{"gene":"LRRTM4","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":2013,"finding":"LRRTM4 binds heparan sulfate proteoglycans (HSPGs) as its presynaptic binding partners, and HSPGs are required to mediate the synaptogenic activity of LRRTM4 — distinct from the neurexin binding used by other LRRTM family members.","method":"Heterologous synapse-formation assays, co-immunoprecipitation, heparan sulfate dependency experiments, LRRTM4 knockout mouse analysis","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Strong — replicated independently in two simultaneous papers (PMID:23911104 and PMID:23911103) using multiple orthogonal methods including synapse-formation assays, co-IP, and in vivo knockout","pmids":["23911104","23911103"],"is_preprint":false},{"year":2013,"finding":"Glypican 4 (GPC4), a heparan sulfate proteoglycan, is a direct receptor for LRRTM4 that interacts in an HS-dependent manner. GPC4 localizes to presynaptic membranes and LRRTM4 to postsynaptic membranes of excitatory synapses, forming a trans-synaptic interaction: LRRTM4 triggers GPC4 clustering in contacting axons and GPC4 induces LRRTM4 clustering in contacting dendrites.","method":"Unbiased proteomics-based receptor identification, co-immunoprecipitation, heterologous cell clustering assays, in vivo localization experiments, HS-dependency assays","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — unbiased proteomics identification plus multiple orthogonal functional assays (clustering, co-IP, in vivo), replicated across two independent labs","pmids":["23911103","23911104"],"is_preprint":false},{"year":2013,"finding":"LRRTM4 is localized specifically at excitatory postsynaptic sites on dentate gyrus granule cells within the hippocampus. LRRTM4 knockout selectively reduces excitatory synapse density and function in dentate gyrus granule cells but not CA1 pyramidal cells, and impairs activity-regulated AMPA receptor trafficking.","method":"Immunolocalization, LRRTM4 knockout mouse electrophysiology, synapse density quantification, AMPA receptor trafficking assays","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean knockout mouse with multiple functional readouts (electrophysiology, synapse density, AMPA receptor trafficking), cell-type specificity confirmed","pmids":["23911104"],"is_preprint":false},{"year":2015,"finding":"Presynaptic PTPσ (receptor protein tyrosine phosphatase) binds glypican 4 (GPC-4) in an HS-dependent manner with nanomolar affinity via its Ig domains, and forms a tripartite complex with GPC-4 and LRRTM4 in rat brains. PTPσ knockdown significantly reduces the synaptogenic activity of LRRTM4 in heterologous synapse-formation assays, and this rescue requires HS-binding-competent PTPσ.","method":"Co-immunoprecipitation from rat brain, heterologous synapse-formation assays, neuronal knockdown, binding affinity measurements, HS-binding-defective mutant rescue experiments","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP from brain tissue, functional synapse-formation assays, mutagenesis of HS-binding domain, single lab with multiple orthogonal methods","pmids":["25624497"],"is_preprint":false},{"year":2020,"finding":"In the mouse retina, LRRTM4 is enriched at GABAergic synapses on axon terminals of rod bipolar cells (RBCs), not at glutamatergic excitatory synapses. Knockout of LRRTM4 reduces RBC axonal GABA-A and GABA-C receptor clustering, disrupts presynaptic inhibition onto RBC terminals, and perturbs stereotyped synaptic ribbon arrangement (shifting from normal dyads toward monads and triads).","method":"Immunofluorescence localization, LRRTM4 knockout mouse electrophysiology, GABA receptor clustering assays, synaptic ribbon morphology analysis","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Moderate — clean knockout mouse with multiple orthogonal readouts (electrophysiology, receptor clustering, synapse morphology) in a focused single-lab study","pmids":["31974009"],"is_preprint":false},{"year":2020,"finding":"LRRTM4 localizes to rod bipolar cell dendritic tips (not ON-cone bipolar cell dendritic tips) and the extracellular domain of LRRTM4 engages in heparan sulfate-dependent binding with pikachurin, implicating LRRTM4 in the GPR179-pikachurin-dystroglycan transsynaptic complex at rod photoreceptor synapses. CRISPR/Cas9 knockout of LRRTM4 in bipolar cells partially reduces GPR179 accumulation at dendritic tips but does not affect TRPM1.","method":"Immunolocalization with two antibodies, CRISPR/Cas9 somatic knockout via subretinal electroporation, heterologous protein binding assay with HS-dependency test","journal":"The Journal of comparative neurology","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — direct binding assay with HS-dependency plus CRISPR knockout with specific protein readouts, single lab","pmids":["32390181"],"is_preprint":false}],"current_model":"LRRTM4 is a postsynaptic adhesion molecule that, unlike other LRRTM family members which bind neurexins, uses heparan sulfate proteoglycans (specifically glypican 4) as its presynaptic trans-synaptic ligand; this LRRTM4–GPC4 interaction is bridged by presynaptic PTPσ in an HS-dependent manner to organize excitatory synapse development on dentate gyrus granule cells and, in the retina, organizes GABAergic inhibitory synapses and synaptic ribbon dyad arrangements at rod bipolar cell axon terminals via HS-dependent interactions with pikachurin."