Impairment of lower jaw growth in developing zebrafish exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin and reduced hedgehog expression.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been shown to cause a multitude of detrimental effects to developing zebrafish (Danio rerio). Previously, we demonstrated that jaw growth was impaired by TCDD exposure, but the exact mechanism underlying these malformations remained unknown. In the present study, we investigated the involvement of hedgehog genes and their downstream signaling in TCDD-mediated jaw malformation. We demonstrate that the developing lower jaw expresses sonic hedgehog a (shha), sonic hedgehog b (shhb) and their receptors, patched1 (ptc1) and patched2 (ptc2), as well as the downstream transcription factors, gli1 and gli2a. Loss of Hh signaling in mutants (sonic you) and larvae treated with a Hh inhibitor (cyclopamine), resulted in similar effects as those caused by TCDD. Moreover, TCDD exposure caused downregulation of shha and shhb in a manner dependent on aryl hydrocarbon receptor 2 (ahr2). Although this suggested an involvement of Hh signaling in TCDD-mediated impairment of jaw growth, we did not observe downregulation of ptc1 and ptc2, receptors dependent on Hh signaling. Furthermore, while the overall occurrence of apoptosis in the developing jaw was minimal, it was significantly increased in larvae treated with cyclopamine. In contrast, both TCDD and cyclopamine markedly reduced immunoreactivity against phosphorylated histone 3, a cell proliferation marker in the developing jaw. Taken together, our data suggest that Ahr2-mediated downregulation of Hh signaling, leading to a failure of cell proliferation, contributes to TCDD induced inhibition of lower jaw growth. TCDD may impair jaw growth through other pathway(s) in addition to Hh signaling.