The role of C-terminial SRC kinase (Csk) in the regulation of N-methyl-D-aspartate receptors.

Description

The N-methyl-D-aspartate receptor (NMDAR) is a major subtype of glutamate receptor that mediates excitatory synaptic transmission in the central nervous system (CNS) and is required for inducing functional plasticity at many synapses. Up-regulation of NMDAR by Src family kinases (SFKs) is a critical mechanism underlying a variety of neuronal functions, and this regulation is controlled by the convergence of many signaling pathways on SFKs. It has been documented that protein tyrosine phosphatase alpha (PTPalpha), which activates SFKs by selectively dephosphorylating the phosphorylated C-tail tyrosine (Tyr527 in chicken c-Src), associates with NMDARs by binding to postsynaptic density 95 (PSD-95), and plays important roles in the initiation and maintenance of NMDAR up-regulation by SFKs. Here, I report that C-terminal Src kinase (Csk), which specifically phosphorylates the regulatory C-tail tyrosine of SFKs and thereby inhibits their kinase activity, targets NMDARs through direct and indirect associations with C-termini of NR2 and NR1 subunits, respectively. As Src activity increases, both associations are enhanced. In vitro pull-down assays show that Csk binds to a peptide corresponding to the C-tail of NR2A through multiple binding sites, but fails to bind to a peptide corresponding to the C-tail of NR1. The Csk-NR2A C-tail interaction can be significantly reduced by the mutation of the Src homology 2 (SH2) domain of Csk or by dephosphorylation of the NR2A C-tail, suggesting the involvement of the classical SH2-phosphotyrosine binding. Furthermore, Csk attenuates the phosphorylation of NR2A subunit mediated by wild-type Src, but not by the mutant Src in which the C-tail tyrosine phosphorylation is blocked. Applying recombinant Csk into hippocampal neurons cultured from embryos or isolated acutely from adult rats inhibits NMDAR-mediated whole-cell currents, and this effect can be occluded by pre-inhibition of SFKs, indicating that Csk down-regulates NMDAR activity via depression of SFKs. Finally, blocking Csk SH2 domain interaction significantly potentiates excitatory synaptic transmission and occludes the tetanic stimulation-induced long-term potentiation (LTP), whereas increasing intracellular Csk inhibits the induction of LTP at Schaffer collateral-CA1 synapses. Thus, it is the first demonstration that Csk dynamically targets NMDARs and negatively regulates NMDAR tyrosine phosphorylation, channel activity and synaptic plasticity.

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