title: Evidence for a role of tight junctions in regulating sodium permeability in zebrafish (Danio rerio) acclimated to ion-poor water.
abstract: 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.
authors: Department of Biology, University of Ottawa, Ottawa, ON, Canada. wkwong@uottawa.ca
authors: Raymond W M
authors: RW
authors: Kwong
authors: Yusuke
authors: Y
authors: Kumai
authors: Steve F
authors: SF
authors: Perry
mesh_terms: D000064:Acclimatization / Q000502:physiology*; D000818:Animals; D015153:Blotting, Western; D057167:Claudins / Q000378:metabolism*; D017931:DNA Primers / Q000235:genetics; D005618:Fresh Water / Q000737:chemistry*; D007150:Immunohistochemistry; D007685:Kidney Tubules, Collecting / Q000378:metabolism; D010539:Permeability; D011092:Polyethylene Glycols; D060888:Real-Time Polymerase Chain Reaction; D012964:Sodium / Q000032:analysis / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D019108:Tight Junctions / Q000378:metabolism / Q000502:physiology*; D015027:Zebrafish / Q000502:physiology*
chemical_list: D057167:Claudins; D017931:DNA Primers; D011092:Polyethylene Glycols; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase