{"gene":"SLC22A5","run_date":"2026-06-10T07:46:32","timeline":{"discoveries":[{"year":1998,"finding":"OCTN2 (SLC22A5) was cloned from human placental trophoblast cells and encodes a 557 amino acid protein with twelve putative transmembrane domains; when heterologously expressed in HeLa cells, it mediates pH-dependent transport of the organic cation tetraethylammonium (TEA), and multiple organic cations (MPP+, MPTP, methamphetamine) compete for OCTN2-mediated transport.","method":"cDNA cloning, heterologous expression in HeLa cells, transport competition assays","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct transport assay in heterologous expression system, foundational cloning paper with functional validation","pmids":["9618255"],"is_preprint":false},{"year":1999,"finding":"OCTN2 functions as a dual-mode transporter: it transports organic cations (e.g., TEA) in a Na+-independent manner and transports L-carnitine and short-chain acylcarnitines in a Na+-dependent manner. Na+ increases affinity for carnitine severalfold without affecting affinity for organic cations. This bifunctionality is conserved in human, rat, and mouse OCTN2.","method":"Heterologous expression in cell lines (HeLa/HEK), radiolabeled substrate uptake assays, Na+ substitution experiments, kinetic analysis","journal":"The Journal of pharmacology and experimental therapeutics","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro transport assays with multiple orthogonal conditions (Na+ dependence, kinetics, substrate competition), replicated across species","pmids":["10454528"],"is_preprint":false},{"year":1999,"finding":"Loss-of-function mutations in OCTN2 (R282X, Y401X, 458X frameshift) abolish carnitine transport in fibroblasts from primary carnitine deficiency patients; transfection of wild-type OCTN2 cDNA partially restores transport, establishing OCTN2 as the causative gene for primary carnitine deficiency (CDSP).","method":"Fibroblast transport assay, OCTN2 cDNA rescue transfection, gene sequencing","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — functional rescue experiment with direct transport measurement, replicated in multiple patients","pmids":["10051646"],"is_preprint":false},{"year":1999,"finding":"Two missense mutations in OCTN2 (M352R/L352R in mouse and P478L in human) associated with primary carnitine deficiency result in loss of carnitine transport function despite normal protein expression levels, indicating that these residues are critical for transport activity rather than protein stability.","method":"Site-directed mutagenesis, heterologous expression, radiolabeled carnitine uptake, Western blot","journal":"The Journal of pharmacology and experimental therapeutics","confidence":"High","confidence_rationale":"Tier 1 / Strong — mutagenesis combined with functional transport assays and protein expression analysis","pmids":["10454528"],"is_preprint":false},{"year":1998,"finding":"The juvenile visceral steatosis (JVS) mouse model of systemic carnitine deficiency carries a missense mutation (L352R) in the sixth transmembrane domain of mouse Octn2, establishing the JVS mouse as an Octn2 loss-of-function model.","method":"DNA sequencing, comparative genomic mapping","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutation identification by sequencing plus positional candidate gene approach; functional confirmation provided by other papers","pmids":["9837751"],"is_preprint":false},{"year":1999,"finding":"The P478L mutation of OCTN2 selectively abolishes carnitine transport while significantly stimulating organic cation (TEA) transport, whereas the L352R mutation abolishes both functions. Studies with human/rat OCTN2 chimeras indicate that the carnitine and organic cation binding sites are not identical. Mutating Y211F also differentially affects the two transport activities.","method":"Site-directed mutagenesis, chimeric transporter construction, heterologous expression, radiolabeled substrate uptake assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — systematic mutagenesis with chimeric constructs and dual-substrate functional assays, multiple orthogonal methods in one study","pmids":["10559218"],"is_preprint":false},{"year":2001,"finding":"OCTN2-mediated Na+-coupled L-carnitine transport is electrogenic with 1:1 Na+:carnitine stoichiometry; transport is driven by an inwardly directed Na+ gradient and is sensitive to membrane potential. OCTN2 protein is localized to the apical membrane of renal tubular epithelial cells, consistent with a role in concentrative carnitine reabsorption.","method":"Plasma membrane vesicle preparation from stably transfected HEK293 cells, radiolabeled carnitine uptake with ion substitution, valinomycin membrane potential manipulation, immunohistochemistry","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted vesicle transport assays with stoichiometry measurement plus direct subcellular localization, multiple orthogonal methods","pmids":["11406104"],"is_preprint":false},{"year":2001,"finding":"OCTN2 mediates TEA transport in jvs mouse embryonic fibroblasts; jvs mice show decreased tissue distribution and renal secretory clearance of TEA. Carnitine and TEA show mutual trans-stimulation in transport, suggesting a carnitine/TEA exchange mechanism. Na+ affects carnitine affinity but not TEA transport.","method":"Pharmacokinetic analysis of [14C]TEA in jvs vs. wild-type mice, fibroblast transport assays, trans-stimulation experiments","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo pharmacokinetics in genetic knockout model plus in vitro mechanistic studies, replicated across multiple assay types","pmids":["11160873"],"is_preprint":false},{"year":2001,"finding":"OCTN2 is expressed in rat and human brain capillary endothelial cells (BCECs) and mediates Na+-dependent, saturable transport of L-carnitine and acetyl-L-carnitine across the blood-brain barrier. Brain distributions of carnitine and acetyl-L-carnitine are reduced in jvs mice with defective OCTN2.","method":"In vivo rat brain perfusion, primary BCEC cultures, RT-PCR, [3H]carnitine uptake assays, jvs mouse in vivo studies","journal":"Journal of neurochemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — combined in vivo genetic model (jvs mouse) and in vitro mechanistic studies with multiple species and orthogonal methods","pmids":["11739607"],"is_preprint":false},{"year":2000,"finding":"Beta-lactam antibiotics with a quaternary nitrogen (cephaloridine, cefoselis, cefepime, cefluprenam) competitively inhibit OCTN2-mediated carnitine transport and are themselves transported by OCTN2. Na+-dependent OCTN2-mediated uptake of cephaloridine was directly demonstrated. Beta-lactams lacking quaternary nitrogen do not interact with OCTN2.","method":"Heterologous expression of human and rat OCTN2 in cell lines, radiolabeled carnitine transport inhibition assays, direct [14C]cephaloridine uptake measurement, competitive inhibition kinetics","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct substrate uptake assay plus competitive inhibition kinetics in heterologous expression system with structural specificity analysis","pmids":["10636865"],"is_preprint":false},{"year":2002,"finding":"The S467C mutation in OCTN2 (Ser in TM domain 11) increases the Km for carnitine ~15-fold without affecting organic cation (TEA) transport. Mutual inhibition kinetics between carnitine and TEA are not completely competitive, suggesting closely situated but non-identical binding sites. Valproate (organic anion) inhibits both carnitine and TEA transport in a Na+-dependent manner, implicating an anion recognition site in TM domain 11 that is altered by S467C.","method":"Site-directed mutagenesis, heterologous expression, kinetic analysis of carnitine/TEA transport, mutual inhibition studies, Na+ activation kinetics","journal":"The Journal of pharmacology and experimental therapeutics","confidence":"High","confidence_rationale":"Tier 1 / Strong — mutagenesis with detailed kinetic analysis and multiple substrate competition experiments","pmids":["12183691"],"is_preprint":false},{"year":2003,"finding":"Tyrosine residues Y447 and Y449 in OCTN2 are required for Na+-stimulated carnitine transport. Y449D increases the Na+ concentration required for half-maximal carnitine transport stimulation; Y447C abolishes carnitine transport and organic cation transport and prevents plasma membrane localization (assessed by GFP-tagged OCTN2 confocal microscopy). Y447F impairs Na+ coupling without affecting membrane localization, while Y449F is functionally normal.","method":"Site-directed mutagenesis, heterologous expression in CHO cells, kinetic analysis of Na+ activation, GFP-fusion protein confocal microscopy for subcellular localization","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — systematic mutagenesis combined with Na+ kinetics and direct subcellular localization using GFP fusion","pmids":["14665638"],"is_preprint":false},{"year":2004,"finding":"PDZK1 directly interacts with the last four amino acids of the C-terminus of OCTN2 (but not basolateral OCT1/OCT2). This interaction stimulates OCTN2-mediated carnitine transport ~6-fold by increasing transport capacity (Vmax), without substantially altering cell-surface expression. PDZK1 and OCTN2 co-localize in brush-border membranes of kidney tubular cells.","method":"Pull-down assay with recombinant C-terminal proteins, yeast two-hybrid, kidney BBM vesicle pull-down, double transfection transport assays, C-terminal deletion mutagenesis, surface biotinylation, immunohistochemistry","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal binding assays (pull-down, Y2H, native tissue), functional transport assay, mutagenesis, and localization in one study","pmids":["15523054"],"is_preprint":false},{"year":2004,"finding":"OCTN2 is localized to the apical membrane of syncytiotrophoblasts in human placenta (confirmed by co-staining with apical marker MRP2 and basal marker OATP-B, and by higher carnitine uptake in apical vs. basal membrane vesicles). OCTN2-mediated carnitine uptake in apical placental vesicles is Na+- and pH-dependent with Km ~21 µM. OCTN2 expression is modulated by trophoblast differentiation.","method":"Confocal immunofluorescence microscopy with marker co-staining, apical/basal membrane vesicle transport assays, real-time PCR, trophoblast differentiation culture","journal":"Drug metabolism and disposition: the biological fate of chemicals","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct subcellular localization with multiple orthogonal markers plus functional transport assays in native tissue vesicles","pmids":["15486076"],"is_preprint":false},{"year":2006,"finding":"OCTN2 is expressed mainly in endothelial cells of the human heart and transports cardiovascular drugs verapamil, spironolactone, and mildronate (as both inhibitors and substrates). OCTN2 expression correlates significantly with ABCB1 (P-glycoprotein) expression in heart; double-transfection studies show functional coupling between OCTN2 and ABCB1 in transcellular transport of verapamil. Beta-blocker administration significantly increases cardiac OCTN2 expression.","method":"Quantitative PCR, in situ hybridization, laser microdissection, immunofluorescence microscopy, MDCKII heterologous expression transport assays, double transfection transcellular transport assay","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (localization, functional transport, transcellular assay), single lab","pmids":["16490820"],"is_preprint":false},{"year":2006,"finding":"OCTN2 is predominantly responsible for apical carnitine uptake in mouse small intestinal