},"narrative":{"mechanistic_narrative":"LRRTM4 is a postsynaptic adhesion molecule that organizes synapse development through heparan sulfate proteoglycan (HSPG) ligands rather than the neurexins used by other LRRTM family members [PMID:23911104, PMID:23911103]. Its principal presynaptic partner is glypican-4 (GPC4), which engages the LRRTM4 ectodomain in a heparan sulfate-dependent manner to form a trans-synaptic adhesion pair: LRRTM4 clusters GPC4 in contacting axons while GPC4 reciprocally clusters LRRTM4 in dendrites [PMID:23911103, PMID:23911104]. This bidirectional interaction is consolidated into a tripartite complex by presynaptic receptor protein tyrosine phosphatase PTPσ, which binds GPC4 with nanomolar affinity via its Ig domains and is required for full LRRTM4 synaptogenic activity in an HS-dependent fashion [PMID:25624497]. In the hippocampus, LRRTM4 resides at excitatory postsynaptic sites of dentate gyrus granule cells, where it is selectively required for excitatory synapse density, function, and activity-regulated AMPA receptor trafficking [PMID:23911104]. In the retina, LRRTM4 adopts distinct roles: it organizes GABAergic inhibitory synapses and stereotyped synaptic ribbon dyad arrangements at rod bipolar cell axon terminals [PMID:31974009], and at rod bipolar cell dendritic tips it engages pikachurin through HS-dependent binding within the GPR179–pikachurin–dystroglycan trans-synaptic complex [PMID:32390181].","teleology":[{"year":2013,"claim":"Established that LRRTM4 uses a fundamentally different presynaptic ligand class than other LRRTMs, defining HSPGs rather than neurexins as the partners required for its synaptogenic activity.","evidence":"Heterologous synapse-formation assays, co-IP, HS-dependency experiments, and LRRTM4 knockout mice","pmids":["23911104","23911103"],"confidence":"High","gaps":["Which specific HSPG species mediate the activity was not fully resolved at this stage","The glycan modifications required were not enumerated"]},{"year":2013,"claim":"Identified glypican-4 as the direct HSPG receptor for LRRTM4, showing a reciprocal trans-synaptic clustering relationship that organizes excitatory synapse assembly.","evidence":"Unbiased proteomics receptor identification, co-IP, heterologous clustering assays, and in vivo localization","pmids":["23911103","23911104"],"confidence":"High","gaps":["Whether other glypicans or HSPGs substitute for GPC4 was not established","Downstream postsynaptic signaling from LRRTM4 clustering was not defined"]},{"year":2013,"claim":"Mapped the in vivo functional requirement for LRRTM4 to dentate gyrus granule cells, demonstrating cell-type-selective control of excitatory synapse number, function, and AMPA receptor trafficking.","evidence":"Immunolocalization and knockout mouse electrophysiology, synapse density, and AMPA trafficking assays","pmids":["23911104"],"confidence":"High","gaps":["The mechanism linking LRRTM4 to activity-regulated AMPA receptor trafficking was not delineated","Why CA1 pyramidal cells are spared was not explained"]},{"year":2015,"claim":"Resolved how the LRRTM4–GPC4 pair couples to presynaptic machinery by identifying PTPσ as an HS-dependent bridging partner that forms a tripartite complex required for full synaptogenic activity.","evidence":"Reciprocal co-IP from rat brain, heterologous synapse-formation assays, neuronal knockdown, affinity measurements, and HS-binding-defective mutant rescue","pmids":["25624497"],"confidence":"High","gaps":["The stoichiometry and structural architecture of the LRRTM4–GPC4–PTPσ complex were not determined","Whether PTPσ phosphatase activity contributes was not addressed"]},{"year":2020,"claim":"Extended LRRTM4 function beyond excitatory synapses by showing it organizes GABAergic inhibitory synapses and synaptic ribbon dyad architecture at retinal rod bipolar cell axon terminals.","evidence":"Immunofluorescence, knockout mouse electrophysiology, GABA receptor clustering, and ribbon morphology analysis","pmids":["31974009"],"confidence":"High","gaps":["The presynaptic ligand at these inhibitory terminals was not identified","How a single adhesion molecule directs ribbon dyad geometry