{"gene":"GSG1L","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":2012,"finding":"GSG1L, a claudin homolog, was identified as a binding protein and auxiliary subunit of AMPA receptors (GluA1-4) through comparative interactome profiling of AMPA and kainate receptor complexes in rat brain, validated by biochemical, cellular, and electrophysiological experiments. GSG1L modulates AMPA-R gating.","method":"Comparative co-purification/interactome (mass spectrometry), biochemical co-IP, cellular and electrophysiological validation","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal biochemical and electrophysiological validation across multiple orthogonal methods in a focused study; independently replicated by subsequent labs","pmids":["22813734"],"is_preprint":false},{"year":2015,"finding":"GSG1L acts as an auxiliary subunit that reduces weighted mean single-channel conductance and calcium permeability of calcium-permeable AMPARs (CP-AMPARs), while increasing polyamine-dependent rectification (promoting inward rectification/polyamine block), in contrast to TARPs which enhance CP-AMPAR function.","method":"Patch-clamp electrophysiology in recombinant systems and native neurons (cerebellar stellate cells, hippocampal pyramidal neurons); shRNA knockdown and overexpression; mEPSC recordings","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal electrophysiological approaches in both recombinant and native neurons, gain- and loss-of-function experiments, replicated in follow-up study (PMID:40185633)","pmids":["26658868"],"is_preprint":false},{"year":2016,"finding":"GSG1L negatively regulates AMPAR-mediated synaptic transmission: overexpression suppresses and GSG1L knockout enhances AMPAR-mediated synaptic transmission in hippocampal CA1 neurons. GSG1L speeds up AMPAR deactivation and desensitization, opposite to effects of TARPs and CNIHs. GSG1L association with AMPARs inhibits CNIH2-induced slowing of receptor kinetics in heterologous cells. Regulation depends on the first extracellular loop domain and the carboxyl-terminus of GSG1L. GSG1L KO rats show deficits in LTP and object recognition.","method":"GSG1L knockout rats, overexpression, whole-cell patch-clamp electrophysiology, heterologous cell expression, domain deletion mutagenesis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — genetic KO, OE, domain mutagenesis, and multiple electrophysiological readouts in a single focused study with clear mechanistic conclusions","pmids":["26932439"],"is_preprint":false},{"year":2016,"finding":"GSG1L regulates the strength of AMPAR-mediated synaptic transmission in hippocampal dentate granule neurons, but does not critically modulate AMPAR deactivation or desensitization kinetics in these cells, demonstrating a neuron-type-specific role.","method":"Electrophysiology (mEPSC recordings), genetic manipulation in hippocampal dentate granule neurons","journal":"Journal of neurophysiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — single lab, two distinct electrophysiological readouts (synaptic strength vs. kinetics) showing cell-type specificity","pmids":["27707810"],"is_preprint":false},{"year":2017,"finding":"GSG1L specifically chaperones GluA4-containing AMPARs during synaptic delivery in a later stage of classical conditioning in brainstem abducens motor neurons, while TARPγ8 chaperones GluA1-containing AMPARs at an earlier stage, revealing subunit-selective auxiliary protein-mediated AMPAR trafficking.","method":"Immunofluorescence colocalization, ex vivo brainstem preparation, classical conditioning paradigm","journal":"Neuroscience letters","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, colocalization-based evidence without biochemical pull-down or functional rescue; indirect inference of chaperoning role","pmids":["28219790"],"is_preprint":false},{"year":2020,"finding":"GSG1L controls short-term plasticity specifically at corticothalamic synapses in anterior thalamus neurons but not at synapses receiving inputs from other pathways. GSG1L KO mice show AT neuron hyperexcitability and increased seizure susceptibility, consistent with GSG1L's negative regulatory role at these synapses.","method":"GSG1L knockout mice, electrophysiology (synaptic responses in anterior thalamus), seizure susceptibility assays","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO with pathway-specific electrophysiological phenotype and in vivo seizure readout, single lab","pmids":["32697982"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures of GluA2 AMPAR in complex with GSG1L reveal that desensitization of GluA2-GSG1L complexes is accompanied by rupture of the