{"gene":"HTR3A","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2002,"finding":"A cation-π interaction between serotonin and Trp183 (loop B) of the 5-HT3A receptor is required for agonist binding, precisely locating the ligand-binding site. The energetic contribution is approximately 4 kcal/mol.","method":"Unnatural amino acid mutagenesis and heterologous expression in Xenopus oocytes (in vitro electrophysiology + mutagenesis)","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted functional assay with unnatural amino acid mutagenesis, quantified energetics","pmids":["12162741"],"is_preprint":false},{"year":2003,"finding":"Arginine 222 (Arg-222) in the pre-transmembrane domain 1 of the 5-HT3A receptor links agonist binding to channel gating; R222A mutation increased agonist potency and efficacy, accelerated activation and desensitization kinetics, and converted an antagonist (apomorphine) to a potent agonist.","method":"Site-directed mutagenesis and whole-cell patch-clamp electrophysiology in HEK293 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro mutagenesis with detailed kinetic analysis","pmids":["12970351"],"is_preprint":false},{"year":2006,"finding":"Arginine 427 in the large cytoplasmic domain (LCD) of the 5-HT3A receptor contributes to receptor desensitization; deletion or point mutation at this position significantly slowed desensitization kinetics, with desensitization rate positively correlated with polarity of the residue at position 427.","method":"Sequential LCD deletion and point mutagenesis with whole-cell patch-clamp in HEK293 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with multiple mutants and mechanistic correlation","pmids":["16754678"],"is_preprint":false},{"year":2000,"finding":"The 4′ lysine residue in the M2 channel-lining domain of the 5-HT3A receptor affects desensitization kinetics but does not form part of the channel lining; mutations at this position slow desensitization without altering single-channel conductance.","method":"Site-directed mutagenesis, whole-cell patch-clamp, and fluctuation analysis in HEK293 cells","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 1 — in vitro mutagenesis with single-channel and whole-cell electrophysiology","pmids":["10639097"],"is_preprint":false},{"year":2001,"finding":"Homomeric murine 5-HT3A receptors require binding of at least three serotonin molecules to open, and channel open probability varies with the number of bound agonist molecules, with reduced open probability for fully liganded receptors; peak open probability exceeds 0.8.","method":"Rapid agonist application, whole-cell and excised outside-out patch clamp, kinetic modeling in HEK293 cells","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 1 — quantitative biophysical analysis with multiple concentrations and state modeling","pmids":["11533135"],"is_preprint":false},{"year":2005,"finding":"The conserved proline P303 in the M2-M3 linker of the human 5-HT3A receptor is important for function (Ca2+ dependence and desensitization rate) but trans-cis isomerization at this proline is not required for agonist-induced channel opening.","method":"Natural amino acid mutagenesis of P303 with two-electrode voltage-clamp in Xenopus oocytes","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with functional electrophysiological characterization","pmids":["19457066"],"is_preprint":false},{"year":2005,"finding":"Aspartate 298 (D298) in the TM2-TM3 extracellular loop of the mouse 5-HT3A receptor participates in channel gating kinetics (desensitization, deactivation, partial agonist efficacy) and is crucial for Ca2+ modulation; charge at this residue determines these properties.","method":"Site-directed mutagenesis, whole-cell patch-clamp with fast agonist application in HEK293 cells","journal":"The Journal of physiology","confidence":"High","confidence_rationale":"Tier 1 — systematic charge-reversal/neutralization mutagenesis with kinetic analysis","pmids":["16096341"],"is_preprint":false},{"year":2005,"finding":"Loop C residues F226, I228, D229, and Y234 of the murine 5-HT3A receptor contribute differentially to ligand binding and gating; I228 and D229 are specific for 5-HT versus mCPBG interactions, while F226 and Y234 are important for both agonists.","method":"Site-directed mutagenesis, radioligand binding, two-electrode voltage-clamp in Xenopus oocytes, homology modeling","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with binding and electrophysiology","pmids":["15966738"],"is_preprint":false},{"year":2007,"finding":"Serotonin and dopamine act as high- and low-efficacy agonists, respectively, at human 5-HT3A receptors with distinct kinetic mechanisms: serotonin shows concentration-dependent activation and sigmoidal recovery from desensitization; dopamine shows concentration-independent activation, faster deactivation, and non-sigmoidal recovery, explained by different rates of channel opening and dissociation from open/desensitized states.","method":"Rapid solution exchange electrophysiology in HEK293 cells with allosteric kinetic modeling","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 — rigorous biophysical characterization with kinetic modeling","pmids":["18045909"],"is_preprint":false},{"year":2007,"finding":"Dynamic modification of the charge at cytoplasmic residue Arg436 regulates single-channel conductance of the 5-HT3A receptor, consistent with cytoplasmic portals that impose a rate-limiting barrier to ion conduction; negatively charged MTS reagents at this position increased conductance while positively charged reagents decreased it.","method":"Cysteine substitution (R436C), MTS reagent modification, outside-out and inside-out patch clamp electrophysiology","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — substituted cysteine accessibility method with bidirectional charge modification","pmids":["17200121"],"is_preprint":false},{"year":2013,"finding":"Multiple residues within the MA (membrane-associated) helix intracellular portals of human 5-HT3A (positions 435, 436, 439, 440) markedly influence single-channel conductance; alanine- and arginine-scanning mutagenesis combined with substituted cysteine accessibility method generated a functional map of the cytoplasmic portals.","method":"Alanine/arginine-scanning mutagenesis and substituted cysteine accessibility method (SCAM) with single-channel electrophysiology","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic SCAM and mutagenesis across MA helix","pmids":["24030822"],"is_preprint":false},{"year":2011,"finding":"Two anionic residues in the extracellular vestibule of the 5-HT3A receptor, Asp113 and Asp127, markedly influence single-channel conductance, Ca2+ permeability relative to Cs+ (PCa/PCs), and Ca2+-mediated suppression of conductance; D127 mutations had larger effects than D113, and both influenced ion selectivity.","method":"Site-directed mutagenesis, single-channel outside-out patch-clamp, permeability ratio measurements in HEK293 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — charge-reversal mutagenesis with quantitative single-channel and selectivity measurements","pmids":["21454663"],"is_preprint":false},{"year":2004,"finding":"The cytoplasmic pore (selectivity filter) of the 5-HT3A receptor maintains a narrow pore in both open and closed states, indicating minimal structural rearrangement at the 2' and -2' positions during channel gating.","method":"Systematic cysteine mutagenesis throughout M1-M2 loop and M2 domain with Cd2+ accessibility probing in open and closed states","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic SCAM with state-dependent metal ion probing","pmids":["15131114"],"is_preprint":false},{"year":2006,"finding":"Granisetron interacts with a binding cavity formed by loop B, C, and E residues of the 5-HT3A receptor; H185A mutation abolished granisetron binding and D189A reduced affinity 22-fold; Y143 and Y153 in loop E contribute via their hydroxyl groups.","method":"Homology modeling, docking, and alanine mutagenesis with radioligand binding","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1/2 — mutagenesis with binding assays, supported by computational model but no structure","pmids":["16430206"],"is_preprint":false},{"year":2006,"finding":"Ginsenoside Rg3 inhibits 5-HT3A receptor channel activity through interactions with residues V291, F292, and I295 in the channel gating region of TM2, acting in the open state at sites distinct from those of TMB-8 and diltiazem.","method":"TM2 mutagenesis (V291A, F292A, I295A) and two-electrode voltage-clamp in Xenopus oocytes","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis identifying specific channel pore residues with functional electrophysiology","pmids":["17257631"],"is_preprint":false},{"year":2008,"finding":"L293 (L15') in the second transmembrane domain (TM2) of the 5-HT3A receptor is a molecular determinant of allosteric modulation by 5-hydroxyindole (5-HI); L293C and L293S mutations abolished 5-HI potentiation and converted 5-HI to a partial agonist, while also altering desensitization.","method":"Site-directed mutagenesis and whole-cell patch-clamp in HEK293 cells and N1E-115 cells","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis identifying a specific TM2 residue for allosteric modulation","pmids":["18436267"],"is_preprint":false},{"year":2010,"finding":"RIC-3 chaperone directly interacts with 5-HT3A, -C, -D, and -E subunits (co-localizes in ER) but predominantly enhances surface expression of homomeric 5-HT3A receptors in HEK293 cells without altering 5-HT potency; increased Emax correlates with increased Bmax and surface expression.","method":"Co-immunoprecipitation, immunocytochemistry (ER co-localization), flow cytometry for surface expression, Ca2+ influx assays, radioligand binding","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-localization, direct interaction assays, and functional consequences with multiple methods","pmids":["20522555"],"is_preprint":false},{"year":2004,"finding":"N-glycosylation at three sites of the murine 5-HT3A receptor (N109, N174, N190) plays distinct roles: N109 is necessary for receptor assembly, while N174 and N190 are important for plasma membrane targeting and ligand binding; all three are required for Ca2+ influx.","method":"Tunicamycin treatment, site-directed mutagenesis, surface expression assays, radioligand binding, Ca2+ influx assays in transfected cells","journal":"Journal of neuroscience research","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis of each glycosylation site with multiple functional readouts","pmids":["15264219"],"is_preprint":false},{"year":2008,"finding":"The C-terminal residue Ala455 of the human 5-HT3A subunit is critical for receptor folding/maturation and membrane expression; deletion of the three C-terminal residues abolished radioligand binding and membrane expression, and Ala455Leu mutation caused 88% reduction.","method":"C-terminal deletion and point mutagenesis, radioligand binding, cell membrane expression assays at 37°C and 27°C","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with binding and expression assays revealing temperature-dependent rescue","pmids":["18786552"],"is_preprint":false},{"year":2004,"finding":"5-HT3A receptor subunit is localized in axonal (presynaptic), somatodendritic, and glial profiles in the medial nucleus