Abnormal sodium stimulation of carnitine transport in primary carnitine deficiency

Organic Cation Transporters in Health and Disease

The organic cation transporters (OCTs) OCT1, OCT2, OCT3, novel OCT (OCTN)1, OCTN2, multidrug and toxin exclusion (MATE)1, and MATE kidney-specific 2 are polyspecific transporters exhibiting broadly overlapping substrate selectivities. They transport organic cations, zwitterions, and some uncharged compounds and operate as facilitated diffusion systems and/or antiporters. OCTs are critically involved in intestinal absorption, hepatic uptake, and renal excretion of hydrophilic drugs. They modulate the distribution of endogenous compounds such as thiamine, L-carnitine, and neurotransmitters. Sites of expression and functions of OCTs have important impact on energy metabolism, pharmacokinetics, and toxicity of drugs, and on drug-drug interactions. In this work, an overview about the human OCTs is presented. Functional properties of human OCTs, including identified substrates and inhibitors of the individual transporters, are described. Sites of expression are compiled, and data on regulation of OCTs are presented. In addition, genetic variations of OCTs are listed, and data on their impact on transport, drug treatment, and diseases are reported. Moreover, recent data are summarized that indicate complex drug-drug interaction at OCTs, such as allosteric high-affinity inhibition of transport and substrate dependence of inhibitor efficacies. A hypothesis about the molecular mechanism of polyspecific substrate recognition by OCTs is presented that is based on functional studies and mutagenesis experiments in OCT1 and OCT2. This hypothesis provides a framework to imagine how observed complex drug-drug interactions at OCTs arise. Finally, preclinical in vitro tests that are performed by pharmaceutical companies to identify interaction of novel drugs with OCTs are discussed. Optimized experimental procedures are proposed that allow a gapless detection of inhibitory and transported drugs.

Intestinal OCTN2- and MCT1-targeted drug delivery to improve oral bioavailability

Various drug transporters are widely expressed throughout the intestine and play important roles in absorbing nutrients and drugs, thus providing high quality targets for the design of prodrugs or nanoparticles to facilitate oral drug delivery. In particular, intestinal carnitine/organic cation transporter 2 (OCTN2) and mono-carboxylate transporter protein 1 (MCT1) possess high transport capacities and complementary distributions. Therefore, we outline recent developments in transporter-targeted oral drug delivery with regard to the OCTN2 and MCT1 proteins in this review. First, basic information of the two transporters is reviewed, including their topological structures, characteristics and functions, expression and key features of their substrates. Furthermore, progress in transporter-targeting prodrugs and nanoparticles to increase oral drug delivery is discussed, including improvements in the oral absorption of anti-inflammatory drugs, antiepileptic drugs and anticancer drugs. Finally, the potential of a dual transporter-targeting strategy is discussed.

Identification and characterization of a carnitine transporter in Acinetobacter baumannii

The opportunistic pathogen Acinetobacter baumannii is able to grow on carnitine. The genes encoding the pathway for carnitine degradation to the intermediate malic acid are known but the transporter mediating carnitine uptake remained to be identified. The open reading frame HMPREF0010_01347 (aci01347) of Acinetobacter baumannii is annotated as a gene encoding a potential transporter of the betaine/choline/carnitine transporter (BCCT) family. To study the physiological function of Aci01347, the gene was deleted from A. baumannii ATCC 19606. The mutant was no longer able to grow on carnitine as sole carbon and energy source demonstrating the importance of this transporter for carnitine metabolism. Aci01347 was produced in Escherichia coli MKH13, a strain devoid of any compatible solute transporter, and the recombinant E. coli MKH13 strain was found to take up carnitine in an energy-dependent fashion. Aci01347 also transported choline, a compound known to be accumulated under osmotic stress. Choline transport was osmolarity-independent which is consistent with the absence of an extended C-terminus found in osmo-activated BCCT. We propose that the Aci01347 is the carnitine transporter mediating the first step in the growth of A. baumannii on carnitine.

OCTN: A Small Transporter Subfamily with Great Relevance to Human Pathophysiology, Drug Discovery, and Diagnostics

OCTN is a small subfamily of membrane transport proteins that belongs to the larger SLC22 family. Two of the three members of the subfamily, namely, OCTN2 and OCTN1, are present in humans. OCTN2 plays a crucial role in the absorption of carnitine from diet and in its distribution to tissues, as demonstrated by the occurrence of severe pathologies caused by malfunctioning or altered expression of this transporter. These findings suggest avoiding a strict vegetarian diet during pregnancy and in childhood. Other roles of OCTN2 are related to the traffic of carnitine derivatives in many tissues. The role of OCTN1 is still unclear, despite the identification of some substrates such as ergothioneine, acetylcholine, and choline. Plausibly, the transporter acts on the control of inflammation and oxidative stress, even though knockout mice do not display phenotypes. A clear role of both transporters has been revealed in drug interaction and delivery. The polyspecificity of the OCTNs is at the base of the interactions with drugs. Interestingly, OCTN2 has been recently exploited in the prodrug approach and in diagnostics. A promising application derives from the localization of OCTN2 in exosomes that represent a noninvasive diagnostic tool.

