title: Calcium-dependent inactivation of recombinant N-methyl-D-aspartate receptors is NR2 subunit specific.
abstract: Intracellular Ca2+ can reversibly reduce the activity of native N-methyl-D-aspartate (NMDA) receptors in hippocampal neurons, a phenomenon termed Ca2+-dependent inactivation. We examined inactivation in heteromeric NMDA receptors expressed in human embryonic kidney (HEK) 293 cells using whole-cell recording. NR1-1a/2A heteromers showed reversible inactivation that was very similar to native NMDA receptors in cultured hippocampal neurons. Inactivation was dependent on the extracellular Ca2+ concentration and the degree of intracellular Ca2+ buffering. In 2 mM extracellular Ca2+, inactivation resulted in a 46.1 +/- 12.6% reduction in the whole-cell current during a 5-sec agonist application. Inactivation of NR1-1a/2A heteromers was unaffected by calcineurin inhibitors, staurosporine, or phalloidin. NR1-1a/2D heteromers also showed a similar degree of inactivation. In contrast, NR1-1a/2B and NR1-1a/2C heteromers showed no significant inactivation. At saturating concentrations of NMDA (1 mM), NR1-1a/2A heteromers also showed Ca- and glycine-independent desensitization, as seen in native hippocampal neurons. Ca(2+)- and glycine-independent desensitization was less pronounced in NR1-1a/2B heteromers and absent in NR1-1a/2C heteromers. Activation of NR1-1a/2C heteromers triggered intracellular Ca2+ transients similar to NR1-1a/2A heteromers as verified by combined Ca2+ imaging and whole-cell recording. Thus differences in Ca2+ permeability were not responsible for the lack of inactivation in NR1-1a/2C heteromers. Our results show that inactivation of recombinant NMDA receptors requires either the NR2A or NR2D subunit, whereas both inactivation and desensitization were absent in NR2C-containing receptors. The gating of inactivating NMDA receptors is more likely to be influenced by ongoing NMDA receptor activity and Ca2+ transients, perhaps consistent with the prominent expression of NR2A in hippocampus and cerebral cortex.
authors.affiliation: Vollum Institute, Oregon Health Sciences University, Portland 97201, USA.
authors.forename: J J
authors.initials: JJ
authors.lastname: Krupp
authors.forename: B
authors.initials: B
authors.lastname: Vissel
authors.forename: S F
authors.initials: SF
authors.lastname: Heinemann
authors.forename: G L
authors.initials: GL
authors.lastname: Westbrook
mesh_terms: D001704:Biopolymers; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors*; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors
chemical_list: D001704:Biopolymers; D016194:Receptors, N-Methyl-D-Aspartate; D011994:Recombinant Proteins; D010749:Phosphoprotein Phosphatases; D002118:Calcium