epithelial cells, as demonstrated by near-absence of saturable apical carnitine transport in jvs (Octn2-deficient) mice. OCTN2 co-localizes with PDZK1 adaptor protein in microvilli of absorptive enterocytes by immunoprecipitation and immunoelectron microscopy.","method":"Ussing-chamber transport assay comparing wild-type vs. jvs mice, isolated enterocyte uptake assays, immunohistochemistry, immunoprecipitation, immunoelectron microscopy","journal":"Molecular pharmacology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic loss-of-function model with direct transport measurement, confirmed by multiple localization methods","pmids":["16754783"],"is_preprint":false},{"year":2006,"finding":"OCTN2 C-terminal deletion mutants lacking the last four amino acids fail to respond to PDZK1 stimulation, confirming that the C-terminal PDZ-binding motif is required for PDZK1 interaction and functional stimulation.","method":"C-terminal deletion mutagenesis, double transfection, transport assays","journal":"Molecular pharmacology","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — mutagenesis with functional transport assay, single lab, single method confirming prior PDZK1 finding","pmids":["15523054"],"is_preprint":false},{"year":2006,"finding":"The Phe17Leu polymorphism of OCTN2 reduces Vmax for L-carnitine transport to ~50% of reference and causes diffuse cytoplasmic retention rather than plasma membrane localization, revealing that defective membrane targeting is a mechanism of reduced transport function.","method":"Heterologous expression in HEK293 cells, radiolabeled carnitine and TEA transport assays, OCTN2-GFP fusion protein confocal microscopy","journal":"Molecular pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional transport assay and direct GFP localization, two orthogonal methods, single lab","pmids":["16931768"],"is_preprint":false},{"year":2007,"finding":"OCTN2 mediates uptake of the Bacillus subtilis quorum-sensing pentapeptide CSF across the apical membrane of intestinal epithelial cells. CSF-induced activation of p38 MAPK, Akt, and cytoprotective heat shock proteins in intestinal cells depends on OCTN2-mediated CSF transport.","method":"Cell-based CSF uptake assays, pharmacological inhibition of OCTN2, signaling pathway analysis (p38, Akt activation), HSP induction assays","journal":"Cell host & microbe","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional link between OCTN2-mediated transport and downstream signaling, single lab, multiple assays","pmids":["18005709"],"is_preprint":false},{"year":2007,"finding":"PPARα activation (by fasting or WY-14643 treatment) increases hepatic OCTN2 mRNA expression and hepatic carnitine accumulation in a PPARα-dependent manner; this effect is absent in PPARα-/- mice, establishing OCTN2 as a PPARα-regulated gene in vivo.","method":"PPARα agonist treatment of wild-type and PPARα knockout mice, OCTN2 mRNA quantification, carnitine/acylcarnitine measurements","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout model with agonist treatment, replicated across fasting and pharmacological conditions","pmids":["17692817"],"is_preprint":false},{"year":2007,"finding":"OCTN2 is localized to the basolateral membrane and perinuclear cytoplasmic region of blood-brain barrier endothelial cells (not exclusively apical), suggesting OCTN2 can also mediate carnitine transport from the brain side.","method":"Real-time PCR, Western blot, immunocytochemistry with confocal Z-axis analysis using P-glycoprotein as apical membrane marker","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization with orthogonal markers in in vitro BBB model, single lab","pmids":["17995936"],"is_preprint":false},{"year":2008,"finding":"OCTN2 is a highly specific carnitine transporter: among multiple tested drugs (cephaloridine, ergothioneine, gabapentin, pyrilamine, quinidine, spironolactone, TEA, verapamil, vigabatrin), only mildronate (a carnitine structural analog) was efficiently transported. TEA and ergothioneine showed minute transport relative to carnitine. OCTN2 is not a general drug transporter.","method":"Inducible expression of human, rat, and chicken OCTN2 orthologs in HEK293 cells, LC/MS-based uptake quantification, parallel carnitine reference assays","journal":"Drug metabolism and disposition: the biological fate of chemicals","confidence":"High","confidence_rationale":"Tier 1 / Strong — rigorous comparative quantitative transport assays with multiple substrates across three species orthologs, LC/MS detection","pmids":["18981167"],"is_preprint":false},{"year":2008,"finding":"PPARα ligand fenofibrate/Wy14643 upregulates rat Octn2 mRNA in primary hepatocytes and in vivo in wild-type but not PPARα knockout mice, increasing hepatic carnitine uptake. Analysis of rOctn2 promoter identified putative PPARα cis elements.","method":"Primary rat hepatocyte culture with PPARα ligands, PPARα knockout mouse experiments, RT-PCR, [3H]carnitine uptake by isolated hepatocytes","journal":"Biological & pharmaceutical bulletin","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout model with pharmacological intervention and functional transport assay confirmation","pmids":["18520060"],"is_preprint":false},{"year":2008,"finding":"PDZK2 (IKEPP) physically interacts with OCTN2 via the C-terminal four amino acids of OCTN2 and stimulates OCTN2-mediated carnitine transport ~2-fold by increasing cell-surface expression of OCTN2. This effect is lost when the last four amino acids of OCTN2 are deleted. PDZK2 and OCTN2 co-localize in a subapical compartment in mouse kidney.","method":"HEK293 co-expression, radiolabeled carnitine uptake, C-terminal deletion mutagenesis, cell-surface expression analysis, immunohistochemistry","journal":"Drug metabolism and disposition: the biological fate of chemicals","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional transport assay with mutagenesis and localization, single lab","pmids":["16896066"],"is_preprint":false},{"year":2008,"finding":"In vivo studies in jvs mice confirm OCTN2 is responsible for distributing carnitine to the heart; OCTN2 protein is localized to the plasma membrane of cardiac muscle cells by immunoelectron microscopy. Quinidine distribution to the heart is not mediated by OCTN2 despite being an OCTN2 substrate.","method":"Integration plot analysis, heart slice [3H]carnitine uptake in wild-type vs. jvs mice, immunoelectron microscopy","journal":"Drug metabolism and pharmacokinetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic loss-of-function model with in vivo pharmacokinetics and direct ultrastructural localization","pmids":["18574325"],"is_preprint":false},{"year":2008,"finding":"PDZK1 knockout mice show reduced apical membrane expression and protein levels of OCTN2 (Slc22a5) in small intestinal epithelial cells and decreased intestinal absorption of carnitine, confirming PDZK1 as an in vivo regulator of OCTN2 apical targeting and transport function.","method":"pdzk1(-/-) knockout mice, oral carnitine absorption studies, immunohistochemistry, Western blot of brush-border membrane fractions, electron microscopy","journal":"Drug metabolism and disposition: the biological fate of chemicals","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo genetic knockout model with multiple orthogonal localization and functional methods","pmids":["18322073"],"is_preprint":false},{"year":2008,"finding":"Omeprazole inhibits OCTN2-mediated carnitine/carnitine antiport reconstituted in liposomes via two mechanisms: covalent reaction with Cys residue(s) (non-competitive, partially reversible by DTE, IC50=5.7 µM) and non-covalent competitive inhibition (IC50=20.4 µM). Inhibition is only from the external face; omeprazole is not itself transported.","method":"OCTN2 reconstitution in liposomes (proteoliposomes), [3H]-carnitine antiport assay, DTE reversibility test, kinetic analysis, sided inhibition studies","journal":"Chemico-biological interactions","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — reconstituted in vitro system with mechanistic inhibitor characterization, single lab","pmids":["19041296"],"is_preprint":false},{"year":2009,"finding":"Mouse OCTN2 is directly transcriptionally regulated by PPARα via a functional PPRE (PPRE1) located in the first intron. PPARα/RXRα heterodimer binds PPRE1 in gel shift assays; selective mutation of PPRE1 abolishes responsiveness to PPARα activation in reporter gene assays.","method":"In silico PPRE identification, reporter gene assays with intronic constructs, selective PPRE mutagenesis, gel shift (EMSA) assays with PPARα/RXRα","journal":"Biochemical pharmacology","confidence":"High","confidence_rationale":"Tier 1 / Strong — EMSA confirming direct protein-DNA binding combined with mutagenesis-based reporter assays, multiple orthogonal methods","pmids":["19819229"],"is_preprint":false},{"year":2009,"finding":"Substrate discrimination between SLC22A5 (OCTN2/CTT, carnitine) and SLC22A4 (OCTN1/ETT, ergothioneine) involves transmembrane segments 5, 7, 8, 9, 10, and 12. Carnitine is excluded from ETT by binding, whereas ergothioneine is excluded from CTT by turnover movement (conformational change pathway), as demonstrated by gain-of-function mutagenesis.","method":"Multiple alignment-guided site-directed mutagenesis, heterologous expression in HEK293 cells, uptake assays for ergothioneine and carnitine","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 1 / Strong — systematic gain-of-function mutagenesis across multiple TM segments identifying distinct mechanistic bases for substrate exclusion","pmids":["19814996"],"is_preprint":false},{"year":2011,"finding":"OCTN2 transports oxaliplatin; HEK293 cells overexpressing human OCTN2 show increased [14C]oxaliplatin uptake and cytotoxicity that are inhibited by L-carnitine. OCTN2 is functionally expressed in rat dorsal root ganglion (DRG) neurons and contributes to neuronal oxaliplatin accumulation, though OCTN1-mediated transport appears to dominate neurotoxicity.","method":"Radiolabeled oxaliplatin uptake in OCTN2-overexpressing HEK293 cells, cytotoxicity assays, competitive inhibition with carnitine/ergothioneine, RT-PCR and functional transport assays in rat DRG","journal":"The Journal of pharmacology and experimental therapeutics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct uptake measurement in overexpression system with competitive inhibition, two cell model systems, single lab","pmids":["21606177"],"is_preprint":false},{"year":2012,"finding":"SLC22A5/OCTN2 expression in breast cancer cells is induced by estrogen via a novel intronic estrogen-response element (ERE) requiring co-recruitment of both estrogen receptor (ER) and NR4A2/Nurr1. siRNA knockdown of either ER or Nurr1 inhibits estrogen induction of SLC22A5; ChIP assays confirm ER and Nurr1 binding to this intronic enhancer. SLC22A5 knockdown inhibits L-carnitine uptake, causes lipid droplet accumulation, and suppresses breast cancer cell proliferation.","method":"Luciferase reporter assays with ERE mutagenesis, siRNA knockdown, ChIP assays, radiolabeled carnitine uptake, lipid staining, cell proliferation assays","journal":"Breast cancer research and treatment","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (reporter assays, ChIP, siRNA, functional transport, cell biology), single lab","pmids":["22212555"],"is_preprint":false},{"year":2013,"finding":"Caveolin-1 directly interacts with OCTN2 in rat astrocytes; PKC activation increases L-carnitine transport, promotes OCTN2 trafficking to cholesterol/sphingolipid-rich plasma membrane rafts, and increases OCTN2-caveolin-1 co-precipitation. The