Root uptake of cationic amino acids by Arabidopsis depends on functional expression of amino acid permease 5

* Specific transporters mediate uptake of amino acids by plant roots. Earlier studies have indicated that the lysine histidine transporter 1 and amino acid permease 1 participate in this process, but although plant roots have been shown to absorb cationic amino acids with high affinity, neither of these transporters seems to mediate transport of L-arginine (L-Arg) or L-lysine (L-Lys). * Here, a collection of T-DNA knockout mutants were screened for alterations in Arabidopsis root uptake rates of L-Arg and it was found that only the AAP5 mutant displayed clear phenotypic divergence on high concentrations of L-Arg. A second screen using low concentrations of (15)N-labelled L-Arg in the growth media also identified AAP5 as being involved in L-Arg acquisition. * Momentaneous root uptake of basic amino acids was strongly affected in AAP5 mutant lines, but their uptake of other types of amino acids was only marginally affected. Comparisons of the root uptake characteristics of AAP5 and LHT1 mutants corroborated the hypothesis that the two transporters have distinct affinity spectra in planta. * Root uptake of all tested amino acids, except L-aspartic acid (L-Asp), was significantly affected in double AAP5*LHT1 mutants, suggesting that these two transporters account for a major proportion of roots' uptake of amino acids at low concentrations.

Twin Arginine Translocation (Tat)-dependent Export in the Apparent Absence of TatABC or TatA Complexes Using Modified Escherichia coli TatA Subunits That Substitute for TatB

The twin arginine translocation pathway exports folded proteins across the cytoplasmic membrane of many bacteria. In Escherichia coli and other Gram-negative bacteria, TatA, TatB, and TatC are all essential for efficient translocation, and current models suggest that separate TatABC and TatA complexes coalesce at the point of translocation. However, other microbes appear only to possess tatA and tatC genes. In Escherichia coli, virtually no translocation is observed when only TatA and TatC are present, but several mutations at the extreme N terminus of TatA were shown to support translocation. Here we show that these apparently bifunctional mutant TatA variants can function as typical TatA components because translocation is observed when they are co-expressed with TatBC, and they assemble into large, heterogeneous complexes that resemble wild type TatA complexes. However, cells expressing TatC plus the mutant TatA variants do not contain complexes that resemble the expected 370-kDa TatABC complex, clearly indicating that the mutant TatA forms cannot assemble efficiently, or stably, into this complex. The simultaneous expression of wild type TatA furthermore blocks translocation activity, suggesting that the mutant TatA forms preferentially bind to other TatA molecules rather than TatC. Surprisingly, we observe translocation in the absence of detectable free TatA, when translational fusions of the mutant TatAs with TatC are expressed. Transport can thus proceed in the simultaneous absence of TatABC and TatA complexes at detectable levels, and we conclude that the active translocon may be formed from dynamic twin arginine translocation complexes, one or more of which may await characterization.

ArgR-dependent repression of arginine and histidine transport genes in Escherichia coli K-12

In Escherichia coli L-arginine is taken up by three periplasmic binding protein-dependent transport systems that are encoded by two genetic loci: the artPIQM-artJ and argT-hisJQMP gene clusters. The transcription of the artJ, artPIQM and hisJQMP genes and operons is repressed by liganded ArgR, whereas argT, encoding the LAO (lysine, arginine, ornithine) periplasmic binding protein, is insensitive to the repressor. Here we characterize the repressible Esigma70 P artJ, P artP and P hisJ promoters and demonstrate that the cognate operators consist of two 18 bp ARG boxes separated by 3 bp. Determination of the energy landscape of the ArgR-operator contacts by missing contact probing and mutant studies indicated that each box of a pair contributes to complex formation in vitro and to the repressibility in vivo, but to a different extent. The organization of the ARG boxes and promoter elements in the control regions of the uptake genes is distinct from that of the arginine biosynthetic genes. The hisJQMP operon is the first member of the E. coli ArgR regulon, directly repressed by liganded ArgR, where none of the core promoter elements overlaps the ARG boxes. Single round in vitro transcription assays and DNase I footprinting experiments indicate that liganded ArgR inhibits P artJ and P artP promoter activity by steric exclusion of the RNA polymerase. In contrast, ArgR-mediated repression of P hisJ by inhibition of RNA polymerase binding appears to occur through topological changes of the promoter region.

Error-free RAD52 pathway and error-prone REV3 pathway determines spontaneous mutagenesis in Saccharomyces cerevisiae

Using the CAN1 gene in haploid cells or heterozygous diploid cells, we characterized the effects of mutations in the RAD52 and REV3 genes of Saccharomyces cerevisiae in spontaneous mutagenesis. The mutation rate was 5-fold higher in the haploid rad52 strain and 2.5-fold lower in rev3 than in the wild-type strain. The rate in the rad52 rev3 strain was as low as in the wild-type strain, indicating the rad52 mutator phenotype to be dependent on REV3. Sequencing indicated that G:C-->T:A and G:C-->C:G transversions increased in the rad52 strain and decreased in the rev3 and rad52 rev3 strains, suggesting a role for REV3 in transversion mutagenesis. In diploid rev3 cells, frequencies of can1Delta::LEU2/can1Delta::LEU2 from CAN1/can1Delta::LEU2 due to recombination were increased over the wild-type level. Overall, in the absence of RAD52, REV3-dependent base-substitutions increased, while in the absence of REV3, RAD52-dependent recombination events increased. We further found that the rad52 mutant had an increased rate of chromosome loss and the rad52 rev3 double mutant had an enhanced chromosome loss mutator phenotype. Taken together, our study indicates that the error-free RAD52 pathway and error-prone REV3 pathway for rescuing replication fork arrest determine spontaneous mutagenesis, recombination, and genome instability.

Identification of a chloride ion binding site in Na+/Cl -dependent transporters

The recent determination of the crystal structure of the leucine transporter from Aquifex aeolicus (aaLeuT) has provided significant insights into the function of neurotransmitter:sodium symporters. Transport by aaLeuT is Cl(-) independent, whereas many neurotransmitter:sodium symporters from higher organisms depend on Cl(-) ions. However, the only Cl(-) ion identified in the aaLeuT structure interacts with nonconserved residues in extracellular loops, and thus the relevance of this binding site is unclear. Here, we use calculations of pK(A)s and homology modeling to predict the location of a functionally important Cl(-) binding site in serotonin transporter and other Cl(-)-dependent transporters. We validate our model through the site-directed mutagenesis of residues predicted to coordinate the Cl(-) ion and through the observation of sequence conservation patterns in other Cl(-)-dependent transporters. The proposed site is located midway across the membrane and is formed by residues from transmembrane helices 2, 6, and 7. It is close to the Na1 sodium binding site, thus providing an explanation for the coupling of Cl(-) and Na(+) ions during transport. Other implications of the model are also discussed.

Effect of ocular hypertension on retinal nitridergic pathway activity

PURPOSE

Understanding the mechanisms of neuronal cell death in glaucoma is important for devising new treatments. Excitatory amino acids, excessive Ca(2+) influx, and formation of nitric oxide (NO) via NO synthase (NOS)-1 could be involved in glaucomatous neuropathy. The purpose of the present study was to examine the retinal nitridergic pathway activity in rats exposed to experimentally elevated intraocular pressure.

METHODS

Weekly injections of HA were performed unilaterally in the rat anterior chamber, whereas the contralateral eye was injected with saline solution. At 3 or 6 weeks of treatment, retinal NOS activity was assessed through the conversion of (3)H-L-arginine to (3)H-L-citrulline, whereas NOS-1, -2, and -3 levels were assessed by Western blotting. L-Arginine uptake was measured using (3)H-l-arginine, whereas mRNA levels of L-arginine transporters were determined by semiquantitative RT-PCR. In addition, cyclic guanosine monophosphate (cGMP) levels were quantified by radioimmunoassay.

