Aryl Hydrocarbon Receptor Activation Mediates Dioxin Toxicity in an Age and Cell Dependent Manner in Zebrafish

Exposure of early life stage fish to 2,3,7,8 tetrachlorodibenzo- p-dioxin (TCDD), and other dioxin like compounds, causes developmental toxicity characterized by pericardial and yolk sac edema, heart malformations, reduced blood flow, hemorrhage and craniofacial malformations. In feral fish populations this assortment of endpoints also characterizes a condition known as blue sac syndrome, which affects early life stage fish. Older fish are resistant to this syndrome. How TCDD causes blue sac syndrome is unknown, as is the reason for the difference in sensitivity across life stages. However, it is well established that TCDD toxicity is mediated by the activation of a cytosolic receptor and transcription factor known as the aryl hydrocarbon receptor (AHR). To better understand the shift in sensitivity across life stages zebrafish were exposed to TCDD at different time points between the embryonic and juvenile stages, and their responses quantitatively and qualitatively assessed. This showed that zebrafish remain sensitive to TCDD well into the larval stage and still exhibit heart malformations after the heart has formed. It's not until they have entered metamorphosis that they begin to show resistance to TCDD, suggesting that some remodeling event associated with metamorphosis may be changing their response to TCDD. It is generally believed that cardiovascular toxicity is responsible for early life stage mortality, and the heart is a likely target organ for TCDD to exert its toxic effects. However, there is also strong evidence to indicate that the vasculature is responsible for the cardiovascular defects. The only way to determine where TCDD is acting is to selectively activate or inhibit its effects in one or the other tissue. To this end a dominant negative AHR (dnAHR) and a constitutively active AHR (caAHR) were constructed and tested in vivo. Expression of the caAHR in only the myocardium of developing zebrafish was able to reproduce most, if not all, of the cardiovascular endpoints of TCDD associated with TCDD exposure in early life stage fish. This establishes that TCDD acting on the heart can sufficiently explain the etiology of blue sac syndrome, and advances our understanding of how dioxin acts to cause toxicity in developing embryos.