caveolin-1 binding motifs on OCTN2 map to amino acids 14-22 and 447-454. Direct OCTN2-caveolin-1 interaction (but not flotillin-1) was confirmed by proximity ligation assay upon PKC activation.","method":"Phorbol ester PKC activation, [14C]L-carnitine transport assays, raft fractionation, co-immunoprecipitation, proximity ligation assay, deletion mutagenesis of caveolin-binding motifs","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proximity ligation assay for direct interaction plus functional and biochemical fractionation data, single lab","pmids":["24349196"],"is_preprint":false},{"year":2014,"finding":"The human, porcine, and bovine OCTN2 genes are directly regulated by PPARα via a conserved functional PPRE in intron 1, confirmed by reporter gene assays and gel shift assays showing PPARα/RXRα binding to this PPRE. This regulatory mechanism is conserved across species.","method":"Positional cloning, reporter gene assays with intron 1 constructs, EMSA gel shift assays","journal":"BMC genetics","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct protein-DNA binding (EMSA) combined with mutagenesis-based reporter assays, replicated in three species","pmids":["25299939"],"is_preprint":false},{"year":2016,"finding":"GM-CSF-induced differentiation of human monocytes to macrophages massively upregulates OCTN2-mediated high-affinity Na+-dependent L-carnitine transport (Km ~4 µM) through the mTOR-STAT3 signaling axis; STAT3 phosphorylation downstream of mTOR drives SLC22A5/OCTN2 transcription.","method":"Human monocyte-to-macrophage differentiation, kinetic [14C]carnitine uptake assays, mTOR/STAT3 inhibitor experiments, RT-PCR and Western blot for OCTN2 and SNAT2","journal":"Journal of leukocyte biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional transport kinetics with pharmacological pathway dissection, single lab","pmids":["27733576"],"is_preprint":false},{"year":2018,"finding":"OCTN2 protein in HEK293-derived exosomes is functional: exosomal OCTN2 reconstituted into proteoliposomes mediates Na+-dependent, pH-dependent [3H]-carnitine transport. Functional OCTN2 is also detected in human urinary exosomes. Pro-inflammatory IFNγ treatment increases exosomal OCTN2 levels.","method":"Exosome isolation, Western blot, reconstitution of exosomal proteins into proteoliposomes, [3H]-carnitine uptake assay, IFNγ treatment","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — reconstituted transport assay in proteoliposomes from exosomal protein, single lab","pmids":["29491466"],"is_preprint":false},{"year":2019,"finding":"A 5'-UTR variant (c.-149G>A) of SLC22A5 introduces an upstream out-of-frame translation initiation codon that suppresses translation from the wild-type ATG, resulting in reduced OCTN2 protein levels and lower carnitine transport activity in patient fibroblasts. This is the most frequent cause of primary carnitine deficiency in a Dutch cohort (allele frequency 24.2%).","method":"Sanger sequencing, luciferase reporter constructs in HeLa cells, tandem mass spectrometry-based carnitine transport assay in fibroblasts","journal":"Human mutation","confidence":"High","confidence_rationale":"Tier 1 / Strong — reporter assay demonstrating upstream ORF mechanism plus direct functional transport measurement in patient fibroblasts, two orthogonal methods","pmids":["31187905"],"is_preprint":false},{"year":2020,"finding":"Proinflammatory cytokines (TNF-α, IL-1β, IFNγ) downregulate OCTN2 expression and reduce L-carnitine content in colon cells via PPARγ/RXRα pathways. OCTN2 silencing reduces colon cell proliferation; OCTN2 overexpression increases it. PPARγ agonist luteolin restores OCTN2 expression and alleviates colonic inflammation in an IBD model.","method":"Cytokine treatment of FHC colon cells, PPARγ/RXRα pathway analysis, siRNA knockdown and OCTN2 overexpression, cell proliferation assays, IBD mouse model with luteolin treatment","journal":"Biochemical pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pathway mechanism established by pharmacological and genetic manipulation with in vitro and in vivo confirmation, single lab","pmids":["32579962"],"is_preprint":false},{"year":2022,"finding":"Systematic functional characterization of 150 OCTN2 variants revealed that 70% reduce carnitine transport and 62% of GFP-tagged variants impair plasma membrane localization in HEK293T cells. Impaired subcellular localization significantly associates with reduced transport function, establishing membrane mistargeting as a major loss-of-function mechanism for OCTN2 variants in Carnitine Transporter Deficiency.","method":"[14C]-carnitine uptake assays for 150 variants expressed in HEK293T cells, GFP-tagged variant subcellular localization by fluorescence microscopy, machine learning variant effect prediction","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — large-scale systematic functional assay with orthogonal localization data, multiple methods across large variant set","pmids":["36343260"],"is_preprint":false},{"year":2004,"finding":"OCTN2 is expressed in brain capillary endothelial cells forming the inner blood-retinal barrier (TR-iBRB2 cells and isolated rat retinal vascular endothelial cells) and mediates Na+-dependent, saturable uptake of L-carnitine and acetyl-L-carnitine (Km ~26-29 µM). OCTN2 substrates and inhibitors block carnitine/acetylcarnitine uptake in retinal endothelial cells.","method":"In vivo retinal uptake index analysis, TR-iBRB2 cell culture transport assays, RT-PCR, inhibitor competition","journal":"Investigative ophthalmology & visual science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo pharmacokinetic measurement plus in vitro cell-based transport with RT-PCR confirmation, single lab","pmids":["19684012"],"is_preprint":false}],"current_model":"SLC22A5/OCTN2 is a plasma membrane transporter with twelve transmembrane domains that operates as a dual-mode carrier: it mediates Na+-independent transport of organic cations (e.g., TEA) and high-affinity, electrogenic, Na+-coupled transport of L-carnitine and acylcarnitines (1:1 Na+:carnitine stoichiometry); carnitine and organic cation binding sites are closely situated but distinct, with TM domains 5, 7–12 and key residues (Y447, Y449, E452, S467) contributing to Na+ coupling and substrate recognition; OCTN2 is localized to apical membranes of renal tubules, intestinal enterocytes, and placental syncytiotrophoblasts (driving carnitine reabsorption, intestinal absorption, and maternofetal transfer), and to cardiac myocyte plasma membranes; its transport capacity is upregulated by direct interaction with PDZ-domain scaffolding proteins PDZK1 and PDZK2 (requiring the C-terminal four amino acids), by PKC-mediated trafficking to caveolin-1-containing lipid rafts, and transcriptionally by PPARα via a conserved intronic PPRE and by estrogen receptor/Nurr1 via an intronic ERE; mildronate is an efficient substrate and competitive inhibitor; loss-of-function mutations (including missense, nonsense, frameshift, and 5'-UTR upstream-ORF variants) cause primary carnitine deficiency with cardiomyopathy and metabolic decompensation."},"narrative":{"mechanistic_narrative":"SLC22A5/OCTN2 is a polyspecific plasma membrane solute carrier whose central physiological role is the high-affinity, Na+-coupled cellular uptake of L-carnitine that sustains systemic carnitine homeostasis [PMID:10454528, PMID:11406104]. Cloned from placental trophoblast as a twelve-transmembrane protein, it operates as a dual-mode transporter: Na+-independent flux of organic cations such as TEA and Na+-dependent, electrogenic transport of L-carnitine and short-chain acylcarnitines with 1:1 Na+:carnitine stoichiometry, where Na+ raises carnitine affinity without affecting organic cation handling [PMID:9618255, PMID:10454528, PMID:11406104]. Mutagenesis and chimera studies place the carnitine and organic cation binding sites in close but non-identical positions, with TM segments 5 and 7–12 and residues including S467, Y447 and Y449 governing Na+ coupling and substrate discrimination, and Y447 additionally required for plasma membrane targeting [PMID:10559218, PMID:12183691, PMID:14665638, PMID:19814996]. OCTN2 is highly carnitine-selective rather than a general drug transporter, though it efficiently carries the carnitine analog mildronate and selected quaternary-nitrogen compounds [PMID:18981167]. It localizes to apical membranes of renal tubules, intestinal enterocytes and placental syncytiotrophoblasts and to cardiac and barrier endothelia, driving carnitine reabsorption, absorption, maternofetal transfer and tissue distribution [PMID:11406104, PMID:15486076, PMID:16754783, PMID:18574325]. Transport capacity is tuned post-translationally by direct interaction of its C-terminal PDZ-binding motif with the scaffolds PDZK1 and PDZK2 and by PKC-driven trafficking into caveolin-1 lipid rafts, and transcriptionally by PPARα via a conserved intronic PPRE and by estrogen receptor/Nurr1 via an intronic enhancer [PMID:15523054, PMID:16896066, PMID:24349196, PMID:19819229, PMID:22212555]. Loss-of-function mutations across the gene—including missense alleles, truncations, and a 5'-UTR upstream-ORF variant—cause primary carnitine deficiency, frequently through impaired plasma membrane targeting [PMID:10051646, PMID:31187905, PMID:36343260].","teleology":[{"year":1998,"claim":"Established the molecular identity of OCTN2 and its capacity to transport organic cations, defining the protein as a polyspecific SLC22 carrier.","evidence":"cDNA cloning from placental trophoblast and TEA transport with cation competition in transfected HeLa cells","pmids":["9618255"],"confidence":"High","gaps":["Carnitine transport not yet identified","No physiological substrate or in vivo role established","No structural model of substrate sites"]},{"year":1999,"claim":"Resolved that OCTN2 is a dual-mode carrier coupling Na+-dependent carnitine transport to Na+-independent organic cation transport, defining its physiological substrate.","evidence":"Radiolabeled uptake with Na+ substitution and kinetics across human, rat, mouse orthologs","pmids":["10454528"],"confidence":"High","gaps":["Stoichiometry and electrogenicity not yet measured","Binding site architecture undefined","Tissue localization not established"]},{"year":1999,"claim":"Established OCTN2 as the causative gene for primary carnitine deficiency by linking patient loss-of-function mutations to abolished transport and rescuing it with wild-type cDNA.","evidence":"Fibroblast transport assays, sequencing of patient alleles, and cDNA rescue transfection","pmids":["10051646","9837751"],"confidence":"High","gaps":["Genotype-phenotype spectrum across diverse variant classes not resolved","Mechanism distinguishing folding vs trafficking vs catalytic defects not yet defined"]},{"year":1999,"claim":"Showed that carnitine and organic cation transport can be genetically uncoupled, demonstrating distinct (non-identical) binding determinants for the two substrate classes.","evidence":"Site-directed and chimeric mutagenesis (P478L, L352R, Y211F) with dual-substrate uptake assays","pmids":["10559218","10454528"],"confidence":"High","gaps":["Precise residues coordinating each substrate not fully mapped","No structural validation of separate binding pockets"]},{"year":2001,"claim":"Defined the bioenergetics of carnitine transport and localized OCTN2 to renal apical membranes, explaining concentrative carnitine reabsorption.","evidence":"Membrane vesicle uptake with ion substitution, membrane-potential manipulation, and renal immunohistochemistry; plus jvs mouse TEA pharmacokinetics and