Neurodevelopment and Thyroid Hormone Synthesis Inhibition in the Rat: Quantitative Understanding Within the Adverse Outcome Pathway Framework

Abstract

Adequate levels of thyroid hormone (TH) are needed for proper brain development, deficiencies may lead to adverse neurologic outcomes in humans and animal models. Environmental chemicals have been linked to TH disruption, yet the relationship between developmental exposures and decline in serum TH resulting in neurodevelopmental impairment is poorly understood. The present study developed a quantitative adverse outcome pathway where serum thyroxin (T4) reduction following inhibition of thyroperoxidase in the thyroid gland are described and related to deficits in fetal brain TH and the development of a brain malformation, cortical heterotopia. Pregnant rats were exposed to 6-propylthiouracil (PTU 0, 0.1, 0.5, 1, 2, or 3 parts per million [ppm]) from gestational days 6-20, sequentially increasing PTU concentrations in maternal thyroid gland and serum as well as in fetal serum. Dams exposed to 0.5 ppm PTU and higher exhibited dose-dependent decreases in thyroidal T4. Serum T4 levels in the dam were significantly decreased with exposure to 2 and 3 ppm PTU. In the fetus, T4 decrements were first observed at a lower dose of 0.5 ppm PTU. Based on these data, fetal brain T4 levels were estimated from published literature sources, and quantitatively linked to increases in the size of the heterotopia present in the brains of offspring. These data show the potential of in vivo assessments and computational descriptions of biologic responses to predict the development of this structural brain malformation and use of quantitative adverse outcome pathway approach to evaluate brain deficits that may result from exposure to other TH disruptors.

Evaluating Chemicals for Thyroid Disruption: Opportunities and Challenges with in Vitro Testing and Adverse Outcome Pathway Approaches

“…For example, AOPs (Figure 2; Table 2) describe chemical inhibition of NIS and TPO leading to reduced TH production and decreased serum T4 that culminate in delayed/arrested amphibian development (AOP 176, AOP 175, respectively) and impaired mammalian neurodevelopment (AOP 134, AOP 42, respectively). However, a quantitative understanding of KE relationships in the TPO- and NIS-related AOPs is limiting, although current efforts are beginning to quantify these interactions (Gilbert and Sui 2008; Hassan et al. 2017; O’Shaughnessy et al.…”

A transient window of hypothyroidism alters neural progenitor cells and results in abnormal brain development

“…With regards to the developing animals, male and female pups from each litter were decapitated on PN0, PN2, and PN6, and whole trunk blood collected for serum TH analysis; if multiple littermates were sampled, then the blood was pooled as a single sample. All serum was isolated from whole blood via a serum separator tube with clot activator gel (Benton Dickinson) according to our previously published methods 14,61 . Isolated serum was aliquoted and stored at -80 °C until analysis.…”

“…Isolated serum was aliquoted and stored at -80 °C until analysis. Total T4 and T3 were analyzed by mass spectrometry (LC/MS/MS) as previously described 14,61 . The lower limit of quantification (LLOQ) for both T4/T3 were 0.1 ng/ml.…”

Development and analysis of an adverse outcome pathway network for human neurotoxicity

“…The only AOP that has chemicals associated with all KEs is AOP ID 42 "Inhibition of Thyroperoxidase and Subsequent Adverse Neurodevelopmental Outcomes in Mammals". Therefore, a qAOP could be derived and was modelled by Hassan et al (2017) for 6-propyl-2-thiouracil which is an enzyme inhibitor that is known to trigger AOP ID 42. Information on all chemicals collected for the AOPs used for modelling the AOP network is provided in the supplementary material.…”