The dual-function glutamate transporters: structure and molecular characterisation of the substrate-binding sites

Neuronal MML-1/MXL-2 regulates systemic aging via glutamate transporter and cell nonautonomous autophagic and peroxidase activity

Accumulating evidence has demonstrated the presence of intertissue-communication regulating systemic aging, but the underlying molecular network has not been fully explored. We and others previously showed that two basic helix-loop-helix transcription factors, MML-1 and HLH-30, are required for lifespan extension in several longevity paradigms, including germlineless Caenorhabditis elegans. However, it is unknown what tissues these factors target to promote longevity. Here, using tissue-specific knockdown experiments, we found that MML-1 and its heterodimer partners MXL-2 and HLH-30 act primarily in neurons to extend longevity in germlineless animals. Interestingly, however, the downstream cascades of MML-1 in neurons were distinct from those of HLH-30. Neuronal RNA interference (RNAi)-based transcriptome analysis revealed that the glutamate transporter GLT-5 is a downstream target of MML-1 but not HLH-30. Furthermore, the MML-1-GTL-5 axis in neurons is critical to prevent an age-dependent collapse of proteostasis and increased oxidative stress through autophagy and peroxidase MLT-7, respectively, in long-lived animals. Collectively, our study revealed that systemic aging is regulated by a molecular network involving neuronal MML-1 function in both neural and peripheral tissues.

Late-onset episodic ataxia associated with SLC1A3 mutation

Episodic ataxia type 6 (EA6) is caused by mutations in SLC1A3 that encodes excitatory amino acid transporter 1 (EAAT1), a glial glutamate transporter. EAAT1 regulates the extent and durations of glutamate-mediated signal by the clearance of glutamate after synaptic release. In addition, EAAT1 also has an anion channel activity that prevents additional glutamate release. We identified a missense mutation in SLC1A3 in a family with EA. The proband exhibited typical EA2-like symptoms such as recurrent ataxia, slurred speech with a duration of several hours, interictal nystagmus and response to acetazolamide, but had late-onset age of sixth decade. Whole-exome sequencing detected a heterozygous c.1177G>A mutation in SLC1A3. This mutation predicted a substitution of isoleucine for a highly conserved valine residue in the seventh transmembrane domain of EAAT1. The mutation was not present in 100 controls, a large panel of in-house genome data and various mutation databases. Most functional prediction scores revealed to be deleterious. Same heterozygous mutation was identified in one clinically affected family member and two asymptomatic members. Our data expand the mutation spectrum of SLC1A3 and the clinical phenotype of EA6.

Intriguing possibilities and beneficial aspects of transporter-conscious drug design

It has been revealed that many types of drugs interact with transporter proteins within an organism. Transporter proteins absorb or excrete materials, including drugs and nutrients, across the cell membrane. Some hydrophobic drugs are excreted from the cell as xenobiotics by ATP-binding cassette (ABC) transporters. However, solute carrier (SLC) transporters are tissue-specifically expressed and have substrate specificities. Thus, transporter-conscious drug design is an excellent method of delivering drugs to pharmaceutical target organs and provides advantages in absorption, distribution, excretion, and toxicity of drugs (ADMET) due to transport systems. In fact, based on this strategy, the bioavailability of prodrugs designed as peptide transporter 1 (PEPT1) substrates was better than that of the corresponding parent compounds due to the transport system in the small intestine. Furthermore, in central nervous system (CNS) drug developing, drug delivery into brain across the blood-brain barrier (BBB) is a serious problem. However, this problem can be also solved by the use of the transport systems at the BBB. Therefore, transporter-consciously designed drugs not only may effectively elicit activity but also may control adverse side effects caused by off-targets and drug-drug interactions and, consequently, may show good performance in clinical trials. In this review, I introduce possibilities and advantages of transporter-conscious drug designs.

