{"gene":"CLDN1","run_date":"2026-06-09T22:57:18","timeline":{"discoveries":[{"year":1999,"finding":"CLDN1 (previously called SEMP1) encodes a protein with 91% identity to mouse claudin-1, a tight junction (TJ)-associated transmembrane protein, placing it in the epithelial membrane protein (EMP) superfamily with roles in cell polarity and paracellular permeability.","method":"cDNA cloning, differential display of mRNA, amino acid sequence analysis","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — sequence identity and domain inference from cloning; foundational identification paper with multiple supporting analyses but no in vitro functional reconstitution","pmids":["9931503"],"is_preprint":false},{"year":2002,"finding":"Re-expression of CLDN1 in CLDN1-negative breast cancer cells (MDA-MB-361 and MDA-MB-435) results in plasma membrane localization at cell-cell contact sites and inhibition of paracellular flux, even in the absence of occludin, demonstrating that CLDN1 alone is sufficient for tight junction gate function.","method":"Retroviral transduction, immunofluorescence, quantitative RT-PCR, paracellular dextran flux assay, Western blot","journal":"Journal of cellular physiology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (localization, flux assay, molecular quantification) in two cell lines; direct loss-of-function context with functional readout","pmids":["11920682"],"is_preprint":false},{"year":2004,"finding":"CLDN1 membrane localization in breast tumor spheroids (3D culture) induces apoptosis; cytosolic/heterogeneous CLDN1 distribution correlates with reduced apoptosis, indicating that membrane-targeted CLDN1 restricts nutrient/growth factor supply and triggers cell death in 3D but not 2D culture.","method":"Retroviral transduction, FACS sorting, apoptosis assays, immunofluorescence, paracellular flux measurement in 2D and 3D spheroid cultures","journal":"International journal of cancer","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods; direct comparison of 2D vs 3D, membrane vs cytosolic localization with functional consequence","pmids":["14648703"],"is_preprint":false},{"year":2016,"finding":"CLDN1 overexpression in cervical cancer cells promotes invasion and metastasis by inducing epithelial-mesenchymal transition (EMT) through interaction with SNAI1, leading to decreased E-cadherin and increased vimentin expression.","method":"Overexpression in SiHa cells, invasion/migration assays, Western blot for EMT markers, xenograft mouse model, co-immunoprecipitation/interaction assay with SNAI1","journal":"Oncotarget","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — functional assays with EMT marker changes and SNAI1 interaction reported; single lab but multiple assays","pmids":["27974683"],"is_preprint":false},{"year":2016,"finding":"CLDN1 promotes TGF-β1-induced migration and EMT in bronchial epithelial cells via the Notch signaling pathway; CLDN1 knockdown reduces NICD and Hes-1 levels and abrogates TGF-β1-induced EMT, while Notch activator Jagged-1 reverses the protective effect of CLDN1 silencing.","method":"siRNA knockdown, Western blot, qRT-PCR, Transwell migration/invasion assays, Notch pathway rescue with Jagged-1","journal":"Molecular and cellular biochemistry","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — epistasis via rescue experiment with Jagged-1; single lab, multiple assays but abstract-level detail","pmids":["28316062"],"is_preprint":false},{"year":2016,"finding":"Nm23H1 regulates CLDN1 expression via the AKT signaling pathway in esophageal squamous cell carcinoma: Nm23H1 knockdown increases AKT phosphorylation and decreases CLDN1 expression, enhancing invasion; AKT inhibitor MK2206 restores CLDN1 expression in Nm23H1-depleted cells.","method":"siRNA knockdown, overexpression, Western blot for pAKT and CLDN1, invasion assays, AKT inhibitor treatment, immunofluorescence","journal":"Oncogenesis","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — pathway placement by pharmacological rescue; single lab with multiple methods","pmids":["27376780"],"is_preprint":false},{"year":2017,"finding":"CLDN1 promotes cisplatin drug resistance in NSCLC by activating autophagy through upregulation of ULK1 phosphorylation.","method":"Western blot, CCK-8 proliferation assay, Transwell assay, confocal microscopy for autophagosomes, CLDN1 knockdown/overexpression","journal":"Medical science monitor","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — mechanistic link between CLDN1, ULK1 phosphorylation, and autophagy established by loss-of-function; single lab","pmids":["28614291"],"is_preprint":false},{"year":2017,"finding":"TMPRSS4 upregulates CLDN1 expression via ERK1/2 signaling in hepatocellular carcinoma, thereby promoting cancer stem cell (CSC) traits including tumorsphere formation.","method":"Overexpression and knockdown studies, Western blot, ERK1/2 pathway analysis, tumorsphere assay","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single approach per mechanism; correlation-based pathway inference with limited mechanistic depth","pmids":["28651932"],"is_preprint":false},{"year":2018,"finding":"IL-33 downregulates CLDN1 expression in keratinocytes via the ERK/STAT3 signaling pathway; STAT3 directly binds the CLDN1 promoter and suppresses transcription, impairing epithelial barrier function.","method":"Western blot, real-time PCR, immunofluorescence, MAPK inhibitors, siRNA, TEER and FITC-dextran flux assays, EMSA for STAT3 promoter binding","journal":"Journal of dermatological science","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — direct promoter binding shown by EMSA, pathway confirmed by siRNA and pharmacological inhibition, functional barrier readout; multiple orthogonal methods","pmids":["29534857"],"is_preprint":false},{"year":2018,"finding":"BHLHE40 suppresses CLDN1 transcription not by directly binding E-box motifs, but by interacting with SP1 and preventing SP1 from binding a specific motif (-233 to -61 bp upstream of the TSS) in the CLDN1 promoter; SP1 is identified as a major direct transcriptional activator of CLDN1.","method":"Reporter/luciferase assays, siRNA knockdown of SP1, co-immunoprecipitation, deletion mutant analysis, invasion assays","journal":"Molecular carcinogenesis","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — promoter mapping with deletion mutants, Co-IP of BHLHE40-SP1 interaction, epistasis by SP1 knockdown; multiple orthogonal methods in single study","pmids":["29704436"],"is_preprint":false},{"year":2019,"finding":"CLDN1 suppresses lung adenocarcinoma metastasis via a feedback loop involving the CLDN1-EPHB6-ERK1/2-SLUG axis: CLDN1 upregulates and activates EPHB6, which suppresses ERK1/2 signaling; DNA hypermethylation of the CLDN1 promoter abrogates SLUG-mediated suppression of CLDN1 in high-metastatic cells.","method":"Immunoblots, immunoprecipitation, methylation-specific PCR, pyrosequencing, chromatin immunoprecipitation, reporter assay, sphere/aldefluor/flow cytometry assays, migration assay, xenograft","journal":"Theranostics","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (Co-IP, ChIP, methylation assays, functional rescue) in one study establishing a defined signaling axis","pmids":["32754286"],"is_preprint":false},{"year":2019,"finding":"Peptides derived from the first extracellular loop of CLDN1 transiently disrupt tight junctions in human lung epithelial cells and delay TJ formation in primary human keratinocytes, demonstrating that the extracellular loop mediates TJ assembly/integrity.","method":"Peptide design based on CLDN1 extracellular loop, TEER measurement, permeability assays, in vivo epicutaneous vaccine model","journal":"Journal of Investigative Dermatology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional disruption of TJs by extracellular loop-derived