Cooperation of the conserved aspartate 439 and bound amino acid substrate is important for high-affinity Na+ binding to the glutamate transporter EAAC1

The neuronal glutamate transporter EAAC1 contains several conserved acidic amino acids in its transmembrane domain, which are possibly important in catalyzing transport and/or binding of co/countertransported cations. Here, we have studied the effects of neutralization by site-directed mutagenesis of three of these amino acid side chains, glutamate 373, aspartate 439, and aspartate 454, on the functional properties of the transporter. Transport was analyzed by whole-cell current recording from EAAC1-expressing mammalian cells after applying jumps in voltage, substrate, or cation concentration. Neutralization mutations in positions 373 and 454, although eliminating steady-state glutamate transport, have little effect on the kinetics and thermodynamics of Na(+) and glutamate binding, suggesting that these two positions do not constitute the sites of Na(+) and glutamate association with EAAC1. In contrast, the D439N mutation resulted in an approximately 10-fold decrease of apparent affinity of the glutamate-bound transporter form for Na(+), and an approximately 2,000-fold reduction in the rate of Na(+) binding, whereas the kinetics and thermodynamics of Na(+) binding to the glutamate-free transporter were almost unchanged compared to EAAC1(WT). Furthermore, the D439N mutation converted l-glutamate, THA, and PDC, which are activating substrates for the wild-type anion conductance, but not l-aspartate, into transient inhibitors of the EAAC1(D439) anion conductance. Activation of the anion conductance by l-glutamate was biphasic, allowing us to directly analyze binding of two of the three cotransported Na(+) ions as a function of time and [Na(+)]. The data can be explained with a model in which the D439N mutation results in a dramatic slowing of Na(+) binding and a reduced affinity of the substrate-bound EAAC1 for Na(+). We propose that the bound substrate controls the rate and the extent of Na(+) interaction with the transporter, depending on the amino acid side chain in position 439.     

Neutralization of the aspartic acid residue Asp-367, but not Asp-454, inhibits binding of Na+ to the glutamate-free form and cycling of the glutamate transporter EAAC1

Substrate transport by the plasma membrane glutamate transporter EAAC1 is coupled to cotransport of three sodium ions. One of these Na(+) ions binds to the transporter already in the absence of glutamate. Here, we have investigated the possible involvement of two conserved aspartic acid residues in transmembrane segments 7 and 8 of EAAC1, Asp-367 and Asp-454, in Na(+) cotransport. To test the effect of charge neutralization mutations in these positions on Na(+) binding to the glutamate-free transporter, we recorded the Na(+)-induced anion leak current to determine the K(m) of EAAC1 for Na(+). For EAAC1(WT), this K(m) was determined as 120 mm. When the negative charge of Asp-367 was neutralized by mutagenesis to asparagine, Na(+) activated the anion leak current with a K(m) of about 2 m, indicating dramatically impaired Na(+) binding to the mutant transporter. In contrast, the Na(+) affinity of EAAC1(D454N) was virtually unchanged compared with the wild type transporter (K(m) = 90 mm). The reduced occupancy of the Na(+) binding site of EAAC1(D367N) resulted in a dramatic reduction in glutamate affinity (K(m) = 3.6 mm, 140 mm [Na(+)]), which could be partially overcome by increasing extracellular [Na(+)]. In addition to impairing Na(+) binding, the D367N mutation slowed glutamate transport, as shown by pre-steady-state kinetic analysis of transport currents, by strongly decreasing the rate of a reaction step associated with glutamate translocation. Our data are consistent with a model in which Asp-367, but not Asp-454, is involved in coordinating the bound Na(+) in the glutamate-free transporter form.     

Individual subunits of the glutamate transporter EAAC1 homotrimer function independently of each other

