Thyroid Hormone Signaling in the Xenopus laevis Embryo Is Functional and Susceptible to Endocrine Disruption

Abstract

Thyroid hormone (TH) is essential for vertebrate brain development. Most research on TH and neuronal development focuses on late development, mainly the perinatal period in mammals. However, in human infants neuromotor development correlates best with maternal TH levels in the first trimester of pregnancy, suggesting that TH signaling could affect early brain development. Studying TH signaling in early embryogenesis in mammals is experimentally challenging. In contrast, free-living embryos, such as Xenopus laevis, permit physiological experimentation independent of maternal factors. We detailed key elements of TH signaling: ligands, receptors (TR), and deiodinases during early X. laevis development, before embryonic thyroid gland formation. Dynamic profiles for all components were found. Between developmental stages 37 and 41 (~48 h after hatching, coincident with a phase of continuing neurogenesis) significant increases in T(3) levels as well as in mRNA encoding deiodinases and TR occurred. Exposure of embryos at this developmental stage for 24 h to either a TH antagonist, NH-3, or to tetrabromobisphenol A, a flame retardant and known TH disruptor, differentially modulated the expression of a number of TH target genes implicated in neural stem cell function or neural differentiation. Moreover, 24-h exposure to either NH-3 or tetrabromobisphenol A diminished cell proliferation in the brain. Thus, these data show first, that TH signaling exerts regulatory roles in early X. laevis neurogenesis and second, that this period represents a potential window for endocrine disruption.

A screening assay for thyroid hormone signaling disruption based on thyroid hormone-response gene expression analysis in the frog Pelophylax nigromaculatus

“…This study aimed to develop a P. nigromaculatus screening assay for TH signaling disruption using additional TH-response genes as molecular markers. We chose four candidate TH-response genes, including TH/bZIP, SHH, MMP2 and BTEB, which have been used as molecular markers to study TH signaling disruption in X. laevis (Fini et al, 2012a;Heimeier et al, 2009;Zhang et al, 2014). The expression changes of the four candidate genes in P. nigromaculatus intestines were found to parallel endogenous TH levels during metamorphosis, which is consistent with previous observations in X. laevis (Heimeier et al, 2010).…”

“…Expression analysis for candidate TH-response genes in the intestine during metamorphosis (SHH) have been demonstrated to be TH-response genes in X. laevis (Fini et al, 2012a;Hasebe et al, 2011;Heimeier et al, 2009). Their expression levels in the intestine parallel endogenous TH levels (Heimeier et al, 2010;Hoopfer et al, 2002;Wang et al, 2008).…”

“…Expression of TH-response genes (TRβ, BTEB, TH/bZIP, MMP2, and SHH) was analyzed by RT-qPCR. Additionally, expression of type 3 iodothyronine deiodinase (DIO3), which was used as a molecular biomarker to assay exogenous TR agonists and antagonists in previous studies using X. laevis (Fini et al, 2012a;Nakajima et al, 2012), was also detected. The experiment was repeated three times using tadpoles from different sets of adults.…”

Molecular characterization and mRNA expression of ribosomal protein L8 in Rana nigromaculata during development and under exposure to hormones

“…In amphibian research assaying thyroid-disrupting effects of chemicals, THs (T3 and T4) are often used as positive reference chemicals, with mRNA expression of TH receptors (TRs, especially TRβ) as molecular biomarkers (Fini et al, 2012;Hinther et al, 2011). To evaluate the stability of rpl8 and ef1a1 mRNA expression under exposure to thyroid-disrupting chemicals, we performed a 2-day experiment, in which R. nigromaculata tadpoles at stage 27 were exposed to a series of concentrations of T4 (0.01, 0.1, 1, 10, 100, 1000 nmol/L) in tanks.…”

“…In addition, amphibians have become a unique biological model for studying thyroid-disrupting effects of EDCs because amphibian J O U R N A L O F E N V I R O N M E N T A L S C I E N C E S 2 6 ( 2 0 1 4 ) 2 3 3 1 -2 3 3 9 metamorphosis is controlled by thyroid hormone (TH) (Berg et al, 2009;Takase et al, 2007). Some endpoints have been well-developed in the amphibian model for studying thyroid disrupting effects, including the time to metamorphosis, limb growth, intestinal remodeling, TH level, and mRNA expression of TH-related genes (Fini et al, 2012;Heimeier and Shi, 2010).…”