Organophosphorus flame retardants are developmental neurotoxicants in a rat primary brainsphere in vitro model

Abstract

Due to regulatory bans and voluntary substitutions, halogenated polybrominated diphenyl ether (PBDE) flame retardants (FR) are increasingly substituted by mainly organophosphorus FR (OPFR). Leveraging a 3D rat primary neural organotypic in vitro model (rat brainsphere), we compare developmental neurotoxic effects of BDE-47—the most abundant PBDE congener—with four OPFR (isopropylated phenyl phosphate—IPP, triphenyl phosphate—TPHP, isodecyl diphenyl phosphate—IDDP, and tricresyl phosphate (also known as trimethyl phenyl phosphate)—TMPP). Employing mass spectroscopy-based metabolomics and transcriptomics, we observe at similar human-relevant non-cytotoxic concentrations (0.1–5 µM) stronger developmental neurotoxic effects by OPFR. This includes toxicity to neurons in the low µM range; all FR decrease the neurotransmitters glutamate and GABA (except BDE-47 and TPHP). Furthermore, n-acetyl aspartate (NAA), considered a neurologic diagnostic molecule, was decreased by all OPFR. At similar concentrations, the FR currently in use decreased plasma membrane dopamine active transporter expression, while BDE-47 did not. Several findings suggest astrogliosis induced by the OPFR, but not BDE-47. At the 5 µM concentrations, the OPFR more than BDE-47 interfered with myelination. An increase of cytokine gene and receptor expressions suggests that exposure to OPFR may induce an inflammatory response. Pathway/category overrepresentation shows disruption in 1) transmission of action potentials, cell–cell signaling, synaptic transmission, receptor signaling, (2) immune response, inflammation, defense response, (3) cell cycle and (4) lipids metabolism and transportation. Taken together, this appears to be a case of regretful substitution with substances not less developmentally neurotoxic in a primary rat 3D model.

Neurodevelopmental toxicity assessment of flame retardants using a human DNT in vitro testing battery

“…Disturbance of NCC migration causes, e.g., cleft palate or loss of functional hearing (Mayor and Theveneau 2013). Our data from human cells are in good agreement with model systems from other species: micromolar concentrations of IPPHP and t-BPDPHP were also toxic for zebrafish (Behl et al 2015;Alzualde et al 2018), Caenorhabditis elegans (Behl et al 2015;Boyd et al 2016), rat cortical neurons (Behl et al 2015), and 3D rat brain spheres (Hogberg et al 2020). t-BPDPHP specifically inhibits neurite outgrowth of rat cortical neurons at 14.9 μM (Behl et al 2015), an effect that we observe at similar concentrations in the NPC4, but not in the UKN4/5 assays.…”

“…Hence, interference with radial glia neural progenitors underlie a number of cortical malformations and cause mental retardation in genetic and infectious diseases (Guerrini and Dobyns 2014;Hu et al 2014;Juric-Sekhar and Hevner 2019). In a recent study, IDDPHP triggered an increase of nestin expression, and this was interpreted as evidence of reactive astrogliosis (Hogberg et al 2020). However, there may be alternative explanations, as changes in nestin may also point to effects on the radial glia and neural stem cell compartments.…”

Beyond Cholinesterase Inhibition: Developmental Neurotoxicity of Organophosphate Ester Flame Retardants and Plasticizers

“…2020 ). Similarly, transcriptomic and metabolomic analyses in a three-dimensional rat primary brainsphere model after OPE exposure showed perturbation in pathways related to neurotransmission, immune response, and cell cycle and fatty acid metabolism ( Hogberg et al. 2021 ).…”

Human Oligodendrocytes and Myelin In Vitro to Evaluate Developmental Neurotoxicity

“…However, OPFRs have still been demonstrated to elicit DNT effects on oligodendrocytes, such as 1,3-dichloro 2-propyl phosphate (TDCPP), which has been studied in zebrafish embryos and is thought to act through thyroid hormone disruption [130]. Furthermore, several OPFRs affected the expression of genes involved in myelination in a 3D rat primary in vitro model [180].…”

“…The same markers can also be studied by gene expression analysis [180]. For example, the gene expressions of different myelin and oligodendrocyte makers were assessed in BrainSpheres after the exposure to different demyelinating compounds [218].…”