Reduced maternal thyroxine alters microRNA expression and cytoarchitecture in the developing rat brain
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While thyroid disease has a complex etiology in patients, exposure to endocrine disrupting chemicals (EDCs) is correlated with reduced serum thyroxine (T4) in some populations. As thyroid hormones control brain development in children, determining the health effects of EDCs is paramount. Unfortunately, evaluating EDCs in animal models is difficult, as neurodevelopmental phenotypes assessed in regulatory studies are often nonspecific. To address these limitations, we hypothesized that differential gene expression could serve as biomarkers of TH action in the developing brain. While we previously investigated mRNA expression, here we performed small RNA-sequencing (RNA-Seq) to next evaluate small noncoding RNAs. As small RNAs (like micro RNAs) often control transcription of multiple genes simultaneously, these molecules can be highly informative. To determine if TH action controls small RNA expression, pregnant rats were administered the goitrogen propylthiouracil (0 or 3 ppm) in their drinking water from gestational day 6 until postnatal day 14 (PN14). Dam serum T4 was reduced by ~50% while triiodothyronine (T3) was comparable to controls, suggesting that the maternal thyroid axis was not severely affected. In contrast, pup serum T4 and T3 were both significantly decreased on PN8 (~94% and ~45%, respectively) as was brain T4/T3 (~94% and ~59%). Next, small RNA-Seq was performed on the PN8 pup forebrain, which identified 6 differentially expressed microRNAs with Q-values<0.05. Gene ontology analyses revealed that Wnt signaling, neuronal differentiation, and cell adhesion are pathways targeted by the differentially expressed microRNAs. To support these RNA-Seq results, immunohistochemistry further demonstrates that cell adhesion in the pup brain is abnormal. Together, these results show that reduced maternal serum T4, even in the absence of reduced T3, is associated with differentially expressed microRNAs and altered cellular morphology in the postnatal brain. Given the stability of microRNAs in tissues, it is possible that targeted assessment of these molecules could serve as a readout of TH action in developmental and reproductive toxicity studies. This work does not reflect US EPA policy