The glutamate-activated anion conductance in excitatory amino acid transporters is gated independently by the individual subunits

Identification of Polymorphisms in EAAT1 Glutamate Transporter Gene SLC1A3 Associated with Reduced Migraine Risk

Dysfunction in ion channels or processes involved in maintaining ionic homeostasis is thought to lower the threshold for cortical spreading depression (CSD), and plays a role in susceptibility to associated neurological disorders, including pathogenesis of a migraine. Rare pathogenic variants in specific ion channels have been implicated in monogenic migraine subtypes. In this study, we further examined the channelopathic nature of a migraine through the analysis of common genetic variants in three selected ion channel or transporter genes: SLC4A4, SLC1A3, and CHRNA4. Using the Agena MassARRAY platform, 28 single-nucleotide polymorphisms (SNPs) across the three candidate genes were genotyped in a case-control cohort comprised of 182 migraine cases and 179 matched controls. Initial results identified significant associations between migraine and rs3776578 (p = 0.04) and rs16903247 (p = 0.05) genotypes within the SLC1A3 gene, which encodes the EAAT1 glutamate transporter. These SNPs were subsequently genotyped in an independent cohort of 258 migraine cases and 290 controls using a high-resolution melt assay, and association testing supported the replication of initial findings-rs3776578 (p = 0.0041) and rs16903247 (p = 0.0127). The polymorphisms are in linkage disequilibrium and localise within a putative intronic enhancer region of SLC1A3. The minor alleles of both SNPs show a protective effect on migraine risk, which may be conferred via influencing the expression of SLC1A3.

Comprehensive review of amino acid transporters as therapeutic targets

The solute carrier (SLC) family, with more than 400 membrane-bound proteins, facilitates the transport of a wide array of substrates such as nutrients, ions, metabolites, and drugs across biological membranes. Amino acid transporters (AATs) are membrane transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs participate in many important physiological functions including nutrient supply, metabolic transformation, energy homeostasis, redox regulation, and neurological regulation. Several AATs have been found to significantly impact the progression of human malignancies, and dysregulation of AATs results in metabolic reprogramming affecting tumor growth and progression. However, current clinical therapies that directly target AATs have not been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, the molecular mechanisms in human diseases such as tumors, kidney diseases, and emerging therapeutic strategies for targeting AATs.

Symport and antiport mechanisms of human glutamate transporters

Excitatory amino acid transporters (EAATs) uptake glutamate into glial cells and neurons. EAATs achieve million-fold transmitter gradients by symporting it with three sodium ions and a proton, and countertransporting a potassium ion via an elevator mechanism. Despite the availability of structures, the symport and antiport mechanisms still need to be clarified. We report high-resolution cryo-EM structures of human EAAT3 bound to the neurotransmitter glutamate with symported ions, potassium ions, sodium ions alone, or without ligands. We show that an evolutionarily conserved occluded translocation intermediate has a dramatically higher affinity for the neurotransmitter and the countertransported potassium ion than outward- or inward-facing transporters and plays a crucial role in ion coupling. We propose a comprehensive ion coupling mechanism involving a choreographed interplay between bound solutes, conformations of conserved amino acid motifs, and movements of the gating hairpin and the substrate-binding domain.

Evaluation of the Key Advantages between Two Modalities of Boronophenylalanine Administration for Clinical Boron Neutron Capture Therapy Using an Animal Model

In clinical boron neutron capture therapy (BNCT), boronophenylalanine (BPA) administrations through one-step infusion (OSI) and two-step infusion (TSI) are the most widely used. This study compared the advantages of OSI and TSI using a human oral squamous cell carcinoma-bearing animal model. OSI was administered at a high-dose rate of 20 mg/kg/min for 20 min (total dose: 400 mg/kg) as the first step infusion. TSI was a prolonged infusion at a low-dose rate of 1.67 mg/kg/min for 15, 30, 45, and 60 min (total dose: 25, 50, 75, and 100 mg/kg) following the first step infusion. The sigmoid Emax model was used to evaluate the boron accumulation effect in the tumor. The advantages of TSI were observed to be greater than those of OSI. The observed advantages of TSI were as follows: a stable level of boron concentration in blood; tumor to blood boron ratio (T/B); tumor to muscle boron ratio (T/M); and skin to blood boron ratio (S/B). The boron accumulation effect in tumors increased to 68.98%. Thus, effective boron concentration in these tumor cells was achieved to enhance the lethal damage in BNCT treatment. Boron concentration in the blood was equal to that in the skin. Therefore, the equivalent dose was accurately estimated for the skin.

Structural basis of ligand binding modes of human EAAT2

In the central nervous system (CNS), excitatory amino acid transporters (EAATs) mediate the uptake of excitatory neurotransmitter glutamate and maintain its low concentrations in the synaptic cleft for avoiding neuronal cytotoxicity. Dysfunction of EAATs can lead to many psychiatric diseases. Here we report cryo-EM structures of human EAAT2 in an inward-facing conformation, in the presence of substrate glutamate or selective inhibitor WAY-213613. The glutamate is coordinated by extensive hydrogen bonds and further stabilized by HP2. The inhibitor WAY-213613 occupies a similar binding pocket to that of the substrate glutamate. Upon association with the WAY-213613, the HP2 undergoes a substantial conformational change, and in turn stabilizes the inhibitor binding by forming hydrophobic interactions. Electrophysiological experiments elucidate that the unique S441 plays pivotal roles in the binding of hEAAT2 with glutamate or WAY-213613, and the I464-L467-V468 cluster acts as a key structural determinant for the selective inhibition of this transporter by WAY-213613.

The archaeal glutamate transporter homologue GltPh shows heterogeneous substrate binding

Integral membrane glutamate transporters couple the concentrative substrate transport to ion gradients. There is a wealth of structural and mechanistic information about this protein family. Recent studies of an archaeal homologue, GltPh, revealed transport rate heterogeneity, which is inconsistent with simple kinetic models; however, its structural and mechanistic determinants remain undefined. Here, we demonstrate that in a mutant GltPh, which exclusively populates the outward-facing state, at least two substates coexist in slow equilibrium, binding the substrate with different apparent affinities. Wild type GltPh shows similar binding properties, and modulation of the substate equilibrium correlates with transport rates. The low-affinity substate of the mutant is transient following substrate binding. Consistently, cryo-EM on samples frozen within seconds after substrate addition reveals the presence of structural classes with perturbed helical packing of the extracellular half of the transport domain in regions adjacent to the binding site. By contrast, an equilibrated structure does not show such classes. The structure at 2.2-Å resolution details a pattern of waters in the intracellular half of the domain and resolves classes with subtle differences in the substrate-binding site. We hypothesize that the rigid cytoplasmic half of the domain mediates substrate and ion recognition and coupling, whereas the extracellular labile half sets the affinity and dynamic properties.

