{"gene":"LRFN3","run_date":"2026-04-28T18:30:27","timeline":{"discoveries":[{"year":2006,"finding":"SALM1 (a family member; the broader SALM/Lrfn family includes LRFN3/SALM4) interacts with PSD-95, SAP102, and SAP97 via its PDZ-binding domain, as shown by co-immunoprecipitation of detergent-solubilized brain; SALM1 also co-immunoprecipitates NMDA receptor NR1 and NR2 subunits and interacts with NR1 through its extracellular or TM1 domains in heterologous cells.","method":"Co-immunoprecipitation from brain membranes and heterologous cells; subcellular fractionation; hippocampal neuron transfection with domain-deletion constructs","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal Co-IP from brain and heterologous cells with domain mapping, but specific to SALM1 (LRFN1), not LRFN3/SALM4 directly","pmids":["16495444"],"is_preprint":false},{"year":2006,"finding":"LRFN3 (Lrfn3) encodes a glycoprotein with LRR-Ig-Fn-transmembrane domain architecture, is expressed predominantly in the brain starting from immature neural cells during development, and its C-terminus does NOT bind the PDZ domains of PSD-95 (unlike Lrfn1, Lrfn2, Lrfn4), and does not redistribute PSD-95 to the cell periphery.","method":"Sequence analysis, expression profiling, transfection of heterologous cells with PSD-95 co-expression and imaging","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 — direct functional transfection assay demonstrating absence of PSD-95 binding for LRFN3 specifically, combined with domain structure characterization","pmids":["16828986"],"is_preprint":false},{"year":2011,"finding":"SALM4/LRFN3, unlike SALM1-3, lacks a C-terminal PDZ-binding motif and does not interact with PSD-95; SALM4 does not form homo- or heteromeric cis-complexes with SALM1-3 but instead participates in homophilic trans-cellular adhesion; SALM4 uniquely increases the number of primary processes from the cell body during neurite outgrowth.","method":"Co-immunoprecipitation, cell-based adhesion assays, hippocampal neuron transfection and morphological analysis","journal":"Seminars in cell & developmental biology","confidence":"Medium","confidence_rationale":"Tier 3 — review synthesizing multiple experimental findings from the field; some data are from original papers cited herein","pmids":["21736948"],"is_preprint":false},{"year":2010,"finding":"SALM4/LRFN3-induced neurite branching in hippocampal neurons is mediated by flotillin-1 (flot-1); knockdown of flot-1 by siRNA prevents SALM4-induced neurite branching; flot-1 signaling depends on amino acids 134-151, lipid raft microdomains, SoHo proteins (for actin cytoskeleton regulation), and the exocyst complex (for membrane delivery to growing neurites).","method":"siRNA knockdown in cultured hippocampal neurons (3-7 DIV), overexpression, domain-deletion constructs, lipid raft disruption","journal":"Molecular and cellular neurosciences","confidence":"Medium","confidence_rationale":"Tier 2 — clean KD/OE with defined cellular phenotype and pathway placement via multiple perturbations","pmids":["20600927"],"is_preprint":false},{"year":2016,"finding":"SALM4/LRFN3 suppresses excitatory synapse development by cis-inhibiting SALM3: SALM4 directly cis-interacts with SALM3, blocking SALM3's trans-synaptic interaction with presynaptic LAR family receptor tyrosine phosphatases and thereby suppressing SALM3-dependent presynaptic differentiation. Salm4-knockout mice show increased hippocampal excitatory synapse numbers, and double knockout of Salm3;Salm4 normalizes this increase.","method":"Co-immunoprecipitation (cis-interaction), genetic epistasis (Salm4-/- and Salm3-/-;Salm4-/- double knockout mice), synapse counting in hippocampus","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1-2 — reciprocal Co-IP for cis-interaction plus rigorous genetic epistasis with double-knockout rescue of synaptic phenotype","pmids":["27480238"],"is_preprint":false},{"year":2021,"finding":"SALM4/LRFN3 negatively regulates GluN2B-containing NMDA receptor (but not AMPA receptor) currents in the hippocampus: Lrfn3-/- mice show increased GluN2B-NMDAR currents, enhanced contextual fear memory consolidation (7-day post-training), and this NMDAR increase requires presynaptic PTPσ. Chronic fluoxetine treatment normalizes both NMDAR function and fear memory consolidation in Lrfn3-/- mice.","method":"Lrfn3 knockout mice, electrophysiology (NMDAR and AMPAR currents), behavioral testing (fear conditioning), pharmacological rescue with fluoxetine and ifenprodil","journal":"Communications biology","confidence":"High","confidence_rationale":"Tier 