Role of the GABA transporter in epilepsy

A review on epilepsy, current treatments, and potential of medicinal plants as an alternative treatment

Epilepsy is considered common neurological diseases that threaten the lives of millions of people all around the world. Since ancient times, different forms of medications have been used to treat this condition. Adverse events associated with treatments and the residence time of available drugs caused to search for safer and more efficient therapies and drugs remain one of the major areas of research interest for scientists. As one of the therapeutics with fewer side effects, plants and their essential oils can be considered replacements for existing treatments. Medicinal plants have proven to be an effective natural source of antiepileptic drugs; most of them have their mechanism of action by affecting GABA receptors in different paths. Cannabis indica and Cymbopogon winterianus are well-known plant species with antiepileptic activities. The current review presenting a list of plants with antiepileptic effects aims to pave the way for finding alternative drugs with fewer side effects for scientists.

1H Magnetic Resonance Spectroscopy to Understand the Biological Basis of ALS, Diagnose Patients Earlier, and Monitor Disease Progression

At present, limited biomarkers exist to reliably understand, diagnose, and monitor the progression of amyotrophic lateral sclerosis (ALS), a fatal neurological disease characterized by motor neuron death. Standard MRI technology can only be used to exclude a diagnosis of ALS, but ¹H-MRS technology, which measures neurochemical composition, may provide the unique ability to reveal biomarkers that are specific to ALS and sensitive enough to diagnose patients at early stages in disease progression. In this review, we present a summary of current theories of how mitochondrial energetics and an altered glutamate/GABA neurotransmitter flux balance play a role in the pathogenesis of ALS. The theories are synthesized into a model that predicts how pathogenesis impacts glutamate and GABA concentrations. When compared with the results of all MRS studies published to date that measure the absolute concentrations of these neurochemicals in ALS patients, results were variable. However, when normalized for neuronal volume using the MRS biomarker N-acetyl aspartate (NAA), there is clear evidence for an elevation of neuronal glutamate in nine out of thirteen studies reviewed, an observation consistent with the predictions of the model of increased activity of glutamatergic neurons and excitotoxicity. We propose that this increase in neuronal glutamate concentration, in combination with decreased neuronal volume, is specific to the pathology of ALS. In addition, when normalized to glutamate levels, there is clear evidence for a decrease in neuronal GABA in three out of four possible studies reviewed, a finding consistent with a loss of inhibitory regulation contributing to excessive neuronal excitability. The combination of a decreased GABA/Glx ratio with an elevated Glx/NAA ratio may enhance the specificity for ¹H-MRS detection of ALS and ability to monitor glutamatergic and GABAergic targeted therapeutics. Additional longitudinal studies calculating the exact value of these ratios are needed to test these hypotheses and understand how ratios may change over the course of disease progression. Proposed modifications to the experimental design of the reviewed ¹H MRS studies may also increase the sensitivity of the technology to changes in these neurochemicals, particularly in early stages of disease progression.

Drug treatment of epilepsy: From serendipitous discovery to evolutionary mechanisms

Epilepsy is a chronic brain disorder caused by abnormal firing of neurons. Up to now, using antiepileptic drugs is the main method of epilepsy treatment. The development of antiepileptic drugs lasted for centuries. In general, most agents entering clinical practice act on the balance mechanisms of brain "excitability-inhibition". More specifically, they target voltage-gated ion channels, GABAergic transmission and glutamatergic transmission. In recent years, some novel drugs representing new mechanisms of action have been discovered. Although there are about 30 available drugs in the market, it is still in urgent need of discovering more effective and safer drugs. The development of new antiepileptic drugs is into a new era: from serendipitous discovery to evolutionary mechanism-based design. This article presents an overview of drug treatment of epilepsy, including a series of traditional and novel drugs.

Selenium chalcogen bonds are involved in protein-carbohydrate recognition: a combined PDB and theoretical study

In this manuscript the ability of selenium carbohydrates to undergo chalcogen bonding (ChB) interactions with protein residues has been studied at the RI-MP2/def2-TZVP level of theory. An inspection of the Protein Data Bank (PDB) revealed SeA (A = O, C and S) intermolecular contacts involving Se-pyranose ligands and ASP, TYR, SER and MET residues. Theoretical models were built to analyse the strength and directionality of the interaction together with "Atoms in Molecules" (AIM), Natural Bonding Orbital (NBO) and Non Covalent Interactions plot (NCIplot) analyses, which further assisted in the characterization of the ChBs described herein. We expect that the results from this study will be useful to expand the current knowledge regarding biological ChBs as well as to increase the visibility of the interaction among the carbohydrate chemistry community.

The Lepidopteran KAAT1 and CAATCH1: Orthologs to Understand Structure-Function Relationships in Mammalian SLC6 Transporters

To the SLC6 family belong 20 human transporters that utilize the sodium electrochemical gradient to move biogenic amines, osmolytes, amino acids and related compounds into cells. They are classified into two functional groups, the Neurotransmitter transporters (NTT) and Nutrient amino acid transporters (NAT). Here we summarize how since their first cloning in 1998, the insect (Lepidopteran) Orthologs of the SLC6 family transporters have represented very important tools for investigating functional–structural relationships, mechanism of transport, ion and pH dependence and substate interaction of the mammalian (and human) counterparts.

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.