RESULTS

At both 3 and 6 weeks of treatment, NOS activity significantly increased in HA-injected eyes although no changes in retinal NOS-1, -2, or -3 levels were observed in eyes injected with HA. L-Arginine influx and mRNA levels of cationic amino acid transporter type (CAT)-1 and -2 significantly increased in retinas from hypertensive eyes. Retinal cGMP levels significantly increased in eyes injected with HA for 3 but not 6 weeks.

CONCLUSIONS

These results suggest a significant activation of the retinal nitridergic pathway in hypertensive eyes.

The amino acid L-lysine blocks the disruptive effect of phencyclidine on prepulse inhibition in mice

RATIONALE

The cognitive and attentional deficits observed in schizophrenic patients are now considered central to the pathophysiology of the disorder. These deficits include an inability to filter sensory input as measured by, e.g., prepulse inhibition (PPI) reflex. Administration of phencyclidine (PCP), a drug that can induce a schizophrenia-like psychosis in humans, disrupts PPI in experimental animals. In rodents, this PCP-induced deficit can be blocked by pretreatment with nitric oxide (NO) synthase inhibitors. This suggests that some of the behavioral effects of PCP are mediated via NO. The substrate for in vivo NO production is L-arginine, and active transport of L-arginine via the cationic amino acid transporter may serve as a regulatory mechanism in NO production.

OBJECTIVES

The aim of the present study was to study the effects of L-arginine transport inhibition, using acute and repeated L-lysine treatment, on PCP-induced disruption of PPI in mice.

RESULTS

Subchronic, and to some extent acute, pretreatment with L-lysine blocked a PCP-induced deficit in PPI without affecting basal PPI.

CONCLUSIONS

L-lysine has been shown to block L-arginine transport in vitro, most likely via a competitive blockade and down regulation of cationic amino acid transporters. However, the importance of L-arginine transport as a regulatory mechanism in NO production in vivo is still not clear. The present results lend further support to the notion that some of the effects of PCP in the central nervous system are mediated via NO and that L-arginine transport may play a role in the regulation of NO production in the brain.

Protein characterization of NA+-independent system L amino acid transporter 3 in mice: a potential role in supply of branched-chain amino acids under nutrient starvation

We recently cloned the human Na(+)-independent system L neutral amino acid transporter LAT3. The aim of the present study was to characterize the molecular nature of mouse LAT3 at the protein level. Isolated mouse LAT3 showed 83% identity to human LAT3. Xenopus oocytes injected with mouse LAT3 cRNA showed the same functional property as human LAT3. Reverse transcriptase-polymerase chain reaction revealed apparent transcripts of mouse LAT3 in the liver, skeletal muscle, and pancreas, an expression pattern identical to that found in humans. Antibody generated against mouse LAT3 detected both approximately 58-kd and 48-kd bands in the sample from liver and only a 48-kd band in skeletal muscle and pancreas. Immunohistochemical study showed its clear localization in the plasma membrane of liver and skeletal muscle, whereas it was only detectable in the endoplasmic reticulum and in crystalline inclusions in pancreatic acinar cells. Starvation induced up-regulation of mouse LAT3 protein and mRNA in both liver and skeletal muscle but not in pancreas. These results suggest that LAT3 may indeed function as an amino acid transporter, transporting branched-chain amino acids from liver and skeletal muscle to the bloodstream and thereby participating in the regulatory system of interorgan amino acid nutrition.

Structure and Function of Cationic Amino Acid Transporters (CATs)

The CAT proteins (CAT for cationic amino acid transporter) are amongst the first mammalian amino acid transporters identified on the molecular level and seem to be the major entry path for cationic amino acids in most cells. However, CAT proteins mediate also efflux of their substrates and thus may also deplete cells from cationic amino acids under certain circumstances. The CAT proteins form a subfamily of the solute carrier family 7 (SLC7) that consists of four confirmed transport proteins for cationic amino acids: CAT-1 (SLC7A1), CAT-2A (SLC7A2A), CAT-2B (SLC7A2B), and CAT-3 (SLC7A3). SLC7A4 and SLC7A14 are two related proteins with yet unknown function. One focus of this review lies on structural and functional differences between the different CAT isoforms. The expression of the CAT proteins is highly regulated on the level of transcription, mRNA stability, translation and subcellular localization. Recent advances toward a better understanding of these mechanisms provide a second focus of this review.

Comprehensive screening of Arabidopsis mutants suggests the lysine histidine transporter 1 to be involved in plant uptake of amino acids

Plant nitrogen (N) uptake is a key process in the global N cycle and is usually considered a "bottleneck" for biomass production in land ecosystems. Earlier, mineral N was considered the only form available to plants. Recent studies have questioned this dogma and shown that plants may access organic N sources such as amino acids. The actual mechanism enabling plants to access amino acid N is still unknown. However, a recent study suggested the Lysine Histidine Transporter 1 (LHT1) to be involved in root amino acid uptake. In this study, we isolated mutants defective in root amino acid uptake by screening Arabidopsis (Arabidopsis thaliana) seeds from ethyl methanesulfonate-treated plants and seeds from amino acid transporter T-DNA knockout mutants for resistance against the toxic D-enantiomer of alanine (Ala). Both ethyl methanesulfonate and T-DNA knockout plants identified as D-Ala resistant were found to be mutated in the LHT1 gene. LHT1 mutants displayed impaired capacity for uptake of a range of amino acids from solutions, displayed impaired growth when N was supplied in organic forms, and acquired substantially lower amounts of amino acids than wild-type plants from solid growth media. LHT1 mutants grown on mineral N did not display a phenotype until at the stage of flowering, when premature senescence of old leaf pairs occurred, suggesting that LHT1 may fulfill an important function at this developmental stage. Based on the broad and unbiased screening of mutants resistant to D-Ala, we suggest that LHT1 is an important mediator of root uptake of amino acids. This provides a molecular background for plant acquisition of organic N from the soil.

Rapamycin stimulates arginine influx through CAT2 transporters in human endothelial cells

In endothelial cells Tumor Necrosis Factor-alpha (TNFalpha) stimulates arginine transport through the increased expression of SLC7A2/CAT2 transcripts. Here we show that also rapamycin, an inhibitor of mTOR kinase, stimulates system y(+)-mediated arginine uptake in human endothelial cells derived from either saphenous (HSVECs) or umbilical veins (HUVECs). When used together with TNFalpha, rapamycin produces an additive stimulation of arginine transport in both cell models. These effects are observed also upon incubation with AICAR, a stimulator of Adenosine-Monophosphate-dependent-Protein Kinase (AMPK) that produces a rapamycin-independent inhibition of the mTOR pathway. Rapamycin increases the V(max) of high affinity arginine transport and causes the appearance of a low affinity component that is particularly evident if the treatment is carried out in the presence of TNFalpha. RT-qPCR studies have demonstrated that these kinetic changes correspond to the induction of both the high affinity transporter CAT2B and the low affinity isoform CAT2A. Western blot and immunocytochemical analyses indicate that, consistently, the expression of CAT2 proteins is also stimulated under the same conditions. These changes are associated with an increase of the intracellular arginine concentration but with a decrease of NO production. Thus, our data suggest that mTOR activity is associated with the repression of CAT2 expression at mRNA and protein level.

