PFHxS and Developmental Neurotoxicity: Does Thyroid Hormone Action Play a Role?

Perfluorohexane sulfonate (PFHxS) is an environmental thyroid disrupting chemical that reduces serum thyroid hormones (THs) in animal models. As thyroid action is required for normal brain development, it is suspected that PFHxS may induce developmental neurotoxicity by endocrine mediated mechanisms. Using a multidisciplinary approach, we investigate whether PFHxS induces abnormal brain development and function associated with TH dysregulation. Pregnant rats were orally dosed (50 mg/kg/day) from gestational day 6 (GD6) to postnatal day 21 (PN21), permitted to give birth, and the offspring analyzed. Results show that PFHxS exposure reduced serum THs in both the dams and pups. Serum total thyroxine (T4) was reduced by approximately 75% in exposed neonates. However remarkably, brain T4 was only significantly reduced in the pup brain on PN0, and not on PN2, PN6, or PN14. Brain triiodothyronine (T3) was also not significantly reduced at any stage tested. We also detected no evidence, transcriptionally or phenotypically, of TH-mediated dysfunction of the cortex. No significant differences in learning and memory were detected in offspring aged to adulthood, as assayed by trace fear conditioning; sensory gating was similarly unaffected. These data suggest that despite significant reductions in serum T4, the neonatal rat brain does not appear to be TH insufficient following PFHxS exposure. These observations may be attributed to chemical action: PFHxS purportedly reduces serum THs by interfering with the function of serum binding proteins. However, unbound (free) THs are transported across the blood brain barrier and into the tissue. Thus, these dynamics in hormone transport may explain why brain TH concentrations are largely unaffected by PFHxS exposure at the stages tested. This hypothesis is consistent with clinical presentations of patients and knockout mice possessing loss of function mutations in serum binding proteins. In the future additional work is warranted to determine if other perfluorinated compounds alter brain TH concentrations at other developmental stages, and/or induce abnormal brain development by other mechanisms. This work does not reflect EPA policy.

Impact/Purpose

Thyroid hormones (TH) are critical for brain development. Many per- and polyfluoroalkyl substances (PFAS) are known to reduce serum THs in rodents (rats and mice), and exposure to various PFAS are also associated with thyroid disorders in some human populations. Furthermore, the published literature has shown that some PFAS can induce developmental neurotoxicity in animals, though the precise mechanism is unknown. This presentation examines how the contaminant perfluorohexane sulfonic acid (PFHxS) affects thyroid function in rat mothers and offspring. Rat offspring born to PFHxS exposed mothers were then evaluated for known indices of developmental neurotoxicity (structural and functional phenotypes), that result from abnormal thyroid hormone action. The multidisciplinary approach employed in this study is a fundamental step in characterizing if, and how, PFHxS induces abnormal brain development. This work 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. 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. Major Observations and Results: Serum total thyroxine (T4) was reduced by approximately 80% in exposed in 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. We also detected no evidence, transcriptionally or phenotypically, of TH-mediated dysfunction of the cortex. Neither were differences detected in neurobehavioral assays designed to assess auditory reflex modification and learning and memory when assessed in offspring tested in adulthood. Impact/Potential Implications of the Findings: These data suggest that despite reductions in serum T4 in dams and offspring, according to these metrics, the developing brain does not appear to be adversely affected. Future studies are needed to determine if PFHxS can induce developmental neurotoxicity in other measures of brain dysfunction by chemical-specific mechanisms that are independent of thyroid action.

Citation

O'Shaughnessy, Katherine, S. Thomas, I. Hassan, J. Ford, C. Wood, W. Oshiro, AND M. Gilbert. PFHxS and Developmental Neurotoxicity: Does Thyroid Hormone Action Play a Role? Society of Toxicology Annual Meeting -Virtual-Webinar, RTP, North Carolina, May 07, 2020.