title: Aquaporin-4 in Astroglial Cells in the CNS and Supporting Cells of Sensory Organs-A Comparative Perspective.
abstract: The main water channel of the brain, aquaporin-4 (AQP4), is one of the classical water-specific aquaporins. It is expressed in many epithelial tissues in the basolateral membrane domain. It is present in the membranes of supporting cells in most sensory organs in a specifically adapted pattern: in the supporting cells of the olfactory mucosa, AQP4 occurs along the basolateral aspects, in mammalian retinal Müller cells it is highly polarized. In the cochlear epithelium of the inner ear, it is expressed basolaterally in some cells but strictly basally in others. Within the central nervous system, aquaporin-4 (AQP4) is expressed by cells of the astroglial family, more specifically, by astrocytes and ependymal cells. In the mammalian brain, AQP4 is located in high density in the membranes of astrocytic endfeet facing the pial surface and surrounding blood vessels. At these locations, AQP4 plays a role in the maintenance of ionic homeostasis and volume regulation. This highly polarized expression has not been observed in the brain of fish where astroglial cells have long processes and occur mostly as radial glial cells. In the brain of the zebrafish, AQP4 immunoreactivity is found along the radial extent of astroglial cells. This suggests that the polarized expression of AQP4 was not present at all stages of evolution. Thus, a polarized expression of AQP4 as part of a control mechanism for a stable ionic environment and water balanced occurred at several locations in supporting and glial cells during evolution. This initially basolateral membrane localization of AQP4 is shifted to highly polarized expression in astrocytic endfeet in the mammalian brain and serves as a part of the neurovascular unit to efficiently maintain homeostasis.
authors: Institute of Clinical Anatomy and Cell Analysis, Eberhard Karls Universität Tübingen, 72074 Tübingen, Germany. corinna.gleiser@klinikum.uni-tuebingen.de.
authors: Corinna
authors: C
authors: Gleiser
authors: Institute of Clinical Anatomy and Cell Analysis, Eberhard Karls Universität Tübingen, 72074 Tübingen, Germany. andreas.wagner@anatom.uni-tuebingen.de.
authors: Andreas
authors: A
authors: Wagner
authors: Institute of Pathology and Neuropathology, Eberhard Karls Universität Tübingen, 72076 Tubingen, Germany. Petra.Fallier-Becker@med.uni-tuebingen.de.
authors: Petra
authors: P
authors: Fallier-Becker
authors: Institute of Pathology and Neuropathology, Eberhard Karls Universität Tübingen, 72076 Tubingen, Germany. h.wolburg@t-online.de.
authors: Hartwig
authors: H
authors: Wolburg
authors: Institute of Clinical Anatomy and Cell Analysis, Eberhard Karls Universität Tübingen, 72074 Tübingen, Germany. bernhard.hirt@uni-tuebingen.de.
authors: Bernhard
authors: B
authors: Hirt
authors: Institute of Clinical Anatomy and Cell Analysis, Eberhard Karls Universität Tübingen, 72074 Tübingen, Germany. mack@anatu.uni-tuebingen.de.
authors: Andreas F
authors: AF
authors: Mack
mesh_terms: D000818:Animals; D051401:Aquaporin 4 / Q000378:metabolism*; D001253:Astrocytes / Q000166:cytology / Q000378:metabolism*; D001921:Brain / Q000166:cytology / Q000378:metabolism*; D063928:Ependymoglial Cells / Q000166:cytology / Q000378:metabolism; D006801:Humans; D009831:Olfactory Mucosa / Q000166:cytology / Q000378:metabolism; D014867:Water / Q000378:metabolism
chemical_list: D051401:Aquaporin 4; D014867:Water
keywords: astroglial evolution; brain water channel; polarization; water homeostasis