The cationic amino acid transporters CAT1 and CAT3 mediate NMDA receptor activation-dependent changes in elaboration of neuronal processes via the mammalian target of rapamycin mTOR pathway

Neuronal activity influences protein synthesis and neuronal growth. Availability of nutrients, especially leucine and arginine, regulates the mammalian target of rapamycin (mTOR) pathway that controls cell growth. We show that NMDA receptor activation markedly reduces arginine transport by decreasing surface expression of the cationic amino acid transporters (CAT) 1 and 3. Depletion of CAT1 and CAT3 by RNA interference blocks influences of NMDA receptor activation on the mTOR pathway and neuronal process formation. Thus, the CATs mediate influences of NMDA receptor activation on the mTOR pathway that regulates neuronal processes.

Multiple Pathways for Cationic Amino Acid Transport in Rat Seminiferous Tubule Cells1

Arginine and ornithine are known to be important for various biological processes in the testis, but the delivery of extracellular cationic amino acids to the seminiferous tubule cells remains poorly understood. We investigated the activity and expression of cationic amino acid transporters in isolated rat Sertoli cells, peritubular cells, pachytene spermatocytes, and early spermatids. We assessed the l-arginine uptake kinetics, Na(+) dependence of transport, profiles of cis inhibition of uptake by cationic and neutral amino acids, and sensitivity to trans stimulation of cationic amino acid transporters, and studied the expression of the genes encoding them by RT-PCR. Our data suggest that l-arginine is taken up by Sertoli cells and peritubular cells, principally via system y(+)L (SLC3A2/SLC7A6) and system y(+) (SLC7A1 and SLC7A2), with system B(0+) making a minor contribution. By contrast, system B(0+), associated with system y(+)L (SLC3A2/SLC7A7 and SLC7A6), made a major contribution to the transport of cationic amino acids in pachytene spermatocytes and early spermatids. Sertoli cells had higher rates of l-arginine transport than the other seminiferous tubule cells. This high efficiency of arginine transport in Sertoli cells and the properties of the y(+)L system predominating in these cells strongly suggest that Sertoli cells play a key role in supplying germ cells with l-arginine and other cationic amino acids. Furthermore, whereas cytokines induce nitric oxide (NO) production in peritubular and Sertoli cells, little or no upregulation of arginine transport by cytokines was observed in these cells. Thus, NO synthesis does not depend on the stimulation of arginine transport in these somatic tubular cells.

Involvement of recognition and interaction of carnitine transporter in the decrease of L-carnitine concentration induced by pivalic acid and valproic acid

PURPOSE

Prodrugs with pivalic acid and valproic acid decrease L-carnitine concentration in plasma and tissues by urinary excretion of acylcarnitine as pivaloylcarnitine (PC) and valproylcarnitine (VC), respectively. We investigated the role of the Na+/L-carnitine cotransporter in the porcine kidney epithelial cell line, LLC-PK1 for the decrease of L-carnitine concentration.

METHODS

The uptake of L-[3H]carnitine, acetyl-L-[3H]carnitine (AC), L-[3H]PC and L-[3H]VC were investigated in LLC-PK1 cells seeded in a 6-well culture plate.

RESULTS

L-Carnitine and AC uptake in LLC-PK1 cells exhibited Na+ dependency. The Km values for L-carnitine and AC uptake were 11.0 and 8.18 microM, respectively. These results indicated expression of Na+/ L-carnitine cotransporter in LLC-PK1 cells. PC and VC inhibited Na+/L-carnitine cotransporter in the competitive (Ki = 90.4 microM) and noncompetitive (Ki = 41.6 microM) manners, respectively. PC and VC uptake by Na+/L-carnitine cotransporter were not observed in LLC-PK1 cells.

CONCLUSIONS

These data suggested that PC and VC formed in the body could not be reabsorbed in the kidney, resulting in the decrease of L-carnitine concentration. In addition, inhibition of L-carnitine reabsorption by VC with lower Ki value could induce the decrease of L-carnitine concentration. Collectively, the recognition and interaction of Na+/L-carnitine cotransporter are important factors for carnitine homeostasis.

Regional distribution of sodium-dependent excitatory amino acid transporters in rat spinal cord

BACKGROUND/OBJECTIVE

The excitatory amino acid transporters (EAATs), or sodium-dependent glutamate transporters, provide the primary mechanism for glutamate removal from the synaptic cleft. EAAT distribution has been determined in the rat brain, but it is only partially characterized in the spinal cord.

METHODS

The regional anatomic distribution of EAATs in spinal cord was assessed by radioligand autoradiography throughout cervical, thoracic, and lumbar cord levels in female Sprague-Dawley rats. EAAT subtype regional distribution was evaluated by inclusion of pharmacologic transport inhibitors in the autoradiography assays and by immunohistochemistry using subtype-specific polyclonal antibodies to rat GLT1 (EAAT2), GLAST (EAAT1), and EAAC1 (EAAT3) rat transporter subtypes.

RESULTS

[3H]-D-Aspartate binding was distributed throughout gray matter at the 3 spinal cord levels, with negligible binding in white matter. Inclusion of pharmacologic transport inhibitors indicates that the EAAT2/ GLT1 subtype represents 21% to 40% of binding. Both EAAT1/GLAST and EAAT3/EAAC1 contributed the remainder of binding. Immunoreactivity to subtype-specific antibodies varied, depending on cord level, and was present in both gray and white matter. All 3 subtypes displayed prominent immunoreactivity in the dorsal horn. EAAT3/EAAC1 and to a lesser extent EAAT1/GLAST immunoreactivity also occurred in a punctate pattern in the ventral horn.

CONCLUSIONS

The results indicate heterogeneity of EAAT distribution among spinal cord levels and regions. The presence of these transporters throughout rat spinal cord suggests the importance of their contributions to spinal cord function.

Increased L-arginine transport via system b0,+ in human proximal tubular cells exposed to albumin

Albumin has complex effects on PTECs (proximal tubular epithelial cells) and is able to stimulate growth or injury depending on its bound moieties. Albumin itself is a mitogen, inducing proliferation through a number of pathways. In PTEC exposed to purified albumin, polyamines are required for entry into the cell cycle and are critical for proliferation. Polyamines are synthesized from L-ornithine (itself derived by the action of arginase on L-arginine), and the transport and availability of L-arginine may thus be important for subsequent polyamine-dependent proliferation. In the present study we investigated radiolabelled cationic amino-acid transport in cultured PTEC exposed to 20 mg/ml ultrapure recombinant human albumin, describing the specific kinetic characteristics of transport and the expression of transporters. L-[3H]Arginine transport capacity in human PTEC is increased after exposure for 24 h to human albumin, mediated by the broad-scope high-affinity system b0,+ and, to a lesser extent, system y+L (but not system y+) transport. Increased transport is associated with increased b0,+-associated transporter expression. Inhibition of phosphoinositide 3-kinase, a key regulator of albumin endocytosis and signalling, inhibited proliferation, but had no effect on the observed increase in transport. PTEC proliferated in response to albumin. L-Lysine, a competitive inhibitor of L-arginine transport, had no effect on albumin-induced proliferation; however, arginine deprivation effectively reversed the albumin-induced proliferation observed. In conclusion, in PTEC exposed to albumin, increased L-arginine transport is mediated by increased transcription and activity of the apical b0,+ transport system. This may make L-arginine available as a substrate for the downstream synthesis of polyamines, but is not critical for cell proliferation.

