Expression of NKCC1 and Aquaporins 4, 7 and 9 in Mouse Choroid Plexus and Ependymal Cells

Abstract

Choroid plexus epithelial cells (CPECs) secrete cerebrospinal fluid (CSF) through poorly understood mechanisms involving Aquaporins (AQPs) and various solute transport proteins like the Na+/K+ pump and the Na+‐K+‐2Cl− cotransporter 1 (NKCC1). Net water movement across CPECs occurs from the basolateral membrane through the apical membrane. However, with the exception of AQP1 in the apical membrane, the molecular pathways for transepithelial water fluxes are either unknown, or their expression is controversial (Damkier et al. 2013, Physiol Rev 93, 1847). Because of their relatively high transcript expression (Cha et al, 2014, FASEB Journal 28 .1 Suppl 1182.3) the present work focuses on the cellular and subcellular localization of AQPs 4, 7 and 9 proteins in mouse CPECs and ependymal cells (EPCs), using immunolabeling with validated antibodies and confocal immuno‐fluorescence. Evidence for AQP4 expression in CPECs is controversial. The present results demonstrate that: 1) AQP 4 is expressed neither in CPECs nor in ependymal cells, but in radial glial endfeet surrounding the basal aspect of EPCs, where it is co‐expressed with NKCC1; the latter was also expressed in the apical membrane of both CPECs and EPCs; 2) AQP 7 expression was found to be restricted to the apical membrane of CPECs and EPCs. 3) AQP9 was expressed neither in CPECs nor in EPCs. The pathways for water influx across the basolateral membrane of CPECs remain to be elucidated. The pattern of expression of NKCC1 and AQP4 suggests a role of these proteins in K+ uptake from the CSF, both in CPECs and EPCs, thus regulating CSF and interstitial brain fluid [K+]. The expression of AQP7 in the apical membrane of CPECs suggests that this protein could work together with AQP1 in CSF secretion.Support or Funding Information(Supported by Wright State University Foundation FOAP 551462)