ligand-binding domain (LBD) dimer interface. GSG1L binds with maximum stoichiometry of two auxiliary subunits per AMPAR tetramer. The structural mechanism of GSG1L-associated desensitization differs from that of type II TARP γ5 (which maintains two-fold LBD dimer symmetry).","method":"Cryo-EM structure determination, electrophysiology (single-channel conductance, desensitization kinetics)","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure with electrophysiological functional validation; clear structural mechanism for desensitization","pmids":["34678168"],"is_preprint":false},{"year":2023,"finding":"GSG1L acts through two evolutionarily conserved allosteric sites on the AMPAR agonist-binding domain: a weak interaction at the TARP/KGK site that slows desensitization, and a stronger interaction at a distinct site that slows recovery from desensitization. GSG1L can assemble as the sole auxiliary subunit or co-assemble with TARPs or CNIHs, constituting ~5% of AMPAR complexes in the adult brain with region-specific and late developmental expression.","method":"Native interactome proteomics, single-molecule imaging, electrophysiology, mutagenesis of allosteric sites","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (proteomics, electrophysiology, mutagenesis), identification of distinct mechanistic sites, single lab but rigorous multi-method approach","pmids":["37884493"],"is_preprint":false},{"year":2023,"finding":"GSG1L and γ-2 (stargazin/TARP) compete for the same binding sites on the GluA1 AMPAR tetramer with comparable affinities (apparent Kd ~2.0–2.5/µm²), suggesting their occupancy is dynamically regulated by relative expression levels.","method":"Three-color single-molecule fluorescence imaging in living cells, quantitative colocalization analysis","journal":"Cellular & molecular biology letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — single-molecule live imaging with quantitative binding model, single lab, no orthogonal biochemical validation in this paper","pmids":["37430208"],"is_preprint":false},{"year":2025,"finding":"Intracellular spermine (polyamine) is required for GSG1L's characteristic suppression of CP-AMPAR single-channel conductance and its slowing of recovery from desensitization. Without intracellular spermine, GSG1L loses these effects. Specific residues in the channel selectivity filter and in GSG1L's C-tail are required for these polyamine-dependent effects. TARPs prevent spermine-mediated conductance reduction whereas GSG1L enhances it.","method":"Patch-clamp electrophysiology with intracellular spermine manipulation, site-directed mutagenesis of channel selectivity filter residues and GSG1L C-tail","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 / Strong — mutagenesis of defined residues combined with pharmacological manipulation and single-channel electrophysiology; mechanistic dissection of polyamine role in a dedicated focused study","pmids":["40185633"],"is_preprint":false}],"current_model":"GSG1L is a claudin-type transmembrane auxiliary subunit of AMPA receptors (AMPARs) that negatively regulates receptor function by reducing single-channel conductance, accelerating deactivation and desensitization, slowing recovery from desensitization, and suppressing synaptic AMPAR currents; it acts through two distinct allosteric sites on the AMPAR ligand-binding domain, requires intracellular spermine (polyamine) for its suppressive effects on calcium-permeable AMPARs, assembles with up to two copies per AMPAR tetramer (competing with TARPs for binding sites), and in vivo controls short-term synaptic plasticity, LTP, and seizure susceptibility in a synapse- and neuron-type-specific manner."},"narrative":{"mechanistic_narrative":"GSG1L is a claudin-homologous transmembrane auxiliary subunit of AMPA receptors (GluA1-4) that, unlike TARPs and CNIHs, acts as a negative regulator of receptor function [PMID:22813734, PMID:26932439]. It reduces the weighted mean single-channel conductance and calcium permeability of calcium-permeable AMPARs while promoting polyamine-dependent inward rectification [PMID:26658868], and it accelerates receptor deactivation and desensitization, thereby suppressing AMPAR-mediated synaptic transmission [PMID:26932439]. Cryo-EM of GluA2-GSG1L complexes shows that GSG1L binds with a maximum stoichiometry of two auxiliary subunits per AMPAR tetramer and that desensitization is accompanied by rupture of the ligand-binding domain dimer interface, a structural mechanism distinct from that of type II TARPs [PMID:34678168]. Functionally GSG1L engages two evolutionarily conserved allosteric sites on the agonist-binding domain — a weak interaction at the TARP/KGK site that slows desensitization and a stronger distinct site that slows recovery from desensitization — and can act as the sole auxiliary subunit or co-assemble with TARPs or CNIHs, while competing with γ-2/stargazin for shared binding sites at comparable affinity [PMID:37884493, PMID:37430208]. Its suppression of CP-AMPAR conductance and slowing of recovery from desensitization require intracellular spermine, acting through defined channel selectivity-filter residues and the GSG1L C-tail [PMID:40185633]. In vivo, GSG1L controls short-term plasticity, LTP, and seizure susceptibility in a synapse- and neuron-type-specific manner: knockout enhances AMPAR transmission and impairs LTP and object recognition in hippocampus [PMID:26932439], shapes corticothalamic short-term plasticity and constrains anterior thalamic excitability [PMID:32697982], yet does not modulate AMPAR kinetics in all neuron types [PMID:27707810].","teleology":[{"year":2012,"claim":"Established GSG1L as a bona fide AMPAR auxiliary subunit, answering whether this claudin homolog physically associates with and modulates native AMPAR complexes.","evidence":"Comparative interactome mass spectrometry of AMPA/kainate receptor complexes from rat brain with biochemical, cellular, and electrophysiological validation","pmids":["22813734"],"confidence":"High","gaps":["Direction and magnitude of gating modulation not yet quantified","Stoichiometry and binding site unknown"]},{"year":2015,"claim":"Defined GSG1L as a negative regulator of calcium-permeable AMPARs, distinguishing its action from the potentiating TARPs.","evidence":"Patch-clamp in recombinant systems and native neurons with shRNA knockdown and overexpression","pmids":["26658868"],"confidence":"High","gaps":["Molecular basis of conductance reduction unresolved","Requirement for polyamines not yet established"]},{"year":2016,"claim":"Demonstrated that GSG1L suppresses synaptic AMPAR transmission and accelerates gating kinetics in vivo, opposing TARP/CNIH effects, and mapped the responsible domains.","evidence":"GSG1L knockout rats, overexpression, domain-deletion mutagenesis, and patch-clamp in hippocampal CA1; behavioral LTP/object recognition assays","pmids":["26932439"],"confidence":"High","gaps":["Structural basis of kinetic acceleration not defined","Generality across neuron types untested"]},{"year":2016,"claim":"Revealed neuron-type specificity by showing GSG1L sets synaptic strength but not gating kinetics in dentate granule neurons.","evidence":"mEPSC recordings and genetic manipulation in hippocampal dentate granule neurons","pmids":["27707810"],"confidence":"Medium","gaps":["Mechanism underlying cell-type divergence unknown","Single lab without orthogonal biochemical correlate"]},{"year":2017,"claim":"Proposed a subunit-selective trafficking role in which GSG1L chaperones GluA4-containing AMPARs during a late stage of conditioning.","evidence":"Immunofluorescence colocalization in ex vivo brainstem during a classical conditioning paradigm","pmids":["28219790"],"confidence":"Low","gaps":["Colocalization-based, no biochemical pull-down or functional rescue of chaperone activity","Subunit selectivity inferred indirectly"]},{"year":2020,"claim":"Linked GSG1L's negative regulation to circuit-level control of short-term plasticity and seizure susceptibility at specific synapses.","evidence":"GSG1L knockout mice with pathway-specific electrophysiology in anterior thalamus and seizure susceptibility assays","pmids":["32697982"],"confidence":"Medium","gaps":["Molecular determinants of pathway specificity unknown","Single lab"]},{"year":2021,"claim":"Provided the structural mechanism of GSG1L-associated desensitization and fixed its stoichiometry at two subunits per tetramer.","evidence":"Cryo-EM of GluA2-GSG1L complexes with single-channel electrophysiology","pmids":["34678168"],"confidence":"High","gaps":["Structure of mixed GSG1L/TARP complexes not resolved","Polyamine contribution not addressed structurally"]},{"year":2023,"claim":"Identified two distinct allosteric sites mediating GSG1L's separable effects on desensitization and recovery, and quantified its prevalence among native AMPAR complexes.","evidence":"Native interactome proteomics, single-molecule imaging, mutagenesis of allosteric sites, and electrophysiology","pmids":["37884493"],"confidence":"High","gaps":["Functional consequences of co-assembly with TARPs/CNIHs incompletely defined","Developmental regulation of expression not mechanistically explained"]},{"year":2023,"claim":"Showed that GSG1L and γ-2/stargazin compete for the same tetramer binding sites