Deletion of the C‐terminal domain of the NR2B subunit alters channel properties and synaptic targeting of N‐methyl‐D‐aspartate receptors in nascent neocortical synapses

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Abstract
Channel properties and synaptic targeting of N‐methyl‐D‐aspartate (NMDA) receptors determine their importance in synaptic transmission, long‐term synaptic plasticity, and developmental reorganization of synaptic circuits. To investigate the involvement of the C‐terminal domain of the NR2B subunit in regulating channel properties and synaptic localization, we analyzed gene‐targeted mice expressing C‐terminally truncated NR2B subunits (NR2BΔC/ΔC mice; Sprengel et al. [1998] Cell 92:279–89). Because homozygous NR2BΔC/ΔC mice die perinatally, we studied embryonic neocortical neurons differentiating in culture. At early stages in vitro, neurons predominantly expressed NR1/NR2B receptors, as shown by the NR2B subunit‐specific antagonist ifenprodil. At these nascent synapses, NMDA excitatory postsynaptic currents (EPSCs) in neurons from NR2BΔC/ΔC mice showed a strong‐amplitude reduction to 20% of control, but AMPA EPSCs were unaltered. Analysis of the MK‐801 block of NMDA receptor‐mediated whole‐cell currents revealed a decreased peak open probability of NMDA receptor channels (to about 60%) in neurons from NR2BΔC/ΔC mice, although their single channel conductance was unchanged. To study effects on synaptic targeting, we determined the fraction of synaptically localized NMDA receptors relative to the whole‐cell NMDA receptor population. In neurons from NR2BΔC/ΔC mice, the synaptic NMDA receptor fraction was drastically reduced, suggesting that the C‐terminal domain of the NR2B subunit plays a major role in synaptic targeting of NMDA receptors at nascent synapses. With increasing time in culture, the reduction in NMDA EPSCs in neurons from NR2BΔC/ΔC mice diminished. This is explained by the expression of additional NMDA receptor subtypes containing NR2A subunits at more mature synapses.
Funding Information
  • Deutsche Forschungsgemeinschaft (Go 710/2-2)