Molecular and cellular diversity of aquaporin in a water-adaptation of the amphibians

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We cloned cDNAs encoding three distinct AQPs (AQP-h1,AQP-h2 and AQP-h3) from the ventral skin of the tree frog, Hyla japonica. AQP-h1 mRNA is expressed in a wide variety of tissues, and AQP-h2 in several tissues including anti-diuretic hormone-dependent tissues, but AQP-h3 mRNA exists only in the ventral skin. An immunofluorescence study showed that the AQP-h3 protein was only localized in the pelvic ventral skin. In the pelvic skin, AQP-h3 protein was more intense in the upper layer of the stratum granulosum, and was localized in both the apical and basolateral plasma membranes of the granular cells. AQP-h3 in the ventral skin may be essential in the land adaptation of the tree frog, but AQP-h2 protein was detected in the same sites as AQP-h3 in the plasma membrane of the granular cells in the adult ventral skin. In the urinary bladder, AQP-h2 protein was observed in the cytoplasm of the granular cells. Incubation of adult ventral skin with vasotocin enhanced the labeling density of both AQP-h2 and AQP-h3 in the apical plasma membrane in the skin. Taken together, the present data suggest that AQP-h3 is an intrinsic AQP in the ventral skin, and that it maintains a high permeability to water in the ventral skin in concert with AQP-h2 under the regulation of vasotocin. In the urinary bladder, AQP-h2 plays a pivotal role in the reabsorption of water. It is of interest that both AQP-h2-and AQP-h3-like proteins are expressed in the ventral skin of the terrestrial frog Bufo japonica and tree-adapted frog Hyla japonica. On the other hand, expression of AQP-h3-like gene in the ventral skins is observed in all the frogs from aquatic species to terrestrial dwellers, but no AQP-h2-like protein is expressed in the ventral skin in the aquatic and semi-terrestrial-adapted frogs.