Glutamate transporters are thought to be assembled as trimers of identical subunits that line a central hole, possibly the permeation pathway for anions. Here, we have tested the effect of multimerization on the transporter function. To do so, we coexpressed EAAC1(WT) with the mutant transporter EAAC1(R446Q), which transports glutamine but not glutamate. Application of 50 microM glutamate or 50 microM glutamine to cells coexpressing similar numbers of both transporters resulted in anion currents of 165 and 130 pA, respectively. Application of both substrates at the same time generated an anion current of 297 pA, demonstrating that the currents catalyzed by the wild-type and mutant transporter subunits are purely additive. This result is unexpected for anion permeation through a central pore but could be explained by anion permeation through independently functioning subunits. To further test the subunit independence, we coexpressed EAAC1(WT) and EAAC1(H295K), a transporter with a 90-fold reduced glutamate affinity as compared to EAAC1(WT), and determined the glutamate concentration dependence of currents of the mixed transporter population. The data were consistent with two independent populations of transporters with apparent glutamate affinities similar to those of EAAC1(H295K) and EAAC1(WT), respectively. Finally, we coexpressed EAAC1(WT) with the pH-independent mutant transporter EAAC1(E373Q), showing two independent populations of transporters, one being pH-dependent and the other being pH-independent. In conclusion, we propose that EAAC1 assembles as trimers of identical subunits but that the individual subunits in the trimer function independently of each other.     

Effect of benzodiazepines on the epithelial and neuronal high-affinity glutamate transporter EAAC1

EAAC1-mediated glutamate transport concentrates glutamate across plasma membranes of brain neurons and epithelia. In brain, EAAC1 provides a presynaptic uptake mechanism to terminate the excitatory action of released glutamate and to keep its extracellular concentration below toxic levels. Here we report the effect of well known anxiolytic compounds, benzodiazepines, on glutamate transport in EAAC1-stably transfected Chinese hamster ovary (CHO) cells and in EAAC1-expressing Xenopus laevis oocytes. Functional properties of EAAC1 agreed well with already reported characteristics of the neuronal high-affinity glutamate transporter (Km D-Asp,CHO cells: 2.23+/-0.15 microM; Km D-Asp,oocytes: 17.01+/-3.42 microM). In both expression systems, low drug concentrations (10-100 microM) activated substrate uptake (up to 200% of control), whereas concentrations in the millimolar range inhibited (up to 50%). Furthermore, the activation was more pronounced at low substrate concentrations (1 microM), and the inhibition was attenuated. The activity of other sodium cotransporters such as the sodium/D-glucose cotransporter SGLT1, stably transfected in CHO cells, was not affected by benzodiazepines. In electrophysiological studies, these drugs also failed to change the membrane potential of EAAC1-expressing Xenopus laevis oocytes. These results suggest a direct action on the glutamate transporter itself without modifying the general driving forces. Thus, in vivo low concentrations of benzodiazepines may reduce synaptic glutamate concentrations by increased uptake, providing an additional mechanism to modulate neuronal excitability.     

On the mechanism of proton transport by the neuronal excitatory amino acid carrier 1

Uptake of glutamate from the synaptic cleft is mediated by high affinity transporters and is driven by Na(+), K(+), and H(+) concentration gradients across the membrane. Here, we characterize the molecular mechanism of the intracellular pH change associated with glutamate transport by combining current recordings from excitatory amino acid carrier 1 (EAAC1)-expressing HEK293 cells with a rapid kinetic technique with a 100-micros time resolution. Under conditions of steady state transport, the affinity of EAAC1 for glutamate in both the forward and reverse modes is strongly dependent on the pH on the cis-side of the membrane, whereas the currents at saturating glutamate concentrations are hardly affected by the pH. Consistent with this, the kinetics of the pre-steady state currents, measured after saturating glutamate concentration jumps, are not a function of the pH. In addition, we determined the deuterium isotope effect on EAAC1 kinetics, which is in agreement with proton cotransport but not OH(-) countertransport. The results can be quantitatively explained with an ordered binding model that includes a rapid proton binding step to the empty transporter followed by glutamate binding and translocation of the proton-glutamate-transporter complex. The apparent pK of the extracellular proton binding site is approximately 8. This value is shifted to approximately 6.5 when the substrate binding site is exposed to the cytoplasm.     

Early intermediates in the transport cycle of the neuronal excitatory amino acid carrier EAAC1

