Inflammatory response of microglia to prions is controlled by sialylation of PrPSc

Abstract

Neuroinflammation is recognized as one of the obligatory pathogenic features of neurodegenerative diseases including Alzheimer’s, Parkinson’s or prion diseases. In prion diseases, space and time correlations between deposition of disease-associated, pathogenic form of the prion protein or PrP Sc and microglial-mediated neuroinflammation has been established. Yet, it remains unclear whether activation of microglia is triggered directly by a contact with PrP Sc, and what molecular features of PrP Sc microglia sense and respond to that drive microglia to inflammatory states. The current study asked the questions whether PrP Sc can directly trigger activation of microglia and whether the degree of microglia response depends on the nature of terminal carbohydrate groups on the surface of PrP Sc particles. PrP Sc was purified from brains of mice infected with mouse-adapted prion strain 22L or neuroblastoma N2a cells stably infected with 22L. BV2 microglial cells or primary microglia were cultured in the presence of purified 22L. We found that exposure of BV2 cells or primary microglia to purified PrP Sc triggered proinflammatory responses characterized by an increase in the levels of TNFα, IL6, nitric oxide (NO) and expression of inducible Nitric Oxide Synthase (iNOS). Very similar patterns of inflammatory response were induced by PrP Sc purified from mouse brains and neuroblastoma cells arguing that microglia response is independent of the source of PrP Sc. To test whether the microglial response is mediated by carbohydrate epitopes on PrP Sc surface, the levels of sialylation of PrP Sc N-linked glycans was altered by treatment of purified PrP Sc with neuraminidase. Partial cleavage of sialic acid residues was found to boost the inflammatory response of microglia to PrP Sc. Moreover, transient degradation of Iκβα observed upon treatment with partially desialylated PrP Sc suggests that canonical NFκB activation pathway is involved in inflammatory response. The current study is the first to demonstrate that PrP Sc can directly trigger inflammatory response in microglia. In addition, this work provides direct evidence that the chemical nature of the carbohydrate groups on PrP Sc surface is important for microglial activation.