{"gene":"TTYH2","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2021,"finding":"Cryo-EM structures of mouse TTYH2 and TTYH3 in lipid nanodiscs revealed a previously unobserved fold with an extended extracellular domain containing a partially solvent-exposed hydrophobic pocket. In the presence of Ca2+, TTYH2 forms homomeric cis-dimers bridged by extracellularly coordinated Ca2+. In the absence of Ca2+, TTYH2 forms trans-dimers spanning opposing membranes across ~130 Å intermembrane space as well as a monomeric state. No ion-conducting pathways were observed in any structure, and no TTYH2-dependent channel activity was detected in cells, indicating TTYHs are not pore-forming subunits of anion channels.","method":"Cryo-EM structural determination in lipid nanodiscs; electrophysiology in cells","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures with functional validation (electrophysiology), multiple states characterized, rigorous controls","pmids":["34824283"],"is_preprint":false},{"year":2025,"finding":"TTYH2 interacts with APOE (apolipoprotein E) as its endogenous binding partner; both proteins colocalize in endosomal compartments. Structural studies identified an epitope in an extended extracellular domain of TTYH2 that faces the endosomal lumen and binds APOE-containing lipoprotein particles. In vitro assays demonstrated that TTYH2 accelerates lipid transfer from APOE-containing lipoproteins into membranes, establishing TTYH2 as a facilitator of lipid extraction and insertion at endosomal membranes.","method":"Pull-down of endogenous proteins; subcellular fractionation; immunocytochemistry; binding assays; structural studies; in vitro lipid transfer assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution (in vitro lipid transfer), structural characterization, multiple orthogonal methods (pull-down, fractionation, binding assays, structure) in single rigorous study","pmids":["40562935"],"is_preprint":false},{"year":2008,"finding":"Nedd4-2 (a HECT-type E3 ubiquitin ligase) binds to TTYH2 via its PY motif ((L/P)PXY consensus), ubiquitinates TTYH2, and this ubiquitination regulates both cell surface and total cellular levels of TTYH2. Endogenous TTYH2 and Nedd4-2 were confirmed as binding partners, and the TTYH2 PY motif was shown to be essential for this interaction. Nedd4-2 does not bind TTYH1, which lacks the PY motif.","method":"Co-immunoprecipitation of endogenous proteins; ubiquitination assays; PY-motif mutagenesis; cell surface expression assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — endogenous co-IP confirmed, mutagenesis of critical motif, ubiquitination assay, surface expression measurement; multiple orthogonal methods in single study","pmids":["18577513"],"is_preprint":false},{"year":2008,"finding":"N-glycosylation of TTYH2 is important but not essential for plasma membrane trafficking; incomplete N-glycosylation mediates reduced expression and increased ubiquitination of TTYH2, but is not the determining factor for TTYH2 trafficking to the plasma membrane. N-glycosylation site mutagenesis supports a five transmembrane domain topology with an extracellular N-terminus and cytoplasmic C-terminus.","method":"N-glycosylation site mutagenesis; glycosylation analysis; cell surface expression assays; ubiquitination assays","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis plus multiple biochemical readouts (surface expression, ubiquitination), single lab","pmids":["18260827"],"is_preprint":false},{"year":2019,"finding":"β-COP, a vesicle transport protein, was identified as a direct binding partner of TTYH2 via yeast two-hybrid screening using the TTYH2 C-terminal region as bait, confirmed by in vitro and in vivo binding assays. Co-expression of β-COP with TTYH2 decreased TTYH2 surface expression and channel activity in heterologous systems. In LoVo colon cancer cells, endogenous β-COP associated with TTYH2, and β-COP overexpression dramatically decreased surface expression and activity of endogenous TTYH2, indicating β-COP regulates TTYH2 trafficking to the plasma membrane.","method":"Yeast two-hybrid screening; in vitro and in vivo binding assays; co-immunoprecipitation of endogenous proteins; surface expression assays; electrophysiology","journal":"BMB reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal binding confirmation (Y2H + co-IP), endogenous interaction, functional readout (surface expression + activity), single lab","pmids":["30670146"],"is_preprint":false},{"year":2019,"finding":"TTYH2 (along with TTYH1) functions as a pore-forming subunit of volume-regulated anion channels (VRACs) in astrocytes. Gene silencing of all three Ttyh1/2/3 eliminated hypo-osmotic-solution-induced Cl- conductance (ICl,swell) in