| The increasing levels of thyroid hormone (TH) during metamorphosis in the frog Xenopus laevis lead to a spatially and temporally controlled balance of adult cell proliferation and larval cell death. TH acts through a pair of highly conserved thyroid hormone receptors (TRs) to initiate gene expression cascades. However, how the hormone regulates these networks is still poorly understood, and is the subject of the experiments I undertook in my thesis work. Here, I have shown that TH response genes during X. laevis metamorphosis can be grouped into three distinct classes based on their response kinetics and sensitivity to protein synthesis inhibitors, the existence of thyroid hormone response element (TRE), and their associated epigenetic changes. These classes are designated as early, early/late and late response genes, reminiscent of ecdysone response gene classes in Drosophila melanogaster. In particular, I studied the regulation of the TRbeta gene as a representative early gene, the TH/bZIP gene as a representative early/late gene and the collagenase-3 as a representative late gene. I demonstrated that two specific histone modification markers, histone H3 acetylated on lysines 9 and 14 and trimethylated on lysine 4, are associated with transcriptionally active genes in X. laevis. The levels of these active gene markers are induced by T3 on two response gene promoters with known TREs, TRbeta and TH/bZIP, but not on the late response gene promoter of collagenase-3. These epigenetic changes are highly correlated with the kinetic responses of these genes to T3, i.e TRbeta, the earliest response gene, appears to be in an active state that can be additionally modified; TH/bZIP, classified as an early/late gene, is highly induced from an inactive chromatin state. However, the late response collagenase-3 gene appears to be regulated by mechanisms highly distinct from early or early/late genes. In addition, I have determined that the activity of two specific serine hydrolases, fibroblast activation protein alpha and carboxylesterase 2, are regulated by TH in resorbing tails during metamorphosis in a manner similar to their regulation in human tumor metastasis. Like other proteolytic enzymes in the tail resorption program such as the matrix metalloproteinases, fibroblast activation protein alpha activity closely follows induction of its corresponding mRNA. However, carboxylesterase 2 activity is repressed during metamorphosis even though its mRNA is slightly induced, indicating that higher levels of post-transcriptional or post-translational regulation need to be considered along with transcriptional control in order to understand TH induced tail resorption. |
