Poly(A)+ RNA from rabbit intestinal mucosa induces b0,+ and y+ amino acid transport activities in Xenopus laevis oocytes

The zebrafish cationic amino acid transporter/glycoprotein-associated family: sequence and spatiotemporal distribution during development of the transport system b0,+ (slc3a1/slc7a9)

System b^0,+ absorbs lysine, arginine, ornithine, and cystine, as well as some (large) neutral amino acids in the mammalian kidney and intestine. It is a heteromeric amino acid transporter made of the heavy subunit SLC3A1/rBAT and the light subunit SLC7A9/b^0,+AT. Mutations in these two genes can cause cystinuria in mammals. To extend information on this transport system to teleost fish, we focused on the slc3a1 and slc7a9 genes by performing comparative and phylogenetic sequence analysis, investigating gene conservation during evolution (synteny), and defining early expression patterns during zebrafish (Danio rerio) development. Notably, we found that slc3a1 and slc7a9 are non-duplicated in the zebrafish genome. Whole-mount in situ hybridization detected co-localized expression of slc3a1 and slc7a9 in pronephric ducts at 24 h post-fertilization and in the proximal convoluted tubule at 3 days post-fertilization (dpf). Notably, both the genes showed co-localized expression in epithelial cells in the gut primordium at 3 dpf and in the intestine at 5 dpf (onset of exogenous feeding). Taken together, these results highlight the value of slc3a1 and slc7a9 as markers of zebrafish kidney and intestine development and show promise for establishing new zebrafish tools that can aid in the rapid screening(s) of substrates. Importantly, such studies will help clarify the complex interplay between the absorption of dibasic amino acids, cystine, and (large) neutral amino acids and the effect(s) of such nutrients on organismal growth.

Expression of proposed methionine transporters along the gastrointestinal tract of pigs and their regulation by dietary methionine sources

BACKGROUND

Given the key role of methionine (Met) in biological processes like protein translation, methylation, and antioxidant defense, inadequate Met supply can limit performance. This study investigated the effect of different dietary Met sources on the expression profile of various Met transporters along the gastrointestinal tract (GIT) of pigs.

METHODS

A total of 27 pigs received a diet supplemented with 0.21% DL-Met, 0.21% L-Met, or 0.31% DL-2-hydroxy-4-(methylthio)butanoic acid (DL-HMTBA). Changes in mRNA expression of B⁰AT1, ATB^0,+, rBAT, ASCT2, IMINO, LAT4, y⁺LAT1, LAT2, and SNAT2 were evaluated in the oral mucosa, cardia, fundus, pylorus, duodenum, proximal jejunum, middle jejunum, ileum, cecum, proximal colon, and distal colon, complemented by protein expression analysis of B⁰AT1, ASCT2, LAT2, and LAT4.

RESULTS

Expression of all investigated transcripts differed significantly along the GIT. B⁰AT1, rBAT, y⁺LAT1, LAT2, and LAT4 showed strongest mRNA expression in small intestinal segments. ASCT2, IMINO, and SNAT2 were similarly expressed along the small and large intestines but expression differed in the oral mucosa and stomach. ATB^0,+ showed highest mRNA expression in large intestinal tissues, cardia, and pylorus. In pigs fed DL-Met, mRNA expression of ASCT2 was higher than in pigs fed DL-HMTBA in small intestinal tissues and mRNA expression of IMINO was lower than in pigs fed L-Met in large intestinal tissues. Dietary DL-HMTBA induced a stronger mRNA expression of basolateral uptake systems either in the small (LAT2) or large (y⁺LAT1) intestine. Protein expression of B⁰AT1 was higher in the middle jejunum and ileum in pigs fed DL-Met when compared with the other Met supplements. LAT4 expression was higher in pigs fed DL-HMTBA when compared with DL-Met (small intestine) and L-Met (small intestine, oral mucosa, and stomach).

CONCLUSION

A high expression of several Met transporters in small intestinal segments underlines the primary role of these segments in amino acid absorption; however, some Met transporters show high transcript and protein levels also in large intestine, oral mucosa, and stomach. A diet containing DL-Met has potential to increase apical Met transport in the small intestine, whereas a diet containing DL-HMTBA has potential to increase basolateral Met transport in the small intestine and, partly, other gastrointestinal tissues.

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.

Mechanistic investigation of capability of enzymatically synthesized polycysteine to cross-link proteins

Background

Previously, we had reported that α-chymotrypsin–catalyzed polymerization of l-cysteine ethyl ester in a frozen buffer provided poly-l-cysteine (PLCys) in good yield, of which degree of polymerization had been determined to be 6–11. Almost all of SH groups in PLCys were in free forms. Such a multi-thiol peptide may cross-link proteins through thiol/disulfide (SH/SS) exchange reactions, considering the knowledge that other synthetic multi-thiol additives changes properties of protein materials.

