{"gene":"SLC13A2","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":1996,"finding":"NaDC-1 (SLC13A2) expressed in COS-7 cells mediates sodium-coupled di- and tricarboxylate (succinate, citrate) transport with a stoichiometry of ~3 Na+ per succinate, pH-dependent citrate transport, and pH-independent succinate transport, consistent with apical membrane renal proximal tubule function.","method":"Transient expression in COS-7 cells, transport kinetics assay","journal":"Pflugers Archiv : European journal of physiology","confidence":"High","confidence_rationale":"Tier 1 — functional reconstitution in heterologous cell system with kinetic characterization","pmids":["8596711"],"is_preprint":false},{"year":1998,"finding":"His-106 of NaDC-1 is required for membrane expression/stability rather than catalytic function; His-153 and His-569 are involved in DEPC binding but not transport activity; no individual histidine residue is essential for transport.","method":"Site-directed mutagenesis of all 11 histidines, expression in Xenopus oocytes, cell surface biotinylation","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis combined with surface biotinylation and transport assay","pmids":["9512488"],"is_preprint":false},{"year":1999,"finding":"Asp-373 (TM8) and Glu-475 (TM9) are critical for NaDC-1 cation recognition and substrate transport: Asp-373 substitutions alter sodium affinity and cation selectivity, while Glu-475 substitutions drastically reduce transport and alter sodium/substrate binding, suggesting these residues form part of the cation- and substrate-binding site in the C-terminal portion of NaDC-1.","method":"Site-directed mutagenesis, expression in Xenopus oocytes, transport kinetics, electrophysiology","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with kinetic and electrophysiological characterization","pmids":["10360950"],"is_preprint":false},{"year":1999,"finding":"No individual cysteine residue is essential for NaDC-1 transport function; however, Cys-227 (TM5) and Cys-476 (TM9) are accessible to the impermeant reagent pCMBS, and cumulative cysteine mutagenesis impairs protein stability/trafficking to the plasma membrane.","method":"Site-directed mutagenesis of all 11 cysteines, pCMBS inhibition assay, expression in Xenopus oocytes","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — complete cysteine-scanning mutagenesis with functional and chemical accessibility assays","pmids":["10548552"],"is_preprint":false},{"year":1999,"finding":"Protein kinase C (PKC) activation by PMA or DOG inhibits NaDC-1 transport activity in Xenopus oocytes by ~95%, partly through endocytosis (reduced plasma membrane NaDC-1 protein by ~30%) and partly through direct transport inhibition; PKA activation has no effect; PKC consensus phosphorylation sites on NaDC-1 are not required for PMA-mediated inhibition.","method":"PKC/PKA activator treatment in Xenopus oocytes expressing NaDC-1, transport assay, surface protein quantification, cytochalasin D treatment, staurosporine blockade","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (transport assay, surface protein quantification, inhibitor studies) in single study","pmids":["10446305"],"is_preprint":false},{"year":2000,"finding":"Lys-84 (cytoplasmic end of TM3) is important for substrate binding in NaDC-1: K84A increases Km for succinate ~6-fold with no plasma membrane targeting defect. Arg-349 (extracellular end of TM7) affects succinate handling (inhibition at high concentrations) but not citrate or Na+ kinetics; both residues are expressed at the plasma membrane.","method":"Site-directed mutagenesis, expression in Xenopus oocytes, transport kinetics","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with kinetic characterization and surface expression controls","pmids":["10970779"],"is_preprint":false},{"year":2000,"finding":"Chronic metabolic acidosis increases NaDC-1 mRNA abundance and apical membrane protein abundance in rat kidney proximal tubule (maximally in S2 segment), correlating with increased Na+/citrate cotransport activity; alkali feeding has no effect on NaDC-1 mRNA or protein.","method":"Immunohistochemistry, immunoblot of brush border membranes, mRNA quantification in rat kidney","journal":"Kidney international","confidence":"High","confidence_rationale":"Tier 2 — in vivo model with protein and mRNA quantification, multiple time points and severity gradations","pmids":["10886565"],"is_preprint":false},{"year":2004,"finding":"NaDC-1 in OKP proximal tubule cells mediates Na+-coupled citrate transport with stoichiometry ≥3 Na+:1 citrate; media acidification increases Na-dependent citrate uptake post-transcriptionally (without changing mRNA abundance), establishing posttranscriptional acid regulation of NaDC-1 activity.","method":"cDNA cloning, Xenopus oocyte expression, [14C]-citrate uptake assay, GFP-NaDC-1 transfection in OKP cells, media acidification experiment","journal":"American journal of physiology. Cell physiology","confidence":"High","confidence_rationale":"Tier 1–2 — functional reconstitution plus cell-based transport assay demonstrating posttranscriptional regulation","pmids":["14973148"],"is_preprint":false},{"year":2010,"finding":"Acid stimulation of NaDC-1 activity in vivo and in vitro requires a functional endothelin B (ETB) receptor: ETB knockout mice show no acid-induced increase in brush border NaDC-1 activity; ETB C-terminal tail is required for ET-1-mediated NaDC-1 stimulation, and the ETB transmembrane domain contributes to this effect.","method":"ETB knockout mouse model, ETB chimeric receptor constructs in OKP cells, [14C]-citrate uptake, brush border membrane vesicle assay, ETB antagonist BQ788","journal":"Kidney international","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout combined with chimeric receptor dissection and functional transport assays","pmids":["20703215"],"is_preprint":false},{"year":2013,"finding":"SLC26A6 (oxalate transporter) and NaDC-1 (SLC13A2) physically interact via the STAS domain of SLC26A6 and the first intracellular loop of NaDC-1; functionally, NaDC-1 enhances SLC26A6 transport activity while SLC26A6 inhibits NaDC-1 transport activity in an activity-dependent manner, forming a regulatory pathway that controls oxalate and citrate homeostasis.","method":"Co-expression in Xenopus oocytes, transport assays, biochemical interaction studies, Slc26a6-null mouse model, domain mapping","journal":"Journal of the American Society of Nephrology : JASN","confidence":"High","confidence_rationale":"Tier 1–2 — physical interaction domain mapping combined with functional reconstitution in oocytes and in vivo knockout validation","pmids":["23833257"],"is_preprint":false},{"year":2016,"finding":"NaDC-1 (SLC13A2) protein localizes exclusively to the apical membrane of the entire proximal tubule in human kidney (including proximal convoluted and straight tubule segments), confirmed by colocalization with the proximal tubule marker NBCe1; NaDC-1 is not detectable in other renal cell types or in renal tumors.","method":"Immunoblot (~61 kDa band), immunohistochemistry, colocalization with NBCe1 in human kidney tissue","journal":"American journal of physiology. Renal physiology","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization in human tissue with protein-level confirmation and colocalization, single study","pmids":["27927654"],"is_preprint":false},{"year":2018,"finding":"Acid stimulation of NaDC-1 activity requires Pyk2 and c-Src kinases (upstream) and