Functional and molecular studies in primary carnitine deficiency

Primary carnitine deficiency is caused by a defect in the OCTN2 carnitine transporter encoded by the SLC22A5 gene. It can cause hypoketotic hypoglycemia or cardiomyopathy in children, and sudden death in children and adults. Fibroblasts from affected patients have reduced carnitine transport. We evaluated carnitine transport in fibroblasts from 358 subjects referred for possible carnitine deficiency. Carnitine transport was reduced to 20% or less of normal in fibroblasts of 140 out of 358 subjects. Sequencing of the 10 exons and flanking regions of the SLC22A5 gene in 95 out of 140 subjects identified causative variants in 84% of the alleles. The missense variants identified in our patients and others previously reported (n = 92) were expressed in CHO cells. Carnitine transport was impaired by 73 out of 92 variants expressed. Prediction algorithms (Polyphen-2, SIFT) correctly predicted the functional effects of expressed variants in about 80% of cases. These results indicate that mutations in the coding region of the SLC22A5 gene cannot be identified in about 16% of the alleles causing primary carnitine deficiency. Prediction algorithms failed to determine the functional effects of amino acid substitutions in this transmembrane protein in about 20% of cases. Therefore, functional studies in fibroblasts remain the best strategy to confirm or exclude a diagnosis of primary carnitine deficiency.

Wide tolerance to amino acids substitutions in the OCTN1 ergothioneine transporter

BACKGROUND

Organic cation transporters transfer solutes with a positive charge across the plasma membrane. The novel organic cation transporter 1 (OCTN1) and 2 (OCTN2) transport ergothioneine and carnitine, respectively. Mutations in the SLC22A5 gene encoding OCTN2 cause primary carnitine deficiency, a recessive disorders resulting in low carnitine levels and defective fatty acid oxidation. Variations in the SLC22A4 gene encoding OCTN1 are associated with rheumatoid arthritis and Crohn disease.

METHODS

Here we evaluate the functional properties of the OCTN1 transporter using chimeric transporters constructed by fusing different portion of the OCTN1 and OCTN2 cDNAs. Their relative abundance and subcellular distribution was evaluated through western blot analysis and confocal microscopy.

RESULTS

Substitutions of the C-terminal portion of OCTN1 with the correspondent residues of OCTN2 generated chimeric OCTN transporters more active than wild-type OCTN1 in transporting ergothioneine. Additional single amino acid substitutions introduced in chimeric OCTN transporters further increased ergothioneine transport activity. Kinetic analysis indicated that increased transport activity was due to an increased V(max), with modest changes in K(m) toward ergothioneine.

CONCLUSIONS

Our results indicate that the OCTN1 transporter is tolerant to extensive amino acid substitutions. This is in sharp contrast to the OCTN2 carnitine transporter that has been selected for high functional activity through evolution, with almost all substitutions reducing carnitine transport activity.

GENERAL SIGNIFICANCE

The widespread tolerance of OCTN1 to amino acid substitutions suggests that the corresponding SLC22A4 gene may have derived from a recent duplication of the SLC22A5 gene and might not yet have a defined physiological role.

Carnitine transport and fatty acid oxidation

Carnitine is essential for the transfer of long-chain fatty acids across the inner mitochondrial membrane for subsequent β-oxidation. It can be synthesized by the body or assumed with the diet from meat and dairy products. Defects in carnitine biosynthesis do not routinely result in low plasma carnitine levels. Carnitine is accumulated by the cells and retained by kidneys using OCTN2, a high affinity organic cation transporter specific for carnitine. Defects in the OCTN2 carnitine transporter results in autosomal recessive primary carnitine deficiency characterized by decreased intracellular carnitine accumulation, increased losses of carnitine in the urine, and low serum carnitine levels. Patients can present early in life with hypoketotic hypoglycemia and hepatic encephalopathy, or later in life with skeletal and cardiac myopathy or sudden death from cardiac arrhythmia, usually triggered by fasting or catabolic state. This disease responds to oral carnitine that, in pharmacological doses, enters cells using the amino acid transporter B(0,+). Primary carnitine deficiency can be suspected from the clinical presentation or identified by low levels of free carnitine (C0) in the newborn screening. Some adult patients have been diagnosed following the birth of an unaffected child with very low carnitine levels in the newborn screening. The diagnosis is confirmed by measuring low carnitine uptake in the patients' fibroblasts or by DNA sequencing of the SLC22A5 gene encoding the OCTN2 carnitine transporter. Some mutations are specific for certain ethnic backgrounds, but the majority are private and identified only in individual families. Although the genotype usually does not correlate with metabolic or cardiac involvement in primary carnitine deficiency, patients presenting as adults tend to have at least one missense mutation retaining residual activity. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

