Microcystin-LR Induces Lipid Metabolism Disorder in Pelophylax nigromaculatus Tadpoles via the Gut–Liver Axis

Disruption of lipid homeostasis in aquatic animals poses serious health risks, including tissue damage and systemic metabolic dysfunction. The precise mechanisms by which microcystin-LR, a potent cyanotoxin, disrupts lipid metabolism in amphibian tadpoles remain unclear. In this study, tadpoles ( Pelophylax nigromaculatus ) were exposed to MC-LR and fecal microbiota transplantation (FMT) experiments were performed to investigate whether or how MC-LR at environmental concentrations interfered with tadpole lipid metabolism from the perspective of the gut microbiota-gut-liver axis. Following exposure, the liver exhibited significant inflammation, hypertrophy, and fibrosis, accompanied by elevated serum lipid levels. Furthermore, the expression levels of the farnesoid X receptor (FXR), a nuclear receptor, were significantly downregulated. Molecular docking and molecular dynamics simulations indicated a strong and stable binding between FXR and MC-LR. Moreover, MC-LR suppressed liver FXR expression or activity, triggering: (1) upregulation of sterol regulatory element-binding protein 1 (SREBP1)-mediated triglyceride (TG) synthesis, (2) inhibition of free fatty acid (FFA) β-oxidation, and (3) activation of SREBP2-dependent bile acid biosynthesis. Moreover, MC-LR altered the composition of gut microbiota and specific bile acid levels (e.g., taurocholic acid and glycochenodeoxycholic acid) in the gut, thereby interfering with hepatic lipid metabolism, as evidenced by FMT-induced hepatic lipid accumulation in recipient tadpoles. These findings identify FXR as a potentially key molecular target for MC-LR and suggest that changes in bile acid levels of intestinal microbiota metabolism also may be an important pathway driving hepatic lipid dysregulation in amphibians exposed to environmental concentrations of MC-LR.