Cloning of a rat kidney cDNA that stimulates dibasic and neutral amino acid transport and has sequence similarity to glucosidases

Absorption of methionine sources in animals-is there more to know?

This literature review evaluates the absorption of methionine (Met) sources such as 2-hydroxy-4-methylthiobutyric acid (HMTBa), its calcium salts (HMTBa-Ca), and DL-methionine (DL-Met) by focusing on the state of knowledge regarding the absorption mechanism, experimental methodology, and factors affecting their absorption. The 2 Met sources differ in mechanism and site of absorption due to differences in their chemical characteristics and enzymatic conversion. This review addresses diffusion- and transport-mediated absorption systems for amino acids and carboxylic compounds, best elucidated by in vitro, ex vivo, and in vivo experimental models. Opportunities and limitations in the use of radioisotopes to depict absorption sites as well as host and microbial metabolism are described. Physiological and environmental conditions that lead to changes in gut absorptive capacity and the impact of Met source absorption are also evaluated. This review concludes that any comparison between HMTBa and DL-Met should consider their different behaviors during the absorption phase. Hence, the chemical characteristics of these 2 molecules entail different absorption sites and mechanisms, from passive absorption in the case of HMTBa and HMTBa-Ca to active transporters for DL-Met, HMTBa, and HMTBa-Ca. In addition, the different conversion modes of these 2 molecules further differentiate their absorption modes. Considering these important differences, it is easier to understand the apparent divergence between the conclusions of existing publications. When comparing these 2 molecules, it is recommended to properly adapt to the conditions under which the absorption of Met sources is evaluated.

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.

Transport of L-Arginine Related Cardiovascular Risk Markers

L-arginine and its derivatives, asymmetric and symmetric dimethylarginine (ADMA and SDMA) and L-homoarginine, have emerged as cardiovascular biomarkers linked to cardiovascular outcomes and various metabolic and functional pathways such as NO-mediated endothelial function. Cellular uptake and efflux of L-arginine and its derivatives are facilitated by transport proteins. In this respect the cationic amino acid transporters CAT1 and CAT2 (SLC7A1 and SLC7A2) and the system y⁺L amino acid transporters (SLC7A6 and SLC7A7) have been most extensively investigated, so far, but the number of transporters shown to mediate the transport of L-arginine and its derivatives is constantly increasing. In the present review we assess the growing body of evidence regarding the function, expression, and clinical relevance of these transporters and their possible relation to cardiovascular diseases.

Sodium-coupled glucose transport, the SLC5 family, and therapeutically relevant inhibitors: from molecular discovery to clinical application

Sodium glucose transporters (SGLTs) belong to the mammalian solute carrier family SLC5. This family includes 12 different members in human that mediate the transport of sugars, vitamins, amino acids, or smaller organic ions such as choline. The SLC5 family belongs to the sodium symporter family (SSS), which encompasses transporters from all kingdoms of life. It furthermore shares similarity to the structural fold of the APC (amino acid-polyamine-organocation) transporter family. Three decades after the first molecular identification of the intestinal Na+-glucose cotransporter SGLT1 by expression cloning, many new discoveries have evolved, from mechanistic analysis to molecular genetics, structural biology, drug discovery, and clinical applications. All of these advances have greatly influenced physiology and medicine. While SGLT1 is essential for fast absorption of glucose and galactose in the intestine, the expression of SGLT2 is largely confined to the early part of the kidney proximal tubules, where it reabsorbs the bulk part of filtered glucose. SGLT2 has been successfully exploited by the pharmaceutical industry to develop effective new drugs for the treatment of diabetic patients. These SGLT2 inhibitors, termed gliflozins, also exhibit favorable nephroprotective effects and likely also cardioprotective effects. In addition, given the recent finding that SGLT2 is also expressed in tumors of pancreas and prostate and in glioblastoma, this opens the door to potential new therapeutic strategies for cancer treatment by specifically targeting SGLT2. Likewise, further discoveries related to the functional association of other SGLTs of the SLC5 family to human pathologies will open the door to potential new therapeutic strategies. We furthermore hope that the herein summarized information about the physiological roles of SGLTs and the therapeutic benefits of the gliflozins will be useful for our readers to better understand the molecular basis of the beneficial effects of these inhibitors, also in the context of the tubuloglomerular feedback (TGF), and the renin-angiotensin system (RAS). The detailed mechanisms underlying the clinical benefits of SGLT2 inhibition by gliflozins still warrant further investigation that may serve as a basis for future drug development.

Dietary Supplementation of dl-Methionine Potently Induces Sodium-Dependent l-Methionine Absorption in Porcine Jejunum Ex Vivo

BACKGROUND

Methionine is an essential amino acid (AA) with many fundamental roles. Humans often supplement l-Met, whereas dl-Met and dl-2-hydroxy-4-(methylthio)butanoic acid (dl-HMTBA) are more frequently used to supplement livestock.

OBJECTIVES

The study aimed to investigate whether dietary Met source alters the absorptive capacity for Met isomers in the small intestine of piglets.

METHODS

A total of 27 male 10-wk-old piglets in 3 feeding groups received a diet supplemented with 0.21% dl-Met, 0.21% l-Met, or 0.31% dl-HMTBA to meet the Met + cystine requirement. After ≥10 d, absorptive fluxes of d-Met or l-Met were measured at a physiological concentration of 50 μM and a high concentration of 5 mM in duodenum, middle jejunum, and ileum ex vivo. Data were compared by 2-factor ANOVA.

RESULTS

Across diets, fluxes of both Met isomers at both tested concentrations increased from duodenum to ileum by a factor of ∼2-5.5 (P < 0.05). Pigs supplemented with dl-Met had greater (P < 0.085) absorptive fluxes at 50 μM l-Met (0.50, 2.07, and 3.86 nmol · cm-2 · h-1) and d-Met (0.62, 1.41, and 1.19 nmol · cm-2 · h-1) than did pigs supplemented with dl-HMTBA (l-Met: 0.28, 0.76, and 1.08 nmol · cm-2 · h-1; d-Met: 0.34, 0.58, and 0.64 nmol · cm-2 · h-1) in duodenum, jejunum, and ileum, respectively. Only in jejunum of dl-Met-fed pigs, fluxes at 50 μM l-Met were reduced by the omission of luminal Na+ (from 3.27 to 0.86 nmol · cm-2 · h-1; P < 0.05) and by a cocktail of 22 luminal AAs (to 1.05 nmol · cm-2 · h-1; P < 0.05).

CONCLUSIONS

Dietary supplementation of dl-Met increases the efficiency of l-Met and d-Met absorption at physiologically relevant luminal Met concentrations along the small intestine of pigs, including a very prominent induction of an Na+-dependent transport system with preference for l-Met in the mid-jejunum. Dietary supplementation with dl-Met could be a promising tool to improve the absorption of Met and other AAs.