Methods

This study explored the capability of PLCys to cross-link proteins using lysozyme as a model protein which has four disulfide bonds but no free SH group. The protein was incubated with PLCys at neutral pH and at below 70 °C to avoid PLCys-independent, β-elimination-mediated cross-linkings. Protein polymerization was analyzed by SDS-PAGE and SEC. PLCys peptides involved in the protein polymer, which were released by reduction with dithiothreitol, were analyzed by RP-HPLC.

Conclusions

Addition of urea and thermal treatment at 60 °C caused PLCys-induced lysozyme polymerization. Compared with free cysteine, a higher level of PLCys was required for the polymerization probably due to its low water solubility. RP-HPLC analyses suggested that PLCys played a role in the protein polymerization as a cross-linker.

General significance

Enzymatically synthesized PLCys shows promise as a peptidic cross-linker for the production of protein polymers with novel physiochemical properties and functionalities.

Genetic and biochemical analysis of protozoal polyamine transporters

Polyamines are aliphatic polycations that function in key cellular processes such as growth, differentiation, and macromolecular biosynthesis. Intracellular polyamines pools are maintained from de novo synthesis and from transport of polyamines from the extracellular milieu. This acquisition of exogenous polyamines is mediated by cell surface transporter proteins. Protozoan parasites are the etiologic agents of a plethora of devastating and often fatal diseases in humans and their domestic animals. These pathogens accommodate de novo and/or salvage mechanisms for polyamine acquisition. Because of its therapeutic relevance, the polyamine biosynthetic pathway has been thoroughly investigated in many genera of protozoan parasites, but the polyamine permeation pathways have generally been ignored. Our group has now identified at the molecular level polyamine transporters from two species of protozoan parasites, Leishmania major and Trypanosoma cruzi, characterized these polytopic proteins with respect to ligand specificities and affinities, and determined the subcellular environments in which these transporters reside.

Evaluation of cystine transport in cultured human kidney cells and establishment of cystinuria type I phenotype by antisense technology

Cystinuria is a rare hereditary disease resulting in recurrent stone formation and the need for repeated invasive interventions. So far, two responsible genes have been identified which encode the two transporters, rBAT and b(0,+)AT forming a heterodimer to transport cystine in proximal tubular cells (PTC) and whose defect results in increased excretion of cystine. A human cell line mimicing the phenotype of cystinuria in vitro is yet to be developed. Human kidney (HK)-2 is a PTC line derived from normal HK. After determining the presence of rBAT gene by RT-PCR and Western blot analysis, radioactively labeled cystine (S(35)) was used to evaluate the functional presence of the amino acid transport in HK-2 cells when cultured in vitro. To achieve a cystinuria type I phenotype in HK-2 cells, the rBAT gene was silenced using antisense oligonucleotides complimentary to human rBAT mRNA. The reduced transport activity of cystine was then determined by radiolabeled cystine uptake measurements. RT-PCR and Western blot confirmed the expression of the rBAT gene in HK-2 cells. Considerable transport of the radio labeled cystine was observed in HK-2 cells and was linearly dependent on the incubation time with the amino acid. The cystine transport in rBAT knockdown cells after incubation with antisense oligonucleotides was significantly lower compared to control (0.76 vs. 0.98%; P=0.0008), proving a transient knock-down of the rBAT gene. This study demonstrates the presence of the b(0,+) amino acid transport system in human proximal tubular HK-2 cells when cultured in vitro. Inhibition of this transport system is possible by using antisense technology. A permanent inhibition of the cystine transport, based on our model, would be useful for the development and evaluation gene therapeutic approaches.

Evidence for significant contribution of a newly identified monoamine transporter (PMAT) to serotonin uptake in the human brain

The high affinity serotonin transporter (SERT) constitutes the principal pathway for removal of serotonin (5-HT) from extracellular fluid of brain, but evidence indicates that other transporters may also be involved in this process. We recently reported the cloning of a novel plasma membrane monoamine transporter (PMAT), which is abundantly expressed in the human brain and avidly transports 5-HT [Engel K, Zhou M, Wang J. Identification and characterization of a novel monoamine transporter in the human brain. J Biol Chem 2004;279:50042-9]. In this study, we evaluated whether PMAT contributes to total human brain uptake of 5-HT using a hybrid depletion approach in Xenopus laevis oocytes. We also examined whether PMAT interacts with selective serotonin reuptake inhibitors (SSRIs) using MDCK cells stably expressing recombinant human PMAT. Microinjection of total human brain poly(A)(+) mRNA into oocytes elicited approximately 2.5-3-fold increase in 5-HT uptake. Pre-hybridization of poly(A)(+) mRNA with PMAT or SERT antisense oligonucleotides significantly reduced mRNA-induced 5-HT uptake. An additive inhibitory effect was observed when poly(A)(+) mRNA was co-hybridized with both PMAT and SERT antisense oligonucleotides. In contrast, mRNA-induced 5-HT uptake was not affected by pre-hybridization with sense oligonucleotides. These data suggest that functional transcripts of PMAT are present in the human brain, and the PMAT transporter may be significantly involved in brain uptake of 5-HT. All five tested SSRIs inhibited PMAT with IC(50) values ranging from 11 to 116 microM, which are much greater than clinically encountered concentrations, suggesting that PMAT activity is minimally affected by SSRI therapies.

