The gene encoding a cationic amino acid transporter (SLC7A4) maps to the region deleted in the velocardiofacial syndrome

Comprehensive review of amino acid transporters as therapeutic targets

The solute carrier (SLC) family, with more than 400 membrane-bound proteins, facilitates the transport of a wide array of substrates such as nutrients, ions, metabolites, and drugs across biological membranes. Amino acid transporters (AATs) are membrane transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs participate in many important physiological functions including nutrient supply, metabolic transformation, energy homeostasis, redox regulation, and neurological regulation. Several AATs have been found to significantly impact the progression of human malignancies, and dysregulation of AATs results in metabolic reprogramming affecting tumor growth and progression. However, current clinical therapies that directly target AATs have not been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, the molecular mechanisms in human diseases such as tumors, kidney diseases, and emerging therapeutic strategies for targeting AATs.

Fetal membranes exhibit similar nutrient transporter expression profiles to the placenta

INTRODUCTION

During pregnancy, the growth of the fetus is supported by the exchange of nutrients, waste, and other molecules between maternal and fetal circulations in the utero-placental unit. Nutrient transfer, in particular, is mediated by solute transporters such as solute carrier (SLC) and adenosine triphosphate-binding cassette (ABC) proteins. While nutrient transport has been extensively studied in the placenta, the role of human fetal membranes (FM), which was recently reported to have a role in drug transport, in nutrient uptake remains unknown.

OBJECTIVES

This study determined nutrient transport expression in human FM and FM cells and compared expression with placental tissues and BeWo cells.

METHODS

RNA sequencing (RNA-Seq) of placental and FM tissues and cells was done. Genes of major solute transporter groups, such as SLC and ABC, were identified. Proteomic analysis of cell lysates was performed via nano-liquid chromatography-tandem mass spectrometry (nanoLC-MS/MS) to confirm expression at a protein level.

RESULTS

We determined that FM tissues and cells derived from the fetal membrane tissues express nutrient transporter genes, and their expression is similar to that seen in the placenta or BeWo cells. In particular, transporters involved in macronutrient and micronutrient transfer were identified in both placental and FM cells. Consistent with RNA-Seq findings, carbohydrate transporters (3), vitamin transport-related proteins (8), amino acid transporters (21), fatty acid transport-related proteins (9), cholesterol transport-related proteins (6) and nucleoside transporters (3) were identified in BeWo and FM cells, with both groups sharing similar nutrient transporter expression.

CONCLUSION

This study determined the expression of nutrient transporters in human FMs. This knowledge is the first step in improving our understanding of nutrient uptake kinetics during pregnancy. Functional studies are required to determine the properties of nutrient transporters in human FMs.

Effects of sex and estrous cycle on the brain and plasma arginine metabolic profile in rats

L-arginine is a versatile amino acid with a number of bioactive metabolites. Increasing evidence implicates altered arginine metabolism in the aging and neurodegenerative processes. The present study, for the first time, determined the effects of sex and estrous cycle on the brain and blood (plasma) arginine metabolic profile in naïve rats. Female rats displayed significantly lower levels of L-arginine in the frontal cortex and three sub-regions of the hippocampus when compared to male rats. Moreover, female rats had significantly higher levels of L-arginine and γ-aminobutyric acid, but lower levels of L-ornithine, agmatine and putrescine, in plasma relative to male rats. The observed sex difference in brain L-arginine appeared to be independent of the enzymes involved in its metabolism, de novo synthesis and blood-to-brain transport (cationic acid transporter 1 protein expression at least), as well as circulating L-arginine. While the estrous cycle did not affect L-arginine and its metabolites in the brain, there were estrous cycle phase-dependent changes in plasma L-arginine. These findings demonstrate the sex difference in brain L-arginine in the estrous cycle-independent manner. Since peripheral blood has been increasingly used to identify biomarkers of brain pathology, the influences of sex and estrous cycle on blood arginine metabolic profile need attention when experimental research involves female rodents.