Amino Acid Transport Across the Mammalian Intestine

The small intestine mediates the absorption of amino acids after ingestion of protein and sustains the supply of amino acids to all tissues. The small intestine is an important contributor to plasma amino acid homeostasis, while amino acid transport in the large intestine is more relevant for bacterial metabolites and fluid secretion. A number of rare inherited disorders have contributed to the identification of amino acid transporters in epithelial cells of the small intestine, in particular cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. These are most readily detected by analysis of urine amino acids, but typically also affect intestinal transport. The genes underlying these disorders have all been identified. The remaining transporters were identified through molecular cloning techniques to the extent that a comprehensive portrait of functional cooperation among transporters of intestinal epithelial cells is now available for both the basolateral and apical membranes. Mouse models of most intestinal transporters illustrate their contribution to amino acid homeostasis and systemic physiology. Intestinal amino acid transport activities can vary between species, but these can now be explained as differences of amino acid transporter distribution along the intestine. © 2019 American Physiological Society. Compr Physiol 9:343-373, 2019.

Remarkable evolutionary relatedness among the enzymes and proteins from the α-amylase family

The α-amylase is a ubiquitous starch hydrolase catalyzing the cleavage of the α-1,4-glucosidic bonds in an endo-fashion. Various α-amylases originating from different taxonomic sources may differ from each other significantly in their exact substrate preference and product profile. Moreover, it also seems to be clear that at least two different amino acid sequences utilizing two different catalytic machineries have evolved to execute the same α-amylolytic specificity. The two have been classified in the Cabohydrate-Active enZyme database, the CAZy, in the glycoside hydrolase (GH) families GH13 and GH57. While the former and the larger α-amylase family GH13 evidently forms the clan GH-H with the families GH70 and GH77, the latter and the smaller α-amylase family GH57 has only been predicted to maybe define a future clan with the family GH119. Sequences and several tens of enzyme specificities found throughout all three kingdoms in many taxa provide an interesting material for evolutionarily oriented studies that have demonstrated remarkable observations. This review emphasizes just the three of them: (1) a close relatedness between the plant and archaeal α-amylases from the family GH13; (2) a common ancestry in the family GH13 of animal heavy chains of heteromeric amino acid transporter rBAT and 4F2 with the microbial α-glucosidases; and (3) the unique sequence features in the primary structures of amylomaltases from the genus Borrelia from the family GH77. Although the three examples cannot represent an exhaustive list of exceptional topics worth to be interested in, they may demonstrate the importance these enzymes possess in the overall scientific context.

Molecular characterization of cystinuria in south-eastern European countries

Cystinuria is an autosomal recessive disorder caused by defective transport of cystine and dibasic amino acids in the proximal renal tubules and small intestine. So far, more than 128 mutations in SLC3A1 gene, and 93 in SLC7A9 gene have been described as a cause of cystinuria. We present a molecular characterization of the cystinuria in 47 unrelated south-east European families. The molecular methodology included direct sequencing, single strand conformational polymorphism, and restriction fragment length polymorphism. A total of 93 (94.9 %) out of 98 unrelated cystinuric chromosomes have been characterized. Mutations in SLC3A1 gene account for 64.3 % and in SLC7A9 gene for 30.6 % of the cystinuric chromosomes. Ten different mutations in SLC3A1 gene were found, and two of them were novel (C242R and L573X), while in SLC7A9 gene seven mutations were found, of which three were novel (G73R, V375I and c.1048_1051delACTC). The most common mutations in this study were T216M (24.5 %), M467T (16.3 %) and R365L (11.2 %) in SLC3A1 and G105R (21.4 %) in SLC7A9 gene. A population specificity of cystinuria mutations was observed; T216M mutation was the only mutation present among Gypsies, G105R was the most common mutation among Albanians and Macedonians, and R365L among Serbs. The results of this study allowed introduction of rapid, simple and cost-effective genetic diagnosis of cystinuria that enables an early preventive care of affected patients and a prenatal diagnosis in affected families.

Expression of intestinal transporter genes in beagle dogs

This study was performed to produce a transcriptional database of the intestinal transporters of beagle dogs. Total RNA was isolated from the duodenum and the expression of various mRNAs was measured using GeneChip(®) oligonucleotide arrays. A total of 124 transporter genes were detected. Genes for fatty acid, peptide, amino acid and glucose and multidrug resistance/multidrug resistance-associated protein (MDR/MRP) transport were expressed at relatively higher levels than the other transporter types. The dogs exhibited abundant mRNA expression of the fatty acid transporters (fatty acid binding proteins, FABPs) FABP1 and FABP2, the ATP-binding cassettes (ABCs) ABCB1A and ABCC2, the amino acid/peptide transporters SLC3A1 and SLC15A1, the glucose transporters SLC5A1, SLC2A2 and SLC2A5, the organic anion transporter SLC22A9 and the phosphate transporters SLC20A1 and SLC37A4. In mice, a similar profile was observed with high expression of the glucose transporters SLC5A1 and SLC2As, the fatty acid transporters FABP1 and FABP2, the MDR/MRP transporters ABCB1A and ABCC2 and the phosphate transporter SLC37A4. However, the overall data reveal diverse transcriptomic profiles of the intestinal transporters of dogs and mice. Therefore, the current database may be useful for comparing the intestinal transport systems of dogs with those of mice to better evaluate xenobiotics.

Functional expression of two system A glutamine transporter isoforms in rat auditory brainstem neurons

Glutamine plays multiple roles in the CNS, including metabolic functions and production of the neurotransmitters glutamate and GABA. It has been proposed to be taken up into neurons via a variety of membrane transport systems, including system A, which is a sodium-dependent electrogenic amino acid transporter system. In this study, we investigate glutamine transport by application of amino acids to individual principal neurons of the medial nucleus of the trapezoid body (MNTB) in acutely isolated rat brain slices. A glutamine transport current was studied in patch-clamped neurons, which had the electrical and pharmacological properties of system A: it was sodium-dependent, had a non-reversing current-voltage relationship, was activated by proline, occluded by N-(methylamino)isobutyric acid (MeAIB), and was unaffected by 2-aminobicyclo-[2.2.1]-heptane-2-carboxylic acid (BCH). Additionally, we examined the expression of different system A transporter isoforms using immunocytochemical staining with antibodies raised against system A transporter 1 and 2 (SAT1 and SAT2). Our results indicate that both isoforms are expressed in MNTB principal neurons, and demonstrate that functional system A transporters are present in the plasma membrane of neurons. Since system A transport is highly regulated by a number of cellular signaling mechanisms and glutamine then goes on to activate other pathways, the study of these transporters in situ gives an indication of the mechanisms of neuronal glutamine supply as well as points of regulation of neurotransmitter production, cellular signaling and metabolism in the native neuronal environment.

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.

Acid-base transport by the renal proximal tubule

One of the major tasks of the renal proximal tubule is to secrete acid into the tubule lumen, thereby reabsorbing approximately 80% of the filtered HCO3- as well as generating new HCO3- for regulating blood pH. This review summarizes the cellular and molecular events that underlie four major processes in HCO3- reabsorption. The first is CO2 entry across the apical membrane, which in large part occurs via a gas channel (aquaporin 1) and acidifies the cell. The second process is apical H+ secretion via Na-H exchange and H+ pumping, processes that can be studied using the NH4+ prepulse technique. The third process is the basolateral exit of HCO3- via the electrogenic Na/HCO3 co-transporter, which is the subject of at least 10 mutations that cause severe proximal renal tubule acidosis in humans. The final process is the regulation of overall HCO3- reabsorption by CO2 and HCO3- sensors at the basolateral membrane. Together, these processes ensure that the proximal tubule responds appropriately to acute acid-base disturbances and thereby contributes to the regulation of blood pH.

