L-glutamate stimulates the efflux of newly taken up glutamine from astroglia but not from synaptosomes of the rat

The Human SLC1A5 (ASCT2) Amino Acid Transporter: From Function to Structure and Role in Cell Biology

SLC1A5, known as ASCT2, is a neutral amino acid transporter belonging to the SLC1 family and localized in the plasma membrane of several body districts. ASCT2 is an acronym standing for Alanine, Serine, Cysteine Transporter 2 even if the preferred substrate is the conditionally essential amino acid glutamine, with cysteine being a modulator and not a substrate. The studies around amino acid transport in cells and tissues began in the '60s by using radiolabeled compounds and competition assays. After identification of murine and human genes, the function of the coded protein has been studied in cell system and in proteoliposomes revealing that this transporter is a Na⁺ dependent antiporter of neutral amino acids, some of which are only inwardly transported and others are bi-directionally exchanged. The functional asymmetry merged with the kinetic asymmetry in line with the physiological role of amino acid pool harmonization. An intriguing function has been described for ASCT2 that is exploited as a receptor by a group of retroviruses to infect human cells. Interactions with scaffold proteins and post-translational modifications regulate ASCT2 stability, trafficking and transport activity. Two asparagine residues, namely N163 and N212, are the sites of glycosylation that is responsible for the definitive localization into the plasma membrane. ASCT2 expression increases in highly proliferative cells such as inflammatory and stem cells to fulfill the augmented glutamine demand. Interestingly, for the same reason, the expression of ASCT2 is greatly enhanced in many human cancers. This finding has generated interest in its candidacy as a pharmacological target for new anticancer drugs. The recently solved 3D structure of ASCT2 will aid in the rational design of such therapeutic compounds.

Protein Kinase C Phosphorylates the System N Glutamine Transporter SN1 (Slc38a3) and Regulates Its Membrane Trafficking and Degradation

The system N transporter SN1 (also known as SNAT3) is enriched on perisynaptic astroglial cell membranes. SN1 mediates electroneutral and bidirectional glutamine transport, and regulates the intracellular as well as the extracellular concentrations of glutamine. We hypothesize that SN1 participates in the glutamate/γ-aminobutyric acid (GABA)-glutamine cycle and regulates the amount of glutamine supplied to the neurons for replenishment of the neurotransmitter pools of glutamate and GABA. We also hypothesize that its activity on the plasma membrane is regulated by protein kinase C (PKC)-mediated phosphorylation and that SN1 activity has an impact on synaptic plasticity. This review discusses reports on the regulation of SN1 by PKC and presents a consolidated model for regulation and degradation of SN1 and the subsequent functional implications. As SN1 function is likely also regulated by PKC-mediated phosphorylation in peripheral organs, the same mechanisms may, thus, have impact on e.g., pH regulation in the kidney, urea formation in the liver, and insulin secretion in the pancreas.

GLAST/EAAT1-induced glutamine release via SNAT3 in Bergmann glial cells: evidence of a functional and physical coupling

Glutamate, the major excitatory transmitter in the vertebrate brain, is removed from the synaptic cleft by a family of sodium-dependent glutamate transporters profusely expressed in glial cells. Once internalized, it is metabolized by glutamine synthetase to glutamine and released to the synaptic space through sodium-dependent neutral amino acid carriers of the N System (SNAT3/slc38a3/SN1, SNAT5/slc38a5/SN2). Glutamine is then taken up by neurons completing the so-called glutamate/glutamine shuttle. Despite of the fact that this coupling was described decades ago, it is only recently that the biochemical framework of this shuttle has begun to be elucidated. Using the established model of cultured cerebellar Bergmann glia cells, we sought to characterize the functional and physical coupling of glutamate uptake and glutamine release. A time-dependent Na⁺-dependent glutamate/aspartate transporter/EAAT1-induced System N-mediated glutamine release could be demonstrated. Furthermore, D-aspartate, a specific glutamate transporter ligand, was capable of enhancing the co-immunoprecipitation of Na⁺-dependent glutamate/aspartate transporter and Na⁺-dependent neutral amino acid transporter 3, whereas glutamine tended to reduce this association. Our results suggest that glial cells surrounding glutamatergic synapses may act as sensors of neuron-derived glutamate through their contribution to the neurotransmitter turnover.