mechanistically was not resolved"]},{"year":2020,"claim":"Implicated LRRTM4 at the opposite (dendritic) pole of the rod synapse, showing HS-dependent binding to pikachurin and a role in GPR179 accumulation within an established trans-synaptic complex.","evidence":"Immunolocalization with two antibodies, CRISPR/Cas9 somatic knockout, and heterologous binding assay with HS-dependency test","pmids":["32390181"],"confidence":"Medium","gaps":["The partial GPR179 reduction indicates additional redundant components not identified","Direct versus indirect nature of the pikachurin interaction within the larger complex was not fully dissected"]},{"year":null,"claim":"The intracellular signaling and structural basis by which LRRTM4 adhesion is transduced into synapse-type-specific assembly across hippocampal and retinal contexts remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of the LRRTM4–GPC4–PTPσ complex","Postsynaptic cytoplasmic effectors of LRRTM4 unidentified","Mechanism distinguishing excitatory versus inhibitory/ribbon organization unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[0,1,4]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0,1,3,5]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,2,4,5]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[2,4]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[1,2]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[1,3]}],"complexes":["LRRTM4-GPC4-PTPσ tripartite trans-synaptic complex","GPR179-pikachurin-dystroglycan trans-synaptic complex"],"partners":["GPC4","PTPRS","PIKACHURIN","GPR179"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q86VH4","full_name":"Leucine-rich repeat transmembrane neuronal protein 4","aliases":[],"length_aa":590,"mass_kda":67.2,"function":"May play a role in the development and maintenance of the vertebrate nervous system. Exhibits strong synaptogenic activity, restricted to excitatory presynaptic differentiation (By similarity)","subcellular_location":"Cell membrane; Postsynaptic cell membrane","url":"https://www.uniprot.org/uniprotkb/Q86VH4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LRRTM4","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/LRRTM4","total_profiled":1310},"omim":[{"mim_id":"610870","title":"LEUCINE-RICH REPEAT TRANSMEMBRANE PROTEIN 4; LRRTM4","url":"https://www.omim.org/entry/610870"},{"mim_id":"610868","title":"LEUCINE-RICH REPEAT TRANSMEMBRANE PROTEIN 2; LRRTM2","url":"https://www.omim.org/entry/610868"},{"mim_id":"610867","title":"LEUCINE-RICH REPEAT TRANSMEMBRANE PROTEIN 1: LRRTM1","url":"https://www.omim.org/entry/610867"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"brain","ntpm":11.3},{"tissue":"retina","ntpm":6.1}],"url":"https://www.proteinatlas.org/search/LRRTM4"},"hgnc":{"alias_symbol":["FLJ12568"],"prev_symbol":[]},"alphafold":{"accession":"Q86VH4","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q86VH4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q86VH4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q86VH4-F1-predicted_aligned_error_v6.png","plddt_mean":74.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LRRTM4","jax_strain_url":"https://www.jax.org/strain/search?query=LRRTM4"},"sequence":{"accession":"Q86VH4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q86VH4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q86VH4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q86VH4"}},"corpus_meta":[{"pmid":"23911104","id":"PMC_23911104","title":"An LRRTM4-HSPG complex mediates excitatory synapse development on dentate gyrus granule cells.","date":"2013","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/23911104","citation_count":154,"is_preprint":false},{"pmid":"23911103","id":"PMC_23911103","title":"Unbiased discovery of glypican as a receptor for LRRTM4 in regulating excitatory synapse development.","date":"2013","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/23911103","citation_count":134,"is_preprint":false},{"pmid":"25624497","id":"PMC_25624497","title":"PTPσ functions as a presynaptic receptor for the glypican-4/LRRTM4 complex and is essential for excitatory synaptic transmission.","date":"2015","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/25624497","citation_count":86,"is_preprint":false},{"pmid":"31974009","id":"PMC_31974009","title":"LRRTM4: A Novel Regulator of Presynaptic Inhibition and Ribbon Synapse Arrangements of Retinal Bipolar Cells.","date":"2020","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/31974009","citation_count":33,"is_preprint":false},{"pmid":"32390181","id":"PMC_32390181","title":"LRRTM4 is a member of the transsynaptic complex between rod photoreceptors and bipolar cells.","date":"2020","source":"The Journal of comparative