{"gene":"HTR3B","run_date":"2026-06-10T01:55:22","timeline":{"discoveries":[{"year":1999,"finding":"The 5-HT3B subunit assembles with 5-HT3A subunits to form heteromeric receptors with a large single-channel conductance (16 pS), low calcium permeability, and a current-voltage relationship resembling native neuronal 5-HT3 channels. Homomeric 5-HT3A receptors have sub-picosiemen conductance. The M2 region of 5-HT3B lacks structural features known to promote conductance in related receptors, indicating an indirect mechanism of conductance enhancement.","method":"Recombinant co-expression of 5-HT3A and 5-HT3B subunits, single-channel electrophysiology, pharmacological profiling","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — foundational reconstitution and single-channel electrophysiology establishing heteromeric assembly and biophysical consequences; widely replicated","pmids":["9950429"],"is_preprint":false},{"year":2003,"finding":"Co-expression of 5-HT3B with 5-HT3A in HEK293 cells reduces 5-HT sensitivity (EC50 shifts from 3 µM to 25 µM, Hill coefficient from 1.8 to 0.9), markedly alters desensitization kinetics (homomeric receptors desensitize via agonist-induced open-channel block whereas heteromeric receptors do not), and accelerates recovery from desensitization.","method":"Whole-cell patch-clamp recordings in HEK293 cells expressing homomeric 5-HT3A or heteromeric 5-HT3AB receptors; kinetic modeling","journal":"Biophysical journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — patch-clamp electrophysiology with kinetic modeling, single lab, multiple orthogonal parameters measured","pmids":["12609874"],"is_preprint":false},{"year":2003,"finding":"Picrotoxin inhibits homomeric 5-HT3A receptors with ~100-fold higher potency than heteromeric 5-HT3A/3B receptors, demonstrating that the 5-HT3B subunit confers reduced sensitivity to this channel blocker and providing a pharmacological tool to distinguish the two receptor isoforms.","method":"Whole-cell patch-clamp recordings in cells expressing homomeric mouse 5-HT3A or heteromeric 5-HT3A/3B receptors","journal":"Brain research. Molecular brain research","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro electrophysiology, single lab, single method","pmids":["14625088"],"is_preprint":false},{"year":2003,"finding":"Transient transfection of recombinant 5-HT3B subunit into NB41A3 neuroblastoma cells (which endogenously express mainly homomeric 5-HT3A receptors) converts native receptors to heteromeric 5-HT3AB receptors, reducing 5-HT potency, altering current kinetics, and abolishing the 5-HT-induced intracellular Ca2+ rise.","method":"RT-PCR for subunit expression, calcium imaging, whole-cell patch-clamp, transient transfection","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Ca2+ imaging, electrophysiology, RT-PCR) in a single lab using neuroblastoma cell model","pmids":["12623220"],"is_preprint":false},{"year":2006,"finding":"5-HT3B subunit protein is absent from the plasma membrane when expressed alone but reaches the cell surface when co-expressed with the 5-HT3A subunit, establishing that 5-HT3A is required for 5-HT3B membrane trafficking.","method":"Immunocytochemistry using a novel anti-5-HT3B polyclonal antibody (pAb77) in transfected HEK cells; Western blot","journal":"BMC neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization experiment with functional consequence (surface trafficking dependent on 5-HT3A co-expression), single lab, two complementary methods","pmids":["16571125"],"is_preprint":false},{"year":2006,"finding":"Two alternative promoters control tissue-specific expression of different HTR3B transcripts: intestinal transcripts initiate upstream (matching genome annotation), while brain transcripts initiate ~4 kb downstream, lacking the first coding exon but containing an upstream-extended exon 2 with a new potential translational start site, implying tissue-specific 5-HT3B isoforms.","method":"Transcription start site analyses, transcript-specific RT-PCR, reporter gene (luciferase) promoter assays","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (TSS mapping, RT-PCR, reporter assay) in a single lab","pmids":["17010535"],"is_preprint":false},{"year":2007,"finding":"5-HT3A and 5-HT3B subunit proteins are co-expressed in human hippocampal pyramidal neurones (CA2, CA3) and large hilar neurones (CA4), as established by subunit-selective polyclonal antibodies and PCR, indicating the capacity to form heteromeric 5-HT3A/3B receptors in human brain.","method":"SDS-PAGE/Western blotting, immunohistochemistry with selective polyclonal antibodies, and PCR on human hippocampal tissue","journal":"Neuropharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct protein localization in human tissue with two orthogonal methods, single lab","pmids":["17327132"],"is_preprint":false},{"year":2008,"finding":"The naturally occurring variant Y129S in the 5-HT3B subunit (HTR3B rs1176744) dramatically augments 5-HT3AB receptor signaling: deactivation kinetics are 20-fold slower, desensitization 