Astrocyte-neuron interplay in maladaptive plasticity

The complexity of neuronal networks cannot only be explained by neuronal activity so neurobiological research in the last decade has focused on different components of the central nervous system: the glia. Glial cells are fundamental elements for development and maintenance of physiological brain work. New data confirm that glia significantly influences neuronal communication through specific molecules, named "gliotransmitters", and their related receptors. This new approach to the traditional model of the way synapses work is also supported by changes occurring in pathological conditions, such as neurodegenerative diseases or toxic/traumatic injury to nervous system. Experimental models have revealed that glial cells are the starting point of damage progression that subsequently involves neurons. The "bedside to bench" approach has demonstrated that clinical phenotypes are strictly related to neuronal death, however it is conceivable that the disease begins earlier, years before clinical onset. This temporal gap is necessary to determine complex changes in the neuro-glial network organization and produce a "maladaptive plasticity". We review the function of glial cells in health and disease, pointing the putative mechanisms of maladaptive plasticity, suggesting that glial cells may represent a fascinating therapeutic target to prevent irreversible neuronal cell death.

Topology of the yeast Ras converting enzyme as inferred from cysteine accessibility studies

The Ras converting enzyme (Rce1p) is an endoprotease that is involved in the post-translational processing of the Ras GTPases and other isoprenylated proteins. Its role in Ras biosynthesis marks Rce1p as an anticancer target. By assessing the chemical accessibility of cysteine residues substituted throughout the Saccharomyces cerevisiae Rce1p sequence, we have determined that yeast Rce1p has eight segments that are protected from chemical modification. Notably, the three residues that are essential for yeast Rce1p function (E156, H194, and H248) are all chemically inaccessible and associated with separate protected segments. By specifically assessing the chemical reactivity and glycosylation potential of the NH2 and COOH termini of Rce1p, we further demonstrate that Rce1p has an odd number of transmembrane spans. Substantial evidence that the most NH2-terminal segment functions as a transmembrane segment with the extreme NH2 terminus projecting into the endoplasmic reticulum (ER) lumen is presented. Because each of the remaining seven segments is too short to contain two spans and is flanked by chemically reactive positions, we infer that these segments are not transmembrane segments but rather represent compact structural features and/or hydrophobic loops that penetrate but do not fully span the bilayer (i.e., re-entrant helices). We thus propose a topological model in which yeast Rce1p contains a single transmembrane helix localized at its extreme NH2 terminus and one or more re-entrant helices and/or compact structural domains that populate the cytosolic face of the ER membrane. Lastly, we demonstrate that the natural cysteine residues of Rce1p are chemically inaccessible and fully dispensable for in vivo enzyme activity, formally eliminating the possibility of a cysteine-based enzymatic mechanism for this protease.

Determinants of coupled transport and uncoupled current by the electrogenic SLC26 transporters

Members of the SLC26 family of anion transporters mediate the transport of diverse molecules ranging from halides to carboxylic acids and can function as coupled transporters or as channels. A unique feature of the two members of the family, Slc26a3 and Slc26a6, is that they can function as both obligate coupled and mediate an uncoupled current, in a channel-like mode, depending on the transported anion. To identify potential features that control the two modes of transport, we performed in silico modeling of Slc26a6, which suggested that the closest potential fold similarity of the Slc26a6 transmembrane domains is to the CLC transporters, despite their minimal sequence identity. Examining the predicted Slc26a6 fold identified a highly conserved glutamate (Glu⁻; Slc26a6(E357)) with the predicted spatial orientation similar to that of the CLC-ec1 E148, which determines coupled or uncoupled transport by CLC-ec1. This raised the question of whether the conserved Glu⁻ in Slc26a6(E357) and Slc26a3(E367) have a role in the unique transport modes by these transporters. Reversing the Glu⁻ charge in Slc26a3 and Slc26a6 resulted in the inhibition of all modes of transport. However, most notably, neutralizing the charge in Slc26a6(E357A) eliminated all forms of coupled transport without affecting the uncoupled current. The Slc26a3(E367A) mutation markedly reduced the coupled transport and converted the stoichiometry of the residual exchange from 2Cl⁻/1HCO₃⁻ to 1Cl⁻/1HCO₃⁻, while completely sparing the current. These findings suggest the possibility that similar structural motif may determine multiple functional modes of these transporters.