The SLC3 and SLC7 families of amino acid transporters

Amino acids are necessary for all living cells and organisms. Specialized transporters mediate the transfer of amino acids across plasma membranes. Malfunction of these proteins can affect whole-body homoeostasis giving raise to diverse human diseases. Here, we review the main features of the SLC3 and SLC7 families of amino acid transporters. The SLC7 family is divided into two subfamilies, the cationic amino acid transporters (CATs), and the L-type amino acid transporters (LATs). The latter are the light or catalytic subunits of the heteromeric amino acid transporters (HATs), which are associated by a disulfide bridge with the heavy subunits 4F2hc or rBAT. These two subunits are glycoproteins and form the SLC3 family. Most CAT subfamily members were functionally characterized and shown to function as facilitated diffusers mediating the entry and efflux of cationic amino acids. In certain cells, CATs play an important role in the delivery of L-arginine for the synthesis of nitric oxide. HATs are mostly exchangers with a broad spectrum of substrates and are crucial in renal and intestinal re-absorption and cell redox balance. Furthermore, the role of the HAT 4F2hc/LAT1 in tumor growth and the application of LAT1 inhibitors and PET tracers for reduction of tumor progression and imaging of tumors are discussed. Finally, we describe the link between specific mutations in HATs and the primary inherited aminoacidurias, cystinuria and lysinuric protein intolerance.

CAT-1-mediated arginine uptake and regulation of nitric oxide synthases for the survival of human breast cancer cell lines

Growth of the human MCF-7 breast cancer cell line is highly dependent on L-arginine. We have reported that L-arginine, released from extracellular substrates by prolactin (PRL)- and 17β-estradiol (E2)-induced carboxypeptidase-D in the cell membrane, promotes nitric oxide (NO) production for MCF-7 cell survival. Arginine uptake is mediated by members of the cationic amino acid transporter (CAT) family and may coincide with induction of nitric oxide synthase (NOS) for the production of NO. The present study investigated the CAT isoforms and PRL/E2 regulation of CAT and NOS in breast cancer cell lines. Using RT-PCR analysis, CAT-1, CAT-2A, and CAT-2B transcripts were detected in MCF-7, T47D, and MDA-MB-231 cells. The CAT-4 transcript was detected in MDA-MB-231 only. CAT-3 was not detected in any of these cells. PRL and E2 did not significantly alter levels of CAT-1 mRNA and protein, nor CAT-2A and CAT-2B mRNAs in MCF-7 and T47D cells. PRL and E2 also had no effect on the overall uptake of L-[2,3,4,5-H(3)] arginine into these cells. However, confocal immunofluorescent microscopy showed that PRL and E2 upregulated eNOS and iNOS proteins, which distributed in the cytoplasm and/or nucleus of MCF-7 cells. Knockdown of CAT-1 gene expression using small interfering RNA significantly decreased L-[2,3,4,5-H(3)]-arginine uptake, decreased viability and increased apoptosis of MCF-7 and T47D cells. In summary, several CAT isoforms are expressed in breast cancer cells. The CAT-1 isoform plays a role in arginine uptake and, together with PRL/E2-induced NOS, contribute to NO production for the survival of MCF-7 and T47D cells.

Unambiguous molecular detections with multiple genetic approach for the complicated chromosome 22q11 deletion syndrome

Background

Chromosome 22q11 deletion syndrome (22q11DS) causes a developmental disorder during the embryonic stage, usually because of hemizygous deletions. The clinical pictures of patients with 22q11DS vary because of polymorphisms: on average, approximately 93% of affected individuals have a de novo deletion of 22q11, and the rest have inherited the same deletion from a parent. Methods using multiple genetic markers are thus important for the accurate detection of these microdeletions.

Methods

We studied 12 babies suspected to carry 22q11DS and 18 age-matched healthy controls from unrelated Taiwanese families. We determined genomic variance using microarray-based comparative genomic hybridization (array-CGH), quantitative real-time polymerase chain reaction (qPCR) and multiplex ligation-dependent probe amplification (MLPA).

Results

Changes in genomic copy number were significantly associated with clinical manifestations for the classical criteria of 22q11DS using MPLA and qPCR (p < 0.01). An identical deletion was shown in three affected infants by MLPA. These reduced DNA dosages were also obtained partially using array-CGH and confirmed by qPCR but with some differences in deletion size.

Conclusion

Both MLPA and qPCR could produce a clearly defined range of deleted genomic DNA, whereas there must be a deleted genome that is not distinguishable using MLPA. These data demonstrate that such multiple genetic approaches are necessary for the unambiguous molecular detection of these types of complicated genomic syndromes.

l-Arginine transporters in cardiovascular disease: A novel therapeutic target

The amino acid l-arginine participates in a variety of key biochemical and physiological activities, including its well-recognized role as the key substrate for nitric oxide (NO) biosynthesis. The current review describes the cellular influences on arginine metabolism with particular focus on the transport of l-arginine in the endothelium. It details the processes by which intracellular and extracellular levels of l-arginine may influence nitric oxide production and further documents the imbalance that is evident in various cardiovascular disease states. In man, impairment of l-arginine transport has been observed in hypertension, heart failure, and renal disease, and it may thus be a relevant therapeutic target for rectification of nitric oxide pathogenesis in these conditions.

