{"gene":"MPP3","run_date":"2026-04-28T18:30:28","timeline":{"discoveries":[{"year":2003,"finding":"MPP3 directly associates with TSLC1 (CADM1) through a PDZ-binding motif at the carboxyl terminus of TSLC1, and both proteins colocalize at cell-cell attachment sites at low and high cell density, placing MPP3 in the same cell-cell interaction cascade as TSLC1 in lung epithelial cells.","method":"Co-immunoprecipitation, GST pulldown, immunofluorescence colocalization","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal Co-IP with PDZ-motif dependence shown, single lab","pmids":["13679854"],"is_preprint":false},{"year":2006,"finding":"MPP3 prevents desensitization of the serotonin 5-HT2C receptor-mediated Ca2+ response and increases cell surface expression of the receptor, opposite to the effect of PSD-95, in both heterologous cells and cortical neurons; this effect is mediated through PDZ-domain interaction with the C-terminus of the receptor.","method":"Ca2+ response assays in HEK293 cells and primary cortical neurons, cell surface expression assays, Tat-peptide inhibitor of PDZ interactions, loss-of-function","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 — functional assay in neurons plus heterologous cells with peptide inhibitor control, single lab","pmids":["16914526"],"is_preprint":false},{"year":2006,"finding":"MPP3 localizes at the photoreceptor synapse and at the subapical region adjacent to adherens junctions at the outer limiting membrane (OLM) of the retina, where it colocalizes with MPP5 and CRB1; MPP3 is recruited to the MPP5 protein scaffold and forms separate complexes with DLG1 at the outer plexiform layer, together with MPP4.","method":"Immunofluorescence localization in mouse and human retinae, co-immunoprecipitation of endogenous complexes in vivo","journal":"The FEBS journal","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization with in vivo Co-IP demonstrating distinct complexes, single lab","pmids":["16519681"],"is_preprint":false},{"year":2013,"finding":"MPP3 is required for maintenance of the apical protein complex and adherens junctions in cortical progenitor cells; conditional knockout of Mpp3 in mouse cortex causes loss of apical complex proteins, disrupted adherens junctions, randomized spindle orientation, ectopically localized mitotic cells, delayed neuronal migration, and defective cortical stratification, without affecting cell division rate or cell cycle exit.","method":"Conditional knockout mouse (Cre-lox), immunofluorescence, confocal microscopy, BrdU/EdU cell cycle analysis, spindle orientation analysis","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — clean conditional KO with multiple orthogonal phenotypic readouts and mechanistic specificity","pmids":["23658188"],"is_preprint":false},{"year":2013,"finding":"MPP3 is required for maintaining proper levels of PALS1 (MPP5) at the subapical region in Müller glia cells of the retina; conditional knockout of Mpp3 causes late-onset retinal degeneration with rosette formation, accelerated by light exposure, and significant loss of PALS1 at the subapical region; loss of MPP3 in Pals1 conditional knockdown retinas significantly accelerates onset of retinal degeneration.","method":"Conditional knockout mouse, electroretinography, electron microscopy ultrastructural localization, immunofluorescence, genetic epistasis (double knockout)","journal":"Glia","confidence":"High","confidence_rationale":"Tier 2 — conditional KO with epistasis experiment and multiple orthogonal readouts, single lab","pmids":["23893895"],"is_preprint":false},{"year":2014,"finding":"MPP3, together with Dlg, forms a multi-protein complex connecting CADM1 to the p85 regulatory subunit of PI3K at the cell periphery; trans-homophilic interaction of CADM1 activates the PI3K pathway, leading to Akt and Rac1 activation and actin cytoskeleton reorganization required for cell spreading and epithelial structure formation.","method":"Cell-based spreading assay with chemical inhibitor screening (LY294002, Akt inhibitor, Rac1 inhibitor), co-immunoprecipitation of CADM1-MPP3-Dlg-p85 complex, confocal microscopy","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP of multi-protein complex combined with functional inhibitor epistasis, single lab with two publications","pmids":["24503895","25268382"],"is_preprint":false},{"year":2015,"finding":"MPP3 forms a stable cell adhesion complex with CADM1 and the actin-binding protein 4.1B at the plasma membrane; FRAP analysis reveals that MPP3 has two distinct fractions — one forming a long-lived complex with CADM1 (τ ~16 min) and a free pool (τ ~40-60 sec) — with CADM1 playing a central role in stabilizing the complex.","method":"FRAP (fluorescence recovery after photobleaching) with exponential curve-fitting, FLIP (fluorescence loss in photobleaching), live-cell imaging","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — quantitative live-cell FRAP with mathematical modeling distinguishing complex fractions, single lab","pmids":["25780926"],"is_preprint":false},{"year":2014,"finding":"MPP3 is significantly upregulated in high-metastatic-potential hepatocellular carcinoma (HCC) cell lines and clinical tissue; forced expression of MPP3 enhances HCC cell migration and invasion, while knockdown inhibits these effects; mechanistically, MPP3 promotes migration and invasion by upregulating matrix metalloproteinase 1 (MMP1).","method":"Forced expression and siRNA knockdown in HCC cell lines, migration/invasion assays (Transwell), qRT-PCR and western blot for MMP1","journal":"Cancer letters","confidence":"Medium","confidence_rationale":"Tier 3 — KD/OE with defined downstream effector (MMP1) but no direct binding or pathway epistasis, single lab","pmids":["24513266"],"is_preprint":false},{"year":2025,"finding":"MPP3 expression in enteric neural crest cells (ENCCs) is downregulated by MIR22HG (delivered via Schwann cell-derived extracellular vesicles) through an m6A-dependent ALKBH5/IGF2BP3 axis; knockdown of MPP3 inhibits ENCC proliferation and migration, and in vivo MPP3 reduction blocks ENCC colonization of the distal colon and disrupts enteric nervous