Selective inhibitors of GABA uptake: synthesis and molecular pharmacology of 4-N-methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol analogues

Regulation of excitation by GABA neurotransmission: focus on metabolism and transport

The vast majority of excitatory synapses in the central nervous system (CNS) utilize glutamate as the neurotransmitter. The level of excitation appears to be under regulatory control by the major inhibitory neurotransmitter GABA, which is synthesized from glutamate by its decarboxylation catalysed by glutamate decarboxylase (GAD). The inactivation of GABA is brought about by high affinity GABA transporters located in the presynaptic GABAergic neurons as well as surrounding astrocytes and subsequently GABA may be metabolized by GABA-transaminase (GABA-T) ultimately allowing the carbon skeleton to enter the tricarboxylic acid (TCA) cycle for oxidative metabolism. In the presynaptic GABAergic neuron, GABA taken up seems, however, preferentially to enter the vesicular GABA pool and hence it is recycled as a transmitter. It has become clear that compounds acting as inhibitors at either the transporters or GABA-T are capable of regulating the inhibitory tonus thus controlling excitation. This has led to development of clinically efficatious antiepileptic drugs. This paper shall review recent progress in targeting these pharmacological entities.

Cloning and Characterization of a Functional Human γ-Aminobutyric Acid (GABA) Transporter, Human GAT-2

Plasma membrane gamma-aminobutyric acid (GABA) transporters act to terminate GABA neurotransmission in the mammalian brain. Intriguingly four distinct GABA transporters have been cloned from rat and mouse, whereas only three functional homologs of these transporters have been cloned from human. The aim of this study therefore was to search for this fourth missing human transporter. Using a bioinformatics approach, we successfully identified and cloned the full-length cDNA of a so far uncharacterized human GABA transporter (GAT). The predicted protein displays high sequence similarity to rat GAT-2 and mouse GAT3, and in accordance with the nomenclature for rat GABA transporters, we therefore refer to the transporter as human GAT-2. We used electrophysiological and cell-based methods to demonstrate that this protein is a functional transporter of GABA. The transport was saturable and dependent on both Na(+) and Cl(-). Pharmacologically the transporter is distinct from the other human GABA transporters and similar to rat GAT-2 and mouse GAT3 with high sensitivity toward GABA and beta-alanine. Furthermore the GABA transport inhibitor (S)-SNAP-5114 displayed some inhibitory activity at the transporter. Expression analysis by reverse transcription-PCR showed that GAT-2 mRNA is present in human brain, kidney, lung, and testis. The finding of the human GAT-2 demonstrates for the first time that the four plasma membrane GABA transporters identified in several mammalian species are all conserved in human. Furthermore the availability of human GAT-2 enables the use of all human clones of the GABA transporters in drug development programs and functional characterization of novel inhibitors of GABA transport.

GABA: Homeostatic and pharmacological aspects

The central nervous system (CNS) operates by a fine-tuned balance between excitatory and inhibitory signalling. In this context, the inhibitory neurotransmission may be of particular interest as it has been suggested that such neuronal pathways may constitute 'command pathways' and the principle of 'dis-inhibition' leading ultimately to excitation may play a fundamental role (Roberts, E. (1974). Adv. Neurol., 5: 127-143). The neurotransmitter responsible for this signalling is gamma-aminobutyrate (GABA) which was first discovered in the CNS as a curious amino acid (Roberts, E., Frankel, S. (1950). J. Biol. Chem., 187: 55-63) and later proposed as an inhibitory neurotransmitter (Curtis, D.R., Watkins, J.C. (1960). J. Neurochem., 6: 117-141; Krnjevic, K., Schwartz, S. (1967). Exp. Brain Res., 3: 320-336). The present review will describe aspects of GABAergic neurotransmission related to homeostatic mechanisms such as biosynthesis, metabolism, release and inactivation. Additionally, pharmacological and therapeutic aspects of this will be discussed.

