Embryonic exposure to excess thyroid hormone causes thyrotrope cell death
Ksenia N. Tonyushkina, … , Theresa Ortiz-Toro, Rolf O. Karlstrom
Ksenia N. Tonyushkina, … , Theresa Ortiz-Toro, Rolf O. Karlstrom
Published December 9, 2013
Citation Information: J Clin Invest. 2014;124(1):321-327. https://doi.org/10.1172/JCI70038.
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Research Article

Embryonic exposure to excess thyroid hormone causes thyrotrope cell death

Abstract

Central congenital hypothyroidism (CCH) is more prevalent in children born to women with hyperthyroidism during pregnancy, suggesting a role for thyroid hormone (TH) in the development of central thyroid regulation. Using the zebrafish embryo as a model for thyroid axis development, we have characterized the ontogeny of negative feedback regulation of thyrotrope function and examined the effect of excess TH on thyrotrope development. We found that thyroid-stimulating hormone β subunit (tshb) and type 2 deiodinase (dio2) are coexpressed in zebrafish thyrotropes by 48 hours after fertilization and that TH-driven negative feedback regulation of tshb transcription appears in the thyroid axis by 96 hours after fertilization. Negative feedback regulation correlated with increased systemic TH levels from the developing thyroid follicles. We used a transgenic zebrafish that expresses GFP under the control of the tshb promoter to follow thyrotrope fates in vivo. Time-lapse imaging revealed that early exposure to elevated TH leads to thyrotrope cell death. Thyrotrope numbers slowly recovered following the removal of excess TH. These data demonstrate that transient TH exposure profoundly impacts the thyrotrope population during a critical period of pituitary development and may have long-term implications for the functional reserve of thyroid-stimulating hormone (TSH) production and the TSH set point later in life.

Authors

Ksenia N. Tonyushkina, Meng-Chieh Shen, Theresa Ortiz-Toro, Rolf O. Karlstrom

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Figure 5

Onset of tshb feedback regulation.

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Onset of tshb feedback regulation. (A–C) Graphs comparing the number o...
(A–C) Graphs comparing the number of tshb-expressing cells detected by ISH (blue) with the numbers of GFP-expressing thyrotropes (green) quantified in Tg(tshb:EGFP) embryos. Cells were counted following a 24-hour exposure to 30, 100, or 300 nM T4 (or DMSO carrier) starting 2 dpf (A), 4 dpf (B), or 7 dpf (C). (A) Exposure to 30 nM T4 from 2 to 3 dpf had no significant effect on tshb gene expression or thyrotrope numbers, while 100- and 300-nM T4 treatments led to a loss of both tshb- and GFP-expressing cells, consistent with cell death. (B) 30-nM T4 treatments from 4 to 5 dpf led to an approximately 50% reduction in tshb-expressing cells, with no significant change in GFP-expressing thyrotropes. Exposure to 100 nM and 300 nM T4 led to similar reductions in both tshb- and GFP-expressing cells, consistent with thyrotrope cell death. (C) 30-, 100-, and 300-nM T4 treatments from 6 to 7 dpf did not affect the number of GFP-expressing thyrotropes. In contrast, the number of tshb-expressing cells was significantly reduced, reflecting negative feedback of T4 on tshb gene expression in the thyrotropes. (D and E) Upregulation of dio3 expression following a 24-hour exposure to 300 nM T4 (E) versus DMSO (D). Pituitary (pit.) is indicated by an arrow, and the areas of dio3 expression in the diencephalic (di) and third ventricular (3rd v) regions are marked by arrowheads. (F) Graph showing expression levels (fold change) of dio3 following a 24-hour exposure to T4 by 3 versus 8 dpf. ##P < 0.01; *** and ###P < 0.001. Scale bars: 25 μm.