Roles of glutamine in neurotransmission

Glutamine (Gln) is found abundantly in the central nervous system (CNS) where it participates in a variety of metabolic pathways. Its major role in the brain is that of a precursor of the neurotransmitter amino acids: the excitatory amino acids, glutamate (Glu) and aspartate (Asp), and the inhibitory amino acid, γ-amino butyric acid (GABA). The precursor-product relationship between Gln and Glu/GABA in the brain relates to the intercellular compartmentalization of the Gln/Glu(GABA) cycle (GGC). Gln is synthesized from Glu and ammonia in astrocytes, in a reaction catalyzed by Gln synthetase (GS), which, in the CNS, is almost exclusively located in astrocytes (Martinez-Hernandez et al., 1977). Newly synthesized Gln is transferred to neurons and hydrolyzed by phosphate-activated glutaminase (PAG) to give rise to Glu, a portion of which may be decarboxylated to GABA or transaminated to Asp. There is a rich body of evidence which indicates that a significant proportion of the Glu, Asp and GABA derived from Gln feed the synaptic, neurotransmitter pools of the amino acids. Depolarization-induced-, calcium- and PAG activity-dependent releases of Gln-derived Glu, GABA and Asp have been observed in CNS preparations in vitro and in the brain in situ. Immunocytochemical studies in brain slices have documented Gln transfer from astrocytes to neurons as well as the location of Gln-derived Glu, GABA and Asp in the synaptic terminals. Patch-clamp studies in brain slices and astrocyte/neuron co-cultures have provided functional evidence that uninterrupted Gln synthesis in astrocytes and its transport to neurons, as mediated by specific carriers, promotes glutamatergic and GABA-ergic transmission. Gln entry into the neuronal compartment is facilitated by its abundance in the extracellular spaces relative to other amino acids. Gln also appears to affect neurotransmission directly by interacting with the NMDA class of Glu receptors. Transmission may also be modulated by alterations in cell membrane polarity related to the electrogenic nature of Gln transport or to uncoupled ion conductances in the neuronal or glial cell membranes elicited by Gln transporters. In addition, Gln appears to modulate the synthesis of the gaseous messenger, nitric oxide (NO), by controlling the supply to the cells of its precursor, arginine. Disturbances of Gln metabolism and/or transport contribute to changes in Glu-ergic or GABA-ergic transmission associated with different pathological conditions of the brain, which are best recognized in epilepsy, hepatic encephalopathy and manganese encephalopathy.

The glutamine/amino acid transporter (ASCT2) reconstituted in liposomes: transport mechanism, regulation by ATP and characterization of the glutamine/glutamate antiport

The glutamine/amino acid transporter solubilized from rat renal apical plasma membrane (brush-border membrane) with C12E8 and reconstituted into liposomes has been previously identified as the ASCT2 transporter. The reconstituted transporter catalyses an antiport reaction in which external glutamine and Na+ are cotransported in exchange with internal glutamine (or other amino acids). The glutamine-Na+ cotransport occurred with a 1:1 stoichiometry. The concentration of Na+ did not influence the Km for glutamine and vice versa. Experimental data obtained by a bi-substrate analysis of the glutamine-Na+ cotransport, together with previous report on the glutamine(ex)/glutamine(in) pseudo bi-reactant analysis, indicated that the transporter catalyses a three-substrate transport reaction with a random simultaneous mechanism. The presence of ATP in the internal compartment of the proteoliposomes led to an increase of the Vmax of the transport and to a decrease of the Km of the transporter for external Na+. The reconstituted glutamine/amino acid transporter was inhibited by glutamate; the inhibition was more pronounced at acidic pH. A kinetic analysis revealed that the inhibition was competitive with respect to glutamine. Glutamate was also transported in exchange with glutamine. The external Km of the transporter for glutamate (13.3 mM) was slightly higher than the internal one (8.3 mM). At acidic pH the external but not the internal Km decreased. According with the Km values, glutamate should be transported preferentially from inside to outside in exchange for external glutamine and Na+.

