Stress responses to warming in Japanese flounder (Paralichthys olivaceus) from different environmental scenarios

Japanese flounder (Paralichthys olivaceus) is one of cold-water species widely farmed in Asia. In recent years, the increased frequency of extreme weather events caused by global warming has led to serious impact on Japanese flounder. Therefore, it is crucial to understand the effects of representative coastal economic fish under increasing water temperature. In this study, we investigated the histological and apoptosis responses, oxidative stress and transcriptomic profile in the liver of Japanese flounder exposed to gradual temperature rise (GTR) and abrupt temperature rise (ATR). The histological results showed liver cells in ATR group were the most serious in all three groups including vacuolar degeneration and inflammatory infiltration, and had more apoptosis cells than GTR group detected by TUNEL staining. These further indicated ATR stress caused more severe damage than GTR stress. Compared with control group, the biochemical analysis showed significantly changes in two kinds of heat stress, including GPT, GOT and D-Glc in serum, ATPase, Glycogen, TG, TC, ROS, SOD and CAT in liver. In addition, the RNA-Seq was used to analyze the response mechanism in Japanese flounder liver after heat stress. A total of 313 and 644 differentially expressed genes (DEGs) were identified in GTR and ATR groups, respectively. Further pathway enrichment of these DEGs revealed that heat stress affected cell cycle, protein processing and transportation, DNA replication and other biological processes. Notably, protein processing pathway in the endoplasmic reticulum (ER) was enriched significantly in KEGG and GSEA enrichment analysis, and the expression of ATF4 and JNK was significantly up-regulated in both GTR and ATR groups, while CHOP and TRAF2 were high expressed in GTR and ATR groups, respectively. In conclusion, heat stress could cause tissue damage, inflammation, oxidative stress and ER stress in the liver of Japanese flounder. The present study would provide insight into the reference for the adaptive mechanisms of economic fish in face of increasing water temperature caused by global warming.