Transmembrane water-flux through SLC4A11: a route defective in genetic corneal diseases

Gonzalo L Vilas,Jun Liu,Andri K Riau,Eranga N Vithana,Sampath K Loganathan,Jodhbir S Mehta,James D Young,Joseph R Casey

doi:10.1093/hmg/ddt307

Abstract

Three genetic corneal dystrophies [congenital hereditary endothelial dystrophy type 2 (CHED2), Harboyan syndrome and Fuchs endothelial corneal dystrophy] arise from mutations of the SLC4a11 gene, which cause blindness from fluid accumulation in the corneal stroma. Selective transmembrane water conductance controls cell size, renal fluid reabsorption and cell division. All known water-channelling proteins belong to the major intrinsic protein family, exemplified by aquaporins (AQPs). Here we identified SLC4A11, a member of the solute carrier family 4 of bicarbonate transporters, as an unexpected addition to known transmembrane water movement facilitators. The rate of osmotic-gradient driven cell-swelling was monitored in Xenopus laevis oocytes and HEK293 cells, expressing human AQP1, NIP5;1 (a water channel protein from plant), hCNT3 (a human nucleoside transporter) and human SLC4A11. hCNT3-expressing cells swelled no faster than control cells, whereas SLC4A11-mediated water permeation at a rate about half that of some AQP proteins. SLC4A11-mediated water movement was: (i) similar to some AQPs in rate; (ii) uncoupled from solute-flux; (iii) inhibited by stilbene disulfonates (classical SLC4 inhibitors); (iv) inactivated in one CHED2 mutant (R125H). Localization of AQP1 and SLC4A11 in human and murine corneal (apical and basolateral, respectively) suggests a cooperative role in mediating trans-endothelial water reabsorption. Slc4a11−/− mice manifest corneal oedema and distorted endothelial cells, consistent with loss of a water-flux. Observed water-flux through SLC4A11 extends the repertoire of known water movement pathways and call for a re-examination of explanations for water movement in human tissues.

Highlights

Human cornea is arranged in five layers: the outer corneal epithelium, Bowman’s layer that maintains the cornea’s shape, the corneal stroma, composed mainly of tightly packed collagen fibrils secreted by keratocytes which occupy about 10% of the layer, the Descemet membrane, a modified basement membrane and the corneal endothelium, which is a monolayer of mitochondria-rich cells
We examined the ability of human SLC4A11 to mediate water-flux, when expressed in Xenopus laevis oocytes and
The functional activity and localization reported here suggest that SLC4A11 provides a pathway for water-flux through the basolateral membrane, which is required for reabsorption of fluid from the corneal stroma. These studies examined the molecular defect present in individuals whose corneal disease arose from mutations of SLC4A11

Summary

Introduction

Human cornea is arranged in five layers: the outer corneal epithelium, Bowman’s layer that maintains the cornea’s shape, the corneal stroma, composed mainly of tightly packed collagen fibrils secreted by keratocytes which occupy about 10% of the layer, the Descemet membrane, a modified basement membrane and the corneal endothelium, which is a monolayer of mitochondria-rich cells. Mutations in SLC4A11 ( called NaBC1 or BTR1) [3,4] were recently identified in a range of corneal endothelial disorders characterized by dysfunction of the endothelial cells forming the inner surface of the cornea [5,6,7]. These diseases are marked by fluid accumulation in the corneal stroma and abnormal deposition of material (gutatta) on the Descemet membrane that underlies the corneal endothelial layer [1].

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