Cloning and expression of a bovine glutamate transporter

Amiloride-insensitive currents of the acid-sensing ion channel-2a (ASIC2a)/ASIC2b heteromeric sour-taste receptor channel

Acid-sensing ion channel-2a (ASIC2a) is an amiloride-blockable proton-gated cation channel, probably contributing to sour-taste detection in rat taste cells. To isolate another subtype of the sour-taste receptor, we screened a rat circumvallate papilla cDNA library and identified ASIC2b, an N-terminal splice variant of ASIC2a. Reverse transcription-PCR analyses confirmed the expression of ASIC2b transcripts in the circumvallate papilla and, moreover, demonstrated its expression in the foliate and fungiform papillae. Immunohistochemical analyses revealed that ASIC2b, as well as ASIC2a, was expressed in a subpopulation of taste cells in the circumvallate, foliate, and fungiform papillae, and some of the cells displayed both ASIC2a and ASIC2b immunoreactivities. Subsequent coimmunoprecipitation studies with circumvallate papillae extracts indicated that ASIC2b associated with ASIC2a to form assemblies and, together with our immunohistochemical findings, strongly suggested that both ASIC2 subunits formed heteromeric channels in taste cells in the circumvallate, foliate, and fungiform papillae. Oocyte electrophysiology demonstrated that the ASIC2a/ASIC2b channel generated maximal inward currents at a pH of < or =2.0, which is in agreement with the in vivo pH sensitivity of rat taste cells, and that the amiloride sensitivity of the heteromer decreased with decreasing pH and was almost completely abolished at a pH of 2.0. These findings provide persuasive explanations for the amiloride insensitivity of acid-induced responses of rat taste cells.

Cloning and functional expression of ASIC-beta2, a splice variant of ASIC-beta

We have isolated a cDNA encoding a splice variant of ASIC (acid-sensing ion channel)-beta from the rat trigeminal ganglion. This clone, designated ASIC-beta2, showed a 342 base deletion just after the first transmembrane domain in ASIC-beta. RT-PCR experiments revealed that ASIC-beta2 was expressed exclusively in the trigeminal ganglion and dorsal root ganglion. In situ hybridization showed that ASIC-beta2 mRNA was concentrated in both small diameter and large diameter neurons and co-localized with ASIC-beta mRNA within single sensory neurons in the trigeminal ganglion. When expressed in Xenopus oocytes, ASIC-beta2 was inactive by itself. However, it associated with ASIC-beta to form heteromers, which display lower affinity for protons than ASIC-beta alone.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Characterization of a mouse colonic system B(0+) amino acid transporter related to amino acid absorption in colon

Previous experiments have shown that an amino acid transport system B(0+) transporter in cultured colonic epithelial cells mediates amino acid absorption. Here we describe the cloning and functional characterization of a system B(0+) transporter selectively expressed in the colon. Using the combination of an expressed sequence tag database search and RT-PCR approaches, we cloned a mouse colonic amino acid transporter, designated mCATB(0+). Northern blot analysis revealed that mCATB(0+) was selectively expressed in the large intestine. In situ hybridization showed the mCATB(0+) mRNA to be localized in absorptive epithelial cells. When expressed in Xenopus oocytes, mCATB(0+) exhibited a Na(+)-dependent stereoselective uptake and a broad specificity for neutral and cationic amino acids, which is characteristic of amino acid transport system B(0+). In vivo [(3)H]glycine uptake assay demonstrated that a system B(0+)-like transporter protein was expressed on the apical surface of the colonic absorptive cells. Our data suggest that a mouse colonic amino acid transporter mCATB(0+) may absorb amino acids from the intestinal contents in the colon.

A single amino acid substitution in MDEG2 specifically alters desensitization of the proton-activated cation current

To clarify functional roles of MDEG2 (mammalian degenerin-2), a modulatory subunit of proton-activated cation channels, in MDEG1/MDEG2 heteromer, we replaced the Gly481 residue in MDEG2 with cysteine or phenylalanine and characterized them electrophysiologically. Expression of MDEG1 in Xenopus oocytes elicited proton-activated cation currents that were rapidly desensitized. Co-expression of MDEG1 and MDEG2 (or MDEG2-G481C) displayed similar current traces as MDEG1 alone. In contrast, co-expression of MDEG1 and MDEG2-G481F dramatically attenuated desensitization of the proton-activated currents. Interestingly, the G481F mutation in MDEG2 did not alter other channel properties including maximal whole-cell currents, ionic selectivity, pH-sensitivity and affinity for amiloride. Thus, Gly481 in MDEG2 specifically controls inactivation process of the MDEG1/MDEG2 channel.

