2000 Global Researcher Conference Proceeding

March 10 - 12, 2000

Conference: 2000 Global Researcher Conference
Title: Nephrogenic Diabetes Insipidus in mice lacking aquaporin-3 water channels
Authors: Ma, Tonghui; Song, M.D., Yuanlin; Yang, Baoxue; Gillespie, Annemarie; Carlson, Elaine J.; Epstein, Charles; Verkman, Alan S.
Institutions: University of California, S.F., University of California, San Francisco, University of California
Ma Aquaporin-3 (AQP3) is a water channel expressed at the basolateral plasma membrane of kidney collecting duct epithelial cells. The mouse AQP3 cDNA was isolated and encodes a 292 amino acid water/glycerol transporting glycoprotein expressed in kidney, large airways, eye, urinary bladder, skin, and gastrointestinal tract. The mouse AQP3 gene was analyzed and AQP3 null mice were generated by targeted gene disruption. The growth and phenotype of AQP3 null mice were grossly normal except for polyuria. Laboratory analysis was unremarkable except for hypotriglyceridemia in the AQP3 null mice. In kidney, AQP3 deletion had little effect on AQP1 or AQP4 protein expression, but decreased AQP2 protein expression particularly in renal cortex. Fluid consumption in AQP3 null mice was more than ten-fold greater than that in wild-type litter mates, and urine osmolality (< 250 mOsm) was much lower than in wild-type mice (> 1200 mOsm). After DDAVP administration or water deprivation the AQP3 null mice were able to partially concentrate their urine to ~30% of that in wild-type mice. Osmotic water permeability of cortical collecting duct basolateral membrane, measured by a spatial filtering optics method, was >3-fold reduced by AQP3 deletion. To test the hypothesis that the residual concentrating ability of AQP3 null mice was due to the inner medullary collecting duct (IMCD) water channel AQP4, AQP3/AQP4 double knockout mice were generated. The double knockout mice had greater impairment of urinary concentrating ability than the AQP3 single knockout mice. AQP3 is expressed strongly in epidermis. Preliminary studies of evaporative water loss and water content in stratum corneum suggest that AQP3 plays a role in maintaining skin moisture. In summary, our findings establish a novel form of nephrogenic diabetes insipidus produced by impaired water permeability in collecting duct basolateral membrane. Basolateral membrane aquaporins may thus provide blood-accessible targets for drug discovery of aquaretic inhibitors.

Aquaporins (AQPs) are a family of proteins that allow water to transport across cell membranes. One result of mutations in aquaporin-2 (AQP2) genes is nephrogenic diabetes insipidus (NDI), a condition wherein the kidneys are unable to concentrate urine. NDI patients exhibit polyuria (excessive urination) and polydipsia (excessive thrist). Ma, et al., generated a line of mice lacking AQP3. These mice exhibited polyuria, polydipsia and dilute urine. They also had fewer AQP2s than normal mice. They responded to injections of DDAVP, a modified form of the antidiuretic hormone, arginine vasopressin (AVP), by showing a slight increase in their ability to concentrate urine. When Ma, et al., generated a line of mice lacking both AQP3 and AQP4, the mice had an even greater inability to concentrate their urine. Whereas AQP2 functions at the top of the membrane of kidney collecting duct principal cells (i.e. the apical membrane), AQP3 operates at the base and sides of the principal cell membrane (i.e. the basolateral membrane).

The researchers, then, have found that a form of NDI is produced when the basolateral membrane’s ability to transport water is impaired. Focusing on basolateral membrane AQPs may provide blood-accessible avenues for drugs that may help treat NDI.