Ah receptor action in developing zebrafish (Danio rerio)
Description
The aryl hydrocarbon receptor (AhR) mediates toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a contaminant found ubiquitously in the environment. Teratogenic and carcinogenic activities of TCDD are well documented, however, little is known about the molecular basis of these actions. A transgenic zebrafish was developed that expressed green fluorescent protein (GFP) in the presence of AhR ligands. The time at which embryos were exposed to TCDD was critical to their survival. Exposure during the first 24 hours of development resulted in 100% mortality by 10 days post-fertilization, compared to 80% survival in control fish (0.01% DMSO). Comparable to controls, embryos exposed to TCDD (1 nM and 10 nM) after the first 15 hours of development exhibited 80% survival at 10 days post-fertilization. TCDD induced cranio-facial abnormalities in exposed animals. TCDD induced jaw dysmorphogenesis in 100% vs. 5% of fish when exposure was initiated at 1 or 15 hours post-fertilization, respectively. Control fish did not exhibit phenotypic aberrations. RTPCR detected mRNA expression for AhR and its dimerization partner, Arnt in unfertilized oocytes and at all developmental stages examined. Cytochrome P450 1A1 (CYP1A1) is an early biochemical marker of AhR action. A zebrafish cDNA fragment of CYP1A1 was cloned and used to detect functional AhR in embryos. TCDD induced CYP1A1 mRNA by 15 hours post-fertilization. CYP1A1 protein and activity were detected at 3 days post-fertilization. Induction was dependent on the age of the embryo and state of hatch. Precocious hatching, induced manually or chemically, did not induce CYP1A1 activity prematurely, and inhibition of hatching prevented induction of activity. Data presented show that CYP1A1 is regulated differently in embryos than adults and that TCDD mediated toxicity was not CYP1A1 dependent. Alcian blue stain indicated that TCDD- and retinoic acid-induced jaw dysmorphogenesis were phenotypically similar and targeted Hox gene-regulated skeletal structures. TCDD-induced Hoxa-1 levels and broadened the region of Hoxb-1 expression as shown by Northern blot analysis and in situ hybridization, respectively. Disruption of Hox gene regulation suggests a novel mechanism by which AhR disrupts morphogenic development following TCDD exposure