Pathogenesis and Treatment of Autosomal-Dominant Nephrogenic Diabetes Insipidus Caused by an Aquaporin 2 Mutation

Title: Pathogenesis and Treatment of Autosomal-Dominant Nephrogenic Diabetes Insipidus Caused by an Aquaporin 2 Mutation
Authors: Sohara, Eisei; Rai, Tatemitsu; Yang, Sung-Sen; Uchida, Keiko; Nitta, Kosaku; Horita, Shigeru; Ohno, Mayuko; Harada, Akihiro; Sasaki, Sei; Uchida, Shinichi
Publisher: Proceedings of the National Academy of Sciences of the U.S.A
Date Published: September 19, 2006
Reference Number: 710
Frame-shift mutations within the C terminus of aquaporin 2 (AQP2) cause autosomal-dominant nephrogenic diabetes insipidus (AD-NDI). To identify the molecular mechanism(s) of this disease in vivo and to test possible therapeutic strategies, we generated a mutant AQP2 (763-772 del) knockin mouse. Heterozygous knockin mice showed a severely impaired urine-concentrating ability. However, they were able to slightly increase urine osmolality after dehydration. This milder phenotype, when compared with autosomal-recessive NDI, is a feature of AD-NDI in humans, thus suggesting successful establishment of an AD-NDI mouse model. Immunofluorescence of collecting duct cells in the AD-NDI mouse revealed that the mutant AQP2 was missorted to the basolateral instead of apical plasma membrane. Furthermore, the mutant AQP2 formed a heterooligomer with wild-type AQP2 and showed a dominant-negative effect on the normal apical sorting of wild-type AQP2 even under dehydration. Using this knockin mouse, we tested several drugs for treatment of AD-NDI and found that rolipram, a phosphodiesterase 4 inhibitor, was able to increase urine osmolality. Phosphodiesterase inhibitors may thus be useful drugs for the treatment of AD-NDI. This animal model demonstrates that a mutant monomer gains a dominant-negative effect that reverses the normal polarized sorting of multimers.
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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)

The aquaporin 2 protein, in response to a molecular sequence initiated when the hormone, vasopressin (AVP) binds with the vasopressin 2 receptor (V2R) on the membrane of the kidney collecting duct principal cells, moves from the cell interior to the top (apical) section of the cell membrane. Once embedded in the apical section of the cell membrane, it acts as a channel through which water can flow into the cell. AQP2, then, plays a vital role in the kidney’s ability to concentrate urine. Mutations in the AQP2 gene can result in NDI. There are two types of NDI caused by AQP2 mutations: autosomal-recessive (AR-NDI) and autosomal-dominant NDI (AD-NDI). In the former, both pair of chromosomes carry a defective AQP2 gene. In the latter, only one of the chromosome pair carries a defective AQP2 gene. Of the two, AD-NDI has less severe NDI symptoms.

Sohara, et al., had previously identified three different AQP2 mutations that resulted in AD-NDI. Each of these AQP2 mutations resulted in AQP2s that had different amino acids than normal in a section of the AQP2 called the C-terminus. The research team had used laboratory cell cultures to determine how AD-NDI mutants could cause NDI. The results were highly suggestive, but they could not be used to accurately model how these mutant AQP2 behaved in living bodies because the levels of normal and mutant AQP2 in the cell cultures did not correspond to the proportions in the bodies of those with AD-NDI.

To better understand the mechanics of how the mutant AQP2s in AD-NDI actually cause the disease in living systems, Sohara, et al., developed a line of mice with AD-NDI. To do this, they created a AQP2 protein that was part mouse and part human, the human part being the C-terminus with an AD-NDI mutation. They injected this into the mice. The resulting mouse line exhibited the same mix of normal and mutant AQP2 proteins as do humans with AD-NDI. Further, these mice reacted to dehydration and had similar urine concentration and urine volume as did humans with AD-NDI. Thus, this mouse line was a good model with which to examine the mechanics of AD-NDI and to test treatment of it.

Upon examination, the researchers found that the mutant AQP2 in their mice did not go to the apical section of the cell membranes of the principal cells in the kidney collecting duct. Instead, they went to the bottom and sides of the membrane, a section called the basolateral membrane. Not only that, the mutant AQP2s would bind with the normal AQP2s and take them to the basolateral membrane.

The research team tested several types of phosphodiesterase (PDE) inhibitorsPDE 3, 4 & 5 - to see if they could increase the concentration of urine and reduce urine volume in the AD-NDI mice. Of the three tested, PDE4, also called rolipram, was able to increase the urine concentration ability of the AD-NDI mice. Rolipram increased the number of AQP2 traveling to the apical membrane. It also increased the levels of cyclic adenosine monophosphate (cAMP) in the principal cells. This increase in cAMP correlated well with the increase in urine concentration. cAMP is required in the molecular sequence that results in urine concentration because it activates protein Kinase A (PKA). PKA adds a phosphate group (in a process called phosphorylation) to the AQP2. This helps AQP2 travel to the apical membrane. The researchers also found that rolipram increased the phosphorylation of AQP2.

In sum, this research group established a mouse line that serves as a model to investigate AD-NDI, discovered that mutant AQP2s in AD-NDI travel to the basolateral membrane instead of the apical membrane, discovered that the mutants bind with normal AQP2s in the cell and misdirect them to the basolateral membrane, and found that PDE4 may be useful in treating AD-NDI.