Molecular Dynamic Simulations to Probe Stereoselectivity of Tiagabine Binding with Human GAT1

The human gamma aminobutyric acid transporter subtype 1 (hGAT1) located in the nerve terminals is known to catalyze the neuronal function by the electrogenic reuptake of γ-aminobutyric acid (GABA) with the co-transport of Na⁺ and Cl⁻ ions. In the past, there has been a major research drive focused on the dysfunction of hGAT1 in several neurological disorders. Thus, hGAT1 of the GABAergic system has been well established as an attractive target for such diseased conditions. Till date, there are various reports about stereo selectivity of –COOH group of tiagabine, a Food and Drug Administration (FDA)-approved hGAT1-selective antiepileptic drug. However, the effect of the stereochemistry of the protonated –NH group of tiagabine has never been scrutinized. Therefore, in this study, tiagabine has been used to explore the binding hypothesis of different enantiomers of tiagabine. In addition, the impact of axial and equatorial configuration of the–COOH group attached at the meta position of the piperidine ring of tiagabine enantiomers was also investigated. Further, the stability of the finally selected four hGAT1–tiagabine enantiomers namely entries 3, 4, 6, and 9 was evaluated through 100 ns molecular dynamics (MD) simulations for the selection of the best probable tiagabine enantiomer. The results indicate that the protonated –NH group in the R-conformation and the –COOH group of Tiagabine in the equatorial configuration of entry 4 provide maximum strength in terms of interaction within the hGAT1 binding pocket to prevent the change in hGAT1 conformational state, i.e., from open-to-out to open-to-in as compared to other selected tiagabine enantiomers 3, 6, and 9.

Betaine/GABA transporter-1 (BGT-1) deficiency in mouse prevents acute liver failure in vivo and hepatocytes apoptosis in vitro

Betaine/γ-aminobutyric acid (GABA) transporter 1 (BGT-1 or Slc6a12) is a transporter for the neurotransmitter GABA and osmolyte betaine. To date, most studies on BGT-1 have focused on its functions in the nervous system and renal osmotic homeostasis. Despite its dominant distribution in the liver, the function of BGT-1 in hepatic physiology or disease remains unknown. Here, we report that BGT-1 was significantly downregulated in patients with liver failure as well as in mice with experimental acute liver failure (ALF). Furthermore, mice deficient in BGT-1 showed significant resistance to ALF compared with wild type (WT) mice, manifesting as improved survival rate, reduced alanine transaminase/aspartate aminotransferase levels, better histopathological symptoms and fewer apoptotic cells in the liver. Similarly, in primary hepatocytes, BGT-1 deficiency or treatment with a BGT-1 inhibitor, NNC 05-2090, attenuated TNF-α mediated apoptosis. In addition, BGT-1 deficiency or dosing with NNC 05-2090 stimulated the expression of the anti-apoptotic gene, c-Met in the liver, suggesting the involvement of c-Met in the function on hepatocytes of BGT-1 apoptosis. Our findings suggest BGT-1 is a promising candidate drug target to prevent and treat hepatocyte apoptosis related diseases, such as ALF.

Age-Dependent Electrocorticogram Dynamics and Epileptogenic Responsiveness in Rats Subjected to Prenatal Hypoxia

Using electrocorticogram (ECoG) analysis, we compared age-related dynamics of general neuronal activity and convulsive epileptiform responsiveness induced by intracortical microinjections of 4-aminopyridine (4-AP) in control Wistar rats and those subjected to prenatal hypoxia (Hx; E14; 7% O2, 3 h). The studies were carried out in three age periods roughly corresponding to childhood (P20-27), adolescence (P30-45), and adulthood (P90-120). It was found that in the process of postnatal development of the control rats, the peak of the ECoG power spectrum density (PSD) of the theta rhythm during wakefulness shifted from the low to the higher frequency, while in the Hx rats this shift had the opposite direction. Moreover, the Hx rats had different frequency characteristics of the ECoG PSD and longer episodes of spike-and-wave discharges caused by 4-AP injections compared to the controls. The total ECoG PSD of slow-wave sleep (1-5 Hz) was also dramatically decreased in the process of development of the Hx rats. Such alterations in PSD could be explained by the changes in balance of the excitation and inhibition processes in the cortical networks. Analyzing protein levels of neurotransmitter transporters in the brain structures of the Hx rats, we found that the content of the glutamate transporter EAAT1 was higher in the parietal cortex in all age groups of Hx rats while in the hippocampus it decreased during postnatal development compared to controls. Furthermore, the content of the vesicular acetylcholine transporter in the parietal cortex, and of the inhibitory GABA transporter 1 in the hippocampus, was also affected by prenatal Hx. These data suggest that prenatal Hx results in a shift in the excitatory and inhibitory balance in the rat cortex towards excitation, making the rat's brain more vulnerable to the effects of proconvulsant drugs and predisposing animals to epileptogenesis during postnatal life.

Thermodynamic first law efficiency of membrane proteins

Proteins are nature's biomolecular machines. Proteins, such as transporters, pumps and motors, have complex function/operating-machinery/mechanisms, comparable to the macro-scaled machines that we encounter in our daily life. These proteins, as it is for their macro-scaled counterparts, convert (part of) other/various forms of energy into work. In this study, we are performing the first law analysis on a set of proteins, including the dopamine transporter, glycine transporters I and II, glutamate transporter, sodium-potassium pump and Ca²⁺ ATPase. Each of these proteins operates on a thermodynamic/mechanic cycle to perform their function. In each of these cycles, they receive energy from a source, convert part of this energy into work and reject the remaining part of the energy to the environment. Conservation of energy principle was applied to the thermodynamic/mechanic cycle of each protein, and thermodynamic first law efficiency was evaluated for each cycle, which shows how much of the energy input per cycle was converted into useful work. Interestingly, calculations based on experimental data indicate that proteins can operate under a range of efficiencies, which vary based on the extracellular and intracellular ion and substrate concentrations. The lowest observed first law efficiency was 50%, which is a very high value if compared to the efficiency of the macro-scaled heat engines we encounter in our daily lives.Communicated by Ramaswamy H. Sarma.

