Endoplasmic reticulum-associated degradation of the NR1 but not the NR2 subunits of the N-methyl-D-aspartate receptor induced by inhibition of the N-glycosylation in cortical neurons.

The N-methyl-D-aspartate receptor (NMDAR) is fundamental to normal and pathological functioning of neurons. The receptor subunits are N-glycosylated proteins synthesized in the endoplasmic reticulum (ER) that fold, mature, and oligomerize as they transit through the secretory pathway. Although the early processes of biogenesis are fundamental to NMDAR expression and function, our knowledge of them is nevertheless limited. Additionally, the investigation of NMDAR synthesis is highly relevant, in that ER dysfunction, frequently associated with acute and degenerative brain diseases, might alter this process. We characterize here the effect of ER stress produced by inhibition of N-glycosylation on NMDAR synthesis and function. We use first heterologous systems of NMDAR expression in which NR1 and NR2A subunits are synthesized in nonneuronal cells. The function of these NMDARs as Ca2+ channels is repressed by tunicamycin, because of the inhibition of NR1, but no NR2A, synthesis. The regulation of NR1 is relevant to the central nervous system, in that a dramatic decrease in synthesis of this subunit and assembly of NMDARs is observed in cortical neurons treated with tunicamycin. The inhibition of NR1 synthesis is not due to changes in levels of mRNA but associated with the earliest stages in NMDAR biogenesis. The inhibition of N-glycosylation activates ER-specific stress responses in neurons, which include the ER-associated degradation (ERAD) mechanism responsible for differential and extremely efficient degradation of nonglycosylated NR1 by the proteasome after ubiquitination. Because this is an obligatory NMDAR component, the significant sensitivity of NR1 to ER stress will have important consequences on receptor function.