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Single cell RNA sequencing detects persistent cell type- and methylmercury exposure paradigm-specific effects in human cortical neurodevelopmental model

ABSTRACT: The developing human brain is uniquely vulnerable to methylmercury (MeHg) intoxication. Given the limited experimental access to MeHg-intoxicated human fetal brain, neither the threshold levels for neurotoxic effects nor the mechanisms underlying neurodevelopmental MeHg toxicity are clear. The observed persistent and latent effects in human and animal exposures and the relatively nonspecific neurodevelopmental changes suggest that MeHg impacts fundamental neurodevelopmental processes. Human cortical development is characterized by the sequential emergence of different neural stem cells, precursors, intermediate progenitors and postmitotic neurons, a sequence that can be replicated in human-induced pluripotent stem cell (hiPSC)-derived differentiating neuronal cultures. As the developing human cortex has been shown to be particularly vulnerable to fetal MeHg exposure, we assessed here by means of single cell RNA sequencing (scRNAseq) MeHg’s effects on different stages of differentiation and cell types present in early cortical cultures differentiated from hiPSC cells. Cortical cultures differentiating from hiPSC were exposed to toxicologically relevant MeHg (0.1 and 1.0 µM) continuously for 6 days at two different time points (days 4-10; and/or days 14-20) of early cortical differentiation under conditions below the cytotoxic threshold. Previous analysis at days 21-23 revealed subtle changes in cortical differentiation markers and cellular energetics, but no effect on glutathione levels, a biomarker of acute MeHg toxicity. Therefore, here we assessed the possibility of persistent effects on neurodevelopment at day 38 of differentiation (18 or 28 days after cessation of MeHg exposure). At this time, we observed subtle but significant changes in the population size of different cell types and the fractions of cells present in the different phases of the cell cycle. The observed MeHg-induced changes were developmental-stage and MeHg paradigm-specific. We further assessed overall differential gene expression, quantifying the number of genes showing significantly up- or down-regulated expression and identifying the genes affected at least partially identified biological processes affected. Importantly, the MeHg-induced changes in gene expression were also developmental-stage and MeHg-exposure paradigm specific. Thus, our studies demonstrate for the first time in a human model that MeHg exposure induces persistent changes in gene expression well after cessation of exposure, these changes are cell type- and developmental stage-specific, and this outcome is highly dependent on the concentration and timing of MeHg exposure. We conclude hiPSC-derived developing neuronal cultures provide an excellent human model to study persistent neurodevelopmental toxicity of MeHg in the developing human cortex.

ORGANISM(S): Homo sapiens

PROVIDER: GSE169751 | GEO | 2021/06/11

REPOSITORIES: GEO

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Project description:Maternal antibodies specific for antigens in the developing brain are implicated as risk factors for neurodevelopmental disorders, but how these antibodies interfere with neurodevelopment remain speculative. It has been postulated that immunoglobulin G-immune complexes (IgG-IC) activate Fc gamma receptors (FcγR) on non-immune cells in the brain, thereby modulating intracellular signaling and/or internalizing function-blocking antibodies specific for intracellular antigens. However, testing this hypothesis has been hindered by the paucity of data regarding FcγR in the developing brain. Thus, we first investigated FcγR expression in the brain of neonatal male and female rats using quantitative PCR analyses. FcgrIa, FcgrIIa, FcgrIIb, FcgrIIIa and Fcgrt transcripts were detectable in the cortex, hippocampus and cerebellum at postnatal days 1 and 7. These transcripts were also present in primary hippocampal and cortical cell cultures, where their expression was upregulated by IFNγ. In order to confirm protein abundance of FcγRIa, FcγRIIb and FcγRIIIa in cultured hippocampal and cortical neurons and astrocytes on the single cell and tissue level we used immunocytochemistry, western blotting, proteotype analysis, and flow cytometry. The data shows that a subpopulation of these cells co-express the excitatory FcγRIa and the inhibitory FcγRIIb. Functional analysis shows that exposure of hippocampal and cortical cell cultures to IgG-IC increased intracellular calcium and Erk phosphorylation, and triggered FcγR-mediated internalization of IgG. Collectively, these data demonstrate that developing neurons and astrocytes express signaling competent FcγR. which could establish a molecular mode of action of maternal antibodies could influence vulnerability to neurodevelopmental disorders via direct interactions with FcγR on non-immune cells in the developing brain. These findings support the hypothesis that maternal antibodies influence vulnerability to neurodevelopmental disorders via direct interactions with FcγR on non-immune cells in the developing brain.

Project description:Methylmercury (MeHg) toxicity in humans manifests deficits in neurological function. Cases of prenatal exposure to mercury have established that the developing nervous system is most highly susceptible to perturbation by MeHg. At a cellular level, MeHg-induced defects result from altered neuronal proliferation, migration and pathfinding. However, the molecular targets of MeHg that give rise to these outcomes are not fully understood. In an overall effort to identify the fundamental molecular targets of MeHg in neural development, we are investigating the effects of MeHg on gene expression and protein function in the Drosophila model. Since the fundamental signaling pathways in development of multicellular animals have been extensively characterized in Drosophila, and demonstrate high degree of conservation in vertebrates, we believe these data will lead us to the most pertinent pathways affected by MeHg and begin to elucidate the mechanism of MeHg neural toxicity relevant to cases of human exposure to this prevalent environmental toxin. Our aim is to identify fundamental signaling pathways in neural development that are targets for MeHg poisoning. Our hypothesis is that MeHg, by direct interaction with cysteine thiol groups, alters the function signaling pathway proteins and subsequently alters transcription of target genes in the respective pathways. Our hypothesis is supported by our recent data demonstrating a direct action of MeHg in activating the Notch receptor pathway and upregulating target gene expression. We anticipate a microarray analysis will elucidate additional fundamental signaling pathways where transcription is affected by this toxin. Fertilized female flies are fed on food containing methylmercury (1-20 micromolar) or solvent control (DMSO) for a period of five days to allow for MeHg penetration in to eggs. Flies are then transfered to a cage for embryo collection for a discrete window of time (e.g. 1hr). Embryos are aged 16-24 hours and collected for total RNA extraction using the Trizol reagent. Total RNA is quantitated by spectrophotometry with a Nanodrop reader. While concentration dependent effects of MeHg are of interest, initial experiments will be conducted on samples exposed to a single concetration known to illicit effect in other bioassays (e.g. 10 micromolar). The goal is to see which genes are up- or down-regulated with MeHg exposure, as compared to control embryos that are not exposed. Emphasis in the analysis stage will be in identifying target genes of known signaling pathways.

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Single cell RNA sequencing detects persistent cell type- and methylmercury exposure paradigm-specific effects in human cortical neurodevelopmental model

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