Methylmercury Rewires Brain Cell Genes at Doses That Leave Cells Alive

Methylmercury, the organic form of mercury that accumulates in fish and seafood, has long been known to attack the nervous system, but the precise molecular choreography of that attack has remained frustratingly opaque. Now a team of Brazilian researchers has mapped, in unprecedented detail, how the gene expression programs of two fundamental brain cell types respond to concentrations of the toxin that are lethal to some cells but leave others standing. The study, published in Discover Toxicology, exposes a startling truth: even at doses that do not kill cells, methylmercury scrambles the transcriptome in ways that echo the cognitive and developmental damage seen in mercury poisoning.

The research team, led by Laís Teixeira Bonfim and Edivaldo Herculano Corrêa de Oliveira, working at the Federal University of Pará and the Evandro Chagas Institute, exposed two rat cell lines to methylmercury for 24 hours. The B103 neuroblastoma line served as a stand-in for neurons, while the C6 glioma line represented glial cells, the support cells that outnumber neurons in the brain. Crucially, the researchers chose concentrations anchored to previously determined LC50 values, the doses that kill half the cells. For neurons, they tested 0.1 micromolar, a low non-lethal dose, and 2.8 micromolar. For glial cells, they used the same low dose alongside 6.3 micromolar. RNA was then extracted and hybridized to Agilent microarrays capable of interrogating more than 44,000 gene transcripts simultaneously, with all experiments performed in triplicate and quality-controlled through principal component analysis and correlation metrics.

The results in neuronal cells defied a simple dose-response logic. At the low 0.1 micromolar concentration, 798 genes shifted their expression, with 644 upregulated and 154 downregulated. Yet at the higher 2.8 micromolar dose, the number of differentially expressed genes actually fell to 572. The authors suggest this inversion may reflect a hormesis effect, a biological phenomenon in which low doses of a stressor provoke defensive signaling while high doses suppress the transcriptional machinery altogether. Seven genes changed more than threefold at the low dose, including Dcx, a marker of migrating neurons, and Trim14, while five genes crossed that threshold at the high dose. Only five overlapping genes appeared in both treatments, and they share a common thread: they participate in building glycosylphosphatidylinositol-anchored proteins, or GPI-APs, molecules that tether signaling proteins to the cell membrane and are essential for adhesion, migration, and immune responses. This is the first evidence that methylmercury disrupts GPI-anchored protein biology, opening a fresh window on how the toxin may sabotage membrane organization and signal transduction.

Functional annotation of the neuronal response revealed further surprises. At the low dose, differentially expressed genes were enriched for steroid dehydrogenase activity, specifically the enzyme 3β-hydroxysteroid dehydrogenase, which converts pregnenolone to progesterone and dehydroepiandrosterone to androstenedione in the brain. These neurosteroids are critical for neurodevelopment and neuroprotection, and the authors propose that methylmercury’s well-documented mitochondrial damage could starve this pathway of substrate, while oxidative stress disrupts the ERK/MAPK signaling that regulates the enzyme’s expression. No prior study had reported effects of methylmercury on 3β-HSD in the brain. The serotonin receptor pathway also emerged as a target, consistent with evidence that methylmercury disrupts neurotransmitter regulation and that mercury compounds reduce serotonin binding to its receptors in vitro.

At the higher neuronal dose, the picture shifted toward ion transport. Genes involved in transporter activity, particularly metal ion transport, dominated the enrichment analysis, alongside terms for ion channel complexes, neurogenesis, and nervous system development. Reactome pathway analysis confirmed that solute carrier proteins, or SLCs, were heavily implicated. These membrane transporters act as sentinels at the blood-brain barrier and maintain homeostasis within neurons and glia. Their disruption could allow metal ions to accumulate inside cells, generating oxidative stress and cascading damage to gene expression, protein synthesis, and enzyme activity. Studies in zebrafish and Nile tilapia have shown that mercury induces similar transporter upregulation in aquatic organisms, suggesting an evolutionarily conserved vulnerability.

The glial cells told a different story. At 0.1 micromolar, 812 genes changed expression, with a striking bias toward downregulation: 612 genes were suppressed against only 200 upregulated. The most prominent pathway affected was Rho-GTPase signaling, a molecular network that governs the actin cytoskeleton and cell morphology. This finding dovetails with prior work showing that methylmercury activates RhoA in astrocytes, causing process retraction and loss of the contact guidance that astrocytes provide to growing axons. At the high 6.3 micromolar dose, 872 genes shifted, and the analysis converged on a single critical molecular function: ATPase activity, specifically the sodium-potassium-exchange-ATPase. This pump is the workhorse of neuronal energy metabolism, maintaining the ion gradients that underpin every action potential. Its inhibition by methylmercury has been linked to oxidative stress, impaired neurotransmitter uptake of dopamine and glutamate, and even teratogenesis.