Electrogenic glutamate transport by the excitatory amino acid carrier 1 (EAAC1) is associated with multiple charge movements across the membrane that take place on time scales ranging from microseconds to milliseconds. The molecular nature of these charge movements is poorly understood at present and, therefore, was studied in this report in detail by using the technique of laser-pulse photolysis of caged glutamate providing a 100-micros time resolution. In the inward transport mode, the deactivation of the transient component of the glutamate-induced coupled transport current exhibits two exponential components. Similar results were obtained when restricting EAAC1 to Na(+) translocation steps by removing potassium, thus, demonstrating (1) that substrate translocation of EAAC1 is coupled to inward movement of positive charge and, therefore, electrogenic; and (2) the existence of at least two distinct intermediates in the Na(+)-binding and glutamate translocation limb of the EAAC1 transport cycle. Together with the determination of the sodium ion concentration and voltage dependence of the two-exponential charge movement and of the steady-state EAAC1 properties, we developed a kinetic model that is based on sequential binding of Na(+) and glutamate to their extracellular binding sites on EAAC1 explaining our results. In this model, at least one Na(+) ion and thereafter glutamate rapidly bind to the transporter initiating a slower, electroneutral structural change that makes EAAC1 competent for further, voltage-dependent binding of additional sodium ion(s). Once the fully loaded EAAC1 complex is formed, it can undergo a much slower, electrogenic translocation reaction to expose the substrate and ion binding sites to the cytoplasm.     

Modulation of the neural glutamate transporter EAAC1 by the addicsin-interacting protein ARL6IP1

Addicsin (Arl6ip5) is a murine homologue of rat glutamate transporter-associated protein 3-18 (GTRAP3-18), a putative negative modulator of Na+-dependent neural glutamate transporter-excitatory amino acid carrier 1 (EAAC1). Here we report that ADP-ribosylation factor-like 6 interacting protein 1 (Arl6ip1) is a novel addicsin-associated partner that indirectly promotes EAAC1-mediated glutamate transport activity in a protein kinase C activity-dependent manner. Like addicsin, Arl6ip1 is expressed in numerous tissues and proved likely to be co-localized with addicsin in certain neurons in the matured brain. Arl6ip1 was not translocated from the subcellular compartments under any of the test conditions and had no association with any molecules on the plasma membrane. Immunoprecipitation assay demonstrated that Arl6ip1 bound directly to addicsin and that the hydrophobic region located at amino acids 103-117 of addicsin was crucial to the formation of the Arl6ip1-addicsin heterodimer and addicsin homodimer. Glutamate transport assay revealed that increasing the expression of Arl6ip1 in C6BU-1 cells markedly enhanced Na+-dependent EAAC1-mediated glutamate transport activity in the presence of 100 nm phorbol 12-myristate 13-acetate. Under these conditions, kinetic analyses demonstrated that EAAC1 altered glutamate transport activity by increasing its glutamate affinity but not its maximal velocity. Meanwhile, increasing expression of addicsin Y110A/L112A mutant lacking binding ability for Arl6ip1 showed no enhancement of EAAC1-mediated glutamate transport activity, regardless of phorbol 12-myristate 13-acetate activation, suggesting that association between addicsin and Arl6ip1 causes altered EAAC1-mediated glutamate transport activity. Our findings suggest that Arl6ip1 is a novel addicsin-associated partner that promotes EAAC1-mediated glutamate transport activity by decreasing the number of addicsin molecules available for interaction with EAAC1.     

Thallium ions can replace both sodium and potassium ions in the glutamate transporter excitatory amino acid carrier 1

The excitatory amino acid carrier EAAC1 belongs to a family of glutamate transporters that use the electrochemical transmembrane gradients of sodium and potassium to mediate uphill transport of glutamate into the cell. While the sites of cation interaction with EAAC1 are unknown, two cation binding sites were observed in the crystal structure of the bacterial glutamate transporter homologue GltPh. Although occupied by Tl(+) in the crystal structure, these sites were proposed to be Na(+) binding sites. Therefore, we tested whether Tl(+) has the ability to replace Na(+) also in the mammalian transporters. Our data demonstrate that Tl(+) can bind to EAAC1 with high affinity and mediate a host of different functions. Tl(+) can functionally replace potassium when applied to the cytoplasm and can support glutamate transport current. When applied extracellularly, Tl(+) induces some behavior that mimics that of the Na(+)-bound transporter, such as activation of the cation-induced anion conductance and creation of a substrate binding site, but it cannot replace Na(+) in supporting glutamate transport current. Moreover, our data show a differential effect of mutations to two acidic amino acids potentially involved in cation binding (D367 and D454) on Na(+) and Tl(+) affinity. Overall, our results demonstrate that the ability of the glutamate transporters to interact with Tl(+) is conserved between GltPh and a mammalian member of the transporter family. However, in contrast to GltPh, which does not bind K(+), Tl(+) is more efficient in mimicking K(+) than Na(+) when interacting with the mammalian protein.     