The role of Pyruvate Dehydrogenase Complex in cardiovascular diseases

The regulation of mammalian myocardial carbohydrate metabolism is complex; many factors such as arterial substrate and hormone levels, coronary flow, inotropic state and the nutritional status of the tissue play a role in regulating mammalian myocardial carbohydrate metabolism. The Pyruvate Dehydrogenase Complex (PDHc), a mitochondrial matrix multienzyme complex, plays an important role in energy homeostasis in the heart by providing the link between glycolysis and the tricarboxylic acid (TCA) cycle. In TCA cycle, PDHc catalyzes the conversion of pyruvate into acetyl-CoA. This review determines that there is altered cardiac glucose in various pathophysiological states consequently causing PDC to be altered. This review further summarizes evidence for the metabolism mechanism of the heart under normal and pathological conditions including ischemia, diabetes, hypertrophy and heart failure.

Primary carnitine deficiency: novel mutations and insights into the cardiac phenotype

Solute carrier family 22 member 5 (SLC22A5) encodes a sodium-dependent ion transporter responsible for shuffling carnitine across the plasma membrane. This process provides energy for the heart, among other organs allowing beta-oxidation of fatty acids. Mutations in SLC22A5 result in primary carnitine deficiency (PCD), a disorder that manifests with cardiac, skeletal, or metabolic symptoms. We hereby describe two novel mutations in SLC22A5 in two Lebanese families associated exclusively with a cardiac phenotype. The frequency of the cardiac, metabolic and skeletal symptoms in PCD patients remains undefined. All the reported eight PCD patients belonging to five different Lebanese families have an exclusive cardiac phenotype. Carnitine levels appear to be directly linked to the type and position of the mutation and the severity of the phenotypic presentation does not seem to be associated with serum carnitine levels. A comprehensive review of 61 literature-reported PCD cases revealed an exclusive cardiac manifestation frequency at 62.3% with a very low likelihood of simultaneous occurrence of cardiac and metabolic manifestation.

Genotype-phenotype correlation in primary carnitine deficiency

Primary carnitine deficiency is caused by defective OCTN2 carnitine transporters encoded by the SLC22A5 gene. Lack of carnitine impairs fatty acid oxidation resulting in hypoketotic hypoglycemia, hepatic encephalopathy, skeletal and cardiac myopathy. Recently, asymptomatic mothers with primary carnitine deficiency were identified by low carnitine levels in their infant by newborn screening. Here, we evaluate mutations in the SLC22A5 gene and carnitine transport in fibroblasts from symptomatic patients and asymptomatic women. Carnitine transport was significantly reduced in fibroblasts obtained from all patients with primary carnitine deficiency, but was significantly higher in the asymptomatic women's than in the symptomatic patients' fibroblasts (P < 0.01). By contrast, ergothioneine transport (a selective substrate of the OCTN1 transporter, tested here as a control) was similar in cells from controls and patients with carnitine deficiency. DNA sequencing indicated an increased frequency of nonsense mutations in symptomatic patients (P < 0.001). Expression of the missense mutations in Chinese hamster ovary (CHO) cells indicated that many mutations retained residual carnitine transport activity, with no difference in the average activity of missense mutations identified in symptomatic versus asymptomatic patients. These results indicate that cells from asymptomatic women have on average higher levels of residual carnitine transport activity as compared to that of symptomatic patients due to the presence of at least one missense mutation.

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.

Effects of low oxygen levels on the expression and function of transporter OCTN2 in BeWo cells

Although hypoxia is normal in early pregnancy, low placental oxygen concentrations later in pregnancy are often linked to complications such as pre-eclampsia and intrauterine growth restriction. The effects of low oxygen levels on drug and nutrient uptake via the organic cation transporter OCTN2 has been studied in BeWo cells, an in-vitro model of human trophoblast. BeWo cells were cultured under 20% (control) or 2% O2 (hypoxia) for 48 h before each experiment. In-vitro hypoxia was also simulated by the addition of CoCl2 to the cell culture medium. RT-PCR indicated increased transcription of OCTN2 in BeWo cells cultured under hypoxia, but Western blots did not show a corresponding increase in the amount of OCTN2 protein in the hypoxic cells compared with control. Hypoxia resulted in significant reductions in OCTN2-mediated carnitine uptake. Decreased placental transport of carnitine may lead to symptoms of carnitine deficiency in infants from hypoxic pregnancies, whether caused by high altitude, pre-eclampsia or other factors. The OCTN1 substrate ergothioneine reversed the effects of hypoxia on carnitine transport, but identical concentrations of N-acetylcysteine, another water-soluble intracellular antioxidant, did not have the same effect.

