{"gene":"GRM2","run_date":"2026-06-10T01:55:21","timeline":{"discoveries":[{"year":1996,"finding":"mGluR2 and mGluR3 are localized in postsynaptic, presynaptic, and glial compartments at excitatory synapses, with presynaptic staining concentrated in mossy fiber and perforant path glutamatergic terminals in the hippocampus, as determined by ultrastructural immunocytochemistry.","method":"Electron microscopy immunocytochemistry with affinity-purified antibody","journal":"Neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct ultrastructural localization replicated across multiple brain regions and independently corroborated by mRNA distribution studies","pmids":["8684625"],"is_preprint":false},{"year":1993,"finding":"mGluR2 mRNA is coupled to the inhibitory cAMP cascade and is expressed predominantly in neurons (not glia) throughout the CNS, with highest expression in cerebellar Golgi cells, mitral cells of the accessory olfactory bulb, and entorhinal/parasubicular cortex neurons.","method":"In situ hybridization","journal":"Neuroscience","confidence":"Medium","confidence_rationale":"Tier 3 / Strong — single method (ISH), replicated distribution findings from multiple brain regions","pmids":["8389425"],"is_preprint":false},{"year":2000,"finding":"PKA directly phosphorylates mGluR2 at a single serine residue (Ser843) on the C-terminal tail, and this phosphorylation inhibits coupling of mGluR2 to GTP-binding proteins; cAMP/PKA activation inhibits presynaptic mGluR2-mediated responses at the medial perforant path synapse.","method":"Site-directed mutagenesis, biochemical phosphorylation assays, in vitro kinase assay, electrophysiology with selective PKA inhibitor","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro phosphorylation assay with mutagenesis identifying specific residue (Ser843), plus functional electrophysiology confirmation","pmids":["10908604"],"is_preprint":false},{"year":2003,"finding":"mGluR2 acts as a postsynaptic receptor at granule cell–Golgi cell synapses in the cerebellum; glutamate from granule cells activates postsynaptic mGluR2, which hyperpolarizes Golgi cells via G protein-coupled inwardly rectifying K+ (GIRK) channels, producing long-lasting silencing dependent on stimulus strength.","method":"Whole-cell recording in GFP-positive Golgi cells of wild-type vs. mGluR2-deficient mice","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct electrophysiology in knockout vs. wild-type mice with clear cellular phenotype and identified downstream channel (GIRK)","pmids":["12948448"],"is_preprint":false},{"year":2002,"finding":"mGlu2 is the major presynaptic group II autoreceptor activated by synaptically released glutamate at perforant path inputs to the dentate gyrus and CA1 stratum lacunosum moleculare; mGlu3 can regulate transmission but contributes less to autoreceptor function at these synapses.","method":"Field EPSP recordings in mGlu2-/- mice vs. wild-type; LY341495-mediated unmasking of autoreceptor activity","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic knockout combined with pharmacological and electrophysiological approaches, clear quantitative differences","pmids":["12213275"],"is_preprint":false},{"year":2007,"finding":"The mGlu2 receptor subtype (not mGlu3) specifically mediates the antipsychotic-like actions of the mGluR2/3 agonist LY379268 in reversing PCP- and amphetamine-evoked hyperactivity, as demonstrated using mGluR2 and mGluR3 knockout mice.","method":"Locomotor activity assays in mGluR2-KO, mGluR3-KO, and wild-type mice with LY379268","journal":"Psychopharmacology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — receptor-specific knockout mice with two behavioral models (PCP and amphetamine hyperactivity), clear genotype-dependent pharmacology","pmids":["18057917"],"is_preprint":false},{"year":2011,"finding":"The 5-HT2A receptor–mGluR2 heteromeric complex is required for the pharmacological and behavioral effects of hallucinogens: mGluR2 knockout abolishes DOI-induced high-affinity binding of DOI to 5-HT2A receptors, eliminates head-twitch behavior, and prevents hallucinogen-specific egr-2 induction.","method":"Radioligand binding ([(3)H]ketanserin displacement), immunofluorescence, behavioral assays in mGluR2-KO mice","journal":"Neuroscience letters","confidence":"High","confidence_rationale":"Tier 2 / Moderate — knockout mouse approach with multiple orthogonal readouts (binding, gene expression, behavior)","pmids":["21276828"],"is_preprint":false},{"year":2012,"finding":"Three specific residues at the intracellular end of transmembrane domain 4 of mGluR2 (Ala-677, Ala-681, Ala-685) are required for assembly of the 5-HT2A–mGluR2 GPCR heteromeric complex in mouse frontal cortex; mutation of these residues abolishes heteromer formation, reduces heteromeric ligand binding, and attenuates hallucinogen-induced psychosis-like effects in mice.","method":"Site-directed mutagenesis, Co-IP, BRET, behavioral assays in mice with viral-mediated expression of mutant mGluR2","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — mutagenesis combined with physical interaction assays and in vivo functional behavioral readout","pmids":["23129762"],"is_preprint":false},{"year":2016,"finding":"In cells co-expressing mGluR2 and 5-HT2A receptors, stimulation of mGluR2 leads to activation of Gq/11 proteins via the 5-HT2A receptor in the heteromeric complex; this cross-G-protein signaling requires one mGluR2 subunit to couple to Gi/o, and this mGluR2-dependent Gq/11 activation is reduced in frontal cortex of 5-HT2A-KO mice and in postmortem schizophrenic brains.","method":"Single-cell BRET signaling experiments with mutant mGluR2 variants, G-protein coupling assays, postmortem brain analysis","journal":"Science signaling","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — multiple orthogonal methods (BRET, mutagenesis, in vivo KO validation, human postmortem tissue)","pmids":["26758213"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures of human mGluR2 bound to heterotrimeric Gi reveal a G-protein-binding site formed by three intracellular loops and helices III and IV, distinct from other GPCR structures. Asymmetric dimerization of the transmembrane domain is crucial for receptor activation and asymmetric signal transduction.","method":"Cryo-electron microscopy structural determination with functional validation","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — near-atomic cryo-EM structure with functional mutagenesis data confirming asymmetric dimerization role","pmids":["34135510"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures of inactive mGluR2 homodimer, agonist/PAM-bound active mGluR2 homodimer, and inactive mGluR2-mGluR7 heterodimer reveal: a unique helix IV-mediated dimer interface in inactive mGluR2 that limits activity; sequential Venus flytrap domain closure followed by transmembrane domain rearrangement from symmetric inactive to asymmetric active conformation; and the mGluR7 subunit dominates G-protein activation in the mGluR2-mGluR7 heterodimer.","method":"Cryo-EM structure determination with functional assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple cryo-EM structures (4 states) with functional validation of key structural elements","pmids":["34135509"],"is_preprint":false},{"year":2023,"finding":"Twelve cryo-EM structures of mGluR2-mGluR3 and mGluR2-mGluR4 heterodimers in inactive, intermediate, and active states reveal sequential Venus flytrap domain conformational changes, substantial transmembrane domain rearrangement from symmetric inactive to asymmetric active dimer, and stability of inactive conformations as a determinant of asymmetric signal transduction. A novel PAM binding site was identified at the asymmetric dimer interface.","method":"Cryo-EM structural determination across multiple conformational states, combined with functional assays","journal":"Cell research","confidence":"High","confidence_rationale":"Tier 1 / Strong — comprehensive structural analysis (12 structures, multiple states) with functional data","pmids":["37286794"],"is_preprint":false},{"year":2009,"finding":"mGluR2 is resistant to homologous desensitization by GRK2 for the cAMP signaling pathway, whereas mGluR3 is fully desensitized by GRK2 and beta-arrestin1. This difference is signal-dependent: GRK2 desensitizes the MAPK pathway mediated by both mGluR2 and mGluR3. In vivo agonist pretreatment desensitizes cAMP response only in mGluR3, not mGluR2, supporting its resistance to tolerance.","method":"HEK293 cell transfection, cAMP assays, MAPK assays, kinase-dead GRK2 mutant, in vivo knockout mouse pharmacology","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — in vitro mutagenesis/overexpression combined with in vivo knockout mouse validation across both