Stimulations of the Culture Medium of Activated Microglia and TNF-Alpha on a Scrapie-Infected Cell Line Decrease the Cell Viability and Induce Marked Necroptosis That Also Occurs in the Brains from the Patients of Human Prion Diseases

Activation of microglia and increased expression of TNF-α are frequently observed in the brains of human and animal prion diseases. As an important cytokine, TNF-α participates in not only pro-inflammatory responses but also in cellular communication, cell differentiation, and cell death. However, the role of TNF-α in the pathogenesis of prion disease remains ambiguous. In this study, the activities of a scrapie-infected cell line SMB-S15 and its normal partner SMB-PS exposed to the supernatant of a LPS-activated microglia cell line BV2 were evaluated. After it was exposed to the LPS-stimulated supernatant of BV2 cells, the cell viability of SMB-S15 cells was markedly decreased, whereas that of the SMB-PS cells remained unchanged. The level of TNF-α was significantly increased in the LPS-stimulated supernatant of BV2 cells. Further, we found that the recombinant TNF-α alone induced the decreased cell viability of SMB-S15 and the neutralizing antibody for TNF-α completely antagonized the decreased cell viability caused by the LPS-stimulated supernatant of BV2 cells. Stimulation with TNF-α induced the remarkable increases of apoptosis-associated proteins in SMB-PS cells, such as cleaved caspase-3 and RIP1, whereas an obvious increase of necroptosis-associated protein in SMB-S15 cells, such as p-MLKL. Meanwhile, the upregulation of caspase-8 activity in SMB-PS cells was more significant than that of SMB-S15 cells. The decreased cell viability of SMB-S15 and the increased expression of p-MLKL induced by TNF-α were completely rescued by Necrostatin-1. Moreover, we verified that removal of PrP^Sc propagation in SMB-S15 cells by resveratrol partially rescues the cell tolerance to the stimulation of TNF-α. These data indicate that the prion-infected cell line SMB-S15 is more vulnerable to the stimulations of activated microglia and TNF-α, which is likely due to the outcome of necroptosis rather than apoptosis. Furthermore, significant upregulation of p-MLKL, MLKL, and RIP3 was detected in the post-mortem cortical brains of the patients of various types of human prion diseases, including sporadic Creutzfeldt-Jakob disease (sCJD), G114 V-genetic CJD (gCJD), and fatal familial insomnia (FFI).