of the solitary tract (mNTS); presynaptically, it associates with synaptic vesicle membranes and extrasynaptic plasma membranes, consistent with modulation of both presynaptic release and postsynaptic responses.","method":"Electron microscopic immunocytochemistry with subcellular localization in rat brain tissue","journal":"Brain research","confidence":"Medium","confidence_rationale":"Tier 2 — direct ultrastructural localization but functional consequence inferred","pmids":["15527741"],"is_preprint":false},{"year":2004,"finding":"Fluorescently tagged YFP-5-HT3A receptor is targeted to plasma membrane, micropodia in HEK293 cells, and dendritic spines in hippocampal neurons, accessible by extracellular probes, in contrast to alpha3beta4-nAChRs which remain predominantly intracellular.","method":"Live fluorescence microscopy with YFP-fusion subunit, extracellular fluorescent probe accessibility, whole-cell patch-clamp in HEK293 cells and hippocampal neurons","journal":"The European journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — direct live-cell imaging with functional validation (current amplitude correlated with surface expression)","pmids":["15009132"],"is_preprint":false},{"year":2000,"finding":"Alternative splicing of the human 5-HT3A gene produces a short truncated variant (h5-HT3AT, 238 aa, single TM domain) and a long variant (h5-HT3AL, 32 extra aa in M2-M3 extracellular loop); neither forms functional homomeric receptors, but h5-HT3AT slows desensitization and greatly increases cation flux when co-expressed with 5-HT3A, while h5-HT3AL reduces cation flux.","method":"CDNA cloning, HEK293 cell transfection, whole-cell patch-clamp electrophysiology, expression in amygdala/hippocampus confirmed by RT-PCR","journal":"Naunyn-Schmiedeberg's archives of pharmacology","confidence":"High","confidence_rationale":"Tier 1 — functional reconstitution of splice variants with electrophysiological characterization","pmids":["11111833"],"is_preprint":false},{"year":2009,"finding":"The M3M4 intracellular loop of the 5-HT3A receptor is not required for receptor assembly or function; however, loop length and amino acid composition affect channel expression and desensitization rate, suggesting the cytoplasmic ends of M3 and M4 undergo conformational changes during gating/desensitization.","method":"M3M4 loop replacement with 1-7 alanine residues, expression in Xenopus oocytes, two-electrode voltage-clamp","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 — systematic loop substitution mutagenesis with functional characterization","pmids":["22539982"],"is_preprint":false},{"year":2012,"finding":"Deletion of 5-HT3A receptor gene in mice abolishes NMDAR-dependent long-term depression (LTD) at hippocampal CA1 synapses and inhibits AMPA receptor internalization following low-frequency stimulation, without altering mGluR-dependent LTD, basal AMPAR surface levels, or synaptic structure.","method":"5-HT3AR knockout mice, hippocampal slice electrophysiology (LTD induction by LFS), AMPAR internalization assay","journal":"Neuroscience","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined synaptic plasticity phenotype and receptor-specific mechanistic dissection","pmids":["25130560"],"is_preprint":false},{"year":2013,"finding":"The 5-HT3A receptor is essential for fear extinction in mice; Htr3a knockout mice show normal fear acquisition and retention but fail to extinguish contextual and tone-cued fear memory.","method":"Htr3a knockout mice, fear conditioning and extinction behavioral paradigms (contextual and cued)","journal":"Learning & memory","confidence":"High","confidence_rationale":"Tier 2 — clean KO with specific behavioral phenotype dissociating acquisition from extinction","pmids":["24344177"],"is_preprint":false},{"year":2021,"finding":"Htr3a knockout mice exhibit autistic-like behaviors; mechanistically, loss of 5-HT3A leads to NMDAR upregulation specifically in parvalbumin-positive (PV+) interneurons, increasing NMDAR current and excitability, which enhances GABAergic transmission to pyramidal neurons and decreases the E/I ratio. NMDAR antagonist memantine rescues GABAergic transmission and autistic-like behaviors.","method":"TALEN-generated Htr3a KO mice, behavioral tests, patch-clamp electrophysiology, transcriptome sequencing, immunoblotting, immunofluorescence, immunoprecipitation, memantine rescue","journal":"Theranostics","confidence":"High","confidence_rationale":"Tier 2 — KO with multiple orthogonal methods identifying specific interneuron mechanism and pharmacological rescue","pmids":["34646371"],"is_preprint":false},{"year":2017,"finding":"5-HT3A receptor is required for normal bladder innervation development; Htr3a knockout male mice exhibit increased voiding frequency and decreased voiding efficiency. Loss of 5-HT3A causes transient imbalance of autonomic neuronal subtype markers in fetal pelvic ganglia and higher density of autonomic/sensory neuronal fibers in bladder smooth muscle in both fetal and adult mice.","method":"Htr3a knockout mice, cystometry, immunohistochemistry for neuronal markers, fiber density quantification","journal":"Frontiers in neuroscience","confidence":"High","confidence_rationale":"Tier 2 — KO with defined functional and developmental phenotypes across multiple timepoints","pmids":["29311772"],"is_preprint":false},{"year":2004,"finding":"Constitutively activating V13'S mutation in the M2 domain of the 5-HT3A receptor (valine to serine) renders the receptor 70-fold more sensitive to serotonin and produces constitutive activity when combined with 5-HT3B; homozygous knock-in mice develop urinary bladder dysfunction and die from obstructive uropathy, suggesting persistent 5-HT3A activation causes excitotoxic neuronal death in bladder.","method":"Targeted exon replacement knock-in mice (V13'S), whole-cell patch-clamp in sympathetic ganglion cells, cystometry, histological analysis","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1/2 — constitutively active point mutant in vivo with electrophysiological and anatomical validation","pmids":["15201326"],"is_preprint":false},{"year":2002,"finding":"Cannabinoids (including anandamide) stereoselectively inhibit currents through human 5-HT3A receptors independently of cannabinoid CB1/CB2 receptors, reducing maximum current without changing EC50 of 5-HT, consistent with allosteric modulatory site action; WIN55,212-2 did not displace [3H]-GR65630 from the 5-HT3 binding site.","method":"Outside-out patch-clamp in HEK293 cells stably transfected with h5-HT3A, radioligand binding competition assays","journal":"British journal of pharmacology","confidence":"High","confidence_rationale":"Tier 2 — functional electrophysiology combined with binding assays, stereoselectivity control","pmids":["12381672"],"is_preprint":false},{"year":2007,"finding":"Anandamide (AEA) inhibits 5-HT3A receptor function by accelerating desensitization in a concentration-dependent manner; the magnitude of inhibition is inversely correlated with surface receptor density and is reduced by agents (5-hydroxyindole, nocodazole) that slow desensitization, indicating desensitization kinetics regulate AEA potency.","method":"Two-electrode voltage-clamp in Xenopus oocytes and whole-cell patch-clamp in HEK293 cells; surface expression manipulation with actinomycin D","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 2 — orthogonal expression systems with pharmacological manipulation of surface density and desensitization","pmids":["17993512"],"is_preprint":false},{"year":2009,"finding":"The lipid-protein interface of the 5-HT3A receptor is defined at the M4 transmembrane helix; hydrophobic photolabel [125I]TID labeled Ser451 within M4, equivalent to the lipid-exposed face of Torpedo nAChR alpha1M4, establishing the lipid-facing surface of the 5-HT3A receptor.","method":"Purification of alphaBgTx-tagged 5-HT3A receptor, hydrophobic photoaffinity labeling with [125I]TID, peptide mapping","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — photoaffinity labeling of purified receptor identifying specific lipid-exposed residue","pmids":["19715355"],"is_preprint":false},{"year":2001,"finding":"The 5-HT3B subunit, when co-expressed with 5-HT3A, alters predominantly biophysical rather than pharmacological properties of the 5-HT3 receptor; homomeric 5-HT3A and heteromeric 5-HT3A/3B receptors show very similar pharmacological profiles.","method":"Heterologous expression in HEK293 cells, radioligand binding, whole-cell electrophysiology comparing homomeric vs. heteromeric receptors","journal":"Neuropharmacology","confidence":"High","confidence_rationale":"Tier 2 — direct comparison of homomeric vs. heteromeric receptors with multiple methods","pmids":["11489465"],"is_preprint":false},{"year":2003,"finding":"Picrotoxin selectively inhibits homomeric 5-HT3A receptors with 100-fold higher potency compared to heteromeric 5-HT3A/3B receptors, providing a pharmacological tool to distinguish receptor composition.","method":"Whole-cell patch-clamp in cells expressing mouse homomeric 5-HT3A vs. heteromeric 5-HT3A/3B receptors","journal":"Brain research. Molecular brain research","confidence":"High","confidence_rationale":"Tier 2 — direct functional comparison demonstrating 100-fold selectivity","pmids":["14625088"],"is_preprint":false},{"year":2007,"finding":"Naturally occurring single nucleotide polymorphisms A33T, S253N, and M257I in the human 5-HT3A receptor reduce maximal 5-HT-induced responses to 3-64% of wild-type without altering 5-HT potency or antagonist affinities; A33T, M257I, and R344H display reduced surface expression (2-4-fold) despite similar total expression, indicating effects on receptor biogenesis/trafficking.","method":"Fluorescence-based calcium influx assays, [3H]GR65630 radioligand binding, surface expression assays in transfected cells expressing 5-HT3A variants","journal":"Pharmacogenetics and genomics","confidence":"High","confidence_rationale":"Tier 2 — multiple functional and trafficking assays across multiple variants","pmids":["17496724"],"is_preprint":false},{"year":2014,"finding":"Partial agonism of tryptamine at the 5-HT3A receptor results from reduced priming (decreased ability to overcome transitions to closed preopen states) rather than channel blockade; in contrast, 2-Me-5HT is not a genuine partial agonist since its low apparent efficacy is mainly due to channel blockade within the activating concentration range.","method":"Single-channel patch-clamp analysis of high-conductance 5-HT3A receptor, kinetic modeling, molecular docking","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 1 — single-channel analysis with full kinetic scheme revealing distinct mechanisms for two partial agonists","pmids":["25505338"],"is_preprint":false},{"year":2012,"finding":"A coupled array of noncovalent interactions governs 5-HT3A receptor function in an agonist-specific manner: cation-π interaction with W183 (loop B), a hydrogen bond from E129 (loop A), and hydrogen bonds from D124 (loop A) to loop B backbone are tightly coupled and function as a unit. mCPBG (partial agonist) shows no cation-π at TrpB and extreme sensitivity to E129 positioning.","method":"Mutant cycle analyses using conventional and unnatural amino acid mutagenesis in Xenopus oocytes","journal":"ACS chemical neuroscience","confidence":"High","confidence_rationale":"Tier 