{"gene":"GRM7","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":1994,"finding":"mGluR7 (metabotropic glutamate receptor) is a G protein-coupled receptor that inhibits forskolin-stimulated cAMP accumulation in response to agonist interaction (L-AP4, L-serine-O-phosphate), demonstrating Gi/o-coupled inhibitory signaling.","method":"Functional assay in transfected CHO cells (cAMP accumulation assay)","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in heterologous expression system with direct functional readout, foundational paper with 419 citations","pmids":["8288585"],"is_preprint":false},{"year":2000,"finding":"PICK1 binds via its PDZ domain to the C-terminus of mGluR7a, forming a complex with PKCα, and PICK1 reduces PKCα-evoked phosphorylation of mGluR7a in vitro, identifying PICK1 as a regulator of mGluR7 phosphorylation.","method":"Yeast two-hybrid, GST pulldown, co-immunoprecipitation from COS-7 cells and rat brain tissue, confocal microscopy in hippocampal neurons, in vitro phosphorylation assay","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including pulldown, native co-IP, in vitro phosphorylation assay, and neuronal colocalization","pmids":["11007882"],"is_preprint":false},{"year":2005,"finding":"mGluR7 acts as a metaplastic switch at hippocampal mossy fiber-stratum lucidum interneuron (MF-SLIN) synapses: activation during high-frequency stimulation induces LTD via a PKC-dependent mechanism suppressing presynaptic release; subsequent mGluR7 internalization unmasks the ability of these synapses to undergo LTP with the same stimulus.","method":"Hippocampal slice electrophysiology, pharmacological manipulation, immunofluorescence for receptor surface expression","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 — clean loss/gain-of-function with defined cellular phenotype, multiple pharmacological controls, published in high-impact journal with 156 citations","pmids":["15820696"],"is_preprint":false},{"year":2006,"finding":"mGluR7 undergoes rapid agonist-induced internalization via a clathrin-independent, Arf6-positive endosomal pathway, distinct from NMDAR clathrin-mediated endocytosis, determining the steady-state amount of receptor on the plasma membrane.","method":"Immunofluorescence internalization assay, live imaging with pHluorin-tagged mGluR7 in hippocampal neurons, dominant-negative Arf6 experiments","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 — two independent assays (immunofluorescence + live pHluorin imaging) with pharmacological controls in neurons","pmids":["16914173","16890965"],"is_preprint":false},{"year":2006,"finding":"Constitutive endocytosis of mGluR7 in neurons proceeds via a clathrin-independent, Arf6-positive pathway (similar to MHC I and GPI-anchored proteins), not through the clathrin pathway used by NMDARs.","method":"Biochemical endocytosis assay in heterologous cells and neurons, dominant-negative constructs for clathrin pathway vs. Arf6 pathway","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 — mechanistic dissection with dominant-negative constructs distinguishing clathrin-dependent vs. independent pathways","pmids":["16890965"],"is_preprint":false},{"year":2008,"finding":"PKC phosphorylation of mGluR7 at Ser862 inhibits calmodulin (CaM) binding, thereby increasing mGluR7 surface expression and PICK1 binding; in PICK1 knockout mice, PKC-dependent increases in mGluR7 phosphorylation and surface expression are diminished, and mGluR7-dependent plasticity at MF-interneuron synapses is impaired.","method":"Phosphorylation-specific antibodies, mutagenesis of Ser862, PICK1 knockout mice, hippocampal slice electrophysiology, surface biotinylation","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis, genetic knockout, multiple biochemical readouts, and physiological validation in hippocampal slices","pmids":["18549785"],"is_preprint":false},{"year":2002,"finding":"PKC, PKA, and PKG each phosphorylate mGluR7 at a single serine residue (Ser862) in the C-terminus, which inhibits calmodulin binding; however, PKC-mediated inhibition of mGluR7-coupled GIRK currents is independent of Ser862 phosphorylation and CaM binding.","method":"In vitro phosphorylation assays, site-directed mutagenesis of Ser862, CaM binding assay, GIRK current recordings in heterologous expression system","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with mutagenesis and multiple kinase assays, electrophysiological validation","pmids":["12021391"],"is_preprint":false},{"year":2007,"finding":"mGluR7 inhibits glutamate release at hippocampal nerve terminals primarily by inhibiting P/Q-type (but not N-type) Ca2+ channels via pertussis toxin-sensitive (Gi/o) G-proteins; a secondary pathway involving cAMP reduction is revealed when cAMP levels are elevated. The mechanism is PKC- and PKA-independent.","method":"Glutamate release assay from rat hippocampal synaptosomes, selective Ca2+ channel blockers (omega-conotoxin-GVIA), pertussis toxin, cAMP measurement, PKC/PKA inhibitors","journal":"The European journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro synaptosomal assay with pharmacological dissection of multiple signaling branches","pmids":["17650109"],"is_preprint":false},{"year":2010,"finding":"Prolonged activation of mGlu7 receptor with L-AP4 at cerebrocortical nerve terminals activates a second signaling pathway (phospholipase C/PIP2 hydrolysis/DAG) that potentiates glutamate release via translocation of the active zone priming protein Munc13-1 from soluble to particulate fractions; this is distinct from the canonical Gi/o-mediated inhibitory pathway.","method":"Glutamate release assay from cortical synaptosomes, pertussis toxin, PLC inhibitors, calphostin C (DAG-binding site antagonist), subcellular fractionation for Munc13-1","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — reconstituted in synaptosomal preparation with multiple pharmacological dissections and protein fractionation","pmids":["20375012"],"is_preprint":false},{"year":2008,"finding":"Intracellular Ca2+ elevation potentiates mGluR7a signaling: physiological levels of [Ca2+]i enhance L-AP4-induced GIRK channel activation via mGluR7a, consistent with mGluR7a acting as a coincidence detector of Ca2+ influx and glutamate release at presynaptic terminals.","method":"Xenopus oocyte expression system with channelrhodopsin-2 (ChR2) for controlled Ca2+ influx, GIRK3.1/3.2 concatemer as readout, Ca2+ chelator controls","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted functional assay with precise Ca2+ manipulation and pharmacological controls","pmids":["18599484"],"is_preprint":false},{"year":2013,"finding":"PP1 (serine/threonine protein phosphatase 1), specifically the PP1γ1 isoform, dephosphorylates mGluR7 at Ser862 and regulates agonist-induced (but not NMDA-activity-induced) dephosphorylation, thereby controlling mGluR7 surface expression.","method":"PP1 inhibitors in neurons, phospho-specific antibody for Ser862, surface biotinylation, isoform-specific knockdown","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — specific pharmacological and genetic dissection of PP1 isoforms with multiple pathway controls","pmids":["23612982"],"is_preprint":false},{"year":2013,"finding":"GRK4 regulates mGluR7 signaling through the adenylyl cyclase and JNK pathways, while GRK2 selectively regulates the ERK1/2 pathway; β-arrestin1 positively modulates ERK1/2 and inhibits JNK, while β-arrestin2 does the opposite; β-arrestin1 amplifies mGluR7-dependent ERK1/2 activation in response to orthosteric agonist L-AP4 but not the allosteric agonist AMN082.","method":"Recombinant mGluR7 expression, GRK/arrestin overexpression and knockdown, signaling pathway assays (adenylyl cyclase, ERK1/2, JNK)","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple signaling pathways dissected in recombinant system, single lab","pmids":["24148810"],"is_preprint":false},{"year":2014,"finding":"Elfn1 (extracellular leucine-rich repeat and fibronectin type III domain-containing 1) recruits mGluR7 in trans to presynaptic sites on somatostatin interneuron synapses; loss of Elfn1 leads to deficits in mGluR7 recruitment and impaired presynaptic plasticity, with missense mutations in ELFN1 clustering in the region required for mGluR7 recruitment.","method":"Immunocytochemistry/immunofluorescence, Elfn1 KO mice, electrophysiology of SOM interneuron synapses, patient mutation analysis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout with specific electrophysiological phenotype, localization studies, and human genetics validation","pmids":["25047565"],"is_preprint":false},{"year":2019,"finding":"Elfn1 produces glutamate-independent (constitutive) activation of mGluR7 via presynaptic clustering at synapses onto cortical somatostatin neurons, reducing initial release probability and generating synaptic facilitation; this represents a deviation from the canonical autoreceptor role of group III mGluRs.","method":"Electrophysiology in cortical slices from WT and mGluR7/Elfn1 manipulated