10-fold slower, mean single-channel open time 7-fold longer, and maximal response to serotonin is substantially increased compared to wild-type.","method":"Fluorescence-based cellular assays, whole-cell patch-clamp electrophysiology, single-channel recordings in cells expressing 5-HT3AB(Y129S) vs. WT receptors","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstitution with mutagenesis, multiple orthogonal electrophysiological methods (whole-cell and single-channel) plus fluorescence assays, single lab","pmids":["18184810"],"is_preprint":false},{"year":2008,"finding":"Co-expression of 5-HT3A and 5-HT3B subunits confers constitutive (agonist-independent) channel opening on the heteromeric 5-HT3AB receptor. The 5-HT3B subunit also alters ligand properties: 5-methoxyindole, a partial agonist at 5-HT3A, becomes a protean agonist (acting as both agonist and inverse agonist) at 5-HT3AB, and 5-hydroxyindole acts as a negative allosteric modulator of the spontaneously active R* conformation but a positive modulator of the ligand-bound AR* conformation.","method":"Whole-cell patch-clamp electrophysiology in HEK293 cells co-expressing 5-HT3A and 5-HT3B","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — rigorous in vitro electrophysiology with multiple ligand tests, single lab, single method type","pmids":["18187416"],"is_preprint":false},{"year":2008,"finding":"The HTR3B variant V183I decreases surface expression of heteromeric 5-HT3A/B receptors, while Y129S and S156R increase maximal 5-HT responses without substantially altering surface expression levels, as established by Ca2+ influx (aequorin) and radioligand binding ([3H]GR65630) assays.","method":"Aequorin-based Ca2+ influx assay, radioligand binding with [3H]GR65630, transient transfection in HEK293 cells","journal":"Pharmacogenetics and genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two orthogonal functional/expression methods, single lab","pmids":["18698232"],"is_preprint":false},{"year":2008,"finding":"The HTR3B variant I143T markedly reduces cell surface expression of both 5-HT3B and 5-HT3A subunits and produces 3-fold lower current densities with otherwise similar macroscopic kinetics; variants S156R, V183I, and A223T do not significantly alter 5-HT3AB receptor expression or signaling.","method":"ELISA and immunocytochemistry for surface expression, whole-cell patch-clamp electrophysiology, membrane potential fluorescence assay in HEK cells","journal":"Pharmacogenetics and genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (ELISA, immunocytochemistry, electrophysiology, fluorescence assay), single lab","pmids":["19008750"],"is_preprint":false},{"year":2008,"finding":"The -100_-102delAAG deletion in the HTR3B promoter region increases promoter activity by 25-43% compared to the insertion allele, and differential binding of nuclear proteins to the polymorphic DNA region was detected (stronger binding to insertion allele), establishing a functional molecular mechanism for this polymorphism.","method":"Electrophoretic mobility shift assay (EMSA), luciferase reporter gene assay in PC-12 and HEK293 cells with native HTR3B promoter and tandem triplication constructs","journal":"Pharmacogenetics and genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two orthogonal methods (EMSA + reporter assay) in two cell lines, single lab","pmids":["18300944"],"is_preprint":false},{"year":2011,"finding":"The 5-HT3B subunit is N-glycosylated at five consensus sites (N31, N75, N117, N147, N182); disruption of each site individually reduces the molecular weight of the subunit by ~2-4 kDa and decreases cell membrane expression of the 5-HT3B subunit when co-expressed with 5-HT3A, establishing that N-glycosylation at all five sites is required for efficient surface trafficking.","method":"Tunicamycin treatment, site-directed mutagenesis (N→S at each glycosylation site), immunocytochemistry, SDS-PAGE/Western blot in HEK293 cells stably expressing 5-HT3A","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis of all five sites with orthogonal biochemical (Western blot) and imaging (immunocytochemistry) validation, single lab with rigorous site-by-site analysis","pmids":["21138434"],"is_preprint":false}],"current_model":"HTR3B (5-HT3B) encodes a subunit that co-assembles with 5-HT3A to form heteromeric pentameric ligand-gated ion channels with a large single-channel conductance (~16 pS), reduced calcium permeability, altered desensitization kinetics, lower 5-HT sensitivity, and constitutive (spontaneous) channel opening; 5-HT3B surface trafficking is strictly dependent on 5-HT3A co-expression and requires N-glycosylation at five consensus sites, while common coding variants (notably Y129S/rs1176744) profoundly alter receptor gating kinetics and maximal serotonin responses, and a promoter deletion (-100_-102delAAG) increases HTR3B transcriptional activity by differentially recruiting nuclear proteins."