Heteroligomerization of an Aquaporin-2 Mutant with Wild-Type Aquaporin-2 and Their Misrouting to Late Endosomes/Lysosomes Explains Dominant Nephrogenic Diabetes Insipidus
| Title: | Heteroligomerization of an Aquaporin-2 Mutant with Wild-Type Aquaporin-2 and Their Misrouting to Late Endosomes/Lysosomes Explains Dominant Nephrogenic Diabetes Insipidus |
|---|---|
| Authors: | Marr, Nannette; Bichet, Daniel G.; Lonergan, Michele; Arthus, Marie-Francoise; Jeck, Nikola; Seyberth, M.D., Hannsjorg W.; Rosenthal, Walter; van Os, Carel; Oksche, Alexander; Deen, Peter M.T. |
| Publisher: | Human Molecular Genetics |
| Date Published: | April 01, 2002 |
| Reference Number: | 561 |
Autosomal nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin, is caused by mutations in the Aquaporin-2 (AQP2) gene. Analysis of a new family with dominant NDI revealed a single nucleotide deletion (727DG) in one AQP2 allele, which encoded an AQP2 mutant with an altered and extended C-terminal tail. When expressed in oocytes, the tetrameric AQP2-727DG was retained within the cell. When co-expressed, AQP2-727DG, but not a mutant in recessive NDI (AQP2-R187C), formed hetero-oligomers with wild-type (wt) AQP2 and reduced the water permeability of these oocytes, because of a reduced plasma membrane expression of wt-AQP2. Expressed in renal epithelial cells, AQP2-727DG predominantly localized to the basolateral membrane and late endosomes/lysosomes, whereas wt-AQP2 was expressed in the apical membrane. Upon co-expressing in these cells, wt-AQP2 and AQP2-727DG mainly co-localized to late endosomes/lysosomes. In conclusion, hetero-oligomerization of AQP2-727DG with wt-AQP2 and consequent mistargeting of this complex to late endosomes/lysosomes results in absence of AQP2 in the apical membrane, which can explain dominant NDI in this family. Together with other mutants in dominant NDI, our data reveal that a misrouting, instead of a lack of function, is a general mechanism for the 'loss of function' phenotype in dominant NDI and visualizes for the first time a mislocalization of a wild-type protein to late endosomes/lysosomes in polarized cells after oligomerization with a mutant protein.
You may, however, read this article at the Human Molecular Genetics website. To return to this page, use your "back" key. |
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)
Marr, et al. report a previously unreported mutated AQP2 gene that is inherited in a dominant fashion. This AQP2 mutation (727DG) was missing a single nucleotide – the base, guanosine at position 727 of the AQP2 gene. The research team, working with AQP2 (727DG) in laboratory cell cultures, was able to determine how the mutation acted in the cell to cause NDI. The AQP2 protein must make its way from the cell interior to a specific part of the cell membrane in order to perform its function as a channel through which water can pass into the cell. It is able to make this journey as a result of a molecular sequence initiated when the hormone, arginine vasopressin (AVP) links with the vasopressin-2 receptor (V2R).
The research team discovered that AQP2-727DG produced an AQP2 protein with a structure different than normal AQP2 proteins. It has a longer tail end (C-terminus) than normal. Since the C-terminus concerns itself with helping steer the AQP2 from the cell interior to the cell membrane, it was not too surprising that the mutant AQP2 protein was found, not in the cell membranes, but in cellular structures within the cell called late endosomes/lysosomes. The researchers found that this AQP2 protein was structurally able to let water channel through it, but unable to get the cell membrane to let the water do so. Its misrouting to the late endosomes/lysomes instead of the cell membrane explains the patient’s NDI symptoms.
The reason AQP2-727DG could cause NDI when the patient’s other, normal AQP2 gene encoded for a normal AQP2 protein is that the mutant AQP2 is able to combine with the normal AQP2. When so combined, the normal AQP2 is directed to the late endosomes/lysosomes. Interestingly, all five forms of NDI caused by dominant AQP2 result in functional AQP2 proteins that are unable to reach the cell membrane. As far as why this particular AQP2 mutant is unable to reach its target, the researchers were able to narrow the reason down to three options:
- The absence of the molecular structure found in normal AQP2s;
- The presence of different molecular structures found in the mutant AQP2 C-terminus;
- A structural change in the mutant AQP2 that doesn’t let it pass through the cell’s quality control system.