Glutamine uptake by neurons: interaction of protons with system a transporters

A conserved Na(+) binding site of the sodium-coupled neutral amino acid transporter 2 (SNAT2)

The SLC38 family of solute transporters mediates the coupled transport of amino acids and Na(+) into or out of cells. The structural basis for this coupled transport process is not known. Here, a profile-based sequence analysis approach was used, predicting a distant relationship with the SLC5/6 transporter families. Homology models using the LeuT(Aa) and Mhp1 transporters of known structure as templates were established, predicting the location of a conserved Na(+) binding site in the center of membrane helices 1 and 8. This homology model was tested experimentally in the SLC38 member SNAT2 by analyzing the effect of a mutation to Thr-384, which is predicted to be part of this Na(+) binding site. The results show that the T384A mutation not only inhibits the anion leak current, which requires Na(+) binding to SNAT2, but also dramatically lowers the Na(+) affinity of the transporter. This result is consistent with a previous analysis of the N82A mutant transporter, which has a similar effect on anion leak current and Na(+) binding and which is also expected to form part of the Na(+) binding site. In contrast, random mutations to other sites in the transporter had little or no effect on Na(+) affinity. Our results are consistent with a cation binding site formed by transmembrane helices 1 and 8 that is conserved among the SLC38 transporters as well as among many other bacterial and plant transporter families of unknown structure, which are homologous to SLC38.

Manganese disrupts astrocyte glutamine transporter expression and function

Glutamine (Gln) plays an important role in brain energy metabolism and as a precursor for the synthesis of neurotransmitter glutamate and GABA. Previous studies have shown that astrocytic Gln transport is impaired following manganese (Mn) exposure. The present studies were performed to identify the transport routes and the respective Gln transporters contributing to the impairment. Rat neonatal cortical primary astrocytes treated with Mn displayed a significant decrease in Gln uptake mediated by the principle Gln transporting systems, N and ASC. Moreover, systems N, ASC and L were less efficient in Gln export after Mn treatment. Mn treatment caused a significant reduction of both in mRNA expression and protein levels of SNAT3 (system N), SNAT2 (system A) and LAT2 (system L), and lowered the protein but not mRNA expression of ASCT2 (system ASC). Mn exposure did not affect the expression of the less abundant systems N transporter SNAT5 and the system L transporter LAT1, at either the mRNA or protein level. Hence, Mn-induced decrease of inward and outward Gln transport can be largely ascribed to the loss of the specific Gln transporters. Consequently, deregulation of glutamate homeostasis and its diminished availability to neurons may lead to impairment in glutamatergic neurotransmission, a phenomenon characteristic of Mn-induced neurotoxicity.

SNAT2 amino acid transporter is regulated by amino acids of the SLC6 gamma-aminobutyric acid transporter subfamily in neocortical neurons and may play no role in delivering glutamine for glutamatergic transmission

System A transporters SNAT1 and SNAT2 mediate uptake of neutral alpha-amino acids (e.g. glutamine, alanine, and proline) and are expressed in central neurons. We tested the hypothesis that SNAT2 is required to support neurotransmitter glutamate synthesis by examining spontaneous excitatory activity after inducing or repressing SNAT2 expression for prolonged periods. We stimulated de novo synthesis of SNAT2 mRNA and increased SNAT2 mRNA stability and total SNAT2 protein and functional activity, whereas SNAT1 expression was unaffected. Increased endogenous SNAT2 expression did not affect spontaneous excitatory action-potential frequency over control. Long term glutamine exposure strongly repressed SNAT2 expression but increased excitatory action-potential frequency. Quantal size was not altered following SNAT2 induction or repression. These results suggest that spontaneous glutamatergic transmission in pyramidal neurons does not rely on SNAT2. To our surprise, repression of SNAT2 activity was not limited to System A substrates. Taurine, gamma-aminobutyric acid, and beta-alanine (substrates of the SLC6 gamma-aminobutyric acid transporter family) repressed SNAT2 expression more potently (10x) than did System A substrates; however, the responses to System A substrates were more rapid. Since ATF4 (activating transcription factor 4) and CCAAT/enhancer-binding protein are known to bind to an amino acid response element within the SNAT2 promoter and mediate induction of SNAT2 in peripheral cell lines, we tested whether either factor was similarly induced by amino acid deprivation in neurons. We found that glutamine and taurine repressed the induction of both transcription factors. Our data revealed that SNAT2 expression is constitutively low in neurons under physiological conditions but potently induced, together with the taurine transporter TauT, in response to depletion of neutral amino acids.

L-Proline transport into renal OK epithelial cells: a second renal proline transport system is induced by amino acid deprivation

Influx of [(3)H]-L-proline into renal OK cells revealed that basal transport was mediated by the transporter SIT1. When cells were submitted for 8 h to amino acid deprivation, uptake of L-proline was now dominated by a low-affinity system with an apparent K (m) of 4.4 +/- 0.6 mM and a V (max) of 10.2 +/- 0.6 nmol/mg of protein/min operating in addition to the high-affinity SIT1 system with a K (m) of 0.12 +/- 0.01 mM and a V (max) of 0.28 +/- 0.04 nmol/mg of protein/min. The low- and high-affinity proline transporting systems were sensitive to inhibitors of JNK and PI-3 kinases, whereas a GSK-3 inhibitor affected only the upregulated transport system. Ion-replacement studies and experiments assessing substrate specificities for both systems provided strong evidence that SNAT2, that showed two- to threefold increased mRNA levels, is the responsible transporter mediating the increased proline influx under conditions of amino acid deprivation.

Complementary expression of SN1 and SAT2 in the islets of Langerhans suggests concerted action of glutamine transport in the regulation of insulin secretion

Insulin and glucagon secretion from the islets of Langerhans is highly regulated. Although an increased plasma glucose level is the major stimulus for insulin exocytosis, roles for glutamine and glutamate have been suggested. Interestingly, the islet cells display elements associated with synaptic transmission. In the central nervous system (CNS), glutamine transport by SN1 and SAT2 sustain the generation of neurotransmitter glutamate. We hypothesized that the same transporters are essential for glutamine transport into the islet cells and for subsequent formation of glutamate acting as an intracellular signaling molecule. We demonstrate that islet cells express several transporters which can mediate glutamine transport. In particular, we show pronounced expression of SN1 and SAT2 in B-cells and A-cells, respectively. The cell-specific expression of these transporters together with their functional characteristics suggest an important role for glutamine in the regulation of insulin secretion.

System A transporter SAT2 mediates replenishment of dendritic glutamate pools controlling retrograde signaling by glutamate

Glutamate mediates several modes of neurotransmission in the central nervous system including recently discovered retrograde signaling from neuronal dendrites. We have previously identified the system N transporter SN1 as being responsible for glutamine efflux from astroglia and proposed a system A transporter (SAT) in subsequent transport of glutamine into neurons for neurotransmitter regeneration. Here, we demonstrate that SAT2 expression is primarily confined to glutamatergic neurons in many brain regions with SAT2 being predominantly targeted to the somatodendritic compartments in these neurons. SAT2 containing dendrites accumulate high levels of glutamine. Upon electrical stimulation in vivo and depolarization in vitro, glutamine is readily converted to glutamate in activated dendritic subsegments, suggesting that glutamine sustains release of the excitatory neurotransmitter via exocytosis from dendrites. The system A inhibitor MeAIB (alpha-methylamino-iso-butyric acid) reduces neuronal uptake of glutamine with concomitant reduction in intracellular glutamate concentrations, indicating that SAT2-mediated glutamine uptake can be a prerequisite for the formation of glutamate. Furthermore, MeAIB inhibited retrograde signaling from pyramidal cells in layer 2/3 of the neocortex by suppressing inhibitory inputs from fast-spiking interneurons. In summary, we demonstrate that SAT2 maintains a key metabolic glutamine/glutamate balance underpinning retrograde signaling by dendritic release of the neurotransmitter glutamate.