A novel missense mutation of SLC7A9 frequent in Japanese cystinuria cases affecting the C-terminus of the transporter

Cystinuria is caused by the inherited defect of apical membrane transport systems for cystine and dibasic amino acids in renal proximal tubules. Mutations in either SLC7A9 or SLC3A1 gene result in cystinuria. The mutations of SLC7A9 gene have been identified mainly from Italian, Libyan Jewish, North American, and Spanish patients. In the present study, we have analyzed cystinuria cases from oriental population (mostly Japanese). Mutation analyses of SLC7A9 and SLC3A1 genes were performed on 41 cystinuria patients. The uptake of 14C-labeled cystine in COS-7 cells was measured to determine the functional properties of mutants. The protein expression and localization were examined by Western blot and confocal laser-scanning microscopy. Among 41 patients analyzed, 35 were found to possess mutations in SLC7A9. The most frequent one was a novel missense mutation P482L that affects a residue near the C-terminus end of the protein and causes severe loss of function. In MDCK II and HEK293 cells, we found that P482L protein was expressed and sorted to the plasma membrane as well as wild type. The alteration of Pro482 with amino acids with bulky side chains reduced the transport function of b(0,+)AT/BAT1. Interestingly, the mutations of SLC7A9 for Japanese cystinuria patients are different from those reported for European and American population. The results of the present study contribute toward understanding the distribution and frequency of cystinuria-related mutations of SLC7A9.

y+ LAT-1 mediates transport of the potent and selective iNOS inhibitor, GW274150, in control J774 macrophages

This study has characterised the transport mechanism(s) for the novel and selective inhibitor of inducible nitric oxide synthase (iNOS), GW274150, in murine macrophage J774 cells. Transport of GW274150 was saturable (K(m) = 0.24 +/- 0.01 mM and V(max) of 8.5 +/- 0.12 pmol.microg protein(-1) min(-1)), pH-insensitive and largely Na(+)-independent. Transport was also susceptible to trans-stimulation and was significantly inhibited by a 10-fold excess of L-arginine, L-lysine, L-leucine, L-methionine, L-glutamine and 6-diazo-5-oxo-L-norleucine but not by other amino acids or by N-ethylmaleimide. More importantly, the inhibitions caused by the neutral amino acids were critically dependent on Na(+). These results strongly implicate system y(+)L in the transport of GW274150. Northern blot analysis confirmed this by revealing the presence of transcripts for y(+)LAT-1 but not y(+)LAT-2. Thus, taken together, our data show for the first time that J774 macrophages express y(+)LAT-1 transporters and that these carriers mediate transport of GW2741500 at least in these cells.

Heterogeneous mutations in the SLC3A1 and SLC7A9 genes in Chinese patients with cystinuria

Cystinuria is a recessively inherited aminoaciduria that leads to recurrent urolithiasis. It is caused by the defective transport of cystine and dibasic amino acids in the proximal renal tubules and intestinal epithelium. Two genes responsible for this, SLC3A1 and SLC7A9, are known. Patients with two SLC3A1 mutations are classified as type A cystinuria, whereas patients with two SLC7A9 mutations are classified as type B cystinuria. Few clinical and molecular data have been reported for Asian cystinuria patients. In this study, we determined the molecular basis of cystinuria in eight unrelated Chinese subjects. Coding exons and flanking introns of the SLC3A1 and SLC7A9 genes were directly sequenced after amplification by polymerase chain reaction. Five different SLC3A1 mutations were found. Two missense mutations, D210G and S547L, were novel. The other three SLC3A1 mutations (IVS6+2T>C, R181Q and R365W) have been described previously. In addition, four novel SLC7A9 mutations, C137R, c.730delG, IVS10+2_3delTG and IVS12+3insT, together with two previously reported mutations (A70V and G195R) were found. All patients except one carried compound heterozygous mutations. IVS12+3insT was detected in patients from two families. This is the first molecular genetic study on Chinese cystinuria patients. Three patients with type A cystinuria, two with type B cystinuria, and three carriers of type B cystinuria were identified. Our results suggest that the molecular basis of cystinuria is heterogeneous in our local population.

A Toxicogenomics Approach to Identify New Plausible Epigenetic Mechanisms of Ochratoxin A Carcinogenicity in Rat

Ochratoxin A (OTA) is a mycotoxin occurring naturally in a wide range of food commodities. In animals, it has been shown to cause a variety of adverse effects, nephrocarcinogenicity being the most prominent. Because of its high toxic potency and the continuous exposure of the human population, OTA has raised public health concerns. There is significant debate on how to use the rat carcinogenicity data to assess the potential risk to humans. In this context, the question of the mechanism of action of OTA appears of key importance and was studied through the application of a toxicogenomics approach. Male Fischer rats were fed OTA for up to 2 years. Renal tumors were discovered during the last 6 months of the study. The total tumor incidence reached 25% at the end of the study. Gene expression profile was analyzed in groups of animals taken in intervals from 7 days to 12 months. Tissue-specific responses were observed in kidney versus liver. For selected genes, microarray data were confirmed at both mRNA and protein levels. In kidney, several genes known as markers of kidney injury and cell regeneration were significantly modulated by OTA. The expression of genes known to be involved in DNA synthesis and repair, or genes induced as a result of DNA damage, was only marginally modulated. Very little or no effect was found amongst genes associated with apoptosis. Alterations of gene expression indicating effects on calcium homeostasis and a disruption of pathways regulated by the transcription factors hepatocyte nuclear factor 4 alpha (HNF4alpha) and nuclear factor-erythroid 2-related factor 2 (Nrf2) were observed in the kidney but not in the liver. Previous data have suggested that a reduction in HNF4alpha may be associated with nephrocarcinogenicity. Many Nrf2-regulated genes are involved in chemical detoxication and antioxidant defense. The depletion of these genes is likely to impair the defense potential of the cells, resulting in chronic elevation of oxidative stress in the kidney. The inhibition of defense mechanism appears as a highly plausible new mechanism, which could contribute to OTA carcinogenicity.

Heterodimeric amino acid transporter glycoprotein domains determining functional subunit association

The heteromeric amino acid transporter glycoprotein subunits rBAT and 4F2hc (heavy chains) form, with different catalytic subunits (light chains), functional heterodimers that are covalently stabilized by a disulphide bridge. Whereas rBAT associates with b(0,+)AT to form the cystine and cationic amino acid transporter defective in cystinuria, 4F2hc associates with other homologous light chains, for instance with LAT1 to form a system L neutral amino acid transporter. To identify within the heavy chains the domain(s) involved in recognition of and functional interaction with partner light chains, chimaeric and truncated forms of rBAT and 4F2hc were co-expressed in Xenopus laevis oocytes with b(0,+)AT or LAT1. Heavy chain-light chain association was analysed by co-immunoprecipitation, and transport function was tested by tracer uptake experiments. The results indicate that the cytoplasmic tail and transmembrane domain of rBAT together play a dominant role in selective functional interaction with b(0,+)AT, whereas the extracellular domain of rBAT appears to facilitate specifically L-cystine uptake. For 4F2hc, functional interaction with LAT1 was mediated by the N-terminal part, comprising cytoplasmic tail, transmembrane segment and neck, even in the absence of the extracellular domain. Alternatively, functional association with LAT1 was also supported by the extracellular part of 4F2hc comprising neck and glycosidase-like domain linked to the complementary part of rBAT. In conclusion, the cytoplasmic tail and the transmembrane segment together play a determinant role for the functional interaction of rBAT with b(0,+)AT, whereas either cytoplasmic or extracellular glycosidase-like domains are dispensable for the functional interaction of 4F2hc with LAT1.

