Date of Award

Spring 5-12-2021

Document Type


Degree Name

Bachelor of Arts



First Advisor

Dr. Wade Powell


Amphibian metamorphosis, and vertebrate development more broadly, is governed by circulating thyroid hormone and glucocorticoids. These endocrine signals are transduced into transcriptional changes by the thyroid hormone receptor (TR) and glucocorticoid receptor (GR). As precisely regulated processes, TR and GR signaling pathways are vulnerable to disruption by environmental signals, such as aryl hydrocarbon receptor (AHR) activation. One classical AHR ligand is the toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a byproduct of many industrial processes and component of Agent Orange that induces a wide array of deleterious effects in vertebrates. The zinc-finger domain transcription factor krüppel-like factor 9 (klf9) is a central regulator of metamorphosis. Klf9 transcription can be altered by the thyroid hormone triiodothyronine (T3), glucocorticoids, and TCDD in several vertebrate species, but the precise mechanism of this regulation remains unclear. The klf9 upstream region includes a highly-conserved enhancer cluster containing a functional glucocorticoid response element (GRE) and a thyroid hormone response element (TRE), but there are many other potential regulatory elements as well. Using transactivation assays, I examined the molecular basis of the TR, GR, and AHR pathway interactions, demonstrating that the three pathways synergize to induce klf9 transactivation, and that a glucocorticoid response can be isolated to a previously-uncharacterized segment of the klf9 regulatory region. These results were then validated by qPCR assays measuring endogenous klf9 induction upon TR, GR, and AHR activation in the XLK-WG cell line and prometamorphic X. laevis tadpoles.

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