{"gene":"SLC23A1","run_date":"2026-06-10T07:46:32","timeline":{"discoveries":[{"year":2000,"finding":"Human SVCT1 (SLC23A1) mediates saturable, concentrative, high-affinity L-ascorbic acid transport (K0.5 = 50–100 µM) that is electrogenic, sodium-dependent, and inhibited by phloretin. The transporter displays exquisite substrate selectivity, greatly favoring L-ascorbic acid over D-isoascorbic acid, dehydroascorbic acid, and 2- or 6-substituted analogues, while excluding glucose and nucleobases.","method":"Radiotracer uptake and voltage-clamp assays in cRNA-injected Xenopus oocytes","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct functional reconstitution in Xenopus oocytes with kinetic and electrophysiological characterization","pmids":["10631088"],"is_preprint":false},{"year":2002,"finding":"Slc23a1 knockout mice have less than 5% of normal ascorbic acid uptake in cultured embryonic fibroblasts, undetectable or markedly reduced ascorbic acid in blood and tissues, and die within minutes of birth with respiratory failure and intraparenchymal brain hemorrhage. Prenatal supplementation failed to elevate fetal blood ascorbic acid, demonstrating that Slc23a1 is required for placental ascorbic acid transport.","method":"Knockout mouse model; ascorbic acid uptake assays in cultured fibroblasts; tissue ascorbic acid measurement; histopathology","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean KO with multiple defined phenotypic readouts and mechanistic follow-up, replicated across tissues","pmids":["11984597"],"is_preprint":false},{"year":2002,"finding":"SVCT1 expression is up-regulated ~4-fold at the mRNA level during post-confluent differentiation of CaCo-2 cells and is selectively sorted to the apical membrane compartment in polarized epithelial monolayers, where it is the sole functional ascorbic acid transporter mediating vectorial uptake across the intestinal barrier.","method":"Real-time quantitative PCR; transport kinetics; apical vs. basolateral functional assays in Transwell filter inserts; RT-PCR","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (qPCR, functional transport assays, polarized localization) in a single study","pmids":["12381735"],"is_preprint":false},{"year":2002,"finding":"High-dose ascorbic acid (4.5 mg/ml for 24 h) significantly reduces SVCT1 mRNA expression (~77%) and L-ascorbic acid uptake (~50%) in Caco-2 TC7 intestinal epithelial cells, indicating that SVCT1 is subject to substrate-dependent transcriptional down-regulation.","method":"RT-PCR for SVCT1 mRNA; radiolabeled 14C-ascorbic acid uptake assay","journal":"The British journal of nutrition","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — two orthogonal methods (RT-PCR and radiotracer uptake), single lab, single cell model","pmids":["11895172"],"is_preprint":false},{"year":2005,"finding":"SVCT1-EGFP localizes predominantly to the apical membrane of confluent Caco-2 and MDCK epithelial cells, and expression of SVCT1 increases transport activity from the apical membrane, establishing a non-redundant apical function distinct from the basolateral function of SVCT2.","method":"EGFP-tagged SVCT expression constructs; confocal microscopy; Transwell ascorbate transport assays in MDCK and Caco-2 cells; SVCT2-knockout enterocytes","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct localization imaging combined with functional transport assays and genetic knockout validation","pmids":["15993839"],"is_preprint":false},{"year":2006,"finding":"UVB irradiation increases vitamin C uptake into keratinocytes in a time- and dose-dependent manner through translocation of SVCT1 from the cytosol to the plasma membrane, thereby enabling vitamin C to suppress UVB-induced IL-8 and MCP-1 production.","method":"Vitamin C uptake assays; subcellular fractionation/translocation imaging; RNase protection assay; ELISA for chemokine protein levels","journal":"The Journal of investigative dermatology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — localization change observed with functional consequence (chemokine suppression), single lab, multiple readouts but limited mechanistic depth","pmids":["17008880"],"is_preprint":false},{"year":2007,"finding":"SVCT1 mediates L-ascorbic acid transport with a Na+:ascorbate coupling ratio of 2:1, with an ordered simultaneous binding mechanism (Na+, L-ascorbic acid, Na+). Pre-steady-state currents in the absence of ascorbate, described by single Boltzmann distributions, indicate that the first Na+ binds partway within the membrane electric field (ion-well effect). A detailed transport model was established.","method":"Simultaneous radiotracer flux and voltage-clamp measurements in Xenopus oocytes; pre-steady-state current analysis; model simulation","journal":"American journal of physiology. Cell physiology","confidence":"High","confidence_rationale":"Tier 1 / Strong — rigorous electrophysiological reconstitution with simultaneous flux/current measurement, kinetic modeling, and stoichiometry determination","pmids":["18094143"],"is_preprint":false},{"year":2009,"finding":"The differential apical (SVCT1) vs. basolateral (SVCT2) membrane targeting in epithelial cells is determined hierarchically: an N-terminal basolateral targeting sequence (BTS) present in SVCT2 but not SVCT1 drives basolateral localization; its destruction redirects SVCT2 apically. A C-terminal sequence in both SVCTs is required for plasma membrane incorporation/retention; its deletion causes intracellular accumulation of both transporters. Default targeting for SVCT is apical.","method":"Domain swaps, deletions, insertions, and point mutations on EGFP-tagged hSVCT1/hSVCT2; stable expression in MDCK cells; confocal microscopy; Transwell ascorbate transport assays","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — systematic mutagenesis combined with direct localization imaging and functional transport assays, multiple orthogonal approaches","pmids":["19216494"],"is_preprint":false},{"year":2010,"finding":"Ascorbic acid depletion in SMP30/GNL knockout mice (unable to synthesize ascorbic acid) enhances SVCT1 mRNA expression in the liver and small intestine, and increases actual ascorbic acid uptake in primary cultured hepatocytes, indicating that intracellular ascorbic acid negatively regulates SVCT1 expression.","method":"SMP30/GNL knockout mice; RT-PCR for SVCT1/SVCT2 mRNA; radiotracer ascorbic acid uptake in primary hepatocytes","journal":"Archives of biochemistry and