An Impaired Routing of Wild-type Aquaporin-2 after Tetramerization with an Aquaporin-2 Mutant Explains Dominant Nephrogenic Diabetes Insipidus

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Title: An Impaired Routing of Wild-type Aquaporin-2 after Tetramerization with an Aquaporin-2 Mutant Explains Dominant Nephrogenic Diabetes Insipidus
Authors: Kamsteeg, Erik-Jan; Wormhoudt, Thera A.M.; Rijss, Johan P.L.; van Os, Carel; Deen, Peter M.T.
Publisher: Embo Journal
Date Published: May 04, 1999
Reference Number: 481
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Autosomal recessive and dominant nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin, are caused by mutations in the aquaporin-2 (AQP2) gene. Missense AQP2 proteins in recessive NDI have been shown to be retarded in the endoplasmic reticulum, whereas AQP2-E258K, an AQP2 mutant in dominant NDI, was retained in the Golgi complex. In this study, we identified the molecular mechanisms underlying recessive and dominant NDI. Sucrose gradient centrifugation of rat and human kidney proteins and subsequent immunoblotting revealed that AQP2 forms homotetramers. When expressed in oocytes, wild-type AQP2 and AQP2-E258K also formed homotetramers, whereas AQP2-R187C, a mutant in recessive NDI, was expressed as a monomer. Upon co-injection, AQP2-E258K, but not AQP2-R187C, was able to heterotetramerize with wild-type AQP2. Since an AQP monomer is the functional unit and AQP2-E258K is a functional but misrouted water channel, heterotetramerization of AQP2-E258K with wild-type AQP2 and inhibition of further routing of this complex to the plasma membrane is the cause of dominant NDI. This case of NDI is the first example of a dominant disease in which the 'loss-of-function' phenotype is caused by an impaired routing rather than impaired function of the wild-type protein.
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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)

A hereditary disease may be inherited in one of two ways: in a recessive or a dominant manner. When a child inherits a disease in a recessive manner, that means he or she must inherit a defective gene from both parents. For example, in autosomal recessive nephrogenic diabetes insipidus, the child must inherit a mutated aquaporin-2 gene from both his mother and father.

When a child inherits a disease in a dominant manner, that means he or she only has to inherit a defective gene from one of the parents in order for symptoms of the disease to manifest. Autosomal nephrogenic diabetes insipidus is caused by mutations of the aquaporin-2 gene that produce defective aquaporin-2 (AQP2) proteins. Autosomal NDI is generally inherited in a recessive manner. But in the particular case of the AQP2 gene mutation labeled AQP2-E258K, NDI is inherited in a dominant fashion. In other words, the NDI patient who inherits an AQP2 gene that results in an AQP2-E258K mutation only has to inherit this mutated gene from one parent in order to manifest the disease.

In this paper, Kamsteeg, et al., report of their research which identified the molecular mechanisms underlying recessive and dominant autosomal NDI. The defective AQP2s produced by recessive NDI are retained in a part of the cell called the endoplasmic reticulum (ER) because they are improperly folded and therefore do not have the proper shape of a normal AQP2. The ER's quality control mechanism does not let the defective AQP2 travel to its job sight on the apical cell membrane. Instead, it keeps it in the ER where the defective AQP2 is broken down. AQP2-E258K, the mutant in dominant NDI, is released by the ER, but is retained in another part of the cell called the Golgi complex. Thus, it too cannot travel to its job site in the apical membrane.

The authors' work showed that a normal AQP2 groups with three other AQP2s to form a larger unit called a homotetramer: a complex comprised of four similar subunits, in this case, four AQP2s. The AQP2-E258K also forms homotetramers, whereas the mutant AQP2s resulting from recessive inheritance (in this study represented by the mutant AQP2-R187C) do not. That is, they do not combine with other AQP2s to form a homotetramer, buy stay as isolated, single units.

When the researchers co-injected AQP2-E258Ks along with normal AQP2s into laboratory cell cultures, they found that AQP2-E258K would form homotetramers with normal AQP2s. They would become part of the unit of four AQP2s and then misroute the entire homotetramer into the Golgi complex, where it would be retained. Upon co-injection of normal AQP2s and AQP2-R187C, the researchers found the AQP2-R187C was not able to form homotetramers with normal AQP2s and thus could not influence their routing to the cell membrane.

Dominant NDI is the first example of a dominant disease in which the disease symptoms are caused by impaired routing rather than impaired function of the normal protein (in this case, the normal AQP2) with which the mutant protein (in this case, AQP2-E258K) is paired.