L-Arginine currents in rat cardiac ventricular myocytes

L-Arginine (L-Arg) is a basic amino acid that plays a central role in the biosynthesis of nitric oxide, creatine, agmantine, polyamines, proline and glutamate. Most tissues, including myocardium, must import L-Arg from the circulation to ensure adequate intracellular levels of this amino acid. This study reports novel L-Arg-activated inward currents in whole-cell voltage-clamped rat ventricular cardiomyocytes. Ion-substitution experiments identified extracellular L-Arg as the charge-carrying cationic species responsible for these currents, which, thus, represent L-Arg import into cardiac myocytes. This result was independently confirmed by an increase in myocyte nitric oxide production upon extracellular application of L-Arg. The inward movement of Arg molecules was found to be passive and independent of Na(2+), K(2+), Ca(2+) and Mg(2+). The process displayed saturation and membrane potential (V(m))-dependent kinetics, with a K(0.5) for l-Arg that increased from 5 mm at hyperpolarizing V(m) to 20 mm at +40 mV. L-Lysine and L-ornithine but not D-Arg produced currents with characteristics similar to that activated by L-Arg indicating that the transport process is stereospecific for cationic L-amino acids. L-Arg current was fully blocked after brief incubation with 0.2 mm N-ethylmaleimide. These features suggest that the activity of the low-affinity, high-capacity CAT-2A member of the y(2+) family of transporters is responsible for L-Arg currents in acutely isolated cardiomyocytes. Regardless of the mechanism, we hypothesize that a low-affinity arginine transport process in heart, by ensuring substrate availability for sustained NO production, might play a cardio-protective role during catabolic states known to increase Arg plasma levels severalfold.

Radiolabeled L-lysine for tumor imaging

RATIONALE AND OBJECTIVES

The aims of this study were to label the versatile amino acid l-lysine with (99m)Tc using 2,3-dimercapto-succinic acid (DMSA) as a chelator, and to assess its tumor imaging feasibility under in vivo and in vitro conditions, and finally to determine the subcellular biodistribution of this radiopharmaceutical.

MATERIALS AND METHODS

DMSA-l-lysine was chemically synthesized and labeled with sodium pertechnetate. Nuclear magnetic resonance (NMR) and mass spectral analysis of DMSA-l-lysine were conducted. Radiochemical purity was determined by thin-layer chromatography (TLC) and paper chromatography. Cellular uptake, competition and subcellular localization studies were performed in rat breast cancer cells (13762). In vivo studies of planar imaging and biodistribution studies were performed on female Fischer 344 rats. Medical Internal Radiation Dose (MIRD) dosimetry estimates were calculated.

RESULTS

Radiochemical purity (determined by radio-TLC and high-performance liquid chromatography) of these compounds was >95%. (99m)Tc-DMSA-l-lysine showed good uptake in in vitro cell culture assays and uptake was reduced in competition studies. (99m)Tc-DMSA-l-lysine accumulates in the nucleus as much as in the cytoplasm and it was also shown that accumulation of the (99m)Tc-DMSA-l-lysine in the nucleus increases as a function of a time. There was an increase in tumor-to-blood and tumor-to-muscle count density ratios. Tumor/background ratios were 5.75 at 1 hour and 6.87 at 2 hours. In vivo tissue distribution studies revealed that radiation dosimetry of blood-forming organs were within radiation dose limits.

CONCLUSION

DMSA-l-lysine kits can be labeled with (99m)Tc easily and efficiently, with high radiochemical purity and cost-effectiveness. In vitro cellular uptake and scintigraphic imaging studies demonstrated the pharmacokinetic distribution and feasibility of using (99m)Tc-DMSA-l-lysine for tumor imaging.