system formation.","method":"Extracellular vesicle treatment of ENCCs, siRNA knockdown of MPP3, proliferation and migration assays, m6A sequencing, in vivo mouse colonization assay","journal":"Communications biology","confidence":"Medium","confidence_rationale":"Tier 3 — KD with defined phenotypic readout and upstream regulatory mechanism identified, single lab","pmids":["41286351"],"is_preprint":false}],"current_model":"MPP3 (MAGUK p55 subfamily member 3) is a scaffolding protein that localizes to cell-cell junctions and apical membrane domains where it organizes multi-protein complexes: it is recruited to the CRB1/MPP5 scaffold at the retinal outer limiting membrane (where it maintains PALS1 levels and photoreceptor-Müller glia adhesion), connects CADM1 to PI3K via a CADM1-MPP3-Dlg-p85 complex to activate PI3K/Akt/Rac1 signaling and actin reorganization, stabilizes the apical junctional complex in cortical progenitors to regulate spindle orientation and neuronal migration, and modulates 5-HT2C receptor desensitization and surface expression through PDZ-domain interactions, with loss of MPP3 causing retinal degeneration and cortical stratification defects."},"narrative":{"teleology":[{"year":2003,"claim":"Identifying MPP3 as a direct PDZ-dependent binding partner of CADM1 (TSLC1) at cell-cell contacts established that this MAGUK scaffold operates in an intercellular adhesion cascade in epithelial cells.","evidence":"Reciprocal Co-IP and GST pulldown with PDZ-motif deletion controls in lung epithelial cells","pmids":["13679854"],"confidence":"Medium","gaps":["No in vivo validation of the CADM1–MPP3 interaction","Downstream signaling consequences of the interaction were unknown"]},{"year":2006,"claim":"Demonstrating that MPP3 localizes to the subapical region and photoreceptor synapse in retina, where it is recruited to the MPP5/CRB1 scaffold, placed MPP3 within the Crumbs polarity complex and suggested tissue-specific complex assembly with DLG1.","evidence":"Immunofluorescence in mouse and human retinae with endogenous Co-IP confirming MPP3–MPP5 and MPP3–DLG1 complexes","pmids":["16519681"],"confidence":"Medium","gaps":["Loss-of-function consequences in the retina were not yet established","Stoichiometry and direct versus bridged interactions within the complex were unresolved"]},{"year":2006,"claim":"Showing that MPP3 opposes PSD-95 by preventing 5-HT2C receptor desensitization and increasing its surface expression revealed a PDZ-dependent role for MPP3 in GPCR trafficking and signaling in neurons.","evidence":"Ca²⁺ response assays in HEK293 cells and primary cortical neurons with Tat-peptide PDZ inhibitor controls","pmids":["16914526"],"confidence":"Medium","gaps":["Mechanism by which MPP3 stabilizes receptor surface expression (endocytic versus recycling) was not delineated","In vivo neuronal phenotype of MPP3 loss on serotonergic signaling was not tested"]},{"year":2013,"claim":"Conditional knockout of Mpp3 in cortical progenitors proved that MPP3 is essential for maintaining the apical protein complex, adherens junctions, and correct spindle orientation, resolving whether it is required or redundant for cortical development.","evidence":"Cre-lox conditional KO in mouse cortex with immunofluorescence, BrdU/EdU cell cycle, and spindle orientation analysis","pmids":["23658188"],"confidence":"High","gaps":["Which specific apical complex proteins are direct MPP3 interactors versus secondarily affected was not distinguished","Postnatal cortical circuit consequences were not assessed"]},{"year":2013,"claim":"Conditional deletion of Mpp3 in retina demonstrated that MPP3 maintains PALS1 levels at the subapical region and that its loss causes light-accelerated retinal degeneration, with genetic epistasis showing synergy with Pals1 haploinsufficiency.","evidence":"Conditional KO mouse with ERG, EM ultrastructure, immunofluorescence, and Mpp3/Pals1 double-knockout epistasis","pmids":["23893895"],"confidence":"High","gaps":["Whether MPP3 stabilizes PALS1 by direct protein-protein interaction or indirectly through complex integrity was not resolved","Contribution of individual retinal cell types (photoreceptors vs. Müller glia) was not fully separated"]},{"year":2014,"claim":"Identification of the CADM1–MPP3–Dlg–p85 quaternary complex linked MPP3 scaffolding to PI3K/Akt/Rac1 activation and actin reorganization, providing a signaling mechanism downstream of trans-homophilic CADM1 adhesion.","evidence":"Co-IP of the multi-protein complex with functional inhibitor epistasis (LY294002, Akt and Rac1 inhibitors) in cell spreading assays","pmids":["24503895","25268382"],"confidence":"Medium","gaps":["Direct binding interfaces between MPP3 and p85 or Dlg were not mapped","In vivo relevance of this signaling axis was not tested"]},{"year":2015,"claim":"Quantitative FRAP analysis showed that MPP3 exists in two kinetically distinct pools — a long-lived CADM1-bound complex (τ ~16 min) and a rapidly exchanging free pool (τ ~40–60 s) — establishing the dynamic behavior of MPP3 at adhesion sites.","evidence":"FRAP and FLIP live-cell imaging with exponential curve-fitting in cultured cells","pmids":["25780926"],"confidence":"Medium","gaps":["Whether the two pools have distinct functional roles was not determined","Dynamics were not measured in polarized epithelial or retinal contexts"]},{"year":2025,"claim":"Demonstration that MPP3 expression in enteric neural crest cells is downregulated by Schwann cell-derived extracellular vesicles carrying MIR22HG, and that MPP3 knockdown blocks ENCC proliferation, migration, and distal colon colonization, extended MPP3's functional importance to enteric nervous system development.","evidence":"siRNA knockdown in ENCCs, extracellular vesicle treatment, m6A sequencing, and in vivo mouse colonization assay","pmids":["41286351"],"confidence":"Medium","gaps":["Downstream effectors of MPP3 in ENCC migration were not identified","Whether known MPP3 complexes (CADM1–Dlg–PI3K) operate in this context is unknown","Single lab; awaits independent replication"]},{"year":null,"claim":"The structural basis of MPP3 scaffold assembly — how its SH3, GUK, and PDZ domains simultaneously engage distinct partners to form tissue-specific complexes — and the full range of physiological contexts requiring MPP3 remain to be determined.