2,7-dibromocarbazole Induces Developmental Toxicity Through Alternation Of Lipid Metabolism In Xenopus Tropicalis Embryos

Polyhalogenated carbazoles(PHCZs)are emerging pollutants,which had been detected in various environmental media.Researchers have found that PHCZs have dioxin-like toxicity and pose a threat to the survival and growth of aquatic organisms.At present,researches on the potential ecological risk of PHCZs are still limited.In particular,the impact of amphibians which are considered as indicators of environmental pollution is still unclear.Studies have shown that amphibians were generally less sensitive to dioxin-like compounds.However,it is still unclear whether the differences of species sensitivity are similar in PHCZs.Therefore,it is urgent to explore the response mechanism of amphibians to such new dioxin-like compounds.In this study,2,7-dibromocarbazole(2,7-DBCZ)was used as the represent of PHCZs,and Xenopus tropicalis was used as the model organism.The bioaccumulation of 2,7-DBCZ was analyzed,and the effects on embryonic development,cardiac function and cardiomyocytes ultrastructure were evaluated.Furthermore,using transcriptome sequencing,lipid metabolism analysis and molecular simulation,the toxic mechanism of 2,7-DBCZ on Xenopus tropicalis embryonic development was systematically analyzed.The results are as follows:(1)The results of exposure experiment showed that 2,7-DBCZ were accumulated in embryos.The environmental concentration of 2,7-DBCZ did not affect the hatching,survival and growth of Xenopus tropicalis embryos.However,2,7-DBCZ could induce pericardiac edema and small eyes of embryos,indicating the toxic effect of 2,7-DBCZ on the early development of amphibians.(2)The analysis of embryonic heart rate and rhythm showed that high concentration of 2,7-DBCZ could significantly reduce the heart rate of embryos and affect normal cardiac function.After exposure to 2,7-DBCZ,mitochondria were observed to be swollen and mitochondrial cristae were disorder,accompanied by more autophagosomes around mitochondria,suggesting that the ultrastructure of cardiomyocytes was damaged.(3)The results of transcriptome sequencing showed that 2,7-DBCZ exposure led to extensive and significant changes in embryonic transcriptome.Differentially expressed genes were mainly enriched in nutrient metabolism-related pathways centered on lipid metabolism and transport,while drug metabolism-related and complement system-related pathways were also enriched.The results of protein interaction analysis showed that there were complex correlations among the related molecules of lipid transport,drug metabolism and complement system signaling pathway,affecting the embryonic development of Xenopus tropicalis.(4)UPLC-MS was used to detect the changes in the types and contents of lipids during embryonic development.The results showed that the exposure of 2,7-DBCZ result in the decrease in the contents of glycerides,phospholipids and cholesterol lipids.Among them,the number of triglycerides,which was the main energy source of early embryonic development decreased most.The content of lysophosphatidylethanolamines decreased most,which was the component of cell membrane.With the prolongation of exposure time,the decrease of glycerides increased,while that of phospholipids decreased.The results of lipid metabolism study showed that 2,7-DBCZ interfered with lipid homeostasis during development of Xenopus tropicalis embryos.(5)qPCR results showed that 2,7-DBCZ could significantly up-regulated the expression level of drug metabolic genes,especially cyp1a1,suggesting the activation of aromatic hydrocarbon receptor signaling pathway.Based on homology modeling and molecular simulation,it was found that 2,7-DBCZ could bind to aromatic hydrocarbon receptor of Xenopus tropicalis.The binding was mainly relied on hydrophobic interaction and additional ?-? stacking.The present study explored the toxic effect and mechanism of 2,7-DBCZ on Xenopus tropicalis embryos.The results showed that 2,7-DBCZ could activate the aromatic hydrocarbon receptor of Xenopus tropicalis,induce the expression level of drug metabolism-related genes such as cyp1a1 to be significantly up-regulated,interfere with lipid metabolism and transport process.The affection on lipid homeostasis and immune regulation in embryonic development could lead to the damage of cardiomyocytes structure and insufficienting the energy supply.It would have toxic effects on the embryonic development of Xenopus tropicalis.The results will provide more comprehension of ecological risk assessment and toxicity mechanism analysis of PHCZs.