Lysine deficiency and feed restriction independently alter cationic amino acid transporter expression in chickens (Gallus gallus domesticus)

The effect of a lysine-deficient diet on cationic amino acid transporter (CAT1-3) mRNA expression was determined in broiler chickens. Chicks consumed a lysine-adequate (LA; 1.3% lysine) or lysine-deficient (LD; 0.7% lysine) diet. Pair-fed chicks consumed the LA diet in an amount equal to that consumed by LD chicks during the previous day (PLA). CAT 1-3 mRNA expression in the liver, pectoralis and bursa of LD chicks were lower than that of LA and PLA chicks (P<0.05), and levels were not detectable in LD chick thymus. High affinity CAT mRNA expression in isolated bursacytes was 16-fold higher in LD chicks than that of LA chicks (P<0.001). Thymocyte high affinity CAT mRNA expression was 5-fold lower than that of LA chicks (P<0.05). The summed amount of high affinity CAT-1 and CAT-3 mRNA expression in chicks fed a lysine adequate diet was highly correlated (r2=0.51; P<0.001) to a tissue's growth during a lysine deficiency or feed restriction. In the thymus and bursa of LD chicks, CAT mRNA levels differed between resident lymphocytes and their surrounding tissues. By expressing high affinity CAT isoforms, developing lymphocytes may have a greater ability to obtain lysine than their surrounding tissue during a lysine deficiency.

A y+LAT-1 mutant protein interferes with y+LAT-2 activity: implications for the molecular pathogenesis of lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is an inherited aminoaciduria caused by defective cationic amino acid (CAA) transport at the basolateral membrane of epithelial cells in the intestine and kidney. The SLC7A7 gene, mutated in LPI, encodes the y(+)LAT-1 protein, which is the light subunit of the heterodimeric CAA transporter in which 4F2hc is the heavy chain subunit. Co-expression of 4F2hc and y(+)LAT-1 induces the y(+)L activity. This activity is also exerted by another complex composed of 4F2hc and y(+)LAT-2, the latter encoded by the SLC7A6 gene and more ubiquitously expressed than SLC7A7. On the basis of both the pattern of expression and the transport activity, y(+)LAT-2 might compensate for CAA transport when y(+)LAT-1 is defective. By expression in Xenopus laevis oocytes and mammalian cells, we functionally analysed two SLC7A7 mutants, E36del and F152L, respectively, the former displaying a partial dominant-negative effect. The results of the present study provide further insight into the molecular pathogenesis of LPI: a putative multiheteromeric structure of both [4F2hc/y(+)LAT-1] and [4F2hc/y(+)LAT-2], and the interference between y(+)LAT-1 and y(+)LAT-2 proteins. This interference can explain why the compensatory mechanism, that is, an increased expression of SLC7A6 as seen in lymphoblasts from LPI patients, may not be sufficient to restore the y(+)L system activity.

Regulation of cationic amino acid transport: the story of the CAT-1 transporter

The discovery of the function of the receptor for the ecotropic retrovirus as a membrane transporter for the essential amino acids lysine and arginine was a landmark finding in the field of molecular nutrition. This finding indicated that cationic amino acid transporters (CATs) act pathologically as viral receptors. The importance of this transporter was further supported by knockout mice that were not viable after birth. CAT-1 was the first amino acid transporter to be cloned; several other CATs were later characterized biochemically and molecularly. These transporters mediate the bidirectional transport of cationic amino acids, thus supporting important metabolic functions, such as synthesis of proteins, nitric oxide (NO) synthesis, polyamine biosynthesis, and interorgan amino acid flow. This review briefly describes the advances in the regulation of cationic amino acid transport, focusing on the molecular mechanisms that regulate the CAT-1 transporter. Of particular interest to this review is the regulation of CAT-1 by nutritional stresses, such as amino acid availability. The studies that are reviewed conclude that the CAT-1 gene is essential for cell survival during stress because it allows cells to resume growth as soon as amino acids become available.

Plasma membrane transporters for arginine

The supply of arginine may become rate limiting for enzymatic reactions that use this semiessential amino acid as a substrate (e.g., nitric oxide, agmatine, creatine, and urea synthesis), particularly under conditions of high demand such as growth, sepsis, or wound healing. In addition, arginine acts as a signaling molecule that regulates essential cellular functions such as protein synthesis, apoptosis, and growth. In the past decade, a number of carrier proteins for amino acids have been identified on the molecular level. They belong to different gene families, exhibit overlapping but distinctive substrate specificities, and can further be distinguished by their requirement for the cotransport or countertransport of inorganic ions. A number of these transporters function as exchangers rather than uniporters. Uptake of amino acids by these transporters therefore depends largely on the intracellular substrate composition. Hence, there is a complex crosstalk between transporters for cationic and neutral amino acids as well as for peptides. This article briefly reviews current knowledge regarding mammalian plasma membrane transporters that accept arginine as a substrate.

