'Trojan-Horse' stress-granule formation mediated by manganese oxide nanoparticles

Exposure to nanomaterials is considered as one of the risk factors for neurodegenerative pathology. In vitro inorganic nanoparticles (NPs) absorb intrinsically disordered proteins, many of which are the constituents of stress-granules (SGs). SGs normally form in response to cellular stress and, here, we addressed whether selected inorganic NPs could trigger SGs formation in cells. To this end, we have tested a series of inorganic NPs for their ability to induce SGs formation in human glioblastoma and fibroblast cell lines. Among tested NPs, only Mn₃O₄ NPs triggered SGs formation in cell-type-specific and metabolic-dependent manner. In human glioblastoma U87 MG cell line, Mn₃O₄ NPs entered cells within minutes and resided inside intracellular vesicles for at least 48 h. Mn₃O₄ NPs induced a strong reduction in oxidative phosphorylation rate, but not glycolysis. We showed that Mn₃O₄ NPs slowly dissolve producing a local net of Mn²⁺ cations, which are known to inhibit oxidative phosphorylation. Indeed, direct incubation of cells with equimolar amounts of Mn²⁺ cations triggered SGs formation and reduced cellular respiration rate. However, while SGs formed in response to Mn₃O₄ NPs persisted for hours, SGs formation by Mn²⁺ peaked and dropped within minutes. Finally, Mn₃O₄ NPs mediated SGs formation via the phosphorylation of eIF2α. Thus, we conclude that exposure of U87 MG cells to Mn₃O₄ NPs caused a 'Trojan-horse' prolonged SGs response.