Cloning of an Aquaporin Homologue Present in Water Channel Containing Endosomes of Toad Urinary Bladder

Title: Cloning of an Aquaporin Homologue Present in Water Channel Containing Endosomes of Toad Urinary Bladder
Authors: Hammond, Timothy G.; Harris, H. William; Siner, J.; Paredes, A.; Hosselet, Christine A.; Strange, Kevin
Publisher: American Journal of Physiology
Date Published: January 01, 1996
Reference Number: 262
Regulation of total body water balance in amphibians by antidiuretic hormone (ADH) contributed to their successful colonization of terrestrial habitats approximately 200-300 million years ago. In the mammalian kidney, ADH modulates epithelial cell apical membrane water permeability (Pf) by fusion and retrieval of cytoplasmic vesicles containing water channel proteins called aquaporins (AQPs). To determine the role of AQPs in ADH-elicited Pf in amphibians, we have identified and characterized a unique AQP from Bufo marinus called AQP toad bladder (AQP-TB). AQP-TB possesses many structural features common to other AQPs, AQP-TB is expressed abundantly in ADH-responsive tissues, including toad urinary bladder and skin as well as lung, skeletal muscle, kidney, and brain. In a manner identical to that reported for the mammalian ADH-elicited water channel AQP2, AQP-TB expression is increased significantly by intervals of dehydration or chronic ADH stimulation. However, expression of AQP-TB protein in Xenopus laevis oocytes does not significantly increase oocyte Pf. The lack of expression of functional AQP-TB water channels in oocytes may result from intracellular sequestration of AQP-TB due to the presence of a YXRF sequence motif present in its carboxyterminal domain.

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)

Overall regulation of body water balance is essential for survival both in mammals and amphibians. In both these groups, the antidiuretic hormone (ADH) plays an important role in body water balance by allowing the body to reabsorb body water and concentrate urine.

ADH accomplishes this by signaling certain aquaporins (AQPs) to insert themselves in specific cell membranes to make them more water permeable. AQPs are proteins that either are located in cell membranes or travel to and insert themselves in membranes. In membranes, they act as channels through which body water can flow, thus making the membranes much more permeable than they would be without them. AQPs are found throughout the mammalian and amphibian body, with specific AQPs locating in specific places. At present, there are eight known AQPs in the human body.

In humans, ADH signals AQP-2 to insert itself in the apical membrane of the principal cells of the kidney collecting duct. Once inserted, they greatly increase the apical membranes permeability, allowing the kidney to reabsorb water and concentrate urine.

Siner, et al., identified and cloned a new AQP found in the toad urinary bladder. This newly discovered AQP, called AQP toad bladder (AQP-TB), possesses all the structural features of the AQP family. That does not mean it is identical to other types of the AQP family. Each AQP type shares the same general structure as the others, but the sequence of proteins that make up each type differs somewhat from the others. One thing about AQP-TB that distinguishes it from other AQP types is an amino acid motif in its tail end. This motif is a sequence of four amino acids, tyrosine-leucine-arginine-phenylalanine (YLRF).

This motif promotes high-efficiency internalization of protein from the plasma membrane. So, other membrane proteins that have this YLRF motif get recycled quickly and efficiently from the cell membrane surface to the interior of the cell.

AQP-TB is found in the toad bladder, skin, lung, skeletal muscle, kidney and brain. AQP-TB in granular cells of the toad urinary bladder functions much as AQP2s do in the principal cells of the kidney collecting duct. Siner, et al., tracked the movement and function of AQP-TB in a laboratory cell culture of a type of cell called oocyte. As does AQP2 in humans, AQP-TB increases in numbers following either periods of dehydration or chronic ADH stimulation. AQP-TB, as does AQP2, resides within the cell in little sacs called water channels vesicles (WCVs).

There is a major difference, however. Whereas AQP2 greatly increases oocyte cell membrane water permeability, AQP-TB does not. The authors conclude that, although AQP-TB is expressed by oocytes, (i.e. these cells can generate AQP-TB once AQP-TB clone cRNA is injected into them), AQP-TB does not function as a plasma membrane water channel. More research will be needed to determine whether AQP-TB is a nonwater channel or whether AQP-TB, due to its YLRF motif, is quickly removed and sequestered from the oocyte plasma membrane before it can function as a water channel.