Isoforms of amino acid transporters in placental syncytiotrophoblast: plasma membrane localization and potential role in maternal/fetal transport

Many cell proteins exist as isoforms arising either from gene duplication or alternate RNA splicing. There is growing evidence that isoforms with different, but closely related, functional characteristics are often directed to discrete cellular locations. Thus, specialized functions may be carried out by proteins of similar evolutionary origin in different membrane compartments. In polarized epithelial cells, this mechanism allows the cell to control amino acid transport independently at each of its specialized apical and basolateral plasma membrane domains. Investigations of isoform localization in these membranes have generally been performed in epithelia other than the placental trophoblast.This review of placental amino acid transporter isoforms first provides an overview of their properties and preliminary plasma membrane localization. We then discuss studies suggesting various roles of isoform localization in trophoblast function. To provide insights into the molecular basis of this localization in trophoblast, we present a review of current knowledge of plasma membrane protein localization as derived from investigations with a widely used epithelial model cell line. Finally, we discuss a potential approach using cultured trophoblast-derived cells for studies of transporter isoform localization and function. We hope that this review will stimulate investigation of the properties of trophoblast transporter isoforms, their membrane localization and their contribution to the cellular mechanism of maternal-fetal nutrient transport.

CATs and HATs: the SLC7 family of amino acid transporters

The SLC7 family is divided into two subgroups, the cationic amino acid transporters (the CAT family, SLC7A1-4) and the glycoprotein-associated amino acid transporters (the gpaAT family, SLC7A5-11), also called light chains or catalytic chains of the hetero(di)meric amino acid transporters (HAT). The associated glycoproteins (heavy chains) 4F2hc (CD98) or rBAT (D2, NBAT) form the SLC3 family. Members of the CAT family transport essentially cationic amino acids by facilitated diffusion with differential trans-stimulation by intracellular substrates. In some cells, they may regulate the rate of NO synthesis by controlling the uptake of l-arginine as the substrate for nitric oxide synthase (NOS). The heterodimeric amino acid transporters are, in contrast, quite diverse in terms of substrate selectivity and function (mostly) as obligatory exchangers. Their selectivity ranges from large neutral amino acids (system L) to small neutral amino acids (ala, ser, cys-preferring, system asc), negatively charged amino acid (system x(c)(-)) and cationic amino acids plus neutral amino acids (system y(+)L and b(0,+)-like). Cotransport of Na(+) is observed only for the y(+)L transporters when they carry neutral amino acids. Mutations in b(0,+)-like and y(+)L transporters lead to the hereditary diseases cystinuria and lysinuric protein intolerance (LPI), respectively.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Expression of solute carrier 7A4 (SLC7A4) in the plasma membrane is not sufficient to mediate amino acid transport activity

Member 4 of human solute carrier family 7 (SLC7A4) exhibits significant sequence homology with the SLC7 subfamily of human cationic amino acid transporters (hCATs) [Sperandeo, Borsani, Incerti, Zollo, Rossi, Zuffardi, Castaldo, Taglialatela, Andria and Sebastio (1998) Genomics 49, 230-236]. It is therefore often referred to as hCAT-4 even though no convincing transport activity has been shown for this protein. We expressed SLC7A4 in Xenopus laevis oocytes, but could not detect any transport activity for cationic, neutral or anionic amino acids or for the polyamine putrescine. In addition, human glioblastoma cells stably overexpressing a fusion protein between SLC7A4 and the enhanced green fluorescent protein (EGFP) did not exhibit an increased transport activity for l-arginine. The lack of transport activity was not due to a lack of SLC7A4 protein expression in the plasma membrane, as in both cell types SLC7A4-EGFP exhibited a similar subcellular localization and level of protein expression as functional hCAT-EGFP proteins. The expression of SLC7A4 can be induced in NT2 teratocarcinoma cells by treatment with retinoic acid. However, also for this endogenously expressed SLC7A4, we could not detect any transport activity for l-arginine. Our data demonstrate that the expression of SLC7A4 in the plasma membrane is not sufficient to induce an amino acid transport activity in X. laevis oocytes or human cells. Therefore, SLC7A4 is either not an amino acid transporter or it needs additional (protein) factor(s) to be functional.

