The molecular basis of neutral aminoacidurias

Unconventional Functions of Amino Acid Transporters: Role in Macropinocytosis (SLC38A5/SLC38A3) and Diet-Induced Obesity/Metabolic Syndrome (SLC6A19/SLC6A14/SLC6A6)

Amino acid transporters are expressed in mammalian cells not only in the plasma membrane but also in intracellular membranes. The conventional function of these transporters is to transfer their amino acid substrates across the lipid bilayer; the direction of the transfer is dictated by the combined gradients for the amino acid substrates and the co-transported ions (Na⁺, H⁺, K⁺ or Cl⁻) across the membrane. In cases of electrogenic transporters, the membrane potential also contributes to the direction of the amino acid transfer. In addition to this expected traditional function, several unconventional functions are known for some of these amino acid transporters. This includes their role in intracellular signaling, regulation of acid–base balance, and entry of viruses into cells. Such functions expand the biological roles of these transporters beyond the logical amino acid homeostasis. In recent years, two additional unconventional biochemical/metabolic processes regulated by certain amino acid transporters have come to be recognized: macropinocytosis and obesity. This adds to the repertoire of biological processes that are controlled and regulated by amino acid transporters in health and disease. In the present review, we highlight the unusual involvement of selective amino acid transporters in macropinocytosis (SLC38A5/SLC38A3) and diet-induced obesity/metabolic syndrome (SLC6A19/SLC6A14/SLC6A6).

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

A non-helical region in transmembrane helix 6 of hydrophobic amino acid transporter MhsT mediates substrate recognition

MhsT of Bacillus halodurans is a transporter of hydrophobic amino acids and a homologue of the eukaryotic SLC6 family of Na+ -dependent symporters for amino acids, neurotransmitters, osmolytes, or creatine. The broad range of transported amino acids by MhsT prompted the investigation of the substrate recognition mechanism. Here, we report six new substrate-bound structures of MhsT, which, in conjunction with functional studies, reveal how the flexibility of a Gly-Met-Gly (GMG) motif in the unwound region of transmembrane segment 6 (TM6) is central for the recognition of substrates of different size by tailoring the binding site shape and volume. MhsT mutants, harboring substitutions within the unwound GMG loop and substrate binding pocket that mimick the binding sites of eukaryotic SLC6A18/B0AT3 and SLC6A19/B0AT1 transporters of neutral amino acids, exhibited impaired transport of aromatic amino acids that require a large binding site volume. Conservation of a general (G/A/C)ΦG motif among eukaryotic members of SLC6 family suggests a role for this loop in a common mechanism for substrate recognition and translocation by SLC6 transporters of broad substrate specificity.

Amino acid transporter B(0)AT1 (slc6a19) and ancillary protein: impact on function

Amino acids play an important role in the metabolism of all organisms. Their epithelial re-absorption is due to specific transport proteins, such as B(0)AT1, a Na(+)-coupled neutral amino acid symporter belonging to the solute carrier 6 family. Here, a recently cloned fish orthologue, from the intestine of Salmo salar, was electrophysiologically characterized with the two-electrode voltage clamp technique, in Xenopus laevis oocytes heterologously expressing the transporter. Substrate specificity, apparent affinities and the ionic dependence of the transport mechanism were determined in the presence of specific collectrin. Results demonstrated that like the human, but differently from sea bass (Dicentrarchus labrax) orthologue, salmon B(0)AT1 needs to be associated with partner proteins to be correctly expressed at the oocyte plasma membrane. Cloning of sea bass collectrin and comparison of membrane expression and functionality of the B(0)AT1 orthologue transporters allowed a deeper investigation on the role of their interactions. The parameters acquired by electrophysiological and immunolocalization experiments in the mammalian and fish transporters contributed to highlight the dynamic of relations and impacts on transport function of the ancillary proteins. The comparative characterization of the physiological parameters of amino acid transporters with auxiliary proteins can help the comprehension of the regulatory mechanism of essential nutrient absorption.