the Raf1-ERK1/2-p90RSK signaling cascade (downstream), but these kinase pathways are not downstream of ET-1/ETB receptor; in Pyk2-null mice, acid loading fails to stimulate brush border NaDC-1 activity.","method":"Kinase inhibitors, Pyk2 knockdown, Pyk2-null mice, [14C]-citrate uptake in OKP cells, immunoblot for ERK1/2, Raf1, p90RSK phosphorylation, brush border membrane vesicle assay","journal":"Journal of the American Society of Nephrology : JASN","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout mouse combined with multiple kinase inhibitors and epistasis analysis with ET-1/ETB pathway","pmids":["29678998"],"is_preprint":false},{"year":2025,"finding":"SLC13A2 promotes liver regeneration by importing citrate into hepatocytes, which serves as substrate for ACLY-dependent acetyl-CoA production, driving de novo cholesterol biosynthesis via SREBP2 cleavage and upregulation of HMGCR and LDLR; liver-specific overexpression or depletion of SLC13A2 promotes or attenuates liver regeneration, respectively, and HMGCR inhibition (lovastatin) or ACLY inhibition abolishes SLC13A2-mediated regeneration.","method":"Liver-specific SLC13A2 overexpression/depletion in mice, partial hepatectomy model, HMGCR inhibitor (lovastatin), ACLY inhibitor, metabolic flux analysis, SREBP2 cleavage assay, cholesterol synthesis measurement","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — in vivo gain/loss-of-function with multiple pharmacological epistasis experiments and metabolic flux analysis","pmids":["39824985"],"is_preprint":false},{"year":2026,"finding":"In hepatocellular carcinoma, SLC13A2 imports citrate to generate acetyl-CoA, which acetylates pyruvate kinase M2 (PKM2) and triggers its degradation, thereby reducing pyruvate supply and impairing glycolysis, amino acid synthesis, and nucleotide metabolism; additionally, SLC13A2-imported citrate increases histone acetylation, providing an epigenetic basis for transcriptional tumor suppression.","method":"AAV-mediated liver-specific knockout/overexpression, metabolic flux analysis, PKM2 acetylation assay, glycolysis/respiration/ATP measurements, histone acetylation profiling in HCC mouse models and human tissue","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1–2 — in vivo genetic models with flux analysis, substrate tracing, and post-translational modification characterization","pmids":["41950310"],"is_preprint":false}],"current_model":"SLC13A2 (NaDC-1) is an apical membrane Na+-dicarboxylate cotransporter (stoichiometry ≥3 Na+:1 substrate) in renal proximal tubules that reabsorbs TCA cycle intermediates (citrate, succinate); its activity is acutely downregulated by PKC-mediated endocytosis, stimulated by acid through an ETB receptor → Pyk2/c-Src → Raf1-ERK1/2-p90RSK signaling cascade, and reciprocally regulated by direct physical interaction with SLC26A6 via its first intracellular loop; in hepatocytes, SLC13A2-imported citrate fuels ACLY-dependent acetyl-CoA production for de novo cholesterol biosynthesis (supporting liver regeneration) and drives PKM2 acetylation/degradation and histone acetylation to suppress tumor growth."},"narrative":{"teleology":[{"year":1996,"claim":"Establishing the basic transport identity of SLC13A2: heterologous expression demonstrated that NaDC-1 is an electrogenic Na⁺-coupled cotransporter for di- and tricarboxylates with ~3 Na⁺ per succinate and pH-dependent citrate selectivity, defining its role as the apical renal citrate/succinate reabsorption pathway.","evidence":"Transient expression in COS-7 cells with radiolabeled substrate uptake and kinetic analysis","pmids":["8596711"],"confidence":"High","gaps":["Three-dimensional structure undetermined","In vivo physiological confirmation not yet performed","Coupling stoichiometry for citrate not precisely defined"]},{"year":1999,"claim":"Identification of the molecular determinants of cation and substrate recognition: systematic mutagenesis revealed that Asp-373 and Glu-475 in TM8–TM9 form part of the Na⁺/substrate binding site, Lys-84 at the cytoplasmic end of TM3 contributes to substrate affinity, and His-106 is required for membrane stability rather than catalysis, mapping functional residues onto the NaDC-1 topology.","evidence":"Site-directed mutagenesis of histidines, cysteines, and charged residues expressed in Xenopus oocytes with transport kinetics, electrophysiology, and surface biotinylation","pmids":["9512488","10360950","10548552","10970779"],"confidence":"High","gaps":["No crystal or cryo-EM structure to confirm spatial arrangement of identified residues","Na⁺ binding order and cooperativity mechanism unresolved"]},{"year":1999,"claim":"Discovery of acute post-translational regulation: PKC activation suppresses NaDC-1 transport by ~95% through a dual mechanism—partial endocytosis (~30% surface protein reduction) plus direct inhibition of remaining surface transporters—independent of PKC consensus sites on NaDC-1 itself.","evidence":"PMA/DOG treatment of NaDC-1-expressing Xenopus oocytes with transport assay, surface protein quantification, cytochalasin D, and staurosporine blockade","pmids":["10446305"],"confidence":"High","gaps":["Direct PKC phosphorylation target mediating the ~65% non-endocytic inhibition is unknown","Physiological trigger for PKC-mediated regulation in vivo not established"]},{"year":2004,"claim":"Acid regulation of NaDC-1 was shown to operate post-transcriptionally: media acidification stimulated citrate transport without changing NaDC-1 mRNA, complementing in vivo evidence that chronic metabolic acidosis increases apical NaDC-1 protein abundance in rat proximal tubule, revealing both transcriptional and post-transcriptional regulatory tiers.","evidence":"In vivo rat acidosis model with immunohistochemistry and mRNA quantification; OKP cell acidification with [14C]-citrate uptake and GFP-NaDC-1 trafficking","pmids":["10886565","14973148"],"confidence":"High","gaps":["Post-transcriptional mechanism (trafficking vs. translational vs. stability) not molecularly defined","Whether acid regulation involves the same pathway in human kidney is untested"]},{"year":2010,"claim":"The endothelin B receptor was identified as a required upstream mediator of acid-stimulated NaDC-1 activity: ETB knockout mice lost the acid-induced increase in brush border NaDC-1 activity, and the ETB C-terminal tail and transmembrane domain were required for ET-1-mediated stimulation.","evidence":"ETB knockout mouse, chimeric ETB receptor constructs in OKP cells, [14C]-citrate uptake, BQ788 antagonist","pmids":["20703215"],"confidence":"High","gaps":["Whether ETB directly signals to NaDC-1 or acts through intermediate effectors was unknown at this stage","Relationship between ETB pathway and PKC inhibitory pathway not clarified"]},{"year":2013,"claim":"NaDC-1 was shown to be reciprocally regulated by direct physical interaction with the oxalate transporter SLC26A6: the STAS domain of SLC26A6 binds the first intracellular loop of NaDC-1, SLC26A6 inhibits NaDC-1 activity in an activity-dependent manner, and NaDC-1 enhances SLC26A6 transport, linking citrate and oxalate homeostasis.","evidence":"Co-expression in Xenopus oocytes, domain mapping, Slc26a6-null mouse model","pmids":["23833257"],"confidence":"High","gaps":["Structural basis of the STAS–intracellular loop interaction is unknown","In vivo consequences for kidney stone formation not directly tested","Whether other SLC26 family members similarly regulate NaDC-1 is unexplored"]},{"year":2016,"claim":"Human