The Role And Mechanism Of PFOS-Induced Hyperthyroidism In Neurotoxicity Of Zebrafish Larvae

Perfluorooctane sulfonate（PFOS）is one kind of persistent pollutant that widely distributed in the water environment and chronically affect the health of human and aquatic organisms.There is a closely relationship between thyroid and the development and function of the nervous system.In addition,evidence shows that PFOS can cause neurotoxicity and thyroid toxicity simultaneously.However,studies on the mechanism of neurotoxicity of PFOS mainly focus on its direct damage to the nervous system,while ignoring its possible influence on the development and function of the nervous system by interfering with the thyroid gland.In this study,zebrafish（Danio rerio）was exposed to PFOS at different concentrations（10,100,1000μg/L）from 6 hpf（hour post-fertilization）to 6 dpf（day post-fertilization）.The neurotoxicity of PFOS was evaluated by detecting behavior,brain apoptosis level,neurodevelopment-related gene expression,dopamine（DA）level,and dopamine-related gene expression.In addition,thyroid toxicity of PFOS was evaluated by detecting thyroid hormone level,thyroid-related gene expression and organ development status.On this basis,the anti-hyperthyroid drug methimazole（MZ）was used to relieve hyperthyroidism caused by PFOS,and the neurotoxicity indexes were tested again to explore the role of hyperthyroidism caused by PFOS in the neurotoxicity of zebrafish larvae.Meanwhile,the molecular mechanism of DNA methylation was explored,which provided new basic data for the neurotoxicity of PFOS.The results of this study were as follows:（1）PFOS could cause neurotoxicity in zebrafish larvae.1000μg/L PFOS caused significantly increased of the spontaneous movement and the movement distance of zebrafish larvae under light and dark stimulation,respectively.In addition,the expressions ofα1-tubulin and mbp were significantly decreased by 100 and 1000μg/L PFOS exposure,while that of gfap was significantly increased following 1000μg/L PFOS treatment.Moreover,1000μg/L PFOS significantly increased the level of DA,while 100μg/L and 1000μg/L PFOS significantly decreased the expressions of dat and mao.（2）PFOS resulted in hyperthyroidism of zebrafish larvae,while MZ alleviated the hyperthyroidism caused by PFOS.1000μg/L PFOS significantly increased T₃ level while had no effect on T₄ level.In addition,the expression of dio1 was down-regulated by all treatments,and the transcript of trb was down-regulated by 100 and 1000μg/L exposure.Meanwhile,1000μg/L PFOS significantly reduced eye length.Nevertheless,MZ addition alleviated the upregulation of T₃,downregulation of dio1 expression and reduction of eye length by PFOS treatment.（3）Hyperthyroidism induced by PFOS leaded to its neurotoxicity in zebrafish larvae.MZ significantly alleviated the hypermotility induced by 1000μg/L PFOS under light and dark stimuli.In addition,MZ significantly alleviated the elevated expression of gfap while partially alleviated the expressions ofα1-tubulin and mbp caused by PFOS exposure.Nevertheless,MZ addition alleviated PFOS induced decreased expressions of dat.（4）PFOS-induced hyperthyroidism may caused the decrease of dat expression through up-regulating DNA methylation in its promoter region.Nevertheless,MZ addition relieved the the significantly upregulated DNA methylation level in the promoter region of dat gene caused by PFOS treatment.In conclusion,PFOS-induced hyperthyroidism may regulate the expression of dat gene through DNA methylation,thus affecting the dopamine system and ultimately leading to neurotoxicity in zebrafish larvae.This study provided a new perspective on the neurotoxic effects and mechanism of PFOS.