A developmental switch in the expression of aquaporin-4 and Kir4.1 from horizontal to Muller cells in mouse retina

PURPOSE. In adult retina, aquaporin-4 (AQP4) and inwardly rectifying K+ (Kir4.1) channels localize to astrocyte and Muller cell membranes facing vascular and vitreous compartments, optimizing clearance of extracellular K+ and water from the synaptic layers. However, it is unknown whether these channels are expressed at early developmental stages, before gliogenesis or angiogenesis take place in the neural retina. This study was conducted to determine the presence of AQP4 and Kir4.1 proteins in the developing mouse retina. METHODS. Simultaneous AQP4 and Kir4.1 immunodetection was performed in postnatal mice 1, 9, 15, and 30 days of age. Confocal microscopy was used to identify the cellular distribution of AQP4 and Kir4.1 proteins, as well as their coexpression with the cell-selective immunomarkers Prox-1, calbindin, and neurofilament. RESULTS. AQP4 and Kir4.1 proteins were coexpressed in calbindin- and Prox1-expressing retinal neurons at birth. These neurons were identified as horizontal cells based on their position and morphology. By P15, when vision starts, AQP4 and Kir4.1 localization coordinately switched from horizontal cells to Muller glial cells. CONCLUSIONS. The findings showed that AQP4 and Kir4.1 protein expression is confined to differentiating horizontal cells before its expression in Muller cells. The finding of AQP4 in neurons is novel, since AQP4 expression within the central nervous system is restricted to glia. Also, the results demonstrated that AQP4 is a horizontal cell-specific immunomarker in neonatal retina. The transitory coexpression of AQP4 and Kir4.1 proteins by differentiating horizontal inter-neurons suggests that these cells mediate K+ and water transcellular uptake until the initiation of phototransduction, when glial cells assume these functions.