The cytotoxicity of zinc oxide nanoparticles to 3D brain organoids results from excessive intracellular zinc ions and defective autophagy

Although the neurotoxicity of ZnO nanoparticles (NPs) has been evaluated in animal and nerve cell culture models, these models cannot accurately mimic human brains. Three-dimensional (3D) brain organoids based on human-induced pluripotent stem cells have been developed to study the human brains, but this model has rarely been used to evaluate NP neurotoxicity. We used 3D brain organoids that express cortical layer proteins to investigate the mechanisms of ZnO NP-induced neurotoxicity. Cytotoxicity caused by high levels of ZnO NPs (64 mu g/mL) correlated with high intracellular Zn ion levels but not superoxide levels. Exposure to a non-cytotoxic concentration of ZnO NPs (16 mu g/mL) increased the autophagy-marker proteins LC3B-II/I but decreased p62 accumulation, whereas a cytotoxic concentration of ZnO NPs (64 mu g/mL) decreased LC3B-II/I proteins but did not affect p62 accumulation. Fluorescence micro-optical sectioning tomography revealed that 64 mu g/mL ZnO NPs led to decreases in LC3B proteins that were more obvious at the outer layers of the organoids, which were directly exposed to the ZnO NPs. In addition to reducing LC3B proteins in the outer layers, ZnO NPs increased the number of micronuclei in the outer layers but not the inner layers (where LC3B proteins were still expressed). Adding the autophagy flux inhibitor bafilomycin A1 to ZnO NPs increased cytotoxicity and intracellular Zn ion levels, but adding the autophagy inducer rapamycin only slightly decreased cellular Zn ion levels. We conclude that high concentrations of ZnO NPs are cytotoxic to 3D brain organoids via defective autophagy and intracellular accumulation of Zn ions.

Automated summary

The neurotoxicity of ZnO nanoparticles on 3D brain organoids was studied. High concentrations of ZnO nanoparticles were found to be cytotoxic to the organoids via defective autophagy and intracellular accumulation of Zn ions.