Abstract
Activation of the transcription factor aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) prevents the formation of the epicardium and leads to severe heart malformations in developing zebrafish (Danio rerio). The downstream genes that cause heart malformation are not known. Because TCDD causes craniofacial malformations in zebrafish by downregulating the sox9b gene, we hypothesized that cardiotoxicity might also result from sox9b downregulation. We found that sox9b is expressed in the developing zebrafish heart ventricle and that TCDD exposure markedly reduces this expression. Furthermore, we found that manipulation of sox9b expression could phenocopy many but not all of the effects of TCDD at the heart. Loss of sox9b prevented the formation of epicardium progenitors comprising the proepicardium on the pericardial wall, and prevented the formation and migration of the epicardial layer around the heart. Zebrafish lacking sox9b showed pericardial edema, an elongated heart, and reduced blood circulation. Fish lacking sox9b failed to form valve cushions and leaflets. Sox9b is one of two mammalian Sox9 homologs, sox9b and sox9a. Knock down of sox9a expression did not cause cardiac malformations, or defects in epicardium development. We conclude that the decrease in sox9b expression in the heart caused by TCDD plays a role in many of the observed signs of cardiotoxicity. We find that while sox9b is expressed in myocardial cells, it is not normally expressed in the affected epicardial cells or progenitors. We therefore speculate that sox9b is involved in signals between the cardiomyocytes and the nascent epicardial cells.
Footnotes
- Received March 26, 2013.
- Accepted June 13, 2013.
This work was supported by the National Institutes of Health National Institute of Environmental Health Sciences [Grant R01 ES012716]; and the University of Wisconsin Sea Grant Institute, National Sea Grant College Program, National Oceanic and Atmospheric Administration, U.S. Department of Commerce [Grant NA 16RG2257], Sea Grant Project R/BT-25.
The contents are solely the responsibility of the authors and do not necessarily represent the official view of the National Institutes of Health National Institute of Environmental Health Sciences. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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- Copyright © 2013 by The American Society for Pharmacology and Experimental Therapeutics
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