trans-stimulation","pmids":["11406104","11160873"],"confidence":"High","gaps":["Conformational cycle of the antiport/symport mechanism not resolved","Regulation of surface expression not yet addressed"]},{"year":2002,"claim":"Mapped a substrate recognition determinant to TM domain 11, refining the model of closely situated but distinct binding sites and revealing an anion-sensitive component.","evidence":"S467C mutagenesis with carnitine/TEA kinetics, mutual inhibition, and valproate Na+-dependent inhibition","pmids":["12183691"],"confidence":"High","gaps":["Atomic basis of TM11 contribution not structurally defined","Physiological relevance of anion recognition unclear"]},{"year":2003,"claim":"Identified tyrosine residues required for Na+ coupling versus membrane targeting, separating ion-coupling defects from trafficking defects.","evidence":"Y447/Y449 mutagenesis with Na+ activation kinetics and GFP-fusion confocal localization in CHO cells","pmids":["14665638"],"confidence":"High","gaps":["Mechanism by which Y447 governs trafficking not defined","No structural model of the Na+ coordination site"]},{"year":2004,"claim":"Demonstrated PDZK1 as a direct C-terminal partner that boosts transport capacity, establishing post-translational scaffold-dependent regulation of OCTN2.","evidence":"Pull-down, yeast two-hybrid, native BBM pull-down, transport assays with C-terminal deletion, and renal co-localization","pmids":["15523054"],"confidence":"High","gaps":["Mechanism of Vmax increase without surface expression change unresolved","In vivo requirement not yet tested"]},{"year":2004,"claim":"Extended OCTN2 carnitine transport to placental and retinal barrier tissues, defining its role in maternofetal and neural carnitine supply.","evidence":"Apical/basal placental vesicle transport with marker co-staining; retinal uptake index and TR-iBRB2 cell transport","pmids":["15486076","19684012"],"confidence":"Medium","gaps":["Quantitative contribution to fetal/retinal carnitine supply in vivo not established","Regulation in these tissues undefined"]},{"year":2006,"claim":"Established OCTN2 roles in intestinal carnitine absorption and cardiac carnitine delivery, and a functional partnership with ABCB1, anchoring its in vivo physiological functions.","evidence":"Ussing-chamber and isolated-cell uptake in jvs mice with immuno-EM; cardiac expression, transcellular double-transfection with ABCB1, immunofluorescence","pmids":["16754783","16490820","15523054"],"confidence":"High","gaps":["Mechanism of OCTN2-ABCB1 functional coupling not detailed","Heart cell-type expression debated (endothelial vs myocyte)"]},{"year":2007,"claim":"Identified OCTN2 as a PPARα-regulated gene in vivo, linking carnitine transport to fatty-acid oxidation transcriptional programs.","evidence":"PPARα agonist and fasting in wild-type vs PPARα-/- mice with OCTN2 mRNA and carnitine measurements","pmids":["17692817","18520060"],"confidence":"High","gaps":["Cis-element location not yet defined (resolved later)","Tissue-specific regulatory differences not addressed"]},{"year":2007,"claim":"Broadened OCTN2 cargo to a bacterial quorum-sensing peptide, linking transport to host intestinal cytoprotective signaling.","evidence":"Cell-based CSF uptake with OCTN2 inhibition and downstream p38/Akt/HSP signaling readouts","pmids":["18005709"],"confidence":"Medium","gaps":["Direct vs indirect peptide transport not fully resolved","In vivo relevance of host-microbe signaling not established"]},{"year":2008,"claim":"Defined OCTN2 as a highly carnitine-selective transporter rather than a general drug carrier, with mildronate as the notable transported analog.","evidence":"LC/MS comparative uptake of multiple drugs across human, rat, chicken orthologs in HEK293","pmids":["18981167"],"confidence":"High","gaps":["Structural basis of carnitine selectivity not resolved","Does not address minor in vivo drug-transport contributions"]},{"year":2008,"claim":"Confirmed PDZK1 as an in vivo regulator of OCTN2 apical targeting and identified PDZK2 as a second scaffold partner, defining a PDZ-protein regulatory module.","evidence":"pdzk1-/- mice with carnitine absorption and BBM expression; PDZK2 co-expression transport with C-terminal deletion and renal co-localization","pmids":["18322073","16896066","15523054"],"confidence":"High","gaps":["Relative contributions of PDZK1 vs PDZK2 in vivo not quantified","Mechanism of differential effect on Vmax vs surface expression unclear"]},{"year":2009,"claim":"Localized the PPARα response to a functional intronic PPRE bound by PPARα/RXRα, establishing direct transcriptional control of OCTN2.","evidence":"Reporter assays with intronic constructs, PPRE mutagenesis, and EMSA in mouse; later extended to human/porcine/bovine","pmids":["19819229","25299939"],"confidence":"High","gaps":["Interplay with other transcription factors not addressed","Chromatin context in native locus not examined"]},{"year":2009,"claim":"Mapped substrate discrimination between OCTN2 (carnitine) and OCTN1 (ergothioneine) to specific TM segments and distinct exclusion mechanisms (binding vs turnover).","evidence":"Alignment-guided gain-of-function mutagenesis across TM 5,7–10,12 with dual-substrate uptake","pmids":["19814996"],"confidence":"High","gaps":["Structural conformations underlying turnover-based exclusion not resolved","No atomic structure"]},{"year":2013,"claim":"Defined PKC-driven trafficking of OCTN2 into caveolin-1 lipid rafts as a dynamic post-translational regulator of carnitine transport.","evidence":"Phorbol ester PKC activation, raft fractionation, co-IP, proximity ligation, caveolin-binding motif mutagenesis in astrocytes","pmids":["24349196"],"confidence":"Medium","gaps":["In vivo significance of raft trafficking unestablished","Single cell-type/lab"]},{"year":2016,"claim":"Linked OCTN2 induction to immune-cell metabolic reprogramming via mTOR-STAT3 signaling during macrophage differentiation.","evidence":"Monocyte-to-macrophage differentiation with carnitine uptake kinetics and mTOR/STAT3 inhibitor dissection","pmids":["27733576"],"confidence":"Medium","gaps":["Direct STAT3 binding to the SLC22A5 promoter not shown","Functional consequence for macrophage biology not defined"]},{"year":2012,"claim":"Identified estrogen receptor/Nurr1 control of SLC22A5 and a pro-proliferative role of carnitine uptake in breast cancer, extending its regulation to hormone signaling.","evidence":"ERE reporter mutagenesis, ChIP, siRNA of ER/Nurr1, carnitine uptake, lipid staining, proliferation assays; plus colon cytokine/PPARγ regulation","pmids":["22212555","32579962"],"confidence":"High","gaps":["Generality across tumor types not established","Mechanistic link between carnitine uptake and proliferation incompletely defined"]},{"year":2019,"claim":"Revealed a 5'-UTR upstream-ORF variant as a prevalent disease mechanism that suppresses translation from the canonical start codon.","evidence":"Sanger sequencing, luciferase reporters, and MS-based carnitine transport in patient fibroblasts","pmids":["31187905"],"confidence":"High","gaps":["Cohort-specific allele frequency may not generalize","Effect on other regulatory variants not addressed"]},{"year":2022,"claim":"Systematically quantified the functional and trafficking consequences of OCTN2 variants, establishing membrane mistargeting as a dominant loss-of-function mechanism in carnitine transporter deficiency.","evidence":"Carnitine uptake and GFP localization for 150 variants in HEK293T with variant-effect prediction","pmids":["36343260"],"confidence":"High","gaps":["Mechanism of mistargeting (folding, chaperone, motif) not resolved per variant","In vivo clinical correlation not directly tested"]},{"year":null,"claim":"An atomic-resolution structure defining the carnitine and organic cation binding pockets, the Na+ coordination geometry, and the conformational cycle underlying transport and substrate exclusion remains to be determined.","evidence":"","pmids":[],"confidence":"High","gaps":["No experimental 3D structure in the corpus","Transport conformational cycle inferred only from mutagenesis","Mechanistic basis of variant-specific trafficking defects unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,1,6,21]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[1,6]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[6,13,15,24]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[34]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[1,6]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[19,30]}],"complexes":[],"partners":["PDZK1","PDZK2","CAV1","ABCB1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O76082","full_name":"Organic cation/carnitine transporter 2","aliases":["High-affinity sodium-dependent carnitine cotransporter","Solute carrier family 22 member 5"],"length_aa":557,"mass_kda":62.8,"function":"Sodium-ion dependent, high affinity carnitine transporter. Involved in the active cellular uptake of carnitine. Transports one sodium ion with one molecule of carnitine (PubMed:10454528, PubMed:10525100, PubMed:10966938, PubMed:17509700, PubMed:20722056, PubMed:33124720). Also transports organic cations such as tetraethylammonium (TEA) without the involvement of sodium. Relative uptake activity ratio of carnitine to TEA is 11.3 (PubMed:10454528, PubMed:10525100, PubMed:10966938). In intestinal epithelia, transports the quorum-sensing pentapeptide CSF (competence and sporulation factor) from B.subtilis which induces cytoprotective heat shock proteins contributing to intestinal homeostasis (PubMed:18005709). May also contribute to regulate the transport of organic compounds in testis across the blood-testis-barrier (Probable) Retained in the ER, unable to perform carnitine uptake","subcellular_location":"Endoplasmic reticulum","url":"https://www.uniprot.org/uniprotkb/O76082/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SLC22A5","classification":"Not Classified","n_dependent_lines":5,"n_total_lines":1208,"dependency_fraction":0.0041390728476821195},"opencell":{"profiled":true,"resolved_as":"SCD","ensg_id":"ENSG00000099194","cell_line_id":"CID000321","localizations":[{"compartment":"vesicles","grade":3},{"compartment":"er","grade":2}],"interactors":[{"gene":"ARHGAP15","stoichiometry":0.2},{"gene":"BCAP31","stoichiometry":0.2},{"gene":"DDOST","stoichiometry":0.2},{"gene":"ESYT1","stoichiometry":0.2},{"gene":"OST4","stoichiometry":0.2},{"gene":"PGRMC1","stoichiometry":0.2},{"gene":"RPN1","stoichiometry":0.2},{"gene":"RPN2","stoichiometry":0.2},{"gene":"RTN4","stoichiometry":0.2},{"gene":"TTC27","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID000321","total_profiled":1310},"omim":[{"mim_id":"618235","title":"MITOCHONDRIAL COMPLEX I DEFICIENCY, NUCLEAR TYPE 13; MC1DN13","url":"https://www.omim.org/entry/618235"},{"mim_id":"607582","title":"SOLUTE CARRIER FAMILY 22 (ORGANIC ANION TRANSPORTER), MEMBER 6; SLC22A6","url":"https://www.omim.org/entry/607582"},{"mim_id":"607562","title":"INTERLEUKIN 23 RECEPTOR; IL23R","url":"https://www.omim.org/entry/607562"},{"mim_id":"606348","title":"INFLAMMATORY BOWEL DISEASE 5; IBD5","url":"https://www.omim.org/entry/606348"},{"mim_id":"603377","title":"SOLUTE CARRIER FAMILY 22 (ORGANIC CATION TRANSPORTER), MEMBER 5; SLC22A5","url":"https://www.omim.org/entry/603377"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Mitochondria","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"skeletal muscle","ntpm":33.7}],"url":"https://www.proteinatlas.org/search/SLC22A5"},"hgnc":{"alias_symbol":["OCTN2","SCD"],"prev_symbol":["CDSP"]},"alphafold":{"accession":"O76082","domains":[{"cath_id":"1.20.1250.20","chopping":"2-42_131-319","consensus_level":"medium","plddt":90.1261,"start":2,"end":319},{"cath_id":"-","chopping":"49-120","consensus_level":"high","plddt":88.6643,"start":49,"end":120},{"cath_id":"1.20.1250.20","chopping":"330-531","consensus_level":"medium","plddt":89.3672,"start":330,"end":531}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O76082","model_url":"https://alphafold.ebi.ac.uk/files/AF-O76082-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O76082-F1-predicted_aligned_error_v6.png","plddt_mean":86.