NMDA receptor autoantibodies primarily impair the extrasynaptic compartment.

Autoantibodies directed against the N-methyl-D-aspartate receptor (NMDAR-Ab) are pathogenic immunoglobulins detected in patients suffering from NMDAR encephalitis. NMDAR-Ab alter the receptor membrane trafficking, synaptic transmission and neuronal network properties, leading to patients' neurological and psychiatric symptoms. Patients often have very little neuronal damage but rapid and massive (treatment-responsive) brain dysfunctions related to unknown early mechanism of NMDAR-Ab. Our understanding of this early molecular cascade remains surprisingly fragmented. Here, we used a combination of single molecule-based imaging of membrane proteins to unveil the spatio-temporal action of NMDAR-Ab onto live hippocampal neurons. We first demonstrate that different clones of NMDAR-Ab primarily affect extrasynaptic -and not synaptic- NMDAR. In the first minutes, NMDAR-Ab increase extrasynaptic NMDAR membrane dynamics, de-clustering its surface interactome. NMDAR-Ab also rapidly reshuffle all membrane proteins located at the extrasynaptic compartment. Consistent with this alteration of multiple proteins, NMDAR-Ab effects were not mediated through the sole interaction between NMDAR and EphB2 receptor. At the long-term, NMDAR-Ab reduce NMDAR synaptic pool by slowing down receptor membrane dynamics in a cross-linking independent manner. Remarkably, exposing only extrasynaptic NMDAR to NMDAR-Ab was sufficient to produce their full-blown effect on synaptic receptors. Collectively, we demonstrate that NMDAR-Ab first impair extrasynaptic proteins, and then the synaptic ones. These data shed thus new, and unsuspected, lights on the mode of action of NMDAR-Ab and likely to our understanding of (extra)synaptopathies.