Decreased NR2B subunit synaptic levels cause impaired long-term potentiation but not long-term depression.

The discovery of the molecular mechanisms regulating the abundance of synaptic NMDA receptors is essential for understanding how synaptic plasticity, as well as excitotoxic events, are regulated. However, a complete understanding of the precise molecular mechanisms regulating the composition of the NMDA receptor complex at hippocampal synapse is still missing. Here, we show that 2 h of CaMKII inhibition leads to a specific reduction of synaptic NR2B-containing NMDA receptors without affecting localization of the NR2A subunit; this molecular event is accompanied by a dramatic reduction in the induction of long-term potentiation (LTP), while long-term depression induction is unaffected. The same molecular and functional results were obtained by disrupting NR2B/PSD-95 complex with NR2B C-tail cell permeable peptide (TAT-2B). These data indicate that NR2B redistribution between synaptic and extrasynaptic membranes represents an important molecular disturbance of the glutamatergic synapse and affects the correct induction of LTP.

Epilepsy-induced abnormal striatal plasticity in Bassoon mutant mice.

Recently, the striatum has been implicated in the spread of epileptic seizures. As the absence of functional scaffolding protein Bassoon in mutant mice is associated with the development of pronounced spontaneous seizures, we utilized this new genetic model of epilepsy to investigate seizure-induced changes in striatal synaptic plasticity. Mutant mice showed reduced long-term potentiation in striatal spiny neurons, associated with an altered N-methyl-D-aspartate (NMDA) receptor subunit distribution, whereas GABAergic fast-spiking (FS) interneurons showed NMDA-dependent short-term potentiation that was absent in wild-type animals. Alterations in the dendritic morphology of spiny neurons and in the number of FS interneurons were also observed. Early antiepileptic treatment with valproic acid reduced epileptic attacks and mortality, rescuing physiological striatal synaptic plasticity and NMDA receptor subunit composition. However, morphological alterations were not affected by antiepileptic treatment. Our results indicate that, in Bsn mutant mice, initial morphological alterations seem to reflect a more direct effect of the abnormal genotype, whereas plasticity changes are likely to be caused by the occurrence of repeated cortical seizures.

A critical interaction between NR2B and MAGUK in L-DOPA induced dyskinesia.

Abnormal function of NMDA receptor has been suggested to be correlated with the pathogenesis of Parkinson's disease (PD) as well as with the development of l-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia. Here we show that NMDA receptor NR2 subunits display specific alterations of their subcellular distribution in striata from unilateral 6-hydroxydopamine-lesioned, L-DOPA-treated dyskinetic, and L-DOPA-treated nondyskinetic rats. Dyskinetic animals have significantly higher levels of NR2A subunit in the postsynaptic compartment than all other experimental groups, whereas NR2B subunit shows a significant reduction in both dopamine-denervated and dyskinetic rats. These events are paralleled by profound modifications of NMDA receptor NR2B subunit association with interacting elements, i.e., members of the membrane-associated guanylate kinase (MAGUK) protein family postsynaptic density-95, synapse-associated protein-97 and synapse-associated protein-102. Treatment of nondyskinetic animals with a synthetic peptide (TAT2B) able to affect NR2B binding to MAGUK proteins as well as synaptic localization of this subunit in nondyskinetic rats was sufficient to induce a shift of treated rats toward a dyskinetic motor behavior. These data indicate abnormal NR2B redistribution between synaptic and extrasynaptic membranes as an important molecular disturbance of the glutamatergic synapse involved in L-DOPA-induced dyskinesia.

Calcium-calmodulin-dependent protein kinase II phosphorylation modulates PSD-95 binding to NMDA receptors.

At the postsynaptic membrane of excitatory synapses, NMDA-type receptors are bound to scaffolding and signalling proteins that regulate the strength of synaptic transmission. The cytosolic tails of the NR2A and NR2B subunits of NMDA receptor bind to calcium-calmodulin-dependent protein kinase II (CaMKII) and to members of the MAGUK family such as PSD-95. In particular, although NR2A and NR2B subunits are highly homologous, the sites of their interaction with CaMKII as well as the regulation of this binding differ. We identified PSD-95 phosphorylation as a molecular mechanism responsible for the dynamic regulation of the interaction of both PSD-95 and CaMKII with the NR2A subunit. CaMKII-dependent phosphorylation of PSD-95 occurs both in vitro, in GST-PSD-95 fusion proteins phosphorylated by purified active CaMKII, and in vivo, in transfected COS-7 as well as in cultured hippocampal neurons. We identified Ser73 as major phosphorylation site within the PDZ1 domain of PSD-95, as confirmed by point mutagenesis experiments and by using a phospho-specific antibody. PSD-95 Ser73 phosphorylation causes NR2A dissociation from PSD-95, while it does not interfere with NR2B binding to PSD-95. These results identify CaMKII-dependent phosphorylation of the PDZ1 domain of PSD-95 as a mechanism regulating the signalling transduction pathway downstream NMDA receptor.