Developmental and regional expression of thyroid hormone receptor genes during Xenopus metamorphosis
Authors: Kawahara Akira , Baker Betty S. , Tata Jamshed R. 
Published: April 2021

ABSTRACT
A characteristic feature of the obligatory control of amphibian metamorphosis by thyroid hormones is the early acquisition of response of tadpole tissues to these hormones well before the latter are secreted, with ‘exponentially’ increasing hormonal sensitivity upon the onset of metamorphosis. We have therefore analyzed the expression of the two thyroid hormone receptor genes (TR α and β) before, during and after metamorphosis in Xenopus tadpoles and froglets. Using non-cross-hybridizing cRNA probes for 5 ′ and 3 ′ sequences of Xenopus TR α and β transcripts for RNAase protection assays, the two mRNAs can be detected in tadpoles as early as stage 39. Their concentration increases abruptly at stage 44 and continues to increase differentially at the onset of metamorphosis (stage 55) and through metamorphic climax at stages 58 –62, after which they decline upon completion of metamorphosis at stage 66. Quantitative densitométrie scanning of autoradiograms showed that, although the concentration of TR β transcripts is about l/30th of that of TR α mRNA at stages 44 –48, depending on the region, it accumulates 3 –10 times more rapidly than does the α isoform during further development. A substantial proportion of the increase in TR β mRNA is localized to the head region of tadpoles. Using the hormone-binding domain (HBD) and 3 ′ end of Xenopus TR α cRNA as probe for in situ hybridization, the highest concentration of TR transcripts in stage 44 tadpoles is seen in the brain and spinal cord. High concentrations of mRNA are also present in the intestinal epithelium and tail tip, tissues programmed for regression. At later stages (55 onwards), strong hybridization signals are also exhibited by hindlimb buds. This pattern persists through metamorphic climax, after which TR mRNAs decline in all tissues to low levels in froglets at stage 66. In developing froglets, TR transcripts were detected in large amounts in the cytoplasm of stage 1 and 2 oocytes but the rate of their accumulation did not increase with further oocyte growth. This observation raises the possibility that the response to thyroid hormones at early stages of tadpoles (42 –44) may be due to TR synthesized on maternally derived mRNA. Exposure of tadpoles at premetamorphic stages (48 –52) to exogenous thyroid hormone (T3) substantially enhanced the accumulation of TR mRNA, especially that of TR β message, which could explain the accelerated increase in sensitivity of tadpoles to thyroid hormones at the onset of natural metamorphosis. This autoinduction by T3 of the mRNA for its own receptor was hormone-specific and could not be induced by ligands of other members of the nuclear receptor family, such as oestrogen (E2) or retinoic acid (RA). We propose that stored maternal mRNA for thyroid hormone receptor and the phenomenon of upregulation of its own receptor may explain the precocious hormonal response and the ‘exponential’ kinetics of metamorphic climax characteristic of this amphibian postembryonic developmental process.