The two-way flux of cationic amino acids across the plasma membrane of mammalian cells is largely explained by a single transport system

Rapid ammonia gas transport accounts for futile transmembrane cycling under NH3/NH4+ toxicity in plant roots

Futile transmembrane NH3/NH4(+) cycling in plant root cells, characterized by extremely rapid fluxes and high efflux to influx ratios, has been successfully linked to NH3/NH4(+) toxicity. Surprisingly, the fundamental question of which species of the conjugate pair (NH3 or NH4(+)) participates in such fluxes is unresolved. Using flux analyses with the short-lived radioisotope (13)N and electrophysiological, respiratory, and histochemical measurements, we show that futile cycling in roots of barley (Hordeum vulgare) seedlings is predominately of the gaseous NH3 species, rather than the NH4(+) ion. Influx of (13)NH3/(13)NH4(+), which exceeded 200 µmol g(-1) h(-1), was not commensurate with membrane depolarization or increases in root respiration, suggesting electroneutral NH3 transport. Influx followed Michaelis-Menten kinetics for NH3 (but not NH4(+)), as a function of external concentration (Km = 152 µm, Vmax = 205 µmol g(-1) h(-1)). Efflux of (13)NH3/(13)NH4(+) responded with a nearly identical Km. Pharmacological characterization of influx and efflux suggests mediation by aquaporins. Our study fundamentally revises the futile-cycling model by demonstrating that NH3 is the major permeating species across both plasmalemma and tonoplast of root cells under toxicity conditions.

Transport of asymmetric dimethylarginine (ADMA) by cationic amino acid transporter 2 (CAT2), organic cation transporter 2 (OCT2) and multidrug and toxin extrusion protein 1 (MATE1)

Asymmetric dimethylarginine (ADMA), inhibiting the nitric oxide (NO) synthesis from L-arginine, is a known cardiovascular risk factor. Our aim was to investigate if ADMA and/or L-arginine are substrates of the human cationic amino acid transporters 2A (CAT2A, SLC7A2A) and 2B (CAT2B, SLC7A2B), the organic cation transporter 2 (OCT2, SLC22A2), and the multidrug and toxin extrusion protein 1 (MATE1, SLC47A1). We systematically investigated the kinetics of ADMA and L-arginine transport in human embryonic kidney (HEK293) cells stably overexpressing CAT2A, CAT2B, OCT2, or MATE1. Vector-only transfected HEK293 cells served as controls. Compared to vector control cells, uptake of ADMA and L-arginine was significantly higher (p < 0.05) in cells expressing CAT2B and OCT2 at almost all investigated concentrations, while cells expressing CAT2A only showed a significant uptake at concentrations above 300 μM. Uptake of MATE1 overexpressing cells was significantly (p < 0.05) higher at pH 7.8 and 8.2 than controls. Apparent V max values (nmol mg protein(-1) min(-1)) for cellular uptake of ADMA and L-arginine were ≈11.8 ± 1.2 and 19.5 ± 0.7 for CAT2A, ≈14.3 ± 1.0 and 15.3 ± 0.4 for CAT2B, and 6.3 ± 0.3 and >50 for OCT2, respectively. Apparent K m values (μmol/l) for cellular uptake of ADMA and L-arginine were ≈3,033 ± 675 and 3,510 ± 419 for CAT2A, ≈4,021 ± 532 and 952 ± 92 for CAT2B, and 967 ± 143 and >10,000 for OCT2, respectively. ADMA and L-arginine are substrates of human CAT2A, CAT2B, OCT2 and MATE1. Transport kinetics of CAT2A, CAT2B, and OCT2 indicate a low affinity, high capacity transport, which may be relevant for renal and hepatic elimination of ADMA or L-arginine.

Molecular organization of Drosophila neuroendocrine cells by Dimmed

BACKGROUND

In Drosophila, the basic-helix-loop-helix protein DIMM coordinates the molecular and cellular properties of all major neuroendocrine cells, irrespective of the secretory peptides they produce. When expressed by nonneuroendocrine neurons, DIMM confers the major properties of the regulated secretory pathway and converts such cells away from fast neurotransmission and toward a neuroendocrine state.