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal structure or cryo-EM model of MPP3 in complex with any partner","No systematic interactome beyond candidate-based approaches","Relationship between retinal, cortical, and enteric functions has not been integrated mechanistically"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,2,5,6]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,5,6]}],"pathway":[{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[0,6]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[0,3,5,6]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,5]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[3,4,8]}],"complexes":["CRB1/MPP5/MPP3 apical complex","CADM1-MPP3-Dlg-p85 signaling complex"],"partners":["CADM1","MPP5","DLG1","CRB1","PIK3R1","HTR2C","MPP4"],"other_free_text":[]},"mechanistic_narrative":"MPP3 is a MAGUK-family scaffolding protein that organizes apical junctional complexes and multi-protein signaling assemblies at cell-cell contact sites in epithelia, retina, and developing cortex. At the retinal outer limiting membrane, MPP3 is recruited to the CRB1/MPP5 scaffold, maintains PALS1 (MPP5) levels in Müller glia, and its conditional loss causes late-onset retinal degeneration accelerated by light exposure [PMID:16519681, PMID:23893895]. In cortical progenitors, MPP3 is required for adherens junction integrity, proper spindle orientation, and neuronal migration; conditional knockout produces ectopic mitoses and defective cortical stratification [PMID:23658188]. MPP3 also bridges CADM1 to PI3K via a CADM1–MPP3–Dlg–p85 complex that activates Akt/Rac1 signaling and actin reorganization for epithelial cell spreading, and it modulates 5-HT2C receptor surface expression and desensitization through its PDZ domain [PMID:24503895, PMID:16914526]."},"prefetch_data":{"uniprot":{"accession":"Q13368","full_name":"MAGUK p55 subfamily member 3","aliases":["Discs large homolog 3","Protein MPP3"],"length_aa":585,"mass_kda":66.2,"function":"Participates in cell spreading through the phosphoinositide-3-kinase (PI3K) pathway by connecting CADM1 to DLG1 and the regulatory subunit of phosphoinositide-3-kinase (PI3K) (PubMed:24503895). Stabilizes HTR2C at the plasma membrane and prevents its desensitization. May participates in the maintenance of adherens junctions (By similarity)","subcellular_location":"Cell membrane; Apical cell membrane; Cell junction, adherens junction","url":"https://www.uniprot.org/uniprotkb/Q13368/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/MPP3","classification":"Not Classified","n_dependent_lines":51,"n_total_lines":1208,"dependency_fraction":0.042218543046357616},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/MPP3","total_profiled":1310},"omim":[{"mim_id":"610973","title":"MEMBRANE PROTEIN, PALMITOYLATED 7; MPP7","url":"https://www.omim.org/entry/610973"},{"mim_id":"606575","title":"MEMBRANE PROTEIN, PALMITOYLATED 4; MPP4","url":"https://www.omim.org/entry/606575"},{"mim_id":"601114","title":"MEMBRANE PROTEIN, PALMITOYLATED 3; MPP3","url":"https://www.omim.org/entry/601114"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"brain","ntpm":28.2},{"tissue":"heart muscle","ntpm":14.5}],"url":"https://www.proteinatlas.org/search/MPP3"},"hgnc":{"alias_symbol":[],"prev_symbol":["DLG3"]},"alphafold":{"accession":"Q13368","domains":[{"cath_id":"-","chopping":"11-59","consensus_level":"medium","plddt":74.6157,"start":11,"end":59},{"cath_id":"-","chopping":"69-113","consensus_level":"medium","plddt":87.9511,"start":69,"end":113},{"cath_id":"2.30.42.10","chopping":"137-216","consensus_level":"high","plddt":88.5095,"start":137,"end":216},{"cath_id":"2.30.30.40","chopping":"230-303","consensus_level":"high","plddt":91.3323,"start":230,"end":303},{"cath_id":"3.40.50.300","chopping":"381-413_478-509_529-585","consensus_level":"medium","plddt":90.2621,"start":381,"end":585}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13368","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q13368-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q13368-F1-predicted_aligned_error_v6.png","plddt_mean":78.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MPP3","jax_strain_url":"https://www.jax.org/strain/search?query=MPP3"},"sequence":{"accession":"Q13368","fasta_url":"https://rest.uniprot.org/uniprotkb/Q13368.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q13368/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13368"}},"corpus_meta":[{"pmid":"15185169","id":"PMC_15185169","title":"Mutations 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sites\",\n      \"method\": \"Co-immunoprecipitation, pulldown, colocalization by immunofluorescence\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP with domain mapping (PDZ-binding motif dependency), replicated in multiple cell contexts\",\n      \"pmids\": [\"13679854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"MPP3 prevents desensitization of the 5-HT2C receptor-mediated Ca2+ response and maintains cell surface expression of the receptor through PDZ domain interaction with the receptor C-terminus, opposing the effect of PSD-95\",\n      \"method\": \"Heterologous cell expression, primary cortical neuron culture, Ca2+ response assay, cell surface expression measurement, Tat-peptide inhibitor of PDZ interactions\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional assay in both heterologous cells and primary neurons, peptide inhibitor controls, multiple orthogonal readouts\",\n      \"pmids\": [\"16914526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"MPP3 is recruited to the MPP5 protein scaffold at the outer limiting membrane (OLM) of the retina, where it colocalizes with MPP5 and CRB1 at the subapical region, and forms separate complexes with DLG1 at the outer plexiform layer\",\n      \"method\": \"Co-immunoprecipitation in vivo, immunofluorescence localization in mouse and human retinae, subcellular fractionation\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP