Aquaporin-4 expression in adult and developing mouse and rat kidney

Aquaporin-4 (AQP4) is a member of the aquaporin water-channel family. AQP4 is expressed primarily in the brain, but it is also present in the collecti ng duct of the kidney, where it is located in the basolateral plasma membra ne of principal cells and inner medullary collecting duct (IMCD) cells. Rec ent studies in the mouse also have reported the presence of AQP4 in the bas olateral membrane of the proximal tubule. The purpose of this study was to establish the pattern of AQP4 expression during kidney development and in t he adult kidney of both the mouse and the rat. Kidneys of adult and 3-, 7-, and 15-d-old mice and rats were preserved for immunohistochemistry and pro cessed using a peroxidase pre-embedding technique. In both the mouse and th e rat., strong basolateral immunostaining was observed in IMCD cells and pr incipal cells in the medullary collecting duct at all ages examined. Labeli ng was weaker in the cortical collecting duct and the connecting tubule, an d there was no labeling of connecting tubule cells in the mouse. In adult m ouse kidney, strong AQP4 immunoreactivity was observed in the S3 segment of the proximal tubule. However, there was little or no labeling in the corte x or around the corticomedullary junction in 3- and 7-d-old mice. Between 7 and 15 d of age, distinct AQP4 immunoreactivity appeared in the S3 segment of the mouse proximal tubule concomitant with the differentiation of this segment of the nephron. Labeling of proximal tubules was never observed in the rat kidney. These results suggest that there are differences in transep ithelial water transport between mouse and rat or that additional, not yet identified water channels exist in the rat proximal tubule.