Effect of sodium lithium and proton concentrations on the electrophysiological properties of the four mouse GABA transporters expressed in Xenopus oocytes

A comparative review on the well-studied GAT1 and the understudied BGT-1 in the brain

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). Its homeostasis is maintained by neuronal and glial GABA transporters (GATs). The four GATs identified in humans are GAT1 (SLC6A1), GAT2 (SLC6A13), GAT3 (SLC6A11), and betaine/GABA transporter-1 BGT-1 (SLC6A12) which are all members of the solute carrier 6 (SLC6) family of sodium-dependent transporters. While GAT1 has been investigated extensively, the other GABA transporters are less studied and their role in CNS is not clearly defined. Altered GABAergic neurotransmission is involved in different diseases, but the importance of the different transporters remained understudied and limits drug targeting. In this review, the well-studied GABA transporter GAT1 is compared with the less-studied BGT-1 with the aim to leverage the knowledge on GAT1 to shed new light on the open questions concerning BGT-1. The most recent knowledge on transporter structure, functions, expression, and localization is discussed along with their specific role as drug targets for neurological and neurodegenerative disorders. We review and discuss data on the binding sites for Na⁺, Cl^-, substrates, and inhibitors by building on the recent cryo-EM structure of GAT1 to highlight specific molecular determinants of transporter functions. The role of the two proteins in GABA homeostasis is investigated by looking at the transport coupling mechanism, as well as structural and kinetic transport models. Furthermore, we review information on selective inhibitors together with the pharmacophore hypothesis of transporter substrates.

Transcriptomic Analysis of the Effect of GAT-2 Deficiency on Differentiation of Mice Naïve T Cells Into Th1 Cells In Vitro

The neurotransmitter γ-aminobutyric acid (GABA) is known to affect the activation and function of immune cells. This study investigated the role of GABA transporter (GAT)-2 in the differentiation of type 1 helper T (Th1) cells. Naïve CD4+ T cells isolated from splenocytes of GAT-2 knockout (KO) and wild-type (WT) mice were cultured; Th1 cell differentiation was induced and transcriptome and bioinformatics analyses were carried out. We found that GAT-2 deficiency promoted the differentiation of naïve T cells into Th1 cells. RNA sequencing revealed 2984 differentially expressed genes including 1616 that were up-regulated and 1368 that were down-regulated in GAT-2 KO cells compared to WT cells, which were associated with 950 enriched Gene Ontology terms and 33 enriched Kyoto Encyclopedia of Genes and Genomes pathways. Notably, 4 signal transduction pathways (hypoxia-inducible factor [HIF]-1, Hippo, phospholipase D, and Janus kinase [JAK]/signal transducer and activator of transcription [STAT]) and one metabolic pathway (glycolysis/gluconeogenesis) were significantly enriched by GAT-2 deficiency, suggesting that these pathways mediate the effect of GABA on T cell differentiation. Our results provide evidence for the immunomodulatory function of GABA signaling in T cell-mediated immunity and can guide future studies on the etiology and management of autoimmune diseases.

Towards a Better Understanding of GABAergic Remodeling in Alzheimer's Disease

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the vertebrate brain. In the past, there has been a major research drive focused on the dysfunction of the glutamatergic and cholinergic neurotransmitter systems in Alzheimer’s disease (AD). However, there is now growing evidence in support of a GABAergic contribution to the pathogenesis of this neurodegenerative disease. Previous studies paint a complex, convoluted and often inconsistent picture of AD-associated GABAergic remodeling. Given the importance of the GABAergic system in neuronal function and homeostasis, in the maintenance of the excitatory/inhibitory balance, and in the processes of learning and memory, such changes in GABAergic function could be an important factor in both early and later stages of AD pathogenesis. Given the limited scope of currently available therapies in modifying the course of the disease, a better understanding of GABAergic remodeling in AD could open up innovative and novel therapeutic opportunities.