mGluR2 and mGluR3 knockouts","pmids":["19164443"],"is_preprint":false},{"year":2018,"finding":"The 5-HT2A receptor promotes phosphorylation of mGluR2 at Ser843 in cells co-expressing both receptors and in mouse prefrontal cortex in vivo; Ser843 phosphorylation is required for efficient Gi/o signaling elicited by either mGluR2 or 5-HT2A receptor stimulation in co-expressing cells, establishing a trans-receptor phosphorylation mechanism underlying 5-HT2A–mGluR2 crosstalk.","method":"Phosphoproteomics, site-directed mutagenesis (Ser843Ala), co-expression in HEK293 cells, prefrontal cortex analysis in 5-HT2A KO mice","journal":"Molecular psychiatry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — mutagenesis identifying specific phosphorylation site combined with in vivo mouse validation","pmids":["29858599"],"is_preprint":false},{"year":2016,"finding":"CB1R- and mGluR2/3-induced presynaptic inhibition of synaptic transmission requires ERK/MAPK-dependent phosphorylation of Munc18-1, which ultimately leads to Munc18-1 degradation via the ubiquitin-proteasome system; blocking ERK-dependent Munc18-1 phosphorylation prevents mGluR2/3-induced synaptic inhibition.","method":"Electrophysiology, biochemistry, pharmacological inhibitors of ERK/MEK, phosphomimetic and phospho-null Munc18-1 mutants","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — multiple orthogonal methods (electrophysiology, mutagenesis, biochemistry) in a single rigorous study","pmids":["27056679"],"is_preprint":false},{"year":2016,"finding":"mGluR2 receptors on astrocytes in the thalamic ventrobasal nucleus reduce inhibitory transmission (miniature IPSC frequency) from the thalamic reticular nucleus and disinhibit thalamocortical sensory responses to vibrissae stimulation in vivo; the glial inhibitor fluorocitrate abolishes the mGluR2 PAM effect, confirming the astrocytic locus of action.","method":"Electrophysiology in thalamic slices (mIPSC recording), calcium imaging of astrocytes, in vivo thalamic neuron recording with vibrissae stimulation, fluorocitrate glial blockade","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (in vitro slice electrophysiology, calcium imaging, in vivo recording, pharmacological dissection of cell type)","pmids":["28416443"],"is_preprint":false},{"year":2011,"finding":"mGlu2 receptors play the predominant role over mGlu3 in the endogenous control of inflammatory pain; the analgesic activity of mixed mGlu2/3 agonists (LY379268) is entirely mediated by mGlu2 subtype activation, as the analgesic effect is absent in mGlu2-/- but preserved (though tolerance develops) in mGlu3-/- mice.","method":"Formalin test in mGlu2-/- and mGlu3-/- knockout mice with systemic LY379268 administration","journal":"Molecular pain","confidence":"High","confidence_rationale":"Tier 2 / Moderate — receptor-specific knockout mice with clear pharmacological phenotype across two receptor subtypes","pmids":["21543601"],"is_preprint":false},{"year":2013,"finding":"mGlu2 receptor-induced hyperthermia is mediated specifically by mGlu2 (not mGlu3) receptors, as the agonist LY379268-induced temperature increase is abolished in mGlu2-/- mice but not mGlu3-/- mice; this in vivo readout predicts antidepressant-like efficacy of mGluR2/3 antagonists via mGlu2.","method":"Body temperature measurement in mGlu2-/-, mGlu3-/-, and mGlu2/3-/- knockout mice; forced swim test","journal":"CNS & neurological disorders drug targets","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple receptor knockout lines with pharmacological validation","pmids":["23574174"],"is_preprint":false},{"year":2019,"finding":"The antidepressant-relevant behavioral effects and gamma-range (30-80 Hz) cortical EEG oscillation increases produced by (2R,6R)-hydroxynorketamine are absent in mGlu2 (Grm2) but not mGlu3 (Grm3) knockout mice, and are blocked by mGlu2/3 agonist pretreatment, demonstrating that (2R,6R)-HNK acts via mGlu2 receptor signaling.","method":"Behavioral assays, quantitative EEG in Grm2-/- and Grm3-/- mice, pharmacological blockade with mGlu2/3 agonist","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (behavior, EEG, knockout genetics, pharmacology) in a rigorous study","pmids":["30867285"],"is_preprint":false},{"year":2019,"finding":"Selective mGlu2 NAM enhances thalamocortical transmission and inhibits long-term depression via presynaptic mechanisms in the PFC, while mGlu3 NAM acts via postsynaptic mechanisms; both NAMs activate distinct PFC pyramidal cell ensembles and produce antidepressant-like effects in chronic stress models.","method":"Electrophysiology in PFC slices, transgenic mice, viral-assisted optogenetics, behavioral tests in chronic stress models","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (electrophysiology, optogenetics, behavior) with mechanistic dissection of pre/postsynaptic contributions","pmids":["31735403"],"is_preprint":false},{"year":2024,"finding":"Influenza virus hemagglutinin (HA) directly interacts with mGluR2, using it as an endocytic receptor to initiate clathrin-mediated endocytosis (CME); mGluR2 then activates KCa1.1, leading to F-actin polymerization, clathrin-coated pit maturation, and completion of CME. mGluR2-knockout mice are significantly more resistant to multiple influenza subtypes.","method":"siRNA screen, Co-IP of HA with mGluR2, mGluR2 knockout mice challenged with influenza, actin polymerization assays","journal":"Nature microbiology","confidence":"High","confidence_rationale":"Tier 2 / Strong — Co-IP demonstrating direct HA-mGluR2 interaction, mechanistic cell biology assays, and in vivo knockout validation","pmids":["38849624"],"is_preprint":false},{"year":2023,"finding":"mGluR2 mediates RABV internalization in vitro and infection in vivo (mGluR2-KO mice survive lethal RABV challenge); transferrin receptor 1 (TfR1) interacts with mGluR2, co-internalizes with mGluR2 and RABV in the same clathrin-coated pit, and TfR1 knockdown blocks agonist-triggered mGluR2 internalization.","method":"Co-IP of mGluR2 with TfR1, knockdown of TfR1, mGluR2-KO in vivo RABV challenge, confocal colocalization","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, knockdown, in vivo KO) establishing TfR1-mGluR2 axis in viral entry","pmids":["36779763"],"is_preprint":false},{"year":2016,"finding":"Glutamate signals through mGluR2 in Schwann cells to induce ERK phosphorylation via ErbB2/3 receptor tyrosine kinase, with Gβγ and Src involved downstream; mGluR2 activation transforms myelinating Schwann cells to a proliferating state, and inhibition of mGluR2 signaling reduces demyelination of injured nerves in vivo.","method":"Cell culture signaling assays (Erk and Akt phosphorylation), ErbB2/3 phosphorylation assays, in vivo nerve injury model with mGluR2 inhibition","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple signaling readouts in vitro plus in vivo validation, single lab","pmids":["27432639"],"is_preprint":false},{"year":2006,"finding":"LY354740-induced c-Fos expression increases in the central amygdala, bed nucleus of stria terminalis, and other limbic/non-limbic structures are abolished by mGlu2 (but not mGlu3) receptor deletion; mGlu3 receptor deletion specifically increases basal hippocampal c-Fos and prevents LY354740-mediated suppression of hippocampal activity.","method":"c-Fos immunohistochemistry in mGlu2-KO and mGlu3-KO mice after LY354740 administration","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — receptor-specific knockout approach with regional brain mapping, single lab","pmids":["16733060"],"is_preprint":false},{"year":2013,"finding":"L-acetylcarnitine (LAC) produces antidepressant effects by epigenetically upregulating mGlu2 receptor expression: LAC increases acetylated H3K27 bound to the Grm2 promoter and acetylation of NF-κB-p65, enhancing Grm2 transcription in hippocampus and prefrontal cortex. LAC has no effect in mGlu2 knockout mice exposed to chronic unpredictable stress.","method":"ChIP (H3K27 acetylation at Grm2 promoter), NF-κB acetylation assays, mGlu2 KO mice behavioral assays, HDAC inhibitor experiments","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Moderate — ChIP demonstrating specific epigenetic modification at Grm2 promoter, confirmed with KO mice and pharmacological tools","pmids":["23382250"],"is_preprint":false},{"year":2013,"finding":"5-HT2A receptor-dependent signaling epigenetically controls mGlu2 transcription: disruption of 5-HT2A signaling in KO mice reduces H3 and H4 acetylation and increases H3K27me3 at the mGlu2 promoter; the transcription factor Egr1 binds the mGlu2 promoter in a 5-HT2A-dependent manner, and viral overexpression of Egr1 increases mGlu2 expression.","method":"ChIP for multiple histone marks at mGlu2 promoter in 5-HT2A KO mice, Egr1 ChIP, viral-mediated