},"narrative":{"mechanistic_narrative":"HTR3B encodes the 5-HT3B subunit, an accessory component of serotonin-gated cation channels that does not function autonomously but co-assembles with the 5-HT3A subunit to build heteromeric pentameric ligand-gated ion channels with biophysical properties matching native neuronal 5-HT3 receptors [PMID:9950429]. Incorporation of 5-HT3B confers a large single-channel conductance (~16 pS versus sub-picosiemen homomeric channels) by an indirect mechanism, since its M2 region lacks the structural features that promote conductance in related receptors, together with reduced calcium permeability [PMID:9950429]. The subunit reshapes receptor gating: it lowers serotonin sensitivity, abolishes the open-channel-block mode of desensitization seen in homomeric receptors, accelerates recovery from desensitization, and endows the channel with constitutive, agonist-independent opening [PMID:12609874, PMID:18187416]. 5-HT3B also reduces sensitivity to the channel blocker picrotoxin, providing a pharmacological signature of heteromeric assembly [PMID:14625088]. The subunit cannot reach the plasma membrane on its own and depends strictly on co-expression with 5-HT3A for surface trafficking [PMID:16571125], a process that further requires N-glycosylation at five consensus sites (N31, N75, N117, N147, N182) [PMID:21138434]. 5-HT3A and 5-HT3B are co-expressed in human hippocampal neurons, supporting heteromeric receptor formation in human brain [PMID:17327132]. Coding and regulatory variation in HTR3B alters receptor behavior: the Y129S variant (rs1176744) dramatically slows deactivation and desensitization, prolongs single-channel open time, and increases maximal serotonin responses [PMID:18184810, PMID:18698232], other variants such as I143T and V183I reduce surface expression [PMID:18698232, PMID:19008750], and a promoter -100_-102delAAG deletion increases transcriptional activity through differential recruitment of nuclear proteins [PMID:18300944].","teleology":[{"year":1999,"claim":"Established that the orphan 5-HT3B subunit is functionally meaningful by showing it confers native-like high conductance only when co-assembled with 5-HT3A, defining its role as a heteromeric channel-shaping subunit.","evidence":"Recombinant co-expression with single-channel electrophysiology and pharmacological profiling","pmids":["9950429"],"confidence":"High","gaps":["Indirect mechanism of conductance enhancement not structurally resolved","Subunit stoichiometry of the heteromer not defined","No evidence whether 5-HT3B forms functional channels with subunits other than 5-HT3A"]},{"year":2003,"claim":"Resolved how 5-HT3B reshapes channel gating, showing it lowers serotonin sensitivity, eliminates the open-channel-block desensitization mode, and speeds recovery, distinguishing heteromeric from homomeric kinetics.","evidence":"Whole-cell patch-clamp with kinetic modeling in HEK293 cells; conversion of native neuroblastoma receptors by transfection; picrotoxin pharmacology","pmids":["12609874","12623220","14625088"],"confidence":"High","gaps":["Kinetic measurements from single labs in heterologous systems","Loss of calcium signaling implications for downstream physiology not addressed","Structural basis of altered desensitization unknown"]},{"year":2006,"claim":"Explained why 5-HT3B is non-functional alone by demonstrating its surface delivery depends on 5-HT3A, and revealed tissue-specific regulation through dual alternative promoters generating distinct transcripts in gut versus brain.","evidence":"Immunocytochemistry and Western blot with a novel anti-5-HT3B antibody; transcription start site mapping, transcript-specific RT-PCR, and luciferase promoter assays","pmids":["16571125","17010535"],"confidence":"Medium","gaps":["Molecular trafficking determinants requiring 5-HT3A not mapped","Predicted tissue-specific protein isoforms not biochemically confirmed","Functional consequences of brain-specific N-terminal isoform untested"]},{"year":2007,"claim":"Provided anatomical support for native heteromeric receptors by showing 5-HT3A and 5-HT3B proteins co-localize in human hippocampal neurons.","evidence":"Western blot, immunohistochemistry with selective antibodies, and PCR on human hippocampal tissue","pmids":["17327132"],"confidence":"Medium","gaps":["Co-expression in same cells inferred but direct heteromer assembly in tissue not demonstrated","Limited to hippocampus; broader CNS distribution not addressed"]},{"year":2008,"claim":"Demonstrated that 5-HT3B endows the receptor with constitutive opening and converts ligand pharmacology, establishing the subunit as a determinant of agonist-independent activity and allosteric ligand behavior.","evidence":"Whole-cell patch-clamp in HEK293 cells with multiple indole ligands","pmids":["18187416"],"confidence":"Medium","gaps":["Single method type, single lab","Physiological relevance of constitutive activity in vivo unknown","Structural states (R*/AR*) inferred from kinetics, not resolved structurally"]},{"year":2008,"claim":"Mapped functional impact of HTR3B coding and promoter variants, showing distinct mechanisms by which natural variation alters receptor