peptides; single lab with multiple readouts","pmids":["31381894"],"is_preprint":false},{"year":2019,"finding":"CLDN1 promotes proliferation and metastasis of esophageal squamous cell carcinoma by triggering autophagy through the AMPK/STAT1/ULK1 signaling pathway; CLDN1 is aberrantly distributed to the nucleus in ESCC tumor cells.","method":"CLDN1 knockdown/overexpression, Western blot, in vitro and in vivo proliferation/metastasis assays, pathway inhibition","journal":"Journal of cellular physiology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — AMPK/STAT1/ULK1 pathway placement by functional assays; single lab with both in vitro and in vivo validation","pmids":["31498437"],"is_preprint":false},{"year":2021,"finding":"EZH2-mediated H3K27me3 suppresses CLDN1 transcription by accumulating at the CLDN1 TSS region, causing epithelial barrier dysfunction; EZH2 inhibition with GSK126 restores CLDN1 expression and barrier function in vivo.","method":"ChIP-qPCR, Western blot, TEER, FITC-dextran flux assay, EZH2 forced expression, in vivo rat EGDA model with EZH2 inhibitor","journal":"Digestive and liver disease","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — direct ChIP evidence for H3K27me3 at CLDN1 TSS, pharmacological rescue in vivo, functional barrier readout; multiple orthogonal methods","pmids":["34789399"],"is_preprint":false},{"year":2021,"finding":"CLDN1 regulates trophoblast cell survival: knockdown inhibits proliferation and induces apoptosis by decreasing BIRC3 expression and increasing cleaved PARP; BIRC3 overexpression rescues the apoptotic phenotype caused by CLDN1 knockdown.","method":"siRNA knockdown, overexpression, RNA-seq, Western blot, proliferation assay, apoptosis assay, BIRC3 rescue experiment","journal":"Reproduction","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — epistatic rescue experiment (BIRC3) establishes pathway position; single lab with multiple methods","pmids":["33784242"],"is_preprint":false},{"year":2022,"finding":"A missense variant in CLDN1 (p.Arg81His) causes decreased CLDN1 expression and mislocalization of the protein in keratinocytes, with 3D protein modeling predicting deleterious conformational changes at Arg81, demonstrating that proper protein folding is required for correct membrane targeting.","method":"Whole exome sequencing, Sanger sequencing, 3D protein modeling, Western blot, immunofluorescence confocal microscopy, HaCaT transfection with mutant vs wildtype CLDN1","journal":"American journal of medical genetics. Part A","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional cell-based validation of missense variant effects on localization; single study with multiple methods","pmids":["35920354"],"is_preprint":false},{"year":2022,"finding":"LncRNA WAKMAR2 directly binds c-Fos protein and recruits it to the CLDN1 promoter to enhance CLDN1 transcription, thereby maintaining keratinocyte barrier function; loss of WAKMAR2 reduces CLDN1 expression and barrier integrity.","method":"RNA pull-down, promoter-reporter assay, chromatin isolation by RNA purification-sequencing (ChIRP-seq), AP-1 inhibitor treatment, in vivo UV-irradiation mouse model","journal":"Contact dermatitis","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — direct RNA-protein interaction (RNA pulldown), promoter activity, ChIRP-seq for genomic binding; multiple orthogonal methods in single study","pmids":["36461623"],"is_preprint":false},{"year":2023,"finding":"LIN28B directly binds and posttranscriptionally regulates CLDN1 mRNA (shown by RNA immunoprecipitation), promoting collective invasion, cell migration, and metastatic liver tumor formation; NOTCH3 is identified as a downstream effector of the LIN28B/CLDN1 axis.","method":"LIN28B knockdown/overexpression, RNA immunoprecipitation, in vitro invasion/migration assays, murine metastatic CRC model, bulk RNA sequencing, NOTCH3 genetic/pharmacological manipulation","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 2 / Strong — RNA immunoprecipitation directly demonstrates LIN28B-CLDN1 mRNA binding; in vitro and in vivo validation; downstream epistasis with NOTCH3; multiple orthogonal methods","pmids":["37318881"],"is_preprint":false},{"year":2023,"finding":"CLDN1 promotes airway smooth muscle cell (ASMC) proliferation, migration, invasion, and inflammation by directly interacting with MMP14; CLDN1 positively regulates MMP14 expression, and MMP14 overexpression reverses the inhibitory effects of CLDN1 silencing.","method":"Co-immunoprecipitation, protein-protein interaction assay, CCK-8, EdU assay, Transwell assay, ELISA for inflammatory factors, CLDN1 knockdown/overexpression","journal":"Autoimmunity","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP establishes direct CLDN1-MMP14 interaction; MMP14 rescue experiment supports epistasis; single lab","pmids":["37964516"],"is_preprint":false},{"year":2023,"finding":"CLDN1 knockdown suppresses trophoblast invasion, migration, and endovascular trophoblast (enEVT) differentiation, decreasing VIM, SNAIL, IL1B, and PECAM1 expression; CLDN1 overexpression promotes these processes, placing CLDN1 upstream of EMT/invasion markers in trophoblast biology.","method":"siRNA knockdown, overexpression, invasion/migration/tube formation assays, Western blot, qRT-PCR for enEVT markers, EOPE mouse model","journal":"Placenta","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — bidirectional manipulation with multiple phenotypic readouts; in vivo mouse model corroborates; single lab","pmids":["37523840"],"is_preprint":false},{"year":2023,"finding":"CAF-derived WNT7A downregulates CLDN1 expression in OSCC cells via AKT signaling; AKT activation (SC79) reduces CLDN1, while AKT inhibitor (MK2206) restores CLDN1 and suppresses cancer cell migration, placing CLDN1 downstream of the WNT7A/AKT axis.","method":"Transwell coculture assay, microarray, WNT7A knockdown, phosphokinase array, AKT inhibitor/agonist treatment, CLDN1 knockdown, migration/invasion assays","journal":"Laboratory investigation","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — pharmacological epistasis with AKT inhibitor/activator; single lab, multiple assays","pmids":["37541622"],"is_preprint":false},{"year":2023,"finding":"CLDN1 upregulation induced by oxaliplatin chemotherapy is mediated at least in part by activation of the MAPKp38/GSK3β/Wnt/β-catenin pathway, and this CLDN1 overexpression confers resistance to apoptosis in CRC cells.","method":"Flow cytometry, immunofluorescence, Western blot, phosphoproteome analysis, proximity ligation assay, luciferase reporter assay, RNAseq, xenograft model","journal":"Cell & bioscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — phosphoproteomics and pathway reporter assays establish mechanism; single lab with multiple orthogonal methods","pmids":["37041570"],"is_preprint":false},{"year":2023,"finding":"CLDN1 p.Arg81His variant causes distorted tight junction ultrastructure in patient skin as visualized by transmission electron microscopy, confirming that Arg81 is required for proper TJ architecture.","method":"Transmission electron microscopy of patient skin, haplotype analysis, immunofluorescence","journal":"Clinical genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct ultrastructural evidence of TJ disruption from a variant study; single study","pmids":["37814412"],"is_preprint":false},{"year":2023,"finding":"SEMP1/CLDN1 overexpression in trophoblast cells promotes proliferation, migration, invasion, and VEGFA secretion (facilitating endothelial tube formation) via PI3K/AKT signaling; PI3K inhibitor LY294002 blocks these effects.","method":"Overexpression and knockdown, cell proliferation/migration/invasion assays, VEGFA ELISA, endothelial tube formation assay, LY294002 pharmacological inhibition","journal":"Molecular