A novel selective γ-aminobutyric acid transport inhibitor demonstrates a functional role for GABA transporter subtype GAT2/BGT-1 in the CNS

The system of GABA transporters in neural cells constitutes an efficient mechanism for terminating inhibitory GABAergic neurotransmission. This transport system is an important therapeutical target in epileptic disorders, but potentially also in other neurological disorders. Thus, selective intervention in GABA uptake has been the subject of extensive research for several decades. In a series of lipophilic diaromatic derivatives of (RS)-3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazole (exo-THPO), N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (EF1502) turned out to be an equipotent inhibitor at the mouse transporters GAT1 and GAT2 (BGT-1) but inactive at GAT3 and GAT4. This novel pharmacological profile among GABA uptake inhibitors prompted a thorough investigation of the in vivo properties of this compound. These investigations have for the first time demonstrated a functional role for GABA transporter subtype GAT2/BGT-1, which points to the therapeutic relevance of inhibiting this transporter subtype. An overview of the development and characterisation of EF1502 is presented here.

Functional characterization of Zn2(+)-sensitive GABA transporter expressed in primary cultures of astrocytes from rat cerebral cortex

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 (GATs). GAT1 mainly expressed in cerebrocortical neurons is thought to play an important role for clearance of GABA in the extracellular fluid, whereas there is a little information available for pharmacological importance for astrocytic GABA transporters. In the present study, we therefore described the functional characterization of GABA transport in primary cultures of astrocytes from rat cerebral cortex and the identification of GABA transporter subtype(s). GABA transport was Na(+) and Cl(-) dependent and saturable with a Michaelis constant (K(t)) of 9.3+/-2.8 microM. Na(+)- and Cl(-)- activation kinetics revealed that the Na(+)-Cl(-)-to-GABA stoichiometry was 2:1:1 and concentrations of Na(+) and Cl(-) necessary for half-maximal transport (K(0.5)(Na) and K(0.5)(Cl)) were 78+/-28 mM and 9.6+/-2.6 mM, respectively. Na(+)-dependent GABA transport was competitively inhibited by various GABA transport inhibitors, especially GAT2- or GAT3-selective inhibitor. In addition, Zn(2+), which has been reported to be a potent inhibitor of GAT3, was found to have a significantly but partially inhibitory effect on the Na(+)-dependent GABA transport in a concentration-dependent manner. Furthermore, reverse transcription-PCR and Western blot analyses revealed that GAT2 and GAT3 are expressed in primary cultures of astrocytes. These results clearly showed that zinc is a useful reagent for separating GAT3 activity from GAT1- and GAT2-activities in CNS. To our knowledge, the present study represents the first report on the inhibitory effect of zinc on the Na(+)-dependent GABA transport in rat cerebrocortical astrocytes.

Novel parent structures for inhibitors of the murine GABA transporters mGAT3 and mGAT4

Searching for potent and subtype selective parent structures of the murine gamma-aminobutyric acid (GABA) transporter subtypes mGAT3 and mGAT4 a series of amino acids was characterised in a uniform [3H]GABA uptake test system based on transiently expressed mGAT1-4. From several potent inhibitors showing IC50 values at mGAT3 and mGAT4 in the low microM range cis-4-aminocrotonic acid and (RS)-2,3-diaminopropionic acid turned out to be most subtype selective for these transporters. With (RS)-isoserine--a compound unknown as GAT inhibitor until now--one of the most potent amino acids selectively inhibiting mGAT3 and mGAT4 was found. Furthermore, (2-amino-1,3-thiazol-4-yl)acetic acid was identified as the first parent structure exhibiting a clear, though still moderate, selective inhibition of GABA uptake at mGAT3.

Effect of Hyperosmotic Conditions on the Expression of the Betaine-GABA-Transporter (BGT-1) in Cultured Mouse Astrocytes

The adaptation of cells to hyperosmotic conditions involves accumulation of organic osmolytes to achieve osmotic equilibrium and maintenance of cell volume. The Na+ and Cl(-)-coupled betaine/GABA transporter, designated BGT-1, is responsible for the cellular accumulation of betaine and has been proposed to play a role in osmoregulation in the brain. BGT-1 is also called GAT2 (GABA transporter 2) when referring to the mouse transporter homologue. Using Western Blotting the expression of the mouse GAT2 protein was investigated in astrocyte primary cultures exposed to a growth medium made hyperosmotic (353+/-2.5 mosmol/kg) by adding sodium chloride. A polyclonal anti-BGT-1 antibody revealed the presence of two characteristic bands at 69 and 138 kDa. When astrocytes were grown for 24 h under hyperosmotic conditions GAT2 protein was up-regulated 2-4-fold compared to the level of the isotonic control. Furthermore, the expected dimer of GAT2 was also up-regulated after 24 h under the hyperosmotic conditions. The [3H]GABA uptake was examined in the hyperosmotic treated astrocytes, and characterized using different selective GABA transport inhibitors. The up-regulation of GAT2 protein was not affecting total GABA uptake but the hyperosmotic condition did change total GABA uptake possibly involving GAT1. Immunocytochemical studies revealed cell membrane localization of GAT2 throughout astroglial processes. Taken together, these results indicate that astroglial GAT2 expression and function may be regulated by hyperosmolarity in cultured mouse astrocytes, suggesting a role of GAT2 in osmoregulation in neural cells.