Vitamin D3 deficiency in puberty rats causes presynaptic malfunctioning through alterations in exocytotic release and uptake of glutamate/GABA and expression of EAAC-1/GAT-3 transporters

Recent experimental and epidemiologic investigations have revealed that the central nervous system is a target for vitamin D3 action and also linked vitamin D3 deficiency to Alzheimer's and Parkinson's disease, autism and dementia. Abnormal homeostasis of glutamate and GABA and signaling disbalance are implicated in the pathogenesis of major neurological diseases. Here, key transport characteristics of glutamate and GABA were analysed in presynaptic nerve terminals (synaptosomes) isolated from the cortex of vitamin D3 deficient (VDD) rats. Puberty rats were kept at the VDD diet up to adulthood. VDD caused: (i) a decrease in the initial rates of L-[¹⁴C]glutamate and [³H]GABA uptake by plasma membrane transporters of nerve terminals; (ii) a decrease in exocytotic release of L-[¹⁴C]glutamate and [³H]GABA; (iii) changes in expression of glutamate (EAAC-1) and GABA (GAT-3) transporters. Whereas, the synaptosomal ambient levels and Ca²⁺-independent transporter-mediated release of L-[¹⁴C]glutamate and [³H]GABA were not significantly altered in VDD. Vitamin D3 is a potent neurosteroid and its nutritional deficiency can provoke development of neurological consequences changing glutamate/GABA transporter expressions and excitation/inhibition balance. Also, changes in glutamate transport can underlie lower resistance to hypoxia/ischemia, larger infarct volumes and worsened outcomes in ischemic stroke patients with VDD.

Expression and functions of glutamate and γ‑aminobutyric acid transporters in ischemic models

Glutamate and γ-aminobutyric acid (GABA) transporters serve central roles in normal neuronal activity and are associated with numerous pathological brain conditions, including ischemia and epilepsy. However, the interplay between these transporters in ischemia remains unclear. In the present study, the expression levels of the excitatory amino acid carrier 1 (EAAC1) and GABA transporter 1 (GAT1) were analyzed in vivo and in vitro within ischemic models by immunofluorescence, western blot and RT-qPCR. Cell survival rates were analyzed following altered expression of these transporters within neuronal cells by flow cytometry. Expression levels of EAAC1 were reduced within the cerebrum of focal cerebral ischemic middle cerebral artery occlusion rat models as well as in primary neurons cultured under hypoxia. However, GAT1 expression levels were slightly elevated under ischemic conditions. The altered expression levels of EAAC1 and GAT1 were combined within neuron cells and the effects were investigated. Apoptotic analysis revealed that EAAC1 suppression and overexpression of GAT1 increased neuronal cell apoptosis under hypoxic conditions; however, EAAC1 overexpression combined with GAT1 knockdown reduced neuronal cell apoptosis under hypoxic conditions. The present study detected the expression levels of the glutamate and GABA transporters under hypoxia, in association with ischemia. The results indicated that, increased expression of EAAC1 combined with GAT1 suppression may provide protective effects in the treatment of epilepsy and ischemia.

Effect of Dimerization on the Dynamics of Neurotransmitter:Sodium Symporters

Dimerization is a common feature among the members of the neurotransmitter:sodium symporter (NSS) family of membrane proteins. Yet, the effect of dimerization on the mechanism of action of NSS members is not fully understood. In this study, we examined the collective dynamics of two members of the family, leucine transporter (LeuT) and dopamine transporter (DAT), to assess the significance of dimerization in modulating the functional motions of the monomers. We used to this aim the anisotropic network model (ANM), an efficient and robust method for modeling the intrinsic motions of proteins and their complexes. Transporters belonging to the NSS family are known to alternate between outward-facing (OF) and inward-facing (IF) states, which enables the uptake and release of their substrate (neurotransmitter) respectively, as the substrate is transported from the exterior to the interior of the cell. In both LeuT and DAT, dimerization is found to alter the collective motions intrinsically accessible to the individual monomers in favor of the functional transitions (OF ↔ IF), suggesting that dimerization may play a role in facilitating transport.

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.

Effects of new fluorinated analogues of GABA, pregabalin bioisosters, on the ambient level and exocytotic release of [3H]GABA from rat brain nerve terminals

Recently, we have shown that new fluorinated analogues of γ-aminobutyric acid (GABA), bioisosters of pregabalin (β-i-Bu-GABA), i.e. β-polyfluoroalkyl-GABAs (FGABAs), with substituents: β-CF₃-β-OH (1), β-CF₃ (2); β-CF₂CF₂H (3), are able to increase the initial rate of [³H]GABA uptake by isolated rat brain nerve terminals (synaptosomes), and this effect is higher than that of pregabalin. So, synthesized FGABAs are structural but not functional analogues of GABA. Herein, we assessed the effects of synthesized FGABAs (100μM) on the ambient level and exocytotic release of [³H]GABA in nerve terminals and compared with those of pregabalin (100μM). It was shown that FGABAs 1-3 did not influence the ambient level of [³H]GABA in the synaptosomal preparations, and this parameter was also not altered by pregabalin. During blockage of GABA transporters GAT1 by specific inhibitor NO-711, FGABAs and pregabalin also did not change ambient [³H]GABA in synaptosomal preparations. Exocytotic release of [³H]GABA from synaptosomes decreased in the presence of FGABAs 1-3 and pregabalin, and the effects of FGABAs 1 &3 were more significant than those of FGABAs 2 and pregabalin. FGABAs 1-3/pregabalin-induced decrease in exocytotic release of [³H]GABA from synaptosomes was not a result of changes in the potential of the plasma membrane. Therefore, new synthesized FGABAs 1 &3 were able to decrease exocytotic release of [³H]GABA from nerve terminals more effectively in comparison to pregabalin. Absence of unspecific side effects of FGABAs 1 &3 on the membrane potential makes these compounds perspective for medical application.

Energy landscape of LeuT from molecular simulations

The bacterial sodium-coupled leucine transporter (LeuT) has been broadly used as a structural model for understanding the structure-dynamics-function of mammalian neurotransmitter transporters as well as other solute carriers that share the same fold (LeuT fold), as the first member of the family crystallographically resolved in multiple states: outward-facing open, outward-facing occluded, and inward-facing open. Yet, a complete picture of the energy landscape of (sub)states visited along the LeuT transport cycle has been elusive. In an attempt to visualize the conformational spectrum of LeuT, we performed extensive simulations of LeuT dimer dynamics in the presence of substrate (Ala or Leu) and co-transported Na(+) ions, in explicit membrane and water. We used both conventional molecular dynamics (MD) simulations (with Anton supercomputing machine) and a recently introduced method, collective MD, that takes advantage of collective modes of motions predicted by the anisotropic network model. Free energy landscapes constructed based on ∼40 μs trajectories reveal multiple substates occluded to the extracellular (EC) and/or intracellular (IC) media, varying in the levels of exposure of LeuT to EC or IC vestibules. The IC-facing transmembrane (TM) helical segment TM1a shows an opening, albeit to a smaller extent and in a slightly different direction than that observed in the inward-facing open crystal structure. The study provides insights into the spectrum of conformational substates and paths accessible to LeuT and highlights the differences between Ala- and Leu-bound substates.