biotechnology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — pharmacological epistasis with PI3K inhibitor; bidirectional manipulation; single lab","pmids":["37277581"],"is_preprint":false},{"year":2024,"finding":"CLDN1 knockout keratinocytes (CRISPR/Cas9) show significantly reduced barrier function in monolayer and organotypic cultures, decreased filaggrin and cytokeratin-10 expression, diminished stratification, reduced stratum granulosum formation, and increased basal layer proliferation, establishing CLDN1 as required for both barrier function and proper epidermal stratification.","method":"CRISPR/Cas9 knockout, TEER measurement, organotypic culture, gene expression analysis, immunofluorescence, morphological analysis","journal":"Experimental dermatology","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean CRISPR KO with multiple orthogonal phenotypic readouts (barrier function, differentiation markers, morphology); rigorous loss-of-function model","pmids":["38711223"],"is_preprint":false},{"year":2024,"finding":"Imatinib blocks peritendon adhesion formation by inhibiting the PDGFRβ/ERK/STAT3/CLDN1 pathway, placing CLDN1 as a downstream effector of PDGFRβ signaling in fibroblast/adhesion biology.","method":"In vivo tendon adhesion model, drug treatment, Western blot, pathway inhibition analysis","journal":"Bioactive materials","confidence":"Low","confidence_rationale":"Tier 3 / Weak — pathway placement inferred from drug treatment in animal model; mechanistic detail limited in abstract","pmids":["38699245"],"is_preprint":false},{"year":2025,"finding":"Genome-wide CRISPR/Cas9 knockout screening identified CLDN1 as a mediator of berberine sensitivity in lung cancer A549 cells; CLDN1 knockout markedly increased sensitivity to berberine, leading to enhanced G1-phase arrest and reduced proliferation, establishing CLDN1 as promoting cellular resistance to berberine.","method":"Genome-wide CRISPR/Cas9 knockout screening, functional validation (cell cycle analysis, proliferation assay)","journal":"Oncology letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — unbiased genome-wide CRISPR screen plus functional validation; single lab","pmids":["41383979"],"is_preprint":false},{"year":2025,"finding":"CLDN1 directly interacts with PMP22 (peripheral myelin protein 22) in MDCKII epithelial cells, identified as a co-immunoprecipitation/mass spectrometry interactor in a cell-type-specific manner.","method":"Co-immunoprecipitation with ALFA-tag/nanobody, mass spectrometry, cross-context validation in multiple cell types","journal":"bioRxiv (preprint)","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single pulldown-MS experiment; preprint, not peer-reviewed; cell-type specificity noted but functional consequence not established","pmids":["bio_10.1101_2025.09.03.673966"],"is_preprint":true},{"year":2026,"finding":"High-molecular-weight hyaluronan (HM-HA) decreases CLDN1 mRNA and protein expression in keratinocytes via the CD44 receptor; knockdown of CD44 suppresses HM-HA-induced CLDN1 downregulation. Additionally, knockdown of HYAL1 (the main HA-degrading enzyme) decreases CLDN1 expression, suggesting HYAL1-mediated HA degradation is required to maintain CLDN1 levels and TJ barrier function.","method":"HM-HA treatment, siRNA knockdown of CD44 and HYAL1, HA-degrading activity assay, Western blot, mRNA expression analysis","journal":"Archives of biochemistry and biophysics","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — siRNA epistasis establishing CD44 as mediator of HM-HA effect on CLDN1; single lab with multiple knockdown conditions","pmids":["41881410"],"is_preprint":false}],"current_model":"CLDN1 is a tetraspan tight junction transmembrane protein whose membrane localization is required for paracellular barrier function (independently of occludin); its transcription is activated by SP1 and repressed by STAT3, BHLHE40 (via SP1 displacement), and EZH2 (via H3K27me3 at the TSS), while upstream regulators including IL-33/ERK, Nm23H1/AKT, WNT7A/AKT, EZH2, and RUNX3/DNA methylation control its expression level; at the signaling level, CLDN1 promotes EMT via SNAI1 interaction and Notch activation, induces autophagy through AMPK/STAT1/ULK1, suppresses metastasis in lung adenocarcinoma via an EPHB6-ERK1/2-SLUG feedback axis, and directly interacts with MMP14 to regulate cell migration, collectively establishing CLDN1 as a context-dependent regulator of epithelial barrier integrity, cell survival, invasion, and chemosensitivity."},"narrative":{"mechanistic_narrative":"CLDN1 is a tetraspan tight junction transmembrane protein that establishes paracellular barrier function in epithelia and contributes to epidermal differentiation [PMID:9931503, PMID:11920682, PMID:38711223]. Re-expression in claudin-negative cells localizes CLDN1 to cell-cell contacts and restores tight junction gate function independently of occludin, with the extracellular loop mediating junction assembly [PMID:11920682, PMID:31381894]. Correct membrane targeting requires proper folding: the p.Arg81His missense variant causes mislocalization and distorted tight junction ultrastructure in keratinocytes [PMID:35920354, PMID:37814412], and CRISPR knockout in keratinocytes impairs barrier function, stratification, and filaggrin/cytokeratin-10 expression [PMID:38711223]. CLDN1 transcription is governed by a defined regulatory network in which SP1 acts as a direct activator and is antagonized by BHLHE40, which sequesters SP1 from the promoter, by STAT3, which binds and represses the promoter downstream of IL-33/ERK signaling, and by EZH2-deposited H3K27me3 at the transcription start site [PMID:29534857, PMID:29704436, PMID:34789399]; positive inputs include c-Fos recruited by the lncRNA WAKMAR2 [PMID:36461623]. Beyond its junctional role, CLDN1 functions as a context-dependent regulator of invasion and cell survival: it drives epithelial-mesenchymal transition through interaction with SNAI1 and activation of Notch signaling [PMID:27974683, PMID:28316062], directly binds MMP14 to promote migration and inflammation [PMID:37964516], suppresses lung adenocarcinoma metastasis via a CLDN1-EPHB6-ERK1/2-SLUG feedback axis [PMID:32754286], and promotes chemoresistance by activating autophagy through ULK1 and AMPK/STAT1/ULK1 signaling [PMID:28614291, PMID:31498437].","teleology":[{"year":1999,"claim":"Established the molecular identity of CLDN1 as a claudin-family tight junction transmembrane protein, framing all subsequent functional work on barrier and polarity.","evidence":"cDNA cloning and sequence analysis of SEMP1/CLDN1","pmids":["9931503"],"confidence":"Medium","gaps":["No functional reconstitution in the cloning study","Tissue distribution and partner proteins not defined"]},{"year":2002,"claim":"Resolved whether CLDN1 alone is sufficient for tight junction gate function, showing it restores paracellular barrier independently of occludin.","evidence":"Retroviral re-expression in claudin-negative breast cancer cells with localization and dextran flux assays","pmids":["11920682"],"confidence":"High","gaps":["Molecular basis of selective permeability not addressed","Did not define which extracellular determinants mediate sealing"]},{"year":2004,"claim":"Linked CLDN1 membrane localization to apoptosis in 3D culture, indicating its barrier role has consequences for nutrient access and cell survival.","evidence":"Comparison of membrane vs cytosolic CLDN1 in 2D and 3D spheroid breast cancer cultures","pmids":["14648703"],"confidence":"High","gaps":["Death-triggering signal mechanistically undefined","Generalizability beyond breast spheroids unknown"]},{"year":2016,"claim":"Connected CLDN1 to EMT and invasion through SNAI1 interaction and Notch activation, establishing a non-junctional pro-invasive function.","evidence":"Overexpression/knockdown with EMT markers, SNAI1 Co-IP, Notch