Expression of recombinant NMDA receptors in hippocampal neurons by adenoviral-mediated gene transfer

Virally mediated knock-down of NR2 subunits ipsilateral to the deprived eye blocks ocular dominance plasticity

NMDA receptors (NMDARs) are important in developmental plasticity in the visual cortex. The NR2A and NR2B subunits of this receptor develop with different time courses, suggesting that they play different roles in plasticity. To understand the role of the NR2B subunit, we knocked-down NR2B gene expression in visual cortex by injecting a recombinant adenovirus containing an antisense NR2B oligonucleotide. To assess knock-down, we injected the recombinant adenovirus into the right visual cortex of rats (p22) or mice (p30). Eight days later we perfused the animals and processed the visual cortex for NMDAR subunit immunoreactivity (IR). NR2B-IR was depleted dramatically in the neuropil near the injection. Depletion was more modest in the neuronal somata. Surprisingly, NR2A-IR was also reduced, but NR1-IR was not reduced. To assess the functional effects of depletion, we measured ocular dominance plasticity with monocular deprivation (MD). We compared mice receiving the NR2B antisense virus with mice receiving virus containing only the GFP sequence and mice receiving no injection. All injections were between p26 and p29 in the right cortex and bilateral recordings were performed 6-8 days later. Animals receiving the antisense virus lost plasticity if the right eye was deprived. If the left eye was deprived, the cortex was normally plastic bilaterally. Injection of control virus had no effect on plasticity. The data indicate that ocular dominance plasticity requires normal NMDARs in the hemisphere ipsilateral to the deprived eye but not in the hemisphere contralateral to the deprived eye.

Transfer of NMDAR2 cDNAs increases endogenous NMDAR1 protein and induces expression of functional NMDA receptors in PC12 cells

The pheochromocytoma cell line (PC12) has been used as a model system for the study of regulation of expression of NMDA receptors. PC12 cells express a substantial amount of NMDAR1 subunit (NR1) mRNA, whereas they express only a small amount of NR1 protein. The level of functional NMDA receptor expression is almost negligible. To test the possibility that NMDAR2 subunits (NR2) control expression of functional NMDA receptors as well as NR1 protein, we transferred NR2A-D cDNAs into PC12 cells using adenovirus vectors. Prominent NMDA receptor-mediated currents were recorded in PC12 cells to which NR2A or NR2B cDNA was delivered without NR1 cDNA. The amplitudes of these responses were similar to those in PC12 cells to which NR1 cDNA was delivered together with NR2A or NR2B cDNA. In cells to which either NR2C or NR2D cDNA alone was delivered, NMDA receptor-mediated currents were also detected, although to a much lesser extent. These results showed that NR2 proteins produced by gene transfer are co-assembled with the endogenous NR1 protein to form functional heteromeric receptors. The delivery of NR2A-D cDNAs also increased the amount of NR1 protein but not that of NR1 mRNA, suggesting that this protein increase is due to post-transcriptional mechanisms. The effects of NR2A-B gene transfer on expression of NR1 protein were much more efficient than those of NR2C-D gene transfer.

Glutamate receptor subunit expression in primary neuronal and secondary glial cultures

We report on the expression of ionotropic glutamate receptor subunits in primary neuronal cultures from rat cortex, hippocampus and cerebellum and of metabotropic glutamate (mGlu) receptor subtypes in these neuronal cultures as well as in cortical astroglial cultures. We found that the NMDA receptor (NR) subunits NR1, NR2A and NR2B were expressed in all three cultures. Each of the three cultures showed also expression of the four AMPA receptor subunits. Although RT-PCR detected mRNA of all kainate (KA) subunits in the three cultures, western blot showed only expression of Glu6 and KA2 receptor subunits. The expression analysis of mGlu receptors indicated the presence of all mGlu receptor subtype mRNAs in the three neuronal cultures, except for mGlu2 receptor mRNA, which was not detected in the cortical and cerebellar culture. mGlu1a/alpha, -2/3 and -5 receptor proteins were present in all three cultures, whereas mGlu4a and mGlu8a receptor proteins were not detected. Astroglial cultures were grown in either serum-containing or chemically defined medium. Only mGlu5 receptor protein was found in astroglial cultures grown in serum-containing medium. When astrocytes were cultured in chemically defined medium, mGlu3, -5 and -8 receptor mRNAs were detected, but at the protein level, still only mGlu5 receptor was found.

Sindbis viral-mediated expression of Ca2+-permeable AMPA receptors at hippocampal CA1 synapses and induction of NMDA receptor-independent long-term potentiation

Gene manipulation in order to artificially express a particular gene in neurons in the central nervous system is a powerful tool for the analysis of brain function. Sindbis viral vectors have been developed to express high levels of foreign genes in postmitotic brain neurons with little transfection of glial cells. In this study, we expressed the gene encoding the unedited GluR2 (GluR-B) subunit of the AMPA-type glutamate receptor that forms inwardly rectifying and Ca2+-permeable channels, in rat CA1 hippocampal neurons in slice cultures using Sindbis viral vectors. The pyramidal cell layer of the CA1 region was injected with recombinant Sindbis viruses encoding both enhanced green fluorescent protein (GFP) and unedited GluR2. The GFP fluorescence from CA1 neurons could be detected as early as 6 h and reached a maximal level about 48 h postinfection. The inwardly rectifying and Ca2+-permeable AMPA receptors were expressed in most CA1 pyramidal cells expressing GFP. These AMPA receptors expressed by gene transfer were involved in fast excitatory neurotransmission elicited by electrical stimulation of the Schaffer collaterals in the stratum radiatum. Tetanic stimulation of Schaffer collaterals induced NMDA receptor-independent, long-term potentiation due to Ca2+ influx through the newly expressed AMPA receptors in the area densely stained with GFP. Thus, the combined use of Sindbis viral vectors with the GFP reporter allowed physiological examination of the roles of a specific gene product in synaptic function in well-characterized brain neurons.