2 — clean KO with electrophysiological readout, behavioral phenotype, presynaptic PTPσ dependency, and pharmacological rescue across multiple orthogonal methods","pmids":["34588597"],"is_preprint":false}],"current_model":"LRFN3/SALM4 is a brain-expressed synaptic adhesion molecule lacking a PDZ-binding domain that negatively regulates excitatory synapse development and function: it cis-inhibits SALM3's trans-synaptic interaction with presynaptic LAR-family phosphatases to suppress excitatory synaptogenesis, suppresses GluN2B-NMDAR currents (requiring presynaptic PTPσ) to limit fear memory consolidation, and promotes neurite branching via a flotillin-1/lipid raft/actin/exocyst pathway."},"narrative":{"teleology":[{"year":2006,"claim":"Establishing the SALM/LRFN family as synaptic adhesion molecules and identifying that LRFN3 uniquely lacks a PDZ-binding domain and does not bind PSD-95, distinguishing it from other family members.","evidence":"Sequence analysis, expression profiling, and heterologous cell co-expression with PSD-95 for LRFN3; co-immunoprecipitation from brain for SALM1","pmids":["16828986","16495444"],"confidence":"Medium","gaps":["No direct interactors identified for LRFN3 itself at this stage","Functional consequence of lacking PSD-95 binding not determined","LRFN3-specific postsynaptic roles not addressed"]},{"year":2010,"claim":"Demonstrating that LRFN3 has an active neurodevelopmental function — promoting neurite branching — and delineating the flotillin-1/lipid raft/actin/exocyst pathway that mediates this effect.","evidence":"siRNA knockdown and overexpression in cultured hippocampal neurons with domain-deletion constructs and lipid raft disruption","pmids":["20600927"],"confidence":"Medium","gaps":["In vivo relevance of neurite branching phenotype not tested","Direct physical interaction between LRFN3 and flotillin-1 not shown by reciprocal pull-down","Whether this pathway operates independently of synaptic adhesion functions unknown"]},{"year":2011,"claim":"Revealing that LRFN3 engages in homophilic trans-cellular adhesion rather than heteromeric cis-complexes with SALM1-3, suggesting a distinct adhesion mode.","evidence":"Cell-based adhesion assays and co-immunoprecipitation","pmids":["21736948"],"confidence":"Medium","gaps":["Data drawn partly from a review synthesis rather than a single primary study","Structural basis of homophilic adhesion not resolved","Physiological relevance of homophilic adhesion in vivo unknown"]},{"year":2016,"claim":"Establishing LRFN3 as a negative regulator of excitatory synaptogenesis through a cis-inhibition mechanism: LRFN3 directly binds SALM3 in cis to block its trans-synaptic interaction with LAR-family phosphatases, and genetic epistasis in double-knockout mice confirmed this pathway.","evidence":"Reciprocal co-immunoprecipitation for cis-interaction; Salm4-knockout and Salm3;Salm4 double-knockout mice with hippocampal synapse quantification","pmids":["27480238"],"confidence":"High","gaps":["Structural interface mediating LRFN3–SALM3 cis-interaction not mapped","Whether LRFN3 cis-inhibits other SALM family members not tested","Behavioral consequences of altered excitatory synapse number in Salm4-knockout mice not reported here"]},{"year":2021,"claim":"Extending LRFN3's inhibitory role to NMDA receptor function: LRFN3 suppresses GluN2B-NMDAR currents via a presynaptic PTPσ-dependent mechanism and limits fear memory consolidation, linking synaptic adhesion to receptor-level and behavioral regulation.","evidence":"Lrfn3-knockout mice; hippocampal electrophysiology (NMDAR/AMPAR currents); contextual fear conditioning; pharmacological rescue with fluoxetine and ifenprodil","pmids":["34588597"],"confidence":"High","gaps":["Molecular mechanism by which LRFN3–PTPσ trans-synaptic signaling regulates GluN2B surface expression or gating not defined","Whether LRFN3-mediated NMDAR suppression is independent of the SALM3 cis-inhibition pathway unclear","Whether LRFN3's homophilic trans-adhesion contributes to NMDAR regulation not addressed"]},{"year":null,"claim":"Key unresolved questions include the structural basis of LRFN3–SALM3 cis-interaction and LRFN3 homophilic trans-adhesion, whether the neurite branching and synapse-suppressive functions are mechanistically linked, and how LRFN3's multiple signaling modes (cis-inhibition, NMDAR regulation, flotillin-1 pathway) are coordinated in vivo.