Effect of External Electric Field on Substrate Transport of a Secondary Active Transporter

Substrate transport across a membrane accomplished by a secondary active transporter (SAT) is essential to the normal physiological function of living cells. In the present research, a series of all-atom molecular dynamics (MD) simulations under different electric field (EF) strengths was performed to investigate the effect of an external EF on the substrate transport of an SAT. The results show that EF both affects the interaction between substrate and related protein's residues by changing their conformations and tunes the timeline of the transport event, which collectively reduces the height of energy barrier for substrate transport and results in the appearance of two intermediate conformations under the existence of an external EF. Our work spotlights the crucial influence of external EFs on the substrate transport of SATs and could provide a more penetrating understanding of the substrate transport mechanism of SATs.

γ-aminobutyric acid transporter-1 is involved in anxiety-like behaviors and cognitive function in knockout mice

The aim of the present study was to investigate the effect of γ-aminobutyric acid transporter-1 (GAT-1) on the anxiety-like behaviors and cognitive function in knockout mice. In total, 20 adult male mice were divided into two groups, namely the GAT-1 knockout (GAT-1-/-) and wild-type (WT) groups. The open field test, elevated 0-maze (EZM) and Morris water maze were used to evaluate changes in anxiety-like behaviors and cognitive function. Compared with the WT mice, GAT-1-/- mice made more entries and spent a longer time within the central area, traveling a greater distance, during the open field test (P<0.05). The EZM revealed that GAT-1-/- mice spent more time in the open sectors and made more total entries when compared with the WT mice (P<0.01). Observations from the two tests indicated reduced anxiety-like behaviors in the GAT-1-/- mice. During the learning session using a Morris water maze, the latency to find the platform was significantly longer in the GAT-1-/- mice when compared with the WT mice (P<0.01). In addition, during the probe test, the GAT-1-/- mice spent less time in the target quadrant and more time in the opposite quadrant when compared with the WT mice (P<0.01); thus, the cognitive function in the GAT-1-/- mice was impaired. Therefore, the results demonstrated that the anxiety-like behaviors were reduced and cognitive function was impaired in GAT-1 knockout mice, indicating that GAT-1 is involved in anxiety and cognitive functions.

Synthesis of new fluorinated analogs of GABA, Pregabalin bioisosteres, and their effects on [(3)H]GABA uptake by rat brain nerve terminals

Fluorinated analogs of natural substances take an essential place in the design of new biologically active compounds. New fluorinated analogs of γ-aminobutyric acid, that is, β-polyfluoroalkyl-GABAs (FGABAs), were synthesized with substituents: β-CF3-β-OH (1), β-CF3 (2); β-CF2CF2H (3). FGABAs are bioisosteres of Pregabalin (Lyrica®, Pfizer's blockbuster drug, β-i-Bu-GABA), and have lipophilicity close to this medicine. The effects of synthesized FGABAs on [(3)H]GABA uptake by isolated rat brain nerve terminals (synaptosomes) were assessed and compared with those of Pregabalin. FGABAs 1-3 (100μM) did not influence the initial velocity of [(3)H]GABA uptake when applied acutely, whereas an increase in this parameter was found after preliminary incubation of FGABAs with synaptosomes. Pregabalin after preliminary incubation with synaptosomes caused unidirectional changes in the initial velocity of [(3)H]GABA uptake. Using specific inhibitors of GAT1 and GAT3, NO-711 and SNAP5114, respectively, the ability of FGABAs 1-3 to influence non-GAT1 and non-GAT3 uptake activity of nerve terminals was analyzed, but no specificity was found. Therefore, new synthesized FGABAs are structural but not functional analogs of GABA (because they did not inhibit synaptosomal [(3)H]GABA uptake). Moreover, FGABAs are able to increase the initial velocity of [(3)H]GABA uptake by synaptosomes, and this effect is higher than that of Pregabalin.

Microseconds simulations reveal a new sodium-binding site and the mechanism of sodium-coupled substrate uptake by LeuT

The bacterial sodium-coupled leucine/alanine transporter LeuT is broadly used as a model system for studying the transport mechanism of neurotransmitters because of its structural and functional homology to mammalian transporters such as serotonin, dopamine, or norepinephrine transporters, and because of the resolution of its structure in different states. Although the binding sites (S1 for substrate, and Na1 and Na2 for two co-transported sodium ions) have been resolved, we still lack a mechanistic understanding of coupled Na(+)- and substrate-binding events. We present here results from extensive (>20 μs) unbiased molecular dynamics simulations generated using the latest computing technology. Simulations show that sodium binds initially the Na1 site, but not Na2, and, consistently, sodium unbinding/escape to the extracellular (EC) region first takes place at Na2, succeeded by Na1. Na2 diffusion back to the EC medium requires prior dissociation of substrate from S1. Significantly, Na(+) binding (and unbinding) consistently involves a transient binding to a newly discovered site, Na1″, near S1, as an intermediate state. A robust sequence of substrate uptake events coupled to sodium bindings and translocations between those sites assisted by hydration emerges from the simulations: (i) bindings of a first Na(+) to Na1″, translocation to Na1, a second Na(+) to vacated Na1″ and then to Na2, and substrate to S1; (ii) rotation of Phe(253) aromatic group to seclude the substrate from the EC region; and (iii) concerted tilting of TM1b and TM6a toward TM3 and TM8 to close the EC vestibule.

