Immunocytochemical studies of aquaporin 4, kir4.1, and α1-syntrophin in the astrocyte endfeet of mouse brain capillaries

Abstract

One of the mostt impoHritsatnoct hpehmyisstiroylo agnidc aCl yr-oles of brain astrocytes is the maintenance of extracellular K+ concentration by adjusting the K+influx and K+ efflux. The inwardly rectifying K+ channel Kir4.1 has been identified as an important member of K+ channels and is highly concentrated in glial endfeet membranes. Aquaporin (AQP) 4 is another abundantly expressed molecule in astrocyte endfeet membranes. We examined the ultrastructural localization of Kir4.1, AQP4, α1-syntrophin, and β-spectrin molecules to understand the functional role(s) of Kir4.1 and AQP4. Immunogold electron microscopy of these molecules showed that the signals of these molecules were present along the plasma membranes of astrocyte endfeet. Double immunogold electron microscopy showed frequent co-localization in the combination of molecules of Kir4.1 and AQP4, Kir4.1 and α1-syntrophin, and AQP4 and α1-syntrophin, but not those of AQP4 and β-spectrin. Our results support biochemical evidence that both Kir4.1 and AQP4 are associated with α1-syntrophin by way of postsynaptic density-95, Drosophila disc large protein, and the Zona occludens protein I proteininteraction domain. Co-localization of AQP4 and Kir4.1 may indicate that water flux mediated by AQP4 is associated with K+ siphoning. © 2010 The Japan Society of Histochemistry and Cytochemistry.