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Title: The molecular mechanisms of thyroid disruption by brominated flame retardants in fish : in vitro and in vivo studies
Author: Parsons, Aoife
ISNI:       0000 0004 7653 296X
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2017
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Fish are particularly vulnerable to the exposure of anthropogenic pollutants, with a vast array of endocrine disrupting chemicals (EDCs) introduced into the aquatic environment via sewage discharge, waste disposal and land runoff. Brominated flame retardants (BFRs) are halogenated flame retardants that are used to effectively inhibit the flammability of various materials including plastic products, electrical appliances, construction materials and textiles. BFRs are ubiquitous environmental contaminants and are known to disrupt thyroid hormone (TH) homeostasis in several vertebrate species, including fish. Given the vital role of THs in a wide range of developmental processes and physiological functions, assessing and identifying thyroid disrupting chemicals is crucial for safe guarding the long-term health of humans and wildlife. In fish the molecular mechanisms underlying TH disruption by BFRs and the effects on TH-sensitive tissues during early life stages remains unclear. This has been limited by the lack of fundamental knowledge on the TH system of fish and the difficulties associated with examining transcriptional changes in discrete embryonic-larval tissues. Here I have established the expression profiles of a suite of genes in the hypothalamic-pituitary-thyroid (HPT) axis of zebrafish (Danio rerio) during embryonic-larval stages and their regulation by the biologically active TH (3, 5, 3′- tri-iodothyronine; T3). Using molecular tools (whole mount is situ hybridisation and RT-PCR), I demonstrate that a number of genes display spatial and temporal expression profiles during embryo/larval development, and their regulation by T3 was tissue- and developmental stage-specific. I subsequently demonstrated that TBBPA and BDE-47, two important BFR compounds, disrupted TH homeostasis at multiple levels of the HPT axis of zebrafish embryo-larvae after short sub-acute exposures. These compounds altered the expression of genes associated with TH conjugation and clearance, thyroid follicle development and TH transport. In addition, we suggest that TH target genes in the brain, liver, pronephric ducts and craniofacial tissues of zebrafish embryo-larvae may be particularly vulnerable to TBBPA and BDE-47 exposure. It has been proposed that environmental pollutants can disrupt TH signalling in wildlife by disrupting the activity of thyroid receptors (TRs), ligand-binding transcription factors, which mediate the genomic actions of THs. The ability of BFRs to disrupt fish TRs has not yet been examined. Here I developed an in vitro reporter gene transcriptional assay for zebrafish thyroid hormone receptors (zfTRα and zfTRβ) in human embryonic kidney cells and investigated their interactions with several BFR compounds. The assays were optimised and validated using the natural TR agonist T3 in cells transiently transfected with two reporter vector constructs, pGL4.24-PAL and pGL4.24-DR4. None of the six brominated flame retardants tested, namely, tetrabromobisphenol A (TBBPA), hexabromocyclododecane (HBCD), 2,2′,4,4′-tetra-bromodiphenyl ether (BDE-47), 2,2′,4,4′,6-penta-bromodiphenyl ether (BDE-100), 2,2′,3,4,4′,5′,6-hepta-bromodiphenyl ether (BDE-183) and deca-bromodiphenyl ether (BDE-209) had an agonistic effect on zfTRα and zfTRβ activity. These results are consistent with our previous finding which suggests that altered TH homeostasis may be a result of increased metabolism and excretion of THs and/or changes in the production of TH by the thyroid follicles. In conclusion, this investigative work aids the understanding of fundamental TH processes in fish, such as gene expression and regulation, and increases our understanding of the mechanisms and potential targets of BFRs in fish.
Supervisor: Tyler, Charles R. ; Tetsu, Kudoh Sponsor: CEFAS ; Defra ; Ministry of the Environment ; Government of Japan
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: endocrine disruption ; fish ; molecular ; thyroid ; pollution ; chemicals ; flame retardants ; development ; aquatic environment ; gene expression ; nuclear receptors