stoichiometry of the complex unknown\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"EP4 was established as the key receptor mediating COX-2–dependent renin secretion by macula densa and PGE2-driven angiogenesis via PKA/Rap1/eNOS, broadening its cardiovascular roles.\",\n      \"evidence\": \"Epistasis panel of COX-2, mPGES1/2, EP2, EP4, IP KO mice with furosemide challenge; siRNA knockdown of EP4/PKA subunits in tube formation, aortic ring, and Matrigel plug assays\",\n      \"pmids\": [\"21835766\", \"21926356\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether EP4 directly signals in juxtaglomerular cells or acts indirectly via macula densa not fully dissected\", \"Relative contribution of eNOS vs Rap1A in angiogenesis not quantified\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Post-transcriptional regulation of EP4 by miR-101 was demonstrated, and EP4 signaling in vascular smooth muscle was linked to MMP-2/IL-6 and abdominal aortic aneurysm progression.\",\n      \"evidence\": \"Luciferase reporter with wild-type and mutant EP4 3′-UTR for miR-101 binding; EP4 antagonist and EP4+/− mice in multiple AAA models\",\n      \"pmids\": [\"22353936\", \"22570740\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether miR-101 regulation of EP4 is tissue-restricted not determined\", \"Direct EP4 target cells in the AAA wall not identified by cell-type-specific KO\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Conditional microglial EP4 deletion in an Alzheimer's disease model revealed EP4 as an anti-inflammatory and pro-phagocytic receptor in microglia, suppressing IRF1/IRF7/NF-κB while enhancing Aβ clearance.\",\n      \"evidence\": \"Microglial EP4 conditional KO in APP-PS1 mice with microarray profiling and Aβ deposition measurement; EP4 agonist in cultured microglia\",\n      \"pmids\": [\"24760848\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling intermediary between EP4 and IRF1/IRF7 not identified\", \"Long-term disease progression effects not reported\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"EP4 was linked to cancer stemness via PI3K/AKT–NOTCH/WNT in breast cancer, to pro-tumorigenic myeloid M1-to-M2 switching via mTOR/ERK in colorectal cancer, and to Th17 differentiation in ankylosing spondylitis, expanding its roles in adaptive immunity and tumor immunology.\",\n      \"evidence\": \"EP4 antagonist/siRNA with PI3K/NOTCH/WNT inhibitors in breast CSC assays and xenografts; myeloid-specific EP4 cKO in ApcMin/+ mice; EP4 agonist with IL-23R/STAT3/FoxO1 analysis in patient Th17 cells\",\n      \"pmids\": [\"27301070\", \"26378024\", \"31253169\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether EP4-driven CSC properties require cell-autonomous signaling or microenvironmental cues not resolved\", \"Th17 study based on pharmacological agonist without genetic validation\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Endothelial-specific EP4 deletion demonstrated that EP4 maintains vascular barrier integrity through Rap1/Rac1 and suppresses NF-κB-driven adhesion molecule expression, providing a mechanistic basis for EP4's protective role in acute lung injury.\",\n      \"evidence\": \"Endothelial EP4 cKO mice in ALI models, Rap1/Rac1 GTPase activity assays, EP4 siRNA and pharmacological inhibition\",\n      \"pmids\": [\"28428256\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether EP4 barrier protection involves junctional complex remodeling beyond VE-cadherin not fully explored\", \"Role of β-arrestin in endothelial EP4 signaling not addressed\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Endothelial EP4 was shown to regulate systemic blood pressure via AMPK-dependent eNOS phosphorylation and to protect against myocardial ischemia-reperfusion injury by maintaining microvascular perfusion, establishing EP4 as a cardioprotective endothelial receptor.\",\n      \"evidence\": \"EC-specific EP4 KO and overexpression mice with BP measurement and eNOS/AMPK pathway analysis; mPges-1 KO and EC-EP4 KO mice in MI/R models with vascular reactivity and leukocyte adhesion assays\",\n      \"pmids\": [\"32641583\", \"31015404\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether AMPK activation by EP4 is cAMP-dependent or involves alternative upstream signals not fully resolved\", \"Specific prostanoid source (COX-1 vs COX-2) feeding endothelial EP4 in different vascular beds not systematically compared\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"EP4 was found to form a complex with Orai1/TRPC1 channels to drive PI3K-dependent calcium influx and ERK-mediated MMP activation in oral cancer migration, revealing a non-canonical ion channel partnership.