Expression and localization of organic cation/carnitine transporter OCTN2 in Caco-2 cells

l-Carnitine is derived both from dietary sources and biosynthesis. Dietary carnitine is absorbed in the small intestine and then distributed to other organs. Previous studies using Caco-2 cells demonstrated that the transport of l-carnitine in the intestine involves a carrier-mediated system. The purpose of this study was to determine whether the uptake of l-carnitine in Caco-2 cells is mediated by the recently identified organic cation/carnitine transporter (OCTN2). Kinetics of l-[(3)H]carnitine uptake were investigated with or without specific inhibitors. l-Carnitine uptake in mature cells was sodium dependent and linear with time. K(m) and V(max) values for saturable uptake were 14.07 +/- 1.70 micro M and 26.3 +/- 0.80 pmol. mg protein(-1). 6 min(-1), respectively. l-carnitine uptake was inhibited (P < 0.05-0.01) by valproate and other organic cations. Anti-OCTN2 antibodies recognized a protein in the brush-border membrane (BBM) of Caco-2 cells with an apparent molecular mass of 60 kDa. The OCTN2 expression was confirmed by double immunostaining. Our results demonstrate that l-carnitine uptake in differentiated Caco-2 cells is primarily mediated by OCTN2, located on the BBM.     

Glycosylation of the OCTN2 carnitine transporter: Study of natural mutations identified in patients with primary carnitine deficiency

Primary carnitine deficiency is caused by impaired activity of the Na⁺-dependent OCTN2 carnitine/organic cation transporter. Carnitine is essential for entry of long-chain fatty acids into mitochondria and its deficiency impairs fatty acid oxidation. Most missense mutations identified in patients with primary carnitine deficiency affect putative transmembrane or intracellular domains of the transporter. Exceptions are the substitutions P46S and R83L located in an extracellular loop close to putative glycosylation sites (N57, N64, and N91) of OCTN2. P46S and R83L impaired glycosylation and maturation of OCTN2 transporters to the plasma membrane. We tested whether glycosylation was essential for the maturation of OCTN2 transporters to the plasma membrane. Substitution of each of the three asparagine (N) glycosylation sites with glutamine (Q) decreased carnitine transport. Substitution of two sites at a time caused a further decline in carnitine transport that was fully abolished when all three glycosylation sites were substituted by glutamine (N57Q/N64Q/N91Q). Kinetic analysis of carnitine and sodium-stimulated carnitine transport indicated that all substitutions decreased the Vmax for carnitine transport, but N64Q/N91Q also significantly increased the Km toward carnitine, indicating that these two substitutions affected regions of the transporter important for substrate recognition. Western blot analysis confirmed increased mobility of OCTN2 transporters with progressive substitutions of asparagines 57, 64 and/or 91 with glutamine. Confocal microscopy indicated that glutamine substitutions caused progressive retention of OCTN2 transporters in the cytoplasm, up to full retention (such as that observed with R83L) when all three glycosylation sites were substituted. Tunicamycin prevented OCTN2 glycosylation, but it did not impair maturation to the plasma membrane. These results indicate that OCTN2 is physiologically glycosylated and that the P46S and R83L substitutions impair this process. Glycosylation does not affect maturation of OCTN2 transporters to the plasma membrane, but the 3 asparagines that are normally glycosylated are located in a region important for substrate recognition and turnover rate.     

Epitope shared by functional variant of organic cation/carnitine transporter, OCTN1, Campylobacter jejuni and Mycobacterium paratuberculosis may underlie susceptibility to Crohn's disease at 5q31

