Altered Expression of Several Molecular Mediators of Cerebrospinal Fluid Production in Hyp Mice

Abstract

X-Linked Hypophosphatemia (XLH) is a genetic disease, causing life-long hypophosphatemia due to overproduction of FGF23. XLH is associated with Chiari malformations, cranial synostosis, and syringomyelia. FGF23 signals through FGFR1c and requires a co-receptor, α-Klotho, which is expressed in the renal distal convoluted tubules and the choroid plexus (ChP). In the ChP, α-Klotho participates in regulating cerebral spinal fluid (CSF) production by shuttling the sodium/potassium adenosine triphosphatase (Na⁺/K⁺-ATPase) to the luminal membrane. The sodium/potassium/chloride cotransporter 1 (NKCC1) also makes a substantial contribution to CSF production. Since CSF production has not been studied in XLH, we sought to determine if there are changes in the expression of these molecules in the ChP of Hyp mice, the murine model of XLH, as a first step towards testing the hypothesis that altered CSF production contributes to the cranial and spinal malformations seen this disease. Real-time PCR (RT-PCR) demonstrated significant upregulation of Klotho transcripts in the fourth ventricle of Hyp mice compared to controls. Transcript levels for Fgfr1c were unchanged in Hyp mice. Atp1a1 transcripts encoding the alpha-1 subunit of Na+/K+-ATPase were significantly downregulated in in the third and lateral ventricles. Expression levels of the Slc12a2 transcript (which encodes NKCC1) were unchanged in Hyp mice compared to controls. In situ hybridization (ISH) was used to obtain anatomical correlates to the changes observed by RT-PCR. ISH confirmed the presence of all four transcripts in the lateral ventricle ChP of both WT and Hyp mice. This is the first study to document a significant change in the level of expression of the molecular machinery required for CSF production in Hyp mice. Whether similar changes occur in patients with XLH, potentially contributing to the cranial and spinal cord abnormalities frequently seen in XLH, remains to be determined.