Capturing Functional Motions of Membrane Channels and Transporters with Molecular Dynamics Simulation

Conformational changes of proteins are involved in all aspects of protein function in biology. Almost all classes of proteins respond to changes in their environment, ligand binding, and interaction with other proteins and regulatory agents through undergoing conformational changes of various degrees and magnitudes. Membrane channels and transporters are the major classes of proteins that are responsible for mediating efficient and selective transport of materials across the cellular membrane. Similar to other proteins, they take advantage of conformational changes to make transitions between various functional states. In channels, large-scale conformational changes are mostly involved in the process of "gating", i.e., opening and closing of the pore of the channel protein in response to various signals. In transporters, conformational changes constitute various steps of the conduction process, and, thus, are more closely integrated in the transport process. Owing to significant progress in developing highly efficient parallel algorithms in molecular dynamics simulations and increased computational resources, and combined with the availability of high-resolution, atomic structures of membrane proteins, we are in an unprecedented position to use computer simulation and modeling methodologies to investigate the mechanism of function of membrane channels and transporters. While the entire transport cycle is still out of reach of current methodologies, many steps involved in the function of transport proteins have been characterized with molecular dynamics simulations. Here, we present several examples of such studies from our laboratory, in which functionally relevant conformational changes of membrane channels and transporters have been characterized using extended simulations.

Dynamics of the extracellular gate and ion-substrate coupling in the glutamate transporter

Glutamate transporters (GluTs) are the primary regulators of extracellular concentration of the neurotransmitter glutamate in the central nervous system. In this study, we have investigated the dynamics and coupling of the substrate and Na(+) binding sites, and the mechanism of cotransport of Na(+) ions, using molecular dynamics simulations of a membrane-embedded model of GluT in its apo (empty form) and various Na(+)- and/or substrate-bound states. The results shed light on the mechanism of the extracellular gate and on the sequence of binding of the substrate and Na(+) ions to GluT during the transport cycle. The results suggest that the helical hairpin HP2 plays the key role of the extracellular gate for the substrate binding site, and that the opening and closure of the gate is controlled by substrate binding. GluT adopts an open conformation in the absence of the substrate exposing the binding sites of the substrate and Na(+) ions to the extracellular solution. Based on the calculated trajectories, we propose that Na1 is the first element to bind GluT, as it is found to be important for the completion of the substrate binding site. The subsequent binding of the substrate, in turn, is shown to result in an almost complete closure of the extracellular gate and the formation of the Na2 binding site. Finally, binding of Na2 locks the extracellular gate and completes the formation of the occluded state of GluT.

Caenorhabditis elegans Glutamate Transporters Influence Synaptic Function and Behavior at Sites Distant from the Synapse

To ensure precise neurotransmission and prevent neurotoxic accumulation, l-glutamate (Glu), the major excitatory neurotransmitter in the brain, is cleared from the synapse by glutamate transporters (GluTs). The molecular components of Glu synapses are highly conserved between Caenorhabditis elegans and mammals, yet the absence of synaptic insulation in C. elegans raises fundamental questions about Glu clearance strategies in the nematode nervous system. To gain insight into how Glu clearance is accomplished and how GluTs impact neurotransmission, we probed expression and function of all 6 GluTs found in the C. elegans genome. Disruption of each GluT impacts multiple Glu-dependent behaviors, with GluT combinations commonly increasing the severity of behavioral deficits. Interestingly, the sole GluT that we find expressed in neurons is localized predominantly in presynaptic neurons, in contrast to the postsynaptic concentration of neuronal GluTs typical in mammals. Moreover, 3 of the 6 GluT genes appear strongly expressed on the capillary excretory canal cell, where they affect Glu-dependent behaviors from positions distal to glutamatergic circuits. Indeed, our focused study of GLT-3, one of the distally expressed GluTs, shows that despite this distance, GLT-3 function can balance the activity mediated by synaptic release and synaptic receptors. The effects of distal GluTs on glutamatergic circuits support that Glu diffusion outside the vicinity of the synapse is a critical factor in C. elegans neurotransmission. Together with the presynaptic localization of neuronal GluTs, these observations suggest an unusual strategy for Glu clearance in C. elegans.