Neuroprotective role of liver growth factor "LGF" in an experimental model of cerebellar ataxia

Cerebellar ataxias (CA) comprise a heterogeneous group of neurodegenerative diseases characterized by a lack of motor coordination. They are caused by disturbances in the cerebellum and its associated circuitries, so the major therapeutic goal is to correct cerebellar dysfunction. Neurotrophic factors enhance the survival and differentiation of selected types of neurons. Liver growth factor (LGF) is a hepatic mitogen that shows biological activity in neuroregenerative therapies. We investigate the potential therapeutic activity of LGF in the 3-acetylpiridine (3-AP) rat model of CA. This model of CA consists in the lesion of the inferior olive-induced by 3-AP (40 mg/kg). Ataxic rats were treated with 5 µg/rat LGF or vehicle during 3 weeks, analyzing: (a) motor coordination by using the rota-rod test; and (b) the immunohistochemical and biochemical evolution of several parameters related with the olivo-cerebellar function. Motor coordination improved in 3-AP-lesioned rats that received LGF treatment. LGF up-regulated NeuN and Bcl-2 protein levels in the brainstem, and increased calbindin expression and the number of neurons receiving calbindin-positive projections in the cerebellum. LGF also reduced extracellular glutamate and GABA concentrations and microglia activation in the cerebellum. In view of these results, we propose LGF as a potential therapeutic agent in cerebellar ataxias.

Perinatal hypoxia: different effects of the inhibitors of GABA transporters GAT1 and GAT3 on the initial velocity of [3H]GABA uptake by cortical, hippocampal, and thalamic nerve terminals

AIM. To analyze the effects of highly selective blocker GAT1, NO-711, and substrate inhibitor GAT3, β-alanine, on the initial velocity of [(3)H]GABA uptake by cortical, hippocampal, and thalamic nerve terminals (synaptosomes) after perinatal hypoxia. METHODS. Animals were divided into two groups: control (n=17) and hypoxia (n=12). Rats in the hypoxia group underwent hypoxia and seizures (airtight chamber, 4% O2 and 96% N2) at the age of 10-12 postnatal days and were used in the experiments 8-9 weeks after hypoxia. RESULTS. In cortical synaptosomes, the effects of NO-711 (30 μΜ) and β-alanine (100 μΜ) on [(3)H]GABA uptake were similar in control and hypoxia groups. In hippocampal synaptosomes, NO-711 inhibited 84.3% of the initial velocity of [(3)H]GABA uptake in normal conditions and 80.1% after hypoxia, whereas the effect of β-alanine was increased after hypoxia from 14.4% to 22.1%. In thalamic synaptosomes, the effect of NO-711 was decreased by 79.6% in controls and by 70.9% in hypoxia group, whereas the effect of β-alanine was increased after hypoxia from 20.2% to 30.2%. CONCLUSIONS. The effectiveness of β-alanine to influence GABA uptake was increased in hippocampal and thalamic nerve terminals as a result of perinatal hypoxia and the effectiveness of NO-711 in thalamic nerve terminals was decreased. These results may indicate changes in the ratio of active GAT1/GAT3 expressed in the plasma membrane of nerve terminals after perinatal hypoxia. We showed a possibility to modulate non-GAT1 GABA transporter activity in different brain regions by exogenous and endogenous β-alanine.

Plasma membrane transporters GAT-1 and GAT-3 contribute to heterogeneity of GABAergic synapses in neocortex

Cortical GABAergic synapses exhibit a high degree of molecular, anatomical and functional heterogeneity of their neurons of origins, presynaptic mechanisms, receptors, and scaffolding proteins. GABA transporters (GATs) have an important role in regulating GABA levels; among them, GAT-1 and GAT-3 play a prominent role in modulating tonic and phasic GABA_AR-mediated inhibition. We asked whether GAT-1 and GAT-3 contribute to generating heterogeneity by studying their ultrastructural localization at cortical symmetric synapses using pre- and post-embedding electron microcopy. GAT-1 and GAT-3 staining at symmetric synapses showed that in some cases the transporters were localized exclusively over axon terminals; in others they were in both axon terminals and perisynaptic astrocytic processes; and in some others GAT-1 and GAT-3 were in perisynaptic astrocytic processes only. Moreover, we showed that the organizational pattern of GAT-1, but not of GAT-3, exhibits a certain degree of specificity related to the post-synaptic target of GABAergic synapses. These findings show that symmetric synapses expressing GAT-1 or GAT-3 are heterogeneous, and indicate that plasma membrane transporters can contribute to synaptic heterogeneity.

The effect of tiagabine on physical development and neurological reflexes and their relationship with the γ-aminobutyric acid switch in the rat cerebral cortex during developmental stages

In the present study, we focused on γ-aminobutyric acid (GABA) signaling through the γ-aminobutyric acid transporter (GAT) in the developing rat cerebral cortex. Tiagabine was used as a GAT inhibitor. The offspring received injections from birth until postnatal day 21 intraperitoneally. Physical development and neurological reflexes were assessed daily. Tiagabine did not influence body weight, the onset and completion of incisor eruption, or the time to appearance of cliff avoidance. However, the onset and completion of eye opening, ear unfolding, and fur growth occurred earlier in treated pups. Further, the slanted board test and righting reflex showed accelerated development (i.e. decreased time to criterion) when compared with the control group. To determine whether the obtained effects are related to the GABA switch, we examined the protein and mRNA expression of the K(+)-Cl(-) cotransporter KCC2 using western blotting and RT-PCR, respectively. Downregulation of KCC2 mRNA and protein levels was observed when GAT was inhibited. The results may indicate a role of GAT in the neurobehavioral changes that accompany the developmental switch in GABA function.

