LETTER TO THE EDITORS

Potential Role of Aquaporins and Atrial Natriuretic Peptides in the Aqueous Humor Dynamics

The Ciliary Body (CB), the site of aqueous humor secretion, is comprised of a bilayered secretory polarized neuroepithelium, the Pigmented (PE) and the Non-Pigmented (NPE) Ciliary Epithelium (CE), overlapping the ciliary muscle and stroma containing blood vessels. Both epithelial cell layers of the CE and the iris share a common embryological origin with the multiple cell layers of the retina and with the Retinal Pigment Epithelium (RPE). These tissues derive from the optic cup during development as four distinct structures: the iris, the CB, the retina and the RPE. The CE is formed at the rim of the optic cup, whereas the retina and RPE are formed around the optic cup. The PE cell layer of the CE is continuous with the RPE, and the NPE cell layer is related to the neural retina. The expression of neuroendocrine peptides in the CE is of particular interest, since many of them are restricted to neuroendocrine cells and tissues around the body. Neuropeptides can function as neurotransmitters, neurohormones, or neuromodulators depending of the distance and effect mediated by the neuropeptide. Their expression in the CE has been confirmed by the detection of their mRNA, by conventional RT-PCR or by Northern blotting analysis, and by radioimmunoassay. Further analysis by indirect immuno-fluorescence had also helped to verify their cellular distribution along the CE.

The discovery in the human ocular ciliary body of glaucoma-associated genes (i.e., MYOC, CYP1B1), neuroendocrine processing enzymes, neuroendocrine peptides, steroid-converting enzymes, glutamate transporters, glutamate-metabolizing enzymes, and anti-angiogenic factors requires a reevaluation of its function on aqueous humor secretion, intraocular pressure and its role in glaucoma. The ciliary body should be considered as a multifunctional and interactive tissue. The intrinsic hypotensive and/or hypertensive biological activities of many of the endocrine peptides released by the ciliary epithelium are best explained within the context of a neuroendocrine system, linking the inflow and the outflow of aqueous humor. This interpretation is consistent with physiological and genetic studies indicating that changes altering the inflow affects intraocular pressure. In the proposed endocrine system, regulatory peptides secreted by the ciliary epithelium may subserve multiple functions in the following: inflow and outflow pathways of aqueous humor, ciliary blood flow, the immune privilege status of the anterior segment and the diurnal circadian rhythms of aqueous humor secretion and intraocular pressure. These previously unsuspected and challenging functions of the ciliary epithelium should be considered when assessing the multifactorial events which lead to the pathophysiology of glaucoma affecting the outflow pathways of aqueous humor. This review highlights published, and ongoing studies on authors’ labs supporting neuroendocrine, steroidogenic and glutamatergic features of the ciliary epithelium and the endocrine communication between the inflow and outflow pathways of aqueous humor. We also discuss how glaucoma-associated genes expressed in the ciliary body and their mutant proteins could influence intraocular pressure, contributing to the development of glaucoma.

This chapter focuses on the novel roles played by aquaporins (AQPs) as gated ion channels. AQPs are the members of the major intrinsic protein (MIP) gene family, a diverse group of water and solute channels found throughout the phyla of plants, animals, and bacteria. Membrane lipid bilayers have an inherently low permeability to water, an attribute that benefits life in aqueous and terrestrial environments. In specialized water-absorbing and secreting cells of mammals, membrane permeability to water is greatly enhanced by the expression of AQPs, a family of membrane proteins that provide channels for osmotically driven water fluxes. An expanding role for mammalian AQPs as pathways for regulated solute transport is becoming evident and these channels are now proposed to mediate transmembrane fluxes for a growing list of other substrates, including ions, glycerol, and carbon dioxide. The existence of selective water channels in biological membranes has been postulated based on physiological evidence before the first molecular identification of AQPs. The first member of the MIP family to be sequenced and characterized was the MIP of bovine lens, now known as “AQP0.” The chapter describes the structural features of MIP channels and summarizes the physiological and pathophysiological significance of aquaporin channel activity.

In plants, as in vertebrates, natriuretic peptide (NP) hormones can influence water and solute homeostasis. Here we demonstrate that a synthetic peptide identical to the C-terminus (amino acids 99–126) of the rat atrial natriuretic peptide (rANP) modulates osmotically induced swelling of mesophyll cell protoplasts (MCPs) in a concentration and time-dependent manner. Osmotically-induced volume changes in MCPs are enhanced by plant extracts with NP immunoreactivity and this effect is concentration-dependent. In contrast, pre-treatment of the plant extracts with rabbit anti-human ANP (99–126) antiserum suppresses enhanced osmoticum-induced swelling. Isolated plant peptides (irPNP) that have been immunoaffinity purified with rabbit anti-human ANP (99–126) antiserum also enhance osmotically-induced swelling. While rANP and irPNP cause increases in cGMP levels in MCPs, elevated cGMP levels do not cause increases in osmoticum-dependent swelling but exert an inhibitory effect. These findings are consistent with a NP-dependent, cGMP-independent effect on plant cell volume regulation and a role in homeostasis for peptides that are recognized by antibodies directed against the C-terminus of vertebrate ANPs.