Identification and characterization of Cathepsins gene family from obscure puffer Takifugu obscurus and their expression pattern in response to diverse pathogens infection.

Cathepsins are a major class of lysosomal proteases that play key roles in various physiological, immune, and pathological processes. Although cathepsins have been reported in some teleost fish, their inflammatory responses have not yet been characterized in Takifugu obscurus. In this study, 14 cathepsin genes were first systematically characterized in T. obscurus, which were further classified into cysteine and aspartic proteases. Phylogenetic analysis revealed that lineage-specific expansion (e.g., Cathepsin B, D, L1) and loss (e.g., Cathepsin A, E, W) might have occurred in the cathepsin family. Structural prediction showed that the cathepsin family has conserved function domains (e.g., inhibitor_I29 and peptidase_C1 domains) and a shared α/β-fold core in the tertiary structures. Tissue distribution analysis revealed high expression of cathepsins in immune-related tissues (liver, head kidney) and mucosal barriers (intestine, skin). Upon stimulation with bacteria (Edwardsiella tarda, Aeromonas hydrophila, Vibrio harveyi) and poly (I:C), cathepsins displayed dynamic and pathogen-specific expression patterns. Several members (e.g., Cathepsin B-like, S and Z) were consistently upregulated, suggesting their potential involvement in antibacterial and antiviral immunity. Collectively, our findings provide insights into the molecular mechanisms of cathepsins in T. obscurus and contribute to understanding the roles of cathepsins in teleost immunity.