Plasma membrane expression of the neuronal glutamate transporter EAAC1 is regulated by glial factors: evidence for different regulatory pathways associated with neuronal maturation

At the glutamatergic synapse the neurotransmitter is removed from the synaptic cleft by high affinity amino acid transporters located on neurons (EAAC1) and astrocytes (GLAST and GLT1), and a coordinated action of these cells is necessary in order to regulate glutamate extracellular concentration. We show here that treatment of neuronal cultures with glial soluble factors (GCM) is associated with a redistribution of EAAC1 and GLAST to the cell membrane and we analysed the effect of membrane cholesterol depletion on this regulation.

In enriched neuronal culture (90% neurons and 10% astrocytes), GCM treatment for 10 days increases EAAC1 and GLAST cell surface expression with no change in total expression. In opposite, GLT1 surface expression is not modified by GCM but total expression is increased. When cholesterol is acutely depleted from the membrane by 10 mM methyl-beta-cyclodextrin (β5-MCD, 30 min), glutamate transport activity and cell surface expressions of EAAC1 and GLAST are decreased in the enriched neuronal culture treated by GCM. In pure neuronal culture addition of GCM also increases EAAC1 cell membrane expression but surprisingly acute treatment with β5-MCD decreases glutamate uptake activity but not EAAC1 cell membrane expression. By immunocytochemistry a modification in the distribution of EAAC1 within neurons was undetectable whatever the treatment but we show that EAAC1 was no more co localized with Thy-1 in the enriched neuronal culture treated by GCM suggesting that GCM have stimulated polarity formation in neurons, an index of maturation.

In conclusion we suggest that different regulatory mechanisms are involved after GCM treatment, glutamate transporter trafficking to and from the plasma membrane in enriched neuronal culture and modulation of EAAC1 intrinsic activity and/or association with regulatory proteins at the cell membrane in the pure neuronal culture. These different regulatory pathways of EAAC1 are associated with different neuronal maturation stages.     

Constitutive endocytosis and recycling of the neuronal glutamate transporter, excitatory amino acid carrier 1

The neuronal glutamate transporter, excitatory amino acid carrier 1 (EAAC1), has a diverse array of physiologic and metabolic functions. There is evidence that there is a relatively large intracellular pool of EAAC1 both in vivo and in vitro, that EAAC1 cycles on and off the plasma membrane, and that EAAC1 cell surface expression can be rapidly regulated by intracellular signals. Despite the possible relevance of EAAC1 trafficking to both physiologic and pathologic processes, the cellular machinery involved has not been defined. In the present study, we found that agents that disrupt clathrin-dependent endocytosis or plasma membrane cholesterol increased steady-state levels of biotinylated EAAC1 in C6 glioma cells and primary neuronal cultures. Acute depletion of cholesterol increased the V(max) for EAAC1-mediated activity and had no effect on Na(+)-dependent glycine transport in the same system. These agents also impaired endocytosis as measured using a reversible biotinylating reagent. Co-expression with dominant-negative variants of dynamin or the clathrin adaptor, epidermal growth factor receptor pathway substrate clone 15, increased the steady-state levels of biotinylated myc-EAAC1. EAAC1 immunoreactivity was found in a subcellular fraction enriched in early endosome antigen 1 (EEA1) isolated by differential centrifugation and partially co-localized with EEA1. Co-expression of a dominant-negative variant of Rab11 (Rab11 S25N) reduced steady-state levels of biotinylated myc-EAAC1 and slowed constitutive delivery of myc-EAAC1 to the plasma membrane. Together, these observations suggest that EAAC1 is constitutively internalized via a clathrin- and dynamin-dependent pathway into early endosomes and that EAAC1 is trafficked back to the cell surface via the endocytic recycling compartment in a Rab11-dependent mechanism. As one defines the machinery required for constitutive trafficking of EAAC1, it may be possible to determine how intracellular signals regulate EAAC1 cell surface expression.     