Revised Ion/Substrate Coupling Stoichiometry of GABA Transporters

The purpose of this review is to highlight recent evidence in support of a 3 Na⁺: 1 Cl^-: 1 GABA coupling stoichiometry for plasma membrane GABA transporters (SLC6A1 , SLC6A11 , SLC6A12 , SLC6A13 ) and how the revised stoichiometry impacts our understanding of the contribution of GABA transporters to GABA homeostasis in synaptic and extrasynaptic regions in the brain under physiological and pathophysiological states. Recently, our laboratory probed the GABA transporter stoichiometry by analyzing the results of six independent measurements, which included the shifts in the thermodynamic transporter reversal potential caused by changes in the extracellular Na⁺, Cl^-, and GABA concentrations, as well as the ratio of charge flux to substrate flux for Na⁺, Cl^-, and GABA under voltage-clamp conditions. The shifts in the transporter reversal potential for a tenfold change in the external concentration of Na⁺, Cl^-, and GABA were 84 ± 4, 30 ± 1, and 29 ± 1 mV, respectively. Charge flux to substrate flux ratios were 0.7 ± 0.1 charges/Na⁺, 2.0 ± 0.2 charges/Cl^-, and 2.1 ± 0.1 charges/GABA. We then compared these experimental results with the predictions of 150 different transporter stoichiometry models, which included 1-5 Na⁺, 0-5 Cl^-, and 1-5 GABA per transport cycle. Only the 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry model correctly predicts the results of all six experimental measurements. Using the revised 3 Na⁺: 1 Cl^-: 1 GABA stoichiometry, we propose that the GABA transporters mediate GABA uptake under most physiological conditions. Transporter-mediated GABA release likely takes place under pathophysiological or extreme physiological conditions.

The betaine/GABA transporter and betaine: roles in brain, kidney, and liver

The physiological roles of the betaine/GABA transporter (BGT1; slc6a12) are still being debated. BGT1 is a member of the solute carrier family 6 (the neurotransmitter, sodium symporter transporter family) and mediates cellular uptake of betaine and GABA in a sodium- and chloride-dependent process. Most of the studies of BGT1 concern its function and regulation in the kidney medulla where its role is best understood. The conditions here are hostile due to hyperosmolarity and significant concentrations of NH₄Cl and urea. To withstand the hyperosmolarity, cells trigger osmotic adaptation, involving concentration of a transcriptional factor TonEBP/NFAT5 in the nucleus, and accumulate betaine and other osmolytes. Data from renal cells in culture, primarily MDCK, revealed that transcriptional regulation of BGT1 by TonEBP/NFAT5 is relatively slow. To allow more acute control of the abundance of BGT1 protein in the plasma membrane, there is also post-translation regulation of BGT1 protein trafficking which is dependent on intracellular calcium and ATP. Further, betaine may be important in liver metabolism as a methyl donor. In fact, in the mouse the liver is the organ with the highest content of BGT1. Hepatocytes express high levels of both BGT1 and the only enzyme that can metabolize betaine, namely betaine:homocysteine –S-methyltransferase (BHMT1). The BHMT1 enzyme removes a methyl group from betaine and transfers it to homocysteine, a potential risk factor for cardiovascular disease. Finally, BGT1 has been proposed to play a role in controlling brain excitability and thereby represents a target for anticonvulsive drug development. The latter hypothesis is controversial due to very low expression levels of BGT1 relative to other GABA transporters in brain, and also the primary location of BGT1 at the surface of the brain in the leptomeninges. These issues are discussed in detail.