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SLC22A5","jax_strain_url":"https://www.jax.org/strain/search?query=SLC22A5"},"sequence":{"accession":"O76082","fasta_url":"https://rest.uniprot.org/uniprotkb/O76082.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O76082/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O76082"}},"corpus_meta":[{"pmid":"9618255","id":"PMC_9618255","title":"cDNA 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cations (MPP+, MPTP, methamphetamine) compete for OCTN2-mediated transport.\",\n      \"method\": \"cDNA cloning, heterologous expression in HeLa cells, transport competition assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct transport assay in heterologous expression system, foundational cloning paper with functional validation\",\n      \"pmids\": [\"9618255\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"OCTN2 functions as a dual-mode transporter: it transports organic cations (e.g., TEA) in a Na+-independent manner and transports L-carnitine and short-chain acylcarnitines in a Na+-dependent manner. Na+ increases affinity for carnitine severalfold without affecting affinity for organic cations. This bifunctionality is conserved in human, rat, and mouse OCTN2.\",\n      \"method\": \"Heterologous expression in cell lines (HeLa/HEK), radiolabeled substrate uptake assays, Na+ substitution experiments, kinetic analysis\",\n      \"journal\": \"The Journal of pharmacology and experimental therapeutics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro transport assays with multiple orthogonal conditions (Na+ dependence, kinetics, substrate competition), replicated across species\",\n      \"pmids\": [\"10454528\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Loss-of-function mutations in OCTN2 (R282X, Y401X, 458X frameshift) abolish carnitine transport in fibroblasts from primary carnitine deficiency patients; transfection of wild-type OCTN2 cDNA partially restores transport, establishing OCTN2 as the causative gene for primary carnitine deficiency (CDSP).\",\n      \"method\": \"Fibroblast transport assay, OCTN2 cDNA rescue transfection, gene sequencing\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — functional rescue experiment with direct transport measurement, replicated in multiple patients\",\n      \"pmids\": [\"10051646\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Two missense mutations in OCTN2 (M352R/L352R in mouse and P478L in human) associated with primary carnitine deficiency result in loss of carnitine transport function despite normal protein expression levels, indicating that these residues are critical for transport activity rather than protein stability.\",\n      \"method\": \"Site-directed mutagenesis, heterologous expression, radiolabeled carnitine uptake, Western blot\",\n      \"journal\": \"The Journal of pharmacology and experimental therapeutics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mutagenesis combined with functional transport assays and protein expression analysis\",\n      \"pmids\": [\"10454528\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The juvenile visceral steatosis (JVS) mouse model of systemic carnitine deficiency carries a missense mutation (L352R) in the sixth transmembrane domain of mouse Octn2, establishing the JVS mouse as an Octn2 loss-of-function model.\",\n      \"method\": \"DNA sequencing, comparative genomic mapping\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutation identification by sequencing plus positional candidate gene approach; functional confirmation provided by other papers\",\n      \"pmids\": [\"9837751\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The P478L mutation of OCTN2 selectively abolishes carnitine transport while significantly stimulating organic cation (TEA) transport, whereas the L352R mutation abolishes both functions. Studies with human/rat OCTN2 chimeras indicate that the carnitine and organic cation binding sites are not identical. Mutating Y211F also differentially affects the two transport activities.\",\n      \"method\": \"Site-directed mutagenesis, chimeric transporter construction, heterologous expression, radiolabeled substrate uptake assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — systematic mutagenesis with chimeric constructs and dual-substrate functional assays, multiple orthogonal methods in one study\",\n      \"pmids\": [\"10559218\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"OCTN2-mediated Na+-coupled L-carnitine transport is electrogenic with 1:1 Na+:carnitine stoichiometry; transport is driven by an inwardly directed Na+ gradient and is sensitive to membrane potential. OCTN2 protein is localized to the apical membrane of renal tubular epithelial cells, consistent with a role in concentrative carnitine reabsorption.\",\n      \"method\": \"Plasma membrane vesicle preparation from stably transfected HEK293 cells, radiolabeled carnitine uptake with ion substitution, valinomycin membrane potential manipulation, immunohistochemistry\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted vesicle transport assays with stoichiometry measurement plus direct subcellular localization, multiple orthogonal methods\",\n      \"pmids\": [\"11406104\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"OCTN2 mediates TEA transport in jvs mouse embryonic fibroblasts; jvs mice show decreased tissue distribution and renal secretory clearance of TEA. Carnitine and TEA show mutual trans-stimulation in transport, suggesting a carnitine/TEA exchange mechanism. Na+ affects carnitine affinity but not TEA transport.\",\n      \"method\": \"Pharmacokinetic analysis of [14C]TEA in jvs vs. wild-type mice, fibroblast transport assays, trans-stimulation experiments\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo pharmacokinetics in genetic knockout model plus in vitro mechanistic studies, replicated across multiple assay types\",\n      \"pmids\": [\"11160873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"OCTN2 is expressed in rat and human brain capillary endothelial cells (BCECs) and mediates Na+-dependent, saturable transport of L-carnitine and acetyl-L-carnitine across the blood-brain barrier. Brain distributions of carnitine and acetyl-L-carnitine are reduced in jvs mice with defective OCTN2.\",\n      \"method\": \"In vivo rat brain perfusion, primary BCEC cultures, RT-PCR, [3H]carnitine uptake assays, jvs mouse in vivo studies\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — combined in vivo genetic model (jvs mouse) and in vitro mechanistic studies with multiple species and orthogonal methods\",\n      \"pmids\": [\"11739607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Beta-lactam antibiotics with a quaternary nitrogen (cephaloridine, cefoselis, cefepime, cefluprenam) competitively inhibit OCTN2-mediated carnitine transport and are themselves transported by OCTN2. Na+-dependent OCTN2-mediated uptake of cephaloridine was directly demonstrated. Beta-lactams lacking quaternary nitrogen do not interact with OCTN2.\",\n      \"method\": \"Heterologous expression of human and rat OCTN2 in cell lines, radiolabeled carnitine transport inhibition assays, direct [14C]cephaloridine uptake measurement, competitive inhibition kinetics\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct substrate uptake assay plus competitive inhibition kinetics in heterologous expression system with structural specificity analysis\",\n      \"pmids\": [\"10636865\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The S467C mutation in OCTN2 (Ser in TM domain 11) increases the Km for carnitine ~15-fold without affecting organic cation (TEA) transport. Mutual inhibition kinetics between carnitine and TEA are not completely competitive, suggesting closely situated but non-identical binding sites. Valproate (organic anion) inhibits both carnitine and TEA transport in a Na+-dependent manner, implicating an anion recognition site in TM domain 11 that is altered by S467C.\",\n      \"method\": \"Site-directed mutagenesis, heterologous expression, kinetic analysis of carnitine/TEA transport, mutual inhibition studies, Na+ activation kinetics\",\n      \"journal\": \"The Journal of pharmacology and experimental therapeutics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mutagenesis with detailed kinetic analysis and multiple substrate competition experiments\",\n      \"pmids\": [\"12183691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Tyrosine residues Y447 and Y449 in OCTN2 are required for Na+-stimulated carnitine transport. Y449D increases the Na+ concentration required for half-maximal carnitine transport stimulation; Y447C abolishes carnitine transport and organic cation transport and prevents plasma membrane localization (assessed by GFP-tagged OCTN2 confocal microscopy). Y447F impairs Na+ coupling without affecting membrane localization, while Y449F is functionally normal.\",\n      \"method\": \"Site-directed mutagenesis, heterologous expression in CHO cells, kinetic analysis of Na+ activation, GFP-fusion protein confocal microscopy for subcellular localization\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — systematic mutagenesis combined with Na+ kinetics and direct subcellular localization using GFP fusion\",\n      \"pmids\": [\"14665638\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"PDZK1 directly interacts with the last four amino acids of the C-terminus of OCTN2 (but not basolateral OCT1/OCT2). This interaction stimulates OCTN2-mediated carnitine transport ~6-fold by increasing transport capacity (Vmax), without substantially altering cell-surface expression. PDZK1 and OCTN2 co-localize in brush-border membranes of kidney tubular cells.\",\n      \"method\": \"Pull-down assay with recombinant C-terminal proteins, yeast two-hybrid, kidney BBM vesicle pull-down, double transfection transport assays, C-terminal deletion mutagenesis, surface biotinylation, immunohistochemistry\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal binding assays (pull-down, Y2H, native tissue), functional transport assay, mutagenesis, and localization in one study\",\n      \"pmids\": [\"15523054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"OCTN2 is localized to the apical membrane of syncytiotrophoblasts in human placenta (confirmed by co-staining with apical marker MRP2 and basal marker OATP-B, and by higher carnitine uptake in apical vs. basal membrane vesicles). OCTN2-mediated carnitine uptake in apical placental vesicles is Na+- and pH-dependent with Km ~21 µM. OCTN2 expression is modulated by trophoblast differentiation.