Soluble Aβ Oligomers and Protofibrils Induce NLRP3 Inflammasome Activation in Microglia.

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder causing memory loss, language problems and behavioural disturbances. AD is associated with the accumulation of fibrillar amyloid-beta (Abeta) and the formation of neurofibrillary tau tangles. Fibrillar Abeta itself represents a danger-associated molecular pattern, which is recognized by specific microglial receptors. One of the key players is formation of the NOD-, LRR- and pyrin domain-containing 3 (NLRP3) inflammasome, whose activation has been demonstrated in AD patient brains and transgenic animal models of AD. Here, we investigated whether Abeta oligomers or protofibrils that represent lower molecular aggregates prior to Abeta deposition are able to activate the NLRP3 inflammasome and subsequent interleukin-1 beta (IL-1beta) release by microglia. In our study, we used Abeta preparations of different sizes: small oligomers and protofibrils of which the structure was confirmed by atomic force microscopy. Primary microglial cells from C57BL/6 mice were treated with the respective Abeta preparations and NLRP3 inflammasome activation, represented by caspase-1 cleavage, IL-1beta production, and apoptosis-associated speck-like protein containing a CARD speck formation was analysed. Both protofibrils and low molecular weight Abeta aggregates induced a significant increase in IL-1beta release. Inflammasome activation was confirmed by apoptosis-associated speck-like protein containing a CARD speck formation and detection of active caspase-1. The NLRP3 inflammasome inhibitor MCC950 completely inhibited the Abeta-induced immune response. Our results show that the NLRP3 inflammasome is activated not only by fibrillar Abeta aggregates as reported before, but also by lower molecular weight Abeta oligomers and protofibrils, highlighting the possibility that microglial activation by these Abeta species may initiate innate immune responses in the central nervous system prior to the onset of Abeta deposition.