{"gene":"PRRT1","run_date":"2026-06-10T06:43:36","timeline":{"discoveries":[{"year":2018,"finding":"SynDIG4/PRRT1 modifies AMPAR gating properties in a subunit-dependent manner; knockout mice show weaker excitatory synapses and complete loss of tetanus-induced LTP; SynDIG4 colocalizes with GluA1 at non-synaptic (extrasynaptic) sites, suggesting it maintains a pool of extrasynaptic AMPARs necessary for synapse development and plasticity.","method":"Immunocytochemistry, electrophysiology (mEPSC recording, LTP induction) in SynDIG4 KO mice; behavioral assays (cognitive tests)","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (electrophysiology, ICC, behavior) in KO mice; independently replicated across related papers","pmids":["29490264"],"is_preprint":false},{"year":2015,"finding":"SynDIG4/PRRT1 is a component of native AMPAR complexes in the brain, is de-enriched at the postsynaptic density, and colocalizes with extrasynaptic GluA1 puncta in primary dissociated neurons, indicating it functions as an auxiliary factor specifically for extrasynaptic GluA1-containing AMPARs.","method":"Immunofluorescence, subcellular fractionation, primary neuron culture imaging","journal":"The Journal of comparative neurology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — localization established by ICC/fractionation in neurons, consistent with proteomic identification of PRRT1 in AMPAR complexes, but no functional manipulation in this study","pmids":["26660156"],"is_preprint":false},{"year":2019,"finding":"PRRT1 controls surface levels of AMPARs; it differentially stabilizes GluA1 phosphorylated at S845 versus S831; PRRT1 is required for NMDA receptor-dependent long-term depression (LTD) and proper NMDA-induced AMPAR trafficking in hippocampal slices, though it is dispensable for basal synaptic transmission.","method":"Electrophysiology (LTD induction) and biochemistry (phospho-specific immunoblotting, surface biotinylation) on acute hippocampal slices from PRRT1 KO mice","journal":"Molecular and cellular neurosciences","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal biochemical and electrophysiological methods in KO slices, single lab but two orthogonal approaches","pmids":["31216424"],"is_preprint":false},{"year":2021,"finding":"PRRT1 physically interacts with all four AMPAR subunits (GluA1–GluA4); only the second hydrophobic segment of PRRT1 spans the membrane completely (membrane topology clarified) and mediates the interaction with AMPARs; PRRT1 also physically interacts with phosphatase PP2B (calcineurin), which dephosphorylates AMPARs during plasticity; PRRT1 localizes to early and recycling endosomes as well as the plasma membrane in neurons, where it associates with AMPARs extrasynaptically.","method":"Co-immunoprecipitation, membrane topology analysis, co-localization in primary neuronal cultures, subcellular fractionation","journal":"Frontiers in synaptic neuroscience","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP and localization experiments, single lab, but multiple subunits tested and topology analyzed with functional interpretation","pmids":["34408636"],"is_preprint":false},{"year":2022,"finding":"Co-expression of SynDIG4/PRRT1 with GluA1 or GluA2 in COS cells leads to mutually dependent clustering: SD4 increases AMPAR puncta area and AMPAR co-localization increases SD4 puncta area; the membrane-bound domain of SD4 alone is sufficient to recapitulate this effect; during chemical LTP (glycine-induced), co-localization of SD4 with GluA1 increases significantly along with GluA1 cluster size.","method":"Heterologous expression in COS cells and primary hippocampal neurons with immunofluorescence; chimeric protein constructs; glycine-induced chemical LTP","journal":"Frontiers in molecular neuroscience","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — heterologous cell and neuron experiments with chimeric constructs, single lab, two cellular systems","pmids":["35465096"],"is_preprint":false},{"year":2025,"finding":"Loss of SynDIG4/PRRT1 does not affect basal AMPAR endocytosis but impairs recycling of GluA1-containing AMPARs; this results in accumulation of internal GluA1/GluA2 in Rab4-positive recycling endosomes and elevated overlap between Rab4- and Rab11-positive vesicles, indicating a block in trafficking between these compartments; surface GluA1 is reduced at synaptic regions.","method":"Antibody-feeding