biophysics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic model combined with functional uptake assays and mRNA quantification, single lab","pmids":["20122894"],"is_preprint":false},{"year":2010,"finding":"Transcriptional regulation of hSVCT1 in human liver epithelial cells (HepG2) requires HNF-1 binding sites in the hSVCT1 promoter for basal activity and for adaptive (ascorbic acid-responsive) regulation; ascorbic acid deprivation increases and supplementation decreases hSVCT1 mRNA, protein, and promoter activity via HNF-1 sites, whereas hSVCT2 promoter/expression is unaffected.","method":"Promoter-reporter constructs; mutational analysis of cis-elements; 14C-ascorbic acid uptake; RT-PCR and protein expression in HepG2 cells under ascorbic acid-deficient/supplemented conditions","journal":"The Journal of nutritional biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — promoter mutagenesis plus functional uptake and expression assays, single lab","pmids":["20471816"],"is_preprint":false},{"year":2012,"finding":"Rab8a co-localizes with hSVCT1 in intestinal cells and is required for proper apical membrane targeting of hSVCT1; knockdown of Rab8a reduces cell-surface expression of hSVCT1 (leading to lysosomal mis-trafficking revealed by LAMP1 co-localization) and significantly inhibits ascorbic acid uptake. Rab8a knockout mice show similarly reduced intestinal ascorbic acid uptake and decreased mSVCT1 protein.","method":"Co-localization by confocal microscopy; siRNA knockdown; cell-surface biotinylation; 14C-ascorbic acid uptake; Rab8a knockout mouse intestinal uptake assays; LAMP1 co-localization","journal":"Digestive diseases and sciences","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (siRNA knockdown, KO mouse, biotinylation, lysosomal trafficking imaging, radiotracer uptake) in single study","pmids":["23014846"],"is_preprint":false},{"year":2013,"finding":"SLC23A1 does not mediate ascorbic acid efflux/release in the proximal renal epithelial cell; using a dual-transporter Xenopus oocyte system and mammalian cells overexpressing SLC23A1, no ascorbate release was detected, establishing that SLC23A1 functions exclusively in ascorbate uptake across the apical membrane of proximal tubule cells.","method":"Dual-transporter Xenopus oocyte efflux assay; mammalian cell overexpression; gene expression profiling of human proximal tubule segments","journal":"Physiological reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional reconstitution in oocytes plus mammalian cell validation; negative result (no efflux) is the mechanistic finding","pmids":["24400138"],"is_preprint":false},{"year":2013,"finding":"Purified human SVCT1 (hSVCT1) exists predominantly as a monomer with a minor dimeric population in detergent solution; chemical crosslinking of isolated oocyte membranes also shows predominantly monomeric and minor dimeric states in lipid bilayers. The protein is glycosylated when expressed in Xenopus oocytes.","method":"Expression in Xenopus oocytes; protein purification; transmission electron microscopy and single-particle analysis; chemical crosslinking","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — structural analysis by EM with crosslinking, but low-resolution and single study","pmids":["24124560"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structures of mouse SVCT1 in apo and substrate-bound states show that SVCT1 forms a homodimer, with each protomer containing a core domain and a gate domain. Vitamin C binds at the core domain of each subunit with two sodium ions coordinated near the binding site. The tightly packed extracellular interfaces stabilize an inward-open conformation. Transport likely proceeds via an elevator mechanism combined with local structural rearrangements.","method":"Cryo-electron microscopy of apo and substrate-bound mouse SVCT1; structural analysis of sodium ion coordination and substrate binding site","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution cryo-EM structures in two states with identification of substrate and ion binding sites, explaining coupling mechanism","pmids":["36914666"],"is_preprint":false},{"year":2023,"finding":"SVCT1 functions as a sodium-dependent low-affinity/high-capacity urate transporter in addition to its ascorbic acid transport role, as demonstrated by mammalian cell-based transport assays for human SVCT1 and mouse Svct1. Svct1 knockout mice in a hyperuricemic background have lower serum urate than controls, suggesting a physiological role for Svct1 in renal urate reuptake.","method":"Mammalian cell-based transport assays (human SVCT1 and mouse Svct1); CRISPR-Cas9 Svct1 knockout mice crossed into hyperuricemic (uricase-deficient) background; serum urate measurement","journal":"Pflugers Archiv : European journal of physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct transport assays plus in vivo genetic model, single lab, novel substrate identification","pmids":["36749388"],"is_preprint":false},{"year":2015,"finding":"Species-specific transcriptional regulation of SVCT1 in rat vs. human hepatoma cells involves distinct cis-regulatory elements: Bach1 and HNF4 binding sites are critical for rat SVCT1 promoter activity but absent at equivalent positions in the human promoter, while HNF1 sites critical for human SVCT1 regulation are present in the rat promoter but do not affect its activity.","method":"Promoter cloning; deletion and mutant reporter constructs; site-directed mutagenesis; transfection into rat H4IIE and human HepG2 hepatoma cells with transcription factor co-expression","journal":"Free radical biology & medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic promoter mutagenesis with functional reporter assays in two species, single lab","pmids":["25933589"],"is_preprint":false},{"year":2011,"finding":"Iron exposure up-regulates SVCT1 protein expression (~24%) in ascorbic acid-deficient Caco-2 cells and correlates with a ~285% increase in ascorbic acid uptake, demonstrating that iron regulates SVCT1 expression and ascorbic acid transport in intestinal epithelial cells.","method":"ELISA and Western blot for SVCT1 protein; radiolabeled ascorbic acid uptake; SVCT1 inhibitor (quercetin) validation in Caco-2 cells on Transwell inserts","journal":"The British journal of nutrition","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — multiple methods (Western blot, ELISA, uptake assay with inhibitor), single lab, limited mechanistic