Pharmacological characterization of human excitatory amino acid transporters EAAT1, EAAT2 and EAAT3 in a fluorescence-based membrane potential assay.

We have expressed the human excitatory amino acid transporters EAAT1, EAAT2 and EAAT3 stably in HEK293 cells and characterized the transporters pharmacologically in a conventional [(3) H]-d-aspartate uptake assay and in a fluorescence-based membrane potential assay, the FLIPR Membrane Potential (FMP) assay. The K(m) and K(i) values obtained for 12 standard EAAT ligands at EAAT1, EAAT2 and EAAT3 in the FMP assay correlated well with the K(i) values obtained in the [(3) H]-d-aspartate assay (r(2) values of 0.92, 0.92, and 0.95, respectively). Furthermore, the pharmacological characteristics of the cell lines in the FMP assay were in good agreement with previous findings in electrophysiology studies of the transporters. The FMP assay was capable of distinguishing between substrates and non-substrate inhibitors and to discriminate between "full" and "partial" substrates at the transporters. Taking advantage of the prolific nature of the FMP assay, interactions of the EAATs with substrates and inhibitors were studied in some detail. This is the first report of a high throughput screening assay for EAATs. We propose that the assay will be of great use in future studies of the transporters. Although conventional electrophysiology set-ups might be superior in terms of studying sophisticated kinetic aspects of the uptake process, the FMP assay enables the collection of considerable amounts of highly reproducible data with relatively little labor. Furthermore, considering that the number of EAAT ligands presently available is limited, and that almost all of these are characterized by low potency and a low degree of subtype selectivity, future screening of compound libraries at the EAAT-cell lines in the FMP assay could help identify structurally and pharmacologically novel ligands for the transporters.