Astroglial glutamine transport by system N is upregulated by glutamate

Release of glutamine from astrocytes is an essential step of the glutamate-glutamine cycle, and hence for the maintenance of neuronal glutamate and gamma-aminobutyric acid (GABA) pools. The glutamine transporter SNAT3 (SN1) has recently been identified as one of the major mediators of glutamine efflux from astrocytes. We investigated the regulation of SNAT3 mediated glutamine transport in cultured astrocytes. Incubation of primary astrocyte cultures with physiological concentrations of glutamate resulted in a rapid, about twofold, upregulation of SNAT3-mediated transport activity. The effect was not mediated by glutamate receptors but required uptake of glutamate into astrocytes. Both net uptake and net efflux increased after treatment of cells with glutamate, excluding an acceleration of the transport by way of an exchange mechanism. Elevated intracellular glutamate most likely reduces the K(m) of SNAT3 for its substrate glutamine. The results suggest that astrocytes respond actively to the release of glutamate by increasing glutamine release and thereby may modulate glutamatergic neurotransmission.

Transfer of glutamine between astrocytes and neurons

The export of glutamine from astrocytes, and the uptake of glutamine by neurons, are integral steps in the glutamate-glutamine cycle, a major pathway for the replenishment of neuronal glutamate. We review here the functional and molecular identification of the transporters that mediate this transfer. The emerging picture of glutamine transfer in adult brain is of a dominant pathway mediated by system N transport (SN1) in astrocytes and system A transport (SAT/ATA) in neurons. The participating glutamine transporters are functionally and structurally related, sharing the following properties: (a) unlike many neutral amino acid transporters which have proven to be obligate exchangers, these glutamine transporters mediate net substrate transfer energized by coupling to ionic gradients; (b) they are sensitive to small pH changes in the physiological range; (c) they are susceptible to adaptive and humoral regulation; (d) they are related structurally to the AAAP (amino acid and auxin permeases) family of transporters. A key difference between SN1 and the SAT/ATA transporters is the ready reversibility of glutamine fluxes via SN1 under physiological conditions, which allows SN1 both to sustain a glutamine concentration gradient in astrocytes and to mediate the net outward flux of glutamine. It is likely that the ASCT2 transporter, an obligate exchanger of neutral amino acids, displaces the SN1 transporter as the main carrier of glutamine export in proliferating astrocytes.

Molecular analysis of system N suggests novel physiological roles in nitrogen metabolism and synaptic transmission

The amino acid glutamine has a central role in nitrogen metabolism. Although the molecular mechanisms responsible for its transport across cell membranes remain poorly understood, classical amino acid transport system N appears particularly important. Using intracellular pH measurements, we have now identified an orphan protein related to a vesicular neurotransmitter transporter as system N. Functional analysis shows that this protein (SN1) involves H+ exchange as well as Na+ cotransport and, under physiological conditions, mediates glutamine efflux as well as uptake. Together with the pattern of SN1 expression, these unusual properties suggest novel physiological roles for system N in nitrogen metabolism and synaptic transmission.

