Up-Regulation of Excitatory Amino Acid Transporters EAAT3 and EAAT4 by Lithium Sensitive Glycogen Synthase Kinase GSK3ß

Background: Cellular uptake of glutamate by the excitatory amino-acid transporters (EAATs) decreases excitation and thus participates in the regulation of neuroexcitability. Kinases impacting on neuronal function include Lithium-sensitive glycogen synthase kinase GSK3ß. The present study thus explored whether the activities of EAAT3 and/or EAAT4 isoforms are sensitive to GSK3ß.

Methods: cRNA encoding wild type EAAT3 (SLC1A1) or EAAT4 (SLC1A6) was injected into Xenopus oocytes without or with additional injection of cRNA encoding wild type GSK3ß or the inactive mutant K85AGSK3ß. Dual electrode voltage clamp was performed in order to determine glutamate-induced current (IEAAT).

Results: Appreciable IEAAT was observed in EAAT3 or EAAT4 expressing but not in water injected oocytes. IEAAT was significantly increased by coexpression of GSK3ß but not by coexpression of K85AGSK3ß. Coexpression of GSK3ß increased significantly the maximal IEAAT in EAAT3 or EAAT4 expressing oocytes, without significantly modifying apparent affinity of the carriers. Lithium (1 mM) exposure for 24 hours decreased IEAAT in EAAT3 and GSK3ß expressing oocytes to values similar to IEAAT in oocytes expressing EAAT3 alone. Lithium did not significantly modify IEAAT in oocytes expressing EAAT3 without GSK3ß.

Conclusions: Lithium-sensitive GSK3ß is a powerful regulator of excitatory amino acid transporters EAAT3 and EAAT4.