RESULTS

We first identified 134 transcripts upregulated by DIMM in embryos and then evaluated them systematically using diverse assays (including embryo in situ hybridization, in vivo chromatin immunoprecipitation, and cell-based transactivation assays). We conclude that of eleven strong candidates, six are strongly and directly controlled by DIMM in vivo. The six targets include several large dense-core vesicle (LDCV) proteins, but also proteins in non-LDCV compartments such as the RNA-associated protein Maelstrom. In addition, a functional in vivo assay, combining transgenic RNA interference with MS-based peptidomics, revealed that three DIMM targets are especially critical for its action. These include two well-established LDCV proteins, the amidation enzyme PHM and the ascorbate-regenerating electron transporter cytochrome b(561-1). The third key DIMM target, CAT-4 (CG13248), has not previously been associated with peptide neurosecretion-it encodes a putative cationic amino acid transporter, closely related to the Slimfast arginine transporter. Finally, we compared transcripts upregulated by DIMM with those normally enriched in DIMM neurons of the adult brain and found an intersection of 18 DIMM-regulated genes, which included all six direct DIMM targets.

CONCLUSIONS

The results provide a rigorous molecular framework with which to describe the fundamental regulatory organization of diverse neuroendocrine cells.

Overexpression of ornithine decarboxylase decreases ventricular systolic function during induction of cardiac hypertrophy

Ornithine decarboxylase (ODC), the first enzyme of polyamine metabolism, is rapidly upregulated in response to agents that induce a pathological cardiac hypertrophy. Transgenic mice overexpressing ODC in the heart (MHC-ODC mice) experience a much more dramatic left ventricular hypertrophy in response to β-adrenergic stimulation with isoproterenol (ISO) compared to wild-type (WT) controls. ISO also induced arginase activity in transgenic hearts but not in controls. The current work studies the cooperation between the cardiac polyamines and L-arginine (L-Arg) availability in MHC-ODC mice. Although ISO-induced hypertrophy is well-compensated, MHC-ODC mice administered L-Arg along with ISO showed a rapid onset of systolic dysfunction and died within 48 h. Myocytes isolated from MHC-ODC mice administered L-Arg/ISO exhibited reduced contractility and altered calcium transients, suggesting an alteration in [Ca(2+)] homeostasis, and abbreviated action potential duration, which may contribute to arrhythmogenesis. The already elevated levels of spermidine and spermine were not further altered in MHC-ODC hearts by L-Arg/ISO treatment, suggesting alternative L-Arg utilization pathways lead to dysregulation of intracellular calcium. MHC-ODC mice administered an arginase inhibitor (Nor-NOHA) along with ISO died almost as rapidly as L-Arg/ISO-treated mice, while the iNOS inhibitor S-methyl-isothiourea (SMT) was strongly protective against L-Arg/ISO. These results point to the induction of arginase as a protective response to β-adrenergic stimulation in the setting of high polyamines. Further, NO generated by exogenously supplied L-Arg may contribute to the lethal consequences of L-Arg/ISO treatment. Since considerable variations in human cardiac polyamine and L-Arg content are likely, it is possible that alterations in these factors may influence myocyte contractility.

Arginine deprivation and argininosuccinate synthetase expression in the treatment of cancer

Arginine, a semi-essential amino acid in humans, is critical for the growth of human cancers, particularly those marked by de novo chemoresistance and a poor clinical outcome. In addition to protein synthesis, arginine is involved in diverse aspects of tumour metabolism, including the synthesis of nitric oxide, polyamines, nucleotides, proline and glutamate. Tumoural downregulation of the enzyme argininosuccinate synthetase (ASS1), a recognised rate-limiting step in arginine synthesis, results in an intrinsic dependence on extracellular arginine due to an inability to synthesise arginine for growth. This dependence on extracellular arginine is known as arginine auxotrophy. Several tumours are arginine auxotrophic, due to variable loss of ASS1, including hepatocellular carcinoma, malignant melanoma, malignant pleural mesothelioma, prostate and renal cancer. Importantly, targeting extracellular arginine for degradation in the absence of ASS1 triggers apoptosis in arginine auxotrophs. Several phase I/II clinical trials of the arginine-lowering drug, pegylated arginine deiminase, have shown encouraging evidence of clinical benefit and low toxicity in patients with ASS1-negative tumours. In part, ASS1 loss is due to epigenetic silencing of the ASS1 promoter in various human cancer cell lines and tumours, and it is this silencing that confers arginine auxotrophy. In relapsed ovarian cancer, this is associated with platinum refractoriness. In contrast, several platinum sensitive tumours, including primary ovarian, stomach and colorectal cancer, are characterised by ASS1 overexpression, which is regulated by proinflammatory cytokines. This review examines the prospects for novel approaches in the prevention, diagnosis and treatment of malignant disease based on ASS1 pathophysiology and its rate-limiting product, arginine.