Effect of intracellular glutamine on the uptake of large neutral amino acids in astrocytes: concentrative Na(+)-independent transport exhibits metastability

To examine whether the concentration gradient of glutamine (Gln) drives concentrative Na(+)-independent uptake of neutral amino acids (NAA) in mouse cerebral astrocytes, uptake was compared in "Gln-depleted" and "Gln-replete" cultures. Uptake (30 min in Na(+)-free buffer) of histidine, kynurenine, leucine, tyrosine, and a model substrate for System L transport was 70-150% greater in Gln-replete cultures. Phenylalanine uptake was not affected. All of these NAA trans-stimulated the export of Gln from astrocytes. However, the increase in NAA uptake was sustained even though the Gln content of Gln-replete cultures declined. Also, uptake of Gln itself was enhanced in Gln-replete cultures. Thus, countertransport of Gln was insufficient to explain the enhancement of NAA uptake. Enhanced uptake was restored, and could be magnified, by reloading Gln-depleted cultures either with Gln or with histidine. It is suggested that substrate-induced asymmetry and molecular hysteresis in the Na(+)-independent carrier could account for the sustained enhancement of NAA uptake. Only histidine and kynurenine were concentrated comparably to Gln (15- to 29-fold at 1 mM in Na(+)-free buffer). The other NAA were four to six times less concentrated. At least two Na(+)-dependent transport systems also supported the concentration gradient of Gln in regular buffer.

Changes in brain ECF amino acids in rats with experimentally induced hyperammonemia

Using microdialysis, we studied brain extracellular fluid (ECF) amino acid metabolism in rats with experimentally induced hyperammonemia and regional elevation of brain ECF ammonia levels. The total brain ECF amino acid level was increased by an elevation of the blood ammonia level. Hyperammonemia elevated brain ECF aromatic amino acids and reduced arterial blood branched chain amino acids. When rats with hyperammonemia were intravenously administered norleucine, the brain ECF norleucine level rose markedly, suggesting increased permeability of the blood-brain barrier. When rats with hyperammonemia were infused with a branched chain amino acid-rich preparation, the elevated brain ECF aromatic amino acids level was not suppressed. Following local intracerebral ammonia infusion, only glutamate levels showed a marked elevation. These results suggest that impairment of the blood-brain barrier related to hyperammonemia increases the inflow of low molecular weight substances including amino acids. Furthermore, the ammonia-induced increase of glutamate in the cerebral ECF suggests that high ammonia levels may increase the excitability of the brain. Thus, ammonia may serve as a key factor in the onset of hepatic encephalopathy.

Characteristics of large neutral amino acid-induced release of preloaded L-glutamine from rat cerebral capillaries in vitro: Effects of ammonia, hepatic encephalopathy, and ?-glutamyl transpeptidase inhibitors

The release of newly loaded L-[14C]glutamine (L-Gln) from rat cerebral cortical capillaries was stimulated by L-transport system substrates: tryptophan (TRY), leucine (leu), and nonlabeled L-Gln, respectively, by 32, 50, and 40% above the basal release resulting from superfusion with standard Krebs-Henseliet buffer. However, no stimulation was observed upon treatment with D-Gln or L-glutamate (L-Glu), which are not the L-system substrates, or with ammonium chloride. The stimulatory effect of TRY was temperature dependent but sodium independent, and was abolished in the presence of a sulfhydryl reagent N-ethylmaleimide (NEM). The results support the view that the L-Gln-stimulated uptake of large neutral amino acids (LNAA) across the blood-brain barrier involves the L-system mediated Gln-LNAA exchange. The TRY-stimulated Gln release was enhanced in vitro by simultaneous addition of ammonium chloride, and in capillaries derived from rats with acute hepatic encephalopathy (HE). These results confirm the role of Gln-LNAA exchange in the excessive accumulation of LNAA in brain observed in a variety of hyperammonemic conditions. Superfusion of L-Gln-loaded capillaries in a buffer containing gamma-glutamyl transpeptidase (GGT) inhibitors, serine borate (SB) or 6-diazo-5-oxo-L-norleucine (DON), increased the basal L-Gln release and made it irresponsive to subsequent treatment with TRY. However, the basal release was also increased by superfusion with serine alone or Leu, and this treatment abolished the subsequent effect of TRY as well. Moreover, DON stimulated L-Gln release from capillaries superfused in a standard way, and the effects of DON and TRY were additive. Hence, in the present conditions, SB and DON acted as L-system substrates rather than as GGT inhibitors. Taken together, the results do not support the concept that GGT mediates the Gln-LNAA exchange.

