| الملخص: |
The choroid plexuses (CPs) are highly vascularized epithelial structures lying in the brain ventricles, forming the blood-cerebrospinal fluid (CSF) barrier. CPs are composed of a monolayer of cuboidal epithelial cells, derived from ependymal cells (EPCs), joined by tight junctions, and surrounding a core of fenestrated capillaries. Choroid plexus epithelial cells (CPECs) secrete most of the CSF through poorly understood mechanisms that involve aquaporins (AQPs) and various solute transport proteins like the Na+-K+-2Cl- cotransporter 1 (NKCC1). It is thought that the concerted action of these membrane proteins directs the transepithelial movement of solutes and water across CPECs. Net water movement across CPECs occurs from the basolateral membrane (blood-facing) through the apical membrane (CSF-facing). However, with the exception of AQP1 in the apical membrane, the molecular pathways for transepithelial water transport are either unknown or their expression is controversial; there are no known AQPs in the basolateral membrane of CPECs to account for the influx of water, and whether AQPs other than AQP1 are expressed in the apical membrane remains to be elucidated. The expression of AQP1 in the basolateral membrane is a subject of debate. Previous work from our lab demonstrated strict apical localization of AQP1; its expression in the basolateral membrane of mouse CPECs is undetectable. Thus, AQPs other than AQP1 are potential molecular substrates for putative basolateral water influx. Further, it is not known if besides AQP1 there are other AQPs in the apical membrane. In previous work from our group, transcripts for several AQPs were detected in whole choroid plexus tissue using RT-PCR. Specifically, mRNAs for AQP water channels 0, 1, 2, 4, aquaglyceroporins 3, 7, 9, and “unorthodox” AQP11 were detected, whereas transcripts for AQPs 5, 6, 8, and 12 were undetectable. Because of their relatively high transcript expression the present work addresses the cellular and subcellular location of AQPs 4, 7 and 9 proteins in CPECs and EP cells, using immunolabeling with validated antibodies and confocal immunofluorescence. AQP4 expression in CPECs is debatable. The present results demonstrate that: 1) AQP 4 is expressed neither in CPECs nor in ependymal cells, but in radial glial end feet surrounding the basal aspect of EPCs. Thus previous RT-PCR mRNA for AQ4 is likely to represent glial contamination during tissue extraction. 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 water pathways for water influx across the CPECs basolateral membrane remain to be elucidated. The functional significance of the present results for CSF secretion and brain extracellular K+ homeostasis is discussed. |