Uncoupling and turnover in a Cl-/H+ exchange transporter

The CLC-family protein CLC-ec1, a bacterial homologue of known structure, stoichiometrically exchanges two Cl⁻ for one H⁺ via an unknown membrane transport mechanism. This study examines mutations at a conserved tyrosine residue, Y445, that directly coordinates a Cl⁻ ion located near the center of the membrane. Mutations at this position lead to “uncoupling,” such that the H⁺/Cl⁻ transport ratio decreases roughly with the volume of the substituted side chain. The uncoupled proteins are still able to pump protons uphill when driven by a Cl⁻ gradient, but the extent and rate of this H⁺ pumping is weaker in the more uncoupled variants. Uncoupling is accompanied by conductive Cl⁻ transport that is not linked to counter-movement of H⁺, i.e., a “leak.” The unitary Cl⁻ transport rate, measured in reconstituted liposomes by both a conventional initial-velocity method and a novel Poisson dilution approach, is ∼4,000 s⁻¹ for wild-type protein, and the uncoupled mutants transport Cl⁻ at similar rates.

Structure and function of sodium-coupled GABA and glutamate transporters

Neurotransmitter transporters are key elements in the termination of the synaptic actions of the neurotransmitters. They use the energy stored in the electrochemical ion gradients across the plasma membrane of neurons and glial cells for uphill transport of the transmitters into the cells surrounding the synapse. Therefore specific transporter inhibitors can potentially be used as novel drugs for neurological disease. Sodium-coupled neurotransmitter transporters belong to either of two distinct families. The glutamate transporters belong to the SLC1 family, whereas the transporters of the other neurotransmitters belong to the SLC6 family. An exciting and recent development is the emergence of the first high-resolution structures of archeal and bacterial members belonging to these two families. In this review the functional results on prototypes of the two families, the GABA transporter GAT-1 and the glutamate transporters GLT-1 and EAAC1, are described and discussed within the perspective provided by the novel structures.

Coupling substrate and ion binding to extracellular gate of a sodium-dependent aspartate transporter

Secondary transporters are integral membrane proteins that catalyse the movement of substrate molecules across the lipid bilayer by coupling substrate transport to one or more ion gradients, thereby providing a mechanism for the concentrative uptake of substrates. Here we describe crystallographic and thermodynamic studies of Glt(Ph), a sodium (Na+)-coupled aspartate transporter, defining sites for aspartate, two sodium ions and d,l-threo-beta-benzyloxyaspartate, an inhibitor. We further show that helical hairpin 2 is the extracellular gate that controls access of substrate and ions to the internal binding sites. At least two sodium ions bind in close proximity to the substrate and these sodium-binding sites, together with the sodium-binding sites in another sodium-coupled transporter, LeuT, define an unwound alpha-helix as the central element of the ion-binding motif, a motif well suited to the binding of sodium and to participation in conformational changes that accompany ion binding and unbinding during the transport cycle.

Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement

L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.

Structural classification and prediction of reentrant regions in alpha-helical transmembrane proteins: application to complete genomes

Alongside the well-studied membrane spanning helices, alpha-helical transmembrane (TM) proteins contain several functionally and structurally important types of substructures. Here, existing 3D structures of transmembrane proteins have been used to define and study the concept of reentrant regions, i.e. membrane penetrating regions that enter and exit the membrane on the same side. We find that these regions can be divided into three distinct categories based on secondary structure motifs, namely long regions with a helix-coil-helix motif, regions of medium length with the structure helix-coil or coil-helix and regions of short to medium length consisting entirely of irregular secondary structure. The residues situated in reentrant regions are significantly smaller on average compared to other regions and reentrant regions can be detected in the inter-transmembrane loops with an accuracy of approximately 70% based on their amino acid composition. Using TOP-MOD, a novel method for predicting reentrant regions, we have scanned the genomes of Escherichia coli, Saccharomyces cerevisiae and Homo sapiens. The results suggest that more than 10% of transmembrane proteins contain reentrant regions and that the occurrence of reentrant regions increases linearly with the number of transmembrane regions. Reentrant regions seem to be most commonly found in channel proteins and least commonly in signal receptors.

