Loss of Calcineurin Aα Results in Altered Trafficking of AQP2 and in Nephrogenic Diabetes Insipidus

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Title: Loss of Calcineurin Aα Results in Altered Trafficking of AQP2 and in Nephrogenic Diabetes Insipidus
Authors: Gooch, Jennifer L.; Guler, Rebecca L.; Barnes, Jeffrey L.; Toro, Juan J.
Publisher: Journal of Cell Science
Date Published: June 15, 2006
Reference Number: 705
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The serine/threonine phosphatase calcineurin is an important signaling molecule involved in kidney development and function. One potential target of calcineurin action is the water channel aquaporin 2 (AQP2). In this study, we examined the effect of loss of calcineurin Aalpha (CnAalpha) on AQP2 function in vivo. CnAalpha null mice were found to have defective post-natal urine-concentrating ability and an impaired urine-concentrating response to vasopressin. Expression of AQP2 is normal but, paradoxically, vasopressin-mediated phosphorylation of the channel is decreased compared with wild-type littermates and there is no accumulation of AQP2 in the apical membrane. Calcineurin protein and activity was found in innermedullary collecting duct vesicles, and loss of calcineurin expression and activity was associated with a loss of AQP2 in the vesicle fraction. As such, the lack of vasopressin-mediated phosphorylation of AQP2 might be the result of a defect in normal trafficking of AQP2 to apical-targeted vesicles. Likewise, treatment of wild-type mice with cyclosporin A to inhibit calcineurin produces a similarly impaired urine-concentrating response to vasopressin and alterations in AQP2 phosphorylation and trafficking. These experiments demonstrate that, CnAalpha is required for normal intracellular trafficking of AQP2 and loss of calcineurin protein or activity disrupts AQP2 function.
http://jcs.biologists.org/cgi/content/full/119/12/2468

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)

Aquaporin 2 (AQP2) is a type of protein that travels from the cell interior - in tiny sacs called vesicles – to the cell membrane. Once there, it acts as a channel through which water can enter the cell. AQP2’s movement to the cell membrane to the cell interior is a process called exocytosis. It is a result of the following chemical sequence: the hormone, arginine vasopressin (AVP) links with a vasopressin 2 receptor (V2R) protein at the cell membrane. This stimulates adenylyl cyclase, and this leads to the cellular levels of cyclic adenosine monophosphate (cAMP) to elevate. This activates protein kinase A (PKA). PKA then attaches a group of phosphates to a specific amino acid residue in the chain of amino acids that comprises the AQP2 (i.e., it phosphorylates the AQP2 at serine amino acid residue that is the 256th amino acid in the chain of amino acids that is AQP2). AQP2 then enters vesicles and travels to the membrane.

Gooch, et al., noted that recent research has revealed that the enzyme, calcineurin (CnAα) plays a part in the cellular movement, called trafficking, of AQP2. Their own research indicates that CnAα and AQP2 are both found in the principal cells of the kidney collecting duct. When the researchers inhibited the function of CnAα by using the drug, cyclosporin, they noted a decrease in phosphorylation of AQP2 and significantly fewer number of AQP2s in the cell membrane.

Gooch, et al.’s, current research was designed to clarify the role of CnAα in AQP2 trafficking in the kidneys of living mice. They experimented on mice designed to have no CnAα in them so they could examine the effect of the loss of CnAα on AQP2 function in the mice. The researchers found that these mice could not concentrate their urine. These mice had normal AQP2 levels, but their AQP2 had decreased levels of phosphorylation. These mice also showed less AQP2 accumulation in the kidney collecting duct cells than normal.

The researchers then took normal mice and treated them with the CnAα inhibitor, cyclosporin A. The treated mice expressed similar symptoms as mice lacking CnAα: impaired urine concentrating ability, reduced AQP2 phosphorylation and reduced numbers of AQP2 in the cell membrane. The research team concluded that CnAα is required for AQP2 to be able to move to the cell interior to the cell membrane, the site where it must be if it is to perform its function as a channel through which water flows into the cell.