Pertussis toxin-sensitive modulation of glutamate transport by endothelin-1 type A receptors in glioma cells

Endothelin-1 (ET-1) is a 21 amino acids peptide that exerts several biological activities through interaction with specific G-protein coupled receptors. Increased ET-1 expression is frequently associated with pathological situations involving alterations in glutamate levels. In the present study, a brief exposure to ET-1 was found to increase aspartate uptake in C6 glioma cells, which endogenously express the neuronal glutamate transporter EAAC1 (pEC50 of 9.89). The stimulatory effect of ET-1 mediated by ETA receptors corresponds to a 62% increase in the Vmax with no modification of the affinity for the substrate. While protein kinase C activity is known to participate in the regulation of EAAC1, the effect of ET-1 on the glutamate uptake was found to be independent of this kinase activation. In contrast, the inactivation of Go/i type G-protein dependent signaling with pertussis toxin was found to impair ET-1-mediated regulation of EAAC1. An examination of the cell surface expression of EAAC1 by protein biotinylation studies or by confocal analysis of immuno-fluorescence staining demonstrated that ET-1 stimulates EAAC1 translocation to the cell surface. Hence, the disruption of the cytoskeleton with cytochalasin D prevented ET-1-stimulated aspartate uptake. Together, the data presented in the current study suggest that ET-1 participates in the acute regulation of glutamate transport in glioma cells. Considering the documented role of glutamate excitotoxicity in the development of brain tumors, endothelinergic system constitutes a putative target for the pharmacological control of glutamate transmission at the vicinity of glioma cells.     

Charge Compensation Mechanism of a Na+-coupled, Secondary Active Glutamate Transporter

Forward glutamate transport by the excitatory amino acid carrier EAAC1 is coupled to the inward movement of three Na(+) and one proton and the subsequent outward movement of one K(+) in a separate step. Based on indirect evidence, it was speculated that the cation binding sites bear a negative charge. However, little is known about the electrostatics of the transport process. Valences calculated using the Poisson-Boltzmann equation indicate that negative charge is transferred across the membrane when only one cation is bound. Consistently, transient currents were observed in response to voltage jumps when K(+) was the only cation on both sides of the membrane. Furthermore, rapid extracellular K(+) application to EAAC1 under single turnover conditions (K(+) inside) resulted in outward transient current. We propose a charge compensation mechanism, in which the C-terminal transport domain bears an overall negative charge of -1.23. Charge compensation, together with distribution of charge movement over many steps in the transport cycle, as well as defocusing of the membrane electric field, may be combined strategies used by Na(+)-coupled transporters to avoid prohibitive activation barriers for charge translocation.     

The conserved histidine 295 does not contribute to proton cotransport by the glutamate transporter EAAC1

Transmembrane glutamate transport by the excitatory amino acid carrier (EAAC1) is coupled to the cotransport of three Na(+) ions and one proton. Previously, we suggested that the mechanism of H(+) cotransport involves protonation of the conserved glutamate residue E373. However, it was also speculated that the cotransported proton is shared in a H(+)-binding network, possibly involving the conserved histidine 295 in the sixth transmembrane domain of EAAC1. Here, we used site-directed mutagenesis together with pre-steady-state electrophysiological analysis of the mutant transporters to test the protonation state of H295 and to determine its involvement in proton transport by EAAC1. Our results show that replacement of H295 with glutamine, an amino acid residue that cannot be protonated, generates a fully functional transporter with transport kinetics that are close to those of the wild-type EAAC1. In contrast, replacement with lysine results in a transporter in which substrate binding and translocation are dramatically inhibited. Furthermore, it is demonstrated that the effect of the histidine 295 to lysine mutation on the glutamate affinity is caused by its positive charge, since wild-type-like affinity can be restored by changing the extracellular pH to 10.0, thus partially deprotonating H295K. Together, these results suggest that histidine 295 is not protonated in EAAC1 at physiological pH and, thus, does not contribute to H(+) cotransport. This conclusion is supported by data from H295C-E373C double mutant transporters which demonstrate that these residues cannot be linked by oxidation, indicating that H295 and E373 are not close in space and do not form a proton binding network. A kinetic scheme is used to quantify the results, which includes binding of the cotransported proton to E373 and binding of a modulatory, nontransported proton to the amino acid side chain in position 295.     

