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

Abstract

BackgroundPolybrominated 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.ObjectiveWe 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.MethodsWe analyzed [3H]ryanodine binding, microsomal Ca2+ fluxes, cellular measurements of Ca2+ homeostasis, and neurotoxicity to define mechanisms and specificity of PBDE-mediated Ca2+ dysregulation.ResultsPBDEs 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.ConclusionsWe 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.

An Extended Structure–Activity Relationship of Nondioxin-Like PCBs Evaluates and Supports Modeling Predictions and Identifies Picomolar Potency of PCB 202 Towards Ryanodine Receptors

“…We demonstrate that in vitro ; however; the total number of PCBs, and other noncoplanar compounds, including PBDEs and hydroxylated-PCBs (Fritsch and Pessah, 2013;Kim et al, 2010;Niknam et al, 2013), that could contribute to RyR mediated toxicity in environmental mixtures, is probably higher (Rayne and Forest, 2010). Work suggests that RyR-active PCBs and PBDEs display additivity at the receptor (Fritsch and Pessah, 2013;Kostyniak et al, 2005;Pessah et al, 2006) demonstrating that the Ca 2þ channel is likely a common target of contaminants mixtures currently found in the environment.…”

Triclosan impairs excitation–contraction coupling and Ca 2+ dynamics in striated muscle

“…Receptor-based screens (12,16) revealed that similar to PCBs (17) and PBDEs (18), TCS has potential for direct interaction with type 1 and type 2 ryanodine receptors (RyR1 and RyR2), supporting a possible convergent mechanism of action among structurally related noncoplanar halogenated persistent organic pollutants. Despite extensive analysis of TCS efficacy and toxicity, the potential of TCS to alter cardiac and skeletal muscle physiology have not been previously reported (2).…”

Isolation of Acetobacterium sp. Strain AG, Which Reductively Debrominates Octa- and Pentabrominated Diphenyl Ether Technical Mixtures

“…Given the high specificity toward para debromination by culture G, dioxin-like PBDE congeners, if there are any, are not likely to form, since both para bromines are indispensable in a dioxin-like structure (42). One recent study suggested that single-substituted para bromine is critical for enhancing ryanodine receptor (RyR) activity in human kidney cells, which is relevant for the neurotoxicity of PBDEs (43). Again, this PBDE structure (single para bromine) could also be avoided in the debromination process by culture G because of its rapid removal of both para bromines.…”