Expression of L-serine biosynthetic enzyme 3-phosphoglycerate dehydrogenase (Phgdh) and neutral amino acid transporter ASCT1 following an excitotoxic lesion in the mouse hippocampus

The nonessential amino acid L-serine functions as a glia-derived trophic factor and strongly promotes the survival and differentiation of cultured neurons. The L-serine biosynthetic enzyme 3-phosphoglycerate dehydrogenase (Phgdh) and the small neutral amino acid transporter ASCT1 are preferentially expressed in specific glial cells in the brain. However, their roles in pathological progression remain unclear. We examined the expression of Phgdh and ASCT1 in kainic acid (KA)-induced neurodegeneration of the mouse hippocampus using immunohistochemistry and Western blots. Our quantitative analysis revealed that Phgdh and ASCT1 were constitutively expressed in the normal brain and transiently upregulated by KA-treatment. At the cellular level, Phgdh was expressed in astrocytes in control and in KA-treated mice while ASCT1 that was expressed primarily in the neurons of the normal brain appeared also in activated astrocytes in KA treated mouse brain. The preferential glial expression of ASCT1 was consistent with that of Phgdh. These results demonstrate injury-induced changes in Phgdh and ASCT1 expression. It is hypothesized that the secretion of L-serine is regulated by astrocytes in response to toxic molecules such as glutamate and free radicals that promote neurodegeneration, and may correspond to the level of L-serine needed for neuronal survival and glial activation during brain insults.

Glutamine uptake contributes to central sensitization in the medullary dorsal horn

Mustard oil application to tooth pulp produces central sensitization in rat medullary dorsal horn (MDH) nociceptive neurons, which has been implicated in persistent pain mechanisms. We found that superfusion onto MDH of methylaminoisobutyric acid, a competitive inhibitor of the neuronal system A transporter for presynaptic uptake of glutamine (a glutamate precursor released from astroglia), significantly depressed development of mustard oil-induced central sensitization in rat MDH nociceptive neurons. This finding indicates that the system A transporter is required for the expression of central sensitization and confirms the important roles of astroglia, glutamine and presynaptic modulation of glutamate release in the development of central sensitization.

Highly conserved asparagine 82 controls the interaction of Na+ with the sodium-coupled neutral amino acid transporter SNAT2

The neutral amino acid transporter 2 (SNAT2), which belongs to the SLC38 family of solute transporters, couples the transport of amino acid to the cotransport of one Na(+) ion into the cell. Several polar amino acids are highly conserved within the SLC38 family. Here, we mutated three of these conserved amino acids, Asn(82) in the predicted transmembrane domain 1 (TMD1), Tyr(337) in TMD7, and Arg(374) in TMD8; and we studied the functional consequences of these modifications. The mutation of N82A virtually eliminated the alanine-induced transport current, as well as amino acid uptake by SNAT2. In contrast, the mutations Y337A and R374Q did not abolish amino acid transport. The K(m) of SNAT2 for its interaction with Na(+), K(Na(+)), was dramatically reduced by the N82A mutation, whereas the more conservative mutation N82S resulted in a K(Na(+)) that was in between SNAT2(N82A) and SNAT2(WT). These results were interpreted as a reduction of Na(+) affinity caused by the Asn(82) mutations, suggesting that these mutations interfere with the interaction of SNAT2 with the sodium ion. As a consequence of this dramatic reduction in Na(+) affinity, the apparent K(m) of SNAT2(N82A) for alanine was increased 27-fold compared with that of SNAT2(WT). Our results demonstrate a direct or indirect involvement of Asn(82) in Na(+) coordination by SNAT2. Therefore, we predict that TMD1 is crucial for the function of SLC38 transporters and that of related families.

Glycinergic transmission shaped by the corelease of GABA in a mammalian auditory synapse

The firing pattern of neurons is shaped by the convergence of excitation and inhibition, each with finely tuned magnitude and duration. In an auditory brainstem nucleus, glycinergic inhibition features fast decay kinetics, the mechanism of which is unknown. By applying glycine to native or recombinant glycine receptors, we show that response decay times are accelerated by addition of GABA, a weak partial agonist of glycine receptors. Systematic variation in agonist exposure time revealed that fast synaptic time course may be achieved with submillisecond exposures to mixtures of glycine and GABA at physiological concentrations. Accordingly, presynaptic terminals generally contained both transmitters, and depleting terminals of GABA slowed glycinergic synaptic currents. Thus, coreleased GABA accelerates glycinergic transmission by acting directly on glycine receptors, narrowing the time window for effective inhibition. Packaging both weak and strong agonists in vesicles may be a general means by which presynaptic neurons regulate the duration of postsynaptic responses.

Roles of l-serine and sphingolipid synthesis in brain development and neuronal survival

Sphingolipids represent a class of membrane lipids that contain a hydrophobic ceramide chain as its common backbone structure. Sphingolipid synthesis requires two simple components: l-serine and palmitoyl CoA. Although l-serine is classified as a non-essential amino acid, an external supply of l-serine is essential for the synthesis of sphingolipids and phosphatidylserine (PS) in particular types of central nervous system (CNS) neurons. l-Serine is also essential for these neurons to undergo neuritogenesis and to survive. Biochemical analysis has shown that l-serine is synthesized from glucose and released by astrocytes but not by neurons, which is the major reason why this amino acid is an essential amino acid for neurons. Biosynthesis of membrane lipids, such as sphingolipids, PS, and phosphatidylethanolamine (PE), in neurons is completely dependent on this astrocytic factor. Recent advances in lipid biology research using transgenic mice have demonstrated that synthesis of endogenous l-serine and neuronal sphingolipids is essential for brain development. In this review, we discuss the metabolic system that coordinates sphingolipid synthesis with the l-serine synthetic pathway between neurons and glia. We also discuss the crucial roles of the metabolic conversion of l-serine to sphingolipids in neuronal development and survival. Human diseases associated with serine and sphingolipid biosynthesis are also discussed.

Beta-amyloid 25-35 peptide reduces the expression of glutamine transporter SAT1 in cultured cortical neurons

β-Amyloid (Aβ) peptides may cause malfunction and death of neurons in Alzheimer’s disease. We investigated the effect of Aβ on key transporters of amino acid neurotransmission in cells cultured from rat cerebral cortex. The cultures were treated with Aβ(25-35) at 3 and 10 μM for 12 and 24 h followed by quantitative analysis of immunofluorescence intensity. In mixed neuronal–glial cell cultures (from P1 rats), Aβ reduced the concentration of system A glutamine transporter 1 (SAT1), by up to 50% expressed relative to the neuronal marker microtubule-associated protein 2 (MAP2) in the same cell. No significant effects were detected on vesicular glutamate transporters VGLUT1 or VGLUT2 in neurons, or on glial system N glutamine transporter 1 (SN1). In neuronal cell cultures (from E18 rats), Aβ(25-35) did not reduce SAT1 immunoreactivity, suggesting that the observed effect depends on the presence of astroglia. The results indicate that Aβ may impair neuronal function and transmitter synthesis, and perhaps reduce excitotoxicity, through a reduction in neuronal glutamine uptake.

The Neurotransmitter Cycle and Quantal Size

Changes in the response to release of a single synaptic vesicle have generally been attributed to postsynaptic modification of receptor sensitivity, but considerable evidence now demonstrates that alterations in vesicle filling also contribute to changes in quantal size. Receptors are not saturated at many synapses, and changes in the amount of transmitter per vesicle contribute to the physiological regulation of release. On the other hand, the presynaptic factors that determine quantal size remain poorly understood. Aside from regulation of the fusion pore, these mechanisms fall into two general categories: those that affect the accumulation of transmitter inside a vesicle and those that affect vesicle size. This review will summarize current understanding of the neurotransmitter cycle and indicate basic, unanswered questions about the presynaptic regulation of quantal size.