Neutral aminoaciduria in cystathionine β-synthase-deficient mice, an animal model of homocystinuria

The kidney is one of the major loci for the expression of cystathionine β-synthase (CBS) and cystathionine γ-lyase (CTH). While CBS-deficient ( Cbs−/−) mice display homocysteinemia/methioninemia and severe growth retardation, and rarely survive beyond the first 4 wk, CTH-deficient ( Cth−/−) mice show homocysteinemia/cystathioninemia but develop with no apparent abnormality. This study examined renal amino acid reabsorption in those mice. Although both 2-wk-old Cbs−/− and Cth−/− mice had normal renal architecture, their serum/urinary amino acid profiles largely differed from wild-type mice. The most striking feature was marked accumulation of Met and cystathionine in serum/urine/kidney samples of Cbs−/− and Cth−/− mice, respectively. Levels of some neutral amino acids (Val, Leu, Ile, and Tyr) that were not elevated in Cbs−/− serum were highly elevated in Cbs−/− urine, and urinary excretion of other neutral amino acids (except Met) was much higher than expected from their serum levels, demonstrating neutral aminoaciduria in Cbs−/− (not Cth−/−) mice. Because the bulk of neutral amino acids is absorbed via a B0AT1 transporter and Met has the highest substrate affinity for B0AT1 than other neutral amino acids, hypermethioninemia may cause hyperexcretion of neutral amino acids.

Functional properties of a newly cloned fish ortholog of the neutral amino acid transporter B0AT1 (SLC6A19)

The functional properties of an ortholog of the B(0)AT1 (SLC6A19) amino acid transporter, cloned from the intestine of the sea bass Dicentrachus labrax, were investigated. The two-electrode voltage-clamp technique was applied to Xenopus laevis oocytes heterologously expressing the transporter in order to measure the currents associated with the transport process in different conditions. In particular the substrate specificity, the ionic requirements, and possible effects of pH were examined. Among the organic substrates, leucine, glycine, serine and valine generated the largest transport currents with apparent affinities in the lower millimolar range. The importance of Na(+) as the driver ion in the transport process is confirmed, although Li(+) is also capable to sustain transport, while K(+) is not. No evidence of a relevant role of Cl(-) in the transport activity was found. Concerning the other two kinds of currents commonly found in electrogenic transporters, very fast pre-steady-state currents were detected in the absence of organic substrate, while lithium-specific leak currents were not observed. The comparison of these properties with those of the mammalian and insect orthologs may give interesting indication for future structure-function studies in this transporter subfamily.

Over-expression in E. coli and purification of the human OCTN2 transport protein

The OCTN2 cDNA amplified from human skin fibroblast was cloned in pET-41a(+) carrying the glutathione S-transferase (GST) gene. The construct pET-41a(+)-hOCTN2 was used to express the GST-hOCTN2 fusion protein in Escherichia coli Rosetta(DE3)pLysS. The best over-expression was obtained after 6 h of induction with IPTG at 28°C. The GST-hOCTN2 polypeptide was collected in the inclusion bodies and showed an apparent molecular mass on SDS-PAGE of 85 kDa. After solubilization with a buffer containing 0.8% sarkosyl and 3 M urea, the fusion protein was applied onto a Ni(2+)-chelating chromatography column. The purified GST-hOCTN2 was treated with thrombin, and the hOCTN2 was separated from the GST by size exclusion chromatography. After the whole procedure, a yield of about 0.2 mg purified protein per liter of cell culture was obtained. To improve the protein yield, hOCTN2 cDNA was subjected to codon bias. The second codon CGG was substituted with AAA; the substitution led to the mutation R2K in the hOCTN2 protein. hOCTN2(R2K) cDNA was cloned in pET-21a(+) carrying a C-terminal 6His tag. The resulting protein was expressed in E. coli Rosetta(DE3)pLysS and purified by Ni(2+)-chelating chromatography. A yield of about 3.5 mg purified protein per liter of cell culture was obtained with this procedure.

Over-expression in Escherichia coli, purification and reconstitution in liposomes of the third member of the OCTN sub-family: the mouse carnitine transporter OCTN3

pET-21a(+)-mOCTN3-6His was constructed and used for over-expression in Escherichia coli Rosetta(DE3)pLysS. After IPTG induction a protein with apparent molecular mass of 53 kDa was collected in the insoluble fraction of the cell lysate and purified by Ni(2+)-chelating chromatography with a yield of 2mg/l of cell culture. The over-expressed protein was identified with mOCTN3 by anti-His antibody and reconstitution in liposomes. mOCTN3 required peculiar conditions for optimal expression and reconstitution in liposomes. The protein catalyzed a time dependent [(3)H]carnitine uptake which was stimulated by intraliposomal ATP and nearly independent of the pH. The K(m) for carnitine was 36 μM. [(3)H]carnitine transport was inhibited by carnitine analogues and some Cys and NH(2) reagents. This paper represents the first outcome in over-expressing, in active form, the third member of the OCTN sub-family, mOCTN3, in E. coli.