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.

Radiolabeled L-lysine for tumor imaging

RATIONALE AND OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Amino acids as modulators of endothelium-derived nitric oxide

To examine the mechanisms whereby amino acids modulate nitric oxide (NO) production and blood flow in the renal vasculature, chemiluminescence techniques were used to quantify NO in the renal venous effluent of the isolated, perfused rat kidney as different amino acids were added to the perfusate. The addition of 10−4or 10−3M cationic amino acids (l-ornithine, l-lysine, or l-homoarginine) or neutral amino acids (l-glutamine, l-leucine, or l-serine) to the perfusate decreased NO and increased renal vascular resistance. Perfusion with anionic amino acids (l-glutamate or l-aspartate) had no effect on either parameter. The effects of the cationic and neutral amino acids were reversed with 10−3M l-arginine and prevented by deendothelialization or NO synthase inhibition. The effects of the neutral amino acids but not the cationic amino acids were dependent on extracellular sodium. Cationic and neutral amino acids also decreased calcimycin-induced NO, as assessed by DAF-FM-T fluorescence, in cultured EA.hy926 endothelial cells. Inhibition of system y+or y+L by siRNA for the cationic amino acid transporter 1 or the CD98/4F2 heavy chain diminished the NO-depleting effects of these amino acids. Finally, transport studies in cultured cells demonstrated that cationic or neutral amino acids in the extracellular space stimulate efflux of l-arginine out of the cell. Thus the present experiments demonstrate that cationic and neutral amino acids can modulate NO production in endothelial cells by altering cellular l-arginine transport through y+and y+L transport mechanisms.

l-Arginine uptake affects nitric oxide production and blood flow in the renal medulla

Experiments were performed to determine whether l-arginine transport regulates nitric oxide (NO) production and hemodynamics in the renal medulla. The effects of renal medullary interstitial infusion of cationic amino acids, which compete with l-arginine for cellular uptake, on NO levels and blood flow in the medulla were examined in anesthetized rats. NO concentration in the renal inner medulla, measured with a microdialysis-oxyhemoglobin trapping technique, was significantly decreased by 26–44% and renal medullary blood flow, measured by laser Doppler flowmetry, was significantly reduced by 20–24% during the acute renal medullary interstitial infusion of l-ornithine, l-lysine, and l-homoarginine (1 μmol·kg−1·min−1each; n = 6–8/group). In contrast, intramedullary infusion of l-arginine increased NO concentration and medullary blood flow. Flow cytometry experiments with 4-amino-5-methylamino-2′,7′-difluorescein diacetate, a fluorophore reactive to intracellular NO, demonstrated that l-ornithine, l-lysine, and l-homoarginine decreased NO by 54–57% of control, whereas l-arginine increased NO by 21% in freshly isolated inner medullary cells (1 mmol/l each, n > 1,000 cells/experiment). The mRNA for the cationic amino acid transporter-1 was predominantly expressed in the inner medulla, and cationic amino acid transporter-1 protein was localized by immunohistochemistry to the collecting ducts and vasa recta in the inner medulla. These results suggest that l-arginine transport by cationic amino acid transport mechanisms is important in the production of NO and maintenance of blood flow in the renal medulla.

l -Arginine Transport in the Human Coronary and Peripheral Circulation

Background— Nitric oxide synthase (NOS) uses arginine for the production of nitric oxide (NO). High intracellular concentrations of arginine suggest that NOS activity should be independent of plasma arginine supply. However, under certain conditions, increased plasma arginine concentrations appear to be associated with increased NOS activity. The purpose of this study was to explore arginine transport within the human coronary and peripheral circulation