Egr1 overexpression","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal ChIP experiments with defined histone marks, plus gain-of-function viral approach","pmids":["23508685"],"is_preprint":false},{"year":2008,"finding":"mGluR2 activation in neuronal cell cultures leads to activation of ERK/MAPK pathways and increased tau phosphorylation, and reduces oxidative stress-mediated cytotoxicity.","method":"Cell culture signaling assays (ERK phosphorylation), tau phosphorylation assays, oxidative stress viability assay","journal":"Brain research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single method per endpoint, no genetic confirmation of mGluR2 specificity","pmids":["19026996"],"is_preprint":false},{"year":2009,"finding":"mGluR2 knockout striata show elevated proportion of dopamine D2(High) receptors (220% increase) and 67-fold functional supersensitivity to the D2 agonist (+)PHNO, suggesting that mGluR2 normally regulates dopamine D2 receptor affinity state.","method":"Radioligand binding (D2(High) proportion measurement), GTPγS incorporation assays in mGluR2-KO and mGluR3-KO striata","journal":"Synapse","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical assays in receptor-specific knockout mice with quantitative receptor pharmacology","pmids":["19084908"],"is_preprint":false},{"year":2012,"finding":"mGluR2 forms a heteromeric complex with 5-HT2A receptor; additionally, mGluR2 forms complexes with 5-HT2B and mGluR5 but not 5-HT2C, indicating selective but not exclusive heterocomplex formation. However, co-expression of mGluR2 and 5-HT2A had no relevant effects on individual receptor signaling (Gq/11, Gi, or cAMP) in HEK-293 cells under the tested conditions.","method":"Quantitative SNAP/CLIP-tag HTRF assay for heterocomplex formation, cAMP and calcium signaling assays in HEK293 cells, radioligand competition binding in medial PFC membranes","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — confirmed heterocomplex formation with quantitative HTRF, but signaling consequence was negative in this system; single lab","pmids":["22300836"],"is_preprint":false},{"year":2003,"finding":"mGluR2/3 receptor activation supports the proliferation of human glioma cells in culture; mGluR2/3 antagonist LY341495 reduces glioma cell proliferation, inhibits EGF-induced MAPK pathway activation, and reduces cyclin D1 induction, with these effects partially reversed by mGluR2/3 agonist co-addition.","method":"[3H]-thymidine incorporation, Western blot for MAPK and cyclin D1, pharmacological antagonism/agonism in primary human glioma cultures","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — functional pharmacological dissection in primary human cells with multiple readouts, single lab","pmids":["12614329"],"is_preprint":false},{"year":2018,"finding":"mGluR2/3 activation in spinal cord glutamatergic nerve endings via mGluR2/3 autoreceptors suppresses glutamate exocytosis; the 5-HT2A receptor co-localizes and co-immunoprecipitates with mGluR2/3 in spinal cord synaptosomes, and 5-HT2A agonist activation antagonizes mGluR2/3-mediated inhibition of glutamate release, while 5-HT2A antagonists potentiate it and increase mGluR2/3 membrane expression.","method":"[3H]D-aspartate release from synaptosomes, Co-IP of mGluR2/3 and 5-HT2A, Western blot, confocal microscopy","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — Co-IP demonstrating protein interaction combined with functional release assays, single lab","pmids":["29499271"],"is_preprint":false}],"current_model":"GRM2 encodes mGluR2, a class C GPCR that couples to Gi/o proteins via a novel intracellular binding site (formed by three intracellular loops and helices III/IV) within an asymmetric homodimer or heterodimer, where activation proceeds through sequential Venus flytrap domain closure and transmembrane domain rearrangement; presynaptically, mGluR2 is the principal autoreceptor for synaptically-released glutamate at multiple forebrain synapses and reduces neurotransmitter release through a mechanism requiring ERK-dependent Munc18-1 phosphorylation; postsynaptically, mGluR2 hyperpolarizes neurons via GIRK channels; mGluR2 function is negatively regulated by PKA phosphorylation at Ser843 (which uncouples the receptor from G proteins) but is resistant to GRK2-mediated homologous desensitization for cAMP signaling; mGluR2 forms functional heteromeric complexes with the 5-HT2A receptor in the frontal cortex, where 5-HT2A-driven phosphorylation of mGluR2 at Ser843 enhances Gi/o signaling and cross-activates Gq/11, and Egr1-mediated transcription of GRM2 is epigenetically regulated by 5-HT2A signaling; additionally, mGluR2 acts as a host endocytic receptor exploited by influenza virus (via direct HA binding) and rabies virus (in cooperation with TfR1) to enter cells via clathrin-mediated endocytosis."},"narrative":{"mechanistic_narrative":"GRM2 encodes mGluR2, a class C G-protein-coupled receptor that serves as the principal presynaptic group II autoreceptor for synaptically released glutamate at multiple forebrain synapses, where it limits neurotransmitter release [PMID:12213275]. Cryo-EM structures establish that mGluR2 signals as an asymmetric dimer: agonist binding drives sequential Venus flytrap domain closure followed by transmembrane domain rearrangement from a symmetric inactive to an asymmetric active conformation, with the resulting Gi heterotrimer engaging a non-canonical intracellular site formed by three intracellular loops and helices III and IV [PMID:34135510, PMID:34135509, PMID:37286794]. Presynaptically, mGluR2/3 inhibits release through ERK/MAPK-dependent phosphorylation and subsequent proteasomal degradation of Munc18-1 [PMID:27056679], and postsynaptically it hyperpolarizes neurons by activating GIRK channels, as shown at cerebellar granule cell–Golgi cell synapses [PMID:12948448]. Receptor output is gated by PKA phosphorylation at Ser843, which uncouples mGluR2 from G proteins [PMID:10908604], while mGluR2 is uniquely resistant to GRK2-mediated homologous desensitization of its cAMP pathway compared with mGluR3 [PMID:19164443]. mGluR2 assembles a functional heteromeric complex with the 5-HT2A receptor in frontal cortex via residues at the intracellular end of transmembrane domain 4 [PMID:23129762]; within this complex 5-HT2A drives trans-receptor phosphorylation of mGluR2 at Ser843 to enhance Gi/o signaling [PMID:29858599] and mGluR2 stimulation cross-activates Gq/11 through the 5-HT2A partner [PMID:26758213], a complex required for the behavioral and binding effects of hallucinogens [PMID:21276828]. GRM2 transcription is epigenetically controlled, including 5-HT2A/Egr1-dependent regulation at the Grm2 promoter [PMID:23508685]. mGluR2 mediates the antipsychotic-like and antidepressant-relevant actions of group II ligands and of (2R,6R)-hydroxynorketamine [PMID:18057917, PMID:30867285, PMID:31735403]. Beyond its neuronal roles, mGluR2 is exploited as a host endocytic receptor: influenza hemagglutinin binds it directly to drive clathrin-mediated endocytosis via KCa1.1-dependent F-actin polymerization [PMID:38849624], and rabies virus uses mGluR2 in cooperation with transferrin receptor 1 [PMID:36779763].","teleology":[{"year":1993,"claim":"Establishing where and how mGluR2 signals required defining its expression and G-protein coupling; this showed mGluR2 is a neuronal receptor coupled to the inhibitory cAMP cascade.","evidence":"In situ hybridization mapping mGluR2 mRNA distribution and cAMP coupling across CNS","pmids":["8389425"],"confidence":"Medium","gaps":["mRNA distribution does not establish subcellular protein localization","functional consequence at synapses not addressed"]},{"year":1996,"claim":"Whether mGluR2 acts pre- or postsynaptically was unresolved; ultrastructural localization placed it at presynaptic, postsynaptic, and glial compartments of excitatory synapses, concentrated at mossy fiber and perforant path terminals.","evidence":"Electron microscopy immunocytochemistry across multiple brain regions","pmids":["8684625"],"confidence":"High","gaps":["does not assign functional role to each compartment","antibody cross-reactivity with mGluR3 not fully separated"]},{"year":2000,"claim":"How mGluR2 output is regulated was unknown; PKA was shown to phosphorylate Ser843 on the C-terminus, uncoupling the receptor from G proteins and inhibiting presynaptic responses.","evidence":"Site-directed mutagenesis, in vitro kinase assays, and electrophysiology with a PKA inhibitor","pmids":["10908604"],"confidence":"High","gaps":["upstream signals driving PKA activation at the receptor not defined","stoichiometry and kinetics of dephosphorylation unaddressed"]},{"year":2003,"claim":"The postsynaptic effector of mGluR2 was undefined; knockout electrophysiology