ACTIVITY-DEPENDENT INTERNALIZATION OF GLUN2B-CONTAINING NMDARS IS REQUIRED FOR SYNAPTIC INCORPORATION OF GLUN2A AND SYNAPTIC PLASTICITY

NMDA-type glutamate receptors (NMDARs) are heterotetrameric complexes composed of two GluN1 and two GluN2 subunits. The precise composition of the GluN2 subunits determines the channel’s biophysical properties and influences its interaction with postsynaptic scaffolding proteins and signaling molecules involved in synaptic physiology and plasticity. Consequently, the precise regulation of NMDAR subunit composition at synapses is crucial for proper synaptogenesis, neuronal circuit development, and synaptic plasticity, a cellular model of memory formation.

In the forebrain during early development, NMDARs contain the GluN2B subunit, which is necessary for proper synaptogenesis and synaptic plasticity. In rodents, GluN2A subunit expression begins in the second postnatal week, replacing GluN2B-containing NMDARs at synapses in an activity- or sensory experience-dependent process. This switch in NMDAR subunit composition at synapses alters channel properties and reduces synaptic plasticity. The molecular mechanism regulating the switch remains unclear.

We have investigated the role of activity-dependent internalization of GluN2B-containing receptors in shaping synaptic NMDAR subunit composition. Using a combination of molecular, pharmacological, and electrophysiological approaches in cultured organotypic hippocampal slices from rats of both sexes, we show that the process of incorporating GluN2A-containing NMDARs receptors requires activity-dependent internalization of GluN2B-containing NMDARs. Interestingly, blockade of GluN2A synaptic incorporation was associated with impaired potentiation of AMPA-mediated synaptic transmission, suggesting a potential coupling between the trafficking of AMPARs into synapses and that of GluN2A-containing NMDARs.

These insights contribute to our understanding of the molecular mechanisms underlying synaptic trafficking of glutamate receptors and synaptic plasticity. They may also have implications for therapeutic strategies targeting NMDAR function in neurological disorders.

SIGNIFICANCE STATEMENT

Synaptic NMDARs play a critical role in synaptogenesis, synaptic stability, and activity-dependent regulation of synaptic strength. The developmental switch in GluN2 subunits composition of synaptic NMDARs is part of normal synapse development and is crucial for proper synaptic physiology, plasticity, and the formation of functional neuronal circuits, though the mechanisms governing it remain unclear.

We show that internalization of GluN2B-containing NMDARs is required for synaptic incorporation of GluN2A-containing receptors. This process can be induced by long-term potentiation and requires Ca ⁺² . Notably, GluN2A trafficking to synapses is linked to the incorporation of AMPA-type glutamate receptors, suggesting a shared pathway for synaptic incorporation.

These findings provide greater insight into the molecular mechanisms behind glutamate receptor trafficking and synaptic plasticity, potentially informing therapeutic strategies for neurological disorders.