1 — double mutant cycle analysis with unnatural amino acid mutagenesis revealing coupling","pmids":["23077719"],"is_preprint":false},{"year":2024,"finding":"FOXD3-mediated transcriptional upregulation of ALKBH5 (m6A demethylase) in injured trigeminal ganglia neurons erases m6A sites on Htr3a mRNA, preventing YTHDF2 binding and mRNA degradation, thereby stabilizing and increasing 5-HT3A protein expression and channel currents, promoting neuropathic pain.","method":"Rat trigeminal nerve injury model, ALKBH5 knockdown/overexpression, ChIP-seq for H3K27ac and FOXD3 binding, m6A-seq, RIP for YTHDF2, patch-clamp for 5-HT3 currents, behavioral pain assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1/2 — mechanistic pathway established with multiple orthogonal methods in vivo and in vitro","pmids":["38285939"],"is_preprint":false},{"year":2021,"finding":"The lncRNA linc01305 stabilizes HTR3A mRNA by interacting with RNA-binding proteins IGF2BP2 and IGF2BP3, promoting proliferation and metastasis of esophageal squamous cell carcinoma.","method":"linc01305 pulldown, mass spectrometry, RNA immunoprecipitation, mRNA stability assays, RNA-seq, FISH, transwell and colony formation assays","journal":"The international journal of biochemistry & cell biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (pulldown+MS+RIP+stability) identifying post-transcriptional regulatory mechanism","pmids":["34022433"],"is_preprint":false},{"year":2012,"finding":"Ethanol consumption induces tissue-specific DNA methylation changes at the Htr3a promoter in mouse brain and blood; reduced methylation in the dorsomedial striatum correlated with increased Htr3a mRNA expression (1.43-fold), suggesting promoter methylation inversely regulates Htr3a expression.","method":"Sequenom MassARRAY methylation quantification across 8-9 CpGs in 9 brain regions and blood, real-time PCR for Htr3a expression","journal":"Alcoholism, clinical and experimental research","confidence":"Medium","confidence_rationale":"Tier 2 — quantitative methylation analysis with expression correlation in same tissue, but single lab study","pmids":["22834954"],"is_preprint":false},{"year":2001,"finding":"A variant C178T (Pro16Ser) in an upstream open reading frame (uORF) of the HTR3A 5' UTR increases reporter gene expression by 245% (C178T alone) or 138% (C178T+C195T) compared to wild-type, indicating this variant affects HTR3A expression at the translational level.","method":"Luciferase reporter constructs containing C178T allele transfected into cells, compared to wild-type","journal":"Pharmacogenetics","confidence":"Medium","confidence_rationale":"Tier 2 — reporter assay with functional allele comparison, single lab","pmids":["11505217"],"is_preprint":false},{"year":2012,"finding":"Loss of 5-HT3A receptor in knockout mice causes exuberant apical dendritic complexity in cortical layer 2/3 pyramidal neurons, which reduces spike frequency adaptation and alters afterhyperpolarization development, demonstrating that 5-HT3A-mediated serotonergic input on Cajal-Retzius cells during early postnatal development shapes dendritic morphology and consequently neuronal excitability.","method":"5-HT3A knockout mice, morphological reconstruction, whole-cell patch-clamp recording of layer 2/3 pyramidal neurons, computational modeling","journal":"Journal of neurophysiology","confidence":"High","confidence_rationale":"Tier 2 — KO with quantitative morphological and electrophysiological analysis validated by computational model","pmids":["22696545"],"is_preprint":false},{"year":2017,"finding":"In Htr3a knockout mice, fluoxetine effects are blunted while citalopram triggers 5-HT1A receptor desensitization in the dorsal raphe at lower doses than in wild-type, suggesting 5-HT3 receptor inactivation promotes SSRI efficacy via altered autoreceptor sensitivity. KO also prevents CSDS-induced cortical changes in CaMKIIa and SOD1.","method":"Htr3a KO mice, behavioral tests (EPM, FST, social interaction), in vitro dorsal raphe electrophysiology, gene expression analysis","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — KO with electrophysiology identifying raphe autoreceptor mechanism, single lab","pmids":["28493335"],"is_preprint":false}],"current_model":"HTR3A encodes the obligatory 5-HT3A subunit of the serotonin type 3 receptor, a pentameric Cys-loop ligand-gated cation channel that is activated by binding of at least three serotonin molecules; agonist recognition involves a cation-π interaction at Trp183 (loop B) coupled to hydrogen bonds from loop A residues E129 and D124, while gating is transduced through pre-TM1 Arg222 and the TM2-TM3 linker (D298), desensitization is governed by the 4′ lysine in M2 and Arg427 in the cytoplasmic domain, single-channel conductance is rate-limited by arginine residues (Arg436, Arg440) in the intracellular MA helix portals and anionic residues in the extracellular vestibule, RIC-3 chaperone selectively enhances surface expression of homomeric 5-HT3A receptors, N-glycosylation at N109/N174/N190 controls assembly and trafficking, and the receptor plays essential roles in fear extinction, NMDAR-dependent LTD via AMPA receptor internalization, cortical interneuron-regulated E/I balance, and bladder innervation development."},"narrative":{"teleology":[{"year":2000,"claim":"Identifying the M2 4′ lysine as a desensitization determinant separable from channel conductance established that gating conformational changes extend beyond the pore-lining face of M2.","evidence":"Site-directed mutagenesis with whole-cell patch-clamp and fluctuation analysis in HEK293 cells","pmids":["10639097"],"confidence":"High","gaps":["No structural basis for how 4′ position influences desensitization without lining the pore","Whether the same residue functions identically in heteromeric 5-HT3A/3B receptors"]},{"year":2000,"claim":"Discovery of alternative splice variants (h5-HT3AT and h5-HT3AL) that modulate desensitization and ion flux when co-expressed with 5-HT3A revealed endogenous mechanisms for tuning receptor properties without requiring heteromeric subunit partners.","evidence":"cDNA cloning, HEK293 electrophysiology, RT-PCR confirmation in amygdala/hippocampus","pmids":["11111833"],"confidence":"High","gaps":["Stoichiometry of splice variant incorporation into pentamers unknown","In vivo functional significance of splice variants not tested"]},{"year":2001,"claim":"Demonstrating that at least three serotonin molecules must bind to open the channel, with concentration-dependent open probability, defined the cooperativity and occupancy requirements for 5-HT3A gating.","evidence":"Rapid agonist application, outside-out patch-clamp, and kinetic modeling in HEK293 cells","pmids":["11533135"],"confidence":"High","gaps":["Which specific binding sites among five subunits are occupied at threshold unknown","Whether heteromeric receptors have the same occupancy requirement"]},{"year":2002,"claim":"Identification of a cation-π interaction between serotonin and Trp183 (loop B) using unnatural amino acid mutagenesis precisely located the agonist-receptor contact and quantified its ~4 kcal/mol contribution, establishing the primary binding mechanism.","evidence":"Unnatural amino acid incorporation and electrophysiology in Xenopus oocytes","pmids":["12162741"],"confidence":"High","gaps":["Whether the same interaction applies to all five binding sites in the pentamer","Structural validation awaited (no crystal/cryo-EM structure at the time)"]},{"year":2003,"claim":"Showing that Arg222 in the pre-TM1 region links binding to gating — its mutation increased potency, altered kinetics, and converted an antagonist to agonist — identified a critical transduction element between the extracellular and transmembrane domains.","evidence":"Site-directed mutagenesis and patch-clamp in HEK293 cells","pmids":["12970351"],"confidence":"High","gaps":["The partner residues that Arg222 interacts with during transduction not identified","No direct structural evidence for conformational change at this position"]},{"year":2004,"claim":"Mapping the M2 pore with systematic SCAM and Cd²⁺ probing in open and closed states revealed that the cytoplasmic selectivity filter maintains a narrow pore in both states, indicating minimal rearrangement at the 2′ and −2′ positions during gating.","evidence":"Cysteine mutagenesis with state-dependent Cd²⁺ accessibility in HEK293 cells","pmids":["15131114"],"confidence":"High","gaps":["Resolution limited to accessibility changes; structural movements below probe size undetectable","Whether this holds for heteromeric receptors"]},{"year":2004,"claim":"Dissecting N-glycosylation sites showed N109 is required for receptor assembly while N174 and N190 control plasma membrane trafficking and ligand binding, establishing that post-translational modifications at distinct sites play non-redundant roles in receptor biogenesis.","evidence":"Site-directed mutagenesis of each glycosylation site with surface expression, radioligand binding, and Ca²⁺ influx assays","pmids":["15264219"],"confidence":"High","gaps":["Glycan composition at each site not determined","Whether glycosylation requirements differ in neurons vs. heterologous cells"]},{"year":2005,"claim":"Identification of D298 in the TM2-TM3 linker as a charge-dependent determinant of gating kinetics and Ca²⁺ modulation, together with loop C residue contributions, expanded the map of conformational coupling from the ligand-binding domain to the channel gate.","evidence":"Charge-reversal/neutralization mutagenesis with fast-application patch-clamp (D298); alanine mutagenesis with binding and electrophysiology (loop C) in oocytes and HEK293 cells","pmids":["16096341","15966738"],"confidence":"High","gaps":["No direct measurement of conformational movements at these positions","Interaction partners of D298 within the linker not mapped"]},{"year":2006,"claim":"Localization of Arg427 in the large cytoplasmic domain as a desensitization determinant, with polarity-dependent kinetics, extended the desensitization mechanism beyond the transmembrane domain into the intracellular region.","evidence":"Sequential LCD deletion and point mutagenesis with whole-cell patch-clamp in HEK293 cells","pmids":["16754678"],"confidence":"High","gaps":["Structural context of Arg427 within the cytoplasmic domain unknown","Whether phosphorylation or protein interactions at this site modulate desensitization in vivo"]},{"year":2007,"claim":"Dynamic charge modification at Arg436 and systematic scanning of MA helix residues (435–440) demonstrated that cytoplasmic portals impose a rate-limiting electrostatic barrier to ion permeation, explaining the characteristically low single-channel conductance of homomeric 5-HT3A receptors.","evidence":"SCAM with MTS reagents, alanine/arginine scanning, single-channel electrophysiology in HEK293 cells","pmids":["17200121","24030822"],"confidence":"High","gaps":["Structural model of portal geometry at atomic resolution not available","How heteromeric subunit composition alters portal electrostatics"]},{"year":2007,"claim":"Kinetic modeling of serotonin vs. dopamine as high- vs. low-efficacy agonists revealed that partial agonism arises from distinct opening rates and dissociation from open/desensitized states, providing a mechanistic framework for efficacy differences at this receptor.","evidence":"Rapid