Biomarkers of Neurodevelopmental Effect Are Necessary to Interpret Chemical Action In Vivo

2022 SOT Symposium Speaker 5: Katherine O’Shaughnessy Biomarkers of Neurodevelopmental Effect Are Necessary to Interpret Chemical Action In Vivo  One of the primary concerns of thyroid disrupting chemicals is their potential impact on neurodevelopment, as thyroid hormones control normal brain patterning and function. As such, some standardized developmental and reproductive toxicity studies suggest or require serum T4 measures in rats. However, the developing brain is not often examined concurrently, so it is unclear if/when a serum T4 deficit is adverse. To address this data gap, we have been characterizing mechanisms of thyroid-dependent neurotoxicity in rats and have identified several potential biomarkers of neurotoxicity. In this seminar this basic science work is discussed, and then presented in the context of a perfluoroalkyl substance, perfluorohexanesulfonate (PFHxS). PFHxS purportedly reduces serum T4 by multiple mechanisms in vivo, although its neurotoxic potential is unclear. We show that a maternal exposure to PFHxS reduces serum T4 in both the exposed dam and in their pups as compared to controls. Surprisingly, brain T4 and T3 in neonates was largely unaffected, and our immunochemical markers of thyroid dysfunction in the neonatal brain appeared normal. This includes a typical appearance of neural progenitors that mediate cell migration and no evidence of one adverse outcome assayed. However, RNA-Sequencing (RNA-Seq) of the developing liver and brain suggests that PFHxS has multiple mechanisms of action in vivo, and the brain may still be affected by a thyroid dysregulation. This work shows that environmental contaminants have complex biological effects and demonstrates how molecular biomarkers may improve chemical assessment. This work does not reflect US EPA policy.

Impact/Purpose

Relevancy to EPA Program/Regional Research Needs/Priorities: This abstract addresses Chemical Safety for Sustainability (CSS) Adverse Outcome Pathway Discovery and Development (AOPDD 17.01). Specifically, the AOPs relevant to this study serve both OPPTS and OW by providing empirical evidence to support regulatory decision making. In example, how serum thyroid hormone concentrations in rats correlate to developmental neurotoxicity. Currently serum TH measures are recommended and/or required endpoints in several OECD testing guidelines, and sound interpretation of this biomarker is critical for chemical evaluation. Study Description: Pregnant rats were orally dosed with PFHxS (50 mg/kg/day) or vehicle only from gestational day 6 (GD6) to postnatal day 21 (PN21), permitted to give birth, and the offspring analyzed. This concentration of PFHxS was chosen as it was not expected to induce overt toxicity in dams or pups but does significantly reduce serum THs.  Dam and pup serum THs (T4, T3, and TSH) were assessed from birth to weaning; brain TH concentrations were also evaluated at several developmental stages. The whole transcriptome (RNA-Sequencing, RNA-Seq) of the developing brain and liver were also analyzed to determine any effects. Major Observations and Results:   Serum total thyroxine (T4) was reduced by approximately 80% in exposed neonates on postnatal day 0 and 2. However, brain T4 was only significantly reduced in the pup brain on PN0, and not on PN2, PN6, or PN14. Brain triiodothyronine (T3) was not significantly reduced at any stage tested. Liver and brain RNA-Seq showed that PFHxS may be affecting thyroid hormone metabolism and distribution in vivo. Impact/Potential Implications of the Findings:  These data suggest that despite brain hormones being largely unaffected during early postnatal development, that PFHxS may still induce neurotoxicity by a thyroid-mediated mechanism. Findings Advancing Existing Scientific Knowledge:  These findings show how complex action of environmental contaminants may produce unexpected apical outcomes in vivo.  

Citation

OShaughnessy, K. Biomarkers of Neurodevelopmental Effect Are Necessary to Interpret Chemical Action In Vivo. Society of Toxicology (SOT) Annual Meeting 2022, Raleigh, NC, March 26 - 30, 2022.