depth","pmids":["21418708"],"is_preprint":false}],"current_model":"SLC23A1 (SVCT1) is an electrogenic, sodium-coupled ascorbic acid cotransporter that uses an ordered binding mechanism (2 Na⁺ : 1 ascorbate, with sequential Na⁺–ascorbate–Na⁺ binding) and an elevator-type transport mechanism, as established by cryo-EM structures and electrophysiological reconstitution; it localizes to the apical membrane of intestinal and renal proximal tubule epithelial cells via a default apical targeting pathway requiring a C-terminal membrane retention sequence—overridden by an N-terminal basolateral targeting sequence in the paralog SVCT2—and is regulated transcriptionally by HNF-1 sites in response to intracellular ascorbic acid levels; additionally, SVCT1 mediates renal ascorbic acid reabsorption (but not basolateral efflux), can transport urate as a secondary substrate, requires Rab8a for proper apical trafficking in intestinal cells, and its deficiency in mice is perinatally lethal due to failure of ascorbic acid delivery to tissues including brain and lung."},"narrative":{"mechanistic_narrative":"SLC23A1 (SVCT1) is an electrogenic, sodium-coupled L-ascorbic acid cotransporter that mediates concentrative vitamin C uptake at the apical membrane of intestinal and renal epithelia and is essential for systemic ascorbate delivery [PMID:10631088, PMID:12381735]. Transport is high-affinity (K0.5 = 50–100 µM), highly selective for L-ascorbic acid, and couples 2 Na⁺ per ascorbate through an ordered, simultaneous binding mechanism (Na⁺, ascorbate, Na⁺) in which the first sodium binds partway within the membrane electric field [PMID:10631088, PMID:18094143]. Cryo-EM structures resolve SVCT1 as a homodimer with each protomer composed of a core domain and a gate domain; ascorbate and two coordinating sodium ions occupy the core domain, and transport proceeds by an elevator-type mechanism with local rearrangements [PMID:36914666]. In polarized epithelia SVCT1 is sorted to the apical membrane by default, a fate established by a C-terminal sequence required for plasma-membrane retention and distinguished from its basolateral paralog SVCT2 by the absence of an N-terminal basolateral targeting sequence [PMID:19216494]; correct apical delivery in intestinal cells additionally requires Rab8a, whose loss diverts SVCT1 to lysosomes and abolishes uptake [PMID:23014846]. Expression is governed transcriptionally by intracellular ascorbate levels acting through HNF-1 sites in the human promoter, such that depletion induces and supplementation represses SVCT1 [PMID:20471816, PMID:20122894]. In the kidney SVCT1 mediates apical ascorbate reabsorption but not basolateral efflux, and it additionally functions as a sodium-dependent low-affinity/high-capacity urate transporter [PMID:24400138, PMID:36749388]. Knockout mice die within minutes of birth from respiratory failure and brain hemorrhage with profound tissue ascorbate depletion, reflecting an essential role in placental and tissue ascorbate provision [PMID:11984597].","teleology":[{"year":2000,"claim":"Established that human SVCT1 is itself a sodium-dependent, electrogenic, high-affinity ascorbate transporter with strict substrate selectivity, defining the protein's core biochemical activity.","evidence":"Radiotracer uptake and voltage-clamp assays in cRNA-injected Xenopus oocytes","pmids":["10631088"],"confidence":"High","gaps":["Stoichiometry and ion-coupling mechanism not yet resolved","No structural basis for substrate selectivity"]},{"year":2002,"claim":"Demonstrated that SVCT1 is physiologically essential, with knockout causing perinatal lethality and failure of placental ascorbate transfer, establishing the transporter's role in systemic vitamin C delivery.","evidence":"Slc23a1 knockout mice with tissue ascorbate measurement, fibroblast uptake assays, and histopathology","pmids":["11984597"],"confidence":"High","gaps":["Tissue-specific contributions to lethality not dissected","Does not address regulation or trafficking"]},{"year":2002,"claim":"Showed SVCT1 is the sole apical ascorbate transporter mediating vectorial uptake across the intestinal barrier and is induced during epithelial differentiation.","evidence":"qPCR, transport kinetics, and apical/basolateral functional assays in polarized Caco-2 monolayers","pmids":["12381735"],"confidence":"High","gaps":["Targeting signals driving apical sorting unknown","Trafficking machinery not identified"]},{"year":2002,"claim":"Provided early evidence that SVCT1 expression is feedback-regulated by its own substrate, with high ascorbate down-regulating mRNA and uptake.","evidence":"RT-PCR and 14C-ascorbate uptake in Caco-2 TC7 cells under high-dose ascorbate","pmids":["11895172"],"confidence":"Medium","gaps":["Cis-elements and transcription factors mediating the response not identified","Single cell model"]},{"year":2007,"claim":"Defined the transport mechanism quantitatively: a 2 Na⁺:1 ascorbate coupling ratio via ordered simultaneous binding with the first Na⁺ sensing the membrane field.","evidence":"Simultaneous radiotracer flux and voltage-clamp with pre-steady-state current analysis and kinetic modeling in Xenopus oocytes","pmids":["18094143"],"confidence":"High","gaps":["Structural basis of ion and substrate sites not yet visualized","Conformational cycle inferred, not observed"]},{"year":2009,"claim":"Resolved how SVCT1 versus SVCT2 achieve opposite polarized targeting, identifying a default apical pathway, a C-terminal retention sequence, and a paralog-specific N-terminal basolateral signal.","evidence":"Systematic domain swaps, deletions, and point mutations on EGFP-tagged SVCTs with confocal imaging and Transwell transport in MDCK cells","pmids":["19216494"],"confidence":"High","gaps":["Trafficking proteins reading these signals not identified","Mechanism of C-terminal retention unknown"]},{"year":2010,"claim":"Connected substrate-dependent regulation to a defined transcriptional mechanism, showing HNF-1 promoter sites mediate both basal and ascorbate-adaptive control of human SVCT1.","evidence":"Promoter-reporter and cis-element mutagenesis with uptake and expression assays in HepG2 cells; SMP30/GNL knockout mice with hepatocyte uptake","pmids":["20471816","20122894"],"confidence":"Medium","gaps":["Signal coupling intracellular ascorbate to HNF-1 activity unknown","Single lab per model"]},{"year":2012,"claim":"Identified Rab8a as a trafficking factor required for apical surface delivery of SVCT1, with loss causing lysosomal mis-sorting