Development and characterization of [123I]iodotiagabine for in-vivo GABA-transporter imaging

The increased knowledge of molecular changes associated with different neurological disorders calls for the development of novel radioligands. Tiagabine (Gabitril) is an anticonvulsive drug that binds selectively to GABA transporter-1 and thereby inhibits GABA uptake. As radioligands for in-vivo imaging of the GABA transporter are not yet available, we radiolabelled tiagabine and assessed its efficacy for in-vivo imaging of these transporters. Tiagabine was first brominated at its vinylic part, which was then exchanged with I. Next, anaesthetized rats received a bolus injection of [I]iodotiagabine in their tail vein, which was immediately followed by acquisition of planar and high-resolution micro-single-photon emission computed tomography (SPECT) images of the total body with special focus on the brain. Uptake in anatomical regions was assessed by coregistration of micro-SPECT with micro-CT images. Tiagabine labelling with I resulted in 50% yield and 99.7% radiochemical purity. Within 3 h after injection, SPECT demonstrated an increased signal-to-background ratio in the nasal mucosa and/or the Harderian glands but not in the brain. In addition we observed an increased signal-to-background ratio in organs such as the thyroid, heart, liver, kidney and bladder. More than 99% pure I-labelled tiagabine can be obtained and applied in animal micro-SPECT studies. However, this new radioligand is not taken up sufficiently by the brain and therefore cannot be used to successfully detect cerebral GABA transporters.

An inverse relationship links temperature and substrate apparent affinity in the ion-coupled cotransporters rGAT1 and KAAT1

The effects of temperature on the operation of two ion-coupled cotransporters of the SLC6A family, namely rat GAT1 (SLC6A1) and KAAT1 (SLC6A19) from Manduca sexta, have been studied by electrophysiological means in Xenopus laevis oocytes expressing these proteins. The maximal transport-associated current (I_max) and the apparent substrate affinity (K₀₅) were measured. In addition to the expected increase in transport rate (Q₁₀ = 3–6), both transporters showed greater K₀₅ values (i.e., a decrease in apparent affinity) at higher temperatures. The transport efficiency, estimated as I_max/K₀₅, increased at negative potentials in both transporters, but did not show statistically significant differences with temperature. The observation that the apparent substrate affinity is inversely related to the transport rate suggests a kinetic regulation of this parameter. Furthermore, the present results indicate that the affinities estimated at room temperature for mammalian cotransporters may not be simply extrapolated to their physiological operating conditions.

The dynamics of changes in hippocampal GABAergic system in rats exposed to early-life hypoxia-induced seizures

Hypoxia-evoked seizures (H/S) early in life lead to multiple chronic neurological deficits. Here, we present the results of studying GABA release and uptake in hippocampal axon terminals of rats exposed to H/S at 10-12 days of age. We characterized (i) exocytotic release of GABA; (ii) the initial rate of GABA uptake; (iii) the regulation of GABA release by presynaptic GABA(B) receptors. Rats were used for experiments 2, 4 and 8 weeks after H/S. We found that exocytotic [(3)H]GABA release was higher in rats exposed to H/S, and a maximal difference in the release was observed between the control and experimental rats tested 2 weeks after H/S. In contrast, the initial rate of GABA uptake decreased with age, and this tendency was more pronounced in rats exposed to H/S. Using (±)-baclofen and SKF 97541 as agonists of GABA(B) receptor, we revealed that a significant difference in the auto-inhibition of exocytotic [(3)H]GABA release was detected only between the control and experimental adult rats (8 weeks after hypoxia). The inhibitory effect dropped dramatically in the control adults, but only slightly decreased in adult rats exposed to H/S, thus becoming threefold more potent after hypoxic injury. Together, the results show that H/S affects the dynamics of age-dependent changes in the GABAergic system, and that the enhanced GABA(B) receptor-mediated auto-inhibition can be an important factor in weakening the postsynaptic inhibition and in the development of hyperexcitability in rats exposed to H/S.

Substrate binds in the S1 site of the F253A mutant of LeuT, a neurotransmitter sodium symporter homologue

LeuT serves as the model protein for understanding the relationships between structure, mechanism and pharmacology in neurotransmitter sodium symporters (NSSs). At the present time, however, there is a vigorous debate over whether there is a single high-affinity substrate site (S1) located at the original, crystallographically determined substrate site or whether there are two high-affinity substrates sites, one at the primary or S1 site and the other at a second site (S2) located at the base of the extracellular vestibule. In an effort to address the controversy over the number of high-affinity substrate sites in LeuT, one group studied the F253A mutant of LeuT and asserted that in this mutant substrate binds exclusively to the S2 site and that 1 mM clomipramine entirely ablates substrate binding to the S2 site. Here we study the binding of substrate to the F253A mutant of LeuT using ligand binding and X-ray crystallographic methods. Both experimental methods unambiguously show that substrate binds to the S1 site of the F253A mutant and that binding is retained in the presence of 1 mM clomipramine. These studies, in combination with previous work, are consistent with a mechanism for LeuT that involves a single high-affinity substrate binding site.

Structures of LeuT in bicelles define conformation and substrate binding in a membrane-like context

Neurotransmitter sodium symporters (NSSs) catalyze the uptake of neurotransmitters into cells, terminating neurotransmission at chemical synapses. Consistent with the role of NSSs in the central nervous system, they are implicated in multiple diseases and disorders. LeuT, from Aquifex aeolicus, is a prokaryotic ortholog of the NSS family and has contributed to our understanding of the structure, mechanism and pharmacology of NSSs. At present, however, the functional state of LeuT in crystals grown in the presence of n-octyl-β-D-glucopyranoside (β-OG) and the number of substrate binding sites are controversial issues. Here we present crystal structures of LeuT grown in DMPC-CHAPSO bicelles and demonstrate that the conformations of LeuT-substrate complexes in lipid bicelles and in β-OG detergent micelles are nearly identical. Furthermore, using crystals grown in bicelles and the substrate leucine or the substrate analog selenomethionine, we find only a single substrate molecule in the primary binding site.

