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Abstract
The thyroperoxidase (TPO) enzyme is expressed by the thyroid follicular cells and is required for thyroid hormone synthesis. In turn, thyroid hormones are essential for brain development, thus inhibition of TPO in early life can have life-long consequences for brain function. If environmental chemicals with the capacity to inhibit TPO in vitro can also alter brain development in vivo through thyroid hormone dependent mechanisms, however, remains unknown. In this study we show that the in vitro TPO inhibiting pesticide amitrole alters neuronal migration and induces periventricular heterotopia; a thyroid hormone dependent brain malformation. Perinatal exposure to amitrole reduced pup serum thyroxine (T4) concentrations to less than 50% of control animals and this insufficiency led to heterotopia formation in the 16-day old pup’s brain. Two other in vitro TPO inhibitors, 2-mercaptobenzimidazole and cyanamide, caused reproductive toxicity and had only minor sporadic effects on the thyroid hormone system; consequently, they did not cause heterotopia. This is the first demonstration of an environmental chemical causing heterotopia, a brain malformation until now only reported for rodent studies with the anti-thyroid drugs propylthiouracil and methimazole. Our results highlight that certain TPO-inhibiting environmental chemicals can alter brain development through thyroid hormone dependent mechanisms. Improved understanding of the effects on the brain as well as the conditions under which chemicals can perturb brain development will be key to protect human health.
Highlights
Proper brain development is critically dependent on thyroid hormones
Compounds with IC50 values > 10 mM were excluded, as were estrogens, aniline derivatives suspected of carcinogenicity, potentially directly neurotoxic compounds (n 1⁄4 44) and compounds with maximal TPO inhibition < 60% in the Toxcast assay (n 1⁄4 38)
After excluding compounds that could be regarded as positive controls (6-propyl-2-thiouracil, 6-methyl-2-thiouracil, methimazole and ethylene thiourea) (n 1⁄4 4), we arrived at 3 suitable test compounds all in current use in the European Union: 2-Mercaptobenzimidazole (MBI), amitrole and cyanamide
Summary
Epidemiological studies show associations between low maternal thyroxine (T4) concentrations and neurodevelopmental effects manifesting as decreased intelligence coefficients (IQ) and cognitive disorders later in the child’s life (Andersen et al, 2018; Fetene et al, 2017; Ghassabian et al, 2014; Gyllenberg et al, 2016; Haddow et al, 1999; Korevaar et al, 2016; Roman et al, 2013) These associations have been confirmed in animal studies, where disruption of thyroid hormone signalling during development led to brain malformations and behavioural effects in the maturing and adult offspring (Akaike et al, 1991; Auso et al, 2004; Axelstad et al, 2008; Gilbert et al, 2016; Gilbert and Sui, 2006; Lavado-Autric et al, 2003; Richard et al, 2020; Sharlin et al, 2008).
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