Severely Impaired Urinary Concentrating Ability in Transgenic Mice Lacking Aquaporin-1 Water Channels

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Title: Severely Impaired Urinary Concentrating Ability in Transgenic Mice Lacking Aquaporin-1 Water Channels
Authors: Ma, Tonghui; Yang, Baoxue; Verkman, Alan S.; Gillespie, Annemarie; Carlson, Elaine J.; Epstein, Charles
Publisher: Journal of Biological Chemistry
Date Published: February 20, 1998
Reference Number: 203
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Water channel aquaporin-1 (AQP1) is strongly expressed in kidney in proximal tubule and descending limb of Henle epithelia and in vasa recta endothelia. The grossly normal phenotype in human subjects deficient in AQP1 (Colton null blood group) and in AQP4 knockout mice has suggested that aquaporins (other than the vasopressin-regulated water channel AQP2) may not be important in mammalian physiology. We have generated transgenic mice lacking detectable AQP1 by targeted gene disruption. In kidney proximal tubule membrane vesicles from knockout mice, osmotic water permeability was reduced 8-fold compared with vesicles from wild-type mice. Although the knockout mice were grossly normal in terms of survival, physical appearance, and organ morphology, they became severely dehydrated and lethargic after water deprivation for 36 h. Body weight decreased by 35 +/- 2%, serum osmolality increased to >500 mOsm, and urinary osmolality (657 +/- 59 mOsm) did not change from that before water deprivation. In contrast, wild-type and heterozygous mice remained active after water deprivation, body weight decreased by 20-22%, serum osmolality remained normal (310-330 mOsm), and urine osmolality rose to >2500 mOsm. Urine [Na+] in water-deprived knockout mice was <10 mM, and urine osmolality was not increased by the V2 agonist DDAVP. The results suggest that AQP1 knockout mice are unable to create a hypertonic medullary interstitium by countercurrent multiplication. AQP1 is thus required for the formation of a concentrated urine by the kidney.
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This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

Aquaporins (AQPs) are a family of water transporting proteins found widely in animals, plants and bacteria. Of the eight AQPs (AQP 1 - 8) that have been cloned, only AQP2 has been shown to have a function that was physiologically important. This was indicated when mutated AQP2s were shown to be the cause of one form of inherited nephrogenic diabetes insipidus (NDI), a disorder rendering the kidneys unable to concentrate urine by reabsorbing body water through the principal cells of the kidney collecting duct.

In contrast, those extremely rare people who lack AQP1 have shown no outward defect as a result. This was surprising as AQP1 is expressed in so many parts of the body: kidney, lung, eye, spleen, heart, blood vessels and gallbladder. Because AQP1 is expressed so densely in areas of the kidney responsible for concentrating urine, Ma, et al., investigated as to whether AQP1 does or does not play an essential role in the kidneys' ability to concentrate urine.

To do so, they generated mice lacking in AQP1. The authors' genetic manipulations produced a line of 166 mice: 45 normal, 84 capable of producing up to half the normal amount of AQP2, and 37 AQP1 knockout mice, i.e. mice lacking any AQP2s. Though the knockout mice appeared normal in terms of their physical appearance, organ shape and survival rate, analysis revealed that their kidney proximal tubule membrane vesicles, cells in a part of the kidney through which body water is reabsorbed, showed severe reductions in their ability to let water permeate through them. This low water permeability indicates that AQP1 is the principal functional water channel for the proximal tubule apical membrane.

Next, the authors sought to determine whether AQP1 is required for the formation of concentrated urine. They subjected the knockout mice, the normal mice and the mice capable of generating one-half the amount of normal AQP1, to a 36 hour period of water deprivation, collecting urine samples before and after the water fast.

After the water fast, the knockout mice became grossly lethargic and had lost 35% of their body weight. Also, their serum osmolality (a measure of the concentration of osmotically active particles in their serum) was high. By contrast, the other mice remained active, lost only 20 - 22% of their body weight, and their serum osmolality was substantially lower than that of the knockout mice.

Knockout mice had very low urine osmolality prior, during and after the water fast. This indicated they could not concentrate urine. Injections of DDAVP, a synthetic derivative of the antidiuretic hormone, arginine vasopressin (AVP), resulted in no increase in the knockout mice's urine osmolality, indicating that the urinary concentrating defect associated with AQP1 knockout involves the kidney.

These test results suggest the kidneys of the AQP1 knockout mice are unable to generate the proper osmotic conditions in their inner tissues which allow urine concentration to occur. This indicates that AQP1 plays an important role in the kidneys' ability to concentrate urine.