solution exchange electrophysiology with allosteric kinetic modeling in HEK293 cells","pmids":["18045909"],"confidence":"High","gaps":["Whether the kinetic scheme generalizes to other partial agonists","No structural basis for differential state-dependent binding"]},{"year":2010,"claim":"Demonstrating that RIC-3 chaperone directly interacts with 5-HT3A (and C/D/E subunits) in the ER and selectively enhances homomeric 5-HT3A surface expression identified an ER-resident quality control step specific to receptor biogenesis.","evidence":"Co-IP, ER co-localization, flow cytometry, Ca²⁺ influx, and radioligand binding in HEK293 cells","pmids":["20522555"],"confidence":"High","gaps":["Molecular interface between RIC-3 and 5-HT3A not mapped","Whether RIC-3 is rate-limiting for 5-HT3A expression in neurons"]},{"year":2012,"claim":"Mutant cycle analysis with unnatural amino acids revealed a tightly coupled array — cation-π at W183, hydrogen bonds from E129 and D124 — that functions as a unit for serotonin recognition but not for the partial agonist mCPBG, establishing agonist-specific binding site mechanics.","evidence":"Double mutant cycle analysis with unnatural amino acid mutagenesis in Xenopus oocytes","pmids":["23077719"],"confidence":"High","gaps":["How coupling energetics translate to conformational changes in the binding domain","Coupling analysis not extended to other agonists beyond 5-HT and mCPBG"]},{"year":2012,"claim":"5-HT3A knockout abolished NMDAR-dependent LTD (but not mGluR-dependent LTD) and blocked activity-dependent AMPA receptor internalization at hippocampal CA1 synapses, establishing a specific role for 5-HT3A in one form of synaptic plasticity.","evidence":"Htr3a KO mice, hippocampal slice LTD protocols, AMPAR internalization assays","pmids":["25130560"],"confidence":"High","gaps":["Cell type (interneuron vs. pyramidal) through which 5-HT3A acts in LTD not determined","Signaling intermediates between 5-HT3A activation and AMPAR endocytosis unknown"]},{"year":2012,"claim":"Loss of 5-HT3A caused exuberant apical dendritic complexity in cortical L2/3 pyramidal neurons with altered excitability, linking serotonergic input on Cajal-Retzius cells during development to cortical circuit maturation.","evidence":"Htr3a KO mice, morphological reconstruction, whole-cell patch-clamp, computational modeling","pmids":["22696545"],"confidence":"High","gaps":["Molecular signals downstream of 5-HT3A in Cajal-Retzius cells not identified","Whether dendritic phenotype is reversible if 5-HT3A is restored postnatally"]},{"year":2013,"claim":"Htr3a KO mice showed normal fear acquisition but failed to extinguish contextual and cued fear, dissociating 5-HT3A function specifically to fear extinction circuitry.","evidence":"Fear conditioning and extinction behavioral paradigms in Htr3a KO mice","pmids":["24344177"],"confidence":"High","gaps":["Brain region and cell type mediating the extinction deficit not identified","Whether pharmacological 5-HT3 blockade recapitulates the extinction deficit"]},{"year":2017,"claim":"5-HT3A loss disrupted bladder innervation development — altering autonomic neuronal subtype balance in fetal pelvic ganglia and increasing fiber density — resulting in voiding dysfunction, establishing a developmental role for 5-HT3A in visceral innervation patterning.","evidence":"Htr3a KO mice, cystometry, immunohistochemistry for neuronal markers across developmental timepoints","pmids":["29311772"],"confidence":"High","gaps":["Mechanism by which 5-HT3A signaling regulates neuronal subtype specification in ganglia unknown","Whether human HTR3A variants cause bladder dysfunction"]},{"year":2021,"claim":"5-HT3A loss caused NMDAR upregulation specifically in PV+ interneurons, increasing their excitability and GABAergic output onto pyramidal neurons, reducing E/I ratio and producing autistic-like behaviors rescuable by memantine — identifying a cell-type-specific mechanism linking 5-HT3A to E/I balance.","evidence":"TALEN-generated Htr3a KO mice, behavioral testing, patch-clamp in PV+ interneurons, transcriptomics, immunoblotting, memantine rescue","pmids":["34646371"],"confidence":"High","gaps":["How 5-HT3A loss leads to NMDAR transcriptional upregulation in PV+ cells not resolved","Whether the E/I imbalance mechanism generalizes to other brain regions"]},{"year":2024,"claim":"FOXD3-driven ALKBH5 upregulation erases m6A marks on Htr3a mRNA, preventing YTHDF2-mediated degradation and stabilizing 5-HT3A expression to promote neuropathic pain — revealing an epitranscriptomic regulatory layer controlling HTR3A abundance.","evidence":"Rat trigeminal nerve injury, ALKBH5 manipulation, ChIP-seq, m6A-seq, RIP, patch-clamp, behavioral pain assays","pmids":["38285939"],"confidence":"High","gaps":["Whether m6A regulation of Htr3a occurs in CNS neurons beyond trigeminal ganglia","Other m6A reader proteins that may also regulate Htr3a stability"]},{"year":null,"claim":"High-resolution structural basis of the full gating cycle (resting → open → desensitized) for homomeric and heteromeric 5-HT3 receptors, the molecular determinants of heteromeric subunit assembly stoichiometry, and the signaling pathways linking 5-HT3A channel activity to AMPAR internalization and fear extinction circuitry remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No full gating-cycle cryo-EM structures capturing all functional states in the timeline","Intracellular signaling cascade from 5-HT3A to AMPAR endocytosis not identified","Cell-type and circuit identity mediating fear extinction role undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[4,9,11]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,35]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[16,17,20]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[16]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[23,24,25,40]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[25,23]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[4,9,11]}],"complexes":["5-HT3 receptor (homomeric pentamer)","5-HT3A/3B heteromeric receptor"],"partners":["HTR3B","RIC3","ALKBH5","IGF2BP2","IGF2BP3","YTHDF2"],"other_free_text":[]},"mechanistic_narrative":"HTR3A encodes the obligatory 5-HT3A subunit of the serotonin type 3 receptor, a pentameric Cys-loop ligand-gated cation channel that mediates fast excitatory serotonergic neurotransmission and regulates synaptic plasticity, fear memory extinction, cortical circuit development, and autonomic innervation. Agonist recognition requires a cation-π interaction at Trp183 (loop B) coupled to hydrogen bonds from loop A residues E129 and D124, with at least three serotonin molecules needed for channel opening; gating is transduced through pre-TM1 Arg222 and TM2-TM3 linker residue D298, while desensitization is governed by the M2 4′ lysine and cytoplasmic Arg427 [PMID:12162741, PMID:23077719, PMID:11533135, PMID:12970351, PMID:16096341, PMID:10639097, PMID:16754678]. Single-channel conductance is rate-limited by arginine residues (Arg436, Arg440) forming electrostatic barriers in the intracellular MA helix portals and by anionic residues (Asp113, Asp127) in the extracellular vestibule, while surface expression depends on RIC-3 chaperone interaction, N-glycosylation at N109/N174/N190, and the C-terminal residue Ala455 [PMID:17200121, PMID:24030822, PMID:21454663, PMID:20522555, PMID:15264219, PMID:18786552]. In vivo, 5-HT3A is essential for NMDAR-dependent hippocampal LTD via AMPA receptor internalization, fear extinction, cortical dendritic patterning through Cajal-Retzius cell signaling, excitation/inhibition balance in parvalbumin-positive interneurons, and normal bladder innervation development [PMID:25130560, PMID:24344177, PMID:22696545, PMID:34646371, PMID:29311772]."},"prefetch_data":{"uniprot":{"accession":"P46098","full_name":"5-hydroxytryptamine receptor 3A","aliases":["5-hydroxytryptamine receptor 3","5-HT-3","5-HT3R","Serotonin receptor 3A","Serotonin-gated ion channel receptor"],"length_aa":478,"mass_kda":55.3,"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/P46098/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/HTR3A","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/HTR3A","total_profiled":1310},"omim":[{"mim_id":"610509","title":"RIC3 ACETYLCHOLINE RECEPTOR CHAPERONE; RIC3","url":"https://www.omim.org/entry/610509"},{"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"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"brain","ntpm":2.3},{"tissue":"intestine","ntpm":2.0},{"tissue":"lymphoid tissue","ntpm":7.5},{"tissue":"pancreas","ntpm":2.7},{"tissue":"salivary gland","ntpm":4.8}],"url":"https://www.proteinatlas.org/search/HTR3A"},"hgnc":{"alias_symbol":["5-HT3R","5-HT3A"],"prev_symbol":["HTR3"]},"alphafold":{"accession":"P46098","domains":[{"cath_id":"2.70.170.10","chopping":"40-241","consensus_level":"high","plddt":94.5796,"start":40,"end":241},{"cath_id":"-","chopping":"243-357_426-478","consensus_level":"medium","plddt":85.7905,"start":243,"end":478}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P46098","model_url":"https://alphafold.ebi.ac.uk/files/AF-P46098-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P46098-F1-predicted_aligned_error_v6.png","plddt_mean":82.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=HTR3A","jax_strain_url":"https://www.jax.org/strain/search?query=HTR3A"},"sequence":{"accession":"P46098","fasta_url":"https://rest.uniprot.org/uniprotkb/P46098.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P46098/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P46098"}},"corpus_meta":[{"pmid":"12162741","id":"PMC_12162741","title":"Cation-pi interactions in ligand recognition by serotonergic (5-HT3A) and nicotinic acetylcholine receptors: the anomalous binding properties of nicotine.","date":"2002","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12162741","citation_count":230,"is_preprint":false},{"pmid":"12381672","id":"PMC_12381672","title":"Direct inhibition by cannabinoids of human 5-HT3A receptors: probable involvement of an allosteric modulatory site.","date":"2002","source":"British journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/12381672","citation_count":160,"is_preprint":false},{"pmid":"17250964","id":"PMC_17250964","title":"Mechanical, thermal and formalin-induced nociception is differentially altered in 5-HT1A-/-, 5-HT1B-/-, 5-HT2A-/-, 5-HT3A-/- and 5-HTT-/- knock-out male mice.","date":"2007","source":"Pain","url":"https://pubmed.ncbi.nlm.nih.gov/17250964","citation_count":124,"is_preprint":false},{"pmid":"11505217","id":"PMC_11505217","title":"Association between the 5' UTR variant C178T of the serotonin receptor gene HTR3A and bipolar affective disorder.","date":"2001","source":"Pharmacogenetics","url":"https://pubmed.ncbi.nlm.nih.gov/11505217","citation_count":100,"is_preprint":false},{"pmid":"12568911","id":"PMC_12568911","title":"Targeted gene deletion of the 5-HT3A receptor subunit produces an anxiolytic phenotype in mice.","date":"2003","source":"European journal of pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/12568911","citation_count":85,"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":"11489465","id":"PMC_11489465","title":"Pharmacological comparison of human homomeric 5-HT3A receptors versus heteromeric 5-HT3A/3B receptors.","date":"2001","source":"Neuropharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/11489465","citation_count":71,"is_preprint":false},{"pmid":"21420406","id":"PMC_21420406","title":"The HTR3A polymorphism c. -42C>T is associated with amygdala responsiveness in patients with irritable bowel syndrome.","date":"2011","source":"Gastroenterology","url":"https://pubmed.ncbi.nlm.nih.gov/21420406","citation_count":68,"is_preprint":false},{"pmid":"14557147","id":"PMC_14557147","title":"A polymorphism in the serotonin receptor 3A (HTR3A) gene and its association with harm avoidance in women.","date":"2003","source":"Archives of general psychiatry","url":"https://pubmed.ncbi.nlm.nih.gov/14557147","citation_count":66,"is_preprint":false},{"pmid":"15515405","id":"PMC_15515405","title":"Expression of 5-HT3A receptors in cells of the immune system.","date":"2004","source":"Scandinavian journal of rheumatology. 