rescue with Jagged-1, xenografts in cervical and bronchial cells","pmids":["27974683","28316062"],"confidence":"Medium","gaps":["Direct binding interface with SNAI1 not mapped","Whether junctional vs cytosolic CLDN1 drives EMT unresolved"]},{"year":2017,"claim":"Identified autophagy activation via ULK1 as a route by which CLDN1 confers chemoresistance, expanding its role into cell survival signaling.","evidence":"CLDN1 manipulation with ULK1 phosphorylation analysis and autophagosome imaging in NSCLC under cisplatin","pmids":["28614291"],"confidence":"Medium","gaps":["Mechanism coupling CLDN1 to ULK1 phosphorylation unknown","Direct vs indirect link not established"]},{"year":2018,"claim":"Defined the core transcriptional control of CLDN1, with SP1 as direct activator antagonized by BHLHE40 and by STAT3 downstream of IL-33/ERK.","evidence":"Promoter mapping, deletion mutants, BHLHE40-SP1 Co-IP, EMSA for STAT3 binding, and barrier assays in keratinocytes","pmids":["29534857","29704436"],"confidence":"High","gaps":["Combinatorial logic among regulators not modeled","Tissue-specific selection of activators vs repressors unclear"]},{"year":2019,"claim":"Demonstrated context-dependent CLDN1 functions: metastasis suppression via an EPHB6-ERK1/2-SLUG axis, autophagy-driven progression via AMPK/STAT1/ULK1, and an extracellular-loop role in TJ assembly.","evidence":"Co-IP, ChIP, methylation assays, peptide TJ-disruption experiments, and xenograft models across lung and esophageal systems","pmids":["32754286","31498437","31381894"],"confidence":"High","gaps":["What dictates pro- vs anti-metastatic output not defined","Nuclear CLDN1 distribution in ESCC mechanistically unexplained"]},{"year":2021,"claim":"Showed CLDN1 transcription is epigenetically repressed by EZH2/H3K27me3 and that CLDN1 supports cell survival via BIRC3, deepening regulatory and pro-survival mechanisms.","evidence":"ChIP-qPCR with EZH2 inhibitor rescue in vivo and BIRC3 rescue of CLDN1-knockdown apoptosis in trophoblasts","pmids":["34789399","33784242"],"confidence":"High","gaps":["How EZH2 is recruited to the CLDN1 TSS not defined","Link between CLDN1 and BIRC3 regulation indirect"]},{"year":2022,"claim":"Established that proper folding underlies CLDN1 membrane targeting and that lncRNA WAKMAR2/c-Fos positively drives CLDN1 transcription to maintain barrier integrity.","evidence":"p.Arg81His variant cell modeling with immunofluorescence; RNA pull-down, promoter reporter, and ChIRP-seq for WAKMAR2","pmids":["35920354","36461623"],"confidence":"Medium","gaps":["Disease mechanism of the variant at organism level not detailed here","Conditions selecting WAKMAR2/c-Fos activation unclear"]},{"year":2023,"claim":"Identified posttranscriptional and physical-partner control of CLDN1 (LIN28B mRNA binding, MMP14 direct interaction) and multiple upstream pathways (WNT7A/AKT, PI3K/AKT, MAPKp38/Wnt) governing CLDN1 in invasion, survival, and chemoresistance.","evidence":"RNA immunoprecipitation, Co-IP, pharmacological epistasis, phosphoproteomics, and in vivo metastasis/chemotherapy models across CRC, OSCC, ASMC, and trophoblast systems","pmids":["37318881","37964516","37541622","37277581","37041570","37523840","37814412"],"confidence":"High","gaps":["Whether AKT acts on CLDN1 transcription or stability not separated","MMP14 and PMP22 binding interfaces unmapped"]},{"year":2024,"claim":"Provided clean loss-of-function evidence that CLDN1 is required for both barrier function and epidermal stratification, and placed it downstream of PDGFRβ signaling in adhesion biology.","evidence":"CRISPR/Cas9 keratinocyte knockout with organotypic and differentiation analyses; pathway analysis in a tendon adhesion model","pmids":["38711223","38699245"],"confidence":"High","gaps":["How CLDN1 loss reprograms differentiation gene expression unknown","PDGFRβ-to-CLDN1 link is correlative"]},{"year":2025,"claim":"Unbiased screening identified CLDN1 as a mediator of drug sensitivity and reported a new physical partner, extending its functional repertoire.","evidence":"Genome-wide CRISPR knockout screen for berberine sensitivity in A549; ALFA-tag Co-IP/mass spectrometry identifying PMP22 in MDCKII cells (preprint)","pmids":["41383979","bio_10.1101_2025.09.03.673966"],"confidence":"Medium","gaps":["Mechanism of berberine resistance via CLDN1 undefined","PMP22 interaction lacks functional validation and reciprocal confirmation"]},{"year":2026,"claim":"Linked extracellular matrix signaling to CLDN1 regulation, showing HM-HA via CD44 and HYAL1-dependent HA turnover control CLDN1 levels and barrier function.","evidence":"HM-HA treatment with CD44 and HYAL1 siRNA knockdown and barrier readouts in keratinocytes","pmids":["41881410"],"confidence":"Medium","gaps":["Signaling cascade from CD44 to CLDN1 transcription not mapped","In vivo relevance to skin barrier disease unaddressed"]},{"year":null,"claim":"It remains unresolved what molecular switch determines whether CLDN1 acts as a barrier-sealing junctional protein versus a pro-invasive/pro-survival signaling effector, and how its subcellular distribution (membrane vs cytosolic/nuclear) is controlled to select these outputs.","evidence":"No single study in the corpus reconciles the opposing tumor-suppressive and tumor-promoting roles","pmids":[],"confidence":"Low","gaps":["No structural model of CLDN1 in distinct signaling complexes","Determinants of nuclear/cytosolic relocalization unknown","Context-dependence across tissues not mechanistically unified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1,11,24]},{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[1,11]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,2,15,22]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[12]}],"pathway":[{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[1,11,24]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[6,12]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[8,9,13,16]}],"complexes":["tight junction"],"partners":["SNAI1","MMP14","EPHB6","PMP22","SP1","LIN28B"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O95832","full_name":"Claudin-1","aliases":["Senescence-associated epithelial membrane protein"],"length_aa":211,"mass_kda":22.7,"function":"Claudins function as major constituents of the tight junction complexes that regulate the permeability of epithelia. While some claudin family members play essential roles in the formation of impermeable barriers, others mediate the permeability to ions and small molecules. Often, several claudin family members are coexpressed and interact with each other, and this determines the overall permeability. CLDN1 is required to prevent the paracellular diffusion of small molecules through tight junctions in the epidermis and is required for the normal barrier function of the skin. Required for normal water homeostasis and to prevent excessive water loss through the skin, probably via an indirect effect on the expression levels of other proteins, since CLDN1 itself seems to be dispensable for water barrier formation in keratinocyte tight junctions (PubMed:23407391) (Microbial infection) Acts as a co-receptor for hepatitis C virus (HCV) in hepatocytes (PubMed:17325668, PubMed:20375010, PubMed:24038151). Associates with CD81 and the CLDN1-CD81 receptor complex is essential for HCV entry into host cell (PubMed:20375010). Acts as a receptor for dengue virus (PubMed:24074594)","subcellular_location":"Cell junction, tight junction; Cell membrane; Basolateral cell membrane","url":"https://www.uniprot.org/uniprotkb/O95832/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CLDN1","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CLDN1","total_profiled":1310},"omim":[{"mim_id":"619677","title":"CLAUDIN DOMAIN-CONTAINING PROTEIN 1; CLDND1","url":"https://www.omim.org/entry/619677"},{"mim_id":"619658","title":"CHOLESTASIS, PROGRESSIVE FAMILIAL INTRAHEPATIC, 7, WITH OR WITHOUT HEARING LOSS; PFIC7","url":"https://www.omim.org/entry/619658"},{"mim_id":"617579","title":"CLAUDIN 10; CLDN10","url":"https://www.omim.org/entry/617579"},{"mim_id":"616671","title":"KERATIN 76, TYPE II; KRT76","url":"https://www.omim.org/entry/616671"},{"mim_id":"615878","title":"CHOLESTASIS, PROGRESSIVE FAMILIAL INTRAHEPATIC, 4; PFIC4","url":"https://www.omim.org/entry/615878"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"liver","ntpm":289.7},{"tissue":"skin 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foundational identification paper with multiple supporting analyses but no in vitro functional reconstitution\",\n      \"pmids\": [\"9931503\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Re-expression of CLDN1 in CLDN1-negative breast cancer cells (MDA-MB-361 and MDA-MB-435) results in plasma membrane localization at cell-cell contact sites and inhibition of paracellular flux, even in the absence of occludin, demonstrating that CLDN1 alone is sufficient for tight junction gate function.