Insights into Brominated Flame Retardant Neurotoxicity: Mechanisms of Hippocampal Neural Cell Death and Brain Region-Specific Transcriptomic Shifts in Mice
Brominated Flame Retardants (BFRs) reduce flammability in a wide range of products including electronics, carpets, and paint, but leach into the environment to result in continuous, population-level exposure. Epidemiology studies have correlated BFR exposure with neurological problems, including alterations in learning and memory. This study investigated the molecular mechanisms mediating BFR-induced cell death in hippocampal cells and clarified the impact of HBCD exposure on gene transcription in the hippocampus, dorsal striatum, and frontal cortex of male mice. Hippocampal derived HT-22 cell exposure to tetrabromobisphenol-A (TBBPA), hexabromocyclododecane (HBCD), or 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) (current, phasing out, and phased out BFRs, respectively) induced time, concentration, and chemical-dependent cellular and nuclear morphological alterations indicative of apoptosis, which aligned with alterations in cell cycle and increases in annexin V staining. All three BFRs increased p53 and p21 expression; however, inhibition of p53 nuclear translocation using pifthrin-a did not decrease cell death. Transcriptomic analysis upon low (10 nM) and cytotoxic (10 µM) BFR exposure indicated that HBCD and BDE-47 altered genes mediating autophagy-related pathways. Further evaluation showed BFR exposure increased LC3-II conversion and autophagosome formation, and co-exposure with the autophagy inhibitor 3-methyladenine (3-MA) attenuated cytotoxicity. Transcriptomic assessment of select brain regions from subchronically HBCD-exposed male mice demonstrated alteration of genes mediating vesicular transport, with greater impact on the frontal cortex and dorsal striatum compared to the dorsal and ventral hippocampus. These data demonstrate that BFRs can induce chemical-dependent autophagy in neural cells in vitro and provide evidence that BFRs induce region-specific transcriptomic effects in the brain. Overall design: To investigate the impact of brominated flame retardants on the brain and neural cell populations, HT-22 cells were treated with 10 nM or 10 µM TBBPA, HBCD, or BDE-47 for 48 hr. Results were compared to cells treated with a DMSO vehicle control. Alternatively, 8 wk old male mice were given peanut butter pellets containing 1 mg/kg HBCD once every 3 days for 28 days (10 doses). Following completion of the dosing period, frontal cortex, dorsal striatum, dorsal hippocampus, ventral hippocampus, liver, and kidney tissues were collected for RNA-Sequencing. Results were compared to peanut butter only vehicle control.