Embryonic exposures to flame retardant tetrabromobisphenol A (TBBPA) disrupts dorsoventral patterning in zebrafish

Tetrabromobisphenol A (TBBPA), a widely used flame retardant in commercial products such as synthetic textiles, plastics, and electronics poses potential toxicity risks through indoor exposure. This study aims to leverage zebrafish as a model to study TBBPA impacts on zebrafish dorsoventral patterning--a process that lays the foundation of an embryos axial determination and localization of specific tissues and organs. Zebrafish embryos were exposed to varying concentrations of TBBPA (0-10 {micro}M) at either 0.75- or 6-hours post-fertilization (hpf) and phenotyped at 8 or 24 hpf. Following this, whole-mount immunohistochemistry (IHC) was conducted out to quantify various proteins important in the BMP signaling pathway, epithelial-to-mesenchymal transition (EMT) and ectoderm and mesoderm germ layers. Importantly, these assessments were done at environmentally relevant concentrations ranging down to nM and pM levels. Our data showed a significant concentration-dependent increase in ventralization phenotypes, marked by enlarged blood island area, coupled with a disruption of the ventral-to-dorsal gradient of pSMAD protein levels, indicating BMP signaling disruptions. Perturbations in epithelial-to-mesenchymal transition (EMT), as evidenced by changes in E- and N-cadherin expression and Snail2 transcription factor levels, indicated impaired cell migration on TBBPA exposures. We then interrogated if TBBPA impacts germ layers and structures derived from specific germ layers. We observed significant concentration-dependent changes in levels of Sox2 and Sox10- both indicators of neural crest cell formation and differentiation from the ectoderm. We also observed a significant reduction of Tbx16- a marker of paraxial mesodermal cells. Collectively, both these data show TBBPA-induced impact on germ layers. Finally, we examined specific cell types derived from ectoderm and mesoderm and showed TBBPA-induced inhibition of cartilage development (derived from ectodermal neural crest cells) and blood cell development (derived from mesodermal cells). Our findings collectively demonstrate that TBBPA-induced disruptions in early developmental signaling and dorsoventral patterning may contribute to systemic toxicity in zebrafish embryos. Importantly, we see these disruptions at environmentally relevant concentrations, reinforcing the importance of continued interrogation of TBBPA in targeting early embryogenesis.