Cellular Physiology and Pathophysiology of EAAT Anion Channels

Excitatory amino acid transporters (EAATs) optimize the temporal resolution and energy demand of mammalian excitatory synapses by quickly removing glutamate from the synaptic cleft into surrounding neuronal and glial cells and ensuring low resting glutamate concentrations. In addition to secondary active glutamate transport, EAATs also function as anion channels. The channel function of these transporters is conserved in all homologs ranging from archaebacteria to mammals; however, its physiological roles are insufficiently understood. There are five human EAATs, which differ in their glutamate transport rates. Until recently the high-capacity transporters EAAT1, EAAT2, and EAAT3 were believed to conduct only negligible anion currents, with no obvious function in cell physiology. In contrast, the low-capacity glutamate transporters EAAT4 and EAAT5 are thought to regulate neuronal signaling as glutamate-gated channels. In recent years, new experimental approaches and novel animal models, together with the discovery of a human genetic disease caused by gain-of-function mutations in EAAT anion channels have enabled identification of the first physiological and pathophysiological roles of EAAT anion channels.

Glutamatergic system components as potential biomarkers and therapeutic targets in cancer in non-neural organs

Glutamate is one of the most abundant amino acids in the blood. Besides its role as a neurotransmitter in the brain, it is a key substrate in several metabolic pathways and a primary messenger that acts through its receptors outside the central nervous system (CNS). The two main types of glutamate receptors, ionotropic and metabotropic, are well characterized in CNS and have been recently analyzed for their roles in non-neural organs. Glutamate receptor expression may be particularly important for tumor growth in organs with high concentrations of glutamate and might also influence the propensity of such tumors to set metastases in glutamate-rich organs, such as the liver. The study of glutamate transporters has also acquired relevance in the physiology and pathologies outside the CNS, especially in the field of cancer research. In this review, we address the recent findings about the expression of glutamatergic system components, such as receptors and transporters, their role in the physiology and pathology of cancer in non-neural organs, and their possible use as biomarkers and therapeutic targets.

Resting and stimulated mouse rod photoreceptors show distinct patterns of vesicle release at ribbon synapses

The vertebrate visual system can detect and transmit signals from single photons. To understand how single-photon responses are transmitted, we characterized voltage-dependent properties of glutamate release in mouse rods. We measured presynaptic glutamate transporter anion current and found that rates of synaptic vesicle release increased with voltage-dependent Ca2+ current. Ca2+ influx and release rate also rose with temperature, attaining a rate of ∼11 vesicles/s/ribbon at -40 mV (35°C). By contrast, spontaneous release events at hyperpolarized potentials (-60 to -70 mV) were univesicular and occurred at random intervals. However, when rods were voltage clamped at -40 mV for many seconds to simulate maintained darkness, release occurred in coordinated bursts of 17 ± 7 quanta (mean ± SD; n = 22). Like fast release evoked by brief depolarizing stimuli, these bursts involved vesicles in the readily releasable pool of vesicles and were triggered by the opening of nearby ribbon-associated Ca2+ channels. Spontaneous release rates were elevated and bursts were absent after genetic elimination of the Ca2+ sensor synaptotagmin 1 (Syt1). This study shows that at the resting potential in darkness, rods release glutamate-filled vesicles from a pool at the base of synaptic ribbons at low rates but in Syt1-dependent bursts. The absence of bursting in cones suggests that this behavior may have a role in transmitting scotopic responses.

Kinetic mechanism of Na+-coupled aspartate transport catalyzed by GltTk

It is well-established that the secondary active transporters Glt_Tk and Glt_Ph catalyze coupled uptake of aspartate and three sodium ions, but insight in the kinetic mechanism of transport is fragmentary. Here, we systematically measured aspartate uptake rates in proteoliposomes containing purified Glt_Tk, and derived the rate equation for a mechanism in which two sodium ions bind before and another after aspartate. Re-analysis of existing data on Glt_Ph using this equation allowed for determination of the turnover number (0.14 s⁻¹), without the need for error-prone protein quantification. To overcome the complication that purified transporters may adopt right-side-out or inside-out membrane orientations upon reconstitution, thereby confounding the kinetic analysis, we employed a rapid method using synthetic nanobodies to inactivate one population. Oppositely oriented Glt_Tk proteins showed the same transport kinetics, consistent with the use of an identical gating element on both sides of the membrane. Our work underlines the value of bona fide transport experiments to reveal mechanistic features of Na⁺-aspartate symport that cannot be observed in detergent solution. Combined with previous pre-equilibrium binding studies, a full kinetic mechanism of structurally characterized aspartate transporters of the SLC1A family is now emerging.

Trinco et al. measure aspartate uptake rates in proteoliposomes containing purified prokaryotic Na⁺-coupled aspartate transporter GltTk. To overcome limitation of protein orientation, they use synthetic nanobody that blocks transporters from outside and reveal mechanistic features of Na⁺-aspartate symport that cannot be observed in detergent solution.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Cryo-EM structures of excitatory amino acid transporter 3 visualize coupled substrate, sodium, and proton binding and transport

Human excitatory amino acid transporter 3 (hEAAT3) mediates glutamate uptake in neurons, intestine, and kidney. Here, we report cryo-EM structures of hEAAT3 in several functional states where the transporter is empty, bound to coupled sodium ions only, or fully loaded with three sodium ions, a proton, and the substrate aspartate. The structures suggest that hEAAT3 operates by an elevator mechanism involving three functionally independent subunits. When the substrate-binding site is near the cytoplasm, it has a remarkably low affinity for the substrate, perhaps facilitating its release and allowing the rapid transport turnover. The mechanism of the coupled uptake of the sodium ions and the substrate is conserved across evolutionarily distant families and is augmented by coupling to protons in EAATs. The structures further suggest a mechanism by which a conserved glutamate residue mediates proton symport.