Cloning, gene structure and dietary regulation of the type-IIc Na/Pi cotransporter in the mouse kidney

We have demonstrated previously that the type-IIc Na/Pi cotransporter is a growth-related renal Na/Pi cotransporter that is highly expressed in kidney of the weaning rat. In the present study, we investigated type-IIc Na/Pi cotransporter function further by cloning the mouse gene and characterizing the corresponding protein. The mouse type-IIc transporter amino acid sequence shows a high degree of similarity to the human (86%) and rat (95%) type-IIc Na/Pi-cotransporters. The mouse gene contained 14 exons and mapped to chromosome 2. The DNA sequence upstream from exon 1 is GC rich. The upstream region does not contain an apparent TATA box, but does contain two dietary Pi-responsive elements, which are potential binding sites for the transcription factor micro E3 (TFE3). Microinjection of mouse type-IIc cRNA into Xenopus oocytes demonstrated sodium-dependent Pi cotransport activity. The affinity for Pi was about 200 microM in 100 mM Na. Feeding adult mice fed a low-Pi diet increased the expression of type-IIc protein in the apical membrane of renal proximal tubular cells. Hybrid depletion studies suggested that the type-IIc transporter contributes to about 30% of Na/Pi cotransport in the kidney of adult mice fed a low-Pi diet. The present study suggests that the type-IIc Na/Pi cotransporter is a functional of renal Pi transporter in adult mice fed a low-Pi diet.

Growth-related renal type II Na/Pi cotransporter

Growth is critically dependent on the retention of a variety of nutrients. The kidney contributes to this positive external balance. In the present study, we isolated a cDNA from the human and rat kidney that encodes a growth-related Na(+)-dependent inorganic phosphate (P(i)) cotransporter (type IIc). Microinjection of type IIc cRNA into Xenopus oocytes demonstrated sodium-dependent P(i) cotransport activity. Affinity for P(i) was 0.07 mm in 100 mm Na(+). The transport activity was dependent on extracellular pH. In electrophysiological studies, type IIc Na/P(i) cotransport was electroneutral, whereas type IIa was highly electrogenic. In Northern blotting analysis, the type IIc transcript was only expressed in the kidney and highly in weaning animals. In immunohistochemical analysis, the type IIc protein was shown to be localized at the apical membrane of the proximal tubular cells in superficial and midcortical nephrons of weaning rat kidney. Hybrid depletion experiments suggested that type IIc could function as a Na/P(i) cotransporter in weaning animals, but its role is reduced in adults. The finding of the present study suggest that the type IIc is a growth-related renal Na/P(i) cotransporter, which has a high affinity for P(i) and is electroneutral.

Growth factor regulation of enterocyte nutrient transport during intestinal adaptation

BACKGROUND

Intestinal adaptation occurs in response to injury or alteration in nutrient availability. It is both morphologic and physiologic in nature and can be mediated by growth factors and nutrients. Pathologic conditions such as short-bowel syndrome and inflammatory bowel disease lead to derangements in nutrient absorption that may exceed the body's regenerative and adaptive capacity. Failure to fully adapt often results in long-term dependence on parenteral nutrition, leading to decreased quality of life and excessive medical expenses. The therapeutic use of appropriate growth factors may increase the adaptive capabilities of the gut.

DATA SOURCE

Medline and current literature review.

CONCLUSIONS

The major known nutrient transporters present in the gut and the mechanisms by which growth factors alter transport activity during intestinal adaptation are summarized. Growth factors have the potential to improve nutrient absorption in some bowel diseases.

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.

Properties of the mutant Ser-460-Cys implicate this site in a functionally important region of the type IIa Na(+)/P(i) cotransporter protein

The substituted cysteine accessibility approach, combined with chemical modification using membrane-impermeant alkylating reagents, was used to identify functionally important structural elements of the rat type IIa Na(+)/P(i) cotransporter protein. Single point mutants with different amino acids replaced by cysteines were made and the constructs expressed in Xenopus oocytes were tested for function by electrophysiology. Of the 15 mutants with substituted cysteines located at or near predicted membrane-spanning domains and associated linker regions, 6 displayed measurable transport function comparable to wild-type (WT) protein. Transport function of oocytes expressing WT protein was unchanged after exposure to the alkylating reagent 2-aminoethyl methanethiosulfonate hydrobromide (MTSEA, 100 microM), which indicated that native cysteines were inaccessible. However, for one of the mutants (S460C) that showed kinetic properties comparable with the WT, alkylation led to a complete suppression of P(i) transport. Alkylation in 100 mM Na(+) by either cationic ([2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET), MTSEA) or anionic [sodium(2-sulfonatoethyl)methanethiosulfonate (MTSES)] reagents suppressed the P(i) response equally well, whereas exposure to methanethiosulfonate (MTS) reagents in 0 mM Na(+) resulted in protection from the MTS effect at depolarized potentials. This indicated that accessibility to site 460 was dependent on the conformational state of the empty carrier. The slippage current remained after alkylation. Moreover, after alkylation, phosphonoformic acid and saturating P(i) suppressed the slippage current equally, which indicated that P(i) binding could occur without cotransport. Pre-steady state relaxations were partially suppressed and their kinetics were significantly faster after alkylation; nevertheless, the remaining charge movement was Na(+) dependent, consistent with an intact slippage pathway. Based on an alternating access model for type IIa Na(+)/P(i) cotransport, these results suggest that site 460 is located in a region involved in conformational changes of the empty carrier.

