Effects of thyroid hormone disruption on the ontogenetic expression of thyroid hormone signaling genes in developing zebrafish (Danio rerio)

Abstract

Thyroid hormones (THs) regulate neurodevelopment, thus TH disruption is widely posited as a mechanism of developmental neurotoxicity for diverse environmental chemicals. Zebrafish have been proposed as an alternative model for studying the role of TH in developmental neurotoxicity. To realize this goal, it is critical to characterize the normal ontogenetic expression profile of TH signaling molecules in the developing zebrafish and determine the sensitivity of these molecules to perturbations in TH levels. To address these gaps in the existing database, we characterized the transcriptional profiles of TH transporters, deiodinases (DIOs), receptors (TRs), nuclear coactivators (NCOAs), nuclear corepressors (NCORs), and retinoid X receptors (RXRs) in parallel with measurements of endogenous TH concentrations and tshβ mRNA expression throughout the first five days of zebrafish development. Transcripts encoding these TH signaling components were identified and observed to be upregulated around 48-72 h post fertilization (hpf) concurrent with the onset of larval production of T4. Exposure to exogenous T4 and T3 upregulated mct8, dio3-b, trα-a, trβ, and mbp-a levels, and downregulated expression of oatp1c1. Morpholino knockdown of TH transporter mct8 and treatment with 6-propyl-2-thiouracil (PTU) was used to reduce cellular uptake and production of TH, an effect that was associated with downregulation of dio3-b at 120 hpf. Collectively, these data confirm that larval zebrafish express orthologs of TH signaling molecules important in mammalian development and suggest that there may be species differences with respect to impacts of TH disruption on gene transcription.

Molecular mechanisms and tissue targets of brominated flame retardants, BDE-47 and TBBPA, in embryo-larval life stages of zebrafish (Danio rerio)

“…Therefore, the increased levels of dio1 and dio2 transcripts observed here provide strong evidence of an induced systemic hypothyroid condition in the TBBPA and BDE-47 exposed zebrafish larvae [76][77][78]. This likelihood is supported by the reduced dio3b mRNA levels observed in whole larval (96 hpf) samples exposed to TBBPA (0.18 and 0.46 µM groups), since D3 is the major inactivating pathway of THs and thyroidal status often parallels hepatic D3 activity, increasing during hyperthyroidism and decreasing during hypothyroidism [33,79].…”

Composition and endocrine effects of water collected in the Kibale national park in Uganda

“…Tight regulation of circulating hormones by the HPT axis is well conserved across vertebrates, and the structure and function of key components TH signalling in teleosts closely resemble those of higher vertebrates. Using zebrafish exposed to exogenous THs or PTU (with or without an mct8 morpholino) at 24, 72 and 120 h post-fertilization (hpf), Walter et al (2019) confirmed that expression of key ortholog genes which mediate TH signalling in mammals appear to be coordinated by THs in D. Rerio. However, in teleosts, thyroid follicles (the functional unit of the gland) are dispersed along the ventral midline of the pharynx instead of being encapsulated in an organized pair of thyroid glands (Alt et al, 2006), which therefore precludes the use of histopathological assessment as is employed in Xenopus and mammals.…”

“…Remarkably, specific biomarkers for thyroid disruption in teleost fishes have yet to be accepted for regulatory purposes due to discrepancies resulting from inconsistent experimental parameters across a limited amount of studies using known TH disruptors, as emphasised by Spaan et al (2019). However, given the relatively recent characterization of the zebrafish HPT axis and intrinsic advantages, the zebrafish is becoming a promising alternative vertebrate model for thyroid signalling, amenable to highthroughput screening owing to their small size, ease of culture and short generation time (Haggard et al, 2018;Kollitz et al, 2018;Marelli and Persani, 2017;Noyes et al, 2018;Sipes et al, 2011;Vancamp et al, 2018;Walter et al, 2019).…”

“…Additionally, a particular vulnerable time windows to THADCs such as the transition from the embryonic to larval stage occurs relatively rapidly, i.e. only 2-3 dpf in zebrafish (Walter et al, 2019), and metamorphosis (larva-to-juvenile transition) initiates after 2 weeks (Sharma et al, 2016). The subsequent generation may be obtained in 2-4 months to provide evidence of transgenerational effects following THADC exposure, whilst also having the advantage of taking into account gender-specific effects (Chen et al, 2018;Cheng et al, 2017;Han et al, 2017;Jianjie et al, 2016;Wang et al, 2015;Yu et al, 2014;Zhao et al, 2016).…”

Ontogenetic expression of thyroid hormone signaling genes: An in vitro and in vivo species comparison

“…To examine the expression of TH signaling molecules at later stages of neurodevelopment, including axonal and dendritic outgrowth, and synaptogenesis, we performed transcriptomic analyses of rat primary neuron-glia co-cultures derived from PND 1 rat cortex that were cultured for up to 21 days in vitro (DIV) [23]. Finally, we also examined the ontogenetic profile of these transcripts in zebrafish larvae since this model has been proposed as an alternative organism for screening the DNT potential of TH active chemical compounds [24,25]. Moreover, zebrafish offer the potential for future studies to integrate molecular effects of TH disruption with physiological and behavioral outcomes.…”