with comparable affinity, framing occupancy as expression-level dependent.","evidence":"Three-color single-molecule fluorescence imaging in living cells with quantitative colocalization analysis","pmids":["37430208"],"confidence":"Medium","gaps":["No orthogonal biochemical validation in this study","Dynamics of in vivo competition untested"]},{"year":2025,"claim":"Established intracellular spermine as a required cofactor for GSG1L's conductance suppression and slowed recovery, dissecting the responsible channel and C-tail residues.","evidence":"Patch-clamp with intracellular spermine manipulation and site-directed mutagenesis of selectivity-filter and GSG1L C-tail residues","pmids":["40185633"],"confidence":"High","gaps":["Structural basis of polyamine-GSG1L cooperativity not resolved","In vivo relevance of polyamine dependence untested"]},{"year":null,"claim":"How GSG1L occupancy, allosteric site usage, and polyamine dependence are integrated to produce neuron-type-specific synaptic outcomes remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No unified model linking molecular stoichiometry to cell-type-specific kinetics","Structures of native mixed auxiliary-subunit complexes lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,2,6,7]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,2,8]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[2,5]}],"complexes":["AMPA receptor complex"],"partners":["GRIA1","GRIA2","GRIA3","GRIA4","CACNG2","CNIH2"],"other_free_text":[]}},"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":159,"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":74,"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":40,"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":12,"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":7300,"output_tokens":2651,"usd":0.030832,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9917,"output_tokens":3101,"usd":0.063555,"stage2_stop_reason":"end_turn"},"total_usd":0.094387,"stage1_batch_id":"msgbatch_01DUPACZi7AAWwuZiUjseQF4","stage2_batch_id":"msgbatch_018yt7chE3f48PEFtDwtj6AE","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2012,\n      \"finding\": \"GSG1L, a claudin homolog, was identified as a binding protein and auxiliary subunit of AMPA receptors (GluA1-4) through comparative interactome profiling of AMPA and kainate receptor complexes in rat brain, validated by biochemical, cellular, and electrophysiological experiments. GSG1L modulates AMPA-R gating.\",\n      \"method\": \"Comparative co-purification/interactome (mass spectrometry), biochemical co-IP, cellular and electrophysiological validation\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal biochemical and electrophysiological validation across multiple orthogonal methods in a focused study; independently replicated by subsequent labs\",\n      \"pmids\": [\"22813734\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"GSG1L acts as an auxiliary subunit that reduces weighted mean single-channel conductance and calcium permeability of calcium-permeable AMPARs (CP-AMPARs), while increasing polyamine-dependent rectification (promoting inward rectification/polyamine block), in contrast to TARPs which enhance CP-AMPAR function.\",\n      \"method\": \"Patch-clamp electrophysiology in recombinant systems and native neurons (cerebellar stellate cells, hippocampal pyramidal neurons); shRNA knockdown and overexpression; mEPSC recordings\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal electrophysiological approaches in both recombinant and native neurons, gain- and loss-of-function experiments, replicated in follow-up study (PMID:40185633)\",\n      \"pmids\": [\"26658868\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GSG1L negatively regulates AMPAR-mediated synaptic transmission: overexpression suppresses and GSG1L knockout enhances AMPAR-mediated synaptic transmission in hippocampal CA1 neurons. GSG1L speeds up AMPAR deactivation and desensitization, opposite to effects of TARPs and CNIHs. GSG1L association with AMPARs inhibits CNIH2-induced slowing of receptor kinetics in heterologous cells. Regulation depends on the first extracellular loop domain and the carboxyl-terminus of GSG1L. GSG1L KO rats show deficits in LTP and object recognition.\",\n      \"method\": \"GSG1L knockout rats, overexpression, whole-cell patch-clamp electrophysiology, heterologous cell expression, domain deletion mutagenesis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — genetic KO, OE, domain mutagenesis, and multiple electrophysiological readouts in a single focused study with clear mechanistic conclusions\",\n      \"pmids\": [\"26932439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GSG1L regulates the strength of AMPAR-mediated synaptic transmission in hippocampal dentate granule neurons, but does not critically modulate AMPAR deactivation or desensitization kinetics in these cells, demonstrating a neuron-type-specific role.