Methods and Results— Mass-labeled 15 N 2 -arginine was infused to steady state before cardiac catheterization in 31 patients. After diagnostic angiography, a catheter was placed in the coronary sinus. The transcardiac concentration gradient (aorta−coronary sinus) of 15 N 2 -arginine was used as a measure of arginine transport at baseline and during infusions of acetylcholine and N G -monomethyl- l -arginine (L-NMMA). No gradient was detected at rest. During the infusion of acetylcholine, a significant gradient was detected (2.5±1.2 μmol/L, P =0.01) corresponding to a fractional extraction of 11.7±7.5%. This is consistent with in vitro studies that suggest that stimulation of NOS induces arginine transport. During the infusion of L-NMMA, the concentration of 15 N 2 -arginine increased in the coronary sinus, producing a gradient of −3.9±1.3 μmol/L ( P =0.0002), corresponding to a fractional production of 20.5±5.0%. This is consistent with in vitro studies that suggest that L-NMMA induces the efflux of arginine from the cell to the extracellular space via transporter-mediated transstimulation.

Conclusions— The use of steady-state 15 N 2 -arginine to examine transorgan l -arginine gradients represents a novel tool for the study of l -arginine transport and the mechanisms of endothelial and NOS dysfunction.

Cationic amino acid transport in the renal medulla and blood pressure regulation

Previous studies have indicated that NO synthesis in isolated inner medullary collecting duct cells is reduced by cationic amino acids that compete with L-arginine for cellular uptake. In the present study, we investigated the effects of chronic renal medullary infusion of cationic amino acids on renal NO concentration and mean arterial pressure (MAP) in Sprague-Dawley rats. Renal medullary infusion of L-ornithine (50 microg/kg per min) or L-lysine (50 microg/kg per min) markedly decreased NO in the medulla (vehicle, 124 +/- 11 nmol/L; L-ornithine, 45 +/- 4 nmol/L; L-lysine, 42 +/- 6 nmol/L) and increased MAP (vehicle, 111 +/- 7 mm Hg; L-ornithine, 143 +/- 6 mm Hg; L-lysine, 148 +/- 3 mm Hg) after 5 days of infusion. In contrast, intravenous infusion of the same dose of L-ornithine or L-lysine for 5 days increased plasma concentration to levels similar to those observed with intramedullary infusion but did not change NO in the medulla or alter MAP. Furthermore, the NO-suppressing and hypertensive effects of medullary interstitial infusion of L-ornithine (50 microg/kg per min) were attenuated by simultaneous infusion of L-arginine (500 microg/kg per min; NO, 97 +/- 10 nmol/L; MAP, 124 +/- 3 mm Hg). A 5-day infusion of an antisense oligonucleotide against CAT-1 (18-mer, 8.3 nmol/h) significantly decreased CAT-1 protein in the medulla, decreased NO in the medulla (scrambled oligo, 124 +/- 10 nmol/L; antisense oligo, 67 +/- 11 nmol/L), and increased MAP (scrambled oligo, 113 +/- 2 mm Hg; antisense oligo, 130 +/- 2 mm Hg). These results suggest that uptake of L-arginine by cationic amino acid transport systems in the renal medulla plays an important role in the regulation of medullary NO and MAP in rats.

Arginine transport through system y(+)L in cultured human fibroblasts: normal phenotype of cells from LPI subjects

In lysinuric protein intolerance (LPI), impaired transport of cationic amino acids in kidney and intestine is due to mutations of the SLC7A7 gene. To assess the functional consequences of the LPI defect in nonepithelial cells, we have characterized cationic amino acid (CAA) transport in human fibroblasts obtained from LPI patients and a normal subject. In both cell types the bidirectional fluxes of arginine are due to the additive contributions of two Na(+)-independent, transstimulated transport systems. One of these mechanisms, inhibited by N-ethylmaleimide (NEM) and sensitive to the membrane potential, is identifiable with system y(+). The NEM- and potential-insensitive component, suppressed by L-leucine only in the presence of Na(+), is mostly due to the activity of system y(+)L. The inward and outward activities of the two systems are comparable in control and LPI fibroblasts. Both cell types express SLC7A1 (CAT1) and SLC7A2 (CAT2B and CAT2A) as well as SLC7A6 (y+LAT2) and SLC7A7 (y+LAT1). We conclude that LPI fibroblasts exhibit normal CAA transport through system y(+)L, probably referable to the activity of SLC7A6/y+LAT2.

