Changes in the expression of certain osmosensitive channel and transporter genes in primary hepatocytes of air-breathing catfish, Clarias magur: A strategy to adapt under osmotic stress

Environmental salinity/osmolarity is an important abiotic ecological factor that affects several metabolic activities of aquatic organisms. Air-breathing magur catfish (Clarias magur) frequently encounter a wide range of external osmolarity changes in their natural habitats as well as internal osmolarity due to the absorption of amino acids and other metabolites through the intestine. The major objective of the present investigation was to demonstrate the influences of anisotonicity exposure on the expression of certain isoforms of aquaporin (AQP) channels (aqp7, 8, and 11), taurine transporter (TauT, slc6a6), urea transporter (UT1, slc14a1) and Na⁺-K⁺-ATPase (NKA, atp1a1, and atp1b1) transporter genes, and their translated protein products in the primary hepatocytes of magur catfish. These genes are known to play major roles in cellular volume regulation and water homeostasis under osmotic stress. Exposure of primary hepatocytes to hypertonic medium (+40 mOsmol/L) for a period of 48 h resulted in a significant increase in the expression of aqp7, aqp8, aqp11, slc6a6, slc14a1, atp1a1, and atp1b1 genes, as evidenced by a rise in the levels of both mRNAs of all the above-mentioned genes, and also their translated protein products compared to isotonic controls. However, exposure of primary hepatocytes to hypotonic medium (_40 mOsmol/L) for the same period led to a significant decrease in mRNA levels for aqp7, aqp8, aqp11, slc6a6, and slc14a1 genes, and also their translated protein products compared to isotonic controls as a consequence of downregulation of corresponding genes. Interestingly, NKA enzyme activity in primary hepatocytes was found to increase significantly after exposure to both hypertonic and hypotonic media. Hypertonicity induced the genes for α- and β-subunits of this enzyme both at transcriptional and translational levels. In contrast, hypotonicity induced only the atp1a1 gene and the expression of corresponding translated protein without causing any changes in the expression of the atp1b1 gene. Thus, it may be contemplated that osmotic stress, which results due to anisotonicity of the external environment or in the body fluids, leads to differential expression of different osmosensitive channels such as AQPs, and transporters such as TauT, UT1, UT2, and NKA in hepatic cells of magur catfish and work in coordination to defend the osmotically-induced changes of cellular volume. These regulatory responses of channels and transporters could be one of the vital molecular mechanisms that may have evolved in this catfish to defend against the osmotically-induced stresses.