{"gene":"GRIA4","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":2025,"finding":"Cryo-EM structures of rat GluA4:TARP-γ2 were solved in active, resting, and desensitized states, covering a full gating cycle. GluA4 alone displays a classical Y-shaped conformation. In resting conditions, GluA4:TARP-γ2 adopts two conformations, one resembling the desensitized state. A regulatory site for TARP-γ2 was identified in the ligand-binding domain that modulates gating kinetics.","method":"Cryo-electron microscopy (cryo-EM) structural determination with functional validation","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures covering full gating cycle with functional validation of TARP-γ2 regulatory site, multiple states resolved in a single rigorous study","pmids":["40954371"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM structures of GluA4 AMPARs reveal a canonical Y-shaped architecture with domain-swapping between ATD and LBD. All four LBDs can be glutamate-bound yet open the ion channel by asymmetric hinging in all channel helices. Conformational plasticity of the glutamate-saturated LBD tunes the ion channel gate, providing a structural basis for subconductance states.","method":"Cryo-electron microscopy (cryo-EM) and single-channel bilayer recordings","journal":"bioRxiv (preprint) / Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures combined with electrophysiology, replicated across two publications (preprint and peer-reviewed journal)","pmids":["40667226","41656278"],"is_preprint":true},{"year":2003,"finding":"Surface expression of homomeric GluA4 (GluR-D) AMPA receptors requires a 14-residue cytoplasmic C-terminal segment that mediates binding to 4.1 family proteins. Co-immunoprecipitation demonstrated GluA4 associates with 4.1 protein(s) in both HEK293 cells and rat brain. GST pull-down confirmed the same segment is critical for 4.1 binding. Point mutations within this segment dramatically decreased surface expression with concomitant loss of 4.1 interaction.","method":"C-terminal deletion analysis, co-immunoprecipitation, GST pull-down, immunofluorescence/ELISA surface expression assay, point mutagenesis","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (co-IP, pulldown, mutagenesis, surface expression ELISA) in one study, validated in both heterologous cells and brain tissue","pmids":["12574408"],"is_preprint":false},{"year":1999,"finding":"The N-terminal X domain of GluA4 (GluRD) forms dimers in solution (shown by hydrodynamic analysis of recombinant fragments), whereas the S1S2 ligand-binding domain is monomeric. The X domain does not bind AMPA or glutamate, nor affect ligand binding properties of the S1S2 domain. This suggests subunit-subunit interactions in AMPA receptors involve the extracellular N-terminal domain.","method":"Recombinant protein expression in insect cells, hydrodynamic analysis, [3H]AMPA/glutamate radioligand binding assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro biochemical reconstitution with binding assays, single study, single lab","pmids":["10506139"],"is_preprint":false},{"year":1996,"finding":"The S1-S2 ligand-binding domain of GluA4 (GluR-D) expressed in E. coli as a soluble periplasmic protein bound [3H]AMPA with high affinity (Kd = 60 nM) and pharmacology typical of native AMPA receptors, demonstrating N-linked glycosylation is not required for ligand-binding site formation. The flip and flop splice variants bind [3H]AMPA with equal affinity; deletion of the C-terminal one-third of S2 abolished binding activity.","method":"Bacterial periplasmic expression, [3H]AMPA radioligand binding assays, deletion mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with quantitative binding assays and mutagenesis, defining structural requirements for ligand binding","pmids":["8663017"],"is_preprint":false},{"year":1998,"finding":"A disulfide bond exists in the ligand-binding domain of GluA4 (GluRD) between conserved cysteines C260 and C315, but is inaccessible to DTT in the intact receptor. Single mutants C260S and C315S show 2-3-fold higher ligand affinity than wild-type. Mutants lacking the native disulfide show non-native oligomerization and dramatically reduced specific activity, indicating the disulfide is required for ligand-binding domain stability.","method":"Biochemical disulfide analysis, site-directed mutagenesis, [3H]AMPA binding assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro biochemical analysis with mutagenesis and quantitative binding assays establishing disulfide bond function","pmids":["9737972"],"is_preprint":false},{"year":2002,"finding":"Site-directed mutagenesis of the helix F region of GluA4 (GluR-D) ligand-binding domain showed that L672 and T677 alanine substitutions severely reduced affinity for all agonists, while mutations at D673, S674, G675, S676, and K678 selectively affected specific agonists. Strikingly, antagonist binding affinities (Ro 48-8587, DNQX) were unaffected by all these mutations, demonstrating selective engagement of helix F side chains in agonist but not antagonist binding.","method":"Site-directed mutagenesis, radioligand binding assays ([3H]AMPA, [3H]Ro 48-8587), ligand docking","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro mutagenesis with quantitative binding assays for multiple ligands, identifying agonist vs antagonist discrimination mechanism","pmids":["12167621"],"is_preprint":false},{"year":2002,"finding":"R507 in GluA4 (GluR-D) is essential for both agonist and antagonist binding (even conservative R507K abolishes binding). E727 is required for agonist binding via ion-pair interaction; the isosteric E727Q mutation abolished all agonist binding but retained high-affinity antagonist binding, indicating that E727 ion-pair interaction is selectively required for agonist activity.","method":"Site-directed mutagenesis, [3H]AMPA and [3H]Ro 48-8587 radioligand binding assays, competition binding, ligand docking","journal":"European journal of biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro mutagenesis with quantitative binding assays for multiple radioligands, mechanistically dissecting agonist vs antagonist binding determinants","pmids":["12473122"],"is_preprint":false},{"year":2010,"finding":"Native GluA4 receptors contain a C-terminal proline that blocks canonical PDZ interactions. Deletion of this proline conferred avid binding to SAP97 in cultured cells by co-immunoprecipitation, whereas wild-type GluA4 did not associate with SAP97. Mass spectrometry confirmed that native GluA4 C-terminus is intact and the single-residue cleavage does not occur to a significant extent in vivo.","method":"Co-immunoprecipitation, mass spectrometry, antibody generation against proline-deleted GluA4 C-terminus","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP combined with mass spectrometry in both heterologous cells and mouse brain, multiple orthogonal methods in single study","pmids":["20090852"],"is_preprint":false},{"year":2014,"finding":"GluA4 expression in immature CA1 pyramidal neurons is sufficient to alter LTP signaling requirements. At immature synapses, PKA activation leads to synaptic potentiation via GluA4 mobilization. GluA4-deficient mice lack neonatal PKA-dependent LTP. Lentiviral expression of GluA4 in CA1 neurons at any developmental stage confers PKA-dependent synaptic potentiation and LTP, demonstrating GluA4 defines the switch in LTP kinase dependency from PKA to CaMKII during synapse maturation.","method":"Electrophysiology (LTP recordings), GluA4 knockout mice, lentiviral GluA4 overexpression, PKA pharmacology","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic loss-of-function and gain-of-function with defined electrophysiological phenotype, multiple orthogonal approaches in one study","pmids":["24599589"],"is_preprint":false},{"year":2016,"finding":"PKA activation leads to synaptic insertion of GluA4 at initially weak or silent CA1 synapses. This effect depends on a novel mechanism involving the extreme C-terminal end of GluA4, which interacts with the membrane-proximal region of its own C-terminal domain to control GluA4 trafficking. In the absence of GluA4, functional maturation of glutamatergic synapses during postnatal development was significantly delayed.","method":"Electrophysiology, C-terminal deletion/mutation constructs, GluA4 knockout mice, PKA pharmacology","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function mice and mutagenesis with defined electrophysiological phenotype, single lab","pmids":["27157711"],"is_preprint":false},{"year":2014,"finding":"GluA4-containing AMPARs in the reticular thalamus mediate cortico-nRT synaptic excitation. In Gria4 knockout mice, synaptic excitation of inhibitory reticular thalamic neurons is specifically reduced at the cortico-nRT projection. Absence seizures (spike-wave discharges) can still be initiated via the cortico-TC-nRT-TC pathway, revealing a bypass mode of corticothalamic transmission.","method":"Electrophysiology in Gria4 knockout mice, optogenetic stimulation of corticothalamic pathways","journal":"PLoS genetics / Nature neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — independent replication across multiple studies using knockout mice combined with electrophysiology and optogenetics","pmids":["21857658","18316356"],"is_preprint":false},{"year":2008,"finding":"A hypomorphic retroviral-like insertion in Gria4 causes absence seizures in C3H/HeJ mice. Gria4 knockout mice have frequent spike-wave discharges, and Gria4(-/-) does not complement the spkw1 locus. In contrast, Gria3 null mutants do not have SWD. In Gria4 mutants, synaptic excitation of inhibitory reticular thalamic neurons is enhanced with increased duration of synaptic responses, consistent with reduction of a kinetically fast AMPA receptor subunit.","method":"EEG recordings, genetic complementation, Gria4 knockout and Gria3 knockout mice, electrophysiology","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis (complementation test), multiple knockout lines, electrophysiological mechanistic data","pmids":["18316356"],"is_preprint":false},{"year":2009,"finding":"TARPs stargazin (γ2) and γ4 slow desensitization of homomeric GluA4 (GluR-D) AMPA receptors, increasing steady-state current. Ethanol concentration-dependently accelerates the rate of GluA4 desensitization, and this effect is enhanced by TARP coexpression. Recovery from desensitization was slowed by γ4 but ethanol did not affect this process.","method":"Whole-cell electrophysiology in HEK293 cells, TARP co-expression, ethanol pharmacology","journal":"Alcohol (Fayetteville, N.Y.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct electrophysiological measurement with pharmacological and genetic manipulation, single lab","pmids":["19560629"],"is_preprint":false},{"year":2019,"finding":"Both stargazin (γ2, type 1a TARP) and γ4 (type 1b TARP) slow the channel-opening rate (kop) and channel-closing rate (kcl) of GluA4 homomeric AMPA receptor channels each by approximately 4-fold and 3-fold respectively, without appreciable change in channel-opening probability. γ4 has a stronger effect on slowing desensitization rate than γ2, while γ2 causes a larger left-shift of the dose-response curve.","method":"Laser-pulse photolysis technique for rapid ligand application, single-channel and macroscopic current analysis","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro kinetic assay with rigorous method (laser-pulse photolysis), single lab, single study","pmids":["31267004"],"is_preprint":false},{"year":2021,"finding":"GluA4 knockout in mice reduces mossy fiber to granule cell synaptic transmission by ~80%. Despite compensatory changes (attenuated tonic inhibition, increased NMDAR transmission), granule cell spike output fidelity was markedly decreased. GluA4 knockout mice failed to form associative memories in delay eyeblink conditioning, while locomotor coordination was generally spared.","method":"Electrophysiology in GluA4 knockout mice, computational network modeling, behavioral conditioning","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — knockout mice with electrophysiology, computational modeling, and behavioral phenotyping, multiple orthogonal methods","pmids":["34219651"],"is_preprint":false},{"year":2013,"finding":"Repeated morphine administration leads to synaptic insertion of GluA4-containing Ca2+-permeable AMPARs in spinal cord dorsal horn laminae III-V. Co-immunoprecipitation suggested increase in GluA4 homomers. EPSC rectification index increased in morphine-treated mice, and infusion of GluA4 antibody through the patch pipette reversed the enhanced Ca2+-permeable AMPAR-mediated EPSCs. Intrathecal Ca2+-permeable AMPAR blocker disrupted morphine-induced mechanical hypersensitivity.","method":"Co-immunoprecipitation, immunohistochemistry, electrophysiology (whole-cell patch clamp, rectification index), intrathecal pharmacology, GluA4 antibody infusion via patch pipette","journal":"Neuropsychopharmacology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (co-IP, electrophysiology, intracellular antibody infusion, pharmacology) in single study establishing synaptic GluA4 role in morphine-induced pain","pmids":["23403695"],"is_preprint":false},{"year":2014,"finding":"Sequential synaptic delivery of GluA4-containing AMPARs during classical conditioning involves SAP97 scaffold. Conditioning induces formation of a SAP97-KSR1/PKC-GluA4 protein complex that delivers GluA4 to the PSD via SAP97-PSD95 interaction. This occurs after an initial PKA-dependent SAP97-AKAP/PKA-GluA1 complex delivers GluA1-containing AMPARs.","method":"Co-immunoprecipitation, in vitro eyeblink classical conditioning model, pharmacological inhibitors","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP demonstrating complex formation combined with functional conditioning assay, single lab","pmids":["24567325"],"is_preprint":false},{"year":2015,"finding":"Loss of GluA4 at corticothalamic L5B-POm synapses almost abolished EPSC amplitude and strongly delayed onset of action potential generation, demonstrating GluA4 is required to produce an EPSC sufficiently large to trigger postsynaptic action potentials within a defined time window after presynaptic input.","method":"Virus-mediated genetic knockdown, whole-cell electrophysiology, direct stimulation of single corticothalamic terminals","journal":"The European journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — viral knockdown with defined electrophysiological phenotype, single lab, single study","pmids":["26390982"],"is_preprint":false},{"year":2016,"finding":"Acoustic trauma decreases GluA4 mRNA and increases GluA1 mRNA in the lateral superior olive (LSO), slowing AMPAR-EPSC decay times. These changes are reversible within two months. Computational modeling confirmed that longer-lasting EPSCs after GluA4 reduction compensate to maintain binaural function with raised auditory thresholds.","method":"Voltage-clamp electrophysiology, mRNA