Upregulation of the glutamine transporter through transactivation mediated by cAMP/protein kinase A signals toward exacerbation of vulnerability to oxidative stress in rat neocortical astrocytes

In the present study, we have evaluated the possible functionality in astrocytes of the glutamine (Gln) transporter (GlnT) known to predominate in neurons for the neurotransmitter pool of glutamate. Sustained exposure to the adenylyl cyclase activator forskolin for 24 h led to a significant increase in mRNA expression of GlnT among different membrane transporters capable of transporting Gln, with an increase in [(3)H]Gln accumulation sensitive to a system A transporter inhibitor, in cultured rat neocortical astrocytes, but not neurons. Forskolin drastically stimulated GlnT promoter activity in a manner sensitive to a protein kinase A (PKA) inhibitor in rat astrocytic C6 glioma cells, while deletion mutation analysis revealed that the stimulation was mediated by a cAMP responsive element (CRE)/activator protein-1 (AP-1) like site located on GlnT gene promoter. Forskolin drastically stimulated the promoter activity in a fashion sensitive to a PKA inhibitor in C6 glioma cells transfected with a CRE or AP-1 reporter plasmid, in association with the phosphorylation of CRE binding protein on serine133. Transient overexpression of GlnT significantly exacerbated the cytotoxicity of hydrogen peroxide in cultured astrocytes. These results suggest that GlnT expression is upregulated by cAMP/PKA signals for subsequent exacerbation of the vulnerability to oxidative stress in astrocytes.

Design, synthesis, and biological activity of novel triazole amino acids used to probe binding interactions between ligand and neutral amino acid transport protein SN1

Novel triazole amino acids were synthesized as probes to investigate ligand-protein binding interactions of the neutral amino acid transporter SN1. The bonding hypothesis to be tested was that the side chains of endogenous substrates are acting as H-bond acceptors. Although limited inhibition of (3)H-L-glutamine uptake by SN1 expressing oocytes was observed, the synthetic compounds show a trend that suggests a hydrogen bond interaction just outside the endogenous ligand binding pocket.

Modulation of epileptiform activity by glutamine and system A transport in a model of post-traumatic epilepsy

Epileptic activity arises from an imbalance in excitatory and inhibitory synaptic transmission. To determine if alterations in the metabolism of glutamate, the primary excitatory neurotransmitter, might contribute to epilepsy we directly and indirectly modified levels of glutamine, an immediate precursor of synaptically released glutamate, in the rat neocortical undercut model of hyperexcitability and epilepsy. We show that slices from injured cortex take up glutamine more readily than control slices, and an increased expression of the system A transporters SNAT1 and SNAT2 likely underlies this difference. We also examined the effect of exogenous glutamine on evoked and spontaneous activity and found that addition of physiological concentrations of glutamine to perfusate of slices isolated from injured cortex increased the incidence and decreased the refractory period of epileptiform potentials. By contrast, exogenous glutamine increased the amplitude of evoked potentials in normal cortex, but did not induce epileptiform potentials. Addition of physiological concentrations of glutamine to perfusate of slices isolated from injured cortex greatly increased abnormal spontaneous activity in the form of events resembling spreading depression, again while having no effect on slices from normal cortex. Interestingly, similar spreading depression like events were noted in control slices at supraphysiological levels of glutamine. In the undercut cortex addition of methylaminoisobutyric acid (MeAIB), an inhibitor of the system A glutamine transporters attenuated all physiological effects of added glutamine suggesting that uptake through these transporters is required for the effect of glutamine. Our findings support a role for glutamine transport through SNAT1 and/or SNAT2 in the maintenance of abnormal activity in this in vitro model of epileptogenesis and suggest that system A transport and glutamine metabolism are potential targets for pharmacological intervention in seizures and epilepsy.

The sodium-coupled neutral amino acid transporter SNAT2 mediates an anion leak conductance that is differentially inhibited by transported substrates

The sodium-coupled neutral amino acid transporter SNAT2 mediates cellular uptake of glutamine and other small, neutral amino acids. Here, we report the existence of a leak anion pathway associated with SNAT2. The leak anion conductance was increased by, but did not require the presence of, extracellular sodium. The transported substrates L-alanine, L-glutamine, and alpha-(methylamino)isobutyrate inhibited the anion leak conductance, each with different potency. A transporter with the mutation H-304A did not catalyze alanine transport but still catalyzed anion leak current, demonstrating that substrate transport is not required for anion current inhibition. Both the substrate and Na+ were able to bind to the SNAT2H-304A transporter normally. The selectivity sequence of the SNAT2H-304A anion conductance was SCN->>NO3->I->Br->Cl->Mes-. Anion flux mediated by the more hydrophobic anion SCN- was not saturable, whereas nitrate flux demonstrated saturation kinetics with an apparent Km of 29 mM. SNAT2, which belongs to the SLC38 family of transporters, has to be added to the growing number of secondary, Na+-coupled transporters catalyzing substrate-gated or leak anion conductances. Therefore, we can speculate that such anion-conducting pathways are general features of Na+-transporting systems.

Methylmercury induces oxidative injury, alterations in permeability and glutamine transport in cultured astrocytes

The neurotoxicity of high levels of methylmercury (MeHg) is well established both in humans and experimental animals. Astrocytes accumulate MeHg and play a prominent role in mediating MeHg toxicity in the central nervous system (CNS). Although the precise mechanisms of MeHg neurotoxicity are ill-defined, oxidative stress and altered mitochondrial and cell membrane permeability appear to be critical factors in its pathogenesis. The present study examined the effects of MeHg treatment on oxidative injury, mitochondrial inner membrane potential, glutamine uptake and expression of glutamine transporters in primary astrocyte cultures. MeHg caused a significant increase in F(2)-isoprostanes (F(2)-IsoPs), lipid peroxidation biomarkers of oxidative damage, in astrocyte cultures treated with 5 or 10 microM MeHg for 1 or 6 h. Consistent with this observation, MeHg induced a concentration-dependant reduction in the inner mitochondrial membrane potential (DeltaPsi(m)), as assessed by the potentiometric dye, tetramethylrhodamine ethyl ester (TMRE). Our results demonstrate that DeltaPsi(m) is a very sensitive endpoint for MeHg toxicity, since significant reductions were observed after only 1 h exposure to concentrations of MeHg as low as 1 microM. MeHg pretreatment (1, 5 and 10 microM) for 30 min also inhibited the net uptake of glutamine ((3)H-glutamine) measured at 1 min and 5 min. Expression of the mRNA coding the glutamine transporters, SNAT3/SN1 and ASCT2, was inhibited only at the highest (10 microM) MeHg concentration, suggesting that the reduction in glutamine uptake observed after 30 min treatment with lower concentrations of MeHg (1 and 5 microM) was not due to inhibition of transcription. Taken together, these studies demonstrate that MeHg exposure is associated with increased mitochondrial membrane permeability, alterations in glutamine/glutamate cycling, increased ROS formation and consequent oxidative injury. Ultimately, MeHg initiates multiple additive or synergistic disruptive mechanisms that lead to cellular dysfunction and cell death.

Evidence for allosteric regulation of pH-sensitive System A (SNAT2) and System N (SNAT5) amino acid transporter activity involving a conserved histidine residue

System A and N amino acid transporters are key effectors of movement of amino acids across the plasma membrane of mammalian cells. These Na+-dependent transporters of the SLC38 gene family are highly sensitive to changes in pH within the physiological range, with transport markedly depressed at pH 7.0. We have investigated the possible role of histidine residues in the transporter proteins in determining this pH-sensitivity. The histidine-modifying agent DEPC (diethyl pyrocarbonate) markedly reduces the pH-sensitivity of SNAT2 and SNAT5 transporters (representative isoforms of System A and N respectively, overexpressed in Xenopus oocytes) in a concentration-dependent manner but does not completely inactivate transport activity. These effects of DEPC were reversed by hydroxylamine and partially blocked in the presence of excess amino acid substrate. DEPC treatment also blocked a reduction in apparent affinity for Na+ (K0.5Na+) of the SNAT2 transporter at low external pH. Mutation of the highly conserved C-terminal histidine residue to alanine in either SNAT2 (H504A) or SNAT5 (H471A) produced a transport phenotype exhibiting reduced, DEPC-resistant pH-sensitivity with no change in K0.5Na+ at low external pH. We suggest that the pH-sensitivity of these structurally related transporters results at least partly from a common allosteric mechanism influencing Na+ binding, which involves an H+-modifier site associated with C-terminal histidine residues.

