Abstract
Two aquaporin (AQP)-type water channels are expressed in mammalian cornea, AQP1 in endothelial cells and AQP5 in epithelial cells. To test whether these aquaporins are involved in corneal fluid transport and transparency, we compared corneal thickness, water permeability, and response to experimental swelling in wild type mice and transgenic null mice lacking AQP1 and AQP5. Corneal thickness in fixed sections was remarkably reduced in AQP1 null mice and increased in AQP5 null mice. By z-scanning confocal microscopy, corneal thickness in vivo was (in microm, mean +/- S.E., n = 5 mice) 123 +/- 1 (wild type), 101 +/- 2 (AQP1 null), and 144 +/- 2 (AQP5 null). After exposure of the external corneal surface to hypotonic saline (100 mosm), the rate of corneal swelling (5.0 +/- 0.3 microm/min, wild type) was reduced by AQP5 deletion (2.7 +/- 0.1 microm/min). After exposure of the endothelial surface to hypotonic saline by anterior chamber perfusion, the rate of corneal swelling (7.1 +/- 1.0 microm/min, wild type) was reduced by AQP1 deletion (1.6 +/- 0.4 microm/min). Base-line corneal transparency was not impaired by AQP1 or AQP5 deletion. However, the recovery of corneal transparency and thickness after hypotonic swelling (10-min exposure of corneal surface to hypotonic saline) was remarkably delayed in AQP1 null mice with approximately 75% recovery at 7 min in wild type mice compared with 5% recovery in AQP1 null mice. Our data indicate that AQP1 and AQP5 provide the principal routes for corneal water transport across the endothelial and epithelial barriers, respectively. The impaired recovery of corneal transparency in AQP1 null mice provides evidence for the involvement of AQP1 in active extrusion of fluid from the corneal stroma across the corneal endothelium. The up-regulation of AQP1 expression and/or function in corneal endothelium may reduce corneal swelling and opacification following injury.
Highlights
The cornea consists of a stromal layer that is covered at its external surface by an epithelium in contact with tear fluid and at its inner surface by an endothelium in contact with aqueous fluid in the anterior chamber
Mice lacking AQP5 manifest impaired fluid secretion by salivary [19] and airway submucosal [20] glands and have reduced alveolar water permeability in lung [21]. It has been postulated without direct evidence that AQP1 and AQP5 are involved in water transport between the corneal stroma and between the tear and aqueous fluids and in the maintenance of corneal transparency [13, 22, 23]
The purpose of this study was to investigate the role of aquaporin water channels in important aspects of corneal physiology that depend on water/fluid transport
Summary
The cornea consists of a stromal layer that is covered at its external surface by an epithelium in contact with tear fluid and at its inner surface by an endothelium in contact with aqueous fluid in the anterior chamber. Mice lacking AQP5 manifest impaired fluid secretion by salivary [19] and airway submucosal [20] glands and have reduced alveolar water permeability in lung [21]. It has been postulated without direct evidence that AQP1 and AQP5 are involved in water transport between the corneal stroma and between the tear and aqueous fluids and in the maintenance of corneal transparency [13, 22, 23]. The marked impairment of corneal recovery in AQP1 null mice has potentially important implications regarding the mechanisms and treatment of corneal edema
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