Cardiomyopathy and carnitine deficiency

Carnitine is essential for the transfer of long-chain fatty acids across the mitochondrial membrane for subsequent beta-oxidation. A defect in the high-affinity carnitine transporter OCTN2 causes autosomal recessive primary carnitine deficiency that can present with hypoketotic hypoglycemia, mainly in infancy or cardiomyopathy. Heterozygotes for primary carnitine deficiency can have mildly reduced plasma carnitine levels and can develop benign cardiac hypertrophy. In animal models, heterozygotes for this disease have a higher incidence of cardiomyopathy with aging. This study tested whether heterozygosity for primary carnitine deficiency was associated with cardiomyopathy. The frequency of mutations in the SLC22A5 gene encoding the OCTN2 carnitine transporter was determined in 324 patients with cardiomyopathy and compared to that described in the normal population. Missense variations identified in normal controls and patients with cardiomyopathy were expressed in Chinese Hamster Ovary cells to confirm a functional effect. Exons 2-10 of the SLC22A5 gene were amplified by PCR in the presence of LCGreen I and analyzed by dye-binding/high-resolution thermal denaturation. Exon 1 of the gene was sequenced in all patients. Heterozygosity for a few variants (L144F, T264M, I312V, E317K, and R488H) was found in 6/324 patients with cardiomyopathy. Expression of these variants in CHO cells indicated that T264M decreased, E317K increased, while L144F, I312V, and R488H did not significantly affect carnitine transport. Expression in CHO cells of all the variants identified in a normal population indicated that only two had a functional effect (L17F and Y449D), while L144F, V481I, V481F, M530V, and P549S did not change significantly carnitine transport. The frequency of variants affecting carnitine transport was 2/324 patients with cardiomyopathy (0.61%) not significantly different from frequency of 3/270 (1.11%) in the general population. These results indicate that heterozygosity for primary carnitine deficiency is not more frequent in patients with unselected types of cardiomyopathy and is unlikely to be an important cause of cardiomyopathy in humans.

Cell free expression and functional reconstitution of eukaryotic drug transporters

Polyspecific organic cation and anion transporters of the SLC22 protein family are critically involved in absorption and excretion of drugs. To elucidate transport mechanisms, functional and biophysical characterization of purified transporters is required and tertiary structures must be determined. Here, we synthesized rat organic cation transporters OCT1 and OCT2 and rat organic anion transporter OAT1 in a cell free system in the absence of detergent. We solubilized the precipitates with 2% 1-myristoyl-2-hydroxy- sn-glycero-3-[phospho- rac-(1-glycerol)] (LMPG), purified the transporters in the presence of 1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) or octyl glucoside, and reconstituted them into proteoliposomes. From 1 mL reaction vessels 0.13-0.36 mg of transporter proteins was purified. Thus, from five to ten 1 mL reaction vessels sufficient protein for crystallization was obtained. In the presence of 1% LMPG and 0.5% CHAPS, OCT1 and OAT1 formed homo-oligomers but no hetero-oligomers. After reconstitution of OCT1, OCT2, and OAT1 into proteoliposomes, similar Michaelis-Menten K m values were measured for uptake of 1-methyl-4-phenylpyridinium and p-aminohippurate (PAH (-)) by the organic cation and anion transporters, respectively, as after expression of the transporters in cells. Using the reconstituted system, evidence was obtained that OAT1 operates as obligatory and electroneutral PAH (-)/dicarboxylate antiporter and contains a low-affinity chloride binding site that stimulates turnover. PAH (-) uptake was observed only with alpha-ketoglutarate (KG (2-)) on the trans side, and trans-KG (2-) increased the PAH (-) concentration in voltage-clamped proteoliposomes transiently above equilibrium. The V max of PAH (-)/KG (2-) antiport was increased by Cl (-) in a manner independent of gradients, and PAH (-)/KG (2-) antiport was independent of membrane potential in the absence or presence of Cl (-).