Elevator-type mechanisms of membrane transport

Membrane transporters are integral membrane proteins that mediate the passage of solutes across lipid bilayers. These proteins undergo conformational transitions between outward- and inward-facing states, which lead to alternating access of the substrate-binding site to the aqueous environment on either side of the membrane. Dozens of different transporter families have evolved, providing a wide variety of structural solutions to achieve alternating access. A sub-set of structurally diverse transporters operate by mechanisms that are collectively named 'elevator-type'. These transporters have one common characteristic: they contain a distinct protein domain that slides across the membrane as a rigid body, and in doing so it 'drags" the transported substrate along. Analysis of the global conformational changes that take place in membrane transporters using elevator-type mechanisms reveals that elevator-type movements can be achieved in more than one way. Molecular dynamics simulations and experimental data help to understand how lipid bilayer properties may affect elevator movements and vice versa.

Molecular Basis of Coupled Transport and Anion Conduction in Excitatory Amino Acid Transporters

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. After its release from presynaptic nerve terminals, glutamate is quickly removed from the synaptic cleft by excitatory amino acid transporters (EAATs) 1–5, a subfamily of glutamate transporters. The five proteins utilize a complex transport stoichiometry that couples glutamate transport to the symport of three Na⁺ ions and one H⁺ in exchange with one K⁺ to accumulate glutamate against up to 10⁶-fold concentration gradients. They are also anion-selective channels that open and close during transitions along the glutamate transport cycle. EAATs belong to a larger family of secondary-active transporters, the SLC1 family, which also includes purely Na⁺- or H⁺-coupled prokaryotic transporters and Na⁺-dependent neutral amino acid exchangers. In recent years, molecular cloning, heterologous expression, cellular electrophysiology, fluorescence spectroscopy, structural approaches, and molecular simulations have uncovered the molecular mechanisms of coupled transport, substrate selectivity, and anion conduction in EAAT glutamate transporters. Here we review recent findings on EAAT transport mechanisms, with special emphasis on the highly conserved hairpin 2 gate, which has emerged as the central processing unit in many of these functions.

Pre-Steady-State Kinetics and Reverse Transport in Rat Glutamate Transporter EAAC1 with an Immobilized Transport Domain

Plasma membrane glutamate transporters move glutamate across the cell membrane in a process that is thought to involve elevator-like movement of the transport domain relative to the static trimerization domain. Conformational changes associated with this elevator-like movement have been blocked by covalent crosslinking of cysteine pairs inserted strategically in several positions in the transporter structure, resulting in inhibition of steady-state transport activity. However, it is not known how these crosslinking restraints affect other partial reactions of the transporter that were identified based on pre-steady-state kinetic analysis. Here, we re-examine two different introduced cysteine pairs in the rat glutamate transporter EAAC1 recombinantely expressed in HEK293 cells, W440C/K268C and K64C/V419C, with respect to the molecular mechanism of their impairment of transporter function. Pre-steady-state kinetic studies of glutamate-induced partial reactions were performed using laser photolysis of caged glutamate to achieve sub-millisecond time resolution. Crosslinking of both cysteine pairs abolished steady-state transport current, as well as the majority of pre-steady-state glutamate-induced charge movements, in both forward and reverse transport mode, suggesting that it is not only the elevator-like movement associated with translocation, but also other transporter partial reactions that are inhibited. In contrast, sodium binding to the empty transporter, and glutamate-induced anion conductance were still intact after the W440C/K268C crosslink. Our results add to the previous mechanistic view of how covalent restraints of the transporter affect function and structural changes linked to individual steps in the transport cycle.

Functional consequences of SLC1A3 mutations associated with episodic ataxia 6

The episodic ataxias (EA) are a group of inherited neurological diseases characterized by paroxysmal cerebellar incoordination. There exist nine forms of episodic ataxia with distinct neurological symptoms and genetic origins. Episodic ataxia type 6 (EA6) differs from other EA forms in long attack duration, epilepsy and absent myokymia, nystagmus, and tinnitus. It has been described in seven families, and mutations in SLC1A3, the gene encoding the glial glutamate transporter EAAT1, were reported in each family. How these mutations affect EAAT1 expression, subcellular localization, and function, and how such alterations result in the complex neurological phenotype of EA6 is insufficiently understood. We here compare the functional consequences of all currently known mutations by heterologous expression in mammalian cells, biochemistry, confocal imaging, and whole-cell patch clamp recordings of EAAT1 transport and anion currents. We observed impairments of multiple EAAT1 properties ranging from changes in transport function, impaired trafficking to increased protein expression. Many mutations caused only slight changes illustrating how sensitively the cerebellum reacts on impaired EAAT1 functions.

Functional (un)cooperativity in elevator transport proteins

The activity of enzymes is subject to regulation at multiple levels. Cooperativity, the interconnected behavior of active sites within a protein complex, directly affects protein activity. Cooperativity is a mode of regulation that requires neither extrinsic factors nor protein modifications. Instead, it allows enzymes themselves to modulate reaction rates. Cooperativity is an important regulatory mechanism in soluble proteins, but also examples of cooperative membrane proteins have been described. In this review, we summarize the current knowledge on interprotomer cooperativity in elevator-type proteins, a class of membrane transporters characterized by large rigid-body movements perpendicular to the membrane, and highlight well-studied examples and experimental approaches.

Structural ensemble of a glutamate transporter homologue in lipid nanodisc environment

Glutamate transporters are cation-coupled secondary active membrane transporters that clear the neurotransmitter L-glutamate from the synaptic cleft. These transporters are homotrimers, with each protomer functioning independently by an elevator-type mechanism, in which a mobile transport domain alternates between inward- and outward-oriented states. Using single-particle cryo-EM we have determined five structures of the glutamate transporter homologue Glt_Tk, a Na⁺- L-aspartate symporter, embedded in lipid nanodiscs. Dependent on the substrate concentrations used, the protomers of the trimer adopt a variety of asymmetrical conformations, consistent with the independent movement. Six of the 15 resolved protomers are in a hitherto elusive state of the transport cycle in which the inward-facing transporters are loaded with Na⁺ ions. These structures explain how substrate-leakage is prevented - a strict requirement for coupled transport. The belt protein of the lipid nanodiscs bends around the inward oriented protomers, suggesting that membrane deformations occur during transport.

