title: Syntrophin-dependent expression and localization of Aquaporin-4 water channel protein.
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 PSD95-Discs large-ZO1 (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, beta-dystroglycan, and syntrophin. AQP4 expression was studied in brain and skeletal muscle of mice lacking alpha-syntrophin (alpha-Syn(-/-)). The total level of AQP4 expression appears normal in brains of alpha-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 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-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 alpha-Syn, and stability of AQP4 in the membrane is increased by the C-terminal PDZ-binding motif.
authors.affiliation: Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Neely
authors.forename: M
authors.initials: M
authors.lastname: Amiry-Moghaddam
authors.forename: O P
authors.initials: OP
authors.lastname: Ottersen
authors.forename: S C
authors.initials: SC
authors.lastname: Froehner
authors.forename: P
authors.initials: P
authors.lastname: Agre
authors.forename: M E
authors.initials: ME
authors.lastname: Adams
mesh_terms: D000818:Animals; D051401:Aquaporin 4; D020346:Aquaporins / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism; D016466:CHO Cells; D002135:Calcium-Binding Proteins; D002460:Cell Line; D006224:Cricetinae; D003598:Cytoskeletal Proteins / Q000378:metabolism; D004285:Dogs; D049030:Dystroglycans; D016189:Dystrophin / Q000031:analogs & derivatives* / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D008562:Membrane Glycoproteins / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D014882:Water-Electrolyte Balance
chemical_list: C496030:AQP4 protein, human; C496031:Aqp4 protein, mouse; C496032:Aqp4 protein, rat; D051401:Aquaporin 4; D020346:Aquaporins; D002135:Calcium-Binding Proteins; D003598:Cytoskeletal Proteins; C483081:DAG1 protein, human; D016189:Dystrophin; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009124:Muscle Proteins; C085339:apo-dystrophin 1; C091825:syntrophin alpha1; D049030:Dystroglycans