Campylobacter jejuni and Mycobacterium paratuberculosis have been implicated in the pathogenesis of Crohn's disease. The presence of bacterial metabolites in the colonic lumen causing a specific breakdown of fatty acid oxidation in colonic epithelial cells has been suggested as an initiating event in inflammatory bowel disease (IBD). l-Carnitine is a small highly polar zwitterion that plays an essential role in fatty acid oxidation and ATP generation in intestinal bioenergetic metabolism. The organic cation/carnitine transporters, OCTN1 and OCTN2, function primarily in the transport of l-carnitine and elimination of cationic drugs in the intestine. High-resolution linkage disequilibrium mapping has identified a region of about 250kb in size at 5q31 (IBD5) encompassing the OCTN1 and -2 genes, to confer susceptibility to Crohn's disease. Recently, two variants in the OCTN1 and OCTN2 genes have been shown to form a haplotype which is associated with susceptibility to Crohn's. We show that OCTN1 and OCTN2 are strongly expressed in target areas for IBD such as ileum and colon. Further, we have now identified a nine amino acid epitope shared by this functional variant of OCTN1 (Leu503Phe) (which decreases the efficiency of carnitine transport), and by C. jejuni (9 aa) and M. paratuberculosis (6 aa). The prevalence of this variant of OCTN1 (Phe503:Leu503) is 3-fold lower in unaffected individuals of Jewish origin (1:3.44) compared to unaffected individuals of non-Jewish origin (1:1). We hypothesize that a specific antibody raised to this epitope during C. jejuni or M. paratuberculosis enterocolitis would cross-react with the intestinal epithelial cell functional variant of OCTN1, an already less efficient carnitine transporter, leading to an impairment of mitochondrial beta-oxidation which may then serve as an initiating event in IBD. This impairment of l-carnitine transport by OCTN1 may respond to high-dose l-carnitine therapy.     

Mutations in novel organic cation transporter (OCTN2), an organic cation/carnitine transporter, with differential effects on the organic cation transport function and the carnitine transport function

Novel organic cation transporter (OCTN2) is an organic cation/carnitine transporter, and two missense mutations, L352R and P478L, in OCTN2 have been identified as the cause for primary carnitine deficiency. In the present study, we assessed the influence of these two mutations on the carnitine transport function and the organic cation transport function of OCTN2. The L352R mutation resulted in a complete loss of both transport functions. In contrast, the P478L mutation resulted in a complete loss of only the carnitine transport function but significantly stimulated the organic cation transport function. Studies with human OCTN2/rat OCTN2 chimeric transporters indicated that the carnitine transport site and the organic cation transport site were not identical. Because carnitine transport is Na(+)-dependent whereas organic cation transport is Na(+)-independent, we investigated the possibility that the P478L mutation affected Na(+) binding. The Na(+) activation kinetics were found to be similar for the P478L mutant and wild type OCTN2. We then mutated nine different tyrosine residues located in or near transmembrane domains and assessed the transport function of these mutants. One of these mutations, Y211F, was found to have differential influence on the two transport activities of OCTN2 as did the P478L mutation. However, the Na(+) activation kinetics were not affected. These findings are of clinical relevance to patients with primary carnitine deficiency because whereas each and every mutation in these patients is expected to result in the loss of the carnitine transport function, all of these mutations may not interfere with the organic cation transport function.     

Functional domains in the carnitine transporter OCTN2, defective in primary carnitine deficiency

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation characterized by hypoketotic hypoglycemia and skeletal and cardiac myopathy. It is caused by mutations in the Na+-dependent organic cation transporter, OCTN2. To define the domains involved in carnitine recognition, we evaluated chimeric transporters created by swapping homologous domains between OCTN1, which does not transport carnitine, and OCTN2. Substitution of the C terminus of OCTN2 (amino acid residues 342-557) with the corresponding residues of OCTN1 completely abolished carnitine transport. The progressive substitution of the N terminus of OCTN2 with OCTN1 resulted in a decrease in carnitine transport associated with a progressive increase in the Km toward carnitine from 3.9 +/- 0.5 to 141 +/- 19 microM. The largest drop in carnitine transport (and increase in Km toward carnitine) was observed with the substitution of residues 341-454 of OCTN2. An additional chimeric transporter (CHIM-9) in which only residues 341-454 of OCTN2 were substituted by OCTN1 had markedly reduced carnitine transport, with an elevated Km toward carnitine (63 +/- 5 microM). Site-directed mutagenesis and introduction of residues nonconserved between OCTN1 and OCTN2 in the OCTN2 cDNA indicated that the R341A, L409W, L424Y, and T429I substitutions significantly decreased carnitine transport. Single substitutions did not increase the Km toward carnitine. By contrast, the combination of three of these substitutions (R341W + L409W + T429I) greatly decreased carnitine transport and increased the Km toward carnitine (20.2 +/- 4.5 microm). The Arg-341, Leu-409, and Thr-429 residues are all located in predicted transmembrane domains. Involvement of these residues in carnitine transport was further supported by the partial restoration of carnitine transport by the introduction of these OCTN2 residues in the OCTN1 portion of CHIM-9. These studies indicate that multiple domains of the OCTN2 transporter are required for carnitine transport and identify transmembrane residues important for carnitine recognition.     

