Syntrophin-dependent expression and localization of Aquaporin-4 water channel protein

The Aquaporin-4 (AQP4) water channel contributes to brain water homeostasis in perivascular astrocyte endfeet where it is concentrated. We postulated that AQP4 is tethered at this site by binding of the AQP4 C terminus to the PSD95-Discs large-ZO1 (PDZ) domain of syntrophin, a component of the dystr ophin protein complex. Chemical cross-linking and coimmunoprecipitations fr om brain demonstrated AQP4 in association with the complex, including dystr ophin, beta -dystroglycan, and syntrophin. AQP4 expression was studied in b rain and skeletal muscle of mice lacking alpha -syntrophin (alpha -Syn(-/-) ). The total level of AQP4 expression appears normal in brains of alpha -Sy n(-/-) mice, but the polarized subcellular localization is reversed. High-r esolution immunogold analyses revealed that AQP4 expression is markedly red uced in astrocyte endfeet membranes adjacent to blood vessels in cerebellum and cerebral cortex of alpha -Syn(-/-) mice, but is present at higher than normal levels in membranes facing neuropil. In contrast, AQP4 is virtually absent from skeletal muscle in alpha -Syn(-/-) mice. Deletion of the PDZ-b inding consensus (Ser-Ser-Val) at the AQP4 C terminus similarly reduced exp ression in transfected cell lines, and pulse-chase labeling demonstrated an increased degradation rate. These results demonstrate that perivascular lo calization of AQP4 in brain requires alpha -Syn, and stability of AQP4 in t he membrane is increased by the C-terminal PDZ-binding motif.