and loss of uptake.","evidence":"Co-localization, siRNA knockdown, surface biotinylation, LAMP1 imaging, and Rab8a knockout mouse intestinal uptake assays","pmids":["23014846"],"confidence":"High","gaps":["Direct Rab8a–SVCT1 interaction not demonstrated","Other trafficking effectors not mapped"]},{"year":2013,"claim":"Clarified the renal role of SVCT1 as apical reabsorptive uptake only, with no basolateral efflux activity.","evidence":"Dual-transporter Xenopus oocyte efflux assay and mammalian overexpression with proximal tubule expression profiling","pmids":["24400138"],"confidence":"Medium","gaps":["Identity of the basolateral efflux pathway unresolved","Negative result depends on assay sensitivity"]},{"year":2013,"claim":"Provided the first biochemical view of the purified, glycosylated transporter, showing predominantly monomeric with minor dimeric states.","evidence":"Purification, single-particle EM, and chemical crosslinking of Xenopus oocyte-expressed hSVCT1","pmids":["24124560"],"confidence":"Medium","gaps":["Low-resolution; oligomeric state later revised by cryo-EM","No atomic detail"]},{"year":2023,"claim":"Delivered atomic-resolution structures explaining ion coupling and the elevator transport mechanism, showing a homodimer with substrate and two sodium ions bound at the core domain.","evidence":"Cryo-EM of apo and substrate-bound mouse SVCT1","pmids":["36914666"],"confidence":"High","gaps":["Outward-open state not captured","Conformational dynamics of the elevator cycle not directly observed"]},{"year":2023,"claim":"Expanded SVCT1's substrate repertoire and physiology by identifying it as a sodium-dependent urate transporter contributing to renal urate reuptake.","evidence":"Mammalian cell transport assays and CRISPR Svct1 knockout in a hyperuricemic mouse background with serum urate measurement","pmids":["36749388"],"confidence":"Medium","gaps":["Structural basis for urate recognition not defined","Relative physiological weight of urate vs ascorbate transport unclear"]},{"year":null,"claim":"How intracellular ascorbate concentration is sensed and transduced to HNF-1-dependent transcriptional and trafficking responses, and the molecular partners reading SVCT1 targeting signals, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No sensor linking ascorbate levels to HNF-1 identified","Direct SVCT1 trafficking interactome incomplete","Basolateral renal efflux transporter unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,2,6,11,14]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[0,6]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,4,5,7,10]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,2,6]}],"complexes":[],"partners":["RAB8A"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9UHI7","full_name":"Solute carrier family 23 member 1","aliases":["Na(+)/L-ascorbic acid transporter 1","Sodium-dependent vitamin C transporter 1","hSVCT1","Yolk sac permease-like molecule 3"],"length_aa":598,"mass_kda":64.8,"function":"Sodium:ascorbate cotransporter. Mediates electrogenic uptake of vitamin C, with a stoichiometry of 2 Na(+) for each ascorbate (PubMed:10556483, PubMed:10556521, PubMed:10631088, PubMed:36749388). Has retained some ancestral activity toward nucleobases such as urate, an oxidized purine. Low-affinity high-capacity sodium:urate cotransporter, may regulate serum urate levels by serving as a renal urate re-absorber (PubMed:36749388) Inactive transporter","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9UHI7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC23A1","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":true,"resolved_as":"SLC23A2","ensg_id":"ENSG00000089057","cell_line_id":"CID001377","localizations":[{"compartment":"membrane","grade":3},{"compartment":"vesicles","grade":3}],"interactors":[{"gene":"GLCCI1","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID001377","total_profiled":1310},"omim":[{"mim_id":"603791","title":"SOLUTE CARRIER FAMILY 23 (NUCLEOBASE TRANSPORTER), MEMBER 2; SLC23A2","url":"https://www.omim.org/entry/603791"},{"mim_id":"603790","title":"SOLUTE CARRIER FAMILY 23 (NUCLEOBASE TRANSPORTER), MEMBER 1; SLC23A1","url":"https://www.omim.org/entry/603790"},{"mim_id":"240400","title":"HYPOASCORBEMIA","url":"https://www.omim.org/entry/240400"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"fallopian tube","ntpm":50.9},{"tissue":"intestine","ntpm":61.5},{"tissue":"kidney","ntpm":37.5},{"tissue":"liver","ntpm":21.3}],"url":"https://www.proteinatlas.org/search/SLC23A1"},"hgnc":{"alias_symbol":["YSPL3","SVCT1"],"prev_symbol":["SLC23A2"]},"alphafold":{"accession":"Q9UHI7","domains":[{"cath_id":"-","chopping":"42-130_144-515","consensus_level":"high","plddt":88.8202,"start":42,"end":515}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UHI7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UHI7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UHI7-F1-predicted_aligned_error_v6.png","plddt_mean":81.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC23A1","jax_strain_url":"https://www.jax.org/strain/search?query=SLC23A1"},"sequence":{"accession":"Q9UHI7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UHI7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UHI7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UHI7"}},"corpus_meta":[{"pmid":"11984597","id":"PMC_11984597","title":"Ascorbic-acid transporter Slc23a1 is essential for vitamin C transport into the brain and for perinatal survival.","date":"2002","source":"Nature medicine","url":"https://pubmed.ncbi.nlm.nih.gov/11984597","citation_count":297,"is_preprint":false},{"pmid":"17541511","id":"PMC_17541511","title":"SVCT1 and SVCT2: key proteins for vitamin C uptake.","date":"2007","source":"Amino acids","url":"https://pubmed.ncbi.nlm.nih.gov/17541511","citation_count":284,"is_preprint":false},{"pmid":"10631088","id":"PMC_10631088","title":"Human vitamin C (L-ascorbic acid) transporter SVCT1.","date":"2000","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/10631088","citation_count":180,"is_preprint":false},{"pmid":"12381735","id":"PMC_12381735","title":"Up-regulation and polarized expression of the sodium-ascorbic acid transporter SVCT1 in post-confluent differentiated CaCo-2 cells.