The yeast PH domain proteins Slm1 and Slm2 are targets of sphingolipid signaling during the response to heat stress

The PH domain-containing proteins Slm1 and Slm2 were previously identified as effectors of the phosphatidylinositol-4,5-bisphosphate (PI4,5P(2)) and TORC2 signaling pathways. Here, we demonstrate that Slm1 and Slm2 are also targets of sphingolipid signaling during the heat shock response. We show that upon depletion of cellular sphingolipid levels, Slm1 function becomes essential for survival under heat stress. We further demonstrate that Slm proteins are regulated by a phosphorylation/dephosphorylation cycle involving the sphingolipid-activated protein kinases Pkh1 and Pkh2 and the calcium/calmodulin-dependent protein phosphatase calcineurin. By using a combination of mass spectrometry and mutational analysis, we identified serine residue 659 in Slm1 as a site of phosphorylation. Characterization of Slm1 mutants that mimic dephosphorylated and phosphorylated states demonstrated that phosphorylation at serine 659 is vital for survival under heat stress and promotes the proper polarization of the actin cytoskeleton. Finally, we present evidence that Slm proteins are also required for the trafficking of the raft-associated arginine permease Can1 to the plasma membrane, a process that requires sphingolipid synthesis and actin polymerization. Together with previous work, our findings suggest that Slm proteins are subject to regulation by multiple signals, including PI4,5P(2), TORC2, and sphingolipids, and may thus integrate inputs from different signaling pathways to temporally and spatially control actin polarization.

Identification of two cationic amino acid transporters required for nutritional signaling during mosquito reproduction

The defining characteristic of anautogenous mosquitoes is their requirement for a blood meal to initiate reproduction. The need for blood drives the association of vector and host, and is the primary reason why anautogenous mosquitoes are effective disease vectors. During mosquito vitellogenesis, a key process in reproduction, yolk protein precursor (YPP) gene expression is activated specifically in the fat body, the insect analogue of the vertebrate liver. We have demonstrated that blood meal derived amino acids (AAs) activate YPP genes via the target of rapamycin (TOR)-signal transduction pathway. Here we show, by stimulating fat bodies with balanced AA solutions lacking individual AAs, that specific cationic and branched AAs are essential for activation of the vitellogenin (vg) gene, the major YPP gene. Treatment of fat bodies with AA uptake inhibitors results in a strong inhibition of AA-induced vg gene expression proving that an active transport mechanism is necessary to transduce the AA signal. We identified two cationic AA transporters (CATs) in the fat body of Aedes aegypti females--Aa slimfast and iCAT2. RNAi knockdown of slimfast and iCAT2 results in a strong decrease in the response to AAs by the vg gene similar to that seen due to TOR inhibition. These data demonstrate that active uptake of specific AAs plays a key role in nutritional signaling during the onset of vitellogenic gene expression in mosquitoes and it is mediated by two cationic AA transporters.

Analysis of a 1-year-old cystinuric patient with recurrent renal stones

Cystinuria is a hereditary disorder of cystine and dibasic amino acids (lysine, arginine, ornithine) transport across the luminal membrane of renal tubules and intestine, resulting in recurrent nephrolithiasis. Cystine stones frequently occur in the second or third decade of life with an occasional occurrence in infancy and in old age. Herein is presented the case of a 1-year-old girl with cystinuria and recurrent urolithiasis; the genetic basis of the disease was investigated by mutational analysis of the SLC3A1 gene. The data show that the present patient has an increased cystine (923.08 microg/mL) level and was heterozygote for M467T mutation.

Regulation of renal amino acid transporters during metabolic acidosis

The kidney plays a major role in acid-base homeostasis by adapting the excretion of acid equivalents to dietary intake and metabolism. Urinary acid excretion is mediated by the secretion of protons and titratable acids, particularly ammonia. NH(3) is synthesized in proximal tubule cells from glutamine taken up via specific amino acid transporters. We tested whether kidney amino acid transporters are regulated in mice in which metabolic acidosis was induced with NH(4)Cl. Blood gas and urine analysis confirmed metabolic acidosis. Real-time RT-PCR was performed to quantify the mRNAs of 16 amino acid transporters. The mRNA of phosphoenolpyruvate carboxykinase (PEPCK) was quantified as positive control for the regulation and that of GAPDH, as internal standard. In acidosis, the mRNA of kidney system N amino acid transporter SNAT3 (SLC38A3/SN1) showed a strong induction similar to that of PEPCK, whereas all other tested mRNAs encoding glutamine or glutamate transporters were unchanged or reduced in abundance. At the protein level, Western blotting and immunohistochemistry demonstrated an increased abundance of SNAT3 and reduced expression of the basolateral cationic amino acid/neutral amino acid exchanger subunit y(+)-LAT1 (SLC7A7). SNAT3 was localized to the basolateral membrane of the late proximal tubule S3 segment in control animals, whereas its expression was extended to the earlier S2 segment of the proximal tubule during acidosis. Our results suggest that the selective regulation of SNAT3 and y(+)LAT1 expression may serve a major role in the renal adaptation to acid secretion and thus for systemic acid-base balance.

Mutation of the 4F2 heavy-chain carboxy terminus causes y+ LAT2 light-chain dysfunction

Heteromeric amino acid transporters are composed of two subunits--a multipass membrane protein called the 'light chain'--and a single pass glycoprotein called the 'heavy chain'. The light chain contains the transport pore, while the heavy chain appears to be necessary for trafficking the light chain to the plasma membrane. In this study, the role of the 4F2hc heavy chain in the function of the y+ LAT2 light chain was investigated. Carboxy terminal truncations and site specific mutants of 4F2hc were co-expressed in Xenopus laevis oocytes with the y+ LAT2 light chain, and the oocytes were analysed for transport activity and surface expression. Truncations of the 4F2hc carboxy terminus ranging between 15 and 404 residues caused a complete loss of light chain function, although all heterodimers were expressed at the cell surface. This indicated that the 15 carboxy-terminal residues of 4F2hc are required for the transport function of the heterodimer. Mutation of the conserved residue leucine 523 to glutamine in the carboxy terminus reduced the Vmax of arginine and leucine uptake. The affinity of the transporter for both arginine and leucine remained unaltered, but the Km-value of Na+, being cotransported with leucine, increased about three-fold. The change of the Na+ Km caused a specific defect of leucine efflux, whereas uptake of leucine at high extracellular NaCl concentration was unaffected.

ADMA metabolism and clearance

The plasma concentration of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, is the resultant of many processes at cellular and organ levels. Post-translational methylation of arginine residues of proteins plays a crucial role in the regulation of their functions, which include processes such as transcription, translation and RNA splicing. Because protein methylation is irreversible, the methylation signal can be turned off only by proteolysis of the entire protein. Consequently, most methylated proteins have high turnover rates. Free ADMA, which is formed during proteolysis, is actively degraded by the intracellular enzyme dimethylarginine dimethylaminohydrolase (DDAH). Some ADMA escapes degradation and leaves the cell via cationic amino acid transporters. These transporters also mediate uptake of ADMA by neighboring cells or distant organs, thereby facilitating active interorgan transport. Clearance of ADMA from the plasma occurs in small part by urinary excretion, but the bulk of ADMA is degraded by intracellular DDAH, after uptake from the circulation. This review discusses the various processes involved in ADMA metabolism: protein methylation, proteolysis of methylated proteins, metabolism by DDAH, and interorgan transport. In addition, the role of the kidney and the liver in the clearance of ADMA is highlighted.

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.

