Methyl-β-Cyclodextrin Induces Vasopressin-Independent Apical Accumulation of Aquaporin-2 in the Isolated, Perfused Rat Kidney
| Title: | Methyl-β-Cyclodextrin Induces Vasopressin-Independent Apical Accumulation of Aquaporin-2 in the Isolated, Perfused Rat Kidney |
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| Authors: | Russo, Leileata M.; McKee, Mary; Brown, Dennis |
| Publisher: | American Journal of Physiology: Renal Physiology |
| Date Published: | July 01, 2006 |
| Reference Number: | 707 |
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)
The aquaporin 2 (AQP2) protein plays a major role in the kidney’s ability to reabsorb water and thereby concentrate urine. Normally, AQP2 is activated by a specific molecular sequence to move from the interior of the principal cells in the kidney collecting duct to the plasma membrane of those cells: the hormone, arginine vasopressin (AVP) links with the vasopressin 2 receptors (V2R) on the cell membrane. This increases cellular level of cyclic adenosine monophosphate (cAMP) , which activates protein kinase A, resulting in the addition of a phosphate group to a specific amino acid residue at a specific location of the AQP2 protein. This is what signals AQP2 to travel to the cell membrane. After AQP2 fulfills its function of allowing water to pass into the cell, it returns to the cell interior (a process called endocytosis). A protein called clathrin plays an essential role in AQP2 endocytosis.
In NDI, either as a result of mutations in the V2R gene or the AQP2 gene, AQP2 is not able to make the trip from the cell interior to the cell membrane. Thus, the kidney is not able to optimally reabsorb water to thereby concentrate urine.
Russo, et al., note that previous research indicates that AQP2 is not completely dependent on the AVP/V2R signaled sequence to make its trip to and from the cell membrane. That is, a number of AQP2s make the round trip even when not signaled to do so by AVP. This team’s previous research showed that significant numbers of AQP2s can be influenced to stay at the cell membrane location through the use of a drug called methyl-B-cyclodextrin (mBCD). mBCD prevents the formation and budding of the clathrin coated pits which normally bring the AQP2 back to the cell interior. This results in the rapid accumulation of AQP2 at the cell membrane.
The researchers' previous research arrived at this result by experimentation with laboratory cell cultures. Here, they sought to find if mBCD would have the same effect in the principal cells of a functioning, intact rat kidney. This did not mean they experimented on live rats because mBCD is toxic. Instead, the researchers used a technique called the isolated, perfused kidney technique which allows the rat kidney to function on an artificial closed circuit.
The team documented that AQP2 in the principal cells of the rat kidney collecting duct shift from being located primarily in the cell interior to being located in the cell membrane following a one hour bath of mBCD. Thus, the researchers could declare that mBCD, by inhibiting the endocytic movement of AQP2, caused a build up of AQP2 in the cell membrane of a functioning, intact rat kidney independent of the molecular sequence initiated through the contact of AVP with V2R. This finding held even when the researchers used kidneys from a type of rat that does not produce AVP at all.
Though challenges remain such as the toxicity of mBCD and the fact that no significant change in urine concentration occurred in the test kidneys despite the APQ2 increase, this research indicates that specifically blocking the return of AQP2 to the cell interior may be a promising form of NDI therapy.