An Aryl Hydrocarbon Receptor from the CaecilianGymnopis multiplicataSuggests Low Dioxin Affinity in the Ancestor of All Three Amphibian Orders

Abstract

AbstractThe aryl hydrocarbon receptor (AHR) plays pleiotropic roles in the development and physiology of vertebrates in conjunction with xenobiotic and endogenous ligands. It is best known for mediating the toxic effects of dioxin-like pollutants such as 2,3,7,8-tetracholordibenzo-p-dioxin (TCDD). While most vertebrates possess at least one AHR that binds TCDD tightly, amphibian AHRs bind TCDD with very low affinity. Previous analyses of AHRs fromXenopus laevis(a frog; order Anura) andAmbystoma mexicanum(a salamander; order Urodela) identified three amino acid residues in the ligand-binding domain (LBD) that underlie low-affinity binding. InX. laevisAHR1β, these are A354, A370, and N325. Here we extend the analysis of amphibian AHRs to the caecilianGymnopis multiplicata, representing the remaining extant amphibian order, Apoda.G. multiplicataAHR groups with the monophyletic vertebrate AHR/AHR1 clade. The LBD includes all three signature residues of low TCDD affinity, and a structural homology model suggests that its architecture closely resembles those of other amphibians. In transactivation assays, the EC50 for reporter gene induction by TCDD was 17.17 nM, comparable toX. laevisAhR1β (26.23 nM) andAmbystomaAHR (34.09 nM) and dramatically higher than mouse AhR (0.13 nM), a trend generally reflected in direct measures of TCDD binding. These shared properties distinguish amphibian AHRs from the high-affinity proteins typical of both more ancient vertebrate groups (teleost fish) and those that appeared more recently (tetrapods). We suggest that AHRs with low TCDD affinity represent a basal characteristic that evolved in a common ancestor of all three extant amphibian groups.Research HighlightsA caecilian aryl hydrocarbon receptor exhibits low dioxin binding and sensitivity.The protein’s ligand-binding domain resembles frog and salamander AHRs in structure and function.AHR with low dioxin affinity likely evolved in a common ancestor of all three extant amphibian groups.Graphical Abstract