BDE-47 and 6-OH-BDE-47 modulate calcium homeostasis in primary fetal human neural progenitor cells via ryanodine receptor-independent mechanisms

Polybrominated diphenyl ethers (PBDEs) are bioaccumulating flame retardants found in rising concentrations in human tissue. Epidemiological and animal studies have raised concern for their potential to induce developmental neurotoxicity (DNT). Considering the essential role of calcium homeostasis in neurodevelopment, PBDE-induced disturbance of intracellular calcium concentration ([Ca²⁺]_i) may underlie PBDE-induced DNT. To test this hypothesis, we investigated acute effects of BDE-47 and 6-OH-BDE-47 on [Ca²⁺]_i in human neural progenitor cells (hNPCs) and unraveled involved signaling pathways. Short-time differentiated hNPCs were exposed to BDE-47, 6-OH-BDE-47, and multiple inhibitors/stimulators of presumably involved signaling pathways to determine possible effects on [Ca ²⁺]_i by single-cell microscopy with the fluorescent dye Fura-2. Initial characterization of calcium signaling pathways confirmed the early developmental stage of hNPCs. In these cells, BDE-47 (2 μM) and 6-OH-BDE-47 (0.2 μM) induce [Ca²⁺]_i transients. This increase in [Ca²⁺]_i is due to extracellular Ca ²⁺ influx and intracellular release of Ca²⁺, mainly from the endoplasmic reticulum (ER). While extracellular Ca²⁺ seems to enter the cytoplasm upon 6-OH-BDE-47 by interfering with the cell membrane and independent of Ca²⁺ ion channels, ER-derived Ca²⁺ is released following activation of protein lipase C and inositol 1,4,5-trisphosphate receptor, but independently of ryanodine receptors. These findings illustrate that immature developing hNPCs respond to low concentrations of 6-OH-BDE-47 by an increase in [Ca²⁺]_i and provide new mechanistic explanations for such BDE-induced calcium disruption. Thus, these data support the possibility of a critical window of PBDE exposure, i.e., early human brain development, which has to be acknowledged in risk assessment.