| While secreted peptides are often studied during early development, small molecules such as estrogens also signal in the embryo. The estrogen pathway includes the aromatase gene, cyp19, which synthesizes estrogen from testosterone, and the estrogen receptors (ESRs), which bind DNA as ligand-dependent transcription factors. In the zebrafish (Danio rerio), the components of the estrogen pathway have not been well-defined. Here, I describe a brain aromatase gene, cyp19b , and an estrogen receptor, ESR2a, during the first three days of development, from fertilization to hatching. Both genes are highly homologous to their human counterparts in amino acid sequence and genomic structure, demonstrating evolutionary conservation for 400 million years. Using a combination of RT-PCR, in situ hybridization, and a transgenic line, I determine the expression patterns of the two genes. Only ESR2a is maternally loaded into the embryo, followed by rapid degradation. Both genes are up-regulated in the embryo between 24 and 48 hours post fertilization (hpf). The location of ESR2a was not able to be determined; however cyp19b is first detected at 48 hpf in the preoptic area, hypothalamus, terminal nerve, and olfactory bulb. A transgenic line driving membrane GFP with a cyp19b promoter region recapitulates this expression pattern. Estrogens do not regulate ESR2a, but are strong regulators of cyp19b. This positive feedback loop enables up-regulation of cyp19b to spread in an autocrine and paracrine manner. Testosterone exposure also induces the cyp19b gene in zebrafish. A majority of this induction is blocked by an estrogen receptor antagonist. I conclude that much of the testosterone effect is accomplished by conversion to estradiol. The expression pattern of cyp19b in the brain and its control by steroid hormones is well conserved among the vertebrate lineage during embryogenesis. The expression of ESR2a also indicates this gene functions in late development. The pleiotropic effects of estrogen signaling in the developing brain could affect neural architecture resulting in morphological and behavioral effects well into adulthood. |
