Molecular characterization and embryonic expression of the family of N-methyl-D-aspartate receptor subunit genes in the zebrafish

We present the cloning of 10 N-methyl-D-aspartate (NMDA) receptor subunits from the zebrafish. These subunits fall into five subtypes, each containing two paralogous genes. Thus, we report two NMDAR1 genes (NR1.1 and NR1.2), and eight NMDAR2 genes, designated NR2A.1 and NR2A.2, NR2B.1 and NR2B.2, NR2C.1 and NR2C.2, and NR2D.1 and NR2D.2. The predicted sequences of the NR1 paralogs display 90% identity to the human protein. The NR2 subunits show less identity, differing most at the N- and C-termini. The NR1. genes are both expressed embryonically, although in a nonidentical manner. NR1.1 is found in brain, retina, and spinal cord at 24 hours postfertilization (hpf). NR1.2 is expressed in the brain at 48 hpf but not in the spinal cord. NR2 developmental gene expression varies: both paralogs of the NR2A are expressed at 48 hpf in the retina, only one paralog of the NR2B is expressed at low levels in the heart at 48 hpf. Neither of the NR2C is expressed embryonically. Both paralogs of the NR2D are expressed: 2D.1 is in the forebrain, retina, and spinal cord at 24 hpf, whereas the 2D.2 is only found in the retina. Our findings demonstrate that the zebrafish can serve as a useful model system for investigating the role of NMDA receptors in the development of the nervous system.