NMDA receptors are selectively partitioned into complexes and supercomplexes during synapse maturation

Area of Science:

• Neuroscience
• Molecular Biology
• Biochemistry

Background:

• Neuronal proteome self-organization is complex and poorly understood.
• Mammalian synapses, particularly NMDA receptors (NMDARs), are key to brain function but their assembly is unclear.

Purpose of the Study:

• To biochemically purify endogenous NMDARs from adult mouse brain.
• To elucidate the assembly mechanism of NMDAR supercomplexes.
• To identify factors regulating NMDAR incorporation into larger synaptic structures.

Main Methods:

• Blue-native polyacrylamide gel electrophoresis (PAGE) for native protein complex separation.
• Gene-tagging of GluN1 for biochemical purification of endogenous NMDARs.
• Analysis of six mouse mutants to investigate NMDAR supercomplex assembly.

Main Results:

• NMDARs exist in two distinct populations: receptor complexes and large (∼1.5 MDa) supercomplexes.
• GluN2B, PSD93, and PSD95 are essential for gating NMDARs into supercomplexes, independent of PDZ-ligands or GluN2A.
• Adult forebrain exhibits a fourfold molar excess of GluN2B over GluN2A.
• NMDAR supercomplex assembly occurs late in postnatal development, linked to synapse maturation, epigenetic, and activity-dependent processes.
• Screening of 60 native proteins revealed numerous discrete supercomplexes within the mammalian synapse.

Conclusions:

• The study provides the first biochemical purification of endogenous NMDARs, revealing their organization into distinct supercomplexes.
• GluN2B, PSD93, and PSD95 play a critical role in the assembly of these NMDAR supercomplexes.
• Synapse maturation, involving epigenetic and activity-dependent mechanisms, triggers the formation of NMDAR supercomplexes during development.