mice, pharmacological dissection with mGluR7 agonist/antagonist","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — electrophysiological epistasis with genetic and pharmacological tools, multiple synaptic preparations","pmids":["30940718"],"is_preprint":false},{"year":2015,"finding":"mGluR7 SUMOylation at Lys889 is required for stable surface expression; agonist treatment reduces SUMOylation; SENP1 (SUMO isopeptidase) reverses SUMOylation and increases mGluR7 internalization; Ser862 phosphorylation facilitates SUMO conjugation of mGluR7.","method":"SUMOylation assay in HEK293T cells, identification of SUMOylated mGluR7 in brain and cortical neurons, site-directed mutagenesis (Lys889), SENP1 overexpression, internalization assay in hippocampal neurons","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 — mutagenesis, multiple cell systems including native neurons, confirmed in brain tissue","pmids":["26631532"],"is_preprint":false},{"year":2019,"finding":"Nedd4 E3 ubiquitin ligase and β-arrestins cooperate to ubiquitinate mGluR7 upon agonist stimulation: β-arrestins recruit Nedd4 to mGluR7, facilitating ubiquitination; Nedd4 and β-arrestins regulate constitutive and agonist-induced mGluR7 endocytosis, are required for mGluR7-dependent MAPK signaling in neurons, and Nedd4-mediated ubiquitination leads to mGluR7 degradation via both the proteasome and lysosomal pathways.","method":"Co-immunoprecipitation, ubiquitination assays in heterologous cells and rat neurons, KD/KO with defined trafficking phenotypes, MAPK signaling readout","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods in both heterologous and native neuronal systems","pmids":["31373553"],"is_preprint":false},{"year":2020,"finding":"N-glycosylation of mGlu7 at four asparagine residues is essential for forward trafficking and surface expression; deglycosylated mGlu7 is retained in the ER and degraded via the autophagolysosomal pathway; N-glycosylation of mGlu7 also promotes its trans-synaptic interaction with Elfn1.","method":"Site-directed mutagenesis of glycosylation sites, ER retention assay, surface biotinylation, co-immunoprecipitation with Elfn1, autophagolysosomal pathway inhibitors","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis with multiple biochemical and cell biological readouts","pmids":["32931036"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures of human mGlu2 and mGlu7 homo- and heterodimers revealed: a subtype-dependent dimerization mode with a unique helix IV-mediated dimer interface in inactive mGlu2 important for limiting receptor activity; the mGlu7 subunit has a dominant role in controlling dimeric association and G-protein activation in the mGlu2-mGlu7 heterodimer.","method":"Cryo-electron microscopy structure determination, functional validation with chimeric receptor studies and G-protein activation assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure with functional validation, high-impact publication","pmids":["34135509"],"is_preprint":false},{"year":2021,"finding":"The GRM7 I154T mutation (within the mGlu7 ligand-binding domain) causes significant loss of mGlu7 protein expression at the posttranscriptional level due to impaired protein folding/trafficking; mGlu7-I154T knock-in mice recapitulate mGlu7-KO phenotypes including reduced motor coordination, deficits in contextual fear learning, and seizures.","method":"Patient mutation identification, functional studies in HEK293A cells and knock-in mice, surface expression analysis, behavioral phenotyping","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 1-2 — mechanistic study combining cell biology, knock-in mouse model, and clinical validation","pmids":["33476302"],"is_preprint":false},{"year":2021,"finding":"Pathogenic GRM7 mutations (I154T and R658W/T675K) lead to mGlu7 protein degradation; R658W/T675K abolishes mGlu7 surface expression; pathogenic mutations impair axon outgrowth through the MAPK-cAMP-PKA signaling pathway during early neuronal development, subsequently reducing presynaptic terminal number.","method":"Mutagenesis in heterologous cells and primary neurons, surface expression assay, axon outgrowth assay, MAPK/PKA pathway pharmacology, presynaptic terminal counting","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — mechanistic pathway dissection with multiple mutations and pharmacological rescue experiments","pmids":["33500274"],"is_preprint":false},{"year":2014,"finding":"mGlu7 activation decreases NMDAR-mediated currents and NR1 surface expression in basal forebrain cholinergic neurons via a mechanism involving cofilin-regulated actin dynamics; Aβ selectively impairs this mGlu7 regulation by increasing p21-activated kinase activity and decreasing cofilin-mediated actin depolymerization through a p75NTR-dependent mechanism.","method":"Whole-cell patch clamp, surface biotinylation, pharmacological dissection (p21-activated kinase inhibitors, cofilin pathway), p75NTR-selective inhibition, actin polymerization assays","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods establishing mechanistic pathway from mGlu7 activation to actin dynamics to NMDAR regulation","pmids":["25297090"],"is_preprint":false},{"year":2015,"finding":"mGluR7 regulates embryonic neurogenesis by directly interacting with calmodulin (CaM), modulating CREB phosphorylation and YAP expression; GRM7 knockdown increases neural progenitor cell proliferation and decreases neuronal differentiation via CyclinD1 regulation; these defects are rescued by Creb or Yap knockdown.","method":"In utero electroporation (gain/loss of function), co-immunoprecipitation of GRM7 with CaM, Western blot for pCREB/YAP/CyclinD1, cortical progenitor proliferation assays","journal":"Stem cell reports","confidence":"Medium","confidence_rationale":"Tier 2 — genetic rescue epistasis and co-IP in single lab study","pmids":["25921811"],"is_preprint":false},{"year":2012,"finding":"Presynaptic mGlu7 receptors exist as heteroreceptors on GABAergic terminals in the mouse hippocampus; their activation (by AMN082 or L-AP4) inhibits GABA release via an adenylyl cyclase-dependent mechanism and they colocalize with syntaxin-1A and VGAT, but do not functionally cross-talk with co-localized GABAB autoreceptors.","method":"Synaptosomal superfusion assay, AMN082/MMPIP pharmacology, confocal microscopy colocalization, cAMP pathway inhibitors","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — synaptosomal functional assay with pharmacological characterization and colocalization, single lab","pmids":["22564442"],"is_preprint":false},{"year":2014,"finding":"The mGlu7 antagonist XAP044 binds within mGlu7's extracellular Venus flytrap domain (not the seven-transmembrane region) and blocks lateral amygdala LTP in an mGlu7-dependent manner; this was demonstrated by chimeric receptor studies showing XAP044's binding site is in the VFTD.","method":"Chimeric receptor studies in recombinant cells, LTP recordings in amygdala brain slices from WT and mGlu7-KO mice","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — chimeric receptor studies establishing binding domain, validated with genetic KO controls in slice physiology","pmids":["24596089"],"is_preprint":false},{"year":2022,"finding":"mGlu7 forms heterodimers with mGlu8 at hippocampal Schaffer collateral-CA1 synapses; mGlu7/8 heterodimers display distinct pharmacology from mGlu7/7 homodimers — ADX71743 blocks both, while MMPIP only antagonizes the homodimer — providing evidence that mGlu7/8 heterodimers are the functionally predominant form at SC-CA1 synapses.","method":"Complemented donor acceptor-resonance energy transfer (CADET) for heterodimer detection in vitro, SC-CA1 electrophysiology in brain slices, pharmacological dissection with selective NAMs","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — novel FRET-based heterodimer detection corroborated by slice electrophysiology, single lab","pmids":["36063995"],"is_preprint":false},{"year":1995,"finding":"mGluR7 is localized presynaptically in axon terminals of primary afferent fibers in laminae I and II of the spinal dorsal horn, where it functions as an autoreceptor; dorsal rhizotomy reduces immunoreactivity in these laminae, confirming the presynaptic location on afferent terminals.","method":"Immunohistochemistry with specific anti-mGluR7 antibody, dorsal rhizotomy lesion experiment, electron microscopy","journal":"Neuroscience letters","confidence":"High","confidence_rationale":"Tier 2 — lesion experiment directly confirming presynaptic localization on afferent terminals","pmids":["8787837"],"is_preprint":false},{"year":1996,"finding":"In the rat retina, mGluR7 is localized both presynaptically (as an autoreceptor at cone bipolar cell ribbon synapses) and postsynaptically in amacrine cells; the presynaptic mGluR7 is restricted to one half of the active zone, facing only one of two postsynaptic processes.","method":"Immunofluorescence with specific antiserum, electron microscopy, double-labeling","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 — electron microscopy for ultrastructural localization with double-labeling controls","pmids":["8764662"],"is_preprint":false},{"year":2003,"finding":"mGluR7a is expressed at high levels in the presynaptic active zones of GABAergic terminals that innervate somatostatin/mGluR1α-positive interneurons in the hippocampal stratum oriens; VIP-positive GABAergic terminals on these cells express mGluR7a at >90%; GABAergic terminals on pyramidal cells are mGluR7a-negative, demonstrating target-cell-specific expression.","method":"Immunocytochemistry (electron microscopy), double-labeling with GAD/GABA and cell-type markers, quantitative analysis","journal":"The European journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — quantitative ultrastructural analysis with multiple cell-type markers in a rigorous multi-site study","pmids":["12823458"],"is_preprint":false},{"year":2005,"finding":"The C-terminal splice variant GluR7a is highly expressed at the plasma membrane through a stretch of positively charged amino acids, while GluR7b is mostly retained in the ER; GluR7a promotes surface expression of ER-retained splice variants when assembled into heteromeric kainate receptors.","method":"Trafficking studies in hippocampal neurons from WT and KAR mutant mice, surface biotinylation, confocal microscopy, mutagenesis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — neuronal trafficking studies with genetic and mutagenesis tools, single lab","pmids":["15805114"],"is_preprint":false},{"year":2017,"finding":"mGlu7 is the predominant mGlu receptor expressed presynaptically at Schaffer collateral-CA1 synapses in adult mice; its activation on GABAergic interneurons is necessary for LTP induction at these synapses; MECP2 mutations reduce mGlu7 protein expression, and positive allosteric modulation of mGlu7 restores LTP and improves learning/memory phenotypes in RTT model mice.","method":"Western blot of RTT patient and mouse tissue, hippocampal slice LTP recordings, PAM pharmacology, behavioral tests","journal":"Science translational medicine","confidence":"High","confidence_rationale":"Tier 2 — mechanistic pathway established with patient tissue, genetic model, and pharmacological rescue","pmids":["28814546"],"is_preprint":false}],"current_model":"mGluR7/GRM7 is a presynaptic Gi/o-coupled class C GPCR that primarily inhibits neurotransmitter release by suppressing P/Q-type Ca2+ channels and reducing cAMP; its surface expression and synaptic localization are tightly regulated by PKC/PKA/PKG phosphorylation at Ser862 (which competitively controls calmodulin vs. PICK1 binding), dephosphorylation by PP1γ1, SUMOylation at Lys889, Nedd4/β-arrestin-mediated ubiquitination and clathrin-independent (Arf6-dependent) endocytosis, and N-glycosylation-dependent ER export; it acts as a metaplastic switch at select hippocampal synapses, can alternatively couple to PLC/DAG/Munc13-1 to potentiate release, functions as a coincidence detector of Ca2+ influx and glutamate via calmodulin, is recruited in trans to somatostatin interneuron synapses by the postsynaptic organizer Elfn1 to constitutively suppress release probability, and forms both homodimers and pharmacologically distinct mGlu7/mGlu8 heterodimers; its cryo-EM structure reveals a unique subtype-specific dimerization mode in which the mGlu7 subunit dominantly controls G-protein activation in heterodimers."},"narrative":{"teleology":[{"year":1994,"claim":"Establishing the fundamental signaling identity of mGluR7 as a Gi/o-coupled receptor that inhibits cAMP accumulation answered the basic question of which second-messenger pathway this orphan group III mGluR engages.","evidence":"cAMP accumulation assay in mGluR7-transfected CHO cells stimulated with L-AP4","pmids":["8288585"],"confidence":"High","gaps":["Downstream effectors at native synapses unknown","No information on splice-variant-specific signaling"]},{"year":1996,"claim":"Ultrastructural localization of mGluR7 to presynaptic active zones — and its asymmetric positioning at retinal ribbon synapses — established that this receptor occupies a privileged position to sense synaptically released glutamate as an autoreceptor.","evidence":"Immunoelectron microscopy in spinal cord (with dorsal rhizotomy lesion) and retina","pmids":["8787837","8764662"],"confidence":"High","gaps":["Mechanism of target-cell-specific presynaptic sorting unknown","Functional consequence of asymmetric active-zone positioning not tested"]},{"year":2000,"claim":"Identification of PICK1 as a PDZ-domain partner that scaffolds PKCα to mGluR7a and modulates its phosphorylation revealed the first molecular mechanism for regulating mGluR7 signaling at the receptor level.","evidence":"Yeast two-hybrid, GST pulldown, co-IP from brain, in vitro phosphorylation assay","pmids":["11007882"],"confidence":"High","gaps":["In vivo relevance of PICK1-PKCα complex not yet shown","Phosphorylation site identity not established"]},{"year":2002,"claim":"Mapping the convergent phosphorylation site Ser862 and showing that PKC, PKA, and PKG all phosphorylate this residue to displace calmodulin defined a single molecular switch governing mGluR7 C-terminal protein interactions.","evidence":"In vitro kinase assays, Ser862 mutagenesis, CaM binding assay, GIRK recordings","pmids":["12021391"],"confidence":"High","gaps":["Which kinase acts in vivo at specific synapses unclear","Phosphatase identity unknown"]},{"year":2003,"claim":"Quantitative ultrastructural demonstration that mGluR7a is selectively concentrated at GABAergic terminals innervating somatostatin interneurons — but absent from terminals on pyramidal cells — established the target-cell-specific expression principle later explained by trans-synaptic recruitment.","evidence":"Immunoelectron microscopy with GAD/cell-type markers in hippocampal stratum oriens","pmids":["12823458"],"confidence":"High","gaps":["Molecular mechanism of target-cell-dependent sorting unknown","Whether pattern extends to cortex not tested"]},{"year":2005,"claim":"Showing that mGluR7 acts as a metaplastic switch at mossy fiber–interneuron synapses — where its activation induces LTD and subsequent internalization unmasks LTP — linked receptor trafficking to bidirectional synaptic plasticity for the first time.","evidence":"Hippocampal slice electrophysiology with pharmacological manipulation and surface-expression immunofluorescence","pmids":["15820696"],"confidence":"High","gaps":["Internalization pathway not identified","Whether metaplastic switch occurs at other synapse types unknown"]},{"year":2006,"claim":"Discovery that mGluR7 endocytosis proceeds through a clathrin-independent, Arf6-positive pathway — distinct from classical NMDAR endocytosis — identified an unconventional trafficking route that governs receptor surface levels both constitutively and upon agonist stimulation.","evidence":"Immunofluorescence and pHluorin live imaging in neurons, dominant-negative Arf6","pmids":["16914173","16890965"],"confidence":"High","gaps":["Recycling vs. degradation fate after Arf6 endocytosis not resolved","Adaptor proteins linking mGluR7 to Arf6 pathway unknown"]},{"year":2007,"claim":"Pharmacological dissection at hippocampal synaptosomes resolved that the primary mechanism of mGluR7-mediated release inhibition is Gi/o-dependent suppression of P/Q-type Ca²⁺ channels, with cAMP reduction as a secondary modulatory pathway.","evidence":"Glutamate release from rat hippocampal synaptosomes with selective Ca²⁺ channel blockers, pertussis toxin, kinase inhibitors","pmids":["17650109"],"confidence":"High","gaps":["Direct G-protein–Ca²⁺ channel coupling vs. membrane-delimited mechanism not distinguished","Contribution at individual synapse types unresolved"]},{"year":2008,"claim":"Two contemporaneous studies resolved complementary aspects of mGluR7 regulation: Ser862 phosphorylation by PKC controls a competitive switch between calmodulin and PICK1 binding to govern surface expression (validated in PICK1-KO mice), while intracellular Ca²⁺ potentiates mGluR7 signaling, establishing a coincidence-detection mechanism for glutamate and Ca²⁺ influx.","evidence":"Phospho-specific antibodies, Ser862 mutagenesis, PICK1-KO slice physiology; Xenopus oocyte GIRK readout with channelrhodopsin-controlled Ca²⁺","pmids":["18549785","18599484"],"confidence":"High","gaps":["Whether CaM mediates the Ca²⁺ coincidence detection in vivo not proven","Relative contributions of CaM vs. PICK1 binding states at native synapses unknown"]},{"year":2010,"claim":"The demonstration that prolonged mGlu7 activation switches from inhibitory Gi/o coupling to a PLC/DAG pathway that translocates Munc13-1 and potentiates release revealed a dual-mode signaling capacity unprecedented for group III mGluRs.","evidence":"Cortical synaptosomal release assay with PLC inhibitors, calphostin C, Munc13-1 subcellular fractionation","pmids":["20375012"],"confidence":"High","gaps":["G-protein identity for the PLC pathway not identified","Duration threshold for pathway switching not