Neuronal-glial glucose oxidation and glutamatergic-GABAergic function

Prior 13C magnetic resonance spectroscopy (MRS) experiments, which simultaneously measured in vivo rates of total glutamate-glutamine cycling (V(cyc(tot))) and neuronal glucose oxidation (CMR(glc(ox), N)), revealed a linear relationship between these fluxes above isoelectricity, with a slope of approximately 1. In vitro glial culture studies examining glutamate uptake indicated that glutamate, which is cotransported with Na+, stimulated glial uptake of glucose and release of lactate. These in vivo and in vitro results were consolidated into a model: recycling of one molecule of neurotransmitter between glia and neurons was associated with oxidation of one glucose molecule in neurons; however, the glucose was taken up only by glia and all the lactate (pyruvate) generated by glial glycolysis was transferred to neurons for oxidation. The model was consistent with the 1:1 relationship between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) measured by 13C MRS. However, the model could not specify the energetics of glia and gamma-amino butyric acid (GABA) neurons because quantitative values for these pathways were not available. Here, we review recent 13C and 14C tracer studies that enable us to include these fluxes in a more comprehensive model. The revised model shows that glia produce at least 8% of total oxidative ATP and GABAergic neurons generate approximately 18% of total oxidative ATP in neurons. Neurons produce at least 88% of total oxidative ATP, and take up approximately 26% of the total glucose oxidized. Glial lactate (pyruvate) still makes the major contribution to neuronal oxidation, but approximately 30% less than predicted by the prior model. The relationship observed between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) is determined by glial glycolytic ATP as before. Quantitative aspects of the model, which can be tested by experimentation, are discussed.

The glutamate/GABA-glutamine cycle: aspects of transport, neurotransmitter homeostasis and ammonia transfer

Neurons are metabolically handicapped in the sense that they are not able to perform de novo synthesis of neurotransmitter glutamate and gamma-aminobutyric acid (GABA) from glucose. A metabolite shuttle known as the glutamate/GABA-glutamine cycle describes the release of neurotransmitter glutamate or GABA from neurons and subsequent uptake into astrocytes. In return, astrocytes release glutamine to be taken up into neurons for use as neurotransmitter precursor. In this review, the basic properties of the glutamate/GABA-glutamine cycle will be discussed, including aspects of transport and metabolism. Discussions of stoichiometry, the relative role of glutamate vs. GABA and pathological conditions affecting the glutamate/GABA-glutamine cycling are presented. Furthermore, a section is devoted to the accompanying ammonia homeostasis of the glutamate/GABA-glutamine cycle, examining the possible means of intercellular transfer of ammonia produced in neurons (when glutamine is deamidated to glutamate) and utilized in astrocytes (for amidation of glutamate) when the glutamate/GABA-glutamine cycle is operating. A main objective of this review is to endorse the view that the glutamate/GABA-glutamine cycle must be seen as a bi-directional transfer of not only carbon units but also nitrogen units.

Functional expression of A glutamine transporter responsive to down-regulation by lipopolysaccharide through reduced promoter activity in cultured rat neocortical astrocytes

The prevailing view is that the glutamine (Gln) transporter (GlnT/ATA1/SAT1/SNAT1) is a member of the system A transporter superfamily with the ability to fuel the glutamate/Gln cycle at nerve terminals in glutamatergic neurons. Semiquantitative reverse transcription-polymerase chain reaction revealed similarly high expression of mRNA for GlnT by rat brain neocortical astrocytes as well as neurons, with progressively lower expression by cerebellar astrocytes, hippocampal astrocytes, and whole-brain microglia in culture. [(3)H]Gln was accumulated in a temperature-dependent manner with a saturable profile in both cultured neocortical neurons and astrocytes, whereas biochemical and pharmacological analyses on [(3)H]Gln accumulation revealed the expression of both system A and system L transporters by cultured neocortical neurons and astrocytes. Exposure to lipopolysaccharide (LPS) for 24 hr resulted in a significant decrease in both GlnT mRNA expression and [(3)H]Gln accumulation, with a concomitant drastic increase in nitrite formation in cultured neocortical astrocytes. Moreover, LPS significantly inhibited the promoter activity of GlnT in the astrocytic cell line C6 glioma cells as well as primary rat neocortical astrocytes in culture. These results suggest that activation by LPS would lead to down-regulation of the expression of GlnT responsible for the incorporation of extracellular Gln into intracellular spaces across plasma membranes through the inhibition of its promoter activity in cultured rat neocortical astrocytes.

Localization of the Na(+)-coupled neutral amino acid transporter 2 in the cerebral cortex

We studied the distribution and cellular localization of Na(+)-coupled neutral amino acid transporter 2, a member of the system A family of amino acid transporters, in the rat and human cerebral cortex using immunocytochemical methods. Na(+)-coupled neutral amino acid transporter 2-positive neurons were pyramidal and non-pyramidal, and Na(+)-coupled neutral amino acid transporter 2/GABA double-labeling studies revealed that Na(+)-coupled neutral amino acid transporter 2 was highly expressed by GABAergic neurons. Double-labeling studies with the synaptophysin indicated that rare axon terminals express Na(+)-coupled neutral amino acid transporter 2. Na(+)-coupled neutral amino acid transporter 2-immunoreactivity was also found in astrocytes, leptomeninges, ependymal cells and choroid plexus. Electron microscopy showed robust Na(+)-coupled neutral amino acid transporter 2-immunoreactivity in the somato-dendritic compartment of neurons and in glial processes, but, as in the case of double-labeling studies, failed to reveal Na(+)-coupled neutral amino acid transporter 2-immunoreactivity in terminals. To rule out the possibility that the absence of Na(+)-coupled neutral amino acid transporter 1- and Na(+)-coupled neutral amino acid transporter 2-positive terminals was due to insufficient antigen detection, we evaluated Na(+)-coupled neutral amino acid transporter 1/synaptophysin and Na(+)-coupled neutral amino acid transporter 2/synaptophysin coexpression using non-standard immunocytochemical procedures and found that Na(+)-coupled neutral amino acid transporter 1 and Na(+)-coupled neutral amino acid transporter 2+ terminals were rare in all conditions. These findings indicate that Na(+)-coupled neutral amino acid transporter 1 and Na(+)-coupled neutral amino acid transporter 2 are virtually absent in cortical terminals, and suggest that they do not contribute significantly to replenishing the Glu and GABA transmitter pools through the glutamate-glutamine cycle. The strong expression of Na(+)-coupled neutral amino acid transporter 2 in the somato-dendritic compartment and in non-neuronal elements that are integral parts of the blood-brain and brain-cerebrospinal fluid barrier suggests that Na(+)-coupled neutral amino acid transporter 2 plays a role in regulating the levels of Gln and other amino acids in the metabolic compartment of cortical neurons.

The glutamine commute: lost in the tube?

The "glutamate-glutamine" cycle appears to have an important, albeit not exclusive role, in the recycling of glutamate (Glu) between neurons and astrocytes. Recent studies show that the efflux of glutamine (Gln) from astrocytes is mediated by SNAT3 (formerly SN1), a system N amino acid transporter localized to perisynaptic astrocytes, whereas its influx into neurons is thought to be mediated by transporters of the system A family, specifically SNAT1 and SNAT2. However, the results of our confocal and electron microscopy immunocytochemical studies of the localization of these transporters in the cerebral cortex show that SNAT1 and SNAT2 are robustly expressed in the somatodendritic domain of cortical neurons, but rarely to axon terminals. To rule out a possible influence of fixation and procedural variables on detection of SNAT1 and SNAT2 immunoreactivity in axon terminals, we used non-conventional immunocytochemical methods, which, in certain cases, improve antigen detection. Though evidencing a slightly increased percentage of axon terminals expressing the two transporters, these techniques demonstrated that SNAT1 and SNAT2 are indeed rarely localized to axon terminals. Our data thus suggest that neither SNAT1 nor SNAT2 meet the criteria for their postulated role in the "glutamate-glutamine" cycle, and indicate that other Gln transporters (either orphan or yet to be identified) must be expressed at axon terminals and sustain the Glu (and gamma-aminobutyric acid) neurotransmitter pool (s).

