MAL Decreases the Internalization of the Aquaporin-2 Water Channel

Title: MAL Decreases the Internalization of the Aquaporin-2 Water Channel
Authors: Kamsteeg, Erik-Jan; Duffield, Amy S.; Konings, Irene B. M.; Spencer, Joanna; Pagel, Phillip; Deen, Peter M.T.; Caplan, Michael J.
Publisher: Proceedings of the National Academy of Sciences of the United States of America
Date Published: October 16, 2007
Reference Number: 726
Body water homeostasis depends critically on the hormonally regulated trafficking of aquaporin-2 (AQP2) water channels in renal collecting duct epithelial cells. Several types of posttranslational modifications are clearly involved in controlling the distribution of AQP2 between intracellular vesicles and the apical plasma membrane. Little is known, however, about the protein interactions that govern the trafficking of AQP2 between these organelles. MAL is a detergent-resistant membrane-associated protein implicated in apical sorting events. We wondered, therefore, whether MAL plays a role in the regulated trafficking of AQP2 between intracellular vesicles and the apical surface. We find that AQP2 and MAL are coexpressed in epithelial cells of the kidney collecting duct. These two proteins interact, both in the native kidney and when expressed by transfection in cultured cells. The S256-phosphorylated form of AQP2 appears to interact more extensively with MAL than does the water channel protein not phosphorylated at this serine. We find that MAL is not involved in detergent-resistant membrane association or apical delivery of AQP2 in LLC-PK(1) renal epithelial cells. Instead, MAL increases the S256 phosphorylation and apical surface expression of AQP2. Furthermore, internalization experiments show that MAL induces surface expression of AQP2 by attenuating its internalization. Thus, the involvement of MAL in the cell surface retention of apical membrane proteins could play an important role in regulated absorption and secretion in transporting epithelia.
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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)

The Aquaporin 2 (AQP2) water channel protein plays a critical role in the kidney’s ability to maintain water balance. When the hormone arginine vasopressin (AVP) forms a bond with the vasopressin 2 receptor (V2R) located on the membrane of the principal cells of the kidney collecting duct, it initiates a molecular cascade that results in AQP2 being transferred in tiny sacs called vesicles from the principal cell interior to a section of the cell membrane called the apical membrane. Once in the apical membrane, AQP2 acts as a channel through which water may enter the cell. When AVP unbinds from V2R, it signals the AQP2 to return to the cell interior from the apical membrane.

Myelin and lymphocyte-associated protein (MAL) is a protein that may be involved in the transfer of proteins from the cell interior to the cell membrane. Caplan, et al., investigated to determine if MAL plays a role in the movement of AQP2 to the apical membrane. First, the researchers confirmed that MAL and AQP2 coexist in the kidney principal cells. Second, they determined that the two proteins interact in the principal cells. Interestingly, MAL prefers to interact with AQP2 that has been phosphorylated, that is, AQP2 that has had a phosphate group added to it, specifically to the serine amino acid residue that is the 256th amino acid residue in the chain of amino acids that comprise the AQP2. Phosphorylation is part of the molecular cascade that occurs when AVP and V2R bind.

By examining AQP2 delivery in cells that do not contain MAL, the research team discovered that AQP2 still was able to transfer from the cell interior to the apical membrane. Nor did the researchers find that MAL helps the AQP2 into the transport vesicles. These findings indicate that MAL is not involved in the translocation of AQP2. However, the researchers did find that when MAL and AQP2 are both expressed in a laboratory cell culture, it results in a two fold increase in AQP2 phosphorylation.

Significantly, the final stages of Caplan, et al’s. research indicated that MAL, though not involved in the delivery of AQP2 to the apical membrane, it slows the rate at which AQP2 returns to cell interior. This allows AQP2 to function as a channel through which water passes for a longer time than it could were MAL not present with it.