Functional consequences of sulfhydryl modification of the γ-aminobutyric acid transporter 1 at a single solvent-exposed cysteine residue

The aims of this study were to optimize the experimental conditions for labeling extracellularly oriented, solvent-exposed cysteine residues of γ-aminobutyric acid transporter 1 (GAT1) with the membrane-impermeant sulfhydryl reagent [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET) and to characterize the functional and pharmacological consequences of labeling on transporter steady-state and presteady-state kinetic properties. We expressed human GAT1 in Xenopus laevis oocytes and used radiotracer and electrophysiological methods to assay transporter function before and after sulfhydryl modification with MTSET. In the presence of NaCl, transporter exposure to MTSET (1–2.5 mM for 5–20 min) led to partial inhibition of GAT1-mediated transport, and this loss of function was completely reversed by the reducing reagent dithiothreitol. MTSET treatment had no functional effect on the mutant GAT1 C74A, whereas the membrane-permeant reagents N-ethylmaleimide and tetramethylrhodamine-6-maleimide inhibited GABA transport mediated by GAT1 C74A. Ion replacement experiments indicated that MTSET labeling of GAT1 could be driven to completion when valproate replaced chloride in the labeling buffer, suggesting that valproate induces a GAT1 conformation that significantly increases C74 accessibility to the extracellular fluid. Following partial inhibition by MTSET, there was a proportional reduction in both the presteady-state and steady-state macroscopic signals, and the functional and pharmacological properties of the remaining signals were indistinguishable from those of unlabeled GAT1. Therefore, covalent modification of GAT1 at C74 results in completely nonfunctional as well as electrically silent transporters.

A novel dominant hyperekplexia mutation Y705C alters trafficking and biochemical properties of the presynaptic glycine transporter GlyT2

Hyperekplexia or startle disease is characterized by an exaggerated startle response, evoked by tactile or auditory stimuli, producing hypertonia and apnea episodes. Although rare, this orphan disorder can have serious consequences, including sudden infant death. Dominant and recessive mutations in the human glycine receptor (GlyR) α1 gene (GLRA1) are the major cause of this disorder. However, recessive mutations in the presynaptic Na(+)/Cl(-)-dependent glycine transporter GlyT2 gene (SLC6A5) are rapidly emerging as a second major cause of startle disease. In this study, systematic DNA sequencing of SLC6A5 revealed a new dominant GlyT2 mutation: pY705C (c.2114A→G) in transmembrane domain 11, in eight individuals from Spain and the United Kingdom. Curiously, individuals harboring this mutation show significant variation in clinical presentation. In addition to classical hyperekplexia symptoms, some individuals had abnormal respiration, facial dysmorphism, delayed motor development, or intellectual disability. We functionally characterized this mutation using molecular modeling, electrophysiology, [(3)H]glycine transport, cell surface expression, and cysteine labeling assays. We found that the introduced cysteine interacts with the cysteine pair Cys-311-Cys-320 in the second external loop of GlyT2. This interaction impairs transporter maturation through the secretory pathway, reduces surface expression, and inhibits transport function. Additionally, Y705C presents altered H(+) and Zn(2+) dependence of glycine transport that may affect the function of glycinergic neurotransmission in vivo.

Using lithium to probe sequential cation interactions with GAT1

Li(+) interacts with the Na(+)/Cl(-)-dependent GABA transporter, GAT1, under two conditions: in the absence of Na(+) it induces a voltage-dependent leak current; in the presence of Na(+) and GABA, Li(+) stimulates GABA-induced steady-state currents. The amino acids directly involved in the interaction with the Na(+) and Li(+) ions at the so-called "Na2" binding site have been identified, but how Li(+) affects the kinetics of GABA cotransport has not been fully explored. We expressed GAT1 in Xenopus oocytes and applied the two-electrode voltage clamp and (22)Na uptake assays to determine coupling ratios and steady-state and presteady-state kinetics under experimental conditions in which extracellular Na(+) was partially substituted by Li(+). Three novel findings are: 1) Li(+) reduced the coupling ratio between Na(+) and net charge translocated during GABA cotransport; 2) Li(+) increased the apparent Na(+) affinity without changing its voltage dependence; 3) Li(+) altered the voltage dependence of presteady-state relaxations in the absence of GABA. We propose an ordered binding scheme for cotransport in which either a Na(+) or Li(+) ion can bind at the putative first cation binding site (Na2). This is followed by the cooperative binding of the second Na(+) ion at the second cation binding site (Na1) and then binding of GABA. With Li(+) bound to Na2, the second Na(+) ion binds more readily GAT1, and despite a lower apparent GABA affinity, the translocation rate of the fully loaded carrier is not reduced. Numerical simulations using a nonrapid equilibrium model fully recapitulated our experimental findings.