Large-scale state-dependent membrane remodeling by a transporter protein

That channels and transporters can influence the membrane morphology is increasingly recognized. Less appreciated is that the extent and free-energy cost of these deformations likely varies among different functional states of a protein, and thus, that they might contribute significantly to defining its mechanism. We consider the trimeric Na⁺-aspartate symporter Glt_Ph, a homolog of an important class of neurotransmitter transporters, whose mechanism entails one of the most drastic structural changes known. Molecular simulations indicate that when the protomers become inward-facing, they cause deep, long-ranged, and yet mutually-independent membrane deformations. Using a novel simulation methodology, we estimate that the free-energy cost of this membrane perturbation is in the order of 6–7 kcal/mol per protomer. Compensating free-energy contributions within the protein or its environment must thus stabilize this inward-facing conformation for the transporter to function. We discuss these striking results in the context of existing experimental observations for this and other transporters.

Excitatory Amino Acid Transporters in Physiology and Disorders of the Central Nervous System

Excitatory amino acid transporters (EAATs) encompass a class of five transporters with distinct expression in neurons and glia of the central nervous system (CNS). EAATs are mainly recognized for their role in uptake of the amino acid glutamate, the major excitatory neurotransmitter. EAATs-mediated clearance of glutamate released by neurons is vital to maintain proper glutamatergic signalling and to prevent toxic accumulation of this amino acid in the extracellular space. In addition, some EAATs also act as chloride channels or mediate the uptake of cysteine, required to produce the reactive oxygen speciesscavenger glutathione. Given their central role in glutamate homeostasis in the brain, as well as their additional activities, it comes as no surprise that EAAT dysfunctions have been implicated in numerous acute or chronic diseases of the CNS, including ischemic stroke and epilepsy, cerebellar ataxias, amyotrophic lateral sclerosis, Alzheimer’s disease and Huntington’s disease. Here we review the studies in cellular and animal models, as well as in humans that highlight the roles of EAATs in the pathogenesis of these devastating disorders. We also discuss the mechanisms regulating EAATs expression and intracellular trafficking and new exciting possibilities to modulate EAATs and to provide neuroprotection in course of pathologies affecting the CNS.

Simultaneous Release of Multiple Vesicles from Rods Involves Synaptic Ribbons and Syntaxin 3B

First proposed as a specialized mode of release at sensory neurons possessing ribbon synapses, multivesicular release has since been described throughout the central nervous system. Many aspects of multivesicular release remain poorly understood. We explored mechanisms underlying simultaneous multivesicular release at ribbon synapses in salamander retinal rod photoreceptors. We assessed spontaneous release presynaptically by recording glutamate transporter anion currents (I_A(glu)) in rods. Spontaneous I_A(glu) events were correlated in amplitude and kinetics with simultaneously measured miniature excitatory postsynaptic currents in horizontal cells. Both measures indicated that a significant fraction of events is multiquantal, with an analysis of I_A(glu) revealing that multivesicular release constitutes ∼30% of spontaneous release events. I_A(glu) charge transfer increased linearly with event amplitude showing that larger events involve greater glutamate release. The kinetics of large and small I_A(glu) events were identical as were rise times of large and small miniature excitatory postsynaptic currents, indicating that the release of multiple vesicles during large events is highly synchronized. Effects of exogenous Ca²⁺ buffers suggested that multiquantal, but not uniquantal, release occurs preferentially near Ca²⁺ channels clustered beneath synaptic ribbons. Photoinactivation of ribbons reduced the frequency of spontaneous multiquantal events without affecting uniquantal release frequency, showing that spontaneous multiquantal release requires functional ribbons. Although both occur at ribbon-style active zones, the absence of cross-depletion indicates that evoked and spontaneous multiquantal release from ribbons involve different vesicle pools. Introducing an inhibitory peptide into rods to interfere with the SNARE protein, syntaxin 3B, selectively reduced multiquantal event frequency. These results support the hypothesis that simultaneous multiquantal release from rods arises from homotypic fusion among neighboring vesicles on ribbons and involves syntaxin 3B.

Paired-Cysteine Scanning Reveals Conformationally Sensitive Proximity between the TM4b-4c Loop and TM8 of the Glutamate Transporter EAAT1

Excitatory amino acid transporters (EAATs) take up the neurotransmitter glutamate from the synaptic cleft and maintain glutamate concentrations below neurotoxic levels. Recently, the crystal structures of thermostable EAAT1 variants have been reported; however, little is understood regarding the functional mechanism of the transmembrane domain (TM) 4b-4c loop, which contains more than 50 amino acids in mammalian EAATs that are absent in prokaryotic homologues. To explore the spatial position and function of TM4 during the transport cycle, we introduced pairwise cysteine substitutions between the TM4b-4c loop and TM8 in a cysteine-less version of EAAT1, CL-EAAT1. We observed pronounced inhibition of transport by Cu(II)(1,10-phenanthroline)₃ (CuPh) for doubly substituted V238C/I469C and A243C/I469C variants, but not for corresponding singly substituted CL-EAAT1 or for more than 20 other double-cysteine variants. Dithiothreitol treatment partially restored the uptake activity of the CuPh-treated V238C/I469C and A243C/I469C doubly substituted variants, confirming that the effects of CuPh on these variants were due to the formation of intramolecular disulfide bonds. Glutamate, KCl, and d,l-threo-β-benzyloxy-aspartate weakened CuPh inhibition of the V238C/I469C variant, but only KCl weakened CuPh inhibition of the V243C/I469C variant, suggesting that the TM4b-4c loop and TM8 are separated from each other in the inward-facing conformations of EAAT1. Our results suggest that the TM4b-4c loop and TM8 are positioned in close proximity during the transport cycle and are less closely spaced in the inward-facing conformation.

Substrate-Induced Motion between TM4 and TM7 of the Glutamate Transporter EAAT1 Revealed by Paired Cysteine Mutagenesis

To maintain efficient synaptic communication, glutamate transporters reuptake glutamate from the synaptic cleft and prevent glutamate concentrations from reaching neurotoxic levels. The number of amino acid residues of the transmembrane (TM) domain 4b-4c loop of mammalian excitatory amino acid transporters (EAATs) is 50 amino acids more than that of the prokaryotic homolog. To investigate the spatial proximity and functional significance of residues in glutamate transporters, cysteine pairs were introduced at positions A243 of the TM4b-4c loop and T396 or A414 of TM7, respectively. The transport activity of double mutants A243C/T396C and A243C/A414C was inhibited by Cu(II) (1,10-phenanthroline)₃ [copper phenanthroline (CuPh)] and cadmium ions, but the uptake activity of corresponding single mutants remained unchanged. Treatment with dithiothreitol after CuPh restored much of the transport activity. The inhibitory effects of CuPh and cadmium could only be detected when cysteine pairs are in the same polypeptide. Therefore, we suggest that the formation of these disulfide bonds occurs intramolecularly. Glutamate, potassium, and DL-threo-β-benzyloxyaspartate facilitated crosslinking in the A243C/T396C transporter and this suggests that the TM4b-4c loop and β-bridge region in TM7 were drawn into close proximity to each other in the inward- and outward-facing conformation of EAAT1. Thus, these data provide evidence that substrate-induced structural rearrangements occur between the TM4b-4c loop and TM7 during the transport cycle.