Molecular basis of essential amino acid transport from studies of insect nutrient amino acid transporters of the SLC6 family (NAT-SLC6)

Two protein families that represent major components of essential amino acid transport in insects have been identified. They are annotated as the SLC6 and SLC7 families of transporters according to phylogenetic proximity to characterized amino acid transporters (HUGO nomenclature). Members of these families have been identified as important apical and basolateral parts of transepithelial essential amino acid absorption in the metazoan alimentary canal. Synergistically, they play critical physiological roles as essential substrate providers to diverse metabolic processes, including generic protein synthesis. This review briefly clarifies the requirements for amino acid transport and a variety of amino acid transport mechanisms, including the aforementioned families. Further it focuses on the large group of Nutrient Amino acid Transporters (NATs), which comprise a recently identified subfamily of the Neurotransmitter Sodium Symporter family (NSS or SLC6). The first insect NAT, cloned from the caterpillar gut, has a broad substrate spectrum similar to mammalian B(0) transporters. Several new NAT-SLC6 members have been characterized in an effort to explore mechanisms for the essential amino acid absorption in model dipteran insects. The identification and functional characterization of new B(0)-like and narrow specificity transporters of essential amino acids in fruit fly and mosquitoes leads to a fundamentally important insight: that NATs evolved and act together as the integrated active core of a transport network that mediates active alimentary absorption and systemic distribution of essential amino acids. This role of NATs is projected from the most primitive prokaryotes to the most complex metazoan organisms, and represents an interesting platform for unraveling the molecular evolution of amino acid transport and modeling amino acid transport disorders. The comparative study of NATs elucidates important adaptive differences between essential amino acid transportomes of invertebrate and vertebrate organisms, outlining a new possibility for selective targeting of essential amino acid absorption mechanisms to control medically and economically important arthropods and other invertebrate organisms.

Reconstitution in liposomes of the functionally active human OCTN1 (SLC22A4) transporter overexpressed in Escherichia coli

The hOCTN1 (human organic cation transporter 1) overexpressed in Escherichia coli and purified by Ni-chelating chromatography has been reconstituted in liposomes by detergent removal with a batch-wise procedure. The reconstitution was optimized with respect to the protein concentration, the detergent/phospholipid ratio and the time of incubation with Amberlite XAD-4 resin. Time-dependent [14C]tetraethylammonium, [3H]carnitine or [3H]ergothioneine uptake was measured in proteoliposomes with activities ratios of 8:1.3:1 respectively. Optimal activity was found at pH 8.0. The transport depended on intraliposomal ATP. [14C]tetraethylammonium transport was inhibited by several compounds. The most effective were acetyl-choline and γ-butyrobetaine, followed by acetylcarnitine and tetramethylammonium. Reagents such as pyridoxal 5-phosphate, MTSES [sodium (2-sulfonatoethyl) methanethiosulfonate] and mercurials strongly inhibited the transport. From kinetic analysis of tetraethylammonium transport a Km of 0.77 mM was calculated. Acetylcholine and γ-butyrobetaine behaved as competitive inhibitors of TEA (tetraethylammonium) transport with Ki values of 0.44 and 0.63 mM respectively.

Metabolomics reveals attenuation of the SLC6A20 kidney transporter in nonhuman primate and mouse models of type 2 diabetes mellitus

To enhance understanding of the metabolic indicators of type 2 diabetes mellitus (T2DM) disease pathogenesis and progression, the urinary metabolomes of well characterized rhesus macaques (normal or spontaneously and naturally diabetic) were examined. High-resolution ultra-performance liquid chromatography coupled with the accurate mass determination of time-of-flight mass spectrometry was used to analyze spot urine samples from normal (n = 10) and T2DM (n = 11) male monkeys. The machine-learning algorithm random forests classified urine samples as either from normal or T2DM monkeys. The metabolites important for developing the classifier were further examined for their biological significance. Random forests models had a misclassification error of less than 5%. Metabolites were identified based on accurate masses (<10 ppm) and confirmed by tandem mass spectrometry of authentic compounds. Urinary compounds significantly increased (p < 0.05) in the T2DM when compared with the normal group included glycine betaine (9-fold), citric acid (2.8-fold), kynurenic acid (1.8-fold), glucose (68-fold), and pipecolic acid (6.5-fold). When compared with the conventional definition of T2DM, the metabolites were also useful in defining the T2DM condition, and the urinary elevations in glycine betaine and pipecolic acid (as well as proline) indicated defective re-absorption in the kidney proximal tubules by SLC6A20, a Na(+)-dependent transporter. The mRNA levels of SLC6A20 were significantly reduced in the kidneys of monkeys with T2DM. These observations were validated in the db/db mouse model of T2DM. This study provides convincing evidence of the power of metabolomics for identifying functional changes at many levels in the omics pipeline.