\",\n      \"method\": \"Electrophysiology (mEPSC recordings), genetic manipulation in hippocampal dentate granule neurons\",\n      \"journal\": \"Journal of neurophysiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — single lab, two distinct electrophysiological readouts (synaptic strength vs. kinetics) showing cell-type specificity\",\n      \"pmids\": [\"27707810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"GSG1L specifically chaperones GluA4-containing AMPARs during synaptic delivery in a later stage of classical conditioning in brainstem abducens motor neurons, while TARPγ8 chaperones GluA1-containing AMPARs at an earlier stage, revealing subunit-selective auxiliary protein-mediated AMPAR trafficking.\",\n      \"method\": \"Immunofluorescence colocalization, ex vivo brainstem preparation, classical conditioning paradigm\",\n      \"journal\": \"Neuroscience letters\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, colocalization-based evidence without biochemical pull-down or functional rescue; indirect inference of chaperoning role\",\n      \"pmids\": [\"28219790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"GSG1L controls short-term plasticity specifically at corticothalamic synapses in anterior thalamus neurons but not at synapses receiving inputs from other pathways. GSG1L KO mice show AT neuron hyperexcitability and increased seizure susceptibility, consistent with GSG1L's negative regulatory role at these synapses.\",\n      \"method\": \"GSG1L knockout mice, electrophysiology (synaptic responses in anterior thalamus), seizure susceptibility assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with pathway-specific electrophysiological phenotype and in vivo seizure readout, single lab\",\n      \"pmids\": [\"32697982\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of GluA2 AMPAR in complex with GSG1L reveal that desensitization of GluA2-GSG1L complexes is accompanied by rupture of the ligand-binding domain (LBD) dimer interface. GSG1L binds with maximum stoichiometry of two auxiliary subunits per AMPAR tetramer. The structural mechanism of GSG1L-associated desensitization differs from that of type II TARP γ5 (which maintains two-fold LBD dimer symmetry).\",\n      \"method\": \"Cryo-EM structure determination, electrophysiology (single-channel conductance, desensitization kinetics)\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure with electrophysiological functional validation; clear structural mechanism for desensitization\",\n      \"pmids\": [\"34678168\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GSG1L acts through two evolutionarily conserved allosteric sites on the AMPAR agonist-binding domain: a weak interaction at the TARP/KGK site that slows desensitization, and a stronger interaction at a distinct site that slows recovery from desensitization. GSG1L can assemble as the sole auxiliary subunit or co-assemble with TARPs or CNIHs, constituting ~5% of AMPAR complexes in the adult brain with region-specific and late developmental expression.\",\n      \"method\": \"Native interactome proteomics, single-molecule imaging, electrophysiology, mutagenesis of allosteric sites\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (proteomics, electrophysiology, mutagenesis), identification of distinct mechanistic sites, single lab but rigorous multi-method approach\",\n      \"pmids\": [\"37884493\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"GSG1L and γ-2 (stargazin/TARP) compete for the same binding sites on the GluA1 AMPAR tetramer with comparable affinities (apparent Kd ~2.0–2.5/µm²), suggesting their occupancy is dynamically regulated by relative expression levels.\",\n      \"method\": \"Three-color single-molecule fluorescence imaging in living cells, quantitative colocalization analysis\",\n      \"journal\": \"Cellular & molecular biology letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — single-molecule live imaging with quantitative binding model, single lab, no orthogonal biochemical validation in this paper\",\n      \"pmids\": [\"37430208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Intracellular spermine (polyamine) is required for GSG1L's characteristic suppression of CP-AMPAR single-channel conductance and its slowing of recovery from desensitization. Without intracellular spermine, GSG1L loses these effects. Specific residues in the channel selectivity filter and in GSG1L's C-tail are required for these polyamine-dependent effects. TARPs prevent spermine-mediated conductance reduction whereas GSG1L enhances it.