Claudin-6, -10d, and -10e contribute to seawater acclimation in the euryhaline puffer fish Tetraodon nigroviridis

Abstract

Abstract Expression profiles of claudin-6, -10d, and -10e in the euryhaline teleost fish Tetraodon nigroviridis revealed claudin-6 in brain, eye, gill, and skin tissue, while claudin-10d and -10e were found in brain, gill and skin only. In fishes, the gill and skin are important tissue barriers that interface directly with surrounding water, but these organs generally function differently in osmoregulation. Therefore, roles for gill and skin claudin-6, -10d, and -10e in the osmoregulatory strategies of Tetraodon were investigated. In the gill epithelium claudin-6, -10d, and -10e co-localized with Na+-K+-ATPase immunoreactive (NKA-ir) ionocytes and differences in sub-cellular localization could be observed in hypo- (freshwater, FW) versus hyperosmotic (seawater, SW) environments. Claudin-10d and -10e abundance increased in the gills of fish acclimated to SW versus FW while claudin-6 abundance decreased in SW. Taken together with our knowledge of claudin-6 and -10 function in other vertebrates, data support the idea that in SW-acclimated Tetraodon, these claudins are abundant in gill ionocytes where they contribute to the formation of a Na+ shunt and 'leaky' epithelium, both of which are characteristic of salt secreting SW fish gills. Skin claudin-10d and -10e abundance also increased in fish acclimated to SW versus those in FW, but so did claudin-6. In skin, claudin-6 was found to co-localize with NKA-ir cells, but claudin-10d and -10e did not. This study provides direct evidence that the gill epithelium contains salinity responsive tight junction proteins that are abundant primarily in ionocytes. These same proteins also appear to play a role in the osmoregulatory physiology of the epidermis.