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal or cryo-EM structure for LRFN3 or its complexes","Relationship between flotillin-1-dependent neurite branching and synapse suppression untested","Human genetic studies linking LRFN3 variants to neuropsychiatric phenotypes lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[2,4]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,2,3]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[4,5]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[3,4]}],"complexes":[],"partners":["SALM3","FLOT1","PTPRS"],"other_free_text":[]},"mechanistic_narrative":"LRFN3 (SALM4) is a brain-enriched synaptic adhesion molecule with LRR-Ig-Fn-transmembrane architecture that lacks a PDZ-binding domain and functions as a negative regulator of excitatory synapse development and NMDA receptor signaling. LRFN3 suppresses excitatory synaptogenesis by cis-interacting with SALM3, thereby blocking SALM3's trans-synaptic engagement of presynaptic LAR-family receptor tyrosine phosphatases; Lrfn3-knockout mice exhibit increased hippocampal excitatory synapse numbers that are normalized by concurrent Salm3 deletion [PMID:27480238]. LRFN3 also restrains GluN2B-containing NMDA receptor currents through a presynaptic PTPσ-dependent mechanism, and its loss enhances contextual fear memory consolidation [PMID:34588597]. During early neuronal development, LRFN3 promotes neurite branching via a flotillin-1/lipid raft/actin/exocyst signaling pathway [PMID:20600927]."},"prefetch_data":{"uniprot":{"accession":"Q9BTN0","full_name":"Leucine-rich repeat and fibronectin type-III domain-containing protein 3","aliases":["Synaptic adhesion-like molecule 4"],"length_aa":628,"mass_kda":66.3,"function":"Cell adhesion molecule that mediates homophilic cell-cell adhesion in a Ca(2+)-independent manner. Promotes neurite outgrowth in hippocampal neurons (By similarity)","subcellular_location":"Cell membrane; Cell projection, axon; Cell projection, dendrite; Synapse; Presynaptic cell membrane; Postsynaptic cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9BTN0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LRFN3","classification":"Not Classified","n_dependent_lines":15,"n_total_lines":1208,"dependency_fraction":0.012417218543046357},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/LRFN3","total_profiled":1310},"omim":[{"mim_id":"612811","title":"LEUCINE-RICH REPEAT AND FIBRONECTIN TYPE III DOMAIN-CONTAINING PROTEIN 5; LRFN5","url":"https://www.omim.org/entry/612811"},{"mim_id":"612809","title":"LEUCINE-RICH REPEAT AND FIBRONECTIN TYPE III DOMAIN-CONTAINING PROTEIN 3; LRFN3","url":"https://www.omim.org/entry/612809"},{"mim_id":"612807","title":"LEUCINE-RICH REPEAT AND FIBRONECTIN TYPE III DOMAIN-CONTAINING PROTEIN 1; LRFN1","url":"https://www.omim.org/entry/612807"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/LRFN3"},"hgnc":{"alias_symbol":["MGC2656","SALM4","FIGLER1"],"prev_symbol":[]},"alphafold":{"accession":"Q9BTN0","domains":[{"cath_id":"2.60.40.10","chopping":"295-385","consensus_level":"high","plddt":94.456,"start":295,"end":385},{"cath_id":"2.60.40.10","chopping":"433-521","consensus_level":"high","plddt":82.8675,"start":433,"end":521}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BTN0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BTN0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BTN0-F1-predicted_aligned_error_v6.png","plddt_mean":77.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LRFN3","jax_strain_url":"https://www.jax.org/strain/search?query=LRFN3"},"sequence":{"accession":"Q9BTN0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BTN0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BTN0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BTN0"}},"corpus_meta":[{"pmid":"16495444","id":"PMC_16495444","title":"A novel family of adhesion-like molecules that interacts with the NMDA receptor.","date":"2006","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/16495444","citation_count":104,"is_preprint":false},{"pmid":"16828986","id":"PMC_16828986","title":"Comparative analysis of structure, expression and PSD95-binding capacity of Lrfn, a novel family of neuronal transmembrane proteins.","date":"2006","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/16828986","citation_count":67,"is_preprint":false},{"pmid":"21736948","id":"PMC_21736948","title":"The SALM/Lrfn family of leucine-rich repeat-containing cell adhesion molecules.","date":"2011","source":"Seminars in cell & developmental biology","url":"https://pubmed.ncbi.nlm.nih.gov/21736948","citation_count":56,"is_preprint":false},{"pmid":"27480238","id":"PMC_27480238","title":"SALM4 suppresses excitatory synapse development by