\",\n      \"method\": \"Patch-clamp electrophysiology with intracellular spermine manipulation, site-directed mutagenesis of channel selectivity filter residues and GSG1L C-tail\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mutagenesis of defined residues combined with pharmacological manipulation and single-channel electrophysiology; mechanistic dissection of polyamine role in a dedicated focused study\",\n      \"pmids\": [\"40185633\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GSG1L is a claudin-type transmembrane auxiliary subunit of AMPA receptors (AMPARs) that negatively regulates receptor function by reducing single-channel conductance, accelerating deactivation and desensitization, slowing recovery from desensitization, and suppressing synaptic AMPAR currents; it acts through two distinct allosteric sites on the AMPAR ligand-binding domain, requires intracellular spermine (polyamine) for its suppressive effects on calcium-permeable AMPARs, assembles with up to two copies per AMPAR tetramer (competing with TARPs for binding sites), and in vivo controls short-term synaptic plasticity, LTP, and seizure susceptibility in a synapse- and neuron-type-specific manner.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"GSG1L is a claudin-homologous transmembrane auxiliary subunit of AMPA receptors (GluA1-4) that, unlike TARPs and CNIHs, acts as a negative regulator of receptor function [#0, #2]. It reduces the weighted mean single-channel conductance and calcium permeability of calcium-permeable AMPARs while promoting polyamine-dependent inward rectification [#1], and it accelerates receptor deactivation and desensitization, thereby suppressing AMPAR-mediated synaptic transmission [#2]. Cryo-EM of GluA2-GSG1L complexes shows that GSG1L binds with a maximum stoichiometry of two auxiliary subunits per AMPAR tetramer and that desensitization is accompanied by rupture of the ligand-binding domain dimer interface, a structural mechanism distinct from that of type II TARPs [#6]. Functionally GSG1L engages two evolutionarily conserved allosteric sites on the agonist-binding domain — a weak interaction at the TARP/KGK site that slows desensitization and a stronger distinct site that slows recovery from desensitization — and can act as the sole auxiliary subunit or co-assemble with TARPs or CNIHs, while competing with γ-2/stargazin for shared binding sites at comparable affinity [#7, #8]. Its suppression of CP-AMPAR conductance and slowing of recovery from desensitization require intracellular spermine, acting through defined channel selectivity-filter residues and the GSG1L C-tail [#9]. In vivo, GSG1L controls short-term plasticity, LTP, and seizure susceptibility in a synapse- and neuron-type-specific manner: knockout enhances AMPAR transmission and impairs LTP and object recognition in hippocampus [#2], shapes corticothalamic short-term plasticity and constrains anterior thalamic excitability [#5], yet does not modulate AMPAR kinetics in all neuron types [#3].\",\n  \"teleology\": [\n    {\n      \"year\": 2012,\n      \"claim\": \"Established GSG1L as a bona fide AMPAR auxiliary subunit, answering whether this claudin homolog physically associates with and modulates native AMPAR complexes.\",\n      \"evidence\": \"Comparative interactome mass spectrometry of AMPA/kainate receptor complexes from rat brain with biochemical, cellular, and electrophysiological validation\",\n      \"pmids\": [\"22813734\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direction and magnitude of gating modulation not yet quantified\", \"Stoichiometry and binding site unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Defined GSG1L as a negative regulator of calcium-permeable AMPARs, distinguishing its action from the potentiating TARPs.\",\n      \"evidence\": \"Patch-clamp in recombinant systems and native neurons with shRNA knockdown and overexpression\",\n      \"pmids\": [\"26658868\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of conductance reduction unresolved\", \"Requirement for polyamines not yet established\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Demonstrated that GSG1L suppresses synaptic AMPAR transmission and accelerates gating kinetics in vivo, opposing TARP/CNIH effects, and mapped the responsible domains.