tissue localization confirmed that NaDC-1 protein is restricted to the apical membrane along the entire proximal tubule and is absent from other nephron segments and renal tumors, solidifying its role as the sole apical dicarboxylate transporter in human kidney.","evidence":"Immunohistochemistry and immunoblot with NBCe1 colocalization in human kidney tissue sections","pmids":["27927654"],"confidence":"Medium","gaps":["Single study with one antibody; independent antibody validation not reported","Functional transport data from human proximal tubule cells not included"]},{"year":2018,"claim":"The intracellular signaling cascade for acid-stimulated NaDC-1 was delineated: Pyk2 and c-Src kinases activate Raf1-ERK1/2-p90RSK to stimulate NaDC-1 activity, and this cascade operates in parallel to (not downstream of) the ETB receptor pathway, as shown by Pyk2-null mice failing to upregulate NaDC-1 upon acid loading.","evidence":"Kinase inhibitors, Pyk2 knockdown/knockout mice, [14C]-citrate uptake in OKP cells, phospho-immunoblots for ERK1/2, Raf1, p90RSK","pmids":["29678998"],"confidence":"High","gaps":["How p90RSK mechanistically increases NaDC-1 surface expression or activity is unknown","Integration point between ETB and Pyk2 pathways remains undefined"]},{"year":2025,"claim":"A hepatic function for SLC13A2 was established: citrate imported by SLC13A2 in hepatocytes fuels ACLY-dependent acetyl-CoA production, SREBP2 cleavage, and de novo cholesterol biosynthesis, which is required for liver regeneration after partial hepatectomy.","evidence":"Liver-specific SLC13A2 overexpression/depletion in mice, partial hepatectomy, lovastatin and ACLY inhibitor epistasis, metabolic flux analysis","pmids":["39824985"],"confidence":"High","gaps":["Whether SLC13A2 is the dominant citrate importer in hepatocytes or acts redundantly with other carriers is untested","Transcriptional or post-translational regulation of hepatic SLC13A2 during regeneration not characterized"]},{"year":2026,"claim":"SLC13A2 was identified as a metabolic tumor suppressor in hepatocellular carcinoma: citrate-derived acetyl-CoA acetylates PKM2, triggering its degradation and suppressing glycolysis, amino acid, and nucleotide metabolism; simultaneously, increased histone acetylation provides an epigenetic basis for transcriptional suppression of tumor programs.","evidence":"AAV liver-specific knockout/overexpression in HCC mouse models, metabolic flux analysis, PKM2 acetylation assay, histone acetylation profiling, human tissue validation","pmids":["41950310"],"confidence":"High","gaps":["Specific acetyltransferase(s) mediating PKM2 acetylation downstream of SLC13A2 not identified","Histone marks and target gene programs driven by SLC13A2-dependent acetylation not fully mapped","Whether SLC13A2 loss is a driver event in human HCC or a consequence of tumor metabolism is correlative"]},{"year":null,"claim":"Key unresolved questions include the three-dimensional structure of NaDC-1, the direct phosphorylation target mediating PKC-dependent transport inhibition, the integration point between the ETB and Pyk2/c-Src signaling arms, and the full spectrum of acetylation targets downstream of SLC13A2-imported citrate in liver.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure available","Direct kinase substrate linking p90RSK to NaDC-1 activation unknown","Relative contribution of SLC13A2 vs. other citrate carriers in hepatocytes not quantified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,7,12,13]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,4,6,10]}],"pathway":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0]}],"complexes":[],"partners":["SLC26A6","ACLY","PKM2","PYK2","EDNRB"],"other_free_text":[]},"mechanistic_narrative":"SLC13A2 (NaDC-1) is an electrogenic Na⁺-dicarboxylate cotransporter that reabsorbs citrate and succinate across the apical membrane of renal proximal tubule cells with a stoichiometry of ≥3 Na⁺ per substrate, and imports citrate into hepatocytes to fuel acetyl-CoA-dependent metabolic and epigenetic programs [PMID:8596711, PMID:27927654, PMID:39824985]. Key determinants of cation and substrate recognition reside in transmembrane helices 8–9 (Asp-373, Glu-475) and the cytoplasmic end of TM3 (Lys-84), while PKC activation inhibits transport through partial endocytosis and direct activity suppression [PMID:10360950, PMID:10970779, PMID:10446305]. Acid stimulation of NaDC-1 activity operates through two parallel pathways—an endothelin B receptor pathway and a Pyk2/c-Src → Raf1-ERK1/2-p90RSK kinase cascade—and is reciprocally modulated by physical interaction with SLC26A6 via the NaDC-1 first intracellular loop [PMID:20703215, PMID:29678998, PMID:23833257]. In the liver, SLC13A2-imported citrate drives ACLY-dependent acetyl-CoA production that supports SREBP2-mediated cholesterol biosynthesis during regeneration and, in hepatocellular carcinoma, promotes PKM2 acetylation/degradation and histone acetylation to suppress tumor growth [PMID:39824985, PMID:41950310]."},"prefetch_data":{"uniprot":{"accession":"Q13183","full_name":"Solute carrier family 13 member 2","aliases":["Na(+)/dicarboxylate cotransporter 1","NaDC-1","Renal sodium/dicarboxylate cotransporter"],"length_aa":592,"mass_kda":64.4,"function":"Low-affinity sodium-dicarboxylate cotransporter, that mediates the entry of citric acid cycle intermediates, such as succinate, citrate, fumarate and alpha-ketoglutarate (2-oxoglutarate) into the small intestine and renal proximal tubule (PubMed:10894787, PubMed:8967342, PubMed:9668069). Transports the dicarboxylate into the cell with a probable stoichiometry of 3 Na(+) for 1 divalent dicarboxylate, rendering the process electrogenic (PubMed:10894787, PubMed:8967342, PubMed:9668069). Citrate is transported in protonated form as a divalent anion, rather than the trivalent form which is normally found in blood (PubMed:10894787). Has a critical role in renal dicarboxylate transport (By similarity)","subcellular_location":"Apical cell membrane","url":"https://www.uniprot.org/uniprotkb/Q13183/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC13A2","classification":"Not Classified","n_dependent_lines":12,"n_total_lines":1208,"dependency_fraction":0.009933774834437087},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SLC13A2","total_profiled":1310},"omim":[{"mim_id":"608305","title":"SOLUTE CARRIER FAMILY 13 (SODIUM-DEPENDENT CITRATE TRANSPORTER), MEMBER 5; SLC13A5","url":"https://www.omim.org/entry/608305"},{"mim_id":"606411","title":"SOLUTE CARRIER FAMILY 13 (SODIUM-DEPENDENT DICARBOXYLATE TRANSPORTER), MEMBER 3; SLC13A3","url":"https://www.omim.org/entry/606411"},{"mim_id":"604148","title":"SOLUTE CARRIER FAMILY 13 (SODIUM-DEPENDENT DICARBOXYLATE TRANSPORTER), MEMBER 2; SLC13A2","url":"https://www.omim.org/entry/604148"},{"mim_id":"603088","title":"SOLUTE CARRIER FAMILY 31 (COPPER TRANSPORTER), MEMBER 2; SLC31A2","url":"https://www.omim.org/entry/603088"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"intestine","ntpm":121.8},{"tissue":"kidney","ntpm":40.2},{"tissue":"salivary gland","ntpm":45.4}],"url":"https://www.proteinatlas.org/search/SLC13A2"},"hgnc":{"alias_symbol":["NaDC-1"],"prev_symbol":[]},"alphafold":{"accession":"Q13183","domains":[{"cath_id":"-","chopping":"32-167_210-393_406-573","consensus_level":"medium","plddt":89.0857,"start":32,"end":573}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13183","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q13183-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q13183-F1-predicted_aligned_error_v6.png","plddt_mean":83.