Analysis of global and gene-specific acetylation of histones in the liver of American bullfrog (Rana catesbeiana) tadpoles acclimated to low temperature

Changes in histone abundance and post-translational modifications in Rana [Lithobates] catesbeiana tissues during temperature-mediated thyroid hormone-induced metamorphosis

Amphibian metamorphosis involves thyroid hormone (TH)-dependent postembryonic remodeling of differentiated tissues as a tadpole transforms into a frog. Rana [Lithobates] catesbeiana (American bullfrog) metamorphosis can be precociously induced in premetamorphic tadpoles by treatment with exogenous TH. However, metamorphosis is temperature-dependent and does not occur at 5 °C, even with TH treatment. Remarkably, exposure to TH at 5 °C establishes a molecular memory that primes the tadpole for an accelerated metamorphosis when shifted to a permissive temperature (24 °C). Previous research suggests that histone post-translational modifications (PTMs) and the incorporation of histone variants are altered upon cold temperature exposure in the presence and absence of TH. Herein, we use mass spectrometry to analyze the histone composition of premetamorphic R. catesbeiana blood, liver, and tailfin following TH injection at a permissive temperature (24 °C), a non-permissive temperature (5 °C), and a shifted condition (5 → 24 °C). Using stringent selection criteria, we identified tissue- and temperature-specific changes in the abundance and putative PTMs of chromatin-binding proteins, including histones. Linker histone variant H1.0 abundance increased in all three tissues following TH treatment under permissive temperatures only. TH-dependent changes in the abundance of methyl-CpG binding protein 2 (MeCP2) and high mobility group (HMG) proteins were temperature- and tissue-specific. PTMs, including variations in H3K27 and H3K36 methylations in the liver, were differentially abundant following TH treatment under the different temperature conditions. We also identified newly discovered HMGB2 methylation marks that exhibited temperature-specific changes in abundance following TH treatment. These observations suggest that epigenetic variations may contribute to early TH signaling events and to molecular memory.

Uncovering early thyroid hormone signalling events through temperature-mediated activation of molecular memory in the cultured bullfrog tadpole tail fin

Thyroid hormone (TH) is a critical signalling molecule for all vertebrate organisms, playing a crucial role in postembryonic development. The best-studied mechanism of TH response is through modulating gene expression, however TH's involvement in coordinating the early steps in the TH signal transduction pathway is still poorly understood. The American bullfrog, Rana [Lithobates] catesbeiana, is a useful model to study these early responses as tadpole post-embryonic development in the form of metamorphosis of the tadpole into a frog can be experimentally induced by TH exposure. The rate of TH-induced metamorphosis can be modulated by temperature where sufficiently cold temperatures (5 °C) completely halt precocious metamorphosis. Interestingly, when premetamorphic tadpoles exposed to exogenous THs at 5 °C are shifted to permissive temperatures (24 °C), their metamorphic rate exceeds that of TH-exposed tadpoles at the permissive temperature. This suggests that a molecular memory of TH exposure is retained at 5 °C even after THs are cleared at this low temperature. However, the molecular memory machinery is poorly understood. Herein we use RNA-seq analysis to identify potential components of the molecular memory in cultured tail fin that allows for the recapitulation of the molecular memory phenomenon. Eighty-one gene transcripts were TH-responsive at 5 °C compared to matched controls indicating that the molecular memory is more complex than previously thought. Many of these transcripts encode transcription factors including thyroid hormone-induced B/Zip, thibz, and a novel krüppel-like factor family member, klfX. Actinomycin D and cycloheximide treatment had no effect on their TH induction suggesting that a change in transcription or translation is not required. Rather a change in RNA stability may be a possible mechanism contributing to the molecular memory. The ability to manipulate temperature and TH response in cultured organs provide an exciting opportunity to further elucidate the early TH signalling mechanisms during postembryonic development.