SARS-CoV-2 spike S1 protein induces microglial NLRP3-dependent neuroinflammation and cognitive impairment in mice

Cognitive impairment is often found at the acute stages and sequelae of coronavirus disease 2019 (COVID-19), and the underlying mechanisms remain unclear. The S1 protein from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) might be a cause of cognitive impairment associated with COVID-19. The nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3 (NLRP3) inflammasome and neuroinflammation play important roles in Alzheimer's disease (AD) with cognitive impairment. However, their roles remain unknown in COVID-19 with cognitive impairment. We stimulated BV2 cells with S1 protein in vitro and injected the hippocampi of wild-type (WT) mice, NLRP3 knockout (KO), and microglia NLRP3 KO mice in vivo with S1 protein to induce cognitive impairment. We assessed exploratory behavior as associative memory using novel object recognition and Morris water maze tests. Neuroinflammation was analyzed using immunofluorescence and western blotting to detect inflammatory markers. Co-localized NLRP3 and S1 proteins were investigated using confocal microscopy. We found that S1 protein injection led to cognitive impairment, neuronal loss, and neuroinflammation by activating NLRP3 inflammation, and this was reduced by global NLRP3 KO and microglia NLRP3 KO. Furthermore, TAK 242, a specific inhibitor of Toll-like receptor-4, resulted in a significant reduction in NLRP3 and pro-IL-1β in BV2 cells with S1 protein stimulation. These results reveal a distinct mechanism through which the SARS-CoV-2 spike S1 protein promotes NLRP3 inflammasome activation and induces excessive inflammatory responses.