\",\n      \"method\": \"Confocal immunofluorescence microscopy with marker co-staining, apical/basal membrane vesicle transport assays, real-time PCR, trophoblast differentiation culture\",\n      \"journal\": \"Drug metabolism and disposition: the biological fate of chemicals\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct subcellular localization with multiple orthogonal markers plus functional transport assays in native tissue vesicles\",\n      \"pmids\": [\"15486076\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OCTN2 is expressed mainly in endothelial cells of the human heart and transports cardiovascular drugs verapamil, spironolactone, and mildronate (as both inhibitors and substrates). OCTN2 expression correlates significantly with ABCB1 (P-glycoprotein) expression in heart; double-transfection studies show functional coupling between OCTN2 and ABCB1 in transcellular transport of verapamil. Beta-blocker administration significantly increases cardiac OCTN2 expression.\",\n      \"method\": \"Quantitative PCR, in situ hybridization, laser microdissection, immunofluorescence microscopy, MDCKII heterologous expression transport assays, double transfection transcellular transport assay\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (localization, functional transport, transcellular assay), single lab\",\n      \"pmids\": [\"16490820\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OCTN2 is predominantly responsible for apical carnitine uptake in mouse small intestinal epithelial cells, as demonstrated by near-absence of saturable apical carnitine transport in jvs (Octn2-deficient) mice. OCTN2 co-localizes with PDZK1 adaptor protein in microvilli of absorptive enterocytes by immunoprecipitation and immunoelectron microscopy.\",\n      \"method\": \"Ussing-chamber transport assay comparing wild-type vs. jvs mice, isolated enterocyte uptake assays, immunohistochemistry, immunoprecipitation, immunoelectron microscopy\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic loss-of-function model with direct transport measurement, confirmed by multiple localization methods\",\n      \"pmids\": [\"16754783\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"OCTN2 C-terminal deletion mutants lacking the last four amino acids fail to respond to PDZK1 stimulation, confirming that the C-terminal PDZ-binding motif is required for PDZK1 interaction and functional stimulation.\",\n      \"method\": \"C-terminal deletion mutagenesis, double transfection, transport assays\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — mutagenesis with functional transport assay, single lab, single method confirming prior PDZK1 finding\",\n      \"pmids\": [\"15523054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The Phe17Leu polymorphism of OCTN2 reduces Vmax for L-carnitine transport to ~50% of reference and causes diffuse cytoplasmic retention rather than plasma membrane localization, revealing that defective membrane targeting is a mechanism of reduced transport function.\",\n      \"method\": \"Heterologous expression in HEK293 cells, radiolabeled carnitine and TEA transport assays, OCTN2-GFP fusion protein confocal microscopy\",\n      \"journal\": \"Molecular pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional transport assay and direct GFP localization, two orthogonal methods, single lab\",\n      \"pmids\": [\"16931768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"OCTN2 mediates uptake of the Bacillus subtilis quorum-sensing pentapeptide CSF across the apical membrane of intestinal epithelial cells. CSF-induced activation of p38 MAPK, Akt, and cytoprotective heat shock proteins in intestinal cells depends on OCTN2-mediated CSF transport.\",\n      \"method\": \"Cell-based CSF uptake assays, pharmacological inhibition of OCTN2, signaling pathway analysis (p38, Akt activation), HSP induction assays\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional link between OCTN2-mediated transport and downstream signaling, single lab, multiple assays\",\n      \"pmids\": [\"18005709\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"PPARα activation (by fasting or WY-14643 treatment) increases hepatic OCTN2 mRNA expression and hepatic carnitine accumulation in a PPARα-dependent manner; this effect is absent in PPARα-/- mice, establishing OCTN2 as a PPARα-regulated gene in vivo.\",\n      \"method\": \"PPARα agonist treatment of wild-type and PPARα knockout mice, OCTN2 mRNA quantification, carnitine/acylcarnitine measurements\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout model with agonist treatment, replicated across fasting and pharmacological conditions\",\n      \"pmids\": [\"17692817\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"OCTN2 is localized to the basolateral membrane and perinuclear cytoplasmic region of blood-brain barrier endothelial cells (not exclusively apical), suggesting OCTN2 can also mediate carnitine transport from the brain side.\",\n      \"method\": \"Real-time PCR, Western blot, immunocytochemistry with confocal Z-axis analysis using P-glycoprotein as apical membrane marker\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization with orthogonal markers in in vitro BBB model, single lab\",\n      \"pmids\": [\"17995936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"OCTN2 is a highly specific carnitine transporter: among multiple tested drugs (cephaloridine, ergothioneine, gabapentin, pyrilamine, quinidine, spironolactone, TEA, verapamil, vigabatrin), only mildronate (a carnitine structural analog) was efficiently transported. TEA and ergothioneine showed minute transport relative to carnitine. OCTN2 is not a general drug transporter.\",\n      \"method\": \"Inducible expression of human, rat, and chicken OCTN2 orthologs in HEK293 cells, LC/MS-based uptake quantification, parallel carnitine reference assays\",\n      \"journal\": \"Drug metabolism and disposition: the biological fate of chemicals\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — rigorous comparative quantitative transport assays with multiple substrates across three species orthologs, LC/MS detection\",\n      \"pmids\": [\"18981167\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PPARα ligand fenofibrate/Wy14643 upregulates rat Octn2 mRNA in primary hepatocytes and in vivo in wild-type but not PPARα knockout mice, increasing hepatic carnitine uptake. Analysis of rOctn2 promoter identified putative PPARα cis elements.\",\n      \"method\": \"Primary rat hepatocyte culture with PPARα ligands, PPARα knockout mouse experiments, RT-PCR, [3H]carnitine uptake by isolated hepatocytes\",\n      \"journal\": \"Biological & pharmaceutical bulletin\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout model with pharmacological intervention and functional transport assay confirmation\",\n      \"pmids\": [\"18520060\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PDZK2 (IKEPP) physically interacts with OCTN2 via the C-terminal four amino acids of OCTN2 and stimulates OCTN2-mediated carnitine transport ~2-fold by increasing cell-surface expression of OCTN2. This effect is lost when the last four amino acids of OCTN2 are deleted. PDZK2 and OCTN2 co-localize in a subapical compartment in mouse kidney.\",\n      \"method\": \"HEK293 co-expression, radiolabeled carnitine uptake, C-terminal deletion mutagenesis, cell-surface expression analysis, immunohistochemistry\",\n      \"journal\": \"Drug metabolism and disposition: the biological fate of chemicals\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional transport assay with mutagenesis and localization, single lab\",\n      \"pmids\": [\"16896066\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"In vivo studies in jvs mice confirm OCTN2 is responsible for distributing carnitine to the heart; OCTN2 protein is localized to the plasma membrane of cardiac muscle cells by immunoelectron microscopy. Quinidine distribution to the heart is not mediated by OCTN2 despite being an OCTN2 substrate.\",\n      \"method\": \"Integration plot analysis, heart slice [3H]carnitine uptake in wild-type vs. jvs mice, immunoelectron microscopy\",\n      \"journal\": \"Drug metabolism and pharmacokinetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic loss-of-function model with in vivo pharmacokinetics and direct ultrastructural localization\",\n      \"pmids\": [\"18574325\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"PDZK1 knockout mice show reduced apical membrane expression and protein levels of OCTN2 (Slc22a5) in small intestinal epithelial cells and decreased intestinal absorption of carnitine, confirming PDZK1 as an in vivo regulator of OCTN2 apical targeting and transport function.\",\n      \"method\": \"pdzk1(-/-) knockout mice, oral carnitine absorption studies, immunohistochemistry, Western blot of brush-border membrane fractions, electron microscopy\",\n      \"journal\": \"Drug metabolism and disposition: the biological fate of chemicals\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo genetic knockout model with multiple orthogonal localization and functional methods\",\n      \"pmids\": [\"18322073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Omeprazole inhibits OCTN2-mediated carnitine/carnitine antiport reconstituted in liposomes via two mechanisms: covalent reaction with Cys residue(s) (non-competitive, partially reversible by DTE, IC50=5.7 µM) and non-covalent competitive inhibition (IC50=20.4 µM). Inhibition is only from the external face; omeprazole is not itself transported.\",\n      \"method\": \"OCTN2 reconstitution in liposomes (proteoliposomes), [3H]-carnitine antiport assay, DTE reversibility test, kinetic analysis, sided inhibition studies\",\n      \"journal\": \"Chemico-biological interactions\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted in vitro system with mechanistic inhibitor characterization, single lab\",\n      \"pmids\": [\"19041296\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Mouse OCTN2 is directly transcriptionally regulated by PPARα via a functional PPRE (PPRE1) located in the first intron. PPARα/RXRα heterodimer binds PPRE1 in gel shift assays; selective mutation of PPRE1 abolishes responsiveness to PPARα activation in reporter gene assays.\",\n      \"method\": \"In silico PPRE identification, reporter gene assays with intronic constructs, selective PPRE mutagenesis, gel shift (EMSA) assays with PPARα/RXRα\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — EMSA confirming direct protein-DNA binding combined with mutagenesis-based reporter assays, multiple orthogonal methods\",\n      \"pmids\": [\"19819229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Substrate discrimination between SLC22A5 (OCTN2/CTT, carnitine) and SLC22A4 (OCTN1/ETT, ergothioneine) involves transmembrane segments 5, 7, 8, 9, 10, and 12. Carnitine is excluded from ETT by binding, whereas ergothioneine is excluded from CTT by turnover movement (conformational change pathway), as demonstrated by gain-of-function mutagenesis.\",\n      \"method\": \"Multiple alignment-guided site-directed mutagenesis, heterologous expression in HEK293 cells, uptake assays for ergothioneine and carnitine\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — systematic gain-of-function mutagenesis across multiple TM segments identifying distinct mechanistic bases for substrate exclusion\",\n      \"pmids\": [\"19814996\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"OCTN2 transports oxaliplatin; HEK293 cells overexpressing human OCTN2 show increased [14C]oxaliplatin uptake and cytotoxicity that are inhibited by L-carnitine. OCTN2 is functionally expressed in rat dorsal root ganglion (DRG) neurons and contributes to neuronal oxaliplatin accumulation, though OCTN1-mediated transport appears to dominate neurotoxicity.