Poster Sessions

Neu-48

Antonio Sanz-Clemente

A. Sanz-Clemente, K. W. Roche

A critical role for Casein Kinase 2 in the developmental switch of NMDA receptors

NMDA receptors (NMDARs) are ion channels that play a key role in synaptic function and neuronal development. They are functional tetramers composed of two NR1 subunits and two NR2A-D and NR3 subunits. Synaptic NMDAR number and subunit composition are strongly regulated by several mechanisms including phosphorylation and interactions with PDZ domain-containing proteins. It is well known that the composition of synaptic NMDARs changes during development. During early stages, NR2B-containing NMDARs are predominant, whereas, NR2A-containing NMDARs are the most abundant at adulthood. However, the precise molecular mechanisms mediating the switch remain obscure. A recent report showed that NR2B is phosphorylated in its PDZ binding domain (S1480) by casein kinase 2 (CK2), which disrupts the binding with PSD-95 and SAP102 and decreases the amount of NR2B at the cell surface. We compared CK2 phosphorylation of NR2B vs NR2A and found that, despite its high homology, NR2A is not phosphorylated by this kinase. This difference means that NR2A and NR2B binding to PDZ domain-containing proteins is differentially regulated, despite the conserved PDZ ligands. CK2 is highly expressed in the brain and increases during development. Therefore, we tested the hypothesis that CK2 might differentially affect NR2A and NR2B synaptic distribution during development. Using biochemical and confocal imaging approaches we observed an increase in the levels of NR2B-S1480 phosphorylation during the period of the developmental NMDAR switch as well as an elevated association of CK2 with the synaptic plasma membrane during the same period. The NMDAR subunit switch may require sequential steps including the synaptic removal of NR2B induced by CK2-mediated endocytosis, which allows the subsequent insertion of NR2A into synapses. Consistent with this model, we found that inhibition of CK2 causes a reduction in the levels of surface NR2A in cortical primary cultures. Since a developmental increase in synaptic NR2A is highly correlated with an enhancement of its total expression, we analyzed NR2A knockout mice. Interestingly, NR2B-S1480 phosphorylation levels and CK2 association with synaptic plasma membrane were reduced in these mice in comparison with wild-type animals, revealing a critical interplay between NR2A expression and CK2 phosphorylation of NR2B. In summary, we show that CK2 plays an important role in the developmental NMDAR switch defining their synaptic localization.