Molecular and functional characterization of organic cation/carnitine transporter family in mice

Carnitine is essential for beta-oxidation of fatty acids, and a defect of cell membrane transport of carnitine leads to fatal systemic carnitine deficiency. We have already shown that a defect of the organic cation/carnitine transporter OCTN2 is a primary cause of systemic carnitine deficiency. In the present study, we further isolated and characterized new members of the OCTN family, OCTN1 and -3, in mice. All three members were expressed commonly in kidney, and OCTN1 and -2 were also expressed in various tissues, whereas OCTN3 was characterized by predominant expression in testis. When their cDNAs were transfected into HEK293 cells, the cells exhibited transport activity for carnitine and/or the organic cation tetraethylammonium (TEA). Carnitine transport by OCTN1 and OCTN2 was Na(+)-dependent, whereas that by OCTN3 was Na(+)-independent. TEA was transported by OCTN1 and OCTN2 but not by OCTN3. The relative uptake activity ratios of carnitine to TEA were 1.78, 11.3, and 746 for OCTN1, -2, and -3, respectively, suggesting high specificity of OCTN3 for carnitine and significantly lower carnitine transport activity of OCTN1. Thus, OCTN3 is unique in its limited tissue distribution and Na(+)-independent carnitine transport, whereas OCTN1 efficiently transported TEA with minimal expression of carnitine transport activity and may have a different role from other members of the OCTN family.     

The glucose transport facilitator GLUT8 is predominantly associated with the acrosomal region of mature spermatozoa

The glucose transporter 8 (GLUT8) is a recently identified member of the family of sugar transport facilitators. In human tissues GLUT8 is predominantly expressed in testis in a gonadotropin-dependent manner. It is shown here that the onset of mRNA synthesis of GLUT8 during the maturation of mouse testis coincides with the appearance of mature spermatozoa. Furthermore, immunohistochemistry with antiserum against the C-terminus of GLUT8 indicated that the protein was associated with spermatozoa within the seminiferous and the epididymal tubules. The GLUT8 immunoreactivity was detected within the head of mouse and human spermatozoa in the acrosomal region, and appeared to be located at the plasma membrane as well as within the cells. This specific expression and localization of GLUT8 suggests that the transport facilitator plays a major role in the fuel supply of mature spermatozoa, and that it is a potential target for inhibition of sperm cell function.     

Characterization of organic cation/carnitine transporter family in human sperm

Spermatozoan maturation, motility, and fertility are, in part, dependent upon the progressive increase in epididymal and spermatozoal carnitine, critical for mitochondrial fatty acid oxidation, as sperm pass from the caput to the cauda of the epididymis. We demonstrate that the organic cation/carnitine transporters, OCTN1, OCTN2, and OCTN3, are expressed in sperm as three distinct proteins with an expected molecular mass of 63 kDa, using Western blot analysis and our transporter-specific antibodies. Carnitine uptake studies in normal control human sperm samples further support the presence of high-affinity (OCTN2) carnitine uptake (K(m) of 3.39+/-1.16 microM; V(max) of 0.23+/-0.14 pmol/min/mg sperm protein; and mean+/-SD; n=12), intermediate-affinity (OCTN3) carnitine uptake (K(m) of 25.9+/-14.7 microM; V(max) of 1.49+/-1.03 pmol/min/mg protein; n=26), and low-affinity (OCTN1) carnitine uptake (K(m) of 412.6+/-191 microM; V(max) of 32.7+/-20.5 pmol/min/mg protein; n=18). Identification of individuals with defective sperm carnitine transport may provide potentially treatable etiologies of male infertility, responsive to L-carnitine supplementation.     

Organic cation/carnitine transporter OCTN3 is present in astrocytes and is up-regulated by peroxisome proliferators-activator receptor agonist