astrocytes. Heterologous expression of TTYH2 in HEK293T or CHO-K1 cells reconstituted ICl,swell with similar pharmacological properties and glutamate permeability as native astrocytic VRACs. Mutagenesis revealed that a positively charged arginine at position 164 in TTYH2 is critical for channel pore formation.","method":"Gene silencing (shRNA/siRNA); heterologous expression; whole-cell patch-clamp electrophysiology; site-directed mutagenesis of pore residue","journal":"Experimental neurobiology","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — electrophysiology with mutagenesis and heterologous reconstitution, single lab; note this finding is directly contradicted by the cryo-EM structural study (PMID 34824283)","pmids":["31138989"],"is_preprint":false},{"year":2019,"finding":"TTYH1 and TTYH2 are critical for LRRC8A-independent VRAC currents in cancer cells. VRAC currents were absent from TTYH1- and TTYH2-deficient SNU-601 gastric cancer cells and restored by expression of either TTYH1 or TTYH2. TTYH2 expression was suppressed by cisplatin resistance and partially restored by histone deacetylase inhibitor treatment.","method":"Gene silencing; heterologous rescue expression; whole-cell patch-clamp electrophysiology; microarray expression profiling","journal":"Cells","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function plus rescue electrophysiology, single lab; note potential contradiction with structural study (PMID 34824283)","pmids":["31181821"],"is_preprint":false},{"year":2021,"finding":"TTYH2 acts as a glycan-dependent binding partner of the SARS-CoV-2 spike protein, interacting via the receptor-binding domain region, as identified by a myeloid cell receptor-focused ectopic expression screen. TTYH2 engagement with SARS-CoV-2 does not support active viral replication but induces proinflammatory responses in myeloid cells.","method":"Ectopic expression screen; binding assays; replication assays; cytokine/inflammatory response measurements","journal":"Immunity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ectopic expression screen with functional validation (binding, replication, inflammatory response), single study","pmids":["34048708"],"is_preprint":false},{"year":2019,"finding":"siRNA-mediated silencing of TTYH2 in U2OS osteosarcoma cells decreased invasion and migration (but not proliferation), and reduced expression of EMT transcription factors Slug and ZEB1, placing TTYH2 upstream of EMT-related signaling in these cells.","method":"siRNA gene silencing; invasion and migration assays; western blot for Slug and ZEB1","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single knockdown approach, limited pathway validation","pmids":["31230749"],"is_preprint":false},{"year":2007,"finding":"siRNA-mediated knockdown of TTYH2 in DLD-1 and Caco-2 colon cancer cell lines significantly inhibited both cell proliferation and cell aggregation/scattering, demonstrating TTYH2 functional roles in these cellular processes.","method":"siRNA knockdown; MTT proliferation assay; cell aggregation assay","journal":"World journal of gastroenterology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single knockdown approach, no pathway mechanism identified","pmids":["17569141"],"is_preprint":false}],"current_model":"TTYH2 is a five-transmembrane domain protein that localizes to the plasma membrane and endosomal compartments; its best-established function, supported by cryo-EM structure and in vitro reconstitution, is facilitating lipid transfer from APOE-containing lipoproteins into endosomal membranes via a Ca2+-regulated conformational switch between cis- and trans-membrane dimers, while its regulation involves Nedd4-2-mediated ubiquitination via a PY motif and β-COP-dependent trafficking control; earlier electrophysiology studies proposed TTYH2 as a pore-forming VRAC subunit, but this is contradicted by structural evidence showing no ion-conducting pathway."},"narrative":{"mechanistic_narrative":"TTYH2 is a five-transmembrane domain protein with an extracellular N-terminus and cytoplasmic C-terminus that localizes to the plasma membrane and endosomal compartments and acts as a facilitator of lipid transfer at endosomal membranes [PMID:40562935, PMID:18260827]. Cryo-EM structures revealed an extended extracellular domain bearing a partially solvent-exposed hydrophobic pocket; in the presence of Ca2+ TTYH2 assembles into cis-dimers bridged by extracellularly coordinated Ca2+, while in its absence it forms trans-dimers spanning opposing membranes across ~130 Å and a monomeric state, providing a Ca2+-regulated conformational basis for membrane bridging [PMID:34824283]. The extended extracellular domain forms an epitope that faces the endosomal lumen and binds