\",\n      \"method\": \"Retroviral transduction, immunofluorescence, quantitative RT-PCR, paracellular dextran flux assay, Western blot\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (localization, flux assay, molecular quantification) in two cell lines; direct loss-of-function context with functional readout\",\n      \"pmids\": [\"11920682\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CLDN1 membrane localization in breast tumor spheroids (3D culture) induces apoptosis; cytosolic/heterogeneous CLDN1 distribution correlates with reduced apoptosis, indicating that membrane-targeted CLDN1 restricts nutrient/growth factor supply and triggers cell death in 3D but not 2D culture.\",\n      \"method\": \"Retroviral transduction, FACS sorting, apoptosis assays, immunofluorescence, paracellular flux measurement in 2D and 3D spheroid cultures\",\n      \"journal\": \"International journal of cancer\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods; direct comparison of 2D vs 3D, membrane vs cytosolic localization with functional consequence\",\n      \"pmids\": [\"14648703\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CLDN1 overexpression in cervical cancer cells promotes invasion and metastasis by inducing epithelial-mesenchymal transition (EMT) through interaction with SNAI1, leading to decreased E-cadherin and increased vimentin expression.\",\n      \"method\": \"Overexpression in SiHa cells, invasion/migration assays, Western blot for EMT markers, xenograft mouse model, co-immunoprecipitation/interaction assay with SNAI1\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — functional assays with EMT marker changes and SNAI1 interaction reported; single lab but multiple assays\",\n      \"pmids\": [\"27974683\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CLDN1 promotes TGF-β1-induced migration and EMT in bronchial epithelial cells via the Notch signaling pathway; CLDN1 knockdown reduces NICD and Hes-1 levels and abrogates TGF-β1-induced EMT, while Notch activator Jagged-1 reverses the protective effect of CLDN1 silencing.\",\n      \"method\": \"siRNA knockdown, Western blot, qRT-PCR, Transwell migration/invasion assays, Notch pathway rescue with Jagged-1\",\n      \"journal\": \"Molecular and cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — epistasis via rescue experiment with Jagged-1; single lab, multiple assays but abstract-level detail\",\n      \"pmids\": [\"28316062\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Nm23H1 regulates CLDN1 expression via the AKT signaling pathway in esophageal squamous cell carcinoma: Nm23H1 knockdown increases AKT phosphorylation and decreases CLDN1 expression, enhancing invasion; AKT inhibitor MK2206 restores CLDN1 expression in Nm23H1-depleted cells.\",\n      \"method\": \"siRNA knockdown, overexpression, Western blot for pAKT and CLDN1, invasion assays, AKT inhibitor treatment, immunofluorescence\",\n      \"journal\": \"Oncogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — pathway placement by pharmacological rescue; single lab with multiple methods\",\n      \"pmids\": [\"27376780\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CLDN1 promotes cisplatin drug resistance in NSCLC by activating autophagy through upregulation of ULK1 phosphorylation.\",\n      \"method\": \"Western blot, CCK-8 proliferation assay, Transwell assay, confocal microscopy for autophagosomes, CLDN1 knockdown/overexpression\",\n      \"journal\": \"Medical science monitor\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — mechanistic link between CLDN1, ULK1 phosphorylation, and autophagy established by loss-of-function; single lab\",\n      \"pmids\": [\"28614291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"TMPRSS4 upregulates CLDN1 expression via ERK1/2 signaling in hepatocellular carcinoma, thereby promoting cancer stem cell (CSC) traits including tumorsphere formation.\",\n      \"method\": \"Overexpression and knockdown studies, Western blot, ERK1/2 pathway analysis, tumorsphere assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single approach per mechanism; correlation-based pathway inference with limited mechanistic depth\",\n      \"pmids\": [\"28651932\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"IL-33 downregulates CLDN1 expression in keratinocytes via the ERK/STAT3 signaling pathway; STAT3 directly binds the CLDN1 promoter and suppresses transcription, impairing epithelial barrier function.\",\n      \"method\": \"Western blot, real-time PCR, immunofluorescence, MAPK inhibitors, siRNA, TEER and FITC-dextran flux assays, EMSA for STAT3 promoter binding\",\n      \"journal\": \"Journal of dermatological science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — direct promoter binding shown by EMSA, pathway confirmed by siRNA and pharmacological inhibition, functional barrier readout; multiple orthogonal methods\",\n      \"pmids\": [\"29534857\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"BHLHE40 suppresses CLDN1 transcription not by directly binding E-box motifs, but by interacting with SP1 and preventing SP1 from binding a specific motif (-233 to -61 bp upstream of the TSS) in the CLDN1 promoter; SP1 is identified as a major direct transcriptional activator of CLDN1.\",\n      \"method\": \"Reporter/luciferase assays, siRNA knockdown of SP1, co-immunoprecipitation, deletion mutant analysis, invasion assays\",\n      \"journal\": \"Molecular carcinogenesis\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — promoter mapping with deletion mutants, Co-IP of BHLHE40-SP1 interaction, epistasis by SP1 knockdown; multiple orthogonal methods in single study\",\n      \"pmids\": [\"29704436\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CLDN1 suppresses lung adenocarcinoma metastasis via a feedback loop involving the CLDN1-EPHB6-ERK1/2-SLUG axis: CLDN1 upregulates and activates EPHB6, which suppresses ERK1/2 signaling; DNA hypermethylation of the CLDN1 promoter abrogates SLUG-mediated suppression of CLDN1 in high-metastatic cells.\",\n      \"method\": \"Immunoblots, immunoprecipitation, methylation-specific PCR, pyrosequencing, chromatin immunoprecipitation, reporter assay, sphere/aldefluor/flow cytometry assays, migration assay, xenograft\",\n      \"journal\": \"Theranostics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (Co-IP, ChIP, methylation assays, functional rescue) in one study establishing a defined signaling axis\",\n      \"pmids\": [\"32754286\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Peptides derived from the first extracellular loop of CLDN1 transiently disrupt tight junctions in human lung epithelial cells and delay TJ formation in primary human keratinocytes, demonstrating that the extracellular loop mediates TJ assembly/integrity.