Neurotoxicity of PBDEs and metabolites : concern for the developing brain?

Abstract

Increasing human exposure to brominated flame retardants (BFRs), including the widely used polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCD), raises concern about possible neurotoxicity in humans, particularly through developmental exposure. Recently, also several hydroxylated PBDEs (OH-PBDEs) have been detected in human serum and cord blood. The aim of this research was to investigate mechanisms underlying previously observed behavioral impairments in rodents after neonatal exposure to BFRs. Effects of neonatal exposure to the environmentally relevant BDE-47 on a form of hippocampal synaptic plasticity, long-term potentiation (LTP), were investigated in mice. After exposure to BDE-47, LTP and several hippocampal postsynaptic proteins were reduced, without presynaptic effects. Nonetheless, modest increases of vesicular catecholamine release as well as basal and depolarization-evoked intracellular Ca2+ ([Ca2+]i) were observed in PC12 cells when acutely exposed to a high concentration of BDE-47, by using amperometry and imaging of the fluorescent Ca2+-sensitive dye Fura-2, respectively. To investigate differences in neurotoxic potential, acute in vitro effects of PBDEs and metabolites as well as HBCD on vesicular catecholamine release and basal and depolarization-evoked [Ca2+]i were also investigated. No effects on basal or depolarization-evoked [Ca2+]i were observed during exposure to BDE-49, BDE-99, BDE-100, BDE-153 or methoxylated BDE-47. Acute exposure of PC12 cells to 6-OH-BDE-47 induced vesicular catecholamine release, which coincided with a transient increase in [Ca2+]i observed shortly after the onset of exposure. To investigate possible structure-activity relationships, the effects of other OH-PBDEs on [Ca2+]i were also investigated in PC12 cells. All OH-PBDEs induced Ca2+ release from intracellular stores, although with different lowest-observed-effect concentrations (LOECs). It was found that the major intracellular Ca2+ sources were either endoplasmic reticulum (ER) or both ER and mitochondria. Lower LOECs were observed for some OH-PBDEs when investigating fluctuations in [Ca2+]i. OH-PBDEs and HBCD dose-dependently inhibited depolarization-evoked [Ca2+]i. For OH-PBDEs, this inhibition was potentiated by preceding increase in basal [Ca2+]i. The inhibition appeared more sensitive to Ca2+ release from intracellular stores than to influx of extracellular Ca2+, suggesting involvement of Ca2+-dependent regulatory processes close to voltage-gated Ca2+ channels. In a heterogeneous in vitro model for neurodevelopment, human neural progenitor cells (hNPCs), BDE-47 and 6-OH-BDE-47 also increase [Ca2+]i. In these cells, LOECs of both BDE-47 and 6-OH-BDE-47 are an order of magnitude lower compared to in PC12 cells, suggesting a higher sensitivity for disturbance of [Ca2+]i in hNPCs. In summary, a reduction of LTP together with changes in postsynaptic proteins in the mouse hippocampus could be related to the neurobehavioral effects of PBDEs. OH-PBDEs are more potent in disturbing [Ca2+]i and neurotransmitter release compared to BDE-47 or methoxylated BDE-47. Shielding of the OH-group on both sides with large atomic groups lowers the potency of OH-PBDEs. Besides increasing basal [Ca2+]i, OH-PBDEs inhibit depolarization-evoked [Ca2+]i, depending on preceding basal [Ca2+]i. HBCD inhibits depolarization-evoked [Ca2+]i and neurotransmitter release. The findings in this research indicate that bioactivation by oxidative metabolism adds considerably to the neurotoxic potential of PBDEs, raising concern for the developing brain. Therefore, oxidative metabolism should be included in the risk assessment for (developmental) neurotoxicity by PBDEs in humans