Amyloid β Clustering Around ASC Fibrils Boost Its Toxicity in Microglia

Abstract

Alzheimer’s Disease (AD) is the world’s most common cause of dementia. The exact mechanisms causing neurodegeneration are not fully understood but implicate the activation of microglia by amyloid-β (Aβ) deposits resulting in neuroinflammation. Aβ triggers the assembling of the microglial NOD-like receptor protein (NLRP3) inflammasome and induces the polymerization of the apoptosis-associated speck-like protein containing CARD (ASC). It has been shown in macrophages that ASC released during cell death propagates from one cell to another in an aggregated and fully functional protein state. We provide evidence that this is likewise true for microglia. Moreover, released ASC specks rapidly bind to Aβ and increase the formation of ASC-Aβ composites. Here, we investigated effects of these newly formed ASC-Aβ composites on microglial inflammasome activation, pyroptosis and Aβ phagocytosis using western blot, enzyme-linked immunosorbent assay (ELISA), fluorescence-activated cell scanning (FACS), viability and cytotoxicity assays etc. Therefore, we exposed primary microglia to ASC, Aβ_1-42 and ASC-Aβ_1-42 composites. We found that the protein composites amplified the expression of active caspase-1 and augment IL-1β levels to a greater extent than sole application of ASC or Aβ, thereby establishing evidence for a feed-forward stimulating vicious cycle. Furthermore, ASC-Aβ composites aggravate microglial pyroptotic cell death involving gasdermin D cleavage. Moreover, FACS analysis revealed that in the presence of extracellular ASC specks, the uptake and degradation of Aβ by microglia is decreased over time. Together, these data shed new light on the propagation of inflammasome activation and the turning point from acute to chronic Aβ related neuroinflammation through the formation of ASC-Aβ composites.

Keywords: ASC, Aβ, microglia, NLRP3 inflammasome, Alzheimer's disease