Pharmacologic Chaperones as a Potential Treatment for X-linked Nephrogenic Diabetes Insipidus

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Title: Pharmacologic Chaperones as a Potential Treatment for X-linked Nephrogenic Diabetes Insipidus
Authors: Bernier, Virginie; Morello, Jean-Pierre; Zarruk, Alexandro; Debrand, Nicolas; Salahpour, Ali; Lonergan, Michele; Arthus, Marie-Francoise; Laperriere, Andre; Brouard, Remi; Bouvier, Michel; Bichet, Daniel G.
Publisher: Journal of American Society of Nephrology
Date Published: January 01, 2006
Reference Number: 701
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In many mendelian diseases, some mutations result in the synthesis of misfolded proteins that cannot reach a transport-competent conformation. In X-linked nephrogenic diabetes insipidus, most of the mutant vasopressin 2 (V2) receptors are trapped in the endoplasmic reticulum and degraded. They are unable to reach the plasma membrane and promote water reabsorption through the principal cells of the collecting ducts. Herein is reported two types of experiments: In vivo studies to assess clinically a short-term treatment with a nonpeptide V1a receptor antagonist (SR49059) and in vitro studies in cultured cell systems. In patients, SR49059 decreased 24- h urine volume (11.9 +/- 2.3 to 8.2 +/- 2.0 L; P = 0.005) and water intake (10.7 +/- 1.9 to 7.2 +/- 1.6 L; P < 0.05). Maximum increase in urine osmolality was observed on day 3 (98 +/- 22 to 170 +/- 52 mOsm/kg; P = 0.05). Sodium, potassium, and creatinine excretions and plasma sodium were constant throughout the study. In vitro studies indicate that the nonpeptide V1a receptor antagonist SR49059 and the V1a/V2 receptor antagonist YM087 (Conivaptan) rescued cell surface expression and function of mutant V2 receptors. Mutant V2 receptors with nonsense mutations were not affected by the treatment. Misfolded V2 receptor mutants were rescued in vitro and also in vivo by nonpeptide antagonists. This therapeutic approach could be applied to the treatment of several hereditary diseases that result from errors in protein folding and kinesis.
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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)

Congenital X-linked Nephrogenic Diabetes Insipidus (NDI) is the result of mutations of the vasopressin 2 receptor (V2R) gene that result in mutant V2R proteins. These are unable to perform their function of providing a receptor site for the anti-diuretic hormone, arginine vasopressin (AVP), to bind to. As a result, the NDI patient experiences a decreased ability to reabsorb water through the kidneys. Clinical symptoms of NDI include excessive thirst and urinating, and dilute urine marked by a low concentration of specific chemical compounds such as sodium and potassium.

More than half of the mutations that result in X-linked NDI are of a type called missense mutations. These mutations result in a different amino acid than normal being placed in the chain of amino acids in the protein that the gene codes for. Most of the mutant V2R proteins are resulting from a missense mutation are misfolded. That is, they do not mature into the shape of a normal V2R. As such, they are trapped in the endoplasmic reticulum (ER), a part of the cell that acts as a quality control center for the cell. The ER holds the mutant V2R, and then dismantles it.

Researchers have found, however, that most of these mutant V2Rs trapped in the ER could actually perform their intended function of acting as a site to which AVP binds if they were only able to escape the ER and travel to the cell membrane. Thus, researchers conceived of the idea of trying to rescue mutant V2Rs from the ER through the use of chemical or pharmacological chaperones, agents that could bind with the mutant V2Rs in the ER, allowing them to pass the quality control specifications of the ER and travel to the cell membrane.

Bernier, et al., undertook two types of experiments that tested the efficacy of a nonpeptide V1a receptor antagonist SR49059 as a pharmacologic chaperone to rescue specific V2R mutants. In their first experiment, the researchers tested five adult male patients. Three had the V2R mutation R137H; one had the W164S V2R mutation; and one had the inframe V2R mutation 185_193del. All had life long severe expressions of the classic NDI symptoms, and all received the same treatment over the three day course of the experiment. Two of the patients received a one week treatment period six weeks after the initial experiment.

The results of the initial experiment indicated that SR49059 significantly decreased 24 hour urine volume and water intake from day 1 to day 3. Plus, the patients’ urine became more concentrated, while the measured variables in the blood chemistry remained constant. The two patients who received the later, one-week treatment experienced reduced urine volumes and increased urine concentrations through the treatment week.

In the second type of experiment devised by the researchers, they used laboratory cell cultures to test the ability of both SR49059 and YM087 to bring two different V2R missense mutations, one in-frame deletion V2R mutation and one nonsense V2R mutation to the cell surface and function there. The research team treated laboratory cell cultures that contained the mutations with with SR49059 for 16 hours. This increased the ability of all the mutations except the nonsense mutation to reach the cell membrane. The three mutations that reached the cell membrane increased the production of cyclic adenosine monophosphate (cAMP). This indicated that the mutations were able to perform their function of binding with AVP, thus indicating that SR49059 does act as a pharmacologic chaperone to the three different mutated V2R proteins.

Because clinical development of SR49059 was halted due to possible side-effects, the researchers tested YM087’s ability as a chaperone on 19 V2R mutants in laboratory cell cultures and found that it promoted cell surface expression and facilitated AVP-mediated cAMP production in ten of them. YM087 could not help the tested nonsense mutations.

At present, there exists only indirect methods to mitigate NDI symptoms. The results of this research presents the possibility of specific treatment. Because approximately 50% of X-linked NDI involves missense mutations, and pharmacologic agents such as SR49059 have a significant positive impact on them, this could indicated a potential general treatment of NDI that results from protein misfoldiing. Bernier, et al., speculate that SR49059 may make its way to the ER. Once there, the misshapen V2R protein may wrap itself around the SR49059 and thus escape the ER.