On the role of NR3A in human NMDA receptors.

In the present paper we describe our on-going project investigating the functional roles of the N-methyl-D-aspartate (NMDA) receptor subunit NR3A. We find that NR3A mRNA is abundant both in embryonic and adult human brain, in contrast to the almost non-existing expression in adult rodent brain. Human NR3A (hNR3A) protein expression is particularly abundant in the cerebral cortex, as shown by western blot using NR3A-specific antibodies. Distribution of hNR3A in adult human brain shows a similar pattern as NR3A in post-natal rodent brain. We have previously reported that NR3A contains a glycine binding site, with similar affinity as the glycine binding site of NR1 subunits. This suggests that NR3A may replace one of the two NR1 subunits in native NMDA receptors. Cloning of hNR3A showed a human-specific polyproline-sequence in the intracellular C-terminus, that may bind to SH3-domains. We hypothesized that the significant differences in expression in the adult human and rodent brain could be due to an atypical interaction of hNR3A with the SH3 domain of the synaptic scaffolding protein PSD-95, that binds to NR2 subunits through its PDZ domains. However, using a number of different protein interaction assays, binding of PSD-95 to hNR3A could no be demonstrated either in vitro or in vivo. To identify intracellular signaling pathways for NR3A-containing NMDA receptors, we screened for proteins interacting with hNR3A and identified three proteins: plectin, CARP-1 and GPS2. The possible physiological roles of these interactions are discussed.