Spatial and temporal dissociation of AQP4 and Kir4.1 expression during induction of refractive errors

Purpose: Spatial co-localization of aquaporin water channels (AQP4) and inwardly rectifying potassium ion channels (Kir4.1) on the endfeet regions of glial cells has been suggested as the basis of functionally interrelated mechanisms of osmoregulation in brain edema. The aim of this study was to investigate the spatial and temporal changes in the expression of AQP4 and Kir4.1 channels in an avascular retina during the first week of the optical induction of refractive errors. Methods: Three-day-old hatchling chicks were randomly assigned to three groups and either did not wear lenses or were monocularly goggled with +/- 10D lenses for varying times up to 7 days before biometric assessment. Retinal tissue was prepared either for western blot analysis to show the presence of the AQP4 and Kir4.1 protein in the chick retina or for immunolocalization using AQP4 and Kir4.1 antibodies to determine the regional distribution and intensity of labeling during the induction of refractive errors. Results: As expected, ultrasonography demonstrated that all eyes showed rapid elongation post hatching. Negative lens-wearing eyes elongated faster than fellow eyes or normal non goggled eyes and became progressively more myopic with time post lensing. Positive lens-wearing eyes showed reduced ocular growth compared to normal controls and developed a hyperopic refraction. Quantitative immunohistochemistry revealed the upregulation of AQP4 channel expression on Muller cells in the retinal nerve fiber layer during the first 2 days of negative lens wear. Kir4.1 channel upregulation in the inner plexiform layer was only found on day 4 of positive lens wear during the development of refractive hyperopia. Conclusions: These results indicate that the expression of AQP4 and Kir4.1 channels on Muller cells is associated with the changes in ocular volume seen during the induction of refractive errors. However, the sites of greatest expression and the temporal pattern of the upregulation of AQP4 and Kir4.1 were dissimilar, indicating a dissociation of AQP4 and Kir4.1 function during refractive error development. Increased AQP4 expression in the nerve fiber layer is suggested to contribute to the rapid axial elongation and movement of fluid into the vitreous cavity in the presence of minus lenses; whereas, upregulation of Kir4.1 channels appears to play a role in limiting axial elongation in the presence of plus lenses.