defined","In vivo relevance at specific circuit level unknown"]},{"year":2013,"claim":"Identification of PP1γ1 as the phosphatase that dephosphorylates Ser862 completed the kinase–phosphatase cycle controlling mGluR7 surface expression, while parallel work delineated biased GRK2/GRK4 and β-arrestin1/2 signaling through ERK and JNK pathways.","evidence":"PP1 isoform-specific knockdown and inhibitors in neurons with phospho-Ser862 readout; GRK/arrestin overexpression/knockdown with pathway-selective signaling assays","pmids":["23612982","24148810"],"confidence":"High","gaps":["Activity-dependent regulation of PP1γ1 at mGluR7-containing synapses not explored","GRK/arrestin findings from recombinant system only"]},{"year":2014,"claim":"Discovery that the postsynaptic adhesion molecule Elfn1 recruits mGluR7 in trans to somatostatin-interneuron synapses provided the molecular explanation for the long-observed target-cell-specific presynaptic expression of mGluR7 and linked it to synapse-type-specific plasticity rules.","evidence":"Elfn1-KO mice, electrophysiology of SOM interneuron synapses, immunocytochemistry, human mutation analysis","pmids":["25047565"],"confidence":"High","gaps":["Direct biophysical interaction interface not mapped","Whether other postsynaptic organizers contribute unknown"]},{"year":2015,"claim":"SUMOylation at Lys889 was identified as a novel post-translational modification stabilizing mGluR7 surface expression, with Ser862 phosphorylation facilitating SUMO conjugation, revealing crosstalk between phosphorylation and SUMOylation in receptor trafficking.","evidence":"SUMO assay in HEK293T cells and brain tissue, Lys889 mutagenesis, SENP1 overexpression, neuronal internalization assay","pmids":["26631532"],"confidence":"High","gaps":["SUMO E3 ligase identity unknown","Functional consequence at synaptic level not tested"]},{"year":2019,"claim":"Two key advances solidified the mGluR7 trafficking and signaling framework: Nedd4/β-arrestin-mediated ubiquitination was shown to drive both constitutive and agonist-induced endocytosis and degradation, while Elfn1 was demonstrated to produce constitutive (glutamate-independent) mGluR7 activation that tonically reduces release probability at somatostatin-interneuron synapses.","evidence":"Ubiquitination assays and trafficking in neurons with Nedd4/β-arrestin manipulation; cortical slice electrophysiology in Elfn1/mGluR7-manipulated mice","pmids":["31373553","30940718"],"confidence":"High","gaps":["Ubiquitin chain type (K48 vs K63) on mGluR7 not determined","Relative contribution of proteasomal vs. lysosomal degradation in vivo unclear"]},{"year":2020,"claim":"N-glycosylation at four asparagine residues was shown to be essential for ER export and surface expression of mGlu7, with deglycosylated receptor shunted to autophagolysosomal degradation; glycosylation also facilitates trans-synaptic Elfn1 interaction.","evidence":"Glycosylation-site mutagenesis, surface biotinylation, ER retention assay, co-IP with Elfn1, autophagolysosome inhibitors","pmids":["32931036"],"confidence":"High","gaps":["Specific glycan structures not characterized","Whether glycosylation defects contribute to disease mutations unknown"]},{"year":2021,"claim":"Cryo-EM structures of mGlu7 homodimers and mGlu2/mGlu7 heterodimers revealed a subtype-specific dimerization mode and demonstrated that the mGlu7 subunit dominantly controls G-protein activation in heterodimers, providing the first atomic-level framework for understanding mGluR7 function and heterodimer pharmacology.","evidence":"Cryo-EM structure determination with chimeric receptor and G-protein activation assays","pmids":["34135509"],"confidence":"High","gaps":["No structure of mGlu7/mGlu8 heterodimer, the predominant form at SC-CA1 synapses","Agonist-bound active-state structure not yet available"]},{"year":2021,"claim":"Pathogenic GRM7 mutations (I154T, R658W/T675K) were shown to cause mGlu7 protein destabilization and loss of surface expression, impairing MAPK-cAMP-PKA signaling and axon outgrowth; I154T knock-in mice recapitulate neurodevelopmental phenotypes including seizures, establishing GRM7 as a neurodevelopmental disease gene.","evidence":"Patient mutation identification, functional studies in HEK293A and neurons, knock-in mouse behavioral phenotyping, pharmacological rescue","pmids":["33476302","33500274"],"confidence":"High","gaps":["Full allelic series and genotype-phenotype correlation in humans incomplete","Whether pharmacological chaperones can rescue folding-deficient mutants unknown"]},{"year":2022,"claim":"Detection of functional mGlu7/mGlu8 heterodimers at Schaffer collateral-CA1 synapses with pharmacology distinct from homodimers raised the possibility that heterodimeric assemblies are the physiologically predominant signaling species at major hippocampal synapses.","evidence":"CADET FRET-based heterodimer detection in vitro, SC-CA1 slice electrophysiology with NAM pharmacology","pmids":["36063995"],"confidence":"Medium","gaps":["Heterodimer stoichiometry at native synapses not quantified","Selective genetic tools to dissect heterodimer vs. homodimer function in vivo lacking","Finding from single lab awaits independent replication"]},{"year":null,"claim":"Key unresolved questions include: how the dual-mode Gi/o-inhibitory versus PLC-facilitatory signaling switch is temporally and spatially regulated at individual synapses in vivo; the structural basis for mGlu7/mGlu8 heterodimer signaling; and whether pharmacological chaperones or allosteric modulators can rescue pathogenic folding-deficient GRM7 mutants for therapeutic purposes.","evidence":"","pmids":[],"confidence":"Low","gaps":["No in vivo signaling-mode switching data at single-synapse resolution","No mGlu7/mGlu8 heterodimer structure","No pharmacological rescue strategy for loss-of-function mutations tested in vivo"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,9]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[7,8,22]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[3,5,14,25,26]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[16,28]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,7,8,9,11]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[2,7,20,29]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[5,6,10,14,15]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[3,4,16]}],"complexes":["mGlu7/mGlu7 homodimer","mGlu7/mGlu8 heterodimer","mGlu2/mGlu7 heterodimer"],"partners":["PICK1","ELFN1","NEDD4","ARRB1","ARRB2","CALM1","GRM8","PPP1CC"],"other_free_text":[]},"mechanistic_narrative":"GRM7 encodes mGluR7, a presynaptic class C G protein-coupled receptor that functions as a central regulator of neurotransmitter release probability and synaptic plasticity at glutamatergic and GABAergic terminals throughout the brain. Canonically coupled to Gi/o, mGluR7 inhibits release by suppressing P/Q-type Ca²⁺ channels and reducing cAMP, but prolonged activation engages an alternative PLC/DAG pathway that recruits Munc13-1 to potentiate release [PMID:8288585, PMID:17650109, PMID:20375012]. Surface expression is governed by a convergent regulatory network at Ser862: PKC/PKA/PKG phosphorylation displaces calmodulin to promote PICK1 binding and surface retention, while PP1γ1-mediated dephosphorylation, SUMO conjugation at Lys889, Nedd4/β-arrestin-dependent ubiquitination, and clathrin-independent Arf6-mediated endocytosis dynamically control receptor density [PMID:18549785, PMID:23612982, PMID:26631532, PMID:31373553, PMID:16890965]. The postsynaptic organizer Elfn1 recruits mGluR7 in trans to somatostatin-interneuron synapses to constitutively suppress release probability, and pathogenic GRM7 mutations (e.g., I154T) that destabilize protein folding cause neurodevelopmental phenotypes including seizures and impaired motor coordination in knock-in mice [PMID:25047565, PMID:30940718, PMID:33476302]."},"prefetch_data":{"uniprot":{"accession":"Q14831","full_name":"Metabotropic glutamate receptor 7","aliases":[],"length_aa":915,"mass_kda":102.3,"function":"G-protein coupled receptor activated by glutamate that regulates axon outgrowth through the MAPK-cAMP-PKA signaling pathway during neuronal development (PubMed:33500274). 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\"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in heterologous expression system with direct functional readout, foundational paper with 419 citations\",\n      \"pmids\": [\"8288585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PICK1 binds via its PDZ domain to the C-terminus of mGluR7a, forming a complex with PKCα, and PICK1 reduces PKCα-evoked phosphorylation of mGluR7a in vitro, identifying PICK1 as a regulator of mGluR7 phosphorylation.