Regulation of L-arginine uptake by Ca(2+) in human platelets

L-Arginine uptake and Ca(2+) changes in unstirred platelets activated by thrombin, collagen and Ca(2+) ionophore A23187 were evaluated. Thrombin did not affect L-arginine uptake at short incubation times (2-15 min), but at prolonged times slowed down the amino acid transport. Collagen was ineffective. A23187 decreased the L-arginine uptake in a dose-dependent manner, producing the maximal inhibition at 5 microM. In FURA 2-loaded platelets collagen did not modify Ca(2+) basal level, thrombin induced a late Ca(2+) rise and A23187 dose-dependently increased cytosolic Ca(2+), eliciting the highest increase at 5 microM. It is likely that L-arginine uptake is inversely modulated by Ca(2+) concentrations and is inhibited during platelet stimulation with agonists which induce cytosolic Ca(2+) elevation.

Control of expression of the gene for the arginine transporter Cat-1 in rat liver cells by glucocorticoids and insulin

Hepatic arginine and lysine uptake is partly regulated by changes in the transport activity of a group of cell surface proteins exhibiting properties of the transport system y+. The Cat-1 gene encodes a sodium-independent high-affinity cationic amino acid transporter of the y+ system which is nearly undetectable in the quiescent liver. In this paper we investigate the regulation of expression of Cat-1 in the quiescent rat liver by glucocorticoids and insulin, two hormones which play a critical role in amino acid dependent pathways of hepatic metabolism. Injection of insulin and glucocorticoids resulted in a rapid (15-30 min, 4-5 fold) increase in transcription which returned to basal levels within 4 hours. In contrast to the rapid single peak of transcriptional induction of the Cat-1 gene, the accumulation of the Cat-1 mRNAs occurred transiently with two peaks, the first at 30 minutes and the second at 2-4 hours following hormone treatment. These data indicate that expression of the Cat-1 gene in the quiescent liver can be transiently induced by both transcriptional and post-transcriptional mechanisms. In FTO2B rat hepatoma cells, expression of the gene is constitutive and accumulation of Cat-1 mRNAs in response to dexamethasone and insulin was dependent on transcription and protein synthesis. Furthermore, the accumulation of the basal level of the Cat-1 mRNAs was reduced by 70%, upon treatment of cells with inhibitors of protein synthesis for 6 h, when the transcription rate of the gene did not decrease significantly. We conclude the following: (i) under normal physiologic conditions, expression of the Cat-1 gene in the quiescent liver is negligible, probably to prevent unnecessary transport and metabolism of arginine by the hepatic arginase in the hepatocytes. (ii) in the cases when hepatic cationic amino acid transport is needed, such as following feeding, cellular growth and illness, glucocorticoids and insulin induce expression of the Cat-1 gene in liver cells through induction of transcription and stabilization of the mRNA. (iii) constitutive Cat-1 mRNA accumulation in rat hepatoma cells depends on protein synthesis through a labile regulated factor. Overall, constitutive expression of Cat-1 is associated with hepatic cellular growth and transformation.

Stress differentially induces cationic amino acid transporter gene expression

The amino acid l-arginine plays a central role in several adaptive metabolic pathways and we postulate that regulated L-arginine transport contributes to important physiological responses. The majority of L-arginine flux is mediated by transport system y+ that is encoded by at least three genes, Cat1, Cat2 and Cat3. Cat2 encodes two distinct protein isoforms (CAT2/CAT2a) that differ by 10-fold in their apparent substrate affinity. Cat2 transcription is controlled by four widely spaced promoters. The expression of CAT2/2a transcripts was tested in skeletal muscle and macrophages following specific stresses or activators. Unexpectedly, CAT2a transcripts accumulated in skeletal muscle in response to surgical trauma (hepatectomy and splenectomy) as well as food deprivation, although neither high affinity CAT2 nor CAT1 were detectably altered. Activated macrophages decreased CAT1 levels, but accumulated CAT2 and iNOS mRNA and protein with parallel kinetics suggesting that CAT2 mediated L-arginine transport might regulate the L-arginine:nitric oxide pathway. In macrophages, liver and skeletal muscle, the most distal CAT2 promoter was predominant. No change in promoter usage was apparent under any stress conditions tested nor was alternate splicing of the CAT2 transcript dictated by promoter usage. The differential regulation of the Cat genes indicates their encoded transporter proteins meet different requirements for cationic amino acids in the intact animal.

