{"gene":"GSG1L","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2012,"finding":"GSG1L was identified as a novel AMPA receptor auxiliary subunit by comparative interactome profiling of AMPA and kainate receptor complexes in rat brain, validated by co-purification and electrophysiological experiments showing it modulates AMPAR gating.","method":"Comparative receptor interactome (co-purification/MS), Co-IP, electrophysiology, biochemical validation","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — reciprocal biochemical and functional validation with multiple orthogonal methods in a highly-cited foundational paper","pmids":["22813734"],"is_preprint":false},{"year":2015,"finding":"GSG1L reduces the weighted mean single-channel conductance and calcium permeability of calcium-permeable AMPARs (CP-AMPARs) while increasing polyamine-dependent inward rectification, opposing the enhancing effects of TARPs such as stargazin.","method":"Single-channel electrophysiology in recombinant systems; shRNA knockdown in cultured hippocampal and cerebellar neurons; mEPSC recordings","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro single-channel recordings combined with neuronal knockdown, multiple orthogonal readouts","pmids":["26658868"],"is_preprint":false},{"year":2016,"finding":"GSG1L negatively regulates AMPAR-mediated synaptic transmission: overexpression suppresses and knockout enhances AMPAR EPSCs in hippocampal CA1 neurons. GSG1L speeds up AMPAR deactivation and desensitization kinetics, opposing the slowing effects of TARPs/CNIHs. The first extracellular loop and C-terminus of GSG1L are required for these functions. GSG1L association with AMPARs inhibits CNIH2-induced slowing. GSG1L KO rats show LTP deficits and impaired object recognition.","method":"Overexpression and knockout (KO rat), patch-clamp electrophysiology, heterologous cell expression, domain deletion analysis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (KO, OE, domain mutants, electrophysiology) in a well-cited study","pmids":["26932439"],"is_preprint":false},{"year":2016,"finding":"In hippocampal dentate granule cells, GSG1L regulates AMPAR synaptic strength but does not modulate AMPAR deactivation or desensitization kinetics, revealing a neuron-type-specific role for GSG1L.","method":"Electrophysiology in dentate granule neurons (overexpression/KO)","journal":"Journal of neurophysiology","confidence":"Medium","confidence_rationale":"Tier 2 — direct electrophysiology with genetic manipulation, single lab","pmids":["27707810"],"is_preprint":false},{"year":2017,"finding":"GSG1L selectively chaperones GluA4-containing AMPARs to synapses during the late phase of classical conditioning, while TARPγ8 chaperones GluA1-containing AMPARs early, demonstrating subunit-selective AMPAR trafficking by auxiliary proteins.","method":"Co-localization immunofluorescence, ex vivo brainstem preparation, conditioning behavioral paradigm","journal":"Neuroscience letters","confidence":"Low","confidence_rationale":"Tier 3 — co-localization without biochemical confirmation of direct interaction; single lab, single method","pmids":["28219790"],"is_preprint":false},{"year":2020,"finding":"GSG1L controls short-term plasticity (suppresses short-term facilitation) specifically at corticothalamic synapses in anterior thalamus neurons. GSG1L KO causes AT neuron hyperexcitability and increased seizure susceptibility, while stargazin co-exists in these neurons but is functionally absent from corticothalamic synapses.","method":"GSG1L knockout mice, electrophysiology at identified synapses, seizure susceptibility assay","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined electrophysiological and behavioral phenotypes, synapse-specific resolution","pmids":["32697982"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures of GluA2 AMPAR in complex with GSG1L and with type II TARP γ5 revealed that desensitization of both GluA2-GSG1L and GluA2-γ5 complexes is accompanied by rupture of the ligand-binding domain (LBD) dimer interface, and that GSG1L (like γ5) accommodates a maximum stoichiometry of two auxiliary subunits per AMPAR tetramer, unlike type I TARPs.","method":"Cryo-EM structural determination, functional electrophysiology","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure with functional validation, published in high-impact journal","pmids":["34678168"],"is_preprint":false},{"year":2023,"finding":"GSG1L acts through two discrete evolutionarily-conserved allosteric sites on the AMPAR agonist-binding domain: a weaker interaction at the TARP/KGK site slows desensitization, and