Identification of a membrane protein, LAT-2, that Co-expresses with 4F2 heavy chain, an L-type amino acid transport activity with broad specificity for small and large zwitterionic amino acids

We have identified a new human cDNA, L-amino acid transporter-2 (LAT-2), that induces a system L transport activity with 4F2hc (the heavy chain of the surface antigen 4F2, also named CD98) in oocytes. Human LAT-2 is the fourth member of the family of amino acid transporters that are subunits of 4F2hc. The amino acid transport activity induced by the co-expression of 4F2hc and LAT-2 was sodium-independent and showed broad specificity for small and large zwitterionic amino acids, as well as bulky analogs (e.g. BCH (2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid)). This transport activity was highly trans-stimulated, suggesting an exchanger mechanism of transport. Expression of tagged N-myc-LAT-2 alone in oocytes did not induce amino acid transport, and the protein had an intracellular location. Co-expression of N-myc-LAT-2 and 4F2hc gave amino acid transport induction and expression of N-myc-LAT-2 at the plasma membrane of the oocytes. These data suggest that LAT-2 is an additional member of the family of 4F2 light chain subunits, which associates with 4F2hc to express a system L transport activity with broad specificity for zwitterionic amino acids. Human LAT-2 mRNA is expressed in kidney >>> placenta >> brain, liver > spleen, skeletal muscle, heart, small intestine, and lung. Human LAT-2 gene localizes at chromosome 14q11.2-13 (13 cR or approximately 286 kb from marker D14S1349). The high expression of LAT-2 mRNA in epithelial cells of proximal tubules, the basolateral location of 4F2hc in these cells, and the amino acid transport activity of LAT-2 suggest that this transporter contributes to the renal reabsorption of neutral amino acids in the basolateral domain of epithelial proximal tubule cells.

Identification and characterization of a membrane protein (y+L amino acid transporter-1) that associates with 4F2hc to encode the amino acid transport activity y+L. A candidate gene for lysinuric protein intolerance

We have identified a new human cDNA (y+L amino acid transporter-1 (y+LAT-1)) that induces system y+L transport activity with 4F2hc (the surface antigen 4F2 heavy chain) in oocytes. Human y+LAT-1 is a new member of a family of polytopic transmembrane proteins that are homologous to the yeast high affinity methionine permease MUP1. Other members of this family, the Xenopus laevis IU12 and the human KIAA0245 cDNAs, also co-express amino acid transport activity with 4F2hc in oocytes, with characteristics that are compatible with those of systems L and y+L, respectively. y+LAT-1 protein forms a approximately 135-kDa, disulfide bond-dependent heterodimer with 4F2hc in oocytes, which upon reduction results in two protein bands of approximately 85 kDa (i.e. 4F2hc) and approximately 40 kDa (y+LAT-1). Mutation of the human 4F2hc residue cysteine 109 (Cys-109) to serine abolishes the formation of this heterodimer and drastically reduces the co-expressed transport activity. These data suggest that y+LAT-1 and other members of this family are different 4F2 light chain subunits, which associated with 4F2hc, constitute different amino acid transporters. Human y+LAT-1 mRNA is expressed in kidney >> peripheral blood leukocytes >> lung > placenta = spleen > small intestine. The human y+LAT-1 gene localizes at chromosome 14q11.2 (17cR approximately 374 kb from D14S1350), within the lysinuric protein intolerance (LPI) locus (Lauteala, T., Sistonen, P. , Savontaus, M. L., Mykkanen, J., Simell, J., Lukkarinen, M., Simmell, O., and Aula, P. (1997) Am. J. Hum. Genet. 60, 1479-1486). LPI is an inherited autosomal disease characterized by a defective dibasic amino acid transport in kidney, intestine, and other tissues. The pattern of expression of human y+LAT-1, its co-expressed transport activity with 4F2hc, and its chromosomal location within the LPI locus, suggest y+LAT-1 as a candidate gene for LPI.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Multiple pathways for L-methionine transport in brush-border membrane vesicles from chicken jejunum

