REGULATION OF NMDA RECEPTOR PHOSPHORYLATION BY ALTERNATIVE SPLICING OF THE C-TERMINAL DOMAIN

THE NMDA (N-methyl D-aspartate) receptors in the brain play a critical role in synaptic plasticity, synaptogenesis and excitotoxicity1-3. Mo lecular cloning has demonstrated that NMDA receptors consist of severa l homologous subunits (NMDAR1, 2A-2D)4-7. A variety of studies have su ggested that protein phosphorylation of NMDA receptors may regulate th eir function7-12 and play a role in many forms of synaptic plasticity such as long-term potentiation13,14. We have examined the phosphorylat ion of the NMDA receptor subunit NMDAR1 (NR1) by protein kinase C (PKC ) in cells transiently expressing recombinant NR1 and in primary cultu res of cortical neurons. PKC phosphorylation occurs on several distinc t sites on the NR1 subunit. Most of these sites are contained within a single alternatively spliced exon in the C-terminal domain, which has previously been proposed to be on the extracellular side of the membr ane4,5,15. These results demonstrate that alternative splicing of the NR1 messenger RNA regulates its phosphorylation by PKC, and that mRNA splicing is a novel mechanism for regulating the sensitivity of glutam ate receptors to protein phosphorylation. These results also provide e vidence that the C-terminal domain of the NR1 protein is located intra cellularly, suggesting that the proposed transmembrane topology model for glutamate receptors may be incorrect.