Assembly and architecture of endogenous NMDA receptors in adult cerebral cortex and hippocampus

The cerebral cortex and hippocampus are crucial brain regions for learning and memory, which depend on activity-induced synaptic plasticity involving N-methyl-ᴅ-aspartate receptors (NMDARs). However, subunit assembly and molecular architecture of endogenous NMDARs (eNMDARs) in the brain remain elusive. Using conformation- and subunit-dependent antibodies, we purified eNMDARs from adult rat cerebral cortex and hippocampus. Three major subtypes of GluN1-N2A-N2B, GluN1-N2B, and GluN1-N2A eNMDARs were resolved by cryoelectron microscopy (cryo-EM) at the resolution up to 4.2 Å. The particle ratio of these three subtypes was 9:7:4, indicating that about half of GluN2A and GluN2B subunits are incorporated into the tri-heterotetramers. Structural analysis revealed the asymmetric architecture of the GluN1-N2A-N2B receptor throughout the extracellular to the transmembrane layers. Moreover, the conformational variations between GluN1-N2B and GluN1-N2A-N2B receptors revealed the distinct biophysical properties across different eNMDAR subtypes. Our findings imply the structural and functional complexity of eNMDARs and shed light on structure-based therapeutic design targeting these eNMDARs in vivo.