Multiple pathways for cationic amino acid transport in rat thyroid epithelial cell line PC Cl3

Information regarding cationic amino acid transport systems in thyroid is limited to Northern blot detection of y+LAT1 mRNA in the mouse. This study investigated cationic amino acid transport in PC cell line clone 3 (PC Cl3 cells), a thyroid follicular cell line derived from a normal Fisher rat retaining many features of normal differentiated follicular thyroid cells. We provide evidence that in PC Cl3 cells plasmalemmal transport of cationic amino acids is Na+independent and occurs, besides diffusion, with the contribution of high-affinity, carrier-mediated processes. Carrier-mediated transport is via y+, y+L, and b0,+systems, as assessed by l-arginine uptake and kinetics, inhibition of l-arginine transport by N-ethylmaleimide and neutral amino acids, and l-cystine transport studies. y+L and y+systems account for the highest transport rate (with y+L > y+) and b0,+for a residual fraction of the transport. Uptake data correlate to expression of the genes encoding for CAT-1, CAT-2B, 4F2hc, y+LAT1, y+LAT2, rBAT, and b0,+AT, an expression profile that is also shown by the rat thyroid gland. In PC Cl3 cells cationic amino acid uptake is under TSH and/or cAMP control (with transport increasing with increasing TSH concentration), and upregulation of CAT-1, CAT-2B, 4F2hc/y+LAT1, and rBAT/b0,+AT occurs at the mRNA level under TSH stimulation. Our results provide the first description of an expression pattern of cationic amino acid transport systems in thyroid cells. Furthermore, we provide evidence that extracellular l-arginine is a crucial requirement for normal PC Cl3 cell growth and that long-term l-arginine deprivation negatively influences CAT-2B expression, as it correlates to reduction of CAT-2B mRNA levels.

Cystinuria-specific rBAT(R365W) mutation reveals two translocation pathways in the amino acid transporter rBAT-b0,+AT

Apical reabsorption of dibasic amino acids and cystine in kidney is mediated by the heteromeric amino acid antiporter rBAT/b(0,+)AT (system b(0,+)). Mutations in rBAT cause cystinuria type A, whereas mutations in b(0,+)AT cause cystinuria type B. b(0,+)AT is the catalytic subunit, whereas it is believed that rBAT helps the routing of the rBAT/b(0,+)AT heterodimeric complex to the plasma membrane. In the present study, we have functionally characterized the cystinuria-specific R365W (Arg(365)-->Trp) mutation of human rBAT, which in addition to a trafficking defect, alters functional properties of the b(0,+) transporter. In oocytes, where human rBAT interacts with the endogenous b(0,+)AT subunit to form an active transporter, the rBAT(R365W) mutation caused a defect of arginine efflux without altering arginine influx or apparent affinities for intracellular or extracellular arginine. Transport of lysine or leucine remained unaffected. In HeLa cells, functional expression of rBAT(R365W)/b(0,+)AT was observed only at the permissive temperature of 33 degrees C. Under these conditions, the mutated transporter showed 50% reduction of arginine influx and a similar decreased accumulation of dibasic amino acids. Efflux of arginine through the rBAT(R365W)/b(0,+)AT holotransporter was completely abolished. This supports a two-translocation-pathway model for antiporter b(0,+), in which the efflux pathway in the rBAT(R365W)/b(0,+)AT holotransporter is defective for arginine translocation or dissociation. This is the first direct evidence that mutations in rBAT may modify transport properties of system b(0,+).

Functional characterization of Caenorhabditis elegans heteromeric amino acid transporters

Mammalian heteromeric amino acid transporters (HATs) are composed of a multi-transmembrane spanning catalytic protein covalently associated with a type II glycoprotein (e.g. 4F2hc, rBAT) through a disulfide bond. Caenorhabditis elegans has nine genes encoding close homologues of the HAT catalytic proteins. Three of these genes (designated AAT-1 to AAT-3) have a much higher degree of similarity to the mammalian homologues than the other six, including the presence of a cysteine residue at the position known to form a disulfide bridge to the glycoprotein partner in mammalian HATs. C. elegans also has two genes encoding homologues of the heteromeric amino acid transporter type II glycoprotein subunits (designated ATG-1 and ATG-2). Both ATG, and/or AAT-1, -2, -3 proteins were expressed in Xenopus oocytes and tested for amino acid transport function. This screen revealed that AAT-1 and AAT-3 facilitate amino acid transport when expressed together with ATG-2 but not with ATG-1 or the mammalian type II glycoproteins 4F2hc and rBAT. AAT-1 and AAT-3 covalently bind to both C. elegans ATG glycoproteins, but only the pairs with ATG-2 traffic to the oocyte surface. Both of these functional, surface-expressed C. elegans HATs transport most neutral amino acids and display the highest transport rate for l-Ala and l-Ser (apparent K(m) 100 microm range). Similar to their mammalian counterparts, the C. elegans HATs function as (near) obligatory amino acid exchangers. Taken together, this study demonstrates that the heteromeric structure and the amino acid exchange function of HATs have been conserved throughout the evolution of nematodes to mammals.

Urinary excretion of total cystine and the dibasic amino acids arginine, lysine and ornithine in relation to genetic findings in patients with cystinuria treated with sulfhydryl compounds

Advances in molecular genetics have brought a deeper understanding of cystinuria. This autosomal recessive disease, which is caused by a defective tubular reabsorption of cystine and the three dibasic amino acids arginine, lysine and ornithine, results in a lifelong risk of renal stone formation because of the low solubility of cystine in urine. Mutations detected within the two genes known to be associated with cystinuria, SLC3A1 (related to type I) and SLC7A9 (related to non-type I), cannot, however, in all cases explain the disease. Inasmuch as a high urinary concentration of cystine is the basis of stone formation in these patients, our aim was to measure urinary total cystine, arginine, lysine and ornithine, in patients currently lacking a full genetic explanation for their disease. Thirty-three patients with cystinuria who were on long-term treatment with tiopronin or D-penicillamine were divided into two groups. Group 1 comprised eight patients who carried mutation in one of the SLC3A1 alleles and two patients who completely lacked mutations both in the SLC3A1 and the SLC7A9 genes, that is genetic findings discordant with the increased urinary excretion of cystine and the dibasic amino acids in these patients. Group 2 comprised 23 patients homozygous for mutations within SLC3A1, that is genetic findings in accordance with the excretion pattern of classic type I cystinuria. When the two groups were compared, Group 1 had a significantly higher total urinary excretion of cystine ( p<0.01) as well as of arginine, lysine and ornithine ( p<0.05) than Group 2. Also, when the two patients without mutations were excluded from the calculations, there still was a significant difference in the urinary excretion of total cystine ( p<0.05). This suggests that the two patients without any detected mutations in the two known cystine transport genes also contributed to the difference. These unexpected findings indicate that an additional gene or genes participate in the urinary cystine reabsorption in the cystinuric patients who currently are without a full genetic explanation for their disease.