a stronger interaction at a distinct site slows recovery from desensitization. GSG1L constitutes ~5% of all AMPAR complexes in adult brain, expresses late in development in a region-specific manner, can co-assemble with TARPs or CNIHs or serve as the sole auxiliary subunit.","method":"Native interactome proteomics (MS), allosteric site mutagenesis, electrophysiology, developmental expression profiling","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 — site-directed mutagenesis of allosteric sites combined with native interactome MS and electrophysiology, multiple orthogonal methods","pmids":["37884493"],"is_preprint":false},{"year":2023,"finding":"GSG1L and γ-2 (stargazin) compete for the same binding sites on the GluA1 AMPAR tetramer with comparable apparent dissociation constants (~2.0–2.5/µm²), implying dynamic regulation of AMPAR auxiliary subunit composition.","method":"Three-color single-molecule imaging in living cells, colocalization analysis, binding affinity modeling","journal":"Cellular & molecular biology letters","confidence":"Medium","confidence_rationale":"Tier 2 — single-molecule live-cell imaging with quantitative modeling, single lab","pmids":["37430208"],"is_preprint":false},{"year":2025,"finding":"Intracellular spermine is required for GSG1L's suppression of CP-AMPAR single-channel conductance and its slowing of recovery from desensitization. Specific residues in the channel's selectivity filter and GSG1L's C-tail mediate these effects. TARPs prevent spermine-mediated conductance reduction while GSG1L enhances it.","method":"Single-channel electrophysiology with intracellular polyamine manipulation, site-directed mutagenesis of channel and GSG1L C-tail residues","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 — in vitro single-channel recordings with mutagenesis of specific functional residues, mechanistically detailed","pmids":["40185633"],"is_preprint":false}],"current_model":"GSG1L is a claudin-family transmembrane auxiliary subunit of AMPA receptors (AMPARs) that assembles with up to two copies per AMPAR tetramer, acts through two discrete allosteric sites on the ligand-binding domain to suppress CP-AMPAR function by reducing single-channel conductance (via enhanced intracellular spermine block dependent on selectivity-filter residues and the GSG1L C-tail), increasing polyamine-dependent rectification, speeding deactivation and desensitization, and slowing recovery from desensitization; in neurons it negatively regulates synaptic AMPAR transmission, controls short-term plasticity at corticothalamic synapses, and is required for normal LTP and seizure threshold."},"narrative":{"teleology":[{"year":2012,"claim":"Identifying GSG1L as an AMPAR auxiliary subunit resolved a gap in the catalog of proteins that directly assemble with and modulate AMPAR gating, establishing it as a previously unrecognized component of native receptor complexes.","evidence":"Comparative interactome profiling (co-purification/MS) of AMPA vs. kainate receptors in rat brain with Co-IP and electrophysiological validation","pmids":["22813734"],"confidence":"High","gaps":["Stoichiometry of GSG1L per AMPAR tetramer unknown","Mechanism of gating modulation unresolved","Brain region and cell-type expression pattern not characterized"]},{"year":2015,"claim":"Demonstrating that GSG1L reduces single-channel conductance and calcium permeability of CP-AMPARs while increasing polyamine-dependent rectification revealed that GSG1L is functionally antagonistic to TARPs at the biophysical level, establishing its role as a negative modulator of channel properties.","evidence":"Single-channel electrophysiology in recombinant systems; shRNA knockdown in cultured hippocampal and cerebellar neurons with mEPSC recordings","pmids":["26658868"],"confidence":"High","gaps":["Molecular basis of conductance reduction unknown","Whether effects depend on intracellular polyamines not tested","In vivo relevance not established"]},{"year":2016,"claim":"Knockout and overexpression studies in hippocampal CA1 neurons demonstrated that GSG1L suppresses synaptic AMPAR transmission, speeds desensitization/deactivation kinetics (opposing TARPs/CNIHs), and is required for normal LTP and object recognition, connecting biophysical modulation to synaptic plasticity and cognition.","evidence":"GSG1L KO rat and overexpression in hippocampal slices, patch-clamp electrophysiology, domain deletion analysis, behavioral testing","pmids":["26932439"],"confidence":"High","gaps":["Neuron-type specificity of kinetic effects not fully mapped","Structural basis of GSG1L–AMPAR interaction