Immunolocalization of aquaporin-9 in rat hepatocytes and Leydig cells

The aquaporin (AQP)-9 gene was recently isolated from human and rat liver c DNA libraries as a member of the water channel family for water and neutral solutes. Although the expression of AQP9 mRNA has been demonstrated in sev eral organs including the liver and testis by Northern blot analysis, the c ellular and subcellular localization of the AQP9 protein remains unclear. I n the present light and electron microscopic immunohistochemical study, the localization of the AQP9 immunoreactivity was examined in fifteen hinds of rat organs using an antibody against rat AQP9 synthetic peptide. The antib ody immunostained a major band of similar to 33 kDa in the liver by Western blot analysis. Immunoreactivity for AQP9 was found exclusively in the live r and testis among the organs examined. In the liver, positive staining app eared selectively along the space of Disse, Immunoelectron microscopy confi rmed the localization of AQP9 on the surface of hepatocyte microvilli facin g the space of Disse. In the testis, the plasma membrane of Leydig cells lo cated between seminiferous tubules was conspicuously immunoreactive to the antibody. Intense mRNA expression was detected in the liver and testis but not in other organs by ribonuclease protection assay. These findings sugges t a specific role for AQP9 in the transport of water and non-charged solute s in hepatocytes and Leydig cells.