Regulation of L-leucine transport in rat kidney by dexamethasone and triiodothyronine

We have investigated the transport mechanisms involved in the stimulation of renal tubular reabsorption of large amino acids by glucocorticoids in vivo through the examination of activity and expression of specific transport systems L and y(+)L for L-leucine in membrane preparations of rat kidneys. Kidneys were removed from adult female Wistar rats treated with dexamethasone or triiodothyronine, and the fractions of brush-border and basolateral membranes were isolated by density gradient centrifugation. Functional analysis of L-leucine uptake using rapid filtration technique revealed induction of a sodium-dependent, arginine-inhibitable system y(+)L transport component in the basolateral membrane in the dexamethasone-treated group. A minor sodium-independent, BCH-inhibitable, system L transport component was unaffected by glucocorticoids. L-leucine uptake remained unaffected in the triiodothyronine-treated group. Expression of both subunits of the system y(+)L transporter was increased in dexamethasone-treated rat kidneys: Western blot analysis showed a significant (46%) increase of 4F2hc protein abundance in the basolateral membrane fraction and competitive RT-PCR revealed an almost 4-times induced expression of y(+)LAT1 mRNA. Our results indicate that system y(+)L in rat kidney is regulated by glucocorticoids. We suggest that enhancement of both 4F2 heavy chain and y(+)LAT1 light chain is necessary for induction of this transport system in the kidney.

Expression of the activity of cystine/glutamate exchange transporter, system x(c)(-), by xCT and rBAT

The expression of the activity of cystine/glutamate exchange transporter, designated system x(c)(-), requires two components, xCT and 4F2 heavy chain (4F2hc) in Xenopus oocytes. rBAT (related to b(0,+) amino acid transporter) has a significant homology to 4F2hc and is known to be located in the apical membrane of epithelial cells. To determine whether xCT can associate with rBAT and express the activity of system x(c)(-), xCT, and rBAT were co-expressed in Xenopus oocytes and in mammalian cultured cells. In the oocytes injected with rBAT cRNA alone, the activities of cystine and arginine transport were induced, indicating that the system b(0,+)-like transporter was expressed by associating the exogenous rBAT with an endogenous b(0,+)AT-like factor as reported previously. In the oocytes injected with xCT and rBAT cRNAs, the activity of cystine transport was further induced. This induced activity of cystine transport was partially inhibited by glutamate or arginine and completely inhibited by adding both amino acids. In these oocytes, the activity of glutamate transport was also induced and it was strongly inhibited by cystine. In NIH3T3 cells transfected with xCT cDNA alone, the activity of cystine transport was significantly increased, and in the cells transfected with both xCT and rBAT cDNAs, the activity of cystine transport was further enhanced. The enhanced activity was Na(+)-independent and was inhibited by glutamate and homocysteate. These results indicate that rBAT can replace 4F2hc in the expression of the activity of system x(c)(-) and suggest that system x(c)(-) activity could be expressed in the apical membrane of epithelial cells.

The ancillary proteins of HATs: SLC3 family of amino acid transporters

The heteromeric amino acid transporters (HATs) are composed of a light and a heavy subunit linked by a disulfide bridge. The heavy subunits are the SLC3 members (rBAT and 4F2hc), whereas the light subunits are members of the SLC7 family of amino acid transporters. SLC3 proteins are type II membrane glycoproteins (i.e., one single transmembrane domain and the C-terminus located outside the cell) with a bulky extracellular domain that shows homology with alpha-glucosidases. rBAT heterodimerizes with b(0,+)AT (SLC7A9) constituting the amino acid transport b(0,+), the main system responsible for the apical reabsorption of cystine in kidney. The defect in this system causes cystinuria, the most common primary inherited aminoaciduria. 4F2hc subserves various amino acid transport systems by dimerization with different SLC7 proteins. The main role of SLC3 proteins is to help routing of the holotransporter to the plasma membrane. A working model for the biogenesis of HATs based on recent data on the rBAT/b(0,+)AT heterodimeric complex is presented. 4F2hc is a multifunctional protein, and in addition to its role in amino acid transport, it may be involved in other cellular functions. Studies on two SLC7 members (Asc-2 and AGT1) demonstrate heterodimerization with unknown heavy subunits.

Oligomerization of the murine fatty acid transport protein 1

The 63-kDa murine fatty acid transport protein 1 (FATP1) was cloned on the basis of its ability to augment fatty acid import when overexpressed in mammalian cells. The membrane topology of this integral plasma membrane protein does not resemble that of polytopic membrane transporters for other substrates. Western blot analysis of 3T3-L1 adipocytes that natively express FATP1 demonstrate a prominent 130-kDa species as well as the expected 63-kDa FATP1, suggesting that this protein may participate in a cell surface transport protein complex. To test whether FATP1 is capable of oligomerization, we expressed functional FATP1 molecules with different amino- or carboxyl-terminal epitope tags in fibroblasts. These epitope-tagged proteins also form apparent higher molecular weight species. We show that, when expressed in the same cells, differentially tagged FATP1 proteins co-immunoprecipitate. The region between amino acid residues 191 and 475 is sufficient for association of differentially tagged truncated FATP1 constructs. When wild type FATP1 and the non-functional s250a FATP1 mutant are co-expressed in COS7 cells, mutant FATP1 has dominant inhibitory function in fatty acid uptake assays. Taken together, these results are consistent with a model in which FATP1 homodimeric complexes play an important role in cellular fatty acid import.

Functional properties of multispecific amino acid transporters and their implications to transporter-mediated toxicity

The absorption, distribution and excretion of most of xenobiotics, drugs, environmental toxins and their metabolites are mediated by membrane transporters. Recent advances in the transporter molecular biology have made it possible to investigate the mechanisms of transport of those exogenous compounds and their transporter-mediated toxicity at the molecular level. Exogenous compounds including drugs and toxic substances occurring in the environment pass through the transporters with broad substrate selectivity, namely "multispecific" transporters, taking advantage of the multispecific nature to exert their toxic effects. The remarkable examples of such transporter-mediated toxicity are 1-methyl-4-phenyl-2,3-dihydropyridinium (MPP+)-neurotoxicity mediated by dopamine transporters, cephaloridine-nephrotoxicity mediated by organic anion transporters and methylmercury-toxicity mediated by system L amino acid transporters. The molecular identification of system L transporter LAT1 (L-type amino acid transporter 1) has lead to the understanding of the mechanisms of their multispecific substrate recognition and revealed their localization at the blood-brain barrier and placental barrier. LAT1 relies on the hydrophobic interaction between substrate amino acid side chains and the substrate binding site, so that many variations are possible for the substrate amino acid side chains, which is the basis of the broad substrate selectivity. System L transporters, thus, function as a path for the membrane permeation of drugs and toxic compounds occurring in the environment with amino acid-related structures. Beside methylmercury-cysteine conjugate, amino acid-related neurotoxins such as beta-N-methylamino-L-alanine, S-(1,2-dichlorovinyl)-L-cysteine and 3-hydroxykynurenine are proposed to pass through system L transporters to exert their toxicity. Because the presence of such transporters is crucial for the manifestation of the organ toxicity, the inhibition of the transporters would be expected to be beneficial to prevent the disorders caused by the transporter-mediated toxicity.