gating, surface expression, and transcription.","evidence":"Patch-clamp and single-channel recordings for Y129S; aequorin Ca2+ influx, radioligand binding, ELISA/immunocytochemistry for variant surface expression; EMSA and luciferase reporter assays for the promoter deletion","pmids":["18184810","18698232","19008750","18300944"],"confidence":"High","gaps":["Nuclear proteins binding the promoter polymorphism not identified","In vivo phenotypic consequences of variants not established","No direct clinical or disease causation demonstrated in these studies"]},{"year":2011,"claim":"Defined the post-translational requirement for surface delivery by showing N-glycosylation at all five consensus sites is needed for efficient 5-HT3B membrane trafficking.","evidence":"Tunicamycin treatment, site-directed mutagenesis of each glycosylation site, Western blot and immunocytochemistry in HEK293 cells stably expressing 5-HT3A","pmids":["21138434"],"confidence":"High","gaps":["Whether glycosylation affects channel gating beyond trafficking not tested","Enzymatic machinery and glycan structures not characterized"]},{"year":null,"claim":"The physiological and behavioral roles of 5-HT3B-containing receptors in vivo, and whether HTR3B variants drive specific clinical phenotypes, remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No in vivo or genetic model linking HTR3B function to organismal physiology in the corpus","No high-resolution structure of the heteromeric channel","No direct disease-causation evidence in the timeline"]}],"mechanism_profile":{"molecular_activity":[],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,12,6]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[0,6]}],"complexes":["5-HT3A/5-HT3B heteromeric receptor"],"partners":["HTR3A"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O95264","full_name":"5-hydroxytryptamine receptor 3B","aliases":["Serotonin receptor 3B"],"length_aa":441,"mass_kda":50.3,"function":"Forms serotonin (5-hydroxytryptamine/5-HT3)-activated cation-selective channel complexes, which when activated cause fast, depolarizing responses in neurons","subcellular_location":"Postsynaptic cell membrane; Cell membrane","url":"https://www.uniprot.org/uniprotkb/O95264/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/HTR3B","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/HTR3B","total_profiled":1310},"omim":[{"mim_id":"610123","title":"5-@HYDROXYTRYPTAMINE RECEPTOR 3E; HTR3E","url":"https://www.omim.org/entry/610123"},{"mim_id":"610122","title":"5-@HYDROXYTRYPTAMINE RECEPTOR 3D; HTR3D","url":"https://www.omim.org/entry/610122"},{"mim_id":"610121","title":"5-@HYDROXYTRYPTAMINE RECEPTOR 3C; HTR3C","url":"https://www.omim.org/entry/610121"},{"mim_id":"604654","title":"5-@HYDROXYTRYPTAMINE RECEPTOR 3B; HTR3B","url":"https://www.omim.org/entry/604654"},{"mim_id":"182139","title":"5-@HYDROXYTRYPTAMINE RECEPTOR 3A; HTR3A","url":"https://www.omim.org/entry/182139"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in single","driving_tissues":[{"tissue":"brain","ntpm":3.9}],"url":"https://www.proteinatlas.org/search/HTR3B"},"hgnc":{"alias_symbol":["5-HT3B"],"prev_symbol":[]},"alphafold":{"accession":"O95264","domains":[{"cath_id":"2.70.170.10","chopping":"34-238","consensus_level":"high","plddt":92.182,"start":34,"end":238},{"cath_id":"1.20.58.390","chopping":"240-330_397-441","consensus_level":"medium","plddt":86.366,"start":240,"end":441}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O95264","model_url":"https://alphafold.ebi.ac.uk/files/AF-O95264-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O95264-F1-predicted_aligned_error_v6.png","plddt_mean":81.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=HTR3B","jax_strain_url":"https://www.jax.org/strain/search?query=HTR3B"},"sequence":{"accession":"O95264","fasta_url":"https://rest.uniprot.org/uniprotkb/O95264.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O95264/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O95264"}},"corpus_meta":[{"pmid":"9950429","id":"PMC_9950429","title":"The 5-HT3B subunit is a major determinant of serotonin-receptor function.","date":"1999","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/9950429","citation_count":465,"is_preprint":false},{"pmid":"16487942","id":"PMC_16487942","title":"Distinguishable haplotype blocks in the HTR3A and HTR3B region in the Japanese reveal evidence of association of HTR3B with female major depression.","date":"2006","source":"Biological psychiatry","url":"https://pubmed.ncbi.nlm.nih.gov/16487942","citation_count":73,"is_preprint":false},{"pmid":"12609874","id":"PMC_12609874","title":"Co-expression of the 5-HT3B serotonin receptor subunit alters the biophysics of the 5-HT3 receptor.","date":"2003","source":"Biophysical journal","url":"https://pubmed.ncbi.nlm.nih.gov/12609874","citation_count":68,"is_preprint":false},{"pmid":"18184810","id":"PMC_18184810","title":"High-frequency HTR3B variant associated with major depression dramatically augments the signaling of the human 5-HT3AB