Prolactin, cortisol, and extracellular osmolality regulate cftr, ostf1, and sgk1 in tilapia ionocytes

Euryhaline fishes can inhabit salinities ranging from fresh water (FW) to seawater (SW), in part because their endocrine system aligns the ion-transporting capacities of branchial ionocytes with the external environment. Ionocytes also directly sense extracellular osmolality and adjust their functions accordingly; therefore, this study investigated the interplay between osmotic conditions and osmoregulatory hormones in Mozambique tilapia (Oreochromis mossambicus) to further elucidate the basis of euryhalinity. We sought to determine whether prolactin (Prl) supports FW acclimation by counteracting osmotic and hormonal signals that initiate branchial responses to SW environments.

We first combined hypophysectomy, hormone replacement, RNA-Seq, and qPCR to identify cystic fibrosis transmembrane conductance regulator (cftr), osmotic stress transcription factor 1 (ostf1), and serum- and glucocorticoid-inducible kinase 1 (sgk1) as Prl-repressed genes. We then leveraged a series of in vivo and in vitro experimental paradigms to characterize their regulation by environmental salinity, Prl, cortisol, and extracellular osmolality.

Our findings indicate that 1) Prl's capacity to antagonize cortisol-stimulated cftr expression depends on extracellular osmotic conditions; 2) Prl and cortisol play opposing roles in regulating branchial ostf1 expression; 3) Sgk1 is expressed in 'SW-type' ionocytes, where interactions among Prl, cortisol, and osmotic conditions influence its expression; and 4) Prl promotes the expression of Na⁺/Cl^- cotransporter 2 and Clc family Cl^- channel 2c, as shown previously.

The combined actions of osmotic stimuli, Prl, and cortisol shape the branchial expression of cftr, ostf1, and sgk1. While hyperosmotic extracellular conditions and cortisol promote the activation of these genes during SW acclimation, Prl supports FW adaptation by suppressing these genes and promoting processes underlying active ion uptake. Thus, euryhaline tilapia adjust the ion-transporting activity of their branchial ionocytes to meet environmental demands by integrating multiple regulatory cues.