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12381735","citation_count":92,"is_preprint":false},{"pmid":"15993839","id":"PMC_15993839","title":"Polarized localization of vitamin C transporters, SVCT1 and SVCT2, in epithelial cells.","date":"2005","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/15993839","citation_count":84,"is_preprint":false},{"pmid":"20519558","id":"PMC_20519558","title":"Genetic variation at the SLC23A1 locus is associated with circulating concentrations of L-ascorbic acid (vitamin C): evidence from 5 independent studies with >15,000 participants.","date":"2010","source":"The American journal of clinical nutrition","url":"https://pubmed.ncbi.nlm.nih.gov/20519558","citation_count":77,"is_preprint":false},{"pmid":"11895172","id":"PMC_11895172","title":"Decreased expression of the vitamin C transporter SVCT1 by ascorbic acid in a human intestinal epithelial cell line.","date":"2002","source":"The British journal of nutrition","url":"https://pubmed.ncbi.nlm.nih.gov/11895172","citation_count":71,"is_preprint":false},{"pmid":"17008880","id":"PMC_17008880","title":"Regulation of UVB-induced IL-8 and MCP-1 production in skin keratinocytes by increasing vitamin C uptake via the redistribution of SVCT-1 from the cytosol to the membrane.","date":"2006","source":"The Journal of investigative dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/17008880","citation_count":62,"is_preprint":false},{"pmid":"16357110","id":"PMC_16357110","title":"Genetic variation in the sodium-dependent vitamin C transporters, SLC23A1, and SLC23A2 and risk for preterm delivery.","date":"2005","source":"American journal of epidemiology","url":"https://pubmed.ncbi.nlm.nih.gov/16357110","citation_count":57,"is_preprint":false},{"pmid":"20122894","id":"PMC_20122894","title":"Ascorbic acid depletion enhances expression of the sodium-dependent vitamin C transporters, SVCT1 and SVCT2, and uptake of ascorbic acid in livers of SMP30/GNL knockout mice.","date":"2010","source":"Archives of biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/20122894","citation_count":50,"is_preprint":false},{"pmid":"15316768","id":"PMC_15316768","title":"Comparison of the genomic structure and variation in the two human sodium-dependent vitamin C transporters, SLC23A1 and SLC23A2.","date":"2004","source":"Human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/15316768","citation_count":48,"is_preprint":false},{"pmid":"23737080","id":"PMC_23737080","title":"Vitamin C transporter gene (SLC23A1 and SLC23A2) polymorphisms, plasma vitamin C levels, and gastric cancer risk in the EPIC cohort.","date":"2013","source":"Genes & nutrition","url":"https://pubmed.ncbi.nlm.nih.gov/23737080","citation_count":40,"is_preprint":false},{"pmid":"18094143","id":"PMC_18094143","title":"Transport model of the human Na+-coupled L-ascorbic acid (vitamin C) transporter SVCT1.","date":"2007","source":"American journal of physiology. 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The transporter displays exquisite substrate selectivity, greatly favoring L-ascorbic acid over D-isoascorbic acid, dehydroascorbic acid, and 2- or 6-substituted analogues, while excluding glucose and nucleobases.\",\n      \"method\": \"Radiotracer uptake and voltage-clamp assays in cRNA-injected Xenopus oocytes\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct functional reconstitution in Xenopus oocytes with kinetic and electrophysiological characterization\",\n      \"pmids\": [\"10631088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Slc23a1 knockout mice have less than 5% of normal ascorbic acid uptake in cultured embryonic fibroblasts, undetectable or markedly reduced ascorbic acid in blood and tissues, and die within minutes of birth with respiratory failure and intraparenchymal brain hemorrhage. Prenatal supplementation failed to elevate fetal blood ascorbic acid, demonstrating that Slc23a1 is required for placental ascorbic acid transport.\",\n      \"method\": \"Knockout mouse model; ascorbic acid uptake assays in cultured fibroblasts; tissue ascorbic acid measurement; histopathology\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean KO with multiple defined phenotypic readouts and mechanistic follow-up, replicated across tissues\",\n      \"pmids\": [\"11984597\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"SVCT1 expression is up-regulated ~4-fold at the mRNA level during post-confluent differentiation of CaCo-2 cells and is selectively sorted to the apical membrane compartment in polarized epithelial monolayers, where it is the sole functional ascorbic acid transporter mediating vectorial uptake across the intestinal barrier.\",\n      \"method\": \"Real-time quantitative PCR; transport kinetics; apical vs. basolateral functional assays in Transwell filter inserts; RT-PCR\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (qPCR, functional transport assays, polarized localization) in a single study\",\n      \"pmids\": [\"12381735\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"High-dose ascorbic acid (4.5 mg/ml for 24 h) significantly reduces SVCT1 mRNA expression (~77%) and L-ascorbic acid uptake (~50%) in Caco-2 TC7 intestinal epithelial cells, indicating that SVCT1 is subject to substrate-dependent transcriptional down-regulation.\",\n      \"method\": \"RT-PCR for SVCT1 mRNA; radiolabeled 14C-ascorbic acid uptake assay\",\n      \"journal\": \"The British journal of nutrition\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — two orthogonal methods (RT-PCR and radiotracer uptake), single lab, single cell model\",\n      \"pmids\": [\"11895172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"SVCT1-EGFP localizes predominantly to the apical membrane of confluent Caco-2 and MDCK epithelial cells, and expression of SVCT1 increases transport activity from the apical membrane, establishing a non-redundant apical function distinct from the basolateral function of SVCT2.\",\n      \"method\": \"EGFP-tagged SVCT expression constructs; confocal microscopy; Transwell ascorbate transport assays in MDCK and Caco-2 cells; SVCT2-knockout enterocytes\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization imaging combined with functional transport assays and genetic knockout validation\",\n      \"pmids\": [\"15993839\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"UVB irradiation increases vitamin C uptake into keratinocytes in a time- and dose-dependent manner through translocation of SVCT1 from the cytosol to the plasma membrane, thereby enabling vitamin C to suppress UVB-induced IL-8 and MCP-1 production.