unknown","Mechanism by which GSG1L opposes CNIH2 not resolved"]},{"year":2016,"claim":"Finding that GSG1L regulates synaptic strength but not desensitization kinetics in dentate granule cells (unlike CA1 neurons) established that GSG1L's functional impact is neuron-type-specific, likely reflecting differences in auxiliary subunit composition.","evidence":"Electrophysiology with overexpression and KO in dentate granule neurons","pmids":["27707810"],"confidence":"Medium","gaps":["Molecular basis of cell-type difference not identified","Whether auxiliary subunit stoichiometry differs across neuron types not tested"]},{"year":2020,"claim":"Showing that GSG1L controls short-term plasticity at corticothalamic synapses and that its loss causes thalamic hyperexcitability and increased seizure susceptibility established a synapse-specific in vivo function for GSG1L beyond the hippocampus.","evidence":"GSG1L knockout mice, electrophysiology at identified thalamic synapses, seizure susceptibility assay","pmids":["32697982"],"confidence":"High","gaps":["Whether GSG1L acts at other thalamic synapse types unknown","Mechanism linking GSG1L to short-term facilitation suppression not fully resolved","Seizure circuit mechanism not delineated"]},{"year":2021,"claim":"Cryo-EM structures of GluA2–GSG1L complexes revealed a maximum stoichiometry of two GSG1L per AMPAR tetramer and showed that desensitization involves LBD dimer interface rupture, providing the first structural framework for GSG1L's modulatory mechanism.","evidence":"Cryo-EM structural determination of GluA2–GSG1L and GluA2–γ5 complexes with functional electrophysiology","pmids":["34678168"],"confidence":"High","gaps":["Allosteric communication pathway from GSG1L to the gating apparatus not resolved","Structures of GSG1L co-assembled with TARPs or CNIHs not available","Conformational dynamics during recovery from desensitization unknown"]},{"year":2023,"claim":"Identification of two discrete allosteric sites on the AMPAR LBD through which GSG1L modulates desensitization and recovery, combined with native proteomics showing ~5% of brain AMPARs contain GSG1L with late developmental onset and co-assembly with TARPs/CNIHs, resolved how GSG1L exerts dual kinetic effects and defined its quantitative niche in the native receptor landscape.","evidence":"Native interactome proteomics (MS), allosteric site mutagenesis, electrophysiology, developmental expression profiling","pmids":["37884493"],"confidence":"High","gaps":["Functional consequences of GSG1L co-assembly with TARPs vs. CNIHs in neurons not tested","Signaling pathways regulating GSG1L expression during development unknown"]},{"year":2023,"claim":"Single-molecule imaging showing that GSG1L and stargazin (γ-2) compete for the same AMPAR binding sites with comparable affinities established a dynamic competition model for auxiliary subunit composition at the cell surface.","evidence":"Three-color single-molecule imaging in living cells with colocalization analysis and binding affinity modeling","pmids":["37430208"],"confidence":"Medium","gaps":["Whether competition occurs at synapses in neurons not shown","Regulatory signals that shift the GSG1L/TARP ratio unknown","Limited to GluA1 homotetramers"]},{"year":2025,"claim":"Demonstrating that intracellular spermine is required for GSG1L's suppression of CP-AMPAR conductance and that specific selectivity-filter residues and the GSG1L C-tail mediate this effect resolved the molecular mechanism by which GSG1L reduces channel conductance—through enhanced polyamine block rather than a direct pore effect.","evidence":"Single-channel electrophysiology with intracellular polyamine manipulation and site-directed mutagenesis of channel and GSG1L C-tail residues","pmids":["40185633"],"confidence":"High","gaps":["Whether spermine-dependent mechanism operates at native synapses in vivo unknown","Structural basis of C-tail interaction with polyamine binding site not resolved","Applicability to GluA2-containing heteromeric AMPARs not tested"]},{"year":null,"claim":"How activity-dependent or developmental signals regulate GSG1L expression and its competitive incorporation into AMPAR complexes at specific synapses in vivo remains unresolved, as does the structural basis for GSG1L C-tail–mediated polyamine enhancement.