Glutamate metabolism in epilepsy: 13C-magnetic resonance spectroscopy observation in the human brain

To clarify changes in glutamate metabolism in the brain with chronic epileptic activities, 13C-magnetic resonance spectroscopy observation of glutamate and glutamine synthesis after oral administration of [1-13C] glucose (Glc C1) (0.75 g/kg) was performed in intractable occipital lobe epilepsy patients (n=5) and controls (n=10). 1H[13C]-spectra were obtained from two voxels of 64 ml placed on the bilateral parieto-occipital lobes of the study participants. Time courses for 13C-incorporation into 4-glutamate and 3-glutamate (Glu C4, C3) and 4-glutamine (Gln C4) were obtained and the concentrations of Glu C4, C3 and Gln C4 at the time between 120 and 150 min after Glc C1 administration was calculated. Concentration of Gln C4 was increased in the epilepsy patients [control: 0.39 mM (SD 0.14), epilepsy: 0.60 mM (SD 0.15), P<0.05], whereas those of Glu C4 and Glu C3 were not. The present study revealed increased glutamine synthesis compared with glutamate formation in a widespread cortical area with sustained epileptiform activities, possibly a result of chronic excessive glutamate release from neurons and subsequent uptake into astrocytes.

The Extracellular pH Dependency of Transport Activity by Human Oligopeptide Transporter 1 (hPEPT1) Expressed Stably in Chinese Hamster Ovary (CHO) Cells: A Reason for the Bell-Shaped Activity versus pH

Human oligopeptide transporter (hPEPT1) translocates di/tri-peptide by coupling to movement of proton down the electrochemical gradient. This transporter has the characteristics that the pH-profile of neutral dipeptide transport shows a bell-shaped curve with an optimal pH of 5.5. In the present study, we examined the reason for the decrease in the acidic region with hPEPT1-transfected CHO cells stably oeverexpressing hPEPT1 (CHO/hPEPT1). The pH profile of the transport activity vs. pH was measured in the presence of nigericin/monensin. Under this condition, the inwardly directed proton concentration gradient was dissipated while the membrane potential remained. As pH increased the activity increased, and the Henderson-Hasselbalch equation with a single pKa was fitted well to the activity curve. The pKa value was estimated to be 6.7+/-0.2. This value strongly suggests that there is a key amino acid residue, which is involved in pH regulation of transport activity. To identify the key amino acid residue, we examined the effects of various chemical modifications on pH-profile of the transport activity. Modification of carboxyl groups or hydroxyl groups had no significant influence on the pH-profile, whereas a chemical modification of histidine residue with diethylpyrocarbonate (DEPC) completely abolished the transport activity in CHO/hPEPT1 cells. On the other hand, this abolishment was almost prevented by the presence of 10 mM Gly-Sar. This protection was observed only in the presence of the substrate of hPEPT1, indicating that the histidine residue is located at the substrate recognition site. The pH-profile of the transport activity in CHO/hPEPT1 cells treated with DEPC in the presence of 10 mM Gly-Sar also showed a bell-shape similar to that in non-treated CHO/hPEPT1 cells. These data stressed that the histidine residue located at or near the substrate binding site is involved in the pH regulation of transport activity.

Kinetics of bidirectional H+ and substrate transport by the proton-dependent amino acid symporter PAT1

PAT1 is a recently identified member of the PAT family of proton/amino acid co-transporters with predominant expression in the plasma membrane of enterocytes and in lysosomal membranes of neurons. Previous studies in Xenopus oocytes expressing PAT1 established proton/substrate co-transport associated with positive inward currents for a variety of small neutral amino acids. Here we provide a detailed analysis of the transport mode of the murine PAT1 in oocytes using the two-electrode voltage-clamp technique to measure steady-state and pre-steady-state currents. The GPC (giant patch clamp) technique and efflux studies were employed to characterize the reversed transport mode. Kinetic parameters [K(m) (Michaelis constant) and I(max) (maximum current)] for transport of various substrates revealed a dependence on membrane potential: hyperpolarization increases the substrate affinity and maximal transport velocity. Proton affinity for interaction with PAT1 is almost 100 nM, corresponding to a pH of 7.0 and is independent of substrate. Kinetic analysis revealed that binding of proton most likely occurs before substrate binding and that the proton and substrate are translocated in a simultaneous step. No evidence for a substrate-uncoupled proton shunt was observed. As shown by efflux studies and current measurements by the GPC technique, PAT1 allows bidirectional amino acid transport. Surprisingly, PAT1 exhibits no pre-steady-state currents in the absence of substrate, even at low temperatures, and therefore PAT1 takes an exceptional position among the ion-coupled co-transporters.

Ngamma-aryl glutamine analogues as probes of the ASCT2 neutral amino acid transporter binding site

Analogues of L-glutamine were designed and synthesized to test a hydrogen-bond hypothesis between ligand and neutral amino acid transporter ASCT2. The key design feature contains a substituted phenyl ring on the amide nitrogen that contains electron withdrawing and electron donating groups that alter the pKa of the amide NH. Through this study a preliminary binding site map has been developed, and a potent commercially available competitive inhibitor of the ASCT2 transporter has been identified.

Differential Regulation of Amino Acid Transporter SNAT3 by Insulin in Hepatocytes

The liver is a metabolism and transfer center of amino acids as well as the prime target organ of insulin. In this report, we characterized the regulation of system N/A transporter 3 (SNAT3) in the liver of dietary-restricted mice and in hepatocytes treated with serum starvation and insulin. The expression of SNAT3 was up-regulated in dietary-restricted mice. The expression of SNAT3 protein was detected on the plasma membrane of hepatocyte-like H2.35 cells with a half-life of 6-8 h. When H2.35 cells were depleted of serum, the expression of SNAT3 was increased. An increased concentration of insulin, however, suppressed SNAT3 expression. Interestingly, the down-regulation of SNAT3 expression by insulin was blocked by the specific phosphoinositide 3-kinase inhibitor LY294002 and mammalian target of rapamycin inhibitor, but not by MAPK inhibitor PD98059, suggesting that insulin exerts its effect on SNAT3 through phosphoinositide 3-kinase-mammalian target of rapamycin signaling. Surface biotinylation assay showed an increased level of SNAT3 on the cell surface after 0.5 h of insulin treatment, although no effect was observed after 24 h of treatment. Consistently, the transport of the substrate l-histidine was increased with short, but not long, treatment by insulin in both H2.35- and SNAT3-transfected COS-7 cells. The L-histidine uptake was inhibited significantly by L-histidine followed by 2-endoamino-bicycloheptane-2-carboxylic acid and L-cysteine and to a lesser extent by L-alanine and aminoisobutyric acid, but was not inhibited by alpha-(methylamino)isobutyric acid, implying that uptake of L-histidine in H2.35 cells is primarily mediated by system N transporters. In conclusion, differential regulation of SNAT3 by insulin and serum starvation reinforces the functional significance of this transporter in liver physiology.

Immunohistochemical localization of the amino acid transporter SNAT2 in the rat brain

SNAT2 is a neutral amino acid carrier that belongs to the system A family. Since its function in the nervous system remains unclear, we have analyzed its distribution in the rat CNS using specific antisera. Although SNAT2 is expressed widely in the CNS, it is enriched in the spinal cord and the brainstem nuclei, especially those of the auditory system. At the cellular level, SNAT2 was preferentially located in neuronal cell bodies and processes, although it was also strongly expressed in the meninges and ependyma. In astrocytes, the localization of SNAT2 was more restricted since it was intensely expressed in the perivascular end-feet, glia limitans, cerebellar astrocytes and Bergmann glia, but it was less intense in astrocytes of the cerebral parenchyma. Among neurons, the primary sensory neurons of the mesencephalic trigeminal nucleus appeared to be those that most strongly express SNAT2, but many other neurons, including cortical pyramidal cells and their dendrites were also intensely stained. In several regions the transporter was detected in axons, especially in the brainstem, and its presence in both dendrites and axons was confirmed by confocal microscopy and ultrastructural studies. However, while SNAT2 was observed in the large principal dendrites and the small distal dendrites, it was only found in axonal shafts and was excluded from terminals. Some glutamatergic neurons were among the more intensely labeled cells whereas SNAT2 was not detected on GABAergic neurons. The expression of SNAT2 partially coincides with that reported for SNAT1, especially in glutamatergic neurons. Hence, both proteins could fulfill complementary roles in replenishing glutamate pools and be differentially regulated under different physiological conditions. They also seem to co-localize in non-neuronal cells probably contributing to amino acid fluxes through the blood-brain barrier.

