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Keywords:

  • Ca2+ influx ;
  • desensitization;
  • excitotoxicity;
  • flip;
  • flop isoform;
  • Na+ overload

Abstract

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) -type glutamate receptors play a critical role in excitotoxicity associated with cerebral hypoxia, ischaemia and other acute brain insults. AMPA receptors are composed of GluR1–GluR4 subunits in homomeric and heteromeric assemblies, forming nonselective cation channels. In addition, each subunit has alternative splice variants, flip and flop forms. Heterologous expression studies showed that the AMPA receptor channels exhibit diverse properties depending on subunit/variant composition. For example, the absence of the GluR2 subunit makes AMPA receptor assemblies Ca2+-permeable. Excitotoxicity induced by activating AMPA receptor channels has been linked to excessive Ca2+ influx through the GluR2-lacking channels. Here we demonstrate that coexpression of the AMPA receptor GluR2flip and GluR4flip subunits exerts a lethal effect on HEK293 cells, whereas no lethal activity is observed in other homomeric or heteromeric combinations of AMPA receptor subunits. Patch clamp recordings and Ca2+ imaging analyses have revealed that this GluR2flip/GluR4flip receptor exhibits a low Ca2+ permeability. This subunit combination, however, showed prolonged Na+ influx following AMPA stimulation, even in the absence of cyclothiazide, which attenuates AMPA receptor desensitization. Furthermore, the GluR2flip/GluR4flip-mediated lethality was potentiated by the interruption of cellular Na+ extrusion mechanisms using ouabain or benzamil. These observations suggest that the GluR2flip/GluR4flip receptor-mediated excitotoxicity is attributed to Na+ overload, but not Ca2+ influx.