\",\n      \"method\": \"Peptide design based on CLDN1 extracellular loop, TEER measurement, permeability assays, in vivo epicutaneous vaccine model\",\n      \"journal\": \"Journal of Investigative Dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional disruption of TJs by extracellular loop-derived peptides; single lab with multiple readouts\",\n      \"pmids\": [\"31381894\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CLDN1 promotes proliferation and metastasis of esophageal squamous cell carcinoma by triggering autophagy through the AMPK/STAT1/ULK1 signaling pathway; CLDN1 is aberrantly distributed to the nucleus in ESCC tumor cells.\",\n      \"method\": \"CLDN1 knockdown/overexpression, Western blot, in vitro and in vivo proliferation/metastasis assays, pathway inhibition\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — AMPK/STAT1/ULK1 pathway placement by functional assays; single lab with both in vitro and in vivo validation\",\n      \"pmids\": [\"31498437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"EZH2-mediated H3K27me3 suppresses CLDN1 transcription by accumulating at the CLDN1 TSS region, causing epithelial barrier dysfunction; EZH2 inhibition with GSK126 restores CLDN1 expression and barrier function in vivo.\",\n      \"method\": \"ChIP-qPCR, Western blot, TEER, FITC-dextran flux assay, EZH2 forced expression, in vivo rat EGDA model with EZH2 inhibitor\",\n      \"journal\": \"Digestive and liver disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — direct ChIP evidence for H3K27me3 at CLDN1 TSS, pharmacological rescue in vivo, functional barrier readout; multiple orthogonal methods\",\n      \"pmids\": [\"34789399\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CLDN1 regulates trophoblast cell survival: knockdown inhibits proliferation and induces apoptosis by decreasing BIRC3 expression and increasing cleaved PARP; BIRC3 overexpression rescues the apoptotic phenotype caused by CLDN1 knockdown.\",\n      \"method\": \"siRNA knockdown, overexpression, RNA-seq, Western blot, proliferation assay, apoptosis assay, BIRC3 rescue experiment\",\n      \"journal\": \"Reproduction\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — epistatic rescue experiment (BIRC3) establishes pathway position; single lab with multiple methods\",\n      \"pmids\": [\"33784242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"A missense variant in CLDN1 (p.Arg81His) causes decreased CLDN1 expression and mislocalization of the protein in keratinocytes, with 3D protein modeling predicting deleterious conformational changes at Arg81, demonstrating that proper protein folding is required for correct membrane targeting.\",\n      \"method\": \"Whole exome sequencing, Sanger sequencing, 3D protein modeling, Western blot, immunofluorescence confocal microscopy, HaCaT transfection with mutant vs wildtype CLDN1\",\n      \"journal\": \"American journal of medical genetics. Part A\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional cell-based validation of missense variant effects on localization; single study with multiple methods\",\n      \"pmids\": [\"35920354\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"LncRNA WAKMAR2 directly binds c-Fos protein and recruits it to the CLDN1 promoter to enhance CLDN1 transcription, thereby maintaining keratinocyte barrier function; loss of WAKMAR2 reduces CLDN1 expression and barrier integrity.\",\n      \"method\": \"RNA pull-down, promoter-reporter assay, chromatin isolation by RNA purification-sequencing (ChIRP-seq), AP-1 inhibitor treatment, in vivo UV-irradiation mouse model\",\n      \"journal\": \"Contact dermatitis\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — direct RNA-protein interaction (RNA pulldown), promoter activity, ChIRP-seq for genomic binding; multiple orthogonal methods in single study\",\n      \"pmids\": [\"36461623\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"LIN28B directly binds and posttranscriptionally regulates CLDN1 mRNA (shown by RNA immunoprecipitation), promoting collective invasion, cell migration, and metastatic liver tumor formation; NOTCH3 is identified as a downstream effector of the LIN28B/CLDN1 axis.\",\n      \"method\": \"LIN28B knockdown/overexpression, RNA immunoprecipitation, in vitro invasion/migration assays, murine metastatic CRC model, bulk RNA sequencing, NOTCH3 genetic/pharmacological manipulation\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — RNA immunoprecipitation directly demonstrates LIN28B-CLDN1 mRNA binding; in vitro and in vivo validation; downstream epistasis with NOTCH3; multiple orthogonal methods\",\n      \"pmids\": [\"37318881\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CLDN1 promotes airway smooth muscle cell (ASMC) proliferation, migration, invasion, and inflammation by directly interacting with MMP14; CLDN1 positively regulates MMP14 expression, and MMP14 overexpression reverses the inhibitory effects of CLDN1 silencing.\",\n      \"method\": \"Co-immunoprecipitation, protein-protein interaction assay, CCK-8, EdU assay, Transwell assay, ELISA for inflammatory factors, CLDN1 knockdown/overexpression\",\n      \"journal\": \"Autoimmunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP establishes direct CLDN1-MMP14 interaction; MMP14 rescue experiment supports epistasis; single lab\",\n      \"pmids\": [\"37964516\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CLDN1 knockdown suppresses trophoblast invasion, migration, and endovascular trophoblast (enEVT) differentiation, decreasing VIM, SNAIL, IL1B, and PECAM1 expression; CLDN1 overexpression promotes these processes, placing CLDN1 upstream of EMT/invasion markers in trophoblast biology.\",\n      \"method\": \"siRNA knockdown, overexpression, invasion/migration/tube formation assays, Western blot, qRT-PCR for enEVT markers, EOPE mouse model\",\n      \"journal\": \"Placenta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — bidirectional manipulation with multiple phenotypic readouts; in vivo mouse model corroborates; single lab\",\n      \"pmids\": [\"37523840\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CAF-derived WNT7A downregulates CLDN1 expression in OSCC cells via AKT signaling; AKT activation (SC79) reduces CLDN1, while AKT inhibitor (MK2206) restores CLDN1 and suppresses cancer cell migration, placing CLDN1 downstream of the WNT7A/AKT axis.\",\n      \"method\": \"Transwell coculture assay, microarray, WNT7A knockdown, phosphokinase array, AKT inhibitor/agonist treatment, CLDN1 knockdown, migration/invasion assays\",\n      \"journal\": \"Laboratory investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — pharmacological epistasis with AKT inhibitor/activator; single lab, multiple assays\",\n      \"pmids\": [\"37541622\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CLDN1 upregulation induced by oxaliplatin chemotherapy is mediated at least in part by activation of the MAPKp38/GSK3β/Wnt/β-catenin pathway, and this CLDN1 overexpression confers resistance to apoptosis in CRC cells.\",\n      \"method\": \"Flow cytometry, immunofluorescence, Western blot, phosphoproteome analysis, proximity ligation assay, luciferase reporter assay, RNAseq, xenograft model\",\n      \"journal\": \"Cell & bioscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — phosphoproteomics and pathway reporter assays establish mechanism; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"37041570\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CLDN1 p.Arg81His variant causes distorted tight junction ultrastructure in patient skin as visualized by transmission electron microscopy, confirming that Arg81 is required for proper TJ architecture.