Amino acid transporters revisited: New views in health and disease

Amino acid transporters (AATs) are membrane-bound transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs have diverse functional roles ranging from neurotransmission to acid-base balance, intracellular energy metabolism, and anabolic and catabolic reactions. In cancer cells and diabetes, dysregulation of AATs leads to metabolic reprogramming, which changes intracellular amino acid levels, contributing to the pathogenesis of cancer, obesity and diabetes. Indeed, the neutral amino acid transporters (NATs) SLC7A5/LAT1 and SLC1A5/ASCT2 are likely involved in several human malignancies. However, a clinical therapy that directly targets AATs has not yet been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, their diverse physiological roles in different tissues and organs, their wide-ranging implications in human diseases and the emerging strategies and tools that will be necessary to target AATs therapeutically.

Both reentrant loops of the sodium-coupled glutamate transporters contain molecular determinants of cation selectivity

In the brain, glutamate transporters terminate excitatory neurotransmission by removing this neurotransmitter from the synapse via cotransport with three sodium ions into the surrounding cells. Structural studies have identified the binding sites of the three sodium ions in glutamate transporters. The residue side-chains directly interact with the sodium ions at the Na1 and Na3 sites and are fully conserved from archaeal to eukaryotic glutamate transporters. The Na2 site is formed by three main-chain oxygens on the extracellular reentrant hairpin loop HP2 and one on transmembrane helix 7. A glycine residue on HP2 is located closely to the three main-chain oxygens in all glutamate transporters, except for the astroglial transporter GLT-1, which has a serine residue at that position. Unlike for WT GLT-1, substitution of the serine residue to glycine enables sustained glutamate transport also when sodium is replaced by lithium. Here, using functional and simulation studies, we studied the role of this serine/glycine switch on cation selectivity of substrate transport. Our results indicate that the side-chain oxygen of the serine residues can form a hydrogen bond with a main-chain oxygen on transmembrane helix 7. This leads to an expansion of the Na2 site such that water can participate in sodium coordination at Na2. Furthermore, we found other molecular determinants of cation selectivity on the nearby HP1 loop. We conclude that subtle changes in the composition of the two reentrant hairpin loops determine the cation specificity of acidic amino acid transport by glutamate transporters.

Cysteine Scanning Mutagenesis of TM4b-4c Loop of Glutamate Transporter EAAT1 Reveals Three Conformationally Sensitive Residues

Glutamatergic synaptic transmitters are cleared from the synaptic cleft through excitatory amino acid transporters (EAATs) that are responsible for recycling glutamate and transporting it into neurons and glial cells. To probe the structural role of the TM4b-4c loop of EAAT1 (Rattus norvegicus), each of the 57 amino acid residues was mutated to cysteine. Thirteen of the single mutants have very low transport activity. Aqueous accessibility of the introduced cysteines from the remaining mutants was then explored by membrane-permeant and membrane-impermeant sulfhydryl reagents in different conditions. F190C, V238C, and A243C were affected by MTSET, whereas Q189C, F190C, V238C, A243C, and L244C were sensitive to MTSEA. Q189C and L244C transport activity was diminished in the presence of potassium, which is expected to favor the inward-facing conformation of the transporter. Inversely, L244C was protected by glutamate. The modification of A243C by MTSEA was enhanced by either potassium and glutamate or dl-threo-β-benzyloxyaspartate. From these results, we suggest that residues F190C, V238C, and A243C may be located near the extracellular surface, and the TM4b-4c loop forms multiple reentrant membrane loops on the cell surface. Alternatively, F190C, V238C, and A243C may function in the transport pathway, which is exposed to MTSET. In addition, Q189C, A243C, and L244C are conformationally sensitive and may play a role in the transport cycle.

Transport mechanism of a glutamate transporter homologue GltPh

Glutamate transporters are responsible for uptake of the neurotransmitter glutamate in mammalian central nervous systems. Their archaeal homologue GltPh, an aspartate transporter isolated from Pyrococcus horikoshii, has been the focus of extensive studies through crystallography, MD simulations and single-molecule FRET (smFRET). Here, we summarize the recent research progress on GltPh, in the hope of gaining some insights into the transport mechanism of this aspartate transporter.

Glial and Neuronal Glutamate Transporters Differ in the Na+ Requirements for Activation of the Substrate-Independent Anion Conductance

Excitatory amino acid transporters (EAATs) are secondary active transporters of L-glutamate and L- or D-aspartate. These carriers also mediate a thermodynamically uncoupled anion conductance that is gated by Na⁺ and substrate binding. The activation of the anion channel by binding of Na⁺ alone, however, has only been demonstrated for mammalian EAAC1 (EAAT3) and EAAT4. To date, no difference has been observed for the substrate dependence of anion channel gating between the glial, EAAT1 and EAAT2, and the neuronal isoforms EAAT3, EAAT4 and EAAT5. Here we describe a difference in the Na⁺-dependence of anion channel gating between glial and neuronal isoforms. Chloride flux through transporters without glutamate binding has previously been described as substrate-independent or "leak" channel activity. Choline or N-methyl-D-glucamine replacement of external Na⁺ ions significantly reduced or abolished substrate-independent EAAT channel activity in EAAT3 and EAAT4 yet has no effect on EAAT1 or EAAT2. The interaction of Na⁺ with the neuronal carrier isoforms was concentration dependent, consistent with previous data. The presence of substrate and Na⁺-independent open states in the glial EAAT isoforms is a novel finding in the field of EAAT function. Our results reveal an important divergence in anion channel function between glial and neuronal glutamate transporters and highlight new potential roles for the EAAT-associated anion channel activity based on transporter expression and localization in the central nervous system.

