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Catherine W. Morgans, Manuel Esguerra, Robert F. Miller; Synaptic Localization of D-Amino Acid Oxidase in the Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4324.
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In the retina, D-serine has been shown to be the predominant co-agonist of NMDA receptors (NMDARs). The co-agonist site is unsaturated under physiological conditions, and modulation of D-serine levels dramatically affects the light responses of retinal ganglion cells. The local concentration of D-serine is determined by the activity and localization of the enzymes that synthesize, degrade and transport D-serine. We show that the D-serine degrading enzyme, D-amino acid oxidase (DAO), is synaptically localized in the retina.
A linked DAO-HRP activity assay was performed on lightly fixed retinal sections from wild-type mice and from mice with a loss-of-function point mutation (G181R) in the DAO gene, ddY/DAO- mice. DAO was localized in wildtype and ddY/DAO- retina sections by immunofluorescence and confocal microscopy. Some sections were double labeled with antibodies against DAO and NMDAR subunits, or against DAO and synaptic ribbon proteins to identify sites of glutamate release.
The DAO assay localized enzymatic activity to the inner plexiform layer (IPL) in the wild-type retina but not in the ddY/DAO- mutant retina. The location of the reaction product corresponded to the localization of DAO immunofluorescence. In both wildtype and ddY/DAO- retina sections, the DAO antibody labeled dense puncta throughout the IPL, indicating that the ddY/DAO- point mutation affects function but not localization of the enzyme. Double labeling revealed close apposition of DAO and NMDARs, as well as synaptic ribbons.
The localization of DAO in the IPL suggests that modulation of DAO activity in the retina will affect synaptic D-serine levels, and hence ganglion cell light responses.
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