Ultrastructural Localization And Function Of Aquaporin1 In The Mouse Inner Ear

The water permeability of biological membranes has been a longstanding problem in physiology, but the proteins responsible for this remained unknown until discovery of the Aquaporin 1 (AQP1) water channel protein. Peter Agre was awarded the 2003 Nobel Prize in Chemistry for this discovery. It is now well agreed that both diffusion and channel-mediated water movements are existed in the water permeability of biological membranes. The aquaporins are a family of at least 11 homologous(AQP0-10) water transporting proteins expressed in many mammalian cell types. Aquaporin-mediated water flow in vivo is directed by osmotic or hydraulic gradients.Water transport between the perilymph and endolymph is important in the regulation of volume and osmotic pressure of the inner ear. The endolymph is located in a closed fluid compartment. Changes in the composition, especially an increase of the volume (hydrops), have dramatic pathologic effects on the inner ear sensory function (Such as in Meniere's disease). Seven subtypes of aquaporins have been identified in the inner ear in mammal. AQP1 is abundant in the inner ear. But its function in the inner ear is currently poorly understood. To explore the function of the AQP1 in the inner ear, the ultrastructural localization of AQP1 in the mouse inner ear was investigated by the technique of immunohistochemical electron microscope.The cellular localization of AOP1 in the mouse inner ear was investigated byimmunocytochemistry. In cochlea, strong signal of AQP1 immunoreactivity was seen in the spiral ligament mostly near the bony shell where it is thought to be at the type III fibrocytes region. AQP1 was also present in the mesothelial cells lining the scala tympani and scala vestibuli. In the endolymphatic sac (ES), strong AQP1 labeling was observed in the sub-epithelial connective tissue. AQP1 expressed in the cochlea seems more abundant than that expressed in the ES by Western blot.The ultrastructural localization of AQP1 in the mouse inner ear was performed by immunogold electron microscopy, which is characterized by cryoprotection and high sensitivity. The right concentration of anti-AQP1 antibody and glutaraldehyde were determined by crossing-matched test for considering both ultrastructure and immunolabeling. AQP1 is localized on the membrane of type III fibrocytes in the spiral ligment, especially on the cell processes of fibrocytes. Gold particles are more abundant on the membrane near the bone shell than that on the membrane near the luminal space. Several gold particles are labeled on the membrane of the fibrocytes in the spiral limbus. Fibroblasts of sub-epithelial connective tissue of the ES also present densely labeling of gold particles. But the epithelial cells of the ES are devoid of labeling. AQP1 was localized on the cell processes of the fibrocytes. AQP1 maybe involved in :Perilymph exchange between the scala tympani and scala vestibule and the structure maintaining of the membranous cochlea canals : Except helicotrema, a new approach of perilymph exchange may exist— AQP1-bearing fibrocytes in the spiral ligment may mediate transfer of water between the scala tympani and scala vestibule, which both belong to the perilymphatic space. This maybe contribute to maintaining the structure of the membranous cochlea canal.Potassium recycling in the cochlea: The spiral ligament is thought to pump...