\",\n      \"method\": \"Yeast two-hybrid, GST pulldown, co-immunoprecipitation from COS-7 cells and rat brain tissue, confocal microscopy in hippocampal neurons, in vitro phosphorylation assay\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including pulldown, native co-IP, in vitro phosphorylation assay, and neuronal colocalization\",\n      \"pmids\": [\"11007882\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"mGluR7 acts as a metaplastic switch at hippocampal mossy fiber-stratum lucidum interneuron (MF-SLIN) synapses: activation during high-frequency stimulation induces LTD via a PKC-dependent mechanism suppressing presynaptic release; subsequent mGluR7 internalization unmasks the ability of these synapses to undergo LTP with the same stimulus.\",\n      \"method\": \"Hippocampal slice electrophysiology, pharmacological manipulation, immunofluorescence for receptor surface expression\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean loss/gain-of-function with defined cellular phenotype, multiple pharmacological controls, published in high-impact journal with 156 citations\",\n      \"pmids\": [\"15820696\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"mGluR7 undergoes rapid agonist-induced internalization via a clathrin-independent, Arf6-positive endosomal pathway, distinct from NMDAR clathrin-mediated endocytosis, determining the steady-state amount of receptor on the plasma membrane.\",\n      \"method\": \"Immunofluorescence internalization assay, live imaging with pHluorin-tagged mGluR7 in hippocampal neurons, dominant-negative Arf6 experiments\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — two independent assays (immunofluorescence + live pHluorin imaging) with pharmacological controls in neurons\",\n      \"pmids\": [\"16914173\", \"16890965\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Constitutive endocytosis of mGluR7 in neurons proceeds via a clathrin-independent, Arf6-positive pathway (similar to MHC I and GPI-anchored proteins), not through the clathrin pathway used by NMDARs.\",\n      \"method\": \"Biochemical endocytosis assay in heterologous cells and neurons, dominant-negative constructs for clathrin pathway vs. Arf6 pathway\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic dissection with dominant-negative constructs distinguishing clathrin-dependent vs. independent pathways\",\n      \"pmids\": [\"16890965\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PKC phosphorylation of mGluR7 at Ser862 inhibits calmodulin (CaM) binding, thereby increasing mGluR7 surface expression and PICK1 binding; in PICK1 knockout mice, PKC-dependent increases in mGluR7 phosphorylation and surface expression are diminished, and mGluR7-dependent plasticity at MF-interneuron synapses is impaired.\",\n      \"method\": \"Phosphorylation-specific antibodies, mutagenesis of Ser862, PICK1 knockout mice, hippocampal slice electrophysiology, surface biotinylation\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis, genetic knockout, multiple biochemical readouts, and physiological validation in hippocampal slices\",\n      \"pmids\": [\"18549785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PKC, PKA, and PKG each phosphorylate mGluR7 at a single serine residue (Ser862) in the C-terminus, which inhibits calmodulin binding; however, PKC-mediated inhibition of mGluR7-coupled GIRK currents is independent of Ser862 phosphorylation and CaM binding.\",\n      \"method\": \"In vitro phosphorylation assays, site-directed mutagenesis of Ser862, CaM binding assay, GIRK current recordings in heterologous expression system\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with mutagenesis and multiple kinase assays, electrophysiological validation\",\n      \"pmids\": [\"12021391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"mGluR7 inhibits glutamate release at hippocampal nerve terminals primarily by inhibiting P/Q-type (but not N-type) Ca2+ channels via pertussis toxin-sensitive (Gi/o) G-proteins; a secondary pathway involving cAMP reduction is revealed when cAMP levels are elevated. The mechanism is PKC- and PKA-independent.\",\n      \"method\": \"Glutamate release assay from rat hippocampal synaptosomes, selective Ca2+ channel blockers (omega-conotoxin-GVIA), pertussis toxin, cAMP measurement, PKC/PKA inhibitors\",\n      \"journal\": \"The European journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro synaptosomal assay with pharmacological dissection of multiple signaling branches\",\n      \"pmids\": [\"17650109\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Prolonged activation of mGlu7 receptor with L-AP4 at cerebrocortical nerve terminals activates a second signaling pathway (phospholipase C/PIP2 hydrolysis/DAG) that potentiates glutamate release via translocation of the active zone priming protein Munc13-1 from soluble to particulate fractions; this is distinct from the canonical Gi/o-mediated inhibitory pathway.\",\n      \"method\": \"Glutamate release assay from cortical synaptosomes, pertussis toxin, PLC inhibitors, calphostin C (DAG-binding site antagonist), subcellular fractionation for Munc13-1\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstituted in synaptosomal preparation with multiple pharmacological dissections and protein fractionation\",\n      \"pmids\": [\"20375012\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Intracellular Ca2+ elevation potentiates mGluR7a signaling: physiological levels of [Ca2+]i enhance L-AP4-induced GIRK channel activation via mGluR7a, consistent with mGluR7a acting as a coincidence detector of Ca2+ influx and glutamate release at presynaptic terminals.\",\n      \"method\": \"Xenopus oocyte expression system with channelrhodopsin-2 (ChR2) for controlled Ca2+ influx, GIRK3.1/3.2 concatemer as readout, Ca2+ chelator controls\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted functional assay with precise Ca2+ manipulation and pharmacological controls\",\n      \"pmids\": [\"18599484\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"PP1 (serine/threonine protein phosphatase 1), specifically the PP1γ1 isoform, dephosphorylates mGluR7 at Ser862 and regulates agonist-induced (but not NMDA-activity-induced) dephosphorylation, thereby controlling mGluR7 surface expression.\",\n      \"method\": \"PP1 inhibitors in neurons, phospho-specific antibody for Ser862, surface biotinylation, isoform-specific knockdown\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — specific pharmacological and genetic dissection of PP1 isoforms with multiple pathway controls\",\n      \"pmids\": [\"23612982\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"GRK4 regulates mGluR7 signaling through the adenylyl cyclase and JNK pathways, while GRK2 selectively regulates the ERK1/2 pathway; β-arrestin1 positively modulates ERK1/2 and inhibits JNK, while β-arrestin2 does the opposite; β-arrestin1 amplifies mGluR7-dependent ERK1/2 activation in response to orthosteric agonist L-AP4 but not the allosteric agonist AMN082.\",\n      \"method\": \"Recombinant mGluR7 expression, GRK/arrestin overexpression and knockdown, signaling pathway assays (adenylyl cyclase, ERK1/2, JNK)\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple signaling pathways dissected in recombinant system, single lab\",\n      \"pmids\": [\"24148810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Elfn1 (extracellular leucine-rich repeat and fibronectin type III domain-containing 1) recruits mGluR7 in trans to presynaptic sites on somatostatin interneuron synapses; loss of Elfn1 leads to deficits in mGluR7 recruitment and impaired presynaptic plasticity, with missense mutations in ELFN1 clustering in the region required for mGluR7 recruitment.\",\n      \"method\": \"Immunocytochemistry/immunofluorescence, Elfn1 KO mice, electrophysiology of SOM interneuron synapses, patient mutation analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout with specific electrophysiological phenotype, localization studies, and human genetics validation\",\n      \"pmids\": [\"25047565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Elfn1 produces glutamate-independent (constitutive) activation of mGluR7 via presynaptic clustering at synapses onto cortical somatostatin neurons, reducing initial release probability and generating synaptic facilitation; this represents a deviation from the canonical autoreceptor role of group III mGluRs.\",\n      \"method\": \"Electrophysiology in cortical slices from WT and mGluR7/Elfn1 manipulated mice, pharmacological dissection with mGluR7 agonist/antagonist\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — electrophysiological epistasis with genetic and pharmacological tools, multiple synaptic preparations\",\n      \"pmids\": [\"30940718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"mGluR7 SUMOylation at Lys889 is required for stable surface expression; agonist treatment reduces SUMOylation; SENP1 (SUMO isopeptidase) reverses SUMOylation and increases mGluR7 internalization; Ser862 phosphorylation facilitates SUMO conjugation of mGluR7.