quantification, auditory brainstem responses, computational modeling","journal":"The Journal of physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — electrophysiology combined with molecular and computational analyses in single study, single lab","pmids":["27104476"],"is_preprint":false},{"year":2017,"finding":"Quantitative freeze-fracture replica immunogold labeling of auditory nerve synapses showed higher number and density of GluA4 subunits at auditory nerve to fusiform cell (AN-FC) synapses compared to bushy cell (AN-BC) synapses, while GluA3 is enriched at AN-BC synapses. GluA4 gold labeling was homogeneously distributed along both synapse types, contrasting with the central distribution of GluA3 at AN-BC synapses.","method":"Quantitative freeze-fracture replica immunogold labeling (FRIL), GluA3 knockout mice","journal":"Brain structure & function","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — quantitative ultrastructural localization with genetic validation, single lab","pmids":["28397107"],"is_preprint":false},{"year":2017,"finding":"De novo heterozygous variants in the SYTANLAAF motif of GRIA4 transmembrane domain M3 (a conserved gating motif) cause intellectual disability with or without seizures. Molecular modeling showed that three variants orient toward the pore center and are predicted to disturb the gating mechanism; a fourth variant in the same motif likely reduces permeability; a fifth extracellular domain variant potentially interferes with monomer binding.","method":"Trio whole-exome sequencing, molecular modeling of gating mechanism","journal":"American journal of human genetics","confidence":"Low","confidence_rationale":"Tier 4 / Weak — genetic identification combined with computational modeling only, no direct functional validation of gating mechanism in this study","pmids":["29220673"],"is_preprint":false},{"year":2004,"finding":"Human GluR4c is an alternative splicing isoform of GluA4 with a 113-bp insert containing a stop codon resulting in a short C terminus. Its expression is widespread in the adult human brain and upregulated with development in cerebellum and cerebral cortex, reaching ~30% of total GluA4 mRNA in adults.","method":"cDNA cloning, RT-PCR expression profiling","journal":"Brain research. Molecular brain research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — characterization of splice variant by cloning and expression analysis, no functional mechanistic data","pmids":["15306133"],"is_preprint":false}],"current_model":"GluA4 (GRIA4) is an AMPA-type ionotropic glutamate receptor subunit with a bilobed extracellular ligand-binding domain (S1S2) that binds AMPA/glutamate with ~60 nM affinity through critical residues R507 and E727, a conserved disulfide bond (C260-C315) required for LBD stability, and an N-terminal domain that drives subunit dimerization; its C-terminal domain mediates surface expression and trafficking via interaction with 4.1 family proteins (requiring a 14-residue cytoplasmic segment) but not canonical PDZ interactions due to a blocking C-terminal proline; cryo-EM structures reveal a Y-shaped architecture where glutamate-bound LBDs open the channel by asymmetric hinging, with conformational plasticity underlying subconductance states, and TARP-γ2 binding to the LBD slows both channel opening and closing rates; at the circuit level, GluA4 is the dominant fast-kinetic AMPA subunit at reticular thalamic, cerebellar mossy fiber–granule cell, and auditory brainstem synapses, and its synaptic recruitment at immature hippocampal synapses is driven by PKA via a novel intramolecular C-terminal mechanism that defines the neonatal form of LTP and facilitates the functional maturation of glutamatergic circuitry."},"narrative":{"mechanistic_narrative":"GRIA4 encodes GluA4 (GluR-D), an AMPA-type ionotropic glutamate receptor subunit that supplies fast-kinetic excitatory transmission at defined central synapses and governs activity-dependent synaptic strengthening during development [PMID:24599589, PMID:21857658, PMID:18316356]. Its extracellular S1S2 ligand-binding domain folds into an autonomous module that binds AMPA with ~60 nM affinity independently of glycosylation, stabilized by a buried C260–C315 disulfide bond, with R507 and E727 forming ion-pair contacts that are selectively required for agonist over antagonist engagement and helix-F residues (L672, T677) tuning agonist affinity [PMID:8663017, PMID:9737972, PMID:12167621, PMID:12473122]. The N-terminal domain mediates subunit dimerization without contributing to ligand binding [PMID:10506139], and cryo-EM resolves a Y-shaped, domain-swapped architecture in which glutamate-saturated LBDs gate the channel through asymmetric hinging, with conformational plasticity underlying subconductance states; an LBD regulatory site for the auxiliary subunit TARP-γ2 slows channel opening and closing and tunes desensitization across the gating cycle [PMID:40954371, PMID:40667226, PMID:41656278, PMID:31267004]. Surface delivery depends on a 14-residue C-terminal cytoplasmic segment that binds 4.1-family proteins, while a terminal proline blocks canonical PDZ binding to SAP97 [PMID:12574408, PMID:20090852]; at immature hippocampal synapses, PKA drives synaptic insertion of GluA4 through an intramolecular C-terminal mechanism that defines neonatal PKA-dependent LTP and the maturation of glutamatergic circuitry [PMID:24599589, PMID:27157711]. At the circuit level GluA4 sets EPSC speed and amplitude at corticothalamic reticular, cerebellar mossy fiber–granule cell, and auditory brainstem synapses, where its loss impairs spike fidelity, associative eyeblink memory, and corticothalamic drive, and Gria4 disruption produces absence seizures [PMID:21857658, PMID:18316356, PMID:34219651, PMID:26390982]. De novo variants in the M3 SYTANLAAF gating motif of GRIA4 cause intellectual disability with or without seizures [PMID:29220673].","teleology":[{"year":1996,"claim":"Establishing that the isolated S1S2 segment is a self-contained ligand-binding module answered whether the AMPA binding site could fold without the rest of the receptor and without glycosylation.","evidence":"bacterial periplasmic expression of S1S2 with [3H]AMPA binding and deletion mutagenesis","pmids":["8663017"],"confidence":"High","gaps":["Does not resolve how the LBD couples to channel gating","flip/flop functional consequences beyond equal binding affinity not addressed"]},{"year":1998,"claim":"Identifying the C260–C315 disulfide as essential for LBD stability explained why correct folding and ligand-binding competence depend on a specific intradomain bond.","evidence":"biochemical disulfide analysis with C260S/C315S mutagenesis and [3H]AMPA binding","pmids":["9737972"],"confidence":"High","gaps":["Effect on full channel gating not measured","redox regulation in vivo unaddressed"]},{"year":1999,"claim":"Showing the N-terminal X domain dimerizes while S1S2 stays monomeric localized subunit-assembly contacts to the extracellular N-terminus.","evidence":"hydrodynamic analysis of recombinant domain fragments with radioligand binding","pmids":["10506139"],"confidence":"Medium","gaps":["In vitro fragment behavior, not full receptor assembly","single lab"]},{"year":2002,"claim":"Pinpointing R507, E727, and helix-F residues distinguished agonist from antagonist binding determinants, dissecting how the LBD discriminates activating from blocking ligands.","evidence":"site-directed mutagenesis with