Functional Rescue of Vasopressin V2 Receptor Mutants in MDCK Cells by Pharmacochaperones: Relevance to Therapy of Nephrogenic Diabetes Insipidus
| Title: | Functional Rescue of Vasopressin V2 Receptor Mutants in MDCK Cells by Pharmacochaperones: Relevance to Therapy of Nephrogenic Diabetes Insipidus |
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| Authors: | Robben, Joris; Sze, Mozes; Knoers, Nine; Deen, Peter M.T. |
| Publisher: | American Journal of Physiology: Renal Physiology |
| Date Published: | August 22, 2006 |
| Reference Number: | 709 |
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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)
When the hormone, arginine vasopressin (AVP), reaches the cell membrane, it attaches to the vasopressin-2 receptor (V2R). This initiates a molecular cascade involving, among other things, cyclic adenosine monophosphate (cAMP). This cascade results in the kidney reabsorbing water. The V2R does not live on the cell membrane, but must develop into a mature and proper form through synthesis in order to travel through the cell interior to the cell membrane where it then performs its function of binding with AVP.
In congenital X-linked NDI, mutations in the V2R gene produce mutant V2R proteins that are unable to perform their function. There are over 200 V2R mutations that result in NDI, and around half of those are of the type called missense mutations. These mutations result in improperly folded, immature V2R proteins which are retained in the cell interior in a section called the endoplasmic reticulum (ER). Missense V2R mutants have been shown to be able to perform their function (i.e., bind with AVP), but because they have not synthesized to full maturity and are misshapen, they are retained in the ER and dismantled by cellular machinery.
Research has shown that there exist a number of cell permeable antagonists called pharmacochaperones that can enter the cell, bind with some types of missense V2R mutations in the ER, and allow them to reach the cell membrane where they are able to perform their function. This research holds promise, but questions remain. Pharmacochaperones must be able to bind with the mutant V2R and bring it to the cell surface. But they also must be able to release their contact with the V2R so that AVP can bind with the V2R in order to initiate the necessary chemical cascade that results in water reabsorption and urine concentration. Further, low concentrations of the pharmacochaperones should be enough to perform the rescue as high concentrations could lead to side effects. Finally, the pharmacochaperones should remain effective in the cell as long as possible.
Robben, et al., performed a series of experiments designed to ascertain which of four promising pharmacochaperones is most likely to relieve NDI in patients by meeting the above requirements. They introduced nine different mutant V2R proteins into cell cultures designed to mimic the principal cells of the kidney collecting duct. The researchers tested each pharmacochaperone to determine the extent to which it could bring the mutants to the cell surface; how well the V2R functioned once on the surface; at what concentration did the chaperones have their effect; and how long the effect lasted.
The researchers found that all four of the pharmacochaperones they tested – SR49049 (SR4), OPC31260 (OPC3), OPC41061(OPC4), and SR121463B (SR1) – were able to help eight out of nine different V2R mutants develop to a maturity and shape that allowed them to move from the ER to the cell membrane. The team observed a correlation between the concentration of the pharmacochaperone used, along with its affinity for the V2R mutant, and the extent to which the V2R mutant matured and reached the cell membrane.
Affinity is the tendency of a compound to combine by chemical reaction with compounds of unlike compositions. To serve as an effective and safe treatment, a pharmacochaperone should have a high affinity for a mutant receptor, i.e., it should have a strong tendency to combine with it. Further, it should be easily displaced by AVP once it has rescued the mutant V2R and helped it get to the cell membrane. At high concentrations, SR4 achieved the greatest functional rescue of the four phamacochaperones, even though it brought the fewest mutant V2Rs to the surface. This was because SR4 was more easily displaced by AVP than the others. In other words, even though SR4 brought fewer mutant V2Rs to the cell surface than the others (because it was easier for AVP to take the place of SR4), more of the chemical cascade required to initiate water reabsorption is able to take place.
However, the picture changes when low concentrations of pharmacochaperones are used on the cell cultures. Under these conditions, OPC3 and OPC4, which have a high affinity for the tested V2R mutants, were able to functionally rescue the mutants whereas SR4 was not.
The researchers conclude that, taking the fact that using low concentrations of these pharmacochaperones is in the best interests of the NDI patient, OPC3 and OPC4 show the most promise as potential treatment options for a significant percentage of X-linked NDI patients.