Identification of Two Mutations (F758W and F758Y) in the N-methyl-D-aspartate Receptor Glycine-binding Site that Selectively Prevent Competitive Inhibition by Xenon without Affecting Glycine Binding

Xenon is a general anesthetic with neuroprotective properties. Xenon inhibition at the glycine-binding site of the N-Methyl-D-aspartate (NMDA) receptor mediates xenon neuroprotection against ischemic injury in vitro. Here we identify specific amino acids important for xenon binding to the NMDA receptor, with the aim of finding silent mutations that eliminate xenon binding but leave normal receptor function intact. Site-directed mutagenesis was used to mutate specific amino-acids in the GluN1 subunit of rat NMDA receptors. Mutant GluN1/GluN2A receptors were expressed in HEK 293 cells and were assessed functionally using patch-clamp electrophysiology. The responses of the mutant receptors to glycine and anesthetics were determined. Mutation of phenylalanine 758 to an aromatic tryptophan or tyrosine left glycine affinity unchanged, but eliminated xenon binding without affecting the binding of sevoflurane or isoflurane. These findings confirm xenon binds to the glycine site of the GluN1 subunit of the NMDA receptor and indicate that interactions between xenon and the aromatic ring of the phenylalanine 758 residue are important for xenon binding. Our most important finding is that we have identified two mutations, F758W and F758Y, that eliminate xenon binding to the NMDA receptor glycine site without changing the glycine affinity of the receptor or the binding of volatile anesthetics. The identification of these selective mutations will allow knock-in animals to be used to dissect the mechanism(s) of xenon's neuroprotective and anesthetic properties in vivo.