Regulation of NMDA receptor trafficking and channel permeability by protein kinases

Abstract

The N-methyl-D-aspartate receptor (NMDAR) is a glutamate-gated ion channel pivotal to synaptogenesis, synaptic remodeling and long-lasting changes in synaptic efficacy such as long-term potentiation (LTP). Protein kinase C (PKC) enhances NMDAR channel opening rate and promotes NMDAR delivery to the cell surface via SNARE-dependent exocytosis. Although the mechanisms of PKC potentiation are established, the molecular target of PKC is unclear. We show that synaptosomal-associated protein of 25 kDa (SNAP-25), a SNARE protein, is functionally relevant to PKC-dependent NMDAR insertion and identify serine residue-187 as the molecular target of PKC phosphorylation. Constitutively active PKC delivered via the patch pipette potentiated NMDA (but not AMPA) whole-cell currents in hippocampal neurons. Expression of mutant SNAP-25(S187A) or acute disruption of SNARE interactions by a blocking peptide abolished NMDAR potentiation. These findings identify SNAP-25 as the target of PKC phosphorylation critical to PKC-dependent incorporation of NMDARs and document a postsynaptic action of this major SNARE protein relevant to synaptic plasticity. Additionally, we show that Ca2+ permeability of NMDARs is under control of the cyclic AMP/protein kinase A (PKA) signaling cascade. PKA blockers reduced the relative fractional Ca2+ influx through NMDARs as determined by Ca2+-reversal potential shift analysis and a combination of electrical recording and Ca2+ influx measurements in hippocampal neurons. PKA regulation of NMDAR-mediated currents was subunit-specific in that the PKA inhibitor H-89 markedly depressed currents mediated by NR2B-containing NMDARs whereas it more modestly depressed NR2A currents. Mutant NR2B(S1166A), but not NR2B(S929A/S930A), attenuated the H-89 effect on NR2B currents, suggesting that Ser1166 in the NR2B C-terminus is a candidate site of PKA phosphorylation. Moreover, H-89 shifted the reversal potential of NMDAR current/voltage relationships mediated by NR2B(WT), but not NR2B(S1166A), receptors in the negative direction. In hippocampal slices, PKA blockers inhibited the NMDAR-mediated Ca 2+ rises in dendritic spines, with little effects on synaptic current. Consistently, PKA blockers depressed the early phase of LTP at Schaffer collateral-CA1 synapses. These data identify Ser1166 in NR2B as an important functional target of PKA phosphorylation and link PKA-dependent synaptic plasticity to Ca 2+ signaling in spines. Regulation of NMDAR-mediated signaling profoundly impacts on activity-dependent modulation of synaptic efficacy and gene expression.