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure of GSG1L C-tail interaction with the channel pore/selectivity filter","Signaling pathways controlling GSG1L expression unknown","No conditional KO studies dissecting developmental vs. acute roles"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,2,7,9]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,6,8]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[1,2,5,7]}],"complexes":["AMPA receptor complex"],"partners":["GRIA1","GRIA2","GRIA4","CNIH2","CACNG2","CACNG5"],"other_free_text":[]},"mechanistic_narrative":"GSG1L is a claudin-family transmembrane auxiliary subunit of AMPA-type glutamate receptors (AMPARs) that negatively regulates synaptic AMPAR function through multiple allosteric and ion-channel-level mechanisms. It assembles with up to two copies per AMPAR tetramer and acts through two discrete sites on the ligand-binding domain to speed deactivation and desensitization, slow recovery from desensitization, reduce single-channel conductance of calcium-permeable AMPARs via enhanced intracellular spermine block (dependent on selectivity-filter residues and the GSG1L C-tail), and increase polyamine-dependent inward rectification—effects that oppose the enhancing actions of TARPs and CNIHs [PMID:26658868, PMID:34678168, PMID:37884493, PMID:40185633]. In neurons, GSG1L constitutes ~5% of adult brain AMPAR complexes with late developmental onset and region-specific expression; it suppresses AMPAR-mediated EPSCs, controls short-term facilitation at corticothalamic synapses, and is required for normal hippocampal LTP and seizure threshold, as GSG1L knockout causes enhanced synaptic transmission, thalamic hyperexcitability, LTP deficits, and increased seizure susceptibility [PMID:26932439, PMID:32697982, PMID:37884493]."},"prefetch_data":{"uniprot":{"accession":"Q6UXU4","full_name":"Germ cell-specific gene 1-like protein","aliases":[],"length_aa":331,"mass_kda":36.8,"function":"As a component of the inner core of AMPAR complex, modifies AMPA receptor (AMPAR) gating","subcellular_location":"Cell membrane; Synapse","url":"https://www.uniprot.org/uniprotkb/Q6UXU4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GSG1L","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/GSG1L","total_profiled":1310},"omim":[{"mim_id":"617161","title":"GSG1-LIKE PROTEIN; GSG1L","url":"https://www.omim.org/entry/617161"},{"mim_id":"143100","title":"HUNTINGTON DISEASE; HD","url":"https://www.omim.org/entry/143100"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"blood vessel","ntpm":18.1},{"tissue":"brain","ntpm":19.8},{"tissue":"heart muscle","ntpm":21.6}],"url":"https://www.proteinatlas.org/search/GSG1L"},"hgnc":{"alias_symbol":["MGC18079","PRO19651","KTSR5831"],"prev_symbol":[]},"alphafold":{"accession":"Q6UXU4","domains":[{"cath_id":"1.20.140","chopping":"1-29_129-259","consensus_level":"medium","plddt":89.5618,"start":1,"end":259}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6UXU4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q6UXU4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q6UXU4-F1-predicted_aligned_error_v6.png","plddt_mean":71.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=GSG1L","jax_strain_url":"https://www.jax.org/strain/search?query=GSG1L"},"sequence":{"accession":"Q6UXU4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q6UXU4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q6UXU4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6UXU4"}},"corpus_meta":[{"pmid":"22813734","id":"PMC_22813734","title":"Differences in AMPA and kainate receptor interactomes facilitate identification of AMPA receptor auxiliary subunit GSG1L.","date":"2012","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/22813734","citation_count":158,"is_preprint":false},{"pmid":"26932439","id":"PMC_26932439","title":"GSG1L suppresses AMPA receptor-mediated synaptic transmission and uniquely modulates AMPA receptor kinetics in hippocampal neurons.","date":"2016","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/26932439","citation_count":73,"is_preprint":false},{"pmid":"26658868","id":"PMC_26658868","title":"Auxiliary Subunit GSG1L Acts to Suppress Calcium-Permeable AMPA Receptor Function.","date":"2015","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/26658868","citation_count":52,"is_preprint":false},{"pmid":"27010727","id":"PMC_27010727","title":"Genome-Wide Pharmacogenomic Study on Methadone Maintenance Treatment Identifies SNP rs17180299 and Multiple Haplotypes on CYP2B6, SPON1, and GSG1L Associated with Plasma Concentrations of Methadone R- and S-enantiomers in Heroin-Dependent Patients.","date":"2016","source":"PLoS genetics","url":"https://pubmed.ncbi.nlm.nih.gov/27010727","citation_count":45,"is_preprint":false},{"pmid":"34678168","id":"PMC_34678168","title":"Structure and desensitization of AMPA receptor complexes with type II TARP γ5 and