Life-Stage and Parasite-Induced Shifts in the Skin Mucus Proteome of Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) skin mucus serves as a critical primary defense against aquatic pathogens, yet its developmental and infection-induced proteomic dynamics remain poorly characterized. This study utilized liquid chromatography–mass spectrometry (LC-MS)-based comparative proteomics to characterize skin mucus profiles across developmental stages and during monogenean Cichlidarus cichlidarum infection. The analysis identified 7950 proteins, including 1192 core proteins shared by all groups, primarily associated with maintaining physical barrier integrity, responses to oxidative stress, and pathogen recognition. We identified 1328 juvenile-specific proteins enriched in the organophosphate metabolic process, cytoskeletal protein binding and hydrolase activity, complement and coagulation cascades, and phagosome pathways, whereas 833 adult-specific proteins were associated with macromolecule catabolic process, NOD-like (Nucleotide-binding Oligomerization Domain-like) receptor signaling, chemokine signaling, leukocyte transendothelial migration and Toll-like receptor signaling. This may indicate that juvenile tilapia skin mucus is characterized by high metabolic activity and innate immune effectors for broad-spectrum defense against environmental microbes, while adults employ integrated innate-adaptive mechanisms for targeted pathogen clearance. Following C. cichlidarum infection, the markedly increased abundance of 1343 proteins in tilapia skin mucus delineates a highly coordinated defense network. Functional enrichment analysis revealed that these proteins are primarily associated with catalytic activity and are significantly involved in key signaling pathways, including phagosome, C-type lectin receptor signaling, PI3K-Akt signaling, chemokine signaling, and the complement and coagulation cascades. Moreover, 64 C. cichlidarum-derived proteins were identified in the mucus of infected tilapias, including HSC (heat shock cognate protein) 70, serine/threonine protein phosphatase, calmodulin, and thioredoxin peroxidase. These proteins are potentially involved in the host invasion or immune evasion of C. cichlidarum. These results provide insights into the adaptability of fish mucosal immunity and suggest targets for future functional characterization.