SLC22A2

SLC22A2 (OCT2) is a polyspecific, electrogenic organic cation transporter that mediates the basolateral uptake step of vectorial renal secretion of cationic drugs and endogenous compounds (PMID:10051314, PMID:20053795). Working coordinately with the apical efflux transporter MATE1 (SLC47A1), OCT2 takes up substrates across the basolateral membrane of proximal tubule cells for subsequent apical extrusion, such that selective MATE1 inhibition causes intracellular accumulation of shared substrates like metformin and TEA (PMID:20053795). Its broad substrate range includes the nephrotoxin cisplatin—where OCT2-mediated accumulation is the major determinant of cisplatin-induced renal tubular toxicity (PMID:16242669)—as well as ranitidine, choline, the survivin suppressant YM155, and the model cation MPP+ (PMID:16141367, PMID:29794161, PMID:19833842). Substrate and inhibitor recognition occurs within a hydrophilic transmembrane cleft in which the conserved transmembrane-10 glutamate E447 governs selectivity: substitutions at E447 shift, reduce, or abolish transport in a substrate-dependent manner (PMID:16087669), and inhibitor binding follows a two-point pharmacophore of an ion-pair and a hydrophobic aromatic site (PMID:19002438). OCT2 displays allosteric, multi-site transport kinetics that depend on plasma membrane cholesterol acting through CARC/CRAC cholesterol-recognition motifs in transmembrane domain 5; cholesterol depletion or mutation of conserved CARC/CRAC residues collapses allosteric kinetics to single-site behavior (PMID:31624079, PMID:34774844). Kidney-restricted expression is enforced epigenetically: CpG methylation of the proximal promoter blocks USF1 binding at an E-box to silence OCT2 in non-renal tissue, while histone modifications and DNA methylation also modulate allelic and disease-associated expression (PMID:18508876, PMID:21523786, PMID:23228442). In rodents, Slc22a2 is subject to imprinted, Air non-coding-RNA-dependent silencing of the paternal allele (PMID:12853484).

1. 1997 Medium

   Establishing the genomic location of human OCT2 anchored it within the SLC22 cation-transporter locus and distinguished it from its paralog OCT1.

   Evidence FISH chromosomal mapping of SLC22A2 to 6q26 alongside SLC22A1

   PMID:9605850

   Open questions at the time

   • No functional or transport data
   • Does not address tissue expression or substrate range
2. 1999 Medium

   Cloning the mouse ortholog defined OCT2 as a 12-TM polyspecific transporter with kidney-restricted expression, separating its physiological niche from liver-expressed OCT1.

   Evidence Cloning, expression profiling, and chromosomal mapping of mouse Slc22a2

   PMID:10051314

   Open questions at the time

   • Substrate spectrum not enumerated
   • Transport mechanism and topology only predicted
3. 2001 Medium

   Genetic complementation excluded Slc22a2 as the gene underlying the t(w73) implantation phenotype, refining the developmental candidate region.

   Evidence YAC transgene rescue and complementation in mice

   PMID:11641723

   Open questions at the time

   • Negative result; does not assign OCT2 a developmental role
   • Does not address renal function
4. 2003 High

   Defining the regulatory logic of rodent Slc22a2 silencing showed its paternal allele is repressed by the Air non-coding RNA independently of Igf2r promoter overlap.

   Evidence Igf2r promoter replacement/deletion mouse models with imprinted-expression analysis

   PMID:12853484

   Open questions at the time

   • Mechanism of Air cis-silencing not resolved
   • Relevance to human OCT2 regulation unclear
5. 2005 High

   Linking OCT2 to cisplatin handling established the transporter as the primary determinant of cisplatin-induced nephrotoxicity, explaining tissue-specific drug toxicity.

   Evidence Stable HEK293 transfection, cytotoxicity/accumulation assays, inhibitor studies, and sex/castration pharmacokinetics in rats

   PMID:16242669

   Open questions at the time

   • Human in vivo confirmation not addressed
   • Structural basis of cisplatin recognition not defined
6. 2005 High

   Site-directed mutagenesis identified E447 in TM10 as a key selectivity determinant, providing the first residue-level model of the substrate-binding cleft.

   Evidence E447 mutant panel with transport assays and homology modeling in rabbit OCT2

   PMID:16087669

   Open questions at the time

   • No experimental structure
   • Roles of other cleft residues inferred from modeling
7. 2005 High

   Comparative substrate testing of H2-antagonists demonstrated OCT2 transports ranitidine and distinguished its selectivity from OCT1.

   Evidence Xenopus oocyte expression with uptake, IC50, trans-stimulation, and electrophysiology

   PMID:16141367

   Open questions at the time

   • Does not generalize to other drug classes
   • No in vivo correlate
8. 2008 Medium

   A two-point pharmacophore relating polar surface area to inhibitory potency provided predictive structure-activity rules for OCT2 inhibitors.