showed mGluR2 hyperpolarizes Golgi cells via GIRK channels to produce long-lasting silencing.","evidence":"Whole-cell recording in mGluR2-deficient vs wild-type mouse cerebellum","pmids":["12948448"],"confidence":"High","gaps":["G-protein subtype linking mGluR2 to GIRK not detailed","generality across brain regions unestablished"]},{"year":2002,"claim":"Distinguishing mGluR2 from mGluR3 autoreceptor roles was needed; genetic and pharmacological dissection identified mGluR2 as the major autoreceptor activated by synaptic glutamate at perforant path synapses.","evidence":"Field EPSP recordings in mGlu2-/- mice with LY341495 unmasking","pmids":["12213275"],"confidence":"High","gaps":["intracellular cascade reducing release not yet identified","contribution at other synapse types varies"]},{"year":2009,"claim":"Whether mGluR2 desensitizes like other GPCRs mattered for tolerance; mGluR2 was found resistant to GRK2 desensitization of its cAMP pathway, unlike mGluR3, in a signal-dependent manner.","evidence":"HEK293 cAMP/MAPK assays with kinase-dead GRK2 plus in vivo knockout pharmacology","pmids":["19164443"],"confidence":"High","gaps":["structural basis for differential GRK2 sensitivity unknown","arrestin engagement on MAPK pathway only partially resolved"]},{"year":2016,"claim":"The presynaptic mechanism coupling mGluR2/3 to reduced release was unknown; ERK-dependent phosphorylation of Munc18-1 followed by its proteasomal degradation was shown to mediate the inhibition.","evidence":"Electrophysiology, biochemistry, and phosphomimetic/phospho-null Munc18-1 mutants","pmids":["27056679"],"confidence":"High","gaps":["link between receptor activation and ERK at the terminal not fully mapped","shared mechanism with CB1R leaves receptor specificity step undefined"]},{"year":null,"claim":"The 5-HT2A–mGluR2 heteromer was progressively defined as physically and functionally coupled and pharmacologically consequential, but how it is regulated and its causal role in human disease remains open.","evidence":"Co-IP/BRET/HTRF interaction mapping, trans-receptor Ser843 phosphorylation, Gq/11 cross-activation, and behavioral knockout studies","pmids":["21276828","23129762","26758213","29858599","22300836","30180227"],"confidence":"High","gaps":["one report found no signaling consequence of co-expression in HEK293 (idx 28)","stoichiometry and dynamics of the heteromer in vivo unresolved","translation to human therapeutics not established by direct causal evidence"]},{"year":2021,"claim":"How mGluR2 transduces signal at atomic resolution was unknown; cryo-EM revealed asymmetric dimer activation through sequential VFT closure and TMD rearrangement and a non-canonical Gi-binding site on helices III/IV and three intracellular loops.","evidence":"Cryo-EM of inactive/active homodimers and heterodimers with functional mutagenesis","pmids":["34135510","34135509","37286794"],"confidence":"High","gaps":["how heteromerization with 5-HT2A maps onto these conformational states is not resolved","dynamics in native membranes not captured"]},{"year":2024,"claim":"Whether mGluR2 has roles beyond glutamatergic signaling was unclear; it was shown to be a host endocytic receptor directly bound by influenza HA driving KCa1.1/F-actin-dependent clathrin-mediated endocytosis, and used by rabies virus in cooperation with TfR1.","evidence":"siRNA screen, Co-IP of HA and TfR1 with mGluR2, actin/CME assays, and in vivo knockout viral challenge","pmids":["38849624","36779763"],"confidence":"High","gaps":["relationship between viral-entry signaling and canonical Gi/o coupling unresolved","structural basis of HA–mGluR2 binding undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[3,4,9]},{"term_id":"GO:0001618","term_label":"virus receptor activity","supporting_discovery_ids":[20,21]},{"term_id":"GO:0038024","term_label":"cargo receptor activity","supporting_discovery_ids":[20,21]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,30]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,8,9,14]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[3,4]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[20,21]}],"complexes":["5-HT2A–mGluR2 heteromer"],"partners":["HTR2A","MUNC18-1","TFR1","GRK2","EGR1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q14416","full_name":"Metabotropic glutamate receptor 2","aliases":[],"length_aa":872,"mass_kda":95.6,"function":"Dimeric G protein-coupled receptor which is activated by the excitatory neurotransmitter L-glutamate (PubMed:37286794). Plays critical roles in modulating synaptic transmission and neuronal excitability. Upon activation by glutamate, inhibits presynaptic calcium channels, reducing further glutamate release and dampening excitatory signaling (By similarity). Mechanistically, ligand binding causes a conformation change that triggers signaling via guanine nucleotide-binding proteins (G proteins) and modulates the activity of down-stream effectors, such as adenylate cyclase. May mediate suppression of neurotransmission or may be involved in synaptogenesis or synaptic stabilization (Microbial infection) Plays an important role in influenza virus internalization (Microbial infection) Acts as a host entry factor for rabies virus that hijacks the endocytosis of GRM2 to enter cells (Microbial infection) Acts as a host entry factor for SARS-CoV-2 that hijacks the endocytosis of GRM2 to enter cells","subcellular_location":"Cell membrane; Synapse; Cell projection, dendrite","url":"https://www.uniprot.org/uniprotkb/Q14416/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/GRM2","classification":"Not 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antidepressant-like efficacy in mice.","date":"2013","source":"CNS & neurological disorders drug targets","url":"https://pubmed.ncbi.nlm.nih.gov/23574174","citation_count":26,"is_preprint":false},{"pmid":"16023151","id":"PMC_16023151","title":"Comparison of c-Fos induction in the brain by the mGlu2/3 receptor antagonist LY341495 and agonist LY354740: evidence for widespread endogenous tone at brain mGlu2/3 receptors in vivo.","date":"2005","source":"Neuropharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/16023151","citation_count":26,"is_preprint":false},{"pmid":"19084908","id":"PMC_19084908","title":"Glutamate receptor mGlu2 and mGlu3 knockout striata are dopamine supersensitive, with elevated D2(High) receptors and marked supersensitivity to the dopamine agonist (+)PHNO.","date":"2009","source":"Synapse (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/19084908","citation_count":26,"is_preprint":false},{"pmid":"27432639","id":"PMC_27432639","title":"Glutamate signals through mGluR2 to control Schwann cell differentiation and proliferation.","date":"2016","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/27432639","citation_count":25,"is_preprint":false},{"pmid":"29858599","id":"PMC_29858599","title":"5-HT2A receptor-dependent phosphorylation of mGlu2 receptor at Serine 843 promotes mGlu2 receptor-operated Gi/o signaling.","date":"2018","source":"Molecular psychiatry","url":"https://pubmed.ncbi.nlm.nih.gov/29858599","citation_count":25,"is_preprint":false},{"pmid":"24206109","id":"PMC_24206109","title":"Stimulating ERK/PI3K/NFκB signaling pathways upon activation of mGluR2/3 restores OGD-induced impairment in glutamate clearance in astrocytes.","date":"2013","source":"The European journal of neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/24206109","citation_count":25,"is_preprint":false},{"pmid":"16469447","id":"PMC_16469447","title":"The mGlu2/3 receptor agonist LY354740 suppresses immobilization stress-induced increase in rat prefrontal cortical BDNF mRNA expression.","date":"2006","source":"Neuroscience letters","url":"https://pubmed.ncbi.nlm.nih.gov/16469447","citation_count":25,"is_preprint":false},{"pmid":"31242055","id":"PMC_31242055","title":"mGluR2 positive allosteric modulators: an updated patent review (2013-2018).","date":"2019","source":"Expert opinion on therapeutic patents","url":"https://pubmed.ncbi.nlm.nih.gov/31242055","citation_count":24,"is_preprint":false},{"pmid":"26292187","id":"PMC_26292187","title":"5-HT2A and mGlu2/3 receptor interactions: on their relevance to cognitive function and psychosis.","date":"2016","source":"Behavioural pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/26292187","citation_count":24,"is_preprint":false},{"pmid":"16178732","id":"PMC_16178732","title":"Positive allosteric modulators of the metabotropic glutamate receptor subtype 2 (mGluR2).","date":"2005","source":"Current topics in medicinal 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Antagonists: Similarity to Ketamine.","date":"2019","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/30871246","citation_count":21,"is_preprint":false},{"pmid":"19477188","id":"PMC_19477188","title":"Use