cis-inhibiting trans-synaptic SALM3-LAR adhesion.","date":"2016","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/27480238","citation_count":29,"is_preprint":false},{"pmid":"33689574","id":"PMC_33689574","title":"Neoantigen-reactive T cells exhibit effective anti-tumor activity against colorectal cancer.","date":"2021","source":"Human vaccines & immunotherapeutics","url":"https://pubmed.ncbi.nlm.nih.gov/33689574","citation_count":25,"is_preprint":false},{"pmid":"31170000","id":"PMC_31170000","title":"SALM4 regulates angiogenic functions in endothelial cells through VEGFR2 phosphorylation at Tyr1175.","date":"2019","source":"FASEB journal : official publication of the Federation of American Societies for Experimental Biology","url":"https://pubmed.ncbi.nlm.nih.gov/31170000","citation_count":17,"is_preprint":false},{"pmid":"20600927","id":"PMC_20600927","title":"Flotillin-1 mediates neurite branching induced by synaptic adhesion-like molecule 4 in hippocampal neurons.","date":"2010","source":"Molecular and cellular neurosciences","url":"https://pubmed.ncbi.nlm.nih.gov/20600927","citation_count":17,"is_preprint":false},{"pmid":"24551291","id":"PMC_24551291","title":"Chromosomal and genetic imbalances in Chinese patients with rhabdomyosarcoma detected by high-resolution array comparative genomic hybridization.","date":"2014","source":"International journal of clinical and experimental pathology","url":"https://pubmed.ncbi.nlm.nih.gov/24551291","citation_count":15,"is_preprint":false},{"pmid":"39684228","id":"PMC_39684228","title":"Genetic Variants Linked to Opioid Addiction: A Genome-Wide Association Study.","date":"2024","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/39684228","citation_count":7,"is_preprint":false},{"pmid":"34588597","id":"PMC_34588597","title":"SALM4 negatively regulates NMDA receptor function and fear memory consolidation.","date":"2021","source":"Communications biology","url":"https://pubmed.ncbi.nlm.nih.gov/34588597","citation_count":5,"is_preprint":false},{"pmid":"41693713","id":"PMC_41693713","title":"Integrative multi-omics analyses identify PKD1 and SLC2A4 as genetically supported glycolysis-related candidate genes for rheumatoid arthritis.","date":"2026","source":"Frontiers in immunology","url":"https://pubmed.ncbi.nlm.nih.gov/41693713","citation_count":1,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7956,"output_tokens":1813,"usd":0.025531},"stage2":{"model":"claude-opus-4-6","input_tokens":5060,"output_tokens":1876,"usd":0.1083},"total_usd":0.133831,"stage1_batch_id":"msgbatch_01Mgbec4vM6c9vXht7JVJzss","stage2_batch_id":"msgbatch_01JkuuH5RB2BdJNsgnhjbC6s","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2006,\n      \"finding\": \"SALM1 (a family member; the broader SALM/Lrfn family includes LRFN3/SALM4) interacts with PSD-95, SAP102, and SAP97 via its PDZ-binding domain, as shown by co-immunoprecipitation of detergent-solubilized brain; SALM1 also co-immunoprecipitates NMDA receptor NR1 and NR2 subunits and interacts with NR1 through its extracellular or TM1 domains in heterologous cells.\",\n      \"method\": \"Co-immunoprecipitation from brain membranes and heterologous cells; subcellular fractionation; hippocampal neuron transfection with domain-deletion constructs\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP from brain and heterologous cells with domain mapping, but specific to SALM1 (LRFN1), not LRFN3/SALM4 directly\",\n      \"pmids\": [\"16495444\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"LRFN3 (Lrfn3) encodes a glycoprotein with LRR-Ig-Fn-transmembrane domain architecture, is expressed predominantly in the brain starting from immature neural cells during development, and its C-terminus does NOT bind the PDZ domains of PSD-95 (unlike Lrfn1, Lrfn2, Lrfn4), and does not redistribute PSD-95 to the cell periphery.\",\n      \"method\": \"Sequence analysis, expression profiling, transfection of heterologous cells with PSD-95 co-expression and imaging\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct functional transfection assay demonstrating absence of PSD-95 binding for LRFN3 specifically, combined with domain structure characterization\",\n      \"pmids\": [\"16828986\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SALM4/LRFN3, unlike SALM1-3, lacks a C-terminal PDZ-binding motif and does not interact with PSD-95; SALM4 does not form homo- or heteromeric cis-complexes with SALM1-3 but instead participates in homophilic trans-cellular adhesion; SALM4 uniquely increases the number of primary processes from the cell body during neurite outgrowth.