Differential expression of multiple glutamine synthetase genes in air-breathing magur catfish, Clarias magur and their induction under hyper-ammonia stress

The present study demonstrates the unique presence of three different gs genes (cmgs01, cmgs02, and cmgs03) in air-breathing ureogenic magur catfish (Clarias magur), which is otherwise reported to be encoded by a single gene in higher vertebrates. Of these three genes, two (cmgs01and cmgs03) were identified as 'liver' form, predominantly expressed in liver cells, and the third one as 'brain' form (cmgs02), expressed chiefly in brain cells. Molecular characterization studies have revealed conservation of homologous active site residues in all the three gs genes. In silico analysis, accompanied by GS enzyme assay and Western blot analysis of different GS isoforms in different subcellular fractions indicated the mitochondrial localization of cmGS01 and cmGS03 in liver and kidney cells and cytosolic localization of cmGS02 in brain cells. Further, exposure of magur catfish to high external ammonia (HEA; 25 mM NH4Cl) led to a significant induction of multiple gs genes as evidenced by higher expression of different gs mRNAs at variable levels in different tissues. The cmgs01 and cmgs03 mRNA levels elevated significantly in liver, kidney, muscle, and gills, whereas the cmgs02 mRNA level increased considerably in the brain after 14 days of exposure to HEA. These increases in mRNA levels were associated with a significant rise in cmGS01 and cmGS03 proteins in liver, kidney, muscle, and gills, and the cmGS02 protein in the brain after 14 days of exposure to HEA. Therefore, it can be concluded that the unique differential expression of three gs genes and their induction under high ammonia level probably helps in detoxification of ammonia to glutamine and further to urea via the ornithine-urea cycle in ureogenic as well as non-ureogenic tissues of these magur catfish.