In the brain β-oxidation, which takes place in astrocytes, is not a major process of energy supply. Astrocytes synthesize important lipid metabolites, mainly due to the processes taking place in peroxisomes. One of the compounds necessary in the process of mitochondrial β-oxidation and export of acyl moieties from peroxisomes is l-carnitine. Two Na-dependent plasma membrane carnitine transporters were shown previously to be present in astrocytes: a low affinity amino acid transporter B^0,+ and a high affinity cation/carnitine transporter OCTN2. The expression of OCTN2 is known to increase in peripheral tissues upon the stimulation of peroxisome proliferators-activator receptor α (PPARα), a nuclear receptor known to up-regulate several enzymes involved in fatty acid metabolism. The present study was focused on another high affinity carnitine transporter—OCTN3, its presence, regulation and activity in astrocytes. Experiments using the techniques of real-time PCR, Western blot and immunocytochemistry analysis demonstrated the expression of octn3 in rat astrocytes and, out of two rat sequences ascribed as similar to mouse OCTN3, XM_001073573 was found in these cells. PPARα activator–2-[4-chloro-6-[(2,3-dimethylphenyl)amino]-2-pyrimidinyl]thio]acetic acid (WY-14,643) stimulated by 50% expression of octn3, while, on the contrary to peripheral tissues, it did not change the expression of octn2. This observation was correlated with an increased Na-independent activity of carnitine transport. Analysis by transmission electron microscopy showed an augmented intracellular localization of OCTN3 upon PPARα stimulation, mainly in peroxisomes, indicating a physiological role of OCTN3 as peroxisomal membrane transporter. These observations point to an important role of OCTN3 in peroxisomal fatty acid metabolism in astrocytes.     

Mouse cysteine-rich secretory protein 4 (CRISP4): a member of the Crisp family exclusively expressed in the epididymis in an androgen-dependent manner

The final maturation of spermatozoa produced in the testis takes place during their passage through the epididymis. In this process, the proteins secreted into the epididymal lumen along with changes in the pH and salt composition of the epididymal fluid cause several biochemical changes and remodeling of the sperm plasma membrane. The Crisp family is a group of cysteine-rich secretory proteins that previously consisted of three members, one of which-CRISP1-is an epididymal protein shown to attach to the sperm surface in the epididymal lumen and to inhibit gamete membrane fusion. In the present paper, we introduce a new member of the Crisp protein family, CRISP4. The new gene was discovered through in silico analysis of the epididymal expressed sequence tag library deposited in the UniGene database. The peptide sequence of CRISP4 has a signal sequence suggesting that it is secreted into the epididymal lumen and might thus interact with sperm. Unlike the other members of the family, Crisp4 is located on chromosome 1 in a cluster of genes encoding for cysteine-rich proteins. Crisp4 is expressed in the mouse exclusively in epithelial cells of the epididymis in an androgen-dependent manner, and the expression of the gene starts at puberty along with the onset of sperm maturation. The identified murine CRISP4 peptide has high homology with human CRISP1, and the homology is higher than that between murine and human CRISP1, suggesting that CRISP4 represents the mouse counterpart of human CRISP1 and could have similar effects on sperm membrane as mouse and human CRISP1.     

beta-lactam antibiotics as substrates for OCTN2, an organic cation/carnitine transporter

Therapeutic use of cephaloridine, a beta-lactam antibiotic, in humans is associated with carnitine deficiency. A potential mechanism for the development of carnitine deficiency is competition between cephaloridine and carnitine for the renal reabsorptive process. OCTN2 is an organic cation/carnitine transporter that is responsible for Na(+)-coupled transport of carnitine in the kidney and other tissues. We investigated the interaction of several beta-lactam antibiotics with OCTN2 using human cell lines that express the transporter constitutively as well as using cloned human and rat OCTN2s expressed heterologously in human cell lines. The beta-lactam antibiotics cephaloridine, cefoselis, cefepime, and cefluprenam were found to inhibit OCTN2-mediated carnitine transport. These antibiotics possess a quaternary nitrogen as does carnitine. Several other beta-lactam antibiotics that do not possess this structural feature did not interact with OCTN2. The interaction of cephaloridine with OCTN2 is competitive with respect to carnitine. Interestingly, many of the beta-lactam antibiotics that were not recognized by OCTN2 were good substrates for the H(+)-coupled peptide transporters PEPT1 and PEPT2. In contrast, cephaloridine, cefoselis, cefepime, and cefluprenam, which were recognized by OCTN2, did not interact with PEPT1 and PEPT2. The interaction of cephaloridine with OCTN2 was Na(+)-dependent, whereas the interaction of cefoselis and cefepime with OCTN2 was largely Na(+)-independent. Furthermore, the Na(+)-dependent, OCTN2-mediated cellular uptake of cephaloridine could be demonstrated by direct uptake measurements. These studies show that OCTN2 plays a crucial role in the pharmacokinetics and therapeutic efficacy of certain beta-lactam antibiotics such as cephaloridine and that cephaloridine-induced carnitine deficiency is likely to be due to inhibition of carnitine reabsorption in the kidney.     