Contributions of phosphorylation to regulation of OCTN2 uptake of carnitine are minimal in BeWo cells

Physiological functions of organic cation transporters (OCTs) in the placenta include transporting essential nutrients from the maternal to fetal circulations. OCTN2 transports carnitine with high affinity, and the transport of several drugs has also been shown to be mediated by this transporter. In this work, the role of phosphorylation and dephosphorylation mechanisms in regulating OCTN2 was investigated by observing the effects of various activators and inhibitors of kinases and phosphatases on the uptake of carnitine in BeWo cells, a human choriocarcinoma trophoblast cell line frequently used as an in vitro model of the rate-limiting barrier for maternal-fetal exchange. Preincubation with genistein resulted in significant increases in both alkaline phosphatase (ALP) activity and carnitine uptake. Levamisole, an ALP inhibitor, caused a more substantial decrease in carnitine uptake than expected from its corresponding decrease in ALP activity. It was determined that levamisole competitively inhibits carnitine uptake, with a K(i) value of 1.01+/-0.05mM, and this effect has a greater role in decreasing carnitine uptake than any indirect effects of ALP inhibition upon OCTN2 function. Progesterone also competitively inhibited carnitine uptake (K(i)=48.6+/-5.0muM), but had no effect on ALP activity in BeWo cells.

Identification of cysteines in rat organic cation transporters rOCT1 (C322, C451) and rOCT2 (C451) critical for transport activity and substrate affinity

Effects of the sulfhydryl reagent methylmethanethiosulfonate (MMTS) on functions of organic cation transporters (OCTs) were investigated. Currents induced by 10 mM choline [ Imax(choline)] in Xenopus laevis oocytes expressing rat OCT1 (rOCT1) were increased four- to ninefold after 30-s incubation with 5 mM MMTS whereas Imax(choline) by rat OCT2 was 70% decreased. MMTS activated the rOCT1 transporter within the plasma membrane without changing stoichiometry between translocated charge and cation. After modification of oocytes expressing rOCT1 or rOCT2 with MMTS, I0.5(choline) values for choline-induced currents were increased. For rOCT1 it was shown that MMTS increased I0.5 values for different cations by different degrees. Mutagenesis of individual cysteine residues in rOCT1 revealed that modification of cysteine 322 in the large intracellular loop, and of cysteine 451 at the transition of the transmembrane α-helix (TMH) 10 to the short intracellular loop between the TMH 10 and 11 is responsible for the observed effects of MMTS. After replacement of cysteine 451 by methionine, the IC50(choline) for choline to inhibit MPP uptake by rOCT1 was increased whereas the I0.5(choline) value for choline-induced current remained unchanged. At variance, in double mutant Cys322Ser, Cys451Met, I0.5(choline) was increased compared with rOCT1 wild-type whereas in the single mutant Cys322Ser I0.5(choline) was not changed. The data suggest that modification of rOCT1 at cysteines 322 and 451 leads to an increase in turnover. They indicate that cysteine 451 in rOCT1 interacts with the large intracellular loop and that cysteine 451 in both rOCT1 and rOCT2 is critical for the affinity of choline.

OCTN2VT, a splice variant of OCTN2, does not transport carnitine because of the retention in the endoplasmic reticulum caused by insertion of 24 amino acids in the first extracellular loop of OCTN2

A novel organic cation transporter OCTN2 is indispensable for carnitine transport across plasma membrane and subsequent fatty acid metabolism in the mitochondria. Here, we report a novel splice variant of OCTN2 (OCTN2VT), in which a 72-base-pair sequence located in the first intron of OCTN2 gene was spliced between exons 1 and 2 of OCTN2, causing the insertion of 24 amino acids in the first extracellular loop of OCTN2. Despite the similarity between OCTN2 and OCTN2VT regarding primary structure and tissue distribution, their biochemical characteristics were significantly different. OCTN2 was expressed on the plasma membrane with robust N-glycosylation, whereas OCTN2VT was retained in the endoplasmic reticulum (ER) with poor N-glycosylation. In addition, the retention in the ER caused no carnitine uptake into the cells. These results demonstrate that the biochemical and functional characteristics of OCTN2VT are distinct from OCTN2 due to the insertion of 24 amino acids in the first extracellular loop.

Polyspecific organic cation transporters: structure, function, physiological roles, and biopharmaceutical implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.

Polyspecific organic cation transporters: structure, function, physiological roles, and biopharmaceutical implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.

