Figure 4. Localization of proteins for ion transporters, channels and associated enzymes and identification of their corresponding genes in adult and immature rat choroid plexus.

CSF secretion results from coordinated intracellular carbonic anhydrase activity and transport of ions and water from basolateral membrane to cytoplasm, then sequentially across apical membrane into the cerebral ventricles (Davson & Segal, 1996; Speake et al. 2001; Praetorius & Damkier, 2017). This process has only been studied in adult choroid plexus (Brown et al. 2004; Praetorius & Damkier, 2017). The membrane and intracellular locations of the ion channels, transporters and enzymes indicated are from Praetorius & Damkier (2017). Data from Liddelow et al. (2013) compares expression of these genes and other functionally related genes in E15 and adult rat lateral ventricular choroid plexus. Blue indicates the genes that are upregulated (enriched) in the adult. Light red indicates genes that are expressed at a higher level at E15. We have assumed the same cellular/membrane location for members of the same gene family. The genes all had substantial but variable transcript numbers in the RNA‐Seq analysis. In some cases where a gene was upregulated in the adult, the transcript number was also high in the embryo, suggesting this transporter or channel was likely to be functionally effective at both ages, e.g. the K⁺ channel Kcnj13 (Kir7.1), Slc12a2 (NKCC1) a Na⁺–K⁺–Cl⁻ exchanger and Slc4a2 (NBCe2) a coupled Na⁺–HCO₃ ⁻ pump. ATPB1 (Atpb1b1) is a Na⁺/K⁺‐ATPase. Green indicates genes that were expressed at similar levels at the two ages. There are many more channels that show age‐related differential expression in choroid plexus, the functions of which are unclear. Redrawn from Liddelow et al. (2016) with additional data from Liddelow et al. (2013).