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of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2002,\n      \"finding\": \"A cation-π interaction between serotonin and Trp183 (loop B) of the 5-HT3A receptor is required for agonist binding, precisely locating the ligand-binding site. The energetic contribution is approximately 4 kcal/mol.\",\n      \"method\": \"Unnatural amino acid mutagenesis and heterologous expression in Xenopus oocytes (in vitro electrophysiology + mutagenesis)\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted functional assay with unnatural amino acid mutagenesis, quantified energetics\",\n      \"pmids\": [\"12162741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Arginine 222 (Arg-222) in the pre-transmembrane domain 1 of the 5-HT3A receptor links agonist binding to channel gating; R222A mutation increased agonist potency and efficacy, accelerated activation and desensitization kinetics, and converted an antagonist (apomorphine) to a potent agonist.\",\n      \"method\": \"Site-directed mutagenesis and whole-cell patch-clamp electrophysiology in HEK293 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro mutagenesis with detailed kinetic analysis\",\n      \"pmids\": [\"12970351\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Arginine 427 in the large cytoplasmic domain (LCD) of the 5-HT3A receptor contributes to receptor desensitization; deletion or point mutation at this position significantly slowed desensitization kinetics, with desensitization rate positively correlated with polarity of the residue at position 427.\",\n      \"method\": \"Sequential LCD deletion and point mutagenesis with whole-cell patch-clamp in HEK293 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with multiple mutants and mechanistic correlation\",\n      \"pmids\": [\"16754678\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The 4′ lysine residue in the M2 channel-lining domain of the 5-HT3A receptor affects desensitization kinetics but does not form part of the channel lining; mutations at this position slow desensitization without altering single-channel conductance.\",\n      \"method\": \"Site-directed mutagenesis, whole-cell patch-clamp, and fluctuation analysis in HEK293 cells\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro mutagenesis with single-channel and whole-cell electrophysiology\",\n      \"pmids\": [\"10639097\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Homomeric murine 5-HT3A receptors require binding of at least three serotonin molecules to open, and channel open probability varies with the number of bound agonist molecules, with reduced open probability for fully liganded receptors; peak open probability exceeds 0.8.\",\n      \"method\": \"Rapid agonist application, whole-cell and excised outside-out patch clamp, kinetic modeling in HEK293 cells\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — quantitative biophysical analysis with multiple concentrations and state modeling\",\n      \"pmids\": [\"11533135\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The conserved proline P303 in the M2-M3 linker of the human 5-HT3A receptor is important for function (Ca2+ dependence and desensitization rate) but trans-cis isomerization at this proline is not required for agonist-induced channel opening.\",\n      \"method\": \"Natural amino acid mutagenesis of P303 with two-electrode voltage-clamp in Xenopus oocytes\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with functional electrophysiological characterization\",\n      \"pmids\": [\"19457066\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Aspartate 298 (D298) in the TM2-TM3 extracellular loop of the mouse 5-HT3A receptor participates in channel gating kinetics (desensitization, deactivation, partial agonist efficacy) and is crucial for Ca2+ modulation; charge at this residue determines these properties.\",\n      \"method\": \"Site-directed mutagenesis, whole-cell patch-clamp with fast agonist application in HEK293 cells\",\n      \"journal\": \"The Journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic charge-reversal/neutralization mutagenesis with kinetic analysis\",\n      \"pmids\": [\"16096341\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Loop C residues F226, I228, D229, and Y234 of the murine 5-HT3A receptor contribute differentially to ligand binding and gating; I228 and D229 are specific for 5-HT versus mCPBG interactions, while F226 and Y234 are important for both agonists.\",\n      \"method\": \"Site-directed mutagenesis, radioligand binding, two-electrode voltage-clamp in Xenopus oocytes, homology modeling\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with binding and electrophysiology\",\n      \"pmids\": [\"15966738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Serotonin and dopamine act as high- and low-efficacy agonists, respectively, at human 5-HT3A receptors with distinct kinetic mechanisms: serotonin shows concentration-dependent activation and sigmoidal recovery from desensitization; dopamine shows concentration-independent activation, faster deactivation, and non-sigmoidal recovery, explained by different rates of channel opening and dissociation from open/desensitized states.\",\n      \"method\": \"Rapid solution exchange electrophysiology in HEK293 cells with allosteric kinetic modeling\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — rigorous biophysical characterization with kinetic modeling\",\n      \"pmids\": [\"18045909\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Dynamic modification of the charge at cytoplasmic residue Arg436 regulates single-channel conductance of the 5-HT3A receptor, consistent with cytoplasmic portals that impose a rate-limiting barrier to ion conduction; negatively charged MTS reagents at this position increased conductance while positively charged reagents decreased it.\",\n      \"method\": \"Cysteine substitution (R436C), MTS reagent modification, outside-out and inside-out patch clamp electrophysiology\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — substituted cysteine accessibility method with bidirectional charge modification\",\n      \"pmids\": [\"17200121\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Multiple residues within the MA (membrane-associated) helix intracellular portals of human 5-HT3A (positions 435, 436, 439, 440) markedly influence single-channel conductance; alanine- and arginine-scanning mutagenesis combined with substituted cysteine accessibility method generated a functional map of the cytoplasmic portals.\",\n      \"method\": \"Alanine/arginine-scanning mutagenesis and substituted cysteine accessibility method (SCAM) with single-channel electrophysiology\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic SCAM and mutagenesis across MA helix\",\n      \"pmids\": [\"24030822\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Two anionic residues in the extracellular vestibule of the 5-HT3A receptor, Asp113 and Asp127, markedly influence single-channel conductance, Ca2+ permeability relative to Cs+ (PCa/PCs), and Ca2+-mediated suppression of conductance; D127 mutations had larger effects than D113, and both influenced ion selectivity.\",\n      \"method\": \"Site-directed mutagenesis, single-channel outside-out patch-clamp, permeability ratio measurements in HEK293 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — charge-reversal mutagenesis with quantitative single-channel and selectivity measurements\",\n      \"pmids\": [\"21454663\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The cytoplasmic pore (selectivity filter) of the 5-HT3A receptor maintains a narrow pore in both open and closed states, indicating minimal structural rearrangement at the 2' and -2' positions during channel gating.\",\n      \"method\": \"Systematic cysteine mutagenesis throughout M1-M2 loop and M2 domain with Cd2+ accessibility probing in open and closed states\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic SCAM with state-dependent metal ion probing\",\n      \"pmids\": [\"15131114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Granisetron interacts with a binding cavity formed by loop B, C, and E residues of the 5-HT3A receptor; H185A mutation abolished granisetron binding and D189A reduced affinity 22-fold; Y143 and Y153 in loop E contribute via their hydroxyl groups.\",\n      \"method\": \"Homology modeling, docking, and alanine mutagenesis with radioligand binding\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1/2 — mutagenesis with binding assays, supported by computational model but no structure\",\n      \"pmids\": [\"16430206\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Ginsenoside Rg3 inhibits 5-HT3A receptor channel activity through interactions with residues V291, F292, and I295 in the channel gating region of TM2, acting in the open state at sites distinct from those of TMB-8 and diltiazem.\",\n      \"method\": \"TM2 mutagenesis (V291A, F292A, I295A) and two-electrode voltage-clamp in Xenopus oocytes\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis identifying specific channel pore residues with functional electrophysiology\",\n      \"pmids\": [\"17257631\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"L293 (L15') in the second transmembrane domain (TM2) of the 5-HT3A receptor is a molecular determinant of allosteric modulation by 5-hydroxyindole (5-HI); L293C and L293S mutations abolished 5-HI potentiation and converted 5-HI to a partial agonist, while also altering desensitization.\",\n      \"method\": \"Site-directed mutagenesis and whole-cell patch-clamp in HEK293 cells and N1E-115 cells\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis identifying a specific TM2 residue for allosteric modulation\",\n      \"pmids\": [\"18436267\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"RIC-3 chaperone directly interacts with 5-HT3A, -C, -D, and -E subunits (co-localizes in ER) but predominantly enhances surface expression of homomeric 5-HT3A receptors in HEK293 cells without altering 5-HT potency; increased Emax correlates with increased Bmax and surface expression.\",\n      \"method\": \"Co-immunoprecipitation, immunocytochemistry (ER co-localization), flow cytometry for surface expression, Ca2+ influx assays, radioligand binding\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-localization, direct interaction assays, and functional consequences with multiple methods\",\n      \"pmids\": [\"20522555\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"N-glycosylation at three sites of the murine 5-HT3A receptor (N109, N174, N190) plays distinct roles: N109 is necessary for receptor assembly, while N174 and N190 are important for plasma membrane targeting and ligand binding; all three are required for Ca2+ influx.