\",\n      \"method\": \"Co-immunoprecipitation, cell-based adhesion assays, hippocampal neuron transfection and morphological analysis\",\n      \"journal\": \"Seminars in cell & developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — review synthesizing multiple experimental findings from the field; some data are from original papers cited herein\",\n      \"pmids\": [\"21736948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"SALM4/LRFN3-induced neurite branching in hippocampal neurons is mediated by flotillin-1 (flot-1); knockdown of flot-1 by siRNA prevents SALM4-induced neurite branching; flot-1 signaling depends on amino acids 134-151, lipid raft microdomains, SoHo proteins (for actin cytoskeleton regulation), and the exocyst complex (for membrane delivery to growing neurites).\",\n      \"method\": \"siRNA knockdown in cultured hippocampal neurons (3-7 DIV), overexpression, domain-deletion constructs, lipid raft disruption\",\n      \"journal\": \"Molecular and cellular neurosciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KD/OE with defined cellular phenotype and pathway placement via multiple perturbations\",\n      \"pmids\": [\"20600927\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SALM4/LRFN3 suppresses excitatory synapse development by cis-inhibiting SALM3: SALM4 directly cis-interacts with SALM3, blocking SALM3's trans-synaptic interaction with presynaptic LAR family receptor tyrosine phosphatases and thereby suppressing SALM3-dependent presynaptic differentiation. Salm4-knockout mice show increased hippocampal excitatory synapse numbers, and double knockout of Salm3;Salm4 normalizes this increase.\",\n      \"method\": \"Co-immunoprecipitation (cis-interaction), genetic epistasis (Salm4-/- and Salm3-/-;Salm4-/- double knockout mice), synapse counting in hippocampus\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reciprocal Co-IP for cis-interaction plus rigorous genetic epistasis with double-knockout rescue of synaptic phenotype\",\n      \"pmids\": [\"27480238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SALM4/LRFN3 negatively regulates GluN2B-containing NMDA receptor (but not AMPA receptor) currents in the hippocampus: Lrfn3-/- mice show increased GluN2B-NMDAR currents, enhanced contextual fear memory consolidation (7-day post-training), and this NMDAR increase requires presynaptic PTPσ. Chronic fluoxetine treatment normalizes both NMDAR function and fear memory consolidation in Lrfn3-/- mice.\",\n      \"method\": \"Lrfn3 knockout mice, electrophysiology (NMDAR and AMPAR currents), behavioral testing (fear conditioning), pharmacological rescue with fluoxetine and ifenprodil\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with electrophysiological readout, behavioral phenotype, presynaptic PTPσ dependency, and pharmacological rescue across multiple orthogonal methods\",\n      \"pmids\": [\"34588597\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"LRFN3/SALM4 is a brain-expressed synaptic adhesion molecule lacking a PDZ-binding domain that negatively regulates excitatory synapse development and function: it cis-inhibits SALM3's trans-synaptic interaction with presynaptic LAR-family phosphatases to suppress excitatory synaptogenesis, suppresses GluN2B-NMDAR currents (requiring presynaptic PTPσ) to limit fear memory consolidation, and promotes neurite branching via a flotillin-1/lipid raft/actin/exocyst pathway.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"LRFN3 (SALM4) is a brain-enriched synaptic adhesion molecule with LRR-Ig-Fn-transmembrane architecture that lacks a PDZ-binding domain and functions as a negative regulator of excitatory synapse development and NMDA receptor signaling. LRFN3 suppresses excitatory synaptogenesis by cis-interacting with SALM3, thereby blocking SALM3's trans-synaptic engagement of presynaptic LAR-family receptor tyrosine phosphatases; Lrfn3-knockout mice exhibit increased hippocampal excitatory synapse numbers that are normalized by concurrent Salm3 deletion [PMID:27480238]. LRFN3 also restrains GluN2B-containing NMDA receptor currents through a presynaptic PTPσ-dependent mechanism, and its loss enhances contextual fear memory consolidation [PMID:34588597]. During early neuronal development, LRFN3 promotes neurite branching via a flotillin-1/lipid raft/actin/exocyst signaling pathway [PMID:20600927].\",\n  \"teleology\": [\n    {\n      \"year\": 2006,\n      \"claim\": \"Establishing the SALM/LRFN family as synaptic adhesion molecules and identifying that LRFN3 uniquely lacks a PDZ-binding domain and does not bind PSD-95, distinguishing it from other family members.