\",\n      \"method\": \"Radiolabeled oxaliplatin uptake in OCTN2-overexpressing HEK293 cells, cytotoxicity assays, competitive inhibition with carnitine/ergothioneine, RT-PCR and functional transport assays in rat DRG\",\n      \"journal\": \"The Journal of pharmacology and experimental therapeutics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct uptake measurement in overexpression system with competitive inhibition, two cell model systems, single lab\",\n      \"pmids\": [\"21606177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SLC22A5/OCTN2 expression in breast cancer cells is induced by estrogen via a novel intronic estrogen-response element (ERE) requiring co-recruitment of both estrogen receptor (ER) and NR4A2/Nurr1. siRNA knockdown of either ER or Nurr1 inhibits estrogen induction of SLC22A5; ChIP assays confirm ER and Nurr1 binding to this intronic enhancer. SLC22A5 knockdown inhibits L-carnitine uptake, causes lipid droplet accumulation, and suppresses breast cancer cell proliferation.\",\n      \"method\": \"Luciferase reporter assays with ERE mutagenesis, siRNA knockdown, ChIP assays, radiolabeled carnitine uptake, lipid staining, cell proliferation assays\",\n      \"journal\": \"Breast cancer research and treatment\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (reporter assays, ChIP, siRNA, functional transport, cell biology), single lab\",\n      \"pmids\": [\"22212555\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Caveolin-1 directly interacts with OCTN2 in rat astrocytes; PKC activation increases L-carnitine transport, promotes OCTN2 trafficking to cholesterol/sphingolipid-rich plasma membrane rafts, and increases OCTN2-caveolin-1 co-precipitation. The caveolin-1 binding motifs on OCTN2 map to amino acids 14-22 and 447-454. Direct OCTN2-caveolin-1 interaction (but not flotillin-1) was confirmed by proximity ligation assay upon PKC activation.\",\n      \"method\": \"Phorbol ester PKC activation, [14C]L-carnitine transport assays, raft fractionation, co-immunoprecipitation, proximity ligation assay, deletion mutagenesis of caveolin-binding motifs\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proximity ligation assay for direct interaction plus functional and biochemical fractionation data, single lab\",\n      \"pmids\": [\"24349196\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The human, porcine, and bovine OCTN2 genes are directly regulated by PPARα via a conserved functional PPRE in intron 1, confirmed by reporter gene assays and gel shift assays showing PPARα/RXRα binding to this PPRE. This regulatory mechanism is conserved across species.\",\n      \"method\": \"Positional cloning, reporter gene assays with intron 1 constructs, EMSA gel shift assays\",\n      \"journal\": \"BMC genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct protein-DNA binding (EMSA) combined with mutagenesis-based reporter assays, replicated in three species\",\n      \"pmids\": [\"25299939\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"GM-CSF-induced differentiation of human monocytes to macrophages massively upregulates OCTN2-mediated high-affinity Na+-dependent L-carnitine transport (Km ~4 µM) through the mTOR-STAT3 signaling axis; STAT3 phosphorylation downstream of mTOR drives SLC22A5/OCTN2 transcription.\",\n      \"method\": \"Human monocyte-to-macrophage differentiation, kinetic [14C]carnitine uptake assays, mTOR/STAT3 inhibitor experiments, RT-PCR and Western blot for OCTN2 and SNAT2\",\n      \"journal\": \"Journal of leukocyte biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional transport kinetics with pharmacological pathway dissection, single lab\",\n      \"pmids\": [\"27733576\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"OCTN2 protein in HEK293-derived exosomes is functional: exosomal OCTN2 reconstituted into proteoliposomes mediates Na+-dependent, pH-dependent [3H]-carnitine transport. Functional OCTN2 is also detected in human urinary exosomes. Pro-inflammatory IFNγ treatment increases exosomal OCTN2 levels.\",\n      \"method\": \"Exosome isolation, Western blot, reconstitution of exosomal proteins into proteoliposomes, [3H]-carnitine uptake assay, IFNγ treatment\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted transport assay in proteoliposomes from exosomal protein, single lab\",\n      \"pmids\": [\"29491466\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"A 5'-UTR variant (c.-149G>A) of SLC22A5 introduces an upstream out-of-frame translation initiation codon that suppresses translation from the wild-type ATG, resulting in reduced OCTN2 protein levels and lower carnitine transport activity in patient fibroblasts. This is the most frequent cause of primary carnitine deficiency in a Dutch cohort (allele frequency 24.2%).\",\n      \"method\": \"Sanger sequencing, luciferase reporter constructs in HeLa cells, tandem mass spectrometry-based carnitine transport assay in fibroblasts\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reporter assay demonstrating upstream ORF mechanism plus direct functional transport measurement in patient fibroblasts, two orthogonal methods\",\n      \"pmids\": [\"31187905\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Proinflammatory cytokines (TNF-α, IL-1β, IFNγ) downregulate OCTN2 expression and reduce L-carnitine content in colon cells via PPARγ/RXRα pathways. OCTN2 silencing reduces colon cell proliferation; OCTN2 overexpression increases it. PPARγ agonist luteolin restores OCTN2 expression and alleviates colonic inflammation in an IBD model.\",\n      \"method\": \"Cytokine treatment of FHC colon cells, PPARγ/RXRα pathway analysis, siRNA knockdown and OCTN2 overexpression, cell proliferation assays, IBD mouse model with luteolin treatment\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pathway mechanism established by pharmacological and genetic manipulation with in vitro and in vivo confirmation, single lab\",\n      \"pmids\": [\"32579962\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Systematic functional characterization of 150 OCTN2 variants revealed that 70% reduce carnitine transport and 62% of GFP-tagged variants impair plasma membrane localization in HEK293T cells. Impaired subcellular localization significantly associates with reduced transport function, establishing membrane mistargeting as a major loss-of-function mechanism for OCTN2 variants in Carnitine Transporter Deficiency.\",\n      \"method\": \"[14C]-carnitine uptake assays for 150 variants expressed in HEK293T cells, GFP-tagged variant subcellular localization by fluorescence microscopy, machine learning variant effect prediction\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — large-scale systematic functional assay with orthogonal localization data, multiple methods across large variant set\",\n      \"pmids\": [\"36343260\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"OCTN2 is expressed in brain capillary endothelial cells forming the inner blood-retinal barrier (TR-iBRB2 cells and isolated rat retinal vascular endothelial cells) and mediates Na+-dependent, saturable uptake of L-carnitine and acetyl-L-carnitine (Km ~26-29 µM). OCTN2 substrates and inhibitors block carnitine/acetylcarnitine uptake in retinal endothelial cells.\",\n      \"method\": \"In vivo retinal uptake index analysis, TR-iBRB2 cell culture transport assays, RT-PCR, inhibitor competition\",\n      \"journal\": \"Investigative ophthalmology & visual science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo pharmacokinetic measurement plus in vitro cell-based transport with RT-PCR confirmation, single lab\",\n      \"pmids\": [\"19684012\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SLC22A5/OCTN2 is a plasma membrane transporter with twelve transmembrane domains that operates as a dual-mode carrier: it mediates Na+-independent transport of organic cations (e.g., TEA) and high-affinity, electrogenic, Na+-coupled transport of L-carnitine and acylcarnitines (1:1 Na+:carnitine stoichiometry); carnitine and organic cation binding sites are closely situated but distinct, with TM domains 5, 7–12 and key residues (Y447, Y449, E452, S467) contributing to Na+ coupling and substrate recognition; OCTN2 is localized to apical membranes of renal tubules, intestinal enterocytes, and placental syncytiotrophoblasts (driving carnitine reabsorption, intestinal absorption, and maternofetal transfer), and to cardiac myocyte plasma membranes; its transport capacity is upregulated by direct interaction with PDZ-domain scaffolding proteins PDZK1 and PDZK2 (requiring the C-terminal four amino acids), by PKC-mediated trafficking to caveolin-1-containing lipid rafts, and transcriptionally by PPARα via a conserved intronic PPRE and by estrogen receptor/Nurr1 via an intronic ERE; mildronate is an efficient substrate and competitive inhibitor; loss-of-function mutations (including missense, nonsense, frameshift, and 5'-UTR upstream-ORF variants) cause primary carnitine deficiency with cardiomyopathy and metabolic decompensation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SLC22A5/OCTN2 is a polyspecific plasma membrane solute carrier whose central physiological role is the high-affinity, Na+-coupled cellular uptake of L-carnitine that sustains systemic carnitine homeostasis [#1, #6]. Cloned from placental trophoblast as a twelve-transmembrane protein, it operates as a dual-mode transporter: Na+-independent flux of organic cations such as TEA and Na+-dependent, electrogenic transport of L-carnitine and short-chain acylcarnitines with 1:1 Na+:carnitine stoichiometry, where Na+ raises carnitine affinity without affecting organic cation handling [#0, #1, #6]. Mutagenesis and chimera studies place the carnitine and organic cation binding sites in close but non-identical positions, with TM segments 5 and 7–12 and residues including S467, Y447 and Y449 governing Na+ coupling and substrate discrimination, and Y447 additionally required for plasma membrane targeting [#5, #10, #11, #28]. OCTN2 is highly carnitine-selective rather than a general drug transporter, though it efficiently carries the carnitine analog mildronate and selected quaternary-nitrogen compounds [#21]. It localizes to apical membranes of renal tubules, intestinal enterocytes and placental syncytiotrophoblasts and to cardiac and barrier endothelia, driving carnitine reabsorption, absorption, maternofetal transfer and tissue distribution [#6, #13, #15, #24]. Transport capacity is tuned post-translationally by direct interaction of its C-terminal PDZ-binding motif with the scaffolds PDZK1 and PDZK2 and by PKC-driven trafficking into caveolin-1 lipid rafts, and transcriptionally by PPARα via a conserved intronic PPRE and by estrogen receptor/Nurr1 via an intronic enhancer [#12, #23, #31, #27, #30]. Loss-of-function mutations across the gene—including missense alleles, truncations, and a 5'-UTR upstream-ORF variant—cause primary carnitine deficiency, frequently through impaired plasma membrane targeting [#2, #35, #37].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Established the molecular identity of OCTN2 and its capacity to transport organic cations, defining the protein as a polyspecific SLC22 carrier.\",\n      \"evidence\": \"cDNA cloning from placental trophoblast and TEA transport with cation competition in transfected HeLa cells\",\n      \"pmids\": [\"9618255\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Carnitine transport not yet identified\", \"No physiological substrate or in vivo role established\", \"No structural model of substrate sites\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Resolved that OCTN2 is a dual-mode carrier coupling Na+-dependent carnitine transport to Na+-independent organic cation transport, defining its physiological substrate.