Coupling modes and stoichiometry of Cl-/HCO3- exchange by slc26a3 and slc26a6

The SLC26 transporters are a family of mostly luminal Cl⁻ and HCO₃⁻ transporters. The transport mechanism and the Cl⁻/HCO₃⁻ stoichiometry are not known for any member of the family. To address these questions, we simultaneously measured the HCO₃⁻ and Cl⁻ fluxes and the current or membrane potential of slc26a3 and slc26a6 expressed in Xenopus laevis oocytes and the current of the transporters expressed in human embryonic kidney 293 cells. slc26a3 mediates a coupled 2Cl⁻/1HCO₃⁻ exchanger. The membrane potential modulated the apparent affinity for extracellular Cl⁻ of Cl⁻/HCO₃⁻ exchange by slc26a3. Interestingly, the replacement of Cl⁻ with NO₃⁻ or SCN⁻ uncoupled the transport, with large NO₃⁻ and SCN⁻ currents and low HCO₃⁻ transport. An apparent uncoupled current was also developed during the incubation of slc26a3-expressing oocytes in HCO₃⁻-buffered Cl⁻-free media. These findings were used to develop a turnover cycle for Cl⁻ and HCO₃⁻ transport by slc26a3. Cl⁻ and HCO₃⁻ flux measurements revealed that slc26a6 mediates a 1Cl⁻/2HCO₃⁻ exchange. Accordingly, holding the membrane potential at 40 and −100 mV accelerated and inhibited, respectively, Cl⁻-mediated HCO₃⁻ influx, and holding the membrane potential at −100 mV increased HCO₃⁻-mediated Cl⁻ influx. These findings indicate that slc26a6 functions as a coupled 1Cl⁻/2HCO₃⁻ exchanger. The significance of isoform-specific Cl⁻ and HCO₃⁻ transport stoichiometry by slc26a3 and slc26a6 is discussed in the context of diseases of epithelial Cl⁻ absorption and HCO₃⁻ secretion.

Sequence and hydropathy profile analysis of two classes of secondary transporters

A structural class in the MemGen classification of membrane proteins is a set of evolutionary related proteins sharing a similar global fold. A structural class contains both closely related pairs of proteins for which homology is clear from sequence comparison and very distantly related pairs, for which it is not possible to establish homology based on sequence similarity alone. In the latter case the evolutionary link is based on hydropathy profile analysis. Here, we use these evolutionary related sets of proteins to analyze the relationship between E-values in BLAST searches, sequence similarities in multiple sequence alignments and structural similarities in hydropathy profile analyses. Two structural classes of secondary transporters termed ST[3], which includes the Ion Transporter (IT) superfamily and ST[4], which includes the DAACS family (TC# 2.A.23) were extracted from the NCBI protein database. ST[3] contains 2051 unique sequences distributed over 32 families and 59 subfamilies. ST[4] is a smaller class containing 399 unique sequences distributed over 2 families and 7 subfamilies. One subfamily in ST[4] contains a new class of binding protein dependent secondary transporters. Comparison of the averaged hydropathy profiles of the subfamilies in ST[3] and ST[4] revealed that the two classes represent different folds. Divergence of the sequences in ST[4] is much smaller than observed in ST[3], suggesting different constraints on the proteins during evolution. Analysis of the correlation between the evolutionary relationship of pairs of proteins in a class and the BLAST E-value revealed that: (i) the BLAST algorithm is unable to pick up the majority of the links between proteins in structural class ST[3], (ii) "low complexity filtering" and "composition based statistics" improve the specificity, but strongly reduce the sensitivity of BLAST searches for distantly related proteins, indicating that these filters are too stringent for the proteins analyzed, and (iii) the E-value cut-off, which may be used to evaluate evolutionary significance of a hit in a BLAST search is very different for the two structural classes of membrane proteins.

Small-scale molecular motions accomplish glutamate uptake in human glutamate transporters

Glutamate transporters remove glutamate from the synaptic cleft to maintain efficient synaptic communication between neurons and to prevent glutamate concentrations from reaching neurotoxic levels. Glutamate transporters play an important role in ischemic neuronal death during stroke and have been implicated in epilepsy and amytropic lateral sclerosis. However, the molecular structure and the glutamate-uptake mechanism of these transporters are not well understood. The most recent models of glutamate transporters have three or five subunits, each with eight transmembrane domains, and one or two membrane-inserted loops. Here, using fluorescence resonance energy transfer (FRET) analysis, we have determined the relative position of the extracellular regions of these domains. Our results are consistent with a trimeric glutamate transporter with a large (>45 A) extracellular vestibule. In contrast to other transport proteins, our FRET measurements indicate that there are no large-scale motions in glutamate transporters and that glutamate uptake is accompanied by relatively small motions around the glutamate-binding sites. The large extracellular vestibule and the small-scale conformational changes could contribute to the fast kinetics predicted for glutamate transporters. Furthermore, we show that, despite the multimeric nature of glutamate transporters, the subunits function independently.