Pertussis toxin-sensitive modulation of glutamate transport by endothelin-1 type A receptors in glioma cells

Endothelin-1 (ET-1) is a 21 amino acids peptide that exerts several biological activities through interaction with specific G-protein coupled receptors. Increased ET-1 expression is frequently associated with pathological situations involving alterations in glutamate levels. In the present study, a brief exposure to ET-1 was found to increase aspartate uptake in C6 glioma cells, which endogenously express the neuronal glutamate transporter EAAC1 (pEC₅₀ of 9.89). The stimulatory effect of ET-1 mediated by ET_A receptors corresponds to a 62% increase in the V_max with no modification of the affinity for the substrate. While protein kinase C activity is known to participate in the regulation of EAAC1, the effect of ET-1 on the glutamate uptake was found to be independent of this kinase activation. In contrast, the inactivation of G_o/i type G-protein dependent signaling with pertussis toxin was found to impair ET-1-mediated regulation of EAAC1. An examination of the cell surface expression of EAAC1 by protein biotinylation studies or by confocal analysis of immuno-fluorescence staining demonstrated that ET-1 stimulates EAAC1 translocation to the cell surface. Hence, the disruption of the cytoskeleton with cytochalasin D prevented ET-1-stimulated aspartate uptake. Together, the data presented in the current study suggest that ET-1 participates in the acute regulation of glutamate transport in glioma cells. Considering the documented role of glutamate excitotoxicity in the development of brain tumors, endothelinergic system constitutes a putative target for the pharmacological control of glutamate transmission at the vicinity of glioma cells.     

Platelet-derived growth factor rapidly increases activity and cell surface expression of the EAAC1 subtype of glutamate transporter through activation of phosphatidylinositol 3-kinase

Na(+)-dependent glutamate transporters are the primary mechanism for removal of excitatory amino acids (EAAs) from the extracellular space of the central nervous system and influence both physiologic and pathologic effects of these compounds. Recent evidence suggests that the activity and cell surface expression of a neuronal subtype of glutamate transporter, EAAC1, are rapidly increased by direct activation of protein kinase C and are decreased by wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K). We hypothesized that this regulation could be analogous to insulin-induced stimulation of the GLUT4 subtype of glucose transporter, which is dependent upon activation of PI3-K. Using C6 glioma, a cell line that endogenously and selectively expresses EAAC1, we report that platelet-derived growth factor (PDGF) increased Na(+)-dependent L-[(3)H]-glutamate transport activity within 30 min. This effect of PDGF was not due to a change in total cellular EAAC1 immunoreactivity but was instead correlated with an increase cell surface expression of EAAC1, as measured using a membrane impermeant biotinylation reagent combined with Western blotting. A decrease in nonbiotinylated intracellular EAAC1 was also observed. These studies suggest that PDGF causes a redistribution of EAAC1 from an intracellular compartment to the cell surface. These effects of PDGF were accompanied by a 35-fold increase in PI3-K activity and were blocked by the PI3-K inhibitors, wortmannin and LY 294002, but not by an inhibitor of protein kinase C. Other growth factors, including insulin, nerve growth factor, and epidermal growth factor had no effect on glutamate transport nor did they increase PI3-K activity. These studies suggest that, as is observed for insulin-mediated translocation of GLUT4, EAAC1 cell surface expression can be rapidly increased by PDGF through activation of PI3-K. It is possible that this PDGF-mediated increase in EAAC1 activity may contribute to the previously demonstrated neuroprotective effects of PDGF.     

Neuronal glutamate transporters vary in substrate transport rate but not in unitary anion channel conductance

Excitatory amino acid transporters (EAATs) not only sustain a secondary active glutamate transport but also function as anion-selective ion channels. The relative proportion of currents generated by glutamate transport or by the chloride conductance varies for each cloned EAAT subtype. For EAAT1, EAAT2, and EAAT3, the anion current is only a small component of the total transporter-associated current amplitude, whereas EAAT4 and EAAT5 transporters mediate predominantly anion currents. We here demonstrate that the distinct current proportions are entirely due to differences in glutamate transport rates. EAAT3 and EAAT4 differ in unitary glutamate transport rates as well as in the voltage and substrate dependence of anion channel opening, but ion conduction properties are very similar. Noise analysis revealed identical unitary current amplitudes and similar absolute open probabilities for the two anion channels. The low glutamate transport rate of EAAT4 allows regulation of cellular excitability without interfering with extracellular glutamate homeostasis and makes this EAAT isoform ideally suited to regulate excitability in dendritic spines of Purkinje neurons.     

