SLC1 glutamate transporters

Abstract

The plasma membrane transporters for the neurotransmitter glutamate belong to the solute carrier 1 family. They are secondary active transporters, taking up glutamate into the cell against a substantial concentration gradient. The driving force for concentrative uptake is provided by the cotransport of Na(+) ions and the countertransport of one K(+) in a step independent of the glutamate translocation step. Due to eletrogenicity of transport, the transmembrane potential can also act as a driving force. Glutamate transporters are expressed in many tissues, but are of particular importance in the brain, where they contribute to the termination of excitatory neurotransmission. Glutamate transporters can also run in reverse, resulting in glutamate release from cells. Due to these important physiological functions, glutamate transporter expression and, therefore, the transport rate, are tightly regulated. This review summarizes recent literature on the functional and biophysical properties, structure-function relationships, regulation, physiological significance, and pharmacology of glutamate transporters. Particular emphasis is on the insight from rapid kinetic and electrophysiological studies, transcriptional regulation of transporter expression, and reverse transport and its importance for pathophysiological glutamate release under ischemic conditions.

Figure 1

Excitatory amino acid transporter (EAAT) stoichiometry (A) and illustration of the glutamate-glutamine cycle (B) (adapted from [68]). (C) Potential multistep kinetic mechanism of glutamate transport.

Figure 2

(A) Concept of the single-turnover K⁺ exchange experiment to determine electrical events associates with K⁺ relocation and/or binding. (B) Outward current induced by external K⁺ application (grey bar), due to negative charge of the reorienting binding site(s). (C-D) Iso-potential planes for hypothesized outward-facing and inward-facing structures. The protein is shown in blue (slice at the depth of the substrate binding site in the left subunit). Conserved, potentially charged residues are overlaid. D454 is omitted due to the possibility of its side chain being neutral. The figure is adapted from [72].

Figure 3

(A) Inverted repeat topology (highlighted by colored triangles) of the SLC1 family members. (B) Proposed major structural change leading to alternating access of the substrate binding site. (C) Structural evidence predicts two Na⁺ binding sites in mammalian EAATs, Na1 and Na2 (yellow). Computational analysis of Na⁺ binding by MD simulations, as well as mutational studies, provided evidence for two other potential Na⁺ binding sites, termed Na3 and Na3′.

Figure 4

Representative structures of the most important classes of transported substrates and inhibitors of glutamate transporters.