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

TLDR

Aquaporin‑4 (AQP4) concentrates in perivascular astrocyte endfeet and regulates brain water homeostasis. The study hypothesizes that AQP4 is tethered to perivascular sites by binding its C‑terminus to the PDZ domain of syntrophin, a dystrophin complex component. AQP4 expression was examined in brain and skeletal muscle of α‑syntrophin knockout mice. Co‑immunoprecipitation shows AQP4 associates with the dystrophin complex; in α‑syntrophin knockout mice, total AQP4 is unchanged but perivascular localization is lost, with reduced endfoot expression and increased neuropil levels, and skeletal muscle lacks AQP4; deleting the C‑terminal PDZ‑binding motif similarly reduces expression and speeds degradation, proving α‑syntrophin and the PDZ motif are essential for perivascular AQP4 localization and membrane stability.

Abstract

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 P SD95- D iscs large- Z O1 (PDZ) domain of syntrophin, a component of the dystrophin protein complex. Chemical cross-linking and coimmunoprecipitations from brain demonstrated AQP4 in association with the complex, including dystrophin, β-dystroglycan, and syntrophin. AQP4 expression was studied in brain and skeletal muscle of mice lacking α-syntrophin (α- Syn −/− ). The total level of AQP4 expression appears normal in brains of α- Syn −/− mice, but the polarized subcellular localization is reversed. High-resolution immunogold analyses revealed that AQP4 expression is markedly reduced in astrocyte endfeet membranes adjacent to blood vessels in cerebellum and cerebral cortex of α- Syn −/− mice, but is present at higher than normal levels in membranes facing neuropil. In contrast, AQP4 is virtually absent from skeletal muscle in α- Syn −/− mice. Deletion of the PDZ-binding consensus (Ser-Ser-Val) at the AQP4 C terminus similarly reduced expression in transfected cell lines, and pulse–chase labeling demonstrated an increased degradation rate. These results demonstrate that perivascular localization of AQP4 in brain requires α-Syn, and stability of AQP4 in the membrane is increased by the C-terminal PDZ-binding motif.