Developmental expression of N-methyl-D-aspartate glutamate receptor 1 splice variants in the chick retina

Neuroprotection by GH against excitotoxic-induced cell death in retinal ganglion cells

Retinal growth hormone (GH) has been shown to promote cell survival in retinal ganglion cells (RGCs) during developmental waves of apoptosis during chicken embryonic development. The possibility that it might also against excitotoxicity-induced cell death was therefore examined in the present study, which utilized quail-derived QNR/D cells as an in vitro RGC model. QNR/D cell death was induced by glutamate in the presence of BSO (buthionine sulfoxamide) (an enhancer of oxidative stress), but this was significantly reduced (P<0.01) in the presence of exogenous recombinant chicken GH (rcGH). Similarly, QNR/D cells that had been prior transfected with a GH plasmid to overexpress secreted and non-secreted GH. This treatment reduced the number of TUNEL-labeled cells and blocked their release of lactate dehydrogenase (LDH). In a further experiment with dissected neuroretinal explants from ED (embryonic day) 10 embryos, rcGH treatment of the explants also reduced (P<0.01) the number of glutamate-BSO-induced apoptotic cells and blocked the explant release of LDH. This neuroprotective action was likely mediated by increased STAT5 phosphorylation and increased bcl-2 production, as induced by exogenous rcGH treatment and the media from GH-overexpressing QNR/D cells. As rcGH treatment and GH-overexpression cells also increased the content of IGF-1 and IGF-1 mRNA this neuroprotective action of GH is likely to be mediated, at least partially, through an IGF-1 mechanism. This possibility is supported by the fact that the siRNA knockdown of GH or IGF-1 significantly reduced QNR/D cell viability, as did the immunoneutralization of IGF-1. GH is therefore neuroprotective against excitotoxicity-induced RGC cell death by anti-apoptotic actions involving IGF-1 stimulation.

Cloning and characterization of the N-methyl-D-aspartate receptor subunit NR1 gene from chum salmon, Oncorhynchus keta (Walbaum, 1792)

Here, we report the information about molecular and expression characterization of NR1 gene in chum salmon for the first time. The complete NR1 subunit showed a large open-reading frame of 2844 bp in the total length of 3193 bp, and this cDNA contained a coding region encoding 948 amino acids and a stop codon. The organization of the NR1 subunit of chum salmon were similar of most other fishes, except C’ terminal. The expression of NR1 subunit was to show higher in the natal river near to the hatchery than near to the coast. We expect that the information reported herein may facilitate further investigations on the relationship between memory factors of natal rivers and homing mechanisms in Salmonidae.

Cloning and Phylogenetic Analysis of NMDA Receptor Subunits NR1, NR2A and NR2B in Xenopus laevis Tadpoles

N-methyl-d-aspartate receptors (NMDARs) play an important role in many aspects of nervous system function such as synaptic plasticity and neuronal development. NMDARs are heteromers consisting of an obligate NR1 and most commonly one or two kinds of NR2 subunits. While the receptors have been well characterized in some vertebrate and invertebrate systems, information about NMDARs in Xenopus laevis brain is incomplete. Here we provide biochemical evidence that the NR1, NR2A and NR2B subunits of NMDARs are expressed in the central nervous system of X. laevis tadpoles. The NR1-4a/b splice variants appear to be the predominant isoforms while the NR1-3a/b variants appear to be expressed at low levels. We cloned the X. laevis NR2A and NR2B subunits and provide a detailed annotation of their functional domains in comparison with NR2A and NR2B proteins from 10 and 13 other species, respectively. Both NR2A and NR2B proteins are remarkably well conserved between species, consistent with the importance of NMDARs in nervous system function.

The N-methyl-d-aspartate receptor in heart development: a gene knockdown model using siRNA

Antagonists of the N-methyl-d-aspartate receptor (NMDAR) may disrupt the development of the cardiac neural crest (CNC) and contribute to conotruncal heart defects. To test this interaction, a loss-of-function model was generated using small interfering RNAs (siRNA) directed against the critical NR1-subunit of this receptor in avian embryos. The coding sequence of the chicken NR1 gene and predicted protein sequences were characterized and found to be homologous with other vertebrate species. Analysis of its spatiotemporal expression demonstrated its expression within the neural tube at pre-migratory CNC sites. siRNA targeted to the NR1-mRNA in pre-migratory CNC lead to a significant decrease in NR1 protein expression. However, embryo survival and heart development were not adversely affected. These results indicate that the CNC may function normally in the absence of functional NMDAR, and that NMDAR antagonists may have a complex impact upon the CNC that transcends impairment of a single receptor type.

Serum differentially modifies the transcription and translation of NMDAR subunits in retinal neurons

The N-methyl-D-aspartate type of glutamate receptor (NMDAR) plays a major role in the vertebrate retina. Expression of NR1 splice-variants and NR2 subunits in the retina differs from that in the brain, suggesting a tissue-specific heteromeric assembly of NMDARs. We previously demonstrated that serum alters retinal glutamate receptor properties. In order to relate this effect to NMDAR subunit composition, we here studied the effect of serum on the expression of NMDAR subunits and splice-variants in chick retinal neurons in primary culture. Our results show that mRNA and protein expression of NR1 alternative splice-variants and NR2 subunits are differentially modified by glutamate contained in serum. Such alteration suggests that NMDAR structure is reversed to embryonic heteromeric composition, through the control of subunit availability. The present findings could be relevant for the understanding of the lack of effect in the retina, of drugs which have been shown to protect cortical neurons from glutamate-induced excitotoxicity in those pathological or clinical conditions in which the retina is exposed to serum.

