| Thyroid Hormone (TH) is a small iodinated molecule that effects gene expression in virtually every type of vertebrate tissue. Appropriate levels of TH are critical for the proper development and maturation of several tissues, including the brain, cochlea, and retina. The objectives of this dissertation were to examine the competence and autonomy of retinal cells to respond to TH signaling during early development. To address these open questions we used a well characterized central nervous system (CNS) tissue, the retina. TH signaling components are expressed during retinal development in dynamic spatial and temporal patterns. First, to probe the competence of retinal cells to mount a transcriptional response to TH, reporters that included thyroid response elements (TREs) were introduced into developing retinal tissue. Some TREs were positively activated by TR+TH in the developing outer nuclear layer (ONL), where photoreceptors reside, as well as in the outer neuroblastic layer (ONBL) where cycling progenitor cells are located. Other TREs were actively repressed by TR+TH in cells of the ONBL. A central-nasal domain was revealed reflective of a global repression in NR signaling which corresponded temporally with the genesis of photoreceptors and spatially with the developing rod-free zone (RFZ). The transcriptional response to these TREs provided a starting point to examine the cell autonomy of the deiodinase enzymes, which provide tissue-specific activation of TH, during development. Dio2 (TH-activating) RNA is restricted to the ONL whereas Dio3 RNA is expressed in progenitor cells. However, it was unknown as to whether Dio2+ cells functioned non-autonomously by activating TH for neighboring cells (as is modeled in the brain), or cell autonomously in only post-mitotic immature photoreceptors. Using a systematic approach of single and double short hairpin RNA (shRNA) knock-down, combined with hormone modulation, we demonstrated a cell autonomous requirement for Dio2 and T3 signaling and that Dio2 is expressed at low, but biologically relevant levels, in a subset of cells outside the ONL. Additionally, these data suggest an under-appreciated requirement for TH signaling in retinal progenitor cells during development. Our experiments help to define the complex roles of the deiodinases and TH signaling during chick retinal development. |