\",\n      \"method\": \"Transmission electron microscopy of patient skin, haplotype analysis, immunofluorescence\",\n      \"journal\": \"Clinical genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct ultrastructural evidence of TJ disruption from a variant study; single study\",\n      \"pmids\": [\"37814412\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SEMP1/CLDN1 overexpression in trophoblast cells promotes proliferation, migration, invasion, and VEGFA secretion (facilitating endothelial tube formation) via PI3K/AKT signaling; PI3K inhibitor LY294002 blocks these effects.\",\n      \"method\": \"Overexpression and knockdown, cell proliferation/migration/invasion assays, VEGFA ELISA, endothelial tube formation assay, LY294002 pharmacological inhibition\",\n      \"journal\": \"Molecular biotechnology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — pharmacological epistasis with PI3K inhibitor; bidirectional manipulation; single lab\",\n      \"pmids\": [\"37277581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CLDN1 knockout keratinocytes (CRISPR/Cas9) show significantly reduced barrier function in monolayer and organotypic cultures, decreased filaggrin and cytokeratin-10 expression, diminished stratification, reduced stratum granulosum formation, and increased basal layer proliferation, establishing CLDN1 as required for both barrier function and proper epidermal stratification.\",\n      \"method\": \"CRISPR/Cas9 knockout, TEER measurement, organotypic culture, gene expression analysis, immunofluorescence, morphological analysis\",\n      \"journal\": \"Experimental dermatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean CRISPR KO with multiple orthogonal phenotypic readouts (barrier function, differentiation markers, morphology); rigorous loss-of-function model\",\n      \"pmids\": [\"38711223\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Imatinib blocks peritendon adhesion formation by inhibiting the PDGFRβ/ERK/STAT3/CLDN1 pathway, placing CLDN1 as a downstream effector of PDGFRβ signaling in fibroblast/adhesion biology.\",\n      \"method\": \"In vivo tendon adhesion model, drug treatment, Western blot, pathway inhibition analysis\",\n      \"journal\": \"Bioactive materials\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — pathway placement inferred from drug treatment in animal model; mechanistic detail limited in abstract\",\n      \"pmids\": [\"38699245\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Genome-wide CRISPR/Cas9 knockout screening identified CLDN1 as a mediator of berberine sensitivity in lung cancer A549 cells; CLDN1 knockout markedly increased sensitivity to berberine, leading to enhanced G1-phase arrest and reduced proliferation, establishing CLDN1 as promoting cellular resistance to berberine.\",\n      \"method\": \"Genome-wide CRISPR/Cas9 knockout screening, functional validation (cell cycle analysis, proliferation assay)\",\n      \"journal\": \"Oncology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — unbiased genome-wide CRISPR screen plus functional validation; single lab\",\n      \"pmids\": [\"41383979\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CLDN1 directly interacts with PMP22 (peripheral myelin protein 22) in MDCKII epithelial cells, identified as a co-immunoprecipitation/mass spectrometry interactor in a cell-type-specific manner.\",\n      \"method\": \"Co-immunoprecipitation with ALFA-tag/nanobody, mass spectrometry, cross-context validation in multiple cell types\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single pulldown-MS experiment; preprint, not peer-reviewed; cell-type specificity noted but functional consequence not established\",\n      \"pmids\": [\"bio_10.1101_2025.09.03.673966\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"High-molecular-weight hyaluronan (HM-HA) decreases CLDN1 mRNA and protein expression in keratinocytes via the CD44 receptor; knockdown of CD44 suppresses HM-HA-induced CLDN1 downregulation. Additionally, knockdown of HYAL1 (the main HA-degrading enzyme) decreases CLDN1 expression, suggesting HYAL1-mediated HA degradation is required to maintain CLDN1 levels and TJ barrier function.\",\n      \"method\": \"HM-HA treatment, siRNA knockdown of CD44 and HYAL1, HA-degrading activity assay, Western blot, mRNA expression analysis\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — siRNA epistasis establishing CD44 as mediator of HM-HA effect on CLDN1; single lab with multiple knockdown conditions\",\n      \"pmids\": [\"41881410\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CLDN1 is a tetraspan tight junction transmembrane protein whose membrane localization is required for paracellular barrier function (independently of occludin); its transcription is activated by SP1 and repressed by STAT3, BHLHE40 (via SP1 displacement), and EZH2 (via H3K27me3 at the TSS), while upstream regulators including IL-33/ERK, Nm23H1/AKT, WNT7A/AKT, EZH2, and RUNX3/DNA methylation control its expression level; at the signaling level, CLDN1 promotes EMT via SNAI1 interaction and Notch activation, induces autophagy through AMPK/STAT1/ULK1, suppresses metastasis in lung adenocarcinoma via an EPHB6-ERK1/2-SLUG feedback axis, and directly interacts with MMP14 to regulate cell migration, collectively establishing CLDN1 as a context-dependent regulator of epithelial barrier integrity, cell survival, invasion, and chemosensitivity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CLDN1 is a tetraspan tight junction transmembrane protein that establishes paracellular barrier function in epithelia and contributes to epidermal differentiation [#0, #1, #24]. Re-expression in claudin-negative cells localizes CLDN1 to cell-cell contacts and restores tight junction gate function independently of occludin, with the extracellular loop mediating junction assembly [#1, #11]. Correct membrane targeting requires proper folding: the p.Arg81His missense variant causes mislocalization and distorted tight junction ultrastructure in keratinocytes [#15, #22], and CRISPR knockout in keratinocytes impairs barrier function, stratification, and filaggrin/cytokeratin-10 expression [#24]. CLDN1 transcription is governed by a defined regulatory network in which SP1 acts as a direct activator and is antagonized by BHLHE40, which sequesters SP1 from the promoter, by STAT3, which binds and represses the promoter downstream of IL-33/ERK signaling, and by EZH2-deposited H3K27me3 at the transcription start site [#8, #9, #13]; positive inputs include c-Fos recruited by the lncRNA WAKMAR2 [#16]. Beyond its junctional role, CLDN1 functions as a context-dependent regulator of invasion and cell survival: it drives epithelial-mesenchymal transition through interaction with SNAI1 and activation of Notch signaling [#3, #4], directly binds MMP14 to promote migration and inflammation [#18], suppresses lung adenocarcinoma metastasis via a CLDN1-EPHB6-ERK1/2-SLUG feedback axis [#10], and promotes chemoresistance by activating autophagy through ULK1 and AMPK/STAT1/ULK1 signaling [#6, #12].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Established the molecular identity of CLDN1 as a claudin-family tight junction transmembrane protein, framing all subsequent functional work on barrier and polarity.\",\n      \"evidence\": \"cDNA cloning and sequence analysis of SEMP1/CLDN1\",\n      \"pmids\": [\"9931503\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No functional reconstitution in the cloning study\", \"Tissue distribution and partner proteins not defined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Resolved whether CLDN1 alone is sufficient for tight junction gate function, showing it restores paracellular barrier independently of occludin.