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC13A2","jax_strain_url":"https://www.jax.org/strain/search?query=SLC13A2"},"sequence":{"accession":"Q13183","fasta_url":"https://rest.uniprot.org/uniprotkb/Q13183.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q13183/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13183"}},"corpus_meta":[{"pmid":"10886565","id":"PMC_10886565","title":"Chronic metabolic acidosis increases NaDC-1 mRNA and protein abundance in rat kidney.","date":"2000","source":"Kidney international","url":"https://pubmed.ncbi.nlm.nih.gov/10886565","citation_count":90,"is_preprint":false},{"pmid":"23833257","id":"PMC_23833257","title":"SLC26A6 and NaDC-1 transporters interact to regulate oxalate and citrate homeostasis.","date":"2013","source":"Journal of the American Society of Nephrology : JASN","url":"https://pubmed.ncbi.nlm.nih.gov/23833257","citation_count":60,"is_preprint":false},{"pmid":"9512488","id":"PMC_9512488","title":"Mutational analysis of histidine residues in the rabbit Na+/dicarboxylate co-transporter NaDC-1.","date":"1998","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/9512488","citation_count":34,"is_preprint":false},{"pmid":"10548552","id":"PMC_10548552","title":"Cysteine residues in the Na+/dicarboxylate co-transporter, NaDC-1.","date":"1999","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/10548552","citation_count":29,"is_preprint":false},{"pmid":"10446305","id":"PMC_10446305","title":"Protein kinase C-mediated regulation of the renal Na(+)/dicarboxylate cotransporter, NaDC-1.","date":"1999","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/10446305","citation_count":28,"is_preprint":false},{"pmid":"10360950","id":"PMC_10360950","title":"Acidic residues involved in cation and substrate interactions in the Na+/dicarboxylate cotransporter, NaDC-1.","date":"1999","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10360950","citation_count":26,"is_preprint":false},{"pmid":"20703215","id":"PMC_20703215","title":"Acid regulation of NaDC-1 requires a functional endothelin B receptor.","date":"2010","source":"Kidney international","url":"https://pubmed.ncbi.nlm.nih.gov/20703215","citation_count":21,"is_preprint":false},{"pmid":"27927654","id":"PMC_27927654","title":"Expression of sodium-dependent dicarboxylate transporter 1 (NaDC1/SLC13A2) in normal and neoplastic human kidney.","date":"2016","source":"American journal of physiology. Renal physiology","url":"https://pubmed.ncbi.nlm.nih.gov/27927654","citation_count":18,"is_preprint":false},{"pmid":"29678998","id":"PMC_29678998","title":"Acid Stimulation of the Citrate Transporter NaDC-1 Requires Pyk2 and ERK1/2 Signaling Pathways.","date":"2018","source":"Journal of the American Society of Nephrology : JASN","url":"https://pubmed.ncbi.nlm.nih.gov/29678998","citation_count":17,"is_preprint":false},{"pmid":"14973148","id":"PMC_14973148","title":"OKP cells express the Na-dicarboxylate cotransporter NaDC-1.","date":"2004","source":"American journal of physiology. Cell physiology","url":"https://pubmed.ncbi.nlm.nih.gov/14973148","citation_count":16,"is_preprint":false},{"pmid":"10970779","id":"PMC_10970779","title":"Role of cationic amino acids in the Na+/dicarboxylate co-transporter NaDC-1.","date":"2000","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/10970779","citation_count":14,"is_preprint":false},{"pmid":"8596711","id":"PMC_8596711","title":"Expression of the renal Na+/dicarboxylate cotransporter, NaDC-1, in COS-7 cells.","date":"1996","source":"Pflugers Archiv : European journal of physiology","url":"https://pubmed.ncbi.nlm.nih.gov/8596711","citation_count":12,"is_preprint":false},{"pmid":"27639591","id":"PMC_27639591","title":"Evidence for epistatic interaction between VDR and SLC13A2 genes in the pathogenesis of hypocitraturia in recurrent calcium oxalate stone formers.","date":"2016","source":"Journal of nephrology","url":"https://pubmed.ncbi.nlm.nih.gov/27639591","citation_count":12,"is_preprint":false},{"pmid":"34458928","id":"PMC_34458928","title":"SLC26A6 and NADC‑1: Future direction of nephrolithiasis and calculus‑related hypertension research (Review).","date":"2021","source":"Molecular medicine reports","url":"https://pubmed.ncbi.nlm.nih.gov/34458928","citation_count":5,"is_preprint":false},{"pmid":"30155711","id":"PMC_30155711","title":"rs11567842 SNP in SLC13A2 gene associates with hypocitraturia in Thai patients with nephrolithiasis.","date":"2018","source":"Genes & genomics","url":"https://pubmed.ncbi.nlm.nih.gov/30155711","citation_count":4,"is_preprint":false},{"pmid":"39824985","id":"PMC_39824985","title":"SLC13A2 promotes hepatocyte metabolic remodeling and liver regeneration by enhancing de novo cholesterol biosynthesis.","date":"2025","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/39824985","citation_count":4,"is_preprint":false},{"pmid":"40868238","id":"PMC_40868238","title":"Investigation of rs11568476 Polymorphism in the SLC13A2 Gene in Turkish Patients with Hypocitraturia and Calcium-Containing Kidney Stones.","date":"2025","source":"Biomedicines","url":"https://pubmed.ncbi.nlm.nih.gov/40868238","citation_count":0,"is_preprint":false},{"pmid":"41950310","id":"PMC_41950310","title":"SLC13A2-transported citrate remodels transcriptional regulation through protein acetylation to suppress tumor growth.","date":"2026","source":"Science advances","url":"https://pubmed.ncbi.nlm.nih.gov/41950310","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":10385,"output_tokens":3554,"usd":0.042232},"stage2":{"model":"claude-opus-4-6","input_tokens":6948,"output_tokens":6181,"usd":0.283898},"total_usd":0.32613,"stage1_batch_id":"msgbatch_011rkcQdVV1RaFq9XhcG6df6","stage2_batch_id":"msgbatch_01RzgrJafdF4EWHhj5mNi6dh","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1996,\n      \"finding\": \"NaDC-1 (SLC13A2) expressed in COS-7 cells mediates sodium-coupled di- and tricarboxylate (succinate, citrate) transport with a stoichiometry of ~3 Na+ per succinate, pH-dependent citrate transport, and pH-independent succinate transport, consistent with apical membrane renal proximal tubule function.\",\n      \"method\": \"Transient expression in COS-7 cells, transport kinetics assay\",\n      \"journal\": \"Pflugers Archiv : European journal of physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — functional reconstitution in heterologous cell system with kinetic characterization\",\n      \"pmids\": [\"8596711\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"His-106 of NaDC-1 is required for membrane expression/stability rather than catalytic function; His-153 and His-569 are involved in DEPC binding but not transport activity; no individual histidine residue is essential for transport.\",\n      \"method\": \"Site-directed mutagenesis of all 11 histidines, expression in Xenopus oocytes, cell surface biotinylation\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis combined with surface biotinylation and transport assay\",\n      \"pmids\": [\"9512488\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Asp-373 (TM8) and Glu-475 (TM9) are critical for NaDC-1 cation recognition and substrate transport: Asp-373 substitutions alter sodium affinity and cation selectivity, while Glu-475 substitutions drastically reduce transport and alter sodium/substrate binding, suggesting these residues form part of the cation- and substrate-binding site in the C-terminal portion of NaDC-1.