Mutations in the transmembrane domain M3 generate spontaneously open orphan glutamate delta 1 receptor

Glutamate delta-1 receptors (GluRδ1) are expressed in the adult hippocampus and inner ear and have recently been shown to be important for high-frequency hearing. Similar to the closest homolog glutamate delta-2 receptor (GluRδ2), no agonist-induced currents are observed from GluRδ1 receptors. In an effort to understand the function of the GluRδ1 subunit, we probed the conserved transmembrane 3 (TM3) region of the GluRδ1 subunit, where the GluRδ2 lurcher mutation is localized. Four mutations in the TM3 domain A650C, L652A, A654C, and F655A resulted in spontaneously open GluRδ1 channels suggesting that GluRδ1 receptors can form homomeric receptors. The leak currents were partially blocked by pentamidine but showed negligible inhibition by NASP. It has been demonstrated that extracellular Ca²⁺ binds and stabilizes the ligand binding domain (LBD) dimer interface leading to potentiation of currents through GluRδ2^Lc channels. We found that extracellular Ca²⁺ potentiated the spontaneous currents through GluRδ1F655A suggesting that extracellular Ca²⁺ may interact with the conserved residues at GluRδ1 LBD dimer interface. A recent study suggested that d-serine and glycine bind to the GluRδ2 LBD and reduce spontaneous currents through the GluRδ2^Lc channels. d-Serine and glycine produced only a modest reduction of spontaneous currents through GluRδ1F655A and had no effect on the spontaneous current through GluRδ1L652A. However, spontaneous currents in a chimeric GluRδ1-δ2^Lc were robustly inhibited by d-serine. These results suggest that the activation gate is conserved in GluRδ1 receptors. Moreover, the conformational changes induced by d-serine and extracellular Ca²⁺ are conserved among GluRδ1 and GluRδ2 receptors.