Aquaporin-4 in the central nervous system: cellular and subcellular distribution and coexpression with KIR4.1.
Aquaporin-4 (AQP4) is the predominant water channel in the neuropil of the central nervous system. It is expressed primarily in astrocytes, but also occurs in ependymocytes and endothelial cells. A striking feature of AQP4 expression is its polarized distribution in brain astrocytes and retinal Muller cells. Thus, immunogold analyses have revealed an enrichment of AQP4 in endfeet membranes in contact with brain microvessels or subarachnoidal space and a low but significant concentration in non-endfeet membranes, including those astrocyte membranes that ensheath glutamate synapses. The subcellular compartmentation of AQP4 mimics that of the potassium channel Kir4.1, which is implicated in spatial buffering of K(+). We propose that AQP4 works in concert with Kir4.1 and the electrogenic bicarbonate transporter NBC and that water flux through AQP4 contributes to the activity dependent volume changes of the extracellular space. Such volume changes are important as they affect the extracellular solute concentrations and electrical fields, and hence neuronal excitability. We conclude that AQP4-mediated water flux represents an integral element of brain volume and ion homeostasis.
Nagelhus
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Nordic Centre of Excellence for Research in Water Imbalance Related Disorders and Centre for Molecular Biology and Neuroscience, Institute of Basic Medical Sciences, University of Oslo, POB 1105 Blindern, N-0317 Oslo, Norway. e.a.nagelhus@medisin.uio.no
Mathiisen
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Ottersen
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D000818:Animals; D051401:Aquaporin 4; D020346:Aquaporins / Q000502:physiology*; D001253:Astrocytes / Q000502:physiology / Q000648:ultrastructure; D001639:Bicarbonates / Q000378:metabolism; D002462:Cell Membrane / Q000502:physiology / Q000648:ultrastructure; D002490:Central Nervous System / Q000502:physiology* / Q000648:ultrastructure; D045604:Extracellular Fluid / Q000502:physiology*; D006801:Humans; D011188:Potassium / Q000502:physiology; D024661:Potassium Channels, Inwardly Rectifying / Q000502:physiology*; D014882:Water-Electrolyte Balance / Q000502:physiology*; D000098854:Kcnj10 Channel
C496030:AQP4 protein, human; D051401:Aquaporin 4; D020346:Aquaporins; D001639:Bicarbonates; D000098854:Kcnj10 Channel; D024661:Potassium Channels, Inwardly Rectifying; D011188:Potassium