Pharmacological rescue of carnitine transport in primary carnitine deficiency

Primary carnitine deficiency is a recessive disorder caused by heterogeneous mutations in the SLC22A5 gene encoding the OCTN2 carnitine transporter. Here we extend mutational analysis to eight new families with this disorder. To determine the mechanism by which missense mutations impaired carnitine transport, the OCTN2 transporter was tagged with the green fluorescent protein and expressed in CHO cells. Analysis by confocal microscopy indicated that several missense mutants (M1I, R169W, T232 M, G242 V, S280F, R282Q, W283R, A301D, W351R, R399Q, T440 M, E452 K, and T468R) matured normally to the plasma membrane. By contrast, other mutations (including R19P, DeltaF22, R83L, S280F, P398L, Y447C, and A142S/R488 H) caused significant retention of the mutant OCTN2 transporter in the cytoplasm. Failed maturation to the plasma membrane is a common mechanism in disorders affecting membrane transporters/ion channels, including cystic fibrosis. To correct this defect, we tested whether drugs reducing the efficiency of protein degradation in the endoplasmic reticulum (ER) (phenylbutyrate, curcumin) or capable of binding the OCTN2 carnitine transporter (verapamil, quinidine) could improve carnitine transport. Prolonged incubation with phenylbutyrate, quinidine, and verapamil partially stimulated carnitine transport, while curcumin was ineffective. These results indicate that OCTN2 mutations can affect carnitine transport by impairing maturation of transporters to the plasma membrane. Pharmacological therapy can be effective in partially restoring activity of mutant transporters.

Validation of dye-binding/high-resolution thermal denaturation for the identification of mutations in the SLC22A5 gene

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation resulting from defective carnitine transport. This disease is caused by mutations in the OCTN2 carnitine transporter encoded by the SLC22A5 gene. Here we validate dye-binding/high-resolution thermal denaturation as a screening procedure to identify novel mutations in this gene. This procedure is based on the amplification of DNA by PCR in capillaries with the dsDNA binding dye LCGreen I. The PCR reaction is then analyzed in the same capillary by high-resolution thermal denaturation. Samples with abnormal melting profiles are sequenced. This technique correctly identified all known patients who were compound heterozygotes for different mutations in the carnitine transporter gene and about 30% of homozygous patients. The remaining 70% of homozygous patients were identified by a second amplification, in which the patient's DNA was mixed with the DNA of a normal control. This screening system correctly identified eight novel mutations and both abnormal alleles in six new families with primary carnitine deficiency. The causative role of the missense mutations identified (c.3G>T/p.M1I, c.695C>T/p.T232M, and c.1403 C>G/p.T468R) was confirmed by expression in Chinese hamster ovary (CHO) cells. These results expand the mutational spectrum in primary carnitine deficiency and indicate dye-binding/high-resolution thermal denaturation as an ideal system to screen for mutations in diseases with no prevalent molecular alteration.

Functional regions of organic cation/carnitine transporter OCTN2 (SLC22A5): roles in carnitine recognition

The organic cation/carnitine transporter OCTN2 transports carnitine in a sodium-dependent manner, whereas it transports organic cations sodium-independently. To elucidate the functional domain in OCTN2, we constructed chimeric proteins of human OCTN2 (hOCTN2) and mouse OCTN3 (mOCTN3) and introduced mutations at several amino acids conserved among human, rat and mouse OCTN2. We found that transmembrane domains (TMD) 1-7 are responsible for organic cation transport and for sodium dependence in carnitine transport. Within TMD1-7, Q180 and Q207 of hOCTN2 are the critical amino acids for the sodium dependence, and double mutation of Q180 and Q207 resulted in minimal change in transport activity when sodium was removed from the uptake medium. We propose that sodium-dependent affinity for carnitine is dependent on sodium recognition by these critical amino acids in hOCTN2, whereas carnitine transport by OCTN2 requires functional linkage between TMD1-7 and TMD11.

Novel organic cation transporter 2-mediated carnitine uptake in placental choriocarcinoma (BeWo) cells

The placental transport of carnitine is significant because the fetus cannot supply itself with adequate amounts of this nutrient. Carnitine deficiencies in infants can lead to symptoms ranging from muscle weakness to sudden infant death. Objectives of this study include the characterization of novel organic cation transporter 2 (OCTN2) function in the BeWo cell line and the inhibition of placental carnitine uptake by amphetamine derivatives. BeWo cells were seeded in 12- or 24-well tissue culture plates and incubated at 37 degrees C until monolayers were confluent. Uptake studies with radiolabeled L-carnitine and inhibitors in Hanks' balanced salt solution were carried out in the plates at 37 degrees C for 30 min. Uptake of L-carnitine in BeWo cells was Na(+)-dependent and saturable (K(m) = 9.8 +/- 2.4 microM, V(max) = 800 +/- 70 pmol/mg of protein/30 min) with a nonsaturable constant of 2.8 +/- 0.3 microl/mg of protein/30 min. Among the amphetamine analogs studied, IC(50) values ranged from 2.3 to 9.2 mM, and the inhibition of carnitine uptake was stronger for compounds having a methyl-substituted nitrogen atom. Lineweaver-Burk plots show that inhibition by tetraethylammonium and valproate was competitive; inhibition by ephedrine was not completely competitive. The observed kinetics, Western blot, and inhibition profiles indicate that high-affinity carnitine uptake in the BeWo cell line is mediated by OCTN2. Inhibition of carnitine transport by amphetamines potentially poses serious consequences for fetal development.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney now allows the development of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