Inhibition of system A-mediated glycine transport in cortical synaptosomes by therapeutic concentrations of clozapine: implications for mechanisms of action

Clozapine is an atypical antipsychotic with particular efficacy in schizophrenia, possibly related to potentiation of brain N-methyl-D-aspartate receptor (NMDAR) -mediated neurotransmission. NMDARs are regulated in vivo by glycine, which is regulated in turn by glycine transporters. The present study investigates transport processes regulating glycine uptake into rat brain synaptosomes, along with effects of clozapine on synaptosomal glycine transport. Amino-acid uptake of amino acids was assessed in rat brain P2 synaptosomal preparations using a radiotransport assay. Synaptosomal glycine transport was inhibited by a series of amino acids and by the selective System A antagonist MeAIB (2-methyl-aminoisobutyric acid). Clozapine inhibited transport of both glycine and MeAIB, but not other amino acids, at concentrations associated with preferential clinical response (0.5-1 microg/ml). By contrast, other antipsychotics studied were ineffective. The novel glycine transport inhibitor N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS) produced biphasic inhibition of [(3)H]glycine transport, with IC(50) values of approximately 25 nM and 25 microM, respectively. NFPS inhibition of [(3)H]MeAIB was monophasic with a single IC(50) value of 31 microM. Clozapine significantly inhibited [(3)H]glycine binding even in the presence of 100 nM NFPS. In conclusion, this study suggests first that System A transporters, or a subset thereof, may play a critical role in regulation of synaptic glycine levels and by extension of NMDA receptor regulation, and second that System A antagonism may contribute to the differential clinical efficacy of clozapine compared with other typical or atypical antipsychotics.

Induction and targeting of the glutamine transporter SN1 to the basolateral membranes of cortical kidney tubule cells during chronic metabolic acidosis suggest a role in pH regulation

During chronic metabolic acidosis (CMA), the plasma levels of glutamine are increased and so is glutamine metabolism in the kidney tubule cells. Degradation of glutamine results in the formation of ammonium (NH(4)(+)) and bicarbonate (HCO(3)(-)) ions, which are excreted in the pre-urine and transported to the peritubular blood, respectively. This process contributes to counteract acidosis and to restore normal pH, but the molecular mechanism, the localization of the proteins involved and the regulation of glutamine transport into the renal tubular cells, remains unknown. SN1, a Na(+)- and H(+)-dependent glutamine transporter has previously been identified molecularly, and its mRNA has been detected in tubule cells in the medulla of the kidney. Now shown is the selective targeting of the protein to the basolateral membranes of the renal tubule cells of the S3 segment throughout development of the normal rat kidney. During CMA, SN1 expression increases five- to six-fold and appears also in cortical tubule cells in parallel with the increased expression and activity of phosphate-activated glutaminase, a mitochondrial enzyme involved in ammoniagenesis. However, SN1 remains sorted to the basolateral membranes. The unique ability of SN1 to change transport direction according to physiologic changes in transmembrane gradients of [glutamine] and pH and its sorting to the basolateral membranes and the presence of a putative pH responsive element in the 3' untranslated region (UTR) of the gene (supported here by the demonstration in CMA kidney of a protein that binds SN1 mRNA) are conducive to the function of this transporter in pH regulation.

Glutamate as a therapeutic target in psychiatric disorders

Glutamate is the primary excitatory neurotransmitter in the mammalian brain. Glutamatergic neurotransmission may be modulated at multiple levels, only a minority of which are currently being exploited for pharmaceutical development. Ionotropic receptors for glutamate are divided into N-methyl-D-aspartate receptor (NMDAR) and AMPA receptor subtypes. NMDAR have been implicated in the pathophysiology of schizophrenia. The glycine modulatory site of the NMDAR is currently a favored therapeutic target, with several modulatory agents currently undergoing clinical development. Of these, the full agonists glycine and D-serine have both shown to induce significant, large effect size reductions in persistent negative and cognitive symptoms when added to traditional or newer atypical antipsychotics in double-blind, placebo-controlled clinical studies. Glycine (GLYT1) and small neutral amino-acid (SNAT) transporters, which regulate glycine levels, represent additional targets for drug development, and may represent a site of action of clozapine. Brain transporters for D-serine have recently been described. Metabotropic glutamate receptors are positively (Group I) or negatively (Groups II and III) coupled to glutamatergic neurotransmission. Metabotropic modulators are currently under preclinical development for neuropsychiatric conditions, including schizophrenia, depression and anxiety disorders. Other conditions for which glutamate modulators may prove effective include stroke, epilepsy, Alzheimer disease and PTSD.

Amino acid transporter SNAT5 localizes to glial cells in the rat brain

The SNAT5 transporter is a neutral amino acid carrier whose function remains unclear. Structural and mechanistically, SNAT5 is closely related to the SNAT3 transporter that mediates the efflux of glutamine from glial cells and that participates in the glutamate-glutamine cycle in the brain. In this study, we have analyzed the distribution of SNAT5 in the rat central nervous system using specific antibodies. Through immunoblotting we observed that SNAT5 is ubiquitously but unevenly distributed in the CNS. It accumulates most intensely in the neocortex, the hippocampus, the striatum, and the spinal cord, whereas moderate levels were found in the thalamus, hypothalamus, and brainstem. Light microscopy revealed that the distribution of SNAT5 paralleled that of the vesicular glutamate transporter vGLUT1 in the forebrain regions, whereas in the diencephalon and brainstem, SNAT5 staining was better correlated with that of vGLUT1 and vGLUT2. However, the cellular localization differed from that of the glutamatergic markers, since SNAT5 was expressed exclusively in astrocyte cell bodies and their processes, ensheathing glutamatergic GABAergic and glycinergic terminals. The presence of SNAT5 in astrocyte processes was confirmed by electron microscopy. They were seen not only to surround different neuronal structures, but they were also found in astrocyte endfeet. Taking into consideration the higher levels of SNAT5 in the neighborhood of glutamatergic terminals and the ability of this transporter family to promote the efflux of amino acids from intracellular stores (including glutamine and perhaps glycine), this transporter is likely to be involved in glutamatergic pathways in the brain.

Intercellular metabolic compartmentation in the brain: past, present and future

The first indication of 'metabolic compartmentation' in brain was the demonstration that glutamine after intracisternal [14C]glutamate administration is formed from a compartment of the glutamate pool that comprises at most one-fifth of the total glutamate content in the brain. This pool, which was designated 'the small compartment,' is now known to be made up predominantly or exclusively of astrocytes, which accumulate glutamate avidly and express glutamine synthetase activity, whereas this enzyme is absent from neurons, which eventually were established to constitute 'the large compartment.' During the following decades, the metabolic compartment concept was refined, aided by emerging studies of energy metabolism and glutamate uptake in cellularly homogenous preparations and by the histochemical observations that the two key enzymes glutamine synthetase and pyruvate carboxylase are active in astrocytes but absent in neurons. It is, however, only during the last few years that nuclear magnetic resonance (NMR) spectroscopy, assisted by previously obtained knowledge of metabolic pathways, has allowed accurate determination in the human brain in situ of actual metabolic fluxes through the neuronal tricarboxylic acid (TCA) cycle, the glial, presumably mainly astrocytic, TCA cycle, pyruvate carboxylation, and the 'glutamate-glutamine cycle,' connecting neuronal and astrocytic metabolism. Astrocytes account for 20% of oxidative metabolism of glucose in the human brain cortex and accumulate the bulk of neuronally released transmitter glutamate, part of which is rapidly converted to glutamine and returned to neurons in the glutamate-glutamine cycle. However, one-third of released transmitter glutamate is replaced by de novo synthesis of glutamate from glucose in astrocytes, suggesting that at steady state a corresponding amount of glutamate is oxidatively degraded. Net degradation of glutamate may not always equal its net production from glucose and enhanced glutamatergic activity, occurring during different types of cerebral stimulation, including the establishment of memory, may be associated with increased de novo synthesis of glutamate. This process may contribute to a larger increase in glucose utilization rate than in rate of oxygen consumption during brain activation. The energy yield in astrocytes from glutamate formation is strongly dependent upon the fate of the generated glutamate.

