Aquaporin-4 expression in the human choroid plexus

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.

Cellular Distribution of Brain Aquaporins and Their Contribution to Cerebrospinal Fluid Homeostasis and Hydrocephalus

“…When this analysis was performed in aged animals, the increase in ventricular size was further enlarged in the wt and AQP1 −/− animals but not in the AQP4 −/− mice, indicating the participation of AQP4 upregulation in the hydrocephalus associated with hypoxia and aging. Recently published experimental studies (107) have demonstrated an increase in AQP4 expression with aging [118]. Several findings from our study are consistent with hypoxia-induced hydrocephalus in aged animals [115,120], similar to the main symptoms of iNPH.…”

“…In situ hybridization and western blot analysis [116,117] has shown the presence of AQP4 in choroidal plexus cells, and our recent results also confirm (using various strategies including AQP4 immunofluorescence) AQP4 expression in the secretory epithelium [96,115]. In the human choroid plexus, recent studies by Deffner et al [118] confirmed the presence of AQP4 expression in samples obtained from elder donors. The authors hypothesized that AQP4 expression in choroid plexus cells was caused by age-related changes.…”

Shifting from ependyma to choroid plexus epithelium and the changing expressions of aquaporin‐1 and aquaporin‐4

“…The inset schematic diagram depicts the transition from the CP to the ventricle lining ependyma in a cross‐section at a level indicated by the white line. The distribution of water channels, as recently reported for the human CP (Deffner et al., 2022), is also shown. C , histological section of the CP and ependymal attachment of the lateral ventricle stained for AQP1 (green) and AQP4 (red).…”

“…First, we found that in the human brain AQP4 was not restricted to ECs but also occurred in the CP epithelium (Fig. 1; Deffner et al., 2022). This immunofluorescence finding was confirmed at the mRNA level by qPCR and at the ultrastructural level by the detection of so‐called orthogonal arrays of particles known to contain AQP4 (Rash et al., 1998; Wolburg et al., 2011).…”

Bioengineering of a human physiologically relevant microfluidic blood–cerebrospinal fluid barrier model

“…16 For example, aquaporin-4 (AQP4), a predominant water-selective transporter that contributes to brain water dynamics, is expressed on the human ChP epithelium but mainly detected in the ependymal and subependymal regions of mice. 17 Current developments of three-dimensional (3D) in vitro models, such as inverted Transwell systems and stem cellderived ChP organoids, advance studies of functional and systemic responses of the BCSFB. [18][19][20] However, existing cultured cell models still fail to reconstitute the in vivo-like fluidic microenvironment, which will limit their utility and translation to the patients.…”