endocytosis/recycling assays in cultured hippocampal neurons from SynDIG4 KO mice; immunofluorescence co-localization with Rab4/Rab11 markers","journal":"Frontiers in pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional recycling assay with KO neurons and Rab-compartment localization, single lab but orthogonal methods","pmids":["40469985"],"is_preprint":false},{"year":2025,"finding":"SynDIG4/PRRT1 possesses a YxxΦ endocytic sorting motif (178-YVPV-181) that binds the AP-2 complex subunit μ2; mutation of this motif (178-AVPA-181) disrupts μ2 binding and causes aberrant accumulation of PRRT1 at the plasma membrane of heterologous cells and primary hippocampal neurons; the endocytic-signal-deficient mutant co-localizes preferentially with GluA1 over GluA2 on the cell surface.","method":"Site-directed mutagenesis of YxxΦ motif, co-immunoprecipitation with AP-2 μ2 subunit, immunofluorescence in heterologous cells and primary neurons","journal":"Frontiers in cellular neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis plus Co-IP plus localization in two cellular systems, single lab","pmids":["39916936"],"is_preprint":false}],"current_model":"PRRT1 (SynDIG4) is a transmembrane AMPAR auxiliary protein whose second hydrophobic domain spans the membrane and directly binds all four AMPAR subunits (GluA1–GluA4); it localizes to extrasynaptic plasma membrane, early endosomes, and recycling endosomes where it maintains a pool of extrasynaptic GluA1-containing AMPARs by promoting their recycling via the Rab4-dependent pathway (using a YxxΦ/AP-2 μ2 endocytic signal), stabilizes the S845 phosphorylation state of GluA1, interacts with phosphatase PP2B, and is required for both LTP and NMDAR-dependent LTD as well as higher-order cognitive function."},"narrative":{"mechanistic_narrative":"PRRT1 (SynDIG4) is a transmembrane auxiliary protein of AMPA-type glutamate receptors that maintains a pool of extrasynaptic GluA1-containing AMPARs and is required for synaptic plasticity and higher-order cognition [PMID:29490264]. It is a component of native brain AMPAR complexes, de-enriched at the postsynaptic density and colocalized with extrasynaptic GluA1 puncta [PMID:26660156], and physically interacts with all four AMPAR subunits (GluA1–GluA4) through its second hydrophobic segment, which is the only domain that fully spans the membrane [PMID:34408636]. This membrane-embedded domain is sufficient to drive mutually dependent clustering of PRRT1 and AMPARs, a co-localization that increases during chemical LTP [PMID:35465096]. Functionally, PRRT1 controls AMPAR surface levels, differentially stabilizes the S845 versus S831 phosphorylation state of GluA1, and interacts with the phosphatase PP2B/calcineurin; it is required for NMDAR-dependent LTD and proper NMDA-induced AMPAR trafficking while being dispensable for basal transmission [PMID:31216424, PMID:34408636]. Mechanistically, PRRT1 acts in the receptor recycling pathway: it does not affect basal endocytosis but is required for recycling of GluA1-containing AMPARs, with its loss causing accumulation of internalized GluA1/GluA2 in Rab4-positive endosomes and a block in Rab4-to-Rab11 trafficking [PMID:40469985]. PRRT1 itself is sorted by a Yxxϕ endocytic motif (178-YVPV-181) that binds the AP-2 μ2 subunit, and disruption of this motif traps PRRT1 at the plasma membrane preferentially with GluA1 [PMID:39916936]. No catalytic activity has been ascribed to PRRT1 in the available corpus; its role is that of a structural/adaptor auxiliary subunit.","teleology":[{"year":2015,"claim":"Established that PRRT1 is not a generic synaptic protein but a dedicated auxiliary factor for extrasynaptic GluA1-containing AMPARs, defining its compartment-specific niche.","evidence":"Immunofluorescence, subcellular fractionation, and primary neuron imaging localizing PRRT1 to extrasynaptic GluA1 puncta and native AMPAR complexes","pmids":["26660156"],"confidence":"Medium","gaps":["No functional manipulation in this study","Direct physical interaction with AMPAR subunits not demonstrated here"]},{"year":2018,"claim":"Showed that PRRT1 is functionally required for synaptic strength and plasticity, answering whether its extrasynaptic localization has physiological consequence.","evidence":"Electrophysiology (mEPSC, LTP), immunocytochemistry, and behavioral assays in SynDIG4 KO