Bidirectional substrate fluxes through the system N (SNAT5) glutamine transporter may determine net glutamine flux in rat liver

System N (SNAT3 and SNAT5) amino acid transporters are key mediators of glutamine transport across the plasma membrane of mammalian cell types, including hepatocytes and astrocytes. We demonstrate that SNAT5 shows simultaneous bidirectional glutamine fluxes when overexpressed in Xenopus oocytes. Influx and efflux are both apparently Na+ dependent but, since they are not directly coupled, the carrier is capable of mediating net amino acid movement across the cell membrane. The apparent Km values for glutamine influx and efflux are similar (approximately 1 mm) and the transporter behaviour is consistent with a kinetic model in which re-orientation of the carrier from outside- to inside-facing conformations (either empty or substrate loaded) is the limiting step in the transport cycle. In perfused rat liver, the observed relationship between influent (portal) glutamine concentration and net hepatic glutamine flux may be described by a simple kinetic model, assuming the balance between influx and efflux through System N determines net flux, where under physiological conditions efflux is generally saturated owing to high intracellular glutamine concentration. SNAT5 shows a more periportal mRNA distribution than SNAT3 in rat liver, indicating that SNAT5 may have particular importance for modulation of net hepatic glutamine flux.

Role of glial amino acid transporters in synaptic transmission and brain energetics

This article reviews how the uptake of neurotransmitter by glial amino acid transporters limits the spatial spread of transmitter to preserve the independent operation of nearby synapses, temporally shapes postsynaptic currents, and regulates the effects of tonic transmitter release. We demonstrate the importance of amino acid uptake and recycling mechanisms for preventing the loss of energetically costly neurotransmitter from the brain, and also examine the suggestion that glutamate uptake into glia plays a key role in regulating the energy production of the brain. Finally, we assess the role of glial amino acid transporters in transmitter recycling pathways.

The proton/amino acid cotransporter PAT2 is expressed in neurons with a different subcellular localization than its paralog PAT1

The new member of the mammalian amino acid/auxin permease family, PAT2, has been cloned recently and represents an electrogenic proton/amino acid symporter. PAT2 and its paralog, PAT1/LYAAT-1, are transporters for small amino acids such as glycine, alanine, and proline. Our immunodetection studies revealed that the PAT2 protein is expressed in spinal cord and brain. It is found in neuronal cell bodies in the anterior horn in spinal cord and in brain stem, cerebellum, hippocampus, hypothalamus, rhinencephalon, cerebral cortex, and olfactory bulb in the brain. PAT2 is expressed in neurons positive for the N-methyl-d-aspartate subtype glutamate receptor subunit NR1. PAT2 is not found in lysosomes, unlike its paralog PAT1, but is present in the endoplasmic reticulum and recycling endosomes in neurons. PAT2 has a high external proton affinity causing half-maximal transport activation already at a pH of 8.3, suggesting that its activity is most likely not altered by physiological pH changes. Transport of amino acids by PAT2 activity is dependent on membrane potential and can occur bidirectionally; membrane depolarization causes net glycine outward currents. Our data suggest that PAT2 contributes to neuronal transport and sequestration of amino acids such as glycine, alanine, and/or proline, whereby the transport direction is dependent on the sum of the driving forces such as substrate concentration, pH gradient, and membrane potential.

Asymmetry of glutamine transporters in cultured neural cells

Transfer of glutamine between astrocytes and neurons is an essential part of the glutamate-glutamine cycle in the brain. Transport of glutamine was investigated in primary cultures of astrocytes and neurons and compared to glutamine transport in cell lines with glial and neuronal properties. Glutamine uptake in astrocytes was mainly mediated by general amino acid transporters with properties similar to ASCT2, LAT1, LAT2, SN1 and y(+)LAT2. In cultured neurons, transport activities were detected consistent with the presence of LAT1, LAT2 and y(+)LAT2, but the most prominent activity was a novel Na(+)-dependent glutamine transporter that could be inhibited by D-aspartate. The mRNA for system A isoforms ATA1 and ATA2 was detected in both neurons and astrocytes, but system A activity was only detected in neurons. ASCT2 on the other hand appeared to be astrocyte-specific. The cell lines F98 and 108CC-15, having astroglial and neuronal properties, respectively, expressed sets of glutamine transporters that were unrelated to those of the corresponding primary culture and are thus of limited use as models to study transfer of glutamine between astrocytes and neurons.

Substrate recognition by the mammalian proton-dependent amino acid transporter PAT1

The PAT family of proton-dependent amino acid transporters has recently been identified at the molecular level. This paper describes the structural requirements in substrates for their interaction with the cloned murine intestinal proton/amino acid cotransporter (PAT1). By using the Xenopus laevis oocytes as an expression system and by combining the two-electron voltage clamp technique with radiotracer flux studies, it was demonstrated that the aliphatic side chain of L-alpha-amino acids substrates can consist maximally of only one CH2-unit for high affinity interaction with PAT1. With respect to the maximal separation between the amino and carboxyl groups, only two CH2-units, as in gamma-aminobutyric acid (GABA), are tolerated. PAT1 displays no or even a reversed stereoselectivity, tolerating serine and cystein only in the form of D-enantiomers. A methyl-substitution of the carboxyl group (e.g. O-methyl-glycine) markedly diminishes substrate affinity and transport rates, whereas methyl-substitutions at the amino group (e.g. sarcosine or betaine) have only minor effects on substrate interaction with the transporter binding site. Furthermore, it has been shown (by kinetic analyses of radiolabelled betaine influx and inhibition studies) that the endogenous PAT system of human Caco-2 cells has very similar transport characteristics to mouse PAT1. In summary, one has defined the structural requirements and limitations thet determine the substrate specificity of PAT1. A critical recognition criterion of PAT1 is the backbone charge separation distance and the side chain size, whereas substitutions on the amino group are well tolerated.

Functional properties and cellular distribution of the system A glutamine transporter SNAT1 support specialized roles in central neurons

Glutamine, the preferred precursor for neurotransmitter glutamate and GABA, is likely to be the principal substrate for the neuronal System A transporter SNAT1 in vivo. We explored the functional properties of SNAT1 (the product of the rat Slc38a1 gene) by measuring radiotracer uptake and currents associated with SNAT1 expression in Xenopus oocytes and determined the neuronal-phenotypic and cellular distribution of SNAT1 by confocal laser-scanning microscopy alongside other markers. We found that SNAT1 mediates transport of small, neutral, aliphatic amino acids including glutamine (K0.5 approximately 0.3 mm), alanine, and the System A-specific analogue 2-(methylamino)isobutyrate. Amino acid transport is driven by the Na+ electrochemical gradient. The voltage-dependent binding of Na+ precedes that of the amino acid in a simultaneous transport mechanism. Li+ (but not H+) can substitute for Na+ but results in reduced Vmax. In the absence of amino acid, SNAT1 mediates Na+-dependent presteady-state currents (Qmax approximately 9 nC) and a nonsaturable cation leak with selectivity Na+, Li+ >> H+, K+. Simultaneous flux and current measurements indicate coupling stoichiometry of 1 Na+ per 1 amino acid. SNAT1 protein was detected in somata and proximal dendrites but not nerve terminals of glutamatergic and GABAergic neurons throughout the adult CNS. We did not detect SNAT1 expression in astrocytes but detected its expression on the luminal membranes of the ependyma. The functional properties and cellular distribution of SNAT1 support a primary role for SNAT1 in glutamine transport serving the glutamate/GABA-glutamine cycle in central neurons. Localization of SNAT1 to certain dopaminergic neurons of the substantia nigra and cholinergic motoneurons suggests that SNAT1 may play additional specialized roles, providing metabolic fuel (via alpha-ketoglutarate) or precursors (cysteine, glycine) for glutathione synthesis.