\",\n      \"method\": \"SUMOylation assay in HEK293T cells, identification of SUMOylated mGluR7 in brain and cortical neurons, site-directed mutagenesis (Lys889), SENP1 overexpression, internalization assay in hippocampal neurons\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis, multiple cell systems including native neurons, confirmed in brain tissue\",\n      \"pmids\": [\"26631532\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Nedd4 E3 ubiquitin ligase and β-arrestins cooperate to ubiquitinate mGluR7 upon agonist stimulation: β-arrestins recruit Nedd4 to mGluR7, facilitating ubiquitination; Nedd4 and β-arrestins regulate constitutive and agonist-induced mGluR7 endocytosis, are required for mGluR7-dependent MAPK signaling in neurons, and Nedd4-mediated ubiquitination leads to mGluR7 degradation via both the proteasome and lysosomal pathways.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assays in heterologous cells and rat neurons, KD/KO with defined trafficking phenotypes, MAPK signaling readout\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in both heterologous and native neuronal systems\",\n      \"pmids\": [\"31373553\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"N-glycosylation of mGlu7 at four asparagine residues is essential for forward trafficking and surface expression; deglycosylated mGlu7 is retained in the ER and degraded via the autophagolysosomal pathway; N-glycosylation of mGlu7 also promotes its trans-synaptic interaction with Elfn1.\",\n      \"method\": \"Site-directed mutagenesis of glycosylation sites, ER retention assay, surface biotinylation, co-immunoprecipitation with Elfn1, autophagolysosomal pathway inhibitors\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis with multiple biochemical and cell biological readouts\",\n      \"pmids\": [\"32931036\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of human mGlu2 and mGlu7 homo- and heterodimers revealed: a subtype-dependent dimerization mode with a unique helix IV-mediated dimer interface in inactive mGlu2 important for limiting receptor activity; the mGlu7 subunit has a dominant role in controlling dimeric association and G-protein activation in the mGlu2-mGlu7 heterodimer.\",\n      \"method\": \"Cryo-electron microscopy structure determination, functional validation with chimeric receptor studies and G-protein activation assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with functional validation, high-impact publication\",\n      \"pmids\": [\"34135509\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The GRM7 I154T mutation (within the mGlu7 ligand-binding domain) causes significant loss of mGlu7 protein expression at the posttranscriptional level due to impaired protein folding/trafficking; mGlu7-I154T knock-in mice recapitulate mGlu7-KO phenotypes including reduced motor coordination, deficits in contextual fear learning, and seizures.\",\n      \"method\": \"Patient mutation identification, functional studies in HEK293A cells and knock-in mice, surface expression analysis, behavioral phenotyping\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mechanistic study combining cell biology, knock-in mouse model, and clinical validation\",\n      \"pmids\": [\"33476302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Pathogenic GRM7 mutations (I154T and R658W/T675K) lead to mGlu7 protein degradation; R658W/T675K abolishes mGlu7 surface expression; pathogenic mutations impair axon outgrowth through the MAPK-cAMP-PKA signaling pathway during early neuronal development, subsequently reducing presynaptic terminal number.\",\n      \"method\": \"Mutagenesis in heterologous cells and primary neurons, surface expression assay, axon outgrowth assay, MAPK/PKA pathway pharmacology, presynaptic terminal counting\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic pathway dissection with multiple mutations and pharmacological rescue experiments\",\n      \"pmids\": [\"33500274\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"mGlu7 activation decreases NMDAR-mediated currents and NR1 surface expression in basal forebrain cholinergic neurons via a mechanism involving cofilin-regulated actin dynamics; Aβ selectively impairs this mGlu7 regulation by increasing p21-activated kinase activity and decreasing cofilin-mediated actin depolymerization through a p75NTR-dependent mechanism.\",\n      \"method\": \"Whole-cell patch clamp, surface biotinylation, pharmacological dissection (p21-activated kinase inhibitors, cofilin pathway), p75NTR-selective inhibition, actin polymerization assays\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods establishing mechanistic pathway from mGlu7 activation to actin dynamics to NMDAR regulation\",\n      \"pmids\": [\"25297090\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"mGluR7 regulates embryonic neurogenesis by directly interacting with calmodulin (CaM), modulating CREB phosphorylation and YAP expression; GRM7 knockdown increases neural progenitor cell proliferation and decreases neuronal differentiation via CyclinD1 regulation; these defects are rescued by Creb or Yap knockdown.\",\n      \"method\": \"In utero electroporation (gain/loss of function), co-immunoprecipitation of GRM7 with CaM, Western blot for pCREB/YAP/CyclinD1, cortical progenitor proliferation assays\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic rescue epistasis and co-IP in single lab study\",\n      \"pmids\": [\"25921811\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Presynaptic mGlu7 receptors exist as heteroreceptors on GABAergic terminals in the mouse hippocampus; their activation (by AMN082 or L-AP4) inhibits GABA release via an adenylyl cyclase-dependent mechanism and they colocalize with syntaxin-1A and VGAT, but do not functionally cross-talk with co-localized GABAB autoreceptors.\",\n      \"method\": \"Synaptosomal superfusion assay, AMN082/MMPIP pharmacology, confocal microscopy colocalization, cAMP pathway inhibitors\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — synaptosomal functional assay with pharmacological characterization and colocalization, single lab\",\n      \"pmids\": [\"22564442\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The mGlu7 antagonist XAP044 binds within mGlu7's extracellular Venus flytrap domain (not the seven-transmembrane region) and blocks lateral amygdala LTP in an mGlu7-dependent manner; this was demonstrated by chimeric receptor studies showing XAP044's binding site is in the VFTD.\",\n      \"method\": \"Chimeric receptor studies in recombinant cells, LTP recordings in amygdala brain slices from WT and mGlu7-KO mice\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — chimeric receptor studies establishing binding domain, validated with genetic KO controls in slice physiology\",\n      \"pmids\": [\"24596089\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"mGlu7 forms heterodimers with mGlu8 at hippocampal Schaffer collateral-CA1 synapses; mGlu7/8 heterodimers display distinct pharmacology from mGlu7/7 homodimers — ADX71743 blocks both, while MMPIP only antagonizes the homodimer — providing evidence that mGlu7/8 heterodimers are the functionally predominant form at SC-CA1 synapses.\",\n      \"method\": \"Complemented donor acceptor-resonance energy transfer (CADET) for heterodimer detection in vitro, SC-CA1 electrophysiology in brain slices, pharmacological dissection with selective NAMs\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — novel FRET-based heterodimer detection corroborated by slice electrophysiology, single lab\",\n      \"pmids\": [\"36063995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"mGluR7 is localized presynaptically in axon terminals of primary afferent fibers in laminae I and II of the spinal dorsal horn, where it functions as an autoreceptor; dorsal rhizotomy reduces immunoreactivity in these laminae, confirming the presynaptic location on afferent terminals.\",\n      \"method\": \"Immunohistochemistry with specific anti-mGluR7 antibody, dorsal rhizotomy lesion experiment, electron microscopy\",\n      \"journal\": \"Neuroscience letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — lesion experiment directly confirming presynaptic localization on afferent terminals\",\n      \"pmids\": [\"8787837\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"In the rat retina, mGluR7 is localized both presynaptically (as an autoreceptor at cone bipolar cell ribbon synapses) and postsynaptically in amacrine cells; the presynaptic mGluR7 is restricted to one half of the active zone, facing only one of two postsynaptic processes.\",\n      \"method\": \"Immunofluorescence with specific antiserum, electron microscopy, double-labeling\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — electron microscopy for ultrastructural localization with double-labeling controls\",\n      \"pmids\": [\"8764662\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"mGluR7a is expressed at high levels in the presynaptic active zones of GABAergic terminals that innervate somatostatin/mGluR1α-positive interneurons in the hippocampal stratum oriens; VIP-positive GABAergic terminals on these cells express mGluR7a at >90%; GABAergic terminals on pyramidal cells are mGluR7a-negative, demonstrating target-cell-specific expression.