Mouse Slc9a8 Mutants Exhibit Retinal Defects Due to Retinal Pigmented Epithelium Dysfunction

In an international effort to assign function to all genes, and to identify new models of human disease, cohorts of mice produced from mutant embryonic stem cells are being screened in large-scale phenotyping pipelines. Hundreds of mouse lines, each mutant for a different gene, have been analyzed for developmental, hematological, immunological, metabolic, neurological, reproductive, and sensory parameters, which, thus, provide a comprehensive assessment of the consequences of gene ablation.1 Included in the sensory analysis is slit-lamp examination of the anterior chamber of the eye, and indirect ophthalmoscopy to view the retina and retinal vessels. The phenotype data can be viewed online and the mice are freely available. We describe here the detailed pathological and functional analysis of a mouse line mutant in the sodium/hydrogen ion exchange protein gene, Slc9a8, which was identified as having a retinal defect and no other abnormality other than male sterility. Sodium/hydrogen ion exchange proteins (NHEs) or cotransporter solute channels are encoded by the Slc9 gene family, comprising nine members: Slc9a1-9, encoding NHE1-9. Proteins NHE1 to 5 are located in the plasma membrane while NHE6 to 9 are found in membranes of cell organelles (reviewed previously2). These proteins contain 12 membrane spanning domains and catalyze the electroneutral transport of cations down a concentration gradient. Typically sodium ions are transported in and hydrogen ions out of the cell or organelle. Simultaneous transport of anions creates osmotic pressure and results in influx of water, regulating the pH and volume of cells and organelles. Sodium/hydrogen ion exchange is involved in aqueous humor dynamics and maintenance of intraocular pressure (IOP). Inhibition of sodium/hydrogen ion exchange in mice3 reduces IOP and at least one cotransporter, NHE1, is found expressed in the ciliary body, where aqueous dynamics are regulated. The single yeast NHE is necessary for correct sorting of proteins to the vacuole4 and, indeed, depletion of NHE8 in mammalian cells disrupts endosome trafficking.5 Six of the Slc9 family members have been mutated in mice and give rise to a range of phenotypes, from hyperactivity or ataxia and seizures (Slc9a1 and 6)6–8 to relatively mild gastric secretion and absorption defects (Slc9a2, 3, and 4).9–11 The gene Slc9a8 is widely expressed in adult mouse tissues, but is enriched in kidney, intestine, muscle, liver, and testes.12,13 The gene is expressed in the kidney proximal tubules, where reabsorption of secreted bicarbonate requires NHE function, although this is mostly via Slc9a3 (NHE3). However, Slc9a8 is expressed at higher levels in the neonatal kidney, and may partially compensate for NHE3 in Slc9a3 knockout animals, which have only a mild absorption defect.14 The Slc9a8 gene previously has been knocked out and the authors report that the mice have normal levels of serum sodium ions. They also report an increase in the length of the intestine, but a reduction in the number of goblet cells in the mutant mice with a concomitant reduction in mucin secretion and pH.13,15 Furthermore, it has been reported recently that the gene is expressed in the epithelial cells of the conjunctiva, cornea, and lacrimal glands and may have a role in the protection of these ocular epithelia.16 The male mutants also are sterile. Here, we identified and characterized a previously undescribed retinal defect of Slc9a8 knockout mice and located the origin of the phenotype to be abnormalities in the retinal pigmented epithelium.