Variability in intracellular localization of D-amino acid oxidase in choroid plexus epithelial cells

D-amino acid oxidase (DAO; also abbreviated as DAAO) in the brain is primarily expressed in glial cells of the cerebellum and brainstem. In glial cells, DAO metabolizes D-serine, which acts as a co-agonist and regulator of glutamate receptors. We previously reported that DAO is also expressed in epithelial cells of the choroid plexus (CP); however, the subcellular localization and functional significance of DAO in the CP remain unclear. In this study, we performed histological analyses to investigate the subcellular localization of DAO in the CP. We first confirmed DAO expression in the mouse CP. Immunostaining revealed vesicle-like DAO signals in the cell bodies of choroid plexus epithelial cells (CPECs). Super-resolution microscopy was then used to evaluate colocalization with intracellular organelle markers. DAO signals partially colocalized with ATP-binding cassette subfamily D member 3 (PMP70) and peroxisomal targeting signal 1 receptor (PEX5). In contrast, they colocalized with Golgin subfamily A member 2 (GM130), trans-Golgi network integral membrane protein 2 (TGN46), adaptor protein complex AP-2 (AP-2), early endosome antigen 1 (EEA1), Ras-related proteins Rab5a, Rab11, CD63 antigen (CD63), tumor susceptibility gene 101 protein (TSG101), lysosome-associated membrane glycoprotein 1 (LAMP-1), LAMP-2, beclin-1 and microtubule-associated protein 1 light chain 3 (LC3). These findings suggest that, in the CP, DAO is transported from the Golgi apparatus to endosomes and subsequently distributed to multiple vesicular compartments. The presence of DAO in peroxisomes, autophagosomes, lysosomes, and exosomes indicated diverse intracellular localization within CPECs. This distribution may enable efficient metabolism of blood-derived D-serine in CPECs.