demonstrating separate complexes in vivo combined with detailed localization studies\",\n      \"pmids\": [\"16519681\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Dlg3 recruits ubiquitin ligases Nedd4 and Nedd4-2 through its PPxY motifs, leading to Dlg3 monoubiquitination, apical membrane recruitment, and tight junction consolidation in polarized epithelial cells\",\n      \"method\": \"Affinity purification of Dlg3 complexes by mass spectrometry, Co-IP, mutagenesis of PPxY motifs, conditional knockout mice, immunofluorescence\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — complex purification by MS, domain mutagenesis, in vivo knockout with specific molecular and cellular readouts\",\n      \"pmids\": [\"21920314\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"MPP3 is required for maintenance of the apical junctional complex and adherens junctions in cortical progenitor cells; conditional knockout of Mpp3 causes loss of apical complex proteins, randomized spindle orientation, ectopic mitotic cells, delayed neuronal migration, and defective cortical stratification\",\n      \"method\": \"Conditional knockout mice (Mpp3 cKO), immunofluorescence, spindle orientation analysis, BrdU cell cycle assay, layer marker analysis\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with multiple specific phenotypic readouts in vivo\",\n      \"pmids\": [\"23658188\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"MPP3 is required for maintaining PALS1 levels at the subapical region of Müller glia in the retina; loss of MPP3 causes significant reduction of PALS1 at adherens junctions, and combined loss of MPP3 and PALS1 accelerates retinal degeneration\",\n      \"method\": \"Conditional knockout mice, immunofluorescence, electroretinography, electron microscopy, double conditional knockout epistasis\",\n      \"journal\": \"Glia\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with genetic epistasis and multiple cellular/functional readouts\",\n      \"pmids\": [\"23893895\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"MPP3 and Dlg form a multi-protein complex with CADM1 and p85 of PI3K at the cell periphery, connecting CADM1 trans-homophilic adhesion to PI3K activation, which drives Akt/Rac1-dependent actin cytoskeleton reorganization and epithelial cell spreading\",\n      \"method\": \"Cell-based spreading assay, chemical inhibitor screen, Co-immunoprecipitation, colocalization by immunofluorescence\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP demonstrating multi-protein complex, functional chemical inhibitor screen with multiple downstream effectors tested, replicated in two papers from same group\",\n      \"pmids\": [\"24503895\", \"25268382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"A duplication variant in the 5' UTR of DLG3 attenuates protein translation without affecting mRNA levels, establishing that the 5' UTR regulates DLG3/SAP102 protein production\",\n      \"method\": \"Dual luciferase reporter assay, Western blot of patient blood cells, RT-PCR\",\n      \"journal\": \"European journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reporter assay plus patient sample validation, but single lab\",\n      \"pmids\": [\"27222290\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"DLG3 negatively regulates RAC1 expression and suppresses PI3K/AKT pathway activation in breast cancer cells; DLG3 overexpression inhibits RAC1 and downstream EMT\",\n      \"method\": \"DLG3 overexpression/knockdown, Western blot, CCK-8, Transwell assay\",\n      \"journal\": \"Evidence-based complementary and alternative medicine\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, overexpression/knockdown with pathway readouts but no direct biochemical mechanism for RAC1 regulation\",\n      \"pmids\": [\"34765009\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"COP1 E3 ubiquitin ligase binds DLG3 protein and promotes its ubiquitination and degradation, facilitating glioma cell proliferation, invasion, and migration\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay with MG132 proteasome inhibitor, siRNA knockdown, rescue experiments\",\n      \"journal\": \"Neurological research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP confirmed binding, ubiquitination assay confirmed modification, functional rescue validated pathway, but single lab\",\n      \"pmids\": [\"37356109\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"MIAT lncRNA binds methylation proteins DNMT1, DNMT3A, and DNMT3B to promote CpG island methylation of the DLG3 promoter, suppressing DLG3 expression; DLG3 in turn binds MST2 and regulates LATS1, preventing nuclear translocation of YAP to activate the Hippo signaling pathway\",\n      \"method\": \"RIP assay, MS-PCR, Co-IP, siRNA knockdown, in vitro and in vivo tumor models\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — RIP and Co-IP for binding, methylation assay, functional rescue in vitro and in vivo, but single lab\",\n      \"pmids\": [\"32593652\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Schwann cell-derived extracellular vesicles transfer MIR22HG to enteric neural crest cells, downregulating MPP3 expression via the m6A/ALKBH5/IGF2BP3 pathway, thereby disrupting enteric neural crest cell colonization and ENS formation\",\n      \"method\": \"EV treatment assays, in vitro proliferation/migration assays, in vivo ENCC colonization assay, m6A pathway inhibition\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, mechanistic pathway partially characterized but MPP3's direct molecular function not defined\",\n      \"pmids\": [\"41286351\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MPP3 (DLG3/SAP102) is a MAGUK-family scaffolding protein that uses its PDZ domains to directly bind the C-termini of transmembrane proteins (including NMDA receptor NR2 subunits, 5-HT2C receptor, and CADM1/TSLC1), recruits PI3K (via p85) and downstream signaling effectors, maintains apical junctional complexes and PALS1 levels in polarized epithelia and retina, undergoes Nedd4/Nedd4-2-mediated monoubiquitination through PPxY motifs for apical membrane trafficking, is subject to COP1-mediated ubiquitin-proteasomal degradation, and is required in cortical progenitors for proper spindle orientation, neuronal migration, and cortical stratification.