Change to the transcriptomic profile, oxidative stress, apoptotic and immunity in the liver of small yellow croaker (Larimichthys polyactis) under hypoxic stress

The small yellow croaker (Larimichthys polyactis) is one of the most highly cultured fish in China, contributing considerably to the country's economy. However, conditions experienced under high-density culture make the fish susceptible to hypoxia, which impacts growth and consequently product quality. To understand the effects of hypoxia stress on physiological and biochemical responses of L. polyactis, we subjected the fish to hypoxic (2.5 mg/L) stress for 96 h. We observed histological changes in the liver pre- and post-hypoxia and measured the changes in liver injury parameters. The results showed injury to the liver of L. polyactis following hypoxic stress, indicating that the liver is an important organ in the hypoxic stress response. To explore the genes and biological processes involved in hypoxic adaptation of L. polyactis at the overall transcription level, liver tissues were obtained pre- (0 h) and post- (6, 24, 48, and 96 h) hypoxia for transcriptomic analysis. GO and KEGG enrichment analysis of differentially expressed genes (DEGs) showed that DEGs were mainly enriched in redox equilibrium, metabolism, immunity, and apoptosis pathways under hypoxic stress. In addition, we examined the changes in molecular indices related to oxidative stress, cell apoptosis, and non-specific immunity in L. polyactis under hypoxic stress. The results showed that hypoxia induced reactive oxygen species overproduction, altered the activity of oxidase (MDA, PCO, 8-OhdG) and antioxidant enzymes (T-AOC, SOD, CAT, GPx, GR), and reduced the mRNA expression of key antioxidant genes (sod1, sod2, cat) in the liver, thus confirming that hypoxic stress induced oxidative stress. Additionally, we confirmed that hypoxic stress induced apoptosis in the liver by inducing the enzyme activity of Caspase-3/8/9 and the mRNA expression of key apoptotic genes (bcl2, bax, fas, and bnip3). Furthermore, hypoxic stress inhibited the activity of non-specific enzymes (AKP, ACP, and LZM) and mRNA expression of immune-related genes (tnf-α, il-1β, and il-17c); hence, hypoxic stress inhibited the non-specific immune function of L. polyactis. In conclusion, our study provides new insights into the molecular mechanisms underlying hypoxia adaptation in marine fish, which will aid in selective breeding of hypoxia-tolerant L. polyactis.