Properties and regulation of glutamine transporter SN1 by protein kinases SGK and PKB

The amino acid transporter SN1 with substrate specificity identical to the amino acid transport system N is expressed mainly in astrocytes and hepatocytes where it accomplishes Na(+)-coupled glutamine uptake and efflux. To characterize properties and regulation of SN1, substrate-induced currents and/or radioactive glutamine uptake were determined in Xenopus oocytes injected with cRNA encoding SN1, the ubiquitin ligase Nedd4-2, and/or the constitutively active serum and glucocorticoid inducible kinase S422DSGK1, its isoform SGK3, and the constitutively active protein kinase B T308D,S473DPKB. The substrate-induced currents were enhanced by increasing glutamine and/or Na(+) concentrations, hyperpolarization, and alkalinization (pH 8.0). They were inhibited by acidification (pH 6.0). Coexpression of Nedd4-2 downregulated SN1-mediated transport, an effect reversed by coexpression of S422DSGK1, SGK3, and T308D,S473DPKB. It is concluded that SN1 is a target for the ubiquitin ligase Nedd4-2, which is inactivated by the serum and glucocorticoid inducible kinase SGK1, its isoform SGK3, and protein kinase B.

Novel neuroglial and glioglial relationships mediated by L-serine metabolism

L-Serine is a non-essential amino acid that can be synthesized in the body. It derives from an intermediate of the glycolytic pathway, 3-phosphoglycerate, and utilized for the syntheses of proteins, other amino acids, membrane lipids, heme, and nucleotides. Emerging evidence indicates that L-serine functions as a glia-derived trophic factor, which strongly promotes the survival and differentiation of cultured neurons. L-Serine biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH) and small neutral amino acid transporter ASCT1 have been revealed to be expressed preferentially in the radial glia-astrocyte lineage and olfactory ensheathing glia of both adult and developing rodent brains. In contrast, these biosynthetic and transporter molecules for L-serine are faint or undetectable in neurons and phagocytic cells. In this review, we summarize recent progress to propose that L-serine synthesis in these glial cells and its supply to nearby neurons and other glia constitute a novel metabolic unit in the brain. Based on these neuroglial and glioglial relationships, glucose in neurons and phogocytes can be strategically used for energy production, while a variety of L-serine-derived biomolecules required for their proliferaton, survival, differentiation, and function are synthesized in and supplied from the radial glia-astrocyte lineage and olfactory ensheathing glia. A transient capillary expression of ASCT1 in fetal and neonatal brains further suggests that, in addition to the glia-borne L-serine, an active transport of blood-borne L-serine would play an essential role in neural development.

Inhibition of glutamine transport depletes glutamate and GABA neurotransmitter pools: further evidence for metabolic compartmentation

The role of glutamine and alanine transport in the recycling of neurotransmitter glutamate was investigated in Guinea pig brain cortical tissue slices and prisms, and in cultured neuroblastoma and astrocyte cell lines. The ability of exogenous (2 mm) glutamine to displace 13C label supplied as [3-13C]pyruvate, [2-13C]acetate, l-[3-13C]lactate, or d-[1-13C]glucose was investigated using NMR spectroscopy. Glutamine transport was inhibited in slices under quiescent or depolarising conditions using histidine, which shares most transport routes with glutamine, or 2-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of the neuronal system A. Glutamine mainly entered a large, slow turnover pool, probably located in neurons, which did not interact with the glutamate/glutamine neurotransmitter cycle. This uptake was inhibited by MeAIB. When [1-13C]glucose was used as substrate, glutamate/glutamine cycle turnover was inhibited by histidine but not MeAIB, suggesting that neuronal system A may not play a prominent role in neurotransmitter cycling. When transport was blocked by histidine under depolarising conditions, neurotransmitter pools were depleted, showing that glutamine transport is essential for maintenance of glutamate, GABA and alanine pools. Alanine labelling and release were decreased by histidine, showing that alanine was released from neurons and returned to astrocytes. The resultant implications for metabolic compartmentation and regulation of metabolism by transport processes are discussed.

Neuronal glutamate uptake Contributes to GABA synthesis and inhibitory synaptic strength

Neurons must maintain a supply of neurotransmitter in their presynaptic terminals to fill synaptic vesicles. GABA is taken up into inhibitory terminals by transporters or is synthesized from glutamate by glutamic acid decarboxylase. Here we report that glutamate transporters supply GABAergic terminals in the hippocampus with glutamate, which is then used to synthesize GABA for filling synaptic vesicles. Glutamate transporter antagonists reduced IPSC and miniature IPSC (mIPSC) amplitudes, consistent with a reduction in the amount of GABA packaged into each synaptic vesicle. This reduction occurred rapidly and independently of synaptic activity, suggesting that modulation of vesicular GABA content does not require vesicle release and refilling. Raising extracellular glutamate levels increased mIPSC amplitudes by enhancing glutamate uptake and, consequently, GABA synthesis. These results indicate that neuronal glutamate transporters strengthen inhibitory synapses in response to extracellular glutamate. This modulation appears to occur under normal conditions and may constitute a negative feedback mechanism to combat hyperexcitability.

The H+-coupled electrogenic lysosomal amino acid transporter LYAAT1 localizes to the axon and plasma membrane of hippocampal neurons

Recent work has identified a lysosomal protein that transports neutral amino acids (LYAAT1). We now show that LYAAT1 mediates H+ cotransport with a stoichiometry of 1 H+/1 amino acid, consistent with a role in the active efflux of amino acids from lysosomes. In neurons, however, LYAAT1 localizes to axonal processes as well as lysosomes. In axons LYAAT1 fails to colocalize with synaptic markers. Rather, axonal LYAAT1 colocalizes with the exocyst, suggesting a role for membranes expressing LYAAT1 in specifying sites for exocytosis. A protease protection assay and measurements of intracellular pH further indicate abundant expression at the plasma membrane, raising the possibility of physiological roles for LYAAT1 on the cell surface as well as in lysosomes.

Highly differential expression of SN1, a bidirectional glutamine transporter, in astroglia and endothelium in the developing rat brain

The transmitters glutamate and GABA also subserve trophic action and are required for normal development of the brain. They are formed from glutamine, which may be synthesized in glia or extracted from the blood. In the adult, the glutamine transporter SN1 is expressed in the astroglia. SN1 works in both directions, depending on the concentration gradients of its substrates and cotransported ions, and is thought to regulate extracellular glutamine and to supply the neurons with the transmitter precursor. In this article, we have quantified the expression and studied the localization of SN1 at different developmental stages. SN1 is expressed in astroglia throughout the CNS from embryonic stages through adulthood. No indication of SN1 staining in neuronal elements has been obtained at any stage. Quantitative immunoblotting of whole brain extracts demonstrates increasing expression of SN1 from P0, reaching a peak at P14, twice the adult level. A moderate and slower rise and fall of the expression levels of SN1 occurs in the cerebellum. Strong transient SN1-like staining is also found in Bergmann glia and vascular endothelium in the first postnatal weeks. Strong intracellular staining in the same time period suggests a high rate of SN1 synthesis in the early postnatal period. This coincides with the increasing levels of glutamate and GABA in the CNS and with the time course of synaptogenesis. This study suggests that the expression of SN1 is highly regulated, correlating with the demand for glutamine during the critical period of development.

Astrocytes and brain injury

Astrocytes are the most numerous cell type in the central nervous system. They provide structural, trophic, and metabolic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions during brain ischemia and other insults can critically influence neuron survival. Astrocyte functions that are known to influence neuronal survival include glutamate uptake, glutamate release, free radical scavenging, water transport, and the production of cytokines and nitric oxide. Long-term recovery after brain injury, through neurite outgrowth, synaptic plasticity, or neuron regeneration, is influenced by astrocyte surface molecule expression and trophic factor release. In addition, the death or survival of astrocytes themselves may affect the ultimate clinical outcome and rehabilitation through effects on neurogenesis and synaptic reorganization.