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V. E. Chankiewitz, B. Krueger, S. Haerteis, M. Zenkel, U. Schloetzer-Schrehardt, F. Kruse, C. Korbmacher; Expression and Localization of Epithelial Sodium Channel (ENaC) Subunits in the Human Eye. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2036.
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The epithelial sodium channel (ENaC) is typically localized in the apical membrane of epithelial cells. Sodium entry via ENaC is the rate-limiting step for transepithelial sodium absorption in various epithelia. There are several reports that ENaC is also present in ocular tissues and possibly linked with glaucoma. However, the precise localization and (patho) physiological relevance of ENaC in the eye are still unclear. ENaC is a heterotrimeric channel composed of three subunits (, β, and γ). In this study we have analyzed the differential expression of these ENaC subunits in the human eye.
Specific antibodies against intracellular domains of the -, β- or γ-subunits of ENaC were generated, tested and used for immunocytochemistry. Expression of the corresponding genes was analyzed by quantitative RT PCR.
Immunohistochemistry revealed that all three ENaC subunits are present in retina, iris, trabecular meshwork, ciliary body, and ciliary muscle. In contrast, the expression pattern is more complicated in cornea and conjunctiva. While keratinocytes contain -, β- and γ- subunits, the corneal epithelium lacks the β-subunit ( and γ are present), and the endothelium exclusively expresses the -subunit. In the limbus the -subunit is missing while the β-subunit is localized in basal and the γ-subunit in suprabasal cells. In the conjunctiva the -subunit is not detectable and the β-subunit is exclusively seen in basal cells. Overall, the quantitative RT PCR results were in good agreement with the immunohistochemistry data.
ENaC is expressed in epithelial and non-epithelial tissues of the human eye with a distinct localization of subunits possibly reflecting different functional states of ENaC. Our findings suggest that ENaC has diverse functions in different ocular tissue. Further studies will be needed to evaluate ENaC expression patterns under various physiological and pathophysiological conditions and to establish its role for ocular function.
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