The substituted aspartate analogue L-beta-threo-benzyl-aspartate preferentially inhibits the neuronal excitatory amino acid transporter EAAT3

The excitatory amino acid transporters (EAATs) play key roles in the regulation of CNS L-glutamate, especially related to synthesis, signal termination, synaptic spillover, and excitotoxic protection. Inhibitors available to delineate EAAT pharmacology and function are essentially limited to those that non-selectively block all EAATs or those that exhibit a substantial preference for EAAT2. Thus, it is difficult to selectively study the other subtypes, particularly EAAT1 and EAAT3. Structure activity studies on a series of beta-substituted aspartate analogues identify L-beta-benzyl-aspartate (L-beta-BA) as among the first blockers that potently and preferentially inhibits the neuronal EAAT3 subtype. Kinetic analysis of D-[(3)H]aspartate uptake into C17.2 cells expressing the hEAATs demonstrate that L-beta-threo-BA is the more potent diastereomer, acts competitively, and exhibits a 10-fold preference for EAAT3 compared to EAAT1 and EAAT2. Electrophysiological recordings of EAAT-mediated currents in Xenopus oocytes identify L-beta-BA as a non-substrate inhibitor. Analyzing L-beta-threo-BA within the context of a novel EAAT2 pharmacophore model suggests: (1) a highly conserved positioning of the electrostatic carboxyl and amino groups; (2) nearby regions that accommodate select structural modifications (cyclopropyl rings, methyl groups, oxygen atoms); and (3) a unique region L-beta-threo-BA occupied by the benzyl moieties of L-TBOA, L-beta-threo-BA and related analogues. It is plausible that the preference of L-beta-threo-BA and L-TBOA for EAAT3 and EAAT2, respectively, could reside in the latter two pharmacophore regions.

How did the neurotransmitter cross the bilayer? A closer view

Plasma membrane neurotransmitter transporters for monoamines, GABA, glycine and excitatory amino acids are homologous to two sizable families of bacterial amino acid transporters. Recently, a high resolution structure was determined for a thermophilic glutamate transporter. Also, a bacterial tryptophan transporter related to the family of biogenic amine neurotransmitter transporters was functionally expressed. Structural insights from these and other bacterial transporters will help to rationalize the mechanisms for the increasingly complex functions that have been described for mammalian transporters, in addition to their modes of regulation. We touch on recent insights into the functions of neurotransmitter transporters in their physiological contexts.

Mutation in the glutamate transporter EAAT1 causes episodic ataxia, hemiplegia, and seizures

BACKGROUND

Transporters, ion pumps, and ion channels are membrane proteins that regulate selective permeability and maintain ionic gradients across cell membranes. Mutations in CACNA1A encoding a neuronal calcium channel and ATP1A2 encoding an ion pump cause episodic ataxia, hemiplegic migraine, and seizures. Mutant gene products of both CACNA1A and ATP1A2 may affect neurotransmission of glutamate, the most abundant excitatory amino acid neurotransmitter.

METHODS

We examined our patient population with episodic ataxia and hemiplegic migraine but with no mutation in either CACNA1A or ATP1A2. We looked for mutations in SLC1A3, which encodes the glutamate transporter excitatory amino acid transporter (EAAT) 1 that is important in removing glutamate from the synaptic cleft.

RESULTS

A patient with episodic ataxia, seizures, migraine, and alternating hemiplegia has a heterozygous mutation in SLC1A3 that is not present in his asymptomatic parents and controls. Expression studies of the mutant EAAT1 showed decreased expression of the protein with a markedly reduced capacity for glutamate uptake. When coexpressed, the mutant EAAT1 decreased the activity of wild-type EAAT1 but not of two other transporters EAAT2 or EAAT3, suggesting that mutant EAAT1 specifically multimerizes with wild-type EAAT1 to exert its dominant negative effect.