\",\n      \"evidence\": \"Retroviral re-expression in claudin-negative breast cancer cells with localization and dextran flux assays\",\n      \"pmids\": [\"11920682\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of selective permeability not addressed\", \"Did not define which extracellular determinants mediate sealing\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Linked CLDN1 membrane localization to apoptosis in 3D culture, indicating its barrier role has consequences for nutrient access and cell survival.\",\n      \"evidence\": \"Comparison of membrane vs cytosolic CLDN1 in 2D and 3D spheroid breast cancer cultures\",\n      \"pmids\": [\"14648703\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Death-triggering signal mechanistically undefined\", \"Generalizability beyond breast spheroids unknown\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Connected CLDN1 to EMT and invasion through SNAI1 interaction and Notch activation, establishing a non-junctional pro-invasive function.\",\n      \"evidence\": \"Overexpression/knockdown with EMT markers, SNAI1 Co-IP, Notch rescue with Jagged-1, xenografts in cervical and bronchial cells\",\n      \"pmids\": [\"27974683\", \"28316062\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct binding interface with SNAI1 not mapped\", \"Whether junctional vs cytosolic CLDN1 drives EMT unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identified autophagy activation via ULK1 as a route by which CLDN1 confers chemoresistance, expanding its role into cell survival signaling.\",\n      \"evidence\": \"CLDN1 manipulation with ULK1 phosphorylation analysis and autophagosome imaging in NSCLC under cisplatin\",\n      \"pmids\": [\"28614291\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism coupling CLDN1 to ULK1 phosphorylation unknown\", \"Direct vs indirect link not established\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Defined the core transcriptional control of CLDN1, with SP1 as direct activator antagonized by BHLHE40 and by STAT3 downstream of IL-33/ERK.\",\n      \"evidence\": \"Promoter mapping, deletion mutants, BHLHE40-SP1 Co-IP, EMSA for STAT3 binding, and barrier assays in keratinocytes\",\n      \"pmids\": [\"29534857\", \"29704436\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Combinatorial logic among regulators not modeled\", \"Tissue-specific selection of activators vs repressors unclear\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrated context-dependent CLDN1 functions: metastasis suppression via an EPHB6-ERK1/2-SLUG axis, autophagy-driven progression via AMPK/STAT1/ULK1, and an extracellular-loop role in TJ assembly.\",\n      \"evidence\": \"Co-IP, ChIP, methylation assays, peptide TJ-disruption experiments, and xenograft models across lung and esophageal systems\",\n      \"pmids\": [\"32754286\", \"31498437\", \"31381894\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"What dictates pro- vs anti-metastatic output not defined\", \"Nuclear CLDN1 distribution in ESCC mechanistically unexplained\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Showed CLDN1 transcription is epigenetically repressed by EZH2/H3K27me3 and that CLDN1 supports cell survival via BIRC3, deepening regulatory and pro-survival mechanisms.\",\n      \"evidence\": \"ChIP-qPCR with EZH2 inhibitor rescue in vivo and BIRC3 rescue of CLDN1-knockdown apoptosis in trophoblasts\",\n      \"pmids\": [\"34789399\", \"33784242\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How EZH2 is recruited to the CLDN1 TSS not defined\", \"Link between CLDN1 and BIRC3 regulation indirect\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Established that proper folding underlies CLDN1 membrane targeting and that lncRNA WAKMAR2/c-Fos positively drives CLDN1 transcription to maintain barrier integrity.\",\n      \"evidence\": \"p.Arg81His variant cell modeling with immunofluorescence; RNA pull-down, promoter reporter, and ChIRP-seq for WAKMAR2\",\n      \"pmids\": [\"35920354\", \"36461623\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Disease mechanism of the variant at organism level not detailed here\", \"Conditions selecting WAKMAR2/c-Fos activation unclear\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified posttranscriptional and physical-partner control of CLDN1 (LIN28B mRNA binding, MMP14 direct interaction) and multiple upstream pathways (WNT7A/AKT, PI3K/AKT, MAPKp38/Wnt) governing CLDN1 in invasion, survival, and chemoresistance.\",\n      \"evidence\": \"RNA immunoprecipitation, Co-IP, pharmacological epistasis, phosphoproteomics, and in vivo metastasis/chemotherapy models across CRC, OSCC, ASMC, and trophoblast systems\",\n      \"pmids\": [\"37318881\", \"37964516\", \"37541622\", \"37277581\", \"37041570\", \"37523840\", \"37814412\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether AKT acts on CLDN1 transcription or stability not separated\", \"MMP14 and PMP22 binding interfaces unmapped\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Provided clean loss-of-function evidence that CLDN1 is required for both barrier function and epidermal stratification, and placed it downstream of PDGFRβ signaling in adhesion biology.\",\n      \"evidence\": \"CRISPR/Cas9 keratinocyte knockout with organotypic and differentiation analyses; pathway analysis in a tendon adhesion model\",\n      \"pmids\": [\"38711223\", \"38699245\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How CLDN1 loss reprograms differentiation gene expression unknown\", \"PDGFRβ-to-CLDN1 link is correlative\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Unbiased screening identified CLDN1 as a mediator of drug sensitivity and reported a new physical partner, extending its functional repertoire.\",\n      \"evidence\": \"Genome-wide CRISPR knockout screen for berberine sensitivity in A549; ALFA-tag Co-IP/mass spectrometry identifying PMP22 in MDCKII cells (preprint)\",\n      \"pmids\": [\"41383979\", \"bio_10.1101_2025.09.03.673966\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of berberine resistance via CLDN1 undefined\", \"PMP22 interaction lacks functional validation and reciprocal confirmation\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Linked extracellular matrix signaling to CLDN1 regulation, showing HM-HA via CD44 and HYAL1-dependent HA turnover control CLDN1 levels and barrier function.\",\n      \"evidence\": \"HM-HA treatment with CD44 and HYAL1 siRNA knockdown and barrier readouts in keratinocytes\",\n      \"pmids\": [\"41881410\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Signaling cascade from CD44 to CLDN1 transcription not mapped\", \"In vivo relevance to skin barrier disease unaddressed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved what molecular switch determines whether CLDN1 acts as a barrier-sealing junctional protein versus a pro-invasive/pro-survival signaling effector, and how its subcellular distribution (membrane vs cytosolic/nuclear) is controlled to select these outputs.\",\n      \"evidence\": \"No single study in the corpus reconciles the opposing tumor-suppressive and tumor-promoting roles\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structural model of CLDN1 in distinct signaling complexes\", \"Determinants of nuclear/cytosolic relocalization unknown\", \"Context-dependence across tissues not mechanistically unified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 11, 24]},\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [1, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 2, 15, 22]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [1, 11, 24]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [6, 12]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [8, 9, 13, 16]}\n    ],\n    \"complexes\": [\"tight junction\"],\n    \"partners\": [\"SNAI1\", \"MMP14\", \"EPHB6\", \"PMP22\", \"SP1\", \"LIN28B\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}