\",\n      \"evidence\": \"Co-immunoprecipitation of EP4–Orai1–TRPC1, siRNA knockdown, calcium imaging, in vivo lung metastasis model\",\n      \"pmids\": [\"31755615\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether EP4–Orai1 interaction is direct or mediated by adaptor proteins not determined\", \"STIM1-independent Orai1 activation mechanism not structurally characterized\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"The cryo-EM structure of EP4 bound to Gs at 3.3 Å provided the first atomic-level view of prostanoid receptor–G protein coupling, revealing a restrained TM6 outward displacement and a Gs C-terminal hook structure mediated by conserved residues Phe54 and Trp327.\",\n      \"evidence\": \"Cryo-EM structure determination at 3.3 Å global resolution\",\n      \"pmids\": [\"33264604\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of EP4 bound to Gαi or β-arrestin available\", \"Ligand-bound inactive state structure not determined\", \"Structural basis for functional selectivity not addressed\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Macrophage-specific PTGER4 deletion revealed EP4/MAPK/CXCL1 as a paracrine axis driving epithelial regeneration in colitis, and TNF-α was shown to impair EP4 signaling via TRAF2-mediated GRK2 recruitment and receptor internalization.\",\n      \"evidence\": \"Csf1r-iCre Ptger4fl/fl conditional KO in DSS colitis with organoid assays and CXCL1 pathway analysis; cAMP FRET biosensor, TRAF2 siRNA, and co-IP of TRAF2-GRK2 in FLS with CIA model validation\",\n      \"pmids\": [\"33558271\", \"33859345\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether TRAF2-GRK2 desensitization mechanism operates in cell types beyond synoviocytes not tested\", \"Epithelial CXCL1 receptor identity not confirmed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Cell-type-specific EP4 functions in bone were dissected: osteoclast EP4 promotes OA via Gαs/PI3K/AKT/MAPK-driven osteoclastogenesis and sensory innervation, while cartilage EP4 antagonism promotes chondrogenesis via cAMP/PKA/CREB/Sox9, and tumor EP4 drives dual pro-inflammatory and immunosuppressive programs.\",\n      \"evidence\": \"Osteoclast-specific EP4 cKO and cartilage-specific EP4 cKO in OA models with pathway analysis; EP2/EP4 antagonists with scRNAseq in tumor models\",\n      \"pmids\": [\"35260562\", \"35256606\", \"35675777\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How EP4 activation produces opposite cAMP/PKA outcomes in osteoclasts vs chondrocytes not mechanistically explained\", \"EP4 vs EP2 contribution to mregDC-Treg axis not fully separated\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Microglial EP4 was identified as a metabolic regulator: it maintains a phagocytic microglial state in the hypothalamus that reduces POMC neurite density and promotes diet-induced obesity, revealing a neuroimmune metabolic role.\",\n      \"evidence\": \"Microglia-specific EP4 knockout mice on high-fat diet with metabolic phenotyping, CD68/phagocytosis assays, and POMC neurite analysis\",\n      \"pmids\": [\"36318114\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether EP4-driven microglial phagocytosis targets POMC neurites directly or acts indirectly not determined\", \"Downstream signaling pathway (cAMP vs PI3K) mediating phagocytic phenotype not dissected\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis for EP4 functional selectivity (no Gαi- or β-arrestin-bound structures), the molecular identity of adaptors linking EP4 to PI3K, and how identical cAMP signaling produces opposing outcomes in different cell types (e.g., osteoclasts vs chondrocytes).\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No EP4–Gαi or EP4–β-arrestin structure available\", \"Adaptor linking EP4 to PI3K unknown\", \"Cell-type-specific signal interpretation mechanisms not characterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 11, 22]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3, 12, 24]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [22, 27, 33]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 6, 11, 21, 22]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [3, 9, 12, 34, 37]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [4, 5, 8, 30, 37]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ARRB1\",\n      \"SRC\",\n      \"ORAI1\",\n      \"TRPC1\",\n      \"GRK2\",\n      \"TRAF2\",\n      \"GNAS\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}