A Novel Mechanism in Recessive Nephrogenic Diabetes Insipidus: Wild-Type Aquaporin-2 Rescues the Apical Membrane Expression of Intracellularly Retained AQP2-P262L

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Title: A Novel Mechanism in Recessive Nephrogenic Diabetes Insipidus: Wild-Type Aquaporin-2 Rescues the Apical Membrane Expression of Intracellularly Retained AQP2-P262L
Authors: de Mattia, Fabrizio; Savelkoul, Paul J.M.; Bichet, Daniel G.; Kamsteeg, Erik-Jan; Konings, Irene B. M.; Marr, Nannette; Arthus, Marie-Francoise; Lonergan, Michele; van Os, Carel; van der Sluijs, Peter; Robertson, Gary; Deen, Peter M.T.
Publisher: Human Molecular Genetics
Date Published: October 27, 2004
Reference Number: 667
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Vasopressin regulates water homeostasis through insertion of homotetrameric Aquaporin-2 (AQP2) water channels in the apical plasma membrane of renal cells. AQP2 mutations cause recessive and dominant Nephrogenic Diabetes Insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin. Until now, all AQP2 mutants in recessive NDI were shown to be misfolded, retained in the endoplasmic reticulum and unable to interact with wild-type (wt) AQP2, whereas AQP2 mutants in dominant NDI are properly-folded and interact with wt-AQP2, but, due to the mutation, cause missorting of the wt-AQP2/mutant complex. Here, patients of two families with recessive NDI appeared compound heterozygotes for AQP2-A190T or AQP2-R187C mutants, together with AQP2-P262L. Since mutations in the AQP2 C-tail, where P262 resides, usually cause dominant NDI, the underlying cell biological mechanism was investigated. Upon expression in oocytes, AQP2-P262L was a properly-folded and functional aquaporin in contrast to the classical mutants, AQP2-R187C and AQP2-A190T. Expressed in polarized cells, AQP2-P262L was retained in intracellular vesicles and did not localize to the endoplasmic reticulum. Upon co-expression, however, AQP2-P262L interacted with wt-AQP2, but not with AQP2-R187C, resulting in a rescued apical membrane expression of AQP2-P262L. In conclusion, our study reveals a novel cellular phenotype in recessive NDI in that AQP2-P262L acts as a mutant in dominant NDI, except for that its missorting is overruled by apical sorting of wt-AQP2. Also, it demonstrates for the first time that the recessive inheritance of a disease involving a channel can be due to two cell-biological mechanisms.
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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)

Congenital NDI is caused by mutations in either the vasopressin type-2 receptor (V2R) gene or the aquaporin-2 (AQP2) gene. The majority of AQP2 mutations resulting in NDI are inherited in a recessive manner. That is, both the mother and father must carry the gene mutation. Sometimes AQP2-caused NDI can be inherited in a dominant manner. That is, only one of the parents need have an AQP2 gene mutation in order for the child to be born with NDI. There are distinct differences between AQP2 mutants in recessive NDI and dominant NDI. The former are misshapen and therefore retained in the quality control section of the cell called the endoplasmic reticulum (ER) where they are unable to interact with normally shaped AQP2. These AQP2 mutants are then dismantled by the cell.

Mutant AQP2 in dominant NDI are properly shaped, are not retained in the ER, and can interact with normally shaped AQP2. But once they do bind with normally shaped AQP2, their mutation causes the mutant/normal AQP2 combination to be misdirected from the cell’s apical membrane – the location of the cell where they must be to perform their function.

de Mattia, et al., examined two NDI families that both carried two different AQP2 gene mutations that resulted in recessive NDI. The interesting thing was while two of the resultant AQP2 mutants – AQP2-A190T and AQP2-R187C – had characteristics that placed them in the classical recessive NDI category, the remaining mutant - AQP2-P262L – fell into the dominant NDI category, yet did not cause NDI in a dominant manner.

The research team conducted a series of laboratory experiments to uncover the cellular dynamics underlying this seeming anomaly. They discovered when AQP2-P262L, recombined with normal AQP2, the mutant’s ability to misdirect the normal AQP2 after it combined with it was overridden by the normal AQP2 targeting capacity. The researchers discovered this by first expressing AQP2-P262L by itself into a laboratory cell culture. Under these conditions most of the AQP2-P262L ended up in unknown intracellular vesicles instead of the cell apical membrane where a normal AQP2 would have ended up. When co-expressed into the laboratory cell culture with normal AQP2, both primarily reached the apical membrane. Further experimental data indicated that the mutant AQP2 recombined with the normal AQP2. Thus, the nature of this mutation is unable to override the targeting ability of the normal AQP2 it combines with. AQP2-P262L is a unique and alternative cell-biological explanation for the occurrence of recessive NDI.