1. The intestinal transport of L-methionine has been investigated in brush-border membrane vesicles isolated from the jejunum of 6-week-old chickens. L-Methionine influx is mediated by passive diffusion and by Na+-dependent and Na+-independent carrier-mediated mechanisms. 2. In the absence of Na+, cis-inhibition experiments with neutral and cationic amino acids indicate that two transport components are involved in L-methionine influx: one sensitive to L-lysine and the other sensitive to 2-aminobicyclo[2.2. 1]heptane-2-carboxylic acid (BCH). The L-lysine-sensitive flux is strongly inhibited by L-phenylalanine and can be broken down into two pathways, one sensitive to N-ethylmaleimide (NEM) and the other to L-glutamine and L-cystine. 3. The kinetics of L-methionine influx in Na+-free conditions is described by a model involving three transport systems, here called a, b and c: systems a and b are able to interact with cationic amino acids but differ in their kinetic characteristics (system a: Km = 2.2 +/- 0.3 microM and Vmax = 0.13 +/- 0.005 pmol (mg protein)-1 (2 s)-1; system b: Km = 3.0 +/- 0.3 mM and Vmax = 465 +/- 4.3 pmol (mg protein)-1 (2 s)-1); system c is specific for neutral amino acids, has a Km of 1.29 +/- 0.08 mM and a Vmax of 229 +/- 5.0 pmol (mg protein)-1 (2 s)-1 and is sensitive to BCH inhibition. 4. The Na+-dependent component can be inhibited by BCH and L-phenylalanine but cannot interact either with cationic amino acids or with alpha-(methylamino)isobutyrate (MeAIB). 5. The kinetic analysis of L-methionine influx under a Na+ gradient confirms the activity of the above described transport systems a and b. System a is not affected by the presence of Na+ while system b shows a 3-fold decrease in the Michaelis constant and a 1.4-fold increase in Vmax. In the presence of Na+, the BCH-sensitive component can be subdivided into two pathways: one corresponds to system c and the other is Na+ dependent and has a Km of 0.64 +/- 0. 013 mM and a Vmax of 391 +/- 2.3 pmol (mg protein)-1 (2 s)-1. 6. It is concluded that L-methionine is transported in the chicken jejunum by four transport systems, one with functional characteristics similar to those of system bo, + (system a); a second (system b) similar to system y+, which we suggest naming y+m to account for its high Vmax for L-methionine transport in the absence of Na+; a third (system c) which is Na+ independent and has similar properties to system L; and a fourth showing Na+ dependence and tentatively identified with system B.

Drug-nutrient interactions: inhibition of amino acid intestinal absorption by fluoxetine

Fluoxetine is one of the most widely used antidepressants and nowadays it is also being used to manage obesity problems. In our laboratory we demonstrated that the drug inhibited sugar absorption (Monteiro et al. 1993). The aim of the present work was to determine the effect of fluoxetine on intestinal leucine absorption. Using a procedure of successive absorptions in vivo the drug diminished amino acid absorption by 30% (P < 0.001). Experiments in vitro in isolated jejunum also revealed a reduction in leucine uptake of 37% (P < 0.001). In both cases fluoxetine only affected mediated transport without altering diffusion. In a preparation enriched in basolateral membrane, fluoxetine inhibited the Na+,K(+)-ATPase (EC 3.6.1.37) activity (55%; P < 0.001) in a non-competitive manner with an inhibition constant (Ki) value of 0.92 mM. Leucine uptake by brush-border membrane vesicles was diminished by the drug (a reduction of 48% was observed at 30s, P < 0.001); only the apical Na(+)-dependent transport system of the amino acid was modified and the inhibition was non-competitive. Leucine uptake in the presence of lysine indicated that transporter B was involved. These results suggest that fluoxetine reduces leucine absorption by its action on the basolateral and apical membrane of the enterocyte; the nutritional status of the patients under drug treatment may be affected as neutral amino acid absorption is decreased.

Role of rBAT gene products in cystinuria

To investigate whether rBAT gene products function as a crystine transporter component or as a transport activator, we microinjected several C-terminal deletion mutants of rBAT cRNA into Xenopus oocytes, and measured transport activity for arginine, leucine and cystine in the presence and absence of sodium. Wild type rBAT significantly stimulated the uptake of all 3 amino acids 10-20 fold compared to control mutants. On the other hand, no mutant, except a Δ511-685 mutant, stimulated the uptake of these amino acids. However, the Δ511-685 mutant significantly increased the uptake of arginine. In the presence of sodium, the Δ511-685 mutant also increased the uptake of leucine. The Δ511-685 mutant did not stimulate crystine uptake in the presence and absence of sodium. Furthermore, inhibition of L-arginine uptake by L-homoserine was seen only in the presence of sodium. These results suggest that mutant rBAT stimulates the endogenous amino acid transport system y+ in oocytes. Finally, rBAT gene products, as the primary cause of cystinuria, may function as activators of the amino acid transport system in renal brush border membrane.