X-ray structures of LeuT in substrate-free outward-open and apo inward-open states

Neurotransmitter sodium symporters are integral membrane proteins that remove chemical transmitters from the synapse and terminate neurotransmission mediated by serotonin, dopamine, noradrenaline, glycine and GABA (γ-aminobutyric acid). Crystal structures of the bacterial homologue, LeuT, in substrate-bound outward-occluded and competitive inhibitor-bound outward-facing states have advanced our mechanistic understanding of neurotransmitter sodium symporters but have left fundamental questions unanswered. Here we report crystal structures of LeuT mutants in complexes with conformation-specific antibody fragments in the outward-open and inward-open states. In the absence of substrate but in the presence of sodium the transporter is outward-open, illustrating how the binding of substrate closes the extracellular gate through local conformational changes: hinge-bending movements of the extracellular halves of transmembrane domains 1, 2 and 6, together with translation of extracellular loop 4. The inward-open conformation, by contrast, involves large-scale conformational changes, including a reorientation of transmembrane domains 1, 2, 5, 6 and 7, a marked hinge bending of transmembrane domain 1a and occlusion of the extracellular vestibule by extracellular loop 4. These changes close the extracellular gate, open an intracellular vestibule, and largely disrupt the two sodium sites, thus providing a mechanism by which ions and substrate are released to the cytoplasm. The new structures establish a structural framework for the mechanism of neurotransmitter sodium symporters and their modulation by therapeutic and illicit substances.

Pre-steady-state and reverse transport currents in the GABA transporter GAT1

The role of internal substrates in the biophysical properties of the GABA transporter GAT1 has been investigated electrophysiologically in Xenopus oocytes heterologously expressing the cotransporter. Increments in Cl(-) and/or Na(+) concentrations caused by intracellular injections did not produce significant effects on the pre-steady-state currents, while a positive shift of the charge-voltage (Q-V) and decay time constant (τ)-voltage (τ-V) curves, together with a slowing of τ at positive potentials, was observed following treatments producing cytosolic Cl(-) depletion. Activation of the reverse transport mode by injections of GABA caused a reduction in the displaced charge. In the absence of external Cl(-), a stronger reduction in the displaced charge, together with a significant increase in reverse transport current, was observed. Therefore, complementarity between pre-steady-state and transport currents, observed in the forward mode, is preserved in the reverse mode. All these findings can be qualitatively reproduced by a kinetic scheme in which, in the forward mode, the Cl(-) ion is released first, after the inward charge movement, while the two Na(+) ions can be released only after binding of external GABA. In the reverse mode, internal GABA must bind first to the empty transporter, followed by internal Na(+) and Cl(-).

GABA reverse transport by the neuronal cotransporter GAT1: influence of internal chloride depletion

The role of intracellular ions on the reverse GABA transport by the neuronal transporter GAT1 was studied using voltage-clamp and [(3)H]GABA efflux determinations in Xenopus oocytes transfected with heterologous mRNA. Reverse transport was induced by intracellular GABA injections and measured in terms of the net outward current generated by the transporter. Changes in various intracellular ionic conditions affected the reverse current: higher concentrations of Na(+) enhanced the ratio of outward over inward transport current, while a considerable decrease of the outward current and a parallel reduction of the transporter-mediated GABA efflux were observed after treatments causing a diminution of the intracellular Cl(-) concentration. Particularly interesting was the impairment of the reverse transport observed after depletion of internal Cl(-) generated by the activity of a coexpressed K(+)-Cl(-) exporter KCC2. This finding suggests that reverse GABA transport may be physiologically regulated during early neuronal development, similarly to the functional alterations seen in GABA receptors caused by KCC2 activity.

Functional and structural determinants of reverse operation in the pH-dependent oligopeptide transporter PepT1

The functional and structural basis of reverse operation of PepT1 has been studied in Xenopus oocytes expressing the wild-type and mutated forms of this protein. Using brief pulses from a negative holding potential, wild-type and Arg282 mutants exhibit outward currents in the presence of Gly-Gln. The reversal potential of these currents is affected by both pH and substrate concentration, confirming coupled transport in the wild type and in the mutants as well. Long-lasting voltage and current-clamp experiments show that the outward currents are only temporary, and reflect accumulation and/or depletion effects near the membrane. The ability to operate in reverse mode was confirmed in all isoforms by intracellular injection of substrate. The role of Arg282 and Asp341 in the reverse transport was also investigated using charged substrates. Positive Lys-Gly (but not Gly-Lys) showed enhanced transport currents in the Arg282 mutants. In contrast, negative Gly-Asp and Asp-Gly elicited modest currents in all isoforms.

Association of a synonymous GAT3 polymorphism with antiepileptic drug pharmacoresistance

It would be likely that the genetic variants of the GTA3 gene encoding GAT-3, an astrocytic GABA transporter, may alter gamma-aminobutyric acid (GABA) neurotransmission in the synaptic cleft in the epileptic brain and cause antiepileptic drugs (AEDs) pharmacoresistance. A candidate gene association analysis with fine mapping was performed to dissect the genetic contributions of GAT3 to AEDs pharmacoresistance. Two independent case sample sets were recruited (Samples 1 and 2), and each set was divided into two groups (drug-resistant and drug-responsive) according to the treatment outcomes with AEDs. Sample1 (n=400) was used for the initial exploratory stage of the study and sample 2 (n=435) was used for confirmation of the genetic association in the replication stage of the study. A GAT3 polymorphism (GAT3 c.1572 C>T, rs2272400) was nominally associated with AEDs pharmacoresistance (P(CC) vs P(CT/TT)=0.012, P(allelic)=0.01). The odds ratio (OR) for AED pharmacoresistance was 1.6 (95% confidence interval (CI), 1.11-2.24; P=0.01) in the additive models of inheritance. The statistical significance remained after we adjusted for a confounding factor, the etiology of epilepsy, at 0.012 (adjusted OR: 1.73, 95% CI: 1.13-2.67) and used Bonferroni's correction for multiple comparisons at 0.048. Importantly, the positive association of c.1572 T was reproduced in the replication stage (P(allelic)=0.037, joint P-value of the replication=0.001). The results suggest that GAT3 c.1572T may be one of the contributing factors with a modest effect on AEDs pharmacoresistance in the epileptic brain, shed light on a better understanding of the underlying mechanisms and serve as an impetus for new avenues of treatment for AEDs pharmacoresistance.