\",\n      \"method\": \"Site-directed mutagenesis, expression in Xenopus oocytes, transport kinetics, electrophysiology\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with kinetic and electrophysiological characterization\",\n      \"pmids\": [\"10360950\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"No individual cysteine residue is essential for NaDC-1 transport function; however, Cys-227 (TM5) and Cys-476 (TM9) are accessible to the impermeant reagent pCMBS, and cumulative cysteine mutagenesis impairs protein stability/trafficking to the plasma membrane.\",\n      \"method\": \"Site-directed mutagenesis of all 11 cysteines, pCMBS inhibition assay, expression in Xenopus oocytes\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — complete cysteine-scanning mutagenesis with functional and chemical accessibility assays\",\n      \"pmids\": [\"10548552\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Protein kinase C (PKC) activation by PMA or DOG inhibits NaDC-1 transport activity in Xenopus oocytes by ~95%, partly through endocytosis (reduced plasma membrane NaDC-1 protein by ~30%) and partly through direct transport inhibition; PKA activation has no effect; PKC consensus phosphorylation sites on NaDC-1 are not required for PMA-mediated inhibition.\",\n      \"method\": \"PKC/PKA activator treatment in Xenopus oocytes expressing NaDC-1, transport assay, surface protein quantification, cytochalasin D treatment, staurosporine blockade\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (transport assay, surface protein quantification, inhibitor studies) in single study\",\n      \"pmids\": [\"10446305\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Lys-84 (cytoplasmic end of TM3) is important for substrate binding in NaDC-1: K84A increases Km for succinate ~6-fold with no plasma membrane targeting defect. Arg-349 (extracellular end of TM7) affects succinate handling (inhibition at high concentrations) but not citrate or Na+ kinetics; both residues are expressed at the plasma membrane.\",\n      \"method\": \"Site-directed mutagenesis, expression in Xenopus oocytes, transport kinetics\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with kinetic characterization and surface expression controls\",\n      \"pmids\": [\"10970779\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Chronic metabolic acidosis increases NaDC-1 mRNA abundance and apical membrane protein abundance in rat kidney proximal tubule (maximally in S2 segment), correlating with increased Na+/citrate cotransport activity; alkali feeding has no effect on NaDC-1 mRNA or protein.\",\n      \"method\": \"Immunohistochemistry, immunoblot of brush border membranes, mRNA quantification in rat kidney\",\n      \"journal\": \"Kidney international\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo model with protein and mRNA quantification, multiple time points and severity gradations\",\n      \"pmids\": [\"10886565\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"NaDC-1 in OKP proximal tubule cells mediates Na+-coupled citrate transport with stoichiometry ≥3 Na+:1 citrate; media acidification increases Na-dependent citrate uptake post-transcriptionally (without changing mRNA abundance), establishing posttranscriptional acid regulation of NaDC-1 activity.\",\n      \"method\": \"cDNA cloning, Xenopus oocyte expression, [14C]-citrate uptake assay, GFP-NaDC-1 transfection in OKP cells, media acidification experiment\",\n      \"journal\": \"American journal of physiology. Cell physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — functional reconstitution plus cell-based transport assay demonstrating posttranscriptional regulation\",\n      \"pmids\": [\"14973148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Acid stimulation of NaDC-1 activity in vivo and in vitro requires a functional endothelin B (ETB) receptor: ETB knockout mice show no acid-induced increase in brush border NaDC-1 activity; ETB C-terminal tail is required for ET-1-mediated NaDC-1 stimulation, and the ETB transmembrane domain contributes to this effect.\",\n      \"method\": \"ETB knockout mouse model, ETB chimeric receptor constructs in OKP cells, [14C]-citrate uptake, brush border membrane vesicle assay, ETB antagonist BQ788\",\n      \"journal\": \"Kidney international\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout combined with chimeric receptor dissection and functional transport assays\",\n      \"pmids\": [\"20703215\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SLC26A6 (oxalate transporter) and NaDC-1 (SLC13A2) physically interact via the STAS domain of SLC26A6 and the first intracellular loop of NaDC-1; functionally, NaDC-1 enhances SLC26A6 transport activity while SLC26A6 inhibits NaDC-1 transport activity in an activity-dependent manner, forming a regulatory pathway that controls oxalate and citrate homeostasis.\",\n      \"method\": \"Co-expression in Xenopus oocytes, transport assays, biochemical interaction studies, Slc26a6-null mouse model, domain mapping\",\n      \"journal\": \"Journal of the American Society of Nephrology : JASN\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — physical interaction domain mapping combined with functional reconstitution in oocytes and in vivo knockout validation\",\n      \"pmids\": [\"23833257\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"NaDC-1 (SLC13A2) protein localizes exclusively to the apical membrane of the entire proximal tubule in human kidney (including proximal convoluted and straight tubule segments), confirmed by colocalization with the proximal tubule marker NBCe1; NaDC-1 is not detectable in other renal cell types or in renal tumors.\",\n      \"method\": \"Immunoblot (~61 kDa band), immunohistochemistry, colocalization with NBCe1 in human kidney tissue\",\n      \"journal\": \"American journal of physiology. Renal physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization in human tissue with protein-level confirmation and colocalization, single study\",\n      \"pmids\": [\"27927654\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Acid stimulation of NaDC-1 activity requires Pyk2 and c-Src kinases (upstream) and the Raf1-ERK1/2-p90RSK signaling cascade (downstream), but these kinase pathways are not downstream of ET-1/ETB receptor; in Pyk2-null mice, acid loading fails to stimulate brush border NaDC-1 activity.\",\n      \"method\": \"Kinase inhibitors, Pyk2 knockdown, Pyk2-null mice, [14C]-citrate uptake in OKP cells, immunoblot for ERK1/2, Raf1, p90RSK phosphorylation, brush border membrane vesicle assay\",\n      \"journal\": \"Journal of the American Society of Nephrology : JASN\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout mouse combined with multiple kinase inhibitors and epistasis analysis with ET-1/ETB pathway\",\n      \"pmids\": [\"29678998\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SLC13A2 promotes liver regeneration by importing citrate into hepatocytes, which serves as substrate for ACLY-dependent acetyl-CoA production, driving de novo cholesterol biosynthesis via SREBP2 cleavage and upregulation of HMGCR and LDLR; liver-specific overexpression or depletion of SLC13A2 promotes or attenuates liver regeneration, respectively, and HMGCR inhibition (lovastatin) or ACLY inhibition abolishes SLC13A2-mediated regeneration.