   Evidence IC50 measurements in OCT2-HEK293 cells with pharmacophore and homology modeling

   PMID:19002438

   Open questions at the time

   • No mutagenesis validation of predicted sites
   • Computational model not experimentally confirmed
9. 2008 High

   Identifying promoter CpG methylation blocking USF1 binding explained the kidney-restricted expression of OCT2.

   Evidence Bisulfite sequencing of kidney vs liver, methylated-promoter reporter assays, and EMSA

   PMID:18508876

   Open questions at the time

   • In vivo demethylation effect not tested
   • Other tissue-specific factors not excluded
10. 2008 Medium

    Demonstrating OCT2 down-regulation in ischemic AKI linked transporter loss to impaired renal clearance of cationic drugs, a clinically relevant consequence.

    Evidence I/R rat AKI model with pharmacokinetics, renal-slice transport, and Western blot

    PMID:18180268

    Open questions at the time

    • Mechanism of down-regulation not defined
    • Human relevance not established
11. 2010 High

    Reconstituting the OCT2–MATE1 relay established the vectorial basolateral-uptake/apical-efflux model of renal cation secretion.

    Evidence Vaccinia expression with single/double-transfected cells and reciprocal inhibition of shared substrates

    PMID:20053795

    Open questions at the time

    • Physical interaction between transporters not addressed
    • In vivo coupling not directly quantified
12. 2010 Medium

    Kinetic characterization of YM155 transport identified OCT2 as the route of this drug into proximal tubule cells, extending its pharmacological substrate range.

    Evidence Comparative uptake/kinetic assays across OCT isoforms in HEK293 cells

    PMID:19833842

    Open questions at the time

    • In vivo renal accumulation not tested
    • Toxicity consequence not assessed
13. 2011 Medium

    Linking placental OCT2 expression to histone marks rather than DNA methylation revealed an additional epigenetic layer governing interindividual and allelic variation.

    Evidence ChIP for H3Ac/H3K9me3 with bisulfite sequencing and qRT-PCR in human placenta

    PMID:21523786

    Open questions at the time

    • Correlative for histone marks, not causal
    • Mechanism of biallelic vs monoallelic switch unresolved
14. 2012 Medium

    Epigenetic suppression of Slc22a2 in renal tumors connected OCT2 silencing to reduced metformin uptake, implicating it in tumor drug responsiveness.

    Evidence 5-aza-dC and trichostatin A reactivation in Tsc1-associated renal tumor cells

    PMID:23228442

    Open questions at the time

    • Specific silencing marks at the locus not mapped
    • In vivo tumor consequence not shown
15. 2013 Medium

    Characterizing drug–transporter interactions defined rilpivirine as an OCT2 inhibitor while showing fluoroquinolones are not, refining the inhibitor/substrate landscape.

    Evidence Xenopus oocyte and cell-based transport inhibition assays

    PMID:23545524 PMID:24002095

    Open questions at the time

    • Clinical relevance limited by sub-threshold plasma levels
    • Negative fluoroquinolone result is single-lab
16. 2018 Medium

    Distinct kinetic signatures for choline versus fluorocholine revealed multiple substrate-binding modes, evidence that OCT2 possesses more than one functional binding site.

    Evidence Influx kinetics and modeling for choline and fluorocholine in OCT2-HEK293 cells

    PMID:29794161

    Open questions at the time

    • Structural identity of the two sites unknown
    • Physiological significance not established
17. 2019 High

    Demonstrating cholesterol-dependent allostery explained the multi-site transport kinetics as a lipid-regulated property of the transporter.

    Evidence Cholesterol depletion/restoration with MPP+ transport assays, cholesterol quantification, and protein-stability analysis

    PMID:31624079

    Open questions at the time

    • Molecular site of cholesterol action not localized in this study
    • In vivo relevance of cholesterol regulation untested
18. 2021 High

    Mapping CARC/CRAC motifs in TM5 localized cholesterol-dependent allostery to specific residues, mechanistically uniting lipid binding with multi-site kinetics.

    Evidence Blind docking, alanine-scanning mutagenesis, MPP+ kinetics, and thermal stability assays

    PMID:34774844

    Open questions at the time

    • No experimental structure of cholesterol-bound OCT2
    • Link between allostery and physiological substrate handling not quantified
19. 2017 Medium

    Functional promoter polymorphisms shown to alter transcriptional output provided a genetic basis for interindividual variation in OCT2 expression.

    Evidence Luciferase reporter assays of rs138765638 and rs59695691 with TF-binding prediction

    PMID:28253084

    Open questions at the time

    • Predicted TF binding not directly confirmed
    • In vivo expression effect not demonstrated

• An experimental high-resolution structure of OCT2 capturing the substrate cleft, the E447 docking site, and bound cholesterol at the TM5 CARC/CRAC motifs is still lacking, leaving the structural basis of polyspecificity and allostery unresolved.
• No experimental 3D structure
• Coupling of transport mechanism to ion/membrane potential not structurally defined
• Physical basis of OCT2–MATE1 functional coupling unknown