Role of aquaporin water channels in eye function

The aquaporins (AQPs) are a family of more than 10 homologous water transporting proteins expressed in many mammalian epithelia and endothelia. At least five AQPs are expressed in the eye: AQP0 (MIP) in lens fiber, AQP1 in cornea endothelium, ciliary and lens epithelia and trabecular meshwork, AQP3 in conjunctiva, AQP4 in ciliary epithelium and retinal Müller cells, and AQP5 in corneal and lacrimal gland epithelia. This cell-specific expression pattern suggests involvement of AQPs in corneal and lens transparency, intraocular pressure (IOP) regulation, retinal signal transduction, and tear secretion. Indeed, humans with mutant AQP0 develop cataracts. Mice lacking AQP1 have reduced IOP and impaired corneal transparency after swelling, and mice lacking AQP4 have reduced light-evoked potentials by electroretinography. There is evidence for impaired cellular processing of AQP5 in lacrimal glands of humans with Sjogren's syndrome. AQPs facilitate fluid secretion and absorption in the eye, and hence are involved in the regulation of pressure, volume and tissue hydration. Pharmacological alteration of AQP function may provide a new approach for therapy of glaucoma, corneal edema, and other diseases of the eye associated with abnormalities in IOP or tissue hydration.