Interactions of postsynaptic density-95 and the NMDA receptor 2 subunit control calpain-mediated cleavage of the NMDA receptor.

The calcium-dependent protease calpain cleaves the NMDA receptor 2 (NR2) subunit of the NMDA receptor both in vitro and in vivo and thus potentially modulates NMDA receptor function and turnover. We examined the ability of postsynaptic density-95 (PSD-95) protein to alter the calpain-mediated cleavage of NR2A and NR2B. Coexpression of PSD-95 with NMDA receptors in human embryonic kidney 293 cells blocked cleavage of NR2A and NR2B by NMDA receptor-activated calpain. NR2A cleavage by calpain occurred in the cell surface and intracellular fractions and required the presence of NR1 subunits. The blocking effect of PSD-95 did not result from decreased calpain activity, lowered intracellular calcium responses, or the blockade of internalization. Instead, this effect was eliminated by deletion of the C-terminal ESDV motif of NR2A or by overexpression of a palmitoylation-deficient PSD-95 mutant lacking the ability to cluster and to interact with NMDA receptors in situ, suggesting a role for association between the C terminus of NR2A and clustered PSD-95. Synapse-associated protein 102, a membrane-associated guanylate kinase interacting with NR2A but lacking palmitoylation motifs and the ability to cluster, did not protect NR2A from cleavage by calpain. Pharmacological inhibition of palmitoylation disrupted the interaction of PSD-95 with NMDA receptors in cortical neurons and allowed NR2A to be cleaved by calpain, whereas NR2A could not be cleaved in untreated neurons. These results indicate that PSD-95 clustering and direct association of NR2A and PSD-95 mediate the blocking effect of PSD-95 on calpain cleavage. PSD-95 could regulate the susceptibility of NMDA receptors to calpain-mediated cleavage during synaptic transmission and excitotoxicity.