The Role of Thioredoxin in Mitigating Ammonia-Induced Oxidative Stress in Nile Tilapia (Oreochromis niloticus)

Thioredoxin (TRX) is an important redox-related protein, which plays a key role in maintaining redox balance in cells. However, the role of TRX in ammonia exposure of Oreochromis niloticus is still unknown. In this study, we successfully cloned the TRX gene (OnTRX) of O. niloticus and performed systematic bioinformatics analysis. The results of multi-sequence alignment and phylogenetic tree analysis showed that OnTRX is highly conserved in vertebrates, but exhibits low sequence similarity with TRX homologs from arthropods (crustaceans). The tissue distribution of OnTRX and its transcriptional response to ammonia challenge were determined by qRT-PCR, and we further investigated the relationship between OnTRX and the response of the Nile tilapia epithelial cell line (TSE-04) to NH4Cl treatment. The results showed that OnTRX exhibited tissue-specific expression, with a relatively high expression level in gill tissue. Ammonia exposure could significantly induce the expression of OnTRX in the gill, head kidney, intestine, skin, liver, and spleen of tilapia. In TSE-04 cells, OnTRX overexpression was associated with reduced NH4Cl-induced morphological damage, a lower proportion of apoptotic cells, and altered transcript levels of several stress-related genes. Collectively, these findings indicate that OnTRX is likely involved in the regulatory response of Nile tilapia to ammonia-induced stress, while the underlying molecular mechanisms thereof remain to be further elucidated.