SLC36A4 (hPAT4) is a high affinity amino acid transporter when expressed in Xenopus laevis oocytes

The SLC36 family of transporters consists of four genes, two of which, SLC36A1 and SLC36A2, have been demonstrated to code for human proton-coupled amino acid transporters or hPATs. Here we report the characterization of the fourth member of the family, SLC36A4 or hPAT4, which when expressed in Xenopus laevis oocytes also encodes a plasma membrane amino acid transporter, but one that is not proton-coupled and has a very high substrate affinity for the amino acids proline and tryptophan. hPAT4 in Xenopus oocytes mediated sodium-independent, electroneutral uptake of [(3)H]proline, with the highest rate of uptake when the uptake medium pH was 7.4 and an affinity of 3.13 μM. Tryptophan was also an excellently transported substrate with a similarly high affinity (1.72 μM). Other amino acids that inhibited [(3)H]proline were isoleucine (K(i) 0.23 mM), glutamine (0.43 mM), methionine (0.44 mM), and alanine (1.48 mM), and with lower affinity, glycine, threonine, and cysteine (K(i) >5 mM for all). Of the amino acids directly tested for transport, only proline, tryptophan, and alanine showed significant uptake, whereas glycine and cysteine did not. Of the non-proteogenic amino acids and drugs tested, only sarcosine produced inhibition (K(i) 1.09 mM), whereas γ-aminobutyric acid (GABA), β-alanine, L-Dopa, D-serine, and δ-aminolevulinic acid were without effect on [(3)H]proline uptake. This characterization of hPAT4 as a very high affinity/low capacity non-proton-coupled amino acid transporter raises questions about its physiological role, especially as the transport characteristics of hPAT4 are very similar to the Drosophila orthologue PATH, an amino acid "transceptor" that plays a role in nutrient sensing.

Pharmaceutical and pharmacological importance of peptide transporters

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many β-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

Evaluation of the proximal tubular function in individuals with primary renal hypouricemia: an NMR-based metabonomic study

Primary renal hypouricemia (PRH) refers to a rare condition of increased renal urate clearance, caused by an isolated inborn error of membrane transport of urate in the renal proximal tubule. Several cases of exercise-induced acute renal failure and urolithiasis have been reported. This is the first study that assessed tubular function in PRH using NMR-based metabonomic urine analysis. The study groups consisted of 36 unrelated asymptomatic subjects with PRH, defined as serum uric acid levels (sUA) <2.5 mg/dL and fractional excretion of uric acid (FEUA) >10%, after exclusion of diseases and drugs that may affect urate homeostasis, and 39 sex and age-matched healthy individuals with normal sUA levels (>4.0 mmol/L) and FEUA<10%. Individuals with primary hypouricemia presented similar biochemical profiles to the controls without significant differences with regard to FE of electrolytes and renal threshold for phosphate excretion. Individuals with primary hypouricemia were differentiated from healthy individuals in the orthogonal signal correction/partial least-squares-discriminant analysis models of the NMR data with a statistically significant separation. The components that contributed to this separation were the lower levels of hippurate, creatinine, and trimethylaminoxide, and the higher levels of phenylalanine, alanine, glycine, glutamate, acetate, and of an unidentified metabolite (3.3 ppm) observed in hypouricemic subjects compared with controls. Primary hypouricemia, though considered an isolated renal tubular defect, is often associated with a more generalized proximal tubular disorder that mimics a partial Fanconi syndrome.