GSG1L.","date":"2021","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/34678168","citation_count":39,"is_preprint":false},{"pmid":"27707810","id":"PMC_27707810","title":"GSG1L regulates the strength of AMPA receptor-mediated synaptic transmission but not AMPA receptor kinetics in hippocampal dentate granule neurons.","date":"2016","source":"Journal of neurophysiology","url":"https://pubmed.ncbi.nlm.nih.gov/27707810","citation_count":22,"is_preprint":false},{"pmid":"32697982","id":"PMC_32697982","title":"AMPA Receptor Auxiliary Subunit GSG1L Suppresses Short-Term Facilitation in Corticothalamic Synapses and Determines Seizure Susceptibility.","date":"2020","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/32697982","citation_count":12,"is_preprint":false},{"pmid":"37884493","id":"PMC_37884493","title":"GSG1L-containing AMPA receptor complexes are defined by their spatiotemporal expression, native interactome and allosteric sites.","date":"2023","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/37884493","citation_count":11,"is_preprint":false},{"pmid":"28219790","id":"PMC_28219790","title":"Subunit-specific synaptic delivery of AMPA receptors by auxiliary chaperone proteins TARPγ8 and GSG1L in classical conditioning.","date":"2017","source":"Neuroscience letters","url":"https://pubmed.ncbi.nlm.nih.gov/28219790","citation_count":8,"is_preprint":false},{"pmid":"37430208","id":"PMC_37430208","title":"γ-2 and GSG1L bind with comparable affinities to the tetrameric GluA1 core.","date":"2023","source":"Cellular & molecular biology letters","url":"https://pubmed.ncbi.nlm.nih.gov/37430208","citation_count":0,"is_preprint":false},{"pmid":"40185633","id":"PMC_40185633","title":"Intracellular Spermine Is a Key Player in GSG1L's Regulation of Calcium-Permeable AMPAR Channel Conductance and Recovery from Desensitization.","date":"2025","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/40185633","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":6531,"output_tokens":2360,"usd":0.027497},"stage2":{"model":"claude-opus-4-6","input_tokens":5692,"output_tokens":2498,"usd":0.136365},"total_usd":0.163862,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2012,\n      \"finding\": \"GSG1L was identified as a novel AMPA receptor auxiliary subunit by comparative interactome profiling of AMPA and kainate receptor complexes in rat brain, validated by co-purification and electrophysiological experiments showing it modulates AMPAR gating.\",\n      \"method\": \"Comparative receptor interactome (co-purification/MS), Co-IP, electrophysiology, biochemical validation\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal biochemical and functional validation with multiple orthogonal methods in a highly-cited foundational paper\",\n      \"pmids\": [\"22813734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GSG1L reduces the weighted mean single-channel conductance and calcium permeability of calcium-permeable AMPARs (CP-AMPARs) while increasing polyamine-dependent inward rectification, opposing the enhancing effects of TARPs such as stargazin.\",\n      \"method\": \"Single-channel electrophysiology in recombinant systems; shRNA knockdown in cultured hippocampal and cerebellar neurons; mEPSC recordings\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro single-channel recordings combined with neuronal knockdown, multiple orthogonal readouts\",\n      \"pmids\": [\"26658868\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GSG1L negatively regulates AMPAR-mediated synaptic transmission: overexpression suppresses and knockout enhances AMPAR EPSCs in hippocampal CA1 neurons. GSG1L speeds up AMPAR deactivation and desensitization kinetics, opposing the slowing effects of TARPs/CNIHs. The first extracellular loop and C-terminus of GSG1L are required for these functions. GSG1L association with AMPARs inhibits CNIH2-induced slowing. GSG1L KO rats show LTP deficits and impaired object recognition.\",\n      \"method\": \"Overexpression and knockout (KO rat), patch-clamp electrophysiology, heterologous cell expression, domain deletion analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (KO, OE, domain mutants, electrophysiology) in a well-cited study\",\n      \"pmids\": [\"26932439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In hippocampal dentate granule cells, GSG1L regulates AMPAR synaptic strength but does not modulate AMPAR deactivation or desensitization kinetics, revealing a neuron-type-specific role for GSG1L.