multiple radioligand binding assays and ligand docking","pmids":["12167621","12473122"],"confidence":"High","gaps":["Binding determinants mapped, but link to gating kinetics not measured directly"]},{"year":2003,"claim":"Mapping a 14-residue C-terminal segment that binds 4.1 proteins answered how GluA4 reaches and is retained at the cell surface.","evidence":"C-terminal deletion, co-IP and GST pull-down in HEK293 and brain, surface ELISA, point mutagenesis","pmids":["12574408"],"confidence":"High","gaps":["Identity of the specific 4.1 family member at synapses not resolved","structural basis of the interaction unknown"]},{"year":2008,"claim":"Genetic complementation linking Gria4 loss to spike-wave discharges established GluA4 as a fast AMPA subunit whose absence dysregulates reticular thalamic excitation and causes absence seizures.","evidence":"EEG, complementation test, Gria4 and Gria3 knockout mice, electrophysiology","pmids":["18316356"],"confidence":"High","gaps":["Molecular basis of prolonged synaptic responses beyond loss of a fast subunit not fully resolved"]},{"year":2010,"claim":"Demonstrating that a terminal proline blocks SAP97 binding clarified why GluA4 does not engage canonical PDZ scaffolds despite a class-I-like C-terminus.","evidence":"reciprocal co-IP and mass spectrometry in cells and mouse brain with proline-deletion constructs","pmids":["20090852"],"confidence":"High","gaps":["Whether any physiological condition relieves this block unaddressed"]},{"year":2014,"claim":"Showing GluA4 defines a PKA-dependent neonatal form of LTP and is required at the cortico-nRT projection connected molecular trafficking to developmental and circuit-level functions.","evidence":"LTP recordings, Gria4 knockout, lentiviral GluA4 expression, PKA pharmacology, optogenetics","pmids":["24599589","21857658"],"confidence":"High","gaps":["Molecular identity of the PKA-driven trafficking step not yet defined here","downstream effectors of GluA4 mobilization unclear"]},{"year":2016,"claim":"Identifying an intramolecular C-terminal interaction controlling PKA-driven insertion defined the molecular switch for GluA4 synaptic delivery and synapse maturation.","evidence":"electrophysiology with C-terminal deletion/mutation constructs and Gria4 knockout mice","pmids":["27157711"],"confidence":"Medium","gaps":["Structural model of the intramolecular contact not resolved","single lab"]},{"year":2017,"claim":"Quantitative kinetic and structural studies of TARP modulation established how auxiliary subunits tune GluA4 gating, and scaffold/co-IP work linked GluA4 delivery to SAP97-KSR1/PKC complexes during conditioning.","evidence":"freeze-fracture immunogold localization, laser-pulse photolysis kinetics, co-IP in conditioning models","pmids":["28397107","24567325"],"confidence":"Medium","gaps":["SAP97 complex evidence is co-IP based without reciprocal in vivo validation","TARP isoform specificity in vivo incompletely mapped"]},{"year":2017,"claim":"Identifying de novo SYTANLAAF (M3) variants linked GRIA4 to intellectual disability with or without seizures, implicating gating disruption in disease.","evidence":"trio whole-exome sequencing with molecular modeling of the gating motif","pmids":["29220673"],"confidence":"Low","gaps":["No direct functional validation of the predicted gating defects in this study","genotype-phenotype mechanism inferred computationally"]},{"year":2021,"claim":"Demonstrating an ~80% loss of mossy fiber–granule cell transmission and failed associative memory in knockouts established GluA4 as essential for cerebellar input fidelity and learning.","evidence":"electrophysiology, computational modeling, and delay eyeblink conditioning in GluA4 knockout mice","pmids":["34219651"],"confidence":"High","gaps":["Compensatory NMDAR/tonic inhibition changes complicate isolating GluA4-specific contribution"]},{"year":2025,"claim":"Full gating-cycle cryo-EM structures with TARP-γ2 and single-channel recordings provided the structural basis for asymmetric LBD-driven gating, subconductance states, and a TARP regulatory site.","evidence":"cryo-EM in active/resting/desensitized states and single-channel bilayer recordings","pmids":["40954371","40667226","41656278"],"confidence":"High","gaps":["Structures of heteromeric native GluA4 receptors not resolved","structural correlates of C-terminal trafficking interactions absent"]},{"year":null,"claim":"How GluA4's intracellular trafficking machinery (4.1 binding, intramolecular C-terminal switch, PKA phosphorylation) maps onto its resolved channel structure and onto heteromeric native assemblies remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No structure of the GluA4 cytoplasmic domain or its scaffold interactions","native subunit composition at each synapse type not structurally defined","in vivo phosphosite-level regulation of trafficking unmapped"]}],"mechanism_profile":{"molecular_activity":[],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,20]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[9,11,15]}],"complexes":["AMPA receptor","GluA4:TARP-γ2 complex"],"partners":["CACNG2","CACNG4","EPB41","DLG1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P48058","full_name":"Glutamate receptor 4","aliases":["AMPA-selective glutamate receptor 4","GluR-D","Glutamate receptor ionotropic, AMPA 4"],"length_aa":902,"mass_kda":100.9,"function":"Ionotropic glutamate receptor that functions as a ligand-gated cation channel, gated by L-glutamate and glutamatergic agonists such as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualic acid, and kainic acid (By similarity). L-glutamate acts as an excitatory neurotransmitter at many synapses in the central nervous system and plays an important role in fast excitatory synaptic transmission (By similarity). Binding of the excitatory neurotransmitter L-glutamate induces a conformation change, leading to the opening of the cation channel, and thereby converts the chemical signal to an electrical impulse upon entry of monovalent and divalent cations such as sodium and calcium. The receptor then desensitizes rapidly and enters a transient inactive state, characterized by the presence of bound agonist (By similarity). In the presence of CACNG8, shows resensitization which is characterized by a delayed accumulation of current flux upon continued application of L-glutamate (PubMed:21172611)","subcellular_location":"Cell membrane; Postsynaptic cell membrane; Cell projection, dendrite; Postsynaptic cell membrane","url":"https://www.uniprot.org/uniprotkb/P48058/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GRIA4","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/GRIA4","total_profiled":1310},"omim":[{"mim_id":"617864","title":"NEURODEVELOPMENTAL DISORDER WITH OR WITHOUT SEIZURES AND GAIT ABNORMALITIES; NEDSGA","url":"https://www.omim.org/entry/617864"},{"mim_id":"138248","title":"GLUTAMATE RECEPTOR, IONOTROPIC, AMPA 1; 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GluA4 alone displays a classical Y-shaped conformation. In resting conditions, GluA4:TARP-γ2 adopts two conformations, one resembling the desensitized state. A regulatory site for TARP-γ2 was identified in the ligand-binding domain that modulates gating kinetics.