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Gabriel E. Romero, Steve J. Sullivan, Robert F. Miller; Postnatal Decline In D-Serine Levels Does Not Depend Solely On D-Amino Acid Oxidase. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1947.
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Results from this laboratory show that D-serine may play a role in controlling the NMDA to AMPA receptor ratios in the retinal ganglion cell. The D-serine degrading enzyme D-amino acid oxidase (DAO) helps limit D-serine from saturating NMDARs in adult retinas. Given the importance of D-serine as a regulating factor for glutamate receptor activity, we evaluated a strain of mice that lack DAO (DAO-), compared them with their wt animals of the same strain, and studied changes from postnatal day one (P1) through adulthood.
Capillary electrophoresis (CE) was performed on retinal homogenates to generate electropherograms and seperate D-serine and L-serine in varying post-natal ages of wt and DAO- mice. Amino acids were labeled with NBD-F and analyzed with a commercial CE instrument fit with laser-induced fluorescence detection. Chiral amino acids were separated using a (2-hydroxypropyl)-β-cyclodextrin buffer and detected by a photomultiplier tube. We used 32 Karat software to generate plots of fluorescence vs time and integrate electropherogram peaks. The concentration of D-serine and L-serine was determined by comparing the samples with known quantities and expressed as nmol/g. Protein determination was based on a commercial BCA protein assay.
Measurements of D-serine and L-serine in both DAO- and wt mice were followed from P1 through adulthood (4 weeks of age). In wt and DAO- retinas, the total amount of both D-serine and L-serine dropped roughly 10-fold between P1 and P16. Thus, during synaptogenesis of the retina, D-serine levels are high in both wt and DAO- mice. However, starting near P16, there was a divergence in the D-serine content of DAO- retinas, which peaked in adulthood. This lead to an increase in D-serine levels in adult DAO- retinas approximately four times higher than wt. Unlike D-serine, L-serine remained consistently similar between genotypes throughout development.
The results from this experiment show that D- and L-serine levels are high in the early postnatal period and drop during early development. While we show DAO is partially responsible for this decline, as is evident by the continued decrease of D-serine in adult wt retinas when compared to DAO-, this study also shows that other factors, such as a decrease in D-serine synthesis, could play a large role in the decline of D-serine observed during early retinal development.
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