Microcystin-LR interferes with the intestinal remodeling during tadpole metamorphosis by disrupting thyroid hormone signaling pathways

Intestinal remodeling during amphibian metamorphosis is essential for adaptation to terrestrial environments. In this study, Pelophylax nigromaculatus tadpoles at developmental stage G26 were exposed to near-environmental concentrations of MC-LR until the end of metamorphosis to investigate whether and how MC-LR affects intestinal remodeling during metamorphosis in tadpoles. Furthermore, parents exposed to MC-LR and those unexposed were cross-matched to determine if tadpoles from the first filial generation (F1) affected intestinal remodeling. MC-LR exposure hindered the proliferation of adult intestinal stem cells and impaired their differentiation into adult epithelial cells during metamorphosis. MC-LR exposure caused pathological damage to thyroid tissue, downregulated thyroid hormone synthesis genes, and reduced blood concentrations of hypothalamic-pituitary-thyroid axis hormones (CRH, TSH, and T3) and transthyretin. Molecular docking analysis suggests that MC-LR may compete with T3 for TTR binding. T3-regulated Notch and Wnt signaling pathways, along with key intestinal remodeling genes (msi, pcna, and lgr5) controlled by these pathways, were downregulated. Thus, MC-LR inhibits Notch and Wnt signaling pathways' responses by interfering with thyroid hormone synthesis and transport, ultimately impairing tadpoles' intestinal remodeling to some extent. Moreover, transgenerational analysis revealed that the disruption of intestinal remodeling also occurred in F1 tadpoles. MC-LR exposure increased DNA methylation levels in parental gonads, as well as in the brains and thyroids of F1 tadpoles. DNA methylation levels negative correlation with the blood T3 concentration and the severity of intestinal remodeling disruption. This research pioneers the investigation of how environmentally relevant concentrations of Microcystin-LR affect intestinal remodeling during amphibian metamorphosis, creating a basis for future assessments of Microcystin-LR exposure risks to amphibian populations.