mice","pmids":["29490264"],"confidence":"High","gaps":["Molecular mechanism linking PRRT1 to LTP not resolved","Direct binding partners not mapped"]},{"year":2019,"claim":"Connected PRRT1 to AMPAR surface regulation and phosphorylation, showing it stabilizes the S845 GluA1 phospho-state and is required for LTD, separating its role from basal transmission.","evidence":"LTD electrophysiology, phospho-specific immunoblotting, and surface biotinylation in PRRT1 KO hippocampal slices","pmids":["31216424"],"confidence":"High","gaps":["Mechanism by which PRRT1 stabilizes S845 phosphorylation unresolved","Phosphatase/kinase partners not identified in this study"]},{"year":2021,"claim":"Defined the physical basis of PRRT1 function: which domain spans the membrane, which AMPAR subunits it binds, and that it associates with the phosphatase PP2B.","evidence":"Co-immunoprecipitation, membrane topology analysis, and subcellular fractionation/colocalization in neurons","pmids":["34408636"],"confidence":"Medium","gaps":["Single-lab Co-IP without reciprocal or structural validation","Stoichiometry of PRRT1–AMPAR complex unknown","Functional consequence of PP2B interaction not directly tested"]},{"year":2022,"claim":"Demonstrated that PRRT1 and AMPARs reciprocally promote each other's clustering via the membrane-bound domain, providing a mechanistic basis for co-trafficking and plasticity-associated reorganization.","evidence":"Heterologous COS-cell and neuronal expression with chimeric constructs and glycine-induced chemical LTP imaging","pmids":["35465096"],"confidence":"Medium","gaps":["Clustering measured by puncta area, not direct binding affinity","Single-lab study","Relationship of clustering to functional receptor delivery not established"]},{"year":2025,"claim":"Placed PRRT1 in the AMPAR recycling pathway, showing it acts downstream of endocytosis at the Rab4-to-Rab11 transition rather than at internalization.","evidence":"Antibody-feeding endocytosis/recycling assays and Rab4/Rab11 colocalization in SynDIG4 KO hippocampal neurons","pmids":["40469985"],"confidence":"Medium","gaps":["Single-lab study","Molecular machinery linking PRRT1 to Rab4/Rab11 vesicle progression unknown"]},{"year":2025,"claim":"Identified the trafficking signal that controls PRRT1's own sorting, a Yxxϕ motif binding AP-2 μ2, explaining how PRRT1 cycles between surface and endosomes.","evidence":"Site-directed mutagenesis of the YVPV motif, Co-IP with AP-2 μ2, and immunofluorescence in heterologous cells and neurons","pmids":["39916936"],"confidence":"Medium","gaps":["Single-lab study","Whether AP-2-mediated PRRT1 sorting directly governs AMPAR trafficking in vivo not tested","Functional link to LTP/LTD phenotypes not established"]},{"year":null,"claim":"How PRRT1 coordinates its AMPAR-binding, PP2B association, and AP-2/Rab-dependent trafficking roles into a single mechanism for activity-dependent receptor delivery remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of the PRRT1–AMPAR complex","Causal chain from PRRT1 phospho-stabilization of GluA1 to plasticity outcomes not reconstituted","In vivo significance of the AP-2 sorting motif for cognition untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[3,6]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[2,3]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,3,6]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[3,5]}],"pathway":[],"complexes":["AMPA receptor complex"],"partners":["GRIA1","GRIA2","GRIA3","GRIA4","PPP3CA","AP2M1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q99946","full_name":"Proline-rich transmembrane protein 1","aliases":["Dispanin subfamily D member 1","DSPD1","Synapse differentiation-induced protein 4","SynDIG4"],"length_aa":306,"mass_kda":31.4,"function":"Required to maintain a pool of extrasynaptic AMPA-regulated glutamate receptors (AMPAR) which is necessary for synapse development and function. Regulates basal AMPAR function and synaptic transmission during development but is dispensable at mature hippocampal synapses. Plays a role in regulating basal phosphorylation levels of glutamate receptor GRIA1 and promotes GRIA1 and GRIA2 cell surface expression","subcellular_location":"Cell membrane; Synapse","url":"https://www.uniprot.org/uniprotkb/Q99946/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PRRT1","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PRRT1","total_profiled":1310},"omim":[{"mim_id":"618297","title":"PROLINE-RICH