Defective proximal tubular fluid reabsorption in transgenic aquaporin-1 null mice

Abstract

To investigate the role of aquaporin-1 (AQP1) water channels in proximal tubule function, in vitro proximal tubule microperfusion and in vivo micropuncture measurements were done on AQP1 knockout mice. The knockout mice were generated by targeted gene disruption and found previously to be unable to concentrate their urine in response to water deprivation. Unanesthetized knockout mice consumed 2.8-fold more f luid than wild-type mice and had lower urine osmolality (505 ؎ 40 vs. 1081 ؎ 68 milliosmolar). An important function of the kidney proximal tubule is the near-isosmolar reabsorption of a significant fraction of fluid that is filtered by the glomerulus. The proximal tubule also reabsorbs nearly all of the filtered glucose, amino acids, and bicarbonate. The apical and basolateral plasma membranes of proximal tubule cells contain water channel protein aquaporin-1 (AQP1), which is thought to provide an important water-selective pathway for transcellular fluid transport (1-3). However, there is conflicting evidence that significant paracellular water transport occurs (4), and it has been suggested that other water channels (AQP7, ref. 5) and transporters (glucose transporter GLUT1, refs. 6, 7; sodium-glucose cotransporter SGLT1, ref. 8) might contribute to transcellular water movement. It is generally believed, but without direct evidence, that high proximal tubule water permeability is important to permit the efficient coupling of solute and water transport to accomplish near-isosmolar fluid absorption.The AQP1 water channel is a water-selective transporter (9, 10) that is found in membranes as tetramers (11) in which each functionally independent monomer (12) contains six transmembrane, tilted helical domains surrounding a putative aqueous pore (13-15). In kidney, AQP1 is strongly expressed in apical and basolateral plasma membranes of epithelial cells in proximal tubule and thin descending limb of Henle and in endothelial cells of descending vasa recta (1-3, 16, 17). Recently, a transgenic AQP1 knockout mouse was generated by targeted gene disruption and shown to manifest a severe defect in urinary concentrating ability (18). When given access to water, the mice appeared grossly normal except for mild growth retardation compared with wild-type mice. When deprived of water, the mice were unable to concentrate their urine and conserve fluid, resulting in marked dehydration and serum hyperosmolality in 1-2 days.The purpose of this study was to define the role of AQP1 in proximal tubule water transport and fluid reabsorption. Isolated tubule microperfusion was used to measure transepithelial osmotic water permeability and fluid absorption under defined in vitro conditions. Free-flow micropuncture was used to determine the in vivo consequences of decreased proximal tubule water permeability. A remarkable decrease in proximal tubule water permeability and fluid reabsorption was found in the AQP1 knockout mice. The results have important implications regarding the mechanisms of proximal tubule fluid reabsorpt...

Dissecting the Roles of Aquaporins in Renal Pathophysiology Using Transgenic Mice

“…1D) (17), indicating that the major pathway for osmotically-driven water transport in this segment is transcellular and AQP1-dependent. Free-flow micropuncture with end-proximal tubule fluid sampling showed ~50% reduced fluid absorption, with little effect on single nephron glomerular filtration rate (18). The data support a 3-compartment model in which mild luminal hypotonicity drives osmotic water movement through highly water permeable cell membranes.…”

Defective water and glycerol transport in the proximal tubules of AQP7 knockout mice

“…In the kidney, AQP7 is expressed on the apical membrane of the proximal straight tubules (S3 segment) (6), where AQP1 is also present (19). Studies of AQP1 knockout mice have shown that AQP1 plays a major role in proximal tubular water transport (15,22). In fact, AQP1 knockout mice were found to develop polyuria due to both a defective water reabsorption mechanism in the proximal tubules and a dysfunctional countercurrent mechanism in the inner medulla (26).…”

Angiotensin-(1-7) serves as an aquaretic by increasing water intake and diuresis in association with downregulation of aquaporin-1 during pregnancy in rats

“…Although aquaretic changes without alterations in collecting duct permeability are not typical, AQP1 knockout mice are unable to concentrate their urine appropriately in response to water deprivation and show decreased transepithelial osmotic water permeability in isolated proximal tubules (33), suggesting that AQP1 can mediate water reabsorption in this part of the nephron. Furthermore, immortalized proximal tubules exposed to ANG-(1-7) showed decreased AQP1 mRNA expression that was blocked by A-779 treatment (18), a finding consistent with the regulation of AQP1 that we found in pregnancy.…”