APOE-containing lipoprotein particles, and TTYH2 accelerates lipid extraction and insertion from these lipoproteins into membranes in reconstituted assays [PMID:40562935]. TTYH2 surface and total abundance are controlled by post-translational and trafficking machinery: Nedd4-2 binds the TTYH2 PY motif and ubiquitinates it to regulate surface and cellular levels [PMID:18577513], N-glycosylation modulates expression and ubiquitination without being essential for plasma-membrane delivery [PMID:18260827], and β-COP binds the C-terminal region to negatively regulate TTYH2 surface trafficking [PMID:30670146]. Although earlier electrophysiology proposed TTYH2 as a pore-forming subunit of volume-regulated anion channels [PMID:31138989, PMID:31181821], the cryo-EM structures showed no ion-conducting pathway and detected no TTYH2-dependent channel activity, arguing against a pore-forming role [PMID:34824283]. TTYH2 also serves as a glycan-dependent binding partner of the SARS-CoV-2 spike receptor-binding domain that drives proinflammatory responses in myeloid cells without supporting viral replication [PMID:34048708].","teleology":[{"year":2007,"claim":"Established an initial cellular phenotype for TTYH2 by asking whether it contributes to cancer cell behavior.","evidence":"siRNA knockdown with proliferation and aggregation assays in DLD-1 and Caco-2 colon cancer cells","pmids":["17569141"],"confidence":"Low","gaps":["No molecular mechanism identified","Single knockdown approach without rescue","Does not connect phenotype to a defined biochemical activity"]},{"year":2008,"claim":"Defined how TTYH2 abundance and surface levels are post-translationally controlled, identifying its first regulatory partner.","evidence":"Endogenous co-IP, PY-motif mutagenesis, and ubiquitination/surface expression assays for Nedd4-2 binding; N-glycosylation site mutagenesis establishing five-TM topology","pmids":["18577513","18260827"],"confidence":"High","gaps":["Did not establish the molecular function being regulated","Physiological context of ubiquitin turnover unclear","N-glycosylation role only partially defined"]},{"year":2019,"claim":"Tested the long-standing hypothesis that TTYH2 is an ion channel and concurrently identified a trafficking regulator.","evidence":"Whole-cell patch-clamp with gene silencing and heterologous reconstitution in astrocytes and cancer cells; Y2H and co-IP for β-COP binding with surface/activity readouts","pmids":["31138989","31181821","30670146"],"confidence":"Medium","gaps":["Channel assignment later contradicted by structural data","Pore residue mutagenesis not reconciled with absence of a conducting pathway","β-COP regulatory mechanism characterized only in heterologous and cancer-cell systems"]},{"year":2019,"claim":"Linked TTYH2 to migratory and EMT-related signaling, extending its cancer-cell role beyond proliferation.","evidence":"siRNA silencing with invasion/migration assays and western blot for Slug and ZEB1 in U2OS cells","pmids":["31230749"],"confidence":"Low","gaps":["Single knockdown approach in one cell line","Mechanism upstream of EMT factors undefined","No direct biochemical link to lipid-transfer function"]},{"year":2021,"claim":"Resolved the structural fold and overturned the channel model, redefining TTYH2 as a Ca2+-regulated membrane-bridging protein.","evidence":"Cryo-EM in lipid nanodiscs capturing cis-dimer, trans-dimer, and monomeric states with cellular electrophysiology controls","pmids":["34824283"],"confidence":"High","gaps":["Functional output of the conformational switch not defined at this stage","Endogenous ligand of the hydrophobic pocket unknown","Physiological trigger for cis/trans transition in cells unresolved"]},{"year":2021,"claim":"Identified an unexpected role for TTYH2 as a glycan-dependent spike-binding partner that drives inflammation.","evidence":"Myeloid receptor-focused ectopic expression screen with binding, replication, and cytokine assays","pmids":["34048708"],"confidence":"Medium","gaps":["Relationship to endogenous lipid-transfer function unclear","Glycan dependence not structurally mapped","Single study without orthogonal in vivo validation"]},{"year":2025,"claim":"Assigned a concrete molecular function by identifying APOE as the endogenous partner and demonstrating lipid transfer.","evidence":"Endogenous pull-down, fractionation, immunocytochemistry, structural mapping of the lumen-facing epitope, and in vitro lipid transfer assays","pmids":["40562935"],"confidence":"High","gaps":["Directionality and physiological cargo of lipid transfer in vivo not fully defined","Connection between Ca2+-driven cis/trans switch and lipid transfer mechanism not explicitly resolved","Downstream consequences of endosomal