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Abstract
The choroid plexus (CP) consists of specialized ependymal cells and underlying blood vessels and stroma producing the bulk of the cerebrospinal fluid (CSF). CP epithelial cells are considered the site of the internal blood-cerebrospinal fluid barrier, show epithelial characteristics (basal lamina, tight junctions), and express aquaporin-1 (AQP1) apically. In this study, we analyzed the expression of aquaporins in the human CP using immunofluorescence and qPCR. As previously reported, AQP1 was expressed apically in CP epithelial cells. Surprisingly, and previously unknown, many cells in the CP epithelium were also positive for aquaporin-4 (AQP4), normally restricted to ventricle-lining ependymal cells and astrocytes in the brain. Expression of AQP1 and AQP4 was found in the CP of all eight body donors investigated (3 males, 5 females; age 74–91). These results were confirmed by qPCR, and by electron microscopy detecting orthogonal arrays of particles. To find out whether AQP4 expression correlated with the expression pattern of relevant transport-related proteins we also investigated expression of NKCC1, and Na/K-ATPase. Immunostaining with NKCC1 was similar to AQP1 and revealed no particular pattern related to AQP4. Co-staining of AQP4 and Na/K-ATPase indicated a trend for an inverse correlation of their expression. We hypothesized that AQP4 expression in the CP was caused by age-related changes. To address this, we investigated mouse brains from young (2 months), adult (12 months) and old (30 months) mice. We found a significant increase of AQP4 on the mRNA level in old mice compared to young and adult animals. Taken together, we provide evidence for AQP4 expression in the CP of the aging brain which likely contributes to the water flow through the CP epithelium and CSF production. In two alternative hypotheses, we discuss this as a beneficial compensatory, or a detrimental mechanism influencing the previously observed CSF changes during aging.
Highlights
The internal environment in the central nervous system (CNS) is separated from blood and surrounding tissues by several barrier-forming structures
To investigate the transition area between the choroid plexus (CP) epithelium and ependyma, we stained for the water channel aquaporin-1 (AQP1) and the extracellular protein laminin, an essential component of the basal lamina (Fig. 1)
The CP epithelium covers underlying blood vessels embedded in connective tissue, collectively called the Tela choroidea
Summary
The internal environment in the central nervous system (CNS) is separated from blood and surrounding tissues by several barrier-forming structures. Besides the blood–brain barrier, there is an outer and inner blood-cerebrospinal fluid barrier (BCSFB), the outer BCSFB formed by the arachnoid cells, the inner by the choroid plexus epithelial cells (CPCs). The choroid plexus (CP) itself is a structure found in all brain ventricles (I-IV) [1] and thought to be the main production site of the cerebrospinal fluid (CSF). The CP consists of blood vessels, the overlaying epithelial cells, and varying amounts of stromal cells and matrix in between. Besides its main role in CSF production, the CP has been attributed with other functions such as water homeostasis, CPCs express ion channels, transport proteins, and tight junction proteins which form the actual BCSBFB, reviewed in [2, 3].
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