D-serine regulates CREB phosphorylation induced by NMDA receptor activation in Müller glia from the retina

D-serine is an N-methyl-D-aspartate (NMDA) receptor coagonist predominantly produced by glial cells in the brain and the retina. Whereas a role for D-serine as a modulator of NMDA receptors in neurons has been suggested, its function in glial cells has not been analyzed. We here show that D-serine modulates gene expression in Müller glial cells from the retina through the induction of transcription factor CREB phosphorylation and the expression of the immediate-early gene c-fos. Pharmacological analysis indicates that D-serine effect involves NMDA receptor activation. Comparison of the effect of D-serine in Müller cells, hippocampal astrocytes and hippocampal neurons suggests that D-serine could function as a retinal NMDA receptor coagonist activating functionally relevant transcription factor pathways in glial cells.

Molecular and functional studies of tilapia (Oreochromis mossambicus) NMDA receptor NR1 subunits

NMDA (N-methyl-d-aspartate) receptor, a subclass of the ionotropic glutamate receptors, participates in synaptic transmission and plays important roles in various higher brain functions in the vertebrate central nervous system. Here, we report the cloning of two NR1 subunits of tilapia (Oreochromis mossambicus). Phylogenetic analysis strongly supports that the two tilapia NR1 genes are paralogous, resulting from a gene duplication event in the teleost lineage. The electrophysiological and pharmacological properties of the tilapia NR1.2 subunit coexpressed with rat NR2B in the Xenopus oocytes are similar to that of the recombinant rat NR1/NR2B. Both tilapia NR1 transcripts are alternatively spliced at the N and C terminal coding regions. The C terminal exons, C1' and C1", originally discovered in the knifefish NR1 gene, are present in the tNR1.1 but not in the tNR1.2. Majorities of the NR1 transcripts expressed in the tilapia and zebrafish brains do not include these alternative splice exons. The splicing patterns of NR1 transcripts differ in various brain subregions. The regional expression patterns of splice variants are not fully preserved between tilapia and zebrafish. Nevertheless, tectum opticum regions of teleost and rat express high levels of NR1 splicing variant with N1 cassette.

Cloning and characterization of the chick NMDA receptor subunit-1 gene

The N-methyl-D-aspartate family of glutamate receptors (NMDARs) are tetrameric cation channels including NR1, NR2, and possibly NR3 subunits. The physiological properties of the receptor are directly related to the subunit composition of the oligomer. Whereas NR1 is essential for the formation of functional channels, NR2 and NR3 play a modulatory role. This work reports, for the first time, the cloning of a non-mammalian NR1 gene, including the 5'-regulatory region. The chick gene spans 31 kb of genomic DNA sequence composed of 22 exons interrupted by 21 introns. The exon/intron organization and the deduced amino acid sequence up to the end of exon 19 showed 85% homology to mammalian NR1 cloned genes. Significant differences from mammals were found at the C-terminal region which in the chick gene, includes a novel exon (exon 20) previously identified at the mRNA level in the chick retina. The basal promoter activity was shown to reside within the proximal 377 bp of 5'-regulatory region. The transcriptional activity of the 5'-flanking region of the chick NR1 gene was shown to be higher in neuronally-differentiated PC12 cells and in chick retinal neurons, than in non-differentiated PC12 cells and Müller glia. Comparison of the 5'-flanking region of chick NR1 gene with mammalian NR1 genes suggests that, in spite of significant differences in the nucleotide sequence, they share common DNA binding sites such as RE1, SP1, AP2, CREB, NFkappaB, and MEF2; therefore, some of the molecular mechanisms involved in transcriptional regulation of NR1 gene expression could be conserved among species.

Glutamate-induced inhibition of D-aspartate uptake in Müller glia from the retina

Müller glial cells from the retina "in situ" and in primary culture, mainly express the high-affinity sodium-coupled glutamate/aspartate transporter GLAST-1, which dominates total retinal glutamate (Glu) uptake, suggesting a major role for these cells in the modulation of excitatory transmission. The possible involvement of ionotropic and metabotropic Glu receptors in the regulation of Glu uptake was studied in primary cultures of Müller glia. We demonstrate that exposure to 1 mM L-Glu induces a time-dependent inhibition of D-aspartate (D-Asp) uptake in a Na+-dependent manner, as a result of a reduction in the number of transporters at the plasma membrane. The inhibition of D-Asp uptake by Glu was not mimicked by agonists or modified by antagonists of ionotropic and metabotropic Glu receptors. In contrast, transport was inhibited by GLAST-1 transportable substrates threo-hydroxyaspartate and aspartate-beta-hydroxamate, but not by the nontransportable inhibitors trans-pyrrolidine dicarboxylate or DL-threo-beta-benzyloxyaspartic acid. Under the same experimental conditions, L-Glu did not affect the sodium-dependent transport systems for glycine or GABA. The present results demonstrate that the specific downregulation of glutamate/aspartate transport by L-Glu is unrelated to Glu receptor activation, and results from the internalization of transporter proteins triggered by the transport process itself. Such negative feedback of Glu on Glu transport, could contribute to retinal toxicity under pathological conditions in which high extracellular concentrations of Glu are reached.