Aryl hydrocarbon receptor activation inhibits regenerative growth

There is considerable literature supporting the conclusion that inappropriate activation of the aryl hydrocarbon receptor (AHR) alters cellular signaling. We have previously established that fin regeneration is specifically inhibited by TCDD in adult zebrafish and have used this in vivo end point to evaluate interactions between AHR and growth controlling pathways. Because there are experimental limitations in studying regeneration in adult animals, we have developed a larval model to evaluate the effect of AHR activation on tissue regeneration. Two day old zebrafish regenerate their amputated caudal fins within 3 days. Here we demonstrate that TCDD specifically blocks regenerative growth in larvae. The AHR pathway in zebrafish is considerably more complex than in mammals, with at least three zebrafish AHR genes (zfAHR1a, zfAHR1b and zfAHR2), and two ARNT genes (zfARNT1 and zfARNT2). Although it was presumed that the block in regeneration was mediated by AHR activation, it had not been experimentally demonstrated. Using antisense morpholinos and mutant fish lines, we report that zfAHR2 and zfARNT1 are the in vivo dimerization partners that are required for inhibition of regeneration by TCDD. Several pathways including FGF signaling are essential for fin regeneration. Even though impaired FGF signaling and TCDD exposure both inhibit fin regeneration, their morphometric response are distinct suggesting that the mechanisms of impairment are different. With the plethora of molecular and genetic techniques that can be applied to larval stage embryos, this in vivo regeneration system can be further exploited to understand cross talk between AHR and other signaling pathways.