Tetrabromobisphenol S (TBBPS)-induced developmental toxicity in zebrafish embryos: impacts on cytoskeletal proteins during gastrulation

3,3',5,5'-Tetrabromobisphenol S (TBBPS) is an emerging brominated flame retardant that is widely detected in the environment, yet its toxic effects remain poorly understood. The objective of this study was to use zebrafish as a model and determine effects of TBBPS exposure on gastrulation stages of embryogenesis. We initiated TBBPS exposures (0 or 40 µM) at 0.75 hours post fertilization (hpf), phenotyped hourly through cleavage, blastula, and gastrula stages, and showed that TBBPS-treated embryos exhibited delay in development beginning at ∼4 hpf, a stage corresponding to early gastrulation, with significant mortality during late-gastrulation. To examine the genetic basis of TBBPS-induced effects, we conducted mRNA sequencing on exposures from 0.75-7 hpf, revealing downregulation of cytoskeletal organization and tight junction assembly. We then fluorescent-stained for multiple cytoskeletal and tight junction proteins (α-tubulin, β-tubulin, ZO-1, F-actin) following exposures to a wide range of TBBPS concentrations (0, 0.004, 0.04, 0.4, 4 and 40 µM). F-actin expression was consistently upregulated across all exposed TBBPS concentrations. To determine whether activation of p38, a key regulator of F-actin polymerization, plays a role in TBBPS-induced F-actin disruption, we co-exposed embryos with TBBPS (0 or 40 µM) and a known p38 inhibitor (5 µM SB 203580) and assessed developmental phenotypes and F-actin levels. Treatment with p38 inhibitor significantly rescued TBBPS-induced effects, with co-exposure groups exhibiting cell stages and F-actin expression similar to controls. Collectively, these findings demonstrate that TBBPS exposures target gastrulation stages and define a novel mechanism in which TBBPS increases F-actin polymerization through p38/MAPK signaling pathways.