Decreased potassium-stimulated release of [3H]D-aspartate from hippocampal slices distinguishes encephalopathy related to acute liver failure from that induced by simple hyperammonemia

The calcium-dependent, high (65 mM) potassium-evoked release of the L-glutamate analogue [3H]D-aspartate (D-Asp) was measured in hippocampal slices derived from rats with (a) hepatic encephalopathy (HE) induced with a hepatotoxin, thioacetamide, (b) hyperammonemia produced by i.p. administration of ammonium acetate, and (c) in normal slices preincubated for 30 min with 1 mM ammonium acetate. HE (variant a) inhibited the release by about 30%, which was interpreted to indicate depressed exocytosis of synaptic glutamate. This phenomenon is likely to lead to a decrease of glutamate-mediated neural excitation, which in turn could contribute to the neural inhibition typical of HE. By contrast, and in agreement with earlier reports, hyperammonemia (variant b) did not affect D-Asp release, whereas in vitro treatment of the slices with ammonium acetate (variant c) resulted in a 60% increase of the release. Hence, impairment of synaptic glutamate exocytosis is the phenomenon that distinguishes HE related to toxic liver failure from simple hyperammonemia. This result emphasizes the role of other factors than ammonia in the pathophysiological mechanism of HE.

Hyperammonaemia does not impair brain function in the absence of net glutamine synthesis

1. It has been established that chronic hyperammonaemia, whether caused by portacaval shunting or other means, leads to a variety of metabolic changes, including a depression in the cerebral metabolic rate of glucose (CMRGlc) increased permeability of the blood-brain barrier to neutral amino acids, and an increase in the brain content of aromatic amino acids. The preceding paper [Jessy, DeJoseph & Hawkins (1991) Biochem. J. 277, 693-696] showed that the depression in CMRGlc caused by hyperammonaemia correlated more closely with glutamine, a metabolite of ammonia, than with ammonia itself. This suggested that ammonia (NH3 and NH4+) was without effect. The present experiments address the question whether ammonia, in the absence of net glutamine synthesis, induces any of the metabolic symptoms of cerebral dysfunction associated with hyperammonaemia. 2. Small doses of methionine sulphoximine, an inhibitor of glutamine synthetase, were used to raise the plasma ammonia levels of normal rats without increasing the brain glutamine content. These hyperammonaemic rats, with plasma and brain ammonia levels equivalent to those known to depress brain function, behaved normally over 48 h. There was no depression of cerebral energy metabolism (i.e. the rate of glucose consumption). Contents of key intermediary metabolites and high-energy phosphates were normal. Neutral amino acid transport (tryptophan and leucine) and the brain contents of aromatic amino acids were unchanged. 3. The data suggest that ammonia is without effect at concentrations less than 1 mumol/ml if it is not converted into glutamine. The deleterious effect of chronic hyperammonaemia seems to begin with the synthesis of glutamine.

L-methionine-DL-sulfoximine induces massive efflux of glutamine from cortical astrocytes in primary culture

L-Methionine-DL-sulfoximine (MSO) is a potent convulsant which metabolically and morphologically primarily affects astroglia, and the links between the gliotoxic and neurophysiological effects of MSO are not clear. A 5 min treatment with 3 mM MSO increased the rate of efflux of newly loaded radiolabelled glutamine (Gln) from rat cortical astrocytes in primary culture to more than 400% of the basal efflux. MSO did not affect the efflux of the neurotransmitter amino acids GABA or D-aspartate (a non-metabolizable analogue of L-glutamate), under the same experimental conditions. MSO-induced overflow with Gln, which according to a recent account strongly interacts with the N-methyl-D-aspartate (NMDA) receptor complex, may contribute to the convulsive action of the drug.