Timing of metamorphosis and the onset of the negative feedback loop between the thyroid gland and the pituitary is controlled by type II iodothyronine deiodinase in Xenopus laevis

Abstract

Two important features of amphibian metamorphosis are the sequential response of tissues to different concentrations of thyroid hormone (TH) and the development of the negative feedback loop between the pituitary and the thyroid gland that regulates TH synthesis by the thyroid gland. At the climax of metamorphosis in Xenopus laevis (when the TH level is highest), the ratio of the circulating precursor thyroxine (T 4) to the active form 3,5,3-triiodothyronine (T3) in the blood is many times higher than it is in tissues. This difference is because of the conversion of T 4 to T3 in target cells of the tadpole catalyzed by the enzyme type II iodothyronine deiodinase (D2) and the local effect (cell autonomy) of this activity. Limb buds and tails express D2 early and late in metamorphosis, respectively, correlating with the time that these organs undergo TH-induced change. T 3 is required to complete metamorphosis because the peak concentration of T 4 that is reached at metamorphic climax cannot induce the final morphological changes. At the climax of metamorphosis, D2 expression is activated specifically in the anterior pituitary cells that express the genes for thyroid-stimulating hormone but not in the cells that express proopiomelanocortin. Physiological concentrations of T 3 but not T4 can suppress thyrotropin subunit ␤ gene expression. The timing and the remarkable specificity of D2 expression in the thyrotrophs of the anterior pituitary coupled with the requirement for locally synthesized T3 strongly support a role for D2 in the onset of the negative feedback loop at the climax of metamorphosis. T he metamorphosis of anurans is controlled by a steadily increasing concentration of thyroid hormone (TH) in tadpoles. TH reaches a peak at the climax of metamorphosis and then falls as the final change, tail resorption, occurs (1, 2). This gradual increase in TH concentration is essential for the sequential development of frog tissues and organs (3, 4). The growth of the hind limbs is the earliest TH-induced morphological modification. In Xenopus laevis, the hind-limb buds form but cannot develop beyond Nieukoop Faber (NF) stage 53 (5) in the absence of TH. TH-induced growth and development of limbs occur when the endogenous TH concentration is low and is completed before the climax of metamorphosis (when TH concentration is the highest). Many metamorphic changes occur in rapid succession at climax. In about 4 days, the intestine remodels and the gills resorb followed by tail resorption, which takes another 3-4 days.A negative feedback loop between the thyroid gland, the pituitary, and the hypothalamus maintains the endogenous TH at a constant level in adults of higher vertebrates (6). The rising TH concentration in X. laevis tadpoles, which is so important for their sequential development, occurs paradoxically as the pituitary content of thyroid-stimulating hormone (TSH) mRNA is increasing also (7). Etkin (8) hypothesized that the rise in TH during the early part of tadpole development was caused by a positive feedba...

Distinctive gene profiles occur at key points during natural metamorphosis in the Xenopus laevis tadpole tail

“…Previous work has shown that this transcript is TH responsive in tail and that overexpression of this enzyme inhibits metamorphosis (Huang et al, 1999). In concordance with previous observations, we did not see an increase in iodothyronine 5Ј-deiodinase II transcript that encodes an "activating" enzyme that converts T 4 to the more bioactive T 3 (data not shown; Huang et al, 2001). Glycoproteins (genes 18 and 19) that are also transiently up-regulated at NF stage 58 have been shown to be expressed in the outer epidermal layer of the tadpole and may play a role in the integrity and function of the larval skin (Furlow et al, 1997).…”

Regulation of pituitary thyrotropin gene expression during Xenopus metamorphosis: negative feedback is functional throughout metamorphosis

“…tissue morphogenesis (Becker et al 1997, Kawahara et al 1999, Huang et al 2001; the presence of DII mRNA at stage 62 of development but not at stage 56 (Huang et al 2001); the observation that TSH mRNA expression levels increase during metamorphosis, peak at climax and then decline rapidly (Buckbinder & Brown 1993); and the claim that T 4 does not downregulate TSH expression (Huang et al 2001). However, as discussed earlier, our study along with several earlier studies clearly show that negative feedback is active early in prometamorphosis.…”

“…More recently, it was shown that treatment of early prometamorphic X. laevis tadpoles with the goitrogen, methimazole, elevated pituitary TSH mRNA levels (Buckbinder & Brown 1993, Huang et al 2001. Taken together, the accumulated data lead to the rejection of the hypothesis that negative feedback is turned on specifically at metamorphic climax (see Huang et al 2001). Huang and colleagues (2001) recently addressed the issue of thyroidal regulation of the pituitary gland during amphibian metamorphosis and the establishment of negative feedback.…”

Dual mechanisms governing muscle cell death in tadpole tail during amphibian metamorphosis

“…However, our result showed muscle cell death at stage 57 when T3 is detectable at first (0.89 nM; Leloup and Buscaglia, 1977). Although the thyroxine concentration is 3.7 nM at this stage, T3 could not be generated from thyroxine in tail, because the tail has almost no mRNA of type II iodothyronine deiodinase (St. Germain et al, 1994;Huang et al, 2001). Therefore, tail muscle cells dying at this stage should be very sensitive to T3.…”

“…6A,B). A decrease of the fluorescence was observed without any morphologic changes of the remaining cells after stage 57 when the endogenous T3 becomes scarcely detectable in plasma (Leloup and Buscaglia, 1977) and type II iodothyronine deiodinase (activating enzyme of thyroxine) is not expressed in tail (Huang et al, 2001). Some GFPpositive cells disappeared between early and late stage 57.…”