\",\n      \"method\": \"Liver-specific SLC13A2 overexpression/depletion in mice, partial hepatectomy model, HMGCR inhibitor (lovastatin), ACLY inhibitor, metabolic flux analysis, SREBP2 cleavage assay, cholesterol synthesis measurement\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo gain/loss-of-function with multiple pharmacological epistasis experiments and metabolic flux analysis\",\n      \"pmids\": [\"39824985\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"In hepatocellular carcinoma, SLC13A2 imports citrate to generate acetyl-CoA, which acetylates pyruvate kinase M2 (PKM2) and triggers its degradation, thereby reducing pyruvate supply and impairing glycolysis, amino acid synthesis, and nucleotide metabolism; additionally, SLC13A2-imported citrate increases histone acetylation, providing an epigenetic basis for transcriptional tumor suppression.\",\n      \"method\": \"AAV-mediated liver-specific knockout/overexpression, metabolic flux analysis, PKM2 acetylation assay, glycolysis/respiration/ATP measurements, histone acetylation profiling in HCC mouse models and human tissue\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vivo genetic models with flux analysis, substrate tracing, and post-translational modification characterization\",\n      \"pmids\": [\"41950310\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC13A2 (NaDC-1) is an apical membrane Na+-dicarboxylate cotransporter (stoichiometry ≥3 Na+:1 substrate) in renal proximal tubules that reabsorbs TCA cycle intermediates (citrate, succinate); its activity is acutely downregulated by PKC-mediated endocytosis, stimulated by acid through an ETB receptor → Pyk2/c-Src → Raf1-ERK1/2-p90RSK signaling cascade, and reciprocally regulated by direct physical interaction with SLC26A6 via its first intracellular loop; in hepatocytes, SLC13A2-imported citrate fuels ACLY-dependent acetyl-CoA production for de novo cholesterol biosynthesis (supporting liver regeneration) and drives PKM2 acetylation/degradation and histone acetylation to suppress tumor growth.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SLC13A2 (NaDC-1) is an electrogenic Na⁺-dicarboxylate cotransporter that reabsorbs citrate and succinate across the apical membrane of renal proximal tubule cells with a stoichiometry of ≥3 Na⁺ per substrate, and imports citrate into hepatocytes to fuel acetyl-CoA-dependent metabolic and epigenetic programs [PMID:8596711, PMID:27927654, PMID:39824985]. Key determinants of cation and substrate recognition reside in transmembrane helices 8–9 (Asp-373, Glu-475) and the cytoplasmic end of TM3 (Lys-84), while PKC activation inhibits transport through partial endocytosis and direct activity suppression [PMID:10360950, PMID:10970779, PMID:10446305]. Acid stimulation of NaDC-1 activity operates through two parallel pathways—an endothelin B receptor pathway and a Pyk2/c-Src → Raf1-ERK1/2-p90RSK kinase cascade—and is reciprocally modulated by physical interaction with SLC26A6 via the NaDC-1 first intracellular loop [PMID:20703215, PMID:29678998, PMID:23833257]. In the liver, SLC13A2-imported citrate drives ACLY-dependent acetyl-CoA production that supports SREBP2-mediated cholesterol biosynthesis during regeneration and, in hepatocellular carcinoma, promotes PKM2 acetylation/degradation and histone acetylation to suppress tumor growth [PMID:39824985, PMID:41950310].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Establishing the basic transport identity of SLC13A2: heterologous expression demonstrated that NaDC-1 is an electrogenic Na⁺-coupled cotransporter for di- and tricarboxylates with ~3 Na⁺ per succinate and pH-dependent citrate selectivity, defining its role as the apical renal citrate/succinate reabsorption pathway.\",\n      \"evidence\": \"Transient expression in COS-7 cells with radiolabeled substrate uptake and kinetic analysis\",\n      \"pmids\": [\"8596711\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Three-dimensional structure undetermined\", \"In vivo physiological confirmation not yet performed\", \"Coupling stoichiometry for citrate not precisely defined\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identification of the molecular determinants of cation and substrate recognition: systematic mutagenesis revealed that Asp-373 and Glu-475 in TM8–TM9 form part of the Na⁺/substrate binding site, Lys-84 at the cytoplasmic end of TM3 contributes to substrate affinity, and His-106 is required for membrane stability rather than catalysis, mapping functional residues onto the NaDC-1 topology.\",\n      \"evidence\": \"Site-directed mutagenesis of histidines, cysteines, and charged residues expressed in Xenopus oocytes with transport kinetics, electrophysiology, and surface biotinylation\",\n      \"pmids\": [\"9512488\", \"10360950\", \"10548552\", \"10970779\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure to confirm spatial arrangement of identified residues\", \"Na⁺ binding order and cooperativity mechanism unresolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Discovery of acute post-translational regulation: PKC activation suppresses NaDC-1 transport by ~95% through a dual mechanism—partial endocytosis (~30% surface protein reduction) plus direct inhibition of remaining surface transporters—independent of PKC consensus sites on NaDC-1 itself.\",\n      \"evidence\": \"PMA/DOG treatment of NaDC-1-expressing Xenopus oocytes with transport assay, surface protein quantification, cytochalasin D, and staurosporine blockade\",\n      \"pmids\": [\"10446305\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct PKC phosphorylation target mediating the ~65% non-endocytic inhibition is unknown\", \"Physiological trigger for PKC-mediated regulation in vivo not established\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Acid regulation of NaDC-1 was shown to operate post-transcriptionally: media acidification stimulated citrate transport without changing NaDC-1 mRNA, complementing in vivo evidence that chronic metabolic acidosis increases apical NaDC-1 protein abundance in rat proximal tubule, revealing both transcriptional and post-transcriptional regulatory tiers.\",\n      \"evidence\": \"In vivo rat acidosis model with immunohistochemistry and mRNA quantification; OKP cell acidification with [14C]-citrate uptake and GFP-NaDC-1 trafficking\",\n      \"pmids\": [\"10886565\", \"14973148\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Post-transcriptional mechanism (trafficking vs. translational vs. stability) not molecularly defined\", \"Whether acid regulation involves the same pathway in human kidney is untested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The endothelin B receptor was identified as a required upstream mediator of acid-stimulated NaDC-1 activity: ETB knockout mice lost the acid-induced increase in brush border NaDC-1 activity, and the ETB C-terminal tail and transmembrane domain were required for ET-1-mediated stimulation.\",\n      \"evidence\": \"ETB knockout mouse, chimeric ETB receptor constructs in OKP cells, [14C]-citrate uptake, BQ788 antagonist\",\n      \"pmids\": [\"20703215\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ETB directly signals to NaDC-1 or acts through intermediate effectors was unknown at this stage\", \"Relationship between ETB pathway and PKC inhibitory pathway not clarified\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"NaDC-1 was shown to be reciprocally regulated by direct physical interaction with the oxalate transporter SLC26A6: the STAS domain of SLC26A6 binds the first intracellular loop of NaDC-1, SLC26A6 inhibits NaDC-1 activity in an activity-dependent manner, and NaDC-1 enhances SLC26A6 transport, linking citrate and oxalate homeostasis.