Beta1 integrins show specific association with CD98 protein in low density membranes

BACKGROUND

The CD98 (4F2, FRP-1) is a widely expressed cell surface protein heterodimer composed of a glycosylated heavy chain and a non-glycosylated light chain. Originally described as a T cell activation antigen, it was later shown to function in amino acid transport, cell fusion and homotypic cell aggregation. Several lines of evidence suggest its functional interaction with integrins but the biochemical basis for this interaction has been unclear.

RESULTS

We demonstrate that CD98 constitutively and specifically associates with beta1 integrins (alpha2beta1,alpha3beta1, alpha5beta1 and alpha6beta1), but minimally with alpha4beta1. Integrin-CD98 association was established by reciprocal immunoprecipitation experiments, and confirmed by CD98-induced clustering of alpha3beta1 but not alpha4beta1 on the surface of rhabdomyosarcoma cells. Integrin-CD98 association is independent of the alpha subunit cytoplasmic tail, is maintained in alpha3beta1 ligand-interaction deficient mutants, and is not inhibited by EDTA. Within the CD98 heavy chain, a C109S mutation (but not a C330S mutation) caused a loss of beta1 integrin association. The same C109S mutation also caused a loss of CD98 light chain association. Importantly, CD98 associated selectively with beta1 integrins present in low density "light membrane" fractions on a sucrose gradient. CD98 was not present in dense fractions that contained the majority of beta1 integrins. Notably, the C109S mutant of CD98, that did not associate with beta1 integrins, showed also a reduced localization into light membrane fractions.

CONCLUSIONS

We demonstrate that CD98 association with beta1 integrins is specific, occurs in the context of low density membranes, and may require the CD98 light chain.

SLC7A9 mutations in all three cystinuria subtypes

BACKGROUND

Cystinuria is an inherited disorder of cystine and dibasic amino acid transport in kidney. Subtypes are defined by the urinary cystine excretion patterns of the obligate heterozygous parents: Type I/N (fully recessive or silent); Type II/N (high excretor); Type III/N (moderate excretor). The first gene implicated in cystinuria (SLC3A1) is associated with the Type I urinary phenotype. A second cystinuria gene (SLC7A9) was recently isolated, and mutations of this gene were associated with dominant (non-Type I) cystinuria alleles. Here we report genotype-phenotype studies of SLC7A9 mutations in a cohort of well-characterized cystinuria probands and their family members.

METHODS

Individual exons of the SLC7A9 gene were screened by single strand conformation polymorphism (SSCP) analysis and sequencing of abnormally migrating fragments.

RESULTS

Seven mutations were identified. A single bp insertion (799insA) was present in four patients: on Type III alleles in two patients and on Type II alleles in two patients. These results suggest that Type II and Type III may be caused by the same mutation and, therefore, other factors must influence urinary cystine excretion. A 4bp deletion in intron 12 (IVS12+4delAGTA) and a missense mutation (1245G-->A, A354T) were identified on Type III alleles. A nonsense codon (1491G-->T, E436X) and a possible splicing mutation (IVS9-17G-->A) were seen in a Type I/III patient, but the mutations could not be assigned to particular alleles. Of additional interest were two missense mutations (316T-->C, I44T and 967C-->T, P261L) linked to Type I alleles.

CONCLUSION

Our results provide evidence that some SLC7A9 mutations may be associated with fully recessive (Type I) forms of cystinuria. We also demonstrate SLC7A9 mutations in dominant Types II and III cystinuria. The finding of SLC7A9 mutations in all three subtypes underscores the complex interactions between specific cystinuria genes and other factors influencing cystine excretion. A simpler phenotypic classification scheme (recessive and dominant) for cystinuria is warranted.

Transport and metabolism of amino acids in placenta

In all mammalian species, the 20 amino acids of the genetic code are required for net protein accretion. The nutritional supply of amino acids for growth is defined as the net umbilical uptake of amino acids, representing the net transfer from maternal circulation, through the placenta and then to the fetus, of essential and non-essential amino acids. In considering the primary role of the placenta in the delivery of amino acids to the fetus for metabolism, it is important to consider the multiplicity of factors that may affect these overall delivery rates, including the activity and location of amino acid transporter systems, changes in these systems as gestation advances, effects of changes in placental surface area, uteroplacental blood flows, and maternal concentrations of amino acids. In this review, we discuss placental amino acid transport, the systems and their associated proteins, umbilical uptake data in animal and human studies, and amino acid transport in fetal growth restriction. Additionally, we discuss the current pool of thought concerning the mechanisms of placental amino acid transport as generated through in vitro vesicle studies and how they relate to the in vivo fluxes of animal studies. Finally, we discuss fetoplacental amino acid metabolism and specifically interorgan exchange.

rBAT-b(0,+)AT heterodimer is the main apical reabsorption system for cystine in the kidney

Mutations in the rBAT and b(0,+)AT genes cause type I and non-type I cystinuria, respectively. The disulfide-linked rBAT-b(0,+)AT heterodimer mediates high-affinity transport of cystine and dibasic amino acids (b(0,+)-like activity) in heterologous cell systems. However, the significance of this heterodimer for cystine reabsorption is unknown, as direct evidence for such a complex in vivo is lacking and the expression patterns of rBAT and b(0,+)AT along the proximal tubule are opposite. We addressed this issue by biochemical means. Western blot analysis of mouse and human kidney brush-border membranes showed that rBAT and b(0,+)AT were solely expressed as heterodimers of identical size and that both proteins coprecipitated. Moreover, quantitative immunopurification of b(0,+)AT followed by SDS-PAGE and mass spectrometry analysis established that b(0,+)AT heterodimerizes exclusively with rBAT. Together with cystine reabsorption data, our results demonstrate that a decreasing expression gradient of heterodimeric rBAT-b(0,+)AT along the proximal tubule is responsible for virtually all apical cystine reabsorption. As a corollary of the above, there should be an excess of rBAT expression over that of b(0,+)AT protein in the kidney. Indeed, complete immunodepletion of b(0,+)AT did not coprecipitate >20-30% of rBAT. Therefore, another rBAT-associated subunit may be present in latter parts of the proximal tubule.

Apical heterodimeric cystine and cationic amino acid transporter expressed in MDCK cells

The luminal uptake of L-cystine and cationic amino acids by (re)absorptive epithelia, as found in the small intestine and the proximal kidney tubule, is mediated by the transport system b(0,+), which is defective in cystinuria. Expression studies in Xenopus laevis oocytes and other nonepithelial cells as well as genetic studies on cystinuria patients have demonstrated that two gene products, the glycoprotein rBAT and the multitransmembrane-domain protein b(0,+)AT, are required for system b(0,+) function. To study the biosynthesis, surface expression, polarity, and function of this heterodimer in an epithelial context, we established stable Madin-Darby canine kidney (MDCK) cell lines expressing rBAT and/or b(0,+)AT. Confocal immunofluorescence microscopy shows that both subunits depend on each other for apical surface expression. Immunoprecipitation of biosynthetically labeled proteins indicates that b(0,+)AT is stable in the absence of rBAT, whereas rBAT is rapidly degraded in the absence of b(0,+)AT. When both are coexpressed, they associate covalently and rBAT becomes fully glycosylated and more stable. Functional experiments show that the expressed transport is of the high-affinity b(0,+)-type and is restricted to the apical side of the epithelia. In conclusion, coexpression experiments in MDCK cell epithelia strongly suggest that the intracellular association of rBAT and b(0,+)AT is required for the surface expression of either subunit, which together form a functional heterocomplex at the apical cell membrane.