Characterization of L-arginine transport in adrenal cells: effect of ACTH

Nitric oxide synthesis depends on the availability of its precursor L-arginine, which could be regulated by the presence of a specific uptake system. In the present report, the characterization of the L-arginine transport system in mouse adrenal Y1 cells was performed. L-arginine transport was mediated by the cationic/neutral amino acid transport system y+L and the cationic amino acid transporter (CAT) y+ in Y1 cells. These Na+-independent transporters were identified by their selectivity for neutral amino acids in both the presence and absence of Na+ and by the effect of N-ethylmaleimide. Transport data correlated to expression of genes encoding for CAT-1, CAT-2, CD-98, and y+LAT-2. A similar expression profile was detected in rat adrenal zona fasciculata. In addition, cationic amino acid uptake in Y1 cells was upregulated by ACTH and/or cAMP with a concomitant increase in nitric oxide (NO) production.

Arabidopsis LHT1 is a high-affinity transporter for cellular amino acid uptake in both root epidermis and leaf mesophyll

Amino acid transport in plants is mediated by at least two large families of plasma membrane transporters. Arabidopsis thaliana, a nonmycorrhizal species, is able to grow on media containing amino acids as the sole nitrogen source. Arabidopsis amino acid permease (AAP) subfamily genes are preferentially expressed in the vascular tissue, suggesting roles in long-distance transport between organs. We show that the broad-specificity, high-affinity amino acid transporter LYSINE HISTIDINE TRANSPORTER1 (LHT1), an AAP homolog, is expressed in both the rhizodermis and mesophyll of Arabidopsis. Seedlings deficient in LHT1 cannot use Glu or Asp as sole nitrogen sources because of the severe inhibition of amino acid uptake from the medium, and uptake of amino acids into mesophyll protoplasts is inhibited. Interestingly, lht1 mutants, which show growth defects on fertilized soil, can be rescued when LHT1 is reexpressed in green tissue. These findings are consistent with two major LHT1 functions: uptake in roots and supply of leaf mesophyll with xylem-derived amino acids. The capacity for amino acid uptake, and thus nitrogen use efficiency under limited inorganic N supply, is increased severalfold by LHT1 overexpression. These results suggest that LHT1 overexpression may improve the N efficiency of plant growth under limiting nitrogen, and the mutant analyses may enhance our understanding of N cycling in plants.

Control of translocations between highly diverged genes by Sgs1, the Saccharomyces cerevisiae homolog of the Bloom's syndrome protein

Sgs1 is a RecQ family DNA helicase required for genome stability in Saccharomyces cerevisiae whose human homologs BLM, WRN, and RECQL4 are mutated in Bloom's, Werner, and Rothmund Thomson syndromes, respectively. Sgs1 and mismatch repair (MMR) are inhibitors of recombination between similar but divergent (homeologous) DNA sequences. Here we show that SGS1, but not MMR, is critical for suppressing spontaneous, recurring translocations between diverged genes in cells with mutations in the genes encoding the checkpoint proteins Mec3, Rad24, Rad9, or Rfc5, the chromatin assembly factors Cac1 or Asf1, and the DNA helicase Rrm3. The S-phase checkpoint kinase and telomere maintenance factor Tel1, a homolog of the human ataxia telangiectasia (ATM) protein, prevents these translocations, whereas the checkpoint kinase Mec1, a homolog of the human ATM-related protein, and the Rad53 checkpoint kinase are not required. The translocation structures observed suggest involvement of a dicentric intermediate and break-induced replication with multiple cycles of DNA template switching.

In hepatocytes the regulation of NOS-2 activity at physiological l-arginine levels suggests a close link to the urea cycle

High-output synthesis of nitric oxide (NO) by the inducible isoform of NO-synthases (NOS-2) plays an important role in hepatic pathophysiological processes and may contribute to both organ protection and organ destruction during inflammatory reactions. As they compete for the same substrate, L-arginine, an interdependence of NOS-2 and arginase-1 has been repeatedly observed in cells where arginase-1 is cytokine-inducible. However, in hepatocytes, arginases are constitutively expressed and thus, their impact on hepatic NOS-2-derived NO synthesis as well as the influence of L-arginine influx via cationic amino acid transporters during inflammatory reactions are still under debate. Freshly isolated rat hepatocytes were cultured for 24h in the presence of various L-arginine concentrations with or without cytokine addition and nitrite and urea accumulation in culture supernatants was measured. We find that both, cytokine-induced NOS-2 and arginase activities strongly depend on extracellular L-arginine concentrations. When we competed for L-arginine influx via the cationic amino acid transporters by addition of L-lysine, we find a 60-70% inhibition of arginase activity without significant loss of NOS-2 activity. Addition of L-valine, as an arginase inhibitor, leads to a 25% increase in NO formation and an 80-90% decrease in arginase activity. Interestingly, product inhibition of arginase and competitive inhibition of CATs through the addition of L-ornithine leads to a highly significant increase in hepatocytic NOS-2 activity with a concomitant and complete abolishment of its dependence on extracellular L-arginine concentrations. In conclusion, hepatocytic NOS-2 activity shows a surprising pattern of dependence on exogenous L-arginine concentrations. Inhibition and competition experiments suggest a relatively tight link of NOS-2 and urea cycle activities. These data stress the hypothesis of a metabolon-like organization of the urea cycle together with NOS-2 in hepatocytes as excess L-ornithine will be metabolized to l-arginine and thereby increases NO production.

Arabidopsis mitochondria have two basic amino acid transporters with partially overlapping specificities and differential expression in seedling development

To shed light on the metabolic role of two mitochondrial transporters for basic amino acids in Arabidopsis, we compared their functional properties in liposomes and expression during germination. Recombinant and purified BAC2, as previously reported for BAC1, transported various basic L-amino acids upon reconstitution in phospholipid vesicles. Both displayed highest affinity for arginine with similar Km and Vmax. However, BAC2 transported citrulline for which BAC1 had little or no affinity. Furthermore, BAC2 was less stereospecific than BAC1, transporting D-arginine and D-lysine at significant rates, and displayed a striking alkaline pH optimum (pH 8.0) whereas BAC1 activity was unaltered from pH 7.0 to 9.0. By semi-quantitative RT-PCR BAC1 transcript levels were found to be higher than those of BAC2 in germinated seeds. However, BAC2 expression transiently increased 2 days after germination. Disruption of the Arabidopsis arginase structural genes (ARGAH1 or ARGAH2) accentuated the increases of transcript levels of BAC1 at germination and of BAC2 2 days after germination and from 6 days on. Early expression of BAC1 and BAC2 is consistent with the delivery of arginine, released from seed reserves, to mitochondrial arginase and the export of ornithine. Increase of BAC2 transcript levels later in seedling development is consistent with roles in NO, polyamine or proline metabolism--processes involving arginine, citrulline and/or ornithine.

A simple method to generate chromosomal mutations in Lactobacillus plantarum strain TF103 to eliminate undesired fermentation products

Gram-positive bacteria have been explored to convert lignocellulosic biomass to biofuel and bioproducts. Our long-term goal is to create genetically engineered lactic acid bacteria (LAB) strains that convert agricultural biomass into ethanol and other value-added products. The immediate approaches toward this goal involve genetic manipulations by either introducing ethanol production pathway genes or inactivating pathways genes that lead to production of undesired byproducts. The widely studied species Lactobacillus plantarum is now considered a model for genetic manipulations of LAB. In this study, L. plantarum TF103 strain, in which two of the chromosomal L-ldh and D-ldh genes are inactivated, was used to introduce additional mutations on the chromosome to eliminate undesired fermentation products. We targeted the acetolactate synthase gene (als) that converts pyruvate to acetolactate, to eliminate the production of acetoin and 2,3-butanodial. A pBluescript derivative containing sections of the als coding region and an erythromycin resistance gene was directly introduced into L. plantarum TF103 cells to create mutations under selection pressure. The resulting erythromycin resistant (Emr) TF103 strain appears to have chromosomal mutations of both the als and the adjacent lysP genes as revealed by polymerase chain reaction and Southern blot analyses. Mutations were thus generated via targeted homologous recombination using a Gram-negative cloning vector, eliminating the use of a shuttle vector. This method should facilitate research in targeted inactivation of other genes in LAB.

