The influence of thyroid hormone and temperature on the transcriptomic response of Rana [Lithobates] catesbeiana tadpole cultured back skin




Evans, Ellis

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Thyroid hormones (THs) are essential signaling molecules for the postembryonic development of all vertebrates. THs are capable of initiating a diverse set of developmental programs across multiple tissues. The role of TH in regulating gene expression is well-known, but the initiation of TH signaling is still not fully understood. In amphibians, THs are the sole hormones required for the metamorphosis from tadpole to juvenile froglet. Amphibians are a useful model for studying TH signaling, as they undergo extensive, tissue-specific response programs in response to exogenous TH. The metamorphosis of the American bullfrog, Rana [Lithobates] catesbeiana is temperature sensitive. R. catesbeiana tadpoles do not undergo metamorphosis at cold temperatures (4-5 °C) even in the presence of THs that should otherwise prompt it. However, tadpoles undergo metamorphosis at an accelerated rate when returned to warm temperatures (24-25 °C) forty days after their initial TH exposure. R. catesbeiana tadpoles possess a “molecular memory” of TH exposure which establishes the TH signal at cold temperatures and prompts accelerated metamorphosis after a return to warmer temperatures. The mechanisms of the molecular memory which allow it to uncouple the initiation of TH signaling from the execution of the TH response program are not fully understood. Previous research has established that transcripts encoding transcription factors are a substantial component of the TH-dependent transcriptomic response of cultured tailfin (C-Fin) at cold temperatures. However, not all of these putative transcripts encoding transcription factors required active transcription and translation for their induction, which suggests that the initiation of a TH signal involves mechanisms other than regulating gene expression. Herein, we used quantitative polymerase chain reaction (qPCR) and RNA-Sequencing (RNA-Seq) to investigate the TH-dependent transcriptomic response of the back skin, a tissue that undergoes extensive remodeling during metamorphosis. Cultured back skin (C-Skin) was TH-responsive in warm, cold and temperature shift conditions. Forty-four transcripts underwent significant changes in abundance in response to TH in cold temperatures under which the molecular memory is established. Seven of these transcripts encoded putative transcription factors. Surprisingly, the only TH-responsive transcript significantly changed at 4 °C in both the C-Skin and the previously studied C-Fin was thyroid hormone-induced basic leucine zipper-containing protein (thibz). Thibz has been found to be TH-responsive at cold temperatures in the liver, lung, liver, brain, tailfin and back skin of whole animals, which suggests it may be an important regulator of initiating TH signaling. The lack of overlap in the transcriptomic responses of C-Skin and C-Fin may suggest that even the early initiation of TH signaling has tissue-specificity. Alternately, the molecular memory may include mechanisms that do not require active transcription and translation. Transcripts associated with epigenetic modifications and post-transcriptional changes to mRNA stability were also significantly expressed at 4 °C within the C-Skin. Previous investigation of the putative transcription factors in C-Fin revealed that active transcription and translation was not always required for changes in transcript abundance. Multiple mechanisms may be at play in the TH response at different temperatures. In cold temperatures, TH may modulate mRNA stability to influence transcript abundance as a part of initiating TH signaling without executing metamorphosis. Further research is needed to explore potential alternative mechanisms of establishing the molecular memory and the accelerated metamorphic response. The temperature sensitivity of R. catesbeiana’s TH response is incredibly valuable in investigating mechanisms of early TH signaling during postembryonic vertebrate development.



Rana catesbeiana, American bullfrog, amphibian metamorphosis, thyroid hormone, molecular memory, transcriptome, temperature