2,3,7,8-Tetrachlorodibenzo-p-dioxin Exposure Prevents Cardiac Valve Formation in Developing Zebrafish

Abstract

Cardiovascular malformations are one of the most common congenital birth defects observed in humans. Defects in cardiac valves disrupt normal blood flow. Zebrafish are an outstanding experimental model for studying the effects that environmental contaminants have on developmental processes. Previous research has shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes blood regurgitation in the heart and reduces peripheral blood flow in embryonic zebrafish, suggesting some form of valve failure. To test this we used video microscopy to examine valve function and structure in developing zebrafish exposed to TCDD. TCDD exposure produced blood regurgitation at both the atrioventricular (AV) and bulboventricular (BV) junctions. In marked contrast to control embryos exposed to the vehicle dimethyl sulfoxide, embryos exposed to TCDD failed to form valve leaflets as the heart matured. In addition, whereas TCDD did not block initial formation of the bulbus arteriosus, we found that TCDD exposure prevented the normal growth and development of this portion of the outflow tract. TCDD altered the localization of endothelial cells at the AV and BV junctions and altered the localized expression of mRNAs bmp4 and notch1b normally associated with the nascent valves. Taken together, our results demonstrate that although TCDD does not prevent the initial specification of the presumptive valve locations, TCDD exposure produces severe alterations in valve development, leading to blood regurgitation and failing circulation in the developing zebrafish.

Retene causes multifunctional transcriptomic changes in the heart of rainbow trout ( Oncorhynchus mykiss ) embryos

“…It is possible that the main factor behind the developmental cardiovascular toxicity and other adverse effects of TCDD is the down-regulated sox9 in epicardium (Mathew et al, 2008,Xiong et al, 2008,Mehta et al, 2008,Hofsteen et al, 2013.…”

“…The AhR agonists disturb the development of the heart and the vasculature in fishes, and cardiovascular pathology is the first sign of dioxin-like toxicity in zebrafish (Danio rerio) and medaka (Oryzias latipes) (Henry et al, 1997,Hornung et al, 1999,Antkiewicz et al, 2005,Carney et al, 2006,Mehta et al, 2008,Plavicki et al, 2013,Scott, 2009). The most studied AhR agonist, TCDD, prevents cardiac valve formation, inhibits epicardial and proepicardial development, causes altered looping of the heart, and reduces the volume and number of cardiomyocytes in fishes (Hornung et al, 1999,Antkiewicz et al, 2005,Carney et al, 2006,Mehta et al, 2008,Plavicki et al, 2013. Similarly, other AhR agonists (e.g., PCB126, retene, benz [a]anthracene) disturb the development of the heart in zebrafish and medaka (Incardona et al, 2006,Grimes et al, 2008,Scott et al, 2011,Scott, 2009).…”

Sox9bIs Required for Epicardium Formation and Plays a Role in TCDD-Induced Heart Malformation in Zebrafish

“…After formation of the epicardial layer, epicardialderived cells migrate into the underlying myocardium and assist development of the cardiac valves (Lie-Venema et al, 2007, 2008. Given that TCDD prevents formation of the valve cushions (Mehta et al, 2008), and reduced levels of sox9b prevent epicardium development, we hypothesized that sox9b may be needed for zebrafish valve development. During the normal progression of valve development, a ring of endothelial cells forms marking the presumptive valve sites at the AV junction and the outflow junction between the ventricle and bulbus arteriosus (Keegan et al, 2002;Bartman et al, 2004).…”

Reproductive and developmental toxicity of dioxin in fish

“…TCDD also inhibited heart valve development (Figure 4C). This was evidenced by a failure of valve cushion and subsequent valve leaflet formation at the atrio-ventricular (AV) and bulbo-ventricular (BV) valve junctions, resulting in blood regurgitation between heart chambers (Figure 4D) (Mehta et al, 2008). TCDD exposed larvae also exhibited abnormal development of the bulbus arteriosus (Mehta et al, 2008).…”