Na(+)-coupled transport of L-carnitine via high-affinity carnitine transporter OCTN2 and its subcellular localization in kidney

The mechanism of Na(+)-dependent transport of L-carnitine via the carnitine/organic cation transporter OCTN2 and the subcellular localization of OCTN2 in kidney were studied. Using plasma membrane vesicles prepared from HEK293 cells that were stably transfected with human OCTN2, transport of L-carnitine via human OCTN2 was characterized. Uptake of L-[(3)H]carnitine by the OCTN2-expressing membrane vesicles was significantly increased in the presence of an inwardly directed Na(+) gradient, with an overshoot, while such transient uphill transport was not observed in membrane vesicles from cells that were mock transfected with expression vector pcDNA3 alone. The uptake of L-[(3)H]carnitine was specifically dependent on Na(+) and the osmolarity effect showed that Na(+) significantly influenced the transport rather than the binding. Changes of inorganic anions in the extravesicular medium and of membrane potential by valinomycin altered the initial uptake activity of L-carnitine by OCTN2. In addition, the fluxes of L-carnitine and Na(+) were coupled with 1:1 stoichiometry. Accordingly, it was clarified that Na(+) is coupled with flux of L-carnitine and the flux is an electrogenic process. Furthermore, OCTN2 was localized on the apical membrane of renal tubular epithelial cells. These results clarified that OCTN2 is important for the concentrative reabsorption of L-carnitine after glomerular filtration in the kidney.     

Novel human cDNAs homologous to Drosophila Orct and mammalian carnitine transporters

The molecular basis of the transport of organic ions (which include such medically important compounds as drugs, toxins, and metabolites) has been intensively studied ever since the identification of the prototypical anion and cation transporters, OAT1 (originally cloned by us as NKT) and OCT1. Here we report the cloning of two novel putative organic ion transporters with 12 predicted membrane spanning segments that are most homologous to mammalian OCTNs (carnitine transporters) and to the Drosophila putative transporter, Orct, an intriguing correspondence that led us to name our sequences Fly-like putative transporters (Flipts). Another transporter we cloned has recently been identified as OAT5. Inclusion of Flipts reveals that the organic ion transporter family tree has trifurcated into three branches, one bearing Flipts, OCTNs, and fly transporters, and the other two bearing OATs and OCTs. Flipts are widely expressed in adult kidney, brain, muscle, and other tissues; in contrast, OAT1 is largely in kidney, and OAT5, in liver. In the embryo as well, Flipts are broadly distributed, whereas OAT5 was found only in fetal liver. Flipt expression patterns resemble those of the phylogenetically related OCTNs, suggesting that Flipts might also participate in carnitine transport, particularly in brain, which has relatively high Flipt expression, including EST matches from amygdala, hippocampus, and hypothalamus.     

Expression of novel organic cation/carnitine transporter (OCTN2) in the mouse pancreas

Among the organic cation transporters, OCTN2 is identified as the most important carnitine transporter owing to the ability to transport carnitine. Although the OCTN2 is previously found in various tissues, there have been no reports showing the OCTN2 in the pancreas. In this study, we examined the expression and localization of OCTN2 in the mouse pancreas by the aid of an in situ hybridization technique and immunohistochemistry with anti-OCTN2 antibody. As a result, the OCTN2 expression was found in the A-cells for the first time. OCTN2 was not expressed in B-cells, notwithstanding that the metabolism of long-chain fatty acids, which are transported into the mitochondria with the help of carnitine, was expected for fatty acid-stimulated insulin secretion. Thus, this study suggests the possibility of carnitine uptake in the pancreatic A-cells through OCTN2 and implies the presence of carnitine transporter(s) other than OCTN2 in the B-cell.     

Mechanism of the inhibitory effect of zwitterionic drugs (levofloxacin and grepafloxacin) on carnitine transporter (OCTN2) in Caco-2 cells

l-Carnitine plays an important role in lipid metabolism by facilitating the transport of long-chain fatty acids across the mitochondrial inner membrane followed by fatty acid beta-oxidation. It is known that l-carnitine exists as a zwitterion and that member of the OCTN family play an important role in its transport. The aims of this study were to characterize l-carnitine transport in the intestine by using Caco-2 cells and to elucidate the effects of levofloxacin (LVFX) and grepafloxacin (GPFX), which are zwitterionic drugs, on l-carnitine uptake. Kinetic analysis showed that the half-saturation Na⁺ concentration, Hill coefficient and K_m value of l-carnitine uptake in Caco-2 cells were 10.3 ± 4.5 mM, 1.09 and 8.0 ± 1.0 μM, respectively, suggesting that OCTN2 mainly transports l-carnitine. LVFX and GPFX have two pK_a values and the existence ratio of their zwitterionic forms is higher under a neutral condition than under an acidic condition. Experiments on the inhibitory effect of LVFX and GPFX on l-carnitine uptake showed that LVFX and GPFX inhibited l-carnitine uptake more strongly at pH 7.4 than at pH 5.5. It was concluded that the zwitterionic form of drugs plays an important role in inhibition of OCTN2 function.     