Sodium-independent lysine uptake by the BeWo choriocarcinoma cell line

Transport of L-lysine by a cultured placental trophoblast cell line was investigated by characterization of L-[3H]lysine uptake. In the mononuclear form of the BeWo clone b30 choriocarcinoma cell, at least two sodium-independent systems are present. Concentration dependence data were fitted by a two system model with Km values (+/- s.e.) of 2 +/- 0.7 and 94 +/- 31 microM and Vmax values (+/- s.e.) of 0.7 +/- 0.3 and 25 +/- 6.0 nM/mg DNA/min. A portion of sodium-independent uptake was inhibited by the sulphydryl modifying reagent N-ethylmaleimide (NEM). Following NEM treatment, the data were fitted by a single system with Km = 10 +/- 2 microM AND Vmax = 5.1 +/- 0.8 nM/mg DNA/min. In the absence of sodium, NEM-resistant uptake was sensitively inhibited by leucine whereas NEM-sensitive uptake was not inhibited by leucine. It is concluded that like placental basal membrane, the mononuclear BeWo cell possesses two sodium-independent L-lysine transport systems. The high-capacity, NEM-sensitive, leucine-insensitive system resembles the widespread system y+. The high-affinity, NEM-resistant, leucine-sensitive system resembles system b(0,+).

A new member of the cationic amino acid transporter family is preferentially expressed in adult mouse brain

We have isolated and characterized a novel member (CAT3) of the cationic amino acid transporter (CAT) family. In oocyte injection assays, CAT3 cRNA exhibited a saturable, sodium ion-independent transport activity with high affinity for L-arginine and L-lysine (Km = 40-60 and 115-165 microM, respectively). Transport of L-arginine was effectively competed only by cationic amino acids in L-form: arginine, lysine, ornithine, and 2,4-diamino-n-butyric acid but not by 2,3-diaminopropionic acid. The presence of L-arginine in the incubation medium stimulated the efflux rate of L-arginine, indicating that CAT3 is subject to trans-stimulation. All these results are consistent with the idea that CAT3, along with CAT1 and CAT2, constitutes the transport activity originally assigned to system y+. Like CAT2, but unlike CAT1, the expression of CAT3 is regulated in a highly tissue-specific manner; when various adult tissues were examined, significant levels of CAT3 transcript were detectable only in brain. In situ hybridization on brain sections revealed that CAT3 transcripts were localized predominantly along the midbrain-thalamus-hypothalamus axis, whereas neither CAT1 nor CAT2 transcripts demonstrated a similar localization. In contrast to its highly localized expression during the primitive streak stage and in the adult stage, CAT3 expression was detected more widely in 13.5 day post-coitum mouse embryos.

The human immunodeficiency virus type 1 (HIV-1) CD4 receptor and its central role in promotion of HIV-1 infection

Interactions between the viral envelope glycoprotein gp120 and the cell surface receptor CD4 are responsible for the entry of human immunodeficiency virus type 1 (HIV-1) into host cells in the vast majority of cases. HIV-1 replication is commonly followed by the disappearance or receptor downmodulation of cell surface CD4. This potentially renders cells nonsusceptible to subsequent infection by HIV-1, as well as by other viruses that use CD4 as a portal of entry. Disappearance of CD4 from the cell surface is mediated by several different viral proteins that act at various stages through the course of the viral life cycle, and it occurs in T-cell lines, peripheral blood CD4+ lymphocytes, and monocytes of both primary and cell line origin. At the cell surface, gp120 itself and in the form of antigen-antibody complexes can trigger cellular pathways leading to CD4 internalization. Intracellularly, the mechanisms leading to CD4 downmodulation by HIV-1 are multiple and complex; these include degradation of CD4 by Vpu, formation of intracellular complexes between CD4 and the envelope precursor gp160, and internalization by the Nef protein. Each of the above doubtless contributes to the ultimate depletion of cell surface CD4, although the relative contribution of each mechanism and the manner in which they interact remain to be definitively established.

