A Medicinal Chemistry Perspective on Excitatory Amino Acid Transporter 2 Dysfunction in Neurodegenerative Diseases

Abstract

The excitatory amino acid transporter 2 (EAAT2) plays a key role in the clearance and recycling of glutamate - the major excitatory neurotransmitter in the mammalian brain. EAAT2 loss/dysfunction triggers a cascade of neurodegenerative events, comprising glutamatergic excitotoxicity and neuronal death. Nevertheless, our current knowledge regarding EAAT2 in neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD), is restricted to post-mortem analysis of brain tissue and experimental models. Thus, detecting EAAT2 in the living human brain might be crucial to improve diagnosis/therapy for ALS and AD. This perspective article describes the role of EAAT2 in physio/pathological processes and provides a structure-activity relationship of EAAT2-binders, bringing two perspectives: therapy (activators) and diagnosis (molecular imaging tools).

Figure 1

Glutamate-glutamine cycle in the healthy brain. (A) Astrocytes are known to provide the glutamine required by neurons to synthesize GABA and glutamate. The glutamine efflux from the astrocytes is mediated via the system N transporter (SN1). Once in the extracellular space, neurons capture glutamine through the system A transporters (SA1 and SA2). In the neuronal intracellular space, glutamine is metabolized into glutamate by the mitochondrial enzyme glutaminase. Glutamate is then packed within synaptic glutamatergic vesicles (VGluT) via a Mg²⁺/ATP-dependent process and released to the extracellular compartment from the vesicles by a Ca²⁺-dependent mechanism. Of note, the glutamate concentration in the intracellular and extracellular space is 10 mM and 10 μM, respectively. Once glutamate is released in the synaptic cleft, it produces an excitatory postsynaptic potential, which is tightly controlled by a wide range of neuronal receptors, including ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs). A balance between glutamate release and clearance is essential. Glutamate in the extracellular space is captured by astrocytes via EAAT2. (B) Topology diagram of EAAT2. The transport domain consists TM3, TM6, TM7, TM8, HP1, HP2 and the connecting HP1 and HP2 loops.

Figure 2

Three-dimensional structure of EAAT2 in complex with glutamate (PDB code 7XR4). Close-ups on the glutamate binding site (BS1). Two peripherical binding sites (BS2 and BS3) are also shown.

Figure 3

EAAT2 translational activators. Compound 11 is also known as LDN/OSU-0212320.

Figure 4

Structures of PAMs.

Figure 5

EAAT2 binders as aspartic acid analogues 17–31.

Figure 6

Binding mode of 26 (WAY213613, green) to EAAT2 (blue cartoon) (PDB ID code: 7XR6).

Figure 7

EAAT2 inhibitors as glutamate analogues 32–39.

Figure 8

Pro-radiotracer approach to increase the BBB permeability for EAAT2 inhibitors. (A) The EAAT2 specific inhibitor developed 24c by Greenfield et al. possesses a free carboxylic acid–a disadvantage for brain penetration. A pro-radiotracer approach may be applied to increase BBB permeability. The ¹⁸F-pro-radiotracer [¹⁸F]41 was developed following the addition of a methyl ester in the C-terminal of the EAAT2 inhibitor. After in vivo administration, the [¹⁸F]41 is hydrolyzed in the brain, producing the EAAT2-specific ¹⁸F-radiotracer [¹⁸F]24c inside the brain. (B) Schematic mechanism of a brain pro-radiotracer.