CONCLUSION

Our data show that a heterozygous mutation in EAAT1 can lead to decreased glutamate uptake, which can contribute to neuronal hyperexcitability to cause seizures, hemiplegia, and episodic ataxia.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

Structure of a glutamate transporter homologue from Pyrococcus horikoshii

Glutamate transporters are integral membrane proteins that catalyse the concentrative uptake of glutamate from the synapse to intracellular spaces by harnessing pre-existing ion gradients. In the central nervous system glutamate transporters are essential for normal development and function, and are implicated in stroke, epilepsy and neurodegenerative diseases. Here we present the crystal structure of a eukaryotic glutamate transporter homologue from Pyrococcus horikoshii. The transporter is a bowl-shaped trimer with a solvent-filled extracellular basin extending halfway across the membrane bilayer. At the bottom of the basin are three independent binding sites, each cradled by two helical hairpins, reaching from opposite sides of the membrane. We propose that transport of glutamate is achieved by movements of the hairpins that allow alternating access to either side of the membrane.

Molecular pharmacology of glutamate transporters, EAATs and VGLUTs

L-Glutamate serves as a major excitatory neurotransmitter in the mammalian central nervous system (CNS) and is stored in synaptic vesicles by an uptake system that is dependent on the proton electrochemical gradient (VGLUTs). Following its exocytotic release, glutamate activates fast-acting, excitatory ionotropic receptors and slower-acting metabotropic receptors to mediate neurotransmission. Na+-dependent glutamate transporters (EAATs) located on the plasma membrane of neurons and glial cells rapidly terminate the action of glutamate and maintain its extracellular concentration below excitotoxic levels. Thus far, five Na+-dependent glutamate transporters (EAATs 1-5) and three vesicular glutamate transporters (VGLUTs 1-3) have been identified. Examination of EAATs and VGLUTs in brain preparations and by heterologous expression of the various cloned subtypes shows these two transporter families differ in many of their functional properties including substrate specificity and ion requirements. Alterations in the function and/or expression of these carriers have been implicated in a range of psychiatric and neurological disorders. EAATs have been implicated in cerebral stroke, epilepsy, Alzheimer's disease, HIV-associated dementia, Huntington's disease, amyotrophic lateral sclerosis (ALS) and malignant glioma, while VGLUTs have been implicated in schizophrenia. To examine the physiological role of glutamate transporters in more detail, several classes of transportable and non-transportable inhibitors have been developed, many of which are derivatives of the natural amino acids, aspartate and glutamate. This review summarizes the development of these indispensable pharmacological tools, which have been critical to our understanding of normal and abnormal synaptic transmission.

Activation of glutamate transporters in rods inhibits presynaptic calcium currents

We found that L-glutamate (L-Glu) inhibits L-type Ca2+currents (ICa) in rod photoreceptors. This inhibition was studied in isolated rods or rods in retinal slices from tiger salamander using perforated patch whole cell recordings and Cl−-imaging techniques. Application of L-Glu inhibitedICaby ∼20% at 0.1 mM and ∼35% at 1 mM. L-Glu also produced an inward current that reversed aroundECl. The metabotropic glutamate receptor (mGluR) agonists t-ADA (Group I), DCG-IV (Group II), and L-AP4 (Group III) had no effect onICa. However, the glutamate transport inhibitor, TBOA (0.1 mM), prevented L-Glu from inhibitingICa. D-aspartate (D-Asp), a glutamate transporter substrate, also inhibitedICawith significantly more inhibition at 1 mM than 0.1 mM. Using Cl−imaging, L-Glu (0.1–1 mM) and D-Asp (0.1–1 mM) were found to stimulate a Cl−efflux from terminals of isolated rods whereas the ionotropic glutamate receptor agonists NMDA, AMPA, and kainate and the mGluR agonist, 1S,3R-ACPD, did not. Glutamate-evoked Cl−effluxes were blocked by the glutamate transport inhibitors TBOA and DHKA. Cl−efflux inhibits Ca2+channel activity in rod terminals (Thoreson et al. (2000),Visual Neuroscience17, 197). Consistent with the possibility that glutamate-evoked Cl−efflux may play a role in the inhibition, reducing intraterminal Cl−prevented L-Glu from inhibitingICa. In summary, the results indicate that activation of glutamate transporters inhibitsICain rods possibly as a consequence of Cl−efflux. The neurotransmitter L-Glu released from rod terminals might thus provide a negative feedback signal to inhibit further L-Glu release.