The use of yeast mitochondria to study the properties of wild-type and mutant human mitochondrial ornithine transporter

Yeast deletion models have general utility for the study of a variety of inherited metabolic disorders. Mutations in the mitochondrial ornithine transporter result in hyperammonemia, hyperornithinemia, homocitrullinuria syndrome, a disorder of the urea cycle. To study the effects of mutations in a model system that more closely resembles the in vivo environment, we have developed an expression system based on a yeast strain lacking its endogenous ornithine transporter homologue. Wild-type human ornithine transporter and a recurrent mutation, DeltaF 188, were expressed and characterized with this system. The wild-type transporter appeared to insert into yeast mitochondria in the same orientation as in mammalian mitochondria. It showed stereospecificity, strong antiport activity and ornithine transport was competed by citrulline and arginine. The DeltaF 188 mutant was not incorporated into the membrane to the same extent as wild type, but retained significant residual activity and lost stereospecificity. In these isolated mitochondria, samarium chloride was found to be a potent blocker of transport compared to previously reported sulfhydryl-based inhibitors. A low-affinity background transport activity that promoted the exchange of ornithine for either acidic or basic amino acids was observed. This yeast model can readily be extended to the study of protein:protein interactions. In this manner, the use of yeast deletion strains can serve as a general framework to perform metabolic pathway analysis.

Co-localization and interaction of b0,+-type amino acid transporter 1 (BAT1) with caveolin-1 in rat kidney

BACKGROUND

Cystinuria has been proposed as an inherited disease causing disorders in renal cystine and basic amino acid transport in the proximal tubules. Although cystinuria-related amino acid transporter gene related to b0,+-type amino acid transporter (rBAT1) and its substrate transport properties have been reported, the functional regulatory mechanisms remain to be elucidated. In this study, protein-protein interaction between rBAT1 and caveolin (Cav)-1 was investigated.

METHODS

The renal distribution of rBAT1, rBAT and Cav-1 were demonstrated by employing reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot analysis. Co-localization of rBAT1 and Cav-1 was observed by immunocytochemistry in primary cultured renal proximal tubule-derived cells using a confocal microscope. This result was confirmed by Western blot analysis of isolated caveolae-rich membrane fraction and immunoprecipitation experiments using respective antibodies.

RESULTS

In the separated rat kidney tissues following the corticomedullary axis, Cav-1 mRNA and protein expressions were increased from the cortex to the inner medulla. rBAT1 mRNA and protein expression were detected mainly in the outer medulla. Confocal microscopic results showed rBAT1 and Cav-1 co-localization in the plasma membrane. This result was confirmed by Western blot analysis of caveolae-rich membrane fraction and immunoprecipitates by respective antibodies. The effect of Cav-1 on rBAT1 function was evaluated using Cav-1 antisense oligodeoxynucleotide (ODN). The [14C] arginine uptake by rBAT1 was unchanged by the treatment with antisense ODN.

CONCLUSIONS

From these results, rBAT1 and Cav-1 share a cellular expression in the segregated caveolae structure. As caveolae are rich in signaling molecules, BAT1 could play a role in diverse pathophysiological processes.

Differential effect of phosphatidylethanolamine depletion on raft proteins

A considerable amount of evidence supports the idea that lipid rafts are involved in many cellular processes, including protein sorting and trafficking. We show that, in this process, also a non-raft lipid, phosphatidylethanolamine (PE), has an indispensable function. The depletion of this phospholipid results in an accumulation of a typical raft-resident, the arginine transporter Can1p, in the membranes of Golgi, while the trafficking of another plasma membrane transporter, Pma1p, is interrupted at the level of the ER. Both these transporters associate with a Triton (TX-100) resistant membrane fraction before their intracellular transport is arrested in the respective organelles. The Can1p undelivered to the plasma membrane is fully active when reconstituted to a PE-containing vesicle system in vitro. We further demonstrate that, in addition to the TX-100 resistance at 4 degrees C, Can1p and Pma1pa exhibit different accessibility to nonyl glucoside (NG), which points to distinct intimate lipid surroundings of these two proteins. Also, at 20 degrees C, these two proteins are extracted by TX-100 differentially. The features above suggest that Pma1p and Can1p are associated with different compartments. This is independently supported by the observations made by confocal microscopy. In addition we show that PE is involved in the stability of Can1p-raft association.

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

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

Distribution of Can1p into stable domains reflects lateral protein segregation within the plasma membrane of living S. cerevisiae cells

Recently, lipid-raft-based subdomains within the plasma membrane of living Saccharomyces cerevisiae cells were visualized using green fluorescent protein fusions, and non-overlapping subdomains containing either Pma1p or Can1p were distinguished. In this study, the long-term stability of the subdomains was investigated. Experiments with latrunculin A and nocodazole ruled out the involvement of cytoskeletal components in the stabilization of the subdomains. Also a putative role of the cell wall was excluded, because protoplasting of the cells changed neither the pattern nor the stability of the subdomains. By contrast, the expected inner dynamics of the membrane subdomains was documented by FRAP experiments. Finally, two other proteins were localized within the frame of the Can1p/Pma1p plasma-membrane partition. We show that Fur4p (another H+ symporter) and Sur7p (a protein of unknown function) occupy the Can1p subdomain.

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

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

Root phloem-specific expression of the plasma membrane amino acid proton co-transporter AAP3

Amino acids are regarded as the nitrogen 'currency' of plants. Amino acids can be taken up from the soil directly or synthesized from inorganic nitrogen, and then circulated in the plant via phloem and xylem. AtAAP3, a member of the Amino Acid Permease (AAP) family, is mainly expressed in root tissue, suggesting a potential role in the uptake and distribution of amino acids. To determine the spatial expression pattern of AAP3, promoter-reporter gene fusions were introduced into Arabidopsis. Histochemical analysis of AAP3 promoter-GUS expressing plants revealed that AAP3 is preferentially expressed in root phloem. Expression was also detected in stamens, in cotyledons, and in major veins of some mature leaves. GFP-AAP3 fusions and epitope-tagged AAP3 were used to confirm the tissue specificity and to determine the subcellular localization of AtAAP3. When overexpressed in yeast or plant protoplasts, the functional GFP-AAP3 fusion was localized in subcellular organelle-like structures, nuclear membrane, and plasma membrane. Epitope-tagged AAP3 confirmed its localization to the plasma membrane and nuclear membrane of the phloem, consistent with the promoter-GUS study. In addition, epitope-tagged AAP3 protein was localized in endodermal cells in root tips. The intracellular localization suggests trafficking or cycling of the transporter, similar to many metabolite transporters in yeast or mammals, for example, yeast amino acid permease GAP1. Despite the specific expression pattern, knock-out mutants did not show altered phenotypes under various conditions including N-starvation. Microarray analyses revealed that the expression profile of genes involved in amino acid metabolism did not change drastically, indicating potential compensation by other amino acid transporters.