Glial soluble factors regulate the activity and expression of the neuronal glutamate transporter EAAC1: implication of cholesterol

A co-ordinated regulation between neurons and astrocytes is essential for the control of extracellular glutamate concentration. Here, we have investigated the influence of astrocytes and glia-derived cholesterol on the regulation of glutamate transport in primary neuronal cultures from rat embryonic cortices. Glutamate uptake rate and expression of the neuronal glutamate transporter EAAC1 were low when neurons were grown without astrocytes and neurons were unable to clear extracellular glutamate. Treatment of the neuronal cultures with glial conditioned medium (GCM) increased glutamate uptake Vmax, EAAC1 expression and restored the capacity of neurons to eliminate extracellular glutamate. Thus, astrocytes up-regulate the activity and expression of EAAC1 in neurons. We further showed that cholesterol, present in GCM, increased glutamate uptake activity when added directly to neurons and had no effect on glutamate transporter expression. Furthermore, part of the GCM-induced effect on glutamate transport activity was lost when cholesterol was removed from GCM (low cholesterol-GCM) and was restored when cholesterol was added to low cholesterol-GCM. This demonstrates that glia-derived cholesterol regulates glutamate transport activity. With these experiments, we provide new evidences for neuronal glutamate transport regulation by astrocytes and identified cholesterol as one of the factors implicated in this regulation.     

Biosynthesis,intracellular targeting,and degradation of the EAAC1 glutamate/aspartate transporter in C6 glioma cells

Rat C6 glioma cells were used as a model system to study the biosynthesis, intracellular targeting, and degradation of the EAAC1 transporter, a sodium-dependent glutamate/aspartate transport protein that encodes System X(-)A,G activity. At steady state, nearly 70% of the EAAC1 transporter was located at the cell surface. The newly synthesized EAAC1 protein was co-translationally N-glycosylated with high mannose oligosaccharide chains that were processed into complex-type sugar chains as the protein matured. The final maturation steps for EAAC1 protein coincided with its plasma membrane arrival, which was first detected at about 45 min after the initial synthesis. The newly synthesized EAAC1 protein was protected from degradation during the maturation and targeting process, as well as during the first 5 h after plasma membrane arrival. After this initial lag period, both the newly synthesized transporter and the total cellular EAAC1 pool were degraded by first order kinetics with a half-life of 6 h. These results represent the first analysis of the synthesis and degradation of the EAAC1 amino acid transporter.     

Different functional roles of arginine residues 39 and 61 and tyrosine residue 98 in transport and channel mode of the glutamate transporter EAAC1

The excitatory amino acid transporter EAAC1 is an electrogenic Na+ - and K+ -gradient-driven transporter. In addition, the transporter mediates in the presence of Na+ and glutamate an anion conductance uncoupled from the transport of the glutamate. The first two N-terminal domains, important for forming the conductance mode, are extracellularly bordered by positively charged arginine residues, R39 and R61, being completely conserved throughout the transporter family. Also the conserved tyrosine residue Y98 could be important for Cl- conductance. We have investigated, by measurements of glutamate uptake and glutamate-induced currents, the effects of mutation of the arginines and the tyrosine to alanine. The mutation R39A hardly affects transport and channel mode. The mutation R61A, on the other hand, reduces the activity of transport but stimulates the channel conductance. In addition, the apparent Km values for glutamate uptake and for the glutamate-activated current are reduced. Glutamate stimulation of current seems to be associated with a voltage-dependent step, and the apparent valence of charge moved during binding is reduced in the R61A mutant. The mutation Y98A leads to reduced function with reduced apparent Km value for glutamate, and with strong reduction of the selectivity ration between NO3- and Cl- of the conductance mode.     

Exchange of cystine and glutamate across plasma membrane of human fibroblasts

It is found that both the inward and outward transport of cystine and glutamate through the plasma membrane of cultured human fibroblasts is mediated mostly by a single transport system. Cystine and glutamate at one side of the membrane stimulate the passage of these amino acids present at the other side of the membrane. When the concentration of intracellular glutamate is reduced to near zero, cystine hardly enters the cell, and likewise the release of glutamate from the cell ceases when cystine is absent in the medium. Homocysteate and alpha-aminoadipate share this transport system and, when added, similarly participate in the transport process. Since the intracellular pool of cystine is negligibly small whereas that of glutamate is very large, the physiologic flows via this system are the entry of cystine and the exodus of glutamate coupled together. Measurements of the rate of uptake of cystine into the cells and the rate of release of glutamate from the cells indicate that the entry of cystine and the exodus of glutamate occur at a ratio close to 1:1. Since cystine is known to behave as an anionic form in this transport, it is concluded that the transport system for cystine and glutamate in plasma membrane of human fibroblasts is a kind of an anion-exchanging agency.