of MGLUR2 and MGLUR3 knockout mice to explore in vivo receptor specificity of the MGLUR2/3 selective antagonist LY341495.","date":"2009","source":"Neuropharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/19477188","citation_count":21,"is_preprint":false},{"pmid":"35934973","id":"PMC_35934973","title":"Where do we go next in antidepressant drug discovery? A new generation of antidepressants: a pivotal role of AMPA receptor potentiation and mGlu2/3 receptor antagonism.","date":"2022","source":"Expert opinion on drug discovery","url":"https://pubmed.ncbi.nlm.nih.gov/35934973","citation_count":20,"is_preprint":false},{"pmid":"31281399","id":"PMC_31281399","title":"Glu-mGluR2/3-ERK Signaling Regulates Apoptosis of Hippocampal Neurons in Diabetic-Depression Model Rats.","date":"2019","source":"Evidence-based complementary and alternative medicine : eCAM","url":"https://pubmed.ncbi.nlm.nih.gov/31281399","citation_count":20,"is_preprint":false},{"pmid":"26791341","id":"PMC_26791341","title":"Presynaptic, release-regulating mGlu2 -preferring and mGlu3 -preferring autoreceptors in CNS: pharmacological profiles and functional roles in demyelinating disease.","date":"2016","source":"British journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/26791341","citation_count":20,"is_preprint":false},{"pmid":"28285123","id":"PMC_28285123","title":"Preclinical predictors that the orthosteric mGlu2/3 receptor antagonist LY3020371 will not engender ketamine-associated neurotoxic, motor, cognitive, subjective, or abuse-liability-related effects.","date":"2017","source":"Pharmacology, biochemistry, and behavior","url":"https://pubmed.ncbi.nlm.nih.gov/28285123","citation_count":20,"is_preprint":false},{"pmid":"36715750","id":"PMC_36715750","title":"mGlu2/3 receptor antagonists for depression: overview of underlying mechanisms and clinical development.","date":"2023","source":"European archives of psychiatry and clinical neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/36715750","citation_count":19,"is_preprint":false},{"pmid":"36779763","id":"PMC_36779763","title":"Transferrin Receptor Protein 1 Cooperates with mGluR2 To Mediate the Internalization of Rabies Virus and 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neuropsychopharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/34534292","citation_count":19,"is_preprint":false},{"pmid":"30240581","id":"PMC_30240581","title":"Chronic methamphetamine self-administration dysregulates 5-HT2A and mGlu2 receptor expression in the rat prefrontal and perirhinal cortex: Comparison to chronic phencyclidine and MK-801.","date":"2018","source":"Pharmacology, biochemistry, and behavior","url":"https://pubmed.ncbi.nlm.nih.gov/30240581","citation_count":19,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":57094,"output_tokens":8019,"usd":0.145784,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":17260,"output_tokens":3621,"usd":0.088413,"stage2_stop_reason":"end_turn"},"total_usd":0.234197,"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\": 1996,\n      \"finding\": \"mGluR2 and mGluR3 are localized in postsynaptic, presynaptic, and glial compartments at excitatory synapses, with presynaptic staining concentrated in mossy fiber and perforant path glutamatergic terminals in the hippocampus, as determined by ultrastructural immunocytochemistry.\",\n      \"method\": \"Electron microscopy immunocytochemistry with affinity-purified antibody\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct ultrastructural localization replicated across multiple brain regions and independently corroborated by mRNA distribution studies\",\n      \"pmids\": [\"8684625\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"mGluR2 mRNA is coupled to the inhibitory cAMP cascade and is expressed predominantly in neurons (not glia) throughout the CNS, with highest expression in cerebellar Golgi cells, mitral cells of the accessory olfactory bulb, and entorhinal/parasubicular cortex neurons.\",\n      \"method\": \"In situ hybridization\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Strong — single method (ISH), replicated distribution findings from multiple brain regions\",\n      \"pmids\": [\"8389425\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PKA directly phosphorylates mGluR2 at a single serine residue (Ser843) on the C-terminal tail, and this phosphorylation inhibits coupling of mGluR2 to GTP-binding proteins; cAMP/PKA activation inhibits presynaptic mGluR2-mediated responses at the medial perforant path synapse.\",\n      \"method\": \"Site-directed mutagenesis, biochemical phosphorylation assays, in vitro kinase assay, electrophysiology with selective PKA inhibitor\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro phosphorylation assay with mutagenesis identifying specific residue (Ser843), plus functional electrophysiology confirmation\",\n      \"pmids\": [\"10908604\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"mGluR2 acts as a postsynaptic receptor at granule cell–Golgi cell synapses in the cerebellum; glutamate from granule cells activates postsynaptic mGluR2, which hyperpolarizes Golgi cells via G protein-coupled inwardly rectifying K+ (GIRK) channels, producing long-lasting silencing dependent on stimulus strength.\",\n      \"method\": \"Whole-cell recording in GFP-positive Golgi cells of wild-type vs. mGluR2-deficient mice\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct electrophysiology in knockout vs. wild-type mice with clear cellular phenotype and identified downstream channel (GIRK)\",\n      \"pmids\": [\"12948448\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"mGlu2 is the major presynaptic group II autoreceptor activated by synaptically released glutamate at perforant path inputs to the dentate gyrus and CA1 stratum lacunosum moleculare; mGlu3 can regulate transmission but contributes less to autoreceptor function at these synapses.\",\n      \"method\": \"Field EPSP recordings in mGlu2-/- mice vs. wild-type; LY341495-mediated unmasking of autoreceptor activity\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout combined with pharmacological and electrophysiological approaches, clear quantitative differences\",\n      \"pmids\": [\"12213275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The mGlu2 receptor subtype (not mGlu3) specifically mediates the antipsychotic-like actions of the mGluR2/3 agonist LY379268 in reversing PCP- and amphetamine-evoked hyperactivity, as demonstrated using mGluR2 and mGluR3 knockout mice.\",\n      \"method\": \"Locomotor activity assays in mGluR2-KO, mGluR3-KO, and wild-type mice with LY379268\",\n      \"journal\": \"Psychopharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — receptor-specific knockout mice with two behavioral models (PCP and amphetamine hyperactivity), clear genotype-dependent pharmacology\",\n      \"pmids\": [\"18057917\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The 5-HT2A receptor–mGluR2 heteromeric complex is required for the pharmacological and behavioral effects of hallucinogens: mGluR2 knockout abolishes DOI-induced high-affinity binding of DOI to 5-HT2A receptors, eliminates head-twitch behavior, and prevents hallucinogen-specific egr-2 induction.\",\n      \"method\": \"Radioligand binding ([(3)H]ketanserin displacement), immunofluorescence, behavioral assays in mGluR2-KO mice\",\n      \"journal\": \"Neuroscience letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knockout mouse approach with multiple orthogonal readouts (binding, gene expression, behavior)\",\n      \"pmids\": [\"21276828\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Three specific residues at the intracellular end of transmembrane domain 4 of mGluR2 (Ala-677, Ala-681, Ala-685) are required for assembly of the 5-HT2A–mGluR2 GPCR heteromeric complex in mouse frontal cortex; mutation of these residues abolishes heteromer formation, reduces heteromeric ligand binding, and attenuates hallucinogen-induced psychosis-like effects in mice.\",\n      \"method\": \"Site-directed mutagenesis, Co-IP, BRET, behavioral assays in mice with viral-mediated expression of mutant mGluR2\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — mutagenesis combined with physical interaction assays and in vivo functional behavioral readout\",\n      \"pmids\": [\"23129762\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In cells co-expressing mGluR2 and 5-HT2A receptors, stimulation of mGluR2 leads to activation of Gq/11 proteins via the 5-HT2A receptor in the heteromeric complex; this cross-G-protein signaling requires one mGluR2 subunit to couple to Gi/o, and this mGluR2-dependent Gq/11 activation is reduced in frontal cortex of 5-HT2A-KO mice and in postmortem schizophrenic brains.