receptor.","date":"2008","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/18184810","citation_count":67,"is_preprint":false},{"pmid":"19713259","id":"PMC_19713259","title":"Do variations in the 5-HT3A and 5-HT3B serotonin receptor genes (HTR3A and HTR3B) influence the occurrence of postoperative vomiting?","date":"2009","source":"Anesthesia and analgesia","url":"https://pubmed.ncbi.nlm.nih.gov/19713259","citation_count":52,"is_preprint":false},{"pmid":"17010535","id":"PMC_17010535","title":"Tissue-specific alternative promoters of the serotonin receptor gene HTR3B in human brain and intestine.","date":"2006","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/17010535","citation_count":50,"is_preprint":false},{"pmid":"15389765","id":"PMC_15389765","title":"Investigation of the human serotonin receptor gene HTR3B in bipolar affective and schizophrenic patients.","date":"2004","source":"American journal of medical genetics. 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The Journal of the Pakistan Medical Association","url":"https://pubmed.ncbi.nlm.nih.gov/29885172","citation_count":1,"is_preprint":false},{"pmid":"40249884","id":"PMC_40249884","title":"Personalized Prophylactic Antiemetic Regimens for Control of Chemotherapy-Induced Nausea and Vomiting by Pharmacogenetic Analysis of Three Receptor Genes: HTR3A, HTR3B, TACR1.","date":"2025","source":"JCO precision oncology","url":"https://pubmed.ncbi.nlm.nih.gov/40249884","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.12.29.24319754","title":"Personalized prophylactic antiemetic regimens for the control of chemotherapy-induced nausea and vomiting by pharmacogenetic analysis of three receptors genes:<i>HTR3A, HTR3B, TACR1</i>","date":"2024-12-30","source":"bioRxiv","url":"https://doi.org/10.1101/2024.12.29.24319754","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":19926,"output_tokens":3461,"usd":0.055847,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10914,"output_tokens":2970,"usd":0.06441,"stage2_stop_reason":"end_turn"},"total_usd":0.120257,"stage1_batch_id":"msgbatch_01DUPACZi7AAWwuZiUjseQF4","stage2_batch_id":"msgbatch_018yt7chE3f48PEFtDwtj6AE","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1999,\n      \"finding\": \"The 5-HT3B subunit assembles with 5-HT3A subunits to form heteromeric receptors with a large single-channel conductance (16 pS), low calcium permeability, and a current-voltage relationship resembling native neuronal 5-HT3 channels. Homomeric 5-HT3A receptors have sub-picosiemen conductance. The M2 region of 5-HT3B lacks structural features known to promote conductance in related receptors, indicating an indirect mechanism of conductance enhancement.\",\n      \"method\": \"Recombinant co-expression of 5-HT3A and 5-HT3B subunits, single-channel electrophysiology, pharmacological profiling\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — foundational reconstitution and single-channel electrophysiology establishing heteromeric assembly and biophysical consequences; widely replicated\",\n      \"pmids\": [\"9950429\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Co-expression of 5-HT3B with 5-HT3A in HEK293 cells reduces 5-HT sensitivity (EC50 shifts from 3 µM to 25 µM, Hill coefficient from 1.8 to 0.9), markedly alters desensitization kinetics (homomeric receptors desensitize via agonist-induced open-channel block whereas heteromeric receptors do not), and accelerates recovery from desensitization.\",\n      \"method\": \"Whole-cell patch-clamp recordings in HEK293 cells expressing homomeric 5-HT3A or heteromeric 5-HT3AB receptors; kinetic modeling\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — patch-clamp electrophysiology with kinetic modeling, single lab, multiple orthogonal parameters measured\",\n      \"pmids\": [\"12609874\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Picrotoxin inhibits homomeric 5-HT3A receptors with ~100-fold higher potency than heteromeric 5-HT3A/3B receptors, demonstrating that the 5-HT3B subunit confers reduced sensitivity to this channel blocker and providing a pharmacological tool to distinguish the two receptor isoforms.\",\n      \"method\": \"Whole-cell patch-clamp recordings in cells expressing homomeric mouse 5-HT3A or heteromeric 5-HT3A/3B receptors\",\n      \"journal\": \"Brain research. Molecular brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro electrophysiology, single lab, single method\",\n      \"pmids\": [\"14625088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Transient transfection of recombinant 5-HT3B subunit into NB41A3 neuroblastoma cells (which endogenously express mainly homomeric 5-HT3A receptors) converts native receptors to heteromeric 5-HT3AB receptors, reducing 5-HT potency, altering current kinetics, and abolishing the 5-HT-induced intracellular Ca2+ rise.