Induction of the high-affinity Na(+)-dependent glutamate transport system XAG- by hypertonic stress in the renal epithelial cell line NBL-1

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Homeostasis of the Intraparenchymal-Blood Glutamate Concentration Gradient: Maintenance, Imbalance, and Regulation

It is widely accepted that glutamate is the most important excitatory neurotransmitter in the central nervous system (CNS). However, there is also a large amount of glutamate in the blood. Generally, the concentration gradient of glutamate between intraparenchymal and blood environments is stable. However, this gradient is dramatically disrupted under a variety of pathological conditions, resulting in an amplifying cascade that causes a series of pathological reactions in the CNS and peripheral organs. This eventually seriously worsens a patient’s prognosis. These two “isolated” systems are rarely considered as a whole even though they mutually influence each other. In this review, we summarize what is currently known regarding the maintenance, imbalance and regulatory mechanisms that control the intraparenchymal-blood glutamate concentration gradient, discuss the interrelationships between these systems and further explore their significance in clinical practice.

Changes in the expression of the glutamate transporter EAAT3/EAAC1 in health and disease

Excitatory amino acid transporters (EAATs) are high-affinity Na(+)-dependent carriers of major importance in maintaining glutamate homeostasis in the central nervous system. EAAT3, the human counterpart of the rodent excitatory amino acid carrier 1 (EAAC1), is encoded by the SLC1A1 gene. EAAT3/EAAC1 is ubiquitously expressed in the brain, mostly in neurons but also in other cell types, such as oligodendrocyte precursors. While most of the glutamate released in the synapses is taken up by the "glial-type" EAATs, EAAT2 (GLT-1 in rodents) and EAAT1 (GLAST), the functional role of EAAT3/EAAC1 is related to the subtle regulation of glutamatergic transmission. Moreover, because it can also transport cysteine, EAAT3/EAAC1 is believed to be important for the synthesis of intracellular glutathione and subsequent protection from oxidative stress. In contrast to other EAATs, EAAT3/EAAC1 is mostly intracellular, and several mechanisms have been described for the rapid regulation of the membrane trafficking of the transporter. Moreover, the carrier interacts with several proteins, and this interaction modulates transport activity. Much less is known about the slow regulatory mechanisms acting on the expression of the transporter, although several recent reports have identified changes in EAAT3/EAAC1 protein level and activity related to modulation of its expression at the gene level. Moreover, EAAT3/EAAC1 expression is altered in pathological conditions, such as hypoxia/ischemia, multiple sclerosis, schizophrenia, and epilepsy. This review summarizes these results and provides an overall picture of changes in EAAT3/EAAC1 expression in health and disease.

Long-term osmotic regulation of amino acid transport systems in mammalian cells

Mammalian cells accumulate organic osmolytes, either to adapt to permanent osmotic changes or to mediate cell volume increase in cell cycle progression. Amino acids may serve as osmolytes in a great variety of cells. System A, a transport system for neutral amino acids, is induced after hypertonic shock by a mechanism which requires protein synthesis and gene transcription. Indirect evidence supports the view that system A activity increases due to the interaction of pre-existing A carriers with putative activating proteins. The intracellular accumulation of most neutral amino acids after hypertonic shock depends, exclusively, on the increase in system A activity. Long-term activation of system A is dependent on the integrity of cytoskeletal structures, but in a different way depending on whether cells are polarized or not.

Regulation of amino acid transport in the renal epithelial cell line NBL-1

The activities of the transport systems A, B° and XAG- are induced by various forms of stress in renal epithelial cells. Amino acid deprivation induces System A and XAG- in a protein-synthesis dependent process. In the case of System XAG- evidence is presented that induction of transport does not involve an increase in the amount of mRNA for the transporter or of the amount of transport protein. Preliminary evidence for the existence of a novel glycoprotein which is induced in parallel to the induction of these transport systems is presented. It is suggested that the induction of amino acid transport proteins and of some of the so-called stress proteins may be triggered by a common molecular mechanism.

Selective induction of glial glutamate transporter GLT-1 by hypertonic stress in C6 glioma cells

Glial glutamate transporter GLT-1 mRNA was selectively induced in C6 glioma cells exposed to hypertonic stress (HS), while the expression of two other subtypes, GLAST and EAAC1, was suppressed. HS increased phosphorylation of the MAPK family, ERK, p38 MAPK, and JNK. Treatment with a PKC inhibitor showed that phosphorylation of both p38 MAPK and JNK is PKC-dependent but ERK phosphorylation is independent. Inhibition of either ERK or p38 MAPK did not abolish GLT-1 mRNA induction. Inhibition of PKC also had no effect. These findings indicate that the induction of GLT-1 mRNA by HS is independent of the MAPK pathways. This is the first report that the expression of glial glutamate transporters is osmotically regulated.

