Evidence for a role of tight junctions in regulating sodium permeability in zebrafish (Danio rerio) acclimated to ion-poor water

Freshwater teleosts are challenged by diffusive ion loss across permeable epithelia including gills and skin. Although the mechanisms regulating ion loss are poorly understood, a significant component is thought to involve paracellular efflux through pathways formed via tight junction proteins. The mammalian orthologue (claudin-4) of zebrafish (Danio rerio) tight junction protein, claudin-b, has been proposed to form a cation-selective barrier regulating the paracellular loss of Na⁺. The present study investigated the cellular localization and regulation of claudin-b, as well as its potential contribution to Na⁺ homeostasis in adult zebrafish acclimated to ion-poor water. Using a green fluorescent protein-expressing line of transgenic zebrafish, we found that claudin-b was expressed along the lamellar epithelium as well as on the filament in the inter-lamellar regions. Co-localization of claudin-b and Na⁺/K⁺-ATPase was observed, suggesting its interaction with mitochondrion-rich cells. Claudin-b also appeared to be associated with other cell types, including the pavement cells. In the kidney, claudin-b was expressed predominantly in the collecting tubules. In addition, exposure to ion-poor water caused a significant increase in claudin-b abundance as well as a decrease in Na⁺ efflux, suggesting a possible role for claudin-b in regulating paracellular Na⁺ loss. Interestingly, the whole-body uptake of a paracellular permeability marker, polyethylene glycol-400, increased significantly after prolonged exposure to ion-poor water, indicating that an increase in epithelial permeability is not necessarily coupled with an increase in passive Na⁺ loss. Overall, our study suggests that in ion-poor conditions, claudin-b may contribute to a selective reduction in passive Na⁺ loss in zebrafish.