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper first, then extract discoveries only from KEEP papers.\n\n**Classification:**\n\nThe gene MPP3 (also known as DLG3/SAP102 in some contexts, but note: MPP3 is MAGUK p55 subfamily member 3, a distinct protein from DLG3/SAP102). \n\nWait - I need to carefully distinguish: MPP3 (MAGUK p55 subfamily member 3, gene symbol MPP3) vs DLG3 (Discs Large Homolog 3, also called SAP102/neuroendocrine-Dlg). These are DIFFERENT genes. MPP3 is on chromosome 17q12-q21 (the original DLG3 paper [10] describes a protein on 17q12-q21 with 66kDa). However, looking at the corpus carefully:\n\n- Papers about DLG3/SAP102 (X-linked, involved in NMDA receptor clustering, mental retardation): EXCLUDE as alias collision - these are about a different gene\n- Papers explicitly about MPP3 as MAGUK p55 subfamily member 3: KEEP\n\nAfter careful review: MPP3 (MAGUK p55 subfamily member 3) is distinct from DLG3 (SAP102). Papers [1],[4],[5],[6],[9],[11],[13],[14],[15],[17],[19],[20],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34] from the main corpus that focus on DLG3/SAP102 as X-linked intellectual disability gene are EXCLUDE.\n\nPapers [2],[3],[7],[8],[16],[18],[21],[22] and [12] from main corpus about MPP3 as MAGUK/MPP5-scaffold member: KEEP.\n\nPaper [12] (secretory MPP3 in hematopoiesis): This uses \"MPP3\" to mean \"multipotent progenitor 3\" - EXCLUDE (alias collision).\nPaper [35] (MIR22HG/MPP3 in ENCC): Uses MPP3 as downstream target in neural crest - KEEP if about the protein MPP3.\nPapers [37],[38] (preprints about MPP3 progenitors): EXCLUDE (hematopoietic progenitor alias).\n\nFrom gene2pubmed additional papers: mostly large interactome/GWAS studies - scan for MPP3-specific mechanistic findings.\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2003,\n      \"finding\": \"MPP3 directly associates with TSLC1 (CADM1) through a PDZ-binding motif at the carboxyl terminus of TSLC1, and both proteins colocalize at cell-cell attachment sites at low and high cell density, placing MPP3 in the same cell-cell interaction cascade as TSLC1 in lung epithelial cells.\",\n      \"method\": \"Co-immunoprecipitation, GST pulldown, immunofluorescence colocalization\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP with PDZ-motif dependence shown, single lab\",\n      \"pmids\": [\"13679854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"MPP3 prevents desensitization of the serotonin 5-HT2C receptor-mediated Ca2+ response and increases cell surface expression of the receptor, opposite to the effect of PSD-95, in both heterologous cells and cortical neurons; this effect is mediated through PDZ-domain interaction with the C-terminus of the receptor.\",\n      \"method\": \"Ca2+ response assays in HEK293 cells and primary cortical neurons, cell surface expression assays, Tat-peptide inhibitor of PDZ interactions, loss-of-function\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional assay in neurons plus heterologous cells with peptide inhibitor control, single lab\",\n      \"pmids\": [\"16914526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"MPP3 localizes at the photoreceptor synapse and at the subapical region adjacent to adherens junctions at the outer limiting membrane (OLM) of the retina, where it colocalizes with MPP5 and CRB1; MPP3 is recruited to the MPP5 protein scaffold and forms separate complexes with DLG1 at the outer plexiform layer, together with MPP4.\",\n      \"method\": \"Immunofluorescence localization in mouse and human retinae, co-immunoprecipitation of endogenous complexes in vivo\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization with in vivo Co-IP demonstrating distinct complexes, single lab\",\n      \"pmids\": [\"16519681\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"MPP3 is required for maintenance of the apical protein complex and adherens junctions in cortical progenitor cells; conditional knockout of Mpp3 in mouse cortex causes loss of apical complex proteins, disrupted adherens junctions, randomized spindle orientation, ectopically localized mitotic cells, delayed neuronal migration, and defective cortical stratification, without affecting cell division rate or cell cycle exit.\",\n      \"method\": \"Conditional knockout mouse (Cre-lox), immunofluorescence, confocal microscopy, BrdU/EdU cell cycle analysis, spindle orientation analysis\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with multiple orthogonal phenotypic readouts and mechanistic specificity\",\n      \"pmids\": [\"23658188\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"MPP3 is required for maintaining proper levels of PALS1 (MPP5) at the subapical region in Müller glia cells of the retina; conditional knockout of Mpp3 causes late-onset retinal degeneration with rosette formation, accelerated by light exposure, and significant loss of PALS1 at the subapical region; loss of MPP3 in Pals1 conditional knockdown retinas significantly accelerates onset of retinal degeneration.\",\n      \"method\": \"Conditional knockout mouse, electroretinography, electron microscopy ultrastructural localization, immunofluorescence, genetic epistasis (double knockout)\",\n      \"journal\": \"Glia\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with epistasis experiment and multiple orthogonal readouts, single lab\",\n      \"pmids\": [\"23893895\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"MPP3, together with Dlg, forms a multi-protein complex connecting CADM1 to the p85 regulatory subunit of PI3K at the cell periphery; trans-homophilic interaction of CADM1 activates the PI3K pathway, leading to Akt and Rac1 activation and actin cytoskeleton reorganization required for cell spreading and epithelial structure formation.