Thyroid and Corticosteroid Signaling in Amphibian Metamorphosis

Abstract

In multicellular organisms, development is based in part on the integration of communication systems. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in orchestrating body morphogenesis. In all vertebrates, the hypothalamic-pituitary-thyroid axis controls thyroid hormone production and release, whereas the hypothalamic-pituitary-adrenal/interrenal axis regulates the production and release of corticosteroids. One of the most salient effects of thyroid hormones and corticosteroids in post-embryonic developmental processes is their critical role in metamorphosis in anuran amphibians. Metamorphosis involves modifications to the morphological and biochemical characteristics of all larval tissues to enable the transition from one life stage to the next life stage that coincides with an ecological niche switch. This transition in amphibians is an example of a widespread phenomenon among vertebrates, where thyroid hormones and corticosteroids coordinate a post-embryonic developmental transition. The review addresses the functions and interactions of thyroid hormone and corticosteroid signaling in amphibian development (metamorphosis) as well as the developmental roles of these two pathways in vertebrate evolution.

Keywords: corticosteroids; development; hormonal crosstalk; metamorphosis; thyroid hormones.

Figure 1

Hypothalamus–pituitary–thyroid and hypothalamus–pituitary–interrenal neuroendocrine axes and their interactions. The main actors of the hypothalamic–pituitary–thyroid (HPT) axis are in blue. Abbreviations: dio, deiodinase; T₄, thyroxine; T₃, tri-iodothyronine; rT₃, reverse triiodothyronine; T₂, diiodothyronine; TR, thyroid hormone receptor; TRH, thyrotropin releasing hormone; TRHR, thyrotropin releasing hormone receptor; TSH, thyrotropin; TSHR, thyrotropin receptor. The action of TRH on TSH in amphibians is observed only in juvenile and adult frogs (purple arrow). In tadpoles, TSH is controlled by CRH (green arrow). The main actors of the corticotropic (hypothalamic–pituitary–adrenal/interrenal, HPA/HPI) axis are in Bordeaux. Abbreviations: ACTH, adrenocorticotropin; CRH, corticotropin-releasing hormone; CRHR1, corticotropin-releasing hormone receptor 1; CRHR2, corticotropin-releasing hormone receptor 2; GR, glucocorticoid receptor; MC2R, melanocortin receptor 2.

Figure 2

TR function during frog development. (Top). Correlations of plasma T₃ and T₄ and mRNA levels of TRα, TRβ, and RXRα with Xenopus laevis development. The embryos hatch around stage NF35, and a feeding stage tadpole is formed by stage NF45. Note that the expression of TRα (solid line) and RXRα (bold line) is activated to high levels when tadpole feeding begins, while high levels of T₃/T₄ and TRβ mRNA (broken line) are present only during metamorphosis. (Bottom). Regulation of T₃-inducible genes in frog development. During embryogenesis, there is little T₃/T₄ or TR expression, and TR target genes are expressed at basal levels independent of TRs or T₃/T₄. During premetamorphosis between stage NF45 but before stage NF54 when there is little T₃/T₄ present, the expression of TRs, mainly TRα and RXRs, causes repression of the genes due to the binding of the TR target genes by unliganded TR/RXR heterodimers that recruit corepressor complexes. The synthesis of endogenous T₃/T₄ after stage NF54 leads to formation of liganded TRs that recruit coactivator complexes for the activation of target genes and metamorphosis.

Figure 3

The expression of TR and RXR genes temporally correlates with organ-specific metamorphosis. (A). Temporal regulation of metamorphic organ transformations in Xenopus laevis. Tadpole developmental stages and ages are based on [51]. The tails at stages NF62-NF66 are drawn to the same scale to show resorption (no tail is left by stage NF66), while the tadpoles, intestinal cross-sections (stages NF54-NF66), and the hindlimbs (stages NF52-NF58) at different stages are not to scale in order to show stage-dependent morphological changes during metamorphosis. Tadpole small intestines have a single epithelial fold, where connective tissue is abundant, while frogs have a multiple-folded intestinal epithelium, with thick connective tissue and muscles. Black dots, proliferating adult intestinal epithelial cells; circles, apoptotic larval intestinal epithelial cells; L, intestinal lumen. (B–D). TR-subtype-dependent temporal regulation of TR and RXR expression in the hindlimb, intestine, and tail. Note that in general, the mRNA levels are high when the organs undergo metamorphosis. During tail resorption (stages NF62-NF66), TRβ expression is strongly upregulated, while TRα expression is only moderately increased (B). In contrast, during limb morphogenesis (stages NF54-NF58), TRα mRNA level is high, while TRβ is low (C). During intestinal remodeling, TRβ expression is strongly upregulated, while TRα mRNA level is slightly increased (D). Adapted from [27,49].

Figure 4

Levels of plasma CORT and klf9 expression in corticosteroid mutant frogs. (A). Compared to wild-type (WT) tadpoles, glucocorticoid receptor knockout (GRKO) tadpoles have high corticosterone (CORT) levels from lack of negative feedback, adrenocorticotropic hormone knockout (ACTHKO) tadpoles have low CORT levels from lack of adrenal stimulation by ACTH, and cyp21a2, encoding 21-hydroxylase, knockout (21OHKO) tadpoles have low CORT levels from lack of a steroidogenic enzyme to make CORT. (B). The expression of klf9 (Krüppel-like factor 9) is directly induced by CORT and by T₃. The expression of klf9 is also indirectly increased by CORT via CORT’s action to increase tissue responsivity to T₃. Thus, compared to WT tadpoles, 21OHKO tadpoles have reduced klf9 levels from reduced CORT signaling caused by lack of CORT but abundant CORT synthesis precursors able to bind CORT receptors with lower affinity. ACTHKO and GRKO have low klf9 levels from minimal to no contribution of CORT/GR signaling to induce klf9 via CORT signaling per se or via an increase in tissue responsivity to T₃.

Figure 5

Schematic representation of changes in baseline thyroid hormones and corticosteroids during post-embryonic development. Blue line—T₄, thyroxine; red line—T₃, triiodothyronine; green line—cortisol/corticosterone, corticosteroids. Thyroid hormone levels are from flounder (teleost fish, [115]), Xenopus (amphibian, [21]), alligator (reptile, [116]), chicken (bird, [117]), and mammals (mouse, [118] and human, [20]). Corticosteroid concentrations are from Wada 2008 [114].