Molecular basis of the differential interaction with lithium of glycine transporters GLYT1 and GLYT2

Glycine synaptic levels are controlled by glycine transporters (GLYTs) catalyzing Na(+)/Cl(-)/glycine cotransport. GLYT1 displays a 2:1 :1 stoichiometry and is the main regulator of extracellular glycine concentrations. The neuronal GLYT2, with higher sodium coupling (3:1 :1), supplies glycine to the pre-synaptic terminal to refill synaptic vesicles. In this work, using structural homology modelling and molecular dynamics simulations of GLYTs, we predict the conservation of the two sodium sites present in the template (leucine transporter from Aquifex aeolicus), and confirm its use by mutagenesis and functional analysis. GLYTs Na1 and Na2 sites show differential cation selectivity, as inferred from the action of lithium, a non-transport-supporting ion, on Na(+)-site mutants. GLYTs lithium responses were unchanged in Na1-site mutants, but abolished or inverted in mutants of Na2 site, which binds lithium in the presence of low sodium concentrations and therefore, controls lithium responses. Here, we report, for the first time, that lithium exerts opposite actions on GLYTs isoforms. Glycine transport by GLYT1 is inhibited by lithium whereas GLYT2 transport is stimulated, and this effect is more evident at increased glycine concentrations. In contrast to GLYT1, high and low affinity lithium-binding processes were detected in GLYT2.

Anionic leak currents through the Na+/monocarboxylate cotransporter SMCT1

SMCT1 is a Na-coupled cotransporter of short chain monocarboxylates, which is expressed in the apical membrane of diverse epithelia such as colon, renal cortex, and thyroid. We previously reported that SMCT1 cotransport was reduced by extracellular Cl− replacement with cyclamate− and that the protein exhibited an ostensible anionic leak current. In this paper, we have revisited the interaction between small monovalent anions and SMCT cotransport and leak currents. We found that the apparent Cl− dependence of cotransport was due to inhibition of this protein by the replacement anion cyclamate, whereas several other replacement anions function as substrates for SMCT1; a suitable replacement anion (MES−) was identified. The observed outward leak currents represented anionic influx and favored larger anions (NO3−>I−>Br−>Cl−); currents in excess of 1 μA (at +50 mV) could be observed and exhibited a quasilinear relationship with anion concentrations up to 100 mM. Application of 25 mM bicarbonate did not produce measurable leak currents. The leak current displayed outward rectification, which disappeared when external Na+ was replaced by N-methyl-d-glucamine+. More precisely, external Na+ blocked the leak current in both directions, but its Ki value rose rapidly when membrane potential became positive. Thus SMCT1 possesses a anionic leak current that becomes significant whenever external Na+ concentration is reduced. The presence of this leak current may represent a second function for SMCT1 in addition to cotransporting short chain fatty acids, and future experiments will determine whether this function serves a physiological role in tissues where SMCT1 is expressed.