Glutamate transporters in kidney and brain

Glutamate transporters play important roles in the termination of excitatory neurotransmission and in providing cells with glutamate for metabolic purposes. In the kidney, glutamate transporters are involved in reabsorption of filtered acidic amino acids, regulation of ammonia and bicarbonate production, and protection of cells against osmotic stress.

Regulation of high-affinity glutamate transport by amino acid deprivation and hyperosmotic stress

High-affinity glutamate transport activity is induced by stress in NBL-1 cells. Exposure of cells to hyperosmotic medium led to an induction of the EAAC1 glutamate transporter, preceded by a large increase in EAAC1 mRNA levels. Culture of cells in amino acid-free medium also caused a protein synthesis-dependent increase in glutamate transport activity, but this was not accompanied by an increase of either EAAC1 mRNA or protein. Indirect evidence suggests that the increase in EAAC1 activity in the latter case may be due to the synthesis of an activator protein in response to decreased intracellular glutamate concentrations.

Hypertonicity stimulates taurine uptake and transporter gene expression in Caco-2 cells

The osmoregulation of taurine transport in intestinal epithelial cells was investigated using human intestinal Caco-2 cells. The activity of taurine transport in the Caco-2 cells was increased by hypertonic conditions. This hypertonicity-induced up-regulation was dependent on both the culturing time and the osmotic pressure. Hypertonicity did not affect the activity of L-leucine, L-lysine, or L-glutamic acid transport, suggesting that osmoregulation was specific to taurine transport. The intracellular taurine content of Caco-2 cells was also increased by culturing in a hypertonic medium. These hypertonicity-induced changes in the intracellular taurine content and transport activity were reversible. A kinetic analysis of taurine transport in the control and hypertonic cells suggested that the up-regulation was associated with an increase in the amount of the taurine transporter. The mRNA level of the taurine transporter in hypertonic cells was markedly higher than that in the control cells, indicating that this osmotic regulation was due to the increased expression of the taurine transporter gene.

Amino acids are compatible osmolytes for volume recovery after hypertonic shrinkage in vascular endothelial cells

The response to chronic hypertonic stress has been studied in human endothelial cells derived from saphenous veins. In complete growth medium the full recovery of cell volume requires several hours and is neither associated with an increase in cell K+ nor hindered by bumetanide but depends on an increased intracellular pool of amino acids. The highest increase is exhibited by neutral amino acid substrates of transport system A, such as glutamine and proline, and by the anionic amino acid glutamate. Transport system A is markedly stimulated on hypertonic stress, with an increase in activity roughly proportional to the extent and the duration of the osmotic shrinkage. Cycloheximide prevents the increase in transport activity of system A and the recovery of cell volume. It is concluded that human endothelial cells counteract hypertonic stress through the stimulation of transport system A and the consequent expansion of the intracellular amino acid pool.

Molecular biology of mammalian plasma membrane amino acid transporters

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

Cellular distribution and kinetic properties of high-affinity glutamate transporters

L-glutamic acid is a key chemical transmitter of excitatory signals in the nervous system. The termination of glutamatergic transmission occurs via uptake of glutamate by a family of high-affinity glutamate transporters that utilize the Na+/K+ electrochemical gradient as a driving force. The stoichiometry of a single translocation cycle is still debatable, although all proposed models stipulate an inward movement of a net positive charge. This electrogenic mechanism is capable of translocating the neurotransmitter against its several thousand-fold concentration gradient, therefore, keeping the resting glutamate concentration below the treshold levels. The five cloned transporters (GLAST/EAAT1, GLT1/EAAT2, EAAC1/EAAT3, EAAT4, and EAAT5) exhibit distinct distribution patterns and kinetic properties in different brain regions, cell types, and reconstitution systems. Moreover, distinct pharmacological profiles were revealed among the species homologues. GLAST and GLT1, the predominant glutamate transporters in the brain, are coexpressed in astroglial processes, whereas neuronal carriers are mainly located in the dendrosomatic compartment. Some of these carrier proteins may possess signal transducing properties, distinct from their transporter activity. Some experimental conditions and several naturally occurring and synthetic compounds are capable of regulating the expression of glutamate transporters. However, selective pharmacological tools interfering with the individual glutamate carriers have yet to be developed.

