Proteomic insights into gill responses to copper oxide nanoparticles in juvenile rainbow trout: Adaptive compensation and structural injury

Copper oxide nanoparticles (CuO NPs) are increasingly considered in aquaculture-related contexts for their antimicrobial properties and potential water-treatment applications. However, sustained waterborne exposure may impose chronic environmental stress on fish gills. The molecular basis underlying the progression from early compensatory responses to clear gill impairment under chronic exposure remains insufficiently resolved. In this study, juvenile rainbow trout ( Oncorhynchus mykiss ) were exposed to different concentrations of CuO NPs and Cu²⁺ for a period of 28 days. The responses of fish gill were evaluated at physiological, histological, and molecular levels. Results indicated that 25 μg/L CuO NPs and Cu²⁺ exhibited similar levels of Cu accumulation in the gills, but their molecular characteristics were different. CuO NPs increased protein Msrb expression by 4.05-fold, and maintained ATP-dependent ionoregulatory function. At 100 μg/L, CuO NPs caused significant gill injury and higher Cu accumulation. Relative to the control, Cu accumulation increased by 45.65%, while ATP content and Ca²⁺-ATPase activity decreased by 53.5% and 60.8%, respectively. These changes were accompanied by the suppression of extracellular matrix-related proteins and obvious histopathological lesions. Proteomic data further revealed non-monotonic responses in a subset of proteins, where their abundance did not increase or decrease monotonically with increasing exposure concentration. Overall, these findings support a concentration-related transition in which low-concentration CuO NPs exposure remains compatible with adaptive compensation, whereas higher exposure shifts gill responses toward energetic decline, ionoregulatory impairment, and structural injury. This study clarifies gill stress responses under chronic, sub-lethal CuO NPs exposure and provides a physiological-proteomic basis to support evidence-based consideration of CuO NPs in aquaculture water management.