\",\n \"method\": \"Cryo-electron microscopy (cryo-EM) structural determination with functional validation\",\n \"journal\": \"Nature structural & molecular biology\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures covering full gating cycle with functional validation of TARP-γ2 regulatory site, multiple states resolved in a single rigorous study\",\n \"pmids\": [\"40954371\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2025,\n \"finding\": \"Cryo-EM structures of GluA4 AMPARs reveal a canonical Y-shaped architecture with domain-swapping between ATD and LBD. All four LBDs can be glutamate-bound yet open the ion channel by asymmetric hinging in all channel helices. Conformational plasticity of the glutamate-saturated LBD tunes the ion channel gate, providing a structural basis for subconductance states.\",\n \"method\": \"Cryo-electron microscopy (cryo-EM) and single-channel bilayer recordings\",\n \"journal\": \"bioRxiv (preprint) / Nature communications\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures combined with electrophysiology, replicated across two publications (preprint and peer-reviewed journal)\",\n \"pmids\": [\"40667226\", \"41656278\"],\n \"is_preprint\": true\n },\n {\n \"year\": 2003,\n \"finding\": \"Surface expression of homomeric GluA4 (GluR-D) AMPA receptors requires a 14-residue cytoplasmic C-terminal segment that mediates binding to 4.1 family proteins. Co-immunoprecipitation demonstrated GluA4 associates with 4.1 protein(s) in both HEK293 cells and rat brain. GST pull-down confirmed the same segment is critical for 4.1 binding. Point mutations within this segment dramatically decreased surface expression with concomitant loss of 4.1 interaction.\",\n \"method\": \"C-terminal deletion analysis, co-immunoprecipitation, GST pull-down, immunofluorescence/ELISA surface expression assay, point mutagenesis\",\n \"journal\": \"The Journal of neuroscience\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (co-IP, pulldown, mutagenesis, surface expression ELISA) in one study, validated in both heterologous cells and brain tissue\",\n \"pmids\": [\"12574408\"],\n \"is_preprint\": false\n },\n {\n \"year\": 1999,\n \"finding\": \"The N-terminal X domain of GluA4 (GluRD) forms dimers in solution (shown by hydrodynamic analysis of recombinant fragments), whereas the S1S2 ligand-binding domain is monomeric. The X domain does not bind AMPA or glutamate, nor affect ligand binding properties of the S1S2 domain. This suggests subunit-subunit interactions in AMPA receptors involve the extracellular N-terminal domain.\",\n \"method\": \"Recombinant protein expression in insect cells, hydrodynamic analysis, [3H]AMPA/glutamate radioligand binding assays\",\n \"journal\": \"The Journal of biological chemistry\",\n \"confidence\": \"Medium\",\n \"confidence_rationale\": \"Tier 1 / Weak — in vitro biochemical reconstitution with binding assays, single study, single lab\",\n \"pmids\": [\"10506139\"],\n \"is_preprint\": false\n },\n {\n \"year\": 1996,\n \"finding\": \"The S1-S2 ligand-binding domain of GluA4 (GluR-D) expressed in E. coli as a soluble periplasmic protein bound [3H]AMPA with high affinity (Kd = 60 nM) and pharmacology typical of native AMPA receptors, demonstrating N-linked glycosylation is not required for ligand-binding site formation. The flip and flop splice variants bind [3H]AMPA with equal affinity; deletion of the C-terminal one-third of S2 abolished binding activity.\",\n \"method\": \"Bacterial periplasmic expression, [3H]AMPA radioligand binding assays, deletion mutagenesis\",\n \"journal\": \"The Journal of biological chemistry\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with quantitative binding assays and mutagenesis, defining structural requirements for ligand binding\",\n \"pmids\": [\"8663017\"],\n \"is_preprint\": false\n },\n {\n \"year\": 1998,\n \"finding\": \"A disulfide bond exists in the ligand-binding domain of GluA4 (GluRD) between conserved cysteines C260 and C315, but is inaccessible to DTT in the intact receptor. Single mutants C260S and C315S show 2-3-fold higher ligand affinity than wild-type. Mutants lacking the native disulfide show non-native oligomerization and dramatically reduced specific activity, indicating the disulfide is required for ligand-binding domain stability.\",\n \"method\": \"Biochemical disulfide analysis, site-directed mutagenesis, [3H]AMPA binding assays\",\n \"journal\": \"The Journal of biological chemistry\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 1 / Strong — in vitro biochemical analysis with mutagenesis and quantitative binding assays establishing disulfide bond function\",\n \"pmids\": [\"9737972\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2002,\n \"finding\": \"Site-directed mutagenesis of the helix F region of GluA4 (GluR-D) ligand-binding domain showed that L672 and T677 alanine substitutions severely reduced affinity for all agonists, while mutations at D673, S674, G675, S676, and K678 selectively affected specific agonists. Strikingly, antagonist binding affinities (Ro 48-8587, DNQX) were unaffected by all these mutations, demonstrating selective engagement of helix F side chains in agonist but not antagonist binding.\",\n \"method\": \"Site-directed mutagenesis, radioligand binding assays ([3H]AMPA, [3H]Ro 48-8587), ligand docking\",\n \"journal\": \"The Journal of biological chemistry\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 1 / Strong — in vitro mutagenesis with quantitative binding assays for multiple ligands, identifying agonist vs antagonist discrimination mechanism\",\n \"pmids\": [\"12167621\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2002,\n \"finding\": \"R507 in GluA4 (GluR-D) is essential for both agonist and antagonist binding (even conservative R507K abolishes binding). E727 is required for agonist binding via ion-pair interaction; the isosteric E727Q mutation abolished all agonist binding but retained high-affinity antagonist binding, indicating that E727 ion-pair interaction is selectively required for agonist activity.\",\n \"method\": \"Site-directed mutagenesis, [3H]AMPA and [3H]Ro 48-8587 radioligand binding assays, competition binding, ligand docking\",\n \"journal\": \"European journal of biochemistry\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 1 / Strong — in vitro mutagenesis with quantitative binding assays for multiple radioligands, mechanistically dissecting agonist vs antagonist binding determinants\",\n \"pmids\": [\"12473122\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2010,\n \"finding\": \"Native GluA4 receptors contain a C-terminal proline that blocks canonical PDZ interactions. Deletion of this proline conferred avid binding to SAP97 in cultured cells by co-immunoprecipitation, whereas wild-type GluA4 did not associate with SAP97. Mass spectrometry confirmed that native GluA4 C-terminus is intact and the single-residue cleavage does not occur to a significant extent in vivo.\",\n \"method\": \"Co-immunoprecipitation, mass spectrometry, antibody generation against proline-deleted GluA4 C-terminus\",\n \"journal\": \"PloS one\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP combined with mass spectrometry in both heterologous cells and mouse brain, multiple orthogonal methods in single study\",\n \"pmids\": [\"20090852\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2014,\n \"finding\": \"GluA4 expression in immature CA1 pyramidal neurons is sufficient to alter LTP signaling requirements. At immature synapses, PKA activation leads to synaptic potentiation via GluA4 mobilization. GluA4-deficient mice lack neonatal PKA-dependent LTP. Lentiviral expression of GluA4 in CA1 neurons at any developmental stage confers PKA-dependent synaptic potentiation and LTP, demonstrating GluA4 defines the switch in LTP kinase dependency from PKA to CaMKII during synapse maturation.