The effects of tidally changing salinities on branchial tight junction protein gene expression in tilapia

To maintain hydromineral balance during transitions between different environmental salinities, euryhaline teleosts adjust the permeability characteristics of their surface epithelia to align with ambient conditions. Tight junctions (TJs), which form the apical-lateral barriers between epithelial cells, control the paracellular movement of solutes and water. Consequently, euryhaline species must efficiently reorganize branchial TJs when acclimating to freshwater (FW) and seawater (SW) environments. However, little is known about how TJs are regulated under tidally changing salinities. In the current study, we employed RNA-Seq to identify several branchial TJ transcripts that are differentially expressed in Mozambique tilapia (Oreochromis mossambicus) acclimated to FW, SW, or a tidal regime (TR). The most abundant and differentially expressed TJ transcripts, which included claudins (cldn-4 like, cldn7b, cldn23a, and cldn-like ZF-A89), occludins (oclna and oclnb), and TJ proteins (tjp1a and tjp3), were further investigated by qPCR. Generally, the expression of TJ transcripts varied to a greater extent between the FW and SW phases of the TR (denoted TF and TS, respectively) than between steady-state FW and SW conditions. Furthermore, TJ transcripts were usually upregulated in TF compared to TS or SW controls. In contrast, the gene expression of branchial ion transporters did not change as markedly under a TR. Together, these results suggest that TJs play a crucial role in regulating the permeability of branchial epithelia by preventing ion loss in hyposmotic conditions, particularly during short-term tidally changing salinities.