Isolation and characterization of the genes up-regulated in isolated neurons by aged garlic extract (AGE)

Aged garlic extract (AGE) produces neurotrophic effects on cultured fetal rat hippocampal neurons. These studies examined the molecular events triggered by AGE that might account for a suppression of neuronal cell death. Genes differentially expressed by the addition of AGE in primary cultured hippocampal neurons isolated from fetal rat brain were screened using mRNA differential display. Four cDNA clones were significantly enhanced at their transcriptional level; they were designated as #24, #110, #153 and #155. Quantitative reverse transcription-polymerase chain reaction (RT-PCR), as well as dot-blot hybridization combined with RT-PCR, confirmed that the transcription from these four genes was elevated at least twofold, particularly the mRNA of #153, which was increased >20 times 72 h after the addition of AGE. A homology search of the respective cDNA sequences in the DNA database revealed that #153 is an alpha 2-microglobulin-related protein (alpha 2MRP) gene. The others genes were not identified. Induction of the alpha 2MRP gene expression occurred within 24 h after addition of AGE. These findings suggest a possible mechanism by which AGE may regulate gene expression and bring about a neurotrophic effect. Further, our results suggest that alpha 2MRP may function at the initial step of the molecular events triggered by AGE and play an important role in the survival of hippocampal neurons.

Inherited defects of sodium-dependent glutamate transport mediated by glutamate/aspartate transporter in canine red cells due to a decreased level of transporter protein expression

Canine red cells have a high affinity Na(+)/K(+)-dependent glutamate transporter. We herein demonstrate that this transport is mediated by the canine homologue of glutamate/aspartate transporter (GLAST), one of the glutamate transporter subtypes abundant in the central nervous system. We also demonstrate that GLAST is the most ubiquitous glutamate transporter among the transporter subtypes that have been cloned to date. The GLAST protein content was extremely reduced in variant red cells, low glutamate transport (LGlut) red cells characterized by an inherited remarkable decrease in glutamate transport activity. All LGluT dogs carried a missense mutation of Gly(492) to Ser (G492S) in either the heterozygous or homozygous state. The GLAST protein with G492S mutation was fully functional in glutamate transport in Xenopus oocytes. However, G492S GLAST exhibited a marked decrease in activity after the addition of cycloheximide, while the wild type showed no significant change, indicating that G492S GLAST was unstable compared with the wild-type transporter. Moreover, LGluT dogs, but not normal dogs, heterozygous for the G492S mutation showed a selective decrease in the accumulation of GLAST mRNA from the normal allele. Based on these findings, we conclude that a complicated heterologous combination of G492S mutation and some transcriptional defect contributes to the pathogenesis of the LGluT red cell phenotype.

Excitatory amino acid transporters: a family in flux

As the most predominant excitatory neurotransmitter, glutamate has the potential to influence the function of most neuronal circuits in the central nervous system. To limit receptor activation during signaling and prevent the overstimulation of glutamate receptors that can trigger excitotoxic mechanisms and cell death, extracellular concentrations of excitatory amino acids are tightly controlled by transport systems on both neurons and glial cells. L-Glutamate is a potent neurotoxin, and the inadequate clearance of excitatory amino acids may contribute to the neurodegeneration seen in a variety of conditions, including epilepsy, ischemia, and amyotrophic lateral sclerosis. To establish the contributions of carrier systems to the etiology of neurological disorders, and to consider their potential utility as therapeutic targets, a detailed understanding of transporter function and pharmacology is required. This review summarizes current knowledge of the structural and functional diversity of excitatory amino acid transporters and explores how they might serve as targets for drug design.