Organic cation transporters

Over the last 15 years, a number of transporters that translocate organic cations were characterized functionally and also identified on the molecular level. Organic cations include endogenous compounds such as monoamine neurotransmitters, choline, and coenzymes, but also numerous drugs and xenobiotics. Some of the cloned organic cation transporters accept one main substrate or structurally similar compounds (oligospecific transporters), while others translocate a variety of structurally diverse organic cations (polyspecific transporters). This review provides a survey of cloned organic cation transporters and tentative models that illustrate how different types of organic cation transporters, expressed at specific subcellular sites in hepatocytes and renal proximal tubular cells, are assembled into an integrated functional framework. We briefly describe oligospecific Na(+)- and Cl(-)-dependent monoamine neurotransmitter transporters ( SLC6-family), high-affinity choline transporters ( SLC5-family), and high-affinity thiamine transporters ( SLC19-family), as well as polyspecific transporters that translocate some organic cations next to their preferred, noncationic substrates. The polyspecific cation transporters of the SLC22 family including the subtypes OCT1-3 and OCTN1-2 are presented in detail, covering the current knowledge about distribution, substrate specificity, and recent data on their electrical properties and regulation. Moreover, we discuss artificial and spontaneous mutations of transporters of the SLC22 family that provide novel insight as to the function of specific protein domains. Finally, we discuss the clinical potential of the increasing knowledge about polymorphisms and mutations in polyspecific organic cation transporters.

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.

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.

L-carnitine transport in kidney of normotensive, Wistar-Kyoto rats: effect of chronic L-carnitine administration

PURPOSE

To examine the effect of long-term administration of L-carnitine on L-carnitine transport in renal brush-border membrane vesicles (BBMVs) from normotensive, Wistar-Kyoto rats.

METHODS

Rats (n = 20) were orally administered 0.2 g carnitine/kg body weight per day for a total period of 8 weeks. Kinetic parameters of L-carnitine uptake were calculated by non-linear regression, and the relative abundance of the carnitine transporter, OCTN2, was determined by Western blot analysis.

RESULTS

Initial rates and maximal overshoot levels of Na+-dependent L-carnitine transport were significantly reduced in BBMVs from L-carnitine-treated rats compared with untreated animals. Similarly, the maximal transport rate (Vmax) of OCTN2 was lower in treated rats. However, no differences were observed in the Michaelis constant (Km) or the diffusion constant (Kd) between the two groups of animals. The amount of OCTN2 protein was also decreased in L-carnitine-fed rats, this reduction being similar to that of the Vmax. These results were accompanied by an increase in the serum levels and also in the renal excretion of both free and esterified carnitine in treated rats, indicating that the long-term administration of L-carnitine leads to increased renal carnitine clearance.

CONCLUSION

These findings suggest a downregulation of OCTN2 at the renal level, in the presence of high levels of carnitine.

Heart metabolic disturbances in cardiovascular diseases

Myocardial function depends on adenosine triphosphate (ATP) supplied by oxidation of several substrates. In the adult heart, this energy is obtained primarily from fatty acid oxidation through oxidative phosphorylation. However, the energy source may change depending on several factors such as substrate availability, energy demands, oxygen supply, and metabolic condition of the individual. Surprisingly, the role of energy metabolism in development of cardiac diseases has not been extensively studied. For instance, alterations in glucose oxidation and transport developed in diabetic heart may compromise myocardial performance under conditions in which ATP provided by glycolysis is relevant, such as in ischemia and reperfusion. In some cardiac diseases such as ischemic cardiomyopathy, heart failure, hypertrophy, and dilated cardiomyopathy, ATP generation is diminished by derangement of fatty acid delivery to mitochondria and by alteration of certain key enzymes of energy metabolism. Shortage of some co-factors such as L-carnitine and creatine also leads to energy depletion. Creatine kinase system and other mitochondrial enzymes are also affected. Initial attempts to modulate cardiac energy metabolism by use of drugs or supplements as a therapeutic approach to heart disease are described.