neurology","url":"https://pubmed.ncbi.nlm.nih.gov/32390181","citation_count":15,"is_preprint":false},{"pmid":"31362389","id":"PMC_31362389","title":"LRRTM4 and PCSK5 Genetic Polymorphisms as Markers for Cognitive Impairment in A Hypotensive Aging Population: A Genome-Wide Association Study in Taiwan.","date":"2019","source":"Journal of clinical medicine","url":"https://pubmed.ncbi.nlm.nih.gov/31362389","citation_count":13,"is_preprint":false},{"pmid":"31850962","id":"PMC_31850962","title":"Cerebrolysin enhances the expression of the synaptogenic protein LRRTM4 in the hippocampus and improves learning and memory in senescent rats.","date":"2020","source":"Behavioural pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/31850962","citation_count":10,"is_preprint":false},{"pmid":"35052406","id":"PMC_35052406","title":"LRRTM4 Terminal Exon Duplicated in Family with Tourette Syndrome, Autism and ADHD.","date":"2021","source":"Genes","url":"https://pubmed.ncbi.nlm.nih.gov/35052406","citation_count":9,"is_preprint":false},{"pmid":"37468908","id":"PMC_37468908","title":"Lnc-LRRTM4 promotes proliferation, metastasis and EMT of colorectal cancer through activating LRRTM4 transcription.","date":"2023","source":"Cancer cell international","url":"https://pubmed.ncbi.nlm.nih.gov/37468908","citation_count":7,"is_preprint":false},{"pmid":"29760528","id":"PMC_29760528","title":"LRRTM4-C538Y novel gene mutation is associated with hereditary macular degeneration with novel dysfunction of ON-type bipolar cells.","date":"2018","source":"Journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/29760528","citation_count":6,"is_preprint":false},{"pmid":"38090526","id":"PMC_38090526","title":"Adjuvant crizotinib treatment selected by patient-derived organoids in a patient with stage IIIA adenocarcinoma with novel LRRTM4-ALK fusion: a case report.","date":"2023","source":"Translational lung cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/38090526","citation_count":4,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.05.12.653402","title":"Genetic disruption of leucine rich repeat transmembrane protein 4 like 1 induces a pro-social behavioural phenotype in zebrafish","date":"2025-05-14","source":"bioRxiv","url":"https://doi.org/10.1101/2025.05.12.653402","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7649,"output_tokens":1839,"usd":0.025266,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8684,"output_tokens":2642,"usd":0.054735,"stage2_stop_reason":"end_turn"},"total_usd":0.080001,"stage1_batch_id":"msgbatch_01BbTCMyrRPyhtEq6b1MdwCN","stage2_batch_id":"msgbatch_011dEed5bbxYvTAaQWKJevxu","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2013,\n      \"finding\": \"LRRTM4 binds heparan sulfate proteoglycans (HSPGs) as its presynaptic binding partners, and HSPGs are required to mediate the synaptogenic activity of LRRTM4 — distinct from the neurexin binding used by other LRRTM family members.\",\n      \"method\": \"Heterologous synapse-formation assays, co-immunoprecipitation, heparan sulfate dependency experiments, LRRTM4 knockout mouse analysis\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — replicated independently in two simultaneous papers (PMID:23911104 and PMID:23911103) using multiple orthogonal methods including synapse-formation assays, co-IP, and in vivo knockout\",\n      \"pmids\": [\"23911104\", \"23911103\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Glypican 4 (GPC4), a heparan sulfate proteoglycan, is a direct receptor for LRRTM4 that interacts in an HS-dependent manner. GPC4 localizes to presynaptic membranes and LRRTM4 to postsynaptic membranes of excitatory synapses, forming a trans-synaptic interaction: LRRTM4 triggers GPC4 clustering in contacting axons and GPC4 induces LRRTM4 clustering in contacting dendrites.\",\n      \"method\": \"Unbiased proteomics-based receptor identification, co-immunoprecipitation, heterologous cell clustering assays, in vivo localization experiments, HS-dependency assays\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — unbiased proteomics identification plus multiple orthogonal functional assays (clustering, co-IP, in vivo), replicated across two independent labs\",\n      \"pmids\": [\"23911103\", \"23911104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"LRRTM4 is localized specifically at excitatory postsynaptic sites on dentate gyrus granule cells within the hippocampus. LRRTM4 knockout selectively reduces excitatory synapse density and function in dentate gyrus granule cells but not CA1 pyramidal cells, and impairs activity-regulated AMPA receptor trafficking.