Mental retardation and inborn errors of metabolism

In countries where clinical phenylketonuria is detected by newborn screening inborn errors of metabolism are rare causes of isolated mental retardation. There is no international agreement about what type of metabolic tests must be applied in patients with unspecific mental retardation. However, and although infrequent, there are a number of inborn errors of metabolism that can present in this way. Because of the high recurrence risk and the possibility of specific therapies, guidelines need to be developed and adapted to different populations. The application of a universal protocol may result in a low diagnostic performance in individual ethnic populations. Consideration of associated signs (extraneurological manifestations, psychiatric signs, autistic traits, cerebellar dysfunction, epilepsy or dysmorphic traits) greatly improves the diagnostic fulfilment.

Over-expression in E. coli and purification of the human OCTN1 transport protein

The hOCTN1 amplified from skin fibroblast RNA was cloned in pET-28a(+) or in pH6EX3 plasmid. The encoded recombinant hOCTN1 resulted in a 6-His tagged fusion protein with a 34 or 21 amino acid extra N-terminal sequence in the pET-28a(+)-hOCTN1 or in the pH6EX3-hOCTN1 constructs, respectively. Both constructs were used to express the hOCTN1 in Escherichia coli Rosetta(DE3)pLysS. The best over-expression was obtained with the pH6EX3-hOCTN1 after 6h of induction with IPTG at 28 degrees C. The expressed protein with an apparent molecular mass of 54kDa, was collected in the insoluble fraction of the cell lysate. Further improvement was obtained using the E. coli RosettaGami2(DE3)pLysS strain to express the protein encoded by pH6EX3-hOCTN1. After 6h of induction with IPTG at 28 degrees C, hOCTN1 accounted for 30% of the total protein in the insoluble pellet. This protein fraction was washed with Triton X-100 and deoxycholate, solubilized with a buffer containing 0.8% Sarkosyl, 3M urea and applied to a Ni2+-chelating chromatography column. The homogeneously purified hOCTN1 was eluted with a buffer containing 50 mM imidazole, 0.1% Triton X-100 and 50 mM 2-mercaptoethanol. A yield of about 3mg purified protein per liter of cell culture was obtained.

Metabonomics and population studies: age-related amino acids excretion and inferring networks through the study of urine samples in two Italian isolated populations

The study of two different Italian isolated populations was combined with a metabonomic approach to better understand tubular handling of amino acids. Levels of amino acids and metabolites have been analyzed by Nucleic Magnetic Resonance and expressed as ratio vs urinary creatinine concentration (mmol/mol). For most of the amino acids there is an age-related U shape pattern of excretion, with the peaks during childhood and old age, and a significant reduction in the adult age. Hierarchical cluster analysis has clearly identified three groups clustering the same amino acids: His, Thr and Ala (group one); Gly and Phe (group two) and a third larger one. Results have been further confirmed by factor and regression analysis, and used to confirm and, in some cases, infer new amino acids networks. As a matter of facts, the identification of strong evidences for clustering of urine excretion of several neutral amino acids suggests the predominant impact of relevant and common transporters.

Synergy and specificity of two Na+-aromatic amino acid symporters in the model alimentary canal of mosquito larvae

The nutrient amino acid transporter (NAT) subfamily is the largest subdivision of the sodium neurotransmitter symporter family (SNF; also known as SLC6; HUGO). There are seven members of the NAT population in the African malaria mosquito Anopheles gambiae, two of which, AgNAT6 and AgNAT8, preferably transport indole- and phenyl-branched substrates, respectively. The relative expression and distribution of these aromatic NATs were examined with transporter-specific antibodies in Xenopus oocytes and mosquito larval alimentary canal, representing heterologous and tissue expression systems, respectively. NAT-specific aromatic-substrate-induced currents strongly corresponded with specific accumulation of both transporters in the plasma membrane of oocytes. Immunolabeling revealed elevated expressions of both transporters in specific regions of the larval alimentary canal, including salivary glands, cardia, gastric caeca, posterior midgut and Malpighian tubules. Differences in relative expression densities and spatial distribution of the transporters were prominent in virtually all of these regions, suggesting unique profiles of the aromatic amino acid absorption. For the first time reversal of the location of a transporter between apical and basal membranes was identified in posterior and anterior epithelial domains corresponding with secretory and absorptive epithelial functions, respectively. Both aromatic NATs formed putative homodimers in the larval gut whereas functional monomers were over-expressed heterologously in Xenopus oocytes. The results unequivocally suggest functional synergy between substrate-specific AgNAT6 and AgNAT8 in intracellular absorption of aromatic amino acids. More broadly, they suggest that the specific selectivity, regional expression and polarized membrane docking of NATs represent key adaptive traits shaping functional patterns of essential amino acid absorption in the metazoan alimentary canal and other tissues.