\",\n      \"method\": \"RT-PCR for subunit expression, calcium imaging, whole-cell patch-clamp, transient transfection\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Ca2+ imaging, electrophysiology, RT-PCR) in a single lab using neuroblastoma cell model\",\n      \"pmids\": [\"12623220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"5-HT3B subunit protein is absent from the plasma membrane when expressed alone but reaches the cell surface when co-expressed with the 5-HT3A subunit, establishing that 5-HT3A is required for 5-HT3B membrane trafficking.\",\n      \"method\": \"Immunocytochemistry using a novel anti-5-HT3B polyclonal antibody (pAb77) in transfected HEK cells; Western blot\",\n      \"journal\": \"BMC neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiment with functional consequence (surface trafficking dependent on 5-HT3A co-expression), single lab, two complementary methods\",\n      \"pmids\": [\"16571125\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Two alternative promoters control tissue-specific expression of different HTR3B transcripts: intestinal transcripts initiate upstream (matching genome annotation), while brain transcripts initiate ~4 kb downstream, lacking the first coding exon but containing an upstream-extended exon 2 with a new potential translational start site, implying tissue-specific 5-HT3B isoforms.\",\n      \"method\": \"Transcription start site analyses, transcript-specific RT-PCR, reporter gene (luciferase) promoter assays\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (TSS mapping, RT-PCR, reporter assay) in a single lab\",\n      \"pmids\": [\"17010535\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"5-HT3A and 5-HT3B subunit proteins are co-expressed in human hippocampal pyramidal neurones (CA2, CA3) and large hilar neurones (CA4), as established by subunit-selective polyclonal antibodies and PCR, indicating the capacity to form heteromeric 5-HT3A/3B receptors in human brain.\",\n      \"method\": \"SDS-PAGE/Western blotting, immunohistochemistry with selective polyclonal antibodies, and PCR on human hippocampal tissue\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct protein localization in human tissue with two orthogonal methods, single lab\",\n      \"pmids\": [\"17327132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The naturally occurring variant Y129S in the 5-HT3B subunit (HTR3B rs1176744) dramatically augments 5-HT3AB receptor signaling: deactivation kinetics are 20-fold slower, desensitization 10-fold slower, mean single-channel open time 7-fold longer, and maximal response to serotonin is substantially increased compared to wild-type.\",\n      \"method\": \"Fluorescence-based cellular assays, whole-cell patch-clamp electrophysiology, single-channel recordings in cells expressing 5-HT3AB(Y129S) vs. WT receptors\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstitution with mutagenesis, multiple orthogonal electrophysiological methods (whole-cell and single-channel) plus fluorescence assays, single lab\",\n      \"pmids\": [\"18184810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Co-expression of 5-HT3A and 5-HT3B subunits confers constitutive (agonist-independent) channel opening on the heteromeric 5-HT3AB receptor. The 5-HT3B subunit also alters ligand properties: 5-methoxyindole, a partial agonist at 5-HT3A, becomes a protean agonist (acting as both agonist and inverse agonist) at 5-HT3AB, and 5-hydroxyindole acts as a negative allosteric modulator of the spontaneously active R* conformation but a positive modulator of the ligand-bound AR* conformation.\",\n      \"method\": \"Whole-cell patch-clamp electrophysiology in HEK293 cells co-expressing 5-HT3A and 5-HT3B\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — rigorous in vitro electrophysiology with multiple ligand tests, single lab, single method type\",\n      \"pmids\": [\"18187416\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The HTR3B variant V183I decreases surface expression of heteromeric 5-HT3A/B receptors, while Y129S and S156R increase maximal 5-HT responses without substantially altering surface expression levels, as established by Ca2+ influx (aequorin) and radioligand binding ([3H]GR65630) assays.\",\n      \"method\": \"Aequorin-based Ca2+ influx assay, radioligand binding with [3H]GR65630, transient transfection in HEK293 cells\",\n      \"journal\": \"Pharmacogenetics and genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal functional/expression methods, single lab\",\n      \"pmids\": [\"18698232\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The HTR3B variant I143T markedly reduces cell surface expression of both 5-HT3B and 5-HT3A subunits and produces 3-fold lower current densities with otherwise similar macroscopic kinetics; variants S156R, V183I, and A223T do not significantly alter 5-HT3AB receptor expression or signaling.\",\n      \"method\": \"ELISA and immunocytochemistry for surface expression, whole-cell patch-clamp electrophysiology, membrane potential fluorescence assay in HEK cells\",\n      \"journal\": \"Pharmacogenetics and genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (ELISA, immunocytochemistry, electrophysiology, fluorescence assay), single lab\",\n      \"pmids\": [\"19008750\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The -100_-102delAAG deletion in the HTR3B promoter region increases promoter activity by 25-43% compared to the insertion allele, and differential binding of nuclear proteins to the polymorphic DNA region was detected (stronger binding to insertion allele), establishing a functional molecular mechanism for this polymorphism.