Molecular Mechanisms of Nuclear Hormone Receptor Transcriptional Synergy and Autoinduction.

Abstract

Thyroid hormone (TH) and glucocorticoids (GC) play critical roles in the development and function of the central nervous system (CNS) by binding to their cognate nuclear hormone receptors (NRs), which function as ligand-activated transcription factors. In my dissertation, I studied the TH and GC-mediated regulation of Krüppel like factor 9 (Klf9/Basic Transcription Element Binding Protein 1; Bteb1), a member of the Sp1/KLF family of zinc finger transcription factors that bind GC rich genomic sequences, and plays an important role in neuronal development and plasticity. In prior work Klf9 was found to be a direct TH receptor (TR) target gene. I showed that Klf9 is also directly targeted by the GC receptor (GR), and that TH and GCs cause synergistic induction of Klf9. This synergistic regulation is phylogenetically ancient, and was likely present in the earliest tetrapods and has been evolutionarily conserved from frogs to mammals.

I identified a genomic region in the 5’ flanking region of the Klf9 gene (the ‘Klf9 synergy module’) that contains TR/GR binding sites and confers synergistic gene regulation by TH and GC. The synergistic effect of TH and GC on Klf9 can be explained by a TR-dependent increase in the recruitment of the GR and enhanced association of stalled RNA polymerase II at the Klf9 synergy module, and an interaction between the synergy module and the Klf9 promoter by chromosomal looping. I also conducted a genome-wide microarray analysis, the first study to identify transcriptional targets that are coordinately regulated by TH and GC in the brain.

Lastly, I demonstrated a role for KLF9 as an accessory transcription factor to support TH-dependent expression of TRβ (autoinduction), a process that is necessary for the progression of amphibian metamorphosis, and normal mammalian brain development. Given the synergistic regulation of the Klf9 gene by TR and GR, and its role in TRβ autoinduction, my findings support that KLF9 is an important intermediate that functions to integrate TH and GC by enhancing the cell sensitivity to hormonal signals. Taken together, my thesis broadens our understanding of the molecular mechanisms of NR cooperativity and autoinduction, with important implications for animal development.