Apical transporters for neutral amino acids: physiology and pathophysiology

Absorption of amino acids in kidney and intestine involves a variety of transporters for different groups of amino acids. This is illustrated by inherited disorders of amino acid absorption, such as Hartnup disorder, cystinuria, iminoglycinuria, dicarboxylic aminoaciduria, and lysinuric protein intolerance, affecting separate groups of amino acids. Recent advances in the molecular identification of apical neutral amino acid transporters has shed a light on the molecular basis of Hartnup disorder and iminoglycinuria.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Aminoacidurias: Clinical and molecular aspects

Inherited aminoacidurias are caused by defective amino-acid transport through renal (reabsorption) and in many cases also small intestinal epithelia (absorption). Recently, many of the genes causing this abnormal transport have been molecularly identified. In this review, we summarize the latest findings in the clinical and molecular aspects concerning the principal aminoacidurias, cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. Signs, symptoms, diagnosis, treatment, causative or candidate genes, functional characterization of the encoded transporters, and animal models are discussed.

Organ specific underexpression renal of Na+-dependent B0AT1 in the SHR correlates positively with overexpression of NHE3 and salt intake

The present study examines the renal and intestinal expression of Na(+)-dependent amino acid transporter B(0)AT1 during the development of hypertension in the spontaneous hypertensive rats (SHR) and its normotensive control (Wistar-Kyoto rat; WKY), and evaluates whether the expression of renal B(0)AT1 correlates with changes in the expression of Na(+) transporters, type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-K(+)-ATPase, known to occur in the SHR. The effect of high salt (HS) intake on the expression of renal and intestinal B(0)AT1 transcript abundance was also evaluated. For this purpose, the cloning of rat homolog of B(0)AT1 was performed. Rat B(0)AT1 shows high sequence homology to the mouse ortholog. Renal B(0)AT1 transcript abundance was lower in SHR than WKY at both 4 and 12 weeks of age. No significant differences between strains were observed in terms of intestinal expression of B(0)AT1. The decreased B(0)AT1 expression in SHR kidney was accompanied with an increase in NHE3 expression, suggesting an impaired Na(+) uptake. HS intake decreased renal B(0)AT1 mRNA in SHR and WKY at 4 weeks of age. In 12-week-old SHR, HS intake increased renal B(0)AT1 transcript abundance. Intestinal B(0)AT1 transcript was significantly increased by HS intake, though the effect was considerably more pronounced in the SHR. It is concluded, that underexpression of B(0)AT1 in the SHR kidney is organ specific, precedes the onset of hypertension and correlates negatively with the renal tubular transport of Na(+). The regulation of B(0)AT1 gene transcription appears to be under the influence of Na(+) delivery, being organ specific.

Underexpression of the Na+-dependent neutral amino acid transporter ASCT2 in the spontaneously hypertensive rat kidney

This study examined the inward transport of l-[14C]alanine, an ASCT2 preferential substrate, in monolayers of immortalized renal proximal tubular epithelial (PTE) cells from Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. The expression of ASCT2 in WKY and SHR PTE cells and kidney cortices from WKY and SHR was also evaluated. l-[14C]alanine uptake was highly dependent on extracellular Na+. Replacement of NaCl by LiCl or choline chloride abolished transport activity in SHR and WKY PTE cells. In the presence of the system L inhibitor BCH, Na+-dependent l-alanine uptake in WKY and SHR PTE cells was inhibited by alanine, serine, and cysteine, which is consistent with amino acid transport through ASCT2. The saturable component of Na+-dependent l-alanine transport under Vmax conditions in SHR PTE cells was one-half of that in WKY PTE cells, with similar Km values. Differences in magnitude of Na+-dependent l-alanine uptake through ASCT2 between WKY and SHR PTE cells correlated positively with differences in ASCT2 protein expression, this being more abundant in WKY PTE cells. Abundance of ASCT2 transcript and protein in kidney cortices of SHR rats was also lower than that in normotensive WKY rats. In conclusion, immortalized SHR and WKY PTE cells take up l-alanine mainly through a high-affinity Na+-dependent amino acid transporter, with functional features of ASCT2 transport. The activity and expression of the ASCT2 transporter were considerably lower in the SHR cells.