Molecular and cellular physiology of sodium-dependent glutamate transporters

Glutamate is the major excitatory transmitter in the vertebrate brain. After its release from presynaptic nerve terminals, it is rapidly taken up by high-affinity sodium-dependent plasma membrane transporters. While both neurons and glial cells express these excitatory amino acid transporters (EAATs), the majority of glutamate uptake is accomplished by astrocytes, which convert synaptically-released glutamate to glutamine or feed it into their own metabolism. Glutamate uptake by astrocytes not only shapes synaptic transmission by regulating the availability of glutamate to postsynaptic neuronal receptors, but also protects neurons from hyper-excitability and subsequent excitotoxic damage. In the present review, we provide an overview of the molecular and cellular characteristics of sodium-dependent glutamate transporters and their associated anion permeation pathways, with a focus on astrocytic glutamate transport. We summarize their functional properties and roles within tripartite synapses under physiological and pathophysiological conditions, exemplifying the intricate interactions and interrelationships between neurons and glial cells in the brain.

Tuning the ion selectivity of glutamate transporter-associated uncoupled conductances

Excitatory amino acid transporters possess a Cl⁻ conductance whose direction is independent of that of the substrate. By mutating an arginine residue in the putative anion permeation pathway, Cater et al. show that a positive charge at this position determines anion selectivity.

The concentration of glutamate within a glutamatergic synapse is tightly regulated by excitatory amino acid transporters (EAATs). In addition to their primary role in clearing extracellular glutamate, the EAATs also possess a thermodynamically uncoupled Cl⁻ conductance. This conductance is activated by the binding of substrate and Na⁺, but the direction of Cl⁻ flux is independent of the rate or direction of substrate transport; thus, the two processes are thermodynamically uncoupled. A recent molecular dynamics study of the archaeal EAAT homologue Glt_Ph (an aspartate transporter from Pyrococcus horikoshii) identified an aqueous pore at the interface of the transport and trimerization domains, through which anions could permeate, and it was suggested that an arginine residue at the most restricted part of this pathway might play a role in determining anion selectivity. In this study, we mutate this arginine to a histidine in the human glutamate transporter EAAT1 and investigate the role of the protonation state of this residue on anion selectivity and transporter function. Our results demonstrate that a positive charge at this position is crucial for determining anion versus cation selectivity of the uncoupled conductance of EAAT1. In addition, because the nature of this residue influences the turnover rate of EAAT1, we reveal an intrinsic link between the elevator movement of the transport domain and the Cl⁻ channel.

Neuronal vs glial glutamate uptake: Resolving the conundrum

Neither normal brain function nor the pathological processes involved in neurological diseases can be adequately understood without knowledge of the release, uptake and metabolism of glutamate. The reason for this is that glutamate (a) is the most abundant amino acid in the brain, (b) is at the cross-roads between several metabolic pathways, and (c) serves as the major excitatory neurotransmitter. In fact most brain cells express glutamate receptors and are thereby influenced by extracellular glutamate. In agreement, brain cells have powerful uptake systems that constantly remove glutamate from the extracellular fluid and thereby limit receptor activation. It has been clear since the 1970s that both astrocytes and neurons express glutamate transporters. However the relative contribution of neuronal and glial transporters to the total glutamate uptake activity, however, as well as their functional importance, has been hotly debated ever since. The present short review provides (a) an overview of what we know about neuronal glutamate uptake as well as an historical description of how we got there, and (b) a hypothesis reconciling apparently contradicting observations thereby possibly resolving the paradox.

Astroglial glutamate transporters coordinate excitatory signaling and brain energetics

In the mammalian brain, a family of sodium-dependent transporters maintains low extracellular glutamate and shapes excitatory signaling. The bulk of this activity is mediated by the astroglial glutamate transporters GLT-1 and GLAST (also called EAAT2 and EAAT1). In this review, we will discuss evidence that these transporters co-localize with, form physical (co-immunoprecipitable) interactions with, and functionally couple to various 'energy-generating' systems, including the Na(+)/K(+)-ATPase, the Na(+)/Ca(2+) exchanger, glycogen metabolizing enzymes, glycolytic enzymes, and mitochondria/mitochondrial proteins. This functional coupling is bi-directional with many of these systems both being regulated by glutamate transport and providing the 'fuel' to support glutamate uptake. Given the importance of glutamate uptake to maintaining synaptic signaling and preventing excitotoxicity, it should not be surprising that some of these systems appear to 'redundantly' support the energetic costs of glutamate uptake. Although the glutamate-glutamine cycle contributes to recycling of neurotransmitter pools of glutamate, this is an over-simplification. The ramifications of co-compartmentalization of glutamate transporters with mitochondria for glutamate metabolism are discussed. Energy consumption in the brain accounts for ∼20% of the basal metabolic rate and relies almost exclusively on glucose for the production of ATP. However, the brain does not possess substantial reserves of glucose or other fuels. To ensure adequate energetic supply, increases in neuronal activity are matched by increases in cerebral blood flow via a process known as 'neurovascular coupling'. While the mechanisms for this coupling are not completely resolved, it is generally agreed that astrocytes, with processes that extend to synapses and endfeet that surround blood vessels, mediate at least some of the signal that causes vasodilation. Several studies have shown that either genetic deletion or pharmacologic inhibition of glutamate transport impairs neurovascular coupling. Together these studies strongly suggest that glutamate transport not only coordinates excitatory signaling, but also plays a pivotal role in regulating brain energetics.

Allosteric Mechanisms of Molecular Machines at the Membrane: Transport by Sodium-Coupled Symporters

Solute transport across cell membranes is ubiquitous in biology as an essential physiological process. Secondary active transporters couple the unfavorable process of solute transport against its concentration gradient to the energetically favorable transport of one or several ions. The study of such transporters over several decades indicates that their function involves complex allosteric mechanisms that are progressively being revealed in atomistic detail. We focus on two well-characterized sodium-coupled symporters: the bacterial amino acid transporter LeuT, which is the prototype for the "gated pore" mechanism in the mammalian synaptic monoamine transporters, and the archaeal GltPh, which is the prototype for the "elevator" mechanism in the mammalian excitatory amino acid transporters. We present the evidence for the role of allostery in the context of a quantitative formalism that can reconcile biochemical and biophysical data and thereby connects directly to recent insights into the molecular structure and dynamics of these proteins. We demonstrate that, while the structures and mechanisms of these transporters are very different, the available data suggest a common role of specific models of allostery in their functions. We argue that such allosteric mechanisms appear essential not only for sodium-coupled symport in general but also for the function of other types of molecular machines in the membrane.