TRANSMEMBRANE PROTEIN 1; PRRT1","url":"https://www.omim.org/entry/618297"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"brain","ntpm":67.2}],"url":"https://www.proteinatlas.org/search/PRRT1"},"hgnc":{"alias_symbol":["NG5","IFITMD7","DSPD1","SynDIG4"],"prev_symbol":["C6orf31"]},"alphafold":{"accession":"Q99946","domains":[{"cath_id":"-","chopping":"226-252_262-301","consensus_level":"medium","plddt":65.1964,"start":226,"end":301}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q99946","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q99946-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q99946-F1-predicted_aligned_error_v6.png","plddt_mean":52.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PRRT1","jax_strain_url":"https://www.jax.org/strain/search?query=PRRT1"},"sequence":{"accession":"Q99946","fasta_url":"https://rest.uniprot.org/uniprotkb/Q99946.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q99946/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q99946"}},"corpus_meta":[{"pmid":"29490264","id":"PMC_29490264","title":"SynDIG4/Prrt1 Is Required for Excitatory Synapse Development and Plasticity Underlying Cognitive Function.","date":"2018","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/29490264","citation_count":57,"is_preprint":false},{"pmid":"26660156","id":"PMC_26660156","title":"Distribution of the SynDIG4/proline-rich transmembrane protein 1 in rat brain.","date":"2015","source":"The Journal of comparative neurology","url":"https://pubmed.ncbi.nlm.nih.gov/26660156","citation_count":23,"is_preprint":false},{"pmid":"31216424","id":"PMC_31216424","title":"PRRT1 regulates basal and plasticity-induced AMPA receptor trafficking.","date":"2019","source":"Molecular and cellular neurosciences","url":"https://pubmed.ncbi.nlm.nih.gov/31216424","citation_count":21,"is_preprint":false},{"pmid":"15213182","id":"PMC_15213182","title":"Identification of a point mutation resulting in loss of cell wall anchoring activity of SrtA of Streptococcus mutans NG5.","date":"2004","source":"Infection and immunity","url":"https://pubmed.ncbi.nlm.nih.gov/15213182","citation_count":15,"is_preprint":false},{"pmid":"34408636","id":"PMC_34408636","title":"Interaction and Subcellular Association of PRRT1/SynDIG4 With AMPA Receptors.","date":"2021","source":"Frontiers in synaptic neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/34408636","citation_count":13,"is_preprint":false},{"pmid":"35465096","id":"PMC_35465096","title":"Mutually Dependent Clustering of SynDIG4/PRRT1 and AMPA Receptor Subunits GluA1 and GluA2 in Heterologous Cells and Primary Neurons.","date":"2022","source":"Frontiers in molecular neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/35465096","citation_count":7,"is_preprint":false},{"pmid":"39773540","id":"PMC_39773540","title":"Uncovering the epigenetic regulatory clues of PRRT1 in Alzheimer's disease: a strategy integrating multi-omics analysis with explainable machine learning.","date":"2025","source":"Alzheimer's research & therapy","url":"https://pubmed.ncbi.nlm.nih.gov/39773540","citation_count":6,"is_preprint":false},{"pmid":"40469985","id":"PMC_40469985","title":"Loss of SynDIG4/PRRT1 alters distribution of AMPA receptors in Rab4- and Rab11-positive endosomes and impairs basal AMPA receptor recycling.","date":"2025","source":"Frontiers in pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/40469985","citation_count":3,"is_preprint":false},{"pmid":"39916936","id":"PMC_39916936","title":"Functional characterization of endocytic signals in the SynDIG/PRRT family members SynDIG1 and SynDIG4 in heterologous cells and neurons.","date":"2025","source":"Frontiers in cellular neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/39916936","citation_count":1,"is_preprint":false},{"pmid":"39764059","id":"PMC_39764059","title":"Loss of SynDIG4/PRRT1 alters distribution of AMPA receptors in Rab4- and Rab11-positive endosomes and impairs basal AMPA receptor recycling.","date":"2024","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/39764059","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7190,"output_tokens":2067,"usd":0.026287,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9009,"output_tokens":2874,"usd":0.058447,"stage2_stop_reason":"end_turn"},"total_usd":0.084734,"stage1_batch_id":"msgbatch_01CNoDbkChHEkgrJnL7BwtGJ","stage2_batch_id":"msgbatch_016Y4x9E9Wg5VQDANj9quSQD","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2018,\n      \"finding\": \"SynDIG4/PRRT1 modifies AMPAR gating properties in a subunit-dependent manner; knockout mice show weaker excitatory synapses and complete loss of tetanus-induced LTP; SynDIG4 colocalizes with GluA1 at non-synaptic (extrasynaptic) sites, suggesting it maintains a pool of extrasynaptic AMPARs necessary for synapse development and plasticity.