\",\n      \"method\": \"Electrophoretic mobility shift assay (EMSA), luciferase reporter gene assay in PC-12 and HEK293 cells with native HTR3B promoter and tandem triplication constructs\",\n      \"journal\": \"Pharmacogenetics and genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal methods (EMSA + reporter assay) in two cell lines, single lab\",\n      \"pmids\": [\"18300944\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The 5-HT3B subunit is N-glycosylated at five consensus sites (N31, N75, N117, N147, N182); disruption of each site individually reduces the molecular weight of the subunit by ~2-4 kDa and decreases cell membrane expression of the 5-HT3B subunit when co-expressed with 5-HT3A, establishing that N-glycosylation at all five sites is required for efficient surface trafficking.\",\n      \"method\": \"Tunicamycin treatment, site-directed mutagenesis (N→S at each glycosylation site), immunocytochemistry, SDS-PAGE/Western blot in HEK293 cells stably expressing 5-HT3A\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis of all five sites with orthogonal biochemical (Western blot) and imaging (immunocytochemistry) validation, single lab with rigorous site-by-site analysis\",\n      \"pmids\": [\"21138434\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"HTR3B (5-HT3B) encodes a subunit that co-assembles with 5-HT3A to form heteromeric pentameric ligand-gated ion channels with a large single-channel conductance (~16 pS), reduced calcium permeability, altered desensitization kinetics, lower 5-HT sensitivity, and constitutive (spontaneous) channel opening; 5-HT3B surface trafficking is strictly dependent on 5-HT3A co-expression and requires N-glycosylation at five consensus sites, while common coding variants (notably Y129S/rs1176744) profoundly alter receptor gating kinetics and maximal serotonin responses, and a promoter deletion (-100_-102delAAG) increases HTR3B transcriptional activity by differentially recruiting nuclear proteins.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"HTR3B encodes the 5-HT3B subunit, an accessory component of serotonin-gated cation channels that does not function autonomously but co-assembles with the 5-HT3A subunit to build heteromeric pentameric ligand-gated ion channels with biophysical properties matching native neuronal 5-HT3 receptors [#0]. Incorporation of 5-HT3B confers a large single-channel conductance (~16 pS versus sub-picosiemen homomeric channels) by an indirect mechanism, since its M2 region lacks the structural features that promote conductance in related receptors, together with reduced calcium permeability [#0]. The subunit reshapes receptor gating: it lowers serotonin sensitivity, abolishes the open-channel-block mode of desensitization seen in homomeric receptors, accelerates recovery from desensitization, and endows the channel with constitutive, agonist-independent opening [#1, #8]. 5-HT3B also reduces sensitivity to the channel blocker picrotoxin, providing a pharmacological signature of heteromeric assembly [#2]. The subunit cannot reach the plasma membrane on its own and depends strictly on co-expression with 5-HT3A for surface trafficking [#4], a process that further requires N-glycosylation at five consensus sites (N31, N75, N117, N147, N182) [#12]. 5-HT3A and 5-HT3B are co-expressed in human hippocampal neurons, supporting heteromeric receptor formation in human brain [#6]. Coding and regulatory variation in HTR3B alters receptor behavior: the Y129S variant (rs1176744) dramatically slows deactivation and desensitization, prolongs single-channel open time, and increases maximal serotonin responses [#7, #9], other variants such as I143T and V183I reduce surface expression [#9, #10], and a promoter -100_-102delAAG deletion increases transcriptional activity through differential recruitment of nuclear proteins [#11].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Established that the orphan 5-HT3B subunit is functionally meaningful by showing it confers native-like high conductance only when co-assembled with 5-HT3A, defining its role as a heteromeric channel-shaping subunit.\",\n      \"evidence\": \"Recombinant co-expression with single-channel electrophysiology and pharmacological profiling\",\n      \"pmids\": [\"9950429\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Indirect mechanism of conductance enhancement not structurally resolved\",\n        \"Subunit stoichiometry of the heteromer not defined\",\n        \"No evidence whether 5-HT3B forms functional channels with subunits other than 5-HT3A\"\n      ]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Resolved how 5-HT3B reshapes channel gating, showing it lowers serotonin sensitivity, eliminates the open-channel-block desensitization mode, and speeds recovery, distinguishing heteromeric from homomeric kinetics.