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First published online December 26, 2008
Journal of Experimental Biology 212, 287-296 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.022822
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Occludin and hydromineral balance in Xenopus laevis

Helen Chasiotis* and Scott P. Kelly

Department of Biology, York University, Toronto, Ontario, Canada M3J 1P3

* Author for correspondence (e-mail: helench{at}yorku.ca)

Accepted 18 November 2008

To investigate the response of the tight junction (TJ) protein occludin to environmental change in an anuran amphibian, we examined occludin tissue distribution, immunolocalization and alterations in mRNA expression in African clawed frogs (Xenopus laevis) acclimated to brackish water (BW) conditions (from freshwater to 2{per thousand}, 5{per thousand} or 10{per thousand} salt water). Occludin mRNA is widely expressed in Xenopus and is abundant in tissues involved in regulating salt and water balance, such as the gastrointestinal (GI) tract, kidney and urinary bladder. Immunohistochemical analyses revealed strong occludin immunolabelling in the apicolateral region of epithelia lining the GI tract and mRNA expression increased along the longitudinal axis of the gut. In kidney tissue, occludin was differentially expressed on the luminal side of the nephron tubule, appearing in the distal tubules and collecting ducts only. In response to BW acclimation, Xenopus exhibited a significant loss of tissue water as well as salinity-dependent elevations in serum osmolality as a result of increased urea levels followed by elevated serum Na+ and Cl– levels. Tissue-specific alterations in the ionomotive enzyme Na+,K+-ATPase were also observed in Xenopus in response to BW acclimation. Most notably, Na+,K+-ATPase activity in the rectum increased in response to elevated environmental salt concentrations while renal activity decreased. Furthermore, acclimation to BW caused tissue-specific and salinity-dependent alterations in occludin mRNA expression within select Xenopus osmoregulatory organs. Taken together, these studies suggest that alterations in occludin, in conjunction with active transport processes, may contribute to amphibian hydromineral homeostasis during environmental change.

Key words: tight junction, occludin, hydromineral balance, Na+,K+-ATPase, paracellular permeability, osmoregulation, amphibian


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