\",\n      \"method\": \"Immunocytochemistry, electrophysiology (mEPSC recording, LTP induction) in SynDIG4 KO mice; behavioral assays (cognitive tests)\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (electrophysiology, ICC, behavior) in KO mice; independently replicated across related papers\",\n      \"pmids\": [\"29490264\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"SynDIG4/PRRT1 is a component of native AMPAR complexes in the brain, is de-enriched at the postsynaptic density, and colocalizes with extrasynaptic GluA1 puncta in primary dissociated neurons, indicating it functions as an auxiliary factor specifically for extrasynaptic GluA1-containing AMPARs.\",\n      \"method\": \"Immunofluorescence, subcellular fractionation, primary neuron culture imaging\",\n      \"journal\": \"The Journal of comparative neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — localization established by ICC/fractionation in neurons, consistent with proteomic identification of PRRT1 in AMPAR complexes, but no functional manipulation in this study\",\n      \"pmids\": [\"26660156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"PRRT1 controls surface levels of AMPARs; it differentially stabilizes GluA1 phosphorylated at S845 versus S831; PRRT1 is required for NMDA receptor-dependent long-term depression (LTD) and proper NMDA-induced AMPAR trafficking in hippocampal slices, though it is dispensable for basal synaptic transmission.\",\n      \"method\": \"Electrophysiology (LTD induction) and biochemistry (phospho-specific immunoblotting, surface biotinylation) on acute hippocampal slices from PRRT1 KO mice\",\n      \"journal\": \"Molecular and cellular neurosciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal biochemical and electrophysiological methods in KO slices, single lab but two orthogonal approaches\",\n      \"pmids\": [\"31216424\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PRRT1 physically interacts with all four AMPAR subunits (GluA1–GluA4); only the second hydrophobic segment of PRRT1 spans the membrane completely (membrane topology clarified) and mediates the interaction with AMPARs; PRRT1 also physically interacts with phosphatase PP2B (calcineurin), which dephosphorylates AMPARs during plasticity; PRRT1 localizes to early and recycling endosomes as well as the plasma membrane in neurons, where it associates with AMPARs extrasynaptically.\",\n      \"method\": \"Co-immunoprecipitation, membrane topology analysis, co-localization in primary neuronal cultures, subcellular fractionation\",\n      \"journal\": \"Frontiers in synaptic neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP and localization experiments, single lab, but multiple subunits tested and topology analyzed with functional interpretation\",\n      \"pmids\": [\"34408636\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Co-expression of SynDIG4/PRRT1 with GluA1 or GluA2 in COS cells leads to mutually dependent clustering: SD4 increases AMPAR puncta area and AMPAR co-localization increases SD4 puncta area; the membrane-bound domain of SD4 alone is sufficient to recapitulate this effect; during chemical LTP (glycine-induced), co-localization of SD4 with GluA1 increases significantly along with GluA1 cluster size.\",\n      \"method\": \"Heterologous expression in COS cells and primary hippocampal neurons with immunofluorescence; chimeric protein constructs; glycine-induced chemical LTP\",\n      \"journal\": \"Frontiers in molecular neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — heterologous cell and neuron experiments with chimeric constructs, single lab, two cellular systems\",\n      \"pmids\": [\"35465096\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Loss of SynDIG4/PRRT1 does not affect basal AMPAR endocytosis but impairs recycling of GluA1-containing AMPARs; this results in accumulation of internal GluA1/GluA2 in Rab4-positive recycling endosomes and elevated overlap between Rab4- and Rab11-positive vesicles, indicating a block in trafficking between these compartments; surface GluA1 is reduced at synaptic regions.