Potassium channel Kir4.1 macromolecular complex in retinal glial cells

A major role for Muller cells in the retina is to buffer changes in the extracellular K+ concentration ([K+](o)) resulting from light-evoked neuronal activity. The primary K+ conductance in Muller cells is the inwardly rectifying K+ channel Kir4.1. Since this channel is constitutively active, K+ can enter or exit Muller cells depending on the state of the [K+](o). This process of [K+](o) buffering by Muller cells ("K+ siphoning") is enhanced by the precise accumulation of these K+ channels at discrete subdomains of Muller cell membranes. Specifically, Kir4.1 is localized to the perivascular processes of Muller cells in animals with vascular retinas and to the endfeet of Muller cells in all species examined. The water channel aquaporin-4 (AQP4) also appears to be important for [K+]. buffering and is expressed in Muller cells in a very similar subcellular distribution pattern to that of Kir4.1. To gain a better understanding of how Muller cells selectively target K and water channels to discrete membrane subdomains, we addressed the question of whether Kir4.1 and AQP4 associate with the dystrophin-glycoprotein complex (DGC) in the mammalian retina. Immunoprecipitation (IP) experiments were utilized to show that Kir4.1 and AQP4 are associated with DGC proteins in rat retina. Furthermore, AQP4 was also shown to co-precipitate with Kir4.1, suggesting that both channels are tethered together by the DGC in Muller cells. This work further defines a subcellular localization mechanism in Muller cells that facilitates [K+], buffering in the retina. (c) 2005 Wiley-Liss, Inc.