Marine medaka responds differently to dioxin compared with its close freshwater relative, the Japanese medaka: the AHR molecular mechanism

The global water pollution now calls for precise risk assessment of chemicals, e.g., dioxins and the emerging dioxin-like compounds (DLCs). The freshwater and marine medaka have been widely implemented in the toxicity testing, and perhaps give mechanistic information for comparative biology. The question that will they report equal results due to their close phylogenetic relation has been raised, therefore, we explored their physiological and molecular responses to dioxin. As the mediator of the dioxin toxicity, the aryl hydrocarbon receptor (AHR) of marine medaka (Oryzias melastigma) has not been functionally characterized and might be species-specific. In terms of sensitivity to dioxin--2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the EC50 values of omeAHR1a (0.16{+/-}0.12 nM), omeAHR1b (2.96{+/-}2.96 nM), omeAHR2a (0.44{+/-}0.30 nM), and omeAHR2b (9.00{+/-}6.88 nM) exhibit marked variations. The omeAHR2a and omeAHR1a display heightened sensitivity compared to the freshwater Japanese medaka (Oryzias latipes) counterparts olaAHR2a and olaAHR1a, respectively. The results indicate the in vitro sensitivity of AHR among species can vary by one or two orders of magnitude. Physiologically, the marine medaka (LC50: 1.64 ng/L (95% CI: 1.05-2.55 ng/L)) also exhibits a pronounced sensitivity to TCDD than Japanese medaka (LC50: 3.42 ng/L (95% CI: 1.37-6.48 ng/L), aligning with the in vitro AHR sensitivity. Further mechanistic investigations using additional ligands and computational modeling reveal that: 1) most of omeAHR2a, olaAHR2a, dreAHR2, and hsaAHR interact with ligands in the affinity order of TCDD > PCB126 > BNF > indole, mirroring their AHR transactivation potency, but the docking poses and dynamics can vary; 2) one AHR subforms high sensitivity to dioxin--TCDD may extend to DLCs but not to other types of ligands. These insights underscore the difference of AHR biology among species even the close relative species, and finger out the necessity for meticulous consideration when evaluating the toxicity of compounds and when extending predictive toxicity assessments to more species.