Year of Publication

2006

Document Type

Dissertation

College

Graduate School

Department

Toxicology

First Advisor

Daniel J. Noonan

Abstract

Section I: This section is an introduction to the field of nuclear receptors. A general overview of nuclear receptor-mediated transcriptional regulation is followed by a review of literature on retinoid and thyroid receptor-mediated signaling. Section II: An introduction to NPDC-1 (neural proliferation, differentiation, and control), its discovery and characterization with regards to developmental expression and cellular localization. In addition NPDC-1 has been found to associate with a number of cell cycle regulatory proteins. NPDC-1 is characterized as a regulator of nuclear receptor-mediated transcriptional regulation. NPDC-1 was also demonstrated to be regulated post-transcriptionally through the ubiquitin/proteosome degradation pathway. Section III: Axolotl NPDC-1 (aNPDC-1) was cloned from axolotl brain and analyzed for homology to NPDC-1 from higher vertebrates. The tissue distribution and developmental expression of axolotl NPDC-1 were also examined. Section IV: The axolotl homolog for RAR (aRAR) was isolated from axolotl brain. Axolotl NPDC-1 and aRAR were then examined in a series of assays for interactions. Axolotl NPDC-1 was found to repress transcription mediated by aRAR to a smaller extent than the repression observed in higher vertebrates. The DNA binding of aRAR-RXR was increased in the presence of aNPDC-1 and complex mobility was also observed. The domain of interaction between aNPDC-1, aRAR and hRXR was localized in the amino terminus of aNPDC-1. Axolotl NPDC-1 was also demonstrated to repress proliferation as measured by reduced [3H] thymidine incorporation. Section V: The axolotl homologs of TR and TR (aTR) genes were isolated and utilized in a series of experiments to demonstrate an interaction between aTRs and aNPDC-1. As observed for RE, aNPDC-1 increases the binding of aTR-RXR heterodimer to xDR4, but no change in the mobility of the complex was observed. Interaction between aNPDC-1, aTR and aTR was localized to the amino terminus of aNPDC-1. In contrast to previous observations for other nuclear receptors, aNPDC-1 was found to stimulate transcription mediated by axolotl TRs, suggesting a role for aNPDC-1 in axolotl metamorphosis. Section VI: A summary of data presented in the previous sections as well as a presentation of future directions and a proposed model for NPDC-1 actions in retinoid and thyroid-receptor mediated signaling in axolotl.

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