High-salt intake and the renal expression of amino acid transporters in spontaneously hypertensive rats

This study evaluated in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) the response to salt loading of the renal dopaminergic system and transcript abundance of Na+-independent (LAT1 and LAT2) and Na+-dependent (ASCT2 and B0AT1) amino acid transporters potentially involved in renal tubular uptake of l-DOPA. Rats were fed normal (NS)- or high (HS; 1% saline as drinking water)-salt intake for 24 h. Transcript abundance of amino acid transporters was age dependent, differently regulated in WKY and SHR and responded differently to salt intake. HS intake similarly increased urinary dopamine in 4-wk-old SHR and WKY. At 12 wk of age, HS intake increased urinary dopamine in SHR, but not in WKY. Changes in urinary dopamine paralleled changes in the uptake of l-DOPA in isolated renal tubules from 4- and 12-wk-old WKY and SHR on NS and HS intake. At 12 wk of age, HS intake was accompanied by decreases in LAT1 and LAT2 transcript abundance in WKY and SHR. ASCT2 and B0AT1 expression was significantly decreased in both 4- and 12-wk-old WKY and in 4-wk-old SHR on HS intake. By contrast, HS intake increased ASCT2 and B0AT1 expression in 12-wk-old SHR. It is concluded that salt-sensitive mechanisms influence LAT1, LAT2, ASCT2, and B0AT1 gene transcription. Differences in urinary dopamine and tubular uptake of l-DOPA between WKY and SHR during HS intake, namely in 12-wk-old animals, may result from increases in the ASCT2 and B0AT1 mRNA levels and less pronounced decreases in LAT2 expression.

Molecular characterization of the first aromatic nutrient transporter from the sodium neurotransmitter symporter family

Nutrient amino acid transporters (NATs, subfamily of sodium neurotransmitter symporter family SNF, a.k.a. SLC6) represent a set of phylogenetically and functionally related transport proteins, which perform intracellular absorption of neutral, predominantly essential amino acids. Functions of NATs appear to be critical for the development and survival in organisms. However, mechanisms of specific and synergetic action of various NAT members in the amino acid transport network are virtually unexplored. A new transporter, agNAT8, was cloned from the malaria vector mosquito Anopheles gambiae (SS). Upon heterologous expression in Xenopus oocytes it performs high-capacity, sodium-coupled (2:1)uptake of nutrients with a strong preference for aromatic catechol-branched substrates, especially phenylalanine and its derivatives tyrosine and L-DOPA,but not catecholamines. It represents a previously unknown SNF phenotype, and also appears to be the first sodium-dependent B0 type transporter with a narrow selectivity for essential precursors of catecholamine synthesis pathways. It is strongly and specifically transcribed in absorptive and secretory parts of the larval alimentary canal and specific populations of central and peripheral neurons of visual-, chemo- and mechano-sensory afferents. We have identified a new SNF transporter with previously unknown phenotype and showed its important role in the accumulation and redistribution of aromatic substrates. Our results strongly suggest that agNAT8 is an important, if not the major, provider of an essential catechol group in the synthesis of catecholamines for neurochemical signaling as well as ecdysozoan melanization and sclerotization pathways, which may include cuticle hardening/coloring, wound curing, oogenesis, immune responses and melanization of pathogens.

Protection of renal tubular cells against the cytotoxicity of cadmium by glycine

Glycine treatment is reported to protect against the nephrotoxicity of cadmium (Cd) in rats. The purpose of the present study was to explore the mechanism of this protection using a renal epithelial cell line, LLC-PK(1). The cells were incubated with 10-30 microM Cd in serum-free DMEM and cytotoxicity was evaluated by LDH leakage into the incubation medium. Under these conditions, 20 and 30 microM Cd concentrations were cytotoxic. As compared to the non-Cd-exposed cells, the LDH release was elevated more than six-fold in cells exposed to 30 microM Cd for 24h. Co-treatment with 5-50mM glycine was cytoprotective in a concentration-dependent manner. Prior treatment with 50 mM glycine for 16 h, or co-treatment for 24h, reduced LDH leakage due to 30 microM Cd exposure by 60 and 66%, respectively. Co-incubation with 50 mM alanine was also protective but only about half as effective as with glycine. During the first 4h, prior to the onset of any significant cell membrane damage, the Cd-exposed cells accumulated 0.55 microg Cd/mg protein. Glycine pre-treatment or co-treatment reduced Cd accumulation by about one-quarter or one-half, respectively. To delineate the mechanism of glycine's effect on Cd accumulation, the efflux of Cd was studied after a 30 min pulse exposure. The results suggested that pre-treatment reduced Cd accumulation by increasing its efflux from the cells. In contrast, co-treatment reduced Cd efflux, suggesting that the co-treatment lowered Cd accumulation by suppressing its uptake. When co-incubated, Cd and glycine formed a complex that was apparently responsible for the marked reduction in Cd uptake. It is concluded that, regardless of the mode of treatment, glycine is cytoprotective against Cd and that it may do so by lowering the intracellular Cd burden.