\",\n \"method\": \"Electrophysiology (LTP recordings), GluA4 knockout mice, lentiviral GluA4 overexpression, PKA pharmacology\",\n \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 2 / Strong — genetic loss-of-function and gain-of-function with defined electrophysiological phenotype, multiple orthogonal approaches in one study\",\n \"pmids\": [\"24599589\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2016,\n \"finding\": \"PKA activation leads to synaptic insertion of GluA4 at initially weak or silent CA1 synapses. This effect depends on a novel mechanism involving the extreme C-terminal end of GluA4, which interacts with the membrane-proximal region of its own C-terminal domain to control GluA4 trafficking. In the absence of GluA4, functional maturation of glutamatergic synapses during postnatal development was significantly delayed.\",\n \"method\": \"Electrophysiology, C-terminal deletion/mutation constructs, GluA4 knockout mice, PKA pharmacology\",\n \"journal\": \"Neuropharmacology\",\n \"confidence\": \"Medium\",\n \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function mice and mutagenesis with defined electrophysiological phenotype, single lab\",\n \"pmids\": [\"27157711\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2014,\n \"finding\": \"GluA4-containing AMPARs in the reticular thalamus mediate cortico-nRT synaptic excitation. In Gria4 knockout mice, synaptic excitation of inhibitory reticular thalamic neurons is specifically reduced at the cortico-nRT projection. Absence seizures (spike-wave discharges) can still be initiated via the cortico-TC-nRT-TC pathway, revealing a bypass mode of corticothalamic transmission.\",\n \"method\": \"Electrophysiology in Gria4 knockout mice, optogenetic stimulation of corticothalamic pathways\",\n \"journal\": \"PLoS genetics / Nature neuroscience\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 2 / Strong — independent replication across multiple studies using knockout mice combined with electrophysiology and optogenetics\",\n \"pmids\": [\"21857658\", \"18316356\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2008,\n \"finding\": \"A hypomorphic retroviral-like insertion in Gria4 causes absence seizures in C3H/HeJ mice. Gria4 knockout mice have frequent spike-wave discharges, and Gria4(-/-) does not complement the spkw1 locus. In contrast, Gria3 null mutants do not have SWD. In Gria4 mutants, synaptic excitation of inhibitory reticular thalamic neurons is enhanced with increased duration of synaptic responses, consistent with reduction of a kinetically fast AMPA receptor subunit.\",\n \"method\": \"EEG recordings, genetic complementation, Gria4 knockout and Gria3 knockout mice, electrophysiology\",\n \"journal\": \"Human molecular genetics\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis (complementation test), multiple knockout lines, electrophysiological mechanistic data\",\n \"pmids\": [\"18316356\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2009,\n \"finding\": \"TARPs stargazin (γ2) and γ4 slow desensitization of homomeric GluA4 (GluR-D) AMPA receptors, increasing steady-state current. Ethanol concentration-dependently accelerates the rate of GluA4 desensitization, and this effect is enhanced by TARP coexpression. Recovery from desensitization was slowed by γ4 but ethanol did not affect this process.\",\n \"method\": \"Whole-cell electrophysiology in HEK293 cells, TARP co-expression, ethanol pharmacology\",\n \"journal\": \"Alcohol (Fayetteville, N.Y.)\",\n \"confidence\": \"Medium\",\n \"confidence_rationale\": \"Tier 2 / Moderate — direct electrophysiological measurement with pharmacological and genetic manipulation, single lab\",\n \"pmids\": [\"19560629\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2019,\n \"finding\": \"Both stargazin (γ2, type 1a TARP) and γ4 (type 1b TARP) slow the channel-opening rate (kop) and channel-closing rate (kcl) of GluA4 homomeric AMPA receptor channels each by approximately 4-fold and 3-fold respectively, without appreciable change in channel-opening probability. γ4 has a stronger effect on slowing desensitization rate than γ2, while γ2 causes a larger left-shift of the dose-response curve.\",\n \"method\": \"Laser-pulse photolysis technique for rapid ligand application, single-channel and macroscopic current analysis\",\n \"journal\": \"Scientific reports\",\n \"confidence\": \"Medium\",\n \"confidence_rationale\": \"Tier 1 / Weak — in vitro kinetic assay with rigorous method (laser-pulse photolysis), single lab, single study\",\n \"pmids\": [\"31267004\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2021,\n \"finding\": \"GluA4 knockout in mice reduces mossy fiber to granule cell synaptic transmission by ~80%. Despite compensatory changes (attenuated tonic inhibition, increased NMDAR transmission), granule cell spike output fidelity was markedly decreased. GluA4 knockout mice failed to form associative memories in delay eyeblink conditioning, while locomotor coordination was generally spared.\",\n \"method\": \"Electrophysiology in GluA4 knockout mice, computational network modeling, behavioral conditioning\",\n \"journal\": \"eLife\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 2 / Strong — knockout mice with electrophysiology, computational modeling, and behavioral phenotyping, multiple orthogonal methods\",\n \"pmids\": [\"34219651\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2013,\n \"finding\": \"Repeated morphine administration leads to synaptic insertion of GluA4-containing Ca2+-permeable AMPARs in spinal cord dorsal horn laminae III-V. Co-immunoprecipitation suggested increase in GluA4 homomers. EPSC rectification index increased in morphine-treated mice, and infusion of GluA4 antibody through the patch pipette reversed the enhanced Ca2+-permeable AMPAR-mediated EPSCs. Intrathecal Ca2+-permeable AMPAR blocker disrupted morphine-induced mechanical hypersensitivity.\",\n \"method\": \"Co-immunoprecipitation, immunohistochemistry, electrophysiology (whole-cell patch clamp, rectification index), intrathecal pharmacology, GluA4 antibody infusion via patch pipette\",\n \"journal\": \"Neuropsychopharmacology\",\n \"confidence\": \"High\",\n \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (co-IP, electrophysiology, intracellular antibody infusion, pharmacology) in single study establishing synaptic GluA4 role in morphine-induced pain\",\n \"pmids\": [\"23403695\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2014,\n \"finding\": \"Sequential synaptic delivery of GluA4-containing AMPARs during classical conditioning involves SAP97 scaffold. Conditioning induces formation of a SAP97-KSR1/PKC-GluA4 protein complex that delivers GluA4 to the PSD via SAP97-PSD95 interaction. This occurs after an initial PKA-dependent SAP97-AKAP/PKA-GluA1 complex delivers GluA1-containing AMPARs.\",\n \"method\": \"Co-immunoprecipitation, in vitro eyeblink classical conditioning model, pharmacological inhibitors\",\n \"journal\": \"The Journal of biological chemistry\",\n \"confidence\": \"Medium\",\n \"confidence_rationale\": \"Tier 2 / Moderate — co-IP demonstrating complex formation combined with functional conditioning assay, single lab\",\n \"pmids\": [\"24567325\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2015,\n \"finding\": \"Loss of GluA4 at corticothalamic L5B-POm synapses almost abolished EPSC amplitude and strongly delayed onset of action potential generation, demonstrating GluA4 is required to produce an EPSC sufficiently large to trigger postsynaptic action potentials within a defined time window after presynaptic input.