Trimeric Subunit Stoichiometry of the Glutamate Transporters fromBacillus caldotenaxandBacillus stearothermophilus†

Catalysis of glutamate transport across cell membranes and coupling of the concentrative transport to sodium, proton, and potassium gradients are processes fundamental to organisms in all kingdoms of life. In bacteria, glutamate transporters participate in nutrient uptake, while in eukaryotic organisms, the transporters clear glutamate from the synaptic cleft. Even though glutamate transporters are crucial to the viability of many life forms, little is known about their structure and quaternary organization. In particular, the subunit stoichiometry of these polytopic integral membrane proteins has not been unequivocally defined. Determination of the native molecular mass of membrane proteins is complicated by their lability in detergent micelles and by their association with detergent and/or lipid molecules. Here we report the purification of glutamate transporters from Bacillus caldotenax and Bacillus stearothermophilus in a monodisperse, detergent-solubilized state. Characterization of both transporters either by chemical cross-linking and mass spectrometry or by size-exclusion chromatography and in-line laser light scattering, refractive index, and ultraviolet absorption measurements shows that the transporters have a trimeric quaternary structure. Limited proteolysis further defines regions of primary structure that are exposed to aqueous solution. Together, our results define the subunit stoichiometry of high-affinity glutamate transporters from B. caldotenax and B. stearothermophilus and localize exposed and accessible elements of primary structure. Because of the close amino acid sequence relationship between bacterial and eukaryotic transporters, our results are germane to prokaryotic and eukaryotic glutamate and neutral amino acid transporters.

Substrate inhibitors and blockers of excitatory amino acid transporters in the treatment of neurodegeneration: critical considerations

Excessive glutamate release (mediated by reversed uptake) or impaired reuptake contributes to the etiopathology of many neurodegenerative disorders. Thus great effort has been devoted to the discovery of agents that can interfere with high-affinity Na+-dependent glutamate transport, with the aim of finding new therapeutics against neurodegenerative diseases. We developed two different 3D-pharmacophore models for substrate inhibitors and blockers, which led to the rational design of novel and potent glutamate and aspartate analogues that selectively interact with excitatory amino acid transporters (EAAT). Our results indicated that all analysed EAAT ligands share the same orientation of the acidic functions and the protonatable nitrogen, even though the distance between the carboxylic carbons varies from 3.7 to 4.9 A. This distance does not discriminate between substrate inhibitors and blockers, but between glutamate and aspartate derivatives. In contrasts differences in the volume distribution of the rest of the molecule with respect to the axis connecting the two carboxylic groups are responsible for the difference in activity between transportable and nontransportable inhibitors. Thus our 3D receptor interaction model for EAAT substrates and nontransportable inhibitors could lead to the rational design of selective EAAT ligands as possible neuroprotective agents. However, some critical points, such as which glutamate transporter is present on glutamatergic nerve terminals and which glutamate transporter mediates reversed glutamate uptake, still remain to be elucidated.

[Pharmacology of excitatory amino acid transporters (EAATs and VGLUTs)]

L-Glutamate is a major excitatory neurotransmitter in the mammalian central nervous system (CNS). It contributes not only to fast synaptic neurotransmission but also to complex physiological processes like plasticity, learning, and memory. Glutamate is synthesized in the cytoplasm and stored in synaptic vesicles by a proton gradient-dependent uptake system (VGLUTs). Following its exocytotic release, glutamate activates different kinds of glutamate receptors and mediates excitatory neurotransmission. To terminate the action of glutamate and maintain its extracellular concentration below excitotoxic levels, glutamate is quickly removed by Na(+)-dependent glutamate transporters (EAATs). Recently, three vesicular glutamate transporters (VGLUT1-3) and five Na(+)-dependent glutamate transporters (EAAT1-5) were identified. VGLUTs and EAATs are thought to play important roles in neuronal disorders, such as amyotrophic lateral sclerosis, Alzheimer's disease, cerebral ischemia, and Huntington's disease. In this review, the development of new compounds to regulate the function of VGLUTs and EAATs will be described.