Epithelial cells isolated from chicken jejunum: an experimental model for the study of the functional properties of amino acid transport system b(0,+)

The transport of lysine has been investigated in epithelial cells isolated from chicken jejunum. The kinetics of lysine transport and the pattern of interaction with zwitterionic amino acids were consistent with system b(0,+) activity, the broad-spectrum and Na(+)-independent amino acid transporter. The half-saturation constant for lysine entry (K(m)+/-S.E.) was 0.029+/-0.002 mM and the flux was not affected significantly by Na(+) replacement with choline. Lysine influx was inhibited by L-leucine both in Na(+) and choline medium with inhibition constants (K(i)+/-S.E.) 0.068+/-0.006 mM (in Na(+)) and 0.065+/-0.009 mM (in choline). Other inhibitory amino acids (K(i)+/-S.E.) were (mM): L-tyrosine (0.073+/-0.018), L-methionine (0.15+/-0.015), L-cystine (0.42+/-0.04), L-cysteine (1.1+/-0.07), L-isoleucine (1.1+/-0.09), L-glutamine (1.8+/-0.16) and L-valine (2.5+/-0.13). Lysine exit was trans-accelerated (approx. 20 fold) by 2 mM L-lysine and L-leucine. The flux was resistant to pretreatment of the cells with p-chloromercuriphenylsulfonate (0.2 mM), which is an inhibitor of system y(+)L, the broad-spectrum and cation-modulated transporter.

The role of CD98 in astrocytic neoplasms

The high expression of CD98 was reported in some normal tissues, including blood brain barrier, activated lymphocytes, the basal layer of skin, proximal tubles of kidney, placenta, testis and a wide variety of tumors. The CD98 complex consists of an 80-85kD heavy chain (4F2hc/FRP-1) and a 40-45kD light chain. CD98hc, 4F2hc, and FRP-1 are the same glycosylated protein each other and define antigenicity of CD98. LAT1, the sodium-independent L-type amino acid transporter 1, has been identified as a light chain of the CD98 heterodimer from C6 glioma cells. LAT1 also corresponds to TA1, an oncofetal antigen that is expressed primarily in fetal tissues and cancer cells such as glioma cells. Increased LAT1 expression was found in various malignancies including human gliomas. Several studies implicated the important role of LAT1 and 4F2hc in malignant transformation and carcinogenesis. The LAT1-CD98 pathway may represent a unique therapeutic target for cancer intervention.

Transporters in the neurohypophysial neuroendocrine system, with special reference to vesicular glutamate transporters (BNPI and DNPI): a review

Recent advances in gene technology have helped to identify novel proteins and allowed study of their distribution and functions in the mammalian brain. One class of these proteins is that of transporters, which exist in plasma and organellar membranes of neurons and other cells to move substances selectively across membranes. These transporters can be categorized further into subclasses by their structural property, substrate selectivity, and site of action. Some of them have been identified in the hypothalamus, which is the only brain site where a neural signal is converted to a humoral one, namely, a hormone for a target organ. This unique neural mechanism has long attracted attention as the neuroendocrine system, part of which has been extensively studied as the hypothalamic-neurohypophysial system involved in secretion of vasopressin and oxytocin. However, transporters in this system have been less well studied. A morphological examination of novel transporters would give us cues to a better understanding of the neuronal organization and function of the system. In this review, we first summarize recent findings on expression of transporter gene and immunoreactivity in the hypothalamus. In the second part, we explain our observations on two vesicular glutamate (inorganic phosphate) transporters (BNPI and DNPI) in the supraoptic and paraventricular nuclei and neurohypophysis. Further study of these and other transporters will provide a basis on which to reevaluate the organization and function of the hypothalamic-neurohypophysial system.

Heteromeric amino acid transporters: biochemistry, genetics, and physiology

The heteromeric amino acid transporters (HATs) are composed of two polypeptides: a heavy subunit (HSHAT) and a light subunit (LSHAT) linked by a disulfide bridge. HSHATs are N-glycosylated type II membrane glycoproteins, whereas LSHATs are nonglycosylated polytopic membrane proteins. The HSHATs have been known since 1992, and the LSHATs have been described in the last three years. HATs represent several of the classic mammalian amino acid transport systems (e.g., L isoforms, y(+)L isoforms, asc, x(c)(-), and b(0,+)). Members of the HAT family are the molecular bases of inherited primary aminoacidurias cystinuria and lysinuric protein intolerance. In addition to the role in amino acid transport, one HSHAT [the heavy subunit of the cell-surface antigen 4F2 (also named CD98)] is involved in other cell functions that might be related to integrin activation. This review covers the biochemistry, human genetics, and cell physiology of HATs, including the multifunctional character of CD98.

Function and structure of heterodimeric amino acid transporters

Heterodimeric amino acid transporters are comprised of two subunits, a polytopic membrane protein (light chain) and an associated type II membrane protein (heavy chain). The heavy chain rbAT (related to b(0,+) amino acid transporter) associates with the light chain b(0,+)AT (b(0,+) amino acid transporter) to form the amino acid transport system b(0,+), whereas the homologous heavy chain 4F2hc interacts with several light chains to form system L (with LAT1 and LAT2), system y(+)L (with y(+)LAT1 and y(+)LAT2), system x (with xAT), or system asc (with asc1). The association of light chains with the two heavy chains is not unambiguous. rbAT may interact with LAT2 and y(+)LAT1 and vice versa; 4F2hc may interact with b(0,+)AT when overexpressed. 4F2hc is necessary for trafficking of the light chain to the plasma membrane, whereas the light chains are thought to determine the transport characteristics of the respective heterodimer. In contrast to 4F2hc, mutations in rbAT suggest that rbAT itself takes part in the transport besides serving for the trafficking of the light chain to the cell surface. Heavy and light subunits are linked together by a disulfide bridge. The disulfide bridge, however, is not necessary for the trafficking of rbAT or 4F2 heterodimers to the membrane or for the functioning of the transporter. However, there is experimental evidence that the disulfide bridge in the 4F2hc/LAT1 heterodimer plays a role in the regulation of a cation channel. These results highlight complex interactions between the different subunits of heterodimeric amino acid transporters and suggest that despite high grades of homology, the interactions between rbAT and 4F2hc and their respective partners may be different.