lipid handling uncharacterized"]},{"year":null,"claim":"How the Ca2+-regulated conformational switch, trafficking control, and APOE-dependent lipid transfer integrate into a defined physiological pathway remains open.","evidence":"","pmids":[],"confidence":"High","gaps":["No in vivo demonstration linking lipid-transfer activity to a tissue-level phenotype","Mechanistic coupling of cis/trans transition to lipid extraction undefined","Reconciliation of cancer-cell phenotypes with the lipid-transfer model unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[1]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,3,4]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[1]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[1]}],"complexes":[],"partners":["APOE","NEDD4L","COPB1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BSA4","full_name":"Protein tweety homolog 2","aliases":["Volume-regulated anion channel subunit TTYH2"],"length_aa":534,"mass_kda":58.8,"function":"Calcium-independent, swelling-dependent volume-regulated anion channel (VRAC-swell) which plays a pivotal role in the process of regulatory volume decrease (RVD) in the brain through the efflux of anions like chloride and organic osmolytes like glutamate (By similarity). Probable large-conductance Ca(2+)-activated chloride channel (PubMed:15010458)","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9BSA4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TTYH2","classification":"Not Classified","n_dependent_lines":41,"n_total_lines":1208,"dependency_fraction":0.03394039735099338},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TTYH2","total_profiled":1310},"omim":[{"mim_id":"608855","title":"TWEETY FAMILY MEMBER 2; TTYH2","url":"https://www.omim.org/entry/608855"},{"mim_id":"605784","title":"TWEETY FAMILY MEMBER 1; TTYH1","url":"https://www.omim.org/entry/605784"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"brain","ntpm":113.1}],"url":"https://www.proteinatlas.org/search/TTYH2"},"hgnc":{"alias_symbol":["C17orf29"],"prev_symbol":[]},"alphafold":{"accession":"Q9BSA4","domains":[{"cath_id":"-","chopping":"13-71_89-422","consensus_level":"high","plddt":93.363,"start":13,"end":422}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BSA4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BSA4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BSA4-F1-predicted_aligned_error_v6.png","plddt_mean":82.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TTYH2","jax_strain_url":"https://www.jax.org/strain/search?query=TTYH2"},"sequence":{"accession":"Q9BSA4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BSA4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BSA4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BSA4"}},"corpus_meta":[{"pmid":"34048708","id":"PMC_34048708","title":"SARS-CoV-2 exacerbates proinflammatory responses in myeloid cells through C-type lectin receptors and Tweety family member 2.","date":"2021","source":"Immunity","url":"https://pubmed.ncbi.nlm.nih.gov/34048708","citation_count":138,"is_preprint":false},{"pmid":"31138989","id":"PMC_31138989","title":"Tweety-homolog (Ttyh) Family Encodes the Pore-forming Subunits of the Swelling-dependent Volume-regulated Anion Channel (VRACswell) in the Brain.","date":"2019","source":"Experimental neurobiology","url":"https://pubmed.ncbi.nlm.nih.gov/31138989","citation_count":43,"is_preprint":false},{"pmid":"11597145","id":"PMC_11597145","title":"TTYH2, a human homologue of the Drosophila melanogaster gene tweety, is located on 17q24 and upregulated in renal cell carcinoma.","date":"2001","source":"Genomics","url":"https://pubmed.ncbi.nlm.nih.gov/11597145","citation_count":33,"is_preprint":false},{"pmid":"17569141","id":"PMC_17569141","title":"TTYH2, a human homologue of the Drosophila melanogaster gene tweety, is up-regulated in colon carcinoma and involved in cell proliferation and cell aggregation.","date":"2007","source":"World journal of gastroenterology","url":"https://pubmed.ncbi.nlm.nih.gov/17569141","citation_count":29,"is_preprint":false},{"pmid":"26752167","id":"PMC_26752167","title":"A Genome-wide study of blood pressure in African Americans accounting for gene-smoking interaction.","date":"2016","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/26752167","citation_count":29,"is_preprint":false},{"pmid":"31181821","id":"PMC_31181821","title":"TTYH1 and TTYH2 Serve as LRRC8A-Independent Volume-Regulated Anion Channels in Cancer Cells.","date":"2019","source":"Cells","url":"https://pubmed.ncbi.nlm.nih.gov/31181821","citation_count":28,"is_preprint":false},{"pmid":"18577513","id":"PMC_18577513","title":"The ubiquitin-protein ligase Nedd4-2 differentially interacts with and regulates members of the Tweety