\",\n      \"method\": \"Cell-based spreading assay with chemical inhibitor screening (LY294002, Akt inhibitor, Rac1 inhibitor), co-immunoprecipitation of CADM1-MPP3-Dlg-p85 complex, confocal microscopy\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP of multi-protein complex combined with functional inhibitor epistasis, single lab with two publications\",\n      \"pmids\": [\"24503895\", \"25268382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"MPP3 forms a stable cell adhesion complex with CADM1 and the actin-binding protein 4.1B at the plasma membrane; FRAP analysis reveals that MPP3 has two distinct fractions — one forming a long-lived complex with CADM1 (τ ~16 min) and a free pool (τ ~40-60 sec) — with CADM1 playing a central role in stabilizing the complex.\",\n      \"method\": \"FRAP (fluorescence recovery after photobleaching) with exponential curve-fitting, FLIP (fluorescence loss in photobleaching), live-cell imaging\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — quantitative live-cell FRAP with mathematical modeling distinguishing complex fractions, single lab\",\n      \"pmids\": [\"25780926\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"MPP3 is significantly upregulated in high-metastatic-potential hepatocellular carcinoma (HCC) cell lines and clinical tissue; forced expression of MPP3 enhances HCC cell migration and invasion, while knockdown inhibits these effects; mechanistically, MPP3 promotes migration and invasion by upregulating matrix metalloproteinase 1 (MMP1).\",\n      \"method\": \"Forced expression and siRNA knockdown in HCC cell lines, migration/invasion assays (Transwell), qRT-PCR and western blot for MMP1\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — KD/OE with defined downstream effector (MMP1) but no direct binding or pathway epistasis, single lab\",\n      \"pmids\": [\"24513266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"MPP3 expression in enteric neural crest cells (ENCCs) is downregulated by MIR22HG (delivered via Schwann cell-derived extracellular vesicles) through an m6A-dependent ALKBH5/IGF2BP3 axis; knockdown of MPP3 inhibits ENCC proliferation and migration, and in vivo MPP3 reduction blocks ENCC colonization of the distal colon and disrupts enteric nervous system formation.\",\n      \"method\": \"Extracellular vesicle treatment of ENCCs, siRNA knockdown of MPP3, proliferation and migration assays, m6A sequencing, in vivo mouse colonization assay\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — KD with defined phenotypic readout and upstream regulatory mechanism identified, single lab\",\n      \"pmids\": [\"41286351\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MPP3 (MAGUK p55 subfamily member 3) is a scaffolding protein that localizes to cell-cell junctions and apical membrane domains where it organizes multi-protein complexes: it is recruited to the CRB1/MPP5 scaffold at the retinal outer limiting membrane (where it maintains PALS1 levels and photoreceptor-Müller glia adhesion), connects CADM1 to PI3K via a CADM1-MPP3-Dlg-p85 complex to activate PI3K/Akt/Rac1 signaling and actin reorganization, stabilizes the apical junctional complex in cortical progenitors to regulate spindle orientation and neuronal migration, and modulates 5-HT2C receptor desensitization and surface expression through PDZ-domain interactions, with loss of MPP3 causing retinal degeneration and cortical stratification defects.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"MPP3 (also called DLG3/SAP102) is a MAGUK-family scaffolding protein that organizes transmembrane protein complexes at cell junctions and the plasma membrane to coordinate adhesion, polarity, and receptor signaling. Its PDZ domain directly binds the C-termini of CADM1/TSLC1 and the 5-HT2C serotonin receptor, maintaining receptor surface expression and linking CADM1 trans-homophilic adhesion to PI3K/Akt/Rac1 signaling and actin cytoskeleton reorganization [PMID:13679854, PMID:16914526, PMID:24503895]. In polarized epithelia and retina, MPP3 is recruited to the CRB1–MPP5/PALS1 apical complex, where Nedd4/Nedd4-2-mediated monoubiquitination via PPxY motifs directs its apical membrane trafficking and tight junction consolidation, and it is required to maintain PALS1 levels at adherens junctions of Müller glia [PMID:16519681, PMID:21920314, PMID:23893895]. Conditional knockout in cortical progenitors causes loss of apical junctional complex proteins, randomized mitotic spindle orientation, ectopic progenitor divisions, delayed neuronal migration, and defective cortical layering, establishing MPP3 as essential for cerebral cortex development [PMID:23658188].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Identification of MPP3 as a PDZ-dependent binding partner of the tumor suppressor CADM1/TSLC1 at cell-cell contacts established MPP3 as a junctional scaffold protein.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation with PDZ-motif deletion controls and colocalization in multiple cell lines\",\n      \"pmids\": [\"13679854\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of the MPP3–CADM1 interaction was not determined\", \"Other PDZ-binding partners not surveyed\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Two studies extended MPP3's scaffolding role to receptor trafficking and retinal polarity: MPP3 maintains 5-HT2C receptor surface expression and opposes desensitization via its PDZ domain, while in the retina MPP3 is recruited to the CRB1–MPP5 apical complex at the outer limiting membrane, revealing tissue-specific complex assembly.\",\n      \"evidence\": \"Ca2+ signaling assays and surface expression measurements in neurons and heterologous cells; reciprocal Co-IP and immunolocalization in mouse/human retinae\",\n      \"pmids\": [\"16914526\", \"16519681\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of MPP3–MPP5 versus MPP3–receptor interactions not resolved\", \"Whether MPP3 affects CRB1 complex function was unknown\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Discovery that Nedd4/Nedd4-2 E3 ligases monoubiquitinate MPP3 through PPxY motifs, directing it to the apical membrane for tight junction consolidation, revealed a post-translational mechanism controlling MPP3 localization and junctional integrity.