The generalized aminoaciduria seen in patients with hepatocyte nuclear factor-1alpha mutations is a feature of all patients with diabetes and is associated with glucosuria

Hepatocyte nuclear factor-1alpha (HNF-1alpha) mutations are the most common cause of maturity-onset diabetes of the young. HNF-1alpha homozygous knockout mice exhibit a renal Fanconi syndrome with glucosuria and generalized aminoaciduria in addition to diabetes. We investigated glucosuria and aminoaciduria in patients with HNF-1alpha mutations. Sixteen amino acids were measured in urine samples from patients with HNF-1alpha mutations, age-matched nondiabetic control subjects, and age-matched type 1 diabetic patients, type 2 diabetic patients, and patients with diabetes and chronic renal failure. The HNF-1alpha patients had glucosuria at lower glycemic control (as shown by HbA1c) than type 1 and type 2 diabetic patients, consistent with a lower renal glucose threshold. The HNF-1alpha patients had a generalized aminoaciduria with elevated levels of 14 of 16 amino acids and an increased mean Z score for all amino acids compared with control subjects (0.66 vs. 0.00; P < 0.0005). Generalized aminoaciduria was also present in type 1 diabetic (Z score, 0.80; P < 0.0001), type 2 diabetic (Z score, 0.71; P < 0.0002), and chronic renal failure (Z score, 0.65; P < 0.01) patients. Aminoaciduria was not associated with microalbuminuria or proteinuria but was associated with glucosuria (1.00 glucosuria vs. 0.19 no glucosuria; P = 0.002). In type 1 diabetic patients, urine samples taken on the same day showed significantly more aminoaciduria when glucosuria was present compared with when it was absent (P < 0.01). In conclusion, HNF-1alpha mutation carriers have a mutation-specific defect of proximal tubular glucose transport, resulting in increased glucosuria. In contrast, the generalized aminoaciduria seen in patients with HNF-1alpha mutations is a general feature of patients with diabetes and glucosuria. Glucose may depolarize and dissipate the electrical gradient of the sodium-dependent amino acid transporters in the proximal renal tubule, causing a reduction in amino acid resorption.

Is the SLC7A10 gene on chromosome 19 a candidate locus for cystinuria?

One of the genes (SLC7A9) that causes cystinuria, an inborn error of amino acid transport, is localized to 19q13. Close examination of human genomic DNA sequences has identified a similar gene (SLC7A10) that also maps to the 19q13.1 region and is highly expressed in kidney. The homologies between SLC7A9 and SLC7A10 are likely the result of gene duplication. SLC7A10 is known to encode a protein with a function similar to that of the SLC7A9 gene product. To determine if mutations in the SLC7A10 gene could also cause cystinuria, we characterized the primary genomic structure and sequenced the 11 exons and surrounding sequences from 10 unrelated patients with cystinuria. We identified one missense mutation which may account for cystinuria in one family. We also observed one intronic change, as well as one silent mutation, that were seen only in cystinuria patients. We therefore suggest that the SLC7A10 gene warrants further investigation as another candidate gene for cystinuria.

Gene expression of VEGF and its receptors Flk-1/KDR and Flt-1 in cultured and transplanted rat islets

BACKGROUND

Vascular endothelial growth factor (VEGF) and its two receptor tyrosine kinases, Flk-1/KDR and Flt-1, may play an important role in mediating the revascularization of transplanted pancreatic islets.

METHODS

Using semiquantitative multiplex reverse-transcribed polymerase chain reaction we determined the gene expression of VEGF and its receptors in cultured and transplanted rat islets.

RESULTS

After exposure of islet cells to hypoxia in vitro, increases were found in the gene expression of the VEGF120 and VEGF164 isoforms, with simultaneous increases in VE-cadherin, Flk-1/KDR, and Flt-1. In vivo studies consisted of analysis of islet grafts transplanted into both normal and diabetic recipients. Expression of both VEGF120 and VEGF164 in grafts was up-regulated for the first 2-3 days after transplantation, with the response being more prolonged in the diabetic rats. These increases were followed by reduced expression of VEGF on days 5, 7, and 9. Increases in the expression of VE-cadherin in islet grafts in normal and diabetic recipients tended to parallel VEGF expression, with the increases in both probably being caused by hypoxia. The early increases of VEGF expression were followed by a rise in the expression of VEGF receptors, which probably represents the early stages of angiogenesis. Graft expression of Flk-1/KDR and Flt-1 was enhanced at 3 and 5 days in the normoglycemic recipients, while in the diabetic recipients increases were found later on days 5, 7, and 14.

CONCLUSIONS

The delayed expression of VEGF receptors in the diabetic recipients could reflect impaired angiogenesis caused by the diabetic milieu; this delay could contribute to the less outcomes of grafts transplanted into a hyperglycemic environment.

Human cystinuria-related transporter: localization and functional characterization

BACKGROUND

Cystinuria has been proposed to be an inherited defect of apical membrane transport systems for cystine and basic amino acids in renal proximal tubules. Although the mutations of the recently identified transporter BAT1/b(0,+)AT have been related to nontype I cystinuria, the function and localization of human BAT1 (hBAT1)/b(0,+)AT have not been well characterized.

METHODS

The cDNA encoding hBAT1 was isolated from human kidney. Fluorescence in situ hybridization was performed to map the hBAT1 gene on human chromosomes. Tissue distribution and localization of expression were examined by Northern blot and immunohistochemical analyses. hBAT1 cDNA was transfected to COS-7 cells with rBAT cDNA, and the uptake and efflux of 14C-labeled amino acids were measured to determine the functional properties. The roles of protein kinase-dependent phosphorylation were investigated using inhibitors or activators of protein kinases.

RESULTS

The hBAT1 gene was mapped to 19q12-13.1 on the human chromosome, which is the locus of nontype I cystinuria. hBAT1 message was expressed predominantly in kidney. hBAT1 protein was localized in the apical membrane of proximal tubules in human kidney. When expressed in COS-7 cells with a type II membrane glycoprotein rBAT (related to b(0,+)-amino acid transporter), hBAT1 exhibited the transport activity with the properties of amino acid transport system b(0,+), which transported cystine as well as basic and neutral amino acids presumably via a substrate exchange mechanism. BAT1-mediated transport was reduced by the protein kinase A activator and enhanced by the tyrosine kinase inhibitor.

CONCLUSIONS

hBAT1 exhibited the properties expected for a transporter subserving the high-affinity cystine transport system in renal proximal tubules. The hBAT1 gene was mapped to the locus of nontype I cystinuria, confirming the involvement of hBAT1 in cystinuria.

Cystinuria type I: identification of eight new mutations in SLC3A1

BACKGROUND

Cystinuria is a heritable disorder of amino acid transport characterized by the defective transport of cystine and the dibasic amino acids through the brush border epithelial cells of the renal tubule and intestine tract. Three types of cystinuria (I, II, and III) have been described based on the urinary excretion of cystine and dibasic amino acids in obligate heterozygotes. The SLC3A1 gene coding for an amino acid transporter named rBAT is responsible for type I cystinuria, whereas the SLC7A9 gene coding for a subunit (b0,+AT) of rBAT is involved in determining non-type I (types II and III) cystinuria.

METHODS

The SLC3A1 gene sequence was investigated in a sample of seven type I/type I, three type I/non-type I, six type I/untyped, and four untyped unrelated cystinuric patients by RNA single-strand conformation polymorphism (RNA-SSCP).

RESULTS

Eight new point mutations (S168X, 765+1G>T, 766-2A>G, R452Q, Y461X, S547W, L564F, and C673W) and seven previously reported mutations were detected. These new mutations increase the number of mutated alleles so far characterized in SLC3A1 to 62.

CONCLUSIONS

We have found SLC3A1 mutations in 0.739 of the type I chromosomes studied. The relatively high proportion of uncharacterized type I chromosomes suggests either that there may be mutations not yet found in SLC3A1 or that many of the assigned type I chromosomes in mixed type I/non-type I patients may have mutations in SLC7A9. If the hypothesis is excluded in the future, we believe that a third gene may be involved in cystinuria.