Cytoskeletal-dependent activation of system A for neutral amino acid transport in osmotically stressed mammalian cells: a role for system A in the intracellular accumulation of osmolytes

System A activity for neutral amino acid transport is increased after hypertonic shock in NBL-1 (an epithelial cell line) and CHO-K1 cells (a nonepithelial cell line) by a mechanism which is consistent with the synthesis of a regulatory protein that activates preexisting system A carrier proteins (Ruiz-Montasell et al., 1994, Proc. Natl. Acad. Sci. USA, 91,9569-9573). In this study, we have further investigated this biological response by determining the role of cytoskeletal structures in system A regulation by hypertonic stress. Using inhibitors of the microfilament and microtubule networks, we show that the increase in system A activity after hypertonic treatment requires the integrity of both cytoskeletal structures in NBL-1 cells, although the increase in system A activity triggered by amino acid starvation is completely insensitive to any of these drugs. In contrast, the enhancement of system A activity in osmotically stressed CHO-K1 cells is not sensitive to inhibitors of the microtubule network. In both cell types, the results suggest that the inhibitors block the increase of system A activity. System A transport decreases when CHO-K1 cells return to isotonic conditions by a mechanism that is insensitive to inhibitors of protein and mRNA synthesis. The increase in system A transport activity is also followed by the accumulation of neutral amino acids (fourfold for alanine), which is totally blocked by the same agents (cycloheximide and actinomycin D) that prevent the increase in system A activity after hypertonic treatment, thus indicating that system A is crucial for maintaining a high concentration of organic osmolytes inside the cell.

Localization of the high-affinity glutamate transporter EAAC1 in rat kidney

Most amino acids filtered by the glomerulus are reabsorbed in the kidney via specialized transport systems. Recently, the cDNA encoding a high-affinity glutamate transporter, EAAC1, has been isolated and shown to be expressed at high levels in the kidney. To determine the potential role of EAAC1 in renal acidic amino acid reabsorption, the distribution of EAAC1 mRNA and protein in rat kidney was examined. In situ hybridization revealed that EAAC1 mRNA is expressed predominantly in S2 and S3 segments of the proximal tubules and at low levels in the inner stripe of outer medulla and inner medulla. Polyclonal antibodies raised against the carboxy terminus of EAAC1 recognized a single band of approximately 70 kDa on Western blots of membrane protein from kidney cortex and medulla. Immunofluorescence microscopy revealed intense signals in the luminal membrane of S2 and S3 segments and weaker signals in S1 segments, descending thin limbs of long-loop nephrons, medullary thick ascending limbs, and distal convoluted tubules. These results are consistent with EAAC1 encoding the previously described apical high-affinity glutamate transporter in the kidney that mediates reabsorption of acidic amino acids in tubules beyond early proximal tubule S1 segments. Potential additional roles of EAAC1 in acid/base balance, cell volume regulation, and amino acid metabolism are discussed.

Molecular cloning of a bovine renal G-protein coupled receptor gene (bRGR): regulation of bRGR mRNA levels by amino acid availability

A cDNA of 3.2 kb, encoding a putative G protein-coupled receptor and hence called bRGR1, has been isolated from a cDNA library generated from the bovine renal epithelial cell line NBL-1. This cDNA consisted of 41 base pairs of 5'-untranslated sequence, an open reading frame of 1083 base pairs, and a 2.07 kb fragment of 3'-untranslated sequence that includes a poly(dA) tail. The coding sequence predicts a protein of 361 residues. The ligand of the bRGR1 protein may be of low molecular weight, as deduced from the analysis of the predicted primary structure of the receptor protein and the comparison with other subtypes of the G protein-coupled receptor family. The amounts of bRGR1 mRNA significantly increase when NBL-1 cells are cultured in an amino acid-depleted medium. This effect can not be caused by a decrease in protein synthesis because cycloheximide did not mimic the increase in bRGR1 mRNA levels triggered by amino acid starvation. These data suggest that bRGR1 may be an amino acid-regulated gene.

