Polystyrene nanoplastics induce transient microglial activation via endolysosomal retention in the mouse cortex

Environmental degradation and accumulation of plastics results in micro- and nanoplastics that are small enough to cross biological barriers, including the blood-brain barrier. Microglia, resident immune cells of the brain, are critical regulators of neuroimmune homeostasis and represent a cellular target of nanoplastic exposure. In this study, we assessed the neurotoxic effects of two sizes of polystyrene nanoplastics (PS-NPs; 100 nm and 500 nm) using integrated in vivo and in vitro exposure and washout paradigms. In vivo exposure in mice (60 days; 1.5 mg/day) showed the presence of both PS-NPs sizes in the cerebral cortex without overt histopathological damage. However, cortical microglia showed pronounced morphological remodeling, assessed by Sholl and Skeleton analyses. Transcriptomic profiling of cortical tissue revealed a strong size-dependent response. The 100 nm PS-NPs group revealed 18 DEGs (|log₂FC= ≥ 2, padj < 0.05), whereas the 500 nm PS-NPs showed more than 4000 DEGs, including upregulation of immune- and microglia-associated genes (CCL5, CXCL10, LCN2, LYZ2) and downregulation of synaptic and neuronal signaling genes (GRIN2B, SYN1, STX1B, MAP1B, ITPR1/2). Using BV2 microglial cells, data indicate size dependent internalization of PS-NPs via the endolysosomal pathway. While both the 100 and 500 nm particles were present in late endosomes, only the 100 nm particles were found in lysosomes. Microglial activation markers (Iba1, CD68) exhibited a transient, size- and concentration-dependent increase, correlated with intracellular particle burden rather than cumulative exposure. Overall, these findings demonstrate that PS-NPs reached cortical regions of the brain, driving size-dependent microglial activation and transcriptomic reprogramming, even after cessation of exposure to PS-NPs.