\",\n      \"method\": \"Vitamin C uptake assays; subcellular fractionation/translocation imaging; RNase protection assay; ELISA for chemokine protein levels\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — localization change observed with functional consequence (chemokine suppression), single lab, multiple readouts but limited mechanistic depth\",\n      \"pmids\": [\"17008880\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"SVCT1 mediates L-ascorbic acid transport with a Na+:ascorbate coupling ratio of 2:1, with an ordered simultaneous binding mechanism (Na+, L-ascorbic acid, Na+). Pre-steady-state currents in the absence of ascorbate, described by single Boltzmann distributions, indicate that the first Na+ binds partway within the membrane electric field (ion-well effect). A detailed transport model was established.\",\n      \"method\": \"Simultaneous radiotracer flux and voltage-clamp measurements in Xenopus oocytes; pre-steady-state current analysis; model simulation\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — rigorous electrophysiological reconstitution with simultaneous flux/current measurement, kinetic modeling, and stoichiometry determination\",\n      \"pmids\": [\"18094143\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The differential apical (SVCT1) vs. basolateral (SVCT2) membrane targeting in epithelial cells is determined hierarchically: an N-terminal basolateral targeting sequence (BTS) present in SVCT2 but not SVCT1 drives basolateral localization; its destruction redirects SVCT2 apically. A C-terminal sequence in both SVCTs is required for plasma membrane incorporation/retention; its deletion causes intracellular accumulation of both transporters. Default targeting for SVCT is apical.\",\n      \"method\": \"Domain swaps, deletions, insertions, and point mutations on EGFP-tagged hSVCT1/hSVCT2; stable expression in MDCK cells; confocal microscopy; Transwell ascorbate transport assays\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — systematic mutagenesis combined with direct localization imaging and functional transport assays, multiple orthogonal approaches\",\n      \"pmids\": [\"19216494\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Ascorbic acid depletion in SMP30/GNL knockout mice (unable to synthesize ascorbic acid) enhances SVCT1 mRNA expression in the liver and small intestine, and increases actual ascorbic acid uptake in primary cultured hepatocytes, indicating that intracellular ascorbic acid negatively regulates SVCT1 expression.\",\n      \"method\": \"SMP30/GNL knockout mice; RT-PCR for SVCT1/SVCT2 mRNA; radiotracer ascorbic acid uptake in primary hepatocytes\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic model combined with functional uptake assays and mRNA quantification, single lab\",\n      \"pmids\": [\"20122894\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Transcriptional regulation of hSVCT1 in human liver epithelial cells (HepG2) requires HNF-1 binding sites in the hSVCT1 promoter for basal activity and for adaptive (ascorbic acid-responsive) regulation; ascorbic acid deprivation increases and supplementation decreases hSVCT1 mRNA, protein, and promoter activity via HNF-1 sites, whereas hSVCT2 promoter/expression is unaffected.\",\n      \"method\": \"Promoter-reporter constructs; mutational analysis of cis-elements; 14C-ascorbic acid uptake; RT-PCR and protein expression in HepG2 cells under ascorbic acid-deficient/supplemented conditions\",\n      \"journal\": \"The Journal of nutritional biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — promoter mutagenesis plus functional uptake and expression assays, single lab\",\n      \"pmids\": [\"20471816\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Rab8a co-localizes with hSVCT1 in intestinal cells and is required for proper apical membrane targeting of hSVCT1; knockdown of Rab8a reduces cell-surface expression of hSVCT1 (leading to lysosomal mis-trafficking revealed by LAMP1 co-localization) and significantly inhibits ascorbic acid uptake. Rab8a knockout mice show similarly reduced intestinal ascorbic acid uptake and decreased mSVCT1 protein.\",\n      \"method\": \"Co-localization by confocal microscopy; siRNA knockdown; cell-surface biotinylation; 14C-ascorbic acid uptake; Rab8a knockout mouse intestinal uptake assays; LAMP1 co-localization\",\n      \"journal\": \"Digestive diseases and sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (siRNA knockdown, KO mouse, biotinylation, lysosomal trafficking imaging, radiotracer uptake) in single study\",\n      \"pmids\": [\"23014846\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SLC23A1 does not mediate ascorbic acid efflux/release in the proximal renal epithelial cell; using a dual-transporter Xenopus oocyte system and mammalian cells overexpressing SLC23A1, no ascorbate release was detected, establishing that SLC23A1 functions exclusively in ascorbate uptake across the apical membrane of proximal tubule cells.\",\n      \"method\": \"Dual-transporter Xenopus oocyte efflux assay; mammalian cell overexpression; gene expression profiling of human proximal tubule segments\",\n      \"journal\": \"Physiological reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional reconstitution in oocytes plus mammalian cell validation; negative result (no efflux) is the mechanistic finding\",\n      \"pmids\": [\"24400138\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Purified human SVCT1 (hSVCT1) exists predominantly as a monomer with a minor dimeric population in detergent solution; chemical crosslinking of isolated oocyte membranes also shows predominantly monomeric and minor dimeric states in lipid bilayers. The protein is glycosylated when expressed in Xenopus oocytes.\",\n      \"method\": \"Expression in Xenopus oocytes; protein purification; transmission electron microscopy and single-particle analysis; chemical crosslinking\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — structural analysis by EM with crosslinking, but low-resolution and single study\",\n      \"pmids\": [\"24124560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structures of mouse SVCT1 in apo and substrate-bound states show that SVCT1 forms a homodimer, with each protomer containing a core domain and a gate domain. Vitamin C binds at the core domain of each subunit with two sodium ions coordinated near the binding site. The tightly packed extracellular interfaces stabilize an inward-open conformation. Transport likely proceeds via an elevator mechanism combined with local structural rearrangements.