Unveiling the Architecture of Native NMDA Receptors in the Mammalian Brain

A research team led by Shujia Zhu from the Chinese Academy of Sciences (CAS) and Yang Li from the Shanghai Institute of Materia Medica has made groundbreaking progress in understanding endogenous N-methyl-D-aspartate receptors (eNMDARs) in the mammalian brain. Published in Cell on January 23, 2025, the study provides detailed insights into the architecture and assembly of eNMDARs in the cerebral cortex and hippocampus, key regions for learning and memory.

The Role of NMDA Receptors

Learning and memory, the foundation of human cognition and perception, rely on synaptic plasticity, which changes over time and with activity. NMDA receptors, part of the ionotropic glutamate receptor family, are critical modulators of these processes. They regulate synaptic connections and play an essential role in advanced brain functions, particularly in higher-order regions like the cerebral cortex and hippocampus.

NMDA receptors are heteromeric tetramers composed of two core GluN1 subunits and two alternative subunits (GluN2 or GluN3). While significant strides have been made in studying these receptors using recombinant systems, research into naturally occurring eNMDARs has been hindered by their scarcity in brain tissue and the lack of effective purification techniques.

Breakthrough Techniques

To overcome these challenges, the team enriched eNMDARs in the brains of adult wild-type rats using a high-affinity antibody with an affinity tag. Cryo-electron microscopy combined with a convolutional network-based model allowed researchers to differentiate eNMDARs from other endogenous proteins. By integrating pharmacological and computational purification techniques, the study achieved atomic-level resolution of natural receptors involved in synaptic plasticity.

Key Findings

The researchers identified three primary eNMDAR subtypes in the cerebral cortex and hippocampus:

• GluN1-N2A-N2B tri-heteromeric receptors (45%)
• GluN1-N2B di-heteromeric receptors (35%)
• GluN1-N2A di-heteromeric receptors (20%)

The structural analysis of the GluN1-N2A-N2B subtype revealed a unique asymmetrical architecture and the functional integration of the GluN2A and GluN2B subunits. Notable conformational differences were also observed in the GluN2B subunit across receptor subtypes, highlighting the functional diversity of eNMDARs.

Broader Implications

The findings provide a molecular basis for understanding how eNMDARs regulate excitatory synaptic transmission and plasticity. Additionally, the team mapped a spatiotemporal atlas of eNMDARs across the brain, shedding light on their variation during development and across different brain regions. This paradigm enhances our understanding of learning and memory and could explain how synaptic plasticity evolves with age.

Future Directions

This study sets the stage for investigating how pathological changes in eNMDARs contribute to neurological and psychiatric diseases. By uncovering the structural and functional diversity of these receptors, the research opens new avenues for exploring their role in brain disorders and developing targeted therapeutic interventions.