Dioxin Disrupts Thyroid Hormone and Glucocorticoid Induction of klf9, a Master Regulator of Frog Metamorphosis

Loss of Krüppel-like factor 9 deregulates both physiological gene expression and development

Krüppel-like factor 9 (Klf9) is a ubiquitously expressed transcription factor that is a feedforward regulator of multiple stress-responsive and endocrine signaling pathways. We previously described how loss of Klf9 function affects the transcriptome of zebrafish larvae sampled at a single time point 5 days post-fertilization (dpf). However, klf9 expression oscillates diurnally, and the sampled time point corresponded to its expression nadir. To determine if the transcriptomic effects of the klf9-/- mutation vary with time of day, we performed bulk RNA-seq on 5 dpf zebrafish embryos sampled at three timepoints encompassing the predawn peak and midmorning nadir of klf9 expression. We found that while the major effects of the klf9-/- mutation that we reported previously are robust to time of day, the mutation has additional effects that manifest only at the predawn time point. We used a published single-cell atlas of zebrafish development to associate the effects of the klf9-/- mutation with different cell types and found that the mutation increased mRNA associated with digestive organs (liver, pancreas, and intestine) and decreased mRNA associated with differentiating neurons and blood. Measurements from confocally-imaged larvae suggest that overrepresentation of liver mRNA in klf9-/- mutants is due to development of enlarged livers.

Short- and Long-Term Effects of Chlorpyrifos on Thyroid Hormone Axis and Brain Development in Xenopus laevis

INTRODUCTION

The extensive use of the insecticide chlorpyrifos (CPF) throughout the world has brought increased scrutiny on its environmental and health impact. CPF is a cholinergic neurotoxicant; however, exposure to low noncholinergic doses is associated with numerous neurodevelopmental effects in animal models. In this study, we aimed to assess CPF for its potential to disrupt thyroid hormone signalling and investigate the short- and long-term effects on neurodevelopment by using Xenopus laevis.

METHODS

The thyroid hormone (TH) disrupting potential of CPF was assessed using TH-sensitive transgenic Tg(thibz:eGFP) tadpoles. The consequences of early embryonic exposure were examined by exposing fertilized eggs for 72 h to environmentally relevant CPF concentrations (10-10 M and 10-8 M). Three endpoints were evaluated: (1) gene expression in whole embryonic brains immediately after exposure, (2) mobility and brain morphology 1 week after exposure, and (3) brain morphology and axon diameters at the end of metamorphosis (2 months after the exposure).

RESULTS

CPF disrupted TH signalling in Tg(thibz:eGFP) tadpoles. The expression of genes klf9, cntn4, oatp1c1, and tubb2b was downregulated in response to CPF. Tadpoles exposed to CPF exhibited increased mobility and altered brain morphology compared to control tadpoles. Early embryonic exposure of CPF affected myelinated axon diameter, with exposed animals exhibiting shifted frequency distributions of myelinated axons diameters towards smaller diameters in the hindbrain of froglets.

DISCUSSION/CONCLUSION

This study provides more evidence of the endocrine and neurodevelopment disrupting activity of CPF. Further experimental and epidemiological studies are warranted to determine the long-term consequences of early CPF exposure on brain development.