Anti-NMDAR and non-anti-NMDAR antibodies promptly modulate NMDAR through p38 and flux-independent signaling: implications for anti-NMDAR encephalitis.

Anti-N-methyl D-aspartic acid receptor (anti-NMDAR) encephalitis is caused by anti-NMDAR antibodies (Abs) that induce neurologic and psychiatric symptoms, explained mainly by NMDAR hypofunction. In the long-term, these Abs decrease surface NMDAR and NMDAR-mediated intracellular Ca2+ ([Ca2+]i) influx. However, there are contradictory findings regarding short-term mechanisms. We investigated NMDAR function in cultured neurons after 60 min treatment with three commercial, rabbit, anti-NMDAR Abs (anti-GluN1 extracellular (EC) domain; anti-GluN2B EC domain; and anti-GluN1 intracellular (IC) domain). The anti-GluN2B and anti-GluN1 IC Abs were previously reported to mimic patient[s] Ab effects in a rodent in vivo model and decreased NMDAR-mediated [Ca2+]i entry after 24 h treatment in our cells. After 60 min incubation with anti-GluN2B or anti-GluN1 IC decreased the NMDAR-mediated [Ca2+]i rise, whereas anti-GluN1 EC slightly increased it. Interestingly, all Abs induced p38 phosphorylation (p-p38). However, surprisingly, it was also elicited by a rabbit Ab directed against a non-NMDAR intracellular epitope, which also reduced NMDAR-mediated [Ca2+]i entry. We further investigated the cellular mechanisms regulated by the anti-GluN2B Ab after 60 min. This Ab did not reduce surface NMDAR and p38 inhibition partially prevented its effect on NMDAR function. This Ab did not elicit per se an [Ca2+]i rise, whereas NMDAR inhibitors 7DCK and MK-801 did not prevent p-p38. Nonetheless, 7DCK prevented NMDAR-mediated [Ca2+]i reduction by the Ab, suggesting a role of GluN1 flux-independent signaling. These data indicate that anti-NMDAR and non-anti-NMDAR Ab modulate NMDAR function distinctly and p38 signaling in the short-term, and a role of a third-party mediator. Finally, our results suggest the involvement of NMDAR flux-independent signaling.