\",\n      \"method\": \"Tunicamycin treatment, site-directed mutagenesis, surface expression assays, radioligand binding, Ca2+ influx assays in transfected cells\",\n      \"journal\": \"Journal of neuroscience research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis of each glycosylation site with multiple functional readouts\",\n      \"pmids\": [\"15264219\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The C-terminal residue Ala455 of the human 5-HT3A subunit is critical for receptor folding/maturation and membrane expression; deletion of the three C-terminal residues abolished radioligand binding and membrane expression, and Ala455Leu mutation caused 88% reduction.\",\n      \"method\": \"C-terminal deletion and point mutagenesis, radioligand binding, cell membrane expression assays at 37°C and 27°C\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with binding and expression assays revealing temperature-dependent rescue\",\n      \"pmids\": [\"18786552\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"5-HT3A receptor subunit is localized in axonal (presynaptic), somatodendritic, and glial profiles in the medial nucleus of the solitary tract (mNTS); presynaptically, it associates with synaptic vesicle membranes and extrasynaptic plasma membranes, consistent with modulation of both presynaptic release and postsynaptic responses.\",\n      \"method\": \"Electron microscopic immunocytochemistry with subcellular localization in rat brain tissue\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct ultrastructural localization but functional consequence inferred\",\n      \"pmids\": [\"15527741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Fluorescently tagged YFP-5-HT3A receptor is targeted to plasma membrane, micropodia in HEK293 cells, and dendritic spines in hippocampal neurons, accessible by extracellular probes, in contrast to alpha3beta4-nAChRs which remain predominantly intracellular.\",\n      \"method\": \"Live fluorescence microscopy with YFP-fusion subunit, extracellular fluorescent probe accessibility, whole-cell patch-clamp in HEK293 cells and hippocampal neurons\",\n      \"journal\": \"The European journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct live-cell imaging with functional validation (current amplitude correlated with surface expression)\",\n      \"pmids\": [\"15009132\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Alternative splicing of the human 5-HT3A gene produces a short truncated variant (h5-HT3AT, 238 aa, single TM domain) and a long variant (h5-HT3AL, 32 extra aa in M2-M3 extracellular loop); neither forms functional homomeric receptors, but h5-HT3AT slows desensitization and greatly increases cation flux when co-expressed with 5-HT3A, while h5-HT3AL reduces cation flux.\",\n      \"method\": \"CDNA cloning, HEK293 cell transfection, whole-cell patch-clamp electrophysiology, expression in amygdala/hippocampus confirmed by RT-PCR\",\n      \"journal\": \"Naunyn-Schmiedeberg's archives of pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — functional reconstitution of splice variants with electrophysiological characterization\",\n      \"pmids\": [\"11111833\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The M3M4 intracellular loop of the 5-HT3A receptor is not required for receptor assembly or function; however, loop length and amino acid composition affect channel expression and desensitization rate, suggesting the cytoplasmic ends of M3 and M4 undergo conformational changes during gating/desensitization.\",\n      \"method\": \"M3M4 loop replacement with 1-7 alanine residues, expression in Xenopus oocytes, two-electrode voltage-clamp\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic loop substitution mutagenesis with functional characterization\",\n      \"pmids\": [\"22539982\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Deletion of 5-HT3A receptor gene in mice abolishes NMDAR-dependent long-term depression (LTD) at hippocampal CA1 synapses and inhibits AMPA receptor internalization following low-frequency stimulation, without altering mGluR-dependent LTD, basal AMPAR surface levels, or synaptic structure.\",\n      \"method\": \"5-HT3AR knockout mice, hippocampal slice electrophysiology (LTD induction by LFS), AMPAR internalization assay\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined synaptic plasticity phenotype and receptor-specific mechanistic dissection\",\n      \"pmids\": [\"25130560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The 5-HT3A receptor is essential for fear extinction in mice; Htr3a knockout mice show normal fear acquisition and retention but fail to extinguish contextual and tone-cued fear memory.\",\n      \"method\": \"Htr3a knockout mice, fear conditioning and extinction behavioral paradigms (contextual and cued)\",\n      \"journal\": \"Learning & memory\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with specific behavioral phenotype dissociating acquisition from extinction\",\n      \"pmids\": [\"24344177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Htr3a knockout mice exhibit autistic-like behaviors; mechanistically, loss of 5-HT3A leads to NMDAR upregulation specifically in parvalbumin-positive (PV+) interneurons, increasing NMDAR current and excitability, which enhances GABAergic transmission to pyramidal neurons and decreases the E/I ratio. NMDAR antagonist memantine rescues GABAergic transmission and autistic-like behaviors.\",\n      \"method\": \"TALEN-generated Htr3a KO mice, behavioral tests, patch-clamp electrophysiology, transcriptome sequencing, immunoblotting, immunofluorescence, immunoprecipitation, memantine rescue\",\n      \"journal\": \"Theranostics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with multiple orthogonal methods identifying specific interneuron mechanism and pharmacological rescue\",\n      \"pmids\": [\"34646371\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"5-HT3A receptor is required for normal bladder innervation development; Htr3a knockout male mice exhibit increased voiding frequency and decreased voiding efficiency. Loss of 5-HT3A causes transient imbalance of autonomic neuronal subtype markers in fetal pelvic ganglia and higher density of autonomic/sensory neuronal fibers in bladder smooth muscle in both fetal and adult mice.\",\n      \"method\": \"Htr3a knockout mice, cystometry, immunohistochemistry for neuronal markers, fiber density quantification\",\n      \"journal\": \"Frontiers in neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined functional and developmental phenotypes across multiple timepoints\",\n      \"pmids\": [\"29311772\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Constitutively activating V13'S mutation in the M2 domain of the 5-HT3A receptor (valine to serine) renders the receptor 70-fold more sensitive to serotonin and produces constitutive activity when combined with 5-HT3B; homozygous knock-in mice develop urinary bladder dysfunction and die from obstructive uropathy, suggesting persistent 5-HT3A activation causes excitotoxic neuronal death in bladder.\",\n      \"method\": \"Targeted exon replacement knock-in mice (V13'S), whole-cell patch-clamp in sympathetic ganglion cells, cystometry, histological analysis\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — constitutively active point mutant in vivo with electrophysiological and anatomical validation\",\n      \"pmids\": [\"15201326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Cannabinoids (including anandamide) stereoselectively inhibit currents through human 5-HT3A receptors independently of cannabinoid CB1/CB2 receptors, reducing maximum current without changing EC50 of 5-HT, consistent with allosteric modulatory site action; WIN55,212-2 did not displace [3H]-GR65630 from the 5-HT3 binding site.\",\n      \"method\": \"Outside-out patch-clamp in HEK293 cells stably transfected with h5-HT3A, radioligand binding competition assays\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional electrophysiology combined with binding assays, stereoselectivity control\",\n      \"pmids\": [\"12381672\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Anandamide (AEA) inhibits 5-HT3A receptor function by accelerating desensitization in a concentration-dependent manner; the magnitude of inhibition is inversely correlated with surface receptor density and is reduced by agents (5-hydroxyindole, nocodazole) that slow desensitization, indicating desensitization kinetics regulate AEA potency.\",\n      \"method\": \"Two-electrode voltage-clamp in Xenopus oocytes and whole-cell patch-clamp in HEK293 cells; surface expression manipulation with actinomycin D\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — orthogonal expression systems with pharmacological manipulation of surface density and desensitization\",\n      \"pmids\": [\"17993512\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The lipid-protein interface of the 5-HT3A receptor is defined at the M4 transmembrane helix; hydrophobic photolabel [125I]TID labeled Ser451 within M4, equivalent to the lipid-exposed face of Torpedo nAChR alpha1M4, establishing the lipid-facing surface of the 5-HT3A receptor.\",\n      \"method\": \"Purification of alphaBgTx-tagged 5-HT3A receptor, hydrophobic photoaffinity labeling with [125I]TID, peptide mapping\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — photoaffinity labeling of purified receptor identifying specific lipid-exposed residue\",\n      \"pmids\": [\"19715355\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The 5-HT3B subunit, when co-expressed with 5-HT3A, alters predominantly biophysical rather than pharmacological properties of the 5-HT3 receptor; homomeric 5-HT3A and heteromeric 5-HT3A/3B receptors show very similar pharmacological profiles.\",\n      \"method\": \"Heterologous expression in HEK293 cells, radioligand binding, whole-cell electrophysiology comparing homomeric vs. heteromeric receptors\",\n      \"journal\": \"Neuropharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct comparison of homomeric vs. heteromeric receptors with multiple methods\",\n      \"pmids\": [\"11489465\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Picrotoxin selectively inhibits homomeric 5-HT3A receptors with 100-fold higher potency compared to heteromeric 5-HT3A/3B receptors, providing a pharmacological tool to distinguish receptor composition.\",\n      \"method\": \"Whole-cell patch-clamp in cells expressing mouse homomeric 5-HT3A vs. heteromeric 5-HT3A/3B receptors\",\n      \"journal\": \"Brain research. Molecular brain research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct functional comparison demonstrating 100-fold selectivity\",\n      \"pmids\": [\"14625088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Naturally occurring single nucleotide polymorphisms A33T, S253N, and M257I in the human 5-HT3A receptor reduce maximal 5-HT-induced responses to 3-64% of wild-type without altering 5-HT potency or antagonist affinities; A33T, M257I, and R344H display reduced surface expression (2-4-fold) despite similar total expression, indicating effects on receptor biogenesis/trafficking.