Nephrogenic Diabetes Insipidus Caused By Mutation of Tyr205: A Key Residue of V2 Vasopressin Receptor Function

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Title: Nephrogenic Diabetes Insipidus Caused By Mutation of Tyr205: A Key Residue of V2 Vasopressin Receptor Function
Authors: Sangkuhl, Katrin; Rompler, Holger; Busch, Wibke; Karges, Beate; Schoneberg, Torsten
Publisher: Human Mutation
Date Published: May 01, 2005
Reference Number: 689
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Abstract • Article • Translation
Mutations in the V2 vasopressin receptor (AVPR2) are the most frequent genetic cause of the inherited nephrogenic diabetes insipidus (NDI). About 50% of all missense mutations found in extracellular loops of AVPR2 introduce additional cysteine residues, e.g. R181C, G185C, and Y205C. To explain the loss of receptor function two mechanistic models were suggested: First, the introduction of an additional extracellular Cys residue disrupts the conserved disulfide bond connecting the first and the second extracellular loop. And second, the mutationally introduced Cys residue forms a second disulfide bond with a free Cys residue within the second exoloop. Herein, we took advantage of a new NDI-causing mutation Y205H which affects a codon frequently found to be mutated to Cys in NDI patients. In contrast to Y205C the two mechanisms described above cannot account for the loss of receptor function of Y205H. In-depth functional characterization of mutant AVPR2 showed that also for Y205C the lack of a Tyr residue at position 205 is responsible for the abolished receptor function rather than the formation of a disastrous second disulfide bond. The concerted experimental and phylogenetic analysis emphasizes that Y205 is a key residue in maintaining the structure of AVPR2 and other members of the vasopressin receptor family.

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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)

Almost 90% of the mutations that result in congenital NDI occur in the vasopressin 2 receptor (V2R). That is, mutations in the V2R gene result in mutant V2R proteins that are unable to perform their function. The V2R is a chain of amino acid residues that intersects the cell membrane seven times. Part of the V2R exists outside the cell forming what are called the three extracellular loops. Part of it exists inside the cell forming what are called the three intracellular loops. And, as mentioned, part of it exists inside the cell membrane forming the seven transmembrane domains. The head of the amino acid residue chain that forms the V2R problem is outside the cell; the tail is inside.

Sangkuhl, et al., discovered a V2R mutation that had never before been recorded in NDI literature. The mutation, Y205H, had a histidine amino acid residue (H) at the 205th amino acid in the V2R amino acid sequence instead of a tyrosine amino acid (Y). This mutation occurred in the second extracellular loop. Only a relative few V2R mutations are located in the extracellular loops, and almost half of them lead to a cysteine (C) residue appearing where another amino acid residue should be.

The research team examined the Y205H mutant to find the mechanics behind its failure to function. They discovered that Y205H only had 40% as much cell surface expression as normal V2Rs, and that those Y205H V2Rs that do reach the cell surface do not bind efficiently with the hormone vasopressin (AVP).

The mutant Y205H V2R results from a mutation of a specific codon in the V2R gene. This same codon, when mutated, gives rise to other mutations that result in a C where another amino acid residue should be. Other researchers have postulated that such a substitution of a C results in V2R loss of function in one of two ways:
  1. The mutationally introduced C residue in an extracellular loop disrupts a necessary disulfide bond that connects the first and second extracellular loop.

  2. The extra C residue forms a second disulfide bond with a free C residue located in the second extracellular loop.
No matter which of these is the case, either would change the shape of the resulting V2R and inhibit its function.

However, Sagkuhl, et al., found that neither of these proposed mechanisms accurately reflected the way in which the mutant Y205H lost its functional capacity. What the research team did find was Y295, the tyrosine amino acid residue that appears as the 295th residue in the V2R protein, is 100% conserved in mammals. That is, the Y295 position has remained constant throughout the course of the evolution of the V2R protein. Other parts of the V2R protein may have changed through mutations through time, but Y205 is one of the aspects of the V2R that hasn’t. This, along with other amino acid residues in the V2R that have remained unchanged throughout the V2R’s evolution, ensures that the V2R will function properly.

This may explain why all Y295 mutations are retained in the interior of the cell by the cell’s quality control mechanism. When the Y residue at 205 is replaced by another amino acid residue, the resulting V2R mutant cannot form the proper shape, probably due to a misfolded binding pocket. Thus, the researchers not only discovered the functional interference that caused Y205H to fail to bind with AVP, they also reviewed Y295’s evolutionary history to determine how important it is to the proper functioning of the V2R.