Disruption of intestinal structure, tight junction complex, immune response and microbiota after chronic exposure to copper in swamp eel (Monopterus albus)

As an essential micronutrient, copper is crucial in aquatic organisms' growth and development. Numerous studies have consistently reported that excessive intake of copper can have harmful effects on organisms. However, there are limited studies on the impact of copper on the intestine of the swamp eel (Monopterus albus). This study aimed to investigate the changes of intestinal histopathology, tight junction complex, immune response, and microbiota in swamp eel treated with 0 mg/L Cu2+, 0.05 mg/L Cu2+, and 0.10 mg/L Cu2+ for 56 d. Intestinal histopathology showed major changes such as the increased number of erythrocytes and goblet cells in the lamina propria, and separation of the lamina propria. The expression of genes involved in tight junction complex (ZO-1, Claudin-3, Claudin-12 and Claudin-15) was significantly changed. In addition, copper exposure significantly increased the mRNA levels of TLR3, TLR7, TLR8, NF-κB, I-κB, TNF-α and IL-8, especially in 0.10 mg/L Cu2+ group. In contrast, the relative expression level of anti-inflammatory cytokine TGF-β was significantly decreased after exposure to copper. Analysis of the intestinal microbiome showed the intestinal microbiota of swamp eels in the control and copper exposure groups were dominated by Firmicutes and Proteobacteria at the phylum level. Notably, copper exposure changed the diversity of the intestinal microbiota and decreased the relative abundance of Firmicutes and Proteobacteria in the intestine of swamp eel. Collectively, this study demonstrates that chronic copper exposure induces intestinal pathologic changes and inflammatory response, disrupts the intestinal microbial diversity and microbiota composition, and decreases intestinal barrier function in swamp eel, which enhances our understanding of copper-induced intestinal toxicity in fish.