First demonstration of a functional role for central nervous system betaine/{gamma}-aminobutyric acid transporter (mGAT2) based on synergistic anticonvulsant action among inhibitors of mGAT1 and mGAT2

In a recent study, EF1502 [N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo [d]isoxazol-3-ol], which is an N-substituted analog of the GAT1-selective GABA uptake inhibitor exo-THPO (4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol), was found to inhibit GABA transport mediated by both GAT1 and GAT2 in human embryonic kidney (HEK) cells expressing the mouse GABA transporters GAT1 to 4 (mGAT1-4). In the present study, EF1502 was found to possess a broad-spectrum anticonvulsant profile in animal models of generalized and partial epilepsy. When EF1502 was tested in combination with the clinically effective GAT1-selective inhibitor tiagabine [(R)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid] or LU-32-176B [N-[4,4-bis(4-fluorophenyl)-butyl]-3-hydroxy-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol], another GAT1-selective N-substituted analog of exo-THPO, a synergistic rather than additive anticonvulsant interaction was observed in the Frings audiogenic seizure-susceptible mouse and the pentylenetetrazol seizure threshold test. In contrast, combination of the two mGAT1-selective inhibitors, tiagabine and LU-32-176B, resulted in only an additive anticonvulsant effect. Importantly, the combination of EF1502 and tiagabine did not result in a greater than additive effect in the rotarod behavioral impairment test. In subsequent in vitro studies conducted in HEK-293 cells expressing the cloned mouse GAT transporters mGAT1 and mGAT2, EF1502 was found to noncompetitively inhibit both mGAT1 and the betaine/GABA transporter mGAT2 (K(i) of 4 and 5 muM, respectively). Furthermore, in a GABA release study conducted in neocortical neurons, EF1502 did not act as a substrate for the GABA carrier. Collectively, these findings support a functional role for mGAT2 in the control of neuronal excitability and suggest a possible utility for mGAT2-selective inhibitors in the treatment of epilepsy.

GABA transporters as drug targets for modulation of GABAergic activity

The identification and subsequent development of the GABA transport inhibitor tiagabine has confirmed the important role that GABA transporters play in the control of CNS excitability. Tiagabine was later demonstrated to be a selective inhibitor of the GABA transporter GAT1. Although selective for GAT1, tiagabine lacks cell type selectivity and is an equipotent inhibitor of neuronal and glial GAT1. To date, four GABA transporters have been cloned, i.e., GAT1-4. The finding that some of these display differential cellular and regional expression patterns suggests that drugs targeting GABA transporters other than GAT1 might offer some therapeutic advantage over GAT1 selective inhibitors. Furthermore, it is particularly interesting that several recently defined GABA transport inhibitors have been demonstrated to display a preferential selectivity for the astrocytic GAT1 transporter. That cellular heterogeneity of GAT1 plays a role in the control of CNS function is confirmed by the demonstration that inhibition of astrocytic GABA uptake is highly correlated to anticonvulsant activity. At the present time, a functional role for the other GABA transporters is less well defined. However, recent findings have suggested a role for the mouse GAT2 (homologous to the human betaine transporter) in the control of seizure activity. In these studies, the non-selective GAT1 and mouse GAT2 transport inhibitor EF1502 (N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol) was found to exert a synergistic anticonvulsant action when tested in combination with the GAT1 selective inhibitors tiagabine and LU-32-176B (N-[4,4-bis(4-fluorophenyl)-butyl]-3-hydroxy-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol). Additional studies will be required to define a role for the other GABA transporters and to further identify the functional importance of their demonstrated cellular and regional heterogeneity. A summary of these and other issues are discussed in this brief review.