Nutrition in early life. How important is it?

While few would argue the importance of nutrition during adult life, temporary excess or deficiency has typically been thought to be of little long-term consequence. Recent data, summarized above, suggests that this may not be the case during in utero life, when alterations in the quantity or quality of nutrients provided may have life-long consequences. Perhaps even more surprisingly, decisions made in the neonatal period, such as whether to breastfeed or bottle feed, may have impacts on later health that, while small individually, have huge public health implications. Clarification of the links between adult health and fetal/neonatal nutrition are clearly required. Prospective studies, though difficult because of the time involved, will play a key role in this process, as will more basic research on the mechanisms underlying both normal and pathologic fetal development.

Expression, regulation and function of carrier proteins for cationic amino acids

Different carrier proteins exhibiting distinct transport properties participate in cationic amino acid transport. There are sodium-independent systems, such as b+, y+, y+L and b0,+, and a sodium-dependent system B0,+, most of which have now been identified at the molecular level. In most non-epithelial cells, members of the cationic amino acid transporter (CAT) family mediating system y+ activity seem to be the major entry pathway for cationic amino acids. CAT proteins underlie complex regulation at the transcriptional, post-transcriptional and activity levels. Recent evidence indicates that individual CAT isoforms are necessary for providing the substrate for nitric oxide synthesis, for example CAT-1 for Ca2+-independent nitric oxide production in endothelial cells and CAT-2B for sustained nitric oxide production in macrophages.

Differential expression of the cationic amino acid transporter 2(B) in the adult rat brain

L-Arginine is a substrate for the synthesis of proteins, nitric oxide (NO), creatine, urea, proline, glutamate, polyamines and agmatine. In the central nervous system (CNS), arginine is extracted from the blood and exchanged by cells through carriers called cationic amino acid transporters (CAT) and belonging to the so-called system y+. In order to better understand the arginine transport in the CNS, we studied in detail the regional distribution of the cells expressing the CAT2(B) transcript in the adult rat brain by non-radioisotopic in situ hybridization. We show that CAT2(B) is expressed in neurons and oligodendrocytes throughout the brain, but is not detected in astrocytes. The pattern of localization of CAT2(B) in the normal adult rat brain fits closely that of CRT1, a specific creatine transporter. Our study demonstrates that the in vivo expression of CAT2(B) differs from that reported in vitro, implying that local cellular interactions should be taken into account in studies of gene regulation of the CAT2(B) gene. Our work suggests that CAT2(B) may play a role in case of increased NO production as well as arginine or creatine deficiency in the brain.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.

Voltage dependence of L-arginine transport by hCAT-2A and hCAT-2B expressed in oocytes from Xenopus laevis

Membrane potential and currents were investigated with the two-electrode voltage-clamp technique in Xenopus laevis oocytes expressing hCAT-2A or hCAT-2B, the splice variants of the human cationic amino acid transporter hCAT-2. Both hCAT-2A- and hCAT-2B-expressing oocytes exhibited a negative extracellular L-arginine concentration ([L-Arg](o))-sensitive membrane potential, additive to the K(+) diffusion potential, when cells were incubated in Leibovitz medium (containing 1.45 mM L-Arg and 0.25 mM L-lysine). The two carrier proteins produced inward and outward currents, which were dependent on the L-Arg gradient and membrane potential. Ion substitution experiments showed that the hCAT-induced currents were independent of external Na(+), K(+), Ca(2+), or Mg(2+). The apparent Michaelis-Menten constant values at -60 mV, obtained from plots of L-Arg-induced currents against [L-Arg](o), were 0.97 and 0.13 mM in oocytes expressing hCAT-2A and hCAT-2B, respectively; maximal currents amounted to -194 +/- 8 and -84 +/- 2 nA, respectively. At saturating [L-Arg](o), the current-voltage relationships of hCAT-2A-expressing oocytes became steeper, yielding an additional conductance up to 2 microS/oocyte, whereas those of hCAT-2B-expressing oocytes were simply shifted to the right, resulting in voltage-independent difference currents. The distinct electrochemical properties of the two isoforms of hCAT-2 are assumed to contribute differentially to the membrane transport and the maintenance of cationic amino acids in various tissues.