Molecular and functional characterization of a family of amino acid transporters from Arabidopsis

More than 50 distinct amino acid transporter genes have been identified in the genome of Arabidopsis, indicating that transport of amino acids across membranes is a highly complex feature in plants. Based on sequence similarity, these transporters can be divided into two major superfamilies: the amino acid transporter family and the amino acid polyamine choline transporter family. Currently, mainly transporters of the amino acid transporter family have been characterized. Here, a molecular and functional characterization of amino acid polyamine choline transporters is presented, namely the cationic amino acid transporter (CAT) subfamily. CAT5 functions as a high-affinity, basic amino acid transporter at the plasma membrane. Uptake of toxic amino acid analogs implies that neutral or acidic amino acids are preferentially transported by CAT3, CAT6, and CAT8. The expression profiles suggest that CAT5 may function in reuptake of leaking amino acids at the leaf margin, while CAT8 is expressed in young and rapidly dividing tissues such as young leaves and root apical meristem. CAT2 is localized to the tonoplast in transformed Arabidopsis protoplasts and thus may encode the long-sought vacuolar amino acid transporter.

INFgamma stimulates arginine transport through system y+L in human monocytes

Freshly isolated human monocytes transport L-arginine mostly through a sodium independent, NEM insensitive pathway inhibited by L-leucine in the presence, but not in the absence of sodium. Interferon-gamma (IFNgamma) stimulates this pathway, identifiable with system y+L, and markedly enhances the expression of SLC7A7, the gene that encodes for system y+L subunit y+LAT1, but not of SLC7A6, that codes for the alternative subunit y+LAT2. System y+ plays a minor role in arginine uptake by monocytes and the expression of system y+-related genes, SLC7A1 and SLC7A2, is not changed by IFNgamma. These results demonstrate that system y+L is sensitive to IFNgamma.

Nitric oxide synthesis requires activity of the cationic and neutral amino acid transport system y+L in human umbilical vein endothelium

L-arginine transport is mediated by the cationic/neutral amino acid transport system y+L and cationic amino acid transporters y+/CATs in human umbilical vein endothelial cells (HUVECs). System y+/CATs activity may be rate-limiting for nitric oxide (NO) synthesis, but no reports have demonstrated system y+L involvement in NO synthesis in endothelium. We investigated the role of system y+L in NO synthesis in HUVECs. Transport of 1.5 microM L-arginine was inhibited (P < 0.05) by L-lysine (K(i), 1.4 micro M), L-leucine (K(i), 1.8 micro M) and L-phenylalanine (K(i), 4.1 microM), but was unaltered (P > 0.05) by L-alanine or L-cysteine. The system y+/CATs inhibitor, N-ethylmaleimide (NEM), did not alter 1.5 microM L-arginine transport, but inhibited (92 +/- 3 %) 100 microM L-arginine transport. L-arginine transport in the presence of NEM was saturable (V(max), 0.37 +/- 0.02 pmol (microg protein)(-1) min(-1); K(m), 1.5 +/- 0.3 microM) and competitively inhibited by L-leucine in the presence of Na+ (V(max), 0.49 +/- 0.06 pmol (microg protein)(-1) min(-1); K(m), 6.5 +/- 0.9 microM). HUVECs express SLC3A2/4F2hc, SLC7A7/4F2-lc2 and SLC7A6/4F2-lc3 genes encoding for the high-affinity transport system y+L. N(G)-Nitro-L-arginine methyl ester and L-leucine, but not NEM, inhibited NO synthesis in medium containing 1.5 microM L-arginine. Cells exposed to 25 mM D-glucose (24 h) exhibited reduced system y+L activity (V(max), 0.15 +/- 0.008 pmol (microg protein)(-1) min(-1); K(m), 1.4 +/- 0.3 microM) and NO synthesis. However, 25 mM D-glucose increased NO synthesis and L-arginine transport via system y+. Thus, L-arginine transport through system y+L plays a role in NO synthesis, which could be a determining factor in pathological conditions where the endothelial L-arginine-NO pathway is altered, such as in diabetes mellitus.

Arginine revisited: Minireview article

Arginine is a precursor of proteins and employed in urea synthesis. It is also the precursor of many other compounds, such as creatine, nitric oxide, polyamines, agmatine, proline. In this review, its transport and that of other basic amino acids are examined, along with its transformation into nitric oxide, agmatine and proline, and the mutual regulation of the individual pathways.

L-Arginine Transport in Disease

The importance of membrane transport in normal physiological cell function is unquestionable. However, to what extent alterations in the transport of amino acids are the cause and/or consequence of pathological changes observed in disease states is a question not yet completely clarified. Kinetic experiments with blood cells provide a simple and useful model for researching alterations in amino acid transport. The cationic amino acid L-arginine is the precursor of nitric oxide (NO), a key second messenger involved in functions such as endothelium-dependent vascular relaxation, immune defence and platelet activation. The transport of L-arginine, being rate-limiting for nitric oxide production, is extremely relevant to pathological conditions where NO synthesis and/or actions are affected. The current review provides an overview of L-arginine transport in disease, specifically in uraemia, heart failure, hypertension, diabetes mellitus, septic shock and sickle cell disease.

L-arginine depletion in preeclampsia orients nitric oxide synthase toward oxidant species

Less nitric oxide (NO)-dependent vasodilation and excess formation of reactive oxygen species could explain poor placenta perfusion in preeclampsia, but the pathways involved are unknown. We tested the hypothesis that reduced NO activity and increased oxidative stress in preeclamptic placenta is related to a low bioavailability of l-arginine. Placental endothelial NO synthase (ecNOS) expression (by immunoperoxidase) and activity (by diaphorase and [(3)H]L-citrulline formation) were comparable in normotensive pregnancy and in preeclampsia, whereas nitrotyrosine staining, a marker of peroxynitrite, was stronger in preeclamptic villi, confirming previously reported data. Oxidative tissue damage was documented in preeclamptic villi by strong 4-hydroxynonenal-lysine staining (by immunoperoxidase), which closely colocalized with nitrotyrosine. Concentration of the NO precursor l-arginine (by HPLC) in umbilical blood and in villous tissue was lower in preeclampsia than in normotensive pregnancy. This was not caused by a defective l-arginine transport, because gene expression of the CAT-1, 4F2hc, and LAT-1 cationic amino acid transporters (by real-time reverse-transcription polymerase chain reaction [RT-PCR]) was normal. Instead, gene expression (by real-time RT-PCR) and protein tissue content (by immunoperoxidase and Western blot) of arginase II-the enzyme that degrades arginine to ornithine-were higher in preeclamptic villi than in normotensive pregnancy. These results provide a biochemical explanation for defective NO activity and increased oxidative stress in preeclamptic placenta. In normal placenta, adequate concentration of l-arginine orients ecNOS toward NO. In preeclampsia, a lower than normal l-arginine concentration caused by arginase II overexpression redirects ecNOS toward peroxynitrite.

Glutamine uptake and expression of mRNA's of glutamine transporting proteins in mouse cerebellar and cerebral cortical astrocytes and neurons

The relative roles of the three sodium-dependent transport systems: A, ASC and N in the uptake of [3H]Gln, and the compatibility of the uptake characteristics with the expression of mRNAs coding for the Gln transporting molecules, were examined in primary cultures of astrocytes and neurons derived from mouse cerebellum, a glutaminergic system-enriched structure, and in cerebral cortex. Gln uptake activity (Vmax) was higher in cerebellar astrocytes or neurons than in their cerebral cortical counterparts. The N-methylamino-isobutyric acid (MeAiB)- and pH-sensitive, system A-mediated component of the uptake, and the uptake of [14C]MeAiB itself, was much more active in neurons than in astrocytes derived from either region. Also, the expression of mRNA for GlnT (SAT1), a system A isoform specific for Gln, was only expressed in neurons derived from both structures, while an alanine (Ala)-preferring system A transporter, SAT2, was expressed in neurons and astrocytes from either region. System ASC-mediated Gln uptake and expression of ASCT2 mRNA were in both structures more pronounced in astrocytes than in neurons, consistent with the postulated role of ASCT2 in the efflux of de novo synthesized Gln from astrocytes. System N-mediated (threonine+MeAiB-inhibitable) Gln uptake showed comparable activities in all four types of cells, which is compatible with the ubiquitous expression of NAT2 mRNA-a mouse brain-specific N-system isoform.