\",\n \"method\": \"Virus-mediated genetic knockdown, whole-cell electrophysiology, direct stimulation of single corticothalamic terminals\",\n \"journal\": \"The European journal of neuroscience\",\n \"confidence\": \"Medium\",\n \"confidence_rationale\": \"Tier 2 / Moderate — viral knockdown with defined electrophysiological phenotype, single lab, single study\",\n \"pmids\": [\"26390982\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2016,\n \"finding\": \"Acoustic trauma decreases GluA4 mRNA and increases GluA1 mRNA in the lateral superior olive (LSO), slowing AMPAR-EPSC decay times. These changes are reversible within two months. Computational modeling confirmed that longer-lasting EPSCs after GluA4 reduction compensate to maintain binaural function with raised auditory thresholds.\",\n \"method\": \"Voltage-clamp electrophysiology, mRNA quantification, auditory brainstem responses, computational modeling\",\n \"journal\": \"The Journal of physiology\",\n \"confidence\": \"Medium\",\n \"confidence_rationale\": \"Tier 2 / Moderate — electrophysiology combined with molecular and computational analyses in single study, single lab\",\n \"pmids\": [\"27104476\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2017,\n \"finding\": \"Quantitative freeze-fracture replica immunogold labeling of auditory nerve synapses showed higher number and density of GluA4 subunits at auditory nerve to fusiform cell (AN-FC) synapses compared to bushy cell (AN-BC) synapses, while GluA3 is enriched at AN-BC synapses. GluA4 gold labeling was homogeneously distributed along both synapse types, contrasting with the central distribution of GluA3 at AN-BC synapses.\",\n \"method\": \"Quantitative freeze-fracture replica immunogold labeling (FRIL), GluA3 knockout mice\",\n \"journal\": \"Brain structure & function\",\n \"confidence\": \"Medium\",\n \"confidence_rationale\": \"Tier 2 / Moderate — quantitative ultrastructural localization with genetic validation, single lab\",\n \"pmids\": [\"28397107\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2017,\n \"finding\": \"De novo heterozygous variants in the SYTANLAAF motif of GRIA4 transmembrane domain M3 (a conserved gating motif) cause intellectual disability with or without seizures. Molecular modeling showed that three variants orient toward the pore center and are predicted to disturb the gating mechanism; a fourth variant in the same motif likely reduces permeability; a fifth extracellular domain variant potentially interferes with monomer binding.\",\n \"method\": \"Trio whole-exome sequencing, molecular modeling of gating mechanism\",\n \"journal\": \"American journal of human genetics\",\n \"confidence\": \"Low\",\n \"confidence_rationale\": \"Tier 4 / Weak — genetic identification combined with computational modeling only, no direct functional validation of gating mechanism in this study\",\n \"pmids\": [\"29220673\"],\n \"is_preprint\": false\n },\n {\n \"year\": 2004,\n \"finding\": \"Human GluR4c is an alternative splicing isoform of GluA4 with a 113-bp insert containing a stop codon resulting in a short C terminus. Its expression is widespread in the adult human brain and upregulated with development in cerebellum and cerebral cortex, reaching ~30% of total GluA4 mRNA in adults.\",\n \"method\": \"cDNA cloning, RT-PCR expression profiling\",\n \"journal\": \"Brain research. Molecular brain research\",\n \"confidence\": \"Low\",\n \"confidence_rationale\": \"Tier 3 / Weak — characterization of splice variant by cloning and expression analysis, no functional mechanistic data\",\n \"pmids\": [\"15306133\"],\n \"is_preprint\": false\n }\n ],\n \"current_model\": \"GluA4 (GRIA4) is an AMPA-type ionotropic glutamate receptor subunit with a bilobed extracellular ligand-binding domain (S1S2) that binds AMPA/glutamate with ~60 nM affinity through critical residues R507 and E727, a conserved disulfide bond (C260-C315) required for LBD stability, and an N-terminal domain that drives subunit dimerization; its C-terminal domain mediates surface expression and trafficking via interaction with 4.1 family proteins (requiring a 14-residue cytoplasmic segment) but not canonical PDZ interactions due to a blocking C-terminal proline; cryo-EM structures reveal a Y-shaped architecture where glutamate-bound LBDs open the channel by asymmetric hinging, with conformational plasticity underlying subconductance states, and TARP-γ2 binding to the LBD slows both channel opening and closing rates; at the circuit level, GluA4 is the dominant fast-kinetic AMPA subunit at reticular thalamic, cerebellar mossy fiber–granule cell, and auditory brainstem synapses, and its synaptic recruitment at immature hippocampal synapses is driven by PKA via a novel intramolecular C-terminal mechanism that defines the neonatal form of LTP and facilitates the functional maturation of glutamatergic circuitry.\"\n}\n```","stage2_raw":"{\n \"mechanistic_narrative\": \"GRIA4 encodes GluA4 (GluR-D), an AMPA-type ionotropic glutamate receptor subunit that supplies fast-kinetic excitatory transmission at defined central synapses and governs activity-dependent synaptic strengthening during development [#9, #11]. Its extracellular S1S2 ligand-binding domain folds into an autonomous module that binds AMPA with ~60 nM affinity independently of glycosylation, stabilized by a buried C260–C315 disulfide bond, with R507 and E727 forming ion-pair contacts that are selectively required for agonist over antagonist engagement and helix-F residues (L672, T677) tuning agonist affinity [#4, #5, #6, #7]. The N-terminal domain mediates subunit dimerization without contributing to ligand binding [#3], and cryo-EM resolves a Y-shaped, domain-swapped architecture in which glutamate-saturated LBDs gate the channel through asymmetric hinging, with conformational plasticity underlying subconductance states; an LBD regulatory site for the auxiliary subunit TARP-\\u03b32 slows channel opening and closing and tunes desensitization across the gating cycle [#0, #1, #14]. Surface delivery depends on a 14-residue C-terminal cytoplasmic segment that binds 4.1-family proteins, while a terminal proline blocks canonical PDZ binding to SAP97 [#2, #8]; at immature hippocampal synapses, PKA drives synaptic insertion of GluA4 through an intramolecular C-terminal mechanism that defines neonatal PKA-dependent LTP and the maturation of glutamatergic circuitry [#9, #10]. At the circuit level GluA4 sets EPSC speed and amplitude at corticothalamic reticular, cerebellar mossy fiber\\u2013granule cell, and auditory brainstem synapses, where its loss impairs spike fidelity, associative eyeblink memory, and corticothalamic drive, and Gria4 disruption produces absence seizures [#11, #12, #15, #18]. De novo variants in the M3 SYTANLAAF gating motif of GRIA4 cause intellectual disability with or without seizures [#21].\",\n \"teleology\": [\n {\n \"year\": 1996,\n \"claim\": \"Establishing that the isolated S1S2 segment is a self-contained ligand-binding module answered whether the AMPA binding site could fold without the rest of the receptor and without glycosylation.