Branchial transcriptional responses of spotted scat, Scatophagus argus, to acute hypo-osmotic stress

The spotted scat (Scatophagus argus) is a euryhaline marine fish with the capability to withstand different salinity levels and can tolerate abrupt transfer from salt water (SW) to fresh water (FW) or vice versa. Differentially expressed genes were identified by constructing suppression subtractive hybridization (SSH) cDNA libraries to explore branchial osmoregulatory mechanisms affected by salinity challenge. After trimming and blasting, 105 uniquely expressed sequence tags were identified. Among them, 24 candidate genes involved in the stress response, metabolism, and the respiratory chain were chosen for further validation by real-time polymerase chain reaction. More than two-fold expression differences were observed in ADP-ribosylation factor 1 (arf1), cyclic AMP-responsive element-binding protein 3-like 4 (creb3l4), EVI5-like protein (evi5l), protocadherin fat 2 (fat2), transferrin receptor protein 2 (tfr2), C-X-C chemokine receptor type 4 (cxcr4), aquaporin-3 (aqp3), sodium/potassium-transporting ATPase subunit beta-233 (nka β₂₃₃), serum and glucocorticoid-regulated kinase 1 (sgk1), and ras GTPase-activating-like protein IQGAP1 (iqgap1). Among these genes, aqp and nka are important osmoregulatory factors. AQP3 protein was observed being localized to the membranes of mitochondrion-rich cells (MRCs) and pillar cells of the gill of S. argus, and NKA β₂₃₃ subunit isoform was only found in MRC membranes by immunostaining. Significant differences in aqp3 and nka β₂₃₃ expression occurred within 24 h after being transferring into the freshwater, and nka β₂₃₃ expression level continued to highly upregulated 2 and 7 days post-transfer (dpt). These results suggest that branchial aqp3 and nka β₂₃₃ play important roles in response to hypo-osmotic stress in S. argus.