Thyroid hormone (TH) is a critical signalling molecule for all vertebrate organisms, playing an especially crucial role in postembryonic development. Given its importance, many studies have focused on further elucidating the initial TH signal response and its method of transduction. Although the primary mechanism of TH response is genomic signalling, alternative mechanisms of early TH signal transduction have been relatively poorly studied. The North American bullfrog, Rana catesbeiana, is a useful model to study these early responses as tadpole post-embryonic development, or metamorphosis, can be experimentally induced through exposure to TH. The experimental induction of the TH signalling program leads to similar morphological endpoints as seen in natural metamorphosis in the transition of a tadpole to a juvenile froglet, such as regression of the tail. This TH-induced developmental program can also be manipulated through temperature where, as temperatures lower, developmental rate is delayed and at 5°C metamorphosis is completely stalled. Interestingly, when tadpoles exposed to TH at 5°C are introduced to permissive temperatures (24°C), an accelerated developmental program ensues, even when no more endogenous TH signal remains. Previous research has shown that this phenomenon can also be seen on the molecular level where only a select few transcripts have been shown to be responsive to TH at 5°C. However, the characteristic, if not augmented, TH response program is seen on the transcriptomic level when tadpoles are shifted to 24°C. This indicates that there is a molecular memory where the TH signal is induced in cold temperatures but not executed until more permissive temperatures arise. The extent and regulation of the transcriptomic program involved in this TH-induced molecular memory has yet to be understood. Herein we use the broader probing technique of RNA-seq analysis to identify potential components of the molecular memory. Eighty-one gene transcripts were TH-responsive at 5°C in cultured R. catesbeiana tail fin indicating that the molecular memory is more complex than previously thought. A number of these transcripts encoded regulators of transcription. Closer examination of select transcripts including a novel krüppel-like factor family member, klfX, at 5oC indicated that not all of the candidate molecular memory transcripts are regulated through active transcription and active translation is not required. When moved into 24°C an accelerated transcriptomic response occurred even when no additional TH is added, suggesting that a priming event occurs by TH exposure at 5°C allowing an accelerated metamorphosis at permissive temperatures. The molecular memory may be used as a means to isolate the initiating TH signalling response and the regulation of this program to allow further elucidation of early TH signalling in post-embryonic development. / Graduate

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