Are vesicular neurotransmitter transporters potential treatment targets for temporal lobe epilepsy?

Abstract

The vesicular neurotransmitter transporters (VNTs) are small proteins responsible for packing synaptic vesicles with neurotransmitters thereby determining the amount of neurotransmitter released per vesicle through fusion in both neurons and glial cells. Each transporter subtype was classically seen as a specific neuronal marker of the respective nerve cells containing that particular neurotransmitter or structurally related neurotransmitters. More recently, however, it has become apparent that common neurotransmitters can also act as co-transmitters, adding complexity to neurotransmitter release and suggesting intriguing roles for VNTs therein. We will first describe the current knowledge on vesicular glutamate transporters (VGLUT1/2/3), the vesicular excitatory amino acid transporter (VEAT), the vesicular nucleotide transporter (VNUT), vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT) and the vesicular γ-aminobutyric acid (GABA) transporter (VGAT) in the brain. We will focus on evidence regarding transgenic mice with disruptions in VNTs in different models of seizures and epilepsy. We will also describe the known alterations and reorganizations in the expression levels of these VNTs in rodent models for temporal lobe epilepsy (TLE) and in human tissue resected for epilepsy surgery. Finally, we will discuss perspectives on opportunities and challenges for VNTs as targets for possible future epilepsy therapies.

Keywords: SLC17; SLC18; SLC32; antiepileptic drugs; epileptogenesis; temporal lobe epilepsy; vesicular neurotransmitter transporters.

Figure 1

Proposed mechanisms of action of currently available antiepileptic drugs (AEDs) at excitatory and inhibitory synapses. (A) Currently available AEDs are thought to target several molecules at the excitatory synapse. These include voltage-gated Na⁺ channels, synaptic vesicle glycoprotein 2A (SV2A), the α 2δ subunit of the voltage-gated Ca²⁺ channel, AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors, and NMDA (N-methyl-d-aspartate) receptors. Many of the AEDs can modulate voltage-gated Na⁺ channels. This would be expected to decrease depolarization-induced Ca²⁺ influx and vesicular release of neurotransmitters. Levetiracetam is the only available drug that binds to SV2A, which might have a role in neurotransmitter release. Gabapentin and pregabalin bind to the α 2δ subunit of voltage-gated Ca²⁺ channels, which is thought to be associated with a decrease in neurotransmitter release. Excitatory neurotransmission at the postsynaptic membrane can be limited by topiramate (acting on AMPA and kainate receptors) and felbamate (acting on NMDA receptors). (B) AED targets at inhibitory synapses have also been proposed. These include the γ-aminobutyric acid (GABA) transporter GAT1 (also known as SLC6A1), which is inhibited by tiagabine, leading to a decrease in GABA uptake into presynaptic terminals and surrounding glia, and GABA transaminase (GABA-T), which is irreversibly inhibited by vigabatrin. This decreases the metabolism of GABA in presynaptic terminals and glial cells. The benzodiazepines, barbiturates, topiramate, and felbamate have been found to enhance inhibitory neurotransmission by allosterically modulating GABAA receptor-mediated Cl⁻ currents. However, the action of each of these drugs is different and is dependent on the subunit conformation of the GABAA receptor complex. GAD, glutamic acid decarboxylase. Figure is modified, with permission, from Bialer and White (2010).

Figure 2

Vesicular neurotransmitter transporters depend differentially on the two components of the electrochemical gradient of H⁺ (Δμ_H+). A V-ATPase generates a Δμ_H+ across the vesicle membranes. The vesicular transporters use this gradient to drive the transport of transmitters into secretory vesicles by coupling the translocation of transmitter to H⁺ running down Δμ_H+. The different vesicular transporters rely to different extents on the two components (Δ pH and Δψ) of this gradient. (A) VMATs and (B) VAChT transport their positively charged substrates coupled to the exchange of two H⁺, and hence rely primarily on Δ pH. (C) GABA and glycine are transported as neutral zwitterions by VGAT, which depends equally on both the chemical and the electrical component of Δμ_H+. (D) VGLUTs transport the negatively charged glutamate and thus rely more on Δψ than Δ pH. [Modified from Chaudhry et al. (2008b) with permission].

Figure 3

A two and three dimensional molecular structure of the members of the SLC17 family. (A,B) The vesicular glutamate transporter 1 (VGLUT1) (Almqvist et al., 2007), (D,E) the vesicular excitatory amino acid transporter (VEAT) (Courville et al., 2010) and (G) the vesicular nucleotide transporter (VNUT) (Sawada et al., 2008). (C) The different families of VGLUT modulators (Pietrancosta et al., 2010). (F) The most bioactive analog of sialic acid: per-O-Ac,9-iodo-Neu5Ac and the novel VEAT ligand identified by virtual high-throughput screening: FR139317 (Pietrancosta et al., 2012). (H) 4,4′-diisothiocyanatostilbene-2,2′-disulfonate, the only known inhibitor of ATP transport in vitro.

Figure 4

(A) Predicted secondary structure of the human vesicular monoamine transporter 2 (VMAT2) and (B) two classical and now commonly used VMAT inhibitors: tetrabenazine and reserpine (Wimalasena, 2010). (C) Three dimensional homology model of the vesicular acetylcholine transporter (VAChT) (Khare et al., 2010) and (D) its most studied inhibitor: vesamicol (Kozaka et al., 2012).

Figure 5

Refined transmembrane topology of the vesicular GABA transporter (VGAT), the only member of the SLC32 gene family (Martens et al., 2008).