\",\n      \"method\": \"Electrophysiology in dentate granule neurons (overexpression/KO)\",\n      \"journal\": \"Journal of neurophysiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct electrophysiology with genetic manipulation, single lab\",\n      \"pmids\": [\"27707810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GSG1L selectively chaperones GluA4-containing AMPARs to synapses during the late phase of classical conditioning, while TARPγ8 chaperones GluA1-containing AMPARs early, demonstrating subunit-selective AMPAR trafficking by auxiliary proteins.\",\n      \"method\": \"Co-localization immunofluorescence, ex vivo brainstem preparation, conditioning behavioral paradigm\",\n      \"journal\": \"Neuroscience letters\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — co-localization without biochemical confirmation of direct interaction; single lab, single method\",\n      \"pmids\": [\"28219790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"GSG1L controls short-term plasticity (suppresses short-term facilitation) specifically at corticothalamic synapses in anterior thalamus neurons. GSG1L KO causes AT neuron hyperexcitability and increased seizure susceptibility, while stargazin co-exists in these neurons but is functionally absent from corticothalamic synapses.\",\n      \"method\": \"GSG1L knockout mice, electrophysiology at identified synapses, seizure susceptibility assay\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined electrophysiological and behavioral phenotypes, synapse-specific resolution\",\n      \"pmids\": [\"32697982\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of GluA2 AMPAR in complex with GSG1L and with type II TARP γ5 revealed that desensitization of both GluA2-GSG1L and GluA2-γ5 complexes is accompanied by rupture of the ligand-binding domain (LBD) dimer interface, and that GSG1L (like γ5) accommodates a maximum stoichiometry of two auxiliary subunits per AMPAR tetramer, unlike type I TARPs.\",\n      \"method\": \"Cryo-EM structural determination, functional electrophysiology\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with functional validation, published in high-impact journal\",\n      \"pmids\": [\"34678168\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GSG1L acts through two discrete evolutionarily-conserved allosteric sites on the AMPAR agonist-binding domain: a weaker interaction at the TARP/KGK site slows desensitization, and a stronger interaction at a distinct site slows recovery from desensitization. GSG1L constitutes ~5% of all AMPAR complexes in adult brain, expresses late in development in a region-specific manner, can co-assemble with TARPs or CNIHs or serve as the sole auxiliary subunit.\",\n      \"method\": \"Native interactome proteomics (MS), allosteric site mutagenesis, electrophysiology, developmental expression profiling\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — site-directed mutagenesis of allosteric sites combined with native interactome MS and electrophysiology, multiple orthogonal methods\",\n      \"pmids\": [\"37884493\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GSG1L and γ-2 (stargazin) compete for the same binding sites on the GluA1 AMPAR tetramer with comparable apparent dissociation constants (~2.0–2.5/µm²), implying dynamic regulation of AMPAR auxiliary subunit composition.\",\n      \"method\": \"Three-color single-molecule imaging in living cells, colocalization analysis, binding affinity modeling\",\n      \"journal\": \"Cellular & molecular biology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — single-molecule live-cell imaging with quantitative modeling, single lab\",\n      \"pmids\": [\"37430208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Intracellular spermine is required for GSG1L's suppression of CP-AMPAR single-channel conductance and its slowing of recovery from desensitization. Specific residues in the channel's selectivity filter and GSG1L's C-tail mediate these effects. TARPs prevent spermine-mediated conductance reduction while GSG1L enhances it.\",\n      \"method\": \"Single-channel electrophysiology with intracellular polyamine manipulation, site-directed mutagenesis of channel and GSG1L C-tail residues\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro single-channel recordings with mutagenesis of specific functional residues, mechanistically detailed\",\n      \"pmids\": [\"40185633\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GSG1L is a claudin-family transmembrane auxiliary subunit of AMPA receptors (AMPARs) that assembles with up to two copies per AMPAR tetramer, acts through two discrete allosteric sites on the ligand-binding domain to suppress CP-AMPAR function by reducing single-channel conductance (via enhanced intracellular spermine block dependent on selectivity-filter residues and the GSG1L C-tail), increasing polyamine-dependent rectification, speeding deactivation and desensitization, and slowing recovery from desensitization; in neurons it negatively regulates synaptic AMPAR transmission, controls short-term plasticity at corticothalamic synapses, and is required for normal LTP and seizure threshold.