family of chloride ion channels.","date":"2008","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/18577513","citation_count":28,"is_preprint":false},{"pmid":"31242033","id":"PMC_31242033","title":"Disruption of the Extracellular Matrix Progressively Impairs Central Nervous System Vascular Maturation Downstream of β-Catenin Signaling.","date":"2019","source":"Arteriosclerosis, thrombosis, and vascular biology","url":"https://pubmed.ncbi.nlm.nih.gov/31242033","citation_count":28,"is_preprint":false},{"pmid":"25541457","id":"PMC_25541457","title":"Characterization of tweety gene (ttyh1-3) expression in Xenopus laevis during embryonic development.","date":"2014","source":"Gene expression patterns : GEP","url":"https://pubmed.ncbi.nlm.nih.gov/25541457","citation_count":25,"is_preprint":false},{"pmid":"34824283","id":"PMC_34824283","title":"Structures of tweety homolog proteins TTYH2 and TTYH3 reveal a Ca2+-dependent switch from intra- to intermembrane dimerization.","date":"2021","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/34824283","citation_count":20,"is_preprint":false},{"pmid":"18260827","id":"PMC_18260827","title":"N-glycosylation analysis of the human Tweety family of putative chloride ion channels supports a penta-spanning membrane arrangement: impact of N-glycosylation on cellular processing of Tweety homologue 2 (TTYH2).","date":"2008","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/18260827","citation_count":19,"is_preprint":false},{"pmid":"31230749","id":"PMC_31230749","title":"Upregulated TTYH2 expression is critical for the invasion and migration of U2OS human osteosarcoma cell lines.","date":"2019","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/31230749","citation_count":14,"is_preprint":false},{"pmid":"34262434","id":"PMC_34262434","title":"The tweety Gene Family: From Embryo to Disease.","date":"2021","source":"Frontiers in molecular neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/34262434","citation_count":14,"is_preprint":false},{"pmid":"35054564","id":"PMC_35054564","title":"The Role of Chloride Channels in the Multidrug Resistance.","date":"2021","source":"Membranes","url":"https://pubmed.ncbi.nlm.nih.gov/35054564","citation_count":12,"is_preprint":false},{"pmid":"30670146","id":"PMC_30670146","title":"Surface expression of TTYH2 is attenuated by direct interaction with β-COP.","date":"2019","source":"BMB reports","url":"https://pubmed.ncbi.nlm.nih.gov/30670146","citation_count":9,"is_preprint":false},{"pmid":"40562935","id":"PMC_40562935","title":"Interactions between TTYH2 and APOE facilitate endosomal lipid transfer.","date":"2025","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/40562935","citation_count":5,"is_preprint":false},{"pmid":"38300446","id":"PMC_38300446","title":"Association Between COVID-19 and Neurological Diseases: Evidence from Large-Scale Mendelian Randomization Analysis and Single-Cell RNA Sequencing Analysis.","date":"2024","source":"Molecular neurobiology","url":"https://pubmed.ncbi.nlm.nih.gov/38300446","citation_count":3,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":10261,"output_tokens":2875,"usd":0.036954,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10115,"output_tokens":2755,"usd":0.059725,"stage2_stop_reason":"end_turn"},"total_usd":0.096679,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of mouse TTYH2 and TTYH3 in lipid nanodiscs revealed a previously unobserved fold with an extended extracellular domain containing a partially solvent-exposed hydrophobic pocket. In the presence of Ca2+, TTYH2 forms homomeric cis-dimers bridged by extracellularly coordinated Ca2+. In the absence of Ca2+, TTYH2 forms trans-dimers spanning opposing membranes across ~130 Å intermembrane space as well as a monomeric state. No ion-conducting pathways were observed in any structure, and no TTYH2-dependent channel activity was detected in cells, indicating TTYHs are not pore-forming subunits of anion channels.\",\n      \"method\": \"Cryo-EM structural determination in lipid nanodiscs; electrophysiology in cells\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures with functional validation (electrophysiology), multiple states characterized, rigorous controls\",\n      \"pmids\": [\"34824283\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"TTYH2 interacts with APOE (apolipoprotein E) as its endogenous binding partner; both proteins colocalize in endosomal compartments. Structural studies identified an epitope in an extended extracellular domain of TTYH2 that faces the endosomal lumen and binds APOE-containing lipoprotein particles. In vitro assays demonstrated that TTYH2 accelerates lipid transfer from APOE-containing lipoproteins into membranes, establishing TTYH2 as a facilitator of lipid extraction and insertion at endosomal membranes.