Choline Transporters in Human Lung Adenocarcinoma： Expression and Functional Implications

Choline is an essential nutrient for cell survival and proliferation, however, the expression and function of choline transporters have not been well identified in cancer. In this study, we detected the mRNA and protein expression of organic cation transporter OCT3, carnitine/cation transporters OCTN 1 and OCTN2, and choline transporter-like protein CTL1 in human lung adenocarcinoma cell lines A549, H 1299 and SPC-A-1. Their expression pattern was further confirmed in 25 human primary adenocarcinoma tissues. The choline uptake in these cell lines was significantly blocked by CTL1 inhibitor, but only partially inhibited by OCT or OCTN inhibitors. The efficacy of these inhibitors on cell proliferation is closely correlated with their abilities to block choline transport. Under the native expression of these transporters, the total choline uptake was notably blocked by specific PI3K/AKT inhibitors. These results describe the expression of choline transporters and their relevant function in cell proliferation of human lung adenocarcinoma, thus providing a potential ＂choline-starvation＂ strategy of cancer interference through targeting choline transporters, especially CTL1.     

Tyrosine residues affecting sodium stimulation of carnitine transport in the OCTN2 carnitine/organic cation transporter

Primary carnitine deficiency is a disorder of fatty acid oxidation caused by mutations in the Na+-dependent carnitine/organic cation transporter OCTN2. Studies with tyrosyl group-modifying reagents support the involvement of tyrosine residues in Na+ binding by sodium-coupled transporters. Here we report two new patients with carnitine deficiency caused by mutations affecting tyrosyl residues (Y447C and Y449D) close to a residue (Glu-452) previously shown to affect sodium stimulation of carnitine transport. Kinetic analysis indicated that the Y449D substitution, when expressed in Chinese hamster ovary cells, increased the concentration of sodium required to half-maximally stimulate carnitine transport from 14.8 +/- 1.8 to 34.9 +/- 5.8 mM (p<0.05), whereas Y447C completely abolished carnitine transport. Substitution of these tyrosine residues with phenylalanine restored normal carnitine transport in Y449F but resulted in markedly impaired carnitine transport by Y447F. This was associated with an increase in the concentration of sodium required to half-maximally stimulate carnitine transport to 57.8 +/- 7.4 mM (p<0.01 versus normal OCTN2). The Y447F and Y449D mutant transporters retained their ability to transport the organic cation tetraethylammonium indicating that their effect on carnitine transport was specific and likely associated with the impaired sodium stimulation of carnitine transport. By contrast, the Y447C natural mutation abolished the transport of organic cations in addition to carnitine. Confocal microscopy of OCTN2 transporters tagged with green fluorescent protein indicated that the Y447C mutant transporters failed to reach the plasma membrane, whereas Y447F, Y449D, and Y449F had normal membrane localization. These natural mutations identify tyrosine residues possibly involved in coupling the sodium electrochemical gradient to transmembrane solute transfer in the sodium-dependent co-transporter OCTN2.     

A third human carnitine/organic cation transporter (OCTN3) as a candidate for the 5q31 Crohn's disease locus (IBD5)

Organic cation transporters function primarily in the elimination of cationic drugs in kidney, intestine, and liver. The murine organic cation/carnitine (Octn) transporter family, Octn1, Octn2, and Octn3 is clustered on mouse chromosome 11 (NCBI Accession No. NW_000039). The human OCTN1 and OCTN2 orthologs map to the syntenic IBD5 locus at 5q31, which has been shown to confer susceptibility to Crohn's disease. We show that the human OCTN3 protein, whose corresponding gene is not yet cloned or annotated in the human reference DNA sequence, does indeed exist and is uniquely involved in carnitine-dependent transport in peroxisomes. Its functional properties and inferred chromosomal location implicate it for involvement in Crohn's disease.