\",\n      \"method\": \"Immunocytochemistry (electron microscopy), double-labeling with GAD/GABA and cell-type markers, quantitative analysis\",\n      \"journal\": \"The European journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — quantitative ultrastructural analysis with multiple cell-type markers in a rigorous multi-site study\",\n      \"pmids\": [\"12823458\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The C-terminal splice variant GluR7a is highly expressed at the plasma membrane through a stretch of positively charged amino acids, while GluR7b is mostly retained in the ER; GluR7a promotes surface expression of ER-retained splice variants when assembled into heteromeric kainate receptors.\",\n      \"method\": \"Trafficking studies in hippocampal neurons from WT and KAR mutant mice, surface biotinylation, confocal microscopy, mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — neuronal trafficking studies with genetic and mutagenesis tools, single lab\",\n      \"pmids\": [\"15805114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"mGlu7 is the predominant mGlu receptor expressed presynaptically at Schaffer collateral-CA1 synapses in adult mice; its activation on GABAergic interneurons is necessary for LTP induction at these synapses; MECP2 mutations reduce mGlu7 protein expression, and positive allosteric modulation of mGlu7 restores LTP and improves learning/memory phenotypes in RTT model mice.\",\n      \"method\": \"Western blot of RTT patient and mouse tissue, hippocampal slice LTP recordings, PAM pharmacology, behavioral tests\",\n      \"journal\": \"Science translational medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic pathway established with patient tissue, genetic model, and pharmacological rescue\",\n      \"pmids\": [\"28814546\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"mGluR7/GRM7 is a presynaptic Gi/o-coupled class C GPCR that primarily inhibits neurotransmitter release by suppressing P/Q-type Ca2+ channels and reducing cAMP; its surface expression and synaptic localization are tightly regulated by PKC/PKA/PKG phosphorylation at Ser862 (which competitively controls calmodulin vs. PICK1 binding), dephosphorylation by PP1γ1, SUMOylation at Lys889, Nedd4/β-arrestin-mediated ubiquitination and clathrin-independent (Arf6-dependent) endocytosis, and N-glycosylation-dependent ER export; it acts as a metaplastic switch at select hippocampal synapses, can alternatively couple to PLC/DAG/Munc13-1 to potentiate release, functions as a coincidence detector of Ca2+ influx and glutamate via calmodulin, is recruited in trans to somatostatin interneuron synapses by the postsynaptic organizer Elfn1 to constitutively suppress release probability, and forms both homodimers and pharmacologically distinct mGlu7/mGlu8 heterodimers; its cryo-EM structure reveals a unique subtype-specific dimerization mode in which the mGlu7 subunit dominantly controls G-protein activation in heterodimers.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"GRM7 encodes mGluR7, a presynaptic class C G protein-coupled receptor that functions as a central regulator of neurotransmitter release probability and synaptic plasticity at glutamatergic and GABAergic terminals throughout the brain. Canonically coupled to Gi/o, mGluR7 inhibits release by suppressing P/Q-type Ca²⁺ channels and reducing cAMP, but prolonged activation engages an alternative PLC/DAG pathway that recruits Munc13-1 to potentiate release [PMID:8288585, PMID:17650109, PMID:20375012]. Surface expression is governed by a convergent regulatory network at Ser862: PKC/PKA/PKG phosphorylation displaces calmodulin to promote PICK1 binding and surface retention, while PP1γ1-mediated dephosphorylation, SUMO conjugation at Lys889, Nedd4/β-arrestin-dependent ubiquitination, and clathrin-independent Arf6-mediated endocytosis dynamically control receptor density [PMID:18549785, PMID:23612982, PMID:26631532, PMID:31373553, PMID:16890965]. The postsynaptic organizer Elfn1 recruits mGluR7 in trans to somatostatin-interneuron synapses to constitutively suppress release probability, and pathogenic GRM7 mutations (e.g., I154T) that destabilize protein folding cause neurodevelopmental phenotypes including seizures and impaired motor coordination in knock-in mice [PMID:25047565, PMID:30940718, PMID:33476302].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Establishing the fundamental signaling identity of mGluR7 as a Gi/o-coupled receptor that inhibits cAMP accumulation answered the basic question of which second-messenger pathway this orphan group III mGluR engages.\",\n      \"evidence\": \"cAMP accumulation assay in mGluR7-transfected CHO cells stimulated with L-AP4\",\n      \"pmids\": [\"8288585\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effectors at native synapses unknown\", \"No information on splice-variant-specific signaling\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Ultrastructural localization of mGluR7 to presynaptic active zones — and its asymmetric positioning at retinal ribbon synapses — established that this receptor occupies a privileged position to sense synaptically released glutamate as an autoreceptor.\",\n      \"evidence\": \"Immunoelectron microscopy in spinal cord (with dorsal rhizotomy lesion) and retina\",\n      \"pmids\": [\"8787837\", \"8764662\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of target-cell-specific presynaptic sorting unknown\", \"Functional consequence of asymmetric active-zone positioning not tested\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Identification of PICK1 as a PDZ-domain partner that scaffolds PKCα to mGluR7a and modulates its phosphorylation revealed the first molecular mechanism for regulating mGluR7 signaling at the receptor level.\",\n      \"evidence\": \"Yeast two-hybrid, GST pulldown, co-IP from brain, in vitro phosphorylation assay\",\n      \"pmids\": [\"11007882\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of PICK1-PKCα complex not yet shown\", \"Phosphorylation site identity not established\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapping the convergent phosphorylation site Ser862 and showing that PKC, PKA, and PKG all phosphorylate this residue to displace calmodulin defined a single molecular switch governing mGluR7 C-terminal protein interactions.\",\n      \"evidence\": \"In vitro kinase assays, Ser862 mutagenesis, CaM binding assay, GIRK recordings\",\n      \"pmids\": [\"12021391\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which kinase acts in vivo at specific synapses unclear\", \"Phosphatase identity unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Quantitative ultrastructural demonstration that mGluR7a is selectively concentrated at GABAergic terminals innervating somatostatin interneurons — but absent from terminals on pyramidal cells — established the target-cell-specific expression principle later explained by trans-synaptic recruitment.\",\n      \"evidence\": \"Immunoelectron microscopy with GAD/cell-type markers in hippocampal stratum oriens\",\n      \"pmids\": [\"12823458\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of target-cell-dependent sorting unknown\", \"Whether pattern extends to cortex not tested\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Showing that mGluR7 acts as a metaplastic switch at mossy fiber–interneuron synapses — where its activation induces LTD and subsequent internalization unmasks LTP — linked receptor trafficking to bidirectional synaptic plasticity for the first time.\",\n      \"evidence\": \"Hippocampal slice electrophysiology with pharmacological manipulation and surface-expression immunofluorescence\",\n      \"pmids\": [\"15820696\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Internalization pathway not identified\", \"Whether metaplastic switch occurs at other synapse types unknown\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Discovery that mGluR7 endocytosis proceeds through a clathrin-independent, Arf6-positive pathway — distinct from classical NMDAR endocytosis — identified an unconventional trafficking route that governs receptor surface levels both constitutively and upon agonist stimulation.\",\n      \"evidence\": \"Immunofluorescence and pHluorin live imaging in neurons, dominant-negative Arf6\",\n      \"pmids\": [\"16914173\", \"16890965\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Recycling vs. degradation fate after Arf6 endocytosis not resolved\", \"Adaptor proteins linking mGluR7 to Arf6 pathway unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Pharmacological dissection at hippocampal synaptosomes resolved that the primary mechanism of mGluR7-mediated release inhibition is Gi/o-dependent suppression of P/Q-type Ca²⁺ channels, with cAMP reduction as a secondary modulatory pathway.