\",\n      \"evidence\": \"Co-expression in Xenopus oocytes, domain mapping, Slc26a6-null mouse model\",\n      \"pmids\": [\"23833257\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the STAS–intracellular loop interaction is unknown\", \"In vivo consequences for kidney stone formation not directly tested\", \"Whether other SLC26 family members similarly regulate NaDC-1 is unexplored\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Human tissue localization confirmed that NaDC-1 protein is restricted to the apical membrane along the entire proximal tubule and is absent from other nephron segments and renal tumors, solidifying its role as the sole apical dicarboxylate transporter in human kidney.\",\n      \"evidence\": \"Immunohistochemistry and immunoblot with NBCe1 colocalization in human kidney tissue sections\",\n      \"pmids\": [\"27927654\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single study with one antibody; independent antibody validation not reported\", \"Functional transport data from human proximal tubule cells not included\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"The intracellular signaling cascade for acid-stimulated NaDC-1 was delineated: Pyk2 and c-Src kinases activate Raf1-ERK1/2-p90RSK to stimulate NaDC-1 activity, and this cascade operates in parallel to (not downstream of) the ETB receptor pathway, as shown by Pyk2-null mice failing to upregulate NaDC-1 upon acid loading.\",\n      \"evidence\": \"Kinase inhibitors, Pyk2 knockdown/knockout mice, [14C]-citrate uptake in OKP cells, phospho-immunoblots for ERK1/2, Raf1, p90RSK\",\n      \"pmids\": [\"29678998\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How p90RSK mechanistically increases NaDC-1 surface expression or activity is unknown\", \"Integration point between ETB and Pyk2 pathways remains undefined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A hepatic function for SLC13A2 was established: citrate imported by SLC13A2 in hepatocytes fuels ACLY-dependent acetyl-CoA production, SREBP2 cleavage, and de novo cholesterol biosynthesis, which is required for liver regeneration after partial hepatectomy.\",\n      \"evidence\": \"Liver-specific SLC13A2 overexpression/depletion in mice, partial hepatectomy, lovastatin and ACLY inhibitor epistasis, metabolic flux analysis\",\n      \"pmids\": [\"39824985\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SLC13A2 is the dominant citrate importer in hepatocytes or acts redundantly with other carriers is untested\", \"Transcriptional or post-translational regulation of hepatic SLC13A2 during regeneration not characterized\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"SLC13A2 was identified as a metabolic tumor suppressor in hepatocellular carcinoma: citrate-derived acetyl-CoA acetylates PKM2, triggering its degradation and suppressing glycolysis, amino acid, and nucleotide metabolism; simultaneously, increased histone acetylation provides an epigenetic basis for transcriptional suppression of tumor programs.\",\n      \"evidence\": \"AAV liver-specific knockout/overexpression in HCC mouse models, metabolic flux analysis, PKM2 acetylation assay, histone acetylation profiling, human tissue validation\",\n      \"pmids\": [\"41950310\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific acetyltransferase(s) mediating PKM2 acetylation downstream of SLC13A2 not identified\", \"Histone marks and target gene programs driven by SLC13A2-dependent acetylation not fully mapped\", \"Whether SLC13A2 loss is a driver event in human HCC or a consequence of tumor metabolism is correlative\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the three-dimensional structure of NaDC-1, the direct phosphorylation target mediating PKC-dependent transport inhibition, the integration point between the ETB and Pyk2/c-Src signaling arms, and the full spectrum of acetylation targets downstream of SLC13A2-imported citrate in liver.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure available\", \"Direct kinase substrate linking p90RSK to NaDC-1 activation unknown\", \"Relative contribution of SLC13A2 vs. other citrate carriers in hepatocytes not quantified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 7, 12, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 4, 6, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"SLC26A6\", \"ACLY\", \"PKM2\", \"Pyk2\", \"EDNRB\"],\n    \"other_free_text\": []\n  }\n}\n```\n\nWait, I need to fix the pathway field - it should use Reactome IDs, not GO terms. Let me correct:\n\n```json\n{\n  \"mechanistic_narrative\": \"SLC13A2 (NaDC-1) is an electrogenic Na⁺-dicarboxylate cotransporter that reabsorbs citrate and succinate across the apical membrane of renal proximal tubule cells with a stoichiometry of ≥3 Na⁺ per substrate, and imports citrate into hepatocytes to fuel acetyl-CoA-dependent metabolic and epigenetic programs [PMID:8596711, PMID:27927654, PMID:39824985]. Key determinants of cation and substrate recognition reside in transmembrane helices 8–9 (Asp-373, Glu-475) and the cytoplasmic end of TM3 (Lys-84), while PKC activation inhibits transport through partial endocytosis and direct activity suppression [PMID:10360950, PMID:10970779, PMID:10446305]. Acid stimulation of NaDC-1 activity operates through two parallel pathways—an endothelin B receptor pathway and a Pyk2/c-Src → Raf1-ERK1/2-p90RSK kinase cascade—and is reciprocally modulated by physical interaction with SLC26A6 via the NaDC-1 first intracellular loop [PMID:20703215, PMID:29678998, PMID:23833257]. In the liver, SLC13A2-imported citrate drives ACLY-dependent acetyl-CoA production that supports SREBP2-mediated cholesterol biosynthesis during regeneration and, in hepatocellular carcinoma, promotes PKM2 acetylation/degradation and histone acetylation to suppress tumor growth [PMID:39824985, PMID:41950310].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Establishing the basic transport identity of SLC13A2: heterologous expression demonstrated that NaDC-1 is an electrogenic Na⁺-coupled cotransporter for di- and tricarboxylates with ~3 Na⁺ per succinate and pH-dependent citrate selectivity, defining its role as the apical renal citrate/succinate reabsorption pathway.\",\n      \"evidence\": \"Transient expression in COS-7 cells with radiolabeled substrate uptake and kinetic analysis\",\n      \"pmids\": [\"8596711\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Three-dimensional structure undetermined\", \"In vivo physiological confirmation not yet performed\", \"Coupling stoichiometry for citrate not precisely defined\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identification of the molecular determinants of cation and substrate recognition: systematic mutagenesis revealed that Asp-373 and Glu-475 in TM8–TM9 form part of the Na⁺/substrate binding site, Lys-84 at the cytoplasmic end of TM3 contributes to substrate affinity, and His-106 is required for membrane stability rather than catalysis, mapping functional residues onto the NaDC-1 topology.