\",\n      \"method\": \"Immunolocalization, LRRTM4 knockout mouse electrophysiology, synapse density quantification, AMPA receptor trafficking assays\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean knockout mouse with multiple functional readouts (electrophysiology, synapse density, AMPA receptor trafficking), cell-type specificity confirmed\",\n      \"pmids\": [\"23911104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Presynaptic PTPσ (receptor protein tyrosine phosphatase) binds glypican 4 (GPC-4) in an HS-dependent manner with nanomolar affinity via its Ig domains, and forms a tripartite complex with GPC-4 and LRRTM4 in rat brains. PTPσ knockdown significantly reduces the synaptogenic activity of LRRTM4 in heterologous synapse-formation assays, and this rescue requires HS-binding-competent PTPσ.\",\n      \"method\": \"Co-immunoprecipitation from rat brain, heterologous synapse-formation assays, neuronal knockdown, binding affinity measurements, HS-binding-defective mutant rescue 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 — reciprocal co-IP from brain tissue, functional synapse-formation assays, mutagenesis of HS-binding domain, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"25624497\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In the mouse retina, LRRTM4 is enriched at GABAergic synapses on axon terminals of rod bipolar cells (RBCs), not at glutamatergic excitatory synapses. Knockout of LRRTM4 reduces RBC axonal GABA-A and GABA-C receptor clustering, disrupts presynaptic inhibition onto RBC terminals, and perturbs stereotyped synaptic ribbon arrangement (shifting from normal dyads toward monads and triads).\",\n      \"method\": \"Immunofluorescence localization, LRRTM4 knockout mouse electrophysiology, GABA receptor clustering assays, synaptic ribbon morphology analysis\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean knockout mouse with multiple orthogonal readouts (electrophysiology, receptor clustering, synapse morphology) in a focused single-lab study\",\n      \"pmids\": [\"31974009\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"LRRTM4 localizes to rod bipolar cell dendritic tips (not ON-cone bipolar cell dendritic tips) and the extracellular domain of LRRTM4 engages in heparan sulfate-dependent binding with pikachurin, implicating LRRTM4 in the GPR179-pikachurin-dystroglycan transsynaptic complex at rod photoreceptor synapses. CRISPR/Cas9 knockout of LRRTM4 in bipolar cells partially reduces GPR179 accumulation at dendritic tips but does not affect TRPM1.\",\n      \"method\": \"Immunolocalization with two antibodies, CRISPR/Cas9 somatic knockout via subretinal electroporation, heterologous protein binding assay with HS-dependency test\",\n      \"journal\": \"The Journal of comparative neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — direct binding assay with HS-dependency plus CRISPR knockout with specific protein readouts, single lab\",\n      \"pmids\": [\"32390181\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LRRTM4 is a postsynaptic adhesion molecule that, unlike other LRRTM family members which bind neurexins, uses heparan sulfate proteoglycans (specifically glypican 4) as its presynaptic trans-synaptic ligand; this LRRTM4–GPC4 interaction is bridged by presynaptic PTPσ in an HS-dependent manner to organize excitatory synapse development on dentate gyrus granule cells and, in the retina, organizes GABAergic inhibitory synapses and synaptic ribbon dyad arrangements at rod bipolar cell axon terminals via HS-dependent interactions with pikachurin.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"LRRTM4 is a postsynaptic adhesion molecule that organizes synapse development through heparan sulfate proteoglycan (HSPG) ligands rather than the neurexins used by other LRRTM family members [#0]. Its principal presynaptic partner is glypican-4 (GPC4), which engages the LRRTM4 ectodomain in a heparan sulfate-dependent manner to form a trans-synaptic adhesion pair: LRRTM4 clusters GPC4 in contacting axons while GPC4 reciprocally clusters LRRTM4 in dendrites [#1]. This bidirectional interaction is consolidated into a tripartite complex by presynaptic receptor protein tyrosine phosphatase PTPσ, which binds GPC4 with nanomolar affinity via its Ig domains and is required for full LRRTM4 synaptogenic activity in an HS-dependent fashion [#3]. In the hippocampus, LRRTM4 resides at excitatory postsynaptic sites of dentate gyrus granule cells, where it is selectively required for excitatory synapse density, function, and activity-regulated AMPA receptor trafficking [#2]. In the retina, LRRTM4 adopts distinct roles: it organizes GABAergic inhibitory synapses and stereotyped synaptic ribbon dyad arrangements at rod bipolar cell axon terminals [#4], and at rod bipolar cell dendritic tips it engages pikachurin through HS-dependent binding within the GPR179–pikachurin–dystroglycan trans-synaptic complex [#5].