\",\n      \"method\": \"Pull-down of endogenous proteins; subcellular fractionation; immunocytochemistry; binding assays; structural studies; in vitro lipid transfer assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution (in vitro lipid transfer), structural characterization, multiple orthogonal methods (pull-down, fractionation, binding assays, structure) in single rigorous study\",\n      \"pmids\": [\"40562935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Nedd4-2 (a HECT-type E3 ubiquitin ligase) binds to TTYH2 via its PY motif ((L/P)PXY consensus), ubiquitinates TTYH2, and this ubiquitination regulates both cell surface and total cellular levels of TTYH2. Endogenous TTYH2 and Nedd4-2 were confirmed as binding partners, and the TTYH2 PY motif was shown to be essential for this interaction. Nedd4-2 does not bind TTYH1, which lacks the PY motif.\",\n      \"method\": \"Co-immunoprecipitation of endogenous proteins; ubiquitination assays; PY-motif mutagenesis; cell surface expression assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — endogenous co-IP confirmed, mutagenesis of critical motif, ubiquitination assay, surface expression measurement; multiple orthogonal methods in single study\",\n      \"pmids\": [\"18577513\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"N-glycosylation of TTYH2 is important but not essential for plasma membrane trafficking; incomplete N-glycosylation mediates reduced expression and increased ubiquitination of TTYH2, but is not the determining factor for TTYH2 trafficking to the plasma membrane. N-glycosylation site mutagenesis supports a five transmembrane domain topology with an extracellular N-terminus and cytoplasmic C-terminus.\",\n      \"method\": \"N-glycosylation site mutagenesis; glycosylation analysis; cell surface expression assays; ubiquitination assays\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis plus multiple biochemical readouts (surface expression, ubiquitination), single lab\",\n      \"pmids\": [\"18260827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"β-COP, a vesicle transport protein, was identified as a direct binding partner of TTYH2 via yeast two-hybrid screening using the TTYH2 C-terminal region as bait, confirmed by in vitro and in vivo binding assays. Co-expression of β-COP with TTYH2 decreased TTYH2 surface expression and channel activity in heterologous systems. In LoVo colon cancer cells, endogenous β-COP associated with TTYH2, and β-COP overexpression dramatically decreased surface expression and activity of endogenous TTYH2, indicating β-COP regulates TTYH2 trafficking to the plasma membrane.\",\n      \"method\": \"Yeast two-hybrid screening; in vitro and in vivo binding assays; co-immunoprecipitation of endogenous proteins; surface expression assays; electrophysiology\",\n      \"journal\": \"BMB reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal binding confirmation (Y2H + co-IP), endogenous interaction, functional readout (surface expression + activity), single lab\",\n      \"pmids\": [\"30670146\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"TTYH2 (along with TTYH1) functions as a pore-forming subunit of volume-regulated anion channels (VRACs) in astrocytes. Gene silencing of all three Ttyh1/2/3 eliminated hypo-osmotic-solution-induced Cl- conductance (ICl,swell) in astrocytes. Heterologous expression of TTYH2 in HEK293T or CHO-K1 cells reconstituted ICl,swell with similar pharmacological properties and glutamate permeability as native astrocytic VRACs. Mutagenesis revealed that a positively charged arginine at position 164 in TTYH2 is critical for channel pore formation.\",\n      \"method\": \"Gene silencing (shRNA/siRNA); heterologous expression; whole-cell patch-clamp electrophysiology; site-directed mutagenesis of pore residue\",\n      \"journal\": \"Experimental neurobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — electrophysiology with mutagenesis and heterologous reconstitution, single lab; note this finding is directly contradicted by the cryo-EM structural study (PMID 34824283)\",\n      \"pmids\": [\"31138989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"TTYH1 and TTYH2 are critical for LRRC8A-independent VRAC currents in cancer cells. VRAC currents were absent from TTYH1- and TTYH2-deficient SNU-601 gastric cancer cells and restored by expression of either TTYH1 or TTYH2. TTYH2 expression was suppressed by cisplatin resistance and partially restored by histone deacetylase inhibitor treatment.