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Like OATs, OCTs belong to the SLC22A gene family. OCTs are responsible for the transport of both endogenous and exogenous cations, which are positively charged at physiological pH. Due to the heterogeneous nature of substrates transported by OCTs, they are classified as polyspecific and have been shown to transport cations

electrogenically using a Na+ independent mechanism (Ciarimboli, 2008). Three human isoforms of OCT exist, OCT1, OCT2 and OCT3, ranging from 550 to 560 amino acids in length. These proteins share similar membrane topology, consisting of 12

26 transmembrane domains, intracellular C- and N-termini, a large glycosylated

extracellular loop between transmembrane domains 1 and 2 and a large intracellular loop with phosphorylation sites between transmembrane domains 6 and 7 (Choi et al., 2008). Despite structural similarities and overlapping substrate specificities, tissue expression of the different OCT isoforms is relatively distinct (Ciarimboli, 2008).

OCT1, encoded by the SLC22A1 gene, is a protein of 553 amino acids in length that is largely expressed in hepatocytes and intestinal enterocytes but has also been detected in placenta, on the apical membrane of ciliated epithelial cells in the lung, and most recently on the apical membrane of brain microvascular endothelial cells (Ciarimboli, 2008; Giacomini et al., 2010). OCT1 has been shown to transport substrates such as tetraethylammonium, N-methylpyridinium, oxaliplatin and metformin, and is inhibited by compounds such as quinine, quinidine and verapamil (Giacomini et al., 2010). Most significantly with regard to epilepsy, OCT1 has recently been shown to mediate the high affinity transport of LTG in an in vitro BBB model and in a stably transfected OCT1 cell line (Dickens et al., 2012).

OCT2, encoded by the SLC22A2 gene, is two amino acids longer than OCT1, has broader tissue expression than its orthologue and has been shown to be expressed in proximal tubule cells of the kidney and has also been detected in the spleen, placenta, small intestine and brain (Ciarimboli, 2008; Giacomini et al., 2010). Substrates of OCT2 include dopamine, N-methylpyridinium, tetraethylammonium, metformin, pindolol, procainamide, ranitidine, amantadine, amiloride, oxaliplatin, varenicline, cisplatin and lamivudine. Transport of these substrates by OCT2 can be inhibited by cimetidine, quinidine, testosterone and pilsicainide (Giacomini et al., 2010).

OCT3, encoded by theSLC22A3 gene,is a 556 amino acid protein that has a wide tissue distribution, including the brain, placenta, heart, skeletal muscle, blood vessels and liver (Choi et al., 2008). Although OCT3 is the most widespread OCT protein, it has been shown to transport a smaller number of substrates than OCT1 and OCT2. These include atropine, dopamine, epinephrine and tetraethylammonium (Klaassen et al., 2010). OCT3 mediated transport of these substances is inhibited by several steroids, and most potently by β-estradiol (Wu et al., 1998).

27 OCTN1 and OCTN2, encoded by the genes SLC22A4 and SLC22A5 respectively, comprise a subfamily of organic carnitine/cation transporters with approximately 30% homology to classical OCT proteins. The OCTNs are so called because of their

predominant endogenous function, which is to transport carnitine for use in fatty acid oxidation (Koepsell et al., 2007). OCTN1 and OCTN2 are 551 and 557 amino acids in length respectively and, like many other members of the SLC superfamily, are predicted to have 12 transmembrane domains. OCTN1 and OCTN2 are ubiquitously expressed, with strongest expression in the kidney (Dresser et al., 2000). OCTNs have a similar but less extensive substrate specificity to OCT1 and OCT2, but exhibit a much lower

affinity for these substrates (Koepsell et al., 2007).

In addition to the OCTs discussed above, there are two additional cation-specific transporters within the SLC transporter superfamily known as the multidrug and toxic compound extrusion (MATE) proteins. These proteins, MATE1 and MATE2, are encoded by the SLC47A1 and SLC47A2 genes respectively. The resulting proteins, which are 570 and 602 amino acids in length respectively, share around 50% sequence homology and consist of 13 transmembrane helices (Zhang et al.; Komatsu et al., 2011). MATE transporters are unusual in comparison to other SLC family members by the fact that they function physiologically as efflux rather than influx transporters. However, depending on cellular pH, MATEs can also function as influx transporters, with a more acidic pH increasing MATE mediated extrusion of substrate drugs and a more alkaline pH promoting MATE mediated uptake (Müller et al., 2013). As their name suggests MATE transporters are involved in the elimination of toxins and xenobiotics from the body, thus they are primarily found in the excretory organs: liver and kidney where they work in concert with OCTs to provide an elimination pathway for organic cations (Ciarimboli, 2008). Nevertheless, expression has also been observed in the CNS (Koepsell et al., 2007). MATEs are known to mediate the transport of over 40

clinically relevant cationic drugs such as metformin, cisplatin and captopril in a cation- proton antiport system, with the extrusion of cations enhanced by an opposing (influx) gradient of protons (Staud et al., 2013).

28 1.7.4Monocarboxylic acid transporters

The MCT family currently contains 14 identified member genes, 7 of which encode functional transporters and 7 of which encode orphan proteins with no known transport role (table 1.6). All MCTs are predicted to have 12 conserved transmembrane domains,

with intracellular C- and N-termini, and a large cytosolic loop between transmembrane segments 6 and 7 (Halestrap et al., 2004; Halestrap, 2013). MCT1 to MCT4, which are encoded by SLC16A1, SLC16A7, SLC16A8 and SLC16A4 respectively,have been well characterised experimentally and shown to transport important monocarboxylic fuels such as lactate, pyruvate and ketone bodies in a proton linked mechanism (Halestrap et al., 1990). More recently, MCT8 (encoded by SLC16A2) and MCT10 (encoded by SLC16A10) have been shown to mediate the high affinity transport of thyroid hormones and aromatic amino acids, respectively. Interestingly transport by these isoforms is thought to be facilitative rather than proton or sodium linked as with other family members (Friesema et al., 2003; Heuer et al., 2009).