\",\n      \"method\": \"Single-cell BRET signaling experiments with mutant mGluR2 variants, G-protein coupling assays, postmortem brain analysis\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — multiple orthogonal methods (BRET, mutagenesis, in vivo KO validation, human postmortem tissue)\",\n      \"pmids\": [\"26758213\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of human mGluR2 bound to heterotrimeric Gi reveal a G-protein-binding site formed by three intracellular loops and helices III and IV, distinct from other GPCR structures. Asymmetric dimerization of the transmembrane domain is crucial for receptor activation and asymmetric signal transduction.\",\n      \"method\": \"Cryo-electron microscopy structural determination with functional validation\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — near-atomic cryo-EM structure with functional mutagenesis data confirming asymmetric dimerization role\",\n      \"pmids\": [\"34135510\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of inactive mGluR2 homodimer, agonist/PAM-bound active mGluR2 homodimer, and inactive mGluR2-mGluR7 heterodimer reveal: a unique helix IV-mediated dimer interface in inactive mGluR2 that limits activity; sequential Venus flytrap domain closure followed by transmembrane domain rearrangement from symmetric inactive to asymmetric active conformation; and the mGluR7 subunit dominates G-protein activation in the mGluR2-mGluR7 heterodimer.\",\n      \"method\": \"Cryo-EM structure determination with functional assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple cryo-EM structures (4 states) with functional validation of key structural elements\",\n      \"pmids\": [\"34135509\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Twelve cryo-EM structures of mGluR2-mGluR3 and mGluR2-mGluR4 heterodimers in inactive, intermediate, and active states reveal sequential Venus flytrap domain conformational changes, substantial transmembrane domain rearrangement from symmetric inactive to asymmetric active dimer, and stability of inactive conformations as a determinant of asymmetric signal transduction. A novel PAM binding site was identified at the asymmetric dimer interface.\",\n      \"method\": \"Cryo-EM structural determination across multiple conformational states, combined with functional assays\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — comprehensive structural analysis (12 structures, multiple states) with functional data\",\n      \"pmids\": [\"37286794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"mGluR2 is resistant to homologous desensitization by GRK2 for the cAMP signaling pathway, whereas mGluR3 is fully desensitized by GRK2 and beta-arrestin1. This difference is signal-dependent: GRK2 desensitizes the MAPK pathway mediated by both mGluR2 and mGluR3. In vivo agonist pretreatment desensitizes cAMP response only in mGluR3, not mGluR2, supporting its resistance to tolerance.\",\n      \"method\": \"HEK293 cell transfection, cAMP assays, MAPK assays, kinase-dead GRK2 mutant, in vivo knockout mouse pharmacology\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — in vitro mutagenesis/overexpression combined with in vivo knockout mouse validation across both mGluR2 and mGluR3 knockouts\",\n      \"pmids\": [\"19164443\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The 5-HT2A receptor promotes phosphorylation of mGluR2 at Ser843 in cells co-expressing both receptors and in mouse prefrontal cortex in vivo; Ser843 phosphorylation is required for efficient Gi/o signaling elicited by either mGluR2 or 5-HT2A receptor stimulation in co-expressing cells, establishing a trans-receptor phosphorylation mechanism underlying 5-HT2A–mGluR2 crosstalk.\",\n      \"method\": \"Phosphoproteomics, site-directed mutagenesis (Ser843Ala), co-expression in HEK293 cells, prefrontal cortex analysis in 5-HT2A KO mice\",\n      \"journal\": \"Molecular psychiatry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — mutagenesis identifying specific phosphorylation site combined with in vivo mouse validation\",\n      \"pmids\": [\"29858599\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CB1R- and mGluR2/3-induced presynaptic inhibition of synaptic transmission requires ERK/MAPK-dependent phosphorylation of Munc18-1, which ultimately leads to Munc18-1 degradation via the ubiquitin-proteasome system; blocking ERK-dependent Munc18-1 phosphorylation prevents mGluR2/3-induced synaptic inhibition.\",\n      \"method\": \"Electrophysiology, biochemistry, pharmacological inhibitors of ERK/MEK, phosphomimetic and phospho-null Munc18-1 mutants\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — multiple orthogonal methods (electrophysiology, mutagenesis, biochemistry) in a single rigorous study\",\n      \"pmids\": [\"27056679\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"mGluR2 receptors on astrocytes in the thalamic ventrobasal nucleus reduce inhibitory transmission (miniature IPSC frequency) from the thalamic reticular nucleus and disinhibit thalamocortical sensory responses to vibrissae stimulation in vivo; the glial inhibitor fluorocitrate abolishes the mGluR2 PAM effect, confirming the astrocytic locus of action.\",\n      \"method\": \"Electrophysiology in thalamic slices (mIPSC recording), calcium imaging of astrocytes, in vivo thalamic neuron recording with vibrissae stimulation, fluorocitrate glial blockade\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (in vitro slice electrophysiology, calcium imaging, in vivo recording, pharmacological dissection of cell type)\",\n      \"pmids\": [\"28416443\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"mGlu2 receptors play the predominant role over mGlu3 in the endogenous control of inflammatory pain; the analgesic activity of mixed mGlu2/3 agonists (LY379268) is entirely mediated by mGlu2 subtype activation, as the analgesic effect is absent in mGlu2-/- but preserved (though tolerance develops) in mGlu3-/- mice.\",\n      \"method\": \"Formalin test in mGlu2-/- and mGlu3-/- knockout mice with systemic LY379268 administration\",\n      \"journal\": \"Molecular pain\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — receptor-specific knockout mice with clear pharmacological phenotype across two receptor subtypes\",\n      \"pmids\": [\"21543601\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"mGlu2 receptor-induced hyperthermia is mediated specifically by mGlu2 (not mGlu3) receptors, as the agonist LY379268-induced temperature increase is abolished in mGlu2-/- mice but not mGlu3-/- mice; this in vivo readout predicts antidepressant-like efficacy of mGluR2/3 antagonists via mGlu2.\",\n      \"method\": \"Body temperature measurement in mGlu2-/-, mGlu3-/-, and mGlu2/3-/- knockout mice; forced swim test\",\n      \"journal\": \"CNS & neurological disorders drug targets\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple receptor knockout lines with pharmacological validation\",\n      \"pmids\": [\"23574174\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The antidepressant-relevant behavioral effects and gamma-range (30-80 Hz) cortical EEG oscillation increases produced by (2R,6R)-hydroxynorketamine are absent in mGlu2 (Grm2) but not mGlu3 (Grm3) knockout mice, and are blocked by mGlu2/3 agonist pretreatment, demonstrating that (2R,6R)-HNK acts via mGlu2 receptor signaling.\",\n      \"method\": \"Behavioral assays, quantitative EEG in Grm2-/- and Grm3-/- mice, pharmacological blockade with mGlu2/3 agonist\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (behavior, EEG, knockout genetics, pharmacology) in a rigorous study\",\n      \"pmids\": [\"30867285\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Selective mGlu2 NAM enhances thalamocortical transmission and inhibits long-term depression via presynaptic mechanisms in the PFC, while mGlu3 NAM acts via postsynaptic mechanisms; both NAMs activate distinct PFC pyramidal cell ensembles and produce antidepressant-like effects in chronic stress models.\",\n      \"method\": \"Electrophysiology in PFC slices, transgenic mice, viral-assisted optogenetics, behavioral tests in chronic stress models\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (electrophysiology, optogenetics, behavior) with mechanistic dissection of pre/postsynaptic contributions\",\n      \"pmids\": [\"31735403\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Influenza virus hemagglutinin (HA) directly interacts with mGluR2, using it as an endocytic receptor to initiate clathrin-mediated endocytosis (CME); mGluR2 then activates KCa1.1, leading to F-actin polymerization, clathrin-coated pit maturation, and completion of CME. mGluR2-knockout mice are significantly more resistant to multiple influenza subtypes.