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S. S. Kumari, R. V. Patil, J. Gao, N. A. Sharif, R. T. Mathias, K. Varadaraj; Differential Expression of Aquaporin and Potassium Channel Genes in the Nonpigmented Ciliary Epithelium: Functional Implications in Aqueous Humor Secretion. Invest. Ophthalmol. Vis. Sci. 2009;50(13):814.
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© ARVO (1962-2015); The Authors (2016-present)
To study the spatial expression of aquaporins (AQP1 and AQP4) and potassium channels (Maxi-K and Kir4.1) in mammalian ciliary epithelial cells in order to elucidate the molecular mechanisms underlying aqueous humor secretion.
Expression and localization of AQP1, AQP4, Maxi-K and Kir4.1 were followed in the epithelial cells of mouse and rabbit ciliary bodies, and human ciliary nonpigmented epithelial cells (hNPE) using immunocytochemistry and Western blotting. Water permeability was determined from the rate of volume change when hNPE cells were immersed in hypotonic solution. Whole-cell-voltage-clamp technique was used to record potassium current due to Kir4.1 in hNPE cells.
Expression profiles of the selected channel proteins in ciliary body were similar in both rabbit and mouse. AQP1 was detected in the apical and basolateral plasma membranes of nonpigmented epithelial (NPE) cells in the anterior pars plicata of ciliary body. However, localization of AQP4 was observed at the basolateral plasma membranes of NPE cells in the anterior, medial and posterior pars plicata. AQP1 or AQP4 expression was absent in the pars plana of ciliary body. Maxi-K and Kir4.1 were more abundant in the pars plicata than in pars plana. Human NPE primary culture cells expressed AQP1, AQP4, Maxi-K and Kir4.1 proteins. Water transport studies showed relatively higher (~3 fold) water permeability for hNPE cells than for cells that do not express AQPs (e.g., mouse Neuroblastoma 2a (N2a) cells). In addition, hNPE cells showed the presence of typical Kir4.1 inwardly rectifying K+-current.
Differential expression of AQP1, AQP4, Maxi-K and Kir4.1 in the plasma membranes of NPE cells suggests that functional coupling of these proteins may play a significant role in aqueous humor secretion and intraocular pressure (IOP) regulation for maintaining homeostasis by facilitating K+-buffering and water movement. These observations match those pertaining to retinal Müller cells and brain glial cells which also co-express AQP4, Maxi-K and Kir4.1 and participate in K+-buffering and water transport. Manipulation of aqueous humor secretion by regulating the kinetics of AQP and K+-channel activation in NPE cells may provide a novel approach to regulate IOP.
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