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

Abstract

All characterized mammalian aquaporins (AQPs) are localized to plasma membranes where they function chief ly to mediate water transport across cells. Here we show that AQP6 is localized exclusively in intracellular membranes in 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 cortex and medulla. Endoglycosidase treatment demonstrated presence of an intracellular high mannose glycan on each subunit. Sequential ultracentrifugation of rat kidney homogenates confirmed that AQP6 resides predominantly in vesicular fractions, and immunohistochemical and immunoelectron microscopic studies confirmed that >98% of AQP6 is located in intracellular membrane vesicles. In glomeruli, AQP6 is present in membrane vesicles within podocyte cell bodies and foot processes. In proximal tubules, AQP6 is also abundant in membrane vesicles within the subapical compartment of segment 2 and segment 3 cells, but was not detected in the brush border or basolateral membranes. In collecting duct, AQP6 resides in intracellular membrane vesicles in apical, mid, and basolateral cytoplasm of type A intercalated cells, but was not observed in the plasma membrane. Unlike other members of the AQP family, the unique distribution in intracellular membrane vesicles in multiple types of renal epithelia indicates that AQP6 is not simply involved in transcellular f luid absorption. Moreover, our studies predict that AQP6 participates in distinct physiological functions such as glomerular filtration, tubular endocytosis, and acid-base metabolism.

Relative CO2/NH3 selectivities of mammalian aquaporins 0–9

“…This channel colocalizes with the H ϩ -ATPase in intracellular vesicles in the renal collecting duct (73). In the present study, the only AQP that is impermeable to H 2 O is rAQP6.…”

Immunolocalization of AQP-5 in rat parotid and submandibular salivary glands after stimulation or inhibition of secretion in vivo

“…For immunoelectron microscopy, the frozen samples were freeze-substituted in a Reichert AFS freeze substitution unit (20,25,27,33). In brief, the samples were sequentially equilibrated over 3 days in methanol containing 0.5% uranyl acetate at temperatures gradually raised from Ϫ80 to Ϫ70°C and, then rinsed in pure methanol for 24 h while the temperature was increased from Ϫ70 to Ϫ45°C.…”

Evidence against Functionally Significant Aquaporin Expression in Mitochondria

“…Indeed, the high surface-to-volume ratio of endosomes and other organelles, generally Ͼ3 ϫ 10 5 cm Ϫ1 (diameter Ͻ 200 nm) predicts very rapid osmotic equilibration times of Ͻ100 ms, even when osmotic water permeability (P f ) is low (Ͻ0.005 cm/s). Other AQPs have been reported in endosomes in some cell types, including AQP6 in the renal proximal tubule and collecting duct epithelial cells (5) and various AQPs in liver cells (6), although data are lacking on their possible cellular functions. Two recent studies reported AQP expression in mitochondria and suggested possible involvement of mitochondrial AQPs in many functions, including metabolism and apoptosis, and in the pathogenesis of neurological diseases such as Parkinson disease.…”