\",\n \"evidence\": \"bacterial periplasmic expression of S1S2 with [3H]AMPA binding and deletion mutagenesis\",\n \"pmids\": [\"8663017\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Does not resolve how the LBD couples to channel gating\", \"flip/flop functional consequences beyond equal binding affinity not addressed\"]\n },\n {\n \"year\": 1998,\n \"claim\": \"Identifying the C260–C315 disulfide as essential for LBD stability explained why correct folding and ligand-binding competence depend on a specific intradomain bond.\",\n \"evidence\": \"biochemical disulfide analysis with C260S/C315S mutagenesis and [3H]AMPA binding\",\n \"pmids\": [\"9737972\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Effect on full channel gating not measured\", \"redox regulation in vivo unaddressed\"]\n },\n {\n \"year\": 1999,\n \"claim\": \"Showing the N-terminal X domain dimerizes while S1S2 stays monomeric localized subunit-assembly contacts to the extracellular N-terminus.\",\n \"evidence\": \"hydrodynamic analysis of recombinant domain fragments with radioligand binding\",\n \"pmids\": [\"10506139\"],\n \"confidence\": \"Medium\",\n \"gaps\": [\"In vitro fragment behavior, not full receptor assembly\", \"single lab\"]\n },\n {\n \"year\": 2002,\n \"claim\": \"Pinpointing R507, E727, and helix-F residues distinguished agonist from antagonist binding determinants, dissecting how the LBD discriminates activating from blocking ligands.\",\n \"evidence\": \"site-directed mutagenesis with multiple radioligand binding assays and ligand docking\",\n \"pmids\": [\"12167621\", \"12473122\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Binding determinants mapped, but link to gating kinetics not measured directly\"]\n },\n {\n \"year\": 2003,\n \"claim\": \"Mapping a 14-residue C-terminal segment that binds 4.1 proteins answered how GluA4 reaches and is retained at the cell surface.\",\n \"evidence\": \"C-terminal deletion, co-IP and GST pull-down in HEK293 and brain, surface ELISA, point mutagenesis\",\n \"pmids\": [\"12574408\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Identity of the specific 4.1 family member at synapses not resolved\", \"structural basis of the interaction unknown\"]\n },\n {\n \"year\": 2008,\n \"claim\": \"Genetic complementation linking Gria4 loss to spike-wave discharges established GluA4 as a fast AMPA subunit whose absence dysregulates reticular thalamic excitation and causes absence seizures.\",\n \"evidence\": \"EEG, complementation test, Gria4 and Gria3 knockout mice, electrophysiology\",\n \"pmids\": [\"18316356\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Molecular basis of prolonged synaptic responses beyond loss of a fast subunit not fully resolved\"]\n },\n {\n \"year\": 2010,\n \"claim\": \"Demonstrating that a terminal proline blocks SAP97 binding clarified why GluA4 does not engage canonical PDZ scaffolds despite a class-I-like C-terminus.\",\n \"evidence\": \"reciprocal co-IP and mass spectrometry in cells and mouse brain with proline-deletion constructs\",\n \"pmids\": [\"20090852\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Whether any physiological condition relieves this block unaddressed\"]\n },\n {\n \"year\": 2014,\n \"claim\": \"Showing GluA4 defines a PKA-dependent neonatal form of LTP and is required at the cortico-nRT projection connected molecular trafficking to developmental and circuit-level functions.\",\n \"evidence\": \"LTP recordings, Gria4 knockout, lentiviral GluA4 expression, PKA pharmacology, optogenetics\",\n \"pmids\": [\"24599589\", \"21857658\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Molecular identity of the PKA-driven trafficking step not yet defined here\", \"downstream effectors of GluA4 mobilization unclear\"]\n },\n {\n \"year\": 2016,\n \"claim\": \"Identifying an intramolecular C-terminal interaction controlling PKA-driven insertion defined the molecular switch for GluA4 synaptic delivery and synapse maturation.\",\n \"evidence\": \"electrophysiology with C-terminal deletion/mutation constructs and Gria4 knockout mice\",\n \"pmids\": [\"27157711\"],\n \"confidence\": \"Medium\",\n \"gaps\": [\"Structural model of the intramolecular contact not resolved\", \"single lab\"]\n },\n {\n \"year\": 2017,\n \"claim\": \"Quantitative kinetic and structural studies of TARP modulation established how auxiliary subunits tune GluA4 gating, and scaffold/co-IP work linked GluA4 delivery to SAP97-KSR1/PKC complexes during conditioning.\",\n \"evidence\": \"freeze-fracture immunogold localization, laser-pulse photolysis kinetics, co-IP in conditioning models\",\n \"pmids\": [\"28397107\", \"24567325\"],\n \"confidence\": \"Medium\",\n \"gaps\": [\"SAP97 complex evidence is co-IP based without reciprocal in vivo validation\", \"TARP isoform specificity in vivo incompletely mapped\"]\n },\n {\n \"year\": 2017,\n \"claim\": \"Identifying de novo SYTANLAAF (M3) variants linked GRIA4 to intellectual disability with or without seizures, implicating gating disruption in disease.\",\n \"evidence\": \"trio whole-exome sequencing with molecular modeling of the gating motif\",\n \"pmids\": [\"29220673\"],\n \"confidence\": \"Low\",\n \"gaps\": [\"No direct functional validation of the predicted gating defects in this study\", \"genotype-phenotype mechanism inferred computationally\"]\n },\n {\n \"year\": 2021,\n \"claim\": \"Demonstrating an ~80% loss of mossy fiber\\u2013granule cell transmission and failed associative memory in knockouts established GluA4 as essential for cerebellar input fidelity and learning.\",\n \"evidence\": \"electrophysiology, computational modeling, and delay eyeblink conditioning in GluA4 knockout mice\",\n \"pmids\": [\"34219651\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Compensatory NMDAR/tonic inhibition changes complicate isolating GluA4-specific contribution\"]\n },\n {\n \"year\": 2025,\n \"claim\": \"Full gating-cycle cryo-EM structures with TARP-\\u03b32 and single-channel recordings provided the structural basis for asymmetric LBD-driven gating, subconductance states, and a TARP regulatory site.\",\n \"evidence\": \"cryo-EM in active/resting/desensitized states and single-channel bilayer recordings\",\n \"pmids\": [\"40954371\", \"40667226\", \"41656278\"],\n \"confidence\": \"High\",\n \"gaps\": [\"Structures of heteromeric native GluA4 receptors not resolved\", \"structural correlates of C-terminal trafficking interactions absent\"]\n },\n {\n \"year\": null,\n \"claim\": \"How GluA4's intracellular trafficking machinery (4.1 binding, intramolecular C-terminal switch, PKA phosphorylation) maps onto its resolved channel structure and onto heteromeric native assemblies remains unresolved.\",\n \"evidence\": \"\",\n \"pmids\": [],\n \"confidence\": \"High\",\n \"gaps\": [\"No structure of the GluA4 cytoplasmic domain or its scaffold interactions\", \"native subunit composition at each synapse type not structurally defined\", \"in vivo phosphosite-level regulation of trafficking unmapped\"]\n }\n ],\n \"mechanism_profile\": {\n \"molecular_activity\": [\n {\"term_id\": \"GO:0005230\", \"supporting_discovery_ids\": [0, 1]}\n ],\n \"localization\": [\n {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 20]}\n ],\n \"pathway\": [\n {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [9, 11, 15]}\n ],\n \"complexes\": [\"AMPA receptor\", \"GluA4:TARP-\\u03b32 complex\"],\n \"partners\": [\"CACNG2\", \"CACNG4\", \"EPB41\", \"DLG1\"],\n \"other_free_text\": []\n }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}