Lysinuric protein intolerance characterized by bone marrow abnormalities and severe clinical course

STUDY OBJECTIVE

To evaluate phenotypic variability of lysinuric protein intolerance in a cohort of nine Italian patients.

DESIGN

Retrospective analysis of patient records.

SUBJECTS

Nine Italian patients (seven independent families), all originating from southern Italy, observed during the last 14 years.

RESULTS

Some of the patients had unique clinical features, including bone marrow abnormalities featuring erythroblastophagocytosis (five patients) and clinical course and the outcome of the disease, have also been observed: respiratory involvement was present in five cases, with a lethal picture of "alveolar proteinosis" in one. Severe kidney involvement, with both glomerular and tubular damage and rapidly progressing to chronic renal failure, has been observed in one case.

CONCLUSION

Lysinuric protein intolerance may cause severe multisystem involvement, which requires early and careful monitoring. Some peculiar clinical findings observed in Italian patients point to a genetic heterogeneity of lysinuric protein intolerance.

Two cationic amino acid transport systems in human placental basal plasma membranes

Transport of cationic amino acids in basal (fetal facing) plasma membranes was investigated by characterization of L-[3H]lysine and L-[3H]arginine uptake in membrane vesicles isolated from term human placentas. At least two Na(+)-independent systems were present. Lysine concentration dependence data were fit by a two-system model with Km values of 1.0 +/- 0.8 and 223 +/- 57 microM and Vmax values of 0.06 +/- 0.03 and 24.0 +/- 5.8 pmol.mg protein-1.min-1. In the presence of either 10 mM L-leucine or Na+ plus 10 mM L-homoserine, the data were fit by single system models with kinetic parameters similar to the higher and lower Km systems seen in the absence of inhibitors. Uptake of 10 or 20 microM L-lysine in the absence of Na+ showed the higher Km system was inhibited completely by L-arginine, L-homoarginine, and L-histidine. In the presence of Na+, the higher Km system was inhibited completely by L-alanine, L-homoserine, L-leucine, L-phenylalanine, and L-norleucine. The lower Km system was inhibited completely by L-arginine, L-homoarginine, L-histidine, L-leucine, and L-methionine. Time course studies of uptake demonstrated that uptake by either system alone filled the total vesicular space. The basal membrane of human placental syncytiotrophoblast possesses two transport systems for lysine and arginine, resembling the ubiquitous y+ system and the bo,+ system previously described in mouse blastocysts. The higher Vmax of the y+ system suggests that in utero it may mediate transfer of cationic amino acids from the syncytiotrophoblast to the fetus. The role of the high-affinity low-capacity bo,+ system remains to be determined.

Influx of L-arginine is an indicator of membrane potential in human fibroblasts

The net influx of L-arginine (JARG) was employed as an indicator of the membrane potential in human fibroblasts. Cell depolarization, obtained by increasing [K+]out, decreased both JARG and the net influx of the lipid soluble cation tetraphenylphosphonium (JTPP), a probe of membrane potential. JTPP, but not JARG, was influenced by the mitochondrial potential and exhibited a component dependent on intracellular and/or extracellular binding. JARG was sensitive to changes in the membrane potential induced by Na+-dependent transport of L-proline or by the activity of Na+-K+-ATPase. In the presence of 50 microM valinomycin, JARG was markedly influenced by the distribution ratio of K+ in a range of [K+]out from 1.5 to 100 mM. In this range of [K+]out, membrane potential (Em) varied from -90 to -23 mV, and calibration of JARG vs. the membrane potential yielded a linear relationship. These results indicate the following: 1) that the net influx of TPP+ is not a reliable indicator of membrane potential in cultured human fibroblasts; 2) that in the same cells the net influx of L-arginine can be employed as an index of membrane potential; 3) that in a range of Em from -23 to -90 mV the activity of system y+ (the membrane agency devoted to L-arginine transport in cultured human fibroblasts) exhibits no saturation of potential-dependent activation of transport.

