G-Protein-Coupled Estrogen Receptor-1 (GPER): A Potential Target For Xenoestrogens During Vertebrate Embryogenesis.

G-protein-coupled estrogen receptor 1 (GPER) is a G-protein-coupled receptor that induces non-genomic signaling in response to some nuclear estrogen receptor ligands. To date, the distribution and functional characterization of GPER within vertebrate organs have been restricted to juvenile and adult animals. However, the role of GPER during vertebrate embryogenesis is currently unknown. Therefore, the overall objectives of this dissertation were to (1) characterize the spatiotemporal expression patterns of gper during vertebrate embryogenesis and (2) identify the potential impacts of ligand-induced GPER activation during vertebrate embryogenesis. Using wildtype zebrafish, we first demonstrated that (1) gper exhibited strong stage-dependent expression patterns during embryogenesis; (2) aberrant GPER activation by a selective GPER agonist (G-1) - but not inhibition by a selective GPER antagonist (G-15) - resulted in developmental malformations; and (3) co-exposure with G-15 blocked G-1-induced teratogenesis, suggesting that G-1 toxicity is mediated via aberrant GPER activation. Based on these data, we hypothesized that the developing cardiovascular system was the primary target organ for G-1-induced toxicity. Using quantitative in vivo imaging assays and transgenic zebrafish (fli1:egfp), we then demonstrated that G-1 exposure resulted in impaired cardiac morphology and performance as well as, at higher concentrations, early circulatory arrest. Similarly, embryonic exposure to reference xenoestrogens - genistein and bisphenol-A (BPA) - with known affinity to human GPER resulted in concentration-dependent effects (albeit with lower potency than G-1) on the developing cardiovascular system. In order to begin revealing mechanisms of G-1-, genistein-, and BPA-induced cardiovascular malformations, we then relied on a battery of inhibitors and antagonists that target signaling events downstream of GPER. Based on these studies, we found that cyclic adenosine monophosphate and nitric oxide were involved in mediating G-1-induced toxicity whereas mitogen-activated protein kinase (MAPK) [but not phosphatidyl-inositol 3-kinase (PI3K)] and epidermal growth factor receptor (EGFR) played a protective role in mitigating G-1-induced toxicity. While genistein and BPA appeared to involve distinct signaling mechanisms compared to G-1, our data suggest that cardiovascular malformations resulting from G-1, genistein, or BPA exposure likely involved EGFR transactivation, MAPK activation, and reduced levels of vascular endothelial growth factor-a (Vegf-a) and Notch ligand delta like 4 (Dll-4) mRNA. Overall, our findings suggest that GPER activation represents a potentially novel and understudied mechanism of cardiovascular toxicity for environmentally relevant chemicals that affect vertebrate embryogenesis.