Transport of L-proline, L-proline-containing peptides and related drugs at mammalian epithelial cell membranes

Membrane transport of L-proline has received considerable attention in basic and pharmaceutical research recently. Of the most recently cloned members of the solute carrier family, two are "proline transporters". The amino acid transporter PAT1, expressed in intestine, kidney, brain and other organs, mediates the uptake of proline and derivatives in a pH gradient-dependent manner. The Na(+)-dependent proline transporter SIT1, cloned in 2005, exhibits the properties of the long-sought classical IMINO system. Proline-containing peptides are of interest for several reasons. Many biologically important peptide sequences contain highly conserved proline residues. Xaa-Pro peptides are very often resistant to enzymatic hydrolysis and display, in contrast to Pro-Xaa peptides, a high affinity to the H(+)/peptide cotransporter PEPT1 which is expressed in intestinal, renal, lung and biliary duct epithelial cells. Furthermore, several orally available drugs are recognized by PEPT1 as Xaa-Pro analogues due to their sterical resemblance to small peptides.

Na+ and pH dependence of proline and beta-alanine absorption in rat small intestine

AIMS

Early characterization of intestinal absorption of imino acids in mammals has demonstrated the existence of a Na+-dependent, Cl- -independent transport system in rat small intestine, which is the only carrier for beta-alanine. Based on the substrate selectivity, it was proposed that the Proton Amino Acid Transporter 1 (PAT1) could be the same as this imino acid carrier. The present study characterizes the pH and Na+ dependence of proline and beta-alanine uptake in rat small intestine.

METHODS

Intestinal uptake of radiolabelled l-proline or beta-alanine was measured in brush border membrane vesicles and everted intestinal rings, in the presence and absence of Na+ and at different pH values.

RESULTS

The existence of an inwardly directed H+ gradient in the absence of Na+ enhanced the initial entry of proline and beta-alanine in brush border membrane vesicles, that reached a transient overshoot with maximal value around 30 s. In the absence of pH gradient, no overshoot was shown. In entire tissue, there was an increase of proline and beta-alanine uptake at acidic pH that was higher in the presence of Na+ than in its absence. This ion dependence and pH effect of the amino acids uptake also increased with the incubation period. Substrate inhibition studies confirmed that intestinal proline absorption in rat occurs mainly by system B and PAT1-like transporter.

CONCLUSIONS

There is a Na+ -independent, H+ -dependent transporter of amino acids at the apical membrane of the rat enterocytes.

The SLC6 orphans are forming a family of amino acid transporters

Transporters in the human genome are grouped in solute carrier families (SLC). The SLC6 family is one of the biggest transporter families in the human genome comprising 20 members. It is usually referred to as the neurotransmitter transporter family because its founding members encode transporters for the neurotransmitters GABA, noradrenaline, serotonin and dopamine. The family also includes a number of 'orphan' transporters, the function of which has remained elusive until recently. Identification of the broadly specific neutral amino acid transporter SLC6A19 (also called B(0)AT1) suggested that all orphan transporters may in fact be amino acid transporters. This was subsequently confirmed by the identification of SLC6A20 as the long-sought IMINO system, a proline transporter found in kidney, intestine and brain. Very recently, SLC6A15 was identified as the neutral amino acid transporter B(0)AT2. All amino acid transporters appear to cotransport only 1Na(+) together with the amino acid substrate. Both, B(0)AT1 and B(0)AT2 are chloride independent, whereas IMINO is chloride dependent. The amino acid transporters of the SLC6 family are functionally and sequence related to the recently crystallized leucine transporter from Aquifex aeolicus. The structure elegantly explains many of the mechanistic features of the SLC6 amino acid transporters.