\",\n      \"method\": \"Cryo-electron microscopy of apo and substrate-bound mouse SVCT1; structural analysis of sodium ion coordination and substrate binding site\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution cryo-EM structures in two states with identification of substrate and ion binding sites, explaining coupling mechanism\",\n      \"pmids\": [\"36914666\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SVCT1 functions as a sodium-dependent low-affinity/high-capacity urate transporter in addition to its ascorbic acid transport role, as demonstrated by mammalian cell-based transport assays for human SVCT1 and mouse Svct1. Svct1 knockout mice in a hyperuricemic background have lower serum urate than controls, suggesting a physiological role for Svct1 in renal urate reuptake.\",\n      \"method\": \"Mammalian cell-based transport assays (human SVCT1 and mouse Svct1); CRISPR-Cas9 Svct1 knockout mice crossed into hyperuricemic (uricase-deficient) background; serum urate measurement\",\n      \"journal\": \"Pflugers Archiv : European journal of physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct transport assays plus in vivo genetic model, single lab, novel substrate identification\",\n      \"pmids\": [\"36749388\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Species-specific transcriptional regulation of SVCT1 in rat vs. human hepatoma cells involves distinct cis-regulatory elements: Bach1 and HNF4 binding sites are critical for rat SVCT1 promoter activity but absent at equivalent positions in the human promoter, while HNF1 sites critical for human SVCT1 regulation are present in the rat promoter but do not affect its activity.\",\n      \"method\": \"Promoter cloning; deletion and mutant reporter constructs; site-directed mutagenesis; transfection into rat H4IIE and human HepG2 hepatoma cells with transcription factor co-expression\",\n      \"journal\": \"Free radical biology & medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic promoter mutagenesis with functional reporter assays in two species, single lab\",\n      \"pmids\": [\"25933589\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Iron exposure up-regulates SVCT1 protein expression (~24%) in ascorbic acid-deficient Caco-2 cells and correlates with a ~285% increase in ascorbic acid uptake, demonstrating that iron regulates SVCT1 expression and ascorbic acid transport in intestinal epithelial cells.\",\n      \"method\": \"ELISA and Western blot for SVCT1 protein; radiolabeled ascorbic acid uptake; SVCT1 inhibitor (quercetin) validation in Caco-2 cells on Transwell inserts\",\n      \"journal\": \"The British journal of nutrition\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — multiple methods (Western blot, ELISA, uptake assay with inhibitor), single lab, limited mechanistic depth\",\n      \"pmids\": [\"21418708\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC23A1 (SVCT1) is an electrogenic, sodium-coupled ascorbic acid cotransporter that uses an ordered binding mechanism (2 Na⁺ : 1 ascorbate, with sequential Na⁺–ascorbate–Na⁺ binding) and an elevator-type transport mechanism, as established by cryo-EM structures and electrophysiological reconstitution; it localizes to the apical membrane of intestinal and renal proximal tubule epithelial cells via a default apical targeting pathway requiring a C-terminal membrane retention sequence—overridden by an N-terminal basolateral targeting sequence in the paralog SVCT2—and is regulated transcriptionally by HNF-1 sites in response to intracellular ascorbic acid levels; additionally, SVCT1 mediates renal ascorbic acid reabsorption (but not basolateral efflux), can transport urate as a secondary substrate, requires Rab8a for proper apical trafficking in intestinal cells, and its deficiency in mice is perinatally lethal due to failure of ascorbic acid delivery to tissues including brain and lung.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SLC23A1 (SVCT1) is an electrogenic, sodium-coupled L-ascorbic acid cotransporter that mediates concentrative vitamin C uptake at the apical membrane of intestinal and renal epithelia and is essential for systemic ascorbate delivery [#0, #2]. Transport is high-affinity (K0.5 = 50\\u2013100 \\u00b5M), highly selective for L-ascorbic acid, and couples 2 Na\\u207a per ascorbate through an ordered, simultaneous binding mechanism (Na\\u207a, ascorbate, Na\\u207a) in which the first sodium binds partway within the membrane electric field [#0, #6]. Cryo-EM structures resolve SVCT1 as a homodimer with each protomer composed of a core domain and a gate domain; ascorbate and two coordinating sodium ions occupy the core domain, and transport proceeds by an elevator-type mechanism with local rearrangements [#13]. In polarized epithelia SVCT1 is sorted to the apical membrane by default, a fate established by a C-terminal sequence required for plasma-membrane retention and distinguished from its basolateral paralog SVCT2 by the absence of an N-terminal basolateral targeting sequence [#7]; correct apical delivery in intestinal cells additionally requires Rab8a, whose loss diverts SVCT1 to lysosomes and abolishes uptake [#10]. Expression is governed transcriptionally by intracellular ascorbate levels acting through HNF-1 sites in the human promoter, such that depletion induces and supplementation represses SVCT1 [#9, #8]. In the kidney SVCT1 mediates apical ascorbate reabsorption but not basolateral efflux, and it additionally functions as a sodium-dependent low-affinity/high-capacity urate transporter [#11, #14]. Knockout mice die within minutes of birth from respiratory failure and brain hemorrhage with profound tissue ascorbate depletion, reflecting an essential role in placental and tissue ascorbate provision [#1].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Established that human SVCT1 is itself a sodium-dependent, electrogenic, high-affinity ascorbate transporter with strict substrate selectivity, defining the protein's core biochemical activity.\",\n      \"evidence\": \"Radiotracer uptake and voltage-clamp assays in cRNA-injected Xenopus oocytes\",\n      \"pmids\": [\"10631088\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and ion-coupling mechanism not yet resolved\", \"No structural basis for substrate selectivity\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstrated that SVCT1 is physiologically essential, with knockout causing perinatal lethality and failure of placental ascorbate transfer, establishing the transporter's role in systemic vitamin C delivery.