\",\n      \"evidence\": \"Sequence analysis, expression profiling, and heterologous cell co-expression with PSD-95 for LRFN3; co-immunoprecipitation from brain for SALM1\",\n      \"pmids\": [\"16828986\", \"16495444\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No direct interactors identified for LRFN3 itself at this stage\",\n        \"Functional consequence of lacking PSD-95 binding not determined\",\n        \"LRFN3-specific postsynaptic roles not addressed\"\n      ]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstrating that LRFN3 has an active neurodevelopmental function — promoting neurite branching — and delineating the flotillin-1/lipid raft/actin/exocyst pathway that mediates this effect.\",\n      \"evidence\": \"siRNA knockdown and overexpression in cultured hippocampal neurons with domain-deletion constructs and lipid raft disruption\",\n      \"pmids\": [\"20600927\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"In vivo relevance of neurite branching phenotype not tested\",\n        \"Direct physical interaction between LRFN3 and flotillin-1 not shown by reciprocal pull-down\",\n        \"Whether this pathway operates independently of synaptic adhesion functions unknown\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Revealing that LRFN3 engages in homophilic trans-cellular adhesion rather than heteromeric cis-complexes with SALM1-3, suggesting a distinct adhesion mode.\",\n      \"evidence\": \"Cell-based adhesion assays and co-immunoprecipitation\",\n      \"pmids\": [\"21736948\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Data drawn partly from a review synthesis rather than a single primary study\",\n        \"Structural basis of homophilic adhesion not resolved\",\n        \"Physiological relevance of homophilic adhesion in vivo unknown\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Establishing LRFN3 as a negative regulator of excitatory synaptogenesis through a cis-inhibition mechanism: LRFN3 directly binds SALM3 in cis to block its trans-synaptic interaction with LAR-family phosphatases, and genetic epistasis in double-knockout mice confirmed this pathway.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation for cis-interaction; Salm4-knockout and Salm3;Salm4 double-knockout mice with hippocampal synapse quantification\",\n      \"pmids\": [\"27480238\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural interface mediating LRFN3–SALM3 cis-interaction not mapped\",\n        \"Whether LRFN3 cis-inhibits other SALM family members not tested\",\n        \"Behavioral consequences of altered excitatory synapse number in Salm4-knockout mice not reported here\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Extending LRFN3's inhibitory role to NMDA receptor function: LRFN3 suppresses GluN2B-NMDAR currents via a presynaptic PTPσ-dependent mechanism and limits fear memory consolidation, linking synaptic adhesion to receptor-level and behavioral regulation.\",\n      \"evidence\": \"Lrfn3-knockout mice; hippocampal electrophysiology (NMDAR/AMPAR currents); contextual fear conditioning; pharmacological rescue with fluoxetine and ifenprodil\",\n      \"pmids\": [\"34588597\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular mechanism by which LRFN3–PTPσ trans-synaptic signaling regulates GluN2B surface expression or gating not defined\",\n        \"Whether LRFN3-mediated NMDAR suppression is independent of the SALM3 cis-inhibition pathway unclear\",\n        \"Whether LRFN3's homophilic trans-adhesion contributes to NMDAR regulation not addressed\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of LRFN3–SALM3 cis-interaction and LRFN3 homophilic trans-adhesion, whether the neurite branching and synapse-suppressive functions are mechanistically linked, and how LRFN3's multiple signaling modes (cis-inhibition, NMDAR regulation, flotillin-1 pathway) are coordinated in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No crystal or cryo-EM structure for LRFN3 or its complexes\",\n        \"Relationship between flotillin-1-dependent neurite branching and synapse suppression untested\",\n        \"Human genetic studies linking LRFN3 variants to neuropsychiatric phenotypes lacking\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 2, 3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [4, 5]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"SALM3\",\n      \"FLOT1\",\n      \"PTPRS\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}