Date of Award

Spring 5-11-2016

Document Type

Thesis

Degree Name

Bachelor of Arts

Department

Biology

First Advisor

Wade Powell

Abstract

Gene duplication confers genetic redundancy that can facilitate subfunctionalization, the partitioning of ancestral functions between each gene. We capitalize on a recent genome duplication in Xenopus laevis (the African clawed frog) in order to interrogate putative functional differentiation of X. laevis aryl hydrocarbon receptors AHR1a and AHR1b. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that induces expression of a battery of detoxification enzymes and other transcriptional targets in response to a diverse cohort of xenobiotic and endogenous agonists. The toxicity of planar aromatic hydrocarbons (PAH) and planar halogenated aromatic hydrocarbons (PHAH), including 2,3,7,8-tetracholordibenzo-p-dioxin (TCDD), is a consequence of AHR-mediated gene expression. However, AHR also coordinates endogenous functions in development and physiology. We aim to discern the partitioned functions of X. laevis AHRs to gain a more comprehensive understanding of the roles of the single human AHR. X. laevis AHR1a and AHR1b are characterized by 86% amino acid identity and differential expression patterns in vivo. We hypothesize that each AHR mediates distinct transcriptional functions in xenobiotic toxicity or development. As one test of this hypothesis, we transfected X. laevis XLK-WG cells with TALENs, generating cell lines lacking functional AHR1a or AHR1b. Transcriptional responsiveness of these mutant strains was measured in order to characterize the functions of each isolated AHR paralog. We find that both AHR1a and AHR1b are required for maximal TCDD induction of the canonical AHR target gene Cytochrome P4501A6 (CYP1A6), despite greater abundance of AHR1a relative to AHR1b in wild-type cells. In response to TCDD, AHR1a alone mediates expression of the more modestly induced target genes Aryl hydrocarbon receptor repressor (AHRR), Spectrin repeat-containing nuclear envelope protein 1 (SYNE-1), and Gap junction protein gamma 1 (GJC1). In contrast, CYP1A6 induction by the endogenous AHR agonist FICZ is mediated solely by AHR1b, and this response is insensitive to AHR1a mutation. While FICZ is not an AHR1a agonist for CYP1A6, AHR1a induces AHRR, SYNE-1, and GJC1 following FICZ treatment; AHR1b mutation did not diminish induction of these targets. These data indicate that AHR1a and AHR1b transcriptional functions are distinct and differentially responsive to specific agonists.

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