\",\n      \"evidence\": \"GSG1L knockout rats, overexpression, domain-deletion mutagenesis, and patch-clamp in hippocampal CA1; behavioral LTP/object recognition assays\",\n      \"pmids\": [\"26932439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of kinetic acceleration not defined\", \"Generality across neuron types untested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Revealed neuron-type specificity by showing GSG1L sets synaptic strength but not gating kinetics in dentate granule neurons.\",\n      \"evidence\": \"mEPSC recordings and genetic manipulation in hippocampal dentate granule neurons\",\n      \"pmids\": [\"27707810\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism underlying cell-type divergence unknown\", \"Single lab without orthogonal biochemical correlate\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Proposed a subunit-selective trafficking role in which GSG1L chaperones GluA4-containing AMPARs during a late stage of conditioning.\",\n      \"evidence\": \"Immunofluorescence colocalization in ex vivo brainstem during a classical conditioning paradigm\",\n      \"pmids\": [\"28219790\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Colocalization-based, no biochemical pull-down or functional rescue of chaperone activity\", \"Subunit selectivity inferred indirectly\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Linked GSG1L's negative regulation to circuit-level control of short-term plasticity and seizure susceptibility at specific synapses.\",\n      \"evidence\": \"GSG1L knockout mice with pathway-specific electrophysiology in anterior thalamus and seizure susceptibility assays\",\n      \"pmids\": [\"32697982\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular determinants of pathway specificity unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Provided the structural mechanism of GSG1L-associated desensitization and fixed its stoichiometry at two subunits per tetramer.\",\n      \"evidence\": \"Cryo-EM of GluA2-GSG1L complexes with single-channel electrophysiology\",\n      \"pmids\": [\"34678168\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of mixed GSG1L/TARP complexes not resolved\", \"Polyamine contribution not addressed structurally\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified two distinct allosteric sites mediating GSG1L's separable effects on desensitization and recovery, and quantified its prevalence among native AMPAR complexes.\",\n      \"evidence\": \"Native interactome proteomics, single-molecule imaging, mutagenesis of allosteric sites, and electrophysiology\",\n      \"pmids\": [\"37884493\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequences of co-assembly with TARPs/CNIHs incompletely defined\", \"Developmental regulation of expression not mechanistically explained\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Showed that GSG1L and γ-2/stargazin compete for the same tetramer binding sites with comparable affinity, framing occupancy as expression-level dependent.\",\n      \"evidence\": \"Three-color single-molecule fluorescence imaging in living cells with quantitative colocalization analysis\",\n      \"pmids\": [\"37430208\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No orthogonal biochemical validation in this study\", \"Dynamics of in vivo competition untested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Established intracellular spermine as a required cofactor for GSG1L's conductance suppression and slowed recovery, dissecting the responsible channel and C-tail residues.\",\n      \"evidence\": \"Patch-clamp with intracellular spermine manipulation and site-directed mutagenesis of selectivity-filter and GSG1L C-tail residues\",\n      \"pmids\": [\"40185633\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of polyamine-GSG1L cooperativity not resolved\", \"In vivo relevance of polyamine dependence untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How GSG1L occupancy, allosteric site usage, and polyamine dependence are integrated to produce neuron-type-specific synaptic outcomes remains unresolved.\",\n      \"evidence\": null,\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No unified model linking molecular stoichiometry to cell-type-specific kinetics\", \"Structures of native mixed auxiliary-subunit complexes lacking\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 2, 6, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 2, 8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [2, 5]}\n    ],\n    \"complexes\": [\"AMPA receptor complex\"],\n    \"partners\": [\"GRIA1\", \"GRIA2\", \"GRIA3\", \"GRIA4\", \"CACNG2\", \"CNIH2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":6,"faith_pct":83.33333333333333}}