Molecular cloning and distinct developmental expression pattern of spliced forms of a novel zinc finger gene wiz in the mouse cerebellum

In the course of a study conducted to identify the mouse homologue of Drosophila eyes absent (eya), we isolated a novel mouse cDNA fragment which show little homology to eya but encodes a protein with Krüppel (C2H2)-type zinc finger motifs. By further screening using this cDNA fragment as a probe, we obtained the short and long forms of full-length cDNAs, which were apparently alternatively spliced products from one gene. Since both mRNAs encode proteins with widely-interspaced zinc finger motifs, we termed this gene wiz and refer to the short and long wiz transcripts as wizS and wizL, respectively. In situ hybridization studies using the probe against the region common to wizS and wizL showed that these mRNAs were expressed abundantly in the granule cell layers of the mouse cerebellum, the olfactory bulb, and the dentate gyrus, whereas the same technique using the probe against only wizL could not detect positive signals in the developing cerebellum, indicating that there is no expression of wizL mRNA there. Northern blot and in situ hybridization analyses demonstrated that the extracerebellar regions expressed both wizS and wizL mRNAs from the midgestational period to adulthood. The finding that two types of wiz transcripts (wizS and wizL) are expressed with different developmental patterns might indicate separate transcription functions in the cerebellar granule cells and the extracerebellar regions.

Molecular biology of mammalian plasma membrane amino acid transporters

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

Cellular distribution and kinetic properties of high-affinity glutamate transporters

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

DL-threo-beta-benzyloxyaspartate, a potent blocker of excitatory amino acid transporters

DL-threo-beta-Benzyloxyaspartate (DL-TBOA), a novel derivative of DL-threo-beta-hydroxyaspartate, was synthesized and examined as an inhibitor of sodium-dependent glutamate/aspartate (excitatory amino acid) transporters. DL-TBOA inhibited the uptake of [14C]glutamate in COS-1 cells expressing the human excitatory amino acid transporter-1 (EAAT1) (Ki = 42 microM) with almost the same potency as DL-threo-beta-hydroxyaspartate (Ki = 58 microM). With regard to the human excitatory amino acid transporter-2 (EAAT2), the inhibitory effect of DL-TBOA (Ki = 5.7 microM) was much more potent than that of dihydrokainate (Ki = 79 microM), which is well known as a selective blocker of this subtype. Electrophysiologically, DL-TBOA induced no detectable inward currents in Xenopus laevis oocytes expressing human EAAT1 or EAAT2. However, it significantly reduced the glutamate-induced currents, indicating the prevention of transport. The dose-response curve of glutamate was shifted by adding DL-TBOA without a significant change in the maximum current. The Kb values for human EAAT1 and EAAT2 expressed in X. laevis oocytes were 9.0 microM and 116 nM, respectively. These results demonstrated that DL-TBOA is, so far, the most potent competitive blocker of glutamate transporters. DL-TBOA did not show any significant effects on either the ionotropic or metabotropic glutamate receptors. Moreover, DL-TBOA is chemically much more stable than its benzoyl analog, a previously reported blocker of excitatory amino acid transporters; therefore, DL-TBOA should be a useful tool for investigating the physiological roles of transporters.

Inhibitory activity of a naturally occurring heterocyclic beta-substituted alanine, beta-(isoxazolin-5-on-4-yl)-L-alanine, on the L-glutamate/L-aspartate transporter (GLAST) expressed in Xenopus oocytes

Excitatory amino acid (EAA) transporters are of physiological importance in the regulation of the extracellular concentration of excitatory amino acids and the neuroexcitation in CNS. Among four identified transporters, the Na+-dependent high-affinity L-glutamate/L-aspartate transporter (GLAST) is highly expressed in glial cells. Here, we report a naturally occurring inhibitor of GLAST, derived from bovine retina, using the Xenopus oocyte expression system. Beta-(isoxazolin-5-on-4-yl)-L-alanine (TAN), an antifungal antibiotic, inhibited [14C]L-glutamate (L-Glu) transport into GLAST-expressing oocytes. TAN also served as a substrate for this transporter in voltage-clamp experiments measuring the current coupled to the EAA transport. The maximum current of TAN itself was approximately 1/3 of that of L-glutamate, and its apparent affinity was almost the same as L-Glu. In combination with L-Glu, TAN antagonized L-glutamate transport. In radioisotope experiments, the inhibitory potency of this compound against [14C]L-Glu uptake into oocytes was approximately 1/6 of that of L-(-)-threo-3-hydroxyaspartate (THA). The glucoside of TAN (TANG), occurring in seedlings of the garden pea, the lentil and some Lathyrus species, did not show any electrophysiological activity nor was it transported into oocytes. It is proposed that TAN is a novel type antagonist of natural origin on GLAST. By affecting such transport system, naturally occurring compounds may affect the regulation of the extracellular level of endogenous EAA.

