TCDD Inhibits Heart Regeneration in Adult Zebrafish

The heart is a target for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity in a number of vertebrate species, including humans (Dalton et al., 2001; Pesatori et al., 1998), nonhuman primates (Allen et al., 1977), rodents (Thackaberry et al., 2005), birds (Ivnitski et al., 2001), and fish (Carney et al., 2006a; Peterson et al., 1993). In zebrafish (Danio rerio), TCDD exposure during the first few days of development causes pericardial edema, decreased cardiomyocyte number, cardiac valve malformation, reduced cardiac output, altered looping, and ventricular standstill (Antkiewicz et al., 2005; Carney et al., 2006a; Henry et al., 1997; Mehta et al., 2008). Although the newly formed zebrafish heart is exquisitely sensitive to TCDD toxicity, this sensitivity disappears with development, until at the end of the larval stage the heart is no longer sensitive to a lethal TCDD exposure (Lanham et al., 2012). In addition to being a model system for studying cardiotoxicity, the zebrafish heart has been the subject of intense interest because of its ability to regenerate myocardium after surgical removal of part of the ventricle (Poss et al., 2002; Wang et al., 2011; Chablais et al., 2011). Damage of human ventricular myocardium is common in response to myocardial infarction; however, unlike adult zebrafish, humans are unable to regenerate the damaged cardiac muscle (Chablais et al., 2011). Following partial surgical amputation of a portion of the adult zebrafish ventricle, a fibrin-rich blood clot is formed. Shortly thereafter, cells proliferate and migrate toward the wound site to form a cluster of cardiac progenitor cells (Jopling et al., 2010; Kikuchi et al., 2010, 2011; Lepilina et al., 2006; Poss et al., 2002; Schnabel et al., 2011). The regenerating tissue goes through a process of sealing the wound, recruitment of cells, differentiation, and organization to eventually form new contractile myocardium that can hardly be distinguished from the original. During zebrafish heart regeneration, the regenerating heart is sometimes said to become “embryonic-like” because genes normally expressed during heart development are found expressed in cells along the outer most layer of the heart, the epicardium (Lepilina et al., 2006; Poss et al., 2002). Among these genes is raldh2, encoding a key enzyme in the synthesis of retinoic acid (RA). The expression of raldh2 is thought to provide a source of RA that assists cardiomyocyte proliferation (Kikuchi et al., 2011; Lepilina et al., 2006). Knowing that TCDD is toxic to embryonic but not adult hearts and that the response to wounding in the adult heart involves the dedifferentiation of cells to an embryonic-like state, we wondered if TCDD exposure would have an impact on the ventricular response to surgical resection. This idea is strengthened by the knowledge that TCDD exposure blocks regeneration of the rat liver (Bauman et al., 1995; Mitchell et al., 2006) and the zebrafish fin (Zodrow and Tanguay, 2003). Here, we report that pre-exposure to TCDD does indeed inhibit regeneration of the heart muscle. TCDD prevents the intense proliferation of cells around the wound site and also alters the pattern of gene expression induced by wounding at the ventricle. We speculate that TCDD exposure inhibits a process that is important for progenitor cells needed for both early heart development and regeneration.