\",\n      \"evidence\": \"Whole-cell patch-clamp with kinetic modeling in HEK293 cells; conversion of native neuroblastoma receptors by transfection; picrotoxin pharmacology\",\n      \"pmids\": [\"12609874\", \"12623220\", \"14625088\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Kinetic measurements from single labs in heterologous systems\",\n        \"Loss of calcium signaling implications for downstream physiology not addressed\",\n        \"Structural basis of altered desensitization unknown\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Explained why 5-HT3B is non-functional alone by demonstrating its surface delivery depends on 5-HT3A, and revealed tissue-specific regulation through dual alternative promoters generating distinct transcripts in gut versus brain.\",\n      \"evidence\": \"Immunocytochemistry and Western blot with a novel anti-5-HT3B antibody; transcription start site mapping, transcript-specific RT-PCR, and luciferase promoter assays\",\n      \"pmids\": [\"16571125\", \"17010535\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Molecular trafficking determinants requiring 5-HT3A not mapped\",\n        \"Predicted tissue-specific protein isoforms not biochemically confirmed\",\n        \"Functional consequences of brain-specific N-terminal isoform untested\"\n      ]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Provided anatomical support for native heteromeric receptors by showing 5-HT3A and 5-HT3B proteins co-localize in human hippocampal neurons.\",\n      \"evidence\": \"Western blot, immunohistochemistry with selective antibodies, and PCR on human hippocampal tissue\",\n      \"pmids\": [\"17327132\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Co-expression in same cells inferred but direct heteromer assembly in tissue not demonstrated\",\n        \"Limited to hippocampus; broader CNS distribution not addressed\"\n      ]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Demonstrated that 5-HT3B endows the receptor with constitutive opening and converts ligand pharmacology, establishing the subunit as a determinant of agonist-independent activity and allosteric ligand behavior.\",\n      \"evidence\": \"Whole-cell patch-clamp in HEK293 cells with multiple indole ligands\",\n      \"pmids\": [\"18187416\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single method type, single lab\",\n        \"Physiological relevance of constitutive activity in vivo unknown\",\n        \"Structural states (R*/AR*) inferred from kinetics, not resolved structurally\"\n      ]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Mapped functional impact of HTR3B coding and promoter variants, showing distinct mechanisms by which natural variation alters receptor gating, surface expression, and transcription.\",\n      \"evidence\": \"Patch-clamp and single-channel recordings for Y129S; aequorin Ca2+ influx, radioligand binding, ELISA/immunocytochemistry for variant surface expression; EMSA and luciferase reporter assays for the promoter deletion\",\n      \"pmids\": [\"18184810\", \"18698232\", \"19008750\", \"18300944\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Nuclear proteins binding the promoter polymorphism not identified\",\n        \"In vivo phenotypic consequences of variants not established\",\n        \"No direct clinical or disease causation demonstrated in these studies\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Defined the post-translational requirement for surface delivery by showing N-glycosylation at all five consensus sites is needed for efficient 5-HT3B membrane trafficking.\",\n      \"evidence\": \"Tunicamycin treatment, site-directed mutagenesis of each glycosylation site, Western blot and immunocytochemistry in HEK293 cells stably expressing 5-HT3A\",\n      \"pmids\": [\"21138434\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether glycosylation affects channel gating beyond trafficking not tested\",\n        \"Enzymatic machinery and glycan structures not characterized\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The physiological and behavioral roles of 5-HT3B-containing receptors in vivo, and whether HTR3B variants drive specific clinical phenotypes, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No in vivo or genetic model linking HTR3B function to organismal physiology in the corpus\",\n        \"No high-resolution structure of the heteromeric channel\",\n        \"No direct disease-causation evidence in the timeline\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005216\", \"supporting_discovery_ids\": [0, 1, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 12, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [0, 6]}\n    ],\n    \"complexes\": [\n      \"5-HT3A/5-HT3B heteromeric receptor\"\n    ],\n    \"partners\": [\n      \"HTR3A\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}