Mechanisms Underlying the Size-Dependent Neurotoxicity of Polystyrene Nanoplastics in Zebrafish

Nanoplastics (NPs) are ubiquitous in the environment, posing significant threats to biological systems, including nervous systems, across various trophic levels. Nevertheless, the molecular mechanisms behind the size-dependent neurotoxicity of NPs remain unclear. Here, we investigated the neurotoxicity of 20 and 100 nm polystyrene NPs (PS-NPs) to zebrafish. Utilizing molecular dynamics simulations and complementary methods, we discovered that PS-NPs initiated neurotoxicity by promoting dimerization of the toll-like receptor 4/myeloid differentiation-2 (TLR4/MD-2) complex. This process involves the binding of PS-NPs to the hydrophobic pocket of MD-2, which induced the flipping of Phe-126 toward the dimer interface and the bending of the C-terminal domain of TLR-4, bringing the two domains into close proximity. Thereafter, the astrocytes and microglia were activated, initiating a cascade of events that include neuroinflammation, central nervous system cell apoptosis, inhibition of motor neuron development, and ultimately alteration of the swimming behavior of zebrafish. Further, 20 nm PS-NPs elicited more severe neurotoxicity than 100 nm PS-NPs, attributed to their higher accumulation in the brain as determined through ¹⁴C-labeled PS-NPs and more effective interaction with the TLR4/MD-2 complex. Overall, our study uncovers the mechanisms underlying the size-dependent neurotoxicity of NPs, which merit attention during their risk assessment and regulation.