\",\n      \"method\": \"Gene silencing; heterologous rescue expression; whole-cell patch-clamp electrophysiology; microarray expression profiling\",\n      \"journal\": \"Cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function plus rescue electrophysiology, single lab; note potential contradiction with structural study (PMID 34824283)\",\n      \"pmids\": [\"31181821\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"TTYH2 acts as a glycan-dependent binding partner of the SARS-CoV-2 spike protein, interacting via the receptor-binding domain region, as identified by a myeloid cell receptor-focused ectopic expression screen. TTYH2 engagement with SARS-CoV-2 does not support active viral replication but induces proinflammatory responses in myeloid cells.\",\n      \"method\": \"Ectopic expression screen; binding assays; replication assays; cytokine/inflammatory response measurements\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ectopic expression screen with functional validation (binding, replication, inflammatory response), single study\",\n      \"pmids\": [\"34048708\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"siRNA-mediated silencing of TTYH2 in U2OS osteosarcoma cells decreased invasion and migration (but not proliferation), and reduced expression of EMT transcription factors Slug and ZEB1, placing TTYH2 upstream of EMT-related signaling in these cells.\",\n      \"method\": \"siRNA gene silencing; invasion and migration assays; western blot for Slug and ZEB1\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single knockdown approach, limited pathway validation\",\n      \"pmids\": [\"31230749\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"siRNA-mediated knockdown of TTYH2 in DLD-1 and Caco-2 colon cancer cell lines significantly inhibited both cell proliferation and cell aggregation/scattering, demonstrating TTYH2 functional roles in these cellular processes.\",\n      \"method\": \"siRNA knockdown; MTT proliferation assay; cell aggregation assay\",\n      \"journal\": \"World journal of gastroenterology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single knockdown approach, no pathway mechanism identified\",\n      \"pmids\": [\"17569141\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"TTYH2 is a five-transmembrane domain protein that localizes to the plasma membrane and endosomal compartments; its best-established function, supported by cryo-EM structure and in vitro reconstitution, is facilitating lipid transfer from APOE-containing lipoproteins into endosomal membranes via a Ca2+-regulated conformational switch between cis- and trans-membrane dimers, while its regulation involves Nedd4-2-mediated ubiquitination via a PY motif and β-COP-dependent trafficking control; earlier electrophysiology studies proposed TTYH2 as a pore-forming VRAC subunit, but this is contradicted by structural evidence showing no ion-conducting pathway.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"TTYH2 is a five-transmembrane domain protein with an extracellular N-terminus and cytoplasmic C-terminus that localizes to the plasma membrane and endosomal compartments and acts as a facilitator of lipid transfer at endosomal membranes [#1, #3]. Cryo-EM structures revealed an extended extracellular domain bearing a partially solvent-exposed hydrophobic pocket; in the presence of Ca2+ TTYH2 assembles into cis-dimers bridged by extracellularly coordinated Ca2+, while in its absence it forms trans-dimers spanning opposing membranes across ~130 Å and a monomeric state, providing a Ca2+-regulated conformational basis for membrane bridging [#0]. The extended extracellular domain forms an epitope that faces the endosomal lumen and binds APOE-containing lipoprotein particles, and TTYH2 accelerates lipid extraction and insertion from these lipoproteins into membranes in reconstituted assays [#1]. TTYH2 surface and total abundance are controlled by post-translational and trafficking machinery: Nedd4-2 binds the TTYH2 PY motif and ubiquitinates it to regulate surface and cellular levels [#2], N-glycosylation modulates expression and ubiquitination without being essential for plasma-membrane delivery [#3], and β-COP binds the C-terminal region to negatively regulate TTYH2 surface trafficking [#4]. Although earlier electrophysiology proposed TTYH2 as a pore-forming subunit of volume-regulated anion channels [#5, #6], the cryo-EM structures showed no ion-conducting pathway and detected no TTYH2-dependent channel activity, arguing against a pore-forming role [#0]. TTYH2 also serves as a glycan-dependent binding partner of the SARS-CoV-2 spike receptor-binding domain that drives proinflammatory responses in myeloid cells without supporting viral replication [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Established an initial cellular phenotype for TTYH2 by asking whether it contributes to cancer cell behavior.