Computational approaches to detect allosteric pathways in transmembrane molecular machines

Many of the functions of transmembrane proteins involved in signal processing and transduction across the cell membrane are determined by allosteric couplings that propagate the functional effects well beyond the original site of activation. Data gathered from breakthroughs in biochemistry, crystallography, and single molecule fluorescence have established a rich basis of information for the study of molecular mechanisms in the allosteric couplings of such transmembrane proteins. The mechanistic details of these couplings, many of which have therapeutic implications, however, have only become accessible in synergy with molecular modeling and simulations. Here, we review some recent computational approaches that analyze allosteric coupling networks (ACNs) in transmembrane proteins, and in particular the recently developed Protein Interaction Analyzer (PIA) designed to study ACNs in the structural ensembles sampled by molecular dynamics simulations. The power of these computational approaches in interrogating the functional mechanisms of transmembrane proteins is illustrated with selected examples of recent experimental and computational studies pursued synergistically in the investigation of secondary active transporters and GPCRs. This article is part of a Special Issue entitled: Membrane Proteins edited by J.C. Gumbart and Sergei Noskov.

Molecular physiology of EAAT anion channels

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. After release from presynaptic nerve terminals, glutamate is quickly removed from the synaptic cleft by a family of five glutamate transporters, the so-called excitatory amino acid transporters (EAAT1-5). EAATs are prototypic members of the growing number of dual-function transport proteins: they are not only glutamate transporters, but also anion channels. Whereas the mechanisms underlying secondary active glutamate transport are well understood at the functional and at the structural level, mechanisms and cellular roles of EAAT anion conduction have remained elusive for many years. Recently, molecular dynamics simulations combined with simulation-guided mutagenesis and experimental analysis identified a novel anion-conducting conformation, which accounts for all experimental data on EAAT anion currents reported so far. We here review recent findings on how EAATs accommodate a transporter and a channel in one single protein.

The Hydroxyl Side Chain of a Highly Conserved Serine Residue Is Required for Cation Selectivity and Substrate Transport in the Glial Glutamate Transporter GLT-1/SLC1A2

Glutamate transporters maintain synaptic concentration of the excitatory neurotransmitter below neurotoxic levels. Their transport cycle consists of cotransport of glutamate with three sodium ions and one proton, followed by countertransport of potassium. Structural studies proposed that a highly conserved serine located in the binding pocket of the homologous GltPh coordinates L-aspartate as well as the sodium ion Na1. To experimentally validate these findings, we generated and characterized several mutants of the corresponding serine residue, Ser-364, of human glutamate transporter SLC1A2 (solute carrier family 1 member 2), also known as glutamate transporter GLT-1 and excitatory amino acid transporter EAAT2. S364T, S364A, S364C, S364N, and S364D were expressed in HEK cells and Xenopus laevis oocytes to measure radioactive substrate transport and transport currents, respectively. All mutants exhibited similar plasma membrane expression when compared with WT SLC1A2, but substitutions of serine by aspartate or asparagine completely abolished substrate transport. On the other hand, the threonine mutant, which is a more conservative mutation, exhibited similar substrate selectivity, substrate and sodium affinities as WT but a lower selectivity for Na(+) over Li(+). S364A and S364C exhibited drastically reduced affinities for each substrate and enhanced selectivity for L-aspartate over D-aspartate and L-glutamate, and lost their selectivity for Na(+) over Li(+). Furthermore, we extended the analysis of our experimental observations using molecular dynamics simulations. Altogether, our findings confirm a pivotal role of the serine 364, and more precisely its hydroxyl group, in coupling sodium and substrate fluxes.

Molecular Determinants of Substrate Specificity in Sodium-coupled Glutamate Transporters

Crystal structures of the archaeal homologue GltPh have provided important insights into the molecular mechanism of transport of the excitatory neurotransmitter glutamate. Whereas mammalian glutamate transporters can translocate both glutamate and aspartate, GltPh is only one capable of aspartate transport. Most of the amino acid residues that surround the aspartate substrate in the binding pocket of GltPh are highly conserved. However, in the brain transporters, Thr-352 and Met-362 of the reentrant hairpin loop 2 are replaced by the smaller Ala and Thr, respectively. Therefore, we have studied the effects of T352A and M362T on binding and transport of aspartate and glutamate by GltPh. Substrate-dependent intrinsic fluorescence changes were monitored in transporter constructs containing the L130W mutation. GltPh-L130W/T352A exhibited an ~15-fold higher apparent affinity for l-glutamate than the wild type transporter, and the M362T mutation resulted in an increased affinity of ~40-fold. An even larger increase of the apparent affinity for l-glutamate, around 130-fold higher than that of wild type, was observed with the T352A/M362T double mutant. Radioactive uptake experiments show that GltPh-T352A not only transports aspartate but also l-glutamate. Remarkably, GltPh-M362T exhibited l-aspartate but not l-glutamate transport. The double mutant retained the ability to transport l-glutamate, but its kinetic parameters were very similar to those of GltPh-T352A alone. The differential impact of mutation on binding and transport of glutamate suggests that hairpin loop 2 not only plays a role in the selection of the substrate but also in its translocation.

The domain interface of the human glutamate transporter EAAT1 mediates chloride permeation

The concentration of glutamate within the glutamatergic synapse is tightly regulated by the excitatory amino-acid transporters (EAATs). In addition to their primary role of clearing extracellular glutamate, the EAATs also possess a thermodynamically uncoupled Cl(-) conductance. Several crystal structures of an archaeal EAAT homolog, GltPh, at different stages of the transport cycle have been solved. In a recent structure, an aqueous cavity located at the interface of the transport and trimerization domains has been identified. This cavity is lined by polar residues, several of which have been implicated in Cl(-) permeation. We hypothesize that this cavity opens during the transport cycle to form the Cl(-) channel. Residues lining this cavity in EAAT1, including Ser-366, Leu-369, Phe-373, Arg-388, Pro-392, and Thr-396, were mutated to small hydrophobic residues. Wild-type and mutant transporters were expressed in Xenopus laevis oocytes and two-electrode voltage-clamp electrophysiology, and radiolabeled substrate uptake was used to investigate function. Significant alterations in substrate-activated Cl(-) conductance were observed for several mutant transporters. These alterations support the hypothesis that this aqueous cavity at the interface of the transport and trimerization domains is a partially formed Cl(-) channel, which opens to form a pore through which Cl(-) ions pass. This study enhances our understanding as to how glutamate transporters function as both amino-acid transporters and Cl(-) channels.