Chloride-dependent amino acid transport in the small intestine: occurrence and significance

The unidirectional influx of amino acids, D-glucose and ions across the brush-border membrane of the small intestine of different species has been measured in vitro with emphasis on characterization of topographic and species differences and on chloride dependence. The regional differences in transport along the small intestine are outlined and shown to be caused by variation in transport capacity, while the apparent affinity constants are unchanged. Rabbit small intestine is unique by exhibiting maximal rates of transport in the distal ileum and a very steep decline in the oral direction from where tissues are normally harvested for preparation of brush-border membrane vesicles. Transport in the guinea pig and rat is much more constant throughout the small intestine. Since the capacity of nutrient carriers is regulated by their substrates it is possible that bacterial breakdown of peptides and proteins in rabbit distal ileum increases the concentration of amino acids leading to an upregulation of the carriers. Chloride dependence is a characteristics of the carrier rather than the transported amino acid, and is used to improve the classification of amino acid carriers in rabbit small intestine. In this species the imino acid carrier, the beta-amino acid carrier, and the beta-alanine carrier, which should be renamed the B0,+ carrier, are chloride-dependent. The steady-state mucosal uptake of classical substrates for these carriers in biopsies from the human duodenum is also chloride-dependent. The carrier of beta-amino acids emerges as ubiquitous and chloride-dependent, and evidence of cotransport with both sodium and chloride is reviewed. A sodium:chloride:2-methyl-aminoisobutyric acid coupling stoichiometry of approx. 2:1:1 is suggested by ion activation studies. Direct measurements of coupled ion fluxes in rabbit distal ileum confirm that sodium, chloride and 2-methyl-aminoisobutyric acid are cotransported on the imino acid carrier with an identical influx stoichiometry. Control experiments and reference to the literature on the electrophysiology of the small intestine exclude alterations of the membrane potential as a feasible explanation of the chloride dependence. Thus, it is concluded that chloride is cotransported with both sodium and 2-methyl-aminoisobutyric acid across the brush-border membrane of rabbit distal ileum.

Sodium-independent currents of opposite polarity evoked by neutral and cationic amino acids in neutral and basic amino acid transporter cRNA-injected oocytes

To elucidate the electrical events associated with the movement of amino acids by the neutral and basic amino acid transporter (NBAT)-encoded protein (Yan, N., Mosckovitz, R., Gerber, L.D., Mathew, S., Murty, V.V. V.S., Tate, S.S., and Udenfriend, S. (1994) Proc. Natl. Acad. Sci. USA 91, 7548-7552), we have investigated the membrane potential and current changes associated with the increased transport of amino acids across the cell membrane of NBAT cRNA-injected Xenopus laevis oocytes. Superfusion of 0.05 mM L-phenylalanine, in current-clamped NBAT-injected oocytes, caused a hyperpolarization (8.5 +/- 0.9 mV), but superfusion of L-arginine caused a depolarization (18.3 +/- 1.3 mV). In voltage-clamped (-60 mV) oocytes, superfusion of L-phenylalanine evoked a sodium- and chloride-independent, saturable (Km = 0.34 +/- 0.02 mM, Imax = 31.3 +/- 0.5 nA), outward current. This outward current was reduced in the presence of high external [K] and was barium-sensitive. Outward currents were also evoked by L-leucine, L-glutamine, L-alanine, D-phenylalanine, and L-beta-phenylalanine. Superfusion of L-arginine evoked a saturable (Km = 0.09 +/- 0.02 mM, Imax = -29.2 +/- 1.3 nA) inward current; L-lysine and D-arginine also evoked inward currents. L-Glutamate and beta-alanine failed to evoke any currents. Effluxes of L-[3H]phenylalanine and L-[3H]arginine were trans-stimulated in the presence of either amino acid. Flux-current comparisons indicated amino acid:charge movement stoichiometry of 1:1 for both neutral and cationic amino acids. These findings indicate that the amino acid transport activity(ies) expressed in NBAT cRNA-injected oocytes is electrogenic by a mechanism including the outward movement of a net positive charge (potassium ion or cationic amino acid) in exchange for uptake of a neutral amino acid.

Drug excretion mediated by a new prototype of polyspecific transporter

Cationic drugs of different types and structures (antihistaminics, antiarrhythmics, sedatives, opiates, cytostatics and antibiotics, for example) are excreted in mammals by epithelial cells of the renal proximal tubules and by hepatocytes in the liver. In the proximal tubules, two functionally disparate transport systems are involved which are localized in the basolateral and luminal plasma membrane and are different from the previously identified neuronal monoamine transporters and ATP-dependent multidrug exporting proteins. Here we report the isolation of a complementary DNA from rat kidney that encodes a 556-amino-acid membrane protein, OCT1, which has the functional characteristics of organic cation uptake over the basolateral membrane of renal proximal tubules and of organic cation uptake into hepatocytes. OCT1 is not homologous to any other known protein and is found in kidney, liver and intestine. As OCT1 translocates hydrophobic and hydrophilic organic cations of different structures, it is considered to be a new prototype of polyspecific transporters that are important for drug elimination.