\",\n      \"evidence\": \"Radiolabeled uptake with Na+ substitution and kinetics across human, rat, mouse orthologs\",\n      \"pmids\": [\"10454528\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and electrogenicity not yet measured\", \"Binding site architecture undefined\", \"Tissue localization not established\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Established OCTN2 as the causative gene for primary carnitine deficiency by linking patient loss-of-function mutations to abolished transport and rescuing it with wild-type cDNA.\",\n      \"evidence\": \"Fibroblast transport assays, sequencing of patient alleles, and cDNA rescue transfection\",\n      \"pmids\": [\"10051646\", \"9837751\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Genotype-phenotype spectrum across diverse variant classes not resolved\", \"Mechanism distinguishing folding vs trafficking vs catalytic defects not yet defined\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Showed that carnitine and organic cation transport can be genetically uncoupled, demonstrating distinct (non-identical) binding determinants for the two substrate classes.\",\n      \"evidence\": \"Site-directed and chimeric mutagenesis (P478L, L352R, Y211F) with dual-substrate uptake assays\",\n      \"pmids\": [\"10559218\", \"10454528\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise residues coordinating each substrate not fully mapped\", \"No structural validation of separate binding pockets\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Defined the bioenergetics of carnitine transport and localized OCTN2 to renal apical membranes, explaining concentrative carnitine reabsorption.\",\n      \"evidence\": \"Membrane vesicle uptake with ion substitution, membrane-potential manipulation, and renal immunohistochemistry; plus jvs mouse TEA pharmacokinetics and trans-stimulation\",\n      \"pmids\": [\"11406104\", \"11160873\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conformational cycle of the antiport/symport mechanism not resolved\", \"Regulation of surface expression not yet addressed\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapped a substrate recognition determinant to TM domain 11, refining the model of closely situated but distinct binding sites and revealing an anion-sensitive component.\",\n      \"evidence\": \"S467C mutagenesis with carnitine/TEA kinetics, mutual inhibition, and valproate Na+-dependent inhibition\",\n      \"pmids\": [\"12183691\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic basis of TM11 contribution not structurally defined\", \"Physiological relevance of anion recognition unclear\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Identified tyrosine residues required for Na+ coupling versus membrane targeting, separating ion-coupling defects from trafficking defects.\",\n      \"evidence\": \"Y447/Y449 mutagenesis with Na+ activation kinetics and GFP-fusion confocal localization in CHO cells\",\n      \"pmids\": [\"14665638\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which Y447 governs trafficking not defined\", \"No structural model of the Na+ coordination site\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstrated PDZK1 as a direct C-terminal partner that boosts transport capacity, establishing post-translational scaffold-dependent regulation of OCTN2.\",\n      \"evidence\": \"Pull-down, yeast two-hybrid, native BBM pull-down, transport assays with C-terminal deletion, and renal co-localization\",\n      \"pmids\": [\"15523054\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of Vmax increase without surface expression change unresolved\", \"In vivo requirement not yet tested\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Extended OCTN2 carnitine transport to placental and retinal barrier tissues, defining its role in maternofetal and neural carnitine supply.\",\n      \"evidence\": \"Apical/basal placental vesicle transport with marker co-staining; retinal uptake index and TR-iBRB2 cell transport\",\n      \"pmids\": [\"15486076\", \"19684012\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Quantitative contribution to fetal/retinal carnitine supply in vivo not established\", \"Regulation in these tissues undefined\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Established OCTN2 roles in intestinal carnitine absorption and cardiac carnitine delivery, and a functional partnership with ABCB1, anchoring its in vivo physiological functions.\",\n      \"evidence\": \"Ussing-chamber and isolated-cell uptake in jvs mice with immuno-EM; cardiac expression, transcellular double-transfection with ABCB1, immunofluorescence\",\n      \"pmids\": [\"16754783\", \"16490820\", \"15523054\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of OCTN2-ABCB1 functional coupling not detailed\", \"Heart cell-type expression debated (endothelial vs myocyte)\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Identified OCTN2 as a PPARα-regulated gene in vivo, linking carnitine transport to fatty-acid oxidation transcriptional programs.\",\n      \"evidence\": \"PPARα agonist and fasting in wild-type vs PPARα-/- mice with OCTN2 mRNA and carnitine measurements\",\n      \"pmids\": [\"17692817\", \"18520060\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cis-element location not yet defined (resolved later)\", \"Tissue-specific regulatory differences not addressed\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Broadened OCTN2 cargo to a bacterial quorum-sensing peptide, linking transport to host intestinal cytoprotective signaling.\",\n      \"evidence\": \"Cell-based CSF uptake with OCTN2 inhibition and downstream p38/Akt/HSP signaling readouts\",\n      \"pmids\": [\"18005709\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct vs indirect peptide transport not fully resolved\", \"In vivo relevance of host-microbe signaling not established\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Defined OCTN2 as a highly carnitine-selective transporter rather than a general drug carrier, with mildronate as the notable transported analog.\",\n      \"evidence\": \"LC/MS comparative uptake of multiple drugs across human, rat, chicken orthologs in HEK293\",\n      \"pmids\": [\"18981167\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of carnitine selectivity not resolved\", \"Does not address minor in vivo drug-transport contributions\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Confirmed PDZK1 as an in vivo regulator of OCTN2 apical targeting and identified PDZK2 as a second scaffold partner, defining a PDZ-protein regulatory module.\",\n      \"evidence\": \"pdzk1-/- mice with carnitine absorption and BBM expression; PDZK2 co-expression transport with C-terminal deletion and renal co-localization\",\n      \"pmids\": [\"18322073\", \"16896066\", \"15523054\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of PDZK1 vs PDZK2 in vivo not quantified\", \"Mechanism of differential effect on Vmax vs surface expression unclear\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Localized the PPARα response to a functional intronic PPRE bound by PPARα/RXRα, establishing direct transcriptional control of OCTN2.\",\n      \"evidence\": \"Reporter assays with intronic constructs, PPRE mutagenesis, and EMSA in mouse; later extended to human/porcine/bovine\",\n      \"pmids\": [\"19819229\", \"25299939\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Interplay with other transcription factors not addressed\", \"Chromatin context in native locus not examined\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Mapped substrate discrimination between OCTN2 (carnitine) and OCTN1 (ergothioneine) to specific TM segments and distinct exclusion mechanisms (binding vs turnover).\",\n      \"evidence\": \"Alignment-guided gain-of-function mutagenesis across TM 5,7–10,12 with dual-substrate uptake\",\n      \"pmids\": [\"19814996\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural conformations underlying turnover-based exclusion not resolved\", \"No atomic structure\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Defined PKC-driven trafficking of OCTN2 into caveolin-1 lipid rafts as a dynamic post-translational regulator of carnitine transport.\",\n      \"evidence\": \"Phorbol ester PKC activation, raft fractionation, co-IP, proximity ligation, caveolin-binding motif mutagenesis in astrocytes\",\n      \"pmids\": [\"24349196\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo significance of raft trafficking unestablished\", \"Single cell-type/lab\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Linked OCTN2 induction to immune-cell metabolic reprogramming via mTOR-STAT3 signaling during macrophage differentiation.\",\n      \"evidence\": \"Monocyte-to-macrophage differentiation with carnitine uptake kinetics and mTOR/STAT3 inhibitor dissection\",\n      \"pmids\": [\"27733576\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct STAT3 binding to the SLC22A5 promoter not shown\", \"Functional consequence for macrophage biology not defined\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identified estrogen receptor/Nurr1 control of SLC22A5 and a pro-proliferative role of carnitine uptake in breast cancer, extending its regulation to hormone signaling.\",\n      \"evidence\": \"ERE reporter mutagenesis, ChIP, siRNA of ER/Nurr1, carnitine uptake, lipid staining, proliferation assays; plus colon cytokine/PPARγ regulation\",\n      \"pmids\": [\"22212555\", \"32579962\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Generality across tumor types not established\", \"Mechanistic link between carnitine uptake and proliferation incompletely defined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Revealed a 5'-UTR upstream-ORF variant as a prevalent disease mechanism that suppresses translation from the canonical start codon.\",\n      \"evidence\": \"Sanger sequencing, luciferase reporters, and MS-based carnitine transport in patient fibroblasts\",\n      \"pmids\": [\"31187905\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cohort-specific allele frequency may not generalize\", \"Effect on other regulatory variants not addressed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Systematically quantified the functional and trafficking consequences of OCTN2 variants, establishing membrane mistargeting as a dominant loss-of-function mechanism in carnitine transporter deficiency.\",\n      \"evidence\": \"Carnitine uptake and GFP localization for 150 variants in HEK293T with variant-effect prediction\",\n      \"pmids\": [\"36343260\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of mistargeting (folding, chaperone, motif) not resolved per variant\", \"In vivo clinical correlation not directly tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"An atomic-resolution structure defining the carnitine and organic cation binding pockets, the Na+ coordination geometry, and the conformational cycle underlying transport and substrate exclusion remains to be determined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental 3D structure in the corpus\", \"Transport conformational cycle inferred only from mutagenesis\", \"Mechanistic basis of variant-specific trafficking defects unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 1, 6, 21]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [1, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [6, 13, 15, 24]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [34]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [19, 30]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"PDZK1\", \"PDZK2\", \"CAV1\", \"ABCB1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}