Activity-Dependent Plasticity of Astroglial Potassium and Glutamate Clearance

Recent evidence has shown that astrocytes play essential roles in synaptic transmission and plasticity. Nevertheless, how neuronal activity alters astroglial functional properties and whether such properties also display specific forms of plasticity still remain elusive. Here, we review research findings supporting this aspect of astrocytes, focusing on their roles in the clearance of extracellular potassium and glutamate, two neuroactive substances promptly released during excitatory synaptic transmission. Their subsequent removal, which is primarily carried out by glial potassium channels and glutamate transporters, is essential for proper functioning of the brain. Similar to neurons, different forms of short- and long-term plasticity in astroglial uptake have been reported. In addition, we also present novel findings showing robust potentiation of astrocytic inward currents in response to repetitive stimulations at mild frequencies, as low as 0.75 Hz, in acute hippocampal slices. Interestingly, neurotransmission was hardly affected at this frequency range, suggesting that astrocytes may be more sensitive to low frequency stimulation and may exhibit stronger plasticity than neurons to prevent hyperexcitability. Taken together, these important findings strongly indicate that astrocytes display both short- and long-term plasticity in their clearance of excess neuroactive substances from the extracellular space, thereby regulating neuronal activity and brain homeostasis.

Excitatory amino acid transporters: recent insights into molecular mechanisms, novel modes of modulation and new therapeutic possibilities

The five excitatory amino acid transporters (EAAT1-5) mediating the synaptic uptake of the major excitatory neurotransmitter glutamate are differently expressed throughout the CNS and at the synaptic level. Although EAATs are crucial for normal excitatory neurotransmission, explorations into the physiological functions mediated by the different transporter subtypes and their respective therapeutic potential have so far been sparse, in no small part due to the limited selection of pharmacological tools available. In the present update, we outline important new insights into the molecular compositions of EAATs and their intricate transport process, the novel approaches to pharmacological modulation of the transporters that have emerged, and interesting new perspectives in EAAT as drug targets proposed in recent years.

Disulfide cross-linking of transport and trimerization domains of a neuronal glutamate transporter restricts the role of the substrate to the gating of the anion conductance

Excitatory amino acid transporters remove synaptically released glutamate and maintain its concentrations below neurotoxic levels. EAATs also mediate a thermodynamically uncoupled substrate-gated anion conductance that may modulate cell excitability. A structure of an archeal homologue, which reflects an early intermediate on the proposed substrate translocation path, has been suggested to be similar to an anion conducting conformation. To probe this idea by functional studies, we have introduced two cysteine residues in the neuronal glutamate transporter EAAC1 at positions predicted to be close enough to form a disulfide bond only in outward-facing and early intermediate conformations of the homologue. Upon treatment of Xenopus laevis oocytes expressing the W441C/K269C double mutant with dithiothreitol, radioactive transport was stimulated >2-fold but potently inhibited by low micromolar concentrations of the oxidizing reagent copper(II)(1,10-phenanthroline)3. The substrate-induced currents by the untreated double mutant, reversed at approximately -20 mV, close to the reversal potential of chloride, but treatment with dithiothreitol resulted in transport currents with the same voltage dependence as the wild type. It appears therefore that in the oocyte expression system the introduced cysteine residues in many of the mutant transporters are already cross-linked and are only capable of mediating the substrate-gated anion conductance. Reduction of the disulfide bond now allows these transporters to execute the full transport cycle. Our functional data support the idea that the anion conducting conformation of the neuronal glutamate transporter is associated with an early step of the transport cycle.

Neurotransmitter transporters: structure meets function

At synapses, sodium-coupled transporters remove released neurotransmitters, thereby recycling them and maintaining a low extracellular concentration of the neurotransmitter. The molecular mechanism underlying sodium-coupled neurotransmitter uptake is not completely understood. Several structures of homologs of human neurotransmitter transporters have been solved with X-ray crystallography. These crystal structures have spurred a plethora of computational and experimental work to elucidate the molecular mechanism underlying sodium-coupled transport. Here, we compare the structures of GltPh, a glutamate transporter homolog, and LeuT, a homolog of neurotransmitter transporters for the biogenic amines and inhibitory molecules GABA and glycine. We relate these structures to data obtained from experiments and computational simulations, to draw conclusions about the mechanism of uptake by sodium-coupled neurotransmitter transporters. Here, we propose how sodium and substrate binding is coupled and how binding of sodium and substrate opens and closes the gates in these transporters, thereby leading to an efficient coupled transport.

Mechanisms of glutamate transport

L-Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and plays important roles in a wide variety of brain functions, but it is also a key player in the pathogenesis of many neurological disorders. The control of glutamate concentrations is critical to the normal functioning of the central nervous system, and in this review we discuss how glutamate transporters regulate glutamate concentrations to maintain dynamic signaling mechanisms between neurons. In 2004, the crystal structure of a prokaryotic homolog of the mammalian glutamate transporter family of proteins was crystallized and its structure determined. This has paved the way for a better understanding of the structural basis for glutamate transporter function. In this review we provide a broad perspective of this field of research, but focus primarily on the more recent studies with a particular emphasis on how our understanding of the structure of glutamate transporters has generated new insights.

The Concise Guide to PHARMACOLOGY 2013/14: transporters

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Transporters are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

SLC1 glutamate transporters

The plasma membrane transporters for the neurotransmitter glutamate belong to the solute carrier 1 family. They are secondary active transporters, taking up glutamate into the cell against a substantial concentration gradient. The driving force for concentrative uptake is provided by the cotransport of Na(+) ions and the countertransport of one K(+) in a step independent of the glutamate translocation step. Due to eletrogenicity of transport, the transmembrane potential can also act as a driving force. Glutamate transporters are expressed in many tissues, but are of particular importance in the brain, where they contribute to the termination of excitatory neurotransmission. Glutamate transporters can also run in reverse, resulting in glutamate release from cells. Due to these important physiological functions, glutamate transporter expression and, therefore, the transport rate, are tightly regulated. This review summarizes recent literature on the functional and biophysical properties, structure-function relationships, regulation, physiological significance, and pharmacology of glutamate transporters. Particular emphasis is on the insight from rapid kinetic and electrophysiological studies, transcriptional regulation of transporter expression, and reverse transport and its importance for pathophysiological glutamate release under ischemic conditions.