Water is the essence of life and every organism is critically dependent upon it for survival and for the efficient and effective performance of numerous cellular functions. Since diffusional movement of water across membranes is very slow, the existence of water transporting proteins was postulated. Indeed, aquaporins (AQPs) are integral membrane protein channels that transport water in most living cells. Five different isoforms of AQPs are expressed in the eye and at least seven isoforms are expressed in ear. AQP0 is present in the lens fiber cells, AQP1 is expressed in corneal endothelium, ciliary epithelium, lens epithelium and trabecular meshwork, AQP3 is localized to conjunctiva, AQP4 is present in nonpigmented ciliary epithelium and Müller cells of the retina, and AQP5 is expressed in corneal epithelium and lacrimal gland. Such diverse and characteristic distribution of AQPs suggests their involvement in the maintenance of water homeostasis necessary for corneal and lens transparency, regulation of intraocular pressure (IOP), retinal signal transduction, and tear production. Pharmacological alteration of AQP function may provide a novel approach to treat several eye disorders such as ocular hypertension and glaucoma, corneal and macular edema, cataracts and dry eye. In the ear, AQPs2-4 are localized in the endolymphatic sac and AQP5 in the cochlea of the inner ear and are thus intimately linked to the auditory processes and body balance. Disturbances of fluid transport within these otic structures can lead to hearing loss, Meniere’s disease and vertigo.
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