\",\n  \"teleology\": [\n    {\n      \"year\": 2013,\n      \"claim\": \"Established that LRRTM4 uses a fundamentally different presynaptic ligand class than other LRRTMs, defining HSPGs rather than neurexins as the partners required for its synaptogenic activity.\",\n      \"evidence\": \"Heterologous synapse-formation assays, co-IP, HS-dependency experiments, and LRRTM4 knockout mice\",\n      \"pmids\": [\"23911104\", \"23911103\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which specific HSPG species mediate the activity was not fully resolved at this stage\", \"The glycan modifications required were not enumerated\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identified glypican-4 as the direct HSPG receptor for LRRTM4, showing a reciprocal trans-synaptic clustering relationship that organizes excitatory synapse assembly.\",\n      \"evidence\": \"Unbiased proteomics receptor identification, co-IP, heterologous clustering assays, and in vivo localization\",\n      \"pmids\": [\"23911103\", \"23911104\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other glypicans or HSPGs substitute for GPC4 was not established\", \"Downstream postsynaptic signaling from LRRTM4 clustering was not defined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Mapped the in vivo functional requirement for LRRTM4 to dentate gyrus granule cells, demonstrating cell-type-selective control of excitatory synapse number, function, and AMPA receptor trafficking.\",\n      \"evidence\": \"Immunolocalization and knockout mouse electrophysiology, synapse density, and AMPA trafficking assays\",\n      \"pmids\": [\"23911104\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The mechanism linking LRRTM4 to activity-regulated AMPA receptor trafficking was not delineated\", \"Why CA1 pyramidal cells are spared was not explained\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Resolved how the LRRTM4–GPC4 pair couples to presynaptic machinery by identifying PTPσ as an HS-dependent bridging partner that forms a tripartite complex required for full synaptogenic activity.\",\n      \"evidence\": \"Reciprocal co-IP from rat brain, heterologous synapse-formation assays, neuronal knockdown, affinity measurements, and HS-binding-defective mutant rescue\",\n      \"pmids\": [\"25624497\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The stoichiometry and structural architecture of the LRRTM4–GPC4–PTPσ complex were not determined\", \"Whether PTPσ phosphatase activity contributes was not addressed\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Extended LRRTM4 function beyond excitatory synapses by showing it organizes GABAergic inhibitory synapses and synaptic ribbon dyad architecture at retinal rod bipolar cell axon terminals.\",\n      \"evidence\": \"Immunofluorescence, knockout mouse electrophysiology, GABA receptor clustering, and ribbon morphology analysis\",\n      \"pmids\": [\"31974009\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The presynaptic ligand at these inhibitory terminals was not identified\", \"How a single adhesion molecule directs ribbon dyad geometry mechanistically was not resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Implicated LRRTM4 at the opposite (dendritic) pole of the rod synapse, showing HS-dependent binding to pikachurin and a role in GPR179 accumulation within an established trans-synaptic complex.\",\n      \"evidence\": \"Immunolocalization with two antibodies, CRISPR/Cas9 somatic knockout, and heterologous binding assay with HS-dependency test\",\n      \"pmids\": [\"32390181\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The partial GPR179 reduction indicates additional redundant components not identified\", \"Direct versus indirect nature of the pikachurin interaction within the larger complex was not fully dissected\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The intracellular signaling and structural basis by which LRRTM4 adhesion is transduced into synapse-type-specific assembly across hippocampal and retinal contexts remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model of the LRRTM4–GPC4–PTPσ complex\", \"Postsynaptic cytoplasmic effectors of LRRTM4 unidentified\", \"Mechanism distinguishing excitatory versus inhibitory/ribbon organization unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [0, 1, 4]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 1, 3, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 2, 4, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [2, 4]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"complexes\": [\n      \"LRRTM4-GPC4-PTPσ tripartite trans-synaptic complex\",\n      \"GPR179-pikachurin-dystroglycan trans-synaptic complex\"\n    ],\n    \"partners\": [\n      \"GPC4\",\n      \"PTPRS\",\n      \"pikachurin\",\n      \"GPR179\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}