Role of Na,K-ATPase α1 and α2 isoforms in the support of astrocyte glutamate uptake

Abstract

Glutamate released during neuronal activity is cleared from the synaptic space via the astrocytic glutamate/Na+ co-transporters. This transport is driven by the transmembrane Na+ gradient mediated by Na,K-ATPase. Astrocytes express two isoforms of the catalytic Na,K-ATPase ∝ subunits; the ubiquitously expressed ∝1 subunit and the ∝2 subunit that has a more specific expression profile. In the brain ∝2 is predominantly expressed in astrocytes. The isoforms differ with regard to Na+ affinity, which is lower for ∝2. The relative roles of the ∝1 and ∝2 isoforms in astrocytes are not well understood. Here we present evidence that the presence of the ∝2 isoform may contribute to a more efficient restoration of glutamate triggered increases in intracellular sodium concentration [Na+]i. Studies were performed on primary astrocytes derived from E17 rat striatum expressing Na,K-ATPase ∝1 and ∝2 and the glutamate/Na+ co-transporter GLAST. Selective inhibition of ∝2 resulted in a modest increase of [Na+] i accompanied by a disproportionately large decrease in uptake of aspartate, an indicator of glutamate uptake. To compare the capacity of ∝1 and ∝2 to handle increases in [Na+]i triggered by glutamate, primary astrocytes overexpressing either a1 or a2 were used. Exposure to glutamate 200 μM caused a significantly larger increase in [Na +]i in ∝1 than in ∝2 overexpressing cells, and as a consequence restoration of [Na+]i, after glutamate exposure was discontinued, took longer time in a1 than in a2 overexpressing cells. Both ∝1 and ∝2 interacted with astrocyte glutamate/Na + co-transporters via the 1st intracellular loop. © 2014 Illarionova et al.