Aquaporin-6: An intracellular vesicle water channel protein in renal epithelia

All characterized mammalian aquaporins (AQPs) are localized to plasma membr anes where they function chiefly to mediate water transport across cells, H ere we show that AQP6 is localized exclusively in intracellular membranes i n renal epithelia, By using a polyclonal antibody to the C terminus of AQP6 , immunoblots revealed a major 30-kDa band in membranes from rat renal cort ex and medulla, Endoglycosidase treatment demonstrated presence of an intra cellular high mannose glycan on each subunit, Sequential ultracentrifugatio n of rat kidney homogenates confirmed that AQP6 resides predominantly in ve sicular fractions, and immunohistochemical and immunoelectron microscopic s tudies confirmed that >98% of AQP6 is located in intracellular membrane ves icles. In glomeruli, AQP6 is present in membrane vesicles within podocyte c ell bodies and foot processes. In proximal tubules, AQP6 is also abundant i n membrane vesicles within the subapical compartment of segment 2 and segme nt 3 cells, but was not detected in the brush border or basolateral membran es. In collecting duct, AQP6 resides in intracellular membrane vesicles in apical, mid, and basolateral cytoplasm of type A intercalated cells, but wa s not observed in the plasma membrane. Unlike other members of the AQP fami ly, the unique distribution in intracellular membrane vesicles in multiple types of renal epithelia indicates that AQP6 is not simply involved in tran scellular fluid absorption. Moreover, our studies predict that AQP6 partici pates in distinct physiological functions such as glomerular filtration, tu bular endocytosis, and acid-base metabolism.