Characterization of cationic amino acid transporters and expression of endothelial nitric oxide synthase in human placental microvascular endothelial cells

We investigated the expression and activity of arginine transporters and endothelial nitric oxide synthase (eNOS) in human placental microvascular endothelial cells (HPMEC). Using RT-PCR amplification products for eNOS, CAT1, CAT2A, CAT2B, CAT4, 4F2hc (CD98), rBAT and the light chains y+LAT1, y+LAT2, and b0+T1 were detected in HPMEC, but not B0+. Immunohistochemistry and Western blotting confirmed the presence of 4F2hc and CAT1 protein in HPMEC. 4F2hc-light chain dimers were indicated by a shift in molecular mass detected under nonreducing conditions. L-Arginine transport into HPMEC was independent of Na+ or Cl- and was inhibited by the neutral amino acid glutamine, but not by cystine. The Ki for glutamine inhibition was greater in the absence of Na+. Kinetic analysis supported a two-transporter model attributed to system y+L and system y+. Expression of eNOS in HPMEC was detectable by immunohistochemistry and ELISA but not by Western blotting. Activity of eNOS in HPMEC, measured over 48 h, either as the basal production of nitric oxide (NO) or as the accumulation of intracellular cGMP was not detectable. We conclude that HPMEC transport cationic amino acids by systems y+ and y+L and that basal eNOS expression and activity in these cells is low.

Identification of a mitochondrial transporter for basic amino acids in Arabidopsis thaliana by functional reconstitution into liposomes and complementation in yeast

We describe the identification and functional characterization of two Arabidopsis mitochondrial basic amino acid carriers (BAC), AtmBAC1 and AtmBAC2, which are related to the yeast ornithine (Orn) carrier Ort1p, also known as Arg11p. The arg11 mutant requires arginine (Arg) supplementation because it fails to export sufficient ornithine from the mitochondrion to the cytosol where it is converted to arginine. AtmBAC1 and, to a lesser extent, AtmBAC2 partially replaced the function of Ort1p in yeast arg11. The more efficient putative carrier, AtmBAC1, was expressed in E. coli, purified, and reconstituted into phospholipid vesicles, where it transported the basic l-amino acids arginine, lysine, ornithine and histidine (in order of decreasing affinity). AtmBAC1 recognized l-histidine whereas both yeast Ort1p and the mammalian ortholog ORNT1p do not. Also different from ORNT1p, AtmBAC1 did not transport citrulline. AtmBAC1 appeared to be more stereospecific than the yeast and mammalian ornithine carriers, exhibiting greater preference for the l-forms of arginine, lysine and ornithine. By RT-PCR, both AtmBAC1 and AtmBAC2 transcripts were detected in stems, leaves, flowers, siliques, and seedlings. Expression of AtmBAC1 in seedlings is consistent with its involvement in Arg breakdown in early seedling development, i.e. delivery of Arg to mitochondrial arginase. The Km (0.19 mm) for Arg uptake by AtmBAC1 was close to the value we previously determined for the saturable component of Arg uptake into intact mitochondria from soybean seedling cotyledons.

Increase in the rate of L-pipecolic acid production using lat-expressing Escherichia coli by lysP and yeiE amplification

Biotransformation of L-lysine (L-Lys) to L-pipecolic acid (L-PA) using lat-expressing Escherichia coli has been reported (Fujii et al., Biosci. Biotechnol. Biochem., 66, 622-627 (2002)). The rate-limiting step of this biotransformation seemes to be the transport of L-Lys into cells. To improve the L-PA production rate, we attempted to increase the rate of L-Lys uptake. E. coli BL21 carrying a plasmid with lat and lysP (pRH125) caused a 5-fold increase in the rate of L-PA production above the level of cells carrying a plasmid with lat (pRH124). Moreover, E. coli BL21 carrying a plasmid with lat, lysP, and yeiE (pRH127) caused a 6.4-fold increase in the rate of L-PA production above the level of cells carrying pRH124. Our results from RT-PCR experiments and the sequence similarity of YeiE to LysR transcriptional regulators suggest the possibility that yeiE expression induces lysP expression. The amplification of lysP, or rather both lysP and yeiE, increases the rate of L-PA production using lat-expressing E. coli.

A glutamine transport gene, glnQ, is required for fibronectin adherence and virulence of group B streptococci

Group B streptococci (GBS) are a leading cause of neonatal sepsis and meningitis. GBS adhere to fibronectin when it is attached to a solid phase. We isolated a Tn917 transposon mutant, COH1-GT1, which shows decreased adherence to fibronectin. COH1-GT1 also shows decreased adherence to and invasion of respiratory epithelial cells in vitro and decreased virulence in vivo. COH1-GT1 contains a Tn917 insertion in a homolog of glnQ, a gene from Escherichia coli which is required for glutamine transport and codes for a cytoplasmic ATP-binding cassette protein. To confirm that the decreased fibronectin adherence of COH1-GT1 was due to the mutation in glnQ, we constructed COH1-GT2, a strain with a nonpolar site-directed mutation in glnQ. COH1-GT2 showed decreased binding to fibronectin. We also demonstrated that complementation of glnQ in trans restored fibronectin adherence to COH1-GT1. COH1-GT1 shows decreased uptake of radiolabeled glutamine and is resistant to the toxic glutamine analog gamma-L-glutamylhydrazide, demonstrating that the glnQ gene is required for glutamine transport in GBS. glnQ lacks a signal sequence and is a cytoplasmic protein in E. coli and thus is unlikely to act as a fibronectin adhesin. glnQ is transcribed in an operon with a putative glutamine permease gene, glnP, which has a novel predicted structure containing three distinct domains linked in a single gene. The first two domains are putative glutamine binding domains with homology to the E. coli periplasmic glutamine binding gene glnH. The third is a putative permease domain with homology to the E. coli glutamine permease gene glnP. RT-PCR analysis demonstrated that glnP and glnQ are contained within a single transcript. Transcription of scpB, encoding the only known fibronectin-binding adhesin of GBS, is unaffected. We speculate that glnQ may regulate expression of fibronectin adhesins by affecting cytoplasmic glutamine levels and that regulation may be posttranscriptional.

Volatile anesthetics affect nutrient availability in yeast

Volatile anesthetics affect all cells and tissues tested, but their mechanisms and sites of action remain unknown. To gain insight into the cellular activities of anesthetics, we have isolated genes that, when overexpressed, render Saccharomyces cerevisiae resistant to the volatile anesthetic isoflurane. One of these genes, WAK3/TAT1, encodes a permease that transports amino acids including leucine and tryptophan, for which our wild-type strain is auxotrophic. This suggests that availability of amino acids may play a key role in anesthetic response. Multiple lines of evidence support this proposal: (i) Deletion or overexpression of permeases that transport leucine and/or tryptophan alters anesthetic response; (ii) prototrophic strains are anesthetic resistant; (iii) altered concentrations of leucine and tryptophan in the medium affect anesthetic response; and (iv) uptake of leucine and tryptophan is inhibited during anesthetic exposure. Not all amino acids are critical for this response since we find that overexpression of the lysine permease does not affect anesthetic sensitivity. These findings are consistent with models in which anesthetics have a physiologically important effect on availability of specific amino acids by altering function of their permeases. In addition, we show that there is a relationship between nutrient availability and ubiquitin metabolism in this response.