Effects of Salinity Stress on Histological Changes, Glucose Metabolism Index and Transcriptomic Profile in Freshwater Shrimp, Macrobrachium nipponense

Salinity is an important factor in the aquatic environment and affects the ion homeostasis and physiological activities of crustaceans. Macrobrachium nipponense is a shrimp that mainly lives in fresh and low-salt waters and plays a huge economic role in China's shrimp market. Currently, there are only a few studies on the effects of salinity on M. nipponense. Therefore, it is of particular importance to study the molecular responses of M. nipponense to salinity fluctuations. In this study, M. nipponense was set at salinities of 0, 8, 14 and 22‱ for 6 weeks. The gills from the control (0‱) and isotonic groups (14‱) were used for RNA extraction and transcriptome analysis. In total, 593 differentially expressed genes (DEGs) were identified, of which 282 were up-regulated and 311 were down-regulated. The most abundant gill transcripts responding to different salinity levels based on GO classification were organelle membrane (cellular component), creatine transmembrane transporter activity (molecular function) and creatine transmembrane transport (biological function). KEGG analysis showed that the most enriched and significantly affected pathways included AMPK signaling, lysosome and cytochrome P450. In addition, 15 DEGs were selected for qRT-PCR verification, which were mainly related to ion homeostasis, glucose metabolism and lipid metabolism. The results showed that the expression patterns of these genes were similar to the high-throughput data. Compared with the control group, high salinity caused obvious injury to gill tissue, mainly manifested as contraction and relaxation of gill filament, cavity vacuolation and severe epithelial disintegration. Glucose-metabolism-related enzyme activities (e.g., pyruvate kinase, hexokinase, 6-phosphate fructose kinase) and related-gene expression (e.g., hexokinase, pyruvate kinase, 6-phosphate fructose kinase) in the gills were significantly higher at a salinity of 14‱. This study showed that salinity stress activated ion transport channels and promoted an up-regulated level of glucose metabolism. High salinity levels caused damage to the gill tissue of M. nipponense. Overall, these results improved our understanding of the salt tolerance mechanism of M. nipponense.