Effects of cyclosporine A on Na,K-ATPase expression in the renal epithelial cell line NBL-1

The bovine renal epithelial cell line NBL-1 has been used to monitor the effects of cyclosporine A (CsA) on Na+,K(+)-ATPase activity and expression. CsA at two single doses (0.6 mg/liter and 2.5 mg/liter) inhibits the ouabain-sensitive component of Rb+ uptake, assumed to be Na+,K(+)-ATPase, but increases the low activity of a furosemide-sensitive component corresponding to a Na+/K+/Cl- cotransporter. CsA addition also induces a slight decrease of alpha 1 subunit mRNA levels, without altering the already low beta 1 subunit mRNA amounts. Hypertonic treatment of NBL-1 cells leads to a significant increase in both Na+,K(+)-ATPase activity and alpha 1 subunit mRNA amounts, but does not modify beta 1 subunit mRNA levels. The differential response of the alpha 1 and beta 1 subunit genes may explain why hypertonic treatment does not result in higher alpha 1 protein expression, and supports the view that increased activity relies upon post-translational events, despite the likely transcriptional activation of the alpha 1 subunit gene. The addition of CsA does not alter the hypertonicity-mediated increase of Na+,K(+)-ATPase activity but blocks the accumulation of alpha 1 subunit mRNA. In conclusion, CsA may compromise the ion handling by renal cells as a result of the inhibition of basal Na+,K(+)-ATPase activity and the stimulation of Na+/K+/Cl- cotransport activity. Moreover, this is the first report showing that CsA may affect the long-term adaptation of the pump by altering its subunit gene expression.

Regulation of Na+,K(+)-ATPase and the Na+/K+/Cl- co-transporter in the renal epithelial cell line NBL-1 under osmotic stress

The long-term adaptation of the Na+,K(+)-ATPase to hypertonicity was studied using the bovine renal epithelial cell line NBL-1. Na+,K(+)-ATPase activity measured in intact cells as the ouabain-sensitive fraction of Rb+ uptake was stimulated (40% above controls) after incubating the cells in hypertonic medium. This stimulation was not correlated with significant changes in the amount of Na+,K(+)-ATPase alpha 1 subunit protein. Nevertheless, the amount of alpha 1 but not beta 1 subunit mRNA progressively increased after hypertonic shock (3-4-fold above basal values). These results suggest that the alpha 1 subunit gene is modulated by medium osmolarity, although this does not necessarily involve enhanced translation of the mRNA into active alpha 1 protein. Indeed, the increase in the biological activity of the Na+,K(+)-ATPase is abolished when the electrochemical Na+ transmembrane gradient is depleted by monensin, which is consistent with a post-translational effect on the activity of the sodium pump. A furosemide-sensitive component of Rb+ uptake, attributable to Na+/K+/Cl- co-transporter activity, was very low when cells were cultured in a regular medium, but was greatly induced after hypertonic shock. This induction could not be blocked by cycloheximide. Colcemide addition slightly reduced the absolute increase in Na+/K+/Cl- co-transporter activity, while cytochalasin B significantly potentiated the effect triggered by hypertonic shock. It is concluded: (i) that in NBL-1 cells the alpha 1 but not the beta 1 subunit of the Na+,K(+)-ATPase is encoded by an osmotically sensitive gene, and (ii) that the Na+/K+/Cl- co-transporter, although an osmotically sensitive carrier, is induced by a mechanism that is independent of protein synthesis but may rely, in an undetermined manner, on the structure of the cytoskeletal network.

Induction of high affinity glutamate transport activity by amino acid deprivation in renal epithelial cells does not involve an increase in the amount of transporter protein

In renal epithelial cells amino acid deprivation induces an increase in L-Asp transport with a doubling of the Vmax and no change in Km (4.5 micronM) in a cycloheximide-sensitive process. The induction of sodium-depending L-aspartate transport was inhibited by single amino acids that are metabolized to produce glutamate but not by those that do not produce glutamate. The transaminase inhibitor aminooxyacetate in glutamine-free medium caused a decrease in cell glutamate content and an induction of glutamate transport. In complete medium aminooxyacetate neither decreased cell glutamate nor increased transport activity. These results are consistent with a triggering of induction of transport by low intracellular glutamate concentrations. High affinity glutamate transport in these cells is mediated by the excitatory amino acid carrier 1 (EAAC1) gene product. Western blotting using antibodies to the C-terminal region of EAAC1 showed that there is no increase in the amount of EAAC1 protein on prolonged incubation in amino acid-free medium. Conversely, the induction of high affinity glutamate transport by hyperosmotic shock was accompanied by an increase in EAAC1 protein. It is proposed that low glutamate levels lead to the induction of a putative protein that activates the EAAC1 transporter. A model illustrating such a mechanism is described.