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S. P. Patel, A. N. Van Hoek, N. C. Joyce, K. Colby; Expression of Aquaporin 4 in Corneal Endothelial Cells. Invest. Ophthalmol. Vis. Sci. 2010;51(13):693.
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© ARVO (1962-2015); The Authors (2016-present)
Aquaporin (AQP) water channels are present in corneal endothelium, but their role in corneal water transport is unidentified. Prior functional studies of AQPs in corneal endothelium have focused on aquaporin 1 (AQP1), a major mammalian water channel. The purpose of this study was to identify and localize aquaporin 4 (AQP4) in corneal endothelial cells in several species. This may provide additional insight into mechanisms of corneal water homeostasis.
Normal human donor corneas were obtained from the National Disease Research Interchange (Philadelphia, PA). Donor confidentiality was maintained according to the Declaration of Helsinki. Fuchs endothelial dystrophy corneas were obtained at the time of penetrating keratoplasty under an institutional review board approved protocol. Human corneal specimens were stored in Optisol GS for transport. Bovine eyes were transported on ice from a local slaughterhouse. Whole rat eyes were fixed in paraformaldehyde. Upon arrival in the lab, tissues were mounted in OCT embedding medium for frozen sectioning. Immunolocalization for AQP4 was performed with an AQP4-specific antibody (Millipore, Billerica, MA). Images were acquired with epifluorescence and confocal microscopy. Immunoblot analysis was performed on normal human corneal tissue to verify the presence of AQP4. The presence of AQP1 by immunolocalization (AQP1 antibody, Millipore) in the corneal endothelium was also evaluated.
AQP4 expression was found in human corneas from both normal and Fuchs’ dystrophy patients, and was also noted in bovine corneal endothelium. Confocal microscopy showed a speckled cytoplasmic localization in endothelial cells for all samples. However, AQP4 was not detected in rat corneal endothelium, but, as has been previously noted, was robustly expressed in non-pigmented ciliary epithelium and retinal Mueller cells. The cytoplasmic localization of AQP4 in human and bovine cells was compared to AQP1. AQP1 was similarly present in a cytoplasmic distribution in human samples. Apical localization was noted in bovine endothelial cells.
This is the first demonstration, to our knowledge, of species-specific expression of AQPs in ocular tissue. Future functional studies of AQPs in corneal endothelium may address the potential contribution of AQP4 in water balance as well as the potential consequences of cytoplasmic localization of AQPs.
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