Perhaps the most compelling thread running through both cell types was the role of microRNAs, the short regulatory RNAs that fine-tune gene expression after transcription. Using the miRTarBase database of experimentally validated microRNA-target interactions, the researchers predicted which microRNAs regulate the differentially expressed genes. In neuronal cells, 23 microRNAs targeted 162 genes at the low dose and 14 targeted 97 genes at the high dose. In glial cells, 23 microRNAs regulated 314 genes at the low dose and 24 regulated 337 at the high dose. When these microRNAs were mapped onto KEGG signaling pathways, one target emerged above all others: the mitogen-activated protein kinase, or MAPK, pathway, which was impaired by methylmercury in both cell lines regardless of dose. The MAPK cascade, encompassing the p38, ERK, and JNK subfamilies, orchestrates cellular responses to stress, proliferation, and apoptosis, and methylmercury is known to hijack it to promote oxidative damage and inflammation.

A specific microRNA family, mmu-miR-466, appeared consistently across conditions, regulating more than ten signaling pathways including unsaturated fatty acid biosynthesis, GABAergic synapse function, and calcium signaling. This finding resonates with decades-old observations that developmental methylmercury exposure disrupts the fatty acid composition of myelin cerebrosides, and with more recent work in migratory birds showing that mercury perturbs lipid metabolism with consequences for flight performance. In glial cells at the high dose, three additional microRNAs, mmu-miR-1192, mmu-miR-495-3p, and mmu-miR-669c-3p, collectively regulated roughly 40 pathways, with axon guidance emerging as the most significant. Within that pathway, the microRNAs targeted receptors in the WNT signaling cascade, which directs growing axons toward their destinations during development.

To connect these molecular perturbations to human disease, the team performed Human Phenotype Ontology enrichment analysis, asking whether the altered genes correlated with documented clinical phenotypes. The results were sobering. Across both cell lines and all doses, the most enriched terms related to abnormalities of the nervous system, skeletal muscle, brain morphology, and higher mental functions. The low-dose glial exposure specifically flagged hypotonia, low intelligence quotient, and aplasia or hypoplasia of the central nervous system, while the high dose pointed to neurodevelopmental delays and disorders of brain morphology. These computational predictions align with the historical record, from the 1971-1972 Iraq poisoning epidemic, which established the exquisite vulnerability of the developing fetus, to modern dose-response analyses showing harm at even low prenatal exposures.

The study’s significance extends beyond the laboratory. Methylmercury enters the food web through microbial methylation of inorganic mercury in aquatic environments, a process amplified by artisanal gold mining, industrial wastewater, and agricultural runoff. Because it crosses both the blood-brain barrier and the placenta, contaminated fish and seafood remain the primary route of human poisoning, with consequences ranging from cerebellar ataxia and tremors to memory loss and impaired cognition. By demonstrating that non-lethal concentrations rewire gene expression in ways that map directly onto neurological disease phenotypes, this work strengthens the scientific case for stringent environmental regulation and public health protections, particularly for pregnant women and young children. It also hands toxicologists a new set of molecular markers, from GPI-anchored protein synthesis to the mmu-miR-466 family, that could serve as early-warning signals of neurotoxicity long before cell death begins.

Subject of Research: Transcriptomic effects of methylmercury exposure on rat neuronal and glial central nervous system cell lines

Article Title: Effects of methylmercury exposure on the transcriptomes of cell lines from the rat central nervous system

Article References: Bonfim, L. T., Amorim, C. K. N., de Oliveira, E. H. C., & Ferreira, W. A. S. (2025). Effects of methylmercury exposure on the transcriptomes of cell lines from the rat central nervous system. Discover Toxicology, 2(1), Article 25. https://doi.org/10.1007/s44339-025-00032-y

Image Credits: AI Generated

DOI: 10.1007/s44339-025-00032-y

Keywords: methylmercury, neurotoxicity, transcriptome, microRNA, glial cells, neurons, MAPK signaling, Na+/K+-ATPase, GPI-anchored proteins, oxidative stress, blood-brain barrier, neurodevelopment