\",\n      \"evidence\": \"Slc23a1 knockout mice with tissue ascorbate measurement, fibroblast uptake assays, and histopathology\",\n      \"pmids\": [\"11984597\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific contributions to lethality not dissected\", \"Does not address regulation or trafficking\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Showed SVCT1 is the sole apical ascorbate transporter mediating vectorial uptake across the intestinal barrier and is induced during epithelial differentiation.\",\n      \"evidence\": \"qPCR, transport kinetics, and apical/basolateral functional assays in polarized Caco-2 monolayers\",\n      \"pmids\": [\"12381735\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Targeting signals driving apical sorting unknown\", \"Trafficking machinery not identified\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Provided early evidence that SVCT1 expression is feedback-regulated by its own substrate, with high ascorbate down-regulating mRNA and uptake.\",\n      \"evidence\": \"RT-PCR and 14C-ascorbate uptake in Caco-2 TC7 cells under high-dose ascorbate\",\n      \"pmids\": [\"11895172\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cis-elements and transcription factors mediating the response not identified\", \"Single cell model\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Defined the transport mechanism quantitatively: a 2 Na\\u207a:1 ascorbate coupling ratio via ordered simultaneous binding with the first Na\\u207a sensing the membrane field.\",\n      \"evidence\": \"Simultaneous radiotracer flux and voltage-clamp with pre-steady-state current analysis and kinetic modeling in Xenopus oocytes\",\n      \"pmids\": [\"18094143\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of ion and substrate sites not yet visualized\", \"Conformational cycle inferred, not observed\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Resolved how SVCT1 versus SVCT2 achieve opposite polarized targeting, identifying a default apical pathway, a C-terminal retention sequence, and a paralog-specific N-terminal basolateral signal.\",\n      \"evidence\": \"Systematic domain swaps, deletions, and point mutations on EGFP-tagged SVCTs with confocal imaging and Transwell transport in MDCK cells\",\n      \"pmids\": [\"19216494\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trafficking proteins reading these signals not identified\", \"Mechanism of C-terminal retention unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Connected substrate-dependent regulation to a defined transcriptional mechanism, showing HNF-1 promoter sites mediate both basal and ascorbate-adaptive control of human SVCT1.\",\n      \"evidence\": \"Promoter-reporter and cis-element mutagenesis with uptake and expression assays in HepG2 cells; SMP30/GNL knockout mice with hepatocyte uptake\",\n      \"pmids\": [\"20471816\", \"20122894\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Signal coupling intracellular ascorbate to HNF-1 activity unknown\", \"Single lab per model\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identified Rab8a as a trafficking factor required for apical surface delivery of SVCT1, with loss causing lysosomal mis-sorting and loss of uptake.\",\n      \"evidence\": \"Co-localization, siRNA knockdown, surface biotinylation, LAMP1 imaging, and Rab8a knockout mouse intestinal uptake assays\",\n      \"pmids\": [\"23014846\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct Rab8a\\u2013SVCT1 interaction not demonstrated\", \"Other trafficking effectors not mapped\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Clarified the renal role of SVCT1 as apical reabsorptive uptake only, with no basolateral efflux activity.\",\n      \"evidence\": \"Dual-transporter Xenopus oocyte efflux assay and mammalian overexpression with proximal tubule expression profiling\",\n      \"pmids\": [\"24400138\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the basolateral efflux pathway unresolved\", \"Negative result depends on assay sensitivity\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Provided the first biochemical view of the purified, glycosylated transporter, showing predominantly monomeric with minor dimeric states.\",\n      \"evidence\": \"Purification, single-particle EM, and chemical crosslinking of Xenopus oocyte-expressed hSVCT1\",\n      \"pmids\": [\"24124560\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Low-resolution; oligomeric state later revised by cryo-EM\", \"No atomic detail\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Delivered atomic-resolution structures explaining ion coupling and the elevator transport mechanism, showing a homodimer with substrate and two sodium ions bound at the core domain.\",\n      \"evidence\": \"Cryo-EM of apo and substrate-bound mouse SVCT1\",\n      \"pmids\": [\"36914666\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Outward-open state not captured\", \"Conformational dynamics of the elevator cycle not directly observed\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Expanded SVCT1's substrate repertoire and physiology by identifying it as a sodium-dependent urate transporter contributing to renal urate reuptake.\",\n      \"evidence\": \"Mammalian cell transport assays and CRISPR Svct1 knockout in a hyperuricemic mouse background with serum urate measurement\",\n      \"pmids\": [\"36749388\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis for urate recognition not defined\", \"Relative physiological weight of urate vs ascorbate transport unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How intracellular ascorbate concentration is sensed and transduced to HNF-1-dependent transcriptional and trafficking responses, and the molecular partners reading SVCT1 targeting signals, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No sensor linking ascorbate levels to HNF-1 identified\", \"Direct SVCT1 trafficking interactome incomplete\", \"Basolateral renal efflux transporter unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 2, 6, 11, 14]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [0, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 4, 5, 7, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 2, 6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"RAB8A\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}