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GSG1L is a claudin-family transmembrane auxiliary subunit of AMPA-type glutamate receptors (AMPARs) that negatively regulates synaptic AMPAR function through multiple allosteric and ion-channel-level mechanisms. It assembles with up to two copies per AMPAR tetramer and acts through two discrete sites on the ligand-binding domain to speed deactivation and desensitization, slow recovery from desensitization, reduce single-channel conductance of calcium-permeable AMPARs via enhanced intracellular spermine block (dependent on selectivity-filter residues and the GSG1L C-tail), and increase polyamine-dependent inward rectification—effects that oppose the enhancing actions of TARPs and CNIHs [PMID:26658868, PMID:34678168, PMID:37884493, PMID:40185633]. In neurons, GSG1L constitutes ~5% of adult brain AMPAR complexes with late developmental onset and region-specific expression; it suppresses AMPAR-mediated EPSCs, controls short-term facilitation at corticothalamic synapses, and is required for normal hippocampal LTP and seizure threshold, as GSG1L knockout causes enhanced synaptic transmission, thalamic hyperexcitability, LTP deficits, and increased seizure susceptibility [PMID:26932439, PMID:32697982, PMID:37884493].\",\n  \"teleology\": [\n    {\n      \"year\": 2012,\n      \"claim\": \"Identifying GSG1L as an AMPAR auxiliary subunit resolved a gap in the catalog of proteins that directly assemble with and modulate AMPAR gating, establishing it as a previously unrecognized component of native receptor complexes.\",\n      \"evidence\": \"Comparative interactome profiling (co-purification/MS) of AMPA vs. kainate receptors in rat brain with Co-IP and electrophysiological validation\",\n      \"pmids\": [\"22813734\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of GSG1L per AMPAR tetramer unknown\", \"Mechanism of gating modulation unresolved\", \"Brain region and cell-type expression pattern not characterized\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrating that GSG1L reduces single-channel conductance and calcium permeability of CP-AMPARs while increasing polyamine-dependent rectification revealed that GSG1L is functionally antagonistic to TARPs at the biophysical level, establishing its role as a negative modulator of channel properties.\",\n      \"evidence\": \"Single-channel electrophysiology in recombinant systems; shRNA knockdown in cultured hippocampal and cerebellar neurons with mEPSC recordings\",\n      \"pmids\": [\"26658868\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of conductance reduction unknown\", \"Whether effects depend on intracellular polyamines not tested\", \"In vivo relevance not established\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Knockout and overexpression studies in hippocampal CA1 neurons demonstrated that GSG1L suppresses synaptic AMPAR transmission, speeds desensitization/deactivation kinetics (opposing TARPs/CNIHs), and is required for normal LTP and object recognition, connecting biophysical modulation to synaptic plasticity and cognition.\",\n      \"evidence\": \"GSG1L KO rat and overexpression in hippocampal slices, patch-clamp electrophysiology, domain deletion analysis, behavioral testing\",\n      \"pmids\": [\"26932439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Neuron-type specificity of kinetic effects not fully mapped\", \"Structural basis of GSG1L–AMPAR interaction unknown\", \"Mechanism by which GSG1L opposes CNIH2 not resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Finding that GSG1L regulates synaptic strength but not desensitization kinetics in dentate granule cells (unlike CA1 neurons) established that GSG1L's functional impact is neuron-type-specific, likely reflecting differences in auxiliary subunit composition.\",\n      \"evidence\": \"Electrophysiology with overexpression and KO in dentate granule neurons\",\n      \"pmids\": [\"27707810\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of cell-type difference not identified\", \"Whether auxiliary subunit stoichiometry differs across neuron types not tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Showing that GSG1L controls short-term plasticity at corticothalamic synapses and that its loss causes thalamic hyperexcitability and increased seizure susceptibility established a synapse-specific in vivo function for GSG1L beyond the hippocampus.