\",\n      \"evidence\": \"siRNA knockdown with proliferation and aggregation assays in DLD-1 and Caco-2 colon cancer cells\",\n      \"pmids\": [\"17569141\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No molecular mechanism identified\", \"Single knockdown approach without rescue\", \"Does not connect phenotype to a defined biochemical activity\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Defined how TTYH2 abundance and surface levels are post-translationally controlled, identifying its first regulatory partner.\",\n      \"evidence\": \"Endogenous co-IP, PY-motif mutagenesis, and ubiquitination/surface expression assays for Nedd4-2 binding; N-glycosylation site mutagenesis establishing five-TM topology\",\n      \"pmids\": [\"18577513\", \"18260827\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the molecular function being regulated\", \"Physiological context of ubiquitin turnover unclear\", \"N-glycosylation role only partially defined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Tested the long-standing hypothesis that TTYH2 is an ion channel and concurrently identified a trafficking regulator.\",\n      \"evidence\": \"Whole-cell patch-clamp with gene silencing and heterologous reconstitution in astrocytes and cancer cells; Y2H and co-IP for β-COP binding with surface/activity readouts\",\n      \"pmids\": [\"31138989\", \"31181821\", \"30670146\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Channel assignment later contradicted by structural data\", \"Pore residue mutagenesis not reconciled with absence of a conducting pathway\", \"β-COP regulatory mechanism characterized only in heterologous and cancer-cell systems\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Linked TTYH2 to migratory and EMT-related signaling, extending its cancer-cell role beyond proliferation.\",\n      \"evidence\": \"siRNA silencing with invasion/migration assays and western blot for Slug and ZEB1 in U2OS cells\",\n      \"pmids\": [\"31230749\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single knockdown approach in one cell line\", \"Mechanism upstream of EMT factors undefined\", \"No direct biochemical link to lipid-transfer function\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Resolved the structural fold and overturned the channel model, redefining TTYH2 as a Ca2+-regulated membrane-bridging protein.\",\n      \"evidence\": \"Cryo-EM in lipid nanodiscs capturing cis-dimer, trans-dimer, and monomeric states with cellular electrophysiology controls\",\n      \"pmids\": [\"34824283\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional output of the conformational switch not defined at this stage\", \"Endogenous ligand of the hydrophobic pocket unknown\", \"Physiological trigger for cis/trans transition in cells unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified an unexpected role for TTYH2 as a glycan-dependent spike-binding partner that drives inflammation.\",\n      \"evidence\": \"Myeloid receptor-focused ectopic expression screen with binding, replication, and cytokine assays\",\n      \"pmids\": [\"34048708\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relationship to endogenous lipid-transfer function unclear\", \"Glycan dependence not structurally mapped\", \"Single study without orthogonal in vivo validation\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Assigned a concrete molecular function by identifying APOE as the endogenous partner and demonstrating lipid transfer.\",\n      \"evidence\": \"Endogenous pull-down, fractionation, immunocytochemistry, structural mapping of the lumen-facing epitope, and in vitro lipid transfer assays\",\n      \"pmids\": [\"40562935\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Directionality and physiological cargo of lipid transfer in vivo not fully defined\", \"Connection between Ca2+-driven cis/trans switch and lipid transfer mechanism not explicitly resolved\", \"Downstream consequences of endosomal lipid handling uncharacterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the Ca2+-regulated conformational switch, trafficking control, and APOE-dependent lipid transfer integrate into a defined physiological pathway remains open.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No in vivo demonstration linking lipid-transfer activity to a tissue-level phenotype\", \"Mechanistic coupling of cis/trans transition to lipid extraction undefined\", \"Reconciliation of cancer-cell phenotypes with the lipid-transfer model unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 3, 4]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"APOE\",\n      \"NEDD4L\",\n      \"COPB1\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}