Development and polarization of cationic amino acid transporters and regulators in the human placenta

We have investigated L-arginine transport systems in the human placental syncytiotrophoblast across gestation using purified microvillous (MVM) and basal (BM) plasma membrane vesicles. In MVM from first-trimester and term placentas, L-arginine transport was by systems y(+) and y(+)L. In BM (term placentas), however, there was evidence for system y(+)L only. The Michaelis constant of system y(+)L was significantly lower (P < 0.05) in first-trimester compared with term MVM and lower in term MVM compared with BM (P < 0.05). There was no functional evidence for system b(0+) in term MVM or BM. Cationic amino acid transporter (CAT) 1, CAT 4, and 4F2hc were detected using RT-PCR in placentas throughout gestation. rBAT was not detected in term placentas. An approximately 85-kDa and an approximately 135-kDa protein was detected by Western blotting in MVM under reducing and nonreducing conditions, respectively, consistent with the 4F2hc monomer and the 4F2hc-light chain dimer, and their expression was significantly higher (P < 0.05) in term compared with first-trimester MVM. These proteins were not detected in BM despite functional evidence for system y(+)L. These data suggest different roles for 4F2hc in the development and polarization of cationic amino acid transporters in the syncytiotrophoblast.

Nutrient transport across the placenta

The placenta forms a selective barrier that functions to transport nutrients that are of critical use to the fetus. Nutrient transport across the placenta is regulated by many different active transporters found on the surface of both maternal and fetal facing membranes of the placenta. The presence of these transporters in the placenta has been implicated in the facilitation of nutrient diffusion and proper fetal growth. In this review, recent developments concerning nutrient transporters that regulate glucose, amino acid, fatty acid, and nucleoside transplacental movement are discussed.

SLC7A7, encoding a putative permease-related protein, is mutated in patients with lysinuric protein intolerance

Lysinuric protein intolerance (LPI, MIM 222700) is an autosomal recessive multisystem disorder found mainly in Finland and Italy. On a normal diet, LPI patients present poor feeding, vomiting, diarrhoea, episodes of hyperammoniaemic coma and failure to thrive. Hepatosplenomegaly, osteoporosis and a life-threatening pulmonary involvement (alveolar proteinosis) are also seen. LPI is caused by defective cationic amino acid (CAA) transport at the basolateral membrane of epithelial cells in kidney and intestine. Metabolic derangement is characterized by increased renal excretion of CAA, reduced CAA absorption from intestine and orotic aciduria. The gene causing LPI was assigned using linkage analysis to chromosome 14q11.2 near the T-cell receptor alpha/delta chains locus, and a critical region has been defined. We have identified two new transcripts (SLC7A8 and SLC7A7) homologous to amino acid transporters, highly expressed in kidney and mapping in the LPI critical region. Mutational analysis of both transcripts revealed that SLC7A7 (for solute carrier family 7, member 7) is mutated in LPI. In five Italian patients, we found either an insertion or deletion in the coding sequence, which provides evidence of a causative role of SLC7A7 in LPI. Furthermore, we detected a splice acceptor change resulting in a frameshift and premature translation termination in four unrelated Finnish patients. This mutation may represent the founder LPI allele in Finland.

Identification of SLC7A7, encoding y+LAT-1, as the lysinuric protein intolerance gene

Lysinuric protein intolerance (LPI; OMIM 222700) is a rare, recessive disorder with a worldwide distribution, but with a high prevalence in the Finnish population; symptoms include failure to thrive, growth retardation, muscle hypotonia and hepatosplenomegaly. A defect in the plasma membrane transport of dibasic amino acids has been demonstrated at the baso-lateral membrane of epithelial cells in small intestine and in renal tubules and in plasma membrane of cultured skin fibroblasts from LPI patients. The gene causing LPI has been assigned by linkage analysis to 14q11-13. Here we report mutations in SLC7A7 cDNA (encoding y+L amino acid transporter-1, y+LAT-1), which expresses dibasic amino-acid transport activity and is located in the LPI region, in 31 Finnish LPI patients and 1 Spanish patient. The Finnish patients are homozygous for a founder missense mutation leading to a premature stop codon. The Spanish patient is a compound heterozygote with a missense mutation in one allele and a frameshift mutation in the other. The frameshift mutation generates a premature stop codon, eliminating the last one-third of the protein. The missense mutation abolishes y+LAT-1 amino-acid transport activity when co-expressed with the heavy chain of the cell-surface antigen 4F2 (4F2hc, also known as CD98) in Xenopus laevis oocytes. Our data establish that mutations in SLC7A7 cause LPI.