\",\n      \"method\": \"Antibody-feeding endocytosis/recycling assays in cultured hippocampal neurons from SynDIG4 KO mice; immunofluorescence co-localization with Rab4/Rab11 markers\",\n      \"journal\": \"Frontiers in pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional recycling assay with KO neurons and Rab-compartment localization, single lab but orthogonal methods\",\n      \"pmids\": [\"40469985\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SynDIG4/PRRT1 possesses a YxxΦ endocytic sorting motif (178-YVPV-181) that binds the AP-2 complex subunit μ2; mutation of this motif (178-AVPA-181) disrupts μ2 binding and causes aberrant accumulation of PRRT1 at the plasma membrane of heterologous cells and primary hippocampal neurons; the endocytic-signal-deficient mutant co-localizes preferentially with GluA1 over GluA2 on the cell surface.\",\n      \"method\": \"Site-directed mutagenesis of YxxΦ motif, co-immunoprecipitation with AP-2 μ2 subunit, immunofluorescence in heterologous cells and primary neurons\",\n      \"journal\": \"Frontiers in cellular neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis plus Co-IP plus localization in two cellular systems, single lab\",\n      \"pmids\": [\"39916936\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PRRT1 (SynDIG4) is a transmembrane AMPAR auxiliary protein whose second hydrophobic domain spans the membrane and directly binds all four AMPAR subunits (GluA1–GluA4); it localizes to extrasynaptic plasma membrane, early endosomes, and recycling endosomes where it maintains a pool of extrasynaptic GluA1-containing AMPARs by promoting their recycling via the Rab4-dependent pathway (using a YxxΦ/AP-2 μ2 endocytic signal), stabilizes the S845 phosphorylation state of GluA1, interacts with phosphatase PP2B, and is required for both LTP and NMDAR-dependent LTD as well as higher-order cognitive function.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PRRT1 (SynDIG4) is a transmembrane auxiliary protein of AMPA-type glutamate receptors that maintains a pool of extrasynaptic GluA1-containing AMPARs and is required for synaptic plasticity and higher-order cognition [#0]. It is a component of native brain AMPAR complexes, de-enriched at the postsynaptic density and colocalized with extrasynaptic GluA1 puncta [#1], and physically interacts with all four AMPAR subunits (GluA1–GluA4) through its second hydrophobic segment, which is the only domain that fully spans the membrane [#3]. This membrane-embedded domain is sufficient to drive mutually dependent clustering of PRRT1 and AMPARs, a co-localization that increases during chemical LTP [#4]. Functionally, PRRT1 controls AMPAR surface levels, differentially stabilizes the S845 versus S831 phosphorylation state of GluA1, and interacts with the phosphatase PP2B/calcineurin; it is required for NMDAR-dependent LTD and proper NMDA-induced AMPAR trafficking while being dispensable for basal transmission [#2, #3]. Mechanistically, PRRT1 acts in the receptor recycling pathway: it does not affect basal endocytosis but is required for recycling of GluA1-containing AMPARs, with its loss causing accumulation of internalized GluA1/GluA2 in Rab4-positive endosomes and a block in Rab4-to-Rab11 trafficking [#5]. PRRT1 itself is sorted by a Yxxϕ endocytic motif (178-YVPV-181) that binds the AP-2 μ2 subunit, and disruption of this motif traps PRRT1 at the plasma membrane preferentially with GluA1 [#6]. No catalytic activity has been ascribed to PRRT1 in the available corpus; its role is that of a structural/adaptor auxiliary subunit.\",\n  \"teleology\": [\n    {\n      \"year\": 2015,\n      \"claim\": \"Established that PRRT1 is not a generic synaptic protein but a dedicated auxiliary factor for extrasynaptic GluA1-containing AMPARs, defining its compartment-specific niche.\",\n      \"evidence\": \"Immunofluorescence, subcellular fractionation, and primary neuron imaging localizing PRRT1 to extrasynaptic GluA1 puncta and native AMPAR complexes\",\n      \"pmids\": [\"26660156\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No functional manipulation in this study\", \"Direct physical interaction with AMPAR subunits not demonstrated here\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showed that PRRT1 is functionally required for synaptic strength and plasticity, answering whether its extrasynaptic localization has physiological consequence.