cDNA cloning of a rat small-intestinal Na+/SO4(2-) cotransporter

We have isolated a cDNA (ileal NaSi-1) from rat small intestine by homology screening with a cDNA (renal NaSi-1) encoding rat kidney cortex Na(+)-SO4(2-) cotransport. Ileal NaSi-1 cRNA specifically stimulates Na(+)-dependent SO4(2-) uptake in a time- and dose-dependent manner in Xenopus laevis oocytes, with kinetic parameters almost identical to those of the renal NaSi-1. Ileal NaSi-1 cDNA contains 2722 base pairs (bp), almost 500 bp more than the renal NaSi-1 cDNA; however, it encodes a protein of 595 amino acids identical to the renal NaSi-1 protein. Northern blot analysis shows strong signals in rat lower small intestine and kidney cortex (2.9 x 10(3) and 2.3 x 10(3) bases), with the ileal NaSi-1 corresponding to the longer transcript. We conclude that we have identified a rat ileal cDNA that encodes a membrane protein most likely involved in brush-border Na(+)-SO4(2-) cotransport. It differs to the renal NaSi-1 only in the length of the 3' untranslated region, suggesting that the major difference lies in the differential use of polyadenylation signals.

Expression of a renal Na(+)-nucleoside cotransport system (N2) in Xenopus laevis oocytes

Xenopus laevis oocytes have been used for the expression of a renal, pyrimidine-selective, Na(+)-nucleoside cotransporter (N2). As compared to its uptake in water-injected oocytes, Na(+)-dependent thymidine uptake was enhanced in a time- and dose-dependent manner in oocytes injected with rat renal cortex total poly(A)+ RNA. An increased uptake was also observed after injection of size fractionated rat renal cortex poly(A)+ RNA (2-3 kb). Consistent with the selectivity of the N2 nucleoside transporter, cytidine significantly inhibited Na(+)-dependent thymidine uptake in oocytes injected with total poly(A)+ RNA whereas guanosine and formycin B did not. Na(+)-dependent thymidine uptake was also enhanced in oocytes injected with size fractionated human renal cortex poly(A)+ RNA (2-3 kb). The above data demonstrate functional expression of renal cortex, Na(+)-nucleoside cotransporters in Xenopus laevis oocytes.

Expression of Na+-independent isoleucine transport activity from rat brain in Xenopus laevis oocytes

Poly(A)+ RNA from C6-BU-1 rat glioma cells and rat astroglial cells induced isoleucine transport activity when injected into Xenopus laevis oocytes. The Na+-independent component of isoleucine transport was inhibited by leucine, phenylalanine and 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) but neither by methylaminoisobutyric acid (MeAIB) nor lysine. A Km value of approx. 100 microM was determined for the Na+-independent transport of isoleucine. These data are in accordance with expression of a system L like transporter. By injection of size fractionated poly(A)+ RNA a length of approx. 1.9 kb was determined for the pertinent mRNA.

Expression of rat ileal Na(+)-sulphate cotransport in Xenopus laevis oocytes: functional characterization

Small-intestinal sulphate absorption is a Na(+)-dependent process having its highest rate in the ileum; it involves brush-border membrane Na(+)-sulphate cotransport. Injection of rat ileal mRNA into Xenopus laevis oocytes induced Na(+)-dependent sulphate uptake in a dose-dependent manner, with no apparent effect on Na(+)-independent sulphate uptake. For mRNA-induced transport, the apparent Km value for sulphate interaction was 0.6 +/- 0.2 mM and that for sodium interaction was 25 +/- 2 mM (Hill coefficient: 2.3 +/- 0.3). mRNA-induced transport, was inhibited by thiosulphate, but not by phosphate or 4,4,'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). Using a rat renal Na(+)-sulphate cotransporter cDNA as a probe [NaSi-1; Markovich et al. (1993) Proc Natl Acad Sci USA 90:8073-8077], the highest hybridization signals (2.3 kb and 2.9 kb) were obtained in size fractions showing the highest expression of Na(+)-dependent sulphate transport in oocytes. Hybrid depletion experiments using antisense oligonucleotides (from the NaSi-1 cDNA sequence), provided further evidence that rat small-intestinal (ileal) Na(+)-sulphate cotransport is closely related to rat proximal-tubular brush-border membrane Na(+)-sulphate cotransport.