\",\n      \"evidence\": \"Affinity purification–mass spectrometry, PPxY motif mutagenesis, conditional knockout mice with junctional phenotypes\",\n      \"pmids\": [\"21920314\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Deubiquitinase counteracting this modification not identified\", \"Whether monoubiquitination also affects MPP3 scaffolding partners was untested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Conditional knockout studies in cortex and retina demonstrated that MPP3 is essential in vivo for maintaining apical junctional complexes: loss in cortical progenitors randomizes spindle orientation and disrupts neuronal migration and cortical layering, while loss in retinal Müller glia depletes PALS1 and accelerates retinal degeneration.\",\n      \"evidence\": \"Mpp3 conditional knockout mice with spindle orientation quantification, layer-marker immunostaining, BrdU analysis (cortex); electroretinography, electron microscopy, double cKO epistasis (retina)\",\n      \"pmids\": [\"23658188\", \"23893895\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream signaling pathways mediating spindle orientation defects not defined\", \"Relative contributions of MPP3 versus PALS1 loss to retinal phenotype not fully delineated\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Linking CADM1 adhesion to intracellular signaling, MPP3 was shown to scaffold a CADM1–Dlg–p85(PI3K) complex that activates Akt/Rac1 and drives actin-dependent cell spreading, establishing MPP3 as a signal-transducing adaptor downstream of adhesion.\",\n      \"evidence\": \"Co-immunoprecipitation of multi-protein complex, chemical inhibitor dissection of PI3K/Akt/Rac1 pathway, cell spreading assays\",\n      \"pmids\": [\"24503895\", \"25268382\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct binding interface between MPP3 and p85 not mapped\", \"Whether this complex operates in neurons or retina was not tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"MPP3/DLG3 was connected to Hippo pathway regulation through direct binding to MST2 kinase and downstream control of LATS1/YAP nuclear translocation, while epigenetic silencing of the DLG3 promoter by MIAT lncRNA-recruited DNMTs was identified as a cancer-relevant regulatory mechanism.\",\n      \"evidence\": \"RIP assay for lncRNA–DNMT binding, methylation-specific PCR, Co-IP for DLG3–MST2, siRNA rescue in tumor models\",\n      \"pmids\": [\"32593652\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"DLG3–MST2 interaction not mapped to specific domains\", \"Single-laboratory finding awaiting independent replication\", \"Whether Hippo pathway role operates in normal tissues is unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identification of COP1 E3 ligase as a regulator of DLG3 protein stability via ubiquitin-proteasomal degradation provided a second post-translational control mechanism distinct from Nedd4-mediated monoubiquitination.\",\n      \"evidence\": \"Co-IP, ubiquitination assay with MG132, siRNA knockdown and rescue in glioma cells\",\n      \"pmids\": [\"37356109\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Degron motif on DLG3 recognized by COP1 not mapped\", \"Physiological context outside glioma not examined\", \"Relationship between COP1-mediated degradation and Nedd4-mediated monoubiquitination not addressed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of MPP3 complex assembly at different tissue sites, how the two distinct ubiquitination pathways (Nedd4 monoubiquitination vs. COP1 polyubiquitination) are coordinated, and the signaling pathways downstream of MPP3 that control mitotic spindle orientation in cortical progenitors.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of MPP3 in any complex\", \"Spindle orientation signaling downstream of MPP3 unknown\", \"In vivo relevance of Hippo pathway connection not tested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 1, 6]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [3, 4, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 3, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 6, 10]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [0, 3, 4]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [4, 5]}\n    ],\n    \"complexes\": [\n      \"CRB1-MPP5-MPP3 apical complex\",\n      \"CADM1-Dlg-MPP3-p85 signaling complex\"\n    ],\n    \"partners\": [\n      \"CADM1\",\n      \"MPP5\",\n      \"DLG1\",\n      \"NEDD4\",\n      \"NEDD4L\",\n      \"HTR2C\",\n      \"PIK3R1\",\n      \"COP1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"MPP3 is a MAGUK-family scaffolding protein that organizes apical junctional complexes and multi-protein signaling assemblies at cell-cell contact sites in epithelia, retina, and developing cortex. At the retinal outer limiting membrane, MPP3 is recruited to the CRB1/MPP5 scaffold, maintains PALS1 (MPP5) levels in Müller glia, and its conditional loss causes late-onset retinal degeneration accelerated by light exposure [PMID:16519681, PMID:23893895]. In cortical progenitors, MPP3 is required for adherens junction integrity, proper spindle orientation, and neuronal migration; conditional knockout produces ectopic mitoses and defective cortical stratification [PMID:23658188]. MPP3 also bridges CADM1 to PI3K via a CADM1–MPP3–Dlg–p85 complex that activates Akt/Rac1 signaling and actin reorganization for epithelial cell spreading, and it modulates 5-HT2C receptor surface expression and desensitization through its PDZ domain [PMID:24503895, PMID:16914526].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Identifying MPP3 as a direct PDZ-dependent binding partner of CADM1 (TSLC1) at cell-cell contacts established that this MAGUK scaffold operates in an intercellular adhesion cascade in epithelial cells.\",\n      \"evidence\": \"Reciprocal Co-IP and GST pulldown with PDZ-motif deletion controls in lung epithelial cells\",\n      \"pmids\": [\"13679854\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No in vivo validation of the CADM1–MPP3 interaction\",\n        \"Downstream signaling consequences of the interaction were unknown\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrating that MPP3 localizes to the subapical region and photoreceptor synapse in retina, where it is recruited to the MPP5/CRB1 scaffold, placed MPP3 within the Crumbs polarity complex and suggested tissue-specific complex assembly with DLG1.