Para- and Ortho -Substitutions Are Key Determinants of Polybrominated Diphenyl Ether Activity toward Ryanodine Receptors and Neurotoxicity

Abstract

Background Polybrominated diphenyl ethers (PBDEs) are widely used flame retardants that bioaccumulate in human tissues. Their neurotoxicity involves dysregulation of calcium ion (Ca²⁺) signaling; however, specific mechanisms have yet to be defined. Objective We aimed to define the structure–activity relationship (SAR) for PBDEs and their metabolites toward ryanodine receptors type 1 (RyR1) and type 2 (RyR2) and to determine whether it predicts neurotoxicity. Methods We analyzed [³H]ryanodine binding, microsomal Ca²⁺ fluxes, cellular measurements of Ca²⁺ homeostasis, and neurotoxicity to define mechanisms and specificity of PBDE-mediated Ca²⁺ dysregulation. Results PBDEs possessing two ortho-bromine substituents and lacking at least one para-bromine substituent (e.g., BDE-49) activate RyR1 and RyR2 with greater efficacy than corresponding congeners with two para-bromine substitutions (e.g., BDE-47). Addition of a methoxy group in the free para position reduces the activity of parent PBDEs. The hydroxylated BDEs 6-OH-BDE-47 and 4′-OH-BDE-49 are biphasic RyR modulators. Pretreatment of HEK293 cells (derived from human embryonic kidney cells) expressing either RyR1 or RyR2 with BDE-49 (250 nM) sensitized Ca²⁺ flux triggered by RyR agonists, whereas BDE-47 (250 nM) had negligible activity. The divergent activity of BDE-49, BDE-47, and 6-OH-BDE-47 toward RyRs predicted neurotoxicity in cultures of cortical neurons. Conclusions We found that PBDEs are potent modulators of RyR1 and RyR2. A stringent SAR at the ortho and para position determined whether a congener enhanced, inhibited, or exerted nonmonotonic actions toward RyRs. These results identify a convergent molecular target of PBDEs previously identified for noncoplanar polychlorinated biphenyls (PCBs) that predicts their cellular neurotoxicity and therefore could be a useful tool in risk assessment of PBDEs and related compounds.