Poly(A)+ RNA from the mucosa of rat jejunum induces novel Na(+)-dependent and Na(+)-independent leucine transport activities in in oocytes of Xenopus laevis

Complementary DNA clones have been isolated recently from rat (D2) and rabbit kidney (rBAT) which induce increased Na(+)-independent Leu and Lys transport activities (System b0, +) when expressed in oocytes of Xenopus laevis. These cDNAs encode type II membrane glycoproteins which show significant homology to the heavy chain of the human and mouse 4F2 surface antigen (4F2hc). Injection of human 4F2hc cRNA into oocytes also results in induction of Leu/Lys transport activity, but with differing cation requirements for the two amino acids (Na(+)-dependent for Leu, Na(+)-independent for Lys: system y+L). System y+L is a newly discovered zwitterionic/cationic amino acid transporter first described in human erythrocytes. Here we have examined the characteristics of Leu transport in Xenopus oocytes microinjected with mRNA from the mucosa of rat jejunum. L-Leu uptake during 10 min (0.2 mM, 20 degrees C) reached 20 pmol/oocyte compared with endogenous uptake by water-injected oocytes of typically 3-4 pmol/oocyte. The expressed transport activity was 80% Na(+)-dependent. The Na(+)-dependent component of the expressed flux was saturable (Km app 0.20 mM) and inhibited by Lys, but not by Ala or Phe. The minor Na(+)-independent component of expressed Leu transport activity was also saturable (Km app 0.10 mM). Amino acid inhibition studies resolved this flux into two main components, one of which was inhibited by Lys, Ala and Phe and another which was only inhibited by Lys. There was a small residual component of Na(+)-independent Leu transport which was insensitive to inhibition by Lys. Experiments utilizing polymerase chain reaction (PCR) demonstrated the presence of both D2 and 4F2hc message in rat jejunum. Hybrid-depletion of jejunal mRNA with an antisense oligonucleotide complementary to D2 had no effect on the expression of Na(+)-linked Leu transport activity, but reduced the smaller Na(+)-independent component of Leu transport by 40%, suggesting only a minor role of D2 in the expression of rat intestinal Leu transport activity. Although the properties of Na(+)-dependent Leu transport were, with the exception of a lack of inhibition by Ala and Phe, consistent with erythrocyte y+L, hybrid-depletion of jejunal mRNA with an antisense oligonucleotide complementary to 4F2hc had no detectable effect on the expressed transport activity. We conclude, therefore, that mRNA from rat jejunum encodes novel Na(+)-dependent and Na(+)-independent transport activities unrelated to the D2/4F2hc glycoproteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Endogenous amino acid transport systems and expression of mammalian amino acid transport proteins in Xenopus oocytes

Oocyte amino acid transport has physiological significance to oocytes and practical importance to molecular biologists and transport physiologists. Expression of heterologous mRNA in Xenopus oocytes is currently being used to help clone cDNAs for amino acid transporters and their effectors. A major question to be resolved in many of these studies is whether the injected mRNA codes for a transporter or an activator of an endogenous system. Nevertheless, the cDNAs of several families of amino acid transporters or their activators appear already to have been cloned. One such transporter is the anion exchanger, band 3, which may also transport glycine and taurine under some important physiological conditions such as hypoosmotic stress. Site-directed mutagenesis of band 3 has already shown that an amino acid residue believed to be at or near the active site nevertheless does not appear to influence Cl- transport in Xenopus oocytes expressing the modified band 3 protein. Continuation of such studies along with examination of transport of all possible substrates of band 3 should yield insight into the relationship between the structure and function of this transporter. Each of three other families not only contains amino acid transporters, but also appears to contain members that serve as transporters of neurotransmitters or their metabolites. Because of the distinct structural differences in the preferred substrates of different transporters within some of these families, elucidation of the tertiary and possibly quaternary structural relationships among the members of such families may reveal transport mechanisms. In addition, the grouping of neurotransmitters or their metabolites according to the family to which their transport systems and transporters belong could yield insight into mechanisms of brain development, function and evolution. Another family of transporters for cationic amino acids also serves, at least in one case, as a viral receptor. Hence, these or other transporters also could conceivably function in eggs as receptors for sperm and, more broadly, in cell-cell interactions as well as in amino acid transport. Moreover, a family of apparent amino acid transport activators are homologous to a family of glycosidases, so these activators could also serve to recognize carbohydrate structures on other cells or the extracellular matrix. Some of these activators appear to increase more than one amino acid transport activity in Xenopus oocytes. In other studies, expression of heterologous mRNA in oocytes has led apparently to detection of inhibitors as well as activators of amino acid transport. Some amino acid transport systems also could conceivably contain nucleic acid as well as glycoprotein components.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of system L and system y+ amino acid transport activity in cultured vascular smooth muscle cells

The uptake of L-leucine and L-lysine into vascular smooth muscle cells cultured from the aortas of rats has been investigated. Both amino acids are taken up by saturable systems that are independent of the presence of a Na+ gradient and can be stimulated in trans by neutral bulky amino acids for leucine and cationic amino acids for lysine. Leucine uptake is inhibited competitively in cis by several neutral amino acids, whereas lysine uptake is inhibited strongly by other cationic amino acids but also significantly by neutral amino acids such as leucine. The leucine inhibition is noncompetitive. Cells preloaded with leucine and lysine could also export these amino acids and the rate of efflux was stimulated by the presence of appropriate amino acids in trans. These data are all consistent with leucine being transported largely if not entirely by System L and lysine by the System y+ transporter.