Studies on functional sites of organic cation/carnitine transporter OCTN2 (SLC22A5) using a Ser467Cys mutant protein

The organic cation/carnitine transporter OCTN2 mediates transport of carnitine and organic cations in Na(+)-dependent and Na(+)-independent manners, respectively. However, the mechanism of molecular recognition of different substrates has not been clarified yet. We previously found a single amino acid change in OCTN2, Ser467Cys (S467C), in the Japanese population and observed a decreased carnitine transport but unchanged organic cation transport compared with wild type. Therefore, we conducted detailed kinetic and functional analyses of the substrate recognition sites of wild-type and S467C-mutant OCTN2. The K(m) value for carnitine of S467C-mutant was increased about 15-fold over that of the wild type. Mutual inhibition kinetics of carnitine and tetraethylammonium (TEA) were not completely competitive, suggesting that the binding sites are very close to each other, but not identical. Several organic anions such as valproate, as well as organic cations, significantly inhibited carnitine and TEA uptake by OCTN2, and valproate showed Na(+)-dependent inhibition of OCTN2-mediated TEA uptake. The Na(+)-activation kinetics of the S467C mutant was similar to that of the wild type. Furthermore, a significant decrease of the TEA uptake-inhibitory potency of valproate was observed in S467C-mutant OCTN2. These observations suggest that the decrease in affinity of S467C-mutant OCTN2 for carnitine was caused by functional alteration of the anion (carboxyl moiety of carnitine) recognition site located in trans-membrane domain 11, which is closely related to the Na(+)-binding site, on OCTN2 protein. These results demonstrate that OCTN2 has functional sites for carnitine and Na(+) and that the carnitine-binding site is involved, in part, in the recognition of organic cations.

Phenotype and genotype variation in primary carnitine deficiency

PURPOSE

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation resulting from defective carnitine transport. This disease is caused by mutations in the carnitine transporter gene SLC22A5. The objective of this study was to extend mutational analysis to four additional families with this disorder and determine whether recurrent mutations could be found.

METHODS

The SLC22A5 gene encoding the OCTN2 carnitine transporter was sequenced, and the missense mutations identified were expressed in Chinese hamster ovary (CHO) cells.

RESULTS

DNA sequencing revealed four novel mutations (Y4X; dup 254-264, 133X; R19P; R399Q). Alleles introducing premature STOP codons reduced the levels of OCTN2 mRNA. Carnitine transport in CHO cells expressing the R19P and R399Q mutations was reduced to < 5% of normal. The 133X mutation was found in two unrelated European families. Two patients within the same family, both homozygous for the same mutation (R399Q) had completely different clinical presentation.

CONCLUSIONS

Heterogeneous mutations in the SLC22A5 gene cause primary carnitine deficiency. Different presentations are observed even in children with identical mutations.

Carnitine transport by organic cation transporters and systemic carnitine deficiency

The intracellular homeostasis is controlled by different membrane transporters. Organic cation transporters function primarily in the elimination of cationic drugs, endogenous amines, and other xenobiotics in tissues such as the kidney, intestine, and liver. Among these molecules, carnitine is an endogenous amine which is an essential cofactor for mitochondrial beta-oxidation. Recently, a new family of transporters, named OCT (organic cation transporters) has been described. In this minireview, we present the recent knowledge about OCT and focus on carnitine transport, more particularly by the OCTN2. The importance of this sodium-dependent carnitine cotransporter, OCTN2, comes from various recently reported mutations in the gene which give rise to the primary systemic carnitine deficiency (SCD; OMIM 212140). The SCD is an autosomal recessive disorder of fatty acid oxidation characterized by skeletal myopathy, progressive cardiomyopathy, hypoglycemia and hyperammonemia. Most of the OCTN2 mutations identified in humans with SCD result in loss of carnitine transport function. Identifying these mutations will allow an easy targeting of the SCD syndrome. The characteristics of the juvenile visceral steatosis (jvs) mouse, an animal model of SCD showing similar symptoms as humans having this genetic disorder, are also described. These mice have a mutation in the gene encoding the mouse carnitine transporter octn2. Although various OCTN carnitine transporters have been identified and functionally characterized, their membrane localization and regulation are still unknown and must be investigated. This knowledge will also help in designing new drugs that regulate carnitine transport activity.

Molecular enzymology of carnitine transfer and transport

Carnitine (L-3-hydroxy-4-N-trimethylaminobutyric acid) forms esters with a wide range of acyl groups and functions to transport and excrete these groups. It is found in most cells at millimolar levels after uptake via the sodium-dependent carrier, OCTN2. The acylation state of the mobile carnitine pool is linked to that of the limited and compartmentalised coenzyme A pools by the action of the family of carnitine acyltransferases and the mitochondrial membrane transporter, CACT. The genes and sequences of the carriers and the acyltransferases are reviewed along with mutations that affect activity. After summarising the accepted enzymatic background, recent molecular studies on the carnitine acyltransferases are described to provide a picture of the role and function of these freely reversible enzymes. The kinetic and chemical mechanisms are also discussed in relation to the different inhibitors under study for their potential to control diseases of lipid metabolism.