\",\n      \"evidence\": \"GSG1L knockout mice, electrophysiology at identified thalamic synapses, seizure susceptibility assay\",\n      \"pmids\": [\"32697982\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether GSG1L acts at other thalamic synapse types unknown\", \"Mechanism linking GSG1L to short-term facilitation suppression not fully resolved\", \"Seizure circuit mechanism not delineated\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Cryo-EM structures of GluA2–GSG1L complexes revealed a maximum stoichiometry of two GSG1L per AMPAR tetramer and showed that desensitization involves LBD dimer interface rupture, providing the first structural framework for GSG1L's modulatory mechanism.\",\n      \"evidence\": \"Cryo-EM structural determination of GluA2–GSG1L and GluA2–γ5 complexes with functional electrophysiology\",\n      \"pmids\": [\"34678168\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Allosteric communication pathway from GSG1L to the gating apparatus not resolved\", \"Structures of GSG1L co-assembled with TARPs or CNIHs not available\", \"Conformational dynamics during recovery from desensitization unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identification of two discrete allosteric sites on the AMPAR LBD through which GSG1L modulates desensitization and recovery, combined with native proteomics showing ~5% of brain AMPARs contain GSG1L with late developmental onset and co-assembly with TARPs/CNIHs, resolved how GSG1L exerts dual kinetic effects and defined its quantitative niche in the native receptor landscape.\",\n      \"evidence\": \"Native interactome proteomics (MS), allosteric site mutagenesis, electrophysiology, developmental expression profiling\",\n      \"pmids\": [\"37884493\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequences of GSG1L co-assembly with TARPs vs. CNIHs in neurons not tested\", \"Signaling pathways regulating GSG1L expression during development unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Single-molecule imaging showing that GSG1L and stargazin (γ-2) compete for the same AMPAR binding sites with comparable affinities established a dynamic competition model for auxiliary subunit composition at the cell surface.\",\n      \"evidence\": \"Three-color single-molecule imaging in living cells with colocalization analysis and binding affinity modeling\",\n      \"pmids\": [\"37430208\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether competition occurs at synapses in neurons not shown\", \"Regulatory signals that shift the GSG1L/TARP ratio unknown\", \"Limited to GluA1 homotetramers\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Demonstrating that intracellular spermine is required for GSG1L's suppression of CP-AMPAR conductance and that specific selectivity-filter residues and the GSG1L C-tail mediate this effect resolved the molecular mechanism by which GSG1L reduces channel conductance—through enhanced polyamine block rather than a direct pore effect.\",\n      \"evidence\": \"Single-channel electrophysiology with intracellular polyamine manipulation and site-directed mutagenesis of channel and GSG1L C-tail residues\",\n      \"pmids\": [\"40185633\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether spermine-dependent mechanism operates at native synapses in vivo unknown\", \"Structural basis of C-tail interaction with polyamine binding site not resolved\", \"Applicability to GluA2-containing heteromeric AMPARs not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How activity-dependent or developmental signals regulate GSG1L expression and its competitive incorporation into AMPAR complexes at specific synapses in vivo remains unresolved, as does the structural basis for GSG1L C-tail–mediated polyamine enhancement.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of GSG1L C-tail interaction with the channel pore/selectivity filter\", \"Signaling pathways controlling GSG1L expression unknown\", \"No conditional KO studies dissecting developmental vs. acute roles\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 2, 7, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 6, 8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [1, 2, 5, 7]}\n    ],\n    \"complexes\": [\"AMPA receptor complex\"],\n    \"partners\": [\"GRIA1\", \"GRIA2\", \"GRIA4\", \"CNIH2\", \"CACNG2\", \"CACNG5\"],\n    \"other_free_text\": []\n  }\n}\n```"}