Comparison of ornithine metabolism in hyperornithinaemia-hyperammonaemia-homocitrullinuria syndrome, lysinuric protein intolerance and gyrate atrophy fibroblasts

We measured L-ornithine oxidation in cultured skin fibroblasts from seven patients with hyperornithinaemia-hyperammonaemia-homocitrullinuria (HHH) syndrome (McKusick 23897), and compared it with oxidation by ornithine aminotransferase deficient gyrate atrophy (McKusick 25887) cells and lysinuric protein intolerance (McKusick 22270) cells in which there is an ornithine transport abnormality at the plasma membrane. Net uptake of ornithine is not abnormal in intact HHH cells. Ornithine oxidation was depressed in HHH and gyrate atrophy cells but not in lysinuric protein intolerance cells; the latter finding suggests there is no significant mitochondrial defect in lysinuric protein intolerance cells. Since HHH cells have intact ornithine aminotransferase, impaired oxidation is compatible with deficient penetration of ornithine into mitochondria in this disease. We could not demonstrate a gene dosage effect in oxidation values.

Lysinuric protein intolerance mutation is not expressed in the plasma membrane of erythrocytes

We measured mediated fluxes of L-lysine and L-ornithine across the plasma membrane of erythrocytes from control subjects and patients homozygous for the lysinuric protein intolerance (LPI) mutation. We found no differences in net uptake or efflux of cationic amino acids in control and LPI cells, contrary to our findings in cultured skin fibroblasts. We conclude that expression of the LPI (y+) transport system for cationic amino acids varies between tissues and that measurements of fluxes in erythrocytes cannot be used for diagnosis of LPI.

Quantitative relationships between the measurable transport resistances of membrane carriers: theory and experiment

A theoretical analysis is made of the possible quantitative relationships between the transport resistances that characterise membrane carrier systems. It is shown that there exist only five possible patterns in which to rank the four transport resistances. Symbolising these as A, B, C and D, the five possible patterns are (i) A = B = C = D; (ii) A = B much greater than C, D; (iii) A = B much greater than C = D; (iv) A = B = 2C = 2D; (v) A = 2B = 2C much greater than D. A survey of the available experimental data shows that pattern (ii) is the most prevalent, pattern (v) is often found and pattern (iii) has been identified. None of the ten transport systems so far analysed experimentally failed to fit one of the predicted patterns.

Transport competence of plasma membrane vesicles from cultured human fibroblasts

We obtained plasma membranes from cultured human skin fibroblasts. The preparation was enriched 10-fold with about 40 percent yield. There was minimal contamination with other cell membranes. Various observations indicated vesicular conformation of a portion of the plasma membranes, notably by electron microscopy and from the effect of osmotic pressure on the distribution of solutes between mass and medium at equilibrium. Other studies indicated that these fibroblast plasma membrane vesicles retained mediated transport processes for a variety of substrates. The evidence included: stereospecific and temperature-dependent uptake of glucose; dependence of L-alanine uptake on sodium ion and an inward-directed transmembrane Na+ gradient; stimulation of L-alanine uptake, with overshoot, by enhancement of the interior-negative transmembrane potential; concentration dependent uptake of methotrexate with apparent competitive inhibition by folinic acid; stimulation of L-lysine uptake by trans-L-arginine. These findings indicate that human fibroblast plasma membrane vesicles could be used to study membrane transport processes and, perhaps, expression of mutant genes that cause inborn errors of transport.

Neutral amino acid transport and in vitro aging

Human fibroblasts possess five distinguishable systems for amino acid transport. Initial rates for three of these decline with in vitro age. The decline most noticeably affects the L system, whose rate for L-phenylalanine transport at 10 microM declines by 44% between the 27th and 39th population doubling levels. Accumulation of L-phenylalanine is similarly affected. The insensitivity of older cells to reduction of the amino acid concentration in culture medium casts doubt on possible interpretation of the transport decline as a causal factor in limiting growth.