Inhibitors of the gamma-aminobutyric acid transporter 1 (GAT1) do not reveal a channel mode of conduction

We expressed the gamma-aminobutyric acid (GABA) transporter GAT1 (SLC6A1) in Xenopus laevis oocytes and performed GABA uptake experiments under voltage clamp at different membrane potentials as well as in the presence of the specific GAT1 inhibitors SKF-89976A and NO-711. In the absence of the inhibitors, GAT1 mediated the inward translocation of 2 net positive charges across the plasma membrane for every GABA molecule transported into the cell. This 2:1 charge flux/GABA flux ratio was the same over a wide range of membrane potentials from -110 mV to +10 mV. Moreover, when GABA-evoked (500 microM) currents were measured at -50 and -90 mV, neither SKF-89976A (5 and 25 microM) nor NO-711 (2 microM) altered the 2:1 charge flux/GABA flux ratio. The results are not consistent with previous hypotheses that (i) GABA evokes an uncoupled channel-mediated current in GAT1, and (ii) GAT1 inhibitors block the putative uncoupled current gated by GABA. Rather, the results suggest tight coupling of GAT1-mediated charge flux and GABA flux.

The importance of company: Na+ and Cl- influence substrate interaction with SLC6 transporters and other proteins

SLC6 transporters, which include transporters for gamma-aminobutyric acid (GABA), norepinephrine, dopamine, serotonin, glycine, taurine, L-proline, creatine, betaine, and neutral cationic amino acids, require Na+ and Cl- for their function, and this review covers the interaction between transporters of this family with Na+ and Cl- from a structure-function standpoint. Because detailed structure-function information regarding ion interactions with SLC6 transporters is limited, we cover other proteins cotransporting Na+ or Cl- with substrate (SLClA2, PutP, SLC5A1, melB), or ion binding to proteins in general (rhodanese, ATPase, LacY, thermolysine, angiotensin-converting enzyme, halorhodopsin, CFTR). Residues can be involved in directly binding Na+ or Cl-, in coupling ion binding to conformational changes in transporter, in coupling Na+ or Cl- movement to transport, or in conferring ion selectivity. Coordination of ions can involve a number of residues, and portions of the substrate and coupling ion binding sites can be distal in space in the tertiary structure of the transporter, with other portions that are close in space thought to be crucial for the coupling process. The reactivity with methanethiosulfonate reagents of cysteines placed in strategic positions in the transporter provides a readout for conformational changes upon ion or substrate binding. More work is needed to establish the relationships between ion interactions and oligomerization of SLC6 transporters.

The SLC6 orphans are forming a family of amino acid transporters

Transporters in the human genome are grouped in solute carrier families (SLC). The SLC6 family is one of the biggest transporter families in the human genome comprising 20 members. It is usually referred to as the neurotransmitter transporter family because its founding members encode transporters for the neurotransmitters GABA, noradrenaline, serotonin and dopamine. The family also includes a number of 'orphan' transporters, the function of which has remained elusive until recently. Identification of the broadly specific neutral amino acid transporter SLC6A19 (also called B(0)AT1) suggested that all orphan transporters may in fact be amino acid transporters. This was subsequently confirmed by the identification of SLC6A20 as the long-sought IMINO system, a proline transporter found in kidney, intestine and brain. Very recently, SLC6A15 was identified as the neutral amino acid transporter B(0)AT2. All amino acid transporters appear to cotransport only 1Na(+) together with the amino acid substrate. Both, B(0)AT1 and B(0)AT2 are chloride independent, whereas IMINO is chloride dependent. The amino acid transporters of the SLC6 family are functionally and sequence related to the recently crystallized leucine transporter from Aquifex aeolicus. The structure elegantly explains many of the mechanistic features of the SLC6 amino acid transporters.

Role of the conserved glutamine 291 in the rat gamma-aminobutyric acid transporter rGAT-1

We investigated the role of the Q291 glutamine residue in the functioning of the rat gamma-aminobutyric acid (GABA) transporter GAT-1. Q291 mutants cannot transport GABA or give rise to transient, leak and transport-coupled currents even though they are targeted to the plasma membrane. Coexpression experiments of wild-type and Q291 mutants suggest that GAT-1 is a functional monomer though it requires oligomeric assembly for membrane insertion. We determined the accessibility of Q291 by investigating the impact of impermeant sulfhydryl reagents on cysteine residues engineered in close proximity to Q291. The effect of these reagents indicates that Q291 faces the external aqueous milieu. The introduction of a steric hindrance close to Q291 by means of [2-(trimethylammonium)ethyl] methanethiosulfonate bromide modification of C74A/T290C altered the affinity of the mutant for cations. Taken together, these results suggest that this irreplaceable residue is involved in the interaction with sodium or in maintaining the cation accessibility to the transporter.