New beta-hydroxyaspartate derivatives are competitive blockers for the bovine glutamate/aspartate transporter

Four subtypes of excitatory amino acid transporters (EAAT1-4) have been identified in the mammalian brain. A number of pharmacological agents have been developed to study their intrinsic properties and function. Up to now, blockers were available only for EAAT2, whereas all the inhibitors of glutamate uptake active on the other subtypes were proved to be substrates of the transporters. We synthesized five new derivatives of DL-threo-beta-hydroxyaspartic acid, a well known general substrate of EAATs, and investigated their potential blocking activity on the cloned bovine EAAT1 expressed in the Xenopus oocyte system, by using radiotracer and voltage-clamp techniques. Two of our derivatives proved to be substrates for bovine EAAT1, with reduced electrogenicity compared with their parent compound, and an affinity of 40 and 64 microM. The last three derivatives displayed a blocking activity on bovine EAAT1. The affinity of DL-threo-beta-benzoyloxyaspartate and DL-threo-beta-(1-naphthoyl)oxyaspartate was determined by Schild analysis as 17.2 and 52.1 microM, respectively. These blockers should help in the better understanding of the key intrinsic properties of EAAT1. Moreover, they appear as good candidates for a general blocking activity on EAATs.

Stimulation of glutamate uptake and Na,K-ATPase activity in rat astrocytes exposed to ischemia-like insults

The postsynaptic actions of glutamate are rapidly terminated by high affinity glutamate uptake into glial cells. In this study we demonstrate the stimulation of both glutamate uptake and Na,K-ATPase activity in rat astrocyte cultures in response to sublethal ischemia-like insults. Primary cultures of neonatal rat cortical astrocytes were subjected to hypoxia, or to serum- and glucose-free medium, or to both conditions (ischemia). Cell death was assessed by propidium iodide staining of cell nuclei. To measure sodium pump activity and glutamate uptake, 3H-glutamate and 86Rb were both simultaneously added to the cell culture in the presence or absence of 2 mM ouabain. Na,K-ATPase activity was defined as ouabain-sensitive 86Rb uptake. Concomitant transient increases (2-3 times above control levels) of both Na,K-ATPase and glutamate transporter activities were observed in astrocytes after 4-24 h of hypoxia, 4 h of glucose deprivation, and 2-4 h of ischemia. A 24 h ischemia caused a profound loss of both activities in parallel with significant cell death. The addition of 5 mM glucose to the cells after 4 h ischemia prevented the loss of both sodium pump activity and glutamate uptake and rescued astrocytes from death observed at the end of 24 h ischemia. Reoxygenation after the 4 h ischemic event caused the selective inhibition of Na,K-ATPase activity. The observed increases in Na,K-ATPase activity and glutamate uptake in cultured astrocytes subjected to sublethal ischemia-like insults may model an important functional response of astrocytes in vivo by which they attempt to maintain ion and glutamate homeostasis under restricted energy and oxygen supply.

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

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

New mutations and phenotypes associated with glutamate and aspartate transport in Chinese hamster ovary (CHO-K1) cells

Two new Chinese hamster ovary cell (CHO-K1) mutants lacking amino acid transport System X-AG activity were isolated by [3H]aspartate suicide selection. These null mutants, Dd-B6 and Dd-B7, were analyzed by somatic cell hybridization, along with previously described partial-function mutants, Ed-A1 and Ed-B8. With respect to System X-AG activity, all four mutations fell into a single complementation group. By quantitative assay, the mutations in Ed-A1 and Ed-B8 behaved as simple recessives in fusions with wild type cells, while those in Dd-B6 and Dd-B7 were codominant. We have discovered that Ed-A1 and Ed-B8 are highly permeable to small neutral molecules. This high permeability phenotype was dominant to wild-type. Northern, Southern, and Western analyses indicated that System X-AG in CHO is not closely related to any of the three well characterized glutamate transporters represented by GLT-1, EAACI or GLAST.