\",\n      \"evidence\": \"Glutamate release from rat hippocampal synaptosomes with selective Ca²⁺ channel blockers, pertussis toxin, kinase inhibitors\",\n      \"pmids\": [\"17650109\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct G-protein–Ca²⁺ channel coupling vs. membrane-delimited mechanism not distinguished\", \"Contribution at individual synapse types unresolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Two contemporaneous studies resolved complementary aspects of mGluR7 regulation: Ser862 phosphorylation by PKC controls a competitive switch between calmodulin and PICK1 binding to govern surface expression (validated in PICK1-KO mice), while intracellular Ca²⁺ potentiates mGluR7 signaling, establishing a coincidence-detection mechanism for glutamate and Ca²⁺ influx.\",\n      \"evidence\": \"Phospho-specific antibodies, Ser862 mutagenesis, PICK1-KO slice physiology; Xenopus oocyte GIRK readout with channelrhodopsin-controlled Ca²⁺\",\n      \"pmids\": [\"18549785\", \"18599484\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CaM mediates the Ca²⁺ coincidence detection in vivo not proven\", \"Relative contributions of CaM vs. PICK1 binding states at native synapses unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The demonstration that prolonged mGlu7 activation switches from inhibitory Gi/o coupling to a PLC/DAG pathway that translocates Munc13-1 and potentiates release revealed a dual-mode signaling capacity unprecedented for group III mGluRs.\",\n      \"evidence\": \"Cortical synaptosomal release assay with PLC inhibitors, calphostin C, Munc13-1 subcellular fractionation\",\n      \"pmids\": [\"20375012\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"G-protein identity for the PLC pathway not identified\", \"Duration threshold for pathway switching not defined\", \"In vivo relevance at specific circuit level unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identification of PP1γ1 as the phosphatase that dephosphorylates Ser862 completed the kinase–phosphatase cycle controlling mGluR7 surface expression, while parallel work delineated biased GRK2/GRK4 and β-arrestin1/2 signaling through ERK and JNK pathways.\",\n      \"evidence\": \"PP1 isoform-specific knockdown and inhibitors in neurons with phospho-Ser862 readout; GRK/arrestin overexpression/knockdown with pathway-selective signaling assays\",\n      \"pmids\": [\"23612982\", \"24148810\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Activity-dependent regulation of PP1γ1 at mGluR7-containing synapses not explored\", \"GRK/arrestin findings from recombinant system only\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Discovery that the postsynaptic adhesion molecule Elfn1 recruits mGluR7 in trans to somatostatin-interneuron synapses provided the molecular explanation for the long-observed target-cell-specific presynaptic expression of mGluR7 and linked it to synapse-type-specific plasticity rules.\",\n      \"evidence\": \"Elfn1-KO mice, electrophysiology of SOM interneuron synapses, immunocytochemistry, human mutation analysis\",\n      \"pmids\": [\"25047565\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct biophysical interaction interface not mapped\", \"Whether other postsynaptic organizers contribute unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"SUMOylation at Lys889 was identified as a novel post-translational modification stabilizing mGluR7 surface expression, with Ser862 phosphorylation facilitating SUMO conjugation, revealing crosstalk between phosphorylation and SUMOylation in receptor trafficking.\",\n      \"evidence\": \"SUMO assay in HEK293T cells and brain tissue, Lys889 mutagenesis, SENP1 overexpression, neuronal internalization assay\",\n      \"pmids\": [\"26631532\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SUMO E3 ligase identity unknown\", \"Functional consequence at synaptic level not tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Two key advances solidified the mGluR7 trafficking and signaling framework: Nedd4/β-arrestin-mediated ubiquitination was shown to drive both constitutive and agonist-induced endocytosis and degradation, while Elfn1 was demonstrated to produce constitutive (glutamate-independent) mGluR7 activation that tonically reduces release probability at somatostatin-interneuron synapses.\",\n      \"evidence\": \"Ubiquitination assays and trafficking in neurons with Nedd4/β-arrestin manipulation; cortical slice electrophysiology in Elfn1/mGluR7-manipulated mice\",\n      \"pmids\": [\"31373553\", \"30940718\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ubiquitin chain type (K48 vs K63) on mGluR7 not determined\", \"Relative contribution of proteasomal vs. lysosomal degradation in vivo unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"N-glycosylation at four asparagine residues was shown to be essential for ER export and surface expression of mGlu7, with deglycosylated receptor shunted to autophagolysosomal degradation; glycosylation also facilitates trans-synaptic Elfn1 interaction.\",\n      \"evidence\": \"Glycosylation-site mutagenesis, surface biotinylation, ER retention assay, co-IP with Elfn1, autophagolysosome inhibitors\",\n      \"pmids\": [\"32931036\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific glycan structures not characterized\", \"Whether glycosylation defects contribute to disease mutations unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Cryo-EM structures of mGlu7 homodimers and mGlu2/mGlu7 heterodimers revealed a subtype-specific dimerization mode and demonstrated that the mGlu7 subunit dominantly controls G-protein activation in heterodimers, providing the first atomic-level framework for understanding mGluR7 function and heterodimer pharmacology.\",\n      \"evidence\": \"Cryo-EM structure determination with chimeric receptor and G-protein activation assays\",\n      \"pmids\": [\"34135509\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of mGlu7/mGlu8 heterodimer, the predominant form at SC-CA1 synapses\", \"Agonist-bound active-state structure not yet available\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Pathogenic GRM7 mutations (I154T, R658W/T675K) were shown to cause mGlu7 protein destabilization and loss of surface expression, impairing MAPK-cAMP-PKA signaling and axon outgrowth; I154T knock-in mice recapitulate neurodevelopmental phenotypes including seizures, establishing GRM7 as a neurodevelopmental disease gene.\",\n      \"evidence\": \"Patient mutation identification, functional studies in HEK293A and neurons, knock-in mouse behavioral phenotyping, pharmacological rescue\",\n      \"pmids\": [\"33476302\", \"33500274\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full allelic series and genotype-phenotype correlation in humans incomplete\", \"Whether pharmacological chaperones can rescue folding-deficient mutants unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Detection of functional mGlu7/mGlu8 heterodimers at Schaffer collateral-CA1 synapses with pharmacology distinct from homodimers raised the possibility that heterodimeric assemblies are the physiologically predominant signaling species at major hippocampal synapses.\",\n      \"evidence\": \"CADET FRET-based heterodimer detection in vitro, SC-CA1 slice electrophysiology with NAM pharmacology\",\n      \"pmids\": [\"36063995\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Heterodimer stoichiometry at native synapses not quantified\", \"Selective genetic tools to dissect heterodimer vs. homodimer function in vivo lacking\", \"Finding from single lab awaits independent replication\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: how the dual-mode Gi/o-inhibitory versus PLC-facilitatory signaling switch is temporally and spatially regulated at individual synapses in vivo; the structural basis for mGlu7/mGlu8 heterodimer signaling; and whether pharmacological chaperones or allosteric modulators can rescue pathogenic folding-deficient GRM7 mutants for therapeutic purposes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No in vivo signaling-mode switching data at single-synapse resolution\", \"No mGlu7/mGlu8 heterodimer structure\", \"No pharmacological rescue strategy for loss-of-function mutations tested in vivo\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 9]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [7, 8, 22]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [3, 5, 14, 25, 26]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [16, 28]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 7, 8, 9, 11]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [2, 7, 20, 29]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [5, 6, 10, 14, 15]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [3, 4, 16]}\n    ],\n    \"complexes\": [\n      \"mGlu7/mGlu7 homodimer\",\n      \"mGlu7/mGlu8 heterodimer\",\n      \"mGlu2/mGlu7 heterodimer\"\n    ],\n    \"partners\": [\n      \"PICK1\",\n      \"ELFN1\",\n      \"NEDD4\",\n      \"ARRB1\",\n      \"ARRB2\",\n      \"CALM1\",\n      \"GRM8\",\n      \"PPP1CC\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}