Mixtures of xenobiotics found in human fluid modify embryonic thyroid hormone signalling and brain development
Jean-Baptiste Fini, Bilal Mughal, Michelle Leemans, Sébastien Le Mével, Petra Spirhanzlova and Barbara A Demeneix
« Evolution des régulations endocriniennes » team
Département Régulations, Développement et Diversité Moléculaire, MNHN, Sorbonne
Endocrine disrupting chemicals were officially named 26 years ago at Wingspread conference. Since then many chemicals were shown to be harmful to both wildlife and human health. Characteristics of EDCs include action at low doses, critical windows of exposure with fetal exposure being a major one. Thyroid hormone (TH) signaling is conserved across vertebrates and can be seen as bridging the environment to gene expression networks. Data suggest a causal link between mixtures of EDCs affecting TH signalling and the increasing incidence of human neurodevelopmental disorders, such as Autism Spectrum Disorders (ASD). Thus, there is an increasingly need to screen chemical mixtures that could be exerting “environment x gene” interactions, thereby contributing to the exponential rise in neurodevelopmental disorders including autism and hyperactivity with their enormous socio-economic costs for individuals and society.
Humans are currently exposed to myriads of chemicals, from early gestation onwards. Whilst data exists for such individual chemicals, few if any studies have focused on effects of a mixture of these compounds. Besides the evolutionary conservation of TH signalling and brain development, the amphibian model provides experimental access to early, critical windows of development that now known to be essential vulnerable exposure periods. In Pr Demeneix team, we studied the potential thyroid disrupting effect of 15 chemicals commonly found in pregnant women in the USA. Using a previously validated in vivo Xenopus assay for identifying potential Thyroid hormone (TH) disrupters, we confirmed that the 9 out of the 15 individual chemicals exerted an inhibiting or activating effect on the Thyroid hormone, T3, signalling pathway.
Application of the mixture of the 15 chemicals together, at concentrations reported in amniotic fluid, we observed a significant and dose-dependent potentialisation of T3 dependent transcription. RT-qPCR analysis on the dissected brain tissue from the mixture-exposed Xenopus embryos revealed modifications of TH related genes including thrb, klf9 and especially the deiodinases (dio1, 2, 3) but also autism related genes such as rora, cntn4, bdnf. Using a locomotor tracking system we observed that tadpoles exposed to increasing concentrations displayed severely and significantly reduced mobility. In order to study the mixture impact on neurogenesis we further subjected the amniotic mixture exposed embryonic brains to immuno-histochemistry. We observed increased proliferation within the developing brain and modification of cell fate (neurons, neuroblasts) when exposed to the mixture. Taken together these results argue for urgent concern from legislative entities in order to protect future generation’s health.