Characterization of thioredoxin domain-containing protein 9 in chub mackerel (Scomber japonicus): Molecular features, antioxidant activities, and involvement of cellular homeostasis.

Thioredoxin domain-containing protein 9 (TXNDC9), a member of the thioredoxin (Trx) superfamily, is known for its role in cellular defense against oxidative stress. However, its functional role of TXNDC9 in immune response and under various stress conditions remains poorly understood. In the present study, a TXNDC9 homolog, SjTXNDC9, was identified in Scomber japonicus. Its open reading frame consists of 669 base pairs, encoding a 222-amino acid protein. The theoretical molecular weight and isoelectric point of the SjTXNDC9 were predicted to be 25.47 kDa and 6.47, respectively. In silico analysis revealed a Trx domain spanning from amino acids 63 to 175, featuring an atypical/monothiol CXXC motif with "T/AFRC" residues. Subcellular localization prediction indicated SjTXNDC9 is found in the cytoplasm and nucleus of fish cells. Spatial mRNA expression analysis demonstrated the highest SjTXNDC9 expression in the brain, followed by the blood, with the lowest expression in the skin. Upon stimulation with polyinosinic:polycytidylic acid (poly I:C) and lipopolysaccharide (LPS), SjTXNDC9 mRNA expression significantly increased in the blood. Recombinant SjTXNDC9 (rSjTXNDC9) displayed insulin disulfide reduction activity, suggesting a contribution to cellular redox homeostasis. SjTXNDC9-overexpressed cells showed lower levels of cellular reactive oxygen species (ROS) upon H2O2-induced oxidative stress. Additionally, SjTXNDC9 exhibited cytoprotective activity against UV-induced apoptosis and aided wound healing. Overall, these findings suggest that SjTXNDC9 plays a critical role in maintaining cellular homeostasis during oxidative stress and immune response in S. japonicus.