SPECIALIZED MEMBRANE DOMAINS FOR WATER TRANSPORT IN GLIAL-CELLS - HIGH-RESOLUTION IMMUNOGOLD CYTOCHEMISTRY OF AQUAPORIN-4 IN RAT-BRAIN

Membrane water transport is critically involved in brain volume homeos tasis and in the pathogenesis of brain edema. The cDNA encoding aquapo rin-4 (AQP4) water channel protein was recently isolated from rat brai n. We used immunocytochemistry and high-resolution immunogold electron microscopy to identify the cells and membrane domains that mediate wa ter flux through AQP4. The AQP4 protein is abundant in glial cells bor dering the subarachnoidal space, ventricles, and blood vessels. AQP4 i s also abundant in osmosensory areas, including the supraoptic nucleus and subfornical organ. Immunogold analysis demonstrated that AQP4 is restricted to glial membranes and to subpopulations of ependymal cells . AQP4 is particularly strongly expressed in glial membranes that are in direct contact with capillaries and pia. The highly polarized AQP4 expression indicates that these cells are equipped with specific membr ane domains that are specialized for water transport, thereby mediatin g the flow of water between glial cells and the cavities filled with C SF and the intravascular space.