Prolactin and cortisol regulate branchial claudin expression in Japanese medaka

Several gill claudin (Cldn) tight junction proteins in Japanese medaka are regulated by salinity (cldn10 paralogs and cldn28b), while others are constitutively expressed (cldn27a, cldn28a and cldn30c). The role of the endocrine system in this regulation has yet to be understood. The in vitro effects of cortisol and prolactin on cldn expression in gill explant cultures were investigated in medaka. ncc2b and cftr were used as markers of specific ionocytes associated with freshwater- and seawater-acclimation, respectively. Concentration-response experiments were performed by overnight incubation with 0, 0.1, 1 and 10μgmL^-1 cortisol or 0, 0.01, 0.1 and 1μgmL^-1 ovine prolactin. Cortisol significantly up-regulated cftr, ncc2b, cldn10 paralogs, cldn27a and cldn30c from 1.2- to 5-fold control levels at 10μgmL^-1. Cortisol had no effect on cldn28a and cldn28b. Prolactin had a concentration-dependent effect, decreasing expression of cftr (1μgmL^-1, 2.2-fold) while increasing ncc2b (from 0.1μgmL^-1, 6-7-fold). Prolactin up-regulated expression of 3 cldns: cldn28b (0.1 and 1μgmL^-1), cldn10c and cldn10f (1μgmL^-1), with up to 2-, 2.5- and 2-fold of control level, respectively. A combination experiment with both hormones showed that they act in synergy on cldn28b and have an additive effect on cftr, ncc2b, cldn10c and cldn10f. Our results showed that cortisol and prolactin are essential to maintain the expression of specific branchial claudins. This work also provides evidence that both hormones act directly on gill of medaka to modulate determinants of paracellular ion movement.