GATMD: γ-aminobutyric acid transporter mutagenesis database

Since the cloning of the first γ-aminobutyric acid (GABA) transporter (GAT1; SLC6A1) from rat brain in 1990, more than 50 published studies have provided structure-function information on investigator-designed rat and mouse GAT1 mutants. To date, more than 200 of 599 GAT1 residues have been subjected to mutagenesis experiments by substitution with different amino acids, and the resulting transporter functional properties have significantly advanced our understanding of the mechanism of Na+- and Cl⁻-coupled GABA transport by this important member of the neurotransmitter:sodium symporter family. Moreover, many studies have addressed the functional consequences of amino acid deletion or insertion at various positions along the primary sequence. The enormity of this growing body of structure-function information has prompted us to develop GABA Transporter Mutagenesis Database (GATMD), a web-accessible, relational database of manually annotated biochemical, functional and pharmacological data reported on GAT1-the most intensely studied GABA transporter isoform. As of the last update of GATMD, 52 GAT1 mutagenesis papers have yielded 3360 experimental records, which collectively contain a total of ∼100 000 annotated parameters. Database URL: http://physiology.sci.csupomona.edu/GATMD/

Coupling electron cryomicroscopy and X-ray crystallography to understand secondary active transport

In the past few years we have seen an amazing increase in the number of high-resolution structures for secondary transporters determined by X-ray crystallography, while 3D data obtained by electron cryomicroscopy (cryo-EM) from two-dimensional (2D) crystals are only available at medium resolutions of about 6-10A. Despite their superior resolution, it turned out that the description of a molecular mechanism of secondary transport could not solely rely on high-resolution X-ray structures and have to be supplemented with biochemical and spectroscopic data. Moreover, the comparison of X-ray structures and 3D EM maps has proved to be an important tool for validating native conformations of several membrane proteins, especially when functional data contradicted predictions based on a crystal structure. In addition, 3D EM maps are better suited to investigate transporter activation because of the lipidic environment.

Characterization of membrane-protein interactions for the leucine transporter from Aquifex aeolicus by molecular dynamics calculations

Multinanosecond molecular dynamics (MD) simulations have been employed to characterize the interaction of an integral membrane protein (IMP), the leucine transmitter from Aquifex aeolicus (Yamashita et al., Nature 2005, 437, 215-223), with hydrated lipid bilayer membranes in their physiologically relevant liquid crystalline phases. Analysis of the MD trajectories for dimyristoyl phosphatidylcholine (DMPC), 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC), and 1-palmitoyl-2-oleoyl phosphatidylethanolamine (POPE) focused on the contacts between aromatic and basic side chains of the IMP with the lipid head groups and water. Structural fluctuations of the IMP were investigated as well as the contact dynamics of neighboring lipids. In characterizing the IMP-membrane systems, the behaviors of the protein's cytoplasmic and periplasmic parts are considered separately. All three lipid membranes show a rather similar overall level of association with the IMP. However, for DMPC there is a better matching of the membrane core to the hydrophobic transmembrane portion of the IMP. The closed cytoplasmic end of the IMP exhibits a higher degree of association with lipids than the more open periplasmic end, an observation which correlates with the more compact structure and a slower dynamics of surrounding lipids.

Emerging structure-function relationships defining monoamine NSS transporter substrate and ligand affinity

Monoamine transporters are a group of transmembrane neurotransmitter sodium symporter (NSS) transporters that play a crucial role in regulating biogenic monoamine concentrations at peripheral and central synapses. Given the key role played by serotonin, dopamine and noradrenaline in addictive and disease states, structure-function studies have been conducted to help guide the development of improved central nervous system therapeutics. Extensive pharmacological, immunological and biochemical studies, in conjunction with three-dimensional homology modeling, have been performed to structurally and functionally characterise the monoamine transporter substrate permeation pathway, substrate selectivity, and binding sites for ions, substrates and inhibitors at the molecular level. However, only recently has it been possible to start to construct an accurate molecular interaction network for the monoamine transporters and their corresponding substrates and inhibitors. Crystal structures of Aquifex aeolicus leucine transporter (LeuT(Aa)), a homologous protein to monoamine transporters that has been experimentally demonstrated to share similar structural folds with monoamine transporters, have been determined in complex with amino acids and inhibitors. The molecular interactions of leucine and tricyclic antidepressants (TCA) has supported many of the predictions based on the mutational studies. Models constructed from LeuT(Aa) are now allowing a rational approach to further clarify the molecular determinants of NSS transporter-ligand complexes, and potentially the ability to better manipulate drug specificity and affinity. In this review, we compare the structure-function relationships of other SLC6 NSS family transporters with monoamine transporters, and discuss possible mechanisms involved in substrate binding and transport, and modes of inhibition by TCAs.

In vitro assessment of paraoxon effects on GABA uptake in rat hippocampal synaptosomes

Treating organophosphate poisoning is achieved mainly using compounds with anticholinergic characteristics. Nevertheless currently the focus of attention is aimed at examining their interference with other neurotransmitter systems. The present investigation studied the potential interactions between paraoxon and GABA uptake in hippocampal synaptosomes. Wistar rats weighing 200-250 g were used. Hippocampal synaptosomes were prepared and incubated with [(3)H] GABA in the presence of different doses of paraoxon for 10 min at 37 degrees C; and were then layered in chambers of a superfusion system and the [(3)H] GABA uptake was measured. Our finding revealed that mean GABA uptake decreased by 21%, 42%, 37%, 20%, and 8% of the corresponding control values in the presence of paraoxon concentrations of 0.01, 0.1, 1, 10, and 100 microM, respectively which was significant at 0.1 and 1 microM of paraoxon (P<0.05). In conclusion, micromolar concentrations of paraoxon were shown to interfere with GABA uptake in hippocampal synaptosomes, which indicates the GABA transporters may play a role in organophosphate-induced convulsions.