The epithelial cells of the choroid plexus are responsible for the majority of cerebrospinal fluid secretion into the ventricles of the brain. The secretion of Na+, Cl– and HCO3– across the choroid plexus epithelium creates an osmotic gradient to drive water movement from the blood into the ventricles. Aquaporins are proteins that mediate the transport of water across cell membranes. Aquaporin 1 (AQP1) protein has previously been identified in rat choroid plexus epithelium (Wu et al. 1998). Furthermore, mRNA encoding AQP4 has been detected in lateral ventricle choroid plexus using in situ hybridisation (Venero et al. 1999). In this study we have examined the expression of two aquaporin channel proteins, AQP1 and AQP4, in rat fourth and lateral ventricle choroid plexus epithelium using western analysis.
Fourth and lateral choroid plexus tissue was removed from male Sprague-Dawley rats humanely killed using an overdose of Fluothane. Crude plasma membrane proteins were prepared from tissue samples following homogenisation and differential centrifugation steps. The purity of the plasma membrane fractions were assessed using western analysis of the α1-subunit of Na+/K+-ATPase and also the AE2 isoform of the Cl–/HCO3– anion exchanger. In choroid plexus epithelial cells, the Na+/K+-ATPase and the Cl–/HCO3– exchanger are expressed in the apical (CSF-facing) and basolateral (blood-facing) membranes, respectively (Masuzawa et al. 1984; Alper et al. 1994). The Na+/K+-ATPase and AE2 antibodies detected bands of the expected size in the fourth and lateral choroid plexus membrane fractions, suggesting that these fractions are enriched for both apical and basolateral membrane proteins.
Western analysis using antibodies for both AQP1 and AQP4 proteins detected a band of the expected size for both proteins in the fourth and lateral choroid plexus plasma membrane samples. We therefore conclude that AQP1 and AQP4 channel protein is expressed in both fourth and lateral ventricle choroid plexus.
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Masuzawa, T., Ohta, T., Kawamura, M., Nakahara, N. & Sato, F. (1984). Brain Res. 302, 357-362.
Venero, J.L., Vizuete, M.L., Ilundain, A.A., Machado, A., Echevarria, M. & Cano, J. (1999). Neuroscience 94, 239-250.
Wu, Q., Delpire, E., Hebert, S.C. & Strange, K. (1998). Am. J. Physiol. 275, C1565-1572.