Department of Neuroscience, University of Minnesota, Minneapolis, Minnesota;
J Neurophysiol. 2013 Dec;110(12):2718-26. doi: 10.1152/jn.00310.2013. Epub 2013 Sep 25.
D-serine is present in the vertebrate retina and serves as a coagonist for the N-methyl-D-aspartate (NMDA) receptors of ganglion cells. Although the enzyme D-amino acid oxidase (DAO) has been implicated as a pathway for d-serine degradation, its role in the retina has not been established. In this study, we investigated the role of DAO in regulating D-serine levels using a mutant mouse line deficient in DAO (ddY/DAO(-)) and compared these results with their wild-type counterparts (ddY/DAO(+)). Our results show that DAO is functionally present in the mouse retina and normally serves to reduce the background levels of D-serine. The enzymatic activity of DAO was restricted to the inner plexiform layer as determined by histochemical analysis. Using capillary electrophoresis, we showed that mutant mice had much higher levels of D-serine. Whole cell recordings from identified retinal ganglion cells demonstrated that DAO-deficient animals had light-evoked synaptic activity strongly biased toward a high NMDA-to-AMPA receptor ratio. In contrast, recordings from wild-type ganglion cells showed a more balanced ratio between the two receptor subclasses. Immunostaining for AMPA and NMDA receptors was carried out to compare the two receptor ratios by quantitative immunofluorescence. These studies revealed that the mutant mouse had a significantly higher representation of NMDA receptors compared with the wild-type controls. We conclude that 1) DAO is an important regulatory enzyme and normally functions to reduce D-serine levels in the retina, and 2) D-serine levels play a role in the expression of NMDA receptors and the NMDA-to-AMPA receptor ratio.
D-丝氨酸存在于脊椎动物的视网膜中，作为神经节细胞的 N-甲基-D-天冬氨酸（NMDA）受体的共激动剂。尽管 D-氨基酸氧化酶（DAO）已被认为是 D-丝氨酸降解的途径之一，但它在视网膜中的作用尚未确定。在这项研究中，我们使用缺乏 DAO 的突变小鼠品系（ddY/DAO(-)）研究了 DAO 在调节 D-丝氨酸水平中的作用，并将这些结果与野生型对照（ddY/DAO(+))进行了比较。我们的结果表明，DAO 在小鼠视网膜中具有功能性，正常情况下可降低 D-丝氨酸的背景水平。通过组织化学分析确定，DAO 的酶活性仅限于内丛状层。使用毛细管电泳，我们表明突变小鼠的 D-丝氨酸水平更高。从鉴定的视网膜神经节细胞进行全细胞记录表明，DAO 缺乏的动物具有强烈偏向高 NMDA 对 AMPA 受体比值的光诱发突触活性。相比之下，来自野生型神经节细胞的记录显示两种受体亚类之间的比值更为平衡。进行 AMPA 和 NMDA 受体免疫染色，通过定量免疫荧光比较两种受体比值。这些研究表明，与野生型对照相比，突变小鼠的 NMDA 受体表达显著增加。我们得出结论，1）DAO 是一种重要的调节酶，正常情况下可降低视网膜中的 D-丝氨酸水平，2）D-丝氨酸水平在 NMDA 受体的表达和 NMDA 对 AMPA 受体的比值中发挥作用。
