Kwong, R.W., and Perry, S.F. (2013) The tight junction protein claudin-b regulates epithelial permeability and sodium handling in larval zebrafish, Danio rerio. Am. J. Physiol. Regul. Integr. Comp. Physiol.. 304(7):R504--513.
The functional role of the tight junction protein claudin-b in larval zebrafish (Danio rerio) was investigated. We showed
that claudin-b protein is expressed at epithelial cell-cell contacts on the skin. Translational gene knockdown of claudin-b
protein expression caused developmental defects, including edema in the pericardial cavity and yolk sac. Claudin-b morphants
exhibited an increase in epithelial permeability to the paracellular marker polyethylene glycol (PEG-4000) and fluorescein
isothiocyanate-dextran (FD-4). Accumulation of FD-4 was confined mainly to the yolk sac and pericardial cavity in the claudin-b
morphants, suggesting these regions became particularly leaky in the absence of claudin-b expression. Additionally, Na+ efflux was substantially increased in the claudin-b morphants which contributed to a significant reduction in whole-body
Na+ levels. These results indicate that claudin-b normally acts as a paracellular barrier to Na+. Nevertheless, the elevated loss of Na+ in the morphants was compensated by an increase in Na+ uptake. Notably, we observed that the increased Na+ uptake in the morphants was attenuated in the presence of the selective Na+/Cl--cotransporter (NCC) inhibitor metolazone, or during exposure to Cl--free water. These results suggested that the increased Na+ uptake in the morphants was at least in part mediated by NCC. Furthermore, treatment with an H+-ATPase inhibitor bafilomycin A1 was found to reduce Na+ uptake in the morphants, suggesting that H+-ATPase activity was essential to provide a driving force for Na+ uptake. Overall, the results suggest that claudin-b plays an important role in regulating epithelial permeability and Na+ handling in zebrafish.