\",\n      \"method\": \"Fluorescence-based calcium influx assays, [3H]GR65630 radioligand binding, surface expression assays in transfected cells expressing 5-HT3A variants\",\n      \"journal\": \"Pharmacogenetics and genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple functional and trafficking assays across multiple variants\",\n      \"pmids\": [\"17496724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Partial agonism of tryptamine at the 5-HT3A receptor results from reduced priming (decreased ability to overcome transitions to closed preopen states) rather than channel blockade; in contrast, 2-Me-5HT is not a genuine partial agonist since its low apparent efficacy is mainly due to channel blockade within the activating concentration range.\",\n      \"method\": \"Single-channel patch-clamp analysis of high-conductance 5-HT3A receptor, kinetic modeling, molecular docking\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — single-channel analysis with full kinetic scheme revealing distinct mechanisms for two partial agonists\",\n      \"pmids\": [\"25505338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A coupled array of noncovalent interactions governs 5-HT3A receptor function in an agonist-specific manner: cation-π interaction with W183 (loop B), a hydrogen bond from E129 (loop A), and hydrogen bonds from D124 (loop A) to loop B backbone are tightly coupled and function as a unit. mCPBG (partial agonist) shows no cation-π at TrpB and extreme sensitivity to E129 positioning.\",\n      \"method\": \"Mutant cycle analyses using conventional and unnatural amino acid mutagenesis in Xenopus oocytes\",\n      \"journal\": \"ACS chemical neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — double mutant cycle analysis with unnatural amino acid mutagenesis revealing coupling\",\n      \"pmids\": [\"23077719\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"FOXD3-mediated transcriptional upregulation of ALKBH5 (m6A demethylase) in injured trigeminal ganglia neurons erases m6A sites on Htr3a mRNA, preventing YTHDF2 binding and mRNA degradation, thereby stabilizing and increasing 5-HT3A protein expression and channel currents, promoting neuropathic pain.\",\n      \"method\": \"Rat trigeminal nerve injury model, ALKBH5 knockdown/overexpression, ChIP-seq for H3K27ac and FOXD3 binding, m6A-seq, RIP for YTHDF2, patch-clamp for 5-HT3 currents, behavioral pain assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — mechanistic pathway established with multiple orthogonal methods in vivo and in vitro\",\n      \"pmids\": [\"38285939\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The lncRNA linc01305 stabilizes HTR3A mRNA by interacting with RNA-binding proteins IGF2BP2 and IGF2BP3, promoting proliferation and metastasis of esophageal squamous cell carcinoma.\",\n      \"method\": \"linc01305 pulldown, mass spectrometry, RNA immunoprecipitation, mRNA stability assays, RNA-seq, FISH, transwell and colony formation assays\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (pulldown+MS+RIP+stability) identifying post-transcriptional regulatory mechanism\",\n      \"pmids\": [\"34022433\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Ethanol consumption induces tissue-specific DNA methylation changes at the Htr3a promoter in mouse brain and blood; reduced methylation in the dorsomedial striatum correlated with increased Htr3a mRNA expression (1.43-fold), suggesting promoter methylation inversely regulates Htr3a expression.\",\n      \"method\": \"Sequenom MassARRAY methylation quantification across 8-9 CpGs in 9 brain regions and blood, real-time PCR for Htr3a expression\",\n      \"journal\": \"Alcoholism, clinical and experimental research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — quantitative methylation analysis with expression correlation in same tissue, but single lab study\",\n      \"pmids\": [\"22834954\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"A variant C178T (Pro16Ser) in an upstream open reading frame (uORF) of the HTR3A 5' UTR increases reporter gene expression by 245% (C178T alone) or 138% (C178T+C195T) compared to wild-type, indicating this variant affects HTR3A expression at the translational level.\",\n      \"method\": \"Luciferase reporter constructs containing C178T allele transfected into cells, compared to wild-type\",\n      \"journal\": \"Pharmacogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reporter assay with functional allele comparison, single lab\",\n      \"pmids\": [\"11505217\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Loss of 5-HT3A receptor in knockout mice causes exuberant apical dendritic complexity in cortical layer 2/3 pyramidal neurons, which reduces spike frequency adaptation and alters afterhyperpolarization development, demonstrating that 5-HT3A-mediated serotonergic input on Cajal-Retzius cells during early postnatal development shapes dendritic morphology and consequently neuronal excitability.\",\n      \"method\": \"5-HT3A knockout mice, morphological reconstruction, whole-cell patch-clamp recording of layer 2/3 pyramidal neurons, computational modeling\",\n      \"journal\": \"Journal of neurophysiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with quantitative morphological and electrophysiological analysis validated by computational model\",\n      \"pmids\": [\"22696545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In Htr3a knockout mice, fluoxetine effects are blunted while citalopram triggers 5-HT1A receptor desensitization in the dorsal raphe at lower doses than in wild-type, suggesting 5-HT3 receptor inactivation promotes SSRI efficacy via altered autoreceptor sensitivity. KO also prevents CSDS-induced cortical changes in CaMKIIa and SOD1.\",\n      \"method\": \"Htr3a KO mice, behavioral tests (EPM, FST, social interaction), in vitro dorsal raphe electrophysiology, gene expression analysis\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with electrophysiology identifying raphe autoreceptor mechanism, single lab\",\n      \"pmids\": [\"28493335\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"HTR3A encodes the obligatory 5-HT3A subunit of the serotonin type 3 receptor, a pentameric Cys-loop ligand-gated cation channel that is activated by binding of at least three serotonin molecules; agonist recognition involves a cation-π interaction at Trp183 (loop B) coupled to hydrogen bonds from loop A residues E129 and D124, while gating is transduced through pre-TM1 Arg222 and the TM2-TM3 linker (D298), desensitization is governed by the 4′ lysine in M2 and Arg427 in the cytoplasmic domain, single-channel conductance is rate-limited by arginine residues (Arg436, Arg440) in the intracellular MA helix portals and anionic residues in the extracellular vestibule, RIC-3 chaperone selectively enhances surface expression of homomeric 5-HT3A receptors, N-glycosylation at N109/N174/N190 controls assembly and trafficking, and the receptor plays essential roles in fear extinction, NMDAR-dependent LTD via AMPA receptor internalization, cortical interneuron-regulated E/I balance, and bladder innervation development.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"HTR3A encodes the obligatory 5-HT3A subunit of the serotonin type 3 receptor, a pentameric Cys-loop ligand-gated cation channel that mediates fast excitatory serotonergic neurotransmission and regulates synaptic plasticity, fear memory extinction, cortical circuit development, and autonomic innervation. Agonist recognition requires a cation-π interaction at Trp183 (loop B) coupled to hydrogen bonds from loop A residues E129 and D124, with at least three serotonin molecules needed for channel opening; gating is transduced through pre-TM1 Arg222 and TM2-TM3 linker residue D298, while desensitization is governed by the M2 4′ lysine and cytoplasmic Arg427 [PMID:12162741, PMID:23077719, PMID:11533135, PMID:12970351, PMID:16096341, PMID:10639097, PMID:16754678]. Single-channel conductance is rate-limited by arginine residues (Arg436, Arg440) forming electrostatic barriers in the intracellular MA helix portals and by anionic residues (Asp113, Asp127) in the extracellular vestibule, while surface expression depends on RIC-3 chaperone interaction, N-glycosylation at N109/N174/N190, and the C-terminal residue Ala455 [PMID:17200121, PMID:24030822, PMID:21454663, PMID:20522555, PMID:15264219, PMID:18786552]. In vivo, 5-HT3A is essential for NMDAR-dependent hippocampal LTD via AMPA receptor internalization, fear extinction, cortical dendritic patterning through Cajal-Retzius cell signaling, excitation/inhibition balance in parvalbumin-positive interneurons, and normal bladder innervation development [PMID:25130560, PMID:24344177, PMID:22696545, PMID:34646371, PMID:29311772].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Identifying the M2 4′ lysine as a desensitization determinant separable from channel conductance established that gating conformational changes extend beyond the pore-lining face of M2.\",\n      \"evidence\": \"Site-directed mutagenesis with whole-cell patch-clamp and fluctuation analysis in HEK293 cells\",\n      \"pmids\": [\"10639097\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural basis for how 4′ position influences desensitization without lining the pore\", \"Whether the same residue functions identically in heteromeric 5-HT3A/3B receptors\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Discovery of alternative splice variants (h5-HT3AT and h5-HT3AL) that modulate desensitization and ion flux when co-expressed with 5-HT3A revealed endogenous mechanisms for tuning receptor properties without requiring heteromeric subunit partners.\",\n      \"evidence\": \"cDNA cloning, HEK293 electrophysiology, RT-PCR confirmation in amygdala/hippocampus\",\n      \"pmids\": [\"11111833\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of splice variant incorporation into pentamers unknown\", \"In vivo functional significance of splice variants not tested\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Demonstrating that at least three serotonin molecules must bind to open the channel, with concentration-dependent open probability, defined the cooperativity and occupancy requirements for 5-HT3A gating.\",\n      \"evidence\": \"Rapid agonist application, outside-out patch-clamp, and kinetic modeling in HEK293 cells\",\n      \"pmids\": [\"11533135\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which specific binding sites among five subunits are occupied at threshold unknown\", \"Whether heteromeric receptors have the same occupancy requirement\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identification of a cation-π interaction between serotonin and Trp183 (loop B) using unnatural amino acid mutagenesis precisely located the agonist-receptor contact and quantified its ~4 kcal/mol contribution, establishing the primary binding mechanism.\",\n      \"evidence\": \"Unnatural amino acid incorporation and electrophysiology in Xenopus oocytes\",\n      \"pmids\": [\"12162741\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the same interaction applies to all five binding sites in the pentamer\", \"Structural validation awaited (no crystal/cryo-EM structure at the time)\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Showing that Arg222 in the pre-TM1 region links binding to gating — its mutation increased potency, altered kinetics, and converted an antagonist to agonist — identified a critical transduction element between the extracellular and transmembrane domains.\",\n      \"evidence\": \"Site-directed mutagenesis and patch-clamp in HEK293 cells\",\n      \"pmids\": [\"12970351\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The partner residues that Arg222 interacts with during transduction not identified\", \"No direct structural evidence for conformational change at this position\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Mapping the M2 pore with systematic SCAM and Cd²⁺ probing in open and closed states revealed that the cytoplasmic selectivity filter maintains a narrow pore in both states, indicating minimal rearrangement at the 2′ and −2′ positions during gating.\",\n      \"evidence\": \"Cysteine mutagenesis with state-dependent Cd²⁺ accessibility in HEK293 cells\",\n      \"pmids\": [\"15131114\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Resolution limited to accessibility changes; structural movements below probe size undetectable\", \"Whether this holds for heteromeric receptors\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Dissecting N-glycosylation sites showed N109 is required for receptor assembly while N174 and N190 control plasma membrane trafficking and ligand binding, establishing that post-translational modifications at distinct sites play non-redundant roles in receptor biogenesis.\",\n      \"evidence\": \"Site-directed mutagenesis of each glycosylation site with surface expression, radioligand binding, and Ca²⁺ influx assays\",\n      \"pmids\": [\"15264219\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Glycan composition at each site not determined\", \"Whether glycosylation requirements differ in neurons vs. heterologous cells\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identification of D298 in the TM2-TM3 linker as a charge-dependent determinant of gating kinetics and Ca²⁺ modulation, together with loop C residue contributions, expanded the map of conformational coupling from the ligand-binding domain to the channel gate.\",\n      \"evidence\": \"Charge-reversal/neutralization mutagenesis with fast-application patch-clamp (D298); alanine mutagenesis with binding and electrophysiology (loop C) in oocytes and HEK293 cells\",\n      \"pmids\": [\"16096341\", \"15966738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No direct measurement of conformational movements at these positions\", \"Interaction partners of D298 within the linker not mapped\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Localization of Arg427 in the large cytoplasmic domain as a desensitization determinant, with polarity-dependent kinetics, extended the desensitization mechanism beyond the transmembrane domain into the intracellular region.\",\n      \"evidence\": \"Sequential LCD deletion and point mutagenesis with whole-cell patch-clamp in HEK293 cells\",\n      \"pmids\": [\"16754678\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural context of Arg427 within the cytoplasmic domain unknown\", \"Whether phosphorylation or protein interactions at this site modulate desensitization in vivo\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Dynamic charge modification at Arg436 and systematic scanning of MA helix residues (435–440) demonstrated that cytoplasmic portals impose a rate-limiting electrostatic barrier to ion permeation, explaining the characteristically low single-channel conductance of homomeric 5-HT3A receptors.\",\n      \"evidence\": \"SCAM with MTS reagents, alanine/arginine scanning, single-channel electrophysiology in HEK293 cells\",\n      \"pmids\": [\"17200121\", \"24030822\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural model of portal geometry at atomic resolution not available\", \"How heteromeric subunit composition alters portal electrostatics\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Kinetic modeling of serotonin vs. dopamine as high- vs. low-efficacy agonists revealed that partial agonism arises from distinct opening rates and dissociation from open/desensitized states, providing a mechanistic framework for efficacy differences at this receptor.\",\n      \"evidence\": \"Rapid solution exchange electrophysiology with allosteric kinetic modeling in HEK293 cells\",\n      \"pmids\": [\"18045909\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the kinetic scheme generalizes to other partial agonists\", \"No structural basis for differential state-dependent binding\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstrating that RIC-3 chaperone directly interacts with 5-HT3A (and C/D/E subunits) in the ER and selectively enhances homomeric 5-HT3A surface expression identified an ER-resident quality control step specific to receptor biogenesis.\",\n      \"evidence\": \"Co-IP, ER co-localization, flow cytometry, Ca²⁺ influx, and radioligand binding in HEK293 cells\",\n      \"pmids\": [\"20522555\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular interface between RIC-3 and 5-HT3A not mapped\", \"Whether RIC-3 is rate-limiting for 5-HT3A expression in neurons\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Mutant cycle analysis with unnatural amino acids revealed a tightly coupled array — cation-π at W183, hydrogen bonds from E129 and D124 — that functions as a unit for serotonin recognition but not for the partial agonist mCPBG, establishing agonist-specific binding site mechanics.\",\n      \"evidence\": \"Double mutant cycle analysis with unnatural amino acid mutagenesis in Xenopus oocytes\",\n      \"pmids\": [\"23077719\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How coupling energetics translate to conformational changes in the binding domain\", \"Coupling analysis not extended to other agonists beyond 5-HT and mCPBG\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"5-HT3A knockout abolished NMDAR-dependent LTD (but not mGluR-dependent LTD) and blocked activity-dependent AMPA receptor internalization at hippocampal CA1 synapses, establishing a specific role for 5-HT3A in one form of synaptic plasticity.\",\n      \"evidence\": \"Htr3a KO mice, hippocampal slice LTD protocols, AMPAR internalization assays\",\n      \"pmids\": [\"25130560\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cell type (interneuron vs. pyramidal) through which 5-HT3A acts in LTD not determined\", \"Signaling intermediates between 5-HT3A activation and AMPAR endocytosis unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Loss of 5-HT3A caused exuberant apical dendritic complexity in cortical L2/3 pyramidal neurons with altered excitability, linking serotonergic input on Cajal-Retzius cells during development to cortical circuit maturation.\",\n      \"evidence\": \"Htr3a KO mice, morphological reconstruction, whole-cell patch-clamp, computational modeling\",\n      \"pmids\": [\"22696545\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular signals downstream of 5-HT3A in Cajal-Retzius cells not identified\", \"Whether dendritic phenotype is reversible if 5-HT3A is restored postnatally\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Htr3a KO mice showed normal fear acquisition but failed to extinguish contextual and cued fear, dissociating 5-HT3A function specifically to fear extinction circuitry.\",\n      \"evidence\": \"Fear conditioning and extinction behavioral paradigms in Htr3a KO mice\",\n      \"pmids\": [\"24344177\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Brain region and cell type mediating the extinction deficit not identified\", \"Whether pharmacological 5-HT3 blockade recapitulates the extinction deficit\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"5-HT3A loss disrupted bladder innervation development — altering autonomic neuronal subtype balance in fetal pelvic ganglia and increasing fiber density — resulting in voiding dysfunction, establishing a developmental role for 5-HT3A in visceral innervation patterning.\",\n      \"evidence\": \"Htr3a KO mice, cystometry, immunohistochemistry for neuronal markers across developmental timepoints\",\n      \"pmids\": [\"29311772\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which 5-HT3A signaling regulates neuronal subtype specification in ganglia unknown\", \"Whether human HTR3A variants cause bladder dysfunction\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"5-HT3A loss caused NMDAR upregulation specifically in PV+ interneurons, increasing their excitability and GABAergic output onto pyramidal neurons, reducing E/I ratio and producing autistic-like behaviors rescuable by memantine — identifying a cell-type-specific mechanism linking 5-HT3A to E/I balance.\",\n      \"evidence\": \"TALEN-generated Htr3a KO mice, behavioral testing, patch-clamp in PV+ interneurons, transcriptomics, immunoblotting, memantine rescue\",\n      \"pmids\": [\"34646371\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How 5-HT3A loss leads to NMDAR transcriptional upregulation in PV+ cells not resolved\", \"Whether the E/I imbalance mechanism generalizes to other brain regions\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"FOXD3-driven ALKBH5 upregulation erases m6A marks on Htr3a mRNA, preventing YTHDF2-mediated degradation and stabilizing 5-HT3A expression to promote neuropathic pain — revealing an epitranscriptomic regulatory layer controlling HTR3A abundance.\",\n      \"evidence\": \"Rat trigeminal nerve injury, ALKBH5 manipulation, ChIP-seq, m6A-seq, RIP, patch-clamp, behavioral pain assays\",\n      \"pmids\": [\"38285939\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether m6A regulation of Htr3a occurs in CNS neurons beyond trigeminal ganglia\", \"Other m6A reader proteins that may also regulate Htr3a stability\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"High-resolution structural basis of the full gating cycle (resting → open → desensitized) for homomeric and heteromeric 5-HT3 receptors, the molecular determinants of heteromeric subunit assembly stoichiometry, and the signaling pathways linking 5-HT3A channel activity to AMPAR internalization and fear extinction circuitry remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No full gating-cycle cryo-EM structures capturing all functional states in the timeline\", \"Intracellular signaling cascade from 5-HT3A to AMPAR endocytosis not identified\", \"Cell-type and circuit identity mediating fear extinction role undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [4, 9, 11]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 35]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [16, 17, 20]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [16]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [23, 24, 25, 40]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [25, 23]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [4, 9, 11]}\n    ],\n    \"complexes\": [\n      \"5-HT3 receptor (homomeric pentamer)\",\n      \"5-HT3A/3B heteromeric receptor\"\n    ],\n    \"partners\": [\n      \"HTR3B\",\n      \"RIC3\",\n      \"ALKBH5\",\n      \"IGF2BP2\",\n      \"IGF2BP3\",\n      \"YTHDF2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}