\",\n      \"evidence\": \"Site-directed mutagenesis of histidines, cysteines, and charged residues expressed in Xenopus oocytes with transport kinetics, electrophysiology, and surface biotinylation\",\n      \"pmids\": [\"9512488\", \"10360950\", \"10548552\", \"10970779\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure to confirm spatial arrangement of identified residues\", \"Na⁺ binding order and cooperativity mechanism unresolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Discovery of acute post-translational regulation: PKC activation suppresses NaDC-1 transport by ~95% through a dual mechanism—partial endocytosis (~30% surface protein reduction) plus direct inhibition of remaining surface transporters—independent of PKC consensus sites on NaDC-1 itself.\",\n      \"evidence\": \"PMA/DOG treatment of NaDC-1-expressing Xenopus oocytes with transport assay, surface protein quantification, cytochalasin D, and staurosporine blockade\",\n      \"pmids\": [\"10446305\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct PKC phosphorylation target mediating the ~65% non-endocytic inhibition is unknown\", \"Physiological trigger for PKC-mediated regulation in vivo not established\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Acid regulation of NaDC-1 was shown to operate post-transcriptionally: media acidification stimulated citrate transport without changing NaDC-1 mRNA, complementing in vivo evidence that chronic metabolic acidosis increases apical NaDC-1 protein abundance in rat proximal tubule, revealing both transcriptional and post-transcriptional regulatory tiers.\",\n      \"evidence\": \"In vivo rat acidosis model with immunohistochemistry and mRNA quantification; OKP cell acidification with [14C]-citrate uptake and GFP-NaDC-1 trafficking\",\n      \"pmids\": [\"10886565\", \"14973148\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Post-transcriptional mechanism (trafficking vs. translational vs. stability) not molecularly defined\", \"Whether acid regulation involves the same pathway in human kidney is untested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The endothelin B receptor was identified as a required upstream mediator of acid-stimulated NaDC-1 activity: ETB knockout mice lost the acid-induced increase in brush border NaDC-1 activity, and the ETB C-terminal tail and transmembrane domain were required for ET-1-mediated stimulation.\",\n      \"evidence\": \"ETB knockout mouse, chimeric ETB receptor constructs in OKP cells, [14C]-citrate uptake, BQ788 antagonist\",\n      \"pmids\": [\"20703215\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ETB directly signals to NaDC-1 or acts through intermediate effectors was unknown at this stage\", \"Relationship between ETB pathway and PKC inhibitory pathway not clarified\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"NaDC-1 was shown to be reciprocally regulated by direct physical interaction with the oxalate transporter SLC26A6: the STAS domain of SLC26A6 binds the first intracellular loop of NaDC-1, SLC26A6 inhibits NaDC-1 activity in an activity-dependent manner, and NaDC-1 enhances SLC26A6 transport, linking citrate and oxalate homeostasis.\",\n      \"evidence\": \"Co-expression in Xenopus oocytes, domain mapping, Slc26a6-null mouse model\",\n      \"pmids\": [\"23833257\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the STAS–intracellular loop interaction is unknown\", \"In vivo consequences for kidney stone formation not directly tested\", \"Whether other SLC26 family members similarly regulate NaDC-1 is unexplored\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Human tissue localization confirmed that NaDC-1 protein is restricted to the apical membrane along the entire proximal tubule and is absent from other nephron segments and renal tumors, solidifying its role as the sole apical dicarboxylate transporter in human kidney.\",\n      \"evidence\": \"Immunohistochemistry and immunoblot with NBCe1 colocalization in human kidney tissue sections\",\n      \"pmids\": [\"27927654\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single study with one antibody; independent antibody validation not reported\", \"Functional transport data from human proximal tubule cells not included\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"The intracellular signaling cascade for acid-stimulated NaDC-1 was delineated: Pyk2 and c-Src kinases activate Raf1-ERK1/2-p90RSK to stimulate NaDC-1 activity, and this cascade operates in parallel to (not downstream of) the ETB receptor pathway, as shown by Pyk2-null mice failing to upregulate NaDC-1 upon acid loading.\",\n      \"evidence\": \"Kinase inhibitors, Pyk2 knockdown/knockout mice, [14C]-citrate uptake in OKP cells, phospho-immunoblots for ERK1/2, Raf1, p90RSK\",\n      \"pmids\": [\"29678998\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How p90RSK mechanistically increases NaDC-1 surface expression or activity is unknown\", \"Integration point between ETB and Pyk2 pathways remains undefined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A hepatic function for SLC13A2 was established: citrate imported by SLC13A2 in hepatocytes fuels ACLY-dependent acetyl-CoA production, SREBP2 cleavage, and de novo cholesterol biosynthesis, which is required for liver regeneration after partial hepatectomy.\",\n      \"evidence\": \"Liver-specific SLC13A2 overexpression/depletion in mice, partial hepatectomy, lovastatin and ACLY inhibitor epistasis, metabolic flux analysis\",\n      \"pmids\": [\"39824985\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SLC13A2 is the dominant citrate importer in hepatocytes or acts redundantly with other carriers is untested\", \"Transcriptional or post-translational regulation of hepatic SLC13A2 during regeneration not characterized\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"SLC13A2 was identified as a metabolic tumor suppressor in hepatocellular carcinoma: citrate-derived acetyl-CoA acetylates PKM2, triggering its degradation and suppressing glycolysis, amino acid, and nucleotide metabolism; simultaneously, increased histone acetylation provides an epigenetic basis for transcriptional suppression of tumor programs.\",\n      \"evidence\": \"AAV liver-specific knockout/overexpression in HCC mouse models, metabolic flux analysis, PKM2 acetylation assay, histone acetylation profiling, human tissue validation\",\n      \"pmids\": [\"41950310\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific acetyltransferase(s) mediating PKM2 acetylation downstream of SLC13A2 not identified\", \"Histone marks and target gene programs driven by SLC13A2-dependent acetylation not fully mapped\", \"Whether SLC13A2 loss is a driver event in human HCC or a consequence of tumor metabolism is correlative\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the three-dimensional structure of NaDC-1, the direct phosphorylation target mediating PKC-dependent transport inhibition, the integration point between the ETB and Pyk2/c-Src signaling arms, and the full spectrum of acetylation targets downstream of SLC13A2-imported citrate in liver.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure available\", \"Direct kinase substrate linking p90RSK to NaDC-1 activation unknown\", \"Relative contribution of SLC13A2 vs. other citrate carriers in hepatocytes not quantified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 5, 7, 12, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 4, 6, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 7, 12, 13]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [12, 13]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [8, 11]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"SLC26A6\", \"ACLY\", \"PKM2\", \"PTK2B\", \"EDNRB\"],\n    \"other_free_text\": []\n  }\n}\n```"}