Aromatase is the only enzyme converting androgens into estrogens, which are key actors in reproductive biology. Teleost fishes have two copies of the cyp19a1 gene that encode two isoforms of aromatase: cyp19a1a encodes ovarian aromatase, while the cyp19a1b gene encodes brain aromatase. Our recent work showed that, in zebrafish, aromatase B is strongly expressed in a unique brain cell type, the radial glial cells (RGC). In mammals, such cells act as stem cells during embryonic development before disappearing in adults. On the contrary, in fishes, RGCs persist in adult where they act as neuronal progenitors allowing the brain to constantly keep growing. We have also shown that, intriguingly, the cyp19a1b gene is very sensitive to estrogens, through a mechanism that involves a well conserved ERE. This feature makes this gene an outstanding biomarker of xeno-estrogen exposure, and we have developed an in vivo assay allowing detection of estrogenic activity with a very high sensitivity. This assay is based on a transgenic zebrafish tg(cyp19a1b-GFP) line that expresses GFP in RGCs. By quantifying GFP expression in live fish, we show that short-term exposure of tg(cyp19a1b-GFP) embryos from 0 to 120 hpf to a variety of well established estrogenic compounds (estradiol, estriol, estrone, ethinylestradiol,, zearalenone and its metabolites, nonyl, octyl and tert-pentylphenol, BPA, benzophenones derivates, etc) turns on GFP expression in a concentration-dependent manner. Overall, we demonstrate the remarkable usefulness of the tg(cyp19a1b-GFP) embryos as a reliable, sensitive and rapid in vivo estrogenic screening assay. As we have also evidenced an effect of estrogens on the neurogenic activity of zebrafish, abnormal exposure of fish embryos to estrogenic endocrine disruptors is likely to affect the neurogenic process. Supported by the ANR NEED and the NEMO programme