Steady-state kinetic characterization of the mouse B(0)AT1 sodium-dependent neutral amino acid transporter

The members of the neurotransmitter transporter family SLC6A exhibit a high degree of structural homology; however differences arise in many aspects of their transport mechanisms. In this study we report that mouse B(0)AT1 (mouse Slc6a19) mediates the electrogenic transport of a broad range of neutral amino acids but not of the chemically similar substrates transported by other SLC6A family members. Cotransport of L: -Leu and Na(+) generates a saturable, reversible, inward current with Michaelis-Menten kinetics (Hill coefficient approximately 1) yielding a K(0.5) for L: -Leu of 1.16 mM and for Na(+) of 16 mM at a holding potential of -50 mV. Changing the membrane voltage influences both substrate binding and substrate translocation. Li(+) can substitute partially for Na(+) in the generation of L: -Leu-evoked inward currents, whereas both Cl(-) and H(+) concentrations influence its magnitude. The simultaneous measurement of charge translocation and L: -Leu uptake in the same cell indicates that B(0)AT1 transports one Na(+) per neutral amino acid. This appears to be accomplished by an ordered, simultaneous mechanism, with the amino acid binding prior to the Na(+), followed by the simultaneous translocation of both co-substrates across the plasma membrane. From this kinetic analysis, we conclude that the relatively constant [Na(+)] along the renal proximal tubule both drives the uptake of neutral amino acids via B(0)AT1 thermodynamically and ensures that, upon binding, these are translocated efficiently into the cell.

Novel properties of a mouse gamma-aminobutyric acid transporter (GAT4)

We expressed the mouse gamma-aminobutyric acid (GABA) transporter GAT4 (homologous to rat/ human GAT-3) in Xenopus laevis oocytes and examined its functional and pharmacological properties by using electrophysiological and tracer uptake methods. In the coupled mode of transport (Na+/ Cl-/GABA cotransport), there was tight coupling between charge flux and GABA flux across the plasma membrane (2 charges/GABA). Transport was highly temperature-dependent with a temperature coefficient (Q10) of 4.3. The GAT4 turnover rate (1.5 s(-l); -50 mV, 21 degrees C) and temperature dependence suggest physiological turnover rates of 15-20 s(-1). No uncoupled current was observed in the presence of Na+. In the absence of external Na+, GAT4 exhibited two distinct uncoupled currents. (i) A Cl- leak current (ICl(leak)) was observed when Na+ was replaced with choline or tetraethylammonium. The reversal potential of (ICl(leak)) followed the Cl- Nernst potential. (ii) A Li+ leak current (ILi(leak)) was observed when Na+ was replaced with Li+. Both leak currents were inhibited by Na+, and both were temperature-independent (Q10 approximately 1). The two leak modes appeared not to coexist, as Li+ inhibited (ICl(leak)). The results suggest the existence of cation- and anion-selective channel-like pathways in GAT4. Flufenamic acid inhibited GAT4 Na+/Cl-/GABA cotransport, ILi(leak), and ICl(leak), (Ki approximately 30 microM), and the voltage-induced presteady-state charge movements (Ki approximately 440 microM). Flufenamic acid exhibited little or no selectivity for GAT1, GAT2, or GAT3. Sodium and GABA concentration jicroumps revealed that slow Na+ binding to the transporter is followed by rapid GABA-induced translocation of the ligands across the plasma membrane. Thus, Na+ binding and associated conformational changes constitute the rate-limiting steps in the transport cycle.

