Microcystin-LR disrupts gut metabolic homeostasis through microbiota-metabolite axis remodeling in grass carp

Microcystin-LR (MC-LR) exerts detrimental effects on gut mucosal integrity and functional homeostasis in aquatic organisms. This study investigated the gut toxicity of MC-LR in grass carp exposed to 5 and 10 μg/L MC-LR through integrated histological, biochemical, molecular, microbiomics, metabolomic, and cellular analyses. MC-LR exposure caused pronounced gut structural damage, characterized by epithelial disorganization, villus shortening, and crypt alteration. MC-LR exposure significantly reduced antioxidant enzyme activities (SOD, CAT, and GSH) and increased MDA levels in the gut, reflecting oxidative imbalance. Expression of inflammatory and apoptotic genes was significantly upregulated, accompanied by the downregulation of tight junction proteins. Immunofluorescence showed that MC-LR exposure elevated inflammatory and apoptotic proteins while reducing antioxidant and tight junction proteins in the intestine. Additionally, MC-LR exposure altered the gut microbiota of grass carp, characterized by reduced diversity and shifts in key genera, such as increased Barnesiellaceae and decreased Ruminococcus gauvreauii group. Metabolomic profiling revealed substantial alterations in amino acid and energy metabolism, particularly involving lysine degradation, pyrimidine metabolism, and apoptosis pathways. Multi-omics correlation analysis revealed that increased abundances of Vibrio and Prevotella_9 were associated with higher 25-hydroxycholesterol levels and lower 1H-indole-3-acetamide levels. Furthermore, MC-LR exposure significantly decreased the viability and antioxidant enzyme activity of primary gut cells. Collectively, these findings demonstrate that MC-LR induces gut injury in grass carp by triggering oxidative stress, inflammation, apoptosis, and microbiota-metabolite dysregulation, providing new insights into the mechanisms of cyanotoxin-induced gut toxicity in fish.