\",\n      \"method\": \"siRNA screen, Co-IP of HA with mGluR2, mGluR2 knockout mice challenged with influenza, actin polymerization assays\",\n      \"journal\": \"Nature microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — Co-IP demonstrating direct HA-mGluR2 interaction, mechanistic cell biology assays, and in vivo knockout validation\",\n      \"pmids\": [\"38849624\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"mGluR2 mediates RABV internalization in vitro and infection in vivo (mGluR2-KO mice survive lethal RABV challenge); transferrin receptor 1 (TfR1) interacts with mGluR2, co-internalizes with mGluR2 and RABV in the same clathrin-coated pit, and TfR1 knockdown blocks agonist-triggered mGluR2 internalization.\",\n      \"method\": \"Co-IP of mGluR2 with TfR1, knockdown of TfR1, mGluR2-KO in vivo RABV challenge, confocal colocalization\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, knockdown, in vivo KO) establishing TfR1-mGluR2 axis in viral entry\",\n      \"pmids\": [\"36779763\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Glutamate signals through mGluR2 in Schwann cells to induce ERK phosphorylation via ErbB2/3 receptor tyrosine kinase, with Gβγ and Src involved downstream; mGluR2 activation transforms myelinating Schwann cells to a proliferating state, and inhibition of mGluR2 signaling reduces demyelination of injured nerves in vivo.\",\n      \"method\": \"Cell culture signaling assays (Erk and Akt phosphorylation), ErbB2/3 phosphorylation assays, in vivo nerve injury model with mGluR2 inhibition\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple signaling readouts in vitro plus in vivo validation, single lab\",\n      \"pmids\": [\"27432639\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"LY354740-induced c-Fos expression increases in the central amygdala, bed nucleus of stria terminalis, and other limbic/non-limbic structures are abolished by mGlu2 (but not mGlu3) receptor deletion; mGlu3 receptor deletion specifically increases basal hippocampal c-Fos and prevents LY354740-mediated suppression of hippocampal activity.\",\n      \"method\": \"c-Fos immunohistochemistry in mGlu2-KO and mGlu3-KO mice after LY354740 administration\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — receptor-specific knockout approach with regional brain mapping, single lab\",\n      \"pmids\": [\"16733060\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"L-acetylcarnitine (LAC) produces antidepressant effects by epigenetically upregulating mGlu2 receptor expression: LAC increases acetylated H3K27 bound to the Grm2 promoter and acetylation of NF-κB-p65, enhancing Grm2 transcription in hippocampus and prefrontal cortex. LAC has no effect in mGlu2 knockout mice exposed to chronic unpredictable stress.\",\n      \"method\": \"ChIP (H3K27 acetylation at Grm2 promoter), NF-κB acetylation assays, mGlu2 KO mice behavioral assays, HDAC inhibitor experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP demonstrating specific epigenetic modification at Grm2 promoter, confirmed with KO mice and pharmacological tools\",\n      \"pmids\": [\"23382250\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"5-HT2A receptor-dependent signaling epigenetically controls mGlu2 transcription: disruption of 5-HT2A signaling in KO mice reduces H3 and H4 acetylation and increases H3K27me3 at the mGlu2 promoter; the transcription factor Egr1 binds the mGlu2 promoter in a 5-HT2A-dependent manner, and viral overexpression of Egr1 increases mGlu2 expression.\",\n      \"method\": \"ChIP for multiple histone marks at mGlu2 promoter in 5-HT2A KO mice, Egr1 ChIP, viral-mediated Egr1 overexpression\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal ChIP experiments with defined histone marks, plus gain-of-function viral approach\",\n      \"pmids\": [\"23508685\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"mGluR2 activation in neuronal cell cultures leads to activation of ERK/MAPK pathways and increased tau phosphorylation, and reduces oxidative stress-mediated cytotoxicity.\",\n      \"method\": \"Cell culture signaling assays (ERK phosphorylation), tau phosphorylation assays, oxidative stress viability assay\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single method per endpoint, no genetic confirmation of mGluR2 specificity\",\n      \"pmids\": [\"19026996\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"mGluR2 knockout striata show elevated proportion of dopamine D2(High) receptors (220% increase) and 67-fold functional supersensitivity to the D2 agonist (+)PHNO, suggesting that mGluR2 normally regulates dopamine D2 receptor affinity state.\",\n      \"method\": \"Radioligand binding (D2(High) proportion measurement), GTPγS incorporation assays in mGluR2-KO and mGluR3-KO striata\",\n      \"journal\": \"Synapse\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical assays in receptor-specific knockout mice with quantitative receptor pharmacology\",\n      \"pmids\": [\"19084908\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"mGluR2 forms a heteromeric complex with 5-HT2A receptor; additionally, mGluR2 forms complexes with 5-HT2B and mGluR5 but not 5-HT2C, indicating selective but not exclusive heterocomplex formation. However, co-expression of mGluR2 and 5-HT2A had no relevant effects on individual receptor signaling (Gq/11, Gi, or cAMP) in HEK-293 cells under the tested conditions.\",\n      \"method\": \"Quantitative SNAP/CLIP-tag HTRF assay for heterocomplex formation, cAMP and calcium signaling assays in HEK293 cells, radioligand competition binding in medial PFC membranes\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — confirmed heterocomplex formation with quantitative HTRF, but signaling consequence was negative in this system; single lab\",\n      \"pmids\": [\"22300836\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"mGluR2/3 receptor activation supports the proliferation of human glioma cells in culture; mGluR2/3 antagonist LY341495 reduces glioma cell proliferation, inhibits EGF-induced MAPK pathway activation, and reduces cyclin D1 induction, with these effects partially reversed by mGluR2/3 agonist co-addition.\",\n      \"method\": \"[3H]-thymidine incorporation, Western blot for MAPK and cyclin D1, pharmacological antagonism/agonism in primary human glioma cultures\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — functional pharmacological dissection in primary human cells with multiple readouts, single lab\",\n      \"pmids\": [\"12614329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"mGluR2/3 activation in spinal cord glutamatergic nerve endings via mGluR2/3 autoreceptors suppresses glutamate exocytosis; the 5-HT2A receptor co-localizes and co-immunoprecipitates with mGluR2/3 in spinal cord synaptosomes, and 5-HT2A agonist activation antagonizes mGluR2/3-mediated inhibition of glutamate release, while 5-HT2A antagonists potentiate it and increase mGluR2/3 membrane expression.\",\n      \"method\": \"[3H]D-aspartate release from synaptosomes, Co-IP of mGluR2/3 and 5-HT2A, Western blot, confocal microscopy\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — Co-IP demonstrating protein interaction combined with functional release assays, single lab\",\n      \"pmids\": [\"29499271\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"GRM2 encodes mGluR2, a class C GPCR that couples to Gi/o proteins via a novel intracellular binding site (formed by three intracellular loops and helices III/IV) within an asymmetric homodimer or heterodimer, where activation proceeds through sequential Venus flytrap domain closure and transmembrane domain rearrangement; presynaptically, mGluR2 is the principal autoreceptor for synaptically-released glutamate at multiple forebrain synapses and reduces neurotransmitter release through a mechanism requiring ERK-dependent Munc18-1 phosphorylation; postsynaptically, mGluR2 hyperpolarizes neurons via GIRK channels; mGluR2 function is negatively regulated by PKA phosphorylation at Ser843 (which uncouples the receptor from G proteins) but is resistant to GRK2-mediated homologous desensitization for cAMP signaling; mGluR2 forms functional heteromeric complexes with the 5-HT2A receptor in the frontal cortex, where 5-HT2A-driven phosphorylation of mGluR2 at Ser843 enhances Gi/o signaling and cross-activates Gq/11, and Egr1-mediated transcription of GRM2 is epigenetically regulated by 5-HT2A signaling; additionally, mGluR2 acts as a host endocytic receptor exploited by influenza virus (via direct HA binding) and rabies virus (in cooperation with TfR1) to enter cells via clathrin-mediated endocytosis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"GRM2 encodes mGluR2, a class C G-protein-coupled