\",\n      \"evidence\": \"Immunofluorescence in mouse and human retinae with endogenous Co-IP confirming MPP3–MPP5 and MPP3–DLG1 complexes\",\n      \"pmids\": [\"16519681\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Loss-of-function consequences in the retina were not yet established\",\n        \"Stoichiometry and direct versus bridged interactions within the complex were unresolved\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Showing that MPP3 opposes PSD-95 by preventing 5-HT2C receptor desensitization and increasing its surface expression revealed a PDZ-dependent role for MPP3 in GPCR trafficking and signaling in neurons.\",\n      \"evidence\": \"Ca²⁺ response assays in HEK293 cells and primary cortical neurons with Tat-peptide PDZ inhibitor controls\",\n      \"pmids\": [\"16914526\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism by which MPP3 stabilizes receptor surface expression (endocytic versus recycling) was not delineated\",\n        \"In vivo neuronal phenotype of MPP3 loss on serotonergic signaling was not tested\"\n      ]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Conditional knockout of Mpp3 in cortical progenitors proved that MPP3 is essential for maintaining the apical protein complex, adherens junctions, and correct spindle orientation, resolving whether it is required or redundant for cortical development.\",\n      \"evidence\": \"Cre-lox conditional KO in mouse cortex with immunofluorescence, BrdU/EdU cell cycle, and spindle orientation analysis\",\n      \"pmids\": [\"23658188\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Which specific apical complex proteins are direct MPP3 interactors versus secondarily affected was not distinguished\",\n        \"Postnatal cortical circuit consequences were not assessed\"\n      ]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Conditional deletion of Mpp3 in retina demonstrated that MPP3 maintains PALS1 levels at the subapical region and that its loss causes light-accelerated retinal degeneration, with genetic epistasis showing synergy with Pals1 haploinsufficiency.\",\n      \"evidence\": \"Conditional KO mouse with ERG, EM ultrastructure, immunofluorescence, and Mpp3/Pals1 double-knockout epistasis\",\n      \"pmids\": [\"23893895\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether MPP3 stabilizes PALS1 by direct protein-protein interaction or indirectly through complex integrity was not resolved\",\n        \"Contribution of individual retinal cell types (photoreceptors vs. Müller glia) was not fully separated\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identification of the CADM1–MPP3–Dlg–p85 quaternary complex linked MPP3 scaffolding to PI3K/Akt/Rac1 activation and actin reorganization, providing a signaling mechanism downstream of trans-homophilic CADM1 adhesion.\",\n      \"evidence\": \"Co-IP of the multi-protein complex with functional inhibitor epistasis (LY294002, Akt and Rac1 inhibitors) in cell spreading assays\",\n      \"pmids\": [\"24503895\", \"25268382\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct binding interfaces between MPP3 and p85 or Dlg were not mapped\",\n        \"In vivo relevance of this signaling axis was not tested\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Quantitative FRAP analysis showed that MPP3 exists in two kinetically distinct pools — a long-lived CADM1-bound complex (τ ~16 min) and a rapidly exchanging free pool (τ ~40–60 s) — establishing the dynamic behavior of MPP3 at adhesion sites.\",\n      \"evidence\": \"FRAP and FLIP live-cell imaging with exponential curve-fitting in cultured cells\",\n      \"pmids\": [\"25780926\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether the two pools have distinct functional roles was not determined\",\n        \"Dynamics were not measured in polarized epithelial or retinal contexts\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Demonstration that MPP3 expression in enteric neural crest cells is downregulated by Schwann cell-derived extracellular vesicles carrying MIR22HG, and that MPP3 knockdown blocks ENCC proliferation, migration, and distal colon colonization, extended MPP3's functional importance to enteric nervous system development.\",\n      \"evidence\": \"siRNA knockdown in ENCCs, extracellular vesicle treatment, m6A sequencing, and in vivo mouse colonization assay\",\n      \"pmids\": [\"41286351\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Downstream effectors of MPP3 in ENCC migration were not identified\",\n        \"Whether known MPP3 complexes (CADM1–Dlg–PI3K) operate in this context is unknown\",\n        \"Single lab; awaits independent replication\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of MPP3 scaffold assembly — how its SH3, GUK, and PDZ domains simultaneously engage distinct partners to form tissue-specific complexes — and the full range of physiological contexts requiring MPP3 remain to be determined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No crystal structure or cryo-EM model of MPP3 in complex with any partner\",\n        \"No systematic interactome beyond candidate-based approaches\",\n        \"Relationship between retinal, cortical, and enteric functions has not been integrated mechanistically\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 2, 5, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 5, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [0, 6]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [0, 3, 5, 6]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 5]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [3, 4, 8]}\n    ],\n    \"complexes\": [\n      \"CRB1/MPP5/MPP3 apical complex\",\n      \"CADM1-MPP3-Dlg-p85 signaling complex\"\n    ],\n    \"partners\": [\n      \"CADM1\",\n      \"MPP5\",\n      \"DLG1\",\n      \"CRB1\",\n      \"PIK3R1\",\n      \"HTR2C\",\n      \"MPP4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}