\",\n      \"evidence\": \"Electrophysiology (mEPSC, LTP), immunocytochemistry, and behavioral assays in SynDIG4 KO mice\",\n      \"pmids\": [\"29490264\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism linking PRRT1 to LTP not resolved\", \"Direct binding partners not mapped\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Connected PRRT1 to AMPAR surface regulation and phosphorylation, showing it stabilizes the S845 GluA1 phospho-state and is required for LTD, separating its role from basal transmission.\",\n      \"evidence\": \"LTD electrophysiology, phospho-specific immunoblotting, and surface biotinylation in PRRT1 KO hippocampal slices\",\n      \"pmids\": [\"31216424\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which PRRT1 stabilizes S845 phosphorylation unresolved\", \"Phosphatase/kinase partners not identified in this study\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined the physical basis of PRRT1 function: which domain spans the membrane, which AMPAR subunits it binds, and that it associates with the phosphatase PP2B.\",\n      \"evidence\": \"Co-immunoprecipitation, membrane topology analysis, and subcellular fractionation/colocalization in neurons\",\n      \"pmids\": [\"34408636\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab Co-IP without reciprocal or structural validation\", \"Stoichiometry of PRRT1–AMPAR complex unknown\", \"Functional consequence of PP2B interaction not directly tested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrated that PRRT1 and AMPARs reciprocally promote each other's clustering via the membrane-bound domain, providing a mechanistic basis for co-trafficking and plasticity-associated reorganization.\",\n      \"evidence\": \"Heterologous COS-cell and neuronal expression with chimeric constructs and glycine-induced chemical LTP imaging\",\n      \"pmids\": [\"35465096\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Clustering measured by puncta area, not direct binding affinity\", \"Single-lab study\", \"Relationship of clustering to functional receptor delivery not established\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Placed PRRT1 in the AMPAR recycling pathway, showing it acts downstream of endocytosis at the Rab4-to-Rab11 transition rather than at internalization.\",\n      \"evidence\": \"Antibody-feeding endocytosis/recycling assays and Rab4/Rab11 colocalization in SynDIG4 KO hippocampal neurons\",\n      \"pmids\": [\"40469985\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab study\", \"Molecular machinery linking PRRT1 to Rab4/Rab11 vesicle progression unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified the trafficking signal that controls PRRT1's own sorting, a Yxxϕ motif binding AP-2 μ2, explaining how PRRT1 cycles between surface and endosomes.\",\n      \"evidence\": \"Site-directed mutagenesis of the YVPV motif, Co-IP with AP-2 μ2, and immunofluorescence in heterologous cells and neurons\",\n      \"pmids\": [\"39916936\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab study\", \"Whether AP-2-mediated PRRT1 sorting directly governs AMPAR trafficking in vivo not tested\", \"Functional link to LTP/LTD phenotypes not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How PRRT1 coordinates its AMPAR-binding, PP2B association, and AP-2/Rab-dependent trafficking roles into a single mechanism for activity-dependent receptor delivery remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of the PRRT1–AMPAR complex\", \"Causal chain from PRRT1 phospho-stabilization of GluA1 to plasticity outcomes not reconstituted\", \"In vivo significance of the AP-2 sorting motif for cognition untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [3, 6]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 3, 6]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [3, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0112316\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-9709957\", \"supporting_discovery_ids\": []}\n    ],\n    \"complexes\": [\"AMPA receptor complex\"],\n    \"partners\": [\"GRIA1\", \"GRIA2\", \"GRIA3\", \"GRIA4\", \"PPP3CA\", \"AP2M1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}