A Role for Extracellular Na⁺in the Channel Gating of Native and Recombinant Kainate Receptors

Abstract

Ionotropic glutamate receptors of the kainate and AMPA subtypes share a number of structural features, both topographical and in terms of stoichiometry. In addition, AMPA and kainate receptors share similar pharmacological and biophysical properties in that they are activated by common agonists and display rapid activation and desensitization characteristics. However, we show here that in contrast to AMPA receptor-mediated responses (native or recombinant GluR3 receptor), the response of native and recombinant (GluR6) kainate receptors to glutamate was drastically reduced in the absence of extracellular Na+ (i.e., when replaced by Cs+). Removal of Na+ increases the rate of desensitization, indicating that external Na+ modulates channel gating. Whereas the size of the substituting cation is important in mimicking the action of Na+ (Li+>K+>Cs+), modulation was voltage independent. These results indicate the existence of different gating mechanisms for AMPA and kainate receptors. By using chimeric AMPA-kainate receptors derived from GluR3 and GluR6, we have identified a key residue in the S2 segment of GluR6 (M770) that is largely responsible for the sensitivity of the receptor to external Na+. Thus, these results show the existence of a specific kainate receptor gating mechanism that requires external Na+ to be operative.

A Mosaic of Functional Kainate Receptors in Hippocampal Interneurons

“…However, functional GluR6 homomers could still coexist with GluR5-GluR6 or GluR6 -KA2 receptors in cells cotransfected with GluR6 and either GluR5 or KA2. We estimated the relative abundance of GluR6 -KA2 heteromers as described previously (i.e., looking at the ratio of responses to AMPA and glutamate) (Paternain et al, 2003) and by the sensitivity of these cells to ATPA (Fig. 1C).…”

“…The GluR5 and KA2 plasmids were cotransfected in 1:1 or 1:3 ratios, whereas a ratio of 1:2.5 was used for GluR6 and GluR5 and 1:10 when GluR6 was cotransfected with KA2. These ratios were sufficient for GluR6 to generate a large proportion of heteromeric receptors with GluR5 and KA2 subunits (Paternain et al, 2000(Paternain et al, , 2003. Indeed, this fact was systematically evaluated by applying AMPA and (RS)-2-␣-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), which activates heteromeric GluR6 -GluR5 or GluR6 -KA2 but not homomeric GluR6.…”

“…Highly fluorescent cells were identified and selected for recording. In our hands, GluR5 (up to 10 g) did not yield any current when transfected alone (Paternain et al, 2000(Paternain et al, , 2003. Therefore, we used HEK293 cells stably expressing human GluR5-1a(Q) to evaluate homomeric GluR5 receptors as described previously (Varming et al, 2001).…”

Atypical Functional Properties of GluK3-Containing Kainate Receptors

“…We chose GluA1 since GluK2 is inactive in the absence of extracellular sodium (Paternain et al, 2003). These cells were bathed in a solution containing the following (in mM): 150 N-methyl glucamine chloride, 10 HEPES, 10 CaCl 2 , equilibrated to pH 7.4 with HCl, 315 mosm.…”

Structure of the kainate receptor subunit GluR6 agonist-binding domain complexed with domoic acid

“…Although no structure has been available for the S1S2 domain of a kainate-selective subunit, the primary sequences of the agonistbinding domains of AMPA and kainate receptor subunits are sufficiently similar (48-55%) that the GluR2 S1S2 structure can be used as a good model. This has enabled directed mutagenesis studies to probe kainate receptor function (9,10). Nonetheless, for kainate receptors, there are limits to the inferences that can be drawn from a straight comparison with the GluR2 structure.…”