receptor that serves as the principal presynaptic group II autoreceptor for synaptically released glutamate at multiple forebrain synapses, where it limits neurotransmitter release [#4]. Cryo-EM structures establish that mGluR2 signals as an asymmetric dimer: agonist binding drives sequential Venus flytrap domain closure followed by transmembrane domain rearrangement from a symmetric inactive to an asymmetric active conformation, with the resulting Gi heterotrimer engaging a non-canonical intracellular site formed by three intracellular loops and helices III and IV [#9, #10, #11]. Presynaptically, mGluR2/3 inhibits release through ERK/MAPK-dependent phosphorylation and subsequent proteasomal degradation of Munc18-1 [#14], and postsynaptically it hyperpolarizes neurons by activating GIRK channels, as shown at cerebellar granule cell–Golgi cell synapses [#3]. Receptor output is gated by PKA phosphorylation at Ser843, which uncouples mGluR2 from G proteins [#2], while mGluR2 is uniquely resistant to GRK2-mediated homologous desensitization of its cAMP pathway compared with mGluR3 [#12]. mGluR2 assembles a functional heteromeric complex with the 5-HT2A receptor in frontal cortex via residues at the intracellular end of transmembrane domain 4 [#7]; within this complex 5-HT2A drives trans-receptor phosphorylation of mGluR2 at Ser843 to enhance Gi/o signaling [#13] and mGluR2 stimulation cross-activates Gq/11 through the 5-HT2A partner [#8], a complex required for the behavioral and binding effects of hallucinogens [#6]. GRM2 transcription is epigenetically controlled, including 5-HT2A/Egr1-dependent regulation at the Grm2 promoter [#25]. mGluR2 mediates the antipsychotic-like and antidepressant-relevant actions of group II ligands and of (2R,6R)-hydroxynorketamine [#5, #18, #19]. Beyond its neuronal roles, mGluR2 is exploited as a host endocytic receptor: influenza hemagglutinin binds it directly to drive clathrin-mediated endocytosis via KCa1.1-dependent F-actin polymerization [#20], and rabies virus uses mGluR2 in cooperation with transferrin receptor 1 [#21].\",\n  \"teleology\": [\n    {\n      \"year\": 1993,\n      \"claim\": \"Establishing where and how mGluR2 signals required defining its expression and G-protein coupling; this showed mGluR2 is a neuronal receptor coupled to the inhibitory cAMP cascade.\",\n      \"evidence\": \"In situ hybridization mapping mGluR2 mRNA distribution and cAMP coupling across CNS\",\n      \"pmids\": [\"8389425\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"mRNA distribution does not establish subcellular protein localization\", \"functional consequence at synapses not addressed\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Whether mGluR2 acts pre- or postsynaptically was unresolved; ultrastructural localization placed it at presynaptic, postsynaptic, and glial compartments of excitatory synapses, concentrated at mossy fiber and perforant path terminals.\",\n      \"evidence\": \"Electron microscopy immunocytochemistry across multiple brain regions\",\n      \"pmids\": [\"8684625\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"does not assign functional role to each compartment\", \"antibody cross-reactivity with mGluR3 not fully separated\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"How mGluR2 output is regulated was unknown; PKA was shown to phosphorylate Ser843 on the C-terminus, uncoupling the receptor from G proteins and inhibiting presynaptic responses.\",\n      \"evidence\": \"Site-directed mutagenesis, in vitro kinase assays, and electrophysiology with a PKA inhibitor\",\n      \"pmids\": [\"10908604\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"upstream signals driving PKA activation at the receptor not defined\", \"stoichiometry and kinetics of dephosphorylation unaddressed\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"The postsynaptic effector of mGluR2 was undefined; knockout electrophysiology showed mGluR2 hyperpolarizes Golgi cells via GIRK channels to produce long-lasting silencing.\",\n      \"evidence\": \"Whole-cell recording in mGluR2-deficient vs wild-type mouse cerebellum\",\n      \"pmids\": [\"12948448\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"G-protein subtype linking mGluR2 to GIRK not detailed\", \"generality across brain regions unestablished\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Distinguishing mGluR2 from mGluR3 autoreceptor roles was needed; genetic and pharmacological dissection identified mGluR2 as the major autoreceptor activated by synaptic glutamate at perforant path synapses.\",\n      \"evidence\": \"Field EPSP recordings in mGlu2-/- mice with LY341495 unmasking\",\n      \"pmids\": [\"12213275\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"intracellular cascade reducing release not yet identified\", \"contribution at other synapse types varies\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Whether mGluR2 desensitizes like other GPCRs mattered for tolerance; mGluR2 was found resistant to GRK2 desensitization of its cAMP pathway, unlike mGluR3, in a signal-dependent manner.\",\n      \"evidence\": \"HEK293 cAMP/MAPK assays with kinase-dead GRK2 plus in vivo knockout pharmacology\",\n      \"pmids\": [\"19164443\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"structural basis for differential GRK2 sensitivity unknown\", \"arrestin engagement on MAPK pathway only partially resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"The presynaptic mechanism coupling mGluR2/3 to reduced release was unknown; ERK-dependent phosphorylation of Munc18-1 followed by its proteasomal degradation was shown to mediate the inhibition.\",\n      \"evidence\": \"Electrophysiology, biochemistry, and phosphomimetic/phospho-null Munc18-1 mutants\",\n      \"pmids\": [\"27056679\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"link between receptor activation and ERK at the terminal not fully mapped\", \"shared mechanism with CB1R leaves receptor specificity step undefined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The 5-HT2A–mGluR2 heteromer was progressively defined as physically and functionally coupled and pharmacologically consequential, but how it is regulated and its causal role in human disease remains open.\",\n      \"evidence\": \"Co-IP/BRET/HTRF interaction mapping, trans-receptor Ser843 phosphorylation, Gq/11 cross-activation, and behavioral knockout studies\",\n      \"pmids\": [\"21276828\", \"23129762\", \"26758213\", \"29858599\", \"22300836\", \"30180227\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"one report found no signaling consequence of co-expression in HEK293 (idx 28)\", \"stoichiometry and dynamics of the heteromer in vivo unresolved\", \"translation to human therapeutics not established by direct causal evidence\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"How mGluR2 transduces signal at atomic resolution was unknown; cryo-EM revealed asymmetric dimer activation through sequential VFT closure and TMD rearrangement and a non-canonical Gi-binding site on helices III/IV and three intracellular loops.\",\n      \"evidence\": \"Cryo-EM of inactive/active homodimers and heterodimers with functional mutagenesis\",\n      \"pmids\": [\"34135510\", \"34135509\", \"37286794\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"how heteromerization with 5-HT2A maps onto these conformational states is not resolved\", \"dynamics in native membranes not captured\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Whether mGluR2 has roles beyond glutamatergic signaling was unclear; it was shown to be a host endocytic receptor directly bound by influenza HA driving KCa1.1/F-actin-dependent clathrin-mediated endocytosis, and used by rabies virus in cooperation with TfR1.\",\n      \"evidence\": \"siRNA screen, Co-IP of HA and TfR1 with mGluR2, actin/CME assays, and in vivo knockout viral challenge\",\n      \"pmids\": [\"38849624\", \"36779763\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"relationship between viral-entry signaling and canonical Gi/o coupling unresolved\", \"structural basis of HA–mGluR2 binding undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [3, 4, 9]},\n      {\"term_id\": \"GO:0001618\", \"supporting_discovery_ids\": [20, 21]},\n      {\"term_id\": \"GO:0038024\", \"supporting_discovery_ids\": [20, 21]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 30]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 8, 9, 14]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [3, 4]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [20, 21]}\n    ],\n    \"complexes\": [\"5-HT2A–mGluR2 heteromer\"],\n    \"partners\": [\"HTR2A\", \"Munc18-1\", \"TfR1\", \"GRK2\", \"EGR1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}