Functional characterization of high-affinity Na(+)/dicarboxylate cotransporter found in Xenopus laevis kidney and heart

The SLC13 gene family includes sodium-coupled transporters for citric acid cycle intermediates and sulfate. The present study describes the sequence and functional characterization of a SLC13 family member from Xenopus laevis, the high-affinity Na(+)/dicarboxylate cotransporter xNaDC-3. The cDNA sequence of xNaDC-3 codes for a protein of 602 amino acids that is approximately 70% identical to the sequences of mammalian NaDC-3 orthologs. The message for xNaDC-3 is found in the kidney, liver, intestine, and heart. The xNaDC-3 has a high affinity for substrate, including a K(m) for succinate of 4 muM, and it is inhibited by the NaDC-3 test substrates 2,3-dimethylsuccinate and adipate. The transport of succinate by xNaDC-3 is dependent on sodium, with sigmoidal activation kinetics, and lithium can partially substitute for sodium. As with other members of the family, xNaDC-3 is electrogenic and exhibits inward substrate-dependent currents in the presence of sodium. However, other electrophysiological properties of xNaDC-3 are unique and involve large leak currents, possibly mediated by anions, that are activated by binding of sodium or lithium to a single site.

GABA transporters in the mammalian cerebral cortex: localization, development and pathological implications

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity, Na+/Cl- dependent transporters. Four distinct genes encoding GABA transporters (GATs), named GAT-1, GAT-2, GAT-3, and BGT-1 have been identified using molecular cloning. Of these, GAT-1 and -3 are expressed in the cerebral cortex. Studies of the cortical distribution, cellular localization, ontogeny and relationships of GATs with GABA-releasing elements using a variety of light and electron microscopic immunocytochemical techniques have shown that: (i) a fraction of GATs is strategically placed to mediate GABA uptake at fast inhibitory synapses, terminating GABA's action and shaping inhibitory postsynaptic responses; (ii) another fraction may participate in functions such as the regulation of GABA's diffusion to neighboring synapses and of GABA levels in cerebrospinal fluid; (iii) GATs may play a role in the complex processes regulating cortical maturation; and (iv) GATs may contribute to the dysregulation of neuronal excitability that accompanies at least two major human diseases: epilepsy and ischemia.

The first external loop of the metal ion transporter DCT1 is involved in metal ion binding and specificity

The yeast null mutant smf1Delta cannot grow on medium containing EGTA. Expression of Smf1p or the mammalian transporter DCT1 (Slc11a2) suppresses the above-mentioned phenotype. Both can also be expressed in Xenopus oocytes, and the uptake activity and their electrophysiological properties can be studied. We used these systems to analyze the properties of mutations in the predicted external loop I of DCT1. The sensitivity of the transporter to amino acid substitutions in this region is manifested by the mutation G119A, which resulted in almost complete inhibition of the metal ion uptake activity and marked changes in the pre-steady-state currents in Xenopus oocytes. The mutation Q126D abolished the uptake and the electrophysiology, but the double mutant D124A/Q126D partially restored it and changed the metal ion specificity in favor of Fe2+. The maximal pre-steady-state currents at negatively imposed potentials shifted to a lower pH of approximately 5. The triple mutant G119A/D124A/Q126D, which has no apparent transport activity, exhibited remarkable pre-steady-state currents at pH 7.5. Moreover, Zn2+ had a dual effect on this mutant; at pH 7.5 it eliminated the pre-steady state without generating steady-state currents, and at pH 5.5 it induced large pre-steady-state currents. The mutant D124A retained appreciable Fe2+ uptake activity but exhibited very little Mn2+ uptake at pH 5.5 and was abolished at pH 6.5. The properties of the various mutants suggest that loop I is involved in the metal ion binding and its coupling to the proton-driving force.